Molecular cloning, cDNA structure, and regulation of the regulatory subunit of type II cAMP-dependent protein kinase from rat ovarian granulosa cells

Protein Kinase A in neurological disorders

Cyclic adenosine 3', 5' monophosphate (cAMP)-dependent Protein Kinase A (PKA) is a multi-functional serine/threonine kinase that regulates a wide variety of physiological processes including gene transcription, metabolism, and synaptic plasticity. Genomic sequencing studies have identified both germline and somatic variants of the catalytic and regulatory subunits of PKA in patients with metabolic and neurodevelopmental disorders. In this review we discuss the classical cAMP/PKA signaling pathway and the disease phenotypes that result from PKA variants. This review highlights distinct isoform-specific cognitive deficits that occur in both PKA catalytic and regulatory subunits, and how tissue-specific distribution of these isoforms may contribute to neurodevelopmental disorders in comparison to more generalized endocrine dysfunction.

A most versatile kinase: The catalytic subunit of PKA in T-cell biology

The cAMP-dependent protein kinase, more commonly referred to as protein kinase A (PKA), is one of the most-studied enzymes in biology. PKA is ubiquitously expressed in mammalian cells, can be activated in response to a plethora of biological stimuli, and phosphorylates more than 250 known substrates. Indeed, PKA is of central importance to a wide range of organismal processes, including energy homeostasis, memory formation and immunity. It serves as the primary effector of the second-messenger molecule 3',5'-cyclic adenosine monophosphate (cAMP), which is believed to have mostly inhibitory effects on the adaptive immune response. In particular, elevated levels of intracellular cAMP inhibit the activation of conventional T cells by limiting signal transduction through the T-cell receptor and altering gene expression, primarily in a PKA-dependent manner. Regulatory T cells have been shown to increase the cAMP levels in adjacent T cells by direct and indirect means, but the role of cAMP within regulatory T cells themselves remains incompletely understood. Paradoxically, cAMP has been implicated in promoting T-cell activation as well, adding another functional dimension beyond its established immunosuppressive effects. Furthermore, PKA can phosphorylate the NF-κB subunit p65, a transcription factor that is essential for T-cell activation, independently of cAMP. This phosphorylation of p65 drastically enhances NF-κB-dependent transcription and thus is likely to facilitate immune activation. How these immunosuppressive and immune-activating properties of PKA balance in vivo remains to be elucidated. This review provides a brief overview of PKA regulation, its ability to affect NF-κB activation, and its diverse functions in T-cell biology.

PRKAR2A deficiency protects mice from experimental colitis by increasing IFN-stimulated gene expression and modulating the intestinal microbiota

Protein kinase A (PKA) plays an important role in regulating inflammation via its catalytic subunits. Recently, PKA regulatory subunits have been reported to directly modulate some signaling pathways and alleviate inflammation. However, the role of PKA regulatory subunits in colonic inflammation remains unclear. Therefore, we conducted this study to investigate the role of the PKA regulatory subunit PRKAR2A in colitis. We observed that PRKAR2A deficiency protected mice from dextran sulfate sodium (DSS)-induced experimental colitis. Our experiments revealed that the intestinal epithelial cell-specific deletion of Prkar2a contributed to this protection. Mechanistically, the loss of PRKAR2A in Prkar2a^-/- mice resulted in an increased IFN-stimulated gene (ISG) expression and altered gut microbiota. Inhibition of ISGs partially reversed the protective effects against DSS-induced colitis in Prkar2a^-/- mice. Antibiotic treatment and cross-fostering experiments demonstrated that the protection against DSS-induced colitis in Prkar2a^-/- mice was largely dependent on the gut microflora. Altogether, our work demonstrates a previously unidentified function of PRKAR2A in promoting DSS-induced colitis.

Mitochondrial cAMP-PKA signaling: What do we really know?

Mitochondria are key organelles for cellular homeostasis. They generate the most part of ATP that is used by cells through oxidative phosphorylation. They also produce reactive oxygen species, neurotransmitters and other signaling molecules. They are important for calcium homeostasis and apoptosis. Considering the role of this organelle, it is not surprising that most mitochondrial dysfunctions are linked to the development of pathologies. Various mechanisms adjust mitochondrial activity according to physiological needs. The cAMP-PKA signaling emerged in recent years as a direct and powerful mean to regulate mitochondrial functions. Multiple evidence demonstrates that such pathway can be triggered from cytosol or directly within mitochondria. Notably, specific anchor proteins target PKA to mitochondria whereas enzymes necessary for generation and degradation of cAMP are found directly in these organelles. Mitochondrial PKA targets proteins localized in different compartments of mitochondria, and related to various functions. Alterations of mitochondrial cAMP-PKA signaling affect the development of several physiopathological conditions, including neurodegenerative diseases. It is however difficult to discriminate between the effects of cAMP-PKA signaling triggered from cytosol or directly in mitochondria. The specific roles of PKA localized in different mitochondrial compartments are also not completely understood. The aim of this work is to review the role of cAMP-PKA signaling in mitochondrial (patho)physiology.

Phosphorylation of protein kinase A (PKA) regulatory subunit RIα by protein kinase G (PKG) primes PKA for catalytic activity in cells

cAMP-dependent protein kinase (PKAc) is a pivotal signaling protein in eukaryotic cells. PKAc has two well-characterized regulatory subunit proteins, RI and RII (each having α and β isoforms), which keep the PKAc catalytic subunit in a catalytically inactive state until activation by cAMP. Previous reports showed that the RIα regulatory subunit is phosphorylated by cGMP-dependent protein kinase (PKG) in vitro, whereupon phosphorylated RIα no longer inhibits PKAc at normal (1:1) stoichiometric ratios. However, the significance of this phosphorylation as a mechanism for activating type I PKA holoenzymes has not been fully explored, especially in cellular systems. In this study, we further examined the potential of RIα phosphorylation to regulate physiologically relevant "desensitization" of PKAc activity. First, the serine 101 site of RIα was validated as a target of PKGIα phosphorylation both in vitro and in cells. Analysis of a phosphomimetic substitution in RIα (S101E) showed that modification of this site increases PKAc activity in vitro and in cells, even without cAMP stimulation. Numerous techniques were used to show that although Ser101 variants of RIα can bind PKAc, the modified linker region of the S101E mutant has a significantly reduced affinity for the PKAc active site. These findings suggest that RIα phosphorylation may be a novel mechanism to circumvent the requirement of cAMP stimulus to activate type I PKA in cells. We have thus proposed a model to explain how PKG phosphorylation of RIα creates a "sensitized intermediate" state that is in effect primed to trigger PKAc activity.

