Subcellular localization and biological actions of activated RSK1 are determined by its interactions with subunits of cyclic AMP-dependent protein kinase

Potential therapeutic applications of AKAP disrupting peptides

The 3'-5'-cyclic adenosine monophosphate (cAMP)/PKA pathway represents a major target for pharmacological intervention in multiple disease conditions. Although the last decade saw the concept of highly compartmentalized cAMP/PKA signaling consolidating, current means for the manipulation of this pathway still do not allow to specifically intervene on discrete cAMP/PKA microdomains. Since compartmentalization is crucial for action specificity, identifying new tools that allow local modulation of cAMP/PKA responses is an urgent need. Among key players of cAMP/PKA signaling compartmentalization, a major role is played by A-kinase anchoring proteins (AKAPs) that, by definition, anchor PKA, its substrates and its regulators within multiprotein complexes in well-confined subcellular compartments. Different tools have been conceived to interfere with AKAP-based protein-protein interactions (PPIs), and these primarily include peptides and peptidomimetics that disrupt AKAP-directed multiprotein complexes. While these molecules have been extensively used to understand the molecular mechanisms behind AKAP function in pathophysiological processes, less attention has been devoted to their potential application for therapy. In this review, we will discuss how AKAP-based PPIs can be pharmacologically targeted by synthetic peptides and peptidomimetics.

Link between steroidogenesis, the cell cycle, and PKA in adrenocortical tumor cells

Adrenocortical tumors (ACTs) frequently cause steroid excess and present cell-cycle dysregulation. cAMP/PKA signaling is involved in steroid synthesis and play a role in cell-cycle regulation. We investigated, by cell synchronization in the different phases of the cell-cycle, the control of steroidogenesis and the contribution of PKA in adrenocortical cells (H295R and culture of primary pigmented nodular adrenocortical disease cells). Cells showed increased steroidogenesis and a maximal PKA activity at G2 phase, and a reduction at G1 phase. PRKACA overexpression, or cAMP stimulation, enhanced PKA activity and induced steroidogenesis in all synchronized groups but is not sufficient to drive cell-cycle progression. PRKAR1A inactivation enhanced PKA activity and induced STAR gene expression, only in cells in G1, and triggered cell-cycle progression in all groups. These findings provide evidence for a tight association between steroidogenesis and cell-cycle in ACTs. Moreover, PRKAR1A is essential for mediating the function of PKA activity on both steroidogenesis and cell-cycle progression in adrenocortical cells.

Cardiac function modulation depends on the A-kinase anchoring protein complex

The A‐kinase anchoring proteins (AKAPs) are a group of structurally diverse proteins identified in various species and tissues. These proteins are able to anchor protein kinase and other signalling proteins to regulate cardiac function. Acting as a scaffold protein, AKAPs ensure specificity in signal transduction by enzymes close to their appropriate effectors and substrates. Over the decades, more than 70 different AKAPs have been discovered. Accumulative evidence indicates that AKAPs play crucial roles in the functional regulation of cardiac diseases, including cardiac hypertrophy, myofibre contractility dysfunction and arrhythmias. By anchoring different partner proteins (PKA, PKC, PKD and LTCCs), AKAPs take part in different regulatory pathways to function as regulators in the heart, and a damaged structure can influence the activities of these complexes. In this review, we highlight recent advances in AKAP‐associated protein complexes, focusing on local signalling events that are perturbed in cardiac diseases and their roles in interacting with ion channels and their regulatory molecules. These new findings suggest that AKAPs might have potential therapeutic value in patients with cardiac diseases, particularly malignant rhythm.

Divergent off-target effects of RSK N-terminal and C-terminal kinase inhibitors in cardiac myocytes

P90 ribosomal S6 kinases (RSK) are ubiquitously expressed and regulate responses to neurohumoral stimulation. To study the role of RSK signalling on cardiac myocyte function and protein phosphorylation, pharmacological RSK inhibitors were tested. Here, the ATP competitive N-terminal kinase domain-targeting compounds D1870 and SL0101 and the allosteric C-terminal kinase domain-targeting FMK were evaluated regarding their ability to modulate cardiac myocyte protein phosphorylation. Exposure to D1870 and SL0101 significantly enhanced phospholamban (PLN) Ser16 and cardiac troponin I (cTnI) Ser22/23 phosphorylation in response to D1870 and SL0101 upon exposure to phenylephrine (PE) that activates RSK. In contrast, FMK pretreatment significantly reduced phosphorylation of both proteins in response to PE. D1870-mediated enhancement of PLN Ser16 phosphorylation was also observed after exposure to isoprenaline or noradrenaline (NA) stimuli that do not activate RSK. Inhibition of β-adrenoceptors by atenolol or cAMP-dependent protein kinase (PKA) by H89 prevented the D1870-mediated increase in PLN phosphorylation, suggesting that PKA is the kinase responsible for the observed phosphorylation. Assessment of changes in cAMP formation by FRET measurements revealed increased cAMP formation in vicinity to PLN after exposure to D1870 and SL0101. D1870 inhibited phosphodiesterase activity similarly as established PDE inhibitors rolipram or 3-isobutyl-1-methylxanthine. Assessment of catecholamine-mediated force development in rat ventricular muscle strips revealed significantly reduced EC₅₀ for NA after D1870 pretreatment (DMSO/NA: 2.33 μmol/L vs. D1870/NA: 1.30 μmol/L). The data reveal enhanced cardiac protein phosphorylation by D1870 and SL0101 that was not detectable in response to FMK. This disparate effect might be attributed to off-target inhibition of PDEs with impact on muscle function as demonstrated for D1870.

