Progesterone photoaffinity labels P-glycoprotein in multidrug-resistant human leukemic lymphoblasts

We show for the first time that [3H]progesterone ([3H]PRG) can directly photoaffinity label membrane proteins prepared from a multidrug-resistant human leukemic lymphoblastic cell line CEM/VLB5K. A 170-kDa protein in CEM/VLB5K cell membranes was specifically labeled by [3H]PRG, which we identified as P-glycoprotein (Pgp) by immunoprecipitation with monoclonal antibody C219. The anticancer drug vinblastine and multidrug resistance reversing agent verapamil as well as several steroidal hormones were examined for their ability to interfere with [3H]PRG binding to Pgp. We found that 200-fold molar excess of vinblastine strongly inhibited (93%) the binding of [3H]PRG to Pgp compared with verapamil (80%), progesterone (78%), testosterone (46%), dexamethasone (25%), and aldosterone (56%). The results of this study provide direct evidence that progesterone can bind to Pgp and support the hypothesis that under physiological conditions Pgp may play a role in the excretion of progesterone from certain cells. Importantly, our results show that under our conditions vinblastine and verapamil are better able to compete with [3H]PRG for binding to Pgp than are other steroids, including testosterone, corticosteroids, and mineralocorticoids.