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.

cAMP-Dependent Protein Kinase and cGMP-Dependent Protein Kinase as Cyclic Nucleotide Effectors

The cAMP-dependent protein kinase (PKA) and the cGMP-dependent protein kinase (PKG) are homologous enzymes with different binding and activation specificities for cyclic nucleotides. Both enzymes harbor conserved cyclic nucleotide-binding (CNB) domains. Differences in amino acid composition of these CNB domains mediate cyclic nucleotide selectivity in PKA and PKG, respectively. Recently, the presence of the noncanonical cyclic nucleotides cCMP and cUMP in eukaryotic cells has been proven, while the existence of cellular cIMP and cXMP remains unclear. It was shown that the main effectors of cyclic nucleotide signaling, PKA and PKG, can be activated by each of these noncanonical cyclic nucleotides. With unique effector proteins still missing, such cross-activation effects might have physiological relevance. Therefore, we approach PKA and PKG as cyclic nucleotide effectors in this chapter. The focus of this chapter is the general cyclic nucleotide-binding properties of both kinases as well as the selectivity for cAMP or cGMP, respectively. Furthermore, we discuss the binding affinities and activation potencies of noncanonical cyclic nucleotides.

Protein kinase A catalytic subunit isoform PRKACA; History, function and physiology

Our appreciation of the scope and influence of second messenger signaling has its origins in pioneering work on the cAMP-dependent protein kinase. Also called protein kinase A (PKA), this holoenzyme exists as a tetramer comprised of a regulatory (R) subunit dimer and two catalytic (C) subunits. Upon binding of two molecules of the second messenger cAMP to each R subunit, a conformational change in the PKA holoenzyme occurs to release the C subunits. These active kinases phosphorylate downstream targets to propagate cAMP responsive cell signaling events. This article focuses on the discovery, structure, cellular location and physiological effects of the catalytic subunit alpha of protein kinase A (encoded by the gene PRKACA). We also explore the potential role of this essential gene as a molecular mediator of certain disease states.

An entirely specific type I A-kinase anchoring protein that can sequester two molecules of protein kinase A at mitochondria

A-kinase anchoring proteins (AKAPs) tether the cAMP-dependent protein kinase (PKA) to intracellular sites where they preferentially phosphorylate target substrates. Most AKAPs exhibit nanomolar affinity for the regulatory (RII) subunit of the type II PKA holoenzyme, whereas dual-specificity anchoring proteins also bind the type I (RI) regulatory subunit of PKA with 10-100-fold lower affinity. A range of cellular, biochemical, biophysical, and genetic approaches comprehensively establish that sphingosine kinase interacting protein (SKIP) is a truly type I-specific AKAP. Mapping studies located anchoring sites between residues 925-949 and 1,140-1,175 of SKIP that bind RI with dissociation constants of 73 and 774 nM, respectively. Molecular modeling and site-directed mutagenesis approaches identify Phe 929 and Tyr 1,151 as RI-selective binding determinants in each anchoring site. SKIP complexes exist in different states of RI-occupancy as single-molecule pull-down photobleaching experiments show that 41 ± 10% of SKIP sequesters two YFP-RI dimers, whereas 59 ± 10% of the anchoring protein binds a single YFP-RI dimer. Imaging, proteomic analysis, and subcellular fractionation experiments reveal that SKIP is enriched at the inner mitochondrial membrane where it associates with a prominent PKA substrate, the coiled-coil helix protein ChChd3.

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.

Overexpression of follicle-stimulating hormone receptor facilitates the development of ovarian epithelial cancer

We previously showed that the expressing level of FSH receptor (FSHR) increased from ovarian epithelial inclusions (OEIs) to benign ovarian epithelial tumors (OETs) and to borderline OETs, whereas FSHR levels decreased with an increase in carcinoma grade. The aim of this study was to investigate the role of FSHR in OET development. MCV152 cells with FSHR overexpression showed an increased cellular proliferation and invasive capacity, which was associated with reduced levels of prohibitin and RII-beta expression and increased levels of HER-2/neu, c-Myc, and EGFR expression. Overexpression of FSHR may be associated with an elevated level of OET cell proliferation via an enhanced activity of potential oncogenic pathways. Therefore, the findings in this study suggest that overexpression of FSHR may play a role in OET development.

Regulation of transforming growth factor beta-induced responses by protein kinase A in pancreatic acinar cells

TGF-beta is an important regulator of growth and differentiation in the pancreas and has been implicated in pancreatic tumorigenesis. We have recently demonstrated that TGF-beta can activate protein kinase A (PKA) in mink lung epithelial cells (Zhang L, Duan C, Binkley C, Li G, Uhler M, Logsdon C, Simeone D. Mol Cell Biol 24: 2169-2180, 2004). In this study, we sought to determine whether TGF-beta activates PKA in pancreatic acinar cells, the mechanism by which PKA is activated, and PKA's role in TGF-beta-mediated growth regulatory responses. TGF-beta rapidly activated PKA in pancreatic acini while having no effect on intracellular cAMP levels. Coimmunoprecipitation experiments demonstrated a physical interaction between a Smad3/Smad4 complex and the regulatory subunits of PKA. TGF-beta also induced activation of the PKA-dependent transcription factor CREB. Both the specific PKA inhibitor H89 and PKI peptide significantly blocked TGF-beta's ability to activate PKA and CREB. TGF-beta-mediated growth inhibition and TGF-beta-induced p21 and SnoN expression in pancreatic acinar cells were blocked by H89 and PKI peptide. This study demonstrates that this novel cross talk between TGF-beta and PKA signaling pathways may play an important role in regulating TGF-beta signaling in the pancreas.

Epac and PKA: a tale of two intracellular cAMP receptors

cAMP-mediated signaling pathways regulate a multitude of important biological processes under both physiological and pathological conditions, including diabetes, heart failure and cancer. In eukaryotic cells, the effects of cAMP are mediated by two ubiquitously expressed intracellular cAMP receptors, the classic protein kinase A (PKA)/cAMP-dependent protein kinase and the recently discovered exchange protein directly activated by camp (Epac)/cAMP-regulated guanine nucleotide exchange factors. Like PKA, Epac contains an evolutionally conserved cAMP binding domain that acts as a molecular switch for sensing intracellular second messenger cAMP levels to control diverse biological functions. The existence of two families of cAMP effectors provides a mechanism for a more precise and integrated control of the cAMP signaling pathways in a spatial and temporal manner. Depending upon the specific cellular environments as well as their relative abundance, distribution and localization, Epac and PKA may act independently, converge synergistically or oppose each other in regulating a specific cellular function.

Systematic interpretation of cyclic nucleotide binding studies using KinetXBase

Functional proteomics aims to describe cellular protein networks in depth based on the quantification of molecular interactions. In order to study the interaction of adenosine-3',5'-cyclic monophosphate (cAMP), a general second messenger involved in several intracellular signalling networks, with one of its respective target proteins, the regulatory (R) subunit of cAMP dependent protein kinase (PKA), a number of different methods was employed. These include fluorescence polarisation (FP), isothermal titration calorimetry (ITC), surface plasmon resonance (SPR), amplified luminescence proximity homogeneous assay (ALPHA-screen), radioligand binding or activity-based assays. Kinetic, thermodynamic and equilibrium binding data of a variety of cAMP derivatives to several cAMP binding domains were integrated in a single database system, we called KinetXBase, allowing for very distinct data formats. KinetXBase is a practical data handling system for molecular interaction data of any kind, providing a synopsis of data derived from different technologies. This supports ongoing efforts in the bioinformatics community to devise formal concepts for a unified representation of interaction data, in order to enable their exchange and easy comparison. KinetXBase was applied here to analyse complex cAMP binding data and highly site-specific cAMP analogues could be identified. The software package is free for download by academic users.

