cAMP export and its regulation by prostaglandin A1

Pediatric Dyslipidemia-Beyond Familial Hypercholesterolemia

Dyslipidemia is seen with increasing prevalence in young Canadians, mainly mild to moderate hypertriglyceridemia secondary to obesity. This review focuses on pediatric dyslipidemias excluding familial hypercholesterolemia (FH), but including both severe and mild to moderate hypertriglyceridemia, combined hyperlipidemia, and elevated lipoprotein(a) [Lp(a)]. We suggest that for Canadian children and adolescents with dyslipidemia, atherosclerotic cardiovascular disease (ASCVD) risk assessment should include both low-density lipoprotein cholesterol and triglyceride measurement. To further stratify risk, determination of non-high-density lipoprotein cholesterol is recommended, for both its ability to predict ASCVD and convenience for the patient because fasting is not required. Similarly, apolipoprotein B measurement (fasting or nonfasting), where available, can be helpful. Lp(a) measurement should not be routine in childhood, but it can be considered in special circumstances. After ruling out secondary causes, the foundation for management of pediatric dyslipidemia includes weight regulation, optimizing diet, and increasing activity level. At present, randomized clinical trial data to guide pharmaceutical management of pediatric hypertriglyceridemia or other non-FH pediatric dyslipidemias are scarce. Pharmaceutical management should be reserved for special situations in which risk of complications such as acute pancreatitis or ASCVD over the intermediate term is high and conservative lifestyle-based interventions have been ineffective.

MRP4/ABCC4 As a New Therapeutic Target: Meta-Analysis to Determine cAMP Binding Sites as a Tool for Drug Design

MRP4 transports multiple endogenous and exogenous substances and is critical not only for detoxification but also in the homeostasis of several signaling molecules. Its dysregulation has been reported in numerous pathological disorders, thus MRP4 appears as an attractive therapeutic target. However, the efficacy of MRP4 inhibitors is still controversial. The design of specific pharmacological agents with the ability to selectively modulate the activity of this transporter or modify its affinity to certain substrates represents a challenge in current medicine and chemical biology. The first step in the long process of drug rational design is to identify the therapeutic target and characterize the mechanism by which it affects the given pathology. In order to develop a pharmacological agent with high specific activity, the second step is to systematically study the structure of the target and identify all the possible binding sites. Using available homology models and mutagenesis assays, in this review we recapitulate the up-to-date knowledge about MRP structure and aligned amino acid sequences to identify the candidate MRP4 residues where cyclic nucleotides bind. We have also listed the most relevant MRP inhibitors studied to date, considering drug safety and specificity for MRP4 in particular. This meta-analysis platform may serve as a basis for the future development of inhibitors of MRP4 cAMP specific transport.

Cellular export of drugs and signaling molecules by the ATP-binding cassette transporters MRP4 (ABCC4) and MRP5 (ABCC5)

Like other members of the multidrug resistance protein (MRP)/ABCC subfamily of ATP-binding cassette transporters, MRP4 (ABCC4) and MRP5 (ABCC5) are organic anion transporters. They have, however, the outstanding ability to transport nucleotides and nucleotide analogs. In vitro experiments using drug-selected or -transfected cells indicated that these transport proteins, when overexpressed, can lower the intracellular concentration of nucleoside/nucleotide analogs, such as the antiviral compounds PMEA (9-(2-phosphonylmethoxyethyl)adenine) or ganciclovir, and of anticancer nucleobase analogs, such as 6-mercaptopurine, after their conversion into the respective nucleotides. This may lead to an impaired ability of these compounds to inhibit virus replication or cell proliferation. It remains to be tested whether antiviral or anticancer chemotherapy based on nucleobase, nucleoside, or nucleotide precursors can be modulated by inhibition of MRP4 and MRP5. MRP4 also seems to be able to mediate the transport of conjugated steroids, prostaglandins, and glutathione. Furthermore, cyclic nucleotides (cyclic adenosine monophosphate and cyclic guanine monophosphate) are exported from cells by MRP4 and MRP5. This may modulate the intracellular concentration of these important mediators, besides the action of phosphodiesterases, as well as provide extracellular nucleotides for a possible paracrine action. In this line, tissue distribution and subcellular localization of MRP4 and MRP5 specifically in smooth muscle cells (MRP5), platelet-dense granules (MRP4), and nervous cells (MRP4 and MRP5), besides the capillary endothelium, point not only to a possible function of these transporters as exporters in cellular defense, but also to a physiological function in signaling processes.

