FAT1 cadherin is multiply phosphorylated on its ectodomain but phosphorylation is not catalysed by the four-jointed homologue

The interaction between the Drosophila cadherins fat and dachsous is regulated by phosphorylation of their respective ectodomains, a process catalysed by the atypical kinase four-jointed. Given that many signalling functions are conserved between Drosophila and vertebrate Fat cadherins, we sought to determine whether ectodomain phosphorylation is conserved in FAT1 cadherin, and also whether FJX1, the vertebrate orthologue of four-jointed, was involved in such phosphorylation events. Potential Fj consensus phosphorylation motifs were identified in FAT1 and biochemical experiments revealed the presence of phosphoserine and phosphothreonine residues in its extracellular domain. However, silencing FJX1 did not influence the levels of FAT1 ectodomain phosphorylation, indicating that other mechanisms are likely responsible.

Fat1 cadherin provides a novel minimal residual disease marker in acute lymphoblastic leukemia

Measurement of minimal residual disease (MRD) maintains an important role in the clinical management of acute lymphoblastic leukemia (ALL). Recently, we identified Fat1 cadherin as a unique and independent prognostic factor for relapse-free and overall survival in pediatric pre-B-ALL. Here, we analyzed Fat1 mRNA for its potential as a novel marker of MRD in cases of pre-B- and T-ALL. Analyses of microarray data from 125 matched diagnosis/relapse samples across three independent datasets indicate that Fat1 mRNA is detectable in an average of 31.3% of diagnosed pre-B-ALL, of which 67.5% of cases remained positive at relapse. Furthermore, some 20% of cases with undetectable levels of Fat1 mRNA at diagnosis became positive upon relapse. T-ALL cases were 83.3% positive for Fat1 expression at diagnosis with 77.7% remaining positive at relapse. Towards proof of concept, we developed a quantitative polymerase chain reaction assay and demonstrate detection of Fat1 mRNA in leukemic cells mixed with normal peripheral blood cells at a sensitivity of 1 in 10 000 to 100 000 cells. Fat1 may therefore provide a new marker of MRD for patients with ALL lacking known genomic aberrations or within a multiplex approach to MRD detection.

Dual processing of FAT1 cadherin protein by human melanoma cells generates distinct protein products

The giant cadherin FAT1 is one of four vertebrate orthologues of the Drosophila tumor suppressor fat. It engages in several functions, including cell polarity and migration, and in Hippo signaling during development. Homozygous deletions in oral cancer suggest that FAT1 may play a tumor suppressor role, although overexpression of FAT1 has been reported in some other cancers. Here we show using Northern blotting that human melanoma cell lines variably but universally express FAT1 and less commonly FAT2, FAT3, and FAT4. Both normal melanocytes and keratinocytes also express comparable FAT1 mRNA relative to melanoma cells. Analysis of the protein processing of FAT1 in keratinocytes revealed that, like Drosophila FAT, human FAT1 is cleaved into a non-covalent heterodimer before achieving cell surface expression. The use of inhibitors also established that such cleavage requires the proprotein convertase furin. However, in melanoma cells, the non-cleaved proform of FAT1 is also expressed at the cell surface together with the furin-cleaved heterodimer. Moreover, furin-independent processing generates a potentially functional proteolytic product in melanoma cells, a persistent 65-kDa membrane-bound cytoplasmic fragment no longer in association with the extracellular fragment. In vitro localization studies of FAT1 showed that melanoma cells display high levels of cytosolic FAT1 protein, whereas keratinocytes, despite comparable FAT1 expression levels, exhibited mainly cell-cell junctional staining. Such differences in protein distribution appear to reconcile with the different protein products generated by dual FAT1 processing. We suggest that the uncleaved FAT1 could promote altered signaling, and the novel products of alternate processing provide a dominant negative function in melanoma.

Palmitoylation of CD36/FAT regulates the rate of its post-transcriptional processing in the endoplasmic reticulum

CD36/FAT is a transmembrane glycoprotein that functions in the cellular uptake of long-chain fatty acids and also as a scavenger receptor. As such it plays an important role in lipid homeostasis and, pathophysiologically, in the progression of type 2 diabetes and atherosclerosis. CD36 expression is tightly regulated at the levels of both transcription and translation. Here we show that its expression and location are also regulated post-translationally, by palmitoylation. Although palmitoylation of CD36 was not required for receptor maturation and cell surface expression, inhibition of palmitoylation either pharmacologically with cerulenin or by mutation of the relevant cysteines delayed processing at the ER and trafficking through the secretory pathway. The absence of palmitoylation also reduced the half life of the CD36 protein. Additionally, the CD36 palmitoylation mutant did not incorporate efficiently into lipid rafts, a site known to be required for its function of fatty acid uptake, and this reduced the efficiency of uptake of oxidized low density lipoprotein. These findings provide an added level of sophistication where translocation of CD36 to the plasma membrane may be physiologically regulated by palmitoylation.

Shed gangliosides provide detergent-independent evidence for Type-3 glycosynapses

Membrane microdomains, or rafts, at the plasma membrane have been invoked to explain many cellular processes. Protein-protein interactions within such microdomains including, for example, the tetraspanin web are reported to provide a scaffold for signal transduction. However, the nature of such protein-protein interactions is not fully elucidated. Hakomori [S.I. Hakomori, The glycosynapse, Proc. Natl. Acad. Sci. USA 99 (2002) 225-232] has advanced the concept that glycosphingolipids, particularly gangliosides, provide the intermediary link between transmembrane receptors and signal transducers and has redefined membrane rafts as Type-1, -2 or -3 glycosynapses. Here, using simple immunofluorescent analysis of the ganglioside complexes naturally released from cellular microprocesses (termed "footprints") we show that the ganglioside can determine the nature of protein-protein associations. Specifically, we demonstrate that CD36 and the tetraspanin CD151, both of which interact with beta1 integrins, associate together only in the presence of the gangliosides GD2/GD3. These results substantiate the glycosynapse hypothesis and suggest that the nature of the tetraspanin web may be determined by gangliosides.

CD36 is a receptor for oxidized high density lipoprotein: Implications for the development of atherosclerosis

Atherosclerotic plaques result from the excessive deposition of cholesterol esters derived from lipoproteins and lipoprotein fragments. Tissue macrophage within the intimal space of major arterial vessels have been shown to play an important role in this process. We demonstrate in a transfection system using two human cell lines that the macrophage scavenger receptor CD36 selectively elicited lipid uptake from Cu(2+)-oxidized high density lipoprotein (HDL) but not from native HDL or low density lipoprotein (LDL). The uptake of oxHDL displayed morphological and biochemical similarities with the CD36-dependent uptake of oxidized LDL. CD36-mediated uptake of oxidized HDL by macrophage may therefore contribute to atheroma formation.

