Functional STAT 1 and 3 signaling by the leptin receptor (OB-R); reduced expression of the rat fatty leptin receptor in transfected cells.

The leptin receptor (OB-R) bears homology to members of the class I cytokine receptor family. We demonstrate that leptin binding to OB-R stimulates formation of STAT-1 and STAT-3 complexes, thereby defining transcriptional motifs for genes that are under leptin control. Transfected fa OB-R bound leptin with equal affinity to that of wild type OB-R. fa OB-R abundance was about 7 fold reduced compared to control cells. Surprisingly, the low level of fa OB-R is fully capable of activating the STAT signal transduction pathway. We discuss plausible explanations for the obese phenotype in Zucker fatty rats.

Cloning of a novel member of the low-density lipoprotein receptor family.

A gene encoding a novel transmembrane protein was identified by DNA sequence analysis within the insulin-dependent diabetes mellitus (IDDM) locus IDDM4 on chromosome 11q13. Based on its chromosomal position, this gene is a candidate for conferring susceptibility to diabetes. The gene, termed low-density lipoprotein receptor related protein 5 (LRP5), encodes a protein of 1615 amino acids that contains conserved modules which are characteristic of the low-density lipoprotein (LDL) receptor family. These modules include a putative signal peptide for protein export, four epidermal growth factor (EGF) repeats with associated spacer domains, three LDL-receptor (LDLR) repeats, a single transmembrane spanning domain, and a cytoplasmic domain. The encoded protein has a unique organization of EGF and LDLR repeats; therefore, LRP5 likely represents a new category of the LDLR family. Both human and mouse LRP5 cDNAs have been isolated and the encoded mature proteins are 95% identical, indicating a high degree of evolutionary conservation.

Characterisation of a chimeric hD3/D2 dopamine receptor expressed in CHO cells.

The D2 dopamine receptor is known to be functionally coupled when expressed in CHO cells, whereas the effector systems for the D3 dopamine receptor remain unclear. A chimeric, human D3/D2 receptor (hD3/D2) was constructed containing the third intracellular loop region of the D2 receptor. CHO cells stably expressing the D2, D3, or hD3/D2 receptors were created and the pharmacology of the receptors was examined. The chimeric hD3/D2 receptor retained D3-like affinities for dopaminergic ligands. However, in contrast to the D2 receptor neither the D3 receptor nor the hD3/D2 receptor could functionally couple to the adenylate cyclase or arachidonic acid release mechanisms.

Differential expression of mRNA for leptin receptor isoforms in the rat brain.

Leptin plays an important role in the control of food intake and energy metabolism by interacting with its receptor (OB-R) in the brain. Several alternatively spliced isoforms of OB-R have been identified. To study the expression patterns and the potential biological function of these OB-Rs in the brain, the distribution of mRNA encoding OB-R isoforms was examined by in situ hybridization. In agreement with previous studies, strong signals for OB-R mRNA were detected in the hypothalamus, thalamus and choroid plexus. In addition, intense signals were observed in several other brain areas including piriform cortex, granule cell layer of the cerebellum and substantia nigra. With isoform-specific probes, a differential expression pattern of OB-Rs was revealed: OB-Ra and OB-Rb, but not OB-Rc and OB-Rf, are abundantly expressed in the hypothalamus, whereas OB-Ra, OB-Rc and OB-Rf, but not OB-Rb, are significantly expressed in the choroid plexus. The preferential expression of OB-Rb in the hypothalamus is in support of its role in mediating the satiety effect of leptin. The co-expression of OB-Ra with OB-Rb in the hypothalamus may suggest a possible interaction between the two isoforms. Finally, the detection of OB-R mRNA in a number of other brain regions may indicate the involvement of leptin in additional as yet undefined physiological functions.

The amino terminal domain of the glucagon-like peptide-1 receptor is a critical determinant of subtype specificity.

