Biomedical subjects

Jack Lawler

Publications and source records attributed to Jack Lawler.

At least 19 recordsLinked to original sources

PubMed2007

Thrombospondin-1 inhibits VEGF levels in the ovary directly by binding and internalization via the low density lipoprotein receptor-related protein-1 (LRP-1).

VEGF is a potent pro-angiogenic factor whose effects are opposed by a host of anti-angiogenic proteins, including thrombospondin-1 (TSP-1). We have previously shown that VEGF has important extravascular roles in the ovary and that VEGF and TSP-1 are inversely expressed throughout the ovarian cycle. To date, however, a causal interaction between TSP-1 and VEGF has not been identified. Here, we show that TSP-1 has a direct inhibitory effect on VEGF by binding the growth factor and internalizing it via LRP-1. Mice lacking TSP-1 are subfertile and exhibited ovarian hypervascularization and altered ovarian morphology. Treatment of ovarian cells with TSP-1 decreased VEGF levels and rendered the cells more susceptible to TNFalpha-induced apoptosis. Knockdown of TSP-1, through RNA interference, resulted in overexpression of VEGF and reduced cytokine-induced apoptosis. In conclusion, we demonstrate a direct inhibitory effect of TSP-1 on VEGF in the ovary. TSP-1's regulation of VEGF appears to be an important mediator of ovarian angiogenesis and follicle development.

Animals↗

PubMed2006

Continuous administration of the three thrombospondin-1 type 1 repeats recombinant protein improves the potency of therapy in an orthotopic human pancreatic cancer model.

Thrombospondin-1 is one of most important natural angiogenic inhibitors. The three thrombospondin-1 type 1 repeats (3TSR), an anti-angiogenic domain of thrombospondin-1, is a promising novel agent for anti-angiogenic treatment. In the present study, we showed 3TSR was biologically stable at least for 7 days in mini-osmotic pumps in vivo, and continuous administration of 3TSR decreased the dosage and improved the potency of therapy in an orthotopic pancreatic cancer model. By using different dosage and delivery routes, we proved that the anti-tumor efficacy of 3TSR was correlated with its anti-angiogenic efficacy. 3TSR treatment also decreased tumor vessel patency and blood flow. The results indicate the advantage of continuous administration of angiogenic inhibitors and provide rationale for using such delivery methods for cancer treatment.

Angiogenesis Inhibitors↗

PubMed2006

The thrombospondin-1 N700S polymorphism is associated with early myocardial infarction without altering von Willebrand factor multimer size.

The N700S polymorphism of thrombospondin-1 (TSP-1) has been identified as a potential genetic risk factor for myocardial infarction (MI). In a large case-control study of 1425 individuals who survived a myocardial infarction prior to age 45, the N700S polymorphism was a significant risk factor for myocardial infarction in both homozygous (odds ratio [OR] 1.9, 95% confidence interval [CI] 1.1-3.3, P = .01) and heterozygous carriers of the S700 allele (OR 1.4, 95% CI 1.1-3.3, P = .01). TSP-1 has been shown to reduce von Willebrand factor (VWF) multimer size, and the domain responsible for VWF-reducing activity has been localized to the calcium-binding C-terminal sequence. As the N700S polymorphism was previously shown to alter the function of this domain, we investigated whether the altered VWF-reducing activity of TSP-1 underlies the observed prothrombotic phenotype. The TSP1 N700S polymorphism did not influence VWF multimer size in patients homozygous for either allele nor was there a significant reduction of VWF multimer size following incubation with recombinant N700S fragments or platelet-derived TSP-1.

Alleles↗

PubMed2006

Halofuginone inhibits tumor growth in the polyoma middle T antigen mouse via a thrombospondin-1 independent mechanism.

