Protein kinase C regulates transcription of the human guanylate cyclase C gene

Abstract

Guanylate cyclase C is the receptor for the bacterial heat-stable enterotoxins and guanylin family of peptides, and mediates its action by elevating intracellular cGMP levels. Potentiation of ligand-stimulated activity of guanylate cyclase C in human colonic T84 cells is observed following activation of protein kinase C as a result of direct phosphorylation of guanylate cyclase C. Here, we show that prolonged exposure of cells to phorbol esters results in a decrease in guanylate cyclase C content in 4β-phorbol 12-myristate 13-acetate-treated cells, as a consequence of a decrease in guanylate cyclase C mRNA levels. The reduction in guanylate cyclase C mRNA was inhibited when cells were treated with 4β-phorbol 12-myristate 13-acetate (PMA) in the presence of staurosporine, indicating that a primary phosphorylation event by protein kinase C triggered the reduction in RNA levels. The reduction in guanylate cyclase C mRNA levels was not due to alterations in the half-life of guanylate cyclase C mRNA, but regulation occurred at the level of transcription of guanylate cyclase C mRNA. Expression in T84 cells of a guanylate cyclase C promoter-luciferase reporter plasmid, containing 1973 bp of promoter sequence of the guanylate cyclase C gene, indicated that luciferase activity was reduced markedly on PMA treatment of cells, and the protein kinase C-responsive element was present in a 129-bp region of the promoter, containing a HNF4 binding element. Electrophoretic mobility shift assays using an oligonucleotide corresponding to the HNF4 binding site, indicated a decrease in binding of the factor to its cognate sequence in nuclear extracts prepared from PMA-treated cells. We therefore show for the first time that regulation of guanylate cyclase C activity can be controlled at the transcriptional level by cross-talk with signaling pathways that modulate protein kinase C activity. We also suggest a novel regulation of the HNF4 transcription factor by protein kinase C.