Metabolic adaptations of cancer in extreme tumor microenvironments

Abstract

Cancer cells are highly heterogeneous to adapt to extreme tumor microenvironments (TMEs). TMEs challenge cancer cells via hypoxia, nutrition starvation, and acidic pH, promoting invasion and metastasis concomitant with genetic, epigenetic, and metabolic alterations. Metabolic adaptation to an extreme TME could allow cancer cells to evade cell death and immune responses, as well as resulting in drug resistance, recurrence, and poor patient prognosis. Therefore, elucidation of the metabolic adaptation of malignant cancer cells within TMEs is necessary, however, most are still elusive. Recently, adaptation of cancer cells within the TME can be analyzed via cell–cell interactions at the single‐cell level. In addition, information into organelle–organelle interactions has recently been obtained. These cell–cell, and organelle–organelle interactions demonstrate the potential as new cancer therapy targets, as they play essential roles in the metabolic adaptation of cancer cells to the TME. In this manuscript, we review (1) metabolic adaptations within tumor microenvironments through (2) cell‐to‐cell, and (3) organelle–organelle metabolic interactions.

Cancer cells are highly heterogeneous to adapt to extreme tumor microenvironments. Tumor microenvironments challenge cancer cells via hypoxia, nutrition starvation, and acidic pH, promoting invasion and metastasis concomitant with genetic, epigenetic, and metabolic alterations. In this manuscript, we review (1) metabolic adaptations within tumor microenvironments through (2) cell‐to‐cell, and (3) organelle–organelle metabolic interactions.

Abbreviations

ACSS2

acyl‐CoA synthetase short‐chain family, member 2

ATF

activating transcription factor

BCAA

branched chain amino acids

ER

endoplasmic reticulum

FAK

focal adhesion kinase

FOXO

Forkhead box O

HIF1α

hypoxia‐inducible factor

IRE1a

inositol requiring enzyme 1a

MMP9

matrix metalloproteinase‐9

ODD

oxygen‐dependent degradation

PERK

PKR‐like endoplasmic reticulum kinase

PHD

prolyl hydroxylase

TCA

tricarboxylic acid

UPR

unfolded protein response

VEGF

vascular endothelial growth factor

1. INTRODUCTION

Cancer cells adapt to extreme conditions within tumor microenvironments (TMEs), such as hypoxia, nutrient starvation, and acidic pH, leading to cancer cell malignancy. The metabolic adaptation of cancer cells was originally reported in the Origin of Cancer by Dr. Otto Warburg, demonstrating that cancer cells acquire metabolic alterations compared with normal cells, in the 1950s. ¹ This phenomenon emphasizes the importance of tumor hypoxia and metabolic adaptations. Indeed, cancer cells in a hypoxic TME exist at an acidic pH, because of the promotion of glycolysis (Warburg effect) via HIF1α, accompanied by lactate and H⁺ extracellular efflux, as well as the accumulation of lactate. ² , ³ , ⁴ In addition to glycolysis, lipid metabolism, one‐carbon metabolism, acetate metabolism, and amino acid metabolism have recently received attention in cancer metabolism. In addition to the glycolytic and lipid metabolic adaptations, metabolic adaptation toward essential amino acids, such as leucine and valine, not only promote cancer progression via the mTOR complex but also cooperate with the stress adaptation mechanisms of HIF1α, SREBP2, and ATF4. ⁵ , ⁶ , ⁷ Amino acid metabolism via the mTOR complex is considered to be the primary signaling cascade. However, mTOR inhibitors, such as rapamycin, are not complete cures for cancer, suggesting the existence of an adaptive amino acid metabolism that is not mediated by the mTOR complex. ⁸ Metabolic signaling by glucose, fatty acids, and amino acids may differ between cell–cell and organelle–organelle interactions. Therefore, a comprehensive understanding of cancer metabolic systems may lead to breakthroughs in understanding cancer pathogenesis. In this review, we discuss (1) the metabolic adaptations of cancer cells in TMEs, (2) the cell–cell metabolic interactions in TMEs, and (3) the organelle–organelle metabolic interactions in TMEs.

