AMP converted from intracellularly transported adenosine upregulates p53 expression to induce malignant pleural mesothelioma cell apoptosis

BACKGROUND/AIMS

The present study investigated adenosine-induced apoptosis in human malignant pleural mesothelioma cells.

METHODS

MTT assay, TUNEL staining, flow cytometry using propidium iodide and annexin V-FITC, real-time RT-PCR, Western blotting, and assay of caspase-3, -8, and -9 activities were carried out using malignant pleural mesothelioma cell lines such as NCI-H28, NCI-H2052, NCI-H2452, and MSTO-211H cells, and p53 or A(3) adenosine receptor was knocked-down by transfecting each siRNA into cells.

RESULTS

Adenosine induced apoptosis in all the malignant pleural mesothelioma cells used here, independently of caspase activation. The adenosine effect was prevented by the adenosine transporter inhibitor dipyridamole, the adenosine kinase inhibitor ABT-702, or the A(3) adenosine receptor inhibitor MRS1191. Adenosine upregulated expression of the p53 mRNA and protein, that is abolished by ABT-702, but not by knocking-down A(3) adenosine receptor. Adenosine-induced apoptosis in NCI-H28 cells was significantly inhibited by knocking-down p53 and in part by knocking-down A(3) adenosine receptor.

CONCLUSION

The results of the present study show that AMP converted from intracellularly transported adenosine upregulates p53 expression to induce caspase-independent apoptosis in malignant pleural mesothelioma cells and that A(3) adenosine receptor also participates partially in the apoptosis by the different mechanism.

Aging of Xenopus tropicalis eggs leads to deadenylation of a specific set of maternal mRNAs and loss of developmental potential

As first shown more than 100 years ago, fertilization of an aged (overripe) egg increases the rate of malformations and embryonic loss in several vertebrates, including possibly humans as well. Since the molecular events in aging eggs may be similar in these species, we established in the frog Xenopus tropicalis a defined protocol for delayed fertilization of eggs. A three-hour delayed fertilization led to a dramatic increase in malformation and mortality. Gene expression profiling revealed that 14% of the polyadenylated maternal transcripts were downregulated upon aging. These transcripts were not degraded, but rather deadenylated as shown for specific maternal mRNAs. The affected transcripts are characterized by a relatively short 3′UTR and a paucity of cytoplasmic polyadenylation elements (CPE) and polyadenylation signals (PAS). Furthermore, maternal mRNAs known to be deadenylated during egg maturation as well as after fertilization were preferentially deadenylated in aged eggs. Taken together our analysis of aging eggs reveals that unfertilized eggs are in a dynamic state that was previously not realized. On the one hand deadenylation of transcripts that are typically deadenylated during egg maturation continues and this implies overripeness of the aged egg in the truest sense of the word. On the other hand transcripts that normally are deadenylated after fertilization loose their poly(A) in the aged egg and this implies that the egg awaiting fertilization starts processes that are normally only observed after fertilization. Based on our novel finding we postulate that the imbalance of the polyadenylated maternal transcripts upon egg aging contributes to the loss of developmental potential. Based on this hypothesis the developmental consequences of downregulation of specific transcripts can be analyzed in future.

Dysregulation of glycogen synthase COOH- and NH2-terminal phosphorylation by insulin in obesity and type 2 diabetes mellitus

CONTEXT

Insulin-stimulated glucose disposal is impaired in obesity and type 2 diabetes mellitus (T2DM) and is tightly linked to impaired skeletal muscle glucose uptake and storage. Impaired activation of glycogen synthase (GS) by insulin is a well-established defect in both obesity and T2DM, but the underlying mechanisms remain unclear.

DESIGN AND PARTICIPANTS

Insulin action was investigated in a matched cohort of lean healthy, obese nondiabetic, and obese type 2 diabetic subjects by the euglycemic-hyperinsulinemic clamp technique combined with muscle biopsies. Activity, site-specific phosphorylation, and upstream signaling of GS were evaluated in skeletal muscle.

RESULTS

GS activity correlated inversely with phosphorylation of GS site 2+2a and 3a. Insulin significantly decreased 2+2a phosphorylation in lean subjects only and induced a larger dephosphorylation at site 3 in lean compared with obese subjects. The exaggerated insulin resistance in T2DM compared with obese subjects was not reflected by differences in site 3 phosphorylation but was accompanied by a significantly higher site 1b phosphorylation during insulin stimulation. Hyperphosphorylation of another Ca(2+)/calmodulin-dependent kinase-II target, phospholamban-Thr17, was also evident in T2DM. Dephosphorylation of GS by phosphatase treatment fully restored GS activity in all groups.

CONCLUSIONS

Dysregulation of GS phosphorylation plays a major role in impaired insulin regulation of GS in obesity and T2DM. In obesity, independent of T2DM, this is associated with impaired regulation of site 2+2a and likely site 3, whereas the exaggerated insulin resistance to activate GS in T2DM is linked to hyperphosphorylation of at least site 1b. Thus, T2DM per se seems unrelated to defects in the glycogen synthase kinase-3 regulation of GS.

Hormonal and purinergic stimulation of bicarbonate secretion in oviducts of rhesus monkey

Because an increase in the HCO(3)(-) concentration of oviductal liquid at midcycle is believed to markedly enhance fertility, we have studied active secretion of HCO(3)(-) across highly differentiated cultures of monkey oviductal epithelium. Cultured cell sheets were mounted in Ussing chambers and bathed in medium containing 25 mM HCO(3)(-). Purinergic agents potently stimulated short-circuit current (I(sc)) with an initial transient response declining within approximately 2 min to a sustained response. The potency sequence of ATP approximately UTP > ADP >> AMP suggested that the I(sc) response was mediated mainly by P2Y(2) receptors. Acetazolamide, an inhibitor of carbonic anhydrase, had little or no effect on baseline I(sc) or the transient response to ATP but abolished the sustained response to ATP. Similar results were obtained on sheets of native epithelium. In pH-stat experiments, the abluminal medium of cell cultures was bathed in HCO(3)(-)-CO(2) medium, and the pH of the unbuffered luminal medium was maintained at approximately 7.4 by addition of strong acid or base. ATP stimulated base secretion, and this was inhibited by acetazolamide. Furthermore, these changes in secretion of base were in good quantitative agreement with the I(sc) responses. When phenol red (an estrogen) was removed from the culture medium, ATP-dependent HCO(3)(-) secretion was markedly reduced but could be restored by treatment with estradiol. Estrogens also markedly increased ciliation of the cultures. These results suggest that the midcycle increase in the HCO(3)(-) concentration of oviductal liquid may be mediated by the effects of estradiol on purinergic pathways or on ATP secretion.

[Regulation of energy metabolism by AMPK: a novel therapeutic approach for the treatment of metabolic and cardiovascular diseases]

The 5' AMP-activated protein kinase (AMPK) is a sensor of cellular energy homeostasis well conserved in all eukaryotic cells. AMPK is activated by rising AMP and falling ATP, either by inhibiting ATP production or by accelerating ATP consumption, by a complex mechanism that results in an ultrasensitive response. AMPK is a heterotrimeric enzyme complex consisting of a catalytic subunit alpha and two regulatory subunits beta and gamma. AMP activates the system by binding to the gamma subunit that triggers phosphorylation of the catalytic alpha subunit by the upstream kinases LKB1 and CaMKKbeta. Once activated, it switches on catabolic pathways (such as fatty acid oxidation and glycolysis) and switches off ATP-consuming pathways (such as lipogenesis) both by short-term effect on phosphorylation of regulatory proteins and by long-term effect on gene expression. Dominant mutations in the regulatory gamma subunit isoforms cause hypertrophy of cardiac and skeletal muscle providing a link in human diseases caused by defects in energy metabolism. As well as acting at the level of the individual cell, the system also regulates food intake and energy expenditure at the whole body level, in particular by mediating the effects of adipokines such as leptin and adiponectin. Moreover, the AMPK system is one of the probable target for the anti-diabetic drug metformin and rosiglitazone. The relationship between AMPK activation and beneficial metabolic effects provides the rationale for the development of new therapeutic strategies. Thus, pharmacological AMPK activation may, through signaling, metabolic and gene expression effects, reduce the risk of Type 2 diabetes, metabolic syndrome and cardiac diseases.

