Sialic acid modification of adiponectin is not required for multimerization or secretion but determines half-life in circulation

The connections of sialic acids and diabetes mellitus: therapeutic or diagnostic value?

Modulation of sialic acids is one of the important pathological consequences of both type 1 and type 2 diabetes mellitus with or without the micro- and macrovascular complications. However, the mechanistic, therapeutic and/or diagnostic implications of these observations are uncoordinated and possibly conflicting. This review critically analyses the scientific investigations connecting sialic acids with diabetes mellitus. Generally, variations in the levels and patterns of sialylation, fucosylation and galactosylation were predominant across various tissues and body systems of diabetic patients, but the immune system seemed to be most affected. These might be explored as a basis for differential diagnosis of various diabetic complications. Sialic acids are predominantly elevated in nearly all forms of diabetic conditions, particularly nephropathy and retinopathy, which suggests some diagnostic value but the mechanistic details were not unequivocal from the available data. The plausible mechanistic explanations for the elevated sialic acids are increased desialylation by sialidases, stimulation of hexosamine pathway and synthesis of acute phase proteins as well as oxidative stress. Additionally, sialic acids are also profoundly associated with glucose transport and insulin resistance in human-based studies while animal-based studies revealed that the increased desialylation of insulin receptors by sialidases, especially NEU1, might be the causal link. Interestingly, inhibition of the diabetes-associated NEU1 desialylation was beneficial in diabetes management and might be considered as a therapeutic target. It is hoped that the article will provide an informed basis for future research activities on the exploitation of sialic acids and glycobiology for therapeutic and/or diagnostic purposes against diabetes mellitus.

The Effects of Brown Algae-Derived Monosaccharide L-Fucose on Lipid Metabolism in C57BL/6J Obese Mice

Obesity is a global public health problem and a risk factor for several metabolic disorders as well as cancer. In this study, we investigated the effects of L-fucose on lipid metabolism through chronic and acute in vivo experiments in mice. In the chronic test, mice were fed a high-calorie diet (HCD) containing 0.0001%, 0.001%, 0.01%, and 0.1% L-fucose for one month. The L-fucose supplementation inhibited body weight and visceral fat mass gain in HCD-fed mice. The results of the acute test showed that L-fucose increased the ratio of serum high molecular weight adiponectin and enhanced glucose and lipid catabolism. Furthermore, L-fucose also decreased the expression of adipogenic genes (peroxisome proliferator-activated receptor γ and cluster of differentiation 36). In conclusion, this study provides a new approach to combat obesity and the related diseases.

Mapping and engineering the interaction between adiponectin and T-cadherin

Adiponectin is a highly abundant protein hormone secreted by adipose tissue. It elicits diverse biological responses, including anti-diabetic, anti-inflammatory, anti-tumor, and anti-atherosclerotic effects. Adiponectin consists of a globular domain and a collagen-like domain, and it occurs in three major oligomeric forms that self-assemble: trimers, hexamers, and high-molecular-weight oligomers. Adiponectin has been reported to bind to two seven-transmembrane domain receptors, AdipoR1 and AdipoR2, as well as to the protein T-cadherin, which is highly expressed in the cardiovascular system and binds only the high-molecular-weight form of adiponectin. The molecular mechanisms underlying this specificity remain unclear. Here we used a combination of X-ray crystallography and protein engineering to define the details of adiponectin's interaction with T-cadherin. We found that T-cadherin binds to the globular domain of adiponectin, relying on structural stabilization of this domain by bound metal ions. Moreover, we show that the adiponectin globular domain can be engineered to enhance its binding affinity for T-cadherin. These results help to define the molecular basis for the interaction between adiponectin and T-cadherin, and our engineered globular domain variants may be useful tools for further investigating adiponectin's functions.

Adiponectin-Consideration for its Role in Skeletal Muscle Health

Adiponectin regulates metabolism through blood glucose control and fatty acid oxidation, partly mediated by downstream effects of adiponectin signaling in skeletal muscle. More recently, skeletal muscle has been identified as a source of adiponectin expression, fueling interest in the role of adiponectin as both a circulating adipokine and a locally expressed paracrine/autocrine factor. In addition to being metabolically responsive, skeletal muscle functional capacity, calcium handling, growth and maintenance, regenerative capacity, and susceptibility to chronic inflammation are all strongly influenced by adiponectin stimulation. Furthermore, physical exercise has clear links to adiponectin expression and circulating concentrations in healthy and diseased populations. Greater physical activity is generally related to higher adiponectin expression while lower adiponectin levels are found in inactive obese, pre-diabetic, and diabetic populations. Exercise training typically restores plasma adiponectin and is associated with improved insulin sensitivity. Thus, the role of adiponectin signaling in skeletal muscle has expanded beyond that of a metabolic regulator to include several aspects of skeletal muscle function and maintenance critical to muscle health, many of which are responsive to, and mediated by, physical exercise.

