The effect of cAMP on tumour promoter responses mediated by C-kinase

The phorbol ester tumour promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) induced several rapid changes in the HEL-37 mouse epidermal cell line. These included an alteration in cell morphology, inhibition of cell-cell communication, inhibition of epidermal growth factor (EGF) binding and a stimulation of phosphatidylcholine (PC) synthesis. The synthetic diacylglycerol sn 1-oleoyl-2-acetylglycerol (OAG) and sn 1,2-dioctanoylglycerol (diC8) caused similar changes, implying an involvement of the Ca2+- and phospholipid-dependent protein kinase (C-kinase). Treatment of the cells with the cAMP-generating agents db-cAMP and isoproterenol together with the phosphodiesterase inhibitors aminophylline and isobutyl-methylxanthine (IBMX) prior to and during TPA, OAG or diC8 treatment protected the cells against the inhibition of both junctional communication and EGF binding. TPA-induced morphological changes and enhanced PC synthesis, however, were unaffected by elevated levels of intracellular cAMP. These experiments provide evidence for the existence of a dual regulatory system controlling some (but not all) tumour promoter effects.