A Protein Kinase Inhibitor Gene Reduces Both Basal and Multihormone-stimulated Prolactin Gene Transcription

CREB regulates the expression of type 1 inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) play a central role in regulating intracellular Ca2+ signals in response to a variety of internal and external cues. Dysregulation of IP3R signaling is the underlying cause for numerous pathological conditions. It is well established that the activities of IP3Rs are governed by several post-translational modifications, including phosphorylation by protein kinase A (PKA). However, the long-term effects of PKA activation on expression of IP3R subtypes remains largely unexplored. In this report, we investigate the effects of chronic stimulation and tonic activity of PKA on the expression of IP3R subtypes. We demonstrate that expression of the type 1 IP3R (IP3R1) is augmented upon prolonged activation of PKA or upon ectopic overexpression of cyclic AMP-response element-binding protein (CREB) without altering IP3R2 and IP3R3 abundance. By contrast, inhibition of PKA or blocking CREB diminished IP3R1 expression. We also demonstrate that agonist-induced Ca2+-release mediated by IP3R1 is significantly attenuated upon blocking of CREB. Moreover, CREB - by regulating the expression of KRAS-induced actin-interacting protein (KRAP) - ensures correct localization and licensing of IP3R1. Overall, we report a crucial role for CREB in governing both the expression and correct localization of IP3R1. This article has an associated First Person interview with the first author of the paper.

The Kinase Specificity of Protein Kinase Inhibitor Peptide

G-protein-coupled-receptor (GPCR) signaling is exquisitely controlled to achieve spatial and temporal specificity. The endogenous protein kinase inhibitor peptide (PKI) confines the spatial and temporal spread of the activity of protein kinase A (PKA), which integrates inputs from three major types of GPCRs. Despite its wide usage as a pharmaceutical inhibitor of PKA, it was unclear whether PKI only inhibits PKA activity. Here, the effects of PKI on 55 mouse kinases were tested in in vitro assays. We found that in addition to inhibiting PKA activity, both PKI (6-22) amide and full-length PKIα facilitated the activation of multiple isoforms of protein kinase C (PKC), albeit at much higher concentrations than necessary to inhibit PKA. Thus, our results call for appropriate interpretation of experimental results using PKI as a pharmaceutical agent. Furthermore, our study lays the foundation to explore the potential functions of PKI in regulating PKC activity and in coordinating PKC and PKA activities.

Prostaglandin E1 Inhibits GLI2 Amplification-Associated Activation of the Hedgehog Pathway and Drug Refractory Tumor Growth

Aberrant activation of the Hedgehog (HH) signaling pathway underlines the initiation and progression of a multitude of cancers. The effectiveness of the leading drugs vismodegib (GDC-0449) and sonidegib (LDE225), both Smoothened (SMO) antagonists, is compromised by acquisition of mutations that alter pathway components, notably secondary mutations in SMO and amplification of GLI2, a transcriptional mediator at the end of the pathway. Pharmacologic blockade of GLI2 activity could ultimately overcome these diversified refractory mechanisms, which would also be effective in a broader spectrum of primary tumors than current SMO antagonists. To this end, we conducted a high-content screening directly analyzing the ciliary translocation of GLI2, a key event for GLI2 activation in HH signal transduction. Several prostaglandin compounds were shown to inhibit accumulation of GLI2 within the primary cilium (PC). In particular, prostaglandin E1 (PGE1), an FDA-approved drug, is a potent GLI2 antagonist that overcame resistance mechanisms of both SMO mutagenesis and GLI2 amplification. Consistent with a role in HH pathway regulation, EP4 receptor localized to the PC. Mechanistically, PGE1 inhibited HH signaling through the EP4 receptor, enhancing cAMP-PKA activity, which promoted phosphorylation and degradation of GLI2 via the ubiquitination pathway. PGE1 also effectively inhibited the growth of drug refractory human medulloblastoma xenografts. Together, these results identify PGE1 and other prostaglandins as potential templates for complementary therapeutic development to circumvent resistance to current generation SMO antagonists in use in the clinic. SIGNIFICANCE: These findings show that PGE1 exhibits pan-inhibition against multiple drug refractory activities for Hedgehog-targeted therapies and elicits significant antitumor effects in xenograft models of drug refractory human medulloblastoma mimicking GLI2 amplification.

Prostaglandin E2 Inhibits Histamine-Evoked Ca2+ Release in Human Aortic Smooth Muscle Cells through Hyperactive cAMP Signaling Junctions and Protein Kinase A

In human aortic smooth muscle cells, prostaglandin E₂ (PGE₂) stimulates adenylyl cyclase (AC) and attenuates the increase in intracellular free Ca²⁺ concentration evoked by activation of histamine H₁ receptors. The mechanisms are not resolved. We show that cAMP mediates inhibition of histamine-evoked Ca²⁺ signals by PGE₂. Exchange proteins activated by cAMP were not required, but the effects were attenuated by inhibition of cAMP-dependent protein kinase (PKA). PGE₂ had no effect on the Ca²⁺ signals evoked by protease-activated receptors, heterologously expressed muscarinic M3 receptors, or by direct activation of inositol 1,4,5-trisphosphate (IP₃) receptors by photolysis of caged IP₃. The rate of Ca²⁺ removal from the cytosol was unaffected by PGE₂, but PGE₂ attenuated histamine-evoked IP₃ accumulation. Substantial inhibition of AC had no effect on the concentration-dependent inhibition of Ca²⁺ signals by PGE₂ or butaprost (to activate EP₂ receptors selectively), but it modestly attenuated responses to EP₄ receptors, activation of which generated less cAMP than EP₂ receptors. We conclude that inhibition of histamine-evoked Ca²⁺ signals by PGE₂ occurs through “hyperactive signaling junctions,” wherein cAMP is locally delivered to PKA at supersaturating concentrations to cause uncoupling of H₁ receptors from phospholipase C. This sequence allows digital signaling from PGE₂ receptors, through cAMP and PKA, to histamine-evoked Ca²⁺ signals.

Protein kinase A can block EphA2 receptor-mediated cell repulsion by increasing EphA2 S897 phosphorylation

The EphA2 receptor plays multiple roles in cancer through two distinct signaling mechanisms. In a novel cross-talk, the β2-adrenoceptor/cAMP/PKA axis can promote EphA2 pro-oncogenic, ligand-independent signaling, blocking cell repulsion induced by ligand-dependent signaling. PKA emerges as a third kinase, besides AKT and RSK, that can regulate EphA2.

The EphA2 receptor tyrosine kinase plays key roles in tissue homeostasis and disease processes such as cancer, pathological angiogenesis, and inflammation through two distinct signaling mechanisms. EphA2 “canonical” signaling involves ephrin-A ligand binding, tyrosine autophosphorylation, and kinase activity; EphA2 “noncanonical” signaling involves phosphorylation of serine 897 (S897) by AKT and RSK kinases. To identify small molecules counteracting EphA2 canonical signaling, we developed a high-content screening platform measuring inhibition of ephrin-A1–induced PC3 prostate cancer cell retraction. Surprisingly, most hits from a screened collection of pharmacologically active compounds are agents that elevate intracellular cAMP by activating G protein–coupled receptors such as the β2-adrenoceptor. We found that cAMP promotes phosphorylation of S897 by protein kinase A (PKA) as well as increases the phosphorylation of several nearby serine/threonine residues, which constitute a phosphorylation hotspot. Whereas EphA2 canonical and noncanonical signaling have been viewed as mutually exclusive, we show that S897 phosphorylation by PKA can coexist with EphA2 tyrosine phosphorylation and block cell retraction induced by EphA2 kinase activity. Our findings reveal a novel paradigm in EphA2 function involving the interplay of canonical and noncanonical signaling and highlight the ability of the β2-adrenoceptor/cAMP/PKA axis to rewire EphA2 signaling in a subset of cancer cells.

Impact of kinase activating and inactivating patient mutations on binary PKA interactions

The second messenger molecule cAMP links extracellular signals to intracellular responses. The main cellular cAMP effector is the compartmentalized protein kinase A (PKA). Upon receptor initiated cAMP-mobilization, PKA regulatory subunits (R) bind cAMP thereby triggering dissociation and activation of bound PKA catalytic subunits (PKAc). Mutations in PKAc or RIa subunits manipulate PKA dynamics and activities which contribute to specific disease patterns. Mutations activating cAMP/PKA signaling contribute to carcinogenesis or hormone excess, while inactivating mutations cause hormone deficiency or resistance. Here we extended the application spectrum of a Protein-fragment Complementation Assay based on the Renilla Luciferase to determine binary protein:protein interactions (PPIs) of the PKA network. We compared time- and dose-dependent influences of cAMP-elevation on mutually exclusive PPIs of PKAc with the phosphotransferase inhibiting RIIb and RIa subunits and the protein kinase inhibitor peptide (PKI). We analyzed PKA dynamics following integration of patient mutations into PKAc and RIa. We observed that oncogenic modifications of PKAc(L206R) and RIa(Δ184-236) as well as rare disease mutations in RIa(R368X) affect complex formation of PKA and its responsiveness to cAMP elevation. With the cell-based PKA PPI reporter platform we precisely quantified the mechanistic details how inhibitory PKA interactions and defined patient mutations contribute to PKA functions.

