Immunoregulation in cancer-associated cachexia

Graphical abstract

Highlights

• •The inflammatory factors and additional mediators contribute to systemic inflammation involved in cachexia.
• •Immune checkpoints exert a modulatory effect in the cachectic development through regulating adipocyte differentiation and systemic metabolic balance.
• •The multilayered insights including the immunometabolic axis, immune-gut axis and immune-nerve axis in cachexia are elaborated.

Abstract

Background

Cancer-associated cachexia is a multi-organ disorder associated with progressive weight loss due to a variable combination of anorexia, systemic inflammation and excessive energy wasting. Considering the importance of immunoregulation in cachexia, it still lacks a complete understanding of the immunological mechanisms in cachectic progression.

Aim of review

Our aim here is to describe the complex immunoregulatory system in cachexia. We summarize the effects and translational potential of the immune system on the development of cancer-associated cachexia and we attempt to conclude with thoughts on precise and integrated therapeutic strategies under the complex immunological context of cachexia.

Key Scientific Concepts of Review

This review is focused on three main key concepts. First, we highlight the inflammatory factors and additional mediators that have been identified to modulate this syndrome. Second, we decipher the potential role of immune checkpoints in tissue wasting. Third, we discuss the multilayered insights in cachexia through the immunometabolic axis, immune-gut axis and immune-nerve axis.

Introduction

As a multiorgan clinical syndrome, cachexia results in involuntary weight loss and systemic inflammation [1]. Cachexia commonly occurs in combination with aggressive cancers and multiple chronic diseases, such as kidney disease, cardiovascular disease, chronic infections, neurological disease or chronic obstructive pulmonary disease [2], [3]. As a multifactorial syndrome, cancer-associated cachexia (CAC) is defined as a continuing loss of skeletal muscle mass (with or without loss of fat mass) that cannot easily be reversed by a sufficient supply of nutrients and is caused by anorexia, excessive catabolism and high energy expenditure [4], [5]. Muscle wasting could be attributed to decreased muscle protein synthesis and increased protein breakdown. The regulation of skeletal muscle mass is a complex process that involves multiple signaling pathways and molecular mechanisms. Myogenesis is the process by which new muscle fibers are formed, and it is regulated by a family of transcription factors known as myogenic regulatory factors (MyoD, Myf5, Myogenin, and MRF4) [6]. Several studies have shown that the expression of MRFs is reduced in cachexia, which contributes to the loss of muscle mass. In addition to its negative mental health implications, severe emaciation caused by cachexia can often progress to a debilitating state in which the patient becomes unable to carry out their basic daily activities. While this definition of cachexia presented here is relevant to CAC and illustrates the distinctive characteristics of cachexia from a clinical and metabolic perspective, it does not mention the immunological aspects of cachexia. In reality, an inflammatory response is clearly responsible for cachexia. In this review, we describe emerging immunological insights into CAC, including immunoregulatory cytokines and peptides, immune checkpoints, immunometabolic imbalances and crosstalk signals between the immune system and other organs. An improved understanding of immune modulation in CAC may guide the design and implementation of new therapeutic approaches. (Fig. 1; Table 1).

Fig. 1.

Immunoregulation in cancer-associated cachexia.

Table 1.

Summary of cytokines involved in cancer cachexia and their underlying mechanisms.

Mediator	ReceptorMain Cellular SourceMechanism			Major Activities	References
IL-1α	IL-1R1	Neutrophil; necrotic cellsIndirect inhibition of melanocortin-4		skeletal muscle proteolysis; Lipolysis; Diminished appetite	12
IL-1β	IL-1R1	MicrogliaActivate the hypothalamic–pituitary–adrenal axis		Muscle atrophy	13
IL-18	IL-18R	Different cell types in theRecruitment of NLRP3 inflammasomes; Impair the periphery;microgliaactivity of Type III GABAergic neurons		Sarcopenia; Inflammatory in the adipose tissues; Reduced ingestive behavior	14,15,16
IL-6	IL-6R	A AMPK nuclear signaling pathway induced by IL6 Tumor cells; adipocyte; B cellsconverges on FoxO3 transcription factor		Lipolysis; muscle atrophy	19,20,21,22,2 3
IL-27	IL-27R	Syncytiotrophoblasts;Induce p38 MAPK-PGC-1α signaling and increase hepatocytes; monocyteUCP1 production		promotes adipocyte thermogenesis and energy expenditure	24
OSM	OSMR	LeucocytesInduce the phosphorylation of STAT3		Lipolysis	25
LIF	LIFR	Tumor cell; glandular and luminal Decrease leptin signaling and trigger G-CSF and IL-6 cells; fibroblastsproduction		Lipolysis	26
IL-20	IL-20R	Monocytes, dendritic cells, epithelial cells, and endothelial cells	HSL and ATGL production	Lipolysis	27,28
TNFα	TNFR	Macrophages and adipocytes	Decrease G0S2 levels; Activate NF-κB; Induce ZIP14	Lipolysis; muscle atrophy; Inhibit myoblast differentiation	14,29–37
TWEAK	Fn14	Adipocytes	Increase the expression of E3 ubiquitin ligase MuRF1	Weight loss; muscle proteolysis	38-40
IFNγ	IFNγR	Immune cells	Induce the phosphorylation of STAT3	Lipolysis; muscle atrophy	20,41–44
DAMPs	PRRs;TLRs;RLRs;NLR s;CLRs	Tumor cells;immune cells	Cause systemic inflammation	Muscle proteolysis; lipolysis	8,9,45–50
TGFβ	TGFβR	Bone matrix cells;tumor cells;adipocyte	Promote Nox4 expression;inflammation-mediated lipolysis	Muscle weakness; lipolysis	52,54,55
TSC22D4		hepatocyte	Impaired lipogenic gene expression	hypo-secretion of hepatic VLDL release and a decrease in body weight	53
GDF8	ActRIIB	Skeletal muscle cells	Inhibit myogenesis	Muscle loss	58,59
GDF11	TGFβR	Cone photoreceptor cells; bipolar cells; horizontal cells; rod photoreceptor cells	Activation of SMAD2; autophagy	Total body wasting; skeletal muscle impairment	56
GDF15	GFRAL	Syncytiotrophoblasts; urothelial cells; club cells; neutrophil	Decrease dietary intake and enhance ATGLdependent sympathetic activation of adipose tissue lipolysis	Weight loss; lipolysis	57,60–62
Activin A	ACVR2A or ACVR2B	wide range of cell types	Cause SMAD2 and SMAD3 transcription factors to dimerize and phosphorylate; Induce the expression of the Noggin	Muscle loss	63–67
PTHrP	PTHR	wide range of cell types	PKA activation; modulate the leptin or hypothalamic feeding-regulated peptides	Adipose tissue browning; muscle loss and adipose loss	71,76–79
leptin	Lepr			Muscle atrophy	82
ZAG		Adipocytes; epithelial cells		lipolysis	80,85
Musclin		skeletal muscle cells	Inhibition of FOXO transcriptional activity	Muscle growth and regeneration	88
Myostatin	ALK4/5ActR2	skeletal muscle cells	Promote phosphorylation of Smad2/3, thereby promoting FOXO transcriptional activity.	Muscle atrophy	89
FGF21	FGFR	liver, skeletal muscle cells	Activate the PI3K/AKT/mTOR signaling pathway	Muscle growth	90,91
Irisin		Irisin		Muscle growth	92,93
ACBP	GABAA	Proximal tubular cells; hepatocytes	Activation of orexigenic neurons by ACBP in the hypothalamus; ACBP binding to acyl-CoAs facilitates FAO	Stimulate fat metabolism and appetite	86-90
PD-L1	PD-1	Cancer-associated adipocytes; DCs	Promote browning and inflammation	Induce lipidolysis and suppress lipogenesis	6,91,92
B7-H3	TLT-2	Progenitors; fibroblasts; immune cells		Control adipogenesis and regulates adipocyte differentiation	105,106
VSIG4	C3b; iC3b	Macrophage		Strongly correlated with physiological frailty index (PFI, a multi-parameter assessment of health)	107–111
ATP	P2X7		Promote a Th17-polarizing microenvironment with high levels of IL-1β, IL-6, and IL-17	Drive the induction of lipidolysis and inflammation	112–114

Cytokines

The inflammatory response is a key contributor to the metabolic changes associated with CAC, which is considered a hallmark of cancer [7]. This systemic inflammation is triggered by a range of cytokines released by both tumour cells and immune cells, including both inflammatory and non-inflammatory mediators. In the following section, we will provide a summary of the current knowledge about the role of cytokines in the development of CAC.

Inflammatory factors

Tumour necrosis factor α (TNFα), interferon γ (IFNγ), interleukin-1 (IL-1) family, IL-6 family, and IL-20 are cytokines that are consistently upregulated in cachexia, according to numerous studies conducted in mouse models and in patients with cachexia [9]]. These inflammatory mediators are expressed abundantly by both immune and non-immune cells when their pattern-recognition receptors are activated by damage-associated molecular patterns and/or pathogen-associated molecular patterns [10], [11]. Activation of the JAK–STAT and NF-κB signalling pathways, and downstream transcriptional regulation by cytokines can induce various catabolic pathways in muscles and adipose tissue [4], [7], [12]. The activation of these signaling pathways can result in mitochondrial dysfunction, promote a burst of reactive oxygen species (ROS), and further activate the NF-κB, NLRP3, JNK, MAPK, and PI3K pathways, leading to sustained inflammation [13]. (Fig. 2).

