Portal vein thrombosis in 33 dogs: 1998-2011

BACKGROUND

Portal vein thrombosis (PVT) has been reported infrequently in dogs.

OBJECTIVES

To characterize the presentation, associated disease conditions, and outcome in dogs with PVT.

ANIMALS

Client-owned dogs with a diagnosis of PVT and a complete medical record.

METHODS

Records were retrospectively analyzed for presentation, history, physical examination, clinicopathologic data, diagnostic imaging, treatment, and outcome.

RESULTS

Thirty-three dogs were included. Common clinical signs were vomiting, diarrhea, abdominal pain, ascites, and signs of hypovolemic shock. Associated disease conditions included hepatic (14/33), neoplastic (7/33), immune (5/33), and infectious (4/33) diseases, protein-losing nephropathy (3/33), hyperadrenocorticism (2/33), protein-losing enteropathy (1/33), and pancreatitis (1/33). Fourteen dogs were receiving glucocorticoids at the time of diagnosis. Twenty-nine dogs had at least 1 predisposing condition for venous thrombosis, and 11 had 2 or more. Thrombocytopenia (24/33), increased liver enzyme activity (23/33), and hypoalbuminemia (20/33) were common laboratory abnormalities. Clinical syndromes at the time of PVT diagnosis included shock (16/33), systemic inflammatory response syndrome (SIRS), (13/33) and disseminated intravascular coagulation (3/33). Twenty-four dogs had acute and 9 had chronic PVT. Multiple thrombi were found in 17/33 dogs. Nineteen dogs survived to discharge. Dogs treated with anticoagulant therapy were more likely, whereas those with acute PVT, multiple thromboses or SIRS were less likely to survive.

CONCLUSIONS AND CLINICAL IMPORTANCE

Hepatic disease is a common pre-existing condition in dogs with PVT. PVT should be considered in dogs with risk factors for venous thrombosis presenting with abdominal pain, ascites, and thrombocytopenia. Studies evaluating anticoagulant therapy in the management of PVT are warranted.

Activation-enhanced alpha(IIb)beta(3)-integrin-cytoskeleton interactions outside of focal contacts require the alpha-subunit

Integrins link the cell's cytoskeleton to the extracellular matrix, as well as to receptors on other cells. These links occur not only at focal contacts but also at smaller integrin-containing protein complexes outside of focal contacts. We previously demonstrated the importance of focal contact-independent integrin-cytoskeleton interactions of beta(2) integrins: activation of adhesion resulted from a release of integrins from cytoskeletal constraints. To determine whether changes in integrin-cytoskeleton interactions were related to activation of the integrin, we used single particle tracking to examine focal contact-independent cytoskeletal associations of alpha(IIb)beta(3)-integrin, in which activation results in a large conformational change. Direct activation of alpha(IIb)beta(3) by mutation did not mimic activation of lymphocytes with phorbol ester, because it enhanced integrin-cytoskeleton interactions, whereas activation of lymphocytes decreased them. Using additional integrin mutants, we found that both alpha- and beta-cytoplasmic domains were required for these links. This suggests that 1) both beta(2)- and beta(3)-integrins interact with the cytoskeleton outside of focal contacts; 2) activation of a cell and activation of an integrin are distinct processes, and both can affect integrin-cytoskeleton interactions; and 3) the role of the alpha-subunit in integrin-cytoskeleton interactions in at least some circumstances is more direct than generally supposed.

The WD protein Rack1 mediates protein kinase C and integrin-dependent cell migration

The scaffolding protein, Rack1, is a seven-WD-domain-containing protein that has been implicated in binding to integrin (β) subunit cytoplasmic domains and to members of two kinase families (src and protein kinase C, PKC) that mediate integrin bidirectional signaling. To explore the role of Rack1 in integrin function we have transfected this protein in Chinese hamster ovary (CHO) cells. We have observed no effect of Rack1 overexpression on inside-out signaling as the ligand binding properties of CHO cells also expressing constitutively active or inactive integrins were not affected. In contrast, we observed that cells stably or transiently overexpressing Rack1 had decreased migration compared to mock transfected cells. Stable Rack1 transfectants also demonstrated an increased number of actin stress fibers and focal contacts. These effects on motility and cytoskeletal organization did not appear to result from Rack1 inhibition of src function as downstream substrates of this kinase were phosphorylated normally. In addition, expression of an active src construct did not reverse the migratory deficit induced by Rack1 overexpression. On the other hand when we overexpressed a Rack1 variant with alanine substitutions in the putative PKC binding site in its third WD domain, we observed no deficit in migration. Thus the ability of Rack1 to bind, localize and stabilize PKC isoforms is likely to be involved in aspects of integrin outside-in signaling.

