Leptin and the regulation of body weight in mammals.

The assimilation, storage and use of energy from nutrients constitute a homeostatic system that is essential for life. In vertebrates, the ability to store sufficient quantities of energy-dense triglyceride in adipose tissue allows survival during the frequent periods of food deprivation encountered during evolution. However, the presence of excess adipose tissue can be maladaptive. A complex physiological system has evolved to regulate fuel stores and energy balance at an optimum level. Leptin, a hormone secreted by adipose tissue, and its receptor are integral components of this system. Leptin also signals nutritional status to several other physiological systems and modulates their function. Here we review the role of leptin in the control of body weight and its relevance to the pathogenesis of obesity.

Acute stimulation of glucose metabolism in mice by leptin treatment.

Leptin is an adipocyte hormone that functions as an afferent signal in a negative feedback loop regulating body weight, and acts by interacting with a receptor in the hypothalamus and other tissues. Leptin treatment has potent effects on lipid metabolism, and leads to a large, specific reduction of adipose tissue mass after several days. Here we show that leptin also acts acutely to increase glucose metabolism, although studies of leptin's effect on glucose metabolism have typically been confounded by the weight-reducing actions of leptin treatment, which by itself could affect glucose homoeostasis. We have demonstrated acute in vivo effects of intravenous and intracerebroventricular administrations of leptin on glucose metabolism. A five-hour intravenous infusion of leptin into wild-type mice increased glucose turnover and glucose uptake, but decreased hepatic glycogen content. The plasma levels of insulin and glucose did not change. Similar effects were observed after both intravenous and intracerebroventricular infusion of leptin, suggesting that effects of leptin on glucose metabolism are mediated by the central nervous system (CNS). These data indicate that leptin induces a complex metabolic response with effects on glucose as well as lipid metabolism. This response is unique to leptin, which suggests that new efferent signals emanate from the CNS after leptin treatment.

Physiological response to long-term peripheral and central leptin infusion in lean and obese mice.

Recent data have identified leptin as an afferent signal in a negative-feedback loop regulating the mass of the adipose tissue. High leptin levels are observed in obese humans and rodents, suggesting that, in some cases, obesity is the result of leptin insensitivity. This hypothesis was tested by comparing the response to peripherally and centrally administered leptin among lean and three obese strains of mice: diet-induced obese AKR/J, New Zealand Obese (NZO), and Ay. Subcutaneous leptin infusion to lean mice resulted in a dose-dependent loss of body weight at physiologic plasma levels. Chronic infusions of leptin intracerebroventricularly (i.c.v.) at doses of 3 ng/hr or greater resulted in complete depletion of visible adipose tissue, which was maintained throughout 30 days of continuous i.c.v. infusion. Direct measurement of energy balance indicated that leptin treatment did not increase total energy expenditure but prevented the decrease that follows reduced food intake. Diet-induced obese mice lost weight in response to peripheral leptin but were less sensitive than lean mice. NZO mice were unresponsive to peripheral leptin but were responsive to i.c.v. leptin. Ay mice did not respond to subcutaneous leptin and were 1/100 as sensitive to i.c.v. leptin. The decreased response to leptin in diet-induced obese, NZO, and Ay mice suggests that obesity in these strains is the result of leptin resistance. In NZO mice, leptin resistance may be the result of decreased transport of leptin into the cerebrospinal fluid, whereas in Ay mice, leptin resistance probably results from defects downstream of the leptin receptor in the hypothalamus.

Leptin activation of Stat3 in the hypothalamus of wild-type and ob/ob mice but not db/db mice.

Leptin, a hormone secreted by adipocytes, regulates the size of the adipose tissue mass through effects on satiety and energy metabolism. Leptin's precise sites of action are not known. The leptin receptor (Ob-R) is found in many tissues in several alternatively spliced forms raising the possibility that leptin exerts effects on many tissues including the hypothalamus. Ob-R is a member of the gp130 family of cytokine receptors which are known to stimulate gene transcription via activation of cytosolic STAT proteins. In order to identify the sites of leptin action in vivo, we assayed for activation of STAT proteins in mice treated with leptin. The STAT proteins bind to phosphotyrosine residues in the cytoplasmic domain of the ligand-activated receptor where they are phosphorylated. The activated STAT proteins dimerize and translocate to the nucleus where they bind DNA and activate transcription. The activation of STAT proteins in response to leptin was assayed in a variety of mouse tissues known to express Ob-R. Leptin injection activated Stat3 but no other STAT protein in the hypothalamus of ob/ob and wild-type mice but not db/db mice, mutants that lack an isoform of the leptin receptor. Leptin did not induce STAT activation in any of the other tissues tested. Activation of Stat3 by leptin was dose dependent and first observed after 15 minutes and maximal at 30 minutes. Our data indicate the hypothalamus is a direct target of leptin action and that this activation is critically dependent on the gp-130-like leptin receptor isoform missing in C57BLKS/J db/db mice. This is the first in vivo demonstration of leptin signal transduction.

