Long-term globular adiponectin administration improves adipose tissue dysmetabolism in high-fat diet-fed Wistar rats

Adiponectin Resistance in Obesity: Adiponectin Leptin/Insulin Interaction

The adiponectin (APN) levels in obesity are negatively correlated with chronic subclinical inflammation markers. The hypertrophic adipocytes cause obesity-linked insulin resistance and metabolic syndrome. Furthermore, macrophage polarization is a key determinant regulating adiponectin receptor (AdipoR1/R2) expression and differential adiponectin-mediated macrophage inflammatory responses in obese individuals. In addition to decrease in adiponectin concentrations, the decline in AdipoR1/R2 messenger ribonucleic acid (mRNA) expression leads to a decrement in adiponectin binding to cell membrane, and this turns into attenuation in the adiponectin effects. This is defined as APN resistance, and it is linked with insulin resistance in high-fat diet-fed subjects. The insulin-resistant group has a significantly higher leptin-to-APN ratio. The leptin-to-APN ratio is more than twofold higher in obese individuals. An increase in expression of AdipoRs restores insulin sensitivity and β-oxidation of fatty acids via triggering intracellular signal cascades. The ratio of high molecular weight to total APN is defined as the APN sensitivity index (ASI). This index is correlated to insulin sensitivity. Homeostasis model of assessment (HOMA)-APN and HOMA-estimated insulin resistance (HOMA-IR) are the most suitable methods to estimate the metabolic risk in metabolic syndrome. While morbidly obese patients display a significantly higher plasma leptin and soluble (s)E-selectin concentrations, leptin-to-APN ratio, there is a significant negative correlation between leptin-to-APN ratio and sP-selectin in obese patients. When comparing the metabolic dysregulated obese group with the metabolically healthy obese group, postprandial triglyceride clearance, insulin resistance, and leptin resistance are significantly delayed following the oral fat tolerance test in the first group. A neuropeptide, Spexin (SPX), is positively correlated with the quantitative insulin sensitivity check index (QUICKI) and APN. APN resistance together with insulin resistance forms a vicious cycle. Despite normal or high APN levels, an impaired post-receptor signaling due to adaptor protein-containing pleckstrin homology domain, phosphotyrosine-binding domain, and leucine zipper motif 1 (APPL1)/APPL2 may alter APN efficiency and activity. However, APPL2 blocks adiponectin signaling through AdipoR1 and AdipoR2 because of the competitive inhibition of APPL1. APPL1, the intracellular binding partner of AdipoRs, is also an important mediator of adiponectin-dependent insulin sensitization. The elevated adiponectin levels with adiponectin resistance are compensatory responses in the condition of an unusual discordance between insulin resistance and APN unresponsiveness. Hypothalamic recombinant adeno-associated virus (rAAV)-leptin (Lep) gene therapy reduces serum APN levels, and it is a more efficient strategy for long-term weight maintenance.

Thrombin-Mediated Formation of Globular Adiponectin Promotes an Increase in Adipose Tissue Mass

A decrease in the circulating levels of adiponectin in obesity increases the risk of metabolic complications, but the role of globular adiponectin, a truncated form produced by proteolytic cleavage, has not been defined. The objective of this investigation was to determine how globular adiponectin is generated and to determine whether this process impacts obesity. The cleavage of recombinant full-length adiponectin into globular adiponectin by plasma in vitro was used to identify Gly-93 as the N-terminal residue after proteolytic processing. The amino acid sequence of the cleavage site suggested thrombin was the protease responsible for cleavage, and inhibitors confirmed its likely involvement. The proteolytic site was modified, and this thrombin-resistant mutant protein was infused for 4 weeks into obese adiponectin-knockout mice that had been on a high-fat diet for 8 weeks. The mutation of the cleavage site ensured that globular adiponectin was not generated, and thus did not confound the actions of the full-length adiponectin. Mice infused with the mutant adiponectin accumulated less fat and had smaller adipocytes compared to mice treated with globular adiponectin, and concurrently had elevated fasting glucose. The data demonstrate that generation of globular adiponectin through the action of thrombin increases both adipose tissue mass and adipocyte size, but it has no effect on fasting glucose levels in the context of obesity.