Sustained Intermittent Hypoxemia Induces Adiponectin Oligomers Redistribution and a Tissue-Specific Modulation of Adiponectin Receptor in Mice

Introduction: Hypoxemia is a critical component of several respiratory diseases and is known to be involved in the processes underlying co-morbidities associated to such disorders, notably at the cardiovascular level. Circulating level of Adiponectin (Ad), known as a metabolic regulator and cardio-protective hormone was previously suggested to be reduced by hypoxia but consequences of such variation are unclear. The evaluation of the specific effect of hypoxemia on Ad forms and receptors could improve the understanding of the involvement of Ad axis in hypoxemia-related diseases.

Methods: Ad-pathway components were investigated in a murine model of sustained intermittent hypoxemia (FiO₂ 10%, 8 h/day, 35 days).

Results: Sustained intermittent hypoxemia (SIH) induced a redistribution of Ad multimers in favor of HMW forms, without change in total plasmatic level. Mice submitted to hypoxia also exhibited tissue-specific modification of adiporeceptor (AdipoR) protein level without mRNA expression change. A decreased AdipoR2 abundance was observed in skeletal muscle and heart whereas AdipoR1 level was only reduced in muscle. No change was observed in liver regarding AdipoR. Lipid profile was unchanged but glucose tolerance increased in hypoxemic mice.

Conclusion: Sustained intermittent hypoxemia, per se, modify Ad oligomerization state as well as AdipoR protein abundance in a tissue-specific way. That suggests alteration in Ad-dependant pathways in pathological conditions associated to SIH. Investigation of Ad-pathway components could therefore constitute useful complementary criteria for the clustering of patients with hypoxemia-related diseases and management of co-morbidities, as well as to develop new therapeutic strategies.

Adiponectin as a Potential Therapeutic Target for Prostate Cancer

Adipokines are bioactive proteins that mediate proliferation, metabolism, inflammation, and angiogenesis. Adiponectin is an important adipokine that exerts multiple key functions via its anti-metabolic syndrome and anti-inflammatory properties. A number of adiponectin receptors, AdipoR1, AdipoR2 and T-cadherin, have been identified. Recent studies have suggested the involvement of adiponectin and receptors in several cancers, including prostate, breast, endometrial, brain, and colon cancer. Altered levels of adiponectin expression, or its interacting receptors, in cancers can lead to dysregulation of signaling pathways. Our current review describes the molecular mechanisms underlying the anti-tumorigenesis activity of adiponectin and the role of its receptors in prostate carcinogenesis, and provides perspectives of adiponectin-mediated signaling as a potential target for therapy.

Collagen overlays can inhibit leptin and adiponectin secretion but not lipid accumulation in adipocytes

Background

White adipose tissue (WAT) is essential for energy storage as well as being an active endocrine organ. The secretion of adipokines by adipocytes can affect whole body metabolism, appetite, and contribute to overall health. WAT is comprised of lipid-laden mature adipocytes, as well as immune cells, endothelial cells, pre-adipocytes, and adipose-derived stem cells. In addition, the presence of extracellular matrix (ECM) proteins in WAT can actively influence adipocyte differentiation, growth, and function. Type I collagen is an abundant fibrous ECM protein in WAT that is secreted by developing adipocytes. However, the extent and overall effect of Type I collagen on adipokine secretion in mature adipocytes when added exogenously has not been established.

Methods

We characterized the effects of Type I collagen overlays prepared using two different buffers on adipocyte physiology and function when added at different times during differentiation. In addition, we compared the effect of collagen overlays when adipocytes were cultured on two different tissue culture plastics that have different adherent capabilities. Triglyceride accumulation was analyzed to measure adipocyte physiology, and leptin and adiponectin secretion was determined to analyze effects on adipokine secretion.

Results

We found that collagen overlays, particularly when added during the early differentiation stage, impaired adipokine secretion from mature adipocytes. Collagen prepared using PBS had a greater suppression of leptin than adiponectin while collagen prepared using HANKS buffer suppressed the secretion of both adipokines. The use of CellBind plates further suppressed leptin secretion. Triglyceride accumulation was not substantially impacted with any of the collagen overlays.

Discussion

Adipokine secretion can be selectively altered by collagen overlays. Thus, it is feasible to selectively manipulate the secretion of adipokines by adipocytes in vitro by altering the composition or timing of collagen overlays. The use of this technique could be applied to studies of adipokine function and secretion in vitro as well as having potential therapeutic implications to specifically alter adipocyte functionality in vivo.