Integrin activation controls metastasis in human breast cancer

Metastasis is the primary cause of death in human breast cancer. Metastasis to bone, lungs, liver, and brain involves dissemination of breast cancer cells via the bloodstream and requires adhesion within the vasculature. Blood cell adhesion within the vasculature depends on integrins, a family of transmembrane adhesion receptors, and is regulated by integrin activation. Here we show that integrin alpha v beta 3 supports breast cancer cell attachment under blood flow conditions in an activation-dependent manner. Integrin alpha v beta 3 was found in two distinct functional states in human breast cancer cells. The activated, but not the nonactivated, state supported tumor cell arrest during blood flow through interaction with platelets. Importantly, activated alpha v beta 3 was expressed by freshly isolated metastatic human breast cancer cells and variants of the MDA-MB 435 human breast cancer cell line, derived from mammary fat pad tumors or distant metastases in severe combined immunodeficient mice. Expression of constitutively activated mutant alpha v beta 3(D723R), but not alpha v beta 3(WT), in MDA-MB 435 cells strongly promoted metastasis in the mouse model. Thus breast cancer cells can exhibit a platelet-interactive and metastatic phenotype that is controlled by the activation of integrin alpha v beta 3. Consequently, alterations within tumors that lead to the aberrant control of integrin activation are expected to adversely affect the course of human breast cancer.

The association of CRKII with C3G can be regulated by integrins and defines a novel means to regulate the mitogen-activated protein kinases

In studies to define mechanisms of ERK activation in Chinese hamster ovary cells, we have observed an inverse correlation between CRKII-C3G complex formation and ERK activity. That is, we were able to coprecipitate the guanine nucleotide exchange factor C3G with the adaptor protein CRKII in lysates from suspended cells that had low ERK activity, but we could not do so or could do so less efficiently in lysates of adherent cells with increased ERK activity. Consistent with the presence of a functional CRKII-C3G complex, we detected more GTP-loaded RAP1 in suspension than adherent lysates. Overexpression of cDNAs encoding B-RAF, CRKII W109L, and PTP1B C215S activated ERK in suspension cells, the latter two constructs also disrupting CRKII-C3G complex formation. Finally, we have also observed that certain integrin alpha subunit cytoplasmic splice variants differentially regulate ERK1/2 but also in a manner that correlated with levels of a CRKII-C3G complex. Thus, these data suggest the involvement of integrins in an ERK suppression pathway mediated by CRKII-C3G complex formation and downstream signaling from activated RAP1.

A single immunoglobulin-like domain of the human neural cell adhesion molecule L1 supports adhesion by multiple vascular and platelet integrins

The neural cell adhesion molecule L1 has been shown to function as a homophilic ligand in a variety of dynamic neurological processes. Here we demonstrate that the sixth immunoglobulin-like domain of human L1 (L1-Ig6) can function as a heterophilic ligand for multiple members of the integrin superfamily including alphavbeta3, alphavbeta1, alpha5beta1, and alphaIIbbeta3. The interaction between L1-Ig6 and alphaIIbbeta3 was found to support the rapid attachment of activated human platelets, whereas a corresponding interaction with alphavbeta3 and alphavbeta1 supported the adhesion of umbilical vein endothelial cells. Mutation of the single Arg-Gly-Asp (RGD) motif in human L1-Ig6 effectively abrogated binding by the aforementioned integrins. A L1 peptide containing this RGD motif and corresponding flanking amino acids (PSITWRGDGRDLQEL) effectively blocked L1 integrin interactions and, as an immobilized ligand, supported adhesion via alphavbeta3, alphavbeta1, alpha5beta1, and alphaIIbbeta3. Whereas beta3 integrin binding to L1-Ig6 was evident in the presence of either Ca2+, Mg2+, or Mn2+, a corresponding interaction with the beta1 integrins was only observed in the presence of Mn2+. Furthermore, such Mn2+-dependent binding by alpha5beta1 and alphavbeta1 was significantly inhibited by exogenous Ca2+. Our findings suggest that physiological levels of calcium will impose a hierarchy of integrin binding to L1 such that alphavbeta3 or active alphaIIbbeta3 > alphavbeta1 > alpha5beta1. Given that L1 can interact with multiple vascular or platelet integrins it is significant that we also present evidence for de novo L1 expression on blood vessels associated with certain neoplastic or inflammatory diseases. Together these findings suggest an expanded and novel role for L1 in vascular and thrombogenic processes.

Integrin signaling: building connections beyond the focal contact?

The integrin family of adhesion receptors is notable for its bi-directional signaling properties, changing extracellular conformation and ligand binding affinity in response to external agonists, and inducing complex intracellular events following ligation. The interaction of integrins with intracellular or transmembrane moieties has been extensively studied in order to define the mechanisms of bi-directional signaling. In particular, the recruitment of integrins to focal contacts puts them in proximity with many cytoskeletal and signaling molecules. The importance of these structures in connecting integrins and signaling pathways has been well described. However, the purpose of this minireview is to explore additional protein interactions and how these might serve as an alternative means in connecting integrins and signaling pathways.

Integrin activation and cytoskeletal interaction are essential for the assembly of a fibronectin matrix

Fibronectin (Fn) matrices are vital to vertebrate development and wound healing and modulate tumorigenesis. We used a recombinant Fn-binding integrin alpha IIb beta3, to define rules for integrin-initiated Fn matrix formation. We report the following. First, multiple Fn-binding integrins can support matrix assembly; their activation state controls fibrillogenesis. Second, Fn binding to cells expressing an activated integrin is necessary but not sufficient for matrix assembly. Additional "postoccupancy" events involving the integrin beta, but not the alpha subunit, cytoplasmic domain are needed. Third, these postoccupancy events require an intact actin cytoskeleton. We propose a model for integrin involvement in Fn fibrillogenesis that reconciles previous paradoxes and suggests novel approaches to the therapeutic control of Fn matrix assembly.

