RCC is a heterogeneous and complex cancer associated with a variety of genetic mutations, epigenetic changes, chromosome abnormality and so on (Hsieh et al. 2017). RCC has multiple subtypes, each with distinctive histological, genetic, molecular characteristics and biological behavior. KIRC, often accompanied with high metastasis rate and mortality, and is significantly resistant to chemoradiotherapy (Padala et al. 2021). There is growing evidence that CLEC2D plays a vital part in the initiation and development of multiple tumors. However, the knowledge of CLEC2D in KIRC is still unclear, which deserves further research and exploration.
In our research, we first analyzed the expression of CLEC2D via TCGA data, and then further verified the expression of CLEC2D using 4 GEO cohorts. Several studies have showed that CLEC2D overexpression has been observed in PC, glioblastoma, B cell lymphomas. Consistent with these studies, our study found similar results in KIRC. Survival analysis of CLEC2D in KIRC showed that cases with low CLEC2D expression had an ideal outcome. Furthermore, multivariate analysis demonstrated that CLEC2D expression could be an independent biomaker of dismal prognosis in KIRC patients. Moreover, the higher CLEC2D expression was contributed to the risk of nodal and distant metastasis, as well as higher grade. Consequently, the CLEC2D has a stronger ability to discriminate patients with KIRC from healthy individuals, implying that CLEC2D may be a prospective and indicator and valid therapeutic target for KIRC.
Sufficient literatures have demonstrated that ncRNAs can participate gene expression regulation by interaction with each other through the ceRNA mechanism (Qi et al. 2015; Smillie et al. 2018). MiRNAs can bind to partially or completely complementary mRNA targets and guide to gene silencing through mRNA degradation or translation inhibition. Previous researchs displayed that miR-30c-2-3p was a latent cancer inhibitor gene in GC, PDAC, and BC, and low expression of which was strongly correlated with inferior prognosis (Tanaka et al. 2020; Tang et al. 2018; Shukla et al. 2015). Shukla et al. have elucidated that miR-30c-2-3p negatively regulated NF-κB signaling and cell cycle progression in BC (Shukla et al. 2015). Mathew and colleagues have reported that miR-30c-2-3p was down regulated in von Hippel-Lindau (VHL)-deficient KIRC (Mathew et al. 2014). The miR-30c-2-3p can specifically bind to and inhibit HIF2α transcripts which functions as cancer suppressors. Consistent with the previous studies, we unravelled that miR-30c-2-3p played an inhibitory role in the regulation of KIRC. After correlation, expression, and survival analysis, miR-30c-2-3p was selected as the most principal upstream inhibitor miRNA of CLEC2D.
LncRNAs were previously identified as "transcriptional noise" without any biological function (Kung et al. 2013). However, existing researchs have proved that LncRNAs played vital parts in numerous physiological and pathophysiological processes, especially tumorigenesis. Through expression, survival and correlation analysis, LINC00894 and LINC01422 might be the potential regulatory lncRNAs of the miR-30c-2-3p/CLEC2D axis in KIRC. LINC00894 is reported to be an oncogene in various tumours, including KIRP, LUAD, and BC (Meng et al. 2021; Zhou et al. 2021). LINC00894 competitively binds to miR-429 to regulate the transcriptional factor Zinc finger E-box binding homeobox 1 expression, leading to BC progression (Meng et al. 2021). Searching for existing articles, we did not retrieve the articles related to LINC01422. Therefore, LINC00894/-miR-30c-2-3p/CLEC2D axis may be considered as latent regulatory pathways of KIRC.
KIRC has long been recognized as an immunogenic tumour. In recent years, the approval of immunotherapeutic drug strategy has changed the outcome of patients with metastatic RCC. PD-1 (Nivolumab, pembrolizumab), PD-L1 (avelumab), and CTLA-4 (ipilimumab) antibody are also approved to treat advanced RCC and are likely to make dramatic changes in the therapeutic landscape. Despite proven benefits, a substantial proportion of patients remain unresponsive to therapy, indicating that there are still unreported mechanisms of immunosuppressive in the tumor and its microenvironment.
Our study indicated that the CLEC2D was negatively associated with the tumor purity, implying that high expression of CLEC2D was correlated with high TIICs. There were differences in the composition of some immune cells between the two CLEC2D subgroups. M1 Macrophages, NK cells, Plasma cells, CD4 + T cells, CD8 + T cells, TFH cells, and Tregs were more enriched in the high-CLEC2D subgroup, while DC, M2 Macrophages, and Neutrophils were more enriched in the low-CLEC2D subgroup. Further work indicated that M1 macrophage markers IRF5 was positively correlated with CLEC2D expression, while markers of M2 macrophage (CD163, VSIG4, MS4A4A) were moderately associated with CLEC2D level, indicating that CLEC2D may play a regulatory role in TAM polarization. M2 macrophage, a predominant subtype of macrophages, has been proved to be conducive to tumour growth and progression of invasive phenotype, and is related to terrible prognosis in RCC, BC, bladder cancer, ovarian cancer, gastric cancer and PC. Intriguingly, Treg cells are the main manipulators of the immunosuppressive TME. Treg cell infiltration in tumor is related to higher pathological stage and terrible outcome of KIRC. DC are necessary for T cell-mediated cancer immunity. Mayoux et al discovered that RCC patients treated with PD-L1 blockade, DC gene signature were closely related to improved OS (Mayoux et al. 2020). Previous researchs have demonstrated that TFH related cells were positively correlated with long term survival of patients with melanoma tumor, breast tumor, and colorectal tumor. Xu et al found that neutrophils inhibit anti-tumor immunity by acting on local angiogenesis, lymphangiogenesis and inflammation, thereby promoting tumor progression (Xu et al. 2020). To summarise, CLEC2D may be an excellent immune index for evaluating the prognosis of KIRC.
There is compelling evidence that CD8 + T and NK cells played an indispensable part in the clearance of immunogenic tumor cells. Our study unraveled that CLEC2D level was prominently associated with CD8 + T cells infiltration and its corresponding biomarkers CD8A and CD8B in KIRC. CD8 + T cells are critical responder cells for tumor immunity treatment. The target cell expression of CLEC2D can inhibit the function of NK cells. Mathewson and Marrufo et al. have demonstrated that blocking CLEC2D-CD161 interaction could enhance T cell-mediated immunity against hematologic malignancies and NK cell mediated killing of triple negative BC cells (Mathewson et al. 2021; Marrufo et al. 2018). Furthermore, as manifested by Roth and Mathew et al. unravelled that glioblastoma and PC cells evade NK cell mediated killing by over-expressing CLEC2D, further confirming the pivotal role of CLEC2D in immunotherapy (Mathew et al. 2016; Roth et al. 2007).
In addition, the efficacy of immunotherapy depends not only on the sufficient TIICs into the TME, but also on the adequate expression of IC. Therefore, we also evaluated the correlation between CLEC2D and IC, and the results unravelled that the CLEC2D expression was closely correlated to PD1, PD-L1 or CTLA-4 in KIRC, meaning that CLEC2D expression may be a latent novel predictor to guide immunotherapy stratification of patients with KIRC, which may be one of the major elements that a panel repeatedly prognosticate for the KIRC cases.