Distribution and regulation of plasminogen activator inhibitor-1 in murine adipose tissue in vivo. Induction by tumor necrosis factor-alpha and lipopolysaccharide

Temporal profiling of the secretome during adipogenesis in humans

Adipose tissue plays a key role as a fat-storage depot and as an endocrine organ. Although mouse adipogenesis has been studied extensively, limited studies have been conducted to characterize this process in humans. We carried out a temporal proteomic analysis to interrogate the dynamic changes in the secretome of primary human preadipocytes as they differentiate into mature adipocytes. Using iTRAQ-based quantitative proteomics, we identified and quantified 420 proteins from the secretome of differentiated human adipocytes. Our results revealed that the majority of proteins showed differential expression during the course of differentiation. In addition to adipokines known to be differentially secreted in the course of adipocyte differentiation, we identified a number of proteins whose dynamic expression in this process has not been previously documented. They include collagen triple helix repeat containing 1, cytokine receptor-like factor 1, glypican-1, hepatoma-derived growth factor, SPARC related modular calcium binding protein 1, SPOCK 1, and sushi repeat-containing protein. A bioinformatics analysis using Human Protein Reference Database and Human Proteinpedia revealed that of the 420 proteins identified, 164 proteins possess signal peptides and 148 proteins are localized to the extracellular compartment. Additionally, we employed antibody arrays to quantify changes in the levels of 182 adipokines during human adipogenesis. This is the first large-scale quantitative proteomic study that combines two platforms, mass spectrometry and antibody arrays, to analyze the changes in the secretome during the course of adipogenesis in humans.

The secretome of adipocytes is regulated during the differentiation of preadipocytes into adipocytes. Using iTRAQ-based quantitative proteomics, we identified and quantified 420 proteins. Additionally, we employed antibody arrays to quantify changes in the levels of 182 adipokines. This is the first large-scale quantitative proteomic study that combines two platforms, mass spectrometry and antibody arrays, to analyze the changes in the secretome during the course of adipogenesis in humans.

Fat tissue, aging, and cellular senescence

Fat tissue, frequently the largest organ in humans, is at the nexus of mechanisms involved in longevity and age-related metabolic dysfunction. Fat distribution and function change dramatically throughout life. Obesity is associated with accelerated onset of diseases common in old age, while fat ablation and certain mutations affecting fat increase life span. Fat cells turn over throughout the life span. Fat cell progenitors, preadipocytes, are abundant, closely related to macrophages, and dysdifferentiate in old age, switching into a pro-inflammatory, tissue-remodeling, senescent-like state. Other mesenchymal progenitors also can acquire a pro-inflammatory, adipocyte-like phenotype with aging. We propose a hypothetical model in which cellular stress and preadipocyte overutilization with aging induce cellular senescence, leading to impaired adipogenesis, failure to sequester lipotoxic fatty acids, inflammatory cytokine and chemokine generation, and innate and adaptive immune response activation. These pro-inflammatory processes may amplify each other and have systemic consequences. This model is consistent with recent concepts about cellular senescence as a stress-responsive, adaptive phenotype that develops through multiple stages, including major metabolic and secretory readjustments, which can spread from cell to cell and can occur at any point during life. Senescence could be an alternative cell fate that develops in response to injury or metabolic dysfunction and might occur in nondividing as well as dividing cells. Consistent with this, a senescent-like state can develop in preadipocytes and fat cells from young obese individuals. Senescent, pro-inflammatory cells in fat could have profound clinical consequences because of the large size of the fat organ and its central metabolic role.

Plasminogen activator inhibitor-1 (PAI-1): a key factor linking fibrinolysis and age-related subclinical and clinical conditions

INTRODUCTION

The close relationship existing between aging and thrombosis has growingly been studied in this last decade. The age-related development of a prothrombotic imbalance in the fibrinolysis homeostasis has been hypothesized as the basis of this increased cardiovascular and cerebrovascular risk. Fibrinolysis is the result of the interactions among multiple plasminogen activators and inhibitors constituting the enzymatic cascade, and ultimately leading to the degradation of fibrin. The plasminogen activator system plays a key role in a wide range of physiological and pathological processes.

METHODS

Narrative review.

RESULTS

Plasminogen activator inhibitor-1 (PAI-1) is a member of the superfamily of serine-protease inhibitors (or serpins), and the principal inhibitor of both the tissue-type and the urokinase-type plasminogen activator, the two plasminogen activators able to activate plasminogen. Current evidence describing the central role played by PAI-1 in a number of age-related subclinical (i.e., inflammation, atherosclerosis, insulin resistance) and clinical (i.e., obesity, comorbidities, Werner syndrome) conditions is presented.

CONCLUSIONS

Despite some controversial and unclear issues, PAI-1 represents an extremely promising marker that may become a biological parameter to be progressively considered in the prognostic evaluation, in the disease monitoring, and as treatment target of age-related conditions in the future.

Regulation of adipocyte differentiation by activation of serotonin (5-HT) receptors 5-HT2AR and 5-HT2CR and involvement of microRNA-448-mediated repression of KLF5

Retrovirus insertion-mediated random mutagenesis was applied in 3T3-L1 preadipocyte cells to better understand the molecular basis of obesity (the expansion of individual adipocytes). We found that tryptophan hydroxylase-1, a rate-limiting enzyme for the synthesis of serotonin (5-HT), is expressed in adipocytes and is required for their differentiation. A 5-HT type 2A receptor (5-HT(2A)R) antagonist, ketanserin, and a 5-HT(2c)R antagonist, SB-242084, inhibited adipocyte differentiation. Because 5-HT(2c)R mRNA levels are up-regulated during adipocyte differentiation and micro-RNA (miR)-448 is located in the fourth intron of Htr2c, we also studied the role of miR-448 in 3T3-L1 cells. Through a bioinformatics approach, Krüppel-like factor 5 (KLF5) was identified as a potential target of miR-448. Using a luciferase reporter assay, we confirmed that miR-448 targets the Klf5 3'-intranslated region. Overexpression of miR-448 reduced the expression of Klf5 and adipocyte differentiation, which was confirmed by the reduced expression of adipogenic genes and triglyceride accumulation. To examine the loss of miR-448 function, we constructed a decoy gene that had tandem complementary sequences for miR-448 in the 3'-untranslated region of a luciferase gene under the control of a cytomegalovirus promoter. When the miR-448 decoy gene was introduced into 3T3-L1 preadipocytes, KLF5 was up-regulated and triglyceride concentration was increased. In this study, we identified the regulation of adipocyte differentiation by 5-HT, 5-HT(2A)R, and 5-HT(2C)R. miR-448-mediated repression of KLF5 was identified as a negative regulator for adipocyte differentiation.

Serum chemerin levels vary with time of day and are modified by obesity and tumor necrosis factor-{alpha}

Chemerin is an adipokine with important regulatory roles in adipogenesis. In humans, serum total chemerin (i.e. prochemerin plus chemerin) levels are positively associated with body mass index and metabolic syndrome. However, the mechanisms that increase serum chemerin concentration are unknown. We hypothesized that chronic low-grade inflammation that occurs in obesity promotes chemerin production by adipocytes. Consistent with this, TNFalpha treatment of 3T3-L1 adipocytes increased bioactive chemerin levels in the cell media as detected using a CMKLR1 cell-based bioassay. This effect was blocked by the protein synthesis inhibitor cycloheximide and protein secretion inhibitor brefeldin A, indicating that TNFalpha may enhance prochemerin synthesis and secretion from adipocytes. In vivo, TNFalpha produced a time-dependent increase in serum total chemerin and bioactive chemerin. Bioactive chemerin was produced by primary mouse adipocytes and hepatocytes. Only primary adipocyte-derived chemerin was responsive to TNFalpha regulation implicating adipocytes as a potential source of elevated serum chemerin after TNFalpha exposure in vivo. In lean mice, serum total chemerin levels oscillated with peak levels occurring during daytime and trough levels at night. Comparatively, leptin- and leptin receptor-deficient obese mice, which have elevated adipose tissue expression of TNFalpha, displayed elevated serum total chemerin levels with an enhanced oscillatory pattern. In summary, our novel results identified TNFalpha as a positive regulator of adipocyte-derived chemerin. We corroborate the finding of elevated chemerin in obese humans by identifying elevated serum levels of total chemerin in two obese mouse models with a corresponding alteration in the rhythmic pattern of serum chemerin levels.