Halofuginone inhibits fibrosis by decreasing type I collagen synthesis and tumor growth through an anti-angiogenic mechanism. In vitro data suggested that halofuginone inhibits angiogenesis through upregulating thrombospondin-1 (TSP-1) expression and by inhibiting cell proliferation. To determine whether thrombospondin-1 (TSP-1) is necessary for inhibition of tumor growth and angiogenesis by halofuginone, we tested the effect of halofuginone on mammary tumor growth in polyoma middle T antigen, TSP-1 null (TSP-1-/-PyT) transgenic mice. After 30 days of treatment, we found a significant decrease in tumor weight in these mice and the extent of tumor growth inhibition was comparable to that found in TSP-1 expressing PyT mice (TSP-1+/+PyT). However, no significant difference in tumor weight was observed after 60 days of halofuginone treatment between control and treated mice in both genotypes. Interestingly, type I collagen level was lower in the halofuginone treated TSP-1+/+PyT tumors at 30 days, but this was not observed in the TSP-1-/-PyT mice. Levels of type I collagen did not correlate with blood vessel number as a decrease in the number of vessels was observed in the halofuginone treated tumors from both the TSP-1+/+PyT and TSP-1-/-PyT mice as compared to control tumors. Because halofuginone has been shown to inhibit type I collagen synthesis by inhibiting the TGF-beta signaling pathway, we measured Smad 2/3 phosphorylation levels and found that halofuginone inhibited Smad 2/3 phosphorylation in cells derived from TSP-1+/+PyT tumors. We also found that it inhibited Smad 2/3 phosphorylation in cells treated with the TGF-beta activating sequence of TSP-1, TSR2+RFK. Our data demonstrate that halofuginone inhibits mammary tumor growth in a transgenic mouse model via a TSP-1 independent pathway, by decreasing tumor angiogenesis and by inhibiting TGF-beta signaling.

Animals↗

PubMed2006

The structures of the thrombospondin-1 N-terminal domain and its complex with a synthetic pentameric heparin.

The N-terminal domain of thrombospondin-1 (TSPN-1) mediates the protein's interaction with (1) glycosaminoglycans, calreticulin, and integrins during cellular adhesion, (2) low-density lipoprotein receptor-related protein during uptake and clearance, and (3) fibrinogen during platelet aggregation. The crystal structure of TSPN-1 to 1.8 A resolution is a beta sandwich with 13 antiparallel beta strands and 1 irregular strand-like segment. Unique structural features of the N- and C-terminal regions, and the disulfide bond location, distinguish TSPN-1 from the laminin G domain and other concanavalin A-like lectins/glucanases superfamily members. The crystal structure of the complex of TSPN-1 with heparin indicates that residues R29, R42, and R77 in an extensive positively charged patch at the bottom of the domain specifically associate with the sulfate groups of heparin. The TSPN-1 structure and identified adjacent linker region provide a structural framework for the analysis of the TSPN domain of various molecules, including TSPs, NELLs, many collagens, TSPEAR, and kielin.

Amino Acid Sequence↗

PubMed2005

Regulation of tumor angiogenesis by thrombospondin-1.

Angiogenesis plays a critical role in the growth and metastasis of tumors. Thrombospondin-1 (TSP-1) is a potent angiogenesis inhibitor, and down-regulation of TSP-1 has been suggested to alter tumor growth by modulating angiogenesis in a variety of tumor types. Expression of TSP-1 is up-regulated by the tumor suppressor gene, p53, and down-regulated by oncogenes such as Myc and Ras. TSP-1 inhibits angiogenesis by inhibiting endothelial cell migration and proliferation and by inducing apoptosis. In addition, activation of transforming growth factor beta (TGF-beta) by TSP-1 plays a crucial role in the regulation of tumor progression. An understanding of the molecular basis of TSP-1-mediated inhibition of angiogenesis and tumor progression will aid in the development of novel therapeutics for the treatment of cancer.

Angiogenesis Inhibitors↗

PubMed2005

Thrombospondin-1 controls vascular platelet recruitment and thrombus adherence in mice by protecting (sub)endothelial VWF from cleavage by ADAMTS13.

The function of thrombospondin-1 (TSP-1) in hemostasis was investigated in wild-type (WT) and Tsp1-/- mice, via dynamic platelet interaction studies with A23187-stimulated mesenteric endothelium and with photochemically injured cecum subendothelium. Injected calcein-labeled WT platelets tethered or firmly adhered to almost all A23187-stimulated blood vessels of WT mice, but Tsp1-/- platelets tethered to 45% and adhered to 25.8% of stimulated Tsp1-/- vessels only. Stimulation generated temporary endothelium-associated ultralarge von Willebrand factor (VWF) multimers, triggering platelet string formation in 48% of WT versus 20% of Tsp1-/- vessels. Injection of human TSP-1 or thrombotic thrombocytopenic purpura (TTP) patient-derived neutralizing anti-ADAMTS13 antibodies corrected the defective platelet recruitment in Tsp1-/- mice, while having a moderate effect in WT mice. Photochemical injury of intestinal blood vessels induced thrombotic occlusions with longer occlusion times in Tsp1-/- venules (1027 +/- 377 seconds) and arterioles (858 +/- 289 seconds) than in WT vessels (559 +/- 241 seconds, P < .001; 443 +/- 413 seconds, P < .003) due to defective thrombus adherence, resulting in embolization of complete thrombi, a defect restored by both human TSP-1 and anti-ADAMTS13 antibodies. We conclude that in a shear field, soluble or local platelet-released TSP-1 can protect unfolded endothelium-bound and subendothelial VWF from degradation by plasma ADAMTS13, thus securing platelet tethering and thrombus adherence to inflamed and injured endothelium, respectively.

