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

Abstract

Elevation of the intracellular cAMP concentration ([cAMP]i ) regulates metabolism, cell proliferation, and differentiation and plays roles in memory formation and neoplastic growth. cAMP mediates its effects mainly through activation of protein kinase A (PKA) as well as Epac1 and Epac2, exchange factors activating the small GTPases Rap1 and Rap2. However, how cAMP utilizes these effectors to induce distinct biological responses is unknown. We here studied the specific roles of PKA and Epac in neuroendocrine PC12 cells. In these cells, elevation of [cAMP] i activates extracellular signal-regulated kinase (ERK) 1/2 and induces low-degree neurite outgrowth. The present study showed that specific stimulation of PKA triggered ERK1/2 activation that was considerably more transient than that observed upon simultaneous activation of both PKA and Epac. Unexpectedly, the PKA-specific cAMP analog induced cell proliferation rather than neurite outgrowth. The proliferative signaling pathway activated by the PKA-specific cAMP analog involved activation of the epidermal growth factor receptor and ERK1/2. Activation of Epac appeared to extend the duration of PKA-dependent ERK1/2 activation and converted cAMP from a proliferative into an anti-proliferative, neurite outgrowthpromoting signal. Thus, the present study showed that the outcome of cAMP signaling can depend heavily on the set of cAMP effectors activated. INTRODUCTIONThe normal development of organisms requires a tight coordination between growth and differentiation. Importantly, the regulation of this decision is altered in disease states, most notably in cancer, where growth signals overcome the physiological processes of differentiation. The differentiation of neuronal cells is important for the regular function of the brain. Thus, there is interest in understanding the molecular signaling mechanisms that regulate cell proliferation and differentiation.In a number of cells, most notably of neuronal origin, G s protein-coupled receptors (GsPCR) are key regulators of cell proliferation, differentiation, and survival and play important roles in numerous biological processes such as neoplasia, learning, and memory formation (Kandel, 2001;Spada and Lania, 2002;Wang et al., 2004). The signaling cascades downstream of GsPCRs to regulate these events include activation of adenylyl cyclase, elevation of the intracellular cAMP concentration ([cAMP] i ), and modulation of the extracellular signal-regulated kinase (ERK) 1/2 pathway (Stork, 2003).cAMP binds to and directly activates protein kinase A (PKA) as well as the cAMP binding guanine nucleotide exchange factors Epac1 and -2, which in turn stimulate the small GTPases Rap1 and Rap2. Together, PKA and Epacs appear to mediate the majority of effects of cAMP in mammalian cells (de Rooij et al., 1998;Kawasaki et al., 1998;Bos, 2003). Nevertheless, most work on cAMP-dependent pathways has been centered on the role of PKA. For instance, a role of PKA in memory formation has been verified in numerous in vitro and in vivo studies t...

Lot1 Is a Key Element of the Pituitary Adenylate Cyclase-activating Polypeptide (PACAP)/Cyclic AMP Pathway That Negatively Regulates Neuronal Precursor Proliferation

“…We additionally found that the PAC1-R/cAMP system, up-regulated by Lot1, induced activation of the MEK pathway, with a consequent increase of c-Fos expression. According to recent studies, PACAP induces PC12 cell neurite outgrowth through either the MEK and/or the Epac pathway, independently of PKA (26,63,64). Thus, although in PC12 cells PACAP-induced neuritogenesis appears to be PKAindependent, according to our study the anti-proliferative action of PACAP is PKA-dependent.…”

Protein Kinase A-mediated Gating of Neuregulin-dependent ErbB2-ErbB3 Activation Underlies the Synergistic Action of cAMP on Schwann Cell Proliferation

“…Even though PKA has been suggested to mimic EGF action and directly trigger tyrosine phosphorylation of EGF receptor (36,45), our results clearly indicate a particular case of PKA-mediated gating in the synergistic enhancement of neuregulin-dependent ErbB2-ErbB3 activation by PKA. To the best of our knowledge, evidence for RTK gating by cAMP has only been reported for TrkB receptors in hippocampal neurons.…”

Urinary EGF Receptor Ligand Excretion in Patients with Autosomal Dominant Polycystic Kidney Disease and Response to Tolvaptan

“…Both cAMP and EGF receptor activation stimulate the proliferation of tubular cells to form cysts by the RAS/RAF/MEK/ERK pathway (23). Cross-talk between these two pathways could exist (i.e., between the EGF receptor and the vasopressin-cAMP pathways [24][25][26]), resulting in an increase in HB-EGF during treatment with the V2RA. Second, V2RA treatment could induce short-term ischemia, causing an increase in HB-EGF excretion.…”