A novel E-cadherin/SOX9 axis regulates cancer stem cells in multiple myeloma by activating Akt and MAPK pathways

Cancer stem cells (CSCs) have been identified in multiple myeloma (MM) and are widely regarded as a key driver of MM initiation and progression. E-cadherin, in addition to its established role as a marker for epithelial-mesenchymal transition, also plays critical roles in controlling the aggressive behaviors of various tumor cells. Here, we show that depletion of E-cadherin in MM cells remarkably inhibited cell proliferation and cell cycle progression, in part through the decreased prosurvival CD138 and Bcl-2 and the inactivated Akt and MAPK pathways. CSC features, including the ability of the cells to form clonogenic colonies indicative of self-renewal and side population, were greatly suppressed upon the depletion of E-cadherin and subsequent loss of SOX9 stem-cell factor. We further provide evidence that SOX9 is a downstream target of E-cadherin-mediated CSC growth and self-renewal-ectopic re-expression of SOX9 in E-cadherin-depleted cells rescued its inhibitory effects on CSC-like properties and survival signaling. Collectively, our findings unveil a novel regulatory mechanism of MM CSCs via the E-cadherin/SOX9 axis, which could be important in understanding the long-term cell survival and outgrowth that leads to relapsed/refractory MM.