The influence of dietary fats on lipid synthesis by human mononuclear cells

Abstract

Various mechanisms have been proposed to explain the divergent effects of dietary saturated, monounsaturated and polyunsaturated fats on plasma lipoprotein concentrations in man. These include (1) changes in cholesterol absorption, (2) synthesis de novo, (3) an increased faecal excretion of cholesterol, (4) enhanced catabolism of low-density lipoproteins with dietary polyunsaturated fats or (5) a redistribution of cholesterol between plasma and peripheral tissue pools. The latter mechanism is also compatible with the higher fractional catabolic rate of low-density lipoproteins seen with polyunsaturated-fat-enriched diets (Shepherd et al., 1980), and would be predicted to result in increased concentrations of tissue cholesterol and a concomitant inhibition of cholesterol biosynthesis de novo. Human mononuclear cells possess lowdensity lipoprotein receptors (Reichl et al., 1978) and are capable of cholesterol and phospholipid biosynthesis. As such, they appear to be an accessible ‘peripheral‘ cell in which to study the relationships between dietor drug-induced changes in plasma lipoproteins and cellular lipid synthesis. In the present study normolipidaemic male subjects were each fed cholesterol-free diets (40% total energy from fat) in which only the percentage composition of dietary fats was varied (‘saturated diet’ contained saturated 50%; monounsaturated 40% and polyunsaturated 10%; ‘monounsaturated diet’ contained saturated lo%, monounsaturated 80%, polyunsaturated 10%; ’polyunsaturated diet’ contained saturated lo%, monounsaturated 40%, polyunsaturated 50%). Each diet period was 4 weeks and each subject was studied on all three diets. Blood samples for studies of lipid synthesis were taken after 2 weeks on each diet. Mononuclear cells were separated on Ficoll (Boyum, 1968), washed with buffered saline and resuspended in homologous serum. Cell viability (assessed by Trypan Blue exclusion) was >95%; the cell population contained 85-90% lymphocytes, 10-15% monocytes and (2% granulocytes. Duplicate portions of the resuspended cells [( 1 2) x 10’ cells1 were incubated at 37OC for 3 h in the presence of 0 . 5 2 , ~ ~ [Me-3H1choline and 1 . 3 , ~ ~ [ l-“C]acetate. After incubation lipids were extracted and portions of the total and nonsaponifiable fractions sepaated by t.1.c. Plasma levels of low-density lipoproteins and the fatty-acid profile changed during the first 2 weeks of each study but remained stable during the last 2 weeks (mean values were: baseline low-density lipoprotein cholesterol, 102 mg/dl; saturated-diet low-density lipoprotein cholesterol, 80mg/dl; monounsaturated-diet low-density lipoprotein cholesterol, 74 mg/dl; polyunsaturated-diet low-density lipoprotein cholesterol, 71 mg/dl). Lipid synthesis in mononuclear cells was therefore studied at a time when both the concentration and fatty-acid composition of plasma lipoproteins had just reached new steady-state levels. As Table 1 illustrates, the synthesis of squalene, lanosterol and cholesterol by human mononuclear cells showed considerable individual variation. There was no significant difference in sterol synthesis between the three dietary periods, although synthesis of lanosterol was lowest on the polyunsaturatedfat-enriched diet (P < 0.10 versus saturated). Synthesis of phosphatidylcholine was significantly ( P < 0.05) higher on the monounsaturated diet than on either the saturated or polyunsaturated phases. There was no correlation between either the concentration of plasma lipids or the percentage of monocytes in the mononuclear-cell preparations and the cellular synthesis of sterols or phosphatidylchlorine. In conclusion, our results reveal no differences in sterol synthesis by freshly isolated human mononuclear cells from subjects on diets relatively enriched in saturated, monounsaturated or (m6)-polyunsaturated fatty acids. Nestel & Ma (1979) have also failed to reveal any differences in the uptake and degradation of control and linoleate-enriched lZJI-labelled low-density lipoproteins by cultured human fibroblasts. Taken together these observations lend no support to the theory that polyunsaturated fatty acids may lower plasma level of lowdensity lipoproteins by increasing removal in non-hepatic tissues.