The association between CD36 and Lyn protein tyrosine kinase is mediated by lipid

CD36 is a transmembrane glycoprotein receptor that engages in signal transduction implicated in important physiological and pathophysiological events. CD36 in platelets has been shown physically and functionally to associate with members of the Src family of protein tyrosine kinases, Fyn, Lyn, and Yes, but the nature of this important association has never been rigorously examined. Here, we show that CD36 does not associate with Lyn through a protein-mediated interaction. In COS cells transfected with both CD36 and Lyn these molecules did not co-precipitate, suggesting a requirement for an intermediary molecule absent from the COS cells. Yeast two-hybrid analysis confirmed that the carboxylterminal cytoplasmic tail of CD36 did not bind Lyn directly, and no Lyn binding protein bound to CD36 in a cDNA library screen. Conversely, when the CD36-Lyn association seen in platelets was analysed by biophysical parameters, dissociation occurred at 37 degrees C and also by solubilisation in octylglucoside, indicative of a lipid-mediated association. Since both CD36 and Lyn are enriched in Triton X-100-insoluble rafts at the plasma membrane, these findings point to the importance of raft-associated lipids in CD36-mediated signal transduction.

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

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

FAT, E-cadherin, beta catenin, HER 2/neu, Ki67 immuno-expression, and histological grade in intrahepatic cholangiocarcinoma

AIM

To identify surrogate prognostic markers in intrahepatic cholangiocarcinoma (IHCC).

METHODS

Thirty one cases of IHCC were graded and immunostained for FAT, Ki67, E-cadherin, beta catenin, and HER 2/neu.

RESULTS

Twenty two cases were high grade and 27 had high Ki67 counts. Strong membranous staining of HER 2/neu was found in 10 tumours and reduced membranous E-cadherin and beta catenin in 19 and 18 tumours, respectively. Nuclear localisation of beta catenin was identified in five tumours and 22 showed weak cytoplasmic staining of FAT. Strong HER 2/neu and weak FAT immuno-expression were significantly correlated with high histological grade (p=0.01) and high Ki67 index (p=0.03). Upregulation of HER 2/neu was also significantly associated with nuclear localisation of beta catenin (p=0.01). Reduced membranous beta catenin was significantly related to reduced membranous E-cadherin (p=0.03), weak staining for FAT (p=0.01), and nuclear translocation of beta catenin (p=0.04).

CONCLUSIONS

Reduced immuno-expression of E-cadherin and FAT at their normal membranous location may be potential prognostic markers, and the overexpression of HER 2/neu and beta catenin nuclear translocation may have a role in cholangiocarcinogenesis.

Cloning and expression of the large zebrafish protocadherin gene, Fat

The cadherin superfamily members play an important role in mediating cell-cell contact and adhesion (Takeichi, M., 1991. Cadherin cell adhesion receptors as a morphogenetic regulator. Science 251, 1451-1455). A distinct subfamily, neither belonging to the classical or protocadherins includes Fat, the largest member of the cadherin super-family. Fat was originally identified in Drosophila. Subsequently, orthologues of Fat have been described in man (Dunne, J., Hanby, A. M., Poulsom, R., Jones, T. A., Sheer, D., Chin, W. G., Da, S. M., Zhao, Q., Beverley, P. C., Owen, M. J., 1995. Molecular cloning and tissue expression of FAT, the human homologue of the Drosophila fat gene that is located on chromosome 4q34-q35 and encodes a putative adhesion molecule. Genomics 30, 207-223), rat (Ponassi, M., Jacques, T. S., Ciani, L., ffrench, C. C., 1999. Expression of the rat homologue of the Drosophila fat tumour suppressor gene. Mech. Dev. 80, 207-212) and mouse (Cox, B., Hadjantonakis, A. K., Collins, J. E., Magee, A. I., 2000. Cloning and expression throughout mouse development of mfat1, a homologue of the Drosophila tumour suppressor gene fat [In Process Citation]. Dev. Dyn. 217, 233-240). In Drosophila, Fat has been shown to play an important role in both planar cell polarity and cell boundary formation during development. In this study we describe the characterization of zebrafish Fat, the first non-mammalian, vertebrate Fat homologue to be identified. The Fat protein has 64% amino acid identity and 80% similarity to human FAT and an identical domain structure to other vertebrate Fat proteins. During embryogenesis fat mRNA is expressed in the developing brain, specialised epithelial surfaces the notochord, ears, eyes and digestive tract, a pattern similar but distinct to that found in mammals.

The LFA-1-associated molecule PTA-1 (CD226) on T cells forms a dynamic molecular complex with protein 4.1G and human discs large

Clustering of the T cell integrin, LFA-1, at specialized regions of intercellular contact initiates integrin-mediated adhesion and downstream signaling, events that are necessary for a successful immunological response. But how clustering is achieved and sustained is not known. Here we establish that an LFA-1-associated molecule, PTA-1, is localized to membrane rafts and binds the carboxyl-terminal domain of isoforms of the actin-binding protein 4.1G. Protein 4.1 is known to associate with the membrane-associated guanylate kinase homologue, human discs large. We show that the carboxyl-terminal peptide of PTA-1 also can bind human discs large and that the presence or absence of this peptide greatly influences binding between PTA-1 and different isoforms of 4.1G. T cell stimulation with phorbol ester or PTA-1 cross-linking induces PTA-1 and 4.1G to associate tightly with the cytoskeleton, and the PTA-1 from such activated cells now can bind to the amino-terminal region of 4.1G. We propose that these dynamic associations provide the structural basis for a regulated molecular adhesive complex that serves to cluster and transport LFA-1 and associated molecules.

The role of the Eph-ephrin signalling system in the regulation of developmental patterning

The Eph and ephrin system, consisting of fourteen Eph receptor tyrosine kinase proteins and nine ephrin membrane proteins in vertebrates, has been implicated in the regulation of many critical events during development. Binding of cell surface Eph and ephrin proteins results in bi-directional signals, which regulate the cytoskeletal, adhesive and motile properties of the interacting cells. Through these signals Eph and ephrin proteins are involved in early embryonic cell movements, which establish the germ layers, cell movements involved in formation of tissue boundaries and the pathfinding of axons. This review focuses on two vertebrate models, the zebrafish and mouse, in which experimental perturbation of Eph and/or ephrin expression in vivo have provided important insights into the role and functioning of the Eph/ephrin system.

A CD44 survival pathway triggers chemoresistance via lyn kinase and phosphoinositide 3-kinase/Akt in colon carcinoma cells

A major obstacle to successful treatment of colorectal cancer is chemotherapy resistance. Enhanced expression of variant CD44 isoforms has been associated with aggressive tumor behavior, prompting the question of whether signaling from this receptor might modulate drug sensitivity. Activation of variant CD44 in colon carcinoma cell lines triggered resistance to the drug 1,3-bis(2-chloroethyl)-1-nitrosurea. Resistance was induced by monoclonal antibodies directed against epitopes independent of the hyaluronate-binding region but was not triggered by identical treatment of a carcinoma line expressing the standard CD44 isoform. We observed that variant CD44 produced activation of the src-family tyrosine kinase lyn. Moreover, overexpression of dominant-active lyn recapitulated chemoresistance via a pathway shown to involve activation of phosphoinositide 3-kinase and Akt. These results establish a novel role for CD44 in determining survival of colon carcinoma cells through lyn kinase and Akt. The ability to suppress apoptosis might play a critical role in the onset and development of colorectal malignancies.

