JAK-STAT signaling pathway


The JAK-STAT signaling pathway is a chain of interactions between proteins in a cell, and is involved in processes such as immunity, cell division, cell death, and tumor formation. The pathway communicates information from chemical signals outside of a cell to the cell nucleus, resulting in the activation of genes through the process of transcription. There are three key parts of JAK-STAT signalling: Janus kinases, signal transducer and activator of transcription proteins, and receptors. Disrupted JAK-STAT signalling may lead to a variety of diseases, such as skin conditions, cancers, and disorders affecting the immune system.

Structure of JAKs and STATs

Main articles: JAKs and STATs
There are four JAK proteins: JAK1, JAK2, JAK3 and TYK2. JAKs contains a FERM domain, an SH2-related domain, a kinase domain and a pseudokinase domain. The kinase domain is vital for JAK activity, since it allows JAKs to phosphorylate proteins.
There are seven STAT proteins: STAT1, STAT2, STAT3, STAT4, STAT5A, STAT5B and STAT6. STAT proteins contain many different domains, each with a different function, of which the most conserved region is the SH2 domain. The SH2 domain is formed of 2 α-helices and a β-sheet and is formed approximately from residues 575–680. STATs also have transcriptional activation domains, which are less conserved and are located at the C-terminus. In addition, STATs also contain: tyrosine activation, amino-terminal, linker, coiled-coil and DNA-binding domains.

Mechanism

The binding of various ligands, usually cytokines, such as interferons and interleukins, to cell-surface receptors, causes the receptors to dimerize, which brings the receptor-associated JAKs into close proximity. The JAKs then phosphorylate each other on tyrosine residues located in regions called activation loops, through a process called transphosphorylation, which increases the activity of their kinase domains. The activated JAKs then phosphorylate tyrosine residues on the receptor, creating binding sites for proteins possessing SH2 domains. STATs then bind to the phosphorylated tyrosines on the receptor using their SH2 domains, and then they are tyrosine-phosphorylated by JAKs, causing the STATs to dissociate from the receptor. At least STAT5 requires glycosylation at threonine 92 for strong STAT5 tyrosine phosphorylation. These activated STATs form hetero- or homodimers, where the SH2 domain of each STAT binds the phosphorylated tyrosine of the opposite STAT, and the dimer then translocates to the cell nucleus to induce transcription of target genes. STATs may also be tyrosine-phosphorylated directly by receptor tyrosine kinases - but since most receptors lack built-in kinase activity, JAKs are usually required for signalling.

Movement of STATs from the cytosol to the nucleus

To move from the cytosol to the nucleus, STAT dimers have to pass through nuclear pore complexes, which are protein complexes present along the nuclear envelope that control the flow of substances in and out of the nucleus. To enable STATs to move into the nucleus, an amino acid sequence on STATs, called the nuclear localization signal, is bound by proteins called importins. Once the STAT dimer enters the nucleus, a protein called Ran binds to the importins, releasing them from the STAT dimer. The STAT dimer is then free in the nucleus.
Specific STATs appear to bind to specific importin proteins. For example, STAT3 proteins can enter the nucleus by binding to importin α3 and importin α6. On the other hand, STAT1 and STAT2 bind to importin α5. Studies indicate that STAT2 requires a protein called interferon regulatory factor 9 to enter the nucleus. Not as much is known about nuclear entrance of other STATs, but it has been suggested that a sequence of amino acids in the DNA-binding domain of STAT4 might allow nuclear import; also, STAT5 and STAT6 can both bind to importin α3. In addition, STAT3, STAT5 and STAT6 can enter the nucleus even if they are not phosphorylated at tyrosine residues.

Role of post-translational modifications

After STATs are made by protein biosynthesis, they have non-protein molecules attached to them, called post-translational modifications. One example of this is tyrosine phosphorylation, but STATs experience other modifications, which may affect STAT behaviour in JAK-STAT signalling. These modifications include: methylation, acetylation and serine phosphorylation.
  • Methylation. STAT3 can be dimethylated on a lysine residue, at position 140, and it is suggested that this could reduce STAT3 activity. There is debate as to whether STAT1 is methylated on an arginine residue, and what the function of this methylation could be.
  • Acetylation. STAT1, STAT2, STAT3, STAT5 and STAT6 have been shown to be acetylated. STAT1 may have an acetyl group attached to lysines at positions 410 and 413, and as a result, STAT1 can promote the transcription of apoptotic genes - triggering cell death. STAT2 acetylation is important for interactions with other STATs, and for the transcription of anti-viral genes.
Acetylation of STAT3 has been suggested to be important for its dimerization, DNA-binding and gene-transcribing ability, and IL-6 JAK-STAT pathways that use STAT3 require acetylation for transcription of IL-6 response genes.
STAT5 acetylation on lysines at positions 694 and 701 is important for effective STAT dimerization in prolactin signalling. Adding acetyl groups to STAT6 is suggested to be essential for gene transcription in some forms of IL-4 signalling, but not all the amino acids which are acetylated on STAT6 are known.
  • Serine phosphorylation. Most of the seven STATs undergo serine phosphorylation. Serine phosphorylation of STATs has been shown to reduce gene transcription. It is also required for the transcription of some target genes of the cytokines IL-6 and IFN- γ. It has been proposed that phosphorylation of serine can regulate STAT1 dimerization, and that continuous serine phosphorylation on STAT3 influences cell division.

