Cyclic AMP-dependent activation of Rap1b

Epac as a tractable therapeutic target

In 1957, cyclic adenosine monophosphate (cAMP) was identified as the first secondary messenger, and the first signaling cascade discovered was the cAMP-protein kinase A (PKA) pathway. Since then, cAMP has received increasing attention given its multitude of actions. Not long ago, a new cAMP effector named exchange protein directly activated by cAMP (Epac) emerged as a critical mediator of cAMP's actions. Epac mediates a plethora of pathophysiologic processes and contributes to the pathogenesis of several diseases such as cancer, cardiovascular disease, diabetes, lung fibrosis, neurological disorders, and others. These findings strongly underscore the potential of Epac as a tractable therapeutic target. In this context, Epac modulators seem to possess unique characteristics and advantages and hold the promise of providing more efficacious treatments for a wide array of diseases. This paper provides an in-depth dissection and analysis of Epac structure, distribution, subcellular compartmentalization, and signaling mechanisms. We elaborate on how these characteristics can be utilized to design specific, efficient, and safe Epac agonists and antagonists that can be incorporated into future pharmacotherapeutics. In addition, we provide a detailed portfolio for specific Epac modulators highlighting their discovery, advantages, potential concerns, and utilization in the context of clinical disease entities.

Rap1 controls epiblast morphogenesis in sync with the pluripotency states transition

The complex architecture of the murine fetus originates from a simple ball of pluripotent epiblast cells, which initiate morphogenesis upon implantation. In turn, this establishes an intermediate state of tissue-scale organization of the embryonic lineage in the form of an epithelial monolayer, where patterning signals delineate the body plan. However, how this major morphogenetic process is orchestrated on a cellular level and synchronized with the developmental progression of the epiblast is still obscure. Here, we identified that the small GTPase Rap1 plays a critical role in reshaping the pluripotent lineage. We found that Rap1 activity is controlled via Oct4/Esrrb input and is required for the transmission of polarization cues, which enables the de novo epithelialization and formation of tricellular junctions in the epiblast. Thus, Rap1 acts as a molecular switch that coordinates the morphogenetic program in the embryonic lineage, in sync with the cellular states of pluripotency.

CAP1 binds and activates adenylyl cyclase in mammalian cells

CAP1 (Cyclase-Associated Protein 1) is highly conserved in evolution. Originally identified in yeast as a bifunctional protein involved in Ras-adenylyl cyclase and F-actin dynamics regulation, the adenylyl cyclase component seems to be lost in mammalian cells. Prompted by our recent identification of the Ras-like small GTPase Rap1 as a GTP-independent but geranylgeranyl-specific partner for CAP1, we hypothesized that CAP1-Rap1, similar to CAP-Ras-cyclase in yeast, might play a critical role in cAMP dynamics in mammalian cells. In this study, we report that CAP1 binds and activates mammalian adenylyl cyclase in vitro, modulates cAMP in live cells in a Rap1-dependent manner, and affects cAMP-dependent proliferation. Utilizing deletion and mutagenesis approaches, we mapped the interaction of CAP1-cyclase with CAP's N-terminal domain involving critical leucine residues in the conserved RLE motifs and adenylyl cyclase's conserved catalytic loops (e.g., C1a and/or C2a). When combined with a FRET-based cAMP sensor, CAP1 overexpression-knockdown strategies, and the use of constitutively active and negative regulators of Rap1, our studies highlight a critical role for CAP1-Rap1 in adenylyl cyclase regulation in live cells. Similarly, we show that CAP1 modulation significantly affected cAMP-mediated proliferation in an RLE motif-dependent manner. The combined study indicates that CAP1-cyclase-Rap1 represents a regulatory unit in cAMP dynamics and biology. Since Rap1 is an established downstream effector of cAMP, we advance the hypothesis that CAP1-cyclase-Rap1 represents a positive feedback loop that might be involved in cAMP microdomain establishment and localized signaling.

Molecular and Pharmacological Characterization of the Interaction between Human Geranylgeranyltransferase Type I and Ras-Related Protein Rap1B

Geranylgeranyltransferase type-I (GGTase-I) represents an important drug target since it contributes to the function of many proteins that are involved in tumor development and metastasis. This led to the development of GGTase-I inhibitors as anti-cancer drugs blocking the protein function and membrane association of e.g., Rap subfamilies that are involved in cell differentiation and cell growth. In the present study, we developed a new NanoBiT assay to monitor the interaction of human GGTase-I and its substrate Rap1B. Different Rap1B prenylation-deficient mutants (C181G, C181S, and ΔCQLL) were designed and investigated for their interaction with GGTase-I. While the Rap1B mutants C181G and C181S still exhibited interaction with human GGTase-I, mutant ΔCQLL, lacking the entire CAAX motif (defined by a cysteine residue, two aliphatic residues, and the C-terminal residue), showed reduced interaction. Moreover, a specific, peptidomimetic and competitive CAAX inhibitor was able to block the interaction of Rap1B with GGTase-I. Furthermore, activation of both Gα_s-coupled human adenosine receptors, A_2A (A_2AAR) and A_2B (A_2BAR), increased the interaction between GGTase-I and Rap1B, probably representing a way to modulate prenylation and function of Rap1B. Thus, A_2AAR and A_2BAR antagonists might be promising candidates for therapeutic intervention for different types of cancer that overexpress Rap1B. Finally, the NanoBiT assay provides a tool to investigate the pharmacology of GGTase-I inhibitors.

Integration of Rap1 and Calcium Signaling

Ca²⁺ is a universal intracellular signal. The modulation of cytoplasmic Ca²⁺ concentration regulates a plethora of cellular processes, such as: synaptic plasticity, neuronal survival, chemotaxis of immune cells, platelet aggregation, vasodilation, and cardiac excitation–contraction coupling. Rap1 GTPases are ubiquitously expressed binary switches that alternate between active and inactive states and are regulated by diverse families of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Active Rap1 couples extracellular stimulation with intracellular signaling through secondary messengers—cyclic adenosine monophosphate (cAMP), Ca²⁺, and diacylglycerol (DAG). Much evidence indicates that Rap1 signaling intersects with Ca²⁺ signaling pathways to control the important cellular functions of platelet activation or neuronal plasticity. Rap1 acts as an effector of Ca²⁺ signaling when activated by mechanisms involving Ca²⁺ and DAG-activated (CalDAG-) GEFs. Conversely, activated by other GEFs, such as cAMP-dependent GEF Epac, Rap1 controls cytoplasmic Ca²⁺ levels. It does so by regulating the activity of Ca²⁺ signaling proteins such as sarcoendoplasmic reticulum Ca²⁺-ATPase (SERCA). In this review, we focus on the physiological significance of the links between Rap1 and Ca²⁺ signaling and emphasize the molecular interactions that may offer new targets for the therapy of Alzheimer’s disease, hypertension, and atherosclerosis, among other diseases.

Regulatory actions of 3',5'-cyclic adenosine monophosphate on osteoclast function: possible roles of Epac-mediated signaling

Alterations in cellular levels of the second messenger 3',5'-cyclic adenosine monophosphate ([cAMP]_i ) regulate a wide range of physiologically important cellular signaling processes in numerous cell types. Osteoclasts are terminally differentiated, multinucleated cells specialized for bone resorption. Their systemic regulator, calcitonin, triggers morphometrically and pharmacologically distinct retraction (R) and quiescence (Q) effects on cell-spread area and protrusion-retraction motility, respectively, paralleling its inhibition of bone resorption. Q effects were reproduced by cholera toxin-mediated G_s -protein activation known to increase [cAMP]_i , unaccompanied by the [Ca²⁺ ]_i changes contrastingly associated with R effects. We explore a hypothesis implicating cAMP signaling involving guanine nucleotide-exchange activation of the small GTPase Ras-proximate-1 (Rap1) by exchange proteins directly activated by cAMP (Epac). Rap1 activates integrin clustering, cell adhesion to bone matrix, associated cytoskeletal modifications and signaling processes, and transmembrane transduction functions. Epac activation enhanced, whereas Epac inhibition or shRNA-mediated knockdown compromised, the appearance of markers for osteoclast differentiation and motility following stimulation by receptor activator of nuclear factor kappa-Β ligand (RANKL). Deficiencies in talin and Rap1 compromised in vivo bone resorption, producing osteopetrotic phenotypes in genetically modified murine models. Translational implications of an Epac-Rap1 signaling hypothesis in relationship to N-bisphosphonate actions on prenylation and membrane localization of small GTPases are discussed.

