A majority of breast tumors express estrogen receptor alpha (ER), and many require ER activity to grow. Endocrine agents that target ER have been successfully used to treat many breast cancer patients. However, de novo and acquired resistance is an important clinical problem that results in tumor progression and patient death. New targeted therapies that can be used to treat or prevent the development of endocrine resistant tumors are therefore needed. Resistance to endocrine therapies can be due to activated signaling pathways that alter the ligand dependence of ER or make cells independent of ER function. Agents that target these signaling pathways might therefore be used to treat or prevent endocrine resistance. However, activation of multiple signaling pathways may contribute to endocrine resistance, and blockade of a single receptor or pathway may only be effective for a subset of tumors. Mixed Lineage Kinases (MLKs) are a family of Map Kinase Kinase Kinases (MAP3Ks) that are capable of activating multiple MAPK pathways including the JNK, p38 and ERK pathways. MLKs function downstream of several cell surface receptors that have been implicated in endocrine resistance, and upstream of several effectors that can impact ER function and cell proliferation. We therefore investigated the effects of an MLK inhibitor (CEP 1347) on human mammary epithelial and cancer cell lines including MCF-7 (ER-positive breast cancer, endocrine sensitive), MCF-7/LCC9 (ER-positive breast cancer, endocrine resistant), MCF-10A (non-tumorigenic) and 184B5 (non-tumorigenic). Short term CEP 1347 treatment resulted in a G2/M arrest and an increase in apoptosis in both tumor cell lines, but not in the non-tumorigenic cells. Similarly, six days of treatment with CEP 1347 caused a significant decrease in viable cells in both breast cancer cell lines, but not in non-tumorigenic cells. Six day CEP 1347 treatment of MCF-7 and MCF-7/LCC9 cells also resulted in polyploidy and increased cell size. Since MLKs can act upstream of multiple MAPKs, we investigated the effects of CEP 1347 on JNK, p38 and ERK activity in the four cell lines. Neither ERK nor p38 activity was affected in any of the cell lines. In contrast, CEP 1347 treatment led to decreased JNK activity in MCF-7 and MCF-7/LCC9 cells, but not in the non-tumorigenic cell lines. Based on these results, we propose that MLK activity is required for JNK activation in ER-positive breast cancer cells, but not in non-tumorigenic mammary epithelial cells. Since JNK activity is required for G2/M progression in these cells, inhibiting MLK activity therefore results in a cell cycle arrest, the accumulation of polyploid cells, and ultimately apoptosis. Since MLKs are not required for JNK activation in non-tumorigenic cells, MLK inhibitors such as CEP 1347 may be selective agents for the treatment of endocrine sensitive and endocrine resistant ER-positive breast tumors.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2807. doi:1538-7445.AM2012-2807