Transforming growth factor-beta1 induces collagen synthesis and accumulation via p38 mitogen-activated protein kinase (MAPK) pathway in cultured L(6)E(9) myoblasts

Transforming growth factor-beta (TGF-beta) plays a pivotal role in the extracellular matrix accumulation observed in chronic progressive tissue fibrosis, but the intracellular signaling mechanism by which TGF-beta stimulates this process remains poorly understood. We examined whether mitogen-activated protein kinase (MAPK) routes were involved in TGF-beta1-induced collagen expression in L(6)E(9) myoblasts. TGF-beta1 induced p38 and extracellular signal-regulated kinase (ERK) 1/2 phosphorylation whereas no effect on Jun N-terminal kinase phosphorylation was observed. Biochemical blockade of p38 but not of the ERK MAPK pathway abolished TGF-beta1-induced alpha(2)(I) collagen mRNA expression and accumulation. These data indicate that TGF-beta1-induced p38 activation is involved in TGF-beta1-stimulated collagen synthesis.

Synergistic cooperation between hypoxia and transforming growth factor-beta pathways on human vascular endothelial growth factor gene expression

Signaling by transforming growth factor (TGF)-beta family members is mediated by Smad proteins that regulate gene transcription through functional cooperativity and association with other DNA-binding proteins. The hypoxia-inducible factor (HIF)-1 is a transcriptional complex that plays a key role in oxygen-regulated gene expression. We demonstrate that hypoxia and TGF-beta cooperate in the induction of the promoter activity of vascular endothelial growth factor (VEGF), which is a major stimulus in the promotion of angiogenesis. This cooperation has been mapped on the human VEGF promoter within a region at -1006 to -954 that contains functional DNA-binding sequences for HIF-1 and Smads. Optimal HIF-1alpha-dependent induction of the VEGF promoter was obtained in the presence of Smad3, suggesting an interaction between these proteins. Consistent with this, co-immunoprecipitation experiments revealed that HIF-1alpha physically associates with Smad3. These results demonstrate that both TGF-beta and hypoxia signaling pathways can synergize in the regulation of VEGF gene expression at the transcriptional level.

Extracellular signal-regulated protein kinase signaling pathway negatively regulates the phenotypic and functional maturation of monocyte-derived human dendritic cells

Dendritic cells (DC) are highly specialized antigen-presenting cells that on activation by inflammatory stimuli (eg, tumor necrosis factor alpha [TNF-alpha] and interleukin-1beta [IL-1beta]) or infectious agents (eg, lipopolysaccharide [LPS]), mature and migrate into lymphoid organs. During maturation, DC acquire the capacity to prime and polarize resting naive T lymphocytes. Maturation of monocyte-derived DC (MDDC) is inhibited by the p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580. This study found that in the presence of the mitogen-activated protein kinase kinase 1-extracellular signal-regulated kinase (ERK) inhibitors PD98059 or U0126, TNF-alpha- and LPS-induced phenotypic and functional maturation is enhanced. ERK pathway inhibitors increased expression of major histocompatibility complex and costimulatory molecules; loss of mannose-receptor-mediated endocytic activity; nuclear factor-kappaB DNA-binding activity; release of IL-12 p40; and allogeneic T-cell proliferation induced by LPS or TNF-alpha. Moreover, PD98059 and U0126 enhanced LPS-triggered production of IL-12 p70. In agreement with the effect of ERK inhibitors, maturation of MDDC was delayed in the presence of serum, an effect that was reversed by U0126. These results indicate that the ERK and p38 MAPK signaling pathways differentially regulate maturation of MDDC and suggest that their relative levels of activation might modulate the initial commitment of naive T-helper (Th) cells toward Th1 or Th2 subsets. The findings also suggest that maturation of MDDC might be pharmacologically modified by altering the relative levels of activation of both intracellular signaling routes.

Identification of a critical Sp1 site within the endoglin promoter and its involvement in the transforming growth factor-beta stimulation

