Hyperinsulinemia is associated with obesity and pancreatic islet hyperplasia, but whether insulin causes these phenomena or is a compensatory response has remained unsettled for decades. We examined the role of insulin hypersecretion in diet-induced obesity by varying the pancreas-specific Ins1 gene dosage in mice lacking Ins2 gene expression in the pancreas, thymus, and brain. Age-dependent increases in fasting insulin and β cell mass were absent in Ins1(+/-):Ins2(-/-) mice fed a high-fat diet when compared to Ins1(+/+):Ins2(-/-) littermate controls. Remarkably, Ins1(+/-):Ins2(-/-) mice were completely protected from diet-induced obesity. Genetic prevention of chronic hyperinsulinemia in this model reprogrammed white adipose tissue to express uncoupling protein 1 and increase energy expenditure. Normalization of adipocyte size and activation of energy expenditure genes in white adipose tissue was associated with reduced inflammation, reduced fatty acid spillover, and reduced hepatic steatosis. Thus, we provide genetic evidence that pathological circulating hyperinsulinemia drives diet-induced obesity and its complications.

exposure to nicotine causes postnatal obesity and altered perivascular adipose tissue function. Obes Res. 2005;13: 687-692. Objective: Recent epidemiological studies have shown that there is an increased risk of obesity and hypertension in children born to women who smoked during pregnancy. The aim of this study was to examine the effect of fetal and neonatal exposure to nicotine, the major addictive component of cigarette smoke, on postnatal adiposity and blood vessel function. Research Methods and Procedures: Female Wistar rats were given nicotine or saline (vehicle) during pregnancy and lactation. Postnatal growth was determined in the male offspring from weaning until 26 weeks of age. At 26 weeks of age, fat pad weight and the function of the perivascular adipose tissue (PVAT) in the thoracic aorta and mesenteric arteries were examined. Results: Exposure to nicotine resulted in increased postnatal body weight and fat pad weight and an increased amount of PVAT in the offspring. Contraction of the aorta induced by phenylephrine was significantly attenuated in the presence of PVAT, whereas this effect was not observed in the aortic rings from the offspring of nicotine-exposed dams. Phenylephrine-induced contraction without PVAT was not different between saline-and nicotine-exposed rats. Transfer of solution incubated with PVAT-intact aorta to PVAT-free aorta induced a marked relaxation response in the rats from saline-exposed dams, but this relaxation response was significantly impaired in the rats from nicotine-exposed dams. Discussion: Our results showed that prenatal nicotine exposure increased adiposity and caused an alteration in the modulatory function of PVAT on vascular relaxation response, thus providing insight into the mechanisms underlying the increased prevalence of obesity and hypertension in children exposed to cigarette smoke in utero.

The proteins that coordinate complex adipogenic transcriptional networks are poorly understood. 14-3-3ζ is a molecular adaptor protein that regulates insulin signalling and transcription factor networks. Here we report that 14-3-3ζ-knockout mice are strikingly lean from birth with specific reductions in visceral fat depots. Conversely, transgenic 14-3-3ζ overexpression potentiates obesity, without exacerbating metabolic complications. Only the 14-3-3ζ isoform is essential for adipogenesis based on isoform-specific RNAi. Mechanistic studies show that 14-3-3ζ depletion promotes autophagy-dependent degradation of C/EBP-δ, preventing induction of the master adipogenic factors, Pparγ and C/EBP-α. Transcriptomic data indicate that 14-3-3ζ acts upstream of hedgehog signalling-dependent upregulation of Cdkn1b/p27Kip1. Indeed, concomitant knockdown of p27Kip1 or Gli3 rescues the early block in adipogenesis induced by 14-3-3ζ knockdown in vitro. Adipocyte precursors in 14-3-3ζKO embryos also appear to have greater Gli3 and p27Kip1 abundance. Together, our in vivo and in vitro findings demonstrate that 14-3-3ζ is a critical upstream driver of adipogenesis.

