Influence of apolipoprotein E on soluble and heparin-immobilized hepatic lipase

Effect of Leptin Administration on Circulating Apolipoprotein CIII levels in Patients With Lipodystrophy

CONTEXT

Apolipoprotein CIII (apoCIII), an inhibitor of lipoprotein lipase, plays an important role in triglyceride metabolism. However, the role of apoCIII in hypertriglyceridemia in lipodystrophy and the effects of leptin replacement on apoCIII levels are unknown.

OBJECTIVE

The objective of the study was to test the hypotheses that apoCIII is elevated in hypertriglyceridemic patients with lipodystrophy and that leptin replacement in these patients lowers circulating apoCIII.

DESIGN, SETTING, STUDY PARTICIPANTS, INTERVENTION, AND OUTCOME MEASURES

Using a post hoc cross-sectional case-control design, we compared serum apoCIII levels from patients with lipodystrophy not associated with HIV (n = 60) and age-, gender-, race-, and ethnicity-matched controls (n = 54) participating in ongoing studies at the National Institutes of Health. In a prospective, open-label, ongoing study, we studied the effects of 6–12 months of leptin replacement on apoCIII in lipodystrophy patients as an exploratory outcome.

RESULTS

ApoCIII was higher in lipodystrophy patients (geometric mean [25th and 75th percentiles]) (23.9 mg/dL [14.6, 40.3]) compared with controls (14.9 mg/dL [12.3, 17.7]) (P < .0001). ApoCIII and triglyceride levels were positively correlated in patients with lipodystrophy (R = 0.72, P < .0001) and healthy controls (R = 0.6, P < .0001). Leptin replacement (6–12 mo) did not significantly alter apoCIII (before leptin: 23.4 mg/dL [14.5, 40.1]; after leptin: 21.4 mg/dL [16.7, 28.3]; P = .34).

CONCLUSIONS

Leptin replacement in lipodystrophy did not alter serum apoCIII levels. Elevated apoCIII may play a role in the hypertriglyceridemia of lipodystrophy independent of leptin deficiency and replacement.

Overexpression of apolipoprotein C-III decreases secretion of dietary triglyceride into lymph

Apolipoprotein C‐III (apoC‐III) is not only predominantly synthesized by the liver but also by the small intestine. Because apoC‐III is secreted from the intestine on the chylomicron along with lipid absorption, we questioned whether apoC‐III might play a role in intestinal lipid absorption and/or transport. Using both wild‐type (WT) and apoC‐III transgenic (apoC‐III Tg) mice, we showed that apoC‐III Tg mice have decreased lymphatic lipid transport compared with WT mice in response to an intraduodenal infusion of radiolabeled lipid. This is associated with accumulation of radiolabeled lipids in the luminal compartment of the apoC‐III Tg mice, indicating delayed lipid uptake from the lumen. The total amount of radioactive lipids in the mucosal compartment did not differ between apoC‐III Tg and WT mice, but the lipid distribution analysis indicated a predominance of free fatty acids and monoacylglycerol in the mucosa of apoC‐III Tg mice, implying impaired esterification capacity. Thus, the mechanisms underlying the reduced lymphatic lipid transport in apoC‐III Tg mice involve both a delayed lipid uptake into enterocytes, as well as impaired esterification to form triglyceride in the mucosa. These data document a novel role for apoC‐III in the uptake, re‐esterification, and lymphatic transport of dietary lipids in the intestine.

The relationship between high density lipoprotein subclass profile and apolipoprotein concentrations

The HDL fraction in human plasma is heterogeneous in terms of size, shape, composition, and surface charge. The HDL subclasses contents were quantified by 2-dimensional non-denaturing gel electrophoresis, immunoblotting, and image analysis. This research review systematically analyzed the relationship between the contents of HDL subclasses and the concentrations and ratios of the 5 major plasma apolipoproteins (apo). As the concentration of apoA-I increases, the contents of all HDL subclasses increase significantly. The most significant association was observed between large-sized HDL2b contents and apoA-I. ApoA-II played a dual function in the contents of HDL subclasses, and both small-sized HDL3b and HDL3a and large-sized HDL2b tended to increase with apoA-II concentration. An increase in the concentrations of apoC-II, C-III, and B-100 resulted in higher levels of small-sized HDL particles and lower levels of large-sized HDL particles. Plasma apoB- 100, apoC-II, and apoC-III appear to play a coordinated role in assembly of HDL particles and the determination of their contents. Higher concentrations of apoA-I could inhibit the reduction in content of large-sized HDL2b effected by apoB-100, C-II, and C-III. The preβ1-HDL contents increased significantly and those of HDL2b declined progressively with an increased apoB-100/apoA-I or a decreased apoC-III/apoC-II ratio. In summary, each apo has distinct but interrelated roles in HDL particle generation and metabolism. ApoA-I and apoC-II concentrations are independent determinants of HDL subtypes in circulation and apoA-I levels might be a more powerful factor to influence HDL subclasses distribution. Moreover, apoB- 100/apoA-I ratio could reliably and sensitively reflect the HDL subclass profile.

