Rack1 mediates Src binding to drug transporter P-glycoprotein and modulates its activity through regulating Caveolin-1 phosphorylation in breast cancer cells

The failure of chemotherapy and the emergence of multidrug resistance (MDR) are the major obstacles for effective therapy in locally advanced and metastatic breast cancer. Overexpression of the drug transporter P-glycoprotein(P- gp) in cancer cells is one of the main causes of MDR due to its ability to efflux anticancer drugs out of cells. Although the signaling node that regulates the expression of P- gphas been intensively investigated; the regulatory mechanism underlying P- gptransport activity remains obscure. Herein, we reported that Rack1 and tyrosine kinase Src confer drug resistance through modulating the transport function of P- gpwithout altering its protein level. We provide evidences that Rack1 and Src regulate P- gpactivity by modulating caveolin-1 (Cav1) phosphorylation. Importantly, Rack1 acts as a signaling hub and mediates Src binding to P- gp, thereby facilitating the phosphorylation of Cav1 by Src and abolishing the inhibitory effect of Cav1 on P- gp. Taken together, our results demonstrate the pivotal roles of Rack1 and Src in modulating P- gpactivity in drug-resistant cells. Our findings also provide novel insights into the mechanism regulating P- gptransport activity. Rack1 may represent a new target for the development of effective therapies for reversing drug resistance.