Structural and functional characterization of the human CD36 gene promoter: identification of a proximal PEBP2/CBF site

The Multifunctionality of CD36 in Diabetes Mellitus and Its Complications-Update in Pathogenesis, Treatment and Monitoring

CD36 is a multiligand receptor contributing to glucose and lipid metabolism, immune response, inflammation, thrombosis, and fibrosis. A wide range of tissue expression includes cells sensitive to metabolic abnormalities associated with metabolic syndrome and diabetes mellitus (DM), such as monocytes and macrophages, epithelial cells, adipocytes, hepatocytes, skeletal and cardiac myocytes, pancreatic β-cells, kidney glomeruli and tubules cells, pericytes and pigment epithelium cells of the retina, and Schwann cells. These features make CD36 an important component of the pathogenesis of DM and its complications, but also a promising target in the treatment of these disorders. The detrimental effects of CD36 signaling are mediated by the uptake of fatty acids and modified lipoproteins, deposition of lipids and their lipotoxicity, alterations in insulin response and the utilization of energy substrates, oxidative stress, inflammation, apoptosis, and fibrosis leading to the progressive, often irreversible organ dysfunction. This review summarizes the extensive knowledge of the contribution of CD36 to DM and its complications, including nephropathy, retinopathy, peripheral neuropathy, and cardiomyopathy.

CD 36: Focus on Epigenetic and Post-Transcriptional Regulation

CD36 is a transmembrane protein involved in fatty acid translocation, scavenging for oxidized fatty acids acting as a receptor for adhesion molecules. It is expressed on macrophages, as well as other types of cells, such as endothelial and adipose cells. CD36 participates in muscle lipid uptake, adipose energy storage, and gut fat absorption. Recently, several preclinical and clinical studies demonstrated that upregulation of CD36 is a prerequisite for tumor metastasis. Cancer metastasis-related research emerged much later and has been less investigated, though it is equally or even more important. CD36 protein expression can be modified by epigenetic changes and post-transcriptional interference from non-coding RNAs. Some data indicate modulation of CD36 expression in specific cell types by epigenetic changes via DNA methylation patterns or histone tails, or through miRNA interference, but this is largely unexplored. The few papers addressing this topic refer mostly to lipid metabolism-related pathologies, whereas in cancer research, data are even more scarce. The aim of this review was to summarize major epigenetic and post-transcriptional mechanisms that impact CD36 expression in relation to various pathologies while highlighting the areas in need of further exploration.

Tet38 Efflux Pump Affects Staphylococcus aureus Internalization by Epithelial Cells through Interaction with CD36 and Contributes to Bacterial Escape from Acidic and Nonacidic Phagolysosomes

We previously reported that the Tet38 efflux pump is involved in internalization of Staphylococcus aureus by A549 lung epithelial cells. A lack of tet38 reduced bacterial uptake by A549 cells to 36% of that of the parental strain RN6390. Using invasion assays coupled with confocal microscopy imaging, we studied the host cell receptor(s) responsible for bacterial uptake via interaction with Tet38. We also assessed the ability of S. aureus to survive following alkalinization of the phagolysosomes by chloroquine. Antibody to the scavenger receptor CD36 reduced the internalization of S. aureus RN6390 by A549 cells, but the dependence on CD36 was reduced in QT7 tet38, suggesting that an interaction between Tet38 and CD36 contributed to S. aureus internalization. Following fusion of the S. aureus-associated endosomes with lysosomes, alkalinization of the acidic environment with chloroquine led to a rapid increase in the number of S. aureus RN6390 bacteria in the cytosol, followed by a decrease shortly thereafter. This effect of chloroquine was not seen in the absence of intact Tet38 in mutant QT7. These data taken together suggest that Tet38 plays a role both in bacterial internalization via interaction with CD36 and in bacterial escape from the phagolysosomes.

Comparative Studies of Vertebrate Platelet Glycoprotein 4 (CD36)

Platelet glycoprotein 4 (CD36) (or fatty acyl translocase [FAT], or scavenger receptor class B, member 3 [SCARB3]) is an essential cell surface and skeletal muscle outer mitochondrial membrane glycoprotein involved in multiple functions in the body. CD36 serves as a ligand receptor of thrombospondin, long chain fatty acids, oxidized low density lipoproteins (LDLs) and malaria-infected erythrocytes. CD36 also influences various diseases, including angiogenesis, thrombosis, atherosclerosis, malaria, diabetes, steatosis, dementia and obesity. Genetic deficiency of this protein results in significant changes in fatty acid and oxidized lipid uptake. Comparative CD36 amino acid sequences and structures and CD36 gene locations were examined using data from several vertebrate genome projects. Vertebrate CD36 sequences shared 53-100% identity as compared with 29-32% sequence identities with other CD36-like superfamily members, SCARB1 and SCARB2. At least eight vertebrate CD36 N-glycosylation sites were conserved which are required for membrane integration. Sequence alignments, key amino acid residues and predicted secondary structures were also studied. Three CD36 domains were identified including cytoplasmic, transmembrane and exoplasmic sequences. Conserved sequences included N- and C-terminal transmembrane glycines; and exoplasmic cysteine disulphide residues; TSP-1 and PE binding sites, Thr92 and His242, respectively; 17 conserved proline and 14 glycine residues, which may participate in forming CD36 'short loops'; and basic amino acid residues, and may contribute to fatty acid and thrombospondin binding. Vertebrate CD36 genes usually contained 12 coding exons. The human CD36 gene contained transcription factor binding sites (including PPARG and PPARA) contributing to a high gene expression level (6.6 times average). Phylogenetic analyses examined the relationships and potential evolutionary origins of the vertebrate CD36 gene with vertebrate SCARB1 and SCARB2 genes. These suggested that CD36 originated in an ancestral genome and was subsequently duplicated to form three vertebrate CD36 gene family members, SCARB1, SCARB2 and CD36.

