Insulin inhibits delta-aminolevulinate synthase gene expression in rat hepatocytes and human hepatoma cells

Insulin has been known to regulate intracellular metabolism by modifying the activity or location of many enzymes but it is only in the past few years that the regulation of gene expression is recognized to be a major action of this hormone. The present work provides evidences that insulin inhibits delta-aminolevulinate synthase (ALA-S) gene expression, the enzyme which governs the rate-limiting step in heme biosynthesis. The addition of 5 nM insulin to hepatocytes culture led to a significant decrease of both basal and phenobarbital-induced ALA-S mRNA in a dose-dependent manner, as measured by Northern and slot-blot analysis. Several clues as to how insulin regulates ALA-S transcription were determined. The inhibitory effect is achieved at physiological concentrations but much higher proinsulin doses are needed. Insulin's effect is rapid, quite specific, and protein synthesis is not required. Moreover, ALA-S mRNA half-life is not modified by the presence of the peptidic hormone. Our results demonstrate that the insulin effect is dominant; it overrides 8-CPT-cAMP plus phenobarbital-mediated induction. Also, insulin requires the activation of protein kinase C to exert its full effect. On the other hand, a 870-bp fragment of the ALA-S promoter region is able to sustain the inhibition of CAT expression in plasmid-transfected HepG2 cells. Thus, these results indicate that insulin plays an important role in regulating ALA-S expression by inhibiting its transcription.