Adipocyte and steroidogenic cell cross-talk in polycystic ovary syndrome

BACKGROUND

Metabolic and endocrine alterations in women with polycystic ovary syndrome (PCOS) affect adipose tissue mass and distribution. PCOS is characterised by hyperandrogenism, obesity and adipocyte dysfunction. Hyperandrogenism in PCOS drives dysfunctional adipocyte secretion of potentially harmful adipocytokines. Glucocorticoids and sex-steroids modulate adipocyte development and function. For their part, adipocyte products interact with adrenal and ovarian steroidogenic cells. Currently, the relationship between adipocyte and steroidogenic cells is not clear, and for these reasons, it is important to elucidate the interrelationship between these cells in women with and without PCOS.

OBJECTIVE AND RATIONALE

This comprehensive review aims to assess current knowledge regarding the interrelationship between adipocytes and adrenal and ovarian steroidogenic cells in animal models and humans with or without PCOS.

SEARCH METHODS

We searched for articles published in English and Portuguese in PubMed. Keywords were as follows: polycystic ovary syndrome, steroidogenesis, adrenal glands, theca cells, granulosa cells, adipocytes, adipocytokines, obesity, enzyme activation, and cytochrome P450 enzymes. We expanded the search into the references from the retrieved articles.

OUTCOMES

Glucocorticoids and sex-steroids modulate adipocyte differentiation and function. Dysfunctional adipocyte products play important roles in the metabolic and endocrine pathways in animals and women with PCOS. Most adipokines participate in the regulation of the hypothalamic-pituitary-adrenal and ovarian axes. In animal models of PCOS, hyperinsulinemia and poor fertility are common; various adipokines modulate ovarian steroidogenesis, depending on the species. Women with PCOS secrete unbalanced levels of adipocyte products, characterised by higher levels of leptin and lower levels of adiponectin. Leptin expression positively correlates with body mass index, waist/hip ratio and levels of total cholesterol, triglyceride, luteinising hormone, oestradiol and androgens. Leptin inhibits the production of oestradiol and, in granulosa cells, may modulate 17-hydroxylase and aromatase enzyme activities. Adiponectin levels negatively correlate with fat mass, body mass index, waist-hip ratio, glucose, insulin and triglycerides, and decrease androgen production by altering expression of luteinising hormone receptor, steroidogenic acute regulatory protein, cholesterol-side-chain cleavage enzyme and 17-hydroxylase. Resistin expression positively correlates with body mass index and testosterone, and promotes the expression of 17-hydroxylase enzyme in theca cells. The potential benefits of adipokines in the treatment of women with PCOS require more investigation.

WIDER IMPLICATIONS

The current data regarding the relationship between adipocyte products and steroidogenic cells are conflicting in animals and humans. Polycystic ovary syndrome is an excellent model to investigate the interrelationship among adipocyte and steroidogenic cells. Women with PCOS manifest some pathological conditions associated with hyperandrogenism and adipocyte products. In animals, cross-talk between cells may vary according to species, and the current review suggests opportunities to test new medications to prevent or even reverse several harmful sequelae of PCOS in humans. Further studies are required to investigate the possible therapeutic application of adipokines in women with obese and non-obese PCOS. Meanwhile, when appropriate, metformin use alone, or associated with flutamide, may be considered for therapeutic purposes.