Adenylate-coupled ion movement. A mechanism for the control of nodule permeability to O2 diffusion

In response to changes in phloem supply, adenylate demand, and oxygen status, legume nodules are known to exercise rapid (seconds to hours) physiological control over their permeability to oxygen diffusion. Diffusion models have attributed this permeability control to the reversible flow of water into or out of intercellular spaces. To test hypotheses on the mechanism of diffusion barrier control, nodulated soybean (Glycine max L. Merr.) plants were exposed to a range of treatments known to alter nodule O2 permeability (i.e. 10% O2, 30% O2, Ar:O2 exposure, and stem girdling) before the nodules were rapidly frozen, freeze dried, and dissected into cortex and central zone (CZ) fractions that were assayed for K, Mg, and Ca ion concentrations. Treatments known to decrease nodule permeability (30% O2, Ar:O2 exposure, and stem girdling) were consistently associated with an increase in the ratio of [K+] in cortex to [K+] in the CZ tissue, whereas the 10% O2 treatment, known to increase nodule permeability, was associated with a decrease in the [K+]cortex:[K+](CZ). When these findings were considered in the light of previous results, a proposed mechanism was developed for the adenylate-coupled movement of ions and water into and out of infected cells as a possible mechanism for diffusion barrier control in legume nodules.

Early rise in exhaled nitric oxide and mast cell activation in repeated low-dose allergen challenge

Repeated low-dose allergen inhalation challenge mimics natural allergen exposure, providing a model for early mechanisms in the triggering of asthma. The current authors performed a controlled study to evaluate the time course of changes in exhaled nitric oxide fraction (F(e,NO)) and urinary biomarkers of airway inflammation. Eight subjects with mild allergic asthma completed two 7-day repeated low-dose challenge periods, with diluent and allergen, respectively. Subjects were symptom free at inclusion and were investigated when not exposed to specific allergen. Pulmonary function and symptoms were followed, and F(e,NO) and urinary mediators were correlated to changes in airway responsiveness to histamine and adenosine. Despite no change in pulmonary function (forced expiratory volume in one second mean+/-sem fall 0.3+/-0.7 versus 0.6+/-1.0%, for diluent and allergen, respectively) and no asthma symptoms, repeated allergen exposure, in contrast to diluent, caused significant increases in histamine responsiveness (2.3 doubling doses), an early and gradual increase in F(e,NO) (up to a doubling from baseline) and a small increase in the mast cell marker 9alpha11beta-prostaglandin F(2) after adenosine challenge. In conclusion, serial measurements of exhaled nitric oxide fraction have the potential to provide a very sensitive strategy for early detection of emerging airway inflammation and subsequent changes in airway hyperresponsiveness to histamine.

N488I mutation of the gamma2-subunit results in bidirectional changes in AMP-activated protein kinase activity

Mutations in the human gene encoding the nucleotide-binding region in the gamma-subunit of AMP-activated protein kinase (AMPK) cause cardiomyopathy with preexcitation syndrome. Mutant AMPK showed reduced binding affinity to nucleotides in vitro raising the possibility that altered regulation of AMPK activity by AMP/ATP could contribute to the disease phenotype. In this study, we determined the sensitivity of AMPK activity to AMP/ATP in the beating hearts using transgenic mice expressing a mutant (N488I, gamma2-mutant) or wild-type gamma2-subunit (gamma2-TG). The [ATP] and [AMP] were unaltered in all hearts but the AMPK activity was increased by 2.5-fold in gamma2-mutant hearts freeze-clamped at normal AMP/ATP compared with nontransgenic (WT) or gamma2-TG. The increased basal AMPK activity was caused by increased Thr-172 phosphorylation of the alpha-subunit (p-AMPK, by 4-fold) at normal [ATP] and was not changed by reducing glycogen content by 60% in the gamma2-mutant hearts. A reversal of AMP/ATP, caused by ATP degradation, increased p-AMPK by 7-fold in WT but caused no change in gamma2-mutant hearts. These results demonstrate that the mutation renders AMPK insensitive to the inhibitory and stimulatory effects of the regulatory nucleotides ATP and AMP, respectively, suggesting that the pathogenesis of the human disease may not be attributable to a simple loss- or gain-of-function.

Pyruvate prevents cardiac dysfunction and AMP-activated protein kinase activation by hydrogen peroxide in isolated rat hearts

Ischemia-reperfusion injury in the heart results in enhanced production of H2O2 and activation of AMP-activated protein kinase (AMPK). Since mutations in AMPK result in cardiovascular dysfunction, we investigated whether the activation of AMPK mediates the H2O2-induced reduction in cardiac mechanical function. Isolated working rat hearts were perfused at 37 degrees C with Krebs-Henseleit solution. Following a 20-minute equilibration period, a single bolus of H2O2 (300 micromol/L) was added and the hearts were perfused for an additional 5 min. H2O2 induced a dramatic and progressive reduction in cardiac function. This was accompanied by rapid and significant activation of AMPK, an increase in Thr-172 phosphorylation of AMPK, and an increase in the creatine to phosphocreatine (Cr/PCr) ratio. Addition of pyruvate (5 mmol/L) to the perfusate prevented the H2O2-mediated reduction in cardiac mechanical dysfunction, activation of myocardial AMPK activity, increase in AMPK phosphorylation and the increase in the Cr/PCr ratio. Hearts challenged with H2O2 (300 micromol/L) in presence of either AMPK inhibitor Compound C (10 micromol/L) or its vehicle (dimethyl sulfoxide (DMSO), 0.1%) showed reduced impairment in cardiac mechanical function. Compound C but not its vehicle significantly inhibited myocardial AMPK activity. Thus, H2O2 induces cardiac dysfunction via both AMPK-dependent and independent mechanisms.

Depression of cell metabolism and proliferation by membrane-permeable and -impermeable modulators: role for AMP-to-ATP ratio

The metabolic and developmental depression commonly observed during natural states of dormancy, such as diapause and quiescence, is typically accompanied by an increase in the intracellular ratio of AMP to ATP. We investigated the impact of artificially increasing the AMP-to-ATP ratio in mouse macrophages. Evidence is presented here that the P2X7 receptor channel can be used as an effective means to load cells with membrane-impermeable compounds. Intracellular loading of adenosine-5'-O-thiomonophosphate (AMPS), a nonhydrolyzable analog of 5'-AMP and potent activator of AMP-activated protein kinase, significantly depresses metabolism and proliferation of macrophages. The intracellular effective AMP-to-ATP ratio obtained (the sum of AMPS plus endogenous 5'-AMP) was 0.073, well above that reported to activate AMP-activated protein kinase in vitro. Optimizing both the conditions under which the P2X7 receptor channel is opened and the duration of opening facilitates high analog uptake and approximately 98% survivorship. An advantage to AMPS is its minimal impact on other components of the nucleotide pool, most notably the unchanged concentration of ADP. An alternative way to shift the effective AMP-to-ATP ratio is by incubation with the membrane-permeable compound 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR), which is phosphorylated intracellularly to form the 5'-AMP analog ZMP. Despite a rapid intracellular accumulation of AICAR, conversion to ZMP was slow and inefficient. Furthermore, AICAR incubation increased cellular ADP, and, although cell proliferation was depressed, the overall cellular energy flow was unchanged. The rapid action of AMPS avoids upregulation of compensatory metabolic pathways and may provide a viable approach for promoting cell stasis.

