Abstract 1586: Breast cancer cells overexpressing EMT-inducing transcription factors mediate metastasis of neighboring tumor cells via secretion of molecules that upregulate Hedgehog signaling

Breast cancers are known to be heterogeneous, and thus it is important to understand how different tumor subpopulations influence each other to mediate metastasis. Epithelial-mesenchymal transition (EMT) is known to facilitate metastasis in breast cancer, via enabling tumor cells to be more motile and invasive (amongst other properties), yet only a small population of cells within a tumor is thought to undergo EMT at any one time. Transcription factors (TFs) such as Twist1, Snail1, and Six1 induce EMT, and are all known to increase metastasis of numerous tumor types, but are not thought to be expressed uniformly throughout tumors. While the cell-autonomous function of these EMT regulators has been extensively studied, how cells expressing these TFs influence neighboring tumor cells remains unknown. We have found that co-culture of HMLER-WT and HMLER-Snail1/Twist1 cells leads to a significant increase in migration of HMLER-WT cells. Importantly, transfer of conditioned media (CM) from HMLER-Twist1 or HMLER-Snail1 cells to HMLER-WT cells leads to increased migration and invasion, and Six1 is required downstream of Twist1 and Snail1 to mediate these non-cell autonomous phenotypes. Similarly, CM from metastatic MCF7-Six1 cells, when placed on MCF7-control (Ctrl) cells, leads to an EMT in the control cells, as observed by downregulation of cytokeratin 18 and membranous E-cadherin and upregulation of fibronectin. Importantly, while MCF7-Ctrl cells are non-metastatic when injected orthotopically into mice, co-injection of metastatic MCF7-Six1 and MCF7-Ctrl cells into nude mice results in increased metastasis of the control cells. These data demonstrate that in a heterogenous tumor where a population of cells express EMT-inducing TFs, these cells can influence their neighbors to also undergo EMT, thus promoting their metastasis. More interestingly, we have found that the non-cell autonomous effects of EMT-inducing TFs Snail1 and Six1 are mediated by Hedgehog signaling, and that the secreted factors that lead to activation of this pathway can be different in different contexts, but all impinge on Gli activation as a critical means by which neighboring cells develop metastatic characteristics. Therefore, our data suggest that treatment of heterogenous tumors with downstream, rather than upstream, inhibitors of the Hedgehog signaling pathway will be more efficacious in treating metastatic progression in breast cancer, as the pathway can be activated by means not dependent on the ligands SHH, IHH, or DHH, nor on the canonical receptors. Citation Format: Hengbo Zhou, Deepika Neelakantan, Heide L. Ford. Breast cancer cells overexpressing EMT-inducing transcription factors mediate metastasis of neighboring tumor cells via secretion of molecules that upregulate Hedgehog signaling. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1586.