Endoglin, a component of the transforming growth factor-beta (TGF-beta) receptor complex expressed on endothelial cells, is involved in cardiovascular morphogenesis and vascular remodeling, as exemplified by the fact that the endoglin gene is the target for the autosomal dominant disorder known as hereditary hemorrhagic telangiectasia type 1. Since haploinsufficiency is the underlying mechanism for hereditary hemorrhagic telangiectasia type 1, understanding the regulation of endoglin gene expression appears to be a crucial step to correct the disease. In this study we have identified an Sp1 site at -37 as a critical element for the basal transcription of the endoglin TATA-less promoter. Since endoglin promoter activity is stimulated by TGF-beta and this stimulation is located at the Sp1-containing proximal region, we have investigated the possible involvement of Sp1 in the TGF-beta-mediated induction. Mutation of the Sp1-binding sequence, or addition of the Sp1 inhibitor WP631, abolished both the basal transcription activity and the TGF-beta responsiveness of the endoglin promoter. Binding of Sp1 and Smad3 to the proximal promoter region -50/-29 was evidenced by electrophoretic mobility shift assays and DNA affinity precipitation studies. Furthermore, synergistic cooperation on the promoter activity between Sp1 and TGF-beta or Smad3 could be demonstrated by co-transfection experiments of reporter promoter constructs. The molecular mechanism underlying this cooperation appears to involve a direct physical interaction between Sp1 and Smad3/Smad4.

Vascular gene transfer driven by endoglin and ICAM-2 endothelial-specific promoters

The involvement of the vascular endothelium in a large number of diseases supports the importance of vascular-specific gene delivery for their treatment. The hereditary hemorrhagic telangiectasia type 1 is an example of a vascular inherited disease (OMIM 187300). This is an autosomal dominant vascular disorder originated by mutations in the endoglin gene and associated with frequent epistaxis, telangiectases, gastrointestinal bleedings, and arteriovenous malformations in brain, lung and liver. Here, we address for the first time the possibility of using in vivo gene transfer to target endoglin expression to the vasculature. The promoter of the endothelial gene, ICAM-2, was used to generate transgenic animals which demonstrated endothelial expression of endoglin. Next, the promoters of the human endothelial genes, endoglin and ICAM-2, were inserted upstream of the human endoglin cDNA, and the resulting constructs were systemically or locally delivered, demonstrating endoglin expression in the vessel walls of liver, lung and skin. These gene transfer experiments represent an initial step in the treatment of the hereditary hemorrhagic telangiectasia type 1 by gene therapy, and suggest that endoglin and ICAM-2 promoters can be used to deliver other genes to the endothelium specifically.

Endoglin expression in human and rat mesangial cells and its upregulation by TGF-beta1

Endoglin is a component of the TGF-beta receptor complex present in the kidney at the human glomerular mesangium. Since the cellular origin of the glomerular endoglin is unknown, in the present study we investigated the expression of endoglin in mesangial cells in culture, as well as their response to TGF-beta1. Western and Northern blot analysis identified the expression of endoglin protein and mRNA transcript in both human and rat mesangial cells. Flow cytometry and immunocytochemistry analyses revealed that endoglin is present on the cell membrane. Exogenous TGF-beta1 stimulated not only the expression of collagen alpha1 (I) I and TGF-beta1, but also that of endoglin. These data provide the first evidence for the expression of endoglin in mesangial cells, as well as its upregulation by TGF-beta1, thus suggesting that endoglin may have a role in modulating the effects of TGF-beta1 on the glomerular mesangium.

Maturation-dependent expression and function of the CD49d integrin on monocyte-derived human dendritic cells

Dendritic cells (DC) are highly specialized APC that are critical for the initiation of T cell-dependent immune responses. DC exert a sentinel function while immature and, after activation by inflammatory stimuli or infectious agents, mature and migrate into lymphoid organs to prime T cells. We have analyzed integrin expression on monocyte-derived DC (MDDC) and found that expression of CD49d integrins (CD49d/CD29 and CD49d/beta7) was induced/up-regulated during TNF-alpha- or LPS-initiated MDDC maturation, reflecting the induction/up-regulation of CD49d and beta7 mRNA. CD49d mRNA steady-state level increased more than 10 times during maturation, with the highest levels observed 24 h after TNF-alpha treatment. CD49d integrin expression conferred mature MDDC with an elevated capacity to adhere to the CS-1 fragment of fibronectin, and also mediated transendothelial migration of mature MDDC. Up-regulation of CD49d integrin expression closely paralleled that of the mature DC marker CD83. CD49d integrin expression was dependent on cell maturation, as its induction was abrogated by N:-acetylcysteine, which inhibits NF-kappaB activation and the functional and phenotypic maturation of MDDC. Moreover, CD49d integrin up-regulation and MDDC maturation were prevented by SB203580, a specific inhibitor of p38 mitogen-activated protein kinase, but were almost unaffected by the mitogen-activated protein/extracellular signal-related kinase kinase 1/2 inhibitor PD98059. Our results support the existence of a link between functional and phenotypic maturation of MDDC and CD49d integrin expression, thus establishing CD49d as a maturation marker for MDDC. The differential expression of CD49d on immature and mature MDDC might contribute to their distinct motility capabilities and mediate mature DC migration into lymphoid organs.

