The role of autophagy in regulating metabolism in the tumor microenvironment

Abstract

Autophagy, as a special programmed cell death, is a critical degradative process that eliminates intracellular abnormal proteins or damage organelles to balance cell energy and favor cell metabolism with autophagy-related (ATG) proteins. Autophagy activation is being increasingly recognized as an essential hallmark in tumorigenesis through influencing the metabolism of stromal cells in the tumor microenvironment (TME) which comprises of tumor cells, cancer-associated fibroblasts (CAFs), cancer-associated endothelial cells (CAEs), immune cells and adipocytes. Tumor cells can reuse autophagy-involved recycling to maintain mitochondrial function and energy supply to meet the metabolic demand of their growth and proliferation. However, the mechanism through which autophagy can promote a crosstalk between tumor and stroma cells is not clear. Reprogramed metabolism is one of the main characteristics of TME leading to higher adaptability of tumor cells with diverse mechanisms. The activation of autophagy has expanded our understanding on the interaction between tumor metabolism and TME. The aim of this review is to report recent advances on the metabolic cross-talk between stromal cells and solid tumor cells induced by autophagy in TME and revealed potential therapeutic targets.

Introduction

Autophagy is a complex catabolic process, which is characterized by encapsulation of dysfunctional proteins and organelles into autophagic vesicles and fusion of these vesicles via the lysosomal pathway to produce energy for maintaining cell metabolism and energetic homeostasis.¹ Autophagy is roughly divided into three types, including macro-autophagy, micro-autophagy, chaperone-mediated autophagy (CMA).² For all types of autophagy, the end products, such as amino acid, lipid and nucleotide synthesis and ATP production, are recycled to participate in various processes.

It was reported autophagy plays a complex role in several stages of tumorigenesis. Although was initially thought to be only a mode of cell death as a tumor-suppressive process, autophagy is now recognized as the pro-survival mechanism in most advanced human tumors.³ According to the current studies, autophagy can promote tumorigenesis through the following ways: (i) maintaining tumor cell survival and energetic homeostasis, (ii) promoting metastasis, (iii) maintaining the stemness of tumor cells, (iv) changing the tumor microenvironment, and (v) reducing immune surveillance.⁴

Tumor microenvironment (TME) is usually a hypoxic, nutrient deficient environment and only tumor cells with reprometabolic ability can survive in this harsh microenvironment. The metabolic substrate produced by the autophagy cycle participates in the production of metabolites to maintain cell growth, which is essential for tumor growth, invasion, metastasis and drug resistance. However, the mechanism is not clear. The aim of our work is to summarize the role of autophagy in the crosstalk between the tumor cells and the TME (Fig. 1). In this review, we will first focus on the core autophagy mechanisms, which regulates the TME metabolism to influence the biological behavior of tumors.

Figure 1.

The relationship of autophagy and tumor in the tumor microenvironment. Autophagy in the tumor microenvironment supports tumor cell growth through supply of nutrients and other factors. Metabolic crosstalk occurs between tumor cells and stromal cells through autophagy, which in turn affects their own development.

Pathways in regulating the autophagy

Taking the macroautophagy as an example, autophagy–lysosome process mainly contains four steps: (i) the isolation membrane formation; (ii) the autophagosomes formation; (iii) the autophagolysosome formation; (iv) the autophagolysosome degradation.⁵ And the process is regulated by a limited number of highly conserved genes named autophagy-related genes (ATGs) such as Atg8/light chain 3 (LC3) and Atg5-12/Atg16L1.⁶ LC3 is activated into LC3-I by the cysteine protease ATG4, then LC3-I combines phosphatidylethanolamine (PE) to generate LC3-II with lipidation reaction, which is the most well characterized to be a mark of autophagosomes formation.⁷

