Fast-multiplying cancer cells mainly draw energy from increasing aerobic glycolysis, which is known as the “Warburg Effect” 15. However, the metabolic alterations of cancer cells primarily involve lipid metabolism 16. Several studies have identified core gene expression signatures for predicting the malignancy of HCC and the outcomes of the patients 17. However, it is difficult to accurately understand the role of lipid metabolism-related gene sets in liver cancer. In this study, for the first time, a nomogram was built, based on lipid metabolism-related genes and clinical features, to predict the prognosis of HCC. The lipid metabolism-related gene set was downloaded using GSEA. Gene expression profiles and clinical characteristics were determined. The 221 patients in the TCGA database were regarded as the training set, whereas another 230 patients in the ICGC database served as the validation set.
First, a series of screenings on the lipid metabolism-related gene sets were conducted and 10 DEGs associated with the prognosis of HCC patients were picked: ACSL3, LCLAT1, LPCAT1, PIGU, PLA2G7, PLEKHA8, PON1, PTPMT1, SOCS2, TBL1XR1. ACSL3 is ubiquitously expressed in the prostate, brain, and other tissues including the liver. The protein belongs to long-chain fatty-acid-coenzyme A ligase family which is essential in lipid biosynthesis and fatty acid degradation. Chang et al. illustrated that endoplasmic reticulum (ER) stress-induced the expression of ACSL3. Meanwhile, ACSL3 shRNA inhibited the induction of lipid accumulation 18. This phenomenon recommended that ACSL3 may be a novel therapeutic target towards lipid dysregulation. Tushiro Migita et al. showed that ACSL3 contributes to the growth of castration-resistant prostate cancer (CRPC) through intratumoral steroidogenesis 19. Haarith Ndiaye et al, utilizing immunohistochemical analyses of HCC tissues and subcellular fractionation of cultured HepG2 cells, discovered the increasing expression of ACSL3 in HCC in contrast to normal liver 20. In our study, HCC patients with ACSL3 high expression encountered a worse survival rate than those with low expression in both TCGA and ICGC databases. Lysocardiolipin acyltransferase 1 (LCLAT1), a cardiolipin-remodeling enzyme of mammalian mitochondrial cardiolipin, modulates mitochondrial membrane potential, cardiolipin remodeling, reactive oxygen species generation, and apoptosis of alveolar epithelial cells 21. One study demonstrated that LCLAT1 causes insulin resistance 22. Another study demonstrated that insulin resistance promotes HCC process. There were no reports about LCLAT1 on tumors. In our study, metformin intake was related to decreased LCLAT1 expression. For this study, high expression of LCLAT1 predicted a poor prognosis in both the training and validation sets. Lysophosphatidylcholine acyltransferase 1 (LPCAT1) participates in phospholipid metabolism, particularly in the process of converting lysophosphatidylcholine into phosphatidylcholine when acyl-CoA exists. Bi et al. identified LPCAT1 as a hub among signaling, tumor growth, and the expression of genetically driven growth factor receptors 23. Mounting evidence suggests that alteration in LPCAT activities is involved in the pathological processes, such as NAFLD, viral infections, and cancer 24. Several studies found that LPCAT1 is upregulated or overexpressed in human cancers, including colorectal, renal, prostate, lung, and breast cancer 25,26. Moreover, LPCAT1 upregulation leads to poor prognosis by promoting progression and recurrence of breast and prostate cancer 27,28. LPCAT1 also stimulates brain metastasis of lung adenocarcinoma 29. For HCC cells cultured in favorable conditions, LPCAT1 modulated phospholipid composition and distribution. Moreover, LPCAT1 overexpression promoted HCC cell proliferation, invasion, and migration. LPCAT1 knockdown produced the opposite effect.30 In our study, the high expression of LPCAT1 resulted in poor prognosis in the training set but not obtained in the validation set. Phosphatidylinositol glycan anchor biosynthesis class U (PIGU), encoding GPI transamidase fifth subunit, was confirmed as an oncogene for bladder cancer 31. For HCC, PIGU was a significant stage-specific DEG 32. Additionally, consistent with our study, PIGU overexpression was reported as an independent predictive factor for poor prognosis in HCC and the incorporation of PIGU expression with a typical TNM stage was thought to elevate prognostic stratification 33. Phospholipase A2 group VII (PLA2G7) catalyzes the activation of the platelet-activating factor. PLA2G7 defects lead to platelet-activating factor acetylhydrolase deficiency. Moreover, knocking out PLA2G7 leads to the absence of the activity of soluble lipoprotein-associated phospholipase A2 34. Most studies involving PLA2G7 focus on the process of inflammatory interaction or lipid