Cytoskeletal rearrangement and caspase activation in sphingosine 1-phosphate-induced lung capillary tube formation

Angiogenesis is a multistep process involving the endothelial cell (EC) cytoskeleton in migration, proliferation, and barrier stabilization. Although precise intracellular pathways by which angiogenic tube formation occurs remain poorly understood, we speculated that interactions between the cytoskeleton and apoptosis are involved and explored cytoskeletal rearrangement and caspase activation in human lung microvascular EC capillary-like tube formation induced by sphingosine 1-phosphate (Sph 1-P) and vascular endothelial growth factor (VEGF). Sph 1-P and VEGF enhance tube formation quantified by a Tube Immaturity Index (TII) generated from the ratio of cell number to tube length, with concomitant morphologic and actomyosin network changes. Angiogenesis was temporally grouped into three stages with early changes characterized by cortical actin localization, whereas midstage tube development demonstrated elongated EC with peripheral actin labeling with transcellular stress fibers. Late tube formation was characterized by broad actin distribution and presence of caspase-positive EC. Phosphorylated MLC immunoreactivity was present at all stages, suggesting that coordinate Rho kinase and MLCK involvement is important to Sph 1-P-induced cell motility; however, chemical inhibition of either MLCK or Rho kinase failed to alter early tube formation. To address whether gaps created by apoptosis expand the lumen, Sph 1-P-induced tubes were differentiated in the presence of caspase inhibitor z-Val-Ala-Asp-fluoromethylketone (zVAD-FMK). Capillary-like tube maturation, but not length, was decreased by zVAD-FMK treatment. These studies suggest that Sph 1-P may induce EC tube formation by regulating early cytoskeletal rearrangement, whereas EC apoptosis within capillary-like tubes is necessary for late stage Sph 1-P-induced tube maturation and lumen formation.