Hypoadiponectinemia--cause or consequence of human "insulin resistance"?

CONTEXT

Adiponectin is a highly abundant plasma protein synthesized nearly exclusively in adipose tissue from the ADIPOQ gene. It has excited intense interest because of robust correlation of its circulating levels with indices of insulin resistance (IR) and risk of type 2 diabetes, and their unusual inverse relationship with fat mass. It has been suggested that pharmacological strategies aimed at augmenting adiponectin levels or action may generate novel insulin-sensitizing drugs.

EVIDENCE ACQUISITION

Relevant publications were identified by searching PubMed, with secondary searches of their bibliographies.

EVIDENCE SYNTHESIS

Rodent studies suggest that adiponectin exerts a direct insulin-sensitizing effect on the liver, consistent with a role in the pathogenesis of prevalent forms of IR and its sequelae. However, the complex higher-order structure of adiponectin and inconsistent reports regarding its putative receptors have complicated efforts to understand the mechanistic basis of this. No proof yet exists that adiponectin modulates insulin sensitivity in humans, and genetic, biochemical, and physiological evidence suggests that low adiponectin levels may be a consequence of IR with compensatory hyperinsulinemia. This suggests that there may be a bidirectional relationship between IR and hypoadiponectinemia in humans.

CONCLUSIONS

The relationship between adiponectin and insulin action in humans is more complex than often suggested. Further investigation of the direction of causality in this relationship, allied to studies of the cellular mechanisms involved, will be central to improving understanding of the physiological role of this enigmatic protein, and to efforts to exploit it for therapeutic benefit.

The role of inflammation and macrophage accumulation in the development of obesity-induced type 2 diabetes mellitus and the possible therapeutic effects of long-chain n-3 PUFA

The WHO estimate that >1 x 10(6) deaths in Europe annually can be attributed to diseases related to excess body weight, and with the rising global obesity levels this death rate is set to drastically increase. Obesity plays a central role in the metabolic syndrome, a state of insulin resistance that predisposes patients to the development of CVD and type 2 diabetes mellitus. Obesity is associated with low-grade chronic inflammation characterised by inflamed adipose tissue with increased macrophage infiltration. This inflammation is now widely believed to be the key link between obesity and development of insulin resistance. In recent years it has been established that activation of pro-inflammatory pathways can cross talk with insulin signalling pathways via a number of mechanisms including (a) down-regulation of insulin signalling pathway proteins (e.g. GLUT4 and insulin receptor substrate (IRS)-1), (b) serine phosphorylation of IRS-1 blocking its tyrosine phosphorylation in response to insulin and (c) induction of cytokine signalling molecules that sterically hinder insulin signalling by blocking coupling of the insulin receptor to IRS-1. Long-chain (LC) n-3 PUFA regulate gene expression (a) through transcription factors such as PPAR and NF-kappaB and (b) via eicosanoid production, reducing pro-inflammatory cytokine production from many different cells including the macrophage. LC n-3 PUFA may therefore offer a useful anti-inflammatory strategy to decrease obesity-induced insulin resistance, which will be examined in the present review.

Role for nuclear factor-kappaB in augmented lung injury because of interaction between hyperoxia and high stretch ventilation

High-tidal-volume mechanical ventilation and hyperoxia used in patients with acute lung injury (ALI) can induce alveolar coagulopathy and fibrin depositions within the airways. Hyperoxia has been shown to increase ventilator-induced lung injury (VILI), but the mechanisms that regulate interaction between high-tidal-volume mechanical ventilation and hyperoxia are unclear. We hypothesized that mechanical stretch with hyperoxia synergistically augmented neutrophil infiltration and production of plasminogen activator inhibitor-1 (PAI-1) via the nuclear factor-kappaB (NF-kappaB) pathway. C57BL/6 mice (n=5 per group) were exposed to high-tidal-volume (30 mL/kg) or low-tidal-volume (6 mL/kg) mechanical ventilation with room air or hyperoxia for 1 to 5h after 2-microg/g NF-kappaB inhibitor (SN-50) administration. Nonventilated mice with room air or hyperoxia served as control groups. Evans blue dye, myeloperoxidase, electrophoretic mobility shifting of nuclear protein, and inflammatory cytokine were measured. The expression of tumor necrosis factor-alpha (TNF-alpha) and PAI-1 were studied by immunohistochemistry. The addition of hyperoxia to high-tidal-volume ventilation-augmented lung injury, as demonstrated by increased microvascular leak, neutrophil migration into the lung, TNF-alpha and active PAI-1 production, DNA binding activity of NF-kappaB, and NF-kappaB activation. No statistically significant increase of neutrophil infiltration and inflammatory cytokine production was found in the mice ventilated at 6 mL/kg using hyperoxia. Hyperoxia-induced augmentation of VILI was attenuated in mice with pharmacologic inhibition of NF-kappaB activity by SN-50. We conclude that hyperoxia increased high-tidal-volume-induced cytokine production and neutrophil influx through activation of the NF-kappaB pathway.

Tissue-specific PAI-1 gene expression and glycosylation pattern in insulin-resistant old rats

Increased levels of plasminogen activator inhibitor-1 (PAI-1) have been associated with obesity, aging, insulin resistance, and type 2 diabetes, conditions that contribute to increased cardiovascular risk. PAI-1 is expressed in a variety of tissues, but the cellular origin of plasma PAI-1 is unknown. To link insulin resistance, aging, and cardiovascular disease, we examined the expression and glycosylation pattern of PAI-1 in liver and white adipose tissue (WAT) from adult (3 mo) and insulin-resistant old (24 mo) Wistar rats. Glycosylated PAI-1 protein was also purified by affinity chromatography from endothelial culture supernatans to analyze its inhibitory activity. We also analyzed the contribution of adipocytes and stromal vascular cells from WAT to PAI-1 levels with aging. Aging caused a significant increase of PAI-1 mRNA (P < 0.001) in WAT that was predominantly due to the adipocytes and not to stroma-vascular cells, while there was no modification in liver from aged rats. Moreover, PAI-1 expression increased during preadipocyte differentiation (P < 0.001). Furthermore, we found a tissue-dependent PAI-1 glycosylation pattern: adipose tissue only expresses the glycosylated PAI-1 form, whereas the liver mainly expresses the nonglycosylated form. Finally, we also found evidences suggesting that the glycosylated PAI-1 form shows higher inhibitory activity than the nonglycosylated. Our data suggest that WAT may be a major source of the elevated plasma levels of PAI-1 in insulin-resistant old rats. Additionally, the high degree of PAI-1 glycosylation and activity, together with the significant increase in visceral fat in old rats, may well contribute to an increased cardiovascular risk associated with insulin-resistant states.

Role of adipose tissue in haemostasis, coagulation and fibrinolysis

Obesity is associated with an increased incidence of insulin resistance (IR), type 2 diabetes mellitus and cardiovascular diseases. The increased risk for cardiovascular diseases could partly be caused by a prothrombotic state that exists because of abdominal obesity. Adipose tissue induces thrombocyte activation by the production of adipose tissue-derived hormones, often called adipokines, of which some such as leptin and adiponectin have been shown to directly interfere with platelet function. Increased adipose tissue mass induces IR and systemic low-grade inflammation, also affecting platelet function. It has been demonstrated that adipose tissue directly impairs fibrinolysis by the production of plasminogen activator inhibitor-1 and possibly thrombin-activatable fibrinolysis inhibitor. Adipose tissue may contribute to enhanced coagulation by direct tissue factor production, but hypercoagulability is likely to be primarily caused by affecting hepatic synthesis of the coagulation factors fibrinogen, factor VII, factor VIII and tissue factor, by releasing free fatty acids and pro-inflammatory cytokines (tumour necrosis factor-alpha, interleukin-1beta and interleukin-6) into the portal circulation and by inducing hepatic IR. Adipose tissue dysfunction could thus play a causal role in the prothrombotic state observed in obesity, by directly and indirectly affecting haemostasis, coagulation and fibrinolysis.

