REGULATION OF THE RAT-LIVER SODIUM-DEPENDENT BILE-ACID COTRANSPORTER GENE BY PROLACTIN - MEDIATION OF TRANSCRIPTIONAL ACTIVATION BY STAT5

The intracellular mechanism(s) underlying the upregulation of the hepa tic Na+/taurocholate cotransporting polypeptide (ntcp) by prolactin (P RL) are unknown. In this report, we demonstrate a time-dependent incre ase in nuclear translocation of phosphorylated liver Stat5 (a member o f the Signal Transducers and Activators of Transcription family) that correlated with suckling-induced increases in serum PRL levels. In ele ctrophoretic mobility gel shift assays, nuclear Stat5 exhibited specif ic DNA-binding ability towards IFN-gamma-activated sequence (GAS)-like elements (GLEs; 5'-TTC/A-PyNPu-G/TAA-3') located in the -937 to -904 bp region of the ntcp promoter. Transient cotransfections in HepG2 cel ls revealed that PRL inducibility (2.5-3-fold) required coexpression o f the long form of the PRL receptor (PRLRL and Stat5. Deletion analysi s mapped the PRL-inducible region to -1237 to -758 bp of the ntcp prom oter. Linking this 0.5-kb region to a heterologous thymidine kinase (t k) promoter, or linking multimerized ntcp GLEs either upstream of the ntcp minimal promoter (-158 to +47 bp) or the heterologous promoter co nferred dose-dependent PRL responsiveness. The short form of the PRL r eceptor failed to transactivate ntcp GLEs. These results indicate that PRL acts via the PRLRL to facilitate Stat5 binding to ntcp-GLEs and t o transcriptionally regulate ntcp.