p38 MAPK activation coupled to endocytosis is a determinant of endothelial monolayer integrity

We show in rat lung microvessel endothelial cells (RLMVEC) that endocytosis is a critical determinant of activation of mitogen-activated protein kinase ( MAPK) and thereby regulates endothelial monolayer integrity. In RLMVEC grown in serum-free medium, we observed that albumin supplementation induced the phosphorylation of p38 MAPK within 30 min, which persisted for up to 2 h. Engagement of the endocytic machinery regulated the activation of p38 MAPK that contributed to endothelial cell proliferation and reduction of apoptosis. We also observed an interaction between the caveolar protein caveolin-1 and p38 MAPK with reciprocal coimmunoprecipitation assays and colocalization using double-label immunofluorescence staining. Knockdown of caveolin-1 expression with small interfering RNA significantly reduced endocytosis and activation of p38 MAPK and interfered with the ability of endothelial cells to form a confluent monolayer. Thus caveolae-mediated endocytosis and concomitant activation of p38 MAPK may help to maintain endothelial monolayer integrity by signaling proliferation and survival of endothelial cells.