Human uraemic plasma stimulates release of leptin and uptake of tumour necrosis factor-alpha in visceral adipocytes

BACKGROUND

End-stage renal disease (ESRD) is commonly associated with anorexia, malnutrition and inflammation. In addition to serving as the primary reservoir for energy storage, adipocytes produce numerous pro- and anti-inflammatory mediators and regulate food intake by releasing the appetite-suppressing (leptin) and appetite-stimulating (adiponectin) hormones. Under normal conditions, release of leptin is stimulated by feeding to prevent excess intake, and release of adiponectin is stimulated by fasting to induce feeding. However, under certain pathological conditions such as inflammation, maladaptive release of these hormones leads to anorexia, wasting and malnutrition and simultaneously intensifies inflammation. Anorexia, malnutrition and inflammation in ESRD are frequently accompanied by hyper-leptinaemia. This study was designed to test the hypothesis that uraemic plasma may stimulate leptin release and suppress adiponectin release in normal adipocytes.

METHODS

Visceral adipose tissue was harvested from normal rats, and adipocytes were isolated and incubated for 2-4 h in media containing 90% plasma from 12 ESRD patients (before and after haemodialysis) and 12 normal control subjects.

RESULTS

The ESRD group had a marked elevation of plasma TNF-alpha, IL-6, IL-8 and leptin concentrations before and after haemodialysis. Incubation in media containing plasma from the ESRD group elicited a much greater leptin release by adipocytes than that containing normal plasma. Post-dialysis plasma evoked an equally intense leptin release. The rise in leptin release was coupled with a parallel fall in TNF-alpha concentration in the incubation media. In contrast to leptin, adiponectin release in the presence of uraemic plasma was similar to that found with the control plasma.

CONCLUSIONS

Exposure to uraemic plasma induces exuberant release of leptin that is coupled with avid uptake of TNF-alpha by visceral adipocytes. These observations confirm the role of TNF-alpha, formerly known as cachexin, in the over-production and release of leptin in patients with ESRD.

Insulin acts through FOXO3a to activate transcription of plasminogen activator inhibitor type 1

Plasminogen activator inhibitor-1 (PAI-1) is an important regulator of fibrinolysis. PAI-1 levels are elevated in type 2 diabetes, and this elevation correlates with macro- and microvascular complications of diabetes. However, the mechanistic link between insulin and up-regulation of PAI-1 is unclear. Here we demonstrate that overexpression of Forkhead-related transcription factor (Fox)O1, FoxO3a, and FoxC1 augment insulin's ability to activate the PAI-1 promoter. In addition, insulin treatment promotes the phosphorylation of nuclear and cytoplasmic Fox03a and an increase of cytoplasmic Fox03a. In contrast, insulin treatment led to the accumulation of phospho-Fox01 only in the cytoplasm. Furthermore, insulin also increased the ability of chimeric LexA-FoxO1, LexA-FoxO3a, and LexA-FoxC1 proteins to increase the activity of a LexA reporter, suggesting that the effect of insulin on FoxO3a was direct. Using small interfering RNA to specifically deplete each of the Fox transcription factors tested, we demonstrate that only reduction of FoxO3a inhibits insulin-increased PAI-1-Luc expression and PAI-1 mRNA accumulation. Finally, chromatin immunoprecipitation assays confirm the presence of FoxO3a on the PAI-1 promoter. These results suggest that FoxO3a mediates insulin-increased PAI-1 gene expression.

Inflammation, coagulation, and the pathway to frailty

There are inevitable physiologic changes associated with advancing age, yet for some people these changes are exaggerated, and as a result a phenotype emerges recognized as "frailty." Why some people become frail and others do not remains incompletely understood. Although chronic illnesses are common among frail elderly persons, some will develop all of the phenotypic features without a diagnosed underlying disease. It has been recognized that certain proinflammatory cytokines and coagulation factors are elevated to a greater extent in those who are frail than in age-matched nonfrail individuals. In this review, we provide an overview of current research in the biology of frailty with particular emphasis on the role of inflammatory pathways and disordered coagulation in its pathogenesis.

Central role of ceramide biosynthesis in body weight regulation, energy metabolism, and the metabolic syndrome

Although obesity is associated with multiple features of the metabolic syndrome (insulin resistance, leptin resistance, hepatic steatosis, chronic inflammation, etc.), the molecular changes that promote these conditions are not completely understood. Here, we tested the hypothesis that elevated ceramide biosynthesis contributes to the pathogenesis of obesity and the metabolic syndrome. Chronic treatment for 8 wk of genetically obese (ob/ob), and, high-fat diet-induced obese (DIO) mice with myriocin, an inhibitor of de novo ceramide synthesis, decreased circulating ceramides. Decreased ceramide was associated with reduced weight, enhanced metabolism and energy expenditure, decreased hepatic steatosis, and improved glucose hemostasis via enhancement of insulin signaling in the liver and muscle. Inhibition of de novo ceramide biosynthesis decreased adipose expression of suppressor of cytokine signaling-3 (SOCS-3) and induced adipose uncoupling protein-3 (UCP3). Moreover, ceramide directly induced SOCS-3 and inhibited UCP3 mRNA in cultured adipocytes suggesting a direct role for ceramide in regulation of metabolism and energy expenditure. Inhibition of de novo ceramide synthesis had no effect on adipose tumor necrosis factor-alpha (TNF-alpha) expression but dramatically reduced adipose plasminogen activator inhibitor-1 (PAI-1) and monocyte chemoattactant protein-1 (MCP-1). This study highlights a novel role for ceramide biosynthesis in body weight regulation, energy expenditure, and the metabolic syndrome.

Hepatic insulin resistance, metabolic syndrome and cardiovascular disease

BACKGROUND

The metabolic syndrome is a constellation of common metabolic disorders that is associated with cardiovascular disease. Insulin resistance has a central role in the pathophysiology of metabolic syndrome.

RECENT ADVANCES

It is now commonly accepted that chronic inflammation associated with visceral obesity induces insulin resistance in the liver. Chronic inflammation is characterized by the production of abnormal adipokines and cytokines such as TNF-alpha, FFA, IL-1, IL-6, leptin and resistin. These factors inhibit insulin signalling in hepatocytes by activating SOCS proteins, several kinases such as JNK, IKK-beta and PKC and protein tyrosine phosphatases such as PTP1B and PTEN, that in turn impair insulin signalling at insulin receptor and insulin receptor substrate (IRS) level. Hepatic insulin resistance in turn causes impaired suppression of glucose production by insulin in hepatocytes leading to hyperglycemia. An important and early complication of hepatic insulin resistance is the induction of hepatic VLDL production, via changes in the rate of apoB synthesis and degradation and de novo lipogenesis, or increased FFA flux from adipose tissue into the liver. Insulin resistance also stimulates the production of CRP and PAI-1, both markers of an inflammatory state. All metabolic abnormalities related to hepatic insulin resistance have been shown to directly or indirectly promote atherosclerosis. Hyperglycemia induces a series of alterations including endothelial dysfunction, cellular proliferation, changes in extracellular matrix conformation and impairment of LDL receptor-mediated uptake decreasing the in vivo clearance of LDL. Small dense LDLs associated with high circulating VLDL have higher affinity to the intimal proteoglycans leading to the penetration of more LDL particles into the arterial wall. CRP can also accelerate atherosclerosis by increasing the expression of PAI-1 and adhesion molecules in endothelial cells, inhibition of nitric oxide formation and increasing LDL uptake into macrophages.

CONCLUSIONS

Overall, growing evidence suggests that hepatic insulin resistance is sufficient to induce several components of the metabolic syndrome and promote progression to cardiovascular disease. Many unresolved questions remain however on the molecular and cellular mechanisms that trigger hepatic insulin resistance and promote the development of clinical metabolic syndrome.

