Roles of Tissue Transglutaminase in Ethanol-induced Inhibition of Hepatocyte Proliferation and α1-Adrenergic Signal Transduction

Abstract

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 (G␣ 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 treatment of hepatocytes with an ␣1 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 ␣1␤ adrenergic receptor (␣1BAR) mRNA, the hepatoma cell lines did not have this mRNA. When the Hep G 2 cell line was stably transduced with an expression vector containing the ␣1BR cDNA, the cell line responded to phenylephrine treatment with enhanced proliferation and with decreased tTG cross-linking activity. Ethanol treatment of the ␣1BAR-transfected cells suppressed the phospholipase C-mediated signaling pathways, as detected in the phenylephrine-induced Ca 2؉ response. These results suggest that phenylephrine stimulation of hepatocyte proliferation appears to be occurring through the ␣1BAR, which is known to be coupled with the tTG G protein moiety, G␣ 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.

An ultrasound and histomorphological analysis of experimental liver cirrhosis in rats

“…Collagen crosslinking makes this molecule more resistant to collagenase degradation, therefore inhibiting fibrosis remodeling. In agreement with other reports (5,20), our results showed that the persistent presence of tTG after withdrawal of toxic agents might determine a point of "no return" for hepatic cirrhosis.…”

“…This reaction leads to production of oxidative radicals that cause lipid peroxidation and, consequently, hepatotoxic injury (7). Ethanol also induces synthesis of type I collagen fibrils (18,19), inhibition of hepatocyte proliferation (20)(21)(22), exacerbation of activity in hepatic stellate cells (20,23), and increased expression of tissue transglutaminase (tTG) (24). Since a mechanism that renders matrix resistant to degradation results from the formation of ε-(γ-glutamyl) lysine cross-links by tissue transglutaminase (5,25,26), use of the hepatotoxic agents in combination could be more effective in maintaining the fibrotic pattern.…”

Increased potency of α1-adrenergic receptors to induce inositol phosphates production correlates with the up-regulation of α1d/Ghα/phospholipase Cδ1 signaling pathway in term rat myometrium

“…One may then postulate that this signaling pathway participates to the myometrial cell hypertrophy or proliferation that occurs during this period. Indeed, Gha protein was shown to mediate a1-AR-induced hepatocyte proliferation (Wu et al 2000) and activation of extracellular signal-regulated kinases in neonatal rat cardiomyocytes (Lee et al 2003). In agreement with these results, we have recently shown that Gha protein participates to the a1-AR-mediated regulation of smooth muscle cell proliferation (Dupuis et al 2004).…”

Post-translational Modification of the Hepatitis C Virus Core Protein by Tissue Transglutaminase

“…tTG Assay-The tTG activity was measured by analyzing the incorporation of [1,[4][5][6][7][8][9][10][11][12][13][14] C]putrescine dihydrochloride into proteins (31)(32)(33). Cells were treated with CalC or serum-deprived and then lysed in Tris-buffered saline containing 0.5% Nonidet P-40 and 1 mM phenylmethylsulfonyl fluoride.…”