High-carbohydrate diets affect the size and composition of plasma lipoproteins in hamsters (Mesocricetus auratus)

High-carbohydrate diets reduce plasma low-density lipoprotein (LDL)-cholesterol but also provoke the appearance of an atherogenic lipoprotein profile (ALP). Characterized by high plasma triglyceride, small dense LDL, and reduced high-density lipoprotein (HDL) cholesterol, an ALP is associated with insulin resistance. Despite extensive use of the fructose-fed hamster as a model of insulin resistance, little is known about changes that occur in the physical properties of circulating lipoproteins. Therefore, we investigated the metabolic and physical properties of lipoproteins in hamsters fed high-carbohydrate diets of varying complexity (60% carbohydrate as chow, cornstarch, or fructose) for 2 wk. Hamsters fed the high-fructose diet showed significantly increased very- low-density lipoprotein (VLDL)-triglyceride (92.3%), free cholesterol (68.6%), and phospholipid (95%), whereas apolipoprotein B levels remained unchanged. Median diameter of circulating VLDL was larger in fructose-fed hamsters (63 nm) than in cornstarch-fed hamsters. Fructose feeding induced a 42.5% increase LDL-triglyceride concurrent with a 20% reduction in LDL-cholesteryl ester. Compositional changes were associated with reduced LDL diameter. In contrast, fructose feeding caused elevations in all HDL fractions. The physical properties of apolipoprotein-B-containing lipoprotein fractions are similar between fructose-fed hamsters and humans with ALP. However, metabolism of high-density lipoprotein appears to differ in the 2 species.

Endothelial lipase-modified high-density lipoprotein exhibits diminished ability to mediate SR-BI (scavenger receptor B type I)-dependent free-cholesterol efflux

Endothelial lipase (EL) is a phospholipase with little triacylglycerol lipase activity. To assess structural and functional properties of EL-HDL (EL-modified high-density lipoprotein), HDL was incubated with conditioned medium from Cos-7 cells infected with adenovirus encoding human EL. After re-isolation of HDL by ultracentrifugation, TLC and HPLC analyses revealed that EL-HDL was markedly depleted in phosphatidylcholine and enriched in lyso-phosphatidylcholine compared with LacZ-HDL (control HDL) incubated with conditioned medium from Cos-7 cells infected with adenovirus encoding beta-galactosidase. The EL-HDL was enriched in non-esterified fatty acids and, as revealed by lipid electrophoresis, was more negatively charged than control HDL. The HDL particle size as well as the total cholesterol, free cholesterol and triacylglycerol content of HDL were not significantly altered after EL modification. The ability of EL-HDL to mediate 3H-cholesterol efflux from SR-BI (scavenger receptor B type I) overexpressing Chinese-hamster ovary cells was impaired and markedly lower compared with LacZ-HDL at HDL concentrations of 100 microg/ml and above. Studies with 125I-labelled HDL showed almost unaltered binding affinity (K(m) values) and a slightly but significantly decreased binding capacity (B(max) values) of EL-HDL to SR-BI, compared with LacZ-HDL. The ATP-binding-cassette transporter A1-dependent cholesterol and phospholipid effluxes were not affected by EL modification. From these results, we concluded that EL modification alters chemical composition and physical properties of HDL, resulting in its decreased binding capacity to SR-BI and a diminished ability to mediate SR-BI-dependent cholesterol efflux.