Mitogen-activated protein kinases (MAPKs) are important signal transducing enzymes, unique to eukaryotes, that are involved in many facets of cellular regulation. Initial research concentrated on defining the components and organization of MAPK signalling cascades, but recent studies have begun to shed light on the physiological functions of these cascades in the control of gene expression, cell proliferation and programmed cell death.

Obesity is closely associated with insulin resistance and establishes the leading risk factor for type 2 diabetes mellitus, yet the molecular mechanisms of this association are poorly understood. The c-Jun amino-terminal kinases (JNKs) can interfere with insulin action in cultured cells and are activated by inflammatory cytokines and free fatty acids, molecules that have been implicated in the development of type 2 diabetes. Here we show that JNK activity is abnormally elevated in obesity. Furthermore, an absence of JNK1 results in decreased adiposity, significantly improved insulin sensitivity and enhanced insulin receptor signalling capacity in two different models of mouse obesity. Thus, JNK is a crucial mediator of obesity and insulin resistance and a potential target for therapeutics.

TNFalpha is a pleiotropic cytokine that induces either cell proliferation or cell death. Inhibition of NF-kappaB activation increases susceptibility to TNFalpha-induced death, concurrent with sustained JNK activation, an important contributor to the death response. Sustained JNK activation in NF-kappaB-deficient cells was suggested to depend on reactive oxygen species (ROS), but how ROS affect JNK activation was unclear. We now show that TNFalpha-induced ROS, whose accumulation is suppressed by mitochondrial superoxide dismutase, cause oxidation and inhibition of JNK-inactivating phosphatases by converting their catalytic cysteine to sulfenic acid. This results in sustained JNK activation, which is required for cytochrome c release and caspase 3 cleavage, as well as necrotic cell death. Treatment of cells or experimental animals with an antioxidant prevents H(2)O(2) accumulation, JNK phosphatase oxidation, sustained JNK activity, and both forms of cell death. Antioxidant treatment also prevents TNFalpha-mediated fulminant liver failure without affecting liver regeneration.

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The proinflammatory cytokine tumor necrosis factor (TNF) alpha signals both cell survival and death. The biological outcome of TNFalpha treatment is determined by the balance between NF-kappaB and Jun kinase (JNK) signaling; NF-kappaB promotes survival, whereas JNK enhances cell death. Critically, identity of a JNK substrate that promotes TNFalpha-induced apoptosis has been outstanding. Here we show that TNFalpha-mediated JNK activation accelerates turnover of the NF-kappaB-induced antiapoptotic protein c-FLIP, an inhibitor of caspase-8. This is not due to direct c-FLIP phosphorylation but depends on JNK-mediated phosphorylation and activation of the E3 ubiquitin ligase Itch, which specifically ubiquitinates c-FLIP and induces its proteasomal degradation. JNK1 or Itch deficiency or treatment with a JNK inhibitor renders mice resistant in three distinct models of TNFalpha-induced acute liver failure, and cells from these mice do not display inducible c-FLIP(L) ubiquitination and degradation. Thus, JNK antagonizes NF-kappaB during TNFalpha signaling by promoting the proteasomal elimination of c-FLIP(L).

Emerging evidence suggests that inflammation provides a link between obesity and insulin resistance. The noncanonical IκB kinases IKKε and TANK-binding kinase 1 (TBK1) are induced in liver and fat after high fat diet by NF-κB activation, and in turn initiate a program of counter-inflammation that preserves energy storage. Here, we report the discovery of a small molecule inhibitor of these kinases called amlexanox. Treatment of obese mice with amlexanox elevates energy expenditure through increased thermogenesis, producing weight loss, improved insulin sensitivity and decreased steatosis in obese mice. Because of its record of safety in patients, amlexanox may be an interesting candidate for clinical evaluation in the treatment of obesity and related disorders.

