Additional key questions with respect to the Jak/STAT signal transduction pathway include: how is signalling terminated and how is it modulated? The following termination mechanisms have been proposed (Figure 24.1):
• tyrosine phosphatases in the nucleus;
• STAT degradation via the nuclear proteasome;
• tyrosine phosphatases dephosphorylate receptor-associated proteins (i.e. Jaks, STATs);
• specific inhibition of STAT proteins by protein inhibitors of activated STATs; and
• synthesis of Jak/STAT feedback inhibitors (suppressors of cytokine signalling, SOCS).
The following studies emphasize the respective importance of two of these -dephosphorylation of Jak/STATs and the synthesis of SOCS. In studies of the former with the gp130 mutant YFYYYY, it is evident that the tyrosine phosphatase SHP2, which is activated through Y759 of gp130, is counteracting Jak/STAT signalling. In Ba/F3 cells stably transfected with the gp130 Y759F mutant, STAT1/3 activation was shown to be prolonged compared with that in the gp130 wild type using both Western blotting and electrophoretic mobility shift assays. Furthermore, acute phase protein gene induction in HepG2 cells after IL-6 stimulation measured by a reporter assay clearly showed that exchange of tyrosine 759 of gp130 with phenylalanine gave rise to higher reporter gene activity than the wild type. In a further series of experiments, using a heterodimeric receptor system in which tyrosine 759 was exchanged by phenylalanine on the second gp130, the authors showed that tyrosine 759 on a single gp130 receptor chain is sufficient to attenuate signalling. The authors next asked the question of whether activation of SHP2 on one gp130 receptor chain was capable of affecting the tyrosine residues responsible for STAT recruitment on the other gp130 molecule. It was found that SHP2 and STAT recruitment sites do not have to be located on the same receptor chain. When two different receptor systems were transfected into HepG2 cells - one which recruits and activates SHP-2 but not STATs, the other unable to recruit SHP2 but able to activate STAT3 - no cross-talk was observed.
SOCS are molecules which have a so-called SOCS box near the C-terminal end and an SH2 domain important for function. Presently, there are seven known members of the SOCS protein family. After IL-6 stimulation of liver cells, SOCS1 and SOCS3 mRNAs are induced and the correspondingproteins act as very efficient feedback inhibitors by interacting with the catalytic domains of Jak1 or Jak2. Recently, the authors discovered that SOCS3 specifically binds to the phosphotyrosine peptide containing the tyrosine 759 motif of gp130. The other gp130 phosphotyrosine motifs do not bind SOCS3. As already mentioned, SHP2 also binds to the tyrosine 759 motif in gp130, i.e. SHP2 and SOCS3, but not SOCS1, can be coimmunoprecipitated together with the phosphotyrosine 759 peptide of gp130. The question then arose of whether SOCS3 was bound directly to phosphotyrosine 759 or whether it was recruited via SHP2. Using cell lysates containing SOCS3, but depleted of SHP2, it was shown that binding of SOCS3 to the phosphotyrosine motif around Y759 of gp130 occurred directly.
In collaboration with Nelson Fausto of the University of Washington, the authors have examined whether SOCS3 induction is a possible mechanism underlying the transient activation of STAT3 observed around 2 hours after partial hep-atectomy of mice. A dramatic increase in SOCS3 mRNA has been observed. Consistent with these findings are reports from Fausto's group that TNF a injection into TNF a receptor 1-deficient or IL-6-deficient mice suppressed liver regeneration and the induction of SOCS3 mRNA. Interestingly, IL-6 injection overcame the defect, i.e. injection of IL-6 into TNFa receptor 1- or IL-6-deficient mice led to a strong SOCS3 induction.
Was this article helpful?