Fig. 2.

Summary of the main altered cytokines identified in cancer cachexia. (A) The body wasting observed in cancer cachexia is triggered by various cytokines that are produced by tumor cells, immune cells, and stromal cells present in the tumor microenvironment. These cytokines, such as TNF-α, IL-20, LIF, OSM, IGF1, PTHrP, and ACBP, can cause tissue breakdown and fat loss by modifying the activity of genes that are involved in fat metabolism. Additionally, certain immunometabolic checkpoints, such as VSIG4, B7-H3, and PD-L1, can also contribute to the loss of body fat. PTHrP and IL27 signals can stimulate the generation of heat in fat cells, which is known as thermogenesis. (B) Muscle wasting in cancer cachexia is a complex process caused by a variety of factors. Some cytokines, such as IL-6, TNFα, TWEAK, and DAMPs, activate specific receptors and promote muscle breakdown. Other cytokines, including IFNγ, TNFα, and members of the TGFβ family (such as GDF8, GDF11, activinA, and TGFβ), can affect muscle growth and development by inhibiting muscle cell differentiation or inducing muscle weakness. Mitochondrial dysfunction is also known to contribute to muscle wasting by increasing oxidative stress and protein degradation. Additionally, some cytokines, such as IL-1β, can activate the HPA xia pathway, resulting in the loss of muscle mass. On the other hand, there are cytokines, such as Musclin, that inhibit muscle atrophy by suppressing the activity of the FOXO transcription factor. In contrast, myostatin promotes muscle atrophy by enhancing FOXO activity. Other cytokines, such as FGF21 and Irisin, can stimulate muscle growth by activating specific signaling pathways. Finally, several cytokines, including IL-1α, IL-18, LCN2, ACBP, and GDF15, can regulate feeding behavior and contribute to cancer cachexia.

IL-1 family. The IL-1 family is a group of 11 cytokines that regulate the immune response and inflammation. IL-1α, IL-1β, and IL-18 have been found to have a relationship with cancer cachexia (CAC). IL-1α and IL-1β are primarily produced by activated macrophages and are involved in various biological activities, including inducing fever, promoting inflammation, and activating the immune system. IL-18 is produced by activated macrophages and dendritic cells and stimulates the production of interferon-gamma (IFN-γ), which plays a crucial role in the immune response to intracellular pathogens. IL-1α and IL-1β exacerbate CAC by triggering a set of common signaling pathways through interaction with IL-1 receptor 1 (IL-1R1), which is expressed on a range of cell types [14]. IL-1α accelerates skeletal muscle proteolysis and fat loss in adipose tissue, leading to diminished appetite, possibly via indirect inhibition of melanocortin-4 (MC4-R) [15]. By activating the hypothalamic–pituitary–adrenal axis and glucocorticoid production in the hypothalamus, IL-1β also engages in excessive catabolism in both muscle tissues and adipose tissues in a manner similar to IL-1α [16]. IL-1β can elicit muscle atrophy when administered intracerebroventricularly. This is associated with upregulated factors that contribute to muscle catabolism, such as forkhead box protein O1 (FOXO1), E3 ubiquitin-protein ligase TRIM63, and muscle atrophy F-box protein 32 (FBXO32), loss of muscle mass and reduction of fiber cross-sectional area [16]. There was no muscle loss in response to an intraperitoneal injection of IL-1β [16], suggesting that IL-1β exerts its effects primarily by stimulating the hypothalamic–pituitary–adrenal axis. Notably, comparison of tissue mass with mice fed in pairs revealed that the reduction in fat mass following IL-1β treatment was linked to food intake, whereas deficiency in muscle mass was unrelated to food intake [16]. Sarcopenic patients have elevated expression of IL-18 in their serum, which belongs to the IL-1 family of cytokines [17]. It has been found that the stimulation of IL-18 triggers the recruitment of NLRP3 inflammasomes in the adipose tissues of patients with cancer cachexia [18]. Additionally, ingestive behaviour is repressed by IL-18 by impairing the activity of Type III GABAergic neurons in the bed nucleus of the stria terminalis in a presynaptic way [19].

IL-6 family. A variety of cytokines from the IL-6 family have been linked to CAC pathophysiology, including IL-6, IL-27, leukaemia inhibitory factor (LIF), and oncostatin M (OSM) [20], [21]. Exercise-induced secretion of IL-6 promotes skeletal muscle differentiation and proliferation, enhancing muscle repair and regeneration [22]. However, in the context of cancer cachexia, IL-6 becomes a major pro-cachectic factor in adipose tissue, muscle, and circulation [23], [24], [25], [26]. Clinical data and mouse models show that tumor tissues expressing increased levels of IL-6 are more aggressive and the tumor hosts have a shorter survival time [24]. Injecting pancreatic tumor cells lacking IL-6 expression into mice reduced fat loss and prevented muscle atrophy [25]. Furthermore, increased UCP1 expression in white adipose tissue (WAT) leads to enhanced lipolysis and energy consumption in cachectic mice exposed to IL-6 [23], [27]. A feedforward loop in the catabolism of IL-6 signalling was also demonstrated in vivo and in vitro between the tumor, adipose tissue, and muscles [25]. Mechanistically, an AMPK nuclear signalling pathway induced by IL6 converges on the FoxO3 transcription factor to expedite muscle atrophy involved in cachexia through the bromodomain and extraterminal domain (BET) protein BRD4. JQ1, an anti-BET inhibitor, reduced the cachectic phenotype [26]. Additionally, IL-27/IL-27Rα signalling can adversely affect energy expenditure in adipose tissues, and IL-27 directly contacts adipocytes, inducing p38 MAPK-PGC-1α signalling and increasing UCP1 production [28]. Furthermore, OSM secreted by leukocytes in cancer-associated adipose tissue induced the phosphorylation of STAT3 in breast cancer cells, as well as the transcription of several S100 family members, including S100A7, S100A8, and S100A9, promoting vascularization of the peritumoral tissue and invasion [29]. Finally, LIF, a tumour-derived factor, induces lipolysis of adipocytes and indirectly decreases leptin signalling, leading to cachexia-like adipose loss [30]. Additionally, tumour-derived LIF accelerates the cachectic process by triggering G-CSF and IL-6 production in the serum [30].

IL-20. IL-20 is a proinflammatory cytokine mainly produced by hematopoietic cells, which facilitates communication with epithelial cells and plays a crucial role in activating the natural defense and repair processes in epithelial surfaces [31]. Pancreatic cancer patients often have high levels of IL-20, which correlates with tumor fibrosis, PDL1 expression, and poor survival in pancreatic ductal adenocarcinoma (PDAC) mouse models [32]. In mouse models of Lewis lung carcinoma (LLC) and PDAC-induced cachexia, anti-IL-20 treatment prevented the reduction in body weight and restrained adipose tissue loss by decreasing hormone-sensitive lipase (HSL) and ATGL expression. However, this treatment did not improve muscle atrophy, suggesting that IL-20 exerts its pro-cachectic effects via its effects on adipose tissue [32].

TNFα. TNFα plays a crucial role in both homeostasis and cachectic pathogenesis. Studies have shown that elevated levels of TNFα in plasma, fat, and skeletal muscle are associated with cachectic progression [17], [33], [34], [35]. Additionally, the promoter region of the TNF-α gene contains multiple polymorphic sites, including the rs1799964 polymorphic site. The rs1799964 creates a stronger binding site for transcription factors, leading to increased TNF-α expression. The rs1799964 polymorphism has been studied in relation to a number of different diseases and conditions, and therein lies a correlation between rs1799964 and cachexia [36]. Experiments conducted in adipocytes have demonstrated that TNFα triggers adipose triglyceride lipase (ATGL)-mediated lipolysis by decreasing G0/G1 switch protein 2 (G0S2) levels, the ATGL-inhibitory protein [37]. Additionally, TNF-induced activation of NF-kappaB inhibits skeletal muscle differentiation by suppressing MyoD mRNA at the posttranscriptional level, leading to protein breakdown and inhibiting myoblast differentiation [38], [39], [40]. Further validation of the results was demonstrated in mice injected with tumour cells overexpressing TNFα, which suffered from severe weight loss, lipolysis, and muscle atrophy [41]. Mechanistically, the metal-ion transporter ZRT- and IRT-like protein 14 (ZIP14) is induced by TNF-α, resulting in a loss of myosin heavy chain in differentiated muscle cells due to the accumulation of zinc [42]. Chronic kidney disease patients exhibit higher TNFα expression due to upregulated TLR4, which triggers muscle inflammation [35]. TWEAK, another pro-cachexia cytokine, is a member of the TNF family. Elevated TWEAK in tumours interacts with its receptor Fn14 to exacerbate cachexia, while the neutralizing antibody against TNFRSF12A resulted in weight loss prevention and extended lifespan in a mouse model [43]. Mechanistically, Fn14 transcription is inhibited by Dnmt3a, yet MAPK signalling stimulates its expression in skeletal muscle. Additionally, the connection between TWEAK and Fn14 induces muscle proteolysis by increasing the expression of the E3 ubiquitin ligase MuRF1 [44], [45]. In vivo and in vitro analysis of cachexia induced by Lewis lung carcinoma showed reduced rates of muscle ATP synthesis and mitochondrial electron flow, accompanied by increased TNF-α levels, which can disrupt mitochondrial function and lead to muscle wasting [46], [47], [48].

