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

Deciphering the role of CD47 in cancer immunotherapy

BACKGROUND

Immunotherapy has emerged as a novel strategy for cancer treatment following surgery, radiotherapy, and chemotherapy. Immune checkpoint blockade and Chimeric antigen receptor (CAR)-T cell therapies have been successful in clinical trials. Cancer cells evade immune surveillance by hijacking inhibitory pathways via overexpression of checkpoint genes. The Cluster of Differentiation 47 (CD47) has emerged as a crucial checkpoint for cancer immunotherapy by working as a "don't eat me" signal and suppressing innate immune signaling. Furthermore, CD47 is highly expressed in many cancer types to protect cancer cells from phagocytosis via binding to SIRPα on phagocytes. Targeting CD47 by either interrupting the CD47-SIRPα axis or combing with other therapies has been demonstrated as an encouraging therapeutic strategy in cancer immunotherapy. Antibodies and small molecules that target CD47 have been explored in pre- and clinical trials. However, formidable challenges such as the anemia and palate aggregation cannot be avoided because of the wide presentation of CD47 on erythrocytes.

AIM OF VIEW

This review summarizes the current knowledge on the regulation and function of CD47, and provides a new perspective for immunotherapy targeting CD47. It also highlights the clinical progress of targeting CD47 and discusses challenges and potential strategies.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This review provides a comprehensive understanding of targeting CD47 in cancer immunotherapy, it also augments the concept of combination immunotherapy strategies by employing both innate and adaptive immune responses.

The Contributions of Thrombospondin-1 to Epilepsy Formation

Epilepsy is a neural network disorder caused by uncontrolled neuronal hyperexcitability induced by an imbalance between excitatory and inhibitory networks. Abnormal synaptogenesis plays a vital role in the formation of overexcited networks. Recent evidence has confirmed that thrombospondin-1 (TSP-1), mainly secreted by astrocytes, is a critical cytokine that regulates synaptogenesis during epileptogenesis. Furthermore, numerous studies have reported that TSP-1 is also involved in other processes, such as angiogenesis, neuroinflammation, and regulation of Ca2+ homeostasis, which are closely associated with the occurrence and development of epilepsy. In this review, we summarize the potential contributions of TSP-1 to epilepsy development.

The molecular mechanism of thrombospondin family members in cardiovascular diseases

Cardiovascular diseases have been identified as vital factors in global morbidity and mortality in recent years. The available evidence suggests that various cytokines and pathological proteins participate in these complicated and changeable diseases. The thrombospondin (TSP) family is a series of conserved, multidomain calcium-binding glycoproteins that cause cell-matrix and cell-cell effects via interactions with other extracellular matrix components and cell surface receptors. The TSP family has five members that can be divided into two groups (Group A and Group B) based on their different structures. TSP-1, TSP-2, and TSP-4 are the most studied proteins. Among recent studies and findings, we investigated the functions of several family members, especially TSP-5. We review the basic concepts of TSPs and summarize the relevant molecular mechanisms and cell interactions in the cardiovascular system. Targeting TSPs in CVD and other diseases has a remarkable therapeutic benefit.

CD47: The Next Frontier in Immune Checkpoint Blockade for Non-Small Cell Lung Cancer

The success of PD-1/PD-L1-targeted therapy in lung cancer has resulted in great enthusiasm for additional immunotherapies in development to elicit similar survival benefits, particularly in patients who do not respond to or are ineligible for PD-1 blockade. CD47 is an immunosuppressive molecule that binds SIRPα on antigen-presenting cells to regulate an innate immune checkpoint that blocks phagocytosis and subsequent activation of adaptive tumor immunity. In lung cancer, CD47 expression is associated with poor survival and tumors with EGFR mutations, which do not typically respond to PD-1 blockade. Given its prognostic relevance, its role in facilitating immune escape, and the number of agents currently in clinical development, CD47 blockade represents a promising next-generation immunotherapy for lung cancer. In this review, we briefly summarize how tumors disrupt the cancer immunity cycle to facilitate immune evasion and their exploitation of immune checkpoints like the CD47-SIRPα axis. We also discuss approved immune checkpoint inhibitors and strategies for targeting CD47 that are currently being investigated. Finally, we review the literature supporting CD47 as a promising immunotherapeutic target in lung cancer and offer our perspective on key obstacles that must be overcome to establish CD47 blockade as the next standard of care for lung cancer therapy.

Checkpoint Inhibitors in Acute Myeloid Leukemia

The prognosis of acute myeloid leukemia (AML) remains unsatisfactory. Among the reasons for the poor response to therapy and high incidence of relapse, there is tumor cell immune escape, as AML blasts can negatively influence various components of the immune system, mostly weakening T-cells. Since leukemic cells can dysregulate immune checkpoints (ICs), receptor-based signal transductors that lead to the negative regulation of T-cells and, eventually, to immune surveillance escape, the inhibition of ICs is a promising therapeutic strategy and has led to the development of so-called immune checkpoint inhibitors (ICIs). ICIs, in combination with conventional chemotherapy, hypomethylating agents or targeted therapies, are being increasingly tested in cases of AML, but the results reported are often conflicting. Here, we review the main issues concerning the immune system in AML, the main pathways leading to immune escape and the results obtained from clinical trials of ICIs, alone or in combination, in newly diagnosed or relapsed/refractory AML.

Controlled extracellular proteolysis of thrombospondins

Limited proteolysis of thrombospondins is a powerful mechanism to ensure dynamic tuning of their activities in the extracellular space. Thrombospondins are multifunctional matricellular proteins composed of multiple domains, each with a specific pattern of interactions with cell receptors, matrix components and soluble factors (growth factors, cytokines and proteases), thus with different effects on cell behavior and responses to changes in the microenvironment. Therefore, the proteolytic degradation of thrombospondins has multiple functional consequences, reflecting the local release of active fragments and isolated domains, exposure or disruption of active sequences, altered protein location, and changes in the composition and function of TSP-based pericellular interaction networks. In this review current data from the literature and databases is employed to provide an overview of cleavage of mammalian thrombospondins by different proteases. The roles of the fragments generated in specific pathological settings, with particular focus on cancer and the tumor microenvironment, are discussed.

The landscape overview of CD47-based immunotherapy for hematological malignancies

Extensive clinical and experimental evidence suggests that macrophages play a crucial role in cancer immunotherapy. Cluster of differentiation (CD) 47, which is found on both healthy and malignant cells, regulates macrophage-mediated phagocytosis by sending a "don't eat me" signal to the signal regulatory protein alpha (SIRPα) receptor. Increasing evidence demonstrates that blocking CD47 interaction with SIRPα can enhance cancer cell clearance by macrophages. Additionally, inhibition of CD47/SIRPα interaction can increase antigen cross-presentation, leading to T-cell priming and an activated adaptive antitumor immune response. Therefore, inhibiting CD47/SIRPα axis has a significant impact on tumor immunotherapy. Studies on CD47 monoclonal antibodies are at the forefront of research, and impressive results have been obtained. Nevertheless, hematotoxicity, especially anemia, has become the most common adverse effect of the CD47 monoclonal antibody. More specific targeted drugs (i.e., bispecific antibodies, SIRPα/Fc fusion protein antibodies, and small-molecule inhibitors) have been developed to reduce hematotoxicity. Here, we review the present usage of CD47 antagonists for the treatment of lymphomas and hematologic neoplasms from the perspectives of structure, function, and clinical trials, including a comprehensive overview of the drugs in development.