Targeting the CaMKII/ERK Interaction in the Heart Prevents Cardiac Hypertrophy

Aims

Activation of Ca²⁺/Calmodulin protein kinase II (CaMKII) is an important step in signaling of cardiac hypertrophy. The molecular mechanisms by which CaMKII integrates with other pathways in the heart are incompletely understood. We hypothesize that CaMKII association with extracellular regulated kinase (ERK), promotes cardiac hypertrophy through ERK nuclear localization.

Methods and Results

In H9C2 cardiomyoblasts, the selective CaMKII peptide inhibitor AntCaNtide, its penetratin conjugated minimal inhibitory sequence analog tat-CN17β, and the MEK/ERK inhibitor UO126 all reduce phenylephrine (PE)-mediated ERK and CaMKII activation and their interaction. Moreover, AntCaNtide or tat-CN17β pretreatment prevented PE induced CaMKII and ERK nuclear accumulation in H9C2s and reduced the hypertrophy responses. To determine the role of CaMKII in cardiac hypertrophy in vivo, spontaneously hypertensive rats were subjected to intramyocardial injections of AntCaNtide or tat-CN17β. Left ventricular hypertrophy was evaluated weekly for 3 weeks by cardiac ultrasounds. We observed that the treatment with CaMKII inhibitors induced similar but significant reduction of cardiac size, left ventricular mass, and thickness of cardiac wall. The treatment with CaMKII inhibitors caused a significant reduction of CaMKII and ERK phosphorylation levels and their nuclear localization in the heart.

Conclusion

These results indicate that CaMKII and ERK interact to promote activation in hypertrophy; the inhibition of CaMKII-ERK interaction offers a novel therapeutic approach to limit cardiac hypertrophy.

In-vivo detection of binary PKA network interactions upon activation of endogenous GPCRs

Membrane receptor-sensed input signals affect and modulate intracellular protein-protein interactions (PPIs). Consequent changes occur to the compositions of protein complexes, protein localization and intermolecular binding affinities. Alterations of compartmentalized PPIs emanating from certain deregulated kinases are implicated in the manifestation of diseases such as cancer. Here we describe the application of a genetically encoded Protein-fragment Complementation Assay (PCA) based on the Renilla Luciferase (Rluc) enzyme to compare binary PPIs of the spatially and temporally controlled protein kinase A (PKA) network in diverse eukaryotic model systems. The simplicity and sensitivity of this cell-based reporter allows for real-time recordings of mutually exclusive PPIs of PKA upon activation of selected endogenous G protein-coupled receptors (GPCRs) in cancer cells, xenografts of mice, budding yeast, and zebrafish embryos. This extends the application spectrum of Rluc PCA for the quantification of PPI-based receptor-effector relationships in physiological and pathological model systems.

Relaxin activates peroxisome proliferator-activated receptor γ (PPARγ) through a pathway involving PPARγ coactivator 1α (PGC1α)

Relaxin activation of its receptor RXFP1 triggers multiple signaling pathways. Previously, we have shown that relaxin activates PPARγ transcriptional activity in a ligand-independent manner, but the mechanism for this effect was unknown. In this study, we examined the signaling pathways of downstream of RXFP1 leading to PPARγ activation. Using cells stably expressing RXFP1, we found that relaxin regulation of PPARγ activity requires accumulation of cAMP and subsequent activation of cAMP-dependent protein kinase (PKA). The activated PKA subsequently phosphorylated cAMP response element-binding protein (CREB) at Ser-133 to activate it directly, as well as indirectly through mitogen activated protein kinase p38 MAPK. Activated CREB was required for relaxin stimulation of PPARγ activity, while there was no evidence for a role of the nitric oxide or ERK MAPK pathways. Relaxin increased the mRNA and protein levels of the coactivator protein PGC1α, and this effect was dependent on PKA, and was completely abrogated by a dominant-negative form of CREB. This mechanism was confirmed in a hepatic stellate cell line stably that endogenously expresses RXFP1. Reduction of PGC1α levels using siRNA diminished the regulation of PPARγ by relaxin. These results suggest that relaxin activates the cAMP/PKA and p38 MAPK pathways to phosphorylate CREB, resulting in increased PGC1α levels. This provides a mechanism for the ligand-independent activation of PPARγ in response to relaxin.

cAMP/PKA signalling reinforces the LATS-YAP pathway to fully suppress YAP in response to actin cytoskeletal changes

Actin cytoskeletal damage induces inactivation of the oncoprotein YAP (Yes-associated protein). It is known that the serine/threonine kinase LATS (large tumour suppressor) inactivates YAP by phosphorylating its Ser127 and Ser381 residues. However, the events downstream of actin cytoskeletal changes that are involved in the regulation of the LATS-YAP pathway and the mechanism by which LATS differentially phosphorylates YAP on Ser127 and Ser381 in vivo have remained elusive. Here, we show that cyclic AMP (cAMP)-dependent protein kinase (PKA) phosphorylates LATS and thereby enhances its activity sufficiently to phosphorylate YAP on Ser381. We also found that PKA activity is involved in all contexts previously reported to trigger the LATS-YAP pathway, including actin cytoskeletal damage, G-protein-coupled receptor activation, and engagement of the Hippo pathway. Inhibition of PKA and overexpression of YAP cooperate to transform normal cells and amplify neural progenitor pools in developing chick embryos. We also implicate neurofibromin 2 as an AKAP (A-kinase-anchoring protein) scaffold protein that facilitates the function of the cAMP/PKA-LATS-YAP pathway. Our study thus incorporates PKA as novel component of the Hippo pathway.

Compound A, a dissociated glucocorticoid receptor modulator, inhibits T-bet (Th1) and induces GATA-3 (Th2) activity in immune cells

Background

Compound A (CpdA) is a dissociating non-steroidal glucocorticoid receptor (GR) ligand which has anti-inflammatory properties exerted by down-modulating proinflammatory gene expression. By favouring GR monomer formation, CpdA does not enhance glucocorticoid (GC) response element-driven gene expression, resulting in a reduced side effect profile as compared to GCs. Considering the importance of Th1/Th2 balance in the final outcome of immune and inflammatory responses, we analyzed how selective GR modulation differentially regulates the activity of T-bet and GATA-3, master drivers of Th1 and Th2 differentiation, respectively.

Results

Using Western analysis and reporter gene assays, we show in murine T cells that, similar to GCs, CpdA inhibits T-bet activity via a transrepressive mechanism. Different from GCs, CpdA induces GATA-3 activity by p38 MAPK-induction of GATA-3 phosphorylation and nuclear translocation. CpdA effects are reversed by the GR antagonist RU38486, proving the involvement of GR in these actions. ELISA assays demonstrate that modulation of T-bet and GATA-3 impacts on cytokine production shown by a decrease in IFN-γ and an increase in IL-5 production, respectively.

Conclusions

Taken together, through their effect favoring Th2 over Th1 responses, particular dissociated GR ligands, for which CpdA represents a paradigm, hold potential for the application in Th1-mediated immune disorders.

Phosphorylation of RGS13 by the cyclic AMP-dependent protein kinase inhibits RGS13 degradation

Regulators of G-protein signaling (RGS) proteins are scaffolds that control diverse signaling pathways by modulating signalosome formation and by accelerating the GTPase activity of heterotrimeric G proteins. Although expression of many RGS proteins is relatively low in quiescent cells, transcriptional and post-translational responses to environmental cues regulate both their abundance and activity. We found previously that RGS13, one of the smallest RGS proteins in the family, inhibited cyclic AMP-dependent protein kinase (PKA)-induced gene expression through interactions with the transcription factor cAMP-response element-binding (CREB) protein. Here, we show that PKA activation also leads to increased steady-state RGS13 expression through RGS13 phosphorylation, which inhibits RGS13 protein degradation. RGS13 turnover was significantly reduced in cells stimulated with cAMP, which was reversed by expression of the PKA-specific inhibitory peptide PKI. RGS13 phosphorylation was diminished by mutation of an N-terminal Thr residue (T41) identified as a phosphorylation site by mass spectrometry. Mutation of Thr41 in RGS13 to Ala (T41A) reduced steady-state RGS13 levels and its ability to inhibit M2 muscarinic receptor-mediated Erk phosphorylation compared with wild-type RGS13 by attenuating the protective effect of cAMP on RGS13 degradation. RGS13 underwent ubiquitylation, indicating that it is a likely target of the proteasome. These studies are the first to demonstrate post-translational mechanisms controlling the expression of RGS13. Stabilization of RGS13 through PKA-mediated phosphorylation could enhance RGS13 functions, providing negative feedback regulation that promotes cellular desensitization.

