Calreticulin Explained

Calreticulin should not be confused with calretinin.

Calreticulin also known as calregulin, CRP55, CaBP3, calsequestrin-like protein, and endoplasmic reticulum resident protein 60 (ERp60) is a protein that in humans is encoded by the CALR gene.^{[1] [2]}

Calreticulin is a multifunctional soluble protein that binds Ca²⁺ ions (a second messenger in signal transduction), rendering it inactive. The Ca²⁺ is bound with low affinity, but high capacity, and can be released on a signal (see inositol trisphosphate). Calreticulin is located in storage compartments associated with the endoplasmic reticulum and is considered an ER resident protein.^[2]

The term "Mobilferrin" is considered to be the same as calreticulin by some sources.^[3]

Function

Calreticulin binds to misfolded proteins and prevents them from being exported from the endoplasmic reticulum to the Golgi apparatus.

A similar quality-control molecular chaperone, calnexin, performs the same service for soluble proteins as does calreticulin, however it is a membrane-bound protein. Both proteins, calnexin and calreticulin, have the function of binding to oligosaccharides containing terminal glucose residues, thereby targeting them for degradation. Calreticulin and Calnexin's ability to bind carbohydrates associates them with the lectin protein family. In normal cellular function, trimming of glucose residues off the core oligosaccharide added during N-linked glycosylation is a part of protein processing. If "overseer" enzymes note that residues are misfolded, proteins within the rER will re-add glucose residues so that other calreticulin/calnexin can bind to these proteins and prevent them from proceeding to the Golgi. This leads these aberrantly folded proteins down a path whereby they are targeted for degradation.

Studies on transgenic mice reveal that calreticulin is a cardiac embryonic gene that is essential during development.^[4]

Calreticulin and calnexin are also integral in the production of MHC class I proteins. As newly synthesized MHC class I α-chains enter the endoplasmic reticulum, calnexin binds on to them retaining them in a partly folded state.^[5] After the β2-microglobulin binds to the peptide-loading complex (PLC), calreticulin (along with ERp57) takes over the job of chaperoning the MHC class I protein while the tapasin links the complex to the transporter associated with antigen processing (TAP) complex. This association prepares the MHC class I to bind an antigen for presentation on the cell surface.

Transcription regulation

Calreticulin is also found in the nucleus, suggesting that it may have a role in transcription regulation. Calreticulin binds to the synthetic peptide KLGFFKR, which is almost identical to an amino acid sequence in the DNA-binding domain of the superfamily of nuclear receptors. The amino terminus of calreticulin interacts with the DNA-binding domain of the glucocorticoid receptor and prevents the receptor from binding to its specific glucocorticoid response element. Calreticulin can inhibit the binding of androgen receptor to its hormone-responsive DNA element and can inhibit androgen receptor and retinoic acid receptor transcriptional activities in vivo, as well as retinoic acid-induced neuronal differentiation. Thus, calreticulin can act as an important modulator of the regulation of gene transcription by nuclear hormone receptors.

Clinical significance

Calreticulin binds to antibodies in certain area of systemic lupus and Sjögren patients that contain anti-Ro/SSA antibodies. Systemic lupus erythematosus is associated with increased autoantibody titers against calreticulin, but calreticulin is not a Ro/SS-A antigen. Earlier papers referred to calreticulin as an Ro/SS-A antigen, but this was later disproven. Increased autoantibody titer against human calreticulin is found in infants with complete congenital heart block of both the IgG and IgM classes.^[6]

In 2013, two groups detected calreticulin mutations in a majority of JAK2-negative/MPL-negative patients with essential thrombocythemia and primary myelofibrosis, which makes CALR mutations the second most common in myeloproliferative neoplasms. All mutations (insertions or deletions) affected the last exon, generating a reading frame shift of the resulting protein, that creates a novel terminal peptide and causes a loss of endoplasmic reticulum KDEL retention signal.^{[7] [8]}

Role in cancer

Calreticulin (CRT) is expressed in many cancer cells and plays a role to promote macrophages to engulf hazardous cancerous cells. The reason why most of the cells are not destroyed is the presence of another molecule with signal CD47, which blocks CRT. Hence antibodies that block CD47 might be useful as a cancer treatment. In mice models of myeloid leukemia and non-Hodgkin lymphoma, anti-CD47 were effective in clearing cancer cells while normal cells were unaffected.^[9]

Interactions

Calreticulin has been shown to interact with Perforin^[10] and NK2 homeobox 1.^[11]

