Response of esophageal cancer cells to epigenetic inhibitors is mediated via altered thioredoxin activity

Abstract

We previously showed that histone deacetylase inhibitor (HDACi) and 5-azacytidine (AZA) treatment selectively induced cell death of esophageal cancer cells. The mechanisms of cancer selectivity, however, remained unclear. Here we examined whether the cancer selectivity of HDACi/AZA treatment is mediated by the thioredoxin (Trx) system and reactive oxygen species (ROS) in esophageal cancer cells. For this, we first analyzed human tissue specimens of 37 esophageal cancer patients by immunohistochemistry for Trx, Trx-interacting protein (TXNIP) and Trx reductase (TXNRD). This revealed a loss or at least reduction of nuclear Trx in esophageal cancer cells, compared with normal epithelial cells (P<0.001). Although no differences were observed for TXNIP, TXNRD was more frequently expressed in cancer cells (P<0.001). In the two main histotypes of esophageal squamous cell carcinomas (ESCCs, n=19) and esophageal adenomcarcinomas (EAC, n=16), similar Trx, TXNIP and TXNRD expression patterns were observed. Also in vitro, nuclear Trx was only detectable in non-neoplastic Het-1A cells, but not in OE21/ESCC or OE33/EAC cell lines. Moreover, the two cancer cell lines showed an increased Trx activity, being significant for OE21 (P=0.0237). After treatment with HDACi and/or AZA, ROS were exclusively increased in both cancer cell lines (P=0.048–0.017), with parallel decrease of Trx activity. This was variably accompanied by increased TXNIP levels upon AZA, MS-275 or MS-275/AZA treatment for 6 or 24 h in OE21, but not in Het-1A or OE33 cells. In summary, this study evaluated Trx and its associated proteins TXNIP and TXNRD for the first time in esophageal cancers. The analyses revealed an altered subcellular localization of Trx and strong upregulation of TXNRD in esophageal cancer cells. Moreover, HDACi and AZA disrupted Trx function and induced accumulation of ROS with subsequent apoptosis in esophageal cancer cells exclusively. Trx function is hence an important cellular mediator conferring non-neoplastic cell resistance for HDACi and/or AZA.