Ethanol causes desensitization of receptor-mediated phospholipase C activation in isolated hepatocytes.
The effect of ethanol on receptor-mediated phospholipase C-linked signal transduction processes was investigated in isolated rat hepatocytes. Pretreatment of the cells with ethanol (6-300 mM) markedly inhibited a subsequent stimulation of phospholipase C by vasopressin, angiotensin II, or epidermal growth factor. By contrast, the effects of the alpha 1-adrenergic agonist phenylephrine and of glucagon were not affected by ethanol pretreatment. Ethanol inhibited the agonist-induced decrease in polyphosphoinositides, the formation of inositol phosphates, and the increase in cytosolic free Ca2+ levels, as detected with the intracellular Ca2+ indicator indo-1. The effects of ethanol were concentration dependent and were pronounced at low concentrations of agonists but were not significant at saturating levels. Pretreatment of the cells with the protein kinase C inhibitor H7 partly prevented the inhibition by ethanol of vasopressin-induced phospholipase C activation. By contrast, pretreatment of the cells with (Rp)-adenosine cyclic 3':5'-phosphorothioate [Rp)-cAMP-S), a competitive inhibitor of protein kinase A, potentiated the inhibitory effect of ethanol on the Ca2+ mobilization by vasopressin. (Rp)-cAMP-S similarly potentiated the inhibition of phospholipase C by the protein kinase C-activating phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). The kinase A inhibitor also made the Ca2+ mobilization by phenylephrine sensitive to ethanol, indicating that the formation of cAMP in the cells played a role in suppressing the sensitivity to ethanol. Pretreatment of the cells with ethanol enhanced the inhibitory effects of TPA on the vasopressin-induced phospholipase C activation at all concentrations of the hormone; however, these synergistic effects were prevented when TPA was added prior to ethanol, a condition that prevents the activation of phospholipase C by ethanol. The data indicate that ethanol causes desensitization of the receptor-mediated phospholipase C secondary to the ethanol-induced activation of phospholipase C and activation of protein kinase C. Ethanol treatment also affects the sensitivity of the phospholipase C system to control by protein kinases A and C. The data indicate that ethanol can affect the control of intracellular signal transduction processes in liver cells under physiologically relevant conditions.
Higashi
K
K
Department of Pathology and Cell Biology, Thomas Jefferson University, Philadelphia, Pennsylvania 19107.
Hoek
J B
JB
D000804:Angiotensin II / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D004789:Enzyme Activation; D004815:Epidermal Growth Factor / Q000494:pharmacology; D000431:Ethanol / Q000494:pharmacology*; D005934:Glucagon / Q000494:pharmacology; D008099:Liver / Q000166:cytology / Q000201:enzymology*; D008297:Male; D010656:Phenylephrine / Q000494:pharmacology; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols / Q000378:metabolism; D047428:Protein Kinase Inhibitors; D011494:Protein Kinases / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D011956:Receptors, Cell Surface / Q000378:metabolism; D015398:Signal Transduction; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D010738:Type C Phospholipases / Q000378:metabolism*; D014667:Vasopressins / Q000494:pharmacology
D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols; D047428:Protein Kinase Inhibitors; D011956:Receptors, Cell Surface; D014667:Vasopressins; D000804:Angiotensin II; D010656:Phenylephrine; D000431:Ethanol; D004815:Epidermal Growth Factor; D005934:Glucagon; D011494:Protein Kinases; D010738:Type C Phospholipases; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium