Purification and characterization of an inactive form of cAMP-dependent protein kinase containing bound cAMP

Calcium-independent phospholipase A(2) mediates CREB phosphorylation and c-fos expression during ischemia

In isolated, perfused adult rat hearts, global ischemia increased the phosphorylation of cAMP response element-binding protein (CREB) relative to control levels, and this phosphorylation was reversed with reperfusion. CREB phosphorylation elicited by 5 min of global ischemia was sensitive to treatments with the calcium-independent phospholipase A(2) (iPLA(2)) inhibitor bromoenol lactone (BEL) and occurred in the absence of increases in myocardial cAMP content. In contrast, CREB phosphorylation elicited by 15 min of global ischemia was likely mediated by elevated cAMP levels. The expression of c-fos, in response to brief myocardial ischemia, was also sensitive to BEL treatment. The induction of iPLA(2)-mediated CREB phosphorylation was further substantiated by the observations that lysoplasmenylcholine increased both the phosphorylation of CREB and the induction of c-fos expression in the absence and presence of BEL. CREB phosphorylation in both ischemic hearts and lysoplasmenylcholine-perfused hearts was inhibited by pretreatment of hearts with the specific cAMP-dependent protein kinase (PKA) inhibitor H-89. Taken together, these data demonstrate that iPLA(2) mediates CREB phosphorylation through a PKA-dependent pathway during brief periods of myocardial ischemia, possibly through the formation of lysophospholipids.

alpha(2A)-adrenergic receptor stimulation potentiates calcium release in platelets by modulating cAMP levels

alpha(2A)-Adrenergic receptor-mediated Ca(2+) signaling and integrin alpha(IIb)beta(3) exposure were investigated in human platelets under conditions where indirect, thromboxane- or ADP-mediated effects were absent. The alpha(2)-adrenergic receptor agonists, UK14304 and epinephrine (EPI), were unable to raise cytosolic levels of inositol 1,4,5-trisphosphate (InsP(3)) or Ca(2+) but potentiated the [Ca(2+)](i) rises evoked by other agonists that act through stimulation of phospholipase C (thrombin or platelet-activating factor) or stimulation of Ca(2+)-induced Ca(2+) release (CICR) in the absence of InsP(3) generation (thimerosal or thapsigargin). In addition, alpha(2)-adrenergic stimulation resulted in a 20% lowering in the cytosolic cAMP level. In platelets treated with G(salpha)-stimulating prostaglandin E(1), EPI increased the Ca(2+) signal evoked by either phospholipase C- or CICR-stimulating agonists mainly through modulation of the cAMP level. The stimulating effects of UK14304 and EPI on platelet Ca(2+) responses, and also on integrin alpha(IIb)beta(3) exposure and platelet aggregation, were abolished by pharmacological stimulation of cAMP-dependent protein kinase, and these effects were mimicked by inhibition of this activity. In permeabilized platelets, UK14304 and EPI potentiated InsP(3)-induced, CICR-mediated mobilization of Ca(2+) from internal stores in a similar way as did inhibition of cAMP-dependent protein kinase. In summary, a G(ialpha)-mediated decrease in cAMP level appears to play a major role in the platelet-activating effects of alpha(2A)-adrenergic receptor stimulation. Thus, in platelets, unlike other cell types, occupation of the G(ialpha)-coupled alpha(2A)-adrenergic receptors does not result in phospholipase C activation but rather in modulation of the Ca(2+) response by relieving cAMP-mediated suppression of InsP(3)-dependent CICR.

Protein kinase A-Ialpha subunit-directed antisense inhibition of ovarian cancer cell growth: crosstalk with tyrosine kinase signaling pathway

Expression of the RIalpha subunit of cAMP-dependent protein kinase type I is increased in human cancers in which an autocrine pathway for epidermal growth factor-related growth factors is activated. We have investigated the effect of sequence-specific inhibition of RIalpha gene expression on ovarian cancer cell growth. We report that RIalpha antisense treatment results in a reduction in RIalpha expression and protein kinase A type I, and inhibition of cell growth. The growth inhibition was accompanied by changes in cell morphology and appearance of apoptotic nuclei. In addition, EGF receptor, c-erbB-2 and c-erbB-3 levels were reduced, and the basal and EGF-stimulated mitogen-activated protein kinase activities were reduced. Protein kinase A type I and EGF receptor levels were also reduced in cells overexpressing EGF receptor antisense cDNA. These results suggest that the antisense depletion of RIalpha leads to blockade of both the serine-threonine kinase and the tyrosine kinase signaling pathways resulting in arrest of ovarian cancer cell growth.

