B-Raf-dependent regulation of the MEK-1/mitogen-activated protein kinase pathway in PC12 cells and regulation by cyclic AMP

Role of A-kinase anchoring proteins in cyclic-AMP-mediated Schwann cell proliferation

Proliferation of Schwann cells during peripheral nerve development is stimulated by the heregulin/neuregulin family of growth factors expressed by neurons. However, for neonatal rat Schwann cells growing in culture, heregulins produce only a weak mitogenic response. Supplementing heregulin with forskolin, an agent that elevates cyclic AMP levels, produces a dramatic increase in the proliferation of cultured Schwann cells. The mechanisms underlying this synergistic effect required for Schwann cell proliferation in vivo is not well established. Characterizing the A-kinase anchoring proteins (AKAPs) in Schwann cells might help identify substrates tethered to and phosphorylated by the cAMP-dependent protein kinase A (PKA). Using an RII overlay assay that detects AKAPs that are bound to the type II regulatory subunits of PKA, we identified AKAP150 in Schwann cells. Western blot analysis revealed that additional AKAPs, specifically AKAP95, and yotiao were also present. Disruption of PKA/AKAP interaction with Ht-31 peptide resulted in an increase in luciferase-conjugated cyclin D3 promoter activity. Transfection with sequence-specific AKAP siRNAs for AKAP150 and AKAP95 produced a marked reduction in cell proliferation. Immunoblot analysis revealed that knock down of AKAP95 protein caused a significant decrease in expression of the cell cycle regulatory proteins cyclin D2, cyclin D3 and the cell survival signal Akt/Protein Kinase B (Akt/PKB). Morphological characterization of Schwann cell AKAPs indicated the presence of nuclear (AKAP95), cytoplasm-associated (AKAP150) and perinuclear (yotiao) A-kinase anchoring proteins. These results indicate a role for AKAP95 and AKAP150 in the synergistic response of Schwann cells to treatment with heregulin and forskolin.

Overcoming Resistance to Therapies Targeting the MAPK Pathway in BRAF-Mutated Tumours

Overactivation of the mitogen-activated protein kinase (MAPK) pathway is an important driver of many human cancers. First line, FDA-approved therapies targeting MAPK signalling, which include BRAF and MEK inhibitors, have variable success across cancers, and a significant number of patients quickly develop resistance. In recent years, a number of preclinical studies have reported alternative methods of overcoming resistance, which include promoting apoptosis, modulating autophagy, and targeting mitochondrial metabolism. This review summarizes mechanisms of resistance to approved MAPK-targeted therapies in BRAF-mutated cancers and discusses novel preclinical approaches to overcoming resistance.

Perspectives of RAS and RHEB GTPase Signaling Pathways in Regenerating Brain Neurons

Cellular activation of RAS GTPases into the GTP-binding “ON” state is a key switch for regulating brain functions. Molecular protein structural elements of rat sarcoma (RAS) and RAS homolog protein enriched in brain (RHEB) GTPases involved in this switch are discussed including their subcellular membrane localization for triggering specific signaling pathways resulting in regulation of synaptic connectivity, axonal growth, differentiation, migration, cytoskeletal dynamics, neural protection, and apoptosis. A beneficial role of neuronal H-RAS activity is suggested from cellular and animal models of neurodegenerative diseases. Recent experiments on optogenetic regulation offer insights into the spatiotemporal aspects controlling RAS/mitogen activated protein kinase (MAPK) or phosphoinositide-3 kinase (PI3K) pathways. As optogenetic manipulation of cellular signaling in deep brain regions critically requires penetration of light through large distances of absorbing tissue, we discuss magnetic guidance of re-growing axons as a complementary approach. In Parkinson’s disease, dopaminergic neuronal cell bodies degenerate in the substantia nigra. Current human trials of stem cell-derived dopaminergic neurons must take into account the inability of neuronal axons navigating over a large distance from the grafted site into striatal target regions. Grafting dopaminergic precursor neurons directly into the degenerating substantia nigra is discussed as a novel concept aiming to guide axonal growth by activating GTPase signaling through protein-functionalized intracellular magnetic nanoparticles responding to external magnets.

Crosstalk from cAMP to ERK1/2 emerges during postnatal maturation of nociceptive neurons and is maintained during aging

Maturation of nociceptive neurons depends on changes in transcription factors, ion channels and neuropeptides. Mature nociceptors initiate pain in part by drastically reducing the activation threshold via intracellular sensitization signaling. Whether sensitization signaling also changes during development and aging remains so far unknown. Using a novel automated microscopy approach, we quantified changes in intracellular signaling protein expression and in their signaling dynamics, as well as changes in intracellular signaling cascade wiring, in sensory neurons from newborn to senescent (24 months of age) rats. We found that nociceptive subgroups defined by the signaling components protein kinase A (PKA)-RIIβ (also known as PRKAR2B) and CaMKIIα (also known as CAMK2A) developed at around postnatal day 10, the time of nociceptor maturation. The integrative nociceptor marker, PKA-RIIβ, allowed subgroup segregation earlier than could be achieved by assessing the classical markers TRPV1 and Na_v1.8 (also known as SCN10A). Signaling kinetics remained constant over lifetime despite in part strong changes in the expression levels. Strikingly, we found a mechanism important for neuronal memory - i.e. the crosstalk from cAMP and PKA to ERK1 and ERK2 (ERK1/2, also known as MAPK3 and MAPK1, respectively) - to emerge postnatally. Thus, maturation of nociceptors is closely accompanied by altered expression, activation and connectivity of signaling pathways known to be central for pain sensitization and neuronal memory formation.

BRAF V600E mutation in papillary thyroid carcinoma: significant association with node metastases and extra thyroidal invasion

B-Raf (BRAF) is the strongest activator in the downstream of MAP kinase signaling. The somatic point mutation of BRAF gene (V600E) is the most common and specific event in papillary thyroid carcinoma (PTC). However, its prevalence is variable among different studies and its association with clinico-pathological features is controversial. This study tests the prevalence of BRAF (V600E) mutation in thyroid cancer patients in Indian subcontinental population. We analyzed 140 thyroid tumor specimens for BRAF gene mutation at codon 600 using mutant-allele-specific amplification, single-strand conformation polymorphism, Mutector assay, and DNA sequencing of the PCR-amplified exon 15. BRAF mutation at codon 600 was detected in 46 of 86 PTC patients (53.4%) from Indian subcontinental cohort. Frequency of mutation varied across the subtypes of PTCs. BRAF (V600E) mutation was more common in the conventional PTC (38 out of 62; 61%) than in the follicular variant of PTC (2 out of 17; 11.7%). None of the 8 follicular thyroid adenomas, 14 follicular thyroid carcinomas, 16 medullary thyroid carcinomas, and 16 benign hyperplasia patients showed any exon 15 mutation. We found significant correlation between BRAF mutation status and extra-thyroidal invasion, lymph node metastasis, and tumor stage. However no correlation was observed with gender, age, and tumor size of the patients. Thus our findings suggest that BRAF (V600E) is a prevalent genetic alteration in adult sporadic PTCs in Indian cohort and it may be responsible for the progression of classic variant of PTC to metastatic and poorly differentiated subtype and likely to have significant impact on its diagnostic and prognostic management.

