Kinetic and molecular species analyses of mitogen-induced increases in diglycerides: evidence for stimulated hydrolysis of phosphoinositides and phosphatidylcholine.
A wide variety of agonist-induced events appear to be mediated through an increase in cellular diglyceride levels. With regard to the ability of diglycerides to mediate these events, three important parameters must be considered: a) the kinetics of diglyceride generation, b) the absolute mass levels, and c) their molecular species. While this increase is often due to a stimulated hydrolysis of phosphoinositides, there is increasing evidence that the stimulated hydrolysis of phosphatidylcholine also contributes to agonist-induced increases in diglyceride levels. The kinetics of mass increases in diglyceride levels stimulated in cultured fibroblasts are agonist-dependent. High concentrations of alpha-thrombin stimulate a biphasic increase in diglyceride levels with the first phase peaking at 15 s and the second phase peaking at 5 min. In contrast, stimulation with epidermal growth factor, or platelet-derived growth factor, results in a monophasic increase in cellular diglyceride levels. Furthermore, the molecular species and phospholipid source of the stimulated diglycerides are also agonist-dependent. While the hydrolysis of phosphoinositides is major source of diglycerides initially generated in response to some agonists (15 s with alpha-thrombin at 500 ng/ml), phosphatidylcholine is hydrolyzed as well. Following longer incubations, or at all times following stimulation by epidermal growth factor or platelet-derived growth factor, phosphatidylcholine hydrolysis is the principal source of the stimulated diglycerides.
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Department of Physiology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
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D002478:Cells, Cultured; D002849:Chromatography, Gas; D004075:Diglycerides / Q000145:classification / Q000378:metabolism*; D004815:Epidermal Growth Factor / Q000494:pharmacology; D005347:Fibroblasts / Q000187:drug effects* / Q000378:metabolism; D006868:Hydrolysis; D007700:Kinetics; D008563:Membrane Lipids / Q000378:metabolism*; D008934:Mitogens / Q000494:pharmacology*; D010713:Phosphatidylcholines / Q000378:metabolism*; D010716:Phosphatidylinositols / Q000378:metabolism*; D010982:Platelet-Derived Growth Factor / Q000494:pharmacology; D015290:Second Messenger Systems; D013917:Thrombin / Q000494:pharmacology
D004075:Diglycerides; D008563:Membrane Lipids; D008934:Mitogens; D010713:Phosphatidylcholines; D010716:Phosphatidylinositols; D010982:Platelet-Derived Growth Factor; D004815:Epidermal Growth Factor; D013917:Thrombin