Hypoxia is an important feature in the TME of solid tumors. With the increasing volume of ovarian cancer itself, the tumor tissue expands away from blood vessels containing nutrients and oxygen, and this lack of blood supply leads to further hypoxia and even necrosis in the central area [24, 25]. The adaptation of tumor cells to the hypoxic environment is the basis for tumor cell survival and proliferation. Tumor cells in the hypoxic region consume glucose by anaerobic glycolysis and release lactic acid, which is then uptaken by tumor cells in the adjacent oxygen-generating tumor region as a raw material for TCA circulation [26]. Multicellular aggregation and hypoxia were reported to reduce the response to the metabolic drugs AICAR (AMPK agonist) and metformin (ETC complex I inhibitor) as well as the chemotherapeutic drugs cisplatin and paclitaxel [23]. In addition, cysteine, which is commonly found in the ascites of patients with advanced ovarian cancer, was revealed to promote ovarian cancer cell adaptation to the hypoxic environment and contributes to platinum-based chemotherapy resistance [27]. Accordingly, hypoxia is an important direction for the development of tumor-targeted therapies.

Hypoxia-inducible factor 1 (HIF-1) is a major regulator of cellular responses to hypoxia. HIF-1 belongs to the basic helix-loop-helix Per–Arnt–Sim (PAS) protein family [28, 29]. It consists of a hypoxia-regulated $\alpha$ subunit and a non-hypoxia-regulated $\beta$ subunit [29]. HIF-1$\alpha$ (hypoxia-inducible factor-1$\alpha$) is degraded in normoxic tissues but highly expressed in hypoxic tumor tissues [30, 31]. In hypoxia, HIF-1$\alpha$ is enriched in the promoter regions of many genes and binds to hypoxia-regulatory elements [25]. It is involved in metabolism, angiogenesis, cell proliferation, apoptosis, and other biological processes [32]. Numerous studies have revealed that high expression of HIF-1$\alpha$ is associated with a poor prognosis and chemotherapy resistance in EOC, and key enzymes regulated by HIF-1$\alpha$ involved in tumor glycolysis are potential therapeutic targets [32, 33]. The inhibition of HIF-1$\alpha$ can improve the sensitivity of drug-resistant ovarian cancer cells to cisplatin [34].

Acidosis in the TME in ovarian cancer develops through various pathways. The acidification of the TME provides a more hostile environment compared to that of non-neoplastic cancers, favoring tumor survival and growth. The production of a large amount of lactic acid in glycolysis and CO2 from the pentose phosphate pathway coupled with the incomplete vasculature around the tumor tissue causes the accumulation of catabolites and contributes to the low pH environment [35, 36]. Lactic acid is regulated by monocarboxylate transporters (MCTs) on the cell membrane [37]. The expression of MCT4 is abnormally increased in hypoxic tumor areas, and lactate is transported out of the cell through MCT4. In tumor regions with normal oxygen supply, the expression of MCT1, which transports lactate from extracellular to intracellular compartments, is increased [38]. In ovarian cancer, MCT1 may be positively correlated with lactic acid [39, 40]. Furthermore, extracellular acidosis leads to reprogramming to a highly plastic tumor phenotype, involving stem-related marker expression, high clonogenicity, and transdifferentiation ability, which can promote invasion, tumor progression, and metastatic disease [41].

Transporters and pumps which contribute to H+ secretion are important targets for tumor therapy [42, 43]. As a classical transport channel, proton pump vacuolar H+–ATPases (V-ATPases) consume ATP to pump protons out of the cell or organelles to avoid self-acidosis [44]. H+ excreted by tumor cells follows a concentration gradient into normal cell tissues. The substantial accumulation of H+ in normal cells activates enzymatic cascade reactions, leading to necrosis or apoptosis, which is conducive to tumor spread and metastasis [42]. Protein carriers, such as V-ATPases, which maintain the extracellular acidic microenvironment and thereby promote tumor malignancy, are also potential therapeutic targets worthy of further attention [45, 46]. Kulshrestha et al. [47] found that targeting the V-ATPase isoform can restore cisplatin activity in drug-resistant ovarian cancer via the ERK/MEK pathway. The application of specific V-ATPase inhibitors, such as bafilomycin and proton pump inhibitors, can reduce tumor acidity, reduce tumor metastasis, influence the survival of tumor cells, and prevent chemotherapy tolerance [42, 43, 44, 45, 48, 49]. Furthermore, various membrane transporters form structural and functional complexes with carbonic anhydrases (CAs), known as transporters [50]. The CA inhibitor topiramate (TPM) can inhibit the proliferation of ovarian cancer cells and induce G1 phase cell cycle arrest, cell stress, and apoptosis via the AKT/mTOR and MAPK pathways. It also exerts anti-metastatic effects by reducing the adhesion and invasion of ovarian cancer cells and affecting the expression of key regulators of the epithelial-mesenchymal transition (EMT) [51]. In tumor cells, these bicarbonate transporters are thought to play roles in pH regulation and cell migration and might become new drug targets for cancer therapy [50, 52, 53].

