Introduction

Aberrant DNA methylation is one of the well-known epigenetic changes in cancer, although its involvement in the chemoresistance remains to be elucidated. In this study, we aimed to unravel the roles played by DNA methylation in chemoresistance in bladder cancer (BCa).

Methods

We established gemcitabine (GEM)-resistant (T24-RG and UMUC3-RG) or cisplatin (CDDP)-resistant (T24-RC and UNUC3-RC) BCa cell lines by continuously treating their parental cells. Genome-wide DNA methylation was assessed by Infinium HumanMethylation450 BeadChip (HM450). To assess the chromatin accessibilities, cells were treated with a CpG methyltransferase M.SssI, after which DNA methylation was analyzed by HM450. To evaluate whether treatment with epigenetic drugs could overcome the chemoresistance in BCa, cells were treated with a DNA methyltransferase (DNMT) inhibitor 5-aza-2’-deoxycytidine (5-Aza-CdR) and/or a histone deacetylase (HDAC) inhibitor suberoylanilide hydroxamic acid (SAHA), after which they were treated with or without GEM or CDDP.

Results

HM450 assays revealed increased levels of methylation at a number of CpG sites in the resistant cells as compared the parental cells (730 sites in T24-RC and 3856 sites in UMUC3-RC, respectively), however, a number of CpG sites remained unmethylated but loss chromatin accessibility in the resistant cells (1391 sites in T24-RG and 1322 sites in UMUC3-RG, respectively. In CDDP-resistant BCa cells, a combination treatment with 5-Aza-CdR and CDDP synergistically suppressed cell proliferation, suggesting that 5-Aza-CdR restored CDDP-sensitivity (Figure A, B). In contrast, 5-Aza-CdR didn’t show any growth inhibitory effects in GEM-resistant cells (Figure C, D). Moreover, a treatment with SAHA with or without 5-Aza-CdR also failed to restore GEM-resistance in BCa cells (Figure E, F).

Conclusions

Our results suggest that epigenetic alteration may be one of key factors for drug resistance, drug resistance cells could be desensitized by DNA methylation inhibitors especially in the CDDP-resistance in BCa cells.

Funding

none