Protein kinase A (PKA) isoform RIIbeta mediates the synergistic killing effect of cAMP and glucocorticoid in acute lymphoblastic leukemia cells

Protein kinase A (PKA) or cAMP-dependent protein kinase (cAPK) mediates the synergistic effects of cAMP- and glucocorticoid (GC)-induced apoptosis in lymphoid cells. Using two human acute lymphoblastic leukemia cell (CEM) clones with respective GC-sensitive and GC-resistant phenotypes, we discovered that the PKA regulatory subunit isoform RII(beta) is preferentially expressed in the GC-sensitive clone C7-14 cells, whereas other intracellular cAMP receptors, including the exchange proteins directly activated by cAMP (Epac), are expressed at similar levels in both GC-sensitive and GC-resistant clones. High RII(beta) expression level in C7-14 cells is associated with elevated total PKA cellular activity and cAMP sensitivity, which consequently lead to an increased basal PKA activity. cAMP analogs that selectively activate type II PKA recapitulate the effects of forskolin of promoting apoptosis and antagonizing AKT/PKB activity in both GC-sensitive and GC-resistant clones, whereas type I PKA-selective agonists do not. Furthermore, down-regulation of RII(beta) leads to increased AKT/PKB activation and enhanced GC resistance in C7-14 cells. These results demonstrate that PKA RII(beta) is responsible for increased GC sensitivity, critical for cAMP-mediated synergistic cell killing in CEM cells, and may represent a novel therapeutic target for GC-resistant lymphoid malignancy.

Distribution of insoluble cAMP-dependent kinase type RI and RII in the lizard and turtle central nervous system

cAMP is a universal second messenger. In eucaryotes it acts mainly via protein kinases composed of regulatory (R) and catalytic subunits; their subcellular distribution may differ according to the cell type. In rodent brain, peculiar detergent-insoluble RIalpha aggregates were previously described in neurons of areas related to the limbic system, while RIIbeta is more evenly distributed also in non-nervous cells. It is unclear whether the regional distribution of regulatory subunits is typical of mammalian brain. Western blots and immunohistochemistry showed that in lizard brains a large fraction of the cAMP-dependent protein kinase regulatory isoforms is insoluble, as in mammals. Insoluble RIalpha and RII regulatory isoforms were not evenly distributed but organized in clearly separated aggregates. Numerous RII aggregates were present in almost all brain regions and were found also in non-nervous cells. As shown by immunohistochemistry and equilibrium binding of fluorescently tagged cAMP, RIalpha aggregates were restricted to neurons of some brain regions: telencephalon, particularly medial cortical areas, dorsal ventricular ridge, olfactory pathways, medial hypothalamus and cerebellar granular layer were intensely labelled. A very weak RIalpha labelling was detected in the brainstem reticular formation, in the periaqueductal gray and in the spinal cord dorsal horn. A similar distribution of RIalpha aggregates was also found in turtle brains. Their distribution is reminiscent of that observed in mammals, although with some differences in relative intensity and persistence. The supramolecular organization of the RIalpha isoform may help in establishing homologies and differences between brain areas involved in visceroemotional control.

PKA activation in concert with ARIS and asterosap induces the acrosome reaction in starfish

The acrosome reaction (AR) is a fundamental event for fertilization, which is induced in concert with acrosome reaction-inducing substance (ARIS) and asterosap, both of which are components of starfish egg jelly (EJ). During the AR, a spermatozoon undergoes a series of physiological changes, such as in intracellular cGMP concentration ([cGMP]i), pHi and intracellular Ca2+ concentration ([Ca2+]i). Affinity purification of cGMP-binding protein resulted in the isolation of a regulatory subunit of the cAMP-dependent protein kinase A (PKA), suggesting the involvement of a cAMP-dependent pathway in the AR. By using a cAMP enzyme immunoassay, [cAMP]i was found to increase in starfish spermatozoa when stimulated with ARIS and asterosap. ARIS could also increase the [cAMP]i in the presence of high pH seawater. Pretreatment of spermatozoa with two specific and cell-permeable PKA inhibitors, H89 and KT5720, prevented the induction of the AR in a concentration-dependent manner. These results suggest that PKA activity participates in the induction of the AR with ARIS and asterosap. To investigate this, we have cloned a gene that encodes a regulatory subunit of PKA that had been identified in starfish spermatozoa.

Analysis of posttranslational modifications exemplified using protein kinase A

With the completion of the major genome projects, one focus in biomedical research has shifted from the analysis of the rather static genome to the highly dynamic proteome. The sequencing of whole genomes did not lead to much anticipated insights into disease mechanisms; however, it paved the way for proteomics by providing the databases for protein identification by peptide mass fingerprints. The relative protein distribution within a cell or tissue is subject to change upon external and internal stimuli. Signal transduction events extend beyond a simple change in protein levels; rather they are governed by posttranslational modifications (PTMs), which provide a quick and efficient way to modulate cellular signals. Because most PTMs change the mass of a protein, they are amenable to analysis by mass spectrometry. Their investigation adds a level of functionality to proteomics, which can be expected to greatly aid in the understanding of the complex cellular machinery involved in signal transduction, metabolism, differentiation or in disease. This review provides an overview on posttranslational modifications exemplified on the model system cAMP-dependent protein kinase. Strategies for detection of selected PTMs are described and discussed in the context of protein kinase function.

A transforming growth factor beta-induced Smad3/Smad4 complex directly activates protein kinase A

Transforming growth factor beta (TGFbeta) interacts with cell surface receptors to initiate a signaling cascade critical in regulating growth, differentiation, and development of many cell types. TGFbeta signaling involves activation of Smad proteins which directly regulate target gene expression. Here we show that Smad proteins also regulate gene expression by using a previously unrecognized pathway involving direct interaction with protein kinase A (PKA). PKA has numerous effects on growth, differentiation, and apoptosis, and activation of PKA is generally initiated by increased cellular cyclic AMP (cAMP). However, we found that TGFbeta activates PKA independent of increased cAMP, and our observations support the conclusion that there is formation of a complex between Smad proteins and the regulatory subunit of PKA, with release of the catalytic subunit from the PKA holoenzyme. We also found that the activation of PKA was required for TGFbeta activation of CREB, induction of p21(Cip1), and inhibition of cell growth. Taken together, these data indicate an important and previously unrecognized interaction between the TGFbeta and PKA signaling pathways.

Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Protein kinases critically regulate synaptic plasticity in the mammalian hippocampus. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of long-term potentiation (LTP), long-term depression (LTD), and hippocampal long-term memory. We review the roles of PKA in activity-dependent forms of hippocampal synaptic plasticity by highlighting particular themes that have emerged in ongoing research. These include the participation of distinct isoforms of PKA in specific types of synaptic plasticity, modification of the PKA-dependence of LTP by multiple factors such as distinct patterns of imposed activity, environmental enrichment, and genetic manipulation of signalling molecules, and presynaptic versus postsynaptic mechanisms for PKA-dependent LTP. We also discuss many of the substrates that have been implicated as targets for PKA's actions in hippocampal synaptic plasticity, including CREB, protein phosphatases, and glutamatergic receptors. Future prospects for shedding light on the roles of PKA are also described from the perspective of specific aspects of synaptic physiology and brain function that are ripe for investigation using incisive genetic, cell biological, and electrophysiological approaches.

