Stimulation of cyclic nucleotide phosphodiesterases from rat brain by activator protein, proteolytic enzymes and a vitamin E derivative

Properties of base-substituted and carboxyl-esterified analogues of griseolic acid, a potent cAMP phosphodiesterase inhibitor

Griseolic acid (GA) is a potent cyclic AMP (cAMP) phosphodiesterase (PDE) inhibitor that has an adenine base and two carboxyl groups in its molecule (Nakagawa F, Okazaki T, Naito A, Iijima Y and Yamazaki M, J Antibiot 38: 823-829, 1985). GA analogues were synthesized in which the adenine group was substituted with guanine (6-deamino-2-amino-6-hydroxygriseolic acid, G-GA) or hypoxanthine (6-deamino-6-hydroxygriseolic acid, H-GA). Their inhibitory activities to cyclic GMP (cGMP) PDE and cAMP PDE were compared with GA. For cGMP PDE from rod outer segments of bovine retina, the IC50 values of GA, G-GA and H-GA were 18, 0.040 and 0.12 microM, respectively, with 0.25 microM cGMP as substrate. For type IV PDE isozyme from mouse 3T3 fibroblast cells, the IC50 values of GA, G-GA and H-GA were 0.021, 15 and 11 microM, respectively, with 0.25 microM cAMP as substrate. Thus, GA and G-GA were found to be base-selective inhibitors of type IV PDE of 3T3 cells and type V PDE of bovine retinas, respectively. Esters of carboxylic acids of GA were synthesized in order to increase permeability into cells, and their efficacy was tested by measuring the accumulation of cAMP in 3T3 cells. The dipivaloyloxymethyl ester of GA was found to increase cAMP levels at 0.1 microM, while GA and 3-isobutyl-1-methylxanthine were active only above 100 microM, and the dimethyl ester of GA was inactive. The dipivaloyloxymethyl ester of GA seems to exert its activity after conversion to GA in the cell, since the pivaloyloxymethyl ester was easily hydrolysed by the enzyme action and the dipivaloyloxymethyl ester of GA itself was much less potent an inhibitor of PDE. The dipivaloyloxymethyl ester of GA inhibited thrombin-induced aggregation of platelets and stimulated lipolysis of adipocytes at low concentrations.

Inhibition of alpha-tocopherol and calcium calmodulin-stimulated phosphodiesterase activity in vitro by anthracyclines

1. The inhibition of alpha-tocopherol and calmodulin-stimulated phosphodiesterase activities was investigated in vitro. 2. Anthracyclines--doxorubicin, daunorubicin and aclacinomycin--inhibited calcium calmodulin-stimulated cyclic 3',5'-AMP (cAMP) nucleotide phosphodiesterase (EC. 3.1.4-17) activity (IC50 = 33.00 +/- 3.50-36.50 +/- 2.75 mumol/l). The stimulation of this enzyme by alpha-tocopherol was also inhibited by doxorubicin (IC50 = 18.50 +/- 4.00 mumol/l). 3. The anthracycline-induced inhibition of the calcium calmodulin and alpha-tocopherol-stimulated phosphodiesterase activity was competitive with calmodulin and alpha-tocopherol respectively. Increasing the concentration of the substrate, cAMP or calcium ions did not attenuate the drug-induced inhibition. The basal activity of the enzyme was not inhibited by concentration of doxorubicin up to 50 mumol/l. 4. In vivo, single dose drug distribution studies of the fluorescence of doxorubicin indicate that in the heart after a cardiotoxic dose (20 mg/kg), myocardial concentrations were achieved which could cause 70-80% inhibition of this phosphodiesterase enzyme. 5. Inhibition of calmodulin function by anthracyclines via direct interaction with calmodulin may contribute significantly to the effects of anthracyclines, such as disturbance in calcium homeostasis as well as acute and chronic deleterious effects on the myocardium. The action of alpha-tocopherol to bind or complex anthracycline may in part contribute to its protection against anthracycline-induced membrane damage and cardiotoxicity.