Phosphorylation promotes binding affinity of Rap-Raf complex by allosteric modulation of switch loop dynamics

The effects of phosphorylation of a serine residue on the structural and dynamic properties of Ras-like protein, Rap, and its interactions with effector protein Ras binding domain (RBD) of Raf kinase, in the presence of GTP, are investigated via molecular dynamics simulations. The simulations show that phosphorylation significantly effects the dynamics of functional loops of Rap which participate in the stability of the complex with effector proteins. The effects of phosphorylation on Rap are significant and detailed conformational analysis suggest that the Rap protein, when phosphorylated and with GTP ligand, samples different conformational space as compared to non-phosphorylated protein. In addition, phosphorylation of SER11 opens up a new cavity in the Rap protein which can be further explored for possible drug interactions. Residue network analysis shows that the phosphorylation of Rap results in a community spanning both Rap and RBD and strongly suggests transmission of allosteric effects of local alterations in Rap to distal regions of RBD, potentially affecting the downstream signalling. Binding free energy calculations suggest that phosphorylation of SER11 residue increases the binding between Rap and Raf corroborating the network analysis results. The increased binding of the Rap-Raf complex can have cascading effects along the signalling pathways where availability of Raf can influence the oncogenic effects of Ras proteins. These simulations underscore the importance of post translational modifications like phosphorylation on the functional dynamics in proteins and can be an alternative to drug-targeting, especially in notoriously undruggable oncoproteins belonging to Ras-like GTPase family.

Cyclase-associated protein 1 (CAP1) is a prenyl-binding partner of Rap1 GTPase

Rap1 proteins are members of the Ras subfamily of small GTPases involved in many biological responses, including adhesion, cell proliferation, and differentiation. Like all small GTPases, they work as molecular allosteric units that are active in signaling only when associated with the proper membrane compartment. Prenylation, occurring in the cytosol, is an enzymatic posttranslational event that anchors small GTPases at the membrane, and prenyl-binding proteins are needed to mask the cytoplasm-exposed lipid during transit to the target membrane. However, several of these proteins still await discovery. In this study, we report that cyclase-associated protein 1 (CAP1) binds Rap1. We found that this binding is GTP-independent, does not involve Rap1's effector domain, and is fully contained in its C-terminal hypervariable region (HVR). Furthermore, Rap1 prenylation was required for high-affinity interactions with CAP1 in a geranylgeranyl-specific manner. The prenyl binding specifically involved CAP1's C-terminal hydrophobic β-sheet domain. We present a combination of experimental and computational approaches, yielding a model whereby the high-affinity binding between Rap1 and CAP1 involves electrostatic and nonpolar side-chain interactions between Rap1's HVR residues, lipid, and CAP1 β-sheet domain. The binding was stabilized by the lipid insertion into the β-solenoid whose interior was occupied by nonpolar side chains. This model was reminiscent of the recently solved structure of the PDEδ-K-Ras complex; accordingly, disruptors of this complex, e.g. deltarasin, blocked the Rap1-CAP1 interaction. These findings indicate that CAP1 is a geranylgeranyl-binding partner of Rap1.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

From Ras to Rap and Back, a Journey of 35 Years

Our laboratory has studied Ras and Ras-like proteins since the discovery of the Ras oncogene 35 years ago. In this review, I will give an account of what we have done in these 35 years and indicate the main papers that have guided our research. Our efforts started with the early analysis of mutant Ras in human tumors followed by deciphering of the role of Ras in signal transduction pathways. In an attempt to interfere in Ras signaling we turned to Rap proteins. These proteins are the closest relatives of Ras and were initially identified as Ras antagonists. However, our studies revealed that the Rap signaling network primarily is involved in spatiotemporal control of cell adhesion, in part through regulation of the actin cytoskeleton. More recently we returned to Ras, trying to interfere in Ras signaling by combinatorial drug testing using the organoid technology.

RAP1B, a DVL2 binding protein, activates Wnt/beta-catenin signaling in esophageal squamous cell carcinoma

RAP1B is a small GTPase, which regulates multiple cellular processes. Up-regulation of RAP1B has been observed in several cancer types. Although previous study has shown that miR-518 inhibited the proliferation and invasion of esophageal squamous cell carcinoma (ESCC) cells possibly by targeting RAP1B, the expression pattern and the functions of RAP1B in ESCC are not fully understood. Here, we have fund that the expression of RAP1B was up-regulated in ESCC clinical samples. Gain-of-function and loss-of-function assays demonstrated that RAP1B promoted the growth, migration and metastasis of the ESCC cells. Moreover, the mechanism study showed that RAP1B interacted with DVL2, an important upstream regulator for beta-catenin/TCF signaling, and activated beta-catenin/TCF signaling. Taken together, our study demonstrated the oncogenic roles of RAP1B in ESCC, and suggested that RAP1B might be a therapeutic target.

Dopamine Receptors and Neurodegeneration

Dopamine (DA) is one of the major neurotransmitters and participates in a number of functions such as motor coordination, emotions, memory, reward mechanism, neuroendocrine regulation etc. DA exerts its effects through five DA receptors that are subdivided in 2 families: D1-like DA receptors (D1 and D5) and the D2-like (D2, D3 and D4). All DA receptors are widely expressed in the central nervous system (CNS) and play an important role in not only in physiological conditions but also pathological scenarios. Abnormalities in the DAergic system and its receptors in the basal ganglia structures are the basis Parkinson's disease (PD), however DA also participates in other neurodegenerative disorders such as Huntington disease (HD) and multiple sclerosis (MS). Under pathological conditions reorganization of DAergic system has been observed and most of the times, those changes occur as a mechanism of compensation, but in some cases contributes to worsening the alterations. Here we review the changes that occur on DA transmission and DA receptors (DARs) at both levels expression and signals transduction pathways as a result of neurotoxicity, inflammation and in neurodegenerative processes. The better understanding of the role of DA receptors in neuropathological conditions is crucial for development of novel therapeutic approaches to treat alterations related to neurodegenerative diseases.

β-Agonist-mediated relaxation of airway smooth muscle is protein kinase A-dependent

Inhaled β-agonists are effective at reversing bronchoconstriction in asthma, but the mechanism by which they exert this effect is unclear and controversial. PKA is the historically accepted effector, although this assumption is made on the basis of associative and not direct evidence. Recent studies have asserted that exchange protein activated by cAMP (Epac), not PKA, mediates the relaxation of airway smooth muscle (ASM) observed with β-agonist treatment. This study aims to clarify the role of PKA in the prorelaxant effects of β-agonists on ASM. Inhibition of PKA activity via expression of the PKI and RevAB peptides results in increased β-agonist-mediated cAMP release, abolishes the inhibitory effect of isoproterenol on histamine-induced intracellular calcium flux, and significantly attenuates histamine-stimulated MLC-20 phosphorylation. Analyses of ASM cell and tissue contraction demonstrate that PKA inhibition eliminates most, if not all, β-agonist-mediated relaxation of contracted smooth muscle. Conversely, Epac knockdown had no effect on the regulation of contraction or procontractile signaling by isoproterenol. These findings suggest that PKA, not Epac, is the predominant and physiologically relevant effector through which β-agonists exert their relaxant effects.

Guidance for life, cell death, and colorectal neoplasia by netrin dependence receptors

This review is focusing on a critical mediator of embryonic and postnatal development with multiple implications in inflammation, neoplasia, and other pathological situations in brain and peripheral tissues. These morphogenetic guidance and dependence processes are involved in several malignancies targeting the epithelial and immune systems including the progression of human colorectal cancers. We consider the most important findings and their impact on basic, translational, and clinical cancer research. Expected information can bring new cues for innovative, efficient, and safe strategies of personalized medicine based on molecular markers, protagonists, signaling networks, and effectors inherent to the Netrin axis in pathophysiological states.

