Abstract 4559: Knockdown of cyclin G2 expression hinders the cell cycle arrest response of MCF-7 cells to estrogen receptor signaling-antagonists and treatment with the antidiabetic metformin

Ligand activated estrogen receptors (ER) drive growth of nearly two-thirds of breast cancers (BC). Acquired BC tumor resistance to endocrine-based therapeutics that inhibit ER signaling poses a significant challenge for long-term abatement of ER-positive BCs. Signaling crosstalk between activated ER and peptide growth factor receptor pathways (e.g. HER2 and IGF-1R) is one adaptive mechanism promoting ER-positive BC tumor resistance to drugs inhibiting estrogen (E2) signaling. Definition of the cell cycle control proteins that modify tumor cell resistance to the ER-antagonists tamoxifen and fulvestrant could improve future therapeutic approaches for control of BC. Cyclin G2 (CycG2) is an unconventional cyclin upregulated during cell cycle arrest responses to a variety of cellular stresses and growth inhibitory conditions. Transcription of the gene encoding CycG2, CCNG2, is directly repressed by E2-bound ER complexes in BC cell lines. Our previous studies showed that blockade of HER2, PI3K and mTOR signaling upregulates CycG2 expression in HER2 overexpressing BC cells, and that ectopic CycG2 expression induces cell cycle arrest in BC cell lines. Here we show that pharmacological blockade of ER-signaling in the E2-dependent BC cell lines MCF-7 and T-47D enhances the expression and nuclear localization of CycG2. shRNA-mediated knockdown of CycG2 in E2-deprived and fulvestrant-treated MCF-7 cells dampened the cell cycle arrest response to these treatments. Our evidence suggests that loss of CycG2 increases phospho-activation of MEK1 and inhibitory phosphorylation of RB. Interestingly our work also indicates that CycG2 can form complexes with CDK10, an orphan CDK linked to inhibition of tamoxifen resistance and modulation of RAF/MEK/MAPK signaling. Recently we reported that signaling through insulin and insulin-like growth factor-1 receptors (IGF-1R) strongly represses CycG2 expression. Current studies suggest that the anti-diabetic drug metformin (MTFN) inhibits BC cell growth by promoting AMPK-mediated suppression of growth factor receptor activation of mTOR. Patients taking MTFN exhibit a dose-dependent reduction in cancer risk and clinical trials indicate that BC therapies including MTFN improve response rates. We found that MTFN treatment stimulates CycG2 expression and potentiates both fulvestrant-mediated upregulation of CycG2 expression and growth-inhibition of MCF-7 cells. Moreover knockdown of CycG2 expression blunts the enhanced anti-proliferative effect of MTFN on fulvestrant treated cells. Importantly, analysis of BC tumor cDNA microarray databases indicates that CCNG2 transcripts are reduced in aggressive, poor-prognosis BCs and that higher levels of CCNG2 expression in the patient samples correlated with longer periods of relapse free survival. Citation Format: Mary C. Horne, Maike Zimmerman, Aruni S. Arachchige Don, Michaela Donaldson, Tommaso Patriarchi. Knockdown of cyclin G2 expression hinders the cell cycle arrest response of MCF-7 cells to estrogen receptor signaling-antagonists and treatment with the antidiabetic metformin. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4559. doi:10.1158/1538-7445.AM2014-4559