Inhibition of the EGF-activated MAP kinase signaling pathway by adenosine 3',5'-monophosphate

Activation and induction of NUR77/NURR1 in corticotrophs by CRH/cAMP: involvement of calcium, protein kinase A, and MAPK pathways

Nur factors are critical for proopiomelanocortin (POMC) induction by CRH in corticotrophs, but the pathways linking CRH to Nur are unknown. In this study we show that in AtT-20 corticotrophs CRH and cAMP induce Nur77 and Nurr1 expression and transcription at the NurRE site by protein kinase A (PKA) and calcium-dependent and -independent mechanisms. Calcium pathways depend on calmodulin kinase II (CAMKII) activity, and calcium-independent pathways are accounted for in part by MAPK activation (Rap1/B-Raf/MAPK-ERK kinase/ERK1/2), demonstrated by the use of molecular and pharmacological tools. AtT-20 corticotrophs express B-Raf, as do other cells in which cAMP stimulates MAPK. CRH/cAMP stimulated ERK2 activity and increased transcriptional activity of a Gal4-Elk1 protein, which was blocked by overexpression of dominant negative mutants and kinase inhibitors and stimulated by expression of B-Raf. The MAPK kinase inhibitors did not affect Nur77 and Nurr1 mRNA induction but blocked CRH or cAMP-stimulated Nur transcriptional activity. Moreover, MAPK stimulated phosphorylation and transactivation of Nur77. The functional impact of these pathways was confirmed at the POMC promoter. In conclusion, in AtT-20 corticotrophs the CRH/cAMP signaling that leads to Nur77/Nurr1 mRNA induction and transcriptional activation, and thus POMC expression, is dependent on protein kinase A and involves calcium/calmodulin kinase II (Nur induction/activation) and MAPK calcium-dependent and -independent (Nur phosphorylation-activation) pathways.

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.

Protein kinase A balances the growth factor-induced Ras/ERK signaling

Protein kinase A (PKA) has been proposed to regulate the signal transduction through the Ras/extracellular-regulated kinase (ERK) pathway. Here we demonstrate that when the PKA activity was inhibited prior to growth factor stimulus the signal flow through the Ras/ERK pathway was significantly increased. Furthermore, the data indicated that this PKA-mediated regulation was simultaneously targeted to the upstream kinase Raf-1 and to the ERK-specific phosphatase mitogen-activated protein kinase phosphatase-1 (MKP-1). Moreover, our data suggested that the level of PKA activity determined the transcription rate of mkp-1 gene, whereas the Ras/ERK signal was required to protect the MKP-1 protein against degradation. These results point to a tight regulatory relationship between PKA and the growth factor signaling, and further suggest an important role for basal PKA activity in such regulation. We propose that PKA adjusts the activity of the Ras/ERK pathway and maintains it within a physiologically appropriate level.

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.

Cyclic AMP blocks cell growth through Raf-1-dependent and Raf-1-independent mechanisms

It is widely accepted that cyclic AMP (cAMP) can block cell growth by phosphorylating Raf-1 on serine 43 and inhibiting signaling to extracellular signal-regulated protein kinase. We show that the suppression of Raf-1 by cAMP is considerably more complex than previously reported. When cellular cAMP is elevated, Raf-1 is phosphorylated on three residues (S43, S233, and S259), which work independently to block Raf-1. Both Ras-dependent and Ras-independent processes are disrupted. However, when cAMP-insensitive versions of Raf-1 are expressed in NIH 3T3 cells, their growth is still strongly suppressed when cAMP is elevated. Thus, although Raf-1 appears to be an important cAMP target, other pathways are also targeted by cAMP, providing alternative mechanisms that lead to suppression of cell growth.

Selective down-regulation of c-jun gene expression by pentoxifylline and c-jun antisense interrupts platelet-derived growth factor signaling: pentoxifylline inhibits phosphorylation of c-Jun on serine 73

Platelet-derived growth factor (PDGF) signals through several pathways, including mitogen-activated protein (MAP) kinase, Jun kinase, and C kinase, and stimulates proliferation of fibroblasts. Pentoxifylline inhibits PDGF-driven proliferation of fibroblasts. We have reported that pentoxifylline did not inhibit binding of PDGF to its specific cell-surface receptors or PDGF receptor phosphorylation. In this study, we investigated the effect of PDGF on the expression of c-fos and c-jun, because c-fos and c-jun form activator protein-1 complexes that stimulate genes involved in proliferation. We determined whether pentoxifylline would alter the expression of c-fos and c-jun. Our results indicate that PDGF induced the expression of both c-fos and c-jun. Pentoxifylline effectively reduced c-jun gene expression, which had been up-regulated by PDGF, but did not alter c-fos gene expression. The lack of effect on c-fos supports other studies from this laboratory, which indicate that pentoxifylline did not inhibit PDGF activation of MAP kinase. Treatment of fibroblasts with a phosphothioate c-jun antisense oligodeoxynucleotide reduced the levels of c-Jun protein and blocked PDGF-stimulated proliferation, suggesting a critical role for c-jun in PDGF-mediated proliferation. Combination of pentoxifylline and c-jun antisense suggested that they were likely inhibiting PDGF-stimulated proliferation at a single site in the PDGF signaling pathway. These results suggest that pentoxifylline inhibits PDGF-stimulated proliferation by selectively decreasing c-jun expression. To further define the mechanism of action of pentoxifylline, we assessed the effect of pentoxifylline on c-Jun and phosphorylated c-Jun immunoreactivity in cells treated with PDGF and cells that were transfected with wild-type c-jun plasmid using immunocytochemistry and Western blot analyses, and our results indicate that pentoxifylline inhibited phosphorylation of c-Jun on serine 73.

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.

Elastase-released epidermal growth factor recruits epidermal growth factor receptor and extracellular signal-regulated kinases to down-regulate tropoelastin mRNA in lung fibroblasts

Elastase/anti-elastase imbalance is a hallmark of emphysema, a chronic obstructive pulmonary disease associated with the rupture and inefficient repair of interstitial elastin. We report that neutrophil elastase (NE) at low physiologic concentrations, ranging from 35 nm to 1 microm, invokes transient, peaking at 15 min, activation of extracellular signal-regulated kinases 1 and 2 (ERK) in elastogenic lung fibroblasts. ERK activation is preceded by the release of soluble 25-26-kDa forms of epidermal growth factor (EGF) and transactivation of EGF receptor (EGFR) in NE-exposed cells. The stimulatory effect of NE on ERK is abrogated in the presence of anti-EGF-neutralizing antibodies, EGFR tyrosine kinase inhibitor (AG1478), and ERK kinase inhibitor (PD98059), as well as abolished in both EGFR-desensitized and endocytosis-arrested fibroblasts. Nuclear accumulation of activated ERK is associated with transient, peaking at 30 min, induction of c-Fos and sustained, observed at 24-48 h, decrease of tropoelastin mRNA levels in NE-challenged cells. Pretreatment of fibroblasts with AG1478 or PD98059 abrogates the NE-initiated tropoelastin mRNA suppression. We conclude that proteolytically released EGF signals directly via EGFR and ERK to down-regulate tropoelastin mRNA in NE-challenged lung fibroblasts.

