Progesterone and its metabolites: the potent inhibitors of the transporting activity of P-glycoprotein in the adrenal gland

P-glycoprotein (P-gp) is a transmembrane glycoprotein responsible for the multidrug resistant (MDR) phenotype in various cancer cells. It has been shown that P-gp transports various kinds of anti-cancer agents as well as hydrophobic chemicals. Although P-gp is also expressed in normal human tissues, such as liver, kidney, and adrenal gland, its function and transporting substrates in these tissues are still unknown. In previous work, we demonstrated that some compounds in human plasma modulate the transporting activity of P-gp. We also found that P-gp is expressed at a high level in the bovine adrenal gland and that this tissue contains large amount of compounds which inhibit the transporting activity of P-gp. We purified such compounds from the adrenal gland by monitoring the ability to enhance the accumulation of [3H]vincristine in MDR cells. Two major compounds were purified and identified as progesterone and pregnenolone by nuclear magnetic resonance (NMR) analysis. Progesterone was the most potent and abundant compound that inhibited the transporting activity of P-gp among the compounds extracted from bovine adrenal gland with methanol. We also found that six authentic progesterone metabolites in the 5 beta-metabolic pathway but none in the 5 alpha-metabolic pathway were able to enhance the accumulation of [3H]vincristine in MDR cells and to inhibit [3H]azidopine photolabeling of P-gp in the adrenal gland. These results indicate that some progesterone metabolites can interact with P-gp and that stereoisomerism around carbon 5 of the progesterone metabolites is important for them to be recognized by P-gp.

Expression of the multidrug transporter, P-glycoprotein, in renal and transitional cell carcinomas

BACKGROUND

Renal cell carcinomas (RCC) respond poorly to anthracyclines, Vinca alkaloids, and other agents. P-glycoprotein is overproduced in multidrug-resistant cells and thought to function as an energy-dependent drug efflux pump. The authors thus examined the expression level of P-glycoprotein in RCC and transitional cell carcinomas (TCC).

METHODS

P-glycoprotein was detected using immunoblotting with a monoclonal antibody against it, C219.

RESULTS

Thirty-three of 38 patients with RCC and 3 of 17 patients with TCC had P-glycoprotein positive tumors. The expression level of P-glycoprotein in most of RCC was lower than that in the normal kidney tissues and that of P-glycoprotein in the TCC was very low. The size of P-glycoprotein in 14 RCC and 3 TCC was 5-10 kilodaltons smaller than in the normal renal tissues. The variation of P-glycoprotein size in the RCC was attributed to differential N-linked glycosylation. P-glycoprotein in a RCC was photolabeled by tritiated azidopine, and the labeling was inhibited by some organic agents. P-glycoprotein distributed on the apical or marginal cell surface of the RCC.

CONCLUSIONS

These data show that P-glycoprotein was expressed in many RCC, and its expression level, glycosylation, and distribution were altered. These data also suggest that the P-glycoprotein in RCC had similar drug binding site(s) to that in multidrug-resistant cells.