Epac- and Ca2+-controlled Activation of Ras and Extracellular Signal-regulated Kinases by Gs-coupled Receptors

Abstract

We have recently reported that two typical G s -coupled receptors, the ␤ 2 -adrenergic receptor and the receptor for prostaglandin E 1 , stimulate phospholipase C-⑀ (PLC- Activation of mitogen-activated protein (MAP) 1 kinases plays a prominent role in many cellular responses to a large variety of membrane receptors. Initially identified as signal transducers of growth factor receptors with intrinsic tyrosine kinase activity, activation of MAP kinases, specifically of the extracellular signal-regulated kinases 1 and 2 (ERK1/2), is now also recognized as a major signal transduction pathway of many receptors coupled to heterotrimeric G proteins. ERK activation by these second messenger-generating receptors is apparently accomplished by diverse molecular mechanisms, depending on the cell types studied as well as the receptor and the heterotrimeric G protein involved (1-5).⑀The cAMP-producing G s -coupled receptors play a rather unique role in ERK activation. These receptors inhibit ERK activation by growth factor receptors in several cell types while stimulating this cellular response in others, most notably in neuronal and endocrine cells. Several models have been proposed to explain these diverse actions of cAMP and G s -coupled receptors on ERK activation, both apparently involving in most cases the principal cAMP target, the cAMP-activated protein kinase (PKA) (6, 7). In particular, PKA-dependent ERK activation in HEK-293 cells by the ␤ 2 -adrenergic receptor (␤ 2 -AR), a prototypical G s -coupled receptor, has been extensively studied. Lefkowitz and coworkers (8) reported that cAMP-activated PKA phosphorylates the ␤ 2 -AR and thereby alters the coupling specificity of the receptor from G s to G i proteins. By this G protein "switching," the ␤ 2 -AR then apparently induces ERK activation by releasing G␤␥ dimers from the pertussis toxin (PTX)-sensitive G i proteins, followed by activation of the cytosolic tyrosine kinase c-Src, the GTPase Ras, and the MAP kinase kinase kinase, Raf-1. Meanwhile, it has been reported that the switching of ␤ 2 -AR from G s to G i proteins is controlled by ␤-arrestin, which recruits the cAMP-degrading phosphodiesterase 4 to the plasma membrane and thereby alters the activity state of PKA (9). In contrast, Schmitt and Stork (10) report that ERK activation by the ␤ 2 -AR in HEK-293 cells, although also PKA-and c-Src-dependent, is PTX-insensitive and thus apparently does not require ␤ 2 -AR switching to G i proteins, a finding recently confirmed by others using a PKAinsensitive ␤ 2 -AR mutant (11). Furthermore, Schmitt and Stork (10) show that ERK activation by ␤ 2 -AR is not mediated by Ras but by the related GTPase Rap1, activating the MAP kinase kinase kinase B-Raf, but not Raf-1. Interestingly, activation of the ␤ 2 -AR also resulted in Ras activation, but this receptor action was apparently independent of cAMP and PKA (10).

Microtubule-Associated Protein 1B-Light Chain 1 Enhances Activation of Rap1 by Exchange Protein Activated by Cyclic AMP but Not Intracellular Targeting

“…The minimal dose of 8CPT-2Me-cyclic AMP required to activate Rap1 in NIH3T3 cells was initially reported to be 10 M (Enserink et al, 2002). However, in a report by Keiper et al (2004) doses of 8CPT-2Me-cyclic AMP as low as 0.1 M were reported to activate extracellular signal-regulated kinase in human embryonic kidney 293 cells (Keiper et al, 2004). The presence of biological enhancers of EPAC function, like LC1 and LC2, in certain cell types could possibly explain this kind of sensitization of EPAC signaling.…”

Epac Activation Converts cAMP from a Proliferative into a Differentiation Signal in PC12 Cells

“…Epacs are guanine nucleotide exchange factors on Rap1 and Rap2 and may thus exert their effects on ERK1/2 through activation of Rap1 or Rap2 (Chen et al, 2004;Keiper et al, 2004). It has been reported that cAMP-induced ERK1/2 activation involves PKA-dependent activation of Rap1, which in turn activates B-Raf in PC12 cells (Vossler et al, 1997).…”

Epac Mediates a cAMP-to-PKC Signaling in Inflammatory Pain: An Isolectin B4(+) Neuron-Specific Mechanism

“…Epac in turn activates a newly identified phospholipase, phospholipase C⑀ (PLC⑀) (Schmidt et al, 2001), and could therefore potentially activate novel PKCs, such as PKC⑀, through phospholipase-produced diacylglycerol (DAG) (Parekh et al, 2000). While the role of Epac in activation of mitogen-activated protein kinases (MAPKs) is a matter of intensive ongoing investigation (Enserink et al, 2002;Keiper et al, 2004), Epac is thus far not known to mediate activation of PKCs.…”