MAP kinase stimulation by cAMP does not require RAP1 but SRC family kinases

Adenosine A2A receptor as a potential target for improving cancer immunotherapy

The adenosine nucleoside performs a wide range of actions on various human tissues by activating four cell surface receptors. Adenosine A_2A receptors (A_2ARs) are widely expressed in the striatum, olfactory bulb, platelets, leukocytes, spleen, and thymus. They promote vasodilatation, platelet antiaggregatory effect, protection from ischemic damage, and regulation of sensorimotor neurons in basal ganglia. Adenosine signaling plays a vital part in modulating in vivo pathophysiological responses. A_2ARs are potent negative regulators of the antitumor and proinflammatory actions of activated T cells. This axis offers several therapeutic targets, the most important of which are A_2ARs, HIF-1α, and CD39/CD73. Downregulation of this axis increases the effectiveness of modern immunotherapeutic approaches against cancer, such as αCTLA-4/αPD-1. These discoveries have led to a promising novel role of antagonists of A_2AR in blocking angiogenesis in immunotherapy of cancer. A small molecule, AZD4635, strongly inhibits A_2AR, lowering cancer volume and increasing anticancer immunity. Deletion of A_2AR with CRISPR/Cas9 in both human and murine CAR T cells produces a substantial increase in the efficiency of these cells. This review asserts that inhibition of the adenosinergic pathway can boost antitumor immunity, and this axis should be a target for future immunotherapeutic strategies.

Proteomic Analysis of Src Family Kinase Phosphorylation States in Cancer Cells Suggests Deregulation of the Unique Domain

The Src family kinases (SFK) are homologs of retroviral oncogenes, earning them the label of proto-oncogenes. Their functions are influenced by positive and negative regulatory tyrosine phosphorylation events and inhibitory and activating intramolecular and extramolecular interactions. This regulation is disrupted in their viral oncogene counterparts. However, in contrast to most other proto-oncogenes, the genetic alteration of these genes does not seem to occur in human tumors and how and whether their functions are altered in human cancers remain to be determined. To look for proteomic-level alterations, we took a more granular look at the activation states of SFKs based on their two known regulatory tyrosine phosphorylations, but found no significant differences in their activity states when comparing immortalized epithelial cells with cancer cells. SFKs are known to have other less well-studied phosphorylations, particularly within their unstructured N-terminal unique domains (UD), although their role in cancers has not been explored. In comparing panels of epithelial cells with cancer cells, we found a decrease in S17 phosphorylation in the UD of Src in cancer cells. Dephosphorylated S17 favors the dimerization of Src that is mediated through the UD and suggests increased Src dimerization in cancers. These data highlight the important role of the UD of Src and suggest that a deeper understanding of proteomic-level alterations of the unstructured UD of SFKs may provide considerable insights into how SFKs are deregulated in cancers. IMPLICATIONS: This work highlights the role of the N-terminal UD of Src kinases in regulating their signaling functions and possibly in their deregulation in human cancers.

Recruitment of endosomal signaling mediates the forskolin modulation of guinea pig cardiac neuron excitability

Forskolin, a selective activator of adenylyl cyclase (AC), commonly is used to establish actions of G protein-coupled receptors (GPCRs) that are initiated primarily through activation of AC/cAMP signaling pathways. In the present study, forskolin was used to evaluate the potential role of AC/cAMP, which is a major signaling mechanism for the pituitary adenylate cyclase-activating polypeptide (PACAP)-selective PAC1 receptor, in the regulation of guinea pig cardiac neuronal excitability. Forskolin (5-10 µM) increases excitability in ~60% of the cardiac neurons. The forskolin-mediated increase in excitability was considered related to cAMP regulation of a cyclic nucleotide gated channel or via protein kinase A (PKA)/ERK signaling, mechanisms that have been linked to PAC1 receptor activation. However, unlike PACAP mechanisms, forskolin enhancement of excitability was not significantly reduced by treatment with cesium to block currents through hyperpolarization-activated nonselective cation channels (I_h) or by treatment with PD98059 to block MEK/ERK signaling. In contrast, treatment with the clathrin inhibitor Pitstop2 or the dynamin inhibitor dynasore eliminated the forskolin-induced increase in excitability; treatments with the inactive Pitstop analog or PP2 treatment to inhibit Src-mediated endocytosis mechanisms were ineffective. The PKA inhibitor KT5702 significantly suppressed the forskolin-induced change in excitability; further, KT5702 and Pitstop2 reduced the forskolin-stimulated MEK/ERK activation in cardiac neurons. Collectively, the present results suggest that forskolin activation of AC/cAMP/PKA signaling leads to the recruitment of clathrin/dynamin-dependent endosomal transduction cascades, including MEK/ERK signaling, and that endosomal signaling is the critical mechanism underlying the forskolin-induced increase in cardiac neuron excitability.

Allosteric mechanisms within the adenosine A2A-dopamine D2 receptor heterotetramer

The structure constituted by a G protein coupled receptor (GPCR) homodimer and a G protein provides a main functional unit and oligomeric entities can be viewed as multiples of dimers. For GPCR heteromers, experimental evidence supports a tetrameric structure, comprised of two different homodimers, each able to signal with its preferred G protein. GPCR homomers and heteromers can act as the conduit of allosteric interactions between orthosteric ligands. The well-known agonist/agonist allosteric interaction in the adenosine A2A receptor (A2AR)-dopamine D2 receptor (D2R) heteromer, by which A2AR agonists decrease the affinity of D2R agonists, gave the first rationale for the use of A2AR antagonists in Parkinson's disease. We review new pharmacological findings that can be explained in the frame of a tetrameric structure of the A2AR-D2R heteromer: first, ligand-independent allosteric modulations by the D2R that result in changes of the binding properties of A2AR ligands; second, differential modulation of the intrinsic efficacy of D2R ligands for G protein-dependent and independent signaling; third, the canonical antagonistic Gs-Gi interaction within the frame of the heteromer; and fourth, the ability of A2AR antagonists, including caffeine, to also exert the same allosteric modulations of D2R ligands than A2AR agonists, while A2AR agonists and antagonists counteract each other's effects. These findings can have important clinical implications when evaluating the use of A2AR antagonists. They also call for the need of monitoring caffeine intake when evaluating the effect of D2R ligands, when used as therapeutic agents in neuropsychiatric disorders or as probes in imaging studies. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Effect of 5-HT7 receptor blockade on liver regeneration after 60-70% partial hepatectomy

Background

Serotonin exhibits a vast repertoire of actions including cell proliferation and differentiation. The effect of serotonin, as an incomplete mitogen, on liver regeneration has recently been unveiled and is mediated through 5-HT₂ receptor. The aim of the present study was to investigate the effect of 5-HT₇ receptor blockade on liver regeneration after partial hepatectomy.

Methods

Male Wistar rats were subjected to 60-70% partial hepatectomy. 5-HT₇ receptor blockade was applied by intraperitoneal administration of SB-269970 hydrochloride two hours prior to and sixteen hours after partial hepatectomy and by intraperitoneal administration of SB-258719 sixteen hours after partial hepatectomy. Animals were sacrificed at different time points until 72 h after partial hepatectomy. Liver regeneration was evaluated by [³H]-thymidine incorporation into hepatic DNA, the mitotic index in hematoxylin-eosin (HE) sections and by immunochemical detection of Ki67 nuclear antigen. Reversion of 5-HT₇ blockade was performed by intraperitoneal administration of AS-19. Serum and liver tissue lipids were also quantified.

Results

Liver regeneration peaked at 24 h ([³H]-thymidine incorporation into hepatic DNA and mitotic index by immunochemical detection of Ki67) and at 32 h (mitotic index in HE sections) in the control group of rats. 5-HT₇ receptor blockade had no effect on liver regeneration when applied 2 h prior to partial hepatectomy. Liver regeneration was greatly attenuated when blockade of 5-HT₇ receptor was applied (by SB-258719 and SB-269970) at 16 h after partial hepatectomy and peaked at 32 h ([³H]-thymidine incorporation into hepatic DNA and mitotic index by immunochemical detection of Ki67) and 40 h (mitotic index in HE sections) after partial hepatectomy. AS-19 administration totally reversed the observed attenuation of liver regeneration.