Taste Receptor Cells in Mice Express Receptors for the Hormone Adiponectin

The metabolic hormone adiponectin is secreted into the circulation by adipocytes and mediates key biological functions, including insulin sensitivity, adipocyte development, and fatty acid oxidation. Adiponectin is also abundant in saliva, where its functions are poorly understood. Here we report that murine taste receptor cells (TRCs) express specific adiponectin receptors and may be a target for salivary adiponectin. This is supported by the presence of all three known adiponectin receptors in transcriptomic data obtained by RNA-seq analysis of purified circumvallate (CV) taste buds. As well, immunohistochemical analysis of murine CV papillae showed that two adiponectin receptors, ADIPOR1 and T-cadherin, are localized to subsets of TRCs. Immunofluorescence for T-cadherin was primarily co-localized with the Type 2 TRC marker phospholipase C β2, suggesting that adiponectin signaling could impact sweet, bitter, or umami taste signaling. However, adiponectin null mice showed no differences in behavioral lick responsiveness compared with wild-type controls in brief-access lick testing. AAV-mediated overexpression of adiponectin in the salivary glands of adiponectin null mice did result in a small but significant increase in behavioral lick responsiveness to the fat emulsion Intralipid. Together, these results suggest that salivary adiponectin can affect TRC function, although its impact on taste responsiveness and peripheral taste coding remains unclear.

A 2 Adenosine Receptors Mediate Whole-Body Insulin Sensitivity in a Prediabetes Animal Model: Primary Effects on Skeletal Muscle

Epidemiological studies showed that chronic caffeine intake decreased the risk of type 2 diabetes. Previously, we described that chronic caffeine intake prevents and reverses insulin resistance induced by hypercaloric diets and aging, in rats. Caffeine has several cellular mechanisms of action, being the antagonism of adenosine receptors the only attained with human coffee consumption. Here, we investigated the subtypes of adenosine receptors involved on the effects of chronic caffeine intake on insulin sensitivity and the mechanisms and sex differences behind this effect. Experiments were performed in male and female Wistar rats fed either a chow or high-sucrose (HSu) diet (35% of sucrose in drinking water) during 28 days, to induce insulin resistance. In the last 15 days of diet the animals were submitted to DPCPX (A₁ antagonist, 0.4 mg/kg), SCH58261 (A_2A antagonist, 0.5 mg/kg), or MRS1754 (A_2B antagonist, 9.5 μg/kg) administration. Insulin sensitivity, fasting glycaemia, blood pressure, catecholamines, and fat depots were assessed. Expression of A₁, A_2A, A_2B adenosine receptors and protein involved in insulin signaling pathways were evaluated in the liver, skeletal muscle, and visceral adipose tissue. UCP1 expression was measured in adipose tissue. Paradoxically, SCH58261 and MRS1754 decreased insulin sensitivity in control animals, whereas they both improved insulin response in HSu diet animals. DPCPX did not alter significantly insulin sensitivity in control or HSu animals, but reversed the increase in total and visceral fat induced by the HSu diet. In skeletal muscle, A₁, A_2A, and A_2B adenosine receptor expression were increased in HSu group, an effect that was restored by SCH58261 and MRS1754. In the liver, A₁, A_2A expression was increased in HSu group, while A_2B expression was decreased, being this last effect reversed by administration of MRS1754. In adipose tissue, A₁ and A_2A block upregulated the expression of these receptors. A₂ adenosine antagonists restored impaired insulin signaling in the skeletal muscle of HSu rats, but did not affect liver or adipose insulin signaling. Our results show that adenosine receptors exert opposite effects on insulin sensitivity, in control and insulin resistant states and strongly suggest that A₂ adenosine receptors in the skeletal muscle are the majors responsible for whole-body insulin sensitivity.

Carotid body: a metabolic sensor implicated in insulin resistance

The carotid body is now looked at as a multipurpose sensor for blood gases, blood pH, and several hormones. The matter of glucose sensing by the carotid body has been debated for several years in the literature, and these days there is a consensus that carotid body activity is modified by metabolic factors that contribute to glucose homeostasis. However, the sensing ability for glucose is still being pondered: are the carotid bodies low glucose sensors or, in contrast, are they overresponsive in high-glucose conditions? Herein, we debate the glucose and insulin sensing capabilities of the carotid body as key early events in the overactivation of the carotid body, which is increasingly recognized as an important feature of metabolic diseases. Additionally, we dedicate a final section to discuss new outside-the-box therapies designed to decrease carotid body activity that may be used for treating metabolic diseases.

