Brain leptin resistance in human obesity revisited

Abstract

Leptin is a 16 kDa peptide predominantly produced by adipocytes. Leptin and its receptor are known to be involved in the regulation of energy balance. The data from animal studies as well as our own observations of leptin overflow from the brain suggest that the central nervous system is a site of leptin synthesis. Using simultaneous arterio-venous blood sampling we here confirm that leptin is released from the brain into the internal jugular vein, and that release is greater in overweight men and in females compared to lean men, 467.3 ng/min ± 160.4 and 1426 ng/min ± 769.3 vs 80.0 ng/min ± 29.3, respectively (P < 0.05). Furthermore, we have examined the gene expression of leptin and its receptor isoforms by reverse transcription-polymerase chain reaction (RT-PCR) in human cadaver hypothalami across a broad range of adiposity. Leptin gene expression was detected in a number of donors; the presence of detectable leptin mRNA was related to the mode of death rather than BMI or gender. We have also demonstrated gene expression of the three leptin receptor isoforms in the human hypothalamus. No relation was observed between the levels of hypothalamic expression of the long signaling form of the leptin receptor and BMI. In summary, this study indicates that it is very difficult to explain human obesity on the basis of central nervous system “leptin resistance”, in that leptin is released in the brain, and at a higher level in the obese, and brain leptin receptor gene expression is not impaired in obesity.

Introduction

Leptin, the adipose tissue-derived hormone, plays an important role in regulating appetite and energy expenditure, by acting on specific receptors in the hypothalamus [1]. It is released by fat cells at levels proportional to body fat stores and plasma leptin levels strongly correlate with the degree of adiposity, with greater levels found in overweight individuals and reduced levels found in lipodystrophic animals. Obese humans and animals have elevated plasma leptin levels; therefore “leptin deficiency” is not the cause of most cases of obesity. Leptin resistance has been suggested as an explanation for why increased plasma leptin concentrations do not have the expected weight reducing effect.

A number of mechanisms have been proposed to explain leptin resistance. The transport of leptin into the central nervous system represents an important step in the regulation of body weight. It has been suggested that one basis for leptin resistance might be failure of leptin to cross the blood–brain barrier. Studies by Banks and Farrell and Schwartz et al. [2], [3] do provide evidence of impaired transport of leptin into the central nervous system in obesity. Previous experimental studies were successful in demonstrating that central leptin administration, as opposed to peripheral, is more effective in reducing appetite and stimulating thermogenesis [4].

When first discovered, leptin was thought to be secreted into the bloodstream exclusively by adipocytes [5]. Later evidence, however, suggests that adipose tissue may not be the only site of leptin production, with demonstration of leptin gene expression in other tissues, including the stomach [6], placenta [7], and skeletal muscles [8]. Previous studies from our laboratory demonstrated a net efflux of leptin from the brain into the internal jugular veins [9], [10], [11]. These observations suggest that perhaps the brain itself produces leptin, which acts locally and is subsequently released into the circulation, contributing to the total leptin plasma pool [9]. In fact, leptin mRNA expression has been demonstrated in the rat pituitary and brain [12].

There are six leptin receptor isoforms, which are splicing variants of a single gene transcript [13], [14]. Regions identical to all receptor isoforms include an extracellular domain, a transmembrane domain, and the first 29 amino acids of the cytoplasmic domain. Based on the length of the intracellular domain, one of the isoforms is “long” (OB-Rb), and four are “short”. These differ in their cytosolic carboxy terminus. In addition, there is one soluble isoform (OB-Re), lacking the transmembrane domain, which may be involved in leptin transport from plasma [15]. When leptin binds to the full-length OB-Rb receptor, it can be demonstrated to activate JAK/STAT (Janus kinase/signal transducer and activator of transcription) and MAPK (mitogen activated protein kinase) signal transduction pathways in a variety of in vitro systems [16]. The first demonstration of brain leptin receptor gene expression came soon after the discovery of leptin [17]. Loss of function mutations, resulting in complete leptin resistance has been demonstrated to cause morbid obesity in some experimental models of rodent obesity [18], [19]. Leptin receptor gene mutations, however, are very rare in humans [1].

Based on previous findings we set out to further investigate whether the human brain is a site of leptin production, and what might be the contribution of brain leptin to the plasma leptin pool. We also investigated the expression of leptin receptor isoforms in the hypothalamus and whether this was related to the degree of adiposity and to gender.