P‐Glycoprotein, Multidrug Resistance and Protein Kinase C

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Abstract
The multidrug resistant (MDR) phenotype is a well‐studied subject that has been recognized as a determinant underlying specific types of drug resistance in human cancer. Although it is clear that the P‐glycoprotein plays a major role in MDR, it is not clear whether post‐translational modifications such as phosphorylation have any major impact on its modulation. The laboratory of Dr. Bruce Chabner was one of the first to describe increased expression and activity of protein kinase C (PKC) associated with the MDR phenotype. Since that time, a similar correlation has been observed in many other MDR cell lines. Most of these studies have been performed with doxorubicin‐selected cells that have acquired MDR and have shown increased PKC activity, mainly for PKC‐α isoenzyme. Intrinsic MDR in human renal cell carcinoma lines has been shown to correlate directly with PKC activity, but further studies with intrinsic MDR cell lines are needed before any conclusions can be drawn. More recent evidence suggests that there is a complex biochemical process by which PKC isoenzymes differentially phosphorylate specific serine residues in the linker region of P‐glycoprotein which may lead to alterations in P‐glycoprotein ATPase and drug‐binding functions. To further complicate matters, PKC plays an important role in anti‐apoptotic pathways, which can confound the dissection and elucidation of drug‐resistance mechanisms. However, these areas are still under active investigation and not fully answered. Further studies are needed to specifically answer the question of whether PKC directly modulates basal and/or drug‐stimulated P‐glycoprotein function. This manuscript reviews the majority of the literature on PKC and MDR, as well as offers caveats for interpretation of these studies to answer the above questions.