%0 Journal Article
%T Suppression of Hypoxia-Inducible Factor 1¦Á (HIF-1¦Á) by Tirapazamine Is Dependent on eIF2¦Á Phosphorylation Rather Than the mTORC1/4E-BP1 Pathway
%A Jun Zhang
%A Ji Cao
%A Qinjie Weng
%A Rui Wu
%A Yan Yan
%A Hui Jing
%A Hong Zhu
%A Qiaojun He
%A Bo Yang
%J PLOS ONE
%D 2012
%I Public Library of Science (PLoS)
%R 10.1371/journal.pone.0013910
%X Hypoxia-inducible factor 1 (HIF-1), a heterodimeric transcription factor that mediates the adaptation of tumor cells and tissues to the hypoxic microenvironment, has attracted considerable interest as a potential therapeutic target. Tirapazamine (TPZ), a well-characterized bioreductive anticancer agent, is currently in Phase II and III clinical trials. A major aspect of the anticancer activity of TPZ is its identity as a tumor-specific topoisomerase II¦Á inhibitor. In the study, for the first time, we found that TPZ acts in a novel manner to inhibit HIF-1¦Á accumulation driven by hypoxia or growth factors in human cancer cells and in HepG2 cell-derived tumors in athymic nude mice. We investigated the mechanism of TPZ on HIF-1¦Á in HeLa human cervical cancer cells by western blot analysis, reverse transcription-PCR assay, luciferase reporter assay and small interfering RNA (siRNA) assay. Mechanistic studies demonstrated that neither HIF-1¦Á mRNA levels nor HIF-1¦Á protein degradation are affected by TPZ. However, TPZ was found to be involved in HIF-1¦Á translational regulation. Further studies revealed that the inhibitory effect of TPZ on HIF-1¦Á protein synthesis is dependent on the phosphorylation of translation initiation factor 2¦Á (eIF2¦Á) rather than the mTOR complex 1/eukaryotic initiation factor 4E-binding protein-1 (mTORC1/4E-BP1) pathway. Immunofluorescence analysis of tumor sections provide the in vivo evidences to support our hypothesis. Additionally, siRNA specifically targeting topoisomerase II¦Á did not reverse the ability of TPZ to inhibit HIF-1¦Á expression, suggesting that the HIF-1¦Á inhibitory activity of TPZ is independent of its topoisomerase II¦Á inhibition. In conclusion, our findings suggest that TPZ is a potent regulator of HIF-1¦Á and provide new insight into the potential molecular mechanism whereby TPZ serves to reduce HIF-1¦Á expression.
%U http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0013910