Sphingosine kinase transmits estrogen signaling in human breast cancer cells

Current understanding of cytoplasmic signaling pathways that mediate estrogen action in human breast cancer is incomplete. Here we report that treatment with 17beta-estradiol (E2) activates a novel signaling pathway via activation of sphingosine kinase (SphK) in MCF-7 breast cancer cells. We found that E2 has dual actions to stimulate SphK activity, i.e. a rapid and transient activation mediated by putative membrane G protein-coupled estrogen receptors (ER) and a delayed but prolonged activation relying on the transcriptional activity of ER. The E2-induced SphK activity consequently activates downstream signal cascades including intracellular Ca2+ mobilization and Erk1/2 activation. Enforced expression of human SphK type 1 gene in MCF-7 cells resulted in increases in SphK activity and cell growth. Moreover, the E2-dependent mitogenesis were highly promoted by SphK overexpression as determined by colony growth in soft agar and solid focus formation. In contrast, expression of SphKG82D, a dominant-negative mutant SphK, profoundly inhibited the E2-mediated Ca2+ mobilization, Erk1/2 activity and neoplastic cell growth. Thus, our data suggest that SphK activation is an important cytoplasmic signaling to transduce estrogen-dependent mitogenic and carcinogenic action in human breast cancer cells.

The protein tyrosine phosphatase Pez is a major phosphatase of adherens junctions and dephosphorylates beta-catenin

Cell-cell adhesion regulates processes important in embryonal development, normal physiology, and cancer progression. It is regulated by various mechanisms including tyrosine phosphorylation. We have previously shown that the protein tyrosine phosphatase Pez is concentrated at intercellular junctions in confluent, quiescent monolayers but is nuclear in cells lacking cell-cell contacts. We show here with an epithelial cell model that Pez localizes to the adherens junctions in confluent monolayers. A truncation mutant lacking the catalytic domain acts as a dominant negative mutant to upregulate tyrosine phosphorylation at adherens junctions. We identified beta-catenin, a component of adherens junctions, as a substrate of Pez by a "substrate trapping" approach and by in vitro dephosphorylation with recombinant Pez. Consistent with this, ectopic expression of the dominant negative mutant caused an increase in tyrosine phosphorylation of beta-catenin, demonstrating that Pez regulates the level of tyrosine phosphorylation of adherens junction proteins, including beta-catenin. Increased tyrosine phosphorylation of adherens junction proteins has been shown to decrease cell-cell adhesion, promoting cell migration as a result. Accordingly, the dominant negative Pez mutant enhanced cell motility in an in vitro "wound" assay. This suggests that Pez is also a regulator of cell motility, most likely through its action on cell-cell adhesion.

The nucleotide-binding site of human sphingosine kinase 1

Sphingosine kinase catalyzes the formation of sphingosine 1-phosphate, a lipid second messenger that has been implicated in a number of agonist-driven cellular responses including mitogenesis, anti-apoptosis, and expression of inflammatory molecules. Despite the importance of sphingosine kinase, very little is known regarding its structure or mechanism of catalysis. Moreover, sphingosine kinase does not contain recognizable catalytic or substrate-binding sites, based on sequence motifs found in other kinases. Here we have elucidated the nucleotide-binding site of human sphingosine kinase 1 (hSK1) through a combination of site-directed mutagenesis and affinity labeling with the ATP analogue, FSBA. We have shown that Gly(82) of hSK1 is involved in ATP binding since mutation of this residue to alanine resulted in an enzyme with an approximately 45-fold higher K(m)((ATP)). We have also shown that Lys(103) is important in catalysis since an alanine substitution of this residue ablates catalytic activity. Furthermore, we have shown that this residue is covalently modified by FSBA. Our data, combined with amino acid sequence comparison, suggest a motif of SGDGX(17-21)K is involved in nucleotide binding in the sphingosine kinases. This motif differs in primary sequence from all previously identified nucleotide-binding sites. It does, however, share some sequence and likely structural similarity with the highly conserved glycine-rich loop, which is known to be involved in anchoring and positioning the nucleotide in the catalytic site of many protein kinases.

A novel mechanism of repression of the vascular endothelial growth factor promoter, by single strand DNA binding cold shock domain (Y-box) proteins in normoxic fibroblasts

Overexpression of vascular endothelial growth factor (VEGF) is implicated in a number of diseases. It is therefore critical that mechanisms exist to strictly regulate VEGF expression. A hypoxia-responsive (HR) region of the VEGF promoter which binds the HIF-1 transcription factor is a target for many signals that up-regulate VEGF transcription. Repressors targeting the HIF-1 transcription factor have been identified but no repressors directly binding the HR promoter region had been reported. We now report a novel mechanism of repression of the VEGF HR region involving DNA binding. We find that single strand DNA-specific cold shock domain (CSD or Y-box) proteins repress the HR region via a binding site downstream of the HIF-1 site. The repressor site is functional in unstimulated, normoxic fibroblasts and represents a novel means to prevent expression of VEGF in the absence of appropriate stimuli. We characterized complexes forming on the VEGF repressor site and identified a previously unreported nuclear CSD protein complex containing dbpA. Nuclear dbpA appears to bind as a dimer and we determined a means by which nuclear CSD proteins may enter double strand DNA to bind to their single strand sites to bring about repression of the VEGF HR region.

The human IL-3 locus is regulated cooperatively by two NFAT-dependent enhancers that have distinct tissue-specific activities

The human IL-3 gene is expressed by activated T cells, mast cells, and eosinophils. We previously identified an enhancer 14 kb upstream of the IL-3 gene, but this element only functioned in a subset of T cells and not in mast cells. To identify additional mechanisms governing IL-3 gene expression, we mapped DNase I hypersensitive (DH) sites and evolutionarily conserved DNA sequences in the IL-3 locus. The most conserved sequence lies 4.5 kb upstream of the IL-3 gene and it encompassed an inducible cyclosporin A-sensitive DH site. A 245-bp fragment spanning this DH site functioned as a cyclosporin A-sensitive enhancer, and was induced by calcium and kinase signaling pathways in both T cells and mast cells via an array of three NFAT sites. The enhancer also encompassed AML1, AP-1, and Sp1 binding sites that potentially mediate function in both T and myeloid lineage cells, but these sites were not required for in vitro enhancer function in T cells. In stably transfected T cells, the -4.5-kb enhancer cooperated with the -14-kb enhancer to activate the IL-3 promoter. Hence, the IL-3 gene is regulated by two enhancers that have distinct but overlapping tissue specificities. We also identified a prominent constitutive DH site at -4.1 kb in T cells, mast cells, and CD34(+) myeloid cells. This element lacked in vitro enhancer function, but may have a developmental role because it appears to be the first DH site to exist upstream of the IL-3 gene during hemopoietic development before IL-3 expression.

Sphingosine kinase interacts with TRAF2 and dissects tumor necrosis factor-alpha signaling

Tumor necrosis factor-alpha (TNF) receptor-associated factor 2 (TRAF2) is one of the major mediators of TNF receptor superfamily transducing TNF signaling to various functional targets, including activation of NF-kappa B, JNK, and antiapoptosis. We investigated how TRAF2 mediates differentially the distinct downstream signals. We now report a novel mechanism of TRAF2-mediated signal transduction revealed by an association of TRAF2 with sphingosine kinase (SphK), a lipid kinase that is responsible for the production of sphingosine 1-phosphate. We identified a TRAF2-binding motif of SphK that mediated the interaction between TRAF2 and SphK resulting in the activation of the enzyme, which in turn is required for TRAF2-mediated activation of NF-kappa B but not JNK. In addition, by using a kinase inactive dominant-negative SphK and a mutant SphK that lacks TRAF2-binding motif we show that the interaction of TRAF2 with SphK and subsequent activation of SphK are critical for prevention of apoptosis during TNF stimulation. These findings show a role for SphK in the signal transduction by TRAF2 specifically leading to activation of NF-kappa B and antiapoptosis.

