Biology:CD47

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Short description: Protein-coding gene in humans


A representation of the 3D structure of the protein myoglobin showing turquoise α-helices.
Generic protein structure example

CD47 (Cluster of Differentiation 47) also known as integrin associated protein (IAP) is a transmembrane protein that in humans is encoded by the CD47 gene. CD47 belongs to the immunoglobulin superfamily[1] and partners with membrane integrins and also binds the ligands thrombospondin-1 (TSP-1) and signal-regulatory protein alpha (SIRPα).[2] CD-47 acts as a don't eat me signal to macrophages of the immune system which has made it a potential therapeutic target in some cancers, and more recently, for the treatment of pulmonary fibrosis.[3]

CD47 is involved in a range of cellular processes, including apoptosis, proliferation, adhesion, and migration. Furthermore, it plays a key role in insulin secretion (DOI: 10.1126/scitranslmed.add2387), as well as immune and angiogenic responses. CD47 is ubiquitously expressed in human cells and has been found to be overexpressed in many different tumor cells.[2][4] Expression in equine cutaneous tumors has been reported as well.[5]

Structure

CD47 is a 50 kDa membrane receptor that has extracellular N-terminal IgV domain, five transmembrane domains, and a short C-terminal intracellular tail. There are four alternatively spliced isoforms of CD47 that differ only in the length of their cytoplasmic tail.[6]

Form 2 is the most widely expressed form that is found in all circulating and immune cells. The second most abundant isoform is form 4, which is predominantly expressed in the brain and in the peripheral nervous system. Only keratinocytes expressed significant amounts of form 1. Little is known about the functional significance of this alternative splicing. However, these isoforms are highly conserved between mouse and man, suggesting an important role for the cytoplasmic domains in CD47 function.[2][6][7]

Interactions

Thrombospondin (TSP)

CD47 is a high affinity receptor for thrombospondin-1 (TSP-1), a secreted glycoprotein that plays a role in vascular development and angiogenesis, and in this later capacity the TSP1-CD47 interaction inhibits nitric oxide signaling at multiple levels in vascular cells.[8] Binding of TSP-1 to CD47 influences several fundamental cellular functions including cell migration and adhesion, cell proliferation or apoptosis, and plays a role in the regulation of angiogenesis and inflammation.[2]

Signal-regulatory protein (SIRP)

CD47 interacts with signal-regulatory protein alpha (SIRPα), an inhibitory transmembrane receptor present on myeloid cells. The CD47/SIRPα interaction leads to bidirectional signaling, resulting in different cell-to-cell responses including inhibition of phagocytosis, stimulation of cell-cell fusion, and T-cell activation.[2][9][10]

Integrins

CD47 interacts with several membrane integrins, most commonly integrin αVβ3. These interactions result in CD47/integrin complexes that affect a range of cell functions including adhesion, spreading and migration.[2][10]

Function

Tumor cells

Due to the ubiquitous expression of CD47, signaling differs according to cell type. It is likely that intracellular and membrane-associated partners are crucial in determining the cellular response of CD47 signaling.

Cell proliferation

The role of CD47 in promoting cell proliferation is heavily dependent on cell type as both activation and loss of CD47 can result in enhanced proliferation.

Activation of CD47 with TSP-1 increases proliferation of human U-87 MG and U373 astrocytoma cells but not normal astrocytes. Additionally, CD47 blocking antibodies inhibit proliferation of unstimulated astrocytoma cells but not normal astrocytes. Though the exact mechanism is unclear, it is likely that CD47 promotes proliferation via the PI3K/Akt pathway in cancerous cells but not normal cells.[11]

Loss of CD47 allows sustained proliferation of primary murine endothelial cells and enables these cells to spontaneously reprogram to form multipotent embryoid body-like clusters. Expression of several stem cell markers, including c-Myc, is elevated in CD47-null endothelial cells and a human T cell line lacking CD47. Activation of CD47 with TSP-1 in wild-type cells inhibits proliferation and reduces expression of stem cell transcription factors.[12]

Cell death

CD47 ligation leads to cell death in many normal and tumor cell lines via apoptosis or autophagy. The activation of CD47 induces rapid apoptosis of T cells. Jurkat cells and peripheral blood mononuclear cells (PBMC) incubated with the monoclonal antibody Ad22 results in apoptosis within 3 hours. However, apoptosis was not observed following culture with other anti-CD47 antibodies. The apoptosis inducing function of CD47 appears to be dependent on activation of specific epitopes on the extracellular domain.[13]

