The JAK/STAT signalingpathwayJason S. Rawlings, Kristin M.Rosler and Douglas A. Harrison*University of Kentucky, Department of Biology, 101T.H. Morgan Bldg., Lexington, KY 40506, USA

*Author for correspondence (e-mail:dough@uky.edu)

Journal of Cell Science 117, 1281-1283Published by The Company of Biologists 2004doi:10.1242/jcs.00963

The Janus kinase/signal transducers andactivators of transcription (JAK/STAT)pathway is one of a handful ofpleiotropic cascades used to transduce amultitude of signals for development andhomeostasis in animals, from humansto flies. In mammals, the JAK/STATpathway is the principal signalingmechanism for a wide array of cytokines

and growth factors. JAK activationstimulates cell proliferation,differentiation, cell migration andapoptosis. These cellular events arecritical to hematopoiesis, immunedevelopment, mammary glanddevelopment and lactation, adipogenesis,sexually dimorphic growth and otherprocesses. Predictably, mutations thatreduce JAK/STAT pathway activityaffect these processes (reviewed byIgaz et al., 2001; O’Shea et al.,2002). Conversely, mutations thatconstitutively activate or fail to regulateJAK signaling properly causeinflammatory disease, erythrocytosis,gigantism and an array of leukemias.Here we present a general overview ofthe JAK/STAT pathway and illustratethe primary mechanisms of activationand regulation of this essential signalingcascade.

Mechanistically, JAK/STAT signaling isrelatively simple, with only a fewprincipal components (for reviews, seeAaronson and Horvath, 2002; Heinrichet al., 2003; Kisseleva et al., 2002;O’Shea et al., 2002). As describedabove, a variety of ligands and theirreceptors stimulate the JAK/STATpathway. Intracellular activation occurswhen ligand binding induces themultimerization of receptor subunits.For some ligands, such as erythropoietinand growth hormone, the receptorsubunits are bound as homodimerswhile, for others, such as interferons andinterleukins, the receptor subunits areheteromultimers. For signal propagationthrough either homodimers orheteromultimers, the cytoplasmicdomains of two receptor subunits mustbe associated with JAK tyrosine kinases.JAKs are distinctive in that they have

Cell Science at a Glance 1281

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Journal of Cell Science 2004 (117, pp. 1281-1283)

The JAK/STAT Signaling PathwayJason S. Rawlings, Kristin M. Rosler and Douglas A. Harrison

STATT

STATYP-YP-Y

Y-PY-P

S-PS-P

S-PS-P

AdPs, adaptor proteins; BC, elongin B-C complex; CNTF, ciliary neurotrophic factor; CSF, colony stimulating factor; CT, cardiotrophin; CTFs, cooperating transcription factors; CUL, cullin 2; DBD, DNA-binding domain; ENH, enhancer; FERM, 4.1/ezrin/radixin/moesin domain; GAP, GTPase-activating protein (RasGAP);GM, granulocyte macrophage; GRB2, growth-factor-receptor-bound protein 2; IFN, interferon; IL, interleukin; JAK, janus kinase (just another kinase); LIF, leukemia inhibitory factor; LIG, ligand; MAPK, mitogen-activated protein kinase (ERK1/2); MEK, MAPK/ERK kinase (MAPKK, MAP kinase kinase); NPI-1, nucleoprotein F,interactor 1 (importin α

, suppressor of cytokine signaling (CIS, cytokine-inducible SH2-containing protein, JAB, JAK-binding protein, SSI, Stat-inducible Stat inhibitor);SOS, Son of Sevenless (GNEF, guanine nucleotide exchange factor); STAM, signal-transducing adapter molecule; StIP, Stat-interacting protein; S/T K, serine/threonine kinase; SUMO, small ubiquitin-related modifier (sentrin); Ub, ubiquitin; Ubc9, ubiquitin conjugase 9

AK JAK JAK Y-PP

STATTP-YP-Y

STATT

STATP-YY

Y-PY-P

GRB2

SOS

Ras RasGDPGDPPGDP GTPGTPPGTP

GAP

Raf

PY PY-PY-P

Y-PY-PY-P GRB2 YP-YY

STATP-YP-Y

Y-PY-P

S-PS-P

S-PS-P

StIP

SOCS

MEKS-PS-P

PYY-PY-P

JAK Y-PP

Y-PY-PY-P

Y-PY-P SrcY-PY-P

STATP-YP-Y

RTKs

S-PP

MAPKT-PT

PY-PY-P

(ERK1/2, p38, JNK,(PKCδ, FRAP)

(SOCS, Nmi, Bcl-XL, p21, MYC, NOS2, etc.)