The Molecular Basis for Specificity at the Level of the Protein Kinase a Catalytic Subunit

Assembly of multi enzyme complexes at subcellular localizations by anchoring- and scaffolding proteins represents a pivotal mechanism for achieving spatiotemporal regulation of cellular signaling after hormone receptor targeting [for review, see (1)]. In the 3' 5'-cyclic adenosine monophosphate (cAMP) dependent protein kinase (PKA) signaling pathway it is generally accepted that specificity is secured at several levels. This includes at the first level stimulation of receptors coupled to heterotrimeric G proteins which through stimulation of adenylyl cyclase (AC) forms the second messenger cAMP. Cyclic AMP has several receptors including PKA. PKA is a tetrameric holoenzyme consisting of a regulatory (R) subunit dimer and two catalytic (C) subunits. The R subunit is the receptor for cAMP and compartmentalizes cAMP signals through binding to cell and tissue-specifically expressed A kinase anchoring proteins (AKAPs). The current dogma tells that in the presence of cAMP, PKA dissociates into an R subunit dimer and two C subunits which are free to phosphorylate relevant substrates in the cytosol and nucleus. The release of the C subunit has raised the question how specificity of the cAMP and PKA signaling pathway is maintained when the C subunit no longer is attached to the R subunit-AKAP complex. An increasing body of evidence points toward a regulatory role of the cAMP and PKA signaling pathway by targeting the C subunits to various C subunit binding proteins in the cytosol and nucleus. Moreover, recent identification of isoform specific amino acid sequences, motifs and three dimensional structures have together provided new insight into how PKA at the level of the C subunit may act in a highly isoform-specific fashion. Here we discuss recent understanding of specificity of the cAMP and PKA signaling pathway based on C subunit subcellular targeting as well as evolution of the C subunit structure that may contribute to the dynamic regulation of C subunit activity.

Relative importance of phosphatidylinositol-3 kinase (PI3K)/Akt and mitogen-activated protein kinase (MAPK3/1) signaling during maturational steroid-induced meiotic G2-M1 transition in zebrafish oocytes

Participation and relative importance of phosphatidylinositol-3 kinase (PI3K) and mitogen-activated protein kinase (MAPK) signalling, either alone or in combination, have been investigated during 17α,20β-dihydroxy-4-pregnen-3-one (DHP)-induced meiotic G2-M1 transition in denuded zebrafish oocyte. Results demonstrate that concomitant with rapid phosphorylation (activation) of Akt (Ser473) and MAPK (ERK1/2) at as early as 15 min of incubation, DHP stimulation promotes enhanced an GVBD response and histone H1 kinase activation between 1 and 5 h in full-grown oocytes in vitro. While p-Akt reaches its peak at 60 to 90 min and undergoes downregulation to the basal level by 240 min, ERK1/2 phosphorylation (activation) increases gradually until 120 min and remains high thereafter. Although, priming with MEK1/2 inhibitor U0126 is without effect, PI3K inhibitors, wortmannin or LY294002, delay the GVBD response significantly (P < 0.001) until 3 h but not at 5 h of incubation. Interestingly, blocking PI3K and MEK function together could abrogate steroid-induced oocyte maturation at all time points tested. While DHP stimulation promotes phospho-PKA catalytic (p-PKAc) dephosphorylation (inactivation) between 30-120 min of incubation, simultaneous inhibition of PI3K and MEK1/2 kinases abrogates DHP action. Conversely, elevated intra-oocyte cAMP, through priming with either adenylyl cyclase (AC) activator forskolin (FK) or dibutyryl cAMP (db-cAMP), abrogates steroid-induced Akt and ERK1/2 phosphorylation. Taken together, these results suggest that DHP-induced Akt and ERK activation precedes the onset of meiosis (GVBD response) in a cAMP-sensitive manner and PI3K/Akt and MEK/MAPK pathways together have a pivotal influence in the downregulation of PKA and resumption of meiotic maturation in zebrafish oocytes in vitro.

t-Darpp stimulates protein kinase A activity by forming a complex with its RI regulatory subunit

t-Darpp is the truncated form of the dopamine- and cAMP-regulated phosphoprotein of 32kDa (Darpp-32) and has been demonstrated to confer resistance to trastuzumab, a Her2-targeted anticancer agent, via sustained signaling through the phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/Akt pathway and activation of protein kinase A (PKA). The mechanism of t-Darpp-mediated PKA activation is poorly understood. In the PKA holoenzyme, when the catalytic subunits are bound to regulatory subunits RI or RII, kinase activity is inhibited. We investigated PKA activity and holoenzyme composition in cell lines overexpressing t-Darpp (SK.tDp) or a T39A phosphorylation mutant (SK.tDp^T39A), as well as an empty vector control cell line (SK.empty). We also evaluated protein-protein interactions between t-Darpp and PKA catalytic (PKAc) or regulatory subunits RI and RII in those cell lines. SK.tDp cells had elevated PKA activity and showed diminished association of RI with PKAc, whereas SK.tDp^T39A cells did not have these properties. Moreover, wild type t-Darpp associates with RI. Concurrent expression of Darpp-32 reversed t-Darrp's effects on PKA holoenzyme state, consistent with earlier observations that Darpp-32 reverses t-Darpp's activation of PKA. Together, t-Darpp phosphorylation at T39 seems to be crucial for t-Darpp-mediated PKA activation and this activation appears to occur through an association with RI and sequestering of RI away from PKAc. The t-Darpp-RI interaction could be a druggable target to reduce PKA activity in drug-resistant cancer.

The many faces of compartmentalized PKA signalosomes

Cellular signal transmission requires the dynamic formation of spatiotemporally controlled molecular interactions. At the cell surface information is received by receptor complexes and relayed through intracellular signaling platforms which organize the actions of functionally interacting signaling enzymes and substrates. The list of hormone or neurotransmitter pathways that utilize the ubiquitous cAMP-sensing protein kinase A (PKA) system is expansive. This requires that the specificity, duration, and intensity of PKA responses are spatially and temporally restricted. Hereby, scaffolding proteins take the center stage for ensuring proper signal transmission. They unite second messenger sensors, activators, effectors, and kinase substrates within cellular micro-domains to precisely control and route signal propagation. A-kinase anchoring proteins (AKAPs) organize such subcellular signalosomes by tethering the PKA holoenzyme to distinct cell compartments. AKAPs differ in their modular organization showing pathway specific arrangements of interaction motifs or domains. This enables the cell- and compartment- guided assembly of signalosomes with unique enzyme composition and function. The AKAP-mediated clustering of cAMP and other second messenger sensing and interacting signaling components along with functional successive enzymes facilitates the rapid and precise dissemination of incoming signals. This review article delineates examples for different means of PKA regulation and for snapshots of compartmentalized PKA signalosomes.