The transcription factor prolactin regulatory element-binding protein mediates prolactin transcription induced by thyrotropin-releasing hormone in GH3 cells

The prolactin regulatory element-binding protein (PREB) is a transcription factor that regulates prolactin (PRL) promoter activity in the rat anterior pituitary. PRL gene expression and secretion are regulated by various hormones and growth factors, including dopamine, epidermal growth factor, and thyrotropin-releasing hormone (TRH). We examined the effect of TRH on PREB expression in pituitary cells. Western blots probed with a PREB-specific antiserum showed that the relative abundance of PREB in GH3 cells increased on treatment with TRH in a dose-dependent manner. The relative abundance of PREB mRNA also increased in a dose-dependent manner after treatment with TRH. TRH induced the expression of the luciferase reporter protein under the PREB promoter control. We used inhibitors of certain signal transduction pathways to show that TRH-induced PREB induction is sensitive to the protein kinase A (PKA) inhibitor. TRH stimulated the activity of the wild-type PRL promoter, whereas mutation of the PREB core-binding element on the PRL promoter reduced this ability. In summary, we have shown that TRH stimulated PREB expression in GH3 cells via the PKA pathway. PREB can function as a transcriptional regulator of PRL promoter activity and might be involved in TRH-induced PRL gene transcription.

Myosin Va cooperates with PKA RIalpha to mediate maintenance of the endplate in vivo

Myosin V motor proteins facilitate recycling of synaptic receptors, including AMPA and acetylcholine receptors, in central and peripheral synapses, respectively. To shed light on the regulation of receptor recycling, we employed in vivo imaging of mouse neuromuscular synapses. We found that myosin Va cooperates with PKA on the postsynapse to maintain size and integrity of the synapse; this cooperation also regulated the lifetime of acetylcholine receptors. Myosin Va and PKA colocalized in subsynaptic enrichments. These accumulations were crucial for synaptic integrity and proper cAMP signaling, and were dependent on AKAP function, myosin Va, and an intact actin cytoskeleton. The neuropeptide and cAMP agonist, calcitonin-gene related peptide, rescued fragmentation of synapses upon denervation. We hypothesize that neuronal ligands trigger local activation of PKA, which in turn controls synaptic integrity and turnover of receptors. To this end, myosin Va mediates correct positioning of PKA in a postsynaptic microdomain, presumably by tethering PKA to the actin cytoskeleton.

Regulation of bovine tumor necrosis factor-alpha-induced protein 6 in ovarian follicles during the ovulatory process and promoter activation in granulosa cells

To study the regulation of bovine TNFalpha-induced protein 6 (TNFAIP6) prior to ovulation, preovulatory follicles obtained after the treatment with human chorionic gonadotropin (hCG) were used. RT-PCR analyses showed that levels of TNFAIP6 mRNA were low before hCG but significantly increased after hCG treatment in follicles. Further analyses and immunohistochemistry indicated that this increase in transcript and protein levels occurred in theca and granulosa cells. To investigate molecular mechanisms involved in TNFAIP6 transactivation, the activity of bovine TNFAIP6 promoter was studied in granulosa cell cultures. Mutant studies identified the minimal region conferring full-length promoter activity, in which activator protein-1 (AP1) and cAMP response element (CRE) elements were required for promoter activity. Overexpression of dominant-negative AP1 and activating transcription factor/cAMP response element-binding protein (CREB) inhibited forskolin-inducible promoter activity. DNA binding assays demonstrated the importance of AP1 and CRE for activity and identified JunD, FosB, Fra2, CREB1, and CREB2 as being part of the AP1 complex, and FosB, Fra2, and CREB1 for the CRE complex. Chromatin immunoprecipitation assays confirmed binding of these proteins with endogenous TNFAIP6 promoter. Treatment with forskolin, prostaglandin E2, and catalytic subunit protein kinase (cPKA) stimulated, but H89, PKA inhibitor peptide, and indomethacin inhibited, TNFAIP6 promoter activity and gene expression in granulosa cells. Collectively, this study is the first to describe that the ovulatory process in cows is associated with a gonadotropin-dependent induction of TNFAIP6 in ovarian follicles and provide the molecular basis through which AP1 and CRE sites and PKA activation played important roles in the regulation of TNFAIP6 in granulosa cells.

Modulation of F-actin rearrangement by the cyclic AMP/cAMP-dependent protein kinase (PKA) pathway is mediated by MAPK-activated protein kinase 5 and requires PKA-induced nuclear export of MK5

The MAPK-activated protein kinases belong to the Ca2+/calmodulin-dependent protein kinases. Within this group, MK2, MK3, and MK5 constitute three structurally related enzymes with distinct functions. Few genuine substrates for MK5 have been identified, and the only known biological role is in ras-induced senescence and in tumor suppression. Here we demonstrate that activation of cAMP-dependent protein kinase (PKA) or ectopic expression of the catalytic subunit Calpha in PC12 cells results in transient nuclear export of MK5, which requires the kinase activity of both Calpha and MK5 and the ability of Calpha to enter the nucleus. Calpha and MK5, but not MK2, interact in vivo, and Calpha increases the kinase activity of MK5. Moreover, Calpha augments MK5 phosphorylation, but not MK2, whereas MK5 does not seem to phosphorylate Calpha. Activation of PKA can induce actin filament accumulation at the plasma membrane and formation of actin-based filopodia. We demonstrate that small interfering RNA-triggered depletion of MK5 interferes with PKA-induced F-actin rearrangement. Moreover, cytoplasmic expression of an activated MK5 variant is sufficient to mimic PKA-provoked F-actin remodeling. Our results describe a novel interaction between the PKA pathway and MAPK signaling cascades and suggest that MK5, but not MK2, is implicated in PKA-induced microfilament rearrangement.

A co-culture system reveals the involvement of intercellular pathways as mediators of the lutropin receptor (LHR)-stimulated ERK1/2 phosphorylation in Leydig cells

Co-cultures of lutropin receptor (LHR) positive and negative Leydig cells were used to test the hypothesis that the LHR provokes phosphorylation of the extracellular regulated kinases (ERK1/2) using intracellular and intercellular pathways. Addition of hCG to MA-10 cells (LHR positive) stimulates phosphorylation of the EGF receptor (EGFR) and ERK1/2 whereas addition of hCG to I-10 cells (LHR negative) does not. Addition of hCG to co-cultures of MA-10 and I-10 cells rapidly stimulates the phosphorylation of the EGFR and ERK1/2 in I-10 cells, however. Transfection of interfering constructs shows that the LHR-mediated activation of Fyn in MA-10 cells is necessary for the phosphorylation of the EGFR and ERK1/2 in I-10 cells. This pathway can also be demonstrated in MA-10 cells but the phosphorylation of ERK1/2 in MA-10 cells also involves a second pathway mediated by protein kinase A (PKA). We propose that the LHR-mediated stimulation of the ERK1/2 cascade in Leydig cells depends on two independent pathways. One is intracellular and is mediated by PKA. The second is mediated by Fyn and it involves the release of soluble factors that act to phosphorylate the EGFR in an autocrine/paracrine fashion.

Guanosine 3',5'-cyclic monophosphate (cGMP)/cGMP-dependent protein kinase induce interleukin-6 transcription in osteoblasts

Natriuretic peptides and nitric oxide (NO) activate the cGMP/cGMP-dependent protein kinase (PKG) signaling pathway and play an important role in bone development and adult bone homeostasis. The cytokine IL-6 regulates bone turnover and osteoclast and osteoblast differentiation. We found that C-type natriuretic peptide and the NO donor Deta-NONOate induced IL-6 mRNA expression in primary human osteoblasts, an effect mimicked by the membrane-permeable cGMP analog 8-chlorophenylthio-cGMP (8-CPT-cGMP). Similar results were obtained in rat UMR106 osteosarcoma cells, where C-type natriuretic peptide and 8-CPT-cGMP stimulated transcription of the human IL-6 promoter and increased IL-6 secretion into the medium. Cotransfection of type I PKG enhanced the cGMP effect on the IL-6 promoter, whereas small interfering RNA-mediated silencing of PKG I expression prevented the cGMP effect on IL-6 mRNA expression. Step-wise deletion of the IL-6 promoter demonstrated a cAMP response element to be critical for transcriptional effects of cGMP, and experiments with dominant interfering proteins showed that cGMP activation of the promoter required cAMP response element binding-related proteins, and, to a lesser extent, proteins of the CAAT enhancer-binding protein and activator protein-1 (Fos/Jun) families. 8-CPT-cGMP induced nuclear translocation of type I PKG and increased cAMP response element binding-related protein phosphorylation on Ser(133). PKG regulation of the IL-6 promoter appeared to be of physiological significance, because inhibitors of the NO/cGMP/PKG signaling pathway largely prevented fluid shear stress-induced increases of IL-6 mRNA in UMR106 cells.

Activation of human period-1 by PKA or CLOCK/BMAL1 is conferred by separate signal transduction pathways

Circadian clocks are self-sustained biochemical oscillators that autonomously generate a near-24 h cycle in the absence of external signals. The process of synchronization to the environment involves the transcriptional activation of several genes. Photic input signals from the retina are transduced via the retinohypothalamic tract to the central pacemaker located in the suprachiasmatic nuclei (SCN) of the hypothalamus. It is known that cells of peripheral organs possess similar molecular organizations, but the signal transductional pathways lack direct light entrainment. It has been assumed that the adaptation of peripheral organs to the SCN phase is achieved by the alternate usage of promoter elements. This question has been addressed by characterizing the signal transductional pathways regulating human Period-1 gene expression in human hepatoma cells (HuH-7). Plasmids coding for key modulators of circadian rhythm, hCLOCK, hBMAL1, and hCRY2 were used to analyze the activation of a human period-1 promoter luciferase (hPER1-luc) construct. Beside classical CLOCK/BMAL1 activation, hPER1-luc was also inducible by the overexpression of the catalytic subunit of PKA (Calpha). The cotransfection of dominant negative constructs to c-FOS, CREB, PKA, and C/EBP were used to characterize both regulatory pathways. It was found that hCLOCK/hBMAL1-mediated hPER1 activation was influenced by AP1, but not significantly by other regulators. Conversely, PKA-induced activation of hPER1 was reduced by the inhibition of CREB and the CCAAT-box binding protein C/EBP, but not by AP1. The present findings imply that CLOCK/BMAL1-mediated activation of hPER1 by AP1 and E-Box elements is distinct from peripheral transcriptional modulation via cAMP-induced CREB and C/EBP.

