Properties of the cyclic GMP phosphodiesterase from rat lung. Inhibition by unsaturated fatty acids

Inhibition of calcineurin by polyunsaturated lipids

From earlier studies on calcineurin, the presence of multiple double bonds in putative inhibitors was hypothesized as critical features for effective inhibition. Polyunsaturated fatty acids were tested as inhibitors of calcineurin and found to inhibit the phosphatase activity of calcineurin although effective inhibition was observed only in the absence of calmodulin. Calmodulin and fatty acids seemed to compete for the enzyme with the activation curve of calmodulin shifted approximately 100-fold in the presence of 50 microM eicosa-11Z,14Z-dienoic acid (20:2, n-6) or 50 microM eicosa-8Z,11Z,14Z-trienoic acid (20:3, n-6). Leukotriene B4 and derivatives also were screened as inhibitors. The most effective inhibition was caused by the 6-trans,12-epi-leukotriene B4 with an IC50 of 16.4 microM for the inhibition of calcineurin with pNPP as the substrate. Lipoxins A4 and B4 likewise caused inhibition in the presence of calmodulin with an IC50 of 42.7 microM for lipoxin B4. There was no protection by calmodulin, as found with the inhibition by the fatty acids. These data support the hypothesis that effective inhibition is bolstered by the presence of conjugated double bonds in the inhibitor. Consideration of cis- and trans-orientation of the double bonds suggests that presentation of the delocalized electron density is also a factor in effective inhibition of calcineurin.

Phosphodiesterases in the vascular system

Cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) are second messengers involved in the intracellular signal transduction of a variety of extracellular stimuli in several tissues. In the vascular system, these nucleotides play important roles in the regulation of vascular tone and in the maintenance of the mature contractile phenotype in smooth muscle cells. Given that cyclic nucleotide signaling regulates a wide variety of cellular functions, it is not surprising that cyclic nucleotide phosphodiesterases (PDEs). In paticular, the accumulating data showing that there are a large number of different PDE isozymes have triggered an equally large increase in interest about these enzymes. At least 11 different gene families of PDEs are currently known to exist in mammalian tissues. Most families contain several distinct genes, and many of these genes are expressed in different tissues as functionally unique alternative splice variants. This article reviews many of the important aspects about the structure, cellular localization, and regulation of each family of PDEs. Particular emphasis is placed on new information obtained in the last few years about vascular disease. The development of novel methods to deliver more potent and selective PDE inhibitors to individual cell types and subcellular locations will lead to new therapeutic uses for this class of drugs in diseases of the vascular system.

Expression of an active, monomeric catalytic domain of the cGMP-binding cGMP-specific phosphodiesterase (PDE5)

Phosphodiesterases (PDEs) comprise a superfamily of phosphohydrolases that degrade 3',5'-cyclic nucleotides. All known mammalian PDEs are dimeric, but the functional significance of dimerization is unknown. A deletion mutant of cGMP-binding cGMP-specific PDE (PDE5), encoding the 357 carboxyl-terminal amino acids including the catalytic domain, has been generated, expressed, and purified. The K(m) of the catalytic fragment for cGMP (5.5 +/- 0. 51 microM) compares well with those of the native bovine lung PDE5 (5.6 microM) and full-length wild type recombinant PDE5 (2 +/- 0.4 microM). The catalytic fragment and full-length PDE5 have similar IC(50) values for the inhibitors 3-isobutyl-1-methylxanthine (20 microM) and sildenafil (Viagra(TM))(4 nM). Based on measured values for Stokes radius (29 A) and sedimentation coefficient (2.9 S), the PDE5 catalytic fragment has a calculated molecular mass of 35 kDa, which agrees well with that predicted by amino acid content (43.3 kDa) and with that estimated using SDS-polyacrylamide gel electrophoresis (39 kDa). The combined data indicate that the recombinant PDE5 catalytic fragment is monomeric, and retains the essential catalytic features of the dimeric, full-length enzyme. Therefore, the catalytic activity of PDE5 holoenzyme requires neither interaction between the catalytic and regulatory domains nor interactions between subunits of the dimer.

