Differential interactions of thrombospondin-1, -2, and -4 with CD47 and effects on cGMP signaling and ischemic injury responses

Integrating bulk RNA-seq and scRNA-seq analyses revealed the function and clinical value of thrombospondins in colon cancer

Background

Acting as mediators in cell-matrix and cell-cell communication, matricellular proteins play a crucial role in cancer progression. Thrombospondins (TSPs), a type of matricellular glycoproteins, are key regulators in cancer biology with multifaceted roles. Although TSPs have been implicated in anti-tumor immunity and epithelial-mesenchymal transition (EMT) in several malignancies, their specific roles to colon cancer remain elusive. Addressing this knowledge gap is essential, as understanding the function of TSPs in colon cancer could identify new therapeutic targets and prognostic markers.

Methods

Analyzing 1981 samples from 10 high-throughput datasets, including six bulk RNA-seq, three scRNA-seq, and one spatial transcriptome dataset, our study investigated the prognostic relevance, risk stratification value, immune heterogeneity, and cellular origin of TSPs, as well as their influence on cancer-associated fibroblasts (CAFs). Utilizing survival analysis, unsupervised clustering, and functional enrichment, along with multiple correlation analyses of the tumor-microenvironment (TME) via Gene Set Variation Analysis (GSVA), spatial localization, Monocle2, and CellPhoneDB, we provided insights into the clinical and cellular implications of TSPs.

Results

First, we observed significant upregulation of THBS2 and COMP in colon cancer, both of which displayed significant prognostic value. Additionally, we detected a significant positive correlation between TSPs and immune cells, as well as marker genes of EMT. Second, based on TSPs expression, patients were divided into two clusters with distinct prognoses: the high TSPs expression group (TSPs-H) was characterized by pronounced immune and stromal cell infiltration, and notably elevated T-cell exhaustion scores. Subsequently, we found that THBS2 and COMP may be associated with the differentiation of CAFs into pan-iCAFs and pan-dCAFs, which are known for their heightened matrix remodeling activities. Moreover, THBS2 enhanced CAFs communication with vascular endothelial cells and monocyte-macrophages. CAFs expressing THBS2 (THBS2+ CAFs) demonstrated higher scores across multiple signaling pathways, including angiogenic, EMT, Hedgehog, Notch, Wnt, and TGF-β, when compared to THBS2- CAFs. These observations suggest that THBS2 may be associated with stronger pro-carcinogenic activity in CAFs.

Conclusions

This study revealed the crucial role of TSPs and the significant correlation between THBS2 and CAFs interactions in colon cancer progression, providing valuable insights for targeting TSPs to mitigate cancer progression.

Engineering Pancreatic Islets to Transiently Codisplay on Their Surface Thrombomodulin and CD47 Immunomodulatory Proteins as a Means of Mitigating Instant Blood-Mediated Inflammatory Reaction following Intraportal Transplantation

Most pancreatic islets are destroyed immediately after intraportal transplantation by an instant blood-mediated inflammatory reaction (IBMIR) generated through activation of coagulation, complement, and proinflammatory pathways. Thus, effective mitigation of IBMIR may be contingent on the combined use of agents targeting these pathways for modulation. CD47 and thrombomodulin (TM) are two molecules with distinct functions in regulating coagulation and proinflammatory responses. We previously reported that the islet surface can be modified with biotin for transient display of novel forms of these two molecules chimeric with streptavidin (SA), that is, thrombomodulin chimeric with SA (SA-TM) and CD47 chimeric with SA (SA-CD47), as single agents with improved engraftment following intraportal transplantation. This study aimed to test whether islets can be coengineered with SA-TM and SA-CD47 molecules as a combinatorial approach to improve engraftment by inhibiting IBMIR. Mouse islets were effectively coengineered with both molecules without a detectable negative impact on their viability and metabolic function. Coengineered islets were refractory to destruction by IBMIR ex vivo and showed enhanced engraftment and sustained function in a marginal mass syngeneic intraportal transplantation model. Improved engraftment correlated with a reduction in intragraft innate immune infiltrates, particularly neutrophils and M1 macrophages. Moreover, transcripts for various intragraft procoagulatory and proinflammatory agents, including tissue factor, HMGB1 (high-mobility group box-1), IL-1β, IL-6, TNF-α, IFN-γ, and MIP-1α, were significantly reduced in coengineered islets. These data demonstrate that the transient codisplay of SA-TM and SA-CD47 proteins on the islet surface is a facile and effective platform to modulate procoagulatory and inflammatory responses with implications for both autologous and allogeneic islet transplantation.

Differential regulation by CD47 and thrombospondin-1 of extramedullary erythropoiesis in mouse spleen

Extramedullary erythropoiesis is not expected in healthy adult mice, but erythropoietic gene expression was elevated in lineage-depleted spleen cells from cd47-/- mice. Expression of several genes associated with early stages of erythropoiesis was elevated in mice lacking CD47 or its signaling ligand thrombospondin-1, consistent with previous evidence that this signaling pathway inhibits expression of multipotent stem cell transcription factors in spleen. In contrast, cells expressing markers of committed erythroid progenitors were more abundant in cd47-/- spleens but significantly depleted in thbs1-/- spleens. Single cell transcriptome and flow cytometry analyses indicated that loss of CD47 is associated with accumulation and increased proliferation in spleen of Ter119-CD34+ progenitors and Ter119+CD34- committed erythroid progenitors with elevated mRNA expression of Kit, Ermap, and Tfrc. Induction of committed erythroid precursors is consistent with the known function of CD47 to limit the phagocytic removal of aged erythrocytes. Conversely, loss of thrombospondin-1 delays the turnover of aged red blood cells, which may account for the suppression of committed erythroid precursors in thbs1-/- spleens relative to basal levels in wild type mice. In addition to defining a role for CD47 to limit extramedullary erythropoiesis, these studies reveal a thrombospondin-1-dependent basal level of extramedullary erythropoiesis in adult mouse spleen.

Emerging functions of thrombospondin-1 in immunity

Thrombospondin-1 is a secreted matricellular glycoprotein that modulates cell behavior by interacting with components of the extracellular matrix and with several cell surface receptors. Its presence in the extracellular matrix is induced by injuries that cause thrombospondin-1 release from platelets and conditions including hyperglycemia, ischemia, and aging that stimulate its expression by many cell types. Conversely, rapid receptor-mediated clearance of thrombospondin-1 from the extracellular space limits its sustained presence in the extracellular space and maintains sub-nanomolar physiological concentrations in blood plasma. Roles for thrombospondin-1 signaling, mediated by specific cellular receptors or by activation of latent TGFβ, have been defined in T and B lymphocytes, natural killer cells, macrophages, neutrophils, and dendritic cells. In addition to regulating physiological nitric oxide signaling and responses of cells to stress, studies in mice lacking thrombospondin-1 or its receptors have revealed important roles for thrombospondin-1 in regulating immune responses in infectious and autoimmune diseases and antitumor immunity.

Thrombospondins modulate cell function and tissue structure in the skeleton

Thrombospondins (TSPs) belong to a functional class of ECM proteins called matricellular proteins that are not primarily structural, but instead influence cellular interactions within the local extracellular environment. The 3D arrangement of TSPs allow interactions with other ECM proteins, sequestered growth factors, and cell surface receptors. They are expressed in mesenchymal condensations and limb buds during skeletal development, but they are not required for patterning. Instead, when absent, there are alterations in musculoskeletal connective tissue ECM structure, organization, and function, as well as altered skeletal cell phenotypes. Both functional redundancies and unique contributions to musculoskeletal tissue structure and physiology are revealed in mouse models with compound TSP deletions. Crucial roles of individual TSPs are revealed during musculoskeletal injury and regeneration. The interaction of TSPs with mesenchymal stem cells (MSC), and their influence on cell fate, function, and ultimately, musculoskeletal phenotype, suggest that TSPs play integral, but as yet poorly understood roles in musculoskeletal health. Here, unique and overlapping contributions of trimeric TSP1/2 and pentameric TSP3/4/5 to musculoskeletal cell and matrix physiology are reviewed. Opportunities for new research are also noted.