Cyclic AMP and adenylate cyclase activators stimulate Trypanosoma cruzi differentiation

A chemically defined in vitro differentiating condition was used to study the potential role of cyclic AMP (cAMP) and adenylate cyclase activators on the transformation of Trypanosoma cruzi epimastigotes to the infective metacyclic trypomastigotes (metacyclogenesis). It was observed that both addition of cAMP analogs or adenylate cyclase activators to the differentiating medium stimulated the transformation of epimastigotes to metacyclic trypomastigotes. These results were further corroborated by showing that inhibitors of cAMP phosphodiesterase were stimulatory while activators of this enzyme inhibited the metacyclogenesis process. On the other hand, inhibitors of calmodulin inhibited the transformation of epimastigotes to metacyclic trypomastigotes, suggesting that T. cruzi adenylate cyclase might be activated by calmodulin. In addition, the results strongly suggest that guanine nucleotide binding proteins are involved in T. cruzi adenylate cyclase activation. This system may be useful for studying cell differentiation mechanisms in eukaryotes.

Characterization of cyclic nucleotide phosphodiesterases in Xenopus laevis ovary

A cGMP-stimulated cyclic nucleotide phosphodiesterase present in cytosol of Xenopus laevis ovary has been purified and characterized. A cAMP-specific phosphodiesterase which is not activated by either cGMP or calmodulin, has also been characterized. Brief exposure of intact oocytes to 10 micro M progesterone results in an increase in activity of the cAMP-specific enzyme. The cGMP-stimulated and the calmodulin-activated phosphodiesterases are not altered. Changes in cyclic nucleotide levels during progesterone-induced maturation of oocytes may be modulated by these isoenzymes.

Evidence for regulatory control of canine platelet phosphodiesterase

Forskolin, epinephrine, and prostaglandin I2 were used to examine the adenylate cyclase-phosphodiesterase system of intact thrombopathic and normal canine platelets. The results provide indirect support for the hypothesis that the elevation of intraplatelet c-AMP in this unique hereditary defect is due to impaired phosphodiesterase activity. The inhibitory (Nj) and stimulatory (Ns) components of adenylate cyclase appeared functionally intact. Cytosolic fractions of normal and thrombopathic platelets had similar cAMP hydrolytic activities. The failure of intact forskolin-stimulated thrombopathic platelets to return elevated cAMP to non-stimulated levels after 15 min, despite significant phosphodiesterase activity in cytosolic fractions, implies that the platelet isoenzymes are under regulatory control.

Protein analyses and reagents: microscale assay of calcium-binding activity of proteins and peptides using a nitrocellulose membrane

A simple and reliable method for the measurement of calcium binding to proteins and peptides was developed. It is composed of two procedures--filtration through a nitrocellulose membrane filter and estimation of 45Ca retained on the membrane. The routine assay was completed within a few minutes, and only microgram amounts of samples were necessary. This method permitted the quantitative determination of the calcium-binding activity of proteins and peptides including one with a molecular weight of as low as 4000. This method also permitted the detection of low-affinity interactions (Kd congruent to 10(-3) M), possibly because the nitrocellulose membrane did not show nonspecific binding of calcium and because a washing step was not employed in the routine assay. Ultrafiltration membranes when used in the apparatus gave no useful data.

Activation of the cyclic nucleotide phosphodiesterase from rat heart cytosol by phospholipase C

Phospholipase C (clostridium perfringens) significantly increased the cyclic nucleotide phosphodiesterase activity of a crude 105 000 g supernatant from rat heart. This activation only concerned the basal activity of cyclic GMP phosphodiesterase determined with 0.25 microM cyclic GMP as substrate, in the presence of EGTA, whereas stimulation was found to be independent of EGTA when phosphodiesterase activity was measured with 0.25 microM cyclic AMP. Similar qualitative results were found for the three cytosolic forms of phosphodiesterase separated from rat heart supernatant by isoelectric focusing. Supplementary experiments provided evidence that the activation of the cyclic AMP-specific phosphodiesterase was attributable to Phospholipase C activity and not to contaminating protease(s). In contrast, the stimulation of the cyclic GMP phosphodiesterase activity appeared to be largely dependent on the proteolytic activity of commercial Phospholipase C. Phosphatidic acid also significantly increased the cyclic AMP phosphodiesterase activity of the rat heart cytosol. These results suggest that the activation of cardiac cyclic AMP phosphodiesterase may be related to changes in phospholipid metabolism, notably the accumulation of phosphatidate, and relevant to physiological regulatory processes.

