Restoring endoplasmic reticulum function by chemical chaperones: an emerging therapeutic approach for metabolic diseases.

The endoplasmic reticulum (ER) is a eukaryotic organelle that plays important roles in protein synthesis, folding and trafficking, calcium homoeostasis and lipid and steroid synthesis. It is the major protein synthesis compartment for secreted, plasma membrane and organelle proteins. Perturbations of ER homeostasis such as the accumulation of unfolded or misfolded proteins cause ER stress. To alleviate this stress, ER triggers an evolutionarily conserved signalling cascade called the unfolded protein response (UPR). As an initial response, the UPR aims at adapting and restoring ER function by translational attenuation, upregulation of ER chaperones and degradation of unfolded proteins. However, if the ER function is severely impaired because of excessive or prolonged exposure to stress, then the inflicted cells may undergo programmed cell death. During ER stress, unstable or partially folded mutant proteins are prevented from trafficking to their proper subcellular localizations and usually rapidly degraded. The small molecules named chemical chaperones help to stabilize these mutant proteins and facilitate their folding and proper trafficking from the ER to their final destinations. Because increasing number of studies suggest that ER stress is involved in a number of disease pathogenesis including neurodegenerative diseases, cancer, obesity, diabetes and atherosclerosis, promoting ER folding capacity through chemical chaperones emerges as a novel therapeutic approach. In this review, we provide insight into the many important functions of chemical chaperones during ER stress, their impact on the ER-stress-related pathologies and their potential as a new drug targets, especially in the context of metabolic disorders.

Inflammation and endoplasmic reticulum stress in obesity and diabetes.

Obesity is associated with chronic low-grade inflammation. Inflammatory signals interfere with insulin action and disrupt metabolic homeostasis. The c-Jun N-terminal kinase (JNK) has been identified as a central mediator of insulin resistance. Recent studies showed that in obesity compromising endoplasmic reticulum (ER) function results in insulin resistance and type 2 diabetes that are dependent on JNK activation. In contrast, enhancing ER function in transgenic mice or by the use of chemical chaperones protects against diet-induced insulin resistance. Hence, ER stress and the related signaling networks present a critical mechanism underlying obesity-induced JNK activity, inflammatory response and insulin resistance.

JNK1 deficiency does not enhance muscle glucose metabolism in lean mice.

Mice deficient in c-jun-NH(2)-terminal kinase 1 (JNK1) exhibit decreased fasting blood glucose and insulin levels, and protection against obesity-induced insulin resistance, suggesting increased glucose disposal into skeletal muscle. Thus, we assessed whether JNK1 deficiency enhances muscle glucose metabolism. Ex vivo insulin or contraction-induced muscle [(3)H]2-deoxyglucose uptake was not altered in JNK1 knockout mice, demonstrating that JNK1 does not regulate blood glucose levels via direct alterations in muscle. In vivo muscle [(3)H]2-deoxyglucose uptake in response to a glucose injection was also not enhanced by JNK1 deficiency, demonstrating that a circulating factor was not required to observe altered muscle glucose uptake in the knockout mice. JNK1 deficiency did not affect muscle glycogen levels or the protein expression of key molecules involved in glucose metabolism. This study is the first to directly demonstrate that enhanced skeletal muscle glucose metabolism does not underlie the beneficial effects of JNK1 deficiency in lean mice.

A genetic variant at the fatty acid-binding protein aP2 locus reduces the risk for hypertriglyceridemia, type 2 diabetes, and cardiovascular disease.

Obesity and the associated pathologies including dyslipidemia, insulin resistance, type 2 diabetes, and cardiovascular disease constitute a major threat to global human health. Yet, the genetic factors that differentially predispose individuals to this cluster of pathologies are unclear. The fatty acid-binding protein aP2 is a cytoplasmic lipid chaperon expressed in adipocytes and macrophages. Mice with aP2 deficiency are partially resistant to obesity-induced insulin resistance and type 2 diabetes, have lower circulating triglycerides, and exhibit marked protection against atherosclerosis. Here, we demonstrate a functionally significant genetic variation at the aP2 locus in humans that results in decreased adipose tissue aP2 expression due to alteration of the CAAT box/enhancer-binding protein binding and reduced transcriptional activity of the aP2 promoter. In population genetic studies with 7,899 participants, individuals that carry this T-87C polymorphism had lower serum triglyceride levels and significantly reduced risk for coronary heart disease and type 2 diabetes compared with subjects homozygous for the WT allele. Taken together, our results indicate that reduction in aP2 activity in humans generate a metabolically favorable phenotype that is similar to aP2 deficiency in experimental models.