Tolerating CD47

Cluster of differentiation 47 (CD47) occupies the outer membrane of human cells, where it binds to soluble and cell surface receptors on the same and other cells, sculpting their topography and resulting in a pleiotropic receptor-multiligand interaction network. It is a focus of drug development to temper and accentuate CD47-driven immune cell liaisons, although consideration of on-target CD47 effects remain neglected. And yet, a late clinical trial of a CD47-blocking antibody was discontinued, existent trials were restrained, and development of CD47-targeting agents halted by some pharmaceutical companies. At this point, if CD47 can be exploited for clinical advantage remains to be determined. Herein an airing is made of the seemingly conflicting actions of CD47 that reflect its position as a junction connecting receptors and signalling pathways that impact numerous human cell types. Prospects of CD47 boosting and blocking are considered along with potential therapeutic implications for autoimmune diseases and cancer.

The role of CD47 in non-neoplastic diseases

CD47 is a 50 kDa five-spanning membrane receptor that plays a crucial role in multiple cellular processes, including myeloid cell activation, neutrophils transmigration, vascular remodeling, leukocyte adhesion and trans-endothelial migration. Recent studies have revealed that CD47 is a highly expressed anti-phagocytic signal in several types of cancer, and therefore, blocking of CD47 has shown an effective therapeutic potential in cancer immunotherapy. In addition, CD47 has been found to be involved in a complex interplay with microglia and other types of cells, and increasing evidence indicates that CD47 can be targeted as part of immune modulatory strategies for non-neoplastic diseases as well. In this review, we focus on CD47 and its role in non-neoplastic diseases, including neurological disorders, atherosclerosis and autoimmune diseases. In addition, we discuss the major challenges and potential remedies associated with CD47-SIRPα-based immunotherapies.

A metabolic role for CD47 in pancreatic β cell insulin secretion and islet transplant outcomes

Diabetes is a global public health burden and is characterized clinically by relative or absolute insulin deficiency. Therapeutic agents that stimulate insulin secretion and improve insulin sensitivity are in high demand as treatment options. CD47 is a cell surface glycoprotein implicated in multiple cellular functions including recognition of self, angiogenesis, and nitric oxide signaling; however, its role in the regulation of insulin secretion remains unknown. Here, we demonstrate that CD47 receptor signaling inhibits insulin release from human as well as mouse pancreatic β cells and that it can be pharmacologically exploited to boost insulin secretion in both models. CD47 depletion stimulated insulin granule exocytosis via activation of the Rho GTPase Cdc42 in β cells and improved glucose clearance and insulin sensitivity in vivo. CD47 blockade enhanced syngeneic islet transplantation efficiency and expedited the return to euglycemia in streptozotocin-induced diabetic mice. Further, anti-CD47 antibody treatment delayed the onset of diabetes in nonobese diabetic (NOD) mice and protected them from overt diabetes. Our findings identify CD47 as a regulator of insulin secretion, and its manipulation in β cells offers a therapeutic opportunity for diabetes and islet transplantation by correcting insulin deficiency.

The TSP1-CD47-SIRPα interactome: an immune triangle for the checkpoint era

The use of treatments, such as programmed death protein 1 (PD1) or cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) antibodies, that loosen the natural checks upon immune cell activity to enhance cancer killing have shifted clinical practice and outcomes for the better. Accordingly, the number of antibodies and engineered proteins that interact with the ligand-receptor components of immune checkpoints continue to increase along with their use. It is tempting to view these molecular pathways simply from an immune inhibitory perspective. But this should be resisted. Checkpoint molecules can have other cardinal functions relevant to the development and use of blocking moieties. Cell receptor CD47 is an example of this. CD47 is found on the surface of all human cells. Within the checkpoint paradigm, non-immune cell CD47 signals through immune cell surface signal regulatory protein alpha (SIRPα) to limit the activity of the latter, the so-called trans signal. Even so, CD47 interacts with other cell surface and soluble molecules to regulate biogas and redox signaling, mitochondria and metabolism, self-renewal factors and multipotency, and blood flow. Further, the pedigree of checkpoint CD47 is more intricate than supposed. High-affinity interaction with soluble thrombospondin-1 (TSP1) and low-affinity interaction with same-cell SIRPα, the so-called cis signal, and non-SIRPα ectodomains on the cell membrane suggests that multiple immune checkpoints converge at and through CD47. Appreciation of this may provide latitude for pathway-specific targeting and intelligent therapeutic effect.

Combining Multikinase Tyrosine Kinase Inhibitors Targeting the Vascular Endothelial Growth Factor and Cluster of Differentiation 47 Signaling Pathways Is Predicted to Increase the Efficacy of Antiangiogenic Combination Therapies

Angiogenesis is a critical step in tumor growth, development, and invasion. Nascent tumor cells secrete vascular endothelial growth factor (VEGF) that significantly remodels the tumor microenvironment through interaction with multiple receptors on vascular endothelial cells, including type 2 VEGF receptor (VEGFR2). The complex pathways initiated by VEGF binding to VEGFR2 lead to enhanced proliferation, survival, and motility of vascular endothelial cells and formation of a new vascular network, enabling tumor growth. Antiangiogenic therapies that inhibit VEGF signaling pathways were among the first drugs that targeted stroma rather than tumor cells. Despite improvements in progression-free survival and higher response rates relative to chemotherapy in some types of solid tumors, the impact on overall survival (OS) has been limited, with the majority of tumors eventually relapsing due to resistance or activation of alternate angiogenic pathways. Here, we developed a molecularly detailed computational model of endothelial cell signaling and angiogenesis-driven tumor growth to investigate combination therapies targeting different nodes of the endothelial VEGF/VEGFR2 signaling pathway. Simulations predicted a strong threshold-like behavior in extracellular signal-regulated kinases 1/2 (ERK1/2) activation relative to phosphorylated VEGFR2 levels, as continuous inhibition of at least 95% of receptors was necessary to abrogate phosphorylated ERK1/2 (pERK1/2). Combinations with mitogen-activated protein kinase/ERK kinase (MEK) and spingosine-1-phosphate inhibitors were found to be effective in overcoming the ERK1/2 activation threshold and abolishing activation of the pathway. Modeling results also identified a mechanism of resistance whereby tumor cells could reduce pERK1/2 sensitivity to inhibitors of VEGFR2 by upregulation of Raf, MEK, and sphingosine kinase 1 (SphK1), thus highlighting the need for deeper investigation of the dynamics of the crosstalk between VEGFR2 and SphK1 pathways. Inhibition of VEGFR2 phosphorylation was found to be more effective at blocking protein kinase B, also known as AKT, activation; however, to effectively abolish AKT activation, simulations identified Axl autophosphorylation or the Src kinase domain as potent targets. Simulations also supported activating cluster of differentiation 47 (CD47) on endothelial cells as an effective combination partner with tyrosine kinase inhibitors to inhibit angiogenesis signaling and tumor growth. Virtual patient simulations supported the effectiveness of CD47 agonism in combination with inhibitors of VEGFR2 and SphK1 pathways. Overall, the rule-based system model developed here provides new insights, generates novel hypothesis, and makes predictions regarding combinations that may enhance the OS with currently approved antiangiogenic therapies.

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.

Targeting the CD47/thrombospondin-1 signaling axis regulates immune cell bioenergetics in the tumor microenvironment to potentiate antitumor immune response

BACKGROUND

CD47 is an integral membrane protein that alters adaptive immunosurveillance when bound to the matricellular glycoprotein thrombospondin-1 (TSP1). We examined the impact of the CD47/TSP1 signaling axis on melanoma patient response to anti-PD-1 therapy due to alterations in T cell activation, proliferation, effector function, and bioenergetics.

METHODS

A syngeneic B16 mouse melanoma model was performed to determine if targeting CD47 as monotherapy or in combination with anti-PD-1 impacted tumor burden. Cytotoxic (CD8+) T cells from Pmel-1 transgenic mice were used for T cell activation, cytotoxic T lymphocyte, and cellular bioenergetic assays. Single-cell RNA-sequencing, ELISA, and flow cytometry was performed on peripheral blood mononuclear cells and plasma of melanoma patients receiving anti-PD-1 therapy to examine CD47/TSP1 expression.

