Cul2 is required for the activity of hypoxia-inducible factor and vasculogenesis

CULLIN 2 (CUL2) is a component of the ElonginB/C-CUL2-RBX-1-Von Hippel-Lindau (VHL) tumor suppressor complex that ubiquitinates and degrades hypoxia inducible factor a (HIFa). HIFa is a transcription factor that mediates the expression of hypoxia-sensitive genes, including vascular endothelial growth factor (VEGF), which in turn regulates vasculogenesis. While CUL2 participates in degradation of HIFa, the potential role of CUL2 in the regulation of other cellular processes is less well established. In the present study, suppression of CUL2 expression by Cul2 siRNA inhibited HIFa transcriptional activation of the VEGF gene in vitro, indicating that CUL2 plays a role distinct from its known function in HIFa degradation. Since ARNT heterodimerizes with HIFa, we assessed whether CUL2 influenced ARNT expression. Cul2 siRNA inhibited the expression of endogenous ARNT. Ectopically expressed ARNT reversed the inhibition of HIF activity by Cul2 siRNA in the VEGF promoter, suggesting that CUL2 regulates HIF activation through ARNT. In 786-O cells lacking VHL, Cul2 siRNA suppressed the expression of both ARNT and VEGF, indicating that CUL2 regulates HIF activity independently of VHL. In transgenic zebrafish expressing GFP driven by the Flk promoter (a known HIF target), zCul2 morpholino blocked embryonic vasculogenesis in a manner similar to that caused by inhibition of VEGF-A. In the zebrafish embryos, zCul2 inhibited the expression of CUL2, VEGF and Flk-GFP protein, indicating that CUL2 is required for expression of other vasculogenic HIF targets. Taken together, CUL2 is required for normal vasculogenesis, at least in part mediated by its regulation of HIF-mediated transcription.