Summary Circulating leptin and insulin convey information regarding energy stores to the central nervous system, particularly the hypothalamus. Hypothalamic pro-opiomelanocortin (POMC) neurons regulate energy balance and glucose homeostasis and express leptin and insulin receptors. However, the physiological significance of concomitant leptin and insulin action on POMC neurons remains to be established. Here we show that mice lacking both insulin and LepRs in POMC neurons (Pomc-Cre, Leprflox/flox IRflox/flox mice) display systemic insulin resistance, which is distinct from the single deletion of either receptor. In addition, Pomc-Cre, Leprflox/flox IRflox/flox female mice display elevated serum testosterone levels and ovarian abnormalities resulting in reduced fertility. We conclude that direct action of insulin and leptin on POMC neurons is required to maintain normal glucose homeostasis and reproductive function.

Type 2 diabetes is a heterogeneous disease characterized by insulin resistance and altered glucose and lipid metabolism in multiple organs. To understand the complex series of events that occur during the development of obesityassociated diabetes, we examined the temporal pattern of changes in insulin action and glucose metabolism in individual organs during chronic high-fat feeding in C57BL/6 mice. Insulin-stimulated cardiac glucose metabolism was significantly reduced after 1.5 weeks of high-fat feeding, and cardiac insulin resistance was associated with blunted Akt-mediated insulin signaling and GLUT4 levels. Insulin resistance in skeletal muscle, adipose tissue, and liver developed in parallel after 3 weeks of high-fat feeding. Diet-induced whole-body insulin resistance was associated with increased circulating levels of resistin and leptin but unaltered adiponectin levels. High-fat feeding caused insulin resistance in skeletal muscle that was associated with significantly elevated intramuscular fat content. In contrast, diet-induced hepatic insulin resistance developed before a marked increase in intrahepatic triglyceride levels. Cardiac function gradually declined over the course of high-fat feeding, and after 20 weeks of high-fat diet, cardiac dysfunction was associated with mild hyperglycemia, hyperleptinemia, and reduced circulating adiponectin levels. Our findings demonstrate that cardiac insulin resistance is an early adaptive event in response to obesity and develops before changes in whole-body glucose homeostasis. This suggests that obesity-associated defects in cardiac function may not be due to insulin resistance per se but may be attributable to chronic alteration in cardiac glucose and lipid metabolism and circulating adipokines. Diabetes 54: 3530 -3540, 2005

. Caveolin-3 knockout mice show increased adiposity and whole body insulin resistance, with ligand-induced insulin receptor instability in skeletal muscle. Am J Physiol Cell Physiol 288: C1317-C1331, 2005. First published February 2, 2005 doi:10.1152/ajpcell.00489.2004 is expressed predominantly in skeletal muscle fibers, where it drives caveolae formation at the muscle cell's plasma membrane. In vitro studies have suggested that Cav-3 may play a positive role in insulin signaling and energy metabolism. We directly address the in vivo metabolic consequences of genetic ablation of Cav-3 in mice as it relates to insulin action, glucose metabolism, and lipid homeostasis. At age 2 mo, Cav-3 null mice are significantly larger than wild-type mice, and display significant postprandial hyperinsulinemia, whole body insulin resistance, and whole body glucose intolerance. Studies using hyperinsulinemiceuglycemic clamps revealed that Cav-3 null mice exhibited 20% and 40% decreases in insulin-stimulated whole body glucose uptake and whole body glycogen synthesis, respectively. Whole body insulin resistance was mostly attributed to 20% and 40% decreases in insulin-stimulated glucose uptake and glucose metabolic flux in the skeletal muscle of Cav-3 null mice. In addition, insulin-mediated suppression of hepatic glucose production was significantly reduced in Cav-3 null mice, indicating hepatic insulin resistance. Insulin-stimulated glucose uptake in white adipose tissue, which does not express Cav-3, was decreased by ϳ70% in Cav-3 null mice, suggestive of an insulin-resistant state for this tissue. During fasting, Cav-3 null mice possess normal insulin receptor protein levels in their skeletal muscle. However, after 15 min of acute insulin stimulation, Cav-3 null mice show dramatically reduced levels of the insulin receptor protein, compared with wild-type mice treated identically. These results suggest that Cav-3 normally functions to increase the stability of the insulin receptor at the plasma membrane, preventing its rapid degradation, i.e., by blocking or slowing ligand-induced receptor downregulation. Thus our results demonstrate the importance of Cav-3 in regulating whole body glucose homeostasis in vivo and its possible role in the development of insulin resistance. These findings may have clinical implications for the early diagnosis and treatment of caveolinopathies.

