Lipid lowering activity of drugs affecting cholesterol absorption

AIM

Dietary cholesterol absorption, endogenous cholesterol synthesis and biliary cholesterol excretion regulate whole body cholesterol balance as a result of biotransformation into bile acids or direct cholesterol excretion. Recent studies have significantly advanced our understanding of intestinal sterol absorption at molecular level. This review concentrates on two major issues: the mechanisms of sterol absorption, and the currently available or experimental drugs that affect this pathway.

DATA SYNTHESIS

Nuclear hormone receptors, such as the liver X, farnesoid X and retinoid X receptors, regulate the absorption of dietary sterols by modulating the transcription of several genes involved in cholesterol metabolism, The ABC proteins transport dietary cholesterol from enterocytes back to the intestinal lumen, thus limiting the amount of absorbed cholesterol. By means of the same mechanism, ABC transporters also provide an efficient barrier against the absorption of plant sterols. Phytosterols, bile acid sequestrants, ezetimibe and ACAT inhibitors are possible means of affecting these pathways.

CONCLUSION

In addition to providing an insight into the molecular mechanisms of sterol absorption, these recent findings may lead to new therapeutic options for the treatment of hypercholesterolemia. This is particularly true in the case of patients at high risk of coronary artery disease requiring aggressive lipid-lowering therapy combining a statin with drugs affecting cholesterol absorption in order to ensure the optimal management of dyslipidemia.

High-density lipoprotein subfraction 3 decreases ADAMTS-1 expression induced by lipopolysaccharide and tumor necrosis factor-α in human endothelial cells

Endothelial expression of matrix metalloproteinases has been implicated in angiogenesis and endothelial cell proliferation. Recently, it has been shown that high-density lipoproteins (HDLs) promote angiogenesis. In the present study, we investigated the effects of native HDLs on the expression of several proteases and their inhibitors in human umbilical vein endothelial cells. We show that ADAMTS-1 (a disintegrin and metalloproteinase with thrombospondin motif) was potently induced by incubation with lipopolysaccharide or tumor necrosis factor-alpha and that the expression was significantly reduced in the presence of HDL subfraction 3. Since ADAMTS-1 has recently been shown to inhibit endothelial cell proliferation, the result of the present work may represent a new mechanism by which HDL could have a positive effect on endothelial cell and vascular wall function.

Gene expression and intracellular pathways involved in endothelial dysfunction induced by VLDL and oxidised VLDL

OBJECTIVES

The molecular mechanisms underlying the relationship between elevated plasma concentrations of triglyceride-rich lipoproteins and coronary artery disease remain uncertain. In the present work, we investigated the gene expression pattern and intracellular pathways in human endothelial cells incubated with very low density lipoproteins (VLDL). Moreover, as VLDL can enter the arterial wall and undergo oxidative modification, we compared the VLDL-induced expression pattern with the one of oxidised VLDL (Ox-VLDL).

METHODS

Total RNA from endothelial cells incubated with 75 microg/ml VLDL or Ox-VLDL and total RNA from endothelial cells under basal conditions were hybridised to identical microarrays containing 8411 genes. Seven clusters of expression profiles were identified. This pattern was validated by quantitative real-time PCR of selected genes. The intracellular pathway involved in VLDL or Ox-VLDL mediated endothelial responses were also investigated.

RESULTS AND CONCLUSION

VLDL predominantly activated the ERK1/2 pathway while P38 MAPK was the main target of Ox-VLDL. CREB and NF-kappa B were activated by both VLDL and Ox-VLDL. Real-time PCR demonstrated that VLDL induced matrix metalloproteinase-2 (5.47+/-1.74 fold), CD38 (2.38+/-0.23) and transforming growth factor-alpha (2.51+/-0.30) expression. Ox-VLDL was found to induce interleukin-15 (2.10+/-0.48) and macrophage migration inhibitory factor (3.19+/-0.07) expression. In addition, several genes implicated in endothelial cell activation and damage/proliferation were identified by the array analysis. Ox-VLDL was found to promote the generation of reactive oxygen species and exert a cytotoxic effect, while VLDL lacks these effects. These findings confirm the involvement of VLDL and Ox-VLDL in endothelial dysfunction and suggest new genes and molecular mechanisms involved in these actions.

Effects of HDL3 on the expression of matrix-degrading proteases in human endothelial cells

Modified lipoproteins have been suggested to modulate the expression of matrix-degrading proteases in the vascular wall. Since oxidized high density lipoprotein (HDL) has been found in atheromatous plaques and receptors for modified HDL are present on endothelial cells, we investigated the role of native and oxidized HDL3 on the expression of 35 proteases and their inhibitors in human endothelial cells using microarray analysis. Matrix metalloproteinase (MMP)-1, -2, -10, -13 and -14, tissue inhibitor of MMP (TIMP)-1, -2 and -3, cathepsin B and D, and cystatin C were expressed under basal conditions, of which MMP-10 and cystatin C expression have not been described before in endothelial cells. Native HDL3 increased MMP-1 and MMP-14 expression and decreased MMP-13 expression, whereas oxidized HDL3 increased PAI-1 and MMP-1 expression. The expression pattern was confirmed by quantitative real-time PCR. In summary, a large repertoire of matrix-degrading proteases is expressed in endothelial cells, an expression that can be modulated by native and oxidized HDL3.

