Molecular mechanisms of weight regulation in obesity and chronic renal failure with special reference to leptin and uncoupling protein 2

Abstract

Obesity, which increases the risk of developing serious medical disorders, such as cardiovascular disease and diabetes, is threatening to become a global epidemic, meaning escalating healthcare costs for society and reduced quality of life for the individual. Thus, there is a large unmet need for improved preventive strategies and effective treatments. This emphasizes the necessity of understanding the molecular mechanisms controlling energy regulation as well as identifying the genes that are important in the development of obesity. The identification of the LEP gene and its product leptin, as well as two novel uncoupling proteins, UCP2 and UCP3, has further elucidated the complex network of mechanisms underlying body weight regulation in animal models and in man.

Several hundred Swedish obese and lean subjects were screened for mutations in the LEP gene using solid-phase sequencing and single strand conformation polymorphism analysis (SSCP), without finding any sequence variations. However, an 80% increase in LEP mRNA levels in obese compared to non-obese subjects was demonstrated in subcutaneous adipose tissue using in situ hybridization (paper 1). LEP expression was 75% higher in obese women than in obese men, indicating a sex dimorphism in the LEP gene regulation.

The role of leptin expression and secretion in the regulation of circulating leptin levels was investigated in obese and non-obese women (paper 11). Both LEP expression and leptin secretion rates were twice as high in obese women, resulting in a five-fold elevation of plasma leptin in the obese subjects. Fat cell volume accounted for 70-80% of secretion rate variations, while leptin mRNA levels accounted for about 40% of the variations in secretion rates and plasma leptin.

Hyperleptinemia may be one of the factors inducing anorexia and weight loss in chronic renal failure (CRF). In paper III, the glomerular filtration rate (GFR) was shown to correlate with serum leptin and the hyperleptinemia, caused by decreased plasma clearance, resulted in downregulation of LEP gene expression. Inflammation stimulated LEP gene expression in patients with CRF, suggesting that the hyperleptinemia induced feedback inhibition of LEP gene expression is overcome by inflammatory cytokines. Following PD treatment, strong positive correlations were seen between changes in LEP expression and changes in both body fat mass and serum leptin.

In paper IV, the importance of aberrations in the regulation of UCP2 and UCP3 for obesity was investigated. In skeletal muscle of obese males, we detected a 28 % decrease in UCP2 mRNA levels, compared to controls.

In paper V, we investigated if the insertion/deletion polymorphism in the UCP2 gene could influence changes in body composition during PD. Following 12 months of PD, a significant increase in body weight and body fat mass was seen in patients who were homozygous for the deletion, while a significant reduction in lean body mass was seen in the heterozygotes. In 790 Swedish subjects, we found a significant association between the UCP2 polymorphism and BMI.

SNP-genotyping of more than 1000 Swedish subjects revealed a significant association between a locus on chromosome 10 and BMI in males, using pyrosequencing for single nucleotide polymorphism (SNP) analysis (paper VI).