Involvement of HP1α protein in irreversible transcriptional inactivation by antiestrogens in breast cancer cells

Resistance to 4-hydroxy-tamoxifen (OHT), which appears in breast cancer cells after long-term antiestrogen treatment, may involve irreversible changes of gene expression. We previously developed a MCF-7 derived cell line (MVLN), in which OHT rapidly and irreversibly inactivates the expression of an estrogen-regulated luciferase transgene (Vit-tk-luciferase). In chromatin immunoprecipitation experiments, heterochromatin protein 1 (HP1α) was found to be associated with the Vit-tk-luciferase transgene, only when it was inactivated by OHT treatment. Chimeras composed of either HP1α or the Krupple-associated box (KRAB) module of KOX-1 protein (known to repress gene expression by recruitment of HP1 proteins), fused to the estrogen receptor (ER)–DNA binding domain (DBD) and the androgen receptor (AR)–ligand binding domain (LBD) were generated and appeared as potent transcriptional repressors. In stably transfected MVLN cells, irreversible inactivation of the luciferase transgene expression obtained with HP1α–ER(DBD)–AR(LBD) was partial, whereas inactivation obtained with KRAB–ER(DBD)–AR(LBD) was comparable to that obtained with OHT, although with a slower kinetics. Altogether, these data suggest that HP1α is involved in the silencing effects associated with long-term OHT treatments.