Molecular mechanisms of endothelial dysfunction in the diabetic heart.

Our observations show that long term hyperglycaemia by the formation of AGE, but also short term hyperglycaemic periods ("glucose spikes") damage the endothelium of the heart in diabetes. The endothelium is exposed to oxidative stress. The simultaneous generation of NO and superoxide anions enables the reaction of both species to form peroxynitrite which has been identified as an important mediator for the transformation of endothelium from an anticoagulant to a procoagulant state. Together with a functional loss of endothelium these processes are assumed to impair the coronary perfusion and to provoke adaptive processes which finally lead to cardiac dysfunction and remodelling of cardiac structure (Figure 6) as it has been described for the heart in diabetes.

Introduction of apoptosis by high proinsulin and glucose in cultured human umbilical vein endothelial cells is mediated by reactive oxygen species.

There is much evidence that diabetes and hyperglycaemia contribute to the impairment of endothelial function and induce severe changes in the proliferation, the adhesive and synthetic properties of endothelial cells. Induction of apoptosis could represent one mechanism to prevent the new accumulation of those vascular defects and to allow generation of vascular endothelium. In this study, we demonstrate that high concentrations of glucose or proinsulin induce apoptosis in human umbilical endothelial cells by three independent methods (DNA fragmentation, fluorescence activated cell sorting analysis, and morphology). The number of apoptotic cells was increased by glucose (30 mmol/l or proinsulin (100 nmol/l) from less than 10% to about 30%. Activation of protein kinase C (PKC) largely prevented the induction of apoptosis, whereas inhibition of PKC further increased the number of apoptotic cells. Similar changes as induced by glucose were also observed after incubation of the cells with the non-metabolisable 3-O-methylglucose. These findings indicate that hyperglycaemic conditions stimulate the induction of apoptosis in endothelial cells by a mechanism which is independent from the formation of diacylglycerol and the activation of PKC. The induction of apoptosis by the non-metabolisable glucose suggests that formation of oxygen derived radicals by autoxidative processes is involved and may lead to an activation of transcription factors such as nuclear transcription factor-kappaB (NF-kappaB) transferring the activation signal into the nucleus and leading to changes in gene expression necessary for induction of apoptosis.