Protection from high fat diet-induced increase in ceramide in mice lacking plasminogen activator inhibitor 1

Obesity increases the risk for metabolic and cardiovascular disease, and adipose tissue plays a central role in this process. Ceramide, the key intermediate of sphingolipid metabolism, also contributes to obesity-related disorders. We show that a high fat diet increased ceramide levels in the adipose tissues and plasma in C57BL/6J mice via a mechanism that involves an increase in gene expression of enzymes mediating ceramide generation through the de novo pathway (e.g. serine palmitoyltransferase) and via the hydrolysis of sphingomyelin (acid sphingomyelinase and neutral sphingomyelinase). Although the induction of total ceramide in response to the high fat diet was modest, dramatic increases were observed for C16, C18, and C18:1 ceramides. Next, we investigated the relationship of ceramide to plasminogen activator inhibitor-1 (PAI-1), the primary inhibitor of plasminogen activation and another key player in obesity. PAI-1 is consistently elevated in obesity and thought to contribute to increased artherothrombotic events and more recently to obesity-mediated insulin resistance. Interestingly, the changes in ceramide were attenuated in mice lacking PAI-1. Mechanistically, mice lacking PAI-1 were protected from diet-induced increase in serine palmitoyltransferase, acid sphingomyelinase, and neutral sphingomyelinase mRNA, providing a mechanistic link for decreased ceramide in PAI-1-/- mice. The decreases in plasma free fatty acids and adipose tumor necrosis factor-alpha in PAI-1-/- mice may have additionally contributed indirectly to improvements in ceramide profile in these mice. This study has identified a novel link between sphingolipid metabolism and PAI-1 and also suggests that ceramide may be an intermediary molecule linking elevated PAI-1 to insulin resistance.

Inflammation, obesity and comorbidities: the role of diet

The adipocyte metabolism has been shown to change during the fat enlargement process associated to obesity. Several procoagulant proteins such as plasminogen activator inhibitor type 1, tissue factor or factor VII and also inducible nitric oxide synthase show higher expression in adipose tissue of obese people in comparison to lean. This overexpression could explain at least a part of the atherogenic and cardiovascular risk associated with obesity. In addition to cytokine secretion, many other features have been observed to be common to adipocyte and monocyte/macrophage lines: for example, phagocytic and microbicidal activities, and possibly a cellular plasticity of adipose precursors. Overweight and obesity are associated with an increased risk of such metabolic abnormalities as dyslipidemia, hypertension or type 2 diabetes mellitus and cardiovascular diseases, common features of the metabolic syndrome. Initially, insulin resistance or hyperinsulinemia was suggested as the origin of these abnormalities. More recent studies indicate that adipokynes have an important role in obesity-associated metabolic complications, and suggest that chronically elevated local or systemic concentrations of adipokynes contribute to the development of complications associated with obesity and metabolic syndrome. Considering all the evidence relating to diet and inflammation, the best diet for protecting against the metabolic derangements associated with obesity and metabolic syndrome would be high in fibre-rich cereals, fruit, vegetables, fish, virgin olive oil and nuts; moderate in wine; and low in meat, processed meat foods and trans-fatty acids.

Insulin resistance and thrombogenesis: recent insights and therapeutic implications

OBJECTIVE

To review the relationship between insulin resistance and thrombogenesis, especially in the context of obesity, diabetes, and cardiovascular disease, and to discuss therapeutic implications.

METHODS

The pertinent peer-reviewed literature was examined for evidence in support of the aforementioned relationship, and the reported efficacy of various therapeutic interventions was assessed.

RESULTS

Robust evidence indicates that insulin resistance and enhanced thrombogenesis are closely related pathophysiologic mechanisms, especially in the presence of obesity. Thus, targeting insulin resistance and thrombogenesis may be of value in the prevention and management of type 2 diabetes and associated cardiovascular morbidity and mortality. Many proven preventive and therapeutic strategies, such as weight loss, exercise, and various pharmaceutical agents, affect both thrombogenesis and insulin resistance.

CONCLUSION

Both insulin resistance and thrombogenesis contribute to the morbidity and mortality associated with obesity, diabetes, and cardiovascular disease. Effective measures for prevention and management of diabetes and cardiovascular disease also tend to improve insulin sensitivity and to ameliorate abnormalities in coagulation, fibrinolysis, and platelet function.

Hypercoagulation and thrombophilia in liver disease

A complex balance exists between endogenous procoagulants and the anticoagulant system in liver disease patients. Hypercoagulable events occur in cirrhosis patients despite the well-known bleeding diathesis of liver disease. These events may be clinically evident, such as in portal vein thrombosis or pulmonary embolism, but these conditions may also be a silent contributor to certain disease states, such as portopulmonary hypertension or parenchymal extinction with liver atrophy as well as thrombosis of extracorporeal circuits in dialysis or liver assist devices. Moreover, liver disease-related hypercoagulability may contribute to vascular disease in the increasingly common condition of non-alcoholic fatty liver disease. Despite the incidence of these problems, there are few widely accessible and practical laboratory tests to evaluate the risk of a hypercoagulable event in cirrhosis patients. Furthermore, there is little research on the use of commonly accepted anticoagulants in patients with liver disease. This article is a result of an international symposium on coagulation disorders in liver disease and addresses several areas of specific interest in hypercoagulation in liver disease. Critical areas lacking clinical information are highlighted and future areas of research interest are defined with an aim to foster clinical research in this field.

Akt kinase targets association of CBP with SMAD 3 to regulate TGFbeta-induced expression of plasminogen activator inhibitor-1

Transforming growth factor-beta (TGFbeta) controls expression of plasminogen activator inhibitor type 1 (PAI-1), which regulates degradation of extracellular matrix proteins in fibrotic diseases. The TGFbeta receptor-specific Smad 3 has been implicated in the PAI-1 expression. The mechanism by which non-Smad signaling contributes to this process is not known. We studied the cross-talk between Smad 3 and PI 3 kinase/Akt signaling in TGFbeta-induced PAI-1 expression in renal mesangial cells. Inhibition of PI 3 kinase and Akt kinase blocked TGFbeta- and Smad 3-mediated expression of PAI-1. In contrast, constitutively active PI 3 kinase and Akt kinase increased PAI-1 expression, similar to TGFbeta. Inhibition of PI 3 kinase and Akt kinase had no effect on TGFbeta-induced Smad 3 phosphorylation and its translocation to the nucleus. Notably, inhibition of PI 3 kinase-dependent Akt kinase abrogated TGFbeta-induced PAI-1 transcription, without affecting binding of Smad 3 to the PAI-1 Smad binding DNA element. However, PI 3 kinase inhibition and dominant negative Akt kinase antagonized the association of the transcriptional coactivator CBP with Smad 3 in response to TGFbeta, resulting in inhibition of Smad 3 acetylation. Together our findings identify TGFbeta-induced PI 3 kinase/Akt signaling as a critical regulator of Smad 3-CBP interaction and Smad 3 acetylation, which cause increased PAI-1 expression.

TNF-alpha and adipocyte biology

Dyslipidemia and insulin resistance are commonly associated with catabolic or lipodystrophic conditions (such as cancer and sepsis) and with pathological states of nutritional overload (such as obesity-related type 2 diabetes). Two common features of these metabolic disorders are adipose tissue dysfunction and elevated levels of tumour necrosis factor-alpha (TNF-alpha). Herein, we review the multiple actions of this pro-inflammatory adipokine on adipose tissue biology. These include inhibition of carbohydrate metabolism, lipogenesis, adipogenesis and thermogenesis and stimulation of lipolysis. TNF-alpha can also impact the endocrine functions of adipose tissue. Taken together, TNF-alpha contributes to metabolic dysregulation by impairing both adipose tissue function and its ability to store excess fuel. The molecular mechanisms that underlie these actions are discussed.

Links Between Adipose Tissue and Thrombosis in the Mouse

Obesity has become a global epidemic and carries a considerable negative impact in regard to quality of life and life expectancy. A primary problem is that obese individuals are at increased risk of suffering from cardiovascular disease complications such as myocardial infarction and stroke. Because fat accumulation is a consistent aspect of obesity, mechanisms that may link adipose tissue to cardiovascular disease complications should be considered. Proteins expressed from adipose tissue, known as adipokines, are hypothesized to have important effects on the progression and incidence of cardiovascular disease complications. This review examines the evidence that adipokines play a direct role in vascular thrombosis, an important event in cardiovascular disease complications.

