PKCδ as a regulator for TGF-β-stimulated connective tissue growth factor production in human hepatocarcinoma (HepG2) cells

CTGF (connective tissue growth factor) is widely regarded as an important amplifier of the profibrogenic action of TGF-beta (transforming growth factor beta) in a variety of tissues, although the precise mechanism of how the TGF-beta signalling pathways modulate CTGF expression remains unclear. In the present study, the role of PKC delta (protein lcinase C delta) in TGF-beta 1-mediated CTGF expression was investigated using HepG2 cells. TGF-beta 1 treatment specifically elevated PKC delta activation and CTGF expression. In contrast, blockade of PKG delta by the selective inhibitor Rottlerin or by siRNA knockdown significantly reduced TGF-beta 1-induced CTGF production. The regulatory mechanism was further demonstrated in HepG2 cells whereby TGF-beta 1-induced PKC delta activation negatively regulated the nuclear levels of PPM1A (protein phosphatase, Mg2+/Mn2+ dependent, 1A) through the RhoA/ROCK (Rho-associated kinase) pathway. Moreover, we showed that both Smad signalling and the PKG delta pathway appeared to be stimulated by TGF-beta 1 in parallel. Time course assessments indicated that PKC delta signalling may have a function in maintaining nuclear phospho-Smads at a maximal level. The collective results of the present study demonstrated that PKG delta-stimulated RhoA/ROCK activation resulted in a reduction in PPM1A, thereby up-regulating Smad-dependent gene induction for extended periods. These findings indicated that PKC delta plays a critical role in TGF-beta 1-induced CTGF production in HepG2 cells.