The integrins alpha3beta1 and alpha6beta1 physically and functionally associate with CD36 in human melanoma cells. Requirement for the extracellular domain OF CD36

Lateral association between different transmembrane glycoproteins can serve to modulate integrin function. Here we characterize a physical association between the integrins alpha(3)beta(1) and alpha(6)beta(1) and CD36 on the surface of melanoma cells and show that ectopic expression of CD36 by CD36-negative MV3 melanoma cells increases their haptotactic migration on extracellular matrix components. The association was demonstrated by co-immunoprecipitation, reimmunoprecipitation, and immunoblotting of surface-labeled cells lysed in Brij 96 detergent. Confocal microscopy illustrated the co-association of alpha(3) and CD36 in cell membrane projections and ruffles. A requirement for the extracellular domain of CD36 in this association was shown by co-immunoprecipitation experiments using surface-labeled MV3 melanoma or COS-7 cells that had been transiently transfected with chimeric constructs between CD36 and intercellular adhesion molecule 1 (ICAM-1) or with a truncation mutant of CD36. CD36 is known to engage in signal transduction and to localize to membrane microdomains or rafts in several cell types. Toward a mechanistic explanation for the functional effects of CD36 expression, we demonstrate that in fractionated Triton X-100 lysates of the MV3 cells stably transfected with CD36, CD36 was greatly enriched with the detergent-insoluble fractions that represent plasma membrane rafts. Significantly, when these fractionated lysates were reprobed for endogenous beta(1) integrin, it was found that a 4-fold increase in the proportion of the mature protein was contained within the detergent-insoluble fractions when extracted from the CD36-transfected cells compared with MV3 cells transfected with vector only. These results suggest that in melanoma cells CD36 expression may induce the sequestration of certain integrins into membrane microdomains and promote cell migration.

CD36 is a ditopic glycoprotein with the N-terminal domain implicated in intracellular transport

The CD36 receptor sequence predicts two hydrophobic domains located at the N- and C-termini of the protein, but there are conflicting reports as to whether the N-terminal uncleaved leader sequence functions as a transmembrane domain. To investigate the topology of CD36, we generated a panel of mutants lacking either one or both hydrophobic regions and analyzed their folding and transport in COS-7 cells. The N- and the C-terminal hydrophobic regions were both sufficient to anchor CD36 in the membrane, and a FLAG epitope inserted at the N-terminus was located intracellularly. These results indicate that CD36 adopts a ditopic configuration. Accordingly, neither N- nor C-terminal truncation mutants were secreted. Analysis with conformation-specific monoclonal antibodies showed that the N-terminal transmembrane domain truncated molecule was slowly transported through the exocytic pathway and largely accumulated intracellularly. Thus, dual membrane insertion dictates the correct topogenesis and seems to be necessary for efficient folding and intracellular transport.

Active Lyn protein tyrosine kinase is selectively enriched within membrane microdomains of resting platelets

Circulating platelets are primed to respond very rapidly to thrombogenic stimuli, but most platelets complete their lifespan without ever becoming activated. Platelet activation is accompanied by waves of sequential tyrosine phosphorylation thought to involve members of the Src family of protein tyrosine kinases (PTKs). We show here that resting platelets contain highly active pp53/56(Lyn) PTK within membrane microdomains (rafts) isolated biochemically with or without the use of detergent. This fraction is also greatly enriched in the transmembrane glycoprotein CD36, known to associate with Lyn PTK, but in transfection studies we could find no evidence to suggest that CD36 affects the distribution or function of Lyn. Upon platelet activation Lyn activity remains constant or diminishes and pp60(c-src) PTK within this fraction becomes highly activated, indicating the dynamic nature of the membrane microdomains. It is suggested that the function of active Lyn PTK in the resting platelet is to allow prolonged survival of this anucleate cell.

Engagement of variant CD44 confers resistance to anti-integrin antibody-mediated apoptosis in a colon carcinoma cell line

The LIM 1863 colon carcinoma cell line grows as structured organoids around a central lumen, and we have previously demonstrated that the three-dimensional arrangement protects the individual cells from apoptosis induced by an anti-alpha v integrin antibody, 23C6 (Bates et al., 1994). Here we show that the intercellular forces which drive spheroid formation can be overcome by exposure of the cells to a collagen substrate, or more specifically through ligation of the CD44 receptor by a monoclonal antibody. Binding to immobilized anti-CD44 antibody induced a monolayer morphology which is accompanied by fibronectin production and secretion, and expression of the integrin alpha v beta 6. Significantly, the cells of the monolayer acquired resistance to 23C6 antibody-mediated apoptosis over time and this property was sustained even after removal from the monolayer. We provide data to show that this resistance is not dependent on monolayer morphology, constant engagement of the CD44 receptor, loss of the 23C6 antigen, or elevation of Bcl-2 or Bcl-XL protein. The CD44 expressed by LIM 1863 is shown to be the metastatic variant of the molecule therefore these results provide a possible explanation for the selective advantages conferred by expression of this variant for metastasizing colon cancer cells. Overall, the findings of this study support a model for the development of malignancy through the production of specific survival and growth signals as a direct consequence of a signaling event induced by stimulation of an epithelial variant of CD44.

Cytochemistry of human T-cell subpopulations

Acid phosphatase and esterase cytochemistry performed on purified normal human T-cell populations showed that both methods produced distinctive localized dot patterns of reactivity in 60-70% of cells. By examination of rosette preparations formed with ox erythrocytes coated with IgM (EAM), with IgG (EAG), or anti-human kappa and lambda light chains, it was shown that this pattern of reactivity was largely restricted to small T lymphocytes possessing receptors for the Fc of IgM (T mu cells). In addition, both B lymphocytes and T cells with receptors for the Fc of IgG (T gamma cells) were larger lymphocytes with more abundant cytoplasm and usually displayed scattered granular acid phosphatase activity; in esterase preparations both cell types were either negative or possessed similar scattered granular positivity. As compared with T mu cells, T gamma cells were seen to form loose spontaneous rosettes with sheep erythrocytes. Combined esterase and acid phosphatase staining showed that both enzyme activities in the T mu cells are localized in the same area, and ultrastructural acid phosphatase cytochemistry established that this was in distinctive lysosomal structures. T mu staining by both esterase and acid phosphatase cytochemistry was greatly reduced after rosetting with EAG, but not after rosette formation with EAM or sheep erythrocytes.