Further reading

• Del Bem LE . The evolutionary history of calreticulin and calnexin genes in green plants . Genetica . 139 . 2 . 225–9 . Feb 2011 . 21222018 . 10.1007/s10709-010-9544-y . 9228786 .
• Coppolino MG, Dedhar S . Calreticulin . The International Journal of Biochemistry & Cell Biology . 30 . 5 . 553–8 . May 1998 . 9693955 . 10.1016/S1357-2725(97)00153-2 .
• Brucato A, Grava C, Bortolati M, Ikeda K, Milanesi O, Cimaz R, Ramoni V, Vignati G, Martinelli S, Sadou Y, Borghi A, Tincani A, Chan EK, Ruffatti A . Congenital heart block not associated with anti-Ro/La antibodies: comparison with anti-Ro/La-positive cases . The Journal of Rheumatology . 36 . 8 . 1744–8 . Aug 2009 . 19567621 . 2798588 . 10.3899/jrheum.080737 .
• Peng RQ, Chen YB, Ding Y, Zhang R, Zhang X, Yu XJ, Zhou ZW, Zeng YX, Zhang XS . Expression of calreticulin is associated with infiltration of T-cells in stage IIIB colon cancer . World Journal of Gastroenterology . 16 . 19 . 2428–34 . May 2010 . 20480531 . 2874150 . 10.3748/wjg.v16.i19.2428 . free .
• Tarr JM, Young PJ, Morse R, Shaw DJ, Haigh R, Petrov PG, Johnson SJ, Winyard PG, Eggleton P . A mechanism of release of calreticulin from cells during apoptosis . Journal of Molecular Biology . 401 . 5 . 799–812 . Sep 2010 . 20624402 . 10.1016/j.jmb.2010.06.064 . 10871/20264 . free .
• Abd Alla J, Reeck K, Langer A, Streichert T, Quitterer U . Calreticulin enhances B2 bradykinin receptor maturation and heterodimerization . Biochemical and Biophysical Research Communications . 387 . 1 . 186–90 . Sep 2009 . 19580784 . 10.1016/j.bbrc.2009.07.011 . 2019-12-09 . 2021-05-03 . https://web.archive.org/web/20210503073327/https://www.zora.uzh.ch/id/eprint/25081/1/Accepted-manuscript-BBRC-387-186-2009.pdf . dead .
• Caramelo JJ, Parodi AJ . Getting in and out from calnexin/calreticulin cycles . The Journal of Biological Chemistry . 283 . 16 . 10221–5 . Apr 2008 . 18303019 . 2447651 . 10.1074/jbc.R700048200 . free .
• Du XL, Yang H, Liu SG, Luo ML, Hao JJ, Zhang Y, Lin DC, Xu X, Cai Y, Zhan QM, Wang MR . Calreticulin promotes cell motility and enhances resistance to anoikis through STAT3-CTTN-Akt pathway in esophageal squamous cell carcinoma . Oncogene . 28 . 42 . 3714–22 . Oct 2009 . 19684620 . 10.1038/onc.2009.237 . 23543648 .
• Gelebart P, Opas M, Michalak M . Calreticulin, a Ca2+-binding chaperone of the endoplasmic reticulum . The International Journal of Biochemistry & Cell Biology . 37 . 2 . 260–6 . Feb 2005 . 15474971 . 10.1016/j.biocel.2004.02.030 .
• Qiu Y, Michalak M . Transcriptional control of the calreticulin gene in health and disease . The International Journal of Biochemistry & Cell Biology . 41 . 3 . 531–8 . Mar 2009 . 18765291 . 10.1016/j.biocel.2008.06.020 .
• Zhu Y, Zhang W, Veerapen N, Besra G, Cresswell P . Calreticulin controls the rate of assembly of CD1d molecules in the endoplasmic reticulum . The Journal of Biological Chemistry . 285 . 49 . 38283–92 . Dec 2010 . 20861015 . 2992262 . 10.1074/jbc.M110.170530 . free .
• Talmud PJ, Drenos F, Shah S, Shah T, Palmen J, Verzilli C, Gaunt TR, Pallas J, Lovering R, Li K, Casas JP, Sofat R, Kumari M, Rodriguez S, Johnson T, Newhouse SJ, Dominiczak A, Samani NJ, Caulfield M, Sever P, Stanton A, Shields DC, Padmanabhan S, Melander O, Hastie C, Delles C, Ebrahim S, Marmot MG, Smith GD, Lawlor DA, Munroe PB, Day IN, Kivimaki M, Whittaker J, Humphries SE, Hingorani AD . Gene-centric association signals for lipids and apolipoproteins identified via the HumanCVD BeadChip . American Journal of Human Genetics . 85 . 5 . 628–42 . Nov 2009 . 19913121 . 2775832 . 10.1016/j.ajhg.2009.10.014 .
