miR-124 represses FZD5 to attenuate P-glycoprotein-mediated chemo-resistance in renal cell carcinoma

Renal cell carcinoma (RCC) is one of most malignant neoplasms, exhibiting poor responsiveness to the conventional chemo-regime. Abnormal expression of P-glycoprotein (P-gp) has been implicated in the emergence of multiple-drug resistance (MDR) by reducing the accumulation of intracellular chemotherapy drugs. Wnt signaling plays critical roles in renal cancer and is triggered by binding of Wnt ligands to Frizzled (FZD) receptor proteins. miR-124 is a tumor suppressor associated with cancer relapse and MDR, whereas its role in P-gp-mediated MDR in refractory RCC is as yet unrevealed. Our study aimed to investigate the roles of miR-124 in chemo-resistant RCC cells and the potential targeted signaling paths in inducing P-gp expression. Doxorubicin (DOX)- and vinblastine (VBL)-resistant Caki-2 cells were developed by exposure of parental Caki-2 cells to the agents over a long period of time. In comparison with their parental cells, miR-124 was downregulated in Caki-2/DOX and Caki-2/VBL cells, accompanied by increased FZD5 and P-gp. IC₅₀ values were reduced significantly after miR-124 mimics were introduced into Caki-2/DOX cells. In addition, miR-124 mimics significantly promoted apoptosis of Caki-2/DOX cells. miR-124 targeted to FZD5 and miR-124 mimics as well as FZD5 siRNA showed significant inhibitory effects on P-gp expression in Caki-2/DOX cells. Furthermore, Wnt-5a dose-dependently stimulated the presentation of p-PKCα/βII and p-CamKII via activating FZD5, which was reversed by FZD5 silencing. Moreover, FZD5/protein kinase C (PKC) signaling is responsible for the elevation of P-gp and cancer cell survival. In conclusion, restoring miR-124 may function as a promising strategy to overcome P-gp-mediated MDR by inhibiting FZD5/PKC signaling.