2. METABOLIC ADAPTATIONS OF CANCER CELLS IN TMEs

Genetic mutations in primary cells causes cancer development. Overall, 5%–10% of all cancers are derived from acquired genetic defects, most of which are a consequence of the environment and lifestyle. ⁹ Druggable targets are, therefore, shifting to tumor microenvironmental factors rather than cancer cells. Such TME conditions include hypoxia, nutrient starvation, and acidic pH microenvironments leading to tumor progression regardless of genetic mutations.

TMEs are thought to be important for cancer malignancy. Distance from functional blood vessels makes a difference to the oxygen supply. The cell layers ~85–100 μm from tumor vessels demonstrate hypoxia (O₂ concentration of less than 1%). ¹⁰ Cancer cells exposed to a hypoxic TME promote tumor angiogenesis to resolve the blood flow insufficiency. Essential factors in this environmental response mechanism include HIF1 and HIF2. Under normoxic conditions, the two proline residues in the oxygen‐dependent degradation domain region of each HIF‐α subunit are hydroxylated by PHDs. E3 ubiquitin ligase then recognizes and binds to HIF‐α on the hydroxylated site, leading to proteasome‐dependent degradation of HIF‐α. Thus, a low intracellular abundance of HIF‐α subunits is maintained under normoxia. ² , ³ , ⁴ Conversely, as the hydroxylation activity of PHD is reduced under hypoxic conditions, the HIF‐α subunit stabilizes and migrates into the nucleus to form a heterodimer with the HIF‐1β subunit. The heterodimer is required for the expression of VEGF. The other HIF‐1β subunits also stabilize and migrate into the nucleus to form heterodimers with HIF‐1β subunits, inducing VEGF and angiogenesis. Furthermore, these heterodimers cause the expression of genes involved in glycolysis, such as LDHA and GLUT1. In addition, HIF is known to induce the expression of genes related to metastasis and invasion, and the hypoxic adaptation response is thought to contribute significantly to the metabolic adaptations of cancer cells ¹¹ , ¹² (Figure 1). We have previously investigated multilayered analyses in hypoxia and nutrient starvation and reported that hypoxia, nutrient starvation, and acidic pH had distinct genetic, epigenetic, and metabolic regulations. ⁸ , ¹³ , ¹⁴

FIGURE 1.

A tumor‐specific microenvironment is established as a result of incomplete vasculature. Nutrients and oxygen are less available to the cells far from blood vessels. Enhanced glycolysis, also known as the Warburg effect, induces lactate efflux leading to acidic pH. Lipid metabolism can be accelerated within the avascular tissues. Cancer cell‐specific metabolic alterations in the extreme microenvironment are also involved in the maintenance of cancer stemness, induction of angiogenesis, and metastasis.

Other tumor microenvironmental stress is known as ER stress. Inactivation of the ER stress response and inactivation of the PI3K–Akt pathway have also been identified as metabolic adaptations to nutrient‐starved environments. ⁵ , ⁶ , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ The ER stress signaling pathway is activated by the activating transcription factor (ATF6). The ER stress signaling pathways, known to include the ATF6, IRE1a, and PERK, are activated by abnormal protein folding due to nutrient starvation. When unfolded proteins accumulate in the ER, the stress response (UPR) from ATF6, IRE1a, and PERK is often activated simultaneously, inducing apoptosis. ¹⁵ , ¹⁶ The PI3K–Akt pathway is inactivated during nutrient starvation, leading to the inactivation of mTOR complex1 (mTORC1) and activation of FOXO family transcription factors. mTORC1 is a serine–threonine kinase that plays a central role in nutrient signaling and regulation of intracellular biosynthesis. It is reported that mTORC1 is inhibited upon nutrient starvation, resulting in the induction of autophagy through dephosphorylation of Unc‐51‐like kinase 1 (ULK1). Upregulated autophagy restores nutrient levels in cells, enabling biosynthesis and ATP production. ¹⁹ The ER stress response is thought to be involved in the adaptation to nutrient starvation, ¹¹ , ¹² , ¹⁵ however, metabolic adaptation although ER stress is largely elusive.