The expression profiles of CD47 in the tumor microenvironment of salivary gland cancers: a next step in histology-driven immunotherapy

BACKGROUND

Salivary gland carcinomas (SGC) are extremely rare malignancies with only limited treatment options for the metastatic phase of the disease. Treatment with anti-CD47 antibodies could represent a potent therapy for SGCs by promoting the phagocytic clearance of tumor cells through various mechanisms. However, the efficacy of anti-CD47 therapy is largely dependent on the expression of CD47 within the tumor microenvironment (TME).

MATERIALS AND METHODS

In 43 patients with SGC, we were the first to investigate the CD47 expression in both tumor cells and tumor-infiltrating immune cells (TIIC) in the center and periphery of primary tumors. We also correlated the data with the clinicopathological variables of the patients and offered novel insights into the potential effectiveness of anti-CD47 therapy in SGCs.

RESULTS

We observed that the CD47⁺ tumor cells are outnumbered by CD47⁺ TIICs in mucoepidermoid carcinoma. In the tumor center, the proportion of CD47⁺ tumor cells was comparable to the proportion of CD47⁺ TIICs in most histological subtypes. In low-grade tumors, significantly higher expression of CD47 was observed in TIICs in the periphery of the tumor as compared to the center of the tumor.

CONCLUSION

The reason for a high expression of 'don't eat me' signals in TIICs in the tumor periphery is unclear. However, we hypothesize that in the tumor periphery, upregulation of CD47 in TIICs could be a mechanism to protect newly recruited leukocytes from macrophage-mediated phagocytosis, while also allowing the removal of old or exhausted leukocytes in the tumor center.

A Mini Review on Molecules Inducing Caspase-Independent Cell Death: A New Route to Cancer Therapy

Most anticancer treatments trigger tumor cell death through apoptosis, where initiation of proteolytic action of caspase protein is a basic need. But under certain circumstances, apoptosis is prevented by the apoptosis inhibitor proteins, survivin and Hsp70. Several drugs focusing on classical programmed death of the cell have been reported to have low anti-tumorogenic potency due to mutations in proteins involved in the caspase-dependent programmed cell death with intrinsic and extrinsic pathways. This review concentrates on the role of anti-cancer drug molecules targeting alternative pathways of cancer cell death for treatment, by providing a molecular basis for the new strategies of novel anti-cancer treatment. Under these conditions, active agents targeting alternative cell death pathways can be considered as potent chemotherapeutic drugs. Many natural compounds and other small molecules, such as inorganic and synthetic compounds, including several repurposing drugs, are reported to cause caspase-independent cell death in the system. However, few molecules indicated both caspase-dependent as well caspase-free cell death in specific cancer lines. Cancer cells have alternative methods of caspase-independent programmed cell death which are equally promising for being targeted by small molecules. These small molecules may be useful leads for rational therapeutic drug design, and can be of potential interest for future cancer-preventive strategies.

CD47 cross-dressing by extracellular vesicles expressing CD47 inhibits phagocytosis without transmitting cell death signals

Transgenic CD47 overexpression is an encouraging approach to ameliorating xenograft rejection and alloresponses to pluripotent stem cells, and the efficacy correlates with the level of CD47 expression. However, CD47, upon ligation, also transmits signals leading to cell dysfunction or death, raising a concern that overexpressing CD47 could be harmful. Here, we unveiled an alternative source of cell surface CD47. We showed that extracellular vesicles, including exosomes, released from normal or tumor cells overexpressing CD47 (transgenic or native) can induce efficient CD47 cross-dressing on pig or human cells. Like the autogenous CD47, CD47 cross-dressed on cell surfaces is capable of interacting with SIRPα to inhibit phagocytosis. However, ligation of the autogenous, but not cross-dressed, CD47 induced cell death. Thus, CD47 cross-dressing provides an alternative source of cell surface CD47 that may elicit its anti-phagocytic function without transmitting harmful signals to the cells. CD47 cross-dressing also suggests a previously unidentified mechanism for tumor-induced immunosuppression. Our findings should help to further optimize the CD47 transgenic approach that may improve outcomes by minimizing the harmful effects of CD47 overexpression.

CD47 in the Brain and Neurodegeneration: An Update on the Role in Neuroinflammatory Pathways

CD47 is a receptor belonging to the immunoglobulin (Ig) superfamily and broadly expressed on cell membranes. Through interactions with ligands such as SIRPα, TSP-1, integrins, and SH2-domain bearing protein tyrosine phosphatase substrate-1 (SHPS-1), CD47 regulates numerous functions like cell adhesion, proliferation, apoptosis, migration, homeostasis, and the immune system. In this aspect, previous research has shown that CD47 modulates phagocytosis via macrophages, the transmigration of neutrophils, and the activation of T-cells, dendritic cells, and B-cells. Moreover, several studies have reported the increased expression of the CD47 receptor in a variety of diseases, including acute lymphoblastic leukemia (ALL), chronic myeloid leukemia, non-Hodgkin’s lymphoma (NHL), multiple myeloma (MM), bladder cancer, acute myeloid leukemia (AML), Gaucher disease, Multiple Sclerosis and stroke among others. The ubiquitous expression of the CD47 cell receptor on most resident cells of the CNS has previously been established through different methodologies. However, there is little information concerning its precise functions in the development of different neurodegenerative pathologies in the CNS. Consequently, further research pertaining to the specific functions and roles of CD47 and SIRP is required prior to its exploitation as a druggable approach for the targeting of various neurodegenerative diseases that affect the human population. The present review attempts to summarize the role of both CD47 and SIRP and their therapeutic potential in neurodegenerative disorders.

CD47 and thrombospondin-1 regulation of mitochondria, metabolism, and diabetes

Thrombospondin-1 (TSP1) is the prototypical member of a family of secreted proteins that modulate cell behavior by engaging with molecules in the extracellular matrix and with receptors on the cell surface. CD47 is widely displayed on many, if not all, cell types and is a high-affinity TSP1 receptor. CD47 is a marker of self that limits innate immune cell activities, a feature recently exploited to enhance cancer immunotherapy. Another major role for CD47 in health and disease is to mediate TSP1 signaling. TSP1 acting through CD47 contributes to mitochondrial, metabolic, and endocrine dysfunction. Studies in animal models found that elevated TSP1 expression, acting in part through CD47, causes mitochondrial and metabolic dysfunction. Clinical studies established that abnormal TSP1 expression positively correlates with obesity, fatty liver disease, and diabetes. The unabated increase in these conditions worldwide and the availability of CD47 targeting drugs justify a closer look into how TSP1 and CD47 disrupt metabolic balance and the potential for therapeutic intervention.

Functions of Thrombospondin-1 in the Tumor Microenvironment

The identification of thrombospondin-1 as an angiogenesis inhibitor in 1990 prompted interest in its role in cancer biology and potential as a therapeutic target. Decreased thrombospondin-1 mRNA and protein expression are associated with progression in several cancers, while expression by nonmalignant cells in the tumor microenvironment and circulating levels in cancer patients can be elevated. THBS1 is not a tumor suppressor gene, but the regulation of its expression in malignant cells by oncogenes and tumor suppressor genes mediates some of their effects on carcinogenesis, tumor progression, and metastasis. In addition to regulating angiogenesis and perfusion of the tumor vasculature, thrombospondin-1 limits antitumor immunity by CD47-dependent regulation of innate and adaptive immune cells. Conversely, thrombospondin-1 is a component of particles released by immune cells that mediate tumor cell killing. Thrombospondin-1 differentially regulates the sensitivity of malignant and nonmalignant cells to genotoxic stress caused by radiotherapy and chemotherapy. The diverse activities of thrombospondin-1 to regulate autophagy, senescence, stem cell maintenance, extracellular vesicle function, and metabolic responses to ischemic and genotoxic stress are mediated by several cell surface receptors and by regulating the functions of several secreted proteins. This review highlights progress in understanding thrombospondin-1 functions in cancer and the challenges that remain in harnessing its therapeutic potential.