A major link between inflammation and cancer is provided by NF-B transcription factors. Ikk␤ ⌬hep mice, which specifically lack IB kinase ␤ (IKK␤), an activator of NF-B, in hepatocytes, are unable to activate NF-B in response to proinflammatory stimuli, such as TNF-␣. Surprisingly, Ikk␤ ⌬hep mice are hypersusceptible to diethylnitrosamine (DEN)-induced hepatocarcinogenesis. Because defective NF-B activation promotes sustained c-Jun N-terminal kinase (JNK) activation in cells exposed to TNF-␣, whose expression is induced by DEN, and JNK activity is required for normal hepatocyte proliferation, we examined whether increased susceptibility to DEN-induced hepatocarcinogenesis in Ikk␤ ⌬hep mice requires JNK activation. Hepatocytes express both JNK1 and JNK2, but previous studies indicate that JNK1 is more important for hepatocyte proliferation. We therefore investigated this hypothesis using mice homozygous for a JNK1 deficiency either in wild-type or Ikk␤ ⌬hep backgrounds. In both cases, mice lacking JNK1 were much less susceptible to DEN-induced hepatocarcinogenesis. This impaired tumorigenesis correlated with decreased expression of cyclin D and vascular endothelial growth factor, diminished cell proliferation, and reduced tumor neovascularization. Whereas hepatocyte-specific deletion of IKK␤ augmented DEN-induced hepatocyte death and cytokine-driven compensatory proliferation, disruption of JNK1 abrogated this response. In addition to underscoring the importance of JNK1-mediated hepatocyte death and compensatory proliferation, these results strongly suggest that the control of tissue renewal through the IKK and JNK pathways plays a key role in liver carcinogenesis.hepatocellular carcinoma H epatocellular carcinoma (HCC), the most common type of liver cancer, is the third leading cause of cancer deaths worldwide (1). Epidemiological studies suggest that the major risk factors for HCC are persistent infection with hepatitis B and C virus (HBV and HCV) and exposure to genotoxic and cytotoxic chemicals, including high levels of ethanol, all of which cause chronic liver injury and inflammation (2). Although relatively uncommon in the U.S., the incidence of HCC has been increasing rapidly in the past decade because of the current HCV epidemic, affecting nearly 2% of the U.S. population. In humans, HCC almost inevitably develops in the setting of chronic hepatitis or cirrhosis, conditions in which hepatocytes are killed and resident inflammatory cells (Kupffer cells), as well as newly recruited inflammatory cells (macrophages, neutrophils, NK and NKT cells), are activated to produce cytokines that drive the compensatory proliferation of surviving hepatocytes. Although the precise molecular role of chronic liver inflammation in the pathogenesis of HCC remains to be fully elucidated, it is likely to promote HCC development through cycles of hepatocyte death and compensatory proliferation (3). For instance, infection with woodchuck hepatitis virus, which eventually culminates in HCC, greatly enhances the proliferative acti...

Insulin stimulates glucose transport by promoting exocytosis of the glucose transporter Glut4 (refs 1, 2). The dynamic processes involved in the trafficking of Glut4-containing vesicles, and in their targeting, docking and fusion at the plasma membrane, as well as the signalling processes that govern these events, are not well understood. We recently described tyrosine-phosphorylation events restricted to subdomains of the plasma membrane that result in activation of the G protein TC10 (refs 3, 4). Here we show that TC10 interacts with one of the components of the exocyst complex, Exo70. Exo70 translocates to the plasma membrane in response to insulin through the activation of TC10, where it assembles a multiprotein complex that includes Sec6 and Sec8. Overexpression of an Exo70 mutant blocked insulin-stimulated glucose uptake, but not the trafficking of Glut4 to the plasma membrane. However, this mutant did block the extracellular exposure of the Glut4 protein. So, the exocyst might have a crucial role in the targeting of the Glut4 vesicle to the plasma membrane, perhaps directing the vesicle to the precise site of fusion.