Activation of JNK by Epac Is Independent of Its Activity as a Rap Guanine Nucleotide Exchanger

Abstract

Guanine nucleotide exchange factors (GEFs) and their associated GTP-binding proteins (G-proteins) are key regulatory elements in the signal transduction machinery that relays information from the extracellular environment into specific intracellular responses. Among them, the MAPK cascades represent ubiquitous downstream effector pathways. We have previously described that, analogous to the Ras-dependent activation of the Erk-1/2 pathway, members of the Rho family of small G-proteins activate the JNK cascade when GTP is loaded by their corresponding GEFs. Searching for novel regulators of JNK activity we have identified Epac (exchange protein activated by cAMP) as a strong activator of JNK-1. Epac is a member of a growing family of GEFs that specifically display exchange activity on the Rap subfamily of Ras small G-proteins. We report here that while Epac activates the JNK severalfold, a constitutively active (G12V) mutant of Rap1b does not, suggesting that Rap-GTP is not sufficient to transduce Epac-dependent JNK activation. Moreover, Epac signaling to the JNKs was not blocked by inactivation of endogenous Rap, suggesting that Rap activation is not necessary for this response. Consistent with these observations, domain deletion mutant analysis shows that the catalytic GEF domain is dispensable for Epac-mediated activation of JNK. These studies identified a region overlapping the Ras exchange motif domain as critical for JNK activation. Consistent with this, an isolated Ras exchange motif domain from Epac is sufficient to activate JNK. We conclude that Epac signals to the JNK cascade through a new mechanism that does not involve its canonical catalytic action, i.e. Rap-specific GDP/GTP exchange. This represents not only a novel way to activate the JNKs but also a yet undescribed mechanism of downstream signaling by Epac.

Prostaglandin E2 Induces Breast Cancer–Related Aromatase Promoters via Activation of p38 and c-Jun NH2-Terminal Kinase in Adipose Fibroblasts

“…Pituitary adenylate cyclase-activating polypeptide stimulates p38 in neuronal cells via cAMP/EPAC (32). cAMP activates JNK in certain cell types, and the Ras exchange motif domain of EPAC has been found to be sufficient to stimulate JNK (31,33). The mechanisms whereby cAMP and PKA stimulate the p38 and JNK cascades, however, remain poorly understood.…”

Epac: A New cAMP-Binding Protein in Support of Glucagon-Like Peptide-1 Receptor-Mediated Signal Transduction in the Pancreatic β-Cell

“…Rap1 will antagonize (6,7) and also the ␤-cell sulfonylurea receptor (SUR1) (5). Epac may also activate c-Jun kinase (JNK) independently of Rap1 (26). Table 1 summarizes signaling pathways that are proposed to be regulated by Epac.…”

Impaired Shc, Ras, and MAPK Activation but Normal Akt Activation in FL5.12 Cells Expressing an Insulin-like Growth Factor I Receptor Mutated at Tyrosines 1250 and 1251

“…This suggests that decreased activity of the MAPK families contributes to the loss of anti-apoptotic activity seen in cells expressing the Y1250F/Y1251F mutant receptor. Ras Activation Is Decreased in FL5.12 Cells Expressing the Y1250F/Y1251F Mutant-Growth factor-mediated activation of the Ras superfamily of GTPases has been shown to contribute to activation of the MAPK and JNK pathways and contribute to growth, survival, transformation, and cytoskeletal organizations associated with cell movement (34,35). Therefore, it was important to determine whether Ras activity was normal in FL5.12 cells expressing the Y1250F/Y1251F mutant.…”