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Context: Adiponectin is a highly abundant plasma protein synthesized nearly exclusively in adipose tissue from the ADIPOQ gene. It has excited intense interest because of robust correlation of its circulating levels with indices of insulin resistance (IR) and risk of type 2 diabetes, and their unusual inverse relationship with fat mass. It has been suggested that pharmacological strategies aimed at augmenting adiponectin levels or action may generate novel insulin-sensitizing drugs., Evidence Acquisition: Relevant publications were identified by searching PubMed, with secondary searches of their bibliographies., Evidence Synthesis: Rodent studies suggest that adiponectin exerts a direct insulin-sensitizing effect on the liver, consistent with a role in the pathogenesis of prevalent forms of IR and its sequelae. However, the complex higher-order structure of adiponectin and inconsistent reports regarding its putative receptors have complicated efforts to understand the mechanistic basis of this. No proof yet exists that adiponectin modulates insulin sensitivity in humans, and genetic, biochemical, and physiological evidence suggests that low adiponectin levels may be a consequence of IR with compensatory hyperinsulinemia. This suggests that there may be a bidirectional relationship between IR and hypoadiponectinemia in humans., Conclusions: The relationship between adiponectin and insulin action in humans is more complex than often suggested. Further investigation of the direction of causality in this relationship, allied to studies of the cellular mechanisms involved, will be central to improving understanding of the physiological role of this enigmatic protein, and to efforts to exploit it for therapeutic benefit.

Growing evidence suggests the tachykinin neurokinin B (NKB) may modulate gonadotrophin secretion and play a role in sex-steroid feedback within the reproductive axis. NKB signalling has recently been identified as being necessary for normal human reproductive function, although the precise mechanisms underpinning this role remain to be established. We have used rodents to explore further the role of NKB within the reproductive axis. In particular, we have studied its interactions with kisspeptin, a neuropeptide essential for reproductive function in rodent and human with close anatomical links to NKB within the hypothalamus. Intraperitoneal administration of NKB (50 nmol) to male mice had no effect on circulating luteinsing hormone (LH) levels and, although i.p. kisspeptin (15 nmol) increased LH five-fold, co-administration of NKB and kisspeptin was indistinguishable from kisspeptin alone. Intracerebroventricular administration of NKB (10 nmol) to male mice also had no effect on LH levels, with 1 nmol kisspeptin i.c.v. significantly increasing LH compared to control (0.37 +/- 0.18 versus 5.11 +/- 0.28 ng/ml, respectively). Interestingly, i.c.v. co-administration of NKB and kisspeptin caused a significant increase in LH concentrations compared to kisspeptin alone (8.96 +/- 1.82 versus 5.11 +/- 0.28 ng/ml respectively). We used hypothalamic explants from rats to assess the effect of NKB on gonadotrpohin-releasing hormone (GnRH) secretion ex vivo. Doses of NKB up to 1000 nm failed to stimulate GnRH secretion, whereas 100 nm kisspeptin robustly increased GnRH secretion. Of note, co-administration of NKB with kisspeptin abrogated the effect of kisspeptin, producing no GnRH release above basal state. Finally, we analysed the expression of Tac2/Tacr3 (genes encoding NKB and NK3R, respectively) within the arcuate nucleus in different nutritional states. After a 48-h fast, the expression of both Tac2 and Tacr3 showed a significant increase, in contrast to levels of Kiss1 and Kiss1r mRNA, which remained unchanged. In male rodent models, NKB and kisspeptin have different effects upon gonadotrophin release and appear to interact in a complex manner.

The hypothalamus integrates multiple environmental and developmental cues relevant to reproductive function, serving to transduce these into a pulsatile output of gonadotropin-releasing hormone (GnRH). Although neuroanatomical and physiological studies have yielded key clues about the functional organisation of the so-called 'GnRH pulse generator', only in the last decade have the molecular components of the circuitry upstream from GnRH begun to be elucidated. A major contributor to this has been human genetics, through identification of mutations causing isolated hypogonadotropic hypogonadism (IHH) without developmental defects. The greatest success of this approach was the finding in 2003 that mutations of KISS1R cause IHH, producing a quantum leap in understanding of regulation of GnRH secretion, and energising the field. New evidence has now emerged that loss of function of neurokinin B (NKB) or its receptor, the neurokinin-3 receptor, produces IHH of similar severity to that caused by KISS1R mutations in humans. Preliminary evidence suggests that the role of NKB in reproductive function differs significantly between humans and rodents, posing challenges for future studies. We review the human genetics of NKB and its receptor, and discuss the future work required to elucidate their precise role in the regulation of human GnRH secretion., (Copyright 2010 S. Karger AG, Basel.)

