Resistance to endocrine therapy occurs in nearly 50% of all patients diagnosed with estrogen receptor ≤ positive breast cancer, thus elucidating mechanism(s) of endocrine resistance is of major importance. To provide a model, we selected MCF7 variants that are estrogen independent and either sensitive (LCC1) or resistant (LCC9) to antiestrogens: Faslodex (FAS; ICI 182, 780) and Tamoxifen (TAM). Antiestrogens induce endoplasmic reticulum stress which, depending on cell type, stimulus, and duration, may subsequently activate the unfolded protein response (UPR), autophagy, and/or apoptosis. Gene microarray analysis of LCC9 cells show high expression levels of X-box binding protein 1 (XBP1), implicating the unfolded protein response (UPR) as a pro-survival mechanism of resistance. In addition to an elevated UPR, autophagy is substantially elevated in response to antiestrogens. To better understand the role of autophagy in antiestrogen responsiveness, we investigated the consequence of knocking down BECLIN-1 in LCC1 and LCC9 cells treated with FAS and TAM. Preliminary results indicate a time-dependent change in autophagy following knock-down of beclin-1 in response to short-term (48h) and long-term (6 day) treatment. At 48h, sensitive LCC1 cells respond to antiestrogens with high levels of autophagy measured by increased LC3BII to LC3BI ratio and decreased expression of p62. Relative cell density (measured by crystal violet assay) decreases by 20% at 48h and by 80% at 6 days in LCC1 cells however, LCC9 cells are minimally affected by antiestrogen treatment. The level of autophagy remains high at 6 days in sensitive cells however levels remain constant in resistant cells. Autophagy is significantly lowered by reduced BECN1 in both sensitive and resistant cells however there is an increase in cell density in sensitive cells transfected with BECN1- shRNA at 6 days, indicating an autophagy associated programmed cell death mechanism in response to antiestrogen treatment. Protein expression of an active form of caspase −12/4 (an initiator caspase activated during endoplasmic reticulum stress) is elevated in sensitive cells at 6 days in response to antiestrogen treatment, also indicating the onset of apoptosis at this time. Elevated expression of Bcl-2 family members in resistant cells may play a more substantial role than BECN1 alone, as the combination of Bcl-2 family protein inhibitors and BECN1-shRNA sensitizes resistant cells to a greater extent. Thus, Bcl-2 family members in resistant cells may play a critical role in managing the balance between cell survival (moderate autophagy) and programmed cell death (excessive autophagy and apoptosis) in response to antiestrogen treatment.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2266. doi:1538-7445.AM2012-2266

©2012 American Association for Cancer Research
2012