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Biomedical subjects

H H Hobbs

Publications and source records attributed to H H Hobbs.

At least 37 records · Page 2Linked to original sources
PubMed1997

Structure of the human gene encoding sterol regulatory element binding protein 2 (SREBF2).

Sterol-regulatory element binding protein (SREBP) 1 and SREBP2 are ubiquitously expressed transcription factors that play key roles in the regulation of cholesterol and fatty acid metabolism. SREBP1 and SREBP2 share approximately 47% sequence identity and map to chromosomes 17 and 22, respectively. The gene encoding SREBP1 (SREBF1) has been cloned and characterized. In this paper we describe the gene structure and 5'-flanking sequence of SREBF2. SREBF2 spans 72 kb and is composed of 19 exons and 18 introns. The locations of the exon/intron boundaries of SREBF2 are remarkably similar to those of SREBF1, but SREBF2 is approximately 2.8 times larger in size. The 5'-flanking regions of SREBF2 and of two alternatively spliced forms of SREBF1, SREBF1a and SREBF1c, were sequenced, and the SREBF2 transcription start site was determined. A perfect 10-bp sterol regulatory element (SRE)-1 sequence was present in the promoter region of SREBF2. No SRE-1 was identified in the 5'-flanking sequences of either SREBF1a or SREBF1c, but several E-box sequences were present in SREBP1c. Thus, analysis of the 5'-flanking regions provides support that these two transcription factors, though similar in their coding sequence and overall gene structure, have different physiological roles. Finally, evidence is presented for the presence of another expressed gene of unknown function located 500 bp upstream of SREBF2.

Amino Acid Sequence↗
PubMed1997

The Apo(a) gene is the major determinant of variation in plasma Lp(a) levels in African Americans.

The distributions of plasma lipoprotein(a), or Lp(a), levels differ significantly among ethnic groups. Individuals of African descent have a two- to threefold higher mean plasma level of Lp(a) than either Caucasians or Orientals. In Caucasians, variation in the plasma Lp(a) levels has been shown to be largely determined by sequence differences at the apo(a) locus, but little is known about either the genetic architecture of plasma Lp(a) levels in Africans or why they have higher levels of plasma Lp(a). In this paper we analyze the plasma Lp(a) levels of 257 sibling pairs from 49 independent African American families. The plasma Lp(a) levels were much more similar in the sibling pairs who inherited both apo(a) alleles identical by descent (IBD) (r = .85) than in those that shared one (r = .48) or no (r = .22) parental apo(a) alleles in common. On the basis of these findings, it was estimated that 78% of the variation in plasma Lp(a) levels in African Americans is attributable to polymorphism at either the apo(a) locus or sequences closely linked to it. Thus, the apo(a) locus is the major determinant of variation in plasma Lp(a) levels in African Americans, as well as in Caucasians. No molecular evidence was found for a common "high-expressing" apo(a) allele in the African Americans. We propose that the higher plasma levels of Lp(a) in Africans are likely due to a yet-to-be-identified trans-acting factor(s) that causes an increase in the rate of secretion of apo(a) or a decrease in its catabolism.

Adolescent↗
PubMed1997

High plasma levels of apo(a) fragments in Caucasians and African-Americans with end-stage renal disease: implications for plasma Lp(a) assay.

Apolipoprotein(a) [apo(a)] is a plasma glycoprotein that is highly polymorphic in size due to differences in the number of a tandemly arrayed cysteine-rich repeat called kringle (K)4 at its N-terminus. Most plasma apo(a) is covalently attached to apolipoprotein B-100 and circulates as part of lipoprotein(a) [Lp(a)]. A fraction of apo(a) circulates free of lipoproteins. Almost all of the free apo(a) consists of fragments containing variable numbers of K4 repeats derived from the N-terminal region. Previously we provided evidence suggesting that the apo(a) fragments present in human plasma are the source of the apo(a) fragments in human urine. If this were the case, it would be expected that plasma levels of fragments would be higher in subjects with end-stage renal disease (ESRD). In this paper we quantified the levels of apo(a) fragments and plasma Lp(a) in 26 Caucasian and 26 African-American subjects with ESRD and 52 healthy subjects matched for race, sex and the size of the apo(a) isoforms. The plasma levels of apo(a) fragments and Lp(a) were both higher in the ESRD subjects. In addition, the ratio of apo(a) fragments to total immunodetectable apo(a) was increased in ESRD. To determine how much the increase in the apo(a) fragments contributed to the increase in plasma Lp(a) in ESRD, the plasma Lp(a) levels were measured employing two different anti-apo(a) enzyme-linked immunoabsorption assays (ELISA). One assay detected both free and bound apo(a), whereas the other assay detected only bound apo(a). Although the plasma levels of apo(a) in the ESRD subjects tended to be higher using the assay that detected both fragments and full-length apo(a), the increase was modest. Thus, although a greater proportion of the apo(a) in ESRD plasma circulates as fragments, most of the elevation in plasma levels of Lp(a) associated with renal insufficiency is due to an increase in intact Lp(a).

