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Biomedical subjects

J M Hoeg

Publications and source records attributed to J M Hoeg.

At least 91 records · Page 5Linked to original sources
PubMed1986

Effectiveness of mevinolin on plasma lipoprotein concentrations in type II hyperlipoproteinemia.

Patients with low-density lipoprotein (LDL) concentrations in the top 10th percentile of the population (type II hyperlipoproteinemia [HLP]) are at increased risk for premature cardiovascular disease; however, the incidence of myocardial infarction and death can be decreased by LDL cholesterol reduction. Mevinolin, an inhibitor of endogenous cholesterol synthesis, has been shown to reduce LDL cholesterol concentrations in a subset of type II patients with heterozygous familial hypercholesterolemia (FH). Using a double-blind, randomized, crossover, placebo-controlled trial, the safety and efficacy of mevinolin were compared in 24 patients with type II HLP with heterozygous FH (n = 6) or without FH type II HLP (n = 18). Compared with placebo treatment, both apolipoprotein B and LDL cholesterol levels were reduced (p less than 0.01) in both FH and non-FH patients by 28 to 34% with mevinolin treatment. In addition, high-density lipoprotein cholesterol levels were significantly increased (p less than 0.001) in both patients with FH (16%) and those with non-FH type II HLP (14%). Patients had no serious or clinically significant adverse effects. Thus, mevinolin is a useful drug for treatment of most patients with elevated plasma LDL cholesterol concentrations.

Adult↗
PubMed1986

Human apolipoprotein A-I. Post-translational modification by fatty acid acylation.

The human apolipoproteins are secretory proteins some of which have been shown to undergo proteolytic processing and post-translational addition of carbohydrate. Apolipoprotein A-I (apo-A-I), the predominant protein associated with high density lipoproteins, undergoes co-translational proteolytic processing as well as post-translational conversion of proapo-A-I to mature apo-A-I following cellular secretion. Utilizing the human hepatoma cell line HEP-G2, we have established that, in addition to proteolytic processing, secreted nascent apo-A-I is acylated with palmitate. Uniformly labeled [14C]palmitate and [1-14C]palmitate were each incorporated into apo-A-I when analyzed by sodium dodecyl sulfate gel electrophoresis and autoradiography. The acylation of apo-A-I with palmitate was confirmed by immunoprecipitation and gas chromatography/mass spectrometry. Hydroxylamine treatment resulted in the deacylation of apo-A-I. Although three of the apo-A-I isoforms analyzed by two-dimensional gel electrophoresis were shown to contain radio-labeled palmitate, 80% of acylated apo-A-I was in the proapolipoprotein A-I isoform. [14C]Oleate was not incorporated in secreted apo-A-I, indicating the specificity of the acylation of apo-A-I. Incubation of [14C] palmitate-acylated apo-A-I in serum and plasma under conditions in which proapo-A-I is proteolytically cleaved to mature apo-A-I did not result in deacylation. These data establish that fatty acid acylation occurs in human secretory proteins in addition to the previously reported acylation of cellular membrane proteins. These results suggest that the covalent linkage of lipids to apolipoproteins may play a critical role in apolipoprotein and lipoprotein metabolism.

Acylation↗
PubMed1986

The expressed human hepatic receptor for low-density lipoproteins differs from the fibroblast low-density lipoprotein receptor.