A point mutant of human sphingosine kinase 1 with increased catalytic activity

Sphingosine kinase (SK) catalyses the formation of sphingosine 1-phosphate, a lipid second messenger that has been implicated in mediating such fundamental biological processes as cell growth and survival. Very little is currently known regarding the structure or mechanisms of catalysis and activation of SK. Here we have tested the functional importance of Gly(113), a highly conserved residue of human sphingosine kinase 1 (hSK), by site-directed mutagenesis. Surprisingly, a Gly(113)-->Ala substitution generated a mutant that had 1.7-fold greater catalytic activity than wild-type hSK (hSK(WT)). Our data suggests that the Gly(113)-->Ala mutation increases catalytic efficiency of hSK, probably by inducing a conformational change that increases the efficiency of phosphoryl transfer. Interestingly, hSK(G113A) activity could be stimulated in HEK293T cells by cell agonists to a comparable extent to hSK(WT).

Time sequence of the inhibition of endothelial adhesion molecule expression by reconstituted high density lipoproteins

We have used discoidal reconstituted high density lipoproteins (rHDL) containing apolipoprotein (apo) A-I and dimyristoyl phosphatidylcholine (DMPC) as a tool to investigate the time sequence of the HDL-mediated inhibition of vascular cell adhesion molecule (VCAM)-1 and E-selectin expression in cytokine-activated human umbilical vein endothelial cells (HUVECs). Specifically, we have asked a few questions - (i) how long do the cells need to be exposed to the rHDL before adhesion molecule expression is inhibited and (ii) how long does the inhibition persist after removing the rHDL from the cells. When the cells were not pre-incubated with the rHDL, there was no inhibition. The magnitude of the inhibition increased progressively with increasing duration of pre-incubation up to 16 h. Inhibition did not require the rHDL to be physically present during the activation of adhesion molecule expression by tumour necrosis factor(TNF)-alpha, excluding the possibility that the rHDL was merely interfering with the interaction between TNF-alpha and the cells. When HUVECs were pre-incubated for 16 h with rHDL, the inhibition remained substantial even if the rHDL were removed from the medium up to 8 h prior to addition of TNF-alpha. The HDL-mediated inhibition of VCAM-1 in HUVECs was unaffected by the presence of puromycin, an inhibitor of protein synthesis, excluding the possibility that HDL may have acted by stimulating the synthesis of a cell protein that itself inhibits adhesion molecule expression. These results have important implications in terms of understanding the mechanism(s) of the HDL-mediated inhibition of endothelial adhesion molecule expression.

Peptide insertions in domain 4 of hbeta(c), the shared signalling receptor subunit for GM-CSF, IL3 and IL5, induce ligand-independent activation

A mutant form of the common beta-subunit of the GM-CSF, interleukin-3 (IL3) and IL5 receptors is activated by a 37 residue duplicated segment which includes the WSXWS motif and an adjacent, highly conserved, aliphatic/basic element. Haemopoietic expression of this mutant, hbeta(c)FIDelta, in mice leads to myeloproliferative disease. To examine the mechanism of activation of this mutant we targetted the two conserved motifs in each repeat for mutagenesis. Here we show that this mutant exhibits constitutive activity in BaF-B03 cells in the presence of mouse or human GM-CSF receptor alpha-subunit (GMRalpha) and this activity is disrupted by mutations of the conserved motifs in the first repeat. In the presence of these mutations the receptor reverts to an alternative conformation which retains responsiveness to human IL3 in a CTLL cell line co-expressing the human IL3 receptor alpha-subunit (hIL3Ralpha). Remarkably, the activated conformation is maintained in the presence of substitutions, deletions or replacement of the second repeat. This suggests that activation occurs due to insertion of extra sequence after the WSXWS motif and is not dependent on the length or specific sequence of the insertion. Thus hbeta(c) displays an ability to fold into functional receptor conformations given insertion of up to 37 residues in the membrane-proximal region. Constitutive activation most likely results from a specific conformational change which alters a dormant, inactive receptor complex, permitting functional association with GMRalpha and ligand-independent mitogenic signalling.

Nucleotide sequence of the mouse VEGF 3'UTR and quantitative analysis of sites of polyadenylation

Sequencing of rat and human vascular endothelial growth factor (VEGF) cDNA clones has previously identified a 3' untranslated region of approximately 1.9 kb, although the apparent site of polyadenylation differed in the two species, despite a high degree of sequence conservation in the region. Neither site is preceded by a canonical AAUAAA polyadenylation signal, a situation frequently found in genes that are subject to alternative polyadenylation. We have sequenced 2.25 kb of the 3' region of the mouse VEGF gene and mapped the usage of potential polyadenylation sites in fibroblasts cultured under both normoxic and hypoxic conditions. We find that two sites for polyadenylation are present in the mouse VEGF gene but the majority of transcripts contain the longer form of the 3'UTR and that their usage is not effected by environmental oxygen tension.

Cold shock domain factors activate the granulocyte-macrophage colony-stimulating factor promoter in stimulated Jurkat T cells

Cold shock domain (CSD) family members have been shown to play roles in either transcriptional activation or repression of many genes in various cell types. We have previously shown that CSD proteins dbpAv and dbpB (also known as YB-1) act to repress granulocyte-macrophage colony-stimulating factor transcription in human embryonic lung (HEL) fibroblasts via binding to single-stranded DNA regions across the promoter. Here we show that the same CSD factors are involved in granulocyte-macrophage colony-stimulating factor transcriptional activation in Jurkat T cells. Unlike the mechanisms of CSD repression in HEL fibroblasts, CSD-mediated activation in Jurkat T cells is not mediated through DNA binding but presumably through protein-protein interactions via the C terminus of the CSD protein with transcription factors such as RelA/NF-kappaB p65. We demonstrate that Jurkat T cells lack truncated CSD factor subtypes present in HEL fibroblasts, which raises the possibility that the cellular content of CSD proteins may determine their final role as activators or repressors of transcription.

An oncogenic role of sphingosine kinase

Sphingosine kinase (SphK) is a highly conserved lipid kinase that phosphorylates sphingosine to form sphingosine-1-phosphate (S1P). S1P/SphK has been implicated as a signalling pathway to regulate diverse cellular functions [1-3], including cell growth, proliferation and survival [4-8]. We report that cells overexpressing SphK have increased enzymatic activity and acquire the transformed phenotype, as determined by focus formation, colony growth in soft agar and the ability to form tumours in NOD/SCID mice. This is the first demonstration that a wild-type lipid kinase gene acts as an oncogene. Using a chemical inhibitor of SphK, or an SphK mutant that inhibits enzyme activation, we found that SphK activity is involved in oncogenic H-Ras-mediated transformation, suggesting a novel signalling pathway for Ras activation. The findings not only point to a new signalling pathway in transformation but also to the potential of SphK inhibitors in cancer therapy.

