Initiation and progression of HCC is considered a multi-step process, including chronic inflammation, hepatic fibrosis and cirrhosis, and involving dynamic interactions between multiple cell types. Cytokine signaling plays a significant role in cell-cell communication, growth, differentiation, and immune function (29). Uncoordinated regulation of cytokine signaling has been linked to a variety of inflammatory and neoplastic diseases. Suppressor of cytokine signaling family, the key negative regulators of cytokines and growth factor signaling, consists of eight structurally similar proteins, SOCS1-7, and cytokine-inducible SH2-containing protein (CISH). Among them, SOCS 1–3 and CISH are strictly associated with the control of cytokine signaling (30). Several studies have reported that some SOCS proteins emerged as potential tumor suppressor-like proteins and immune checkpoint molecules, suggesting that SOCS family may modulate tumor progression and immunotherapeutic effect (31). However, the involvement of different SOCS members in tumor progression, especially in HCC is still unclear.
In this study, by integrating the expression profiling from TCGA-LIHC dataset and GSE94660 dataset, we demonstrated that SOCS2 and SOCS3 were significantly downregulated, while SOCS5 and SOCS7 were upregulated in HCC tissues. Furthermore, survival analysis revealed only SOCS2 mRNA had significant prognostic value in terms of OS and DFS, and patients with higher expression level of SOCS2 had improved OS and DFS. These findings were consistent with the study by Xinyu et al., which detected SOCS2 level in 106 HCC patients and found reduced SOCS2 expression correlated with tumor progression (32). However, the biological function and the downregulation mechanism of SOCS2 remain to be investigated.
We next analyzed the association between SOCS2 expression and HCC clinical and immunological features by TISIDB. As tumor stage and grade increased, the mRNA expression of SOCS2 tended to be lower. The mRNA expression of SOCS2 in stage Ⅲ seemed to be lower than that in stage Ⅳ, possibly due to the small sample size (only six HCC patients were at stage Ⅳ). Meanwhile, SOCS2 expression significantly correlated with different immune subtypes and molecular subtypes. Concretely, the highest SOCS2 expression was observed in immune subtype C3 and iCluster2, which were recognized as good prognostic indicators (33, 34), suggesting SOCS2 potentially functioned as a tumor suppressor in HCC.
We then evaluated the effects of SOCS2 on biological processes. By analyzing the most significantly differentially expressed 500 genes in SOCS2high versus SOCS2low patients, the results of functional enrichment indicated SOCS2 may contribute to RNA metabolism, which plays an essential role in the regulation of immune responses through its effects on cytokine production (35, 36). Besides, accumulating evidences have revealed SOCS family proteins affected the behavior and activation state of many immune cell types (37), we further explored the association between SOCS2 level and infiltration of various immune cell subtypes. The results showed that HCC patients with higher SOCS2 mRNA level had higher resting memory CD4 T cells and γδ T cells infiltration but lower follicular helper T cells, regulatory T cells and M0 macrophages infiltration. These immune subtypes were important components of the tumor microenvironment. Memory CD4 T cells and γδ T cells are reported to have anti-tumor response while regulatory T cells are known as one key immunosuppressive subset (38–40). Follicular helper T cells are considered to possess both oncogenic and tumor suppressor properties (41, 42). So are M0 macrophages, which can exert different biological effects by M1/M2 polarization (43). Taken together, these findings indicated that SOCS2 may act as a potent tumor suppressor by altering RNA metabolism and immune responses.
Finally, we investigated the molecular mechanism by which SOCS2 is down-regulated in HCC. In terms of genetic alteration, only one case with missense mutation with unknown significance was observed in the TCGA-LIHC cohort. Since several studies had reported that promoter methylation was associated with the downregulation of some SOCS proteins (12, 44), we then evaluated the methylation levels of SOCS2’s promoter in HCC but found SOCS2 expression was independent of the methylation level. In addition, there was no significant difference in promoter methylation of SOCS2 between HCC tissues and normal ones, which didn’t seem to affect the OS of patients with HCC. Thus, we suspect that this process could involve microRNA. The analysis of three datasets suggested negative association between miR-7-5p and miR-655-3p with SOCS2. Further comparison showed only miR-7-5p was upregulated in HCC tissues, and patients with lower miR-7-5p level had longer OS. All of these results indicated that miR-7-5p may be a potent regulator of the low expression of SOCS2 in HCC.
There were some limitations in our study need to be recognized. All the data analyzed in this study was retrieved from the online databases, and further in vitro and in vivo studies should be performed to validate our results.