Beta 3 integrin-mediated fibrin clot retraction by nucleated cells: differing behavior of alpha IIb beta 3 and alpha v beta 3

Fibrin clot retraction may be important in resolution of thrombi and, in platelets, is mediated by integrin alpha IIb beta 3 (GPIIb-IIIa). Nucleated cells that lack alpha IIb beta 3 can retract fibrin clots, and we now report that integrin alpha v beta 3 can support this process. In addition, we compared the capacities of recombinant beta 3 integrins to mediate clot retraction in Chinese hamster ovary and M21 melanoma cells. We found that alpha v beta 3, but not alpha IIb beta 3, could spontaneously support retraction. Transferring the cytoplasmic domain of alpha v to alpha IIb enabled the resulting chimeric alpha IIb beta 3 to support clot retraction. The capacity of the alpha v cytoplasmic domain to support clot retraction was not caused by activation of the ligand binding function of alpha IIb beta 3 or by enhancement of alpha IIb beta 3's capacity to stimulate the formation of focal adhesions or the tyrosine phosphorylation of pp125FAK. These experiments define requirements for alpha IIb beta 3-mediating clot retraction, establish the capacity of alpha v beta 3 to mediate this process, and suggest differing functional roles of the alpha v and alpha IIb cytoplasmic domains.

The conserved membrane-proximal region of an integrin cytoplasmic domain specifies ligand binding affinity

Integrin affinities for ligands can change markedly via a process termed inside-out signaling. We expressed several truncations of the beta 3 cytoplasmic domain in conjunction with an "activating" alpha subunit chimera, alpha IIb alpha 6B. Deletion of the 4 C-terminal residues of the beta 2 tail blocked inside-out signaling as assessed by the binding of an activation-specific antibody, PAC1. Several additional truncations remained in the low affinity state, but complete truncation (beta 3 delta 717) caused PAC1 binding. Activation by this truncation mutant did not depend on the alpha subunit cytoplasmic domain and was resistant to inhibitors of cellular metabolism and the over-expression of an isolated beta 3 cytoplasmic domain. Since deletion of beta 3(Leu717-Asp723) results in a constitutively activated integrin, this membrane-proximal seven amino acids of the beta 3 cytoplasmic domain is required to maintain alpha IIb beta 3 in a default low affinity state. The amino acid sequence of this region is conserved among integrins. Moreover, the conserved membrane-proximal sequence in alpha subunit tails seems to serve a similar function. Consequently, the conserved membrane-proximal regions of both integrin cytoplasmic domains control the ligand binding affinity of the extracellular domain.

Regulation of integrin function and cellular adhesion

The integrins are a family of adhesion receptors involved in many physiological functions. These molecules are characterized by an ability to dynamically regulate their ligand binding affinity. Several integrins become "activated" or achieve the high affinity state in response to extracellular agonists or signals. High affinity ligand binding does not result from an increase in receptor number or from changes in the receptor microenvironment. Rather, evidence suggests these altered affinity states result from the varied conformations of these molecules. Understanding how these conformational changes are achieved remains an area of great interest in the field. In this review, we will discuss several means and potential mechanisms of integrin activation. First, we will focus on "activators" such as antibodies, peptides, and cations. For the most part, these agents can be viewed as nonphysiological activators that directly effect integrin conformational changes. Later we will discuss how conformational changes are achieved in a physiological sense. Many physiological activators stimulate signal transduction pathways inside the cell and are believed to transmit these signals outward to effect conformational changes. An understanding of integrin activation mechanisms is important as it might suggest ways to regulate cell adhesion in pathology and disease.

Mutation of the cytoplasmic domain of the integrin beta 3 subunit. Differential effects on cell spreading, recruitment to adhesion plaques, endocytosis, and phagocytosis

The cytoplasmic domain of the beta subunit of the alpha IIb beta 3 integrin is required for cell spreading on fibrinogen. Here we report that deletion of six amino acids from the COOH terminus of the beta 3 (I757TYRGT) totally abolished cell spreading and formation of adhesion plaques, whereas retaining Ile757 partially preserved these functions. We further found that substitution of Tyr747 with Ala also abolished alpha IIb beta a-mediated cell spreading. The effects of these and other mutations on additional functions of alpha IIb beta 3 were also studied. Progressive truncations of beta 3, in which stop codons were inserted at amino acid positions 759-756, caused partial defects in the recruitment of alpha IIb beta 3 to preestablished adhesion plaques and a gradual decrease in the ability of alpha IIb beta 3 to mediate internalization of fibrinogen-coated particles. The Tyr747-->Ala substitution mutant was almost totally inactive in both of these assays. Point mutations at Tyr759, and at a conserved area close to the transmembrane domain of beta 3, decreased integrin recruitment to preestablished adhesion plaques but allowed alpha IIb beta 3-mediated formation of these structures and partial cell spreading. Deletion of the cytoplasmic domain of beta 3 did not affect the constitutive endocytosis of alpha IIb beta 3.