Reconstitution of T cell-specific transcription directed by composite NFAT/Oct elements

The complex nature of most promoters and enhancers makes it difficult to identify key determinants of tissue-specific gene expression. Furthermore, most tissue-specific genes are regulated by transcription factors that have expression profiles more widespread than the genes they control. NFAT is an example of a widely expressed transcription factor that contributes to several distinct patterns of cytokine gene expression within the immune system and where its role in directing specificity remains undefined. To investigate distinct combinatorial mechanisms employed by NFAT to regulate tissue-specific transcription, we examined a composite NFAT/AP-1 element from the widely active GM-CSF enhancer and a composite NFAT/Oct element from the T cell-specific IL-3 enhancer. The NFAT/AP-1 element was active in the numerous cell types that express NFAT, but NFAT/Oct enhancer activity was T cell specific even though Oct-1 is ubiquitous. Conversion of the single Oct site in the IL-3 enhancer to an AP-1 enabled activation outside of the T cell lineage. By reconstituting the activities of both the IL-3 enhancer and its NFAT/Oct element in a variety of cell types, we demonstrated that their T cell-specific activation required the lymphoid cofactors NIP45 and OCA-B in addition to NFAT and Oct family proteins. Furthermore, the Oct family protein Brn-2, which cannot recruit OCA-B, repressed NFAT/Oct enhancer activity. Significantly, the two patterns of combinatorial regulation identified in this study mirror the cell-type specificities of the cytokine genes that they govern. We have thus established that simple composite transcription factor binding sites can indeed establish highly specific patterns of gene expression.

Expression of a catalytically inactive sphingosine kinase mutant blocks agonist-induced sphingosine kinase activation. A dominant-negative sphingosine kinase

Sphingosine kinase (SK) catalyzes the formation of sphingosine 1-phosphate (S1P), a lipid messenger that plays an important role in a variety of mammalian cell processes, including inhibition of apoptosis and stimulation of cell proliferation. Basal levels of S1P in cells are generally low but can increase rapidly when cells are exposed to various agonists through rapid and transient activation of SK activity. To date, elucidation of the exact signaling pathways affected by these elevated S1P levels has relied on the use of SK inhibitors that are known to have direct effects on other enzymes in the cell. Furthermore, these inhibitors block basal SK activity, which is thought to have a housekeeping function in the cell. To produce a specific inhibitor of SK activation we sought to generate a catalytically inactive, dominant-negative SK. This was accomplished by site-directed mutagenesis of Gly(82) to Asp of the human SK, a residue identified through sequence similarity to the putative catalytic domain of diacylglycerol kinase. This mutant had no detectable SK activity when expressed at high levels in HEK293T cells. Activation of endogenous SK activity by tumor necrosis factor-alpha (TNFalpha), interleukin-1beta, and phorbol esters in HEK293T cells was blocked by expression of this inactive sphingosine kinase (hSK(G82D)). Basal SK activity was unaffected by expression of hSK(G82D). Expression of hSK(G82D) had no effect on TNFalpha-induced activation of protein kinase C and sphingomyelinase activities. Thus, hSK(G82D) acts as a specific dominant-negative SK to block SK activation. This discovery provides a powerful tool for the elucidation of the exact signaling pathways affected by elevated S1P levels following SK activation. To this end we have employed the dominant-negative SK to demonstrate that TNFalpha activation of extracellular signal-regulated kinases 1 and 2 (ERK1,2) is dependent on SK activation.

Angiopoietin-1 is an antipermeability and anti-inflammatory agent in vitro and targets cell junctions

Inflammation is a basic pathological mechanism that underlies many diseases. An important component of the inflammatory response is the passage of plasma components and leukocytes from the blood vessel into the tissues. The endothelial monolayer lining blood vessels reacts to stimuli such as thrombin or vascular endothelial growth factor by changes in cell-cell junctions, an increase in permeability, and the leakage of plasma components into tissues. Other stimuli, such as tumor necrosis factor-alpha (TNF-alpha), are responsible for stimulating the transmigration of leukocytes. Here we show that angiopoietin-1, a cytokine essential in fetal angiogenesis, not only supports the localization of proteins such as platelet endothelial cell adhesion molecule-1 (PECAM-1) into junctions between endothelial cells and decreases the phosphorylation of PECAM-1 and vascular endothelial cadherin, but it also strengthens these junctions, as evidenced by a decrease in basal permeability and inhibition of permeability responses to thrombin and vascular endothelial growth factor. Furthermore, angiopoietin-1 inhibits TNF-alpha-stimulated leukocyte transmigration. Angiopoietin-1 may thus have a major role in maintaining the integrity of endothelial monolayers.

Human sphingosine kinase: purification, molecular cloning and characterization of the native and recombinant enzymes

Sphingosine 1-phosphate (S1P) is a novel lipid messenger that has important roles in a wide variety of mammalian cellular processes including growth, differentiation and death. Basal levels of S1P in mammalian cells are generally low, but can increase rapidly and transiently when cells are exposed to mitogenic agents and other stimuli. This increase is largely due to increased activity of sphingosine kinase (SK), the enzyme that catalyses its formation. In the current study we have purified, cloned and characterized the first human SK to obtain a better understanding of its biochemical activity and possible activation mechanisms. The enzyme was purified to homogeneity from human placenta using ammonium sulphate precipitation, anion-exchange chromatography, calmodulin-affinity chromatography and gel-filtration chromatography. This resulted in a purification of over 10(6)-fold from the original placenta extract. The enzyme was cloned and expressed in active form in both HEK-293T cells and Escherichia coli, and the recombinant E. coli-derived SK purified to homogeneity. To establish whether post-translational modifications lead to activation of human SK activity we characterized both the purified placental enzyme and the purified recombinant SK produced in E. coli, where such modifications would not occur. The premise for this study was that post-translational modifications are likely to cause conformational changes in the structure of SK, which may result in detectable changes in the physico-chemical or catalytic properties of the enzyme. Thus the enzymes were characterized with respect to substrate specificity and kinetics, inhibition kinetics and various other physico-chemical properties. In all cases, both the native and recombinant SKs displayed remarkably similar properties, indicating that post-translational modifications are not required for basal activity of human SK.

Translocation of protein tyrosine phosphatase Pez/PTPD2/PTP36 to the nucleus is associated with induction of cell proliferation

Pez is a non-transmembrane tyrosine phosphatase with homology to the FERM (4.1, ezrin, radixin, moesin) family of proteins. The subcellular localisation of Pez in endothelial cells was found to be regulated by cell density and serum concentration. In confluent monolayers Pez was cytoplasmic, but in cells cultured at low density Pez was nuclear, suggesting that it is a nuclear protein in proliferating cells. This notion is supported by the loss of nuclear Pez when cells are serum-starved to induce quiescence, and the rapid return of Pez to the nucleus upon refeeding with serum to induce proliferation. Vascular endothelial cells normally exist as a quiescent confluent monolayer but become proliferative during angiogenesis or upon vascular injury. Using a ‘wound’ assay to mimic these events in vitro, Pez was found to be nuclear in the cells that had migrated and were proliferative at the ‘wound’ edge. TGFbeta, which inhibits cell proliferation but not migration, inhibited the translocation of Pez to the nucleus in the cells at the ‘wound’ edge, further strengthening the argument that Pez plays a role in the nucleus during cell proliferation. Together, the data presented indicate that Pez is a nuclear tyrosine phosphatase that may play a role in cell proliferation.

Phospholipid composition of reconstituted high density lipoproteins influences their ability to inhibit endothelial cell adhesion molecule expression

The ability of different phosphatidylcholine (PC) species to inhibit cytokine-induced expression of vascular cell adhesion molecule 1 (VCAM-1) in human umbilical vein endothelial cells (HUVECs) was investigated. PC species containing palmitoyl- in the sn-1 position and palmitoyl- (DPPC), arachidonyl- (PAPC), linoleoyl- (PLPC) or oleoyl- (POPC) in the sn-2 position were compared. These PC species were studied as components of reconstituted high density lipoproteins (rHDL) (containing apolipoprotein A-I [apoA-I] as the sole protein) or as small unilamellar vesicles (SUVs). The rHDL containing PLPC and PAPC inhibited VCAM-1 expression in activated HUVECs by 95 and 70%, respectively, at an apoA-I concentration of 16 micrometer. At this concentration of apoA-I, POPC rHDL inhibited by only 16% and DPPC rHDL did not inhibit at all. These differences could not be explained by differential binding of the rHDL to HUVECs. The same hierarchy of inhibitory activity was observed when these PC species were presented to the cells as SUVs but only when the SUVs also contained an antioxidant. It was concluded that rHDL PC is responsible for their inhibitory activity and that this varies widely with different PC species.