RESULTS

Human malignant melanoma tissue had increased CD47 and TSP1 expression within the tumor microenvironment compared with benign tissue. Due to the negative implications CD47/TSP1 can have on antitumor immune responses, we targeted CD47 in a melanoma model and observed a decrease in tumor burden due to increased tumor oxygen saturation and granzyme B secreting CD8+ T cells compared with wild-type tumors. Additionally, Pmel-1 CD8+ T cells exposed to TSP1 had reduced activation, proliferation, and effector function against B16 melanoma cells. Targeting CD47 allowed CD8+ T cells to overcome this TSP1 interaction to sustain these functions. TSP1 exposed CD8+ T cells have a decreased rate of glycolysis; however, targeting CD47 restored glycolysis when CD8+ T cells were exposed to TSP1, suggesting CD47 mediated metabolic reprogramming of T cells. Additionally, non-responding patients to anti-PD-1 therapy had increased T cells expressing CD47 and circulating levels of TSP1 compared with responding patients. Since CD47/TSP1 signaling axis negatively impacts CD8+ T cells and non-responding patients to anti-PD-1 therapy have increased CD47/TSP1 expression, we targeted CD47 in combination with anti-PD-1 in a melanoma model. Targeting CD47 in combination with anti-PD-1 treatment further decreased tumor burden compared with monotherapy and control.

CONCLUSION

CD47/TSP1 expression could serve as a marker to predict patient response to immune checkpoint blockade treatment, and targeting this pathway may preserve T cell activation, proliferation, effector function, and bioenergetics to reduce tumor burden as a monotherapy or in combination with anti-PD-1.

Single vesicle analysis of CD47 association with integrins and tetraspanins on extracellular vesicles released by T lymphoblast and prostate carcinoma cells

CD47 regulates the trafficking of specific coding and noncoding RNAs into extracellular vesicles (EVs), and the RNA contents of CD47+ EVs differ from that of CD63+ EVs released by the same cells. Single particle interferometric reflectance imaging sensing combined with immunofluorescent imaging was used to analyse the colocalization of tetraspanins, integrins, and CD47 on EVs produced by wild type and CD47-deficient Jurkat T lymphoblast and PC3 prostate carcinoma cell lines. On Jurkat cell-derived EVs, β1 and α4 integrin subunits colocalized predominantly with CD47 and CD81 but not with CD63 and CD9, conserving the known lateral interactions between these proteins in the plasma membrane. Although PC3 cell-derived EVs lacked detectable α4 integrin, specific association of CD81 with β1 and CD47 was preserved. Loss of CD47 expression in Jurkat cells significantly reduced β1 and α4 levels on EVs produced by these cells while elevating CD9+ , CD63+ , and CD81+ EVs. In contrast, loss of CD47 in PC3 cells decreased the abundance of CD63+ and CD81+ EVs. These data establish that CD47+ EVs are mostly distinct from EVs bearing the tetraspanins CD63 and CD9, but CD47 also indirectly regulates the abundance of EVs bearing these non-interacting tetraspanins via mechanisms that remain to be determined.

CD47 as a promising therapeutic target in oncology

CD47 is ubiquitously expressed on the surface of cells and plays a critical role in self-recognition. By interacting with SIRPα, TSP-1 and integrins, CD47 modulates cellular phagocytosis by macrophages, determines life span of individual erythrocytes, regulates activation of immune cells, and manipulates synaptic pruning during neuronal development. As such, CD47 has recently be regarded as one of novel innate checkpoint receptor targets for cancer immunotherapy. In this review, we will discuss increasing awareness about the diverse functions of CD47 and its role in immune system homeostasis. Then, we will discuss its potential therapeutic roles against cancer and outlines, the possible future research directions of CD47- based therapeutics against cancer.

Neuronal GPR81 regulates developmental brain angiogenesis and promotes brain recovery after a hypoxic ischemic insult

Perinatal hypoxic/ischemic (HI) brain injury is a major clinical problem with devastating neurodevelopmental outcomes in neonates. During HI brain injury, dysregulated factor production contributes to microvascular impairment. Glycolysis-derived lactate accumulated during ischemia has been proposed to protect against ischemic injury, but its mechanism of action is poorly understood. Herein, we hypothesize that lactate via its G-protein coupled receptor (GPR81) controls postnatal brain angiogenesis and plays a protective role after HI injury. We show that GPR81 is predominantly expressed in neurons of the cerebral cortex and hippocampus. GPR81-null mice displayed a delay in cerebral microvascular development linked to reduced levels of various major angiogenic factors and augmented expression of anti-angiogenic Thrombospondin-1 (TSP-1) in comparison to their WT littermates. Coherently, lactate stimulation induced an increase in growth factors (VEGF, Ang1 and 2, PDGF) and reduced TSP-1 expression in neurons, which contributed to accelerating angiogenesis. HI injury in GPR81-null animals curtailed vascular density and consequently increased infarct size compared to changes seen in WT mice; conversely intracerebroventricular lactate injection increased vascular density and diminished infarct size in WT but not in GPR81-null mice. Collectively, we show that lactate acting via GPR81 participates in developmental brain angiogenesis, and attenuates HI injury by restoring compromised microvasculature.

[Research Progress of CD47-Related Signaling Pathway and the Role of CD47 in Pathogenic Infection]

CD47, a transmembrane glycoprotein widely expressed on the cell surface, is one of the important checkpoints through which cells escape innate immune surveillance. The important role of CD47-related signaling pathway and changes in expression level in immune regulation, pathogen infection and anti-tumor immunity has gradually come to be recognized. We reviewed herein the structure and biological characteristics of CD47, the interaction and the downstream signaling of CD47 with integrin, thrombospondin 1, and signal regulatory protein, and the upregulated expression of CD47 induced by the infection of different pathogens and the role of CD47 in different types of immune response to infection. Discussions were made regarding the prospective application of CD47 targeted immunotherapy in pathogenic infection-related cancers, intending to provide guidance for future research.

Fatty Acids, CD36, Thrombospondin-1, and CD47 in Glioblastoma: Together and/or Separately?

Glioblastoma (GBM) is one of the most aggressive tumors of the central nervous system, characterized by a wide range of inter- and intratumor heterogeneity. Accumulation of fatty acids (FA) metabolites was associated with a low survival rate in high-grade glioma patients. The diversity of brain lipids, especially polyunsaturated fatty acids (PUFAs), is greater than in all other organs and several classes of proteins, such as FA transport proteins (FATPs), and FA translocases are considered principal candidates for PUFAs transport through BBB and delivery of PUFAs to brain cells. Among these, the CD36 FA translocase promotes long-chain FA uptake as well as oxidated lipoproteins. Moreover, CD36 binds and recognizes thrombospondin-1 (TSP-1), an extracellular matrix protein that was shown to play a multifaceted role in cancer as part of the tumor microenvironment. Effects on tumor cells are mediated by TSP-1 through the interaction with CD36 as well as CD47, a member of the immunoglobulin superfamily. TSP-1/CD47 interactions have an important role in the modulation of glioma cell invasion and angiogenesis in GBM. Separately, FA, the two membrane receptors CD36, CD47, and their joint ligand TSP-1 all play a part in GBM pathogenesis. The last research has put in light their interconnection/interrelationship in order to exert a cumulative effect in the modulation of the GBM molecular network.

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.

Novel Pharmaceutical Strategy for Selective Abrogation of TSP1-Induced Vascular Dysfunction by Decoy Recombinant CD47 Soluble Receptor in Prophylaxis and Treatment Models

Elevated thrombospondin 1 (TSP1) is a prevalent factor, via cognate receptor CD47, in the pathogenesis of cardiovascular conditions, including ischemia-reperfusion injury (IRI) and pulmonary arterial hypertension (PAH). Moreover, TSP1/CD47 interaction has been found to be associated with platelet hyperaggregability and impaired nitric oxide response, exacerbating progression in IRI and PAH. Pathological TSP1 in circulation arises as a target of our novel therapeutic approach. Our “proof-of-concept” pharmacological strategy relies on recombinant human CD47 peptide (rh-CD47p) as a decoy receptor protein (DRP) to specifically bind TSP1 and neutralize TSP1-impaired vasorelaxation, strongly implicated in IRI and PAH. The binding of rh-CD47p and TSP1 was first verified as the primary mechanism via Western blotting and further quantified with modified ELISA, which also revealed a linear molar dose-dependent interaction. Ex vivo, pretreatment protocol with rh-CD47p (rh-CD47p added prior to TSP1 incubation) demonstrated a prophylactic effect against TSP1-impairment of endothelium-dependent vasodilation. Post-treatment set-up (TSP1 incubation prior to rh-CD47p addition), mimicking pre-existing excessive TSP1 in PAH, reversed TSP1-inhibited vasodilation back to control level. Dose titration identified an effective molar dose range (approx. ≥1:3 of tTSP1:rh-CD47p) for prevention of/recovery from TSP1-induced vascular dysfunction. Our results indicate the great potential for proposed novel decoy rh-CD47p-therapy to abrogate TSP1-associated cardiovascular complications, such as PAH.

