Abstract 4364: Characterization of mucin-like 1 (MUCL1) in breast cancer and its novel role as a potent activator of cell proliferation

Abstract

Abstract MUCL1 was first identified as a breast-specific gene in 2001. Based on its highly restricted mRNA expression in normal tissues, and its continued expression during breast tumorigenesis and progression (expressed in >90% of breast cancers), MUCL1 is an attractive tumor associated antigen as a potential therapeutic target. However, very little is known about the cellular location and biological functions of the MUCL1 protein, which will have a major impact on its druggability. Here we describe our efforts to fully characterize the cellular localization of MUCL1 and discover its functional roles in breast cancer. Protein sequence analysis yielded an ambiguous prediction as MUCL1 contains a peptide signal sequence for targeting to the ER/Golgi secretory pathway, but it could also act as a weak transmembrane domain. Flow cytometry and cell surface protein biotinylation studies were performed to assess whether it could be a surface membrane protein. The MUCL1 protein was found in both whole cell lysates and conditioned mediums of native cancer cell lines or ectopically expressing cell lines. However, cell surface expression was not detectable in either system. Interestingly, intracellular protein expression did not necessarily correspond with levels of secreted protein. Phenotypic assays revealed a strong dependence on MUCL1 for cell proliferation, but not cell migration/invasion, in several breast cancer cell lines. We further explored the mechanism by which MUCL1 regulated cell growth by carrying out cell cycle analysis and assessing expression of cell cycle markers. Knockdown of MUCL1 by siRNA triggered a G1/S phase growth arrest along with decreased Cyclin D1 expression and increased expression of the cyclin-dependent kinase inhibitors (CDKIs) p21 and p27. Finally, we investigated the impact of MUCL1 loss on kinase signaling pathways in breast cancer cells through phospho-kinase array profiling. Strong phospho-FAK, JNK and c-Jun signals were detected in control cell lines which were abrogated by MUCL1 siRNA and confirmed by Western blot. Our data indicate that MUCL1 is not a membrane bound protein, but is secreted by some breast cancer cells, while others only express high levels of intracellular MUCL1. We are the first to identify an important role for MUCL1 in the proliferation of breast cancer cells, likely mediated via the FAK/JNK signaling pathway. These data suggest a potential use for targeting this protein in breast cancer. Citation Format: Sarah J. Conley, Emily Bosco, David Tice, Robert Hollingsworth, Ronald Herbst, Zhan Xiao. Characterization of mucin-like 1 (MUCL1) in breast cancer and its novel role as a potent activator of cell proliferation. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4364. doi:10.1158/1538-7445.AM2015-4364