Multidrug resistance is the phenomenon by which, after exposure to a single chemotherapeutic agent, cancer cells evade the agent’s cytotoxic effects as well as become resistant to several classes of diverse drugs. ATP-binding cassette (ABC) transporters are a family of transporter proteins that contribute to drug resistance via an ATP-dependent drug efflux pump. P-glycoprotein (P-gp) is a prominent ABC superfamily protein encoded by the mdr gene which has the ability to mediate the cellular extrusion of xenobiotics and anticancer drugs from tumor cells. Exclusively expressed P-gp cells from the human colon cancer HCT15/DOX line showed resistance to doxorubicin while parental HCT15 cells treated with doxorubicin displayed typical signs of apoptosis. In order to verify the hypothesis that expression of MDR is controlled in part, by protein kinase C (PKC), expression patterns of different PKC isoforms were examined in both cell lines. Of the PKC isoforms evaluated, the membrane translocation and expression levels of PKC? were strikingly increased in HCT15/ DOX cells. PKC? reversed doxorubicin-induced apoptosis through the scavenging of ROS as well as inhibition of PARP cleavage. In addition, inhibition of PKC? with Go6976, a specific inhibitor of classical PKC, led to re-duced MDR expression and increased doxorubicin-induced apoptosis. Knockdown of PKC? by siRNA diminished the protective effects of PKC? for doxorubicin-induced apoptosis. These results suggested that over-expression and activity of PKC? is closely as-sociated with the regulation of the MDR phenotype in human colon cancer HCT15 cells and provided insight into a new strategy for inhibiting doxorubicin resistance in human cancers.

Keywords: apoptosis, doxorubicin, multidrug resistance, PKC alpha?