Transforming growth factor β and tumor necrosis factor α inhibit both apoptosis and proliferation of activated rat hepatic stellate cells

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Abstract
Transforming growth factor β (TGF-β) as well as tumor necrosis factor α (TNF-α) gene expression are up-regulated in chronically inflamed liver. These cytokines were investigated for their influence on apoptosis and proliferation of activated hepatic stellate cells (HSCs). Spontaneous apoptosis in activated HSC was significantly down-regulated by 53% ± 8% (P < .01) under the influence of TGF-β and by 28% ± 2% (P < .05) under the influence of TNF-α. TGF-β and TNF-α significantly reduced expression of CD95L in activated HSCs, whereas CD95 expression remained unchanged. Furthermore, HSC apoptosis induced by CD95-agonistic antibodies was reduced from 96% ± 2% to 51 ± 7% (P < .01) by TGF-β, and from 96% ± 2% to 58 ± 2% (P < .01) by TNF-α, suggesting that intracellular antiapoptotic mechanisms may also be activated by both cytokines. During activation, HSC cultures showed a reduced portion of cells in the G0/G1phase and a strong increment of G2-phase cells. This increment was significantly inhibited (G1 arrest) by administration of TGF-β and/or TNF-α to activated cells. In liver sections of chronically damaged rat liver (CCl4 model), using desmin and CD95L as markers for activated HSC, most of these cells did not show apoptotic signs (TUNEL-negative). Taken together, these findings indicate that TGF-β and/or TNF-α both inhibit proliferation and also apoptosis in activated HSC in vitro. Both processes seem to be linked to each other, and their inhibition could represent the mechanism responsible for prolonged survival of activated HSC in chronic liver damage in vivo.

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