In vitro regulation of low-density lipoprotein receptor interaction by fatty acids

Dietary menhaden and corn oils and the red blood cell membrane lipid composition and fluidity in hyper- and normocholesterolemic miniature swine

Fatty acids in the diet are readily incorporated into lipids in various tissues. However, it is not clear whether all tissues have the same level of incorporation. Second, (n-6) unsaturated fatty acids increase the fluidity of membranes, but this has not been shown for (n-3) fatty acids. In this study, we measured the incorporation of (n-6) and (n-3) fatty acids into erythrocyte membrane lipids and studied their effects on the fluidity of erythrocyte membranes. One group of female miniature swine was made hypercholesterolemic by feeding the swine cholesterol and lard for 2 mo; the other group served as controls and was fed a stock diet. Both groups were then fed either corn oil or menhaden oil or a mixture of the two for 23 additional weeks. Blood was collected at 0, 2, 4, 12 and 23 wk after initialization of the experimental diets, and fatty acid composition of phospholipids was assessed. Membrane phospholipids of pigs fed menhaden oil had elevated (n-3) fatty acids (20:5 and 22:6), and lower 18:2 than those fed corn oil. There was no difference in 20:4 content. The fatty acid changes occurred as early as 2 wk after consumption of the corn oil or menhaden oil in pigs previously fed a stock diet, but it took longer in pigs previously fed lard + cholesterol, indicating residual effects of pretreatment. Menhaden oil increased anisotropy (indicating decreased fluidity) more than corn oil for the nonpolar probe diphenylhexatriene (DPH) at earlier time points, but not at 23 wk. Erythrocyte membrane fluidity was significantly related to membrane polyunsaturate content, with (n-6) fatty acids having a greater influence than (n-3) fatty acids. A comparison of the present red blood cell fatty acid compositions with brain synaptosome fatty acid compositions for the same animals showed poor correlations for some of the fatty acids. There was no significant direct relationship between docosahexaenoate (DHA) concentrations in erythrocyte membranes with DHA concentrations in brain synaptosomes from cerebellum, forebrain and caudate nucleus.

High fat, high cholesterol diets alter low density lipoprotein size and binding affinity in monkeys

The purpose of this study was to examine the effects of various dietary fats on low density lipoprotein (LDL) binding in an in vitro system where receptor number is not regulated. Cynomolgus monkeys were fed diets containing 37% of energy from fat, with various degrees of saturation, and 0.4 mg/kcal cholesterol or low-fat (13% of energy), low cholesterol (0.03 mg/kcal) chow. Plasma LDL was isolated after 16 weeks. The fatty acid composition of LDL showed enrichment corresponding to the dietary fats consumed, and the high fat, high cholesterol diets produced marked hypercholesterolemia compared to chow feeding. Of those fed the high fat diets, monkeys fed the fish oil diet had the highest LDL cholesterol concentrations, 13.25 +/- 0.77 mmol/l, while those fed the safflower oil diet had the lowest, 7.51 +/- 3.31. LDL from chow fed monkeys had the lowest binding affinity; the Kd was 26.2 +/- 8.7 microg/ml, nearly twice that of the high fat diets (P = 0.003). No significant differences in binding were found between the different high fat diets, although there was a trend toward lower affinity in the diets enriched in polyunsaturated fat. LDL size was affected by diet with chow fed monkeys having the smallest average LDL, 259.3 +/- 1.7 A compared to the other groups (P = 0.03). Monkeys fed the fish oil diet tended to have smaller LDL, but this was not significantly different from the other high fat diets. Binding affinity was correlated with LDL size, r = 0.54, P < 0.01. LDL composition, as measured by apo B/cholesterol ratio, was altered by feeding a high fat, high cholesterol diet. The ratio was reduced in the LDL samples from monkeys fed the high fat diets compared to those fed chow, but this ratio was not significantly correlated with binding. Thus, it appears that increasing dietary fat and cholesterol intake increases LDL size and binding affinity, such that LDL metabolism may be altered independently from effects on receptor number; the type of dietary fat does not seem to influence this process when fat and cholesterol content is very high.

Fatty acid modification of membrane fluidity in Chinese hamster ovary (TR715-19) cells

Dietary saturated fatty acids, especially lauric (12:0), myristic (14:0) and palmitic (16:0) acids, which are hypercholesterolemic, influence cell membrane fatty acid composition and affect LDL receptor function. When membrane phospholipid fatty acids in Chinese hamster ovary cells, containing the human LDL receptor, were modified (Hannah J. S. et al., 1995 Metabolism 44, 1428-1434), LDL receptor function was affected, but correlations with DPH-determined membrane fluidity were weak. The role of fluidity in various membrane domains with respect to the LDL receptor is examined here. Membrane fluidity was assessed by measuring steady-state fluorescence polarization of diphenylhexatriene (DPH) and its polar propionic acid (DPH-PA) and trimethylammonium (TMA-DPH) derivatives from 38 to 4 degrees C in fatty acid modified Chinese hamster ovary cells. Fatty acid changes modulated mid-bilayer fluidity as determined with DPH, but fluidity in phospholipid headgroup domains, assessed with DPH-PA and TMA-DPH, was independent of fatty acyl composition. The DPH fluidity was related to membrane unsaturation (P < 0.02), oleate contents (P < 0.009) in particular, but inversely related (P < 0.0002) to the longer chain (> or = 20 C atoms) unsaturated fatty acids with from four to six double bonds. The LDL binding was independent of fluidity, but there were weak relations between LDL internalization and DPH-PA anisotropy and between LDL degradation and TMA-DPH anisotropy. It was concluded that LDL binding was not related to mid-bilayer fluidity, but the results with the polar probes suggest a role of fluidity in modulating vertical displacement of the LDL/LDL receptor complex across the plasma membrane.