Rap1 and its regulatory proteins: the tumor suppressor, oncogene, tumor suppressor gene axis in head and neck cancer

Squamous cell carcinoma of the head and neck (SCCHN) is the sixth most common cancer, globally. Previously, we showed that Rap1GAP is a tumor suppressor gene that inhibits tumor growth, but promotes invasion in SCCHN. In this work, we discuss the role of Rap1 and Rap1GAP in SCCHN progression in the context of a microRNA-oncogene-tumor suppressor gene axis, and investigate the role of Rap1GAP in EZH2-mediated invasion. Loss of expression of microRNA-101 in SCCHN leads to upregulation of EZH2, a histone methyltransferase. Overexpression of EZH2 silences Rap1GAP via methylation, thereby promoting activation of its target, Rap1. This microRNA-controlled activation of Rap1, via EZH2-mediated silencing of Rap1GAP, is a novel mechanism of Rap1 regulation. In two independent SCCHN cell lines, downregulation of EZH2 inhibits proliferation and invasion. In both cell lines, stable knockdown of EZH2 (shEZH2) recovers Rap1GAP expression and inhibits proliferation. However, siRNA-mediated knockdown of Rap1GAP in these cells rescues proliferation but not invasion. Thus, EZH2 promotes proliferation and invasion via Rap1GAP-dependent and -independent mechanisms, respectively. Although the studies presented here are in the context of SCCHN, our results may have broader implications, given that Rap1GAP acts as a tumor suppressor in pancreatic cancer, thyroid cancer, and melanoma.

Glycosides, depression and suicidal behaviour: the role of glycoside-linked proteins

Nowadays depression and suicide are two of the most important worldwide public health problems. Although their specific molecular mechanisms are still largely unknown, glycosides can play a fundamental role in their pathogenesis. These molecules act presumably through the up-regulation of plasticity-related proteins: probably they can have a presynaptic facilitatory effect, through the activation of several intracellular signaling pathways that include molecules like protein kinase A, Rap-1, cAMP, cADPR and G proteins. These proteins take part in a myriad of brain functions such as cell survival and synaptic plasticity. In depressed suicide victims, it has been found that their activity is strongly decreased, primarily in hippocampus and prefrontal cortex. These studies suggest that glycosides can regulate neuroprotection through Rap-1 and other molecules, and may play a crucial role in the pathophysiology of depression and suicide.

Phosphoproteome reveals an atlas of protein signaling networks during osteoblast adhesion

Cell adhesion on surfaces is a fundamental process in the emerging biomaterials field and developmental events as well. However, the mechanisms regulating this biological process in osteoblasts are not fully understood. Reversible phosphorylation catalyzed by kinases is probably the most important regulatory mechanism in eukaryotes. Therefore, the goal of this study is to assess osteoblast adhesion through a molecular prism under a peptide array technology, revealing essential signaling proteins governing adhesion-related events. First, we showed that there are main morphological changes on osteoblast shape during adhesion up to 3 h. Second, besides classical proteins activated upon integrin activation, our results showed a novel network involving signaling proteins such as Rap1A, PKA, PKC, and GSK3beta during osteoblast adhesion on polystyrene. Third, these proteins were grouped in different signaling cascades including focal adhesion establishment, cytoskeleton rearrangement, and cell-cycle arrest. We have thus provided evidence that a global phosphorylation screening is able to yield a systems-oriented look at osteoblast adhesion, providing new insights for understanding of bone formation and improvement of cell-substratum interactions. Altogether, these statements are necessary means for further intervention and development of new approaches for the progress of tissue engineering.

Phosphorylation-induced conformational changes in Rap1b: allosteric effects on switch domains and effector loop

Rap1b has been implicated in the transduction of the cAMP mitogenic response. Agonists that increase intracellular cAMP rapidly activate (i.e. GTP binding) and phosphorylate Rap1b on Ser(179) at its C terminus. cAMP-dependent protein kinase (PKA)-mediated phosphorylation of Rap1b is required for cAMP-dependent mitogenesis, tumorigenesis, and inhibition of AKT activity. However, the role of phosphorylation still remains unknown. In this study, we utilized amide hydrogen/deuterium exchange mass spectroscopy (DXMS) to assess potential conformational changes and/or mobility induced by phosphorylation. We report here DXMS data comparing exchange rates for PKA-phosphorylated (Rap1-P) and S179D phosphomimetic (Rap1-D) Rap1b proteins. Rap1-P and Rap1-D behaved exactly the same, revealing an increased exchange rate in discrete regions along the protein; these regions include a domain around the phosphorylation site and unexpectedly the two switch loops. Thus, local effects induced by Ser(179) phosphorylation communicate allosterically with distal domains involved in effector interaction. These results provide a mechanistic explanation for the differential effects of Rap1 phosphorylation by PKA on effector protein interaction.

Adenylyl cyclases types 1 and 8 promote pro-survival pathways after ethanol exposure in the neonatal brain

Although a wide range of developmental disabilities following fetal alcohol exposure are observed clinically, the molecular factors that determine the severity of these sequelae remain undefined. In mice exposed to ethanol, deletion of adenylyl cyclases (ACs) 1 and 8 exacerbates the neuroapoptosis that occurs in a prolonged post-treatment period; however, it remains unclear whether AC1 and AC8 are critical to the primary or secondary mechanisms underlying ethanol-induced neurodegeneration. Here we demonstrate that mice lacking AC1 and AC8 (DKO) display significantly increased apoptosis in the striatum, a region sensitive to neuroapoptosis in the acute post-treatment period, compared to WT controls. The enhanced neuroapoptotic response observed in the striatum of DKO mice is accompanied by significant reductions in phosphorylation of known pro-survival proteins, insulin receptor substrate-1 (IRS-1), Akt and extracellular signal-regulated kinases (ERKs). These data suggest that AC1/AC8 are crucial activators of cell survival signaling pathways acutely following ethanol exposure and represent molecular factors that may directly modulate the severity of symptoms associated with Fetal Alcohol Syndrome.

pathFinder: a static network analysis tool for pharmacological analysis of signal transduction pathways

The study of signal transduction is becoming a de facto part of the analysis of gene expression and protein profiling techniques. Many online tools are used to cluster genes in various ways or to assign gene products to signal transduction pathways. Among these, pathFinder is a unique tool that can find signal transduction pathways between first, second, or nth messengers and their targets within the cell. pathFinder can identify qualitatively all possible signal transduction pathways connecting any starting component and target within a database of two-component pathways (directional dyads). One or more intermediate pathway components can be excluded to simulate the use of pharmacological inhibitors or genetic deletion (knockout). Missing elements in a pathway connecting the activator or initiator and target can also be inferred from a null pathway result. The value of this static network analysis tool is illustrated by the predication from pathFinder analysis of a novel cyclic AMP-dependent, protein kinase A-independent signaling pathway in neuroendocrine cells, which has been experimentally confirmed.

Pituitary adenylate cyclase-activating polypeptide stimulates glial fibrillary acidic protein gene expression in cortical precursor cells by activating Ras and Rap1

Pituitary adenylate cyclase-activating polypeptide (PACAP) acts on cortical precursor cells to trigger glial fibrillary acidic protein (GFAP) gene expression and astrocyte differentiation by stimulation of intracellular cAMP production. Here, we show that as expected, PACAP activates cAMP-dependent protein kinase A. However, inhibition of protein kinase A does not prevent PACAP-induced GFAP gene expression or astrocytogenesis. PACAP also activates the small GTPases Rap1 and Ras, but either activation of Rap1 alone by selective stimulation of the guanine nucleotide exchange factor Epac, or expression of a constitutively active form of Ras, do not induce GFAP gene expression. Ras is activated by PACAP in a cAMP-dependent manner, and inhibition of Ras and/or Rap1 decreases PACAP-induced GFAP promoter stimulation. Thus, cAMP-dependent PACAP-induced GFAP expression during astrocytogenesis involves the coordinated activation of both Ras and Rap1, but activation of either one of them in isolation is not sufficient to trigger this response.

