Mutations in the Human Leptin and Leptin Receptor Genes as Models of Serum Leptin Receptor Regulation Najiba Lahlou, 1 Tarik Issad, 2 Yves Lebouc, 3 Jean-Claude Carel, 4 Luc Camoin, 2 Marc Roger, 1 and Jean Girard 1,5 A part of serum Ob leptin, an adipocyte-secreted peptide, is bound to a soluble Ob receptor (sObR). Immunoreactive sObR was measured in 125 lean or obese control subjects (group 1), 18 individuals with a mutation in the leptin gene impairing leptin secre- tion (group 2), and 10 individuals with a mutation in the ObR gene, leading to production of a truncated ObR not anchored to cell membranes (group 3). In group 1, sObR levels were negatively correlated with age and BMI in children and with BMI in adults. sObR levels were also negatively correlated with leptin levels. Leptin binding activity and sObR levels coe- luted in gel-filtration chromatography. In group 2, sObR levels did not differ from those in lean control subjects and were not correlated with BMI. A single peak was detected in chromatographic fractions. In group 3, sObR levels were high and positively corre- lated with BMI. Immunoreactive sObR coeluted with leptin binding activity. These data demonstrate that leptin is not needed for ObR gene expression, and they suggest that leptin plays a role in receptor downregulation because sObR levels are negatively correlated with leptin levels and BMI in control sub- jects, whereas sObR levels are not depressed in obese leptin-deficient or leptin receptor– deficient individu- als. Diabetes 51:1980 –1985, 2002 L eptin exerts its biological activity through a mem- brane receptor (ObR) located in various tissues, including the brain (rev. in 1). The lack of leptin due to mutations in the leptin gene and the lack of functional leptin receptors result in early-onset obesity and various endocrine disturbances in both rodents and humans (2–5). In the leptin-deficient ob/ob mice as well as in human subjects harboring a mutation in the leptin gene that results in the absence of leptin production, parenteral administration of recombinant leptin reduces food intake and adiposity (6,7). These observations are consistent with an inhibitory role of leptin on food intake and fat mass development (rev. in 8). Serum leptin levels are strongly correlated with BMI or fat mass, as evidenced by a number of studies. That led to the concept of an apparent resistance to leptin in common obesity, since high leptin levels appear unable to reduce fat storage, most likely through a defect in leptin transport to the brain (9). In serum, leptin circulates mainly as a 16-kDa free peptide and also as a large protein complex (10). The leptin binding proteins were hypothesized to play a role in the availability of leptin to its target cells (11). A soluble form of the extracellular domain of the leptin receptor (sObR) accounted for the majority of the serum leptin binding activity (12,13). Very high levels of leptin binding activity have been reported by us in individuals who were either heterozygous or homozygous for a mutation in the leptin receptor gene responsible for the production of a truncated extracellular receptor not anchored to the cell membrane (14). On the other hand, leptin levels were undetectable or very low in individuals with a mutation in the leptin gene (3,4,15). Serum leptin binding activity has not been reported in these situations. Because leptin- and leptin receptor– deficient human subjects provide unique models which could, when com- pared to normal subjects, shed light on the regulatory mechanisms governing the production of the leptin recep- tor, we investigated the relationship between sObR levels, leptin levels, and BMI in three groups of individuals: subjects with a mutation in the leptin gene or in the ObR gene and lean and overweight control subjects. RESEARCH DESIGN AND METHODS Subjects. The subjects in group 1 were 125 individuals of normal weight or with common obesity: 31 girls and 23 boys aged 3–15 years, 37 female subjects aged 17–59 years, and 34 male subjects aged 15– 61 years (Table 1). All patients and/or their parents gave informed consent for this study, which was approved by the local ethical committee. The serum leptin receptor was also assayed in all serum chromatographic fractions from eight obese and four nonobese normal female subjects. In group 2, the subjects were 18 individuals from a consanguineous Turkish family with a mutation in the leptin gene, which impairs the normal processing of leptin through the secretory pathway (4). Of these 18 subjects, 4 (1 male and 3 female subjects) were homozygous for the mutation, 13 (6 male and 7 female) were heterozygous for the mutation, and 1 girl was homozygous for the wild-type gene (Table 2). Immunoreactive sObR was also measured in all chromatographic fractions from two homozygotes and two heterozygotes for the mutation. Group 3 consisted of 10 individuals from a consanguineous Kabylian family with a mutation in the leptin receptor gene leading to the production of a truncated receptor not anchored to the cell membrane (5,14). Of these 10 subjects, 3 female subjects were homozygous for the mutation, 3 female subjects were heterozygous, 1 female subject was homozygous for the wild-type gene, 2 male subjects were heterozygous, and 1 male subject had the wild-type gene (Table 3). Immunoreactive sObR was also measured in all chromatographic fractions from three homozygotes, five heterozygotes for the mutation, and the wild- From the 1 Hormone Biology, Saint-Vincent-de-Paul Hospital, Paris, France; 2 National Center for Scientific Research, Cochin Hospital, Paris, France; the 3 Laboratory for Hormone Investigations, Trousseau Hospital, Paris, France; 4 Pediatric Endocrinology, Saint-Vincent-de-Paul Hospital, Paris, France; and the 5 Cochin Institute for Molecular Genetics, Paris, France. Address correspondence and reprint requests to Dr. Najiba Lahlou, Biologie Hormonale, Ho ˆ pital Saint-Vincent-de-Paul, 82, avenue Denfert-Rochereau, 75014 Paris, France. E-mail: najiba.lahlou@svp.ap-hop-paris.fr. Received for publication 6 February 2002 and accepted in revised form 13 March 2002. CSF, cerebrospinal fluid. 1980 DIABETES, VOL. 51, JUNE 2002 Downloaded from http://diabetesjournals.org/diabetes/article-pdf/51/6/1980/651058/db0602001980.pdf by guest on 04 November 2022