Weight-reducing effects of the plasma protein encoded by the obese gene.

The gene product of the ob locus is important in the regulation of body weight. The ob product was shown to be present as a 16-kilodalton protein in mouse and human plasma but was undetectable in plasma from C57BL/6J ob/ob mice. Plasma levels of this protein were increased in diabetic (db) mice, a mutant thought to be resistant to the effects of ob. Daily intraperitoneal injections of either mouse or human recombinant OB protein reduced the body weight of ob/ob mice by 30 percent after 2 weeks of treatment with no apparent toxicity but had no effect on db/db mice. The protein reduced food intake and increased energy expenditure in ob/ob mice. Injections of wild-type mice twice daily with the mouse protein resulted in a sustained 12 percent weight loss, decreased food intake, and a reduction of body fat from 12.2 to 0.7 percent. These data suggest that the OB protein serves an endocrine function to regulate body fat stores.

Orphan receptor HNF-4 and bZip protein C/EBP alpha bind to overlapping regions of the apolipoprotein B gene promoter and synergistically activate transcription.

As the sole protein component of low density lipoproteins, apolipoprotein B (apoB) plays an important role in cholesterol metabolism. Previously, we found that the proximal promoter region of apoB (-81 to -52 relative to the start site) played a critical role in hepatocyte-specific gene expression and that that region contained overlapping binding sites for nuclear factors AF-1 (-81 to -62) and C/EBP (-69 to -52) (Metzger, S., Leff, T., and Breslow, J. L. (1990) J. Biol. Chem. 265, 9978-9983). In this study, we show that HNF-4, a member of the steroid hormone receptor superfamily, binds the AF-1 site on the apoB promoter and through it activates transcription in transient transfection assays in both liver and non-liver cell lines, HepG2 and HeLa, respectively. Mutational analysis of the AF-1/HNF-4 binding site indicated a correlation of HNF-4 binding and transcriptional activity. In addition, transient co-transfection experiments with HNF-4 and C/EBP alpha expression vectors showed that the two factors can synergistically activate transcription to levels more than 3-fold above the sum of either factor alone. Finally, using gel retardation analysis we show that purified HNF-4 and C/EBP proteins can concurrently occupy their overlapping binding sites on the apoB promoter in vitro. However, since the same system showed a lack of cooperative binding, we argue that an alternative mechanism is responsible for the synergistic effect of HNF-4 and C/EBP alpha on apoB gene transcription.

An apolipoprotein CIII haplotype protective against hypertriglyceridemia is specified by promoter and 3' untranslated region polymorphisms.

Five DNA polymorphisms were detected in the promoter of the apolipoprotein CIII gene of a type III hyperlipidemic subject with severe hypertriglyceridemia (HTG). The polymorphic sites were C-641-->A, G-630--> A, T-625-->deletion, C-482-->T, and T-455-->C, with the previously reported base at each site designated allele 1 and the variant base designated allele 2. The sites were in strong linkage disequilibrium with each other and with a polymorphic Sst I site in the apolipoprotein CIII 3' untranslated region whose presence (S2 allele) has previously been shown to be associated with HTG. The distribution of haplotypes of the form -625 -482 Sst I among 78 normolipidemic adults and 79 adults with severe HTG was estimated by maximum likelihood analysis. The 211 haplotype was estimated to be 3.8-fold more common in normal subjects than in HTG subjects (estimated proportions, 0.186 and 0.049, respectively). This haplotype was associated with reduced HTG risk (relative risk, 0.28; P = 0.005) when compared with other haplotypes lacking the Sst I site (S1 allele). The 222 haplotype was estimated to be present on 48 of the 54 S2-containing chromosomes observed and was associated with increased risk for HTG (relative risk, 3.14; P < 0.0025). These results support the existence of apolipoprotein CIII promoter/Sst I haplotypes conferring either protection against or susceptibility to severe HTG.