(SHP-2,HP 2Shc, etc.)

Activation Inactivation

AdPsPYY-PY-P

NPI-1

P-YP-Y

JAKP-YPSOCS

SOCS

JAK

PIAS PIASUbc9

SUMOSSUMOSUMO

SUMOSUMO

P-YP-YSOCSBC

BC Ubb

LIG

LIG

P-YP-Y

JAKP-YP

STATT

STATP-YY

Y-PY-P

S-PS-P

S-PS-P

Ran

PY-PY-P

STAM

STAM

SSH2-BSH2-BYP-YP-Y

S/T K

SHP-1

SHP-1

SHP-1

YY

Transmembranedomain

Box 1Box 2

Phosphorylationtarget/docking site

{JAKbinding

JAK-bindingreceptors

Janus kinases (JAKs)

Signal transducers andactivators of transcription (STATs)

Receptorbinding

Phosphotyrosinebinding

Non-catalytickinase-like

domain(regulatory)

Catalyticdomain

Y S

Oligomerizationdomain

DNA-bindingdomain

Phosphotyrosine binding (recruitment

and dimerization)

Transcriptionalactivation domain

Coiled coil(protein interaction)

FERM Ψ-Kinase Kinase

gOligo DBD ADCoil SH2

Suppressors ofcytokine signaling (SOCS)

SOCS Box(ubiquitin ligase

binding)Phosphotyrosinebinding

(recruitment)

Kinase inhibitorydomain

KI SH2 SOCS

Protein inhibitorsof activated STATs (PIAS)

SAP domain(target binding)

RING fingerdomain

(SUMO transfer)

Serine/threonine-rich domain

(target binding)

SAP Zn-RF S/T

SH2

STATYP-YP-Y

Y-PY-P

S-PS-P

(p48, Sp1, Jun,Fos, GR, NF-κB,

SMAD, etc.)

Cofactors (p300/CBP, BRCA1,Nmi-1, MCM5, etc.)

CTFs

Mam als DrDroro o ilala aa s DiDi um *iga ds 2-3 ( pd, Upd , U d2?) 0 0

Rec pto s 1-2 (Dome, C 14 25?) 0 0

4

s 1 3 ( -c)

S 8 o )

? Function in JAK/STAT signaling not yet established

7 (STA TAT6)

Many (below)

Many (below)

IFN- //β IFN-γ IL-10C G-CSF, LIF O M I -12 Le tin

IL 3 IL-5GM- S

I -2 IL-I IL- 5 IL-4 I -1

Gr wthHormone Pr lac in

Th om o-p ietin

An io-tensi er to in

1

2ST T1

a/bST T6

GP RsIn ro s s

Abbreviations and synonyms

jcs.biologists.org

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tandem kinase-homologous domains atthe C-terminus. The first is a non-catalytic regulatory domain, whereasthe second has tyrosine kinase activity.In mammals, the JAK familycomprises four members: JAK1, JAK2,JAK 3 and Tyk2. JAK activationoccurs upon ligand-mediated receptormultimerization because two JAKs arebrought into close proximity, allowingtrans-phosphorylation. The activatedJAKs subsequently phosphorylateadditional targets, including both thereceptors and the major substrates,STATs. STATs are latent transcriptionfactors that reside in the cytoplasm untilactivated. The seven mammalian STATsbear a conserved tyrosine residue nearthe C-terminus that is phosphorylated byJAKs. This phosphotyrosine permitsthe dimerization of STATs throughinteraction with a conserved SH2domain. Phosphorylated STATs enterthe nucleus by a mechanism that isdependent on importin α-5 (also callednucleoprotein interactor 1) and the Rannuclear import pathway. Once in thenucleus, dimerized STATs bind specificregulatory sequences to activate orrepress transcription of target genes.Thus the JAK/STAT cascade provides adirect mechanism to translate anextracellular signal into a transcriptionalresponse.

