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Receptor-associated constitutive protein tyrosine phosphatase activity controls the kinase function of JAK1
1Department of Cancer Biology, Research Institute, Cleveland Clinic Foundation, Cleveland, OH 44195, USA. haquej@cesmtp.ccf.org
Abstract:
Exposure of cells to protein tyrosine phosphatase (PTP) inhibitors causes an increase in the phosphotyrosine content of many cellular proteins. However, the level at which the primary signaling event is affected is still unclear. We show that Jaks are activated by tyrosine phosphorylation in cells that are briefly exposed to the PTP inhibitor pervanadate (PV), resulting in tyrosine phosphorylation and functional activation of Stat6 (in addition to other Stats). Mutant cell lines that lack Jak1 activity fail to support PV-mediated [or interleukin 4 (IL-4)-dependent] activation of Stat6 but can be rescued by complementation with functional Jak1. The docking sites for both Jak1 and Stat6 reside in the cytoplasmic domain of the IL-4 receptor alpha-chain (IL-4Ralpha). The glioblastoma-derived cell lines T98G, GRE, and M007, which do not express the IL-4Ralpha chain, fail to support Stat6 activation in response to either IL-4 or PV. Complementation of T98G cells with the IL-4Ralpha restores both PV-mediated and IL-4-dependent Stat6 activation. Murine L929 cells, which do not express the gamma common chain of the IL-4 receptor, support PV-mediated but not IL-4-dependent Stat6 activation. Thus, Stat6 activation by PV is an IL-4Ralpha-mediated, Jak1-dependent event that is independent of receptor dimerization. We propose that receptor-associated constitutive PTP activity functions to down-regulate persistent, receptor-linked kinase activity. Inhibition or deletion of PTP activity results in constitutive activation of cytokine signaling pathways.
Insights
Protein tyrosine phosphatase (PTP) inhibitors like pervanadate activate Janus kinases (Jaks), leading to Stat6 activation. This process is mediated by the IL-4 receptor alpha-chain and Jak1, independent of receptor dimerization.
Area of Science:
- Cellular signaling pathways
- Molecular biology
- Immunology
Background:
- Protein tyrosine phosphatases (PTPs) regulate cellular signaling by dephosphorylating phosphotyrosine residues.
- The precise upstream events targeted by PTP inhibitors remain incompletely understood.
- Cytokine signaling pathways, including those involving Janus kinases (Jaks) and Signal transducers and activators of transcription (Stats), are critical for immune responses.
Purpose of the Study:
- To elucidate the primary signaling events affected by PTP inhibitors.
- To investigate the role of Jaks and the IL-4 receptor alpha-chain (IL-4Ralpha) in PTP inhibitor-induced Stat6 activation.
- To determine whether PTP inhibition leads to constitutive activation of cytokine signaling.
Main Methods:
- Treatment of cell lines with pervanadate (PV), a PTP inhibitor.
- Analysis of tyrosine phosphorylation and activation of Stat6 and other Stats.
- Utilizing mutant cell lines lacking specific Jak activity or receptor components.
- Complementation assays with functional Jak1 or IL-4Ralpha.
Main Results:
- Pervanadate (PV) exposure activates Jaks, leading to Stat6 tyrosine phosphorylation and activation.
- Jak1 activity is essential for both PV-mediated and IL-4-dependent Stat6 activation.
- The IL-4 receptor alpha-chain (IL-4Ralpha) is necessary for PV-mediated Stat6 activation.
- PV-induced Stat6 activation is independent of receptor dimerization but dependent on IL-4Ralpha and Jak1.
Conclusions:
- Stat6 activation by PTP inhibition is an IL-4Ralpha-mediated, Jak1-dependent event.
- Constitutive PTP activity normally down-regulates persistent receptor-linked kinase activity.
- Inhibition of PTP activity results in the constitutive activation of cytokine signaling pathways.