Autophagy is a complex process which can be regulated via various signaling pathways including mammalian target of rapamycin (mTOR), the phosphoinositide 3-kinase (PI3K)/Akt and Adenosine 5′-monophosphate-activated protein kinase (AMPK).² mTOR is activated with stimulus (amino acids and growth factors) to inhibit autophagy, which not only phosphorylates the downstream ATG proteins, but also phosphorylates eukaryotic translation initiation factor 4E binding protein 1 (4E-BP1) and p70 ribosomal protein S6 kinase (p70S6K).^8,9 On the contrary, autophagy is induced by nutrient deficiency or after rapamycin (mTOR inhibitor) treatment. In addition, the activated AMPK induces autophagy by suppressing the activity of mTORC1, and then indirectly allows activation of ULK1 (including ULK1, ULK2, ATG13, and FIP200)¹⁰ (Fig. 2).

Figure 2.

The pathways enrolled in autophagy. Autophagy can induce the cargo degradation to fuel cellular metabolism during stressors such as hypoxia, oxidative stress, nutrient limitation, low glucose and so on. Autophagy is activated by the reduced activity of the mTOR complex due to activated AMPK, p53 signaling and decreased upstream growth signaling such as PI3K/AKT pathway, and MAPK signaling.

Autophagy in tumor cells is induced to recycle metabolites as an energy supply to increase cell survival. Thus, autophagy may be a protection mechanism which helps tumor cells cope with many stressors.¹¹ Autophagy inhibitors, such as chloroquine (CQ), which prevents autophagosomes fusion with lysosomes and decreases lysosomal acidification,¹² have shown effect in malign tumors like glioblastoma (GBM). Glioma initiating cells (GICs) play an important role to tumorigenesis though autophagy. Studies showed that chloroquine increases antitumor effects of temozolomide (TMZ) by inhibiting the GICs in G2/M phase (DNA replication).¹³ Interestingly, ATG9A is the only upregulated ATG in GBM cells.¹⁴ Although a previous study argued that autophagy blocked migration and invasion of GBM by down regulation of SNAIL and SLUG (two master regulators of the EMT).¹⁵

Autophagy is crucial for the interactions between tumor cells and stroma cells. Tumor cells induce and exploit increased level of autophagy in stromal cells, in turn, autophagic stromal cells can supply the demands for tumor cellular energy and biomacromolecules synthesis by scavenging own damaged organelles. Namely, the stroma leads to a negative energy balance by autophagy, in favor of the tumor cells. In acid-conditioned stromal cells, the expression of ATG5 and BNIP3 increased, which suggests that autophagy is a survival mechanism in TME.¹⁶ When tumor cells are in coculture with stromal cells, they usually gain faster growth rates which is confirmed in AML cells or colon cancer cells.^17,18

Autophagy-activating conditions are often detected in tumor stroma, such as metabolic stress, oxidative stress and hypoxia. Reactive oxygen species (ROS) is elevated in almost all tumors, which are not restricted to the tumor cells but are observed in the stromal cells. High levels of ROS are detected in cryosectioned patient tumor biopsies (such as breast cancer), suggesting extracellular oxidative stress.¹⁹ Many evidences have supported the opinion excess ROS can induce tumor cell growth through autophagy. ROS-mediated autophagy is related to the redox-regulation of the Akt-mTOR and AMPK pathway.²⁰ Hypoxia involves in inhibiting mTORC1 activity although the mechanism has remained completely unresolved. Brugarolas et al suggested that the TSC1/2 complex and the REDD1 gene might provide an underlying mechanism to regulate mTORC1 activity.²¹ Hypoxia-inducible factors (HIFs) play a key role in adaptive mechanisms triggered in response to hypoxia-induced autophagy, such as HIF-1α. In most cancers, HIF-1α is closely associated with cellular survive.²² Interestingly, HIF-1α can regulate the autophagic process by the metabolic reprogramming of cancer.²³