Collagen beta (1-O) galactosyltransferase 1 (GLT25D1) is required for the secretion of high molecular weight adiponectin and affects lipid accumulation

Secretion of high molecular weight (HMW) adiponectin is dependent on post-translational modification (PTM) of conserved lysines in the collagenous domain. The present study aims to characterize the enzymes responsible for the PTM of conserved lysines which leads to HMW adiponectin secretion, and to define its significance in relation to obesity. Collagen beta (1-O) galactosyltransferase 1 (GLT25D1) was knocked down in HEK cells modified for the stable expression of adiponectin (adiponectin expressing human embryonic kidney cells, Adipo-HEK) as well as in Simpson Golabi-Behmel-Syndrome (SGBS) adipocytes. Knockdown of GLT25D1 caused a significant decrease in HMW adiponectin in Adipo-HEK cells with no change in total adiponectin. Knockdown in the SGBS cells caused an increase in lipid accumulation yet inhibited adipogenesis. Co-immunoprecipitation with adiponectin and mass spectrometry showed that adiponectin formed a protein complex with lysyl hydroxylase 3 (LH3) and GLT25D1. Transient overexpression of GLT25D1 showed that the intracellular retention of LH3 was dependent on GLT25D1. To determine whether changes in GLT25D1 were significant in obesity, mice were fed a standard chow or high-fat diet (HFD) for 5 weeks. GLT25D1 was significantly decreased in mice fed HFD which coincided with a decrease in HMW adiponectin. We conclude that GLT25D1 regulates HMW adiponectin secretion and lipid accumulation, consistent with changes in mice after high-fat feeding. These results suggest a novel function of GLT25D1 leading to decreased HMW adiponectin secretion in early obesity.

Establishment of a fluorescence-based method to evaluate endocytosis of desialylated glycoproteins in vitro

Insufficient sialylation can result in rapid clearance of therapeutic glycoproteins by intracellular degradation, which is mainly mediated by asialoglycoprotein receptors (ASGPRs) on hepatic cells. In contrast, for glycoproteins, a long half-life is often related to high level of terminal sialic acid. These could be extremely important for insufficient sialylated biomedicines in clinic, and development of therapeutic glycoproteins in laboratory. However, how the desialylated glycoproteins are removed and how to evaluate the ASGPRs mediated endocytosis in vitro needs further investigate. Herein we described an integrative characterization of ASGPRs in vitro to elucidate its endocytosis properties. The endocytosis was determined by a fluorescence-based quantization method. The results showed that the ASGPRs could bind to poorly sialylated glycoproteins including asialofetuin and low sialylated recombinant Factor VIIa with a relatively higher ASGPRs binding affinity, and induce a more rapid endocytosis in vitro. Moreover, the mechanism under the internalization of ASGPRs was also investigated, which was found to depend on clathrin and caveolin. Utilizing the relative fluorescence quantification can be suitable for measurement of insufficient sialylated glycoprotein endocytosis and quality control of therapeutic glycoproteins, which could be useful for the understanding of the development of therapeutic glycoproteins.

Insights into the Impact of Heterogeneous Glycosylation on the Pharmacokinetic Behavior of Follistatin-Fc-Based Biotherapeutics

Follistatin 315 heparan sulfate-binding deficient mutant human IgG4 Fc fusion (FST-ΔHBS-Fc) is a follistatin (FST) based Fc fusion protein currently being developed as a novel therapy for several potential indications, including muscle wasting. Previous assessments of the pharmacokinetics and therapeutic activity of FST-ΔHBS-Fc have shown a close association of the exposure-response relationship. The current work builds upon these initial studies by investigating the glycosylation characteristics of FST-ΔHBS-Fc after recombinant expression and its impact on the pharmacokinetics in mice and Cynomolgus monkeys. The data presented indicate that FST-ΔHBS-Fc is heterogeneously glycosylated at the three putative sites in FST when recombinantly expressed in stably transfected Chinese hamster ovary cells. Such carbohydrate heterogeneity, especially with regards to sialic acid incorporation, directly results in sugar-dependent clearance in both mice and Cynomolgus monkeys. Examination of the pharmacokinetics of FST-ΔHBS-Fc molecules containing variable sialic acid content in asialoglycoprotein receptor 1 (ASPGR-1) knockout mice supports the receptor's role as part of the clearance mechanism of the molecules. Based on the evaluation of several variably sialylated lots of material in pharmacokinetic assessments, we define specifications for average sialic acid incorporation into FST-ΔHBS-Fc that result in limited sugar-mediated clearance. Taken together, these studies highlight the importance of establishing an early understanding of the glycosylation/pharmacokinetic relationships of FST-ΔHBS-Fc, which will provide a basis for future application toward optimal systemic drug delivery and dosing strategies.

The diverse roles of adiponectin in non-small-cell lung cancer: current data and future perspectives

In recent years, there is growing research interest for the biological role of adipose tissue-derived bioactive factors, mainly including adipokines, in various forms of cancer. Adiponectin (APN) is the most abundant circulating adipokine, and a key mediator of several cancer-related processes, such as cell proliferation, apoptosis, regulation of tumor cell invasion and angiogenesis. In this review we summarize and critically discuss the published literature on the diverse roles of APN in non-small-cell lung cancer, including its implication in lung cancer development, its use as a diagnostic and prognostic biomarker, and its correlation with cancer-related cachexia. The main challenges and future perspectives, mainly with regard to the potential development of APN-targeted therapeutic agents in cancer therapeutics, are also briefly presented and discussed.