• Taner SB, Pando MJ, Roberts A, Schellekens J, Marsh SG, Malmberg KJ, Parham P, Brodsky FM . Interactions of NK cell receptor KIR3DL1*004 with chaperones and conformation-specific antibody reveal a functional folded state as well as predominant intracellular retention . Journal of Immunology . 186 . 1 . 62–72 . Jan 2011 . 21115737 . 3129036 . 10.4049/jimmunol.0903657 .
• Tarr JM, Winyard PG, Ryan B, Harries LW, Haigh R, Viner N, Eggleton P . Extracellular calreticulin is present in the joints of patients with rheumatoid arthritis and inhibits FasL (CD95L)-mediated apoptosis of T cells . Arthritis and Rheumatism . 62 . 10 . 2919–29 . Oct 2010 . 20533543 . 10.1002/art.27602 . 10871/13850 . free .
• Kepp O, Gdoura A, Martins I, Panaretakis T, Schlemmer F, Tesniere A, Fimia GM, Ciccosanti F, Burgevin A, Piacentini M, Eggleton P, Young PJ, Zitvogel L, van Endert P, Kroemer G . Lysyl tRNA synthetase is required for the translocation of calreticulin to the cell surface in immunogenic death . Cell Cycle . 9 . 15 . 3072–7 . Aug 2010 . 20699648 . 10.4161/cc.9.15.12459 . 1963601 .
• Sato H, Azuma Y, Higai K, Matsumoto K . Altered expression of glycoproteins on the cell surface of Jurkat cells during etoposide-induced apoptosis: shedding and intracellular translocation of glycoproteins . Biochimica et Biophysica Acta (BBA) - General Subjects . 1790 . 10 . 1198–205 . Oct 2009 . 19524015 . 10.1016/j.bbagen.2009.05.019 .
• Hong C, Qiu X, Li Y, Huang Q, Zhong Z, Zhang Y, Liu X, Sun L, Lv P, Gao XM . Functional analysis of recombinant calreticulin fragment 39-272: implications for immunobiological activities of calreticulin in health and disease . Journal of Immunology . 185 . 8 . 4561–9 . Oct 2010 . 20855873 . 10.4049/jimmunol.1000536 . free .
• Alur M, Nguyen MM, Eggener SE, Jiang F, Dadras SS, Stern J, Kimm S, Roehl K, Kozlowski J, Pins M, Michalak M, Dhir R, Wang Z . Suppressive roles of calreticulin in prostate cancer growth and metastasis . The American Journal of Pathology . 175 . 2 . 882–90 . Aug 2009 . 19608864 . 2716982 . 10.2353/ajpath.2009.080417 .
• Bailey SD, Xie C, Do R, Montpetit A, Diaz R, Mohan V, Keavney B, Yusuf S, Gerstein HC, Engert JC, Anand S . Variation at the NFATC2 locus increases the risk of thiazolidinedione-induced edema in the Diabetes REduction Assessment with ramipril and rosiglitazone Medication (DREAM) study . Diabetes Care . 33 . 10 . 2250–3 . Oct 2010 . 20628086 . 2945168 . 10.2337/dc10-0452 .
• Nabi MO, Mirabzadeh A, Feizzadeh G, Khorshid HR, Karimlou M, Yeganeh MZ, Asgharian AM, Najmabadi H, Ohadi M . Novel mutations in the calreticulin gene core promoter and coding sequence in schizoaffective disorder . American Journal of Medical Genetics Part B . 153B . 2 . 706–9 . Mar 2010 . 19760677 . 10.1002/ajmg.b.31036 . 6959934 .
• Schardt JA, Eyholzer M, Timchenko NA, Mueller BU, Pabst T . Unfolded protein response suppresses CEBPA by induction of calreticulin in acute myeloid leukaemia . Journal of Cellular and Molecular Medicine . 14 . 6B . 1509–19 . Jun 2010 . 19659458 . 3829017 . 10.1111/j.1582-4934.2009.00870.x .