Ovarian cancer tumor cells consume glucose for growth and survival and are much more sensitive to “glucose deprivation” than are normal cells in the body [54]. Glucose absorption occurs through glucose transporters (GLUTs). GLUT1 and GLUT3 are highly expressed in ovarian cancer. GLUT1 is ubiquitously distributed in cells, and GLUT3 shows a high affinity for glucose and may promote uptake in tumors with strong glucose needs [55]. Related study has confirmed that GLUT1 expression is variable among different types and stages of ovarian cancer. The mean expression level of GLUT1 is higher in high grade serous ovarian cancer (HGSOC); meanwhile, it was also significantly higher in advanced (III/IV) ovarian cancer than in early (I/II) disease [56].

Pyruvate kinase M2 (PKM2), an isoform of pyruvate kinase (PK), regulates glycolysis and OXPHOS by controlling pyruvate production in the final step of glycolysis [57]. PKM2 activity is reduced in tumor cells, indirectly facilitating the redirection of glycolytic intermediates to the biosynthesis of other biomolecules and promoting tumor anabolism [58]. PKM2 is expressed only in the cytoplasm of ovarian cancer cells [59]. PKM2 inhibitors can significantly inhibit glycolysis and disrupt the Warburg effect according to the glucose consumption level of cells, thereby inhibiting ovarian cancer progression and cell migration in vitro [57, 60]. Targeting PKM2 is a promising treatment strategy for patients with ovarian cancer [61].

Lactate dehydrogenase A (LDHA) functions in the final step of glycolysis, converting pyruvate to lactate. Patients with ovarian cancer often have significantly higher serum LDH levels than normal due to upregulation of LDHA gene expression [62]. This high level of LDHA expression contributes to drug resistance in ovarian cancer. Xiang J found that the high level of LDHA affected the inhibitory effect of PARP inhibitors on wild-type BRCA ovarian cancer and that inhibition of LDHA significantly promoted the inhibitory effect of PARP inhibitors on A2780 and SKOV3 cells [63].

Endothelial cells, which form a monolayer to line blood vessels, not only separate circulating blood from tissues but also offer maintain metabolic homeostasis, deliver immune cells, and participate in the formation of new blood vessels [65]. In addition, tumor cells connect to ECs, altering the endothelial barrier and thereby facilitating cancer cell migration, invasion, and metastasis [9]. To meet tumor metabolic demands, ECs have to adapt to the environment. The upregulation of the glucose transporter GLUT1 in the tumor endothelium [64, 66] and the ability of tumor-derived VEGF to activate phosphofructokinase-2/fructose-2,6-bisphosphatase 3 (PFKFB3) expression [67] suggest that ECs have enhanced lactate dehydrogenase B expression and are likely to further boost glycolysis. To guarantee efficient energy generation essential for tumor cell motility and invasion, ECs co-compartmentalize glycolytic enzymes with actin-rich areas in invading structures, such as filopodia. Although the functional significance of EC glutamine metabolism and the level of importance of the two GLS isoforms (GLS1 vs. GLS2) are unknown, blocking GLS causes decreased EC proliferation and increased senescence [68], implying that glutamine plays a critical role in tumor vessel sprouting.

CAFs, derived from different tissue-resident fibroblasts in the TME, have the ability to produce a wide range of extracellular components, such as growth factors, cytokines, and ECM components, all of which contribute to tumor progression, metastasis, neoangiogenesis, ECM remodeling, and immunosuppression [65].

Adipocytes are unique in the TME because they can manage the energy balance and store excess energy as fat by secreting metabolites, enzymes, hormones, growth factors, and cytokines. Metalloproteases, such as MMP-1, MMP-7, MMP-10, MMP-11, and MMP-14, are secreted by adipocytes and play a crucial role in ECM changes [65]. In ovarian cancer, the presence of adipose-derived stem cells (ADSCs) in the TME and their transformation into CAFs are essential to promote growth, survival, EMT, and the acquisition of a tumor stem cell-like phenotype [69].

