Connective tissue growth factor is a Smad2 regulated amplifier of transforming growth factor beta actions in hepatocytes--but without modulating bone morphogenetic protein 7 signaling.
In vivo knockdown of connective tissue growth factor (CTGF/CCN2) was recently shown to attenuate the formation of experimental liver fibrosis. The secreted, cysteine-rich growth factor is proposed to adversely modulate the binding of profibrogenic transforming growth factor beta (TGF-beta) and its natural antagonist bone morphogenetic protein (BMP) to their cognate receptors in several cellular systems, but the functionality of CTGF in modulation of the TGF-beta/BMP signaling pathways is still unknown. This study aims at characterizing a potentially differential modulating role of CTGF on TGF-beta- and BMP7-dependent transactivation of reporter gene [Ad-(CAGA)(12)-MLP-luc, Ad-hCTGF-luc, and Ad-(BRE)(2)-luc reporter gene] expression in rat hepatocytes. In this context, emphasis is also placed on the differential roles of Smad2 and Smad3 in the TGF-beta-dependent transactivation of the endogenous CTGF gene and the CTGF gene reporter, as investigated following adenoviral infection of wild-type and dominant negative Smad2/3 or treatment with the specific inhibitor of Smad3 or ALK5-specific (SB-431542) inhibitor. In this analysis, we found (1) a selective transcriptional activation of the CTGF promoter by Smad2 (but not Smad3); (2) the failure of BMP7 to inhibit the transcriptional activation of the Smad3-selective (CAGA)(12)-luc reporter by TGF-beta, as well as the failure of TGF-beta to inhibit the transcriptional activation of the Smad5-selective (BRE)(2)-luc reporter by BMP7; and (3) the sensitization of hepatocytes toward TGF-beta type I receptor (ALK5)/Smad2 and Smad3-mediated TGF-beta signaling by CTGF, whereas BMP type I receptor (ALK1)/Smad5-mediated BMP7 signaling is not modulated.


CONCLUSIONS
CTGF acts as a Smad2-dependent sensitizer of TGF-beta actions that does not influence BMP7 signaling in hepatocytes.
Gressner
Olav A
OA
Institute of Clinical Chemistry and Pathobiochemistry, RWTH-University Hospital, Aachen, Germany. ogressner@ukaachen.de
Lahme
Birgit
B
Siluschek
Monika
M
Rehbein
Katharina
K
Weiskirchen
Ralf
R
Gressner
Axel M
AM
D000818:Animals; D055419:Bone Morphogenetic Protein 7 / Q000502:physiology*; D002478:Cells, Cultured; D055513:Connective Tissue Growth Factor / Q000502:physiology*; D022781:Hepatocytes / Q000502:physiology*; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction / Q000502:physiology; D051899:Smad2 Protein / Q000502:physiology*; D016212:Transforming Growth Factor beta / Q000378:metabolism*
D055419:Bone Morphogenetic Protein 7; D051899:Smad2 Protein; D016212:Transforming Growth Factor beta; D055513:Connective Tissue Growth Factor