Regulation of adiponectin in adipocytes upon exposure to HIV-1.

OBJECTIVES: Adipose dysregulation, dyslipidemia, and insulin resistance are hallmarks of HIV-related lipodystrophy. The precise mechanisms behind these disturbances are unknown. In HIV-infected patients, we previously demonstrated a strong relationship between lipodystrophy and levels of adiponectin, an adipose peptide implicated in regulation of glucose and lipid metabolisms. In this study we investigated the effect of HIV on adipocytes, to determine whether HIV can directly infect adipocytes and/or alter the regulation and secretion of the adipocyte-derived hormone adiponectin. METHODS: Human subcutaneous preadipocytes and adipocytes were exposed to HIV-1 under various conditions. Adiponectin was measured in supernatants and cell lysates. RESULTS: Although adipocytes expressed CD4, the major HIV receptor, they could not be infected in vitro. However, exposure to HIV dramatically increased the secretion of adiponectin from human adipocytes, in the absence of infection. This was exacerbated with sustained exposure to HIV in a transwell assay. Further, human peripheral mononuclear cells also produced adiponectin, but this was largely dependent upon T-cell activation. CONCLUSIONS: We propose that the stimulation of adiponectin production by HIV can perturb adiponectin regulation, leading to substantially decreased levels upon viral suppression by antiretroviral therapy. These data suggest a potential molecular mechanism of adiponectin regulation in HIV-infected patients.

Inflammatory pathways and insulin action.

Obesity and type 2 diabetes are associated with a state of abnormal inflammatory response. While this correlation has also been recognized in the clinical setting, its molecular basis and physiological significance are not yet fully understood. Studies in recent years have provided important insights into this curious phenomenon. The state of chronic inflammation typical of obesity and type 2 diabetes occurs at metabolically relevant sites, such as the liver, muscle, and most interestingly, adipose tissues. The biological relevance of the activation of inflammatory pathways became evident upon the demonstration that interference with these pathways improve or alleviate insulin resistance. The abnormal production of tumor necrosis factor alpha (TNF-alpha) in obesity is a paradigm for the metabolic significance of this inflammatory response. When TNF-alpha activity is blocked in obesity, either biochemically or genetically, the result is improved insulin sensitivity. Studies have since focused on the identification of additional inflammatory mediators critical in metabolic control and on understanding the molecular mechanisms by which inflammatory pathways are coupled to metabolic control. Recent years have seen a critical progress in this respect by the identification of several downstream mediators and signaling pathways, which provide the crosstalk between inflammatory and metabolic signaling. These include the discovery of c-Jun N-terminal kinase (JNK) and I kappa beta kinase (I kappa K) as critical regulators of insulin action activated by TNF-alpha and other inflammatory and stress signals, and the identification of potential targets. Here, the role of the JNK pathway in insulin receptor signaling, the impact of blocking this pathway in obesity and the mechanisms underlying JNK-induced insulin resistance will be discussed.

Effects of obesity and weight loss on the expression of proteins involved in fatty acid metabolism in human adipose tissue.

OBJECTIVE: Disturbances in adipocyte lipolysis in obesity may contribute to elevated circulating non-esterified fatty acid (NEFA) concentrations and insulin resistance. In experimental models, NEFA metabolism is influenced by adipocyte proteins such as adipocyte and keratinocyte lipid binding proteins (aP2/ALBP and mal1/KLBP) and fatty acid translocase (CD36). We investigated the effect of obesity and weight loss on the expression of these proteins in human subcutaneous adipose tissue. STUDY DESIGN AND SUBJECTS: Subcutaneous adipose tissue was obtained from 12 obese (body mass index (BMI) 42.4+/-1.6 kg/m(2)) and 12 lean (23.4+/-0.6 kg/m(2)) subjects. The obese subjects underwent gastric banding and biopsies were taken again after 2 y following a significant weight reduction (BMI 32.8+/-1.4 kg/m(2)). Adipose tissue proteins were quantified by Western blotting. RESULTS: Differential expression of ALBP, KLBP and CD36 was observed in lean and weight-reduced subjects compared with obese individuals. This resulted in a significantly lower ALBP/KLBP ratio in lean and weight-reduced individuals compared to obese subjects. Furthermore there was a significant influence of gender on this ratio. Moreover, the commonly used internal standard protein actin was expressed significantly higher in lean compared to obese individuals. CONCLUSION: The relative content of ALBP and KLBP in human adipose tissue changes with obesity, weight loss and gender indicating differential regulation. Differing responses in the expression patterns of adipose tissue proteins capable of binding NEFAs in response to weight changes suggest a potential importance in the development of obesity-associated complications.