Polyomavirus enhancer-binding protein 2/core binding factor/acute myeloid leukemia factors contribute to the cell type-specific activity of the CD11a integrin gene promoter

The CD11a/CD18 leukocyte integrin (LFA-1; also known as alphaL/beta2) mediates leukocyte transendothelial migration during immune and inflammatory responses and participates in lymphoma metastasis. CD11a/CD18 leukocyte-restricted expression is controlled by the CD11a gene promoter, which confers tissue-specific expression to reporter genes in vitro and in vivo. DNase I protection analysis of the CD11a proximal gene promoter revealed DNA-protein interactions centered at position -110 (CD11a-110). Disruption of CD11a-110 reduced CD11a promoter activity in a cell type-specific manner, as it reduced its activity by 70% in Jurkat lymphoid cells, whereas the effect was considerably lower in K562 and HepG2 cells. Electrophoretic mobility shift assays showed evidence of cell type-specific differences in CD11a-110 binding and indicated its specific recognition by members of the polyomavirus enhancer-binding protein 2/core binding factor (CBF)/acute myeloid leukemia (AML) family of transcription factors. AML1B/CBFbeta transactivated the CD11a promoter, with AML1B/CBFbeta-mediated transactivation being completely dependent on the integrity of the CD11a-110 element. Therefore, CBF/AML factors play a role in the cell type-restricted transcription of the CD11a integrin gene through recognition of CD11a-110. The involvement of CBF/AML factors in CD11a expression raises the possibility that CD11a/CD18 expression might be deregulated in acute myeloid and B-lineage acute lymphoblastic leukemias, thus contributing to their altered adhesion and metastatic potential.

c-Myc inhibits CD11a and CD11c leukocyte integrin promoters

The c-Myc transcription factor is an important regulator of cell growth and differentiation, and its gene repression ability seems to play a key role in Myc-mediated cellular transformation. Since Myc overexpression has been associated with reduced expression of beta1 and beta2 integrins, we have investigated the role of c-Myc on CD11a and CD11c transcription. c-Myc inhibited CD11a and CD11c integrin promoter activity in co-transfection experiments, and similar repression was obtained in cells where c-Myc expression (KmycB) or activity (Rat-1 c-MycER) is inducible. The c-Myc repression on the CD11c promoter was independent of the USF-binding site (USF-150), other putative Myc-binding elements, or the integrity of the initiator (Inr)-like sequence present at the major transcriptional start site. Analysis of deletion and mutant promoter constructs revealed that, in the absence of additional upstream cisacting elements, an AP-1-binding site at -60 (AP1-60) is required for c-Myc repressor activity. The c-Myc repressor activity on both integrin promoters was abrogated by deletion of c-Myc residues 106-143, a domain involved in Inr-dependent transcriptional repression. These results demonstrate a direct effect of c-Myc on integrin gene transcription and suggest the existence of a c-Myc-dependent mechanism for coupling leukocyte integrin expression to the cell proliferative state.

Identification of a functional NF-kappa B site in the platelet endothelial cell adhesion molecule-1 promoter

Platelet endothelial cell adhesion molecule-1 (PECAM-1) is a type I transmembrane adhesion protein of 130 kDa that belongs to a subgroup of the Ig gene superfamily, characterized by the presence of immunoreceptor tyrosine-based inhibitory motifs. PECAM-1 is expressed in circulating platelets, monocytes, neutrophils, a selective subgroup of T cells, and in endothelial cells, where it is preferentially located at intercellular junctions and participates in leukocyte transmigratory processes. The identification of two consensus NF-kappa B sites within the PECAM-1 promoter led us to analyze their possible involvement in the PECAM-1 expression regulated by inflammatory stimuli. We found that surface expression and promoter activity of PECAM-1 in myeloid cells are regulated by modulators of NF-kappa B, including TNF-alpha, PMA, and pyrrolidine dithiocarbamate. Mobility shifts assays identified a specific NF-kappa B-binding element at +110/+120, whose mutation abolished the basal promoter activity of PECAM-1 and decreased NF-kappa B-dependent responses of the PECAM-1 gene promoter. Furthermore, cotransfection experiments with an expression vector encoding the p65 subunit of NF-kappa B showed transactivation of the PECAM-1 promoter. These results demonstrate that NF-kappa B can regulate the transcriptional activity of PECAM-1.