An ordered array of cold shock domain repressor elements across tumor necrosis factor-responsive elements of the granulocyte-macrophage colony-stimulating factor promoter

The tumor necrosis factor-alpha-responsive region of the human granulocyte-macrophage colony-stimulating factor (GM-CSF) promoter (-114 to -31) encompasses binding sites for NF-kappaB, CBF, AP-1, ETS, and NFAT families of transcription factors. We show both here and previously that mutation of any one of these binding sites greatly reduces tumor necrosis factor-alpha induction of the GM-CSF promoter. Interspersed between these elements are sequences that when mutated lead to an increase in GM-CSF promoter activity. We have previously shown that two of these repressor elements bind proteins known as cold shock domain (CSD) factors and that overexpression of CSD proteins leads to repression of GM-CSF promoter activity in fibroblasts. CSD proteins are single strand DNA- and RNA-binding proteins that contact 5'-CCTG-3' sequences in the GM-CSF repressor elements. We show here that two newly identified repressor sequences in the proximal promoter can also bind CSD proteins. We have characterized the CSD-containing protein complexes that bind to the GM-CSF promoter and identified a novel protein related to mitochondrial single strand binding protein that forms part of one of these complexes. The four CSD-binding sites on the promoter occur in pairs on opposite strands of the DNA and appear to form an ordered array of binding elements. A similar ordered array of CSD sites are present in the promoters of the granulocyte colony-stimulating factor and interleukin-3 genes, implying a common mechanism for negative regulation of these myeloid growth factors.

The solution structure of the cytokine-binding domain of the common beta-chain of the receptors for granulocyte-macrophage colony-stimulating factor, interleukin-3 and interleukin-5

The haemopoietic cytokines, granulocyte-macrophage colony-stimulating factor, interleukin-3 and interleukin-5 bind to cell-surface receptors comprising ligand-specific alpha-chains and a shared beta-chain. The beta-chain is the critical signalling subunit of the receptor and its fourth domain not only plays a critical role in interactions with ligands, hence in receptor activation, but also contains residues whose mutation can lead to ligand-independent activation of the receptor. We have determined the NMR solution structure of the isolated human fourth domain of the beta-chain. The protein has a fibronectin type III fold with a well-defined hydrophobic core and is stabilised by an extensive network of pi-cation interactions involving Trp and Arg side-chains, including two Trp residues outside the highly conserved Trp-Ser-Xaa-Trp-Ser motif (where Xaa is any amino acid) that is found in many cytokine receptors. Most of the residues implicated in factor-independent mutants localise to the rigid core of the domain or the pi-cation stack. The loops between the B and C, and the F and G strands, that contain residues important for interactions with cytokines, lie adjacent at the membrane-distal end of the domain, consistent with their being involved cooperatively in binding cytokines. The elucidation of the structure of the cytokine-binding domain of the beta-chain provides insight into the cytokine-dependent and factor-independent activation of the receptor.

The human granulocyte-macrophage colony-stimulating factor gene is autonomously regulated in vivo by an inducible tissue-specific enhancer

The granulocyte-macrophage colony-stimulating factor (GM-CSF) gene is part of a cytokine gene cluster and is directly linked to a conserved upstream inducible enhancer. Here we examined the in vitro and in vivo functions of the human GM-CSF enhancer and found that it was required for the correctly regulated expression of the GM-CSF gene. An inducible DNase I-hypersensitive site appeared within the enhancer in cell types such as T cells, myeloid cells, and endothelial cells that express GM-CSF, but not in nonexpressing cells. In a panel of transfected cells the human GM-CSF enhancer was activated in a tissue-specific manner in parallel with the endogenous gene. The in vivo function of the enhancer was examined in a transgenic mouse model that also addressed the issue of whether the GM-CSF locus was correctly regulated in isolation from other segments of the cytokine gene cluster. After correction for copy number the mean level of human GM-CSF expression in splenocytes from 11 lines of transgenic mice containing a 10.5-kb human GM-CSF transgene was indistinguishable from mouse GM-CSF expression (99% +/- 56% SD). In contrast, a 9.8-kb transgene lacking just the enhancer had a significantly reduced (P = 0.004) and more variable level of activity (29% +/- 89% SD). From these studies we conclude that the GM-CSF enhancer is required for the correct copy number-dependent expression of the human GM-CSF gene and that the GM-CSF gene is regulated independently from DNA elements associated with the closely linked IL-3 gene or other members of the cytokine gene cluster.

Activation of sphingosine kinase by tumor necrosis factor-alpha inhibits apoptosis in human endothelial cells

Human umbilical vein endothelial cells (HUVEC), like most normal cells, are resistant to tumor necrosis factor-alpha (TNF)-induced apoptosis in spite of TNF activating sphingomyelinase and generating ceramide, a known inducer of apoptosis. Here we report that TNF activates another key enzyme, sphingosine kinase (SphK), in the sphingomyelin metabolic pathway resulting in production of sphingosine-1-phosphate (S1P) and that S1P is a potent antagonist of TNF-mediated apoptosis. The TNF-induced SphK activation is independent of sphingomyelinase and ceramidase activities, suggesting that TNF affects this enzyme directly other than through a mass effect on sphingomyelin degradation. In contrast to normal HUVEC, in a spontaneously transformed endothelial cell line (C11) TNF stimulation failed to activate SphK and induced apoptosis as characterized by morphological and biochemical criteria. Addition of exogenous S1P or increasing endogenous S1P by phorbol ester markedly protected C11 cell line from TNF-induced apoptosis. Conversely, N, N-dimethylsphingosine, an inhibitor of SphK, profoundly sensitized normal HUVEC to killing by TNF. Thus, we demonstrate that the activation of SphK by TNF is an important signaling for protection from the apoptotic effect of TNF in endothelial cells.

High density lipoproteins (HDL) interrupt the sphingosine kinase signaling pathway. A possible mechanism for protection against atherosclerosis by HDL

The ability of high density lipoproteins (HDL) to inhibit cytokine-induced adhesion molecule expression has been demonstrated in their protective function against the development of atherosclerosis and associated coronary heart disease. A key event in atherogenesis is endothelial activation induced by a variety of stimuli such as tumor necrosis factor-alpha (TNF), resulting in the expression of various adhesion proteins. We have recently reported that sphingosine 1-phosphate, generated by sphingosine kinase activation, is a key molecule in mediating TNF-induced adhesion protein expression. We now show that HDL profoundly inhibit TNF-stimulated sphingosine kinase activity in endothelial cells resulting in a decrease in sphingosine 1-phosphate production and adhesion protein expression. HDL also reduced TNF-mediated activation of extracellular signal-regulated kinases and NF-kappaB signaling cascades. Furthermore, HDL enhanced the cellular levels of ceramide which in turn inhibits endothelial activation. Thus, the regulation of sphingolipid signaling in endothelial cells by HDL provides a novel insight into the mechanism of protection against atherosclerosis.

