The liver may regulate glucose homeostasis by modulating the sensitivity/resistance of peripheral tissues to insulin, by way of the production of secretory proteins, termed hepatokines. Here, we demonstrate that selenoprotein P (SeP), a liver-derived secretory protein, causes insulin resistance. Using serial analysis of gene expression (SAGE) and DNA chip methods, we found that hepatic SeP mRNA levels correlated with insulin resistance in humans. Administration of purified SeP impaired insulin signaling and dysregulated glucose metabolism in both hepatocytes and myocytes. Conversely, both genetic deletion and RNA interference-mediated knockdown of SeP improved systemic insulin sensitivity and glucose tolerance in mice. The metabolic actions of SeP were mediated, at least partly, by inactivation of adenosine monophosphate-activated protein kinase (AMPK). In summary, these results demonstrate a role of SeP in the regulation of glucose metabolism and insulin sensitivity and suggest that SeP may be a therapeutic target for type 2 diabetes.

Recently, nonalcoholic steatohepatitis (NASH) was found to be correlated with cardiovascular disease events independently of the metabolic syndrome. The aim of this study was to investigate whether an atherogenic (Ath) diet induces the pathology of steatohepatitis necessary for the diagnosis of human NASH and how cholesterol and triglyceride alter the hepatic gene expression profiles responsible for oxidative stress. We

Running title: oxidative stress induced by high-fat diet Key words: insulin resistance, oxidative stress, β-oxidation, NADPH oxidase Reprint requests to corresponding author. 1The abbreviations used are the following: HFD, high-fat diet; ROS, reactive oxygen species; PPARα, peroxisome proliferator-activated receptor-α; Nox, NADPH oxidase; FFAs, free fatty acids; Acox, acyl-CoA oxidase; CPT-1a, carnitine palmitoyltransferase 1a; CYP2E1, cytochrome P450 2E1; GTT, glucose tolerance test; Gpx, glutathione peroxidase; ITT, insulin tolerance test. 2 AbstractInsulin resistance is a key pathophysiological feature of metabolic syndrome. However, the initial events triggering the development of insulin resistance and its causal relations with dysregulation of glucose and fatty acids metabolism remain unclear. We investigated biological pathways that have the potential to induce insulin resistance in mice fed a high-fat diet (HFD). We demonstrate that the pathways for reactive oxygen species (ROS) production and oxidative stress are coordinately up-regulated in both the liver and adipose tissue of mice fed a HFD prior to the onset of insulin resistance through discrete mechanism. In the liver, a HFD up-regulated genes involved in sterol regulatory element binding protein-1c (SREBP-1c)-related fatty acid synthesis and peroxisome proliferator-activated receptor-α (PPARα)-related fatty acid oxidation. In the adipose tissue, however, the HFD down-regulated genes involved in fatty acid synthesis and up-regulated NADPH oxidase (Nox) complex. Furthermore, increased ROS production preceded the elevation of TNF-α and free fatty acids (FFAs) in the plasma and liver. ROS may be an initial key event triggering HFD-induced insulin resistance.

Visceral adiposity in obesity causes excessive free fatty acid (FFA) flux into the liver via the portal vein and may cause fatty liver disease and hepatic insulin resistance. However, because animal models of insulin resistance induced by lipid infusion or a high fat diet are complex and may be accompanied by alterations not restricted to the liver, it is difficult to determine the contribution of FFAs to hepatic insulin resistance. Therefore, we treated H4IIEC3 cells, a rat hepatocyte cell line, with a monounsaturated fatty acid (oleate) and a saturated fatty acid (palmitate) to investigate the direct and initial effects of FFAs on hepatocytes. We show that palmitate, but not oleate, inhibited insulin-stimulated tyrosine phosphorylation of insulin receptor substrate 2 and serine phosphorylation of Akt, through c-Jun NH 2 -terminal kinase (JNK) activation. Among the well established stimuli for JNK activation, reactive oxygen species (ROS) played a causal role in palmitate-induced JNK activation. In addition, etomoxir, an inhibitor of carnitine palmitoyltransferase-1, which is the rate-limiting enzyme in mitochondrial fatty acid ␤-oxidation, as well as inhibitors of the mitochondrial respiratory chain complex (thenoyltrifluoroacetone and carbonyl cyanide m-chlorophenylhydrazone) decreased palmitateinduced ROS production. Together, our findings in hepatocytes indicate that palmitate inhibited insulin signal transduction through JNK activation and that accelerated ␤-oxidation of palmitate caused excess electron flux in the mitochondrial respiratory chain, resulting in increased ROS generation. Thus, mitochondria-derived ROS induced by palmitate may be major contributors to JNK activation and cellular insulin resistance.

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