Crosstalk between oxidative phosphorylation and immune escape in cancer: a new concept of therapeutic targets selection

Abstract

Background

Cancer is increasingly recognized as a metabolic disease, with evidence suggesting that oxidative phosphorylation (OXPHOS) plays a significant role in the progression of numerous cancer cells. OXPHOS not only provides sufficient energy for tumor tissue survival but also regulates conditions for tumor proliferation, invasion, and metastasis. Alterations in OXPHOS can also impair the immune function of immune cells in the tumor microenvironment, leading to immune evasion. Therefore, investigating the relationship between OXPHOS and immune escape is crucial in cancer-related research. This review aims to summarize the effects of transcriptional, mitochondrial genetic, metabolic regulation, and mitochondrial dynamics on OXPHOS in different cancers. Additionally, it highlights the role of OXPHOS in immune escape by affecting various immune cells. Finally, it concludes with an overview of recent advances in antitumor strategies targeting both immune and metabolic processes and proposes promising therapeutic targets by analyzing the limitations of current targeted drugs.

Conclusions

The metabolic shift towards OXPHOS contributes significantly to tumor proliferation, progression, metastasis, immune escape, and poor prognosis. A thorough investigation of concrete mechanisms of OXPHOS regulation in different types of tumors and the combination usage of OXPHOS-targeted drugs with existing immunotherapies could potentially uncover new therapeutic targets for future antitumor therapies.

Introduction

In recent decades, increasing evidence suggested that cancer is a metabolic disease [1, 2]. As the energy center of cells, mitochondria play an important role in producing sufficient energy and holding multiple metabolic reactions to meet the needs of cell survival [3, 4]. For tumor cells that are characterized by rapid and aberrant proliferation, mitochondrial metabolism is apparently indispensable in tumor survival and growth [5]. Almost 100 years ago, Warburg purposed the hypothesis that due to the irreversible damage to the respiration of tumor cells, tumor cells tend to undergo aerobic glycolysis rather than the more efficient oxidative phosphorylation (OXPHOS) process even in the presence of oxygen to ensure rapid production of abundant energy [6, 7], which is well-known as “Warburg effect”. However, the past two decades have witnessed a shift in the perception of metabolism in tumor cells. Recent research revealed other metabolic patterns, such as the reverse Warburg effect, which implies that aerobic glycolysis seems to take place in cancer-associated fibroblasts (CAFs) rather than cancer cells themselves [8, 9]. It has also been found that OXPHOS seemed to take a major charge in numerous cancer cells. OXPHOS not only supports tumor cells by producing enough energy, but also takes part in many facets of cancer progression due to its vital position in regulated cell death (RCD), and its ability to produce reactive oxygen species (ROS), which can promote malignant transformation of normal cells [10]. Taken together, mitochondrial OXPHOS confers plasticity to cancer cells, allowing them to better adapt to the variable tumor microenvironment (TME) [11].

The phenomenon of immune escape to evade immune attack on tumor cells is considered to be a vital link in tumor growth and development. On the one hand, a variety of metabolism-related factors, such as the hypoxic microenvironment, act on immune cells due to the metabolic reprogramming of tumor cells, forming an immunosuppressive environment that affects the antitumor effect of immune cells [12, 13]. On the other hand, alterations in the metabolism of the immune cell itself lead to changes in their immune function [14]. From this point of view, a more thorough study of OXPHOS and immune escape is an important entry point for cancer-related research. In this review, we will discuss the relationship between OXPHOS and immune escape in tumor, and we will also present an overview of recent advances in antitumor strategies targeting both immune and metabolic processes, and thus suggest possible therapeutic targets for future antitumor therapies based on the modulation of mitochondrial metabolism.

The role of OXPHOS in the progression of cancer

OXPHOS in different types of cancer

Tumor cells often undergo metabolic reprogramming to maintain their growth and adapt to environmental changes (Table 1). The conventional view is that tumor cells tend to enhance glycolysis for rapid production of sufficient energy to adapt to growth under low oxygen concentration. The affecting function of the electron transport chain (ETC) is one of the approaches to impairing mitochondrial function. The electron transport chain, also called the respiratory chain, consists mainly of four protein complexes located on the inner mitochondrial membrane (Complex I-IV). In mitochondria, nicotinamide adenine dinucleotide (NADH) and reduced flavine adenine dinucleotide (FADH₂), which are responsible for electron transfer, are oxidized through the electron transfer chain to generate water, and this process is accompanied by the release of energy that drives the phosphorylation of ADP to generate ATP, which is the OXPHOS process. Impairment of ETC causes mitochondrial dysfunction. Research shows that under a hypoxia microenvironment, OMA1, an ATP-independent zinc metalloprotease located at the mitochondrial inner membrane can damage ETC assembly, inhibit OXPHOS and increase mitochondrial ROS (mtROS) production, thereby promoting glycolysis and facilitating colorectal cancer development [15]. Mitochondrial ribosomes are the main site of mitochondrial-associated protein synthesis and are essential for the proper functioning of mitochondria. Degradation of the mitochondrial ribosomal protein S23 (MRPS23) inhibits OXPHOS with elevated mtROS level, inducing increased breast cancer cell invasion and metastasis [16, 17]. In addition, increasing glycolytic enzyme expression, decreasing or inhibiting mitochondrial oxidase and transport enzyme expression, decreasing mitochondrial number, and metabolite level interactions and regulation can also reduce OXPHOS and shift cells to glycolysis for energy [18–22]. However, unlike the mitochondrial dysfunction and increased aerobic glycolysis in cancer cells described by Warburg, in recent years, many studies have shown that tumor cells are highly heterogeneous and that energy metabolism in many tumor cells is switched toward OXPHOS, which is critical for the biological behavior of tumor survival, proliferation, invasion, and metastasis. Many cancers, including lung adenocarcinoma, colorectal cancer, acute lymphoblastic leukemia (ALL), ovarian cancer, prostate cancer, and head and neck cancer, have increased mitochondrial DNA (mtDNA) content with relatively higher OXPHOS activity than normal tissue [23]. Several cancer cells, especially cancer stem cells with high metastatic and tumorigenesis potential dependent more on OXPHOS than the bulk. Loss of retinoblastoma (RB1) tumor suppressor in breast cancer was shown to induce mitochondrial protein translation (MPT) and OXPHOS, which play a central role in enhancing anabolic metabolism and cell proliferation, as well as cancer stemness and metastasis [24]. Cancer cells without mtDNA lose the ability to form tumors unless they restore the OXPHOS activity by depriving mitochondria of the stroma. Meanwhile, an increasing number of studies have demonstrated that OXPHOS inhibition is effective in targeting cancer heavily reliant on OXPHOS [25].