Cyclic AMP-dependent kinase regulates Raf-1 kinase mainly by phosphorylation of serine 259

The Raf-1 kinase activates the ERK (extracellular-signal-regulated kinase) pathway. The cyclic AMP (cAMP)-dependent protein kinase (PKA) can inhibit Raf-1 by direct phosphorylation. We have mapped all cAMP-induced phosphorylation sites in Raf-1, showing that serines 43, 259, and 621 are phosphorylated by PKA in vitro and induced by cAMP in vivo. Serine 43 phosphorylation decreased the binding to Ras in serum-starved but not in mitogen-stimulated cells. However, the kinase activity of a RafS43A mutant was fully inhibited by PKA. Mutation of serine 259 increased the basal Raf-1 activity and rendered it largely resistant to inhibition by PKA. cAMP increased Raf-1 serine 259 phosphorylation in a PKA-dependent manner with kinetics that correlated with ERK deactivation. PKA also decreased Raf-1 serine 338 phosphorylation of Raf-1, previously shown to be required for Raf-1 activation. Serine 338 phosphorylation of a RafS259A mutant was unaffected by PKA. Using RafS259 mutants we also demonstrate that Raf-1 is the sole target for PKA inhibition of ERK and ERK-induced gene expression, and that Raf-1 inhibition is mediated mainly through serine 259 phosphorylation.

On the mitogenic properties of Rap1b: cAMP-induced G(1)/S entry requires activated and phosphorylated Rap1b

We have shown that the small GTPase Rap1b, a protein known to antagonize the mitogenic and transforming activity of Ras, is endowed with both mitogenic and tumorigenic properties. Rap1b can be activated by cAMP, an intracellular message known to either stimulate or inhibit cell proliferation. The oncogenic property of Rap1b was revealed in a model system in which cAMP stimulates cell proliferation and was linked to Rap's ability to promote S phase entry. We have now tested the significance of the mitogenic action of Rap1b in a physiologically relevant model, the differentiated thyroid follicular cells, a system that requires thyroid-stimulating hormone (TSH), acting via cAMP, to mediate a full mitogenic response. Here we report that cAMP-dependent hormonal stimulation of DNA synthesis requires Rap1b in a manner dependent on its phosphorylation by protein kinase A.

Role of tyrosine kinase and p44/42 MAPK in D(2)-like receptor-mediated stimulation of Na(+), K(+)-ATPase in kidney

Our laboratory has shown that dopamine D(2)-like receptor activation causes stimulation of Na(+), K(+)-ATPase (NKA) activity in the proximal tubules of the rat kidney. The present study was designed to investigate the cellular signaling mechanisms mediating this response to D(2)-like receptor activation. We measured the stimulation of NKA activity by bromocriptine (D(2)-like receptor agonist) in the absence and presence of PD-98059 [p44/42 mitogen-activated protein kinase (MAPK) kinase inhibitor] and genistein (tyrosine kinase inhibitor) in renal proximal tubules. Both agents inhibited bromocriptine-mediated stimulation of NKA, suggesting the involvement of p44/42 MAPK and tyrosine kinase in this response. Additionally, we found that bromocriptine increased the phosphorylation of p44/42 MAPK in the proximal tubules, which was blocked by PD-98059 and genistein. These results show that D(2)-like receptor activation causes stimulation of NKA activity by means of a tyrosine kinase-p44/42 MAPK pathway in the proximal tubules of the kidney.

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.

PAK1 primes MEK1 for phosphorylation by Raf-1 kinase during cross-cascade activation of the ERK pathway

The serine/threonine kinase Raf-1 acts downstream of Ras in the MAPK pathway leading to ERK activation in response to mitogens. Raf-1 has oncogenic potential, but is normally controlled by a complex interplay of inhibitory and activating mechanisms. Although Raf-1 is phosphorylated in unstimulated cells, mitogens cause its membrane recruitment by Ras and subsequent phosphorylation on additional sites. Some of these events modulate Raf-1 kinase activity while others determine interactions with other proteins. These changes regulate the ability of Raf-1 to phosphorylate its downstream targets MEK1 and MEK2. Rho family small G proteins act synergistically with Raf-1 to stimulate the ERK pathway by a cross-cascade mechanism that enhances MEK phosphorylation by Raf-1. Here we show that both Raf-1 and MEK1 are phosphorylated by PAK1 and that mutations at PAK1 phosphorylation sites in either protein prevent cross-cascade activation. In contrast, MEK1 activation by constitutively-active Raf-1 is refractory to mutations at PAK1 phosphorylation sites. Phosphorylation of MEK1 on serine 298 does not appear to regulate the interaction between Raf-1 and MEK1, but rather the ability of Raf-1 to phosphorylate MEK1 with which it is complexed in vivo. Our findings indicate that PAK1 primes MEK1 for activation by Raf-1 and imply another level of regulation in the ERK cascade.

ERK activation by G-protein-coupled receptors in mouse brain is receptor identity-specific

In transfected cells and non-neuronal tissues many G-protein-coupled receptors activate p44/42 MAP kinase (ERK), a kinase involved in both hippocampal synaptic plasticity and learning and memory. However, it is not clear to what degree these receptors couple to ERK in brain. G(s)-coupled beta-adrenergic receptor activation of ERK in neurons is critical in the regulation of synaptic plasticity in area CA1 of the hippocampus. In addition, alpha(1)- and alpha(2)-adrenergic receptors, present in CA1, could potentially activate ERK. We find that, like the beta-adrenergic receptor, the G(q)-coupled alpha(1)AR activates ERK in adult mouse CA1. However, activation of the G(i/o)-coupled alpha(2)AR does not activate ERK, nor does activation of a homologous G(i/o)-coupled receptor enriched in adult mouse CA1, the 5HT(1A) receptor. In contrast, the nonhomologous G(i/o)-coupled gamma-aminobutyric acid type B receptor does activate ERK in adult mouse CA1. Surprisingly, activation of alpha(2)ARs in CA1 from immature animals where basal phospho-ERK is low induces ERK phosphorylation. These data suggest that although most G-protein-coupled receptor subtypes activate ERK in non-neuronal cells, the coupling of G(i/o) to ERK is tightly regulated in brain.

Adenovirus-directed expression of Q227L-G alpha(s) inhibits growth of established tumors of later-stage human breast cancer cells in athymic mice

Elevation of cAMP inhibits proliferation and expression of the transformed phenotype in several cell types. We studied the effects of elevation of cAMP and expression of mutant (Q227L) activated G alpha(s) on the proliferation and tumorigenic capability of the later stage, metastatic estrogen-independent human breast cancer cell lines MDA-231 and MDA-435. Our studies show that 8Br-cAMP inhibits proliferation of these cells in culture and their ability to form colonies in soft agar. This inhibition may occur by different mechanisms in the two cell types. In MDA-231 cells, cAMP elevation results in sustained expression of the cell cycle inhibitor p27kip1 and inhibition of CDK2 activity, whereas in MDA-435 cells inhibition of mitogen-activated protein (MAP) kinase 1,2 activity is observed. We tested whether these effects in culture could be translated into inhibition of tumor growth in vivo. Tumors were developed in athymic (Nu/Nu) mice by injections of MDA-231 or MDA-435 cells. Injection of Q227L-G alpha(s) expressing adenoviral vector into these established tumors inhibited further tumor growth under conditions where tumors injected with either saline or adenoviral vector containing beta-galactosidase grew up to four to five times their original size. These results raise the possibility that sustained elevation of cAMP may have therapeutic value in the treatment of estrogen-resistant later stage breast cancers.