    Recruitment of co-activators

Like many other transcription factors, STATs are capable of recruiting co-activators such as CBP and p300, and these co-activators increase the rate of transcription of target genes. The coactivators are able to do this by making genes on DNA more accessible to STATs and by recruiting proteins needed for transcription of genes. The interaction between STATs and coactivators occurs through the transactivation domains of STATs. The TADs on STATs can also interact with histone acetyltransferases ; these HATs add acetyl groups to lysine residues on proteins associated with DNA called histones. Adding acetyl groups removes the positive charge on lysine residues, and as a result there are weaker interactions between histones and DNA, making DNA more accessible to STATs and enabling an increase in the transcription of target genes.

Integration with other signalling pathways

JAK-STAT signalling is able to interconnect with other cell-signalling pathways, such as the PI3K/AKT/mTOR pathway. When JAKs are activated and phosphorylate tyrosine residues on receptors, proteins with SH2 domains are able bind to the phosphotyrosines, and the proteins can carry out their function. Like STATs, the PI3K protein also has an SH2 domain, and therefore it is also able to bind to these phosphorylated receptors. As a result, activating the JAK-STAT pathway can also activate PI3K/AKT/mTOR signalling.
JAK-STAT signalling can also integrate with the MAPK/ERK pathway. Firstly, a protein important for MAPK/ERK signalling, called Grb2, has an SH2 domain, and therefore it can bind to receptors phosphorylated by JAKs. Grb2 then functions to allow the MAPK/ERK pathway to progress. Secondly, a protein activated by the MAPK/ERK pathway, called MAPK, can phosphorylate STATs, which can increase gene transcription by STATs. However, although MAPK can increase transcription induced by STATs, one study indicates that phosphorylation of STAT3 by MAPK can reduce STAT3 activity.
One example of JAK-STAT signalling integrating with other pathways is Interleukin-2 receptor signaling in T cells. IL-2 receptors have γ chains, which are associated with JAK3, which then phosphorylates key tyrosines on the tail of the receptor. Phosphorylation then recruits an adaptor protein called Shc, which activates the MAPK/ERK pathway, and this facilitates gene regulation by STAT5.

Alternative signalling pathway

An alternative mechanism for JAK-STAT signalling has also been suggested. In this model, SH2 domain-containing kinases, can bind to phosphorylated tyrosines on receptors and directly phosphorylate STATs, resulting in STAT dimerization. Therefore, unlike the traditional mechanism, STATs can be phosphorylated not just by JAKs, but by other receptor-bound kinases. So, if one of the kinases cannot function, signalling may still occur through activity of the other kinase. This has been shown experimentally.

Role in cytokine receptor signalling

Given that many JAKs are associated with cytokine receptors, the JAK-STAT signalling pathway plays a major role in cytokine receptor signalling. Since cytokines are substances produced by immune cells that can alter the activity of neighbouring cells, the effects of JAK-STAT signalling are often more highly seen in cells of the immune system. For example, JAK3 activation in response to IL-2 is vital for lymphocyte development and function. Also, one study indicates that JAK1 is needed to carry out signalling for receptors of the cytokines IFNγ, IL-2, IL-4 and IL-10.
The JAK-STAT pathway in cytokine receptor signalling can activate STATs, which can bind to DNA and allow the transcription of genes involved in immune cell division, survival, activation and recruitment. For example, STAT1 can enable the transcription of genes which inhibit cell division and stimulate inflammation. Also, STAT4 is able to activate NK cells, and STAT5 can drive the formation of white blood cells. In response to cytokines, such as IL-4, JAK-STAT signalling is also able to stimulate STAT6, which can promote B-cell proliferation, immune cell survival, and the production of an antibody called IgE.