Antibody Binding to Individual Short Consensus Repeats of Decay-Accelerating Factor Enhances Enterovirus Cell Attachment and Infectivity

Decay-accelerating factor (DAF), a widely expressed membrane complement-regulatory protein, is utilized as a cellular receptor by many human enteric pathogens. We show here that the binding of two enteroviruses to individual short consensus repeats (SCR) of DAF on the cell surface is greatly augmented by mAb binding to an alternate SCR: Coxsackievirus A21 binding to the SCR1 of DAF is increased by Ab binding to SCR3 and, conversely, Echovirus 7 binding to SCR3 is enhanced severalfold by Ab binding to SCR1. These Ab-induced increases in viral binding also resulted in increased viral infectivity. Using purified soluble DAF in a solid phase assay it was found that Ab binding to SCR1 is increased greatly in the presence of an Ab against SCR3 and, reciprocally, Ab against SCR1 greatly increases Ab binding to SCR3. In contrast to the results obtained with the larger viral particles, however, this reciprocal Ab-induced enhancement of binding is not seen when measuring Ab binding to membrane-bound DAF SCR on the cell surface. These findings provide a possible explanation for functional differences between membrane-bound and soluble DAF with implications for a potential role for DAF-binding molecules in regulating DAF function. This is the first demonstration of enhancement of viral infectivity mediated by Ab against the viral receptor.

Antibody binding to individual short consensus repeats of decay-accelerating factor enhances enterovirus cell attachment and infectivity

Decay-accelerating factor (DAF), a widely expressed membrane complement-regulatory protein, is utilized as a cellular receptor by many human enteric pathogens. We show here that the binding of two enteroviruses to individual short consensus repeats (SCR) of DAF on the cell surface is greatly augmented by mAb binding to an alternate SCR: Coxsackievirus A21 binding to the SCR1 of DAF is increased by Ab binding to SCR3 and, conversely, Echovirus 7 binding to SCR3 is enhanced severalfold by Ab binding to SCR1. These Ab-induced increases in viral binding also resulted in increased viral infectivity. Using purified soluble DAF in a solid phase assay it was found that Ab binding to SCR1 is increased greatly in the presence of an Ab against SCR3 and, reciprocally, Ab against SCR1 greatly increases Ab binding to SCR3. In contrast to the results obtained with the larger viral particles, however, this reciprocal Ab-induced enhancement of binding is not seen when measuring Ab binding to membrane-bound DAF SCR on the cell surface. These findings provide a possible explanation for functional differences between membrane-bound and soluble DAF with implications for a potential role for DAF-binding molecules in regulating DAF function. This is the first demonstration of enhancement of viral infectivity mediated by Ab against the viral receptor.

CD36 forms covalently associated dimers and multimers in platelets and transfected COS-7 cells

CD36 is a transmembrane glycoprotein expressed on the surface of a number of cell types. The analysis of CD36 from platelets using immunoblotting, gel filtration, and native PAGE indicated the presence of high molecular complexes exceeding the Mr of monomeric CD36. Experiments using transfected COS-7 cells revealed these complexes were homodimers and -multimers of CD36. The multimers could be dissociated by treatment with a reducing agent, indicating they were formed by intermolecular cysteine-bridging. Mutagenesis of the cDNA for CD36 implicated the cysteines in the extracellular domain of the molecule. The potential physiological roles of CD36 multimerisation are discussed.

TLiSA1 (PTA1) activation antigen implicated in T cell differentiation and platelet activation is a member of the immunoglobulin superfamily exhibiting distinctive regulation of expression

T lineage-specific activation antigen 1 (TLiSA1) antigen was initially described as a T lineage-specific activation antigen involved in the differentiation of human cytotoxic T cells. Subsequently, the antigen was identified on platelets and was shown to be involved in platelet activation, hence it was renamed platelet and T cell antigen 1 (PTA1), although identity between the two antigens was not established. In the present study we have cloned the cDNA encoding TLiSA1 from Jurkat cells and show it to be a novel member of the immunoglobulin superfamily with the unusual structure of two V domains only. Identity between TLiSA1 and platelet PTA1 is established by immunological criteria, by internal peptide sequences obtained from the purified platelet glycoprotein and by sequencing the platelet transcript after reverse transcriptase-polymerase chain reaction. In Jurkat cells, TLiSA1/PTA1 mRNA and surface protein expression is greatly stimulated by treatment of the cells with phorbol ester, but the T cell proliferative signal of phorbol ester and ionophore combined greatly reduces or abrogates this response, and this suppressive effect of the ionophore is not reversed by incorporating FK506 to inhibit calcineurin. Together with the known signaling role of PTA1, these data substantiate the notion that this molecule is implicated in T cell differentiation, perhaps by engagement of an adhesive ligand.

Coxsackievirus A21 binds to decay-accelerating factor but requires intercellular adhesion molecule 1 for cell entry

It is becoming increasingly apparent that many viruses employ multiple receptor molecules in their cell entry mechanisms. The human enterovirus coxsackievirus A21 (CAV21) has been reported to bind to the N-terminal domain of intercellular adhesion molecule 1 (ICAM-1) and undergo limited replication in ICAM-1-expressing murine L cells. In this study, we show that in addition to binding to ICAM-1, CAV21 binds to the first short consensus repeat (SCR) of decay-accelerating factor (DAF). Dual antibody blockade using both anti-ICAM-1 (domain 1) and anti-DAF (SCR1) monoclonal antibodies (MAbs) is required to completely abolish binding and replication of high-titered CAV21. However, the binding of CAV21 to DAF, unlike that to ICAM-1, does not initiate a productive cell infection. The capacity of an anti-DAF (SCR3) MAb to block CAV21 infection but not binding, coupled with immunoprecipitation data from chemical cross-linking studies, indicates that DAF and ICAM-1 are closely associated on the cell surface. It is therefore suggested that DAF may function as a low-affinity attachment receptor either enhancing viral presentation or providing a viral sequestration site for subsequent high-affinity binding to ICAM-1.

Formation of spheroid structures in a human colon carcinoma cell line involves a complex series of intercellular rearrangements

The structural remodelling of tissues that occurs in vivo during animal morphogenesis can often prove difficult to study. Here we investigate the organizational processes of the LIM 1863 colon carcinoma cell line as it transforms from a single-cell stage into multicellular spherical structures called 'organoids'. The organoids can be dissociated into a viable single-cell suspension when cultured in calcium-depleted medium, and then induced to reform the organoid structure by the readdition of calcium. Previous studies have shown that initial cell attachment under these conditions is characterized by a novel mechanism of cell engulfment termed 'clutching'. This investigation reveals the subsequent appearance of junctional complexes between groups of 'clutched' cells prior to lumen formation, and the ultimate 'declutching' of entrapped cells as a means of cell rearrangement. Intact actin filaments but not microtubules were required for the initial clutching events, while inhibition of microtubule polymerization resulted in aberrant apical protein polarization, but did not affect the development of a luminal space within the spheroids. Single cells exhibited pools of intracellular microvilli contained in vacuolar apical compartments, which were resistant to the effects of cytoskeleton-disrupting drugs. However, these structures did not seem to be responsible for the swift development of the luminal surface observed in these cells. Two other cell lines, MDCK and DU 4475, were found to exhibit similar clutching conformations when induced to form three-dimensional structures, suggesting that this may be a widespread mechanism of cell rearrangement that reflects the process of organ morphogenesis in vivo.

