Physiological Modulation of the Reverse Cholesterol Transport Pathway in Vivo

Atherosclerosis is a progressive disease characterized by the appearance of inflamed lesions in the arterial wall. The main component of an atherosclerotic lesion is a cholesterol-filled macrophage (a foam cell). In addition, the inflamed intima contains numerous mast cells which, upon activation, acutely secrete serine proteases and other mediators that can influence the progression of atherosclerotic lesions. HDL-mediated removal of cholesterol from the lipid-filled macrophages and its transfer to the liver and feces, a process termed macrophagereverse cholesterol transport (m-RCT), is an important anti-atherogenic mechanism. The multi-step m-RCT pathway appears to be modulated at the various steps. Mast cell-derived proteases, by degrading HDL lipoproteins, may affect the early steps of mRCT, a possibility that has not been investigated in vivo. Psychological stress, an established risk factor for cardiovascular diseases and a potent activator of mast cells, might also interfere with m-RCT. With an aim to answer the question whether cholesterol flux through the m-RCT pathway could be physiologically modulated by mast cell activation and stress, this thesis assessed the functionality of the various components of the m-RCT pathway using the mouse as the experimental model. In the first study, a short-term m-RCT in vivo analysis was validated and performed to investigate the consequence of local mast cell activation for the functionality of HDL in m-RCT. The following study utilized the same method to address the effects of acute psychological stress on m-RCT. In the third study, the effects of stress on m-RCT were assessed in a chronic setting. An inhibitory role of peritoneal mast cell activation in vivo on the initial step of the m-RCT was established. Conversely, stress exposure, both acute and repetitive, induced multiple m-RCT-promoting responses in the liver and intestine. Mice exposed to acute psychophysical stress exhibited accelerated m-RCT due to compromised intestinal absorption of cholesterol, uncovering a novel functional connection between the stress hormone corticosterone and m-RCT. Repeated exposure to the same stressor resulted in increased fecal excretion of bile acids which also stimulated the rate of m-RCT. Altogether, the results presented in this thesis demonstrate that the m-RCT pathway is effectively modulated by two physiological factors, psychological stress and mast cell activation, which are involved in the pathology of