JAK/STAT

Cytokine receptors are a group of structurally related receptors, which couple to the JAK-STAT pathway. Cytokine receptors function as homodimers or heterooligomers. They are divided into two main subclasses, class I, which contains receptors for a variety of hematopoietic growth factors and interleukins and class II, which contains receptors for interferons and interleukins 10, 20/24 and 22. 

Immune Defense JAK-STAT Pathway PIAS proteins Map Kinase Cascades Toll-like Receptors Growth Factors... 

JAK-STAT Pathway. Figure 1 Activating and inhibitory mechanisms of the JAK-STAT pathway. 

Components of the JAK-STAT signaling pathway represent novel targets for pharmacological interventions [4]. Recently, a specific and orally active JAK3 antagonist was identified from screening of a chemical library for inhibitors of in vitro JAK3 kinase activity. The most effective compound, CP-690,550, was shown... 

The cytokine leptin is secreted by adipocytes (fat cells) in proportion to the size of the adipose dq>ot and circulates via the bloodstream to the brain, where it ultimately affects feeding behavior, endocrine systems including reproductive function and, at least in rodents, energy expenditure. The major effect of Lqrtin is on the hy-pothalamous, where it suppresses appetite and hence food intake. Leptin exerts its effects via binding to the leptin receptor in the brain (specifically in the hypothalamus), which activates the JAK-STAT Pathway. 

Cytokine receptors that couple to the JAK-STAT Pathway decode the signaling though hematopoietic cytokines (erythropoietin, thrombopoietin, colony-stimulating factors), prolactin, growth hormone, the a-, (3- and y- interferons, and a number of immunomodulatory interleukins [3], They form homodimetic or heterodimeric receptor complexes, which after ligandbinding recruit and activate isotypes of Janus kinases (JAKs). Activated JAKs in turn... 

In chronic myelogenous leukemia (CML) as well as in a subset of acute lymphoblastic leukemia (ALL) Bcr-Abl, a fusion protein of c-Abl and the breakpoint cluster region (bcr), is expressed in the cytosol of leukemic cells. This fusion protein forms homo-oligomeric complexes that display elevated kinase activity and is the causative molecular abnormality in CML and certain ALL. The transforming effect of Bcr-Abl is mediated by numerous downstream signaling pathways, including protein kinase C (PKC), Ras-Raf-ERK MAPK, JAK-STAT (see below), and PI3-kinase pathways. 

On binding to its receptor, the IFN molecule is sandwiched between the two chains of the receptor, thus forming a ternary complex that activates the canonical Jak/Stat pathway through activation of the transduction elements located in the intracyto-plasmic tail of each receptor subunit (Fig. 1). 

Darnell JE Jr, Kerr IM, Stark GR Jak-STAT pathways and transcriptional activation in response to IFNs and other extracellular signaling proteins. Science 1994 264 l4l5. 

Vila-Coro AJ, Rodriguez-Frade JM, Martin De Ana A, Moreno-Ortiz MC, Martinez AC, Mellado M. The chemokine SDF-lalpha triggers CXCR4 receptor dimerization and activates the JAK/STAT pathway. FASEB J 1999 13(13) 1699-1710. 

The intracellular events triggered upon binding of type I or II interferons to their respective receptors are quite similar. The sequence of events, known as the JAK-STAT pathway, has been elucidated over the last few years. It has quickly become apparent that this pathway plays a prominent role in mediating signal transduction, not only for interferon, but also of many cytokines. 

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