Tahar Hajri

Saturated fatty acids and LDL receptor modulation in humans and monkeys

It has been known for 40 years that dietary saturated fat (SAT FAT) increases plasma cholesterol, including LDL-C and HDL-C. In humans, where LDL-C is typically > 90 mg/dl this SAT FAT effect largely reflects changes in LDL-C pool size. The original human studies suggested that LDL-C expansion during SAT FAT consumption reflected reduced LDL clearance (LDL receptor activity) in hyperlipemics and increased LDL production rates in normolipemics (LDL-C < 100 mg/dl) . This dual explanation is supported by data from several animal models where specific saturated fatty acids (SFAs) have been the focus. However, the situation is complicated by the fact that polyunsaturated fatty acids (PUFAs) oppose SFAs, i.e. PUFAs decrease LDL-C and increase LDL receptor (LDLr) activity, so the effect of SAT FAT intake may represent the combined influence of increased SFAs and decreased PUFAs. In fact, careful scrutiny of primate data suggests a negligible effect of saturated fat on LDL clearance (and receptor activity) in the absence of dietary cholesterol when PUFA intake is adequate (5-10%en) and the lipoprotein profile is relatively normal (LDL-C < 90 mg/dl), i.e. normolipemic situations at the time of dietary intervention. In such cases increases in LDL-C due to SFAs (particularly 12:0+14:0) appear to reflect LDL overproduction associated with a shift in cholesterol from tissues to the plasma cholesteryl ester (CE) pool (both LDL-C and HDL-C) without altering whole-body cholesterol balance. The reason for this shift, which is accompanied by an increase in the plasma oleic/linoleic CE ratio, is unknown but may reflect a decreased rate of CE hydrolysis by the liver. When individuals or animals are rendered hyperlipemic by other factors (e.g. chronic caloric and dietary cholesterol excesses in humans or by cholesterol feeding in animals) specific SFAs (particularly 16:0) can contribute to decreased LDLr activity initiated by a primary factor, such as dietary cholesterol. However, LDLr down-regulation by dietary cholesterol greatly exceeds any contribution from SFAs.

A unique amyloidogenic apolipoprotein serum amyloid A (apoSAA) isoform expressed by the amyloid resistant CE/J mouse strain exhibits higher affinity for macrophages than apoSAA1 and apoSAA2 expressed by amyloid susceptible CBA/J mice

CBA/J and other inbred strains of mice that express the amyloidogenic apolipoprotein serum amyloid A (apoSAA) apoSAA2, together with apoSAA1, are susceptible to amyloid A (AA) amyloidosis, whereas CE/J mice that express a single unique isoform, apoSAACEJ, are resistant. Studies indicate that CBA/JxCE/J hybrid mice that express apoSAA2 in the presence of apoSAACEJ are protected from amyloidogenesis. To define a mechanism by which expression of apoSAACEJ may protect from AA formation in the presence of apoSAA2, binding of recombinant apoSAA (r-apoSAA) isoforms, validated by N-terminal sequencing, to a murine macrophage cell line was investigated. Maximal specific binding occurred after incubation of radiolabeled apoSAA with IC-21 macrophages (1x105 cells/ml) for 30 min at 4 degreesC. The binding of 125I-r-apoSAA1, 125I-r-apoSAA2 and 125I-r-apoSAACEJ was specific and saturable, with an affinity (Kd) of about 2.8, 3.2 and 1.3 nM, respectively, and approximately 2-4x106 sites per cell. Competitive binding experiments indicate apoSAACEJ binds with higher affinity to macrophages than does either apoSAA1 or apoSAA2. We suggest that greater cellular affinity of apoSAACEJ compared to apoSAA2 may contribute to protection from AA amyloid in certain CBA/JxCE/J hybrid mice by interfering with interaction of apoSAA2 by macrophages and hence either membrane associated or intracellular degradation.

Catabolism of HDL1 cholesteryl ester in the rat. Effect of ethinyl estradiol treatment

The present study was performed in control and ethinyl estradiol-treated rats in order to determine the mechanisms involved in the catabolism of HDL1 cholesteryl ester. Ligand blottings on liver membranes showed that purified HDL1, containing about 70% apolipoprotein E and 10% apolipoprotein AI, bind to the LDL receptor (130 kDa) and not to HB2 (100 kDa) or SR-BI (82 kDa), candidate HDL receptors. Immunoblots showed that the treatment increased the hepatic level of the LDL receptor five- to ten-fold, strongly decreased that of SRBI and did not change that of HB2. An in vivo kinetic study showed that the turnover of HDL1 cholesteryl ester is more rapid in treated than control rats. The liver participation (60%) in this clearance was not modified by the treatment. Therefore, it can be concluded that the catabolism of HDL1 cholesteryl ester, in control as in treated rats, is essentially ensured by the uptake of entire particles in the hepatocytes via LDL receptors.

