The high-fat high-fructose hamster as an animal model for niacin's biological activities in humans.

OBJECTIVE: Niacin has been used for more than 50 years to treat dyslipidemia, yet the mechanisms underlying its lipid-modifying effects remain unknown, a situation stemming at least in part from a lack of validated animal models. The objective of this study was to determine if the dyslipidemic hamster could serve as such a model. MATERIALS/METHODS: Dyslipidemia was induced in Golden Syrian hamsters by feeding them a high-fat, high-cholesterol, and high-fructose (HF/HF) diet. The effect of high-dose niacin treatment for 18 days and 28 days on plasma lipid levels and gene expression was measured. RESULTS: Niacin treatment produced significant decreases in plasma total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), triglycerides (TG), and free fatty acids (FFA), but had no measureable effect on high-density lipoprotein cholesterol (HDL-C) in the dyslipidemic hamster. Niacin treatment also produced significant increases in hepatic adenosine ATP-Binding Cassette A1 (ABCA1) mRNA, ABCA1 protein, apolipoprotein A-I (Apo A-I) mRNA, and adipose adiponectin mRNA in these animals. CONCLUSIONS: With the exception of HDL-C, the lipid effects of niacin treatment in the dyslipidemic hamster closely parallel those observed in humans. Moreover, the effects of niacin treatment on gene expression of hepatic proteins related to HDL metabolism are similar to those observed in human cells in culture. The HF/HF-fed hamster could therefore serve as an animal model for niacin's lowering of proatherogenic lipids and mechanisms of action relative to lipid metabolism.

Antioxidant N-acetylcysteine inhibits vasoactive agents-potentiated mitogenic effect of mildly oxidized LDL on vascular smooth muscle cells.

Mildly oxidized LDL (mox-LDL) has been shown to induce monocyte-endothelial interactions and vascular smooth muscle cell (VSMC) proliferation, key events in the formation of the atherosclerotic lesion. Growth factors and vasoactive peptides are also thought to play a major role in atherogenesis. We examined the interaction between mox-LDL and well-known vasoactive agents such as serotonin (5-HT), angiotensin II (Ang-II), endothelin-1 (ET-1), or urotensin II (U-II) in inducing DNA synthesis in VSMCs. Growth-arrested VSMCs were incubated with different concentrations of native LDL, mox-LDL, or highly oxidized LDL (ox-LDL) with 5-HT, Ang-II, ET-1, or U-II in the absence or presence of N-acetylcysteine (NAC), an intracellular free radical scavenger. DNA synthesis in VSMCs was examined by [3H]thymidine incorporation into cellular DNA. Mox-LDL and ox-LDL stimulated [3H]thymidine incorporation with a maximal effect at 5 microg/ml (211%, 154%), which values were significantly greater than that for native LDL (128%). 5-HT, Ang-II, ET-1, or U-II also stimulated [3H]thymidine incorporation in a dose-dependent manner. 5-HT had a maximal stimulatory effect at a concentration of 50 micromol/l (205%), Ang-II at 1.75 micromol/l (202%), ET-1 at 0.1 micromol/l (205%), and U-II at 0.05 micromol/l (161%). When added together, mox-LDL (100 ng/ml)-induced [3H]thymidine incorporation was potentiated by low concentrations of 5-HT (1 micromol/l), Ang-II (0.5 micromol/l), ET-1 (1 nmol/l), or U-II (10 nmol/l) (114% to 330%, 325%, 338%, or 345%, respectively). Synergistic interactions of mox-LDL with 5-HT, Ang-II, ET-1, or U-II were significantly inhibited by NAC (400 micromol/l). Our results suggest that mild oxidation of LDL may enhance its atherogenic potential and exert a synergistic interaction with vasoactive agents in inducing DNA synthesis via the generation of reactive oxygen species in VSMCs.