[Activation of the alternative oxidase of Yarrowia lipolytica by adenosine 5'-monophosphate]

The study of the effect of nucleoside phosphates on the activity of cyanide-resistant oxidase in the mitochondria and the submitochondrial particles of Yarrowia lipolytica showed that adenosine monophosphate (5'-AMP, AMP) did not stimulate the respiration of the intact mitochondria. The incubation of the mitochondria at room temperature (25 degrees C) for 3-5 h or their treatment with ultrasound, phospholipase A, and detergent Triton X-100 at a low temperature inactivated the cyanide-resistant alternative oxidase. The inactivated alternative oxidase could be reactivated by AMP. The reactivating effect of AMP was enhanced by azolectin. Some other nucleoside phosphates also showed reactivating ability in the following descending order. AMP = GMP > GDP > GTP > XMP > IMP. The apparent reaction rate constant Km for AMP upon the reactivation of the alternative oxidase of mitochondria treated with Triton X-100 or incubated at 25 degrees C was 12.5 and 20 microM, respectively. The Km for AMP upon the reactivation of the alternative oxidase of submitochondrial particles was 15 microM. During the incubation of yeast cells under conditions promoting the development of alternative oxidase, the content of adenine nucleotides (AMP, ADP, and ATP) in the cells and their respiration tended to decrease. The subsequent addition of cyanide to the cells activated their respiration, diminished the intracellular content of ATP three times, and augmented the content of AMP five times. These data suggest that the stimulation of cell respiration by cyanide may be due to the activation of alternative oxidase by AMP.

Metabolic analysis of senescent human fibroblasts reveals a role for AMP in cellular senescence

Cellular senescence is considered a major tumour-suppressor mechanism in mammals, and many oncogenic insults, such as the activation of the ras proto-oncogene, trigger initiation of the senescence programme. Although it was shown that activation of the senescence programme involves the up-regulation of cell-cycle regulators such as the inhibitors of cyclin-dependent kinases p16INK4A and p21CIP-1, the mechanisms underlying the senescence response remain to be resolved. In the case of stress-induced premature senescence, reactive oxygen species are considered important intermediates contributing to the phenotype. Moreover, distinct alterations of the cellular carbohydrate metabolism are known to contribute to oncogenic transformation, as is best documented for the phenomenon of aerobic glycolysis. These findings suggest that metabolic alterations are involved in tumourigenesis and tumour suppression; however, little is known about the metabolic pathways that contribute to these processes. Using the human fibroblast model of in vitro senescence, we analysed age-dependent changes in the cellular carbohydrate metabolism. Here we show that senescent fibroblasts enter into a metabolic imbalance, associated with a strong reduction in the levels of ribonucleotide triphosphates, including ATP, which are required for nucleotide biosynthesis and hence proliferation. ATP depletion in senescent fibroblasts is due to dysregulation of glycolytic enzymes, and finally leads to a drastic increase in cellular AMP, which is shown here to induce premature senescence. These results suggest that metabolic regulation plays an important role during cellular senescence and hence tumour suppression.

Isoform-specific regulation of 5' AMP-activated protein kinase in skeletal muscle from obese Zucker (fa/fa) rats in response to contraction

Glucose transport can be activated in skeletal muscle in response to insulin via activation of phosphoinositide (PI) 3-kinase and in response to contractions or hypoxia, presumably via activation of 5' AMP-activated protein kinase (AMPK). We determined the effects of insulin and muscle contraction/hypoxia on PI 3-kinase, AMPK, and glucose transport activity in epitrochlearis skeletal muscle from insulin-resistant Zucker (fa/ fa) rats. Insulin-stimulated glucose transport in isolated skeletal muscle was reduced 47% in obese versus lean rats, with a parallel 42% reduction in tyrosine-associated PI 3-kinase activity. Contraction and hypoxia elicited normal responses for glucose transport in skeletal muscle from insulin-resistant obese rats. Isoform-specific AMPK activity was measured in skeletal muscle in response to insulin, contraction, or hypoxia. Contraction increased AMPKalpha1 activity 2.3-fold in lean rats, whereas no effect was noted in obese rats. Hypoxia increased AMPKalpha1 activity to a similar extent (more than sixfold) in lean and obese rats. Regardless of genotype, contraction, and hypoxia, each increased AMPKalpha2 activity more than fivefold, whereas insulin did not alter either AMPKalpha1 or -alpha2 activity in skeletal muscle. In conclusion, obesity-related insulin resistance is associated with an isoform-specific impairment in AMPKalpha1 in response to contraction. However, this impairment does not appear to affect contraction-stimulated glucose transport. Activation of AMPKalpha2 in response to muscle contraction/ exercise is associated with a parallel and normal increase in glucose transport in insulin-resistant skeletal muscle.

Evolving concepts on the value of adenosine hyperresponsiveness in asthma and chronic obstructive pulmonary disease

Adenosine is a purine nucleoside which mediates a variety of cellular responses relevant to asthma and COPD through interaction with specific receptors. Administration of adenosine by inhalation to patients with asthma and COPD is known to cause concentration related bronchoconstriction. Responses elicited by this purine derivative in asthma and COPD should not be considered as a mere reflection of non-specific airways hyperresponsiveness. Evaluation of airways responsiveness by adenosine induced bronchoconstriction may be valuable in differentiating asthma from COPD, monitoring of anti-inflammatory treatment in asthma, surveying disease progression, and assessing disease activity in relation to allergic airways inflammation.

A potential role for AMP-activated protein kinase in meiotic induction in mouse oocytes

Cyclic adenosine monophosphate (cAMP) has been implicated as an important regulator of meiotic maturation in mammalian oocytes. A decrease in cAMP, brought about by the action of cAMP phosphodiesterase (PDE), is thought to initiate germinal vesicle breakdown (GVB) by the inactivation of cAMP-dependent protein kinase. However, the product of PDE activity, 5'-AMP, is a potent activator of an important regulatory enzyme, AMP-activated protein kinase (AMPK). The aim of this study was to evaluate a possible role for AMPK in meiotic induction, using oocytes obtained from eCG-primed, immature mice. Alpha-1 and -2 isoforms of the catalytic subunit of AMPK were detected in both oocytes and cumulus cells. When 5-aminoimidazole-4-carboxamide 1-beta-d-ribofuranoside (AICA riboside), an activator of AMPK, was tested on denuded oocytes (DO) and cumulus cell-enclosed oocytes (CEO) maintained in meiotic arrest by dbcAMP or hypoxanthine, GVB was dose-dependently induced. Meiotic induction by AICA riboside in dbcAMP-supplemented medium was initiated within 3 h in DO and 4 h in CEO and was accompanied by increased AMPK activity in the oocyte. AICA riboside also triggered GVB when meiotic arrest was maintained with hypoxanthine, 8-AHA-cAMP, guanosine, or milrinone, but was ineffective in olomoucine- or roscovitine-arrested oocytes, indicating that it acts upstream of maturation-promoting factor. Adenosine monophosphate dose-dependently stimulated GVB in DO when meiotic arrest was maintained with dbcAMP or hypoxanthine. This effect was not mimicked by other monophosphate or adenosine nucleotides and was not affected by inhibitors of ectophosphatases. Combined treatment with adenosine and deoxycoformycin, an adenosine deaminase inhibitor, stimulated GVB in dbcAMP-arrested CEO, suggesting AMPK activation due to AMP accumulation. It is concluded that phosphodiesterase-generated AMP may serve as a transducer of the meiotic induction process through activation of AMPK.

