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

Abstract

Hypoxia-inducible factor-1 alpha (HIF-1 alpha) plays a central role in oxygen homeostasis. Previously, we reported that the orphan nuclear receptor Nur77 functions in stabilizing HIF-1 alpha. 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 alpha as well as vascular endothelial growth factor (VEGF) in a dose- and time-dependent manner. The induction of HIF-1 alpha 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 alpha 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 alpha protein. The fact that 6-MP decreased the association of HIF-1 alpha with von Hippel - Lindau protein and the acetylation of HIF-1 alpha, may explain how 6-MP induced stability of HIF-1 alpha. Further, 6-MP induced the transactivation function of HIF-1 alpha by recruiting co-activator cyclic-AMP-response-element- binding protein. Finally, 6-MP enhanced the expression of HIF-1 alpha 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-1a and imply a potential application of 6-MP in hypoxia-associated human vascular diseases.