C/EBPbeta associates with caspase 8 complex proteins and modulates apoptosis in hepatic stellate cells.
OBJECTIVE
To analyze the role of C/EBPbeta phosphorylation on hepatic stellate cell survival/cell death.


BACKGROUND
Activation and survival of stellate cells is critical for the development of liver fibrosis. C/EBPbeta phosphorylation regulates stellate cell survival by affecting caspase 8 activation. The mechanisms responsible for these effects are unknown.


METHODS
We study the effects of caspase 8 activators signaling through death receptors. In addition, we assess the role of C/EBPbeta phosphorylation on the susceptibility of stellate cells to apoptotic stimuli. Finally, we investigated whether C/EBPbeta is associated with the caspase 8 complex protein FLIP, a critical inhibitor of caspase 8.


RESULTS
Primary mouse stellate cells from C/EBPbeta wild type and the phosphorylation mimic C/EBPbetaGlu transgenic mice were treated with lipopolysaccharide [an inducer of tumor necrosis factor-alpha (TNF-alpha)], FAS, or TNF-alpha. Stellate cell apoptosis was determined by assessing the binding of annexin-V to exposed phosphatidylserine of plasma membranes. TNF-alpha and FAS, but not lipopolysaccharide, induced annexin-V binding at 6 hours in C/EBPbeta wild type stellate cell. However, the stimulation of apoptosis by TNF-alpha and FAS was markedly blocked in C/EBPbetaGlu stellate cells (P<0.001). Stellate cells activated on a collagen type 1 matrix expressed both C/EBPbeta and FLIPL. Treatment of stellate cells with a MAP kinase kinase1 (MEK1) inhibitor blocked FLIPL cellular localization, suggesting that MEK1 signaling through C/EBPbeta modulates FLIP activity. The colocalization of C/EBPbeta and FLIPL was disrupted by activation of the FAS receptor, by blocking the association of C/EBPbeta with the long form of FLIP, FLIPL.


CONCLUSIONS
The MAPK-RSK-C/EBPbeta signaling may modulate stellate cell survival through caspase 8-associated protein FLIPL. This step is critical for liver fibrosis and if blocked with competitor peptides may prevent fibrogenesis.
Buck
Martina
M
Department of Medicine and Cancer Center, Veterans Healthcare Medical Center and University of California, San Diego, CA 92161, USA.
Chojkier
Mario
M
D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D002070:Butadienes / Q000494:pharmacology; D053446:CASP8 and FADD-Like Apoptosis Regulating Protein / Q000378:metabolism*; D022782:CCAAT-Enhancer-Binding Protein-beta / Q000235:genetics / Q000378:metabolism*; D053181:Caspase 8 / Q000378:metabolism*; D002470:Cell Survival; D002478:Cells, Cultured; D024042:Collagen Type I / Q000378:metabolism; D008070:Lipopolysaccharides / Q000494:pharmacology; D008099:Liver / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism* / Q000473:pathology; D048369:MAP Kinase Kinase 1 / Q000037:antagonists & inhibitors / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D009570:Nitriles / Q000494:pharmacology; D010766:Phosphorylation; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects; D013997:Time Factors; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism; D019014:fas Receptor / Q000378:metabolism
D002070:Butadienes; D053446:CASP8 and FADD-Like Apoptosis Regulating Protein; D022782:CCAAT-Enhancer-Binding Protein-beta; D024042:Collagen Type I; D008070:Lipopolysaccharides; D009570:Nitriles; D047428:Protein Kinase Inhibitors; D014409:Tumor Necrosis Factor-alpha; C113580:U 0126; D019014:fas Receptor; D048369:MAP Kinase Kinase 1; C505526:Casp8 protein, mouse; D053181:Caspase 8