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 inhibits the cellular response to subsequent stimulation (A(2A) desensitization). We have reported previously that activation of phosphodiesterase (PDE) contributes to A(2A) desensitization in PC12 cells. In the present study, we show that a type IV PDE (PDE4)-selective inhibitor (Ro 20-1724) effectively blocks the increase in PDE activity in desensitized cells. Thus, PDE4 appears to be the PDE specifically activated during A(2A) desensitization in PC12 cells. Prolonged treatment of PC12 cells with an A(2A)-selective agonist (CGS21680) leads to increased PDE4 activity in a dose-dependent manner, which can be blocked by an A(2A)-selective antagonist [8-(3-chlorostyryl)caffeine]. Using two PDE4 antibodies, we were able to demonstrate that the levels of two PDE4-immunoreactive bands (72 and 79 kDa) were increased significantly during A(2A) desensitization. Prolonged treatment with forskolin to elevate intracellular cyclic AMP contents also resulted in increased PDE4 activity. In addition, activation of PDE4 activity during A(2A) desensitization could be blocked by a protein kinase A (PKA)- selective inhibitor (H89) and was not observed in a PKA-deficient PC12 cell line (A123). Taken together, activation of PDE4 via a cyclic AMP/PKA- dependent pathway plays a critical role in dampening the signal of the A(2A) receptor.