Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

STAT

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_636

Synonyms

Historical Background

The signal transducer and activator of transcription (STAT) proteins were first identified as signaling proteins that function as second messengers and transcription factors in response to cytokines and growth factors (Santos and Costa-Pereira 2011; Li 2008). Mammals have seven STAT genes, namely, STAT1, STAT2, STAT3, STAT4, STAT5A, STAT5B, and STAT6 (Santos and Costa-Pereira 2011). All the STAT proteins are highly conserved and contain six domains including the SH2, linker, coiled -coil, and DNA- binding domain (Bromberg and Darnell 2000). Traditionally, unphosphorylated STAT is believed to reside in the cytoplasm in an inactive form; upon phosphorylation by JAK or another tyrosine kinase, the phosphorylated (active) STAT translocates into the nucleus to induce transcription of target genes (Bromberg and Darnell 2000). However, recent studies have prompted revisions to this paradigm. In addition to the canonical signaling pathway, there is increasing evidence that unphosphorylated STAT shuttles back and forth between the nucleus and cytoplasm (Santos and Costa-Pereira 2011). In this noncanonical mode of signaling, unphospho-STAT seems to carry additional functions, such as chromatin remodeling and heterochromatin dynamics (Li 2008).

STAT’s Protein Structure and Isoforms

STAT proteins are 750–850 amino acids long and possess six functionally conserved domains (Fig. 1). The crystal structures of both phospho- and unphospho-STAT dimers have been solved for STAT1; both of them adopt a nutcracker-like structure (Lim and Cao 2006). The six conserved domains have been extensively studied and can have overlapping functions. Although most of the domains are functionally conserved across species, the differences within the conserved domains among the different STAT proteins allow specificity in their responses in signaling. There are also multiple isoforms and posttranslational modifications that can determine the specificity of STAT regulation.
STAT, Fig. 1

Phospho- and unphospho-STATs in signal transduction and gene regulation (STAT). The structural organization of STAT proteins. All the STATs have six main domains, namely, the NTD at the N-terminal end of the peptide, coiled-coil domain (CCD), DNA-binding domain (DBD), linker domain (LD), the Src homology 2 domain (SH2D), and transactivation domain (TAD) at the very carboxyl-terminal end. Phosphorylated sites are as indicated

The N-terminal domain (NTD) is required for dimerization of unphosphorylated STATs and tetramerization between STAT1, STAT3, and STAT5. In addition, the NTD can also recruit transcriptional co-activators, such as CREB-binding protein (CBP)/p300, by promoting protein-protein interaction, and is also involved in tyrosine phosphorylation and dephosphorylation of STATs. The coiled-coil domain (CCD) is important for the interaction of STATs with other proteins to mediate transcription. For example, STAT3 interacts with c-Jun to induce transcription of target genes in response to IL-6 induction and binds to epidermal growth factor (EGF) receptors via its CCD domain. Besides, a leucine-rich segment is located within the CCD domain, which also regulates nuclear translocation of STAT proteins. Even though the DNA-binding domain (DBD) is highly conserved across the protein family, the small differences in the amino acid sequences of DBD across different STATs are responsible for their different DNA-binding specificities. Upon cytokine stimulation, the heterodimers formed by phosphorylated STAT1 and STAT3 require the DBD for nuclear translocation as well. Next to the DBD is the α-helical linker domain (LD). It links two crucial domains in STAT, namely, DBD and the Src-homology 2 (SH2) domain (Santos and Costa-Pereira 2011). Additionally, LD is also crucial for the transcriptional activation of STAT1 following IFN-γ stimulation and has been implicated in the constitutive nucleocytoplasmic shuttling of unphosphorylated STATs in resting cells (Yang et al. 1999; Marg et al. 2004). The 100-amino-acid-long SH2 domain, located at the C-terminal end of the peptide, is probably the most well-conserved domain in STAT; it plays a crucial role in protein-protein interactions between STATs and other proteins (Santos and Costa-Pereira 2011). SH2 domain recognizes phosphorylated tyrosine residue and binds to phosphorylated Janus kinase (JAK) or STAT (Huang et al. 2008; Watanabe and Arai 1996). Moreover, a number of residues within this domain are important for STAT to mediate cellular functions. In particular, the phosphorylation of STAT1 on tyrosine 701 and its subsequent homodimerization are dependent on the arginine 602 residue within the SH2 domain (Li 2008); when proline 630 is mutated, tyrosine phosphorylation of STAT2 is impaired, and it no longer dimerizes with STAT1 (Santos and Costa-Pereira 2011). The last domain on the C-terminal end is the transactivation domain (TAD). In addition to its function in activating transcription, TAD is used to interact with other proteins as well. For instance, TAD is required for the interaction of STAT1 with breast cancer 1 (BRCA1) and CBP/p300 (Santos and Costa-Pereira 2011).

