| dc.description.abstract | The primary cause of mortality in developed countries is cardiovascular disease. Development of the disease is regulated by atherosclerosis, a progressive inflammatory reaction that takes place in the intima of the arterial wall. This reaction is characterized by activation of endothelial cells, proliferation of smooth muscle cells and macrophages, accumulation of lipoproteins, deposition of extracellular matrix components and enhanced lipolytic enzyme activity. Later, as the damage in intima expands, the endothelium breaks down and platelets aggregate. Finally, the development of atherosclerosis ends in plaque rupture and closure of a blood-bearing artery.
Circulating low-density lipoproteins (LDL) and their enzymatic lipolytic degradation are essential for the development of atherosclerosis. One candidate enzyme for enzyme modification of lipoproteins is phospholipase A2 (PLA2). PLA2 is a family of enzymes (see chapter 6.2) releasing unsaturated fatty acids in the sn-2 position of membrane phospholipids. Upon release, arachidonic acid (AA) is further metabolized into lipid mediators, the eicosanoids. Eicosanoids are lipid hormones that regulate physiological and pathological processes, including inflammation (see chapter 6.3). Hence, enzymatically modification of LDL by PLA2 generates eicosanoids that may participate in activation of intimal endothelial cells, macrophages or smooth muscle cells.
It is important to clarify the contribution of lipid mediated proinflammatory activation of cells during atherosclerotic plaque development in order to identify targets for development of therapeutic strategies and subsequent treatment of the disease. This thesis focuses on understanding the mechanisms of lipid mediated activation of human monocytes. In more detail we have investigated the ability of lysophosphatidylcholine (lysoPC), LDL and enzymatic modified LDL to stimulate monocyte proinflammatory activation. We found that both LDL and lysoPC, a product of LDL modification, initiate intracellular signal transduction pathways leading to activation of PLA2 enzymes. PLA2 activation was increased when LDL was rendered more proatherogenic by secretory non-pancreatic (snp)PLA2 and sphingomyelinase (SMase) modification.
Both secretory PLA2 (sPLA2) and cytosolic PLA2 were stimulated in human-derived monocytes by enzymatically modified LDL. PLA2 activation was initiated at the cellular surface by aggregated LDL partially mediated via scavenger receptor CD36. The biologic consequence of enzymatic modified LDL stimulation of monocytes was changed cytokine expression, confirming the proinflammatory activation. LysoPC also activated sPLA2 and cPLA2 enzymes in human-derived monocytes, partly mediated by a Gi-protein dependent pathway. Components involved in lysoPC triggered activation of cPLA2 were investigated and compared to the components involved in platelet-activating factor (PAF) induced cPLA2 activation. We found that lysoPC and PAF differ in activation mechanism of cPLA2. PAF activates sPLA2, PLD and p38 in a sequential manner, by a traditional PTX sensitive pathway initiated by the PAF receptor. LysoPC stimulated two pathways leading to activation of cPLA2. One pathway was PTX sensitive and included PLD activation, the other pathway was PTX insensitive and included p38 phosphorylation, both controlling cPLA2 activation. The sPLA2 enzyme was not controlled by the G-protein coupled receptor but participated in AA release in response to lysoPC.
In conclusion, LDL and lysoPC mediated proinflammatory AA release involves activation of both secretory and cytosolic PLA2 enzymes stimulated by specific intracellular pathways in monocytes. This is a confirmation of our hypothesis that lipoproteins and products from enzymatically modification of lipoproteins may induce proinflammatory activation of human monocytes. This work gives important contributions to resolving the molecular mechanisms regulating proinflammatory activation of human-derived monocytes during early atherosclerosis development. | nb_NO |