Regulation of integrin affinity states through an NPXY motif in the beta subunit cytoplasmic domain

The ligand binding affinities of the integrins are regulated through their cytoplasmic domains. To identify specific residues that are involved in this process, we have generated mutants in the beta 1 and beta 3 tails and coexpressed them in Chinese hamster ovary cells with constitutively active alpha subunits. These alpha subunits are chimera of extra-cellular and transmembrane alpha IIb joined to the cytoplasmic domains of alpha 5, alpha 6A, or alpha 6B and confer an energy-dependent high affinity state when expressed in Chinese hamster ovary cells. The affinity state of these transfectants was determined by analyzing the binding of PAC1, an antibody that specifically recognizes the activated form of the reporter group, extracellular alpha IIb beta 3. We have identified point mutants in several areas of the beta tails, which result in a reduced ability to bind ligand. Complete abolition of PAC1 binding was obtained with mutants in an NPXY motif found in many integrin beta subunits and implicated in the internalization of other cell surface receptors. Similar effects on PAC1 binding were observed whether coexpression was with alpha chimera containing alpha 5, alpha 6A, or alpha 6B cytoplasmic sequences. These studies identify a novel role for the NPXY motif in the regulation of integrin binding affinity.

Modulation of cell adhesion by changes in alpha L beta 2 (LFA-1, CD11a/CD18) cytoplasmic domain/cytoskeleton interaction

The integrin alpha L beta 2 (leukocyte function-associated molecule 1, CD11a/CD18) mediates activation-dependent adhesion of leukocytes. The cytoplasmic domains of alpha L beta 2 have been demonstrated to modulate adhesiveness of alpha L beta 2. Affinity changes of alpha L beta 2 for its ligand or postreceptor events can be responsible for this modulation of adhesiveness. To investigate the possible role of the alpha L beta 2 cytoplasmic domains in postreceptor events we constructed cDNA encoding chimeric proteins with intracellular alpha L beta 2 domains, which are responsible for alpha L beta 2 specific intracellular interactions, and extracellular alpha IIIb beta 3 (GP IIb/IIIa) domains, which allow the assessment of the receptor affinity state. The cDNA was stably transfected in Chinese hamster ovary cells and chimeric heterodimer formation proven by immunoprecipitations and flow cytometry. The chimeric receptors mediate adhesion to immobilized fibrinogen, and this adhesion is increased by phorbol myristate acetate and abolished by cytochalasin D. However, neither treatment affects the affinity state of the chimeric receptor, suggesting involvement of the cytoskeleton in the regulation of alpha L beta 2 mediated cell adhesion. To exclude the possibility of postoccupancy affinity changes of the chimeric receptors, we locked the receptors into a high affinity state by creating a deletion variant. The region deleted (VGFFK) is highly conserved in integrin alpha subunit cytoplasmic domains. Cotransfection of this deletion variant with a beta subunit truncation (beta 3 delta 724) and a triple mutation at 758-760 (TTT to AAA) of beta 2 abolishes adhesion without changing the affinity state. A single mutation (TTT to TAT) reduces adhesion by half without affinity change. Scanning electron microscopy reveals impaired spreading of these truncated/mutated chimeras. Immunofluorescence microscopy demonstrates a correlation between impaired adhesion and a decrease in the ability to form focal adhesions and to organize the cytoskeleton into stress fibers. These results describe the integrin/cytoskeleton interaction, the organization of the cytoskeleton, and cell spreading as postreceptor events modulating alpha L beta 2 cytoplasmic domain mediated cell adhesion. Furthermore, we demonstrate that the cytoplasmic domain of the beta 2 subunit, and within it the TTT region, are required for these postreceptor events. Additionally, we present a new approach, using deletion variants to lock integrins in a high affinity state without interfering with the investigated integrin/cytoskeleton interaction. This approach may be generally useful to investigate the role of postreceptor events in integrin-mediated cell adhesion and migration.

A point mutation in the integrin beta 3 cytoplasmic domain (S752-->P) impairs bidirectional signaling through alpha IIb beta 3 (platelet glycoprotein IIb-IIIa)

Agonist-induced inside-out signaling results in an increased affinity of integrin alpha IIb beta 3 (platelet glycoprotein IIb-IIIa) for soluble ligands (fibrinogen [Fg] and PAC1). Ligand binding to integrins initiates outside-in signaling that leads to cellular responses such as cell spreading and focal adhesion formation. A point mutation in the beta 3 cytoplasmic domain (S752-->P) is associated with blocked inside-out alpha IIb beta 3 signaling in a variant Glanzmann's thrombasthenia. This mutation was introduced into beta 3 and cotransfected into Chinese hamster ovary cells with a chimeric alpha subunit consisting of the alpha IIb extracellular and transmembrane domains and the alpha 6B cytoplasmic domain. The substitution of the alpha IIb cytoplasmic domain with that of alpha 6 led to activation of alpha IIb beta 3 to bind PAC1, mimicking inside-out signaling. This effect was reversed by the S752-->P mutation, indicating a disruption of inside-out signaling by the mutation. In addition, transfectants expressing this beta 3 variant showed reduced alpha IIb beta 3-mediated cell spreading on immobilized Fg, focal adhesion, and fibrin clot retraction, suggesting an impairment in outside-in alpha IIb beta 3 signaling. Therefore, a single point mutation in the beta 3 cytoplasmic domain impaired bidirectional signaling through alpha IIb beta 3.