Deficiency of plasminogen activator inhibitor-2 impairs nutritionally induced murine adipose tissue development

BACKGROUND

A functional role for several components of the fibrinolytic (plasminogen/plasmin) system in development of adipose tissue has been demonstrated. No information is available, however, on a potential role of plasminogen activator inhibitor-2 (PAI-2) in obesity.

METHODS

In vitro, 3T3-F442A murine pre-adipocytes were differentiated into mature adipocytes. In vivo, 5-week-old male PAI-2-deficient (PAI-2(-/-)) mice and wild-type (WT) controls of the same genetic background (C57Bl/6) were kept on a high fat diet (HFD, caloric value of 20.1 kJ g(-1)) for 15 weeks.

RESULTS

Semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) revealed expression of PAI-2 mRNA during in vitro differentiation of pre-adipocytes and in vivo in s.c. and gonadal (GON) adipose tissues of WT mice, where it was localized both in the stromal/vascular cell fraction and in adipocytes. During HFD feeding, food intake and body weight gain were comparable for WT and PAI-2(-/-) mice. Subcutaneous plus GON fat mass was, however, significantly lower in PAI-2(-/-) mice (3.15 +/- 0.21 vs. 3.91 +/- 0.18 g; P < 0.05). Immunohistochemical analysis of adipose tissues revealed significant adipocyte hypotrophy in s.c. fat of PAI-2(-/-) mice (about 25% reduction in size; P < 0.01). Blood vessel density, normalized to adipocyte number, was comparable in s.c. fat, but was lower (P < 0.05) in GON fat of PAI-2(-/-) mice. Adipose tissue-associated fibrinolytic activity was not affected by PAI-2 deficiency.

CONCLUSION

PAI-2 promotes adipose tissue development in mice via a mechanism independent of its antifibrinolytic effect.

Plasminogen activator inhibitor-1 contributes to the deleterious effect of obesity on the outcome of thrombotic ischemic stroke in mice

BACKGROUND

It is widely accepted that obesity is a risk factor for ischemic heart disease, but the association with stroke is less clear. Adipose tissue is an important source of plasminogen activator inhibitor-1 (PAI-1), the main inhibitor of plasminogen activation.

OBJECTIVE

To test the hypothesis that elevated PAI-1 levels associated with obesity negatively affect the outcome of thrombotic ischemic stroke.

METHODS

Middle cerebral artery (MCA) occlusion was induced photochemically in mice with nutritionally induced or genetically determined obesity and their lean counterparts.

RESULTS

The MCA occlusion time (to obtain complete occlusion) was significantly shorter in obese (nutritionally induced) than in lean wild-type (WT) C57Bl/6 mice, whereas the infarct size was significantly larger and intracranial hemorrhage (ICH) was enhanced (all P < 0.05). Similar observations were made in genetically obese ob/ob mice, as compared to lean WT littermates. In both strains, obesity was associated with markedly elevated circulating PAI-1 levels, probably originating from the fat tissue. In contrast, PAI-1-deficient lean and obese mice did not display significant differences in MCA occlusion time, infarct volume or ICH.

CONCLUSIONS

Plasminogen activator inhibitor-1 may play a functional role in the deleterious effect of obesity on the outcome of thrombotic ischemic stroke in mice.

Chemerin, a novel adipokine that regulates adipogenesis and adipocyte metabolism

Obesity is an alarming primary health problem and is an independent risk factor for type II diabetes, cardiovascular diseases, and hypertension. Although the pathologic mechanisms linking obesity with these co-morbidities are most likely multifactorial, increasing evidence indicates that altered secretion of adipose-derived signaling molecules (adipokines; e.g. adiponectin, leptin, and tumor necrosis factor alpha) and local inflammatory responses are contributing factors. Chemerin (RARRES2 or TIG2) is a recently discovered chemoattractant protein that serves as a ligand for the G protein-coupled receptor CMKLR1 (ChemR23 or DEZ) and has a role in adaptive and innate immunity. Here we show an unexpected, high level expression of chemerin and its cognate receptor CMKLR1 in mouse and human adipocytes. Cultured 3T3-L1 adipocytes secrete chemerin protein, which triggers CMKLR1 signaling in adipocytes and other cell types and stimulates chemotaxis of CMKLR1-expressing cells. Adenoviral small hairpin RNA targeted knockdown of chemerin or CMKLR1 expression impairs differentiation of 3T3-L1 cells into adipocytes, reduces the expression of adipocyte genes involved in glucose and lipid homeostasis, and alters metabolic functions in mature adipocytes. We conclude that chemerin is a novel adipose-derived signaling molecule that regulates adipogenesis and adipocyte metabolism.

Mechanism of inactivation of plasminogen activator inhibitor-1 by a small molecule inhibitor

The inactivation of plasminogen activator inhibitor-1 (PAI-1) by the small molecule PAI-1 inhibitor PAI-039 (tiplaxtinin) has been investigated using enzymatic analysis, direct binding studies, site-directed mutagenesis, and molecular modeling studies. Previously PAI-039 has been shown to exhibit in vivo activity in various animal models, but the mechanism of inhibition is unknown. PAI-039 bound specifically to the active conformation of PAI-1 and exhibited reversible inactivation of PAI-1 in vitro. SDS-PAGE indicated that PAI-039 inactivated PAI-1 predominantly through induction of PAI-1 substrate behavior. Preincubation of PAI-1 with vitronectin, but not bovine serum albumin, blocked PAI-039 activity while analysis of the reciprocal experiment demonstrated that preincubation of PAI-1 with PAI-039 blocked the binding of PAI-1 to vitronectin. Together, these data suggest that the site of interaction of the drug on PAI-1 is inaccessible when PAI-1 is bound to vitronectin and may overlap with the PAI-1 vitronectin binding domain. This was confirmed by site-directed mutagenesis and molecular modeling studies, which suggest that the binding epitope for PAI-039 is localized adjacent to the previously identified interaction site for vitronectin. Thus, these studies provide a detailed characterization of the mechanism of inhibition of PAI-1 by PAI-039 against free, but not vitronectin-bound PAI-1, suggesting for the first time a novel pool of PAI-1 exists that is vulnerable to inhibition by inactivators that bind at the vitronectin binding site.

Impact of weight reduction on production of platelet-derived microparticles and fibrinolytic parameters in obesity

INTRODUCTION

Generation of platelet-derived microparticle (PMP) is implicated in cardiovascular disease (CVD). However, the influence of adiposity and weight reduction on PMP generation remains to be fully elucidated. We compared PMP generation and fibrinolytic parameters between 49 non-diabetic obese (obese group) and 37 age-matched non-obese subjects (control group), and compared the effects of weight reduction on the parameters between a 12-week calorie restricted diet and diet with aerobic exercise in obese subjects.

MATERIALS AND METHODS

PMP, plasma levels of plasminogen activator inhibitor-1 (PAI-1) activity and tissue-type plasminogen activator (t-PA) antigen were measured before and after intervention.

RESULTS

Before intervention, PMP, PAI-1 activity and t-PA antigen values were elevated in the obese group compared with the control group. In all 86 subjects of both groups, these three parameters correlated with body mass index, waist circumference and fat tissue mass. There was a positive correlation between plasma levels of fibrinolytic parameters and visceral fat area (VFA). PMP values correlated with subcutaneous fat area (SFA). The intervention significantly reduced PMP, PAI-1 activity and t-PA antigen levels. There was a significant correlation between percentages of changes in PMP values and those in BMI, fat tissue mass and VFA in the obese group. No additional effect of exercise on PMP or fibrinolytic parameters was observed.

CONCLUSIONS

Overproduction of PMP and fibrinolytic abnormalities may be associated with excessive adipose tissue. Weight reduction by either calorie restriction with or without exercise improves fibrinolytic abnormalities and PMP overproduction, probably through reduction of adipose tissue.

