Keywords

FormalPara Key Points

  • Metabolic cross talk between CAFs and cancer can be a metabolic target for cancer therapy.

  • The reverse Warburg effects can be targeted via disruption of the “lactate shuttle” by MCT1/MCT 4 inhibitors.

  • Metformin can be used to inhibit glycolysis and block the function of CAFs, which promote cancer cell growth.

  • Blocking the glutamine uptake of cancer cells from CAFs is a strategy in targeting glutaminolysis.

  • Ketone bodies and ketosis in CAFs can be targeted for cancer treatment.

  • Fatty acid metabolism from cancer-associated adipocytes (CAAs) serves as a nutrient reservoir for cancer cell growth and is another target for cancer therapy.

1 Introduction

Although cancer has classically been regarded as a genetic disease of uncontrolled cell growth, the importance of the tumor microenvironment (TME) [1, 2] is continuously emphasized by the accumulating evidence that cancer growth is not simply dependent on the cancer cells themselves [3, 4] but also dependent on angiogenesis [5,6,7,8], inflammation [9, 10], and the supporting roles of cancer-associated fibroblasts (CAFs) [11,12,13]. After the discovery that CAFs are able to remodel the tumor matrix within the TME and provide the nutrients and chemicals to promote cancer cell growth [14], many studies have aimed to uncover the cross talk between cancer cells and CAFs. Moreover, a new paradigm in cancer metabolism shows how cancer cells act like “metabolic parasites” to take up the high-energy metabolites, such as lactate, ketone bodies, free fatty acids, and glutamine from supporting cells, including CAFs and cancer-associated adipocytes (CAAs) [15, 16]. This chapter provides an overview of the metabolic coupling between CAFs and cancer cells to further define the therapeutic options to disrupt the CAF-cancer cell interactions.

2 Overview of the Metabolism of CAFs in Solid Tumors

Pathological analysis shows that CAFs either are located in the tumor margin or infiltrate the tumor mass, indicating that CAFs and cancer cells are physically and functionally connected to each other [17, 18]. Of note, other than their locations, the physiological roles of CAFs depend on the existence of neighboring cancer cells [19], leading Madar et al. to propose a new concept: a “CAF state” instead of “cell type” [20].

Studying cancer metabolism is difficult due to the dynamic and rapid metabolic influx/efflux of heterogeneous cancer cells [2]. However, it is clear that the reprogramming of energy metabolism is one of the hallmarks of cancer [21]. Thus, researchers have sought to identify the metabolic vulnerabilities of cancer cells and exploit them for therapy [22, 23]. Cancer-friendly fibroblasts are the most abundant noncancerous cells in solid tumors, and they promote cancer cell growth and induce chemotherapy resistance [24]. Unfortunately, the underlying mechanism of how CAFs help in tumor cell growth remains unclear. However, recent progress in metabolic technologies, including stable isotope-resolved metabolomics, NMR-based metabolomics, and fluorescence-activated cell sorting [25,26,27,28,29], is deepening our insight into the metabolic cross talk between CAFs and cancer cells in the context of metabolic alterations. Using these advanced technologies, CAF-cancer cell interactions have been investigated in various types of cancers, including breast cancer [30], prostate cancer, ovarian cancer, lymphomas [31], non-small cell lung cancer, and head and neck cancers [24]. For instance, CAF produces and releases lactate to the TME, while cancer cells simultaneously utilize this lactate for mitochondrial oxidative phosphorylation (OXPHOS) in order to produce ATP efficiently and rapidly (Fig. 1). Interestingly, not only do CAFs produce these metabolites to help cancer cells grow, but cancer cells also release epidermal growth factor (EGF) to enhance the production and secretion of leptin by CAFs, which eventually leads to tumor progression [32]. Additionally, tumor cells express pro-inflammatory genes, including nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and interleukin-1 (IL-1), so that normal fibroblasts can be guided by cancer cells to become pro-inflammatory CAFs [33] (Fig. 1).