The adipocyte-derived protein adiponectin is highly heritable and inversely associated with risk of type 2 diabetes mellitus (T2D) and coronary heart disease (CHD). We meta-analyzed 3 genome-wide association studies for circulating adiponectin levels (n = 8,531) and sought validation of the lead single nucleotide polymorphisms (SNPs) in 5 additional cohorts (n = 6,202). Five SNPs were genome-wide significant in their relationship with adiponectin (P< or =5x10(-8)). We then tested whether these 5 SNPs were associated with risk of T2D and CHD using a Bonferroni-corrected threshold of P< or =0.011 to declare statistical significance for these disease associations. SNPs at the adiponectin-encoding ADIPOQ locus demonstrated the strongest associations with adiponectin levels (P-combined = 9.2x10(-19) for lead SNP, rs266717, n = 14,733). A novel variant in the ARL15 (ADP-ribosylation factor-like 15) gene was associated with lower circulating levels of adiponectin (rs4311394-G, P-combined = 2.9x10(-8), n = 14,733). This same risk allele at ARL15 was also associated with a higher risk of CHD (odds ratio [OR] = 1.12, P = 8.5x10(-6), n = 22,421) more nominally, an increased risk of T2D (OR = 1.11, P = 3.2x10(-3), n = 10,128), and several metabolic traits. Expression studies in humans indicated that ARL15 is well-expressed in skeletal muscle. These findings identify a novel protein, ARL15, which influences circulating adiponectin levels and may impact upon CHD risk., Competing Interests: The authors have declared that no competing interests exist.

Context: The neurokinin B (NKB) receptor, encoded by TACR3, is widely expressed within the central nervous system, including hypothalamic nuclei involved in regulating GnRH release. We have recently reported two mutations in transmembrane segments of the receptor and a missense mutation in NKB in patients with normosmic isolated hypogonadotropic hypogonadism (nIHH)., Patients and Methods: We sequenced the TACR3 gene in a family in which three siblings had nIHH. The novel mutant receptor thus identified was studied in a heterologous expression system using calcium flux as the functional readout., Results: All affected siblings were homozygous for the His148Leu mutation, in the first extracellular loop of the NKB receptor. The His148Leu mutant receptor exhibited profoundly impaired signaling in response to NKB (EC(50) = 3 +/- 0.1 nm and >5 microm for wild-type and His148Leu, respectively). The location of the mutation in an extracellular part of the receptor led us also to test whether senktide, a synthetic NKB analog, may retain ability to stimulate the mutant receptor. However, the signaling activity of the His148Leu receptor in response to senktide was also severely impaired (EC(50) = 1 +/- 1 nm for wild-type and no significant response of His148Leu to 10 microm)., Conclusions: Homozygosity for the TACR3 His148Leu mutation leads to failure of sexual maturation in humans, whereas signaling by the mutant receptor in vitro in response to either NKB or senktide is severely impaired. These observations further strengthen the link between NKB, the NKB receptor, and regulation of human reproductive function.

Lipodystrophic syndromes are characterized by adipose tissue deficiency. Although rare, they are of considerable interest as they, like obesity, typically lead to ectopic lipid accumulation, dyslipidaemia and insulin resistant diabetes. In this paper we describe a female patient with partial lipodystrophy (affecting limb, femorogluteal and subcutaneous abdominal fat), white adipocytes with multiloculated lipid droplets and insulin-resistant diabetes, who was found to be homozygous for a premature truncation mutation in the lipid droplet protein cell death-inducing Dffa-like effector C (CIDEC) (E186X). The truncation disrupts the highly conserved CIDE-C domain and the mutant protein is mistargeted and fails to increase the lipid droplet size in transfected cells. In mice, Cidec deficiency also reduces fat mass and induces the formation of white adipocytes with multilocular lipid droplets, but in contrast to our patient, Cidec null mice are protected against diet-induced obesity and insulin resistance. In addition to describing a novel autosomal recessive form of familial partial lipodystrophy, these observations also suggest that CIDEC is required for unilocular lipid droplet formation and optimal energy storage in human fat.