metabolism in Coronary heart disease, stroke, diabetes, and obesity, but few on tumors 35–38. Pleckstrin homology domain-containing A8 (PLEKHA8), also known as FAPP2, participates in vesicle maturation and promotes cytoplasmic lipid transfer. Chen et al. demonstrated that PLEKHA8 overexpression promotes human colon cancer cell growth via an active Wnt signaling 39. Paraoxonase 1 (PON1) is a restricted expression toward the liver, which exhibits lactonase and ester hydrolase activity. The enzyme is synthesized in the liver and kidney and binds to high-density lipoprotein (HDL) particles after being secreted into circulation, and hydrolyzes thiolactones and xenobiotics. Sun et al. reported that serum PON1 level could be used to distinguish early hepatocellular carcinoma from liver cirrhosis with a sensitivity of 71.4% and 95.2% and a specificity of 94.7% and 78.9%, respectively 40. Ding et al. found that the serum level of PON1 was better than AFP for microvascular invasion prediction and did not fluctuate significantly with the change of tumor size in HCC patients 41. In our study, PON1 showed a significant predictive capability for survival rate. PON1 low expression indicated a better prognosis. Protein tyrosine phosphatase mitochondrial 1 (PTPMT1) was a crucial intermediate in cardiolipin biosynthesis and hematopoietic stem cell differentiation 42,43. In pancreatic beta cells, the downregulation of PTPMT1 led to an elevation of insulin production and cellular ATP levels 44. A study reported that PTPMT1 downregulation promoted cancer cell death 45. Another reported modulating PTPMT1 alternative splicing would ameliorate cancer cell radioresistance 46. Suppressor of cytokine signaling 2 (SOCS2) encodes SOCS2 family proteins, which are negative regulators of cytokine receptor signaling via JAK/SATA pathway. SOCS2 is a well-known cancer suppressor. It inhibits the progression and metastasis in colon, breast, and lung cancer 47–49. An experiment in mice indicated that SOCS2 is a modulator of obesity via regulating the metabolic pathways depending on adipocytes’ size. Moreover, SOCS2 also serves as an inflammatory regulator through controlling cell differentiation or recruitment into adipose tissue and cytokines release during the progression of obesity 50. Another study proved that SOCS2 plays a protective role in acute liver injury through balancing immune response and oxidative stress 51. Chen et al. elucidated a mechanism of epigenetic alteration in HCC; SOCS2 expression was suppressed by methyltransferase-like 3 (METTL3) via an m6A-YTHDF2-dependent process 52. Ren et al. concluded that high expression of SOCS2 inhibits HCC progression via the JAK/STAT pathway related to downregulating miR-196a or miR-196b.53 In our results, the high expression of SOCS2 also displayed a protective effect for HCC patients. Transducing (beta)-like 1X-linked receptor 1 (TBL1XR1), belonging to WD40 repeat-containing family, presents the sequence identity of TBL1X and is required for transcriptional activation. Mutations or recurrent translocations in this gene have been frequently observed in intellectual disability and infrequently in some tumors. Several studies showed that the upregulation of TBL1XR1 not only promotes cancer cells (including lung, cervical, ovarian, breast and gastric cancer) proliferation, migration, invasion, and metastasis,54–57 but also suppresses chemotherapy sensitivity in nasopharyngeal carcinoma 58; therefore causing a bad outcome to cancer patients. Guo et al. demonstrated that TBLR1 was a pivotal oncogene of HCC. Synchronous exhibition about cell proliferation, antiapoptosis, and angiogenesis were observed in both HCC cell lines in vitro and samples in vivo when the TBLR1 gene is silenced 59. For our study, high expression of TBL1XR1 was statistically related to poor survival. The predictive capacity of all ten DEGs in HCC prognosis was demonstrated regardless of clinical features. Therefore, a risk score based on ten DEGs was calculated.
A nomogram integrating the risk score and clinicopathological features was later constructed. Then, ROC curves were carried out to compare the prognostic values among the nomogram, risk score, age, sex, and TNM stage. The result suggested that the nomogram could better predict HCC prognostic process. Furthermore, we found that Metformin intake was associated with decreased LCLAT1, PIGU, and PTPMT1 expression and increased PON1 expression. These trends matched the calculated prognosis trends. Therefore, this study speculates that the four genes may offer an effective therapeutic target of HCC with abnormal lipid metabolism. Of note, the lipid metabolism-related risk genes remained an independent prognostic factor even with the exclusion of clinical features. So, combining the risk score and other proven features could produce a better prediction of HCC.