Fig. 1
figure 1

Overview of CAF metabolism in solid tumors

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Moreover, several studies have identified CAF gene expression profiles, including certain extracellular matrix (ECM) components and several matrix metalloproteases (MMP2, MMP11, and MMP14). These results suggest that ECM biosynthesis and remodeling is one of the critical features of the interplay between CAFs and cancer [34,35,36]. For instance, in ovarian and small cell lung cancers, many ECM genes remarkably elevate their expression levels in chemotherapy-treated cancer cells to induce chemoresistance [37, 38]. Additionally, CAFs bypass and help tumor cells resist anti-vascular endothelial growth factor (VEGF) treatment caused by angiogenesis inhibition by activating the platelet-derived growth factor C (PDGF-C) pathway [39], and stroma cells mediate RAF inhibitor resistance in BRAF-mutant melanoma through human growth hormone (HGH) secretion [40]. Even though gene expression profiles provide a method to predict the risks of metabolic coupling between cancer cells and CAFs, gene expression levels do not always correlate with metabolic changes. Therefore, measuring the metabolite levels in a sample could be a more accurate method to predict the risk of the aforementioned metabolic interplay.

3 Targeting the Metabolic Exchanges Between CAFs and Cancer Cells

3.1 Targeting the Reverse Warburg Effect via Disruption of the “Lactate Shuttle” by MCT1/MCT4 Inhibitors

Glycolysis, the process of converting glucose to pyruvate, is an essential metabolic pathway to produce energy in the form of ATP in cells. In the 1920s, Otto Warburg found that cancer cells preferably produce energy by converting glucose to lactic acid, even in aerobic conditions, to generate ATP rapidly. This is known as the Warburg effect [41, 42]. Interestingly, it was suggested that the reverse Warburg effect is the result of fibroblasts secreting lactate/pyruvate and epithelial cancer cells simultaneously taking up the energy-rich metabolites to utilize in the tricarboxylic acid (TCA) cycle and promote energy production for their growth [43]. In this hypothesis, cancer cells first guide the normal stromal cells to become CAFs, providing a tumor-friendly microenvironment to activate tumor growth. Next, lactate from CAFs is directly absorbed by the cancer cells as fuel for OXPHOS after the conversion of lactate to pyruvate [44]. Accordingly, the expression levels of glycolytic enzymes, such as lactate dehydrogenase and pyruvate kinase isozymes M2, and the lactate transporter, monocarboxylate transporter 4 (MCT4), are elevated in CAFs within breast and lung cancer tumors [24, 45]. Of note, lactate plays an important role in generating energy for the brain and the heart [46,47,48,49] and serves as an energy interplay shuttle between stromal cells and various types of cancer cells [45, 49, 50]. In this scenario, surrounding CAFs can communicate with cancer cells through direct cell-to-cell contact by releasing an exosome packaged with CAF-produced metabolites [51]. This coincides with neovascularization, inflammatory cell infiltration, and extensive remodeling of extracellular matrix in the TME [52]. Evidence supporting the “lactate shuttle” in human cancers [16, 53, 54] further shows that lactate can be directly transferred from CAFs to adjacent tumor cells under the premise that (1) CAFs overexpress the transmembrane monocarboxylate transporter 4 (MCT4) for lactate efflux from CAFs to cancer cells [53], (2) cancer cells overexpress monocarboxylate transporter 1 (MCT1) for lactate influx into cancer cells [45, 55], and (3) cancer cells finally utilize lactate as fuel for producing ATP via the TCA cycle [56,57,58]. Of note, MCT1 and MCT4, the main transporters of lactate, are key modulators of lactate homeostasis [59]. The elevated expression levels of genes involved in the lactate shuttle system, including high expression levels of MCT4, are associated with poor prognosis in the prostate, pancreas, and triple-negative breast cancers [59,60,61]. Consequently, accumulating evidence suggests that MCT1 and MCT4 transporters could be promising targets for cancer therapy.