Tre-2, BUB2, CDC16, 1 domain family member 4 (TBC1D4) (AS160) is a Rab-GTPase activating protein implicated in insulin-stimulated glucose transporter 4 (GLUT4) translocation in adipocytes and myotubes. To determine whether loss-of-function mutations in TBC1D4 might impair GLUT4 translocation and cause insulin resistance in humans, we screened the coding regions of this gene in 156 severely insulin-resistant patients. A female presenting at age 11 years with acanthosis nigricans and extreme postprandial hyperinsulinemia was heterozygous for a premature stop mutation (R363X) in TBC1D4. After demonstrating reduced expression of wild-type TBC1D4 protein and expression of the truncated protein in lymphocytes from the proband, we further characterized the biological effects of the truncated protein in 3T3L1 adipocytes. Prematurely truncated TBC1D4 protein tended to increase basal cell membrane GLUT4 levels (P = 0.053) and significantly reduced insulin-stimulated GLUT4 cell membrane translocation (P < 0.05). When coexpressed with wild-type TBC1D4, the truncated protein dimerized with full-length TBC1D4, suggesting that the heterozygous truncated variant might interfere with its wild-type counterpart in a dominant negative fashion. Two overweight family members with the mutation also manifested normal fasting glucose and insulin levels but disproportionately elevated insulin levels following an oral glucose challenge. This family provides unique genetic evidence of TBC1D4 involvement in human insulin action.

The timely secretion of gonadal sex steroids is essential for the initiation of puberty, the postpubertal maintenance of secondary sexual characteristics and the normal perinatal development of male external genitalia. Normal gonadal steroid production requires the actions of the pituitary-derived gonadotropins, luteinizing hormone and follicle-stimulating hormone. We report four human pedigrees with severe congenital gonadotropin deficiency and pubertal failure in which all affected individuals are homozygous for loss-of-function mutations in TAC3 (encoding Neurokinin B) or its receptor TACR3 (encoding NK3R). Neurokinin B, a member of the substance P-related tachykinin family, is known to be highly expressed in hypothalamic neurons that also express kisspeptin, a recently identified regulator of gonadotropin-releasing hormone secretion. These findings implicate Neurokinin B as a critical central regulator of human gonadal function and suggest new approaches to the pharmacological control of human reproduction and sex hormone-related diseases.

Metabolic dyslipidemia is characterized by high circulating triglyceride (TG) and low HDL cholesterol levels and is frequently accompanied by hepatic steatosis. Increased hepatic lipogenesis contributes to both of these problems. Because insulin fails to suppress gluconeogenesis but continues to stimulate lipogenesis in both obese and lipodystrophic insulin-resistant mice, it has been proposed that a selective postreceptor defect in hepatic insulin action is central to the pathogenesis of fatty liver and hypertriglyceridemia in these mice. Here we show that humans with generalized insulin resistance caused by either mutations in the insulin receptor gene or inhibitory antibodies specific for the insulin receptor uniformly exhibited low serum TG and normal HDL cholesterol levels. This was due at least in part to surprisingly low rates of de novo lipogenesis and was associated with low liver fat content and the production of TG-depleted VLDL cholesterol particles. In contrast, humans with a selective postreceptor defect in AKT2 manifest increased lipogenesis, elevated liver fat content, TG-enriched VLDL, hypertriglyceridemia, and low HDL cholesterol levels. People with lipodystrophy, a disorder characterized by particularly severe insulin resistance and dyslipidemia, demonstrated similar abnormalities. Collectively these data from humans with molecularly characterized forms of insulin resistance suggest that partial postreceptor hepatic insulin resistance is a key element in the development of metabolic dyslipidemia and hepatic steatosis.

Context: Lipodystrophy is a heterogeneous condition characterized by an inherited or acquired deficiency in the number of adipocytes required for the storage of energy as triglycerides. Acquired lipodystrophy is frequently associated with other autoimmune disorders. One well-studied form is characterized by the selective loss of upper body fat in association with activation of the alternative complement pathway by C3 nephritic factor, low complement factor C3, and mesangiocapillary glomerulonephritis., Objective: We now describe an immunologically distinct form of acquired generalized lipodystrophy, with evidence of activation of the classical complement pathway (low C4) and autoimmune hepatitis. Patients and Research Design: Three unrelated patients with acquired lipodystrophy and low complement C4 levels are described. In vitro analysis of the complement pathway was undertaken to determine the reason for the low C4 complement levels. Biopsies were obtained from liver, bone marrow, and adipose tissue for histological analysis., Results: All three patients manifested near-total lipodystrophy, chronic hepatitis with autoimmune features, and low C4 complement levels. Additional autoimmune diseases, including severe hemolytic anemia, autoimmune thyroid disease, and polyneuropathy, were variably present. Detailed studies of complement pathways suggested constitutive classical pathway activation., Conclusions: Although the previously described syndrome, which typically results in a cephalad pattern of partial lipodystrophy, results from activation of the alternative complement pathway, this form, in which lipodystrophy is generalized, is associated with activation of the classical pathway. Future therapeutic approaches to these disorders may benefit from being tailored to their distinct immunopathogenesis.

