Utpal B. Pajvani

COMPLEX DISTRIBUTION, NOT ABSOLUTE AMOUNT OF ADIPONECTIN, CORRELATES WITH THIAZOLIDINEDIONE-MEDIATED IMPROVEMENT IN INSULIN SENSITIVITY

Adiponectin is an adipocyte-specific secretory protein that circulates in serum as a hexamer of relatively low molecular weight (LMW) and a larger multimeric structure of high molecular weight (HMW). Serum levels of the protein correlate with systemic insulin sensitivity. The full-length protein affects hepatic gluconeogenesis through improved insulin sensitivity, and a proteolytic fragment of adiponectin stimulates β oxidation in muscle. Here, we show that the ratio, and not the absolute amounts, between these two oligomeric forms (HMW to LMW) is critical in determining insulin sensitivity. We define a new index, SA, that can be calculated as the ratio of HMW/(HMW + LMW). db/db mice, despite similar total adiponectin levels, display decreased SA values compared with wild type littermates, as do type II diabetic patients compared with insulin-sensitive individuals. Furthermore, SA improves with peroxisome proliferator-activated receptor-γ agonist treatment (thiazolidinedione; TZD) in mice and humans. We demonstrate that changes in SA in a number of type 2 diabetic cohorts serve as a quantitative indicator of improvements in insulin sensitivity obtained during TZD treatment, whereas changes in total serum adiponectin levels do not correlate well at the individual level. Acute alterations in SA (ΔSA) are strongly correlated with improvements in hepatic insulin sensitivity and are less relevant as an indicator of improved muscle insulin sensitivity in response to TZD treatment, further underscoring the conclusions from previous clamp studies that suggested that the liver is the primary site of action for the full-length protein. These observations suggest that the HMW adiponectin complex is the active form of this protein, which we directly demonstrate in vivo by its ability to depress serum glucose levels in a dose-dependent manner.

Adiponectin acts in the brain to decrease body weight.

Adiponectin (ADP) is an adipocyte hormone involved in glucose and lipid metabolism. We detected a rise in ADP in cerebrospinal fluid after intravenous (i.v.) injection, consistent with brain transport. In contrast to leptin, intracerebroventricular (i.c.v.) administration of ADP decreased body weight mainly by stimulating energy expenditure. Full-length ADP, mutant ADP with Cys39 replaced with serine, and globular ADP were effective, whereas the collagenous tail fragment was not. Lep ob/ob mice were especially sensitive to i.c.v. and systemic ADP, which resulted in increased thermogenesis, weight loss and reduction in serum glucose and lipid levels. ADP also potentiated the effect of leptin on thermogenesis and lipid levels. While both hormones increased expression of hypothalamic corticotropin-releasing hormone (CRH), ADP had no substantial effect on other neuropeptide targets of leptin. In addition, ADP induced distinct Fos immunoreactivity. Agouti (A y/a) mice did not respond to ADP or leptin, indicating the melanocortin pathway may be a common target. These results show that ADP has unique central effects on energy homeostasis.

Mice Lacking Adiponectin Show Decreased Hepatic Insulin Sensitivity and Reduced Responsiveness to Peroxisome Proliferator-activated Receptor γ Agonists

The adipose tissue-derived hormone adiponectin improves insulin sensitivity and its circulating levels are decreased in obesity-induced insulin resistance. Here, we report the generation of a mouse line with a genomic disruption of the adiponectin locus. We aimed to identify whether these mice develop insulin resistance and which are the primary target tissues affected in this model. Using euglycemic/insulin clamp studies, we demonstrate that these mice display severe hepatic but not peripheral insulin resistance. Furthermore, we wanted to test whether the lack of adiponectin magnifies the impairments of glucose homeostasis in the context of a dietary challenge. When exposed to high fat diet, adiponectin null mice rapidly develop glucose intolerance. Specific PPARγ agonists such as thiazolidinediones (TZDs) improve insulin sensitivity by mechanisms largely unknown. Circulating adiponectin levels are significantly up-regulated in vivo upon activation of PPARγ. Both TZDs and adiponectin have been shown to activate AMP-activated protein kinase (AMPK) in the same target tissues. We wanted to address whether the ability of TZDs to improve glucose tolerance is dependent on adiponectin and whether this improvement involved AMPK activation. We demonstrate that the ability of PPARγ agonists to improve glucose tolerance in ob/ob mice lacking adiponectin is diminished. Adiponectin is required for the activation of AMPK upon TZD administration in both liver and muscle. In summary, adiponectin is an important contributor to PPARγ-mediated improvements in glucose tolerance through mechanisms that involve the activation of the AMPK pathway.