Similarly, CD47 ligation rapidly induces apoptosis in B-cell chronic lymphocytic leukemia (CLL) cells. Treatment with a disulfide-linked antibody dimer induces apoptosis of CD47-positive primary B-CLL and leukemic cells (MOLT-4 and JOK-1). In addition, administration of the antibody prolonged survival of SCID mice implanted with JOK-1 cells. Apoptosis induction appears to be regulated by the hypoxia inducible factor 1α (HIF-1α) pathway.[14]

The RAS-transformed cell lines MDFB6 and B6ras show near complete loss of TSP-1 expression. Activation of CD47 with TSP-1 results in loss of viability in these RAS-expressing cells. Affected cells do not exhibit hallmarks of apoptosis but rather autophagy as seen by staining with acridine orange and immunoreactivity for LC3.[15]

Migration

Cell migration appears to be universally stimulated by CD47 ligation and activation. The role of CD47 in cell migration was first demonstrated for neutrophils, where CD47 blocking antibodies inhibited transmigration of neutrophils and monocytes through the endothelium. These effects were shown to be dependent on avb3 integrins, which interact with and are activated by CD47 at the plasma membrane.[2][10]

Originally discovered by Irving Weissman’s group at Stanford University, blocking CD47 function has been shown to inhibit migration and metastasis in a variety of tumor models. Blockade of CD47 by neutralizing antibodies reduced migration and chemotaxis in response to collagen IV in melanoma, prostate cancer and ovarian cancer-derived cells.[16] In a mouse model of multiple myeloma, tumor metastasis to bone was decreased in CD47-deficient mice compared with wild type controls.[17] In work led by Mark Chao and Ash Alizadeh, at Stanford, mice were xenografted with human non-Hodgkin lymphoma (NHL) cells; blocking CD47 function with shRNA or antibodies led to a dramatic reduction in metastasis to major organs.[18]

Stromal cells

Angiogenesis

Loss of CD47 promotes proliferation and increases asymmetric division of primary murine endothelial cells.[12] Additionally, activation of CD47 with TSP-1 in wild-type primary mouse cerebral endothelial cells induces cytotoxicity, which is significantly decreased in cerebral endothelial cells derived from CD47 knockout mice.[19]

CD47 signaling may suppress angiogenesis as TSP-1 activation significantly inhibited endothelial cell migration and tube formation in vitro.[19] In vivo, injections of TSP-1 in mice after hindlimb ischemia induces a significant decrease of blood flow recovery.[20] The mechanism of the anti-angiogenic activity of CD47 is not fully understood, but introduction of CD47 antibodies and TSP-1 have been shown to inhibit nitric oxide (NO)-stimulated responses in both endothelial and vascular smooth muscle cells.[8] CD47 signaling influences the SDF-1 chemokine pathway, which plays a role in angiogenesis.[20]

Inflammatory response

Interactions between endothelial cell CD47 and leukocyte SIRPγ regulate T cell transendothelial migration (TEM) at sites of inflammation. CD47 knockout mice show reduced recruitment of blood T cells as well as neutrophils and monocytes in areas of inflammation.[21] CD47 also functions as a marker of self on murine red blood cells which allows RBC to avoid phagocytosis. Red blood cells that lack CD47 are rapidly cleared from the bloodstream by macrophages, a process that is mediated by interaction with SIRPα.[22] Mouse hematopoietic stem cells (HSCs) and progenitors transiently upregulate CD47 during their migratory phase, which reduces macrophage engulfment in vivo.[23]

Tumor cells can also evade macrophage phagocytosis through the expression of CD47.[4] CD47 is highly expressed in bladder tumor initiating cells (T-ICs) compared with the rest of the tumor. Blockade of CD47 with a monoclonal antibody results in macrophage engulfment of bladder cancer cells in vitro.[24] CD47 is also upregulated in mouse and human myeloid leukemias, and overexpression of CD47 on a myeloid leukemia line allows these cells to evade phagocytosis.[23]

Insulin secretion

CD47 receptor signaling inhibits insulin release from human as well as mouse pancreatic β cells and that it can be pharmacologically blocked to boost insulin secretion in both models (https://www.science.org/doi/full/10.1126/scitranslmed.add2387).

Clinical significance

CD47 was first identified as a tumor antigen on human ovarian cancer in the 1980s. Since then, CD47 has been found to be expressed on multiple human tumor types including acute myeloid leukemia (AML), chronic myeloid leukemia, acute lymphoblastic leukemia (ALL), non-Hodgkin's lymphoma (NHL), multiple myeloma (MM), bladder cancer, and other solid tumors.[4] CD47 is also highly expressed on pediatric and adult brain tumors.[25]

High levels of CD47 allows cancer cells to avoid phagocytosis despite having a higher level of calreticulin - the dominant pro-phagocytic signal.[26] This is due to engagement of the SIRP-α of macrophage by CD47. Engagement of SIRP-α leads to inhibition of phagocytosis. Thus blocking CD47 with antibody turns off “don’t eat me” signal and favors phagocytosis.