Inhibition of glia maturation factor-induced mitogenesis in glioblasts by calmodulin antagonists

The growth inhibitory activity of calmodulin antagonist, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) and trifluoperazine (TFP), was analyzed by the use of rat fetal glioblasts stimulated by glia maturation factor (GMF) or rat astrocytoma cells (C6). The inhibitory effect of W-7 on GMF-induced DNA synthesis of glioblasts was apparent when the drug was added within 10 h after the stimulation by GMF (late G1 phase), but was not shown when W-7 was added at 12 h or later (S phase). The intracellular calmodulin content was built up concurrently with the increase in the DNA synthesis in S phase. The half-maximal inhibition (ID50) of GMF-induced DNA synthesis in glioblasts was observed at 16.5 microM of W-7 or 9.0 microM of TFP. ID50 of DNA synthesis in exponentially growing C6 cells was approximately 3 times higher than that in glioblasts: 24 microM of TFP and as high as 40 microM of W-7. ID50 of growth rate of C6 cells was 15 microM of TFP which was comparable to the ID50 dose for the inhibition of DNA synthesis. Both calmodulin antagonists and W-5, a dechlorinated analog of W-7, however, elicited a curious activation of DNA synthesis of glioblasts at low concentrations (lower than 10 microM of W-7 and W-5, or lower than 5 microM of TFP), indicating non-specific effects of calmodulin antagonists on DNA synthesis. These results suggest that calmodulin antagonists have two conflicting effects on DNA synthesis: the stimulation of DNA synthesis at lower concentrations, and inhibition at higher concentrations.

Purification, characterization and production of rabbit antibodies to rat liver particulate, high-affinity, cyclic AMP phosphodiesterase

The cyclic nucleotide phosphodiesterase (EC 3.4.16) activities of a rat liver particulate fraction were analyzed after solubilization by detergent or by freeze-thawing. Analysis of the two extracts by DEAE-cellulose chromatography revealed that they contain different complements of phosphodiesterase activities. The detergent-solubilized extract contained a cyclic GMP phosphodiesterase, a low affinity cyclic nucleotide phosphodiesterase whose hydrolysis of cyclic AMP was activated by cyclic GMP and a high affinity cyclic AMP phosphodiesterase. The freeze-thaw extract contained a cyclic GMP phosphodiesterase and two high affinity cyclic AMP phosphodiesterase, but no low affinity cyclic nucleotide phosphodiesterase. The cyclic AMP phosphodiesterase activities from the freeze-thaw extract and from the detergent extract all had negatively cooperative kinetics. One of the cyclic AMP phosphodiesterases from the freeze-thaw extract (form A) was insensitive to inhibition by cyclic GMP; the other freeze-thaw solubilized cyclic AMP phosphodiesterase (form B) and the detergent-solubilized cyclic AMP phosphodiesterase were strongly inhibited by cyclic GMP. The B enzyme appeared to be converted into the A enzyme when the particulate fraction was stored for prolonged periods at -20 degrees C. The B form was purified extensively, using DEAE-cellulose, a guanine-Sepharose column and gel filtration. The enzyme retained its negatively cooperative kinetics and high affinity for both cyclic AMP and cyclic GMP throughout the purification, although catalytic activity was always much greater for cyclic AMP. Rabbit antiserum was raised against the purified B enzyme and tested via a precipitin reaction against other forms of phosphodiesterase. The antiserum cross-reacted with the A enzyme and the detergent-solubilized cyclic AMP phosphodiesterase from rat liver. It did not react with the calmodulin-activated cyclic GMP phosphodiesterase of rat brain, the soluble low affinity cyclic nucleotide phosphodiesterase of rat liver or a commercial phosphodiesterase preparation from bovine heart. These results suggest a possible interrelationship between the high affinity cyclic nucleotide phosphodiesterase of rat liver.

Cyclic nucleotide phosphodiesterase activities from pig coronary arteries. Lack of interconvertibility of major forms