The intersection of tumor metabolism and autophagy

Recently, there has been a growing interest in reprogrammed metabolism for tumors. With the rapid growth and limited nutrients, tumor can reprogram the cellular metabolism pathways, including anabolism and catabolism, and alter nutritional intake mechanism to support uncontrolled proliferation. Metabolic alterations were associated with the hallmarks of tumor tissues. Tumor cells under metabolic stresses created by extreme conditions such as hypoxia, low pH in the tumor microenvironment.¹ Generally, tumor cells use the aerobic glycolysis to sustain cell survival and proliferation, rather than oxidative phosphorylation in the mitochondria. The metabolic phenomenon of aerobic glycolysis metabolism is known as the “Warburg effect”, the metabolic switch. And Warburg attributed this metabolic shift to mitochondrial irreversible dysfunction in tumor cells.²⁴ Further research found that tumor cells with mitochondrial dysfunction showed the more aggressive phenotype.²⁵ The “Warburg effect” has been confirmed in various tumors, such as breast cancer. Breast cancer cells reprogrammed the cell metabolism to maintain their survival and growth in a harsh microenvironment. Ruprecht et al provided evidence for breast tumors acquiring lapatinib resistance by glycolytic posttranslational regulation and phosphorylation-mediated changes in glycolysis.²⁶ During aberrant glycolysis metabolism, glucose not only converts glycolysis into lactic acid to produce ATP, but also produces abundant precursors to meet the needs of tumor cells to synthesize biological macromolecules. In addition to abnormal glycolysis, other metabolisms are also altered.

Studies have shown that the TME and tumor metabolism are interconnected. The TME can regulate the pathway of tumor metabolism. Given that tumor cell metabolism is still exuberant and the proliferation is rapid, this adaptation is achieved by changing the way of tumor cells metabolize energy, which is called metabolism reprogramming.²⁷ With further researches, tumor metabolism is also related to other non-tumor cells in adjacent microenvironment, including cancer-associated fibroblasts (CAFs), endothelial cells (ECs), and immune cells. Stromal cellular state and phenotype are also changed because of aberrant tumor metabolism, which eventually lead to metabolic reprogramming of tumor microenvironment.²⁸ For example, the aberrant metabolism of breast cancer cells reshapes TME, promotes the production of vascular endothelial cells for angiogenesis and destroys the immunity system, thereby accelerating the development of the tumor.²⁹ Therefore, targeting tumor metabolism is potentially major advances in tumor therapy.

The role of autophagy for tumor metabolism is essential, although autophagy also contributes to other aspects for tumorigenesis. When tumor cells are metabolically stressed, it induces the autophagy of stromal cells to provide recycled nutrients and amino acids for tumor growth.³⁰ Autophagy can participate in various metabolic alternations, providing tumor cells with huge metabolic plasticity. Autophagy in the TME affects mitochondrial metabolism, including glycolysis, lipid metabolism, amino acid recycling for the tumor survival in Ras-driven cancer, in order to adapt nutrient deprivation and other stresses of tumor metabolism.¹

Autophagy in regulating the metabolism of CAFs

CAFs are the most important components of the TME and are associated with autophagy. It is clear that CAFs can promote many aspects of tumor progression including tumor growth, metabolism, invasion and metastasis.³¹ CAF can exert its tumorigenic function directly (intercellular communication) and indirectly (soluble factors). Loosely arranged fibroblasts communicate with adjacent stromal cells such as ECs, immune cells to regulate various tumor-promoting processes.³² On the other hand, fibroblasts secrete essential growth factors especially the ligands of tyrosine kinase receptors, to enhance the malignant phenotype of tumors. For example, CAFs can enhance breast cancer cells invasion and migration abilities by secreting MFAP5 and activating the Notch1/Slug pathway.³³

Autophagy is believed to be a hallmark of tumor development. Many evidences suggest that the expression of ATGs (such as LC3, Beclin-1 and SQSTM1/p62) in CAFs might be promising indicators of tumorigenesis and prognosis.³⁴ Deletion of essential ATGs impairs metabolism, survival and malignancy of tumors in spontaneous cancer models.³⁵ In an indirect co-culture system, fibroblasts in TME influence the metabolism and growth of colorectal cancer cells via autophagy and oxidative stress pathway.¹⁷ Besides, autophagy is reported to link with cellular secretion. Upregulated autophagy promotes CAF secreting IL-6 and IL-8 in neck squamous cell cancer (HNSCC).³⁶