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 effects of cell surface CD47 downregulation on ischaemia-reperfusion injury during pig liver transplantation

This study aimed to investigate the effect of cell surface CD47 downregulation on ischaemia-reperfusion injury (IRI) during pig liver transplantation. Blood samples were collected from healthy miniature Bama pigs randomly and equally divided into CD47 antagonism (group A), without CD47 antagonism (group B) and a sham group (group C). Blood samples were collected from groups A and B at 0, 8 and 48 hours after establishment of the new liver through an indwelling tube in the right internal jugular vein. Blood samples were collected at the same time points after liver dissociation from group C. The expression of interleukin-6 (IL-6) and tumour necrosis factor-α (TNF-α) was detected by enzyme-linked immunosorbent assay (ELISA); CD47 expression was detected by Western blot; and liver function indices, including alanine aminotransferase (ALT) and aspartic transaminase (AST), were directly read by an automatic biochemical analyzer. The concentrations of both receptors in group A were significantly lower than groups B and C, at 8 and 48 hours after establishment of blood flow. At 8 and 48 hours in the new liver stage, the values of CD47 levels, ALT and AST in group A were significantly reduced compared to groups B and C (P < 0.05). The levels of CD47 in the three groups were consistent with the trends of the aforementioned observation indicators. The liver functions of the recipients were significantly improved by reducing the release of the inflammatory mediators. This study provides new ideas and intervention approaches for the treatment of IRI in liver transplantation.

Endothelial Cell Behavior Is Determined by Receptor Clustering Induced by Thrombospondin-1

The thrombospondins (TSPs) are a family of multimeric extracellular matrix proteins that dynamically regulate cellular behavior and response to stimuli. In so doing, the TSPs directly and indirectly affect biological processes such as embryonic development, wound healing, immune response, angiogenesis, and cancer progression. Many of the direct effects of Thrombospondin 1 (TSP-1) result from the engagement of a wide range of cell surface receptors including syndecans, low density lipoprotein receptor-related protein 1 (LRP1), CD36, integrins, and CD47. Different or even opposing outcomes of TSP-1 actions in certain pathologic contexts may occur, depending on the structural/functional domain involved. To expedite response to external stimuli, these receptors, along with vascular endothelial growth factor receptor 2 (VEGFR2) and Src family kinases, are present in specific membrane microdomains, such as lipid rafts or tetraspanin-enriched microdomains. The molecular organization of these membrane microdomains and their constituents is modulated by TSP-1. In this review, we will describe how the presence of TSP-1 at the plasma membrane affects endothelial cell signal transduction and angiogenesis.

Inhibiting Th1/2 cells influences hepatic capillarization by adjusting sinusoidal endothelial fenestrae through Rho-ROCK-myosin pathway

CD4⁺ T cells are considered to be vital in chronic liver diseases, but their exact roles in hepatic capillarization, the typical characteristic of liver fibrosis, are poorly understood. This study aimed to assess the roles of typical subtype of CD4⁺ T cells, named T helper 1 (Th1) and Th2 cells in liver fibrosis. Taking advantage of well established fibrotic rat model, we conducted in vitro and in vivo experiments to explore the interactions between liver sinusoidal endothelial cells (LSECs) and Th1/2 cells; meanwhile we evaluated the degree of hepatic capillarization when inhibiting these interactions with inhibitory antibodies. Our results showed that prohibiting interactions between Th2 cells and LSECs caused the restoration of fenestrae, increased cytokine level of Th1 cells and reduction of hepatic capillarization; inhibiting the interaction between Th1 cells and LSECs produced the opposite effects. Moreover, increased Rho and myosin light chain phosphorylation were observed when Th1 cells were inhibited with the corresponding inhibitory antibody; Th2 cell inhibition yielded the opposite results. This study indicated that Th1/2 cells steer the capillarization process in different directions and this effect is probably mediated by the Rho-Rho kinase (ROCK)-myosin signaling pathway.

CD47 (Cluster of Differentiation 47)

CD47, also known as integrin-associated protein, is a constitutively and ubiquitously expressed transmembrane receptor. CD47 is conserved across amniotes including mammals, reptiles, and birds. Expression is increased in many cancers and, in non-malignant cells, by stress and with aging. The up-regulation of CD47 expression is generally epigenetic, whereas gene amplification occurs with low frequency in some cancers. CD47 is a high affinity signaling receptor for the secreted protein thrombospondin-1 (THBS1) and the counter-receptor for signal regulatory protein-α (SIRPA, SIRPα) and SIRPγ (SIRPG). CD47 interaction with SIRPα serves as a marker of self to innate immune cells and thereby protects cancer cells from phagocytic clearance. Consequently, higher CD47 correlates with a poor prognosis in some cancers, and therapeutic blockade can suppress tumor growth by enhancing innate antitumor immunity. CD47 expressed on cytotoxic T cells, dendritic cells, and NK cells mediates inhibitory THBS1 signaling that further limits antitumor immunity. CD47 laterally associates with several integrins and thereby regulates cell adhesion and migration. CD47 has additional lateral binding partners in specific cell types, and ligation of CD47 in some cases modulates their function. THBS1-CD47 signaling in non-malignant cells inhibits nitric oxide/cGMP, calcium, and VEGF signaling, mitochondrial homeostasis, stem cell maintenance, protective autophagy, and DNA damage response, and promotes NADPH oxidase activity. CD47 signaling is a physiological regulator of platelet activation, angiogenesis and blood flow. THBS1/CD47 signaling is frequently dysregulated in chronic diseases.

Aging-associated changes in CD47 arrangement and interaction with thrombospondin-1 on red blood cells visualized by super-resolution imaging

CD47 serves as a ligand for signaling regulatory protein α (SIRPα) and as a receptor for thrombospondin-1 (TSP-1). Although CD47, TSP-1, and SIRPα are thought to be involved in the clearance of aged red blood cells (RBCs), aging-associated changes in the expression and interaction of these molecules on RBCs have been elusive. Using direct stochastic optical reconstruction microscopy (dSTORM)-based imaging and quantitative analysis, we can report that CD47 molecules on young RBCs reside as nanoclusters with little binding to TSP-1, suggesting a minimal role for TSP-1/CD47 signaling in normal RBCs. On aged RBCs, CD47 molecules decreased in number but formed bigger and denser clusters, with increased ability to bind TSP-1. Exposure of aged RBCs to TSP-1 resulted in a further increase in the size of CD47 clusters via a lipid raft-dependent mechanism. Furthermore, CD47 cluster formation was dramatically inhibited on thbs1-/- mouse RBCs and associated with a significantly prolonged RBC lifespan. These results indicate that the strength of CD47 binding to its ligand TSP-1 is predominantly determined by the distribution pattern and not the amount of CD47 molecules on RBCs, and offer direct evidence for the role of TSP-1 in phagocytosis of aged RBCs. This study provides clear nanoscale pictures of aging-associated changes in CD47 distribution and TSP-1/CD47 interaction on the cell surface, and insights into the molecular basis for how these molecules coordinate to remove aged RBCs.

