Minireview: The adipocyte--at the crossroads of energy homeostasis, inflammation, and atherosclerosis

Ciliary neurotrophic factor influences endocrine adipocyte function: inhibition of leptin via PI 3-kinase

Ciliary neurotrophic factor (CNTF), originally known for its involvement in the modulation of neuronal growth, has been discovered to exert anorexigenic effects and is currently being investigated in clinical studies for the treatment of obesity and insulin resistance. This neuropeptide acts on the central nervous system. However, we have recently demonstrated direct peripheral effects on adipocyte signalling and thermogenesis. Given the emerging endocrine role of adipose tissue in the regulation of energy homeostasis and insulin resistance, we investigated potential effects of CNTF on leptin expression and secretion. Our study demonstrates a direct inhibition of leptin expression and secretion by acute and chronic CNTF treatment. Furthermore, we demonstrate a differentiation- and Janus kinase 2 (JAK2)-independent, but phosphatidylinositol 3-kinase-dependent signalling pathway mediating this negative effect. These results provide novel evidence for a role of CNTF in the selective modulation of adipocyte endocrine function which may have important implications for the regulation of energy homeostasis.

Adiposity, dyslipidemia, and insulin resistance in mice with targeted deletion of phospholipid scramblase 3 (PLSCR3)

The phospholipid scramblases (PLSCR1 to PLSCR4) are a structurally and functionally unique class of proteins, which are products of a tetrad of genes conserved from Caenorhabditis elegans to humans. The best characterized member of this family, PLSCR1, is implicated in the remodeling of the transbilayer distribution of plasma membrane phospholipids but is also required for normal signaling through select growth factor receptors. Mice with targeted deletion of PLSCR1 display perinatal granulocytopenia due to defective response of hematopoietic precursors to granulocyte colony-stimulating factor and stem cell factor. To gain insight into the biologic function of another member of the PLSCR family, we investigated mice with targeted deletion of PLSCR3, a protein that like PLSCR1 is expressed in many blood cells but which, by contrast to PLSCR1, is also highly expressed in fat and muscle. PLSCR3(-/-) mice at 2 months of age displayed aberrant accumulation of abdominal fat when maintained on standard rodent chow, which was accompanied by insulin resistance, glucose intolerance, and dyslipidemia. Primary adipocytes and cultured bone-marrow-derived macrophages from PLSCR3(-/-) mice were engorged with neutral lipid, and adipocytes displayed defective responses to exogenous insulin. Plasma of PLSCR3(-/-) mice was elevated in non-high-density lipoproteins, cholesterol, triglycerides, nonesterified fatty acids, and leptin, whereas adiponectin was low. These data suggest that the expression of PLSCR3 may be required for normal adipocyte and/or macrophage maturation or function and raise the possibility that deletions or mutations affecting the PLSCR3(-/-) gene locus may contribute to the risk for lipid-related disorders in humans.

Assessment of the influence of fatty acids on indices of insulin sensitivity and myocellular lipid content by use of magnetic resonance spectroscopy in cats

OBJECTIVE

To determine whether dietary fatty acids affect indicators of insulin sensitivity, plasma insulin and lipid concentrations, and lipid accumulation in muscle cells in lean and obese cats.

ANIMALS

28 neutered adult cats.

PROCEDURE

IV glucose tolerance tests and magnetic resonance imaging were performed before (lean phase) and after 21 weeks of ad libitum intake of either a diet high in omega-3 polyunsaturated fatty acids (3-PUFAs; n = 14) or high in saturated fatty acids (SFAs; 14).

RESULTS

Compared with the lean phase, ad libitum food intake resulted in increased weight, body mass index, girth, and percentage fat in both groups. Baseline plasma glucose or insulin concentrations and glucose area under the curve (AUC) were unaffected by diet. Insulin AUC values for obese and lean cats fed 3-PUFAs did not differ, but values were higher in obese cats fed SFAs, compared with values for lean cats fed SFAs and obese cats fed 3-PUFAs. Nineteen cats that became glucose intolerant when obese had altered insulin secretion and decreased glucose clearance when lean. Plasma cholesterol, triglyceride, and non-esterified fatty acid concentrations were unaffected by diet. Ad libitum intake of either diet resulted in an increase in both intra- and extramyocellular lipid. Obese cats fed SFAs had higher glycosylated hemoglobin concentration than obese cats fed 3-PUFAs.

