Abstract
Tumor necrosis factor-alpha (TNF) is an inflammatory cytokine that induces context-dependent proliferation, survival, and apoptosis responses in hepatocytes. TNF stimulates and enhances growth factor-mediated hepatocyte proliferation and survival following partial hepatectomy, but also acts in concert with other inflammatory cytokines of the innate immune response during viral infection to induce apoptosis in hepatocytes. In other epithelial cell types, TNF has recently been shown to stimulate autocrine release of transforming growth factor-alpha (TGF-alpha) and interleukin-1 (IL-1) family ligands. Here, we examine the role of these autocrine ligands in modulating TNF-induced proliferation and apoptosis in primary hepatocytes. We show that TNF-induced hepatocyte proliferation is regulated by an inducible, coupled, and self-antagonizing autocrine cascade involving the pro-proliferative TGF-alpha and IL-1 receptor antagonist (IL-1ra) ligands and antiproliferative IL-1alpha/beta ligands. Moreover, cooperative stimulation of hepatocyte proliferation by combined TNF and TGF-alpha treatment is self-limited through antiproliferative autocrine IL-1alpha/beta feedback. We show that TNF potently induces apoptosis of adenovirus-infected hepatocytes in a manner similarly determined through the integrated activity of a coupled TGF-alpha-IL-1alpha/beta-IL-1ra autocrine cascade. Exogenous TGF-alpha can either enhance or diminish apoptosis in adenoviral vector-treated and TNF-treated hepatocytes, in a biphasic relationship also mediated by autocrine IL-1alpha/beta feedback. CONCLUSION: We demonstrate that TNF-induced hepatocyte proliferation and apoptosis are both governed by a self-antagonizing TGF-alpha-IL-1alpha/beta-IL-1ra autocrine cascade in vitro, and thus identify multiple molecular targets for control of TNF-regulated hepatocyte phenotypic responses related to liver regeneration and adenoviral gene therapy.