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Liver Int. 2008 Nov;28(9):1207-16. doi: 10.1111/j.1478-3231.2008.01729.x. Epub 2008 Apr 7.

Intracrine signalling of activin A in hepatocytes upregulates connective tissue growth factor (CTGF/CCN2) expression.

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  • 1Institute of Clinical Chemistry and Pathobiochemistry, RWTH-University Hospital, Aachen, Germany. ogressner@ukaachen.de

Abstract

BACKGROUND/AIMS:

Up to now, the effect of activin A on the expression of the important transforming growth factor (TGF)-beta downstream modulator connective tissue growth factor (CTGF) is not known, but might be of relevance for the functional effects of this cytokine on several liver cell types.

METHODS:

In this study, activin A-dependent CTGF expression in hepatocytes (PC) primed by exogenous activin A and in PC maintained under complete activin-free culture conditions was analysed by Western blots, metabolic labelling, gene silencing, reverse transcriptase-polymerase chain reaction (RT-PCR) and CTGF reporter gene assays. This study was supplemented by immunocytochemical staining of activin A and CTGF in PC of injured liver.

RESULTS:

Using alkaline phosphatase alpha-alkaline phosphatase staining, it is demonstrated that activin A becomes increasingly detectable during the course of CCl(4)-liver damage. Addition of activin A to cultured PC induced CTGF protein expression via phosphorylation of Smad2 and Smad3. This induction can be inhibited by the antagonist follistatin and alpha-activin A antibody respectively. When PC were cultured under serum(i.e. activin A)-free culture conditions, a time-dependent increase of activin expression during the course of the culture was proven by RT-PCR. Silencing of inhibin beta(A) gene expression under serum-free conditions by small interfering RNAs greatly suppressed CTGF synthesis and the phosphorylations of Smad2 and Smad3. However, both the extracellularly acting follistatin and the alpha-activin A antibody could not inhibit spontaneous CTGF expression, which, however, was achieved by the cell-permeable TGF-beta Alk4/Alk5 receptor-kinase-inhibitor SB431542.

CONCLUSIONS:

In conclusion, the results point to activin A as an inducer of CTGF synthesis in PC. Intracellular activin A contributes to spontaneous CTGF expression in PC independent of exogenous activin A, which is proposed to occur via Alk4/Alk5-receptors. The findings might be important for many actions of activin A on the liver.

PMID:
18397232
[PubMed - indexed for MEDLINE]
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