Epigenetic regulation of microRNAs in breast epithelial cells derived from progenitor cells exposed to environmental estrogen

Abstract

5060

Prolonged exposure to environmental endocrine disruptors is known to increase the risk of developing breast cancer. These hormonally active agents may disrupt normal development of mammary epithelia and support the growth of breast tumors. Animal and epidemiological studies further suggest an imprinting phenomenon by which early exposure to estrogenic chemicals potentiates a carcinogenic process of breast cancer. Furthermore, in vivo studies provide strong evidence that estrogen imprinting can be mediated by epigenetic changes. Here, we propose that long-lived, self-regenerated stem and progenitor cells are more susceptible to exposure injuries than terminally differentiated epithelial cells in the breast duct. Mammospheres, containing enriched breast progenitors, are used as an in vitro system to investigate the long term significance of environmental exposure by diethylstilbestrol (DES) in differentiated progeny cells. Recent studies suggest that microRNAs (miRNAs, small non-coding RNAs that post-transcriptionally regulate gene expression) are targets of epigenetic modifications such as DNA methylation. miRNAs regulate a large number of cellular processes, including tumorigenesis, and are therefore potential targets of DES perturbation. We used a microRNA microarray to profile the expression of breast epithelial cells derived from DES-exposed progenitor cells. Our data revealed that the expression of a subset of miRNAs (38 out of 884) was reduced in response to DES treatment in comparison to untreated cells. The genomic locations of three of these miRNAs (hsa-miR-92b, hsa-miR-433, and hsa-miR-320) is close to annotated CpG islands whereas the remaining miRNAs are over 1.5-kb from CG-rich regions. Interestingly, some of these down-regulated miRNAs (e.g., miR-20a, miR-16-1, and miR-15) are capable of suppressing Bcl-2 activities while others (e.g., miR-106a) are involved in modulating Rb expression. We validated a subset of these DES-perturbed targets using quantitative RT-PCR and are currently evaluating the methylation status in and around these miRNAs and identifying their functions in DES-exposed breast progenitor cells.