Glycogen-dependent effects of 5-aminoimidazole-4-carboxamide (AICA)-riboside on AMP-activated protein kinase and glycogen synthase activities in rat skeletal muscle

5'-AMP-activated protein kinase (AMPK) functions as a metabolic switch in mammalian cells and can be artificially activated by 5-aminoimidazole-4-carboxamide (AICA)-riboside. AMPK activation during muscle contraction is dependent on muscle glycogen concentrations, but whether glycogen also modifies the activation of AMPK and its possible downstream effectors (glycogen synthase and glucose transport) by AICA-riboside in resting muscle is not known. Thus, we have altered muscle glycogen levels in rats by a combination of swimming exercise and diet and investigated the effects of AICA-riboside in the perfused rat hindlimb muscle. Two groups of rats, one with super-compensated muscle glycogen content (approximately 200-300% of normal; high glycogen [HG]) and one with moderately lowered muscle glycogen content (approximately 80% of normal; low glycogen [LG]), were generated. In both groups, the degree of activation of the alpha2 isoform of AMPK by AICA-riboside depended on muscle type (white gastrocnemius >> red gastrocnemius > soleus). Basal and AICA-riboside-induced alpha2-AMPK activity were markedly lowered in the HG group (approximately 50%) compared with the LG group. Muscle 2-deoxyglucose uptake was also increased and glycogen synthase activity decreased by AICA-riboside. Especially in white gastrocnemius, these effects, as well as the absolute activity levels of AMPK-alpha2, were markedly reduced in the HG group compared with the LG group. The inactivation of glycogen synthase by AICA-riboside was accompanied by decreased gel mobility and was eliminated by protein phosphatase treatment. We conclude that acute AICA-riboside treatment leads to phosphorylation and deactivation of glycogen synthase in skeletal muscle. Although the data do not exclude a role of other kinases/phosphatases, they suggest that glycogen synthase may be a target for AMPK in vivo. Both basal and AICA-riboside-induced AMPK-alpha2 and glycogen synthase activities, as well as glucose transport, are depressed when the glycogen stores are plentiful. Because the glycogen level did not affect adenine nucleotide concentrations, our data suggest that glycogen may directly affect the activation state of AMPK in skeletal muscle.

Adenosine inhibits thrombin-induced expression of tissue factor on endothelial cells by a nitric oxide-mediated mechanism

Enhanced expression of tissue factor (TF) is associated with the occurrence of coronary disease, strokes and arterial thrombosis. We demonstrated previously that adenosine inhibits TF expression in human umbilical vein endothelial cells (HUVECs) stimulated with inflammatory mediators. In the present study, we evaluated the mechanism of adenosine-induced inhibition of TF expression in HUVECs. The adenosine inhibitory activity on thrombin-induced TF expression in HUVECs was potentiated by the NO precursor, l-arginine, but it was significantly suppressed by the NO scavenger, 2(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide, and by endothelial NO synthase inhibitors, N(G)-monomethyl-l-arginine and N(G)-nitro-l-arginine methyl ester, in a dose-dependent manner. The concentrations of nitrites, cGMP and cAMP in the culture medium of HUVECs treated with a mixture of thrombin and adenosine were significantly higher compared with the culture medium of HUVECs treated with thrombin alone. Northern blotting showed that thrombin decreases and adenosine increases the eNOS mRNA expression in HUVECs. A cAMP-dependent protein kinase inhibitor suppressed NO-mediated TF inhibition in a dose-dependent manner. Overall, these results suggest that adenosine inhibits thrombin-induced TF expression in endothelial cells by a NO-mediated mechanism, and that increased intracellular formation of cAMP is implicated in this inhibitory activity of NO.

The influence of glutamate receptor 2 expression on excitotoxicity in Glur2 null mutant mice

AMPA receptor (AMPAR)-mediated ionic currents that govern gene expression, synaptic strength, and plasticity also can trigger excitotoxicity. However, native AMPARs exhibit heterogeneous pharmacological, biochemical, and ionic permeability characteristics, which are governed partly by receptor subunit composition. Consequently, the mechanisms governing AMPAR-mediated excitotoxicity have been difficult to elucidate. The GluR2 subunit is of particular interest because it influences AMPAR pharmacology, Ca(2+) permeability, and AMPAR interactions with intracellular proteins. In this paper we used mutant mice lacking the AMPAR subunit GluR2 to study AMPAR-mediated excitotoxicity in cultured cortical neurons and in hippocampal neurons in vivo. We examined the hypothesis that in these mice the level of GluR2 expression governs the vulnerability of neurons to excitotoxicity and further examined the ionic mechanisms that are involved. In cortical neuronal cultures AMPAR-mediated neurotoxicity paralleled the magnitude of kainate-evoked AMPAR-mediated currents, which were increased in neurons lacking GluR2. Ca(2+) permeability, although elevated in GluR2-deficient neurons, did not correlate with excitotoxicity. However, toxicity was reduced by removal of extracellular Na(+), the main charge carrier of AMPAR-mediated currents. In vivo, the vulnerability of CA1 hippocampal neurons to stereotactic kainate injections and of CA3 neurons to intraperitoneal kainate administration was independent of GluR2 level. Neurons lacking the GluR2 subunit did not demonstrate compensatory changes in the distribution, expression, or function of AMPARs or of Ca(2+)-buffering proteins. Thus GluR2 level may influence excitotoxicity by effects additional to those on Ca(2+) permeability, such as effects on agonist potency, ionic currents, and synaptic reorganization.

[Physiology and pharmacology of lacrymal and salivary secretions]

Salivary and lacrymal glands have secretory mechanisms similar to those of other exocrine glands. Saliva results from two different but integrated processes i.e. hydroelectrolytic transport and protein secretion by regulated exocytosis. Both cellular processes are regulated by the autonomic nervous system with complementary effects without antagonism, and parasympathetic innervation predominates. Signal transduction mechanisms in salivary cells include: increases in cytosolic calcium, cyclic AMP and cyclic GMP. The tear film consists of three layers: mucous inner layer, middle aqueous layer, and outer lipid layer. Each layer secretion is strongly regulated. Aqueous layer secretion is controlled by autonomic nervous system and signal transduction depends from cyclic AMP and intracellular calcium levels. A review of drugs used in France modulating lacrymal and salivary secretions is proposed.

