Abstract 3259: Giant obscurins: Novel tumor and metastasis suppressors in breast cancer

Abstract Obscurins, encoded by the single OBSCN gene, are giant cytoskeletal proteins containing tandem adhesion and signaling domains, including an active RhoGEF motif that directly binds and activates RhoA. The OBSCN gene is highly mutated in breast cancer resulting in a 2-fold reduction of its mRNA levels. Consistent with this, obscurin proteins are nearly lost from breast cancer cell lines and human biopsies, independently of their hormonal status or molecular differentiation. Loss of giant obscurins from breast epithelial cells confers them with a survival and growth advantage, following exposure to current chemotherapies. Moreover, obscurin-depleted breast epithelial cells fail to form adhesion junctions, undergo epithelial-to-mesenchymal transition and generate primary and secondary mammospheres bearing markers of cancer-initiating cells. In line with these phenotypic alterations, obscurin-deficient cells display markedly increased motility as a sheet in 2-dimensional (2D) substrata and individually in confined spaces, and invasion in 3D matrices. More importantly, loss of giant obscurins from breast epithelial cells promotes primary tumor formation and lung colonization in vivo. Obscurin depletion leads to >50% reduction in RhoA activity, as well as decreased phosphorylation of RhoA effectors, including myosin light chain phosphatase, myosin light chain, lim kinase, and cofilin, in both attached and suspended cells. These molecular alterations manifest as decreased actomyosin contractility, allowing suspended cells to escape detachment-induced apoptosis. Moreover, ∼40% of obscurin-depleted cells extend microtentacles, tubulin-based projections that mediate the attachment of circulating tumor cells to endothelium, an advantage that persists even after paclitaxel exposure. Collectively, our findings reveal that loss of giant obscurins from breast epithelium results in disruption of cell-cell contacts and acquisition of a mesenchymal phenotype that leads to enhanced tumorigenesis, migration and invasiveness in vitro and in vivo by affecting RhoA-mediated processes. Moreover, our data suggest that loss of obscurins may represent a substantial selective advantage for breast epithelial cells during metastasis, and that treatment with paclitaxel may exacerbate this advantage by preferentially allowing obscurin-deficient, stem-like cells to attach to the endothelium of distant sites, a first step towards colonizing metastatic tumors. Citation Format: Aikaterini Kontrogianni-Konstantopoulos, Marey Shriver, Nicole Perry, Kimberly Stroka, Michele Vitole, David Huso, Stuart Martin, Konstantinos Konstantopoulos. Giant obscurins: Novel tumor and metastasis suppressors in breast cancer. [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 3259. doi:10.1158/1538-7445.AM2015-3259