STAT phosphorylated

Binding of type I interferons to the IFN-a/ S (type I) receptor results in the phosphorylation and hence activation of two members of the JAK family Tyk2 and JAK 1. These kinases then phosphorylate STAT la (also called STAT 91), STAT (STAT 84) and STAT 2 (STAT 113). 

The phosphorylated Stat proteins form homodimeric or heterodimeric complexes and are transported as such into the nucleus (Fig. 11.7). In the nucleus, they bind to corresponding DNA elements in the promoter region of cytokine responsive genes. Stimulation of transcription takes place in cooperation with other proteins such as p300, CBP (see 1.4.6), glucocorticoid receptors and c-jim. 

Fig. 10.5 (a) Homodrmenc, phosphorylated STAT 1 and STAT 1/STAT 2 heterodimers bind in response to interferorr-yto the same DIMA sequence element, GAS. (b) Interferon-a/p activates the transcription factor complex containing p48/ISGF3 and STAT 1/STAT 2 heterodimers. This transcription-factor complex addresses the interferon-stimulated regulatory element, ISRE. 

Fig. 2. Immunohistochemical detection of STAT-3 and pSTAT-3 in the basal hypothalamus of the mouse. Photomicrographs of 40-p.m coronal slices through the ob/ob mouse brain at the level of the arcuate nucleus and median eminence. Brains were collected 30 min after i.v. injeetion of phosphate buffer (PB) or Axokine (0.1 mg/kg) and adjaeent brain slices were processed for detection of STAT-3 (A,C) or phosphorylated STAT-3 (B,D).

Fig. 3. Assessment of signaling in peripheral tissue after systemic administration of Axokine . The left hand group of panels show Western Blot analysis for phosphorylated STAT-3 (upper panel) and STAT-3 (lower panel) in muscle (A), liver (B) and adipose (C) tissues from normal mice 30 min after i.p. injection of Axokine (0.1-1.0 mg/kg), IL-6 (1 mg/kg) or Vehicle. The right hand group of panels shows Northern Blot analysis of mRNA for SOCS-3 (upper panel) in muscle (D), liver (E) and adipose (F) tissues from normal mice 30 or 120 min after i.p. injection of Axokine (0.1-1.0 mg/kg), IL-6 (1 mg/kg) or vehicle.

These equations describe the yield and loss of the different components. For example, Eq. (1) states, that the unphosphorylated STAT-monomer x is reduced, expressed by the minus sign, with a certain rate fq due to the interaction of the STAT-monomer with the activated receptor described by %i EpoRA. Since this interaction leads to the phosphorylated STAT-monomer x2, the same term as in Eq. (1), but with positive sign appears in Eq. (2). The second part of Eq. (2) describes the loss of the phosphorylated STAT-monomer x2 by dimerization with rate constant k2. This term appears in Eq. (3) with the factor of 0.5 since two monomers form one dimer. The second term in Eq. (3) and the right-hand side of Eq. (4), finally, describe the transport of the dimer into the nucleus. 

To first gain insights into the properties of this system, a simulation study is performed. Therefore, all parameters are set to 1, and an artificial Epo-receptor time course is chosen. The dynamical model is solved numerically and the observation equations are evaluated. The resulting time courses for the phosphorylated STAT-5 in the cytoplasm yi and the total amount of STAT-5 in the cytoplasm y2 are displayed in Fig. 17.1-2. 

The qualitative behavior is identical for all parameter settings The phosphorylated STAT-5 in the cytoplasm shows a biphasic behavior, the total amount of STAT-5 in the cytoplasm decreases monotonically. However, the quantitative behavior depends on the parameters. Thus, if simulated model predictions are compared to experimental data, it is difficult to decide whether discrepancies between simulated and measured data result from inadequate parameters or from an insufficient model. To resolve this simulation dilemma [29], we will estimate the parameters from the experimental data. Mathematically, the equations of the system under investigation can be summarized as ... 

Fig. 17.1-2 Results of a simulation study for y (phosphorylated STAT-5 in the cytoplasm, red) and y2 (total STAT-5 in the cytoplasm, blue). Initially, upper left, all parameters are set to 1, for the other plots parameters /ci to k3 are set to 10.

For the phosphorylated STAT-5 in the cytoplasm, the model does not capture the plateau between 10 and 30 min and the behavior of total STAT-5... 

STATs are also inactivated. The protein inhibitors of activated STAT (PIAS) family of proteins bind to phosphorylated STATs and prevent their dimerization or promote the dissociation of STAT dimers. STATs also may be inactivated by dephosphorylation, although the specific phosphatases have not yet been identified, or by targeting activated STATs for proteolytic degradation. 

Fig. 44.16. C54okine signaling through the JAK/STAT pathway. 1. Cytokine binding to receptors initiates dimerization and activation of the JAK kinase, which phosphorylates the receptor on tyrosine residues. 2. STAT proteins bind to the activated receptors and are themselves phosphorylated. 3. Phosphorylated STAT proteins dimerize, travel to the nucleus, and initiate gene transcription. 4. One of the proteins whose s5mthesis is stimulated by STATs is SOCS (suppressor of cytokine signaling), which inhibits further activation of STAT proteins (circle 5) by a variety of mechanisms.

Proteins phosphorylation (STAT, small G-proteins, MAPK)... 

Fig. 3. IFN-y signal transduction. IFN-y binds to its receptor, triggering JAKl/JAK2-mediated tyrosine phosphorylation. The JAKs phosphorylate STAT-1 molecules, which form a homodimer through cognate phosphotyrosine-SH2 domain interactions. STAT-1 homodimers translocate to the nucleus, where they bind IFN-y activation sequence (GAS) elements present in the promoters of IFN-y-inducible genes...

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