Regulation of hypoxia inducible factor-1α expression by the alteration of redox status in HepG2 cells

Hypoxia inducible factor-1 (HIF-1) has been considered as a critical transcriptional factor in response to hypoxia. It can increase P-glycoprotein (P-Gp) thus generating the resistant effect to chemotherapy. At present, the mechanism regulating HIF-1 alpha is still not fully clear in hypoxic tumor cells. Intracellular redox status is closely correlated with hypoxic micro-environment, so we investigate whether alterations in the cellular redox status lead to the changes of HIF-1 alpha expression. HepG2 cells were exposed to Buthionine sulphoximine (BSO) for 12 h prior to hypoxia treatment. The level of HIF-1 alpha expression was measured by Western blot and immunocytochemistry assays. Reduce glutathione (GSH) concentrations in hypoxic cells were determined using glutathione reductase/5,5'-dithiobis-(2-nitrob-enzoic acid) (DTNB) recycling assay. To further confirm the effect of intracellular redox status on HIF-1 alpha expression, N-acetylcysteine (NAC) was added to culture cells for 8 h before the hypoxia treatment. The levels of multidrug resistance gene-1 (MDR-1) and erythropoietin (EPO) mRNA targeted by HIF-1 alpha in hypoxic cells were further determined with RT-PCR, and then the expression of P-Gp protein was observed by Western blotting. The results showed that BSO pretreatment down-regulated HIF-1 alpha and the effect was concentration-dependent, on the other hand, the increases of intracellular GSH contents by NAC could partly elevate the levels of HIF-1 alpha expression. The levels of P-Gp (MDR-1) and EPO were concomitant with the trend of HIF-1 alpha expression. Therefore, our data indicate that the changes of redox status in hypoxic cells may regulate HIF-1 alpha expression and provide valuable information on tumor chemotherapy.