Stimulation of platelet activation and aggregation by a carboxyl-terminal peptide from thrombospondin binding to the integrin-associated protein receptor

Thrombospondin, a major secretory product of the alpha-granules of activated platelets, is a large trimeric glycoprotein that plays an important role in platelet aggregation. On resting platelets, thrombospondin binds to a single receptor in a cation-independent manner, but upon platelet activation it binds at least two further, distinct receptors that are both dependent upon divalent cations. Each of these receptors on the platelet surface binds to different regions of the thrombospondin molecule, and such binding may be responsible for the multifunctional role of thrombospondin in aggregation. We show here that a peptide from the carboxyl terminus of thrombospondin, RFYVVMWK, directly and specifically induces the activation and aggregation of washed human platelets from different donors at concentrations of 5-25 microM. At lower concentrations the peptide synergizes with suboptimal concentrations of ADP to induce aggregation. Peptide affinity chromatography and immunoprecipitation with a monoclonal antibody were used to identify the receptor for the carboxyl-terminal peptide as the integrin-associated protein. The integrin-associated protein remained bound to the RFYVVMWK-containing peptide column when washed with a scrambled peptide in the presence of 5 mM EDTA, indicating a divalent cation-independent association. It is suggested that integrin-associated protein is the primary receptor for thrombospondin on the surface of resting platelets and is implicated in potentiating the platelet aggregation response.

Mouse cells expressing human intercellular adhesion molecule-1 are susceptible to infection by coxsackievirus A21

Competitive viral binding assays have revealed previously that coxsackievirus A21 (CAV21) and human rhinovirus 14 (HRV14) share a common cell surface receptor. More recently, intercellular adhesion molecule-1 (ICAM-1) has been identified as the cellular receptor for HRV-14. Also, anti-ICAM-1 monoclonal antibodies (MAbs) blocked infection by HRV14, CAV13, CAV18, and CAV21, suggesting that these viruses share this receptor; however, this has never been established by more direct methods. In this study we show conclusively that CAV21 binds to ICAM-1 and that MAbs directed against the N-terminal domain of the molecule inhibit this attachment. Furthermore, we show that the specific interaction between ICAM-1 and 160S CAV21 virions induces formation of 135S A particles. Finally, we show transfection of normally nonsusceptible mouse L cells with human ICAM-1 cDNA renders them susceptible to infection by CAV21.

Biochemical isolation of a membrane microdomain from resting platelets highly enriched in the plasma membrane glycoprotein CD36

Here we describe the isolation and characterization of a Triton X-100-insoluble fraction isolated from lysates of platelets by flotation in sucrose gradients. Transmission electron microscopy of the insoluble material revealed a heterogeneous population of vesicles ranging in size from 20 to 1000 nm, and Western blot analyses of platelet lysates for the caveolae structural coat protein, caveolin/VIP21, were negative. Biochemical characterization of the Triton X-100-insoluble fraction showed it to be cholesterol-rich, greatly and specifically enriched in the plasma membrane glycoprotein CD36, and also to contain Src and the Src-related kinase, Lyn. CD36 within this fraction is shown to be palmitoylated, but the fraction itself is not generally enriched in palmitoylated platelet proteins. These results suggest that this fraction represents caveolin-negative, CD36-rich microdomains in the resting platelet membrane. CD36 can form associations with certain Src-related kinases and can signal to activate platelets. These results suggest the possibility that such microdomains are implicated in platelet activation.

Factor H co-purifies with thrombospondin isolated from platelet secretate

Thrombospondin is a trimeric glycoprotein that has several known functions, including roles in platelet aggregation, phagocytosis and an inhibitor of angiogenesis. Typically the molecule is isolated from platelet secretate by heparin affinity followed by sizing chromatography. In this study, purity is analysed by 7.5% SDS-PAGE under reducing conditions when thrombospondin monomers run as a band at around 180 kDa. Under nonreducing conditions of 7.5% SDS-PAGE, thrombospondin does not penetrate beyond the stacking gel; however, under these conditions a major contaminating band can be seen which, upon reduction, merges into the thrombospondin band. Further purification of this contaminating protein was achieved by DEAE chromatography and it was identified as Factor H by peptide sequencing and immunoblotting. Factor H function was demonstrated by the ability of the protein to function as a cofactor in the Factor-I-mediated cleavage of C3b. Since Factor H has several known functions, such contamination could confound functional studies of thrombospondin thus purified and a pre-elution step of the heparin affinity column is recommended.

CD36 is spatially associated with glycoprotein IIb-IIIa (alpha IIb beta 3) on the surface of resting platelets

Platelet activation and aggregation induced by agonists such as thrombin are accompanied by the phosphorylation of several proteins on tyrosine. Such tyrosine phosphorylation is dependent upon activation and ligand engagement of the major integrin receptor on the surface of platelets, glycoprotein (GP) IIb-IIIa (alpha IIb beta 3), but how this is accomplished is not known. The only platelet membrane GP known to associated with non receptor tyrosine kinases is CD36 (GPIV) which forms associations with pp60Fyn, pp62Yes, and pp54/58Lyn, and antibodies directed against CD36 activate platelets in a process dependent upon GPIIb-IIIa. These and other data suggest a physical association between the two membrane GPs, IIb-IIIa and CD36. By the use of immunoprecipitation of lysates of platelets that have been surface labeled and chemically crosslinked we show here that CD36 and GPIIb-IIIa are spatially associated on the surface of resting platelets.

A common epitope on platelet integrin alpha IIb beta 3 (glycoprotein IIbIIIa; CD41b/CD61) and alpha M beta 2 (Mac-1; CDIIb/CD18) detected by a monoclonal antibody

Evidence is presented that the mAb 25E11, directed against the platelet integrin alpha IIb beta 3 (glycoprotein IIbIIIa;CD41b/CD61) also binds the distinct myeloid cell integrin alpha M beta 2 (Mac-1;CDIIb/CD18). The Ab is shown to identify only the alpha IIb beta 3 integrin complex and not the individual subunits in crossed Ab immunoelectrophoresis of platelet lysate. From cultured human macrophages, sequential immunoprecipitation of labeled glycoproteins indicated that 25E11 also bound the Mac-1 (CD11b/CD18) complex. This was confirmed using COS-7 and WOP cells doubly transfected with alpha M (CD11b) and beta 2 (CD18) or with alpha L (CD11a) and beta 2 when it was found that the Ab bound only the alpha M beta 2 transfectants. Studies with these cells and the RC2A myeloid cell line stimulated with tetradecanoyl phorbol acetate or FMLP indicated that the 25E11 epitope on Mac-1 did not depend on cell activation for its expression. The rationale for this cross-reactivity is not known, but since the 25E11 Ab inhibits the function of both platelets and myeloid cells, it is likely that this shared epitope is important to integrin function. Given the expression of this epitope on IIbIIIa and Mac-1, the dominant integrins of platelets and granulocyte/macrophage cells, but not on other integrins, a role of this epitope in the early events of inflammation is suggested.