γ-Enolase C-terminal peptide promotes cell survival and neurite outgrowth by activation of the PI3K/Akt and MAPK/ERK signalling pathways

γ-Enolase, a glycolytic enzyme, is expressed specifically in neurons. It exerts neurotrophic activity and has been suggested to regulate growth, differentiation, survival and regeneration of neurons. In the present study, we investigated the involvement of γ-enolase in PI3K (phosphoinositide 3-kinase)/Akt and MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) signalling, the two pathways triggered predominantly by neurotrophic factors. Whereas the PI3K/Akt pathway, rather than the MAPK/ERK pathway, is involved in γ-enolase-enhanced cell survival, γ-enolase-stimulated neurite outgrowth requires both pathways, i.e. the activation of both PI3K and ERK1/2, leading to subsequent expression of the growth-cone-specific protein GAP-43 (growth-associated protein of 43 kDa). MEK (MAPK/ERK kinase) and PI3K inhibition blocked or attenuated the neurite outgrowth associated with dynamic remodelling of the actin-based cytoskeleton. We show that γ-enolase-mediated PI3K activation regulates RhoA kinase, a key regulator of actin cytoskeleton organization. Moreover, the inhibition of RhoA downstream effector ROCK (Rho-associated kinase) results in enhanced γ-enolase-induced neurite outgrowth, accompanied by actin polymerization and its redistribution to growth cones. Our results show that γ-enolase controls neuronal survival, differentiation and neurite regeneration by activating the PI3K/Akt and MAPK/ERK signalling pathways, resulting in downstream regulation of the molecular and cellular processes of cytoskeleton reorganization and cell remodelling, activation of transcriptional factors and regulation of the cell cycle.

Identification of BRAF as a new interactor of PLCε1, the protein mutated in nephrotic syndrome type 3

Steroid-resistant nephrotic syndrome is a malfunction of the kidney glomerular filter that leads to proteinuria, hypoalbuminemia, edema, and renal failure. Recently, we identified recessive mutations in the phospholipase C epsilon 1 gene ( PLCE1) as a new cause of early-onset nephrotic syndrome and demonstrated interaction of PLCε1 with IQGAP1. To further elucidate the mechanism by which PLCE1 mutations cause nephrotic syndrome, we sought to identify new protein interaction partners of PLCε1. We utilized information from the genetic interaction network of C. elegans. It relates the PLCE1 ortholog ( plc-1) to the C. elegans ortholog ( lin-45) of human BRAF ( v-raf murine sarcoma viral oncogene homolog B1). We hypothesized that this may indicate a functional protein-protein interaction. Using GST pull down of HEK293T cell lysates in vitro and coimmunoprecipation of mouse kidney lysates in vivo, we show that BRAF interacts with PLCε1. By immunohistochemistry in rat kidney, we demonstrate that both proteins are coexpressed and colocalize in developing and mature glomerular podocytes, reporting for the first time the expression of BRAF in the glomerular podocyte.

KSR and CNK: two scaffolds regulating RAS-mediated RAF activation

The RAS-RAF-MEK-extracellular-regulated kinase (RAS/ERK) pathway is a major intracellular route used by metazoan cells to channel to downstream targets a diverse array of signals, including those controlling cell proliferation and survival. Recent findings suggest that the pathway is assembled by specific scaffolding proteins that in turn regulate the efficiency, the location and/or the duration of signal transmission. Here, through the angle of studies conducted in Drosophila and C. elegans, we present two such proteins, the kinase suppressor of RAS (KSR) and connector enhancer of KSR (CNK) scaffolds, and highlight their implication in a novel mechanism regulating RAS-mediated RAF activation. Based on recent findings, we discuss the possibility that KSR, a RAF-like protein, does not solely act as a scaffold, but directly induces RAF catalytic function by a kinase-independent mechanism apparently shared by RAF-like proteins.

Ethanol and the tobacco-specific carcinogen, NNK, contribute to signaling in immortalized human pancreatic duct epithelial cells

OBJECTIVES

Smoking is a well-documented risk factor for pancreatic cancer. The tobacco-specific nitrosamine, NNK (4-[methylnitrosamino]-1-[3-pyridyl]-1-butanone), significantly induces pancreatic ductal adenocarcinomas in laboratory rodents. Recent observations suggest that ethanol enhances the tumorigenic effects of smoking. Ethanol consumption is associated with the development of chronic pancreatitis, also considered a predisposing factor for pancreatic ductal adenocarcinoma. Because the precise role of ethanol in pancreatic carcinogenesis is not known, this study sought to elucidate the cumulative effects of ethanol and NNK on particular signal transduction pathways that might play a role in cell proliferation in immortalized human pancreatic duct epithelial cells.

METHODS

The HPDE6-c7 cells are developed from pancreatic duct epithelial cells, which are the putative cells of origin of pancreatic ductal adenocarcinoma. Cell proliferation assays, Western blot, and cyclic adenosine monophosphate assays were used to demonstrate the effects of ethanol and NNK treatments on these cells.

RESULTS

Ethanol cotreatments enhanced the NNK-induced proliferation of these cells. This response was inhibited by the adenylyl cyclase, protein kinase A, mitogen-activated protein kinase (p42/p44), and epidermal growth factor receptor-specific tyrosine kinase inhibitors. Cotreatments of NNK and ethanol also increased cyclic adenosine monophosphate accumulation, cAMP response element-binding family of proteins and mitogen-activated protein kinase phosphorylation, and protein kinase A activation.

CONCLUSIONS

These findings suggest a potential role for these pathways contributing to the development of smoking- and alcohol-related pancreatic carcinogenesis.

AKAP signaling complexes: getting to the heart of the matter

Subcellular compartmentalization of protein kinases and phosphatases through their interaction with A-kinase anchoring proteins (AKAPs) provides a mechanism to control signal transduction events at specific sites within the cell. Recent findings suggest that these anchoring proteins dynamically assemble different cAMP effectors to control the cellular actions of cAMP spatially and temporally. In the heart, signaling events such as the onset of cardiac hypertrophy are influenced by muscle-specific mAKAP signaling complexes that target protein kinase A (PKA), the cAMP-responsive guanine-nucleotide exchange factor EPAC and cAMP-selective phosphodiesterase 4 (PDE4). Mediation of signaling events by AKAPs might also have a role in the control of lipolysis in adipocytes, where insulin treatment reduces the association of AKAPs with G-protein-coupled receptors. These are only two examples of how AKAPs contribute to specificity in cAMP signaling. This review will explore recent development that illustrates the role of multiprotein complexes in the regulation of cAMP signaling.

The mAKAP signaling complex: integration of cAMP, calcium, and MAP kinase signaling pathways

Following its production by adenylyl cyclases, the second messenger cAMP is in involved in pleiotrophic signal transduction. The effectors of cAMP include the cAMP-dependent protein kinase (PKA), the guanine nucleotide exchange factor Epac (exchange protein activated by cAMP), and cAMP-dependent ion channels. In turn, cAMP signaling is attenuated by phosphodiesterase-catalyzed degradation. The association of cAMP effectors and the enzymes that regulate cAMP concentration into signaling complexes helps to explain the differential signaling initiated by members of the G(s)-protein coupled receptor family. The signal transduction complex formed by the scaffold protein mAKAP (muscle A kinase-anchoring protein) at the nuclear envelope of both striated myocytes and neurons contains three cAMP-binding proteins, PKA, Epac1, and the phosphodiesterase PDE4D3. In addition, the mAKAP complex also contains components of the ERK5 MAP kinase signaling pathway, the calcium release channel ryanodine receptor and the phosphatases PP2A as well as calcineurin. Analysis of the mAKAP complex illustrates how a macromolecular complex can serve as a node in the intracellular signaling network of cardiac myocytes to integrate multiple cAMP signals with those of calcium and MAP kinases to regulate the hypertrophic actions of several hormones.

