John Bullen

Elevated leptin fragments in renal failure correlate with BMI and haematopoiesis and are normalized by haemodialysis

objective  Leptin is an adipocyte hormone important in appetite, energy homeostasis, neuroendocrine and haematopoietic function. Patients with renal failure often have elevated total and free leptin levels. Biologically active leptin fragments (leptin22−56 and leptin57−92) have been identified, but whether these fragments are affected by renal failure and/or haemodialysis (HD) is not known.

Regulation of mouse hepatic genes in response to diet induced obesity, insulin resistance and fasting induced weight reduction

BackgroundObesity is associated with insulin resistance that can often be improved by caloric restriction and weight reduction. Although many physiological changes accompanying insulin resistance and its treatment have been characterized, the genetic mechanisms linking obesity to insulin resistance are largely unknown. We used DNA microarrys and RT-PCR to investigate significant changes in hepatic gene transcription in insulin resistant, diet-induced obese (DIO)-C57/BL/6J mice and DIO-C57/BL/6J mice fasted for 48 hours, whose weights returned to baseline levels during these conditions.ResultsTranscriptional profiling of hepatic mRNA revealed over 1900 genes that were significantly perturbed between control, DIO, and fasting/weight reduced DIO mice. From this set, our bioinformatics analysis identified 41 genes that rigorously discriminate these groups of mice. These genes are associated with molecular pathways involved in signal transduction, and protein metabolism and secretion. Of particular interest are genes that participate in pathways responsible for modulating insulin sensitivity. DIO altered expression of genes in directions that would be anticipated to antagonize insulin sensitivity, while fasting/ weight reduction partially or completely normalized their levels. Among these discriminatory genes, Sh3kbp1 and RGS3, may have special significance. Sh3kbp1, an endogenous inhibitor of PI-3-kinase, was upregulated by high-fat feeding, but normalized to control levels by fasting/weight reduction. Because insulin signaling occurs partially through PI-3-kinase, increased expression of Sh3kbp1 by DIO mice may contribute to hepatic insulin resistance via inhibition of PI-3-kinase. RGS3, a suppressor of G-protein coupled receptor generation of cAMP, was repressed by high-fat feeding, but partially normalized by fasting/weight reduction. Decreased expression of RGS3 may augment levels of cAMP and thereby contribute to increased, cAMP-induced, hepatic glucose output via phosphoenolpyruvate carboxykinase (PCK1), whose mRNA levels were also elevated.ConclusionThese findings demonstrate that hepatocytes respond to DIO and weight reduction by controlling gene transcription in a variety of important molecular pathways. Future studies that characterize the physiological significance of the identified genes in modulating energy homeostasis could provide a better understanding of the mechanisms linking DIO with insulin resistance.

Linking hepatic transcriptional changes to high-fat diet induced physiology for diabetes-prone and obese-resistant mice.

Insulin resistance is characterized by high insulin levels and decreased responsiveness of tissues to the clearance of glucose from the bloodstream. This study maintained the diabetes-prone C57BL/6J and obese-resistant A/J mice strains on a high-fat diet for 12 weeks to transcriptionally profile the liver for changes caused by high fat diet. In the 8th week of the experiment, the C57BL/6J mice began exhibiting signs of insulin resistance, while the A/J mice did not show any such indications during the course of the experiment. A regression model of partial least squares between serum insulin measurements and the liver gene expression profile for the C57BL/6J mice on a high-fat diet was constructed in an effort to quantitatively link the physiological measurement with the gene expressions. A series of discriminating genes between high fat and chow fed mice was generated for both the C57BL/6J and A/J strains. These discriminatory genes contain information about the mechanisms responsible for the development of insulin resistance, and the compensation for a high fat diet, respectively. The results identified several genes involved in the development of insulin resistance and serve as a framework for other studies involving other organs affected by this systemic disease.

Altered Levels of Adiponectin and Adiponectin Receptors May Underlie the Effect of Ciliary Neurotrophic Factor (CNTF) to Enhance Insulin Sensitivity in Diet-induced Obese Mice

Ciliary neurotrophic factor (CNTF), a neuronal growth factor orginially studied in amyotrophic lateral sclerosis, induces weight loss and improves insulin resistances in humans and rodents (1). It has been recently shown that CNTF induces hypothalamic neurogenesis and that it signals through the CNTFRX-IL-6R-gp130β receptor complex to increase fatty-acid oxidation and to reduce insulin resistance in skeletal muscle by activating AMP-activated protein kinase (AMPK) in peripheral tissues (2–4). Adiponectin, a hormone secreted by adipose tissue, controls glucose and lipid metabolism, prevents hepatic steatosis, and plays an important role in the regulation of insulin resistance and energy homeostasis by binding and activating tow cell membrane receptors. Adiponectin receptor 1 (AdipoR1) is most abundantly expressed in muscle, and adiponectin receptor 2 (AdipoR2), most predominantly in liver (5). The adiponectin receptors can ligand-dependently and dose-dependently activate AMPK, p38 mitogen-activated protein kinase, and PPARa, thus stimulating fatty-acid oxidation and glucose uptake in hepatocytes and myocytes, limiting deposition of fat, and maintaining insulin responsiveness in liver and muscle. In addition, the weight-reducing effects of CNTF appear to result from direct effects on skeletal muscle and may not require central signaling, in contrast to the effects of leptin (3,4). Although accumulating evidence suggests that adiponectin and its receptors play an important role in obesity and insulin resistance, no previous study has investigated whether administration of CNTFAx15 may improve insulin resistance by altering expression of adiponectin and its receptors above and beyond what would be expected based on reduction of caloric intake or body weight alone. We hypothesize that the effect of CNTFAx15 on insulin resistance could be mediated, in part, through alterations of secretion- and expression patterns of adiponectin and its receptors. Therefore we studied the effect of treatment with CNTFAx15 vs. placebo treatment and hypocaloric pair feeding for 7 days on circulating adiponectin and leptin levels as well as expression patterns of adiponectin and its receptors in white adipose tissue, liver and muscle in insulin- and leptin-resistant, hyperinsulinemic diet-induced obese (DIO) C57Bl/6J mice, the mouse model most closely associated with human obesity (1).