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.

Although elevated plasma plasminogen activator inhibitor 1 (PAI-1) is associated with obesity, very little is known about its tissue or cellular origin, or about the events that lead to increased PAI-1 levels under obese conditions. Since TNF-␣ is increased in rodents both during obesity and in response to endotoxin treatment, we examined the effects of these agents on PAI-1 gene expression in the adipose tissue of CB6 mice. In untreated mice, PAI-1 mRNA was detected in both mature adipocytes and in stromal vascular cells. Both TNF-␣ and endotoxin significantly increased PAI-1 mRNA in the adipose tissue, peaking at 3-8 h. In situ hybridization analysis of adipose tissue from untreated mice revealed a weak signal for PAI-1 mRNA only in the smooth muscle cells within the vascular wall. In contrast, after endotoxin or TNF-␣ treatment, PAI-1 mRNA also was detected in adipocytes and in adventitial cells of vessels. Endotoxin also induced PAI-1 in endothelial cells, while TNF-␣ additionally induced it in smooth muscle cells. Mature 3T3-L1 adipocytes in culture also expressed PAI-1 mRNA, and its rate of synthesis was also upregulated by TNF-␣ . These studies suggest that the adipose tissue itself may be an important contributor to the elevated PAI-1 levels observed in the plasma under obese conditions. (J. Clin. Invest. 1996. 97:37-46.)

TNF-alpha contributes to the elevated TGF-beta expression demonstrated in the adipose tissue of obese mice. A potential role for TGF-beta in the increased PAI-1 and vascular pathologies associated with obesity/NIDDM is suggested.

Clinical and epidemiological studies support a connection between obesity and thrombosis, involving elevated expression of the prothrombotic molecules plasminogen activator inhibitor-1 and tissue factor (TF) and increased platelet activation. Cardiovascular diseases and metabolic syndrome–associated disorders, including obesity, insulin resistance, type 2 diabetes, and hepatic steatosis, involve inflammation elicited by infiltration and activation of immune cells, particularly macrophages, into adipose tissue. Although TF has been clearly linked to a procoagulant state in obesity, emerging genetic and pharmacologic evidence indicate that TF signaling via G protein-coupled protease-activated receptors (PAR2, PAR1) additionally drives multiple aspects of the metabolic syndrome. TF–PAR2 signaling in adipocytes contributes to diet-induced obesity by decreasing metabolism and energy expenditure, whereas TF–PAR2 signaling in hematopoietic and myeloid cells drives adipose tissue inflammation, hepatic steatosis, and insulin resistance. TF-initiated coagulation leading to thrombin–PAR1 signaling also contributes to diet-induced hepatic steatosis and inflammation in certain models. Thus, in obese patients, clinical markers of a prothrombotic state may indicate a risk for the development of complications of the metabolic syndrome. Furthermore, TF-induced signaling could provide new therapeutic targets for drug development at the intersection between obesity, inflammation, and thrombosis.

Our results suggest that elevated PAI-1 associated with obesity may result in part from insulin-induced induction of PAI-1 specifically by adipocytes within the fat itself.

Tissue factor (TF), the initiator of the coagulation cascade, mediates coagulation factor VIIa-dependent activation of protease activated receptor-2 (PAR2). Here we delineate an unexpected role for coagulation signaling in obesity and its complications. Mice lacking PAR2 (F2rl1) or the cytoplasmic domain of TF (F3) are protected from high fat diet (HFD) induced weight gain and insulin resistance. In hematopoietic cells, genetic deletion of TF-PAR2 signaling reduces adipose tissue macrophage inflammation and specific pharmacological inhibition of macrophage TF signaling rapidly ameliorates insulin resistance. In contrast, non-hematopoietic cell TF-VIIa-PAR2 signaling specifically promotes obesity. Mechanistically, adipocyte TF cytoplasmic domain dependent VIIa signaling suppresses Akt phosphorylation with concordant adverse transcriptional changes of key regulators of obesity and metabolism. Pharmacological blockade of adipocyte TF in vivo reverses these effects of TF-VIIa signaling and rapidly improves energy expenditure. Thus, TF signaling is a potential therapeutic target to improve impaired metabolism and insulin resistance in obesity.

