Biomedical subjects

M C de Beer

Publications and source records attributed to M C de Beer.

At least 19 recordsLinked to original sources

PubMed2003

Anticipating rotavirus vaccines: epidemiology and surveillance of rotavirus in South Africa.

Rotavirus infection is associated with acute infantile gastroenteritis in infants and young children globally. In South Africa, rotavirus infection has been shown to be associated with approximately one-quarter of all diarrhoeal admissions to hospital. Rotavirus infection predominantly occurs in infants less than 12 months of age (75%) and has a peak of shedding during the cooler, drier months of the year. A secondary peak during the spring has been observed. Multiple infections with rotavirus and at least one other microbial agent are common. The circulating VP7 serotypes and VP4 genotypes have been determined in various regions of South Africa and show a geographic specific distribution. A decade previously, P[8]G1 or G4 strains predominated, and P[4]G2 strains occurred in an epidemic pattern in one region. More recently, rotavirus strains with P[6] genotype have become common and novel VP7/VP4 genotype combinations are occurring across the country. G9 strains have been reported from Cape Town to Vendaland. The circulating rotavirus types observed in this study add to the knowledge of the natural history of rotavirus infection and provide the groundwork to consider future vaccine strategies.

Child, Preschool↗

PubMed2002

Epidemiology of astrovirus infection in Zaria, Nigeria.

Astrovirus has been shown to be an important aetiological agent associated with gastroenteritis in children, although few studies have been conducted in Africa. In this study, stool specimens were obtained from 375 young children less than 5 years of age with acute gastroenteritis presenting at Ahmadu Bello University Hospital, and from a control group of 122 children without diarrhoeal illness. The specimens were examined for the presence of human astroviruses using a monoclonal antibody-based ELISA (Astrovirus IDEIATM, Dako, UK). Negative staining electron microscopy was performed on specimens to confirm the presence of astrovirus particles. Astrovirus was detected in 6.7 per cent (25/375) of the diarrhoeal stools compared to 5.7 per cent (7/122) of the control specimens. Astrovirus seemed to infect older children and more than half the children were between 1 and 4 years of age (15/25). Only four children were less than 6 months old. A winter peak of shedding was observed.

Astroviridae Infections↗

PubMed2001

Apolipoprotein A-II modulates the binding and selective lipid uptake of reconstituted high density lipoprotein by scavenger receptor BI.

High density lipoprotein (HDL) represents a mixture of particles containing either apoA-I and apoA-II (LpA-I/A-II) or apoA-I without apoA-II (LpA-I). Differences in the function and metabolism of LpA-I and LpA-I/A-II have been reported, and studies in transgenic mice have suggested that apoA-II is pro-atherogenic in contrast to anti-atherogenic apoA-I. The molecular basis for these observations is unclear. The scavenger receptor BI (SR-BI) is an HDL receptor that plays a key role in HDL metabolism. In this study we investigated the abilities of apoA-I and apoA-II to mediate SR-BI-specific binding and selective uptake of cholesterol ester using reconstituted HDLs (rHDLs) that were homogeneous in size and apolipoprotein content. Particles were labeled in the protein (with (125)I) and in the lipid (with [(3)H]cholesterol ether) components and SR-BI-specific events were analyzed in SR-BI-transfected Chinese hamster ovary cells. At 1 microg/ml apolipoprotein, SR-BI-mediated cell association of palmitoyloleoylphosphatidylcholine-containing AI-rHDL was significantly greater (3-fold) than that of AI/AII-rHDL, with a lower K(d) and a higher B(max) for AI-rHDL as compared with AI/AII-rHDL. Unexpectedly, selective cholesterol ester uptake from AI/AII-rHDL was not compromised compared with AI-rHDL, despite decreased binding. The efficiency of selective cholesterol ester uptake in terms of SR-BI-associated rHDL was 4-5-fold greater for AI/AII-rHDL than AI-rHDL. These results are consistent with a two-step mechanism in which SR-BI binds ligand and then mediates selective cholesterol ester uptake with an efficiency dependent on the composition of the ligand. ApoA-II decreases binding but increases selective uptake. These findings show that apoA-II can exert a significant influence on selective cholesterol ester uptake by SR-BI and may consequently influence the metabolism and function of HDL, as well as the pathway of reverse cholesterol transport.