H-89 potentiates adipogenesis in 3T3-L1 cells by activating insulin signaling independently of protein kinase A

Among four kinds of protein kinase A (PKA) inhibitors tested, H-89 exhibited a unique action to remarkably enhance adipocyte differentiation of 3T3-L1 cells, whereas the other three PKA inhibitors, PKA inhibitor Fragment 14-22 (PKI), Rp-cAMP, and KT 5720, did not enhance adipocyte differentiation. H-85, which is an inactive form of H-89, exhibited a similar enhancing effect on adipocyte differentiation. H-89 also potentiated the phosphorylation of Akt and extracellular signal-regulated kinase (ERK) 1/2 in 3T3-L1 cells, which function as downstream signaling of insulin. Phosphoinositide 3-kinase (PI3K) inhibitor wortmannin and mitogen-activated protein kinase kinase (MEK) inhibitor PD 98059 suppressed both the H-89-induced promotion of adipocyte differentiation and the H-89-induced potentiation of phosphorylation of Akt and ERK1/2. Rho kinase inhibitor Y-27632 also promoted the phosphorylation of both Akt and ERK1/2 and enhanced adipocyte differentiation, although its effect was somewhat less than that of H-89. Even when cells were treated with a mixture of Y-27632 and H-89, the additive enhancing effects on both the insulin signaling and adipocyte differentiation were not detected. Therefore, it is suggested that the major possible mechanism whereby H-89 potentiates adipocyte differentiation of 3T3-L1 cells is activation of insulin signaling that is elicited mostly by inhibiting Rho/Rho kinase pathway.

Gonadotropin-regulated testicular RNA helicase (GRTH/Ddx25) is a transport protein involved in gene-specific mRNA export and protein translation during spermatogenesis

Gonadotropin-regulated testicular RNA helicase (GRTH/Ddx25), a member of the DEAD-box protein family, is essential for completion of spermatogenesis. GRTH is present in the cytoplasm and nucleus of meiotic spermatocytes and round spermatids and functions as a component of mRNP particles, implicating its post-transcriptional regulatory roles in germ cells. In this study, GRTH antibodies specific to N- or C-terminal sequences showed differential subcellular expression of GRTH 56- and 61-kDa species in nucleus and cytoplasm, respectively, of rodent testis and transfected COS1 cells. The 56-kDa nuclear species interacted with CRM1 and participated in mRNA transport. The phosphorylated cytoplasmic 61-kDa species was associated with polyribosomes. Confocal studies on COS-1 cells showed that GRTH-GFP was retained in the nucleus by treatment with a RNA polymerase inhibitor or the nuclear protein export inhibitor. This indicated that GRTH is a shuttling protein associated with RNA export. The N-terminal leucine-rich region (61-74 amino acids) was identified as the nuclear export signal that participated in CRM1-dependent nuclear export pathway. Deletion analysis identified a 14-amino acid GRTH sequence (100-114 amino acids) as a nuclear localization signal. GRTH selectively regulated the translation of specific genes including histone 4 and HMG2 in germ cells. In addition, GRTH participated in the nuclear export of RNA messages (PGK2, tACE, and TP2) in a gene-specific manner. These studies strongly indicate that the mammalian GRTH/Ddx25 gene is a multifunctional RNA helicase that is an essential regulator of sperm maturation.

GATA-3 regulates the transcriptional activity of tyrosine hydroxylase by interacting with CREB

The zinc finger transcription factor GATA-3 is a master regulator of type 2 T-helper cell development. Interestingly, in GATA-3-/- mice, noradrenaline (NA) deficiency is a proximal cause of embryonic lethality. However, neither the role of GATA-3 nor its target gene(s) in the nervous system were known. Here, we report that forced expression of GATA-3 resulted in an increased number of tyrosine hydroxylase (TH) expressing neurons in primary neural crest stem cell (NCSC) culture. We also found that GATA-3 transactivates the promoter function of TH via specific upstream sequences, a domain of the TH promoter residing at -61 to -39 bp. Surprisingly, this domain does not contain GATA-3 binding sites but possesses a binding motif, a cAMP response element (CRE), for the transcription factor, CREB. In addition, we found that site-directed mutation of this CRE almost completely abolished transactivation of the TH promoter by GATA-3. Furthermore, protein-protein interaction assays showed that GATA-3 is able to physically interact with CREB in vitro as well as in vivo. Based on these results, we propose that GATA-3 may regulate TH gene transcription via a novel and distinct protein-protein interaction, and directly contributes to NA phenotype specification.

Strain-induced fetal type II epithelial cell differentiation is mediated via cAMP-PKA-dependent signaling pathway

The signaling pathways by which mechanical forces modulate fetal lung development remain largely unknown. In the present study, we tested the hypothesis that strain-induced fetal type II cell differentiation is mediated via the cAMP signaling pathway. Freshly isolated E19 fetal type II epithelial cells were cultured on collagen-coated silastic membranes and exposed to mechanical strain for varying intervals, to simulate mechanical forces during lung development. Unstretched samples were used as controls. Mechanical strain activated heterotrimeric G-protein alpha(s) subunit, cAMP, and the transcription factor cAMP response element binding protein (CREB). Incubation of E19 cells with the PKA inhibitor H-89 significantly decreased strain-induced CREB phosphorylation. Moreover, adenylate cyclase 5 and CREB genes were also mechanically induced. In contrast, components of the PKA-independent (Epac) pathway, including Rap-1 or B-Raf, were not phosphorylated by strain. The addition of forskolin or dibutyryl cAMP to unstretched E19 monolayers markedly upregulated expression of the type II cell differentiation marker surfactant protein C, whereas the Epac agonist 8-pCPT-2'-O-Me-cAMP had no effect. Furthermore, incubation of E19 cells with the PKA inhibitor Rp-2'-O-monobutyryladenosine 3',5'-cyclic monophosphorothioate or transient transfection with plasmid DNA containing a PKA inhibitor expression vector significantly decreased strain-induced surfactant protein C mRNA expression. In conclusion, these studies indicate that the cAMP-PKA-dependent signaling pathway is activated by force in fetal type II cells and participates in strain-induced fetal type II cell differentiation.

The role of prostaglandin E receptor-dependent signaling via cAMP in Mdr1b gene activation in primary rat hepatocyte cultures

Multidrug resistance (mdr) proteins of the mdr1 type function as multispecific xenobiotic transporters in hepatocytes. In the liver, mdr1 overexpression occurs during regeneration, cirrhosis, and hepatocarcinogenesis and may contribute to primary chemotherapy resistance. Cultured rat hepatocytes exhibit a time-dependent "intrinsic" increase in functional mdr1b expression, which depends on cyclooxygenase-catalyzed prostaglandin E(2) release. In the present study, the prostaglandin E (EP) receptor agonist misoprostol (1-10 microg/ml) further enhanced intrinsic mdr1b mRNA expression in primary rat hepatocytes. On the other hand, [1alpha(z),2beta,5alpha]-(+)-7-[5-[1,1'-(biphenyl)-4-yl]methoxy]-2-(4-morpholinyl)-3-oxocyclopentyl]-4-heptenoic acid (AH23848B) (30 microM), an antagonist of the cAMP-coupled EP4 receptor, and the protein kinase A (PKA) inhibitor, N-(2-[bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide (H89) (10 nM), repressed intrinsic mdr1b mRNA up-regulation, whereas the stable cAMP analog 8-bromo-cAMP (10 microM) and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) (100 microM) further enhanced intrinsic mdr1b expression. Primary rat hepatocytes, transiently transfected with reporter gene constructs controlled by mdr1b 5'-gene-flanking regions [-1074 to +154 base pairs (bp) or -250 to +154 bp], demonstrated pronounced mdr1b promoter activity, already without the addition of exogenous modulators. Nevertheless, activity was further stimulated by misoprostol, 8-bromo-cAMP, or IBMX. Cotransfection with expression vectors for PKI, an inhibitor protein of cAMP-dependent PKA, or KCREB, a dominant-negative mutant of the cAMP-responsive element-binding protein (CREB), decreased high-intrinsic mdr1b promoter activity. KCREB also counteracted misoprostol-induced mdr1b promoter activation. In conclusion, these data provide evidence for a pivotal role of EP receptor-stimulated, cAMP-dependent activation of PKA and CREB or CREB-related proteins in mdr1b gene activation in primary rat hepatocytes. Thus, these data might offer potential new target structures for the reversal of primary drug resistance, for example, of liver tumors.

