NOS2 and S-nitrosothiol signaling induces DNA hypomethylation and LINE-1 retrotransposon expression

Significance

Nitric oxide is a multifaceted signaling molecule that affects multiple pathways and cellular systems. Here, we report that inducible nitric oxide synthase expression, which is strongly correlated with inflammation and poor outcomes in patients with cancer, substantially alters DNA methylation to regulate cellular plasticity. Our data connect inflammation mechanistically with stress signals, DNA demethylation, and genotoxic retrotransposon expression. Passive DNA demethylation occurs during conditions of reduced DNA (cytosine-5)–methyltransferase 1 (DNMT1) activity; however, the cellular pathways that control passive demethylation are not clear. Our results show that sustained cellular stress signals result in DNMT1 protein loss and DNA hypomethylation, similar to DNMT1 inhibition by 5-azacytidine. This implies that chronic inflammation drives cellular transformation via DNA hypomethylation and retrotransposon activation.

Abstract

Inducible nitric oxide synthase (NOS2) produces high local concentrations of nitric oxide (NO), and its expression is associated with inflammation, cellular stress signals, and cellular transformation. Additionally, NOS2 expression results in aggressive cancer cell phenotypes and is correlated with poor outcomes in patients with breast cancer. DNA hypomethylation, especially of noncoding repeat elements, is an early event in carcinogenesis and is a common feature of cancer cells. In addition to altered gene expression, DNA hypomethylation results in genomic instability via retrotransposon activation. Here, we show that NOS2 expression and associated NO signaling results in substantial DNA hypomethylation in human cell lines by inducing the degradation of DNA (cytosine-5)–methyltransferase 1 (DNMT1) protein. Similarly, NOS2 expression levels were correlated with decreased DNA methylation in human breast tumors. NOS2 expression and NO signaling also resulted in long interspersed noncoding element 1 (LINE-1) retrotransposon hypomethylation, expression, and DNA damage. DNMT1 degradation was mediated by an NO/p38-MAPK/lysine acetyltransferase 5–dependent mechanism. Furthermore, we show that this mechanism is required for NO-mediated epithelial transformation. Therefore, we conclude that NOS2 and NO signaling results in DNA damage and malignant cellular transformation via an epigenetic mechanism.