Animals↗

PubMed2001

Apolipoprotein A-I conformation markedly influences HDL interaction with scavenger receptor BI.

Apolipoprotein A-I (apoA-I) is an important ligand for the high density lipoprotein (HDL) scavenger receptor class B type I (SR-BI). SR-BI binds both free and lipoprotein-associated apoA-I, but the effects of particle size, composition, and apolipoprotein conformation on HDL binding to SR-BI are not understood. We have studied the effect of apoA-I conformation on particle binding using native HDL and reconstituted HDL particles of defined composition and size. SR-BI expressed in transfected Chinese hamster ovary cells was shown to bind human HDL(2) with greater affinity than HDL(3), suggesting that HDL size, composition, and possibly apolipoprotein conformation influence HDL binding to SR-BI. To discriminate between these factors, SR-BI binding was studied further using reconstituted l-alpha-palmitoyloleoyl-phosphatidylcholine-containing HDL particles having identical components and equal amounts of apoA-I, but differing in size (7.8 vs. 9.6 nm in diameter) and apoA-I conformation. The affinity of binding to SR-BI was significantly greater (50-fold) for the larger (9.6-nm) particle than for the 7.8-nm particle. We conclude that differences in apoA-I conformation in different-sized particles markedly influence apoA-I recognition by SR-BI. Preferential binding of larger HDL particles to SR-BI would promote productive selective cholesteryl ester uptake from larger cholesteryl ester-rich HDL over lipid-poor HDL.

Apolipoprotein A-I↗

PubMed2001

CD36 does not play a direct role in HDL or LDL metabolism.

CD36 and scavenger receptor class B, type I (SR-BI) are both class B scavenger receptors that recognize a broad variety of ligands, including oxidized low density lipoprotein (oxLDL), HDL, anionic phospholipids, and apoptotic cells. In this study we investigated the role of mouse CD36 (mCD36) as a physiological lipoprotein receptor. We compared the association of various lipoprotein particles with mCD36 and mSR-BI expressed in COS cells by adenovirus-mediated gene transfer. mCD36 bound human oxLDL and mouse HDL with high affinity. Human LDL bound poorly to mCD36, indicating that mCD36 is unlikely to play a significant role in LDL metabolism. The ability of mCD36 to mediate the selective uptake of cholesteryl esters (CE) from receptor-bound HDL was assessed. In comparison with mSR-BI, mCD36 inefficiently mediated the selective uptake of CE. Hepatic overexpression of mCD36 in C57BL/6 mice by adenovirus-mediated gene transfer did not result in significant alterations in plasma LDL and HDL levels. We conclude that mCD36, while able to bind HDL with high affinity, does not contribute significantly to HDL or LDL metabolism.

Adenoviridae↗

PubMed2000

Expression of mouse acute-phase (SAA1.1) and constitutive (SAA4) serum amyloid A isotypes: influence on lipoprotein profiles.

The serum amyloid A (SAA) family of proteins consists of inducible acute-phase members and a constitutive member that are minor apolipoproteins of normal high density lipoprotein (HDL). During inflammation, HDL cholesterol and apolipoprotein A-I (apoA-I) protein are decreased, and these changes are thought to be partly related to the increase in acute-phase SAA proteins that associate with the HDL particle to become the major apolipoprotein species. To determine the specific role of SAA in the alteration of HDL in the absence of a generalized acute-phase response, acute-phase Saa1.1 transgene expression was directed via an inducible mouse metallothionein promoter. Elevated levels of SAA1.1 (28+/-9 mg/dL) comparable to a moderate acute-phase response were achieved over a 5-day period. SAA association with the HDL particles at this concentration did not significantly alter the apoA-I or HDL cholesterol levels or change the lipoprotein profiles in the transgenic mice compared with wild-type mice. In addition, we used adenoviral vectors to increase the SAA expression to levels seen in a major acute-phase response. Injection of adenovirus expressing the mouse SAA1.1 protein resulted in high-level expression (72+/-8 mg/dL) but did not alter apoA-I levels. However, the SAA associated with the HDL particle gave rise to significantly larger HDL particles ( approximately 10%). Adenoviral expression of the constitutive SAA4 protein resulted in an increase in HDL size ( approximately 10%) and an increase in very low density lipoprotein levels (20-fold) and triglyceride levels (1.7-fold). These studies suggest that increases in acute-phase SAA proteins alone are insufficient to alter HDL cholesterol or apoA-I levels during inflammation. A role for constitutive SAA4 in HDL-very low density lipoprotein interactions should be considered.

