Attenuation by gemfibrozil of expression of plasminogen activator inhibitor type 1 induced by insulin and its precursors

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.

De novo synthesis of diacylglycerol in endothelium may mediate the association between PAI-1 and the insulin resistance syndrome

Increased free fatty acid flux, giving rise to increased de novo synthesis of diacylglycerol (DAG) and activation of protein kinase C (PKC) in vascular endothelium, may be largely responsible for the endotheliopathy and increased vascular risk associated with insulin resistance syndrome. This mechanism may also mediate, in large part, the increase in plasminogen activator inhibitor-1 (PAI-1) observed in this syndrome. PKC activation promotes transcription of PAI-1 in endothelial cells and other tissues, apparently by boosting the activity of Sp1 transcription factors that bind to the PAI-1 promoter. Plasma PAI-1 correlates inversely with the ability of insulin infusion to suppress free fatty acid levels. Moreover, infusion of triglycerides with heparin - inducing a marked increase in free fatty acids - has been shown to induce a rapid increase in plasma PAI-1. Alternatively, hyperinsulinemia and hypertriglyceridemia have been suggested as mediators of PAI-1 excess in insulin resistance, inasmuch as insulin and VLDL can stimulate PAI-1 production in cell cultures. However, plasma PAI-1 tends to decline in response to hyperinsulinemic clamps and insulin treatment of type 2 diabetes, and gemfibrozil treatment of hypertriglyceridemia does not decrease PAI-1 - suggesting that elevations of insulin or triglycerides are not likely to mediate PAI-1 excess in vivo. Hypertrophied adipose mass can secrete PAI-1, and is likely to contribute to the plasma PAI-1 pool in obese insulin-resistant subjects, but current evidence suggests that this is not likely to be the primary source of the elevated plasma PAI-1 in insulin resistance syndrome. Plasma PAI-1 can be decreased in insulin resistant subjects by improving adipocyte insulin sensitivity (with weight loss and thiazolidinediones), by consuming a very-low-fat diet that minimizes postprandial free fatty acid flux, and by administering activators of AMP-activated kinase (e.g., metformin), which can be expected to lessen tissue DAG synthesis.

Platelet function, coagulopathy, and impaired fibrinolysis in diabetes

Diabetes and the metabolic syndrome, including insulin resistance, that underlies it are hyper-coagulable states. Increased platelet reactivity,augmented activity of the coagulation system,and impaired fibrinolysis are characteristic and understood to a remarkable extent. In aggregate,these derangements contribute to accelerated atherosclerosis, premature coronary artery dis-ease, and a profound toll from both.

Obesity and impaired fibrinolysis: role of adipose production of plasminogen activator inhibitor-1

Obesity is the central promoter of the metabolic syndrome which also includes disturbed fibrinolysis in addition to hypertension, dyslipidaemia and impaired glucose tolerance/type 2 diabetes mellitus. Plasminogen activator inhibitor-1 (PAI-1) is the most important endogenous inhibitor of tissue plasminogen activator and uro-plasminogen activator, and is a main determinant of fibrinolytic activity. There is now compelling evidence that obesity and, in particular, an abdominal type of body fat distribution are associated with elevated PAI-1 antigen and activity levels. Recent studies established that PAI-1 is expressed in adipose tissue. The greater the fat cell size and the adipose tissue mass, the greater is the contribution of adipose production to circulating PAI-1. Experimental data show that visceral adipose tissue has a higher capacity to produce PAI-1 than subcutaneous adipose tissue. Studies in human adipocytes indicate that PAI-1 synthesis is upregulated by insulin, glucocorticoids, angiotensin II, some fatty acids and, most potently, by cytokines such as tumour necrosis factor-alpha and transforming growth factor-beta, whereas catecholamines reduce PAI-1 production. Interestingly, pharmacological agents such as thiazolidinediones, metformin and AT(1)-receptor antagonists were found to reduce adipose expression of PAI-1. In addition, weight loss by dietary restriction or comprehensive lifestyle modification is effective in lowering PAI-1 plasma levels. In conclusion, impaired fibrinolysis in obesity is probably also due to an increased expression of PAI-1 in adipose tissue. An altered function of the endocrine system and an impaired auto-/paracrine function at the fat cell levels may mediate this disturbance of the fibrinolytic system and thereby increase the risk for cardiovascular disease..

Rosiglitazone inhibits the insulin-mediated increase in PAI-1 secretion in human abdominal subcutaneous adipocytes

OBJECTIVE

The aim of this study was to investigate the effect of insulin and an insulin-sensitizing agent, rosiglitazone (RSG), on the production of plasminogen-activator inhibitor-1 (PAI-1) in isolated subcutaneous abdominal adipocytes. Human tissue-type plasminogen activator (t-PA) was also measured to assess changes in overall thrombotic risk.

