Inactivation of TGF-β signaling in hepatocytes results in an increased proliferative response after partial hepatectomy

Abstract

The transforming growth factor b (TGF-b) signaling pathway, which is activated by the TGF-b receptor complex consisting of type I and type II TGF-b receptors (TGFBR1 and TGFBR2), regulates cell growth and death. TGF-b and components of its signaling pathway, particularly TGFBR2, have been implicated as tumor suppressor genes and important antimitogenic factors in the gastrointestinal tract and liver. An in vivo approach to study these effects has been hindered by the embryonic lethality of Tgfbr2 À/À mice and poor viability of the Tgfb1 À/À mice. Consequently, we have developed a hepatocyte-specific Tgfbr2 knockout mouse, the Alb-cre Tgfbr2 flx/flx mouse, to study the physiologically relevant effects of TGF-b signaling on epithelial cell proliferation in vivo. After 70% hepatectomy, we observed increased proliferation and an increased liver mass : body weight ratio in the Alb-cre Tgfbr2 flx/flx mice compared to Tgfbr2 flx/flx mice. We also observed decreased expression and increased phosphorylation of p130 in the livers from the Alb-cre Tgfbr2 flx/flx mice as well as increased expression of cyclin E, which is transcriptionally regulated, in part, by p130:E2F4. Consistent with these results, in a hepatocyte cell line derived from the Tgfbr2 flx/flx mice, we found that TGF-b increases the nuclear localization of E2F4, and presumably the transcriptional repression of the p130:E2F4 complex. Thus, we have demonstrated that TGF-b signaling in vivo regulates the mitogenic response in the regenerating liver, affecting the liver mass : body weight ratio after partial hepatectomy, and that these mitogenic responses are accompanied by alterations in p130 expression and phosphorylation, implicating p130 as one of the proteins regulated in vivo by TGF-b during liver regeneration.

Impaired liver regeneration and increased oval cell numbers following T cell–mediated hepatitis

“…[35][36][37] In contrast, other cytokines, including TGF␤ 1 and IFN␥, serve to inhibit the regenerative response of hepatocytes. 23,38 TGF␤ signaling through TGF␤ receptor 2 dampens the early regenerative response of the liver through the inhibition of cyclin expression and a subsequent G1-to-S transition. 23 Furthermore, the deletion of Smad2, a downstream signaling molecule of TGF␤, significantly enhances the proliferative response of hepatocytes in vitro.…”

“…23,38 TGF␤ signaling through TGF␤ receptor 2 dampens the early regenerative response of the liver through the inhibition of cyclin expression and a subsequent G1-to-S transition. 23 Furthermore, the deletion of Smad2, a downstream signaling molecule of TGF␤, significantly enhances the proliferative response of hepatocytes in vitro. 22 Our studies demonstrate the ability of T cell-mediated hepatitis to both decrease the expression of the pro-mitogenic cytokine IL6 and inhibit Stat3 activation while simultaneously increasing the expression of IFN␥ and TGF␤ as well as the phosphorylation of Smad2.…”

“…22,23 To determine the significance of the early enhancements in TGF␤ production, an analysis of downstream Smad activation was assessed. As shown in Figure 4E, T cell-mediated hepatitis induces a significant increase in Smad-2/3 phosphorylation at 6 hours after a hepatectomy in hepatocytes in comparison with vehicle-treated controls.…”

Transforming Growth Factor β Controls the Directional Migration of Hepatocyte Cohorts by Modulating Their Adhesion to Fibronectin

“…Ectopic expression of the active form of TGF-␤1 gives rise to liver fibrosis through the differentiation of hepatic stellate cells into myofibroblasts and stimulation of collagen synthesis and deposition (Sanderson et al, 1995). During liver regeneration, TGF-␤ acts to control the transient stimulation of stellate cells and, through its growth inhibitory action on hepatocytes, the extent of hepatocyte regenerative proliferation (Romero-Gallo et al, 2005). Primary hepatocytes in culture are also sensitive to TGF-␤: in this setting the cytokine provokes a strong apoptotic response of both mature and fetal hepatocytes (Fabregat et al, 1996;Gressner et al, 1997), whereas early hepatic precursors survive the same treatment .…”

The molecular mechanism underlying the liver mass optimization rule

“…Therefore, there remains the possibility that neither TGF-β nor activin is a major factor in terminating regeneration. Similar hepatocyte-specific Tgfβr2 KO mice, Alb-cre Tgfβr2 flx/flx mice, were created [17]. Their conclusion about the role of TGF-β signaling in the termination of liver regeneration appeared not to be similar to that obtained with R2LivKO mice.…”