Prostaglandin E2 promotes colon cancer cell growth through a Gs-axin-beta-catenin signaling axis

How cyclooxygenase-2 (COX-2) and its proinflammatory metabolite prostaglandin E2 (PGE2) enhance colon cancer progression remains poorly understood. We show that PGE2 stimulates colon cancer cell growth through its heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptor, EP2, by a signaling route that involves the activation of phosphoinositide 3-kinase and the protein kinase Akt by free G protein betagamma subunits and the direct association of the G protein alphas subunit with the regulator of G protein signaling (RGS) domain of axin. This leads to the inactivation and release of glycogen synthase kinase 3beta from its complex with axin, thereby relieving the inhibitory phosphorylation of beta-catenin and activating its signaling pathway. These findings may provide a molecular framework for the future evaluation of chemopreventive strategies for colorectal cancer.

Type I collagen structure regulates cell morphology and EGF signaling in primary rat hepatocytes through cAMP-dependent protein kinase A

Adhesion to type 1 collagen elicits different responses dependent on whether the collagen is in fibrillar (gel) or monomeric form (film). Hepatocytes adherent to collagen film spread and proliferate, whereas those adherent to collagen gel remain rounded and growth arrested. To explore the role of potential intracellular inhibitory signals responsible for collagen gel-mediated growth arrest, cAMP-dependent protein kinase A (PKA) was examined in hepatocytes adherent to collagen film or gel. PKA activity was higher in hepatocytes on collagen gel than on film during G1 of the hepatocyte cell cycle. Inhibition of PKA using H89 increased cell spreading on collagen gel in an EGF-dependent manner, whereas activation of PKA using 8-Br-cAMP decreased cell spreading on collagen film. PKA inhibition also restored ERK activation, cyclin D1 expression and G1-S progression on collagen gel, but had no effect on cells adherent to collagen film. Analysis of EGF receptor phosphorylation revealed that adhesion to collagen gel alters tyrosine phosphorylation of the EGF receptor, leading to reduced phosphorylation of tyrosine residue 845, which was increased by inhibition of PKA. These results demonstrate that fibrillar type 1 collagen can actively disrupt cell cycle progression by inhibiting specific signals from the EGF receptor through a PKA-dependent pathway.

Pituitary Adenylyl Cyclase-activating Polypeptide 38 Reduces Astroglial Proliferation by Inhibiting the GTPase RhoA

Pituitary adenylyl cyclase-activating polypeptide 38 (PACAP38) plays an important role in the proliferation and differentiation of neural cells. In the present study, we have investigated how PACAP38 inhibits the proliferation of cultured neocortical astroglial cells. When applied to synchronized cells during the G(1) phase of the cell cycle, PACAP38 diminished the subsequent nuclear uptake of bromodeoxyuridine. When applied for 2 days, it reduced the cell number. PACAP38 did not exert its antiproliferative effect by activating protein kinase A. It also did not reduce the activity of mitogen-activated protein kinases essential for G(1) phase progression. Instead, PACAP38 acted on a member of the Rho family of small GTPases. It reduced the activity of RhoA as was shown with a Rhotekin pull-down assay. The decrease in endogenous RhoA activity induced by treatment of the cells with C3 exotoxin or by expression of dominant negative RhoA also reduced the nuclear uptake of bromodeoxyuridine. In contrast, expression of constitutively active RhoA prevented the effect of PACAP38. Our data show a novel signal transduction pathway by which the neuropeptide influences cell proliferation.

Role of upstream stimulatory factor phosphorylation in the regulation of the prostaglandin G/H synthase-2 promoter in granulosa cells

To investigate the role of USF phosphorylation in the regulation of the PGHS-2 promoter in granulosa cells, promoter activity assays were performed in primary cultures of bovine granulosa cells transfected with the chimeric PGHS-2 promoter/luciferase (LUC) construct -149/-2PGHS-2.LUC. Transfections were done in the absence or presence of forskolin; the protein kinase A (PKA) inhibitor H-89; or an expression vector encoding USF1, USF2, the catalytic subunit of PKA (cPKA), or a PKA inhibitor protein (PKI). Electrophoretic mobility shift assays were performed to study USF/DNA interactions using granulosa cell nuclear extracts and a 32P-labeled proximal PGHS-2 promoter fragment containing the E-box element. The results show that forskolin stimulation and cPKA overexpression caused a marked and significant increase in USF-dependent DNA binding and PGHS-2 promoter activities (p < 0.05). In contrast, both activities were decreased by H-89 treatment or PKI overexpression. Reverse transcription-PCR analyses revealed that these treatments had similar effects on endogenous PGHS-2 mRNA levels in granulosa cells. Cotransfection studies with a USF2 mutant lacking N-terminal activation domains (U2Delta1-220) repressed forskolin-, cPKA-, and USF-dependent PGHS-2 promoter activities. Electrophoretic mobility shift assays showed that U2Delta1-220 was able to compete with full-length USF proteins and to saturate the E-box element. Immunoprecipitation/Western blot analyses revealed an increase in the levels of phosphorylated USF1 and USF2 after forskolin treatment, whereas chromatin immunoprecipitation assays showed that binding of USF proteins to the endogenous PGHS-2 promoter was stimulated by forskolin. Site-directed mutagenesis of a consensus PKA phosphorylation site within USF proteins abolished their transactivating capacity. Collectively, these results characterize the role of USF phosphorylation in PGHS-2 expression and identify the phosphorylation-dependent increase in USF binding to the E-box as a putative molecular basis for the increase in PGHS-2 promoter transactivation in granulosa cells.

Follicle-stimulating hormone induction of ovarian insulin-like growth factor-binding protein-3 transcription requires a TATA box-binding protein and the protein kinase A and phosphatidylinositol-3 kinase pathways

The current study was done to elucidate the mechanism of the FSH stimulation of IGF-binding protein 3 (IGFBP-3) expression and map the FSH response element on the pig IGFBP-3 promoter. Forskolin induced IGFBP-3 reporter activity in transiently transfected granulosa cells. The protein kinase A (PKA) inhibitor [N-[2-(p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide, 2HCl] (and cotransfection with a PKA inhibitor expression vector), the phosphatidylinositol-3 kinase inhibitor [2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one], and the ERK inhibitor [1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene], all blocked FSH stimulation. Use of serial deletion constructs and site-directed mutagenesis show that a TATA box-binding protein site is required for FSH stimulation and that a specific protein 1 (Sp1) site is required for basal transcription. Gel shift assays of nuclear protein with a -61/-25 probe detected four protein-DNA complexes, with bands I and II having significantly higher intensities in FSH-treated cells than in controls. Mutation of the Sp1 site prevented formation of bands I and II whereas mutation of the TATA box-binding protein site prevented formation of band IV. Use of specific antibodies showed that Sp1 participates in formation of band I, Sp3 band II, and p300 in both I and II. Band III was nonspecifically competed out. We conclude that FSH stimulation of IGFBP-3 transcription is mediated by cAMP via the PKA pathway and requires the P1-3 kinase and likely the MAPK pathways.

Prostanoid receptor expression by human airway smooth muscle cells and regulation of the secretion of granulocyte colony-stimulating factor

The prostanoid receptors on human airway smooth muscle cells (HASMC) that augment the release by IL-1β of granulocyte colony-stimulating factor (G-CSF) have been characterized and the signaling pathway elucidated. PCR of HASM cDNA identified products corresponding to EP2, EP3, and EP4receptor subtypes. These findings were corroborated at the protein level by immunocytochemistry. IL-1β promoted the elaboration of G-CSF, which was augmented by PGE2. Cicaprost (IP receptor agonist) was approximately equiactive with PGE2, whereas PGD2, PGF2α, and U-46619 (TP receptor agonist) were over 10-fold less potent. Neither SQ 29,548 nor BW A868C (TP and DP1receptor antagonists, respectively) attenuated the enhancement of G-CSF release evoking any of the prostanoids studied. With respect to PGE2, the EP receptor agonists 16,16-dimethyl PGE2(nonselective), misoprostol (EP2/EP3selective), 17-phenyl-ω-trinor PGE2(EP1selective), ONO-AE1-259, and butaprost (both EP2selective) were full agonists at enhancing G-CSF release. AH 6809 (10 μM) and L-161,982 (2 μM), which can be used in HASMC as selective EP2and EP4receptor antagonists, respectively, failed to displace to the right the PGE2concentration-response curve that described the augmented G-CSF release. In contrast, AH 6809 and L-161,982 in combination competitively antagonized PGE2-induced G-CSF release. Augmentation of G-CSF release by PGE2was mimicked by 8-BrcAMP and abolished in cells infected with an adenovirus vector encoding an inhibitor protein of cAMP-dependent protein kinase (PKA). These data demonstrate that PGE2facilitates G-CSF secretion from HASMC through a PKA-dependent mechanism by acting through EP2and EP4prostanoid receptors and that effective antagonism is realized only when both subtypes are blocked concurrently.

