Marked overproduction of low density lipoprotein apolipoprotein B in a subject with heterozygous familial hypercholesterolemia. Effect of portacaval shunting

In vivo metabolism of apolipoprotein A-I in a patient with homozygous familial hypercholesterolemia

Familial hypercholesterolemia (FH), caused by a defect in the low density lipoprotein (LDL) receptor, results in high plasma concentrations of LDL cholesterol due to both overproduction and delayed catabolism of LDL. FH is also associated with significantly lower levels of plasma high density lipoprotein cholesterol and apolipoprotein (apo) A-I in both heterozygous and homozygous patients. However, the metabolic basis of the hypoalphalipoproteinemia in FH has not been elucidated. We investigated the kinetics of apo A-I in a homozygous FH patient and two normal control subjects by using endogenous labeling with a stable isotopically labeled amino acid. Study subjects were administered a primed constant infusion of 13C6-phenylalanine for 12 hours. Apolipoproteins were isolated from plasma drawn at selected time points and analyzed for their isotopic enrichment by gas chromatography-mass spectrometry. The fractional catabolic rate of apo A-I in the FH subject was found to be substantially increased (0.38 day-1) compared with that of the normal subjects (mean, 0.26 day-1). In addition, the apo A-I production rate was decreased in the FH subject (6.5 mg/kg.day-1) compared with the normal subjects (mean, 11.1 mg/kg.day-1). In conclusion, the low levels of high density lipoprotein cholesterol and apo A-I in this homozygous FH patient are due to the combined metabolic defects of increased apo A-I catabolism and decreased apo A-I production.

Effect of portacaval anastomosis on plasma lipoproteins of lean and obese Zucker rats

The distribution of cholesterol (C), triglycerides (TG), phospholipids (PL) and proteins in the different lipoproteins was studied in lean and obese Zucker rats under 2 conditions: non-operated and after portacaval anastomosis (PCA). All lipoproteins, but mainly the d less than 1.006 g/ml (chylomicrons + VLDL) fraction, were increased in obese rats. They were hypertriglyceridemic (+ 675%) and hypercholesterolemic (+ 207%). PCA decreased cholesterolemia by about 50% and triglyceridemia by 50-70% in both lean and obese rats. Except for VLDL, enriched in TG in obese rats, proportions of C, TG, PL and proteins in lipoproteins were about the same in obese and lean Zucker rats and were nearly unchanged by PCA, suggesting an increased number of particles in obese rats and a decreased number after PCA. Lipoproteins of obese rats were enriched in apo C and proportions of apo E and apo B were decreased in the d less than 1.006 g/ml fraction. PCA decreased the proportion of apo C in all lipoproteins.

Lipoprotein catabolism in rats with portacaval anastomosis on various diets

The catabolism of lipoproteins was measured in rats with a portacaval anastomosis and in intact control rats. Radioiodinated rat high density lipoproteins or human low density lipoproteins, the major cholesterol-bearing lipoproteins in rat or man respectively, were injected intravenously into rats. More than 90% of trace amounts of these lipoproteins were removed from plasma of rats with portacaval anastomosis and controls on standard chow at closely similar rates within 24 h. Also, [125I]high density lipoproteins left the plasma at comparable rates in controls and rats with portacaval anastomosis, whether fed with a cholesterol-free chow or a carbohydrate-rich lard chow. These dietary regimens were employed to avoid artifacts through a different development of the body weight in operated and control rats. A standard laboratory chow ad libitum led to weight loss in rats with portacaval anastomosis. Pair-fed with a cholesterol-free chow both groups of rats kept the same weight, but only with a carbohydrate-rich lard chow could the natural weight gain be achieved. In all rats with portacaval anastomosis liver weights were reduced and serum cholesterol decreased by 21-31% with the major change in high density lipoproteins. The findings suggest that cholesterol concentrations are not likely to be lowered in rats with portacaval anastomosis by enhanced lipoprotein catabolism.