"Inside-out" signal transduction inhibited by isolated integrin cytoplasmic domains

The affinities of integrin alpha beta heterodimers for extracellular ligands are important regulators of cell adhesion. Intracellular signals provoke changes in the integrin extracellular domain resulting in "activation," as manifested by an increase in affinity. Interactions of integrin cytoplasmic domains with intracellular elements may mediate this "inside-out signaling." Here we report that overexpression of chimeras of the cytoplasmic domain of integrin beta 3 or beta 1 subunits, joined to the extracellular and transmembrane domains of the Tac subunit of the interleukin-2 receptor, reduced integrin affinity. In contrast, chimeras containing the cytoplasmic domain of alpha 5 or alpha IIb or of beta 3 bearing a mutation that disrupts inside-out signaling lacked inhibitory activity. These data suggest that limiting quantities of intracellular factors bind to integrin beta 3 and beta 1 cytoplasmic domains to modulate ligand binding affinity. Structural mimics of these domains may provide a novel means to alter cell adhesion.

The inner world of cell adhesion: integrin cytoplasmic domains

Many of the interactions between cells and their environment are mediated by the integrin family of heterodimeric transmembrane receptors. The past decade has been a broad-based effort to decipher the rules by which integrins function. Integrins bind both intracellular and extracellular ligands and thus transfer signals across the membrane in both directions. The cytoplasmic domains of these receptors play a key role in this bidirectional flow of information and in the formation of direct physical linkages between protein structures on the inside and outside of the cell.

Integrin cytoplasmic domains mediate inside-out signal transduction

We analyzed the binding of fibronectin to integrin alpha 5 beta 1 in various cells; in some cells fibronectin bound with low affinity (e.g., K562 cells) whereas in others (e.g., CHO), it bound with high affinity (Kd approximately 100 nM) in an energy-dependent manner. We constructed chimeras of the extracellular and transmembrane domains of alpha IIb beta 3 joined to the cytoplasmic domains of alpha 5 beta 1. The affinity state of these chimeras was assessed by binding of fibrinogen or the monoclonal antibody, PAC1. The cytoplasmic domains of alpha 5 beta 1 conferred an energy-dependent high affinity state on alpha IIb beta 3 in CHO but not K562 cells. Three additional alpha cytoplasmic domains (alpha 2, alpha 6A, alpha 6B) conferred PAC1 binding in CHO cells, while three others (alpha M, alpha L, alpha v) did not. In the high affinity alpha chimeras, cotransfection with a truncated (beta 3 delta 724) or mutated (beta 3(S752-->P)) beta 3 subunit abolished high affinity binding. Thus, both cytoplasmic domains are required for energy-dependent, cell type-specific affinity modulation. In addition, mutations that disrupted a highly conserved alpha subunit GFFKR motif, resulted in high affinity binding of ligands to alpha IIb beta 3. In contrast to the chimeras, the high affinity state of these mutants was independent of cellular metabolism, cell type, and the bulk of the beta subunit cytoplasmic domain. Thus, integrin cytoplasmic domains mediate inside-out signaling. Furthermore, the highly conserved GFFKR motif of the alpha subunit cytoplasmic domain maintains the default low affinity state.

Distinct functions of integrin alpha and beta subunit cytoplasmic domains in cell spreading and formation of focal adhesions

Integrin-mediated cell adhesion often results in cell spreading and the formation of focal adhesions. We exploited the capacity of recombinant human alpha IIb beta 3 integrin to endow heterologous cells with the ability to adhere and spread on fibrinogen to study the role of integrin cytoplasmic domains in initiation of cell spreading and focal adhesions. The same constructs were also used to analyze the role of the cytoplasmic domains in maintenance of the fidelity of the integrin repertoire at focal adhesions. Truncation mutants of the cytoplasmic domain of alpha IIb did not interfere with the ability of alpha IIb beta 3 to initiate cell spreading and form focal adhesions. Nevertheless, deletion of the alpha IIb cytoplasmic domain allowed indiscriminate recruitment of alpha IIb beta 3 to focal adhesions formed by other integrins. Truncation of the beta 3 subunit cytoplasmic domain abolished cell spreading mediated by alpha IIb beta 3 and also abrogated recruitment of alpha IIb beta 3 to focal adhesions. This truncation also dramatically impaired the ability of alpha IIb beta 3 to mediate the contraction of fibrin gels. In contrast, the beta 3 subunit cytoplasmic truncation did not reduce the fibrinogen binding affinity of alpha IIb beta 3. Thus, the integrin beta 3 subunit cytoplasmic domain is necessary and sufficient for initiation of cell spreading and focal adhesion formation. Further, the beta 3 cytoplasmic domain is required for the transmission of intracellular contractile forces to fibrin gels. The alpha subunit cytoplasmic domain maintains the fidelity of recruitment of the integrins to focal adhesions and thus regulates their repertoire of integrins.

