Inhibition by cAMP of Ras-dependent activation of Raf

The phosphodiesterase inhibitor SQ 20006 selectively blocks mitogen activation of p70S6k and transition to S phase of the cell division cycle without affecting the steady state phosphorylation of eIF-4E

In quiescent cells high levels of protein synthesis are required in order to re-enter the cell cycle upon stimulation. Initiation of polypeptide synthesis is the step most often subject to regulation, controlled in part by phosphorylation of 40 S ribosomal protein S6 and a number of initiation factors. The kinase responsible for S6 phosphorylation is p70S6k. We now show that the p70S6k pathway can be selectively blocked by the aminopurine analogue, SQ 20006. This agent is known to raise cAMP levels, resulting in activation of protein kinase A. We present evidence that the increase in cAMP is not responsible for the inhibitory effect observed. We also show that SQ 20006 can prevent the activation of p70S6k in a rapid and reversible manner. The compound does not exert its inhibitory activity on p70S6k but can inhibit in vitro two protein kinase C isozymes (alpha and gamma). In a B lymphoblastoid cell line, treatment with SQ 20006 results in inhibition of protein synthesis at the initiation stage. In contrast, when tested directly upon the translational machinery in the reticulocyte lysate, inhibition is manifest at both the level of initiation and elongation. The role of protein kinase A in the modulation of p70S6k and the rate of translation is discussed.

Hypertrophic agonists stimulate the activities of the protein kinases c-Raf and A-Raf in cultured ventricular myocytes

We detected expression of two Raf isoforms, c-Raf and A-Raf, in neonatal rat heart. Both isoforms phosphorylated, activated, and formed complexes with mitogen-activated protein kinase kinase 1 in vitro. However, these isoforms were differentially activated by hypertrophic stimuli such as peptide growth factors, endothelin-1 (ET1), or 12-O-tetradecanoylphorbol-13-acetate (TPA) that activate the mitogen-activated protein kinase cascade. Exposure of cultured ventricular myocytes to acidic fibroblast growth factor activated c-Raf but not A-Raf. In contrast, TPA produced a sustained activation of A-Raf and only transiently activated c-Raf. ET1 transiently activated both isoforms. TPA and ET1 were the most potent activators of c-Raf and A-Raf. Both utilized protein kinase C-dependent pathways, but stimulation by ET1 was also partially sensitive to pertussis toxin pretreatment. cRaf was inhibited by activation of cAMP-dependent protein kinase although A-Raf was less affected. Fetal calf serum, phenylephrine, and carbachol were less potent activators of c-Raf and A-Raf. These results demonstrate that A-Raf and c-Raf are differentially regulated and that A-Raf may be an important mediator of mitogen-activated protein kinase cascade activation when cAMP is elevated.

pp60c-src is a positive regulator of growth factor-induced cell scattering in a rat bladder carcinoma cell line

The NBT-II rat carcinoma cell line exhibits two mutually exclusive responses to FGF-1 and EGF, entering mitosis at cell confluency while undergoing an epithelium-to-mesenchyme transition (EMT) when cultured at subconfluency. EMT is characterized by acquisition of cell motility, modifications of cell morphology, and cell dissociation correlating with the loss of desmosomes from cellular cortex. The pleiotropic effects of EGF and FGF-1 on NBT-II cells suggest that multiple signaling pathways may be activated. We demonstrate here that growth factor activation is linked to at least two intracellular signaling pathways. One pathway leading to EMT involves an early and sustained stimulation of pp60c-src kinase activity, which is not observed during the growth factor-induced entry into the cell cycle. Overexpression of normal c-src causes a subpopulation of cells to undergo spontaneous EMT and sensitizes the rest of the population to the scattering activity of EGF and FGF-1 without affecting their mitogenic responsiveness. Addition of cholera toxin, a cAMP-elevating agent, severely perturbs growth factor induction of EMT without altering pp60c-src activation, therefore demonstrating that cAMP blockade takes place downstream or independently of pp60c-src. On the other hand, overexpression of a mutated, constitutively activated form of pp60c-src does not block cell dispersion while strongly inhibiting growth factor-induced entry into cell division. Moreover, stable transfection of a dominant negative mutant of c-src inhibits the scattering response without affecting mitogenesis induced by the growth factors. Altogether, these results suggest a role for pp60c-src in epithelial cell scattering and indicate that pp60c-src might contribute unequally to the two separate biological activities engendered by a single signal.

Dual effect of beta-adrenergic receptors on mitogen-activated protein kinase. Evidence for a beta gamma-dependent activation and a G alpha s-cAMP-mediated inhibition

The enzymatic activity of mitogen-activated protein kinases (MAP kinases) increases in response to agents acting on a variety of cell surface receptors, including receptors linked to heterotrimeric G proteins of the Gi and Gq family. Recently, it has been shown that stimulation of beta-adrenergic receptors, which are typical of those that act through Gs to activate adenylyl cyclases, potently activates MAP kinases in the heart, resulting in the hypertrophy of the cardiac muscle (Lazou, A., Bogoyevitch, M.A., Clerk, A., Fuller, S.J., Marshall, C.J., and Sudgen, P.H. (1994) Circ. Res. 75, 938-941). We have observed that exposure of COS-7 cells to a beta-adrenergic agonist, isoproterenol, raises intracellular levels of cAMP and effectively activates protein kinase A (PKA) and an epitope-tagged MAP kinase. However, MAP kinase stimulation by isoproterenol was neither mimicked by expression of an activated mutant of G alpha s, nor by treatment with PKA-stimulating agents. Moreover, pretreatment of COS-7 with a permeable cAMP analog, 8-Br-cAMP, markedly decreased MAP kinase activation by either isoproterenol or epidermal growth factor. Thus, in COS-7 cells cAMP and PKA do not appear to mediate MAP kinase activation by beta-adrenergic receptors. Signaling from beta-adrenergic receptors to MAP kinase was inhibited by transfection of a chimeric molecule consisting of the CD8 receptor and the carboxyl terminus of the beta-adrenergic receptor kinase, which includes the beta gamma-binding domain. MAP kinase activation by isoproterenol was not affected by depletion of protein kinase C, but it was completely abolished by expression of Ras-inhibiting molecules. We conclude that signaling from beta-adrenergic receptors to MAP kinase involves an activating signal mediated by beta gamma subunits acting on a Ras-dependent pathway and a G alpha s-induced inhibitory signal mediated by cAMP and PKA. The balance between these two opposing mechanisms of regulation would be expected to control the MAP kinase response to beta-adrenergic agonists as well as to other biologically active agents known to act on Gs coupled receptors, including a number of hormones, neurotransmitters, and lipid mediators.

Insulin activates nuclear factor kappa B in mammalian cells through a Raf-1-mediated pathway

We examined the effect of insulin on nuclear factor kappa B (NF-kappa B) activity in Chinese ovary (CHO) cells overexpressing wild-type (CHO-R cells) or -defective insulin receptors mutated at Tyr1162 and Tyr1163 autophosphorylation sites (CHO-Y2 cells). In CHO-R cells, insulin caused a specific, time-, and concentration-dependent activation of NF-kappa B. The insulin-induced DNA-binding complex was identified as the p50/p65 heterodimer. Insulin activation of NF-kappa B: 1) was related to insulin receptor number and tyrosine kinase activity since it was markedly reduced in parental CHO cells which proved to respond to insulin growth factor-1 and phorbol 12-myristate 13-acetate (PMA) activation, and was dramatically decreased in CHO-Y2 cells; 2) persisted in the presence of cycloheximide and was blocked by pyrrolidine dithiocarbamate, aspirin and sodium salicylate, three compounds interfering with I kappa B degradation and/or NF-kappa B.I kappa B complex dissociation; 3) was independent of both PMA-sensitive and atypical (zeta) protein kinases C; and 4) was dependent on Raf-1 kinase activity since insulin-stimulated NF-kappa B DNA binding activity was inhibited by 8-bromo-cAMP, a Raf-1 kinase inhibitor. Moreover, insulin activation of NF-kappa B-driven luciferase reporter gene expression was blocked in CHO-R cells expressing a Raf-1 dominant negative mutant. This is the first evidence that insulin activates NF-kappa B in mammalian cells through a post-translational mechanism requiring both insulin receptor tyrosine kinase and Raf-1 kinase activities.

