Year: 2015
Type: dissertation
Abstract: Atherosclerosis is an inflammatory disease characterized by the accumulation of cholesterol in the arterial intima and consequently the formation of atherosclerotic plaques. Formation of these plaques is initiated by the appearance of macrophage foam cell in the arterial intima. Foam cells are formed as excessive cholesterol accumulates in the cytosol of macrophages and finally the net influx exceeds the efflux of cholesterol. Excessive accumulation of chemically modified cholesterol in foam cells finally leads to apoptosis and contributes to the formation of the lipid core in atherosclerotic plaques. The efflux of cholesterol from foam cells is essential for preventing the progression of atherosclerosis. The only unidirectional transporters ABCA1 and ABCG1, expressed on macrophages, transport intracellular cholesterol actively to distinct subpopulations of HDL. ApoA-I, the most important structural and functional component of nascent preβ-migrating HDL particles, receives cholesterol from ABCA1. Lipid-free HDL and apoA-I are sensitive to proteolytic modification leading to loss of function of these molecules. Functional apoA-I is essential for removal of cellular cholesterol and for cholesterol homeostasis. Cholesterol efflux initiates reverse cholesterol transport (RCT) which is the pathway for removal of cholesterol from the periphery for its final excretion into feces. The tiny fraction of total body-RCT that originates from the cholesterol-loaded macrophage foam cells located in the intima, is considered the only RCT component directly involved in atherosclerosis. Mast cells are bone marrow-derived inflammatory cells that are able to activate and secrete various mast cell mediators. Mast cells infiltrate the inflamed arterial intima where they can be activated through several stimuli present in the atherosclerotic intima. Mast cells release several inflammatory compounds of which histamine is probably the best known for its notorious effects in anaphylaxis. In addition to histamine and other vasoactive compounds, such as serotonin and bradykinin, mast cells release upon activation their unique serine proteases, tryptase and chymase. Chymase involvement in the progression of atherosclerosis has been suggested in a number of studies. Chymase is an enzyme capable of degrading LDL and HDL components leading to increased uptake of the modified LDL by macrophage foam cells or resulting in diminished cholesterol efflux, respectively. The purpose of this study was to evaluate whether mast cell-dependent HDL and apoA-I proteolysis would occur in vivo and whether such modification would alter the cholesterol efflux capacities of these cholesterol acceptors and finally affect the macrophage-specific RCT (mRCT). In the present study it was demonstrated for the first time that mast cell activation in vivo resulted in HDL proteolysis. Systemic mast cell activation led to the degradation of lipoprotein particles present in HDL and the entire preβ-HDL and α-HDL subpopulations were reduced in mouse serum following systemic mast cell activation. Systemic activation of mast cells in mice blunted the ability of serum and intraperitoneal fluid to promote cholesterol efflux from macrophage foam cells in vitro. Rat cardiac mast cell activation ex vivo led to the production of truncated apoA-I. ApoA-I was cleaved at the carboxyl-terminal region at Phe229 and Tyr192 or only at Tyr192 depending on the mast cell stimulus. Local peritoneal mast cell activation led to decreased ability of intraperitoneally injected apoA-I to promote macrophage cholesterol efflux in vitro. Furthermore treatment with intact lipid-free apoA-I but not chymase-treated apoA-I increased the overall mRCT from the peritoneal cavity to the intestinal contents within 3 hours. Importantly such an increase was fully blocked by the mast cell-specific degranulating compound 48/80 in mast cell-competent mice but not in mast cell-deficient mice. Interestingly local mast cell activation in the skin was able to promote mRCT from skin to feces. This was due to increased vascular permeability and influx of plasma HDL particles to skin consequently leading to increased mRCT. This stimulatory effect could be reproduced by the sole administration of the mast cell mediators, histamine, serotonin, and bradykinin. Importantly histamine treatment in apoA-I deficient mice was unable to promote mRCT. In conclusion, mast cell chymase is able to proteolyze HDL and lipid-free apoA-I reducing their abilities to promote cellular cholesterol efflux. Proteolysis of lipid-free apoA-I by rat cardiac mast cell chymase can occur within minutes and results in the formation of carboxyl-terminally truncated apoA-I. ApoA-I proteolysis in vivo results in reduced mRCT. Local mast cell activation in the skin results in increased mRCT due to increased availability of cholesterol acceptors in the vicinity of macrophage foam cells. These two seemingly opposite results underline the pleiotropic role of mast cells in the development of atherosclerosis
Source: Työväentutkimus Vuosikirja
Author: Ilona Kareinen
Language: English
Subfield: Biochemistry
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W1817129849