Adiponectin-SOGA Dissociation in Type 1 Diabetes

CONTEXT

Circulating adiponectin is elevated in human type 1 diabetes (T1D) and nonobese diabetic (NOD) mice without the expected indications of adiponectin action, consistent with tissue resistance.

OBJECTIVE

Adiponectin stimulates hepatocyte production of the suppressor of glucose from autophagy (SOGA), a protein that inhibits glucose production. We postulated that due to tissue resistance, the elevation of adiponectin in T1D should fail to increase the levels of a surrogate marker for liver SOGA, the circulating C-terminal SOGA fragment.

MAIN OUTCOME MEASURES

Liver and plasma SOGA were measured in NOD mice (n = 12) by Western blot. Serum adiponectin and SOGA were measured in T1D and control (Ctrl) participants undergoing a three-stage insulin clamp for the Coronary Artery Calcification in T1D study (n = 20). Glucose turnover was measured using 6,6[(2)H2]glucose (n = 12).

RESULTS

In diabetic NOD mice, the 13%-29% decrease of liver SOGA (P = .003) and the 30%-37% reduction of circulating SOGA (P < .001) were correlated (r = 0.826; P = .001). In T1D serum, adiponectin was 50%-60% higher than Ctrl, SOGA was 30%-50% lower and insulin was 3-fold higher (P < .05). At the low insulin infusion rate (4 mU/m(2)·min), the resulting glucose appearance correlated negatively with adiponectin in T1D (r = -0.985, P = .002) and SOGA in Ctrl and T1D (r = -0.837, P = .001). Glucose disappearance correlated with adiponectin in Ctrl (r = -0.757, P = .049) and SOGA in Ctrl and T1D (r = -0.709, P = .010). At 40 mU/m(2)·min, the lowered glucose appearance was similar in Ctrl and T1D. Glucose disappearance increased only in Ctrl (P = .005), requiring greater glucose infusion to maintain euglycemia (8.58 ± 1.29 vs 3.09 ± 0.87 mg/kg·min; P = .009).

CONCLUSIONS

The correlation between liver and plasma SOGA in NOD mice supports the use of the latter as surrogate marker for liver concentration. Reduced SOGA in diabetic NOD mice suggests resistance to adiponectin. The dissociation between adiponectin and SOGA in T1D raises the possibility that restoring adiponectin signaling and SOGA might improve the metabolic response to insulin therapy.

Glycoengineering of Chinese hamster ovary cells for enhanced erythropoietin N-glycan branching and sialylation

Sialic acid, a terminal residue on complex N-glycans, and branching or antennarity can play key roles in both the biological activity and circulatory lifetime of recombinant glycoproteins of therapeutic interest. In order to examine the impact of glycosyltransferase expression on the N-glycosylation of recombinant erythropoietin (rEPO), a human α2,6-sialyltransferase (ST6Gal1) was expressed in Chinese hamster ovary (CHO-K1) cells. Sialylation increased on both EPO and CHO cellular proteins as observed by SNA lectin analysis, and HPLC profiling revealed that the sialic acid content of total glycans on EPO increased by 26%. The increase in sialic acid content was further verified by detailed profiling of the N-glycan structures using mass spectra (MS) analysis. In order to enhance antennarity/branching, UDP-N-acetylglucosamine: α-1,3-D-mannoside β1,4-N-acetylglucosaminyltransferase (GnTIV/Mgat4) and UDP-N-acetylglucosamine:α-1,6-D-mannoside β1,6-N-acetylglucosaminyltransferase (GnTV/Mgat5), was incorporated into CHO-K1 together with ST6Gal1. Tri- and tetraantennary N-glycans represented approximately 92% of the total N-glycans on the resulting EPO as measured using MS analysis. Furthermore, sialic acid content of rEPO from these engineered cells was increased ∼45% higher with tetra-sialylation accounting for ∼10% of total sugar chains compared to ∼3% for the wild-type parental CHO-K1. In this way, coordinated overexpression of these three glycosyltransferases for the first time in model CHO-K1 cell lines provides a mean for enhancing both N-glycan branching complexity and sialylation with opportunities to generate tailored complex N-glycan structures on therapeutic glycoproteins in the future.

