The initial step of the metastatic cascade in most carcinomas occurs when tumor cells break through the basement membrane and invade locally into the surrounding microenvironment. This microenvironment is now appreciated to be an active participant in driving the oncogenic behavior of cancer cells. In particular, evidence is mounting that carcinoma-associated fibroblasts (CAFs) in the microenvironment are a critical player in the promotion of tumor progression and metastasis. However, the precise mechanisms utilized by CAFs to enhance malignancy are only beginning to be understood. Given the importance of CAFs in promoting tumor progression and the fact that cancer cells lack normal extracellular matrix (ECM) attachment after breaking through the basement membrane, we hypothesized that CAFs may be actively involved in the inhibition of anoikis (ECM-detachment-induced cell death). To investigate this question, we utilized NIH-3T3 mouse fibroblasts lacking the caveolin-1 gene (KO 3T3s), a cell line that has previously been demonstrated to share many similarities to human CAFs. Interestingly, the addition of media conditioned by KO 3T3s led to robust anoikis inhibition in detached MCF-10A cells, suggesting that factors secreted by CAFs can block anoikis. We validated these findings by demonstrating that factors secreted from patient-derived human CAFs can inhibit anoikis to a similar degree. Moreover, the ability of CAF-conditioned media to block anoikis was not limited to MCF-10A cells as we obtained similar results in two additional non-tumorigenic cell lines: BPH-1 and HMEC. In addition, when investigating the mechanism by which CAFs block anoikis, we discovered that the release of mitochondrial cytochrome c into the cytosol was robustly inhibited by CAF-conditioned media in MCF-10A cells. Surprisingly, this inhibition was independent of signaling through PI(3)K or MAPK and did not involve alterations in Bim protein levels. To examine if the observed differences in anoikis induction caused by secreted factors from CAFs manifest in a more physiologically relevant context, we employed a 3-dimensional (3D) cell culture model of mammary acinus development. MCF-10A acini exposed to secreted factors from CAFs were more prone to have filled lumen suggesting that factors secreted by CAFs can promote the survival of ECM-detached cells in the luminal space. In aggregate, these data identify a novel mechanism by which CAFs contribute to tumorigenesis (the inhibition of anoikis) and suggest that targeting factors secreted from CAFs could be a novel therapeutic approach to eliminate ECM-detached cells through the selective induction of anoikis.

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 2083. doi:1538-7445.AM2012-2083