Using large samples from multiple sources, this study comprehensively identified different ITGAV expression in SCLC and revealed the conspicuous prognosis and identification values of ITGAV expression in this disease. Several TFs that may regulate ITGAV expression in SCLC were also identified, which has not been reported before. Focusing on TME, the study explored the molecular mechanisms of ITGAV in SCLC. Further, the extensive pan-cancer analysis, with tens of thousands of samples, verified differential expression of ITGAV and its clinical significance in multiple cancers.
Differential expression of ITGAV in cancer is common, and its high expression is predominant. Upregulated ITGAV expression has been identified in multiple cancers, such as gastric cancer cells, breast cancer, and hepatocellular carcinoma. Downregulated ITGAV expression has also been emphasized in epithelial ovarian cancer. Thus, it is necessary to comprehensively explore ITGAV expression in many cancers. In this study, among 35 cancers (including SCLC), 80% (28/35) demonstrated different expression levels between cancer groups and non-cancer groups, and high expression was observed in 68% (16/28) of the latter. One point that should be noted was that low-ITGAV mRNA and high-ITGAV protein levels were detected. These seemingly inconsistent results were in fact reasonable, since (1) polypeptides can be produced by an mRNA molecule, but their translation rates may vary in space and time; (2) mRNA molecules from the same gene may feasibly produce various amounts of protein due to translational heterogeneity; (3) the protein product of a gene tended to be more stable than its mRNA, as the latter degrades more easily. Therefore, inconsistent ITGAV mRNA and protein levels are still rational in some conditions.
Low-ITGAV mRNA expression in SCLC likely resulted from downregulated expression of three TFs—ZEB2, IK2F1, and EGR2. The three TFs may be regulators of ITGAV expression, as they not only demonstrated decreased expression and close positive relationships with ITGAV expression in SCLC, but also had ChIP-Seq peaks upstream of the transcription start site of ITGAV. Previous reports have demonstrated that some of the three TFs are important factors in SCLC. For instance, Wang et al. found that ZEB2 participated in promoting the occurrence of extracellular matrix in SCLC, thus contributing to the progression of the disease. However, to the best of our knowledge, no relevant research on the regulation of ZEB2, IK2F1, and EGR2 for ITGAV exists, which to some extent indicates the novelty of this study.
ITGAV expression demonstrated conspicuous clinical significance in quite a few cancers. The gene has been recognized as having an important risk role in cancer progression. For example, Cheuk et al. showed high-ITGAV expression causing breast cancer metastasis; Kemper et al. identified that ITGAV expression contributed to PAAD; Loeser et al. revealed the relationship between ITGAV expression and unfavorable prognosis for patients with esophageal adenocarcinoma. However, no similar research has been reported for SCLC. Although reduced ITGAV mRNA expression was detected in SCLC, we believe that the roles of ITGAV in SCLC resulted from its high protein levels for several reasons.
First, a gene’s function is usually due to its coding protein rather than its mRNA. Second, its association with SCLC’s poor prognosis (found in the current study) supported ITGAV as a risk factor in SCLC. Further pan-cancer analyses demonstrated that ITGAV expression was related to poor OS of patients with LGG, glioma, LIHC, mesothelioma, stomach adenocarcinoma, PAAD, stomach, and esophageal carcinoma, LUAD, and kidney renal papillary cell carcinoma; moreover, it was also associated with shorter disease-free survival of PAAD patients. Among these cancers, ITGAV has been identified as a risk factor in LIHC, mesothelioma, and PAAD, while no reports about the remaining six cancers indicated novelty of our study. The finding that ITGAV expression makes it feasible to distinguish multiple cancer tissues from their controls suggests the potential of ITGAV expression in screening cancers. ITGAV may serve as an essential marker of prognosis and identification of multiple cancers.
Through disease ontology, ITGAV-PRLEGs are involved in some diseases, including small cell lung carcinoma and non-small cell carcinoma, suggesting associations between ITGAV and lung cancers. For ITGAV-PRLEGs, the keywords for gene ontology were “adhesion,” “extracellular matrix,” and “immunity.” Cell-cell interactions and cell adhesion are key mediators of lung cancer progression, including immune evasion and metastatic events. Integrins take part in cell surface adhesion and signaling and have essential functions in cancer progression. The protein product encoded by ITGAV is a subunit of integrins (the other is the β subunit), and thus participates in cancer development[6, 7]. For example, upregulated ITGAV stimulated the synergistic effect of integrin and selectin, which promoted adhesion between PAAD and peritoneal mesothelial cells, finally leading to the growth of pancreatic cancer. It can be seen that the classic role of ITGAV in cell adhesion may be one of its potential mechanisms in SCLC. Little is known about its immune-related role in cancers, although this finding was clear in this study.
ITGAV’s roles in cancers may be linked to TME. In our study, signaling pathways of KEGG, Reactome, and GSEA consistently demonstrated that ITGAV-PRLEGs were clustered in immune-related pathways. Moreover, ITGAV expression was positively associated with scores for all immune stromal cells, immune cells, and estimated scores (tumor purity), suggesting its close associations with TME. Of the multiple components of TME, immune cells were recognized as predominant factors in regulating cancer progression. Positive associations between ITGAV expression and resting CD4 memory T cells, M1 macrophages, and M2 macrophages were observed in more than 10 cancers. Macrophages demonstrated a dual role in tumor progression. On one hand, based on proinflammatory cytokines and cytotoxic activities, they tend to inhibit tumor growth; on the other hand, they are more likely to stimulate tumor proliferation, angiogenesis, and metastasis, and as a result trigger tumor progression. Thus, a positive correlation between ITGAV expression and macrophage infiltration levels may support its relationship with poor prognosis in cancer patients.
More importantly, negative relationships between ITGAV expression and infiltration levels of memory B cells, CD8 T cells, and activated NK cells were observed. Interestingly, both CD8 T cells and activated NK cells were prominent factors (particularly CD8 T cells) in controlling tumor progression[34, 35], and their negative relationship with ITGAV supported the risk roles ITGAV played in most of the cancers. However, the immune-related mechanism of ITGAV is complex, which is indicated in the negative correlation of ITGAV with memory B cells, follicular helper T cells, and regulatory T cells (the three were considered to play a dual role in immune response). Taken together, the correlation between ITGAV and immune response (at least with immune cell infiltration levels) is clear, but more efforts are needed to investigate the mechanism.
ITGAV has potential for immunotherapy, and immune checkpoint inhibitor therapy is one of the most promising cancer treatment methods available. Studies have shown that immune checkpoints are thought to be involved in immunosuppression, thereby preventing immune cells from eliminating cancer cells. Dysregulated expression of checkpoints in the TME is common. ITGAV is related to a variety of immune checkpoints and is mainly positively correlated, which suggests its potential for use in immunotherapy.
Some limitations of this study should be noted: (1) there was a lack of in-house samples for exploring ITGAV protein levels in pan-cancer; (2) no pure body fluid samples were used to verify the ability of ITGAV to screen cancer; and (3) the complicated molecular mechanisms of ITGAV on cancers (such as immune infiltration levels) still need experimental verification in future research.