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.

Phosphodiesterase activity in intrapulmonary arteries and veins of perinatal lambs

The transition from fetal to newborn life is marked by a reduction in pulmonary vascular tone mediated by the intracellular second messengers, cGMP and cAMP. We have compared the rates of phosphodiesterase (PDE)-catalyzed hydrolysis of cGMP and cAMP in intrapulmonary vessels of fetal (146 +/- 2 days gestation) and newborn (3-7-day-old) lambs, each n = 6. Lung vessels of second to sixth generations were dissected and cytosol was prepared by differential centrifugation. PDE activity in cytosol was determined by radiometric assay of the hydrolysis of exogenous nucleotides at 30 degrees C for 10 min. Rates of hydrolysis (pmol/min/mg protein) of cGMP were 225 +/- 38 in fetal arteries and different from 151 +/- 7 in veins. In newborn vessels, the rates were 155 +/- 49 and 63 +/- 13 in arteries and veins, respectively. Rates of cAMP hydrolysis by the fetus were 80 +/- 11 in arteries and 45 +/- 16 veins. In newborn lambs the rates were 69 +/- 10 in arteries and different from 18 +/- 4 in veins. Inhibition of PDE activity by zaprinast, a cGMP-specific PDE inhibitor, and rolipram, a cAMP-specific PDE inhibitor, was more in veins of fetal and newborn lambs. Our data show that rates of hydrolysis of the cyclic nucleotides were faster in fetal vessels than in the newborn. We speculate that this would result in a greater accumulation of the cyclic nucleotides in newborn vessels, particularly the veins, and therefore endow the veins with less vascular tone.

Activation of myocardial cAMP-dependent protein kinase by lysoplasmenylcholine

Plasmalogen-specific, calcium-independent phospholipase A2 (iPLA2) is activated during myocardial ischemia. Accordingly, we have assessed the activation of myocardial protein kinases by the iPLA2 product, lysoplasmenylcholine. Lysoplasmenylcholine-activated protein kinase activity from heart cytosol fractionated on a DE-52 column was identified as cAMP-dependent protein kinase (PKA) based on the following: (1) protein kinase activity stimulated by cAMP and lysoplasmenylcholine co-eluted on sequential chromatographic steps; (2) lysoplasmenylcholine-activated protein kinase activity was inhibited by the PKA inhibitor, PKI; and (3) the unprimed PKA form generated from the primed form of PKA was activated by cAMP and lysoplasmenylcholine. These results demonstrate a novel mechanism for PKA activation by lysoplasmenylcholine.

Activation by cyclic GMP binding causes an apparent conformational change in cGMP-dependent protein kinase

Cyclic nucleotide binding activates cyclic nucleotide-dependent protein kinases, but the molecular mechanism is unknown. In the present studies, cGMP binding to type Ialpha or type Ibeta cGMP-dependent protein kinase (PKG) caused (i) a large electronegative charge shift of each enzyme on ion exchange chromatography, (ii) an increase in the Stokes radius (>3 A) of each enzyme, and (iii) a decreased mobility of type Ibeta PKG on native gel electrophoresis. These physical changes were not detected in the monomeric form of type Ibeta PKG upon activation by cGMP. However, the results of partial proteolysis of type Ialpha PKG revealed some degree of cGMP-induced conformational change within the PKG-monomer, since cGMP binding protects the PKG-monomer against chymotryptic cleavage. The altered sensitivity to proteolysis occurs at Met-200, which is located between the B and C alpha-helices in the high affinity site (site A), and implies that the cGMP-induced structural perturbations in this region may participate in activation of dimeric PKG. The cGMP-induced conformational effects observed using the physical separation methods are likely to reflect altered interactions within the dimeric PKG that are caused by structural alterations within the subunits.