As a potential drug target

Anti-CD47 antibody–mediated phagocytosis of cancer by macrophages can initiate an antitumor T-cell immune response. Noteworthy, anti-CD47 antibody treatment not only enables macrophage phagocytosis of cancer, but also fosters the activation of cancer-specific lymphocytes: cancer cells display mutant proteins to which the immune system can now react.[27][28] Based on significant activity in preclinical models and in synergistic combinations with other antibodies,[29] Humanized anti-CD47 antibody is being evaluated for the treatment of various cancers, e.g. diffuse large B-cell lymphoma (DLBCL) and follicular lymphoma (FL).[30]

See also

References

  1. ↑ "Entrez Gene: CD47 CD47 molecule". https://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=961. 
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 "CD47 update: a multifaceted actor in the tumour microenvironment of potential therapeutic interest". Br. J. Pharmacol. 167 (7): 1415–30. December 2012. doi:10.1111/j.1476-5381.2012.02099.x. PMID 22774848. 
  3. ↑ "Fibrosis reversed when 'don't eat me' signal blocked, study finds". https://medicalxpress.com/news/2017-04-fibrosis-reversed-dont-blocked-stanford.html. 
  4. ↑ 4.0 4.1 4.2 "The CD47-SIRPα pathway in cancer immune evasion and potential therapeutic implications". Curr. Opin. Immunol. 24 (2): 225–32. April 2012. doi:10.1016/j.coi.2012.01.010. PMID 22310103. 
  5. ↑ "CD47 expression in cryopreserved equine cutaneous masses and normal skin". Journal of Veterinary Diagnostic Investigation 28 (4): 408–13. July 2016. doi:10.1177/1040638716643352. PMID 27154320. 
  6. ↑ 6.0 6.1 "In vivo expression of alternatively spliced forms of integrin-associated protein (CD47)". J. Cell Sci. 108 (Pt 11): 3419–25. November 1995. doi:10.1242/jcs.108.11.3419. PMID 8586654. 
  7. ↑ "CD47". UCSD Nature Molecule Pages. February 2010. doi:10.1038/mp.a002870.01. 
  8. ↑ 8.0 8.1 "CD47 is necessary for inhibition of nitric oxide-stimulated vascular cell responses by thrombospondin-1". J. Biol. Chem. 281 (36): 26069–80. September 2006. doi:10.1074/jbc.M605040200. PMID 16835222. 
  9. ↑ Barclay AN (February 2009). "Signal regulatory protein alpha (SIRPα)/CD47 interaction and function". Curr. Opin. Immunol. 21 (1): 47–52. doi:10.1016/j.coi.2009.01.008. PMID 19223164. 
  10. ↑ 10.0 10.1 10.2 "Integrin-associated protein (CD47) and its ligands". Trends Cell Biol. 11 (3): 130–5. March 2001. doi:10.1016/S0962-8924(00)01906-1. PMID 11306274. 
  11. ↑ "Activation of CD47 receptors causes proliferation of human astrocytoma but not normal astrocytes via an Akt-dependent pathway". Glia 59 (2): 308–19. February 2011. doi:10.1002/glia.21102. PMID 21125662. 
  12. ↑ 12.0 12.1 "Thrombospondin-1 Signaling through CD47 Inhibits Self-renewal by Regulating c-Myc and Other Stem Cell Transcription Factors". Sci Rep 3: 1673. April 2013. doi:10.1038/srep01673. PMID 23591719. Bibcode: 2013NatSR...3E1673K. 
  13. ↑ "CD47 signals T cell death". J. Immunol. 162 (12): 7031–40. June 1999. doi:10.4049/jimmunol.162.12.7031. PMID 10358145. 
  14. ↑ "A new disulfide-linked dimer of a single-chain antibody fragment against human CD47 induces apoptosis in lymphoid malignant cells via the hypoxia inducible factor-1α pathway". Cancer Sci. 102 (6): 1208–15. June 2011. doi:10.1111/j.1349-7006.2011.01925.x. PMID 21401803. 
  15. ↑ "Thrombospondin-1 Receptor Mediates Autophagy of RAS-expressing Cancer Cells and Triggers Tumour Growth Inhibition". Anticancer Res. 33 (4): 1429–38. April 2013. PMID 23564783. 