A common epitope on platelet integrin alpha IIb beta 3 (glycoprotein IIbIIIa; CD41b/CD61) and alpha M beta 2 (Mac-1; CDIIb/CD18) detected by a monoclonal antibody

Evidence is presented that the mAb 25E11, directed against the platelet integrin alpha IIb beta 3 (glycoprotein IIbIIIa;CD41b/CD61) also binds the distinct myeloid cell integrin alpha M beta 2 (Mac-1;CDIIb/CD18). The Ab is shown to identify only the alpha IIb beta 3 integrin complex and not the individual subunits in crossed Ab immunoelectrophoresis of platelet lysate. From cultured human macrophages, sequential immunoprecipitation of labeled glycoproteins indicated that 25E11 also bound the Mac-1 (CD11b/CD18) complex. This was confirmed using COS-7 and WOP cells doubly transfected with alpha M (CD11b) and beta 2 (CD18) or with alpha L (CD11a) and beta 2 when it was found that the Ab bound only the alpha M beta 2 transfectants. Studies with these cells and the RC2A myeloid cell line stimulated with tetradecanoyl phorbol acetate or FMLP indicated that the 25E11 epitope on Mac-1 did not depend on cell activation for its expression. The rationale for this cross-reactivity is not known, but since the 25E11 Ab inhibits the function of both platelets and myeloid cells, it is likely that this shared epitope is important to integrin function. Given the expression of this epitope on IIbIIIa and Mac-1, the dominant integrins of platelets and granulocyte/macrophage cells, but not on other integrins, a role of this epitope in the early events of inflammation is suggested.

Involvement of integrins in cell survival

Apoptosis is a regulated process of cell death by which cells actively participate in their own destruction. In multicellular organisms, the balance between cell proliferation and apoptosis provides homeostatic control, and a regulatory failure of either event can contribute to oncogenesis. The extracellular matrix (ECM) is known to play a regulatory role in cellular growth and differentiation, but only more recently has it been recognized as a regulator of apoptosis. In these processes the major transmitters of ECM-derived signals to the cell are members of the integrin family, although the mechanical process of cell spreading also plays a role. Both in vivo and in vitro the loss of adhesion to specific components of the ECM can lead to cell death, and such apoptosis can be induced experimentally by blocking integrin binding. Heterotypic and homotypic cell-cell adhesion can also protect from adhesion-dependent apoptosis and there is evidence to suggest that this too in integrin mediated. In addition, some integrin mediated signaling appears to promote apoptosis. The downstream mechanisms of integrin signaling causing cell death have not been greatly explored, but there is evidence from two different systems that the induction of ICE transcription and nuclear translocation of p53 are candidate processes. Alterations in integrin expression or signaling therefore are likely to contribute to tumor development by enabling escape from apoptosis. Also, the recognition of the importance of cell-cell adhesion in tumor cell survival offers the potential of developing improved drug regimes for the treatment of malignancy.

Macrophage colony-stimulating factor induces thrombospondin 1 production by cultured human macrophages

The role of colony-stimulating factors (CSFs) in regulating the synthesis of thrombospondin 1 (TSP1) by cultured human macrophages is investigated. Macrophage (M)-CSF is shown rapidly and transiently to induce two predominant species of TSP1 mRNA. One of these species was 3.2 kb in size and appeared to be specific to M-CSF-stimulated macrophages. Adherent M-CSF-treated macrophages are also shown to express abundant surface cell-associated TSP rapidly when examined by indirect immunofluorescence staining. Granulocyte-macrophage (GM)-CSF induced TSP1 mRNA at a later time point, and this was attributable to the effects of endogenous M-CSF induced by the GM-CSF; the GM-CSF-treated cells did not display surface-associated TSP after 3 hr of treatment. Analysis of the TSP1 protein synthesised by the M-CSF-treated macrophages revealed the expected trimeric form of the molecule. In addition, an unidentified 95-kDa protein was found to be covalently associated with immunoreactive TSP1, and this appeared to be specific to the macrophages as it was not found in TSP1 precipitated from other cell types. It is suggested that the induction of TSP1 by M-CSF may play an important role in the major physiological functions of macrophages.

Capture by chemical crosslinkers provides evidence that integrin alpha IIb beta 3 forms a complex with protein tyrosine kinases in intact platelets

Platelet activation is accompanied by a cascade of kinase reactions in which numerous specific proteins are phosphorylated on tyrosine. These events are strictly dependent upon functional activation of an integrin receptor, generally alpha IIb beta 3 (also known as glycoprotein IIb-IIIa). It is not known how alpha IIb beta 3 regulates protein tyrosine kinase activation and, in particular, neither this nor any other integrin has been shown to associate with a protein tyrosine kinase. We employed chemical crosslinking of intact platelets with the bifunctional reagents dithiobis(succinimidyl propionate) (DSP) (lipid-soluble) and dithiobis(sulphosuccinimidyl propionate) (DTSSP) (lipid-insoluble) followed by in vitro kinase assays of immunoprecipitated proteins to identify kinase activity associated with alpha IIb beta 3 in intact platelets. It was found that DSP but not DTSSP crosslinked kinase activity to alpha IIb beta 3, suggesting an internal association. In these immunoprecipitates from DSP-crosslinked platelets, the in vitro kinase reaction revealed a complex of several phosphoproteins in association with alpha IIb beta 3. This association was not seen when the resting platelets were lysed before crosslinking, indicating the specificity of the reaction in crosslinking only molecules in preformed spatial association. The beta 3 subunit of alpha IIb beta 3 was identified as one of the phosphoproteins in the complex obtained after subjecting anti-beta 3 immunoprecipitates from lysates of DSP-treated platelets to an in vitro kinase reaction and SDS/PAGE analysis. Phosphorylation of this subunit is shown to be predominantly on tyrosine. Affinity purification of the crosslinked phosphoprotein complex with anti-beta 3 followed by elution and re-precipitation identified pp60c-src and pp54/58c-lyn as two protein tyrosine kinases associating with the integrin. These results suggest that, upon platelet activation, alpha IIb beta 3 may provide a transmembrane focus for proteins involved in signal transduction.

Coxsackieviruses B1, B3, and B5 use decay accelerating factor as a receptor for cell attachment

Receptor binding and subsequent cell-mediated internalization or disassembly are the initial steps in virus replication. Cell surface molecules that participate in this process are the primary determinants of virus tissue tropism. Monoclonal antibody blockade, immunoprecipitation, and DNA transfection were used to identify decay accelerating factor as a major cell attachment receptor for coxsackieviruses B1, B3, and B5. However, expression of human decay acceleration factor on the surface of nonpermissive murine fibroblasts led only to virus attachment without subsequent replication, and it was concluded that an additional cellular cofactor(s) is required to facilitate cell entry and subsequent replication.

