A high-fat diet causes activation of the regulatory protein c-Jun NH ₂ -terminal kinase 1 (JNK1) and triggers development of insulin resistance. JNK1 is therefore a potential target for therapeutic treatment of metabolic syndrome. We explored the mechanism of JNK1 signaling by engineering mice in which the Jnk1 gene was ablated selectively in adipose tissue. JNK1 deficiency in adipose tissue suppressed high-fat diet–induced insulin resistance in the liver. JNK1-dependent secretion of the inflammatory cytokine interleukin-6 by adipose tissue caused increased expression of liver SOCS3, a protein that induces hepatic insulin resistance. Thus, JNK1 activation in adipose tissue can cause insulin resistance in the liver.

PDK1 functions as a master kinase, phosphorylating and activating PKB/Akt, S6K and RSK. To learn more about the roles of PDK1, we generated mice that either lack PDK1 or possess PDK1 hypomorphic alleles, expressing only ∼10% of the normal level of PDK1. PDK1−/− embryos die at embryonic day 9.5, displaying multiple abnormalities including lack of somites, forebrain and neural crest derived tissues; however, development of hind‐ and midbrain proceed relatively normally. In contrast, hypomorphic PDK1 mice are viable and fertile, and insulin injection induces the normal activation of PKB, S6K and RSK. Nevertheless, these mice are 40–50% smaller than control animals. The organ volumes from the PDK1 hypomorphic mice are reduced proportionately. We also establish that the volume of a number of PDK1‐deficient cells is reduced by 35–60%, and show that PDK1 deficiency does not affect cell number, nuclear size or proliferation. We provide genetic evidence that PDK1 is essential for mouse embryonic development, and regulates cell size independently of cell number or proliferation, as well as insulin's ability to activate PKB, S6K and RSK.

Human arginase I is a potential target for therapeutic intervention in diseases linked to compromisedl-arginine homeostasis. Here, we report high-affinity binding of the reaction coordinate analogue inhibitors 2(S)-amino-6-boronohexanoic acid (ABH,K_d= 5 nM) andS-(2-boronoethyl)-l-cysteine (BEC,K_d= 270 nM) to human arginase I, and we report x-ray crystal structures of the respective enzyme–inhibitor complexes at 1.29- and 1.94-Å resolution determined from crystals twinned by hemihedry. The ultrahigh-resolution structure of the human arginase I–ABH complex yields an unprecedented view of the binuclear manganese cluster and illuminates the structural basis for nanomolar affinity: bidentate inner-sphere boronate–manganese coordination interactions and fully saturated hydrogen bond networks with inhibitor α-amino and α-carboxylate groups. These interactions are therefore implicated in the stabilization of the transition state forl-arginine hydrolysis. Electron density maps also reveal that active-site residue H141 is protonated as the imidazolium cation. The location of H141 is such that it could function as a general acid to protonate the leaving amino group ofl-ornithine during catalysis, and this is a revised mechanistic proposal for arginase. This work serves as a foundation for studying the structural and chemical biology of arginase I in the immune response, and we demonstrate the inhibition of arginase activity by ABH in human and murine myeloid cells.

We employed Cre/loxP technology to generate mPDK1−/− mice, which lack PDK1 in cardiac muscle. Insulin did not activate PKB and S6K, nor did it stimulate 6‐phosphofructo‐2‐kinase and production of fructose 2,6‐bisphosphate, in the hearts of mPDK1−/− mice, consistent with PDK1 mediating these processes. All mPDK1−/− mice died suddenly between 5 and 11 weeks of age. The mPDK1−/− animals had thinner ventricular walls, enlarged atria and right ventricles. Moreover, mPDK1−/− muscle mass was markedly reduced due to a reduction in cardiomyocyte volume rather than cardiomyocyte cell number, and markers of heart failure were elevated. These results suggested mPDK1−/− mice died of heart failure, a conclusion supported by echocardiographic analysis. By employing a single‐cell assay we found that cardiomyocytes from mPDK1−/− mice are markedly more sensitive to hypoxia. These results establish that the PDK1 signalling network plays an important role in regulating cardiac viability and preventing heart failure. They also suggest that a deficiency of the PDK1 pathway might contribute to development of cardiac disease in humans.