Positive Regulation der Plasminogen-Aktivator-Inhibitor-1-Genexpression durch Insulin und Glucagon in primären Rattenhepatozyten (English shop)

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Serine protease inhibitors such as plasminogen activator inhibitor 1 (PAI-1) are involved in the regulation of many important biological processes, including blood coagulation, fibrinolysis and remodelling processes of the extracellular matrix. PAI-1 plays a particular role in the regulation of endogenous fibrinolysis by inhibiting the activity of the plasminogen activators t-PA and u-PA. Elevated PAI-1 concentrations are associated with thromboembolic, cardio- and cerebrovascular diseases. Clinical studies have shown that hyperinsulinaemia in patients with type II diabetes mellitus, particularly in the presence of obesity and arterial hypertension, led to elevated PAI-1 concentrations in the blood. Furthermore, insulin induced PAI-1 expression in vitro in various cell types such as primary cultured human hepatocytes, the human hepatoma cell line HepG2, and arterial endothelial and vascular smooth muscle cells.

The present study aimed to examine whether insulin induces PAI-1 gene expression in primary rat hepatocytes via the PI3K/PKB signalling pathway. Since glucagon is the classical insulin antagonist, it was to be investigated whether glucagon suppresses PAI-1 gene expression in primary rat hepatocytes and whether the possible glucagon effect on PAI-1 gene regulation is mediated via the cAMP/PKA signal transduction pathway. By means of transfection of rat PAI-1 promoter-Luc gene constructs, possible insulin- or glucagon-responsive PAI-1 gene elements were also to be identified.

In primary cultured rat hepatocytes, a positive regulation of plasminogen activator inhibitor 1 (PAI-1) gene expression by insulin and glucagon could be demonstrated. Furthermore, it could be shown that the glucagon-dependent induction of PAI-1 gene expression is mediated via the second messenger cAMP.

In transfection experiments with PAI-1 gene constructs, it could be shown for the first time that the two hypoxia-responsive elements (HRE-1 and 2) of the PAI-1 promoter are involved in the insulin and glucagon-mediated PAI-1 induction independently of the pO2.

It could furthermore be shown that the PI3K/PKB signalling pathway is probably involved in both the insulin- and the hypoxia-dependent induction of PAI-1 gene expression, via the activation of hypoxia-inducible factor 1. In contrast, no involvement of cAMP in the hypoxia-dependent induction of PAI-1 expression could be demonstrated.

On the basis of the present results, the existence of further PAI-1 promoter elements in close proximity to the two HRE, mediating a cAMP-dependent induction of PAI-1 expression, is conceivable. Likewise, with regard to the induction of PAI-1 expression by glucagon and cAMP demonstrated here, the more recent investigations into further possible activation pathways of the PI3K/PKB and MAP kinase cascades via G protein-coupled receptors and cAMP may be of particular importance. PKA, as the classical target protein of cAMP, also appears to be involved in the glucagon-dependent PAI-1 induction.