Portacaval shunt as treatment for hypercholesterolemia. Metabolic and morphological effects in a swine model

Since 1973 the portacaval shunt has been used as a treatment for homozygous familial hypercholesterolemia. Favorable results have been reported, but the mechanism or reduction of cholesterol is not clear. The objective of this research was to evaluate mechanisms of lipid alterations after portacaval shunting in Yucatan miniature swine. The animals were fed a high-fat diet, similar in composition to the average American diet, with or without added cholesterol. Controls were fed the atherogenic diet (+ cholesterol) for 8 months. Pigs were fed atherogenic or American diets for 8 months, then surgery (shunt or sham) was performed. They were continued on the diets for another 8 months. The vascular system was examined for the distribution and severity of atherosclerotic disease. Blood lipids and numerous biochemical indices were measured. Progression of atherosclerosis was slowed by portacaval shunting. Low density lipoprotein (LDL) concentrations were positively and high density lipoprotein (HDL) concentrations were negatively correlated with severity of atherosclerosis. Serum insulin concentrations were positively correlated with atherosclerosis. Cholesterol synthesis was increased by the shunting and decreased by cholesterol feeding. The cholesterol-fed swine is not an adequate model for familial hypercholesterolemia, but the results are consistent with inhibition of the atherosclerotic process and the involvement of lipoproteins and insulin in the mechanisms.

The effect of portacaval shunt on hepatic lipoprotein metabolism in familial hypercholesterolemia

The hyperlipidemia observed in familial hypercholesterolemia can be reduced by portacaval anastomosis. We report the effects of a portacaval shunt on hepatic morphology and biosynthetic pathways crucial to hepatic cholesterol homeostasis in homozygous receptor-negative familial hypercholesterolemia. Portacaval anastomosis was associated with a dramatic change in hepatocyte morphology, 28% reduction in plasma low-density lipoprotein concentration, and a decrease in hepatic total and free cholesterol content by 27 and 75%, respectively. Furthermore, the rate-limiting enzyme in cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A reductase was decreased by 56%. Finally, the reduced binding of low-density lipoproteins to hepatic membranes preoperatively was increased following the portacaval shunt. These combined results indicate that the changes in circulating lipoprotein concentrations observed after portacaval shunt are due to alterations in the metabolic consequences of the defective recognition of low-density lipoproteins by the liver of familial hypercholesterolemic subjects.

Regulation of the production and catabolism of plasma low density lipoproteins in hypertriglyceridemic subjects. Effect of weight loss

In subjects with hypertriglyceridemia, plasma concentrations of low density lipoprotein (LDL) cholesterol are often normal or reduced. Perturbations that alter plasma very low density lipoprotein (VLDL) concentrations are associated with opposite changes in plasma LDL levels. To determine the mechanisms regulating plasma LDL levels, we used 131I-VLDL and 125I-LDL to measure the fractional catabolic rates (FCR), production rates (PR), and rates of interconversion of apoprotein B (apo B) in VLDL, intermediate density lipoprotein, and LDL in six hypertriglyceridemic subjects pre- and post-weight reduction. [2-3H]glycerol was used to quantitate VLDL triglyceride PR. All data are presented as mean +/- SD. Percent ideal body weight fell from 132 +/- 17.9 to 119 +/- 15.9% in the group, P less than 0.05. After weight loss, plasma VLDL triglyceride (486.0 +/- 364.1 vs. 191.3 +/- 65.4 mg/dl, P less than 0.05) and VLDL apo B (32.2 +/- 12.0 vs. 14.8 +/- 6.8 mg/dl, P less than 0.05) concentrations were reduced. VLDL triglyceride PR also fell after weight reduction (56.6 +/- 39.0 vs. 28.6 +/- 23.1 mg/kg per h, P less than 0.05), as did VLDL apo B PR (47.9 +/- 41.4 vs. 19.0 +/- 14.1 mg/kg per d, P less than 0.05). Pre-weight loss, plasma LDL cholesterol and apo B levels were low-normal or reduced (64.0 +/- 12.6 and 58.4 +/- 11.9 mg/dl, respectively) despite normal or elevated LDL apo B PR (17.4 +/- 7.2 mg/kg per d). The reduced cholesterol and apo B levels were associated with increased FCRs (0.68 +/- 0.29 d-1) and reduced cholesterol/protein ratios (1.01 +/- 0.18) in LDL. The plasma levels of LDL cholesterol and apo B rose after weight reduction (84.8 +/- 24.9, P less than 0.05; and 69.5 +/- 14.3 mg/dl, P less than 0.05, respectively, vs. base line). These increased concentrations resulted from a combination of events. First, the FCR for LDL apo B fell in five of six subjects with a significant reduction for the group as a whole (0.48 +/- 0.11 d-1, P less than 0.05 vs. base line). Second, the cholesterol/protein ratio increased in all six subjects with a significantly greater mean after weight loss (1.25 +/- 0.27, P less than 0.05 vs. base line). In contrast, the LDL apo B PR fell or was essentially unchanged in the six subjects after weight loss (mean, 14.4 +/- 2.8 mg/kg per d; NS vs. pre-weight loss). The changes in LDL catabolism and composition were associated with changes in the source of LDL apo B. Pre-weight loss, 73.3% of LDL was derived from VLDL, while 26.7% was directly secreted into plasma. Post-weight reduction, VLDL-derived LDL fell to 46.8% of total, while direct secretion accounted for 53.2% of LDL production. These changes were significant; P < 0.95. Thus, all subjects had direct secretion of LDL apo B and the magnitude of this source of VLDL triglyceride secretion. These results indicate that the regulation of plasma LDL levels in hypertriglyceridemic subjects is quite complex and that the rise in LDL levels after weight loss results from reduction in the fractional catabolism of this lipoprotein. The fall in the FCR is associated with changes in the source of LDL and in its composition.

