Deletion of endothelial cell-specific liver kinase B1 increases angiogenesis and tumor growth via vascular endothelial growth factor.

Liver kinase B1 (LKB1) is a serine/threonine protein kinase ubiquitously expressed in mammalian cells. It was first identified in Peutz-Jeghers syndrome as a tumor suppressor gene. Whether endothelial LKB1 regulates angiogenesis and tumor growth is unknown. In this study, we generated endothelial cell-specific LKB1-knockout (LKB1) mice by crossbreeding vascular endothelial-cadherin-Cre mice with LKB1 mice. Vascular endothelial growth factor (VEGF) level was highly co-stained in endothelial cells but not in macrophages in LKB1 mice. Consistently, LKB1 mouse tissues including the lung, skin, kidney and liver showed increased vascular permeability. Tumors implanted in LKB1 mice but not macrophage-specific LKB1-knockout mice grew faster and showed enhanced vascular permeability and increased angiogenesis as compared with those implanted in wild-type mice. Injection of VEGF-neutralizing antibody but not the isotype-matched control antibody decreased endothelial-cell angiogenesis and tumor growth in vivo. Furthermore, LKB1 deletion enhanced mouse retinal and cell angiogenesis, and knockdown of VEGF by small-interfering RNA decreased endothelial cell proliferation and migration. Re-expression of LKB1 or knockdown of VEGF receptor 2 decreased the overproliferation and -migration observed in LKB1 cells. Mechanistically, LKB1 could bind to the VEGF transcription factor, specificity protein 1 (Sp1), which then inhibited the binding of Sp1 to the VEGF promoter to reduce VEGF expression. Endothelial LKB1 may regulate endothelial angiogenesis and tumor growth by modulating Sp1-mediated VEGF expression.