Abstract P2-06-19: The effect of UBE2C expression on intrinsic chemosensitivity in breast cancer cell lines

Abstract

Abstract Background: The ubiquitin-proteasome pathway plays a crucial role in cancer-related processes by inducing cell cycle arrest through the degradation of mitotic cyclins and other cell cycle regulatory proteins. We recently showed that elevated levels of ubiquitin-conjugating enzyme E2C (UBE2C) were associated with aggressive tumor features and unfavorable clinical outcome in breast carcinoma (BC). UBE2C suppression has been achieved using the FDA-approved proteasome inhibitor bortezomib (VELCADE®) in colorectal carcinoma, but little is known about the efficacy of UBE2C-targeted therapy with proteasome inhibitors in breast cancer. Methods: Cell viability assays were used to determine the intrinsic chemosensitivity of five BC cell lines (MCF-7, MDA-MB-436, HCC38, HCC1395, and ZR-75-30; stratified by UBE2C expression and ER status) and the MCF-10A epithelial cell line to proteasome inhibitors (n=8), mitosis inhibitors (n=2), and platinum agents (n=3). UBE2C expression analysis was performed using quantitative real-time PCR and Western blot. IC50 values and growth inhibition metrics (GR50 and GRmax) were calculated for each compound to determine drug potency and efficiency after 24 hour treatment. Proteasome activity was assessed using bortezomib-treated cells. Results: Heterogeneous UBE2C expression levels were found in the different cell lines, with higher UBE2C levels in ER-negative BC cell lines (HCC38, HCC1395, MDA-MB-436) than ER-positive BC (MCF-7 and ZR-75-30) and MCF-10A control cells (ER-negative). Proteasome inhibition levels close to 50% and 100% were seen in all cell lines after 10 nM and 100 - 1000 nM bortezomib, respectively. As expected, bortezomib blocked cell cycle progression by inducing G2/M phase arrest in HCC38 cells. Due to differences in cell growth rates, calculation of the IC50 value was an ineffective method to determine drug potency. In contrast, the normalized growth rate inhibition method with GR50 and GRmax values demonstrated a correlation between sensitivity to proteasome inhibitors in ER-negative BC cell lines and high UBE2C expression levels. However, MDA-MB-436 cells (GR50, range 1.8-286.1 nM; GRmax, range -0.42- -0.93) were generally less sensitive to proteasome inhibitors than HCC38 cells (GR50, range 8.2-936.8 nM; GRmax, range -0.97- -0.99) though both cell lines were ER-negative, which was possibly due to the lower expression of UBE2C in MDA-MB-436 cells. Compared with the other tested drugs, no cell line was sensitive to the mitosis inhibitors and platinum agents were most effective in HCC38 cells. Conclusions: Taken together, these findings suggest an association between UBE2C expression and response to proteasome inhibition, regardless of ER status. Citation Format: Parris TZ, Larsson P, Biermann J, Engqvist H, Werner-Rönnerman E, Kovács A, Karlsson P, Helou K. The effect of UBE2C expression on intrinsic chemosensitivity in breast cancer cell lines [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P2-06-19.