Emergence of antiestrogen-resistant cells in MCF-7 cells during suppression of estrogen signaling is a widely accepted model of acquired breast cancer resistance to endocrine therapy. To obtain insight into the genomic basis of endocrine therapy resistance, we characterized MCF-7 monoclonal sublines that survived 21-day exposure to tamoxifen (T-series sublines) or fulvestrant (F-series sublines) and sublines unselected by drugs (U-series). All T/Fsublines were resistant to the cytocidal effects of both tamoxifen and fulvestrant. However, their responses to the cytostatic effects of fulvestrant varied greatly, and their remarkably diversified morphology showed no correlation with drug resistance. mRNA expression profiles of the U-sublines differed significantly from those of the T/F-sublines, whose transcriptomal responsiveness to fulvestrant was largely lost. A set of genes strongly expressed in the U-sublines successfully predicted metastasis-free survival of breast cancer patients. Most T/F-sublines shared highly homogeneous genomic DNA aberration patterns that were distinct from those of the U-sublines. Genomic DNA of the U-sublines harbored many aberrations that were not found in the T/F-sublines. These results suggest that the T/F-sublines are derived from a common monoclonal progenitor that lost transcriptomal responsiveness to antiestrogens as a consequence of genetic abnormalities many population doublings ago, not from the antiestrogen-sensitive cells in the same culture during the exposure to antiestrogens. Thus, the apparent acquisition of antiestrogen resistance by MCF-7 cells reflects selection of preexisting drug-resistant subpopulations without involving changes in individual cells. Our results suggest the importance of clonal selection in endocrine therapy resistance of breast cancer. acquired resistance ͉ clonal selection ͉ DNA copy number ͉ fulvestrant ͉ tumor heterogeneity A pproximately 50% to 70% of breast cancers are estrogen receptor (ER␣) positive without amplification of the ERBB2/HER2 gene (1). Such ER ϩ /HER2Ϫ breast cancers typically respond well to endocrine therapy (ET), which suppresses estrogen signaling in cancer cells and halts their proliferation (cytostatic effect) and/or induces apoptosis (cytocidal effect). Although ET is proven effective, its clinical benefit is seriously limited by drug resistance. Approximately 50% of ER ϩ metastatic breast cancers do not respond to ET at all (de novo resistance); even when they initially respond to ET, most eventually become resistant during the course of therapy (acquired resistance). Nearly 40% of early-stage breast cancers treated with ET relapse with ET-resistant disease. Thus, ET resistance is an important clinical challenge in breast cancer treatment.The MCF-7 cell line is an extensively studied cell culture model of ER ϩ /HER2 Ϫ breast cancer (2). Because proliferation and survival of MCF-7 cells in culture or as xenograft are strictly dependent on estrogen (3-5), this cell line has been widely used to study mechanisms of ET resis...

Human embryonic stem cells (hESCs) can be captured in a primed state in which they resemble the postimplantation epiblast, or in a naive state where they resemble the preimplantation epiblast. Naive-cell-specific culture conditions allow the study of preimplantation development ex vivo but reportedly lead to chromosomal abnormalities, which compromises their utility in research and potential therapeutic applications. Although MEK inhibition is essential for the naive state, here we show that reduced MEK inhibition facilitated the establishment and maintenance of naive hESCs that retained naive-cell-specific features, including global DNA hypomethylation, HERVK expression, and two active X chromosomes. We further show that hESCs cultured under these modified conditions proliferated more rapidly; accrued fewer chromosomal abnormalities; and displayed changes in the phosphorylation levels of MAPK components, regulators of DNA damage/repair, and cell cycle. We thus provide a simple modification to current methods that can enable robust growth and reduced genomic instability in naive hESCs.