The protein kinase A anchoring protein mAKAP coordinates two integrated cAMP effector pathways

Cyclic adenosine 3', 5'-monophosphate (cAMP) is a ubiquitous mediator of intracellular signalling events. It acts principally through stimulation of cAMP-dependent protein kinases (PKAs) but also activates certain ion channels and guanine nucleotide exchange factors (Epacs). Metabolism of cAMP is catalysed by phosphodiesterases (PDEs). Here we identify a cAMP-responsive signalling complex maintained by the muscle-specific A-kinase anchoring protein (mAKAP) that includes PKA, PDE4D3 and Epac1. These intermolecular interactions facilitate the dissemination of distinct cAMP signals through each effector protein. Anchored PKA stimulates PDE4D3 to reduce local cAMP concentrations, whereas an mAKAP-associated ERK5 kinase module suppresses PDE4D3. PDE4D3 also functions as an adaptor protein that recruits Epac1, an exchange factor for the small GTPase Rap1, to enable cAMP-dependent attenuation of ERK5. Pharmacological and molecular manipulations of the mAKAP complex show that anchored ERK5 can induce cardiomyocyte hypertrophy. Thus, two coupled cAMP-dependent feedback loops are coordinated within the context of the mAKAP complex, suggesting that local control of cAMP signalling by AKAP proteins is more intricate than previously appreciated.

Integrating signals between cAMP and the RAS/RAF/MEK/ERK signalling pathways. Based on the anniversary prize of the Gesellschaft für Biochemie und Molekularbiologie Lecture delivered on 5 July 2003 at the Special FEBS Meeting in Brussels

One of the hallmarks of cAMP is its ability to inhibit proliferation in many cell types, but stimulate proliferation in others. Clearly cAMP has cell type specific effects and the outcome on proliferation is largely attributed to crosstalk from cAMP to the RAS/RAF/mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK) kinase (MEK)/ERK pathway. We review the crosstalk between these two ancient and conserved pathways, describing the molecular mechanisms underlying the interactions between these pathways and discussing their possible biological importance.

cAMP blocks MAPK activation and sclerotial development via Rap-1 in a PKA-independent manner in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a filamentous ascomycete phytopathogen able to infect an extremely wide range of cultivated plants. Our previous studies have shown that increases in cAMP levels result in the impairment of the development of the sclerotium, a highly differentiated structure important in the disease cycle of this fungus. cAMP also inhibits the activation of a S. sclerotiorum mitogen-activated protein kinase (MAPK), which we have previously shown to be required for sclerotial maturation; thus cAMP-mediated sclerotial inhibition is modulated through MAPK. However, the mechanism(s) by which cAMP inhibits MAPK remains unclear. Here we demonstrate that a protein kinase A (PKA)-independent signalling pathway probably mediates MAPK inhibition by cAMP. Expression of a dominant negative form of Ras, an upstream activator of the MAPK pathway, also inhibited sclerotial development and MAPK activation, suggesting that a conserved Ras/MAPK pathway is required for sclerotial development. Evidence from bacterial toxins that specifically inhibit the activity of small GTPases, suggested that Rap-1 or Ras is involved in cAMP action. The Rap-1 inhibitor, GGTI-298, restored MAPK activation in the presence of cAMP, further suggesting that Rap-1 is responsible for cAMP-dependent MAPK inhibition. Importantly, inhibition of Rap-1 is able to restore sclerotial development blocked by cAMP. Our results suggest a novel mechanism involving the requirement of Ras/MAPK pathway for sclerotial development that is negatively regulated by a PKA-independent cAMP signalling pathway. Cross-talk between these two pathways is mediated by Rap-1.

Melanocortinergic modulation of cholecystokinin-induced suppression of feeding through extracellular signal-regulated kinase signaling in rat solitary nucleus

Signals from the gut and hypothalamus converge in the caudal brainstem to control ingestive behavior. We have previously shown that phosphorylation of ERK1/2 in the solitary nucleus (NTS) is necessary for food intake suppression by exogenous cholecystokinin (CCK). Here we test whether this intracellular signaling cascade is also involved in the integration of melanocortin-receptor (MCR) mediated inputs to the caudal brainstem. Using fourth ventricular-cannulated rats and Western blotting of NTS tissue, we show that the MC4R agonist melanotan II (MTII) rapidly and dose-dependently increases phosphorylation of both ERK1/2 and cAMP response element-binding protein (CREB). Sequential administration of fourth ventricular MTII and peripheral CCK at doses that alone produced submaximal stimulation of pERK1/2 produced an additive increase. Prior fourth ventricular administration of the MC4R antagonist SHU9119 completely abolished the CCK-induced increases in pERK and pCREB and, in freely feeding rats, SHU9119 significantly increased meal size and satiety ratio. Prior administration of the MAPK kinase inhibitor U0126 abolished the capacity of MTII to suppress 2-h food intake and significantly decreased MTII-induced ERK phosphorylation in the NTS. Furthermore, pretreatment with the cAMP inhibitor, cAMP receptor protein-Rp isomer, significantly attenuated stimulation of pERK induced by either CCK or MTII. The results demonstrate that activation of the ERK pathway is necessary for peripheral CCK and central MTII to suppress food intake. The cAMP-->ERK-->CREB cascade may thus constitute a molecular integrator for converging satiety signals from the gut and adiposity signals from the hypothalamus in the control of meal size and food intake.

Adenosine and cAMP are potent inhibitors of the NF-kappa B pathway downstream of immunoreceptors

Anergic B lymphocytes exert compromised signal transduction towards the activation of NF-kappa B in response to B cell antigen receptor (BCR) triggering, whereas activation of the ERK pathway appears normal. How this differential down-regulation of the NF-kappa B pathway is regulated remains still elusive. Here, we demonstrate that stimuli known to enhance 3',5'-cyclic adenosine monophosphate (cAMP) are capable of selectively suppressing the activation both of NF-kappa B downstream of the BCR and Toll-like receptor 4 in splenic B lymphocytes and of the high-affinity receptor for IgE in BM-derived mast cells. This suppression is accomplished by blocking phosphorylation and subsequent degradation of the inhibitor of NF-kappa B. A cAMP-dependent protein kinase (PKA) inhibitor reverses this suppressive effect, indicating that PKA is a downstream effector of cAMP in this process. Importantly, not only drugs that artificially elevate intracellular cAMP levels, but also the nucleoside adenosine, which is known to be a mediator of cellular distress, inhibit the NF-kappa B pathway. This suggests that adenosine-mediated signals represent an important step in the molecular decision process controlling inflammation versus anergic immune responses.

Multiple, PKA-dependent and PKA-independent, signals are involved in cAMP-induced PRL expression in the eosinophilic cell line Eol-1

Besides its pivotal role in reproduction, the polypeptide hormone prolactin (PRL) has been attributed an immunomodulatory function. Extrapituitary PRL expression is regulated differently from that in the pituitary, due to the use of an alternative promoter. In leukocytes, cAMP is an important regulator of PRL expression. We report that in the human eosinophilic cell line Eol-1, cAMP-induced PRL expression is partially abrogated by two protein kinase A (PKA) inhibitors (H89, PKI) and by the p38 inhibitor SB203580. Phosphorylation of p38 was PKA-independent and could be stimulated by a methylated cAMP analogue, which specifically activates the exchange factor directly activated by cAMP (EPAC). Furthermore, cAMP induced a PKA-dependent phosphorylation of cAMP-responsive element binding protein (CREB). We postulate that cAMP induces PRL expression via two different signalling pathways: a PKA-dependent pathway leading to the phosphorylation of CREB, and a PKA-independent pathway leading to the phosphorylation of p38.