Type 2 diabetes and cardiovascular disease: getting to the fat of the matterThis paper is one of a selection of papers published in this Special Issue, entitled Young Investigators' Forum

The increasing national prevalence of obesity is a major public health concern and a substantial burden on the health care resources of Canada. In addition to the direct health impact of obesity, this condition is a well-established risk factor for the development of various prevalent comorbidities including type 2 diabetes, hypertension, and cardiovascular disease. Historically, adipose tissue has been regarded primarily as an organ for energy storage. However, the discovery of leptin in the mid 1990’s revolutionized our understanding of this tissue and has focused attention on the endocrine function of adipose tissue as a source of secreted bioactive peptides. These compounds, collectively termed adipokines, regulate a number of biological functions including appetite and energy balance, insulin sensitivity, lipid metabolism, blood pressure, and inflammation. The physiological importance of adipokines has led to the hypothesis that changes in the synthesis and secretion of these compounds in the obese are a causative factor contributing to the development of obesity and obesity-related diseases in these individuals. Following from this it has been proposed that pharmacologic manipulation of adipokine levels may provide novel effective therapeutic strategies to treat and prevent obesity, type 2 diabetes, and cardiovascular disease.

Adipose tissue-derived factors: impact on health and disease

The endocrine functions of the adipose organ are widely studied at this stage. The adipose organ, and in particular adipocytes, communicate with almost all other organs. Although some adipose tissue pads assume the functions as distinct "miniorgans," adipocytes can also be present in smaller numbers interspersed with other cell types. Although fat pads have the potential to have a significant systemic impact, adipocytes may also affect neighboring tissues through paracrine interactions. These local or systemic effects are mediated through lipid and protein factors. The protein factors are commonly referred to as adipokines. Their expression and posttranslational modifications can undergo dramatic changes under different metabolic conditions. Due to the fact that none of the mutations that affect adipose tissue trigger embryonic lethality, the study of adipose tissue physiology lends itself to genetic analysis in mice. In fact, life in the complete absence of adipose tissue is possible in a laboratory setting, making even the most extreme adipose tissue phenotypes genetically amenable to be analyzed by disruption of specific genes or overexpression of others. Here, we briefly discuss some basic aspects of adipocyte physiology and the systemic impact of adipocyte-derived factors on energy homeostasis.

Secretome of primary cultures of human adipose-derived stem cells: modulation of serpins by adipogenesis

Studies of adipogenic protein induction have led to a new appreciation of the role of adipose tissue as an endocrine organ. Adipocyte-derived "adipokines" such as adiponectin, leptin, and visceral adipose tissue-derived serine protease inhibitor (vaspin) exert hormone-like activities at the systemic level. In this study, we examined the secretome of primary cultures of human subcutaneous adipose-derived stem cells as an in vitro model of adipogenesis. Conditioned media obtained from four individual female donors after culture in uninduced or adipogenic induced conditions were compared by two-dimensional gel electrophoresis and tandem mass spectrometry. Over 80 individual protein features showing > or =2-fold relative differences were examined. Approximately 50% of the identified proteins have been described previously in the secretome of murine 3T3-L1 preadipocytes or in the interstitial fluid derived from human mammary gland adipose tissue. As reported by others, we found that the secretome included proteins such as actin and lactate dehydrogenase that do not display a leader sequence or transmembrane domain and are classified as "cytoplasmic" in origin. Moreover we detected a number of established adipokines such as adiponectin and plasminogen activator inhibitor 1. Of particular interest was the presence of multiple serine protease inhibitors (serpins). In addition to plasminogen activator inhibitor 1, these included pigment epithelium-derived factor (confirmed by Western immunoblot), placental thrombin inhibitor, pregnancy zone protein, and protease C1 inhibitor. These findings, together with the recent identification of vaspin, suggest that the serpin protein family warrants further proteomics investigation with respect to the etiology of obesity and type 2 diabetes.

Bone Marrow Plasminogen Activator Inhibitor-1 Influences the Development of Obesity

Plasma levels of plasminogen activator inhibitor-1 (PAI-1) are elevated in obesity and correlate with body mass index. The increase in PAI-1 associated with obesity likely contributes to increased cardiovascular risk and may predict the development of type 2 diabetes mellitus. Although adipocytes are capable of synthesizing PAI-1, the bulk of evidence indicates that cells residing in the stromal fraction of visceral fat are the primary source of PAI-1. We hypothesized that bone marrow-derived PAI-1, e.g. derived from macrophages located in visceral fat, contributes to the development of diet-induced obesity. To test this hypothesis, male C57BL/6 wild-type mice and C57BL/6 PAI-1 deficient mice were transplanted with either PAI-1(-/-), PAI-1(+/-), or PAI-1(+/+) bone marrow. The transplanted animals were subsequently fed a high fat diet for 24 weeks. Our findings show that only the complete absence of PAI-1 protects from the development of diet-induced obesity, whereas the absence of bone marrow-derived PAI-1 protects against expansion of the visceral fat mass. Remarkably, there is a link between the PAI-1 levels, the degree of inflammation in adipose tissue, and the development of obesity. Based on these findings we suggest that bone marrow-derived PAI-1 has an effect on the development of obesity through its effect on inflammation.

PAI-1 and the Metabolic Syndrome

The link between plasminogen activator inhibitor (PAI)-1 and the metabolic syndrome with obesity was established many years ago. Increased PAI-1 level can be now considered a true component of the syndrome. The metabolic syndrome is associated with an increased risk of developing cardiovascular disease, and PAI-1 overexpression may participate in this process. The mechanisms of PAI-1 overexpression during obesity are complex, and it is conceivable that several inducers are involved at the same time at several sites of synthesis. Interestingly, recent in vitro and in vivo studies showed that besides its role in atherothrombosis, PAI-1 is also implicated in adipose tissue development and in the control of insulin signaling in adipocytes. These findings suggest PAI-1 inhibitors serve in the control of atherothrombosis and insulin resistance.

Differences in the association of PAI-1 activity with the metabolic syndrome between African and Caucasian women

BACKGROUND AND AIMS

The association between PAI-1(act) and markers of the metabolic syndrome is well established in Caucasian populations, but data on African subjects is lacking. The aim of this study was to investigate possible differences between the association of PAI-1(act) and markers of the metabolic syndrome in Caucasian and African women.

METHODS AND RESULTS

Cross-sectional data were collected from 95 African and 114 Caucasian women in the Potchefstroom district of the North West Province, South Africa. Plasma PAI-1(act) was almost twice as high in Caucasians compared to Africans (10.2 versus 5.2 U/mL, p<0.001). Correlations between markers of the metabolic syndrome and PAI-1(act) were remarkably stronger in Caucasians than in Africans. In multivariate regression analyses 56% of the variance of PAI-1(act) could be explained by metabolic syndrome variables in the Caucasian group compared to 12% in the African women. Waist circumference was the strongest independent predictor of PAI-1(act) in both groups.

CONCLUSION

This study showed lower PAI-1(act) in African than in Caucasian women, along with less associations of PAI-1(act) with markers of the metabolic syndrome in the African than in the Caucasian women. The role of PAI-1(act) in the metabolic syndrome may be less prominent in Africans than in Caucasians.

Plasminogen activator inhibitor-1 expression and secretion are stimulated by growth hormone and interleukin-6 in 3T3-L1 adipocytes

Various adipocytokines have been described which influence insulin sensitivity and vascular function profoundly and might, therefore, potentially link obesity, insulin resistance, and atherosclerosis. Among those, plasminogen activator inhibitor (PAI)-1 is an adipose-secreted factor upregulated in obesity and insulin resistance that inhibits fibrinolysis. Furthermore, recent studies in knockout mice suggest that PAI-1 directly impairs insulin sensitivity. In the current study, the impact of growth hormone (GH) and interleukin (IL)-6 on PAI-1 mRNA synthesis and secretion was determined in 3T3-L1 adipocytes. Interestingly, 500 ng/ml GH and 30 ng/ml IL-6 increased PAI-1 secretion five-fold and 3.6-fold, respectively. Furthermore, GH and IL-6 induced PAI-1 mRNA by up to 7.3-fold, and 3.6-fold, respectively, in a time-dependent fashion with significant stimulation seen at concentrations as low as 5 ng/ml GH and 10 ng/ml IL-6. Other insulin resistance-inducing hormones which stimulated PAI-1 synthesis included insulin, TNFalpha, and dexamethasone. Studies using pharmacological inhibitors suggested that basal and GH-induced PAI-1 synthesis were at least in part mediated by p44/42 mitogen-activated protein kinase but not janus kinase 2 and phosphatidylinositol 3-kinase. Taken together, our results show a differential regulation of PAI-1 mRNA by insulin resistance-inducing hormones including GH and IL-6.