The 10q26 Risk Haplotype of Age-Related Macular Degeneration Aggravates Subretinal Inflammation by Impairing Monocyte Elimination

A minor haplotype of the 10q26 locus conveys the strongest genetic risk for age-related macular degeneration (AMD). Here, we examined the mechanisms underlying this susceptibility. We found that monocytes from homozygous carriers of the 10q26 AMD-risk haplotype expressed high amounts of the serine peptidase HTRA1, and HTRA1 located to mononuclear phagocytes (MPs) in eyes of non-carriers with AMD. HTRA1 induced the persistence of monocytes in the subretinal space and exacerbated pathogenic inflammation by hydrolyzing thrombospondin 1 (TSP1), which separated the two CD47-binding sites within TSP1 that are necessary for efficient CD47 activation. This HTRA1-induced inhibition of CD47 signaling induced the expression of pro-inflammatory osteopontin (OPN). OPN expression increased in early monocyte-derived macrophages in 10q26 risk carriers. In models of subretinal inflammation and AMD, OPN deletion or pharmacological inhibition reversed HTRA1-induced pathogenic MP persistence. Our findings argue for the therapeutic potential of CD47 agonists and OPN inhibitors for the treatment of AMD.

Role of CD47 in Hematological Malignancies

CD47, or integrin-associated protein, is a cell surface ligand expressed in low levels by nearly all cells of the body. It plays an integral role in various immune responses as well as autoimmunity, by sending a potent "don't eat me" signal to prevent phagocytosis. A growing body of evidence demonstrates that CD47 is overexpressed in various hematological malignancies and its interaction with SIRPα on the phagocytic cells prevents phagocytosis of cancer cells. Additionally, it is expressed by different cell types in the tumor microenvironment and is required for establishing tumor metastasis. Overexpression of CD47 is thus often associated with poor clinical outcomes. CD47 has emerged as a potential therapeutic target and is being investigated in various preclinical studies as well as clinical trials to prove its safety and efficacy in treating hematological neoplasms. This review focuses on different therapeutic mechanisms to target CD47, either alone or in combination with other cell surface markers, and its pivotal role in impairing tumor growth and metastatic spread of various types of hematological malignancies.

Preclinical and Clinical Development of Therapeutic Antibodies Targeting Functions of CD47 in the Tumor Microenvironment

CD47 is a ubiquitously expressed cell surface glycoprotein that functions as a signaling receptor for thrombospondin-1 and as the counter-receptor for signal regulatory protein-α (SIRPα). Engaging SIRPα on macrophages inhibits phagocytosis, and CD47 thereby serves as a physiological marker of self. However, elevated CD47 expression on some cancer cells also protects tumors from innate immune surveillance and limits adaptive antitumor immunity via inhibitory SIRPα signaling in antigen presenting cells. CD47 also mediates inhibitory thrombospondin-1 signaling in vascular cells, T cells, and NK cells, and blocking inhibitory CD47 signaling on cytotoxic T cells directly increases tumor cell killing. Therefore, CD47 functions as an innate and adaptive immune checkpoint. These findings have led to the development of antibodies and other therapeutic approaches to block CD47 functions in the tumor microenvironment. Preclinical studies in mice demonstrated that blocking CD47 can limit the growth of hematologic malignancies and solid tumors and enhance the efficacy of conventional chemotherapy, radiation therapy, and some targeted cancer therapies. Humanized CD47 antibodies are showing promise in early clinical trials, but side effects related to enhanced phagocytic clearance of circulating blood cells remain a concern. Approaches to circumvent these include antibody preloading strategies, development of antibodies that recognize tumor-specific epitopes of CD47, SIRPα antibodies, and bivalent antibodies that restrict CD47 blockade to specific tumor cells. Preclinical and clinical development of antibodies and related biologics that inhibit CD47/SIRPα signaling are reviewed, including strategies to combine these agents with various conventional and targeted therapeutics to improve patient outcome for various cancers.

Effect of Curcumol on the Fenestrae of Liver Sinusoidal Endothelial Cells Based on NF-κB Signaling Pathway

Objective

To study the effect of curcumol on liver sinusoidal endothelial cells (LSECs) and to analyze the mechanism of antihepatic fibrosis.

Methods

The effects of drug intervention on cell proliferation rates were detected by MTT assay. The expression of NF-κB was detected by RT-PCR and WB. The NF-κB expression and entry into the nucleus were detected by immunofluorescence; scanning electron microscopy was used to observe the changes of LSECs fenestrae.

Results

MTT results showed that the interference of cell proliferation in each group was small. RT-PCR showed that the expression of NF-κB in the curcumol intervention group was significantly lower than that in the positive control group (P < 0.05). The WB detection found that, in the curcumol intervention group, the expression of pNF-κB in the NF-κB signaling pathway was significantly lower than that in the positive control group (P < 0.05). Scanning electron microscopy showed that the LSEC fenestrae were significantly improved compared with the positive control group.

Conclusion

Curcumol may be one of the mechanisms of antihepatic fibrosis by inhibiting the activity of the NF-κB signaling pathway and increasing the fenestrae of LSECs.

Thrombospondin-1 in maladaptive aging responses: a concept whose time has come

Numerous age-dependent alterations at the molecular, cellular, tissue and organ systems levels underlie the pathophysiology of aging. Herein, the focus is upon the secreted protein thrombospondin-1 (TSP1) as a promoter of aging and age-related diseases. TSP1 has several physiological functions in youth, including promoting neural synapse formation, mediating responses to ischemic and genotoxic stress, minimizing hemorrhage, limiting angiogenesis, and supporting wound healing. These acute functions of TSP1 generally require only transient expression of the protein. However, accumulating basic and clinical data reinforce the view that chronic diseases of aging are associated with accumulation of TSP1 in the extracellular matrix, which is a significant maladaptive contributor to the aging process. Identification of the relevant cell types that chronically produce and respond to TSP1 and the molecular mechanisms that mediate the resulting maladaptive responses could direct the development of therapeutic agents to delay or revert age-associated maladies.

Thrombospondin-1 mediates Drp-1 signaling following ischemia reperfusion in the aging heart

BACKGROUND

Ischemia reperfusion (IR) injury leads to activation of dynamin-related protein (Drp-1), causing mitochondrial fission and generation of reactive oxygen species (ROS), but the molecular mechanisms that activate Drp-1 are not known. The purpose of this study was to establish a link between Thbs-1 and fission protein (Drp-1) through Pgc-1α following IR in advancing age.

METHODS

Female Fischer-344 rats were divided into four groups: Young Control, Young + IR, Old Control, and Old + IR. Heart function and coronary flow were evaluated at baseline and 72 hours after IR, hearts were explanted and mitochondrial ROS generation was measured using MitoPY1, as well as protein levels of Thbs-1, Pgc-1α, and Drp-1. In vitro, rat aortic endothelial cells (RAEC) were treated with siRNA or plasmid for Pgc-1α to evaluate Pgc-1α effect on Drp-1.

RESULTS

Mitochondrial ROS generation in heart tissue increased in both age groups following IR. Old animals exhibited diastolic dysfunction at baseline; after IR they displayed reduced systolic function and exacerbated diastolic dysfunction compared to young controls. IR increased Thbs-1 and Drp-1 expression in young and old hearts compared to control. siRNA to Pgc-1α enhanced levels of Drp-1 in RAECs and increased ROS generation after hypoxia, while Pgc-1α plasmid ameliorates Drp-1 expression in the presence of exogenous Thbs-1.

CONCLUSION

These results highlight a novel signaling pathway by which Thbs-1 regulates mitochondrial fission protein (Drp-1) and ROS generation during hypoxia, and presumably, following IR. Inhibiting Thbs-1 immediately after IR may prevent Drp-1-mediated mitochondrial fission and is likely to improve the diastolic function of the heart by reducing ROS-mediated cardiomyocyte damage in the aged population.

Macrophage checkpoint blockade: results from initial clinical trials, binding analyses, and CD47-SIRPα structure-function

The macrophage checkpoint is an anti-phagocytic interaction between signal regulatory protein alpha (SIRPα) on a macrophage and CD47 on all types of cells - ranging from blood cells to cancer cells. This interaction has emerged over the last decade as a potential co-target in cancer when combined with other anti-cancer agents, with antibodies against CD47 and SIRPα currently in preclinical and clinical development for a variety of hematological and solid malignancies. Monotherapy with CD47 blockade is ineffective in human clinical trials against many tumor types tested to date, except for rare cutaneous and peripheral lymphomas. In contrast, pre-clinical results show efficacy in multiple syngeneic mouse models of cancer, suggesting that many of these tumor models are more immunogenic and likely artificial compared to human tumors. However, combination therapies in humans of anti-CD47 with agents such as the anti-tumor antibody rituximab do show efficacy against liquid tumors (lymphoma) and are promising. Here, we review such trials as well as key interaction and structural features of CD47-SIRPα.