Over the last decade, there has been significant progress in understanding the roles of the TME in tumorigenesis and the development of effective strategies for cancer therapy. In order to disrupt the metabolic bridge in CAF-cancer cell interactions through glycolysis and lactate metabolism inhibition, three potential strategies have been proposed. First, elevated expression of the lactate transporter MCT1 in cancer cells is a potential target for blocking cellular uptake of two types of mitochondrial fuels, ketone bodies and lactate [62]. MCT1 and MCT2 inhibitors can block the influx and efflux of lactate produced by either CAFs or cancer cells. Thus, due to the rapid accumulation of lactate and protons within cancer cells by inhibiting lactate influx/efflux, rapid acidification can occur in cancer cells and the TME, resulting in lactic acidosis [63,64,65]. For instance, alpha-cyano-4-hydroxy-cinnamic acid (ACCA), an MCT inhibitor, not only inhibits lactic acid efflux in glycolytic gliomas but also disrupts redox hemostasis and enhances radiosensitivity [66, 67]. AZD3965, an MCT1 inhibitor, is currently being tested in phase I clinical trial in solid tumors, including lymphoma, prostate cancer, and gastric cancer (NCT01791595) (Fig. 2). However, there are concerns over the alternative effects of MCT1 inhibitors, which include the blockage of lactate transport in muscles, gastrointestinal tract, and liver [68, 69].

Fig. 2
figure 2

Therapies targeting metabolic cross talk between cancer cells and cancer-associated fibroblasts

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3.2 Blocking the Function of CAFs by Metformin (Fig. 2)

Metformin, a drug that has been widely used for type 2 diabetes treatment, has found new applications as an anticancer drug for its glucose-targeting effects. Metformin activates the AMPK pathway and simultaneously inhibits cancer cell growth through the inhibition of glycolysis by facilitating the trafficking of glucose transporters 1 and 4 [70, 71]. Recent studies have also shown the therapeutic potential of metformin in blocking the function of CAFs [72]. In other words, metformin is sufficient to reverse the effects of CAFs on cancer cell growth [72], providing a rationale for why metformin is actively being tested in multiple clinical trials in solid tumors and lymphoma (NCTNCT00659568, NCT00881725, NCT00984490, and NCT00909506).

4 Targeting the Glutamine Uptake of Cancer Cells from CAFs

Glutamine addiction is a physiological phenomenon where cancer cells rely on the presence of exogenous glutamine to be used as a fuel for the TCA cycle and as a nitrogen donor for nucleotide and amino acid synthesis [73,74,75,76]. It was recently revealed that CAFs produce and release glutamine, while cancer cells take up and utilize this glutamine from the TME as an alternative carbon and nitrogen source [51, 77]. This explains why glutamine transporters, alanine, serine, cysteine preferring transporter 2 (ASCT2), and solute carrier family 38 member 5 (SLC38A5) are usually overexpressed in breast and prostate cancers [78,79,80]. Of note, ASCT2 mediates the uptake of glutamine, an essential amino acid in triple-negative basal-like breast cancer [80]. And ASCT2 inhibitors, such as benzylserine and L-γ-glutamyl-p-nitroanilide, have been shown to inhibit glutamine uptake and cell growth in melanoma and endometrial carcinoma [81,82,83] (Fig. 2). Additionally, FDA-approved tamoxifen and raloxifene also suppress estrogen receptor-negative breast cancer growth by inhibiting glutamine uptake [84].