METHODS

The mean depot-specific expression of PAI-1 and t-PA mRNA (n = 42) in subcutaneous abdominal (n = 21), omental (n = 10) and thigh (n = 11) adipose tissue depots was examined. Furthermore, subcutaneous adipocytes were treated with insulin, RSG and insulin in combination with RSG (10-8 m) for 48 h. Conditioned media were collected and enzyme-linked immunosorbent assays performed for PAI-1 and t-PA (n = 12) antigen. PAI-1 and t-PA mRNA levels were also assessed.

RESULTS

PAI-1 mRNA levels were significantly higher in subcutaneous and omental abdominal tissue than in thigh fat (p = 0.037 and p = 0.014). No change in t-PA mRNA expression between the adipose tissue depots was observed. Insulin stimulated PAI-1 protein secretion in a concentration-dependent manner in adipocytes (control: 68.3 +/- 1.2 ng/ml (s.e.m.); 10 nm insulin: 73.7 +/- 3.8 ng/ml upward arrow; 100 nm insulin: 86.8 +/- 4.1 ng/ml upward arrow **; 1000 nm insulin: 102.0 +/- 4.8 ng/ml upward arrow ***; **p < 0.01, ***p < 0.001). In contrast, insulin + RSG (10-8 m) reduced PAI-1 production relative to insulin alone (***p < 0.001), whilst RSG alone reduced PAI-1 protein secretion in a concentration-dependent manner (RSG at 10-10 m: 50.4 +/- 2.87 ng/ml downward arrow ***; RSG at 10-5 m: 30.3 +/- 2.0 ng/ml downward arrow ***; p < 0.001). No difference was observed between control and treatments for t-PA secretion (range 7-11 ng/ml).

CONCLUSIONS

Insulin stimulated PAI-1 secretion, whilst RSG reduced both PAI-1 secretion alone and in combination with insulin. These data suggest that adipose tissue may contribute significantly to the elevated circulating PAI-1 in obesity. Therefore, RSG's effects on PAI-1 production in adipose tissue may contribute to the fall in circulating PAI-1 levels observed in patients receiving RSG therapy.

Differential regulation by troglitazone of plasminogen activator inhibitor type 1 in human hepatic and vascular cells

Troglitazone, a novel oral insulin sensitizer, normalizes increased plasma activity of plasminogen activator inhibitor type 1 (PAI-1) in hyperinsulinemic patients such as women with polycystic ovary syndrome and patients with type 2 diabetes mellitus. However, underlying mechanisms have not yet been fully elucidated. Human hepatic and vascular cells, the main sources of circulating PAI-1, were studied in cell culture. In human hepatic cells, PAI-1 accumulated in conditioned medium by 23% within 24 h after exposure to 3 microg/mL troglitazone (P = 0.001). The accumulation depended on the concentration of troglitazone, but not that of insulin (known to stimulate PAI-1 synthesis). By contrast, in human aortic smooth muscle cells, 3 microg/mL troglitazone decreased basal PAI-1 expression by 23% (P = 0.037) and decreased transforming growth factor-beta-induced expression by 34% (P = 0.026). Concomitant insulin had no effect. Tissue-type plasminogen activator was decreased by 38% (P = 0.002). In human endothelial cells, PAI-1 was diminished by 32% (P < 0.001), whereas tissue-type plasminogen activator was unaffected. The results suggest that the reduction in plasma activity of PAI-1 induced by troglitazone in patients may reflect both directly mediated diminution of its elaboration from vessel walls and indirectly mediated reduction of its hepatic synthesis secondary to attenuation of hyperinsulinemia (known to increase the hepatic synthesis of PAI-1).

Effects of gemfibrozil on insulin sensitivity and on haemostatic variables in hypertriglyceridemic patients

In order to assess the efficacy of gemfibrozil on lipid and haemostatic parameters in patients with plurimetabolic syndrome, a multicenter double-blind placebo controlled, parallel study was carried out in 56 patients with primary hypertriglyceridemia and glucose intolerance. These patients had elevated PAI activity and antigen and t-PA antigen levels at rest and after venous occlusion. Gemfibrozil reduced plasma triglyceride levels (P<0.001), whereas it increased free fatty acids (P<0.05) and high density lipoprotein cholesterol levels (P<0.05). In those patients reaching normalization of plasma triglyceride levels (triglyceride reduction > or =50%) (n=15), insulin levels (P<0.05) as well as the insulin resistance index were reduced by gemfibrozil treatment, suggesting an improvement of the insulin resistance index in this patient subgroup. Gemfibrozil treatment did not affect plasma fibrinolysis or fibrinogen levels, despite marked reduction of plasma triglycerides and improvement of the insulin sensitivity associated with triglyceride normalization.