ADAMTS13 Protein↗

PubMed2005

Antiangiogenic treatment with three thrombospondin-1 type 1 repeats versus gemcitabine in an orthotopic human pancreatic cancer model.

PURPOSE: In this study, we investigated the antitumor efficacy of thrombospondin-1 three type 1 repeats (3TSR), the antiangiogenic domain of thrombospondin-1, in comparison and in combination with gemcitabine, in an orthotopic pancreatic cancer model. EXPERIMENTAL DESIGN: Human pancreatic cancer cells were injected into the pancreas of severe combined immunodeficient mice. The animals were treated with 3TSR, gemcitabine, 3TSR plus gemcitabine, or vehicle for 3 weeks. Subsequently, the effects of 3TSR and/or gemcitabine on tumor growth, tumor necrosis, microvessel density, cancer cell proliferation, apoptosis, and endothelial cell apoptosis were analyzed. RESULTS: After 3 weeks of treatment, 3TSR reduced tumor volume by 65%, and gemcitabine by 84%. Tumor volume was not statistically different between gemcitabine group and combinatorial treatment group. Extensive necrotic areas were observed in tumors from 3TSR-treated mice, whereas tumors from gemcitabine and combinatorially treated mice were less necrotic than control tumors. 3TSR reduced tumor microvessel density and increased tumor blood vessel endothelial cell apoptosis. In contrast, gemcitabine induced apoptosis and inhibited proliferation of cancer cells. CONCLUSION: 3TSR, the antiangiogenic domain of thrombospondin-1, showed comparable antitumor efficacy to gemcitabine in a human pancreatic cancer orthotopic mouse model. No synergistic effect was found when the two drugs were combined and possible reasons are discussed in detail. A delicate balance between normalization and excessive regression of tumor vasculature is important when initiating alternative combinatorial regimens for treatment of patients with pancreatic cancer.

Amino Acid Sequence↗

PubMed2005

Cartilage oligomeric matrix protein/thrombospondin 5 supports chondrocyte attachment through interaction with integrins.

Cartilage oligomeric matrix protein/thrombospondin 5 (COMP/TSP5) is a major component of the extracellular matrix of the musculoskeletal system. Although COMP/TSP5 abnormalities are associated with several pathological conditions, its normal function remains unclear. This study was undertaken to delineate the function(s) of COMP/TSP5 in cartilage, especially regarding its interaction with chondrocytes. We show that COMP/TSP5 can support chondrocyte attachment and that the RGD sequence in COMP/TSP5 and the integrin receptors alpha5beta1 and alphaVbeta3 on the chondrocytes are involved in mediating this attachment. The interactions of COMP/TSP5 with the integrins are dependent on COMP/TSP5 conformation. Chondrocyte attachment to COMP/TSP5 in the calcium-replete conformation was inhibited by function-blocking integrin alpha5 and beta1 antibodies, suggesting the involvement of the alpha5beta1 integrin. Under this condition, a function-blocking antibody against alphaVbeta3 did not have any effect on cell attachment. On the other hand, chondrocyte attachment to reduced COMP/TSP5 was instead sensitive to alphaVbeta3 function-blocking antibodies, suggesting that COMP/TSP5 mediates attachment through chondrocyte alphaVbeta3 integrin under this condition. Cell attachment to reduced COMP/TSP5 was not inhibited by beta1 antibodies. These data indicate that COMP/TSP5 in different conformations can utilize different integrin receptors. These results are the first to demonstrate that COMP/TSP5 can mediate chondrocyte attachment through interactions with integrins. Through these interactions, COMP/TSP5 may be able to regulate cellular activities and respond to environment in the surrounding cartilage matrix.

Calcium↗

PubMed2005

Critical role of endogenous thrombospondin-1 in preventing expansion of healing myocardial infarcts.

