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IL-3 Receptor John W. Schrader* Department of Medicine, Biomedical Research Center, University of British Columbia, 2222 Health Science Mall, Vancouver, British Columbia, Canada V6T 1Z3 *corresponding author tel: 604-822-7810, fax: 604-822-7815, e-mail: [email protected] DOI: 10.1006/rwcy.2000.20001. SUMMARY commoninthehuman,oreitherAIC2AorAIC2Bin the mouse, to generate a high-affinity IL-3 receptor. The receptor for IL-3 is made up of two subunits, both members of the cytokine receptor superfamily. Alternative names The smaller (cid:11) subunit has a low affinity for IL-3 and the complex of IL-3 and the (cid:11) subunit binds to the (cid:12) AIC2A and AIC2B in the mouse refer respectively subunit to form a high-affinity signaling complex. to the murine product of the duplicated (cid:12) common The (cid:12) subunit is shared with the receptors for GM- gene that binds IL-3 with low affinity, and the CSF and IL-5, hence its name, (cid:12) common ((cid:12)c), and ortholog of (cid:12) common in the human which can alone,hasnoaffinityforanyofthesethreecytokines. interact with the (cid:11) chains of the IL-3, GM-CSF or In the mouse, however, there is an additional, IL-3- IL-5 receptors. specific (cid:12) subunit, which is encoded by a duplication of the (cid:12)c gene and which has itself a low affinity for IL-3. Signaling downstream of the receptor complex Structure involves activation of JAK2 kinase, tyrosine phos- phorylationof(cid:12)candtheinitiationofsignalingpaths The three-dimensional structures of the (cid:11) and (cid:12) regulating growth, survival, and differentiation. subunits of the IL-3 receptor are not yet available. In the case of both the (cid:11) and (cid:12) subunits, the deduced BACKGROUND proteinsequencesoftheextracellulardomainspredict structures that are homologous to those of other Discovery receptors in this family, for which structures are available such as the receptors for growth hormone, prolactin, and erythropoietin. The first IL-3-binding protein cloned was the murine AIC2A protein (Itoh et al., 1990). This bound IL-3 with low affinity and was homologous with a closely Main activities and related molecule, AIC2B (Gorman et al., 1990). pathophysiological roles AIC2A is the result of the recent duplication in the mouse of the AIC2B gene, which is the murine ortholog of the (cid:12) common chain ((cid:12)c). (cid:12)c is shared The only known activity of the (cid:11) unit of the IL-3 between the receptors for IL-3, GM-CSF, and IL-5 receptor is to bind IL-3. As noted above, the (cid:12) and interacts with complexes of GM-CSF, IL-3, or common chain is able to interact with complexes of IL-5withtheirrespectivelow-affinity(cid:11)subunits.The the IL-3, GM-CSF, and IL-5 receptors, each with (cid:11) subunit of the IL-3 receptor was subsequently their respective (cid:11) subunits. There is no evidence that discovered (Kitamura et al., 1991). It was shown to complexesoftheIL-3receptor(cid:11)and(cid:12) subunitsexist bindIL-3withlowaffinityandthentointeractwith(cid:12) in the absence of IL-3. In contrast, in the case of the 1900 John W. Schrader relatedGM-CSFreceptor,thereisevidencethatthe(cid:11) the characteristic distribution of cysteines, and the and (cid:12) subunits of the receptor are associated before WSXWSmotifcharacteristicofreceptorsofthecyto- the ligand binds. kine family. The cytoplasmic domain of the human IL-3 receptor (cid:11) subunit is relatively short and contains no tyrosines (the murine ortholog has one) GENE but does contain a proline-rich motif (‘Box 1’ motif) characteristic of the cytoplasmic domain of receptors Accession numbers of this family. The extracellular domains of (cid:12)c (or AIC2A) again showthecharacteristicfeaturesofthemembersofthe Mouse (cid:11) subunit: X64534 cytokine receptor family but are larger than that of Human (cid:11) subunit: M74782 the (cid:11) subunit as a result of a duplication of the basic Human (cid:12)c receptor subunit: A39255 cytokine receptor domain. The cytoplasmic domains Mouse (cid:12)c receptor subunit: P26955 (AIC2B), of (cid:12)c or AIC2A have multiple tyrosines, many of AAA39295 (AIC2A) which have been shown by mutational studies to be relevant to receptor function and to be phosphory- Sequence lated following binding of IL-3. The murine IL-3 receptor (cid:11) subunit gene spans 10kb and