Hypotonic cell swelling stimulates permeability to cAMP in a rat colonic cell line

This study characterized the membrane permeability to cAMP in a cell line derived from the rat colon (CC531(mdr+)) by comparison of fluxes of 3H-cAMP, 3H-8-bromo-cAMP, 3H-taurine, 3H-adenosine and 3H-5'AMP under various experimental conditions including cell membrane depolarization and hypotonic cell swelling. Cell volume was modified by changing the osmolality and composition of the extracellular medium. Incubation in iso- and hypotonic KCl media induced graded increases in cell volume and stable activation of volume-sensitive channels that was reflected in an increased efflux of 3H-taurine. Incubation in hypotonic KCl solution also enhanced the efflux of 3H-8-Br-cAMP (a non-hydrolysable analogue of cAMP). Both the efflux of 3H-taurine and of 3H-8-Br-cAMP were inhibited by 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB, 100 microM) suggesting the involvement of volume-sensitive anion channels. To gain further insight into the route mediating cAMP permeability, the uptakes of 3H-cAMP, 3H-8-Br-cAMP and 3H-taurine were determined over short (5-min) periods. Uptakes of these substrates demonstrated close similarities: comparable increases were observed that correlated with the increases in cell volume in iso- and hypoosmotic KCl media; they were inhibited strongly by NPPB (100 microM) and metabolic inhibitors (deoxyglucose, 20 mM together with the mitochondrial uncoupler carbonylcyanide p-(trifluoromethoxy)phenylhydrazone, FCCP, 10 microM) while barely reduced by dipyridamole (100 microM) and they were not affected by adenosine (1 mM). In contrast, the uptakes of 3H-adenosine and 3H-5'AMP had strikingly different properties; they were insensitive to cell swelling; barely inhibited by NPPB (100 microM) and metabolic inhibitors (deoxyglucose and FCCP) while strongly reduced by dipyridamole (100 micro M). Unlike the uptakes of 3H-cAMP, 3H-8-Br-cAMP and 3H-taurine, the uptakes of 3H-adenosine and 3H-5'AMP were reduced in Na(+)-free media, suggesting the presence in this cell line of two different adenosine carriers, one sodium-dependent and one sodium-independent. Taken together the present data show that in this rat colonic cell line, cAMP permeability is increased by cell swelling in hypotonic KCl medium and inhibited by NPPB and metabolic inhibitors. The similarity of these characteristics to those of taurine permeability suggests the involvement of a volume-sensitive anion pathway.

MRP8, ATP-binding cassette C11 (ABCC11), is a cyclic nucleotide efflux pump and a resistance factor for fluoropyrimidines 2',3'-dideoxycytidine and 9'-(2'-phosphonylmethoxyethyl)adenine

MRP8 (ABCC11) is a recently identified cDNA that has been assigned to the multidrug resistance-associated protein (MRP) family of ATP-binding cassette transporters, but its functional characteristics have not been determined. Here we examine the functional properties of the protein using transfected LLC-PK1 cells. It is shown that ectopic expression of MRP8 reduces basal intracellular levels of cAMP and cGMP and enhances cellular extrusion of cyclic nucleotides in the presence or absence of stimulation with forskolin or SIN-1A. Analysis of the sensitivity of MRP8-overexpressing cells revealed that they are resistant to a range of clinically relevant nucleotide analogs, including the anticancer fluoropyrimidines 5'-fluorouracil (approximately 3-fold), 5'-fluoro-2'-deoxyuridine (approximately 5-fold), and 5'-fluoro-5'-deoxyuridine (approximately 3-fold), the anti-human immunodeficiency virus agent 2',3'-dideoxycytidine (approximately 6-fold) and the anti-hepatitis B agent 9'-(2'-phosphonylmethoxynyl)adenine (PMEA) (approximately 5-fold). By contrast, increased resistance was not observed for several natural product chemotherapeutic agents. In accord with the notion that MRP8 functions as a drug efflux pump for nucleotide analogs, MRP8-transfected cells exhibited reduced accumulation and increased efflux of radiolabeled PMEA. In addition, it is shown by the use of in vitro transport assays that MRP8 is able to confer resistance to fluoropyrimidines by mediating the MgATP-dependent transport of 5'-fluoro-2'-deoxyuridine monophosphate, the cytotoxic intracellular metabolite of this class of agents, but not of 5'-fluorouracil or 5'-fluoro-2'-deoxyuridine. We conclude that MRP8 is an amphipathic anion transporter that is able to efflux cAMP and cGMP and to function as a resistance factor for commonly employed purine and pyrimidine nucleotide analogs.

Characterization of the MRP4- and MRP5-mediated transport of cyclic nucleotides from intact cells