Activation of glucose transport by AMP-activated protein kinase via stimulation of nitric oxide synthase

Glucose transport in skeletal muscle is stimulated by two distinct stimuli, insulin and exercise. The mechanism by which exercise stimulates glucose transport is not known, although it is distinct from the insulin-mediated pathway. Recently, it has been shown that AMP-activated protein kinase (AMPK) is activated by exercise in skeletal muscle, whereas pharmacological activation of AMPK by 5-amino-4-imidazolecarboxamide riboside (AICAR) leads to increased glucose transport. It has been postulated, therefore, that AMPK may be the link between exercise and glucose transport. To address this, we have examined the signaling pathway involved in the stimulation of glucose uptake after activation of AMPK. Here we show that activation of AMPK by AICAR in rat muscle and mouse H-2Kb muscle cells activates glucose transport approximately twofold. AMPK in H-2Kb cells is also activated by hyperosmotic stress and the mitochondrial uncoupling agent, dinitrophenol, both of which lead to increased glucose transport. In contrast, insulin, which activates glucose transport two- to-threefold in both rat muscle and H-2Kb cells, has no effect on AMPK activity. A previous study has shown that AMPK phosphorylates and activates endothelial nitric oxide synthase (NOS). We show here that NOS activity in H-2Kb cells is activated after stimulation of AMPK by AICAR. Treatment of H-2Kb cells or rat muscle with NOS inhibitors completely blocks the increase in glucose transport after activation of AMPK. In addition, an inhibitor of guanylate cyclase also blocks activation of glucose transport by AICAR in H-2Kb cells. These results indicate that activation of AMPK in muscle cells stimulates glucose transport by activation of NOS coupled to downstream signaling components, including cyclic GMP.

AMP decreases the efficiency of skeletal-muscle mitochondria

Mitochondrial proton leak in rat muscle is responsible for approx. 15% of the standard metabolic rate, so its modulation could be important in regulating metabolic efficiency. We report in the present paper that physiological concentrations of AMP (K(0.5)=80 microM) increase the resting respiration rate and double the proton conductance of rat skeletal-muscle mitochondria. This effect is specific for AMP. AMP also doubles proton conductance in skeletal-muscle mitochondria from an ectotherm (the frog Rana temporaria), suggesting that AMP activation is not primarily for thermogenesis. AMP activation in rat muscle mitochondria is unchanged when uncoupling protein-3 is doubled by starvation, indicating that this protein is not involved in the AMP effect. AMP activation is, however, abolished by inhibitors and substrates of the adenine nucleotide translocase (ANT), suggesting that this carrier (possibly the ANT1 isoform) mediates AMP activation. AMP activation of ANT could be important for physiological regulation of metabolic rate.

AMP-activated protein kinase counteracted the inhibitory effect of glucose on the phosphoenolpyruvate carboxykinase gene expression in rat hepatocytes

The effect of AMP-activated protein kinase (AMPK) in the regulation of the phosphoenolpyruvate carboxykinase (PEPCK) gene expression was studied in isolated rat hepatocytes. Activation of AMPK by AICAR counteracted the inhibitory effect of glucose on the PEPCK gene expression, both at the mRNA and the transcriptional levels. It is proposed that a target for AMPK is involved in the inhibitory effect of glucose on PEPCK gene transcription.

cAMP-dependent vasodilators cross-activate the cGMP-dependent protein kinase to stimulate BK(Ca) channel activity in coronary artery smooth muscle cells

cAMP-dependent vasodilators are used to treat a variety of cardiovascular disorders; however, the signal transduction pathways and effector mechanisms stimulated by these agents are not fully understood. In the present study we demonstrate that cAMP-stimulating agents enhance the activity of the large-conductance, calcium-activated potassium (BK(Ca)) channel in single myocytes from coronary arteries by "cross-activation" of the cGMP-dependent protein kinase (protein kinase G, PKG). Single-channel patch-clamp data revealed that 10 micromol/L isoproterenol, forskolin, or dopamine opens BK(Ca) channels in coronary myocytes and that this effect is attenuated by inhibitors of PKG (KT5823; Rp-8-pCPT-cGMPS), but not by inhibiting the cAMP-dependent protein kinase (protein kinase A, PKA). In addition, a membrane-permeable analog, CPT-cAMP, also opened BK(Ca) channels in these myocytes, and this effect was reversed by KT5823. Direct biochemical measurement confirmed that dopamine or forskolin stimulates PKG activity in coronary arteries but does not elevate cGMP. Finally, the stimulatory effect of cAMP on BK(Ca) channels was reconstituted in a cell-free, inside-out patch by addition of purified PKG activated by either cGMP or cAMP. In contrast, channel gating was unaffected by exposure to the purified catalytic subunit of PKA. In summary, findings from on-cell and cell-free patch-clamp experiments provide direct evidence that cAMP-dependent vasodilators open BK(Ca) channels in coronary myocytes by cross-activation of PKG (but not via PKA). Biochemical assay confirmed this cross-activation mechanism of cAMP action in these arteries. This signaling pathway is a novel mechanism for regulation of potassium channel activity in vascular smooth muscle and other cells.

Intracellular cyclic AMP inhibits native and recombinant volume-regulated chloride channels from mammalian heart

1. ClC-3 encodes a volume-regulated Cl- channel (ICl,vol) in heart. We studied the regulation of native and recombinant cardiac ICl,vol by intracellular cyclic AMP (cAMPi). 2. Symmetrical high Cl- concentrations were used to effectively separate outwardly rectifying ICl,vol from other non-rectifying Cl- currents, such as the cystic fibrosis transmembrane conductance regulator (CFTR) and Ca2+-activated Cl- currents (ICl,CFTR and ICl,Ca, respectively), which are concomitantly expressed in cardiac myocytes. 3. 8-Bromo-cyclic AMP (8-Br-cAMP) significantly inhibited ICl,vol in most guinea-pig atrial myocytes. In approximately 30 % of the atrial myocytes examined, 8-Br-cAMP increased macroscopic Cl- currents. However, the 8-Br-cAMP-stimulated difference currents exhibited a linear current-voltage (I-V ) relation, consistent with activation of ICl,CFTR, not ICl,vol. 4. In canine atrial myocytes, isoprenaline (1 microM) consistently reduced ICl,vol in Ca2+-free hypotonic bath solutions with strong intracellular Ca2+ (Ca2+i) buffering. In Ca2+-containing hypotonic bath solutions with weak Ca2+i buffering, however, isoprenaline increased net macroscopic Cl- currents. Isoprenaline-stimulated difference currents were not outwardly rectifying, consistent with activation of ICl,Ca, not ICl, vol. 5. In NIH/3T3 cells transfected with gpClC-3 (the gene encoding ICl,vol), 8-Br-cAMP consistently inhibited ICl,ClC-3. These effects were prevented by a protein kinase A (PKA) inhibitor, KT5720, or by mutation of a single consensus protein kinase C (PKC) phosphorylation site (S51A) on the N-terminus of ClC-3, which also mediates PKC inhibition of ICl,ClC-3. 6. We conclude that cAMPi causes inhibition of ICl,vol in mammalian heart due to cross-phosphorylation of the same PKC consensus site on ClC-3 by PKA. Our results suggest that contamination of macroscopic ICl,vol by ICl,CFTR and/or ICl,Ca may account for some of the inconsistent and controversial effects of cAMPi on ICl,vol previously reported in native cardiac myocytes.

AMP is a partial agonist at the sheep cardiac ryanodine receptor

We have investigated the ability of AMP to modulate the native sheep cardiac ryanodine receptor (RyR) channel at various cytosolic [Ca2+]. Channels were incorporated into planar phospholipid bilayers and current fluctuations through the bilayer were monitored under voltage clamp conditions. We demonstrate that AMP only exhibits agonist activity if the cytosolic [Ca2+] is sufficiently high. Even in the presence of a high cytosolic [Ca2+] (65 microM), AMP cannot fully open the channel and the maximum open probability (Po) observed is approximately 0.3 at 2 mM AMP. Concentrations of AMP above the maximally activating level cause inactivation of the channel. Our experiments indicate that AMP is an agonist with such low efficacy at the ATP sites on the cardiac RyR that it is effectively an antagonist of ATP-induced increases in Po. Our study demonstrates that the number of phosphates attached to the 5'-carbon of the ribose ring of adenine-based compounds determines the efficacy of the ligand to increase the Po of the cardiac RyR. Substitution of groups at this position may lead to the identification of potent antagonists at ATP sites on RyR.