Cyclic AMP-increasing agents interfere with chemoattractant-induced respiratory burst in neutrophils as a result of the inhibition of phosphatidylinositol 3-kinase rather than receptor-operated Ca2+ influx

Superoxide anion and arachidonic acid were produced in guinea pig neutrophils in response to a chemotactic peptide formyl-methionyl-leucyl-phenylalanine (fMLP). Both responses were markedly, but the former response to a phorbol ester was not at all, inhibited when the cellular cAMP level was raised by prostaglandin E1 combined with a cAMP phosphodiesterase inhibitor. Increasing cAMP was also inhibitory to fMLP-induced activation of phosphatidylinositol (PI) 3-kinase and Ca2+ influx without any effect on the cation mobilization from intracellular stores. The fMLP-induced respiratory burst was abolished when PI 3-kinase was inhibited by wortmannin or LY294002, but was not affected when Ca2+ influx was inhibited. On the contrary, fMLP released arachidonic acid from the cells treated with the PI 3-kinase inhibitors as well as from non-treated cells, but it did not so when cellular Ca2+ uptake was prevented. The chemotactic peptide activated PI 3-kinase even in cells in which the receptor-mediated intracellular Ca2+ mobilization and respiratory burst were both abolished by exposure of the cells to a permeable Ca(2+)-chelating agent. Thus, stimulation of fMLP receptors gave rise to dual effects, activation of PI 3-kinase and intracellular Ca2+ mobilization; both effects were necessary for the fMLP-induced respiratory burst. Increasing cellular cAMP inhibited the respiratory burst and arachidonic acid release as a result of the inhibitions of PI 3-kinase and Ca2+ influx, respectively, in fMLP-treated neutrophils.

Differential regulation of Raf-1 and B-Raf and Ras-dependent activation of mitogen-activated protein kinase by cyclic AMP in PC12 cells

Growth factor stimulation of the mitogen-activated protein (MAP) kinase pathway in fibroblasts is inhibited by cyclic AMP (cAMP) as a result of inhibition of Raf-1. In contrast, cAMP inhibits neither nerve growth factor-induced MAP kinase activation nor differentiation in PC12 pheochromocytoma cells. Instead, in PC12 cells cAMP activates MAP kinase. Since one of the major differences between the Ras/Raf/MAP kinase cascades of these cell types is the expression of B-Raf in PC12 cells, we compared the effects of cAMP on Raf-1 and B-Raf. In PC12 cells maintained in serum-containing medium, B-Raf was refractory to inhibition by cAMP, whereas Raf-1 was effectively inhibited. In contrast, both B-Raf and Raf-1 were inhibited by cAMP in serum-starved PC12 cells. The effect of cAMP is thus dependent upon growth conditions, with B-Raf being resistant to cAMP inhibition in the presence of serum. These results were extended by studies of Rat-1 fibroblasts into which B-Raf had been introduced by transfection. As in PC12 cells, B-Raf was resistant to inhibition by cAMP in the presence of serum, whereas Raf-1 was effectively inhibited. In addition, the expression of B-Raf rendered Rat-1 cells resistant to the inhibitory effects of cAMP on both growth factor-induced activation of MAP kinase and mitogenesis. These results indicate that Raf-1 and B-Raf are differentially sensitive to inhibition by cAMP and that B-Raf expression can contribute to cell type-specific differences in the regulation of the MAP kinase pathway. In contrast to the situation in PC12 cells, cAMP by itself did not stimulate MAP kinase in B-Raf-expressing Rat-1 cells. The activation of MAP kinase by cAMP in PC12 cells was inhibited by the expression of a dominant negative Ras mutant, indicating that cAMP acts on a target upstream of Ras. Thus, it appears that a signaling component upstream of Ras is also require for cAMP stimulation of MAP kinase in PC12 cells.

Lithium-stimulated proliferation and alteration of phosphoinositide metabolites in MCF-7 human breast cancer cells

Lithium, which is used to treat bipolar psychiatric disorders, can stimulate proliferation of a number of cells in tissue culture. Proliferation of MCF-7 human breast cancer cells, which also respond to EGF and estrogens, was stimulated by LiCl (1-5 mM) within the concentration range that is encountered during human therapy with lithium. Stimulation of growth was specific for lithium; rubidium, potassium, and sodium showed no such effect. In the presence of antiestrogen, lithium stimulated the growth of hormone-dependent breast cancer cells MCF-7, ZR-75-1, and T47D but not hormone-independent MDA-MB-231 cells or an estrogen-independent clone of MCF-7 cells. Lithium-stimulated proliferation was limited by cytotoxicity which could be moderated by added potassium chloride (5-20 mM) in the medium. Each of the mitogens lithium, 17 beta-estradiol, and EGF increased the rate of uptake of myo-inositol into MCF-7 cells. Whether normalized to inositol lipids, to protein, or to DNA, steady-state levels of inositol phosphates were elevated by each of the mitogens including lithium, which inhibits the breakdown of inositol phosphates in the phosphoinositide signaling pathway. These data indicate that therapeutic concentrations of lithium can stimulate the proliferation of human breast cancer cells by a mechanism that may involve the phosphoinositide pathway.

Inhibition of MAP kinase by prostaglandin E2 and forskolin in rat renal mesangial cells

Activation of the mitogen-activated protein (MAP) kinase pathway is believed to play a critical role in normal and pathophysiological proliferation of mesangial cells. Recent studies have shown that MAP kinase activation by growth factors in other cell types involves activation of the low-molecular-weight G protein Ras and the protooncogene serine kinase c-Raf-1. In this study, the role of this pathway in rat renal mesangial cells was assessed. Platelet-derived growth factor (PDGF), epidermal growth factor (EGF), as well as phorbol esters (PMA) rapidly activated MAP kinase three- to fourfold in these cells. PDGF and EGF, but not PMA, were able to activate c-Raf-1 and Ras activity. Stimulation of mesangial cells with the inflammatory mediator prostaglandin E2 (PGE2) or elevation of intracellular adenosine 3',5'-cyclic monophosphate (cAMP) by treatment with forskolin markedly blunted activation of MAP kinase induced by PDGF and EGF, but not by PMA. Consistent with this observation, PGE2 abolished growth factor-induced activation of c-Raf-1. However, Ras activation induced by growth factors was not affected by PGE2 and forskolin. These results suggest that MAP kinase activation can occur by at least two separate pathways in mesangial cells. Tyrosine kinase receptors activate MAP kinase through activation of Ras and Raf. This pathway can be blocked by PGE2 and elevation of cAMP, presumably by interfering with the ability of Ras to activate Raf. In addition, activation of protein kinase C by phorbol esters can activate MAP kinase in a Ras/Raf-independent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of Ca2+/calmodulin-stimulated cyclic AMP phosphodiesterase (PDE1) activity in Chinese hamster ovary cells (CHO) by phorbol 12-myristate 13-acetate and by the selective overexpression of protein kinase C isoforms

The cAMP phosphodiesterase (PDE) activity of CHO cells was unaffected by the addition of Ca2+ +calmodulin (CaM), indicating the absence of any PDE1 (Ca2+/CaM-stimulated PDE) activity. Treatment with the tumour promoting phorbol ester phorbol 12-myristate 13-acetate (PMA) led to the rapid transient induction of PDE1 activity which attained a maximum value after about 13 h before slowly decreasing. Such induction was attenuated by actinomycin D. PCR primers were designed to hybridize with two regions identified as being characteristic of PDE1 forms found in various species and predicted to amplify a 601 bp fragment. RT-PCR using degenerate primers allowed an approx. 600 bp fragment to be amplified from RNA preparations of rat brain but not from CHO cells unless they had been treated with PMA. CHO cells transfected to overexpress protein kinase C (PKC)-alpha and PKC-epsilon, but not those transfected to overexpress PKC-beta I or PKC-gamma, exhibited a twofold higher PDE activity. They also expressed a PDE1 activity, with Ca2+/CaM effecting a 1.8-2.8-fold increase in total PDE activity. RT-PCR, with PDE1-specific primers, identified an approx. 600 bp product in CHO cells transfected to overexpress PKC-alpha and PKC-epsilon, but not in those overexpressing PKC-beta I or PKC-gamma. Treatment of PKC-alpha transfected cells with PMA caused a rapid, albeit transient, increase in PDE1 activity, which reached a maximum some 1 h after PMA challenge, before returning to resting levels some 2 h later. The residual isobutylmethylxanthine (IBMX)-insensitive PDE activity was dramatically reduced (approx. 4-fold) in the PKC-gamma transfectants, suggesting that the activity of the cyclic AMP-specific IBMX-insensitive PDE7 activity was selectively reduced by overexpression of this particular PKC isoform. These data identify a novel point of 'cross-talk' between the lipid and cyclic AMP signalling systems where the action of specific PKC isoforms is shown to cause the induction of Ca2+/CaM-stimulated PDE (PDE1) activity. It is suggested that this protein kinase C-mediated process might involve regulation of PDE1 gene expression by the AP-1 (fos/jun) system.

