Oxygen therapy accelerates apoptosis induced by selenium compounds via regulating Nrf2/MAPK signaling pathway in hepatocellular carcinoma.

Selenium has good antitumor effects in vitro, but the hypoxic microenvironment in solid tumors makes its clinical efficacy unsatisfactory. We hypothesized that the combination with oxygen therapy might improve the treatment efficacy of selenium in hypoxic tumors through the changes of redox environment. In this work, two selenium compounds, Na2SeO3 and CysSeSeCys, were selected to interrogate their therapeutic effects on hepatocellular carcinoma (HCC) under different oxygen levels. In tumor-bearing mice, both selenium compounds significantly inhibited the tumor growth, and combined with oxygen therapy further reduced the tumor volume about 50 %. In vitro HepG2 cell experiments, selenium induced autophagy and delayed apoptosis under hypoxia (1 % O2), while inhibited autophagy and accelerated apoptosis under hyperoxia (60 % O2). We found that, in contrast to hypoxia, the hyperoxic environment facilitated the H2Se, produced by the selenium metabolism in cells, to be rapidly oxidized to generate H2O2, leading to inhibit the expression level of Nrf2 and to increase that of phosphorylation of p38 and MKK4, resulting in inhibiting autophagy and accelerating apoptosis. Once the Nrf2 gene was knocked down, selenium compounds combined with hyperoxia treatment would further activate the MAPK signaling pathway and further increase apoptosis. These findings highlight oxygen can significantly enhance the anti-HCC effect of selenium compounds through regulating the Nrf2 and MAPK signaling pathways, thus providing novel therapeutic strategy for the hypoxic tumors and pave the way for the application of selenium in clinical treatment.