Stimulation of P‐glycoprotein‐mediated drug transport by prazosin and progesterone

Abstract

P-glycoprotein is a plasma membrane protein of mammalian cells that confers multidrug resistance by acting as a broad-specificity, ATP-dependent efflux transporter of diverse lipophilic neutral or cationic compounds. Previously, we identified two positively cooperative drug-binding sites of P-glycoprotein involved in transport [Shapiro, A. B. & Ling, V. (1997) Eur. J. Biochem. 250, 130-137]. The H site is selective for Hoechst 33342 and colchicine. The R site is selective for rhodamine 123 and anthracyclines. Substrate binding to one site stimulates transport by the other. In this paper, we show that prazosin and progesterone stimulate the transport of both Hoechst 33342 and rhodamine 123. Rhodamine 123 and prazosin (or progesterone) in combination stimulate Hoechst 33342 transport in an additive manner. In contrast, Hoechst 33342 and either prazosin or progesterone interfere with each other, so that the stimulatory effect of the combination on rhodamine 123 transport is less than that of each individually. Non-P-glycoprotein-specific effects of prazosin on membrane fluidity and permeability were excluded. These results indicate the existence of a third drug-binding site on P-glycoprotein with a positive allosteric effect on drug transport by the H and R sites. This allosteric site appears to be one of the sites of photoaffinity labeling of P-glycoprotein by [125I]iodoarylazidoprazosin [Safa, A. R., Agresti, M., Bryk, D. & Tamai, I. (1994) Biochemistry 33, 256-265] and is likely not to be capable of drug transport.

Functional expression of P-glycoprotein in primary cultures of human cytotrophoblasts and BeWo cells☆

“…The results further emphasize some of the biochemical similarities of the BeWo cells and the primary cultures of normal human cytotrophoblasts and affirm the use of the cell line for some investigations of trans-trophoblast transport of nutrients and drugs. Recent reports suggest that endogenous steroids including the hormone of pregnancy, progesterone, may regulate P-gp function [33]. Therefore, our results will form the basis for further investigation of the role of efflux systems in regulating the distribution of endogenous steroids and xenobiotics across the placenta, and their possible role in regulating P-gp function in the human trophoblast.…”

Role of P-glycoprotein in transplacental transfer of methadone

“…Two or more P-gp transporter sites have been identified [38]. Verapamil, a substrate of one of the P-gp transporter sites [19] was used to determine whether methadone is a ligand of the same binding site, while progesterone binds to an allosteric modulatory site and is not transported [39].…”

Location of contact residues in pharmacologically distinct drug binding sites on P-glycoprotein

“…The authors developed the nomenclature of H (Hoechst) and R (Rhodamine123) sites on the protein. Subsequent studies revealed the presence of a third (P) site that was capable of transporting prazosin [11]. In addition, these studies demonstrated that some sites are capable of binding both substrates and inhibitors.…”