Evidence for a bifurcation of the mitogenic signaling pathway activated by Ras and phosphatidylcholine-hydrolyzing phospholipase C

NIH 3T3 cells stably transfected with the gene encoding phosphatidylcholine-hydrolyzing phospholipase C (PC-PLC) from Bacillus cereus display a chronic elevation of intracellular diacylglycerol levels and a transformed phenotype. We have used such PC-PLC-transformed cells to evaluate the roles of the cytoplasmic serine/threonine kinases Raf-1, zeta protein kinase C (zeta PKC) and protein kinase A (PKA) in oncogenesis and mitogenic signal transduction elicited by phosphatidylcholine hydrolysis. We demonstrate here that stable expression of dominant negative mutants of both zeta PKC and Raf-1 lead to reversion of PC-PLC-transformed cells. Interestingly, expression of kinase defective zeta PKC also reverted NIH 3T3 cells transformed by the v-Ha-ras oncogene. Activation of PKA in response to elevation of cAMP levels also lead to reversion of PC-PLC-induced transformation, implicating PKA as a negative regulator acting downstream of PC-PLC. On the other hand, inhibition or depletion of phorbol ester responsive PKCs attenuated but did not block the ability of PC-PLC-transformed cells to induce DNA synthesis in the absence of growth factors. These results clearly implicate both Raf-1 and zeta PKC as necessary downstream components for transduction of the mitogenic/oncogenic signal generated by PLC-mediated hydrolysis of phosphatidylcholine and suggest, together with other recent evidence, a bifurcation in the signaling pathway downstream of PC-PLC.

Dissociation of cAMP-stimulated mitogenesis from activation of the mitogen-activated protein kinase cascade in Swiss 3T3 cells

Elevation of intracellular cAMP by forskolin, 8-bromoadenosine 3':5'-cyclic monophosphate, and prostaglandin E1, in synergy with insulin, stimulated DNA synthesis in quiescent Swiss 3T3 cells to the same level achieved by platelet-derived growth factor (PDGF) or bombesin. Both forskolin and 8-bromoadenosine 3':5'-cyclic monophosphate stimulated a significant increase in cell number which, in the presence of insulin, reached the same levels achieved with PDGF. Treatment with either PDGF or bombesin caused a marked and persistent stimulation of p42MAPK and p44MAPK. In striking contrast, no activation was seen with mitogenic combinations of cAMP as shown by three different assays. Swiss 3T3 cells stably transfected with a constitutively activated Gs alpha subunit were 100-fold more sensitive to the mitogenic effects of forskolin but in this distinct cellular model forskolin did not activate p42MAPK. Swiss 3T3 cells stably transfected with interfering mutants of MEK-1 showed a 60% decrease in PDGF-stimulated p42 MAPK activation, but there was no inhibition of the mitogenic effect of forskolin in these cells. Furthermore, the upstream kinases MEK-1/MEK-2 and p74raf-1 were not activated by mitogenic combinations of cAMP while PDGF caused marked stimulation of their activity. Treatment of 3T3 cells with forskolin attenuated PDGF-stimulated p74raf-1 and p42MAPK activation but enhanced the mitogenic effects of this agent. Mitogenic combinations of cAMP strongly stimulated the phosphorylation and activation of p70s6k an effect that was inhibited by rapamycin. This agent markedly inhibited cAMP-stimulated DNA synthesis suggesting a critical role for p70s6k in cAMP mitogenic signaling. These results demonstrate that cAMP-induced mitogenesis can be dissociated from activation of the mitogen-activated protein kinase cascade and that this is not an obligatory point of convergence in mitogenic signaling in Swiss 3T3 cells.

Distinct characteristics of the basal activities of adenylyl cyclases 2 and 6

Regulation of basal activities of adenylyl cyclase (AC) 2 and 6, expressed in Sf9 cells by infection with recombinant baculovirus, was studied. An antipeptide antibody that recognizes AC2 and AC6 with equal sensitivity was used to establish that equivalent levels were expressed. Basal activities of AC2 and AC6 were compared at varying concentrations of Mg2+ or Mn2+ ions; AC2 had 15- and 10-fold greater activity than AC6, respectively. At 20 mM Mg2+, the Km values for ATP were 88 and 39 microM for AC2 and AC6, respectively, whereas their Vmax values were 281 and 11 pmol/mg protein.min. With 100 microM forskolin and either Mg2+ or Mn2+, the difference in activities between AC2 and AC6 was reduced to approximately 2-fold. Forskolin stimulated AC6 greater than 40-fold at 0.5-2 mM Mg2+, whereas AC2 was stimulated 4-6-fold. At 20 mM Mg2+, AC2 was stimulated 2-fold by forskolin, whereas AC6 was stimulated 18-fold. With Mg2+ alone, activities of AC2 and AC6 were not saturable up to 20 mM and yielded curvilinear Hofstee transformations. With forskolin, activities of both AC2 and AC6 were saturable by 10 mM Mg2+ and yielded linear Hofstee transformations. These data indicate that there are substantial differences in the basal enzymatic activities of adenylyl cyclase isoforms, due to differential regulation by Mg2+ ions rather than intrinsic catalytic capabilities. Thus the presence and relative abundance of adenylyl cyclase subtypes could greatly affect the resting cellular cAMP levels with consequent effects on important biological functions, such as differentiation and proliferation.

Platelet-derived growth factor. Distinct signal transduction pathways associated with migration versus proliferation

Figure 2 summarizes our current interpretation of data concerning signals from the activated PDGF receptor involved in directed migration and proliferation of human arterial SMC. Binding of PDGF (PDGF-BB or PDGF-AA) causes PDGF-receptor dimerization, tyrosine autophosphorylation, and subsequent binding of several molecules containing SH2 domains to the activated receptor. Binding and activation of PLC gamma by the PDGF receptor leads to PIP2 hydrolysis, resulting in generation of diacylglycerol (DAG) and IP3. Subsequently, intracellular levels of calcium are elevated as a result of IP3-mediated calcium release from intracellular compartments. The decreased levels of PIP2 and increased levels of calcium both favor actin-filament disassembly by inducing capping of actin-filament barbed ends and actin-monomer sequestration. A localized, and transient, actin-filament disassembly enables the cell to extend filopodia towards PDGF, thereby enabling chemotaxis to take place. At a later time and/or in a different compartment, actin-filament assembly is promoted by PDGF by a mechanism that is not completely understood, but that may involve small GTP-binding proteins, such as Rho, and formation of DAG. Migration on collagen requires functional alpha 2 beta 1 integrins, which may either constitute a permissive state required for a cell to migrate, or which may be actively involved in intracellular signals leading to migration. PDGF-induced DNA synthesis and proliferation involves activation of Ras, MAP kinase kinase, and MAP kinase. Cross-talk between PKA signaling and tyrosine-kinase receptor signaling results in PKA inhibition of the MAP kinase cascade, probably at the level of Raf. Activation of PI 3-kinase, or a PI 3-kinase-like enzyme, is also likely to contribute to the mitogenic effects of PDGF in these cells (Bornfeldt, unpublished observation). What determines if a SMC will migrate and/or proliferate in response to PDGF? Results are starting to emerge that show regulation of expression of molecules involved in intracellular signaling with different phenotypic states of SMC. For example, expression of PLC gamma is very low in intact vascular wall (where SMC show a "contractile phenotype"), and induced when SMC are converted to a "synthetic phenotype" in culture. Proliferation and expression of MAP kinase, but not calcium signaling, appear to be regulated by the extracellular matrix, and the profile of integrin expression is different in SMC in culture compared to SMC in the vascular wall. Thus, the relation between expression of signaling molecules involved in migration and signaling molecules involved in proliferation, as well as cross-talk between different signal-transduction pathways, may determine the net effect of PDGF.