Cyclic nucleotides are known to be effluxed from cultured cells or isolated tissues. Two recently described members of the multidrug resistance protein family, MRP4 and MRP5, might be involved in this process, because they transport the 3',5'-cyclic nucleotides, cAMP and cGMP, into inside-out membrane vesicles. We have investigated cGMP and cAMP efflux from intact HEK293 cells overexpressing MRP4 or MRP5. The intracellular production of cGMP and cAMP was stimulated with the nitric oxide releasing compound sodium nitroprusside and the adenylate cyclase stimulator forskolin, respectively. MRP4- and MRP5-overexpressing cells effluxed more cGMP and cAMP than parental cells in an ATP-dependent manner. In contrast to a previous report we found no glutathione requirement for cyclic nucleotide transport. Transport increased proportionally with intracellular cyclic nucleotide concentrations over a calculated range of 20-600 microm, indicating low affinity transport. In addition to several classic inhibitors of organic anion transport, prostaglandins A(1) and E(1), the steroid progesterone and the anti-cancer drug estramustine all inhibited cyclic nucleotide efflux. The efflux mediated by MRP4 and MRP5 did not lead to a proportional decrease in the intracellular cGMP or cAMP levels but reduced cGMP by maximally 2-fold over the first hour. This was also the case when phosphodiesterase-mediated cyclic nucleotide hydrolysis was inhibited by 3-isobutyl-1-methylxanthine, conditions in which efflux was maximal. These data indicate that MRP4 and MRP5 are low affinity cyclic nucleotide transporters that may at best function as overflow pumps, decreasing steep increases in cGMP levels under conditions where cGMP synthesis is strongly induced and phosphodiesterase activity is limiting.

Extracellular cAMP inhibits proximal reabsorption: are plasma membrane cAMP receptors involved?

Glucagon binding to hepatocytes has been known for a long time to not only stimulate intracellular cAMP accumulation but also, intriguingly, induce a significant release of liver-borne cAMP in the blood. Recent experiments have shown that the well-documented but ill-understood natriuretic and phosphaturic actions of glucagon are actually mediated by this extracellular cAMP, which inhibits the reabsorption of sodium and phosphate in the renal proximal tubule. The existence of this "pancreato-hepatorenal cascade" indicates that proximal tubular reabsorption is permanently influenced by extracellular cAMP, the concentration of which is most probably largely dependent on the insulin-to-glucagon ratio. The possibility that renal cAMP receptors may be involved in this process is supported by the fact that cAMP has been shown to bind to brush-border membrane vesicles. In other cell types (i.e., adipocytes, erythrocytes, glial cells, cardiomyocytes), cAMP eggress and/or cAMP binding have also been shown to occur, suggesting additional paracrine effects of this nucleotide. Although not yet identified in mammals, cAMP receptors (cARs) are already well characterized in lower eukaryotes. The amoeba Dictyostelium discoideum expresses four different cARs during its development into a multicellular organism. cARs belong to the superfamily of seven transmembrane domain G protein-coupled receptors and exhibit a modest homology with the secretin receptor family (which includes PTH receptors). However, the existence of specific cAMP receptors in mammals remains to be demonstrated. Disturbances in the pancreato-hepatorenal cascade provide an adequate pathophysiological understanding of several unexplained observations, including the association of hyperinsulinemia and hypertension, the hepatorenal syndrome, and the hyperfiltration of diabetes mellitus. The observations reviewed in this paper show that cAMP should no longer be regarded only as an intracellular second messenger but also as a first messenger responsible for coordinated hepatorenal functions, and possibly for paracrine regulations in several other tissues.

Transport of cyclic nucleotides and estradiol 17-beta-D-glucuronide by multidrug resistance protein 4. Resistance to 6-mercaptopurine and 6-thioguanine

Human multidrug resistance protein 4 (MRP4) has recently been determined to confer resistance to the antiviral purine analog 9-(2-phosphonylmethoxyethyl)adenine and methotrexate. However, neither its substrate selectivity nor physiological functions have been determined. Here we report the results of investigations of the in vitro transport properties of MRP4 using membrane vesicles prepared from insect cells infected with MRP4 baculovirus. It is shown that expression of MRP4 is specifically associated with the MgATP-dependent transport of cGMP, cAMP, and estradiol 17-beta-D-glucuronide (E(2)17 beta G). cGMP, cAMP, and E(2)17 beta G are transported with K(m) and V(max) values of 9.7 +/- 2.3 microm and 2.0 +/- 0.3 pmol/mg/min, 44.5 +/- 5.8 microm and 4.1 +/- 0.4 pmol/mg/min, and 30.3 +/- 6.2 microm and 102 +/- 16 pmol/mg/min, respectively. Consistent with its ability to transport cyclic nucleotides, it is demonstrated that the MRP4 drug resistance profile extends to 6-mercaptopurine and 6-thioguanine, two anticancer purine analogs that are converted in the cell to nucleotide analogs. On the basis of its capacity to transport cyclic nucleotides and E(2)17 beta G, it is concluded that MRP4 may influence diverse cellular processes regulated by cAMP and cGMP and that its substrate range is distinct from that of any other characterized MRP family member.

