IDENTIFICATION AND FUNCTIONAL-CHARACTERIZATION OF THE PROMOTER REGIONOF THE HUMAN ORGANIC ANION TRANSPORTING POLYPEPTIDE GENE

The organic anion transporting polypeptide (OATP) of the basolateral h epatocyte membrane mediates multispecific uptake of anionic and other amphipathic substrates from sinusoidal blood plasma, To investigate th e mechanisms controlling OATP expression, the 5'-flanking region of th e human OATP gene was isolated from a P1-derived artificial chromosome genomic clone. Sequence analysis of the OATP promoter showed a number of consensus binding sites for both ubiquitous and liver-enriched tra nscription factors. Transfection of HepG2 cells with a series of 5'-de leted promoter-luciferase constructs identified the minimal promoter r egion within 91 base pairs relative to the transcription initiation si te, A putative silencer element was localized in the -662/-440 region. The minimal promoter was also active in Chang liver, Madin-Darby cani ne kidney, and Chinese hamster ovary cells, indicating that basal prom oter function is independent of liver-specific regulatory mechanisms. In transfected HepG2 cells, taurocholate (100 mu mol/L) stimulated and triiodothyronine (1 mu mol/L) inhibited OATP promoter activity, where as hydrocortisone, dexamethasone, beta-estradiol, estrone-3-sulfate, a nd testosterone had no significant effect. Reverse-transcription polym erase chain reaction analysis showed an increase in OATP messenger RNA in the livers of four patients with chronic cholestatic liver disease compared with three noncholestatic controls. The up-regulation of OAT P expression by taurocholate could serve to enhance the sinusoidal eff lux of toxic intracellular compounds during chronic cholestasis.