Metabolism of atherogenic lipoproteins by smooth muscle cells of different phenotype in culture.

Smooth muscle cells of the rabbit aorta, when grown in vitro, express three distinguishable forms of phenotype (contractile, reversible synthetic, and irreversible synthetic). We compared the interactions of these three smooth muscle phenotypes with rabbit very low density lipoprotein (VLDL), low density lipoprotein (LDL), and very low density lipoprotein from cholesterol-fed rabbits (beta-VLDL). beta-VLDL showed saturable. high-affinity binding characteristics with each phenotype predominantly through the B/E receptor. The irreversible synthetic cells displayed the greatest binding capacity and the contractile cells, the least. Binding and degradation of normal VLDL was less than that of beta-VLDL and higher than that of LDL. Only the irreversible synthetic cells showed substantial (about threefold) cholesteryl ester formation and cholesterol accumulation, and then only when incubated with beta-VLDL. Substantial stainable lipid, shown chemically to include triglyceride, cholesterol and cholesteryl ester, was also observed only when irreversible synthetic cells were exposed to beta-VLDL. The high capacity of irreversible synthetic-state, smooth muscle cells to bind and accumulate beta-VLDL in contrast to the relative immunity of contractile cells may be relevant to the genesis of atherosclerosis in the rabbit and possibly also in humans.

Lipoprotein predictors of the severity of coronary artery disease in men and women.

In this study we examined the relationships between levels of several components of plasma lipoproteins and severity of coronary artery disease in 65 men and 42 women who underwent coronary arteriography for suspected coronary disease. Severity of coronary atherosclerosis was scored as the extent of disease seen at arteriography. Univariate analyses of the relationships between the plasma lipoprotein parameters and score for severity of atherosclerosis revealed a marked difference between men and women. In men, the score for severity of atherosclerosis was strongly related to the low-density lipoprotein (LDL) cholesterol and apolipoprotein B concentrations, whereas in women it was related to the triglyceride concentrations in plasma intermediate-density lipoprotein (IDL) and LDL and to the cholesterol and apolipoprotein B concentrations in IDL. The significance of these correlations was not negated by possible confounding factors such as alcohol intake, diabetes, and treatment with thiazides and beta-adrenergic blockers. Stepwise regression analyses of data adjusted for weight and age indicated that 22% of the variation in the score for severity of atherosclerosis could be accounted for by levels of LDL cholesterol in men. No other lipoprotein parameter could account for any further variation. In contrast, cholesterol did not account for any variation in the score for severity of atherosclerosis in women, whereas plasma triglyceride accounted for 16% of the observed variation in this group. No relationships were found between score for severity of atherosclerosis and high-density lipoprotein cholesterol or plasma apolipoprotein A-I concentrations in either group.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression by diets rich in fish oil of very low density lipoprotein production in man.

The highly polyunsaturated fatty acids in fish oils lower the plasma triglyceride concentration. We have studied the effect of a diet rich in fish oil on the rate of production of the triglyceride-transporting very low density lipoprotein (VLDL). Seven subjects, five normal and two with hypertriglyceridemia received up to 30% of daily energy needs from a fish oil preparation that was rich in eicosapentaenoic acid and docosahexaenoic acid, omega-3 fatty acids with five and six double bonds, respectively. Compared with a diet similarly enriched with safflower oil (in which the predominant fatty acid is the omega-6 linoleic acid, with two double bonds), the fish oil diet lowered VLDL lipids and B apoprotein concentrations profoundly. High density lipoprotein lipids and A1 apoprotein were also lowered, but the effect on low density lipoprotein (LDL) concentration was inconsistent. The daily production or flux of VLDL apoprotein B, calculated from reinjected autologous 125I-labeled lipoprotein, was substantially less in six subjects studied after 3 wk of fish oil, compared with after safflower oil. This effect on flux was more consistent than that on the irreversible fractional removal rate, which was increased in the four normolipidemic but inconsistent in the hypertriglyceridemic subjects. This suggests that fish oil reduced primarily the production of VLDL. The daily production of VLDL triglyceride, calculated from the kinetics of the triglyceride specific radioactivity-time curves after [3H]glycerol was injected, also showed very substantial reductions in five subjects studied. The marked suppression in VLDL apoprotein B and VLDL triglyceride formation was found not to be due to diminished plasma total free fatty acid or plasma eicosapentaenoic flux, calculated during constant infusions of [14C]eicosapentaenoic acid and [3H]oleic acid in four subjects. In two subjects there was presumptive evidence for substantial independent influx of LDL during the fish oil diet, based on the precursor-product relationship between the intermediate density lipoprotein and LDL apoprotein B specific radioactivity-time curves.

