PDE4A and chronic obstructive pulmonary disease: Abbreviations: cAMP, cyclic adenosine monophosphate; BALF, Bronchoalveolar fluid; cGMP, cyclic Guanosine Monophosphate; DMSO, Dimethyl Sulfoxide; COPD, Chronic Obstructive Pulmonary Disease; FEV1, Forced Expiratory Volume in 1 s; FVC, Forced Vital Capacity; LABA, Long-acting β2-agonist; PDE4, Phosphodiesterases 4.