Apolipoprotein C-II deficiency. The role of apolipoprotein C-II in the hydrolysis of triacylglycerol-rich lipoproteins

PPARalpha in atherosclerosis and inflammation

Peroxisome proliferator-activated receptor (PPAR)alpha is a nuclear receptor activated by natural ligands such as fatty acids as well as by synthetic ligands such as fibrates currently used to treat dyslipidemia. PPARalpha regulates the expression of genes encoding proteins that are involved in lipid metabolism, fatty acid oxidation, and glucose homeostasis, thereby improving markers for atherosclerosis and insulin resistance. In addition, PPARalpha exerts anti-inflammatory effects both in the vascular wall and the liver. Here we provide an overview of the mechanisms through which PPARalpha affects the initiation and progression of atherosclerosis, with emphasis on the modulation of atherosclerosis-associated inflammatory responses. PPARalpha activation interferes with early steps in atherosclerosis by reducing leukocyte adhesion to activated endothelial cells of the arterial vessel wall and inhibiting subsequent transendothelial leukocyte migration. In later stages of atherosclerosis, evidence suggests activation of PPARalpha inhibits the formation of macrophage foam cells by regulating expression of genes involved in reverse cholesterol transport, formation of reactive oxygen species (ROS), and associated lipoprotein oxidative modification among others. Furthermore, PPARalpha may increase the stability of atherosclerotic plaques and limit plaque thrombogenicity. These various effects may be linked to the generation of PPARalpha ligands by endogenous mechanisms of lipoprotein metabolism. In spite of this dataset, other reports implicate PPARalpha in responses such as hypertension and diabetic cardiomyopathy. Although some clinical trials data with fibrates suggest that fibrates may decrease cardiovascular events, other studies have been less clear, in terms of benefit. Independent of the clinical effects of currently used drugs purported to achieve PPARalpha, extensive data establish the importance of PPARalpha in the transcriptional regulation of lipid metabolism, atherosclerosis, and inflammation.

Lipid binding of apolipoprotein CII is required for stimulation of lipoprotein lipase activity against apolipoprotein CII-deficient chylomicrons

Human apolipoprotein CII (apo CII) consists of 79 amino acid residues. The amino-terminal two thirds of the molecule binds to lipid through the formation of amphipathic helixes, while the carboxy-terminal third is engaged in activation of lipoprotein lipase (LPL). On the basis of studies in model systems, it was previously concluded that fragments of apo CII spanning residues 51-79 were sufficient for activation, although they do not bind to lipid. In the present study, we used chylomicrons from an apo CII-deficient patient to reinvestigate this possibility, with a physiologically relevant substrate. Human LPL expressed very low activity against these chylomicrons. Addition of apo CII caused an immediate > 100-fold increase in lipase activity. The apo CII fragment 50-79 caused very little stimulation, though with some synthetic lipid substrates, this fragment was fully effective. LPL bound to the chylomicrons even in the absence of apo CII but apparently in a nonproductive manner. In accord with this finding, the main effect of apo CII was on the VMAX for the reaction, with little or no change in the apparent K(M). We conclude that the lipid-binding part of apo CII is needed for activity of LPL against chylomicrons. This idea is in accord with previous studies with lipid monolayers, which showed that the lipid-binding part is necessary for activation of the enzyme at high surface pressures.

Effect of gemfibrozil on lipids, apoproteins, and postheparin lipolytic activities in normolipidemic subjects

The lipid lowering agent Gemfibrozil was tested in 8 normolipidemic subjects during a three-month intake. Plasma triglycerides decreased by 41% and Very Low Density Lipoprotein (VLDL) triglycerides decreased by 54%. The reduction of plasma cholesterol, less marked (by 10%), was due to a decrease of Low Density Lipoprotein by 20% while High Density Lipoprotein (HDL) increased up to 30%. The separation of HDL demonstrated that only HDL3 were increased. The determination of the apoproteins in plasma and lipoprotein fractions showed similar results with a decrease of apo B (by 20%) and an increase of apo A-I and apo A-II, mainly in the HDL3 fraction. Plasma postheparin lipolytic activities (PHLA) were not influenced by the therapy and no correlation was found between these activities and any of the plasma or lipoprotein lipids. The apo C-III/apo C-II ratio in VLDL decreased by 30%; however, no correlation was found between this ratio in plasma as well as VLDL and triglycerides. In addition, the Intra Venous Fat Tolerance Test did not demonstrate any improvement of the clearance of exogenous fat. The lipid lowering efficacy of Gemfibrozil, its collateral effects, and the possible mechanisms of action are discussed.

The effect of etophylline clofibrate on HDL subfractions

The effect of etophylline clofibrate on lipids and apolipoproteins of the high density lipoprotein (HDL) subfractions HDL2 and HDL3 as well as on very low density (VLDL) and low density lipoproteins (LDL) and the post heparin lipolytic activities (PHLA) of lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) has been studied in 14 patients with type II hyperlipoproteinemia (HLP). The study was preceded by a 4-week washout phase, followed by a 6-week placebo period. During the next 12 weeks, the patients received 750 mg etophylline clofibrate per day. Then the drug was again replaced by placebo for another 6 weeks. During the study the patients were on a low fat diet poor in cholesterol with a P/S ratio over 1.0. HDL cholesterol and apoproteins increased significantly during treatment. In the first verum phase this effect was related to the rise in HDL2 components with minor changes in HDL3 concentrations, whereas in the second verum period a distinct increase of the HDL3 components could be detected. This development was accompanied by a significant increase of the LPL activities during the first 6 weeks of treatment, followed by a decrease to initially measured values after 12 weeks. The drug lowered plasma- and LDL-cholesterol levels by 19% and 22%, and plasma and VLDL triglycerides by 22% and 25%, respectively. VLDL-C apoproteins (C-I, C-II, C-III) declined by 31% with a percentage increase of apo C-II compared with apo C-I and apo C-III.