IFNγ. IFNγ is a crucial contributor to the progression of cachexia. Studies have shown that IFNγ is highly expressed in patients or mice with CAC [49], [50], [51]. During cancer cachexia, there is a significant increase in IFNγ and its receptors, IFNγR1 and IFNγR2, in mesenteric white adipose tissue (WAT) in mouse models [51]. Notably, mouse models with tumour cell lines overexpressing IFNγ experience severe muscle loss, whereas blocking IFNγ improves cachectic phenotypes [24]. In addition, osteopontin, a cytokine, enhances IFNγ expression. In a prostate cancer (PCa) mouse model, the expression system for the signalling adaptor p62/Sqstm1 is specifically inhibited in adipocytes, and p62 deficiency in adipocytes promotes the secretion of osteopontin, which contributes to aggressive metastatic PCa in vivo through fatty acid oxidation and invasion. Moreover, impaired p62 in adipocytes inhibits mTORC1 activity, reducing general energy expenditure, and supporting cancer cells in gaining access to nutrients. These findings indicate that p62 is a key factor in adipocyte-tumour symbiosis, which facilitates cancer metabolic fitness [52].

DAMPs. In cancer, as well as in the immune system, the release of danger-associated molecular patterns (DAMPs) can occur as a result of injury. These DAMPs include molecules such as high mobility group Box 1 (HMGB1), DNA, RNA, mitochondrial DNA (mtDNA), and heat shock proteins [10], [53]. Additionally, extracellular vesicles (EVs) can be packaged with DAMPs to facilitate interorgan or intercellular communication [4], [10]. Several endogenous pattern recognition receptors (PRRs), including Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs), and C-type lectin receptors (CLRs), can identify these DAMPs [11]. In cancer cachexia, the release of DAMPs into the bloodstream can cause muscle proteolysis by directly interacting with TLRs on muscle cells or acting on the TLR4 receptor on immune cells, resulting in systemic inflammation and indirectly stimulating muscle proteolysis [54]. TLR4 is an isoform that is primarily responsible for LLC-cancer-related muscle proteolysis [55]. Consequently, a low skeletal muscle index and weight loss are significantly associated with high TLR4 expression in the skeletal muscles of cancer patients [36]. Interestingly, TLRs are relatively isoform- and disease-specific players in cancer-induced muscle catabolism. For example, activation of TLR7 in the muscles by tumour-secreted microvesicles can result in necrosis of skeletal muscle cells [56], while the activation of TLR7 induces CD8 + T cells in the tumour stroma, leading to reduced tumour size, improved cachexia, and better survival [57], [58].

Several catabolic processes have been demonstrated to be stimulated by proinflammatory cytokines across multiple organs in experimental models of cancer-associated cachexia (CAC). However, clinical trials aimed at targeting these cytokines have yielded unsatisfactory outcomes. It seems that cachexia is not solely caused by cytokines but rather a response to a combined immune and metabolic disturbance, which is part of a more intricate coordinated reaction.

Noninflammatory factors

Based on current research, it has been found that besides inflammatory mediators, other molecules involved in the immune system are also associated with metabolic abnormalities in patients with chronic inflammatory conditions. These molecules are not solely produced by immune cells but are also released by other types of cells such as adipocytes, intestinal cells, hepatocytes, and cardiomyocytes under conditions of inflammation [1]. In this discussion, we will examine the current understanding of these peptides that affect food intake or energy balance. (Fig. 2).

TGFβ family. Several aspects of cancer are influenced by the activation of transforming growth factor-β (TGFβ) signalling, including immunoregulation, cell cycle arrest, and tumour metastasis [59]. TGFβ also promotes the expression of NADPH oxidase 4 (Nox4) in cancer-associated muscle weakness, enhancing the oxidation of the calcium (Ca2 + ) release channel (RyR1) and ryanodine receptor. RyR1 oxidation causes Ca2 + leakage and impaired muscle contraction. Therefore, dysfunctional TGFβ inhibits Ca2 + -induced muscle force production and exacerbates muscle weakness [60]. The hepatic metabolic dysfunction induced by tumour cells is thought to contribute to cancer cachexia phenotypes, including a reduction in very-low-density-lipoprotein (VLDL) secretion and hypobetalipoproteinemia. The liver of cachectic mice exhibits elevated transcription factor beta 1-stimulated clone (TSC) 22 D4 levels, leading to hyposecretion of hepatic VLDL release and a decrease in body weight [61]. Through the enhancement of TGFβ in a mouse model and clinical trials, fibroblasts may be transdifferentiated into myofibroblasts, adipogenesis can be inhibited, and lipofibrosis can be facilitated, consequently inducing uncontrolled inflammation-mediated lipolysis [62], [63]. Growth differentiation factors (GDFs), members of the TGFβ family, regulate nutritional imbalance during cachectic pathogenesis through GDF8, GDF11 and GDF15 [64], [65], [66]. It is thought that myostatin (GDF8) plays a role in muscle loss [66]. It mediates the signalling process by binding to activin receptor type IIB (ActRIIB). Cancer cachexia was eliminated in tumor-bearing mice following treatment with a soluble form of ActRIIB to block GDF8 and other TGFβ family members [67]. Additionally, GDF11 exposure in mice causes total body wasting and profound cardiac and skeletal muscle impairment, dysfunction, and death through activation of SMAD2, the ubiquitin–proteasome pathway, and autophagy [64]. A pro-cachexia mechanism is mediated by GDF15 binding to its receptor GDNF family receptor α-like (GFRAL), only found in neurons of the area postrema and nucleus tractus solitarius in the brainstem [65]. Patients with CAC have elevated levels of GDF15, which contributes to weight loss by decreasing dietary intake and enhancing ATGL-dependent sympathetic activation of adipose tissue lipolysis through ATGL [9], [68], [69]. Furthermore, the therapeutic antagonistic monoclonal antibody 3P10 targets GFRAL to reduce inordinate lipolysis and defend against cancer-associated cachexia in cancer-bearing mice. Mechanistically, activating the GFRAL–Ret proto-oncogene (RET) pathway in adipose tissue induces the expression of genes involved in lipid metabolism, and both adipose triglyceride lipase deficiency and chemosurgical peripheral sympathectomy protect mice against weight loss caused by GDF15 [70]. The growth factor Activin A, a member of the TGFβ superfamily, is produced both in tumors and immune cells [9]. Upon interaction between Activin A and type II receptors, primarily myostatin and ACVR2B in skeletal muscle, SMAD2 and SMAD3 transcription factors are phosphorylated and dimerized, then translocated from the cytoplasm to the nucleus to regulate gene expression [71], [72]. In cultured myotubes, application of Activin A results in atrophy, while in a mouse model with Activin A overexpression, weight loss and loss of skeletal muscle occur [72]. Patients with CAC have elevated levels of circulating Activin A, leading to muscle wasting [72], [73]. Interestingly, Activin A can synergistically accelerate CAC by promoting IL-6 expression and secretion in an autocrine manner [74]. Activin A and IL-6 induce the expression of Noggin in muscle, which can block the effects of bone morphogenetic protein (BMP) on motor nerves and muscle fibers, causing disruption of the neuromuscular junction (NMJ), denervation, and muscle degradation [75].

PTHrP. Parathyroid hormone-related protein (PTHrP) is a small molecular peptide that shares similar biological activities and homology with the amino terminus of PTH [76]. In patients with CAC, PTHrP is commonly expressed as a part of the KRAS amplicon or as an activator of the epidermal growth factor receptor (EGFR) [77], [78]. PTHrP, together with its downstream target osteopontin promotes tumor growth in both primary and metastatic tumors in mice, with an enriched expression pattern in pancreatic ductal adenocarcinomas [78]. Interestingly, PTHrP can be secreted by tumor cells in extracellular vesicles [79]. The role of PTHrP in the immune response remains unknown and requires further investigation. PTHrP and vitamin D synergistically increase cathelicidin and immune defense against infection [80]. In melanoma, PTHrP is associated with hypercalcemia and immune checkpoint inhibitor (ICI) failure, which can be treated with dacarbazine [81]. Tumor cells expressing increased levels of interleukin-6 (IL-6) are more aggressive and show increased serum PTHrP levels, which independently predict cancer-associated weight loss [82]. The parathyroid hormone type 1 receptor (PTHR), which is the canonical G protein-coupled receptor (GPCR) for PTHrP and PTH, is one of the main regulators of bone turnover and calcium homeostasis. PTHR is essential for human health for the maintenance of Ca2 + homeostasis and exhibits several unusual signaling features, including endosomal cAMP signaling [83]. An adipocyte-specific knockout of PTHR prevents browning and wasting of adipose tissue mediated by PTH. Surprisingly, fat loss and increased strength were also associated with the loss of PTHR in fat tissue. Similarly, tumour-induced cachexia was not observed in PTHR knockout mice. It has been hypothesized that both PTHrP and PTH promote wasting via a common pathway involving PTHR, as well as an unexpected cross-talk mechanism between wasting of skeletal muscle tissues and fat tissues [84]. Tumor-derived PTHrP stimulates adipose tissue browning via PKA activation in cachectic mice with Lewis lung carcinoma and clear cell renal cell carcinoma [79], [85], [86]. Moreover, PTHrP induces cachectic syndromes by independently modulating leptin or hypothalamic feeding-regulated peptides [87].