PC12-E2 cells: a stable variant with altered responses to growth factor stimulation

A variant cell line, designated E2, characterized by more rapid responses to nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) and markedly more robust responses to interleukin-6 and 8-Br-cAMP, has been subcloned from the rat PC12 cell line. The enhanced responsiveness to NGF in E2 cells is not due to receptor overexpression as judged by TrkA protein levels and tyrosine kinase activity, but may be associated with the increased and prolonged tyrosine phosphorylation of ERK1 (extracellular signal regulated kinase 1) and ERK2. The rapid morphological differentiation induced by different growth factors in E2 cells is mediated in a transcription-independent manner suggesting that E2 cells may constitutively express some differentiation-associated molecules that allow direct entry into the neuronal program.

A molecular model for bipolar affective disorder

The biological basis of bipolar disorder is not known. Models for the illness have been proposed that were based on the neurobiological effects of pharmacological agents that affect mood. Although of great interest, these models have not adequately explained the striking clinical pattern of illness in which patients may experience either unipolar episodes or bipolar cycles of mania and depression. We now present a new model suggesting that the unique clinical heterogeneity found in patients with bipolar disorder could be explained by a defect in a 'downstream' portion of a signal transduction pathway that can regulate two or more neurotransmitter systems that have opposite effects on neuronal activity. This model may target specific candidate genes for involvement in bipolar disorder.

AVP inhibits EGF-stimulated MAP kinase cascade in Madin-Darby canine kidney cells

We investigated the effects of epidermal growth factor (EGF) and arginine vasopressin (AVP) on Raf-1-MAP kinase cascade, including Raf-1-kinase (Raf-1-K), MAP kinase kinase (MAPKK), MAP kinase (MAPK) and S6 kinase (S6K) in Madin-Darby canine kidney (MDCK) cells. In a dose-dependent manner (10(-10) M to 10(-6) M), EGF increased autophosphorylation of Raf-1-K and activated MAPKK, MAPK and S6K. Sequential activation of these kinases was indicated by their peak times of activation (Raf-1-K 5 min; MAPKK 10 min; MAPK 15 min; and S6K 30 min). AVP (10(-9) M to 10(-6) M) inhibited EGF-stimulated MAP kinase cascade. 8-Bromo-cyclic AMP (cAMP) could mimic the inhibitory effect of AVP on EGF-stimulated MAP kinase cascade. These results were confirmed using H-89, an inhibitor of protein kinase A (PKA) that blocked the effect of AVP on EGF-stimulated MAPK activity. We conclude that AVP inhibits EGF-stimulated Raf-1-K, MAPKK, MAPK, and S6K activity via cAMP in MDCK cells. Our results indicate that MAP kinase cascade may play an important role in integrating the effects of AVP and EGF on distal tubule function.

Mitogen-activated protein kinase activation requires two signal inputs from the human anaphylatoxin C5a receptor

The anaphylatoxin C5a receptor activates the Ras/Raf/mitogen-activated protein (MAP) kinase pathway in human neutrophils. The signal pathways involved in Ras/Raf/MAP kinase activation in response to C5a and other chemoattractant receptors is poorly understood. Stimulation of the C5a receptor expressed in HEK293 cells results in modest MAP kinase activation, which is inhibited by pertussis toxin-catalyzed ADP-ribosylation of G(i). Coexpression of the C5a receptor and the G16 alpha subunit (alpha 16) results in the G16-mediated activation of phospholipase C beta and a robust MAP kinase activation. Pertussis toxin treatment of C5a receptor/alpha 16-cotransfected cells inhibits C5a stimulation of MAP kinase activity approximately 60% relative to the control response. Similarly, the protein kinase C inhibitor, GF109203X inhibits activation of MAP kinase activation in C5a receptor/alpha 16-cotransfected cells by 60%; the protein kinase C inhibitor does not affect the modest C5a receptor response in the absence of alpha 16 expression. These results demonstrate that two independent signals are required for the maximal activation of MAP kinase by G protein-coupled receptors.

Histamine antagonizes serotonin and growth factor-induced mitogen-activated protein kinase activation in bovine tracheal smooth muscle cells

We examined the effects of the bronchoconstrictor agonists serotonin (5-hydroxytryptamine; 5-HT) and histamine on mitogen-activated protein (MAP) kinase activation in cultured bovine tracheal myocytes. Kinase renaturation assays demonstrated activation of the 42- and 44-kDa MAP kinases within 2 min of 5-HT exposure. MAP kinase activation was mimicked by alpha-methyl-5-HT and reduced by pretreatment with either phorbol 12,13-dibutyrate or forskolin, suggesting activation of the 5-HT2 receptor, protein kinase C, and Raf-1, respectively. Raf-1 activation was confirmed by measurement of Raf-1 activity, and the requirement of Raf-1 for 5-HT-induced MAP kinase activation was demonstrated by transient transfection of cells with a dominant-negative allele of Raf-1. Histamine pretreatment significantly inhibited 5-HT and insulin-derived growth factor-1-induced MAP kinase activation. Attenuation of MAP kinase activation was reversed by cimetidine, mimicked by forskolin, and accompanied by cAMP accumulation and inhibition of Raf-1, suggesting activation of the H2 receptor and cAMP-dependent protein kinase A. However, histamine treatment inhibited Raf-1 but not MAP kinase activation following treatment with either platelet-derived growth factor or epidermal growth factor, implying a Raf-1-independent MAP kinase activation pathway. In summary, our data suggest a model whereby 5-HT activates MAP kinase via a protein kinase C/Raf-1 pathway, and histamine attenuates MAP kinase activation by serotonin via activation of cAMP-dependent protein kinase A and inhibition of Raf-1.

Effect of glucagon on insulin receptor substrate-1 (IRS-1) phosphorylation and association with phosphatidylinositol 3-kinase (PI 3-kinase)

In the present study we have examined the levels and phosphorylation state of the insulin receptor and insulin receptor substrate 1 (IRS-1) as well as the association between IRS-1 and phosphatidylinositol 3-kinase (PI 3-kinase) in the liver and muscle of rats treated with glucagon. There was a decrease in the insulin-stimulated receptor and IRS-1 phosphorylation levels which was paralleled by a reduced association between IRS-1 and PI 3-kinase in vivo in the liver and muscle of glucagon-treated rats. These observations suggest that glucagon, probably acting through cAMP, may impair insulin signaling in the three early steps in insulin action after binding.

Theophylline, a new inducer of apoptosis in B-CLL: role of cyclic nucleotides

We report a case of indolent B-chronic lymphocytic leukaemia (B-CLL) in a stage A patient, treated for 10 years only by theophylline for bronchial asthma. As suggested by the spontaneous apoptosis in the patient's blood (10%), theophylline at 50 micrograms/ml increased spontaneous apoptosis after 72 h in culture by a mean percentage of 90% (range 79-98%) in six B-CLL cases studied in vitro. This effect was partially reversed with Rp-cAMP, a cAMP antagonist, which implies a potent role for this second messenger. We describe a new property of theophylline, which might be an alternative treatment in B-CLL.

Ras-effector interactions, the problem of specificity

Ras plays the role of a molecular switch in many cellular signalling pathways. The Raf-kinase has been identified as the direct target molecule of Ras in mammalian cells. However, in recent reports other proteins have been characterised as putative Ras effectors which have neither a functional nor a structural relationship to each other. In addition it has been shown that also other members of the Ras family like Rap and R-Ras can interact with some of these proteins. To address the problem of specificity and of biological relevance of the interactions, they have to be carefully quantified and the cellular localisation of the proteins involved taken into account.