Neutrophil-derived 5'-adenosine monophosphate promotes endothelial barrier function via CD73-mediated conversion to adenosine and endothelial A2B receptor activation

During episodes of inflammation, polymorphonuclear leukocyte (PMN) transendothelial migration has the potential to disturb vascular barrier function and give rise to intravascular fluid extravasation and edema. However, little is known regarding innate mechanisms that dampen fluid loss during PMN-endothelial interactions. Using an in vitro endothelial paracellular permeability model, we observed a PMN-mediated decrease in endothelial paracellular permeability. A similar decrease was elicited by cell-free supernatants from activated PMN (FMLP 10(-6) M), suggesting the presence of a PMN-derived soluble mediator(s). Biophysical and biochemical analysis of PMN supernatants revealed a role for PMN-derived 5'-adenosine monophosphate (AMP) and its metabolite, adenosine, in modulation of endothelial paracellular permeability. Supernatants from activated PMN contained micromolar concentrations of bioactive 5'-AMP and adenosine. Furthermore, exposure of endothelial monolayers to authentic 5'-AMP and adenosine increased endothelial barrier function more than twofold in both human umbilical vein endothelial cells and human microvascular endothelial cells. 5'-AMP bioactivity required endothelial CD73-mediated conversion of 5'-AMP to adenosine via its 5'-ectonucleotidase activity. Decreased endothelial paracellular permeability occurred through adenosine A2B receptor activation and was accompanied by a parallel increase in intracellular cAMP. We conclude that activated PMN release soluble mediators, such as 5'-AMP and adenosine, that promote endothelial barrier function. During inflammation, this pathway may limit potentially deleterious increases in endothelial paracellular permeability and could serve as a basic mechanism of endothelial resealing during PMN transendothelial migration.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

Adenosine monophosphate as a mediator of ATP effects at P1 purinoceptors

1. When perfused with a medium containing no added magnesium and 4-aminopyridine (4AP) (50 microM) hippocampal slices generated epileptiform bursts of an interictal nature. We have shown in a previous study that adenosine 5'-triphosphate (ATP) depressed epileptiform activity and that this effect was blocked by the adenosine A1 receptor antagonist cyclopentyltheophylline but was not affected by adenosine deaminase. This implied that ATP might act indirectly at P1 receptors or at a xanthine-sensitive P2 receptor. The aim of the present study was to investigate further the action of ATP on epileptiform activity. 2. ATP can be metabolized by ecto-nucleotidases to adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP) and adenosine, respectively. Each of these metabolites can activate receptors in its own right: P2 receptors for ADP and P1 receptors for AMP and adenosine. 3. We now show that both AMP and ATP (50 microM) significantly decrease epileptiform discharge rate in a rapid and reversible manner. 5'Adenylic acid deaminase (AMP deaminase, AMPase) (0.2 u ml(-1)), when perfused alone did not significantly alter the discharge rate over the 10 min superfusion period used for drug application. When perfused concurrently with AMP (50 microM), AMP deaminase prevented the depressant effect of AMP on discharge rate. 4. AMP deaminase, at a concentration of 0.2 u ml(-1) which annulled the effect of AMP (50 microM), prevented the inhibitory activity of ATP (50 microM). A higher concentration of ATP (200 microM) depressed the frequency of spontaneous bursts to approximately 30% control and this response was also prevented by AMP deaminase. 5. Superfusion of the slices with 5'-nucleotidase also prevented the inhibitory activity of ATP on epileptiform discharges. 6. The results suggest that AMP mediates the inhibitory effects of ATP on epileptiform activity, a conclusion which can explain the earlier finding that cyclopentyltheophylline but not adenosine deaminase inhibited the effect of ATP. A corollary to this is that, when examining the pharmacology of ATP, care must be taken to inactivate AMP with AMP deaminase, as well as adenosine with adenosine deaminase, before a direct action of ATP on P1 receptors can be postulated. Failure to do so may have led to erroneous conclusions in some previous studies of nucleotide activity on nucleotide receptors.

Phosphatase-mediated enhancement of cardiac cAMP-activated Cl-conductance by a Cl- channel blocker, anthracene-9-carboxylate

An aromatic carboxylate, anthracene-9-carboxylic acid (9-AC), is known as a Cl- channel blocker. However, variable 9-AC effects have hitherto been reported on the cardiac cAMP-activated Cl- conductance, when applied extracellularly. We have reexamined the 9-AC effect on the Cl-conductance activated by isoproterenol or forskolin in guinea pig ventricular myocytes under whole-cell patch-clamp conditions. The inward current was blocked by 9-AC at > or = 0.5 mmol/L, but in contrast, the outward current was enhanced at much lower concentrations (ED50, approximately 13 mumol/L). 9-AC applied by the intracellular perfusion technique increased both the inward and outward currents. In the presence of intracellular 9-AC, deactivation of the conductance after washout of isoproterenol or forskolin was largely prevented. 9-AC produced an enhancing effect, even after inhibiting the deactivation process by okadaic acid (OA), whereas it failed to produce additional-effects in the presence of orthovanadate. Intracellular application of 9-AC together with OA virtually abolished the current deactivation. The 9-AC effects on the Cl-conductance were not dependent on intracellular Ca2+ or pH. Putative inhibitors of alkaline (bromotetramisole) and acid phosphatases (tartrate) were without effect. 9-AC failed to inhibit the activities of purified protein phosphatase (PP)-1, -2A, and -2C. In the extract of guinea pig ventricle, 9-AC (> or = 10 mumol/L for full action) significantly inhibited a fraction of endogenous phosphatase activity that was sensitive to orthovanadate but not to OA, bromotetramisole, and tartrate. It is concluded that 9-AC blocks cardiac cAMP-activated (cystic fibrosis transmembrane conductance regulator) Cl- conductance from the extracellular side but enhances the conductance from the intracellular side by inhibiting an orthovanadate-sensitive phosphatase distinct from PP-1, -2A, -2B, or -2C and alkaline or acid phosphatase.

Second messengers in the cochlea

Second messengers are vital to the regulation of nearly every aspect of cellular physiology. Evidence is reviewed for the role of second messengers in cochlear fluid homeostasis, hair cell adaptation, motility and synaptic activity. We suggest that the elucidation of the role of second messengers in the regulation of cochlear physiology will be important for the therapeutic management of otopathologies and the side effects of ototoxic agents.

Hormonal regulation of mitogen-activated protein kinase activity in bovine adrenocortical cells: cross-talk between phosphoinositides, adenosine 3',5'-monophosphate, and tyrosine kinase receptor pathways

Angiotensin-II (AII), which stimulates steroidogenesis in bovine adrenocortical (BAC) cells through the phosphoinositides pathway, activates p42-p44 mitogen-activated protein kinases (MAPKs) after 5 min of treatment (EC50 = 0.1 nM). This activation is 1) completely inhibited by the AII receptor AT1 subtype antagonist Dup 753 (10 microM), but unaffected by the AT2 antagonist PD 123177; 2) not reproduced by the AT2 agonist CGP 42112A; 3) insensitive to pretreatment with pertussis toxin; and 4) abolished by a 48-h preexposure of the cells to the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA; 1 microM), which down-regulates protein kinase-C activity. Fibroblast growth factor-2, a potent mitogen for BAC cells, which acts through its tyrosine kinase receptor, also activates MAPK (EC50 = 0.3 in a TPA-insensitive manner, while exhibiting no detectable effect on BAC cell steroidogenesis. In contrast, ACTH, which stimulates steroidogenesis via cAMP and inhibits BAC cell proliferation, does not stimulate MAPK. Indeed, ACTH completely blocks (IC50 = 0.01 nM) the stimulation of MAPK by AII, fibroblast growth factor-2, or TPA. Therefore, bovine adrenocortical cells provide an example of positive and negative hormonal regulation of MAPK activity through a cross-talk between the inositide-, cAMP-, and growth factor-activated tyrosine kinase pathways.