In addition to the principal componentsof the pathway, other effector proteinshave been identified that contribute to atleast a subset of JAK/STAT signalingevents. STAMs (signal-transducingadapter molecules) are adaptermolecules with conserved VHS andSH3 domains (Lohi and Lehto, 2001).STAM1 and STAM2A can bephosphorylated by JAK1-JAK3 in amanner that is dependent on a thirddomain present in some STAMs, theITAM (inducible tyrosine-basedactivation motif). Through a poorlyunderstood mechanism, the STAMsfacilitate the transcriptional activation ofspecific target genes, including MYC.A second adapter that facilitatesJAK/STAT pathway activation is StIP(stat-interacting protein), a WD40protein. StIPs can associate with bothJAKs and unphosphorylated STATs,perhaps serving as a scaffold to facilitatethe phosphorylation of STATs by JAKs.A third class of adapter with function

in JAK/STAT signaling is theSH2B/Lnk/APS family. These proteinscontain both pleckstrin homology andSH2 domains and are also substrates forJAK phosphorylation. Both SH2-Bβ andAPS associate with JAKs, but the formerfacilitates JAK/STAT signaling whilethe latter inhibits it. The degree towhich each of these adapter familiescontributes to JAK/STAT signaling isnot yet well understood, but it is clearthat various proteins outside thebasic pathway machinery influenceJAK/STAT signaling.

In addition to JAK/STAT pathwayeffectors, there are three major classes ofnegative regulator: SOCS (suppressorsof cytokine signaling), PIAS (proteininhibitors of activated stats) andPTPs (protein tyrosine phosphatases)(reviewed by Greenhalgh and Hilton,2001). Perhaps the simplest are thetyrosine phosphatases, which reversethe activity of the JAKs. The bestcharacterized of these is SHP-1, theproduct of the mouse motheatengene.SHP-1 contains two SH2 domains andcan bind to either phosphorylated JAKsor phosphorylated receptors to facilitatedephosphorylation of these activatedsignaling molecules. Other tyrosinephosphatases, such as CD45, appear tohave a role in regulating JAK/STATsignaling through a subset of receptors.

SOCS proteins are a family of at leasteight members containing an SH2domain and a SOCS box at the C-terminus (reviewed by Alexander,2002). In addition, a small kinaseinhibitory region located N-terminal tothe SH2 domain has been identified forSOCS1 and SOCS3. The SOCScomplete a simple negative feedbackloop in the JAK/STAT circuitry:activated STATs stimulate transcriptionof the SOCS genes and the resultingSOCS proteins bind phosphorylatedJAKs and their receptors to turn off thepathway. The SOCS can affect theirnegative regulation by three means.First, by binding phosphotyrosines onthe receptors, SOCS physically block therecruitment of signal transducers, suchas STATs, to the receptor. Second,SOCS proteins can bind directly to JAKsor to the receptors to specifically inhibitJAK kinase activity. Third, SOCSinteract with the elongin BC complex

and cullin 2, facilitating theubiquitination of JAKs and, presumably,the receptors. Ubiquitination of thesetargets decreases their stability bytargeting them for proteasomaldegradation.

The third class of negative regulator isthe PIAS proteins: PIAS1, PIAS3,PIASx and PIASy. These proteins havea Zn-binding RING-finger domain inthe central portion, a well-conservedSAP (SAF-A/Acinus/PIAS) domain atthe N-terminus, and a less-well-conserved carboxyl domain. The latterdomains are involved in target proteinbinding. The PIAS proteins bind toactivated STAT dimers and preventthem from binding DNA. Themechanism by which PIAS proteins actremains unclear. However, PIASproteins have recently beendemonstrated to associate with the E2conjugase Ubc9 and to have E3conjugase activity for sumoylation thatis mediated by the RING finger domain(reviewed by Jackson, 2001). Althoughthere is evidence that STATs can bemodified by sumoylation (Rogers et al.,2003), the function of that modificationin negative regulation is not yet known.