The role of the cellular receptor for the low-density lipoproteins (LDL) in cholesterol transport was initially defined through the study of nonhepatic cells in vitro. Since the liver is central in plasma lipoprotein metabolism, an investigation of hepatic lipoprotein receptors is important for understanding normal lipoprotein transport. Utilizing human hepatic and fibroblast membranes, the characteristics of receptors for LDL from hepatic and nonhepatic tissues were directly compared. Human hepatic membranes reversibly bound LDL within 5 min. Although both fibroblast and hepatic membranes saturably bound LDL at 37 degrees C, the fibroblast LDL receptor affinity (Kd = 2.5 X 10(-8) M) and number (5.5 X 10(12) sites/mg membrane protein) were greater than the hepatic receptor affinity (Kd = 10.8 X 10(-8) M) and number (0.5 X 10(12) sites/mg membrane protein). In contrast to the fibroblast LDL receptor which was unable to bind LDL in the presence of EDTA, the hepatic LDL receptor binding of LDL was only partially blocked by EDTA. The binding of LDL to its hepatic receptor is highly temperature-dependent, and studies utilizing both radiolabeled LDL and colloidal gold-labeled LDL indicate that little, if any, binding of LDL hepatic membranes occur at 0-4 degrees C. The hepatic membrane receptor(s) (Mr approximately equal to 270 000 and 330 000) differ from that of the fibroblast LDL receptor (Mr approximately equal to 130 000) and these proteins are present in hepatic membranes from a patient lacking the fibroblast LDL receptor. These data indicate that an expressed hepatic LDL receptor has unique properties different from those of the fibroblast LDL receptor and that the expressed protein(s) is genetically distinct from the fibroblast receptor.

Biological Transport, Active↗
PubMed1986

Cultured human hepatocytes. Evidence for metabolism of low density lipoproteins by a pathway independent of the classical low density lipoprotein receptor.

Studies of low density lipoprotein (LDL) metabolism in nonhuman model systems have indicated that the mammalian liver has dual mechanisms for the uptake and regulation of the concentration of plasma LDL. Heretofore, direct evaluation of lipoprotein uptake mechanisms in human hepatocytes has not been possible. In order to compare hepatocyte LDL uptake with fibroblast LDL metabolism, human hepatocytes were isolated and cultured from small biopsy specimens obtained from normolipidemic and homozygous familial hypercholesterolemic patients. Cells cultured in serum-free culture medium retained the morphological and biochemical characteristics of hepatocytes for at least 7 days. The uptake and degradation of LDL by hepatocytes was compared to that of the cultured human fibroblasts. Like fibroblasts, hepatocytes bound, internalized, and degraded LDL. In both cell types, uptake approached saturation at a concentration of 50 micrograms of LDL protein/ml. Competition for LDL binding by LDL, high density lipoprotein, and modified LD revealed that the hepatocyte binding was specific for LDL. Cellular cholesterol loading by incubation in LDL-enriched culture medium resulted in diminished LDL uptake in both cell types. Chemical modification of LDL by acetoacetylation, acetylation, and reductive methylation abolished LDL uptake and degradation by fibroblasts. However, hepatocytes bound and degraded the modified LDL at 30-50% the level of native LDL. Homozygous familial hypercholesterolemic hepatocytes were devoid of the LDL receptor pathway but metabolized native LDL to the extent observed with modified LDL uptake by normal hepatocytes. In contrast to the classic LDL receptor pathway, the second or alternate pathway does not lead to regulation of 3-hydroxy-3-methylglutaryl-CoA reductase activity. These findings indicate the presence of two separate pathways of LDL uptake in human hepatocytes which have different effects on hepatocytic cholesterol metabolism.

Cell Separation↗
PubMed1986

The effects of mevinolin and neomycin alone and in combination on plasma lipid and lipoprotein concentrations in type II hyperlipoproteinemia.

The reduction of elevated low density lipoprotein (LDL) cholesterol concentrations in patients with Type II hyperlipoproteinemia leads to improved cardiovascular morbidity and mortality. Two agents which may be of value in treating hypercholesterolemia are mevinolin and neomycin. Since these drugs lower cholesterol levels through complementary mechanisms, we evaluated the effects of mevinolin and combined mevinolin-neomycin treatment on plasma lipoprotein concentrations in 21 type II hyperlipoproteinemic patients. Mevinolin reduced total and LDL cholesterol concentrations by 24% and 31% respectively (P less than 0.001) and 81% of the patients reduced their LDL cholesterol levels to less than 200 mg/dl. Although the addition of neomycin to mevinolin treatment further lowered total (5%) and LDL (4%) cholesterol concentrations, it also reduced HDL cholesterol levels (19%) (P less than 0.05). Therefore mevinolin normalizes the plasma lipid concentrations in patients with type II hyperlipoproteinemia and combined mevinolin and neomycin treatment offers no advantage over mevinolin-only therapy. In addition, these findings emphasize the importance of determining the HDL cholesterol level to fully evaluate the effects of hypolipidemic therapy.