The role of CD47 in pathogenesis and treatment of renal ischemia reperfusion injury

Ischemia reperfusion (IR) injury is a process defined by the temporary loss of blood flow and tissue perfusion followed later by restoration of the same. Brief periods of IR can be tolerated with little permanent deficit, but sensitivity varies for different target cells and tissues. Ischemia reperfusion injuries have multiple causes including peripheral vascular disease and surgical interventions that disrupt soft tissue and organ perfusion as occurs in general and reconstructive surgery. Ischemia reperfusion injury is especially prominent in organ transplantation where substantial effort has been focused on protecting the transplanted organ from the consequences of IR. A number of factors mediate IR injury including the production of reactive oxygen species and inflammatory cell infiltration and activation. In the kidney, IR injury is a major cause of acute injury and secondary loss of renal function. Transplant-initiated renal IR is also a stimulus for innate and adaptive immune-mediated transplant dysfunction. The cell surface molecule CD47 negatively modulates cell and tissue responses to stress through limitation of specific homeostatic pathways and initiation of cell death pathways. Herein, a summary of the maladaptive activities of renal CD47 will be considered as well as the possible therapeutic benefit of interfering with CD47 to limit renal IR.

Epithelial CD47 is critical for mucosal repair in the murine intestine in vivo

CD47 is a ubiquitously expressed transmembrane glycoprotein that regulates inflammatory responses and tissue repair. Here, we show that normal mice treated with anti-CD47 antibodies, and Cd47-null mice have impaired intestinal mucosal wound healing. Furthermore, intestinal epithelial cell (IEC)-specific loss of CD47 does not induce spontaneous immune-mediated intestinal barrier disruption but results in defective mucosal repair after biopsy-induced colonic wounding or Dextran Sulfate Sodium (DSS)-induced mucosal damage. In vitro analyses using primary cultures of CD47-deficient murine colonic IEC or human colonoid-derived IEC treated with CD47-blocking antibodies demonstrate impaired epithelial cell migration in wound healing assays. Defective wound repair after CD47 loss is linked to decreased epithelial β1 integrin and focal adhesion signaling, as well as reduced thrombospondin-1 and TGF-β1. These results demonstrate a critical role for IEC-expressed CD47 in regulating mucosal repair and raise important considerations for possible alterations in wound healing secondary to therapeutic targeting of CD47.

The role of the transmembrane glycoprotein CD47 in healing injured intestinal mucosa is unclear. Here, the authors show that selective loss of CD47 in the murine intestinal epithelium results in defective mucosal repair after colonic wounding, with suggested impaired cell migration in vitro.

CD47: role in the immune system and application to cancer therapy

BACKGROUND

CD47 is a widely expressed cellular receptor well known for its immunoregulatory functions. By interacting with its ligands, including thrombospondin-1 (TSP-1), signal regulatory protein α (SIRPα), integrins, and SH2-domain bearing protein tyrosine phosphatase substrate-1 (SHPS-1), it modulates cellular phagocytosis by macrophages, transmigration of neutrophils and activation of dendritic cells, T cells and B cells. Ample studies have shown that various types of cancer express high levels of CD47 to escape from the immune system. Based on this observation, CD47 is currently considered as a prominent target in cancer therapy.

CONCLUSIONS

Here, we review the role of CD47 in the maintenance of immune system homeostasis. We also depict three emerging CD47-targeting strategies for cancer therapy, including the use of mimicry peptides, antibodies, and gene silencing strategies. Among these approaches, the most advanced one is the use of anti-CD47 antibodies, which enhances cancer cell phagocytosis via inhibition of the CD47-SIRPα axis. These antibodies can also achieve higher anti-cancer efficacies when combined with chemotherapy and immunotherapy and hold promise for improving the survival of patients with cancer.

Deficiency in SIRP-α cytoplasmic recruitment confers protection from acute kidney injury

Acute kidney injury (AKI) remains an important source of progressive chronic kidney injury. Loss of renal blood flow with subsequent restoration, termed ischemia reperfusion (IR), is a common cause of AKI. The cell surface receptor signal regulatory protein α (SIRP-α) is expressed on macrophages and limits inflammation and phagocytosis. SIRP-α has recently been found to have wider cell-based expression and play a role in renal IR. We have explored this in a genetic model of deficient SIRP-α signaling. Mice lacking SIRP-α cytoplasmic signaling (SIRP-α^mut) and wild-type (WT) littermate controls underwent renal ischemia and reperfusion. Chimeric mice transplanted with WT or SIRP-α^mut bone marrow were similarly challenged following engraftment. Molecular and immunohistochemical analysis of renal function, tissue damage, and key molecular targets was performed. SIRP-α^mut mice were protected from renal IR compared with WT animals, demonstrating improved serum creatinine, less histologic damage, reduced proinflammatory cytokine production, and diminished production of reactive oxygen species (ROS). Resistance to renal IR in SIRP-α^mut occurred alongside down-regulation of CD47 and thrombospondin-1, which are known to exert SIRP-α crosstalk and also promote IR. In chimeric mice, lack of SIRP-α signaling conferred protection to IR regardless of the genotype of circulating cells. Renal tubular epithelial cells from SIRP-α^mut mice produced fewer ROS and proinflammatory cytokines in vitro. These results identify parenchymal SIRP-α as an independent driver of IR-mediated AKI and a potential therapeutic target.-Ghimire, K., Chiba, T., Minhas, N., Meijles, D. N., Lu, B., O'Connell, P., Rogers, N. M. Deficiency in SIRP-α cytoplasmic recruitment confers protection from acute kidney injury.

Quantitative high-throughput screening assays for the discovery and development of SIRPα-CD47 interaction inhibitors

CD47 is an immune checkpoint molecule that downregulates key aspects of both the innate and adaptive anti-tumor immune response via its counter receptor SIRPα, and it is expressed at high levels in a wide variety of tumor types. This has led to the development of biologics that inhibit SIRPα engagement including humanized CD47 antibodies and a soluble SIRPα decoy receptor that are currently undergoing clinical trials. Unfortunately, toxicological issues, including anemia related to on-target mechanisms, are barriers to their clinical advancement. Another potential issue with large biologics that bind CD47 is perturbation of CD47 signaling through its high-affinity interaction with the matricellular protein thrombospondin-1 (TSP1). One approach to avoid these shortcomings is to identify and develop small molecule molecular probes and pretherapeutic agents that would (1) selectively target SIRPα or TSP1 interactions with CD47, (2) provide a route to optimize pharmacokinetics, reduce on-target toxicity and maximize tissue penetration, and (3) allow more flexible routes of administration. As the first step toward this goal, we report the development of an automated quantitative high-throughput screening (qHTS) assay platform capable of screening large diverse drug-like chemical libraries to discover novel small molecules that inhibit CD47-SIRPα interaction. Using time-resolved Förster resonance energy transfer (TR-FRET) and bead-based luminescent oxygen channeling assay formats (AlphaScreen), we developed biochemical assays, optimized their performance, and individually tested them in small-molecule library screening. Based on performance and low false positive rate, the LANCE TR-FRET assay was employed in a ~90,000 compound library qHTS, while the AlphaScreen oxygen channeling assay served as a cross-validation orthogonal assay for follow-up characterization. With this multi-assay strategy, we successfully eliminated compounds that interfered with the assays and identified five compounds that inhibit the CD47-SIRPα interaction; these compounds will be further characterized and later disclosed. Importantly, our results validate the large library qHTS for antagonists of CD47-SIRPα interaction and suggest broad applicability of this approach to screen chemical libraries for other protein-protein interaction modulators.