Fatty acid binding protein expression in different adipose tissue depots from lean and obese individuals.

AIMS/HYPOTHESIS: This study investigated the expression of adipose tissue fatty acid binding proteins (FABPs) in subcutaneous and visceral human adipose tissue depots from lean and obese individuals. METHODS: Adipocyte lipid binding protein (ALBP) and keratinocyte lipid binding protein (KLBP) expression was quantified by western blot in subcutaneous and omental adipose tissue from 20 obese and 9 lean individuals. RNA expression was quantified by Northern blot in the obese subjects. RESULTS: In the obese subjects, ALBP protein and RNA expression was higher in subcutaneous compared with omental adipose tissue (increases of 31 +/- 14 % and 40 +/- 13 % respectively, both p < 0.05), whereas in the lean group, KLBP protein levels were 32 +/- 9 % lower in subcutaneous fat (p < 0.03). However, the ALBP/KLBP ratio was greater in subcutaneous compared to omental adipose tissue from both lean and obese subjects: increases of 187 +/- 71 % (p = 0.01) and 52 +/- 23 % (p = 0.17) respectively for the protein ratio, and 21 +/- 6 % for RNA (p = 0.01, obese individuals). In lean subjects, insulin concentrations correlated positively with the ALBP/KLBP protein ratio in both depots (both p < or = 0.03). CONCLUSION/INTERPRETATION: There are regional differences in adipose tissue FABP expression, which could be influenced by obesity. However, the ALBP/KLBP ratio is greater in subcutaneous than visceral adipose tissue in lean as well as in obese subjects. Investigation of adipose tissue FABPs could further our understanding of the role of fatty acids in the insulin resistance syndrome.

Signaling pathways utilized by tumor necrosis factor receptor 1 in adipocytes to suppress differentiation.

Tumor necrosis factor-alpha (TNFalpha) has profound effects on cultured adipocytes, one of which is the inhibition of terminal differentiation. Previous studies in TNF receptor (TNFR)-deficient preadipocytes have demonstrated that the anti-adipogenic effect of both secreted and transmembrane TNFalpha is mediated solely by TNFR1. In this study, we performed a structure-function analysis of the intracellular domains of TNFR1 and investigated the signaling pathway(s) involved in TNFR1-mediated inhibition of adipocyte differentiation. Our results show that repression of adipogenesis required the juxtamembrane and death domains and was independent of the pathways involving nuclear factor kappaB and neutral sphingomyelinase.

SOCS-3 inhibits insulin signaling and is up-regulated in response to tumor necrosis factor-alpha in the adipose tissue of obese mice.

SOCS (suppressor of cytokine signaling) proteins are inhibitors of cytokine signaling involved in negative feedback loops. We have recently shown that insulin increases SOCS-3 mRNA expression in 3T3-L1 adipocytes. When expressed, SOCS-3 binds to phosphorylated Tyr(960) of the insulin receptor and prevents Stat 5B activation by insulin. Here we show that in COS-7 cells SOCS-3 decreases insulin-induced insulin receptor substrate 1 (IRS-1) tyrosine phosphorylation and its association with p85, a regulatory subunit of phosphatidylinositol-3 kinase. This mechanism points to a function of SOCS-3 in insulin resistance. Interestingly, SOCS-3 expression was found to be increased in the adipose tissue of obese mice, but not in the liver and muscle of these animals. Two polypeptides known to be elevated during obesity, insulin and tumor necrosis factor-alpha (TNF-alpha), induce SOCS-3 mRNA expression in mice. Insulin induces a transient expression of SOCS-3 in the liver, muscle, and the white adipose tissue (WAT). Strikingly, TNF-alpha induced a sustained SOCS-3 expression, essentially in the WAT. Moreover, transgenic ob/ob mice lacking both TNF receptors have a pronounced decrease in SOCS-3 expression in the WAT compared with ob/ob mice, providing genetic evidence for a function of this cytokine in obesity-induced SOCS-3 expression. As SOCS-3 appears as a TNF-alpha target gene that is elevated during obesity, and as SOCS-3 antagonizes insulin-induced IRS-1 tyrosine phosphorylation, we suggest that it is a player in the development of insulin resistance.

Plasma insulin, leptin, and soluble TNF receptors levels in relation to obesity-related atherogenic and thrombogenic cardiovascular disease risk factors among men.