CONCLUSION AND CLINICAL RELEVANCE

In obese cats, a diet high in 3-PUFAs appeared to improve long-term glucose control and decrease plasma insulin concentration. Obesity resulted in intra- and extramyocellular lipid accumulations (regardless of diet) that likely modulate insulin sensitivity.

The forkhead transcription factor FoxC2 inhibits white adipocyte differentiation

In this study, we show that expression of FoxC2 blocks the capacity of 3T3-L1 preadipocytes to undergo adipogenesis in the presence of dexamethasone, isobutylmethylxanthine, and insulin. This block is characterized by an extensive decrease in the expression of proteins associated with the function of the mature fat cell, most notably C/EBPalpha, adiponectin, perilipin, and the adipose-specific fatty acid-binding protein, FABP4/aP2. Since the expression of these proteins lies downstream of PPARgamma, we overexpressed PPARgamma in Swiss mouse fibroblasts to promote adipocyte differentiation. We show that FoxC2 blocks the ability of PPARgamma to induce adipogenic gene expression in response to exposure of the cells to dexamethasone, isobutylmethylxanthine, insulin, and a PPARgamma ligand. Interestingly, the expression of aP2 escapes the inhibitory action of FoxC2 under conditions that promote maximum PPARgamma activity. In contrast, FoxC2 inhibits the expression of C/EBPalpha, perilipin, and adiponectin even in the presence of potent PPARgamma ligands. Finally, we show that FoxC2 does not affect the ability of PPARgamma to bind to or transactivate from a PPARgamma response element. These data suggest that FoxC2 blocks adipogenesis by inhibiting the capacity of PPARgamma to promote the expression of a subset of adipogenic genes.

Insulin resistance and obesity

Obesity, particularly central obesity, is associated with insulin resistance. Much research has focused on mechanisms that link obesity to insulin resistance, including lipid accumulation in muscle and liver and the recently discovered adipocytokines. Insulin resistance is an important feature of a number of common conditions, including type 2 diabetes and polycystic ovary syndrome and is associated with rarer disorders, such as inherited insulin receptoropathies and genetic and acquired lipodystrophies. Despite its use for a number of years, metformin's role as an insulin sensitizer has only recently been appreciated. The discovery of a new class of insulin sensitizing agents, the thiazolidinediones, represents a major advance in the understanding of the etiology of insulin resistance, particularly in relation to adipocyte biology and possibly, its inflammatory origins.

A family of Acrp30/adiponectin structural and functional paralogs

Biochemical, genetic, and animal studies in recent years have established a critical role for the adipokine Acrp30/adiponectin in controlling whole-body metabolism, particularly by enhancing insulin sensitivity in muscle and liver, and by increasing fatty acid oxidation in muscle. We describe a widely expressed and highly conserved family of adiponectin paralogs designated as C1q/tumor necrosis factor-alpha-related proteins (CTRPs) 1-7. In the present study, we focus on mCTRP2, the mouse paralog most similar to adiponectin. At nanomolar concentrations, bacterially produced mCTRP2 rapidly induced phosphorylation of AMP-activated protein kinase, acetyl-CoA carboxylase, and mitogen-activated protein kinase in C2C12 myotubes, which resulted in increased glycogen accumulation and fatty acid oxidation. The discovery of a family of adiponectin paralogs has implications for understanding the control of energy homeostasis and could provide new targets for pharmacologic intervention in metabolic diseases such as diabetes and obesity.