Differential transcription factor use by the KIR2DL4 promoter under constitutive and IL-2/15-treated conditions

KIR2DL4 is unique among human KIR genes in expression, cellular localization, structure, and function, yet the transcription factors required for its expression have not been identified. Using mutagenesis, EMSA, and cotransfection assays, we identified two redundant Runx binding sites in the 2DL4 promoter as essential for constitutive 2DL4 transcription, with contributions by a cyclic AMP response element (CRE) and initiator elements. IL-2- and IL-15-stimulated human NK cell lines increased 2DL4 promoter activity, which required functional Runx, CRE, and Ets sites. Chromatin immunoprecipitation experiments show that Runx3 and Ets1 bind the 2DL4 promoter in situ. 2DL4 promoter activity had similar transcription factor requirements in T cells. Runx, CRE, and Ets binding motifs are present in 2DL4 promoters from across primate species, but other postulated transcription factor binding sites are not preserved. Differences between 2DL4 and clonally restricted KIR promoters suggest a model that explains the unique 2DL4 expression pattern in human NK cells.

Variations in host genes encoding adhesion molecules and susceptibility to falciparum malaria in India

Background

Host adhesion molecules play a significant role in the pathogenesis of Plasmodium falciparum malaria and changes in their structure or levels in individuals can influence the outcome of infection. The aim of this study was to investigate the association of SNPs of three adhesion molecule genes, ICAM1, PECAM1 and CD36, with severity of falciparum malaria in a malaria-endemic and a non-endemic region of India.

Methods

The frequency distribution of seven selected SNPs of ICAM1, PECAM1 and CD36 was determined in 552 individuals drawn from 24 populations across India. SNP-disease association was analysed in a case-control study format. Genotyping of the population panel was performed by Sequenom mass spectroscopy and patient/control samples were genotyped by SNaPshot method. Haplotypes and linkage disequilibrium (LD) plots were generated using PHASE and Haploview, respectively. Odds-ratio (OR) for risk assessment was estimated using EpiInfo™ version 3.4.

Results

Association of the ICAM1 rs5498 (exon 6) G allele and the CD36 exon 1a A allele with increased risk of severe malaria was observed (severe versus control, OR = 1.91 and 2.66, P = 0.02 and 0.0012, respectively). The CD36 rs1334512 (-53) T allele as well as the TT genotype associated with protection from severe disease (severe versus control, TT versus GG, OR = 0.37, P = 0.004). Interestingly, a SNP of the PECAM1 gene (rs668, exon 3, C/G) with low minor allele frequency in populations of the endemic region compared to the non-endemic region exhibited differential association with disease in these regions; the G allele was a risk factor for malaria in the endemic region, but exhibited significant association with protection from disease in the non-endemic region.

Conclusion

The data highlights the significance of variations in the ICAM1, PECAM1 and CD36 genes in the manifestation of falciparum malaria in India. The PECAM1 exon 3 SNP exhibits altered association with disease in the endemic and non-endemic region.

Transcriptional regulation of fatty acid translocase/CD36 expression by CCAAT/enhancer-binding protein alpha

Fatty acid translocase (FAT/CD36) plays an important role in facilitating long chain fatty acid transport. FAT/CD36 gene deletion protects mice from high fat diet-induced obesity. In this study we have investigated the regulatory mechanism of FAT/CD36 expression at the transcription level. FAT/CD36 expression was activated during 3T3-L1 adipocyte differentiation, and FAT/CD36 protein levels were positively correlated with CCAAT/enhancer-binding protein alpha (C/EBPalpha) and peroxisome proliferator-activated receptor gamma. However, a negative correlation was detected between FAT/CD36 and C/EBPbeta. Overexpression of C/EBPalpha or C/EBPbeta increased FAT/CD36 mRNA and protein levels in several types of cells. Restoration of C/EBPalpha or C/EBPbeta expression in C/EBPalpha- or C/EBPbeta-deficient mouse embryonic fibroblasts increased FAT/CD36 expression. However, in mouse embryonic fibroblasts C/EBPalpha was a more potent activator of FAT/CD36 expression than was C/EBPbeta. Expression of C/EBPalpha robustly increased FAT/CD36 proximal promoter-directed luciferase expression in human embryonic kidney 293 cells. A C/EBP-responsive element was identified in the FAT/CD36 promoter by using 5' and specific site mutations. The binding of C/EBPalpha in the FAT/CD36 promoter was detected by chromatin immunoprecipitation in 3T3-L1 adipocytes. These results demonstrated that C/EBPalpha regulates FAT/CD36 gene expression at the transcriptional level.

Expression of both N- and C-terminal GFP tagged huCD36 and their discrepancy in OxLDL and pRBC binding on CHO cells

CD36, an 88 kDa membrane glycoprotein, is found in several cell types and it has been characterized to have two hydrophobic domains at their N- and C-termini which are essential for protein folding and targeting. In this study, we first tagged the green fluorescent protein (GFP) to both the N- and C-termini of huCD36 and investigated their cellular expression and influences on lipoprotein and plasmodium falciparium parasitized erythrocytes (pRBC) binding. Our work revealed that huCD36 proteins are expressed normally irrespective of the GFP tag presence at either the N- or C-termini. However, the two recombinant proteins showed discrepancy in uptake and surface-binding of OxLDL but they did not affect pRBC binding. These results suggested that the interaction between oxLDL and CD36 could be blocked using recombinant proteins and this may be useful in potential control of the trafficking of modified lipoproteins into monocytes leading to atherogenesis.

Molecular basis of human CD36 gene mutations

CD36 is a transmembrane glycoprotein of the class B scavenger receptor family. The CD36 gene is located on chromosome 7 q11.2 and is encoded by 15 exons. Defective CD36 is a likely candidate gene for impaired fatty acid metabolism, glucose intolerance, atherosclerosis, arterial hypertension, diabetes, cardiomyopathy, Alzheimer disease, and modification of the clinical course of malaria. Contradictory data concerning the effects of antiatherosclerotic drugs on CD36 expression indicate that further investigation of the role of CD36 in the development of atherosclerosis may be important for the prevention and treatment of this disease. This review summarizes current knowledge of CD36 gene structure, splicing, and mutations and the molecular, metabolic, and clinical consequences of these phenomena.

