On the differential suppression of cholesterol synthesis by low density lipoprotein in B and T lymphocytes

Regulation of bile acid synthesis. Isolation and characterization of microsomal phosphatases

In preliminary experiments, we have shown that rat liver microsomes possess phosphatase activity which was inhibited in the presence of sodium fluoride. We have now separated six microsomal phosphatase fractions appearing to be isoenzymes. They all possess different kinetic constants and are not equally inhibited by tartrate and fluoride ions, inhibitors of phosphatase activity. One phosphatase fraction, in fact, is almost completely unaffected by fluoride ion. More pertinent to our interest, these isoenzymes exhibit differing abilities to modulate the activities of hydroxymethylglutaryl CoA reductase, acyl-CoA:cholesterol O-acetyltransferase, and cholesterol 7 alpha-hydroxylase. Interaction of four of the fractions with rat liver microsomes resulted in a decrease in cholesterol 7 alpha-hydroxylase activity; two were without effect.

Regulation of indices of cholesterol synthesis in human mononuclear leukocytes by dietary cholesterol and fat saturation

The responses of 2 indices of cholesterol synthesis, 3-hydroxy-3-methylglutaryl-CoA reductase activity and incorporation of [14C]acetate into sterols, in mononuclear leukocytes freshly isolated from peripheral blood to variation in the ratio of saturated to unsaturated fat (S:U) and the amount of cholesterol absorbed from the diet were examined in 24 free-living men. Increasing S:U was associated with increasing plasma cholesterol level (r = 0.27, P = 0.03) and increasing reductase activity in leukocytes (r = 0.60, p less than 0.001). This finding is consistent with the hypothesis that saturated fat decreases the flux of cholesterol from plasma into cells thereby releasing reductase from product feedback inhibition. Reductase activity, after controlling for the effect of S:U, was negatively correlated with absorbed cholesterol from sources other than eggs (r = 0.42, P = 0.02). Surprisingly, change in reductase activity was positively correlated with change in absorbed cholesterol upon eating eggs (r = 0.49, P = 0.008). Sterol labeling was negatively correlated with absorbed cholesterol from all sources including eggs (r = -0.64, P less than 0.001) and was uncorrelated with S:U. Reductase activity and sterol labeling responded in parallel to cholesterol in foods other than eggs but not to egg feeding nor to S:U, thus it is unclear which test best reflects endogenous sterol synthesis in these cells.

Effect of insulin on low-density-lipoprotein metabolism in human lymphocytes in vitro

The metabolism of low-density lipoproteins (LDL) in vitro in the presence of insulin was studied in freshly isolated human peripheral-blood lymphocytes. Insulin appeared to decrease the binding affinity of 125I-LDL to its cell-surface receptor, without any change in apparent Vmax or in the number of LDL receptors. As a consequence, the absolute amounts of 125I-LDL internalized and degraded were lower in the presence of insulin than in its abscence, although the fraction of internalized 125I-LDL degraded in either instance was quite similar. 3-Hydroxy-3-methylglutaryl-CoA reductase activity, and hence cholesterol synthesis, were stimulated by insulin. This effect of insulin was independent of the inhibitory effect of LDL on cholesterol synthesis. At the same time, acid cholesterol esterase and acyl-CoA: cholesterol O-acetyltransferase activities were lower in cells incubated with insulin than in controls. The net effect of these metabolic alterations seems to be that cells accumulate greater quantities of free and esterified cholesterol when treated with insulin.

Differences in the binding, internalization and catabolism of low-density lipoprotein between normal human T and B lymphocytes

Studies in human peripheral blood B and T lymphocytes show that high-affinity binding of low-density lipoprotein (LDL) to the cell surface receptor and the kinetics of binding are comparable between B and T lymphocytes, but the internalization of receptor-bound LDL in B cells appears deficient. Yet, the fraction of internalized LDL degraded by both B and T lymphocytes is of similar magnitude. Moreover, the lysosomal acid cholesterol ester hydrolase and acyl-CoA:cholesterol acyltransferase activities in B cell were about one-third of those in T lymphocytes. These data suggest deficient LDL catabolism in B lymphocytes relative to that in T lymphocytes.

Differential suppression of lymphocyte cholesterol synthesis by low density lipoprotein and erythrocyte insulin receptors in normolipidemic subjects

The rates of cholesterol synthesis from acetate were studied in freshly isolated peripheral blood lymphocytes in 41 age- and sex-matched subjects with essentially normal serum lipid profiles. Insulin binding to erythrocytes obtained from the same blood samples was also studied simultaneously. From the data on lymphocyte cholesterol synthesis in the absence or presence of low density lipoprotein in the medium, an index, LDL50, was calculated for each subject. This is the concentration of LDL cholesterol (nmol/ml) in the medium necessary to reduce cholesterol synthesis to 50% of that in the absence of LDL. On the basis of LDL50, the subjects could be segregated into three distinct groups, I, II, and III, with LDL50 of 6.5, 23.3, and 77.0 nmol/ml, respectively. This grouping was independent of the serum lipid profiles, age or sex. Insulin binding studies showed that the amount of insulin specifically bound and the number of insulin receptors per cell were inversely correlated with LDL50. LDL50 was also determined for 4 subjects with clinically manifested consequences of familial hypercholesterolemia. The LDL50 values for these individuals corresponded to values obtained for subjects in group III. The number of insulin receptors and the amount of insulin bound in these patients were correspondingly low. These results suggest that LDL50 may be useful in discerning abnormal cellular cholesterol metabolism in subjects with or without accompanying hyperlipidemias.

The effect of human T and B lymphocytes on low-density lipoprotein catabolism by cultured fibroblasts

Freshly isolated human peripheral blood B lymphocytes degrade LDL via the LDL receptor pathway to a considerably greater extent than do freshly isolated T lymphocytes. In this study, B and T lymphocytes were compared for their ability to modulate LDL degradation in fibroblasts to which they were exposed. B and T lymphocytes were purified by sheep red blood cells rosette formation and passage through a nylon column. Lymphocytes and skin fibroblasts were co-cultured for 48 h in 10% lipoprotein-deficient serum before 125I-labeled LDL was added for measurement of its degradation. Co-culture with B lymphocytes increased LDL degradation by fibroblasts in a dose-dependent manner, but co-culture with T lymphocytes, did not. B lymphocytes had the same effects on human smooth muscle cells. The enhancement of LDL degradation in fibroblasts also was observed when fibroblasts were exposed to conditioned medium from B lymphocytes. The effects of B lymphocytes were not reproduced by exposure of fibroblasts to purified immunoglobulins or to B lymphocytes that had been freeze-thawed. These findings suggest that a secretory product of B lymphocytes can increase LDL receptor activity in other cells.

Positive selection of mouse B and T lymphocytes and analysis of isolated populations by flow cytometry

Mouse spleen cells were separated into Ig+ and Ig- populations by positive selection using petri plates coated with rabbit anti-mouse immunoglobulin. The Ig- cells were subsequently incubated with mouse monoclonal alloantisera to Thy1.2 prior to a second positive selection. The adherent populations were characterized as B (Ig+) or T (Thy1.2+) lymphocytes on the basis of surface immunofluorescence and mitogen-induced proliferation. Analysis of the 2 isolated populations by flow cytometry showed that B and T lymphocytes could be distinguished by their forward light scatter as well as their fluorescence after incubation with fluorescein diacetate.