Table 1.

Metabolic reprogramming and specific mechanisms in different types of cancer

Cancer	Subtype	OXPHOS	Associated mechanism	Ref.
Lung cancer	Non-small cell lung cancer (NSCLC) H1299 cells	↑	PGC-1α ↑	[22]
Lung cancer	H460 cells, A549 cells	↓	AMPK ↑	[20]
Lung cancer	Kras mutation and wild-type human lung cancer cells	↑	AIF ↑	[26]
Lung cancer	Small cell lung CLCs	↑	Maturity of mitochondria	[27]
Lung cancer	NCI-H82 cells	↑	Mitochondria-related genes ↑ (CO1, ND4, NDUFA3, ATP6, UQCRC2, SDHA, etc.), NNMT ↓, DNMT1 ↑	[28]
Lewis lung carcinoma	P29 cells, A11 cells	↓	G13997A and 13885insC mutations in ND6 gene in mtDNA	[29]
Pancreatic ductal adenocarcinoma (PDAC)	Pancreatic cancer stem cells (PaCSCs)	↑	MYC ↓, PGC-1α ↑, ISG15/ISGylation ↓	[30, 31]
PDAC	Pancreatic precursor lesions (PPL)	↑	PGC-1α ↑	[32]
Breast cancer	Triple-negative breast cancer (TNBC)	↑	RB1 ↓	[24]
Breast cancer	MCF7-3 × Flag-PRMT7 cells	↓	Methylated MRPS23	[16]
Breast cancer	Neu4145 cancer cells	↑	LDH-A activity ↑	[18]
Breast cancer	TNBC	↑	Mitochondrial genes encoding Complex I ↑	[33]
Breast cancer	MDA-MB-468 cells, MCF-7 cells	↑	PDHA ↑, PGC-1α ↑	[34]
Breast cancer	Basal-like breast cancer cells	↓	PCK2 ↓	[21]
Breast cancer	TNBC CSCs	↑	MYC ↑, MCL1 ↑, HIF-1α↑	[35]
Breast cancer	MCF-7 cells	↑	UGCG ↑	[36]
Liver cancer	H1299 cells, HepG2 cells	↑	PKM2 ↑	[37]
Prostate cancer	CWR22Rv1 cells, PC3 cells	↑	TEAD4 ↑	[38]
Prostate cancer	PC3 cells	↑	Survivin ↑	[39]
Colorectal cancer (CRC)	HCT116 cells	↓	OMA1 ↑	[15]
CRC	LS174T cells, HT29 cells	↑	SCL25A1 ↑	[40]
CRC	HCT116 cells	↑	IDH2 ↑	[41]
Colon cancer	HT29 cells, SW480 cells	↑	FXR ↓, DHRS9 ↓	[42]
Neuroblastoma	MYCN-driven neuroblastoma	↑	DLST ↑	[43]
Cholangiocarcinoma	Cholangiocarcinoma cancer stem cells	↑	PGC-1α ↑	[44]
Head and neck squamous cell carcinoma (HNSC)	Recurrent human papillomavirus (HPV)-related HNSC	↑	Nrf2 ↑	[45]
Oral squamous cell carcinoma (OSCC)	CAFs	↑	ITGB2 ↑, NADH ↑	[46]
Melanoma brain metastases (MBM)	A375 cells, MEWO cells, CHL1 cells	↑	PGC-1α ↑, IDH3A ↑, COX4l1 ↑, LDHB ↑, NDUFA5 ↑	[47]
Hepatocellular carcinoma (HCC)	Hypoxia HCC cells	↑	UQCC3 ↑	[48]
HCC	SNU-387 cells, SNU-398 cells	↑	SALL4 ↑	[49]
Melanoma	Mel-RM cells, A375 cells	↑	Genes encoding OXPHOS enzymes ↑	[50]
Melanoma	Primary melanoma	↑	PGC-1α ↑	[51]
Ovarian cancer	High-grade serous ovarian cancer (HGSOC)	↑	PML1 ↑, PGC-1α↑	[52]
Ovarian cancer	Ovarian cancer stem cells	↑	NDUFS6 ↑, NDUFA11 ↑	[53]
Ovarian cancer	HGSOC	↑	Ct-OATP1B3 ↑	[54]
Gastric cancer	Gastric cancer stem cells	↑	FoxM1 ↑, Prx3 ↑	[55]
Classical Hodgkin’s lymphoma (cHL)	Hodgkin-Reed-Sternberg (HRS) cells	↑	NFκB ↑	[56]
Acute myelogenous leukemia (AML)	ROS-low leukemia stem cells (LSCs)	↑	BCL-2 ↑	[57]
Urothelial bladder cancer	RT4, and 5637cells	↑ (for RT4 cells)/↓ (for 5637 cells)	Mutation in TP53	[58]
Clear cell renal cell carcinoma (ccRCC)	786-0 cells	↑	PTPRG ↑	[59]
Brain tumor	Neural stem cells of Drosophila	↑	-	[60]
Rhabdoid tumor	G-401 cells	↑	Mitochondria-related genes ↑ (CO1, ND4, NDUFA3, ATP6, UQCRC2, SDHA, etc.), NNMT ↓, DNMT1 ↑	[28]
Human diffuse large B-cell lymphoma	WSU-DLCL2 cells	↑	Mitochondria-related genes ↑ (CO1, ND4, NDUFA3, ATP6, UQCRC2, SDHA, etc.), NNMT ↓, DNMT1 ↑	[28]