Homozygous familial hypercholesterolemia occurring with apoprotein E3 deficiency. Report of two cases.

This is the first report of homozygous familial hypercholesterolemia (FH) occurring together with dysbetalipoproteinemia. The former was demonstrated by deficiency of specific receptors for apoprotein B of low density lipoproteins and the latter by isoelectric focusing of the E isoapoproteins and the presence of a broad-beta band on electrophoresis. Two young boys of Lebanese extraction had extensive tuberous and tendinous xanthomata, serum cholesterol concentrations of 29.9 and 28.4 mmol/liter, respectively, and mildly raised serum triglycerides due to an accumulation of lipoprotein remnant particles. Homozygosity for FH was demonstrated in both boys by the deficiency of specific binding of low density lipoprotein to cultured skin fibroblasts (less than 15% and less than 10% of normal, respectively). The E apoprotein phenotypes showed E3/E2 in one boy and E2/E2 in the other. The treatment of both boys with cholestyramine and probucol reduced the serum cholesterol concentration to between 15 and 18 mmol/liter and dramatically lessened the severity of xanthomatosis.

A new model of human VLDL metabolism based on simultaneous studies of its APOB and triglyceride.

Previous studies which separately examined the behavior of the B apolipoprotein and of the triglyceride of VLDL had raised the possibility that each was metabolized in a different manner. The present studies have explored this possibility by simultaneously studying each of these VLDL constituents in thirteen humans. 125I-labeled Sf 60-400 lipoproteins were injected. The fate of their apo B (a reflection of the metabolism of the lipoprotein particle) was followed in this, and in the smaller Sf 12-60 lipoproteins. In everyone, the larger particle was catabolized to the smaller one. Furthermore, the smaller particle was derived exclusively from the larger one. The behavior of the lipoproteins' triglyceride was examined by following the specific activity of 3H-triglyceride which had been endogenously labelled by injecting 2-3H-glycerol. Some of the larger lipoproteins' triglyceride appeared in the smaller particles. However, in contrast to the apo B, the triglyceride in the smaller particle was not derived exclusively from that in the larger particle. In eleven subjects, the triglyceride specific activity curves above demonstrated that some of the triglyceride entered the smaller circulating particle from a source other than the large particle.

Roles of lipoprotein lipase and hepatic triglyceride lipase in the catabolism in vivo of triglyceride-rich lipoproteins.

