Role of Sox9 in cancer stemness of hepatocellular carcinoma

Abstract

In spite of the great effort invested in research and invention of advanced therapeutic treatments, HCC remains one of the deadliest cancers worldwide with high relapse rate, chemoresistance and metastatic potential. It is suggested that the existence of cancer stem cells (CSCs) may account for the aggressive nature of HCC and the failure of current treatments. The hypothesis of CSC proposed a hierarchy in the heterogeneous tumor bulk. Only a small subset of malignant cells has the capability to initiate tumor formation and bears stem cell-like features such as self-renewal and chemoresistance. For this reason targeting CSC may underlie novel therapeutic strategies for eliminating HCC.

Accumulating evidence suggests that dysregulation of development-related genes may promote carcinogenesis. As a transcription factor of the sex-determining region Y-related high-mobility-goup Box (Sox) family, Sox9 participates in embryogenesis and stem/progenitor cell maintenance. It was demonstrated to be upregulated and serve oncogenic functions in various human malignancies including HCC. However, its in vivo function was not validated in primary liver cancer. In addition, Sox9 expression has been observed in putative hepatic progenitor cells and contributes to self-renewal of these adult progenitors. Therefore, we hypothesized that Sox9 may promote the acquirement of stemness properties in human HCC.

In the current project, we first investigated the clinical significance of Sox9 in human HCC samples followed by in vitro and in vivo functional roles of this Sox protein in human HCC cells. Our findings presented that Sox9 was upregulated in human HCC tumor tissues when compared to their corresponding non-tumorous counterparts. Such upregulation was correlated with poorer cellular differentiation and presence of venous invasion, implying an oncogenic role of Sox9 in HCC aggressiveness and metastasis. Sox9 was also found co-expressed with hepatic progenitor marker CK19 in some of human HCC tissues, suggesting that Sox9 may promote stemness traits in HCC. Using shRNA stable knockdown approach, our in vitro studies presented downregulation of a panel of stem cell-related genes and attenuation of cell proliferation upon knocking down Sox9. More importantly, we have provided the first piece of evidence that silencing Sox9 could greatly inhibit in vitro self-renewal and in vivo tumor initiation and progression in HCC. Furthermore, our functional studies demonstrated that knockdown of Sox9 could led to suppression of in vitro migratory ability and invasiveness as well as epithelial-mesenchymal transition. These findings further supported the notion that Sox9 may contribute to the augmentation of cancer stemness and metastatic potential in HCC.

In conclusion, this study presented upregulation of Sox9 in human HCC samples and provided evidence of the oncogenic role of Sox9 in promoting cancer stemness in HCC. It implied that targeting this Sox protein may present a new therapeutic strategy that aids elimination of the tumor-initiating hepatic CSC population and eventually cures HCC.