Activation of the cAMP-dependent protein kinase signaling pathway by luteinizing hormone in trout theca layers

In the fish ovary, LH is the main factor regulating the production of steroids during the periovulatory period and its effects are believed to be mediated, at least partially, through the cAMP-dependent protein kinase (PKA) signaling pathway. However, there is no direct evidence for the presence of PKA in the fish ovary nor on the regulation of its activity by fish LH. Here, we show the identification of regulatory (R) and catalytic (C) subunits of PKA in trout theca cells by immunoblotting. DEAE-cellulose chromatography of theca cell extracts indicated the presence of PKA type I and II and showed that trout theca cells display PKA-specific phosphotransferase and cAMP-binding activities. Salmon LH (sLH) stimulated PKA activity and increased the levels of immunoreactive RIIalpha, RIIbeta and C subunits in trout theca layers. These observations, coupled with the sLH-dependent decrease in the half-life of the C subunit, as shown by pulse-chase experiments, strongly suggest that sLH activates PKA in trout theca cells. Furthermore, our results suggest that ovarian PKA activity and its regulation by LH has been well conserved from fish to humans.

Related protein-protein interaction modules present drastically different surface topographies despite a conserved helical platform

The subcellular localization of cAMP-dependent protein kinase (PKA) occurs through interaction with A-Kinase Anchoring Proteins (AKAPs). AKAPs bind to the PKA regulatory subunit dimer of both type Ialpha and type IIalpha (RIalpha and RIIalpha). RIalpha and RIIalpha display characteristic localization within different cell types, which is maintained by interaction of AKAPs with the N-terminal dimerization and docking domain (D/D) of the respective regulatory subunit. Previously, we reported the solution structure of RIIa D/D module, both free and bound to AKAPs. We have now solved the solution structure of the dimerization and docking domain of the type Ialpha regulatory dimer subunit (RIalpha D/D). RIalpha D/D is a compact docking module, with unusual interchain disulfide bonds that help maintain the AKAP interaction surface. In contrast to the shallow hydrophobic groove for AKAP binding across the surface of the RIIalpha D/D dimeric interface, the RIalpha D/D module presents a deep cleft for proposed AKAP binding. RIalpha and RIIalpha D/D interaction modules present drastically differing dimeric topographies, despite a conserved X-type four-helix bundle structure.

Intracellular targeting of protein kinases and phosphatases

Compartmentalization of kinases and phosphatases is a key determinant in the specificity of second messenger-mediated signaling events. Localization of the cAMP-dependent protein kinase (PKA) and other signaling enzymes is mediated by interaction with A-kinase anchoring proteins (AKAPs). This study focused on recent advances that further our understanding of AKAPs, with particular emphasis on the bidirectional regulation of signaling events by AKAP signaling complexes and their contribution to the control of actin reorganization events.

Coupling cAMP signaling to transcription in the liver: pivotal role of CREB and CREM

Transcriptional factors binding to cAMP-responsive elements (CREs) in the promoters of various genes belong to the basic domain-leucine zipper superfamily and are composed of three genes in mammals, CREB, CREM, and ATF-1. A large number of CREB, CREM, and ATF-1 proteins are generated by posttranscriptional events, mostly alternative splicing, and regulate gene expression by acting as activators or repressors. Activation is classically brought about by signaling-dependent phosphorylation of a key acceptor site (Ser133 in CREB) by a number of possible kinases, including PKA, CamKIV, and Rsk-2. Phosphorylation is the prerequisite for the interaction of CBP (CREB-binding protein), a co-activator that has also histone acetyltransferase activity. Repression may involve dynamic dephosphorylation of the activators and thus decreased association with CBP. Another pathway of transcriptional repression on CRE sites implicates the inducible repressor ICER (inducible cAMP early repressor), a product of the CREM gene. Being an inducible repressor, ICER is involved in autoregulatory feedback loops of transcription that govern the down-regulation of early response genes, such as the proto-oncogene c-fos. The liver represents a remarkable physiological setting where cAMP-responsive signaling plays a major role. Indeed, a finely tuned program of gene expression is triggered by partial hepatectomy, so that through specific checkpoints a coordinated regeneration of the tissue is obtained. Temporal kinetics of transcriptional activation after hepatectomy reveals a pattern of early induction for several genes, some of them controlled by the CREB/CREM transcription factors. An important role of CREM in liver physiology was suggested by the robust induction of ICER after partial hepatectomy. The delay in tissue regeneration in CREM-deficient mice confirmed the important function of this factor in regulating hepatocyte proliferation. As gene induction is accompanied by critical changes in chromatin organization, the deciphering of the specific modification codes that histones display during liver regeneration and physiology will provide exciting new insights into the dynamics of chromatin architecture.

AKAP mediated signal transduction

Compartmentalization of cyclic AMP-dependent protein kinase (PKA) is achieved through association with A-kinase anchoring proteins (AKAPs). AKAPs are a group of structurally diverse proteins with the common function of binding to the regulatory subunit of PKA and confining the holoenzyme to discrete locations within the cell. This mode of regulation ensures that PKA is exposed to isolated cAMP gradients, which allows for efficient catalytic activation and accurate substrate selection. Several AKAPs coordinate multiple members of signaling cascades, effectively assembling upstream activators and downstream effectors within the same macromolecular complex. Consequently, AKAPs may serve as points of integration for numerous signaling pathways. This review details the most recent advances in our understanding of the various biological functions dependent upon AKAP-anchored signaling complexes.

Differential effects of haloperidol and clozapine on [(3)H]cAMP binding, protein kinase A (PKA) activity, and mRNA and protein expression of selective regulatory and catalytic subunit isoforms of PKA in rat brain

The present study was undertaken to examine whether the mechanism of action of typical and atypical antipsychotics is related in their ability to regulate key phosphorylating enzyme of adenylyl cyclase-cAMP pathway, i.e., protein kinase A (PKA). For this purpose, regulatory (R) and catalytic (Cat) activities of PKA and expression of various isoforms of regulatory and catalytic subunits were examined in rat brain after single or chronic (21-day) treatment with haloperidol (HAL, 1 mg/kg) or clozapine (CLOZ, 20 mg/kg). It was observed that chronic but not acute treatment of CLOZ significantly decreased [(3)H]cAMP binding to the regulatory subunit of PKA as well as catalytic activity of PKA in particulate and cytosol fractions of the rat cortex, hippocampus, and striatum. In these fractions, CLOZ significantly decreased protein levels of selective RII alpha-, RII beta-, and Cat beta-subunit isoforms of PKA. These decreases were accompanied by decreases in their respective mRNA expression. In contrast, chronic but not acute treatment of HAL significantly increased [(3)H]cAMP binding and the catalytic activity of PKA in particulate and cytosol fractions of only the striatum brain area. In addition, chronic treatment of HAL significantly increased mRNA and protein levels of RII alpha- and RII beta-subunit isoforms in the striatum. None of the antipsychotics caused any change in the expression of the Cat alpha-, RI alpha-, or RI beta-subunit isoform. These results, thus, suggest that HAL and CLOZ differentially regulate PKA catalytic and regulatory activities and the expression of selective catalytic and regulatory subunit isoforms of PKA, which may be associated with their mechanisms of action.

