Heparin-induced lipolysis in hypertriglyceridemic subjects results in the formation of atypical HDL particles

New Insights Into the Regulation of HDL Metabolism and Reverse Cholesterol Transport

The metabolism of high-density lipoproteins (HDL), which are inversely related to risk of atherosclerotic cardiovascular disease, involves a complex interplay of factors regulating HDL synthesis, intravascular remodeling, and catabolism. The individual lipid and apolipoprotein components of HDL are mostly assembled after secretion, are frequently exchanged with or transferred to other lipoproteins, are actively remodeled within the plasma compartment, and are often cleared separately from one another. HDL is believed to play a key role in the process of reverse cholesterol transport (RCT), in which it promotes the efflux of excess cholesterol from peripheral tissues and returns it to the liver for biliary excretion. This review will emphasize 3 major evolving themes regarding HDL metabolism and RCT. The first theme is that HDL is a universal plasma acceptor lipoprotein for cholesterol efflux from not only peripheral tissues but also hepatocytes, which are a major source of cholesterol efflux to HDL. Furthermore, although efflux of cholesterol from macrophages represents only a tiny fraction of overall cellular cholesterol efflux, it is the most important with regard to atherosclerosis, suggesting that it be specifically termed macrophage RCT. The second theme is the critical role that intravascular remodeling of HDL by lipid transfer factors, lipases, cell surface receptors, and non-HDL lipoproteins play in determining the ultimate metabolic fate of HDL and plasma HDL-c concentrations. The third theme is the growing appreciation that insulin resistance underlies the majority of cases of low HDL-c in humans and the mechanisms by which insulin resistance influences HDL metabolism. Progress in our understanding of HDL metabolism and macrophage reverse cholesterol transport will increase the likelihood of developing novel therapies to raise plasma HDL concentrations and promote macrophage RCT and in proving that these new therapeutic interventions prevent or cause regression of atherosclerosis in humans.

Effects of Heparin Administration on Trypanosoma brucei gambiense Infection in Rats

We examined whether heparin administration influences in vivo trypanosome proliferation in infected rats. Administration of heparin every 8 hr via cardiac catheter inhibited growth of Trypanosoma brucei gambiense and prolonged survival of treated rats. Heparin administration increased lipoprotein lipase activity, high-density lipoprotein (HDL) concentration in the blood, and haptoglobin messenger RNA content of the liver. The presence of heparin in culture media did not directly affect proliferation of trypanosomes in vitro. However, the addition of plasma from infected rats treated with heparin to culture media decreased the number of trypanosomes. This effect was decreased by incubating the trypanosomes with benzyl alcohol, a known inhibitor of receptor-mediated endocytosis of lipoprotein. These data suggested that heparin administration reduced the number of trypanosomes in infected rats. Trypanosome lytic factor, a HDL and haptoglobin-related protein, protects humans and some animals from infection by Trypanosoma brucei brucei. In rats, increases in HDL and haptoglobin may affect the proliferation of T. b. gambiense.

Expression of human hepatic lipase in the rabbit model preferentially enhances the clearance of triglyceride-enriched versus native high-density lipoprotein apolipoprotein A-I

BACKGROUND

We have shown previously that triglyceride (TG) enrichment of HDL, as occurs in hypertriglyceridemic states, contributes to HDL lowering in humans by enhancing the clearance of HDL apolipoprotein (apo) A-I from the circulation. In the New Zealand White rabbit, an animal naturally deficient in hepatic lipase (HL), we demonstrated that TG enrichment of HDL per se is not sufficient to enhance HDL clearance in the absence of ex vivo lipolysis by HL. Here, we examined in the rabbit the interaction between in vivo HL lipolytic action and HDL TG enrichment on the subsequent metabolic clearance of HDL apoA-I.

METHODS AND RESULTS

The clearance of HDL, TG-enriched with human VLDL (12% mass TG), was compared with a simultaneously injected native rabbit HDL tracer (8% TG) 5 to 7 days after injection of recombinant (r) adenovirus expressing either the human HL or lacZ transgene (n=6 animals each). In rHL-Adv rabbits, HL activity levels were 2- to 7-fold higher (versus rlacZ-Adv controls; P<0.01), and there were significant (P<0.05) reductions in HDL TG (-18%), cholesterol (-21%), cholesteryl ester (-24%), and phospholipid (-14%). Moreover, the clearance of TG-enriched versus native HDL was significantly greater (by 50%; 0.122+/-0.022 versus 0.081+/-0.015 pools/h; P<0.01) in rHL-Adv rabbits but not in controls.

CONCLUSIONS

These studies have shown that TG enrichment of HDL in the presence but not in the absence of in vivo expression of moderate levels of lipolytically active HL results in enhanced HDL clearance, demonstrating the important interaction between TG enrichment and HL action in the pathogenesis of HDL lowering in hypertriglyceridemic states.

