EFFECT OF ETHANOL ON P36 PROTEIN-KINASE SUBSTRATE AND INSULIN-RECEPTOR SUBSTRATE-1 EXPRESSION AND TYROSYL PHOSPHORYLATION IN HUMAN HEPATOCELLULAR-CARCINOMA CELLS

Ethanol inhibits insulin (IN) and epidermal growth factor (EGF)-induce d hepatocyte DNA synthesis. Growth factor receptor kinases, such as IN and EGF, phosphorylate insulin receptor substrate (IRS-1) and p36 pro tein kinase substrate, respectively, on tyrosine residues. IRS-1 and p 36 are thought to be important intracellular signal transduction molec ules involved in the regulation of cell growth. These investigations e xplored the effect of ethanol additions on the expression and tyrosyl phosphorylation (TP) of p36 and IRS-1 in a human hepatocellular carcin oma cell line (FOCUS) in relationship to cell proliferation induced by IN and serum growth factor stimulation. It was found that p36 was con stitutively and highly expressed in serum-starved cells and protein, a nd mRNA levels did not change with cell proliferation induced by growt h factors, However, exposure of FOCUS cells to ethanol additions subst antially inhibited TP of p36. The early TP of IRS-1 induced by IN stim ulation was also reduced by ethanol additions, Finally, there was a pa rallel decrease of FOCUS cell proliferation in ethanol-exposed culture s. These studies suggest that one possible mechanism of ethanol inhibi tory effect on cell proliferation is through reduced TP of putative in tracellular signal transduction molecules, such as p36 and IRS-1.