Generally speaking, immune cells in TME include macrophages, dendritic cells, neutrophils, mast cells, myeloid-derived suppressor cells (MDSCs), and lymphocytes [9]. They play vital roles in both tumor progression and tumor suppression through various signalling molecules, such as EGF, VEGF, IFNs, and ILs.

‘T cells exhaustion’ has been observed in a variety of tumors, which characterized by a non-responsiveness and loss of the effector function state caused by constant antigen exposure, and exhibit the upregulation of the expression of the immune-inhibitory programmed death receptor 1 (PD1) and the cytotoxic T-lymphocyte antigen-4 (CTLA4) [70]. On the one hand, hypoxia stimulates PD-L1 expression in tumor cells, tumor-associated macrophages (TAMs), and MDSCs, leading to T cell incompetence via the PD-L1/PD-1 axis; on the other hand, the hypoxic and acidic environment stimulates the immunosuppressive properties of regulatory T cells (Tregs) and TAMs, which actively inhibit the functions of T cells [71]. During activation of T cells, metabolic reprogramming imprints distinct functional differentiation. Aerobic glycolysis promotion is observed during differentiation to effectors [70]. In addition, with PD-1 ligation, activated T cells are unable to engage in glycolysis or amino acid metabolism which decrease the anti-tumor ability [72]. Due to the up-regulation of glycolysis, large amounts of lactic acid is accumulated in TME. lactic acid suppresse the proliferation of Th1 and Th17 cells and their anti-tumor effect [73].

Glycolysis, however, is not the only process required for T-cell activation. c-Myc-dependent glutaminolysis is also required for appropriate T-cell effector function because it leads to the creation of nucleotides and polyamines, which are required for cell proliferation [70]. Accumulation of the dicarbonyl radical methylglyoxal, a by-product of glycolysis produced by cysteine-sensitive amine oxidase, leads to a metabolic phenotype of MDSCs and MDSC-mediated paralysis of CD8+ T cells; the neutralization of dicarbonyl compound activity overcomes MDSC-mediated T cell suppression and improves the efficacy of cancer immunotherapy in conjunction with checkpoint inhibition in a mouse cancer model [74].

Tregs are a group of tumor-infiltrating immunosuppressive CD4+ T cells that rely primarily on fatty acid oxidation (FAO) for energy, enabling survival in the nutrient-depleted TME [75]. In the oxygen-depleted TME, HIF-1$\alpha$ directs glucose away from the mitochondria, promoting Treg mitochondrial metabolism and enhancing the suppression of CD8+ T cells [75]. Forkhead box protein P3 (FoxP3) enhances fatty acid uptake, OXPHOS, and FAO by Tregs, promoting survival in the TME and facilitating tumor growth and immune escape [75].

Additionally, T cells can interact with fibroblasts to impact ovarian tumor chemotherapeutic responses. It has been shown that gamma-glutamyltransferase (GGT) is activated by CD8+ T cell-derived IFN$\gamma$, which enhances extracellular glutathione (GSH) breakdown and reduces fibroblast cysteine production, and both GSH and cysteine from fibroblasts reduce cisplatin accumulation in ovarian cancer cells, resulting in resistance to first-line chemotherapy [76]. Studies of TAMs have shown that depending on their polarization (M1 versus M2 phenotype), they utilize arginine in the tumor stroma in a very different way [77]. M1 macrophages inhibit tumor development by producing NO, which is harmful to cancer cells, using arginine and inducible nitric oxide synthase (iNOS). M2 macrophages promote tumor development by using arginase 1 (ARG1) to convert arginine to ornithine, which can feed on cancer cell proliferation. Furthermore, because M2 macrophages primarily receive energy through FAO and OXPHOS, they can block glycolysis by mTOR/HIF-1 inactivation [78]. Exosomes from M2 macrophages enriched in miRNAs (including miR-29a-3p and miR-21-5p) induce a Treg/Th17 imbalance, generating an immunosuppressive microenvironment that affects the EOC grade and patient survival [79].

In terms of non-cellular components of the TME, the ECM, which is composed of collagen, fibronectin, elastin, and laminin, provides a physical scaffold for cells but also plays a vital role in promoting tumor metastasis [65]. Additionally, exosomes, including proteins, RNA, DNA, and lipids, which are derived from cellular parts of the TME, are the bridge between cancer cells and stromal cells [9]. Non-tumor cell metabolic interactions in the ovarian cancer microenvironment was shown in Fig. 3.

Fig. 3.

