Forskolin suppresses insulin gene transcription in islet β-cells through a protein kinase A-independent pathway

This work was designed to evaluate the effect of cAMP on insulin gene regulation. We studied the effects of forskolin on insulin gene transcription in the INS-1 β-cell line, confirming key results in primary cultures of human islet cells. Forskolin increased intracellular cAMP and cAMP-responsive element-binding activity. Insulin gene transcription was studied using a reporter construct in which the human insulin promoter was fused to luciferase. When cells were treated with forskolin for 12 h, insulin promoter activity was decreased 2- to 3-fold, whereas islet amyloid polypeptide promoter activity was significantly increased. This effect of forskolin on the insulin gene was time- and concentration-dependent, and was mimicked by 8-bromo-cAMP. Mutagenesis of the CRE-like elements in the insulin promoter had no effect on the forskolin-induced suppression, but dramatically decreased basal insulin promoter activity. Inhibition of PKA with H-89 also did not reverse the forskolin-induced suppression of insulin transcription. However, this effect was completely reversed by inhibition of cellular MAP kinase activity with PD98059 or U0126. These results demonstrate that forskolin suppresses insulin transcription in INS-1 cells through a PKA-independent mechanism that probably involves MAP kinase signalling.