Insulin resistance and type 2 diabetes mellitus are associated with impaired postprandial secretion of glucagon-like peptide-1 (GLP-1), a potent insulinotropic hormone. The direct effects of insulin and insulin resistance on the L cell are unknown. We therefore hypothesized that the L cell is responsive to insulin and that insulin resistance impairs GLP-1 secretion. The effects of insulin and insulin resistance were examined in well-characterized L cell models: murine GLUTag, human NCI-H716, and fetal rat intestinal cells. MKR mice, a model of chronic hyperinsulinemia, were used to assess the function of the L cell in vivo. In all cells, insulin activated the phosphatidylinositol 3 kinase-Akt and MAPK kinase (MEK)-ERK1/2 pathways and stimulated GLP-1 secretion by up to 275 +/- 58%. Insulin resistance was induced by 24 h pretreatment with 10(-7) m insulin, causing a marked reduction in activation of Akt and ERK1/2. Furthermore, both insulin-induced GLP-1 release and secretion in response to glucose-dependent insulinotropic peptide and phorbol-12-myristate-13-acetate were significantly attenuated. Whereas inhibition of phosphatidylinositol 3 kinase with LY294002 potentiated insulin-induced GLP-1 release, secretion was abrogated by inhibiting the MEK-ERK1/2 pathway with PD98059 or by overexpression of a kinase-dead MEK1-ERK2 fusion protein. Compared with controls, MKR mice were insulin resistant and displayed significantly higher fasting plasma insulin levels. Furthermore, they had significantly higher basal GLP-1 levels but displayed impaired GLP-1 secretion after an oral glucose challenge. These findings indicate that the intestinal L cell is responsive to insulin and that insulin resistance in vitro and in vivo is associated with impaired GLP-1 secretion.

G lucagon-like peptide 1 (GLP-1) is an intestinal hormone that exerts profound effects in the regulation of glycemia, stimulating glucose-dependent insulin secretion, proinsulin gene expression, and ␤-cell proliferative and anti-apoptotic pathways, as well as inhibiting glucagon release, gastric emptying, and food intake (1). The demonstrated success of GLP-1 to lower glycemia has led to approval of the GLP-1 receptor agonist exendin-4 (Byetta) for the treatment of patients with type 2 diabetes (2). Studies using GLP-1 receptor antagonists as well as GLP-1 receptor null mice have demonstrated that GLP-1 makes an essential contribution to the "incretin" effect after a meal (3,4). However, GLP-1 secretion is reduced in patients with type 2 diabetes (5-7), and this may contribute in part to the reduced incretin effect and the hyperglycemia that is observed in these individuals (8). Thus, interest has now focused on the factors that regulate the release of this peptide after nutrient ingestion. Many different GLP-1 secretagogues have been described in the literature over the past few decades, including nutrients, neurotransmitters, neuropeptides, and peripheral hormones (rev. in 9,10). However, the specific receptors, ion channels, and intracellular signaling proteins expressed by the GLP-1-producing intestinal L-cell have only recently begun to be characterized. The purpose of this review is to integrate the literature regarding the signal transduction pathways used by the major GLP-1 secretagogues. An improved understanding of the mechanisms regulating GLP-1 secretion may lead to novel approaches to enhance GLP-1 levels in vivo, thereby providing an alternative approach to the use of this peptide in the treatment of patients with type 2 diabetes. SYNTHESIS AND DEGRADATION OF GLP-1Although the proglucagon gene is expressed in enteroendocrine L-cells and pancreatic ␣-cells (11), GLP-1 is synthesized by posttranslational processing of proglucagon only in the intestine. The L-cells are predominantly located in the ileum and colon and have been identified as open-type epithelial cells that are in direct contact with nutrients in the intestinal lumen (12). Furthermore, L-cells are located in close proximity to both neurons and the microvasculature of the intestine (13,14), which allows the L-cell to be affected by both neural and hormonal signals. In fetal rat intestinal L-cell cultures and immortalized murine L-cells, proglucagon gene expression is enhanced by activation of the protein kinase A (PKA) pathway (15,16). Although originally thought to be mediated through CREB binding to the cAMP response element in the proglucagon gene, recent studies have demonstrated that cAMP-dependent proglucagon gene expression in the L-cell occurs via ␤-catenin-mediated activation of the bipartite transcription factor, TCF4 (17). Interestingly, a very recent report has linked variants of TCF4 to the risk for development of type 2 diabetes (18), implicating GLP-1 in the etiology of this disease.Tissue-specific expression of prohormone...