Non-tumor cell metabolic interactions in the ovarian cancer microenvironment. CAF, Cancer-associated fibroblast; EC, Endothelial cell; MDSC, Myeloid-derived suppressor cell; NO, nitric oxide; Th17, IL-17-producing T helper; Treg, Regulatory T cell.

The abnormally high metabolic rate and nutrient depletion of tumor cells may affect competition with neighboring T cells, leading to a decrease in the glucose concentration in the ECM, which may lead to T cell metabolic depletion and consequently affect the function of T cells [80, 81]. Song et al. [82] have shown that ovarian cancer ascites restrict glucose uptake, thereby causing CD4+ T cell IRE1$\alpha$/XBP-mediated mitochondrial dysfunction. The anti-tumor T cell response can be restored by blocking glucose uptake by tumor cells and re-supplying glucose to infiltrating T cells. The inhibition of glycolysis can enhance T cell-mediated anti-tumor immune effects [83]. Toll-like receptors (TLRs) can recognize a variety of pathogen-related molecular patterns that link innate immunity and specific immunity [84]. Among these TLR signals, TLR8, which is highly expressed on Tregs, specifically inhibits glucose utilization and glycolysis in human Treg cells, and its ligand can reverse the immunosuppressive function of Tregs [84]. In vitro studies of CD4+ Treg cells in the ovarian cancer cell co-culture microenvironment have shown that ovarian cancer cells have certain effects on the glucose metabolism and function of CD4+ Treg cells, which can be reversed by the regulation of TLR8 [85]. Lysophosphatidic acid (LPA) is a lipid growth factor in the TME of ovarian cancer ascites. An autocrine stimulatory loop of LPA synthesize and release was observed in ovarian cancer cells in vitro, which lead to the transition of normal ovarian fibroblasts to CAFs via HIF-1$\alpha$, even under normoxic conditions [86].

In the TME, lactic acid exerts multiple effects on various cell types, including effects on local pH and regulatory effects on cell metabolism and/or the redox status. The reverse Warburg effect complements the Warburg effect in terms of energy metabolism, where the interaction between cancer cells and adjacent stromal fibroblasts relies on lactate shuttle (MCT1 and MCT4), through which high-energy metabolites are transported from cells exhibiting aerobic glycolysis to OXPHOS-increasing cancer cells [87, 88]. Lactate secreted by cancer cells increases the synthesis of IL-17A, which inhibits the anti-tumor processes mediated by T cells [73]. Moreover, lactate can recruit and polarize immunosuppressive cells, such as regulatory T cells, TAMs, and MDSCs, further suppressing antitumor immune responses [89]. Lactic acid also blocks the differentiation of dendritic cells, limits their function, and leads to tumor escape from immune monitoring [90]. Tumor cell-derived lactic acid promotes the differentiation and polarization of TAMs, which has an important signaling role in the induction of polarization and the subsequent promotion of tumor growth [91]. M2-like TAMs produce immunosuppressive cytokines (such as IL10), chemokines, enzymes, and exosomes, which promote tumor invasion and immunosuppression, and enable cancer cells to survive, proliferate, and metastasize [92]. Decreased lactic acid levels can improve the efficacy of anti-tumor immunotherapy [93].

Glutamine is a dispensable amino acid in the human body [94]. However, for proliferative tumors, it is the second most essential nutrient after glucose and is stably converted to glutamate by phosphate-dependent glutamine enzymes (GLS) located in the inner membrane of mitochondria [95, 96, 97]. In low glutamine conditions, glutamine dependence drove CAFs migration toward free glutamine and facilitate the invasion of tumor cells [98]. Cancer cells influence CAFs metabolism to maximize Gln synthesis, and in a symbiotic manner (sustaining nucleotide generation and OXPHOS), CAF-derived Gln influences cancer cell metabolism and growth [13, 99]. Shen et al. [100] have shown that GLS inhibitor therapy effectively treats chemotherapy-resistant ovarian cancer, especially in cases with high GLS expression. Therefore, blocking glutamine secretion by CAFs or increasing glutamine decomposition in tumor cells is a potential target for the treatment of ovarian cancer.