Cancer cells in hypoxic regions survive on anaerobic glycolysis rather than oxidative phosphorylation (Figure 1). The resulting accumulation of lactate and protons in the TME causes an acidic microenvironment with a pH of less than 6.8. ²⁰ , ²¹ Extracellular acidity promotes cytoskeletal remodeling and Rho‐GTPase activation via β1‐integrin‐activated FAK signaling, ²² increases the production of MMP9, ²³ affects the subcellular distribution of lysosomes ²⁴ and increases invasive capacity through aggressive extracellular matrix (ECM) degradation and the acquisition of migratory capacity. Recently, it has been reported that targeting tumor acidity by inhibiting glycolysis or monocarboxylate transporters (MCTs) boosts the therapeutic effect of immune‐based cancer therapy, ²⁵ , ²⁶ further demonstrating the significance of acidic microenvironment on tumor progression.

Multiple studies, including ours, have reported the production of energy, acetate, and glutamine metabolism in addition to glycolysis. ²⁷ , ²⁸ , ²⁹ , ³⁰ Also, several metabolic pathways have been reported to contribute to cancer cell survival and malignant transformation. ³¹ , ³² , ³³ Recently, it has been shown that under nutrient‐deprived conditions, the source of acetyl‐CoA is changed from citrate to acetate in cancer cells by the enzyme ACSS2, which produces acetyl‐CoA from acetate ³⁴ , ³⁵ , ³⁶ , ³⁷ (Figure 2). We also reported that glutamine deprivation makes cancer cells resistant to the RNA polymerase I inhibitor through an impaired p53 activation pathway. ¹³ Furthermore, it has been shown that SREBP2, a regulator of cholesterol metabolism, is activated in acidic pH environments, which also induces metabolic shifts. ¹⁴ These findings suggest that cancer cells maintain proliferation by skillfully shifting metabolic pathways under adverse conditions, such as hypoxia, nutrient starvation, and acidic pH. This makes factors related to cancer metabolism new targets for drug discovery.

FIGURE 2.

Distinct epigenetic metabolic regulations of cancer cells within the tumor microenvironments. (A) Epigenetic regulations of cancer cells were investigated in PANC1 cells for hypoxia, nutrient starvation, and acidic pH. Open chromatin region from top 2000 peaks were also distinct between the conditions. (B) Acidic pH tumor microenvironment stimulated cholesterol biosynthesis and acetate switching compared with glycolysis under hypoxia, and amino acid metabolism under nutrient starvation.

3. CELL–CELL METABOLIC ADAPTATIONS IN TMEs

The metabolic adaptations within the TME are not only dependent on the response of cancer cells to nutrient or oxygen availability but are also shaped by the interactions between different types of cells, including cancer‐associated fibroblasts (CAFs) and immune cells ³⁸ (Figure 3). It is widely recognized that cancer cells and stromal cells support tumor growth, whereas cytotoxic T cells suppress tumor progression. Immune cells, such as myeloid cells and neutrophils, have both a tumor‐promoting group and a tumor‐suppressing group, and therefore they are classified into subtypes according to their distinct functions. ³⁹ Central carbon pathways, such as glycolysis and TCA cycle, are essential, not only for cancer cell proliferation, but also for stromal cells, myeloid cells, and T cells to maintain their cellular function. ⁴⁰ , ⁴¹ , ⁴² Thus, different types of cells compete for essential nutrients inside the TME. For example, increased methionine consumption by cancer cells results in methionine depletion in cytotoxic T cells, leading to impaired T‐cell function. ⁴³ Although glucose is the primary energy source used by most cells, a recent study highlighted that glucose is not limiting within the TME. ⁴⁴ Conversely, certain amino acids can be limiting in the TME, including tryptophan, alanine, arginine, serine, and glycine, which are needed not only for cancer cell proliferation but also for cytotoxic T‐cell function. ⁴⁵ Known as a survival mechanism for nutrient‐deprived cancer cells, autophagy in cancer cells also orchestrates the immunosuppressive environment in tumors. ⁴⁶ , ⁴⁷

FIGURE 3.

TME consists of different types of cells including endothelial cells, cancer cells, cancer‐associated fibroblasts (CAFs) and immune cells. (A) Abnormal vasculature results in a limited nutrient source in the tumor center. Therefore, different cells need to compete for nutrients essential for growth and cellular function. Cancer cells reprogram metabolism in adjacent stromal cells or immune cells, causing the latter to provide nutrients, such as amino acids, that can be utilized for cancer cell growth or secrete signaling metabolites that maintain an immunosuppressive microenvironment. (B) Hypoxic tumor microenvironment triggers differential gene expression pathways in cancer cells, fibroblasts and monocytes/macrophages. Under hypoxia, HIF1ɑ‐mediated pathways such as glycolysis are universally upregulated in all three cell types, but HIF1ɑ independent pathway is uniquely upregulated in monocytes/macrophages.

