Aquaporin-1 Facilitates Angiogenic Invasion in the Pathological Neovasculature that Accompanies Cirrhosis

Increasing evidence suggests that hepatic fibrosis and pathological angiogenesis are interdependent processes that occur in parallel. Endothelial cell invasion is requisite for angiogenesis, and thus studies of the mechanisms governing liver endothelial cell (LEC) invasion during cirrhosis are of great importance. Emerging research implicates amoeboid-type motility and membrane blebbing as features that may facilitate invasion through matrix-rich microenvironments. Aquaporins (AQPs) are integral membrane water channels, recognized for their importance in epithelial secretion and absorption. However, recent studies also suggest links between water transport and cell motility or invasion. Therefore, the purpose of this study was to test the hypothesis that AQP-1 is involved in amoeboid motility and angiogenic invasion during cirrhosis. AQP-1 expression and localization was examined in normal and cirrhotic liver tissues derived from human and mouse. AQP-1 levels were modulated in LEG using retroviral overexpression or small interfering RNA (siRNA) knockdown and functional effects on invasion, membrane blebbing dynamics, and osmotic water permeability were assayed. Results demonstrate that AQP-1 is up-regulated in the small, angiogenic, neovasculature within the fibrotic septa of cirrhotic liver. AQP-1 overexpression promotes fibroblast growth factor (FGF)-induced dynamic membrane blebbing in LEG, which is sufficient to augment invasion through extracellular matrix. Additionally, AQP-1 localizes to plasma membrane blebs, where it increases osmotic water permeability and locally facilitates the rapid, trans-membrane flux of water. Conclusion: AQP-1 enhances osmotic water permeability and FGF-induced dynamic membrane blebbing in LEG and thereby drives invasion and pathological angiogenesis during cirrhosis. (HEPATOLOGY 2010;52:238-248)