Differential effects on cAMP on the MAP kinase cascade: evidence for a cAMP-insensitive step that can bypass Raf-1

Because cAMP exerts opposite effects on cell proliferation in different cell types, we undertook to study its effect on the mitogen-activated protein kinase (MAPK) pathway in three cell lines (Rat-1, Swiss-3T3, and COS-7) chosen for their different mitogenic responses to cAMP. We measured the effect of cAMP on MAPK, MEK, and Raf-1 activities after stimulation by agonists acting through a tyrosine kinase receptor (epidermal growth factor) or a G protein-coupled receptor (lysophosphatidic acid). In Rat-1 cells we found that cAMP strongly inhibited all three activities (MAPK, MEK, and Raf-1), in good agreement with its effect on cell proliferation in these cells. In Swiss-3T3 and COS-7 cells, on the contrary, cAMP did not inhibit epidermal growth factor- and lysophosphatidic acid-induced stimulation of MAPK and MEK activities, and even stimulated MAPK activity slightly on its own. Again these results are in good agreement with the proliferative effect of cAMP in Swiss-3T3 cells. Raf-1 activity on the hand, was inhibited by cAMP in Swiss-3T3 and COS-7 as it was in Rat-1 cells. This result indicates that signaling pathways in Swiss-3T3 and COS-7 cells can activate MEK and MAPK in a Raf-1-independent and cAMP-insensitive manner. Our results add to growing evidence for the existence of Ras- and/or Raf-1-independent pathways leading to MEK and MAPK activation.

c-Jun N-terminal kinase but not mitogen-activated protein kinase is sensitive to cAMP inhibition in T lymphocytes

The molecular mechanism underlying the cAMP inhibition of nuclear activation events in T lymphocytes is unknown. Recently, the activation of fibroblasts and muscle cells are shown to be antagonized by cAMP through the inhibition of mitogen-activated protein (MAP) kinases signaling pathway. Whether a similar antagonism may account for the late inhibitory effect of cAMP in T cell was examined. Surprisingly, extracellular signal regulated kinase 2 (ERK1, ERK2, and ERK3) of MAP kinase were poorly inhibited by cAMP. High concentration of cAMP also only weakly antagonized Raf-1 in T cells. The resistance of ERK and Raf-1 to cAMP clearly distinguishes T cells from fibroblasts. In contrast, another MAP kinase homologue c-Jun N-terminal kinase (JNK) was inhibited by cAMP in good correlation with that of IL-2 suppression. Moreover, JNK was antagonized by a delayed kinetics which is characteristic of cAMP inhibition. Despite that both ERK and JNK are essential for T cell activation, selective inhibition by cAMP further supports the specific role of JNK in T cell activation.

Signal transduction by cell adhesion receptors

Over the last few years, it has become clear that cell adhesion receptors function in signal transduction processes leading to the regulation of cell growth and differentiation. Signal transduction by both integrins and CAMs has been shown to involve activation of tyrosine kinases, while CAM signaling in neural cells involves G proteins as well. In the case of integrins, some of the downstream signaling events intersect with the Ras pathway, particularly the activation of MAP kinases. In fibroblasts, integrin mediated anchorage to the substratum regulates cell cycle traverse, while in epithelial cells, loss of anchorage can trigger programmed cell death. In many cell types, but particularly monocytic cells, integrin ligation has a profound impact on gene expression. Preliminary evidence also implicates CAMs and selectins in gene regulation. A consistent theme in signal transduction mediated by adhesion receptors concerns the role of the cytoskeleton. Integrin mediated signaling processes are interrupted by cytoskeletal disassembly. Identification of the APC and neurofibromatosis type 2 tumor suppressors suggest that cytoskeletal complexes also play a key role in signaling by cadherins and CD44, respectively. Thus, signaling by cell adhesion receptors may involve aspects that impinge on previously known signaling pathways including the RTK/Ras pathway and serpentine receptor/G protein pathways. However, novel aspects of signal transduction involving cytoskeletal assemblies may also be critical.

Different developmental pattern of N-ras and unr gene expression in mouse gametogenic and somatic tissues

Accumulation of transcripts from N-ras and unr genes was comparatively analyzed during the development of germline and somatic tissues. Northern blots on fetal and postnatal samples from somatic tissues, including brain, skeletal muscle, liver, kidney, small intestine and heart were studied together with ovaries and testis. While the expression of N-ras was rather stable all along the development of the different tissues analyzed, the expression of unr exhibited a specific pattern in some tissues. Specifically, in testis, there is a developmental regulation of the relative accumulation of the three alternative transcripts. Unr has a relative high expression in testes and heart but the accumulation seems to be different for the different size transcripts in each case. However, the expression in small intestine is practically absent in adults. From the comparative analysis of the expression of both genes, N-ras and unr, we propose that the regulation of N-ras is not directly coordinated with unr expression during the development. However, the expression of unr and its alternative transcripts is developmentally and differentially regulated in small intestine, heart and testis. The change in the pattern of accumulation during testis development from long to small alternative transcripts, could be interpreted in terms of possible alleviation of transcription interference of N-ras.

Differential calcium dependence in the activation of c-Jun kinase and mitogen-activated protein kinase by muscarinic acetylcholine receptors in rat 1a cells

Carbachol stimulation of the muscarinic acetylcholine m1 receptor (m1R), stably expressed in Rat 1a fibroblasts, resulted in a calcium-dependent activation of c-Jun kinase (JNK). Stimulation of the muscarinic acetylcholine m2 receptor (m2R), stably expressed in Rat 1a fibroblasts, resulted in a G1-mediated activation of JNK that was weak relative to that observed with the m1R. Chelation of calcium inhibited the m2R-mediated activation of JNK but not the robust m2R stimulation of mitogen-activated protein kinase (MAPK) activity. These findings demonstrate a role for the second messenger, calcium, in the differential regulation of the activity of JNK and MAPK in Rat 1a cells.

The regulation of tyrosine kinase signalling pathways by growth factor and G-protein-coupled receptors

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Regulation of multidrug resistance through the cAMP and EGF signalling pathways

The development of cross-resistance to many natural product anticancer drugs, termed multidrug resistance (MDR), is a serious limitation to cancer chemotherapy. MDR is often associated with overexpression of the MDR1 gene product, P-glycoprotein, a multifunctional drug transporter. Understanding the mechanisms that regulate the transcriptional activation of MDR1 may afford a means of reducing or eliminating MDR. We have found that MDR1 expression can be modulated by type I cAMP-dependent protein kinase (PKA). This suggests that MDR may be modulated by selectively downregulating PKA activity to effect inhibition of PKA-dependent trans-activating factors which may be involved in MDR1 transcription. High levels of type I PKA occur in primary breast carcinomas and patients exhibiting this phenotype show decreased survival. The selective type I PKA inhibitors, 8-Cl-cAMP and Rp8-Cl-cAMP[S], may be particularly useful for downregulating PKA, and inhibit transient expression of a reporter gene under the control of MDR1 promoter elements. Thus, investigations of the signalling pathways involved in transcriptional regulation of MDR1 may lead to a greater understanding of the mechanisms governing the expression of MDR and provide a focus for pharmacological intervention.

Expression of GTPase-deficient Ras inhibits vasopressin signaling in cultured cortical collecting duct cells

Cross-talk between signaling pathways is increasingly recognized as integral to cellular function. We investigated whether the mitogen-activated protein kinase (MAPK) pathway alters vasopressin (AVP) stimulation of protein kinase A (PKA) by specifically studying the role of Ras. Mouse cortical collecting duct cells (M-1) were transfected with a cDNA encoding oncogenic Ras. Transfection was confirmed by Western blot analysis and functionally by enhanced basal MAPK activity. When compared with basal MAPK activity of 26.4 +/- 6.6 pmol/mg/min in controls, basal MAPK activity varied widely in Ras-transfected clones from 29.0 +/- 6.6 to 96.6 +/- 13.4 pmol/mg/min. Clones that functionally expressed activated Ras displayed complete abolition of AVP-stimulated PKA activity, whereas those that failed to express elevated basal MAPK activity showed intact AVP-stimulated PKA. The correlation between expression of high basal MAPK activity and inhibition of AVP-induced PKA yielded a correlation coefficient of -0.92 (P = 0.009). Exposure to 10 microM forskolin or 1 microgram/ml cholera toxin resulted in comparable activation of PKA in all clones. We found no correlation between PKC activity of the clones and PKA inhibition. To assess whether the observed effect was due to one known Ras target, cells were transfected with constitutively activated Raf. M-1 cells expressing activated Raf exhibited elevated MAPK activity. The Raf clones showed no impairment of AVP-stimulated PKA activity. We conclude that expression of activated Ras is inhibitory of AVP-induced PKA activation in the M-1 cortical collecting duct cell line at a site proximal to G alpha s protein. The failure of Raf to influence AVP signaling indicates that the action of Ras is through a pathway independent of this Ras target.