Glucagon-like peptide-1 (GLP-1) and glucagon are peptide hormones involved in glucose homeostasis. The ligands are closely related (48% identical) and bind with different affinities to distinct, although highly homologous (47% identical), G protein coupled receptors on the surface of cells. By these criteria, glucagon and GLP-1 receptors can be considered receptor subtypes. A series of chimeric receptors in which 4-6 amino acids in the N-terminal extracellular domain of the human GLP-1 receptor were replaced with the analogous region of the human glucagon receptor were constructed and expressed in COS-7 cells. One of these chimeric receptors, C29-32 displays a 7 to 10-fold decrease in affinity for GLP-1 and the GLP-1 antagonist exendin 9-39 amide and a concomitant 7 to 9-fold increase in its affinity for glucagon. This change in affinity results in a 50-fold decrease in the selectivity of this receptor for GLP-1 over glucagon. Thus, the substitution of as few as four residues of the GLP-1 receptor profoundly affects its selectivity for the homologous peptide agonists GLP-1 and glucagon. These results suggest the extracellular N terminal domain of the GLP-1 receptor harbours molecular determinants for both agonist binding affinity and selectivity.

Cloning and functional expression of a human glucagon-like peptide-1 receptor.

A human glucagon-like 1 peptide receptor has been cloned from the gastric tumor cell line HGT-1. The cDNA clone encodes a protein of 463 amino acids and is a member of the superfamily of seven transmembrane domain G protein coupled receptors. Transfection of the human GLP-1 receptor into COS-7 cells confers upon them high affinity binding for [125I] GLP-1 (7-36) amide. In membranes prepared from COS-7 cells transfected with the human GLP-1 receptor, the binding of [125I] GLP-1 (7-36) amide is inhibited with the rank order of potency GLP-1 (7-36) amide > glucagon > secretin, characteristic of a GLP-1 receptor. The human GLP-1 receptor is functionally coupled to increases in intracellular cAMP in these cells: incubation of COS-7 cells expressing the human GLP-1 receptor with GLP-1 (7-36) amide gives rise to a 4-fold increase in cyclic AMP over basal levels, with an EC50 of 25pM. Glucagon is also a full agonist but is 200-fold less potent than GLP-1 (7-36) amide in stimulating the human GLP-1 receptor.

Cloning and expression of a human glucagon receptor.

A human glucagon receptor has been cloned from human liver tissue. The 1578-bp cDNA clone encodes a protein of 477 amino acids with 82% identity to the rat glucagon receptor. The predicted secondary structure and homology to known proteins places this receptor within the superfamily of seven transmembrane domain G protein coupled receptors. Transfection of the human glucagon receptor into COS-7 cells confers upon them high affinity binding for [125I] glucagon. In membranes prepared from COS-7 cells transfected with the human glucagon receptor, the binding of [125I] glucagon is inhibited with the rank order of potency glucagon > oxyntomodulin > glucagon-like peptide 1 (7-36) amide >> glucagon-like peptide 2 = gastric inhibitory peptide = secretin.

Molecular cloning and pharmacological characterization of the canine B1 and B2 bradykinin receptors.

The dog is a valuable animal model in the study of the physiological role of both the B1 and B2 bradykinin receptors. To more thoroughly characterize the pharmacological properties of the canine kinin receptors we isolated the cDNA sequence encoding the B1 and B2 bradykinin receptor subtypes and overexpressed them in Chinese hamster ovary (CHO) cells. The cDNA sequence of the canine B1 bradykinin receptor encodes a protein comprised of 350 amino acids that is 76% identical to the human B1 bradykinin receptor. The cDNA sequence of the canine B2 bradykinin receptor encodes a protein of 392 amino acids that is 81% identical to the human B2 bradykinin receptor. The amino acid sequence of the canine B1 and B2 receptors are 35% identical. Pharmacological studies of the cloned receptors revealed that the agonist affinity of the dog B1 receptor is similar to the rodent B1 receptors, and differs from the human form in that there is no preference for the presence of the N-terminal Lys residue of [des-Arg10]Lys-bradykinin. Significantly, the B1 receptor antagonist [des-Arg9,Leu8]BK behaves as partial agonist on the cloned dog B1 receptor. The dog B2 receptor exhibits the 'classical' pharmacological properties of this receptor subtype.