B-Raf contributes to sustained extracellular signal-regulated kinase activation associated with interleukin-2 production stimulated through the T cell receptor

A T cell receptor (TCR) recognizes and responds to an antigenic peptide in the context of major histocompatibility complex-encoded molecules. This provokes T cells to produce interleukin-2 (IL-2) through extracellular signal-regulated kinase (ERK) activation. We investigated the roles of B-Raf in TCR-mediated IL-2 production coupled with ERK activation in the Jurkat human T cell line. We found that TCR cross-linking could induce up-regulation of both B-Raf and Raf-1 activities, but Raf-1 activity was decreased rapidly. On the other hand, TCR-stimulated kinase activity of B-Raf was sustained. Expression of a dominant-negative mutant of B-Raf abrogated sustained but not transient TCR-mediated MEK/ERK activation. The inhibition of sustained ERK activation by either expression of a dominant-negative B-Raf or treatment with a MEK inhibitor resulted in a decrease of the TCR-stimulated nuclear factor of activated T cells (NFAT) activity and IL-2 production. Collectively, our data provide the first direct evidence that B-Raf is a positive regulator of TCR-mediated sustained ERK activation, which is required for NFAT activation and the full production of IL-2.

cAMP-dependent protein kinase type I regulates ethanol-induced cAMP response element-mediated gene expression via activation of CREB-binding protein and inhibition of MAPK

We have shown that the two types of cAMP-dependent protein kinase (PKA) in NG108-15 cells differentially mediate forskolin- and ethanol-induced cAMP response element (CRE)-binding protein (CREB) phosphorylation and CRE-mediated gene transcription. Activated type II PKA is translocated into the nucleus where it phosphorylates CREB. By contrast, activated type I PKA does not translocate to the nucleus but is required for CRE-mediated gene transcription by inducing the activation of other transcription cofactors such as CREB-binding protein (CBP). We show here that CBP is required for forskolin- and ethanol-induced CRE-mediated gene expression. Forskolin- and ethanol-induced CBP phosphorylation, demonstrable at 10 min, persists up to 24 h. CBP phosphorylation requires type I PKA but not type II PKA. In NG108-15 cells, ethanol and forskolin activation of type I PKA also inhibits several components of the MAPK pathway including B-Raf kinase, ERK1/2, and p90RSK phosphorylation. As a result, unphosphorylated p90RSK no longer binds to nor inhibits CBP. Moreover, MEK inhibition by PD98059 induces a significant increase of CRE-mediated gene activation. Taken together, our findings suggest that inhibition of the MAPK pathway enhances cAMP-dependent gene activation during exposure of NG108-15 cells to ethanol. This mechanism appears to involve type I PKA-dependent phosphorylation of CBP and inhibition of MEK-dependent phosphorylation of p90RSK. Under these conditions p90RSK is no longer bound to CBP, thereby promoting CBP-dependent CREB-mediated gene expression.

Differential regulation of mesangial cell mitogenesis by cAMP phosphodiesterase isozymes 3 and 4

Mesangial cell (MC) mitogenesis is regulated through "negative cross talk" between cAMP-PKA and ERK signaling. Although it is widely accepted that cAMP inhibits mitogenesis through PKA-mediated phosphorylation of Raf-1, recent studies have indicated that cAMP-mediated inhibition of mitogenesis may occur independently of Raf-1 phosphorylation or without inhibiting ERK activity. We previously showed that MCs possess functionally compartmentalized intracellular pools of cAMP that are differentially regulated by cAMP phosphodiesterases (PDE); an intracellular pool directed by PDE3 but not by PDE4 suppresses mitogenesis. We therefore sought to determine whether there was a differential effect of PDE3 vs. PDE4 inhibitors on the Ras-Raf-MEK-ERK pathway in cultured MC. Although PDE3 and PDE4 inhibitors activated PKA and modestly elevated cAMP levels to a similar extent, only PDE3 inhibitors suppressed MC mitogenesis (-57%) and suppressed Raf-1 kinase and ERK activity (-33 and -68%, respectively). Both PDE3 and PDE4 inhibitors suppressed B-Raf kinase activity. PDE3 inhibitors increased phosphorylation of Raf-1 on serine 43 and serine 259 and decreased phosphorylation on serine 338; PDE4 inhibitors were without effect. Overexpression of a constitutively active MEK-1 construct reversed the antiproliferative effect of PDE3 inhibitors. PDE3 inhibitors also reduced cyclin A levels (-27%), cyclin D and cyclin E kinase activity (-30 and -50%, respectively), and induced expression of the cell cycle inhibitor p21 (+90%). We conclude that the antiproliferative effects of PDE3 inhibitors are mechanistically related to inhibition of the Ras-Raf-MEK-ERK pathway. Additional cell cycle targets of PDE3 inhibitors include cyclin A, cyclin D, cyclin E, and p21.

Calcium restriction allows cAMP activation of the B-Raf/ERK pathway, switching cells to a cAMP-dependent growth-stimulated phenotype

cAMP can be either mitogenic or anti-mitogenic, depending on the cell type. We demonstrated previously that cAMP inhibited the proliferation of normal renal epithelial cells and stimulated the proliferation of cells derived from the cysts of polycystic kidney disease (PKD) patients. The protein products of the genes causing PKD, polycystin-1 and polycystin-2, are thought to regulate intracellular calcium levels, suggesting that abnormal polycystin function may affect calcium signaling and thus cause a switch to the cAMP growth-stimulated phenotype. To test this hypothesis, we disrupted intracellular calcium mobilization by treating immortalized mouse M-1 collecting duct cells and primary cultures of human kidney epithelial cells with calcium channel blockers and by lowering extracellular calcium with EGTA. Calcium restriction for 3-5 h converted both cell types from a normal cAMP growth-inhibited phenotype to an abnormal cAMP growth-stimulated phenotype, characteristic of PKD. In M-1 cells, we showed that calcium restriction was associated with an elevation in B-Raf protein levels and cAMP-stimulated, Ras-dependent activation of B-Raf and ERK. Moreover, the activity of Akt, a negative regulator of B-Raf, was decreased by calcium restriction. Inhibition of Akt or phosphatidylinositol 3-kinase also allowed cAMP-dependent activation of B-Raf and ERK in normal calcium. These results suggest that calcium restriction causes an inhibition of the phosphatidylinositol 3-kinase/Akt pathway, which relieves the inhibition of B-Raf to allow the cAMP growth-stimulated phenotypic switch. Finally, M-1 cells stably overexpressing an inducible polycystin-1 C-terminal cytosolic tail construct were shown to exhibit a cAMP growth-stimulated phenotype involving B-Raf and ERK activation, which was reversed by the calcium ionophore A23187. We conclude that disruption of calcium mobilization in cells that are normally growth-inhibited by cAMP can derepress the B-Raf/ERK pathway, thus converting these cells to a phenotype that is growth-stimulated by cAMP.