The multidrug resistance protein 5 functions as an ATP-dependent export pump for cyclic nucleotides

Cellular export of cyclic nucleotides has been observed in various tissues and may represent an elimination pathway for these signaling molecules, in addition to degradation by phosphodiesterases. In the present study we provide evidence that this export is mediated by the multidrug resistance protein isoform MRP5 (gene symbol ABCC5). The transport function of MRP5 was studied in V79 hamster lung fibroblasts transfected with a human MRP5 cDNA. An MRP5-specific antibody detected an overexpression of the glycoprotein of 185 +/- 15 kDa in membranes from MRP5-transfected cells and a low basal expression of hamster Mrp5 in control membranes. ATP-dependent transport of 3',5'-cyclic GMP at a substrate concentration of 1 micrometer was 4-fold higher in membrane vesicles from MRP5-transfected cells than in control membranes. This transport was saturable with a K(m) value of 2.1 micrometer. MRP5-mediated transport was also detected for 3',5'-cyclic AMP at a lower affinity, with a K(m) value of 379 micrometer. A potent inhibition of MRP5-mediated transport was observed by several compounds, known as phosphodiesterase modulators, including trequinsin, with a K(i) of 240 nm, and sildenafil, with a K(i) value of 267 nm. Thus, cyclic nucleotides are physiological substrates for MRP5; moreover, MRP5 may represent a novel pharmacological target for the enhancement of tissue levels of cGMP.

Characterization of cyclic AMP efflux from swine adipocytes in vitro

OBJECTIVE

A variety of cell types transport cyclic AMP (cAMP) to the extracellular fluid; the purpose of this study was to determine if and how this process occurs in adipocytes.

RESEARCH METHODS AND PROCEDURES

Adipocytes were isolated from 3-month-old swine and incubated with stimulators of adenylate cyclase for 2 to 120 minutes to promote cAMP synthesis and efflux. Efflux was characterized in the presence of agents that inhibit ATP production, anion transport, intracellular cAMP metabolism, and extracellular cAMP metabolism. Extracellular cAMP was measured by enzyme immunoassay, then corrected for cell lysis by measuring lactate dehydrogenase release.

RESULTS

cAMP efflux averaged 24.7 fmol/min/cm2 adipocyte surface area, was linear for 2 hours, and was proportional to adipocyte surface area (r=0.94, p<0.05). Efflux was reduced by approximately 35% in cells incubated with 1 microM antimycin, an inhibitor of ATP synthesis (p<0.05), and by approximately 55% in cells incubated with 2 mM probenecid, an anion-specific transport blocker (p<0.05). Extracellular cAMP levels more than doubled by the addition of 1 microM 1,3-dipropyl-8-p-sulfophenylxanthine, a purported inhibitor of extracellular phosphodiesterase.

DISCUSSION

Our data demonstrate that cAMP is transported from swine adipocytes by an energy-dependent anion transporter and can be metabolized extracellularly. Future studies will evaluate extracellular cAMP as a potential source of extracellular adenosine, a potent inhibitor of adipocyte lipolysis.

Glucose-dependent, cAMP-mediated ATP efflux from Saccharomyces cerevisiae

Extracellular ATP plays an important role in the physiology of multicellular organisms; however, it is unknown whether unicellular organisms such as yeast also release ATP extracellularly. Experiments are described here which show that Saccharomyces cerevisiae releases ATP to the extracellular fluid. This efflux required glucose and the rate was increased dramatically by the proton ionophores nigericin, monensin, carbonyl cyanide m-chlorophenylhydrazone and carbonyl cyanide p-(trifluoromethoxy)-phenylhydrazone; ATP efflux was also increased by the plasma membrane proton pump inhibitor diethylstilbestrol. The increase in the concentration of extracellular ATP was not due to cell lysis or general disruption of plasma membrane integrity as measured by colony-forming and methylene-blue-staining assays. ATP efflux was strictly correlated with a rise in intracellular cAMP; therefore, the cAMP pathway is likely to be involved in triggering ATP efflux. These results demonstrate that yeast cells release ATP in a regulated manner.

ATP transport and ABC proteins

ATP can be exported into the extracellular space, where it has important biological effects. Recent evidence shows that direct ATP export across the plasma membrane is associated with the presence of ABC proteins. Do the ABC proteins pump ATP as well as their other substrates, and if so, why?

Pre-steady-state kinetic analysis of cAMP-dependent protein kinase using rapid quench flow techniques

The phosphorylation of a peptide substrate by the catalytic subunit of cAMP-dependent protein kinase was monitored over short time periods (2-1000 ms) using a rapid quench flow mixing device and a radioactive assay. The production of phosphokemptide [LRRAS(P)LG] as a function of time is characterized by a rapid "burst" phase (250 s-1) followed by a slower, linear phase (L/[E]t = 21 s-1) at 100 microM Kemptide. The amplitude of this "burst" phase varies linearly with the enzyme concentration and represents approximately 100% of the total enzyme concentration, indicating that the "burst" phase is not due to product inhibition. The observed rate constants for the "burst" and linear phases and the "burst" amplitude vary hyperbolically with the substrate concentration. From these dependencies, a maximum "burst" rate constant of 500 +/- 60 s-1 and a Km and Kd for Kemptide of 4.9 +/- 1.4 and 200 +/- 60 microM were determined. The kcat and Km data extracted from the linear portion of the rapid quench flow transients are indistinguishable from those obtained by standard steady-state kinetic analyses using low catalytic subunit concentrations and a spectrophotometric, coupled enzyme assay. Both rate constants for the "burst" and linear phases decreased in the presence of Mn2+. The data imply that the phosphorylation of Kemptide by the catalytic subunit occurs by a mechanism in which the substrate is loosely bound, is rapidly phosphorylated at the active site, and is released at a steady-state rate that is likely controlled by the dissociation rate constant for ADP. The combined pre-steady-state kinetic data establish a comprehensive, kinetic mechanism that predicts all the steady-state kinetic and viscosometric data. This study represents the first chemical observation and characterization of phosphoryl transfer at the active site of a protein kinase and will be useful for further structure-function studies on this and other protein kinases.