Adolescent↗
PubMed1996

Kringle-containing fragments of apolipoprotein(a) circulate in human plasma and are excreted into the urine.

Apolipoprotein(a) [apo(a)] contains multiple kringle 4 repeats and circulates as part of lipoprotein(a) [Lp(a)]. Apo(a) is synthesized by the liver but its clearance mechanism is unknown. Previously, we showed that kringle 4-containing fragments of apo(a) are present in human urine. To probe their origin, human plasma was examined and a series of apo(a) immunoreactive peptides larger in size than urinary fragments was identified. The concentration of apo(a) fragments in plasma was directly related to the plasma level of Lp(a) and the 24-h urinary excretion of apo(a). Individuals with low (< 2 mg/dl) plasma levels of Lp(a) had proportionally more apo(a) circulating as fragments in their plasma. Similar apo(a) fragments were identified in baboon plasma but not in conditioned media from primary cultures of baboon hepatocytes, suggesting that the apo(a) fragments are generated from circulating apo(a) or Lp(a). When apo(a) fragments purified from human plasma were injected intravenously into mice, a species that does not produce apo(a), apo(a) fragments similar to those found in human urine were readily detected in mouse urine. Thus, we propose that apo(a) fragments in human plasma are derived from circulating apo(a)/Lp(a) and are the source of urinary apo(a).

Animals↗
PubMed1996

Regulation of scavenger receptor, class B, type I, a high density lipoprotein receptor, in liver and steroidogenic tissues of the rat.

The scavenger receptor, class B, type I (SR-BI) binds HDL and mediates the selective transfer of cholesteryl esters from HDL to cultured cells. The tissue distribution of SR-BI in mice suggests that this receptor may deliver HDL-cholesterol to the liver and to nonplacental steroidogenic tissues. To examine the role of SR-BI in vivo, we determined its tissue and cell type-specific expression pattern and regulation in rats. High levels of immunodetectable SR-BI were present in the adrenal gland, ovary, and liver. In pregnant animals, the mammary gland also expressed high levels of the protein. SR-BI was localized by immunofluorescence to the surfaces of steroidogenic cells in the zona fasciculata and zona reticularis of the adrenal gland and to the corpus luteal cells of the ovary. High-dose estrogen treatment dramatically reduced SR-BI in the liver and increased SR-BI in the adrenal gland and corpus luteal cells of the ovary. These estrogen-induced increases in SR-BI in the adrenal gland and ovary were accompanied by enhanced in vivo uptake of fluorescent lipid from HDL. The administration of human chorionic gonadotropin induced a dramatic increase in SR-BI in the steroidogenic Leydig cells of the testes. These findings suggest that SR-BI mediates physiologically relevant uptake of cholesterol from HDL to nonplacental steroidogenic tissues in vivo.

Adrenal Glands↗
PubMed1996

Apolipoprotein(a) kringle 4-containing fragments in human urine. Relationship to plasma levels of lipoprotein(a).

Apo(a) is a large glycoprotein of unknown function that circulates in plasma as part of lipoprotein(a). Apo(a) is structurally related to plasminogen and contains at least 10 kringle (K)4 repeats (type 1-10), a K5 repeat and sequences similar to the protease domain of plasminogen. Plasminogen generates two biologically active peptides: plasmin and angiostatin, a kringle-containing peptide. As a first step in determining if apo(a) generates a similar kringle-containing peptide, human urine was immunologically examined. Fragments ranging in size from 85 to 215 kD were immunodetected using antibodies directed against epitopes in the K4-type 2 repeat, but not the K4-type 9 repeat or protease domain, NH2-terminal sequence analysis revealed sequences specific for the K4-type 1 repeat, confirming that the fragments are from the NH2 terminus of the K4 array. The amount of urinary apo(a) rose in proportion to the plasma lipoprotein(a) concentration. Even individuals with trace to no apo(a) in plasma had immunodetectable apo(a) fragments in their urine. Intravenous administration of the human urinary apo(a) into mice resulted in the urine. These findings suggest that the apo(a) fragments found in urine are formed extrarenally and then excreted by the kidney.

