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Regulation of the multidrug resistance transporter P‐glycoprotein in multicellular tumor spheroids by hypoxia‐inducible factor‐1 and reactive oxygen species

Hypoxia in tumors is generally associated with chemoresistance and radioresistance. However, the correlation between the heterodimeric hypoxia‐inducible factor‐1 (HIF‐1) and the multidrug resistance transporter P‐glycoprotein (P‐gp) has not been investigated. Herein, we demonstrate that with increasing size of DU‐145 prostate multicellular tumor spheroids the pericellular oxygen pressure and the generation of reactive oxygen species decreased, whereas the α‐subunit of HIF‐1 (HIF‐1α) and P‐gp were up‐regulated. Furthermore, P‐gp was up‐regulated under experimental physiological hypoxia and chemical hypoxia induced by either cobalt chloride or desferrioxamine. The pro‐oxidants H2O2 and buthionine sulfoximine down‐regulated HIF‐1α and P‐gp, whereas up‐regulation was achieved with the radical scavengers dehydroascorbate, N‐acetylcysteine, and vitamin E. The correlation of HIF‐1α and P‐gp expression was validated by the use of hepatoma tumor spheroids that were either wild type (Hepa1) or mutant (Hepa1C4) for aryl hydrocarbon receptor nuclear translocator (ARNT), i.e., HIF‐1β. Chemical hypoxia robustly increased HIF‐1α as well as P‐gp expression in Hepa1 tumor spheroids, whereas no changes were observed in Hepa1C4 spheroids. Hence, our data demonstrate that expression of P‐gp in multicellular tumor spheroids is under the control of HIF‐1.