BACKGROUND: Matricellular proteins are extracellular matrix proteins that do not contribute directly to tissue integrity but are capable of modulating cell function. We hypothesized that the matricellular protein thrombospondin (TSP)-1, a potent inhibitor of angiogenesis and activator of transforming growth factor (TGF-beta), is induced in healing myocardial infarcts and plays a role in suppressing the postinfarction inflammatory response, inhibiting local angiogenesis, and limiting expansion of granulation tissue into the noninfarcted area. METHODS AND RESULTS: We used a canine and a murine model of reperfused infarction. TSP-1 mRNA was induced in canine infarcts after 1 hour of ischemia and 3 to 7 days of reperfusion. TSP-1 protein showed a strikingly selective localization in the extracellular matrix, microvascular endothelium, and a subset of mononuclear cells of the infarct border zone after 5 to 28 days of reperfusion. Isolated canine venous endothelial cells showed low-level constitutive expression of TSP-1 mRNA, which was markedly induced by TGF-beta, and basic fibroblast growth factor. Murine infarcts also had marked TSP-1 deposition in the border zone. Infarcted TSP-1-/- mice exhibited sustained upregulation of the chemokines monocyte chemoattractant protein-1, macrophage inflammatory protein-1alpha, and interferon-gamma-inducible protein-10/CXCL10 and the cytokines interleukin-1beta, interleukin-6, and TGF-beta, suggesting an enhanced and prolonged postinfarction inflammatory response. In addition, TSP-1-/- mice had markedly increased macrophage and myofibroblast density in infarcts and in remodeling noninfarcted myocardial areas neighboring the myocardial scar, suggesting expansion of granulation tissue formation into the noninfarcted territory. TSP-1-/- animals had more extensive postinfarction remodeling than wild-type mice, although infarct size was similar in both groups. CONCLUSIONS: The infarct border zone may be capable of modulating the healing process through its unique extracellular matrix content. The selective endogenous expression of TSP-1 in the infarct border zone may serve as a "barrier," limiting expansion of granulation tissue and protecting the noninfarcted myocardium from fibrotic remodeling.

Animals↗

PubMed2005

Type 3 repeat/C-terminal domain of thrombospondin-1 triggers caspase-independent cell death through CD47/alphavbeta3 in promyelocytic leukemia NB4 cells.

By means of its antiangiogenic activity, thrombospondin-1 (TSP-1) exerts indirect antitumoral action on solid tumors. Here, we investigated potential antitumor action in an in vitro cell model for promyelocytic leukemia (NB4-LR1), resistant to retinoid maturation. Purified soluble TSP-1 added to cultures induced a strong dose-dependent growth inhibition and a slowly developing maturation-independent cell death. Recombinant fragments of TSP-1 allowed mapping of these activities to its type 3 repeat/C-terminal domain, features that are distinct from those of TSP-1 action on solid tumors, previously ascribed to the type 1 repeat domain. Cell death in leukemia was characterized as a caspase-independent mechanism, without DNA fragmentation, but phosphatidylserine externalization followed by membrane permeabilization. Mitochondria membrane depolarization was inherent to TSP-1 action but did not produce release of death-promoting proteins (eg, noncaspase apoptosis regulators, apoptosis-induced factor [AIF], endonuclease G, or Omi/HtrA2 or the caspase regulators, cytochrome c or second mitochondrial activator of caspase/direct inhibitor of apoptosis protein-binding protein with low isoelectric point [Smac/DIABLO]). Although detected, reactive oxygen species (ROS) production was likely not involved in the death process. Finally, receptor agonist RFYVVM and RGD peptides indicated that TSP-1 death effects are mediated by membrane receptors CD47 and alphavbeta3. These results demonstrated a new domain-specific antitumoral activity of TSP-1 on a leukemia cell line, which extends TSP-1 therapeutic potential outside the area of vascularized solid tumors.

Amino Acid Sequence↗

PubMed2005

Endogenous thrombospondin-1 is not necessary for proliferation but is permissive for vascular smooth muscle cell responses to platelet-derived growth factor.

We have reexamined the role of endogenous thrombospondin-1 (TSP1) in growth and motility of vascular smooth muscle cells (SMCs). Based on the ability of aortic-derived SMCs isolated from TSP1 null mice and grown in the absence of exogenous TSP1 to grow at comparable rates and to a slightly higher density than equivalent cells from wild-type mice, TSP1 is not necessary for their growth. Low concentrations of exogenous TSP1 stimulate growth of TSP1 null SMCs, but higher doses of TSP1 or its C-terminal domain are inhibitory. However, SMCs from TSP1 null mice are selectively deficient in chemotactic and proliferative responses to platelet-derived growth factor and in outgrowth in three-dimensional cultures. Recombinant portions of the N- and C-terminal domains of TSP1 stimulate SMC chemotaxis through different integrin receptors. Based on these data, the relative deficiency in SMC outgrowth during an ex vivo angiogenic response of muscle tissue from TSP1 null mice is probably due to restriction of platelet-derived growth factor dependent SMC migration and/or proliferation.

