P-glycoprotein mediated steroid transport in drug-sensitive and multidrug-resistant cells

P-glycoprotein (Pgp), product of the MDR1 gene, is a 170 kDa transmembrane phosphoglycoprotein responsible for the multidrug-resistant (MDR) phenotype in various cancer cells. Pgp is an ATP-dependent drug efflux pump which transports various chemotherapeutic agents and other hydrophobic chemicals across the cell membrane. Although Pgp is also expressed in normal human tissues, such as the adrenal gland, liver, and kidney, its endogenous function is unknown. We hypothesized (1) that Pgp in the adrenal serves to transport newly-synthesized steroids across the cell membrane to the extracellular space and (2) that Pgp function is modulated by Pgp phosphorylation. The effect of Pgp on the accumulation of fifteen steroids in multidrug-resistant human colon carcinoma SW620 Ad300 cells was compared to the accumulation in drug-sensitive parental SW620 cells. The accumulation of three steroids in multidrug resistant cells expressing Pgp was not altered compared to parental cells. However, the accumulation of twelve other steroids was reduced by as much as 50% and was shown to correlate significantly with decreased hydrophobicity. For example, the hydrophilic steroid cortisol was shown to be efficiently transported by Pgp whereas the hydrophobic steroid progesterone was not transported by Pgp. In H295 and SW620 Ad300 cells, progesterone and progesterone-like compounds acted as potent inhibitors of Pgp-mediated vinblastine efflux. The potency of this antagonism correlated with increased steroid hydrophobicity and decreased with increased steroid transport. In H295 human adrenocortical carcinoma cells treatment with calphostin C, a PKC inhibitor which decreases Pgp phosphorylation, caused an increase in steroid accumulation which significantly correlated with reduced hydrophobicity. Treatment with calphostin C increased progesterone efflux in SW620 Ad300 cells, modulated Pgp antagonism by steroids, and inhibited photoaffinity labeling of Pgp by progesterone. We conclude that (1) Pgp can mediate the transport of steroids, (2) Pgp-mediated transport can be antagonized by a variety of steroids, (3) these properties strongly correlate with steroid hydrophobicity, and (4) the state of phosphorylated Pgp affects steroid transport and steroid antagonism of Pgp-mediated transport. Our data suggest that one endogenous function of P-glycoprotein in the adrenal is the transport of hydrophilic steroids, primarily cortisol and aldosterone.