Dystrophin Dp71 is required for neurite outgrowth in PC12 cells

To determine the role of Dp71 in neuronal cells, we generated PC12 cell lines in which Dp71 protein levels were controlled by stable transfection with either antisense or sense constructs. Cells expressing the antisense Dp71 RNA (antisense-Dp71 cells) contained reduced amounts of the two endogenous Dp71 isoforms. Antisense-Dp71 cells exhibited a marked suppression of neurite outgrowth upon the induction with NGF or dibutyryl cyclic AMP. Early responses to NGF-induced neuronal differentiation, such as the cessation of cell division and the activation of ERK1/2 proteins, were normal in the antisense-Dp71 cells. On contrary, the induction of MAP2, a late differentiation marker, was disturbed in these cells. Additionally, the deficiency of Dp71 correlated with an altered expression of the dystrophin-associated protein complex (DAPC) members alpha and beta dystrobrevins. Our results indicate that normal expression of Dp71 is essential for neurite outgrowth in PC12 cells and constitute the first direct evidence implicating Dp71 in a neuronal function.

Melanocortin receptor signaling through mitogen-activated protein kinase in vitro and in rat hypothalamus

The central melanocortin system has emerged as a potential regulator of food intake. This action of melanocortins appears to occur through intrahypothalamic, melanocortin-containing projections, including those from the arcuate to the paraventricular nucleus (PVN). Although the complexity of feeding behavior and the long duration of the effects of melanocortins on food intake suggest changes in gene expression, the mechanism by which such changes occur has been elusive. In the present report, we describe experiments using in vitro and in vivo approaches to demonstrate melanocortin-induced phosphorylation (activation) of members of the mitogen-activated protein kinase (MAPK) family of transcription factors. First, application of the melanocortin agonist MTII to COS-1 cells resulted in an increase in phosphorylated MAPK after the cells were transfected with the melanocortin type 4 receptor (MC4-R), but not the type 3 receptor. Formation of cAMP, however, was observed when either receptor subtype was transfected. Subsequent experiments revealed that the effect of MTII on MAPK activation in MC4-R-transfected cells was dose-dependent and was maximal after 10 min of MTII exposure. Second, central injections of MTII increased the number of phospho-MAPK-immunoreactive cells in the rat PVN compared to vehicle-injected animals. When coupled with immunohistochemical identification of PVN neurons containing oxytocin, a clear segregation was apparent, allowing for a precise anatomical description of the pattern of activated MAPK within the PVN. These data are the first to suggest a differential coupling of MC4-R and may describe a mechanism through which the long-term and persistent behavioral actions of melanocortins are mediated.

Signalling from adenosine receptors to mitogen-activated protein kinases

The purine nucleoside adenosine acts via four distinct adenosine receptor subtypes: the adenosine A(1), A(2A), A(2B), and A(3) receptor. They are all G protein-coupled receptors (GPCR) coupling to classical second messenger pathways such as modulation of cAMP production or the phospholipase C (PLC) pathway. In addition, they couple to mitogen-activated protein kinases (MAPK), which could give them a role in cell growth, survival, death and differentiation. Although each of the adenosine receptors can activate one or more of the MAPKs, the mechanisms appear to differ substantially, both between receptor subtypes in the same cell type and between the same receptor in different cell types.

Cyclic AMP-independent involvement of Rap1/B-Raf in the angiotensin II AT2 receptor signaling pathway in NG108-15 cells

The angiotensin II (Ang II) type 2 (AT(2)) receptor is an atypical seven-transmembrane domain receptor. Controversy surrounding this receptor concerns both the nature of the second messengers produced as well as its associated signaling mechanisms. Using the neuronal cell line NG108-15, we have reported previously that activation of the AT(2) receptor induced morphological differentiation in a p21(ras)-independent, but p42/p44(mapk)-dependent mechanism. The activation of p42/p44(mapk) was delayed, sustained, and had been shown to be essential for neurite elongation. In the present report, we demonstrate that activation of the AT(2) receptor rapidly, but transiently, activated the Rap1/B-Raf complex of signaling proteins. In RapN17- and Rap1GAP-transfected cells, the effects induced by Ang II were abolished, demonstrating that activation of these proteins was responsible for the observed p42/p44(mapk) phosphorylation and for morphological differentiation. To assess whether cAMP was involved in the activation of Rap1/B-Raf and neuronal differentiation induced by Ang II, NG108-15 cells were treated with stimulators or inhibitors of the cAMP pathway. We found that dibutyryl cAMP and forskolin did not stimulate Rap1 or p42/p44(mapk) activity. Furthermore, adding H-89, an inhibitor of protein kinase A, or Rp-8-Br-cAMP-S, an inactive cAMP analog, failed to impair p42/p44(mapk) activity and neurite outgrowth induced by Ang II. The present observations clearly indicate that cAMP, a well known stimulus of neuronal differentiation, did not participate in the AT(2) receptor signaling pathways in the NG108-15 cells. Therefore, the AT(2) receptor of Ang II activates the signaling modules of Rap1/B-Raf and p42/p44(mapk) via a cAMP-independent pathway to induce morphological differentiation of NG108-15 cells.

Cell condition-dependent regulation of ERK5 by cAMP

ERK5 activity is increased by agents known to activate receptor tyrosine kinases, G-protein coupled receptors, and stress response pathways. We now find a role for cAMP in the regulation of ERK5. ERK5 is activated by forskolin, isoproterenol, and epinephrine in NIH3T3 cells and C2C12 myoblasts. ERK1/2 are also activated by cAMP in NIH3T3 cells, but not in C2C12 myoblasts, demonstrating differential regulation of ERK5 and ERK1/2 by cAMP. We examined the effect of cell context on activation of ERK5 and discovered ERK5 activity is inhibited, rather than activated, by cAMP in confluent, serum-deprived NIH3T3 cells and C2C12 myoblasts. Our results suggest that regulation of MAP kinase pathways by cAMP is not only dictated by cell type, but also by cell context.

Identification of a novel testis-specific member of the phosphatidylethanolamine binding protein family, pebp-2

The phosphatidylethanolamine binding proteins (pebps) are an evolutionarily conserved family of proteins recently implicated in mitogen-activated protein (MAP) kinase pathway regulation, where they are called raf kinase inhibitory proteins. Here, we describe the cloning, cellular localization, and partial characterization of a new member, pebp-2, with potential roles in male fertility. Expression data show that pebp-2 is a testis-specific 21-kDa protein found within late meiotic and haploid germ cells in a stage-specific pattern that is temporally distinct from that of pebp-1. Sequence analyses suggest that pebp-2 forms a distinct subset of the pebp family within mammals. Database analyses revealed the existence of a third subset. Analysis suggests that the specificity/regulation of the distinct pebps subsets is likely to be determined by the amino terminal 40 amino acids or the 3' untranslated region, where the majority of sequence differences occur. Protein homology modeling suggests that pebp-2 protein is, however, topologically similar to other pebps and composed of Greek key fold motifs, a dominant beta-sheet formed from five anti-parallel beta strands forming a shallow groove associated with a putative phosphatidylethanolamine binding site. The pebp-2 gene is intronless and data suggest that it is a retrogene derived from pebp-1. Further, pebp-2 colocalizes with members of the MAP kinase pathway in late spermatocytes and spermatids and on the midpiece of epididymal sperm. These data raise the possibility that pebp-2 is a novel participant in the MAP kinase signaling pathway, with a role in spermatogenesis or posttesticular sperm maturation.