Regulation of Raf-1 activation and signalling by dephosphorylation

The Raf-1 kinase is regulated by phosphorylation, and Ser259 has been identified as an inhibitory phosphorylation site. Here we show that the dephosphorylation of Ser259 is an essential part of the Raf-1 activation process, and further reveal the molecular role of Ser259. The fraction of Raf-1 that is phosphorylated on Ser259 is refractory to mitogenic stimulation. Mutating Ser259 elevates kinase activity because of enhanced binding to Ras and constitutive membrane recruitment. This facilitates the phosphorylation of an activating site, Ser338. The mutation of Ser259 also increases the functional coupling to MEK, augmenting the efficiency of MEK activation. Our results suggest that Ser259 regulates the coupling of Raf-1 to upstream activators as well as to its downstream substrate MEK, thus determining the pool of Raf-1 that is competent for signalling. They also suggest a new model for Raf-1 activation where the release of repression through Ser259 dephosphorylation is the pivotal step.

Apelin (65-77) activates extracellular signal-regulated kinases via a PTX-sensitive G protein

We report here that apelin (65-77) induces activation of extracellular-regulated kinases (ERKs) in Chinese hamster ovary (CHO) cells expressing the msr/apj receptor. This concentration-dependent activation was transient, peaking at 5 min. Pretreatment of CHO cells with pertussis toxin fully abrogated ERK phosphorylation, whereas overexpression of the beta-adrenergic receptor kinase-1 C-terminal fragment did not alter ERK activation. Transfection with a dominant-negative mutant of Ras was without effect on ERK activation, whereas an inhibitor of many protein kinase C isoforms, GF109203X, strongly decreased it. These results demonstrate that stimulation of the murine msr/apj receptor promotes ERK activation via the alpha subunit of a pertussis toxin-sensitive protein in a Ras-independent pathway.

PKA phosphorylation of Src mediates cAMP's inhibition of cell growth via Rap1

In fibroblast cells, cAMP antagonizes growth factor activation of ERKs and cell growth via PKA and the small G protein Rap1. We demonstrate here that PKA's activation of Rap1 was mediated by the Rap1 guanine nucleotide exchange factor C3G, the adaptor Crk-L, the scaffold protein Cbl, and the tyrosine kinase Src. Src was required for cAMP activation of Rap1 and the inhibition of ERKs and cell growth. PKA activated Src both in vitro and in vivo by phosphorylating Src on serine 17 within its amino terminus. This phosphorylation was required for cAMP's activation of Src and Rap1, as well as cAMP's inhibition of ERKs and cell proliferation. This study identifies an antiproliferative role for Src in the physiological regulation of cell growth by cAMP.

Diversity of G protein-coupled receptor signaling pathways to ERK/MAP kinase

One of the most intriguing examples of cross talk between signaling systems is the interrelationship between G protein-coupled receptor and growth factor receptor pathways leading to activation of the ERK/MAP kinase phosphorylation cascade. This review focuses on the mechanism of this cross talk, denoting primarily signaling components known to occur in the G protein-coupled receptor branch of the MAP kinase pathways in neural cells. Recent evidence is presented on the existence of a plethora of pathways, due to the multiplicity of G protein-coupled receptors, their differential interaction with heterotrimeric G protein isoforms, various effectors and second messengers. In light of this rich diversity, the review will discuss different points of convergence of G protein-coupled receptor and growth factor receptor pathways that may feature a requirement for growth factor receptor transactivation, receptor internalization and scaffolds to assemble receptor, adaptor and anchoring proteins into multiprotein complexes.

Molecular mechanisms for protein kinase A-mediated modulation of immune function

Protein kinase A (PKA) is a serine/threonine kinase that regulates a number of cellular processes important for immune activation and control. Modulation of signal transduction by PKA is a complex and diverse process, and differential isozyme expression, holoenzyme composition and subcellular localization contribute specificity to the PKA signalling pathway. In lymphocytes, phosphorylation by PKA has been demonstrated to regulate antigen receptor-induced signalling both by altering protein-protein interactions and by changing the enzymatic activity of target proteins. PKA substrates involved in immune activation include transcription factors, members of the MAP kinase pathway and phospholipases. The ability of PKA type I to regulate activation of signalling components important for formation of the immunological synapse, demonstrates that the cAMP signalling pathway can directly modulate proximal events in lymphocyte activation. Furthermore, the recent discovery that PKA regulates Src kinases through modulation of Csk, supports the notion that PKA is involved in the fine-tuning of immune receptor signalling in lipid rafts.

Estrogen action via the G protein-coupled receptor, GPR30: stimulation of adenylyl cyclase and cAMP-mediated attenuation of the epidermal growth factor receptor-to-MAPK signaling axis

Estrogen triggers rapid yet transient activation of the MAPKs, extracellular signal-regulated kinase (Erk)-1 and Erk-2. We have reported that this estrogen action requires the G protein-coupled receptor, GPR30, and occurs via Gbetagamma-subunit protein-dependent transactivation of the epidermal growth factor (EGF) receptor through the release of pro-heparan-bound EGF from the cell surface. Here we investigate the mechanism by which Erk-1/-2 activity is rapidly restored to basal levels after estrogen stimulation. Evidence is provided that attenuation of Erk-1/-2 activity by estrogen occurs via GPR30-dependent stimulation of adenylyl cyclase and cAMP-dependent signaling that results in Raf-1 inactivation. We show that 17beta-E2 represses EGF-induced activation of the Raf-to-Erk pathway in human breast carcinoma cells that express GPR30, including MCF-7 and SKBR3 cells which express both or neither, ER, respectively. MDA-MB-231 cells, which express ERbeta, but not ERalpha, and low levels of GPR30 protein, are unable to stimulate adenylyl cyclase or promote estrogen-mediated blockade of EGF-induced activation of Erk-1/-2. Pretreatment of MDA-MB-231 cells with cholera toxin, which ADP-ribosylates and activates Galphas subunit proteins, results in G protein-coupled receptor (GPCR)-independent adenylyl cyclase activity and suppression of EGF-induced Erk-1/-2 activity. Transfection of GPR30 into MDA-MB-231 cells restores their ability to stimulate adenylyl cyclase and attenuate EGF-induced activation of Erk-1/-2 by estrogen. Moreover, GPR30-dependent, cAMP-mediated attenuation of EGF-induced Erk-1/-2 activity was achieved by ER antagonists such as tamoxifen or ICI 182, 780; yet not by 17alpha-E2 or progesterone. Thus, our data delineate a novel mechanism, requiring GPR30 and estrogen, that acts to regulate Erk-1/-2 activity via an inhibitory signal mediated by cAMP. Coupled with our prior findings, these current data imply that estrogen balances Erk-1/-2 activity through a single GPCR via two distinct G protein-dependent signaling pathways that have opposing effects on the EGF receptor-to-MAPK pathway.

TNF-alpha enhances intracellular glucocorticoid availability

For understanding the mechanism(s) relating inflammation to corticosteroid action, the effect of tumour necrosis factor-alpha (TNF-alpha) on 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2), the enzyme regulating access of 11beta-hydroxycorticosteroids to receptors, was studied in LLC-PK(1) cells. We observed (i) NAD-dependent enzyme activity and mRNA for 11beta-HSD2, but not 11beta-HSD1, (ii) increasing 11beta-HSD2 activity with increasing degree of differentiation and (iii) a concentration-dependent down-regulation by TNF-alpha, phorbol myristate acetate (PMA) or glucose of activity and mRNA of 11beta-HSD2. The decrease of activity and mRNA by glucose and PMA, but not that by TNF-alpha, was abrogated by the protein kinase C inhibitor GF-109203X. The effect of TNF-alpha on 11beta-HSD2 was reversed by inhibiting the mitogen-activated protein kinases ERK with PD-098050 and p38 by SB-202190, or by activating protein kinase A with forskolin. Overexpression of MEK1, an ERK activator, down-regulated the 11beta-HSD2 activity. In conclusion, TNF-alpha decreases 11beta-HSD2 activity and thereby enhances glucocorticoid access to glucocorticoid receptors to modulate the inflammatory response.