Cancer cells utilize metabolic crosstalk between different cell types to obtain essential nutrients for proliferation and/or survival. Stromal cells, such as CAFs, as one of the most abundant components within the TME, are a potential source of nutrient providers for cancer cell growth. Recently it has been reported that, in pancreatic tumors, stroma‐associated pancreatic stellate cells secrete alanine to support TCA cycle metabolism in cancer cells. ⁴⁸ Meanwhile, another group has reported that pancreatic tumors reprogram the branch‐chain amino acid metabolism in stromal cells, revealing that CAF‐derived branched chain amino acids rescue pancreatic cancer cells that heavily rely on branch‐chain keto acids for carbon and nitrogen demand. ⁴⁹ Furthermore, in ovarian cancer, an enhanced level of glutamine synthesis gene GLUL in CAFs is coupled with an enhanced level of glutaminases in cancer cells, demonstrating that CAFs support cancer proliferation through the supply of glutamine. ⁵⁰ In addition, adipocytes provide fatty acids for tumor progression in ovarian cancer. ⁵¹ By contrast, cancer cell‐derived metabolites, such as kynurenine and lactate, enhance immune‐suppressive functions in myeloid cells and regulatory T cells, contributing to tumor growth. ⁵² Finally, metabolic impairment of T cells in the TME is caused by chronic antigen stimulation and immune inhibitory receptors. ⁵³ However, the mechanism underlying multicellular metabolic crosstalk in the TME is not entirely understood (Figure 3A). Also, a current limitation in examining in vivo cancer metabolism is the inability to determine whether the metabolite signals detected come from the cancer cells or other cells present within the TME. Our group is investigating the metabolic alterations depending on the cell‐specific manner (Figure 3B). Such limitations can be overcome in the near future by the development of single‐cell resolution metabolic analyses in combination with single‐cell RNA epigenetic analyses.

4. ORGANELLE–ORGANELLE METABOLIC ADAPTATIONS IN TMEs

Metabolic adaptations can be observed, not only within tissues and multicellular communications, but also in organelle communication. Organelles regulate diverse physiological processes and play a central part in intercellular and intracellular metabolic adaptations. For example, lysosomes are proteolytic and multifunctional organelles that regulate cellular material degradation, autophagy, antigen processing, cellular transport, plasma membrane repair, and exosome biosynthesis. ⁵⁴ They are also known to localize mTOR, an amino acid‐sensing mechanism. ⁷ , ⁵⁵ Mitochondria are dynamic organelles that undergo fusion and fission, are sites of ATP biosynthesis, serve as transient intracellular stores of Ca²⁺, and regulate apoptosis and cell metabolism. ⁵⁶ , ⁵⁷ , ⁵⁸ The Golgi apparatus consists of the cis‐, medial‐, and trans‐Golgi apparatus, which jointly perform quality control of secreted proteins through the glycosylation and funneling of secreted proteins to carriers transported to various intracellular locations. ⁵⁹ , ⁶⁰ The ER is the site of protein biosynthesis and folding of all secretory proteins. The ER is also crucial for unprotected protein reactions and communicates with nearly all other organelles through interorganelle interactions. ⁶¹ , ⁶² Recently, it has been reported that the disruption of ER morphology is strongly associated with obesity‐induced metabolic diseases. ⁶³ Endocytosis is essential for cell survival, and various receptors and some pathogens are taken up by intracellular vesicles using endocytic pathways to access the cytoplasm, which also results in various cellular effects, such as signal transduction and metabolic regulation. ⁶⁴ , ⁶⁵ , ⁶⁶ Endosomes formed by endocytosis interact with various organelles and regulate their functions. Therefore, organelles play a crucial role in intercellular and intracellular communication, which, if dysregulated, can lead to diverse pathophysiologies, including cancer, metabolic, cardiovascular, and neurodegenerative diseases.