Notes and References

1. McCauliffe DP, Zappi E, Lieu TS, Michalak M, Sontheimer RD, Capra JD . A human Ro/SS-A autoantigen is the homologue of calreticulin and is highly homologous with onchocercal RAL-1 antigen and an aplysia "memory molecule" . The Journal of Clinical Investigation . 86 . 1 . 332–5 . Jul 1990 . 2365822 . 296725 . 10.1172/JCI114704 .
2. Web site: Entrez Gene: calreticulin.
3. Beutler E, West C, Gelbart T . HLA-H and associated proteins in patients with hemochromatosis . Molecular Medicine . 3 . 6 . 397–402 . June 1997 . 10.1007/BF03401686 . 9234244 . 2230203 .
4. Michalak M, Lynch J, Groenendyk J, Guo L, Robert Parker JM, Opas M . Calreticulin in cardiac development and pathology . Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics . 1600 . 1–2 . 32–7 . Nov 2002 . 12445456 . 10.1016/S1570-9639(02)00441-7 .
5. Book: Murphy K . Janeway's Immunobiology . 2011 . Taylor & Francis . Oxford . 978-0815342434 . 8th.
6. Web site: Entrez Gene: CALR calreticulin.
7. Nangalia J, Massie CE, Baxter EJ, Nice FL, Gundem G, Wedge DC, Avezov E, Li J, Kollmann K, Kent DG, Aziz A, Godfrey AL, Hinton J, Martincorena I, Van Loo P, Jones AV, Guglielmelli P, Tarpey P, Harding HP, Fitzpatrick JD, Goudie CT, Ortmann CA, Loughran SJ, Raine K, Jones DR, Butler AP, Teague JW, O'Meara S, McLaren S, Bianchi M, Silber Y, Dimitropoulou D, Bloxham D, Mudie L, Maddison M, Robinson B, Keohane C, Maclean C, Hill K, Orchard K, Tauro S, Du MQ, Greaves M, Bowen D, Huntly BJ, Harrison CN, Cross NC, Ron D, Vannucchi AM, Papaemmanuil E, Campbell PJ, Green AR . Somatic CALR mutations in myeloproliferative neoplasms with nonmutated JAK2 . The New England Journal of Medicine . 369 . 25 . 2391–405 . Dec 2013 . 24325359 . 3966280 . 10.1056/NEJMoa1312542 .
8. Klampfl T, Gisslinger H, Harutyunyan AS, Nivarthi H, Rumi E, Milosevic JD, Them NC, Berg T, Gisslinger B, Pietra D, Chen D, Vladimer GI, Bagienski K, Milanesi C, Casetti IC, Sant'Antonio E, Ferretti V, Elena C, Schischlik F, Cleary C, Six M, Schalling M, Schönegger A, Bock C, Malcovati L, Pascutto C, Superti-Furga G, Cazzola M, Kralovics R . Somatic mutations of calreticulin in myeloproliferative neoplasms . The New England Journal of Medicine . 369 . 25 . 2379–90 . Dec 2013 . 24325356 . 10.1056/NEJMoa1311347 . free .
9. Chao MP, Jaiswal S, Weissman-Tsukamoto R, Alizadeh AA, Gentles AJ, Volkmer J, Weiskopf K, Willingham SB, Raveh T, Park CY, Majeti R, Weissman IL . Calreticulin is the dominant pro-phagocytic signal on multiple human cancers and is counterbalanced by CD47 . Science Translational Medicine . 2 . 63 . 63ra94 . Dec 2010 . 21178137 . 4126904 . 10.1126/scitranslmed.3001375.
   • Web site: Christopher Vaughan . December 22, 2010 . Many cancer cells found to have an 'eat me' signal in study . Stanford School of Medicine . https://web.archive.org/web/20131016145928/http://med.stanford.edu/ism/2010/december/crt-signal.html . 2013-10-16.
10. Andrin C, Pinkoski MJ, Burns K, Atkinson EA, Krahenbuhl O, Hudig D, Fraser SA, Winkler U, Tschopp J, Opas M, Bleackley RC, Michalak M . Interaction between a Ca2+-binding protein calreticulin and perforin, a component of the cytotoxic T-cell granules . Biochemistry . 37 . 29 . 10386–94 . Jul 1998 . 9671507 . 10.1021/bi980595z .
11. Perrone L, Tell G, Di Lauro R . Calreticulin enhances the transcriptional activity of thyroid transcription factor-1 by binding to its homeodomain . The Journal of Biological Chemistry . 274 . 8 . 4640–5 . Feb 1999 . 9988700 . 10.1074/jbc.274.8.4640 . free .