Abstract
Alveolar capillary barrier disruption induces local edema and inflammation that impairs pulmonary function and promotes alveolar destruction in COPD. This study aimed to determine how cigarette smoke modulated the serine-threonine phosphatase protein phosphatase 2 A (PP2A) to alter the barrier function of human lung microvascular endothelial cells (HLMVECs). Cigarette smoke exposure lowered overall PP2A activity and enhanced endothelial permeability in HLMVECs. However, directly decreasing PP2A activity with Fostriecin significantly reduced endothelial cell permeability. Protein fractionation studies determined that cigarette smoke diminished cytosolic PP2A activity but increased membrane and cytoskeletal activity. These changes coincided with the translocation of PP2A to the membrane, which reduced occludin phosphorylation in the membrane. Cigarette smoke decreased protein tyrosine phosphatase 1B (PTP1B) activity, a PP2A activator which also counters calcium intracellular influx. The decrease in PTP1B activity correlated with reduced calcium efflux in endothelial cells and these changes in calcium flux regulated PP2A activity. Indeed, culturing endothelial cells in low calcium medium prevented the decrease in cytosolic PP2A activity mediated by cigarette smoke. Together, these findings outline a mechanism whereby cigarette smoke acts via calcium to traffic PP2A from the cytosol to the membrane where it dephosphorylates occludin to increase endothelial cell permeability.