Animals↗

PubMed2005

Antiangiogenic treatment with the three thrombospondin-1 type 1 repeats recombinant protein in an orthotopic human pancreatic cancer model.

PURPOSE: This study investigates the antiangiogenesis and antitumor efficacy of a recombinant protein composed of the three type 1 repeats (3TSR) of thrombospondin-1 in an orthotopic human pancreatic cancer model and provides useful preclinical data for pancreatic cancer treatment. EXPERIMENTAL DESIGN: Human pancreatic cancer cells (AsPC-1) were injected into the pancreas of severe combined immunodeficient mice. The animals were treated with 3TSR (3 mg per kg per day) or PBS for 3 weeks. Subsequently, the effects of 3TSR on tumor growth, microvessel density, cancer cell proliferation, apoptosis, and endothelial cell apoptosis were analyzed. The in vitro effects of 3TSR on human pancreatic cancer cells were also studied. RESULTS: 3TSR treatment significantly reduced angiogenesis and tumor growth of orthotopic pancreatic cancer. 3TSR-treated mice had a 69% reduction in tumor volume (316.6 +/- 79.3 versus 1,012.2 +/- 364.5 mm(3); P = 0.0001), and a significant increase in tumor necrotic area. After 3TSR treatment, both the vessel number and average microvessel size were significantly decreased, and microvessel density was decreased from 8.0% to 3.7% (P < 0.0001). The apoptotic rate of tumoral endothelial cells in 3TSR-treated tumors increased to 14.7% comparing to 4.2% in control tumors (P < 0.0001). 3TSR showed no direct effects on pancreatic cancer cell proliferation or apoptosis either in vivo or in vitro. CONCLUSION: 3TSR, a domain of a natural occurring angiogenesis inhibitor, showed potent therapeutic effect in pancreatic cancer by inhibiting tumor angiogenesis and may prove to be a promising agent for clinical pancreatic cancer treatment.

Angiogenesis Inhibitors↗

PubMed2005

Function of endogenous inhibitors of angiogenesis as endothelium-specific tumor suppressors.

Disruption of the systemic angiogenesis balance to favor enhanced angiogenesis is speculated to represent a key step in the growth of tumors. Although a major emphasis has been placed on the increase of angiogenesis stimulators, such as VEGF, on the disruption of the angiogenic balance, the potential role of the physiological levels of endogenous inhibitors of angiogenesis on tumor growth is poorly understood. Here, we use three independent lines of mice deficient in tumstatin, endostatin, or thrombospondin-1 (TSP-1), to address the role that these endogenous angiogenesis inhibitors play in tumor growth. Our experiments demonstrate that normal physiological levels of these inhibitors serve to retard the growth of tumors, and that their absence leads to enhanced angiogenesis and a 2- to 3-fold increase in tumor growth. The tumor-suppressive action of TSP-1, endostatin, and tumstatin correlates with expression of CD36 receptor, alpha5beta1 integrin, and alphavbeta3 integrin on proliferating endothelial cells, respectively. Moreover, tumors grow 2-fold faster in the tumstatin/TSP-1 double-knockout mice, compared with either the tumstatin- or the TSP-1-deficient mice, strongly suggesting that ceiling rate of cancer growth is not completely dependent on the genetic defects of cancer cells but also depends on the host-derived tumor microenvironment. Additionally, tumor growth in transgenic mice overproducing endostatin specifically in the endothelial cells (a 1.6-fold increase in the circulating levels; mimicking Down's syndrome condition) is 3-fold slower than the tumor growth in wild-type mice. Collectively, our data suggest that physiological levels of endogenous inhibitors of angiogenesis can serve as endothelium-specific tumor suppressors.

Angiogenesis Inhibitors↗

PubMed2005

Inhibition of endothelial cell migration by thrombospondin-1 type-1 repeats is mediated by beta1 integrins.