Acute-Phase Reaction↗

PubMed2000

Comparative analysis of lipid composition of normal and acute-phase high density lipoproteins.

In the acute-phase response and in diseases with prolonged acute phases, normal HDL (NHDL) is converted into acute-phase HDL (APHDL) and becomes proinflammatory and unable to protect LDL against oxidative modification. Earlier work had demonstrated that these changes are associated with alterations in apolipoprotein composition and enzymatic activity of APHDL, but the effect of the acute-phase condition on the lipid composition of APHDL had remained obscure. The present study shows marked quantitative differences in lipid composition between NHDL and APHDL. Specifically, APHDL contained 25% less total lipid per milligram of protein. Up to 50% of cholesteryl ester in the lipid core of APHDL was replaced by triacylglycerol; however, the total phospholipid/total neutral lipid ratios were the same as in NHDL, both lipoproteins giving similar calculated lipid core radii. Furthermore, the phosphatidylcholine/sphingomyelin ratio in APHDL was nearly double that in NHDL, indicating a relative loss of sphingomyelin. A decrease was also seen in diacyl and alkenylacyl glycerophosphatidylethanolamine as well as in phosphatidylinositol of APHDL when compared with NHDL. APHDL contained proportionally more saturated and less polyunsaturated and isoprostane-containing species of phosphatidylcholine, as well as more saturated than unsaturated cholesteryl esters. APHDL also contained significantly more free fatty acids, lysophosphatidylcholine, and free cholesterol. These changes in the lipid composition of HDL are consistent with the alterations in the apoprotein composition and enzymatic activity of APHDL and indicate proinflammatory and proatherogenic roles for APHDL.

Acute-Phase Proteins↗

PubMed2000

HDL modification by secretory phospholipase A(2) promotes scavenger receptor class B type I interaction and accelerates HDL catabolism.

During inflammatory states plasma levels of high density lipoprotein (HDL) cholesterol and apolipoprotein A-I (apoA-I) are reduced. Secretory group IIa phospholipase A(2) (sPLA(2)) is a cytokine-induced acute-phase enzyme associated with HDL. Transgenic mice overexpressing sPLA(2) have reduced HDL levels. Studies were performed to define the mechanism for the HDL reduction in these mice. HDL isolated from sPLA(2) transgenic mice have a significantly lower phospholipid content and greater triglyceride content. In autologous clearance studies, (125)I-labeled HDL from sPLA(2) transgenic mice was catabolized significantly faster than HDL from control mice (4.24 +/- 1.16 vs. 2.84 +/- 0.1 pools per day, P < 0.008). In both sPLA(2) transgenic and control mice, the cholesteryl ester component of HDL was more rapidly catabolized than the protein component, indicating a selective uptake mechanism. In vitro studies using CHO cells transfected with scavenger receptor class B type I (SR-BI) showed that sPLA(2)-modified HDL was nearly twice as efficient as a substrate for cholesteryl ester transfer. These data were confirmed in in vivo selective uptake experiments using adenoviral vector overexpression of SR-BI. In these studies, increased hepatic selective uptake was associated with increased (125)I-labeled apolipoprotein uptake in the kidney. We conclude that during inflammation sPLA(2) hydrolysis of HDL phospholipids alters the lipid composition of the particle, allowing for more efficient SR-BI-mediated selective cholesteryl ester uptake. This enhanced SR-BI activity generates HDL remnants that are preferentially catabolized in the kidney.

Animals↗

PubMed1999

Role of group II secretory phospholipase A2 in atherosclerosis: 1. Increased atherogenesis and altered lipoproteins in transgenic mice expressing group IIa phospholipase A2.

