HIF-1 (HIF-1α and HIF-1β) was first reported by Semenza and colleagues in 1995 as an oxygen-sensitive transcription factor capable of responding to decreases in oxygen levels in the environment. Under normoxic conditions, the proline residues in HIF-1α are hydroxylated by the HIF-prolyl hydroxylase domain enzyme (PHD) and then degraded by ubiquitination of the von Hippel-Lindau protein (pVHL) (24–26). Under hypoxic conditions, PHD activity is inhibited, and HIF-1α is thus able to stabilize and translocate into the nucleus, where it binds to specific DNA hypoxia response elements (27) and activates the transcription of a large number of genes. The hypoxic environment to which tumor tissues are exposed leads to an upregulation of HIF-α expression in tumor cells (28). Furthermore, in addition to being induced by hypoxia, mutations in proto-oncogenes and tumor suppressors can also cause elevated HIF-1α expression (29). The upregulation of HIF-1α in clear stromal renal cell carcinoma results from VHL tumor suppressor, not hypoxia (30).

HIG-2 a newly identified lipolytic regulator of cellular response to hypoxia in recent years, is encoded by Hilpda, a target gene of HIF-1. HIG-2 is expressed in various cells, including cancer cells, hepatocytes, adipocytes, and immune cells. Analysis of HIG-2 expression in the Cancer Genome Atlas database and the Pan-Cancer Data Center revealed that Hilpda is a marker of poor prognosis in tumor patients and positively correlates with increased infiltration of tumor-associated macrophages and immunosuppressive genes in the tumor microenvironment (31). Similarly, in pancreatic cancer cells, the bioinformatic analysis also revealed elevated expression of hypoxic lipid droplet-associated proteins (10). Moreover, in pancreatic ductal adenocarcinoma (PDAC) mouse model, HIG-2 was also found to increase the accumulation of LDs in cancer cells and promote the growth of cancer cells (32). Functionally, HIG-2 regulates fat storage in response to insufficient oxygen supply outside the cell or excess fatty acid content in the cell. The function of this protein is partially dependent on triglyceride hydrolase ATGL and triglyceride synthase DGAT. In a mouse hepatoma cell model, HIG-2 increased triglyceride synthesis and storage by stimulating DGAT1 (33). In another mouse model of colorectal cancer, knockdown of HIG-2 enhanced LDs degradation in cancer cells and promoted apoptosis in a Reactive oxygen species (ROS)-dependent manner, ultimately inhibiting the growth of tumor explants in mice. This enhanced effect of lipid degradation could be reversed by co-ablation of ATGL (34). It was confirmed by immunoprecipitation that HIG-2 could physically bind to ATGL (34). Further studies showed that both the interaction and co-localization of HIG-2 with ATGL on the LDs surface require a segment of the hydrophobic domain, LY(V/L) LG, which is also possessed by G0S2, a direct inhibitor of ATGL (34, 35). The above results suggest the presence of hypoxia-induced HIF-1/HIG-2/ATGL axis in tumor cells. Hence, we speculate that, possibly, tumor cells reduce Fatty acid β-oxidation through this axis to reduce oxygen consumption under hypoxic conditions. Figure 2 illustrates the ATGL-mediated lipid metabolism pathway regulation by HIF-1 under hypoxia conditions.

G0S2 is the gene that controls the shift of the monocyte cycle from the G0 phase to the G1 phase (36, 37). The gene is highly conserved and has 78% identity in humans and mice. G0S2 was found to be a multifunctional protein involved in various intracellular biological pathways, including apoptosis, inflammation, metabolism, oxidative phosphorylation, and possibly even antitumor effect (38–42). G0S2 has been identified as a direct intracellular inhibitor of ATGL and is abundantly expressed in metabolically active tissues, such as adipose tissue, heart, skeletal muscle, etc. (43). G0S2 achieves triglyceride regulation by binding to ATGL to inhibit its hydrolysis of triglycerides. G0S2 inhibits the activity of ATGL even in the presence of the ATGL coactivator ABHD5, and the inhibitory effect of G0S2 on ATGL is dose-dependent and noncompetitive. Mechanistically, G0S2 possesses the same hydrophobic domain (HD) as HIG-2 and can bind to the patatin-like region of ATGL to inhibit its activity. In addition, tumor necrosis factor alpha (TNF-α) was found to stimulate ATGL-mediated lipolysis, which was also inhibited by G0S2 (44). In cancer cells, G0S2 can similarly regulate LDs lipolysis and promote LDs accumulation in cancer cells by inhibiting ATGL activity. Multiple bioinformatics analyses showed that G0S2 was highly expressed in pancreatic, breast, and rectal adenocarcinomas (10, 45, 46). In contrast, double knockout of Pnpla2 and Lipe induced liposarcoma in mice, and abnormal expression of G0S2 was observed in double knockout liposarcoma tissues (47). Notably, knockdown of G0S2 produced tumor suppression, but this effect did not seem to be dependent on the ATGL inhibitory effect of G0S2 (42, 48).