Relations between the mitogen-activated protein kinase and the cAMP-dependent protein kinase pathways: comradeship and hostility

Inter- and intracellular communications and responses to environmental changes are pivotal for the orchestrated and harmonious operation of multi-cellular organisms. These well-tuned functions in living organisms are mediated by the action of signal transduction pathways, which are responsible for receiving a signal, transmitting and amplifying it, and eliciting the appropriate cellular responses. Mammalian cells posses numerous signal transduction pathways that, rather than acting in solitude, interconnect with each other, a phenomenon referred to as cross-talk. This allows cells to regulate the distribution, duration, intensity and specificity of the response. The cAMP/cAMP-dependent protein kinase (PKA) pathway and the mitogen-activated protein kinase (MAPK) cascades modulate common processes in the cell and multiple levels of cross-talk between these signalling pathways have been described. The first- and best-characterized interconnections are the PKA-dependent inhibition of the MAPKs ERK1/2 mediated by RAF-1, and PKA-induced activation of ERK1/2 interceded through B-RAF. Recently, novel interactions between components of these pathways and new mechanisms for cross-talk have been elucidated. This review discusses both known and novel interactions between compounds of the cAMP/PKA and MAPKs signalling pathways in mammalian cells.

Gonadotropins and ovarian cancer

Ovarian epithelial cancer (OEC) accounts for 90% of all ovarian cancers and is the leading cause of death from gynecological cancers in North America and Europe. Despite its clinical significance, the factors that regulate the development and progression of ovarian cancer are among the least understood of all major human malignancies. The two gonadotropins, FSH and LH, are key regulators of ovarian cell functions, and the potential role of gonadotropins in the pathogenesis of ovarian cancer is suggested. Ovarian carcinomas have been found to express specific receptors for gonadotropins. The presence of gonadotropins in ovarian tumor fluid suggests the importance of these factors in the transformation and progression of ovarian cancers as well as being prognostic indicators. Functionally, there is evidence showing a direct action of gonadotropins on ovarian tumor cell growth. This review summarizes the key findings and recent advances in our understanding of these peptide hormones in ovarian cancer development and progression and their role in potential future cancer therapy. We will first discuss the supporting evidence and controversies in the "gonadotropin theory" and the use of animal models for exploring the involvement of gonadotropins in the etiology of ovarian cancer. The role of gonadotropins in regulating the proliferation, survival, and metastasis of OEC is next summarized. Relevant data from ovarian surface epithelium, which is widely believed to be the precursor of OEC, are also described. Finally, we will discuss the clinical applications of gonadotropins in ovarian cancer and the recent progress in drug development.

Rap1 and p38 MAPK mediate 8-chloro-cAMP-induced growth inhibition in mouse fibroblast DT cells

8-Cl-cAMP, which is known to induce differentiation, growth inhibition, and apoptosis in various cancer cells, has been investigated as a putative anti-cancer drug. Previously, we reported that 8-Cl-cAMP and its metabolite 8-Cl-adenosine induce growth inhibition and apoptosis through p38 mitogen-activated protein kinase (MAPK) activation. To further investigate the signal mechanisms that regulate the cellular effects of 8-Cl-cAMP, we focused on a small GTPase Rap1 that is known to be involved in growth inhibition and reverse-transformation. 8-Cl-cAMP and 8-Cl-adenosine could increase Rap1 activity, which was blocked by ABT702-an adenosine kinase inhibitor. This suggests that 8-Cl-cAMP-induced Rap1 activation is also dependent on the metabolic degradation of 8-Cl-cAMP. Overexpression of a constitutively active mutant form of Rap1 (Rap1V12) attenuated cellular growth and soft-agar colony formation, which was basically the same effect as that observed with the 8-Cl-cAMP treatment. Furthermore, the Rap1V12 transfectant showed a high level of p38 MAPK activation. However, 8-Cl-cAMP-induced Rap1 activation was not diminished by SB203580, a p38 MAPK inhibitor, suggesting that Rap1 activation might act upstream of p38 MAPK activation during 8-Cl-cAMP-induced growth inhibition.

Endocrine signaling in ovarian surface epithelium and cancer

Ovarian cancer is the sixth most common cancer and the fifth leading cause of cancer-related death among women in developed countries. Greater than 85% of human ovarian cancer arises within the ovarian surface epithelium (OSE), with the remainder derived from granulosa cells or, rarely, stroma or germ cells. The pathophysiology of ovarian cancer is the least understood among all major human malignancies because of a poor understanding of the aetiological factors and mechanisms of ovarian cancer progression. There is increasing evidence suggesting that several key reproductive hormones, such as GnRH, gonadotrophins and sex steroids, regulate the growth of normal OSE and ovarian cancer cells. The objective of this review was to highlight the effects of these endocrine factors on ovarian cancer cell growth and to summarize the signalling mechanisms involved in normal human OSE and its neoplastic counterparts.

Cyclic GMP modulates store-operated calcium entry inducing phosphatidylserine translocation at the surface of megakaryocytic cells

When subjected to stimulation, cells from the vascular compartment show a spontaneous collapse of the plasma membrane phospholipid asymmetry and phosphatidylserine is exposed at the external leaflet. Thus, phosphatidylserine externalization is essential for normal hemostasis and phagocytosis. The mechanism governing the migration of phosphatidylserine to the exoplasmic leaflet is not yet fully understood. We have proposed that store-operated calcium entry (SOCE) constitutes a key step of this process. Here, interaction of [Ca(2+)](i), cAMP and cGMP pathways and phosphatidylserine exposure was examined in human megakaryocytic cells. The membrane permeable cAMP and cGMP analogues, pCPT-cAMP and pCPT-cGMP, enhanced the Ca(2+) signal induced by ionophore and SOCE. Responses to pCPT-cAMP and pCPT-cGMP were independent of protein kinase A, protein kinase G (PKG) or ERK pathways. Inhibition of small G-proteins reduced or abolished the increase of [Ca(2+)](i) induced by pCPT-cAMP or pCPT-cGMP, respectively. pCPT-cGMP but not pCPT-cAMP enhanced the ability of cells to expose phosphatidylserine. This effect was not prevented by the inhibition of PKG or small G-proteins. These results show the differential role of cyclic nucleotides in the Ca(2+)-dependent membrane remodeling. Hence, pCPT-cGMP is another regulatory element for the completion of SOCE-induced phosphatidylserine transmembrane redistribution in HEL cells through a mechanism implicating small G-proteins.

Rap1-mediated activation of extracellular signal-regulated kinases by cyclic AMP is dependent on the mode of Rap1 activation

Like other small G proteins of the Ras superfamily, Rap1 is activated by distinct guanine nucleotide exchange factors (GEFs) in response to different signals to elicit cellular responses. Activation of Rap1 by cyclic AMP (cAMP) can occur via cAMP-dependent protein kinase A (PKA)-independent and PKA-dependent mechanisms. PKA-independent activation of Rap1 by cAMP is mediated by direct binding of cAMP to Rap1-guanine nucleotide exchange factors (Rap1-GEFs) Epac1 (exchange protein directly activated by cAMP 1) and Epac2 (Epac1 and Epac2 are also called cAMP-GEFI and -GEFII). The availability of cAMP analogues that selectively activate Epacs, but not PKA, provides a specific tool to activate Rap1. It has been argued that the inability of these analogues to regulate extracellular signal-regulated kinases (ERKs) signaling despite activating Rap1 provides evidence that Rap1 is incapable of regulating ERKs. We confirm that the PKA-independent activation of Rap1 by Epac1 activates a perinuclear pool of Rap1 and that this does not result in ERK activation. However, we demonstrate that this inability to regulate ERKs is not a property of Rap1 but is rather a property of Epacs themselves. The addition of a membrane-targeting motif to Epac1 (Epac-CAAX) relocalizes Epac1 from its normal perinuclear locale to the plasma membrane. In this new locale it is capable of activating ERKs in a Rap1- and cAMP-dependent manner. Rap1 activation by Epac-CAAX, but not wild-type Epac, triggers its association with B-Raf. Therefore, we propose that its intracellular localization prevents Epac1 from activating ERKs. C3G (Crk SH3 domain Guanine nucleotide exchanger) is a Rap1 exchanger that is targeted to the plasma membrane upon activation. We show that C3G can be localized to the plasma membrane by cAMP/PKA, as can Rap1 when activated by cAMP/PKA. Using a small interfering RNA approach, we demonstrate that C3G is required for the activation of ERKs and Rap1 by cAMP/PKA. This activation requires the GTP-dependent association of Rap1 with B-Raf. These data demonstrate that B-Raf is a physiological target of Rap1, but its utilization as a Rap1 effector is GEF specific. We propose a model that specific GEFs activate distinct pools of Rap1 that are differentially coupled to downstream effectors.