Statins and oxidative stress during atherogenesis

Oxidised low-density lipoprotein (LDL) is believed to be the most atherogenic form of LDL. However, while a number of experimental data support this concept, the protective role of antioxidants that may prevent LDL oxidation in atherosclerosis is only partially confirmed by studies in man. Observational and epidemiological data as well as randomised trials failed to provide clear-cut indications, because of mixed results on the protective role of antioxidants against cardiovascular diseases. In spite of the lack of a general consensus, recent data reinforce the concept that a regular intake of antioxidants present in food blocks the progression of atherosclerosis and that the reduced ability of LDL to oxidise may represent a good marker to follow the action of antioxidants. Among their properties statins also possess antioxidant activities and the aim of this paper is to review the scientific evidence for such an effect and its possible clinical relevance.

Apoptosis and proliferation of endothelial cells in early atherosclerotic lesions: possible role of oxidised LDL

BACKGROUND AND AIM

Evidence for apoptosis has been found in advanced atherosclerotic lesions, but the factors triggering it are poorly understood. Oxidised low-density lipoproteins (LDLs) are cytotoxic to a variety of cells and induce the apoptosis of smooth muscle cells (SMC), fibroblast, macrophages and endothelial cells in vitro. The aim of this study was to investigate apoptotic cell death in the early phases of aortic atherosclerosis in rabbits, and whether oxidised LDLs colocalize ex vivo with apoptotic cells in atherosclerotic lesions in cholesterol-fed rabbits.

METHODS AND RESULTS

Male albino New Zealand rabbits were fed a standard diet or a diet containing 1.2% cholesterol for 60 days. The aortic arch of each animal was sectioned and stained with antibodies against SMC, endothelial cells, macrophages and oxidised LDLs or for proteins involved in apoptotic pathways such as Fas, Bax, Bcl2, and caspase 3. The nuclei in adjacent sections were stained with Hoechst 33258, TUNEL and for the proliferating cell nuclear antigen (PCNA). Early atherosclerotic lesions were characterised by intimal thickening and the presence of SMC and macrophages. The percentage of apoptotic cells, calculated as the ratio of TUNEL-positive nuclei to total nuclei was 32.6 +/- 3.73% in the lesions and 55.9 +/- 2.36% in the endothelium. As it has been reported that nuclei undergoing active gene transcription can be TUNEL positive, we evaluated the percentage of PCNA-positive cells, which proved to be 45.2 +/- 4.68% along the endothelium and 22.3 +/- 2.7% in the intima. The true percentage of apoptotic cells was therefore about 10% in both cases. Fas, Bax and caspase3 signals were mainly located in the endothelium and SMC proximal to the lumen, whereas Bcl2 colocalized with macrophages and SMC deeper in the lesions. Abundant oxidised LDL epitopes were detected in areas of lipid accumulation and along the endothelium, mainly in the areas in which TUNEL and PCNA-positive cells were localised.

CONCLUSIONS

Our findings may be taken as ex vivo indications of an apoptotic and proliferating role of oxidised LDLs as previously shown in vitro, and may at least partially account for the endothelial dysfunction that can be rapidly induced by hypercholesterolemia.

Overexpression of inducible heat shock protein 70 in Cos-1 cells fails to protect from cytotoxicity of oxidized ldls

Oxidized low density lipoproteins (OxLDLs) are believed to play a central role in atherogenesis and to possess a wide variety of biological properties; among them, OxLDLs are cytotoxic to cultured vascular cells in that they induce necrosis and apoptosis. Moreover, OxLDLs are known to induce the expression of heat shock protein 70 (Hsp70), a protein that protects cells from several cytotoxic stimuli. To determine whether Hsp70 can protect cells against OxLDL-induced cytotoxicity, COS-1 cells were transfected with a construct containing human Hsp70. A number of cell lines permanently expressing Hsp70 were obtained, 1 of which (cos-Hsp70/10, with high Hsp70 expression) was selected for further studies. Hsp70 overexpression protected cells from toxic stimuli, such as H(2)O(2), UV irradiation, and heat shock, suggesting that the overexpressed protein was functional. When incubated with OxLDLs, however, the clone overexpressing Hsp70 showed a significant decrease in viability, as determined by the [(3)H]adenine release assay (319.8+/-3.16% of control for transfected cells versus 217.6+/-6.08% for control cells exposed to 100 microgram protein/mL of OxLDL), 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay (12.5+/-0.9% versus 28.9+/-1.99% of control, respectively), and LDH release (48.4+/-0.04% versus 15.2+/-0.06% of control cells). The increased expression of BAX and the decreased expression of Bcl-2 (a proapoptotic and an antiapoptotic protein, respectively) in cos-Hsp70/10 cells and in control cells on incubation with OxLDLs suggested that overexpression of Hsp70 did not confer protection against apoptosis induced by OxLDLs. The analysis of nucleosome content and the nuclear staining with Hoechst 33258 confirmed this finding. These data suggest that overexpression of Hsp70 not only fails to protect COS-1 cells against OxLDL-induced apoptosis but rather confers a higher sensitivity to the cytotoxic action of these lipoproteins. Thus, the Hsp70 response, although induced by OxLDLs, cannot protect cells from lipoprotein toxicity.

Oxidized lipoproteins and endothelium

Endothelium is an early target of pro-atherosclerotic events, which may result in functional and morphological perturbations. Oxidized low density lipoproteins, an atherogenic factor with strong cytotoxicity, may potentially contribute to altered endothelial function through the activation of a stress response, which would rescue cells to full vitality, or, conversely, by leading to cell death. Evidence is presented here for the ability of chemically oxidized low density lipoproteins to induce the synthesis of the inducible form of heat shock protein 70 in cultured human endothelial cells, and for the association of epitopes of these modified lipoproteins with apoptotic endothelial cells in aortic sections from hypercholesterolemic rabbits.