The Phenomenon of the Cross-Resistance of Breast Cancer to Target and Hormonal Drugs: The Role of Epigenetic Reconstruction

The rearrangement of molecular pathways and the activation of bypass signaling determine the progression of tumor cell resistance to various drugs that specifically block target signaling proteins. The present work was performed on the MCF-7 breast cancer cells and established sublines, resistant to mTOR inhibitor rapamycin or antiestrogen tamoxifen, developed under prolonged cell treatment with rapamycin or tamoxifen, respectively. We have shown that both resistant sublines demonstrate the cross-resistance to rapamycin and tamoxifen and are characterized with the common signaling changes, namely—blocking of the estrogen receptor α (ERα) transcriptional activity and constitutive activation of Akt signaling. Analysis of the epigenetic machinery revealed the drastic suppression of the level of DNA methyltransferase 3A (DNMT3A) in both the resistant sublines that were correlated with the demethylation of the LINE-1 repeats. Knockdown of the DNMT3A via siRNA results in the progression of partial resistance of MCF-7 cells to both tamoxifen and rapamycin, supporting the important role of DNA methylation in the formation of the resistant phenotype. Totally, the results obtained highlight the possible mechanism of the tumor cell resistance to targeting/hormonal drugs based on the rearrangement of DNA methylation profile and activation of the bypass signaling pathways.