Omental adipose stromal cells in the TME also communicate with ovarian cancer cells via arginine metabolism. Ovarian cancer cells use omental adipose stromal cell-secreted arginine and, in turn, secrete citrulline in the TME by the conversion of L-arginine to nitric oxide (NO) and citrulline by inducing nitric oxide synthase (NOS). NO promotes glycolysis and tumor cell proliferation, while citrulline is captured by stromal adipocytes and is converted into arginine. Secreted arginine is used by tumor cells to form a symbiotic cycle of metabolism [101, 102]. Arginine succinate synthase 1 (ASS1) is a rate-limiting enzyme in arginine synthesis in the urea cycle. Cells lacking ASS1 expression show arginine auxotrophy and sensitivity to arginine depletion. Tumors lack ASS1 and cannot synthesize endogenous arginine, which is completely dependent on exogenous sources. In ovarian cancer cell lines, ASS1 silencing resulted in resistance to platinum-based drugs and sensitivity to arginine deficiency [103]. T cells are sensitive to low-arginine environments. Intracellular L-arginine concentrations directly affect metabolic processes and the survival capacity of T cells. Therefore, supplementation with arginine and the prevention of arginine degradation are emerging strategies for enhancing the function of T cells [104, 105]. In addition to these, the indoleamine 2,3-dioxygenase (IDO) pathway is activated in ovarian cancer [106]. IDO promotes the local suppression of effector T cells by metabolically depleting tryptophan from the TME, producing tryptophan metabolites and generating broader systemic tolerance by activating circulating Treg cells [107]. The inhibition of targets in the IDO pathway may be an effective strategy for the treatment of ovarian cancer.

Fatty acids are required for the malignant progression of cancer, and rate-limiting enzymes for lipid metabolism may be therapeutic targets [108]. Ovarian cancer cells prefer to metastasize to the omentum, which is rich in fat cells and provides fatty acids for tumor cell growth [109]. Adipose cells and adipose tissue directly mediate the tumor-promoting effects of ovarian cancer. Fatty acids are used in the synthesis of biological macromolecules via $\beta$-oxidation. Adipocytes are an extracellular lipid source in tumor cells [108]. Co-cultured adipocytes are lipolyzed, providing free fatty acids that are taken up by ovarian cancer cells and inducing intracellular $\beta$-oxidation, thereby accelerating tumor cell proliferation and metastasis to the peritoneal omentum [110]. Adipocytes and secreted arachidonic acid (AA) act directly on ovarian cancer cells to activate Akt and inhibit cisplatin-induced apoptosis. Blocking the production of AA or blocking their anti-apoptotic function may reverse chemoresistance in patients with ovarian cancer [111].

Extracellular vesicles (EVs) are released by tumor cells or non-tumor cells. EVs mediate the transfer of proteins, lipids, and nucleic acids between tumor or non-tumor cells to alter the function and phenotype of recipient cells. Molecules that were not previously thought to be exchanged between cells (such as mRNAs and microRNAs) are transferred by EVs too [12]. Paracrine secretion is a channel for macromolecular communication between ovarian cancer cells and the TME, such that ovarian cancer cells and CAFs maintain a glycolytic phenotype by the paracrine long noncoding RNA LINC00092 [112]. Moreover, ovarian cancer cells secrete lipid membrane vesicles (known as exosomes), which contribute to the invasive phenotype of ovarian cancer cells by activating intracellular signaling pathways that generate tumor-associated myofibroblasts from MSCs in the tumor mesenchyme [113]. Ovarian cancer cells regulate fibroblast behavior by releasing EVs that induce phenotypic and functional changes in normal stromal fibroblasts, resulting in a CAF-like state, and EVs released from a highly aggressive cell line (SKOV3) appear to be more effective in stimulating certain processes than those of a less aggressive cell line (CABA I) [114]. In a ovarian cancer mouse model, ovarian cancer cells secrete EVs carrying ARG1; these EVs are taken up by dendritic cells, inhibit the proliferation of antigen-specific T cells, and promote ovarian cancer progression [115]. Autophagy may be an important mechanism underlying the transport of nutrients from CAFs to tumor cells [116]. Increasing evidence strongly suggests that the molecules carried by EVs, like exosomes, promote the development of platinum resistance in EOC tissues, which is conducive to the adaptive pathological response of cancer cells to drugs [117].

Metabolic interactions between ovarian cancer cells and various non-tumor cells present in the TME are summarized in Fig. 4.

Fig. 4.

Metabolic interactions between ovarian cancer cells and non-tumor cells present in the TME. Namely tumor-associated macrophages (TAM), cancer-associated fibroblasts (CAF), adipose stromal cells, effector T cells (Teff), regulatory T cells (Treg) and dendritic cell. Metabolites are shown in red. The metabolic inhibitors are shown in the yellow box. Gln, glutamine; NO, nitric oxide; MCT, monocarboxylate transporters; TLR, Toll-like receptors; EVs, extracellular vesicles; ARG1, arginase 1.