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

Abstract

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-6R␣/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.

The Cleaved Cytoplasmic Tail of Polycystin-1 Regulates Src-Dependent STAT3 Activation

“…In the absence of PC1-p30, cAMP suppresses STAT3 activation caused by EGFR signaling ( Figure 6C), IL-6 signaling ( Figure 2C), or signaling by membraneanchored PC1 ( Figure 2C). Because all of these mechanisms are sensitive to suppression by SOCS3, and because cAMP is known to lead to EPAC-mediated upregulation of SOCS3 expression, 29,30 we suggest that increased levels of cAMP in normal renal epithelial cells dampen STAT3 signaling in this way. EPAC signaling in response to cAMP agonists has previously been demonstrated in renal epithelial cells.…”

“…1 In other cell types, increased cAMP levels have been shown to lead to exchange protein activated by cAMP (EPAC)-dependent stimulation of SOCS3 expression, 29,30 which would be expected to lead to inhibition of JAK2-dependent STAT3 activation. We found that treatment with the adenylate cyclase activator forskolin strongly inhibits STAT3 activation that is mediated by IL6 or by the membrane-anchored PC1 tail ( Figure 2C), both of which signal via JAK.…”

“…44 cAMP may inhibit STAT3 activity by EPACdependent upregulation of SOCS3 expression. 29 On the other hand, cAMP may increase STAT3 activity by PKA-dependent phosphorylation and activation of Src. 27,45,46 As expected, EGFR expression leads to STAT3 transcriptional activity, which is further enhanced by Src expression ( Figure 6A).…”

Molecular Mechanism of Lipopolysaccharide-Induced SOCS-3 Gene Expression in Macrophages and Microglia

“…A large number of stimuli are able to induce SOCS-3 expression in a cell type-specific manner, including erythropoietin, IFN-␥, IFN-␤, LPS, oncostatin M, growth hormone, IL-4, IL-21, IGF, IL-27, LIF, IL-6, and cAMP (15)(16)(17)(18)(19)(20)(21)(22)(23). LIF activation of STAT-3 leads to SOCS-3 expression in corticotroph AtT-20 cells (16).…”

Regulation of PC12 Cell Differentiation by cAMP Signaling to ERK Independent of PKA: Do All the Connections Add Up?

“…The development of a specific agonist for EPAC, 8-CPT-2Me-cAMP, facilitated the study of this noncanonical cAMP signaling pathway (3). Several groups have since used this compound or its derivatives to probe the involvement of EPAC in various physiological and signaling processes, including β-adrenergic receptor-mediated calcium release in cardiac myocytes (4), leukocyte adhesion and chemotaxis (5), anti-inflammatory effects of cytokines in endothelial cells (6), activation of R-Ras and H-Ras (7,8), modulation of Ca 2+ -regulated potassium channels through p38 MAPK (9), and activation of ERK (8,(10)(11)(12). However, it must be pointed out that EPAC remains only a candidate for noncanonical cAMP signaling to ERK in PC12 and neuronal cells: Direct application of EPAC-specific agonists fails to activate ERK in either PC12 cells (3,10,13) or primary locus ceruleus neurons (10).…”