Amino Acid Sequence↗
PubMed1996

Identification of scavenger receptor SR-BI as a high density lipoprotein receptor.

High density lipoprotein (HDL) and low density lipoprotein (LDL) are cholesterol transport particles whose plasma concentrations are directly (LDL) and inversely (HDL) correlated with risk for atherosclerosis. LDL catabolism involves cellular uptake and degradation of the entire particle by a well-characterized receptor. HDL, in contrast, selectively delivers its cholesterol, but not protein, to cells by unknown receptors. Here it is shown that the class B scavenger receptor SR-BI is an HDL receptor. SR-BI binds HDL with high affinity, is expressed primarily in liver and nonplacental steroidogenic tissues, and mediates selective cholesterol uptake by a mechanism distinct from the classic LDL receptor pathway.

Adrenal Glands↗
PubMed1996

Genetic factors that contribute to interindividual variations in plasma low density lipoprotein-cholesterol levels.

The interplay of multiple genes and environmental factors generates interindividual variation in plasma low density lipoprotein-cholesterol (LDL-C) concentrations. As a result, it has been difficult to identify individual genes that contribute to variation in plasma LDL-C levels using classical linkage analysis. We have exploited a genetic defect in the gene encoding the LDL receptor that is associated with a dramatically elevated plasma LDL-C level to unmask an allele at another locus that lowers plasma LDL-C levels. The existence of such an allele was implied by the analysis of a human pedigree with familial hypercholesterolaemia in which a third of the familial hypercholesterolaemia heterozygotes had normal levels of LDL-C. To develop an animal model of this LDL-C lowering effect and to identify genes that modify the plasma LDL-C level, we crossed LDL receptor-deficient mice with other strains of mice.

Alleles↗
PubMed1996

Expression of the VLDL receptor in endothelial cells.

In this article we describe the cellular distribution of the very low density lipoprotein receptor (VLDLR), a transmembrane protein that is expressed at high concentrations in skeletal muscle, heart, adipose tissue, and brain but in only trace amounts in the liver. Indirect immunofluorescence localization studies were performed in murine and bovine tissues using a rabbit polyclonal anti-human VLDLR antibody. Immunoreactive VLDLR protein was detected in the endothelium of capillaries and small arterioles but not in veins or venules of bovine skeletal muscle, heart, ovary, and brain. In the liver, there was intense staining of the capillaries and arterioles that supply the capsule and hepatic vessels but no staining of the sinusoidal surfaces. We failed to detect any signal from nonendothelial cells in the liver or peripheral organs. The VLDLR was also expressed at high levels on the endothelial surface of bovine coronary arteries; in contrast, little or no staining was seen in aortic endothelium. Antibody staining of cultured bovine coronary artery endothelial cells demonstrated punctate cell-surface staining, as well as staining of large and small cytoplasmic vesicles. This tissue and cell pattern of expression suggests that the VLDLR plays a role in the transport of VLDL or another plasma constituent from the vascular compartment to adjacent tissues.

Animals↗
PubMed1996

Relation between number of apolipoprotein(a) kringle 4 repeats and mobility of isoforms in agarose gel: basis for a standardized isoform nomenclature.

The size of apolipoprotein(a) [apo(a)] varies from 187 to 648 kDa because of interallelic differences in the number of kringle 4-encoding sequences in the apo(a) gene. Plasma apo(a) isoform sizes were determined by a high-resolution sodium dodecyl sulfate-agarose gel electrophoretic method, followed by immunoblotting. The relation between the number of apo(a) kringle 4-encoding sequences in the apo(a) gene, as assessed by pulsed-field gel electrophoresis and genomic blotting, and the mobility of apo(a) isoforms in agarose gel was evaluated in 29 individuals who had 48 expressed apo(a) isoforms that contained between 12 and 41 kringle 4 repeats. The log of the kringle number was linearly related to the distance the isoform migrated on the agarose gel (r= 0.999). A nomenclature is proposed to designate apo(a) isoforms by the respective number of kringle 4 repeats.

