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Purpose: Solid tumors encounter a growth-limiting hypoxic microenvironment as they develop. Hypoxia-inducible factors (HIF) play important roles in hypoxia-associated tumor development and therapeutic resistance. Targeting the HIF pathway (especially HIF-1α) represents a promising cancer treatment strategy. Here, we report a novel class of HIF-1α inhibitors and the possible molecular basis of inhibition.Experimental Design: We analyzed the inhibitory effects of clinically used topoisomerase II (TOP2)–targeting drugs on HIF-1α expression with a primary focus on mitoxantrone. The potential role of TOP2 in mitoxantrone-inhibited HIF-1α expression was studied using pharmacologic inhibition, a knockdown approach, and TOP2 mutant cells. Moreover, involvement of mitoxantrone in proteasome-mediated degradation, transcription, and translation of HIF-1α was examined.Results: The TOP2-targeting mitoxantrone, but neither doxorubicin nor etoposide (VP-16), strongly inhibited HIF-1α expression under hypoxic conditions in a dose- and time-dependent manner. Surprisingly, the mitoxantrone-mediated inhibition of HIF-1α expression was largely independent of two TOP2 isozymes, proteasomal degradation, and transcription. Furthermore, mitoxantrone inhibited HIF-1α expression and function in a similar fashion as cycloheximide, suggesting that mitoxantrone might inhibit HIF-1α via a blockage at its translation step. In vitro translation experiments using HIF-1α mRNA further confirmed inhibition of HIF-1α translation by mitoxantrone. Interestingly, levels of the polysome-bound HIF-1α and VEGF-A mRNA were elevated and decreased after mitoxantrone treatment, respectively.Conclusions: We have identified the TOP2-targeting compound, mitoxantrone, as an HIF-1α inhibitor possibly through a translation inhibition mechanism, suggesting the possibility of an additional anticancer activity for mitoxantrone. Clin Cancer Res; 17(15); 5026–37. ©2011 AACR.

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Purpose: Solid tumors encounter a growth-limiting hypoxic microenvironment as they develop. Hypoxia-inducible factors (HIF) play important roles in hypoxia-associated tumor development and therapeutic resistance. Targeting the HIF pathway (especially HIF-1α) represents a promising cancer treatment strategy. Here, we report a novel class of HIF-1α inhibitors and the possible molecular basis of inhibition. Experimental Design: We analyzed the inhibitory effects of clinically used topoisomerase II (TOP2)–targeting drugs on HIF-1α expression with a primary focus on mitoxantrone. The potential role of TOP2 in mitoxantrone-inhibited HIF-1α expression was studied using pharmacologic inhibition, a knockdown approach, and TOP2 mutant cells. Moreover, involvement of mitoxantrone in proteasome-mediated degradation, transcription, and translation of HIF-1α was examined. Results: The TOP2-targeting mitoxantrone, but neither doxorubicin nor etoposide (VP-16), strongly inhibited HIF-1α expression under hypoxic conditions in a dose- and time-dependent manner. Surprisingly, the mitoxantrone-mediated inhibition of HIF-1α expression was largely independent of two TOP2 isozymes, proteasomal degradation, and transcription. Furthermore, mitoxantrone inhibited HIF-1α expression and function in a similar fashion as cycloheximide, suggesting that mitoxantrone might inhibit HIF-1α via a blockage at its translation step. In vitro translation experiments using HIF-1α mRNA further confirmed inhibition of HIF-1α translation by mitoxantrone. Interestingly, levels of the polysome-bound HIF-1α and VEGF-A mRNA were elevated and decreased after mitoxantrone treatment, respectively. Conclusions: We have identified the TOP2-targeting compound, mitoxantrone, as an HIF-1α inhibitor possibly through a translation inhibition mechanism, suggesting the possibility of an additional anticancer activity for mitoxantrone. Clin Cancer Res; 17(15); 5026–37. ©2011 AACR.