Endoglin is expressed in the chicken vasculature and is involved in angiogenesis

Endoglin is a component of the transforming growth factor beta (TGF-beta) receptor complex, highly expressed by endothelial cells. Mutations in the endoglin gene are responsible for hereditary hemorrhagic telangiectasia type 1 (HHT1), an autosomal dominant vascular disorder caused by a haploinsufficiency mechanism. Vascular lesions (telangiectasia and arteriovenous malformations) in HHT1 are associated with loss of the capillary network, suggesting the involvement of endoglin in vascular repair processes. Using the chick chorioallantoic membrane (CAM) as an angiogenic model, we have analyzed the expression and function of chicken endoglin. A pan-specific polyclonal antibody (pAb) recognized chicken endoglin as demonstrated by immunostaining and Western blot analysis. In ovo treatment of chicken embryos with this pAb resulted in a significantly increased area of CAM. This effect was likely mediated by modulation of the ligand binding to endoglin as this pAb was able to inhibit TGF-beta1 binding. These results support the involvement of endoglin in the angiogenic process.

Endoglin overexpression modulates cellular morphology, migration, and adhesion of mouse fibroblasts

Endoglin is the gene mutated in hereditary hemorrhagic telangiectasia type 1 (HHT1), a dominantly inherited vascular disorder. Endoglin glycoprotein is a component of the transforming growth factor type beta (TGF-beta) receptor system which is highly expressed by endothelial cells, and at lower levels on fibroblasts and smooth muscle cells, suggesting the involvement of these lineages in the HHT1 vascular dysplasia. Overexpression of endoglin in mouse NCTC929 fibroblasts led to decreased migration in chemotactic and wound healing assays, as well as changes in the cellular morphology. When plated on uncoated surfaces, endoglin transfectants formed intercellular clusters, endoglin being not specifically localized to the cell-cell junctions, but homogenously distributed on the cellular surface. Although the expression of alpha5beta1 integrin and of an activation epitope of beta1 integrin were unchanged, a polyclonal antibody to alpha5beta1 integrin was able to inhibit cluster formation, suggesting the involvement of integrin ligand/s. In fact, coating with fibronectin, laminin, or an RGD-containing 80 kDa fragment of fibronectin were able to prevent the cellular clustering. Furthermore, synthesis of plasminogen activator inhibitor 1 (PAI-1), and to a weak extent that of fibronectin, were inhibited in endoglin transfectants. Thus, the presence of endoglin in mouse NCTC929 fibroblasts is associated with reduced production of certain extracellular matrix (ECM) components, which might explain their altered morphology, migration and intercellular cluster formation.

Expression of normal and truncated forms of human endoglin

Endoglin is a transmembrane glycoprotein 633 residues in length expressed at the surface of endothelial cells as a disulphide-linked homodimer; the specific cysteine residues involved in endoglin dimerization are unknown. Mutations in the coding region of the endoglin gene are responsible for hereditary haemorrhagic telangiectasia type 1 (HHT1), a dominantly inherited vascular disorder. Many of these mutations, if translated, would lead to truncated forms of the protein. It is therefore of interest to assess the protein expression of different truncated forms of endoglin. Infections in vitro or in vivo with recombinant vaccinia virus, as well as transient transfections with expression vectors, were used to express normal and truncated forms of endoglin. Truncated mutants could be classified into three different groups: (1) those that did not produce stable transcripts; (2) those that produced stable transcripts but did not secrete the protein; and (3) those that secreted a soluble dimeric protein. This is the first time that a recombinant truncated form of endoglin has been found to be expressed in a soluble form. Because a chimaeric construct encoding the N-terminal sequence of platelet/endothelial cell adhesion molecule (PECAM-1) antigen fused to residues Ile281-Ala658 of endoglin also yielded a dimeric surface protein, these results suggest that cysteine residues contained within the fragment Cys330-Cys412 are involved in disulphide bond formation. Infection with vaccinia recombinants encoding an HHT1 mutation did not affect the expression of the normal endoglin, and did not reveal an association of the recombinant soluble form with the transmembrane endoglin, supporting a haploinsufficiency model for HHT1.