Isolation and characterization of LRP6, a novel member of the low density lipoprotein receptor gene family.

A novel member of the low density lipoprotein receptor (LDLR) gene family has been identified and characterized. This gene, termed LDL receptor-related protein 6 (LRP6), encodes a transmembrane protein which has 71% identity and is structurally similar to the protein encoded by LRP5, a proposed candidate gene for type 1 diabetes located on human chromosome 11q13. LRP6 maps to human chromosome 12p11-p13. Mouse Lrp6 encodes a protein that has 98% identity to human LRP6 and maps to chromosome 6. Unlike other members of the LDLR family, LRP6 and LRP5 display a unique pattern of four epidermal growth factor (EGF) and three LDLR repeats in the extracellular domain. The cytoplasmic domain of LRP6 is not similar to other members of the LDLR family, while comparison with LRP5 reveals proline-rich motifs that may mediate protein-protein interactions. Thus, it is likely that LRP6 and LRP5 comprise a new class of the LDLR family.

The sequence and gene characterization of a 400-kb candidate region for IDDM4 on chromosome 11q13.

Type 1 diabetes is a complex disorder with interaction of both genetic and environmental factors. One of the loci, IDDM4, has been mapped to chromosome 11q13, with evidence of association to two markers, D11S1917 and H0570polyA. To identify putative candidate genes for IDDM4, we have constructed a 400-kb clone contig in this region and sequenced the clones. We have also sequenced the orthologous DNA from mouse. Previously, we identified a cDNA for the low-density lipoprotein receptor-related protein 5 gene (LRP5) 3 kb distal to H0570polyA. We have now determined the exon-intron structure of this gene. Detailed sequence analysis has identified a further three genes in this region: the CGI-85 gene (previously identified by W.-C. Lin) and two novel genes, C11orf24 and C11orf23. The C11orf24 gene has no known similarity to other genes, and its function is unknown. C11orf23 has similarity to the SIT4 (sporulation-induced transcript 4)-associated protein (SAP) family of yeast proteins, which are involved in regulation of the cell cycle. The full-length C11orf23 cDNA is the first mammalian orthologue of the yeast SAP family to be identified. Identification of these four genes in a 400-kb region of the IDDM4 region underpins our strategy to identify the IDDM4 locus.

Down-regulation of the expression of the obese gene by an antidiabetic thiazolidinedione in Zucker diabetic fatty rats and db/db mice.

Obese (ob) is a recently identified gene involved in the regulation of energy balance in the mouse. We report here that AD-5075, a potent thiazolidinedione which lowered plasma glucose and triglyceride in Zucker diabetic fatty (ZDF) rats and db/db mice, decreased the expression of the ob gene in these animal models of obesity and non-insulin-dependent diabetes mellitus. The level of adipose ob mRNA in ZDF rats was 3-fold greater than that detected in the Zucker lean littermates. Chronic treatment with AD-5075 elicited a 67 and 70% reduction of ob mRNA in ZDF and control lean rats, respectively. Furthermore, the amount of adipose ob mRNA in db/db mice was 7 times higher than that detected in lean littermates. Treatment of db/db mice with AD-5075 resulted in a 78% reduction of the level of ob mRNA with parallel changes in circulating level of the ob gene product, leptin. The reduction of the ob mRNA in the Zucker lean rats was accompanied by significantly greater food intake and weight gain. However, in ZDF rats and db/db mice, there was profound increase in body weight without hyperphagia. The results demonstrate that the expression of the ob gene is up-regulated in these two rodent models of diabetes compared to their lean counterparts and that such overexpression is attenuated by treatment with an agent that improves insulin sensitivity and glucose homeostasis in vivo.