Cell cycle-dependent subcellular localization of exchange factor directly activated by cAMP

Epac belongs to a new family of proteins that can directly mediate the action of the intracellular second messenger cAMP by activating a downstream small GTPase Rap1. The Epac/Rap1 pathway represents a novel cAMP-signaling cascade that is independent of the cAMP-dependent protein kinase (PKA). In this study, we have used fluorescence microscopy to probe the intracellular targeting of Epac during different stages of the cell division cycle and the structural features that are important for Epac localization. Our results suggest Epac, endogenous or expressed as a green fluorescent protein fusion protein, is mainly localized to the nuclear membrane and mitochondria during interphase in COS-7 cells. Deletion mutagenesis analysis reveals that whereas the DEP domain is responsible for membrane association, the mitochondrial-targeting sequence is located at the N terminus. Although Epac predominantly exhibits perinuclear localization in interphase, the subcellular localization of Epac is cell cycle-dependent. Epac disassociates from the nuclear membrane and localizes to the mitotic spindle and centrosomes in metaphase. At the end of the cell cycle, Epac is observed to reassociate with the nuclear envelope and concentrate around the contractile ring. Furthermore, overexpression of Epac in COS-7 cells leads to an increase in multinuclear cell populations. These results suggest that Epac may play an important role in mitosis.

New signaling pathway for parathyroid hormone and cyclic AMP action on extracellular-regulated kinase and cell proliferation in bone cells. Checkpoint of modulation by cyclic AMP

cAMP signaling, activated by extracellular stimuli such as parathyroid hormone, has cell type-specific effects important for cellular proliferation and differentiation in bone cells. Recent evidence of a second enzyme target for cAMP suggests divergent effects on extracellular-regulated kinase (ERK) activity depending on Epac/Rap1/B-Raf signaling. We investigated the molecular mechanism of the dual functionality of cAMP on cell proliferation in clonal bone cell types. MC3T3-E1 and ATDC5, but not MG63, express a 95-kDa isoform of B-Raf. cAMP stimulated Ras-independent and Rap1-dependent ERK phosphorylation and cell proliferation in B-Raf-expressing cells, but inhibited growth in B-Raf-lacking cells. The mitogenic action of cAMP was blocked by the ERK pathway inhibitor PD98059. In B-Raf-transduced MG63 cells, cAMP stimulated ERK activation and cell proliferation. Thus, B-Raf is the dominant molecular switch that permits differential cAMP-dependent regulation of ERK with important implications for cell proliferation in bone cells. These findings might explain the dual functionality of parathyroid hormone on osteoblastic cell proliferation.

Hypertonicity increases cAMP in PMN and blocks oxidative burst by PKA-dependent and -independent mechanisms

Hypertonic stress (HS) suppresses neutrophil (PMN) functions. We studied the underlying mechanism and found that HS rapidly (<1 min) increased intracellular cAMP levels by up to sevenfold. cAMP levels correlated with applied hypertonicity and the degree of neutrophil suppression. HS and cAMP-elevating drugs (forskolin and dibutyryl cAMP-acetoxymethyl ester) similarly suppressed extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase activation and superoxide formation in response to N-formylmethionyl-leucyl-phenylalanine (fMLP) stimulation. Inhibition of cAMP-dependent protein kinase A (PKA) with H-89 abrogated the suppressive effects of HS, restoring fMLP-induced ERK and p38 activation and superoxide formation. Inhibition of phosphodiesterase with 3-isobutyl-1-methylxanthine augmented cAMP accumulation and the suppressive effects of HS, while inhibition of adenylyl cyclase with MDL-12330A abolished these effects. These findings suggest that HS-activated cAMP/PKA signaling inhibits superoxide formation by intercepting fMLP-induced activation steps upstream of ERK and p38. In contrast to its effects in the presence of moderate hypertonicity levels (40 mM), H-89 was unable to rescue neutrophil functions from suppression by higher hypertonicity levels (100 mM), indicating that more severe HS suppresses neutrophils via secondary PKA-independent mechanisms.

Stimulation of alpha2-adrenergic receptor inhibits cholangiocarcinoma growth through modulation of Raf-1 and B-Raf activities

Growth factor signaling, mediated by the mitogen-activated protein kinase (MAPK) cascade, induces cell mitosis. Adenosine 3',5'-monophosphate (cAMP) may inhibit or stimulate mitosis (depending on the cell type) through the activation of MAPK and Raf proteins. Among Raf proteins, Raf-1 and B-Raf differentially regulate mitosis. Our aims were to evaluate the role and mechanisms of action of the alpha(2)-adrenergic agonist UK14,304 in the regulation of growth of the human cholangiocarcinoma cell line Mz-ChA-1. Immunocytochemistry and immunoblotting for alpha(2A)-, alpha(2B)-, or alpha(2C)-adrenergic receptor subtypes showed positive reaction in Mz-ChA-1 cells. We found that physiological concentrations of UK14,304 increased cAMP levels and inhibited proliferation and MAPK activity in Mz-ChA-1 cells. Mz-ChA-1 cells expressed Raf-1 and B-Raf. Epidermal growth factor (EGF) immediately and transiently stimulated Raf-1 activity, whereas B-Raf activity was increased with prolonged EGF stimulation. EGF-stimulated Raf-1 and B-Raf activities were both inhibited by UK14,304. UK14,304 did not affect Ras activity. In Mz-ChA-1 cells, alpha(2)-adrenoreceptor stimulation causes up-regulation of cAMP, which inhibits EGF-induced MAPK activity through an acute increase of Raf-1 and sustained activation of B-Raf. In conclusion, because alpha(2)-AR inhibition of growth occurred downstream of Ras, adrenergic stimulation or other stimulants of cAMP may overcome the Ras mutations and offer a new therapeutic approach for patients with cholangiocarcinoma.

Crosstalk between cAMP and MAP kinase signaling in the regulation of cell proliferation

Hormonal stimulation of cyclic adenosine monophosphate (cAMP) and the cAMP-dependent protein kinase PKA regulates cell growth by multiple mechanisms. A hallmark of cAMP is its ability to stimulate cell growth in many cell types while inhibiting cell growth in others. In this review, the cell type-specific effects of cAMP on the mitogen-activated protein (MAP) kinase (also called extracellular signal-regulated kinase, or ERK) cascade and cell proliferation are examined. Two basic themes are discussed. First, the capacity of cAMP for either positive or negative regulation of the ERK cascade accounts for many of the cell type-specific actions of cAMP on cell proliferation. Second, there are several specific mechanisms involved in the inhibition or activation of ERKs by cAMP. Emerging new data suggest that one of these mechanisms might involve the activation of the GTPase Rap1, which can activate or inhibit ERK signaling in a cell-specific manner.

Differential signaling of cyclic AMP: opposing effects of exchange protein directly activated by cyclic AMP and cAMP-dependent protein kinase on protein kinase B activation

The recent discovery of Epac, a novel cAMP receptor protein, opens up a new dimension in studying cAMP-mediated cell signaling. It is conceivable that many of the cAMP functions previously attributed to cAMP-dependent protein kinase (PKA) are in fact also Epac-dependent. The finding of an additional intracellular cAMP receptor provides an opportunity to further dissect the divergent roles that cAMP exerts in different cell types. In this study, we probed cross-talk between cAMP signaling and the phosphatidylinositol 3-kinase/PKB pathways. Specifically, we examined the modulatory effects of cAMP on PKB activity by monitoring the specific roles that Epac and PKA play individually in regulating PKB activity. Our study suggests a complex regulatory scheme in which Epac and PKA mediate the opposing effects of cAMP on PKB regulation. Activation of Epac leads to a phosphatidylinositol 3-kinase-dependent PKB activation, while stimulation of PKA inhibits PKB activity. Furthermore, activation of PKB by Epac requires the proper subcellular targeting of Epac. The opposing effects of Epac and PKA on PKB activation provide a potential mechanism for the cell type-specific differential effects of cAMP. It is proposed that the net outcome of cAMP signaling is dependent upon the dynamic abundance and distribution of intracellular Epac and PKA.

