The janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling pathway was discovered by Darnell in 1994 and is an important pathway for cytokine signaling. It is activated by a variety of cytokines, growth factors and receptors, and is involved in cell proliferation, differentiation, apoptosis, angiogenesis and immune regulation, and plays an important role in the occurrence and development of tumors. Abnormal activation of this pathway leads to clonal proliferation and tumor formation.
Figure 1. Model of JAK/STAT Signaling in the Regulation of Whole-Body Metabolism
JAK/STAT Family Member
JAK is a class of non-receptor tyrosine kinases, including JAK1, JAK2, JAK3 and TYK2. There are 7 domains of JH1-7 in the molecular structure. JH1 has catalytic function and JH2 participates in the catalytic function of JH1. However, the function of JH3-JH7 is currently underreported, suggesting that it may play a role in the coupling of receptors to JAKs. Shan et al. found that the JAKs family has a conserved tyrosine kinase domain at the C-terminus and is highly conserved in evolution. JAK1, JAK2, and TYK2 expression were detected in most tissue cells, and JAK3 was expressed only in bone marrow and lymphocytes.
STATs mainly include STAT1, 2, 3, 4, 5, and 6, wherein STAT5 is further divided into two subtypes, STAT5a and STAT5b. STAT has been found to have six functional domains: DNA binding domain, amino terminal domain, coiled-coil domain, ligation domain, SH2/tyrosine phosphorylation domain, and transcriptional activation domain at the carboxy terminus. The DNA binding domain and the SH2 domain are highly similar, and this similarity contributes to the formation of dimers and the activation of STATs.
JAK/STAT Signaling and Regulation
When a cytokine binds to a receptor, the receptor dimerizes and the JAK coupled thereto is phosphorylated for activation. Phosphorylation of tyrosine residues on activated JAK-catalyzed receptors, phosphorylated tyrosine forms a stop with the surrounding amino acid sequence containing the SH2 domain, and STAT binds to the receptor at this site simultaneously by JAK Phosphorylated. Phosphorylated STAT enters the nucleus to bind to target genes to activate gene transcription and protein expression. Springuel et al. found that many extracellular signals can be signal transduced through the JAK/STAT pathway. These extracellular signals include the interferon family, the glycoprotein 130 family (Glycoprotein 130, gp130), the single-chain family, and the receptor tyrosine kinase.
Studies have shown that the activity of JAK/STAT signaling pathway is regulated by negative regulation mechanisms, such as SOCS (suppressors of cytokine signaling) protein, PIAS (protein inhibitors of activated STATs) protein and PTP (protein tyrosine phosphatases) protein. Slattery et al. believe that SOCS protein can block the activation of signaling pathway by binding to phosphorylated JAK molecules. In recent years, studies have found that methylation inactivation of SOCS gene is closely related to tumorigenesis. PIAS proteins block binding of STAT to target genes primarily by binding to activated STAT dimers. The most studied PTP protein is SH2 domain-containing protein tyrosine phosphates (SHP1). It is reported that SHP1 can dephosphorylate and inactivate JAK, thereby blocking the activation of downstream molecules and blocking the JAK/STAT pathway.
JAK/STAT and Other Signaling Pathways
Although the JAK/STAT signaling pathway is theoretically simple. However, it can interact with other pathways to produce complex biological effects. The relationship between these pathways is very complex, and their paths intersect at multiple levels and are mutually activated. Fahmi et al. found that activated JAKs phosphorylate the tyrosine of their related receptors, making them a stop for SH2 domain proteins from other pathways, including SHP-2 and Src homology collagen protein, SHCP), etc., they can recruit Grb2 to activate the Ras signaling pathway.
Figure 2. Modes of gp130 dimerization and signaling in JAK/STAT signaling pathway.
Studies have found that the JAK/STAT pathway activates the Ras pathway through SOCS3 indirectly. SOCS3 binds to RasGAP and attenuates its activity, which is a negative regulator of the Ras pathway, thereby promoting activation of the Ras pathway. Numerous studies have shown that STATs can be activated by receptor tyrosine kinases (RTKs) instead of JAKs. RTK can activate JAK/STAT signaling through two mechanisms: 1. Activation of some RTKs, including epidermal growth factor receptors (EGFR) and platelet-derived growth factor receptor (PDGFR), which cause STATs tyrosine phosphorylation via Src kinase independent of JAK; 2. RTK/Ras pathway Activation leads to up-regulation of mitogen activated protein kinase (MAPK) activation, which specifically phosphorylates a serine Ser at the STAT C-terminus, and Ser phosphorylation greatly enhances the transcriptional activity of STATs.