Cholesterol, HDL↗
PubMed1986

Evaluation of urinary cells in acid cholesteryl ester hydrolase deficiency.

Deficiency in the lysosomal enzyme responsible for cholesteryl ester hydrolysis, acid cholesteryl ester hydrolase (E.C. 3.1.1.13), leads to two clinically recognized diseases: Wolman disease and cholesteryl ester storage including leukocytes, fibroblasts and liver. Analysis of urinary sediment from well characterized cases of Wolman disease and CESD also revealed the shedding of lipid enriched renal tubular cells. Morphologic, enzymic and lipid compositional studies of these cells indicate that the enzyme deficiency observed in fibroblasts and leukocytes from these individuals are reflected in these cells shed in the urine. These findings in renal tubular cells confirm and extend those made in other cell types. These studies indicate that analysis of urinary sediment in suspected cases of acid cholesteryl ester deficiency may provide a meaningful approach for monitoring therapeutic attempts involving enzyme infusion and gene therapy.

Carboxylic Ester Hydrolases↗
PubMed1986

Analysis of the apolipoprotein B gene and messenger ribonucleic acid in abetalipoproteinemia.

The apolipoprotein B-100 (apoB-100) gene in leukocytes and the apoB-100 messenger RNA (mRNA) and translated apolipoprotein in the livers from normal and abetalipoproteinemic individuals were evaluated. Four complementary DNA probes for apoB-100 covering the 5', middle, and 3' regions of the apoB-100 mRNA were utilized and Southern blot analysis indicated that the apoB-100 gene is present in abetalipoproteinemia without major insertions or deletions. Polyadenylated hepatic apoB-100 mRNA from two abetalipoproteinemic patients was normal in size, and the concentration of apoB-100 mRNA was increased sixfold compared with control hepatic apoB-100 mRNA levels. ApoB-100 was detected in hepatocytes of abetalipoproteinemic patients by immunohistochemical techniques. These results indicate that the biochemical defect in abetalipoproteinemic patients studied is most consistent with a posttranslational defect in apoB-100 processing or secretion with an up-regulation of the apoB-100 mRNA.

Abetalipoproteinemia↗
PubMed1986

Thrombin binding and response in platelets from patients with dyslipoproteinemias: increased stimulus-response coupling in type II hyperlipoproteinemia.

Platelets were obtained from patients with various hyperlipidemias [type II, type V, lecithin-cholesterol acyltransferase (LCAT) deficiency] and hypolipidemias (abetalipoproteinemia, Tangier disease) to ascertain relationships among plasma lipids, platelet lipids, thrombin binding and thrombin-induced platelet aggregation, and to compare these data with those previously obtained on stimulus-response coupling in platelets following in vitro modification of membrane microviscosity. Washed platelets were studied for their ability to bind 125I-thrombin in the range of 10(-10) to 10(-6) mol/L (10 mU/mL to 100 U/mL) and to aggregate with thrombin at concentrations less than 10(-9) mol/L (100 mU/mL). The values for binding and aggregation in eight patients from six kindred with familial hypercholesterolemia, taken as a group, fell in the low normal range. If divided into two groups, patients with overt cardiovascular disease bound normal amounts of thrombin but were more responsive to it, whereas patients without overt cardiovascular disease bound lower amounts of thrombin but gave an aggregation response in the normal range. These results suggest that platelet hyperresponsiveness in familial hypercholesterolemia arises from an alteration in the coupling mechanism between thrombin binding and response such that platelets from patients with familial hypercholesterolemia are able to respond with lower receptor occupancy than is the case with normal platelets. Thrombin binding and aggregation were within normal ranges for platelets from abetalipoproteinemia patients (N = 4) and type V hyperlipoproteinemia (N = 2), although in the latter case the response appeared to be less at very low thrombin concentrations (less than 30 mU/mL). Thrombin binding was elevated in Tangier disease (N = 3) but with lower responsiveness at lower thrombin concentrations. Thrombin binding was also elevated in LCAT deficiency (N = 2), and one patient showed increased and another showed decreased aggregation responses. In general, increased plasma cholesterol levels resulted in increased stimulus-response coupling (type II), whereas increased triglyceride levels resulted in decreased coupling (type V, Tangier), and there was no apparent alteration in the coupling mechanism with overall reduction in plasma lipid levels as in abetalipoproteinemia.