Fat apoptosis through targeted activation of caspase 8: a new mouse model of inducible and reversible lipoatrophy

We describe the generation and characterization of the first inducible fatless model system, the FAT-ATTAC mouse (fat apoptosis through targeted activation of caspase 8). This transgenic mouse develops identically to wild-type littermates. Apoptosis of adipocytes can be induced at any developmental stage by administration of a FK1012 analog leading to the dimerization of a membrane-bound, adipocyte-specific caspase 8–FKBP fusion protein. Within 2 weeks of dimerizer administration, FAT-ATTAC mice show near-knockout levels of circulating adipokines and markedly reduced levels of adipose tissue. FAT-ATTAC mice are glucose intolerant, have diminished basal and endotoxin-stimulated systemic inflammation, are less responsive to glucose-stimulated insulin secretion and show increased food intake independent of the effects of leptin. Most importantly, we show that functional adipocytes can be recovered upon cessation of treatment, allowing the study of adipogenesis in vivo, as well as a detailed examination of the importance of the adipocyte in the regulation of multiple physiological functions and pathological states.

Selective Downregulation of the High–Molecular Weight Form of Adiponectin in Hyperinsulinemia and in Type 2 Diabetes: Differential Regulation From Nondiabetic Subjects

OBJECTIVE—Adiponectin is an adipocyte-specific secretory protein found in circulation in several different forms and is present at significantly lower levels in the plasma of diabetic patients compared with that of insulin-sensitive individuals. We wanted to test whether insulin per se is a contributing factor toward lower plasma adiponectin concentrations and, if so, whether the splanchnic bed contributes to this phenomenon. RESEARCH DESIGN AND METHODS—We sampled femoral artery and hepatic venous samples and measured the high–molecular weight (HMW) and low–molecular weight (LMW) fractions of adiponectin in 11 type 2 diabetic and 7 nondiabetic subjects matched for age, sex, and BMI during basal conditions and during a hyperglycemic (∼9.5 mmol/l) hyperinsulinemic (∼700 pmol/l) clamp. RESULTS—Under these conditions, total arterial adiponectin, HMW, and the ratio of HMW to total adiponectin all were lower (P < 0.01) in the diabetic versus nondiabetic subjects, whereas the LMW form did not significantly differ. Under hyperinsulinemic conditions, total adiponectin levels dropped, primarily due to a reduction of the HMW form, whereas LMW forms were not significantly affected. CONCLUSIONS—HMW adiponectin and the ratio of HMW to total adiponectin are lower in individuals with diabetes than in nondiabetic subjects. We conclude that HMW adiponectin is downregulated in hyperinsulinemia and type 2 diabetes.

Thiazolidinediones improve insulin sensitivity in adipose tissue and reduce the hyperlipidaemia without affecting the hyperglycaemia in a transgenic model of type 2 diabetes

Aim/hypothesisThe aim of this study was to examine the effects of thiazolidinediones on the MKR mouse model of type 2 diabetes.MethodsSix-week-old wild-type (WT) and MKR mice were fed with or without rosiglitazone or pioglitazone for 3 weeks. Blood was collected from the tail vein for serum biochemistry analysis. Hyperinsulinaemic–euglycaemic clamp analysis was performed to study effects of thiazolidinediones on insulin sensitivity of tissues in MKR mice. Northern blot analysis was performed to measure levels of target genes of PPAR γ agonists in white adipose tissue and hepatic gluconeogenic genes.ResultsThiazolidinedione treatment of MKR mice significantly lowered serum lipid levels and increased serum adiponectin levels but did not affect levels of blood glucose and serum insulin. Hyperinsulinaemic–euglycaemic clamp showed that whole-body insulin sensitivity and glucose homeostasis failed to improve in MKR mice after rosiglitazone treatment. Insulin suppression of hepatic endogenous glucose production failed to improve in MKR mice following rosiglitazone treatment. This lack of change in hepatic insulin insensitivity was associated with no change in the ratio of HMW : total adiponectin, hepatic triglyceride content, and sustained hepatic expression of PPAR γ and stearoyl-CoA desaturase 1 mRNA. Interestingly, rosiglitazone markedly enhanced glucose uptake by white adipose tissue with a parallel increase in CD36, aP2 and GLUT4 gene expression.Conclusions/interpretationThese data suggest that potentiation of insulin action on tissues other than adipose tissue is required to mediate the antidiabetic effects of thiazolidinediones in our MKR diabetic mice.