Type III cGMP-inhibited cyclic nucleotide phosphodiesterases (PDE3 gene family)

Seven different but related cyclic nucleotide phosphodiesterase (PDE) gene families have been identified. Type III cGMP-inhibited (cGI) PDEs, the PDE3 gene family, are found in many tissues. cGI PDEs exhibit a high affinity for both cAMP and cGMP, and are selectively and relatively specifically inhibited by certain agents which augment myocardial contractility, promote smooth muscle relaxation and inhibit platelet aggregation. Adipocyte, platelet, and hepatocyte cGI PDE activities are regulated by cAMP-dependent phosphorylation. Insulin-induced phosphorylation/activation of adipocyte and hepatocyte cGI PDEs is thought to be important in acute regulation of triglyceride and glycogen metabolism by insulin. Two distinct cGI PDE subfamilies, products of distinct but related genes, have been identified. They exhibit the domain structure common to PDEs with a carboxyterminal region, conserved catalytic domain and divergent regulatory domain. In their catalytic domains cGI PDEs contain a 44 amino acid insertion not found in other PDE families. The expression of cGIP1 and cGIP2 mRNAs differs in different rat tissues, suggesting distinct functions for the two cGI PDE subfamilies, i.e., cGIP1 in adipose tissue, liver, testis and cGIP2 in myocardium, platelets and smooth muscle.

Inhibitors of cyclic nucleotide phosphodiesterase isozymes type-III and type-IV suppress mitogenesis of rat mesangial cells

We studied interactions between the mitogen-activated protein kinase (MAPK) signalling pathway and cAMP-protein kinase (PKA) signaling pathway in regulation of mitogenesis of mesangial cells (MC) determined by [3H]thymidine incorporation, with or without added EGF. Forskolin or dibutyryl cAMP strongly (by 60-70%) inhibited [3H]thymidine incorporation into MC. Cilostamide, lixazinone or cilostazol selective inhibitors of cAMP-phosphodiesterase (PDE) isozyme PDE-III, inhibited mitogenesis to similar extent as forskolin and DBcAMP and activated in situ PKA, but without detectable increase in cAMP levels. Cilostamide and cilostazol were more than three times more effective at inhibiting mesangial mitogenesis than rolipram and denbufylline, inhibitors of isozyme PDE-IV, even though PDE-IV was two times more abundant in MC than was PDE-III. On the other hand, when incubated with forskolin, rolipram-enhanced cAMP accumulation was far greater (10-100x) than with cilostamide. EGF increased MAPK activity (+300%); PDE isozyme inhibitors which suppressed mitogenesis also inhibited MAPK. PDE isozyme inhibitors also suppressed PDGF-stimulated MC proliferation. We conclude that cAMP inhibits the mitogen-dependent MAPK-signaling pathway probably by decreasing the activity of Raf-1 due to PKA-catalyzed phosphorylation. Further, we surmise that minor increase in the cAMP pool metabolized by PDE-III is intimately related to regulation of mesangial proliferation. Thus, PDE isozyme inhibitors have the potential to suppress MC proliferation by a focused effect upon signaling pathways.

Dissociation of mitogen-activated protein kinase activation from the oxidative burst in differentiated HL-60 cells and human neutrophils

In human polymorphonuclear leukocytes (PMNs), mitogen-activated protein kinases (MAPKs), also known as extracellular signal-regulated kinases (Erks), are activated within minutes upon stimulation with either chemoattractant formyl-Met-Leu-Phe (fMLP) or phorbol 12-myristate 13-acetate (PMA). This activation of MAPKs coincides with the formation of superoxide anion, which occurs through the activation of a multiple-component NADPH oxidase pathway. MAPKs have thus been suggested to be involved in signal transduction leading to the oxidative burst. To investigate whether MAPK activation plays a central role in the oxidative burst, we evaluated the effect of cAMP on MAPK activation induced by fMLP and PMA. cAMP inhibits many PMN functional responses, including the oxidative burst, and has recently been shown to reduce growth factor- and PMA-induced MAPK activities in a variety of cells. We found that in differentiated, neutrophil-like HL-60 cells, while cAMP reduced PMA-induced MAPK activation, it had no effect on fMLP-induced MAPK activation. Despite the presence of unchanged levels of activated MAPKs, the fMLP-induced oxidative burst was substantially diminished by cAMP. By contrast, O2-production induced by PMA remained the same even though MAPK activation was inhibited. In PMNs, although the levels of O2-induced by either 10 ng/ml or 100 ng/ml PMA were similar, only 100 ng/ml could stimulate MAPK activation, suggesting that the oxidative burst could occur in the absence of detectable activation of MAPKs. As in HL-60 cells, cAMP inhibited the O2-production in fMLP-stimulated PMNs but had no effect on MAPK activity. These results demonstrate that, while MAPK activation coincides with PMN activation, it can be dissociated from the oxidative burst.

Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase gene expression in FRTL-5 cells. II. Down-regulation by v-K-ras oncogene

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity and mRNA levels were significantly reduced in FRTL-5 cells transformed with the Kirsten-Moloney sarcoma virus (KiMol); these cells have lost thyrotropin dependence and express high levels of p21ras. FRTL-5 cells, transformed with a temperature-sensitive mutant of the v-K-ras oncogene (Ats cells: 33 degrees C, permissive; 39 degrees C, nonpermissive), showed significant reduction of HMG-CoA reductase expression when exposed to 33 degrees C. In KiMol cells, as well as in Ats cells at 33 degrees C, the transcription driven by cAMP-responsive element was probed by measuring chloramphenicol acetyl transferase (CAT) levels after transfection with a chimeric plasmid containing the reporter gene linked to the rat reductase promoter. Basal CAT activity in KiMol cells transfected with wild-type promoter was lower than in FRTL-5 cells but was increased by forskolin to the levels attained in thyrotropin-stimulated FRTL-5 cells. Forskolin failed to increase CAT activity in KiMol cells transfected with the plasmid harboring a reductase promoter in which the cAMP-responsive element octamer was mutated to a nonpalindromic sequence. The effect of v-K-ras could be mimicked in FRTL-5 cells by tetradecanoyl phorbol acetate and reverted in KiMol and Ats cells, expressing active Ras protein, by increasing intracellular cAMP and/or by protein kinase C inhibition. The data are consistent with the contention that v-K-ras, through protein kinase C and depletion of intracellular cAMP, is inhibitory for the protein kinase A pathway. This is the first demonstration that active v-K-ras down-regulates HMG-CoA reductase expression.

A direct interaction between G-protein beta gamma subunits and the Raf-1 protein kinase

Raf-1 is a serine/threonine protein kinase positioned downstream of Ras in the mitogen-activated protein kinase cascade. Using a yeast two-hybrid strategy to identify other proteins that interact with and potentially regulate Raf-1, we isolated a clone encoding the carboxyl-terminal half of the G beta 2 subunit of heterotrimeric G-proteins. In vitro, purified G beta gamma subunits specifically bound to a GST fusion protein encoding amino acids 1-330 of Raf-1 (Raf/330). Binding assays with truncation mutants of GST-Raf indicate that the region located between amino acids 136 and 239 is a primary determinant for interaction with G beta gamma. In competition experiments, the carboxyl terminus of beta-adrenergic receptor kinase (beta ARK) blocked the binding of G beta gamma to Raf/330; however, the Raf-1-binding proteins, Ras and 14-3-3, had no effect. Scatchard analysis of in vitro binding between Raf/330 and G beta gamma revealed an affinity of interaction (Kd = 163 +/- 36 nM), similar to that seen between G beta gamma and beta ARK (Kd = 87 +/- 24 nM). The formation of native heterotrimeric G alpha beta gamma complexes, as measured by pertussis toxin ADP-ribosylation of G alpha, could be disrupted by increasing amounts of Raf/330, with an EC50 of approximately 200 nM, in close agreement with the estimated binding affinity. In vivo complexes of Raf-1 and G beta gamma were isolated from human embryonic kidney 293-T cells transfected with epitope-tagged G beta 2. The identification and characterization of this novel interaction raises several possibilities for signaling cross-talk between growth factor receptors and those receptors coupled to heterotrimeric G-proteins.