The contributors to OXPHOS regulation in cancer

Transcriptional regulation of OXPHOS in cancer

Modulating the expression of genes associated with OXPHOS and thereby influencing ETC components to alter the OXPHOS process is a frequently employed mechanism for regulating mitochondrial metabolic rewiring [33, 34, 61] (Fig. 2). Peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) is known as a master regulator of mitochondrial biogenesis and function [51, 62]. In pancreatic and cholangiocarcinoma cancer stem cells (CSCs), PGC-1α is highly expressed and induces potentiated OXPHOS system and downregulation of glycolysis pathway, which is mandatory for full CSC stemness and functionality [30, 44]. For ovarian cancer, activated PGC-1α subsequently affects ETC gene expression and mitochondrial respiration, and for patients with HGSOC, high-OXPHOS status is always associated with a better prognosis for its enhancement in chemosensitivity [52]. PGC-1α is also found to promote OXPHOS and thus the display of stemness features in PPLs [32]. This phenomenon also exists in co-culture systems.

Fig. 2.

Transcriptional and metabolic regulation of OXPHOS in cancer cells. Numerous regulators are associated with mitochondrial metabolism in cancer cells. In general, most of them directly affect the OXPHOS process by interacting with complexes of the ETC on the inner mitochondrial membrane, while others regulate OXPHOS by acting on mtDNA or TCA cycle. The regulation of OXPHOS on transcriptional and metabolic levels can have an impact on tumor proliferation, progression, metastasis, poor prognosis, and cell stemness

In addition to PGC-1α, numerous regulatory factors also play a role in regulating OXPHOS in tumor cells. For instance, the expression of TEAD can be targeted by the epigenetic regulator arginine, which at last modulates OXPHOS in prostate cancer and has an impact on tumor recurrence and survival rate [38, 63]. Additionally, B-cell lymphoma 2 (BCL-2), an anti-apoptotic member which is always amplified in cancers, is also a regulator of mitochondrial OXPHOS [64]. The upregulation of BCL-2 in AML cells is closely related to the increase of mitochondrial respiration, and the inhibition of BCL-2 in AML cells can cause a reduction of OXPHOS in an amino acid-dependent way in vivo in AML mice xenograft model [57, 65]. Moreover, heme, an essential cofactor for ETC and ATP synthesis, can destabilize Bach1 which is in charge of several genes important for ETC, enhancing the synthesis of heme suppresses OXPHOS and reduces the proliferation of both ovarian and breast cancer cells [66, 67]. In TNBC, overexpression of MYC induces OXPHOS in mitochondria through mitochondrial biogenesis [35]. Furthermore, Nrf2 is closely related to the expression of Complex IV, and Nrf2-upregulated HNSC cells are heavily reliant on OXPHOS for growth and survival [45]. In the co-culture system of OSCC and CAFs, CAFs exhibit higher ITGB2 expression and secret more lactate into TME, which can be oxidized by OSCC cells to generate NADH, and thus promote OXPHOS and tumor proliferation [46]. In addition, UQCC3 is a Complex III assembly factor, higher expression of UQCC3 contributes to metabolic reprogramming in HCC, maintains mitochondrial homeostasis and OXPHOS in hypoxic HCC, and is critical to hepatocarcinogenesis and HCC progression [48]. Apoptosis-inducing factor (AIF) determines the rate of OXPHOS via regulation of Complex I, high AIF expression in lung cancer patients was associated with poor prognosis [26]. For melanoma cells, activation of genes encoding OXPHOS enzymes induces a switch to mitochondrial OXPHOS, which is essential to the survival of quiescent cells [50]. Increasing OXPHOS activity regulated by overexpression of OXPHOS genes also plays a pivotal role in ovarian cancer and gastric cancer stem cell enrichment and colorectal cancer cell growth and survival [40, 53, 55]. Taken together, these findings suggest that limiting the expression of OXPHOS-related genes may help in antitumor therapy.

Mitochondrial genetic regulation of OXPHOS in cancer cells

Besides the guidance of nuclear DNA, the assembly and functioning of the mitochondrial OXPHOS system are based on the normal expression of mtDNA. Mutations in mitochondrial genes have been observed in tumor cells, which may have an impact on the transformation of mitochondrial metabolic activity and mitochondrial plasticity, which in turn affects the tumor development process [68]. Due to a lack of histones protection, mtDNA point mutation in human tumors was reported as high as 72% under a comprehensive analysis [69]. In Lewis lung carcinoma cells, mutations in the ND6 gene in mtDNA produce a deficiency in Complex I and induce the overproduction of ROS, which contributes to higher metastatic potential of tumor cells. In cytoplasmic hybrid cells with mutant mtDNA derived from osteosarcoma cell line 143B, OXPHOS-deficient cells displayed increasing properties in motility and migration [70]. However, accumulating mtDNA damage will ultimately produce excessive mtROS, which will in turn lead to irreversible cell death. To protect cells from extreme damage, mitochondrial integrity will be managed through mitochondrial biogenesis [71]. Mitochondrial transfer is an important way for cell communication at the level of mitochondrial genes [25]. Tumor cells without mtDNA exhibit a pattern of delayed growth, while tumor cells that acquired mtDNA from host cells show restoration in mitochondrial respiration, suggesting that mtDNA is necessary for cancer cell metabolism [72]. In the meantime, the whole functional mitochondria from adjacent cells which are finally received by cancer cells are found to induce enhancement of mitochondrial respiration by activating well-known OXPHOS-related pathways like SIRT1/PCG-1α axis, driving an OXPHOS alteration on transcriptional level [73]. By treatment with ethidium bromide to construct cells without mtDNA (ρ⁰ cells), we have already demonstrated that cancer cells will lose their tumorigenicity with the destruction of mtDNA integrity and the loss of basic mitochondrial OXPHOS functions. Under a co-culture system with MSCs or human skin fibroblasts, A549 ρ⁰ human adenocarcinoma cells and 143B ρ⁰ osteosarcoma cells will restore the basic mitochondrial activities through the acquisition of the mitochondria from donor cells under the formation of a tunneling nanotube (TNT) [74]. In vivo experiment, Dong et al. showed that the generation of tumors in syngeneic mice by cells devoid of mtDNA is linked to the acquisition of the host mtDNA, which provide functional evidence for an essential role of oxidative phosphorylation in cancer [75]. In the follow-up study, authors have further shown that the latent dihydroorotate dehydrogenase (DHODH), while not mitochondrial ATP generation, in mtDNA-deficient cells is fully activated with the restoration of Complex III/IV activity and coenzyme Q redox-cycling after mitochondrial transfer, which is an essential pathway linking respiration to tumorigenesis [25]. Besides, the whole mitochondrial DNA transferred through several structures like extracellular vesicles (EVs), gap junctions (GJs), and exosomes have also been reported in related studies [76] (Fig. 1).

