Nutritional and insulin regulation of leptin gene expression

Leptin and its metabolic interactions: an update

Obesity results from an abnormal accumulation of fat in the white adipose tissue. Recent research utilizing genetic models of obesity in rodents has implicated a major role of leptin as a controller of obesity. Leptin is a 167-amino acid peptide hormone encoded by the obesity gene (ob), which is secreted by adipocytes and plays an important role in regulating food intake, energy expenditure and adiposity. Leptin receptors (OB-R) are expressed in the central nervous system mainly in afferent satiety centres of hypothalamus and in peripheral organs such as adipose tissues, skeletal muscles, pancreatic beta-cells and liver, thus indicating the autocrine and paracrine role of leptin in energy regulation. In human beings, a highly organized circadian pattern of leptin secretion is observed with peak levels in the midnight probably resulting from cumulative hyperinsulinemia of entire day. Leptin has a dual role in weight maintenance. Leptin reflects total body adipose tissue mass whereas in conditions of negative and positive energy balance, the dynamic changes in plasma leptin concentration function as a sensor of energy balance and influence the efferent energy regulation pathways. Many effects of leptin on metabolism are mediated by interaction with Insulin and also by synergistic action with cholecystokinin. Besides physiological roles, leptin may influence pathological conditions like obesity-associated atherosclerosis, oxidative stress and cancers. The purpose of the present review is to summarize the important aspects of the biology, actions, and regulation of leptin and to serve as an update of new information.

Regulation and function of collagenous repeat containing sequence of 26-kDa protein gene product "cartonectin"

OBJECTIVE

Collagenous repeat containing sequence of 26-kDa protein (CORS-26) was identified as a new gene transcript expressed in cartilage with unknown function. It was the aim of this study to investigate expression, regulation, and function of CORS-26 in adipocytes.

RESEARCH METHODS AND PROCEDURES

CORS-26 mRNA and protein expression was studied by reverse transcriptase-polymerase chain reaction, Western blot analysis, and quantitative real-time polymerase chain reaction. Transcriptional regulation was studied by electrophoretic mobility shift assay and luciferase reporter gene assay. The adipocytic secretion of adiponectin and resistin was measured by enzyme-linked immunosorbent assay.

RESULTS

CORS-26 mRNA is absent in 3T3-L1 preadipocytes and adipocytes after 48 hours of differentiation. CORS-26 mRNA was induced from Day 4 to Day 9 of adipocyte differentiation. CORS-26 protein was induced in mature adipocytes. Peroxisome proliferator-activated receptor (PPAR) gamma (but not PPARalpha) in nuclear extracts prepared from adipocytes was shown to bind specifically to a putative peroxisome proliferator response element-one-half-site located at -641/-596 bp. Increasing doses of the ligands troglitazone (1, 10, 20 microM) and fenofibrate (50, 100, 200 microM) but not 15-deoxy-prostaglandin (J(2)) (0.5, 1.0, 2.5 microM) resulted in a significant reduction of both promoter activity and the amount of mRNA expression. Recombinant CORS-26 significantly stimulated the adipocytic secretion of adiponectin and resistin in a dose-dependent manner.

DISCUSSION

The mRNA and protein expression profile puts CORS-26 in the adipocytokine family. Cartonectin is negatively regulated by exogenous, but not endogenous, PPARgamma ligands. Because CORS-26 up-regulates adipokine secretion, it might be involved in metabolic and immunologic pathways.

Insulin and glucose play a role in foam cell formation and function

Background

Foam cell formation in diabetic patients often occurs in the presence of high insulin and glucose levels. To test whether hyperinsulinemic hyperglycemic conditions affect foam cell differentiation, we examined gene expression, cytokine production, and Akt phosphorylation in human monocyte-derived macrophages incubated with two types of oxidized low density lipoprotein (LDL), minimally modified LDL (mmLDL) and extensively oxidized LDL (OxLDL).

Methods and results

Using Affymetrix GeneChip^® arrays, we found that several genes directly related to insulin signaling were changed. The insulin receptor and glucose-6-phosphate dehydrogenase were upregulated by mmLDL and OxLDL, whereas insulin-induced gene 1 was significantly down-regulated. In hyperinsulinemic hyperglycemic conditions, modified LDL upregulated Akt phosphorylation and expression of the insulin-regulated aminopeptidase. The level of proinflammatory cytokines, IL-lβ, IL-12, and IL-6, and of a 5-lipoxygenase eicosanoid, 5-hydroxyeicosatetraenoic acid (5-HETE), was also increased.

Conclusion

These results suggest that the exposure of macrophages to modified low density lipoproteins in hyperglycemic hyperinsulinemic conditions affects insulin signaling and promotes the release of proinflammatory stimuli, such as cytokines and eicosanoids. These in turn may contribute to the development of insulin resistance.

Update on adipocyte hormones: regulation of energy balance and carbohydrate/lipid metabolism

