Genomic and epigenetic characterization of the arsenic-induced oncogenic microRNA-21.

As one of the first identified oncogenic microRNAs, the precise details concerning the transcriptional regulation and function of microRNA-21 (miR-21) are still not completely established. The miR-21 gene is situated on chromosome 17q23.2, positioned at the 3'-UTR of the gene that encodes vacuole membrane protein-1 (VMP1). In this current study, we presented evidence indicating that miR-21 possesses its own gene promoter, which can be found in the intron 10 of the VMP1 gene. Chromatin immunoprecipitation followed by global DNA sequencing (ChIP-seq) revealed the presence of a broad H3K4me3 peak spanning the entire gene body of the primary miR-21 and the existence of super-enhancer clusters in the close proximity to both the miR-21 gene promoter and the transcription termination site in arsenic (As3+)-induced cancer stem-like cells (CSCs) and human induced pluripotent stem cells (hiPSCs). In non-transformed human bronchial epithelial cells (BEAS-2B), As3+ treatment enhanced Nrf2 binding to both the host gene VMP1 of miR-21 and the miR-21 gene. Knockout of Nrf2 inhibited both the basal and As3+-induced expressions of miR-21. Furthermore, the As3+-enhanced Nrf2 peaks in ChIP-seq fully overlap with these super-enhancers enriched with H3K4me1 and H3K27ac in the miR-21 gene, suggesting that Nrf2 may coordinate with other transcription factors through the super-enhancers to regulate the expression of miR-21 in cellular response to As3+. These findings demonstrate the unique genetic and epigenetic characteristics of miR-21 and may provide insights into understanding the novel mechanisms linking environmental As3+ exposure and human cancers.

Tumor suppressive activity of AHR in environmental arsenic-induced carcinogenesis.

The aryl hydrocarbon receptor (AHR) is a highly conserved pleiotropic transcription factor that senses environmental pollutants, microbial products, and endogenous ligands. The transcriptional targets of AHR include phase I and phase II detoxification enzymes, as well as numerous signaling molecules that affect a wide spectrum of biological and biochemical processes in a manner of cellular context-dependent. In this review, we systematically assess the latest discoveries of AHR in carcinogenesis with an emphasis on its tumor suppressor-like property that represses the expression of genes in oncogenic signaling pathways. Additionally, we outline recent progress in our studies on the interaction among AHR, TGFb and NRF2 in cellular responses to arsenic and malignant transformation. Our findings indicate that AHR antagonized TGFb and NRF2, suggesting that AHR could serve as a potential tumor suppressor in arsenic-induced carcinogenesis. Notably, while AHR can exhibit both oncogenic and tumor-suppressive properties in cancer development and the generation of the cancer stem-like cells (CSCs), the tumor suppressor-like effect of AHR warrants further extensive exploration for the prevention and clinical treatment of cancers.