Although the mechanism of JAK/STATsignaling is relatively simple in theory,the biological consequences of pathwayactivation are complicated byinteractions with other signalingpathways (reviewed by Heinrich et al.,2003; Rane and Reddy, 2000; Shuai,2000). An understanding of this cross-talk is only beginning to emerge, but thebest characterized interactions of theJAK/STAT pathway are with thereceptor tyrosine kinase (RTK)/Ras/MAPK (mitogen-activated proteinkinase) pathway. The relationshipbetween these cascades is complex andtheir paths cross at multiple levels, eachenhancing activation of the other. First,activated JAKs can phosphorylatetyrosines on their associated receptorsthat can serve as docking sites for SH2-containing adapter proteins from othersignaling pathways. These includeSHP-2 and Shc, which recruit theGRB2 adapter and stimulate the Rascascade. The same mechanismstimulates other cascades, such as therecruitment and JAK phosphorylationof insulin receptor substrate (IRS) and

Journal of Cell Science 117 (8)

p85, which results in the activation ofthe phosphoinositide 3-kinase (PI3K)pathway [for more on PI3K signaling,see Foster et al. (Foster et al., 2003)].JAK/STAT signaling also indirectlypromotes Ras signaling through thetranscriptional activation of SOCS3.SOCS3 binds RasGAP, a negativeregulator of Ras signaling, and reducesits activity, thereby promotingactivation of the Ras pathway.Reciprocally, RTK pathway activitypromotes JAK/STAT signaling by atleast two mechanisms. First, theactivation of some RTKs, includingEGFR and PDGFR, results in the JAK-independent tyrosine phosphorylationof STATs, probably by the Src kinase.Second, RTK/Ras pathway stimulationcauses the downstream activationof MAPK. MAPK specificallyphosphorylates a serine near the C-terminus of most STATs. While notabsolutely necessary for STATactivity, this serine phosphorylationdramatically enhances transcriptionalactivation by STAT. In addition to RTKand PI3K interactions with JAK/STATsignaling, multiple levels of cross-talkwith the TGF-β signaling pathway havebeen recently reported [for a reviewof TGF-β, see (Moustakas, 2002)].Furthermore, the functions of activatedSTATs can be altered throughassociation with other transcriptionfactors and cofactors that are regulatedby other signaling pathways. Thus theintegration of input from manysignaling pathways must be consideredif we are to understand the biologicalconsequences of cytokine stimulation.

The JAK/STAT pathway is ubiquitousamongst vertebrates, but can also befound as an intact pathway in some, butnot all, other metazoans. A completeJAK/STAT signaling pathway requiredfor an abundance of biological functionsis found in Drosophila(for reviews, seeHombria and Brown, 2002; Zeidler etal., 2000). There are single homologuesfor JAK, STAT and PIAS, as well asthree SOCS homologues. Each of thesecomponents acts in the same fashion asits mammalian counterpart, although nomutants in the fly SOCS genes areyet available. Furthermore, there is asingle JAK/STAT pathway receptor,Domeless, that shows weak similaritiesto the cytokine-binding modules of the

vertebrate interleukin 6 (IL-6) receptorfamily. A predicted gene, CG14225,encodes a putative protein withsimilarity to Domeless, but no functionalanalyses have been reported. Onlymembers of the Unpaired (Upd) familyof secreted ligands, Upd and Upd3, havebeen shown to activate the JAK/STATpathway in flies (Agaisse et al., 2003;Harrison et al., 1998). Activation by athird homologue, Upd2, has yet to bereported. Although the Unpaireds bearno sequence similarity to cytokines, thepredicted highly α-helical nature isconsistent with an overall structure thatcould be similar to cytokines.Sequencing of the Drosophila genomehas uncovered potential homologues ofother JAK/STAT pathway molecules,including STAM, StIP and SH2-B/APS/Lnk family adapters. Nomutations of these genes are yetavailable to test their function inJAK/STAT signaling. For some otherorganisms, such as C. elegans andDictyostelium, there is not a completefunctional JAK/STAT cassette, buthomologues of some JAK/STATpathway proteins can be found (Dearolf,1999; Hombria and Brown, 2002). Forinstance, STAT homologues inDictyostelium are necessary forchemotaxis, prestalk cell movementduring differentiation, and repression ofstalk-specific genes. However, there areno JAKs in Dictyostelium, but ratherSTAT activation is mediated by G-protein-coupled receptors (GPCRs)(reviewed by Williams, 2000). Althoughthe link is still controversial and themechanism unclear, there is mountingevidence that GPCRs also signal throughSTATs in mammals. This potentialcommonality in mechanism suggeststhat portions of the JAK/STAT pathwaymay have arisen even prior to theappearance of metazoans. Genomesequences from additional organismsand mutants in identifiable homologueswill greatly aid elucidation of themechanism of evolution of theJAK/STAT signaling pathway.

