Alternatively spliced mRNA molecules of the thrombospondin receptor (CD36) in human PBMC

Plasmodium falciparum malaria and the immunogenetics of ABO, HLA, and CD36 (platelet glycoprotein IV)

Plasmodium falciparum malaria has long been a killer of the young, and has selected for polymorphisms affecting not only erythrocytes, but the immunogenetics of three histocompatibility systems: ABO, human leukocyte antigen (HLA), and CD36. The ABO system is important because the original allele, encoding glycosylation with the A sugar, acts as an adhesion ligand with infected red blood cells (iRBC), thereby promoting vasoocclusion. The prevalence of blood group O, which reduces this cytoadhesion, has increased in endemic areas. Other adaptations which could mitigate A-mediated rosetting include weaker A expression and increased soluble A secretion. The role of the HLA system in malaria has been harder to verify. Although HLA-B53 and DRB1*04 may be associated with clinical outcome, HLA studies are challenged by numerous comparisons in this most polymorphic of systems, and confounded by increasingly heterogeneous populations. Certain HLA markers may also reflect linkage artefact with other malaria-relevant polymorphisms. HLA may be less important because the parasite predominantly invades a compartment which does not express HLA. Adhesion of iRBCs is also mediated by CD36, expressed on platelets, monocytes, and microvascular endothelium. CD36 on monocytes is involved in clearing iRBC, while CD36 on platelets and the endothelium may play a role in tissue sequestration. The genetics of CD36 expression are complex, and recent research is fraught with inconsistent results. The solution may lie in examining genotype-phenotype correlations, zygosity effects on differential tissue expression, or other mechanisms altering CD36 tissue expression. Carefully designed prospective studies should bridge the gap between in-vitro observations and clinical outcomes.

Quantitation of CD36 (platelet glycoprotein IV) expression on platelets and monocytes by flow cytometry: application to the study of Plasmodium falciparum malaria

BACKGROUND

The expression of CD36 (platelet glycoprotein IV) is variable among different individuals and cannot be determined by gene analysis. Previous studies suggest that CD36 expression plays a central role in the pathophysiology of Plasmodium falciparum malaria, a disease of global significance.

METHODS

We developed a flow cytometric method to quantitatively measure CD36 on monocytes and platelets from whole blood using antibodies to CD36, CD14, and CD61 directly conjugated to different fluorochromes. Commercially available fluorescent beads were used to quantify CD36 expression.

RESULTS

The assay was successfully run at three different centers. African-Americans (n = 57), nonAfrican-Americans (n = 33), individuals with and without hemoglobin S (n = 15 and n = 12), and children with P falciparum malaria (n = 97) were tested. Platelet-monocyte aggregates, present to varying degrees in different anticoagulants, were eliminated from final analysis. The median fluorescence intensity (MFI) of CD36 among different subjects followed a log-normal distribution. Among African-Americans, 5% were CD36-deficient (logMFI < 1.5; MFI < 32). Expression of platelet CD36 paralleled monocyte CD36.

CONCLUSIONS

Flow cytometry can be used to quantify the expression of CD36 of platelets and monocytes in EDTA whole blood. The assay will allow investigation of the relationship between CD36 and clinical outcome in malaria and other disease states.

Clinical pharmacology and therapeutic use of antioxidant vitamins

The clinical use of antioxidants has gained considerable interest during the last decade. It was suggested from epidemiological studies that diets high in fruits and vegetables might help decrease the risk of cardiovascular disease. Therefore, supplements of vitamins C and E were applied through protocols aimed to prevent diseases such as atherosclerosis, preeclampsia or hypertension, thought to be mediated by oxidative stress. Despite the biological properties of these vitamins could account for an effective protection, as shown by several clinical and experimental studies, their efficacy remains controversial in the light of some recent clinical trials and meta-analyses. However, the methodology of these studies, criteria for selection of patients, the uncertain extent of progression of the disease when initiating supplementation, the lack of mechanistic studies containing basic scientific aspects, such as the bioavailability, pharmacokinetic properties, and the nature of the antioxidant sources of vitamins, could account for the inconsistency of the various clinical trials and meta-analyses assessing the efficacy of these vitamins to prevent human diseases. This review presents a survey of the clinical use of antioxidant vitamins E and C, proposing study models based on the biological effects of these compounds likely to counteract the pathophysiological mechanisms able to explain the structural and functional organ damage.

