Characterization of inhibitors of phosphodiesterase 1C on a human cellular system

Different inhibitors of the Ca2+/calmodulin‐stimulated phosphodiesterase 1 family have been described and used for the examination of phosphodiesterase 1 in cellular, organ or animal models. However, the inhibitors described differ in potency and selectivity for the different phosphodiesterase family enzymes, and in part exhibit additional pharmacodynamic actions. In this study, we demonstrate that phosphodiesterase 1C is expressed in the human glioblastoma cell line A172 with regard to mRNA, protein and activity level, and that lower activities of phosphodiesterase 2, phosphodiesterase 3, phosphodiesterase 4 and phosphodiesterase 5 are also present. The identity of the phosphodiesterase 1C activity detected was verified by downregulation of the mRNA and protein through human phosphodiesterase 1C specific small interfering RNA. In addition, the measured Km values (cAMP, 1.7 µm; cGMP, 1.3 µm) are characteristic of phosphodiesterase 1C. We demonstrate that treatment with the Ca2+ ionophore ionomycin increases intracellular Ca2+ in a concentration‐dependent way without affecting cell viability. Under conditions of enhanced intracellular Ca2+ concentration, a rapid increase in cAMP levels caused by the adenylyl cyclase activator forskolin was abolished, indicating the involvement of Ca2+‐activated phosphodiesterase 1C. The reduction of forskolin‐stimulated cAMP levels was reversed by phosphodiesterase 1 inhibitors in a concentration‐dependent way. Using this cellular system, we compared the cellular potency of published phosphodiesterase 1 inhibitors, including 8‐methoxymethyl‐3‐isobutyl‐1‐methylxanthine, vinpocetine, SCH51866, and two established phosphodiesterase 1 inhibitors developed by Schering‐Plough (named compounds 31 and 30). We demonstrate that up to 10 µm 8‐methoxymethyl‐3‐isobutyl‐1‐methylxanthine and vinpocetine had no effect on the reduction of forskolin‐stimulated cAMP levels by ionomycin, whereas the more selective and up to 10 000 times more potent phosphodiesterase 1 inhibitors SCH51866, compound 31 and compound 30 inhibited the ionomycin‐induced decline of forskolin‐induced cAMP at nanomolar concentrations. Thus, our data indicate that SCH51866 and compounds 31 and 30 are effective phosphodiesterase 1 inhibitors in a cellular context, in contrast to the weakly selective and low‐potency phosphodiesterase inhibitors 8‐methoxymethyl‐3‐isobutyl‐1‐methylxanthine and vinpocetine. A172 cells therefore represent a suitable system in which to study the cellular effect of phosphodiesterase 1 inhibitors. 8‐Methoxymethyl‐3‐isobutyl‐1‐methylxanthine and vinpocetine seem not to be suitable for the study of phosphodiesterase 1‐mediated functions.