Conclusions

In conclusion, 5-HT₇ receptor is a novel type of serotonin receptor implicated in hepatocyte proliferation.

Intracellular calcium levels determine differential modulation of allosteric interactions within G protein-coupled receptor heteromers

The pharmacological significance of the adenosine A2A receptor (A2AR)-dopamine D2 receptor (D2R) heteromer is well established and it is being considered as an important target for the treatment of Parkinson’s disease and other neuropsychiatric disorders. However, the physiological factors that control its distinctive biochemical properties are still unknown. We demonstrate that different intracellular Ca2+ levels exert a differential modulation of A2AR-D2R heteromer-mediated adenylyl-cyclase and MAPK signaling in striatal cells. This depends on the ability of low and high Ca2+ levels to promote a selective interaction of the heteromer with the neuronal Ca2+-binding proteins NCS-1 and calneuron-1, respectively. These Ca2+-binding proteins differentially modulate allosteric interactions within the A2AR-D2R heteromer, which constitutes a unique cellular device that integrates extracellular (adenosine and dopamine) and intracellular (Ca+2) signals to produce a specific functional response.

Immunity, inflammation and cancer: a leading role for adenosine

Cancer is a complex disease that is dictated by both cancer cell-intrinsic and cell-extrinsic processes. Adenosine is an ancient extracellular signalling molecule that can regulate almost all aspects of tissue function. As such, several studies have recently highlighted a crucial role for adenosine signalling in regulating the various aspects of cell-intrinsic and cell-extrinsic processes of cancer development. This Review critically discusses the role of adenosine and its receptors in regulating the complex interplay among immune, inflammatory, endothelial and cancer cells during the course of neoplastic disease.

Activation of cAMP-response element-binding protein is positively regulated by PKA and calcium-sensitive calcineurin and negatively by PKC in insect

The cAMP response element binding protein, CREB, is a G protein-coupled receptor (GPCR) signal-activated transcription factor implicated in the control of many biological processes. In the current study, we constructed a cAMP response element (CRE)-driven luciferase assay system for GPCR characterization in insect cells. Our results indicated that Gs-coupled Bombyx adipokinetic hormone receptor (AKHR) and corazonin receptor could effectively initiate CRE-driven luciferase transcription, but forskolin, a reagent widely used to activate adenylyl cyclase in mammalian systems, failed to induce luciferase activity in insect cells co-transfected with a CRE-driven reporter construct upon agonist treatment. Further investigation revealed that the specific protein kinase C (PKC) inhibitors exhibited stimulatory effects on CRE-driven reporter transcription, and blockage of Ca(2+) signals and inhibition of Ca(2+)-dependent calcineurin resulted in a significant decrease in the luciferase activity. Taken together, these results suggest that PKC likely acts as a negative regulator to modulate CREB activation; in contrast, Ca(2+) signals and Ca(2+)-dependent calcineurin, in addition to PKA, essentially contribute to the positive regulation of CREB activity. This study presents evidence to elucidate the underlying molecular mechanism by which CREB activation is regulated in insects.

Reengineering the collision coupling and diffusion mode of the A2A-adenosine receptor: palmitoylation in helix 8 relieves confinement

The A(2A)-adenosine receptor undergoes restricted collision coupling with its cognate G protein G(s) and lacks a palmitoylation site at the end of helix 8 in its intracellular C terminus. We explored the hypothesis that there was a causal link between the absence of a palmitoyl moiety and restricted collision coupling by introducing a palmitoylation site. The resulting mutant A(2A)-R309C receptor underwent palmitoylation as verified by both mass spectrometry and metabolic labeling. In contrast to the wild type A(2A) receptor, the concentration-response curve for agonist-induced cAMP accumulation was shifted to the left with increasing expression levels of A(2A)-R309C receptor, an observation consistent with collision coupling. Single particle tracking of quantum dot-labeled receptors confirmed that wild type and mutant A(2A) receptor differed in diffusivity and diffusion mode; agonist activation resulted in a decline in mean square displacement of both receptors, but the drop was substantially more pronounced for the wild type receptor. In addition, in the agonist-bound state, the wild type receptor was frequently subject to confinement events (estimated radius 110 nm). These were rarely seen with the palmitoylated A(2A)-R309C receptor, the preferred diffusion mode of which was a random walk in both the basal and the agonist-activated state. Taken together, the observations link restricted collision coupling to diffusion limits imposed by the absence of a palmitoyl moiety in the C terminus of the A(2A) receptor. The experiments allowed for visualizing local confinement of an agonist-activated G protein-coupled receptor in an area consistent with the dimensions of a lipid raft.

Ser/ Thr residues at α3/β5 loop of Gαs are important in morphine-induced adenylyl cyclase sensitization but not mitogen-activated protein kinase phosphorylation

The signaling switch of β2-adrenergic and μ(1) -opioid receptors from stimulatory G-protein (G(αs) ) to inhibitory G-protein (G(αi) ) (and vice versa) influences adenylyl cyclase (AC) and extracellular-regulated kinase (ERK)1/2 activation. Post-translational modifications, including dephosphorylation of G(αs) , enhance opioid receptor coupling to G(αs) . In the present study, we substituted the Ser/Thr residues of G(αs) at the α3/β5 and α4/β6 loops aiming to study the role of G(αs) lacking Ser/Thr phosphorylation with respect to AC sensitization and mitogen-activated protein kinase activation. Isoproterenol increased the cAMP concentration (EC(50) = 22.8 ± 3.4 μm) in G(αs) -transfected S49 cyc- cells but not in nontransfected cells. However, there was no significant difference between the G(αs) -wild-type (wt) and mutants. Morphine (10 μm) inhibited AC activity more efficiently in cyc- compared to G(αs) -wt introduced cells (P < 0.05); however, we did not find a notable difference between G(αs) -wt and mutants. Interestingly, G(αs) -wt transfected cells showed more sensitization with respect to AC after chronic morphine compared to nontransfected cells (101 ± 12% versus 34 ± 6%; P < 0.001); μ1-opioid receptor interacted with G(αs) , and both co-immunoprecipitated after chronic morphine exposure. Furthermore, mutation of T270A and S272A (P < 0.01), as well as T270A, S272A and S261A (P < 0.05), in α3/β5, resulted in a higher level of AC supersensitization. ERK1/2 phosphorylation was rapidly induced by isoproterenol (by 9.5 ± 2.4-fold) and morphine (22 ± 2.2-fold) in G(αs) -transfected cells; mutations of α3/β5 and α4/β6 did not affect the pattern or extent of mitogen-activated protein kinase activation. The findings of the present study show that G(αs) interacts with the μ1-opioid receptor, and the Ser/Thr mutation to Ala at the α3/β5 loop of G(αs) enhances morphine-induced AC sensitization. In addition, G(αs) was required for the rapid phosphorylation of ERK1/2 by isoproterenol but not morphine.

Control of neurite outgrowth by RhoA inactivation

cAMP induces neurite outgrowth in the rat pheochromocytoma cell line 12 (PC12). In particular, di-butyric cAMP (db-cAMP) induces a greater number of primary processes with shorter length than the number induced by nerve growth factor (NGF). db-cAMP up- and down-regulates GTP-RhoA levels in PC12 cells in a time-dependent manner. Tat-C3 toxin stimulates neurite outgrowth, whereas lysophosphatidic acid (LPA) and constitutively active (CA)-RhoA reduce neurite outgrowth, suggesting that RhoA inactivation is essential for the neurite outgrowth from PC12 cells stimulated by cAMP. In this study, the mechanism by which RhoA is inactivated in response to cAMP was examined. db-cAMP induces phosphorylation of RhoA and augments the binding of RhoA with Rho guanine nucleotide dissociation inhibitor (GDI). Moreover, RhoA (S188D) mimicking phosphorylated RhoA induces greater neurite outgrowth than RhoA (S188A) mimicking dephosphorylated form does. Additionally, db-cAMP increases GTP-Rap1 levels, and dominant negative (DN)-Rap1 and DN-Rap-dependent RhoGAP (ARAP3) block neurite outgrowth induced by db-cAMP. DN-p190RhoGAP and the Src inhibitor PP2 suppress neurite outgrowth, whereas transfection of c-Src and p190RhoGAP cDNAs synergistically stimulate neurite outgrowth. Taken together, RhoA is inactivated by phosphorylation of itself, by p190RhoGAP which is activated by Src, and by ARAP3 which is activated by Rap1 during neurite outgrowth from PC12 cells in response to db-cAMP.

