ARTICLE ABSTRACTApproximately 30% of aromatase-inhibitor–resistant, estrogen receptor–positive patients with breast cancer benefit from treatment with estrogen. This enigmatic estrogen action is not well understood and how it occurs remains elusive. Studies indicate that the unfolded protein response and apoptosis pathways play important roles in mediating estrogen-triggered apoptosis. Using MCF7:5C cells, which mimic aromatase inhibitor resistance, and are hypersensitive to estrogen as evident by induction of apoptosis, we define increased global protein translational load as the trigger for estrogen-induced apoptosis. The protein kinase RNA-like endoplasmic reticulum kinase pathway was activated followed by increased phosphorylation of eukaryotic initiation factor-2 alpha (eIF2α). These actions block global protein translation but preferentially allow high expression of specific transcription factors, such as activating transcription factor 4 and C/EBP homologous protein that facilitate apoptosis. Notably, we recapitulated this phenotype of MCF7:5C in two other endocrine therapy–resistant cell lines (MCF7/LCC9 and T47D:A18/4-OHT) by increasing the levels of phospho-eIF2α using salubrinal to pharmacologically inhibit the enzymes responsible for dephosphorylation of eIF2α, GADD34, and CReP. RNAi-mediated ablation of these genes induced apoptosis that used the same signaling as salubrinal treatment. Moreover, combining 4-hydroxy tamoxifen with salubrinal enhanced apoptotic potency.
These results not only elucidate the mechanism of estrogen-induced apoptosis but also identify a drugable target for potential therapeutic intervention that can mimic the beneficial effect of estrogen in some breast cancers.