Effect of TGF-beta on differentiated organoids of the colon carcinoma cell line LIM 1863

The LIM 1863 colon carcinoma cell line grows in suspension as morphologically and functionally organized organoids in serum-containing medium. Addition of TGF-beta caused the organoids to adhere and inhibited DNA synthesis. A 20 min incubation with TGF-beta was sufficient to induce adherence and this could be inhibited by cycloheximide. The adhesion and DNA synthesis inhibition were demonstrated to be separate events. We were not able to detect any changes in matrix or cell membrane antigens. Similarly there were no changes in synthesized proteins (by two-dimensional gel electrophoresis), and no upregulation of proteoglycan. When adhered organoids were lysed from the tissue culture plastic surface, untreated organoids adhered to this surface. This 'conditioned' surface was destroyed by trypsin but not collagenase or medium from normal LIM 1863 cultures. However, the adherent phenotype was prevented when organoids were treated with transforming growth factor-beta (TGF-beta) in the presence of medium conditioned by normal LIM 1863 cultures rather than in fresh medium. The adhesion process was inhibited by an antibody (QE2E5) against beta 1 integrin although no quantitative changes in integrins were observed (by immunoprecipitation or RNA analysis). A second anti-beta 1 integrin antibody (61.2C4) inhibited LIM 1863 adhesion to collagen but not TGF-beta induced adhesion, implying that TGF-beta induced a specific conformational change or interaction of a beta 1 integrin. In this morphologically structured system TGF-beta induced a number of subtle effects including formation of new extracellular matrix and conformational change of a beta 1 integrin, rather than the major quantitative changes in cell/matrix molecules reported previously.

A fibroblast elongation factor purified from colon carcinoma cells shares sequence identity with TIMP-1

We have previously reported that human colon cancer cells secrete a factor(s) which induces elongation of colon fibroblasts in vitro. Isolation of this factor led to the identification of a 55kD protein with fibroblast stretching activity. Two internal amino acid sequences identified in this protein (YEI; GFQALGDAADI) share complete homology with tissue inhibitor of metalloproteinase (TIMP-1).

Apoptosis induced by inhibition of intercellular contact

The LIM 1863 colon carcinoma cell line grows as structural organoids of goblet and columnar cells around a central lumen and provides a model for the development of stem cells in the normal colon. The organoid structure can be disrupted by removal of calcium from the medium, resulting in a suspension of single cells. Upon readdition of calcium, the cells reform the organoid structure over a period of 24 h, and ultrastructural examination of the reforming cells reveals that this involves a complex process that we have termed clutching. To determine the adhesion molecules involved in organoid formation we attempted to block this process by single cell suspensions of LIM 1863 reseeded in the presence of monoclonal antibodies. An anti-integrin antibody directed against a conformational epitope on the alpha v subunit totally inhibited organoid reformation. As a consequence of this inhibition of cell contact the colon carcinoma cells rapidly underwent apoptosis. Investigations of the apoptotic pathway involved suggested an induction mechanism since the onset of apoptosis in the contact-inhibited cells showed specific increased synthesis of 68- and 72-kD proteins. In addition, immunoblotting of cytosolic and nuclear extracts of the cells revealed the rapid translocation of the tumor suppressor gene product, p53 to the cell nucleus upon induction of apoptosis. These results suggest that cell-cell adhesion may be a vital regulator of colon development overcome in tumor cells by loss of adhesion molecules or of functional p53 protein.

Phorbol ester-induced transcription of an immediate-early response gene by human T cells is inhibited by co-treatment with calcium ionophore

Human T cells require two discrete signals to initiate their proliferation. In Jurkat T cells the first signal can be provided by the phorbol ester TPA and the second by the calcium ionophore A23187. We have isolated a cDNA from Jurkat T cells representing mRNA induced by TPA but inhibited by simultaneous treatment of the cells with antibody, lectin, or A23187. Sequencing revealed identity of the Jurkat clone to a cDNA, termed ETR101, recently isolated from HL60 promyelocytic leukaemia cells and shown to be an immediate early gene expressed upon TPA stimulation of these cells [Shimizu et al.: J Biol Chem 266:12157, 1991]. The gene is also induced very rapidly upon TPA treatment of Jurkat cells and is superinduced by co-treatment with cycloheximide. The predicted amino acid sequence encoded by ETR101 has weak homology to JunB and JunD, therefore it is of some interest that these three genes share the chromosomal localization, 19p13.2. The divergent effects of TPA treatment upon cell proliferation and differentiation in different circumstances allow some speculation about a possible role for the ETR101 gene product upon cellular differentiation.

Scanning tunnelling microscopy of collagen I reveals filament bundles to be arranged in a left-handed helix

Collagen I is the most abundant protein found in the body. Its quaternary structure has been extensively characterised and consists of filaments arranged into bundles. In this study we used scanning tunnelling microscopy to image the collagen I filaments and fibrils directly. The images suggested that filaments are arranged into a left-handed helical structure with a periodicity of about 10nm. Such a structure potentially adds increased tensile strength to the collagen fibrils.

A re-examination of the molecular basis of cell movement

A model for cell movement is presented. It is suggested that cells do not migrate on collagen using their VLA (very late antigen) integrins that bind this extracellular matrix protein. Rather, the cells utilize alpha v integrins to bind endogenously produced fibronectin, which binds to the underlying collagen. It is envisaged that cells proceed by a process of engagement and disengagement of alpha v integrins to the extracellular matrix, somewhat analogous to the motion of a monkey climbing a tree. Secretion of isoforms of the adhesion modulator, thrombospondin, regulates disengagement of the integrin from its ligand in migrating cells. The integrin disengagement signal is mediated by thrombospondin cross-linking the alpha v integrin to an integrin accessory molecule and thus activating protein kinases. The cross-linked receptor complex undergoes recycling back along actin stress fibres, guided by the integrin beta-subunit. After endocytosis and protein sorting the alpha v integrin is transported back to the leading edge off migrating cells in vesicles guided by the tubulin-binding capabilities of an integrin accessory molecule. Direct attachment to collagen required for processes, such as matrix contraction, is mediated by VLA integrins which displace alpha v integrins from points of attachment during integrin recycling, possibly through an alpha v beta 1 intermediary receptor.

Granulocyte-macrophage and macrophage colony-stimulating factors differentially regulate alpha v integrin expression on cultured human macrophages

The colony-stimulating factors (CSFs) greatly influence mature macrophage function in vitro: macrophage (M)-CSF induces maturation of monocytes and enhances differentiated cell function; granulocyte-macrophage (GM)-CSF stimulates a variety of antimicrobial functions. In vivo M-CSF is thought to promote differentiation, and GM-CSF is thought to potentiate the inflammatory response. One mechanism by which these differential effects may be achieved is through the receptor-mediated interaction of macrophages with their extracellular matrix. Here we show that M-CSF induces specifically the expression of the alpha v beta 5 integrin receptor, whereas GM-CSF rapidly induces mRNA and surface expression of the alpha v beta 3 integrin. The M-CSF-treated cells acquire a flattened epitheloid phenotype, and on vitronectin the alpha v beta 5 is located in adhesion plaques. These cells do not bind collagen or laminin. In contrast, cells treated with GM-CSF adopt an elongated phenotype on a number of substrates, including collagen and laminin, and express alpha v beta 3 at the leading edge of cells on vitronectin. These results suggest that a primary means by which the CSFs exert their individual effects on mature cells may be through regulating integrin expression.

