Biology of a Novel Organic Solute and Steroid Transporter,OSTΑ-OSTβ

Abstract

Using a comparative approach, recent studies have identified and functionally characterized a new type of organic solute and steroid transporter (OST) from skate, mouse, rat, and human genomes. In contrast to all other organic anion transporters identified to date, transport activity requires the coexpression of two distinct gene products, a predicted 340–amino acid, seven-transmembrane (TM) domain protein (OSTΑ) and a putative 128–amino acid, single-TM domain ancillary polypeptide (OSTβ). When OSTΑ and OSTβ are coexpressed in Xenopus oocytes, they are able to mediate transport of estrone 3-sulfate, dehydroepiandrosterone 3-sulfate, taurocholate, digoxin, and prostaglandin E₂, indicating a role in the disposition of key cellular metabolites or signaling molecules. OSTΑ and OSTβ are expressed at relatively high levels in intestine, kidney, and liver, but they are also expressed at lower levels in many human tissues. Indirect immunofluorescence microscopy revealed that intestinal OSTΑ and OSTβ proteins are localized to the baso-lateral membrane of mouse enterocytes. In MDCK cells, mouse OstΑ–Ostβ mediated the vectorial movement of taurocholate from the apical to the basolateral membrane, but not in the opposite direction, indicating basolateral efflux of bile acids. Overall, these findings indicate that OSTΑ-OSTβ is a heteromeric transporter that is localized to the basolateral membrane of specific epithelial tissues and serves to regulate the export and disposition of bile acids and structurally related compounds from the cell. If confirmed, this model would have important implications for the body's handling of various steroid-derived molecules and may provide a new pharmacologic target for altering sterol homeostasis.