Some observations have suggested that the extracellular group IIa phospholipase A2 (sPLA2), previously implicated in chronic inflammatory conditions such as arthritis, may contribute to atherosclerosis. We have examined this hypothesis by studying transgenic mice expressing the human enzyme. Compared with nontransgenic littermates, the transgenic mice exhibited dramatically increased atherosclerotic lesions when maintained on a high-fat, high-cholesterol diet. Surprisingly, the transgenic mice also exhibited significant atherosclerotic lesions when maintained on a low-fat chow diet. Immunohistochemical staining indicated that sPLA2 was present in the atherosclerotic lesions of the transgenic mice. On both chow and atherogenic diets, the transgenic mice exhibited decreased levels of HDLs and slightly increased levels of LDLs compared with nontransgenic littermates. These data indicate that group IIa sPLA2 may promote atherogenesis, in part, through its effects on lipoprotein levels. These data also provide a possible mechanism for the observation that there is an increased incidence of coronary artery disease in many chronic inflammatory diseases.

Animals↗

PubMed1998

Structure, organization, and chromosomal mapping of the gene encoding macrosialin, a macrophage-restricted protein.

Murine macrosialin and its human homologue CD68 are heavily glycosylated transmembrane proteins expressed specifically in macrophages and macrophage-related cells. Macrosialin is predominantly a late endosomal protein but is also found on the cell surface where it binds oxidized LDL, an important factor in atherogenesis. We have cloned and sequenced the murine macrosialin gene (Cd68) and localized it by linkage analysis to chromosome 11. The gene is 1908 nucleotides long from the start site of transcription to the end of the 3'UTR. It has six exons, which range in size from 79 to 434 nucleotides. The promoter lacks a classical TATA box but contains other protein binding sites consistent with preferential monocyte/macrophage gene expression. Although the function of macrosialin is unknown, it might play a role in lipoprotein regulation given its binding of oxidized LDL in vitro and its colocalization to a region on chromosome 11 involved in the control of HDL levels.

Animals↗

PubMed1998

Lipoproteins are substrates for human secretory group IIA phospholipase A2: preferential hydrolysis of acute phase HDL.

Group IIA secretory phospholipase A2 is an acute phase enzyme, co-expressed with serum amyloid A protein. Both are present in atherosclerotic lesions. We report that human normal and acute phase high density lipoproteins and low density lipoprotein are effective substrates for human group IIA phospholipase A2. The enzyme hydrolyzed choline and ethanolamine glycerophospholipids at the sn -2 position resulting in an accumulation of the corresponding lysophospholipids, including the unhydrolyzed alkyl and alkenyl ether derivatives. The hydrolysis of acute phase high density lipoprotein was 2- to 3-fold more rapid and intensive than of normal high density lipoprotein. The hydrolysis of lipoproteins was noted at enzyme concentration as low as 0.05 microgram/mg protein, which was within the range observed in the circulation in acute and chronic inflammatory diseases. The enzyme hydrolyzed the different molecular species of the residual glycerophospholipids in proportion to their mass, showing no preference for the release of arachidonic acid. Group IIA phospholipase A2 preferentially attacked the hydroxy and hydroperoxy linoleates and possibly other oxygenated fatty acids, which were released from the glycerophospholipids at early times of incubation. There was no effect on the content or molecular species composition of the sphingomyelins or neutral lipids of the lipoproteins. In conclusion, human plasma lipoproteins are the first reported natural biological substrates for human group IIA phospholipase A2. The enhanced hydrolysis of acute phase high density lipoproteins is probably due to its association with serum amyloid A protein, which enhances the activity of the enzyme and may promote its penetration to the lipid monolayer. As sPLA2-induced hydrolysis of the lipoproteins leads to accumulation of lysophosphatidylcholine and potentially toxic oxygenated fatty acids, overexpression of this enzyme may be proatherogenic.

Acute-Phase Reaction↗

PubMed1997

Adenoviral vector-mediated overexpression of serum amyloid A in apoA-I-deficient mice.