Signal integration and coincidence detection in the mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) cascade: concomitant activation of receptor tyrosine kinases and of LRP-1 leads to sustained ERK phosphorylation via down-regulation of dual specificity phosphatases (DUSP1 and -6)

Diverse stimuli can feed into the MAPK/ERK cascade; this includes receptor tyrosine kinases, G protein-coupled receptors, integrins, and scavenger receptors (LDL receptor-related protein (LRP)). Here, we investigated the consequence of concomitant occupancy of the receptor tyrosine kinases (by EGF, basic FGF, VEGF, etc.) and of LRP family members (by LDL or lactoferrin). The simultaneous stimulation of a receptor tyrosine kinase by its cognate ligand and of LRP-1 (by lactoferrin or LDL) resulted in sustained activation of ERK, which was redirected to the cytoplasm. Accordingly, elevated levels of active cytosolic ERK were translated into accelerated adhesion to vitronectin. The sustained ERK response was seen in several cell types, but it was absent in cells deficient in LRP-1 (but not in cells lacking the LDL receptor). This response was also contingent on the presence of urokinase (uPA) and its receptor (uPAR), because it was absent in uPA^−/− and uPAR^−/− fibroblasts. Combined stimulation of the EGF receptor and of LRP-1 delayed nuclear accumulation of phosphorylated ERK. This shift in favor of cytosolic accumulation of phospho-ERK was accounted for by enhanced proteasomal degradation of dual specificity phosphatases DUSP1 and DUSP6, which precluded dephosphorylation of cytosolic ERK. These observations demonstrate that the ERK cascade can act as a coincidence detector to decode the simultaneous engagement of a receptor tyrosine kinase and of LRP-1 and as a signal integrator that encodes this information in a spatially and temporally distinct biological signal. In addition, the findings provide an explanation of why chronic elevation of LRP-1 ligands (e.g. PAI-1) can predispose to cancer.

Functional selectivity of adenosine receptor ligands

Adenosine receptors are plasma membrane proteins that transduce an extracellular signal into the interior of the cell. Basically every mammalian cell expresses at least one of the four adenosine receptor subtypes. Recent insight in signal transduction cascades teaches us that the current classification of receptor ligands into agonists, antagonists, and inverse agonists relies very much on the experimental setup that was used. Upon activation of the receptors by the ubiquitous endogenous ligand adenosine they engage classical G protein-mediated pathways, resulting in production of second messengers and activation of kinases. Besides this well-described G protein-mediated signaling pathway, adenosine receptors activate scaffold proteins such as β-arrestins. Using innovative and sensitive experimental tools, it has been possible to detect ligands that preferentially stimulate the β-arrestin pathway over the G protein-mediated signal transduction route, or vice versa. This phenomenon is referred to as functional selectivity or biased signaling and implies that an antagonist for one pathway may be a full agonist for the other signaling route. Functional selectivity makes it necessary to redefine the functional properties of currently used adenosine receptor ligands and opens possibilities for new and more selective ligands. This review focuses on the current knowledge of functionally selective adenosine receptor ligands and on G protein-independent signaling of adenosine receptors through scaffold proteins.

Src family kinase inhibitor PP2 efficiently inhibits cervical cancer cell proliferation through down-regulating phospho-Src-Y416 and phospho-EGFR-Y1173

Tyrosine (Y) kinases inhibitors have been approved for targeted treatment of cancer. However, their clinical use is limited to some cancers and the mechanism of their action remains unclear. Previous study has indicated that PP2, a selective inhibitor of the Src family of non-receptor tyrosine kinases (nRTK), efficiently repressed cervical cancer growth in vitro and in vivo. In this regard, our aims are to explore the mechanism of PP2 on cervical cancer cell growth inhibition by investigating the suppressive divergence among PP1, PP2, and a negative control compound PP3. MTT results showed that three compounds had different inhibitory effects on proliferation of two cervical cancer cells, HeLa and SiHa, and PP2 was most efficient in a time- and dose-dependent manner. Moreover, we found 10 μM PP2 down-regulated pSrc-Y416 (P < 0.05), pEGFR-Y845 (P < 0.05), and -Y1173 (P < 0.05) expression levels, while 10 μM PP1 down-regulated pSrc-Y416 (P < 0.05) and pEGFR-Y845 (P < 0.05), but not pEGFR-Y1173; 10 μM PP3 down-regulated only pEGFR-Y1173 (P < 0.05). PP2 could modulate cell cycle arrest by up-regulating p21(Cip1) and p27(Kip1) in both HeLa and SiHa cells and down-regulating expression of cyclin A, and cyclin dependent kinase-2, -4 (Cdk-2, -4) in HeLa and of cyclin B and Cdk-2 in SiHa. Our results indicate that Src pathway and EGFR pathway play different roles in the proliferation of cervical cancer cells and PP2 efficiently reduces cervical cancer cell proliferation by reduction of both Src and EGFR activity.

A unique interplay between Rap1 and E-cadherin in the endocytic pathway regulates self-renewal of human embryonic stem cells

Regulatory mechanisms pertaining to the self-renewal of stem cells remain incompletely understood. Here, we show that functional interactions between small GTPase Rap1 and the adhesion molecule E-cadherin uniquely regulate the self-renewal of human embryonic stem cells (hESCs). Inhibition of Rap1 suppresses colony formation and self-renewal of hESCs, whereas overexpression of Rap1 augments hESC clonogenicity. Rap1 does not directly influence the expression of the pluripotency genes Oct4 and Nanog. Instead, it affects the endocytic recycling pathway involved in the formation and maintenance of E-cadherin-mediated cell-cell cohesion, which is essential for the colony formation and self-renewal of hESCs. Conversely, distinct from epithelial cells, disruption of E-cadherin mediated cell-cell adhesions induces lysosome delivery and degradation of Rap1. This in turn leads to a further downregulation of E-cadherin function and a subsequent reduction in hESC clonogenic capacity. These findings provide the first demonstration that the interplay between Rap1 and E-cadherin along the endocytic recycling pathway serves as a timely and efficient mechanism to regulate hESC self-renewal. Given the availability of specific activators for Rap1, this work provides a new perspective to enable better maintenance of human pluripotent stem cells.

Ligand-directed signalling at beta-adrenoceptors

beta-Adrenoceptors (ARs) classically mediate responses to the endogenous ligands adrenaline and noradrenaline by coupling to Gsalpha and stimulating cAMP production; however, drugs designed as beta-AR agonists or antagonists can activate alternative cell signalling pathways, with the potential to influence clinical efficacy. Furthermore, drugs acting at beta-ARs have differential capacity for pathway activation, described as stimulus trafficking, biased agonism, functional selectivity or ligand-directed signalling. These terms refer to responses where drug A has higher efficacy than drug B for one signalling pathway, but a lower efficacy than drug B for a second pathway. The accepted explanation for such responses is that drugs A and B have the capacity to induce or stabilize distinct active conformations of the receptor that in turn display altered coupling efficiency to different effectors. This is consistent with biophysical studies showing that drugs can indeed promote distinct conformational states. Agonists acting at beta-ARs display ligand-directed signalling, but many drugs acting as cAMP antagonists are also able to activate signalling pathways central to cell survival and proliferation or cell death. The observed complexity of drug activity at beta-ARs, prototypical G protein-coupled receptors, necessitates rethinking of the approaches used for screening and characterization of novel therapeutic agents. Most studies of ligand-directed signalling employ recombinant cell systems with high receptor abundance. While such systems are valid for examining upstream signalling events, such as receptor conformational changes and G protein activation, they are less robust when comparing downstream signalling outputs as these are likely to be affected by complex pathway interactions.

