PKD2 mediates multi-drug resistance in breast cancer cells through modulation of P-glycoprotein expression

Multi-drug resistance (MDR) represents a major obstacle for chemotherapeutic treatment of a wide variety of human cancers. Increased expression of drug efflux pumps, such as the P-glycoprotein (P-gp) have been linked to development of MDR. Herein, we have identified protein kinase D isoform 2 (PKD2) as an important regulator of MDR and P-gp expression in paclitaxel-treated breast cancer cell lines. PKD2 was expressed with the highest phosphorylated activation status in the MDA-MB-231 cell line. MDA-MB-231 cells were also found to exhibit the highest level of resistance to an array of chemotherapeutic drugs. To further characterize the relationship between PKD2 activation and MDR, we next focused on the effects of the chemotherapeutic agent paclitaxel in MDA-MB-231 cells. Treatment with paclitaxel was shown to induce both PKD2 phosphorylation and P-gp expression in a time-dependent manner. Importantly, shRNA-mediated knockdown of PKD2 in MDA-MB-231 cells resulted in a significant decrease in resistance to paclitaxel, evident as significant decreases in both the IC₅₀ value and the resistance index (RI). Concurrent with the decrease in drug resistance, paclitaxel-induced expression of P-gp was also significantly reduced in PKD knockdown cells. These results indicate that PKD2 is required for paclitaxel-induced MDR and expression of P-gp. Therefore, modulation of PKD2 activity represents an attractive therapeutic strategy for improvement of the clinical effectiveness of chemotherapy.