Adipokines. Cancer-associated adipocytes (CAAs) have been shown to alter their secretion profile, thereby remodeling the immune microenvironment in cancer cachexia [7], [12], [88]. The adipokines secreted by CAAs, such as leptin, asprosin, resistin, apelin, and zinc-α2-glycoprotein (ZAG), have been extensively reviewed in the context of CAC [89]. Leptin, for instance, has been found to be augmented in sarcopenic patients [17]. Studies on zebrafish hepatocellular carcinoma (HCC) models driven by KRAS have also shown that leptin promotes muscle wasting [90]. However, a systematic review revealed no significant difference in leptin levels between cancer cachexia patients and noncachectic or healthy individuals [34]. Moreover, restoring insulin-like growth factor 1 (IGF1) to mice with a leptin-mutant background was found to alleviate the cachexia-like phenotype without affecting tumor growth. Additionally, the Stat3/Socs3 inhibitor napabucasin reversed insulin resistance and prevented wasting in non-tumor tissues [91]. These findings suggest that leptin and other cytokines work synergistically in the treatment of CAC. Furthermore, ZAG levels have been found to be elevated in both animals and cancer patients during cancer cachexia [92]. ZAG, produced by adipose tissue, is inversely related to adiposity and stimulates lipolysis in white adipose tissue (WAT) [88], [93].

Myokines. A variety of myokines can be produced by skeletal muscle cells, and they exert their biological functions via endocrine, autocrine, and paracrine signaling pathways. Studies have shown that myokines such as musclin, myostatin, Fibroblast Growth Factor 21 (FGF21) and irisin are dysregulated in cachexia and contribute to muscle wasting. In animal studies, a decrease in musclin levels has been observed during cancer cachexia, suggesting that this myokine may play a protective role against muscle wasting. Additionally, administering musclin to mouse models of cancer cachexia has been demonstrated to improve muscle mass and function, potentially via its ability to inhibit the expression of genes associated with muscle atrophy [94]. Conversely, myostatin, a member of the TGF-β family, binds to the activin receptor type IIB (ActRIIB) on muscle cells, leading to the phosphorylation of smad2/3 and the expression of genes associated with skeletal muscle atrophy [95]. While the liver is typically recognized as the primary site of FGF21 production, there is evidence to suggest that FGF21 may also be produced in muscle tissues in response to stress. FGF21 can activate the AMPK pathway, which plays a critical role in regulating cellular energy metabolism. This activation leads to increased glucose uptake and fatty acid oxidation in muscle cells, which can help to provide the energy needed for muscle regeneration [96], [97]. Irisin is a protein produced by skeletal muscle cells in response to exercise, derived from the cleavage of Fibronectin Type III Domain Containing 5 (FNDC5). While its exact mechanism for regulating muscle cachexia is not yet fully understood, it appears to involve inhibition of inflammatory cytokines and activation of the AMPK pathway, resulting in increased mitochondrial function that may prevent muscle mass and strength loss [98], [99]. Besides, skeletal muscle extracellular matrix (ECM) also plays a critical role in regulating muscle mass. The ECM is a complex network of proteins that provide structural support to muscle fibers and facilitate communication between cells. Changes in the composition and organization of the ECM can impact muscle function and contribute to the development of cachexia. For example, increased fibrosis and deposition of collagen in the ECM have been observed in cachexia, which can impair muscle function and contribute to muscle wasting [100], [101]. Growth factors such as insulin-like growth factor 1 (IGF-1), hepatocyte growth factor (HGF) and platelet-derived growth factor (PDGF) also play critical roles in skeletal muscle regulation and myogenesis. As discussed previously, dysregulation of these growth factors can contribute to the development of cachexia [102], [103].

ACBP. Acyl-CoA-binding protein (ACBP), also known as diazepam-binding inhibitor (DBI), has been found to stimulate both fat metabolism and appetite [104]. The activation of orexigenic neurons in the hypothalamus by ACBP, released from tissues in an autophagic manner and elevated in plasma, leads to enhanced feeding behavior [104]. Functionally, ACBP binds to acyl-CoAs, facilitating fatty acid oxidation (FAO), and subsequently promoting glioblastoma cell proliferation [105]. Additionally, CRISPR-based loss-of-function screens have demonstrated that ACBP is a positive regulator engaged in the proliferation and activation of primary human CD4 + and CD8 + T cells [106]. Low levels of ACBP are commonly observed in dyscratic mice and in patients with anorexia, often associated with CAC [107]. Furthermore, cancer patients with advanced disease, as well as those receiving chemotherapy, often experience undernutrition due to elevated levels of circulating ACBP [108]. Knockout or neutralization of ACBP has been shown to result in excessive lipidolysis [107]. Taken together, ACBP analogues may prove to be practical drugs for treating patients with CAC.

After reviewing the current evidence, it appears that the induction of cytokines in cachexia is highly context-dependent and may contribute to the development of this condition. However, the precise mechanisms by which pro-cachectic and anti-cachectic cytokines are involved in cachexia remain an area of active research and are not yet fully understood. Further investigation is needed to gain a deeper understanding of the roles of these cytokines in cachexia.

Immunometabolic checkpoints

Recent studies have suggested that immune checkpoints play a role in adipocyte differentiation and maintaining systemic metabolic balance in addition to cytokines derived from immune and nonimmune cells. In this section, we will discuss the role of immune checkpoints and their potential contribution to cachexia. (Fig. 2).

PD-L1. Antitumor immunity is influenced by the interaction between programmed death-ligand 1 (PD-L1) and programmed cell death protein 1 (PD-1), which are key targets for checkpoint blockade immunotherapy. PD-L1 is expressed in cancer-associated adipocytes that exhibit a browning/beige phenotype and enhances various catabolic processes as a precursor [7], [109]. Ablation of PD-L1 in the whole body results in increased inflammation and exacerbates diet-induced obesity [110], [111]. Interestingly, PD-L1 is specifically distributed in brown adipocytes and activates them to accelerate lipidolysis and energy expenditure independently in response to cold exposure or β-adrenergic activation [112]. Similarly, pharmacological inhibition of adipogenesis selectively reduces adipose tissue expression of PD-L1 [109]. This probably explains why PD-L1 deletion in the whole body leads to weight gain, in which PD-L1 may induce lipidolysis and suppress lipogenesis. In fat tissues, the interaction between dendritic cells (DCs), type 2 innate lymphoid cells (ILC2s), and T cells through the PD-1/PD-L1 axis promotes type 2 polarization and inhibits T helper type 1 proliferation. Functional ablation of PD-L1 on DCs increases weight and promotes inflammation during diet-induced obesity [111]. Therefore, immune cells such as DCs regulate adipose tissue homeostasis by expressing PD-L1. Clinical trials have shown that obese patients respond better to PD-1/PD-L1 inhibitors than those with cachexia [113], [114], [115], [116], [117]. In a mouse model of breast cancer, researchers found that anti-PD1 therapy in combination with exercise training can restore skeletal muscle mitochondrial number and function, thereby improving cancer cachexia [118]. However, treatment with checkpoint inhibitors exacerbates weight loss in obese patients or mice and may cause lipodystrophy [111], [119], [120]. Similarly, treatment with immune checkpoint inhibitors can lead to myositis, including myocarditis and myasthenia gravis [121], [122]. A potential mechanism is that PD-1 blockers reshape the composition of the gut microbiome, inducing systemic metabolic changes and weight loss [123]. Given the complexity of checkpoint inhibition in systemic therapy, alternative approaches are being explored to reduce the unwanted effects of PD-L1/PD-1 inhibitors in normal tissues.

B7-H3. Upon examination, it appears that B7-H3 (CD276) is a type I transmembrane protein that belongs to the B7 family. In humans, it consists of parallel repeats of IgV-IgC domains, while in mice, it is made up of one pair of immunoglobulin C (IgC) and IgV ectodomains [124]. While B7-H3 protein expression is low in a small fraction of cells, including progenitors, fibroblasts, and immune cells [124], human and mouse adipose tissues express the immune checkpoint B7-H3 extensively, with a preference for adipocyte progenitors (APs). It has been observed that a deficiency in mitochondrial function and an increase in lipid storage occur among adipocytes derived from B7-H3-depleted progenitors. In addition, in B7-H3 knockout mice, spontaneous obesity occurs, along with a loss of metabolic and immune homeostasis [125]. B7-H3, which is similar to PD-L1, is an immune checkpoint that controls adipogenesis and regulates adipocyte differentiation in an intrinsic pathway. Therefore, B7-H3 is potentially conducive to cachectic pathophysiology by mediating fat tissue homeostasis.

