Janus kinase 3

Tyrosine-protein kinase JAK3 is an is_associated_with::enzyme that in humans is encoded by the JAK3 is_associated_with::gene.

Janus kinases
is_associated_with::Janus kinase 3 is a tyrosine kinase that belongs to the janus family of kinases. Other members of the Janus family include is_associated_with::JAK1, is_associated_with::JAK2 and is_associated_with::TYK2. is_associated_with::Janus kinases (JAKs) are relatively large kinases of approximately 1150 amino acids with apparent molecular weights of 120-130 kDa. They are cytosolic tyrosine kinases that are specifically associated with cytokine receptors. Since cytokine receptor proteins lack enzymatic activity, they are dependent upon JAKs to initiate signaling upon binding of their ligands (e.g. is_associated_with::cytokines). The cytokine receptors can be divided into five major subgroups based on their different domains and activation motifs. JAK3 is required for signaling of the type I receptors that use the is_associated_with::common gamma chain (γc).

Function
In contrast to the relatively ubiquitous expression of is_associated_with::JAK1, is_associated_with::JAK2 and Tyk2, JAK3 is predominantly expressed in is_associated_with::hematopoietic lineage such as is_associated_with::NK cells, is_associated_with::T cells and is_associated_with::B cells and intestinal is_associated_with::epithelial cells. JAK3 functions in signal transduction and interacts with members of the STAT (signal transduction and activators of transcription) family. JAK3 is predominantly expressed in immune cells and transduces a signal in response to its activation via tyrosine phosphorylation by interleukin receptors. Mutations that abrogate Janus kinase 3 function cause an autosomal SCID (is_associated_with::severe combined immunodeficiency disease).

Jak3 interacts with actin-binding protein villin, thereby facilitating is_associated_with::cytoskeletal remodeling and mucosal wound repair. Structural determinants that regulate the interactions between Jak3 and cytoskeletal proteins of the is_associated_with::villin / is_associated_with::gelsolin family have also been characterized. Functional reconstitution of kinase activity by recombinant Jak3 using Jak3-wt or villin/gelsolin-wt as substrate showed that Jak3 autophosphorylation was the rate-limiting step during interactions between Jak3 and cytoskeletal proteins. Kinetic parameters showed that phosphorylated (P) Jak3 binds to P-is_associated_with::villin with a dissociation constant (Kd) of 23 nM and a Hill's coefficient of 3.7. Pairwise binding between Jak3 mutants and villin showed that the is_associated_with::FERM domain of Jak3 was sufficient for binding to P-villin with a Kd of 40.0 nM. However, the is_associated_with::SH2 domain of Jak3 prevented P-villin from binding to the FERM domain of nonphosphorylated protein. The intramolecular interaction between the FERM and SH2 domains of nonphosphorylated Jak3 prevented Jak3 from binding to villin and tyrosine autophosphorylation of Jak3 at the SH2 domain decreased these intramolecular interactions and facilitated binding of the FERM domain to villin. These demonstrate the molecular mechanism of interactions between Jak3 and cytoskeletal proteins where tyrosine phosphorylation of the SH2 domain acted as an intramolecular switch for the interactions between Jak3 and cytoskeletal proteins.

Sustained damage to the mucosal lining in patients with is_associated_with::inflammatory bowel disease (IBD) facilitates translocation of intestinal microbes to submucosal immune cells leading to chronic inflammation. IL-2 plays a role in intestinal epithelial cell (IEC) homeostasis through concentration-dependent regulation of IEC proliferation and cell death. Activation by IL-2 led to tyrosine phosphorylation-dependent interactions between Jak3 and p52ShcA only at lower concentrations. Higher concentrations of IL-2 decreased the phosphorylation of Jak3, disrupted its interactions with p52ShcA, redistributed Jak3 to the nucleus, and induced apoptosis in IEC. IL-2 also induced dose-dependent downregulation of jak3-mRNA. Constitutive overexpression and mir-shRNA-mediated knockdown studies showed that expression of Jak3 was necessary for IL-2-induced proliferation of IEC. Additionally, IL-2-induced downregulation of jak3-mRNA was responsible for higher IL-2-induced apoptosis in IEC. Thus IL-2-induced mucosal homeostasis through posttranslational and transcriptional regulation of Jak3.

Jak3 is also implicated in mucosal differentiation and predisposition to inflammatory bowel disease in mice model. These studies show that Jak3 is expressed in colonic mucosa of mice, and the loss of mucosal expression of Jak3 results in reduced expression of differentiation markers for the cells of both enterocytic and secretory lineages. Jak3 KO mice showed reduced expression of colonic villin, carbonic anhydrase, secretory mucin muc2, and increased basal colonic inflammation reflected by increased levels of pro-inflammatory cytokines IL-6 and IL-17A in colon along with increased colonic myeloperoxidase activity. The inflammations in KO mice were associated with shortening of colon length, reduced cecum length, decreased crypt heights, and increased severity toward dextran sulfate sodium-induced colitis. In differentiated human colonic epithelial cells, Jak3 redistributed to basolateral surfaces and interacted with adherens junction (AJ) protein β-catenin. Jak3 expression in these cells was essential for AJ localization of β-catenin and maintenance of epithelial barrier functions. Collectively, these results demonstrate the essential role of Jak3 in the colon where it facilitated mucosal differentiation by promoting the expression of differentiation markers and enhanced colonic barrier functions through AJ localization of β-catenin. Though constitutive activation of Janus kinase 3 (Jak3) leads to different cancers, the mechanism of trans-molecular regulation of Jak3 activation is only recently reported. This study showed that Jak3 auto-phosphorylation was the rate limiting step during Jak3 trans-phosphorylation of Shc where Jak3 directly phosphorylated (P) two tyrosine residues in SH-2-domain, and one tyrosine residue each in CH-1, and PID domains of Shc. Direct interactions between mutants of Jak3 and Shc showed that while FERM domain of Jak3 was sufficient for binding to Shc, CH-1 and PID domains of Shc were responsible for binding to Jak3. Functionally, Jak3 was auto-phosphorylated under IL-2 stimulation in epithelial cells. However, Shc recruited tyrosine phosphatase SHP-2 and PTP-1B to Jak3 and thereby dephosphorylate Jak3. Thus the study not only characterized Jak3 interaction with Shc, but also demonstrated the mechanism of intracellular regulation of Jak3 activation where Jak3 interactions with Shc acted as a regulator of Jak3 dephosphorylation through direct interactions of Shc with both Jak3 and tyrosine phosphatases.