Characterization of a novel potent and specific inhibitor of type V phosphodiesterase

Guanosine cyclic 3':5'-monophosphate (cGMP) plays a crucial role in regulating vascular smooth muscle contractile state. In rat aortic smooth muscle cells (RSMC) three isozymes of phosphodiesterase (PDE) may be involved in the degradation of cGMP, namely PDE I, PDE III, and PDE V. To study the effective contribution of PDE V to the control of intracellular cGMP levels, a specific and potent PDE V inhibitor 1,3-dimethyl-6-(2-propoxy-5-methanesulfonylamidophenyl)pyrazolo[3, 4d]- pyrimidin-4-(5H)-one (DMPPO) was synthesized. DMPPO is a competitive inhibitor with respect to cGMP (Ki = 3 nM) and displayed high selectivity for PDE V as compared to other PDE isozymes. DMPPO strongly potentiated the cGMP response of atrial natriuretic peptide- or sodium nitroprusside-treated RSMC (EC50 = 0.5 microM). In addition, similar intracellular cGMP levels were obtained in the presence of a saturating concentration of DMPPO or 3-isobutyl-1-methylxanthine, a nonspecific PDE inhibitor, suggesting that cGMP is almost exclusively hydrolyzed by PDE V in RSMC. Stimulation of RSMC with atrial natriuretic factor resulted in accumulation of cGMP in the extracellular media. This egression was shown to be proportional to the intracellular level of cGMP and a first-order rate constant of 0.04 min-1 was determined for the egression process. DMPPO did not interfere with the efflux and allowed us to show that intracellular cGMP levels are mainly controlled by PDE V, rather than by egression in RSMC. DMPPO is, therefore, a useful tool for determining the role of PDE V in the control of cGMP levels in living cells and tissues.

Agouti antagonism of melanocortin binding and action in the B16F10 murine melanoma cell line

Several dominant mutations at the murine agouti locus result in the expression of a number of phenotypic changes, including a predominantly yellow coat color, obesity, and hyperinsulinemia. The mutants exhibit ectopic overexpression of normal agouti protein, suggesting that agouti regulates coat coloration by direct antagonism of the alpha-melanocyte-stimulating hormone receptor. We have tested this hypothesis by examining agouti inhibition of both melanocortin-stimulated cyclic adenosine monophosphate production and the binding of a radioactive melanocortin analog in the murine B16F10 melanoma cell line. Inhibition of melanocortin-induced cyclic nucleotide accumulation did not require preincubation of the cells with agouti and was independent of the agonist used. Furthermore, inhibition of both agonist binding to and activation of melanocortin receptor could be described by a simple competitive model with similar inhibition constants of 1.9 and 0.9 nM, respectively. The mutually exclusive binding of agouti and melanocortin was verified by cross-linking experiments using a radiolabeled alpha-melanocyte-stimulating hormone analog. Competitive inhibition of alpha-melanocyte-stimulating hormone binding can account for the effects of agouti on coat coloration and suggests the possibility that the other phenotypic changes observed on agouti overexpression may be due to direct action of agouti at a novel melanocortin receptor(s).

Cyclic AMP export from lymphocytes in hypertension

While the importance of receptor-mediated intracellular cyclic AMP in blood pressure regulation is well documented, few studies have evaluated the physiologic relevance of cyclic AMP exported from cells. We report evidence of a relationship between blood pressure and the transport of intracellular cyclic AMP from lymphocytes. Twenty-eight hypertensive and 56 normotensive white and black volunteers (mean age 40 years) were studied. Both intra- and extracellular concentrations of cyclic AMP were determined in lymphocytes following incubation with 10(-5) M isoproterenol. Compared to normotensives, hypertensives (p = 0.001), particularly white hypertensives (p = 0.023) had higher levels of exported cyclic AMP. These values were independent of intracellular concentrations of cyclic AMP, which were similar across the groups. Exported cyclic AMP was independent of both sodium excretion and beta-adrenergic receptor sensitivity, the latter being lower in white hypertensives (p = 0.024). Across all subjects, exported cyclic AMP was correlated with MAP (r = .39, p < 0.001). These findings indicate that the active transport of cyclic AMP may be enhanced in hypertension and suggest a possible pathway which might explain existing data of increased cyclic AMP levels in hypertension.

