Immunopharmacological Potential of Selective Phosphodiesterase Inhibition. II. Evidence for the Involvement of an Inhibitory-κB/Nuclear Factor-κB-Sensitive Pathway in Alveolar Epithelial Cells

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Abstract
In this report we investigated the immunopharmacological role of selective and nonselective phosphodiesterase (PDE) inhibition in regulating the inhibitory-κB (IκB-α)/nuclear factor-κB (NF-κB) signaling transduction pathway. In fetal alveolar type II epithelial cells, PDE blockade at the level of the diverging cAMP/cGMP pathways differentially regulated the phosphorylation and degradation of IκB-α, the major cytosolic inhibitor of NF-κB. Whereas selective inhibition of PDEs 1, 3, and 4, by the action of 8-methoxymethyl-3-isobutyl-1-methylxanthine, amrinone, and rolipram, respectively, exhibited a tendency to augment the translocation of NF-κB1 (p50), RelA (p65), RelB (p68), and c-Rel (p75), selective blockade of PDE 5, 6, and 9, by the action of 4-{[3′,4′-(methylenedioxy)benzyl]amino}-6-methoxyquinazoline and zaprinast, attenuated lipopolysaccharide-endotoxin (LPS)-mediated NF-κB translocation. Pentoxifylline, a nonspecific PDE inhibitor, reversed the excitatory effect of LPS on NF-κB subunit nuclear localization, in a dose-dependent manner. Furthermore, analysis of NF-κB activation under the same conditions revealed a biphasic effect mediated by LPS. PDEs 1, 3, and 4 inhibition was associated with up-regulating NF-κB transcriptional activity. In contrast, blockading the activity of PDEs 5, 6, and 9 negatively attenuated LPS-mediated NF-κB activation, similar to the effect of 3,7-dihydro-3,7-dimethyl-1-(5-oxohexyl)-1H-purine-2,6-dione (pentoxifylline). These results indicate that selective and nonselective interference with the control of the dynamic equilibrium of cyclic nucleotides via PDE isoenzyme regulation represents an immunoregulatory mechanism that requires the differential, biphasic targeting of the IκB-α/NF-κB pathway.