The common risk factors for atherosclerosis, such as obesity, high cholesterol levels, sedentary lifestyle, diabetes and high alcohol intake, only explain approximately 50% of cardiovascular disease events. It is thereby important to identify new mechanisms that can stimulate the process of atherosclerosis. During the past decades, a wide range of investigations have demonstrated connections between infections by the respiratory bacterium Chlamydia pneumoniae and atherosclerosis. Earlier studies have focused on the interaction between C. pneumoniae and monocytes/macrophages, T-lymphocytes, smooth muscle cells and endothelial cells, which are present in the atherosclerotic plaque. However, another important player in atherosclerosis and which is also present in the plaques is the platelet. Activation of platelets can stimulate both initiation and progression of atherosclerosis and thrombosis, which is the ultimate endpoint of the disease. The aim of the present thesis was to investigate the capacity of C. pneumoniae to activate platelets and its role in atherosclerosis. The results show that C. pneumoniae at low concentrations binds to platelets and stimulates platelet aggregation, secretion, reactive oxygen species (ROS) production and oxidation of low-density lipoproteins (LDL), and that these effects are mediated by lipopolysaccharide (LPS). Activation of protein kinase C, nitric oxide synthase and 12-lipoxygenase (12-LOX) was required for platelet ROS production, whereas platelet aggregation was dependent on activation of GpIIb/IIIa. Pharmacological studies showed that the C. pneumoniae-induced platelet activation is prevented by inhibitors against 12-LOX, platelet activating factor (PAF) and the purinergic P2Y1 and P2Y12 receptors, but not against cyclooxygenase (COX). These findings were completely opposite to the effects of these inhibitors on collagen-stimulated platelets. We also present data from a clinical study indicating that percutaneous coronary intervention (PCI or balloon dilatation) leads to release of C. pneumoniae into the circulation, which causes platelet activation and LDL oxidation. In conclusion, these data support a role for C. pneumoniae-induced platelet activation in the process of atherosclerosis. Stimulation of platelets by C. pneumoniae leads to release of growth factors and cytokines, oxidation of LDL and platelet aggregation, which are processes that can stimulate both atherosclerosis and thrombosis. Development of novel drugs that prevent C. pneumoniae-platelet interaction by inhibiting 12-LOX and/or PAF, may be important in the future treatment of cardiovascular disease.

The aim of this thesis is to investigate molecular mechanism of reactive oxygen species production by flavin dehydrogenases mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) and succinate dehydrogenase (SDH). Together, they represent important source of reactive oxygen species in mammalian mitochondria, but the mechanism of electron leak is still poorly understood. Because mechanisms of reactive oxygen species production by other complexes of respiratory chain are better characterized, they can serve as case studies to get insight into mechanisms of reactive oxygen species by flavin dehydrogenases. Relevant knowledge is therefore summarized in the first part of the thesis. To study the production of reactive oxygen species by the isolated flavin dehydrogenases, we used brown adipose tissue mitochondria solubilized by digitonin as a model. Enzyme activity measurements, hydrogen peroxide production studies by Amplex UltraRed fluorescence and luminol luminescence revealed flavin as the most likely source of electron leak in SDH under in vivo conditions, while we propose coenzyme Q binding site as the site of reactive oxygen species production in the case of mGPDH. Distinct mechanism of this production by the two dehydrogenases is also apparent from induction of reactive oxygen species...