Modulation of transforming growth factor β receptors of rat lipocytes during the hepatic wound healing response: Enhanced binding and reduced gene expression accompany cellular activation in culture and in vivo

Activation of lipocytes, characterized by increased proliferation and fibrogenesis, is a central feature of the hepatic wound healing response. We have examined whether modulation of receptors for transforming growth factor β (TGF-β) contributes to the fibrogenic behavior of activated lipocytes. Isolated lipocytes were maintained in a quiescent state by culturing the cells in suspension, where they displayed minimal specific binding for TGF- β1 and only a small amount of type III (betaglycan) receptor by affinity labeling. In contrast, lipocytes activated by growth on uncoated plastic displayed saturable binding of TGF-β1 (K_d = 28 _pM, 7,730 receptors/ cell), and receptors types I, II, and III. Binding activity in quiescent and activated cells correlated with responsiveness to TGF-β1; TGF-β1 induced cellular fibronectin mRNA expression only in activated and not quiescent cells. Despite the absence of binding in quiescent cells, type II receptor was detectable by immunoblot. By RNase protection assay, mRNAs for receptor types II and III were greater in quiescent than activated cells. In freshly isolated lipocytes from animals with liver injury caused by the administration of carbon tetrachloride, a rapid but transient increase in mRNA for receptor types I (∼3.2-fold), II (∼1.5-fold), and III (∼3-fold) was observed, with peaks at 12 h for type I receptor, 1 h for type II receptor, and 6 h for type III receptor; mRNA induction was followed by down-regulation for all receptors. The modest changes in mRNAs compared with marked alterations in binding activity during mesenchymal cell activation suggest that TGF-β receptors may be regulated in vivo in part by a post-translational mechanism.