Chicken Is a Useful Model to Investigate the Role of Adipokines in Metabolic and Reproductive Diseases

Reproduction is a complex and essential physiological process required by all species to produce a new generation. This process involves strict hormonal regulation, depending on a connection between the hypothalamus-pituitary-gonadal axis and peripheral organs. Metabolic homeostasis influences the reproductive functions, and its alteration leads to disturbances in the reproductive functions of humans as well as animals. For a long time, adipose tissue has been recognised as an endocrine organ but its ability to secrete and release hormones called adipokines is now emerging. Adipokines have been found to play a major role in the regulation of metabolic and reproductive processes at both central and peripheral levels. Leptin was initially the first adipokine that has been described to be the most involved in the metabolism/reproduction interrelation in mammals. In avian species, the role of leptin is still under debate. Recently, three novel adipokines have been discovered: adiponectin (ADIPOQ, ACRP30), visfatin (NAMPT, PBEF), and chemerin (RARRES2, TIG2). However, their mode of action between mammalian and nonmammalian species is different due to the different reproductive and metabolic systems. Herein, we will provide an overview of the structure and function related to metabolic and reproductive mechanisms of the latter three adipokines with emphasis on avian species.

Sexual Dimorphism in the Age-Induced Insulin Resistance, Liver Steatosis, and Adipose Tissue Function in Rats

Age-linked metabolic disturbances, such as liver steatosis and insulin resistance, show greater prevalence in men than in women. Thus, our aim was to analyze these sex-related differences in male and female Wistar rats (aged 26 days and 3, 7, and 14 months), and to assess their potential relationship with alterations in the capacity of adipose tissue expansion and the dysregulation of the main adipokines produced by the adipose tissue, leptin and adiponectin. Adiposity-related parameters, blood parameters, the expression of genes related to expandability and inflammation (WAT), lipid metabolism (liver), and leptin and insulin signaling (both tissues) were measured. In females, adiposity index and WAT DNA content gradually increased with age, whereas males peaked at 7 months. A similar sex-dependent pattern was observed for leptin expression in WAT, while Mest expression levels decreased with age in males but not in females. Females also showed increased expression of the proliferation marker PCNA in the inguinal WAT compared to males. In males, leptin/adiponectin ratio greatly increased from 7 to 14 months in a more acute manner than in females, along with an increase in HOMA-IR index and hepatic triacylglyceride content, while no changes were observed in females. In liver, 14-month-old males displayed decreased mRNA levels of Insr, Ampkα2, and Cpt1a compared with levels at 7 months. Males also showed decreased mRNA levels of Obrb (both tissues), and increased expression levels of Cd68 and Emr1 (WAT) with age. In conclusion, females are more protected from age-related metabolic disturbances, such as insulin resistance, hepatic lipid deposition, and WAT inflammation compared to males. This may be related to their greater capacity for WAT expansion-reflected by a greater Mest/leptin mRNA ratio-and to their ability to maintain adiponectin levels and preserve leptin sensitivity with aging.

Omentin treatment of epicardial fat improves its anti-inflammatory activity and paracrine benefit on smooth muscle cells

OBJECTIVE

Epicardial adipose tissue (EAT) in coronary artery disease is insulin resistant and has a proinflammatory profile. This study examined the regulation of EAT by exogenous omentin and its consequence on vascular cells.

METHODS

Stromal vascular cells (SC) of EAT and subcutaneous adipose tissue (SAT) from patients who underwent heart surgery were cultured and exposed to adipogenic factors with or without omentin. Proinflammatory cytokine regulation by omentin was analyzed in SC and mature adipocytes. Glucose uptake by EAT and SAT explants was determined after insulin, omentin, or combined treatment. Human vascular cells were exposed to secretomes of SC, with and without omentin treatment. Migration of smooth muscle cells and expression of adhesion molecules were determined by wound healing or real-time polymerase chain reaction, respectively.