5 Targeting Ketone Bodies and Ketosis in CAFs

Ketone bodies such as acetoacetate, β-hydroxybutyrate (βOHB), and acetone are produced through fatty acid metabolism in the liver [85]. Liver hepatocytes convert fatty acids into ketone bodies and release them into the bloodstream, especially under starvation conditions or after excessive exercises. Consequently, through ketolysis, a process of the conversion of ketone bodies into acetyl-CoA, they feed into the TCA cycle or OXPHOS to generate ATP [86]. Recent studies have shown that CAFs secrete ketone bodies and cancer cells utilize them as energy sources [77, 85, 87]. Furthermore, Bonuccelli et al. observed that ketone bodies, especially βOHB, serve as a more powerful energy source for cancer cell growth in comparison to lactate [88]. Genes associated with ketolysis and ketogenesis in CAFs, including 3-hydroxybutyrate dehydrogenase 1 (BDH1) and 3-hydroxy-3-methylglutaryl-CoA synthase 2 (HMGCS2), were overexpressed [89, 90]. Specifically, BDH1 catalyzes the conversion of acetoacetate to βOHB. HMGCS2, a member of the 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase family, generates HMG-CoA [86, 91]. In contrast to the gene expression profile of the surrounding epithelial cells, cancer cells themselves have upregulated gene expressions associated with ketone reutilization (acetyl-CoA acetyltransferase, ACAT1) and mitochondrial biogenesis (heat-shock protein 60, HSP60) [89]. Moreover, ketone bodies can be a source of lactate and pyruvate because acetone—an end product of ketosis—can be metabolized to lactate and pyruvate [92, 93]. Taken together, this suggests that the ketone bodies produced by CAFs can serve as an energy fuel for tumor growth and have further implications as a potential therapeutic target for cancer therapy. Furthermore, ketone bodies, including βOHB, are transported by the monocarboxylate transporters (MCT1 and MCT2), which also transport them across the blood-brain barrier [91, 94]. Accordingly, treatments targeting MCT1 and MCT2 are currently being tested in phase I clinical trials in solid tumor cancers (NCT01791595) [95]. Thus, the MCT1 and MCT2 inhibitors may effectively block the transport of lactate and ketone bodies, both generated by CAFs (Fig. 2).

6 Targeting Fatty Acid, a Nutrient Reservoir for Cancer Cell Growth, from Cancer-Associated Adipocytes (CAAs)

Cancer-associated adipocytes (CAAs) play an important tumorigenic role in fatty acid metabolism in the TME. For instance, omental adipocytes promote the migration and invasion of ovarian cancer cells to the omentum [96, 97]. It is known that omental adipocytes generate free fatty acids (FFAs) that are further transferred to cancer cells to generate ATP via β-oxidation [98] (Fig. 1). Therefore, in order to utilize FFAs, a subset of cancer cells overexpress the fatty acid-binding protein 4 (FABP4), which plays a key role in fatty acid transport for ovarian and breast cancer metastasis [15, 96, 99] (Fig. 1). It has been shown that a FABP4 inhibitor, which binds long-chain fatty acids, reduces metastasis of prostate cancer and regulates fatty acid production in ovarian and prostate cancers [96, 100] (Fig. 2). Because adipocytes are a major component of the TME in breast and ovarian cancers, it may be a rationale for FABP4’s effectiveness in those cancers [96, 101]. Accumulation of fatty acids in the TME could serve as a nutrient reservoir for cancer cell growth during nutrient deprivation. Taken together, stromal catabolites, such as free fatty acids, promote tumor growth, and act as chemoattractants to metastasizing cancer cells in the omentum.

7 Conclusion

The TME is comprised of cancer cells, CAFs, immune cells, adipocytes, and other supporting cells. Of these, CAFs are one of the key regulators of tumorigenesis. Within the TME of solid tumors, heterogeneous cancer cells and CAFs interact by transferring their metabolites, including amino acids, fatty acids, ketone bodies, cytokines, and growth factors, which reciprocally facilitate the growth of cancer cells. Moreover, CAFs provide not only a structural matrix for providing a tumor-friendly microenvironment to cancer cells but also nutrients for cancer cells. As such, the metabolic interplay between CAFs and cancer cells is considered as an area of vulnerability among cancer cells given that (1) cancer cells release HGH and induce pro-inflammatory gene expression in CAFs, (2) CAFs produce and release high-energy metabolites to the TME, and (3) cancer cells take up those metabolites to produce energy for cancer growth. Therefore, it is widely accepted that CAF-mediated metabolism plays a key role in tumorigenesis and that targeting the metabolic cross talk between cancer cells and CAFs can serve as a potential target for cancer therapy. Consequently, researchers have made continuous efforts to exploit areas of metabolic vulnerability by targeting (1) glycolysis and lactate metabolism, (2) glutaminolysis, (3) ketone bodies and ketosis, and (4) fatty acid metabolism.