PDE4-regulated cAMP degradation controls the assembly of integrin-dependent actin adhesion structures and REF52 cell migration

Plating of REF52 cells onto extracellular matrix components leads to the formation of integrin-dependent actin adhesion microspikes. We show that the formation of these structures is sensitive to chemical agents that regulate cAMP levels, such as forskolin and IBMX. In particular, by using the specific inhibitor rolipram, we identify the PDE4 family of cAMP-specific phosphodiesterases as critical regulators of this process. The effect of PDE4 on microspike formation is mediated by actions exerted through the activation of PKA - rather than through the alternative cAMP effector, Epac. We provide evidence that peripheral microspikes are RhoA-, ROCK- and myosin-dependent, and that this pathway is suppressed by PDE4 inhibition. In addition, PDE4 inhibition impairs cell locomotion that requires dynamic protrusion and retraction of peripheral spike structures. Our data demonstrate that PDE4 activity is a key modulator of integrin-induced actin assembly at the cell periphery which, in turn, controls cell migration.

Synergistic activation of CREB-mediated transcription by forskolin and phorbol ester requires PKC and depends on the glutamine-rich Q2 transactivation domain

Recruitment of a RNA polymerase II complex by the glutamine-rich Q2 domain of cAMP response element-binding protein (CREB) allows basal transcriptional activity, while recruitment of CBP/p300 through signal-induced phosphorylation of the kinase-inducible domain at serine-133 enhances CREB-dependent transcription. Here we demonstrate that co-administration of forskolin and phorbol ester TPA to NIH3T3 cells provoked a dose-dependent increase in phosphoserine-133. CREB- and Q2-dependent transcription, as well as transcription by other glutamine-rich transcription factors, but not by transcription factors lacking glutamine-rich regions, augmented synergistically in the presence of both stimuli. Synergistic activation was abograted by specific inhibition of protein kinase C (PKC), but not of PKA. Co-stimulation increased the basal activity of a minimal, CREB-independent promoter. Therefore, Q2, which directly interacts with the RNA polymerase II initiation complex, may transmit the increased basal promoter activity provoked by these stimuli to CREB, thereby contributing to synergistic activation of CREB-mediated transcription. This synergism may have important implications on glutamine-rich transcription factor-target genes.

Expression of the neurotrophin receptor trkB is regulated by the cAMP/CREB pathway in neurons

trkB as receptor for neurotrophins brain-derived neurotrophic factor (BDNF)/neurotrophin (NT)-4/5 plays a crucial role during development, maintenance of the adult brain, and its adaptation to injury or pathological conditions. In spite of this, very little is known about the mechanisms that regulate its expression. Here, we show that forskolin (Fk) rapidly stimulates the expression of both the full-length and truncated trkB isoforms in primary cultures of cortical neurons. Gel shift assays and transient transfection experiments demonstrate that this activation occurs via a protein kinase A (PKA)/cyclic AMP-responsive element-binding protein (CREB)-dependent mechanism. Activated CREB binds to the second cyclic AMP (cAMP)-responsive element (CRE) of the two CRE sites located within the P2 promoter of the trkB gene, which is able to confer cAMP responsiveness to a heterologous promoter. Our results illustrate that the trkB gene is a target for CREB regulation and explain the increase of trkB expression produced in different adaptative responses of the nervous system where CREB is participating.

Identification in human airways smooth muscle cells of the prostanoid receptor and signalling pathway through which PGE2 inhibits the release of GM-CSF

1. The prostanoid receptor(s) on human airways smooth muscle (HASM) cells that mediates the inhibitory effect of PGE(2) on interleukin (IL)-1 beta-induced granulocyte/macrophage colony-stimulating factor (GM-CSF) release has been classified. 2. IL-1 beta evoked the release of GM-CSF from HASM cells, which was suppressed by PGE(2), 16,16-dimethyl PGE(2) (nonselective), misoprostol (EP(2)/EP(3)-selective), ONO-AE1-259 and butaprost (both EP(2)-selective) with pIC(50) values of 8.61, 7.13, 5.64, 8.79 and 5.43, respectively. EP-receptor agonists that have selectivity for the EP(1)-(17-phenyl-omega-trinor PGE(2)) and EP(3)-receptor (sulprostone) subtypes as well as cicaprost (IP-selective), PGD(2), PGF(2 alpha) and U-46619 (TP-selective) were poorly active or inactive at concentrations up to 10 microM. 3. AH 6809, a drug that can be used to selectively block EP(2)-receptors in HASM cells, antagonised the inhibitory effect of PGE(2), 16,16-dimethyl PGE(2) and ONO-AE1-259 with apparent pA(2) values of 5.85, 6.09 and 6.1 respectively. In contrast, the EP(4)-receptor antagonists, AH 23848B and L-161,982, failed to displace to the right the concentration-response curves that described the inhibition of GM-CSF release evoked by PGE(2) and ONO-AE1-259. 4. Inhibition of GM-CSF release by PGE(2) and 8-Br-cAMP was abolished in cells infected with an adenovirus vector encoding an inhibitor protein of cAMP-dependent protein kinase (PKA) but not by H-89, a purported small molecule inhibitor of PKA. 5. We conclude that prostanoid receptors of the EP(2)-subtype mediate the inhibitory effect of PGE(2) on GM-CSF release from HASM cells by recruiting a PKA-dependent pathway. In addition, the data illustrate that caution should be exercised when using H-89 in studies designed to assess the role of PKA in biological processes.

Adenovirus-mediated delivery and expression of a cAMP-dependent protein kinase inhibitor gene to BEAS-2B epithelial cells abolishes the anti-inflammatory effects of rolipram, salbutamol, and prostaglandin E2: a comparison with H-89

cAMP-elevating drugs are thought to mediate their biological effects by activating the cAMP/cAMP-dependent protein kinase (PKA) cascade. However, this hypothesis is difficult to confirm due to a lack of selective inhibitors. Here, we have probed the role of PKA in mediating inhibitory effects of several cAMP-elevating drugs in BEAS-2B epithelial cells using an adenovirus vector encoding a PKA inhibitor protein (PKIalpha) and have compared it to H-89, a commonly used small molecule PKA inhibitor. Initial studies established efficient gene transfer and confirmed functionality of PKIalpha 48 h after virus infection. All cAMP-elevating drugs tested promoted the phosphorylation of cAMP response element-binding protein (CREB), activated a cAMP response element (CRE)-driven luciferase reporter gene, and suppressed both granulocyte/macrophage colony-stimulating factor (GM-CSF) generation and [(3)H]arachidonic acid (AA) release in response to interleukin-1beta and monocyte chemotactic protein (MCP)-1, respectively. These effects were abolished by PKIalpha. In contrast, H-89 behaved unpredictably under the same conditions. Thus, although CREB phosphorylation evoked by a range of cAMP-elevating drugs was abolished by H-89, neither activation of the CRE-dependent luciferase reporter gene construct nor the inhibition of GM-CSF generation was inhibited. Paradoxically, H-89 antagonized MCP-1-induced [(3)H]AA release and enhanced the inhibitory effect of submaximal concentrations of rolipram and 8-bromo-cAMP. We suggest that expression of PKIalpha in susceptible cells provides a simple and unambiguous way to assess the role of PKA in cAMP signaling and to probe the mechanism of action of other drugs and cAMP-dependent responses where the participation of PKA is equivocal. Furthermore, these data suggest that H-89 is not a selective inhibitor of PKA and should be avoided.

Dopamine D1 receptors mediate CREB phosphorylation via phosphorylation of the NMDA receptor at Ser897-NR1

Addictive drugs such as amphetamine and cocaine stimulate the dopaminergic system, activate dopamine receptors and induce gene expression throughout the striatum. The signal transduction pathway leading from dopamine receptor stimulation at the synapse to gene expression in the nucleus has not been fully elucidated. Here, we present evidence that D1 receptor stimulation leads to phosphorylation of the transcription factor Ca2+ and cyclic AMP response element binding protein (CREB) in the nucleus by means of NMDA receptor-mediated Ca2+ signaling. Stimulation of D1 receptors induces the phosphorylation of Ser897 on the NR1 subunit by protein kinase A (PKA). This phosphorylation event is crucial for D1 receptor-mediated CREB phosphorylation. Dopamine cannot induce CRE-mediated gene expression in neurons transfected with a phosphorylation-deficient NR1 construct. Moreover, stimulation of D1 receptors or increase in cyclic AMP levels leads to an increase in cytosolic Ca2+ in the presence of glutamate, but not in the absence of glutamate, indicating the ability of dopamine and cyclic AMP to facilitate NMDA channel activity. The recruitment of the NMDA receptor signal transduction pathway by D1 receptors may provide a general mechanism for gene regulation that is fundamental for mechanisms of drug addiction and long-term memory.

The lutropin/choriogonadotropin receptor-induced phosphorylation of the extracellular signal-regulated kinases in leydig cells is mediated by a protein kinase a-dependent activation of ras

The pathways involved in activation of the ERK1/2 cascade in Leydig cells were examined in MA-10 cells expressing the recombinant human LH receptor (hLHR) and in primary cultures of rat Leydig cell precursors. In MA-10 cells expressing the recombinant hLHR, human choriogonadotropin-induced activation of ERK1/2 is effectively inhibited by overexpression of a cAMP phosphodiesterase (a manipulation that blunts the human choriogonadotropin-induced cAMP response), by addition of H89 (a selective inhibitor of protein kinase A), or by overexpression of the heat-stable protein kinase A inhibitor, but not by overexpression of an inactive mutant of this inhibitor. Stimulation of hLHR did not activate Rap1, but activated Ras in an H89-sensitive fashion. Addition of H89 to MA-10 cells that had been cotransfected with a guanosine triphosphatase-deficient mutant of Ras almost completely inhibited the hLHR-mediated activation of ERK1/2. We also show that 8-bromo-cAMP activates Ras and ERK1/2 in MA-10 cells and in primary cultures of rat Leydig cells precursors in an H89-sensitive fashion, whereas a cAMP analog 8-(4-chloro-phenylthio)-2'-O-methyl-cAMP (8CPT-2Me-cAMP) that is selective for cAMP-dependent guanine nucleotide exchange factor has no effect. Collectively, our results show that the hLHR-induced phosphorylation of ERK1/2 in Leydig cells is mediated by a protein kinase A-dependent activation of Ras.

