Down-regulation of lncRNA MEG3 promotes chronic low dose cadmium exposure-induced cell transformation and cancer stem cell-like property

Abstract

Cadmium (Cd) is a toxic heavy metal and one of carcinogens that cause lung cancer. However, the exact mechanism of Cd carcinogenesis remains unclear. To investigate the mechanism of Cd carcinogenesis, we exposed human bronchial epithelial cells (BEAS-2B) to a low dose of Cd (2.5 μM, CdCl2) for 9 months, which caused cell malignant transformation and generated cancer stem cell (CSC)-like cells. The goal of this study is to investigate the underlying mechanism. The long non-coding RNA (lncRNA) microarray analysis showed that the expression level of a tumor suppressive lncRNA maternally expressed 3 (MEG3) is significantly down-regulated in Cd-transformed cells, which is confirmed by further q-PCR analysis. Mechanistically, it was found that chronic Cd exposure up-regulates the levels of DNA methyltransferases (DNMTs), which increases the methylation of the differentially methylated region (DMR) 1.5 kb upstream of MEG3 transcription start site to reduce MEG3 expression. Functional studies showed that stably overexpressing MEG3 in Cd-transformed cells significantly reduces their transformed phenotypes. Moreover, stably overexpressing MEG3 in parental non-transformed human bronchial epithelial cells significantly impaired the capability of chronic Cd exposure to induce cell transformation and CSC-like property. Further mechanistic studies revealed that the cell cycle inhibitor p21 level is reduced and retinoblastoma protein (Rb) phosphorylation is increased in Cd-transformed cells to promote cell cycle progression. In addition, Cd-transformed cells also expressed higher levels of Bcl-xL and displayed apoptosis resistance. In contrast, stably overexpressing MEG3 increased p21 levels and reduced Rb phosphorylation and Bcl-xL levels in Cd-exposed cells and reduced their cell cycle progression and apoptosis resistance. Together, these findings suggest that MEG3 down-regulation may play important roles in Cd-induced cell transformation and CSC-like property by promoting cell cycle progression and apoptosis resistance.