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(endo−/−)) mice by crossbreeding vascular endothelial-cadherin-Cre mice with LKB1(flox/flox) mice. Vascular endothelial growth factor (VEGF) level was highly co-stained in endothelial cells but not macrophages in LKB1(endo−/−) mice. Consistently, LKB1(endo−/−) mouse tissues including the lung, skin, kidney, and liver showed increased vascular permeability. Tumors implanted in LKB1(endo−/−) 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 over-proliferation and -migration observed in LKB1(endo−/−) 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.