Regulation of phosphatidylcholine synthesis and the activity of CTP:cholinephosphate cytidylyltransferase in myoblasts by 12-O-tetradecanoylphorbol-13-acetate

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Abstract
The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) increases rate of incorporation of 32P into phosphatidylcholine and phosphatidylethanolamine about twofold in a line of rat skeletal myoblasts (L6). This effect is specific for TPA and is not caused by the nontumor-promoting analogue 4α-phorbol didecanoate (α-PDD). Cycloheximide and actinomycin D have no effect on the TPA-dependent increase in labelling. There is no significant difference in the rate of disappearance of [14C]glycerol from phospholipids in cells exposed to TPA. Several enzymes of phospholipid synthesis in myoblasts exposed to TPA are unaltered in activity, except CTP:cholinephosphate cytidylyltransferase (EC 2.7.7.15) whose activity is enhanced 1.5- to 2-fold compared with cells exposed to α-PDD. Exposure of cells for only 15 min to a concentration of 1 × 10−6 M TPA is sufficient to cause maximum activation of the enzyme. The cytidylyltransferase activity is associated largely with the particulate fraction in the cell-free extracts of myoblasts. Following gel filtration on Bio-Gel A-1.5m, the enzyme activity appears in the void volume and smaller molecular weight forms are not discernible. The kinetic parameters of cytidylyltransferase in preparations from treated cells are unaltered except for an increased Vmax. The apparent Km value for CTP for the enzyme from treated and untreated cells is about 0.9 mM. Both enzyme preparations are weakly inhibited by lysophosphatidylcholine, but are unaffected by several other phospholipids.