includes 12exons. The general genomic structure Relevant homologies and species is similar to that of other genes in the cytokine differences receptor family, consistent with their common phy- logeneticorigin(Miyajimaetal.,1995).Thepromoter of the IL-3 receptor (cid:11) subunit gene in the mouse has As noted only in the mouse, are there two (cid:12) subunits potential sites for GATA, Ets, c-myb, p1, and AP-2. of the IL-3 receptor, the additional (cid:12) chain AIC2A having arisen by duplication of the (cid:12)c gene, and evolved the capacity to itself bind IL-3. Chromosome location and linkages Inthemouse,thegenefortheIL-3receptor(cid:11)subunit Cell types and tissues expressing is on chromosome 14, whereas that encoding the the receptor corresponding (cid:11) subunit for the GM-CSF receptor is onchromosome19.Inthehuman,thegenesforthe(cid:11) subunits of both the IL-3 receptor and the GM-CSF TheIL-3receptor(cid:11)subunitisexpressedonthebroad receptor are tightly linked and are on the pseudo- rangeofcellsofhematopoieticoriginuponwhichitis autosomal regions of the X (Xp22.3) and Y (Yp11.3) alive. It is lacking on mature T and B lymphocytes chromosomes. and neutrophils but present on most other nucleated cells derived from the pluripotential hematopoietic stem cells. There is some evidence for expression of PROTEIN the IL-3 receptor on endothelial cells. The (cid:12)c subunit is expressed more widely, being also present on Accession numbers neutrophils. Mouse (cid:11) subunit: X64534 Regulation of receptor expression Human (cid:11) subunit: M74782 Human (cid:12)c receptor subunit: A39255 Mouse (cid:12)c receptor subunit: P26955 (AIC2B), Binding of its ligand results in internalization and AAA39295 (AIC2A) downregulation of the levels of the IL-3 receptor on the cell surface. There is some evidence for upregulation of expression of mRNA encoding the IL-3 Description of protein receptor (cid:11) subunit by IL-3. A genetic mutation leads todecreasedexpressionoftheIL-3receptor(cid:11)subunit The(cid:11)subunitoftheIL-3receptorisamemberofthe and thus diminished responses to IL-3 in certain cytokine receptor family. Its deduced protein strains of mice (e.g. A/J) (Ichihara et al., 1995; Leslie sequence has 378 amino acids in the human and has et al., 1996). IL-3 Receptor 1901 SIGNAL TRANSDUCTION Elk-1. IL-3 also activates members of the other two familiesoftheMAPkinasesuperfamily,thep38MAP Associated or intrinsic kinases kinases, and the JNK/stress-activated kinases. Attheheadofanotherpathwayactivatedbyphos- phorylation of the activated IL-3 receptor complex is JAK2 kinase appears to be associated with the active the lipid kinase PI-3 kinase. The SH2 domains of the ligand-bound complex of the IL-3 receptor (cid:11) subunit p85 subunit of PI-3 kinase bind to phosphotyrosines and (cid:12)c or AIC2A. There is some evidence suggesting on SHP2andIRS2, anotherofthe proteinsthat bind that JAK2 kinase may associate constitutively with to the receptor complex via a PTB domain and the (cid:12)c. The cytoplasmic domain of the IL-3 receptor themselves become phosphorylated on tyrosine. This (cid:11) subunit is absolutely required for IL-3-mediated approximation of PI-3 kinase with its lipid substrates signaling and one hypothesis is that the cytoplasmic in the membrane results in increased levels of the domain of the IL-3 receptor (cid:11) subunit is involved in products of its action. These phospholipids in turn recruiting JAK kinase to the heterodimeric, ligand- activate the PH domains of two enzymes PDK1 and induced complex of the (cid:11) and (cid:12) subunits. PDK2, which phosphorylate and activate two serine/ threonine protein kinases, PKB and p70 S6 kinase. PI-3 kinase activity is absolutely required for IL-3- Cytoplasmic signaling cascades induced increases in levels of c-Myc mRNA, but p70 S6kinaseactivityisnot,suggestingthatPKBactivity An early event is activation of the JAK2 protein is required for IL-3-induced upregulation of c-Myc. tyrosinekinase.Asnotedabove,thismayresult from Also docking onto the activated IL-3 receptor apposition of two JAK2 kinase molecules brought complex are molecules with counter-regulatory activ- together by ligand-induced approximation of the (cid:11) ity. These include the tyrosine phosphotase SHP2, and (cid:12) subunits. In keeping with this notion, there is and the lipid phosphatase SHIP1, which