Serum amyloid A (SAA) is an acute phase reactant that can become the predominant apolipoprotein of high density lipoprotein (HDL) during severe inflammatory states. However, the function of SAA is unknown. To study the ability of SAA to form HDL in the absence of apolipoprotein A-I, we expressed the mouse SAA pI 6.15 (CE/J) isoform in apolipoprotein A-I knock-out (apoA-I (-/-)) mice using a recombinant adenovirus. As a control, apoA-I (-/-) mice were injected with an adenovirus expressing human apoA-I. High level expression of plasma SAA was obtained in the absence of any endogenous acute phase SAA production. SAA expression increased plasma HDL cholesterol levels about 2-fold, but to a lesser extent than the expression of apoA-I (about 10-fold). The HDL particles isolated by density ultracentrifugation from SAA-expressing mice were heterogeneous in size and composition and rich in free cholesterol as well as apoE and apoA-IV. Of the SAA expressed in the plasma, only a small fraction (4%) was associated with HDL particles in contrast to expressed apoA-I, of which 62% was associated with HDL. We conclude that SAA is unable to substitute for apoA-I in HDL particle formation.

Acute-Phase Reaction↗

PubMed1997

Secretory non-pancreatic phospholipase A2: influence on lipoprotein metabolism.

Lipoprotein metabolism is markedly altered during inflammation. The concentration of human secretory phospholipase A2 (sPLA2) can increase hundreds of fold in inflammatory fluids and in the circulation. It was detected in atherosclerotic lesions where many inflammatory genes are induced. As sPLA2 has been reported to act on lipoproteins as substrates, lipoprotein profiles in transgenic mice expressing sPLA2 were studied. HDL levels were markedly decreased in transgenic mice overexpressing sPLA2. HDL in the transgenics were smaller in size, with a significant decrease (11%) in phospholipid content compared to nontransgenic littermates. In sPLA2 transgenic mice and transgenic mice expressing both sPLA2 and human apolipoprotein B (apoB), the concentrations of apoB-containing lipoproteins were not altered. We conclude that sPLA2 alters HDL metabolism and could be responsible for the depressed levels of HDL that exist during chronic inflammatory diseases.

Animals↗

PubMed1996

Structure of the mouse Saa4 gene and its linkage to the serum amyloid A gene family.

The serum amyloid A (SAA) proteins are a polymorphic family of apolipoproteins associated with high-density lipoproteins (HDL). Three distinct subfamilies have been identified: (i) a cytokine-induced acute phase subfamily that is hepatically produced and can become the major apolipoprotein on HDL (SAA1, SAA2); (ii) a peripherally produced acute phase SAA3 that is only a minor HDL apolipoprotein; and (iii) a constitutive subfamily (SAA4) that is a minor normal HDL apolipoprotein comprising more than 90% of the SAA during homeostasis. Here we define the structure of the Saa4 gene. Similar to other Saa family members, it has four exons and three introns. It is 4588 bp long from the transcription start site to the end of the 3'-untranslated region and is approximately 20% larger than other Saa genes. We have located Saa4 11 kb upstream from Saa3 and 5 kb downstream from Saa1, with the pseudogene approximately 1 kb from the 5' end of Saa4. Saa4 has the same orientation as most other Saa family members, with only Saa2 having an opposing orientation. These data promote our understanding of the evolution of the Saa family. They enhance our ability to develop the mouse as a transgenic and gene deletion model to advance the understanding of the function of these apolipoproteins.

Animals↗

PubMed1995

Serum amyloid A protein enhances the activity of secretory non-pancreatic phospholipase A2.

The acute-phase proteins serum amyloid A protein (SAA) and secretory phospholipase A2 (sPLA2) are simultaneously expressed during inflammatory conditions. SAA associates with high-density lipoprotein (HDL) altering its physicochemical composition. We found that purified acute-phase SAA, but not the constitutive form, markedly enhances the lipolytic activity of sPLA2 in a dose-related manner with phosphatidylcholine/lysophosphatidylcholine or phosphatidylethanolamine/lysophosphatidylethanolamine liposomal substrates. Normal HDL was found to reduce activity of sPLA2 in a dose-dependent manner, but when acute-phase HDL containing 27% SAA was tested, it enhanced sPLA2 activity. Immunopurified monospecific antibodies against SAA completely abolished the enhancing activity of SAA and acute-phase HDL. Given the central role of HDL in lipoprotein metabolism, the interaction between HDL, SAA and sPLA2 may account for changes detected in lipoprotein metabolism during the acute phase.