Alleles↗
PubMed1995

Structure of the human gene encoding sterol regulatory element binding protein-1 (SREBF1) and localization of SREBF1 and SREBF2 to chromosomes 17p11.2 and 22q13.

Sterol regulatory element binding protein-1 (SREBP1) and SREBP2 are structurally related proteins that control cholesterol homeostasis by stimulating transcription of sterol-regulated genes, including those encoding the low-density lipoprotein (LDL) receptor and 3-hydroxy-3-methylglutaryl CoA synthase. SREBP1 and SREBP2 are 47% identical, and they share a novel structure comprising a transcriptionally active NH2-terminal basic helix-loop-helix-leucine zipper (bHLH-Zip) domain followed by a membrane attachment domain. Cleavage by a sterol-regulated protease frees the bHLH-Zip domain from the membrane and allows it to enter the nucleus. SREBP1 exists in several forms, possibly as a result of alternative splicing at both the 5' and the 3' ends of the mRNA. The genes for SREBP1 (SREBF1) and SREBP2 (SREBF2) have not been studied. In this paper we describe the cloning and characterization of the human SREBF1 gene. The gene is 26 kb in length and has 22 exons and 20 introns. The 5' and 3' sequences that differ between the two SREBP1 cDNAs are encoded by discrete exons, confirming the hypothesis that they result from alternative splicing. The chromosomal locations of human SREBF1 and SREBF2 were determined by analysis of human-rodent somatic cell hybrids and fluorescence in situ hybridization. The SREBF1 gene mapped to the proximal short arm of chromosome 17 (17p11.2), and the SREBF2 gene was localized to the long arm of chromosome 22 (22q13).

Alternative Splicing↗
PubMed1995

Sequence polymorphisms in the apo(a) gene associated with specific levels of Lp(a) in plasma.

Most of the interindividual variations in plasma levels of lipoprotein(a) [Lp(a)] can be attributed to sequence differences linked to the apolipoprotein(a) [apo(a)] locus. Plasma levels of Lp(a) tend to be inversely related to the number of kringle 4 (K4)-encoding sequences in the apo(a) gene, but there are several exceptions to this general trend. Other aspects of the apo(a) gene, in addition to the number of K4 repeats, affect plasma levels of Lp(a). To identify sequences in the apo(a) gene that contribute to plasma Lp(a) levels, we characterized the relationship between a length polymorphism [(TTTTA)n] located 1.3 kb 5' of the first exon of the apo(a) gene, the number of K4 repeats in the gene, and the plasma levels of Lp(a). There was significant linkage disequilibrium between the number of TTTTA repeats and the number of K4 repeats. All of the apo(a) alleles with 11 TTTTA repeats contained fewer than 24 K4 repeats and were paradoxically associated with low plasma Lp(a) levels (< or = mg/dl). To determine whether this association was due to the effect of the 11 TTTTA copies on apo(a) gene transcription, we measured the ability of fragments containing 11 or eight TTTTA repeats to promote transcription when introduced into cultured human hepatocarcinoma cells. No difference was found in the transcriptional activity of the two fragments. The TTTTA repeat constitutes the first sequence polymorphism at the apo(a) locus, other than the number of K4 repeats, which is associated with plasma concentrations of Lp(a).

Alleles↗
PubMed1995

Sequence microheterogeneity in apolipoprotein(a) gene repeats and the relationship to plasma Lp(a) levels.

Lipoprotein(a) [Lp(a)] is a cholesterol ester-rich atherogenic lipoprotein that is composed of a particle of low density lipoprotein and a large glycoprotein, apolipoprotein(a) [apo(a)]. Apolipoprotein(a) varies in size over a approximately 500 kDa range due to inter-allelic differences in the number of tandemly repeated kringle 4 (K4)-encoding 5.5 kb sequences in the apo(a) gene. Only one of the 10 different types of K4 repeats in the apo(a) gene, the so-called type 2 K4 repeats, vary in number between apo(a) alleles. In this paper, we show that there is microheterogeneity within the sequence of the type 2 K4 repeat. DraIII restriction digestion and genomic blotting revealed that a subset of the type 2 K4-encoding sequences contained a DraIII site (K4-D). The proportion of apo(a) alleles that had at least one K4-D repeat ranged from 25% in Caucasians to 50% in the Chinese. K4-D repeats were clustered at the end(s) of the type 2 K4 tandem array and the number and patterns of the K4-D repeats were in linkage disequilibrium with flanking sequence polymorphisms; these features are remarkably similar to the minisatellite variant repeats (MVRs) found in variable number of tandem repeat sequences (VNTRs). In addition, a DraIII pattern that comprised 9% of the sample was invariably associated with low plasma levels of Lp(a) in Caucasians.