Crilvastatin, a new 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor, inhibits cholesterol absorption in genetically hypercholesterolemic rats

Crilvastatin is a new drug from the pyrrolidone family, which acts as a non-competitive inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase. The long-term effects of oral crilvastatin treatment (200 mg per day per kg body weight for 4 and 10 weeks) were investigated on in vivo cholesterogenesis in male adult normocholesterolemic (SW) and genetically hypercholesterolemic (RICO) rats. In both strains of rats, the treatment had no effect on the plasma cholesterol level, but efficiently inhibited cholesterol synthesis in liver and intestine, as shown by the decreased incorporation of exogenous [14C]acetate into hepatic (3.5-fold in SW, 1.7-fold in RICO rats) and intestinal (2.5-fold in SW, 3.3-fold in RICO rats) sterols. In RICO rats in which the dietary cholesterol absorption coefficient was two-fold lower in treated (38%) than in untreated (78%) rats, this drug reduced intestinal cholesterol absorption. As a result, the total plasma cholesterol input (absorption + synthesis), measured by isotope analysis in RICO rats, was markedly lower in treated (11.3 mg per day) than in untreated animals (28.8 mg per day).

Linoleic acid-rich diet increases hepatic taurine and cholesterol 7α-hydroxylase activity in conjunction with altered bile acid composition and conjugation in gerbils

Abstract Although dietary fatty acid saturation is an effective modulator of plasma cholesterol, its effects on bile acid metabolism are not well elucidated. In fact, it is possible that primary changes in bile acid metabolism account for the response in plasma lipids. Thus, the aim of this study was to compare the effect of a diet rich in specific saturated fatty acids [lauric and myristic acid (12:0 + 14:0)] with a diet containing a specific unsaturate, linoleic acid (18:2n6) on factors affecting bile acid metabolism in gerbils. Gerbils were fed a high-fat diet (40% of energy) based on either coconut oil (12:0 + 14:0-rich) or safflower oil (18:2n6-rich). After 4 weeks, plasma sterols, the gallbladder bile acid profile, and fecal bile acid excretion were analyzed along with hepatic concentrations of taurine and cholesterol. Hepatic activities of acyl-CoA cholesterol acyltransferase (ACAT) and cholesterol 7α-hydroxylase were also measured. Exchange of 29% energy as 18:2n6 for 12:0 + 14:0 increased the proportion of saturated and monounsaturated fatty acids at the expense of polyunsaturated fatty acids in plasma, hepatic microsomal membranes, and bile phospholipids. Concentrations of plasma cholesterol and 27-hydroxycholesterol were greatly increased by the 12:0 + 14:0-rich diet. This effect was coupled with significant decreases in hepatic taurine concentration and the activities of ACAT and cholesterol 7α-hydroxylase, as well as fecal bile acid excretion. Analysis of gallbladder bile acids revealed that the 12:0 + 14:0-rich diet decreased the cholate:chenodeoxycholate and primary:secondary bile acid ratios as well as the ratio of taurine:glycine conjugates. In essence, the exchange of dietary lauric and myristic acids for linoleic acid was associated with decreased production and excretion of bile acids concurrent with reduced hepatic taurine and taurine-conjugated bile acids.

Dietary modulation of apolipoprotein serum amyloid A (apoSAA) metabolism and prevention of amyloidosis in aging C57BL/6J and SJL/J mice

Abstract The specific effect of acute inflammation on apoSAA and lipoprotein metabolism was determined in two strains of mice that are prone to develop spontaneous age-associated systemic amyloidosis. C57BL 6J and SJL J mice were maintained on purified diets, relatively rich in fat and differing only with respect to protein constituents, i.e., casein (20%) versus soy protein (20%). After 18 months, aging mice of either strain and on either diet were injected subcutaneously with AgNO3 thus inducing a powerful acute phase response as evidenced by increases in plasma LDL and apoSAA-rich HDL. None of the mice in each of the dietary groups developed amyloidosis. Cholesterol levels were elevated during the acute phase response in both mouse strains, although the hypercholesterolemia was less pronounced in C57BL 6J mice and in SJL J mice fed soy protein compared with casein. Control plasma triglyceride levels were lower in C57BL 6J mice in both dietary groups compared to the SJL J strain. By contrast, C57BL 6J mice fed either protein increased the triglyceride concentration, whereas in SJL J mice, triglyceride levels were not altered by casein but were decreased significantly by soy protein diets. Oir findings show for the first time that dietary protein intake modulates the acute phase response in rodents. Furthermore, the data support the concept that alterations in lipid metabolism during the acute phase response may represent a protective mechanism whereby HDL-cholesterol and phospholipids are directed to sites of inflammation for connective tissue repair.