On the one hand, G0S2 is involved in regulating cellular lipid metabolism by inhibiting its activity through direct binding to ATGL. On the other hand, G0S2 expression is also regulated by PPAR (PPAR-α, PPAR-β/δ, and PPAR-γ), a group of nuclear receptor proteins that play an essential role in regulating differentiation, development, metabolism, and tumorigenesis in higher organisms. PPAR-α and PPAR-γ have been extensively studied and found to be elevated in various cancer cells compared to normal tissues, such as breast cancer, colon cancer, liposarcoma, pancreatic cancer, and hepatocellular carcinoma (49). G0S2 is a target gene of PPAR that was first identified by Fokko ZANDBERGEN et al. (50). In 3T3-L1 fibroblasts, the G0S2 promoter was found to contain a functional PPAR response element, leading to its expression being regulated by PPAR. G0S2 was identified as a direct target gene of PPAR-γ by transactivation, gel shift, and chromatin immunoprecipitation assays. The stimulation of lipolysis in adipocytes by TNF-α was also caused by decreased G0S2 expression due to PPAR-γ inhibition (51). In addition, when Hep-G2 cells were treated with palmitate, PPAR-γ and G0S2 expression simultaneously elevated similarly (52).

A critical PPAR-γ super-enhancer was found in human and mouse adipocytes near the HIG-2 gene, containing multiple conserved PPAR-γ binding sites (53). In contrast, in hepatocytes, the regulation of HIG-2 by PPAR-α was also demonstrated by transactivation and chromatin immunoprecipitation assays. Although these results were not confirmed in cancer cell lines, we believe that PPAR-α and PPAR-γ in cancer cells play a crucial role in hypoxia-induced reprogramming of lipid metabolism. Based on the studies of PPAR-α, PPAR-γ, G0S2, and ATGL in tumors, we speculate that under the induction of hypoxia in the tumor microenvironment, cancer cells increase the expression of PPAR-α and PPAR-γ, which in turn promote the expression of their target genes G0S2, to avoid the occurrence of required FAO pathway. As a direct inhibitor of ATGL, the high expression of G0S2 makes ATGL functionally inactivated in cancer cells, ultimately leading to the accumulation of triglycerides and LDs in cancer cells ( Figure 3 ). We also found that although PPAR-γ expression is upregulated in many cancer cell lines, PPAR-γ coactivator Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and PPAR-γ agonists have tumor-suppressive effects, both of which were able to upregulate FAO as well as mitochondrial oxidative phosphorylation processes. In addition, it is noteworthy that G0S2 can also promote F0F1-ATP synthase activity and increase cellular adenosine triphosphate (54) content (41). This may compensate for decreased fatty acid oxidation and decreased ATP content by increased glycolysis rate under hypoxia. In tumor cells, especially in cancer cell lines like breast cancer cells and lipoma cells where G0S2 expression is significantly upregulated compared to normal cells, it is not elucidated whether G0S2 also has a promoting effect on F0F1-ATP synthase. However, we believe that this effect is also present in cancer cells under a hypoxic microenvironment and positively impacts the survival of cancer cells.

Both the HIF-1/HIG-2/ATGL axis and the PPAR-γ/G0S2/ATGL axis ultimately affect the release of fatty acids (FAs) from LDs by inhibiting the activity of ATGL proteins, and no studies have demonstrated that Pnpla2, the gene responsible for encoding ATGL, is in this process is regulated in this process. There are also no studies that clearly describe the regulation of Pnpla2 in tumor cells. However, we found that in adipocytes, FSP-27 (also known as CIDEC) can cooperate with EGR-1, also known as ZNF268 (zinc finger protein 268) or NGFI-A (nerve growth factor-induced protein A) to suppress the expression of ATGL (55). In addition, FSP-27 is also able to reduce the release of FAs in LDs by directly inhibiting ATGL protein activity as G0S2 does with HIG-2 (56).

