ABSTRACT
UNLABELLED: Fas/CD95-induced apoptosis of hepatocytes in vivo proceeds through the so-called type II pathway, requiring the proapoptotic BH3-only Bcl-2 family member Bid for mitochondrial death signaling. Consequently, Bid-deficient mice are protected from anti-Fas antibody injection induced fatal hepatitis. We report the unexpected finding that freshly isolated mouse hepatocytes, cultured on collagen or Matrigel, become independent of Bid for Fas-induced apoptosis, thereby switching death signaling from type II to type I. In such in vitro cultures, Fas ligand (FasL) activates caspase-3 without Bid cleavage, Bax/Bak activation or cytochrome c release, and neither Bid ablation nor Bcl-2 overexpression is protective. The type II to type I switch depends on extracellular matrix adhesion, as primary hepatocytes in suspension die in a Bid-dependent manner. Moreover, the switch is specific for FasL-induced apoptosis as collagen-plated Bid-deficient hepatocytes are protected from tumor necrosis factor alpha/actinomycin D (TNFalpha/ActD)-induced apoptosis. CONCLUSION: Our data suggest a selective crosstalk between extracellular matrix and Fas-mediated signaling that favors mitochondria-independent type I apoptosis induction.
Subject(s)
Apoptosis/physiology , Hepatocytes/metabolism , Signal Transduction/physiology , fas Receptor/metabolism , Animals , BH3 Interacting Domain Death Agonist Protein/metabolism , Caspase 3/metabolism , Cell Adhesion/physiology , Cells, Cultured , Extracellular Matrix/metabolism , Fas Ligand Protein/metabolism , Hepatocytes/cytology , Mice , Mice, Knockout , Mitochondria, Liver/metabolism , Models, Animal , Tumor Necrosis Factor-alpha/metabolism , bcl-2 Homologous Antagonist-Killer Protein/metabolismABSTRACT
Glycogen synthase kinase-3 (GSK-3) is known to modulate cell survival and apoptosis through multiple intracellular signaling pathways. However, its hepatoprotective function and its role in activation of NF-kappaB and anti-apoptotic factors are poorly understood and remain controversial. Here we investigated whether inhibition of GSK-3 could induce apoptosis in the presence of TNF-alpha in primary mouse hepatocytes. We show that pharmacological inhibition of GSK-3 in primary mouse hepatocytes does not lead to TNF-alpha-induced apoptosis despite reduced NF-kappaB activity. Enhanced stability of IkappaB-alpha appears to be responsible for lower levels of nuclear NF-kappaB and hence reduced transactivation. Additionally, inhibition of GSK-3 was accompanied by marked upregulation of beta-catenin, AP-1, and CREB transcription factors. Stimulation of canonical Wnt signaling and CREB activity led to elevated levels of anti-apoptotic factors. Hence, survival of primary mouse hepatocytes may be caused by the activation and/or upregulation of other key regulators of liver homeostasis and regeneration. These signaling molecules may compensate for the compromised anti-apoptotic function of NF-kappaB and allow survival of hepatocytes in the presence of TNF-alpha and GSK-3 inhibition.