Abetalipoproteinemia↗
PubMed1986

The effect of neomycin on plasma alpha tocopherol levels in type II hyperlipoproteinemia.

Use of oral neomycin as a hypocholesterolemic agent was found to decrease plasma alpha tocopherol levels during a 9-month, randomized, crossover placebo-controlled clinical trial with controlled diet. Pooled data from each treatment sequence showed a placebo alpha-tocopherol level of 1,896 +/- 643 micrograms/dl, and a neomycin alpha-tocopherol concentration of 1,449 +/- 451 micrograms/dl (p less than .01). When all subjects were on neomycin simultaneously, the level increased to 1,665 +/- 614 micrograms/dl (p less than .05 from placebo). Normal plasma alpha-tocopherol ranges from 400-1,000 micrograms/dl. Neomycin treatment reduces but does not normalize elevated levels of alpha-tocopherol found in Type II hyperlipoproteinemia.

Cholesterol, LDL↗
PubMed1985

Estimation of VLDL cholesterol in hyperlipidemia.

Lipoprotein data from 10947 fasting blood samples drawn between 1968 and 1982 in the Molecular Disease Branch at the National Institutes of Health were used to test the generalizability of estimating very low density lipoprotein cholesterol (VLDL-C) from plasma triglyceride (TG). Patient samples with total cholesterol levels over 500 mg/l and triglyceride values in the 0-100 000 mg/1 range were included in this study. A previously defined linear relationship VLDL-C = 0.20 (TG) was observed in the past by Friedewald and collaborators, allowing estimation of low density lipoprotein cholesterol (LDL-C) without ultracentrifugation for TG values up to 4 000 mg/1. The results from this report extend the use of the Friedewald relationship to higher TG levels, and to various dyslipidemic states. As the VLDL-C estimates become increasingly imprecise for TG values greater than 10 000 mg/l, caution should be exercised using the estimate in the higher TG ranges. Comparisons with an alternative equation VLDL-C = 0.166 (TG) showed equal or improved accuracy with this estimation procedure, particularly at high TG levels.

Adolescent↗
PubMed1985

Effects of combination cholestyramine-neomycin treatment on plasma lipoprotein concentrations in type II hyperlipoproteinemia.

Recent prospective clinical trials have established that cholesterol reduction in patients with elevated (upper 90th percentile) concentrations of low-density lipoproteins (LDL) reduces the incidence of myocardial infarction and sudden death. Because the level of protection from these cardiovascular sequelae is directly related to the degree of LDL reduction, combination therapy using different hypolipidemic agents has been used in patients with type II hyperlipoproteinemia (HLP). Neomycin is as effective as cholestyramine in reducing LDL levels and combination neomycin-niacin treatment normalizes the plasma lipoproteins in 92% of patients with type II HLP. Because neomycin could theoretically ameliorate some of the gastrointestinal side effects of cholestyramine in addition to further affecting cholesterol levels, the effects of combination cholestyramine-neomycin treatment on the plasma lipoprotein were assessed in 18 patients with type II HLP in a 9-month clinical trial. Compared with diet-only treatment, cholestyramine reduced total and LDL cholesterol levels by 77 mg/dl (22%) and 78 mg/dl (31%), respectively. In addition to relieving cholestyramine-induced constipation, neomycin further reduced the total cholesterol level by 20 mg/dl (6%). However, this further reduction in total cholesterol concentration was the result of a decrease in the concentration of high-density lipoprotein cholesterol. These findings indicate that combination therapy does not have an additive LDL cholesterol-lowering effect and that neomycin and cholestyramine is not a useful drug combination. In addition, these results illustrate the importance of determining the high-density lipoprotein cholesterol concentration to fully interpret the effects of hypolipidemic treatment.