To define the roles, in vivo, of hepatic triglyceride lipase and lipoprotein lipase in the catabolism of triglyceride-rich lipoproteins, we investigated the relationship between the activities of the above enzymes in postheparin plasma and the fractional removal rates of very low density lipoproteins (VLDL) and VLDL remnant particles. In 22 patients, the fractional removal rates of VLDL and VLDL-remnant particles were determined from analyses of the disappearance of radioiodinated Sf 60-400 and Sf 12-60 lipoprotein B apoprotein. The maximal activities of hepatic triglyceride lipase and lipoprotein lipase were determined in plasma samples drawn 2-60 minutes after heparin injection (60 U/kg). A positive correlation was observed between the fractional removal rate of VLDL and postheparin plasma lipoprotein lipase activity (r = 0.65). When all 22 patients were considered together, no relationship was demonstrable between remnant fractional removal and postheparin plasma lipoprotein lipase activity. However, humans may be subdivided with respect to the way in which they catabolize remnants. In some, all remnant may be catabolized to form LDL. In others, some of the remnant may also be directly removed from the circulation. Those subjects in whom previous studies indicate that all remnant is converted to LDL demonstrated a positive correlation between remnant fractional removal rate and postheparin plasma lipoprotein lipase activity (n = 8, r - 0.83). No correlations between postheparin plasma hepatic triglyceride lipase activity and any of the fractional removal rates were found. These data are consistent with the following: 1) lipoprotein lipase plays a key regulatory role in the catabolism of triglyceride-rich lipoproteins; 2) this role applies only to those catabolic involving the formation of particles of higher density VLDL remnants and low density lipoprotein; and 3) hepatic triglyceride lipase plays no rate-limiting role in the catabolism of VLDL or VLDL-remnant particles.

The independent synthesis of intermediate density lipoproteins in type III hyperlipoproteinemia.

The kinetics of VLDL (Sf 60-400) and IDL (Sf 12-60) B apoprotein were examined in five type III hyperlipoproteinemic subjects. The production rates for B apoprotein were significantly greater in the IDL fraction than in VLDL. Precursor-product relationships between VLDL and IDL B apoprotein illustrated that a significant proportion of IDL B apoprotein was derived from some source other than VLDL catabolism. These observations indicated that, in the five individuals studied, an 'unusual' B apoprotein synthetic pathway operated whereby B apoprotein was directly entering the IDL fraction. Furthermore, this second pathway resulted in an overproduction of IDL B apoprotein and possibly was the major defect leading to the development of the type III hyperlipoproteinemia lipid profile. In two subjects who were restudied following hormonal treatment (estrogen or thyroxine replacement) and in whom the type III hyperlipoproteinemia lipid profile no longer existed, it was found that the pathway for direct synthesis of IDL B apoprotein had been abolished. From these studies we have concluded that a pathway for the direct synthesis of IDL B apoprotein operates in type III hyperlipoproteinemics and it is a major causative factor in the development of the IDL accumulation characteristic of this metabolic disorder.

Improved method for quantitation of B apoprotein in plasma lipoproteins by electroimmunoassay.

The B apoprotein occurs in a wide range of plasma lipoproteins, which are heterogenous both with respect to size and to composition. Immunochemical recognition of the apoprotein is influenced by the nature of the particle in which the apoprotein is found, presumably due to the masking of the antigenic sites by lipoprotein lipid. Consequently, it is difficult to provide suitable standards for use in routine electroimmunoassay procedures of the B apoprotein, particularly for triacylglycerol-rich lipoproteins. We have devised a procedure whereby lipoprotein samples, independent of their initial size and composition, are reduced, by use of a bacterial lipase, to a common size and composition, which is almost identical to that of the standard. This permits an assay that may be easily used routinely. It provides a far more reliable estimate of the absolute B apoprotein mass in plasma lipoproteins of the very-low- and low-density lipoprotein spectrum than has been previously available.

Catabolism of very low density lipoprotein B apoprotein in man.

