Low-density lipoprotein turnover in inbred strains of rabbits hypo- or hyperresponsive to dietary cholesterol

Modulating effects of cholesterol feeding and simvastatin treatment on platelet-activating factor acetylhydrolase activity and lysophosphatidylcholine concentration

BACKGROUND

Platelet-activating factor acetylhydrolyse (PAF-AH) is an enzyme that degrades PAF and bioactive oxidized lipids. However, it has been reported to be a risk factor for coronary heart disease. The present study examined the effects of cholesterol feeding and simvastatin treatment on plasma PAF-AH activity.

METHODS

Japanese White rabbits (n=22) were fed a diet containing 0.3% cholesterol and 3% corn oil for 1 month, and then divided into two groups that continued to receive this diet with (treated) or without (control) treatment with simvastatin (0.01%) for another 2 months.

RESULTS

Cholesterol feeding increased and simvastatin treatment decreased apolipoprotein (apo) B-containing lipoprotein subfractions as characterized by capillary isotachophoresis, serum levels of total cholesterol, phospholipids, LDL-C, apoE, plasma and LDL-associated PAF-AH (LDL-PAF-AH) activities, and plasma lyso-PC concentration. Cholesterol feeding also increased apoB levels but decreased the LDL-PAF-AH/LDL-C ratio and did not change the plasma PAF-AH/lyso-PC ratio. Simvastatin treatment did not affect apoB levels and only slightly increased the LDL-PAF-AH/LDL-C ratio. Secretion of PAF-AH activity from monocyte-derived macrophages was increased by cholesterol feeding but not affected by simvastatin treatment. These results indicate that PAF-AH activity is increased by cholesterol feeding due to increased secretion of PAF-AH activity from macrophages and that PAF-AH activity is decreased by simvastatin due to increased removal of lipid and enzyme contents of LDL particles.

CONCLUSION

Cholesterol elevation by cholesterol feeding and cholesterol-lowering by simvastatin modulate plasma PAF-AH activity by different mechanisms.

Cavitary tuberculosis produced in rabbits by aerosolized virulent tubercle bacilli

Liquefaction of solid caseous tuberculous lesions and the subsequent cavity formation are probably the most dangerous processes in the pathogenesis of human pulmonary tuberculosis. In liquefied caseum, the tubercle bacilli grow extracellularly for the first time since the onset of the disease and can reach such large numbers that mutants with antimicrobial resistance may develop. From a cavity, the bacilli enter the bronchial tree and spread to other parts of the lung and also to other people. Of the commonly used laboratory animals, the rabbit is the only one in which cavitary tuberculosis can be readily produced. This report is the first to describe and analyze the complete course of cavitary tuberculosis, produced by aerosolized virulent bovine-type tubercle bacilli in commercially available New Zealand white rabbits. After the inhalation of 220 to 880 bacillary units, all of the rabbits were overtly well until they were sacrificed at 33 weeks. After the inhalation of 3,900 to 5,800 bacillary units, half of the rabbits died of progressive tuberculosis between 5 and 9 weeks and the other half lived until they were sacrificed at 18 weeks. Pulmonary cavities developed in both low- and high-dose groups, some beginning as early as 6 weeks. Bacilli from primary cavities sometimes caused nearby secondary cavities, but more frequently, they ascended the bronchial escalator, were swallowed, and caused secondary tubercles in the lymphoid tissue of the appendix and ileocecal junction. Histologically, and by culture, the number of bacilli found in the liquefied caseum varied from many to comparatively few. Strong tuberculin reactions at 4 weeks after infection were associated with fewer primary lesions, while strong tuberculin reactions at 33 weeks were associated with more cavitary lesions. In the tuberculous granulation tissue surrounding caseous and liquefied pulmonary foci and cavities, we found many mature epithelioid macrophages that contained high levels of the proteinase cathepsin D. Therefore, cathepsin D probably plays a major role in the liquefaction of solid caseous material and in the subsequent cavity formation.

Plasma activities of lecithin:cholesterol acyltransferase, lipid transfer proteins and post-heparin lipases in inbred strains of rabbits hypo- or hyper-responsive to dietary cholesterol

Plasma lipoproteins, plasma activities of lecithin:cholesterol acyltransferase (LCAT), phospholipid transfer protein (PLTP), cholesteryl ester transfer protein (CETP) and post-heparin lipases were measured before and after cholesterol challenge in two inbred strains of rabbits with either a high (hyper-responders) or a low (hyporesponders) response of plasma cholesterol to dietary cholesterol. The purpose of this study was to provide clues about the mechanisms underlying the effect of dietary cholesterol on lipoprotein levels and composition, and particularly those underlying the strain difference of this effect. Cholesterol feeding (0.15 g of cholesterol/100 g of diet) caused increased plasma total cholesterol concentrations and an increased ratio of cholesteryl esters:triacylglycerol in all lipoprotein particles in both strains; these effects were significantly greater in hyper- than hypo-responsive rabbits. Feeding on the high-cholesterol diet lowered plasma triacylglycerols in hyper-responders, but caused increased plasma triacylglycerol levels in hyporesponders. This was accompanied by significantly greater increases in the activities of hepatic triacylglycerol lipase and lipoprotein lipase in hyper- than in hypo-responders. Both strains showed a dietary-cholesterol-induced rise in plasma CETP as well as in PLTP activity. The increase in PLTP activity was greater in the hyper-responders, but that of CETP was less. There was no effect of dietary cholesterol on LCAT activity. It is hypothesized that the lipases are involved in the removal of cholesterol-rich lipoproteins.