Fig. 1.

Mitochondrial genetic changes in cancer cells. Changes in mtDNA include mutation (a), deletion (b), damage (c), and transfer (d), in which mtDNA transfer can be achieved by structures such as EVs and GJs. Mutation, deletion, damage, or transfer of mtDNA can alter the mitochondrial metabolic process in cancer cells, leading to diverse metabolic patterns and different levels of metabolites, which may in turn affect tumor biological behavior

Metabolic regulation of OXPHOS in cancer

Recent studies show that cancer cells rely more heavily on mitochondrial functions than previously thought, which may play a key role in controlling cancer cells’ life and proliferation. In addition to the central role in bioenergetics and biosynthesis, mitochondria also regulate calcium homeostasis, generation of ROS, production of oncoproteins and oncometabolites, and then affect the biological process by modulating biosynthetic pathways and cell signaling pathways [77] (Fig. 2). PKM2, the key enzyme of the rate-limiting step for glycolysis, has been demonstrated to induce the expression of the OXPHOS Complex IV subunit, COXII by interacting with ERK and promoting the c-JUN binding activity on the gene promoter region, which in turn rescuing the autophagic cell death via up-regulating AKT/mTOR pathway in prostate cancer cells [78]. While a novel mitochondrial lactate dehydrogenase (LDH), which could promote cancer cell progression by converting the mitochondrial lactate into pyruvate with increased mitochondrial activity, can be guided as indirect evidence for the importance of oxidative phosphorylation on cancer progression [79]. As an epigenetic regulator to modulate histone acetylation, arginine activates the expression of lysine acetyl-transferases and increases overall levels of acetylated histones and acetyl-CoA, facilitating TEAD4 recruitment, leading to global upregulation of nuclear-encoded OXPHOS genes, and then suppresses prostate cancer cell growth in vitro and in vivo [38]. Under governing the entry of glutamine into the tricarboxylic acid (TCA) cycle for oxidative decarboxylation by converting into succinyl-CoA, producing NADH for OXPHOS, metabolic enzyme DLST play a central role in promoting high-risk neuroblastoma aggression on rescuing growth arrest and apoptosis [43]. By regulating the c-Myc/PGC-1 pathway, Leptin-treated HCT116 and MCF-7 cells distinctly up-regulated the expression levels of carnitine palmitoyl transferase 1 A (CPT1A), pyruvate dehydrogenase, succinate dehydrogenase subunit A and mitochondrial respiratory chain-associated proteins NADH dehydrogenase 1 (ND1), NADH:ubiquinone oxidoreductase subunit B8 (NDUFB8), and mitochondrial transcription factor A (TFAM), which is of utmost importance in promoting cell proliferation and maintaining high adenosine triphosphate levels [80]. By regulating the PI3K/AKT/mTOR axis, ITGB2 high pro-tumoral CAFs promote tumor proliferation in OSCC by NADH oxidation in the mitochondrial oxidative phosphorylation system [46].

Effects of mitochondrial dynamics on OXPHOS in cancer cells

Mitochondrial dynamics, including fusion, fission, and mitophagy, are required to maintain mitochondrial function and respond to changes in cell physiology [81]. In pancreatic cancer stem cells, ISG15 and ISGylation ensure efficient mitochondrial OXPHOS by regulating mitophagy, which results in the maintenance of CSCs stemness [31]. PKM2, the M2 isoform of pyruvate kinase, shows the ability to promote mitochondrial fusion, which modulates the switch between OXPHOS and glycolysis in H1299 cells and enhances their growth [37]. The uncontrolled mitochondrial fusion in Drosophila neural stem cells induces the immortalization of tumor-initiating cells [82]. Exogenous mitochondrial trafficking from bone marrow stromal cells to AML cells as well as endogenous mitochondrial fission and mitophagy is enough to reverse the inhibition of OXPHOS under the influence of a novel Complex I OXPHOS inhibitor, IACS-010759 [83]. Meanwhile, autophagy is necessary for maintaining pancreatic cancer cell growth, the expression of oncogenic activation mutation in GTPase Kras markedly promotes basal autophagy and stimulates OXPHOS through an autophagy-dependent mechanism, which is an important reminder that approaches aimed to suppress OXPHOS or autophagy could be beneficial in treating pancreatic cancer [84]. Other regulators of mitochondrial function including mitochondrial mass and participants of OXPHOS are also indispensable in the complex metabolic reprogramming of tumor cells [43, 85]. Together they provide a novel perspective of screening possible therapeutic targets for tumors.

The role of OXPHOS in immune escape

Alterations in mitochondrial metabolism not only affect the survival and growth of tumor cells, but also directly or indirectly affect the immune response to tumors. In tumor tissues, tumors can escape from immune surveillance by altering their own antigenicity, affecting antigen presentation and immune cell recruitment, promoting the expression of immunosuppressive phenotypes in immune cells or affecting effector functions (Fig. 3). Numerous studies have demonstrated that the mitochondrial metabolic reprogramming that occurs in tumor cells and immune cells can largely influence the above processes and provide favorable conditions for tumor growth.

Fig. 3.

