1. MiR-1825 was abundant in exosomes from ovarian cancer cells and associated with improved prognosis.
We explored expression patterns during four group samples using the GSE76449 dataset:1) normal ovarian cells;2) exosomes outflowed from normal ovarian cells;3) ovarian cancer cells and 4) exosomes outflowed from ovarian cancer cells. Then identified DEMIs between the four groups of samples. Nineteen microRNAs were screened out, The result indicated that microRNAs exhibited a specific distribution pattern in exosomes and their original cells. The expression pattern of 11/19 DEMIs (miR-30c-5p, miR-99b-5p, miR-10a-5p, miR-125a-5p, miR-27a-5p, miR-151b, let-7e-5p, miR-151a-5p, miR-4521, miR-15a-5p, and miR-28-5p) was not correlated with cancer, both high expression in no matter cancer cells or normal cells. The expression pattern of miR-6216, miR-1246, miR-122-5p, and miR-1290 was irrelevant with cancer too, they were always highly expressed in exosomes which outflowed no matter from OC cells or normal ovarian cells. (Fig. 1A) This indicated that their expression modes were cell-specific or exosome-specific but not ovarian cancer exosome-specific. Interestingly, miR-1281, miR-1825, miR-6877-3p, and miR-3921 expressions were upregulated in exosomes outflowed from ovarian cancer cells, and significantly downregulated in the other three samples (Fig. 1B), as supported by a previous study.[43]
Further, the prognostic significance of miR-1281, miR-1825, and miR-6877-3p was evaluated in the high versus low expression groups using TCGA survival data. MiR-3921 was ruled out due to the absence of MiR-3921 expression in most OC samples. (Fig. 1C) In KM survival analysis, miR-1825 upregulation in OC cells was correlated with improved overall survival (P < 0.05) (Fig. 1D). It might act as a tumor suppressor in OC.
2. CLEC5A was the target gene of miR-1825 and had a prognostic value
We performed target gene predictions to identify miR-1825 targets using TargetScan7.2, miRDB, TarBase v.8, miRmap, and miRwalk3.0. The results displayed 52 genes (Fig. 2A). Differential expression analysis for 52 target genes using limma method revealed that the mRNA expression levels of CACNB2 and KIT were downregulated and those of N4BP3, CLEC5A, NOTUM, and DCDC2 were upregulated in the tumor group from TCGA database in comparison with normal ovarian samples from the GTEx database (|logFC| >3 and adjusted p value < 0.001) (Fig. 2B).
Due to the tumor-suppressive effect of miR-1825, the prognostic potential of the four elevated genes (N4BP3, CLEC5A, NOTUM, and DCDC2) was assessed. KM curves showed that among the four genes, CLEC5A appeared to be a prognostic potential marker for OC, as validated using the validation cohort (GSE9891 and ICGC dataset OV-AU) (Fig. 2C), and was significantly associated with worse overall survival of OC patients (P < 0.05). Moreover, CLEC5A could serve as a prognostic biomarker independent of clinicopathological parameters (age, clinical stage, and histologic grade), as validated using GSE9891 and OV-AU validation cohorts (Fig. 2D). In IHC assays, CLEC5A protein expression was significantly increased in OC versus normal ovarian tissues (Fig. 2E).
3.CLEC5A was related to Immune Microenvironment
To clarify the functions of CLEC5A expression signatures associated with OC, we performed biological processes of GO and KEGG pathways enrichment analyses of genes highly correlated to CLEC5A based on the TCGA dataset. As shown in Fig. 3A-B, the intersection of GO terms and KEGG pathways showed that a proportion of highly correlated genes was associated with immune response that plays an important role in tumor inhibition and promotion. These results suggested that immune alterations might contribute to OC occurrence and development. However, the mechanism by which CLEC5A plays in immunity in OC is not clear.
To further verify the results of GO/KEGG enrichment analyses, we conducted a GSEA with the low and high CLEC5A expression datasets. GSEA analyses showed significant differences (p < 0.05) in the enrichment of immune-related pathways and biological processes (Fig. 3C-D). Interestingly, CLEC5A also seems to activate the PI3K − Akt pathway in OC which has been confirmed by experiments in brain glioblastoma by Hong-Wei Fan et al. Given the importance of the PI3K − Akt signaling pathway and the impact of immune on OC, we speculate that activated PI3K − Akt signaling pathway may contribute in part to the development of immune environment in OC.[22]
4.High level expression of CLEC5A led to enhanced M2 macrophage infiltration
To investigate in which way the interaction between CLEC5A and immune microenvironment plays in OC, R package ESTIMATE was used to examine correlations between immune cell/stromal cells infiltration levels and CLEC5A level. Notably, we observed a positive correlation between CLEC5A level and immune cell/stromal cell infiltration. A similar result was obtained based on the GSE9891 dataset (Fig. 4A-B). These results indicated that CLEC5A might play a role in mediating the immune response and immune cells/stromal cells infiltration in ovarian tumors. To further explore which immune cell subtype correlated to CLEC5A expression, we utilized CIBERSORT to calculate the infiltration levels of 22 immune cell subtypes, including B cells (naive B cell, memory B cell, and plasma cell), T cells [CD8 T cell, naive CD4 T cell, memory resting CD4 T cell, memory activated CD4 T cell, follicular helper T cell (Tfh), regulatory T cells (Tregs), and gamma delta T cell], natural killer (NK) cells (resting NK T cell and activated NK cell), and myeloid subsets [monocyte, MO macrophage, M1 macrophage, M2 macrophage, and resting dendritic cell (DC), activated DC, resting mast cell, activated mast cell, eosinophils, and neutrophil]. After integrating the CIBERSORT’s results (p < 0.05) based on the TCGA and GEO databases, we found that the proportions of M2 macrophages were higher in the CLEC5A high expression group than those in the CLEC5A low expression group. In contrast, the plasma cells and Tfh cells were lower in the CLEC5A high expression group (Fig. 4C).
The accumulation of M2 macrophages has been reported as a pivotal part in promoting tumor progression.[44] CLEC5A expression level may affect the function of M2 tumor-associated macrophages (TAMs). We thus analyzed the correlations between CLEC5A expression level and 16 factors involved in TAMs-assisting solid tumors. In accordance with our presumption, CLEC5A was found to be positively correlated with inhibiting factors based on the TCGA and GEO database (Fig. 4D). These results suggest that raising CLEC5A expression can promote tumors via TAMs.
Collectively, CLEC5A was strongly correlated with the immune environment in OC and its expression level could affect the type of immune cells infiltrated in the tumor, such as significant accumulation of M2 macrophages followed with increased CLEC5A expression. One possible explanation for the accumulation of M2 macrophages might be that activated PI3K − Akt signaling pathway could cause macrophage M2 polarization.[45]
5.new Suggestions For Clinical Drug Sensitivity
We then evaluated the responses of CLEC5A high expression group and low expression group to 5 chemotherapeutic drugs that were included in the list of built-in R package pRRophetic and recommended chemotherapeutic regimens named cisplatin, docetaxel, doxorubicin, gemcitabine, and paclitaxel. Interestingly, the high CLEC5A expression group showed more sensitivity to paclitaxel, gemcitabine, and docetaxel with lower IC50. To the contrary, cisplatin and doxorubicin were more sensitive in the low CLEC5A expression group (Fig. 4E). This finding gives physicians a hint about offering OC patients individualized therapy.