Knockdown of RIPK1 Markedly Exacerbates Murine Immune-Mediated Liver Injury through Massive Apoptosis of Hepatocytes, Independent of Necroptosis and Inhibition of NF-?B.

ABSTRACT: Receptor-interacting protein kinase (RIPK)1 has an essential role in the signaling pathways triggered by death receptors through activation of NF-?B and regulation of caspase-dependent apoptosis and RIPK3/mixed lineage kinase domain-like protein (MLKL)-mediated necroptosis. We examined the effect of RIPK1 antisense knockdown on immune-mediated liver injury in C57BL/6 mice caused by ?-galactosylceramide (?GalCer), a specific activator for invariant NKT cells. We found that knockdown of RIPK1 markedly exacerbated ?GalCer-mediated liver injury and induced lethality. This was associated with increased hepatic inflammation and massive apoptotic death of hepatocytes, as indicated by TUNEL staining and caspase-3 activation. Pretreatment with zVAD.fmk, a pan-caspase inhibitor, or neutralizing Abs against TNF, almost completely protected against the exacerbated liver injury and lethality. Primary hepatocytes isolated from RIPK1-knockdown mice were sensitized to TNF-induced cell death that was completely inhibited by adding zVAD.fmk. The exacerbated liver injury was not due to impaired hepatic NF-?B activation in terms of I?B? phosphorylation and degradation in in vivo and in vitro studies. Lack of RIPK1 kinase activity by pretreatment with necrostatin-1, a RIPK1 kinase inhibitor, or in the RIPK1 kinase-dead knock-in (RIPK1D138N) mice did not exacerbate ?GalCer-mediated liver injury. Furthermore, RIPK3-knockout and MLKL-knockout mice behaved similarly as wild-type control mice in response to ?GalCer, with or without knockdown of RIPK1, excluding a switch to RIPK3/MLKL-mediated necroptosis. Our findings reveal a critical kinase-independent platform role for RIPK1 in protecting against TNF/caspase-dependent apoptosis of hepatocytes in immune-mediated liver injury.

SUBMITTER: Suda J 

PROVIDER: S-EPMC5101131 | biostudies-literature | 2016 Oct

REPOSITORIES: biostudies-literature