UBC Theses and Dissertations
The effect of variations in HDL composition on components of reverse cholesterol transport Sparks, Daniel Leslie
This thesis is concerned with the mechanism which underlies the apparent anti-atherogenic capacity of high density lipoproteins (HDL). Specifically, the relationship between HDL composition, cholesteryl ester transfer protein (CETP) and lecithin: cholesterol acyltransferase (LCAT) was investigated. A novel assay was developed which allowed for the determination of the rate of transfer of [³H]cholesteryl ester from agarose-bound HDL to the endogenous lipoproteins of a plasma sample. Comparison of this assay with an established method, which utilized exogenous lipoprotein substrates, indicated a significant positive correlation (r = 0.960) between the assays for the rates measured for normolipidemic subjects. When the same relationship was studied in 30 patients with lipid metabolic disorders, however, no such correlation could be demonstrated (r=0.147). This suggests that lipoprotein composition and content may not affect total cholesteryl ester transfer activity in normolipidemic, but may markedly affect the function of CETP in the plasma of hyperlipidemic patients. When cholesteryl ester transfer was determined in plasma of 50 patients with defined primary disorders of lipid metabolism, transfer to the plasma HDL pool was shown to be significantly reduced in almost all patient groups. Reduced transfer to HDL occurred in samples with altered HDL composition; particularly where HDL-triglyceride was significantly increased and HDL-cholesteryl esters were reduced. Transfer to LDL and VLDL was increased in patients with dysbetalipoproteinemia and hypoalphalipoproteinemia. In addition, HDL unesterified cholesterol content was significantly increased and cholesteryl ester transfer to HDL₃ was significantly reduced in patients with documented evidence of vascular disease. These findings indicate that impaired interaction of CETP with the HDL pool may contribute to the risk of coronary heart disease in patients with specific plasma lipid abnormalities. The mechanism underlying the results described above was investigated in vitro. Incubations utilized recombinant high density lipoproteins (rHDL) prepared by short duration co-sonication. Increasing the triglyceride content, relative to cholesteryl ester, significantly decreased the ability of the particles to accept cholesteryl esters transferred by CETP. When the free cholesterol content was increased relative to phospholipid, the ability of the particles to accept cholesteryl esters was also decreased in a similar manner. Since this corroborates the clinical observations, these findings indicate that altered HDL composition may have marked effects on the transfer and equilibration of cholesteryl esters within this lipoprotein pool. In addition, when rHDL were characterized as substrates for purified human LCAT, it was shown that increasing the triglyceride content relative to cholesteryl ester in rHDL also markedly decreased the maximum catalytic potential. These findings suggest that altered HDL composition results in abnormal interactions between HDL, CETP and LCAT. This may impair the equilibration of cholesteryl esters and their subsequent removal from the HDL pool. The consequence may be increased transfer and accumulation of cholesteryl esters in lower density lipoproteins; an event which promotes atherogenesis.
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