Tumorigenesis is a result of dysregulation of various signal pathways. DOK family members exist in multiple tumor types via providing a docking platform for the assembly of multi-molecular signal transduction. The roles of DOKs in tumorigenesis are still emerging and controversial. Some researches indicated that DOKs functionally act as anti-oncogenes that positively correlated with cancer patient survival[17, 18]. Conversely, others reported that DOKs perform as oncogenes in tumor progression and motility. Tong et al. demonstrated that DOK1 was significantly higher in EBV + gastric cancer tissues compared with EBV- tissues. And higher DOK4 mRNA was found in clear cell renal cell carcinoma biopsies compared with matched normal tissue. Little work, as far as we know, has been done on the effects and mechanisms of DOKs in PC. The present research revealed, in both the transcription and protein levels, that all 7 DOKs family members were over-expressed in PC tissues. Furthermore, DOKs mRNA levels were higher in PC cell lines than that in normal cells. The protein expression was consistent with mRNA expression level, suggesting DOKs may serve as oncogenes promoting PC carcinogenesis and progression. High expression of DOK1/3/6 mRNA forecasted a poor prognosis in PC, but increased of DOK4 expression indicated good prognosis. Notably, DOK4 protein did not show visible relations with survival. Further investigation of DOK4 is needed for validation.
Mounting evidence suggests that DOKs negatively modulate the biological activity of immune cells in the context of physiology in human. The specific subtypes of TIICs in TME may affect tumor cell survival, metastasis and resistance to treatments[21, 22]. Nevertheless, little is known about the impact of these DOKs on tumor immune infiltration, which dictates the functional orientation of DOKs. To determine why DOKs predicted poor survival, we performed enrichment analysis of DOKs and relevant 20 genes in PC which were mainly involved in immunomodulatory. Next, the relationships between DOKs and TIICs were anatomized. Results implied that DOK1/2/3/5/6 were prominently associated with CD8 + T cell, Treg, T exhaustion, macrophages and dendritic cells (DCs) in PC. T cell immune response is the central event in antitumour immunity. Studies have illustrated that TIICs (especially CD8 + T cells) are associated with better outcomes in multiple malignancies, such as breast, lung, melanoma, colorectal, and brain cancers[24, 25]. Although DOK1/2/3/5/6 was strongly associated with CD8 + T cell, high expression of DOKs contributed to poor prognosis. It is feasible that DOKs were mainly related to T cell exhaustion which presented a state of cytotoxic CD8 + T dysfunction in the TME. Moreover, our results also confirmed this hypothesis. Abundant Treg cell infiltration into TME is associated with poor clinical outcomes in PC. Treg cell depletion in combination with conventional chemotherapy has been demonstrated to enhance CD8 + T activation. DCs are took into account the most efficient antigen-presenting cells (APCs) and the only type of cells at stimulating CD8 + T cells. Despite the presence of DCs in TME and their potential to generate anti-tumor immunity, tumor-infiltrating DCs (TIDCs) often exhibit impaired or dysfunctional functions[29, 30]. DOKs may also negatively regulate TIICs in PC, leading to immune evasion and tumor progression.
Epidemiological evidence suggested that tumor-associated macrophages (TAMs) were often associated with poor outcomes in different forms of cancer (e.g. pancreatic and breast cancer)[31, 32]. TAMs were the most common TIICs in PC, which can be divided into M1 and M2 subtypes according to their state. M1 cells secrete pro-inflammatory factors to activate the inflammatory response and anti-tumor effects. TGF-β, IL-6 and other immunosuppressive factors secreted by M2 cells induce the dysregulation of T cell receptor signaling pathway, thereby exerting a tumor-promoting effect. Interestingly, in animal models of PC, M2 cells were the major source of PD-L1 in TME and negatively regulated CD8 + T cells. Our results confirmed that DOK1/2/3/5/6 emerged a significant correlation with M2, however, it was not associated with M1. DOKs may promote PC development and metastasis by promoting macrophages polarization from M1 to M2 phenotype. However, causality between DOKs and TAMs cannot be inferred in this study and the specific mechanism needs to be further explored.
Numerous studies have revealed that bright prospects of immune checkpoint blockers (ICBs), such as CTLA-4 and PDL-1 blockade, to promote T-cell responses by preventing T-cell exhaustion, have bright prospects. However, only part of patients with PC can benefit from ICBs clinically. ICBs exert a crucial part in the activation and infiltration of cytotoxic CD8 + T cells after tumor antigen recognition. The classical TLA-4 and PD-1/PDL-1 axis modulate different inhibitory pathways and have non-overlapping mechanisms. Clinical trials have shown that more than 80% of patients with advanced melanoma have a better outcome after ICBs combination therapy. Therefore, it is worth studying the immunoserotyping and regulation mechanisms of PC. In our research, DOK1/2/3/5/6 were co-expressed with CD247 and PDCD1, and high DOK1/2/3/5/6 expression in tumor accompanied with a high positive rate of PDL-1 protein. DOK1/2/3/5/6 may have the potential to be a new immune checkpoint molecule similar to PDL-1, but its role requires further research in future.