Recent studies have highlighted the metabolism alternation in CAFs which explored its effects on tumor development via autophagy. High ROS or other stressors might lead to such metabolism alternation. CAFs are forced to shift their metabolism toward glycolysis and provide energy-rich nutrients to promote tumor growth, including l-lactate, ketone bodies, glutamine and free fatty acids for neighboring tumor cells. The aberrant glycolysis is termed the “reverse Warburg effect”.³⁷ Intermediate metabolites also can cause tumor recurrence. A study found that some intermediate metabolites (such as β-hydroxybutyrate) in CAFs can promote irradiated lung cancer relapse though autophagy.³⁸ The same mechanism was reported for breast cancer.³⁹ Cav-1 is the principal structural protein of caveolae. Loss of Cav-1 in CAFs is shown to be a hallmark of poor clinical prognosis in breast cancer, pancreatic cancer and lung cancer.40, 41, 42 Cav-1^−/− fibroblasts can upregulate the expression of glycolytic enzymes (such as PKM2 and LDH-B), which are reported to be novel biomarkers for the “reverse Warburg effect”.⁴³ What is the mechanism that induce stromal Cav-1 decreasing to influence the metabolic reprogramming of CAFs? Autophagy plays a role. In a novel co-culture breast tumor model, Cav-1 downregulation is mediated by autophagic/lysosomal degradation, and lysosome-specific inhibitors or chloroquine can reverse Cav-1 expression.⁴⁴ Low Cav-1-driven autophagic CAFs also secret more ROS, l-lactate and MCT-4 involving mitochondrial metabolism and anabolic growth of OSCC.⁴⁵

Beyond glycolysis, autophagy increases amino acids production in CAFs. The amino acids (like arginine) are provided to tumor cells in a paracrine manner to maintain their growth.⁴⁶ Pancreatic stellate cells (PSCs) are the precursors of CAFs, which are critical for pancreatic ductal adenocarcinoma (PDAC) metabolism by secreting nonessential amino acids (NEAA). A study clarifies that alanine generated supplies carbon for the TCA cycle, involving in oxygen consumption and lipid synthesis in associated tumor cells via autophagy.⁴⁷ Autophagy-dependent CAFs further alter the actual level of autophagy and metabolism in adjacent tumor cells. This condition is well documented in pancreatic and ovarian cancer studies.^47,48

Autophagy-mediated regulation in macrophages metabolism

Macrophages are the dominating immune cells in most solid tumors, changes in TME can alter the phenotype and metabolism of macrophages. The metabolic characteristics of M2 macrophages are fatty acid oxidation (FAO) and TCA cycle while M1 macrophages show a glycolytic profile by increasing levels of some glycolytic regulators and lactate. In a HIF-1-dependent pathway, autophagy is related to macrophage polarization via NIX-dependent mitophagy pathway, promoting a glycolytic switch.⁴⁹ CQ can transform TAMs from M2-to M1-type. CQ increases lysosomal pH in TAMs and activates calcium dependent transcription factor EB (TFEB), which reprograms the TAMs metabolism from oxidative phosphorylation to glycolysis.⁵⁰ CQ-induced TAMs repolarization plays a key role in tumor growth inhibition and immune microenvironment amelioration.⁵¹ However, lactate, a metabolite, activated mTORC1 that suppresses the expression of TFEB-mediated ATP6V0d2 (vacuolar ATPase subunit). In a in vivo study, Atp6v0d2^−/− mice acquire more protumoral phenotype. One of the major mechanisms of immune suppression is the content and the functional activity of arginase in TAMs. Some researches show that arginase downregulates autophagy by inhibiting proliferation, NO and ROS production in macrophages.⁵² Therefore, the role of autophagy induced metabolic reprogramming in macrophages depend on cellular types and surrounding microenvironment. Metabolism alterations could be a new direction for regulating cancer immunotherapy.