Anti-insulin antibodies have been described in two contexts: in insulin-naive individuals (so-called 'insulin autoimmune syndrome') and in patients with insulin-treated diabetes, in whom antibodies are rarely of clinical significance. We report the case of an 68-year-old woman who exhibited a local allergic reaction to subcutaneous insulin followed by severe insulin resistance, evidenced by poor glycaemic control despite treatment with > 3.5 U/kg of insulin per day. She was found to have circulating polyclonal anti-insulin antibodies of the IgG subtype and responded clinically to a course of plasma exchange and immunosuppression with mycophenolate mofetil and, subsequently, intravenous immunoglobulin. Falling titres of antibodies on this regimen correlated with improved glycaemic control. This case suggests that clinicians should be alert to the possibility of insulin resistance due to anti-insulin antibodies and that immunosuppression in this situation may be a valuable therapeutic option.

Although mice lacking the architectural DNA binding factor HMGA1 are diabetic and express very low levels of the insulin receptor, they have increased insulin sensitivity. A study in BMC Biology now suggests that changes in circulating retinol binding protein partly account for this paradox.

Context: Despite the very high prevalence of the polycystic ovary syndrome (PCOS), the underlying pathogenetic mechanism has remained obscure., Objective: To determine the cause of two sisters' PCOS associated with severe insulin resistance., Design: Clinical case report. Methods Two sisters who presented with hyperandrogenism and menstrual disorders in the context of PCOS, and were subsequently found to be severely insulin resistant. Physical examination revealed muscular hypertrophy with a paucity of fat in the extremities, trunk and gluteal regions, in spite of excess fat deposits in the face, neck and dorsocervical region. Known genes involved in familial partial lipodystrophy were screened. At the same time, metformin (1700 mg/day) was commenced. After 2-3 years of uninterrupted therapy, lack of clinical improvement led to the introduction of pioglitazone (30 mg/day)., Results: Both sisters were found to be heterozygous for the R482Q mutation in the lamin A/C gene (LMNA) gene, establishing the definitive diagnosis as Dunnigan-type familial partial lipodystrophy complicated by severe insulin resistance and secondary PCOS. Treatment with pioglitazone resulted in progressive amelioration of insulin resistance, hyperinsulinaemia and hyperandrogenaemia. Menses also improved, with restoration of a eumenorrhoeic pattern, and the framework of ultrasound PCO was in complete remission., Conclusions: Assessment of insulin sensitivity and adipose tissue topography should be a key part of the initial evaluation of patients with PCOS. Identifying such forms of PCOS with monogenic insulin resistance as the primary pathogenic abnormality may have practical implications for therapy, since they respond to thiazolidinediones, but not to metformin.

Objective: Berardinelli-Seip congenital lipodystrophy type 2 (BSCL2) is a recessive disorder featuring near complete absence of adipose tissue. Remarkably, although the causative gene, BSCL2, has been known for several years, its molecular function and its role in adipose tissue development have not been elucidated. Therefore, we examined whether BSCL2 is involved in the regulation of adipocyte differentiation and the mechanism whereby pathogenic mutations in BSCL2 cause lipodystrophy., Research Design and Methods: Following the characterization of BSCL2 expression in developing adipocytes, C3H10T1/2 mesenchymal stem cells were generated in which BSCL2 expression was knocked down using short hairpin RNA (shRNA). These cells were used to investigate whether BSCL2 is required for adipogenesis. BSCL2 constructs harboring pathogenic mutations known to cause lipodystrophy were also generated and characterized., Results: BSCL2 expression was strongly induced during adipocyte differentiation, and the induction of BSCL2 expression was essential for adipogenesis to occur. The initial induction of key adipogenic transcription factors, including peroxisome proliferator-activated receptor (PPAR)gamma and CAAT/enhancer-binding protein-alpha, was preserved in cells lacking BSCL2. However, the expression of these critical factors was not sustained, suggesting that the activity of PPARgamma was impaired. Moreover, expression of key genes mediating triglyceride synthesis, including AGPAT2, lipin 1, and DGAT2, was persistently reduced and lipid accumulation was inhibited. Analysis of pathogenic missense mutants of BSCL2 revealed that the amino acid substitution A212P causes aberrant targeting of BSCL2 within the cell, suggesting that subcellular localization of BSCL2 may be critical to its function., Conclusions: This study demonstrates that BSCL2 is an essential, cell-autonomous regulator of adipogenesis.