The α2,3-sialyltransferase encoded by myxoma virus is a virulence factor that contributes to immunosuppression

Myxoma virus (MYXV) induces a lethal disease called Myxomatosis in European rabbits. MYXV is one of the rare viruses that encodes an α2,3-sialyltransferase through its M138L gene. In this study, we showed that although the absence of the enzyme was not associated with any in vitro deficit, the M138L deficient strains are highly attenuated in vivo. Indeed, while all rabbits infected with the parental and the revertant strains died within 9 days post-infection from severe myxomatosis, all but one rabbit inoculated with the M138L deficient strains survived the infection. In primary lesions, this resistance to the infection was associated with an increased ability of innate immune cells, mostly neutrophils, to migrate to the site of virus replication at 4 days post-infection. This was followed by the development of a better specific immune response against MYXV. Indeed, at day 9 post-infection, we observed an important proliferation of lymphocytes and an intense congestion of blood vessels in lymph nodes after M138L knockouts infection. Accordingly, in these rabbits, we observed an intense mononuclear cell infiltration throughout the dermis in primary lesions and higher titers of neutralizing antibodies. Finally, this adaptive immune response provided protection to these surviving rabbits against a challenge with the MYXV WT strain. Altogether, these results show that expression of the M138L gene contributes directly or indirectly to immune evasion by MYXV. In the future, these results could help us to better understand the pathogenesis of myxomatosis but also the importance of glycans in regulation of immune responses.

Assembly of adiponectin oligomers

Adiponectin is among the most studied adipokines, the collection of molecules secreted from adipose tissue. It is also one of the most architecturally complex adipokines with its various oligomeric states that include trimers, hexamers, nonamers (9mers), dodecamers (12mers), and octadecamers (18mers). The importance of adiponectin in metabolic regulation is underscored by its strong positive associations with improvement in insulin action and also decreased risks for developing type 2 diabetes. Understanding the mechanisms involved in maintaining the steady-state concentrations of adiponectin oligomers in circulation is therefore likely to provide important insight into the development of insulin resistance. This review will discuss the current state of knowledge regarding the biochemical composition of adiponectin oligomers, the commonly used techniques to analyze them, and the known post-translational modifications that affect their assembly. Evidence based on in vitro oligomer assembly reactions in support of a "cystine ratchet" model of adiponectin oligomer formation will be considered along with limitations of the evidence. Secretory pathway proteins that have been shown to affect the distribution of adiponectin oligomers will also be discussed along with hypotheses regarding their potential involvement in the cystine ratchet model of adiponectin oligomerization.

Back to the heart: the protective role of adiponectin

Cardiovascular disease (CVD) is the leading cause of death worldwide and the prevalence of obesity and diabetes are increasing. In obesity, adipose tissue increases the secretion of bioactive mediators (adipokines) that may represent a key mechanism linking obesity to CVD. Adiponectin, extensively studied in metabolic diseases, exerts anti-diabetic, anti-atherogenic and anti-inflammatory activities. Due to these positive actions, the role of adiponectin in cardiovascular protection has been evaluated in recent years. In particular, for its potential therapeutic benefits in humans, adiponectin has become the subject of intense preclinical research. In the cardiovascular context, understanding of the cellular and molecular mechanisms underlying the adiponectin system, throughout its secretion, regulation and signaling, is critical for designing new drugs that target adiponectin system molecules. This review focused on recent advances regarding molecular mechanisms related to protective effects of the adiponectin system on both cardiac and vascular compartments and its potential use as a target for therapeutic intervention of CVD.

Multifaceted roles of adiponectin in cancer

Obesity is linked to increased cancer risk. Pathological expansion of adipose tissue impacts adipocyte function and secretion of hormonal factors regulating tissue homeostasis and metabolism. Adiponectin is an adipocyte-secreted, circulating hormone with pleiotropic functions in lipid and glucose metabolism, and beneficial roles in cardiovascular functions and inflammation. In obesity, decreased Adiponectin plasma levels correlate with tumor development and progression. The association of Adiponectin with potential tumor-limiting functions has raised significant interest in exploring this adipokine as a target for cancer-diagnostic and therapeutic applications. Recent studies, however, also implicate Adiponectin in supporting malignancy. This review highlights the evidence that links Adiponectin signaling to either cancer-protective or cancer-supporting functions. In this context, we discuss Adiponectin interactions with its receptors and associated signaling pathways. Despite significant advances in understanding Adiponectin functions and signaling mechanisms, its role in cancer remains multifaceted and subject to controversy.

Inhibition of ERK1/2 pathway suppresses adiponectin secretion via accelerating protein degradation by Ubiquitin-proteasome system: relevance to obesity-related adiponectin decline

OBJECTIVE

Predominantly secreted by adipose tissue, adiponectin possesses insulin-sensitizing, anti-atherogenic, anti-inflammatory, and anti-angiogenic properties. Paradoxically, obesity is associated with declined plasma adiponectin levels; however, the underlying mechanisms remain elusive. In this study, we investigated the mechanistic involvement of MEK/ERK1/2 pathway in obesity-related adiponectin decrease.

MATERIALS/METHODS

C57 BL/6 mice exposed to a high-fat diet (HFD) were employed as animal obesity model. Both fully-differentiated 3T3-L1 and mouse primary adipocytes were used in the in vitro experiments.