Drosophila-raf acts to elaborate dorsoventral pattern in the ectoderm of developing embryos

In the early Drosophila embryo the activity of the EGF-receptor (Egfr) is required to instruct cells to adopt a ventral neuroectodermal fate. Using a gain-of-function mutation we showed that D-raf acts to transmit this and other late-acting embryonic Egfr signals. A novel role for D-raf was also identified in lateral cell development using partial loss-of-function D-raf mutations. Thus, we provide evidence that zygotic D-raf acts to specify cell fates in two distinct pathways that generate dorsoventral pattern within the ectoderm. These functional requirements for D-raf activity occur subsequent to its maternal role in organizing the anterioposterior axis. The consequences of eliminating key D-raf regulatory domains and specific serine residues in the transmission of Egfr and lateral epidermal signals were also addressed here.

Follicle-stimulating hormone stimulates protein kinase A-mediated histone H3 phosphorylation and acetylation leading to select gene activation in ovarian granulosa cells

We examined the phosphorylation and acetylation of histone H3 in ovarian granulosa cells stimulated to differentiate by follicle-stimulating hormone (FSH). We found that protein kinase A (PKA) mediates H3 phosphorylation on serine 10, based on inhibition exclusively by PKA inhibitors. FSH-stimulated H3 phosphorylation in granulosa cells is not downstream of mitogen-activated protein kinase/extracellular signal-regulated kinase, ribosomal S6 kinase-2, mitogen- and stress-activated protein kinase-1, p38 MAPK, phosphatidylinositol-3 kinase, or protein kinase C. Transcriptional activation-associated H3 phosphorylation on serine 10 and acetylation of lysine 14 leads to activation of serum glucocorticoid kinase, inhibin alpha, and c-fos genes. We propose that phosphorylation of histone H3 on serine 10 by PKA in coordination with acetylation of H3 on lysine 14 results in reorganization of the promoters of select FSH responsive genes into a more accessible configuration for activation. The unique role of PKA as the physiological histone H3 kinase is consistent with the central role of PKA in initiating granulosa cell differentiation.

Cyclic AMP inhibits extracellular signal-regulated kinase and phosphatidylinositol 3-kinase/Akt pathways by inhibiting Rap1

Cyclic AMP inhibited both ERK and Akt activities in rat C6 glioma cells. A constitutively active form of phosphatidylinositol 3-kinase (PI3K) prevented cAMP from inhibiting Akt, suggesting that the inactivation of Akt by cAMP is a consequence of PI3K inhibition. Neither protein kinase A nor Epac (Exchange protein directly activated by cAMP), two known direct effectors of cAMP, mediated the cAMP-induced inhibition of ERK and Akt phosphorylation. Cyclic AMP inhibited Rap1 activation in C6 cells. Moreover, inhibition of Rap1 by a Rap1 GTPase-activating protein-1 also resulted in a decrease in ERK and Akt phosphorylation, which was not further decreased by cAMP, suggesting that cAMP inhibits ERK and Akt by inhibiting Rap1. The role of Rap1 in ERK and Akt activity was further demonstrated by our observation that an active form of Epac, which activated Rap1 in the absence of cAMP, increased ERK and Akt phosphorylation. Inhibition of ERK and/or PI3K pathways mediated the inhibitory effects of cAMP on insulin-like growth factor-I (IGF-I) and IGF-binding protein-3 gene expression. Moreover, cAMP, as well as ERK and PI3K inhibitors produced equivalent stimulation and inhibition, respectively, of p27(Kip1) and cyclin D2 protein levels, potentially explaining the observation that cAMP prevented C6 cells from entering S phase.

Mitogen-activated protein kinase phosphorylates and targets inducible cAMP early repressor to ubiquitin-mediated destruction

Inducible cAMP early repressor (ICER) is an important mediator of cAMP antiproliferative activity that acts as a putative tumor suppressor gene product. In this study, we examined the regulation of ICER protein by phosphorylation and ubiquitination in human choriocarcinoma JEG-3 and mouse pituitary AtT20 cells. We found that cAMP stabilized ICER protein by inhibiting the mitogen-activated protein kinase (MAPK) cascade. Activation of the MAPK pathway increased ICER phosphorylation. ICER phosphorylation was abrogated by inhibition of the MAPK pathway either by cAMP or directly by the MAPK inhibitor PD098059. The MAPKs extracellular signal-regulated kinases 1 and 2 physically interact with ICER and mediated the phosphorylation of ICER on a critical serine residue (Ser-41). A mutant form of ICER in which Ser-41 was substituted by alanine had a half-life 4-5 h longer than its wild-type counterpart. This alteration in stability was due to the inability of the Ser-41-mutant ICER to be efficiently ubiquitinated and degraded via the ubiquitin-proteasome pathway. These results present a novel cell signaling cross-talk mechanism at the cell nucleus between the MAPK and cAMP pathways, whereby MAPK targets a repressor of the cAMP-dependent gene expression for ubiquitination and proteasomal degradation.

Regulation of Raf by Akt controls growth and differentiation in vascular smooth muscle cells

The stimulation of platelet-derived growth factor (PDGF) receptors shifts vascular smooth muscle (VSM) cells toward a more proliferative phenotype. Thrombin activates the same signaling cascades in VSM cells, namely the Ras/Raf/MEK/ERK and the phosphatidylinositol 3-kinase (PI 3-kinase)/Akt pathways. Nonetheless, thrombin was not mitogenic, but rather increased the expression of the smooth muscle-specific myosin heavy chain (SM-MHC) indicative of an in vitro re-differentiation of VSM cells. A more detailed analysis of the temporal pattern and relative signal intensities revealed marked differences. The strong and biphasic phosphorylation of ERK1/2 in response to thrombin correlated with its ability to increase the activity of the SM-MHC promoter whereas Akt was only partially and transiently phosphorylated. By contrast, PDGF, a potent mitogen in VSM cells, induced a short-lived ERK1/2 phosphorylation but a complete and sustained phosphorylation of Akt. The phosphorylated form of Akt physically interacted with Raf. Moreover, Akt phosphorylated Raf at Ser(259), resulting in a reduced Raf kinase activity and a termination of MEK and ERK1/2 phosphorylation. Disruption of the PI 3-kinase signaling prevented the PDGF-induced Akt and Raf-Ser(259) phosphorylation. Under these conditions, PDGF elicited a more sustained MEK and ERK phosphorylation and increased SM-MHC promoter activity. Consistently, in cells that express dominant negative Akt, PDGF increased SM-MHC promoter activity. Furthermore, expression of constitutively active Akt blocked the thrombin-stimulated SM-MHC promoter activity. Thus, we present evidence that the balance and cross-regulation between the PI 3-kinase/Akt and Ras/Raf/MEK signaling cascades determine the temporal pattern of ERK1/2 phosphorylation and may thereby guide the phenotypic modulation of vascular smooth muscle cells.