7-Benzylamino-6-chloro-2-piperazino-4-pyrrolidino-pteridine, a potent inhibitor of cAMP-specific phosphodiesterase, enhancing nuclear protein binding to the CRE consensus sequence in human tumour cells

The cAMP-specific phosphodiesterase isoenzyme family PDE4 represents the highest cAMP-hydrolysing activity in many human cancer cell lines including the human large cell lung carcinoma cell line LXFL529L. Treatment of LXFL529L cells with the potent PDE4 inhibitor 7-benzylamino-6-chloro-2-piperazino-4-pyrrolidino-pteridine (DC-TA-46) induces dose-dependent growth inhibition. Cells are arrested in the G(1)-phase of the cell cycle and the induction of apoptosis is observed. In this study, we investigated the effect of DC-TA-46 on downstream elements of the cAMP-pathway. DC-TA-46 mediated inhibition of PDE4 activity in LXFL529L cells resulted in an increase of the intracellular cAMP level and significant induction of the activity of protein kinase A (PKA). The regulatory PKA subunit RIalpha was predominantly expressed in LXFL529L cells. In contrast to effects induced by cAMP analogues like 8-Cl-cAMP, the expression of the regulatory subunits of PKA remained unaffected by DC-TA-46. Treatment of LXFL529L cells with DC-TA-46 enhanced the binding of nuclear proteins to the cAMP-responsive element (CRE) consensus sequence TGACGTCA in a time- and dose-dependent manner, indicating the activation of transcription factors by PKA phosphorylation.

Differential binding of cAMP-dependent protein kinase regulatory subunit isoforms Ialpha and IIbeta to the catalytic subunit

Limited trypsin digestion of type I cAMP-dependent protein kinase holoenzyme results in a proteolytic-resistant Delta(1-72) regulatory subunit core, indicating that interaction between the regulatory and catalytic subunits extends beyond the autoinhibitory site in the R subunit at the NH(2) terminus. Sequence alignment of the two R subunit isoforms, RI and RII, reveals a significantly sequence diversity at this specific region. To determine whether this sequence diversity is functionally important for interaction with the catalytic subunit, specific mutations, R133A and D328A, are introduced into sites adjacent to the active site cleft in the catalytic subunit. While replacing Arg(133) with Ala decreases binding affinity for RII, interaction between the catalytic subunit and RI is not affected. In contrast, mutant C(D328A) showed a decrease in affinity for binding RI while maintaining similar affinities for RII as compared with the wild-type catalytic subunit. These results suggest that sequence immediately NH(2)-terminal to the consensus inhibition site in RI and RII interacts with different sites at the proximal region of the active site cleft in the catalytic subunit. These isoform-specific differences would dictate a significantly different domain organization in the type I and type II holoenzymes.

Visualization of detergent insoluble cyclic AMP-dependent protein kinase RIalpha aggregates in the rat brain

Regulatory subunits of the cAMP dependent protein kinases are the most abundant receptor for cAMP in eukaryotic cells. Four isoforms of regulatory subunits (RIalpha and -beta, RIIalpha and -beta) have been distinguished. Distribution of the most abundant RII isoforms has been extensively studied in the brain, by immunohistochemistry and biochemical fractionation, while the least abundant RI isoforms have been neglected. In neurons most regulatory subunits are bound to the cytoskeleton. A protocol is presented that allows immunohistochemical and biochemical characterization of detergent-insoluble RI isoforms in the brain.

New signaling pathways for hormones and cyclic adenosine 3',5'-monophosphate action in endocrine cells

The glycoprotein hormones, ACTH, TSH, FSH, and LH regulate diverse functions in endocrine cells. Although cAMP and PKA have long been shown to mediate specific intracellular signaling events including the transcription of specific genes via the CREB-CBP complex, recent observations have indicated that PKA does not account for all of the intracellular targets of cAMP. For example, TSH stimulation of thyroid cell proliferation is not completely blocked by PKA inhibitors. TSH and FSH can stimulate PKB phosphorylation by a PKAindependent but PI3-K/PDK1-dependent pathway. An FSH inducible kinase, Sgk, has recently been shown to be a close relative of PKB. Sgk is also a target of PI3-K-PDK1 pathway, indicating that some effects previously ascribed to PKB may be mediated by this inducible kinase. The identification of novel cAMP-binding proteins that exhibit guanine nucleotide exchange (GEF) activity (cAMP-GEFS; Epacs) has open new doors for cAMP action that include activation of small GTPases such as Rap1a, Rap2, and possibly Ras. These GTPases are known activators of downstream kinase cascades, including p38MAPK and Erk1/2 as well as PI3-K. Thus, FSH and TSH activation of PKB and Sgk may occur via this alternative cAMP pathway that involves cAMP-GEFs and the activation of the PI3-K/PDK1 pathway.

Type II beta regulatory subunit of cAMP-dependent protein kinase: purification strategies to optimize crystallization

To elucidate the structural basis for important differences between types I and II regulatory subunit isoforms (RI and RII) of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase, the full-length RII beta isoform and five RII beta deletion mutants were constructed, expressed, purified, and screened for crystallization. Only one of these six proteins yielded diffraction quality crystals. Crystals were grown of the RII beta deletion mutant (delta 1-111) monomer potentially in complex with two cAMP molecules. X-ray diffraction quality data were obtained only after significant modification to existing purification procedures. Modifications required a Sepharose, not agarose, support for cAMP affinity chromatography followed by rapid, quantitative removal of free cAMP by size-exclusion chromatography under reducing conditions. Data to 2.4 A resolution were collected at 29 degrees C using synchrotron radiation on a single crystal measuring 0.2 x 0.3 x 1.2 mm(3). Data were 99% complete. The hexagonal crystal belonged to space group P6((1)) or P6((5)) with unit cell dimensions a = b = 161.62 A and c = 39.66 A.

The localization and activity of cAMP-dependent protein kinase affect cell cycle progression in thyroid cells

cAMP signals are received and transmitted by multiple isoforms of cAMP-dependent protein kinases (PKAs), typically determined by their specific regulatory subunits. We describe changes in the cAMP signal transduction pathway during cell cycle progression in synchronized rat thyroid cells. Both PKA type II (PKAII) localization and nuclear cAMP signaling are significantly modified during G(0) and G(1)-S transitions. G(1) is characterized by PKA activation and amplified cAMP signal transduction. This is associated with a decrease in the concentration of RI and RII regulatory subunits and enhanced anchoring of PKAII to the Golgi-centrosome region. Just prior to S, the cAMP pathway is depressed. Up-regulation of the pathway by exogenous cAMP in G(1) inhibited the subsequent decay of the Cdk inhibitor p27 and delayed the onset of S phase. Forced translocation of endogenous PKAII to the cytosol down-regulated cAMP signaling, advancing the timing of p27 decay and inducing premature exit from G(1). These data indicate that membrane-bound PKA amplifies the transduction of cAMP signals in G(1) and that the length of G(1) is influenced by cAMP-PKA.