The extracellular matrix: A key player in the pathogenesis of hematologic malignancies

Hematopoietic stem and progenitor cells located in the bone marrow lay the foundation for multiple lineages of mature hematologic cells. Bone marrow niches are architecturally complex with specific cellular, physiochemical, and biomechanical factors. Increasing evidence suggests that the bone marrow microenvironment contributes to the pathogenesis of hematological neoplasms. Numerous studies have deciphered the role of genetic mutations and chromosomal translocations in the development hematologic malignancies. Significant progress has also been made in understanding how the cellular components and cytokine interactions within the bone marrow microenvironment promote the evolution of hematologic cancers. Although the extracellular matrix is known to be a key player in the pathogenesis of various diseases, it's role in the progression of hematologic malignancies is less understood. In this review, we discuss the interactions between the extracellular matrix and malignant cells, and provide an overview of the role of extracellular matrix remodeling in sustaining hematologic malignancies.

Thrombospondin-1 as a Potential Therapeutic Target: Multiple Roles in Cancers

Thrombospondin-1, an extracellular matrix protein, is the first identified natural angiogenesis inhibitor. Thrombospondin-1 participates in a great number of physiological and pathological processes, including cell-cell and cell-matrix interactions via a number of cell receptors, including CD36 and CD47, which plays a vital role in mediating inflammation and performs a promoting effect in pulmonary arterial vasculopathy and diabetes. Thrombospondin-1 consists of six domains, which combine with different molecules and participate in various functions in cancers, serving as a critical member in diverse pathways in cancers. Thrombospondin-1 works as a cancer promotor in some pathways but as a cancer suppressor in others, which makes it highly possible that its erroneous functioning might lead to opposite effects. Therefore, subdividing the roles of thrombospondin-1 and distinguishing them in cancers are necessary. Complex structure and multiple roles take disadvantage of the research and application of thrombospondin-1. Compared with the whole thrombospondin-1 protein, each thrombospondin- 1 active peptide performs an uncomplicated structure and, nevertheless, a specific role. In other words, various thrombospondin-1 active peptides may function differently. For instance, thrombospondin-1 could both promote and inhibit glioblastoma, which is significantly inhibited by the three type I repeats, a thrombospondin-1 active peptide but promoted by the fragment 167-569, a thrombospondin-1 active peptide consisting of the procollagen homology domain and the three type I repeats. Further studies of the functions of thrombospondin-1 active peptides and applying them reasonably are necessary. In addition to mediating cancerogenesis, thrombospondin-1 is also affected by cancer development, as reflected by its expression in plasma and the cancer tissue. Therefore, thrombospondin-1 may be a potential biomarker for pre-clinical and clinical application. This review summarizes findings on the multiple roles of thrombospondin-1 in cancer processes, with a focus on its use as a potential therapeutic target.

CD47 (Cluster of differentiation 47): an anti-phagocytic receptor with a multitude of signaling functions

CD47 is a tumor-associated antigen best known for its ability to bind counter-receptors on the surface of professional phagocytes as an immune-evasion strategy. ⁣Recently, CD47 has been shown to play a role as a signaling receptor, involving a number of cell physiological processes.⁣⁣ This review provides a comprehensive survey of the signaling pathways triggered by CD47 ligand-mediated cell death in tumor cells. Such an understanding should lead to improvement of CD47-targeted anti-tumor therapeutics able to both neutralize the anti-phagocytic role and trigger autonomous tumor cell death.

Role of thrombospondin‑1 and thrombospondin‑2 in cardiovascular diseases (Review)

Thrombospondin (TSP)-1 and TSP-2 are matricellular proteins in the extracellular matrix (ECM), which serve a significant role in the pathological processes of various cardiovascular diseases (CVDs). The multiple effects of TSP-1 and TSP-2 are due to their ability to interact with various ligands, such as structural components of the ECM, cytokines, cellular receptors, growth factors, proteases and other stromal cell proteins. TSP-1 and TSP-2 regulate the structure and activity of the aforementioned ligands by interacting directly or indirectly with them, thereby regulating the activity of different types of cells in response to environmental stimuli. The pathological processes of numerous CVDs are associated with the degradation and remodeling of ECM components, and with cell migration, dysfunction and apoptosis, which may be regulated by TSP-1 and TSP-2 through different mechanisms. Therefore, investigating the role of TSP-1 and TSP-2 in different CVDs and the potential signaling pathways they are associated with may provide a new perspective on potential therapies for the treatment of CVDs. In the present review, the current understanding of the roles TSP-1 and TSP-2 serve in various CVDs were summarized. In addition, the interacting ligands and the potential pathways associated with these thrombospondins in CVDs are also discussed.

Advances in Anti-Tumor Treatments Targeting the CD47/SIRPα Axis

CD47 is an immunoglobulin that is overexpressed on the surface of many types of cancer cells. CD47 forms a signaling complex with signal-regulatory protein α (SIRPα), enabling the escape of these cancer cells from macrophage-mediated phagocytosis. In recent years, CD47 has been shown to be highly expressed by various types of solid tumors and to be associated with poor patient prognosis in various types of cancer. A growing number of studies have since demonstrated that inhibiting the CD47-SIRPα signaling pathway promotes the adaptive immune response and enhances the phagocytosis of tumor cells by macrophages. Improved understanding in this field of research could lead to the development of novel and effective anti-tumor treatments that act through the inhibition of CD47 signaling in cancer cells. In this review, we describe the structure and function of CD47, provide an overview of studies that have aimed to inhibit CD47-dependent avoidance of macrophage-mediated phagocytosis by tumor cells, and assess the potential and challenges for targeting the CD47-SIRPα signaling pathway in anti-cancer therapy.

Thrombospondin-1, Free Radicals, and the Coronary Microcirculation: The Aging Conundrum

SIGNIFICANCE

Successful matching of cardiac metabolism to perfusion is accomplished primarily through vasodilation of the coronary resistance arterioles, but the mechanism that achieves this effect changes significantly as aging progresses and involves the contribution of reactive oxygen species (ROS). Recent Advances: A matricellular protein, thrombospondin-1 (Thbs-1), has been shown to be a prolific contributor to the production and modulation of ROS in large conductance vessels and in the peripheral circulation. Recently, the presence of physiologically relevant circulating Thbs-1 levels was proven to also disrupt vasodilation to nitric oxide (NO) in coronary arterioles from aged animals, negatively impacting coronary blood flow reserve.

CRITICAL ISSUES

This review seeks to reconcile how ROS can be successfully utilized as a substrate to mediate vasoreactivity in the coronary microcirculation as "normal" aging progresses, but will also examine how Thbs-1-induced ROS production leads to dysfunctional perfusion and eventual ischemia and why this is more of a concern in advancing age.

FUTURE DIRECTIONS

Current therapies that may effectively disrupt Thbs-1 and its receptor CD47 in the vascular wall and areas for future exploration will be discussed. Antioxid. Redox Signal. 27, 785-801.