Evidence for an extra-cellular function for protein kinase A

In addition to its intra-cellular functions, cAMP-dependent protein kinase (PKA) may well have an extra-cellular regulatory role in blood. This suggestion is based on the following experimental findings: (a) Physiological stimulation of blood platelets brings about a specific release of PKA, together with its co-substrates ATP and Mg++; (b) In human serum, an endogenous phosphorylation of one protein (p75, M(r) 75 kDa) occurs; this phosphorylation is enhanced by addition of cAMP and blocked by the Walsh-Krebs specific PKA inhibitor; (c) No endogenous phosphorylation of p75 occurs in human plasma devoid of platelets, but the selective labeling of p75 can be reproduced by adding to plasma the pure catalytic subunit of PKA; (d) p75 was shown to be vitronectin (V), a multifunctional protein implicated in processes associated with platelet activation, and thus a protein whose function may require modulation for control; (e) The phosphorylation of vitronectin occurs at one site (Ser378) which, at physiological pH, is buried in its two-chain form (V65 + 10) but it becomes 'exposed' in the presence of glycosaminoglycans (GAGs) e.g. heparin or heparan sulfate. Such a transconformation may be used for targeting the PKA phosphorylation to vitronectin molecules bound to GAGs, for example in the extracellular matrix or on cell surfaces; (f) From the biochemical point of view (Km values and physiological concentrations) the phosphorylation of vitronectin can take place at the locus of a hemostatic event; (g) The phosphorylation of Ser378 in vitronectin alters its function, since it significantly reduces its ability to bind the inhibitor-1 of plasminogen activator(s) (PAI-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 1-adrenoceptor-mediated inhibition of cellular cAMP accumulation in neonatal rat ventricular myocytes

We studied adrenergic regulation of cellular cAMP in neonatal rat ventricular myocytes. Since cAMP content depends on synthesis, breakdown and egress, the contribution of each of these mechanisms was assessed. In the presence of the phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine, cAMP accumulation stimulated by the beta-adrenoceptor agonist (-)-isoprenaline was diminished when the mixed alpha + beta adrenoceptor agonist (-)-noradrenaline was coincubated with (-)-isoprenaline. Moreover, adenylyl cyclase activation stimulated by (-)-isoprenaline was decreased by (-)-noradrenaline and by the selective alpha 1-adrenoceptor agonists (-)-phenylephrine and methoxamine, suggesting that alpha-adrenoceptor agonism regulates cAMP metabolism through its effect on the synthetic pathway. Evidence for alpha 1-adrenoceptor mediation of this response was enhancement of (-)-noradrenaline-induced cAMP generation by the selective alpha 1-adrenoceptor antagonist terazosin (10 nmol/l). The selective alpha 2-adrenoceptor antagonist yohimbine (10 nmol/l) had no effect. The alpha 1-adrenoceptor mediated depression of (-)-isoprenaline-stimulated cAMP generation and adenylyl cyclase activation was prevented by terazosin and in separate experiments markedly enhanced by pertussis toxin pretreatment, suggesting involvement of a guanine-nucleotide regulatory protein in this process. Occupation of the alpha 1-adrenoceptor by (-)-noradrenaline did not accelerate the rate of cAMP breakdown in the absence of phosphodiesterase inhibition. Furthermore, there was no enhancement of total phosphodiesterase activity by (-)-noradrenaline in the presence of (-)-propranolol. By contrast, pertussis toxin pretreatment augmented phosphodiesterase activity. Neither pertussis toxin nor (-)-noradrenaline increased cAMP egress. We conclude that in rat neonatal cardiac myocytes agonist occupation of the alpha 1-adrenoceptor inhibits beta-adrenoceptor stimulated cAMP accumulation most likely by coupling to a guanine nucleotide inhibitory protein.

Atrial natriuretic peptide is phosphorylated by intact cells through cAMP-dependent ecto-protein kinase