Taurine prevents streptozotocin impairment of hormone-stimulated glucose uptake in rat adipocytes

Streptozotocin-treated rats were used as models of type 1 diabetes to study the effects of dietary taurine on insulin- and adrenergic-stimulated 2-deoxyglucose uptake by isolated adipocytes. In addition to the well-established impairment of basal and insulin-stimulated 2-deoxyglucose uptakes in adipocytes prepared from streptozotocin-diabetic rats, the alpha-(phenylephrine) and beta-(isoproterenol) adrenergic stimulations of glucose uptake were also abolished. The insulin stimulation of glucose uptake in adipocytes was selectively abolished by the phosphatidylinositol 3-kinase inhibitor wortmannin, whereas that by the adrenergic agonists, phenylephrine and isoproterenol, was inhibited by prazosin and propranolol, respectively. Dietary taurine, 4 weeks before and 4 weeks after streptozotocin administration, prevented the loss of both insulin and adrenergic agonist stimulations of 2-deoxyglucose uptake, without affecting hyperglycaemia. Because insulin and adrenergic activations of glucose transport by adipocytes are coupled to different signalling pathways, it is unlikely that these effects of taurine are related to these disparate postreceptor mechanisms.

Regulation of resistin expression and circulating levels in obesity, diabetes, and fasting

Resistin was originally reported as an adipose tissue-specific hormone that provided a link between obesity and diabetes. Resistin protein level was elevated in obese mice and decreased by insulin-sensitizing thiazolidinediones. Immunoneutralization of resistin improved insulin sensitivity in diet-induced obese mice, while the administration of exogenous resistin induced insulin resistance. More recently, we have shown that ablation of the resistin gene in mice decreased fasting glucose through impairment of gluconeogenesis, while resistin treatment in these knockout mice increased hepatic glucose production. However, the link between resistin and glucose homeostasis has been questioned by studies demonstrating reduced, rather than increased, resistin mRNA expression in obese and diabetic mice. To better understand the regulation of resistin, we developed a sensitive and specific RIA resistin that could accurately measure serum resistin levels in several mouse models. We show that while resistin mRNA is indeed suppressed in obese mice, the circulating resistin level is significantly elevated and positively correlated with insulin, glucose, and lipids. Both resistin mRNA expression and protein levels in Lep(ob/ob) mice are suppressed by leptin treatment in parallel with reductions in glucose and insulin. In wild-type mice, serum resistin increases after nocturnal feeding, concordant with rising levels of insulin. Resistin mRNA and protein levels decline in parallel with glucose and insulin during fasting and are restored after refeeding. We performed clamp studies to determine whether resistin is causally related to insulin and glucose. Adipose resistin expression and serum resistin increased in response to hyperinsulinemia and further in response to hyperglycemia. Taken together, these findings suggest that the nutritional regulation of resistin and changes in resistin gene expression and circulating levels in obesity are mediated, at least in part, through insulin and glucose.

Disulfide-dependent multimeric assembly of resistin family hormones

Resistin, founding member of the resistin-like molecule (RELM) hormone family, is secreted selectively from adipocytes and induces liver-specific antagonism of insulin action, thus providing a potential molecular link between obesity and diabetes. Crystal structures of resistin and RELMbeta reveal an unusual multimeric structure. Each protomer comprises a carboxy-terminal disulfide-rich beta-sandwich "head" domain and an amino-terminal alpha-helical "tail" segment. The alpha-helical segments associate to form three-stranded coiled coils, and surface-exposed interchain disulfide linkages mediate the formation of tail-to-tail hexamers. Analysis of serum samples shows that resistin circulates in two distinct assembly states, likely corresponding to hexamers and trimers. Infusion of a resistin mutant, lacking the intertrimer disulfide bonds, in pancreatic-insulin clamp studies reveals substantially more potent effects on hepatic insulin sensitivity than those observed with wild-type resistin. This result suggests that processing of the intertrimer disulfide bonds may reflect an obligatory step toward activation.

Hormone Therapy: Physiological Complexity Belies Therapeutic Simplicity

The results of the Women's Health Initiative, a study anticipated to provide definitive answers about health benefits and risks of postmenopausal hormone therapy, have generated debate and confusion among clinicians, researchers, and the lay public. The ovarian hormones estrogen and progesterone, which decline at menopause, normally elicit complex tissue-specific responses throughout the body. Major advances are providing a detailed molecular definition of how that differential action is achieved. Here we review estrogen and progestin actions, discuss how effectively knowledge of steroid hormone endocrinology has been incorporated into clinical studies, and consider the impact on modern hormone therapy protocols and pharmaceutical development.