Expression pattern of fatty acid transport protein-1 (FATP-1), FATP-4 and heart-fatty acid binding protein (H-FABP) genes in human term placenta

Placental tissue from five women undergoing elective caesarean did not present differences in fatty acids or mRNA expression of FATP-1, FATP-4 and H-FABP in different placental locations. mRNA expression of FATP-1 and FATP-4 was significantly higher than H-FABP. The expression of L-FABP was too low in placenta for accurate quantification.

Novel Immunoblotting Monoclonal Antibodies Against Human and Rat CD36/Fat Used to Identify an Isoform of CD36 in Rat Muscle

CD36, a surface membrane glycoprotein, functions as a class B scavenger receptor that binds to several distinct ligands. Included among these is oxidized low-density lipoprotein (Ox-LDL), a major trigger of atherosclerotic lesions, and the levels of CD36 activity and Ox-LDL uptake may have an impact on coronary artery disease. In addition, recent studies in rodents have shown that CD36, also known as FAT, controls the levels of free fatty acids and triglycerides in plasma, and is an important regulator of the metabolic pathways involved in insulin resistance. Despite the importance of measuring CD36 expression in different tissues there is a paucity of good immunoblotting antibodies, particularly for rodent tissue. Here, using GST-fusion proteins incorporating the cysteine cluster encoded by exons VIII, IX, and X of the CD36 gene as immunogen, we have generated a panel of monoclonal antibodies that are excellent blotting reagents for human and rat CD36. With these reagents we were able to visualize an additional, faster migrating CD36 band in rat muscle, likely representing a minor splice variant of CD36 (CD36var.1) hitherto seen only in the human HEL cell line.

Alternative promoter usage of the membrane glycoprotein CD36

BACKGROUND

CD36 is a membrane glycoprotein involved in a variety of cellular processes such as lipid transport, immune regulation, hemostasis, adhesion, angiogenesis and atherosclerosis. It is expressed in many tissues and cell types, with a tissue specific expression pattern that is a result of a complex regulation for which the molecular mechanisms are not yet fully understood. There are several alternative mRNA isoforms described for the gene. We have investigated the expression patterns of five alternative first exons of the CD36 gene in several human tissues and cell types, to better understand the molecular details behind its regulation.

RESULTS

We have identified one novel alternative first exon of the CD36 gene, and confirmed the expression of four previously known alternative first exons of the gene. The alternative transcripts are all expressed in more than one human tissue and their expression patterns vary highly in skeletal muscle, heart, liver, adipose tissue, placenta, spinal cord, cerebrum and monocytes. All alternative first exons are upregulated in THP-1 macrophages in response to oxidized low density lipoproteins. The alternative promoters lack TATA-boxes and CpG islands. The upstream region of exon 1b contains several features common for house keeping gene and monocyte specific gene promoters.

CONCLUSION

Tissue-specific expression patterns of the alternative first exons of CD36 suggest that the alternative first exons of the gene are regulated individually and tissue specifically. At the same time, the fact that all first exons are upregulated in THP-1 macrophages in response to oxidized low density lipoproteins may suggest that the alternative first exons are coregulated in this cell type and environmental condition. The molecular mechanisms regulating CD36 thus appear to be unusually complex, which might reflect the multifunctional role of the gene in different tissues and cellular conditions.

Anti-atherosclerotic effects of vitamin E--myth or reality?

Atherosclerosis and its complications such as coronary heart disease, myocardial infarction and stroke are the leading causes of death in the developed world. High blood pressure, diabetes, smoking and a diet high in cholesterol and lipids clearly increase the likelihood of premature atherosclerosis, albeit other factors, such as the individual genetic makeup, may play an additional role. Several epidemiological studies and intervention trials have been performed with vitamin E, and some of them showed that it prevents atherosclerosis. For a long time, vitamin E was assumed to act by decreasing the oxidation of LDL, a key step in atherosclerosis initiation. However, at the cellular level, vitamin E acts by inhibition of smooth muscle cell proliferation, platelet aggregation, monocyte adhesion, oxLDL uptake and cytokine production, all reactions implied in the progression of atherosclerosis. Recent research revealed that these effects are not the result of the antioxidant activity of vitamin E, but rather of precise molecular actions of this compound. It is assumed that specific interactions of vitamin E with enzymes and proteins are at the basis of its non-antioxidant effects. Vitamin E influences the activity of several enzymes (e.g. PKC, PP2A, COX-2, 5-lipooxygenase, nitric oxide synthase, NADPH-oxidase, superoxide dismutase, phopholipase A2) and modulates the expression of genes that are involved in atherosclerosis (e.g. scavenger receptors, integrins, selectins, cytokines, cyclins). These interactions promise to reveal the biological properties of vitamin E and allow designing better strategies for the protection against atherosclerosis progression.

