In consideration of the importance of epithelial-mesenchymal transition in malignancy metastasis(19), developing meaningful gene signatures to monitor the EMT status and motility of tumor cells in cancer patients is significant not only to identify reliable prognostic biomarkers but, if applied properly, to stratify patients at high-risk of recurrence, metastasis or chemoresistance who might benefit from additional therapy. In the present study, we validated a prognostic signature, based on four ERGs, which proved to be a reliable indicator of EMT status and could identify CRC patients with unfavorable prognosis. Moreover, our prognostic signature can further stratify patients sharing specific clinicopathological factors (e.g., age, gender and disease stage) into subgroups with different survival outcomes. Integrating these findings, we constructed a nomogram, incorporating the ERGs signature’s risk score and clinical characteristics, which showed good performance for predicting survival in patients with CRC.
Although several gene signatures representative of tumor EMT status have been proposed with potential clinical applicability in several cancers(16, 20), the impact of EMT status in CRC progression and prognosis prediction remains to be fully explored. Subgroup analysis combining our ERGs signature risk score and clinicopathological parameters revealed that a higher risk score was more strongly correlated with poorer prognosis. This is owing to high risk score reflecting the possibilities of enhancing the motility and invasiveness of tumor cells contributing to tumor progression and metastasis. Noteworthy, we also confirmed that the ERG signature remained an independent prognostic factor with multivariate analysis after adjusting for clinicopathological variables.
Cancer progression and metastasis is often linked to altered plasticity of tumor cells(21, 22), while EMT plays an unignorable role in this process(23). Accordingly, aberrant expression of transcription factors, cytokines, epithelial surface markers and mesenchymal features has been shown to contribute to the migration of CRC cells(24). According to previous studies, the four ERGs in our novel CRC signature encode proteins with active participation in angiogenesis, developing resistance and chemotaxis, increasing cancer cell growth, proliferation, survival, migration and epithelial to mesenchymal transition, and decreasing anoikis, relapse and chemotherapeutic sensitivity.
Brain-derived neurotrophic factor (BDNF) is highly expressed in brain as member of neurotrophins which were initially known to function as growth factors in nervous system(25). However, recent studies have discovered that BDNF and its receptor tropomyosin receptor kinase B (TrkB) were upregulated in a wide range of cancers including CRC(26). Growing evidence showed that BDNF/TrkB pathway was closely involved in CRC proliferation, invasion, migration and poor prognosis(27–29). Besides, positive results render BDNF/TrkB pathway to be a promising therapeutic target for CRC treatment(27, 30).
Oxytocin (OT) is viewed as a hypothalamic neuropeptide and classically functions in uterine contraction and milk ejection(31). Interestingly, some findings suggested that the gene encoding OT receptor (OXTR) also expressed in several types of cancer cells including breast, ovarian and prostate cancer regulating proliferation(32, 33). The relation between OXTR and CRC hasn’t been validated though it was reported that oxytocin receptors were expressed throughout gastrointestinal tract(34).
Plasminogen activator inhibitor 1 & 2 (PAI-1 & PAI-2), also known as serpin family E member 1 & 2 (SERPINE1 & SERPINB2), are the main inhibitor of urokinase plasminogen activator (uPA) and tissue plasminogen activator (tPA) which constitute plasminogen activator (PA) system. PA system in turn acts crucially in extracellular matrix (ECM) remodeling and further plays an important role in tumor progression(35). However, SERPINE1 was found associating with tumor progression and metastasis and identified as an indicator of poor prognosis in a series of cancers(36–38). Similarly, the character of SERPINE1 (or PAI-1) in colorectal cancer was also identified, as a prognostic biomarker(39, 40).
PCOLCE2 encodes procollagen C-proteinase enhancer 2 which is collagen-binding protein capable of binding at multiple sites on the triple helical portions of fibrillar collagens and competing for such binding with procollagen C-proteinases(41). In consistent with our research, PCOLCE2 has already been identified as a diagnostic and prognostic biomarker in CRC patients(42). However, the underlying mechanism of PCOLCE2 engaging in the progression and metastasis of CRC remains to be elucidated and we for the first time propose the idea that PCOLCE2 is associated with EMT process of CRC cancer cells.
Accumulating evidence shows that tumor cells undergoing EMT or acquiring mesenchymal phenotype might have contribution to the component of tumor infiltrating immune cells (TIICs) shifting to an immunosuppressive pattern resulting in immune escape and immune resistance(43, 44). Studies have also showed cross-talk between EMT and different immune cells render the availability to predict the presentation of TIICs by assessing EMT status(20, 45–47). In this study, ERG signature showed a positive correlation with immune cells including CD4+ T cells, dendritic cells and macrophages. In the light of our knowledge, CD4+ T cells and macrophages contribute greatly to the formation of immunosuppressive microenvironment, however, different subpopulations of these immune cells exhibit diverse functions and prognostic roles in CRC, thus immune cells can’t be simply regarded as risk or protective factors to CRC(46). Nevertheless, our study showed a certain change in infiltrating immune cells regarding to the ERGs, indicating the potential value of further investigating the relationship between the ERGs and subpopulations of different tumor-associated immune cells and their prognostic and therapeutic role in CRC.