Identification of critical determinants for autoinhibition in the pseudosubstrate region of type I alpha cAMP-dependent protein kinase

The consensus substrate site for cAMP-dependent protein kinase (PKA) is Arg-Arg-Xaa-Ser(P)-Xaa and the autoinhibitory domain of the PKA type I alpha regulatory subunit (RI subunit) contains a similar sequence, Arg92-Arg-Arg-Arg-Gly-Ala-Ile-Ser-Ala-Glu. The italicized amino acids form a putative pseudosubstrate site (Ser is replaced with Ala), which together with adjacent residues could competitively inhibit substrate phosphorylation by the PKA catalytic subunit (C subunit). The present studies determine the contributions of Arg92-95, Ile98, and Glu101 to inhibitory potency. Amino-terminal truncation of RI subunit through Arg92 (delta1-92) or Arg93 (delta1-93) had no detectable effect on inhibition of C subunit. Truncation through Arg94 (delta1-94), or point mutation of Arg95 within truncated mutants (delta1-93.R95A or delta1-92.R95A), caused a dramatic reduction in inhibitory potency. Truncation through Arg95 (delta1-95) had a greater effect than did replacement or deletion of Arg94 or Arg95 alone. Using full-length RI subunit, the inhibitory potency was reduced by replacing Ile98 with Ala, Gly, or Gln, but not by replacing it with Val. The inhibitory potency of RI subunit was unchanged when Glu101 was replaced with Ala or Gln. It is concluded that Arg94, Arg95 and, to a lesser extent, Ile98 are vital constituents of PKA autoinhibition by type I alpha R subunit.

Overexpression of RII beta regulatory subunit of protein kinase A in human colon carcinoma cell induces growth arrest and phenotypic changes that are abolished by site-directed mutation of RII beta

LS-174T human colon carcinoma cells that contain approximately equal amounts of cAMP-dependent protein kinase (PKA) isozymes, PKA-I and PKA-II, were infected with retroviral vectors coding for regulatory (R) and catalytic (C) subunits of human PKA. In cells overexpressing RII alpha, RII beta and RII beta-P (a RII beta mutant at the autophosphorylation site), PKA-II levels increased while PK-A levels decreased. PKA-I was almost completely eliminated in cells overexpressing RII beta or RII beta-P. In contrast, overexpression of either RI alpha or C alpha had little or no effect on PKA isozyme levels. Although all infectants expressed high levels of PKA subunit mRNAs in accordance with gene introduction, the R subunit protein expression was reflected in PKA isozyme levels rather than in subunit mRNA levels. Only RII beta infectants demonstrated marked growth inhibition in monolayer culture, reduced thymidine incorporation into DNA, and inability to grow in semisolid medium or in serum-free medium. Conversely, all other infectants displayed growth properties similar to uninfected parental cells. The growth-retardation properties of RII beta infectants were reflected in their altered phenotypic appearances. Our findings that the mutant RII beta-P could not mimic the growth-inhibitory effect of RII beta suggest the functional importance of the authophosphorylation site in RII beta. Our results suggest a role for RII beta in the suppression of neoplastic cell growth, and thus abnormal expression of R subunit isoforms of PKA may be involved in neoplastic transformation.

A single-injection protein kinase A-directed antisense treatment to inhibit tumour growth

Expression of the RI alpha subunit of cAMP-dependent protein kinase type I is enhanced in human cancer cell lines, in primary tumours, in cells after transformation and in cells upon stimulation of growth. We have investigated the effect of sequence-specific inhibition of RI alpha gene expression on in vivo tumour growth. We report that single injection RI alpha antisense treatment results in a reduction in RI alpha expression and inhibition of tumour growth. Tumour cells behaved like untransformed cells by making less protein kinase type I. The RI alpha antisense, which produces a biochemical imprint for growth control, requires infrequent dosing to halt neoplastic growth in vivo.

Fluorescence resonance energy transfer within a heterochromatic cAMP-dependent protein kinase holoenzyme under equilibrium conditions: new insights into the conformational changes that result in cAMP-dependent activation

Previous studies of the ligand regulation of the cAMP-dependent protein kinase have demonstrated the cAMP-mediated dissociation of the holoenzyme by using nonequilibrium techniques; i.e., gel filtration, ion-exchange chromatography, and differential centrifugation. While physically mild, these could have caused weakly associated species to dissociate, thereby providing a potentially flawed interpretation of the mechanism of activation of the protein kinase. To assess this, the activation of the cAMP-dependent protein kinase has been monitored under equilibrium conditions using dipolar fluorescence energy transfer to measure changes in the proximity relations between the catalytic (C) and regulatory (R) subunits that compose the holoenzyme. Specifically, we prepared a heterochromatically labeled protein kinase type II holoenzyme, with the regulatory and catalytic subunits labeled with sulforhodamine and carboxyfluorescein, respectively, and monitored the exchange of electronic excitation energy between the C and R subunits by both donor lifetime and steady-state fluorescence. Biochemically, the heterochromatic holoenzyme was closely identical to the native protein with regard to cAMP-induced increase in catalytic activity, reassociation of C and R subunits, inhibition of catalytic activity by the specific protein kinase inhibitor (PKI), and observed dissociation examined by gel filtration upon cAMP addition. However, under equilibrium conditions, the energy-transfer measurements revealed that the addition of cAMP to this heterochromatic reporter complex promoted an estimated 10-A increase in the distance between the derivatization sites on C and R but not a dissociation of these subunits. Addition of PKI plus cAMP promoted full dissociation of the two subunits.(ABSTRACT TRUNCATED AT 250 WORDS)