Thrombospondin-1 interactions regulate eicosanoid metabolism and signaling in cancer-related inflammation

The metabolism of arachidonic acid and other polyunsaturated fatty acids produces eicosanoids, a family of biologically active lipids that are implicated in homeostasis and in several pathologies that involve inflammation. Inflammatory processes mediated by eicosanoids promote carcinogenesis by exerting direct effects on cancer cells and by affecting the tumor microenvironment. Therefore, understanding how eicosanoids mediate cancer progression may lead to better approaches and chemopreventive strategies for the treatment of cancer. The matricellular protein thrombospondin-1 is involved in processes that profoundly regulate inflammatory pathways that contribute to carcinogenesis and metastatic spread. This review focuses on interactions of thrombospondin-1 and eicosanoids in the microenvironment that promote carcinogenesis and how the microenvironment can be targeted for cancer prevention to increase curative responses of cancer patients.

Thrombospondin Type 1 Domain-Containing 7A Localizes to the Slit Diaphragm and Stabilizes Membrane Dynamics of Fully Differentiated Podocytes

BACKGROUND

About 3%-5% of adults with membranous nephropathy have autoantibodies directed against thrombospondin type 1 domain-containing 7A (THSD7A), a podocyte-expressed transmembrane protein. However, the temporal and spatial expression of THSD7A and its biologic function for podocytes are unknown, information that is needed to understand the effects of THSD7A autoantibodies in this disease.

METHODS

Using a variety of microscopic techniques, we analyzed THSD7A localization in postnatal, adult, and autoantibody-injected mice as well as in human podocytes. We also analyzed THSD7A function in human podocytes using confocal microscopy; Western blotting; and adhesion and migration assays.

RESULTS

We found that THSD7A expression begins on glomerular vascularization with slit diaphragm formation in development. THSD7A localizes to the basal aspect of foot processes, closely following the meanders of the slit diaphragm in human and mice. Autoantibodies binding to THSD7A localize to the slit diaphragm. In human podocytes, THSD7A expression is accentuated at filopodia and thin arborized protrusions, an expression pattern associated with decreased membrane activity of cytoskeletal regulators. We also found that, phenotypically, THSD7A expression in human podocytes is associated not only with increases in cell size, enhanced adhesion, and reduced detachment from collagen type IV-coated plates but also, with decreased ability to migrate.

CONCLUSIONS

Our findings suggest that THSD7A functions as a foot process protein involved in the stabilization of the slit diaphragm of mature podocytes and that autoantibodies to THSD7A, on the basis of their localization, might structurally and functionally alter the slit diaphragm's permeability to protein.

Thrombospondin-2 regulates extracellular matrix production, LOX levels, and cross-linking via downregulation of miR-29

Collagen fibrillogenesis and crosslinking have long been implicated in extracellular matrix (ECM)-dependent processes such as fibrosis and scarring. However, the extent to which matricellular proteins influence ECM protein production and fibrillar collagen crosslinking has yet to be determined. Here we show that thrombospondin 2 (TSP2), an anti-angiogenic matricellular protein, is an important modulator of ECM homeostasis. Specifically, through a fractionated quantitative proteomics approach, we show that loss of TSP2 leads to a unique ECM phenotype characterized by a significant decrease in fibrillar collagen, matricellular, and structural ECM protein production in the skin of TSP2 KO mice. Additionally, TSP2 KO skin displays decreased lysyl oxidase (LOX), which manifests as an increase in fibrillar collagen solubility and decreased levels of LOX-mediated fibrillar collagen crosslinking. We show that these changes are indirectly mediated by miR-29, a major regulator of ECM proteins and LOX, as miR-29 expression is increased in the TSP2 KO. Altogether, these findings indicate that TSP2 contributes to ECM production and assembly by regulating miR-29 and LOX.

Contribution of Human Thrombospondin-1 to the Pathogenesis of Gram-Positive Bacteria

A successful colonization of different compartments of the human host requires multifactorial contacts between bacterial surface proteins and host factors. Extracellular matrix proteins and matricellular proteins such as thrombospondin-1 play a pivotal role as adhesive substrates to ensure a strong interaction with pathobionts like the Gram-positive Streptococcus pneumoniae and Staphylococcus aureus. The human glycoprotein thrombospondin-1 is a component of the extracellular matrix and is highly abundant in the bloodstream during bacteremia. Human platelets secrete thrombospondin-1, which is then acquired by invading pathogens to facilitate colonization and immune evasion. Gram-positive bacteria express a broad spectrum of surface-exposed proteins, some of which also recognize thrombospondin-1. This review highlights the importance of thrombospondin-1 as an adhesion substrate to facilitate colonization, and we summarize the variety of thrombospondin-1-binding proteins of S. pneumoniae and S. aureus.

The calcium-binding type III repeats domain of thrombospondin-2 binds to fibroblast growth factor 2 (FGF2)

Thrombospondin (TSP)-1 and TSP-2 share similar structures and functions, including a remarkable antiangiogenic activity. We have previously demonstrated that a mechanism of the antiangiogenic activity of TSP-1 is the interaction of its type III repeats domain with fibroblast growth factor-2 (FGF2), affecting the growth factor bioavailability and angiogenic activity. Since the type III repeats domain is conserved in TSP-2, this study aimed at investigating whether also TSP-2 retained the ability to interact with FGF2. The FGF2 binding properties of TSP-1 and TSP-2 and their recombinant domains were analyzed by solid-phase binding and surface plasmon resonance assays. TSP-2 bound FGF2 with high affinity (Kd = 1.3 nM). TSP-2/FGF2 binding was inhibited by calcium and heparin. The FGF2-binding domain of TSP-2 was located in the type III repeats and the minimal interacting sequence was identified as the GVTDEKD peptide in repeat 3C, corresponding to KIPDDRD, the active sequence of TSP-1. A second putative FGF2 binding sequence was also identified in repeat 11C of both TSPs. Computational docking analysis predicted that both the TSP-2 and TSP-1-derived heptapeptides interacted with FGF2 with comparable binding properties. Accordingly, small molecules based on the TSP-1 active sequence blocked TSP-2/FGF2 interaction. Binding of TSP-2 to FGF2 impaired the growth factor ability to interact with its cellular receptors, since TSP-2-derived fragments prevented the binding of FGF2 to both heparin (used as a structural analog of heparan sulfate proteoglycans) and FGFR-1. These findings identify TSP-2 as a new FGF2 ligand that shares with TSP-1 the same molecular requirements for interaction with the growth factor and a comparable capacity to block FGF2 interaction with proangiogenic receptors. These features likely contribute to TSP-2 antiangiogenic and antineoplastic activity, providing the rationale for future therapeutic applications.

Combination of anthracyclines and anti-CD47 therapy inhibit invasive breast cancer growth while preventing cardiac toxicity by regulation of autophagy

BACKGROUND

A perennial challenge in systemic cytotoxic cancer therapy is to eradicate primary tumors and metastatic disease while sparing normal tissue from off-target effects of chemotherapy. Anthracyclines such as doxorubicin are effective chemotherapeutic agents for which dosing is limited by development of cardiotoxicity. Our published evidence shows that targeting CD47 enhances radiation-induced growth delay of tumors while remarkably protecting soft tissues. The protection of cell viability observed with CD47 is mediated autonomously by activation of protective autophagy. However, whether CD47 protects cancer cells from cytotoxic chemotherapy is unknown.

METHODS

We tested the effect of CD47 blockade on cancer cell survival using a 2-dimensional high-throughput cell proliferation assay in 4T1 breast cancer cell lines. To evaluate blockade of CD47 in combination with chemotherapy in vivo, we employed the 4T1 breast cancer model and examined tumor and cardiac tissue viability as well as autophagic flux.

RESULTS

Our high-throughput screen revealed that blockade of CD47 does not interfere with the cytotoxic activity of anthracyclines against 4T1 breast cancer cells. Targeting CD47 enhanced the effect of doxorubicin chemotherapy in vivo by reducing tumor growth and metastatic spread by activation of an anti-tumor innate immune response. Moreover, systemic suppression of CD47 protected cardiac tissue viability and function in mice treated with doxorubicin.

CONCLUSIONS

Our experiments indicate that the protective effects observed with CD47 blockade are mediated through upregulation of autophagic flux. However, the absence of CD47 in did not elicit a protective effect in cancer cells, but it enhanced macrophage-mediated cancer cell cytolysis. Therefore, the differential responses observed with CD47 blockade are due to autonomous activation of protective autophagy in normal tissue and enhancement immune cytotoxicity against cancer cells.