RESULTS

Obesity and plasma adiponectin decline induced by prolonged HFD exposure were associated with suppressed ERK1/2 activation in adipose tissue. In adipocytes, specific inhibition of MEK/ERK1/2 pathway decreased intracellular and secretory adiponectin levels, whereas adiponectin gene expression was increased, suggesting that MEK/ERK1/2 inhibition may promote adiponectin protein degradation. Cycloheximide (CHX)-chase assay revealed that MEK/ERK1/2 inhibition accelerated adiponectin protein degradation, which was prevented by MG132, a potent proteasome inhibitor. Immunoprecipitation assay showed that intracellular MEK/ERK1/2 activity was negatively associated with ubiquitinated adiponectin protein levels. Consistently, long-term HFD feeing in mice increased ubiquitinated adiponectin levels in the epididymal fat pads.

CONCLUSIONS

Adipose tissue MEK/ERK1/2 activity can differentially regulate adiponectin gene expression and protein abundance and its suppression in obesity may play a mechanistic role in obesity-related plasma adiponectin decline.

Eicosanoids and adipokines in breast cancer: from molecular mechanisms to clinical considerations

Chronic inflammation is one of the foremost risk factors for different types of malignancies, including breast cancer. Additional risk factors of this pathology in postmenopausal women are weight gain, obesity, estrogen secretion, and an imbalance in the production of adipokines, such as leptin and adiponectin. Various signaling products of transcription factor, nuclear factor-kappaB, in particular inflammatory eicosanoids, reactive oxygen species (ROS), and cytokines, are thought to be involved in chronic inflammation-induced cancer. Together, these key components have an influence on inflammatory reactions in malignant tissue damage when their levels are deregulated endogenously. Prostaglandins (PGs) are well recognized in inflammation and cancer, and they are solely biosynthesized through cyclooxygenases (COXs) from arachidonic acid. Concurrently, ROS give rise to bioactive isoprostanes from arachidonic acid precursors that are also involved in acute and chronic inflammation, but their specific characteristics in breast cancer are less demonstrated. Higher aromatase activity, a cytochrome P-450 enzyme, is intimately connected to tumor growth in the breast through estrogen synthesis, and is interrelated to COXs that catalyze the formation of both inflammatory and anti-inflammatory PGs such as PGE(2), PGF(2α), PGD(2), and PGJ(2) synchronously under the influence of specific mediators and downstream enzymes. Some of the latter compounds upsurge the intracellular cyclic adenosine monophosphate concentration and appear to be associated with estrogen synthesis. This review discusses the role of COX- and ROS-catalyzed eicosanoids and adipokines in breast cancer, and therefore ranges from their molecular mechanisms to clinical aspects to understand the impact of inflammation.

Ratiometric Measurements of Adiponectin by Mass Spectrometry in Bottlenose Dolphins (Tursiops truncatus) with Iron Overload Reveal an Association with Insulin Resistance and Glucagon

High molecular weight (HMW) adiponectin levels are reduced in humans with type 2 diabetes and insulin resistance. Similar to humans with insulin resistance, managed bottlenose dolphins (Tursiops truncatus) diagnosed with hemochromatosis (iron overload) have higher levels of 2 h post-prandial plasma insulin than healthy controls. A parallel reaction monitoring assay for dolphin serum adiponectin was developed based on tryptic peptides identified by mass spectrometry. Using identified post-translational modifications, a differential measurement was constructed. Total and unmodified adiponectin levels were measured in sera from dolphins with (n = 4) and without (n = 5) iron overload. This measurement yielded total adiponectin levels as well as site specific percent unmodified adiponectin that may inversely correlate with HMW adiponectin. Differences in insulin levels between iron overload cases and controls were observed 2 h post-prandial, but not during the fasting state. Thus, post-prandial as well as fasting serum adiponectin levels were measured to determine whether adiponectin and insulin would follow similar patterns. There was no difference in total adiponectin or percent unmodified adiponectin from case or control fasting animals. There was no difference in post-prandial total adiponectin levels between case and control dolphins (mean ± SD) at 763 ± 298 and 727 ± 291 pmol/ml, respectively (p = 0.91); however, percent unmodified adiponectin was significantly higher in post-prandial cases compared to controls (30.0 ± 6.3 versus 17.0 ± 6.6%, respectively; p = 0.016). Interestingly, both total and percent unmodified adiponectin were correlated with glucagon levels in controls (r = 0.999, p  < 0.001), but not in cases, which is possibly a reflection of insulin resistance. Although total adiponectin levels were not significantly different, the elevated percent unmodified adiponectin follows a trend similar to HMW adiponectin reported for humans with metabolic disorders.