Single amino acids determine specificity of binding of protein kinase A regulatory subunits by protein kinase A anchoring proteins

Cyclic AMP-dependent protein kinase is tethered to protein kinase A anchoring proteins (AKAPs) through regulatory subunits (R) by RIalpha-specific, RIIalpha-specific, or RIalpha/RIIalpha dual-specific binding. Ala- and Val-scanning mutagenesis determined that hydrophobic amino acids at three homologous positions are required for binding of RIalpha to FSC1/AKAP82 domain B and RIIalpha to AKAP Ht31. A mutation at the middle position reversed the binding specificity of both AKAPs, and mutations at this same position of the dual-specific domain A of FSC1/AKAP82 converted it into either an RIalpha or RIIalpha binding domain. This suggests that hydrophobic amino acids at three conserved positions within the primary sequence and an amphipathic helix of AKAPs are required for cyclic AMP-dependent protein kinase binding, with the size of the aliphatic side chain at the middle position determining RIalpha or RIIalpha binding specificity.

Cyclic nucleotide-dependent protein kinases: intracellular receptors for cAMP and cGMP action

Intracellular cAMP and cGMP levels are increased in response to a variety of hormonal and chemical stimuli; these nucleotides play key roles as second messenger signals in modulating myriad physiological processes. The cAMP-dependent protein kinase and cGMP-dependent protein kinase are major intracellular receptors for these nucleotides, and the actions of these enzymes account for much of the cellular responses to increased levels of cAMP or cGMP. This review summarizes many studies that have contributed significantly to an improved understanding of the catalytic, regulatory, and structural properties of these protein kinases. These accumulated findings provide insights into the mechanisms by which these enzymes produce their specific physiological effects and are helpful in considering the actions of other protein kinases as well.

Diminished Levels of Protein Kinase A RIα and RIβ Transcripts and Proteins in Systemic Lupus Erythematosus T Lymphocytes

Deficient type I protein kinase A phosphotransferase activity occurs in the T cells of 80% of subjects with systemic lupus erythematosus (SLE). To investigate the mechanism of this deficient isozyme activity, we hypothesized that reduced amounts of type I regulatory (RI) isoform transcripts, RIα and RIβ, may be associated with a diminution of RIα and/or RIβ protein. Sixteen SLE subjects with a mean (±1 SD) SLE disease activity index of 12.4 ± 7.2 were studied. Controls included 16 normal subjects, six subjects with primary Sjögren’s syndrome (SS), and three subjects with SS/SLE overlap. RT-PCR revealed that normal, SS, SS/SLE, and SLE T cells expressed mRNAs for all seven R and catalytic (C) subunit isoforms. Quantification of mRNAs by competitive PCR revealed that the ratio of RIα mRNA to RIβ mRNA in normal T cells was 3.4:1. In SLE T cells there were 20 and 49% decreases in RIα and RIβ mRNAs (RIβ; p = 0.008), respectively, resulting in an RIα:RIβ mRNA of 5.3:1. SS/SLE T cells showed a 72.5% decrease in RIβ mRNA compared with normal controls (p = 0.01). Immunoblotting of normal T cell RIα and RIβ proteins revealed a ratio of RIα:RIβ of 3.2:1. In SLE T cells, there was a 30% decrease in RIα protein (p = 0.002) and a 65% decrease in RIβ protein (p < 0.001), shifting the ratio of RIα:RIβ protein to 6.5:1. T cells from 25% of SLE subjects lacked any detectable RIβ protein. Analysis of several lupus T cell lines demonstrated a persistent deficiency of both proteins, excluding a potential effect of disease activity. In conclusion, reduced expression of RIα and RIβ transcripts is associated with a decrement in RIα and RIβ proteins and may contribute to deficient type I protein kinase A isozyme activity in SLE T cells.

Cyclic-AMP-dependent protein kinase (PKA) in testicular cells. Cell specific expression, differential regulation and targeting of subunits of PKA

LH and FSH regulate via cyclic adenosine 3'5' cyclic monophosphate (cAMP) and cAMP-dependent protein kinase (PKA), steroid biosynthesis is Leydig and Sertoli cells, respectively. Cyclic AMP also regulates a number of different cellular processes such as cell growth and differentiation, ion channel conductivity, synaptic release of neurotransmitters, and gene transcription. The principle intracellular target for cAMP in mammalian cells is the PKA. The fact that this broad specificity protein kinase mediates a number of discrete physiological responses following cAMP engagement, has raised the question of how specificity is maintained in the cAMP/PKA system. Here we describe features of this signaling pathway that may contribute to explain how differential effects of cAMP may be contributed to features of the PKA signaling pathway.

Molecular cloning of cAMP-dependent protein kinase catalytic subunit isoforms from the lone star tick, Amblyomma americanum (L.)

The salivary glands of ixodid ticks are central to tick feeding and to survival during off-host periods. They produce and secrete a number of molecules critical to maintaining the complex host-vector interface and to maintaining osmotic balance. We have previously shown that a cyclic AMP-dependent protein kinase (cAPK) is involved in the mechanism of salivary gland secretion. We have now cloned cDNAs encoding three isoforms of the catalytic subunit (cAPK-C) of the cAPK from Amblyomma americanum, which are probably produced from alternative RNA processing of a single cAPK-C gene. The cDNAs contain unique N-termini of variable lengths that are linked to a common region containing the alpha A helix, catalytic core, and a C-terminal tail. The common region is highly similar to both insect and vertebrate cAPK-Cs. We have examined mRNA profiles in whole ticks and in isolated salivary glands throughout feeding and find that a single cAPK-C isoform is expressed in the salivary glands of both unfed and feeding females.

Identification of tethering domains for protein kinase A type Ialpha regulatory subunits on sperm fibrous sheath protein FSC1

The fibrous sheath is a unique cytoskeletal structure in the sperm flagellum believed to modulate sperm motility. FSC1 is the major structural protein of the fibrous sheath. The yeast two-hybrid system was used to identify other proteins that contribute to the structure of the fibrous sheath or participate in sperm motility. When FSC1 was used as the bait to screen a mouse testis cDNA library, two clones were isolated encoding the type Ialpha regulatory subunit (RIalpha) of cAMP-dependent protein kinase. Deletion analysis using the yeast two-hybrid system and in vitro binding assays with glutathione S-transferase-FSC1 fusion proteins identified two RIalpha tethering domains on FSC1. A domain located at residues 219-232 (termed domain A) corresponds to the reported tethering domain for a type II regulatory subunit (RII) of cAMP-dependent protein kinase, indicating that this binding domain has dual specificity to RI and RII. Another RIalpha tethering site (termed domain B) at residues 335-344 shows specific binding of RIalpha and had no significant sequence homology with known RII tethering domains. However, helical wheel projection analysis indicates that domain B is likely to form an amphipathic helix, the secondary structure of RII tethering domains of protein kinase A anchoring proteins. This was supported by the finding that site-directed mutagenesis to disrupt the amphipathic helix eliminated RIalpha binding. This is apparently the first report of an RIalpha-specific protein kinase A anchoring protein tethering domain.

