Kruppel-like Factor 2 Inhibits Hypoxia-inducible Factor 1α Expression and Function in the Endothelium

Hypoxia-inducible factor 1 (HIF-1) is a central regulator of the hypoxic response in many cell types. In endothelial cells, HIF-1 induces the expression of key proangiogenic factors to promote angiogenesis. Recent studies have identified Kruppel-like factor 2 (KLF2) as a potent inhibitor of angiogenesis. However, the role of KLF2 in regulating HIF-1 expression and function has not been evaluated. KLF2 expression was induced acutely by hypoxia in endothelial cells. Adenoviral overexpression of KLF2 inhibited hypoxia-induced expression of HIF-1 alpha and its target genes such as interleukin 8, angiopoietin-2, and vascular endothelial growth factor in endothelial cells. Conversely, knockdown of KLF2 increased expression of HIF-1 alpha and its targets. Furthermore, KLF2 inhibited hypoxia-induced endothelial tube formation, whereas endothelial cells from mice with haploinsufficiency of KLF2 showed increased tube formation in response to hypoxia. Consistent with this ex vivo observation, KLF2 heterozygous mice showed increased microvessel density in the brain. Mechanistically, KLF2 promoted HIF-1 alpha degradation in a von Hippel-Lindau protein-independent but proteasome-dependent manner. Finally, KLF2 disrupted the interaction between HIF-1 alpha and its chaperone Hsp90, suggesting that KLF2 promotes degradation of HIF-1 alpha by affecting its folding and maturation. These observations identify KLF2 as a novel inhibitor of HIF-1 alpha expression and function. Therefore, KLF2 may be a target for modulating the angiogenic response in disease states.