VSIG4. V-set immunoglobulin domain-containing 4 (VSIG4), also known as CRIg/Z39Ig, is a multifunctional cell surface protein that plays a crucial role in innate and adaptive immunity. It is an immunosuppressive and anti-inflammatory protein and is expressed on tissue-specific macrophages, serving as a marker for these cells [126], [127]. Studies have shown that VSIG4 expression is elevated in mice and humans with various inflammatory diseases and cancers. As an immune checkpoint regulator, VSIG4 impairs the function of T-lymphocytes and macrophages, which can increase the risk of cancer development and progression [128], [129]. Age-related regulation of VSIG4 expression has been identified in white adipose tissue surrounding the perigonads (gWAT) in two mouse strains. There was a fourfold increase in VSIG4 + adipose tissue macrophages (ATMs)(13%-52%), which was the main cause of the enrichment of VSIG4. In a longitudinal study, male and female mice showed a strong correlation between VSIG4 expression within gWAT and age, and VSIG4 expression was strongly correlated with the physiological frailty index (PFI), a multiparameter assessment of health, in male mice. VSIG4 has been identified as a novel biomarker of old murine adipose tissue macrophages (ATMs). There is evidence that upregulated VSIG4 expression occurs in other aging tissues, such as the thymus, and in tumour-adjacent stroma in xenograft and spontaneous lung cancer models. Recent studies have linked VSIG4 expression with several inflammatory diseases and cancer, providing clinical diagnostic and prognostic information for both humans and mice [130]. Further research is needed to determine whether VSIG4-positive macrophages participate in systemic inflammation or the reduction of body fat during age-related cachexia.

Adenosine. Adenosine is a crucial immunometabolic mediator that binds to various adenosine receptors, exerting extensive physiological and pathophysiological effects, including immunosuppression and metabolic remodelling [131]. Extracellular adenosine mainly originates from the degradation of extracellular ATP, NAD+, and cGAMP, as previously reviewed [131]. ATP can bind to P2 purinergic receptors (P2Rs) involved in multiple physiological and pathophysiological responses, such as inflammation, hormone release and metabolism [132]. Through activation of the P2X7 receptor, ATP drives the induction of lipidolysis and inflammation in visceral adipose tissue (VAT) by promoting a Th17-polarizing microenvironment with high levels of IL-1β, IL-6, and IL-17. However, the transmembrane enzyme CD39 hydrolyzes ATP into extracellular ADP and AMP, and CD4 + CD39 + effector T cells degrade ATP to protect against ATP-induced adipolysis [133]. Similarly, CD73, a glycophosphatidylinositol-anchored enzyme, converts AMP to adenosine. CD73 is also a differentiation marker of mesenchymal stromal/stem cells (MSCs) widely distributed in adipose, muscle, and peripheral blood [134]. CD73 ablation in mice results in significantly lower body weight and decreases in superficial white fat content but increases in serum and intramyocellular free fatty acids and triglycerides [135]. Importantly, adenosine is generated depending on the activity of CD38 using NAD + as a substrate. CD38 is also regarded as a marker of preadipocytes, and these CD38 + adipose progenitors display a reduced proliferative potential but greater adipogenic potential. An increase in the number of CD38 + adipose progenitors in intra-abdominal fat is accompanied by obesity development [136]. In a mouse model of muscular dystrophy, deletion of CD38 immensely improved skeletal muscle performance and fully restored heart function, which was associated with a reduction in inflammation [137]. Through the reversal of the age-related decline of NAD+, 78c, a highly specific and potent CD38 inhibitor, can significantly enhance cardiovascular function, muscle function and exercise capacity [138].

According to current research, immune checkpoints can function both as pro-cachectic and anti-cachectic effectors. These checkpoints not only directly affect the function of muscle and adipose tissue, but also impact immune cell populations and cytokine production, thereby indirectly promoting cachexia progression.

Immune-metabolic axis

During cachexia, a complex metabolic process characterized by muscle wasting and weight loss, the body's energy reserves are depleted, leading to the release of metabolic substrates that may affect systemic inflammation, metabolism, and immune cell activation [2]. This underscores the importance of immunometabolism in the context of cachexia. To better understand the potential mechanisms involved in cachexia, we will examine the current concepts of immunometabolism, with a particular focus on carbon metabolism, including carbohydrate, lipid, and protein metabolism, within adipose tissue, liver, and muscle tissue. (Fig. 3).

Fig. 3.

Inter-organ communication influences metabolic homeostasis in cancer cachexia.When fat is broken down, free fatty acids are released, which can cause inflammation in local tissues and affect the function of immune cells. In turn, the immune system can also regulate the breakdown of fat and muscle tissue by interacting with nerve cells in response to NE. However, if the immune system is not functioning properly, it can lead to problems with the gut barrier and an imbalance in gut bacteria. This can result in malnutrition, diarrhea, and inflammation, which can ultimately lead to cancer cachexia. Additionally, hormonal changes in the gut can also contribute to liver problems.

Carbon metabolism

A measurement of respiratory exchange ratios reveals that cachexia results in a shift in metabolic dependence from carbohydrates towards fat utilization [23], [139]. This is due to hyperlipolysis of adipose tissue, which leads to elevated glycerol and FFA levels in the blood that can be utilized by other organs [140]. However, mitochondrial dysfunction affects the glycolysis and oxidative phosphorylation pathways, inhibiting energy metabolism and ATP production. Consequently, glucose derived from digestive absorption and glycogenolysis fails to meet energy consumption requirements. In cancer-associated adipose tissues, adipocytes passively release large amounts of lactate, pyruvate, FFAs and ketone bodies, which impact the differentiation and function of various immune cells [7], [12]. These changes in metabolism may be a reaction to increased exposure of cells to lipid substrates, or they may be adaptive processes, depending on the cell type involved.

Innate immune cells, such as macrophages and neutrophils, show metabolic disturbance and play a crucial role in lipid metabolism during cachexia development. FFAs have properties that serve an immunomodulatory function. The saturated fatty acid palmitate polarizes macrophages towards a proinflammatory phenotype via PPARγ and p62 [141]. In contrast, polyunsaturated fatty acids such as ω-3 fatty acids bind to their receptor G protein-coupled receptor 120 (GPR120) and GPR40 in macrophages to exert an anti-inflammatory effect by preventing NLRP3 inflammasome activation [142], [143]. It appears that the functions of innate immune cells are tightly regulated by fatty acid-related metabolites in the case of cachexia-induced adipose tissue lipolysis, which could profoundly affect immunopathology in chronic illness. In addition, macrophages in adipose tissue protect against CAC [144]. In that study, myeloid inflammation deficiency contributed to a reduction in macrophages in visceral fat tissue but unexpectedly enhanced cachexia-associated fat loss [144]. Conversely, macrophages appear to contribute to cancer cachexia in the form of tissue catabolism [32], [145]. The results showed that depletion of macrophages by anti-IL-20 treatment or clodronate increased body weight by reducing the loss of fat and muscle mass [32], [145]. Possible explanations for this apparent discrepancy include the origin or polarization of macrophages in different tissue environments. There is a mixed origin of macrophages in adipose tissues, and these cells perform a variety of functions in tissue metabolism and inflammation. Tissue-resident macrophages in fat tissue originate from embryonic precursors or adult hematopoietic stem cells (HSCs) and restrain adipogenesis. Conversely, macrophages derived from monocytes mainly accumulate in cancer-associated adipose tissues and stimulate lipolysis via diverse approaches.

Cancer-associated cachexia has been extensively studied in relation to adaptive immunity involving DCs, T cells, and B cells. Initially, DCs acquire the ability to take up lipids by FFAs by enhancing Msr1 expression. However, the accumulation of lipids impairs their antigen-presenting abilities and inhibits T cell activity, which could contribute to weight loss [146]. Depletion of PD-L1 on DCs promotes Th1 proliferation by terminating the suppression of PD-L1 in T cells, which is conducive to weight gain [111]. Therefore, the activated DCs in tumours may play a protective role against CAC development. In addition, increased generation of FFAs within cachectic fat loss can induce differentiation and survival of Tregs by elevating their lipid availability [147]. FFA binding proteins were found to be more abundant in intratumoral Tregs than in Tregs present in normal tissues or peripheral circulation [148]. Therefore, Tregs within tumours have been shown to participate in lipid metabolism, which could potentially worsen cachexia pathophysiology. CD8 + effector T cells (Teff) play a pivotal role in cancer. Several metabolic pathways are necessary to maintain Teff cell function and survival, including cell expansion, memory formation, and exhaustion. For example, the proliferation and function of Teff cells flourish in FFA-enriched medium [149], and spare respiratory capacity (SRC) and FAO are abundant in the mitochondria of CD8 + central memory T cells (Tcms), as well as a major increase in the expression of carnitine palmitoyltransferase 1α (CPT1α), one of the enzymes that limits FAO [150]. Teff cells are known to drive cachexia. In a cachexia model, a reduction in lipid stores was observed in adipose tissue when virus-specific CD8 + T cells invaded [151]. Furthermore, adoptive chimeric antigen receptor (CAR) T-cell therapies for haematological malignancies must maximize CAR T proliferation and persist for a prolonged period of time after infusion. An increase in CAR-T-cell activity enhanced by CD40 in animal models was associated with cachexia, in line with the production of high levels of human cytokines [152]. Similarly, fibroblast activation protein (FAP) has been proposed as a potential universal target antigen, and significant cachexia was observed following the adoption of T cells engineered with a FAP-reactive CAR [153]. Therefore, it is expected that systematic evaluations of T-cell function will be able to reveal the correlation between high FFA levels, T-cell functionality and cachexia.

