Sustained induction of epithelial to mesenchymal transition activates DNA methylation of genes silenced in basal-like breast cancers

Abstract

The active acquisition of epigenetic changes is a poorly understood but important process in development, differentiation, and disease. Our work has shown that repression of the p16/pRb pathway in human epithelial cells, a condition common to stem cells and many tumor cells, induces dynamic epigenetic remodeling resulting in the targeted methylation of a selected group of CpG islands. We hypothesized that cells in this epigenetically plastic state could be programmed by the microenvironment to acquire epigenetic changes associated with tumorigenesis. Here, we describe an in vitro model system where epigenetically plastic cells were placed in an environment that induced epithelial to mesenchymal transition (EMT) and led to a program of acquired de novo DNA methylation at targeted sites. In this model, we found that repression of E-cadherin transcription preceded the subsequent acquisition of methylated CpG sites. Furthermore, the induction of EMT was accompanied by de novo methylation of several other gene promoters, including those of the estrogen receptor and Twist. These data demonstrate that signals from the microenvironment can induce phenotypic and gene expression changes associated with targeted de novo epigenetic alterations important in tumor progression, and that these alterations occur through a deterministic, rather than stochastic, mechanism. Given the dynamic epigenetic reprogramming that occurs in these cells, DNA methylation profiles observed in human tumors may reflect the history of environmental exposures during the genesis of a tumor.epigenetic remodeling ͉ human mammary epithelial cells ͉ microenvironment ͉ ras T he heritable regulation of gene expression changes that are critical to processes such as differentiation and disease can be controlled by epigenetic modifications of proteins and DNA sequences. We recently reported that the repression of p16 INK4A in primary human mammary epithelial cells (HMEC) activates an E2F-mediated increase in proteins that remodel chromatin and causes targeted de novo DNA methylation at a non-random collection of loci (1). These studies show that cells can acquire epigenetic plasticity by altering the p16/pRb pathway, and that this program of acquired de novo methylation has a deterministic (predictable) rather than stochastic (random) pattern. Furthermore, the coordinated set of de novo DNA methylation events are preceded by, and dependent upon, the repression of gene expression. Thus, during cancer progression, one may envision that tumor cells can acquire epigenetic plasticity through repression of the p16/pRb pathway via mutations, deletions, or methylation (2), which then provides the potential for programming epigenetic events. These observations are reminiscent of studies that show the acquisition of promoter hypermethylation upon modulation of estrogen or retinoic acid signaling (3, 4). In these cell population-based studies it is unclear whether the nonrandom hypermethylation events observed are due to induction or selection. To explore this question...

Molecular insights on basal-like breast cancer

“…Otherwise, Dumont et al [41] proposed that DNA methylation profiles observed in BC may reflect the history of environmental exposures based on the induction of p16/Rb pathway and impact on epigenetic changes resulting from methylation of CpG islands associated with tumorigenesis [36,41]. Elsheik et al…”

Tumor microenvironment and breast cancer progression

“…68 In the same line, Hu applied a methylation-specific digital karyotyping technique to epithelial and myoepithelial cells and stromal fibroblasts from normal breast tissue and from in situ and invasive breast carcinomas. 69 He found distinct epigenetic alterations in all three cell types during breast tumorigenesis in a tumor stage-and cell type-specific manner, suggesting that epigenetic changes play a role in maintaining the abnormal cellular microenvironment in breast cancer.…”

Regulation of Axl receptor tyrosine kinase expression by miR-34a and miR-199a/b in solid cancer

“…Ninety percent of patient deaths in solid tumors are due to metastasis, therefore, an understanding of the process and pathogenesis at the systemic, cellular and molecular levels is one of the most ambitious goals in cancer research (Gupta and Massague, 2006;Esteller, 2007). Epigenetic changes in the promoters of classical and small non-coding (miRNA) genes has a pivotal role in the acquisition of tumorigenic and metastatic properties (Feinberg and Tycko, 2004;Dumont et al, 2008;Lujambio et al, 2008). Methylation occurs in specific genomic areas called CpG islands, especially within the promoter region of a gene, thereby preventing gene expression (Weber et al, 2007).…”