Autophagy-mediated regulation in T cells metabolism

When T cells are recruited to the tumor microenvironment to kill tumor cells, metabolic signaling, a metabolic immune checkpoint, will inhibit their normal function, such as CTLA-4, PD-1, and PD- L1. Activated CD8⁺ T cells undergo metabolic shift and switch from oxidative phosphorylation to aerobic glycolysis. This transition favors growth and differentiation of cytotoxic T cells, which produces inflammatory cytokines.⁵³ Simultaneously, the role of autophagy in T cells metabolism is also highlighted. Decreased glucose can downregulate IFN-γ transcription and inhibit T cell proliferation, causing their weakened mTOR activity.⁵⁴ These metabolic checkpoint blockade antibodies increase glucose in TME, promoting T cells glycolysis and IFN-γ production.⁵⁵ Zhang et al demonstrate that CQ can inhibit TGF-β secretion, reprograms metabolism and boosts the immune system in mice via upregulating CD8⁺ T cell.⁵⁶ Lactate blocks the differentiation and activation of T cells to promote tumors. Recent studies have suggested that lactate directly impairs nuclear factor of activated T-cells 1 (NFAT) activity, which decreases IFN-γ production.⁵⁷ In addition, hypoxia influence the synthesis and function of T cells through autophagy. These metabolites and stressors in TME might be the mechanism of tumor cells metabolism reprogramming to mediate immune escape. Autophagy regulates tumor cells and immune cells in the tumor microenvironment in both directions. For example, autophagy in hepatocyte lead to accumulation of intracellular lipid droplets, the release of linoleic acid and the depletion of liver CD4⁺ T cells to induce immunosuppressive microenvironment and promote tumor progression.⁵⁸

Autophagy-mediated regulation in DCs metabolism

DCs are the crucial target for the tolerance in tumor antigens. Recent studies have showed lipid metabolism is complex with the regulation of DC function. Unexpectedly, DCs inhibit T cells function and promote tumor progression, causing oxidized lipids accumulation.⁵⁹ Accumulated fatty acids support FAO, which plays an important role for tolerogenic DCs. AMPK promotes autophagy and antagonizes mTORC by increased PGC-1α level,⁶⁰ an essential regulator of energy metabolism involved in catabolic metabolism by oxidizing fatty acids, amino acids, and glucose. Increased expression of the IL-10 via activating the AMPK pathway⁶¹ leads to high levels of the adiponectin receptors (AdipoR1) in DCs and inhibit DC activation. Generally, these studies show that the balance of DC immunogenicity and tolerance reflects the balance of anabolic and catabolism related with autophagy.

Autophagy-mediated regulation of neutrophil differentiation and metabolism

Neutrophils are the main effector cells in human blood participating in various immune processes like immune response in tumor, pathogens, etc. Major mechanisms of neutrophil induced immune response including cell degranulation, phagocytosis function, ROS producing and production of neutrophil extracellular traps (NETs).⁶² Up-regulation of autophagy can maintain mitochondrial stability and the survival of neutrophils in the TME. Autophagy is important for neutrophil differentiation through some metabolites. In the early stage of neutrophil differentiation, autophagy restricts glucose uptake and pyruvate is converted to lactic acid for mitochondrial respiration through the Cori cycle. Neutrophils play a role in preventing endogenous cell apoptosis through the release of Cytochrome c. However, in order to meet the energy requirements of neutrophil production, neutrophil differentiation relies on free fatty acids provided by autophagy, which can support mitochondrial respiration.^63,64 Thus, the metabolism type of earlier neutrophil transfers from glycolysis to fatty acid oxidation. Thomas and his colleagues agreed with this view.⁶⁴ Atg7-deficient neutrophil precursors have increased glycolysis, accumulated lipid droplet, and consumed ATP. Besides, it has been reported that deficient autophagy in neutrophils can suppress NADPH oxidase, causing lower level of neutrophil degranulation.⁶⁵ NADPH is also importantly involved in cell apoptosis related to NETs. Suppression of autophagy or NADPH oxidase activity and influence cell apoptosis strategy, reflected by more apoptosis rather than NETosis.⁶⁶ NETs formation is also relied on high glucose and glutamine.⁶⁷ However, inhibition of ATP synthase has almost no effect on NET formation, and glycolytic bypass has little effect on NETosis.⁶⁷ Autophagy's role in maintaining cell glucose/glycogen and other metabolites is also related metabolic mechanism nets formation.