Cloning of the promoter region of human endoglin, the target gene for hereditary hemorrhagic telangiectasia type 1

Endoglin (CD105) is a cell surface component of the transforming growth factor-beta (TGF-beta) receptor complex highly expressed by endothelial cells. Mutations in the endoglin gene are responsible for the hereditary hemorrhagic telangiectasia type 1 (HHT1), also known as Osler-Weber-Rendu syndrome (OMIM 187300). This is an autosomal dominant vascular disorder probably caused by a haploinsufficiency mechanism displaying low levels of the normal protein. To understand the mechanisms underlying the regulated expression of endoglin, a genomic DNA clone containing 3.3 kb of the 5'-flanking sequence of the human endoglin gene has been isolated. The 5'-flanking region of the endoglin gene lacks consensus TATA and CAAT boxes, but contains two GC-rich regions and consensus motifs for Sp1, ets, GATA, AP-2, NFkappaB, and Mad, as well as TGF-beta-, glucocorticoid-, vitamin D-, and estrogen-responsive elements. As determined by primer extension and 5' RACE experiments, a cluster of transcriptional start sites was found to be located 350 bp upstream from the translation initiation codon. To analyze the endoglin promoter activity, the upstream -400/+341 fragment was fused to the luciferase gene and transient transfections were conducted in several cell types. This construct displayed a tissue-specific activity in human and bovine endothelial cells. Analysis of various deletion constructs showed the existence of a basal promoter region within the -81/+350 fragment as well as major transcriptional regulatory elements within the -400/-141 fragment. Electrophoretic mobility shift assays demonstrated the specific interaction of a member of the ets family with a consensus motif located at position -68. A promoter construct mutated at this ets sequence showed a much reduced activity as compared with the wild-type construct, supporting the involvement of this ets motif in the basal activity of the promoter. The endoglin promoter exhibited inducibility in the presence of TGF-beta1, suggesting possible therapeutic treatments in HHT1 patients, in which the expression level of the normal endoglin allele might not reach the threshold required for its function. Isolation and characterization of the human endoglin promoter represents an initial step in elucidating the controlled expression of the endoglin gene.

Endoglin, a component of the TGF-beta receptor system, is a differentiation marker of human choriocarcinoma cells

Endoglin is an integral membrane glycoprotein that binds transforming growth factor-beta1 (TGF-beta1) with high affinity and it is strongly expressed on syncytiotrophoblasts throughout pregnancy. Here, we describe the expression of endoglin by the choriocarcinoma cell line JAR as evidenced by flow cytometry, immunoprecipitation, Western blot and reverse transcriptase polymerase chain reaction analyses. Cross-linking experiments of [125I]-labeled TGF-beta1 to JAR cells indicated that endoglin expressed at the surface of these cells binds TGF-beta. Furthermore, staining of human choriocarcinoma tissue sections with a polyclonal antibody to endoglin demonstrated a high expression of endoglin in syncytiotrophoblast-like areas, as opposed to a negative staining of cytotrophoblast-like cells. This pattern of endoglin expression was confirmed by experiments with methotrexate, an inducer of giant, multinucleated, non-proliferative cells, morphologically indistinguishable from the naturally occurring syncytiotrophoblasts. Thus, treatment of the JAR and JEG-3 choriocarcinoma cell lines with methotrexate led to an increase in endoglin expression, as demonstrated by Western and Northern blot analyses. Taken together, our results suggest that endoglin, in addition to being involved in placental development, may also be a cellular differentiation marker.

Cloning of the human platelet endothelial cell adhesion molecule-1 promoter and its tissue-specific expression. Structural and functional characterization

Platelet endothelial cell adhesion molecule-1 (PECAM-1; CD31) is a cell adhesion molecule involved in transendothelial migration and expressed by hemopoietic and endothelial cells. To understand the mechanisms underlying its regulated expression, a genomic clone containing 1555 bp of the 5'-flanking region and the first exon of the human PECAM-1 gene has been isolated. The 5'-flanking region of the PECAM-1 gene lacks a consensus TATA box, but contains consensus motifs for Sp1, EGR1, ets, helix-loop-helix (HLH) box, GATA, AP-2, C/EBP, YY1, CCACC, LyF-1, imperfect octamer, heptamer, high mobility group proteins (HMG) box, and nuclear factor-kappaB, as well as shear stress-, retinoic acid-, glucocorticoid-, and acute phase-responsive elements, and an Alu sequence. Successive 5' to 3' or 3' to 5' deletions revealed tissue-specific promoter activity within the two contiguous 0.22-kb NheI/BglII and 0.44-kb BglII/PstI fragments. The transcriptional activity displayed by the 0.22-kb NheI/BglII fragment was specific for the myeloid lineage, whereas the promoter activity of the 0.44-kb BglII/PstI fragment was apparently restricted to endothelial cells. The transcriptional activity of the 0.22-kb NheI/BglII fragment was confirmed by 5' RACE (rapid amplification of 5' cDNA ends) and S1 nuclease protection experiments that revealed previously unidentified transcription start sites. The 0.22-kb NheI/BglII promoter exhibited PMA inducibility in myeloid cells and contained a PMA-responsive element recognized by Sp1 and EGR-1 transcription factors. Isolation and characterization of the human PECAM-1 promoter represent an initial step in elucidating the controlled expression of the PECAM-1 gene.