Dopamine D(2) receptor activation causes mitogenesis via p44/42 mitogen-activated protein kinase in opossum kidney cells

This study was conducted to determine the expression of dopamine D(2)-like receptors in opossum kidney (OK) cells and to examine the potential role of these receptors in mitogenesis. First, the presence of D(2)-like receptor binding sites in OK cell membranes was demonstrated by radioligand binding, using [(3)H]spiperone. The D(2)-like receptor subtypes expressed in OK cells were subsequently demonstrated, by Western blotting, to be D(2), D(3), and D(4) receptors. OK cells were stimulated with bromocriptine, (+/-)-2-(N-phenylethyl-N-propyl)amino-5-hydroxytetralin hydrochloride, (R)-(+)-2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide, or PD 168,077 maleate (D(2)-like, D(2), D(3), and D(4) receptor agonists, respectively), and mitogenesis was measured as a function of [(3)H]thymidine incorporation. It was observed that, whereas bromocriptine and (+/-)-2-(N-phenylethyl-N-propyl)amino-5-hydroxytetralin hydrochloride produced increases in [(3)H]thymidine incorporation, (R)-(+)-2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide and PD 168,077 maleate did not produce such an effect, indicating the involvement of D(2) receptors in the mitogenic response. Pertussis toxin and PD 98059 blocked the mitogenesis caused by bromocriptine, suggesting a role for G(i) or G(o) proteins and p44/42 mitogen-activated protein kinase (MAPK), respectively. Furthermore, it was observed that bromocriptine produced a time-dependent increase in the phosphorylation (activation) of p44/42 MAPK, which was blocked by domperidone, pertussis toxin, or PD 98059. Therefore, this study demonstrates that, although OK cells express D(2), D(3), and D(4) receptors, activation of only D(2) receptors causes mitogenesis via phosphorylation of p44/42 MAPK. Furthermore, the cellular mechanisms contributing to D(2) receptor-mediated phosphorylation of p44/42 MAPK seem to involve the tyrosine kinase, phosphatidylinositol-3-kinase, and protein kinase C pathways. It is likely that bromocriptine and other preferential D(2) receptor agonists might provide protection against ischemic reperfusion injury in renal proximal tubular cells, by increasing the survival rates for ischemic cells.

cAMP potentiates H(2)O(2)-induced ERK1/2 phosphorylation without the requirement for MEK1/2 phosphorylation

In Jurkat T lymphocytes, hydrogen peroxide (H(2)O(2)) potentiates the phosphorylation level of extracellular signal-regulated kinase 1 and 2 (ERK1/2) caused by T cell receptor (TCR) stimulation with anti-CD3 and anti-CD28 or anti-CD3 alone. Submillimolar concentrations of H(2)O(2)-induced phosphorylation of ERK1/2 and MAP/ERK kinase 1 and 2 (MEK1/2) without antigenic stimulation. H(2)O(2) also induced the electrophoretic mobility shift of Lck from 56 to 60 kDa. The MEK inhibitor, PD98059 attenuated ERK1/2 and MEK1/2 phosphorylation, as well as the migration shift of Lck induced by H(2)O(2). The phospholipase C (PLC) inhibitor, U73122, and EGTA reduced the phosphorylation of both ERK1/2 and MEK1/2 induced by H(2)O(2). Interestingly, an increase of intracellular cAMP level with forskolin or 8-(4-chlorophenylthio)-cAMP augmented ERK1/2 phosphorylation by H(2)O(2), while inhibiting MEK1/2 phosphorylation by H(2)O(2). These results demonstrate an alternative pathway that results in augmentation of ERK1/2 phosphorylation without concomitant MEK1/2 phosphorylation in T cells.

Glucagon receptor activates extracellular signal-regulated protein kinase 1/2 via cAMP-dependent protein kinase

We prepared a stable cell line expressing the glucagon receptor to characterize the effect of G(s)-coupled receptor stimulation on extracellular signal-regulated protein kinase 1/2 (ERK1/2) activity. Glucagon treatment of the cell line caused a dose-dependent increase in cAMP concentration, activation of cAMP-dependent protein kinase (PKA), and transient release of intracellular calcium. Glucagon treatment also caused rapid dose-dependent phosphorylation and activation of mitogen-activated protein kinase kinase/ERK kinase (MEK1/2) and ERK1/2. Inhibition of either PKA or MEK1/2 blocked ERK1/2 activation by glucagon. However, no significant activation of several upstream activators of MEK, including Ras, Rap1, and Raf, was observed in response to glucagon treatment. In addition, chelation of intracellular calcium reduced glucagon-mediated ERK1/2 activation. In transient transfection experiments, glucagon receptor mutants that bound glucagon but failed to increase intracellular cAMP and calcium concentrations showed no glucagon-stimulated ERK1/2 phosphorylation. We conclude that glucagon-induced MEK1/2 and ERK1/2 activation is mediated by PKA and that an increase in intracellular calcium concentration is required for maximal ERK activation.

Serum- and glucocorticoid-inducible kinase SGK phosphorylates and negatively regulates B-Raf

Phosphorylation can both positively and negatively regulate activity of the Raf kinases. Akt has been shown to phosphorylate and inhibit C-Raf activity. We have recently reported that Akt negatively regulates B-Raf kinase activation by phosphorylating multiple residues within its amino-terminal regulatory domain. Here we investigated the regulation of B-Raf by serum and glucocorticoid-inducible kinase, SGK, which shares close sequence identity with the catalytic domain of Akt but lacks the pleckstrin homology domain. We observed that SGK inhibits B-Raf activity. A comparison of substrate specificity between SGK and Akt indicates that SGK is a potent negative regulator of B-Raf. In contrast to Akt, SGK negatively regulates B-Raf kinase activity by phosphorylating only a single Akt consensus site, Ser(364). Under similar experimental conditions, SGK displays a measurably stronger inhibitory effect on B-Raf kinase activity than Akt, whereas Akt exhibits a more inhibitory effect on the forkhead transcription factor, FKHR. The selective substrate specificity is correlated with an enhanced association between Akt or SGK and their preferred substrates, FKHR and B-Raf, respectively. These results indicate that B-Raf kinase activity is negatively regulated by Akt and SGK, suggesting that the cross-talk between the B-Raf and other signaling pathways can be mediated by both Akt and SGK.

ET(B) receptor activates adenylyl cyclase via a c-PLA(2)-dependent mechanism: a novel counterregulatory mechanism of ET-induced contraction in airway smooth muscle

Endothelin-1 (ET-1) contracted the rabbit tracheal smooth muscle (RTSM), yielding a bell-shaped tension-concentration curve. Moreover, ET-1 induced concentration- and time-dependent increases in cAMP concentrations in RTSM (EC(50), 58 nM; t(1/2), 2.4 min). Pretreatment with the AC inhibitors, SQ-22536, or 2'-5'-dideoxyadenosine, enhanced contraction to ET-1 and converted its bell-shaped tension curve into a sigmoidal one, but left contraction to carbachol and KCl unaltered. The potent ET(B)-receptor agonists, ET-3 or sarafotoxin-c, mimicked ET-1's effects on cAMP levels (EC(50) values 55 and 50 nM). Further, cAMP formation by ETs was inhibited by BQ-788 (selective ET(B) receptor blocker; IC(50), 8 nM), but not by BQ-610 (selective ET(A) receptor blocker). Removal of the epithelium did not prevent ET-induced increases in cAMP levels. Unlike isoproterenol, ETs failed to activate AC in membrane fractions from RTSM. In intact RTSM, the c-PLA(2) inhibitor, AACOCF3, and the cyclooxygenase inhibitor, indomethacin, blocked ET-induced increases in cAMP levels. These findings reveal a novel, nonepithelial, c-PLA(2)-mediated, regulatory mechanism downstream from ET(B) receptors.