Lysophosphatidylcholine is a major contributor to the synergistic effect of mildly oxidized low-density lipoprotein with endothelin-1 on vascular smooth muscle cell proliferation.

Endothelin-1 (ET-1) and oxidized low-density lipoprotein (ox-LDL) are associated with atherosclerosis and essential hypertension. We assessed the effect of mildly oxidized LDL (mox-LDL) and ox-LDL and their major oxidative components, i.e., reactive oxygen species (ROS), lysophosphatidylcholine (LPC), and 4-hydroxy-2-nonenal (HNE) and their interaction with ET-1 on vascular smooth muscle cell (VSMC) proliferation. Growth-arrested VSMCs isolated from the rabbit aorta were incubated with different concentrations of LDL, mox-LDL, ox-LDL, hydrogen peroxide (H(2)O(2)) (a donor of ROS), LPC, or HNE with or without ET-1. DNA synthesis in VSMCs was measured by [(3)H] thymidine incorporation. Mox-LDL, ox-LDL, H(2)O(2), LPC, HNE, or ET-1 stimulated DNA synthesis in a dose-dependent manner. Maximal effect was observed at 5 microg/ml for mox-LDL (162%) or ox-LDL (154%), 15 microM LPC (156%), 5 microM H2O2 (177%), 1 microM HNE (144%), and 0.1 microM ET-1 (195%). By contrast, LDL was without any significant effect. When added together, there was no synergistic effect of LDL, H2O2, or HNE with ET-1 on DNA synthesis. However, the effect of mox-LDL (0.1 microg/ml), ox-LDL (0.5 microg/ml), or LPC (10 microM) was potentiated by ET-1 (114%-338%, 133%-425%, 118%-333%, respectively). The mitogenic effect of mox-LDL, ox-LDL, or LPC and their interaction with ET-1 were inhibited by defatted albumin (10 microg/ml), antioxidant N-acetylcysteine (400 microM), the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor diphenylene iodonium (1 microM). The ET(A/B) receptor antagonist TAK044 (1 microM) or the MAPK kinase inhibitor PD098059 (10 microM) inhibited the mitogenic effect of ET-1 and its interaction with mox-LDL, ox-LDL, or LPC. The synergistic interaction of mox-LDL, ox-LDL, or LPC with ET-1 was completely reversed by the combined use of N-acetylcysteine and TAK044. Our results suggest that mox-LDL, ox-LDL, and their major phospholipid component LPC act synergistically with ET-1 in inducing VSMC proliferation by way of the activation of redox-sensitive and MAPK pathways.

Induction of endothelial cell proliferation by angiogenic factors released by activated monocytes.

INTRODUCTION: Cell-cell interaction is an essential component of atherosclerotic plaque development. Activated monocytes appear to play a central role in the development of atherosclerosis, not only through foam cell formation but also via the production of various growth factors that induce proliferation of different cell types that are involved in the plaque development. Using serum free co-culture method, we determined the effect of monocytes on endothelial cell proliferation. METHODS: Endothelial cell proliferation is determined by the amount of [3H]thymidine incorporated in to the DNA. Basic fibroblast growth factor (b-FGF), vascular endothelial growth factor (VEGF) and interleukin-8 (IL-8) levels in the conditioned medium were determined by ELISA. RESULTS: Conditioned medium from unactivated monocytes partially inhibited endothelial cell proliferation, whereas conditioned medium from activated monocytes promoted endothelial cell proliferation. The mitogenic effect of conditioned medium derived from activated monocytes is due to the presence of b-FGF, VEGF and IL-8. Neutralizing antibodies against b-FGF, VEGF and IL-8 partially reversed the mitogenic effect of conditioned medium derived from activated monocytes. When b-FGF, VEGF and IL-8 were immunoprecipitated from conditioned medium derived from activated monocytes, it is less mitogenic to endothelial cells. CONCLUSION: Activated monocytes may play an important role in the development of atherosclerotic plaque by producing endothelial cell growth factors.