Snail destabilizes cell surface Crumbs3a

During epithelial to mesenchymal transition (EMT), cells modulate expression of proteins resulting in loss of apical-basal polarity. Effectors of this EMT switch target the polarity protein Crumbs3a, a small transmembrane protein that is essential for generation of the apical membrane and tight junctions of mammalian epithelial cells. We previously showed that the Crumbs3 gene is a direct target of transcriptional regulation by Snail, a potent inducer of EMT. However, Snail has also been shown to have multiple non-transcriptional roles, including regulation of cell adhesion, proliferation and survival. Using SNAP-tag labeling, we determined that cell surface Crumbs3a has a half-life of approximately 3 h and that this cell surface half-life is significantly reduced when EMT is induced by Snail. We further observe that Snail induces differential glycosylation of Crumbs3a, including sialylation, suggesting a mechanism by which Crumbs3a may be destabilized. These results indicate that Crumbs3a is a post-translational target of Snail, in addition to being a transcriptional target. We conclude that Snail's ability to post-translationally modify and destabilize Crumbs3a augments the depolarizing process of EMT.

Adiponectin and cardiovascular health: an update

The global epidemic of obesity is accompanied by an increased prevalence of cardiovascular disease (CVD), in particular stroke and heart attack. Dysfunctional adipose tissue links obesity to CVD by secreting a multitude of bioactive lipids and pro-inflammatory factors (adipokines) with detrimental effects on the cardiovascular system. Adiponectin is one of the few adipokines that possesses multiple salutary effects on insulin sensitivity and cardiovascular health. Clinical investigations have identified adiponectin deficiency (hypoadiponectinaemia) as an independent risk factor for CVD. In animals, elevation of plasma adiponectin by either pharmacological or genetic approaches alleviates obesity-induced endothelial dysfunction and hypertension, and also prevents atherosclerosis, myocardial infarction and diabetic cardiomyopathy. Furthermore, many therapeutic benefits of the peroxisome-proliferator activated receptor gamma agonists, the thiazolidinediones, are mediated by induction of adiponectin. Adiponectin protects cardiovascular health through its vasodilator, anti-apoptotic, anti-inflammatory and anti-oxidative activities in both cardiac and vascular cells. This review summarizes recent findings in the understanding of the physiological role and clinical relevance of adiponectin in cardiovascular health, and in the identification of the receptor and postreceptor signalling events that mediate the cardiovascular actions of adiponectin. It also discusses adiponectin-targeted drug discovery strategies for treating obesity, diabetes and CVD.

LINKED ARTICLES

This article is part of a themed section on Fat and Vascular Responsiveness. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-3.

Cross-talk between adipose tissue and vasculature: role of adiponectin

Adipose tissue is a highly dynamic endocrine organ, secreting a number of bioactive substances (adipokines) regulating insulin sensitivity, energy metabolism and vascular homeostasis. Dysfunctional adipose tissue is a key mediator that links obesity with insulin resistance, hypertension and cardiovascular disease. Obese adipose tissue is characterized by adipocyte hypertrophy and infiltration of inflammatory macrophages and lymphocytes, leading to the augmented production of pro-inflammatory adipokines and vasoconstrictors that induce endothelial dysfunction and vascular inflammation through their paracrine and endocrine actions. By contrast, the secretion of adiponectin, an adipokine with insulin sensitizing and anti-inflammatory activities, is decreased in obesity and its related pathologies. Emerging evidence suggests that adiponectin is protective against vascular dysfunction induced by obesity and diabetes, through its multiple favourable effects on glucose and lipid metabolism as well as on vascular function. Adiponectin improves insulin sensitivity and metabolic profiles, thus reducing the classical risk factors for cardiovascular disease. Furthermore, adiponectin protects the vasculature through its pleiotropic actions on endothelial cells, endothelial progenitor cells, smooth muscle cells and macrophages. Data from both animal and human investigations demonstrate that adiponectin is an important component of the adipo-vascular axis that mediates the cross-talk between adipose tissue and vasculature. This review highlights recent work on the vascular protective activities of adiponectin and discusses the molecular pathways underlying the vascular actions of this adipokine.

Distinct adiponectin profiles might contribute to differences in susceptibility to type 2 diabetes in dogs and humans

Dogs develop obesity-associated insulin resistance but not type 2 diabetes mellitus. Low adiponectin is associated with progression to type 2 diabetes in obese humans. The aims of this study were to compare total and high molecular weight (HMW) adiponectin and the ratio of HMW to total adiponectin (S(A)) between dogs and humans and to examine whether total or HMW adiponectin or both are associated with insulin resistance in naturally occurring obese dogs. We compared adiponectin profiles between 10 lean dogs and 10 lean humans and between 6 lean dogs and 6 age- and sex-matched, client-owned obese dogs. Total adiponectin was measured with assays validated in each species. We measured S(A) with velocity centrifugation on sucrose gradients. The effect of total and HMW adiponectin concentrations on MINMOD-estimated insulin sensitivity was assessed with linear regression. Lean dogs had total and HMW adiponectin concentrations three to four times higher than lean humans (total: dogs 32 ± 5.6 mg/L, humans 10 ± 1.3 mg/L, P<0.001; HMW: dogs 25 ± 4.5 mg/L, humans 6 ± 1.3 mg/L, P<0.001) and a higher S(A) (dogs: 0.78 ± 0.05; humans: 0.54 ± 0.08, P = 0.002). Adiponectin concentrations and S(A) were not lower in obese dogs (0.76 ± 0.05 in both groups; P=1). Total adiponectin, HMW adiponectin, and S(A) were not associated with insulin sensitivity in dogs. We propose that differences in adiponectin profiles between humans and dogs might contribute to the propensity of humans but not dogs to develop type 2 diabetes. Dogs with chronic, naturally occurring obesity do not have selectively reduced HMW adiponectin, and adiponectin does not appear to be important in the development of canine obesity-associated insulin resistance.