Ribosomal S6 kinase (RSK) modulators: a patent review

INTRODUCTION

The p90 ribosomal S6 kinases (RSK) are a family of Ser/Thr protein kinases that are downstream effectors of MEK1/2-ERK1/2. Increased RSK activation is implicated in the etiology of multiple pathologies, including numerous types of cancers, cardiovascular disease, liver and lung fibrosis, and infections.

AREAS COVERED

The review summarizes the patent and scientific literature on small molecule modulators of RSK and their potential use as therapeutics. The patents were identified using World Intellectual Property Organization and United States Patent and Trademark Office databases. The compounds described are predominantly RSK inhibitors, but a RSK activator is also described. The majority of the inhibitors are not RSK-specific.

EXPERT OPINION

Based on the overwhelming evidence that RSK is involved in a number of diseases that have high mortalities it seems surprising that there are no RSK modulators that have pharmacokinetic properties suitable for in vivo use. MEK1/2 inhibitors are in the clinic, but the efficacy of these compounds appears to be limited by their side effects. We hypothesize that targeting the downstream effectors of MEK1/2, like RSK, are an untapped source of drug targets and that they will generate less side effects than MEK1/2 inhibitors because they regulate fewer effectors.

Pharmacological targeting of AKAP-directed compartmentalized cAMP signalling

The second messenger cyclic adenosine monophosphate (cAMP) can bind and activate protein kinase A (PKA). The cAMP/PKA system is ubiquitous and involved in a wide array of biological processes and therefore requires tight spatial and temporal regulation. Important components of the safeguard system are the A-kinase anchoring proteins (AKAPs), a heterogeneous family of scaffolding proteins defined by its ability to directly bind PKA. AKAPs tether PKA to specific subcellular compartments, and they bind further interaction partners to create local signalling hubs. The recent discovery of new AKAPs and advances in the field that shed light on the relevance of these hubs for human disease highlight unique opportunities for pharmacological modulation. This review exemplifies how interference with signalling, particularly cAMP signalling, at such hubs can reshape signalling responses and discusses how this could lead to novel pharmacological concepts for the treatment of disease with an unmet medical need such as cardiovascular disease and cancer.

Neurochondrin is an atypical RIIα-specific A-kinase anchoring protein

Protein kinase activity is regulated not only by direct strategies affecting activity but also by spatial and temporal regulatory mechanisms. Kinase signaling pathways are coordinated by scaffolding proteins that orchestrate the assembly of multi-protein complexes. One family of such scaffolding proteins are the A-kinase anchoring proteins (AKAPs). AKAPs share the commonality of binding cAMP-dependent protein kinase (PKA). In addition, they bind further signaling proteins and kinase substrates and tether such multi-protein complexes to subcellular locations. The A-kinase binding (AKB) domain of AKAPs typically contains a conserved helical motif that interacts directly with the dimerization/docking (D/D) domain of the regulatory subunits of PKA. Based on a pull-down proteomics approach, we identified neurochondrin (neurite-outgrowth promoting protein) as a previously unidentified AKAP. Here, we show that neurochondrin interacts directly with PKA through a novel mechanism that involves two distinct binding regions. In addition, we demonstrate that neurochondrin has strong isoform selectivity towards the RIIα subunit of PKA with nanomolar affinity. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases.

Interactions between the regulatory subunit of type I protein kinase A and p90 ribosomal S6 kinase1 regulate cardiomyocyte apoptosis

Cardiomyocyte apoptosis contributes toward the loss of muscle mass in myocardial pathologies. Previous reports have implicated type I cAMP-dependent protein kinase (PKA) and p90 ribosomal S6 kinase (RSK) in cardiomyocyte apoptosis. However, the precise mechanisms and the isoform of RSK involved in this process remain undefined. Using adult rat ventricular myocytes and mouse-derived cardiac HL-1 cardiomyocytes, we demonstrate that hypoxia/reoxygenation (H/R)-induced apoptosis is accompanied by a decrease in the type I PKA regulatory subunit (PKARIα) and activation of RSK1. As previously described by us for other cell types, in cardiomyocytes, inactive RSK1 also interacts with PKARIα, whereas the active RSK1 interacts with the catalytic subunit of PKA. Additionally, small interfering (siRNA)-mediated silencing of PKARIα or disrupting the RSK1/PKARIα interactions with a small, cell-permeable peptide activates RSK1 and recapitulates the H/R-induced apoptosis. Inhibition of RSK1 or siRNA-mediated silencing of RSK1 attenuates H/R-induced apoptosis, demonstrating the role of RSK1 in cardiomyocyte apoptosis. Furthermore, silencing of RSK1 decreases the H/R-induced phosphorylation of sodium-hydrogen exchanger 1 (NHE1), and inhibition of NHE1 with 5'-N-ethyl-N-isopropyl-amiloride blocks H/R induced apoptosis, indicating the involvement of NHE1 in apoptosis. Overall, our findings demonstrate that H/R-mediated decrease in PKARIα protein levels leads to activation of RSK1, which via phosphorylation of NHE1 induces cardiomyocyte apoptosis.

Ribosomal S6 Kinase 2 (RSK2) maintains genomic stability by activating the Atm/p53-dependent DNA damage pathway

Ribosomal S6 Kinase 2 (RSK2) is a member of the p90^RSK family of serine/threonine kinases, which are widely expressed and respond to many growth factors, peptide hormones, and neurotransmitters. Loss-of function mutations in the RPS6KA3 gene, which encodes the RSK2 protein, have been implicated in Coffin-Lowry Syndrome (CLS), an X-linked mental retardation disorder associated with cognitive deficits and behavioral impairments. However, the cellular and molecular mechanisms underlying this neurological disorder are not known. Recent evidence suggests that defective DNA damage signaling might be associated with neurological disorders, but the role of RSK2 in the DNA damage pathway remains to be elucidated. Here, we show that Adriamycin-induced DNA damage leads to the phosphorylation of RSK2 at Ser227 and Thr577 in the chromatin fraction, promotes RSK2 nuclear translocation, and enhances RSK2 and Atm interactions in the nuclear fraction. Furthermore, using RSK2 knockout mouse fibroblasts and RSK2-deficient cells from CLS patients, we demonstrate that ablation of RSK2 impairs the phosphorylation of Atm at Ser1981 and the phosphorylation of p53 at Ser18 (mouse) or Ser15 (human) in response to genotoxic stress. We also show that RSK2 affects p53-mediated downstream cellular events in response to DNA damage, that RSK2 knockout relieves cell cycle arrest at the G2/M phase, and that an increased number of γH2AX foci, which are associated with defects in DNA repair, are present in RSK2-deficient cells. Taken together, our findings demonstrated that RSK2 plays an important role in the DNA damage pathway that maintains genomic stability by mediating cell cycle progression and DNA repair.