Autophagy-mediated regulation in endothelial cells metabolism

In addition to CAFs and immune cells, ECs in TME are also known as an essential element for angiogenesis in TME.⁶⁸ Angiogenesis not only supplies oxygen and nutrients for tumor growth but also provides a gateway for metastasis via various complex ways.⁶⁹ The unbalance between pro- and anti-angiogenic signals, exacerbates new abnormal blood vessel. ECs, the major components of the vasculature, are activated into CAEs, which are exposed to harsh TME conditions (such as nutrient-deprivation). Besides, ECs are one of the main sources of CAFs. CAEs are characterized with high permeability and low maturation, and weak structure to further aggravate hypoxia, acidity, and inflammation.⁷⁰ Emerging evidence suggests that autophagy is upregulated in CAEs. CAEs have higher expression of BNIP3 than normal ECs, indicating a more obvious autophagic phenotype.⁷¹ Correspondingly, CQ can promote vasculature normalization and reduced maintain redox state in tumors, improving vessel structure and maturation. These mechanisms are associated with VEGF receptor (VEGFR)2 phosphorylation and angiopoietin-1 (Angpt1) unregulation.⁷² The adherence and tight junctions between ECs with other stromal cells cause weak structure in tumor blood vessels. Mase et al suggest CQ can improve this phenomenon via endothelial surface CDH5 expression to decrease tumor cells intravasation and metastasis⁷³ although ATG5-deficiency in ECs do not improve these features.⁷² In a primary mouse melanoma tumor model, BECN1^+/− mice showed an aggressive phenotype with increased angiogenesis compared to the wild-type mice.⁷⁴ These studies clarified autophagy may regulate other pathways instead of ATG5 and BECN1 dependent pathways. Thus, to get further insights into the connection between autophagy and ECs in TME is essential for tumor cells.

In addition, to keep redox homeostasis and regulating ECs permeability, autophagy also meets the increased metabolic demands in the hyper-proliferating CAEs under the metabolic stressors in TME. Hypoxia in CAEs stimulates autophagy and stabilizes the α-subunit of the HIF-1α in ECs. HIF-1α can be translocated to the nucleus to increase the expression of VEGF and glycolytic related genes.⁷⁰ An experimental study showed that glycolysis could play an important role in ECs metabolism due to generating more glucose, glutamine, or fatty acids.⁷⁵ In the hypoxic coculture conditions, lactate and glycolysis-related ATP are increased in c-HUVECs, which indicates glycolysis could be activated in c-HUVECs but suppressed in c-T24 cells in bladder cancer.⁷⁶ Both glycolysis-derived lactate and hexosamine biosynthesis pathway (HBP) play a key role in angiogenesis. For example, HBP involves in VEGF2 glycosylation.⁷⁷ Lactate inhibits the prolyl hydroxylase 2 and activating HIF1α and NF-κB to promote angiogenic phenotype formation and VEGF secretion.⁷⁸ It was reported that glutamine exerted the same effects as lactate to promote angiogenesis.^78,79 Glutamine metabolism is also considered to be an ECs proliferation promoter but not migration.⁸⁰ Previous studies clarified lipid metabolism could be important fuels for ECs. Autophagy supports metabolic rewiring by degrading lipid droplets. And FAO helps de novo nucleotide synthesis for DNA replication in ECs. CPT1A (a rate-limiting enzyme of FAO) inhibits pathological angiogenesis.⁸¹ Interestingly, cholesterol metabolism is a key modulator in ECs. Inhibiting cholesterol trafficking causes the dissociation of mTOR from the lysosomes, further inhibiting its downstream signaling.⁸² ECs metabolism alterations via autophagy related to tumor progression provide novel insight into anticancer therapies.