dephos- evidence that the formation of simple heterodimers phorylates products of PI-3 kinase action. of the cytoplasmic domains of the (cid:11) and (cid:12) subunits is both sufficient and necessary for mitogenesis (Orban et al., 1999). However, higher order com- DOWNSTREAM GENE plexes involving multiple (cid:12) chains may also be ACTIVATION formed. The cytoplasmic domain of (cid:12)c or AIC2A have multiple tyrosines, many of which have been Transcription factors activated shown by mutational studies to be relevant to receptorfunctionandtobephosphorylatedfollowing bindingofIL-3.Thesephosphotyrosineresiduesserve IL-3-induced activation of tyrosine kinases leads to as docking sites. The PTB domain of the adapter activation of STAT5a and STAT5b transcription protein Shc links the activated receptor via Grb2 to factors. These cytoplasmic factors dock onto phos- the Ras exchange factor mSOS1. Another potential photyrosinesonthe(cid:12) subunitandthemselvesbecome linktotheRaspathwayisprovidedbythebindingto phosphorylated on tyrosine by JAK2 kinase. This (cid:12)c via its SH2 domain of the tyrosine phosphatase results in dimerization and translocation to the nu- SHP2, which itself becomes tyrosine phosphorylated cleus,wheretheybindpromoterswithGASelements. and serves as a further docking site for complexes of Grb2andmSOS1.ThetranslocationofmSOS1tothe membranebringsitintoproximitywithitssubstrates, BIOLOGICAL CONSEQUENCES the small GTPases of the Ras family. IL-3 binding OF ACTIVATING OR INHIBITING results in activation of M-Ras, a 29kDa relative of RECEPTOR AND p21rasandprobablyalsoofp21ras,althoughthishas not been demonstrated formally. PATHOPHYSIOLOGY Activation of the Ras family leads to activation of the MAP kinase family through cascades of serine/ Unique biological effects of threonine kinases. Activation of p21ras leads to activating the receptors activation of Raf-1, and in turn MEK1 and MEK2 and then the ERK1 and ERK2 kinases. One of the substrates of the ERK kinases is Stathmin, a protein There is little evidence for any unique biological involvedinregulationofthestabilityofmicrotubules. effects downstream of the IL-3 receptor. Many of its Others include transcription factors such as c-Myc or best-studied effects, such as stimulation of growth 1902 John W. Schrader and survival, involve paths activated by many recep- Ichihara,M.,Hara,T.,Takagi,M.,Cho,L.C.,Gorman,D.M., torssuchastheRas/MAPkinases,PI-3kinase,Bcl-2, and Miyajima, A. (1995). Impaired interleukin-3 (IL-3) responseoftheA/Jmouseiscausedbyabranchpointdeletion etc. STAT5 is also activated downstream of other in the IL-3 receptor alpha subunit gene. EMBO J. 14, 939– cytokine receptors (e.g. those for erythropoietin or 950. IL-2) and any unique effects are likely to result from Itoh, N., Yonehara, S., Schreurs, J., Gorman, D. M., combinatorial and quantitative balances of signals Maruyama, K., Ishii, A., Yahara, I., Arai, K., and also involved in signaling from other receptors. Miyajima, A. (1990). Cloning of an interleukin-3 receptor gene: a member of a distinct receptor gene family. Science 247,324–327. Phenotypes of receptor knockouts Kitamura, T., Sato, N., Arai, K., and Miyajima, A. (1991). Expression cloning of the human IL-3 receptor cDNA reveals and receptor overexpression mice asharedbetasubunitforthehumanIL-3andGM-CSFrecep- tors.Cell66,1165–1174. The knockout of the (cid:12)c gene results in a phenotype Leslie, K. B., Jalbert, S., Orban, P., Welham, M., Duronio, V., and Schrader, J. W. (1996). Genetic basis of hypo-responsive- similar to that seen in the GM-CSF knockout. nessofA/Jmicetointerleukin-3.Blood87,3186–3194. Miyajima,I.,Levitt,L.,Hara,T.,Bedell,M.A.,Copeland,N.G., Jenkins,N.A.,andMiyajima,A.(1995).Themurineinterleu- References kin-3 receptor alpha subunit gene: chromosomal localization, genomic structure, and promoter function. Blood 85, 1246– Gorman,D.M.,Itoh,N.,Kitamura,T.,Schreurs,J.,Yonehara,S., 1253. Yahara, I., Arai, K., and Miyajima, A. (1990). Cloning and Orban, P. C., Levings, M. K., and Schrader, J. W. (1999). expression of a gene encoding an interleukin 3 receptor-like Heterodimerization of the alpha and beta chains of the inter- protein:identificationofanothermemberofthecytokinerecep- leukin-3 (IL-3) receptor is necessary and sufficient for IL-3- torgenefamily.Proc.NatlAcad.Sci.USA87,5459–5463. inducedmitogenesis.Blood94,1614–1622.