Development of a prosaposin-derived therapeutic cyclic peptide that targets ovarian cancer via the tumor microenvironment

The vast majority of ovarian cancer-related deaths are caused by metastatic dissemination of tumor cells, resulting in subsequent organ failure. However, despite our increased understanding of the physiological processes involved in tumor metastasis, there are no clinically approved drugs that have made a major impact in increasing the overall survival of patients with advanced, metastatic ovarian cancer. We identified prosaposin (psap) as a potent inhibitor of tumor metastasis, which acts via stimulation of p53 and the antitumorigenic protein thrombospondin-1 (TSP-1) in bone marrow-derived cells that are recruited to metastatic sites. We report that more than 97% of human serous ovarian tumors tested express CD36, the receptor that mediates the proapoptotic activity of TSP-1. Accordingly, we sought to determine whether a peptide derived from psap would be effective in treating this form of ovarian cancer. To that end, we developed a cyclic peptide with drug-like properties derived from the active sequence in psap. The cyclic psap peptide promoted tumor regression in a patient-derived tumor xenograft model of metastatic ovarian cancer. Thus, we hypothesize that a therapeutic agent based on this psap peptide would have efficacy in treating patients with metastatic ovarian cancer.

Original insights on thrombospondin-1-related antireceptor strategies in cancer

Thrombospondin-1 (TSP-1) is a large matricellular glycoprotein known to be overexpressed within tumor stroma in several cancer types. While mainly considered as an endogenous angiogenesis inhibitor, TSP-1 exhibits multifaceted functionalities in a tumor context depending both on TSP-1 concentration as well as differential receptor expression by cancer cells and on tumor-associated stromal cells. Besides, the complex modular structure of TSP-1 along with the wide variety of its soluble ligands and membrane receptors considerably increases the complexity of therapeutically targeting interactions involving TSP-1 ligation of cell-surface receptors. Despite the pleiotropic nature of TSP-1, many different antireceptor strategies have been developed giving promising results in preclinical models. However, transition to clinical trials often led to nuanced outcomes mainly due to frequent severe adverse effects. In this review, we will first expose the intricate and even sometimes opposite effects of TSP-1-related signaling on tumor progression by paying particular attention to modulation of angiogenesis and tumor immunity. Then, we will provide an overview of current developments and prospects by focusing particularly on the cell-surface molecules CD47 and CD36 that function as TSP-1 receptors; including antibody-based approaches, therapeutic gene modulation and the use of peptidomimetics. Finally, we will discuss original approaches specifically targeting TSP-1 domains, as well as innovative combination strategies with a view to producing an overall anticancer response.

CD47 agonist peptides induce programmed cell death in refractory chronic lymphocytic leukemia B cells via PLCγ1 activation: evidence from mice and humans

BACKGROUND

Chronic lymphocytic leukemia (CLL), the most common adulthood leukemia, is characterized by the accumulation of abnormal CD5+ B lymphocytes, which results in a progressive failure of the immune system. Despite intense research efforts, drug resistance remains a major cause of treatment failure in CLL, particularly in patients with dysfunctional TP53. The objective of our work was to identify potential approaches that might overcome CLL drug refractoriness by examining the pro-apoptotic potential of targeting the cell surface receptor CD47 with serum-stable agonist peptides.

METHODS AND FINDINGS

In peripheral blood samples collected from 80 patients with CLL with positive and adverse prognostic features, we performed in vitro genetic and molecular analyses that demonstrate that the targeting of CD47 with peptides derived from the C-terminal domain of thrombospondin-1 efficiently kills the malignant CLL B cells, including those from high-risk individuals with a dysfunctional TP53 gene, while sparing the normal T and B lymphocytes from the CLL patients. Further studies reveal that the differential response of normal B lymphocytes, collected from 20 healthy donors, and leukemic B cells to CD47 peptide targeting results from the sustained activation in CLL B cells of phospholipase C gamma-1 (PLCγ1), a protein that is significantly over-expressed in CLL. Once phosphorylated at tyrosine 783, PLCγ1 enables a Ca2+-mediated, caspase-independent programmed cell death (PCD) pathway that is not down-modulated by the lymphocyte microenvironment. Accordingly, down-regulation of PLCγ1 or pharmacological inhibition of PLCγ1 phosphorylation abolishes CD47-mediated killing. Additionally, in a CLL-xenograft model developed in NOD/scid gamma mice, we demonstrate that the injection of CD47 agonist peptides reduces tumor burden without inducing anemia or toxicity in blood, liver, or kidney. The limitations of our study are mainly linked to the affinity of the peptides targeting CD47, which might be improved to reach the standard requirements in drug development, and the lack of a CLL animal model that fully mimics the human disease.

CONCLUSIONS

Our work provides substantial progress in (i) the development of serum-stable CD47 agonist peptides that are highly effective at inducing PCD in CLL, (ii) the understanding of the molecular events regulating a novel PCD pathway that overcomes CLL apoptotic avoidance, (iii) the identification of PLCγ1 as an over-expressed protein in CLL B cells, and (iv) the description of a novel peptide-based strategy against CLL.

Members of the thrombospondin gene family bind stromal interaction molecule 1 and regulate calcium channel activity

The thrombospondins (TSPs) are a family of matricellular proteins that regulate cellular phenotype through interactions with a myriad of other proteins and proteoglycans. We have identified a novel interaction of the members of the TSP gene family with stromal interaction molecule 1 (STIM1). This association is robust since it is preserved in Triton X-100, can be detected with multiple anti-TSP-1 and anti-STIM1 antibodies, and is detected in a wide range of cell types. We have also found that STIM1 co-immunoprecipitates with TSP-4 and cartilage oligomeric matrix protein (COMP), and that a recombinant version of the N-terminal domain of STIM1 binds to the signature domain of TSP-1 and COMP. The association of the TSPs with STIM1 is observed in both the presence and absence of calcium indicating that the calcium-dependent conformation of the signature domain of TSPs is not required for binding. Thus, this interaction could occur in the ER under conditions of normal or low calcium concentration. Furthermore, we observed that the expression of COMP in HEK 293 cells decreases STIM1-mediated calcium release activated calcium (CRAC) channel currents and increases arachidonic acid calcium (ARC) channel currents. These data indicate that the TSPs regulate STIM1 function and participate in the reciprocal regulation of two channels that mediate calcium entry into the cell.

Thrombospondin-1 and CD47 regulation of cardiac, pulmonary and vascular responses in health and disease

Cardiovascular homeostasis and health is maintained through the balanced interactions of cardiac generated blood flow and cross-talk between the cellular components that comprise blood vessels. Central to this cross-talk is endothelial generated nitric oxide (NO) that stimulates relaxation of the contractile vascular smooth muscle (VSMC) layer of blood vessels. In cardiovascular disease this balanced interaction is disrupted and NO signaling is lost. Work over the last several years indicates that regulation of NO is much more complex than previously believed. It is now apparent that the secreted protein thrombospondin-1 (TSP1), that is upregulated in cardiovascular disease and animal models of the same, on activating cell surface receptor CD47, redundantly inhibits NO production and NO signaling. This inhibitory event has implications for baseline and disease-related responses mediated by NO. Further work has identified that TSP1-CD47 signaling stimulates enzymatic reactive oxygen species (ROS) production to further limit blood flow and promote vascular disease. Herein consideration is given to the most recent discoveries in this regard which identify the TSP1-CD47 axis as a major proximate governor of cardiovascular health.