Lipolytically modified triglyceride-enriched HDLs are rapidly cleared from the circulation

The precise biochemical mechanisms underlying the reduction of HDL levels in hypertriglyceridemic states are currently not known. In humans, we showed that triglyceride (TG) enrichment of HDL, as occurs in hypertriglyceridemic states, enhances the clearance of HDL-associated apolipoprotein A-I (apoA-I) from the circulation. In the New Zealand White rabbit (an animal model naturally deficient in hepatic lipase [HL]), however, TG enrichment of HDL is not sufficient to alter the clearance of either the protein or lipid moieties of HDL. In the present study, therefore, we determined in the New Zealand White rabbit the combined effects of ex vivo TG enrichment and lipolytic transformation of HDL by HL on the subsequent metabolic clearance of HDL apoA-I. Results of the in vivo kinetic studies (n=18 animals) showed that apoA-I associated with TG-enriched rabbit HDL modified ex vivo by catalytically active HL was cleared 22% more rapidly versus TG-enriched HDL incubated with heat-inactivated HL, and 26% more rapidly than fasting (TG-poor) HDL incubated with active HL (P<0.05 for both). Furthermore, a strong correlation was observed between the HDL TG content and apoA-I fractional catabolic rate (0.59, P<0.05) in the combined active HL groups. These data establish that TG enrichment of HDL with subsequent lipolysis by HL enhances HDL apoA-I clearance, but neither TG enrichment of HDL without HL lipolysis nor HL lipolysis in the absence of previous TG enrichment of HDL is sufficient to enhance HDL clearance. These data further support the important interaction between HDL TG enrichment and HL action in the pathogenesis of HDL lowering in hypertriglyceridemic states.

HDL metabolism in hypertriglyceridemic states: an overview

Reduced plasma high-density lipoprotein (HDL) cholesterol levels have been recognized as a highly significant independent risk factor for atherosclerotic cardiovascular disease. HDL levels are also inversely related to plasma triglyceride levels and there is a dynamic interaction between HDL and triglyceride (TG) rich lipoproteins in vivo. The mechanisms underlying the lowering of HDL in hypertriglyceridemic states have not been fully elucidated, but there is evidence to suggest that triglyceride enrichment of HDL, a common metabolic consequence of hypertriglyceridemia, may play an important role in this process. There is accumulating evidence to suggest that the primary mechanisms leading to reduced plasma HDL cholesterol levels and HDL particle number in hypertriglyceridemic states may be due to any one or a combination of the following possibilities: (1) small HDL particles, which are the product of the intravascular lipolysis of triglyceride-enriched HDL, may be cleared more rapidly from the circulation, (2) triglyceride-enriched HDL may be intrinsically more unstable in the circulation, with apo A-I loosely bound, (3) the lipolytic process itself of triglyceride-enriched HDL may lower HDL particle number by causing apo A-I to be shed from the HDL particles and cleared from the circulation, (4) a dysfunctional lipoprotein lipase or reduced LPL activity may contribute to the lowering of HDL levels by reducing the availability of surface constituents of triglyceride-rich lipoproteins that are necessary for the formation of nascent HDL particles. This review summarizes the evidence that triglyceride-enrichment of HDL is an important factor determining the rate at which HDL is catabolized, a mechanism which could explain, at least in part, the reduced plasma HDL cholesterol levels and particle number frequently observed in hypertriglyceridemic states.

Production of small high-density lipoprotein particles after stimulation of in vivo lipolysis in hypertriglyceridemic individuals: studies before and after triglyceride-lowering therapy

In hypertriglyceridemic states, triglyceride enrichment of high-density lipoprotein (HDL) may play an important role in decreasing the HDL cholesterol and apolipoprotein (apo) A-1 plasma concentration. We have shown previously that HDL particles are transformed into small HDLs when lipolysis is stimulated in vivo or in vitro, and this process is more marked if the HDL is triglyceride-rich. The present study was conducted to determine whether the susceptibility of HDL to transformation can be altered by triglyceride-lowering therapy in humans. Seventeen moderately hypertriglyceridemic individuals (nine with type II diabetes mellitus and eight moderately hypertriglyceridemic nondiabetic subjects) were studied before and after 3 months of triglyceride-lowering therapy with gemfibrozil. Since no significant differences in postprandial and postheparin HDL metabolism were detected between type II diabetic and nondiabetic subjects, results are reported for the two groups combined (N = 17). Fasting HDL was triglyceride-rich with a preponderance of HDL3, and became more enriched with triglycerides postprandially. Heparin administration resulted in a rapid decrease in plasma and HDL triglycerides and an increase in plasma and HDL free fatty acids (FFAs). Postheparin, there was a reduction in HDL size and an increase in the proportion of small (HDL3c) HDL particles (HDL3c constituted 7.1% +/- 1.8% of total HDL preheparin and 26.6% +/- 3.8% postheparin, P < .001). Triglyceride-lowering treatment resulted in a decrease in fasting triglycerides (-54%, P < .001) and HDL triglyceride content (-36%, P = .002), an increase in fasting HDL cholesterol (19%, P = .004), and proportionately fewer (13.2% +/- 2.1%, P < .001) HDL3c particles formed postheparin. Postheparin HDL size correlated inversely with the fasting triglyceride level (r = -.55, P < .001) and HDL triglyceride concentration (r = -.34, P = .02). These results show that the postprandial increase in triglyceride levels in hypertriglyceridemic subjects is associated with increased production of small HDL particles when lipolysis is stimulated, and that lipid-lowering therapy can contribute to favorably reduce this postprandial production of small HDL particles. Further studies are needed to clarify how these abnormalities ultimately lead to a decrease of plasma HDL cholesterol and apo A-1 in hypertriglyceridemic states.