Characterization of distinct subpopulations of hepatic macrophages in HFD/obese mice.

The current dogma is that obesity-associated hepatic inflammation is due to increased Kupffer cell (KC) activation. However, recruited hepatic macrophages (RHMs) were recently shown to represent a sizable liver macrophage population in the context of obesity. Therefore, we assessed whether KCs and RHMs, or both, represent the major liver inflammatory cell type in obesity. We used a combination of in vivo macrophage tracking methodologies and adoptive transfer techniques in which KCs and RHMs are differentially labeled with fluorescent markers. With these approaches, the inflammatory phenotype of these distinct macrophage populations was determined under lean and obese conditions. In vivo macrophage tracking revealed an approximately sixfold higher number of RHMs in obese mice than in lean mice, whereas the number of KCs was comparable. In addition, RHMs comprised smaller size and immature, monocyte-derived cells compared with KCs. Furthermore, RHMs from obese mice were more inflamed and expressed higher levels of tumor necrosis factor-α and interleukin-6 than RHMs from lean mice. A comparison of the MCP-1/C-C chemokine receptor type 2 (CCR2) chemokine system between the two cell types showed that the ligand (MCP-1) is more highly expressed in KCs than in RHMs, whereas CCR2 expression is approximately fivefold greater in RHMs. We conclude that KCs can participate in obesity-induced inflammation by causing the recruitment of RHMs, which are distinct from KCs and are not precursors to KCs. These RHMs then enhance the severity of obesity-induced inflammation and hepatic insulin resistance.

G protein-coupled receptor 21 deletion improves insulin sensitivity in diet-induced obese mice.

Obesity-induced inflammation is a key component of systemic insulin resistance, which is a hallmark of type 2 diabetes. A major driver of this inflammation/insulin resistance syndrome is the accumulation of proinflammatory macrophages in adipose tissue and liver. We found that the orphan GPCR Gpr21 was highly expressed in the hypothalamus and macrophages of mice and that whole-body KO of this receptor led to a robust improvement in glucose tolerance and systemic insulin sensitivity and a modest lean phenotype. The improvement in insulin sensitivity in the high-fat diet-fed (HFD-fed) Gpr21 KO mouse was traced to a marked reduction in tissue inflammation caused by decreased chemotaxis of Gpr21 KO macrophages into adipose tissue and liver. Furthermore, mice lacking macrophage expression of Gpr21 were protected from HFD-induced inflammation and displayed improved insulin sensitivity. Results of in vitro chemotaxis studies in human monocytes suggested that the defect in chemotaxis observed ex vivo and in vivo in mice is also translatable to humans. Cumulatively, our data indicate that GPR21 has a critical function in coordinating macrophage proinflammatory activity in the context of obesity-induced insulin resistance.

Hepatocyte IKK2 protects Mdr2-/- mice from chronic liver failure.

UNLABELLED: Mice lacking the Abc4 protein encoded by the multidrug resistance-2 gene (Mdr2(-/-)) develop chronic periductular inflammation and cholestatic liver disease resulting in the development of hepatocellular carcinoma (HCC). Inhibition of NF-κB by expression of an IκBα super-repressor (IκBαSR) transgene in hepatocytes was shown to prevent HCC development in Mdr2(-/-) mice, suggesting that NF-κB acts as a tumour promoter in this model of inflammation-associated carcinogenesis. On the other hand, inhibition of NF-κB by hepatocyte specific ablation of IKK2 resulted in increased liver tumour development induced by the chemical carcinogen DEN. To address the role of IKK2-mediated NF-κB activation in hepatocytes in the pathogenesis of liver disease and HCC in Mdr2(-/-) mice, we generated Mdr2-deficient animals lacking IKK2 specifically in hepatocytes using the Cre-loxP system. Mdr2(-/-) mice lacking IKK2 in hepatocytes developed spontaneously a severe liver disease characterized by cholestasis, major hyperbilirubinemia and severe to end-stage fibrosis, which caused muscle wasting, loss of body weight, lethargy and early spontaneous death. Cell culture experiments showed that primary hepatocytes lacking IKK2 were more sensitive to bile acid induced death, suggesting that hepatocyte-specific IKK2 deficiency sensitized hepatocytes to the toxicity of bile acids under conditions of cholestasis resulting in greatly exacerbated liver damage. Mdr2(-/-)IKK2(Hep-KO) mice remarkably recapitulate chronic liver failure in humans and might be of special importance for the study of the mechanisms contributing to the pathogenesis of end-stage chronic liver disease or its implications on other organs. CONCLUSION: IKK2-mediated signaling in hepatocytes protects the liver from damage under conditions of chronic inflammatory cholestasis and prevents the development of severe fibrosis and liver failure.

p38 alpha MAPK inhibits JNK activation and collaborates with IkappaB kinase 2 to prevent endotoxin-induced liver failure.

Activation of c-Jun amino-terminal kinase (JNK) facilitates tumour necrosis factor (TNF)-induced cell death. The p38 mitogen-activated protein kinase pathway is induced by TNF stimulation, but it has not been implicated in TNF-induced cell death. Here, we show that hepatocyte-specific ablation of p38alpha in mice results in excessive activation of JNK in the liver after in vivo challenge with bacterial lipopolysaccharide (LPS). Despite increased JNK activity, p38alpha-deficient hepatocytes were not sensitive to LPS/TNF toxicity showing that JNK activation was not sufficient to mediate TNF-induced liver damage. By contrast, LPS injection caused liver failure in mice lacking both p38alpha and IkappaB kinase 2 (IKK2) in hepatocytes. Therefore, when combined with partial nuclear factor-kappaB inhibition, p38alpha deficiency sensitizes the liver to cytokine-induced damage. Collectively, these results reveal a new function of p38alpha in collaborating with IKK2 to protect the liver from LPS/TNF-induced failure by controlling JNK activation.

IKK1 and IKK2 cooperate to maintain bile duct integrity in the liver.

Inflammatory destruction of intrahepatic bile ducts is a common cause of vanishing bile duct syndrome and cholestasis, often progressing to biliary cirrhosis and liver failure. However, the molecular mechanisms underlying the pathogenesis of inflammatory biliary disease are poorly understood. Here, we show that the two IkappaB kinases, IKK1/IKKalpha and IKK2/IKKbeta, display distinct collaborative and specific functions that are essential to protect the liver from cytokine toxicity and bile duct disease. Combined conditional ablation of IKK1 and IKK2, but not of each kinase alone, sensitized the liver to in vivo LPS challenge, uncovering a redundant function of the two IkappaB kinases in mediating canonical NF-kappaB signaling in hepatocytes and protecting the liver from TNF-induced failure. Unexpectedly, mice with combined ablation of IKK1 and IKK2 or IKK1 and NEMO spontaneously developed severe jaundice and fatal cholangitis characterized by inflammatory destruction of small portal bile ducts. This bile duct disease was caused by the combined impairment of canonical NF-kappaB signaling together with inhibition of IKK1-specific functions affecting the bile-blood barrier. These results reveal a novel function of the two IkappaB kinases in cooperatively regulating liver immune homeostasis and bile duct integrity and suggest that IKK signaling may be implicated in human biliary diseases.