Rapgef2 connects GPCR-mediated cAMP signals to ERK activation in neuronal and endocrine cells

G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor (GPCR)-mediated increases in the second messenger cyclic adenosine monophosphate (cAMP) activate the mitogen-activated protein kinase (MAPK) extracellular signal-regulated kinase (ERK), and in neuroendocrine cells, this pathway leads to cAMP-dependent neuritogenesis mediated through Rap1 and B-Raf. We found that the Rap guanine nucleotide exchange factor Rapgef2 was enriched from primary bovine neuroendocrine cells by cAMP-agarose affinity chromatography and that it was specifically eluted by cAMP. With loss-of-function experiments in the rat neuronal cell line Neuroscreen-1 (NS-1) and gain-of-function experiments in human embryonic kidney 293T cells, we demonstrated that Rapgef2 connected GPCR-dependent activation of adenylate cyclase and increased cAMP concentration with the activation of ERK in neurons and endocrine cells. Furthermore, knockdown of Rapgef2 blocked cAMP- and ERK-dependent neuritogenesis. Our data are consistent with a pathway involving the cAMP-mediated activation of Rapgef2, which then stimulates Rap1, leading to increases in B-Raf, MEK, and ERK activity.

Paxillin phosphorylation counteracts proteoglycan-mediated inhibition of axon regeneration

In the adult central nervous system, the tips of axons severed by injury are commonly transformed into dystrophic endballs and cease migration upon encountering a rising concentration gradient of inhibitory proteoglycans. However, intracellular signaling networks mediating endball migration failure remain largely unknown. Here we show that manipulation of protein kinase A (PKA) or its downstream adhesion component paxillin can reactivate the locomotive machinery of endballs in vitro and facilitate axon growth after injury in vivo. In dissociated cultures of adult rat dorsal root ganglion neurons, PKA is activated in endballs formed on gradients of the inhibitory proteoglycan aggrecan, and pharmacological inhibition of PKA promotes axon growth on aggrecan gradients most likely through phosphorylation of paxillin at serine 301. Remarkably, pre-formed endballs on aggrecan gradients resume forward migration in response to PKA inhibition. This resumption of endball migration is associated with increased turnover of adhesive point contacts dependent upon paxillin phosphorylation. Furthermore, expression of phosphomimetic paxillin overcomes aggrecan-mediated growth arrest of endballs, and facilitates axon growth after optic nerve crush in vivo. These results point to the importance of adhesion dynamics in restoring endball migration and suggest a potential therapeutic target for axon tract repair.

Anchored p90 ribosomal S6 kinase 3 is required for cardiac myocyte hypertrophy

RATIONALE

Cardiac myocyte hypertrophy is the main compensatory response to chronic stress on the heart. p90 ribosomal S6 kinase (RSK) family members are effectors for extracellular signal-regulated kinases that induce myocyte growth. Although increased RSK activity has been observed in stressed myocytes, the functions of individual RSK family members have remained poorly defined, despite being potential therapeutic targets for cardiac disease.

OBJECTIVE

To demonstrate that type 3 RSK (RSK3) is required for cardiac myocyte hypertrophy.

METHODS AND RESULTS

RSK3 contains a unique N-terminal domain that is not conserved in other RSK family members. We show that this domain mediates the regulated binding of RSK3 to the muscle A-kinase anchoring protein scaffold, defining a novel kinase anchoring event. Disruption of both RSK3 expression using RNA interference and RSK3 anchoring using a competing muscle A-kinase anchoring protein peptide inhibited the hypertrophy of cultured myocytes. In vivo, RSK3 gene deletion in the mouse attenuated the concentric myocyte hypertrophy induced by pressure overload and catecholamine infusion.

CONCLUSIONS

Taken together, these data demonstrate that anchored RSK3 transduces signals that modulate pathologic myocyte growth. Targeting of signaling complexes that contain select kinase isoforms should provide an approach for the specific inhibition of cardiac myocyte hypertrophy and for the development of novel strategies for the prevention and treatment of heart failure.

Regulation and function of the RSK family of protein kinases

The RSK (90 kDa ribosomal S6 kinase) family comprises a group of highly related serine/threonine kinases that regulate diverse cellular processes, including cell growth, proliferation, survival and motility. This family includes four vertebrate isoforms (RSK1, RSK2, RSK3 and RSK4), and single family member orthologues are also present in Drosophila and Caenorhabditis elegans. The RSK isoforms are downstream effectors of the Ras/ERK (extracellular-signal-regulated kinase) signalling pathway. Significant advances in the field of RSK signalling have occurred in the past few years, including several new functions ascribed to the RSK isoforms, the discovery of novel protein substrates and the implication of different RSK isoforms in cancer. Collectively, these new findings increase the diversity of biological functions regulated by RSK, and highlight potential new directions of research. In the present paper, we review the structure, expression and activation mechanisms of the RSK isoforms, and discuss their physiological roles on the basis of established substrates and recent discoveries.

Localization and retention of p90 ribosomal S6 kinase 1 in the nucleus: implications for its function

Ribosomal S6 kinase 1 (RSK1), which plays a critical role in cell survival and proliferation, contains a bipartite nuclear localization sequence that permits its entry into the nucleus. RSK1 is retained in the nucleus via its indirect interactions with AKAP95. Interference with its nuclear entry or retention decreases DNA synthesis.

