PDE4A and asthma: Furthermore, phosphodiesterase 4 (PDE4) inhibitors, such as roflumilast, which can inhibit the degradation of cAMP to increase the phosphorylation of PKA in cells, has been widely used in the treatment of COPD and asthma in clinic recently [44,45] and it is well-known that the main therapeutic effects of β2 receptor agonists, such as salmeterol and salbutamol, also result from of the activation of the cAMP/PKA signal pathway [46].