Obesity is related to cardiovascular disease (CVD) morbidity and mortality, however, the mechanisms for the development of obesity-induced CVD risk remain unclear. Hyperinsulinemia and insulin resistance are considered key components in the metabolic cardiovascular syndrome and as independent risk factors for CVD. Plasma leptin and tumor necrosis factor-alpha (TNF-alpha), two adipocyte products, are also proposed to be associated with the development of CVD risk. The purpose of this study is to evaluate the association of plasma leptin, soluble TNF receptors (sTNF-R), and insulin levels as possible mediators of the effect of obesity on atherogenic and thrombogenic CVD risk factors among men. From the Health Professionals Follow-up Study (HPFS), we selected 268 men, aged 47--83 years, who were free of CVD, diabetes, and cancer (except non-melanoma skin cancer), and who had provided a fasting blood sample in 1994. We measured plasma insulin and leptin levels by radioimmunoassay and sTNF-R levels by ELISA. Men in the highest quintile of body mass index (BMI, mean=30.5 kg/m(2)) were less physically active and had a more adverse cardiovascular lipid and homeostatic profile, as indicated by levels of insulin, triglyceride (TG), tissue plasminogen activator (t-PA) antigen levels, and apolipoprotein A1 (Apo-A1). In a multivariate regression model controlling for age, smoking, alcohol intake, physical activity and diet, BMI was inversely associated with HDL-cholesterol (HDL-C) and Apo-A1 and positively associated with TG, Apo-B and t-PA antigen levels. The associations between BMI and these CVD risk factors were only slightly changed after adjusting for leptin and/or sTNF-R; but were substantially attenuated after controlling for insulin levels. These data suggest that the association between obesity and biological predictors of CVD may be mediated through changes in plasma insulin, rather than leptin or sTNF-R levels. However, plasma leptin may still play a role in CVD through independent effects on lipid metabolism.

Adipose tissue resistin expression is severely suppressed in obesity and stimulated by peroxisome proliferator-activated receptor gamma agonists.

Elevated levels of the hormone resistin, which is secreted by fat cells, are proposed to cause insulin resistance and to serve as a link between obesity and type 2 diabetes. In this report we show that resistin expression is significantly decreased in the white adipose tissue of several different models of obesity including the ob/ob, db/db, tub/tub, and KKA(y) mice compared with their lean counterparts. Furthermore, in response to several different classes of antidiabetic peroxisome proliferator-activated receptor gamma agonists, adipose tissue resistin expression is increased in both ob/ob mice and Zucker diabetic fatty rats. These data demonstrate that experimental obesity in rodents is associated with severely defective resistin expression, and decreases in resistin expression are not required for the antidiabetic actions of peroxisome proliferator-activated receptor gamma agonists.

Plasma leptin concentrations and four-year weight gain among US men.

OBJECTIVE: Leptin, a primarily adipose tissue-derived protein product of the obesity (ob) gene, is an important regulator of energy metabolism. The strong association between body fat mass and elevated circulating leptin levels in humans suggests that leptin resistance, rather than leptin production, may contribute to the development of obesity and associated disorders. The purpose of this study is to evaluate the relationship between circulating plasma leptin levels and regulation of body weight over time among US men. DESIGN: Four-year prospective study. SUBJECTS: A total of 247 men from the Health Professionals Follow-up Study, who at baseline (1994), were 47-64 y of age, were free of cardiovascular diseases, diabetes mellitus and malignant neoplasmas, and completed a detailed lifestyle questionnaire. In addition, all participants completed a follow-up questionnaire in 1998. MEASUREMENTS: Baseline plasma leptin levels and 4-y weight change. RESULTS: At the start of follow-up, men in the highest quintile for plasma leptin (mean=12.1 ng/ml) weighed more, were less physically active, and had higher circulating insulin levels than men in the lowest quintile (mean=2.7 ng/ml). After adjustments for baseline age, weight, height, smoking status, alcohol intake, and physical activity, each 10 ng/ml increase in plasma leptin concentration was associated with a 1.68 kg (95% CI 0.14-3.18 kg) weight gain over the 4-y follow-up period. The observed association between leptin level and weight gain was limited to men with a baseline body mass index (BMI) of > or =25 kg/m2, in whom a 10 ng/ml higher baseline leptin was associated with a 2.45 kg (95% CI 0.73-4.18-kg) weight gain. Further adjustments for baseline total energy intake, plasma insulin and soluble tumor necrosis factor receptors levels did not appreciably alter these results. Plasma insulin level was not independently associated with subsequent weight gain. CONCLUSION: These results suggest that elevated plasma leptin concentrations among overweight men may be a marker of leptin resistance and subsequent weight gain.