Ribosomal S6 kinase 1 (RSK1) belongs to a family of proteins with two kinase domains. Following activation in the cytoplasm by extracellular signal-regulated kinases (ERK1/2), it mediates the cell-proliferative, cell-growth, and survival-promoting actions of a number of growth factors and other agonists. These diverse biological actions of RSK1 involve regulation of both cytoplasmic and nuclear events. However, the mechanisms that permit nuclear accumulation of RSK1 remain unknown. Here, we show that phosphorylation of RSK1 on S221 is important for its dissociation from the type Iα regulatory subunit of protein kinase A (PKA) in the cytoplasm and that RSK1 contains a bipartite nuclear localization sequence that is necessary for its nuclear entry. Once inside, the active RSK1 is retained in the nucleus via its interactions with PKA catalytic subunit and AKAP95. Mutations of RSK1 that do not affect its activity but disrupt its entry into the nucleus or expression of AKAP95 forms that do not enter the nucleus inhibit the ability of active RSK1 to stimulate DNA synthesis. Our findings identify novel mechanisms by which active RSK1 accumulates in the nucleus and also provide new insights into how AKAP95 orchestrates cell cycle progression.

Isoform-specific targeting of PKA to multivesicular bodies

PKA RIα subunit is localized to MVBs by the A-kinase–anchoring protein AKAP11 when disassociated from the PKA catalytic subunit.

Although RII protein kinase A (PKA) regulatory subunits are constitutively localized to discrete cellular compartments through binding to A-kinase–anchoring proteins (AKAPs), RI subunits are primarily diffuse in the cytoplasm. In this paper, we report a novel AKAP-dependent localization of RIα to distinct organelles, specifically, multivesicular bodies (MVBs). This localization depends on binding to AKAP11, which binds tightly to free RIα or RIα in complex with catalytic subunit (holoenzyme). However, recruitment to MVBs occurs only with the release of PKA catalytic subunit (PKAc). This recruitment is reversed by reassociation with PKAc, and it is disrupted by the presence of AKAP peptides, mutations in the RIα AKAP-binding site, or knockdown of AKAP11. Cyclic adenosine monophosphate binding not only unleashes active PKAc but also leads to the targeting of AKAP11:RIα to MVBs. Therefore, we show that the RIα holoenzyme is part of a signaling complex with AKAP11, in which AKAP11 may direct RIα functionality after disassociation from PKAc. This model defines a new paradigm for PKA signaling.

Activation and function of the MAPKs and their substrates, the MAPK-activated protein kinases

The mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs by relaying extracellular signals to intracellular responses. In mammals, there are more than a dozen MAPK enzymes that coordinately regulate cell proliferation, differentiation, motility, and survival. The best known are the conventional MAPKs, which include the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinases 1 to 3 (JNK1 to -3), p38 (α, β, γ, and δ), and ERK5 families. There are additional, atypical MAPK enzymes, including ERK3/4, ERK7/8, and Nemo-like kinase (NLK), which have distinct regulation and functions. Together, the MAPKs regulate a large number of substrates, including members of a family of protein Ser/Thr kinases termed MAPK-activated protein kinases (MAPKAPKs). The MAPKAPKs are related enzymes that respond to extracellular stimulation through direct MAPK-dependent activation loop phosphorylation and kinase activation. There are five MAPKAPK subfamilies: the p90 ribosomal S6 kinase (RSK), the mitogen- and stress-activated kinase (MSK), the MAPK-interacting kinase (MNK), the MAPK-activated protein kinase 2/3 (MK2/3), and MK5 (also known as p38-regulated/activated protein kinase [PRAK]). These enzymes have diverse biological functions, including regulation of nucleosome and gene expression, mRNA stability and translation, and cell proliferation and survival. Here we review the mechanisms of MAPKAPK activation by the different MAPKs and discuss their physiological roles based on established substrates and recent discoveries.

Activation and Function of the MAPKs and Their Substrates, the MAPK-Activated Protein Kinases

The mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs by relaying extracellular signals to intracellular responses. In mammals, there are more than a dozen MAPK enzymes that coordinately regulate cell proliferation, differentiation, motility, and survival. The best known are the conventional MAPKs, which include the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinases 1 to 3 (JNK1 to -3), p38 (α, β, γ, and δ), and ERK5 families. There are additional, atypical MAPK enzymes, including ERK3/4, ERK7/8, and Nemo-like kinase (NLK), which have distinct regulation and functions. Together, the MAPKs regulate a large number of substrates, including members of a family of protein Ser/Thr kinases termed MAPK-activated protein kinases (MAPKAPKs). The MAPKAPKs are related enzymes that respond to extracellular stimulation through direct MAPK-dependent activation loop phosphorylation and kinase activation. There are five MAPKAPK subfamilies: the p90 ribosomal S6 kinase (RSK), the mitogen- and stress-activated kinase (MSK), the MAPK-interacting kinase (MNK), the MAPK-activated protein kinase 2/3 (MK2/3), and MK5 (also known as p38-regulated/activated protein kinase [PRAK]). These enzymes have diverse biological functions, including regulation of nucleosome and gene expression, mRNA stability and translation, and cell proliferation and survival. Here we review the mechanisms of MAPKAPK activation by the different MAPKs and discuss their physiological roles based on established substrates and recent discoveries.

Cross-talk between protein kinase A and the MAPK-activated protein kinases RSK1 and MK5

Typical mammalian mitogen-activated protein kinase (MAPK) pathways consist of a cascade of three consecutive phosphorylation events exerted by a MAPK kinase kinase (MAPKKK), a MAPK kinase (MAPKK), and finally a MAPK. MAPKs not only target non-protein kinase substrates, they can also phosphorylate other protein kinases designated as MAPK-activated protein kinases (MAPKAPK). The MAPKAPK family includes the ribosomal-S6-kinases (RSK1-4), the MAPK-interacting kinases (MNK1 and 2), the mitogen-and stress-activated kinases (MSK1 and 2), and the MAPKAPK (MK2, 3, and 5) subfamilies. Although several reports indicate extensive cross-talk between the MAPK and protein kinase A (PKA) pathways, evidence of a direct interaction at the level of the MAPKAPK only appeared recently. The MAPKAPKs RSK1 and MK5 can bind to PKA, but the features of these interactions are distinct. This review discusses the different characteristics of regulating the activity and subcellular localization of MK5 and RSK1 by PKA and the functional implications of these interactions.