Matricellular protein thrombospondin-1 in pulmonary hypertension: multiple pathways to disease

Matricellular proteins are secreted molecules that have affinities for both extracellular matrix and cell surface receptors. Through interaction with structural proteins and the cells that maintain the matrix these proteins can alter matrix strength. Matricellular proteins exert control on cell activity primarily through engagement of membrane receptors that mediate outside-in signaling. An example of this group is thrombospondin-1 (TSP1), first identified as a component of the secreted product of activated platelets. As a result, TSP1 was initially studied in relation to coagulation, growth factor signaling and angiogenesis. More recently, TSP1 has been found to alter the effects of the gaseous transmitter nitric oxide (NO). This latter capacity has provided motivation to study TSP1 in diseases associated with loss of NO signaling as observed in cardiovascular disease and pulmonary hypertension (PH). PH is characterized by progressive changes in the pulmonary vasculature leading to increased resistance to blood flow and subsequent right heart failure. Studies have linked TSP1 to pre-clinical animal models of PH and more recently to clinical PH. This review will provide analysis of the vascular and non-vascular effects of TSP1 that contribute to PH, the experimental and translational studies that support a role for TSP1 in disease promotion and frame the relevance of these findings to therapeutic strategies.

The Most N-Terminal Region of THSD7A Is the Predominant Target for Autoimmunity in THSD7A-Associated Membranous Nephropathy

Background Thrombospondin type 1 domain-containing 7A (THSD7A) has been identified as a pathogenic autoantigen in membranous nephropathy (MN). However, the THSD7A epitopes targeted by patient autoantibodies are unknown.Methods We performed an in silico analysis of the THSD7A multidomain structure, expressed the folded domains in HEK293 cells, and tested for domain reactivity with 31 serum samples from patients with THSD7A-associated MN using Western and native blotting. Immunogenicity of the antigen domains was further investigated by cDNA immunization of rabbits and mice.Results We characterized the extracellular topology of THSD7A as a tandem string of 21 thrombospondin type 1 domains. Overall, 28 serum samples (90%) recognized multiple epitope domains along the molecule. Detailed epitope mapping revealed that the complex consisting of the first and second N-terminal domains (amino acids 48-192) was recognized by 27 of 31 patient serum samples (87%). Serum recognizing one or two epitope domains showed lower anti-THSD7A antibody levels than serum recognizing three or more epitope domains. During follow-up, a loss of epitope recognition was observed in seven of 16 patients, and it was accompanied by decreasing antibody levels and remission of proteinuria. In four of 16 patients, epitope recognition patterns changed during follow-up. Notably, immunization experiments in rabbits and mice revealed that induced antibodies, like patient autoantibodies, preferentially bound to the most N-terminal domains of THSD7A.Conclusions Our data show that the immune response in THSD7A-associated MN is polyreactive and that autoantibodies predominantly target the most N-terminal part of THSD7A.

A function-blocking CD47 antibody modulates extracellular vesicle-mediated intercellular signaling between breast carcinoma cells and endothelial cells

Tumor cells release extracellular vesicles (EVs) into the tumor microenvironment that may facilitate malignant progression and metastasis. Breast carcinoma EVs express high levels of the thrombospondin-1 and signal regulatory protein-α receptor CD47, which is the target of several experimental therapeutics currently in clinical trials. We analyzed changes in gene expression and function in human umbilical vein endothelial cells (HUVEC) induced by treatment with EVs derived from breast carcinoma cells and the effects of the function-blocking CD47 antibody B6H12 on the resulting intercellular communication. CD47⁺ EVs exhibited greater uptake by HUVEC compared to CD47^- EVs, but the CD47 antibody did not inhibit their uptake. Global and targeted analyses of transcripts demonstrated that treatment of HUVEC with EVs derived from MDA-MB-231 breast carcinomas cells altered pathways associated with tumor necrosis factor-α signaling, angiogenesis, lymphangiogenesis, endothelial-mesenchymal transition, and extracellular matrix. EVs from triple-negative MDA-MB-231 cells were more active than EVs from less metastatic breast carcinoma cell lines. Treatment with MDA-MB-231 EVs down-regulated VEGFR2 mRNA expression and tyrosine phosphorylation while enhancing phosphorylation of the tyrosine phosphatase SHP2. VEGFR2 expression and phosphorylation in HUVEC was further inhibited by the CD47 antibody. Consistent with the observed changes in endothelial-mesenchymal transition genes and SHP2, treatment with MDA-MB-231-derived EVs decreased Zeb1 protein levels in HUVEC, whereas the CD47 antibody increased Zeb1 levels. The induction of E-selectin and other known targets of tumor necrosis factor-α signaling by EVs was also enhanced by the CD47 antibody, and E-selectin was the most up-regulated transcript following CD47 antibody treatment alone. These studies reveal several mechanisms by which therapeutics targeting CD47 could modulate tumor growth by altering the cross talk between cancer-derived EVs and nonmalignant cells in the tumor stroma.

Regulation of Cellular Redox Signaling by Matricellular Proteins in Vascular Biology, Immunology, and Cancer

SIGNIFICANCE

In contrast to structural elements of the extracellular matrix, matricellular proteins appear transiently during development and injury responses, but their sustained expression can contribute to chronic disease. Through interactions with other matrix components and specific cell surface receptors, matricellular proteins regulate multiple signaling pathways, including those mediated by reactive oxygen and nitrogen species and H2S. Dysregulation of matricellular proteins contributes to the pathogenesis of vascular diseases and cancer. Defining the molecular mechanisms and receptors involved is revealing new therapeutic opportunities. Recent Advances: Thrombospondin-1 (TSP1) regulates NO, H2S, and superoxide production and signaling in several cell types. The TSP1 receptor CD47 plays a central role in inhibition of NO signaling, but other TSP1 receptors also modulate redox signaling. The matricellular protein CCN1 engages some of the same receptors to regulate redox signaling, and ADAMTS1 regulates NO signaling in Marfan syndrome. In addition to mediating matricellular protein signaling, redox signaling is emerging as an important pathway that controls the expression of several matricellular proteins.

CRITICAL ISSUES

Redox signaling remains unexplored for many matricellular proteins. Their interactions with multiple cellular receptors remains an obstacle to defining signaling mechanisms, but improved transgenic models could overcome this barrier.

FUTURE DIRECTIONS

Therapeutics targeting the TSP1 receptor CD47 may have beneficial effects for treating cardiovascular disease and cancer and have recently entered clinical trials. Biomarkers are needed to assess their effects on redox signaling in patients and to evaluate how these contribute to their therapeutic efficacy and potential side effects. Antioxid. Redox Signal. 27, 874-911.

Hypoxia and Redox Signaling on Extracellular Matrix Remodeling: From Mechanisms to Pathological Implications

SIGNIFICANCE

The extracellular matrix (ECM) is an essential modulator of cell behavior that influences tissue organization. It has a strong relevance in homeostasis and translational implications for human disease. In addition to ECM structural proteins, matricellular proteins are important regulators of the ECM that are involved in a myriad of different pathologies. Recent Advances: Biochemical studies, animal models, and study of human diseases have contributed to the knowledge of molecular mechanisms involved in remodeling of the ECM, both in homeostasis and disease. Some of them might help in the development of new therapeutic strategies. This review aims to review what is known about some of the most studied matricellular proteins and their regulation by hypoxia and redox signaling, as well as the pathological implications of such regulation.

CRITICAL ISSUES

Matricellular proteins have complex regulatory functions and are modulated by hypoxia and redox signaling through diverse mechanisms, in some cases with controversial effects that can be cell or tissue specific and context dependent. Therefore, a better understanding of these regulatory processes would be of great benefit and will open new avenues of considerable therapeutic potential.

FUTURE DIRECTIONS

Characterizing the specific molecular mechanisms that modulate matricellular proteins in pathological processes that involve hypoxia and redox signaling warrants additional consideration to harness the potential therapeutic value of these regulatory proteins. Antioxid. Redox Signal. 27, 802-822.