Determination of adhesion force between single cell pairs generated by activated GpIIb-IIIa receptors

A biophysical approach was used to directly determine the avidity of the junction between two Chinese hamster ovary (CHO) cells bearing recombinant GpIIb-IIIa in the presence and absence of fibrinogen. Micromanipulation was used to induce conjugation of the cell pairs with or without activating the GpIIb-IIIa molecules with monoclonal antibody (MoAb) 62. Activation of GpIIb-IIIa caused an increase in the force required to separate the conjugates. The molecular bonding force between cells bearing activated GpIIb-IIIa and fibrinogen molecules was found to be 2.1 x 10(-7) dyne, which is 3.7 times higher than that between nonactivated GpIIb-IIIa and fibrinogen (5.7 x 10(-8) dyne). The results provide a quantitative assessment of the molecular bonding force between fibrinogen and the GpIIb-IIIa expressed on cell surface. The findings indicate that the activation of GpIIb-IIIa leads to an increase in the adhesive force in CHO cell aggregation by increasing the strength of the GpIIb-IIIa-fibrinogen bonds rather than the number of these bonds.

Different specificities of platelet-associated and plasma autoantibodies to platelet GPIIb-IIIa in patients with chronic immune thrombocytopenic purpura

Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disorder due to antiplatelet autoantibodies, many of which are directed against platelet membrane glycoprotein (GP) IIb-IIIa or GPIb-IX. In a recent study, we described plasma autoantibodies from 13 selected ITP patients, which required the presence of the putative GPIIIa cytoplasmic region for antibody binding. Since this region may not be available for antibody binding under physiologic conditions, we evaluated the frequency of binding to this or other regions of GPIIb-IIIa by platelet-associated and plasma autoantibody from a group of chronic ITP patients. We studied platelet-associated autoantibodies in 27 patients and plasma antibodies in 21 patients; in 15 patients, both were studied. To determine if autoantibodies were directed to the cytoplasmic portion of GPIIIa or to another portion of the GPIIb-IIIa molecule, antibody eluted from patient platelets or plasma antibody was tested in an antigen capture assay for binding to GPIIb-IIIa obtained from Chinese hamster ovary (CHO) cells transfected with GPIIb and either intact GPIIIa or GPIIIa lacking the carboxy terminal 35 residues. Of the 21 plasma autoantibodies tested, 13 bound primarily to the carboxy terminus of GPIIIa and eight to other epitopes. Conversely, all 26 platelet-associated autoantibodies, including eight of the 13 with anti-carboxy terminus antibodies, bound to epitopes in other regions of GPIIb-IIIa. Comparison of the degree of antibody adsorption by intact or lysed platelets indicated that epitopes on the c-terminal region of GPIIIa are relatively inaccessible on the surface of intact washed platelets when compared with other epitopes. We conclude that the importance of plasma autoantibodies in chronic ITP patients should be interpreted cautiously, since their specificity may differ from that of antibodies bound to the platelet. Whether antibodies against the c-terminus of GPIIIa are of pathogenetic importance remains to be determined, although patients with these antibodies have particularly severe disease.

Modulation of the affinity of integrin alpha IIb beta 3 (GPIIb-IIIa) by the cytoplasmic domain of alpha IIb

Intracellular signaling alters integrin adhesive functions in inflammation, immune responses, hemostasis, thrombosis, and retinal development. By truncating the cytoplasmic domain of alpha IIb, the affinity of integrin alpha IIb beta 3 for ligand was increased. Reconstitution with the cytoplasmic domain from integrin alpha 5 did not reverse the increased affinity. Thus, the cytoplasmic domain of the alpha subunit of GPIIb-IIIa controls ligand binding affinity, which suggests mechanisms for inside-out transmembrane signaling through integrins. These findings imply the existence of hitherto unappreciated hereditary and acquired thrombotic disorders in humans.

Platelet glycoprotein IIb-IIIa (alpha IIb beta 3 integrin) confers fibrinogen- and activation-dependent aggregation on heterologous cells

To analyze molecular mechanisms of platelet aggregation, we have studied the aggregation of Chinese hamster ovary (CHO) cells expressing between 1 and 4 x 10(5) recombinant human glycoprotein (GP) IIb-IIIa molecules per cell (A5 cells). These cells aggregated as measured by the disappearance of single cells during rotary agitation. Aggregation was dependent on the presence of extracellular fibrinogen (approximately 500 nmol/L) and divalent cations, and required prior activation of the GPIIb-IIIa. A synthetic peptide (GRGDSP) and monoclonal anti-GPIIb-IIIa antibody (2G12) that block platelet aggregation also blocked aggregation of these cells. Parent CHO cells or those expressing recombinant GPIIb-IIIa containing a point mutation that causes variant thrombasthenia both failed to aggregate when stimulated in the presence of fibrinogen. These data show that GPIIb-IIIa is the only unique platelet surface component required for aggregation.

Autoantibodies to the presumptive cytoplasmic domain of platelet glycoprotein IIIa in patients with chronic immune thrombocytopenic purpura

Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disorder due to autoantibodies against platelets that result in their destruction. In some patients, these autoantibodies bind to platelet glycoprotein (GP) IIIa. With the aim of better defining the antigenic epitopes, plasma from 13 selected patients with chronic ITP known to have anti-GPIIb/IIIa autoantibodies was tested for reactivity with nine synthetic peptides corresponding to different regions of the GP IIIa molecule. Of these plasmas, five bound significantly (P less than .001) with either peptide 8 (amino acids 721-744) or peptide 9 (amino acids 742-762), which together form most of the carboxyterminal region presumed to be the cytoplasmic domain. Three of these positive plasmas, were tested further. In two of these positive plasmas, the anti-peptide antibodies represented greater than 80% of the detectable circulating autoantibody. To further evaluate the importance of the carboxyterminal region as an antigenic site, the chronic ITP plasmas were tested against Chinese hamster ovary cells transfected with GPIIb and either whole GPIIIa or GPIIIa lacking amino acids 728 to 762. Ten of the 13 plasmas required the presence of this region for significant autoantibody binding. We conclude that the carboxyterminal region is an important area for stimulating antiplatelet autoantibody formation in some patients with chronic ITP. It is not known whether these autoantibodies to the presumed cytoplasmic domain play an important role in the pathogenesis of the disease or occur as a secondary phenomenon during the course of platelet destruction.