Serine residue 115 of MAPK-activated protein kinase MK5 is crucial for its PKA-regulated nuclear export and biological function

The mitogen-activated protein kinase-activated protein kinase-5 (MK5) resides predominantly in the nucleus of resting cells, but p38^MAPK, extracellular signal-regulated kinases-3 and -4 (ERK3 and ERK4), and protein kinase A (PKA) induce nucleocytoplasmic redistribution of MK5. The mechanism by which PKA causes nuclear export remains unsolved. In the study reported here we demonstrated that Ser-115 is an in vitro PKA phosphoacceptor site, and that PKA, but not p38^MAPK, ERK3 or ERK4, is unable to redistribute MK5 S115A to the cytoplasm. However, the phosphomimicking MK5 S115D mutant resides in the cytoplasm in untreated cells. While p38^MAPK, ERK3 and ERK4 fail to trigger nuclear export of the kinase dead T182A and K51E MK5 mutants, S115D/T182A and K51E/S115D mutants were able to enter the cytoplasm of resting cells. Finally, we demonstrated that mutations in Ser-115 affect the biological properties of MK5. Taken together, our results suggest that Ser-115 plays an essential role in PKA-regulated nuclear export of MK5, and that it also may regulate the biological functions of MK5.

p90 ribosomal S6 kinase 1 (RSK1) and the catalytic subunit of protein kinase A (PKA) compete for binding the pseudosubstrate region of PKAR1alpha: role in the regulation of PKA and RSK1 activities

Previously we showed that the inactive form of p90 ribosomal S6 kinase 1 (RSK1) interacts with the regulatory subunit, PKARIalpha, of protein kinase A (PKA), whereas the active RSK1 interacts with the catalytic subunit (PKAc) of PKA. Herein, we demonstrate that the N-terminal kinase domain (NTK) of RSK1 is necessary for interactions with PKARIalpha. Substitution of the activation loop phosphorylation site (Ser-221) in the NTK with the negatively charged Asp residue abrogated the association between RSK1 and PKARIalpha. This explains the lack of an interaction between active RSK1 and PKARIalpha. Full-length RSK1 bound to PKARIalpha with an affinity of 0.8 nm. The NTK domain of RSK1 competed with PKAc for binding to the pseudosubstrate region (amino acids 93-99) of PKARIalpha. Overexpressed RSK1 dissociated PKAc from PKARIalpha, increasing PKAc activity, whereas silencing of RSK1 increased PKAc/PKARIalpha interactions and decreased PKAc activity. Unlike PKAc, which requires Arg-95 and -96 in the pseudosubstrate region of PKARIalpha for their interactions, RSK1/PKARIalpha association requires all four Arg residues (Arg-93-96) in the pseudosubstrate site of PKARIalpha. A peptide (Wt-PS) corresponding to residues 91-99 of PKARIalpha competed for binding of RSK1 with PKARIalpha both in vitro and in intact cells. Furthermore, peptide Wt-PS (but not control peptide Mut-PS), by dissociating RSK1 from PKARIalpha, activated RSK1 in the absence of any growth factors and protected cells from apoptosis. Thus, by competing for binding to the pseudosubstrate region of PKARIalpha, RSK1 regulates PKAc activity in a cAMP-independent manner, and PKARIalpha by associating with RSK1 regulates its activation and its biological functions.

Mechanisms of protein kinase A anchoring

The second messenger cyclic adenosine monophosphate (cAMP), which is produced by adenylyl cyclases following stimulation of G-protein-coupled receptors, exerts its effect mainly through the cAMP-dependent serine/threonine protein kinase A (PKA). Due to the ubiquitous nature of the cAMP/PKA system, PKA signaling pathways underlie strict spatial and temporal control to achieve specificity. A-kinase anchoring proteins (AKAPs) bind to the regulatory subunit dimer of the tetrameric PKA holoenzyme and thereby target PKA to defined cellular compartments in the vicinity of its substrates. AKAPs promote the termination of cAMP signals by recruiting phosphodiesterases and protein phosphatases, and the integration of signaling pathways by binding additional signaling proteins. AKAPs are a heterogeneous family of proteins that only display similarity within their PKA-binding domains, amphipathic helixes docking into a hydrophobic groove formed by the PKA regulatory subunit dimer. This review summarizes the current state of information on compartmentalized cAMP/PKA signaling with a major focus on structural aspects, evolution, diversity, and (patho)physiological functions of AKAPs and intends to outline newly emerging directions of the field, such as the elucidation of AKAP mutations and alterations of AKAP expression in human diseases, and the validation of AKAP-dependent protein-protein interactions as new drug targets. In addition, alternative PKA anchoring mechanisms employed by noncanonical AKAPs and PKA catalytic subunit-interacting proteins are illustrated.

Glycogen synthase kinase 3beta interaction protein functions as an A-kinase anchoring protein

A-kinase anchoring proteins (AKAPs) include a family of scaffolding proteins that target protein kinase A (PKA) and other signaling proteins to cellular compartments and thereby confine the activities of the associated proteins to distinct regions within cells. AKAPs bind PKA directly. The interaction is mediated by the dimerization and docking domain of regulatory subunits of PKA and the PKA-binding domain of AKAPs. Analysis of the interactions between the dimerization and docking domain and various PKA-binding domains yielded a generalized motif allowing the identification of AKAPs. Our bioinformatics and peptide array screening approaches based on this signature motif identified GSKIP (glycogen synthase kinase 3beta interaction protein) as an AKAP. GSKIP directly interacts with PKA and GSK3beta (glycogen synthase kinase 3beta). It is widely expressed and facilitates phosphorylation and thus inactivation of GSK3beta by PKA. GSKIP contains the evolutionarily conserved domain of unknown function 727. We show here that this domain of GSKIP and its vertebrate orthologues binds both PKA and GSK3beta and thereby provides a mechanism for the integration of PKA and GSK3beta signaling pathways.