Roles of cAMP in regulating microtubule sliding and flagellar bending in demembranated hamster spermatozoa

To understand the mechanism regulating spermatozoa motility, it is important to investigate the mechanism regulating the conversion of microtubule sliding into flagellar bending. Therefore, we analyzed microtubule sliding and its conversion into flagellar bending using a demembranated spermatozoa model in which microtubule sliding and flagellar bending could be analyzed separately by treating the demembranated spermatozoa with and without dithiothreitol, respectively. Using this model, we examined the roles of cAMP and its target molecules in regulating flagellar bending and microtubule sliding. Although flagellar bending did not occur in the absence of cAMP, microtubule extrusion occurred without it, suggesting that cAMP is necessary for the conversion of microtubule sliding into flagellar bending, but not for microtubule sliding itself. The target of cAMP for regulating flagellar bending was not cAMP-dependent protein kinase (PKA), since flagellar bending was still observed in the spermatozoa treated with a PKA-specific inhibitor. Alternatively, the Epac/Rap pathway may be the target. Epac2 and Rap2 were detected in hamster spermatozoa using immunoblotting. Since Rap2 is a GTPase, we investigated the flagellar bending of demembranated spermatozoa treated with GTPgammaS. The treatment markedly increased the beat frequency and bending rate. These results suggest that cAMP activates the Epac/Rap pathway to regulate the conversion of microtubule sliding into flagellar bending.

Epac activation converts cAMP from a proliferative into a differentiation signal in PC12 cells

Elevation of the intracellular cAMP concentration ([cAMP]i) regulates metabolism, cell proliferation, and differentiation and plays roles in memory formation and neoplastic growth. cAMP mediates its effects mainly through activation of protein kinase A (PKA) as well as Epac1 and Epac2, exchange factors activating the small GTPases Rap1 and Rap2. However, how cAMP utilizes these effectors to induce distinct biological responses is unknown. We here studied the specific roles of PKA and Epac in neuroendocrine PC12 cells. In these cells, elevation of [cAMP]i activates extracellular signal-regulated kinase (ERK) 1/2 and induces low-degree neurite outgrowth. The present study showed that specific stimulation of PKA triggered ERK1/2 activation that was considerably more transient than that observed upon simultaneous activation of both PKA and Epac. Unexpectedly, the PKA-specific cAMP analog induced cell proliferation rather than neurite outgrowth. The proliferative signaling pathway activated by the PKA-specific cAMP analog involved activation of the epidermal growth factor receptor and ERK1/2. Activation of Epac appeared to extend the duration of PKA-dependent ERK1/2 activation and converted cAMP from a proliferative into an anti-proliferative, neurite outgrowth-promoting signal. Thus, the present study showed that the outcome of cAMP signaling can depend heavily on the set of cAMP effectors activated.

Selective activation of mitogen-activated protein (MAP) kinase kinase 3 and p38alpha MAP kinase is essential for cyclic AMP-dependent UCP1 expression in adipocytes

The sympathetic nervous system regulates the activity and expression of uncoupling protein 1 (UCP1) through the three beta-adrenergic receptor subtypes and their ability to raise intracellular cyclic AMP (cAMP) levels. Unexpectedly, we recently discovered that the cAMP-dependent regulation of multiple genes in brown adipocytes, including Ucp1, occurred through the p38 mitogen-activated protein kinases (MAPK) (W. Cao, K. W. Daniel, J. Robidoux, P. Puigserver, A. V. Medvedev, X. Bai, L. M. Floering, B. M. Spiegelman, and S. Collins, Mol. Cell. Biol. 24:3057-3067, 2004). However, no well-defined pathway linking cAMP accumulation or cAMP-dependent protein kinase (PKA) to p38 MAPK has been described. Therefore, in the present study using both in vivo and in vitro models, we have initiated a retrograde approach to define the required components, beginning with the p38 MAPK isoforms themselves and the MAP kinase kinase(s) that regulates them. Our strategy included ectopic expression of wild-type and mutant kinases as well as targeted inhibition of gene expression using small interfering RNA. The results indicate that the beta-adrenergic receptors and PKA lead to a highly selective activation of the p38alpha isoform of MAPK, which in turn promotes Ucp1 gene transcription. In addition, this specific activation of p38alpha relies solely on the presence of MAP kinase kinase 3, despite the expression in brown fat of MKK3, -4, and -6. Finally, of the three scaffold proteins of the JIP family expressed in brown adipocytes, only JIP2 co-immunoprecipitates p38alpha MAPK and MKK3. Therefore, in the brown adipocyte the recently described scaffold protein JIP2 assembles the required factors MKK3 and p38alpha MAPK linking PKA to the control of thermogenic gene expression.

8-Chloro-cyclic AMP-induced growth inhibition and apoptosis is mediated by p38 mitogen-activated protein kinase activation in HL60 cells

8-Chloro-cyclic AMP (8-Cl-cAMP), which is known to induce growth inhibition, apoptosis, and differentiation in various cancer cell lines, has been studied as a putative anticancer drug. However, the mechanism of anticancer activities of 8-Cl-cAMP has not been fully understood. Previously, we reported that the 8-Cl-cAMP-induced growth inhibition is mediated by protein kinase C (PKC) activation. In this study, we found that p38 mitogen-activated protein kinase (MAPK) also plays important roles during the 8-Cl-cAMP-induced growth inhibition and apoptosis. SB203580 (a p38-specific inhibitor) recovered the 8-Cl-cAMP-induced growth inhibition and apoptosis, whereas other MAPK inhibitors, such as PD98059 (an extracellular signal-regulated kinase-specific inhibitor) and SP600125 (a c-Jun NH2-terminal kinase-specific inhibitor), had no effect. The phosphorylation (activation) of p38 MAPK was increased in a time-dependent manner after 8-Cl-cAMP treatment. Furthermore, SB203580 was able to block PKC activation induced by 8-Cl-cAMP. However, PKC inhibitor (GF109203x) could not attenuate p38 activation, indicating that p38 MAPK activation is upstream of PKC activation during the 8-Cl-cAMP-induced growth inhibition. 8-Chloro-adenosine, a metabolite of 8-Cl-cAMP, also activated p38 MAPK and this activation was blocked by adenosine kinase inhibitor. These results suggest that 8-Cl-cAMP exerts its anticancer activity through p38 MAPK activation and the metabolite(s) of 8-Cl-cAMP mediates this process.

Regulation of actin-based cell migration by cAMP/PKA

A wide variety of soluble signaling substances utilize the cyclic AMP-dependent protein kinase (PKA) pathway to regulate cellular behaviors including intermediary metabolism, ion channel conductivity, and transcription. A growing literature suggests that integrin-mediated cell adhesion may also utilize PKA to modulate adhesion-associated events such as actin cytoskeletal dynamics and migration. PKA is dynamically regulated by integrin-mediated cell adhesion to extracellular matrix (ECM). Furthermore, while some hallmarks of cell migration and cytoskeletal organization require PKA activity (e.g. activation of Rac and Cdc42; actin filament assembly), others are inhibited by it (e.g. activation of Rho and PAK; interaction of VASP with the c-Abl tyrosine kinase). Also, cell migration and invasion can be impeded by either inhibition or hyper-activation of PKA. Finally, a number of A-kinase anchoring proteins (AKAPs) serve to associate PKA with various components of the actin cytoskeleton, thereby enhancing and/or specifying cAMP/PKA signaling in those regions. This review discusses the growing literature that supports the hypothesis that PKA plays a central role in cytoskeletal regulation and cell migration.

