ACTIVATION OF PHOSPHODIESTERASE-IV DURING DESENSITIZATION OF THE A(2A) ADENOSINE RECEPTOR-MEDIATED CYCLIC-AMP RESPONSE IN RAT PHEOCHROMOCYTOMA (PC12) CELLS

Prolonged activation of an A(2A) adenosine receptor significantly inhi bits the cellular response to subsequent stimulation (A(2A) desensitiz ation). We have reported previously that activation of phosphodiestera se (PDE) contributes to A(2A) desensitization in PC12 cells. In the pr esent study, we show that a type IV PDE (PDE4)-selective inhibitor (Ro 20-1724) effectively blocks the increase in PDE activity in desensiti zed cells. Thus, PDE4 appears to be the PDE specifically activated dur ing A(2A) desensitization in PC12 cells. Prolonged treatment of PC12 c ells with an A(2A)-selective agonist (CGS21680) leads to increased PDE 4 activity in a dose-dependent manner, which can be blocked by an A(2A )-selective antagonist [8-(3-chlorostyryl) caffeine]. Using two PDE4 a ntibodies, we were able to demonstrate that the levels of two PDE4-imm unoreactive bands (72 and 79 kDa) were increased significantly during A(2A) desensitization. Prolonged treatment with forskolin to elevate i ntracellular cyclic AMP contents also resulted in increased PDE4 activ ity. In addition, activation of PDE4 activity during A(2A) desensitiza tion could be blocked by a protein kinase A (PKA)-selective inhibitor (H89) and was not observed in a PKA-deficient PC12 cell line (A123). T aken together, activation of PDE4 via a cyclic AMP/PKA-dependent pathw ay plays a critical role in dampening the signal of the A(2A) receptor .