Salbutamol inhibits the proliferation of human airway smooth muscle cells grown in culture: relationship to elevated cAMP levels

The link between increased usage of beta-adrenoceptor agonists and worsening of asthma symptoms has raised interest in the effects of agents such as salbutamol on airway wall remodelling, and particularly airway smooth muscle proliferation. In the present study we have investigated the role of increases in intracellular cAMP in the inhibitory effect of salbutamol on airway smooth muscle proliferation. The inhibitory effects of a combination of submaximally effective concentrations of salbutamol (10 nM) and the non-selective phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX, 100 microM) on thrombin (0.3 U/mL)-induced mitogenesis in human cultured airway smooth muscle cells was greater than that for either agent alone. In addition, agents known to increase cAMP-dependent protein kinase activity including forskolin (10 microM), 8-bromoadenosine-3',5'-cyclic monophosphate (100 microM), and prostaglandin E2 (1 microM) have an inhibitory effect on thrombin (0.3 U/mL)-induced induced proliferation. Furthermore, the cAMP antagonist, 8-bromoadenosine-3',5'-cyclic monophosphorothioate, Rp-isomer (300 microM) significantly reduced the inhibitory effect of salbutamol (10 nM) on thrombin (0.3 U/mL)-induced DNA synthesis. In IBMX (100 microM)-pretreated cells, salbutamol (100 nM) increased intracellular cAMP levels via stimulation of a beta 2-adrenoceptor. Salbutamol (10 microM), at concentrations supramaximally effective for inhibition of mitogenesis, had no effect on thrombin (0.3 U/mL)-induced increases in intracellular calcium levels. Therefore, our results suggest that the previously reported inhibition of mitogen-induced proliferation in human cultured airway smooth muscle cells by the beta 2-adrenoceptor agonist, salbutamol (100 nM), is at least partly due to elevation of intracellular cAMP, while there is no effect of salbutamol on initial mitogen-induced increases in intracellular calcium.

Influence of cell type on the steroidogenic potential and basal cyclic AMP levels of ras-oncogene-transformed rat cells

Transformation with ras oncogenes causes loss, maintenance or modulation of differentiation, depending on the developmental history of the target cells. In the present study, we examined steps in signal transduction that may underlie some of this variation, using steroidogenic cells of adult rats as the model system. Steroidogenesis in normal cells is regulated by cyclic AMP and protein kinase A (the cAMP/PKA pathway). We showed previously that transformation with v-Ki-ras induces constitutive progesterone secretion in ovarian and adrenocortical cells that are normally steroidogenic (ovarian granulosa and adrenal glomerulosa cells) and also in developmentally related cells that are normally nonsteroidogenic (ovarian surface epithelium and adrenal capsular fibroblasts), but not in unrelated nonsteroidogenic cells, such as muscle fascia fibroblasts and peritoneal mesothelium. In the present study, basal cAMP levels in all transformed ovarian and adrenal cell-lines were increased over basal levels in normal cells, and of transformed muscle fascia and mesothelial cell-lines. As in normal cells, transformation-induced steroidogenesis was stimulated by cAMP and was PKA dependent. A comparison of malignancy-related characteristics showed that transformed cells from nonsteroidogenic organs were more tumorigenic in vivo and less sensitive to growth inhibition by cAMP in vitro than transformed ovarian and adrenocortical cells. The results show that the abnormal, constitutive steroidogenesis induced by the viral form of the Kirsten ras oncogene (v-Ki-ras) in certain cell types is associated with tissue-specific increases in basal cAMP levels. Thus, although the ras oncogenes function primarily through other signal transduction pathways, transformation with ras oncogenes alters PKA-mediated signal transduction in a manner that is developmentally determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression of endothelin-1-induced mitogenic responses of human aortic smooth muscle cells by interleukin-1 beta

When applied to quiescent human aortic smooth muscle cells (AOSMC), endothelin-1 (ET-1) caused significant increases in mitogen-activated protein kinase (MAPK) activity, [3H]thymidine incorporation, and cell proliferation, confirming an activity of ET-1 as a potent mitogen on AOSMC. As an in vitro model to evaluate the significance of the mitogenic activity of ET-1 on smooth muscle cells during atherogenesis, we studied possible modulations of the responsiveness of the cells by treatment with various cytokines (IL-1 beta, IL-8, TNF alpha, and TGF beta). Of the four cytokines tested, we found that the treatment of the cells with IL-1 beta dramatically reduced the responsiveness of the cells to ET-1; IL-1 beta treatment at the concentration of 0.2 ng/ml for 8 h completely abolished the activity of ET-1 to induce the mitogenic responses. IL-1 beta treatment caused no changes in the responses induced by EGF, basic fibroblast growth factor, or PDGF. Studies on ET-1-induced intracellular signaling events in IL-1 beta-treated cells revealed that the failure of ET-1 to induce mitogenic responses was due to an increase in cAMP formation secondary to ET-1-induced activation of prostanoid metabolism. These findings on AOSMC in vitro raise the possibility that, under some inflammatory conditions in vivo, ETs may work as a negative modulator of smooth muscle cell proliferation.

Adenosine and adenine nucleotides inhibit the autonomous and epidermal growth factor-mediated proliferation of cultured human keratinocytes

Previous investigations have shown disparate effects of adenine nucleotides on epidermal cell proliferation. Our present study demonstrates that adenosine and its related nucleotides (ATP, ADP, AMP) are antiproliferative for normal human epidermal keratinocytes cultured in the absence or presence of exogenous epidermal growth factor. Furthermore, the inhibitory effects of these compounds occur at concentrations less than 100 microM, are reversible, and do not affect the viability of the keratinocyte cultures. Our current investigation also demonstrates that both selective and nonselective adenosine receptor agonists are themselves approximately as potent as keratinocyte proliferation inhibitors, but are all less potent inhibitors than adenosine. These observations are consistent with the theory that adenosine mediates its antiproliferative response via a novel or more poorly characterized adenosine purinoreceptor subclass. Moreover, our present study demonstrates that ATP and ATP-gamma-S are significantly more potent antiproliferative agents than either alpha,beta-methylene ATP or beta,gamma-methylene ATP. Based on previous studies that have demonstrated that P2y purinoreceptors possess this type of ligand specificity and that the P2y purinoreceptor may be expressed by keratinocyte cultures, we propose that ATP may mediate its antiproliferative effects via this purinoreceptor. Collectively, our results indicate that adenosine and adenine nucleotides abrogate exogenous epidermal growth factor-dependent and -independent keratinocyte proliferation at submillimolar concentrations and may be important physiologic regulators of keratinocyte growth in vivo. Further, these results suggest that these or related compounds may have application as treatments for epidermal growth factor receptor-signaling pathway has been activated.

Molecular cloning of a novel splice variant of human type IVA (PDE-IVA) cyclic AMP phosphodiesterase and localization of the gene to the p13.2-q12 region of human chromosome 19 [corrected]

We have isolated from a human T-cell Jurkat cDNA library a novel human cDNA (2EL) that is closely related to the human type-IV PDE splice variant family 'A' (PDE-IVA) cDNA characterized previously by us [Sullivan, Egerton, Shakur, Marquardsen and Houslay (1994) Cell. Signalling 6, 793-812]; (h6.1, PDE-IVA/h6.1; HSPDE4A7). (PDE stands for cyclic nucleotide phosphodiesterase). The novel cDNA 2EL (PDE-IVA/2EL; HSPDE4A8) contains two regions of unique sequence not found in PDE-IVA/h6.1. These are a distinct 5'-end and a 34 bp insert which occurs within a domain thought to encode the type-IV PDE catalytic site and which can be expected to result in premature truncation of any expressed protein. HSPDE4A8 appeared to be catalytically inactive. Isolation and characterization of a human genomic cosmid clone revealed that 2EL and h6.1 represent alternative splice variants of the human PDE-IVA gene. Using a unique sequence found at the 5'-end of the 2EL cDNA, a probe was generated which was used to screen the DNA of human-hamster hybrids. This located the human gene for PDE-IVA to human chromosome 19. Through both the analysis of genomic DNAs from a human-hamster somatic cell hybrid panel and also using fluorescent in situ hybridization, it was shown that the human PDE-IVA gene is located on human chromosome 19, between p13.2 [corrected] and q12. This region on chromosome 19 has been shown to be related to genetic diseases such as the autosomal dominant cerebrovascular disease CADASIL, susceptibility to late-onset Alzheimer's disease and changes seen in benign pituitary and thyroid adenomas.