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.

Activation of SRC kinase and phosphorylation of signal transducer and activator of transcription-5 are required for decidual transformation of human endometrial stromal cells

Progesterone induces decidual transformation of estrogen-primed human endometrial stromal cells (hESCs), critical for implantation and maintenance of pregnancy, through activation of many signaling pathways involving protein kinase A and signal transducer and activator of transcription (STAT)-5. We have previously shown that kinase activation of v-src sarcoma (Schmidt-Ruppin A-2) viral oncogene homolog (SRC) kinase is closely associated with decidualization and that SRC is indispensable for maximal decidualization in mice. To address whether SRC kinase activity is essential for decidualization in humans, hESCs were infected with adenoviruses carrying enhanced green fluorescent protein alone (Ad-EGFP), a kinase-inactive dominant-negative mutant (Ad-SRC/K295R), or an inactive autophosphorylation site mutant (Ad-SRC/Y416F). The cells were cultured in the presence of estradiol and progesterone (EP) to induce decidualization and subjected to RT-PCR, immunoblot, and ELISA analyses. Ad-EGFP-infected hESCs exhibited decidual transformation and up-regulation of decidualization markers including IGF binding protein 1 and prolactin in response to 12-d treatment with EP. In contrast, hESCs infected with Ad-SRC/K295R remained morphologically fibroblastoid without production of IGF binding protein 1 and prolactin even after EP treatment. Ad-SRC/Y416F displayed similar but less inhibitory effects on decidualization, compared with Ad-SRC/K295R. During decidualization, STAT5 was phosphorylated on tyrosine 694, a well-known SRC phosphorylation site. Phosphorylation was markedly attenuated by Ad-SRC/K295R but not Ad-EGFP. These results indicate that the SRC-STAT5 pathway is essential for decidualization of hESCs.

Na/H exchange regulatory factor 1, a novel AKT-associating protein, regulates extracellular signal-regulated kinase signaling through a B-Raf-mediated pathway

Na/H exchange regulatory factor 1 (NHERF1) is a scaffolding protein that regulates signaling and trafficking of several G protein-coupled receptors (GPCRs), including the parathyroid hormone receptor (PTH1R). GPCRs activate extracellular signal-regulated kinase (ERK)1/2 through different mechanisms. Here, we characterized NHERF1 regulation of PTH1R-stimulated ERK1/2. Parathyroid hormone (PTH) stimulated ERK1/2 phosphorylation by a protein kinase A (PKA)-dependent, but protein kinase C-, cyclic adenosine 5'-monophosphate-, and Rap1-independent pathway in Chinese hamster ovary cells stably transfected with the PTH1R and engineered to express NHERF1 under the control of tetracycline. NHERF1 blocked PTH-induced ERK1/2 phosphorylation downstream of PKA. This suggested that NHERF1 inhibitory effects on ERK1/2 occur at a postreceptor locus. Forskolin activated ERK1/2, and this effect was blocked by NHERF1. NHERF1 interacted with AKT and inhibited ERK1/2 activation by decreasing the stimulatory effect of 14-3-3 binding to B-Raf, while increasing the inhibitory influence of AKT negative regulation on ERK1/2 activation. This novel regulatory mechanism provides a new model by which cytoplasmic adapter proteins modulate ERK1/2 activation through a receptor-independent mechanism involving B-Raf.

The A(2A)-adenosine receptor: a GPCR with unique features?

The A(2A)-adenosine receptor is a prototypical G(s)-coupled receptor. However, the A(2A)-receptor has several structural and functional characteristics that make it unique. In contrast to the classical model of collision coupling described for the beta-adrenergic receptors, the A(2A)-receptor couples to adenylyl cyclase by restricted collision coupling and forms a tight complex with G(s). The mechanistic basis for this is not clear; restricted collision coupling may arise from the interaction of the receptor with additional proteins or due to the fact that G protein-coupling is confined to specialized membrane microdomains. The A(2A)-receptor has a long C-terminus (of >120 residues), which is for the most part dispensable for coupling to G(s). It was originally viewed as the docking site for kinases and the beta-arrestin family to initiate receptor desensitization and endocytosis. The A(2A)-receptor is, however, fairly resistant to agonist-induced internalization. Recently, the C-terminus has also been appreciated as a binding site for several additional 'accessory' proteins. Established interaction partners include alpha-actinin, ARNO, USP4 and translin-associated protein-X. In addition, the A(2A)-receptor has also been reported to form a heteromeric complex with the D(2)-dopamine receptor and the metabotropic glutamate receptor-5. It is clear that (i) this list cannot be exhaustive and (ii) that all these proteins cannot bind simultaneously to the receptor. There must be rules of engagement, which allow the receptor to elicit different biological responses, which depend on the cellular context and the nature of the concomitant signal(s). Thus, the receptor may function as a coincidence detector and a signal integrator.

Restricted collision coupling of the A2A receptor revisited: evidence for physical separation of two signaling cascades

The A(2A)-adenosine receptor is a prototypical G(s) protein-coupled receptor but stimulates MAPK/ERK in a G(s)-independent way. The A(2A) receptor has long been known to undergo restricted collision coupling with G(s); the mechanistic basis for this mode of coupling has remained elusive. Here we visualized agonist-induced changes in mobility of the yellow fluorescent protein-tagged receptor by fluorescence recovery after photobleaching microscopy. Stimulation with a specific A(2A) receptor agonist did not affect receptor mobility. In contrast, stimulation with dopamine decreased the mobility of the D(2) receptor. When coexpressed in the same cell, the A(2A) receptor precluded the agonist-induced change in D(2) receptor mobility. Thus, the A(2A) receptor did not only undergo restricted collision coupling, but it also restricted the mobility of the D(2) receptor. Restricted mobility was not due to tethering to the actin cytoskeleton but was, in part, related to the cholesterol content of the membrane. Depletion of cholesterol increased receptor mobility but blunted activation of adenylyl cyclase, which was accounted for by impaired formation of the ternary complex of agonist, receptor, and G protein. These observations support the conclusion that the A(2A) receptor engages G(s) and thus signals to adenylyl cyclase in cholesterol-rich domains of the membrane. In contrast, stimulation of MAPK by the A(2A) receptor was not impaired. These findings are consistent with a model where the recruitment of these two pathways occurs in physically segregated membrane microdomains. Thus, the A(2A) receptor is the first example of a G protein-coupled receptor documented to select signaling pathways in a manner dependent on the lipid microenvironment of the membrane.

When a G protein-coupled receptor does not couple to a G protein

Classically, G protein-coupled receptors (GPCRs) relay signals by directly activating heterotrimeric guanine nucleotide-binding proteins (G proteins). Increasing evidence indicates that GPCRs may also signal through G protein-independent pathways. JAK/STATs, Src-family tyrosine kinases, GRKs/beta-arrestins, and PDZ domain-containing proteins have been suggested to directly relay signals from GPCRs independent of G proteins. In addition, our laboratory recently reported that the beta(2) adrenergic receptor (beta(2)AR) could switch from G protein-coupled to G protein-independent ERK (extracellular signal-regulated kinase) activation in an agonist dosage-dependent manner. This finding provides a novel mechanism for G protein-independent GPCR signaling. This review focuses on recent progress in understanding the mechanisms by which G protein-independent GPCR signaling occurs.