Ligands "activate" integrin alpha IIb beta 3 (platelet GPIIb-IIIa)

Integrin alpha IIb beta 3 (platelet GPIIb-IIIa) binds fibrinogen via recognition sequences such as Arg-Gly-Asp (RGD). Fibrinogen binding requires agonist activation of platelets, whereas the binding of short synthetic RGD peptides does not. We now find that RGD peptide binding leads to changes in alpha IIb beta 3 that are associated with acquisition of high affinity fibrinogen-binding function (activation) and subsequent platelet aggregation. The structural specificities for peptide activation and for inhibition of ligand binding are similar, indicating that both are consequences of occupancy of the same site(s) on alpha IIb beta 3. Thus, the RGD sequence is a trigger of high affinity ligand binding to alpha IIb beta 3, and certain RGD-mimetics are partial agonists as well as competitive antagonists of integrin function.

Affinity modulation of the alpha IIb beta 3 integrin (platelet GPIIb-IIIa) is an intrinsic property of the receptor

To analyze the basis of affinity modulation of integrin function, we studied cloned stable Chinese hamster ovary cell lines expressing recombinant integrins of the beta 3 family (alpha IIb beta 3 and alpha v beta 3). Antigenic and peptide recognition specificities of the recombinant receptors resembled those of the native receptors found in platelets or endothelial cells. The alpha IIb beta 3-expressing cell line (A5) bound RGD peptides and immobilized fibrinogen (Fg) but not soluble fibrinogen or the activation-specific monoclonal anti-alpha IIb beta 3 (PAC1), indicating that it was in the affinity state found on resting platelets. Several platelet agonists failed to alter the affinity state of ("activate") recombinant alpha IIb beta 3. The binding of soluble Fg and PAC1, however, was stimulated in both platelets and A5 cells by addition of IgG papain-digestion products (Fab) fragments of certain beta 3-specific monoclonal antibodies. These antibodies stimulated PAC1 binding to platelets fixed under conditions rendering them unresponsive to other agonists. Addition of these antibodies to detergent-solubilized alpha IIb beta 3 also stimulated specific Fg binding. These data demonstrate that certain anti-beta 3 antibodies activate alpha IIb beta 3 by acting directly on the receptor, possibly by altering its conformation. Furthermore, they indicate that the activation state of alpha IIb beta 3 is a property of the receptor itself rather than of the surrounding cell membrane microenvironment.

A beta 3 integrin mutation abolishes ligand binding and alters divalent cation-dependent conformation

The ligand-binding function of integrin adhesion receptors depends on divalent cations. A mutant alpha IIb beta 3 integrin (platelet gpIIb/IIIa) that lacks ligand recognition shows immunologic evidence of a perturbed interaction with divalent cations. This was found to be caused by a G----T mutation that resulted in an Asp119----Tyr119 substitution in the beta 3 subunit. This residue is proximal to bound ligand and is in a conserved region among integrins that are enriched in oxygenated residues. The spacing of these residues aligns with the calcium-binding residues in EF hand proteins, suggesting interaction with receptor-bound divalent cation as a mechanism of ligand binding common to all integrins.

Efficient surface expression of platelet GPIIb-IIIa requires both subunits

Platelet membrane GPIIb-IIIa is a member of the integrin family of heterodimeric adhesion receptors. Processing and export of certain leukocyte and melanoma integrins is disrupted in cells lacking one subunit. We found that surface expression of GPIIb-IIIa, measured by fluorescent activated cell sorting or by surface labeling, required cotransfection of both subunits. In contrast, surface expression was not detected when the subunits were transfected individually. Immunoprecipitation of metabolically labeled transfected cells confirmed the presence of comparable levels of intracellular protein in all cases. When both subunits were transfected, post-translational cleavage of Pro-GPIIb to yield GPIIb heavy chain was also seen, while transfection with GPIIb alone resulted in coprecipitation of Pro-GPIIb with a second band that may be an endogenous beta subunit. Pro-GPIIb in these transfectants was not processed to yield GPIIb heavy chain. When transfected into COS cells alone, transiently expressed GPIIIa remained intracellular and did not appear to complex with any endogenous proteins. Thus, surface expression of processed GPIIb-IIIa depends on the presence of both subunits; the coordinate reduction of both subunits observed in some cases of Glanzmann's thrombasthenia may result from mutation affecting only one.