Txr1: an important factor in oxaliplatin resistance in gastric cancer

Oxaliplatin-based chemotherapy is the main treatment regimen for gastric cancer (GC), but can fail because of drug resistance. We investigated the role of a recently identified drug-resistance gene, taxol-resistant gene 1 (Txr1), in oxaliplatin resistance. A retrospective study based on banked tissue was carried out. We collected clinical data from 95 patients with stage II-III GC who were treated with radical D2 surgery and standardized first-line chemotherapy with oxaliplatin; paraffin blocks of their tumor specimens were prepared for a tissue microarray in which Txr1 expression was analyzed immunohistochemically and compared with their clinical data and their 3-year disease-free survival (DFS) rate. The human GC cell line, SGC7901, was developed into the oxaliplatin-resistant cell line, SGC7901/L-OHP, using slowly increased oxaliplatin concentrations over 6 months. The relationship between Txr1 expression and drug-resistance of oxaliplatin in GC was studied with drug intervention, gene silencing technology, real-time PCR and Western blot analysis. Of the 95 patients with GC, those with TXR1(-) GC had longer postoperative 3-year DFS (77.8 %) than those with TXR1(+) GC (52.9 %). In oxaliplatin-resistant SGC7901/L-OHP cells, the main expression location of Txr1 shifted from the nucleus to cytoplasm, and both the mRNA and protein expression of Txr1 were higher than that of the parental cells, whereas expression of thrombospondin-1 (TSP1) decreased. When the Txr1 gene was silenced, TSP1 expression increased and the oxaliplatin resistance was significantly reduced in SGC7901/L-OHP cells. Changed Txr1 expression in GC affects the efficacy of oxaliplatin-based chemotherapy. Increased Txr1 expression decreases TSP1 expression and inhibits apoptosis. Txr1 could be a target in reversing oxaliplatin resistance in GC.

Thrombospondins: old players, new games

PURPOSE OF REVIEW

Thrombospondins (TSPs) are secreted extracellular matrix (ECM) proteins from TSP family, which consists of five homologous members. They share a complex domain structure and have numerous binding partners in ECM and multiple cell surface receptors. Information that has emerged over the past decade identifies TSPs as important mediators of cellular homeostasis, assigning new important roles in cardiovascular pathology to these proteins.

RECENT FINDINGS

Recent studies of the functions of TSP in the cardiovascular system, diabetes and aging, which placed several TSPs in a position of critical regulators, demonstrated the involvement of these proteins in practically every aspect of cardiovascular pathophysiology related to atherosclerosis: inflammation, immunity, leukocyte recruitment and function, function of vascular cells, angiogenesis, and responses to hypoxia, ischemia and hyperglycemia. TSPs are also critically important in the development and ultimate outcome of the complications associated with atherosclerosis--myocardial infarction, and heart hypertrophy and failure. Their expression and significance increase with age and with the progression of diabetes, two major contributors to the development of atherosclerosis and its complications.

SUMMARY

This overview of recent literature examines the latest information on the newfound functions of TSPs that emphasize the importance of ECM in cardiovascular homeostasis and pathology. The functions of TSPs in myocardium, vasculature, vascular complications of diabetes, aging and immunity are discussed.

Thrombospondin-1 in urological cancer: pathological role, clinical significance, and therapeutic prospects

Angiogenesis is an important process for tumor growth and progression of various solid tumors including urological cancers. Thrombospondins (TSPs), especially TSP-1, are representative “anti”-angiogenic molecules and many studies have clarified their pathological role and clinical significance in vivo and in vitro. In fact, TSP-1 expression is associated with clinicopathological features and prognosis in many types of cancers. However, TSP-1 is a multi-functional protein and its biological activities vary according to the specific tumor environments. Consequently, there is no general agreement on its cancer-related function in urological cancers, and detailed information regarding regulative mechanisms is essential for a better understanding of its therapeutic effects and prognostic values. Various “suppressor genes” and “oncogenes” are known to be regulators and TSP-1-related factors under physiological and pathological conditions. In addition, various types of fragments derived from TSP-1 exist in a given tissue microenvironment and TSP-1 derived-peptides have specific activities. However, a detailed pathological function in human cancer tissues is not still understood. This review will focus on the pathological roles and clinical significance of TSP-1 in urological cancers, including prostate cancer, renal cell carcinoma, and urothelial cancer. In addition, special attention is paid to TSP-1-derived peptide and TSP-1-based therapy for malignancies.

CD47: A Cell Surface Glycoprotein Which Regulates Multiple Functions of Hematopoietic Cells in Health and Disease

Interactions between cells and their surroundings are important for proper function and homeostasis in a multicellular organism. These interactions can either be established between the cells and molecules in their extracellular milieu, but also involve interactions between cells. In all these situations, proteins in the plasma membranes are critically involved to relay information obtained from the exterior of the cell. The cell surface glycoprotein CD47 (integrin-associated protein (IAP)) was first identified as an important regulator of integrin function, but later also was shown to function in ways that do not necessarily involve integrins. Ligation of CD47 can induce intracellular signaling resulting in cell activation or cell death depending on the exact context. By binding to another cell surface glycoprotein, signal regulatory protein alpha (SIRPα), CD47 can regulate the function of cells in the monocyte/macrophage lineage. In this spotlight paper, several functions of CD47 will be reviewed, although some functions may be more briefly mentioned. Focus will be on the ways CD47 regulates hematopoietic cells and functions such as CD47 signaling, induction of apoptosis, and regulation of phagocytosis or cell-cell fusion.

Thrombospondin-1 Silencing Down-Regulates Integrin Expression Levels in Human Anaplastic Thyroid Cancer Cells with BRAF(V600E): New Insights in the Host Tissue Adaptation and Homeostasis of Tumor Microenvironment

BACKGROUND AND RATIONALE

Anaplastic thyroid cancer (ATC) is characterized by pleomorphic cells, has a poor prognosis, is highly devastating disease, and is not curable. No reliable biomarkers of metastatic potential, helpful for early diagnosis of ATC and therapeutic response have been found yet. Thrombospondin-1 (TSP-1) plays a fundamental role in cancer progression by regulating cell stromal cross-talk in the tumor microenvironment.

GOALS

Our goal was to understand whether TSP-1 could affect protein levels of its integrin receptors (e.g., ITGα3, α6, and β1) and cell morphology in BRAF(V600E)-ATC cells in vitro and in vivo.

EXPERIMENTAL DESIGN

Anaplastic thyroid cancer-derived cell cultures and western blotting were used to assess integrin protein expression upon TSP-1 silencing. Immunohistochemistry was performed on orthotopic primary human ATC and metastatic ATC in lung tissue to compare TSP-1 and integrin protein expression levels.

RESULTS

TSP-1 knock-down down-regulates ITGα3, α6, and β1 in BRAF(V600E)-human ATC cells. BRAF(V600E)-ATC cells with TSP-1 knock-down were rounded compared to control cells, which displayed a spread morphology. TSP-1 knock-down also reduced TSP-1, ITGα3, α6, and β1 protein expression levels in vivo in the ATC microenvironment, which is enriched in stromal and inflammatory cells.

CONCLUSION

TSP-1 silencing causes changes in ITG levels and ATC cell morphology. The assessment of TSP-1 and ITG levels might contribute to earlier metastatic potential of BRAF(V600E)-positive aggressive thyroid cancers, and allow improved patient selection for clinical trials.