Cyclic AMP signaling and gene regulation

Cyclic adenosine monophosphate (cAMP) is a ubiquitous second messenger produced in cells in response to hormones and nutrients. The production of cAMP is dependent upon the actions of many different proteins that affect its synthesis and degradation. An important function of cAMP is to activate the phosphorylating enzyme, protein kinase A. The key roles of cAMP and protein kinase A in the phosphorylation and regulation of enzyme substrates involved in intermediary metabolism are well known. A newly discovered role for protein kinase A is in the phosphorylation and activation of transcription factors that are critical for the control of the transcription of genes in response to elevated levels of cAMP.

Characterization of PKA isoforms and kinase-dependent activation of chloride secretion in T84 cells

Chloride exit across the apical membranes of secretory epithelial cells is acutely regulated by the cAMP-mediated second messenger cascade. To better understand the regulation of transepithelial chloride secretion, we have characterized the complement of cAMP-dependent protein kinase (PKA) isoforms present in the human colonic epithelial cell line T84. Our results show that both type I and type II PKA are present in T84 cells. Immunoprecipitation of 8-azido-[32P]cAMP-labeled cell lysates revealed that the major regulatory subunits of PKA were RIalpha and RIIalpha. In addition, immunogold electron microscopy showed that RIIalpha labeling was found on membranes of the trans Golgi network and on apical plasma membrane. In contrast, RIalpha was randomly distributed throughout the cytoplasm, with no discernible membrane association. Northern blot analysis of T84 RNA revealed that Calpha was the predominantly expressed catalytic subunit. Short-circuit current measurements were performed in the presence of combinations of site-selective cAMP analog pairs to preferentially activate either PKA type I or PKA type II in intact T84 cell monolayers. Maximal levels of chloride secretion (approximately 100 microA/cm2) were observed for both type I and type II PKA-selective analog pairs. Subsequent addition of forskolin was unable to further increase chloride secretion. Thus activation of either type I or type II PKA is able to maximally stimulate chloride secretion in T84 colonic epithelial cells.

Cisplatin resistance in cyclic AMP-dependent protein kinase mutants

The emergence of cisplatin resistance poses a major problem to the successful treatment of a variety of human malignancies. Therefore, understanding the molecular mechanisms that underlie cisplatin resistance could significantly improve the clinical efficacy of this cytotoxic agent. Various studies have described that cellular sensitivity to cisplatin can be influenced by several signal transduction pathways. In this review, we examine the role of the cyclic AMP-dependent protein kinase (PKA) in the modulation of drug resistance in cancer. By a somatic mutant genetic approach, the role of PKA in the development of resistance to chemotherapeutic agents has been investigated. A series of mutants with decreased PKA activity was examined for their sensitivity to cisplatin. PKA mutants with defective regulatory (RIalpha) subunits, but not altered catalytic (C) subunits, exhibit increased resistance to cisplatin, as well as other DNA-damaging agents. Furthermore, since RIalpha subunit mutants show enhanced DNA repair we, therefore, hypothesize that functional inactivation of PKA may result in increased recognition and repair of cisplatin lesions. Alternatively, it seems likely that mutation of the RIalpha subunit may affect cellular sensitivity to various anticancer drugs, suggesting that the RIalpha subunit may have other physiological functions in addition to inhibiting the kinase activity of the C subunit. Therefore, exploitation of cyclic AMP levels or functional alteration of the R subunit may potentiate the cytotoxicity of chemotherapeutic agents and circumvent drug resistance in cancer. More importantly, the altered pattern and mechanism of drug resistance may offer the opportunity to investigate novel regulatory functions of the RIalpha subunit of PKA.

Cyclic AMP-dependent protein kinase (PKA) gene expression is developmentally regulated in fetal lung

We characterized the ontogeny of cAMP-dependent protein kinase (PKA) enzymatic activity and PKA subunit mRNA expression in developing lung. The lungs of fetal Sprague-Dawley rat pups were removed after 16, 18, or 20 days of gestation and at term. PKA activity was greatest in the 18- and 20-day gestation lungs. Tissue cAMP levels were lowest in the 16-day lungs and increased with lung maturity. We were able to detect only low levels of mRNA for the C beta subunit of PKA by northern blot analysis of total lung RNA and we were able to detect mRNA for the RI beta and RII beta subunits only by RT-PCR. Therefore, we limited our analysis of PKA subunit mRNA levels to those for C alpha, RI alpha and RII alpha. The mRNA levels for C alpha, were highest in the 16-day lung, decreased at 18 and 20 days, were lower in the newborn and lowest in the adult lung. RI alpha mRNA levels were also highest at 16 days and lowest in the adult lung. However, RII alpha mRNA levels were similar in the 18-day, 20-day and newborn lungs. Dexamethasone treatment of fetal lung explants resulted in a small decrease in RI alpha mRNA levels but was not associated with a change in PKA activity. We conclude that PKA activity and PKA subunit mRNA expression are developmentally regulated in fetal lung. Such regulation results in optimal PKA activity at the time of type II alveolar cell differentiation, presumably in preparation for air breathing. The absence of an effect of glucocorticoid on PKA activity suggests that glucocorticoids are not responsible for the increase in PKA activity which accompanies this critical time in lung maturation.

Functional interactions, phosphorylation, and levels of 3',5'-cyclic adenosine monophosphate-regulatory element binding protein and steroidogenic factor-1 mediate hormone-regulated and constitutive expression of aromatase in gonadal cells

The proximal promoter of the rat aromatase CYP19 gene contains two functional regions that, by 5'-deletion analyses, have been shown to confer hormone/ cAMP inducibility to chimeric genes in primary cultures of rat granulosa cells and constitutive expression in R2C Leydig cells. Promoter region A binds Steroidogenic Factor-1 (SF-1); region B binds cAMP-regulatory element binding protein (CREB) and two other factors (designated X and Y). Mutations were generated within the context of the intact promoter to selectively eliminate the binding of either SF-1, CREB, CREB plus factors X and Y, or all of the above. When expression vectors that failed to bind either CREB alone or CREB plus factors X and Y were transfected into granulosa cells, cAMP-dependent chloramphenicol acetyltransferase (CAT) activity was reduced 65% indicating that CREB alone, and not factors X and Y, mediates the cAMP response of this cAMP response element-like domain. Similarly, cAMP-dependent CAT activity was reduced 50% in constructs that failed to bind SF-1 and was abolished with vectors that were unable to bind either factor. In R2C Leydig cells, the absence of either CREB or SF-1 binding resulted in an almost complete loss in CAT activity. Both immunoreactive CREB and phosphorylated CREB (phospho-CREB) were present in extracts and nuclei of R2C cells. Immunoreactive phosphoCREB was low in granulosa cell extracts and nuclei but increased rapidly (90 min) in response to FSH/cAMP and was highest at 48 h, at a time when SF-1 was also phosphorylated and expression of the endogenous gene was elevated. Although the amount of CREB and SF-1 remained unchanged in response to FSH, LH mediated a rapid decrease in the amount of SF-1 (but not CREB) that is coincident with decreased aromatase mRNA in luteinizing granulosa cells. Taken together, the data indicate that expression of the aromatase gene is dependent on the additive interactions of regions A and B of the aromatase promoter in granulosa cells and the synergistic interactions of these same regions in R2C cells and that these interactions are dependent, in turn, on the phosphorylation of CREB and SF-1 and the content of these factors, as well as the presence of putative coregulatory molecules.