Overview of immune cells in TME. In tumor tissues, tumor cells always try their best to escape from immune surveillance by altering their own antigenicity, affecting antigen presentation and immune cell recruitment, promoting the expression of immunosuppressive phenotypes in immune cells, or attenuating effector functions. Different ways and detailed mechanisms in which tumor cells affect immune cells and escape from immune surveillance have been found. (a) Cancer cells can affect the activation of APCs in the TME by upregulating FAO and releasing DAMPs, which affect the recruitment of effector cells by DCs and the phagocytic activity of Mφs. (b) Cancer cells can reprogram the metabolism of immunosuppression cells to inhibit the immune-killing activity of effector cells. (c) Metabolic interactions, such as mitochondrial transfer and hypoxia, exist between cancer cells and effector cells (such as T cells and NK cells). These interactions greatly impair effector cell function through lack of mitochondria, weakened mitochondrial function, and mitochondrial fission, providing a basis for tumor immune escape

On antigen-presenting cells (APCs)

In the organism, to exclude the interference of foreign substances, the immune system can generate specific immune responses to antigenic stimulation. As the starting point of the antitumor-specific immune process, antigen presentation and immune cell recruitment lay the foundation for the activation and function of effector cells [86]. Dendritic cells (DCs) are a powerful class of specialized APCs that directly activate initial T cells and induce the formation of cytotoxic T lymphocytes (CTLs), which are one of the major effector cells that exert antitumor immune effects. CSCs are known for their capacity of treatment resistance, tumor metastasis, and recurrence, and eradicating CSCs may be a means of antitumor therapy. Research on the Arf1 pathway suggested that ablating of Arf1 in mice caused mitochondrial defects and induced the dying CSCs to release damage-associated molecular patterns (DAMPs), which activated DCs and further enhanced T cells infiltration and function in TME [87]. Another group of vital APCs in the immune system is macrophages, which are responsible for non-specific phagocytosis and the processing of antigens. For glioblastoma multiforme (GBM), enhancement of mitochondrial fatty acid oxidation (FAO) enzymes not only can meet the demand for tumor cell proliferation, but also upregulate immune checkpoint CD47 expression, which has anti-phagocytosis function and protects tumor cells from macrophage-mediated immune surveillance [88]. The phagocytic machinery of macrophages can also be dictated by mitochondrial dynamics. Li et al. demonstrated that efficient clearance of tumor cells by macrophages required mitochondrial fission in mice bearing MC38 cells [89]. Unlike classically activated macrophages (M1) which are pro-inflammatory, there is a different phenotype of alternatively activated (or wound-healing) macrophages (M2) which are in charge of immune suppression to avoid excessive tissue impairment. It has been proved that blockage of cell-autonomous generation of nicotinamide adenine dinucleotide (NAD⁺) suppresses mitochondrial respiration, resulting in defect of macrophage phagocytosis and change in macrophage polarization state [90]. For example, itaconic acid is highly upregulated in peritoneal tissue-resident macrophages (pResMφ), which can alter mitochondrial metabolism, induce an increase in OXPHOS, glycolysis, and ROS production in pResMφ and thus promote peritoneal tumor growth [91]. Regulatory T (Treg) cells in TME are found to preferentially promote M2-like tumor-associated macrophage (TAM) phenotype in melanoma by limiting CD8⁺T cell function and maintaining the mitochondrial integrity of TAM [92].

On effector cells

Affecting immune effector cell survival and function through mitochondrial metabolic reprogramming is also a common method of tumor immune escape. Breast cancer cells have been observed to hijack mitochondria from CD3⁺T cells, impairing immune cells’ respiration and thereby affecting their survival and full defensive function. In advanced NSCLC, Sirt2 suppresses T cell metabolism by targeting enzymes involved in glycolysis, TCA cycle, FAO, and glutaminolysis. In addition, Sirt2-deficient murine T cells exhibit increased glycolysis and OXPHOS, resulting in enhanced proliferation and superior antitumor activity. A decline of NAD + in T cells disrupt mitochondrial glycolysis regulation, block ATP synthesis, and dampen the T cell receptor downstream signaling cascade, which contributes to the repression of T cell activation in ovarian tumor [93]. Natural killer (NK) cells have critical roles in tumor surveillance against cancer. Early natural immune surveillance leads to damage to transformed cells, but as tumors progress, tumor cells can always find ways to escape from immune detection [94]. In liver cancer tissue, the hypoxic tumor microenvironment induces excessive mitochondrial fission into fragments in tumor-infiltrating NK cells, resulting in increased electron leakage during OXPHOS in mitochondria, and this respiratory failure in NK cells then leads to apoptosis and loss of antitumor function. It has also been found that HCC patients with a hypoxic microenvironment tend to have poor survival [95]. A few pathways associated with mitochondrial metabolism do have a significant influence on NK cell proliferation and killing efficiency. Activation of PPARγ in NK cells inhibits the mTOR pathway, representing a type of metabolic paralysis and impairment of antitumor responses in vivo [96]. IRE1α-XBP1, a pathway related to unfolded protein response, is associated with NK cell OXPHOS and proliferation. Mice with IRE1 and XBP1 ablation in NK cells failed to control tumor growth, suggesting that IRE1α-XBP1 plays a crucial role in NK cell immune function [97].

On immunosuppression cells

The formation of an immunosuppressive environment in the tumor is not only dependent on inefficient immune cells, but also induced by immunosuppression cells such as myeloid-derived suppressor cells (MDSCs) and Treg cells. Metabolic switch in MDSCs increases FAO and mitochondrial biogenesis, inducing inhibition of T cell proliferation and maintenance of immunosuppressive microenvironment in NSCLC [98]. The expression of Gpx4, a regulator of lipid peroxidation in intratumoral Treg cells can markedly sustain Treg cell suppression of antitumor immune responses and enhance the proportions of Treg cells in melanomas [99]. Genetic ablation of PPARγ, which is associated with lipid metabolism in group 2 innate lymphoid cells (ILC2s), significantly suppresses tumor growth in vivo, supporting the pro-tumorigenic role of ILC2s in CRC [100].