The anti-angiogenic effect of thrombospondin-1 has been shown to be mediated through binding of the type-1 repeat (TSR) domain to the CD36 transmembrane receptor. We now report that the TSR domain can inhibit VEGF-induced migration in human umbilical vein endothelial cells (HUVEC), cells that lack CD36. Moreover, we identified beta1 integrins as a critical receptor in TSR-mediated inhibition of migration in HUVEC. Using pharmacological inhibitors of downstream VEGF receptor effectors, we found that phosphoinositide 3-kinase (PI3k) was essential for TSR-mediated inhibition of HUVEC migration, but that neither PLCgamma nor Akt was necessary for this response. Furthermore, beta1 integrins were critical for TSR-mediated inhibition of microvascular endothelial cells, cells that express CD36. Together, our results indicate that beta1 integrins mediate the anti-migratory effects of TSR through a PI3k-dependent mechanism.

CD36 Antigens↗

PubMed2005

Thrombospondins are astrocyte-secreted proteins that promote CNS synaptogenesis.

The establishment of neural circuitry requires vast numbers of synapses to be generated during a specific window of brain development, but it is not known why the developing mammalian brain has a much greater capacity to generate new synapses than the adult brain. Here we report that immature but not mature astrocytes express thrombospondins (TSPs)-1 and -2 and that these TSPs promote CNS synaptogenesis in vitro and in vivo. TSPs induce ultrastructurally normal synapses that are presynaptically active but postsynaptically silent and work in concert with other, as yet unidentified, astrocyte-derived signals to produce functional synapses. These studies identify TSPs as CNS synaptogenic proteins, provide evidence that astrocytes are important contributors to synaptogenesis within the developing CNS, and suggest that TSP-1 and -2 act as a permissive switch that times CNS synaptogenesis by enabling neuronal molecules to assemble into synapses within a specific window of CNS development.

Animals↗

PubMed2005

Genetic heterogeneity of the vasculogenic phenotype parallels angiogenesis; Implications for cellular surrogate marker analysis of antiangiogenesis.

Development of antiangiogenic therapies would be significantly facilitated by quantitative surrogate pharmacodynamic markers. Circulating peripheral blood endothelial cells (CECs) and/or their putative progenitor subset (CEPs) have been proposed but not yet fully validated for this purpose. Herein, we provide such validation by showing a striking correlation between highly genetically heterogeneous bFGF- or VEGF-induced angiogenesis and intrinsic CEC or CEP levels measured by flow cytometry, among eight different inbred mouse strains. Moreover, studies using genetically altered mice showed that levels of these cells are affected by regulators of angiogenesis, including VEGF, Tie-2, and thrombospondin-1. Finally, treatment with a targeted VEGFR-2 antibody caused a dose-dependent reduction in viable CEPs that precisely paralleled its previously and empirically determined antitumor activity.

Angiogenesis Inhibitors↗

PubMed2004

Identification of novel beta1 integrin binding sites in the type 1 and type 2 repeats of thrombospondin-1.

In addition to the three known beta(1) integrin recognition sites in the N-module of thrombospondin-1 (TSP1), we found that beta(1) integrins mediate cell adhesion to the type 1 and type 2 repeats. The type 1 repeats of TSP1 differ from typical integrin ligands in that recognition is pan-beta(1)-specific. Adhesion of cells that express one dominant beta(1) integrin on immobilized type 1 repeats is specifically inhibited by antagonists of that integrin, whereas adhesion of cells that express several beta(1) integrins is partially inhibited by each alpha-subunit-specific antagonist and completely inhibited by combining the antagonists. beta(1) integrins recognize both the second and third type 1 repeats, and each type 1 repeat shows pan-beta(1) specificity and divalent cation dependence for promoting cell adhesion. Adhesion to the type 2 repeats is less sensitive to alpha-subunit antagonists, but a beta(1) blocking antibody and two disintegrins inhibit adhesion to immobilized type 2 repeats. beta(1) integrin expression is necessary for cell adhesion to the type 1 or type 2 repeats, and beta(1) integrins bind in a divalent cation-dependent manner to a type 1 repeat affinity column. The widely used TSP1 function blocking antibody A4.1 binds to a site in the third type 2 repeat. A4.1 proximally inhibits beta(1) integrin-dependent adhesion to the type 2 repeats and indirectly inhibits integrin-dependent adhesion mediated by the TSP1 type 1 repeats. Although antibody A4.1 is also an antagonist of CD36 binding to TSP1, these data suggest that some biological activities of A4.1 result from antagonism of these novel beta(1) integrin binding sites.

Antibodies, Monoclonal↗