A retroviral-derived immunosuppressive peptide activates mitogen-activated protein kinases

The highly conserved region within the retroviral transmembrane envelope proteins has been implicated in a number of retrovirus-associated mechanisms of immunosuppression. CKS-17, a synthetic peptide representing the prototypic sequence of the immunosuppressive domain, has been found to suppress numerous immune functions, disregulate cytokines, and elevate intracellular cAMP. In this report we show that using a human monocytic cell line THP-1, CKS-17 activates mitogen-activated protein (MAP) kinases extracellular signal-regulated kinase 1 and 2 (ERK1/2). Kinetic studies show that CKS-17 induces an acute increase of ERK1/2 activity followed by a rapid decrease and then a second sustained increase of ERK1/2. CKS-17 also activates MAP kinase/ERK kinase (MEK) with a similar induction pattern. Mutant THP-1 cells isolated in our laboratory, in which CKS-17 exclusively fails to activate cAMP, did not show the transient decrease of CKS-17-induced ERK1/2 phosphorylation. Pretreatment of THP-1 cells or mutant THP-1 cells with cAMP analog or forskolin followed by treatment with CKS-17 showed no activation of MEK or ERK1/2. These results indicate that CKS-17 activates the MEK/ERK cascade and that there is a cross-talk between CKS-17-mediated MEK/ERK cascade and cAMP in that the MEK/ERK cascade is negatively regulated by cAMP. These data present a novel molecular mechanism(s) by this highly conserved retroviral immunosuppressive component.

The ERK-dependent signalling is stage-specifically modulated by FSH, during primary Sertoli cell maturation

Primary cultures of Sertoli cells provide an interesting model to study how signalling pathways induced by a single hormone in a single cell type evolve, depending on the developmental stage. In vivo, follicle-stimulating hormone (FSH) induces proliferation of Sertoli cells in neonate and controls the subsequent differentiation of the entire population. Molecular mechanisms underlying Sertoli cell pleiotropic responses to FSH have long been investigated. But to date, only cAMP-dependent kinase (PKA) activation has been reported to account for most FSH biological activities in male. Here, we demonstrate that FSH activates the ERK MAP kinase pathway following dual coupling of the FSH-R both to Gs and to Gi heterotrimeric proteins, in a PKA- and also Src-dependent manner. This activation is required for FSH-induced proliferation of Sertoli cells isolated 5 days after birth. Consistently, we show that the ERK-mediated FSH mitogenic effect triggers upregulation of cyclin D1. In sharp contrast, at 19 days after birth, as cells proceed through their differentiation program, the ERK pathway is dramatically inhibited by FSH treatment. Taken together, these results show that FSH can exert opposite effects on the ERK signalling cascade during the maturation process of Sertoli cells. Thus, signalling modules triggered by the FSH-R evolve dynamically throughout development of FSH natural target cells.

Platelet-derived growth factor stimulates phospholipase C-gamma 1, extracellular signal-regulated kinase, and arachidonic acid release in rat myometrial cells: contribution to cyclic 3',5'-adenosine monophosphate production and effect on cell proliferation

In the present study, we examined downstream signaling events that followed exposure of cultured rat myometrial cells to platelet-derived growth factor (PDGF) and their effect on cell proliferation. PDGF-BB induced tyrosine phosphorylation of PDGF-beta receptors and increased inositol trisphosphate production via the tyrosine phosphorylation of phospholipase (PL)C-gamma 1. PDGF-BB also increased cAMP synthesis. This increase was potentiated by forskolin and reduced by indomethacin, a cyclooxygenase inhibitor, reflecting a Gs protein-mediated process via prostaglandin biosynthesis. The prostaglandin produced by PDGF was characterized as prostacyclin (PGI(2)). PDGF-BB increased arachidonic acid (AA) release, which, similarly to cAMP accumulation, was abolished in the presence of AACOCF3, a cytosolic PLA(2) inhibitor, and in the absence of Ca(2+). U-73122, a potent inhibitor of PLC activity, blocked both the production of inositol phosphates and the AA release triggered by PDGF-BB. Extracellular signal-regulated kinases (ERKs) 1 and 2 are expressed in myometrial cells, and PDGF-BB selectively activated ERK2. PD98059, an inhibitor of the ERK-activating kinase, blocked PDGF-BB-mediated ERK2 activation, AA release, and cAMP production. The results demonstrate that PDGF-BB stimulated cAMP formation through both PLC activation and ERK-dependent AA release and PGI(2) biosynthesis. PDGF-BB also increased cell proliferation and [(3)H]thymidine incorporation. This was abolished by PD98059, demonstrating that the ERK cascade is required for the mitogenic effect of PDGF-BB. Forskolin, which potentiated the cAMP response to PDGF-BB, attenuated both DNA synthesis and ERK activation triggered by PDGF-BB, suggesting the presence of a negative feedback regulation.

Evidence for a novel thrombopoietin signalling event: activation of protein kinase A in human megakaryoblastic CMK cells

Thrombopoietin (TPO) plays a crucial role in megakaryocyte development. TPO signalling, which is mediated by its receptor Mpl, includes Janus kinase, (JAK) signal transducer and activator of transcription (STAT) and Shc/Ras/mitogen-activated protein kinase (MAPK) pathways. The precise nature of these signalling routes has not been clarified in detail up until now. We investigated the effect of TPO on activation of cAMP-dependent protein kinase (PKA) and its involvement in MAPK signalling in human megakaryoblastic leukaemia CMK cells. For estimation of PKA activity, phosphorylation of a PKA-specific peptide substrate was assayed in CMK cell lysates. Since activation of PKA is associated with translocation of its catalytic subunit alpha (C-PKA) into the cell nucleus, Western blot analysis of nuclear fractions with an anti-C-PKA antibody was additionally performed. The activation of TPO-induced MAPK activation and the effect of the PKA inhibitor H-89 was measured using immunoblotting with a monoclonal anti-pERK antibody. TPO enhanced cAMP and induced activation of PKA in CMK cells. In addition, H-89 partly blocked TPO-induced MAPK activation in CMK cells. Our results indicate a novel TPO-triggered signalling event, activation of the cAMP/PKA pathway in human megakaryoblastic CMK cells. This signal transduction route seems to be involved in TPO-induced MAPK signaling.

Positive and negative regulation of Raf kinase activity and function by phosphorylation

Activating and inhibitory phosphorylation mechanisms play an essential role in regulating Raf kinase activity. Here we demonstrate that phosphorylation of C-Raf in the kinase activation loop (residues T491 and S494) is necessary, but not sufficient, for activation. C-Raf has additional activating phosphorylation sites at S338 and Y341. Mutating all four of these residues to acidic residues, S338D/Y341D/T491E/S494D (DDED), in C-Raf results in constitutive activity. However, acidic residue substitutions at the corresponding activation loop sites in B-Raf are sufficient to confer constitutive activity. B-Raf and C-Raf also utilize similar inhibitory phosphorylation mechanisms to regulate kinase activity. B-Raf has multiple inhibitory phosphorylation sites necessary for full kinase inhibition where C-Raf requires only one. We examined the functional significance of these inhibitory and activating phosphorylations in Caenorhabditis elegans lin-45 Raf. Eliminating the inhibitory phosphorylation or mimicking activating phosphorylation sites is sufficient to confer constitutive activity upon lin-45 Raf and induce multi-vulva phenotypes in C.elegans. Our results demonstrate that different members of the Raf family kinases have both common and distinct phosphorylation mechanisms to regulate kinase activity and biological function.