Cloning of the human platelet endothelial cell adhesion molecule-1 promoter and its tissue-specific expression. Structural and functional characterization

Platelet endothelial cell adhesion molecule-1 (PECAM-1; CD31) is a cell adhesion molecule involved in transendothelial migration and expressed by hemopoietic and endothelial cells. To understand the mechanisms underlying its regulated expression, a genomic clone containing 1555 bp of the 5'-flanking region and the first exon of the human PECAM-1 gene has been isolated. The 5'-flanking region of the PECAM-1 gene lacks a consensus TATA box, but contains consensus motifs for Sp1, EGR1, ets, helix-loop-helix (HLH) box, GATA, AP-2, C/EBP, YY1, CCACC, LyF-1, imperfect octamer, heptamer, high mobility group proteins (HMG) box, and nuclear factor-kappaB, as well as shear stress-, retinoic acid-, glucocorticoid-, and acute phase-responsive elements, and an Alu sequence. Successive 5' to 3' or 3' to 5' deletions revealed tissue-specific promoter activity within the two contiguous 0.22-kb NheI/BglII and 0.44-kb BglII/PstI fragments. The transcriptional activity displayed by the 0.22-kb NheI/BglII fragment was specific for the myeloid lineage, whereas the promoter activity of the 0.44-kb BglII/PstI fragment was apparently restricted to endothelial cells. The transcriptional activity of the 0.22-kb NheI/BglII fragment was confirmed by 5' RACE (rapid amplification of 5' cDNA ends) and S1 nuclease protection experiments that revealed previously unidentified transcription start sites. The 0.22-kb NheI/BglII promoter exhibited PMA inducibility in myeloid cells and contained a PMA-responsive element recognized by Sp1 and EGR-1 transcription factors. Isolation and characterization of the human PECAM-1 promoter represent an initial step in elucidating the controlled expression of the PECAM-1 gene.

Endoglin modulates cellular responses to TGF-beta 1

Endoglin is a homodimeric membrane glycoprotein which can bind the beta 1 and beta 3 isoforms of transforming growth factor-beta (TGF-beta). We reported previously that endoglin is upregulated during monocyte differentiation. We have now observed that TGF-beta itself can stimulate the expression of endoglin in cultured human monocytes and in the U-937 monocytic line. To study the functional role of endoglin, stable transfectants of U-937 cells were generated which overexpress L- or S- endoglin isoforms, differing in their cytoplasmic domain. Inhibition of cellular proliferation and downregulation of c-myc mRNA which are normally induced by TGF-beta 1 in U-937 cells were totally abrogated in L-endoglin transfectants and much reduced in the S-endoglin transfectants. Inhibition of proliferation by TGF-beta 2 was not altered in the transfectants, in agreement with the isoform specificity of endoglin. Additional responses of U-937 cells to TGF-beta 1, including stimulation of fibronectin synthesis, cellular adhesion, platelet/endothelial cell adhesion molecule 1 (PECAM-1) phosphorylation, and homotypic aggregation were also inhibited in the endoglin transfectants. However, modulation of integrin and PECAM-1 levels and stimulation of mRNA levels for TGF-beta 1 and its receptors R-I, R-II, and betaglycan occurred normally in the endoglin transfectants. No changes in total ligand binding were observed in L-endoglin transfectants relative to mock, while a 1.5-fold increase was seen in S-endoglin transfectants. The degradation rate of the ligand was the same in all transfectants. Elucidating the mechanism by which endoglin modulates several cellular responses to TGF-beta 1 without interfering with ligand binding or degradation should increase our understanding of the complex pathways which mediate the effects of this factor.

