Locating a low density lipoprotein-targeting domain of human apolipoprotein B-100 by expressing a minigene construct in transgenic mice

Structural peculiarities of the binding of very low density lipoproteins and low density lipoproteins to the LDL receptor in hypertriglyceridemia: role of apolipoprotein E

Very low (VLDL) and low density lipoproteins (LDL) were isolated from plasma of patients with the E3/3 phenotype which were divided into three groups based on their plasma triglyceride content: low (TG<200 mg/dl, TG(l)), intermediate (200<300 mg/dl, TG(i)300 mg/dl, TG(h)). The protein density (PD) on the VLDL and LDL surface was calculated from lipoprotein composition and protein location was studied by tryptophan fluorescence quenching by I(-) anions at 25 degrees C and 40 degrees C. A comparison of the TG(h) with the TG(l) group revealed a significant (<0.05) increase of the PD parameter as much as 21% for VLDL, but not for LDL where this parameter did not change for any group; generally, PD(LDL) values were 3.2-3.8-fold lower than PD(VLDL). In accordance with this difference, the tryptophan accessibility f in VLDL vs. LDL was lower at both temperatures. There were temperature-induced changes of the f parameter in opposite directions for these lipoproteins. The difference in f value gradually decreased for VLDL in the direction TG(l)TG(i)TG(h) while for LDL there was a U-shaped dependence for these groups. The Stern-Volmer quenching constant K(S-V) which is sensitive to both temperature and viscosity, did not change for VLDL, but K(S-V)(LDL) was 2-3-fold higher for the TG(i) group compared to the other two. The efficiencies of VLDL and LDL binding to the LDL receptor (LDLr) in vitro were compared by solid-phase assay free of steric hindrance observed in cell binding. The maximal number of binding sites did not change for either type of particles and between groups. The association constant K(a) and apolipoprotein (apo) E/apoB mole ratio values all increased significantly for VLDL, but not for LDL, in comparison of the TG(i+h) with the TG(l) group. Based on VLDL and LDL concentrations in serum and on the affinity constant values obtained in an in vitro assay, VLDL concentrations corresponding to 50% inhibition of LDL binding (IC(50)) were calculated in an assumption of the competition of both ligands for LDLr in vivo; the mean values of IC(50) decreased 2-fold when plasma TG exceeded 200 mg/dl. The functional dependences of K(a)(VLDL), IC(50) and apoE content in VLDL (both fractional and absolute) and in serum on TG content in the whole concentration range studied were fitted to a saturation model. For all five parameters, the mean half-maximum values TG(1/2) were in the range 52-103 mg/dl. The efficiency of protein-protein interactions is suggested to differ in normolipidemic vs. HTG-VLDL and apoE content and/or protein density on VLDL surface may be the primary determinant(s) of the increased binding of HTG-VLDL to the LDL receptor. ApoCs may compete with apoE for the binding to the VLDL lipid surface as plasma triglyceride content increases. The possible competition of VLDL with LDL for the catabolism site(s) in vivo, when plasma TG increases, could explain the atherogenic action of TG-rich lipoproteins. Moreover, the 'dual action' hypothesis on anti-atherogenic action of apoE-containing high density lipoproteins (HDL) in vivo is suggested: besides the well-known effect of HDL as cholesteryl ester catabolic outway, the formation of a transient complex of apoE-containing discs appearing at the site of VLDL TG hydrolysis by lipoprotein lipase with VLDL particles proposed in our preceding paper promotes the efficient uptake of TG-rich particles; in hypertriglyceridemia due to the diminished HDL content this uptake seems to be impaired which results in the increased accumulation of the remnants of TG-rich particles. This explains the observed increase in cholesterol and triglyceride content in VLDL and LDL, respectively, due to the CETP-mediated exchange of cholesteryl ester and triglyceride molecules between these particles.

Liver LDL receptor mRNA expression is decreased in human ApoB/CETP double transgenic mice and is regulated by diet as well as the cytokine oncostatin M

We have investigated liver LDL receptor mRNA expression in nontransgenic, human cholesteryl ester transfer protein (CETP) transgenic, and human apolipoprotein (Apo) B/CETP double transgenic mice fed a normal chow diet and a high fat, high cholesterol diet (HFHC). Three weeks of HFHC feeding increased total serum cholesterol 1.5-fold in the nontransgenic, 3.1-fold in the CETP transgenic, and 3.4-fold in the ApoB/CETP double transgenic mice. To examine the liver LDL receptor mRNA expression among the different groups of mice fed the normal diet or fed the HFHC diet, we developed a quantitative reverse-transcribed polymerase chain reaction assay in which the LDL receptor mRNA level was normalized with the beta-actin mRNA. The results show that on the normal chow diet, the LDL receptor mRNA expression levels were lower in the ApoB/CETP mice than in the nontransgenic mice and the human CETP transgenic mice. Liver LDL receptor gene expression was lower in all groups of mice fed the HFHC diet, with the lowest level of expression in the ApoB/CETP mice. Similar results were obtained by Northern blot analysis. In addition, we have previously shown that the cytokine oncostatin M (OM) increases LDL receptor gene expression in HepG2 cells. In this study, we used the ApoB/CETP mice as the model system to examine the in vivo activity of OM on liver LDL receptor gene expression. Our data show that OM increased the level of liver LDL receptor mRNA up to 80% to 90% when the animals were fed the HFHC diet. The results from these studies demonstrate that the expression of the liver LDL receptor in the ApoB/CETP mice is suppressed compared with nontransgenic mice and that the expression of the hepatic LDL receptor gene in these mice is subjected to the normal cholesterol feedback regulation. In addition, LDL receptor gene expression in these mice is also inducible by a positive regulator.

RNA editing: exploring one mode with apolipoprotein B mRNA

RNA editing is a newly described genetic phenomenon. It encompasses widely different molecular mechanisms and events. According to the specific RNA modification, RNA editing can be broadly classified into six major types. Type II RNA editing occurs in plants and mammals; it consists predominantly in cytidine to uridine conversions resulting from deamination/transamination or transglycosylation, although in plants other mechanisms have not been excluded. Apolipoprotein B mRNA editing is the only well-documented editing phenomenon in mammals. It is an intranuclear event that occurs posttranscriptionally, coincident with splicing and polyadenylation. Recent observations indicate that the tissue- and sequence-specific process is mediated by an enzyme that has separate domains for editing and sequence recognition. The presence of apolipoprotein B mRNA editing activity in tissues that do not produce the protein suggests that other RNAs may be edited and RNA editing may be a genetic phenomenon of general biological importance to the cell.

Transgenic animal models of cardiovascular disease

Transgenic experimentation has become a crucial part of hypertension and atherosclerosis research, and is growing more important in several other areas of cardiovascular disease. It has recently made a particular contribution to understanding the role of the renin-angiotensin system in controlling hypertension. The study of blood pressure regulation, cardiac hypertrophy, atherogenesis and thrombosis are also benefiting from the transgenic approach.