Loss of MEG3 contributes to the enhanced migration and invasion in arsenic-induced carcinogenesis through NQO1/FSCN1 pathway.

Arsenic ranks at the top among all toxic metals and poses a serious threat to human health. Inorganic arsenite and arsenate compounds have been classified as human carcinogens in various types of cancers. Maternally expressed gene 3 (MEG3), a tumor suppressor that is commonly lost in cancer, was investigated in this study for its role in the migration and invasion of arsenic-transformed cells. Our results showed that MEG3 was downregulated in both arsenic-transformed cells (As-T) and cells treated with low doses of arsenic for three months (As-treated). The analysis using TCGA dataset revealed that MEG3 expression was significantly reduced in the tumor tissues from human lung adenocarcinoma (LUAD) and lung squamous cell carcinoma (LUSC) compared to normal lung tissues. The results from the methylation-specific PCR (MSP) assay demonstrated enhanced methylation in the MEG3 promoters in both As-T and As-treated cells, indicating that increased methylation of the MEG3 promoter caused MEG3 downregulation in these cells. Moreover, As-T cells displayed increased migration and invasion and higher levels of NAD(P)H quinone dehydrogenase 1 (NQO1) and fascin actin-bundling protein 1 (FSCN1). Consistently, the results from immunohistochemistry staining showed that both NQO1 and FSCN1 are highly expressed in human lung squamous cell carcinoma tissues compared to those in normal lungs. Knockdown of MEG3 in normal BEAS-2B cells also led to increased migration and invasion, along with elevated levels of NQO1 and FSCN1. The negative regulation of MEG3 on FSCN1 was restored by NQO1 overexpression in both As-T and BEAS-2B cells. The results from immunoprecipitation assays confirmed the direct binding of NQO1 to FSCN1. Overexpression of NQO1 increased migration and invasion abilities in BEAS-2B cells, while knockdown of NQO1 by its shRNA reduced these two hallmarks of cancer. Interestingly, the reduced migration and invasion by NQO1 knockdown were restored by FSCN1. Collectively, the loss of MEG3 upregulated NQO1, which in turn stabilized FSCN1 protein through its direct binding, resulting in elevated migration and invasion in arsenic-transformed cells.