Stem cell factor enhances erythropoietin-mediated transactivation of signal transducer and activator of transcription 5 (STAT5) via the PKA/CREB pathway

OBJECTIVE

To define whether the observed synergistic effects of erythropoietin (EPO) and stem cell factor (SCF) on erythroid cells can, in part, be mediated by the signal transducer and activator of transcription 5 (STAT5).

METHODS

STAT5 activation was examined in erythroid cell lines by analyzing the effects of EPO and SCF on STAT5 tyrosine phosphorylation, serine phosphorylation, DNA binding, and STAT5-mediated gene transactivation.

RESULTS

EPO induced a 5.0-fold+/-0.4-fold increase in STAT5 transactivation, which could be further enhanced by SCF. SCF pretreatment followed by EPO stimulation resulted in a 9.0-fold+/-0.9-fold increase in STAT5 transactivation, while SCF alone did not increase STAT5 transactivation. This costimulatory effect of SCF was not mediated by increased STAT5 tyrosine or serine phosphorylation or increased STAT5 DNA binding. In addition, enhanced STAT5 transactivation was independent of the phosphatidyl inositol 3-kinase and MAPK(p42/p44) pathways. Instead, the protein kinase A (PKA) inhibitor protein PKI and the PKA inhibitor H89 prevented the costimulatory SCF effect. Furthermore, the PKA target CREB showed a strongly increased and prolonged serine-133 phosphorylation after costimulation with SCF + EPO. The involvement of CREB in STAT5 transactivation was demonstrated by overexpression of serine-133-mutated CREB, which completely blocked the SCF effect. In addition, the CREB-binding protein CBP/p300 was shown to be essential for EPO- and SCF-mediated STAT5 transactivation.

CONCLUSION

SCF enhances the EPO-mediated STAT5 transactivation by triggering a PKA/CREB-dependent pathway.

Secretin activation of chromogranin A gene transcription. Identification of the signaling pathways in cis and in trans

Secretin evokes catecholamine secretion from PC12 pheochromocytoma cells. We tested whether secretin activates transcription of the major vesicular core protein chromogranin A (CgA). Secretin stimulated both endogenous CgA gene transcription (approximately 4-6-fold) as well as transfected CgA promoter activity (approximately 8-10-fold; EC50, approximately 7 nm) in PC12 cells. Studies on CgA promoter 5'-deletion mutant/luciferase reporter constructs, point mutations of the CgA cAMP response element (CRE), and their transfer to a heterologous promoter implicated CRE in cis as both necessary and sufficient for secretin-stimulated CgA gene transcription. Secretin-induced CgA gene transcription was inhibited/abolished by cytosolic Ca2+ chelation, chemical blockade of phospholipase C, protein kinase A (PKA), or mitogen-activated protein (MAP) kinase extracellular signal regulated kinase (ERK) 1/2 and the expression of dominant negative mutants of ERK1/2, CRE binding protein (CREB) kinase RSK2, or CREB. Secretin also augmented (approximately 4-fold) phosphorylation of ERK1/2. Trans-activation (approximately 21-fold) of GAL4-CREB fusion protein by secretin indicates involvement of CREB in secretin signaling to gene transcription. Electrophoretic mobility shift assays also identified CREB as the mediator of secretin-induced CgA gene transcription, and pCREB supershifts indicated Ser-133 as the active CREB moiety in vitro. This conclusion was reinforced in vivo by results of chromatin pCREB immunoprecipitation assays. We conclude that secretin signals to CgA gene transcription through the CRE domain in cis and through cAMP, Ca2+, PKA, MAP kinase, and the transcription factor CREB in trans. Thus, multiple signal transduction pathways seem to subserve the function of stimulus-transcription coupling after this peptidergic stimulus to chromaffin cells.

Examining the mechanism of Erk nuclear translocation using green fluorescent protein

In neuronal cells, the mitogen-activated protein kinase (MAP kinase) cascade is an important mediator of neurotrophin signaling from cell surface receptors to the nucleus, resulting in changes in gene expression. Nuclear localization of Erk is thought to be required for these effects. To examine the mechanism and regulation of Erk nuclear translocation, we have created a green fluorescent protein (GFP)-labeled Erk2 construct, which provides a sensitive means to follow the movement of Erk from the cytoplasm to the nucleus following receptor-mediated MAP kinase activation. Using this system in PC12 cells, we have examined a number of mechanisms that have been implicated in regulating the translocation of Erk. In PC12 cells, NGF and EGF induce a rapid translocation of GFP-Erk that requires Ras and Mek. We have found that prolonged phosphorylation of Erk is not required for the rapid and early influx of Erk into the nucleus following growth factor stimulation. Furthermore, following influx, GFP-Erk rapidly returned to the cytoplasm regardless of its phosphorylation state. The release of Erk from its cytoplasmic activator, Mek, followed by the dimerization of Erk, was sufficient to stimulate nuclear uptake, whereas Erk kinase activity was dispensable. PKA activity has been reported to be required for Erk translocation in PC12 cells. However, PKA activity was also not necessary for the early translocation of Erk into the nucleus by NGF or Ras, but it was able to induce a small influx of Erk that could be measured with GFP-Erk2.

Local activation of protein kinase A inhibits morphogenetic movements during Xenopus gastrulation

cAMP-dependent protein kinase (PKA) has various biological roles in many organisms. However, little is known about its role in the developmental processes of vertebrates. In this study, we describe the functional analysis of PKA during gastrulation movements in Xenopus laevis. Overexpression of constitutively active PKA (cPKA) in the dorsal equatorial region of the embryo affects morphogenetic movement during gastrulation. We also show that intrinsic differences of PKA activities along the dorsoventral axis are set up and the level of PKA activity on the dorsal region is lower than that on the ventral region from late blastula to gastrula stages. In addition, PKA activation in animal explants inhibits activin-induced elongation. In cPKA-injected embryos, there were no changes in the expressions of markers involved in mesoderm specification, although the correct expression domains of these genes were altered. The effects of PKA activation can be restored by coexpression of PKI, a pseudosubstrate of PKA. We further analyzed the effects of PKA activation on the behavior of migratory gastrulating cells in vitro. Expression of cPKA in head mesoderm cells causes less polarized and/or randomized migration as demonstrated by a directional cell migration assay. Finally, we show that RhoA GTPase lies downstream of PKA, affecting activin-induced convergent extension movements. Taken together, these results suggest that overexpressed PKA can modulate a pathway responsible for morphogenetic movements during Xenopus gastrulation.

Gonadotropin-releasing hormone-induced stimulation of the rat secretogranin II promoter involves activation of CREB

To investigate the events involved in regulation of the secretogranin II (SgII) gene, luciferase reporter constructs were transfected into gonadotrope-derived, alphaT3-1 cells. DNA between -91 and -60 relative to the transcription start site was found to be required for GnRH induced SgII reporter gene activation. This region contains a consensus cAMP response element (CRE) and disruption of this CRE reduced GnRH responsiveness of the SgII promoter. CREB was shown to bind to the SgII CRE and transfection studies with a dominant-negative CREB mutant provided evidence that CREB is required for GnRH responsiveness of the SgII promoter. An expression vector for an inhibitor of the cAMP-dependent protein kinase was found to reduce the ability of cAMP or GnRH to activate the SgII-luciferase reporter gene. These studies offer evidence that GnRH-induced activation of the SgII promoter in the alphaT3-1 cell line requires cAMP-dependent protein kinase activity and a functional CRE within the 5'-flanking region of the gene.

Mechanisms of basal and kinase-inducible transcription activation by CREB

The cAMP response element (CRE)-binding protein (CREB) stimulates basal transcription of CRE-containing genes and mediates induction of transcription upon phosphorylation by protein kinases. The basal activity of CREB maps to a carboxy-terminal constitutive activation domain (CAD), whereas phosphorylation and inducibility map to a central, kinase-inducible domain (KID). The CAD interacts with and recruits the promoter recognition factor TFIID through an interaction with a specific TATA-binding-protein-associated factor (TAF), dTAFII110/ hTAFII135. Interaction between the TAF and the CAD is mediated by a central cluster of hydrophobic amino acids, mutation of which disrupts TAF binding, polymerase recruitment, and transcription activation. Assessment of the contributions of the CAD and KID to recruitment of the polymerase complex versus enhancement of subsequent reaction steps (isomerization, promoter clearance, and reinitiation) showed that the CAD and P-KID act in a concerted mechanism to stimulate transcription. The CAD, but not the KID, mediated recruitment of a complex containing components of a transcription initiation complex, including pol II, IIB, and IID. However, the CAD was relatively ineffective in stimulating subsequent steps in the reaction mechanism. In contrast, phosphorylation of the KID in CREB effectively stimulated isomerization of the recruited polymerase complex and multiple-round transcription. A model for the activation of transcription by phosphorylated CREB is proposed, in which the polymerase is recruited by interaction of the CAD with TFIID and the recruited polymerase is activated further by phosphorylation of the KID in CREB.