Adenosine A2A receptor occupancy stimulates collagen expression by hepatic stellate cells via pathways involving protein kinase A, Src, and extracellular signal-regulated kinases 1/2 signaling cascade or p38 mitogen-activated protein kinase signaling pathway

Prior studies indicate that adenosine and the adenosine A2A receptor play a role in hepatic fibrosis by a mechanism that has been proposed to involve direct stimulation of hepatic stellate cells (HSCs). The objective of this study was to determine whether primary hepatic stellate cells produce collagen in response to adenosine (via activation of adenosine A2A receptors) and to further determine the signaling mechanisms involved in adenosine A2A receptor-mediated promotion of collagen production. Cultured primary HSCs increase their collagen production after stimulation of the adenosine A2A receptor in a dose-dependent fashion. Likewise, LX-2 cells, a human HSC line, increases expression of procollagen alphaI and procollagen alphaIII mRNA and their translational proteins, collagen type I and type III, in response to pharmacological stimulation of adenosine A2A receptors. Based on the use of pharmacological inhibitors of signal transduction, adenosine A2A receptor-mediated stimulation of procollagen alphaI mRNA and collagen type I collagen expression were regulated by signal transduction involving protein kinase A, src, and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (erk), but surprisingly, adenosine A2A receptor-mediated stimulation of procollagen alphaIII mRNA and collagen type III protein expression depend on the activation of p38 mitogen-activated protein kinase (MAPK), findings confirmed by small interfering RNA-mediated knockdown of src, erk1, erk2, and p38 MAPK. These results indicate that adenosine A2A receptors signal for increased collagen production by multiple signaling pathways. These results provide strong evidence in support of the hypothesis that adenosine receptors promote hepatic fibrosis, at least in part, via direct stimulation of collagen expression and that signaling for collagen production proceeds via multiple pathways.

Stress hormones regulate interleukin-6 expression by human ovarian carcinoma cells through a Src-dependent mechanism

Recent studies have demonstrated that chronic stress promotes tumor growth, angiogenesis, and metastasis. In ovarian cancer, levels of the pro-angiogenic cytokine, interleukin 6 (IL-6), are known to be elevated in individuals experiencing chronic stress, but the mechanism(s) by which this cytokine is regulated and its role in tumor growth remain under investigation. Here we show that stress hormones such as norepinephrine lead to increased expression of IL-6 mRNA and protein levels in ovarian carcinoma cells. Furthermore, we demonstrate that norepinephrine stimulation activates Src tyrosine kinase and this activation is required for increased IL-6 expression. These results demonstrate that stress hormones activate signaling pathways known to be critical in ovarian tumor progression.

The Novel Cellular Mechanism of Human 5-HT6 Receptor through an Interaction with Fyn

The human 5-HT(6) receptor (5-HT(6)R) is one of the latest cloned receptors among the known 5-HT receptors. Its abundant distribution in the limbic region, which participates in the control of mood and emotion and is involved in nervous system diseases such as depression and Alzheimer disease, has caused it to generate much interest. However, the cellular mechanisms of 5-HT(6)R are poorly understood. In the present study we found, using a yeast two-hybrid assay, that the carboxyl-terminal region of 5-HT(6)R interacts with the Fyn-tyrosine kinase. We also determined using a glutathione S-transferase pulldown assay that this interaction was mediated through the SH3 domain of Fyn and confirmed this by co-immunoprecipitation assays in two different transfected cell lines as well as in adult rat brains. Immunocyto(histo)chemistry also showed prominent co-localization between 5-HT(6)R and Fyn in transfected cells and a similar distribution between 5-HT(6)R and Fyn in the rat brain. Based on this interaction, we further examined the modulation of 5-HT(6)R by Fyn and vice versa. In addition, we demonstrated that the activation of 5-HT(6)R activated the extracellular signal-regulated kinase1/2 via an Fyn-dependent pathway. These findings suggest that Fyn may play an important role in 5-HT(6)R- mediated signaling pathways in the central nervous system.

Dosage-dependent switch from G protein-coupled to G protein-independent signaling by a GPCR

G-protein-coupled receptors (GPCRs) mostly signal through heterotrimeric G proteins. Increasing evidence suggests that GPCRs could function in a G-protein-independent manner. Here, we show that at low concentrations of an agonist, beta(2)-adrenergic receptors (beta(2)-ARs) signal through Galpha(s) to activate the mitogen-activated protein kinase pathway in mouse embryonic fibroblast cells. At high agonist concentrations, signals are also transduced through beta(2)-ARs via an additional pathway that is G-protein-independent but tyrosine kinase Src-dependent. This new dosage-dependent switch of signaling modes of GPCRs has significant implications for GPCR intrinsic properties and desensitization.

Epac- and Rap- independent ERK1/2 phosphorylation induced by Gs-coupled receptor stimulation in HEK293 cells

Serotonin activates Ras and Ras-dependent ERK1/2 phosphorylation in HEK293 cells expressing G(s)-coupled 5-HT(4) or 5-HT(7) serotonin receptors through unknown mechanisms. Both Epac/Rap-dependent and -independent pathways for Ras-dependent ERK1/2 activation have been suggested. Epac overexpression or Epac-specific 8-CPT-2'-O-Me-cAMP did not cause ERK1/2 phosphorylation, despite Rap activation. The data did not support a role for PLCepsilon or DAG-dependent Ras GEFs of the Ras-GRP family in Ras-dependent ERK1/2 phosphorylation. However, serotonin stimulated phosphorylation of endogenous and recombinant Ras-GRF1, increased [Ca(2+)](i) and caused Ca(2+)- and calmodulin-dependent ERK1/2 phosphorylation. Different signalling pathways seem to be utilised by G(s)-coupled receptors in various isolates of HEK293 cells.

Independent regulation of Rap1 and mitogen-activated protein kinase by the alpha chain of Go

Receptors coupled to G(i/o) proteins stimulate the mitogen-activated protein kinase (MAPK) cascade. The intracellular pathways linking the alpha chains of these G proteins to MAPK activation are not completely understood. One of the signaling molecules which has been suggested to act downstream of Galpha(i/o) is the small G protein Rap1. We investigated the role of Rap1 in MAPK stimulation by Galpha(o) in Chinese hamster ovary (CHO) cells. Our previous results have shown that in this cell system activated Galpha(o) strongly potentiates the MAPK response to the epidermal growth factor (EGF) receptor. Rap1 regulation was examined in cells transfected with Rap1 and wild-type Galpha(o) or the activated mutant Galpha(o)-Q205L. Immunocytochemical analysis detected both Rap1 and the Galpha(o) subunit at the plasma membrane as well as on perinuclear cytoplasmic vesicles. Expression of wild-type Galpha(o) had no significant effect on the levels of activated Rap1. In contrast, Galpha(o)-Q205L virtually abolished the activation of Rap1 induced by EGF. Further experiments showed that MAPK stimulation by EGF was greatly inhibited by expression of activated Rap1, suggesting that Rap1 inhibition could mediate the effect of Galpha(o) on the MAPK cascade. However, Galpha(o)-Q205L efficiently inhibited the activation of Rap1 induced by fibroblast growth factor (FGF). We have previously found that the ability of FGF to activate MAPK is not modified by Galpha(o). In addition, expression of the GAP protein RAP1GAPII blocked Rap1 activation without affecting EGF- or FGF-dependent MAPK stimulation. These findings provide evidence for independent regulation of Rap1 and MAPK by the G(o )alpha chain.

A tail of two signals: the C terminus of the A(2A)-adenosine receptor recruits alternative signaling pathways

G protein-coupled receptors are endowed with carboxyl termini that vary greatly in length and sequence. In most instances, the distal portion of the C terminus is dispensable for G protein coupling. This is also true for the A(2A)-adenosine receptor, where the last 100 amino acids are of very modest relevance to G(s) coupling. The C terminus was originally viewed mainly as the docking site for regulatory proteins of the beta-arrestin family. These beta-arrestins bind to residues that have been phosphorylated by specialized kinases (G protein-coupled receptor kinases) and thereby initiate receptor desensitization and endocytosis. More recently, it has become clear that many additional "accessory" proteins bind to C termini of G protein-coupled receptors. The article by Sun et al. in the current issue of Molecular Pharmacology identifies translin-associated protein-X as yet another interaction partner of the A(2A) receptor; translin-associated protein allows the A(2A) receptor to impinge on the signaling mechanisms by which p53 regulates neuronal differentiation, but the underlying signaling pathways are uncharted territory. With a list of five known interaction partners, the C terminus of the A(2A) receptor becomes a crowded place. Hence, there must be rules that regulate the interaction. This allows the C terminus to act as coincidence detector and as signal integrator. Despite our ignorance about the precise mechanisms, the article has exciting implications: the gene encoding for translin-associated protein-X maps to a locus implicated in some forms of schizophrenia; A(2A) receptor agonists are candidate drugs for the treatment of schizophrenic symptoms. It is of obvious interest to explore a possible link.

