36, 37 We aimed to gain more insight into the biological significance of shedding of the TNFR1 ectodomain in these pathologies by studying the extent to which ectodomain shedding of the TNFR1 controls the initiation
and progression of NAFLD towards NASH and the development of insulin resistance. Using knockin mice expressing a mutated nonsheddable TNFR1,29 we demonstrated for the first time that ectodomain shedding of the TNFR1 is not an essential feedback mechanism in preventing the development of hepatic steatosis and insulin resistance. However, this mechanism of the TNFα-inflammatory loop is pivotal for protecting against the transition from “simple steatosis” towards NASH. We have shown that p55Δns/Δns mice on a normal R428 chow diet do not develop hepatic steatosis, www.selleckchem.com/products/DAPT-GSI-IX.html despite increased hepatic inflammation (Fig. 2E). Moreover, 12 weeks of HF feeding did
not exacerbate hepatic lipid levels, nor alter the zonal distribution or severity of microvesicular steatosis in p55Δns/Δns mice compared to controls (Fig. 2D-F), suggesting that shedding of TNFR1 does not prevent the development of hepatic steatosis. It was known that the shedding of TNFR1 ectodomains attenuates the inflammatory response induced by TNFα,23 but our data now show that persistent TNFR1 signaling is not involved in the initiation of NAFLD. Consistent with this, mice with a genetic deletion of TNFα or TNFR1 are not protected against developing obesity-induced hepatic steatosis.10, 13, 14 However, TNFα has been shown to be a potent lipid metabolism regulator38 and many studies in rodents have described a role for TNFα in the development of hepatic steatosis.8, 33 Most of these have studied the effects of TNFα within 24 hours of a high PI-1840 dose of human recombinant TNFα. Although administration of TNFα induces acute hepatitis, it does not mimic the chronic low-grade
inflammation associated with obesity. The inflammatory gene expression seen in livers from p55Δns/Δns mice was approximately 10- to 100-fold lower than that seen after a single injection with TNFα (Supporting Fig. 1); it thus led to a more physiologically relevant situation of chronic low-grade hepatic inflammation in our study. Although our data do not support a role for shedding of TNFR1 in the initiation of steatosis, we did see an advanced NASH-like phenotype in the livers of p55Δns/Δns mice fed an HFD compared to wildtype mice. This included the presence of inflammatory infiltrates, apoptotic hepatocytes, and large areas of hepatocellular necrosis surrounded by neutrophils and lymphocytes (Fig. 3A,B). Because our data indicated an important role for TNFR1 in the progression of NAFLD towards NASH, we investigated the effect of ectodomain shedding of TNFR1 on hepatic fibrosis, an advanced hallmark of NASH. P55Δns/Δns mice demonstrated increased levels of collagen staining, as detected by Masson’s Trichrome staining (Fig. 4E).