Requirement of the Caenorhabditis elegans RapGEF pxf-1 and rap-1 for epithelial integrity

The Rap-pathway has been implicated in various cellular processes but its exact physiological function remains poorly defined. Here we show that the Caenorhabditis elegans homologue of the mammalian guanine nucleotide exchange factors PDZ-GEFs, PXF-1, specifically activates Rap1 and Rap2. Green fluorescent protein (GFP) reporter constructs demonstrate that sites of pxf-1 expression include the hypodermis and gut. Particularly striking is the oscillating expression of pxf-1 in the pharynx during the four larval molts. Deletion of the catalytic domain from pxf-1 leads to hypodermal defects, resulting in lethality. The cuticle secreted by pxf-1 mutants is disorganized and can often not be shed during molting. At later stages, hypodermal degeneration is seen and animals that reach adulthood frequently die with a burst vulva phenotype. Importantly, disruption of rap-1 leads to a similar, but less severe phenotype, which is enhanced by the simultaneous removal of rap-2. In addition, the lethal phenotype of pxf-1 can be rescued by expression of an activated version of rap-1. Together these results demonstrate that the pxf-1/rap pathway in C. elegans is required for maintenance of epithelial integrity, in which it probably functions in polarized secretion.

Nitric oxide activates Rap1 and Ral in a Ras-independent manner

Rap1 and Ral, the small GTPases belonging to the Ras superfamily, have recently attracted much attention; Ral because of Ral-specific guanine nucleotide exchange factors which are regulated by direct binding to Ras and Rap1 because of its proposed role as an antagonist of Ras signaling. We have previously demonstrated that nitric oxide (NO) activates Ras and proposed the structural basis of interaction between NO and Ras. In the present study we have shown that NO activates Rap1 and Ral in a time- and concentration-dependent manner. Using activation-specific probes for Rap1 and Ral, it was found that the NO-generating compounds SNP and SNAP could activate both Rap1 and Ral in Jurkat and PC12 cell lines. To investigate the involvement of Ras in NO mediated activation of Rap1 and Ral, we used PC12 cell lines expressing either the Ras mutant C118S (Cys118 mutated to Ser) or N17 (GDP-locked and inactive). We had previously shown that NO fails to activate Ras in these mutant cell lines. However, here it was found that Rap1 and Ral were activated by NO in these cell lines. The evidence presented in this study unambiguously demonstrates the existence of Ras-independent pathways for NO mediated activation of Rap1 and Ral.

cAMP-dependent oncogenic action of Rap1b in the thyroid gland

cAMP signaling leads to activation and phosphorylation of Rap1b. Using cellular models where cAMP stimulates cell proliferation, we have demonstrated that cAMP-mediated activation, as well as phosphorylation of Rap1b, is critical for cAMP stimulation of DNA synthesis. To determine whether Rap1b stimulates mitogenesis in vivo, we have constructed a transgenic mouse where a constitutively active G12V-Rap1b, flanked by Cre recombinase LoxP sites, is followed by the dominant negative S17N mutant. Employing this novel mouse model, we have switched, in a tissue-specific (thyroid) and temporally controlled manner, the expression of Rap1b from a stimulatory to an inhibitory form. These experiments provide conclusive evidence that Rap1b is oncogenic in the thyroid in ways linked to transduction of the cAMP mitogenic signal.

Impact of the mitogen-activated protein kinase pathway on parathyroid hormone-related protein actions in osteoblasts

Parathyroid hormone-related protein (PTHrP) regulates proliferation and differentiation of osteoblastic cells via binding to the parathyroid hormone receptor (PTH-1R). The cAMP-dependent protein kinase A pathway governs the majority of these effects, but recent evidence also implicates the MAPK pathway. MC3T3-E1 subclone 4 cells (MC4) were treated with the MAPK inhibitor U0126 and PTHrP. In differentiated MC4 cells, osteocalcin and bone sialoprotein gene expression were both down-regulated by PTHrP and also by inhibition of the MAPK pathway. PTHrP-mediated down-regulation of PTH-1R mRNA and up-regulation of c-fos mRNA were MAPK-independent, whereas PTHrP stimulation of fra-2 and interleukin-6 (IL-6) mRNA was MAPK-dependent. Luciferase promoter assays revealed that regulation of IL-6 involved the cAMP-dependent protein kinase A and MAPK pathways with a potential minor role of the protein kinase C pathway, and a promoter region containing an activator protein-1 site was necessary for PTHrP-induced IL-6 gene transcription. An alternative pathway, through cAMP/Epac/Rap1/MAPK, mediated ERK phosphorylation but was not sufficient for IL-6 promoter activation. Phosphorylation of the transcription factor CREB was also necessary but not sufficient for PTHrP-mediated IL-6 promoter activity. Most interesting, a bidirectional effect was found with PTHrP increasing phosphorylated ERK in undifferentiated MC4 cells but decreasing phosphorylated ERK in differentiated cells. These data indicate that inactivation of the MAPK pathway shows differential regulation of PTHrP-stimulated activator protein-1 members, blocks PTHrP-stimulated IL-6, and synergistically down-regulates certain osteoblastic markers associated with differentiation. These novel findings indicate that the MAPK pathway plays a selective but important role in the actions of PTHrP.

Cyclic AMP inhibits TGFbeta1-induced cell-scattering and invasiveness in murine-transformed keratinocytes

Mouse-transformed keratinocytes cultured in the presence of transforming growth factor beta1 (TGF-beta1) acquire an array of morphologic and functional properties that give rise to a migratory phenotype that expresses mesenchymal molecular markers. This cellular conversion involves activation of the Ras-ERK pathway, enhancement of urokinase (uPA) and matrix metalloproteinase-9 (MMP-9) expression and induction of invasiveness. In our present work, we demonstrate that cAMP and forskolin are able to prevent the expression of these mesenchymal properties, probably due to blockade of the Ras-ERK pathway. Our results also show that cAMP and forskolin are able to abolish the TGF-beta1-induced reorganization of the actin cytoskeleton that is characteristic of the mesenchymal phenotype and also inhibits the disruption of the E-cadherin cell to cell interactions. The latter responses seem to depend on the activity of protein kinase A, as demonstrated by the activation of the Ras-ERK pathway by specific protein kinase A inhibitors.

G-protein-coupled receptor-mediated activation of rap GTPases: characterization of a novel Galphai regulated pathway

Ras proteins mediate the proliferative effects of G-protein-coupled receptors (GPCRs), but the role of Rap proteins in GPCR signaling is unclear. We have developed a novel cellular proliferation assay for examining signal transduction to Rap utilizing Ras-rap chimeras that respond selectively to Rap-specific exchange factors, but which stimulate cellular proliferation through Ras effectors. Both the D1 dopamine receptor (Gs-coupled) and the 5HT1E serotonin receptor (Gi-coupled) mediated cellular proliferation in a Ras/rap chimera-dependent manner. Responses to both receptors were PKA-independent. Both receptors activated Ras/rap and full-length Rap as measured by activation-specific probes. Pertussis toxin blocked Ras/rap-dependent responses to 5HT1E but not D1. Ras/rap-dependent responses to both receptors were insensitive to beta-gamma scavengers. Responses to 5HT1E, but not D1, were sensitive to inhibition by a dominant-negative C3G fragment, by the Src-like kinase inhibitors PP1 and PP2, and by a dominant-negative mutant of Src. Very similar data were obtained for two other Gi-coupled receptors, the D2 dopamine receptor and the alpha2C adrenergic receptor. A constitutively active mutant of Galphai2 also mediated Ras/rap-dependent responses. These data indicate that GPCRs coupled to pertussis-toxin-sensitive G-proteins activate Rap through a Galpha subunit, C3G, and Src-dependent pathway.

The polybasic region of Ras and Rho family small GTPases: a regulator of protein interactions and membrane association and a site of nuclear localization signal sequences

Many small GTPases in the Ras and Rho families have a C-terminal polybasic region (PBR) comprised of multiple lysines or arginines. The PBR controls diverse functions of these small GTPases, including their ability to associate with membranes, interact with specific proteins, and localize in subcellular compartments. Different signaling pathways mediated by Ras and Rho family members may converge when the small GTPases are directed by their PBRs to shared binding sites in specific proteins or at cell membranes. The PBR promotes the interactions of small GTPases with SmgGDS, which is a nucleocytoplasmic shuttling protein that stimulates guanine nucleotide exchange by small GTPases. The PBR of Rac1 was recently found to have a functional nuclear localization signal (NLS) sequence, which enhances the nuclear accumulation of protein complexes containing SmgGDS and Rac1. Sequence analysis demonstrates that canonical NLS sequences (K-K/R-x-K/R) are present in the PBRs of additional Ras and Rho family members, and are evolutionarily conserved across several phyla. These findings suggest that the PBR regulates the nucleocytoplasmic shuttling of some Ras and Rho family members when they are in protein complexes that are too large to diffuse through nuclear pores. These diverse functions of the PBR indicate its critical role in signaling by Ras and Rho family GTPases.

