Immunosuppressant inhibition of P-glycoprotein function is independent of drug-induced suppression of peptide-prolyl isomerase and calcineurin activity

Purpose: P-glycoprotein is a 170-kDa plasma membrane multidrug transporter that actively exports cytotoxic substances from cells. Overexpression of P- glycoprotein by tumor cells is associated with a multidrug-resistant phenot ype. Immunosuppressive agents such as cyclosporins and macrolides, have bee n shown to attenuate P-glycoprotein activity. However. the mechanism by whi ch some immunosuppressants inhibit P-glycoprotein function has not been det ermined, Since cyclosporin and macrolide immunosuppressants inhibit calcine urin (CaN) phosphatase and FKBP12 peptide-prolyl isomerase (FKBP12 PPI) act ivity, studies were conducted to determine if these effects are directly re lated to the inhibitory effects these immunosuppressants have on P-glycopro tein function. Methods: Western blot analysis was performed to assess CaN a nd FKBP12 protein levels in P-glycoprotein-negative (MCF-7) and -positive ( MCF-7/Adr) breast cancer cell lines. P-glycoprotein function was determined by intracellular doxorubicin accumulation and:or cytotoxicity assays befor e and after CaN and FKBP12 were independently inhibited by pharmacological antagonists. Results: CaN and FKBP12 levels were similar in MCF-7 and MCF-7 /Adr cells. P-glycoprotein function was not affected by treatment of P-glyc oprotein-espressing MCF-7/Adr cells with CaN and FKBP12 antagonists. Conclu sions: These results demonstrate that the inhibitory effects of immunosuppr essive agents on P-glycoprotein function are independent of CaN or FKBP12 P PI activity.