Abstract 4311: Targeting specific regulatory subunits of protein phosphatase 1 phenocopies estrogen-induced apoptosis

Abstract The growth of estrogen receptor positive (ER+) breast cancers is considered to be dependent on estrogen and its actions can be blocked by selective estrogen receptor modulators (SERMS), like tamoxifen, or by inhibiting the synthesis of estrogen, by aromatase inhibitors (AIs). Both of these classes of drug are used to treat ER+ breast cancers in the clinic. Paradoxically, before the discovery of SERMs and AIs, high dose estrogen was the choice of endocrine therapy to treat post-menopausal breast cancers. Recent clinical trials have confirmed a 30% clinical benefit rate with high, as well as low, doses of estrogen therapy in AI- resistant breast cancers. However, the underlying mechanism by which estrogen triggers the tumor regression remains unknown. Using an in vitro cell model MCF7:5C cells, that undergo estrogen-induced apoptosis, we demonstrate that sustained phosphorylation of eukaryotic initiation factor 2-alpha (eIF2-α), a downstream target of PERK activation, is crucial in estrogen-induced apoptosis. Growth arrest and DNA damage inducible-34 (GADD34) and constitutive repressor of eIF2α phosphorylation (CReP) are two regulatory subunits of protein phosphatase 1 (PP1) complex, that provides substrate specificity towards eIF2α, and are responsible for its de-phosphorylation. Our results show that pharmacological inhibition of GADD34 and CReP, or their genetic depletion, can enhance the phospho-eIF2α (p-eIF2α) levels and promote apoptosis. Elevated levels of p-eIF2α attenuated global translation but preferentially allowed high expression of activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP) that are involved in apoptosis. Both estrogen and inhibition of GADD34 and CReP promote apoptosis by an identical mechanism. Importantly, inhibition of GADD34 and CReP induced apoptosis in another estrogen-independent breast cancer cell line, LCC9, which is resistant to both tamoxifen and fulvestrant but does not undergo estrogen mediated apoptosis. Overall, our study provides crucial evidence that inhibiting specific regulatory subunits (GADD34 and CReP) of PP1 complex, can mimic estrogen-induced apoptosis in breast cancer cells that are not susceptible to estrogen mediated apoptosis. Therefore, GADD34 and CReP can be targeted for potential therapeutic intervention in endocrine therapy resistant breast cancers. Citation Format: Catherine Sevigny, Surojeet Sengupta, Kerrie B. Bouker, Robert Clarke. Targeting specific regulatory subunits of protein phosphatase 1 phenocopies estrogen-induced apoptosis [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 4311. doi:10.1158/1538-7445.AM2017-4311