CD47 deficiency confers cell and tissue radioprotection by activation of autophagy

Accidental or therapeutic exposure to ionizing radiation has severe physiological consequences and can result in cell death. We previously demonstrated that deficiency or blockade of the ubiquitously expressed receptor CD47 results in remarkable cell and tissue protection against ischemic and radiation stress. Antagonists of CD47 or its ligand THBS1/thrombospondin 1 enhance cell survival and preserve their proliferative capacity. However the signaling pathways that mediate this cell-autonomous radioprotection are unclear. We now report a marked increase in autophagy in irradiated T-cells and endothelial cells lacking CD47. Irradiated T cells lacking CD47 exhibit significant increases in formation of autophagosomes comprising double-membrane vesicles visualized by electron microscopy and numbers of MAP1LC3A/B(+) puncta. Moreover, we observed significant increases in BECN1, ATG5, ATG7 and a reduction in SQSTM1/p62 expression relative to irradiated wild-type T cells. We observed similar increases in autophagy gene expression in mice resulting from blockade of CD47 in combination with total body radiation. Pharmacological or siRNA-mediated inhibition of autophagy selectively sensitized CD47-deficient cells to radiation, indicating that enhanced autophagy is necessary for the prosurvival response to CD47 blockade. Moreover, re-expression of CD47 in CD47-deficient T cells sensitized these cells to death by ionizing radiation and reversed the increase in autophagic flux associated with survival. This study indicates that CD47 deficiency confers cell survival through the activation of autophagic flux and identifies CD47 blockade as a pharmacological route to modulate autophagy for protecting tissue from radiation injury.

Thrombospondin-1 regulates blood flow via CD47 receptor-mediated activation of NADPH oxidase 1

OBJECTIVE

Although the matricellular protein thrombospondin-1 (TSP1) is highly expressed in the vessel wall in response to injury, its pathophysiological role in the development of vascular disease is poorly understood. This study was designed to test the hypothesis that TSP1 stimulates reactive oxygen species production in vascular smooth muscle cells and induces vascular dysfunction by promoting oxidative stress.

METHODS AND RESULTS

Nanomolar concentrations of TSP1 found in human vascular disease robustly stimulated superoxide (O(2)(•-)) levels in vascular smooth muscle cells at both cellular and tissue level as measured by cytochrome c and electron paramagnetic resonance. A peptide mimicking the C terminus of TSP1 known to specifically bind CD47 recapitulated this response. Transcriptional knockdown of CD47 and a monoclonal inhibitory CD47 antibody abrogated TSP1-triggered O(2)(•-) in vitro and ex vivo. TSP1 treatment of vascular smooth muscle cells activated phospholipase C and protein kinase C, resulting in phosphorylation of the NADPH oxidase organizer subunit p47(phox) and subsequent Nox1 activation, leading to impairment of arterial vasodilatation ex vivo. Further, we observed that blockade of CD47 and NADPH oxidase 1 gene silencing in vivo in rats improves TSP1-induced impairment of tissue blood flow after ischemia reperfusion.

CONCLUSIONS

Our data suggest a highly regulated process of reactive oxygen species stimulation and blood flow regulation promoted through a direct TSP1/CD47-mediated activation of Nox1. This is the first report, to our knowledge, of a matricellular protein acting as a ligand for NADPH oxidase activation and through specific engagement of integrin-associated protein CD47.

Activated CD47 regulates multiple vascular and stress responses: implications for acute kidney injury and its management

Ischemia-reperfusion injury (IRI) remains a significant source of early and delayed renal transplant failure. Therapeutic interventions have yet to resolve this ongoing clinical challenge although the reasons for this remain unclear. The cell surface receptor CD47 is widely expressed on vascular cells and in tissues. It has one known soluble ligand, the stress-released matricellular protein thrombospondin-1 (TSP1). The TSP1-CD47 ligand receptor axis controls a number of important cellular processes, inhibiting survival factors such as nitric oxide, cGMP, cAMP, and VEGF, while activating injurious pathways such as production of reactive oxygen species. A role of CD47 in renal IRI was recently revealed by the finding that the TSP1-CD47 axis is induced in renal tubular epithelial cells (RTEC) under hypoxia and following IRI. The absence of CD47 in knockout mice increases survival, mitigates RTEC damage, and prevents subsequent kidney failure. Conversely, therapeutic blockade of TSP1-CD47 signaling provides these same advantages to wild-type animals. Together, these findings suggest an important role for CD47 in renal IRI as a proximate promoter of injury and as a novel therapeutic target.

An armed oncolytic herpes simplex virus expressing thrombospondin-1 has an enhanced in vivo antitumor effect against human gastric cancer

Advanced gastric cancer is a common disease, but the conventional treatments are unsatisfactory because of the high recurrence rate. One of the promising new therapies is oncolytic virotherapy, using oncolytic herpes simplex viruses (HSVs). Thrombospondin-1 (TSP-1) suppresses tumor progression via multiple mechanisms including antiangiogenesis. Our approach to enhance the effects of oncolytic HSVs is to generate an armed oncolytic HSV that combines the direct viral oncolysis with TSP-1-mediated function for gastric cancer treatment. Using the bacterial artificial chromosome (BAC) system, a 3rd generation oncolytic HSV (T-TSP-1) expressing human TSP-1 was constructed for human gastric cancer treatment. The enhanced efficacy of T-TSP-1 was determined in both human gastric cancer cell lines in vitro and subcutaneous tumor xenografts of human gastric cancer cells in vivo. In addition, we examined the apoptotic effect of T-TSP-1 in vitro, and the antiangiogenic effect of T-TSP-1 in vivo compared with a non-armed 3rd generation oncolytic HSV, T-01. No apparent apoptotic induction by T-TSP-1 was observed for human gastric cancer cell lines TMK-1 cells but for MKN1 cells in vitro. Arming the viruses with TSP-1 slightly inhibited their replication in some gastric cancer cell lines, but the viral cytotoxicity was not attenuated. In addition, T-TSP-1 exhibited enhanced therapeutic efficacy and inhibition of angiogenesis compared with T-01 in vivo. In this study, we established a novel armed oncolytic HSV, T-TSP-1, which enhanced the antitumor efficacy by providing a combination of direct viral oncolysis with antiangiogenesis. Arming oncolytic HSVs may be a useful therapeutic strategy for gastric cancer therapy.

Programmed cell removal: a new obstacle in the road to developing cancer

The development of cancer involves mechanisms by which aberrant cells overcome normal regulatory pathways that limit their numbers and their migration. The evasion of programmed cell death is one of several key early events that need to be overcome in the progression from normal cellular homeostasis to malignant transformation. Recently, we provided evidence in mouse and human cancers that successful cancer clones must also overcome programmed cell removal. In this Opinion article, we explore the role of programmed cell removal in both normal and neoplastic cells, and we place this pathway in the context of the initiation of programmed cell death.