The Src family kinase, Lyn, is activated in pancreatic acinar cells by gastrointestinal hormones/neurotransmitters and growth factors which stimulate its association with numerous other signaling molecules

Src family kinases (SFK) play a central signaling role for growth factors, cytokines, G-protein-coupled receptors and other stimuli. SFKs play important roles in pancreatic acinar cell secretion, endocytosis, growth, cytoskeletal integrity and apoptosis, although little is known of the specific SFKs involved. In this study we demonstrate the SFK, Lyn, is present in rat pancreatic acini and investigate its activation/signaling. Ca(2+)-mobilizing agents, cAMP-mobilizing agents and pancreatic growth factors activated Lyn. CCK, a physiological regulator of pancreatic function, rapidly activated Lyn. The specific SFK inhibitor, PP2, decreased Lyn activation; however, the inactive analogue, PP3, had no effect. Inhibition of CCK-stimulated changes in [Ca(2+)](i) decreased Lyn activation by 55%; GFX, a PKC inhibitor by 36%; and the combination by 95%. CCK activation of Lyn required stimulation of high and low affinity CCK(A) receptor states. CCK stimulated an association of Lyn with PKC-delta, Shc, p125(FAK) and PYK2 as well as with their autophosphorylated forms, but not with Cbl, p85, p130(CAS) or ERK 1/2. These results show Lyn is activated by diverse pancreatic stimulants. CCK's activation of Lyn is likely an important mediator of its ability to cause tyrosine phosphorylation of numerous important cellular mediators such as p125(FAK), PYK2, PKC-delta and Shc, which play central roles in CCK's effects on acinar cell function.

Vaccination with cetuximab mimotopes and biological properties of induced anti-epidermal growth factor receptor antibodies

BACKGROUND

The monoclonal antibody cetuximab (IMC-225, Erbitux) inhibits epidermal growth factor receptor (EGFR) signaling and has been approved for metastatic colon cancer therapy. However, to achieve effective titers, passive antibody therapies must be repeatedly administered over long periods. To overcome this limitation, we aimed to generate a vaccine inducing continuously available "cetuximab-like" antibodies in vivo using the mimotope approach.

METHODS

We used the phage display technique to identify four peptides structurally mimicking the cetuximab epitope. We coupled two of these peptides to an immunogenic carrier protein, and we vaccinated four groups (n = 8) of BALB/c mice intraperitoneally with 10 microg of the mimotope conjugates, a control peptide conjugate, or the carrier protein alone. We assessed antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity mediated by the induced antibodies against EGFR-overexpressing human A431 carcinoma cells. We then tested receptor internalization capacity of the induced antibodies with fluorescently labeled EGFR, and we assayed their growth inhibitory potential toward A431 cells with a [3H]thymidine proliferation assay.

RESULTS

Mimotope-induced antibodies recognized EGFR, and both types of antibody-mediated cytotoxic effects were elicited by these antibodies. In both cellular cytotoxicity assays, the mimotope-induced antibodies exhibited specific lysis of more than 50%. The induced antibodies caused internalization of the receptor from the cell surface into endocytic vesicles and inhibited growth of EGFR-expressing cells to a similar extent as cetuximab [67% (95% confidence interval {CI} = 55% to 79%) and 69% (95% CI = 55% to 84%), respectively].

CONCLUSIONS

Epitope-specific immunization is feasible for active anti-EGFR immunotherapy. The in vitro biologic features of mimotope-induced antibodies are similar to those of the monoclonal antibody cetuximab.

Vasoactive intestinal peptide upregulates MUC2 intestinal mucin via CREB/ATF1

VIP exerts a spectrum of effects as a potent anti-inflammatory factor. In addition, VIP increases expression of MUC2, a major intestinal secretory mucin. We therefore investigated the effects of VIP on the promoter activity of the 5'-flanking region of the MUC2 gene. VIP activated MUC2 transcription in human colonic epithelial cells via cAMP signaling to ERK and p38. cAMP/Epac/Rap1/B-Raf signaling was not involved in MUC2 reporter activation. Furthermore, activation of MUC2 transcription was independent of many of the reported downstream effectors of G protein-coupled receptors, such as PKC, Ras, Raf, Src, calcium, and phosphoinositide 3-kinase. VIP induced cAMP response element-binding protein (CREB)/ATF1 phosphorylation, and this was prevented by treatment with inhibitors of either MEK or p38 and by PKA and MSK1 inhibitor H89. CREB/ATF1 and c-Jun were shown to bind to an oligonucleotide encompassing a distal, conserved CREB/AP1 site in the 5'-flanking region of the MUC2 gene, and this cis element was shown to mediate promoter reporter activation by VIP. This study has identified a new, functional cis element within the MUC2 promoter and also a new pathway regulating MUC2 expression, thus providing further insight into the molecular mechanism of VIP action in the colon. These findings are relevant to the normal biology of the colonic mucosa as well as to the development of VIP as a therapeutic agent for treatment of inflammatory bowel disease.

Regulation of CXCR4-Mediated Nuclear Translocation of Extracellular Signal-Related Kinases 1 and 2

Activation of the chemokine receptor CXCR4 by its agonist stromal cell-derived factor 1 (SDF-1) has been associated with cell migration and proliferation in many cell types, but the intracellular signaling cascades are incompletely defined. Here we show that CXCR4-dependent extracellular signal-regulated kinases 1 and 2 (ERK1/2) phosphorylation was mediated through the Ras/Raf pathway, as demonstrated with a dominant-negative Ras mutant and pharmacological inhibitors. The Src inhibitor 4-amino-5-methylphenyl-7-(t-butyl)pyrazolo[3,4-d] pyrimidine (PP1) and the Rho-kinase (ROCK) inhibitor N-(4-pyridyl)-4-(1-aminoethyl)cyclohexanecarboxamide dihydrochloride (Y27632) also attenuated SDF-1-induced ERK1/2 phosphorylation. Involvement of Src could furthermore be demonstrated by Src phosphorylation and by the shortened ERK1/2 phosphorylation in SYF cells, which are Src/Yes/Fyn-deficient compared with Src-reconstituted Src(++) cells. Membrane translocation of RhoA could be detected similarly. A large portion of the SDF-1-mediated ERK phosphorylation was detected in the nucleus, as shown by Western blotting and confocal microscopy, and resulted in the phosphorylation of the transcription factor Elk. It is interesting that the nuclear accumulation of ERK1/2 and Elk phosphorylation was completely blocked by dominant-negative Rho, Y27632, PP1, and latrunculin B, indicating that the Rho/ROCK pathway, Src kinase, and the actin cytoskeleton were required in this process. In accordance, neither nuclear ERK phosphorylation nor Elk phosphorylation were observed in SYF cells stimulated with SDF-1 but were reconstituted in Src(++) cells. In summary, these results demonstrate that Src, Rho/ROCK, and an intact cytoskeleton contribute to overall ERK1/2 activation in SDF-1-stimulated cells and are indispensable for nuclear translocation of ERK1/2 and activation of transcription factors.