Stat6 activation is essential for interleukin-4 induction of P-selectin transcription in human umbilical vein endothelial cells

Chronic upregulation of P-selectin expression on the surface of the endothelium has been observed in and likely contributes to a number of chronic inflammatory diseases, including atherosclerosis. Agonists of P-selectin expression fall into 2 categories: those that induce a very rapid, transient increase, lasting only hours, and those that induce prolonged upregulation lasting days. It is the latter group, which includes interleukin-4 (IL-4), that is likely to be a mediator of chronic P-selectin upregulation. The increase in P-selectin expression induced by IL-4 results from increased transcriptional activation of the P-selectin gene. The aim of this study was to deduce the postreceptor signaling pathway(s) giving rise to the prolonged increase in P-selectin expression induced by IL-4. We demonstrate the existence of 2 functional signal transducer and activator of transcription 6 (Stat6) binding sites on the P-selectin promoter and further demonstrate, by functional analysis of the P-selectin promoter, that binding of activated Stat6 to at least 1 site is essential for IL-4-induction of P-selectin transcription. Site 1 (nucleotide[nt] -142) bound Stat6 with a higher affinity than did site 2 (nt -229), and this difference was reflected functionally as constructs in which only site 1 was functional showed full IL-4 inducibility, whereas constructs in which only site 2 was functional showed only 40% of maximal IL-4 inducibility. IL-4 also induced prolonged activation of Stat6, which was contingent on the continuous presence of IL-4. The sustained activation of Stat6 induced by IL-4 is likely to be a key factor leading to the prolonged activation of the P-selectin promoter, thereby resulting in prolonged P-selectin upregulation.

Cloning, characterization, and chromosomal location of a novel human K+-Cl- cotransporter

Differential display polymerase chain reaction has been used to isolate genes regulated in vascular endothelial cells by the angiogenic factor vascular endothelial cell growth factor (VEGF). Analysis of one of the bands consistently up-regulated by VEGF led us to the identification of a cDNA from a human umbilical vein endothelial cell library that is 77% identical to the human K+-Cl- cotransporter1 (KCC1). We have referred to the predicted protein as K+-Cl- cotransporter 3 (KCC3). Hydrophobicity analysis of the KCC3 amino acid sequence showed an almost identical pattern to KCC1, suggesting 12 membrane-spanning segments, a large extracellular loop with potential N-glycosylation sites, and cytoplasmic N- and C-terminal regions. The KCC3 mRNA was highly expressed in brain, heart, skeletal muscle, and kidney, showing a distinct pattern and size from KCC1 and KCC2. The KCC3 mRNA level in endothelial cells increased on treatment with VEGF and decreased with the proinflammatory cytokine tumor necrosis factor alpha, whereas KCC1 mRNA levels remained unchanged. Stable overexpression of KCC3 cDNA in HEK293 cells produced a glycoprotein of approximately 150 kDa, which was reduced to 120 kDa by glycosidase digestion. An increased initial uptake rate of 86Rb was seen in clones with high KCC3 expression, which was dependent on extracellular Cl- but not Na+ and was inhibitable by the loop diuretic agent furosemide. The KCC3 genomic localization was shown to be 15q13 by fluorescence in situ hybridization. Radiation hybrid analysis placed KCC3 within an area associated with juvenile myoclonic epilepsy. These results suggest KCC3 is a new member of the KCC family that is under distinct regulation from KCC1.

Hypoxic regulation of vascular endothelial growth factor mRNA stability requires the cooperation of multiple RNA elements

Vascular endothelial growth factor (VEGF) is a key regulator of developmental, physiological, and tumor angiogenesis. Upregulation of VEGF expression by hypoxia appears to be a critical step in the neovascularization of solid cancers. The VEGF mRNA is intrinsically labile, but in response to hypoxia the mRNA is stabilized. We have systematically analyzed the regions in the VEGF mRNA that are responsible for its lability under normoxic conditions and for stabilization in response to hypoxia. We find that the VEGF mRNA not only contains destabilizing elements in its 3' untranslated region (3'UTR), but also contains destabilizing elements in the 5'UTR and coding region. Each region can independently promote mRNA degradation, and together they act additively to effect rapid degradation under normoxic conditions. Stabilization of the mRNA in response to hypoxia is completely dependent on the cooperation of elements in each of the 5'UTR, coding region, and 3'UTR. Combinations of any of two of these three regions were completely ineffective in responding to hypoxia, whereas combining all three regions allowed recapitulation of the hypoxic stabilization seen with the endogenous VEGF mRNA. We conclude that multiple regions in the VEGF mRNA cooperate both to ensure the rapid degradation of the mRNA under normoxic conditions and to allow stabilization of the mRNA in response to hypoxia. Our findings highlight the complexity of VEGF gene expression and also reveal a mechanism of gene regulation that could become the target for strategies of therapeutic intervention.

Ability of reconstituted high density lipoproteins to inhibit cytokine-induced expression of vascular cell adhesion molecule-1 in human umbilical vein endothelial cells

Previous studies have shown that both high density lipoproteins (HDL) isolated from human plasma and reconstituted HDL (rHDL) are effective inhibitors of adhesion molecule expression in human endothelial cells. In this study rHDL have been used to investigate whether HDL particle shape, size, apolipoprotein composition or lipid composition are important determinants of the ability of HDL to inhibit the TNF-alpha induced expression of vascular cell adhesion molecule-1 (VCAM-1) in human umbilical vein endothelial cells (HUVECs). On the basis of these studies it is possible to draw several firm conclusions. i) Neither phospholipid-containing vesicles nor lipid-free apolipoprotein (apo) A-I inhibit VCAM-1 expression in HUVECs. ii) Simple discoidal complexes containing only phospholipid and apoA-I (discoidal (A-I)rHDL) are sufficient to inhibit the TNF-alpha-induced expression of VCAM-1 in HUVECs. iii) Spherical apoA-I-containing rHDL (spherical (A-I)rHDL) are superior to discoidal (A-I)rHDL as inhibitors. iv) The particle size of spherical (A-I)rHDL has no influence on the inhibition. v) Spherical rHDL that contain apoA-I are superior as inhibitors of VCAM-1 to those containing apoA-II when the rHDL preparations are equated for apolipoprotein molarity. However, when compared at equivalent particle molarities, this difference is no longer apparent. vi) Replacement of cholesteryl esters with triglyceride in the core of spherical (A-I)rHDL has no effect on the ability of these particles to inhibit VCAM-1 expression. From these results it is tempting to speculate that variations in inhibitory activity may contribute to the variations observed in the anti-atherogenicity of different HDL subpopulations.-Baker, P. W., K-A. Rye, J. R. Gamble, M. A. Vadas, and P. J. Barter. Ability of reconstituted high density lipoproteins to inhibit cytokine-induced expression of vascular cell adhesion molecule-1 in human umbilical vein endothelial cells.

Dysregulated hematopoiesis and a progressive neurological disorder induced by expression of an activated form of the human common beta chain in transgenic mice

Previously we described activating mutations of hbetac, the common signaling subunit of the receptors for the hematopoietic and inflammatory cytokines, GM-CSF, IL-3, and IL-5. The activated mutant, hbetacFIDelta, is able to confer growth factor-independent proliferation on the murine myeloid cell line FDC-P1, and on primary committed myeloid progenitors. We have used this activating mutation to study the effects of chronic cytokine receptor stimulation. Transgenic mice were produced carrying the hbetacFIDelta cDNA linked to the constitutive promoter derived from the phosphoglycerate kinase gene, PGK-1. Transgene expression was demonstrated in several tissues and functional activity of the mutant receptor was confirmed in hematopoietic tissues by the presence of granulocyte macrophage and macrophage colony-forming cells (CFU-GM and CFU-M) in the absence of added cytokines. All transgenic mice display a myeloproliferative disorder characterized by splenomegaly, erythrocytosis, and granulocytic and megakaryocytic hyperplasia. This disorder resembles the human disease polycythemia vera, suggesting that activating mutations in hbetac may play a role in the pathogenesis of this myeloproliferative disorder. In addition, these transgenic mice develop a sporadic, progressive neurological disease and display bilateral, symmetrical foci of necrosis in the white matter of brain stem associated with an accumulation of macrophages. Thus, chronic hbetac activation has the potential to contribute to pathological events in the central nervous system.

