NEMO Prevents Steatohepatitis and Hepatocellular Carcinoma by Inhibiting RIPK1 Kinase Activity-Mediated Hepatocyte Apoptosis

Abstract

SummaryIκB kinase/nuclear factor κB (IKK/NF-κB) signaling exhibits important yet opposing functions in hepatocarcinogenesis. Mice lacking NEMO in liver parenchymal cells (LPC) spontaneously develop steatohepatitis and hepatocellular carcinoma (HCC) suggesting that NF-κB prevents liver disease and cancer. Here, we show that complete NF-κB inhibition by combined LPC-specific ablation of RelA, c-Rel, and RelB did not phenocopy NEMO deficiency, but constitutively active IKK2-mediated NF-κB activation prevented hepatocellular damage and HCC in NEMOLPC-KO mice. Knock-in expression of kinase inactive receptor-interacting protein kinase 1 (RIPK1) prevented hepatocyte apoptosis and HCC, while RIPK1 ablation induced TNFR1-associated death domain protein (TRADD)-dependent hepatocyte apoptosis and liver tumors in NEMOLPC-KO mice, revealing distinct kinase-dependent and scaffolding functions of RIPK1. Collectively, these results show that NEMO prevents hepatocarcinogenesis by inhibiting RIPK1 kinase activity-driven hepatocyte apoptosis through NF-κB-dependent and -independent functions.

More to Life than NF-κB in TNFR1 Signaling

“…NEMO deficiency in intestinal epithelial cells (IEC) led to RIPK1 kinase-dependent death and subsequent microbiota-driven colitis [12]. The same group also reported an NF-κB-independent role of NEMO in protecting liver parenchymal cells from RIPK1 kinase-mediated apoptosis independently of death receptor activation [56]. These in vivo results are consistent with earlier observations obtained from a NEMO-deficient cell line as discussed above [10].…”

“…This would however render infected cells sensitive to RIPK1 kinase-dependent cell death and thereby facilitate elimination of the infected cells. Furthermore, RIPK1 kinase-dependent cell death has been shown to be inflammatory serving to further alert the immune response [4, 12, 53, 56, 70]. To date, the few concrete examples of a beneficial role for TNF-mediated cytotoxicity in anti-microbial responses are limited to viruses that encode caspase inhibitors and activate necroptosis [71].…”

Complex Pathologic Roles of RIPK1 and RIPK3: Moving Beyond Necroptosis

“…For example, in some cases, activation of RIPK1 kinase-dependent apoptosis, has been primarily linked to the development of the injury, while in other cases a combination of apoptosis and necroptosis was observed. Curiously, this is sometimes observed when activation of RIPK1 signaling is mediated by the same event but in different tissues, as has been reported upon deletion of NEMO in intestinal epithelial cells versus hepatocytes [41, 105]. In some instances, preferential activation of a particular response was traced to the deficiency in caspase-8 activation or low expression of MLKL or RIPK3, but it seems likely that additional signals that affect the direction of Complex IIb responses may exist.…”

“…Curiously, while NEMO loss in the intestine led to the mixture of necroptosis and apoptosis, loss of this protein in the liver parenchymal cells (LPC) led to the development of steatohepatitis and hepatocellular carcinoma (HCC), which were mediated exclusively by RIPK1 kinase-dependent apoptosis and not necroptosis [105]. However, the reasons for preferential activation of RIPK1-dependent apoptosis vs. necroptosis in different tissues are not yet known.…”

NEMO regulates a cell death switch in TNF signaling by inhibiting recruitment of RIPK3 to the cell death-inducing complex II

“…However, we have found that the expression of NEMO-A323P prevents RIPK1 degradation. As the RIPK1 scaffolding role and kinase activity are both important in preventing apoptosis in NEMO-deficient hepatocytes, 42 we speculate that the ubiquitin-binding mutant NEMO-A323P is unable to bind to ubiquitinated RIPK1 at complex I but that it can still interact with RIPK1 via its pseudo- α -helix. 4, 25, 27 This interaction could in turn enhance the RIPK3 kinase activity and also explain the exacerbated cell death we observed in NEMO-A323P cells, when an inhibition of Caspase-8 activity occurs.…”