A mutation within the obese gene was recently identified as the genetic basis for obesity in the ob/ob mouse. The obese gene product, leptin, is a 16-kDa protein expressed predominantly in adipose tissue. Consistent with leptin's postulated role as an extracellular signaling protein, human embryonic kidney 293 cells transfected with the obese gene secreted leptin with minimal intracellular accumulation. Upon differentiation of 3T3-L1 preadipocytes into adipocytes, the leptin mRNA was expressed concomitant with mRNAs encoding adipocyte marker proteins. A factor(s) present in calf serum markedly activated expression of leptin by fully differentiated 3T3-L1 adipocytes. A 16-hr fast decreased (by -85%) the leptin mRNA level of adipose tissue of lean (ob/+ or +/+) mice but had no effect on the -4-fold higher level in obese (ob/ob) litterniates. Since the mutation at the ob locus fails to produce the functional protein, yet its cognate mRNA is overproduced, it appears that leptin is necessary for its own downregulation. Leptin mRNA was also suppressed in adipose tissue of rats during a 16-hr fast and was rapidly induced during a 4-hr refeeding period. Insulin deficiency provoked by streptozotocin also markedly down-regulated leptin mRNA and this suppression was rapidly reversed by insulin. These results suggest that insulin may regulate the expression of leptin.The ob/ob mouse has been widely investigated as a model of genetic obesity (1, 2) since the discovery of the ob mutation in 1950 (3). The phenotype of mice that are homozygous for the ob mutation is characterized by hyperinsulinemia, hyperglucocorticoidemia, hyperglycemia, insulin resistance, altered central nervous system activity, hyperphagia, reduced metabolic rate of brown adipose tissue, and a massive increase in white adipose tissue (reviewed in refs. 1, 2, and 4). As many of these phenotypic characteristics, including adipocyte hypertrophy and hyperplasia, develop in ob/ob mice before the onset of hyperphagia, it is evident that initiation of obesity in this model does not result solely from elevated energy intake. Indeed, obesity occurs even in ob/ob mice pair-fed to the level of energy intake of their lean littermates. Parabiosis experiments with ob/ob mice indicate that a factor circulating in the bloodstream of lean (ob/ob littermates) or db/db mice reverses the effects of the ob mutation (5, 6). More recently, Friedman's group (7) mapped the ob locus, cloned the ob gene, and showed that the gene is primarily, if not exclusively, expressed in adipose tissue of normal rodents. The deduced amino acid sequence of the gene product, referred to hereafter as leptin (8), specifies a 16-kDa protein that possesses a leader sequence and presumably is secreted. The apparent secretory nature of leptin is consistent with a large body of evidence linking the dysfunction (or dysregulation) in ob/ob mice, including feeding behavior, to the hypothalamus (2, 4). As the effects of mutations at the ob locus are pleiotropic, it is possible that leptin also intera...

Adipose tissue has long been known to house the largest energy reserves in the animal body. Recent research indicates that in addition to this role, the adipocyte functions as a global regulator of energy metabolism. Adipose tissue is exquisitely sensitive to a variety of endocrine and paracrine signals, e.g. insulin, glucagon, glucocorticoids, and tumor necrosis factor (TNF), that combine to control both the secretion of other regulatory factors and the recruitment and differentiation of new adipocytes. The process of adipocyte differentiation is controlled by a cascade of transcription factors, most notably those of the C/EBP and PPAR families, which combine to regulate each other and to control the expression of adipocyte-specific genes. One such gene, i.e. the obese gene, was recently identified and found to encode a hormone, referred to as leptin, that plays a major role in the regulation of energy intake and expenditure. The hormonal and transcriptional control of adipocyte differentiation is discussed, as is the role of leptin and other factors secreted by the adipocyte that participate in the regulation of adipose homeostasis.

ABSTRACT

Like other adipocyte genes that are transcriptionally activated by CCAAT/enhancer binding protein a (C/EBPa) during preadipocyte differentiation, expression of the mouse obese (ob) gene is immediately preceded by the expression of C/EBPa. While the 5' flanking region of the mouse ob gene contains several consensus C/EBP binding sites, only one of these sites appears to be functional. DNase I cleavage inhibition patterns (footprinting) of the ob gene promoter revealed that recombinant C/EBPa, as well as a nuclear factor present in fully differentiated 3T3-L1 adipocytes, but present at a much lower level in preadipocytes, protects the same region between nucleotides -58 and -42 relative to the transcriptional start site. Electrophoretic mobility-shift analysis using nuclear extracts from adipose tissue or 3T3-L1 adipocytes and an oligonucleotide probe corresponding to a consensus C/EBP binding site at nucleotides -55 to -47 generated a specific protein-oligonucleotide complex that was supershifted by antibody against C/EBPa. Probes corresponding to two Since the discovery of the obese (ob) mutation (1), the ob/ob mouse has been widely investigated as a model of genetic obesity and type II diabetes. Recessive mutations at the ob locus produce a phenotype characterized by obesity, hyperglycemia, hyperinsulinemia, hyperphagia, and lowered basal metabolic rate (reviewed in refs. 2-5). The obese gene, which was recently identified by positional cloning (6), encodes a 16-kDa secretory protein, referred to as leptin (or OB protein), that is expressed only in white and brown adipose tissue (6-9). Mutation of the obese gene in ob/ob mice gives rise to premature termination of translation of the obese message, producing a nonfunctional protein (6).The db/db mouse exhibits an obese phenotype similar to that of the ob/ob mouse (10). Parabiosis experiments with db/db, ob/ob, and lean mice showed that a blood-borne factor produced by lean mice and overproduced by db/db mice can reverse the effects of the obese mutation in ob/ob mice (10,11). This led to the hypothesis that ob/ob mice lacked the factor, while db/db mice lacked the ability to respond to this factor. Consistent with this hypothesis, the effects of administration of recombinant leptin to ob/ob and db/db mice areThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. strikingly different. Administration of recombinant leptin causes a dramatic decrease in food intake and a profound loss of body weight by ob/ob mice but is without effect on db/db mice (12-15). Thus, db/db mice appear to lack either the receptor or another component of the signal transmission system for leptin. As might be expected, treatment of lean mice with leptin also reduces food intake and body weight (12,13,16).Relevant to transmission of the leptin signal is the observation that ob/ob mice overexpress the obese message (6, 7), while ...