Death receptor-independent FADD signalling triggers hepatitis and hepatocellular carcinoma in mice with liver parenchymal cell-specific NEMO knockout

Abstract

Hepatocellular carcinoma (HCC) usually develops in the context of chronic hepatitis triggered by viruses or toxic substances causing hepatocyte death, inflammation and compensatory proliferation of liver cells. Death receptors of the TNFR superfamily regulate cell death and inflammation and are implicated in liver disease and cancer. Liver parenchymal cell-specific ablation of NEMO/IKKc, a subunit of the IjB kinase (IKK) complex that is essential for the activation of canonical NF-jB signalling, sensitized hepatocytes to apoptosis and caused the spontaneous development of chronic hepatitis and HCC in mice. Here we show that hepatitis and HCC development in NEMO LPC-KO mice is triggered by death receptor-independent FADD-mediated hepatocyte apoptosis. TNF deficiency in all cells or conditional LPC-specific ablation of TNFR1, Fas or TRAIL-R did not prevent hepatocyte apoptosis, hepatitis and HCC development in NEMO LPC-KO mice. To address potential functional redundancies between death receptors we generated and analysed NEMO LPC-KO mice with combined LPC-specific deficiency of TNFR1, Fas and TRAIL-R and found that also simultaneous lack of all three death receptors did not prevent hepatocyte apoptosis, chronic hepatitis and HCC development. However, LPC-specific combined deficiency in TNFR1, Fas and TRAIL-R protected the NEMOdeficient liver from LPS-induced liver failure, showing that different mechanisms trigger spontaneous and LPS-induced hepatocyte apoptosis in NEMO LPC-KO mice. In addition, NK cell depletion did not prevent liver damage and hepatitis. Moreover, NEMO LPC-KO mice crossed into a RAG-1-deficient genetic background-developed hepatitis and HCC. Collectively, these results show that the spontaneous development of hepatocyte apoptosis, chronic hepatitis and HCC in NEMO LPC-KO mice occurs independently of death receptor signalling, NK cells and B and T lymphocytes, arguing against an immunological trigger as the critical stimulus driving hepatocarcinogenesis in this model.

The interplay of IKK, NF‐κB and RIPK1 signaling in the regulation of cell death, tissue homeostasis and inflammation

“…37,39 Consistently, an independent study showed that LPC-specific knockout of caspase-8 also protected NEMO LPC-KO mice from hepatocyte apoptosis and the development of liver cancer. 37,39 Consistently, an independent study showed that LPC-specific knockout of caspase-8 also protected NEMO LPC-KO mice from hepatocyte apoptosis and the development of liver cancer.…”

“…37,39 Consistently, an independent study showed that LPC-specific knockout of caspase-8 also protected NEMO LPC-KO mice from hepatocyte apoptosis and the development of liver cancer. 39 In light of an earlier study reporting that systemic TNFR1 deficiency largely ameliorated liver damage and tumorigenesis, 41 we also generated and analyzed NEMO LPC-KO Tnfr1 −/− mice but found that systemic lack of TNFR1 did not prevent liver damage and only had a mild effect in slowing down tumor progression. Surprisingly NEMO LPC-KO mice with hepatocyte-specific deficiency of TNFR1, Fas or TRAILR, individually or combined, were not protected from the liver pathology, suggesting that these death receptors are not essential drivers of hepatocyte apoptosis in this model.…”

RIPK1 protects from TNF-α-mediated liver damage during hepatitis

“…44, 51, 52 Surprisingly, apoptosis is strongly prevented by crossing NEMO LPC-KO mice with RIPK1 kinase dead mice while poorly with Ripk1 LPC-KO . 44 As NEMO LPC-KO mice presented elevated TNF- α in the liver compared with WT mice, 51 additional RIPK1 deficiency could induce hepatocyte apoptosis in a TNF- α -dependent manner.…”

Kinase-independent functions of RIPK1 regulate hepatocyte survival and liver carcinogenesis

“…NF-κB signaling regulates immune and inflammatory responses and cell survival (12,13) and is implicated in the pathogenesis of hepatitis and HCC (14,15). Mice with liver parenchymal cell-specific (LPC-specific) ablation of NF-κB essential modulator (NEMO)/IκB kinase γ (IKKγ), the regulatory subunit of the IKK complex that is required for canonical NF-κB activation, developed spontaneously chronic hepatitis and HCC due to RIPK1 kinase activity-driven, FAS-associated death domain (FADD)/ caspase-8-mediated hepatocyte apoptosis (16)(17)(18)(19). In contrast, LPC-specific NF-κB deficiency did not cause spontaneous development of severe chronic liver disease and cancer, suggesting that NEMO regulates liver homeostasis by NF-κB-dependent and -independent functions (16).…”