Stress responses are critical for estrogen (E2) to induce apoptosis in E2-deprived breast cancer cells. Nuclear factor-kappa B (NF-κB) is well known as a therapeutic target to prevent stress responses in chronic inflammatory diseases including cancer. However, whether E2 activates NF-κB to participate in stress-associated apoptosis in E2-deprived breast cancer cells is unclear. We demonstrated that E2 differentially modulates NF-κB activity in E2-deprived breast cancer cells according to the treatment time. Because E2 initially has significant potential to down modulate the NF-κB activation, it completely suppresses the tumor necrosis factor alpha (TNFα)-induced NF-κB activation. We found that E2 preferentially and constantly enhances the expression of transcription factor CCAAT/enhancer binding protein beta (C/EBPβ) which is responsible for suppression of NF-κB activation by E2 in MCF-7:5C cells. The mTOR signaling pathway promotes repression of NF-κB by C/EBPβ which is confirmed by the evidence that inhibition of mTOR is synergistic with E2 to upregulate NF-κB-dependent genes, such as TNFα. Interestingly, NF-κB p65 activity is upregulated when E2-treatment is administered for 48 hours, leading to induction of TNFα. Blocking the nuclear translocation of NF-κB completely prevents E2 from induction of TNFα and apoptosis. Importantly, protein kinase RNA-like endoplasmic reticulum kinase (PERK), a stress sensor of unfolded protein response, is activated by E2 and plays an essential role in increasing NF-κB p65 DNA binding through the activation of STAT3, independently of canonical IκBα signal pathway. Thus, inhibition of PERK kinase activity completely blocks nuclear activation of NF-κB and NF-κB-dependent induction of TNFα, thereby preventing E2-induced apoptosis. All of these findings illustrate a crucial role for the novel PERK/NF-κB/TNFα axis in E2-induced apoptosis which is integrally modulated by the stress responsive transcription factor C/EBPβ and endoplasmic reticulum stress. This study provides an important rationale for exercising caution in clinical trials when considering targeting PERK or NF-κB following the development of acquired resistance to aromatase inhibitors whereas mTOR may be a target to enhance the therapeutic effects of E2 in antihormone resistant breast cancer.
Citation Format: Ping Fan, Amit K. Tyagi, Fadeke A. Agboke, Niranjana Pokharel, V. Craig Jordan. Integral modulation of nuclear factor-kappa B activation by C/EBPβ and the endoplasmic reticulum stress sensor PERK to mediate estrogen-induced apoptosis in estrogen-deprived breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2332. doi:10.1158/1538-7445.AM2017-2332