IκB Kinaseα/β Control Biliary Homeostasis and Hepatocarcinogenesis in Mice by Phosphorylating the Cell-Death Mediator Receptor-Interacting Protein Kinase 1

The I kappa B-Kinase (IKK) complex-consisting of the catalytic subunits, IKK alpha and IKK beta, as well as the regulatory subunit, NEMO-mediates activation of the nuclear factor kappa B (NF-kappa B) pathway, but previous studies suggested the existence of NF-kappa B-independent functions of IKK subunits with potential impact on liver physiology and disease. Programmed cell death is a crucial factor in the progression of liver diseases, and receptor-interacting kinases (RIPKs) exerts strategic control over multiple pathways involved in regulating novel programmed cell-death pathways and inflammation. We hypothesized that RIPKs might be unrecognized targets of the catalytic IKK-complex subunits, thereby regulating hepatocarcinogenesis and cholestasis. In this present study, mice with specific genetic inhibition of catalytic IKK activity in liver parenchymal cells (LPCs; IKK alpha/beta(LPC-KO)) were intercrossed with RIPK1(LPC-KO) or RIPK3(-/-) mice to examine whether RIPK1 or RIPK3 might be downstream targets of IKKs. Moreover, we performed in vivo phospho-proteome analyses and in vitro kinase assays, mass spectrometry, and mutagenesis experiments. These analyses revealed that IKKa and IKK beta-in addition to their known function in NF-kappa B activation-directly phosphorylate RIPK1 at distinct regions of the protein, thereby regulating cell viability. Loss of this IKK alpha/beta-dependent RIPK1 phosphorylation in LPCs inhibits compensatory proliferation of hepatocytes and intrahepatic biliary cells, thus impeding HCC development, but promoting biliary cell paucity and lethal cholestasis. Conclusions: IKK-complex subunits transmit a previously unrecognized signal through RIPK1, which is fundamental for the long-term consequences of chronic hepatic inflammation and might have potential implications for future pharmacological strategies against cholestatic liver disease and cancer.