Cholesterol, HDL↗
PubMed1985

Distinct hepatic receptors for low density lipoprotein and apolipoprotein E in humans.

Since the liver is a central organ for lipid and lipoprotein synthesis and catabolism, hepatic receptors for specific apolipoproteins on plasma lipoproteins would be expected to modulate lipid and lipoprotein metabolism. The role of hepatic receptors for low density lipoproteins and apolipoprotein E-containing lipoproteins was evaluated in patients with complementary disorders in lipoprotein metabolism: abetalipoproteinemia and homozygous familial hypercholesterolemia. In addition, hepatic membranes from a patient with familial hypercholesterolemia were studied and compared before and after portacaval shunt surgery. The results establish that the human liver has receptors for apolipoproteins B and E. Furthermore, in the human, hepatic receptors for low density lipoproteins and apolipoprotein E are genetically distinct and can undergo independent control.

Abetalipoproteinemia↗
PubMed1985

Isoforms of apolipoprotein A-II in human plasma and thoracic duct lymph. Identification of proapolipoprotein A-II and sialic acid-containing isoforms.

Proapolipoprotein (apo-) A-II and several isoforms of apo-A-II including sialylated isoforms were identified in human plasma and thoracic duct lymph. Proapo-A-II secreted by HepG2 cells was identified by a combination of immunoblots and [14C]arginine incorporation. Proapo-A-II which contains 2 arginine residues could be readily differentiated from mature apo-A-II which contains no arginine. The pI of proapo-A-II is 6.79, whereas the pI of the major apo-A-II isoform in plasma and lymph is 4.90. Minor apo-A-II isoforms have pI values of 5.17, 4.68, 4.42, and 4.20, respectively. Sialoisoforms of apo-A-II were identified, which had a higher apparent molecular weight on sodium dodecyl sulfate-gel electrophoresis than the major isoform and disappeared following neuraminidase treatment. The relative quantity of proapo-A-II was relatively constant in lymph very low density lipoproteins, lymph high density lipoproteins, and plasma high density lipoproteins, whereas the sialoforms and the other minor isoforms of apo-A-II were greater in lymph very low density lipoproteins and the lowest in plasma high density lipoproteins.

Apolipoprotein A-II↗
PubMed1985

In vivo alteration of a mutant human protein using the free thiol cysteamine.

Inborn errors of metabolism in which there is a mutant protein due to a cysteine for arginine substitution may be amenable to treatment with the free thiol cysteamine. Evidence for this derives from patients with type III hyperlipoproteinemia, who are homozygous for apolipoprotein E2, which differs in charge and in vitro function based on a single such amino acid substitution. The plasma of a type III hyperlipoproteinemic patient, when made at least 50 microM with respect to cysteamine in vitro, demonstrated a charge shift of the apolipoprotein E isoelectric focusing pattern from the E2 to the normal E3 and E4 positions. Two children treated for cystinosis with cysteamine each exhibited some charge alteration of their apoE3 to a form migrating in the apoE4 position. The use of thiol reagents such as cysteamine to specifically alter selected mutant human proteins, such as antithrombin III Toyama, may be added to our therapeutic armamentarium in the treatment of life-threatening metabolic disorders.

Apolipoprotein E2↗