Existing antitumor treatments targeting mitochondrial metabolism and immunity

Treatments targeting mitochondrial OXPHOS in tumor cells

There are several strategies for inhibiting mitochondrial metabolism, especially for inhibiting OXPHOS in mitochondria that range from inhibition of mitochondrial transfer and mitochondrial dynamics, targeted drugs that decrease mitochondrial function, drugs that affect respiratory-related regulators, or inhibitors of respiratory chain complexes (Table 2). Inhibitors that can partially block nanotube formation like L-778,123 have the ability to inhibit intercellular communication and thus cut off mitochondrial transfer to heterogeneous tumor cells [101]. Mitochondrial transcription can be specifically blocked by LDC203974 (IMT1B), leading to a clear reduction of tumor volume in mice containing human ovarian cancer and colon carcinoma xenografts [102]. Coenzyme Q10, a cofactor in OXPHOS, acts as an electron carrier in oxidation-reduction cycles and affects cancer cell survival via metabolic reprogramming [103]. As the main metabolic activity that takes place in mitochondria, OXPHOS is also reportedly inhibited by ouabain, MLN4924, resveratrol, BZL101, BA6 (heteronemin), atovaquone and benzene-1.4-disulfonamides both in vitro and in vivo [104–110]. Gossypol, an ALDH-targeted inhibitor, selectively induces a reduction of ATP production level through decreasing NADH, leading cancer cells to death [111]. Specifical suppression of mitochondrial membrane potential (MMP) by verteporfin results in glioma stem cell death after inhibition of OXPHOS [112]. The use of these inhibitors exhibits significant antitumor effects in tumors, suggesting that modulating OXPHOS is key for regulating the fate of tumor cells. Clearance of OXPHOS-related regulators is another method of disrupting mitochondrial metabolism. SR18292, a selective PGC1α inhibitor, represses the specific metabolic and stemness features of PPLs and effectively represses tumor growth [32]. Arf1 inhibitor GCA/BFA significantly causes mitochondrial defects in liver cancer cells by disrupting lipid metabolism, which triggers T cell infiltration and activity, resulting in the reduction of tumors and prolonged survival in mice [87]. Treatment with metformin for thyroid cancer mouse model is proved to have an association with cancer growth inhibition, which is dependent on mitochondrial glycerophosphate dehydrogenase (mGPDH), a flavin-linked respiratory chain dehydrogenase that has the ability to modulate mitochondrial OXPHOS in cancer cells [113].

Table 2.

 A non-exhaustive list of drugs targeted the ETC and OXPHOS under study or in the clinic as anticancer therapeutics

Compound	Preliminary clinical use	Indications	Targets & Evaluation	Clinical trials	Phase	Ref.
Metformin	Diabetes	Prostate cancer, oral cancer, solid tumor, thoracic neoplasm, breast cancer…	Complex I; inhibits tumor growth in several cancer	Over 400 ongoing clinical trials	Phase 1–4	[30, 44, 113, 114]
Phenformin	Diabetes	Pancreatic cancer, stomach cancer, NSCLC, melanoma	Complex I; inhibits tumor growth	NCT02475499, NCT03026517	Phase 1	[111]
IACS-010759	Experimental	Brain cancer, AML, neuroblastoma, TNBC, advanced malignant solid neoplasm…	Complex I; inhibits tumor growth	NCT02882321, NCT03291938	Phase 1	[33, 43, 45, 47, 119]
LDC203974 (IMT1B)	Experimental	Ovarian cancer, colon carcinoma	Mitochondrial transcription; reduces tumor volume	-	-	[102]
Coenzyme Q10	Cardiovascular diseases	Breast cancer, pancreatic cancer, HCC, gliosarcoma, rectal cancer, lung cancer, ccRCC…	OXPHOS; improves prognosis, improves the quality of life…	NCT00976131, NCT02650804, NCT00096356, NCT01964001…	Phase 1–3	[103]
SR-18,292	Experimental	Cholangiocarcinoma, pancreatic ductal adenocarcinoma	Complex I, PGC1α; inhibits tumor growth, inhibits stemness features of PPLs	-	-	[32, 44]
Marizomib (NPI-0052)	Multiple myeloma	NSCLC, pancreatic cancer, TNBC, lymphomas, melanoma, glioblastoma…	Complex II; inhibits tumor growth and induces apoptosis	NCT00629473, NCT00667082, NCT00396864, NCT04341311, NCT02903069…	Phase 1–3	[116]
Ouabain	Cardiovascular diseases	Lung cancer, breast cancer	OXPHOS; cancer cell cytotoxicity	-	-	[104]
MLN4924	Experimental	Breast cancer, NSCLC, solid tumor, lymphoma, leukemia, multiple myeloma…	Mitochondria fission-to-fusion, neddylation; inhibits tumor growth	NCT01862328, NCT03228186, NCT00677170, NCT04800627, NCT02122770…	Phase 1–3	[105]
Resveratrol	Dietary supplement	Breast cancer, colon cancer, multiple myeloma…	OXPHOS; inhibits cancer cell proliferation	NCT00256334, NCT00578396, NCT00920803, NCT00433576…	Phase 1–4	[106]
Bezielle (BZL101)	Experimental	Breast cancer	OXPHOS; inhibits cancer cell growth	NCT00454532, NCT00907959	Phase 1–2	[107]
ME-344	Experimental	Lung cancer, AML, breast cancer	ETC; induces cancer cell apoptosis	NCT02100007, NCT01544322, NCT02806817	Phase 1–2	[120–122]
Compound 6c	Experimental	CRC	Complex III; suppresses tumor growth and metastasis	-	-	[41]
PEG-GO@XN	Experimental	Breast cancer	Complex I; suppresses metastasis of breast cancer cells	-	-	[123]
BA6 (heteronemin)	Experimental	Lung cancer	OXPHOS; enhances cancer cell apoptosis	-	-	[108]
Gossypol	contraception	Pancreatic cancer, stomach cancer, lung cancer, prostate cancer…	ALDH; suppresses tumor growth	NCT00988169, NCT01977209, NCT00544596, NCT00666666, NCT00773955…	Phase 1–3	[111]
IR-26	Experimental	AML	Complex II& V; kills AML cells	-	-	[117]
Veteporfin	Ocular vascular disorder	GBM	MMP; kills glioma stem cells	-	-	[112]
Atovaquone	Antiparasitic drug	AML, lung cancer	Complex II; reduces disease burden and prolongs survival	NCT04648033, NCT02628080, NCT03568994	Phase 1	[109]
Benzene-1,4-disulfonamides	Experimental	Pancreatic cancer	Complex I, ATP production; inhibits tumor growth	-	-	[110]
VLX600	Experimental	Neuroblastoma, gastrointestinal stromal cancer, colon cancer, refractory cancer	OXPHOS; increases cancer cell apoptosis	NCT02222363	Phase 1	[124–126]
BAY 87 − 2243	Experimental	Melanoma, HCC, neoplasms	Complex I; inhibits tumor growth	NCT01297530	Phase 1	[127, 128]
Gboxin	Experimental	GBM	Complex V; inhibits mouse and human GBM growth	-	-	[118]