The Eck fistula in animals and humans

In all species so far studied, including man, portacaval shunt causes the same changes in liver morphology, including hepatocyte atrophy, fatty infiltration, deglycogenation, depletion and disorganization of the rough endoplasmic reticulum (RER) and its lining polyribosomes, and variable but less specific damage to other organelles. Many, perhaps all, biosynthetic processes are quickly depressed, largely secondary to the selective damage to the RER, which is the "factory" of the cell. These structural and metabolic changes in the liver after portal diversion are caused by the diversion around the liver of the hepatotrophic substances in portal venous blood, of which endogenous insulin is the most important. In experimental animals, the injury of Eck's fistula can be prevented by infusing insulin into the tied-off hilar portal vein. The subtle but far-reaching changes in hepatic function after portal diversion have made it possible to use this procedure in palliating three inborn errors of metabolism: glycogen storage disease, familial hypercholesterolemia, and alpha 1-antitrypsin deficiency. In these three diseases, the abnormalities caused by portal diversion have counteracted abnormalities in the patients that were caused by the inborn errors. In these diseases, amelioration of the inborn errors depends on the completeness of the portal diversion. In contrast, total portal diversion to treat complications of portal hypertension is undesirable and always will degrade hepatic function if a significant amount of hepatopetal portal venous blood is taken from the liver. When total portal diversion is achieved (and this is to be expected after all conventional shunts), the incidence of hepatic failure and hepatic encephalopathy is increased. If portal diversion must be done for the control of variceal hemorrhage, a selective procedure such as the Warren procedure is theoretically superior to the completely diverting shunt. In practice, better patient survival has not been achieved after selective shunts than after conventional shunts, but the incidence of hepatic encephalopathy has been less.

Portacaval shunt in patients with familial hypercholesterolemia

Portacaval shunt was performed in ten patients with homozygous and two with heterozygous familial hypercholesterolemia (FH). Total serum cholesterol was lowered by 20% to 55.4% during follow-up periods of 14 months to almost 9 years, with commensurate decreases in LDL cholesterol. The effect on HDL cholesterol and triglyceride levels was variable. Tendinocutaneous xanthomas diminished or disappeared. Growth and development in children proceeded or accelerated. There was no detectable emotional or intellectual deterioration. Hepatic failure did not occur, although blood ammonia concentrations and serum alkaline phosphatase levels increased relative to preoperative values. Cardiac symptoms were often improved, but evidence of reversal of cardiovascular lesions was inconclusive. Three patients with pre-existing heart disease died of cardiac complications after 4 months, 18 1/2 months, and 30 months. Portacaval shunt has been effective therapy for patients with FH who were refractory or intolerant to medical treatment; it should be performed before the development of irreversible cardiovascular damage.

Treatment of familial hypercholesterolemia by portacaval anastomosis: effect on cholesterol metabolism and pool sizes

Measurements of the key parameters of cholesterol homeostasis and the mass of the body pools of cholesterol were carried out in two patients with familial hypercholesterolemia (FH), one homozygote and one heterozygote, before and 28 and 18 months, respectively, after portacaval anastomosis (PCA). In both patients the procedure significantly reduced the plasma concentrations of total and low density lipoprotein cholesterol and the daily rate of whole body cholesterol and bile acid synthesis. In addition, PCA caused a net efflux of accumulated tissue cholesterol as demonstrated by reductions in the rapidly exchangeable and total exchangeable masses of body cholesterol. Shunt patency was verified by demonstration of increased bile acids in serum from fasting patients and from patients 2 hr after a meal and by increased plasma glucagon before and after arginine infusion. Other than a persistently increased level of serum alkaline phosphatase, liver function tests have fallen within the normal range in both patients; there has been no evidence of hepatic encephalopathy. In the homozygous patient there has also been a striking resolution in xanthoma size and distribution. These multiple effects on cholesterol homeostasis and pool sizes strongly suggest that PCA can reverse the progressive accumulation of cholesterol in body tissues of FH patients.