The recognition of N-glycans by the lectin ArtinM mediates cell death of a human myeloid leukemia cell line

ArtinM, a d-mannose-binding lectin from Artocarpus heterophyllus (jackfruit), interacts with N-glycosylated receptors on the surface of several cells of hematopoietic origin, triggering cell migration, degranulation, and cytokine release. Because malignant transformation is often associated with altered expression of cell surface glycans, we evaluated the interaction of ArtinM with human myelocytic leukemia cells and investigated cellular responses to lectin binding. The intensity of ArtinM binding varied across 3 leukemia cell lines: NB4>K562>U937. The binding, which was directly related to cell growth suppression, was inhibited in the presence of Manα1-3(Manα1-6)Manβ1, and was reverted in underglycosylated NB4 cells. ArtinM interaction with NB4 cells induced cell death (IC₅₀ = 10 µg/mL), as indicated by cell surface exposure of phosphatidylserine and disruption of mitochondrial membrane potential unassociated with caspase activation or DNA fragmentation. Moreover, ArtinM treatment of NB4 cells strongly induced reactive oxygen species generation and autophagy, as indicated by the detection of acidic vesicular organelles in the treated cells. NB4 cell death was attributed to ArtinM recognition of the trimannosyl core of N-glycans containing a ß1,6-GlcNAc branch linked to α1,6-mannose. This modification correlated with higher levels of N-acetylglucosaminyltransferase V transcripts in NB4 cells than in K562 or U937 cells. Our results provide new insights into the potential of N-glycans containing a β1,6-GlcNAc branch linked to α1,6-mannose as a novel target for anti-leukemia treatment.

Therapeutic Targeting of CD47 to Modulate Tissue Responses to Ischemia and Radiation

CD47 is a widely expressed cell surface receptor that serves as a counter-receptor for signal regulatory protein-α and as a receptor for the secreted matricellular protein thrombospondin-1. Thrombospondin-1 signaling through CD47 regulates cellular signaling pathways that control cell survival, growth, motility, mitochondrial biogenesis, arterial vasoactive responses to physiologic vasodilators and blood flow, and responsiveness to growth factors. Studies employing mice lacking either thrombospondin-1 or CD47 have revealed an important role for this receptor-ligand interaction in tissue responses to injury and stress. These null mice show enhanced recovery from soft tissue fixed ischemic injuries, ischemia reperfusion injuries, and radiation injuries. These studies have led to development of antisense strategies to locally or globally suppress CD47 gene expression. A translation-blocking CD47 morpholino improves tissue survival in skin flap and hindlimb fixed ischemia models, full thickness skin grafts, and a liver ischemia/reperfusion model of organ transplantation in mice. Furthermore, the benefits of morpholino treatment extend to aged mice and mice with dysregulated fat metabolism that characteristically exhibit impaired recovery from ischemic injuries. Activity of the morpholino was also demonstrated for treatment of ischemic injury in miniature pigs. Treatment with the CD47 morpholino protects mice from major effects of ionizing radiation including alopecia, deterioration of muscle function, soft tissue and cutaneous fibrosis, and loss of hematopoietic stem cells in bone marrow. Remarkably, the same treatment does not protect tumors but instead enhances their ablation by irradiation. We discuss prospects for further development of CD47 antisense therapeutics for clinical applications including reconstructive surgery, organ transplantation, angioplasty, and cancer.

CD47 knockout mice exhibit improved recovery from spinal cord injury

Recent data have implicated thrombospondin-1 (TSP-1) signaling in the acute neuropathological events that occur in microvascular endothelial cells (ECs) following spinal cord injury (SCI) (Benton et al., 2008b). We hypothesized that deletion of TSP-1 or its receptor CD47 would reduce these pathological events following SCI. CD47 is expressed in a variety of tissues, including vascular ECs and neutrophils. CD47 binds to TSP-1 and inhibits angiogenesis. CD47 also binds to the signal regulatory protein (SIRP)α and facilitates neutrophil diapedesis across ECs to sites of injury. After contusive SCI, TSP-1(-/-) mice did not show functional improvement compared to wildtype (WT) mice. CD47(-/-) mice, however, exhibited functional locomotor improvements and greater white matter sparing. Whereas targeted deletion of either CD47 or TSP-1 improved acute epicenter vascularity in contused mice, only CD47 deletion reduced neutrophil diapedesis and increased microvascular perfusion. An ex vivo model of the CNS microvasculature revealed that CD47(-/-)-derived microvessels (MVs) prominently exhibit adherent WT or CD47(-/-) neutrophils on the endothelial lumen, whereas WT-derived MVs do not. This implicates a defect in diapedesis mediated by the loss of CD47 expression on ECs. In vitro transmigration assays confirmed the role of SIRPα in neutrophil diapedesis through EC monolayers. We conclude that CD47 deletion modestly, but significantly, improves functional recovery from SCI via an increase in vascular patency and a reduction of SIRPα-mediated neutrophil diapedesis, rather than the abrogation of TSP-1-mediated anti-angiogenic signaling.

Thrombospondin-1 as a Paradigm for the Development of Antiangiogenic Agents Endowed with Multiple Mechanisms of Action

Uncontrolled neovascularization occurs in several angiogenesis-dependent diseases, including cancer. Neovascularization is tightly controlled by the balance between angiogenic growth factors and antiangiogenic agents. The various natural angiogenesis inhibitors identified so far affect neovascularization by different mechanisms of action. Thrombospondin-1 (TSP-1) is a matricellular modular glycoprotein that acts as a powerful endogenous inhibitor of angiogenesis. It acts both indirectly, by sequestering angiogenic growth factors and effectors in the extracellular environment, and directly, by inducing an antiangiogenic program in endothelial cells following engagement of specific receptors including CD36, CD47, integrins and proteoglycans (all involved in angiogenesis ). In view of its central, multifaceted role in angiogenesis, TSP-1 has served as a source of antiangiogenic tools, including TSP-1 fragments, synthetic peptides and peptidomimetics, gene therapy strategies, and agents that up-regulate TSP-1 expression. This review discusses TSP-1-based inhibitors of angiogenesis, their mechanisms of action and therapeutic potential, drawing our experience with angiogenic growth factor-interacting TSP-1 peptides, and the possibility of exploiting them to design novel antiangiogenic agents.

Thrombospondin-1/CD47 blockade following ischemia-reperfusion injury is tissue protective

BACKGROUND

Nitric oxide has prosurvival effects that can limit ischemia-reperfusion injuries. However, the matrix glycoprotein thrombospondin-1 is induced following ischemia-reperfusion injury and limits nitric oxide signaling by engaging its cell surface receptor CD47. In this article, the authors examine whether postinjury blocking of this inhibitory signal can protect from ischemia-reperfusion injury in a rat flap model.

METHODS

A total of 40 tissue flaps were created in rats based solely on the deep inferior epigastric vessels. Microvascular clamps were used to create 45 minutes of ischemia time to the flaps. The flaps were then treated using a monoclonal antibody to CD47 or an isotype-matched control immunoglobulin G1 5 or 30 minutes after clamp removal. Twenty-four or 72 hours postoperatively, the necrotic area of the flap was determined, and serum, deep inferior epigastric vessels, and flaps were harvested for analysis from five rats in each respective group.

RESULTS

Treatment with a CD47 antibody 5 minutes after reperfusion significantly reduces flap necrosis compared with immunoglobulin G1 control (9 percent versus 43 percent; p < 0.01). The protective effect is even more dramatic when treatment is delayed until 30 minutes after reperfusion (10 percent versus 88 percent for control; p < 0.01). Markers of neutrophil and endothelial cell activation along with total leukocytes are reduced in CD47 antibody-treated flaps, as are tissue malondialdehyde levels. Levels of cyclic guanosine monophosphate are elevated 72 hours postoperatively in the CD47 antibody-treated deep inferior epigastric vessels versus the control flaps.