cAMP protection of pancreatic cancer cells against apoptosis induced by ERK inhibition

Large increases in cAMP concentration inside the cell are generally growth inhibitory for most cell lines of mesenchymal and epithelial origin. Moreover, recent data suggest a role of cAMP in survival of different cell types. Herein, the ability of forskolin (an adenylyl cyclase activator) and IBMX (3-isobutyl-1-methylxanthine) (a phosphodiesterase inhibitor) to modulate cell cycle progression and survival of human pancreatic cancer cells was evaluated. We showed that forskolin + IBMX inhibited serum-induced ERK activities, Rb hyperphosphorylation, Cdk2 activity, and p27(Kip1) downregulation and caused G1 arrest in MIA PaCa-2 cells. Furthermore, forskolin + IBMX protected pancreatic cells against apoptosis induced by prolonged inhibition of ERK activities by preventing Bcl-X(L) downregulation, activation of caspases 3, 6, 8, and 9, and PARP cleavage and by inducing Bad phosphorylation (ser112). Taken together, our data demonstrate for the first time that cAMP is an inhibitor of cell cycle progression and apoptosis in human pancreatic cancer cells.

Mitogenic G(i) protein-MAP kinase signaling cascade in MC3T3-E1 osteogenic cells: activation by C-terminal pentapeptide of osteogenic growth peptide [OGP(10-14)] and attenuation of activation by cAMP

In osteogenic and other cells the mitogen-activated protein (MAP) kinases have a key role in regulating proliferation and differentiated functions. The osteogenic growth peptide (OGP) is a 14 mer mitogen of osteogenic and fibroblastic cells that regulates bone turnover, fracture healing, and hematopoiesis, including the engraftment of bone marrow transplants. It is present in the serum and extracellular fluid either free or complexed to OGP-binding proteins (OGPBPs). The free immunoreactive OGP consists of the full length peptide and its C-terminal pentapeptide OGP(10-14). In the present study, designed to probe the signaling pathways triggered by OGP, we demonstrate in osteogenic MC3T3 E1 cells that mitogenic doses of OGP(10-14), but not OGP, enhance MAP kinase activity in a time-dependent manner. The OGP(10-14)-induced stimulation of both MAP kinase activity and DNA synthesis were abrogated by pertusis toxin, a G(i) protein inhibitor. These data offer direct evidence for the occurrence in osteogenic cells of a peptide-activated, mitogenic Gi protein-MAP kinase-signaling cascade. Forskolin and dBu(2)-cAMP abrogated the OGP(10-14)-stimulated proliferation, but induced only 50% inhibition of the OGP(10-14)-mediated MAP kinase activation, suggesting additional MAP kinase-dependent, OGP(10-14)-regulated, cellular functions. Finally, it is demonstrated that OGP(10-14) is the active form of OGP, apparently generated proteolytically in the extracellular milieu upon dissociation of OGP-OGPBP complexes.

Extracellular ATP stimulates an inhibitory pathway towards growth factor-induced cRaf-1 and MEKK activation in astrocyte cultures

ATP, acting via P2Y, G protein-coupled receptors (GPCRs), is a mitogenic signal and also synergistically enhances fibroblast growth factor-2 (FGF-2)-induced proliferation in astrocytes. Here, we have examined the effects of ATP and FGF-2 cotreatment on the main components of the extracellular-signal regulated protein kinase (ERK) cascade, cRaf-1, MAPK/ERK kinase (MEK) and ERK, key regulators of cellular proliferation. Surprisingly, ATP inhibited activation of cRaf-1 by FGF-2 in primary cultures of rat cortical astrocytes. The inhibitory effect did not diminish MEK and ERK activation; indeed, cotreatment resulted in a greater initial activation of ERK. ATP inhibition of cRaf-1 activation was not mediated by an increase in cyclic AMP levels or by protein kinase C activation. ATP also inhibited the activation of cRaf-1 by other growth factors, epidermal growth factor and platelet-derived growth factor, as well as other MEK1 activators stimulated by FGF-2, MEK kinase 1 (MEKK1) and MEKK2. Serotonin, an agonist of another GPCR coupled to ERK, did not inhibit FGF-2-induced cRaf-1 activation, thereby indicating specificity in the ATP-induced inhibitory cross-talk. These findings suggest that ATP stimulates an inhibitory activity that lays upstream of MEK activators and inhibits growth factor-induced activation of cRaf-1 and MEKKS: Such a mechanism might serve to integrate the actions of receptor tyrosine kinases and P2Y-GPCRS:

Role of cAMP-dependent protein kinase A activity in endothelial cell cytoskeleton rearrangement

To examine signaling mechanisms relevant to cAMP/protein kinase A (PKA)-dependent endothelial cell barrier regulation, we investigated the impact of the cAMP/PKA inhibitors Rp diastereomer of adenosine 3',5'-cyclic monophosphorothioate (Rp-cAMPS) and PKA inhibitor (PKI) on bovine pulmonary artery and bovine lung microvascular endothelial cell cytoskeleton reorganization. Rp-cAMPS as well as PKI significantly increased the formation of actin stress fibers and intercellular gaps but did not alter myosin light chain (MLC) phosphorylation, suggesting that the Rp-cAMPS-induced contractile phenotype evolves in an MLC-independent fashion. We next examined the role of extracellular signal-regulated kinases (ERKs) in Rp-cAMPS- and PKI-induced actin rearrangement. The activities of both ERK1/2 and its upstream activator Raf-1 were transiently enhanced by Rp-cAMPS and linked to the phosphorylation of the well-known ERK cytoskeletal target caldesmon. Inhibition of the Raf-1 target ERK kinase (MEK) either attenuated or abolished Rp-cAMPS- and PKI-induced ERK activation, caldesmon phosphorylation, and stress fiber formation. In summary, our data elucidate the involvement of the p42/44 ERK pathway in cytoskeletal rearrangement evoked by reductions in PKA activity and suggest the involvement of significant cross talk between cAMP- and ERK-dependent signaling pathways in endothelial cell cytoskeletal organization and barrier regulation.

Differential cAMP gating of glutamatergic signaling regulates long-term state changes in the suprachiasmatic circadian clock

We investigated a role for cAMP/protein kinase A (PKA) in light/glutamate (GLU)-stimulated state changes of the mammalian circadian clock in the suprachiasmatic nucleus (SCN). Nocturnal GLU treatment elevated [cAMP]; however, agonists of cAMP/PKA did not mimic the effects of light/GLU. Coincident activation of cAMP/PKA enhanced GLU-stimulated state changes in early night but blocked light/GLU-induced state changes in the late night, whereas inhibition of cAMP/PKA reversed these effects. These responses are distinct from those mediated by mitogen-activated protein kinase (MAPK). MAPK inhibitors attenuated both GLU-induced state changes. Although GLU induced mPer1 mRNA in both early and late night, inhibition of PKA blocked this event only in early night, suggesting that cellular mechanisms regulating mPer1 are gated by the suprachiasmatic circadian clock. These data support a diametric gating role for cAMP/PKA in light/GLU-induced SCN state changes: cAMP/PKA promotes the effects of light/GLU in early night, but opposes them in late night.