Recently we demonstrated the presence of cell-surface-located cAMP-dependent protein kinase (ecto-PK A) activity in a number of different cell types [Kübler, D., Pyerin, W., Bill, O., Hotz, A., Sonka, J. and Kinzel, V. (1989) J. Biol. Chem. 264, 14549-14555]. The question of the physiological role of externally directed kinase activity prompted a search for potential natural substrates present in the intercellular fluid. In the present study we have investigated the phosphorylation by ecto-PK A of the human atrial natriuretic peptide ANP99-126, a hormone released by cardiac cells. This 28-amino-acid peptide carries the phosphorylation consensus sequence Arg-Arg-Ser-Ser for the PK A. Incubation of various cell lines (including epithelial, epidermal, myoblast and lymphoma cells) or freshly isolated blood cells (macrophages, erythrocytes and platelets) with ANP in the presence of low micromolar concentrations of ATP resulted in the phosphorylation of ANP at Ser residues. The ANP phosphorylation reaction proved strictly dependent on cAMP; cAMP could not be replaced by cGMP. The phosphorylation was inhibited by the PK A-specific inhibitory peptide and increased linearily for up to 15 min and with a Km value of 3-5 microM for ANP. At higher ATP concentrations (greater than 100 microM) the incorporation rates amounted to about 0.3 mmol P (mol ANP)-1 min-1. The rise of intracellular cAMP in HEL30 (an epidermal cell line) after application of the beta-adrenergic receptor agonist isoproterenol led to an approximately three-fold stimulation of ANP phosphorylation which appears to be brought about by an efflux of intracellular cAMP. Employing cell supernatant fluids and cell sonicates, it could be shown that the phosphorylation of ANP results from the ecto-PK A. Comparison of ANP with ANP phosphorylated in vitro using purified catalytic subunit of PK A showed that phosphorylation is accompanied by certain changes in the average solution conformation of the peptide, consistent with the changes known to occur in its biological activity. Our results demonstrate cAMP-dependent phosphorylation of the peptide hormone analogue ANP99-126 by intact cells through ecto-PK A, an intriguing mechanism for post-translational processing of ANP.

A cAMP-binding ectoprotein in the yeast Saccharomyces cerevisiae

Purified plasma membranes from the yeast Saccharomyces cerevisiae bind about 1.2 pmol of cAMP/mg of protein with high affinity (Kd = 6 nM). By using photoaffinity labeling with 8-N3-[32P]cAMP, we have identified in plasma membrane vesicles a cAMP-binding protein (Mr = 54,000) that is present also in bcy1 disruption mutants, lacking the cytoplasmic R subunit of protein kinase A (PKA). This argues that it is genetically unrelated to PKA. Neither high salt, nor alkaline carbonate, nor cAMP extract the protein from the membrane, suggesting that it is not peripherally bound. The observation that (glycosyl)phosphatidylinositol-specific phospholipases (or nitrous acid) release the amphiphilic protein from the membrane, thereby converting it to a hydrophilic form, indicates anchorage by a glycolipidic membrane anchor. Treatment with N-glycanase reduces the Mr to 44,000-46,000 indicative of a modification by N-linked carbohydrate side chain(s). In addition to the action of a phospholipase, the efficient release from the membrane requires the removal of the carbohydrate side chain(s) or the presence of high salt or methyl alpha-mannopyranoside, suggesting complex interactions with the membrane involving not only the glycolipidic anchor but also the glycan side chain(s). Topological studies show that the protein is exposed to the periplasmic space, raising intriguing questions for the function of this protein.

Involvement of a cell surface protein and an ecto-protein kinase in myogenesis

Myogenic differentiation is composed of a sequential cascade of multiple steps leading to the formation of multinucleated myotubes. The interference with any one step would abolish myogenesis. The present investigation examined the cell surface components which might be involved in myogenesis. Studies with subconfluent day 2 cultures of rat L6 myoblasts revealed that a cell surface 112 kDa protein was phosphorylated by a Ca(2+)-, F(-)- and Mg(2+)-dependent ecto-protein kinase [Chen & Lo (1991) Biochem. J. 279, 467-474]. We have shown in the present investigation that adequate ATP was present on the cell surface for efficient functioning of this ecto-protein kinase. The phosphorylation of the 112 kDa protein by this ecto-protein kinase was decrease dramatically in confluent cells and in multinucleated myotubes. The following evidence suggests that both the 112 kDa protein and the ecto-protein kinase may play important roles in myogenesis. (i) The highest phosphorylation activity was observed in subconfluent cultures, i.e. before the onset of morphological differentiation. (ii) Treatment of cells with chemical reagents resulted in a corresponding decrease in the ecto-protein kinase, the 112 kDa protein, the phosphorylated 112 kDa protein (p112) and the ability to form myotubes. (iii) The level of p112 in a conditional myogenesis-defective mutant corresponded with the cells' eventual ability to differentiate. (iv) A mutant defective in the ecto-protein kinase was impaired in the phosphorylation of the 112 kDa protein and in myogenesis. (v) A mutant containing only residual levels of the 112 kDa protein was deficient in both p112 and myogenesis. (vi) Since the level of p112 was normal in another myogenesis-defective mutant, the phosphorylation of this protein was not likely to be a consequence of myogenic differentiation. The above findings suggest that the ecto-protein kinase and the 112 kDa protein may directly or indirectly be associated with the myogenic pathway. Since the levels of the ecto-protein kinase, the 112 kDa protein and p112 decreased dramatically upon the formation of myotubes, these proteins were probably not required once morphological differentiation had been initiated.