Regulation of protein kinase A activity by p90 ribosomal S6 kinase 1

Previously, we reported that the catalytic subunit of cAMP-dependent protein kinase (PKAc) binds to the active p90 ribosomal S6 kinase 1 (RSK1) (Chaturvedi, D., Poppleton, H. M., Stringfield, T., Barbier, A., and Patel, T. B. (2006) Mol. Cell. Biol. 26, 4586-4600). Herein, by overexpressing hemagglutinin-tagged RSK1 fragments in HeLa cells we have identified the region of RSK1 that is responsible for the interaction with PKAc. PKAc bound to the last 13 amino acids of RSK1, which overlaps the Erk1/2 docking site. This interaction between PKAc and RSK1 required the phosphorylation of Ser-732 in the C terminus of RSK1. Depending upon its phosphorylation status, RSK1 switched interactions between Erk1/2 and PKAc. In addition, a peptide corresponding to the last 13 amino acids of RSK1 with substitution of Ser-732 with Glu (peptide E), but not Ala (peptide A), decreased interactions between endogenous active RSK1 and PKAc. RSK1 attenuated the ability of cAMP to activate PKA in vitro and this modulation was abrogated by peptide E, but not by peptide A. Similarly, in intact cells, cAMP-mediated phosphorylation of Bcl-xL/Bcl-2-associated death promoter on Ser-115, the PKA site, was reduced when RSK1 was activated by epidermal growth factor, and this effect was blocked by peptide E, but not by peptide A. These findings demonstrate that interactions between endogenous RSK1 and PKAc in intact cells regulate the ability of cAMP to activate PKA and identify a novel mechanism by which PKA activity is regulated by the Erk1/2 pathway.

The PKARIalpha subunit of protein kinase A modulates the activation of p90RSK1 and its function

Previously, we showed that interactions between p90(RSK1) (RSK1) and the subunits of type I protein kinase A (PKA) regulate the activity of PKA and cellular distribution of active RSK1 (Chaturvedi, D., Poppleton, H. M., Stringfield, T., Barbier, A., and Patel, T. B. (2006) Mol. Cell Biol. 26, 4586-4600). Here we examined the role of the PKARIalpha subunit of PKA in regulating RSK1 activation and cell survival. In mouse lung fibroblasts, silencing of the PKARIalpha increased the phosphorylation and activation of RSK1, but not of RSK2 and RSK3, in the absence of any stimulation. Silencing of PKARIalpha also decreased the nuclear accumulation of active RSK1 and increased its cytoplasmic content. The increased activation of RSK1 in the absence of any agonist and changes in its subcellular redistribution resulted in increased phosphorylation of its cytoplasmic substrate BAD and increased cell survival. The activity of PKA and phosphorylation of BAD (Ser-155) were also enhanced when PKARIalpha was silenced, and this, in part, contributed to increased cell survival in unstimulated cells. Furthermore, we show that RSK1, PKA subunits, D-AKAP1, and protein phosphatase 2A catalytic subunit (PP2Ac) exist in a complex, and dissociation of RSK1 from D-AKAP1 by either silencing of PKARIalpha, depletion of D-AKAP1, or by using a peptide that competes with PKARIalpha for binding to AKAPs, decreased the amount of PP2Ac in the RSK1 complex. We also demonstrate that PP2Ac is one of the phosphatases that dephosphorylates RSK, but not ERK1/2. Thus, in unstimulated cells, the increased phosphorylation and activation of RSK1 after silencing of PKARIalpha or depletion of D-AKAP1 are due to decreased association of PP2Ac in the RSK1 complex.

Rapamycin induces transactivation of the EGFR and increases cell survival

The mammalian target of rapamycin (mTOR) signaling network regulates cell growth, proliferation and cell survival. Deregulated activation of this pathway is a common event in diverse human diseases such as cancers, cardiac hypertrophy, vascular restenosis and nephrotic hypertrophy. Although mTOR inhibitor, rapamycin, has been widely used to inhibit the aberrant signaling due to mTOR activation that plays a major role in hyperproliferative diseases, in some cases rapamycin does not attenuate the cell proliferation and survival. Thus, we studied the mechanism(s) by which cells may confer resistance to rapamycin. Our data show that in a variety of cell types the mTOR inhibitor rapamycin activates extracellularly regulated kinases (Erk1/2) signaling. Rapamycin-mediated activation of the Erk1/2 signaling requires (a) the epidermal growth factor receptor (EGFR), (b) its tyrosine kinase activity and (c) intact autophosphorylation sites on the receptor. Rapamycin treatment increases tyrosine phosphorylation of EGFR without the addition of growth factor and this transactivation of receptor involves activation of c-Src. We also show that rapamycin treatment triggers activation of cell survival signaling pathway by activating the prosurvival kinases Erk1/2 and p90RSK. These studies provide a novel paradigm by which cells escape the apoptotic actions of rapamycin and its derivatives that inhibit the mTOR pathway.

The RSK family of kinases: emerging roles in cellular signalling

The 90 kDa ribosomal S6 kinase (RSK) family of proteins is a group of highly conserved Ser/Thr kinases that regulate diverse cellular processes, such as cell growth, cell motility, cell survival and cell proliferation. RSKs are downstream effectors of the Ras-extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) signalling cascade. Significant advances in the field of RSK and ERK/MAPK signalling have occurred in the past few years, including biological insights and the discovery of novel substrates and new RSK regulatory mechanisms. Collectively, these data expand the current models of RSK signalling and highlight potential directions of research in RSK-mediated survival, growth, proliferation and migration.

Codependent functions of RSK2 and the apoptosis-promoting factor TIA-1 in stress granule assembly and cell survival

Stress granules aid cell survival in response to environmental stressors by acting as sites of translational repression. We report an unanticipated link between stress granules and the serine/threonine kinase RSK2. In stressed breast cells, endogenous RSK2 colocalizes in granules with TIA-1 and poly(A)-binding protein 1, and the sequestration of RSK2 and TIA-1 exhibits codependency. The RSK2 N-terminal kinase domain controls the direct interaction with the prion-related domain of TIA-1. Silencing RSK2 decreases cell survival in response to stress. Mitogen releases RSK2 from the stress granules and permits its nuclear import via a nucleocytoplasmic shuttling sequence in the C-terminal domain. Nuclear accumulation is dependent on TIA-1. Surprisingly, nuclear localization of RSK2 is sufficient to enhance proliferation through induction of cyclin D1, in the absence of other active signaling pathways. Hence, RSK2 is a pivotal factor linking the stress response to survival and proliferation.