  16. ↑ "Regulation of tumor cell chemotaxis by type IV collagen is mediated by a Ca(2+)-dependent mechanism requiring CD47 and the integrin alpha(V)beta(3)". J. Biol. Chem. 275 (7): 4796–802. February 2000. doi:10.1074/jbc.275.7.4796. PMID 10671513. 
  17. ↑ "CD47 regulates bone mass and tumor metastasis to bone". Cancer Res. 69 (7): 3196–204. April 2009. doi:10.1158/0008-5472.CAN-08-3358. PMID 19276363. 
  18. ↑ "Extranodal dissemination of non-Hodgkin lymphoma requires CD47 and is inhibited by anti-CD47 antibody therapy". Blood 118 (18): 4890–901. November 2011. doi:10.1182/blood-2011-02-338020. PMID 21828138. 
  19. ↑ 19.0 19.1 "Neurovascular effects of CD47 signaling: promotion of cell death, inflammation, and suppression of angiogenesis in brain endothelial cells in vitro". J. Neurosci. Res. 87 (11): 2571–7. August 2009. doi:10.1002/jnr.22076. PMID 19360900. 
  20. ↑ 20.0 20.1 "Thrombospondin-1 is a plasmatic marker of peripheral arterial disease that modulates endothelial progenitor cell angiogenic properties". Arterioscler. Thromb. Vasc. Biol. 31 (3): 551–9. March 2011. doi:10.1161/ATVBAHA.110.220624. PMID 21148423. 
  21. ↑ "Endothelial CD47 promotes vascular endothelial-cadherin tyrosine phosphorylation and participates in T cell recruitment at sites of inflammation in vivo". J. Immunol. 189 (5): 2553–62. September 2012. doi:10.4049/jimmunol.1103606. PMID 22815286. 
  22. ↑ "Role of CD47 as a marker of self on red blood cells". Science 288 (5473): 2051–4. June 2000. doi:10.1126/science.288.5473.2051. PMID 10856220. Bibcode: 2000Sci...288.2051O. 
  23. ↑ 23.0 23.1 "CD47 is upregulated on circulating hematopoietic stem cells and leukemia cells to avoid phagocytosis". Cell 138 (2): 271–85. July 2009. doi:10.1016/j.cell.2009.05.046. PMID 19632178. 
  24. ↑ "Identification, molecular characterization, clinical prognosis, and therapeutic targeting of human bladder tumor-initiating cells". Proc. Natl. Acad. Sci. U.S.A. 106 (33): 14016–21. August 2009. doi:10.1073/pnas.0906549106. PMID 19666525. Bibcode: 2009PNAS..10614016C. 
  25. ↑ "Disrupting the CD47-SIRPα anti-phagocytic axis by a humanized anti-CD47 antibody is an efficacious treatment for malignant pediatric brain tumors". Science Translational Medicine 9 (381): eaaf2968. March 2017. doi:10.1126/scitranslmed.aaf2968. PMID 28298418. https://authors.library.caltech.edu/75387/2/aaf2968_SM.pdf. 
  26. ↑ "Calreticulin is the dominant pro-phagocytic signal on multiple human cancers and is counterbalanced by CD47". Sci Transl Med 2 (63): 63ra94. December 2010. doi:10.1126/scitranslmed.3001375. PMID 21178137. 
  27. ↑ "Anti-CD47 antibody-mediated phagocytosis of cancer by macrophages primes an effective antitumor T-cell response". Proc. Natl. Acad. Sci. U.S.A. 110 (27): 11103–8. July 2013. doi:10.1073/pnas.1305569110. PMID 23690610. Bibcode: 2013PNAS..11011103T. 
  28. ↑ Unanue ER (July 2013). "Perspectives on anti-CD47 antibody treatment for experimental cancer". Proc. Natl. Acad. Sci. U.S.A. 110 (27): 10886–7. doi:10.1073/pnas.1308463110. PMID 23784781. Bibcode: 2013PNAS..11010886U. 
  29. ↑ Chao, Mark P.; Alizadeh, Ash A.; Tang, Chad; Myklebust, June H.; Varghese, Bindu; Gill, Saar; Jan, Max; Cha, Adriel C. et al. (2010-09-03). "Anti-CD47 antibody synergizes with rituximab to promote phagocytosis and eradicate non-Hodgkin lymphoma". Cell 142 (5): 699–713. doi:10.1016/j.cell.2010.07.044. ISSN 1097-4172. PMID 20813259. 
  30. ↑ Researchers Report Early Clinical Promise for Macrophage Checkpoint Blockade Dec 2018

Gholamin S, Youssef OA, Rafat M, et al. Irradiation or temozolomide chemotherapy enhances anti-CD47 treatment of glioblastoma. Innate Immun. 2020;26(2):130‐137. doi:10.1177/1753425919876690

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