Organelle morphology, function, and homeostasis reflect the metabolic state of the cell. Our group has discovered a novel transcriptional regulatory mechanism via organelle morphological changes in which Golgi–ER fusion activates cholesterol biosynthesis in the TME (Nakahara et al., unpublished data). In addition, recently, it has been reported that mitochondrial fission is regulated by phosphoinositide, PI4P, and metabolic conversion through the Golgi or ER. ⁶⁷ , ⁶⁸ However, a mechanism for mitochondrial fusion is still elusive. Impaired mitochondrial homeostasis may result in a marked decrease in cellular function and metabolism (Figure 4). Thus, organelle homeostasis reflects the metabolic dysfunction of the cell.

FIGURE 4.

Organelles could be a new therapeutic target for cancer. Organelles play a central role in intercellular and intracellular metabolic communications. (A) The organelle–organelle metabolic communication maintains cellular homeostasis. Disruption of the metabolic communication between organelles leads to pathophysiological conditions, including cancer. Understanding organelle–organelle metabolic communication may lead to therapeutics against adaptive cancer cells within the tumor microenvironment. (B) Mitochondria fusion mechanism although metabolic interaction is still elusive.

Organelles in cancer may play an essential role in regulating metabolic adaptation to the TME. Recently, researchers have focused on the investigation of organelle communication, as this may reflect the features of cancer, such as cell survival, proliferation, and adaptation, to reduced oxygen and nutrients, as well as chemotherapeutic drug treatments. ⁶⁹ , ⁷⁰ Chemotherapy in organelle‐level fidelity requires an understanding of the physiological mechanisms different from cancer. Investigating the differences in organelle communications between normal and cancer cells is essential. Therapeutics that target organelle function by regulating metabolism, therefore, have the potential to introduce novel understanding and treatment of cancer.

5. CONCLUSION

Recent advances in metabolome analyses have revealed metabolic adaptations of cancer cells in TMEs. For example:

(1) Cancer cells demonstrated metabolic adaptations to extreme TMEs, including hypoxia, nutrient starvation, and acidic pH. Metabolic adapted cancers can be new targets for drug discovery.

(2) Cancer cells utilize metabolic crosstalk between different cell types to obtain nutrients essential for cancer progression and survival. Cancer‐associated fibroblasts are one of the most abundant components within the TME and may be a source of nutrients leading to the metabolic adaptations of cancer cells, necessary for tumor progression.

(3) Organelle–organelle metabolic communications could be involved in essential features of cancer, including cell proliferation, survival, and metabolic adaptation to extreme TMEs. Exposure to therapeutic agents and organelle‐level faithful cancer therapy requires a combination of physiological mechanisms of intracellular membrane transport and an understanding of the unique properties of organelles is necessary. A better understanding of the metabolic adaptations within TMEs at both the single‐cell and organelle levels may lead to the development of new targets or strategies for the treatment of cancer.

AUTHOR CONTRIBUTIONS

T.O. supervised the study. R.N., K.M., S.A., and T.O. wrote the paper.

FUNDING INFORMATION

This work was supported by Grant‐in‐Aid for Scientific Research (B) (19H03496, T.O.), Grant‐in‐Aid for Scientific Research on Innovative Areas (20H04834, T.O.) and Grant‐in‐Aid for challenging Exploratory Research (19K22553, 21K19399, T.O.) also from JSPS KAKENHI Grant AdAMS (22H04922) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, The Sumitomo Foundation (T.O.), The Simazu Science Foundation (T.O.), The Kurata Grant (T.O.), The Naito Foundation (T.O.), The Uehara Memorial Foundation (T.O.), The SGH Foundation (T.O.), The Koyanagi Foundation (T.O.), The Takeda Foundation (S.A., T.O.), the Project for Cancer Research And Therapeutic Evolution (P‐PROMOTE), from Japan Agency for Medical Research and development, AMED (T.O.).

CONFLICT OF INTEREST

The authors declare no competing interests.

ETHICS STATEMENT

Approval of the research protocol by an Institutional Reviewer Board. N/A.

Informed consent. N/A.

Registry and the Registration No. of the study/trial. N/A.

Animal studies. N/A.

Nakahara R, Maeda K, Aki S, Osawa T. Metabolic adaptations of cancer in extreme tumor microenvironments. Cancer Sci. 2023;114:1200‐1207. doi: 10.1111/cas.15722