Thyroid-stimulating hormone/cAMP-mediated proliferation in thyrocytes

Current research on thyrotropin-activated proliferation in the thyrocyte needs to be aimed at a better understanding of crosstalk and negative-feedback mechanisms with other proliferative pathways, especially the insulin/IGF-1-induced phosphoinositol-3 kinase pathway and the serum-induced MAPK or Wnt pathways. Convergence of proliferative pathways in mTOR is a hotspot of current research, and combined treatment using double class inhibitors for thyroid cancer may bring some success. New thyroid-stimulating hormone receptor (TSHR)-interacting proteins, a better picture of cAMP targets, a deeper knowledge of the action of the protein kinase A regulatory subunits, especially their interactions with the replication machinery, and a further understanding of mechanisms that lead to cell cycle progression through G₁/S and G₂/M checkpoints are areas that need further elucidation. Finally, massive information coming from microarray data analysis will prompt our understanding of thyroid-stimulating hormone-promoted thyrocyte proliferation in health and disease.

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.

A novel role of Sprouty 2 in regulating cellular apoptosis

Sprouty (SPRY) proteins modulate receptor-tyrosine kinase signaling and, thereby, regulate cell migration and proliferation. Here, we have examined the role of endogenous human SPRY2 (hSPRY2) in the regulation of cellular apoptosis. Small inhibitory RNA-mediated silencing of hSPRY2 abolished the anti-apoptotic action of serum in adrenal cortex adenocarcinoma (SW13) cells. Silencing of hSPRY2 decreased serum- or epidermal growth factor (EGF)-elicited activation of AKT and ERK1/2 and reduced the levels of EGF receptor. Silencing of hSPRY2 also inhibited serum-induced activation of p90RSK and decreased phosphorylation of pro-apoptotic protein BAD (BCL2-antagonist of cell death) by p90RSK. Inhibiting both the ERK1/2 and AKT pathways abolished the ability of serum to protect against apoptosis, mimicking the effects of silencing hSPRY2. Serum transactivated the EGF receptor (EGFR), and inhibition of the EGFR by a neutralizing antibody attenuated the anti-apoptotic actions of serum. Consistent with the role of EGFR and perhaps other growth factor receptors in the anti-apoptotic actions of serum, the tyrosine kinase binding domain of c-Cbl (Cbl-TKB) protected against down-regulation of the growth factor receptors such as EGFR and preserved the anti-apoptotic actions of serum when hSpry2 was silenced. Additionally, silencing of Spry2 in c-Cbl null cells did not alter the ability of serum to promote cell survival. Moreover, reintroduction of wild type hSPRY2, but not its mutants that do not bind c-Cbl or CIN85 into SW13 cells after endogenous hSPRY2 had been silenced, restored the anti-apoptotic actions of serum. Overall, we conclude that endogenous hSPRY2-mediated regulation of apoptosis requires c-Cbl and is manifested by the ability of hSPRY2 to sequester c-Cbl and thereby augment signaling via growth factor receptors.

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.

cAMP and protein kinase A in endocrine (and other) tumors

Studies of the biological role of cAMP have indicated dual and often opposing effects on proliferation and differentiation. Elevation of the intracellular cAMP in normal and transformed cells may lead to cell proliferation; in other cells, it induces changes in morphology, apoptosis and/or differentiation. The best known mediator of cAMP action in the cell is cAMP-dependent protein kinase or protein kinase A (PKA). PKA exists as two different isozymes, designated type I (PKA-I) and type II (PKA-II); the two isoforms are essentially distinct in their physicochemical properties. The relative ratio of PKA-I and PKA-II varies throughout the cell cycle in cells of the same type, it changes significantly during development and follows different patterns in the various tissues. Disruption of the apparently fine balance between the main two PKA isozymes is strongly associated with tumorigenesis and tumor growth, and vice versa. The enormous variety of cAMP/PKA functions and the net effect of this signaling system on cellular growth, proliferation and differentiation have been the subject of debate for more than 30 years among investigators in the field. The relatively recent identification of PRKAR1A mutations and PKA-I deficiency as a cause of endocrine and other tumors in human and mice was instrumental in advancing our understanding of how cAMP and PKA work in regulating the cell cycle. This article reviews the current state of knowledge in the field; the use of pharmacologic modulation of the cAMP/PKA system with the goal of treating certain tumors appears to be near, although very little has been accomplished so far, at least in terms of studies on humans.

A RSK(y) relationship with promiscuous PKA

Where, when, and with "whom" do molecules interact? Such relations in space and time are key concepts that currently engage investigators of cellular signaling processes. The notion of compartmentalized signaling grew out of studies of adenosine 3',5'-monophosphate (cAMP) signaling processes, and this area continues to generate exciting new paradigms. Distinct clouds of cAMP are formed and shaped within cells by tethered cAMP phosphodiesterases (PDEs). AKAPosomes, formed from distinct subpopulations of cAMP-dependent protein kinase (PKA) tethered to anchoring proteins (AKAPs) together with specific substrate molecules, interpret these gradients to generate individualized responses. PKA activity is also regulated by the interaction of other proteins with the regulatory (R) or catalytic (C) subunits of PKA, and a mechanism has been uncovered in which ribosomal S6 kinase (RSK1) interacts with either PKA subunit, depending on whether RSK1 has been phosphorylated and activated by extracellular signal-regulated kinase (ERK). Thus, inactive RSK1 binds the RI subunit of PKA to sensitize it to activation, whereas activated RSK1 binds the C subunit to desensitize PKA to cAMP activation. Cross-talk between the key cAMP and ERK signaling pathways provides a mechanism that, along with distinct mechanisms of both positive and negative attenuation provided by Raf and PDE4 isoforms, can be tailored on a cell type-specific basis.