Nerve growth factor signaling, neuroprotection, and neural repair

Nerve growth factor (NGF) was discovered 50 years ago as a molecule that promoted the survival and differentiation of sensory and sympathetic neurons. Its roles in neural development have been characterized extensively, but recent findings point to an unexpected diversity of NGF actions and indicate that developmental effects are only one aspect of the biology of NGF. This article considers expanded roles for NGF that are associated with the dynamically regulated production of NGF and its receptors that begins in development, extends throughout adult life and aging, and involves a surprising variety of neurons, glia, and nonneural cells. Particular attention is given to a growing body of evidence that suggests that among other roles, endogenous NGF signaling subserves neuroprotective and repair functions. The analysis points to many interesting unanswered questions and to the potential for continuing research on NGF to substantially enhance our understanding of the mechanisms and treatment of neurological disorders.

Phosphorylation by PKA of a site unique to B-Raf kinase

The Raf kinases serve as central intermediates to relay signals from Ras to ERK. Cell-specific effects of these signals on growth, differentiation and survival can be observed due to the recruitment of different isoenzymes of the Raf family. The in vitro phosphorylation of a site unique to B-Raf (Ser429) has been proposed to be responsible for the negative regulation of the isoenzyme by Akt. Using phosphopetide mapping and site-directed mutagenesis we showed that Ser429 is phosphorylated upon cAMP elevation in PC12 cells and proposed that PKA is a major kinase phosphorylating the B-Raf-specific site in vivo.

A CalDAG-GEFI/Rap1/B-Raf cassette couples M(1) muscarinic acetylcholine receptors to the activation of ERK1/2

In this study we examine signaling pathways linking the M(1) subtype of muscarinic acetylcholine receptor (M(1) mAChR) to activation of extracellular signal-regulated kinases (ERK) 1 and 2 in neuronal PC12D cells. We first show that activation of ERK1/2 by the M(1) mAChR agonist carbachol takes place primarily via a Ras-independent pathway that depends largely upon Rap1, another small GTP-binding protein in the Ras family. Rap1 in turn activates B-Raf, an upstream activator of ERK1/2. Consistent with these results, carbachol was found to activate Rap1 more potently than Ras. Similar to other small GTP-binding proteins, activation of Rap1 requires a guanine nucleotide exchange factor (GEF) to promote its conversion from the GDP- to GTP-bound form. Using specific antibodies, we show that a recently identified Rap1 GEF, calcium- and diacylglycerol-regulated guanine nucleotide exchange factor I (CalDAG-GEFI), is expressed in PC12D cells and that carbachol stimulates the formation of a complex containing CalDAG-GEFI, Rap1, and activated B-Raf. Finally, we show that expression of CalDAG-GEFI antisense RNA largely blocks carbachol-stimulated activation of hemagglutinin (HA)1-tagged B-Raf and formation of the CalDAG-GEFI/Rap1/HA1-tagged B-Raf complex. Together, these data define a novel signaling pathway for M(1) mAChR, where increases in Ca(2+) and diacylglycerol stimulate the sequential activation of CalDAG-GEFI, Rap1, and B-Raf, resulting in the activation of MEK and ERK1/2.

beta-Adrenergic stimulation of rat cardiac fibroblasts promotes protein synthesis via the activation of phosphatidylinositol 3-kinase

beta -adrenergic agonists stimulate neonatal rat cardiac fibroblast growth, albeit the identity of the signaling event(s) remains equivocal. Isoproterenol (ISO) treatment increased intracellular cyclic AMP levels; however, cyclic AMP-elevating agents had no effect on protein synthesis. The tyrosine kinase inhibitor tyrphostin A25, and the inhibition of ras processing by the farnesyltransferase inhibitor BMS-191563 attenuated ISO-stimulated protein synthesis. Concomitant with increased protein synthesis, ISO stimulated extracellular signal-regulated protein kinase (ERK) and phosphatidylinositol 3-kinase (PI3-K) activity. The MEK1/2 inhibitor PD098059 abrogated ISO-stimulated ERK activity, albeit the increase in protein synthesis was unaffected. By contrast, LY294002 inhibited both ISO-stimulated PI3-K activity, and protein synthesis. ISO treatment did not increase the expression of transforming growth factor-beta(1)(TGF-beta(1)) mRNA, whereas a significant decrease in the steady-state mRNA level of TGF- beta(3)was observed. This latter effect was mimicked by cyclic AMP-elevating agents. Angiotensin II (AII) activation of the AT(1)receptor increased protein synthesis, but in contrast to ISO, the growth response was not inhibited by either tyrphostin A25 or BMS-191563, and was associated with the concomitant expression of both TGF-beta(1)and TGF-beta(3)mRNAs. Analogous to ISO, AII treatment increased ERK and PI3-K activity, and PI3-K was required for protein synthesis. These findings are the first to highlight the activation of PI3-K by a Gs(alpha)-coupled receptor, and its essential role in beta -adrenergic as well as AT(1)receptor-mediated protein synthesis in neonatal rat cardiac fibroblasts. However, despite the conserved role of PI3-K, additional disparate signaling pathways are recruited by ISO and AII, which may differentially influence fibroblast phenotype.

Coupling of cAMP/PKA and MAPK signaling in neuronal cells is dependent on developmental stage

Neurite formation, an essential feature of neuronal development, is believed to involve participation of the ras-mitogen-activated protein kinase (MAPK) and cAMP-dependent protein kinase A (cAMP/PKA)-mediated signaling pathways. These pathways have been studied extensively in the rat pheochromocytoma cell line PC12, and current hypotheses suggest a single effector mechanism resulting from the convergence of cAMP/PKA and MAPK signaling. However, based on observations using a central neuronal progenitor cell line, AS583-8, there also exists evidence that the two signaling pathways may act independently resulting in neurites with differing dynamic features. In the present study, the upstream components of cAMP/PKA signaling were examined in AS583-8 cells as well as possible interactions with the MAPK pathway. We found that activation of PKA is both necessary and sufficient for the elaboration of rapidly forming processes, typical of the cAMP response. In addition, blockade of the MAPK pathway has no effect on the cAMP response, suggesting that activation of the cAMP/PKA pathway can stimulate neurite formation independent of the MAPK pathway. In order to evaluate which cell line model, PC12 vs AS583-8, best reflects the signaling features of developing central neurons, we examined interactions between cAMP/PKA and MAPK signaling in primary neuronal cultures from several brain regions. In immature cultures (1-day-old), at a point where the initiation of neurite formation is maximal, no interaction was observed. In more mature cultures (7 days old), where synaptic contacts have been established, we found a weak but reproducible activation of MAPK following stimulation of the cAMP/PKA pathway. These results suggest that cAMP/PKA and MAPK signaling act independently at the initiation of neuritogenesis but become coupled during later stages of neuronal development. Therefore, the interaction of the two pathways may be cell stage (younger vs older) specific and may participate in cellular functions that take place after initial neurite formation.