Activation of intestinal Na-K-2Cl cotransport by 5'-AMP requires F-actin remodeling

BACKGROUND

Although cyclic adenosine monophosphate (cAMP)-dependent intestinal chloride ion (Cl-) secretion is regulated primarily at the level of apical Cl- channels, cAMP also elicits basolateral microfilament remodeling and activates basolateral sodium-potassium-2 chloride (Na-K-2Cl) cotransport. Without these additional events, secretion is inhibited. However, it is unclear whether microfilament-dependent activation of Na-K-2Cl cotransport is a direct effect of cAMP or a secondary response to the opening of apical Cl- channels.

METHODS

Using the human intestinal epithelial cell line T84, we examined Cl- secretion elicited by 5'-adenosine monophosphate (5'-AMP), a novel agonist that activates apical Cl- channels without elevation of intracellular cAMP.

RESULTS

5'-AMP was found to activate basolateral Na-K-2Cl cotransport, but such regulation was abolished by the actin stabilizer, phalloidin.

CONCLUSIONS

Basolateral Na-K-2Cl cotransport appears to be regulated, at least in part, as an indirect response to activation of apical Cl- channels, a pathway of regulation which may require cytoskeletal remodeling.

A.E. Bennett Research Award 1993. Olfactory neuroblasts from Alzheimer donors: studies on APP processing and cell regulation

Cell lines of continuously dividing human olfactory neuroblasts can be propagated using olfactory epithelium obtained from human donors at biopsy or autopsy. The expression of neuronal proteins in these cells, such as neurofilament protein and tau protein, can be increased using a combination of factors including nerve growth factor, fibroblast growth factor, interleukin 1 and interleukin 6. These cells also express aspects of human disease. Olfactory neuroblasts generated from donors with the common, sporadic forms of Alzheimer's disease, show elevated levels of the direct precursor to beta-amyloid, the amyloid precursor protein C-terminal derivative (CTD). When treated with the lysosomal inhibitor chloroquine, immunoblots of Alzheimer olfactory neuroblasts show seven-fold higher levels of CTDs than immunoblots from age-matched control neuroblasts. The disease related increases in CTDs can be reversed by treatment with agents that increase intracellular cyclic adenosine monophosphate (cAMP), such as dibutyryl-cyclic-AMP, theophylline, and isoproterenol.

5'-adenosine monophosphate is the neutrophil-derived paracrine factor that elicits chloride secretion from T84 intestinal epithelial cell monolayers

Neutrophil transmigration across intestinal epithelia is thought to contribute to epithelial dysfunction and characterizes many inflammatory intestinal diseases. Neutrophils activated by factors, normally present in the lumen, release a neutrophil-derived secretagogue activity to which intestinal epithelia respond with an electrogenic chloride secretion, the transport event which underlies secretory diarrhea. Using sequential ultrafiltration, column chromatographic, and mass and Raman spectroscopic techniques, neutrophil-derived secretagogue was identified as 5'-AMP. Additional studies suggested that neutrophil-derived 5'-AMP is subsequently converted to adenosine at the epithelial cell surface by ecto-5'-nucleotidase and that adenosine subsequently activates intestinal secretion through adenosine receptors on the apical membrane of target intestinal epithelial cells. These findings suggest that this ATP metabolite may serve as a neutrophil-derived paracrine mediator that contributes to secretory diarrhea in states of intestinal inflammation.

Purine-induced block to mouse embryo cleavage is reversed by compounds that elevate cyclic adenosine monophosphate

The second or third cleavage of mouse embryos from several strains of females is blocked in the presence of hypoxanthine. To begin to determine the mechanism of the block, we studied several aspects of cell metabolism in blocked embryos, including the stage of the cell cycle and the levels of transcription, translation, and protein phosphorylation. In addition, we attempted to reverse the block by transfer of cytoplasm, transfer of RNAs from dividing cells, and co-culture with compounds that elevate cAMP. Our results indicate the following: 1) that the embryos were blocked in interphase; 2) the expression of lac Z linked to SV40 promoter was depressed, but not blocked; 3) overall protein synthesis was depressed but the appearance of early embryonic proteins was not blocked; 4) overall phosphorylation of proteins was not affected; 5) microinjection of additional cytoplasm from non-blocking embryos did not reverse the block; and 6) compounds that elevate cAMP did reverse the block. Thus, the purines apparently do not prevent early embryonic gene expression or most phosphorylation events, but do inhibit a critical cell process occurring during interphase of the cell cycle that can be compensated by elevations in cAMP.

AMP-activated protein kinase - An archetypal protein kinase cascade?

Mammalian AMP-activated protein kinase is the central component of a protein kinase cascade which inactivates three key enzymes involved in the synthesis or release of free fatty acids and cholesterol inside the cell. The kinase cascade is activated by elevation of AMP, and perhaps also by fatty acid and cholesterol metabolites. The system may fulfil a protective function, preventing damage caused by depletion of ATP or excessive intracellular release of free lipids, a type of stress response. Recent evidence suggests that it may have been in existence for at least a billion years, since a very similar protein kinase cascade is present in higher plants. This system therefore represents an early eukaryotic protein kinase cascade, which is unique in that it is regulated by intracellular metabolites rather than extracellular signals or cell cycle events.

The effect of AMP on the NAD-dependent glutamate dehydrogenase during activation and morphogenesis in the cellular slime moulds

In extracts from vegetative Dictyostelium discoideum V12 the basal NAD-dependent glutamate dehydrogenase (NAD-GDH) activity was low, but it increased on standing at 4 degrees C. When 0.1 mM-AMP was included in the assay mix, enzyme activity was stimulated nearly 30-fold. As the extract was allowed to age, the enzyme rapidly lost its ability to be stimulated by AMP. The response of NAD-GDH to AMP was also dependent on the stage of morphogenesis. The ratios of NAD-GDH activity assayed with and without AMP (+AMP/-AMP ratios) in freshly prepared extracts from cells at 0, 4, 8 and 12 h of development were similar, but declined later in morphogenesis. The +AMP/-AMP ratio decreased sharply during activation at 4 degrees C in extracts from cells at 0, 4, 16 and 20 h of development. By contrast, extracts from cells starved for 8 and 12 h remained more responsive to AMP throughout activation. Analysis of Western blots showed that vegetative NAD-GDH did not undergo any detectable proteolytic cleavage during 96 h of activation at 4 degrees C. Also, no change in molecular mass appeared to take place within the cells until culmination (20-24 h), when some breakdown products appeared. Activation of NAD-GDH also occurred in D. discoideum strains NC4 and AX3, and in D. mucoroides. In addition, the enzyme from these four strains was stimulated by AMP and the +AMP/-AMP ratio declined with similar kinetics during activation. The enzyme from Polysphondylium violaceum was not activated on standing, but it was stimulated by AMP. The effect of activation of NAD-GDH is discussed in relation to a postulated catabolic role for this enzyme.