Numerous compounds have the ability to directly inhibit OXPHOS by blocking respiratory chain complexes. The biguanide antidiabetics metformin has been proven to inhibit Complex I and mitochondrial oxygen consumption in pancreatic cancer, cholangiocarcinoma, and bladder cancer cells, and the growth of tumors was significantly reduced under metformin treatment [30, 44, 114]. Other drugs that targeted Complex I with high affinity such as IACS-010759, SR-18,292, BAY87-2243, and second-generation isoflavone ME-344 also present excellent antitumor capacity in vivo [33, 43, 45, 47, 115]. Marizomib, also known as NPI-00052, is a molecule proteasome inhibitor that inhibits multiple proteasome catalytic sites in myeloma and solid tumors [116]. As a Complex III inhibitor, compound 6c, a polyamine-vectorized flavone-naphthalimide conjugate, inhibits cell growth, migration, and invasion of CRC cells [41]. IR-26, one of the heptamethine cyanine dyes, causes mitochondrial OXPHOS impairment by inhibiting the activity of Complex II and V, showing a targeted killing effect of AML cells in vitro and in vivo [117]. The novel small molecule Gboxin, which targets Complex V, has been demonstrated to inhibit the growth of GBM by disrupting GBM cell metabolism without damaging normal cells [118]. Some of these inhibitors are under clinical trials to test their clinical efficacy in human. Although further clinical optimization is required for the development of such agents due to the results of those trials, these drugs provide a novel prospect for designing targeted antitumor therapies via precise blocking of mitochondrial metabolism.

Metabolism-shifting treatments targeting the immune system

As with other cellular life-sustaining activities, the full immune function of immune cells depends heavily on the proper running of mitochondrial metabolism. Nowadays, lots of efforts have been put into immunotherapy, which is to harness the body’s immune system itself to treat cancer[129]. Among these ideas, immunotherapies focusing on modifying the function of effector cells, especially T cells, have always been in the limelight. Small molecule drugs that regulate mitochondrial metabolism can shift effector cells’ metabolism, and thus enhance their immune function. The use of Sirt2 inhibition AGF2 and Thiomyristoyl (TM) enhances tumor-infiltrating lymphocytes (TILs)’ cytotoxic activity when co-cultured with autologous NSCLC cells ex vivo by increasing aerobic glycolysis, OXPHOS and IFN-γ production of TILs[130]. PC7ANP, a sting-activating nanovaccine, can increase the ability of cytosolic delivery and cross-presentation of antigens in APCs, especially DCs, inducing stronger tumor-specific CTL response and inhibition of tumor growth [131]. The PPARγ inhibitor T0070907 targets ILC2’s mitochondrial function and suppresses its pro-tumor capacity, bringing a promising reduction of both tumor volume and weight of CRC tumor-bearing mice [100]. The blocking of CPT1b by etomoxir increases glycolysis and restores cytotoxicity of NK cells, therefore increasing tumor killing in melanoma [96]. As an effective approach to specifically striking against tumors, the rising therapy named adoptive cell transfer (ACT), in which patients are infused with expanded populations of tumor-reactive T cells that are extracted from tumor masses and expanded in vitro, has been approved for clinical use in several tumor types [132]. It has been reported that transient glucose restriction (TGR) treatment monitored CD8⁺ effector T cell proliferation and cytokine production by enhancing glucose metabolism, which could hence benefit the in vivo functional capacity of T cells for ACT in melanoma [133]. Another form of ACT, in which transferred T cells are genetically modified with chimeric antigen receptors (CARs), is known as CAR T cell therapy. CAR T cells containing 4-1BB signaling domains display enhanced OXPHOS and promoted memory T cell differentiation, leading to superior persistence in vivo [134, 135]. In CAR T cell therapy for GBM, IL15 enhances T cell survival through OXPHOS promotion, which results in superior killing capacity in vitro and better persistence in vivo [136].

Combination of OXPHOS inhibitors and immunotherapies

The combination of metabolic regulators and immunotherapy is expected to exert synergistic antitumor effects. Combined use of etomoxir and anti-CD47 antibody forms a dual blockage in the FAO-CD47 axis, which may eliminate cancer cells equipped with a boosted cellular energy fuel supply and CD47-mediated protection against macrophagic attack, and is observed to mediate a noteworthy reduction in tumor volume in mice [88]. The FAO inhibitor etomoxir which targets fatty acid uptake and ATP production can increase the efficacy of cyclophosphamide + ACT combined therapy with a higher number of adoptively transferred OT-1 T cells and an increased number of cells producing IFNγ [98]. Adding of the dual- PI3Kδ/γ inhibitor duvelisib during the manufacture of CART cells may increase mitochondrial mass and epigenetic modifications that correlate with enhanced antileukemia efficacy in vivo [137]. Inhibition of MEK is reported to augment metabolic fitness in CD8⁺T cells for ACT, which at last decreases melanoma growth rate and prolongs tumor-bearing mice survival [138]. Immune checkpoint blockade, a widely used immunotherapy for cancer in recent years, can also be improved with the cooperation of metabolic modification. The combination of farnesyltransferase and geranyl transferase 1 inhibitor L-778,123 and PD1 inhibitor shows significant tumor growth inhibition and increase in survival in vivo by inhibiting both mitochondrial hijackings from immune cells to cancer cells and immune checkpoint [101]. NAD + supplementation, SREBP1 inhibitor fatostatin, or Complex I inhibitor IACS-010759, together with anti-PD-1 treatment, are all proven to have the ability to delay tumor growth, promote immune response to tumor, and extend survival time for tumor-bearing mice [92, 93, 139].