Functional regulation of platelet/endothelial cell adhesion molecule-1 by TGF-beta 1 in promonocytic U-937 cells

Platelet endothelial cell adhesion molecule-1 (PECAM-1) is a widely distributed cell adhesion molecule present on monocytes, macrophages, and monocytic cell lines. Treatment of the promonocytic cell line U-937 with TGF-beta 1 induces homotypic cellular aggregations, simultaneous with an increase in surface expression and specific transcripts of PECAM-1. The TGF-beta-induced cell adhesion phenomena are not dependent on LFA-1, intercellular adhesion molecule-1 (ICAM-1), very late Ag-4 (VLA-4), or very late Ag-5 (VLA-5). However, the phenomena seem to be directly mediated by PECAM-1 because 1) it is inhibited by the addition of Abs or an antisense oligonucleotide specific for PECAM-1; and 2) TGF-beta 1-treated U-937 cells bind to PECAM-1-expressing mouse transfectant fibroblasts, but not to mock transfectants. In addition, this aggregation phenomena are divalent cation-dependent and requires the integrity of the cytoskeleton. Analysis of the intracellular signaling pathways indicates that TGF-beta 1 induces protein kinase C activity, as well as PECAM-1 phosphorylation and association with cytoskeletal components. Furthermore, in this model, an autocrine mechanism for releasing the bioactive form of TGF-beta 1 operates, allowing PECAM-1 activation. These results provide evidence that TGF-beta 1 regulates PECAM-1 function by increasing the expression and activating the adhesion of PECAM-1 in monocytic cells. These two mechanisms seem to be necessary for adhesion because independent inhibition of either expression or activation of PECAM-1 leads to abrogation of cellular aggregation.

Functional regulation of platelet/endothelial cell adhesion molecule-1 by TGF-beta 1 in promonocytic U-937 cells

Platelet endothelial cell adhesion molecule-1 (PECAM-1) is a widely distributed cell adhesion molecule present on monocytes, macrophages, and monocytic cell lines. Treatment of the promonocytic cell line U-937 with TGF-beta 1 induces homotypic cellular aggregations, simultaneous with an increase in surface expression and specific transcripts of PECAM-1. The TGF-beta-induced cell adhesion phenomena are not dependent on LFA-1, intercellular adhesion molecule-1 (ICAM-1), very late Ag-4 (VLA-4), or very late Ag-5 (VLA-5). However, the phenomena seem to be directly mediated by PECAM-1 because 1) it is inhibited by the addition of Abs or an antisense oligonucleotide specific for PECAM-1; and 2) TGF-beta 1-treated U-937 cells bind to PECAM-1-expressing mouse transfectant fibroblasts, but not to mock transfectants. In addition, this aggregation phenomena are divalent cation-dependent and requires the integrity of the cytoskeleton. Analysis of the intracellular signaling pathways indicates that TGF-beta 1 induces protein kinase C activity, as well as PECAM-1 phosphorylation and association with cytoskeletal components. Furthermore, in this model, an autocrine mechanism for releasing the bioactive form of TGF-beta 1 operates, allowing PECAM-1 activation. These results provide evidence that TGF-beta 1 regulates PECAM-1 function by increasing the expression and activating the adhesion of PECAM-1 in monocytic cells. These two mechanisms seem to be necessary for adhesion because independent inhibition of either expression or activation of PECAM-1 leads to abrogation of cellular aggregation.

Therapeutic effect of recombinant vaccinia virus expressing the 60-kd heat-shock protein on adjuvant arthritis

OBJECTIVE

To analyze the effects of recombinant viruses for 2 heat-shock proteins in the treatment of adjuvant arthritis.

METHODS

Virus vaccinia recombinant for mycobacterial heat-shock protein 65 (hsp65-VV) and human hsp60 (hsp60-VV) were administered to rats during different stages of adjuvant arthritis. Arthritis score and immunity to the recombinant virus were analyzed.

RESULTS

When delivered at the pre-arthritis stage, both constructs ameliorated arthritis; greater protection was observed with hsp60-VV. A specific T cell response to the recombinant proteins was detected. Furthermore, hsp60-VV displayed a clear therapeutic effect on established arthritis.

CONCLUSION

Our results suggest novel avenues of therapeutic intervention in autoimmune arthritis associated with immunity to hsp60.

Phosphorylation of the human-transforming-growth-factor-beta-binding protein endoglin

Endoglin is an homodimeric membrane antigen with capacity to bind transforming growth factor-beta (TGF-beta). Phosphorylation of human endoglin was demonstrated in endothelial cells as well as in mouse fibroblast transfectants expressing two isoforms, L-endoglin or S-endoglin, with distinct cytoplasmic domains. The extent of L-endoglin phosphorylation was found to be 8-fold higher than that of S-endoglin, and phosphopeptide analyses revealed at least three different phosphorylation sites for L-endoglin, whereas S-endoglin produces only one phosphopeptide. The immunoprecipitated L-endoglin was found to be phosphorylated mainly on serine, and, to a minor extent, on threonine, residues. Treatment of the cells with TGF-beta 1 or the protein kinase C inhibitor H-7 resulted in a reduction of the levels of endoglin phosphorylation.