In cells, FSP-27 is mainly involved in LDs morphology regulation (57–59). Depletion of FSP-27 in adipocytes leads to fragmentation of LDs (60, 61). In contrast, when FSP-27 is overexpressed, it promotes the fusion of LDs (62) and the exchange of lipids between LDs (63), leading to a decrease in the number of LDs and an increase in LDs volume (57, 58, 61). Currently, FSP-27 has been shown to have anti-lipolytic activity (56–58, 61, 64–66). In cancer cells, FSP-27 is also altered due to changes in lipid metabolism and is prognostic for lung and pancreatic adenocarcinoma (10, 67–70). Furthermore, Vishwajeet Puri and his team found that FSP-27 both directly inhibits ATGL ground activity and with synergistic EGR-1 inhibits Pnpla2 expression (55, 56). The study found that FSP-27 binds directly to ATGL through its core structural domain, aa120-220, inhibiting its triglyceride degrading function and promoting triglyceride storage. On the other hand, FSP-27 enhanced the binding of EGR-1 in the -46/-34 region upstream of Pnpla2 and, by doing so, inhibited Pnpla2 expression. Although the above two studies did not use tumor cell lines, combined with the existing results of altered FSP-27 expression in tumor cells, it is reasonable to believe that the FSP-27/EGR-1/ATGL axis is also present in some cancer cells ( Figure 4 ). It also needs to be further investigated whether hypoxia induces altered FSP-27 expression, to what extent it is involved in the regulation of FSP-27 expression, what role HIF-1 and HIG-2 play in the altered FSP-27 expression, and whether FSP-27 expression in tumor cells correlates with the availability of tumor cells to lipids.

Based on the previous description, the expression of ATGL in tumor cells in a hypoxic microenvironment is usually decreased to reduce the intracellular FFAs. In contrast, low intracellular FFAs in human cells are generally associated with reduced FAO. Therefore cells are likely to respond to the hypoxic environment in this way. FAO and ATGL have often been found to have a promotional effect on breast cancer. The pro-oncogenic effect of FAO suggests that breast cancer cells require more FAs (112). In addition, the activation of FAO is often associated with an increase in the metastatic capacity of breast cancer cells (100). Breast cancer cells can induce the release of large amounts of FAs from nearby adipocytes, which they then store in the form of triglyceride, and later promote the occurrence of non-coupled FAO through the high expression of ATGL, ultimately increasing their metastatic ability (100). It seems that a large amount of lipid is prepared for the upcoming cancer cells at the site of breast cancer spread. In a mouse model of breast cancer lung metastasis, ATGL expression is suppressed in neutrophils located in the lungs of mice, leading to lipid accumulation in neutrophils (97). Lipids stored in neutrophils are transferred to breast cancer cells that have spread there via the giant cellular drinking-lysosome pathway, promoting their proliferation and survival (97). Mechanistically, Src is involved in FAO-induced metastasis in breast cancer cells (113). In addition, mitochondrial carnitine palmitoyltransferase 1a (CPT1A) was also aberrantly expressed in breast cancer cells (114, 115). ATGL is not the only lipase upregulated in breast cancer cells, where intracellular Lipase member H (LIPH) was also highly expressed in cancer cells from breast cancer patients (116). LIPH has also been found to have the ability to increase the metastatic ability of breast cancer cells (117). However, one study found that when G0S2 was knocked down in MDA-MB-231 cells, the proliferation, metastasis, and invasive ability of cancer cells were reduced (118). In triple-negative breast cancer (TNBC), sPLA can induce polyunsaturated fatty acids (PUFAs) to generate triglycerides or LDs, thus avoiding oxidative stress caused by PUFAs and thus promoting cancer cell survival. sPLA can also inhibit the activity of ATGL and prevent the degradation of PUFAs from triglycerides (99).

