Abstract 406: Novel therapeutic combination targets the growth of letrozole resistant breast cancer through decreased cyclin B1
Authors: Banjara Bipika , Patel Jankiben R. , Davidson A Michael , Tilghman Syreeta L. 
Published: April 2023

Abstract
As breast cancer cells transition from letrozole-sensitive to letrozole resistance they over-express EGFR, MAPK, and HER2 and acquire enhanced motility and EMT-like characteristics that are attenuated and reversed by glyceollin treatment, respectively. Since previous reports from our lab demonstrate that the combination of lapatinib, a dual EGFR and HER2 inhibitor, and glyceollin, a novel phytochemical, induce apoptosis in hormone-dependent aromatase inhibitor (AI)-resistant breast cancer cells, we hypothesized that combination therapy could reverse motility in hormone independent letrozole-resistant breast cancer cells (LTLT-Ca) compared to AI-sensitive breast cancer cells (AC-1) by altering proteins involved in motility and cell cycle progression. We compared the effects of 10 μM glyceollin ± 5 μM lapatinib treatment on both cell lines and evaluated cell viability, cell migration, cell cycle analysis, and protein expression of cell cycle regulators. Glyceollin ± lapatinib treatment caused a time-dependent decrease in cell viability which was comparable between each cell line. When wound healing assays were conducted, glyceollin prevented the migration in the AC-1 and LTLT-Ca cell lines by approximately 55% and 51% respectively, while combination treatment exhibited synergistic inhibitory properties by preventing 88% and 93% wound closure in the AC-1 cells and LTLT-Ca cells, respectively. To determine if the glyceollin + lapatinib-induced decrease in cell viability was a consequence of cell cycle dysregulation, flow cytometric analyses were performed and drug treatment had no effect on the AC-1 cell cycle distribution however, combination therapy reduced the number of LTLT-Ca cells in the G1/G0 phase, while causing accumulation of the cells in both the S phase and G2/M phase. Since combination therapy altered the cell cycle distribution of the LTLT-Ca, we measured the effect of glyceollin ± lapatinib treatment on the expression of key cell cycle regulators such as p21, p27, cyclin B1, cdk1, and cdk2. Interestingly, while there there was no effect on protein expression in the AC-1 cells, glyceollin ± lapatinib treatment selectively decreased the expression of cyclin B1 in the LTLT-Ca cells, without significantly altering the expression of the other proteins. These results suggest that while both AI-sensitive and AI-resistant cells respond favorably to the growth inhibitory and anti-migratory properties of glyceollin + lapatinib, the LTLT-Ca cells exhibit increased sensitivity to cell cycle progression, which may represent an opportunity to exploit and refine second-line therapies for AI-resistant cancers. This work was supported by NIH grant #1SC1GM126617 and U54MD007582.
Citation Format: Bipika Banjara, Jankiben R. Patel, A Michael Davidson, Syreeta L. Tilghman. Novel therapeutic combination targets the growth of letrozole resistant breast cancer through decreased cyclin B1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 406.