Autophagy-mediated regulation in adipocytes cells

Adipocytes are the main site for energy storage in TME, the role has not been elucidated in tumors until recent years. These stromal adipocytes can contribute to the tumor development in a variety of ways.⁶⁸ Adipocytes produce various factors, such as adipokines, growth factors, fatty acids, and some metabolites into the TME, all of which affect tumor growth, metastasis, and drug resistance.^83,84 Adipocytes are transformed by neighboring tumor cells into cancer-associated adipocytes (CAAs) to acquire an activated phenotype. Growing evidence shows that adipocytes play an essential part in breast cancer. A cross-talk between tumor cells and adipocytes is conformed in the breast solid tumor that leads to acquisition of malignant phenotype. In a co-culture system of adipocytes and breast cells, adipocytes are activated and release proinflammatory cytokines (IL6, IL8) and MMPs (MMP9, MMP11) to the TME. Alternatively, these cells alter their metabolism to promote tumor cell invasion and metastasis. In addition to provide abundant lipid, elevated fatty acid levels caused by abnormal adipocytes metabolism causes the dyslipidemia syndrome,⁸⁵ which is related to increased risk factors for breast cancer. MMP9 and IL6 in adipocytes are also reported as main modulators for prostate cancer cells proliferation, angiogenesis and motility.⁸⁶ Another study using an ovarian cancer and adipocyte co-culture system showed that adipocytes promote homing, metastasis of ovarian cancer cells by secreting adipokines and IL8.⁸⁷

As early as the 1960s, it was discovered that mutations in lysosomal acid lipase (LAL) caused lipid accumulation in many tissues. It showed that autophagy was related to adipose tissues. Accumulating evidence suggests that autophagy is deeply involved in adipocyte differentiation. When mouse embryonic fibroblast cells (MEFs) are induced to differentiate into adipocytes, autophagy is activated and some cytoplasmic components including mitochondria are engulfed in autophagosomes. It means autophagy involves in cytoplasm remodeling during adipogenesis. Another study shows that autophagy-deficient (Atg7^−/−) MEFs form adipocytes at a significantly lower rate than normal MEFs, and differentiation is stagnated in the early adipogenesis stag, formed much small lipid droplets (LDs).⁸⁸ In vivo experiment, loss of Atg5 in late embryos and newborn mice have less subcutaneous 3T3-L1 adipocytes than wild type mice.⁸⁸ Current finding reveals that differentiation of 3T3-L1 pre-adipocytes cells is related to activated mTOR pathway.⁸⁹

Autophagy and adipocytes metabolism are intertwined in TME, co-regulating the energy balance in the microenvironment, which contributes to tumorigenesis. During nutrient deprivation, the energy stored in adipocytes in the form of triglycerides is hydrolyzed into free fatty acids (FFAs) through adipose triglyceride lipase (ATGL), which transported to various tissues and organs through the blood transport to support the cells growth. Simultaneously, LC3 binds to autophagy-mediated LDs to start the autophagy process.⁹⁰ However, inhibition of autophagy in cultured hepatocytes increased triglyceride storage.⁹⁰ Research shows, increased size and number of adipocytes cause instability of the TME and hypoxia, which further enhances autophagy. Multiple inflammatory factors (TNFα, IL-6, IL-1β, and MMP-11) released by CAAs can affect cellular metabolism. For example, TNF-α affects lipid metabolism under pathological conditions by regulating autophagy levels by secreting Atg5, Atg7 and BECN1 in adipocytes. Maresin1 (MaR1) could ease these alterations on lipolysis and autophagy in adipocytes by inhibiting the TNF-α-induced glycerol release.⁹¹