Rap1, a small GTP-binding protein is upregulated during arrest of proliferation in human keratinocytes

Rap1 is a small GTP-binding protein (SMG) that exists in two 95% homologous isoforms, rap1A and rap1B. The functions of the rap1 proteins are not well understood. In this report we examined expression and function of rap1 in primary (HOKs) and immortalized (IHOKs) human oral keratinocytes under different growth conditions. In HOKs, rap1 increased with passage number, suggesting a role in differentiation and arrest of proliferation. Similarly, when inhibition of proliferation and differentiation were induced in HOKs by 1.2 mM CaCl2, both rap1 and involucrin increased with increasing concentrations of CaCl2. However, when similar experiments were done with IHOKs, which continue to proliferate in the presence of 1.2 mM CaCl2, the increase in involucrin expression was similar to HOKs but there was no substantial increase in rap1, suggesting that increased expression of rap1 is linked to inhibition of proliferation rather than differentiation of keratinocytes. Upon transfection of immortalized keratinocytes with rapGAP, which inactivates both isoforms of endogenous active rap1, enhanced proliferation was observed. Thus, we conclude that rap1 inhibits proliferation in keratinocytes.

Profiling follicle stimulating hormone-induced gene expression changes in normal and malignant human ovarian surface epithelial cells

Epidemiological data have implicated the pituitary gonadotropin follicle stimulating hormone (FSH) as both a risk factor for and a protective agent against epithelial ovarian cancer. Yet, little is known about how this hormone could play such opposing roles in ovarian carcinogenesis. Complementary DNA microarrays containing 2400 named genes were used to examine FSH-induced gene expression changes in ovarian cancer (OC) and immortalized normal human ovarian surface epithelial (HOSE) cell lines. Two-way t-statistics analyses of array data identified two distinct sets of FSH-regulated genes in HOSE and in established OC cell lines established from patients (OVCA cell lines). Among the HOSE cell lines, FSH increased expression of 57% of the 312 genes and downregulated 43%. In contrast, FSH diminished expression of 92% of the 177 genes in the OVCA cell lines. All but 18 of the genes affected by FSH in HOSE cell lines were different from those altered in OVCA cell lines. Among the 18 overlapping genes, nine genes exhibited the same direction of change following FSH challenge, while the other nine showed discordance in response between HOSE and OVCA cell lines. The FSH-induced differential expression of seven out of nine genes was confirmed by real-time RT-PCR. Gene-specific antisense oligonuleotides (ODNs) were used to inhibit the expression of genes encoding GTPase activating protein (rap1GAP), neogenin, and restin in HOSE and OVCA cells. Antisense ODNs to neogenin and restin, but not an antisense ODN to rap1GAP, were effective in inhibiting OVCA cell growth, diminishing proliferating cell nuclear antigen expression, and increasing caspase 3 activities. Furthermore, the ODN to rap1GAP was further shown to be ineffective in altering migration properties of OVCA cell lines. HOSE cell proliferation was not affected by treatment with any of the antisense ODNs. In summary, gene profiling data reveal for the first time that FSH may exert different biological actions on OVCA cells than on HOSE cells, by differential regulation of a set of putative oncogenes/tumor suppressors. Specifically, neogenin and restin were found to exhibit proproliferation/survival action on OC cells.

Insulin induces expression of adenosine kinase gene in rat lymphocytes by signaling through the mitogen-activated protein kinase pathway

The activity of adenosine kinase (AK) was significantly impaired in splenocytes isolated from diabetic rats. Administration of insulin to diabetic animals restored AK activity, protein, and mRNA levels in diabetic splenocytes. Experiments performed on cultured rat lymphocytes demonstrated that insulin did not change the stability of AK mRNA. Insulin induced AK gene expression in a dose- and time-dependent manner. Maximal increases in AK mRNA (3.9-fold) and activity level (3.7-fold) were observed at the fourth and fifth hours of cell incubation with 10 nM insulin, respectively. The insulin effect on AK expression was not influenced by dibutyryl cAMP (dcAMP). On the other hand dcAMP weakly increased (1.7-fold) basal expression of AK. Exposure of rat lymphocytes to wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3K), or rapamycin, an inhibitor of mTOR, did not affect the ability of insulin to stimulate expression of AK. Prior treatment of the cells with 10 microM PD98059, an inhibitor of mitogen-activated protein kinase (MAPK) kinase (MEK) completely blocked insulin-stimulated expression of AK gene. Insulin produced a significant transient increase in the tyrosine phosphorylation of ERK1/2, and PD98059 inhibited this phosphorylation. Furthermore exposure of cells to insulin has resulted in transient phosphorylation of Elk-1 on Ser-383 and sustained elevation of c-Jun and c-Fos protein. The maximal phosphorylation of Elk-1 was observed at 15 min, and was blocked by PD98059. We concluded that insulin stimulates AK gene expression through a series of events occurring sequentially. This includes activation of the MAPK cascade and subsequent phosphorylation of Elk-1 followed by increased expression of c-fos and c-jun genes.

Does Rap1 deserve a bad Rap?

The Ras superfamily of small G proteins is remarkable for both its diversity and physiological functions. One member, Rap1, has been implicated in a particularly wide range of biological processes, from cell proliferation and differentiation to cell adhesion. But the diversity of Rap1 has lead to contradictory reports of its effects. Originally identified as an antagonist of Ras-induced transformation, Rap1 can oppose other actions of Ras including regulation of cell growth and differentiation, integrin-dependent responses and synaptic plasticity. Furthermore, recent evidence confirms that Rap1, like Ras, can activate the MAP kinase cascade (ERK) in several cell types. These diverse functions of Rap1 underscore that the activation and action of Rap1 are regulated by complex factors that are cell-type specific.

Follicle-stimulating hormone activates extracellular signal-regulated kinase but not extracellular signal-regulated kinase kinase through a 100-kDa phosphotyrosine phosphatase

In this report we sought to elucidate the mechanism by which the follicle-stimulating hormone (FSH) receptor signals to promote activation of the p42/p44 extracellular signal-regulated protein kinases (ERKs) in granulosa cells. Results show that the ERK kinase MEK and upstream intermediates Raf-1, Ras, Src, and L-type Ca(2+) channels are already partially activated in vehicle-treated cells and that FSH does not further activate them. This tonic stimulatory pathway appears to be restrained at the level of ERK by a 100-kDa phosphotyrosine phosphatase that associates with ERK in vehicle-treated cells and promotes dephosphorylation of its regulatory Tyr residue, resulting in ERK inactivation. FSH promotes the phosphorylation of this phosphotyrosine phosphatase and its dissociation from ERK, relieving ERK from inhibition and resulting in its activation by the tonic stimulatory pathway and consequent translocation to the nucleus. Consistent with this premise, FSH-stimulated ERK activation is inhibited by the cell-permeable protein kinase A-specific inhibitor peptide Myr-PKI as well as by inhibitors of MEK, Src, a Ca(2+) channel blocker, and chelation of extracellular Ca(2+). These results suggest that FSH stimulates ERK activity in immature granulosa cells by relieving an inhibition imposed by a 100-kDa phosphotyrosine phosphatase.

