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pubmed:abstractText |
The classic pathway for agonist-induced generation of diacylglycerol is via activation of a phospholipase C-mediated hydrolysis of the "phosphoinositides." We now report findings from a variety of cell types, which indicate that tumor-promoting phorbol diesters, serum, and platelet-derived growth factor activate within seconds the hydrolysis of phosphatidylcholine, as detected by the formation of diacylglycerol and phosphocholine. It is known that phorbol diesters do not stimulate hydrolysis of the phosphoinositides. Yet, in cells prelabeled with either [14C]oleate or [32P]orthophosphate, addition of the tumor promoter phorbol dibutyrate (PBt2) resulted in the rapid generation of both diacylglycerol and phosphatidate in a time- and dose-dependent manner. The fatty acid composition of the phosphatidate most resembled the fatty acid profile of phosphatidylcholine from the same cell type. Taken together, these findings suggested a role for protein kinase C in the generation of diacylglycerol (and phosphatidate) from phosphatidylcholine. To define further the pathways involved, the metabolism of cellular phosphatidylcholine was studied. In cells prelabeled with [3H]choline, addition of PBt2, but not 4 alpha-phorbol, stimulated the formation of intracellular phosphocholine within 45 sec. Furthermore, addition of platelet-derived growth factor (PDGF) or serum to "serum-starved" cells prelabeled with [3H]choline resulted in increased levels of intracellular phosphocholine within 15-30 sec. Thus, the data suggest that agonists that stimulate protein kinase C either directly (e.g., PBt2) or indirectly via activation of phosphoinositide hydrolysis (e.g., PDGF and serum) may stimulate degradation of phosphatidylcholine by phospholipase C in intact cells. However, prior down-regulation of protein kinase C by prolonged pretreatment of cells with PBt2 almost totally abolished subsequent stimulation of phosphatidylcholine degradation by PBt2 but only partially attenuated subsequent stimulation by PDGF and serum. These observations suggest that PDGF and serum act, at least partially, through a protein kinase C-independent mechanism. Lastly, the size of the cellular choline and CDP-choline pools were shown to be small and relatively insensitive to agonist addition, as compared to the size and behavior of the phosphocholine pool. Thus, the rapidly increased levels of phosphocholine (and diacylglycerol) arising in response to agonist addition appear to be derived directly from phosphatidylcholine by a phospholipase C-mediated mechanism.
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