Molecular basis of human CD36 gene mutations

CD36 is a transmembrane glycoprotein of the class B scavenger receptor family. The CD36 gene is located on chromosome 7 q11.2 and is encoded by 15 exons. Defective CD36 is a likely candidate gene for impaired fatty acid metabolism, glucose intolerance, atherosclerosis, arterial hypertension, diabetes, cardiomyopathy, Alzheimer disease, and modification of the clinical course of malaria. Contradictory data concerning the effects of antiatherosclerotic drugs on CD36 expression indicate that further investigation of the role of CD36 in the development of atherosclerosis may be important for the prevention and treatment of this disease. This review summarizes current knowledge of CD36 gene structure, splicing, and mutations and the molecular, metabolic, and clinical consequences of these phenomena.

CD36: implications in cardiovascular disease

CD36 is a broadly expressed membrane glycoprotein that acts as a facilitator of fatty acid uptake, a signaling molecule, and a receptor for a wide range of ligands, including apoptotic cells, modified forms of low density lipoprotein, thrombospondins, fibrillar beta-amyloid, components of Gram positive bacterial walls and malaria infected erythrocytes. CD36 expression on macrophages, dendritic and endothelial cells, and in tissues including muscle, heart, and fat, suggest diverse roles, and indeed, this is truly a multi-functional receptor involved in both homeostatic and pathological conditions. Despite an impressive increase in our knowledge of CD36 functions, in depth understanding of the mechanistic aspects of this protein remains elusive. This review focuses on CD36 in cardiovascular disease-what we know, and what we have yet to learn.

RUNX3 negatively regulates CD36 expression in myeloid cell lines

CD36 is a member of the scavenger receptor type B family implicated in the binding of lipoproteins, phosphatidylserine, thrombospondin-1, and the uptake of long-chain fatty acids. On mononuclear phagocytes, recognition of apoptotic cells by CD36 contributes to peripheral tolerance and prevention of autoimmunity by impairing dendritic cell (DC) maturation. Besides, CD36 acts as a coreceptor with TLR2/6 for sensing microbial diacylglycerides, and its deficiency leads to increased susceptibility to Staphylococcus aureus infections. The RUNX3 transcription factor participates in reprogramming DC transcription after pathogen recognition, and its defective expression leads to abnormally accelerated DC maturation. We present evidence that CD36 expression is negatively regulated by the RUNX3 transcription factor during myeloid cell differentiation and activation. In molecular terms, RUNX3 impairs the activity of the proximal regulatory region of the CD36 gene in myeloid cells through in vitro recognition of two functional RUNX-binding elements. Moreover, RUNX3 occupies the CD36 gene proximal regulatory region in vivo, and its overexpression in myeloid cells results in drastically diminished CD36 expression. The down-regulation of CD36 expression by RUNX3 implies that this transcription factor could impair harmful autoimmune responses by contributing to the loss of pathogen- and apoptotic cell-recognition capabilities by mature DCs.

Adverse prognostic impact of CD36 and CD2 expression in adult de novo acute myeloid leukemia patients

BACKGROUND AND OBJECTIVES

A consecutive series of acute myeloid leukemias (AML) patients was analyzed in conditions which reduce the inter-assay variations (the same flow cytometer, the same observers and the same panel of monoclonal antibodies) in order to investigate the prognostic information provided by flow cytometry.

DESIGN AND METHODS

Two hundred and sixty-six bone marrow (BM) samples from 326 patients enrolled in the LMA-99 protocol from the CETLAM group were studied by multiparametric flow cytometry. Immunophenotyping studies were performed on erythrocyte-lysed BM samples. Antigen expression of leukemic cells was analyzed using triple stainings with fluorochrome-conjugated combinations of monoclonal antibodies.

RESULTS

CD2 was positive in 21 cases (8%); an associated inv(16) was detected in eight CD2+ cases (38%). Two-year overall survival (OS) rate for CD2+/inv(16)+ patients was 75%, whereas it was 0% for CD2+/inv(16)- patients and 47% for CD2- patients (p=0.0001). CD36 was expressed in 37% of patients (n=98). Two-year leukemia-free survival (LFS) rate was 34% for CD36+ patients and 55% for CD36- patients (p=0.001). In the multivariate analysis, CD2+ (RR=8.4; p=0.0001) and adverse karyotype (RR=10.2; p=0.0001) were associated with a lower CR rate, CD36+ (RR=1.5; p=0.03), CD2+ (RR=2; p=0.04) and adverse karyotype (RR=4; p=0.0001) were associated with a lower OS and CD36+ (RR=2; p=0.002) and adverse karyotype (RR=3.5; p=0.005) predicted a lower LFS.

CONCLUSIONS

CD2+ patients had a very poor OS when CD2/inv(16)+ cases were excluded. CD36 and CD2 expression at diagnosis can provide prognostically important information in adult de novo AML.

Cytomegalovirus stimulated mRNA accumulation and cell surface expression of the oxidized LDL scavenger receptor, CD36

OBJECTIVE

Cytomegalovirus (CMV) has been epidemiologically associated with multiple disease processes including coronary, carotid and cardiac graft atherosclerosis. An early initiating event in atherogenesis is the uptake by macrophages of oxidized low-density lipoproteins (OxLDL) via the scavenger receptor, CD36. Because CMV can activate host-cell gene transcription, we hypothesized that CMV may upregulate CD36 expression.

