Abstract 1337: MEK1:MAPK1/2 targeting attenuates pro-survival autophagy and enhances antiestrogen-induced apoptosis in breast cancer cells via a cathepsin L-dependent mechanism

Antiestrogen resistance is still a major clinical challenge preventing the eradication of estrogen receptor positive (ER+) breast cancer. A major goal of our laboratory is to identify targeted therapies that can be combined with antiestrogen treatment to block the emergence of antiestrogen resistant breast cancer. Toward this goal, we previously determined that the catabolic process of autophagy facilitates the emergence of antiestrogen resistance (Sammadar et al., Molecular Cancer Ther. 9, 2008) and identified cathepsin L as a key lysosomal protease required for antiestrogen induced pro-survival autophagy ER+ MCF-7cells (Periyasamy-Thandavan et al., Autophagy. 6:19-35, 2010). Thus, we hypothesize that blocking cathepsin L expression / activity is one approach to targeting pro-survival autophagy during antiestrogen treatment of breast cancer. In support of this hypothesis, we now show that antiestrogen resistant TR5 cells show increased levels of active cathepsin L compared to the levels in the parent antiestrogen sensitive MCF-7 cells. The upregulation of cathepsin L expression is consistent with the fact that TR5 cells utilized autophagy to survive long-term 4-hydroxytamoxifen (4-OHT) selection. Withdrawal of 4-OHT selection during TR5 passage does not lead to a reduction in cathepsin L levels, suggesting that the increased expression of cathepsin L is a stable genetic or epigenetic alteration. Elevated expression of active (phosphorylated) MAPK1/2 is also present in TR5 cells and targeting MEK1/MAPK1/2 with the selective inhibitor U0126 consistently reduced the levels of active cathepsin L. We further established a key role for MEK1/MAPK1/2 in the regulation of cathepsin L in the parent MCF-7 cells by performing cathepsin L activity assays. Cells treated with 4-OHT showed increased cathepsin L activity compared to E2-treated control cells; whereas, cathepsin L activity in cells treated with 4-OHT + UO126 was significantly reduced compared to the levels in 4-OHT-treated cells. This reduction in cathepsin L by U0126-mediated blockade of MEK1/MAPK1/2 correlated directly to “impaired” autophagic flux and increased BimEL-dependent apoptosis in the antiestrogen-treated cell populations and was mediated, at least in part, via transcriptional regulation of cathepsin L as determined by quantitative PCR. Further, MEK1/MAPK1/2 up-regulation of cathepsin L is selective, as cathepsin B activity is not reduced by MEK1 targeting. These studies provide evidence that the targeting MEK1/MAPK1/2 in combination with antiestrogen treatment has the potential to reduce cathepsin-L mediated pro-survival autophagy in ER+ breast cancer. Citation Format: Annie Liu, Carol Joseph, Jesse Wayson, Timothy Summers, Haifeng Cai, Patricia V. Schoenlein. MEK1:MAPK1/2 targeting attenuates pro-survival autophagy and enhances antiestrogen-induced apoptosis in breast cancer cells via a cathepsin L-dependent mechanism [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1337.