Abstract 955: Role of cMYC as a critical determinant of estrogen-independent growth of estrogen deprivation-resistant breast cancer cells

Abstract

Abstract Around two-third of all breast cancers express estrogen receptor alpha (ER), which depend upon estrogen for proliferation of the ER positive breast cancer cells. Aromatase inhibitors (AI) are used extensively to treat postmenopausal patients, which block the synthesis of estrogen and render an “estrogen-deprived” environment. Unfortunately, long-term treatment with AIs is associated with “AI-resistant” breast cancers, which by-passes the dependency on estrogen for the growth of the resistant breast cancer cells. To mimic these resistant breast cancers we have developed an estrogen-deprivation (ED) resistant cell line known as MCF7:5C cells from the parental ER positive MCF7 cells by propagating them in estrogen-free conditions for more than a year. MCF7:5C cells are ER positive and grow spontaneously in vitro as well as form xenograft tumors in the athymic mice in absence of estrogen (ref: Lewis et al., JNCI, 2005; 97(23):1746-59). To understand the underlying mechanism and identify the molecular determinant responsible for the estrogen-independent growth of these breast cancer cells we found that a critical estrogen-regulated gene, cMYC, is upregulated in these ED-resistant MCF7:5C cells. Basal levels of cMYC mRNA as well as the protein level were elevated three to four fold as compared to parental MCF7 cells which are dependent on estrogen for growth. Cell cycle analysis revealed twice as many “S” phase MCF7:5C cells as compared to MCF7 cells under basal condition. Treating the cells with a cMYC inhibitor, 10058-F4, decreased the number of ‘S’ phase cells and increased the cells in ‘G1′ phase in MCF7:5C cells but not in parental MCF7 cells, in a dose dependent manner. Interestingly, treatment with fulvestrant, a complete anti-estrogen, which inhibits the growth of the MCF7:5C cells by 50% also decreased the cMYC mRNA levels by similar extent. To understand the precise mechanism by which high levels of cMYC transcripts are synthesized in MCF7:5C cells we investigated the promoter of the cMYC gene and found that under basal conditions the recruitment of phosphor-serine-2-RNA polymeraseII, which is a marker of RNA elongation, was significantly higher in MCF7:5C cells as compared to the parental MCF7 cells. This study strongly indicates cMYC as a critical determinant responsible for estrogen-independent growth of ED-resistant breast cancer cells. Grant Support: This work (VCJ) was supported by the Department of Defense Breast Program under Award number W81XWH-06-1-0590 Center of Excellence; the Susan G Komen For The Cure Foundation under Award number SAC100009. 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 955. doi:1538-7445.AM2012-955