Alterations in the main steps of reverse cholesterol transport in male patients with primary hypertriglyceridemia and low HDL-cholesterol levels

doi:10.1016/S0021-9150(99)00452-9

Atherosclerosis

Volume 152, Issue 1, 1 September 2000, Pages 181-192

https://doi.org/10.1016/S0021-9150(99)00452-9 Get rights and content

Abstract

Hypertriglyceridemia is a complex pathological entity strongly connected to low HDL-C levels but controversially related to the risk of coronary artery disease. In this study, we evaluated the main steps of the antiatherogenic pathway called reverse cholesterol transport in a group of patients with primary hypertriglyceridemia and low HDL-C levels in comparison to normotriglyceridemic subjects with or without hypoalphalipoproteinemia. In patients with primary hypertriglyceridemia, low HDL-C levels were accompanied by decreased apo A-I and apo A-II concentrations. These reductions were manifested by a selective reduction in LpA-I:A-II particles. In addition, apo C-III Lp non B was found to be elevated and HDL lipid percentage composition showed a triglyceride enrichment and cholesterol depletion. The capacity of serum samples from hypertriglyceridemic patients to promote cellular cholesterol efflux was reduced, as evidenced by using two different cellular models, Fu5AH and J774 cells. This impaired cholesterol efflux promotion was also corroborated by incubations of isolated HDL fractions with Fu5AH cells. Lecithin:cholesterol acyltransferase (LCAT) activity, the driving force of reverse cholesterol transport, showed a tendency towards lower values in hypertriglyceridemic patients, but this difference was not statistically significant. Additionally, cholesteryl ester transfer protein (CETP) activity was increased in this group of patients. Therefore, hypertriglyceridemia was found to induce quantitative and qualitative alterations in HDL and its subclasses and, consequently, in some steps of reverse cholesterol transport. The abnormalities found in this antiatherogenic pathway and its promoters could constitute a possible connection between hypertriglyceridemia and atherosclerosis.

Introduction

Hypertriglyceridemia, defined as an increase in plasma triglyceride levels over 200 mg/dl [1], is a complex entity which can be of primary origin or secondary to any other factor or pathological condition capable of producing a lipid disorder.

Plasma triglyceride levels have been related to the concentration of cholesterol transported in high density lipoproteins (HDL-C) and to the risk of coronary artery disease (CAD) [2], [3]. HDL-C levels and CAD risk show a strong negative association; triglyceride concentration and CAD risk exhibit a weak positive relation; and HDL-C and triglyceride levels are connected in a solid inverse way. Thus, while HDL-C stands as a manifest antiatherogenic factor [4], the direct intervention of triglycerides in the genesis of atherosclerosis still remains controversial. However, results from a meta-analysis based on 17 different studies suggest that triglycerides are a risk factor for CAD, independent of HDL-C [5]. More recently, a consensus about the treatment of hypertriglyceridemia also highlighted the strong evidence which associates hypertriglyceridemia and increased CAD risk [6].

In hypertriglyceridemia, diverse lipoprotein particles seem to be affected. The increase in plasma triglyceride levels reflects an accumulation of two overlapping lipoprotein families, which are comprised within chylomicron and VLDL flotation densities: those containing apolipoprotein (apo) B and apo C-III (LpB:C-III) and those with apo B and apo E (LpB:E). Furthermore, not only the concentration, but also the lipid and apolipoprotein composition of these triglyceride rich lipoproteins were proven to be abnormal [7]. On the other hand, hypertriglyceridemia may also influence any of the two HDL subclasses: those which contain apo A-I without apo A-II (LpA-I) and those with apo A-I and apo A-II (LpA-I:A-II), two different metabolic entities [8]. HDL subclasses may also be classified according to their apo C-III or apo E content, and are identified as apo C-III Lp non B or apo E Lp non B.