Assignment of the human endoglin gene (END) to 9q34-->qter

Endoglin is a homodimeric membrane glycoprotein primarily associated with human vascular endothelium. It is also found on bone marrow proerythroblasts, activated monocytes and on lymphoblasts in childhood leukemia. Endoglin has recently been described as a component of the transforming growth factor-beta (TGF-beta) receptor system as it can bind TGF-beta 1 with high affinity. We now report on the localization of the human endoglin gene (END) to human chromosome 9, by Southern blot analysis of BglII fragments of DNA from human-hamster somatic cell hybrids. This chromosomal localization was confirmed by fluorescent in situ hybridization coupled with Distamicin A (DA)/4',6-diamidino-2-phenylindole (DAPI) banding on human chromosomes. The regional localization was assigned to 9q34-->qter by GTG-banding (G-banding by trypsin using Giemsa stain), indicating a telomeric position with respect to the Philadelphia breakpoint.

Identification and expression of two forms of the human transforming growth factor-beta-binding protein endoglin with distinct cytoplasmic regions

Endoglin is an homodimeric membrane antigen with capacity to bind transforming growth factor-beta (TGF-beta) and whose expression is up-regulated on myeloid cells upon differentiation to macrophages. We have isolated full-length cDNA clones from a lambda gt 10 library, prepared from phorbol 12-myristate 13-acetate-differentiated HL60 cells by screening with an endoglin-specific cDNA probe from endothelial cells. Sequencing of the largest clone (3073 bp), revealed that the leader sequence contains 25 residues and that the 586 amino acids of the extracellular and transmembrane domains were identical to those described for endothelial endoglin. However, the cytoplasmic tail encoded by this cDNA clone contains only 14 amino acids as opposed to the 47 residues previously reported, suggesting the existence of two alternative endoglin variants. The expression of these isoforms was demonstrated by polymerase chain reaction analyses on endothelial cells, myelomonocytic cell lines HL-60 and U-937, and placenta. Independent cDNA constructs corresponding to both forms were transfected into mouse fibroblasts leading to the expression of two distinct endoglin molecules. Both forms were shown to bind TGF-beta 1 and, when overexpressed in transfected mouse fibroblasts, to form disulfide-linked homodimers, indicating that the cysteine residues present in the extracellular domain are responsible for the dimerization.

Aggregated human immunoglobulins bind to modified proteins

Human immunoglobulins treated at 55 degrees C in vitro are able to interact with maleylated bovine serum albumin (mBSA), but not with unmodified BSA. Gel filtration experiments demonstrated that the mBSA binding is associated with a high molecular weight complex of aggregated IgG. This aggregated IgG with binding capacity for mBSA could also be generated in vitro by treatment of human IgG at 37 degrees C or 40 degrees C and by incubation with human neutrophils. Furthermore, IgG aggregates with binding activity for mBSA could be detected in untreated synovial fluids from rheumatoid arthritis patients, indicating that these complexes occur in vivo. The phenomenon of binding to aggregated IgG was extended to other modified proteins such as maleylated human serum albumin (mHSA), acetyl low density lipoprotein (Ac-LDL) and BSA reacted with oxidized linolenic acid. Soluble forms of these modified proteins were able to compete for the interaction between aggregated IgG and surface-bound mBSA. We also found that aggregated IgG enhanced the Ac-LDL-dependent foam cell formation. These findings suggest a role for aggregated IgG in the metabolism of oxidized proteins.

Assignment of the human endoglin gene (END) to 9q34-->qter

Endoglin is a homodimeric membrane glycoprotein primarily associated with human vascular endothelium. It is also found on bone marrow proerythroblasts, activated monocytes and on lymphoblasts in childhood leukemia. Endoglin has recently been described as a component of the transforming growth factor-beta (TGF-beta) receptor system as it can bind TGF-beta 1 with high affinity. We now report on the localization of the human endoglin gene (END) to human chromosome 9, by Southern blot analysis of BglII fragments of DNA from human-hamster somatic cell hybrids. This chromosomal localization was confirmed by fluorescent in situ hybridization coupled with Distamicin A (DA)/4',6-diamidino-2-phenylindole (DAPI) banding on human chromosomes. The regional localization was assigned to 9q34-->qter by GTG-banding (G-banding by trypsin using Giemsa stain), indicating a telomeric position with respect to the Philadelphia breakpoint.