STAT in the Canonical JAK-STAT Pathway

In the canonical pathway, when a ligand (e.g., a cytokine or growth factor) binds to its transmembrane receptor, conformational changes lead to receptor dimerization and cross-activation of the receptor-associated tyrosine kinase, Janus kinase (JAK). As a result, multiple phosphorylation events occur, and the phospho-tyrosines on the cytoplasmic tail of the receptor serve as docking sites for the binding of latent STATs residing in the cytoplasm (Fig. 2). Once bound, JAK can then phosphorylate associated STATs and result in dimerization of the STAT proteins via phosphorylated tyrosine residue located in the SH2 domain. These activated STAT dimers then translocate into the nucleus to regulate transcription of STAT target genes in response to cytokine or growth factor stimulation (Li 2008).
STAT, Fig. 2

Phospho- and unphospho-STATs in signal transduction and gene regulation (STAT). The canonical JAK/STAT signaling pathway. Canonically, unphosphorylated STATs maintain their latency and reside in the cytoplasm. When ligand binds to the transmembrane receptors, they homo-dimerize and are phosphorylated by JAK, a tyrosine kinase. Following that, a series of phosphorylation events result in conformational changes on the receptor that open up binding sites for recruiting STATs. Consequently, JAK can then phosphorylate STATs. Phospho-STATs then dimerize and translocate into the nucleus to mediate transcription of target genes

STAT is involved in regulating many cellular processes such as cell proliferation, apoptosis, growth, hematopoiesis, antiviral responses, and immune systems (O’Shea et al. 2004). Typically, STAT binds to well-defined DNA consensus sequences and initiates transcription of interferon (IFN)-stimulated genes (ISGs). It is also known to regulate the transcription of many genes including the B-cell lymphoma 2 (Bcl-2) family proteins, cyclin D1, Myc, and many genes implicated in angiogenesis or metastasis (Li 2008). In addition to the consensus sequence, STAT tetramer complexes can also recognize and bind weakly to non-consensus regions via the NTD domain for added specificity in its regulation. Besides inducing transcription, STAT can also act as a negative regulator of transcription in some instances (Boucheron et al. 1998). As an example, STAT3β, a naturally occurring splice variant of STAT3, inhibits the transcription of STAT3’s target genes (Caldenhoven et al. 1996).

