Biochemical characteristics and cellular regulation of phosphodiesterase IV

Second messengers and their importance for novel drug treatments of patients with bipolar disorder

Second messenger systems, like the cyclic nucleotide, glycogen synthase kinase-3β, phosphoinositide, and arachidonic acid cascades, are involved in bipolar disorder (BD). We investigated their role on the development of novel therapeutic drugs using second messenger mechanisms. PubMed search and narrative review. We used all relevant keywords for each second messenger cascade combining it with BD and related terms and combined all with novel/innovative treatments/drugs. Our search produced 31 papers most were reviews, and focussed on the PI3K/AKT-GSK-3β/Nrf2-NF-ĸB pathways. Only two human randomized clinical trials were identified, of ebselen, an antioxidant, and celecoxib, a cyclooxygenase-2 inhibitor, both with poor unsatisfactory results. Despite the fact that all second messenger systems are involved in the pathophysiology of BD, there are few experiments with novel drugs using these mechanisms. These mechanisms are a neglected and potentially major opportunity to transform the treatment of bipolar illness.

cAMP-specific phosphodiesterase HSPDE4D3 mutants which mimic activation and changes in rolipram inhibition triggered by protein kinase A phosphorylation of Ser-54: generation of a molecular model

Ser-13 and Ser-54 were shown to provide the sole sites for the protein kinase A (PKA)-mediated phosphorylation of the human cAMP-specific phosphodiesterase isoform HSPDE4D3. The ability of PKA to phosphorylate and activate HSPDE4D3 was mimicked by replacing Ser-54 with either of the negatively charged amino acids, aspartate or glutamate, within the consensus motif of RRES54. The PDE4 selective inhibitor rolipram ¿4-[3-(cyclopentoxy)-4-methoxyphenyl]-2-pyrrolidone¿ inhibited both PKA-phosphorylated HSPDE4D3 and the Ser-54-->Asp mutant, with an IC50 value that was approximately 8-fold lower than that seen for the non-PKA-phosphorylated enzyme. Lower IC50 values for inhibition by rolipram were seen for a wide range of non-activated residue 54 mutants, except for those which had side-chains able to serve as hydrogen-bond donors, namely the Ser-54-->Thr, Ser-54-->Tyr and Ser-54-->Cys mutants. The Glu-53-->Ala mutant exhibited an activity comparable with that of the PKA phosphorylated native enzyme and the Ser-54-->Asp mutant but, in contrast to the native enzyme, was insensitive to activation by PKA, despite being more rapidly phosphorylated by this protein kinase. The activated Glu-53-->Ala mutant exhibited a sensitivity to inhibition by rolipram which was unchanged from that of the native enzyme. The double mutant, Arg-51-->Ala/Arg-52-->Ala, showed no change in either enzyme activity or rolipram inhibition from the native enzyme and was incapable of providing a substrate for PKA phosphorylation at Ser-54. No difference in inhibition by dipyridamole was seen for the native enzyme and the Ser-54-->Asp and Ser-54-->Ala mutants. A model is proposed which envisages that phosphorylation by PKA triggers at least two distinct conformational changes in HSPDE4D3; one of these gives rise to enzyme activation and another enhances sensitivity to inhibition by rolipram. Activation of HSPDE4D3 by PKA-mediated phosphorylation is suggested to involve disruption of an ion-pair interaction involving the negatively charged Glu-53. The increase in susceptibility to inhibition by rolipram upon PKA-mediated phosphorylation is suggested to involve the disruption of a hydrogen-bond involving the side-chain hydroxy group of Ser-54.

Receptor-mediated stimulation of lipid signalling pathways in CHO cells elicits the rapid transient induction of the PDE1B isoform of Ca2+/calmodulin-stimulated cAMP phosphodiesterase

Chinese hamster ovary cells (CHO cells) do not exhibit any Ca2+/calmodulin-stimulated cAMP phosphodiesterase (PDE1) activity. Challenge of CHO cells with agonists for endogenous P2-purinoceptors, lysophosphatidic acid receptors and thrombin receptors caused a similar rapid transient induction of PDE1 activity in each instance. This was also evident on noradrenaline challenge of a cloned CHO cell line transfected so as to overexpress alpha 1B-adrenoceptors. This novel PDE1 activity appeared within about 15 min of exposure to ligands, rose to a maximum value within 30 min to 1 h and then rapidly decreased. In each case, the expression of novel PDE1 activity was blocked by the transcriptional inhibitor actinomycin D. Challenge with insulin of either native CHO cells or a CHO cell line transfected so as to overexpress the human insulin receptor failed to induce PDE1 activity. Reverse transcriptase-PCR analyses, using degenerate primers able to detect the PDE1C isoform, did not amplify any fragment from RNA preparations of CHO cells expressing PDE1 activity, although they did so from the human thyroid carcinoma FTC133 cell line. Reverse transcriptase-PCR analyses, using degenerate primers able to detect the PDE1A and PDE1B isoforms, successfully amplified a fragment of the predicted size from RNA preparations of both CHO cells expressing PDE1 activity and human Jurkat T-cells. Sequencing of the PCR products, generated using the PDE1A/B primers, yielded a novel sequence which, by analogy with sequences reported for bovine and murine PDE1B forms, suggests that the PDE1 species induced in CHO cells through protein kinase C activation and that expressed in Jurkat T-cells are PDE1B forms.