Promising therapeutic targets for more effective antitumor therapies

Shortcomings of current targeted drugs

Tumor cells are notorious for their versatile nature. Although many proven drugs can alter mitochondrial metabolism and thus assist cells in metabolic reprogramming, tumor cells often dent to develop therapeutic resistance after a period of time in order to escape immune surveillance [140]. Mitochondria as the central cellular energy powerhouse are capable of rapid sensing energy availability and intra- and extracellular stimulation [141]. In TME, the mitochondrial plasticity of tumor cells provides the enhancement of tumor metastasis and the development of resistance to antitumor drugs, meanwhile, the downregulation of mitochondrial plasticity in immune cells fails to promote the generation of effector and memory T cells, making it harder to fight against tumor through immune responses [142]. What’s more, the genetic heterogeneity of cancer may be a contributing factor to the inevitability of drug resistance and confounding of clinical predictors or biomarkers, which ultimately shortens the duration of disease control achieved with targeted agents or conventional immunotherapies [143–145]. Thorough exploration of the heterogeneity and plasticity of the tumor ecosystem is a prerequisite for the future development of effective and durable personalized targeted antitumor drugs [146]. Identifying relatively stable therapeutic targets to better stratify the patients is essential in this pursuit as well. The biotoxicity of targeted drugs is also an aspect that cannot be ignored in the clinical application of anticancer drugs. In the clinical trials that have been conducted, some drugs have exhibited varying degrees of side effects, thus limiting their entry into clinical use [147]. As an instance, a phase I trial in patients with AML and solid tumors shows that the development of the potent Complex I inhibitor IACS-010759 may have to be halted due to its narrow therapeutic index with dose-limiting toxicities, including evaluated blood lactate and neurotoxicity, that prevent its progress to the clinic [148, 149]. Likewise, these toxicities have also constrained the clinical translation of other OXPHOS inhibitors for cancer therapy, including ASP4132, phenformin, and BAY 87-2243 [150, 151]. In addition, since TME is a complex biological microenvironment, the crosstalk between its components needs to be further investigated, and the therapeutic effects of drugs on a single cellular component may not be extended to tumor tissue as a whole, which creates new challenges for the development of antitumor therapeutic regimens [152, 153].

Promising therapeutic targets

As noted previously, mitochondria are characterized by dynamic alterations, which makes it a priority to pay more attention to mitochondrial morphological changes in tumor-associated cells and explore mitochondrial-dynamics-targeting agents [141]. The mechanism of intercellular metabolic communication in TME, represented by mitochondrial transfer, also remains to be elucidated, and the related targets are expected to open up new avenues for antitumor therapy[154]. Since mtDNA abnormalities and mitochondria-related metabolic enzymes have key roles in tumor development, it is believed that fully exploring their mode of function will be helpful in the exploration of new antitumor therapies. At the same time, it is important to note that many of the therapies mentioned in the previous Sect. 3 and much of the literature involve non-specific drugs like metformin, which makes it difficult to attribute their antitumor effects to specific metabolic controls. More drug delivery vehicles such as PEGylated graphene oxide, which can precisely target mitochondria need to be explored and tested [123]. In addition, the antitumor properties of many therapies are characterized in immunodeficient xenograft models, and whether their antitumor effects can be properly exerted in normal, immunocompetent mice remains to be proven, and there is still a long way to go before they enter clinical trials and eventually apply in the clinic. Meanwhile, some of the targeted agents failed to enter the clinic because of the intolerable adverse events under adequate dosing, but recent studies have shown that certain combination therapies can enhance the antitumor efficacy of drugs while mitigating side effects [155, 156]. This provides new avenues for future research. Therefore, more specific mitochondrial-targeted compounds and their combination usage with existing immunotherapies must be investigated more thoroughly.

Summary and conclusion

Many recent studies have demonstrated that OXPHOS tends to take major charge in a variety of cancers. The metabolic shift towards OXPHOS is achieved through pathways including genetic, transcriptional, metabolic, and mitochondrial dynamic regulation, contributing to tumor proliferation, progression, metastasis, and poor prognosis. Changes in mitochondrial metabolism can also affect the immune response to tumors. By altering cancer cells’ own antigenicity, affecting antigen presentation and immune cell recruitment, and promoting the expression of immunosuppressive phenotypes in immune cells, the effector function of immune cells in TME can be attenuated, which in turn help cancers to escape immune surveillance. In response to this feature of metabolic reprogramming of cancers, many drugs targeting OXPHOS have been developed. Anticancer strategies that combine classic immunotherapies with OXPHOS inhibitors for synergistic effect are also in practice.

Although tumor metabolic reprogramming has been reported many times in recent years, the concrete mechanisms of OXPHOS regulation in some types of tumors are still unclear, and the effect of OXPHOS changes on tumor behavior in vivo is not clear either, which needs to be further elucidated. As for current OXPHOS-targeted drugs, targeted drugs based on various pathways should continue to be explored, and their safety and efficacy in vivo are worth testing, more specific mitochondrial-targeted compounds and their combination usage with existing immunotherapies need to be more thoroughly investigated as well.

Authorship

X.T.Q: Writing - Original Draft; Y.L: Writing - Reviewing & Editing; Z.Y.Z: Conceptualization, Writing - Reviewing & Editing.

Funding

Open Access funding enabled and organized by Projekt DEAL. This work was financially supported by the National Natural Science Foundation of China (Grant No. 82103460 and No. 81972546), the Fundamental Research Funds for Central Universities.

Data availability

Not applicable.

Declarations

Ethics approval

No potential ethical issues were disclosed.

Conflict of interest

No potential conflicts of interest were disclosed.