Context: Berardinelli-Seip congenital lipodystrophy (BSCL) is a rare recessive disease characterized by near absence of adipose tissue, resulting in severe dyslipidemia and insulin resistance. In most reported cases, BSCL is due to alterations in either seipin, of unknown function, or 1-acylglycerol-3-phosphate acyltransferase-beta (AGPAT2), which catalyzes the formation of phosphatidic acid., Objective: We sought to determine the genetic origin of the unexplained cases of BSCL. We thus sequenced CAV1, encoding caveolin-1, as a candidate gene involved in insulin signaling and lipid homeostasis. CAV1 is a key structural component of plasma membrane caveolae, and Cav1-deficient mice display progressive loss of adipose tissue and insulin resistance., Design: We undertook phenotyping studies and molecular screening of CAV1 in four patients with BSCL with no mutation in the genes encoding either seipin or AGPAT2., Results: A homozygous nonsense mutation (p.Glu38X) was identified in CAV1 in a patient with BSCL born from a consanguineous union. This mutation affects both the alpha- and beta-CAV1 isoforms and ablates CAV1 expression in skin fibroblasts. Detailed magnetic resonance imaging of the proband confirmed near total absence of both sc and visceral adipose tissue, with only vestigial amounts in the dorsal sc regions. In keeping with the lack of adipose tissue, the proband was also severely insulin resistant and dyslipidemic. In addition, the proband had mild hypocalcemia likely due to vitamin D resistance., Conclusions: These findings identify CAV1 as a new BSCL-related gene and support a critical role for caveolins in human adipocyte function.

Fundamental processes influencing human growth can be revealed by studying extreme short stature. Using genetic linkage analysis, we find that biallelic loss-of-function mutations in the centrosomal pericentrin (PCNT) gene on chromosome 21q22.3 cause microcephalic osteodysplastic primordial dwarfism type II (MOPD II) in 25 patients. Adults with this rare inherited condition have an average height of 100 centimeters and a brain size comparable to that of a 3-month-old baby, but are of near-normal intelligence. Absence of PCNT results in disorganized mitotic spindles and missegregation of chromosomes. Mutations in related genes are known to cause primary microcephaly (MCPH1, CDK5RAP2, ASPM, and CENPJ).

The burden of type 2 diabetes and its associated premature morbidity and mortality is rapidly growing, and the need for novel efficacious treatments is pressing. We report here that serotonin 2C receptor (5-HT(2C)R) agonists, typically investigated for their anorectic properties, significantly improve glucose tolerance and reduce plasma insulin in murine models of obesity and type 2 diabetes. Importantly, 5-HT(2C)R agonist-induced improvements in glucose homeostasis occurred at concentrations of agonist that had no effect on ingestive behavior, energy expenditure, locomotor activity, body weight, or fat mass. We determined that this primary effect on glucose homeostasis requires downstream activation of melanocortin-4 receptors (MC4Rs), but not MC3Rs. These findings suggest that pharmacological targeting of 5-HT(2C)Rs may enhance glucose tolerance independently of alterations in body weight and that this may prove an effective and mechanistically novel strategy in the treatment of type 2 diabetes.

Severe insulin resistance resulting from known or putative genetic defects affecting the insulin receptor or post-insulin receptor signalling represents a clinical spectrum ranging from Donohue's and Rabson-Mendenhall syndrome, where the genetic defect is identified, through to the milder phenotype of type A insulin resistance, where a genetic defect can only be detected in around 10% of cases. Paradoxically, subjects with these conditions may present with hypoglycaemia due to mismatch of post-prandial glucose excursion and compensatory hyperinsulinaemia. Ultimately, treatment with insulin and insulin sensitisers will be unsuccessful and subjects may succumb to diabetes or its complications. Recombinant human IGF-I alone or combined with its binding protein (IGFBP-3) provides an alternative therapy as IGF-I receptor shares structural and functional homology with the insulin receptor and recombinant human insulin-like growth factor I (rhIGF-I) therapy could improve glucose disposal by signalling through the IGF-I receptor, whilst reducing the adverse effects of high insulin concentrations. There are also data which indicate that IGF-I signalling through the IGF-I receptor on the pancreatic beta-cell may be important in maintaining insulin secretion. Pilot studies confirmed that rhIGF-I could reduce glucose and insulin levels in subjects with type A insulin resistance and those with Rabson-Mendenhall syndrome with sustained beneficial effects on HbA1c. Continued study has confirmed efficacy of rhIGF-I when combined with IGFBP-3 in the treatment of Donohue's and type A insulin resistance subjects. Observations that IGF-I treatment can improve C-peptide levels in these subjects may indicate that it might be more valuable as a first line intervention to preserve beta-cell function, rather than its current use as a medication of last resort in subjects where all other therapies have failed.

