Roles of tissue transglutaminase in ethanol-induced inhibition of hepatocyte proliferation and alpha 1-adrenergic signal transduction

The mechanisms by which ethanol inhibits hepatocyte proliferation have been a source of some considerable investigation. Our studies have suggested a possible role for tissue transglutaminase (tTG) in this process. Others hav e shown that tTG has two distinctly different functions: it catalyzes prote in cross-linking, which can lead to apoptosis and enhancement of extracellu lar matrix stability, and it can function as a G protein (G alpha(h)) Under that circumstance, we speculated that the cross-linking activity would be decreased and that it would function to enhance hepatocyte proliferation in response to adrenergic stimulation. Ethanol treatment inhibited hepatocyte proliferation and led to enhanced tTG crosslinking activity, whereas treat ment of hepatocytes with an alpha 1 adrenergic agonist, phenylephrine, enha nced hepatocyte proliferation while decreasing tTG cross-linking, However, phenylephrine treatment of several hepatoma cell lines had no effect on cel lular proliferation or tTG cross-linking activity, and of note, Northern bl ot analysis demonstrated that whereas primary hepatocytes had high levels o f the alpha 1 beta adrenergic receptor (alpha 1BAR) mRNA, the hepatoma cell lines did not have this mRNA. When the Hep G(2) cell line was stably trans duced with an expression vector containing the alpha 1BR cDNA, the cell lin e responded to phenylephrine treatment with enhanced proliferation and with decreased tTG cross-linking activity. Ethanol treatment of the alpha 1BAR- transfected cells suppressed the phospholipase C-mediated signaling pathway s, as detected in the phenylephrine-induced Ca2+ response. These results su ggest that phenylephrine stimulation of hepatocyte proliferation appears to be occurring through the alpha 1BAR, which is known to be coupled with the tTG G protein moiety, G alpha(h), and that tTG appears to play a significa nt role in either enhancing or inhibiting hepatocyte proliferation, dependi ng on its cellular location and on whether it functions as a cross-linking enzyme or a G protein.