Cell physiology of cAMP sensor Epac
Abstract
Epac is an acronym for the exchange proteins activated directly by cyclic AMP, a family of cAMP-regulated guanine nucleotide exchange factors (cAMPGEFs) that mediate protein kinase A (PKA)-independent signal transduction properties of the second messenger cAMP. Two variants of Epac exist (Epac1 and Epac2), both of which couple cAMP production to the activation of Rap, a small molecular weight GTPase of the Ras family. By activating Rap in an Epac-mediated manner, cAMP influences diverse cellular processes that include integrin-mediated cell adhesion, vascular endothelial cell barrier formation, and cardiac myocyte gap junction formation. Recently, the identification of previously unrecognized physiological processes regulated by Epac has been made possible by the development of Epac-selective cyclic AMP analogues (ESCAs). These cell-permeant analogues of cAMP activate both Epac1 and Epac2, whereas they fail to activate PKA when used at low concentrations. ESCAs such as 8-pCPT-2′-O-Me-cAMP and 8-pMeOPT-2′-O-Me-cAMP are reported to alter Na, K, Ca and Cl channel function, intracellular [Ca], and Na–H transporter activity in multiple cell types. Moreover, new studies examining the actions of ESCAs on neurons, pancreatic beta cells, pituitary cells and sperm demonstrate a major role for Epac in the stimulation of exocytosis by cAMP. This topical review provides an update concerning novel PKA-independent features of cAMP signal transduction that are likely to be Epac-mediated. Emphasized is the emerging role of Epac in the cAMP-dependent regulation of ion channel function, intracellular Ca signalling, ion transporter activity and exocytosis.
The discovery and characterization of a novel cAMP signal transduction mechanism that uses the Epac family of cAMP ‘sensors’ to regulate multiple cellular functions has dramatically reinvigorated interest in cyclic nucleotide research (Bos, 2003). Actions of cAMP, which at one time were thought to be mediated exclusively by protein kinase A (PKA), must now be re-evaluated in the light of an accumulating body of evidence that indicates a likely role for Epac in cell physiology (Holz, 2004; Seino & Shibasaki, 2005). By serving as a cAMP-binding protein with intrinsic guanine nucleotide exchange factor (GEF) activity, Epac couples cAMP production to the activation of Rap, a small molecular weight GTPase of the Ras family (Fig. 1). Cellular processes stimulated as a consequence of the Epac-mediated activation of Rap include integrin-mediated cell adhesion (Rangarajan et al. 2003), vascular endothelial cell barrier formation (Fukuhara et al. 2005; Kooistra et al. 2005), cardiac gap junction formation (Somekawa et al. 2005), mitogen-activated protein kinase (MAPK) signalling (Wang et al. 2006), hormone gene expression (Gerlo et al. 2006; Lotfi et al. 2006), and phospholipase C-epsilon (PLC-ɛ) activation (Schmidt et al. 2001). Thus, Epac is an exchange protein activated directly by cyclic AMP (de Rooij et al. 1998; Rehman et al. 2006), or in an alternative terminology, a cyclic AMP-regulated guanine nucleotide exchange factor (cAMPGEF) (Kawasaki et al. 1998; Ozaki et al. 2000).
When bound to cAMP, Epac catalyses the exchange of GDP for GTP on Rap GTPase. The activated form of Rap-GTP is then capable of promoting integrin-mediated cell adhesion, gap junction formation and ERK1/2 MAPK-mediated protein phosphorylation. Activated Rap also stimulates phospholipase C-epsilon (PLC-ɛ) which hydrolyses PIP2 to generate diacylglycerol (DAG), and the Ca-mobilizing second messenger IP3. As illustrated, some actions of Epac may also be Rap independent. These actions of Epac may involve its interaction with microtubule-associated proteins, the Ras GTPases, secretory granule-associated proteins (Rim2, Piccolo), and the SUR1 subunit of KATP channels. Abbreviations: G protein coupled receptor, GPCR; cytoskeletal protein-associated with the active zone, CAZ; ATP-sensitive K channel, KATP.
The Rap GTPases are not the only interesting molecules with which Epac interacts (Fig. 1). Epac is also reported to interact with Ras GTPases (Li et al. 2006; De Jesus et al. 2006), microtubule-associated proteins (Yarwood, 2005), secretory granule-associated proteins such as Rim2 and Piccolo (Ozaki et al. 2000; Fujimoto et al. 2002; Shibasaki et al. 2004a,b), and the sulphonylurea receptor-1 (SUR1), a subunit of ATP-sensitive K channels (Ozaki et al. 2000; Shibasaki et al. 2004a,b; Kang et al. 2006). Some of these interactions may underlie the recruitment of Epac to an intracellular compartment that is rich in Rap GTPase. Alternatively, Epac may act as a multifunctional protein, one in which cAMP exerts its effects not simply by promoting guanyl nucleotide exchange on Rap, but by allosterically regulating key molecules involved in cell physiology. Intriguingly, newly published findings demonstrate Epac-mediated actions of cAMP that influence Na, K, Ca, and Cl channel function, [Ca]i, Na–H and Na–K transporter activity, and exocytosis in multiple cell types (see below).
Acknowledgments
G.G.H. acknowledges the support of the NIH (R01-DK45817) and the American Diabetes Association (Research Grant Award). M.W.R. acknowledges the support of the NIH (R01-DK074966; R21-DK068822).