¹Center for Hematology and Oncology Molecular Therapeutics, Taussig Cancer Center, Cleveland Clinic Foundation, Cleveland, OH, USA

Correspondence: Dr EC Borden, Center for Hematology and Oncology Molecular Therapeutics, Taussig Cancer Center, Cleveland Clinic Foundation, 9500 Euclid Avenue, R-40, Cleveland, OH 44195, USA. E-mail: bordene@ccf.org

²These authors contributed equally to the work.

Received 16 May 2006; Revised 7 June 2007; Accepted 7 June 2007; Published online 23 July 2007.

Abstract

Human melanoma cell lines, SK-MEL-3 and SK-MEL-28, despite induction of the proapoptotic cytokine, Apo2L/TRAIL, did not undergo apoptosis in response to interferons (IFN-2b or IFN-). Postulating that genes important for apoptosis induction by IFNs might be silenced by methylation, the DNA demethylating agent 5-aza-2'-deoxycytidine (5-AZAdC) was assessed. DR4 (TRAIL-R1) was identified as one of the genes reactivated by 5-AZAdC with a >3-fold increase in 8 of 10 melanoma cell lines. Pretreatment with 5-AZAdC sensitized SK-MEL-3 and SK-MEL-28 cells to apoptosis induced by IFN-2b and IFN-; methylation-specific PCR and bisulfite sequencing confirmed demethylation of 5'CpG islands of DR4 and flow cytometry showed an increase in DR4 protein on the cell surface. In cells with reactivated DR4, neutralizing mAB to TRAIL reduced apoptosis in response to IFN- or Apo2L/TRAIL. To further confirm the role of DR4, it was expressed by retroviral vector in SK-MEL-3 and SK-MEL-28 cells with reversal of resistance to IFN- and Apo2L/TRAIL. Thus, reexpressing DR4 by 5-AZAdC or retroviral transfection in melanoma cell in which promoter methylation had suppressed its expression, potentiated apoptosis by IFN-2b, IFN- and Apo2L/TRAIL. Reactivation of silenced proapoptotic genes by inhibitors of DNA methylation may enhance clinical response to IFNs or Apo2L/TRAIL.