Unphosphorylated STATs in Gene Regulation

The canonical JAK/STAT pathway had to be revised when much evidence has suggested that at least a fraction of the unphosphorylated STATs may have biological functions and shuttle continuously into the nucleus, instead of maintaining its latency in the cytoplasm (Li 2008). Work done in Drosophila has shown that loss of STAT strongly suppresses position-effect variegation (PEV), a heterochromatin-dependent gene-silencing process (Li 2008). Further investigation has shown that unphospho-STAT associates with heterochromatin protein 1 (HP1) in the nucleus to maintain heterochromatin stability (Li 2008; Shi et al. 2008). However, upon phosphorylation, STAT disperses from heterochromatin, causing HP1 displacement and heterochromatin destabilization (Shi et al. 2008). Consistent with the observation in Drosophila, mammalian unphospho-STATs, STAT3 and STAT5A in particular, have also been shown to localize at the nucleus as well (Liu et al. 2005). Mammalian STAT5 also associates with HP1 and localizes at heterochromatin (Hu et al. 2013); unphospho-STATs in mammals clearly regulate gene transcription by a mechanism distinct from the canonical JAK-STAT signaling pathway (Yang and Stark 2008). Due to their effects on chromatin structure, it is not surprising that STATs are crucial for gene regulation in stem cells and during organisms’ development (Boyer et al. 2006; Hochedlinger and Jaenisch 2006). In addition, it has recently been shown that unphospho-STAT5 in the nucleus directly binds to and represses differentiation genes in hematopoietic progenitor cells (HPCs) (Park et al. 2015). Therefore, as opposed to the traditional view of STAT, unphospho-STATs are not completely dormant and may play important roles in the regulation of gene transcription and tumor suppression (Fig. 3).
STAT, Fig. 3

A model of heterochromatin dynamics in determining cancer phenotypes. Oncogenic mutations upregulate cancer-promoting genes in part by reducing heterochromatin levels at their promoters, allowing access to transcription factors (orange); HP1 and U-STAT proteins in excess can restore heterochromatin levels at the cancer gene promoters, thereby repressing their expression and suppressing tumor growth, even in the presence of oncogenic mutations

The Role of STATs in Cancer

STAT1, STAT3, and STAT5 are often misregulated in many cancer cells, especially STAT3, which is a known oncogene (Silva 2004). STAT3 is believed to contribute to cancer cell transformation via its inflammatory response (Yu and Jove 2004). In most cases, the abnormal activities of the upstream kinases are the cause of STAT signaling misregulation in cancer cells. In fact, a well-established tumor model in Drosophila involves a hyperactive JAK kinase called hop Tumorous-L (hop Tum-L) that causes STAT over-activation and over-proliferation of blood cells, analogous to human leukemia (Li 2008; Shi et al. 2006). The role of STAT activation in these pathological conditions has so far been attributed mostly to over-activation of the canonical JAK-STAT pathway that upregulates STAT target genes. In addition to the canonical pathway, disruption of STAT’s function in stabilizing heterochromatin may also contribute to tumorigenesis since heterochromatin has been implicated in tumor suppression. Unphospho-STAT5A has been shown to be prominently present in the nucleus of cultured human cells and plays a role in heterochromatin stability and tumor suppression (Hu et al. 2013). Therefore, respective contributions of canonical and noncanonical JAK-STAT signaling in cancer development are currently under investigation.

Summary

In conclusion, STAT is a crucial signaling molecule in animal cells that translates extracellular signals into changes in gene transcription. In the canonical JAK-STAT pathway, only phosphorylated STATs translocate into the nucleus and act as transcription factors, whereas unphosphorylated STATs are assumed to remain latent in the cytoplasm. In the noncanonical mode of JAK-STAT signaling, unphosphorylated STATs are also found, at least in fruit flies, to localize in the nucleus and play important roles in regulating gene expression by modulating heterochromatin formation. It has been shown that mammalian JAK2 plays a role in regulating heterochromatin formation (Dawson et al. 2009). And STAT’s function in heterochromatin seems conserved in mammals (Hu et al. 2013). Proper function of STAT is essential for different stages of development, and maintaining the health of individual organisms and misregulation of STAT has been implicated in multiple human diseases. STAT thus poses as an attractive potential therapeutic target, because it mediates a rather straightforward signaling pathway, and the phospho-tyrosine residue can conceivably be targeted. However, since STAT has proven to be versatile and involved in multiple functions via distinct mechanisms, targeting STAT may cause unexpected cellular responses. Therefore, the biological functions and cellular effects of STAT proteins must be thoroughly investigated even though STAT remains a potential therapeutic target.

References

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.University of Rochester Medical CenterRochesterUSA
  2. 2.Department of MedicineUniversity of California San DiegoLa JollaUSA