METHODS AND RESULTS

THP-1 monocyte/macrophage cells were treated with Davis strain CMV and cell surface CD36 expression measured by flow cytometry. Virus challenge increased the percentage of cells expressing CD36 from 21.8 +/- 1.7 to 48.2 +/- 4.0% (mean +/- S.D. for three experiments, P=0.0005); CD36 mRNA accumulation was increased by CMV treatment as determined by reverse transcription-polymerase chain reaction. Viral challenge also upregulated the mitogen-activated protein kinase p38; further, the specific p38 inhibitor, SB203580, reversed the CMV-induced CD36 cell surface expression from 57.2% of cells to baseline levels (29.0 and 30.1% for SB203580 treated and control cells, respectively; P=0.001). Treatment with virus also stimulated uptake of OxLDL: microscopically, virus-treated cells had a mean of 32 +/- 4.0 lipid vacuoles compared with 20 +/- 1.3 for control cells (P=0.01).

CONCLUSIONS

These findings suggest that CMV-induced CD36 expression is one mechanism through which CMV may promote atherosclerosis. Other CMV-associated atherogenic mechanisms may exist; additional investigation is necessary.

Tyrosine phosphorylation controls Runx2-mediated subnuclear targeting of YAP to repress transcription

Src/Yes tyrosine kinase signaling contributes to the regulation of bone homeostasis and inhibits osteoblast activity. Here we show that the endogenous Yes-associated protein (YAP), a mediator of Src/Yes signaling, interacts with the native Runx2 protein, an osteoblast-related transcription factor, and suppresses Runx2 transcriptional activity in a dose-dependent manner. Runx2, through its PY motif, recruits YAP to subnuclear domains in situ and to the osteocalcin (OC) gene promoter in vivo. Inhibition of Src/Yes kinase blocks tyrosine phosphorylation of YAP and dissociates endogenous Runx2-YAP complexes. Consequently, recruitment of the YAP co-repressor to subnuclear domains is abrogated and expression of the endogenous OC gene is induced. Our results suggest that Src/Yes signals are integrated through organization of Runx2-YAP transcriptional complexes at subnuclear sites to attenuate skeletal gene expression.

Statins upregulate CD36 expression in human monocytes, an effect strengthened when combined with PPAR-γ ligands Putative contribution of Rho GTPases in statin-induced CD36 expression

Scavenger receptor CD36 plays important roles in atherosclerosis, inflammation, thrombosis, and angiogenesis. Statins besides lowering serum cholesterol levels, exhibit a variety of effects on inflammation, coagulation and atherosclerosis lesion stability. PPAR-gamma ligands influence macrophage responses to many inflammatory stimuli. Herein, we investigated in human monocytes the effect of statins alone, and in combination with PPAR-gamma ligands on CD36 expression, as well as the molecular mechanisms underlying the regulatory action of statins. Our results demonstrate that statins upregulate both CD36 surface protein and mRNA by potentiating the transcription of the CD36 gene. Furthermore, the combination of statins and PPAR-gamma ligands has an additive effect on CD36 expression. Effects of statins on CD36 expression were prevented by mevalonate and geranylgeraniol, indicating the requirement of geranylgeranylated proteins for CD36 regulation. Rho GTPases inhibitor C3 exoenzyme reproduced the effect of statins, while Rho activator lysophosphatidic acid downregulated CD36. Transient expression of dominant-negative mutants of RhoA and RhoB induced a significant increased in CD36 promoter activity. Finally, the actin cytoskeleton disrupter cytochalasin D upregulated CD36. These data indicate that Rho proteins are important modulators of CD36 expression, and strongly suggest that statins increased CD36 expression by disrupting cytoskeleton organization by inactivating Rho GTPases. These features prompt to investigate the roles of Rho GTPases and actin cytoskeleton modulators on monocytic functions affected by statins.

AML1 interconnected pathways of leukemogenesis

The AML1 transcription factor, identified by the cloning of the translocation t(8;21) breakpoint, is one of the most frequent targets for chromosomal translocations in leukemia. Furthermore, polysomies and point mutations can also alter AML1 function. AML1, also called CBF alpha 2, PEBP alpha 2 or RUNX1, is thus implicated in a great number of acute leukemias via a variety of pathogenic mechanisms and seems to act either as an oncogene or a tumor suppressor gene. Characterization of AML1 knockout mice has shown that AML1 is necessary for normal development of all hematopoietic lineages and alterations in the overal functional level of AML1 can have a profound effect on hematopoiesis. Numerous studies have shown that AML1 plays a vital role in the regulation of expression of many genes involved in hematopoietic cell development, and the impairment of AML1 function disregulates the pathways leading to cellular proliferation and differentiation. However, heterozygous AML1 mutations alone may not be sufficient for the development of leukemia. A cumulative process of mutagenesis involving additional genetic events in functionally related molecules, may be necessary for the development of leukemia and may determine the leukemic phenotype. We review the known AML1 target genes, AML1 interacting proteins, AML1 gene alterations and their effects on AML1 function, and mutations in AML1-related genes associated with leukemia. We discuss the interconnections between all these genes in cell signaling pathways and their importance for future therapeutic developments.

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 AML1-ETO fusion gene promotes extensive self-renewal of human primary erythroid cells

The t(8;21) translocation, which encodes the AML1-ETO fusion protein (now known as RUNX1-CBF2T1), is one of the most frequent translocations in acute myeloid leukemia, although its role in leukemogenesis is unclear. Here, we report that exogenous expression of AML1-ETO in human CD34(+) cells severely disrupts normal erythropoiesis, resulting in virtual abrogation of erythroid colony formation. In contrast, in bulk liquid culture of purified erythroid cells, we found that while AML1-ETO initially inhibited proliferation during early (erythropoietin [EPO]-independent) erythropoiesis, growth inhibition gave way to a sustained EPO-independent expansion of early erythroid cells that continued for more than 60 days, whereas control cultures became growth arrested after 10 to 13 days (at the EPO-dependent stage of development). Phenotypic analysis showed that although these cells were CD13(-) and CD34(-), unlike control cultures, these cells failed to up-regulate CD36 or to down-regulate CD33, suggesting that expression of AML1-ETO suppressed the differentiation of these cells and allowed extensive self-renewal to occur. In the early stages of this expansion, addition of EPO was able to promote both phenotypic (CD36(+), CD33(-), glycophorin A(+)) and morphologic differentiation of these cells, almost as effectively as in control cultures. However, with extended culture, cells expressing AML1-ETO became refractory to addition of this cytokine, suggesting that a block in differentiation had been established. These data demonstrate the capacity of AML1-ETO to promote the self-renewal of human hematopoietic cells and therefore support a causal role for t(8;21) translocations in leukemogenesis.