Very-low-density lipoprotein binding to the apolipoprotein E receptor 2 is enhanced by lipoprotein lipase, and does not require apolipoprotein E

The apolipoprotein (apo)E receptor 2 (apoER2) is a recently cloned member of the low-density lipoprotein (LDL) receptor (LDLR) family, showing a high homology with both the LDLR and the very-low-density lipoprotein (VLDL) receptor (VLDLR). In the present study, the binding characteristics of the apoER2 with respect to apoE and lipoprotein lipase (LPL) were investigated. VLDL was isolated from both apoE-deficient mice and mice expressing the human APOE2 (Arg(158)-->Cys) and APOE3-Leiden isoforms on an Apoe(-/-),Ldlr(-/-) double knock-out background. apoE-rich rabbit beta-VLDL was used as a positive control for binding. Binding experiments performed with Chinese hamster ovary cells expressing the human apoER2 showed that the receptor was able to bind VLDL containing either of the apoE isoforms, as well as the apoE-deficient VLDL. Hence, in contrast with the VLDLR, the apoER2 is not strictly dependent on apoE for VLDL binding. Since LPL has been shown to enhance the binding of lipoproteins to several members of the LDLR family, including the LDLR-related protein, VLDL receptor, gp330 and the LDLR itself, VLDL binding experiments were performed in the presence of LPL. Addition of LPL resulted in a significant increase in apoER2 binding for all VLDL fractions used in this study. In conclusion, lipoprotein binding of VLDL to the apoER2 is enhanced in the presence of LPL, and is not restricted to apoE-containing lipoproteins.

Chylomicronemia due to apolipoprotein CIII overexpression in apolipoprotein E-null mice. Apolipoprotein CIII-induced hypertriglyceridemia is not mediated by effects on apolipoprotein E

The mechanism of apolipoprotein (apo) CIII-induced hypertriglyceridemia remains uncertain. We crossed apoCIII transgenic and apoE gene knockout (apoE0) mice, and observed severe hypertriglyceridemia with plasma triglyceride levels of 4,521+/-6, 394 mg/dl vs. 423+/-106 mg/dl in apoE0 mice, P < 0.00001 for log(triglycerides [TG]). Cholesterols were 1,181+/-487 mg/dl vs. 658+/-151 mg/dl, P < 0.0001. Lipoprotein fractionation showed a marked increase in triglyceride-enriched chylomicrons+VLDL. This increase was limited to the lowest density (chylomicrons and Sf 100-400) subfractions. Intermediate density lipoproteins (IDL)+LDL increased moderately, and HDL decreased. There was no significant increase in triglyceride production in apoCIII transgenic/apoE0 mice. The clearance of VLDL triglycerides, however, was significantly decreased. Lipoprotein lipase in postheparin plasma was elevated, but activation studies suggested LPL inhibition by both apoCIII transgenic and apoCIII transgenic/apoE0 plasma. ApoCIII overexpression also produced a marked decrease in VLDL glycosaminoglycan binding which was independent of apoE. The predominant mechanism of apoCIII-induced hypertriglyceridemia appears to be decreased lipolysis at the cell surface. The altered lipoprotein profile that was produced also allowed us to address the question of the direct atherogenicity of chylomicrons and large VLDL. Quantitative arteriosclerosis studies showed identical results in both apoCIII transgenic/apoE0 and apoE0 mice, supporting the view that very large triglyceride-enriched particles are not directly atherogenic.

Overexpression of apolipoprotein CII causes hypertriglyceridemia in transgenic mice

We have generated transgenic mice expressing the human apolipoprotein CII (apoCII) gene under the transcriptional control of the human cytochrome P-450 IA1 (CYPIA1) promoter. Human apoCII transgenic (HuCIITg) mice exhibited significant basal expression of the transgene (plasma apoCII level = 26.1 +/- 4 mg/dl) and showed further induction of transgene expression after treatment with beta-naphthoflavone. Unexpectedly, HuCIITg mice were hypertriglyceridemic and human apoCII levels correlated strongly to triglyceride levels (R = 0.89, P < 0.0001). Triglyceride levels (mg/dl +/- SEM) were elevated compared to controls in both the fed (804 +/- 113 vs 146 +/- 18, P < 0.001) and fasted (273 +/- 39 vs 61 +/- 4, P < 0.001) states. HuCIITg mice accumulated triglyceride-rich very low density lipoproteins (VLDL) with an increased apoC/apoE ratio. Tracer kinetic studies indicated delayed clearance of VLDL-triglyceride, and studies using Triton inhibition of VLDL clearance showed no increase in VLDL production. Plasma from these mice activated mouse lipoprotein lipase normally and radiolabeled VLDL were normally hydrolyzed. However, HuCIITg VLDL showed markedly decreased binding to heparin-Sepharose, suggesting that apoCII-rich, apoE-poor lipoprotein may be less accessible to cell surface lipases or receptors within their glycosaminoglycan matrices. HuCIITg mice are a promising model of hypertriglyceridemia that suggests a more complex role for apoCII in the metabolism of plasma triglycerides.