Hormones produced by adipose tissue play a critical role in the regulation of energy intake, energy expenditure, and lipid and carbohydrate metabolism. This review will address the biology, actions, and regulation of three adipocyte hormones-leptin, acylation stimulating protein (ASP), and adiponectin-with an emphasis on the most recent literature. The main biological role of leptin appears to be adaptation to reduced energy availability rather than prevention of obesity. In addition to the well-known consequences of absolute leptin deficiency, subjects with heterozygous leptin gene mutations have low circulating leptin levels and increased body adiposity. Leptin treatment dramatically improves metabolic abnormalities (insulin resistance and hyperlipidemia) in patients with relative leptin deficiency due to lipoatrophy. Leptin production is primarily regulated by insulin-induced changes of adipocyte metabolism. Dietary fat and fructose, which do not increase insulin secretion, lead to reduced leptin production, suggesting a mechanism for high-fat/high-sugar diets to increase energy intake and weight gain. ASP increases the efficiency of triacylglycerol synthesis in adipocytes leading to enhanced postprandial lipid clearance. In mice, ASP deficiency results in reduced body fat, obesity resistance, and improved insulin sensitivity. Adiponectin production is stimulated by thiazolidinedione agonists of peroxisome proliferator-activated receptor-gamma and may contribute to increased insulin sensitivity. Adiponectin and leptin cotreatment normalizes insulin action in lipoatrophic insulin-resistant animals. These effects may be mediated by AMP kinase-induced fat oxidation, leading to reduced intramyocellular and liver triglyceride content. The production of all three hormones is influenced by nutritional status. These hormones, the pathways controlling their production, and their receptors are promising targets for managing obesity, hyperlipidemia, and insulin resistance.

Response of Plasma Leptin Concentration to Jugular Infusion of Glucose or Lipid Is Dependent on the Stage of Lactation of Holstein Cows

In this study we investigated the hormonal and metabolite responses to isoenergetic jugular infusions of glucose or lipid in early- and late-lactation Holstein cows. Six Holstein cows were used in a replicated Latin square design with jugular infusions of either 1) control (CON; saline), 2) glucose (GLU; 50% dextrose) or 3) lipid (LIP; 20% Intralipid). Treatments did not affect dry matter intake, with the exception of a hypophagic effect of LIP in late lactation. The GLU-induced hyperglycemia and hyperinsulinemia were greater in late-lactation than in early-lactation cows. The GLU treatment did not affect plasma leptin and insulin-like growth factor-1 (IGF-1) concentrations in early-lactation cows, but it increased them in late-lactation cows. The LIP treatment did not affect plasma leptin, insulin and IGF-1 concentrations in early-lactation cows, despite a marked LIP-induced increase in plasma nonesterified fatty acid and beta-hydroxybutyrate concentrations and a reduction in growth hormone (GH) concentration. Compared with the delayed leptin response to GLU, the stimulatory effect of LIP on leptin secretion in late-lactation cows was relatively rapid and occurred in the absence of any significant changes in plasma insulin, IGF-1 or GH. We propose that insulin-mediated glucose metabolism may be involved in the stimulatory effects of glucose on leptin secretion in late-lactation animals but that the stimulatory effects of lipid are independent of insulin or IGF-1. In early-lactation animals a strong inhibitory effect of GH on leptin expression and release, in addition to low adipose reserves and/or energy balance, might override any short-term stimulatory effect of glucose or lipid on leptin secretion.

Comparing mRNA levels of genes encoding leptin, leptin receptor, and lipoprotein lipase between dairy and beef cattle

Body weight and fat mass vary distinctly between German Holstein (dairy cattle) and Charolais (beef cattle). The aim of this study was to determine whether the expression of the obese (Ob) gene and lipoprotein lipase (LPL) gene in fat tissues and expression of the long isoform leptin receptor (Ob-Rb) gene in the hypothalamus were different between these two cattle breeds. Body weight and the area of longissimus muscle cross-section of German Holstein were lower (P<0.001), while body fat content, as well as the omental and perirenal fat mass were higher (P<0.001), compared to Charolais. Plasma insulin and leptin levels between two cattle breeds were determined by radioimmunoassay. Compared to Charolais, plasma insulin concentrations were significantly higher (P<0.01), and plasma leptin levels were tended to be higher (P<0.1) in German Holstein. Ob mRNA levels in subcutaneous and perirenal fat depots, but not in the omental fat depot, were significantly higher (P<0.05) in German Holstein than in Charolais. LPL mRNA expression in the perirenal fat depot of German Holstein was greater in abundance than that of Charolais. No significantly different LPL mRNA levels were found in subcutaneous and omental fat depots, and Ob-Rb mRNA levels in the hypothalamus between these two cattle breeds (P<0.05). Both Ob and LPL expression was greater in perirenal and omental fat depots than in the subcutaneous fat depot (P<0.05). Data indicated that in bovine the Ob and LPL gene expression levels in perirenal fats are an important index that is associated with body fat content, while Ob-Rb in hypothalamus is not.

Polyunsaturated fatty acid regulation of gene transcription: a molecular mechanism to improve the metabolic syndrome

This review addresses the hypothesis that polyunsaturated fatty acids (PUFA), particularly those of the (n-3) family, play pivotal roles as "fuel partitioners" in that they direct fatty acids away from triglyceride storage and toward oxidation, and that they enhance glucose flux to glycogen. In doing this, PUFA may protect against the adverse symptoms of the metabolic syndrome and reduce the risk of heart disease. PUFA exert their beneficial effects by up-regulating the expression of genes encoding proteins involved in fatty acid oxidation while simultaneously down-regulating genes encoding proteins of lipid synthesis. PUFA govern oxidative gene expression by activating the transcription factor peroxisome proliferator-activated receptor alpha. PUFA suppress lipogenic gene expression by reducing the nuclear abundance and DNA-binding affinity of transcription factors responsible for imparting insulin and carbohydrate control to lipogenic and glycolytic genes. In particular, PUFA suppress the nuclear abundance and expression of sterol regulatory element binding protein-1 and reduce the DNA-binding activities of nuclear factor Y, Sp1 and possibly hepatic nuclear factor-4. Collectively, the studies discussed suggest that the fuel "repartitioning" and gene expression actions of PUFA should be considered among criteria used in defining the dietary needs of (n-6) and (n-3) and in establishing the dietary ratio of (n-6) to (n-3) needed for optimum health benefit.