Stimulation of the ERK pathway by GTP-loaded Rap1 requires the concomitant activation of Ras, protein kinase C, and protein kinase A in neuronal cells

The small GTPases Ras or Rap1 were suggested to mediate the stimulatory effect of some G protein-coupled receptors on ERK activity in neuronal cells. Accordingly, we reported here that pituitary adenylate cyclase-activating polypeptide (PACAP), whose G protein-coupled receptor triggers neuronal differentiation of the PC12 cell line via ERK1/2 activation, transiently activated Ras and induced the sustained GTP loading of Rap1. Ras mediated peak stimulation of ERK by PACAP, whereas Rap1 was necessary for the sustained activation phase. However, PACAP-induced GTP-loading of Rap1 was not sufficient to account for ERK activation by PACAP because 1) PACAP-elicited Rap1 GTP-loading depended only on phospholipase C, whereas maximal stimulation of ERK by PACAP also required the activity of protein kinase A (PKA), protein kinase C (PKC), and calcium-dependent signaling; and 2) constitutively active mutants of Rap1, Rap1A-V12, and Rap1B-V12 only minimally stimulated the ERK pathway compared with Ras-V12. The effect of Rap1A-V12 was dramatically potentiated by the concurrent activation of PKC, the cAMP pathway, and Ras, and this potentiation was blocked by dominant-negative mutants of Ras and Raf. Thus, this set of data indicated that GPCR-elicited GTP loading of Rap1 was not sufficient to stimulate efficiently ERK in PC12 cells and required the permissive co-stimulation of PKA, PKC, or Ras.

cAMP inhibits both Ras and Rap1 activation in primary human T lymphocytes, but only Ras inhibition correlates with blockade of cell cycle progression

Cyclic adenosine monophosphate (cAMP) is a negative regulator of T-cell activation. However, the effects of cAMP on signaling pathways that regulate cytokine production and cell cycle progression remain unclear. Here, using primary human T lymphocytes in which endogenous cAMP was increased by the use of forskolin and 3-isobutyl-1-methylxanthine (IBMX), we show that increase of cAMP resulted in inhibition of T-cell receptor (TCR)/CD3 plus CD28-mediated T-cell activation and cytokine production and blockade of cell cycle progression at the G(1) phase. Increase of cAMP inhibited Ras activation and phosphorylation of mitogen-induced extracellular kinase (MEK) downstream targets extracellular signal-related kinase 1/2 (ERK1/2) and phosphatidylinositol-3-kinase (PI3K) downstream target protein kinase B (PKB; c-Akt). These functional and biochemical events were secondary to the impaired activation of ZAP-70 and phosphorylation of LAT and did not occur when cells were stimulated with phorbol ester, which bypasses the TCR proximal signaling events and activates Ras. Increase of cAMP also inhibited activation of Rap1 mediated by TCR/CD3 plus CD28. Importantly, inhibition of Rap1 activation by cAMP was also observed when cells were stimulated with phorbol ester, although under these conditions Ras was activated and cells progressed into the cell cycle. Thus, TCR plus CD28-mediated activation of ERK1/2 and PKB, cytokine production, and cell cycle progression, all of which are inhibited by cAMP, require activation of Ras but not Rap1. These results indicate that signals that regulate cAMP levels after encounter of T cells by antigen will likely determine the functional fate toward clonal expansion or repression of primary T-cell responses.

A growing family of guanine nucleotide exchange factors is responsible for activation of Ras-family GTPases

GTPases of the Ras subfamily regulate a diverse array of cellular-signaling pathways, coupling extracellular signals to the intracellular response machinery. Guanine nucleotide exchange factors (GEFs) are primarily responsible for linking cell-surface receptors to Ras protein activation. They do this by catalyzing the dissociation of GDP from the inactive Ras proteins. GTP can then bind and induce a conformational change that permits interaction with downstream effectors. Over the past 5 years, approximately 20 novel Ras-family GEFs have been identified and characterized. These data indicate that a variety of different signaling mechanisms can be induced to activate Ras, enabling tyrosine kinases, G-protein-coupled receptors, adhesion molecules, second messengers, and various protein-interaction modules to relocate and/or activate GEFs and elevate intracellular Ras-GTP levels. This review discusses the structure and function of the catalytic or CDC25 homology domain common to almost all Ras-family GEFs. It also details our current knowledge about the regulation and function of this rapidly growing family of enzymes that include Sos1 and 2, GRF1 and 2, CalDAG-GEF/GRP1-4, C3G, cAMP-GEF/Epac 1 and 2, PDZ-GEFs, MR-GEF, RalGDS family members, RalGPS, BCAR3, Smg GDS, and phospholipase C(epsilon).

Cyclic AMP induces transactivation of the receptors for epidermal growth factor and nerve growth factor, thereby modulating activation of MAP kinase, Akt, and neurite outgrowth in PC12 cells

In PC12 cells, a well studied model for neuronal differentiation, an elevation in the intracellular cAMP level increases cell survival, stimulates neurite outgrowth, and causes activation of extracellular signal-regulated protein kinase 1 and 2 (ERK1/2). Here we show that an increase in the intracellular cAMP concentration induces tyrosine phosphorylation of two receptor tyrosine kinases, i.e. the epidermal growth factor (EGF) receptor and the high affinity receptor for nerve growth factor (NGF), also termed Trk(A). cAMP-induced tyrosine phosphorylation of the EGF receptor is rapid and correlates with ERK1/2 activation. It occurs also in Panc-1, but not in human mesangial cells. cAMP-induced tyrosine phosphorylation of the NGF receptor is slower and correlates with Akt activation. Inhibition of EGF receptor tyrosine phosphorylation, but not of the NGF receptor, reduces cAMP-induced neurite outgrowth. Expression of dominant-negative Akt does not abolish cAMP-induced survival in serum-free media, but increases cAMP-induced ERK1/2 activation and neurite outgrowth. Together, our results demonstrate that cAMP induces dual signaling in PC12 cells: transactivation of the EGF receptor triggering the ERK1/2 pathway and neurite outgrowth; and transactivation of the NGF receptor promoting Akt activation and thereby modulating ERK1/2 activation and neurite outgrowth.

Glucose-dependent insulinotropic polypeptide activates the Raf-Mek1/2-ERK1/2 module via a cyclic AMP/cAMP-dependent protein kinase/Rap1-mediated pathway

The gastrointestinal hormone, glucose-dependent insulinotropic polypeptide (GIP), is one of the most important regulators of insulin secretion following ingestion of a meal. GIP stimulates insulin secretion from the pancreatic beta-cell via its G protein-coupled receptor activation of adenylyl cyclase and other signal transduction pathways, but there is little known regarding subsequent protein kinase pathways that are activated. A screening technique was used to determine the relative abundance of 75 protein kinases in CHO-K1 cells expressing the GIP receptor and in two pancreatic beta-cell lines (betaTC-3 and INS-1 (832/13) cells). This information was used to identify kinases that are potentially regulated following GIP stimulation, with a focus on GIP regulation of the ERK1/2 MAPK pathway. In CHO-K1 cells, GIP induced phosphorylation of Raf-1 (Ser-259), Mek1/2 (Ser-217/Ser-221), ERK1/2 (Thr-202 and Tyr-204), and p90 RSK (Ser-380) in a concentration-dependent manner. Activation of ERK1/2 was maximal at 4 min and was cAMP-dependent protein kinase-dependent and protein kinase C-independent. Studies using a beta-cell line (INS-1 clone 832/13) corroborated these findings, and it was also demonstrated that the ERK1/2 module could be activated by GIP in the absence of glucose. Finally, we have shown that GIP regulation of the ERK1/2 module is via Rap1 but does not involve Gbetagamma subunits nor Src tyrosine kinase, and we propose that cAMP-based regulation occurs via B-Raf in both CHO-K1 and beta-cells. These results establish the importance of GIP in the cellular regulation of the ERK1/2 module and identify a role for cAMP in coupling its G protein-coupled receptors to ERK1/2 activity in pancreatic beta-cells.

cAMP inhibition of Akt is mediated by activated and phosphorylated Rap1b

Rap1b has been implicated in the transduction of the cAMP mitogenic signal. Rap1b is phosphorylated and activated by cAMP, and its expression in cells where cAMP is mitogenic leads to an increase in G(1)/S phase entry and tumor formation. The PCCL3 thyroid follicular cells represent a differentiated and physiologically relevant system that requires thyrotropin (TSH), acting via cAMP, for a full mitogenic response. In this model system, cAMP stimulation of DNA synthesis requires activation and phosphorylation of Rap1b by the cAMP-dependent protein kinase A (PKA). This scenario presents the challenge of identifying biochemical processes involved in the phosphorylation-dependent Rap1b mitogenic action. In thyroid cells, Akt has been implicated in the stimulation of cell proliferation by TSH and cAMP. However, the mechanism(s) by which cAMP regulates Akt activity remains unclear. In this study we show that in PCCL3 cells 1) TSH inhibits Akt activity via cAMP and PKA; 2) Rap1b is required for cAMP inhibition of Akt; and 3) transduction of the cAMP signal into Akt requires activation as well as phosphorylation of Rap1b by PKA.