Concerning the physiological function that HDL has in cholesterol transport within the organism, Glomset [9] was the first to recognize its participation in the antiatherogenic process called reverse cholesterol transport. This metabolic pathway is responsible for the movement of excess cholesterol from peripheral tissues to the liver for lipoprotein recycling or excretion and could be defined as a progression of closely interconnected events [10]. Among them, four steps are pointed out as the most relevant ones: (1) free cholesterol efflux from extrahepatic cells and its uptake by initial acceptors [11]; (2) free cholesterol esterification by lecithin:cholesterol acyltransferase (LCAT); (3) transfer of newly synthesized cholesteryl esters from HDL to apo B-containing lipoproteins and interchange with triglycerides, carried out by the cholesteryl ester transfer protein (CETP); and (4) hepatic uptake of cholesteryl esters so formed [12].

In hypertriglyceridemic patients, different authors have described quantitative and qualitative variations in lipids and apolipoproteins transported in HDL and its subpopulations [13], [14], [15]. Moreover, in a study carried out in type 2 diabetic patients with moderate hypertriglyceridemia [16], we found an abnormal reverse cholesterol transport both in fasting and postprandial states. Nevertheless, we were not able to find out if the described disorders were due to hypertriglyceridemia itself or to the alterations associated with the diabetic condition. Evidence then is lacking about the different steps of reverse cholesterol transport in primary hypertriglyceridemia in which no additional factors can affect the lipoprotein spectrum. While it has been suggested that LCAT and CETP activities could be determinant factors for HDL levels in hypertriglyceridemic patients [17], cholesterol efflux promotion has not been fully examined before. If hypertriglyceridemia demonstrably affected the whole reverse cholesterol transport, the protective role of this pathway would be deteriorated, thus contributing to the understanding of the controversial relationship between hypertriglyceridemia and atherogenicity.

In view of these considerations, the aim of the present study was to explore the first three steps of reverse cholesterol transport and especially the capacity to promote cholesterol efflux from two different cellular models in primary hypertriglyceridemic patients. We also characterized the lipoprotein, apolipoprotein and lipoprotein particle environment concerned in this antiatherogenic pathway.

Section snippets

Subjects

We studied 36 male subjects aged between 21 and 65 years old. Subjects were recruited consecutively during a period of about 6 months from Hospital de Clı́nicas José de San Martı́n, University of Buenos Aires. Subjects were included in the present study when they satisfied the following criteria previously described [7]: (1) lack of abnormalities in carbohydrate metabolism evidenced by plasma levels of fasting glucose, HbA_1c, insulin and an oral glucose tolerance test; (2) normal thyroid

Results

In this study, we evaluated a group of patients with primary hypertriglyceridemia and low HDL-C levels (group 1, n=12). Results were analyzed in comparison both to a group of subjects who exhibited normal plasma triglyceride and low HDL-C levels (group 2, n=12) and to normotriglyceridemic subjects with normal HDL-C levels (group 3, controls, n=12).

All the subjects were of similar age (47±11, 40±14, 41±12 years, mean±S.D.; groups 1, 2 and 3, respectively). The body mass index was moderately

Discussion

In this study, we evaluated diverse parameters involved in reverse cholesterol transport and the first three steps that make up this antiatherogenic pathway in a group of patients with primary hypertriglyceridemia and low HDL-C levels (group 1), in comparison to normotriglyceridemic subjects with or without hypoalphalipoproteinemia (groups 2 and 3, respectively). Hypertriglyceridemia was found to induce quantitative and qualitative alterations in HDL and its subclasses and, consequently, in all

Acknowledgements

Carla D. Bonavita is a Research Fellow from University of Buenos Aires. This work was supported by a grant from the same university (FA 085) and it was part of the INSERM-CONICET International Cooperation Program.

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On the contrary, increased levels of triglycerides in diabetes may produce a compensatory effect of maintaining ABCA-1 mediated efflux, possibly through the actions of CETP to liberate prebeta-1 HDL. In one past study, hypertriglyceridemia did not alter cholesterol efflux out of J774 macrophages when these macrophages were not induced by c-AMP to express ABCA-1 [17]. Thus, the effect of hypertriglyceridemia is ABCA-1 mediated, likely involving an interaction with lipid-poor or prebeta-1 HDL.
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