Diabetic heart failure may be causally associated with alterations in cardiac energy metabolism and insulin resistance. Mice with heart-specific overexpression of peroxisome proliferator-activated receptor (PPAR)␣ showed a metabolic and cardiomyopathic phenotype similar to the diabetic heart, and we determined tissuespecific glucose metabolism and insulin action in vivo during hyperinsulinemic-euglycemic clamps in awake myosin heavy chain (MHC)-PPAR␣ mice (12-14 weeks of age). Basal and insulin-stimulated glucose uptake in heart was significantly reduced in the MHC-PPAR␣ mice, and cardiac insulin resistance was mostly attributed to defects in insulin-stimulated activities of insulin receptor substrate (IRS)-1-associated phosphatidylinositol (PI) 3-kinase, Akt, and tyrosine phosphorylation of signal transducer and activator of transcription 3 (STAT3). Interestingly, MHC-PPAR␣ mice developed hepatic insulin resistance associated with defects in insulin-mediated IRS-2-associated PI 3-kinase activity, increased hepatic triglyceride, and circulating interleukin-6 levels. To determine the underlying mechanism, insulin clamps were conducted in 8-week-old MHC-PPAR␣ mice. Insulin-stimulated cardiac glucose uptake was similarly reduced in 8-week-old MHC-PPAR␣ mice without changes in cardiac function and hepatic insulin action compared with the age-matched wild-type littermates. Overall, these findings indicate that increased activity of PPAR␣, as occurs in the diabetic heart, leads to cardiac insulin resistance associated with defects in insulin signaling and STAT3 activity, subsequently leading to reduced cardiac function. Additionally, ageassociated hepatic insulin resistance develops in MHC-PPAR␣ mice that may be due to altered cardiac metabolism, functions, and/or inflammatory cytokines. Diabetes 54:2514 -2524, 2005 C ardiovascular disease is the leading cause of mortality in type 2 diabetes (1,2). Although the etiology of diabetic heart failure is poorly understood, there is a growing body of evidence (3-5) indicating that alterations in cardiac energy metabolism may precede and be causally associated with the development of cardiomyopathy in the diabetic heart. The peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors belonging to the nuclear receptor superfamily, and of the three identified mammalian PPAR subtypes (␣, ␥, and ␦), PPAR␣ regulates nuclear expression of genes involved in lipid metabolism in various cell types including heart (6,7). Since normal cardiac function is dependent on a constant rate of ATP resynthesis predominantly driven by mitochondrial fatty acid oxidation, activation of PPAR␣ by endogenous ligands, such as fatty acids, plays an important role in cardiac energy metabolism and functions (8,9). Mice with heart-specific overexpression of PPAR␣ (myosin heavy chain [MHC]-PPAR␣) were recently shown to exhibit increased rates of myocardial lipid oxidation, consistent with the role of PPAR␣ (10). Importantly, MHC-PPAR␣ mice developed cardiomyopathy with enh...

Recent studies demonstrated a role for hypothalamic insulin and leptin action in the regulation of glucose homeostasis. This regulation involves proopiomelanocortin (POMC) neurons because suppression of phosphatidyl inositol 3-kinase (PI3K) signaling in these neurons blunts the acute effects of insulin and leptin on POMC neuronal activity. In the current study, we investigated whether disruption of PI3K signaling in POMC neurons alters normal glucose homeostasis using mouse models designed to both increase and decrease PI3K-mediated signaling in these neurons. We found that deleting p85alpha alone induced resistance to diet-induced obesity. In contrast, deletion of the p110alpha catalytic subunit of PI3K led to increased weight gain and adipose tissue along with reduced energy expenditure. Independent of these effects, increased PI3K activity in POMC neurons improved insulin sensitivity, whereas decreased PI3K signaling resulted in impaired glucose regulation. These studies show that activity of the PI3K pathway in POMC neurons is involved in not only normal energy regulation but also glucose homeostasis.

Adiponectin overexpression in mice increases insulin sensitivity independent of adiposity. Here, we combined stable isotope infusion and in vivo measurements of lipid flux with transcriptomic analysis to characterize fatty acid metabolism in transgenic mice that overexpress adiponectin via the aP2-promoter (ADNTg). Compared with controls, fasted ADNTg mice demonstrated a 31% reduction in plasma free fatty acid concentrations (P = 0.008), a doubling of ketones (P = 0.028), and a 68% increase in free fatty acid turnover in plasma (15.1 ± 1.5 vs. 25.3 ± 6.8 mg/kg · min, P = 0.011). ADNTg mice had 2-fold more brown adipose tissue mass, and triglyceride synthesis and turnover were 5-fold greater in this organ (P = 0.046). Epididymal white adipose tissue was slightly reduced, possibly due to the approximately 1.5-fold increase in the expression of genes involved in oxidation (peroxisome proliferator-activated receptor α, peroxisome proliferator-activated receptor-γ coactivator 1α, and uncoupling protein 3). In ADNTg liver, lipogenic gene expression was reduced, but there was an unexpected increase in the expression of retinoid pathway genes (hepatic retinol binding protein 1 and retinoic acid receptor beta and adipose Cyp26A1) and liver retinyl ester content (64% higher, P < 0.02). Combined, these data support a physiological link between adiponectin signaling and increased efficiency of triglyceride synthesis and hydrolysis, a process that can be controlled by retinoids. Interactions between adiponectin and retinoids may underlie adiponectin's effects on intermediary metabolism.