Regulation of adenosine 3′,5′-cyclic monophosphate (cAMP) accumulation in UMR-106 osteoblast-like cells:: Role of cAMP-phosphodiesterase and cAMP efflux

The present study aimed to define the role of adenosine 3',5'-cyclic monophosphate (cAMP)-phosphodiesterase (PDE) activity and the possible involvement of cAMP efflux on parathyroid hormone (PTH) stimulated intracellular cAMP accumulation in cultured osteoblast like UMR-106 cells. Treatment of the cells with 10 nM PTH (1-84) rapidly increased the level of intracellular cAMP. PTH stimulation also increased the cAMP efflux rate. The efflux of cAMP could only account for a minor part of the decrease in intracellular cAMP. Six peaks of cAMP-hydrolyzing PDE activity were separated by Q-Sepharose chromatography The first peak to elute was stimulated by Ca2+/calmodulin and provided less than 2% of the total eluted cAMP-PDE activity. The second peak, providing less than 4% of the cAMP-PDE activity, was stimulated 3-fold by 4 mu M cyclic GMP (cCMP) and was sensitive to the PDE2 isoenzyme-selective inhibitor erythro 9-(2-hydroxy-3-nonyl) adenine (EHNA). The third peak, providing less than 10% of the cAMP-PDE activity, was insensitive to rolipram, EHNA, Ca2+/calmodulin, and cGMP. Peaks 4, 5 and 6 were sensitive to rolipram (IC50 < 0.1 mu M) and provided approximately 85% of the total cAMP-hydrolyzing activity. It is concluded that cAMP PDE activity in UMR-106 cells plays a major role in the control of intracellular cAMP accumulation, whereas only moderate amounts of cAMP are extruded from the cells through cAMP efflux. The main cAMP-hydrolyzing PDE isozyme is cAMP-specific/rolipram-sensitive. Ca2+/calmodulin-stimulated PDE, cGMP stimulated PDE, and presently unidentified cAMP-specific/rolipram- insensitive PDE are also present in UMR-106 cells. (C) 1999 Elsevier Science Inc.