INDUCTION OF MOUSE RETINOL-BINDING PROTEIN GENE-EXPRESSION BY CYCLIC-AMP IN HEPA-1-6 CELLS

Retinol binding protein (RBP) is the primary circulating transport mol ecule for retinol, facilitating its transport to target tissues and in fluencing target cell uptake. Specific signals and molecular mechanism s that regulate REP gene expression are poorly understood. Using the m ouse hepatoma cell line (Hepa 1-6), we examined the role of cAMP in th e molecular regulation of REP. Dibutyryl cAMP (dbcAMP) or the adenylat e cyclase activator, forskolin, increased REP mRNA levels >6-fold at 2 4 h. Increases in REP mRNA were dose dependent over the range of 10 mu M-1 mM for dbcAMP and 0.5-10 mu M for forskolin, 8-Bromo cAMP, a nonh ydrolyzable analog, over the range of 0.01-0.5 mM, increased REP mRNA levels 9.2-fold at 24 h. Induction of REP transcripts by analogs also resulted in a comparable increase in intracellular REP protein. Cycloh eximide (10 mu g/ml) did not prevent cAMP-mediated induction of REP mR NA, indicating that de novo protein synthesis is not required for cAMP -mediated induction of REP transcription. These studies demonstrate th at cAMP, or agents which elevate intracellular cAMP, increase REP tran script levels. The time course and extent of REP mRNA induction and th e resultant increase in REP protein support the concept that cAMP regu lation of REP gene expression may be physiologically relevent. Given t he ubiquitous nature of cAMP as a second messenger, and the several me chanisms by which cAMP regulates gene expression, studies are in progr ess to define molecular mechanisms by which cAMP regulates REP gene ex pression. (C) 1998 Academic Press.