Identification of a peroxisome proliferator responsive element (PPRE)-like cis-element in mouse plasminogen activator inhibitor-1 gene promoter

PAI-1 is expressed and secreted by adipose tissue which may mediate the pathogenesis of obesity-associated cardiovascular complications. Evidence is presented in this report that PAI-1 is not expressed by preadipocyte, but significantly induced during 3T3-L1 adipocyte differentiation and the PAI-1 expression correlates with the induction of peroxisome proliferator-activated receptor gamma (PPARgamma). A peroxisome proliferator responsive element (PPRE)-like cis-element (-206TCCCCCATGCCCT-194) is identified in the mouse PAI-1 gene promoter by electrophoretic mobility shift assay (EMSA) combined with transient transfection experiments; the PPRE-like cis-element forms a specific DNA-protein complex only with adipocyte nuclear extracts, not with preadipocyte nuclear extracts; the DNA-protein complex can be totally competed away by non-labeled consensus PPRE, and can be supershifted with PPARgamma antibody. Mutation of this PPRE-like cis-element can abolish the transactivation of mouse PAI-1 promoter mediated by PPARgamma. Specific PPARgamma ligand Pioglitazone can significantly induce the PAI-1 expression, and stimulate the secretion of PAI-1 into medium.

TNFalpha up-regulates apelin expression in human and mouse adipose tissue

We have recently identified apelin as a novel adipokine up-regulated by insulin and obesity. Since obesity and insulin resistance are associated with chronically elevated levels of both insulin and TNFalpha, the present study was performed to investigate a putative regulation of apelin expression in adipocytes by TNFalpha. Herein, we report a tight correlation between apelin and TNFalpha expression in adipose tissue of lean and obese humans. Apelin regulation by TNFalpha was further studied in cultured explants of human adipose tissue. The endogenous expression of TNFalpha in adipocytes isolated from the explants was accompanied by a 6-9 h subsequent increase of apelin expression in adipocytes. This increase was reversed by inhibiting TNFalpha expression with 100 microM isobutylmethylxanthine. In different mouse models of obesity, expression of both TNFalpha and apelin was also significantly increased in adipocytes of obese mice. Furthermore, short-term exposure to an i.p. injection of TNFalpha in C57Bl6/J mice induced an increase of apelin expression in adipose tissue as well as apelin plasma levels. Finally, a direct positive effect of TNFalpha has been shown in differentiated 3T3F442A adipocytes on apelin expression and secretion. The signaling pathways of TNFalpha for the induction of apelin were dependent of PI3-kinase, c-Jun NH2-terminal kinase (JNK), and MAPK but not PKC activation. All together, these findings suggest that apelin might be a candidate to better understand potential links between obesity and associated disorders such as inflammation and insulin resistance.

[Relationship between obesity, inflammation and insulin resistance: new concepts]

White adipose tissue is the main site of energy storage, but it is now recognized as an active participant in regulating physiologic and pathologic processes including immunity and inflammation. It has an endocrine function by secreting at least two main hormones, leptin and adiponectin. It can secrete other products, named adipokines, including cytokines and chemokines, involved in inflammation process. The release of adipokines by either adipocytes or adipose tissue infiltrated macrophages lead to a chronic sub-inflammatory state that could play a central role in cardiovascular complications linked to obesity and insulin resistance, a risk factor to develop type-2 diabetes.

Autoamplification of tumor necrosis factor-alpha: a potential mechanism for the maintenance of elevated tumor necrosis factor-alpha in male but not female obese mice

Although tumor necrosis factor-alpha (TNF-alpha) is elevated in adipose tissue in obesity and may contribute to the cardiovascular and metabolic risks associated with this condition, the mechanisms leading to elevated TNF-alpha remain elusive. We hypothesized that autoamplification of TNF-alpha contributes to the maintenance of elevated TNF-alpha in obesity. Treatment of 3T3-L1 adipocytes with TNF-alpha, or injection of TNF-alpha into C57BL/6J mice, up-regulated TNF-alpha mRNA in adipocytes and in adipose tissues, respectively. Ob/ob male but not female mice lacking TNF-alpha receptors showed significantly lower levels of adipose TNF-alpha mRNA when compared with TNF-alpha receptor-expressing ob/ob mice. Thus, the lack of endogenous TNF-alpha signaling reduced adipose TNF-alpha mRNA in ob/ob male mice. Additionally, hyperinsulinemia potentiated this TNF-alpha-mediated autoamplification response in adipose tissues and in adipocytes in a synergistic and dose-dependent manner. Studies in which TNF-alpha was injected into lean mice lacking individual TNF-alpha receptors indicated that TNF-alpha autoamplification in adipose tissues was mediated primarily via the p55 TNF-alpha receptor whereas the p75 TNF-alpha receptor appeared to augment this response. Finally, TNF-alpha autoamplification in adipocytes occurred via the protein kinase C signaling pathway and the transcription factor nuclear factor-kappaB. Thus, TNF-alpha can positively autoregulate its own biosynthesis in adipose tissue, contributing to the maintenance of elevated TNF-alpha in obesity.

Foxc2 Is a Common Mediator of Insulin and Transforming Growth Factor β Signaling to Regulate Plasminogen Activator Inhibitor Type I Gene Expression

Elevated plasma levels of plasminogen activator inhibitor type I (PAI-1), a significant risk factor of ischemic heart disease, are associated with insulin resistance in which insulin and transforming growth factor (TGF)-beta play a pivotal role in regulating PAI-1 production. Forkhead transcription factor FOXC2 is an important regulator of insulin resistance. However, the underlying molecular mechanisms to link FOXC2 to PAI-1 levels in insulin resistance remain to be elucidated. Here, we demonstrate that Foxc2 is a common transcriptional activator of insulin and TGF-beta signaling to directly regulate PAI-1 expression via 2 distinct target sites, an insulin response element (IRE) and a novel forkhead-binding element (FBE), adjacent to a Smad-binding site. We found that in adipocytes and endothelial cells Foxc2 mediates insulin action competing with another Forkhead protein, FOXO1, via the insulin response element, and simultaneously cooperate with the TGF-beta/Smad pathway to transactivate PAI-1. Importantly, Foxc2 haploinsufficiency in mice significantly attenuates TGF-beta1-induced PAI-1 expression in the cardiovascular system and adipose tissue. Taken together, we propose that Foxc2 is a key molecule to regulate PAI-1 gene expression.

Obesity and the role of adipose tissue in inflammation and metabolism

Recent discoveries, notably of the hormones leptin and adiponectin, have revised the notion that adipocytes are simply a storage depot for body energy. Instead, adipocytes are also endocrine organs, with multiple metabolic roles in regulating whole-body physiology. Small adipocytes in lean individuals promote metabolic homeostasis; the enlarged adipocytes of obese individuals recruit macrophages and promote inflammation and the release of a range of factors that predispose toward insulin resistance. Exercise activates the AMP-activated protein kinase (AMPK) in muscle and other tissues, a pathway that increases fat oxidation and glucose transport. Importantly, the adipocyte hormones leptin and adiponectin also activate AMPK; remarkably, the same pathway is activated by certain antidiabetic agents such as thiazolidinediones. Increasingly, our understanding of the adipocyte as an endocrine organ is leading to new insights into obesity and health.

