Incubation of HeLa cells for 24 hr with [3H]choline resulted in extensive labeling of the phosphorylcholine and phosphatidylcholine pools. 12-O-Tetradecanoylphorbol-13-acetate (TPA), other phorbol ester tumor promoters, and mezerein stimulated the release of [3H]choline and [3H]phosphorylcholine from such prelabeled cells. The release was accompanied by decreased radioactivity in the phosphorylcholine pool, raising the possibility that the released materials were derived by leakage from this pool. However, TPA did not induce the release of radioactivity from cells containing a prelabeled nucleotide pool. Similarly, the TPA-stimulated release of radioactivity from prelabeled cells closely paralleled the label present in the phospholipid pool rather than the phosphocholine pool. Consequently, it is suggested that the primary source of the released material is phosphatidylcholine acting as a substrate for a phospholipase C enzyme. TPA also stimulated the incorporation of [3H]choline into phospholipids, but a timecourse study indicated that phospholipase C activation preceded this event. This was supported by the observation that incorporation of [3H]choline was also stimulated by exogenously added phospholipase C.
This study was supported by the Australian Research Grants Committee, the National Health and Medical Research Council, and the Universities of South Australia Anti-Cancer Foundation.