Rapid protein kinase A--mediated activation of cyclic AMP-phosphodiesterase by parathyroid hormone in UMR-106 osteoblast-like cells.

Parathyroid hormone (PTH) plays an essential role in osteoblast proliferation and differentiation. The effects of PTH are known to be mediated by cyclic adenosine monophosphate (cAMP) and calcium and by the activation of protein kinase C (PKC). cAMP is hydrolyzed to the inactive form 5' AMP by cyclic nucleotide phosphodiesterases (PDEs). We have investigated the role of PTH on PDE regulation in UMR-106 osteoblast-like cells. Treatment with 10 nM PTH caused a 3-fold increase in the PDE activity. The activation of PDE could be seen within 2 minutes and reached maximal levels after 20 minutes. The PTH effect was dose dependent with a half-maximal dose of 2 nM. The effect of PTH could be mimicked by the cAMP analogs Bt2 cAMP and forskolin, but not by PTH fragment 3-34, calcium ionophore A23187, or by the PKC activator phorbol 12-myristate 13-acetate. The PDE activity stimulated by PTH could be abolished by the PKA inhibitor H-8. The PDE activated by PTH was inhibitable by low concentrations of the cAMP-PDE-specific inhibitor RO 20-1724 (IC50 = 0.2 microM), but not by low concentrations of the inhibitors of cGMP-stimulated and cGMP-inhibited PDEs MEP-1 and milrinone (IC50 for both compounds > 30 microM). The PTH-stimulated cAMP accumulation was potentiated about 7-fold in the presence of RO 20-1724. H-8 potentiated the PTH-stimulated cAMP accumulation about 4-fold. Our results show that PTH rapidly stimulates the activity of cAMP-PDE in UMR-106 cells. The PDE activation involves cAMP and PKA. Inhibition of PKA can abolish the PTH-stimulated PDE activation and leads to increased accumulation of intracellular cAMP.