A novel function of necroptosis in Non-Alcoholic Steatohepatitis

Non-alcoholic fatty liver disease represents the most common liver disease in Western countries and often progresses to non-alcoholic steatohepatitis (NASH) leading ultimately to liver fibrosis and hepatocellular carcinoma. The occurrence of hepatocyte cell death – so far characterized as hepatocyte apoptosis – represents a fundamental difference between benign steatosis and progressive steatohepatitis. Recently, it was discovered that RIP3-dependent “necroptosis” represents a novel programmed cell-death-pathway activated in chronically inflamed tissues. This novel pathway is counterbalancing the function of Caspase-8 and apoptosis. Given previous studies suggesting a potential function of apoptosis in NASH, we investigated if necroptosis might play a role in murine and human NASH.

Using liver biopsies from well-characterized NASH cohorts from Germany and the UK, we show here that RIP3 is upregulated in human NASH, while – surprisingly – apoptosis of hepatocytes was detected on a very low level in human NASH livers. In the mouse dietary NASH model of Methionin-Cholin-Deficient (MCD) Diet (which was previously used to examine the role of apoptosis in NASH fibrosis), activation of RIP3 promotes liver injury, inflammation, induction of hepatic progenitor cells and ultimately liver fibrosis. Importantly, this deleterious pathway is suppressed by Caspase-8, since mice with deletion of Caspase-8 in parenchymal liver cells showed a massive increase of liver injury and fibrosis, which was mediated through RIP3-hyperactivation. This novel function of RIP3 is mediated by activation of Jun-(N)-terminal Kinase (JNK). Inhibition of JNK reduces intra-hepatic level of RIP3 and subsequently liver fibrosis, providing evidence that a positive feedback-loop exists in liver cells between RIP3 and JNK. Moreover, JNK-activation promotes the release of pro-inflammatory mediators like MCP-1 (CCL-2), thereby attracting macrophages to the injured liver and further augmenting RIP3-dependent signaling leading to cell death and subsequently liver fibrosis.

Previous studies on the role of apoptosis in the MCD NASH model led to clinical trials on apoptosis inhibitors in human NASH. Our present data provide evidence that necroptosis, a pathway counterbalancing apoptosis, and RIP3 as its master regulator promote NASH-induced liver fibrosis in the well-established MCD model and are upregulated in human NASH patients. This pathway might represent a novel and specific target for pharmacological strategies in the context of fatty liver disease. Moreover, our data provide evidence that targeting Caspase-8 in this context might aggravate liver damage and fibrogenesis since its main function is to counterbalance activation of necroptosis.

Corresponding author: Luedde, Tom

E-Mail: [e-mail]