The role of phosphodiesterase 3, phosphodiesterase 5, and the inhibitory γ subunit of the retinal cyclic GMP phosphodiesterase, in pulmonary hypertension
Chronic treatment of rats (to induce pulmonary hypertension, PHT) for 14 days increased cGMP-inhibited. cAMP specific phosphodiesterase (PDE3), and cGMP binding, cGMP specific phosphodiesterase (PDE5) activities in selected branches of the pulmonary artery (MacLean et al., 1997). The objective of this study was to establish the molecular basis for these changes in both animal and cell models of PHT, and also to investigate the effect the PDE3 inhibitor SKF94836, and the PDE5 inhibitor slideafil, on isolated pulmonary arteries from normoxic and hypoxic rats. It was shown that PDE3A/B gene transcription was increased in the main, first, intrapulmonary and resistance pulmonary arteries. Transcript and protein levels of PDE5A2 in the main and first branch pulmonary arteries (PAs) were also increased by chronic hypoxia. In addition, the expression of PDE3A was increased in cultured human pulmonary smooth muscle cells (hPASMC) maintained under chronic hypoxic conditions for 14 days, and this may be mediated via a protein kinase A-dependent mechanism. The treatment of cells with 8-Br-cAMP mimicked chronic hypoxia, inducing increased PDE3A expression, while treatment with the protein kinase A selective inhibitor, H8 peptide, abolished chronic hypoxia-induced expression of PDE3A. Finally, the treatment of cultured hPASMC, with the inhibitor of NF-kB degradation Tosyl-Leucyl-Chloro-Ketone (TLCK, 100mM), substantially reduced PDE5 transcript levels, suggesting a role for this transcription factor in the regulation of PDE5 gene expression. This is of interest because NF-kB is activated by hypoxia (Muraoka et al. 2000, Aziz et al., 1997). Taken together, our results show that phenotypic changes in the expression of PDE3 and PDE5 might provide an explanation for some of the changes in vascular reactivity of pulmonary vessels from rats with PHT. Both SKF94836 (PDE3 inhibitor), and sildenafil (PDE5 inhibitor) were effective in producing a concentration-dependent relaxation in isolated PAs.