Integration of Runx and Smad regulatory signals at transcriptionally active subnuclear sites

Runx factors control lineage commitment and are transcriptional effectors of Smad signaling. Genetic defects in these pathways interfere with normal development. The in situ localization of Runx and Smad proteins must impact the mechanisms by which these proteins function together in gene regulation. We show that the integration of Runx and Smad signals is mediated by in situ interactions at specific foci within the nucleus. Activated Smads are directed to these subnuclear foci only in the presence of Runx proteins. Smad-Runx complexes are associated in situ with the nuclear matrix, and this association requires the intranuclear targeting signal of Runx factors. The convergence of Smad and Runx proteins at these sites supports transcription as reflected by BrUTP labeling and functional cooperativity between the proteins. Thus, Runx-mediated intranuclear targeting of Smads is critical for the integration of two distinct pathways essential for fetal development.

Cloning and characterization of a novel apolipoprotein A-I binding protein, AI-BP, secreted by cells of the kidney proximal tubules in response to HDL or ApoA-I

Apolipoprotein A-I (apoA-I) is the major apolipoprotein of high-density lipoproteins (HDL) and has an important role in the regulation of the stability, lipid transport, and metabolism of HDL particles. To identify novel proteins that are involved in HDL metabolism, we used mature apoA-I (amino acids 25-267) as a bait for the screening of a human liver two-hybrid cDNA library. Among the identified genes, several encoded known proteins, including serum amyloid A(2a) (SAA(2a)), apoC-I, and phosphodiesterase HCAM1 (PDE1A), found to interact with apoA-I. In addition, we have cloned a novel 29 kDa apoA-I interacting protein, which we named AI-BP (apoA-I binding protein). The AI-BP encoding gene, APOA1BP, which is located on chromosome 1q21, is composed of six exons and five introns and spans 2.5 kb. Northern blot analysis demonstrated ubiquitous expression of the APOA1BP mRNA with the highest expression in kidney, heart, liver, thyroid gland, adrenal gland, and testis. AI-BP protein is not detectable in serum of healthy probands, but serum samples of patients with septic syndromes may contain elevated levels of AI-BP. Significant amounts of AI-BP protein are found in cerebrospinal fluid and urine of healthy probands. The stimulation of cells derived from the kidney proximal tubules with apoA-I or HDL induces a concentration-dependent secretion of AI-BP indicating an important role for AI-BP, in the renal tubular degradation or resorption of apoA-I.

Peroxisome-proliferator-activated-receptor gamma (PPARgamma) independent induction of CD36 in THP-1 monocytes by retinoic acid

Retinoic acid (RA) has been shown to regulate cellular growth and differentiation of a variety of cell types, including cells of the myelomonocytic lineage. We used the monocytic leukaemia cell line THP-1, which differentiates to macrophages in response to phorbol 12-myristate 13-acetate (PMA), to investigate the regulation by RA of genes in the scavenger receptor type B family (CD36) in human monocyte/macrophages. Reverse transcription-polymerase chain reaction and flow cytometry demonstrated that, like PMA and the natural peroxisome-proliferator-activated receptor-gamma (PPARgamma) ligand 15d-PGJ2, RA induced CD36 gene expression in these cells. Moreover, RA plus 15d-PGJ2 further enhanced CD36 protein and mRNA levels over that seen with the RA or PPARgamma compounds alone. The PPARgamma antagonist GW9662 was shown to block completely PPARgamma-ligand induction of CD36 gene expression, but had little effect on the action of RA. Our data indicated that RXR- and RAR-specific ligands (LG153 and TTNPB, respectively) were each alone able to increase CD36 mRNA and surface protein levels. By using calphostin C, a specific protein kinase C (PKC) inhibitor, we demonstrated that induction of CD36 by PMA, as well as by PPARgamma and RXR ligands were dependent upon PKC activation. In contrast, activation of CD36 through the RAR pathway was not affected by inhibition of PKC activity. Taken together, these data demonstrate that RA can up-regulate CD36 expression in human monocytes/macrophages. This regulation appears to be predominantly mediated through the RAR/RXR pathway of action and, unlike previously described methods of CD36 modulation, is independent of PPARgamma and PKC signalling. This study suggests a possible role for RA in physiological processes involving the scavenger receptor function in cells of the monocyte/macrophage lineage.

Oct-2 regulates CD36 gene expression via a consensus octamer, which excludes the co-activator OBF-1

The POU domain transcription factor, Oct-2, is essential for the B cell-specific expression of CD36 in mouse B cells. In order to determine how Oct-2 mediates expression of CD36 in B cells, we cloned and analysed the mouse CD36 promoter. In contrast to the human CD36 promoter, the mouse promoter contains a consensus octamer element of the type ATGCTAAT. This octamer element can be bound by either Oct-1 or Oct-2 but requires the expression of Oct-2 to activate transcription in B cells. Mutation of the octamer element renders the CD36 promoter refractory to activation by Oct-2. Furthermore, we demonstrate that the CD36 octamer element does not support recruitment of the B cell-specific co-activator OBF-1 and that CD36 expression is unaffected in primary B cells derived from obf-1(-/-) mice. We conclude that Oct-2 activates CD36 gene expression in mouse B cells via the octamer element in the promoter. Our data also demonstrate that CD36 is the first example of an Oct-2-dependent gene whose expression in B cells is independent of OBF-1. These findings support the notion that Oct-2 regulates gene transcription by both OBF-1-dependent and -independent mechanisms.

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.