cAMP-dependent protein kinase isozymes in porcine follicles and corpora lutea

Our purpose was to identify regulatory (R) subunits and their associations with catalytic (C) subunits to form cAMP-dependent protein kinase (A-kinase) holoenzymes in select porcine ovarian tissues during follicular differentiation. Soluble extracts of small and preovulatory follicles and corpora lutea (CL) were separated on DEAE-cellulose chromatography. R subunits were labeled with 8-N3[32P]cAMP or with [gamma-32P]ATP under RII autophosphorylation conditions and were identified by molecular weight (Mr) determination on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) as well as cross-reactivity of unlabeled subunits with anti-R antibodies. A-kinase holoenzymes and C subunit-free R (free R) subunits were distinguished on the basis of DEAE elution position and sedimentation position on sucrose density gradient centrifugation of phosphotransferase and [3H]cAMP binding activities. In small and preovulatory follicles and CL we identified a minor peak of type I A-kinase containing RI alpha (Mr = 47,000) and a major peak of type II A-kinase holoenzyme containing two RII isoforms (Mr = 52,000 and 56,000). Notable amounts of free RI alpha eluted between the type I and II holoenzymes in all three tissues. Neither of the holoenzymes nor free RI alpha was regulated as a function of follicular differentiation or CL formation. In contrast, free RII subunits were moderately increased in preovulatory follicles relative to levels in small follicles and CL. We conclude that only the RII subunits are hormonally regulated in developing follicles, and in tissues which express both RI and RII subunits, the RII subunits preferentially associate with C subunits to form the dominant holoenzyme despite the presence of significant amounts of RI.

Role of site-selective cAMP analogs in the control and reversal of malignancy

Two isoforms of cAMP receptor protein, RI and RII, the regulatory subunits of cAMP-dependent protein kinase, transduce opposite signals, the RI being stimulatory and the RII being inhibitory of cell proliferation. In normal cells RI and RII exist at a specific physiological ratio whereas in cancer cells such physiological balance of these receptor proteins is disrupted. Reversal and suppression of malignancy can be achieved when the physiologic ratio of these intracellular signal transducers of cAMP is restored as shown by the use of site-selective cAMP analogs, antisense oligodeoxynucleotides or gene transfer, suggesting new approaches to cancer control.

Cyclic nucleotide-dependent protein kinases

The actions of several hormones and neurotransmitters evoke signal transduction pathways which rapidly elevate the cytosolic concentrations of the intracellular messengers, cAMP and cGMP. The cyclic-nucleotide dependent protein kinases, cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG), are the major intracellular receptors of cAMP and cGMP. These enzymes become active upon binding respective cyclic nucleotides and modulate a diverse array of biochemical events through the phosphorylation of specific substrate proteins. The focus of this review is to describe the progress made in understanding the structure and function of both PKA and PKG.

Protein kinases associated with proliferation and differentiation in murine erythroleukaemic cells

1. The changes in electrophoretic distribution of cytosolic protein kinases have been studied in relation to proliferation, differentiation and transformation in murine erythroleukaemic cells, using a non-denaturing polyacrylamide gel electrophoresis system. 2. Native molecular masses of the major forms were determined by Ferguson plots. 3. A two dimensional electrophoresis method was developed for determination of the subunit molecular masses. 4. These studies suggest that the major bands of activity contain components which may correspond to cGMP dependent protein kinase, cAMP dependent protein kinase and protein kinase C. 5. On hexamethylene bisacetamide induced differentiation of the cells, changes in proportions of the different forms were observed.