A potential mechanism for copper amplification of prostaglandin E2 action: attenuation of prostaglandin E2-induced efflux of cyclic AMP from median eminence explants

It is known that prostaglandin E2 (PGE2) stimulation of LHRH release from the median eminence-arcuate nucleus (MEA) is mediated by the cAMP pathway, and a short pretreatment with copper markedly amplifies this release process. Because stimulation of cAMP accumulation is accompanied by cAMP efflux in many tissues, we considered the possibility that attenuation of cAMP efflux is one mechanism by which copper can enhance PGE2 action. When rat MEA explants were incubated in vitro, PGE2 induced a rapid (less than 2.5 min) and sustained (15 min) increase in cAMP efflux, the degree of which was a function of [PGE2]: by 5 min exposure to 10 microM PGEs2, efflux was 8-fold greater than the control (no PGE2) and it accounted for 12.4% of the total (tissue + medium) cAMP. Unlike the dramatic increase in cAMP efflux, PGE2 induced a moderate increase in cAMP content (49%) and in the incorporation of [3H] adenine into [3H] cAMP (78%); this increase was transient: it was evident after a 2.5 but not after a 5 min period of PGE2 exposure. Copper pretreatment did not alter this PGE2-induced increase in tissue cAMP content. In contrast, copper markedly inhibited (by 49%-66%) PGE2-induced cAMP efflux and this inhibition was noted regardless of the length of PGE2 exposure and PGE2 concentration. There was no evidence for hydrolysis of [3H]3',5'-cAMP included in the medium during the incubation with PGE2 with and without copper pretreatment. In summary, copper attenuated PGE2-induced cAMP efflux from MEA explants and this attenuation is not a consequence of a reduction in the availability of intracellular cAMP nor of hydrolysis of cAMP extracellularly.

Contraluminal p-aminohippurate transport in the proximal tubule of the rat kidney. VII. Specificity: cyclic nucleotides, eicosanoids

Using the stop-flow peritubular capillary microperfusion method the inhibitory potency (apparent Ki values) of cyclic nucleotides and prostanoids against contraluminal p-aminohippurate (PAH), dicarboxylate and sulphate transport was evaluated. Conversely the contraluminal transport rate of labelled cAMP, cGMP, prostaglandin E2, and prostaglandin D2 was measured and the inhibition by different substrates was tested. Cyclic AMP and its 8-bromo and dibutyryl analogues inhibited contraluminal PAH transport with an app. Ki,PAH of 3.4, 0.63 and 0.52 mmol/l. The respective app. Ki,PAH values of cGMP and its analogues are with 0.27, 0.04 and 0.05 mmol/l, considerably lower. None of the cyclic nucleotides tested interacted with contraluminal dicarboxylate, sulphate and N1-methylnicotinamide transport. ATP, ADP, AMP, adenosine and adenine as well as GTP, GDP, GMP, guanosine and guanine did not inhibit PAH transport while most of the phosphodiesterase inhibitors tested did. Time-dependent contraluminal uptake of [3H]cAMP and [3H]cGMP was measured at different starting concentrations and showed facilitated diffusion kinetics with the following parameters for cAMP: Km = 1.5 mmol/l, Jmax = 0.34 pmol S-1 cm-1, r (extracellular/intracellular amount at steady state) = 0.91; for cGMP: Km = 0.29 mmol/l, Jmax = 0.31 pmol S-1 cm-1, r = 0.55. Comparison of app. Ki,cGMP with app. Ki,PAH of ten substrates gave a linear relation with a ratio of 1.83 +/- 0.5. All prostanoids applied inhibited the contraluminal PAH transport; the prostaglandins E1, F1 alpha, A1, B1, E2, F2 alpha, D2, A2 and B2 with an app. Ki,PAH between 0.08 and 0.18 mmol/l. The app. Ki of the prostacyclins 6,15-diketo-13,14-dihydroxy-F1 alpha (0.22 mmol/l) and Iloprost (0.17 mmol/l) as well as that of leukotrienes B4 (0.2 mmol/l) was in the same range, while the app. Ki,PAH of the prostacyclins PGI2 (0.55 mmol/l), 6-keto-PGF1 alpha (0.77 mmol/l) and 2,3-dinor-6-keto-PGF1 alpha (0.57 mmol/l) as well as that of thromboxane B2 (0.36 mmol/l) was somewhat higher. None of these prostanoids inhibited contraluminal dicarboxylate transport and only PGB1, E2 and D2 inhibited contraluminal sulphate transport (app. Ki,SO4(2-) 5.4, 11.0, 17.9 mmol/l respectively). Contraluminal influx of labelled PGE2 showed complex transport kinetics with a mixed Km = 0.61 mmol/l and Jmax of 4.26 pmol S-1 cm-1. It was inhibited by probenecid, sulphate and indomethacin. Contraluminal influx of PGD2, however, was only inhibited by probenecid. The data indicate that cyclic nucleotides as well as prostanoids are transported by the contraluminal PAH transporter. For prostaglandin E2 a significant uptake through the sulphate transporter occurs in addition.(ABSTRACT TRUNCATED AT 400 WORDS)