Colon cancer usually occurs at the junction of the rectum and the sigmoid colon, mostly in people aged 40–50. With the development of society, people’s diet and living habits have also undergone tremendous changes. Their preference for heavy flavors, high-sugar and high-fat diets, disordered hunger and fullness, unclean diet, and staying up late make the incidence of colon cancer more and more youthful[13]. More than 30 years ago, Halpern et al. found that there is a methionine-dependent metabolic phenotype in tumor cells. Kano Y et al. subsequently found that except for lymphocytes, normal cells can grow normally under methionine starvation, which supports this theory[14]. Studies on colon cancer have shown that supplementation of methionine in the diet can stimulate malignant changes in the intestinal tissues of mice[15, 16]. Hanley et al. and Komninou et al. successively found that restricting methionine intake inhibited the progression of colorectal cancer[17, 18]. At present, the drug for the metabolism of methionine is methioninase, which can catalyze the α and γ-cleavage reactions of methionine to produce α-butyric acid, methyl mercaptan and ammonia. In the study of the drug, methioninase can cooperate with traditional chemotherapy drugs to treat colon cancer and show a better therapeutic effect[19–22]. The study of Machover et al. showed that the possible mechanism of methioninase's tumor suppressor is that it can induce a decrease in the methylation level of the proto-oncogene cyclin dependent kinase inhibitor 2A and an increase in its expression. We studied the effect of methionine restriction at the cell level on colon cancer cells[23]. The results show that methionine restriction can inhibit the proliferation, metastasis and invasion of colon cancer cells, and promote apoptosis, which also reflects the methionine-dependent phenotype of colon cancer cells.
Methionine restriction is a promising means of suppressing cancer, and there are few reports on the analysis of the potential mechanism of methionine inhibiting the growth of colon cancer. Using public data, we analyzed the genes that may be regulated by methionine restriction and performed an enrichment analysis on these genes. Among the up-regulated genes, we focus on MAP kinase phosphatase activity and protein tyrosine/threonine phosphatase activity. The MAPK cascade is a signal transduction component that plays a key role in converting extracellular stimuli into cellular responses through phosphorylation of different substrates. The MAPK signaling pathway controls various cellular processes, such as growth, differentiation, proliferation, survival and death. The activation of this signal depends on the simultaneous phosphorylation of both threonin (T) and tyrosin (Y) sites[24]. For the study of methionine on MAPK signaling, Lin et al. used proteomics methods to find in gastric cancer that the expression of p38 increased after methionine restriction[25]. P38 can induce cell cycle arrest that has undergone malignant transformation, and even enter a state of senescence or apoptosis, which is an important factor in inhibiting tumor formation and development. Among the down-regulated genes, we observed that they were enriched in glycolysis process, ATPase activity, Drug metabolism, etc. Tumor cells and normal cells also have differences in energy metabolism. They may preferentially generate large amounts of lactic acid through aerobic glycolysis to obtain more energy. By inhibiting the energy production of tumor cells, it can effectively play an anti-tumor effect. In the study of gastric cancer, methionine restriction can reduce the activity of aerobic glycolysis and induce tumor cell apoptosis[26]. The reduction of tumor productivity leads to the inactivation of many ATPases. It is important that drug metabolism depends on a variety of ATP-dependent transport enzymes. Among them, P-gp glycoprotein plays an important role in tumor resistance, and it uses ATP to supply energy to metabolize cell-energy drugs. The study of Xin et al. found that methionine restriction induces down-regulation of the expression of resistance-related protein P-gp, which enhances the drug sensitivity of gastric cancer[27].
Based on bioinformatics analysis, we identified that methionine restriction affects the important transcription factor E2F1 and the key lncRNA MIR17HG. In addition, four independent prognostic genes of colon cancer regulated by methionine restriction were identified based on Lasso and multivariate Cox regression, among which FANCI and HJURP can be considered as risk genes. E2F1 is a member of the E2F transcription factor family, which can bind to DNA with dimer partner (DP) protein through the E2 recognition site 5'-TTTC [CG] CGC-3' found in the promoter region of various genes. E2F1 is directly involved in several types of cancer with poor prognosis, and has been shown to be a key cancer biomarker[28, 29]. E2F1 not only promotes the proliferation and metastasis of rectal cancer, but also contributes to aerobic glycolysis, repressurization and oxidative metabolism, and promotes anabolic metabolism[30]. MIR17HG is a miR-17-92 cluster host gene lncRNA, which participates in cell proliferation and growth by regulating cell growth phenotype. Studies on cervical cancer, non-small cell carcinoma and osteosarcoma have shown that MIR17HG acts as an oncogene in tumors, inhibiting its expression, resulting in tumor cell proliferation, metastasis, and drug resistance[31–33]. Xu et al.'s research in colon cancer showed that MIR17HG is closely related to tumor immunity. First, it competitively sponges microRNA miR-375, thereby increasing the expression of nuclear factor kappa-B/ RELA, and then up-regulating programmed death receptor 1 expression, RELA can also directly bind to the MIR17HG promoter region to form a positive feedback loop to activate MIR17HG transcription[34]. FANCI is a key gene to maintain chromosome stability and plays an important role in repairing DNA double-strand breaks by homologous recombination.
It repairs inter-strand DNA crosslinks through homologous recombination and induces FANCL to promote FANCD2 monoubiquitination, and participates in the recruitment of DNA repair sites[35]. HJURP is a histone chaperone involved in the recruitment of de novo histone H3 variants CenH3 (CENP-A) and CENP-C in nucleosomes, which can regulate the amplification of centromeric chromatin and chromosomal stability[36]. HJURP has been confirmed as a key prognostic-related gene in breast cancer, hepatocellular carcinoma, advanced serous ovarian cancer, colorectal cancer and prostate cancer[37–41]. There are reports showing that HJURP promotes tumor cell proliferation. For example, Wang et al. showed that HJURP activates MDM2 transcription by regulating the recruitment of H3K4me2 in its promoter region to inhibit the expression of p53 and promote the growth of pancreatic cancer cells[42]. Chen et al. found that HJURP destabilizes p21 through MAPK/ERK1/2 and Protein Kinase B/Glycogen Synthase Kinase-3β pathways, thereby regulating nuclear cytoplasmic translocation and ubiquitin-mediated p21 degradation, and promoting tumor cell proliferation[43]. In addition, studies have shown that HJURP can promote epithelial-mesenchymal transition of tumor cells by regulating the Wnt/β-catenin pathway and SPHK1[44, 45]. Not only that, studies by Yuan et al. and Cao et al. have successively confirmed that HJURP affects the oxidative stress and cell cycle arrest of tumor cells by regulating the peroxisome proliferator-activated receptorγ-sirtuin1 feedback loop[46, 47]. In our research, we confirmed that E2F1, MIR17HG, FANCI, and HJURP are highly expressed in colon cancer tissues. Moreover, its expression decreased significantly after methionine restriction, suggesting that methionine restriction may exert a tumor suppressor effect through the above pathways.
In general, based on bioinformatics, we have analyzed the underlying mechanism of methionine restriction affecting colon cancer as a whole, and identified some key genes that methionine restriction affects the progression of colon cancer. Furthermore, we confirmed in vitro experiments that methionine restriction can inhibit the proliferation, metastasis and invasion of colon cancer cells, and induce apoptosis, and verified the expression of key genes after methionine restriction.