Abstract
BackgroundIn addition to the activation of hepatic stellate cells TGF-β govern apoptosis and growth control of hepatocytes in liver injury. In non-parenchymal cells, TGF-β induces plasminogen activator inhibitor 1 (PAI-1) and connective tissue growth factor (CTGF) expression, which are involved in extra cellular matrix formation. Both genes were also regulated by glucocorticoids, which in certain cases showed antagonistic effects to the TGF-β-Smad 3 pathway. The purpose of our work was to investigate the influence of TGF-β and dexamethasone on PAI-1 and CTGF expression and secretion in primary hepatocytes.ResultsBy examining PAI-1 and CTGF mRNA and protein expression in cell lysates and cell-conditioned media under the influence of TGF-β and dexamethasone, we analysed signalling pathways controlling their expression. TGF-β and dexamethasone significantly co-induce PAI-1 and CTGF protein expression. On the other hand, we showed that TGF-β diminished a glucocorticoid receptor dependent luciferase reporter signal in Hep-G2. Inhibition of Erk downstream activation decreased TGF-β induced CTGF and PAI-1 expression to a basal level. PAI-1 was directly secreted by hepatocytes, whereas secretion of CTGF was retarded.ConclusionThe data provide evidence that beside the TGF-β-Smad 3 pathway CTGF and PAI-1 expression is additionally dependent on Erk activity in hepatocytes giving new insights into regulation of the profibrogenic proteins.
Highlights
In addition to the activation of hepatic stellate cells TGF-β govern apoptosis and growth control of hepatocytes in liver injury
plasminogen activator inhibitor 1 (PAI-1) and connective tissue growth factor (CTGF) expression after TGF-β dexamethasone costimulation Western blot analysis showed an up-regulation of PAI-1 after TGF-β incubation and a strong enhancement six hours after a TGF-β-dexamethasone costimulation of hepatocytes (Fig. 1A, 1B)
Dexamethasone promotes CTGF expression in comparison to untreated hepatocytes and a combined incubation with TGF-β leads to a significant "super induction" of CTGF
Summary
In addition to the activation of hepatic stellate cells TGF-β govern apoptosis and growth control of hepatocytes in liver injury. In non-parenchymal cells, TGF-β induces plasminogen activator inhibitor 1 (PAI-1) and connective tissue growth factor (CTGF) expression, which are involved in extra cellular matrix formation. Both genes were regulated by glucocorticoids, which in certain cases showed antagonistic effects to the TGF-β-Smad 3 pathway. Glucocorticoids govern crucial functions in hepatocytes such as glucose metabolism, cell proliferation and differentiation. Once bound to their cytoplasmic receptors (GR) the glucocorticoid-receptor-complex is translocated to the nucleus and binds after dimerization to a cis-active response element to regulate target genes [1,2]. Active TGF-β binds and phosphorylates transmembrane receptors with a serine-threonine kinase activity, which in turn propagates the signal via Smads to the (page number not for citation purposes)
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