Combinations of DNA methyltransferase and histone deacetylase inhibitors induce DNA damage in small cell lung cancer cells: correlation of resistance with IFN-stimulated gene expression.

Because epigenetic inhibitors can reduce cancer cell proliferation, we tested the hypothesis that concurrent inhibition of histone acetylation and DNA methylation could synergistically reduce the viability of small cell lung cancer (SCLC) cells. Sub-IC(50) concentrations of the DNA methyltransferase (DNMT) inhibitor decitabine (5-AZA-dC) and the histone deacetylase (HDAC) inhibitors (LBH589 or MGCD0103) synergistically reduced the proliferation of five of nine SCLC cell lines. Loss of viability of sensitive SCLC cells did not correlate with the inhibition of either DNMT1 or HDACs, suggesting nonepigenetic mechanisms for synergy between these two classes of epigenetic modulators. Because combinations of 5-AZA-dC and HDAC inhibitors had marginal effects on the apoptosis index, Comet assay was undertaken to assess DNA damage. MGCD0103 and 5AZA-dC cotreatment augmented DNA damage in SCLC cells, resulting in increased tail length and moment in Comet assays by 24 hours in sensitive cell lines (P < 0.01). Consistent with augmented DNA damage, combination of a DNMT and HDAC inhibitor markedly increased the levels of phospho-H2A.X in sensitive cells but not in resistant ones. Comparison of basal gene expression between resistant and sensitive cells identified markedly higher basal expression of IFN-stimulated genes in the resistant cell lines, suggesting that IFN-stimulated gene expression may determine SCLC cell sensitivity to epigenetic modulators or other DNA damaging agents.