RESULTS

Omentin treatment raised adipogenesis-induced adiponectin levels on SC of EAT and reduced TNF-α expression levels (0.58 ± 0.14-fold change; P = 0.034) in mature adipocytes. Omentin improved the insulin activity of EAT and SAT explants from cardiovascular disease patients. Finally, secretomes of SC under omentin treatment reduced the migration of smooth muscle cells.

CONCLUSIONS

Exogenous omentin might support a cardioprotective role through its effect on EAT regarding glucose uptake, anti-inflammatory response, and its paracrine role on smooth muscle cells.

Adiponectin improves endothelial function in mesenteric arteries of rats fed a high-fat diet: role of perivascular adipose tissue

BACKGROUND AND PURPOSE

Adiponectin, the most abundant peptide secreted by adipocytes, is involved in the regulation of energy metabolism and vascular physiology. Here, we have investigated the effects of exogenous administration of adiponectin on metabolism, vascular reactivity and perivascular adipose tissue (PVAT) of mesenteric arteries in Wistar rats fed a high-fat diet.

EXPERIMENTAL APPROACH

The effects of adiponectin on NO-dependent and independent vasorelaxation were investigated in isolated mesenteric arteries from 12-month-old male Wistar rats (W12m) fed a high-fat diet (HFD) for 4 months and compared with those from age-matched rats given a control diet. Adiponectin ((96 μg·day^-1 ) was administered by continuous infusion with a minipump, implanted subcutaneously, for 28 days.

KEY RESULTS

Chronic adiponectin treatment reduced body weight, total cholesterol, free fatty acids, fasting glucose and area under the curve of intraperitoneal glucose tolerance test, compared with HFD rats. It also normalized NO-dependent vasorelaxation increasing endothelial NO synthase (eNOS) phosphorylation in mesenteric arteries of HFD rats. In PVAT from aged (W12m) and HFD rats there was increased expression of chemokines and pro-inflammatory adipokines, the latter being important contributors to endothelial dysfunction. Infusion of adiponectin reduced these changes.

CONCLUSIONS AND IMPLICATIONS

Adiponectin normalized endothelial cell function by a mechanism that involved increased eNOS phoshorylation and decreased PVAT inflammation. Detailed characterization of the adiponectin signalling pathway in the vasculature and perivascular fat is likely to provide novel approaches to the management of atherosclerosis and metabolic disease.

LINKED ARTICLES

This article is part of a themed section on Molecular Mechanisms Regulating Perivascular Adipose Tissue - Potential Pharmacological Targets? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.20/issuetoc.

Functional abolition of carotid body activity restores insulin action and glucose homeostasis in rats: key roles for visceral adipose tissue and the liver

AIMS/HYPOTHESIS

We recently described that carotid body (CB) over-activation is involved in the aetiology of insulin resistance and arterial hypertension in animal models of the metabolic syndrome. Additionally, we have demonstrated that CB activity is increased in animal models of insulin resistance, and that carotid sinus nerve (CSN) resection prevents the development of insulin resistance and arterial hypertension induced by high-energy diets. Here, we tested whether the functional abolition of CB by CSN transection would reverse pre-established insulin resistance, dyslipidaemia, obesity, autonomic dysfunction and hypertension in animal models of the metabolic syndrome. The effect of CSN resection on insulin signalling pathways and tissue-specific glucose uptake was evaluated in skeletal muscle, adipose tissue and liver.

METHODS

Experiments were performed in male Wistar rats submitted to two high-energy diets: a high-fat diet, representing a model of insulin resistance, hypertension and obesity, and a high-sucrose diet, representing a lean model of insulin resistance and hypertension. Half of each group was submitted to chronic bilateral resection of the CSN. Age-matched control rats were also used.

RESULTS

CSN resection normalised systemic sympathetic nervous system activity and reversed weight gain induced by high-energy diets. It also normalised plasma glucose and insulin levels, insulin sensitivity lipid profile, arterial pressure and endothelial function by improving glucose uptake by the liver and perienteric adipose tissue.

CONCLUSIONS/INTERPRETATION

We concluded that functional abolition of CB activity restores insulin sensitivity and glucose homeostasis by positively affecting insulin signalling pathways in visceral adipose tissue and liver.