ReferencesAaronson, D. S. and Horvath, C. M.(2002). Aroad map for those who know JAK-STAT. Science296, 1653-1655.Agaisse, H., Petersen, U. M., Boutros, M.,Mathey-Prevot, B. and Perrimon, N. (2003).Signaling role of hemocytes in Drosophila

JAK/STAT-dependent response to septic injury.Dev. Cell5, 441-450.Alexander, W. S.(2002). Suppressors of cytokinesignalling (SOCS) in the immune system. Nat. Rev.Immunol.2, 410-416.Dearolf, C. R. (1999). JAKs and STATs ininvertebrate model organisms. Cell. Mol. Life Sci.55, 1578-1584.Foster, F. M., Traer, C. J., Abraham, S. M. andFry, M. J. (2003). The phosphoinositide (PI) 3-kinase family. J. Cell Sci.116, 3037-3040.Greenhalgh, C. J. and Hilton, D. J. (2001).Negative regulation of cytokine signaling. Journalof Leukocyte Biology70, 348-356.Harrison, D. A., McCoon, P. E., Binari, R.,Gilman, M. and Perrimon, N. (1998). Drosophilaunpaired encodes a secreted protein that activatesthe JAK signaling pathway. Genes Dev. 12, 3252-3263.Heinrich, P. C., Behrmann, I., Haan, S.,Hermanns, H. M., Muller-Newen, G. andSchaper, F.(2003). Principles of interleukin (IL)-6-type cytokine signalling and its regulation.Biochemical J.374, 1-20.Hombria, J. C. and Brown, S.(2002). The fertilefield of Drosophila Jak/STAT signalling. Curr.Biol. 12, R569-R575.Igaz, P., Toth, S. and Falus, A.(2001). Biologicaland clinical significance of the JAK-STATpathway; lessons from knockout mice. Inflamm.Res.50, 435-441.Jackson, P. K.(2001). A new RING for SUMO:wrestling transcriptional responses into nuclearbodies with PIAS family E3 SUMO ligases. GenesDev. 15, 3053-3058.Kisseleva, T., Bhattacharya, S., Braunstein, J.and Schindler, C. W. (2002). Signaling throughthe JAK/STAT pathway, recent advances andfuture challenges. Gene285, 1-24.Lohi, O. and Lehto, V.-P. (2001).STAM/EAST/Hbp adapter proteins – integratorsof signalling pathways. FEBS Lett.508, 287-290.Moustakas, A. (2002). Smad signalling network.J. Cell Sci.115, 3355-3356.O’Shea, J. J., Gadina, M. and Schreiber, R. D.(2002). Cytokine signaling in 2002: new surprisesin the Jak/Stat pathway. Cell 109 Suppl. S121-S131.Rane, S. G. and Reddy, E. P.(2000). Januskinases: components of multiple signalingpathways. Oncogene19, 5662-5679.Rogers, R. S., Horvath, C. M. and Matunis, M.J. (2003). SUMO modification of STAT1 and itsrole in PIAS-mediated inhibition of geneactivation. J. Biol. Chem.278, 30091-30097.Shuai, K. (2000). Modulation of STAT signalingby STAT-interacting proteins. Oncogene19, 2638-2644.Williams, J. G. (2000). STAT signalling in cellproliferation and in development. Curr. Opin.Genet. Dev.10, 503-507.Zeidler, M. P., Bach, E. A. and Perrimon, N.(2000). The roles of the Drosophila JAK/STATpathway. Oncogene19, 2598-2606.

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