Hydrolysis of phosphatidylcholine by phospholipase D is a common response to mitogens which stimulate inositol lipid hydrolysis in Swiss 3T3 fibroblasts

The stimulated hydrolysis of inositol lipids and phosphatidylcholine (PtdCho) by bombesin, Arg8]vasopressin (Arg8]Vp) and prostaglandin F2 alpha (PGF2 alpha) was analysed in Swiss 3T3 cells pre-labelled to isotopic equilibrium with either methyl-3H]choline, myo-2-3H]inositol or 9,10 (n)-3H]palmitic acid. All three agonists activated the phospholipase D-catalysed hydrolysis of PtdCho as determined by the release of 3H]choline (Cho) and the formation of 3H]phosphatidylbutanol (PtdBut). The release of 3H]choline by each agonist exhibited similar sensitivity to prolonged pre-exposure to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). The release of 3H]choline exhibited the same dose dependency as the production of total inositol phosphates for each mitogen suggesting that the two responses might be mediated through identical receptors. Acute pre-treatment with TPA allowed the dissociation of inositol lipid hydrolysis from PtdCho breakdown, since it inhibited inositol phosphate accumulation but stimulated choline generation. The loss of mitogen stimulated choline release in cells pre-treated with the phorbol ester for 48 h was not due to loss of stimulated inositol phosphate production which was reproducibly enhanced in these 'down-regulated' cells.