Decrease of estrogen receptor expression and associated ERE-dependent transcription in MCF-7 breast cancer cells after oligomycin treatment

Abstract

Oligomycin at 0.01 μM produces very rapid decrease of [ 3 H ]estradiol (E 2)-binding capacity in MCF-7 cells maintained in culture in glucose- and serum-free medium. Loss of binding capacity was associated with elimination of the estrogen receptor (ER) as well as a decrease of basal expression of ERE-luciferase reporter gene. These effects were not due to major cell death as shown by MTT assay. Hence, the inhibition of ATP synthesis produced by oligomycin seems to influence ER turnover, resulting in very rapid loss of receptor. Withdrawal of oligomycin and maintenance of glucose in the medium led to only a partial reappearance of ER and failed to restore optimal ERE-dependent transcription. Oligomycin significantly down-regulated progesterone receptor (PR) level and partially abrogated E 2-induced PR up-regulation, indicating that this drug also affects other nuclear receptors. Treatment of cytosol from MCF-7 cells with acid and alkaline phosphatases decreased [ 3 H ]E 2-binding capacity, indicating the requirement of ER phosphorylation for optimal hormone binding. On the other hand, oligomycin-induced ER loss was partly compensated by E 2 and partial anti-estrogens (AEs; 4-OH-TAM or RU 39 411); i.e. oligomycin failed to improve the E 2-induced ER down-regulation and very weakly suppressed partial AE-induced receptor up-regulation. The known ability of these ligands to stabilize ER in the cell nucleus before regulating ER level may explain this phenomenon since such antagonism was not recorded with pure AE RU 58 668, which is known to impede nuclear translocation of the receptor. Interestingly, ligands able to down-regulate ER (i.e. E 2 or RU 58 668) increased ER phosphorylation while 4-OH-TAM which up-regulate the receptor had little effect in this regard. Oligomycin failed to strongly affect such phosphorylation enhancements while it produced a weak decrease of basal phosphorylation level. Hence, phosphorylations/dephosphorylations of specific sites on ER and/or co-regulators seem to govern its turnover.