Activation of phospholipase D by 8-Br-cAMP occurs through novel pathway involving Src, Ras, and ERK in human endometrial stromal cells

We investigated the mechanism of 8-Br-cAMP-mediated phospholipase D (PLD) activation using a primary cell culture system of human endometrial stromal cells (ES cells). PLD activity was increased by the treatment of ES cells with 8-Br-cAMP, maximally at 5 min. To determine whether the effects of 8-Br-cAMP on PLD occurred as a consequence of PKC activation, ES cells were preincubated for 15 min with RO320432 (1 microM) and GF109203X (1 microM), the PKC inhibitors, or they were pretreated for 24h with phorbol myristate acetate (100 nM) to downregulate PKC. However, these treatments had no effects on PLD activation induced by 8-Br-cAMP. Furthermore, 8-Br-cAMP had no effects on the subcellular distribution of PKC alpha and PKC betaI, confirming no involvement of PKC. 8-Br-cAMP activated ERK1/2, maximally at 5 min, and PD98059 (MEK inhibitor: 50 microM) and transfection of ES cells with dominant negative (DN)-MEK completely inhibited 8-Br-cAMP-induced PLD activation, suggesting that ERK1/2 mediates the PLD activation. To investigate the involvement of protein kinase A (PKA), Src, and Ras in 8-Br-cAMP-induced PLD activation, we used PKA inhibitor, H89 and Rp-cAMPs, and transfections of DN-Src and DN-Ras. H-89 and Rp-cAMPs completely blocked 8-Br-cAMP-mediated PLD and ERK activation, implying the involvement of PKA in this PLD activation. In addition, transfection of ES cells with DN-Src, or DN-Ras partially inhibited 8-Br-cAMP-induced ERK1/2 and consequently PLD activation, whereas cotransfection of DN-Src and DN-Ras completely inhibited ERK1/2 and PLD activation, suggesting that Src and Ras independently regulate ERK/PLD activation. Taken together, these results demonstrate a novel pathway in ES cells that 8-Br-cAMP activate PLD through PKA and ERK1/2 and this ERK/PLD activation by 8-Br-cAMP is mediated by Src and Ras, separately.

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

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

Characterization of cAMP-dependent proteolysis of GATA-6

Cyclic AMP-dependent proteolysis of GATA-6(Delta50) was characterized using inhibitors for intracellular signaling pathways. Among these kinase inhibitors, only H-89 and K252a inhibited the proteolysis induced by dbcAMP, a membrane permeable cAMP analogue, others such as PD98059, SB203580, calphostine C, PP1, and KN-93 did not do so. These results suggest that A-kinase, but not C-kinase, MEK, P38 MAP-kinases or Src kinase, could participate in the observed phenomenon. We further demonstrated that an inhibitor for ubiquitin isopeptidase (Delta12-PGJ2) inhibited the degradation of GATA-6(Delta50) in the presence of dbcAMP, suggesting that the cAMP-dependent proteolysis could be mediated through the ubiquitin-proteasome pathway, although proteasome activity did not change significantly during dbcAMP treatment. The full-length GATA-6 was also responsive to the induced degradation. Furthermore, mutation of a potential phosphorylation site (Ser-290-->Ala) for A- and C-kinases, and deletion of the PEST sequence of GATA-6 did not abolish the degradation. All these results suggest that cellular factor(s) may play a crucial role in mediating the activation of the cAMP-dependent process.

Heterotrimeric G protein-independent signaling of a G protein-coupled receptor. Direct binding of ARNO/cytohesin-2 to the carboxyl terminus of the A2A adenosine receptor is necessary for sustained activation of the ERK/MAP kinase pathway

The A2A adenosine receptor is a prototypical G(s)-coupled receptor, but it also signals, e.g. to mitogen-activated protein (MAP) kinase, via a pathway that is independent of heterotrimeric G proteins. Truncation of the carboxyl terminus affects the strength of the signal through these alternative pathways. In a yeast two-hybrid interaction hunt, we screened a human brain library for proteins that bound to the juxtamembrane portion of the carboxyl terminus of the A2A receptor. This approach identified ARNO/cytohesin-2, a nucleotide exchange factor for the small (monomeric) G proteins of the Arf (ADP-ribosylation factor) family, as a potential interaction partner. We confirmed a direct interaction by mutual pull down (of fusion proteins expressed in bacteria) and by immunoprecipitation of the proteins expressed in mammalian cells. To circumvent the long term toxicity associated with overexpression of ARNO/cytohesin-2, we created stable cell lines that stably expressed the A2A receptor and where ARNO/cytohesin-2 or the dominant negative version E156K-ARNO/cytohesin-2 was inducible by mifepristone. Cyclic AMP accumulation induced by an A2A-specific agonist was neither altered by ARNO/cytohesin-2 nor by the dominant negative version. This was also true for agonist-induced desensitization. In contrast, expression of dominant negative E156K-ARNO/cytohesin-2 and of dominant negative T27N-Arf6 abrogated the sustained phase of MAP kinase stimulation induced by the A2A receptor. We therefore conclude that ARNO/cytohesin-2 is required to support the alternative, heterotrimeric G protein-independent, signaling pathway of A2A receptor, which is stimulation of MAP kinase.

Tannic acid inhibits cholangiocyte proliferation after bile duct ligation via a cyclic adenosine 5',3'-monophosphate-dependent pathway

Chronic cholestatic diseases are characterized by morphological changes involving cholangiocyte proliferation and functional alterations of secretory capacity. The plant polyphenol tannic acid inhibits the growth of malignant human cholangiocytes. However, the mechanisms by which tannic acid limits excessive cholangiocyte proliferation are unknown. In this study we assessed the effect of tannic acid on cholangiocyte proliferation after bile duct ligation in rats. Tannic acid feeding decreased cholangiocyte proliferation and ductal mass in vivo after bile duct ligation. These changes were associated with functional changes in bile secretion and with decreases of intracellular cyclic adenosine 5',3'-monophosphate. The anti-proliferative effect of tannic acid was associated with a reduction of ERK1,2 phosphorylation. Additionally, tannic acid feeding decreased protein kinase A phosphorylation and activity. Similar changes were observed in isolated cholangiocytes during in vitro incubation with tannic acid. Furthermore, forskolin abolished the anti-proliferative effect of tannic acid on cholangiocyte proliferation after bile duct ligation. In conclusion, the anti-proliferative effects of tannic acid in cholangiocytes involve modulation of ERK1,2 by a cyclic adenosine 5',3'-monophosphate-protein kinase A-dependent pathway. These data suggest that tannic acid may be useful in limiting excessive cholangiocyte proliferation and modulating secretion during cholestasis.

High-molecular-weight melanoma-associated antigen mimotope immunizations induce antibodies recognizing melanoma cells

Size and posttranslational modifications are obstacles in the recombinant expression of high-molecular-weight melanoma-associated antigen (HMW-MAA). Creating a tumor antigen mimic via the phage display technology may be a means to overcome this problem for vaccine design. In this study, we aimed to generate an immunogenic epitope mimic of HMW-MAA. Therefore we screened a linear 9mer phage display peptide library, using the anti-HMW-MAA monoclonal antibody (mAb) 225.28S. This antibody mediates antibody-dependent cellular cytotoxicity (ADCC) and has already been used for anti-idiotype therapy trials. Fifteen peptides were selected by mAb 225.28S in the biopanning procedure. They share a consensus sequence, but show only partial homology to the amino acid sequence of the HMW-MAA core protein, indicating mimicry with a conformational epitope. One mimotope was chosen to be fused to albumin binding protein (ABP) as an immunogenic carrier. Immunoassays with 225.28S indicated that the mimotope fusion protein was folded correctly. Subsequently, the fusion protein was tested for immunogenicity in BALB/c mice. The induced anti-mimotope antibodies recognized HMW-MAA of 518A2 human melanoma cells, whereas sera of mice immunized with the carrier ABP alone showed no reactivity. These anti-mimotope antibodies were capable of inducing specific lysis of 518A2 melanoma cells in ADCC assays with murine effector cells. In conclusion, the presented data indicate that mimotopes fused to an immunogenic carrier are suitable tools to elicit epitope-specific anti-melanoma immune responses.