Transcriptional mechanisms regulating myeloid-specific genes

Myeloid blood cells comprise an important component of the immune system. Proper control of both lineage- and stage-specific gene expression is required for normal myeloid cell development and function. In recent years, a relatively small number of critical transcriptional regulators have been identified that serve important roles both in myeloid cell development and regulation of lineage-restricted gene expression in mature myeloid cells. This review summarizes our current understanding of the regulation of lineage- and stage-restricted transcription during myeloid cell differentiation, how critical transcriptional regulators control myeloid cell development, and how perturbations in transcription factor function results in the development of leukemia.

CCAAT/enhancer-binding proteins (C/EBP) beta and delta activate osteocalcin gene transcription and synergize with Runx2 at the C/EBP element to regulate bone-specific expression

CCAAT/enhancer-binding proteins (C/EBP) are critical determinants for cellular differentiation and cell type-specific gene expression. Their functional roles in osteoblast development have not been determined. We addressed a key component of the mechanisms by which C/EBP factors regulate transcription of a tissue-specific gene during osteoblast differentiation. Expression of both C/EBPbeta and C/EBPdelta increases from the growth to maturation developmental stages and, like the bone-specific osteocalcin (OC) gene, is also stimulated 3-6-fold by vitamin D(3), a regulator of osteoblast differentiation. We characterized a C/EBP enhancer element in the proximal promoter of the rat osteocalcin gene, which resides in close proximity to a Runx2 (Cbfa1) element, essential for tissue-specific activation. We find that C/EBP and Runx2 factors interact together in a synergistic manner to enhance OC transcription (35-40-fold) in cell culture systems. We show by mutational analysis that this synergism is mediated through the C/EBP-responsive element in the OC promoter and by a direct interaction between Runx2 and C/EBPbeta. Furthermore, we have mapped a domain in Runx2 necessary for this interaction by immunoprecipitation. A Runx2 mutant lacking this interaction domain does not exhibit functional synergism. We conclude that, in addition to Runx2 DNA binding functions, Runx2 can also form a protein complex at C/EBP sites to regulate transcription. Taken together, our findings indicate that C/EBP is a principal transactivator of the OC gene and the synergism with Runx2 suggests that a combinatorial interaction of these factors is a principal mechanism for regulating tissue-specific expression during osteoblast differentiation.

Structural and functional characterization of the mouse fatty acid translocase promoter: activation during adipose differentiation

Fatty acid translocase (FAT/CD36) is a cell-surface glycoprotein that functions as a receptor/transporter for long-chain fatty acids (LCFAs), and interacts with other protein and lipid ligands. FAT/CD36 is expressed by various cell types, including platelets, monocytes/macrophages and endothelial cells, and tissues with an active LCFA metabolism, such as adipose, small intestine and heart. FAT/CD36 expression is induced during adipose cell differentiation and is transcriptionally up-regulated by LCFAs and thiazolidinediones in pre-adipocytes via a peroxisome-proliferator-activated receptor (PPAR)-mediated process. We isolated and analysed the murine FAT/CD36 promoter employing C(2)C(12)N cells directed to differentiate to either adipose or muscle. Transient transfection studies revealed that the 309 bp upstream from the start of exon 1 confer adipose specific activity. Sequence analysis of this DNA fragment revealed the presence of two imperfect direct repeat-1 elements. Electrophoretic mobility-shift assay demonstrated that these elements were peroxisome-proliferator-responsive elements (PPREs). Mutagenesis and transfection experiments indicated that both PPREs co-operate to drive strong promoter activity in adipose cells. We conclude that murine FAT/CD36 expression in adipose tissue is dependent upon transcriptional activation via PPARs through binding to two PPREs located at -245 to -233 bp and -120 to -108 bp from the transcription start site.

Variability of the CD36 gene in West Africa

Studying 12 selected individuals from a malaria-endemic area in West Africa, 24 variants of the CD36 gene were found, 21 of them novel ones. These included three single-nucleotide substitutions causing non-conservative amino acid exchanges E123K, T174A, and I271T as well as a three base pair (bp) insertion resulting in the addition of an asparagine residue (N232-233ins). The E123K variant was located within the putative ligand-binding domain for oxidized low density lipoprotein, while the other substitutions resided outside any of the binding sites for reaction partners mapped on CD36 so far. Twelve single-nucleotide polymorphisms (SNPs) were identified in untranslated parts of the exons and in introns. Five additional SNPs were located in the promoter region whereby -144G-->T, -53G-->T, and -2A-->G alter putative binding sites for the transcription factors purine factor (PuF), phorbol ester-responsive element AP-2, and CCAAT/enhancer-binding protein. A G-->T exchange at position -50 appears to introduce a new recognition site for PuF. Calculations of nucleotide diversity revealed extraordinarily high numbers for all parts of the gene, which may, however, to some extent be due to the selection of individuals studied.

Expression and function of a stem cell promoter for the murine CBFalpha2 gene: distinct roles and regulation in natural killer and T cell development

The Runt family transcription factor CBFalpha2 (AML1, PEBP2alphaB, or Runx1) is required by hematopoietic stem cells and expressed at high levels in T-lineage cells. In human T cells CBFalpha2 is usually transcribed from a different promoter (distal promoter) than in myeloid cells (proximal promoter), but the developmental and functional significance of this promoter switch has not been known. Here, we report that both coding and noncoding sequences of the distal 5' end are highly conserved between the human and the murine genes, and the distal promoter is responsible for the overwhelming majority of CBFalpha2 expression in murine hematopoietic stem cells as well as in T cells. Distal promoter activity is maintained throughout T cell development and at lower levels in B cell development, but downregulated in natural killer cell development. The distal N-terminal isoform binds to functionally important regulatory sites from known target genes with two- to threefold higher affinity than the proximal N-terminal isoform. Neither full-length isoform alters growth of a myeloid cell line under nondifferentiating conditions, but the proximal isoform selectively delays mitotic arrest of the cell line under differentiating conditions, resulting in the generation of greater numbers of neutrophils.