Human adenine phosphoribosyltransferase. Identification of allelic mutations at the nucleotide level as a cause of complete deficiency of the enzyme

This study reports the first demonstration of specific mutations leading to human adenine phosphoribosyltransferase (APRT) deficiency. The molecular basis of the deficiency was investigated by determining the sequence of both alleles of a patient with a complete deficiency in APRT activity. A trinucleotide deletion, corresponding to phenylalanine on the deduced amino acid sequence, was confirmed on one allele. A single nucleotide insertion, immediately adjacent to the splice site at the 5' end of the fourth intervening sequence, was confirmed on the other allele. This insertion lead to aberrant splicing, as was demonstrated by the absence of exon 4 in the complementary DNA sequence and by altered RNase mapping analysis of the abnormal messenger RNA.

Human adenine phosphoribosyltransferase. Complete amino acid sequence of the erythrocyte enzyme

We defined the amino acid sequence of adenine phosphoribosyltransferase isolated from human erythrocytes. Peptide fragments formed by cleavage at arginine, lysine, glutamic acid, and methionine were purified by high pressure liquid chromatography and sequenced by manual Edman degradation. The complete primary structure of human adenine phosphoribosyltransferase was established by sequence analysis of 19 peptide fragments. Presumed homology between the human and rodent enzymes was used to order fragments that had inadequate overlapping sequences. The enzyme has 179 residues with a calculated subunit molecular weight of 19,481. Mass spectrometry indicated that the NH2-terminal residue is acetylated. Human adenine phosphoribosyltransferase has sequence homology with xanthine-guanine phosphoribosyltransferase from Escherichia coli in 110-amino acid region encompassing the NH2-terminal section of the enzyme.

Human adenine phosphoribosyltransferase: characterization from subjects with a deficiency of enzyme activity

Adenine phosphoribosyltransferase (APRT) was characterized with respect to specific activity and immunoreactive protein (CRM) levels in hemolysate from 18 members of an APRT-deficient kindred. In addition, lymphoblastoid cell lines were established from six of these subjects and APRT from these cells was characterized in a similar fashion. Levels of specific activity and CRM in patients homozygous for the deficiency were less than 1% of normal. Heterozygous subjects had higher levels of activity and CRM in lymphoblasts than in erythrocytes and, in all cases, the APRT present was normal in terms of isoelectric point, subunit molecular weight, and heart stability. The higher levels of activity and CRM found in lymphoblasts may be due either to expression of a mutant gene product stabilized in a normal:mutant dimer or to autologous regulation.

Cytokinetic evaluation of the four-drug combination of bleomycin, vincristine, mitomycin C, and methotrexate (BOMM) in cultured Burkitt's lymphoma cells and human bone marrow

The four-drug combination of bleomycin, vincristine, mitomycin C and methotrexate produces a high response rate in patients with squamous cell carcinoma. In this study we have examined the cytokinetic effects of this drug combination in vitro and in human bone marrow in vivo. The in vitro analysis revealed that mitomycin C produces a concentration dependent slowing of S-phase transit with partial G2/M and G1/S blocks in cell cycle progression. A partially synchronized S-wave occurs 4-8 and 16-20 hours following a two-hour drug exposure. Bleomycin produces a dose dependent G2/M block during a 14-hour drug exposure. Drug removal did not result in appreciable cell cycle synchrony. In vitro exposure to the two drug combination resulted in loss of both the marked G2/M accumulation seen with bleomycin, and the partially synchronized S-phase waves seen following mitomycin C exposure. The sequential changes in bone marrow cytokinetics were determined in eight patients during and following administration of vincristine, mitomycin C and a continuous four-day infusion of bleomycin. The major cytokinetic changes observed were an increase in G2/M phase cells 18 hours following vincristine, and an increase in thymidine incorporation and S-phase cells 24 and 48 hours following the end of the bleomycin infusion. The clinical course of 24 patients was reviewed. Eleven patients who received methotrexate 36 to 42 hours following bleomycin had a subsequent median leukocyte nadir of 1.8 x 10(3)/mm3. Thirteen patients receiving methotrexate 60 to 72 hours after bleomycin had median leukocyte nadir of 3.9 x 10(3)/mm3. The objective response rates in the two groups was 75% and 80%, respectively. This study demonstrates that bone marrow cytokinetic analysis may allow schedule modification to avoid myelosuppression without loss of therapeutic activity.

Glucocorticoid receptor activation and inactivation in cultured human lymphocytes

Although glucocorticoids are not cytolytic for and do not inhibit the growth of the IM-9 line of cultured human lymphoblasts, these cells have a high steroid-binding capacity. We have used IM-9 cells in order to examine whether unoccupied glucocorticoid receptors are inactivated and activated in intact cells. when IM-9 cells are incubated in glucose-free medium in a nitrogen atmosphere, both their ability to bind triamcinolone acetonide and their ATP levels decline and, when glucose and oxygen are reintroduced, ATP levels and receptor activity return. The specific glucocorticoid-binding activity of cytosol prepared from cells exposed to various degrees of energy limitation is directly correlated with the ATP content. Receptor activation in intact cells is rapid and independent of protein synthesis. Cytosol prepared from inactivated cells cannot be activated by addition of ATP. The inactivation of glucocorticoid receptors that occurs when cytosol from normal IM-9 cells is incubated at 25 degrees C is inhibited by molybdate, vanadate, fluoride, ATP, and several other nucleotides. The experiments with intact human lymphoblasts suggest that assays of specific glucocorticoid-binding capacity do not necessarily reflect the cellular content of receptor protein.