Tumor necrosis factor-alpha induces adhesion molecule expression through the sphingosine kinase pathway

The signaling pathways that couple tumor necrosis factor-alpha (TNFalpha) receptors to functional, especially inflammatory, responses have remained elusive. We report here that TNFalpha induces endothelial cell activation, as measured by the expression of adhesion protein E-selectin and vascular adhesion molecule-1, through the sphingosine kinase (SKase) signaling pathway. Treatment of human umbilical vein endothelial cells with TNFalpha resulted in a rapid SKase activation and sphingosine 1-phosphate (S1P) generation. S1P, but not ceramide or sphingosine, was a potent dose-dependent stimulator of adhesion protein expression. S1P was able to mimic the effect of TNFalpha on endothelial cells leading to extracellular signal-regulated kinases and NF-kappaB activation, whereas ceramide or sphingosine was not. Furthermore, N, N-dimethylsphingosine, an inhibitor of SKase, profoundly inhibited TNFalpha-induced extracellular signal-regulated kinases and NF-kappaB activation and adhesion protein expression. Thus we demonstrate that the SKase pathway through the generation of S1P is critically involved in mediating TNFalpha-induced endothelial cell activation.

Recruitment and activation of SHP-1 protein-tyrosine phosphatase by human platelet endothelial cell adhesion molecule-1 (PECAM-1). Identification of immunoreceptor tyrosine-based inhibitory motif-like binding motifs and substrates

Stimulation of platelet aggregation leads to tyrosine phosphorylation of a number of receptors and signaling molecules including platelet endothelial cell adhesion molecule-1 (PECAM-1). In this report, we demonstrate that both protein-tyrosine phosphatases SHP-1 and SHP-2 physically associate with different kinetics of assembly with tyrosine-phosphorylated human PECAM-1 during integrin alphaIIbbeta3-mediated platelet aggregation. Peptido-precipitation analysis revealed that tyrosine-phosphorylated peptides encompassing residues 658-668 and 681-691 of PECAM-1 bound specifically to both protein-tyrosine phosphatases SHP-1 and SHP-2. We further show that the association of SHP-1 with PECAM-1 occurs through the direct interaction of the src homology region 2 domains of SHP-1 with two highly conserved phosphotyrosine binding motifs within PECAM-1 having the sequences NSDVQpY663TEVQV and DTETVpY686SEVRK (where pY represents phosphotyrosine). In vitro dephosphorylation experiments using phosphotyrosyl PECAM-1 peptides encompassing either Tyr-663 or Tyr-686 revealed induction of SHP-1 catalytic activity, suggesting that PECAM-1 serves as a SHP-1 substrate. Surface plasmon resonance studies reveal that recombinant SHP-2 binds PECAM-1 phosphopeptides with 5-fold higher affinity than recombinant SHP-1. These data suggest that in hematopoietic cells such as platelets, PECAM-1 cellular signaling is regulated by the selective recruitment and activation of two distinct protein-tyrosine phosphatases, SHP-1 and SHP-2, via a common immunoreceptor tyrosine-based inhibitory-like motif.

The vascular endothelial growth factor mRNA contains an internal ribosome entry site

Vascular endothelial growth factor (VEGF), an essential regulator of angiogenesis during early development as well as during the growth of solid tumours, bears an unusually large 5' untranslated region (5'-UTR) in the mRNA of over 1000 nucleotides. We found that the VEGF 5'-UTR, despite being GC-rich and containing an upstream short open reading frame, promotes efficient translation of a luciferase reporter. The VEGF 5'-UTR also allowed translation of luciferase from a dicistronic mRNA when placed between the two cistrons, demonstrating that it contains an internal ribosome entry site. Deletion analysis indicated that the IRES resides towards the 3' end of the 5'-UTR.

Early events in angiogenesis: cloning an alpha-prolyl 4-hydroxylase-like gene

To gain further insight into angiogenesis we sought to clone genes which are actively expressed during this complex process. Using the Matrigel-induced in vitro model we were able to show that although several cell-types form reticular arrays of cells on the gels (align), only endothelial cells were able to go on and form the capillary-like structures reminiscent of patent vessels. Although this alignment process did not require gene activation we show that tube formation was ultimately dependent upon gene expression occuring during the first few hours that cells are seeded onto Matrigel. We generated a cDNA library enriched for the expression of those genes and have sequenced an alpha-prolyl 4-hydroxylase-like clone (angio 0.9). This clone shares 66% overall homology to the carboxy-terminal 106 amino-acids of the published human sequence. In the region corresponding to the co-factor binding domains, His 1 and His 2, angio 0.9 has >90% homology to the published sequence. Using an RNAse protection assay we show that the level of expression of the message of this clone is five fold elevated in endothelial cells which have aligned on Matrigel. The dependence of collagen, and collagen hydroxylation in angiogenesis is well documented. Thus, our results are demonstrable proof that the principle of this approach has the potential to generate novel discoveries.

Factors influencing the ability of HDL to inhibit expression of vascular cell adhesion molecule-1 in endothelial cells

We have previously reported that high density lipoproteins (HDLs) inhibit the cytokine-induced expression of adhesion molecules in endothelial cells. Here we investigate whether different preparations of HDLs vary in their ability to inhibit the expression of vascular cell adhesion molecule-1 (VCAM-1) in human umbilical vein endothelial cells (HUVECs) activated by tumor necrosis factor-alpha (TNF-alpha). HDLs collected from a number of different human subjects all inhibited VCAM-1 expression in a concentration-dependent manner, although the extent of inhibition varied widely between subjects. The inhibitory activities of the HDL2 and HDL3 subfractions isolated from individual subjects also differed. Whether equated for concentrations of apolipoprotein (apo) A-I or cholesterol, the inhibitory activity of HDL3 was superior to that of HDL2. This difference remained apparent even when the HDL subfractions were present only during preincubations with the HUVECs and were removed before activation by TNF-alpha. To determine whether the inhibitory effect of HDL3 was influenced by apolipoprotein composition, preparations of HDL3 were modified by replacing all of their apo A-I with apo A-II. This change in apolipoprotein composition had no effect on the ability of the HDL3 to inhibit endothelial VCAM-1 expression. Thus, it has been shown that different preparations of HDLs differ markedly in their abilities to inhibit VCAM-1 expression in cytokine-activated HUVECs. The mechanism underlying the differences remains to be determined.

Lumen formation during angiogenesis in vitro involves phagocytic activity, formation and secretion of vacuoles, cell death, and capillary tube remodelling by different populations of endothelial cells

BACKGROUND

We have utilised an in vitro model of angiogenesis to investigate the morphological changes which occur during the formation of a lumen in capillary tubes.

METHODS AND RESULTS

On collagen 1 gel in the presence of phorbol myristate acetate (PMA) and anti-alpha 2 beta 1 antibody, cell aggregation and alignment takes place within two hours of plating. The initial apparently homogeneous population of endothelial cells (EC) actually display at least three distinct phenotypes. One population, characterised by a phagocytic phenotype, migrated through the gel creating channels and defines the extent of the capillary network. These are later enveloped by a second population of cells characterised by intracellular vacuoles. The ultimate fate of these vacuoles is fusion with the plasma membrane. By 12 hours the original phagocytic cell population undergoes cell death, which morphologically appears apoptotic in nature. A consequence of the secretion of vacuoles and programmed cell death is the extensive remodelling of the capillary tubes, resulting in expansion of the intercellular space into a lumen. The remodelling results in 45% of the EC membrane contacting the lumenal surface at the expense of EC-EC and EC-matrix contact. A third population of cells implant between the EC involved in lumen formation and thus expand the size of the capillary tube.

CONCLUSION

Thus, in the formation of a mature multicellular lumen we have identified a number of key events. First, cell-cell contact is essential in order to define the intercellular space. Second, at least three morphologically distinct subpopulations of ECs are involved. Third, vacuole formation and programmed cell death are required for expansion of the intercellular space which ultimately becomes the lumen.