Dietary and lifestyle factors in relation to plasma leptin concentrations among normal weight and overweight men.

OBJECTIVE: Leptin, the product of the obesity (ob) gene, is a multi-functional polypeptide that is important in energy metabolism, which is strongly correlated with body fat mass and body mass index (BMI). In a recent prospective study, we found that leptin was positively associated with 4 y weight gain among overweight and obese men. This suggests that leptin resistance, marked by hyperleptinemia among obese subjects, may be an important marker for weight gain. The purpose of this study is to evaluate whether modifiable dietary and lifestyle factors are associated with plasma leptin concentrations among US men. METHODS: We included 268 men aged 47--83 y (who were free of cardiovascular disease, diabetes mellitus and cancer, except nonmelanoma skin cancer) from the ongoing Health Professionals Follow-up Study. These subjects completed a detailed dietary and lifestyle questionnaire (including cigarette smoking, alcohol drinking and physical activity) and provided a fasting venous blood sample in 1994. All blood samples were stored in a deep freeze (-70 degrees C) for 4--5 y before being analyzed. Plasma leptin concentrations were measured by radioimmunoassay. RESULTS: Men in the highest quintile of plasma leptin (mean=14.4 ng/ml) weighed more, were less physically active, and had higher total and saturated fat and cholesterol intake than men in the lowest quintile (mean=3.0 ng/ml). Physical activity and current smoking were inversely associated with plasma leptin concentrations (P<0.001). A 20 MET difference in physical activity per week (equivalent to approximately 3 h of jogging) was associated with 0.38--0.58 ng/ml lower plasma leptin concentrations for normal weight and overweight men after adjusting for total energy and fat intake, BMI and other confounding variables. Total fat and monounsaturated fat intakes were positively associated with plasma leptin concentrations even after adjusting for BMI and other confounding variables; however, this association was limited to men of normal weight (BMI<25 kg/m(2)). CONCLUSION: These data suggest that physical activity may be a significant determinant of plasma leptin concentrations in men. Increasing physical activity is associated with lower plasma leptin concentrations even after adjusting for BMI. Physical activity may lower leptin concentrations not only due to decreased body fat mass, but potentially through an increase in leptin sensitivity. International Journal of Obesity (2001) 25, 106-114

Molecular mechanisms of insulin resistance and the role of the adipocyte.

Insulin resistance is a common feature of obesity and predisposes the affected individuals to a variety of diseases, including hypertension, dyslipidemias, cardiovascular problems and type 2 diabetes mellitus. However, the molecular mechanisms underlying abnormal insulin action and these other pathological states are not well understood. We have been focusing on cytokines, particularly TNFalpha and fatty acid binding proteins, as potential sites to study the molecular basis of these disorders. The role of TNFalpha in insulin resistance and other pathologies associated with obesity, have been examined in several experimental systems including obese mice with homozygous null mutations at the TNFalpha or TNF receptor loci. Analysis of these animals demonstrated that the genetic absence of TNF signaling in obesity: (i) significantly improves insulin receptor signaling capacity and consequently insulin sensitivity; (ii) prevents brown adipose tissue atrophy and beta3-adrenoreceptor deficiency and improves thermo-adaptive responses, (iii) decreases the elevated PAI-1 and TGFbeta production; and (iv) lowers hyperlipidemia and hyperleptinemia. Hence, abnormal TNFalpha action in adipocytes disturbs many aspects of metabolic homeostasis in obesity.

Function of GATA transcription factors in preadipocyte-adipocyte transition.

Genes that control the early stages of adipogenesis remain largely unknown. Here, we show that murine GATA-2 and GATA-3 are specifically expressed in white adipocyte precursors and that their down-regulation sets the stage for terminal differentiation. Constitutive GATA-2 and GATA-3 expression suppressed adipocyte differentiation and trapped cells at the preadipocyte stage. This effect is mediated, at least in part, through the direct suppression of peroxisome proliferator-activated receptor gamma. GATA-3-deficient embryonic stem cells exhibit an enhanced capacity to differentiate into adipocytes, and defective GATA-2 and GATA-3 expression is associated with obesity. Thus, GATA-2 and GATA-3 regulate adipocyte differentiation through molecular control of the preadipocyte-adipocyte transition.

