Regulation of 4-integrin expression by epigenetic modifications in the mammary gland and during the epithelial-to-mesenchymal transition

The 4 integrin is expressed in epithelial cells, a few other cell types and in some carcinomas. Despite this restricted expression pattern and the functional importance of 4 integrin in epithelial and carcinoma biology, little is known about how its expression is regulated. Here, we assessed the epigenetic regulation of 4 integrin based on the presence of a large CpG island in the 4-integrin gene promoter. We separated basal (4⁺) and luminal (4^-) epithelial cells from the mammary glands of K14-eGFP mice and demonstrated that the 4-integrin promoter is unmethylated in basal cells and methylated in luminal cells. We also observed that expression of 4 integrin and E-cadherin is lost during the epithelial-to-mesenchymal transition (EMT) of mammary gland cells induced by transforming growth factor beta (TGF), which is coincident with de novo DNA methylation, a decrease in active histone modifications (H3K9Ac and H3K4me3) and an increase in the repressive histone modification H3K27me3. Furthermore, TGF withdrawal promotes a mesenchymal-to-epithelial transition (MET) and triggers the re-expression of 4 integrin and E-cadherin. Intriguingly, demethylation at either promoter is not obligatory for transcriptional reactivation after TGF withdrawal. However, both H3K9Ac and H3K4me3 modifications are restored during the MET, and H3K27me3 is reduced, strongly suggesting that reversible histone modifications rather than DNA demethylation are the predominant factors in reactivating expression of these genes. Our data indicate that complex epigenetic modifications contribute to the regulation of the 4 integrin and E-cadherin.