The turnover and the catabolic fate of the B apoprotein of very low density lipoprotein (VLDL-B) was studied in 15 normal and hyperlipidemic subjects using reinjected autologous VLDL labeled with radioiodine. The specific radioactivity-time curve of the B apoprotein in total VLDL (S(f)20-400) was multiexponential but conformed to a two-pool model during the first 48 h of catabolism. The flux was highest in several hypertriglyceridemic subjects. The mass of pool A exceeded the intravascular content of VLDL-B by 30% on average, indicating extravascular metabolism of VLDL. The two-pool model might reflect the input of several populations of particles or heterogeneity of catabolic processes or pools. The flux of B apoprotein was also measured in several subclasses of VLDL, in smaller intermediate density lipoproteins, and in low density lipoproteins (LDL). In three subjects the flux was similar in S(f) 60-400 and in S(f) 12-60 lipoproteins, suggesting that VLDL was catabolized at least to a particle in the density range S(f) 12-60. Subsequent catabolism appeared to proceed by two pathways: in normotriglyceridemic subjects, B apoprotein flux in the S(f) 20-400 and in S(f) 12-20 lipoproteins was similar, whereas in hypertriglyceridemic subjects flux through S(f) 12-20 accounted for only part of the VLDL-B flux. The flux of low density lipoprotein B apoprotein (LDL-B), which is believed to be derived from VLDL catabolism, was calculated from the area between the specific activity time curves of VLDL-B and LDL-B. In subjects with normal plasma triglyceride concentration, LDL-B flux was from 91% to 113% of that of VLDL-B; but in three hypertriglyceridemic subjects showing high rates of VLDL-B transport, LDL-B flux was only one-third that of VLDL-B. This suggests that when VLDL-B flux is high, VLDL is substantially catabolized by a route other than through LDL and possibly leaves the circulation as a particle in the S(f) 20-60 density range.

Plasma post-heparin lipolytic activity and triglyceride clearance in uremic and hemodialysis patients and renal allograft recipients.

In order to study mechanisms in the pathogenesis of hypertriglyceridemia in uremia, maintenance hemodialysis, and post-renal transplantation, plasma post-heparin lipolytic acitivity was measured and the kinetics of triglyceride removal were determined during infusions of triglyceride in patients from each of these groups and in healthy control subjects. In addition, plasma lipolytic activity was measured both during the course of a single hemodialysis treatment and in response to daily hemodialysis over a five-day period. The mean serum triglyceride level was significantly elevated and the mean plasma post-heparin lipolytic activity significantly reduced in all three groups. Post-heparin lipolytic activity in transplant recipients with a normal serum creatinine concentration was not significantly different from that in control subjects. Those transplant recipients with mildly impaired graft function had levels of post-heparin lipolytic activity comparable to those in patients with end-stage renal failure. Triglyceride clearance was significantly reduced in both the transplant recipients and in the uremic patients. During a single hemodialysis treatment with systemic heparinization plasma lipolytic activity decreased after the first hour. With daily hemodialysis, predialysis post-heparin lipolytic activity progressively declined after the second day. It is concluded that reduced post-heparin lipolytic activity and decreased triglyceride clearance contribute to the hypertriglyceridemia seen not only in uremic patients and in patients on maintenance hemodialysis but also in renal allograft recipients. Diminished lipolytic activity in hemodialysis patients may be in part due to heparin-induced depletion.

Dependence of rates of lipolysis, esterification, and free fatty acid release in isolated fat cells on age, cell size, and nutritional state.

Rates of lipolysis, esterification, and free fatty acid release were estimated in isolated epididymal fat cells prepared from rats fed either ad lib. or with a restricted caloric intake. Basal and epinephrine- or theophylline-stimulated rates of lipolysis correlated positively with cell size in the ad lib.-fed group only. Rates of esterification, both basal and epinephrine-stimulated, correlated positively with cell size in the ad lib.-fed group but negatively in the caloric-restricted group. These findings indicate that nutritional factors can modify any possible influence of adipose cell size on lipolysis and esterification. On the other hand, in both groups of rats, epinephrine- and theophylline-stimulated rates of lipolysis correlated positively with the basal rates of lipolysis. Also, rates of esterification in the presence of epinephrine correlated positively with the basal rates of esterification, suggesting that stimulated rates of lipolysis and esterification are at least partly determined by the basal rates regardless of nutritional status. The activity of glycerokinase measured in homogenates of isolated fat cells, if applicable to intact fat cells, was sufficient to cause considerable underestimations of the basal rates of lipolysis (using glycerol production as an index). When lipolysis was stimulated, the potential errors of estimating lipolysis by glycerol production alone were negligible.