Abstract 1279: Caspase-4 is critical in estrogen-induced apoptosis in antihormone-resistant breast cancer

Acquired resistance to long-term antihormonal therapy in breast cancer evolves through two Phases over 5 years. Phase I develops within a year, and tumor growth occurs with either estradiol or tamoxifen. However, Phase II resistance develops after 5 years of therapy. Tamoxifen continues to stimulate tumor growth, however the resistant tumor cell evolves so that estradiol is no longer a survival signal, but instead induces apoptosis. This is the basis for the therapeutic use of estrogen to treat breast cancer. Unfortunately only 30% of tumors respond, so laboratory studies were designed to seek mechanisms to enhance apoptotic responses. We have compared the actions of estradiol in MCF-7 variants that are invulnerable to estradiol-induced apoptosis (wild-type WS8), rapidly induce apoptosis (5C) or exhibit delayed apoptosis (2A). Using Agilent 4×44k human oligonucleotide microarrays, gene expression profiling of these cell lines treated with and without E 2 was conducted over a 96 hour time course. Additionally, a long-term 9 day time course of 2A cells was conducted to capture its delayed apoptotic response. A statistically rigorous method was developed to analyze the time course expression profiles in which the area under the curve (AUC) was computed for all genes and compared between cell lines. Focusing on apoptotic genes, caspase-4 was the earliest caspase significantly induced in the 5C cells from 24-96 h, and in the 2A cells from 6-9 days. Caspase-4 is specifically associated with the accumulation of unfolded proteins and protein aggregation in endoplasmic reticulum stress-induced apoptosis. Consistent with this mechanism of apoptosis, the observed gene expression profiles indicated a lack of induction in both resistant cell lines compared to wild-type cells of genes involved in protein folding such as chaperones (e.g. HSPA13, HSP70 member 13), co-chaperones (e.g. AHSA1, activator of HSP90 protein ATPase homolog 1) and chaperonins (HSP60/HPSD1; TCP1, T-complex 1). Additionally, the pro-apoptotic factor Bim, which is also associated with endoplasmic reticulum stress, was induced in both resistant cell lines compared to wild-type cells. Caspase-4 induction was confirmed by quantitative PCR and immunoblotting. The specific caspase-4 inhibitor Z-LEVD-FMK not only reduced cleaved caspase 4, it also completely blocked PARP cleavage, and estradiol-induced apoptosis changes in morphology and growth. Based on these data, a strategy to sensitize cells to estradiol-induced apoptosis could be applied to improve clinical response rates in Phase II acquired resistance to antihormone therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1279.