Nuclear receptor corepressor RIP140 regulates fat accumulation

Nuclear receptors and their coactivators have been shown to function as key regulators of adipose tissue biology. Here we show that a ligand-dependent transcriptional repressor for nuclear receptors plays a crucial role in regulating the balance between energy storage and energy expenditure. Mice devoid of the corepressor protein RIP140 are lean, show resistance to high-fat diet-induced obesity and hepatic steatosis, and have increased oxygen consumption. Although the process of adipogenesis is unaffected, expression of certain lipogenic enzymes is reduced. In contrast, genes involved in energy dissipation and mitochondrial uncoupling, including uncoupling protein 1, are markedly increased. Therefore, the maintenance of energy homeostasis requires the action of a transcriptional repressor in white adipose tissue, and ligand-dependent recruitment of RIP140 to nuclear receptors may provide a therapeutic target in the treatment of obesity and related disorders.

From blood monocytes to adipose tissue-resident macrophages: induction of diapedesis by human mature adipocytes

Obesity has been suggested to be a low-grade systemic inflammatory state, therefore we studied the interaction between human adipocytes and monocytes via adipose tissue (AT)-derived capillary endothelium. Cells composing the stroma-vascular fraction (SVF) of human ATs were characterized by fluorescence-activated cell sorter (FACS) analysis and two cell subsets (resident macrophages and endothelial cells [ECs]) were isolated using antibody-coupled microbeads. Media conditioned by mature adipocytes maintained in fibrin gels were applied to AT-derived ECs. Thereafter, the expression of endothelial adhesion molecules was analyzed as well as the adhesion and transmigration of human monocytes. FACS analysis showed that 11% of the SVF is composed of CD14(+)/CD31(+) cells, characterized as resident macrophages. A positive correlation was found between the BMI and the percentage of resident macrophages, suggesting that fat tissue growth is associated with a recruitment of blood monocytes. Incubation of AT-derived ECs with adipocyte-conditioned medium resulted in the upregulation of EC adhesion molecules and the increased chemotaxis of blood monocytes, an effect mimicked by recombinant human leptin. These results indicate that adipokines, such as leptin, activate ECs, leading to an enhanced diapedesis of blood monocytes, and suggesting that fat mass growth might be linked to inflammatory processes.

Leptin increases cardiomyocyte hyperplasia via extracellular signal-regulated kinase- and phosphatidylinositol 3-kinase-dependent signaling pathways

Obesity is a major risk factor for the development of heart failure. Importantly, it is now appreciated that a change in the number of myocytes is one of multiple structural and functional alterations (remodeling) leading to heart failure. Here we investigate the effect of leptin, the product of the obese (ob) gene, on proliferation of human and murine cardiomyocytes. Leptin caused a time- and dose-dependent significant increase in proliferation of HL-1 cells that was inhibited by preincubation with PD98059 and LY294002, suggesting that leptin mediated proliferation via extracellular signal-regulated kinase-1/2- and phosphatidylinositol-3-kinase-dependent signaling pathways. We confirmed that leptin activates both extracellular signal-regulated kinase-1/2 phosphorylation and association of phosphatidylinositol-3-kinase (regulatory p85 subunit) with phosphotyrosine immunoprecipitates. We also examined bromodeoxyuridine incorporation as a measure of new DNA synthesis and demonstrated a stimulatory effect of leptin in both HL-1 cells and human cardiomyocytes. Bromodeoxyuridine incorporation in HL-1 cells was inhibited by PD98059 and LY294002. Our results establish a mitogenic effect of leptin in cardiomyocytes and provide additional evidence for a potential direct link between leptin and cardiac remodeling in obesity.

Regulation of fasted blood glucose by resistin

The association between obesity and diabetes supports an endocrine role for the adipocyte in maintaining glucose homeostasis. Here we report that mice lacking the adipocyte hormone resistin exhibit low blood glucose levels after fasting, due to reduced hepatic glucose production. This is partly mediated by activation of adenosine monophosphate-activated protein kinase and decreased expression of gluconeogenic enzymes in the liver. The data thus support a physiological function for resistin in the maintenance of blood glucose during fasting. Remarkably, lack of resistin diminishes the increase in post-fast blood glucose normally associated with increased weight, suggesting a role for resistin in mediating hyperglycemia associated with obesity.