DEAE-cellulose chromatography, with or without dithiothreitol and over a pH range of 6.0 to 8.5, resolved two phosphodiesterase activities (peaks I and II) from the soluble fraction of pig coronary arteries. The activity of peak I was increased by calmodulin (3-7-fold), whereas that of peak II was not. Chromatography of peak I on Biol-Gel A-0.5 m columns resolved two peaks of phosphodiesterase activity (peaks Ia and Ib). Peak Ia was eluted in the presence or absence of 0.1 M KCl and was relatively insensitive to calmodulin. Peak Ib was eluted only in the presence of KCl and was sensitive to calmodulin. The substrate specificity and kinetic behavior were the same for peaks I, Ia, and Ib. Repeated gel chromatography of either peak Ia or Ib, under appropriate conditions, yielded a mixture of peaks Ia and Ib. Peak Ia appears to be a reversible aggregate of peak Ib. Gel chromatography of peak II resolved only one phosphodiesterase activity, which was eluted without KCl, was highly specific for cyclic AMP, was not sensitive to calmodulin and migrated differently on the gel column than either peak Ia or Ib. Sucrose density gradient centrifugation of the soluble fraction from pig coronary arteries in the presence or absence of dithiothreitol resolved two peaks of phosphodiesterase activity (6.6 S and 3.6 S) which were similar to peaks I and II separated by DEAE-cellulose chromatography with regard to their substrate specificity and their sensitivity to calmodulin. Upon recentrifugation, each of the two peaks of phosphodiesterase activity gave a single peak of activity which migrated with the same S value as did its parent. These results indicate that the two major forms of phosphodiesterase of pig coronary arteries, which are representative of those found in many tissues, are not interconvertible in cell-free systems.

Activation of mammalian cyclic AMP phosphodiesterases by trypsin

BHK fibroblasts contain two forms of cyclic AMP phosphodiesterase 3':5'-cyclic nucleotide 5'-nucleotidohydrolase EC 3.1.4.17) as analyzed by linear sucrose gradient fractionation; a 3.6-S form (peak I) and a 6.7-S form (peak II). Peak I is specific for cyclic AMP as substrate and displays Michaelis-Menten kinetics with an apparent Km of 2--3 micrometer. Peak II hydrolyzes cyclic GMP and displays anomalous kinetics for cyclic AMP hydrolysis. The activity of isolated peak II for cyclic AMP is increased by storage at 4 degrees C, treatment with trypsin, or treatment with rat brain and BHK fibroblast activator proteins. The activity of isolated peak I is unaffected by these conditions. Linear sucrose gradient fractionation demonstrates that activation of peak II by trypsin leads to the formation of a 3.6-S cyclic AMP-specific enzyme form, possibly peak I. In contrast to BHK fibroblasts (and most other mammalian tissues), rat uterus contains only one form of cyclic nucleotide phosphodiesterase on linear sucrose gradients, a 7-S form capable of hydrolyzing both cyclic AMP and cyclic GMP. Treatment of rat uterine supernatant with trypsin leads to the appearance of a 4-S, cyclic AMP-specific form with properties similar to that of BHK peak I. These data suggest that the kinetically complex, higher molecular weight cyclic nucleotide phosphodiesterases may consist of more than one catalytically active site and that multiple forms of the enzyme arise through dissociative mechanisms, possibly as a means of in vivo regulation.

Increased activity of cyclic AMP phosphodiesterase from frozen-thawed rat liver. A role of lysosomal protease in enzyme activation

The activity of cyclic AMP phosphodiesterase (3':5'-cyclic-nucleotide 5'-nucleotidohydrolase, EC 3.1.4.17) in 105 000 X g supernatant fraction from frozen-thawed rat liver was 2.5 times higher than the corresponding preparation from fresh liver. This increased activity of frozen liver enzyme was accompanied by a decreased sensitivity of the enzyme to known activators such as alpha-tocopheryl phosphate and trypsin. Neither membrane-bound cyclic AMP phosphodiesterase, nor supernatant cyclic GMP phosphodiesterase increased in frozen liver preparation. It is unlikely that the activator protein of phosphodiesterase participated in the observed change of enzyme activity. Among rat tissues so far tested, the increased level of cyclic AMP phosphodiesterase was noted only in tissues rich in lysosome content. In the recombination experiment where phosphodiesterase from fresh liver was incubated with lysosomal fraction, stimulation of the enzyme activity was observed with a concomitant loss of sensitivity to above-mentioned activators. Since the stimulation by lysosomal fraction was effectively inhibited by cathepsin B1 inhibitors, leupeptin and antipain, it was deduced cathepsin-B1 (EC 3.4.12.3) type protease(s) was the main causative of activating the cyclic AMP phosphodiesterase. The freezing-thawing process of rat liver made the lysosomal membrane more permeable, and hence lysosomal proteases were released into soluble fraction during phosphodiesterase preparation. These results provide a warning not to use frozen liver for phosphodiesterase preparation, otherwise altered properties of the enzymes will be seen.