Se-methylselenocysteine sensitizes hypoxic tumor cells to irinotecan by targeting hypoxia-inducible factor 1α

Purpose Hypoxic tumor cells overexpressing hypoxia-inducible factor 1alpha (HIF-1 alpha) are generally resistant to chemo/radiotherapy. We have reported that Se-methylselenocysteine (MSC) therapeutically enhances the efficacy and selectivity of irinotecan against human tumor xenografts. The aim of this study was to delineate the mechanism responsible for the observed efficacy targeting on HIF-1 alpha and its transcriptionally regulated genes VEGF and CAIX. Methods We investigated the mechanism of HIF-1 alpha inhibition by MSC and its critical role in the therapeutic outcome by generating HIF-1 alpha stable knockdown (KD) human head and neck squamous cell carcinoma, FaDu by transfecting HIF-1 alpha short hairpin RNA. Results While cytotoxic efficacy in combination with methylselenic acid (MSA) with SN-38 (active metabolites of MSC and irinotecan) could not be confirmed in vitro against normoxic tumor cells, the hypoxic tumor cells were more sensitive to the combination. Reduction in HIF-1 alpha either by MSA or shRNA knockdown resulted in significant increase in cytotoxicity of SN38 in vitro against hypoxic, but not the normoxic tumor cells. Similarly, in vivo, either MSC in combination with irinotecan treatment of parental xenografts or HIF-1 alpha KD tumors treated with irinotecan alone resulted in comparable therapeutic response and increase in the long-term survival of mice bearing FaDu xenografts. Conclusions Our results show that HIF-1 alpha is a critical target for MSC and its inhibition was associated with enhanced antitumor activity of irinotecan. Inhibition of HIF-1 alpha appeared to be mediated through stabilization of PHD2, 3 and downregulation of ROS by MSC. Thus, our findings support the development of MSC as a HIF-1 alpha inhibitor in combination chemotherapy.