Adiponectin-Resistance in Obesity

The decrease in adiponectin levels are negatively correlated with chronic subclinical inflammation markers in obesity. The hypertrophic adipocytes cause obesity-linked insulin resistance and metabolic syndrome. Furthermore, macrophage polarization is a key determinant regulating adiponectin receptor (AdipoR1/R2) expression and differential adiponectin-mediated macrophage inflammatory responses in obese individuals. In addition to decrease in adiponectin concentrations, the decline in AdipoR1/R2 mRNA expression leads to a decrement in adiponectin binding to cell membrane, and this turns into attenuation in the adiponectin effects. Within the receptor complex, adaptor protein-containing pleckstrin homology domain, phosphotyrosine-binding domain, and leucine zipper motif 1 (APPL1) is the intracellular binding partner of AdipoR1 and AdipoR2. The expression levels of APPL1 or APPL2 lead to an altered adiponectin activity. Despite normal or high adiponectin levels, an impaired post receptor signaling due to APPL1/APPL2 may alter adiponectin efficiency and activity. However, APPL2 blocks adiponectin signaling through AdipoR1 and AdipoR2 by competitive inhibition of APPL1. APPL1 is also an important mediator of adiponectin dependent insulin sensitization. In this context, adiponectin resistance is associated with insulin resistance and is thought to be partly due to the down-regulation of the AdipoRs in high-fat diet fed subjects. Actually, adiponectin resistance occurs very rapidly after saturated fatty acid feeding, this metabolic disturbance is not due to a decrease in AdipoR1 protein content. Intra-abdominal adipose tissue-AdipoR2 expression is reduced in obesity, whereas AdipoR1 expression is not changed. Adiponectin resistance together with insulin resistance forms a vicious cycle. The elevated adiponectin levels with adiponectin resistance is a compensatory response in the condition of an unusual discordance between insulin resistance and adiponectin unresponsiveness.Additionally, different mechanisms are involved in vascular adiponectin resistance at different stages of obesity. Nevertheless, diet-induced hyperlipidemia is the leading cause of vascular adiponectin resistance. Leptin/adiponectin imbalance may also be an important marker of the elevated risk of developing abdominal obesity-associated cardiovascular diseases.

Reduced expression of collagen VI alpha 3 (COL6A3) confers resistance to inflammation-induced MCP1 expression in adipocytes

OBJECTIVE

Collagen VI alpha 3 (COL6A3) is associated with insulin resistance and adipose tissue inflammation. In this study, the role of COL6A3 in human adipocyte function was characterized.

METHODS

Immortalized human preadipocyte cell lines stably expressing control or COL6A3 shRNA were used to study adipocyte function and inflammation.

RESULTS

COL6A3 knockdown increased triglyceride content, lipolysis, insulin-induced Akt phosphorylation, and mRNA expression of key adipogenic genes (peroxisome proliferator-activated receptor-γ, glucose transporter, adiponectin, and fatty acid binding protein), indicating increased adipocyte function and insulin sensitivity. However, COL6A3 knockdown decreased basal adipocyte chemokine (C-C motif) ligand 2 [CCL2, monocyte chemoattractant protein (MCP1)] mRNA expression, reduced secreted protein levels, and abrogated tumor necrosis factor-α- and lipopolysaccharide-induced MCP1 mRNA expression. In addition, while control adipocytes co-cultured with THP1 macrophages showed a threefold increase in adipocyte MCP1 mRNA expression, in COL6A3 knockdown adipocytes MCP1 mRNA expression was unaltered by co-culturing. Lastly, in normal differentiated adipocytes, matrix metalloproteinase-11 treatment reduced expression of COL6A3 protein, MCP1 mRNA, MCP1 secretion, and abrogated tumor necrosis factor-α- and lipopolysaccharide-induced MCP1 mRNA expression and protein secretion.

CONCLUSIONS

COL6A3 knockdown in adipocytes leads to the development of a unique state of inflammatory resistance via suppression of MCP1 induction.