Adhesion to thrombospondin by human embryonic fibroblasts is mediated by multiple receptors and includes a role for glycoprotein 88 (CD36)

Fetal embryonic fibroblasts attach and spread on thrombospondin (TSP). Adhesion is tight and focal adhesion plaques and "spots" are formed. We have investigated the receptors responsible for this adhesion. Unstimulated cells express the vitronectin receptor on their surface and this beta 3 integrin molecule contributes to adhesion. Another putative receptor for TSP, termed glycoprotein (GP) 88, which exists as a cytoplasmic pool in unstimulated cells becomes surface expressed when these cells are plated on TSP and localizes to areas of cell adhesion. Western blot analysis of cell lysate confirms GP88 as a TSP binding protein. Studies with fucoidan indicate that the heparan sulfate proteoglycan, known to function as a receptor for TSP, appears to contribute substantially to the TSP attachment of these cells and may be the receptor most important in the initial phases of TSP interaction.

Arg-Gly-Asp-containing peptides expose novel collagen receptors on fibroblasts: implications for wound healing

Integrins are a family of cell-surface receptors intimately involved in the interactions of cells with their extracellular matrix. These receptors comprise an alpha and beta subunit in noncovalent association and many have been shown to recognize and bind an arginine-glycine-aspartate (RGD) sequence contained within their specific extracellular matrix ligand. Fibroblasts express integrin receptors belonging to two major subfamilies. Some of the members within the subfamily defined by beta 1 (VLA) are receptors for collagen but, perhaps surprisingly, the other major subfamily of integrins on fibroblasts--that defined by the alpha chain of the vitronectin receptor, alpha v--all appear to bind primarily vitronectin and/or fibronectin. In the present study we show that RGD-containing peptides expose cryptic binding sites on the alpha v-associated integrins enabling them to function as collagen receptors. The addition of RGD-containing peptides to fibroblasts cultured on type I collagen induced dramatic cell elongation and, when the cells were contained within collagen matrices, the peptides induced marked contraction of the gels. These processes were inhibited by Fab fragments of a monoclonal antibody against an alpha v integrin. Also, alpha v-associated integrins from cell lysates bound to collagen I affinity columns in the presence, but not in the absence, of RGD-containing peptides. These data suggest a novel regulatory control for integrin function. In addition, because the cryptic collagen receptors were shown to be implicated in the contraction of collagen gels, the generation of such binding forces suggests that this may be the major biological role for these integrins in processes such as wound healing.

A monoclonal antibody to a 67 kD cell membrane glycoprotein directly induces persistent platelet aggregation independently of granule secretion

The transition from reversible to persistent platelet aggregation has been difficult to study because of interference both from preceding primary aggregation and from the events associated with granule secretion during secondary aggregation. As a result it remains unclear whether the persistence of aggregation involves some secretion-independent specific platelet surface reactions. Here we show that a monoclonal antibody (MAb), LeoAl, against a newly described 67 kD platelet membrane glycoprotein induced active platelet aggregation consisting of two distinct phases. The first secretion-independent phase was in several respects (extracellular protein, divalent cation, and pH dependence) different from primary aggregation, but closely resembled the transition from primary to secondary aggregation observed at certain concentrations of physiological agonists. The second, faster phase was indistinguishable from secretion-dependent aggregation to various stimulants. It was shown that p67, GPIIb-IIIa and FC gamma RII are all involved in the observed aggregation, probably through their close topographical association. It is suggested that LeoAl-induced aggregation can be used as a model to study the receptors, ligands and metabolic pathways specifically involved in the transition from reversible to persistent platelet aggregation.

Individual embryonic fibroblasts express multiple beta chains in association with the alpha v integrin subunit. Loss of beta 3 expression with cell confluence

The alpha chain of the vitronectin receptor, alpha v, has been found in association with the integrin subunits beta 1, beta 3, or beta 5 on different cell types. We show here that cultured embryonic fibroblasts simultaneously display alpha v beta 3, alpha v beta 1, and alpha v in association with two other beta subunits, one of which is probably beta 5. Polymerase chain reaction analysis of single cells isolated by micromanipulation identified mRNA for alpha v, beta 1, beta 3, and beta 5 in six of eight clones. Immunoprecipitation of iodinated cell surface proteins with a monoclonal antibody to alpha v indicated that the relative proportions of the different beta chains in association with alpha v varied, particularly between two different cell lines. The cytokines platelet-derived growth factor, transforming growth factor beta 1, and tumor necrosis factor alpha did not appear to alter this ratio although tumor necrosis factor alpha increased the surface expression of the alpha v-associated integrins; but overnight culture in basic fibroblast growth factor caused a lower expression of alpha v beta 1 and alpha v beta 5 with no reduction in alpha v beta 3 expression. When the cell cultures were grown to complete confluence, surface expression of beta 3 was abolished, and the expression of an unknown beta chain (beta u) became more prominent. This effect was not overcome by culturing confluent cells with basic fibroblast growth factor. Affinity column chromatography showed that alpha v beta 5 bound to vitronectin but alpha v beta 1 did not, whereas alpha v beta 1 but not alpha v beta 5 bound to fibronectin. These results suggest that, on individual cells, the beta subunits found in association with alpha v may vary according to the proliferative capacity of the cell and that the promiscuous beta 3 subunit is progressively replaced by beta subunits of individual ligand specificity.

GMP-140 binding to neutrophils is inhibited by sulfated glycans

GMP-140 is a 140-kDa granule membrane glycoprotein localized to the alpha-granules of platelets and the Weibel-Palade bodies of endothelial cells. Expression of GMP-140 on the activated cell surface has been shown to mediate the adhesion of thrombin-activated platelets to neutrophils and monocytes and the transient adhesion of neutrophils to endothelium. In contrast, fluid-phase GMP-140 strongly inhibits the CD18-dependent adhesion of tumor necrosis factor alpha-activated neutrophils to endothelium suggesting that GMP-140 can also serve an anti-adhesive function. In the present report, it is demonstrated that fluid-phase GMP-140 which exists predominantly as a tetramer binds to a single class of high affinity receptor on neutrophils and HL60 cells. Binding of 125I-labeled GMP-140 to neutrophils and HL60 cells and the rosetting of neutrophils and HL60 cells by thrombin-activated platelets were inhibited by EDTA, excess unlabeled fluid-phase GMP-140, Fab fragments of an affinity-purified rabbit anti-GMP-140 antibody, and by the murine anti-GMP-140 monoclonal antibody, AK 4. Both neutrophil and HL60 GMP-140 binding and platelet rosetting were strongly inhibited by heparin, fucoidin, and dextran sulfate 500,000, were partially inhibited by dextran sulfate 5,000 and lambda- and kappa-carrageenan, but were not inhibited by chondroitins 4- and 6-sulfate. Since this sulfated glycan specificity is identical to that previously reported by us for GMP-140, the present results suggest that the sulfated glycan binding site and the neutrophil receptor binding site on GMP-140 are either identical or proximal.