Abstract LB-029: Negative transcriptional and epigenetic regulation of DNA repair pathways by the heavy metals nickel and arsenic

Abstract

Abstract Environmental exposure to certain heavy metals, such as nickel and arsenic, has been implicated in a variety of human cancers, including lung, skin, digestive track, and bladder cancers. Importantly, the mechanism underlying the carcinogenicity of nickel and arsenic remains poorly understood as they do not induce direct DNA mutagenesis. However, they do lead to global changes in chromatin structure and transcription, many similar to the effects of hypoxia. Since hypoxia is known to regulate many different DNA repair pathways, we investigated whether nickel and arsenic may similarly affect cellular DNA repair. We discovered that nickel and arsenic can lead to alterations in DNA repair gene expression, stable gene silencing, and decreased DNA repair capacity. First, we measured protein and mRNA levels of different DNA repair genes after NiCl2 or NaAsO2 treatment. We found that both metals induced down-regulation of BRCA1, FANCD2, and MLH1 over 24 to 48 hours at both the protein and mRNA levels. These results were observed in several different cells lines (HeLa, MCF7, BEAS-2B) with one notable exception that high dose arsenic induced up-regulation of BRCA1, FANCD2, and MLH1 in lung cancer-derived cell lines (A549, HCC827, NCI-H460). Next, to study the impact of long-term heavy metal exposure on DNA repair gene expression, we utilized an MLH1 promoter reporter construct that allows selection of cells harboring a silenced MLH1 promoter with ganciclovir. RKO cells stably expressing this construct were grown in the presence of 100 μM NiCl2, 0.5 μM NaAsO2, 1% oxygen, or control conditions. After 3 weeks, we observed that arsenic treatment, like hypoxia, led to a significant increase in promoter silencing compared to control cells, peaking at about 3.7-fold after 4 weeks. Nickel did not increase silencing, which may indicate a different mechanism of gene regulation. Finally, we used a luciferase assay to measure the effect of nickel and arsenic on the two primary DNA double-strand break (DSB) repair mechanisms, homologous recombination (HR) and non-homologous end joining (NHEJ). BEAS-2B cells pretreated with 250 μM NiCl2 or 5 μM NaAsO2 were transfected with a digested, inactive luciferase plasmid and allowed to conduct DSB repair to reactivate luciferase expression. We found that nickel and arsenic led to a 40-50% reduction in cellular HR capacity with no significant effect on NHEJ. To further pursue these results, we are performing chromatin immunoprecipitation studies to identify transcriptional or epigenetic factors mediating nickel and arsenic-induced down-regulation of DNA repair genes. In addition, we are using chromosomal-based assays to further characterize the impact of nickel and arsenic on DNA repair capacity. In conclusion, we have found that nickel and arsenic negatively regulate cellular DNA repair pathways, identifying a novel way in which heavy metals may contribute to carcinogenesis. Citation Format: Susan E. Scanlon, Christine D. Scanlon, Denise C. Hegan, Parker Sulkowski, Peter M. Glazer. Negative transcriptional and epigenetic regulation of DNA repair pathways by the heavy metals nickel and arsenic. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-029.