Regulation of leucocyte-endothelial interactions of special relevance to atherogenesis

1. The cellular events underlying atherosclerosis include the accumulation of lipid-laden monocytes in the neointima. This process is associated with the expression of adhesion proteins and chemokines by the endothelium, in a manner similar to that seen after the administration of pro-inflammatory cytokines to endothelial cells. 2. The processes that limit endothelial responses to proinflammatory cytokines are, therefore, the subject of this paper. Evidence is presented that the cytokine TGF-beta exerts a tonic inhibitory influence on endothelial responses. Furthermore, the smooth muscle cells adjacent to endothelial cells have a similar effect to exogenous TGF-beta and this suggests that these two cells form a functional interactive unit. Finally, the atheroprotective lipid fraction, high-density lipoproteins (HDL), also inhibits endothelial activation. The mechanism of effect of HDL that appears separate from its traditional role in cholesterol transport may yield novel insights into atheroprotection.

Roles of the N and C terminal domains of the interleukin-3 receptor alpha chain in receptor function

The interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor, and IL-5 receptor alpha chains are each composed of three extracellular domains, a transmembrane domain and a short intracellular region. Domains 2 and 3 constitute the cytokine receptor module (CRM), typical of the cytokine receptor superfamily; however, the function of the N-terminal domain is not known. We have investigated the functions of the N-terminal and C-terminal domains of the IL-3 receptor (IL-3R) alpha chain. We find that cells transfected with the receptor beta chain (h beta c) and a truncated IL-3R alpha that is devoid of the intracellular region fail to proliferate or to activate STAT5 in response to human IL-3, despite binding the IL-3 with affinity indistinguishable from that of full-length receptor. In addition, IL-3-induced phosphorylation of h beta c was not detected. Thus, the IL-3R alpha intracellular region does not contribute detectably to stabilization of the receptor/ligand complex, but is essential for signal propagation. In contrast, a truncated IL-3R alpha with the N-terminal domain deleted interacts functionally with the beta chain; mouse cells transfected with these receptor chains proliferate in response to human IL-3 and STAT5 transcription factor is activated. High- and low-affinity binding sites are retained, although the affinity for IL-3 is decreased 15-fold, indicating a significant role for the N-terminal domain in IL-3 binding.

A T cell-specific enhancer in the interleukin-3 locus is activated cooperatively by Oct and NFAT elements within a DNase I-hypersensitive site

Interleukin-3 (IL-3) is a cytokine that is expressed primarily in activated T cells. Here we identified an inducible T cell-specific enhancer 14 kb upstream of the IL-3 gene that responded to activation of T cell receptor signaling pathways. The IL-3 enhancer spanned an inducible cyclosporin A-sensitive DNase I-hypersensitive site found only in T cells. Four NFAT-like elements exist within the enhancer. The two most active NFAT-like elements were located at the center of the DNase I-hypersensitive site. One of these NFAT-like elements encompassed overlapping Oct- and NFATp/c-binding sites, which functioned in a highly synergistic manner. We suggest that the T cell-specific expression of the IL-3 gene is partly controlled through the enhancer by cooperation between Oct and NFAT family proteins.

Signals for activation of the GM-CSF promoter and enhancer in T cells

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is one of the many cytokines produced following T-cell activation. It is also produced in a variety of other cell types, in particular following activation by inflammatory mediators. Changes in the rate of transcription are important in the control of GM-CSF expression in T cells and in fibroblasts and endothelial cells. The GM-CSF gene contains two distinct transcriptional control regions. These are the proximal promoter consisting of the first 120 bp from the transcription start site and an enhancer located approximately 3 kb upstream from the proximal promoter. Distinct regions of the proximal promoter respond to a wide array of signals such as phorbol myristate acetate (PMA) and Ca2+ ionophore or phytohemaglutinin (PHA), CD28 activation, human T leukemia virus (HTLV)-1 tax, TNF, and interleukin 1 (IL-1). The transcription factors that mediate these responses have mainly been defined, with the major inducible proteins being the NF-kappa B/rel and AP-I families of transcription factors. In contrast to the promoter, the enhancer responds only to PMA and Ca2+ ionophore signals and binds NFAT/AP-1 complexes that appear to mediate its function.

A discontinuous eight-amino acid epitope in human interleukin-3 binds the alpha-chain of its receptor

We have previously reported that, within the first helix of human interleukin (IL)-3, residues Asp21 and Glu22 are important for interaction with the alpha- and beta-chains of the IL-3 receptor, respectively. In order to define more precisely the sites of interaction with the receptor, we have performed molecular modeling of the helical core of IL-3 and single amino acid substitution mutagenesis of residues predicted to lie on the surfaces of the A, C, and D helices. The resulting analogues were characterized for their abilities to stimulate proliferation of TF-l cells and for binding to the high affinity (alpha- and beta-chain; IL-3Ralpha/Rbeta) or low affinity (alpha-chain alone; IL-3Ralpha) IL-3 receptor. We found that in addition to Asp21, residues Ser17, Asn18, and Thr25 within the A helix and Arg108, Phe113, Lys116, and Glu119 within the D helix of IL-3 were important for biological activity. Analysis of their binding characteristics revealed that these analogues were deficient in binding to both the IL-3Ralpha/Rbeta and the IL-3Ralpha forms of the receptor, consistent with a selective impairment of interaction with IL-3Ralpha. Molecular modeling suggests that these eight amino acid residues are adjacent in the tertiary structure, consistent with a discontinuous epitope interacting selectively with IL-3Ralpha. On the other hand, Glu22 of IL-3 was found to interact preferentially with the beta-chain with bulky and positively charged substitutions causing greater than 10,000-fold reduction in biological activity. These results show fundamental differences between IL-3 and granulocyte-macrophage colony-stimulating factor in the structural basis for recognition of their receptors that has implications for the construction of novel analogues and our understanding of receptor activation.

Differential regulation of the stability of cytokine mRNAs in lipopolysaccharide-activated blood monocytes in response to interleukin-10

Adenosine-uridine (AU) instability elements, found in the 3'-untranslated regions of numerous mRNAs, target these mRNAs for rapid degradation. In addition, the degradation rate of some mRNAs that contain AU instability elements can change. This modulation of mRNA stability is an important component in the regulation of expression of many of the cytokines that control the production and function of blood cells. However, it has not been clear whether the stabilities of individual cytokine mRNAs that contain AU instability elements are coordinately regulated or whether different mRNAs can be independently regulated. We have investigated the influence of the cytokine synthesis inhibitory factor interleukin (IL)-10 on the turnover of granulocyte-colony stimulating factor (G-CSF), granulocyte macrophage-colony stimulating factor (GM-CSF), and IL-10 mRNAs in human blood monocytes stimulated with lipopolysaccharide. We find that all three mRNAs are destabilized in response to IL-10 but at different times. The G-CSF and GM-CSF mRNAs respond similarly, being rapidly destabilized, consistent with a direct influence of IL-10 receptor-mediated signals on the stability of these mRNAs. In contrast the IL-10 mRNA became unstable only after several hours of treatment with IL-10, suggesting that the IL-10 mRNA, although it also contains AU instability elements, is not co-regulated with the G-CSF and GM-CSF mRNAs but is regulated by a secondary factor produced in response to IL-10.

