Preferential reduction of very low density lipoprotein-1 particle number by fenofibrate in type IIB hyperlipidemia: consequences for lipid accumulation in human monocyte-derived macrophages

Abstract

The combined (mixed) type IIB phenotype is typically associated with premature atherosclerosis and characterised by concomitant elevation of plasma levels of atherogenic triglyceride-rich lipoproteins, consisting of very low density lipoprotein (VLDL)-1 (Sf 60–400), VLDL-2 (Sf 20–60), and intermediate density lipoprotein (IDL) (Sf 12–20), as well as small dense LDL. After dietary stabilisation, type IIB patients received micronised fenofibrate (267 mg/day) for up to 12 months. At baseline ( T0), patients ( n=11) displayed fasting triglyceride, cholesterol and apoB levels of 308±13, 350±17 and 187±9 mg/dl, respectively. Micronised fenofibrate (M-fenofibrate) induced marked reductions in plasma triglyceride (TG) (−61%, P<0.0001), total cholesterol (−32%, P=0.0005) and apolipoprotein (apo) B (−33%, P<0.001) at 12 months ( T12); similar effects were seen after 3 months ( T3) of treatment. These changes resulted from significant reductions in VLDL-1 (−75%, P=0.00001), VLDL-2 (−46%, P=0.002) and LDL (−33%, P<0.0003); IDL concentrations were unchanged. At baseline, VLDL-1 constituted the major TG-rich lipoprotein (TRL) fraction (50% of total mass), but only 25% at T12. These drug effects were accompanied by marked increase in HDL-C (+20%, P=0.018). Quantitative changes in triglyceride-rich lipoproteins were accompanied by significant qualitative modifications in particle size and chemical composition (VLDL-1: TG, −10.7%, P<0.001; FC, +59%, P=0.0002; PL, +19%, P=0.033; VLDL-2: FC, +11%, P=0.027; IDL: FC, +14%, P=0.0004; PL, +12%, P=0.002). Reduction in the TG content of VLDL-1 was reflected in a shift of particle size distribution to smaller diameters (mean 45.4 and 42.3 nm, respectively, at T0 and T12). We evaluated the relative atherogenicity of TRL subfractions by determining their capacity, when normalised to equal particle numbers (as apoB 100 content), to induce lipid accumulation in human monocyte-derived macrophages. Among TRL subfractions, VLDL-1 (100 μg apoB/ml) possessed the highest capacity to induce macrophage lipid loading (up to sevenfold increase in TG content, P<0.001; free cholesterol, up to 1.7-fold; P<0.05). At 100 μg apoB/ml, cellular TG loading from VLDL-1 was twofold greater than that for VLDL-2 ( P<0.01), and fivefold greater than for IDL ( P<0.01). Despite drug-induced changes in the qualitative properties of TRL subfractions, the activity of VLDL-1, VLDL-2 and IDL as ligands which lead to induction of macrophage lipid accumulation, at equivalent particle numbers, was not detectably altered. By contrast, the fibrate-mediated reduction in the number of circulating VLDL-1 and VLDL-2 particles (four and twofold, respectively) resulted in marked decrease in cellular lipid loading. Considered together, these findings suggest that fenofibrate may act at systemic and arterial levels to reduce the cardiovascular risk associated with VLDL subfractions in patients with a combined hyperlipidemic (type IIB) phenotype. Indeed, we speculate that reductions in circulating levels of VLDL-1 and VLDL-2 may diminish intimal penetration of these particles and thus their propensity to enhance arterial macrophage lipid accumulation and foam cell formation. Finally, fenofibrate further attenuated the atherogenic lipid profile in these patients by inducing marked reduction in LDL and elevation in cardioprotective HDL.