Intracellular localization of the PDE4A cAMP-specific phosphodiesterase splice variant RD1 (RNPDE4A1A) in stably transfected human thyroid carcinoma FTC cell lines

Cells of two human follicular thyroid carcinoma cell lines (FTC133, FTC236) were stably transfected with a cDNA encoding the PDE4A cAMP-specific phosphodiesterase (PDE) splice variant RD1 (RNPDE4A1A) so as to generate the cloned cell lines, FTC133A and FTC236A. This allowed the expression of a novel rolipram-inhibited cAMP-specific PDE activity in these cells. Unlike the parent cell lines in which Ca2+/calmodulin caused a profound activation (approx. 3-4-fold) of homogenate PDE activity, no such stimulation was evident in the RD1-expressing cell lines, indicating loss of PDE1 activity. Reverse transcriptase-PCR analysis indicated that this was due to the down-regulation of the PDE1C isoform. The novel PDE4 activity in transfected cells was located exclusively in the membrane fraction, as was immunoreactive RD1. Low concentrations of the detergent Triton X-100, but not high NaCl concentrations, allowed RD1 to be solubilized. Laser scanning confocal immunofluorescence analyses identified RD1 immunoreactivity in a discrete perinuclear region of these RD1-expressing transfected cell lines. A similar pattern of labelling was observed using the antiserum Tex1, which specifically identified the Golgi apparatus. Treatment of FTC133A cells with the Golgi-perturbing agents monensin and brefeldin A led to a similar redistribution of immunoreactive species detected using both the Tex1 and anti-RD1 antisera. It is suggested that the PDE4A splice variant RD1 contains a membrane-association signal which allows the targeted expression of RD1 within the Golgi complex of these human follicular thyroid carcinoma cell lines.

Phosphodiesterase 4 in macrophages: relationship between cAMP accumulation, suppression of cAMP hydrolysis and inhibition of [3H]R-(-)-rolipram binding by selective inhibitors

A perplexing phenomenon identified in several tissues is the lack of correlation between inhibition of phosphodiesterase 4 (PDE4) and certain functional responses such as smooth muscle relaxation, gastric acid secretion and cAMP accumulation. Interpretation of these data is complicated further by the finding that function correlates with the ability of PDE4 inhibitors to displace [3H]rolipram [4-(3-cyclopentenyloxy-4-methoxyphenyl)-2-pyrrolidone] from a high-affinity site in rat brain that is apparently distinct from the catalytic centre of the enzyme. We have investigated this discrepancy by using guinea pig macrophages as a source of PDE4 and have confirmed that the ability of a limited range of structurally dissimilar PDE inhibitors (Org 20241, nitraquazone and the enantiomers of rolipram and benafentrine) to increase cAMP content did not correlate with their potency as inhibitors of partly purified PDE4, whereas a significant linear and rank order correlation was found when cAMP accumulation was related to the displacement of [3H]R-(-)-rolipram from a specific site identified in macrophage lysates. An explanation for these data emerged from the finding that the IC50 values and rank order of potency of these compounds for inhibition of partly purified PDE4 and the native (membrane-bound) form of the same enzyme were distinct. Similarly, no correlation was found when membrane-bound PDE4 was compared with the same enzyme that had been solubilized with Triton X-100. These unexpected results were attributable to a selective decrease in the potency of those inhibitors [nitraquazone, R-(-)- and S-(+)-rolipram] that interacted preferentially with the rolipram binding site. Indeed, if membrane-bound PDE4 was used as the enzyme preparation, excellent linear and rank order correlations between inhibition of cAMP hydrolysis, displacement of [3H]R-(-)-rolipram and cAMP accumulation were found, which improved further in the presence of the vanadyl (Vo)/2. GSH complex. Moreover, using Vo/2.GSH-treated membranes, the IC50 values of nitraquazone and the enantiomers of rolipram for the inhibition of PDE4 approached their affinity for the rolipram binding site. Collectively, these data suggest that the rolipram binding site and the catalytic domain on CPPDE4 might represent part of the same entity. In addition, these results support the concept that PDE4 can exist in different conformational states [Barnett, Manning, Cieslinski, Burman, Christensen and Torphy (1995) J. Pharmcol. Exp. Ther. 273, 674-679] and provide evidence that the cAMP content in macrophages is regulated primarily by a conformer of PDE4 for which rolipram has nanomolar affinity.

Cyclic nucleotide phosphodiesterase isoenzymes and asthma--outstanding issues

Cyclic nucleotide phosphodiesterase isoenzymes hydrolyse and thus inactivate the intracellular second messengers cyclic AMP and cyclic GMP. Inhibitors of these isoenzymes modulate tissue function by reducing the rate of breakdown of the cyclic nucleotides. Eukaryotic cells contain multiple forms of phosphodiesterase with differing regulatory characteristics and substrate specificities. At present, the majority of the identified isoenzymes can be grouped into five families (PDE I-PDE V). These isoenzymes have differing distributions and play differing relative roles in the hydrolysis of cyclic nucleotides between tissues. Thus, isoenzyme selective inhibitors may selectively modulate tissue function. Drugs which are therapeutically useful in asthma either bronchodilate or reduce the underlying inflammatory condition. Inhibitors of PDE III, PDE IV and PDE V relax airways smooth muscle. Inhibitors of PDE IV attenuate the activation of pro-inflammatory cells, an effect which in some assays is potentiated by additional PDE III inhibition. PDE V inhibition has not been shown to result in potentially useful anti-inflammatory activity. Despite various degrees of tissue selectivity, the possible side effect profile of isoenzyme selective phosphodiesterase inhibitors requires elucidation before these agents can be proposed as selective anti-asthma drugs. However, it is apparent that selective inhibitors of PDE III, PDE IV and PDE V may be useful bronchodilators, whilst PDE IV and PDE III/IV inhibitors also possess potentially useful anti-inflammatory activity. Such compounds require further evaluation in animals and man to clarify their full potential as therapeutic agents for the treatment of asthma.