Brief treatments with forskolin enhance s-phase entry in balance epithelia from the ears of rats

In the ears of mammals, hair cell loss results in permanent hearing and balance deficits, whereas in fish, amphibians, and birds, the production of replacement hair cells can restore those modalities. In avian ears, continuous exposures to forskolin trigger cell proliferation and the regeneration of hair cells, so we investigated the effect of forskolin on sensory epithelia cultured from the ears of mammals. Continuous 72 hr exposures to forskolin failed to induce proliferation in neonatal rat utricles, but brief (</=1 hr) exposures to forskolin or Br-cAMP did. Proliferation occurred only in media that contained serum. Forskolin also augmented the mitogenic effects of glial growth factor 2. The S-phase entry induced by forskolin was blocked by monensin and bafilomycin, two compounds that can inhibit the recycling of membrane receptors. The results are consistent with the hypothesis that in mammalian vestibular epithelia elevated cAMP induces S-phase entry by increasing the number of growth factor receptors at the plasma membrane.

Gbeta gamma mediate differentiation of vascular smooth muscle cells

Proliferation and subsequent dedifferentiation of vascular smooth muscle (VSM) cells contribute to the pathogenesis of atherosclerosis and postangioplastic restenosis. The dedifferentiation of VSM cells in vivo or in cell culture is characterized by a loss of contractile proteins such as smooth muscle-specific alpha-actin and myosin heavy chain (SM-MHC). Serum increased the expression of contractile proteins in neonatal rat VSM cells, indicating a redifferentiation process. RNase protection assays defined thrombin as a serum component that increases the abundance of SM-MHC transcripts. Additionally, serum and thrombin transiently elevated cytosolic Ca(2+) concentrations, led to a biphasic extracellular signal-regulated kinase (ERK) phosphorylation, up-regulated a transfected SM-MHC promoter construct, and induced expression of the contractile proteins SM-MHC and alpha-actin. Pertussis toxin, N17-Ras/Raf, and PD98059 prevented both the serum- and thrombin-induced second phase ERK phosphorylation and SM-MHC promoter activation. Constitutively active Galpha(q), Galpha(i), Galpha(12), and Galpha(13) failed to up-regulate SM-MHC transcription, whereas Gbetagamma concentration-dependently increased the SM-MHC promoter activity. Furthermore, the Gbetagamma scavenger beta-adrenergic receptor kinase 1 C-terminal peptide abolished the serum-mediated differentiation. We conclude that receptor-mediated differentiation of VSM cells requires Gbetagamma and an intact Ras/Raf/MEK/ERK signaling.

Peptides derived from pro-growth hormone-releasing hormone activate p38 mitogen-activated protein kinase in GH3 pituitary cells

Posttranslational processing of the pro-growth hormone-releasing hormone (proGHRH) peptide can result in the formation of at least two peptide products: GHRH and the C-terminal peptide, GHRH-related peptide (GHRH-RP). While cyclic adenosine monophosphate transduces many of the actions of GHRH, other pathways also have been implicated in its actions. The aims of this study were to examine and characterize the activation of mitogen-activated protein kinase (MAPK) pathways by GHRH, and GHRH-RP in pituitary-derived GH3 cells, as well as the activation of the transcription factors that serve as substrates for these kinases. GHRH rapidly increased p44/p42 MAPK activity in GH3 cells in a protein kinase A-dependent and a protein kinase C-independent manner and stimulated the activation of the transcription factor Elk-1. By contrast, GHRH-RP and p75-92NH2 had no effect on p44/p42 MAPK phosphorylation in these cells. Additionally, we determined that all three peptides, GHRH, GHRH-RP, and p75-92NH2, rapidly and specifically increase phosphorylation of p38 MAPK and stimulate the activation of the nuclear factor CHOP. These are the first studies to demonstrate the activation of Elk-1 by GHRH and the activation of p38 MAPK and CHOP by GHRH, GHRH-RP, and p75-92NH2. We conclude that members of the GHRH family of peptides differentially activate multiple intracellular signaling pathways and suggest that the biologic actions of GHRH may be far more diverse than previously thought.

Cyclic AMP-mediated inhibition of cell growth requires the small G protein Rap1

In many normal and transformed cell types, the intracellular second messenger cyclic AMP (cAMP) blocks the effects of growth factors and serum on mitogenesis, proliferation, and cell cycle progression. cAMP exerts these growth-inhibitory effects via inhibition of the mitogen-activated protein (MAP) kinase cascade. Here, using Hek293 and NIH 3T3 cells, we show that cAMP's inhibition of the MAP kinase cascade is mediated by the small G protein Rap1. Activation of Rap1 by cAMP induces the association of Rap1 with Raf-1 and limits Ras-dependent activation of ERK. In NIH 3T3 cells, Rap1 is required not only for cAMP's inhibition of ERK activation but for inhibition of cell proliferation and mitogenesis as well.

Initiation of cell locomotility is a morphogenetic checkpoint in thyroid epithelial cells regulated by ERK and PI3-kinase signals

Epithelial locomotility is a fundamental determinant of tissue patterning that is subject to strict physiological regulation. The current study sought to identify cellular signals that initiate cell migration in cultured thyroid epithelial cells. Porcine thyroid cells cultured as 3-dimensional follicles convert to 2-dimensional monolayers when deprived of agents that stimulate cAMP/PKA signaling. This morphogenetic event is driven by the activation of cell-on-substrate locomotility, providing a convenient assay for events that regulate the initiation of locomotion. In this system, the extracellular signal regulated kinase (ERK) pathway became activated as follicles converted to monolayer, as demonstrated by immunoblotting for activation-specific phosphorylation and nuclear accumulation of ERK. Inhibition of ERK activation using the drug PD98059 effectively prevented cells from beginning to migrate. PD98059 inhibited cell spreading, actin filament reorganization and the assembly of focal adhesions, cellular events that mediate the initiation of thyroid cell locomotility. Akt (PKB) signaling was also activated during follicle-to-monolayer conversion and the phosphoinositide 3-kinase (PI3-kinase) inhibitor, wortmannin, also blocked the initiation of cell movement. Wortmannin did not, however, block activation of ERK signaling. These findings, therefore, identify the ERK and PI3-kinase signaling pathways as important stimulators of thyroid cell locomotility. These findings are incorporated into a model where the initiation of thyroid cell motility constitutes a morphogenetic checkpoint regulated by coordinated changes in stimulatory (ERK, PI3-kinase) and tonic inhibitory (cAMP/PKA) signaling pathways.

Constitutive association of c-N-Ras with c-Raf-1 and protein kinase C epsilon in latent signaling modules

Phorbol ester stimulation of the MAPK cascade is believed to be mediated through the protein kinase C (PKC)-dependent activation of Raf-1. Although several studies suggest that phorbol ester stimulation of MAPK is insensitive to dominant-negative Ras, a requirement for Ras in Raf-1 activation by PKC has been suggested recently. We now demonstrate that in normal, quiescent mouse fibroblasts, endogenous c-N-Ras is constitutively associated with both c-Raf-1 and PKC epsilon in a biochemically silent, but latent, signaling module. Chemical inhibition of novel PKCs blocks phorbol 12-myristate 13-acetate (PMA)-mediated activation of MAPKs. Down-regulation of PKC epsilon protein levels by antisense oligodeoxyribonucleotides blocks MAPK activation in response to PMA stimulation, demonstrating that PKC epsilon activity is required for MAPK activation by PMA. c-Raf-1 activity in immunoprecipitated c-N-Ras.c-Raf-1.PKC epsilon complexes is stimulated by PMA and is inhibited by GF109203X, thereby linking c-Raf-1 activation in this complex to PKC activation. These observations suggest that in quiescent cells Ras is organized into ordered, inactive signaling modules. Furthermore, the regulation of the MAPK cascade by both Ras and PKC is intimately linked, converging at the plasma membrane through their association with c-Raf-1.