CD36 polymorphism is associated with protection from cerebral malaria

The human protein CD36 is a major receptor for Plasmodium falciparum-infected erythrocytes and contributes to the pathology of P. falciparum malaria. We performed variation screening of the CD36 gene and examined the possible association between CD36 polymorphisms and the severity of malaria in 475 adult Thai patients with P. falciparum malaria. Accordingly, we identified nine CD36 polymorphisms with a high-frequency (>15%) minor allele. Of these, the frequencies of the -14T-->C allele in the upstream promoter region and the -53G-->T allele in the downstream promoter region were significantly decreased in patients with cerebral malaria compared to those with mild malaria (P=.016 for -14T-->C and P=.050 for -53G-->T). The analysis of linkage disequilibrium (LD) between the nine common polymorphisms revealed that there are two blocks with strong LD in the CD36 gene and that the -14T-->C and -53G-->T polymorphisms are within the upstream block of 35 kb from the upstream promoter to exon 8. Further association testing after the second variation screening in the upstream block indicated that the in3(TG)(12) (i.e., 12 TG repeats in intron 3) allele is most strongly associated with the reduction in the risk of cerebral malaria (odds ratio 0.59; 95% confidence interval 0.40-0.87; P=.0069). We found, by reverse-transcriptase PCR amplification, that in3(TG)(12) is involved in the nonproduction of the variant CD36 transcript that lacks exons 4 and 5. Since exon 5 of the gene is known to encode the ligand-binding domain for P. falciparum-infected erythrocytes, in3(TG)(12) itself or a primary variant on the haplotype with in3(TG)(12) may be responsible for protection from cerebral malaria in Thailand. Results of the present study suggest that LD mapping has potential for detecting a disease-associated variant on the basis of haplotype blocks.

The European perspective on vitamin E: current knowledge and future research

Vitamin E is indispensible for reproduction in female rats. In humans, vitamin E deficiency primarily causes neurologic dysfunctions, but the underlying molecular mechanisms are unclear. Because of its antioxidative properties, vitamin E is believed to help prevent diseases associated with oxidative stress, such as cardiovascular disease, cancer, chronic inflammation, and neurologic disorders. However, recent clinical trials undertaken to prove this hypothesis failed to verify a consistent benefit. Given these findings, a group of European scientists met to analyze the most recent knowledge of vitamin E function and metabolism. An overview of their discussions is presented in this article, which includes considerations of the mechanisms of absorption, distribution, and metabolism of different forms of vitamin E, including the alpha-tocopherol transfer protein and alpha-tocopherol-associated proteins; the mechanism of tocopherol side-chain degradation and its putative interaction with drug metabolism; the usefulness of tocopherol metabolites as biomarkers; and the novel mechanisms of the antiatherosclerotic and anticarcinogenic properties of vitamin E, which involve modulation of cellular signaling, transcriptional regulation, and induction of apoptosis. Clinical trials were analyzed on the basis of the selection of subjects, the stage of disease, and the mode of intake, dosage, and chemical form of vitamin E. In addition, the scarce knowledge on the role of vitamin E in reproduction was summarized. In conclusion, the scientists agreed that the functions of vitamin E were underestimated if one considered only its antioxidative properties. Future research on this essential vitamin should focus on what makes it essential for humans, why the body apparently utilizes alpha-tocopherol preferentially, and what functions other forms of vitamin E have.

Novel 5' exon of scavenger receptor CD36 is expressed in cultured human vascular smooth muscle cells and atherosclerotic plaques

CD36, a member of the scavenger receptor family, is centrally involved in the uptake of oxidized low density lipoproteins (oxLDLs) from the bloodstream. During the atherosclerotic process, the lipid cargo of oxLDL accumulates in macrophages and smooth muscle cells (SMCs), inducing their pathological conversion to foam cells. Increased expression of CD36 occurs in human atherosclerotic lesions, and CD36 knockout mice show reduced uptake of modified LDLs and reduced atherosclerosis. Here, we describe a novel exon 1b and extended CD36 promoter in human SMCs. Exon 1b is specifically transcribed in activated aortic SMCs and mainly expressed in atherosclerotic plaques. Thus, switching to exon 1b transcription may be an important step for the activation of SMCs and their conversion to foam cells. Using an antisense oligonucleotide to exon 1b, we inhibit CD36 translation and highly reduce oxLDL uptake. The antisense to exon 1b does not affect CD36 in cell lines not expressing the new exon. The possibility of a novel antiatherosclerotic therapy and the use of exon 1b as a marker of atherosclerosis are discussed.