The role of PPAR-gamma in macrophage differentiation and cholesterol uptake

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma), the transcription factor target of the anti-diabetic thiazolidinedione (TZD) drugs, is reported to mediate macrophage differentiation and inflammatory responses. Using PPAR-gamma-deficient stem cells, we demonstrate that PPAR-gamma is neither essential for myeloid development, nor for such mature macrophage functions as phagocytosis and inflammatory cytokine production. PPAR-gamma is required for basal expression of CD36, but not for expression of the other major scavenger receptor responsible for uptake of modified lipoproteins, SR-A. In wild-type macrophages, TZD treatment divergently regulated CD36 and class A macrophage-scavenger receptor expression and failed to induce significant cellular cholesterol accumulation, indicating that TZDs may not exacerbate macrophage foam-cell formation.

Vitamin E reduces the uptake of oxidized LDL by inhibiting CD36 scavenger receptor expression in cultured aortic smooth muscle cells

BACKGROUND

Vitamin E is well known as an antioxidant, and numerous studies suggest that it has a preventive role in atherosclerosis, although the mechanism of action still remains unclear.

METHODS AND RESULTS

The original aim of this study was to establish whether alpha-tocopherol (the most active form of vitamin E) acts at the earliest events on the cascade of atherosclerosis progression, that of oxidized LDL (oxLDL) uptake and foam-cell formation. We show here that the CD36 scavenger receptor (a specific receptor for oxLDL) is expressed in cultured human aortic smooth muscle cells (SMCs). Treatment of SMCs and HL-60 macrophages with alpha-tocopherol (50 micromol/L, a physiological concentration) downregulates CD36 expression by reducing its promoter activity. Furthermore, we find that alpha-tocopherol treatment of SMCs leads to a reduction of oxLDL uptake.

CONCLUSIONS

This study indicates that CD36 is expressed in cultured human SMCs. In these cells, CD36 transports oxLDL into the cytosol. alpha-Tocopherol inhibits oxLDL uptake by a mechanism involving downregulation of CD36 mRNA and protein expression. Therefore, the beneficial effect of alpha-tocopherol against atherosclerosis can be explained, at least in part, by its effect of lowering the uptake of oxidized lipoproteins, with consequent reduction of foam cell formation.

A review and proposed nomenclature for major proteins of the milk-fat globule membrane

The characteristics and possible functions of the most abundant proteins associated with the bovine milk-fat globule membrane are reviewed. Under the auspices of the Milk Protein Nomenclature Committee of the ADSA, a revised nomenclature for the major membrane proteins is proposed and discussed in relation to earlier schemes. We recommend that proteins be assigned specific names as they are identified by molecular cloning and sequencing techniques. The practice of identifying proteins according to their Mr, electrophoretic mobility, or staining characteristics should be discontinued, except for uncharacterized proteins. The properties and amino acid sequences of the following proteins are discussed in detail: MUC1, xanthine dehydrogenase/oxidase, CD36, butyrophilin, adipophilin, periodic acid Schiff 6/7 (PAS 6/7), and fatty acid binding protein. In addition, a compilation of less abundant proteins associated with the bovine milk-fat globule membrane is presented.

Scavenger receptors and oxidized low density lipoproteins

Oxidized LDL has been shown to exhibit a number of potentially proatherogenic actions and properties, including receptor-mediated uptake and lipid accumulation within macrophages. It has been postulated that rapid, unregulated uptake of oxidatively modified LDL could account for the transformation of monocyte-derived macrophages to foam cells in atherosclerotic lesions. In support of this hypothesis, oxidized LDL and lipid peroxidation products have been shown to exist in atheromas in vivo. Furthermore, a number of cell membrane proteins that can bind oxidized LDL with high affinity have been identified on the surface of macrophages, endothelial cells and smooth muscle cells. One characteristic that almost all of these 'scavenger receptors' share is the ability to bind with high affinity to a broad spectrum of structurally unrelated ligands. Of all of the different classes of scavenger receptors that have been identified, the scavenger receptor class A type I/II (SR-AI/II) has received the most attention. Studies with macrophages from mice deficient in the gene for SR-AI/II provide direct evidence that a receptor other than the SR-AI/II is responsible for most of the uptake of oxidized LDL in murine macrophages. This article provides an overview of the characterization and functions of the scavenger receptors that have been shown to interact with oxidized LDL, including SR-AI/II, CD36, SR-BI, macrosialin/CD68, LOX-1, and SREC. Isolation and characterization of these and other scavenger receptors has increased our understanding of their role in the uptake of oxidized LDL and the pathogenesis of atherosclerosis.

Lovastatin decreases the receptor-mediated degradation of acetylated and oxidized LDLs in human blood monocytes during the early stage of differentiation into macrophages

3-Hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors are used therapeutically to upregulate the LDL receptor-mediated removal of plasma cholesterol by the liver. Several lines of evidence indicate that these drugs also exert direct effects on the metabolism of native and modified LDL in extrahepatic cells. We studied the effects of lovastatin (LOV) on the degradation of native, acetylated, and oxidized LDL, and on levels of mRNA encoding for the LDL, types I and II class A macrophage scavenger, and CD36 receptors in human blood monocytes at different stages of their maturation into adherent macrophages. LOV (10 micromol/L) reduced the degradation of acetylated LDL when added to freshly isolated cells cultured for 2 (81+/-4% of control, P<0.05) and 5 (76+/-6%, of control, P<0.05) days. The degradation of oxidized LDL was also reduced in cells treated with LOV for 2 days after seeding (51+/-3% of control, P<0. 001) but not in 5-day-old cells. LOV had no significant effect on the degradation of either acetylated or oxidized LDL when added to fully matured macrophages allowed to differentiate under control conditions for 7 days before incubations with 10 micromol/L LOV for an additional 2 days. In contrast, LOV increased the degradation of native LDL in these cells at all 3 stages of cell differentiation. LOV also reduced class A types I and II macrophage scavenger receptor and CD36 mRNA levels in 2- and 5-day-old cells but not in the more mature macrophages. These data suggest that 3-hydroxy-3-methylglutaryl-coenzyme A inhibitors may reduce the expression and function of the class A types I and II macrophage scavenger receptor and CD36 in monocytes, during the early stages of their differentiation into adherent macrophages. These effects, if operative in vivo, may slow down the development of the atherosclerotic plaque and thus contribute to the beneficial effects of these drugs.