Based on immunometabolism research, it has been demonstrated that high lipid content can have a direct impact on immune cell function. This effect can occur through two pathways: lipids can act as either biosynthetic or energy substrates for immune cells. Additionally, fatty acid metabolites can function as signaling molecules that regulate transcription factors or bind to receptors. Further research is necessary to determine whether lipids derived from wasted adipose tissue might influence immune activation during cachexia.

Protein metabolism

Studies have shown that mitochondrial dysfunction can lead to a decrease in mitochondrial biogenesis, impaired oxidative phosphorylation, and increased production of reactive oxygen species (ROS), which can induce protein catabolism as a compensatory mechanism [154]. This can cause a change in the amino acid profile within the microenvironment and circulation of tumors that induce cachexia [155]. These changes can lead to massive protein breakdown in muscles due to altered levels of circulating amino acids. For instance, in cachectic mice and patients, metabolic dysfunction is characterized by dramatically increased amounts of arginine, lysine, tyrosine, and proline, but decreased levels of aspartate and glutamate [156], [157]. Such fluctuations in circulating amino acid concentrations may be determined by the rate of amino acid release or by the uptake of amino acids by tumor cells.

Innate immune cells such as macrophages, muscle-resident immune cells, and neutrophils play a crucial role in tumour-induced muscle wasting by serving as immunometabolic modulators. Studies have shown that there is a negative correlation between the extent of CD163 + M2-like macrophages and muscle fibre density in cross-sectional areas in muscle biopsy samples [145]. M2-polarized macrophages and tumour-conditioned medium were found to reduce the thickness of myotubes and protein content while elevating the expression of proteasomal degradation markers in myotubes [145]. Similarly, muscle atrophy in cachexia can be attributed to inflammation caused by skeletal muscle-resident immune cells. Induction of cachexia in C57BL/6 mice shows a significant reduction in myotube diameter due to an increase in activated skeletal muscle-resident degranulating mast cells. Likewise, patients with cachectic symptoms show an upregulation of mast cells [158]. It is thought that cachectic muscles contain enriched activated skeletal muscle-resident mast cells that could function as biomarkers and mediators to improve the diagnosis and prognosis of patients who suffer from cachexia. Finally, cachexia is strongly correlated with a high ratio of neutrophils to lymphocytes in circulation [159].

In cancer patients, there is a significant correlation between T cell populations and muscle strength, performance, and body composition. Studies have shown an association between increased muscle mass and the number of recent thymic migrant and effector memory CD8 + T cells in the circulation. In addition, a negative correlation was observed between muscle mass and Tregs and central memory T cells [160]. Additionally, a correlation has been described between the abundance of CD3 + CD4- cells (presumably CD8 + T cells) and the cross-sectional area of muscle fibres in different cancers, the majority of which were gastrointestinal cancers [161]. According to these studies, a strong and efficient antitumour CD8 + T-cell response during cancer-associated cachexia (CAC) can help prevent catabolic muscle tissue, whereas immune suppression could be detrimental. The reason for this protection is still unknown, as there is no established mechanism that dissociates the tumour-fighting ability of the T cells from their direct catabolic action on muscles. It is likely that muscle atrophy may be prevented primarily by the clearance of tumours by T cells. For example, by adoptively transferring CD4 + CD44 + T cells to the spleen and lymph nodes of mice suffering from LLC, these increased cells were found to reduce the risk of cachexia and muscle atrophy [162]. In contrast, CD4 + T cells have been implicated as a possible mechanism for reducing cachexia and improving survival through the adoptive transfer of CD4 + FOXP3 + Treg cells into mice [163]. There may be another mechanism through which immune cell composition within the tissue microenvironment modifies the function and cell type-specific secretory profiles, resulting in muscle atrophy. The prognosis of cancer patients can also be analysed by measuring the circulating immune cells, where the number of immune cells compared to one another may play a role. The competition for amino acids between immune cells and tumour cells modulates immune cell activation status to meet their high anabolic requirements [164]. For instance, arginine, serine, and glutamine are taken up by tumour cells for growth, which limits their availability for T-cell receptor signalling, proliferation and differentiation [165], [166], [167]. Utilizing an antagonist that targets the tumour microenvironment to block glutamine uptake inhibits tumour growth and metabolism while enhancing antitumour T-cell responses [168]. IL-2-mediated activation and T-cell priming can trigger the production of amino acid transporters, such as SLC7A5, which facilitate the importation of large neutral amino acids, including leucine and phenylalanine [169]. When SLC7A5 is absent, CD4 + and CD8 + effector T cells are impaired, but CD4 + Treg cell development remains unaffected [169]. This could be partly due to the disruption in mTORC1 activity, which results in an inability of mTOR to fine-tune T-cell responses to nutrients. These studies and others highlight the significant impact that amino acids have on T-cell function. Investigating the bidirectional nature of this relationship in animal models, such as by modifying T-cell activation states and examining amino acid profiles, could provide further insights into this area.

In summary, depletion of muscles associated with cachexia releases a spectrum of amino acids into the bloodstream. To gain a deeper understanding of the metabolic, inflammatory, and disease-related consequences of cachexia, it would be valuable to profile and analyze the levels of systemic and local amino acids in both mouse models and cachexia patients. Such research could shed light on the mechanisms underlying cachexia and potentially identify new therapeutic targets.

Immune-gut axis

Intestinal barrier disruption

In cancer patients, dysfunction of the gut barrier is a common occurrence, which can be caused by chemotherapy or radiotherapy, as well as by the tumor itself [170], [171]. This syndrome leads to an inflammatory response when the gut epithelial barrier breaks down, allowing intact bacteria or components of the bacterial cell wall (such as endotoxin or lipopolysaccharide) to enter the bloodstream. Gastrointestinal permeability can also be affected by macrophage infiltration, which modifies the tight junctions of the epithelial layer [170]. As part of maintaining the barrier, resident γδ T cells in the gut protect the mucosal layer from bacterial invasion through the mucosa. Mechanistically, IL-17-producing γδ T (Th17) cells promote the expression of occludin, a protein that prevents excessive permeability and maintains the integrity of the barrier [172]. However, dysfunction of Th17 cells leads directly to the disruption of the epithelial barrier through the release of IFNγ, which modulates tight junctions [173]. Similarly, the activation of Teff cells leads to a reduction in tight junction proteins such as ZO1 and occludin, which in turn compromises intestinal barrier integrity, increases gut permeability and allows multiple immune mediators to enter the lymphatic system [174]. Patients with cancer who have a dysfunctional gut barrier also suffer from complications such as nutrient malabsorption, diarrhea, and other symptoms that can create a negative energy balance [175]. (Fig. 3).

Gut microbiota

Cancer cachexia has been linked to the gut microbiota, which plays a critical role in host immunity and metabolism [176]. Numerous studies have shown that gut microbial dysbiosis exists in mice and patients with CAC [175], [177], [178], [179], [180]. In experimental models of cancer cachexia, an increase in lipopolysaccharide synthesis was observed in the gut microbiota [177]. Additionally, dysbiosis of the gut microbiota was observed in mice with cancer cachexia, including a reduction in Lachnospiraceae and an enrichment in Enterobacteriaceae [178]. Interestingly, the number and composition of gut mycobiota were also found to differ significantly in cachectic patients compared to healthy controls [181]. n clinical trials, specific bacteria such as Proteobacteria and Veillonella were more commonly found in cachectic cancer patients [180]. Metabolites produced by gut microbiota impact energy expenditure in skeletal muscle and adipose tissue [175], [177], [178], [180]. Cachectic cancer patients tend to have lower levels of short-chain fatty acids (SCFAs), which are metabolites of microbial fermentation, but acetate levels are the only SCFAs that are significantly reduced [180]. Similarly, two short-chain fatty acids, acetate and butyrate, were also discovered to be decreased, and a bacterium of the Ruminococcaceae family was found to be primarily responsible for the drop in butyrate [175]. Moreover, the gut microbiota contributes to muscle metabolism by influencing amino acid bioavailability as well as the release of metabolites such as bile acids. The expression of BA synthesis enzymes in the livers of mice with cancer cachexia was suppressed, whereas the quantity of total BAs increased. There is a significant elevation in conjugated BAs to unconjugated BAs in the liver of cancer cachexia mice, and an increase in BA conjugation is detected in the serum of mice with cancer cachexia [178]. Finally, patients with cachexia had significantly depleted plasma levels of branched-chain amino acids (BCAAs), methylhistamines, and vitamins, corresponding to insufficient functional pathways of the gut microbiota [177]. There is evidence that intestinal bacteria release several compounds, such as LPS, peptidoglycan, and flagellin, that activate TLR/NF-κB signalling in adipose and skeletal muscle and may contribute to cachexia [176]. These studies highlight the changes in gut microbiota composition and microbial metabolites that occur in patients with cancer-associated cachexia and their impact on the disease. Further research is needed to investigate how the gut microbiota contributes to cancer cachexia and its potential as a target for cancer treatment. (Fig. 3).