Activation of Ras, Raf-1 and protein kinase C in differentiating human neuroblastoma cells after treatment with phorbolester and NGF

The human neuroblastoma cell line SH-SY5Y/TrkA differentiates in vitro and acquires a sympathetic phenotype in response to phorbolester (activator of protein kinase C, PKC) in the presence of serum or growth factors, or nerve growth factor (NGF). We have now investigated to what extent phorbolester and NGF cause activation of Ras and Raf-1 and the involvement of PKC in this response in differentiating SH-SY5Y/TrkA cells. NGF stimulated increased accumulation of Ras-GTP and a threefold activation of Raf-1. In contrast, 12-O-tetradecanoylphorbol-13-acetate (TPA) had no effect on the amount of Ras-GTP but led to a smaller activation of Raf-1. NGF caused a limited increase in phosphorylation of Raf-1 compared with TPA, and NGF-induced Raf activity was independent of PKC. Analysis of phosphorylation of the endogenous PKC substrate myristoylated alanine-rich C-kinase substrate (MARCKS), and of subcellular distribution of PKC-alpha, -delta, and -epsilon revealed that NGF only caused a very small activation of PKC in SH-SY5Y/TrkA cells. The results identify Raf-1 as a target for both TPA- and NGF-induced signals in differentiating SH-SY5Y/TrkA cells and demonstrate that signalling to Raf-1 was mediated via distinct mechanisms.

Promotion of neurite outgrowth by protein kinase inhibitors and ganglioside GM1 in neuroblastoma cells involves MAP kinase ERK1/2

To investigate mechanisms of neurite outgrowth, murine Neuro-2a neuroblastoma cells were exposed to ganglioside GM1 in the presence or absence of specific protein kinase inhibitors. Isoquinolinesulfonamide (H-89), an inhibitor of cyclic AMP dependent protein kinase A (PKA), and bisindolylmaleimide I (BIM), which inhibits protein kinase C, each stimulated neurite outgrowth in a dose-dependent manner in the absence of exogenous GM1. Minimally effective (threshold) concentrations of H-89 or BIM potentiated outgrowth when they were used in combination with GM1. To search for a shared component in the mechanisms of GM1, H-89 and BIM, phosphorylation of ERK1/2 was examined. Inhibition of the activation of extracellular signal regulated kinases (ERK1/2) by U0126, prevented neuritogenesis of Neuro-2a by all the three agents. Pretreatment of serum-depleted Neuro-2a cultures with GM1 or BIM enhanced ERK1/2 phosphorylation when the serum level was restored to 10%. In contrast, H-89 did not alter the serum-mediated response. In cells exposed to GM1 or BIM without additional serum, a transitory decrease in ERK phosphorylation occurred. These data suggest that GM1 influences two neuritogenic pathways, one modulated by PKC and the other regulated by PKA. Therefore, GM1 may have the potential to stimulate alternate pathways resulting in outgrowth.

Cyclic AMP-elevating agents induce the expression of MAP kinase phosphatase-1 in PC12 cells

Stimulation of pheochromocytoma PC12 cells by cAMP-elevating agents caused the induction of the immediate early gene 3CH134, which encodes MAP kinase phosphatase-1 (MKP-1). Forskolin was as potent as serum in stimulating MKP-1 gene expression, whereas dibutyryl-cAMP and neuropeptide PACAP were less effective. Induction of the MKP-1 gene was accompanied by neo-synthesis of MKP-1 protein. MAP kinase activation was not involved in the cAMP-induced MKP-1 gene expression. The MAP kinase inactivation, that would result from MKP-1 induction in response to increased intracellular cAMP level, contributes to explain how hormones or neurotransmitters signaling through cAMP influence cell growth and differentiation.

Stimulatory and inhibitory effects of adenosine A(2A) receptors on nerve growth factor-induced phosphorylation of extracellular regulated kinases 1/2 in PC12 cells

Effects of nerve growth factor (NGF), adenosine and an adenosine A(2A) receptor agonist (CGS 21680) on the phosphorylation of extracellular-regulated kinases 1/2 (ERK1/2) were examined in PC12 cells. Adenosine and CGS 21680stimulated ERK1/2, but inhibited the phosphorylation of ERK1/2 induced by a 10 min incubation with NGF. Longer treatment with CGS 21680 and NGF (1-2h) resulted in an additive effect on the activation of ERK1/2. Forskolin exerted the same effects, suggesting that they are mediated by cyclic AMP. These results indicate that adenosine A(2A) receptor induced increases in cyclic AMP can stimulate ERK1/2 phosphorylation per se, inhibit the initial and enhance the late NGF-induced activation of ERK1/2. These results may be explained by the fact that NGF action is mediated via different pathways at early and late time points.

Cell type-specific regulation of B-Raf kinase by cAMP and 14-3-3 proteins

Cyclic AMP can either activate or inhibit the mitogen-activated protein kinase (MAPK) pathway in different cell types; MAPK activation has been observed in B-Raf-expressing cells and has been attributed to Rap1 activation with subsequent B-Raf activation, whereas MAPK inhibition has been observed in cells lacking B-Raf and has been attributed to cAMP-dependent protein kinase (protein kinase A)-mediated phosphorylation and inhibition of Raf-1 kinase. We found that cAMP stimulated MAPK activity in CHO-K1 and PC12 cells but inhibited MAPK activity in C6 and NB2A cells. In all four cell types, cAMP activated Rap1, and the 95- and 68-kDa isoforms of B-Raf were expressed. cAMP activation or inhibition of MAPK correlated with activation or inhibition of endogenous and transfected B-Raf kinase. Although all cell types expressed similar amounts of 14-3-3 proteins, approximately 5-fold less 14-3-3 was associated with B-Raf in cells in which cAMP was inhibitory than in cells in which cAMP was stimulatory. We found that the cell type-specific inhibition of B-Raf could be completely prevented by overexpression of 14-3-3 isoforms, whereas expression of a dominant negative 14-3-3 mutant resulted in partial loss of B-Raf activity. Our data suggest that 14-3-3 bound to B-Raf protects the enzyme from protein kinase A-mediated inhibition; the amount of 14-3-3 associated with B-Raf may explain the tissue-specific effects of cAMP on B-Raf kinase activity.

Phosphorylation of serine 43 is not required for inhibition of c-Raf kinase by the cAMP-dependent protein kinase

The activity of the serine/threonine kinase c-Raf (Raf) is inhibited by increased intracellular cAMP. This is believed to require phosphorylation with the cAMP-dependent protein kinase (PKA), although the mechanism by which PKA inhibits Raf is controversial. We investigated the requirement for PKA phosphorylation using Raf mutants expressed in HEK293 or NIH 3T3 cells. Phosphopeptide mapping of (32)P-labeled Raf (WT) or a mutant lacking a putative PKA phosphorylation site (serine to alanine, S43A) confirmed that serine 43 (Ser(43)) was the major cAMP (forskolin)-stimulated phosphorylation site in vivo. Interestingly, the EGF-stimulated Raf kinase activity of the S43A mutant was inhibited by forskolin equivalently to that of the WT Raf. Forskolin also inhibited the activation of an N-terminal deletion mutant Delta5-50 Raf completely lacking this phosphorylation site. Although WT Raf was phosphorylated by PKA, phosphorylation did not inhibit Raf catalytic activity in vitro, nor did forskolin treatment inhibit the activity of an N-terminally truncated Raf protein (Raf 22W) or a full-length Raf protein (Raf-CAAX) expressed in NIH 3T3 cells. In contrast, forskolin inhibited the EGF-dependent activation of a Raf isoform (B-Raf), lacking an analogous phosphorylation site to Ser(43). Thus, these results demonstrate that PKA exerts its inhibitory effects independently of direct Raf phosphorylation and suggests instead that PKA prevents an event required for the EGF-dependent activation of Raf.