The cAMP signalling pathway activates CREB through PKA, p38 and MSK1 in NIH 3T3 cells

Cyclic adenosine 3',5'-monophosphate (cAMP) was originally shown to induce gene transcription through activation of cAMP-dependent protein kinase (PKA), and subsequent phosphorylation of the transcription factor cAMP response element-binding protein, CREB, at serine-133. However, elevated cAMP levels may activate multiple signalling pathways with protein kinases that can phosphorylate CREB at serine-133. We analysed the pathways involved in CREB phosphorylation and activation in NIH 3T3 cells exposed to the cAMP elevating agent forskolin. PKA represented the predominant pathway during the burst phase, while the mitogen-activated protein kinase p38 pathway became activated in a PKA-dependent fashion in forskolin treated cells. The phosphorylation kinetics of p38 was delayed compared to PKA activation. Activated p38 stimulated CREB-mediated transcription and potentiated the transcriptional strength of CREB provoked by forskolin. The p38-mediated activation of CREB was inhibited by dominant negative mutants of MSK-1 and by the PKA/MSK-1 inhibitor H89, but not by dominant negative mutants of MSK-2/RSK-B and MAPKAPK2. Our results suggest that forskolin-induced CREB phosphorylation and activation in NIH 3T3 cells is mediated directly by PKA and by a time-delayed PKA-dependent p38/MSK-1 pathway. This bifurcation and time-dependent regulation of the cAMP-responsive signalling pathways may enable the cell to endure and/or enforce a cellular response provoked by a cAMP-elevating stimulus.

Regulation of Ultraviolet B-induced Phosphorylation of Histone H3 at Serine 10 by Fyn Kinase

Ultraviolet B (UVB) induces phosphorylation of histone H3 at serine 10, and mitogen-activated protein kinases are involved in this signal transduction pathway. Here we provide evidence that Fyn kinase, a member of the Src kinase family, is involved in the UVB-induced phosphorylation of histone H3 at serine 10. UVB distinctly increased Fyn kinase activity and phosphorylation. Fyn kinase inhibitors 4-amino-5-(4-chlorophenyl)-7(t-butyl)pyrazol(3,4-d)pyramide and leflunomide, an Src kinase inhibitor, suppressed both UVB-induced phosphorylation of histone H3 at serine 10 and Fyn kinase activity and phosphorylation. UVB-induced phosphorylation of histone H3 at serine 10 was blocked by either a dominant-negative mutant of Fyn (DNM-Fyn) kinase or small interfering RNA of Fyn kinase. UVB-induced phosphorylation and activities of ERKs and protein kinase B/Akt were markedly inhibited by DNM-Fyn kinase. However, DNM-Fyn kinase did not inhibit UVB-induced phosphorylation of p38 MAPK or c-Jun N-terminal kinases. Active Fyn kinase phosphorylated histone H3 at serine 10 in vitro, and the phosphorylated Fyn kinase could translocate into the nucleus of HaCaT cells. These results indicate that Fyn kinase plays a key role in the UVB-induced phosphorylation of histone H3 at serine 10.

PKA phosphorylation of Src mediates Rap1 activation in NGF and cAMP signaling in PC12 cells

Recent studies suggest that the tyrosine kinase Src plays an important role in the hormonal regulation of extracellular signal-regulated kinases (ERKs) via cyclic AMP (cAMP). Src has also been proposed to mediate signals downstream of nerve growth factor (NGF). Here, we report that the cAMP-dependent protein kinase A (PKA) induced the phosphorylation of Src at residue serine17 (S17) in multiple cell types including PC12, Hek293, AtT-20 and CHO cells. In PC12 cells, Src phosphorylation on S17 participates in the activation of the small G protein Rap1 by both cAMP and NGF. In these cells, Rap1 is required for cAMP/PKA signaling to ERKs and also for the sustained activation of ERKs by NGF. The activation of Rap1 by both cAMP and NGF was blocked by PP2, an inhibitor of Src family kinases, and by a Src mutant incapable of being phosphorylated by PKA (SrcS17A), consistent with the requirement of PKA phosphorylation of Src at S17 in these actions. PP2 and SrcS17A also inhibited the Rap1-dependent activation of ERKs by both agents. These results strongly indicate that PKA phosphorylation of Src at S17 is essential for cAMP and NGF signaling in PC12 cells and identify PKA as an important downstream target of NGF. PKA phosphorylation of Src may therefore be required for Rap1 activation in PC12 cells.

Extracellular calcium induces COX-2 in osteoblasts via a PKA pathway

We have shown that extracellular calcium [Ca(+2)](e) induces cyclooxygenase-2 (COX-2) expression and prostaglandin E(2) (PGE(2)) production via an ERK signaling pathway in osteoblasts. In this study, we examined the roles of protein kinase C (PKC) and A (PKA) signaling pathways in the [Ca(+2)](e) induction of COX-2 in primary calvarial osteoblasts from mice transgenic for -371 bp of the COX-2 promoter fused to a luciferase reporter. Neither PKC specific inhibitors nor downregulation of the PKC pathway by phorbol myristate acetate (PMA) affected the [Ca(+2)](e) stimulation of COX-2 mRNA or promoter activity. In contrast, PKA inhibitors, used at doses that inhibited forskolin-stimulated luciferase activity by 90%, reduced [Ca(+2)](e)-stimulated COX-2 mRNA expression and promoter activity by 80-90%. [Ca(+2)](e) also stimulated a 2- to 3-fold increase in cAMP production. Hence, the [Ca(+2)](e) induction of COX-2 mRNA expression and promoter activity was independent of the PKC pathway and dependent on the PKA signaling pathway.

Prostacyclin receptor-mediated activation of extracellular signal-regulated kinases 1 and 2

The prostacyclin mimetic cicaprost increased phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) in Chinese hamster ovary cells transiently expressing human (hIP-CHO) or mouse prostacyclin (mIP-CHO) receptors, but not in human neuroblastoma SK-N-SH cells or rat/mouse neuroblastoma-glioma NG108-15 cells which endogenously express IP receptors. Cicaprost stimulated ERK1/2 activity in hIP-CHO and mIP-CHO cells with EC50 values of 60 and 83 nM, respectively, and this response was significantly inhibited by protein kinase C inhibitors and agents which elevate cyclic AMP. A poor correlation was discovered between the level of ERK1/2 activity and the ability of agents to increase or decrease cyclic AMP production. The potent inhibitory effect of 3-isobutyl-1-methyl xanthine on cicaprost-stimulated phospho-ERK1/2 may be due to inhibition of phosphoinositide 3-kinase. Therefore, IP receptor-mediated activation of ERK1/2 in CHO cells occurs through a Gq/11/protein kinase C-dependent and a phosphoinoside 3-kinase-dependent process which is insensitive to IP receptor-generated cyclic AMP.

Generation of Peptide mimics of the epitope recognized by trastuzumab on the oncogenic protein Her-2/neu

Immunizations with the oncogenic protein Her-2/neu elicit Abs exerting diverse biological effects--depending on epitope specificity, tumor growth may be inhibited or enhanced. Trastuzumab (herceptin) is a growth-inhibitory humanized monoclonal anti-Her-2/neu Ab, currently used for passive immunotherapy in the treatment of breast cancer. However, Ab therapies are expensive and have to be repeatedly administered for long periods of time. In contrast, active immunizations produce ongoing immune responses. Therefore, the study aims to generate peptide mimics of the epitope recognized by trastuzumab for vaccine formulation, ensuring the subsequent induction of tumor growth inhibitory Abs. We used the phage display technique to generate epitope mimics, mimotopes, complementing the screening Ab trastuzumab. Five candidate mimotopes were isolated from a constrained 10 mer library. These peptides were specifically recognized by trastuzumab, and showed distinctive mimicry with Her-2/neu in two experimental setups. Subsequently, immunogenicity of a selected mimotope was examined in BALB/c mice. Immunizations with a synthetic mimotope conjugated to tetanus toxoid resulted in Abs recognizing Her-2/neu in a blotted cell lysate as well as on the SK-BR-3 cell surface. Analogous to trastuzumab, the induced Abs caused internalization of the receptor from the cell surface to endosomal vesicles. These results indicate that the selected mimotopes are suitable for formulation of a breast cancer vaccine because the resulting Abs show similar biological features as trastuzumab.

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

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