Depression and bipolar disorder: relationships to impaired fatty acid and phospholipid metabolism and to diabetes, cardiovascular disease, immunological abnormalities, cancer, ageing and osteoporosis. Possible candidate genes

Depression and bipolar disorder are two of the commonest illnesses in the developed world. While some patients can be treated effectively with available drugs, many do not respond, especially in the depression related to bipolar disorder. Depression is associated with diabetes, cardiovascular disease, immunological abnormalities, multiple sclerosis, cancer, osteoporosis and ageing: in each case depressed individuals have a worse outcome than non-depressed individuals. In all of these conditions there is now evidence of impaired phospholipid metabolism and impaired fatty acid-related signal transduction processes. Impaired fatty acid and phospholipid metabolism may be a primary cause of depression in many patients and may explain the interactions with other diseases. Several novel gene candidates for involvement in depression and bipolar disorder are proposed.

Receptors for oxidized low density lipoprotein

An increasing body of evidence indicates that oxidized low density lipoprotein (LDL) is involved in the pathogenesis of atherosclerosis. One of the first biologic actions of oxidized LDL to be identified in vitro was its ability to interact with the 'acetyl LDL receptor' discovered by Goldstein and Brown. Over the past decade, considerable progress has been made in identifying and characterizing cell-surface receptors for oxidized LDL. Most of these receptors are thought to be multifunctional because they interact with several structurally different ligands, and accordingly have been termed 'scavenger receptors'. The objective of this article is to review the most important publications dealing with structure, ligand specificity, regulation, and function of scavenger receptors.

Native and modified low density lipoproteins increase the functional expression of the macrophage class B scavenger receptor, CD36

The uptake of oxidized low density lipoprotein (OxLDL) by macrophages is a key event implicated in the initiation and development of atherosclerotic lesions. Two macrophage surface receptors, CD36 (a class B scavenger receptor) and the macrophage scavenger receptor (a class A scavenger receptor), have been identified as the major receptors that bind and internalize OxLDL. Expression of CD36 in monocyte/macrophages in tissue culture is dependent both on the differentiation state as well as exposure to soluble mediators (cytokines and growth factors). The regulatory mechanisms of this receptor in vivo are undetermined as is the role of lipoproteins themselves in modulating CD36 expression. We studied the effect of lipoproteins, native LDL and modified LDL (acetylated LDL (AcLDL) and OxLDL) on the expression of CD36 in J774 cells, a murine macrophage cell line. Exposure to lipoproteins resulted in a marked induction of CD36 mRNA expression (4-8-fold). Time course studies showed that maximum induction was observed 2 h after treatment with AcLDL and at 4 h with LDL and OxLDL. Increased expression of CD36 mRNA persisted for 24 h with each treatment group. Induction of CD36 mRNA expression was paralleled by an increase in CD36 protein as determined by Western blot with the greatest induction by OxLDL (4-fold). In the presence of actinomycin D, treatment of macrophages with LDL, AcLDL, or OxLDL did not affect CD36 mRNA stability, implying that CD36 mRNA was transcriptionally regulated by lipoproteins. To determine the mechanism(s) by which lipoproteins increased expression of CD36 we evaluated the effects of lipoprotein components on CD36 mRNA expression. ApoB 100 increased CD36 mRNA expression significantly, whereas phospholipid/cholesterol liposomes had less effect. Incubation of macrophages with bovine serum albumin or HDL reduced expression of CD36 mRNA in a dose-dependent manner. Finally, to evaluate the in vivo relevance of the induction of CD36 mRNA expression by lipoproteins, peritoneal macrophages were isolated from mice following intraperitoneal injection of lipoproteins. Macrophage expression of CD36 mRNA was significantly increased by LDL, AcLDL, or OxLDL in relation to mice infused with phosphate-buffered saline, with OxLDL causing the greatest induction (8-fold). This is the first demonstration that exposure to free and esterified lipids augments functional expression of the class B scavenger receptor, CD36. These data imply that lipoproteins can further contribute to foam cell development in atherosclerosis by up-regulating a major OxLDL receptor.

Embryonic lethality and impairment of haematopoiesis in mice heterozygous for an AML1-ETO fusion gene

Acute myeloid leukaemia (AML) is a major haematopoietic malignancy characterized by the proliferation of a malignant clone of myeloid progenitor cells. A reciprocal translocation, t(8;21)(q22;q22), observed in the leukaemic cells of approximately 40% of patients with the M2 subtype of AML disrupts both the AML1 (CBFA2) gene on chromosome 21 and the ETO (MTG8) gene on chromosome 8 (refs 3-5). A chimaeric protein is synthesized from one of the derivative chromosomes that contains the N terminus of the AML1 transcription factor, including its DNA-binding domain, fused to most of ETO, a protein of unknown function. We generated mice that mimic human t(8;21) with a "knock-in' strategy. Mice heterozygous for an AML1-ETO allele (AML1-ETO/+) die in midgestation from haemorrhaging in the central nervous system and exhibit a severe block in fetal liver haematopoiesis. This phenotype is very similar to that resulting from homozygous disruption of the AML1 (Cbfa2) or Cbfb genes, indicating that AML1-ETO blocks normal AML1 function. However, yolk sac cells from AML1-ETO/+ mice differentiated into macrophages in haematopoietic colony forming unit (CFU) assays, unlike Cbfa2-/- or Cbfb-/-cells, which form no colonies in vitro. This indicates that AML1-ETO may have other functions besides blocking wild-type AML1, a property that may be important in leukaemogenesis.