Expression of plasminogen activator inhibitor-1 in human adipose tissue: a role for TNF-α?
Introduction
Type-1 plasminogen activator inhibitor (PAI-1) is the main circulating inhibitor of fibrinolysis [1]. Elevated levels of PAI-1 have been previously associated with increased risk for both venous [2], [3], [4] and arterial [5], [6], [7], [8], [9] thrombosis. However, there is now agreement that elevated PAI-1 levels are not a proven pathogenic cause of venous thrombosis, whilst arguments in favour of a role of PAI-1 in arteriosclerosis and arterial thrombosis (smooth muscle cell migration, neointima formation, tissue remodeling, vascular wound healing, reaction to arterial injury) have recently been brought up by Carmeliet et al. [10], [11], [12]
Obesity, and particularly abdominal obesity, is an independent risk factor for cardiovascular disease [13]. Increased visceral fat, with or without obesity, is part of a metabolic syndrome including insulin resistance, impaired glucose tolerance or type-2-diabetes mellitus, hypertriglyceridemia and low HDL-cholesterol, hypertension [13], [14], [15].
Recently, increased PAI-1 activity has been associated with the metabolic syndrome [16], [17], [18] and independent, highly significant correlations between direct measurements of visceral fat and plasma PAI-1 levels have been demonstrated in humans [19], [20], suggesting a direct role of adipose tissue in the determination of plasma PAI-1 levels. The hypothesis that increased plasma PAI-1 levels in obese human subjects may be the result of PAI-1 release from the increased mass of adipose tissue is very attractive and is supported by studies in animals and in cell lines in vitro. In fact, previous studies on the tissue distribution of PAI-1 in mice showed high concentrations of PAI-1 mRNA in the epididymal fat pad [21], [22]; furthermore, both PAI-1 mRNA and protein were found to be actively produced by 3T3-L1 cells after differentiation into adipocytes [20], [23]. PAI-1 expression in human adipose tissue has been recently documented as well [24].
In murine fat tissue, PAI-1 expression is markedly influenced by the cytokine tumor necrosis factor-alpha (TNF-α), as shown by in vivo and in vitro studies [22], while no effect of TNF-α could be found on human adipose tissue PAI-1 production [24].
The presence of TNF-α in adipose tissue has recently received a great deal of attention. This cytokine is expressed in human adipocytes of both normal and, at higher level, obese subjects [25], [26], [27]. TNF-α influences the lipid and glucose metabolism in fat cells and causes insulin resistance [25], [28] by inhibiting very early steps of insulin action at post-receptor-binding level [29]. As a result, TNF-α is now considered a key component of the obesity–diabetes link [30]. To confirm this, protection from obesity-induced insulin resistance has been recently demonstrated in TNF-α-deficient obese mice [31].
On the basis of findings reported in literature, we hypothesized that TNF-α may influence PAI-1 expression in human adipose tissue, the cytokine being a possible patho–physiological link in the clinical relationship between insulin-resistance syndrome and increased PAI-1 plasma levels. Therefore, the present study aimed at verifying whether TNF-α can influence PAI-1 production in human adipose tissue.
Section snippets
Subjects and tissue sample acquisition
Forty non-neoplastic, overweight (BMI 28±5 Kg/mq) subjects were recruited to obtain fat biopsies. Twelve were surgical patients (explorative laparotomy, gallbladder disease, abdominal hernia) and from seven of them both subcutaneous and omental adipose tissue could be obtained. The remaining 28 subjects were ambulatory patients from our metabolic unit and, in that case, only subcutaneous fat tissue was taken by needle biopsy, according to Kolaczynski et al. [32]. All were non-diabetic as
Production of PAI-1 protein in culture
When human subcutaneous and omental adipose tissue fragments were incubated in basal medium, PAI-1 protein level in conditioned medium increased with incubation time, as shown in Fig. 1, left panel (after 1 h incubation: 0.28±0.07 ng/ml; after 8 h incubation: 6.22±2.10 ng/ml; P<0.05, n=7). Therefore, the rate of PAI-1 release increased during 8 h incubation from 0.28±0.07 ng/ml/h (mean±S.D.) per gram adipose tissue in the first hour, to an average of 0.78±0.28 ng/ml/h in 8 h. The effect of
Discussion
This study confirms that human adipose tissue from obese individuals expresses PAI-1 mRNA and releases PAI-1 protein in culture. Similar results were previously obtained in vitro and in vivo in animals [21], [22], in 3T3-L1 preadipocytes cell line, capable of developing into mature adipocytes under proper stimulation [20], [23] and, recently, in human adipose tissue [24]. The demonstration that human adipose tissue produces PAI-1 can contribute to explain the pathophysiology of the elevation of
Acknowledgements
The authors are indebted to Dr Marina Botto (Department of Rheumatology, Hammersmith Hospital, London), for help during all of the steps of this study. The authors also express their gratitude to Professor Elena Tremoli and Dr Luciana Mussoni of the Institute of Pharmacological Sciences, University of Milan, for their constructive advice and the PAI-1 cDNA probe; to Dr Giovanna Agostino for preliminary mRNA analysis and, together with Dr Ketti Boschini, for assistance in our metabolic ward; to
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