Total plasma adiponectin and high-molecular weight (HMW) polymeric adiponectin are strongly positively correlated with insulin sensitivity. However, we have recently reported paradoxical hyperadiponectinemia in patients with severe insulin resistance due to genetically defective insulin receptors. This implies either that the insulin receptor has a critical physiological role in controlling adiponectin production and/or clearance or that constitutive insulin receptor dysfunction influences adiponectin levels through developmental effects. The aim of the current study was to distinguish between these possibilities using a human model of reversible antibody-mediated insulin receptor dysfunction and to refine the previous observations by determining adiponectin complex distribution. Cross-sectional and longitudinal determination of fasting plasma adiponectin and adiponectin complex distribution was undertaken in patients with extreme insulin resistance due to insulin receptor mutations, anti-insulin receptor antibodies (type B insulin resistance), or an undefined cause. Despite extreme insulin resistance, patients with type B insulin resistance (all women; mean age 42 years [range 12-54]) had dramatically elevated total plasma adiponectin compared with the general population (mean 43.0 mg/l [range 31.3-54.2] vs. 8.9 mg/l [1.5-28.5 for BMI <25 kg/m(2)]), which was accounted for largely by HMW polymers. Hyperadiponectinemia resolved in parallel with reduction of insulin receptor antibodies and clinical resolution of insulin resistance. Although the well-established inverse relationship between plasma insulin and adiponectin levels may, in part, reflect positive effects of adiponectin on insulin sensitivity, these data suggest that the magnitude of the effect of insulin action on adiponectin levels may have been underestimated.

Aims/hypothesis: Adiponectin is an adipokine with insulin-sensitising and anti-atherogenic properties. Several reports suggest that genetic variants in the adiponectin gene are associated with circulating levels of adiponectin, insulin sensitivity and type 2 diabetes risk. Recently two receptors for adiponectin have been cloned. Genetic studies have yielded conflicting results on the role of these genes and type 2 diabetes predisposition. In this study we aimed to evaluate the potential role of genetic variation in these genes in syndromes of severe insulin resistance, type 2 diabetes and in related metabolic traits in UK Europid populations., Materials and Methods: Exons and splice junctions of the adiponectin receptor 1 and 2 genes (ADIPOR1; ADIPOR2) were sequenced in patients from our severe insulin resistance cohort (n=129). Subsequently, 24 polymorphisms were tested for association with type 2 diabetes in population-based type 2 diabetes case-control studies (n=2,127) and with quantitative traits in a population-based longitudinal study (n=1,721)., Results: No missense or nonsense mutations in ADIPOR1 and ADIPOR2 were detected in the cohort of patients with severe insulin resistance. None of the 24 polymorphisms (allele frequency 2.3-48.3%) tested was associated with type 2 diabetes in the case-control study. Similarly, none of the polymorphisms was associated with fasting plasma insulin, fasting and 2-h post-load plasma glucose, 30-min insulin increment or BMI., Conclusions/interpretation: Genetic variation in ADIPOR1 and ADIPOR2 is not a major cause of extreme insulin resistance in humans, nor does it contribute in a significant manner to type 2 diabetes risk and related traits in UK Europid populations.

Objective/patients: Rabson-Mendenhall syndrome (RMS) is a rare, recessively inherited disorder of extreme insulin resistance due to mutations in the insulin receptor gene. We have identified a pair of siblings with RMS attributable to compound heterozygosity for two insulin receptor mutations, one previously unreported, and have characterized the novel receptor mutation functionally., Measurements: Insulin receptor sequencing was performed to identify the mutations. Expression levels of the mature receptor were determined in lymphoblastoid cells from the affected subjects. Further studies of immortalized cell lines transfected with mutant and wild type (WT) receptors were undertaken to characterize the effects of the novel mutation on [(125)I]-labelled insulin binding, proreceptor processing and insulin-stimulated receptor autophosphorylation., Results: Sequencing of the insulin proreceptor coding sequence revealed both siblings to be compound heterozygotes for the missense mutations Arg209His and Gly359Ser in the mature insulin receptor. The former mutation has been described in homozygous form in Donohue syndrome, while the latter is novel. Insulin receptor expression in lymphoblastoid cell lines was present at only 10-30% of that in control cells; studies of immortalized cells transfected with mutant and WT receptors confirmed the reduced expression of the mutant. The degree of impairment of insulin binding and insulin-stimulated receptor autophosphorylation were commensurate with the decrease in expression of the mature receptor., Conclusions: Loss of function of the novel insulin receptor (INSR) G359S variant is largely accounted for by aberrant proreceptor processing rather than intrinsically impaired signal transduction by the mutant receptor.