Relationship of growth stimulated by lithium, estradiol, and EGF to phospholipase C activity in MCF-7 human breast cancer cells

Lithium-stimulated MCF-7 cell proliferation was compared to proliferation stimulated by other mitogens for this cell line-estradiol (E2) and epidermal growth factor (EGF)-and lithium was found to be effective within a narrow concentration range. Mitogenic effects of lithium on proliferation stimulated by E2 and EGF were additive below maximum, but were not synergistic. The phosphoinositide pathway is a cell signaling system involved in cell proliferation, within which phospholipase C (PLC)-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] leads to the production of the second messengers inositol-1,4,5-trisphosphate [Ins(1,4,5)P3] and diacylglycerol (DAG), as well as to calcium mobilization. At mitogen concentrations which maximally stimulated cell growth, estradiol stimulated both growth and PLC activity, while EGF and lithium stimulated cell growth but had little effect on the activity of the enzyme. Dose-responses with EGF revealed that a low concentration (0.1 ng/ml, 0.017 nM) of EGF appeared to stimulate both PLC activity and cell growth, but that higher concentrations of EGF which stimulated greater proliferation inhibited PLC activity. Steady-state levels of inositol phosphates including inositol trisphosphate were increased by all three mitogens. In growth assays, the phorbol ester phorbol 12-myristate-13-acetate (PMA), which mimics the actions of DAG, stimulated some cell growth, but dioctanoylglycerol, an additional DAG analog, and the calcium ionophore A23187, alone or with the DAG analogs, had no effect. These results suggest that PLC-mediated PtdIns(4,5)P2 hydrolysis is not primarily associated with signaling proliferation by lithium or EGF in MCF-7 breast cancer cells.

K252a-potentiation of EGF-induced neurite outgrowth from PC12 cells is not mimicked or blocked by other protein kinase activators or inhibitors

Epidermal growth factor (EGF) has recently been shown to cause certain strains of PC12 cells to extend short neurites. This EGF-induced differentiation of PC12 was found to be potentiated by the protein kinase inhibitor, K252a, in that PC12 cells treated with both EGF and K252a extended long branched neurites similar to those induced by nerve growth factor (NGF). As reported here no other protein kinase inhibitor or activator mimicked or blocked the effect of K252a on EGF-induced PC12 differentiation. Cyclic adenosine 3',5'-monophosphate (cAMP) also potentiated EGF-induced neurite outgrowth from PC12 cells, but the mechanism of this potentiation was different from that of K252a. Cells that had been exposed to EGF and then stripped of their neurons extended neurites again when retreated with EGF in the absence of RNA synthesis or when treated with NGF in the absence of RNA synthesis. Thus EGF can prime PC12 cells for either EGF or for NGF, a finding that further suggests that EGF and NGF use similar signaling pathways to induced neuronal differentiation of PC12.

Cyclic AMP-dependent activation of Rap1b

Rap1 proteins belong to the Ras superfamily of small molecular weight GTP-binding proteins. Although Rap1 and Ras share approximately 50% overall amino acid sequence identity, the effector domains of the two proteins are identical, suggesting either similar or antagonistic signaling roles. Several pathways leading to Ras activation have been defined, including those initiated by agonist binding to tyrosine kinase or Gi-coupled receptors. Nothing is known about such events for Rap1 proteins. The cAMP-mediated inhibition of Ras-dependent MAP kinase activation is well documented and resembles that caused by expression of GTPase-deficient Rap1. We have developed a system whereby signals leading to Rap1b activation, i.e. an increase in Rap1b-bound GTP/GDP ratio, can be measured. We report here that treatment of cells with agents that elevate intracellular cAMP levels result in Rap1b activation. These results demonstrate for the first time agonist-dependent activation of Rap1 proteins.

Okadaic acid, cAMP, and selected nutrients inhibit hepatocyte proliferation at different stages in G1: modulation of the cAMP effect by phosphatase inhibitors and nutrients

The protein phosphatase inhibitor okadaic acid (> 100 nM) caused an abrupt and complete cessation of primary rat hepatocyte cell cycle progression at the restriction point in late G1. A decline in the G1/S transition rate was observed in response to elevated cAMP, excess selected nutrients, and okadaic acid (< 100 nM). Excess nutrients (40 mM glucose +/- 5 mM dihydroxyacetone) acted by imposing an incomplete block in early G1. The cAMP action was potentiated by the phosphatase inhibitor microcystin, which in itself did not affect DNA replication. This suggests that cAMP acted by phosphorylating substrate(s) that is dephosphorylated by a microcystin-sensitive phosphatase. The additive effects of submaximal concentrations of okadaic acid and cAMP analogs indicated that okadaic acid and cAMP acted via different pathways. In conclusion, okadaic acid, cAMP, and excess nutrients, acting through distinct pathways, inhibited hepatocytes in different parts of the G1 phase.

Compartmentalization of autocrine signal transduction pathways in Sis-transformed NIH 3T3 cells

The transforming protein of simian sarcoma virus is homologous to the platelet-derived growth factor (PDGF) B-chain. Fibroblasts transformed with simian sarcoma virus constitutively produce a growth factor that stimulates the endogenous tyrosine kinase of PDGF receptors in an autocrine manner. Autophosphorylation of PDGF receptors upon ligand stimulation provides binding sites for Src homology 2 domains of intracellular signaling molecules, which thereby become activated. We have characterized the PDGF receptor-mediated signal transduction in NIH 3T3 cells transformed with a PDGF B-chain cDNA (Sis 3T3 cells) in the absence and presence of suramin, a polyanionic compound that quenches PDGF-induced mitogenicity and reverts the transformed phenotype of the Sis 3T3 cells. Our data show that in the presence of suramin the general level of tyrosine phosphorylation was decreased. Nevertheless, autophosphorylated receptors complexed with substrates persisted in the cells. Suramin had no effect on activation of phosphatidylinositol 3'-kinase or on tyrosine phosphorylation of phospholipase C-gamma and GTPase-activating protein of Ras. On the other hand, kinase activation of Src and Raf-1, phosphorylation of protein-tyrosine phosphatase 1D/Syp and Shc, and complex formation with Grb2 were greatly diminished by suramin. A possible explanation for our findings is that different PDGF receptor-coupled signaling pathways are active in different structural or functional compartments in the cell. Those pathways that are not affected by suramin might elicit distinct cellular responses, which are not sufficient for growth and transformation.

cAMP-dependent protein kinase is necessary for increased NF-E2.DNA complex formation during erythroleukemia cell differentiation

When murine erythroleukemia (MEL) cells are induced to differentiate by hexamethylene bisacetamide (HMBA), erythroid-specific genes are transcriptionally activated; however, transcriptional activation of these genes is severely impaired in cAMP-dependent protein kinase (protein kinase A)-deficient MEL cells. The transcription factor NF-E2, composed of a 45-kDa (p45) and an 18-kDa (p18) subunit, is essential for enhancer activity of the globin locus control regions (LCRs). DNA binding of NF-E2 and alpha-globin LCR enhancer activity was significantly less in HMBA-treated protein kinase A-deficient cells compared to cells containing normal protein kinase A activity; DNA binding of several other transcription factors was the same in both cell types. In parental cells, HMBA treatment and/or prolonged activation of protein kinase A increased the amount of NF-E2.DNA complexes without change in DNA binding affinity; the expression of p45 and p18 was the same under all conditions. p45 and p18 were phosphorylated by protein kinase A in vitro, but the phosphorylation did not affect NF-E2.DNA complexes, suggesting that protein kinase A regulates NF-E2.DNA complex formation indirectly, e.g. by altering expression of a regulatory factor(s). Thus, protein kinase A appears to be necessary for increased NF-E2.DNA complex formation during differentiation of MEL cells and may influence erythroid-specific gene expression through this mechanism.