Roles of tissue transglutaminase in ethanol-induced inhibition of hepatocyte proliferation and α1-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 have shown that tTG has two distinctly different functions: it catalyzes protein cross-linking, which can lead to apoptosis and enhancement of extracellular matrix stability, and it can function as a G protein (Gα(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 cross-linking activity, whereas treatment of hepatocytes with an α1 adrenergic agonist, phenylephrine, enhanced hepatocyte proliferation while decreasing tTG cross-linking. However, phenylephrine treatment of several hepatoma cell lines had no effect on cellular proliferation or tTG cross-linking activity, and of note, Northern blot analysis demonstrated that whereas primary hepatocytes had high levels of the α1β adrenergic receptor (α1BAR) mRNA, the hepatoma cell lines did not have this mRNA. When the Hep G₂ cell line was stably transduced with an expression vector containing the α1BR cDNA, the cell line responded to phenylephrine treatment with enhanced proliferation and with decreased tTG cross-linking activity. Ethanol treatment of the α1BAR-transfected cells suppressed the phospholipase C-mediated signaling pathways, as detected in the phenylephrine-induced Ca²⁺ response. These results suggest that phenylephrine stimulation of hepatocyte proliferation appears to be occurring through the α1BAR, which is known to be coupled with the tTG G protein moiety, Gα(h), and that tTG appears to play a significant role in either enhancing or inhibiting hepatocyte proliferation, depending on its cellular location and on whether it functions as a cross-linking enzyme or a G protein.