Doctor of Philosophy
School of Health Sciences
Cheung, Ching-Loong (Yvonne), An investigation of the atherogenicity of intermediate density lipoprotein in people with and without type 2 diabetes mellitus, Doctor of Philosophy thesis, School of Health Sciences, University of Wollongong, 2009. http://ro.uow.edu.au/theses/3066
It is well-accepted that people with type 2 diabetes mellitus (T2DM) tend to have more adverse lipid profile than those without the disease. There is a lot of information on the adverse role played by low density lipoprotein (LDL) in atherosclerosis, however, much less is known on the intermediate density lipoprotein although both epidemiological evidence and animal and in vitro studies suggests it plays a role in atherosclerosis as well. Hence this thesis examines and compares the atherogenicity of intermediate density lipoprotein (IDL) between people with an without T2DM.
In the first study the ability of IDL isolated from people with-and without-T2DM was investigated and the ability of IDL to induce lipid accumulation was compared. The results show that unlike LDL, IDL could be taken up by the macrophages in its unmodified form and the uptake was not likely to be receptor-mediated as it is mostly triglyceride (TG) (as opposed to CE) that was accumulated in the cells. It is believed that the triglyceride (TG) in IDL was first hydrolyzed into free fatty acids (FFA) by lipoprotein lipase outside the cells and these FFA were re-esterified to TG once inside the cells. Although there was no difference in the lipid accumulation between the T2DM and control groups, both groups accumulated significant amount of TG in the cells when their IDL was incubated with macrophages in vitro (5830±966 vs 5727±1241, sum of area under curve adjusted for cell protein, p>0.05). On the other hand, only negligible amount of cholesterol ester was accumulated in the cells in both groups and the cholesterol concentrations were similar to the concentration found in the non-loaded cells (21± 5 vs 19±3 vs 16±8, p>0.05).
The second study investigated the atherogenicity of IDL by assessing the affinity of IDL to chondroitin sulphate (CS), which is a major proteoglycan found in vessel walls. The results show that IDL isolated from people with and without T2DM both bound to CS at similar affinities (86±0.2 % vs 90±2.0 %) and there were no differences in the composition of the unbound or bound fractions IDL between the two groups. The results also show that IDL bound to CS at two different affinities: lower affinity (low- bound IDL) and higher affinity (high-bound IDL). The high-bound IDL fraction contained significantly lower TG (0.64±0.06 vs 0.39±0.05 mmolL-1/mg, p<0.01) and phospholipids (0.04±0.01 vs 0.03±0.01 mgdl-1/mg, p<0.05) than the low-bound IDL fraction but both fractions contained similar concentration of total cholesterol (0.57±0.07 vs 0.44±0.07 mmolL-1/mg, NS). This study have demonstrated that the IDL that bound to CS with a higher affinity was also the one with smaller size compared to the IDL bound to CS with lower affinity. Similar to LDL, IDL consists of different sizes and the smaller IDL particles bound to CS at a higher affinity than the larger IDL particles.
In the third study, the concentration of glycation in IDL was assessed to determine the atherogenicity of IDL. The concentration of advanced glycation endproducts (AGE) accumulated in the IDL was also compared in T2DM and control groups. The concentration of two AGE, i.e. N-epsilon- (carboxyethyl)lysine (CML) and S-(carboxymethyl)cysteine (CMC), were measured respectively and the results indicate that the concentration of each was similar in both groups (CML: 0.005±0.003 vs 0.002±0.001 nmol/mg NS; CMC: 1.48±0.14 vs 1.45±0.23 nmol/mg, NS). The results also show that CMC in the IDL fraction was positively correlated with both plasma TG concentration (r=0.39, p=0.04) and body mass index (r=0.46, p=0.02). These findings suggest that the IDL is possibly be more glycated when the risk of diabetes increases. Interestingly the same data show that the CML concentration was similar in the T2DM and control groups. Together, this thesis demonstrated that IDL is atherogenic but the atherogenicity was not increased in the people with diabetes whose blood glucose concentrations were well controlled.
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