Prostaglandin synthesis is suppressed by progesterone in rat preovulatory follicles in vitro

The inducible form of prostaglandin endoperoxide-2 (PGS-2) is transiently induced by activators of the protein kinase A and protein kinase C systems in rat preovulatory (PO) granulosa cells. This induction is suggested to play an important role in the ovulatory process, which shares many of the characteristics of an inflammatory reaction. The purpose of the present study was to explore the role of progesterone (P4) as an "anti-inflammatory" steroid for the regulation of PGS-2 and the synthesis of prostaglandins in the PO follicle. Isolated rat PO follicles were preincubated with different amounts of exogenous P4 before the addition of luteinizing hormone (LH) and 3-isobutyl-1-methylxanthine (IBMX) (LH + 1). Medium levels of prostaglandin E2 (PGE2) were measured by RIA and the protein contents of PGS-1 and PGS-2 were determined by immunoblotting. LH + I. Both the content of PGS-2 and the synthesis of PGE2 were decreased. The content of PGS-1 demonstrated only minor changes in response to P4. These results showed a dual regulation of PGS-2 in the rat PO follicle with both stimulatory and inhibitory pathways. One of the "anti-inflammatory" actions exerted by P4 in the present study was to reduce the expression of PGS-2 and the follicular production of prostaglandins. This action might be of importance for restriction and control of the inflammatory response in the ovulatory process in vivo.

Expression of aromatase in the ovary: down-regulation of mRNA by the ovulatory luteinizing hormone surge

Aromatase (CYP19) mRNA is induced by follicle-stimulating hormone (FSH) in granulosa cells of preovulatory follicles and subsequently is rapidly diminished as a consequence of the luteinizing hormone (LH) surge. Primary cultures of rat granulosa cells were used to identify some of the cellular mechanisms by which FSH increases and LH decreases steady-state levels of aromatase mRNA. Induction of aromatase mRNA by FSH was increased by cycloheximide but was blocked by alpha-amanitin and the C-kinase activators gonadotropin-releasing hormone (GnRH) and phorbol 12-myristate 13-acetate (PMA). In contrast, the decrease in steady-state levels of aromatase mRNA by LH was mimicked by A-kinase (forskolin) and C-kinase (PMA or GnRH) activators. The decrease in aromatase mRNA was associated with decreased amounts of mRNA and protein for steroidogenic factor-1 (SF-1), a nuclear orphan receptor that binds and trans-activates the aromatase promoter, and with the A-kinase subunit type II (RII beta), which is required for mediating cAMP action in these cells. The down-regulation of aromatase, SF-1, and RII beta by each kinase activator and alpha-amanitin was prevented by cycloheximide when the drug was added in combination with the activator. If, however, cycloheximide was added 2 h after PMA (or LH), the drug did not prevent the rapid loss of mRNA. When granulosa cells were transfected with an aromatase CAT transgene, CAT activity was stimulated 10- to 20-fold by FSH and forskolin but not by PMA. Taken together, these results indicate that the A-kinase but not the C-kinase pathway can trans-activate the aromatase gene in immature granulosa cells, whereas the C-kinase, as well as A-kinase pathways, mimic the LH surge to decrease aromatase mRNA in preovulatory cells. By increasing degradation of aromatase mRNA and by inhibiting transcription, the LH surge rapidly terminates the granulosa cell pattern of gene expression while reprogramming the cells to express genes associated with ovulation and luteinization.

The v-Ki-Ras oncogene alters cAMP nuclear signaling by regulating the location and the expression of cAMP-dependent protein kinase IIbeta

The v-Ki-Ras oncoprotein dedifferentiates thyroid cells and inhibits nuclear accumulation of the catalytic subunit of cAMP-dependent protein kinase. After activation of v-Ras or protein kinase C, the regulatory subunit of type II protein kinase A, RIIbeta, translocates from the membranes to the cytosol. RIIbeta mRNA and protein were eventually depleted. These effects were mimicked by expressing AKAP45, a truncated version of the RII anchor protein, AKAP75. Because AKAP45 lacks membrane targeting domains, it induces the translocation of PKAII to the cytoplasm. Expression of AKAP45 markedly decreased thyroglobulin mRNA levels and inhibited accumulation of C-PKA in the nucleus. Our results suggest that: 1) The localization of PKAII influences cAMP signaling to the nucleus; 2) Ras alters the localization and the expression of PKAII; 3) Translocation of PKAII to the cytoplasm reduces nuclear C-PKA accumulation, resulting in decreased expression of cAMP-dependent genes, including RIIbeta, TSH receptor, and thyroglobulin. The loss of RIIbeta permanently down-regulates thyroid-specific gene expression.

An E-box region within the prostaglandin endoperoxide synthase-2 (PGS-2) promoter is required for transcription in rat ovarian granulosa cells

The prostaglandin endoperoxide synthase-2 (PGS-2) gene encodes an isoform of prostaglandin synthase that is transiently induced by protein kinase A (luteinizing hormone/cAMP) and protein kinase C (gonadotropin-releasing hormone) agonists in granulosa cells of ovulating follicles. The promoter of the rat PGS-2 gene contains a CAAT enhancer-binding protein consensus site (CAAT box) which can confer hormone inducibility to a PGS-2.CAT reporter gene, as well as a putative E-box region. To determine if the E-box region was involved in hormone induced trans-activation of the rat PGS-2 gene, constructs with the CAAT box and E-box regions (-192 PGS-2.CAT), only the putative E-box (-110 PGS-2.CAT), or neither region (-52 PGS-2.CAT) were transiently transfected into rat granulosa cell cultures. CAT activity was induced in both the -192 and -110 PGS-2*CAT vectors by luteinizing hormone (10-fold) and gonadotropin-releasing hormone (6-fold), whereas CAT activity of the -52 PGS-2.CAT construct did not differ from the promoterless vector (pCAT-Basic). Deletion of 1 base pair from the E-box within the -110 PGS-2.CAT construct, as well as point mutations within the CAAT box, E-box, or both regions of the -192 PGS-2.CAT construct, demonstrated that the E-box is critical for basal transcription, and that regions, in addition to the CAAT box, are involved in hormone induction of the PGS-2 gene. An oligonucleotide spanning the rat PGS-2 E-box bound two specific protein complexes which were supershifted in the presence of antibody specific for the upstream stimulatory factor. Thus, in rat granulosa cells, the PGS-2 E-box region appears to interact with upstream cis-acting elements other than the CAAT box to confer hormonal regulation of the gene. The E-box region of the rat PGS-2 promoter does not contain ATF/CRE activity found in the human and mouse PGS-2 promoters, but is critical for basal transcription of the PGS-2 gene in rat granulosa cells and binds the upstream stimulatory factor (as do E-box regions of other genes regulated in the ovary).