Apolipoproteins↗
PubMed1995

Relative contributions of apolipoprotein(a) and apolipoprotein-B to the development of fatty lesions in the proximal aorta of mice.

Transgenic mice expressing transgenes for both human apolipoprotein B-100 (h-apoB) and apolipoprotein(a) [apo(a)] were fed a high-fat, atherogenic diet for 14 weeks to examine the effect of lipoprotein(a) [Lp(a)]on the development of aortic fatty lesions. The extent of lesions in the proximal region of the aorta of Lp(a) mice was measured by use of a computer-assisted image analysis of 20 sections per animal and compared with that of nontransgenic mice as well as mice expressing either the apo(a) or h-apoB transgene. The control (n = 23) and apo(a) (n = 22) transgenic mice had very small mean lesions areas (607 versus 128 microns2 per section). The h-apoB-expressing mice (n = 20) had significantly higher mean lesion areas (3288 microns2 per section) than either the control or apo(a) transgenic animals. Coexpression of apo(a) and h-apoB transgenes resulted in only a modest increase in lesion area (4678 microns2 per section, n = 19). Thus, the expression of human apo(a) in C57BL/6/SJL hybrid mice fed an atherogenic diet failed to significantly potentiate the development of aortic fatty lesions in the absence or presence of high levels of h-apoB.

Animals↗
PubMed1995

No association between plasma lipoprotein(a) concentrations and the presence or absence of coronary atherosclerosis in African-Americans.

Elevated plasma concentrations of lipoprotein(a) [Lp(a)] are associated with coronary atherosclerosis in Caucasians. Although African-Americans have a higher median plasma Lp(a) concentration than Caucasians, they do not have a greater incidence of coronary atherosclerosis. This study was performed to determine whether the plasma concentration of Lp(a) is associated with coronary atherosclerosis in African-Americans. The fasting plasma concentrations of Lp(a) and lipoproteins were measured in 140 African-American subjects (62 men, 78 women, aged 31 to 80 years) 18 +/- 16 months (mean +/- SD) after they underwent coronary angiography: 72 had angiographically normal coronary arteries and 68 had > 70% luminal diameter narrowing of one or more major epicardial coronary arteries. The groups were similar in age, sex, and other risk factors for atherosclerosis. The subjects with coronary artery disease had higher plasma concentrations of total cholesterol, triglycerides, and VLDL and LDL cholesterol (P = .04) and lower concentrations of HDL cholesterol (P = .0001) than subjects without coronary artery disease, but there was no significant difference in the plasma concentration of Lp(a). The distribution of apolipoprotein(a) alleles by size was also not significantly different between the two groups. These results suggest that the plasma concentration of Lp(a) is not an independent risk factor for coronary artery disease in African-Americans.

Adult↗
PubMed1994

Regulation of the very low density lipoprotein receptor by thyroid hormone in rat skeletal muscle.

A new member of the low density lipoprotein receptor gene family that binds and internalizes very low density lipoprotein (VLDL) particles was previously cloned and characterized from the rabbit and human. The physiological role of this putative VLDL receptor is not known, but its tissue distribution and ligand specificity suggest a possible role in the delivery of triglycerides to peripheral tissue. To learn more about the potential function of this receptor, we measured the changes in VLDL receptor mRNA and protein in various tissues following dietary or hormonal manipulation of rats. No significant changes in the VLDL receptor mRNA or protein were seen after a 48-h fast and subsequent to refeeding. A striking change in receptor mRNA and protein was observed in skeletal muscle of hypothyroid and hyperthyroid rats. In hypothyroid rats, the amount of immunodetectable VLDL receptor was reduced by 80%, while in the hyperthyroid animals it was increased by 300%. These maneuvers did not affect VLDL receptor mRNA or protein levels in adipose tissue or heart. The changes in VLDL receptor mRNA in muscle were opposite to those observed with lipoprotein lipase. These studies suggest that the VLDL receptor plays a role in a metabolic process in muscle that is regulated by thyroid hormone.

Amino Acid Sequence↗