Agonist-induced glycogenolysis in rabbit retinal slices and cultures

1. The effects of different putative retinal transmitters and/or modulators on glycogenolysis in rabbit retinal slices and in retinal Müller cell cultures were examined. 2. Incubation of rabbit retinal slices or primary retinal cultures (either 3-5 day-old or 25-30 day-old) in a buffer solution containing [3H]-glucose resulted in the accumulation of newly synthesized [3H]-glycogen. 3. Noradrenaline (NA), isoprenaline, vasoactive intestinal peptide (VIP), 5-hydroxytryptamine (5-HT) and 8-hydroxy-dipropylaminetetralin (8-OH-DPAT) stimulated the hydrolysis of this newly formed 3H-polymer. The potency order of maximal stimulations was: VIP greater than NA greater than isoprenaline greater than 5-HT greater than 8-OH-DPAT. 4. The putative retinal transmitters, dopamine, gamma-aminobutyric acid (GABA), glycine and taurine and the muscarinic agonist carbachol (CCh) had no effect on [3H]-glycogen content. 5. The glycogenolytic effects of NA/isoprenaline and 5-HT/8-OH-DPAT appear to be mediated by beta-adrenoceptors and 5-HT1 receptors (possibly 5-HT1A), respectively while the VIP-induced response involved another receptor subtype. 6. Agonists which mediated [3H]-glycogen hydrolysis also stimulated an increase in adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation. Both responses are blocked to a similar extent by the same antagonists and so are probably mediated via the same receptor subtypes. Moreover, dibutyryl cyclic AMP (db cyclic AMP) promoted tritiated glycogen breakdown in the three retinal preparations. 7. Not all receptors linked to cyclic AMP production however promote glycogenolysis. Dopamine and apomorphine stimulated cyclic AMP formation via D1-receptors without influencing glycogenolysis. These receptors are exclusively associated with neurones.

Two different intrachain cAMP sites in the cAMP-dependent protein kinase of the dimorphic fungus Mucor rouxii

cAMP sites of the cAMP-dependent protein kinase from the fungus Mucor rouxii have been characterized through the study of the effects of cAMP and of cAMP analogs on the phosphotransferase activity and through binding kinetics. The tetrameric holoenzyme, which contains two regulatory (R) and two catalytic (C) subunits, exhibited positive cooperativity in activation by cAMP, suggesting multiple cAMP-binding sites. Several other results indicated that the Mucor kinase contained two different cooperative cAMP-binding sites on each R subunit, with properties similar to those of the mammalian cAMP-dependent protein kinase. Under optimum binding conditions, the [3H]cAMP dissociation behavior indicated equal amounts of two components which had dissociation rate constants of 0.09 min-1 (site 1) and 0.90 min-1 (site 2) at 30 degrees C. Two cAMP-binding sites could also be distinguished by C-8 cAMP analogs (site-1-selective) and C-6 cAMP analogs (site-2-selective); combinations of site-1- and site-2-selective analogs were synergistic in protein kinase activation. The two different cooperative binding sites were probably located on the same R subunit, since the proteolytically derived dimeric form of the enzyme, which contained one R and one C component, retained the salient properties of the untreated tetrameric enzyme. Unlike any of the mammalian cyclic-nucleotide-dependent isozymes described thus far, the Mucor kinase was much more potently activated by C-6 cAMP analogs than by C-8 cAMP analogs. In the ternary complex formed by the native Mucor tetramer and cAMP, only the two sites 1 contained bound cAMP, a feature which has also not yet been demonstrated for the mammalian cAMP-dependent protein kinase.

An assessment of phosphodiesterase activity in situ after treatment of hepatocytes with hormones

The role of phosphodiesterase activation in controlling adenosine 3',5'-cyclic monophosphate (cAMP) levels within hepatocytes was investigated by preloading hepatocytes with the hydrolyzable cAMP analogue 8-para-chlorophenylthio-cAMP (8-pCl phi S-cAMP) and measuring disappearance of the analogue after treating the cells with various hormones. Incubation of hepatocytes with 15 nM 8-pCl phi S-cAMP increased the intracellular concentration of the analogue at 0.5 and 2 min, but by 5 min the concentration plateaued and remained constant or declined slightly at 7 and 10 min. Treatment of hepatocytes with 5 nM glucagon led to a rapid 50% decline in intracellular concentration of the analogue. However, 6 nM insulin produced no detectable change in analogue concentration, and a combination of 5 nM glucagon and 6 nM insulin produced no greater lowering of 8-pCl phi S-cAMP than did glucagon alone. Treatment of hepatocytes with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (50 microM) blocked approximately 30% of the glucagon-mediated decrease in 8-pCl phi S-cAMP concentration, and in separate cell incubations, it blocked 50% of the cAMP lowering produced by 125 nM 8-pCl phi S-cAMP. Treatment of analogue-preloaded hepatocytes with effective concentrations of phenylephrine, vasopressin, or angiotensin resulted in no change in intracellular analogue or cAMP concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)