As JAK3 is expressed in is_associated_with::hematopoietic cells, its role in cytokine signaling is thought to be more restricted than other JAKs. It is most commonly expressed in is_associated_with::T cells and is_associated_with::NK cells, but has been induced in other is_associated_with::leukocytes, including is_associated_with::monocytes. JAK3 is involved in signal transduction by receptors that employ the is_associated_with::common gamma chain (γc) of the is_associated_with::type I cytokine receptor family (e.g. IL-2R, IL-4R, IL-7R, IL-9R, IL-15R, and IL-21R). Mutations of JAK3 result in is_associated_with::severe combined immunodeficiency (SCID). Mice that do not express JAK3 have is_associated_with::T-cells and is_associated_with::B-cells that fail to respond to many cytokines.

In addition to its well-known roles in is_associated_with::T cells and is_associated_with::NK cells, JAK3 has been found to mediate IL-8 stimulation in human is_associated_with::neutrophils. IL-8 primarily functions to induce is_associated_with::chemotaxis in is_associated_with::neutrophils and is_associated_with::lymphocytes, and JAK3 silencing severely inhibits IL-8-mediated chemotaxis.

Signal transduction model


JAK3 is activated only by is_associated_with::cytokines whose receptors contain the is_associated_with::common gamma chain (γc) subunit: IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21. Cytokine binding induces the association of separate cytokine receptor subunits and the activation of the receptor-associated JAKs. In the absence of cytokine, JAKs lack protein is_associated_with::tyrosine kinase activity. Once activated, the JAKs create docking sites for the STAT transcription factors by phosphorylation of specific tyrosine residues on the cytokine receptor subunits. STATs (signal transduction and activators of transcription) are members of a family of transcription factors, and they have src homology 2 (SH2) domains that allow them to bind to these phosphorylated tyrosine residues. After undergoing JAK-mediated phosphorylation, the STAT transcription factors dimerize, translocate to the nucleus, bind DNA at specific elements and induce expression of specific genes. Cytokine receptors selectively activate particular JAK-STAT pathways to induce transcription of different genes. IL-2 and IL-4 activate JAK1, JAK3 and is_associated_with::STAT5.

Disease relevance
Activating mutations in JAK1, JAK2, and JAK3 have been identified as causes of hematological cancers, while inactivating mutations of JAK3 are known causes of immune deficiency. Mutations in the is_associated_with::common gamma chain (γc) result in X-linked severe combined immunodeficiency (is_associated_with::X-SCID). Since γc specifically associates with JAK3, mutations in JAK3 also result in SCID. Deficiency of JAK3 blocks signaling of the following cytokines and their effects: Overall, JAK3 deficiency results in the phenotype of SCID characterized by T&minus;B+NK&minus;, which indicates the absence of is_associated_with::T cells and is_associated_with::NK cells. Although is_associated_with::B cells are present, they are non-functional due to defective B cell activation and impaired antibody class switching.
 * IL-2 - T cell proliferation and maintenance of is_associated_with::peripheral tolerance
 * IL-4 - differentiation of is_associated_with::Th2 cells
 * IL-7 - is_associated_with::thymocyte development in the thymus
 * IL-9 - survival signal for various is_associated_with::hematopoietic cells
 * IL-15 - is_associated_with::NK cell development
 * IL-21 - regulation of is_associated_with::immunoglobulin class switching in is_associated_with::B cells

Since JAK3 is required for immune cell development, targeting JAK3 could be a useful strategy to generate a novel class of is_associated_with::immunosuppressant drugs. Moreover, unlike other JAKs, JAK3 is primarily expressed in hematopoietic cells, so a highly specific JAK3 inhibitor should have precise effects on immune cells and minimal pleiotropic defects. The selectivity of a JAK3 inhibitor would also have advantages over the current widely used immunosuppressant drugs, which have abundant targets and diverse side effects. A JAK3 inhibitor could be useful for treating is_associated_with::autoimmune diseases, especially those in which a particular cytokine receptor has a direct role on disease pathogenesis. For example, signaling through the IL-15 receptor is known to be important in the development is_associated_with::rheumatoid arthritis, and the receptors for IL-4 and IL-9 play roles in the development of allergic responses.

A selective JAK3 inhibitor, designated CP-690550, has been developed and shown promise in clinical trials. This drug has nanomolar potency against JAK3 and was shown to be effective in preventing transplant rejection in a nonhuman primate renal transplant model. CP-690550 also demonstrated immunosuppressive activity in phase I and II clinical trials of is_associated_with::rheumatoid arthritis, is_associated_with::psoriasis and organ is_associated_with::transplant rejection. CP-690550 (is_associated_with::Tofacitinib) is currently being market by is_associated_with::Pfizer as is_associated_with::Xeljanz for the treatment of rheumatoid arthritis.

Interactions
Janus kinase 3 has been shown to interact with is_associated_with::CD247, is_associated_with::TIAF1 and is_associated_with::IL2RG.