HIF-1α is believed to promote tumor growth and metastasis, and many efforts have been made to develop new anticancer agents based on HIF-1α inhibition. YC-1 is a widely used HIF-1α inhibitor both in vitro and in vivo, and is being developed as a novel class of anticancer drug. However, little is known about the mechanism by which YC-1 degrades HIF-1α. As the first step for understanding the mechanism of action of YC-1, we here identified the HIF-1α domain responsible for YC-1-induced protein degradation. YC-1 blocked the HIF-1α induction by hypoxia, iron chelation, and proteasomal inhibition and also degraded ectopically expressed HIF-1α. In deletion analyses, C-terminal HIF-1α was found to be sensitively degraded by YC-1. Using a GFP-fusion method, the YC-1-induced degradation domain was identified as the aa. 720-780 region of HIF-1α. We next tested the possible involvement of HDAC7 or OS-9 in YC-1-induced HIF-1α degradation. However, their binding to HIF-1α was not affected by YC-1, suggesting that they are not involved in the YC-1 action. It is also suggested that YC-1 targets a novel pathway regulating HIF-1α stability.

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