Interestingly, adipocytes are known as a source of CAFs like endothelial cells.⁹² The reverse Warburg effect has also been inducted in CAAs.⁹³ Tumor cells rely on CAAs to regulating cellular energy and adapting to harsh TME. As mentioned before, active CAAs can enhance their lipid synthesis and lipolysis to release more FFAs.⁹⁴ The tumor cells uptake these FFAs from surrounding microenvironment for producing ATP and lactate through fatty acid β-oxidation (FAO) pathway or saving them in LDs as triglycerides. The citrate derived forms glycolysis and glutaminolysis is involved in lipid and cholesterol synthesis for energy storage. During hypoxia, tumor cells acquire FFAs by increasing the expression of CD36 (the fatty acid translocase). Omental is the most important site of ovarian cancer metastasis, which is an adipocyte-rich microenvironment. Adipocytes alter tumor metabolism by upregulating CD36 levels in ovarian cancer cells. Co-culturing with adipocytes, the ovarian cancer cells express the fatty acid-binding protein 4 (FABP4).⁸⁷ A recent study shows that FABP4 plays an essential role in mediating lipolysis and FA metabolism.⁹⁵ In addition to FFAs, inhibition of CD36 decreases cholesterol accumulation and intracellular ROS content.⁹⁶ ROS production in mature adipocytes can induce mitosis in tumor cells.⁹⁷ Alternatively, carnitine palmitoyl transferase 1 (CPT1) protein is a rate-limiting protein of FAO to transport of acyl-CoAs into the mitochondria. Tumor cells are stimulated to produce more CPT1, enhancing FAO rates under metabolic stress.⁹⁴

Conclusions and perspectives

In the past, researchers paid more attention to the role of tumor cells themselves and ignored the influence of TME. The TME is not a passive onlookers but an active participant during tumor development. TME is characterized by hypoxia and acidity, contributing to an altered cellular metabolism. Various evidence has demonstrated the importance of autophagy in the metabolism in TME. Cytokines, growth factors, and nutrients secreted by autophagy are released and utilized by surrounding tumor cells. Future research may selectively target stromal cells metabolism to treat cancer (Fig. 3). However, the role of autophagy is dual, and the reliance on autophagy is different in stromal cell types, marking it hard to evaluate the outcome of autophagy inhibition in tumor. Thus, we need clarify the potential research on autophagy and complex microenvironment in more detail to guide the design of future clinical studies.

Figure 3.

The role of autophagy in tumor microenvironment. Tumors are composed of tumor cells, stromal cells, and extracellular matrix. Most of them interact with each other through indirect cell contact. In the tumor microenvironment, cells often face varying degrees of oxygen and nutrient deficiency, which can change the metabolism of tumor cells and stromal cells to induce tumor reprogramming. At the same time, nutritional stress can also trigger autophagy to ensure cell survival, or exit cell death if overwhelmed. Autophagy can add the cross-talk between cancer cells and stromal cells via the multiple metabolic compartments (such as fatty acid, FAO, linoleic acid) in tumor microenvironment. Moreover, autophagy in tumor cells can lead to the secretion of different factors (inflammatory cytokines, growth factors and ECM-modulating factors), which influences macrophage polarization, the activation of CAFs, CAEs and the recruitment of NK and T cells. Finally, autophagy in TME can maintain tumor cell growth by recovering various intracellular components (such as amino acids, glucose) through various signaling modification.

Author contributions

Panpan Zhang drafted- original draft, Huijuan Dai revised the manuscript, Shanshan Cheng and Xiaonan Sheng drew the figure. Yueyao Du and Kang He provided direction and revised the manuscript, funding acquisition, writing review & editing.

Conflict of interests

Authors declare no conflict of interests.

Funding

This study was supported by the Multidisciplinary Cross Research Foundation of Shanghai Jiao Tong University, China (No. YG2019QNA26), the National Natural Science Foundation of China, China (No. 82002777) and the Project of the Shanghai Municipal Health Commission, China (No. 20204Y0012).