Obesity is associated with a cluster of abnormalities, including hypertension, insulin resistance, hyperinsulinemia, and elevated levels of both plasminogen activator inhibitor 1 (PAI-1) and transforming growth factor ␤ (TGF-␤). Although these changes may increase the risk for accelerated atherosclerosis and fatal myocardial infarction, the underlying molecular mechanisms remain to be defined. Although tumor necrosis factor ␣ (TNF-␣) has been implicated in the insulin resistance associated with obesity, its role in other disorders of obesity is largely unknown. In this report, we show that in obese (ob͞ob) mice, neutralization of TNF-␣ or deletion of both TNF receptors (TNFRs) results in significantly reduced levels of plasma PAI-1 antigen, plasma insulin, and adipose tissue PAI-1 and TGF-␤ mRNAs. Studies in which exogenous TNF-␣ was infused into lean mice lacking individual TNFRs indicate that TNF-␣ signaling of PAI-1 in adipose tissue can be mediated by either the p55 or the p75 TNFR. However, TNF-␣ signaling of TGF-␤ mRNA expression in adipose tissue is mediated exclusively via the p55 TNFR. Our results suggest that TNF-␣ is a common link between the insulin resistance and elevated PAI-1 and TGF-␤ in obesity. The chronic elevation of TNF-␣ in obesity thus may directly promote the development of the complex cardiovascular risk profile associated with this condition.

P lasminogen activator inhibitor 1 is the primary physiological inhibitor of plasminogen activation in vivo, and elevations in plasma PAI-1 appear to compromise normal fibrin clearance mechanisms and promote thrombosis. PAI-1 is dramatically upregulated in obesity, a complex condition associated with increased risk for myocardial infarction, accelerated atherosclerosis, hypertension, glucose intolerance, insulin resistance, hyperinsulinemia, and NIDDM. In spite of the apparent link between elevated PAI-1 levels and thrombotic disease, little is known about the origin of this plasma inhibitor in obesity/NIDDM or about the signals that control its biosynthesis. Potential insights into the underlying molecular events have come from recent studies of genetically obese mice and of cultured adipocytes. These studies are reviewed here.* They emphasize the key role played by the adipocyte, a cell whose numbers, size, and metabolic activity are grossly altered in obesity/NIDDM. They also suggest that multiple cytokines, hormones, and growth factors may be involved, and they raise the possibility that the abnormal expression of other hemostatic genes by adipocytes in obesity/NIDDM may also contribute to the cardiovascular complications of this disorder. In this regard, our preliminary studies indicate that TF gene expression is elevated in the adipose tissues of the obese mouse. Properties of PAI-1PAI-1 appears to be the primary physiological inhibitor of plasminogen activation in blood, since it is the only PAI found complexed with single-chain tissue-type PA in carefully collected human plasma, and the second-order rate constant for its interaction with tissue-type PA and urokinase-type PA (Ϸ3.5ϫ10 7 [mol/L] Ϫ1 ⅐ s Ϫ1 ) is at least two orders of magnitude higher than that of other PAIs. [1][2][3][4] The normal concentration of PAI-1 protein in human plasma ranges from 6 to 80 ng/mL with a geometric mean at 24 ng/mL, whereas that of tissuetype PA is 5 to 10 ng/mL. Abnormalities in the concentration of PAI-1 are frequently associated with vascular disease. For example, the inhibitor is elevated in a variety of thrombotic conditions, including myocardial infarction and deep venous thrombosis. Elevated PAI-1 also correlates with thrombosis in animal models, and transgenic mice that overexpress PAI-1 have been reported to develop venous thrombosis. 5 On the other hand, the absence of PAI-1 in humans leads to life-long bleeding problems presumably resulting from the development of a hyperfibrinolytic state, and disruption of the PAI-1 gene in mice is also associated with a mild hyperfibrinolytic state as manifested by increased resistance to endotoxin-induced thrombosis. 5 Finally, neutralizing plasma PAI-1 activity with specific antibodies or by the use of PAI-1 inhibitors 4,6 enhances spontaneous or tissue-type PA-mediated thrombolysis. These observations emphasize that imbalances in the PA/PAI-1 ratio are likely to promote either thrombosis or bleeding. As summarized in the next section, this balance is severely disturbed ...