Transforming growth factor-beta 1 inhibits the production of IL-8 and the transmigration of neutrophils through activated endothelium

A central mechanism of inflammation is the activation of vascular endothelium by the inflammatory cytokines TNF-alpha and IL-1. These cytokines induce the expression of adhesion molecules, the elaboration of chemokines, and the transendothelial migration of white cells. TGF-beta 1 has anti-inflammatory properties, is expressed in the vessel wall, and has previously been shown to inhibit leukocyte adhesiveness to the endothelium at least in part by inhibiting the expression of E-selectin. We now show that TGF-beta 1 also inhibits the migration of neutrophils through endothelial monolayers activated by TNF-alpha. At a dose of 10 U/ml TNF-alpha, the transmigration of neutrophils was inhibited 42.7 +/- 7.9% (n = 8) by 0.2 ng/ml TGF-beta 1. Furthermore, TGF-beta 1 inhibited, in a time- and dose-dependent fashion, the elaboration of IL-8 by TNF-activated endothelial cells by between 33 and 78% (TNF doses from 100 down to 0.1 U/ml) and the elaboration of mRNA for IL-8 by 69%. TGF-beta 1 treatment did not significantly alter the TNF-induced IL-8 mRNA stability, suggesting that the mechanism of action of TGF-beta 1 is on gene transcription. Neutrophil transmigration through cytokine-activated endothelium involves both IL-8-dependent and IL-8-independent mechanisms. Using an anti-IL-8 Ab, we show that TGF-beta 1 inhibits only the IL-8-dependent pathway, but does not affect the IL-8-independent transendothelial migration mechanism. These and our previous results show that TGF-beta1, achieves its anti-inflammatory properties by inhibiting the expression of at least two genes, E-selectin and IL-8, which are essential in the inflammatory pathway.

Transforming growth factor-beta 1 inhibits the production of IL-8 and the transmigration of neutrophils through activated endothelium

A central mechanism of inflammation is the activation of vascular endothelium by the inflammatory cytokines TNF-alpha and IL-1. These cytokines induce the expression of adhesion molecules, the elaboration of chemokines, and the transendothelial migration of white cells. TGF-beta 1 has anti-inflammatory properties, is expressed in the vessel wall, and has previously been shown to inhibit leukocyte adhesiveness to the endothelium at least in part by inhibiting the expression of E-selectin. We now show that TGF-beta 1 also inhibits the migration of neutrophils through endothelial monolayers activated by TNF-alpha. At a dose of 10 U/ml TNF-alpha, the transmigration of neutrophils was inhibited 42.7 +/- 7.9% (n = 8) by 0.2 ng/ml TGF-beta 1. Furthermore, TGF-beta 1 inhibited, in a time- and dose-dependent fashion, the elaboration of IL-8 by TNF-activated endothelial cells by between 33 and 78% (TNF doses from 100 down to 0.1 U/ml) and the elaboration of mRNA for IL-8 by 69%. TGF-beta 1 treatment did not significantly alter the TNF-induced IL-8 mRNA stability, suggesting that the mechanism of action of TGF-beta 1 is on gene transcription. Neutrophil transmigration through cytokine-activated endothelium involves both IL-8-dependent and IL-8-independent mechanisms. Using an anti-IL-8 Ab, we show that TGF-beta 1 inhibits only the IL-8-dependent pathway, but does not affect the IL-8-independent transendothelial migration mechanism. These and our previous results show that TGF-beta1, achieves its anti-inflammatory properties by inhibiting the expression of at least two genes, E-selectin and IL-8, which are essential in the inflammatory pathway.

Cold shock domain proteins repress transcription from the GM-CSF promoter

The human granulocyte-macrophage colony stimulating factor (GM-CSF) gene promoter binds a sequence-specific single-strand DNA binding protein termed NF-GMb. We previously demonstrated that the NF-GMb binding sites were required for repression of tumor necrosis factor-alpha (TNF-alpha) induction of the proximal GM-CSF promoter sequences in fibroblasts. We now describe the isolation of two different cDNA clones that encode cold shock domain (CSD) proteins with NF-GMb binding characteristics. One is identical to the previously reported CSD protein dbpB and the other is a previously unreported variant of the dbpA CSD factor. This is the first report of CSD factors binding to a cytokine gene. Nuclear NF-GMb and expressed CSD proteins have the same binding specificity for the GM-CSF promoter and other CSD binding sites. We present evidence that CSD factors are components of the nuclear NF-GMb complex. We also demonstrate that overexpression of the CSD proteins leads to complete repression of the proximal GM-CSF promoter containing the NF-GMb/CSD binding sites. Surprisingly, we show that CSD overexpression can also directly repress a region of the promoter which apparently lacks NF-GMb/CSD binding sites. NF-GMb/CSD factors may hence be acting by two different mechanisms. We discuss the potential importance of CSD factors in maintaining strict regulation of the GM-CSF gene.

Extracellular truncations of h beta c, the common signaling subunit for interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF), and IL-5, lead to ligand-independent activation

The hypothesis that extracellular truncation of the common receptor subunit for interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor, and IL-5 (h beta c) can lead to ligand-independent activation was tested by infecting factor-dependent hematopoietic cell lines with retroviruses encoding truncated forms of h beta c. A truncation, resembling that in v-Mpl, and retaining 45 h beta c-derived extracellular residues, led to constitutive activation in the murine myeloid cell line, FDC-P1. However, infection of cells with retrovirus encoding a more severely truncated receptor, retaining only 7 h beta c-derived extracellular residues, did not confer factor independence on these cells. These experiments show that truncation activates the receptor and define a 37-amino acid segment of h beta c (H395-A431) which contains two motifs conserved throughout the cytokine receptor superfamily (consensus Y/H XX R/Q VR and WSXWS), as essential for factor-independent signaling. The mechanism of activation was also investigated in less severe truncations. A receptor that retains the entire membrane-proximal domain (domain 4) also conferred factor independent growth on FDC-P1 cells; however, a retrovirus encoding a truncated form of h beta c having two intact membrane proximal domains did not have this ability, suggesting that domain 3 may have an inhibitory role in h beta c. The ability of these receptors to confer factor independence was cell specific as demonstrated by their inability to confer factor-independent growth when introduced into the murine IL-3-dependent pro-B cell line BaF-B03. These results are consistent with a model in which activation requires unmasking of an interactive receptor surface in domain 4 and association with a myeloid-specific receptor or accessory component. We suggest that in the absence of ligand intramolecular interactions prevent inappropriate signaling.

Chronic expression of P-selectin on endothelial cells stimulated by the T-cell cytokine, interleukin-3

P-selectin expressed on the surface of endothelium mediates leukocyte adhesion in vitro and rolling in vivo. Several inducers of cell-surface P-selectin expression on endothelial cells (EC) have previously been identified, all of which yield transient cell-surface expression of P-selectin lasting minutes to a few hours. We now show that a T-lymphocyte product, interleukin-3 (IL-3), stimulates the long-term endothelial cells (HUVEC). IL-3 induced cell-surface P-selectin expression in two phases. An initial peak at 10 minutes was followed by a prolonged upregulation beginning 16 hours after IL-3 addition and lasting at least 4 days. The level of P-selectin expression induced by IL-3 added for 48 hours was similar to that induced by treatment of HUVEC for 10 minutes with thrombin, and the effect of adding IL-3 for 48 hours followed by thrombin for 10 minutes was additive. Induction of cell-surface P-selectin expression by IL-3 was blocked by pretreatment of EC with a blocking monoclonal antibody against the IL-3 receptor alpha-chain. Lipopolysaccharide (LPS), tumor necrosis factor alpha (TNF alpha) and a mutant form of IL-3 with decreased potency did not induce cell-surface P-selectin expression after 48 hours' incubation with HUVEC, suggesting that the effect was specific to IL-3. The increase in cell-surface P-selectin expression occurring after 16 hours of incubation with IL-3 was accompanied by a similar prolonged increase in the steady-state mRNA level that was not observed at 10 minutes after IL-3 addition. As T-lymphocyte infiltration is a hallmark of chronic inflammation, our observations suggest that the secretion of IL-3 by T lymphocytes may serve to maintain the inflammatory state during chronic inflammation.