Glycemic status and soluble tumor necrosis factor receptor levels in relation to plasma leptin concentrations among normal weight and overweight US men.

OBJECTIVE: Leptin, an adipocyte-derived protein product of the obesity (ob) gene, is a multifunctional polypeptide associated with the development of obesity-related disorders in humans. There is considerable inter-individual variation in plasma leptin even among subjects with comparable obesity levels, which suggests that factors other than adipose mass may be involved in the regulation of leptin expression and/or production. The purpose of this study was to evaluate the potential role of glycemic status and adipose-derived cytokines in regulating plasma leptin levels among normal and overweight men. DESIGN: Cross-sectional study. SUBJECTS AND MEASUREMENTS: We measured plasma leptin, insulin, c-peptide and plasma soluble tumor necrosis factor receptor (sTNF-R) concentrations in 178 men. The subjects were selected from the Health Professionals Follow-up Study (HPFS), and aged 47-64 y in 1994, were free of cardiovascular disease, diabetes mellitus, malignant neoplasms, and had provided a fasting blood sample and a detailed lifestyle questionnaire. RESULTS: Men in the highest quintile of plasma leptin (mean = 12.7 ng/ml) weighed more, were less physically active and had higher circulating insulin, c-peptide, sTNF-R1 and sTNF-R2 concentrations than men in the lowest quintile (mean = 2.8 ng/ml). We found a significant correlation between plasma insulin, c-peptide, glycosylated hemoglobin (HbA1c), and sTNF-R1 on leptin concentrations (with Spearman correlation coefficients ranging from 0.17 to 0.48 and all P < 0.05). Only HbA1c and sTNF-R1 were independently and positively associated with plasma leptin after further adjusting for body mass index and other metabolic parameters of interest. Interestingly, these observed associations were limited to men with a BMI > or = 25 kg/m2. CONCLUSION: Our results suggest that glucose homeostasis and the activity of the TNF system may modulate leptin secretion and production among overweight men. Glucose homeostasis and TNF-alpha is important in metabolic disorders related to hyperleptinemia.

Improved glucose and lipid metabolism in genetically obese mice lacking aP2.

Adipocyte fatty acid-binding protein, aP2, is a member of the intracellular fatty acid binding protein family. Previously, studies have shown increased insulin sensitivity in aP2-deficient mice with dietary obesity. Here, we asked whether aP2-related alterations in lipolytic response and insulin production are features of obesity-induced insulin resistance and investigated the effects of aP2-deficiency on glucose homeostasis and lipid metabolism in ob/ob mice, a model of extreme obesity. ob/ob mice homozygous for the aP2 null allele (ob/ ob-aP2-/-) became more obese than ob/ob mice as indicated by significantly increased body weight and fat pad size but unaltered body length. However, despite their extreme adiposity, ob/ob-aP2-/- animals were more insulin-sensitive compared with ob/ob controls, as demonstrated by significantly lower plasma glucose and insulin levels and better performance in both insulin and glucose tolerance tests. These animals also showed improvements in dyslipidemia and had lower plasma triglyceride and cholesterol levels. Lipolytic response to beta-adrenergic stimulation and lipolysis-associated insulin secretion was significantly reduced in ob/ob-aP2-/- mice. Interestingly, glucose-stimulated insulin secretion, while virtually abolished in ob/ob controls, was significantly improved in ob/ob-aP2-/- animals. There were no apparent morphological differences in the structure or size of the pancreatic islets between genotypes. Taken together, the data indicate that in obesity, aP2-deficiency not only improves peripheral insulin resistance but also preserves pancreatic beta cell function and has beneficial effects on lipid metabolism.

Tumor necrosis factor alpha mediates apoptosis of brown adipocytes and defective brown adipocyte function in obesity.

Severe quantitative and qualitative brown adipocyte defects are common in obesity. To investigate whether aberrant expression of tumor necrosis factor alpha (TNF-alpha) in obesity is involved in functional brown fat atrophy, we have studied genetically obese (ob/ob) mice with targeted null mutations in the genes encoding the two TNF receptors. The absence of both TNF receptors or p55 receptor alone resulted in a significant reduction in brown adipocyte apoptosis and an increase in beta(3)-adrenoreceptor and uncoupling protein-1 expression in obese mice. Increased numbers of multilocular functionally active brown adipocytes, and improved thermoregulation was also observed in obese animals lacking TNF-alpha function. These results indicate that TNF-alpha plays an important role in multiple aspects of brown adipose tissue biology and mediates the abnormalities that occur at this site in obesity.