Extreme forms of insulin resistance are a rare cause of type 2 diabetes. However, individuals with severe insulin resistance pose unique diagnostic and therapeutic challenges, and have often acted as 'experiments of nature' providing important novel information regarding endocrine physiology and mechanistic insights relevant to the study of more common disorders. Progress in understanding the molecular pathogenesis of such syndromes is also beginning to yield novel therapeutic options. Severe insulin resistance typically presents in 1 of 3 ways: (1) disordered glucose metabolism including both diabetes and/or paradoxical hypoglycaemia; (2) acanthosis nigricans, a velvety hyperpigmentation of axilliary and flexural skin often associated with skin tags; or (3) hyperandrogenism in girls (hirsutism, oligo-/amenorrhoea and polycystic ovaries). Lipodystrophy is a major cause of severe insulin resistance and needs to be looked for very carefully, particularly in the patients with significant dyslipidaemia and fatty liver. Specific treatments are now available for some forms of severe insulin resistance; for example, leptin replacement in patients with generalized lipodystrophy. In the absence of a specific diagnosis and therapy, metformin is a useful insulin sensitizer and should be used in conjunction with aggressive diet and exercise interventions.

Context: Adiponectin has been suggested to play a role in the etiopathogenesis of at least some forms of insulin resistance, in part based on a strong correlation between plasma levels of adiponectin and measures of insulin sensitivity., Objective: The objective of the study was to establish whether this relationship is maintained at extreme levels of insulin resistance., Design/setting: This was a cross-sectional study in a university teaching hospital of subjects recruited from the United Kingdom and the United States., Participants: Participants included 75 subjects with a range of syndromes of severe insulin resistance and 872 nondiabetic controls., Outcome Measures: Fasting plasma insulin, adiponectin, and leptin were measured., Results: Unexpectedly, subjects with mutations in the insulin receptor, despite having the most severe degree of insulin resistance, had elevated plasma adiponectin [median 24.4 mg/liter; range 6.6-36.6 (normal adult range for body mass index 20 kg/m(2) = 3-19 mg/liter)], whereas all other subjects had low adiponectin levels (median 2.0 mg/liter; range 0.12-11.2). Plasma leptin in all but one subject with an insulin receptoropathy was low or undetectable [median 0.5 ng/ml; range 0-16: normal adult range for body mass index of < 25 kg/m(2) = 2.4-24.4 (female) and 0.4-8.3 ng/ml (male)]., Conclusions: We conclude that the relationship between plasma adiponectin and insulin sensitivity is complex and dependent on the precise etiology of defective insulin action and that the combination of high plasma adiponectin with low leptin may have clinical utility in patients with severe insulin resistance as a marker of the presence of a genetic defect in the insulin receptor.

The nuclear receptor family of PPARs was named for the ability of the original member to induce hepatic peroxisome proliferation in mice in response to xenobiotic stimuli. However, studies on the action and structure of the 3 human PPAR isotypes (PPARalpha, PPARdelta, and PPARgamma) suggest that these moieties are intimately involved in nutrient sensing and the regulation of carbohydrate and lipid metabolism. PPARalpha and PPARdelta appear primarily to stimulate oxidative lipid metabolism, while PPARgamma is principally involved in the cellular assimilation of lipids via anabolic pathways. Our understanding of the functions of PPARgamma in humans has been increased by the clinical use of potent agonists and by the discovery of both rare and severely deleterious dominant-negative mutations leading to a stereotyped syndrome of partial lipodystrophy and severe insulin resistance, as well as more common sequence variants with a much smaller impact on receptor function. These may nevertheless have much greater significance for the public health burden of metabolic disease. This Review will focus on the role of PPARgamma in human physiology, with specific reference to clinical pharmacological studies, and analysis of PPARG gene variants in the abnormal lipid and carbohydrate metabolism of the metabolic syndrome.

Several missense mutations in the ligand-binding domain of human peroxisome proliferator-activated receptor (PPAR)gamma have been described in subjects with dominantly inherited severe insulin resistance associated with partial lipodystrophy, hypertension, and dyslipidemia. These mutant receptors behave as dominant-negative inhibitors of PPARgamma signaling when studied in transfected cells. The extent to which such dominant-negative effects extend to signaling through other coexpressed PPAR isoforms has not been evaluated. To examine these issues further, we have created a PPARalpha mutant harboring twin substitutions, Leu459Ala and Glu462Ala, within the ligand binding domain (PPARalpha(mut)), examined its signaling properties, and compared the effects of dominant-negative PPARalpha and PPARgamma mutants on basal and ligand-induced gene transcription in adipocytes and hepatocytes. PPARalpha(mut) was transcriptionally inactive, repressed basal activity from a PPAR response element-containing promoter, inhibited the coactivator function of cotransfected PPAR-gamma coactivator 1alpha, and strongly inhibited the transcriptional response to cotransfected wild-type receptor. In contrast to PPARgamma, wild-type PPARalpha failed to recruit the transcriptional corepressors NCoR and SMRT. However, PPARalpha(mut) avidly recruited these corepressors in a ligand-dissociable manner. In hepatocytes and adipocytes, both PPARalpha(mut) and the corresponding PPARgamma mutant were capable of inhibiting the expression of genes primarily regulated by PPARalpha, -gamma, or -delta ligands, albeit with some differences in potency. Thus, dominant-negative forms of PPARalpha and PPARgamma are capable of interfering with PPAR signaling in a manner that is not wholly restricted to their cognate target genes. These findings may have implications for the pathogenesis of human syndromes resulting from mutations in this family of transcription factors.