Molecular mechanisms of signal transduction via adiponectin and adiponectin receptors

The adipocytokine adiponectin and its receptor (AdipoR) comprise a new receptor-ligand system that is involved in a variety of clinically important morbidities such as obesity, type 2 diabetes and cardiovascular diseases. Adiponectin exerts a multitude of beneficial and tissue specific effects depending on its unique, tightly regulated multimerization behavior. Post-translational modifications are essential for the multimer assembly before secretion and protein stability in the circulation. AdipoR1 and 2 have been discovered as a new class of heptahelix receptors structurally and functionally distinct from G-protein-coupled receptors. Both AdipoRs bind adiponectin and the downstream signaling of both AdipoRs is mediated mainly by phosphorylation of AMPK and activation of peroxisome proliferator-activated receptor α, which influence the lipid and glucose metabolism of skeletal muscle and liver cells as well as inflammatory processes and vascular endothelial integrity. Several intracellular binding partners of the AdipoR N-terminus such as APPL1, CK2ß; and ERp46 have been identified and shown to control receptor signaling. Adiponectin has also been reported to modulate the dimerization and internalization of AdipoRs, which provides new insights into the molecular characteristics of this unusual receptor. The understanding of the functional mechanisms of adiponectin signal transduction is critical to benefit from the full therapeutic potential of the adiponectin-AdipoR system.

Adiponectin profiles are affected by chronic and acute changes in carbohydrate intake in healthy cats

Adiponectin is a key adipokine that regulates carbohydrate and lipid metabolism. It circulates in stable low (LMW) and high molecular weight (HMW) forms. The aims of this study were to characterize baseline adiponectin profiles (total, LMW and HMW multimers) in healthy cats and to assess the effects of varying dietary carbohydrate content on adiponectin profiles. Cats were maintained on a diet with moderate carbohydrate content (37% metabolisable energy [ME]) for 4 weeks and then randomly allocated to either a low carbohydrate (19% ME) or high carbohydrate (52% ME) diet for 4 weeks. Fasting and postprandial plasma adiponectin profiles were measured by ELISA and sucrose gradient/Western blot. After consuming the moderate carbohydrate diet for 4 weeks, fasting total, HMW and LMW plasma adiponectin concentrations were 5.0±0.6, 2.5±0.5 and 2.6±0.2 μg/mL, respectively. After changing to the low carbohydrate diet, fasting total adiponectin was unchanged but HMW adiponectin increased and LMW adiponectin decreased. No significant postprandial changes were observed. Cats consuming the high carbohydrate diet had increased fasting total and LMW adiponectin with no change in HMW adiponectin. In the postprandial state total adiponectin was reduced and there was a trend towards a decrease in HMW (p=0.086) but not LMW multimers. These data indicate that feline adiponectin multimer profiles are similar to those reported in other species and demonstrate that changes in plasma adiponectin occur in response to chronic and acute carbohydrate intake and these reflect differential changes in adiponectin multimers.

Synergistic effects of ascorbic acid and thiazolidinedione on secretion of high molecular weight adiponectin from human adipocytes

AIM

To test the hypothesis that ascorbic acid (AA) and thiazolidinedione (TZD) would have additive effects on HMW adiponectin secretion by virtue of different modes of action.

METHODS

We determined the effects of supplementation of AA and/or TZD on expression and secretion of total and HMW adiponectin from human Simpson-Golabi-Behmel syndrome (SGBS) adipocytes in the absence or presence of the proinflammatory cytokine TNFα.

RESULTS

AA supplementation significantly increased secretion of HMW adiponectin (1.7-fold) without altering adiponectin expression or total adiponectin secretion. TZD significantly increased expression (3-fold) and secretion of total (1.4-fold) but not HMW adiponectin. Combined supplementation resulted in a significant increase in expression (3-fold) and secretion of total (1.8-fold) and HMW (5-fold) adiponectin. Similar results were seen in cells co-treated with TNFα.

CONCLUSIONS

These data show that AA and TZD have synergistic rather than simple additive effects on secretion of HMW adiponectin from human adipocytes and raise the possibility that differences in AA levels may contribute to the variability in adiponectin multimer profiles and efficacy of TZD in humans. Our results also provide a rationale for longitudinal clinical trials investigating the effects of AA supplementation with or without TZD on adiponectin and metabolic profiles.