Gut hormones

The gastrointestinal tract releases several hormones that control metabolism, appetite, and immunity. One such hormone is ghrelin, which is known as a hunger hormone and stimulates food consumption while reducing energy expenditure, affecting the body's energy status [182]. Although unacylated ghrelin does not have an effect on appetite and body weight, it has been reported to promote muscle regeneration by enhancing satellite cell function, inhibiting muscle apoptosis, and facilitating adipogenesis in a receptor-independent manner. Ghrelin also plays a crucial role in both muscle function and fat storage. It can alleviate cancer-induced adipose tissue atrophy and inflammation through ghrelin receptor-dependent and -independent pathways [183]. Additionally, ghrelin inhibits cancer-induced apoptosis in myoblasts and atrophy in myotubes [184]. Ghrelin and its receptor are important regulatory mechanisms for immunometabolism. For instance, ghrelin modulates the activation of peritoneal macrophages by inhibiting IL-1β and TNF-α and increasing the production of IL12 [185]. Similarly, the interaction between ghrelin and GHS-R specifically impairs the expression of proinflammatory anorectic cytokines in T lymphocytes and monocytes, such as IL-1β, IL-6, and TNF-α [186]. In contrast, the dystrophic thymus results in a reduction of ghrelin, further upregulating inflammatory and fibrotic markers and marked metabolic breakdown, worsening muscle atrophy. In clinical settings, there is an increase in plasma ghrelin levels in cachectic patients with neuroendocrine [187], gastric [188] and lung [189] tumors. Ghrelin is found to increase in lung cancer patients with anorexia, leading to lower hypothalamic activity, suggesting central control of appetite dysregulation during CAC [189]. However, a systematic review found no significant difference in ghrelin levels between individuals with cancer cachexia and other patients or healthy individuals [34]. Thus, alteration of ghrelin within cancer cachexia may depend on the tumour type or tumor stage. During cancer cachexia, ghrelin resistance occurs due to the hypothalamic circuitry controlling food intake becoming resistant to ghrelin, which is why cancer patients are given extremely high doses of ghrelin in clinical studies [190]. In summary, ghrelin and its receptor agonists are promising therapeutic agents to ameliorate cancer cachexia syndrome. (Fig. 3).

Immune-neuron axis

Central nervous system

Cytokines derived from immune cells or immunoinduced stromal cells are key signals in the brain that regulate appetite and energy metabolism in cancer cachexia [1]. It is widely accepted that the hypothalamus/orexin area is a target of immune-mediated metabolic regulation during cachexia due to its rapid response to changes in systemic nutritional status. Systemic inflammatory responses, administered through proinflammatory cytokines, initiate a hypothalamic inflammatory response. There is evidence that inflammation within the hypothalamus affects hypothalamic nucleus activity involved in regulating energy homeostasis [191]. For example, in a study based on the PDAC mouse model of cachexia, myeloid immune cells progressively infiltrated into regions of the brain related to food intake and/or energy metabolism [192]. Furthermore, a higher number of microglia in the hypothalamus was related to increased IL-1β and arginine 1 secretion in this tumour model [193]. It is thought that IL-1β promotes insulin secretion through neuronal transmission, and disordered IL-1β signaling during obesity leads to impaired insulin secretion in the cephalic phase [194]. Treatment with a CSF1R inhibitor resulted in reductions in microglial cell numbers and aggravation of the cachectic phenotype, causing dietary intake and muscle atrophy but no significant impacts on tumour growth [193]. There is therefore evidence that microglial cells protect against cachexia during pancreatic tumour development. In addition to controlling anorexia, the hypothalamus affects the metabolism involved in cancer cachexia. For instance, IL-1β triggers glucocorticoid release from the hypothalamic–pituitary–adrenal axis, which contributes to the degradation of skeletal muscle protein [16]. Additionally, the hypothalamus plays a role in muscle wasting by stimulating neuronal output through the melanocortin system [195].

Several brain regions are involved in regulating energy balance, and proopiomelanocortin (POMC) neurons play a crucial role in this process as metabolic sensors that are affected by variations in nutrient supply. The master cristae-remodelling protein OPA1 in POMC neurons mediates adipogenesis in white adipose tissue (WAT), and inactivation of OPA1 or POMC neurons can accelerate WAT lipolysis [196]. Furthermore, there are two types of orexin receptors: orexin type 1 (OX1), which is mainly expressed in the ventral raphe nuclei, and orexin type 2 (OX2), which is primarily expressed in the dorsal raphe nuclei. Inhibition of OX1 receptors in cells expressing serotonin transporters led to a reduction in glucose usage in skeletal muscles and brown adipose tissue in mice. In addition, selective inactivation of OX2 improved glucose utilization by reducing hepatic gluconeogenesis [197].

In addition to the systemic transmitters previously mentioned, certain immune mediators involved in cancer-associated cachexia (CAC) can also impact hypocretin/orexin levels. Lipocalin 2 (LCN2), a factor secreted by neutrophils, has recently been identified as a regulator of the feeding response in CAC. While LCN2 was previously thought to have an antibacterial function and was associated with the innate immune system, recent studies have demonstrated that it exerts neurotoxic effects and affects the central nervous system [198]. Furthermore, pancreatic and breast cancers have been found to activate STAT3 in neutrophils, leading to increased LCN2 production [199], [200]. Once LCN2 enters the central nervous system, it binds to MC4R in the paraventricular and ventromedial neurons of the hypothalamus, which play a role in appetite regulation [201]. In mice with pancreatic ductal adenocarcinoma (PDAC) experiencing cachexia, deleting LCN2 can reverse tumor-associated cachexia, similar to the effect of pharmacological inhibition of the melanocortin receptor in anorexia-bearing mice [202], [203]. Notably, LCN2 activity in the CNS also affects adipose and skeletal muscle metabolism [199], suggesting that its role extends beyond appetite regulation, which previous clinical studies have identified as a key process in cachexia, distinct from the weight loss resulting from chronic diseases. Thus, LCN2 and MC4R are two potential therapeutic targets that expand the list of treatment options for cancer-related anorexia cachexia. (Fig. 3).

