Exchange Protein Activated by Cyclic AMP (Epac)-Mediated Induction of Suppressor of Cytokine Signaling 3 (SOCS-3) in Vascular Endothelial Cells

Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, United Kingdom.
Molecular and Cellular Biology (Impact Factor: 4.78). 10/2006; 26(17):6333-46. DOI: 10.1128/MCB.00207-06
Source: PubMed


Here, we demonstrate that elevation of intracellular cyclic AMP (cAMP) in vascular endothelial cells (ECs) by either a direct activator of adenylyl cyclase or endogenous cAMP-mobilizing G protein-coupled receptors inhibited the tyrosine phosphorylation of STAT proteins by an interleukin 6 (IL-6) receptor trans-signaling complex (soluble IL-6Ralpha/IL-6). This was associated with the induction of suppressor of cytokine signaling 3 (SOCS-3), a bona fide inhibitor in vivo of gp130, the signal-transducing component of the IL-6 receptor complex. Attenuation of SOCS-3 induction in either ECs or SOCS-3-null murine embryonic fibroblasts abolished the inhibitory effect of cAMP, whereas inhibition of SHP-2, another negative regulator of gp130, was without effect. Interestingly, the inhibition of STAT phosphorylation and SOCS-3 induction did not require cAMP-dependent protein kinase activity but could be recapitulated upon selective activation of the alternative cAMP sensor Epac, a guanine nucleotide exchange factor for Rap1. Consistent with this hypothesis, small interfering RNA-mediated knockdown of Epac1 was sufficient to attenuate both cAMP-mediated SOCS-3 induction and inhibition of STAT phosphorylation, suggesting that Epac activation is both necessary and sufficient to observe these effects. Together, these data argue for the existence of a novel cAMP/Epac/Rap1/SOCS-3 pathway for limiting IL-6 receptor signaling in ECs and illuminate a new mechanism by which cAMP may mediate its potent anti-inflammatory effects.

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Available from: William A Sands
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    • "We therefore transfected HUVECs with cDNAs encoding wild type human c-Jun (c-Jun WT) or human c-Jun, where Ser63 and Ser73 had been mutated to alanine (c-Jun AA; Fig. 3a). Cells were then stimulated with F/R, in the presence or absence of the proteasome inhibitor, MG132, to inhibit proteolytic digestion of SOCS3 protein following its synthesis, thereby stabilising its expression, as described [1]. We found that F/R provoked a robust induction of SOCS3 protein in the presence of MG132, which was unaffected by transfection of cells with Fig. 2. Activation of PKA, but not EPAC1, promotes AP1-associated c-Jun activation in HUVECs. "
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    ABSTRACT: The cyclic AMP sensor, EPAC1, activates AP1-mediated transcription in HUVECs. Correspondingly, induction of the SOCS3 minimal promoter by EPAC1 requires a single AP1 site that constitutively binds phosphorylated (Ser63) c-Jun in DNA-pull-down assays. c-Jun (Ser63) becomes further phosphorylated following cyclic AMP stimulation and specific activation of protein kinase A (PKA), but not through selective activation of EPAC1. Moreover, despite a requirement for c-Jun for SOCS3 induction in fibroblasts, phospho-null c-Jun (Ser63/73Ala) had little effect on SOCS3 induction by cyclic AMP in HUVECs. AP1 activation and SOCS3 induction by EPAC1 in HUVECs therefore occur independently of c-Jun phosphorylation on Ser63.
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    • "SOCS-dependent and SOCS-independent mechanisms have been described. In HUVEC, EPAC, activated by cAMP induces SOCS3 via Rap1 activation in [118] [119] which counteracts IL-6 signaling by inhibiting JAK kinases. In primary fibroblasts and hepatocytes cAMP (induced by prostaglandin E1 or glucagon, respectively) inhibits specifically IL-6-dependent MAPK activation without affecting STAT3 activation by SOCS3-independent mechanisms. "
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    • "We found that CNTF caused STAT3 phosphorylation in hypothalamic slices even after Fsk/Lep pretreatment of the slices (Figure S4A). cAMP signaling has been reported to interfere with interleukin 6 (IL-6)-STAT3 signaling in different cell lines (Bousquet et al., 2001; Delgado and Ganea, 2000; Fasshauer et al., 2002; Gasperini et al., 2002; Sands et al., 2006). Thus, we tested whether elevation of cAMP impaired IL6-induced STAT3 phosphorylation in our system. "
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