Diet and obesity

PURPOSE OF REVIEW

Obesity continues to increase in the United States and worldwide. There is controversy surrounding different dietary patterns used to promote weight loss, and none has emerged as clearly more effective. This paper briefly reviews the factors that influence energy intake and dietary treatments used to promote weight loss.

RECENT FINDINGS

Increasing portion size, eating away from home, and consuming a variety of high-energy dense foods appear to increase energy intake. Hormonal influences on diet continue to be explored. Very-low-calorie diets and low-carbohydrate diets lead to greater initial weight loss, but long-term results are no better than more moderate calorie-restricted diets. A program using meal replacements appears to lead to weight loss slightly greater than calorie-restricted diets and offers one option to treat obesity. Dietary patterns low in energy density and glycemic index have potential in treating obesity and should be studied further.

SUMMARY

Clearly, a dietary pattern that prescribes a lower total energy intake is necessary for weight loss, and this pattern should be sustainable to maintain weight loss. Although many dietary programs can achieve short-term loss of weight, dietary treatment should be recommended that emphasizes lifestyle changes and is consistent with other dietary guidelines to promote long-term health. Features consistent with this are a dietary pattern low in total calories, saturated fat, and refined carbohydrate; moderate in whole grains; and high in low-energy dense vegetables and fruits. Future studies should explore dietary strategies and combination therapies that contribute to weight loss, long-term weight maintenance, and improved health.

Obesity: an endocrine tumor?

Obesity is one of the most common disorders in clinical practice. The prevalance of obesity has increased by more than 60% since 1990. Adipose tissue acts as an endocrine organ secreting many factors into the blood, known as adipokines, including leptin, adipsin, acylation-stimulating protein, adiponectin, etc. This article examines the hypothesis that obesity may be evaluated as an endocrine tumor, regarding its genetic basis, hyperplasia and hypertrophy of adipocytes, neovascularisation within the adipose tissue associated with growth, and beneficisal metabolic effects of surgical removal of excess adipose tissue by liposuction. Assuming obesity as an endocrine tumor may bring out new treatment modalities. Liposuction as "cytoreductive surgery", antiangiogenic teraphy or anti-neoplastic drugs may be important components of obesity treatment in future.

A transgenic mouse with a deletion in the collagenous domain of adiponectin displays elevated circulating adiponectin and improved insulin sensitivity

Adiponectin is a plasma protein expressed exclusively in adipose tissue. Adiponectin levels are linked to insulin sensitivity, but a direct effect of chronically elevated adiponectin on improved insulin sensitivity has not yet been demonstrated. We identified a dominant mutation in the collagenous domain of adiponectin that elevated circulating adiponectin values in mice by 3-fold. Adiponectinemia raised lipid clearance and lipoprotein lipase activity, and suppressed insulin-mediated endogenous glucose production. The induction of adiponectin during puberty and the sexual dimorphism in adult adiponectin values were preserved in these transgenic animals. As a result of elevated adiponectin, serum PRL values and brown adipose mass both increased. The effects on carbohydrate and lipid metabolism were associated with elevated phosphorylation of 5'-AMP-activated protein kinase in liver and elevated expression of peroxisomal proliferator-activated receptor gamma2, caveolin-1, and mitochondrial markers in white adipose tissue. These studies strongly suggest that increasing endogenous adiponectin levels has direct effects on insulin sensitivity and may induce similar physiological responses as prolonged treatment with peroxisomal proliferator-activated receptor gamma agonists.

Minireview: From anorexia to obesity--the yin and yang of body weight control

Over the past decade, there has been a tremendous increase in the understanding of the molecular and neural mechanisms that control food intake and body weight. Yet eating disorders and cachexia are still common, and obesity cases are rising at alarming rates. Thus, despite recent progress, an increased understanding of the molecular and neural substrates that control body weight homeostasis is a major public health goal. In this review, we discuss the mechanisms by which metabolic signals interact with key behavioral, neuroendocrine, and autonomic regulatory regions of the central nervous system. Additionally, we offer a model in which hormones such as leptin and ghrelin interact with similar central nervous system circuits and engage them in such a way as to maintain an appropriate and tight regulation of body weight and food intake. Our model predicts that overstimulation or understimulation of these central pathways can result in obesity, anorexia, or cachexia.