Improved post-ischemic ventricular recovery in the absence of changes in energy metabolism in working rat hearts following heat-shock

We have previously demonstrated that induction of the heat-shock response in rats results in improved recovery of isolated Langendorff-perfused rat hearts subjected to low-flow ischemia followed by reperfusion (Currie et al., 1988). The mechanisms underlying this protective effect of heat-shock are uncertain although the protection was associated with enhanced content of the antioxidant enzyme catalase but not superoxide dismutase or glutathione peroxidase (Currie et al., 1988). Various investigators have suggested the importance of improved energy metabolism in determining recovery following ischemia (Pasque and Wechsler, 1984; Haas et al., 1984; Devous and Lewandowski, 1987). We therefore examined, using a working rat heart model subjected to 10 or 15 min zero flow ischemia whether changes in energy metabolites could account for the protective effect of the heat-shock response. Hearts perfused 24 h after induction of heat-shock failed to demonstrate significant improvement of recovery following 10 min ischemia, however recovery was significantly enhanced in hearts reperfused after 15 min ischemia. Ischemia produced a depression in both ATP and creatine phosphate (CP) content whereas a moderate elevation in ADP and AMP and a marked increase in tissue lactate were evident. These changes were unaffected by prior heat-shock treatment. For both durations of ischemia tissue metabolites were determined during early (5 min) and late (30 min) reperfusion. Although partial recovery in high energy phosphates and a return of ADP, AMP and lactate to near-normal levels were evident, no differences in energy products were observed between hearts from normal or heat-shocked animals, in spite of significantly enhanced recovery.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of AMP on the activation of glycogen synthase and phosphorylase by adenosine, fructose, and glutamine in rat hepatocytes

The mechanism for glycogen synthesis stimulation produced by adenosine, fructose, and glutamine has been investigated. We have analyzed the relationship between adenine nucleotides and glycogen metabolism rate-limiting enzymes upon hepatocyte incubation with these three compounds. In isolated hepatocytes, inhibition of AMP deaminase with erythro-9-(2-hydroxyl-3nonyl)adenine further increases the accumulation of AMP and the activation of glycogen synthase and phosphorylase by fructose. This ketose does not increase cyclic AMP or the activity of cyclic AMP-dependent protein kinase. Adenosine raises AMP and ATP concentration. This nucleotide also activates glycogen synthase and phosphorylase by covalent modification. The correlation coefficient between AMP and glycogen synthase activity is 0.974. Nitrobenzylthioinosine, a transport inhibitor of adenosine, blocks (by 50%) the effect of the nucleoside on AMP formation and glycogen synthase but not on phosphorylase. 2-Chloroadenosine and N6-phenylisopropyladenosine, nonmetabolizable analogues of adenosine, activate phosphorylase (6-fold) without increasing the concentration of adenine nucleotides or the activity of glycogen synthase. Cyclic AMP is not increased by adenosine in hepatocytes from starved rats but is in cells from fed animals. [Ethylenebis (oxyethylenenitrilo)]tetraacetic acid (EGTA) blocks by 60% the activation of phosphorylase by adenosine but not that of glycogen synthase. Glutamine also increases AMP concentration and glycogen synthase and phosphorylase activities, and these effects are blocked by 6-mercaptopurine, a purine synthesis inhibitor. Neither adenosine nor glutamine increases glucose 6-phosphate. It is proposed that the observed efficient glycogen synthesis from fructose, adenosine, and glutamine is due to the generation of AMP that activates glycogen synthase probably through increases in synthase phosphatase activity. It is also concluded that the activation of phosphorylase by the above-mentioned compounds can be triggered by metabolic changes.

Phosphofructokinase from Dirofilaria immitis: effect of fructose 2,6-bisphosphate and AMP on the non-phosphorylated and phosphorylated forms of the enzyme

The enzyme responsible for the synthesis of fructose 2,6-bisphosphate (Fru-2,6-P2), 6-phosphofructo-2-kinase, was shown to be present in the heart worm, Dirofilaria immitis. The level of Fru-2,6-P2 was determined to be 4 +/- 0.3 nmol(g wet weight)-1 in the tissues of the filariid. Fru-2,6-P2 stimulated the activity of both the non-phosphorylated and phosphorylated forms of the D. immitis phosphofructokinase (PFK). The Kact values for Fru-2,6-P2 were 378 +/- 18 nM and 65 +/- 6 nM for the non-phosphorylated and phosphorylated forms, respectively, at 1 mM fructose 6-phosphate (Fru-6-P) and 1 mM ATP at pH 6.8. AMP also stimulated the activity of both forms of the enzyme with Kact values of 230 +/- 10 microM and 37.3 +/- 6.1 microM for the non-phosphorylated and phosphorylated forms, respectively. In the absence of any effectors, the S0.5 values for Fru-6-P were 17.4 mM and 11.0 mM for the non-phosphorylated and phosphorylated forms, respectively, of the D. immitis PFK at 1 mM ATP, pH 6.8. These S0.5 values were lowered to 0.03 mM by the combined effects of saturating levels of Fru-2,6-P2 and AMP. A physiological assay was developed based on the level of metabolites in the parasite that influence the activity of PFK. This assay contained the known effectors of the PFK at concentrations approximating those found in the parasite. Under these conditions the KFru-6-P values were 153 microM and 60 microM for the non-phosphorylated and phosphorylated forms of the PFK, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

[Effects of various physiologic adenine derivatives on the secretion of acid in isolated gastric glands in rabbits]

The physiological adenine derivatives, adenosine (ADO), adenosine 5'-monophosphate (AMP), adenosine 5'-diphosphate (ADP) and adenosine 5'-triphosphate (ATP) at concentrations ranging from 10 microM to 1 mM caused concentration-related modifications on gastric H+ secretion, as measured by the aminopyrine accumulation method, in resting and histamine-stimulated rabbit gastric glands. In resting glands, ADO caused significant concentration-related increases of the basal H+ secretion, whereas no changes were obtained in response to the other purines tested. In histamine-stimulated glands, ADO and AMP caused concentration-related potentiation of the histamine-raised H+ secretory rate, while ATP and ADP induced graded inhibition. The results suggest the involvement of purinergic mechanisms in the physiological regulation of the gastric acid secretory process.

Glycogenolysis during recovery from muscular work. The time course of phosphorylase activity is dependent on Pi concentration in the abdominal muscle of the shrimp Crangon crangon

1) Glycogen is degraded in the abdominal muscle of the shrimp Crangon crangon (Decapoda, Crustacea) during the recovery period following work. The regulation of post-exercise glycogen breakdown and the properties of glycogen phosphorylase (EC 2.4.1.1) have been studied: 2) Glycogen phosphorylase exists as unphosphorylated b-form and phosphorylated a-form, the latter contains 1 molecule phosphate/subunit. Both forms of phosphorylase are dimers, isoenzymes have not been detected. 3) The purified b-form is inactive in absence of AMP and has very low affinities for AMP and Pi. For half-maximum activation 0.33 +/- 0.04 mM AMP is necessary, and the Km-value for Pi at 1 mM AMP is 48 +/- 5 mM. IMP does not affect the activity of the b-form. 4) The a-form is active without effectors, its Km-value for Pi is 5.3 +/- 1.5 mM. The proportion of phosphorylase a increases in vivo, from about 25% at rest, to approximately 90% upon work and remains at this high level during the first minutes of recovery. 5) It is concluded that the glycogenolytic flux in the abdominal muscle of the shrimp even during post-exercise periods depends on the level of the a-form the activity of which is restricted in time and extent by the cytoplasmic Pi concentration (Kamp, G. & Juretschke, H. P. (1987) Biochim. Biophys. Acta 929, 121-127).