Induction of PAI-1 expression by tumor necrosis factor alpha in endothelial cells is mediated by its responsive element located in the 4G/5G site

Plasminogen activator inhibitor type 1 (PAI-1) is induced by many proinflammatory and pro-oxidant factors. Among them, tumor necrosis factor alpha (TNFalpha), a pivotal early mediator that regulates and amplifies the development of inflammation, is one of the strongest PAI-1 synthesis activators. Location of the TNFalpha response element in the PAI-1 promoter is still ambiguous. In this study, we attempted to evaluate the significance of the element located in the 4G/5G site of the PAI-1 promoter in the TNFalpha stimulation of PAI-1 expression in endothelial cells. PAI-1 expression was monitored at: (a) the level of mRNA using real-time PCR, (b) PAI-1 gene transcription by transfection reporter assays, and (c) protein synthesis using the enzyme immunoassay. NF-kappaB activity was monitored using the electrophoretic mobility shift assay. Its activity was modified by either antisense oligonucleotides or transfection of endothelial cells with the wild-type or mutated IkappaBalpha. We have shown that TNFalpha-induced expression and gene transcription of PAI-1 involves a regulatory region present in segment -664/-680 of the PAI-1 promoter. This reaction involves the TNFalpha-induced generation of superoxide leading to activation of NF-kappaB, and can be abolished by antioxidants and by overexpression of a super-suppressor phosphorylation-resistant IkappaBalpha. Stimulation of PAI-1 under these conditions involves the motif of the PAI-1 promoter adjacent to the 4G/5G site, which can directly interact with NF-kappaB. We show that activation of PAI-1 gene by TNFalpha and reactive oxygen species is mediated by interaction of NF-kappaB with the cis-acting element located in the -675 4G/5G insertion/deletion in the PAI-1 promoter.

Modulation of angiogenesis during adipose tissue development in murine models of obesity

Development of vasculature and mRNA expression of 17 pro- or antiangiogenic factors were studied during adipose tissue development in nutritionally induced or genetically determined murine obesity models. Subcutaneous (SC) and gonadal (GON) fat pads were harvested from male C57Bl/6 mice kept on standard chow [standard fat diet (SFD)] or on high-fat diet for 0-15 wk and from male ob/ob mice kept on SFD. Ob/ob mice and C57Bl/6 mice on high-fat diet had significantly larger SC and GON fat pads, accompanied by significantly higher blood content, increased total blood vessel volume, and higher number of proliferating cells. mRNA and protein levels of angiopoietin (Ang)-1 were down-regulated, whereas those of thrombospondin-1 were up-regulated in developing adipose tissue in both obesity models. Ang-1 mRNA levels correlated negatively with adipose tissue weight in the early phase of nutritionally induced obesity as well as in genetically determined obesity. Placental growth factor and Ang-2 expression were increased in SC adipose tissue of ob/ob mice, and thrombospondin-2 was increased in both their SC and GON fat pads. mRNA levels of vascular endothelial growth factor (VEGF)-A isoforms VEGF-B, VEGF-C, VEGF receptor-1, -2, and -3, and neuropilin-1 were not markedly modulated by obesity. This modulation of angiogenic factors during development of adipose tissue supports their important functional role in obesity.

Reduction of macrophage infiltration and chemoattractant gene expression changes in white adipose tissue of morbidly obese subjects after surgery-induced weight loss

In human obesity, the stroma vascular fraction (SVF) of white adipose tissue (WAT) is enriched in macrophages. These cells may contribute to low-grade inflammation and to its metabolic complications. Little is known about the effect of weight loss on macrophages and genes involved in macrophage attraction. We examined subcutaneous WAT (scWAT) of 7 lean and 17 morbidly obese subjects before and 3 months after bypass surgery. Immunomorphological changes of the number of scWAT-infiltrating macrophages were evaluated, along with concomitant changes in expression of SVF-overexpressed genes. The number of scWAT-infiltrating macrophages before surgery was higher in obese than in lean subjects (HAM56+/CD68+; 22.6 +/- 4.3 vs. 1.4 +/- 0.6%, P < 0.001). Typical "crowns" of macrophages were observed around adipocytes. Drastic weight loss resulted in a significant decrease in macrophage number (-11.63 +/- 2.3%, P < 0.001), and remaining macrophages stained positive for the anti-inflammatory protein interleukin 10. Genes involved in macrophage attraction (monocyte chemotactic protein [MCP]-1, plasminogen activator urokinase receptor [PLAUR], and colony-stimulating factor [CSF]-3) and hypoxia (hypoxia-inducible factor-1alpha [HIF-1alpha]), expression of which increases in obesity and decreases after surgery, were predominantly expressed in the SVF. We show that improvement of the inflammatory profile after weight loss is related to a reduced number of macrophages in scWAT. MCP-1, PLAUR, CSF-3, and HIF-1alpha may play roles in the attraction of macrophages in scWAT.

Obesity enhances the induction of plasminogen activator inhibitor-1 by restraint stress: a possible mechanism of stress-induced renal fibrin deposition in obese mice

BACKGROUND AND OBJECTIVES

Cardiovascular/thrombotic diseases are frequently induced by a variety of stressors. Obese patients are susceptible to thrombotic diseases associated with stress, but the underlying mechanism is still unknown. We have begun to investigate the expression of a primary inhibitor of fibrinolysis, plasminogen activator inhibitor-1 (PAI-1), in association with tissue thrombosis, using restraint-stressed obese mice.

METHODS AND RESULTS

We analyzed the expression of PAI-1 after restraint (immobilization) stress in genetically obese mice in comparison with their lean counterparts. Dramatic increases in PAI-1 antigen in plasma and in tissue extracts were observed in the obese mice exposed to restraint stress. The induction of PAI-1 mRNA by stress in the tissues was also pronounced in the stressed obese mice as compared with the lean mice, especially in the hearts and adipose tissues. In situ hybridization analysis revealed that strong signals for PAI-1 mRNA were localized in the adipocytes, cardiovascular endothelial cells, and renal glomerular cells of the stressed obese mice. Histological examination revealed that renal glomerular fibrin deposition was detected only in the obese mice after 2 h of restraint stress.

CONCLUSIONS

Obesity enhances the stress-mediated PAI-1 induction in the blood and tissues. This phenomenon may be associated with the increased risk of stress-induced renal fibrin deposition in obese subjects.

Molecular mechanisms of fibrinolysis

The molecular mechanisms that finely co-ordinate fibrin formation and fibrinolysis are now well defined. The structure and function of all major fibrinolytic proteins, which include serine proteases, their inhibitors, activators and receptors, have been characterized. Measurements of real time, dynamic molecular interactions during fibrinolysis of whole blood clots can now be carried out in vitro. The development of gene-targeted mice deficient in one or more fibrinolytic protein(s) has demonstrated expected and unexpected roles for these proteins in both intravascular and extravascular settings. In addition, genetic analysis of human deficiency syndromes has revealed specific mutations that result in human disorders that are reflective of either fibrinolytic deficiency or excess. Elucidation of the fine control of fibrinolysis under different physiological and pathological haemostatic states will undoubtedly lead to novel therapeutic interventions. Here, we review the fundamental features of intravascular plasmin generation, and consider the major clinical syndromes resulting from abnormalities in fibrinolysis.

Molecular mechanisms of tumor necrosis factor-alpha-mediated plasminogen activator inhibitor-1 expression in adipocytes

Increased expression of plasminogen activator inhibitor -1 (PAI-1) in adipose tissues is thought to contribute to both the cardiovascular and metabolic complications associated with obesity. Tumor necrosis factor alpha (TNF-alpha) is chronically elevated in adipose tissues of obese rodents and humans and has been directly implicated to induce PAI-1 in adipocytes. In this study, we used 3T3-L1 adipocytes to examine the mechanism by which TNF-alpha up-regulates PAI-1 in the adipocyte. Acute (3 h) and chronic (24 h) exposure of 3T3-L1 adipocytes to TNF-alpha induces PAI-1 mRNA by increasing the rate of transcription of the PAI-1 gene, and de novo protein synthesis is not required for this process. Although the p44/42 and PKC signaling pathways appear to be significant in the induction of PAI-1 mRNA in response to acute treatment with TNF-alpha, the more dramatic induction of PAI-1 mRNA observed in response to chronic exposure of adipocytes to TNF-alpha was mediated by these and additional signaling molecules, including p38, PI3-kinase, tyrosine kinases, and the transcription factor NF-kappaB. Moreover, the dramatic increase in PAI-1 observed after chronic exposure of adipocytes to TNF-alpha was accompanied by increased metabolic insulin resistance. Finally, we demonstrate that the PKC pathway is also central for PAI-1 induction in response to insulin and transforming growth factor-beta (TGF-beta), two additional molecules which are elevated in obesity and shown to directly induce PAI-1 in the adipocyte. The understanding of the mechanism of regulating PAI-1 expression in the adipocytes at the molecular level provides new insight to help identify novel targets in fighting the pathological complications of obesity.