Acute-Phase Proteins↗

PubMed1995

Association of apolipoprotein E with murine amyloid A protein amyloid.

BACKGROUND: Experimental amyloid A protein (AA) amyloidosis in mice is the most rapid type of amyloid formed, thus providing a valuable model to study amyloid formation. Recent studies have suggested the importance of apolipoprotein E (apoE) in Alzheimer's disease and systemic amyloidoses. To help understand the role of apoE in amyloidoses, we examined amyloid tissue for the presence of apoE in mouse AA amyloid. EXPERIMENTAL DESIGN: Mice were injected with amyloid-enhancing factor and silver nitrate to induce amyloid deposition. Spleens were examined for Congo red staining and serum amyloid A, serum amyloid P component, and apoE immunostaining. In addition, RNA analysis was performed to measure the expression of apoE in various tissues after amyloid induction. RESULTS: We have found that apoE is associated with mouse amyloid. Ab to apoE consistently detected the presence of increased levels of apoE in amyloid tissue. Immunohistochemical analyses confirmed that the apoE immunoreactivity co-associated with AA and serum amyloid P in the amyloid fibrils. Northern blot analysis of amyloid tissue showed an increase in apoE messenger RNA compared with control tissue. CONCLUSIONS: This is the first demonstration of apoE in mouse amyloid tissue. The data presented here, along with previous studies, suggest that apoE may be involved in amyloidogenesis. These studies validate the mouse model for studying the role of apoE in amyloid fibrillogenesis. The use of transgenic and gene-inactivated mice will help to elucidate the role and mechanism of apoE in amyloid formation.

Amyloidosis↗

PubMed1995

Serum amyloid A (SAA): influence on HDL-mediated cellular cholesterol efflux.

Normal high density lipoprotein (N-HDL) is remodeled during acute phase (AP) reactions by the association of serum amyloid A (SAA) and the depletion of apolipoprotein (apo) A-I. To determine the impact of this remodeling on HDL function, the capacities of N-HDL and AP-HDL to associate with and promote cholesterol efflux from human monocytic THP-1 cells were compared. THP-1 cells preferentially bound AP-HDL compared with N-HDL. Examination of the AP-HDL particles bound to THP-1 cells revealed a disproportionate association of an apoSAA-enriched, apoA-I-depleted subpopulation compared with the composition of the starting material. However, N-HDL and AP-HDL promoted cholesterol efflux from THP-1 cells equally efficiently and in a dose-dependent manner. When N-HDL was experimentally remodeled with apoSAA to achieve an apoprotein composition similar to that of the preferentially bound particles, cellular cholesterol efflux was reduced by 30%. The remodelling of HDL with apoSAA during the acute phase reaction alters cholesterol efflux only when apoSAA constitutes more than 50% of the HDL protein.

Apolipoprotein A-I↗

PubMed1995

Characterization of constitutive human serum amyloid A protein (SAA4) as an apolipoprotein.

Serum amyloid A proteins (SAAs), a family of homologous molecules, are apolipoproteins of high density lipoprotein (HDL). They can be divided into two groups. The first group comprises the well-characterized acute phase SAAs that associate with HDL during inflammation, thereby remodeling the HDL particle by displacing apolipoprotein (apo)A-I. The second group consists of the recently discovered constitutive SAAs, mouse SAA5 and human SAA4. They exist as minor apolipoproteins on HDL but constitute more than 90% of the total SAA during homeostasis. We have characterized human SAA4 as an apolipoprotein. During homeostasis, SAA4 is synthesized only in the liver. Purification of SAA4 has been described and its plasma concentration has been established at 55 +/- 13 micrograms/ml in 26 healthy individuals. It was present on all HDL density classes and very low density lipoprotein (VLDL) but was absent from low density lipoprotein (LDL). Using two-dimensional electrophoresis and phosphorimaging, SAA4 was found to be associated with a specific subpopulation of only three HDL particles, not involved in the initial cholesterol transfer from cells.

Acute-Phase Reaction↗