6-Mercaptopurine, an activator of Nur77, enhances transcriptional activity of HIF-1α resulting in new vessel formation

Hypoxia-inducible factor-1 α (HIF-1 α ) plays a central role in oxygen homeostasis. Previously, we reported that the orphan nuclear receptor Nur77 functions in stabilizing HIF-1 α . Here, we demonstrate that 6-mercaptopurine (6-MP), an activator of the NR4A family members, enhances transcriptional activity of HIF-1. 6-MP enhanced the protein-level of HIF-1 α as well as vascular endothelial growth factor (VEGF) in a dose- and time-dependent manner. The induction of HIF-1 α was abolished by the transfection of either a dominant-negative Nur77 mutant or si-Nur77, indicating a critical role of Nur77 in the 6-MP action. The HIF-1 α protein level remained up to 60 min in the presence of 6-MP when de novo protein synthesis was blocked by cycloheximide, suggesting that 6-MP induces stabilization of the HIF-1 α protein. The fact that 6-MP decreased the association of HIF-1 α with von Hippel–Lindau protein and the acetylation of HIF-1 α , may explain how 6-MP induced stability of HIF-1 α . Further, 6-MP induced the transactivation function of HIF-1 α by recruiting co-activator cyclic-AMP-response-element-binding protein. Finally, 6-MP enhanced the expression of HIF-1 α and VEGF, and the formation of capillary tubes in human umbilical vascular endothelial cells. Together, our results provide a new insight for 6-MP action in the stabilization of HIF-1 α and imply a potential application of 6-MP in hypoxia-associated human vascular diseases.