20 years of leptin: what we know and what the future holds.

This special issue of Journal of Endocrinology celebrates the 20th anniversary of the discovery of leptin, a hormone produced by adipose tissue, which provides critical signals to the organism regarding the status of its energy stores. The discovery of leptin not only revolutionised our understanding of endocrine physiology but has also resulted in a registered medicinal product which is already improving the health of patients with serious metabolic diseases. In this issue, we have gathered together a group of essays by some of the world leaders in leptin research, including an overview by Dr Jeffrey Friedman who, in his seminal article in December 1994, described the adipocyte-derived hormone, the lack of which was responsible for the severe obesity in ob/ob mice and suggested that it should be named leptin.

The architecture of gene regulatory variation across multiple human tissues: the MuTHER study.

While there have been studies exploring regulatory variation in one or more tissues, the complexity of tissue-specificity in multiple primary tissues is not yet well understood. We explore in depth the role of cis-regulatory variation in three human tissues: lymphoblastoid cell lines (LCL), skin, and fat. The samples (156 LCL, 160 skin, 166 fat) were derived simultaneously from a subset of well-phenotyped healthy female twins of the MuTHER resource. We discover an abundance of cis-eQTLs in each tissue similar to previous estimates (858 or 4.7% of genes). In addition, we apply factor analysis (FA) to remove effects of latent variables, thus more than doubling the number of our discoveries (1,822 eQTL genes). The unique study design (Matched Co-Twin Analysis--MCTA) permits immediate replication of eQTLs using co-twins (93%-98%) and validation of the considerable gain in eQTL discovery after FA correction. We highlight the challenges of comparing eQTLs between tissues. After verifying previous significance threshold-based estimates of tissue-specificity, we show their limitations given their dependency on statistical power. We propose that continuous estimates of the proportion of tissue-shared signals and direct comparison of the magnitude of effect on the fold change in expression are essential properties that jointly provide a biologically realistic view of tissue-specificity. Under this framework we demonstrate that 30% of eQTLs are shared among the three tissues studied, while another 29% appear exclusively tissue-specific. However, even among the shared eQTLs, a substantial proportion (10%-20%) have significant differences in the magnitude of fold change between genotypic classes across tissues. Our results underline the need to account for the complexity of eQTL tissue-specificity in an effort to assess consequences of such variants for complex traits.

Assessment of acute and chronic pharmacological effects on energy expenditure and macronutrient oxidation in humans: responses to ephedrine.

Evidence of active brown adipose tissue in human adults suggests that this may become a pharmacological target to induce negative energy balance. We have explored whole-body indirect calorimetry to detect the metabolic effects of thermogenic drugs through administration of ephedrine hydrochloride and have assessed ephedrine's merits as a comparator compound in the evaluation of novel thermogenic agents. Volunteers randomly given ephedrine hydrochloride 15 mg QID (n = 8) or placebo (n = 6) were studied at baseline and after 1-2 and 14-15 days of treatment. We demonstrate that overnight or 23-hour, 2% energy expenditure (EE) and 5% fat (FO) or CHO oxidation effects are detectable both acutely and over 14 days. Compared to placebo, ephedrine increased EE and FO rates overnight (EE 63 kJ day 2, EE 105 kJ, FO 190 kJ, day 14), but not over 23 h. We conclude that modest energy expenditure and fat oxidation responses to pharmacological interventions can be confidently detected by calorimetry in small groups. Ephedrine should provide reliable data against which to compare novel thermogenic compounds.

Defective peroxisomal proliferators activated receptor gamma activity due to dominant-negative mutation synergizes with hypertension to accelerate cardiac fibrosis in mice.

AIMS: Humans with inactivating mutations in peroxisomal proliferators activated receptor gamma (PPARgamma) typically develop a complex metabolic syndrome characterized by insulin resistance, diabetes, lipodystrophy, hypertension, and dyslipidaemia which is likely to increase their cardiovascular risk. Despite evidence that the activation of PPARgamma may prevent cardiac fibrosis and hypertrophy, recent evidence has suggested that pharmacological activation of PPARgamma causes increased cardiovascular mortality. In this study, we investigated the effects of defective PPARgamma function on the development of cardiac fibrosis and hypertrophy in a murine model carrying a human dominant-negative mutation in PPARgamma. METHODS AND RESULTS: Mice with a dominant-negative point mutation in PPARgamma (P465L) and their wild-type (WT) littermates were treated with either subcutaneous angiotensin II (AngII) infusion or saline for 2 weeks. Heterozygous P465L and WT mice developed a similar increase in systolic blood pressure, but the mutant mice developed significantly more severe cardiac fibrosis to AngII that correlated with increased expression of profibrotic genes. Both groups similarly increased the heart weight to body weight ratio compared with saline-treated controls. There were no differences in fibrosis between saline-treated WT and P465L mice. CONCLUSION: These results show synergistic pathogenic effects between the presence of defective PPARgamma and AngII-induced hypertension and suggest that patients with PPARgamma mutation and hypertension may need more aggressive therapeutic measures to reduce the risk of accelerated cardiac fibrosis.

Cell proliferation activities on skin fibroblasts from a short child with absence of one copy of the type 1 insulin-like growth factor receptor (IGF1R) gene and a tall child with three copies of the IGF1R gene.

The type 1 IGF receptor (IGF1R) is required for normal embryonic and postnatal growth. The aim of this study was to determine whether we could detect abnormal IGF1R function in skin fibroblasts from children with an abnormal copy number of the IGF1R gene. We report two children with altered copy number of the IGF1R gene who presented with abnormal growth. Case 1 is a girl with intrauterine growth retardation, postnatal growth failure, and recurrent hypoglycemia. Pituitary function tests were normal. Routine karyotype analysis identified a deletion on 15q26.2, and a fluorescence in situ hybridization study using IGF1R probes showed only a single IGF1R gene. Case 2 was large for gestational age, with birth weight and length at or above 97th percentile, and showed rapid early postnatal growth. He was found to have a recombinant chromosome 15 containing a partial duplication at 15q (q25-qter). A fluorescence in situ hybridization study using the same probes showed three copies of the IGF1R gene. In a mitochondrial activity assay, skin fibroblasts from the subject with only one copy of IGF1R showed slower growth, whereas cells from the subject with three copies of IGF1R showed accelerated growth compared with controls. IGF1R phosphorylation, as assessed by Western blot, and IGF1R binding studies were decreased compared with controls in the child with one copy of the IGF1R and increased in the child with three copies of the gene. Our data are consistent with the concept that IGF1R gene copy number is of functional and clinical importance in humans.

Leptin: defining its role in humans by the clinical study of genetic disorders.

Extremely unusual genetic conditions can reveal normal processes governing physiologic regulation and metabolism. Children with rare homozygous mutations in the leptin gene and complete leptin deficiency develop extreme hyperphagia and obesity soon after birth but respond with normal eating and a selective loss of excess body fat upon being given small amounts of leptin. Heterozygote relatives have 30% more fat than predicted and relatively low leptin levels. This demonstrates leptin's fundamental involvement in maintaining energy balance. Leptin also seems to act as a metabolic gate allowing children to enter puberty.