Impact of soluble Endoglin on TGF-β1 mediated immediate early responses in HepG2 cells

Introduction: Expression of Endoglin has been demonstrated in all cells of the liver except hepatocytes [1]. However, data published with respect to the hepatic function of Endoglin is patchy [2]. Nevertheless, there is a wealth of data detecting the soluble form of this receptor in the serum under different pathologic situations affecting the liver, e.g. cystic fibrosis associated liver disease [3]. We have previously shown that Endoglin is highly expressed in hepatic stellate cells [4]. Upon shedding from the HSC surface, this receptor might act not only in an autocrine but also in a paracrine fashion on signalling processes mediated by TGF-β1 in hepatocytes. Methods and results: Transient transfection of HepG2 with a expression vector encoding soluble rat endoglin (solEng) results in secretion of high amounts of dimeric solEng in the cell culture supernatant. solEng can be co-precipitated with TGF-β1. As a mutual consequence of ligand binding/sequestration, the luciferase activity in response to TGF-β1 (measured by the (CAGA)12-MLP Luc reporter) and to BMP-2 (measured by the (BRE)2-MLP Luc reporter) is reduced. In addition, we find in Western blot experimentation that the expression of the TGF-β1 target genes connective tissue growth factor (CTGF), JunB, p21, Id1, and Id2 is differentially modulated under basal and TGF-β1-induced conditions. Upstream, the TGF-β1 type II receptor as well as the activation of the two Smad branches, i.e. Smad2 and Smad1/Smad5, but not Smad3 is affected by solEng. Conclusion: These results underscore the relevance of solEng in the liver not only as a serum marker but also as a modulator of TGF-β superfamily ligand-signaling in hepatocytes. The fact that TGF-β1 responses are modulated differentially points towards a more complex function of soling in addition to ligand sequestration.