THE ALPHA-ISOFORM OF PROTEIN-KINASE-C INHIBITS HISTAMINE-STIMULATED ADENYLATE-CYCLASE ACTIVITY IN A PARTICULATE FRACTION OF THE HUMAN GASTRIC-CANCER CELL-LINE HGT-1

The isoform of protein kinase C responsible for the inhibition of histamine-stimulated adenylate cyclase by the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), has been investigated in a particulate fraction prepared from the human gastric cancer cell line HGT-1. The alpha and epsilon isoforms of protein kinase C were detected in HGT-1 cells and in a 40,000 x g particulate fraction by immunoblotting procedures. The inhibitory effect of TPA on histamine-stimulated adenylate cyclase was enhanced by the presence of Ca2+, but decreased in a concentration-dependent manner by anti-peptide antibody to protein kinase C or, but not to protein kinase C E. Addition of Ca2+ and TPA to the 40,000 x g particulate fraction stimulated the phosphorylation of the protein kinase C substrate myelin basic peptide 4-14. Protein kinase C a is probably the isoform responsible for inhibition of histamine-stimulated adenylate cyclase in HGT-1 cells.