Autophagy is a highly conserved evolutionary process in eukaryotic cells, which is involved in a series of cell homeostasis processes. There are three types of autophagy: megaautophagy, microautophagy and chaperone mediated autophagy. Megaautophagy is the only autophagy that can degrade organelles, which we usually call autophagy. The autophagy process can be summarized as two basic steps: first, the cytoplasmic material is wrapped by the autophagy body of the double membrane, and transported to the lysosome to form autophagy lysosome. Then, lysosomal enzymes are used to degrade the substances in the cells, so as to realize the metabolism of the cells and the renewal of some organelles. Autophagy related genes LC3, Beclin 1 and ATG5 are all biomarkers of autophagy, which are involved in autophagy regulation [16–18].
Many studies have shown that autophagy protein is closely related to the prognosis of GC patients. The high expression of ATG5 is closely related to the poor prognosis and drug resistance of gastric cancer [19]. It has been found that the disease-free survival rate and the overall survival rate of patients in the Beclin 1 high expression group are significantly increased [20]. However, some studies have come to the opposite conclusion [21]. These findings suggest that the abnormal expression of Beclin 1 is closely related to the prognosis of gastric cancer, but the divergence of the total and disease-free survival may be related to the follow-up degree of the researchers, the size of the sample size and the heterogeneity of the patients. Considering the importance of autophagy in gastric cancer, we can reasonably speculate that autophagy related genes have broad prospects in the prognosis evaluation of gastric cancer, and the multi gene signature generated by various algorithms will be better than a single molecule in the prediction of GC OS.
In this study, we analyzed the mRNA expression of 222 ATGs in the TCGA gastric cancer dataset. Single factor Cox regression analysis showed that 10 genes were related to the survival of STAD. We used LASSO regression to develop eight prognostic markers for the TCGA-STAD cohort. Finally, the signature of four genes was established by multivariate Cox regression. The risk score of each patient can be obtained by calculating the mRNA expression level and risk coefficient of the selected gene. In the TCGA-STAD cohort, risk scores significantly stratified patient outcomes. More importantly, in two independent geo gastric cancer datasets within the STAD, the prognostic power of the 4-gene signature was verified. Gene signature is often applied to forcast the prognosis of a variety of tumors in the past few years[22], which is even better than TNM staging and histopathological diagnosis in some extent [23]. Gene signatures based on ATGs have been reported in a variety of cancers, such as serous ovarian cancer, breast cancer, colon cancer and glioma [24–27]. For example, Liu and colleagues recently reported a 14 autophagy related signature (NRG1, itga3, map1lc3a) based on relapse free survival in patients with non-small cell lung cancer [28].
Bioinformatics enrichment analysis showed that 38 differentially expressed autophagy related genes(DE-ATGs) were mainly related to cell growth, positive regulation of cell protein localization, neuron death, regulation of cell growth, platinum drug resistance, apoptosis and p53 signaling pathway in STAD. Interestingly, Huang's study found that autophagy plays a vital role in the platinum drug resistance of tumor cells [29]. In tumor treatment, apoptosis tolerance is an important mechanism for tumor drug resistance. Autophagy can prevent apoptosis induced by antitumor drugs and promote tumor drug resistance. However, autophagy cell death may be a death mode of apoptosis tolerant tumor cells, Autophagy has double effects on drug resistance of tumor cells [30]. There is also a lot of evidence implying the interaction between autophagy and apoptosis [31]. Autophagy may promote or hinder apoptosis.
Autophagy inhibited apoptosis when the environmental conditions were less affected. However, when autophagy causes excessive consumption of intracellular proteins and organelles, resulting in the inability of cells to survive, the cells will turn into apoptosis. In some cases, autophagy can also cause cell death. It is worth mentioning that autophagy and apoptosis involve many apoptosis related proteins, such as p53 and BH3 only proteins [32]. In the early stage of cancer cell formation, autophagy can inhibit the formation of cancer cells; after cancer cell formation, cancer cells use autophagy to promote the survival of cancer cells and inhibit cell apoptosis, which may lead to the resistance of cancer cells to chemotherapy drugs. Therefore, if we inhibit autophagy during chemotherapy, it will be beneficial to enhance the therapeutic effect.
Finally, we developed a nomogram to forecast individual clinical outcomes. Nomogram, which is based on multi factor regression analysis, uses multiple clinical indicators or biological attributes, and then uses line segments with scores, so as to achieve the setting purpose: based on the value of multiple variables to predict a certain clinical outcome or the probability of a certain type of event [33]. The nomogram transforms the complex regression equation into a simple and visual graph, which makes the prediction model more readable and valuable. This advantage makes the nomogram get more attention and application in medical research and clinical practice. Although traditional clinical and pathological features (e.g., TNM staging, tumor size, and histological subtypes) stemmed from gene signatures, risk scores can also be accepted into a predictive model nomogram to better forecast clinical outcomes [34]. Wang et al. Reported a nomogram to predict the recurrence free survival rate of malignant glioma, including the prognosis score calculated according to the characteristics of autophagy gene [35]. Compared with TNM staging, signature based on multi RNA showed a higher accuracy of prognosis. [36] in addition, as shown in the calibration curve, the 3-year and 5-year survival rates can be better predicted by using the nomogram integrating risk index and conventional prognostic factors [37]. Consistently, we show that the nomogram including four-ATG-signature can well predict the 3-year and 5-year survival probability of STAD patients. However, due to the lack of sufficient cases, we were unable to assess the predictive power of autophagy gene signatures in other independent gastric cancer data sets. In addition, other potential prognostic variables related to OS in GC, such as neutrolphil to lymphocyte ratio(NLR) should be investigated. Moreover, the expression of these 4 genes in gastric tissue and their prognostic effects need further study.