CONCLUSIONS

Therapies targeting the thrombospondin-1 receptor CD47 offer potential for increasing tissue survival in ischemia-reperfusion injuries. The ability to protect when given after ischemia-reperfusion injury enables a broader clinical applicability.

Neurovascular effects of CD47 signaling: promotion of cell death, inflammation, and suppression of angiogenesis in brain endothelial cells in vitro

The concept of the neurovascular unit emphasizes that common signals and substrates underlie the physiology and pathophysiology of neuronal and endothelial compartments in brain. Recent data suggest that activation of the integrin-associated protein CD47 promotes neuronal cell death. Is it possible that CD47 may also negatively affect cerebral endothelial cells? Exposure of wild-type primary mouse cerebral endothelial cells to the CD47 ligand thrombospondin 1 (TSP-1) induced an increasing amount of cell death, whereas cytotoxicity was significantly decreased in cerebral endothelial cells derived from CD47 knockout mice. The specific CD47-activating peptide, 4N1K, similarly induced cell death in human brain microvascular endothelial cells. Promotion of inflammation was also involved because lower TSP-1 was able to up-regulate the adhesion molecules intercellular adhesion molecule-1 and vascular cell adhesion molecule-1. Finally, CD47 signaling may suppress angiogenesis because 4N1K significantly inhibited endothelial cell migration and tube formation in vitro. We conclude that CD47 signaling can negatively affect the viability and function of cerebral endothelial cells, further supporting the notion that CD47 may be a potential neurovascular target for stroke and brain injury.

All-trans retinoic acid induces Thrombospondin-1 expression in acute promyelocytic leukemia cells though down-regulation of its transcription repressor, c-MYC oncoprotein

Thrombospondin-1 (TSP-1) was found to mediate the therapeutic effects of all-trans retinoic acid (ATRA) for leukemia. The aim of the present study was to evaluate the role of c-MYC, a key transcription factor that contributes to the genesis of many human tumors, in TSP-1 induction by ATRA in acute promyelocytic leukemia (APL). ATRA treatment markedly increased TSP-1 level and inhibited c-MYC expression in NB4 APL leukemic cells compared with controls. Promoter assays indicated that c-MYC responsive element is functional relevant to the induction of TSP-1 promoter activity by ATRA. c-MYC recruitment to TSP-1 promoter was dramatically decreased in NB4 cells following ATRA treatment. shRNA-mediated inhibition of c-MYC resulted in a marked up-regulation of endogenous TSP-1 expression. Moreover, transient over-expression of c-MYC totally abolished TSP-1 induction by ATRA in NB4 cells. Collectively, our results indicate that ATRA induces TSP-1 expression in APL cells though down-regulation of its transcription repressor, c-MYC oncoprotein.

Role of oxidative stress and caspase 3 in CD47-mediated neuronal cell death

CD47 or integrin-associated protein promotes cell death in blood and tumor cells. Recently, CD47 signaling has been identified in neurons as well. In this study, we investigated the role of CD47 in neuronal cell death. Exposure of primary mouse cortical neurons to the CD47 ligand thrombospondin-1 or the specific CD47-activating peptide 4N1K induced cell death. Activation of CD47 elevated levels of active caspase 3 and increased the generation of reactive oxygen species (ROS) in a time-dependent manner. Both ROS scavengers and caspase inhibitors attenuated cell death. But ROS scavenging did not reduce the activation of caspase 3, and combination treatments with a caspase inhibitor plus free radical scavenger did not yield additive protection. Taken together, these data suggest that parallel and redundant pathways of oxidative stress and caspase-mediated cell death are involved. We conclude that CD47 mediates neuronal cell death through caspase-dependent and caspase-independent pathways.

Thrombospondin 1 binding to calreticulin-LRP1 signals resistance to anoikis

Anoikis, apoptotic cell death due to loss of cell adhesion, is critical for regulation of tissue homeostasis in tissue remodeling. Fibrogenesis is associated with reduced fibroblast apoptosis. The matricellular protein thrombospondin 1 (TSP1) regulates cell adhesion and motility during tissue remodeling and in fibrogenesis. The N-terminal domain of TSP1 binds to the calreticulin-LRP1 receptor co-complex to signal down-regulation of cell adhesion and increased cell motility through focal adhesion disassembly. TSP1 signaling through calreticulin-LRP1 activates cell survival signals such as PI3-kinase. Therefore, we tested the hypothesis that TSP1 supports cell survival under adhesion-independent conditions to facilitate tissue remodeling. Here, we show that platelet TSP1, its N-terminal domain (NoC1) as a recombinant protein, or a peptide comprising the calreticulin-LRP1 binding site [amino acids 17-35 (hep I)] in the N-terminal domain promotes fibroblast survival under anchorage-independent conditions. TSP1 activates Akt and decreases apoptotic signaling through caspase 3 and PARP1 in suspended fibroblasts. Inhibition of PI3K/Akt activity blocks TSP1-mediated anchorage-independent survival. Fibroblasts lacking LRP1 or expressing calreticulin lacking the TSP1 binding site do not respond to TSP1 with anchorage-independent survival. These data define a novel role for TSP1 signaling through the calreticulin/LRP1 co-complex in tissue remodeling and fibrotic responses through stimulation of anoikis resistance.-Pallero, M. A., Elzie, C. A., Chen, J., Mosher, D. F., Murphy-Ullrich, J. E. Thrombospondin 1 binding to calreticulin-LRP1 signals resistance to anoikis.

TSP-1 secreted by bone marrow stromal cells contributes to retinal ganglion cell neurite outgrowth and survival

Background

Bone marrow stromal cells (BMSCs) are pluripotent and thereby a potential candidate for cell replacement therapy for central nervous system degenerative disorders and traumatic injury. However, the mechanism of their differentiation and effect on neural tissues has not been fully elucidated. This study evaluates the effect of BMSCs on neural cell growth and survival in a retinal ganglion cell (RGCs) model by assessing the effect of changes in the expression of a BMSC-secreted protein, thrombospondin-1 (TSP-1), as a putative mechanistic agent acting on RGCs.

Methods and Findings

The effect of co-culturing BMSCs and RGCs in vitro was evaluated by measuring the following parameters: neurite outgrowth, RGC survival, BMSC neural-like differentiation, and the effect of TSP-1 on both cell lines under basal secretion conditions and when TSP-1 expression was inhibited. Our data show that BMSCs improved RGC survival and neurite outgrowth. Synaptophysin, MAP-2, and TGF-β expression are up-regulated in RGCs co-cultured with BMSCs. Interestingly, the BMSCs progressively displayed neural-like morphology over the seven-day study period. Restriction display polymerase chain reaction (RD-PCR) was performed to screen for differentially expressed genes in BMSCs cultured alone or co-cultured with RGCs. TSP-1, a multifactorial extracellular matrix protein, is critically important in the formation of neural connections during development, so its function in our co-culture model was investigated by small interfering RNA (siRNA) transfection. When TSP-1 expression was decreased with siRNA silencing, BMSCs had no impact on RGC survival, but reduced neurite outgrowth and decreased expression of synaptophysin, MAP-2 and TGF-β in RGCs. Furthermore, the number of BMSCs with neural-like characteristics was significantly decreased by more than two-fold using siRNA silencing.

Conclusions

Our data suggest that the TSP-1 signaling pathway might have an important role in neural-like differentiation in BMSCs and neurite outgrowth in RGCs. This study provides new insights into the potential reparative mechanisms of neural cell repair.