To identify potential diagnostic and prognostic markers of OSCC, we constructed a prognostic model based on FRGs. By applying univariate Cox regression and multivariate Cox regression analyses to 103 FRGs that were downloaded from databases, we found 4 FRGs (FTH1, FLT3, CDKN2A, and DDIT3) that were related to the OS of OSCC patients. The FTH1 protein expression was significantly upregulated in breast cancer cells (37) and the epigenetic silencing of FTH1 and TFRC that is induced by estrogen, reduced liver cancer cell growth and survival (38). FLT3 is a receptor tyrosine kinase that plays a crucial role in the development of hematopoietic progenitor cells (39). Furthermore, FLT3 genetic alterations occurred in up to 30% of cases with acute myelogenous leukemia, and patients with FLT3 mutations, have poor outcomes (40). CDKN2A is a tumor suppressor gene that was reported to be frequently altered in OSCC progression (41). CDKN2A low gene expression is associated with the recurrence of disease in oral cancer patients and could be used as a prognostic marker for OSCC (42). DDIT3 is an endoplasmic reticulum stress-responsive transcription factor which plays an important role in apoptotic execution pathways that are induced by the endoplasmic reticulum stress. The speckle-type POZ protein contributes to prostate cancer by targeting DDIT3 (43). Besides, DDIT3 acts as a transcription factor that enhances the expression of TNFRSF10A and TNFRSF10B, resulting in the initiation of ER stress-mediated apoptosis in human lung cancer cells (44). In summary, all 4 FRGs have been reported to be closely correlated with various cancers.
The prognostic model was constructed based on these 4 FRGs. The patients were separated into high risk and low risk groups, according to the threshold of the median risk score. As mentioned above, the patients with higher expression of CDKN2A/FLT3 genes and lower expression of DDIT3/FTH1 genes, were more likely to be in the low risk group, suggesting that FRGs CDKN2A/FLT3 genes may work as tumor suppressor genes. This result is consistent with previous research results (40) (41) (45). The prognostic value of the 4-FRG risk signature was evaluated using the K-M and Log-rank methods. There were significant differences in the survival curves of patients in the two groups of patients. The prediction capability of the specificity and sensitivity of the FRG risk model was assessed by calculating the AUC of the risk score. Moreover, we found that the ferroptosis-related risk score was an independent prognostic indicator for overall survival when considering conventional clinical characteristics. The results indicated that this risk score model is a firm prognostic tool that can be used to classify patients and guide future targeted therapies.
For further understanding of the biological functions of DEGs, between different risk groups, we performed functional enrichment analyses. The results showed significant enrichment in process, including NADH regeneration, glucose catabolic process to pyruvate, canonical glycolysis, and glycolytic process through fructose-6-phosphate, in the biological process category. NADH regeneration is a metabolic process that consumes NAD + to generate a pool of NADH, which is important to the immune system. It is indicated that NAD + promotes the differentiation of CD4 + T cell without antigens. Furthermore, without relying on antigen-presenting cells, NAD + can also regulate the fate of CD4 + T cell (46). Pro-inflammatory stimuli induce the NAD activation of macrophages and dendritic cells, resulting in a metabolic switch towards glycolysis (33) (34), while inflammatory macrophages depend on NAD + salvage, resulting from ROS-mediated DNA damages (35). For KEGG, 11 pathways, including Glycolysis/Gluconeogenesis, Glutathione metabolism, and the HIF-1 signaling pathway were identified. Several types of cancer, including OSCC highly depend on glycolysis for ATP generation. Zheng et al. (47) found that zeste homolog2 can regulate STAT3 and FoxO1 signaling in human OSCC cells and promote invasion and tumor glycolysis. Another study demonstrated that circMDM2 could promote the proliferation and glycolysis of human OSCC cells by acting as ceRNAs to sponge miR-532-3p (48). The HIF-1 signaling pathway, as a cancer therapy glycolytic target is involved in the regulation of glycolysis at pre-clinical and clinical stages (49) (50). Our results indicated that the DEGs, between the two different groups, may affect OSCC progression by altering these immunity-related biological processes or metabolic pathways.
We also focused on investigating the difference in the TME between the two different risk groups. To this end, we explored the correlation between OSCC and tumor microenvironment. The immune score and stromal score calculated based on the ESTIMATE algorithm can help quantify the immune and stromal components in tumors. The immune scores and ESTIMATE scores of the low risk group were significantly higher than those in the high-risk group; however, no significant differences were found in the stromal scores. A high fraction of T cells gamma delta, Macrophages M1, B cell naive, T cells CD4 memory activated, T cells CD8, T cells regulatory, T cells follicular helper, and mast cells resting, mainly infiltrated the tumors of the low risk OSCC patients. A recent study suggested that the cell density of the high parenchymal CD8+, at the invading tumor edge, was associated with improved overall survival, and therefore could be used as an independent favorable prognostic marker for OSCC (51). Moreover, OSSC patients with high levels of CD4 + CD25highCD127low regulatory T cells (Tregs), were found to have a better survival probability compared to patients with lower Tregs. This result indicated that immune cells might have an important effect on the OSCC TME. What’s more, we analyzed the expression of HLA-related genes, important to the immune system, in the two different risk groups, and found that the expression of most HLAs were significantly higher in the low-risk group, demonstrating that higher immune status was related to the prognosis of OSCC. The HLA molecules on the surface of tumor cells can help T cells recognize new antigens to create opportunities for anti-cancer immune responses (52). The expression of the immune checkpoints IDO1, LAG3, PDCD1, and TIGIT significantly increased in the low risk group. Foy et al. (53) found that OSCC tissues are characterized by a higher level of intratumor T-cells, overexpression of PD-L1 and IDO1, and a higher score of response signature to pembrolizumab, suggesting the inhibition of IDO1 and PD-L1 may have good clinical significance for OSCC. Another research indicated that several immune checkpoint receptors (TIM3, LAG3, IDO, PDL1 and CTLA4) could be considered as biomarkers that reflect the immune status in OSCC patients’ TME during nimotuzumab therapy (54). T cells from peripheral blood mononuclear cells, that were collected from OSCC donors, possessed a high expression level of TIGIT. Moreover, TIGIT blockade can promote the in vitro proliferative ability and effective cytokine secretion capacity of CD4 + T cells and CD8 + T cells isolated from OSCC patients (55). These results provided support for the hypothesis that OSCC patients with lower risk score (patients with higher expression of CDKN2A/FLT3 genes and lower expression of DDIT3/FTH1 genes) might respond better to the IDO1, LAG3, PDCD1 and TIGHT inhibitors.
For the first time, we assessed the effects of ferroptosis-related genes on the prognosis of OSCC and constructed an effective prognostic model to reveal the involved biological processes. We also proved that this model can be used as a criterion for determining whether a patient is suitable for immunotherapy. It is reasonable to believe that ferroptosis will potentially serve as a novel strategy for cancer treatment. However, more data from in vivo/in vitro experiments and clinical trials, are needed to elucidate the mechanisms between FRGs and tumor immunity in OSCC.