HNSC is the sixth most common non-skin cancer worldwide, with an overall incidence of 600,000 cases per year and a mortality rate of 50%[29]. STC2 gene encodes a secreted glycoprotein abundant in various tissues with an autocrine/paracrine function. Yoshiaki Kita et al. suggested that high STC2 expression in tumor tissues sensitively predicted shorter five-year survival [16]. The upregulation of STC2 expression has been proven in other tumor types, for example, colorectal and prostate cancers [30, 31]. It may be associated with clinical features of HNSC progression, such as tumor proliferation and lymph node invasion, and a worse prognosis [16]. All these are consistent with our findings. Our results also demonstrated that the STC2 gene was differentially expressed between normal and stage 1–4 HNSC patients, respectively, and this difference was more evident in patients with higher clinical stage. Therefore, we speculated that STC2 was involved in HNSC progression.
TP53 protein regulates the expressions of a large number of target genes and encodes p53. P53 has been considered an oncogene early on as it is preferentially expressed in cancerous cells [32]. Since 1989 when Levine et al. found wild-type p53 as a tumor suppressor gene [33], it has captured more attention and has been confirmed as a regulator of several cellular activities, such as apoptosis, proliferation, differentiation, and cell-cycle control [34]. Its multi-function role has been observed in DNA damage, hypoxia, oxidative stress, DNA mutations, nutrient deprivation, and other stress responses. Alterations in TP53 gene expression, needless to say, are most certain to be detected in a significant proportion of human cancers. Most studies identified TP53 mutations in exons 5–8 with missense changes, but some ascertained functional mutations in exons 5–8 of TP53, which actively contributed to cancer progression [35, 36]. Evidence about TP53 mutations generally supports their associations with shorter recurrence-free survival or OS of HNSC patients [37–39]. This relationship has subsequently been ascertained by a clinical trial comparing TP53-mutated versus wild-type TP53 patients with HNSC [39]. Our study found that the frequency of TP53 mutations in HNSC was approximately 70%, and STC2 expression was pronouncedly increased in the mutated TP53 group. Therefore, the association between TP53 mutations and high STC2 expression is more likely to affect HNSC occurrence and development.
Mammalian DNA methylation is essential in the epigenetic regulation of gene transcription. Dysregulation of genomic DNA methylation has been found to participate in carcinogenesis [40]. A previous study reported that STC2 silencing in human cancer cells could be associated with the methylation of CpG islands of STC2. Methylation of the STC2 promoter is a potential mechanism to hinder STC2 expression in cancer cells[41]. Hence, we explored the relationship between STC2 expression and methylation in HNSC, and consistently, the results suggested a negative correlation between them. Moreover, the eight CpG sites revealing a positive and negative correlation with STC2 expression were classified into two groups, respectively. Kaplan-Meier survival curves for the negative-correlation group indicated the association between a lower methylation level and high STC2 expression. The survival curves for the positive-correlation group showed CpG hypermethylation predicted higher STC2 expression, suggesting worse OS of patients.
Numerous studies of STC2 expression within the tumor microenvironment found that STC2 enhanced tumor cell migration and invasion [42, 43]. Clinical studies confirmed STC2 as a new biomarker of multiple cancers due to its involvement in tumor neovascularization [44, 45]. STC is known for its potent anti-inflammatory effect [46, 47], of which STC1 exerts the effect via inducing uncoupling proteins to reduce oxidative stress [48]. STC2, homologous to STC1, is activated by oxidative stress to forestall cell apoptosis. The present study revealed a positive correlation between STC2 expression and the infiltration levels of CAFs, indicating that STC2 may promote fibrogenesis in HNSC. Previous studies have reported that STC2, as an immune-related protein, is a potential oncoprotein in the process of hepatocarcinogenesis and is considered a promising biomarker and molecular target[49, 50]. It is reported that STC2 can inhibit CD 8 + T cell infiltration [51]. These are consistent with our conclusions.
Our study offered KEGG and GO enrichment analyses based on initially screened genes related to aberrant STC2 expression in HNSC and identified “metabolic pathways” and “biosynthesis of antibodies” as highly relevant pathway terms of STC2’s role in head and neck carcinogenesis. Also, “oxidation-reduction process” and “positive regulation of cell proliferation” were highly associated with the functions of the STC2 gene in this process.
Besides, three limitations are deserving to be discussed. First, inevitably, the raw data from the GEO and TCGA databases were originally used for other purposes. So we have been unable to validate the prognostic value of STC2 gene knockdown or protein downexpression in HNSC. Second, although we preliminarily analyzed the biological processes of STC2 in HNSC using enrichment analysis, the specific mechanism of how STC2 expression/methylation patterns affect HNSC progression requires more research support. Finally, the details of how STC2 promotes immune cell infiltration in HNSC are worthy of standardized investigation. However, the current results are encouraging, considering STC2 as a promising biomarker for the predicting HNSC prognosis.