Abstract

We analyzed whether the cyclic AMP induced down-regulation of the nitric oxide (NO) receptor soluble guanylyl cyclase (sGC) is mediated by the mRNA-protecting protein HuR. Exposure (up to 24 h) of isolated rat aortic segments to the activator of adenylyl cyclase, forskolin (10 μM), and to both activators of cAMP-stimulated protein kinase (PKA), 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole-3′,5′-cyclic monophosphorothioate, Spisomer (Sp-5,6-DCl-cBIMPS; 400 nM), and N⁶-phenyl-cAMP (10 μM), strongly reduced sGCα₁β₁ and HuR protein and mRNA expression in a time-dependent and actinomycin D (10 μM)-sensitive fashion. In vitro degradation of sGCα₁ and β₁ poly(A)⁺ mRNA by native rat aortic protein was markedly increased by pretreatment of intact aortas with forskolin. Native protein extract from rat aorta shifted the electrophoretic mobility of biotin-labeled riboprobes from the 3′-untranslated region of sGCα₁ and β₁ mRNA, and these bands was supershifted by a monoclonal antibody directed against the mRNA-stabilizing protein HuR. Forskolin decreased the HuR-sGCα₁ and β₁ mRNA interaction and HuR protein expression in rat aorta, and this was prevented by the PKA inhibitory cAMP analog 3′,5′-cyclic monophosphorothioate, Rp-isomer (Rp-cAMPS). In cultured smooth muscle cells from rat aorta, forskolin induced a rapid increase in Fos/p-Fos protein levels and activator protein 1 (AP-1) binding activity. Inhibition of this transcription factor by an AP-1 decoy prevented the forskolin-induced down-regulation of HuR. We conclude that forskolin/cAMP decrease the expression of heterodimeric sGC in rat aortic smooth muscle cells via activation of Fos/AP-1, which decreases the expression of HuR and thus destabilizes the sGCα₁ and β₁ mRNA.