In lung cancer, the effect of ATGL in cancer cells is ambiguous. Triglyceride accumulation in LDs and higher levels of cellular phospholipids and bioactive lipid species (lysophospholipids and ether phospholipids) were found in A549 lung cancer cells with knockdown of ATGL (104). A549 cancer cells with concomitant knockdown of ATGL had a more remarkable ability to migrate, associated with elevated levels of phosphorylated Proto-oncogene Src. This mechanism was also observed in breast cancer cells (113). Although different from the elevation of ATGL in breast cancer cells, elevated lipid levels are a common feature of both. However, in a mouse model of lung cancer, high expression of ATGL can promote tumor development (103, 119). The reason for this inconsistent result at the cellular and animal level may be the difference in the microenvironment in which the tumor cells are located. Compared to 2D cultured cells, the metabolic profile of 3D cultured ATGL-KO A549 cells and their microenvironment more closely resembled the actual situation in vivo (9).

ATGL in hepatocellular carcinoma cells (HCC) appears to be more involved in cancer cell proliferation than metastasis. Overexpression of ATGL in hepatocellular carcinoma cells promotes phosphorylation of p-Akt, which promotes cancer cell proliferation but does not affect the metastatic ability of cancer cells (21). Similarly, the long non-coding RNA NEAT1 driven cancer cell proliferation lipolytically by inducing ATGL expression in a mouse model of in situ hepatocellular carcinoma (109). However, in another human-derived HCC sample and induced mouse liver cancer model, ATGL expression was lower in the human HCC and mouse liver cancer models than in control tissues. The proliferation rate of HCC was negatively correlated with ATGL expression. This phenomenon was attributed to the upregulation of the tumor suppressor p53 due to ATGL upregulation, mediated by the PPAR-α/p300 axis (108).

In pancreatic cancer, tumor progression is closely related to lipid content and accessibility. Immunohistochemical analysis of 44 tissue samples from patients with PDAC revealed that ATGL expression was elevated in the tumor stroma of obese patients with PDAC. Still, ATGL content did not significantly correlate with tumor size and histological grade, so increased ATGL might be a critical factor in obesity-induced PDAC (8). In addition, increased adipocyte infiltration in the pancreas and hypertrophy of peritumor adipocytes, and increased levels of tumor tissue LDs formation-related proteins Mannose-6-phosphate receptor binding protein 1 (TIP-47) and Adipose differentiation-related protein (ADRP) were also found in EL-KrasG12D/PEDF deficient mice (120). EL-KrasG12D/PEDF deficient mice develop a more aggressive phenotype of PDAC. Notably, ATGL levels decrease with PEDF knockdown, suggesting that ATGL in pancreatic cancer may not have the same pro-metastatic ability as in breast cancer. Similarly, in another PDAC mouse model, inhibition of ATGL did not alter the difference in triglyceride abundance between Hilpda wild-type and knockdown cells (32). Therefore, in PDAC, ATGL may not be a critical factor in tumor proliferation and metastasis but might be a key factor in tumor formation.

Like its role in breast cancer, ATGL can produce tumor-promoting effects in colorectal cancer. In colon cancer cells and colon cancer stem cells, obesity can promote ATGL-mediated LDs utilization for tumor development (106). Mechanistically, it was found that the promotional effect of ATGL on colorectal cancer was achieved by degrading triglycerides and encouraging the expression of genes related to sphingolipid metabolism and CoA biosynthesis (20). Although ATGL is associated with sphingolipid metabolism, ATGL does not regulate the expression of proteins related to sphingolipid metabolism.

ATGL has no beneficial effect on prostate cancer. DGAT1, ABHD5, and ATGL are overexpressed in prostate cancer cells compared to peripheral blood mononuclear cells, and inhibition of DGAT1 and ABHD5 was found to lead to prostate cancer cell death (107). In contrast, ABHD5 has also been found to have an inhibitory effect on the proliferation and invasion of prostate cancer cells (111), and they also noted that ABDH5 regulates prostate cancer cells independently of ATGL (121). The role of ATGL in prostate cancer cells is also regulated by ephrin B2 receptor (EPHB2). EPHB2 acts as EPHB2, a suppressor of prostate cancer cells, is able to exert its tumor suppressive effect by inhibiting the activity of lipogenic factors DGAT1, DGAT2 and promoting the lipolytic factor ATGL, PEDF (7). The development of prostate cancer cells may also be related to the surrounding environment. It has been found that the expression of ATGL and PEDF is lower in prostate cancer-associated fibroblasts compared to primary human normal prostate fibroblasts (110). It suggests that there may be more lipids stored in prostate cancer-associated fibroblasts to facilitate the development of prostate cancer cells.