Prostaglandin-E2 enhances EPO-mediated STAT5 transcriptional activity by serine phosphorylation of CREB

Erythroid colony formation in response to erythropoietin (EPO) stimulation is enhanced by costimulating the cells with prostaglandin-E2 (PGE2). The present study further analyzed the underlying mechanisms and demonstrated that EPO-mediated STAT5 transactivation in the erythroid AS-E2 cell line was enhanced 6-fold by PGE2 (10 microM), without affecting the STAT5 tyrosine phosphorylation or STAT5-DNA binding. Moreover, the PGE2-enhancing effect was independent of STAT5 serine phosphorylation. In AS-E2 cells STAT5 is constitutively phosphorylated on Ser780 (STAT5A) and EPO-dependently phosphorylated on Ser726/731 (STAT5A/STAT5B), but overexpression of STAT5 serine mutants did not affect STAT5 transactivation. In addition, PGE2 did not affect STAT5 serine phosphorylation. Instead, the stimulatory effect of PGE2 on STAT5 signaling could be mimicked by dibutyryl-cyclic adenosine monophosphate (cAMP) and the phosphodiesterase inhibitor IBMX, suggesting that the effect was mediated by cAMP. Activation of the cAMP pathway resulted in cAMP-response element binding protein (CREB) phosphorylation, which was sustained in the presence of EPO plus PGE2 and transient on EPO stimulation alone. The costimulatory effect of PGE2 on EPO-mediated STAT5 transactivation was inhibited by overexpression of serine-dead CREB or protein kinase A (PKA) inhibitor (PKI), in contrast to EPO-mediated transactivation, which was PKA independent. Furthermore, CREB-binding protein (CBP)/p300 was shown to be involved in EPO-mediated STAT5 transactivation, and a CBP mutant with increased affinity for CREB resulted in an additional enhancement of the PGE2 effect. Finally, we demonstrated that the STAT5 target genes Bcl-X, SOCS2, and SOCS3 were up-regulated by costimulation with PGE2. In summary, these studies demonstrate that PGE2 enhancement of EPO-induced STAT5 transactivation is mediated by the cAMP/PKA/CREB pathway.

The protein kinase A inhibitor H89 acts on cell morphology by inhibiting Rho kinase

The small GTPase RhoA can retract cell extensions by acting on two Rho kinases (ROCKs). Activated protein kinase A (PKA) inhibits RhoA and induces extensions. The isoquinoline H89 inhibits PKA and thus should prevent the inactivation of RhoA. In kinase assays, H89 has been recently found to inactivate a ROCK-II also. Because H89 may be able to exert opposite effects on cell extensions, we have studied the effects of H89 on neurite formation in the neuroblastoma-glioma line NG 108-15, which expresses ROCK-I and ROCK-II. We found that H89 can indeed inhibit ROCKs and PKA. Because ROCKs act downstream of RhoA, the inhibitory effect of H89 on ROCKs is most prominent. The data indicate that H89 may not be used as an antagonist of PKA in systems in which ROCKs play a role.

Thyrotropin-releasing hormone-induced down-regulation of pyroglutamyl aminopeptidase II activity involves L-type calcium channels and cam kinase activities in cultures of adenohypophyseal cells

Released thyrotropin-releasing hormone (TRH) is inactivated by a narrow specificity ectopeptidase, pyroglutamyl aminopeptidase II (PPII), present in brain and lactotrophs. Various hypothalamic/paracrine factors, including TRH, slowly (in hours) regulate the activity of PPII on the surface of adenohypophyseal cells. TRH-induced down-regulation was mimicked by protein kinase C (PKC) activation but was not affected by inhibition of PKC. Adenylate cyclase activation can also down-regulate PPII. The purpose of this study was to identify elements of the transduction pathway used by TRH to regulate PPII activity. In primary cultures of female adenohypophyseal cells, activation of the stimulatory G protein or adenylate cyclase produced an effect additive to that of TRH; inhibition of protein kinase A activity did not interfere with TRH action. However, regulation of PPII activity by TRH was inhibited by a phospholipase C beta inhibitor or chelation of intracellular calcium. L-type calcium channels (LCC) agonists mimicked TRH action and their effect was not additive with that of TRH. Antagonists of LCC channels and inhibitors of calmodulin or calcium/calmodulin-dependent protein kinase blocked TRH action. Therefore, TRH-induced calcium entry through L-type calcium channels and the activity of calcium/calmodulin-dependent protein kinase are required for TRH effect on PPII activity in primary cultures of adenohypophyseal cells. This pathway may coregulate PPII and prolactin biosynthesis in response to TRH.

Vascular endothelial cells express isoforms of protein kinase A inhibitor

The expression and function of the endogenous inhibitor of cAMP-dependent protein kinase (PKI) in endothelial cells are unknown. In this study, overexpression of rabbit muscle PKI gene into endothelial cells inhibited the cAMP-mediated increase and exacerbated thrombin-induced decrease in endothelial barrier function. We investigated PKI expression in human pulmonary artery (HPAECs), foreskin microvessel (HMECs), and brain microvessel endothelial cells (HBMECs). RT-PCR using specific primers for human PKI alpha, human PKI gamma, and mouse PKI beta sequences detected PKI alpha and PKI gamma mRNA in all three cell types. Sequencing and BLAST analysis indicated that forward and reverse DNA strands for PKI alpha and PKI gamma were of >96% identity with database sequences. RNase protection assays showed protection of the 542 nucleotides in HBMEC and HPAEC PKI alpha mRNA and 240 nucleotides in HBMEC, HPAEC, and HMEC PKI gamma mRNA. Western blot analysis indicated that PKI gamma protein was detected in all three cell types, whereas PKI alpha was found in HBMECs. In summary, endothelial cells from three different vascular beds express PKI alpha and PKI gamma, which may be physiologically important in endothelial barrier function.

The cytoplasmic tail of the D1A receptor subtype: identification of specific domains controlling dopamine cellular responsiveness

In this study the rat D1A receptor (wild-type, WT) and truncation mutants thereof, are utilized to delineate specific cytoplasmic tail (CT) domains responsible for regulating ligand binding and receptor-mediated adenylyl cyclase activation. In human embryonic kidney (HEK) cells, all truncation mutants of the D1A receptor (Delta425, Delta379, Delta351) display cell surface localization and express at high but different receptor numbers. Binding studies suggest that residues located between Cys(351) and Asp(425) may serve to restrain the agonist binding conformation of the D1A receptor. This contention is supported by the observation that the constitutive activation of Delta351 is significantly increased in comparison with WT, Delta425 and Delta379. Furthermore, we demonstrate that the extent of dopamine-mediated maximal activation of adenylyl cyclase is significantly augmented in cells expressing Delta351 when compared with WT or mutants harboring shorter truncations. These results suggest that in addition to restraining receptor conformation, determinants located downstream of Cys(351) may act as negative regulators of the G protein coupling efficiency and adenylyl cyclase activation. Interestingly, all truncated receptors used in the present study display a decrease in dopamine potency when compared with WT. We show that inhibition of protein kinase A (PKA) activity leads also to a reduction in dopamine potency in cells expressing WT but not Delta351 receptors. These results hint at a potential previously unanticipated role for PKA in facilitating D1A receptor coupling efficiency in HEK cells. Overall, the present study has uncovered specific CT domains involved in regulating discrete aspects of the D1A receptor signaling.

Ligand-independent activation of pituitary ER: dependence on PKA-stimulated pathways

In pituitary and other target tissues, estrogen acts through ERs, which are ligand-activated nuclear transcription factors. ERs can also be activated by intracellular signaling pathways in a ligand-independent manner in some cells. Because the pituitary is the target of several cAMP-activating factors, we examined the ability of cAMP to activate ERs in the alphaT3 gonadotrope cell line. Forskolin, 8-bromo-cAMP, and pituitary adenylate cyclase-activating polypeptide all enhanced ER-dependent promoter activity, which was inhibited by antiestrogen or a pituitary-specific inhibitory ER variant. Activation was PKA dependent and was blocked by the PKA inhibitor H89 or cotransfection of the inhibitor PKI. Although cAMP activated MAPK in alphaT3 cells, inhibition of MAPK with the MEK inhibitor PD98059 did not prevent forskolin-induced ER activation. Similarly, epidermal growth factor did not stimulate ER activity, although it increased MAPK activation. Forskolin-induced activation of ER was enhanced by cotransfection of steroid receptor coactivator-1 and was inhibited by the repressor of ER action, suggesting that cAMP does not alter the normal interactions between ER and cofactors. In contrast to results with estrogen, cAMP treatment did not decrease ER protein levels. These results demonstrate that in the pituitary, cAMP activates ER in a ligand-independent manner exclusively through PKA.