JAK/STAT Signaling Pathway and Tumor
The appearance of tumors is closely related to inadequate and/or lack of immunosurveillance. Immunosurveillance relies on the expression of MHCI, MHC II by cells. MHCI molecules express tumor-associated antigen (TAA) to CD8+ cytotoxic T cells, and MHCII expression TAA is presented to CD4+ helper T cells. In nucleated cells, both MHC I and MHC II can be expressed on the cell surface by interferon gamma (IFN-γ) and cascaded through the JAK/STAT signaling pathway. IFN-γ binds to IFN-γR1 and activates JAK1, JAK2, which in turn activates STAT1, forms a STAT1 homodimer (called gamma activator, GAF). GAF translocates to the nucleus and binds to the promoter of the γ-activated sequence (GAS) interferon response element IRF1, IRF2, then transcribes to generate IRF1, IRF2 and form a dimer, which in turn binds to the promoter of class II transactivator IV (CIITA). CIITA is required but not sufficient for MHCII transcription and lacks DNA intrinsic binding ability. MHCII transcription requires an enhancer complex to bind CIITA. The enhancer consists of cAMP regulatory element binding protein (CREB), nuclear factor Y (NFY), and regulatory factor X (RFX). When CIITA binds to an enhancer, it allows RNA polymerase II to bind, resulting in MHCII transcription.
When Osborn et al. studied malignant melanoma, it was found that MHCII was expressed in malignant melanoma cells in the growth phase and vertical growth phase. In the metastatic growth phase, not only MHCII is absent, but also the transcription factor interferon response factor (IRF) and its upstream activator STAT are absent. Therefore, the cause of malignant melanoma is that inhibition of STAT1 expression leads to down-regulation of IRF expression, making MHCII unable to express on the cell surface.
JAK/STAT aberrant activation is involved in many physiological processes of cancer cell proliferation and survival. Activation of STAT3 signaling pathways in many cancers such as hematopoietic tumors can promote cancer development. The JAK/STAT signaling pathway may be required for the differentiation of the acute promyelocytic leukemia cell line HT93A. Sen et al. have shown that JAK1 /2 inhibitor AZD1480 can inhibit STAT3 expression, thereby inhibiting the proliferation of head and neck squamous cell carcinoma cell lines, down-regulating the expression of phosphorylated STAT3 in a dose-dependent manner. AZD1480 reduces tumor growth rate in patient-derived xenograft head and neck squamous cell carcinoma models. JAK2 gene V617 activating mutation was found in approximately 96% of polycythemia vera, and JAK2 overactivation spontaneously activates downstream signaling pathways, resulting in non-regulated tumor-like hematopoietic activity. V617-mutant JAK2 is also found in hematological malignancies, causing non-synonymous amino acid substitutions that can result in the change of JAK signaling pathway function.
Figure 3. Mechanisms of constitutive JAK-STAT pathway activation in hematologic malignancies
Zhou et al. found that EPO activates JAK/STAT signaling pathways in breast tumor blasts (TICs), promotes tumorigenesis, and promotes TIC self-renewal. Since the JAK/STAT signaling pathway is involved in the development and progression of tumors, treatment for tumors can also be achieved by affecting the JAK/STAT signaling pathway. Li et al. artificially aggregated macrophage colony-stimulating factor (GMCSF) receptor and interleukin 9 (IL-9) receptor, which are not related to each other, into a bifunctional cytokine named GIFT9. The experiment confirmed that GIFT9 can transfer the phosphorylation of JAK2 bound to the GMCSF receptor to the STAT1 bound to the IL-9 receptor when processing the MC-9 cell line; Similarly, JAK1 /JAK3 binding to the IL-9 receptor increases STAT5 phosphorylation at the GMCSF receptor. A new complex of two functionally unrelated receptors that has been artificially engineered to exhibit functional coordination through the JAK/STAT signaling pathway. This experiment uses artificial modulation of cell physiology, such as GIFT9 fusion, to open a new door to the treatment of diseases such as tumors.
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