Abstract 4604: Roscovitine modulates ERα/ERβ ratio: A novel means to target hormonal therapy resistance

Abstract

Abstract Current endocrine therapy for ER+ve breast cancer involves modulating ERα-pathway using either ERα-antagonists or aromatase inhibitors. Despite the positive effects, de novo and/or acquired resistance to endocrine therapies frequently occur. Although mechanisms for hormonal therapy resistance remains elusive, emerging data implicate ER-growth factor signaling cross talk, and alteration in ER subtypes as the major causes of resistance. Most downstream events in the resistance pathways converge upon modulation of cell cycle regulatory proteins; the most conspicuous of which is the activation of Cyclin Dependent Kinase 2 (CDK2) pathway. Roscovitine is one of the most frequently studied and used CDK2 inhibitor. In this study, we examined whether roscovitine confers tumor suppressive advantage to therapy resistant breast epithelial cells by inhibiting CDK functions using three therapy resistant model cells; (a) MCF-7-Tam (acquired Tamoxifen resistance model); (b) MCF-7-CA-LTLT (acquired Letrozole resistance model); (c) MCF-7-HER2 (ER-growth factor signaling cross talk model). Roscovitine at 10 μM concentration substantially reduced the growth of all three resistant model cells in cell proliferation and foci formation assays. FACS analysis revealed that roscovitine treatment increased proportion of cells in G2-M. To increase the drug efficacy, we have developed PLGA nanoparticles containing roscovitine and this mode of delivery significantly reduced the dosage needed to see the growth inhibitory effect in therapy resistant cells to 500 nM. To determine the mechanism, we have determined the relationship between CDK activity, expression of ERα, β subtypes, ER-coregulators, and expression of ER target genes in the presence or absence of roscovitine in resistant cells. Roscovitine substantially affected CDK2 activity and decreased cyclin A levels in the model cells. These studies also revealed an unexpected discovery that CDK inhibitor roscovitine has potential to alter the ratio of ER isoforms with preferential upregulation of ERβ expression with concomitant down regulation of ERα, its coregulators including AIB1 and PELP1. Since many advanced and therapy resistant tumors exhibit loss or reduced expression of ERβ, dual action of roscovitine will provide a novel drug to inhibit both CDKs and to increase the expression of tumor suppressor ERβ. Since deregulation of cell cycle machinery and estrogen receptor signaling contributes to resistance, roscovitine a drug that attacks both these pathways will serve as a double-edged sword to interfere with the resistance mechanisms. 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 4604.