Suppression of hypoxia-inducible factor 1? (HIF-1?) by tirapazamine is dependent on eIF2? phosphorylation rather than the mTORC1/4E-BP1 pathway.

ABSTRACT: 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? inhibitor. In the study, for the first time, we found that TPZ acts in a novel manner to inhibit HIF-1? 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? 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? mRNA levels nor HIF-1? protein degradation are affected by TPZ. However, TPZ was found to be involved in HIF-1? translational regulation. Further studies revealed that the inhibitory effect of TPZ on HIF-1? protein synthesis is dependent on the phosphorylation of translation initiation factor 2? (eIF2?) 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? did not reverse the ability of TPZ to inhibit HIF-1? expression, suggesting that the HIF-1? inhibitory activity of TPZ is independent of its topoisomerase II? inhibition. In conclusion, our findings suggest that TPZ is a potent regulator of HIF-1? and provide new insight into the potential molecular mechanism whereby TPZ serves to reduce HIF-1? expression.

SUBMITTER: Zhang J 

PROVIDER: S-EPMC2976688 | biostudies-literature | 2010 Nov

REPOSITORIES: biostudies-literature