Peripheral nervous system

In addition to the central nervous system, neuropils and neural factors within the peripheral region can function as immunomodulators in the development of cancer dyscrasia [204]. Cancer-mediated factors, such as Activin A and IL-6, can inhibit BMP action on muscle fibers and nerve terminals by expressing Noggin. This can lead to denervation, muscle atrophy, and rupture of the neuromuscular junction (NMJ). However, tumour-bearing mice with enhanced BMP signalling in the muscles can maintain NMJ function and prevent muscle wasting. Several pathogenic mechanisms associated with muscle wasting due to tumour growth have been identified, including denervation of muscle fibers and altered BMP signaling [75]. Leptin, an adipokine, can stimulate sympathetic innervation in adipose tissue by activating agouti-related peptide and proopiomelanocortin neurons in the paraventricular nucleus of the hypothalamus [205]. Similarly, the neurotrophic factor neurotrophin 3 (NT-3) is enriched in brown/beige adipocytes and serves as an adipokine to enhance the formation of sympathetic neurons and beige adipocytes by binding to the receptor tropomyosin receptor kinase C (TRKC) [206]. As a result, brown adipocytes express calsyntenin 3β to enhance functional sympathetic innervation through secretion of S100B, which stimulates neurite outgrowth from sympathetic neurons [207]. Subsequently, adipose tissue is functionally intertwined with neuron outgrowth and β-adrenergic activation. Local production of catecholamines by intra-adipose tissue neurons is responsible for increased lipolysis and initiation of the browning process in cachectic WAT of mice and humans. Interestingly, alternatively activated macrophages accumulate in neuron arborization to generate a neuroprotective environment covering the neurotoxic proinflammatory signals produced by cachexigenic tumours. In accordance with this concept, tumour-bearing mice that lack the interleukin-4 (IL-4) receptor exhibited a diminished type-2 immune response, attenuated neuronal outgrowth, and reduced browning of WAT compared to tumour-bearing wild-type mice [208]. Deficient IL-4 receptors prevented alternative macrophage activation, diminished sympathetic activity, and decreased WAT browning, thus preventing cancer-induced browning of WAT and adipose atrophy [209]. Cachectic WAT produces a neuroprotective environment by activating type 2 immunity and increasing peripheral sympathetic activity. In turn, sympathetic stimulation leads to enhanced neuronal catecholamine production and secretion, β-adrenergic activation of adipocytes, and augmentation of WAT browning. Specifically, macrophages in adipose tissues have been demonstrated to exhibit a multifarious subpopulation [210], and the cytokines derived from diverse macrophage subtypes enable modulation of the immune-adipose interplay. For example, a cytokine secreted by adipose tissue macrophages is Slit3. By binding to ROBO1 on sympathetic neurons, Slit3 increases Ca2+/calmodulin-dependent protein kinase II signalling and the release of norepinephrine, boosting adipocyte thermogenesis [211]. A specific cholinergic adipose macrophage (ChAM) is responsible for secreting acetylcholine to regulate thermogenic activation in subcutaneous fat [212]. Additionally, sympathetic neuron-associated macrophages (SAMs) play a crucial role in clearing norepinephrine (NE) from the extracellular space. SAMs achieve this by expressing solute carrier family 6 member 2 (SLC6A2), which facilitates the transport of NE, and monoamine oxidase A (MAOA), which acts as a degradation enzyme. When the sympathetic nervous system (SNS) is activated, NE uptake by SAMs increases, leading to a proinflammatory response in SAM profiles. In contrast, a procedure that destroys SAMs results in an increase in brown adipose tissue (BAT), browning of white fat, and elevated thermogenesis, leading to substantial and sustained weight loss [213]. Studies have also shown that deficiencies in the nuclear transcription regulator Mecp2 in a BAT-resident Cx3Cr1 + macrophage subpopulation can result in abnormal homeostatic thermogenesis and spontaneous obesity. Mechanistically, mutant macrophages with impaired BAT exhibit reduced sympathetic innervation and local norepinephrine levels, resulting in decreased expression of thermogenic factors by adipocytes. Furthermore, overexpression of the signaling receptor and ligand PlexinA4 in mutant macrophages suggests that they may contribute to the phenotype by repelling sympathetic axons that express Sema6A. These findings collectively suggest that macrophages located in BAT play a vital role in maintaining tissue innervation homeostasis [214]. Aside from macrophages, other innate immune cells are also involved in the immune-nerve circuit. For instance, group 2 innate lymphoid cells (ILC2s) are controlled by a sympathetic aorticorenal circuit through β2-adrenergic receptor-mediated glial-derived neurotrophic factor (GDNF) expression. ILC2 activity in gonadal fat leads to energy expenditure [215]. Eosinophils also play a regulatory role in nerve plasticity in white adipose tissue (WAT) by producing nerve growth factor (NGF). The release of IL-33 in stromal cells promotes the upregulation of ILC2-produced IL-5, which facilitates eosinophil accretion [216]. (Fig. 3).

Conclusions

In recent years, there has been significant progress in understanding cachexia, a complex syndrome that can lead to death at advanced stages. However, it remains challenging to classify cachexia as a maladaptive syndrome when considering metabolic adaptation. Complications related to cachexia can also make it difficult to determine whether an event is due to cachexia or other underlying illnesses. Evolutionarily, the stages of cachexia suggest a link to the adaptive state of cachexia due to the interplay between interaction mechanisms and immune reactions fighting the underlying disease.

Cancer cachexia is a multifaceted syndrome that involves functional, metabolic, and immune problems, as well as treatment-related toxicity effects. The treatment of cancer cachexia typically involves increasing food intake and normalizing metabolic disturbances through nutritional support, exercise, and orexigenic drugs. However, metabolic dysfunction remains the most challenging aspect of therapeutic intervention. To address this, targeting cytokines, including proinflammatory cytokines and tumour-specific factors, has been proposed as a potential approach. TLR agonist and antagonist drugs, thalidomide, and activin-βC have shown promising results in inhibiting the synthesis and secretion of inflammatory cytokines, reversing muscle wasting, and increasing lean body mass. Other potential therapeutic strategies include using activin-A and myostatin signaling ligand traps, soluble receptor ligand traps targeting activin A, and FFA1 receptor agonism. Additionally, inhibitors of epidermal growth factor receptor and β-adrenergic blockade have shown potential in ameliorating the severity of cachexia. Developing appropriate scoring systems to accurately stage the syndrome is crucial for effective treatment Table 2 [217], [218], [219], [220], [221], [222], [223], [224], [225], [226].

Table 2.

Summary of the development in the treatments of cancer cachexia.

Therapy(mechanism)	Therapeutic approach	Results	Study
FFA1 receptor agonism	Anabolic	Rescue of body weight loss and reestablishement of reskeletal muscle mass	216
Erlotinib(EGFR inhibitor)	Anabolic	Ameliorate wasting syndrome and muscle strength	76
β-adrenergic blockade	Anabolic	Rescue WAT loss and muscular dystrophy	22
LY2495655(anti-myostatin antibody) (NCT01505530)	Anabolic	No significant effect	217
STM 434(ligand-traps); activin-βC(activin-A antagonist) (NCT02262455)	Anabolic	Increased total lean body mass	218,219
TLR agonist; TLR antagonist;Thalidomide (TNFα synthesis inhibitor)	Anti-inflammatory	Avoid weight loss	56,57,220
AICAR(AMPK agonist)	Anti-inflammatory	Reduced muscle atrophy and preserved muscle mass	49
MC4R antagonists (NCT02896192 and NCT03287960)	Appetite-modifying	Increasement of food intake, promotion of protein anabolism and energy storage	221
Ghrelin receptor agonists (NCT01387269, NCT01387282 and ONO-7643)	Appetite-modifying	Increasement of food intake, promotion of protein anabolism and energy storage	222,223
Ureolithin A;UAS03	Enhancement of the gut barrier integrity	Increased lean body mass	224
Leucine	Nutritional	Improvement of muscle mass and protein content	225
TUDCA	Nutritional	Ameliorates the loss of body weight, muscle mass and liver and heart atrophy	178
Lactobacillus and Bifidobacterium strains	Other	Enhancement of muscle mass and physical performance	178
Physical activity (NCT02873676)	Other	Increasement of muscle mass	17

Cachexia research faces challenges in understanding the role of tissue-resident immune cells and their response to altered nutrient levels and inflammation. Supply and demand of substrates and energy must be monitored through ongoing feedback loops between the immune system and tissue. Nutritional competition occurs within the tissue microenvironment, and immune modulation can be achieved through regulating energy and substrate availability. To overcome these challenges, a more systematic integration of immunology and systems biology tools is needed, along with synergistic animal models to mimic the cachectic state. A holistic understanding of the immunology of cachexia is imperative for developing effective therapeutic strategies.

Future perspective

In the future, a key focus of cachexia research will be to understand the role of tissue-resident immune cells and their response to altered nutrient levels and inflammation. Ongoing feedback loops between the immune system and tissue must be closely monitored to ensure that substrates and energy are properly supplied and demanded. Nutritional competition in the tissue microenvironment and immune modulation through the regulation of energy and substrate availability are important areas for future investigation. To develop more effective therapeutic strategies for cachexia, a systematic integration of immunology and systems biology tools will be necessary. Additionally, synergistic animal models that better mimic the cachectic state will be needed. Overall, a holistic understanding of the immunology of cachexia will be essential for addressing this complex syndrome and improving patient outcomes.

Compliance with ethics requirements

This article does not involve any studies with human or animal subjects.

CRediT authorship contribution statement

Qi Wu: Investigation, Writing – original draft. Zhou Liu: Investigation, Writing – original draft. Bei Li: Visualization. Yu-e Liu: Visualization. Ping Wang: Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Biographies

Wu Qi received his Ph.D. from Wuhan University and was co-trained at Paris-Saclay University for 2 years. He has published more than 20 research papers as the first author or co-author in Mol Cancer、Signal Transduct Target Ther、J Hematol Oncol、J Immunother Cancer、Autophagy and Cancer Communication. He is currently working at the Institute of Oncology, Tongji University, Shanghai, and his main research direction is tumor metabolism and immunity.

Liu Zhou received his bachelor's degree from Zhejiang University School of Medicine and is currently studying for a master's degree in surgery at Wuhan University. His main research area is tumor metabolic immunity. After a long period of study, he has acquired a certain clinical work ability. In addition, he focuses on scientific research and has the ability to independently conduct experiments.

Li Bei received his Ph.D. from Wuhan University and is currently working in the Department of Pathology, People's Hospital of Wuhan University. His main research direction is tumor metabolism and immunity. Her study was supported by the grant from the National Natural Science Foundation of China and Several high-level papers have been published.

Liu Yu-e is a doctoral student in basic medicine, whose main research interests are mitochondrial metabolism and tumors. She has published professional academic papers in STTT, Medcomm as the first author or co-first author. She is also the secretary of the Cancer Branch of the Chinese Society of Cell Biology and the secretary of the Youth Committee of the Tumor Microenvironment Special Committee of the Chinese Anti-Cancer Association.

Wang Ping holds a Ph. D. from Shanghai Institute of Biochemistry and Cell Biology, Chinese Academy of Sciences. He is The Yangtze River scholar Professor and is currently the Vice Dean of Tongji University School of Medicine. His main research direction is the basic and translational research of tumor microenvironment.