Cyclic GMP and regulation of cyclic nucleotide hydrolysis

Several of the different PDE isozyme families have the ability in vitro to hydrolyze cGMP. In particular they include the CaM-dependent PDEs, the cGMP-stimulated PDEs, and the cGMP binding, cGMP-specific PDEs. Existing evidence suggests or demonstrates that in different cell types, each of these can be important determinants for the control of cGMP steady-state levels. Each of these enzymes is differentially expressed and regulated; moreover, the amount of the enzyme expressed and the mode of regulation determine to a large extent the rate of rise, maximal level, rate of fall, and duration of the cGMP signal in the cell. In addition to enzymes that function to degrade cGMP at least two also are regulated by cGMP both in vitro and in the intact cell. The cGMP-stimulated PDE has the ability to decrease cAMP levels in response to cGMP and the cGMP-inhibited PDE can increase cAMP levels in response to cGMP. We are just beginning to define how many different isozymes of PDE exist in mammalian tissues, where they are located, and how they are regulated. Selective inhibitors to each are being developed and studies designed to define structural features that determine the mechanisms of action and regulation of the PDEs have been initiated. It is expected that in the next few years more PDEs will be discovered and the functions of the new an existing ones with be more clearly defined.

Phosphatidic acid stimulates the rolipram-sensitive cyclic nucleotide phosphodiesterase from rat thymocytes

The role of phospholipid metabolites in the modulation of cyclic AMP degradation during the early response of rat thymic lymphocytes to mitogenic stimulation was investigated by measuring their in vitro effect on the activity of five different cyclic nucleotide phosphodiesterase forms separated from thymocyte cytosol by means of an HPLC technique. Arachidonic acid was found to markedly inhibit four of the enzyme forms, with IC50 ranging from 14 to 60 microM, while its hydroperoxy and hydroxy derivatives proved inefficient. Dioctanoylglycerol, a biologically active diacylglycerol, was weakly inhibitory while phosphatidic acid, the diacylglycerol phosphorylated derivative, markedly stimulated the two cyclic-AMP-specific type-IV forms identified in thymocyte cytosol, by 50 and 70%. In intact cells labelled with tritiated arachidonate, the mitogenic lectin concanavalin A induced a rapid 4-5-fold increase in radiolabelled phosphatidic acid which peaked at 1 min, and remained elevated for at least 30 min. These observations suggest that phosphatidic acid formed during the mitogenic stimulation of T-cells might be responsible for an early activation of cyclic AMP degradation with, as a consequence, a lowering of cyclic AMP level, which is reported to be necessary for the occurrence of the first steps of mitogenesis.

Occurrence of the methylisobutylxanthine-stimulated cyclic GMP binding protein in various rat tissues

A new type of cGMP binding protein, the activity of which is characteristically stimulated by methylisobutylxanthine, has been previously discovered in rat lung and platelets (Hamet, P. and Coquil, J.F. (1978) J. Cyclic Nucleotide Res. 4, 281-290). In the present study, we demonstrate the occurrence of this protein in soluble extracts of a variety of rat tissues fractionated by a DEAE-Sepharose chromatography. In several tissues (spleen, lung and brain) the binding activity of this protein was of the same order of magnitude as that of the cGMP-dependent protein kinase.

Cyclic AMP phosphodiesterase and cyclic GMP phosphodiesterase activities of rat mammary tissue

Cyclic nucleotide phosphodiesterase activity in mammary tissue from rats in midlactation was resolved by DEAE-cellulose chromatography into three functionally distinct fractions: a Ca2+/calmodulin-stimulated cyclic GMP phosphodiesterase, a cyclic GMP-stimulated low-affinity cyclic nucleotide phosphodiesterase, and a high-affinity cyclic AMP-specific phosphodiesterase. The absolute activities and relative proportions of high- and low-affinity enzymes resemble those found, for example, in liver, as distinct from those in excitable tissues. Three functional characteristics are described which are peculiar to mammary-tissue phosphodiesterases. Firstly, the concentration of free Ca2+ required to achieve half-maximal activation of the Ca2+/calmodulin-stimulated phosphodiesterase is somewhat higher than for the analogous enzyme in other tissues; secondly, the activity of this enzyme towards cyclic AMP relative to that towards cyclic GMP is unusually low, and thirdly, the low-affinity cyclic nucleotide phosphodiesterase is inhibited by low concentrations of free Ca2+.