It has recently been shown that loss-of-function mutations of the G protein-coupled receptor (GPR)54 lead to isolated hypogonadotropic hypogonadism (IHH) in mice and humans. Such mutations are thought to be rare, even within the clinical IHH population, and only a handful of alleles have been described, making further screening of IHH populations imperative. We examined the genes encoding GPR54 and its putative endogenous ligand, kisspeptin-1, for mutations in a cohort of 30 patients with normosmic HH or delayed puberty. One subject with HH, of mixed Turkish-Cypriot and Afro-Caribbean ancestry, was found to be a compound heterozygote for two previously undescribed missense mutations in GPR54: cysteine 223 to arginine (C223R) in the fifth transmembrane helix and arginine 297 to leucine (R297L) in the third extracellular loop. Assessed in vitro using a previously described sensitive signaling assay in cells stably expressing GPR54, the C223R variant was found to exhibit profoundly impaired signaling, whereas the R297L variant showed a mild reduction in ligand-stimulated activity across the ligand dose range. These novel mutations provide further evidence that human HH may be caused by loss-of-function mutations in GPR54.

Thiazolidinedione drugs are in widespread use for the treatment of type 2 diabetes. In addition to improving insulin sensitivity, they generally result in a modest elevation of plasma HDL cholesterol. We report three patients, all of whom had preexisting diabetic dyslipidemia, who showed a profound reduction in plasma HDL cholesterol and apolipoprotein AI levels soon after the initiation of rosiglitazone therapy. In all three patients, HDL cholesterol levels returned to normal following drug withdrawal. The fact that this phenomenon was not seen in >1,400 patients studied in clinical trials indicates that it is likely to be rare and idiosyncratic. Until the frequency of this adverse reaction is clearer, it would seem advisable to ensure that plasma HDL cholesterol is documented before and rechecked after commencement of thiazolidinedione therapy.

The putative transcriptional corepressor ETO/MTG8 has been extensively studied due to its involvement in a chromosomal translocation causing the t(8;21) form of acute myeloid leukemia. Despite this, the role of ETO in normal physiology has remained obscure. Here we show that ETO is highly expressed in preadipocytes and acts as an inhibitor of C/EBPbeta during early adipogenesis, contributing to its characteristically delayed activation. ETO prevents both the transcriptional activation of the C/EBPalpha promoter by C/EBPbeta and its concurrent accumulation in centromeric sites during early adipogenesis. ETO expression rapidly reduces after the initiation of adipogenesis, and this is essential to the normal induction of adipogenic gene expression. These findings define, for the first time, a molecular role for ETO in normal physiology as an inhibitor of C/EBPbeta and a novel regulator of early adipogenesis.

Peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC1alpha) is an accessory protein which can potentiate the transcriptional activation function of many nuclear hormone receptors. Its tissue distribution and physiological studies suggest that its principal in vivo roles are to promote cold-induced thermogenesis, mitochondrial biogenesis, hepatic gluconeogenesis, and fatty acid beta-oxidation. It is expressed in the white adipose tissue of both humans and rodents, and in rodents it has been suggested to mediate in part the leptin-induced conversion of white adipocytes from fat storing to fat oxidising cells. In this study, quantitative real-time PCR has been used in human tissue to demonstrate that (1) PGC1alpha mRNA levels in subcutaneous fat are three-fold lower in morbidly obese than in slim subjects; (2) there are no differences in PGC1alpha mRNA between omental and subcutaneous mature adipocytes; (3) there is a robust induction of PGC1alpha expression during subcutaneous human preadipocyte differentiation ex vivo. Whether low PGC1alpha expression is a prelude to the development of obesity, or a consequence of that obesity, attempts to upregulate endogenous white adipose tissue expression may prove a valuable new avenue to explore in obesity therapy.