Inhibitory roles for SHP-1 and SOCS-3 following pituitary proopiomelanocortin induction by leukemia inhibitory factor
Corinne Bousquet, Christiane Susini, Shlomo Melmed
Corinne Bousquet, Christiane Susini, Shlomo Melmed
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Inhibitory roles for SHP-1 and SOCS-3 following pituitary proopiomelanocortin induction by leukemia inhibitory factor

Abstract

Leukemia inhibitory factor (LIF) is a pleiotropic cytokine that stimulates the hypothalamo-pituitary-adrenal (HPA) axis through JAK-STAT activation. We show here that LIF-induced JAK2 and STAT3 tyrosine phosphorylation is transient, disappearing within 20 and 40 minutes, respectively. LIF activates the SH2 domain–containing tyrosine phosphatase, SHP-1, with maximal stimulation observed at 30 minutes. SHP-1 is constitutively associated with JAK2, and LIF induces recruitment of phosphorylated STAT3 to this complex. Overexpression of wild-type or dominant negative forms of SHP-1 shows decreased or increased LIF-induced proopiomelanocortin (POMC) promoter activity, respectively. LIF-induced JAK2 and STAT3 dephosphorylation is delayed until after 60 minutes in cells that overexpress the mutant SHP-1. In addition, SOCS-3, a negative regulator of LIF signaling, binds to JAK2 after 60 minutes of LIF stimulation, after which the complex is degraded by the proteasome. SOCS-3 overexpression blocks LIF-induced JAK2 tyrosine phosphorylation, confirming a role for SOCS-3 in deactivating JAK2 by direct association. Using SOCS-3 fusion proteins, we also define regions of the SOCS-3 protein that are critical for inhibition of LIF-induced POMC promoter activity. Corticotrophic signaling by LIF is thus subject to 2 forms of negative autoregulation: dephosphorylation of JAK2 and STAT3 by the SHP-1 tyrosine phosphatase, and SOCS-3–dependent inactivation of JAK2.

Authors

Corinne Bousquet, Christiane Susini, Shlomo Melmed

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Model for SHP-1 and SOCS-3 regulation of LIF transduction signal. (a) SH...
Model for SHP-1 and SOCS-3 regulation of LIF transduction signal. (a) SHP-1 is constitutively associated with JAK2 in a tyrosine phosphorylation–independent manner. (b) After LIF stimulation, heterodimerization occurs between LIFRα and gp130 subunits, which activates JAK2 activity. JAK2 tyrosine phosphorylates itself as well as gp130, which allows recruitment of STAT3 through its SH2 domain onto gp130 docking sites. STAT3 is also tyrosine-phosphorylated by JAK2 and therefore activated. STAT3 then homodimerizes or heterodimerizes with STAT1, which accounts for its nuclear translocation. (c) SHP-1 might be activated because its partners (JAK2 and STAT3) become tyrosine-phosphorylated. Increased tyrosine phosphatase activity of SHP-1 directly dephosphorylates JAK2, therefore inhibiting its kinase activity and resulting in dephosphorylation of its substrates. Once gp130, JAK2, and STAT3 are dephosphorylated, STAT3 leaves the receptor complex, which also returns to the basal state. Activation of SHP-1 is maximal at 30 minutes of LIF stimulation. (d) Sustained deactivation of LIF signaling also occurs through LIF-induced SOCS-3 protein synthesis and binding to JAK2, which appears maximally between 40 and 60 minutes of LIF stimulation. SOCS-3 inhibits JAK2 kinase activity and is then degraded after 90 minutes.