Non-Canonical Regulation and Function of Anti-Apoptotic Protein c-FLIP

The metastasis of cancer cells to distant sites accounts for over 90% of cancer related deaths. In order to metastasize, cancer cells must be able to survive during extracellular matrix (ECM) detachment, namely by blocking anoikis (a caspase-dependent cell death) and restoring metabolic deficiencies. Such barriers during tumorigenesis are abrogated through activation of oncogenic signaling, and/or inhibition of tumor suppressors. Furthermore, the tumor microenvironment (TME), comprised of a heterogeneous population of transformed and non-transformed cells, plays a crucial role in cancer growth. Studies have shown that there is continuous bidirectional communication between cancer cells and the different cell populations within the TME. This communication can either work to promote tumor growth, as seen with carcinoma associated fibroblasts (CAFs), or inhibit tumor growth, as observed with tumor infiltrating lymphoid cells. In this study, we investigate the context dependent role of cellular FLICE-like Inhibitory Protein (c-FLIP), a protein well known for antagonizing receptor-mediated apoptosis, in modulating the survival of cancer cells. We demonstrate that normal mammary fibroblasts (fibroblasts isolated from normal breast tissue) secrete soluble factors that induce caspase-mediated cell death in a variety of cancer cell lines. We observed a substantive increase in the cleavage of caspase 8 that is mediated by the modulation of c-FLIP levels by stress-activated MAPK signaling. Furthermore, our investigation has revealed a surprising role of c-FLIP in antagonizing the viability of ECM-detached cancer cells. Additionally, we demonstrated that signaling emanating from oncogenic growth factor receptors results in the downregulation of c-FLIP expression providing additional evidence that cancer cells may benefit from diminished FLIP levels. Interestingly, this repression of c-FLIP expression by activated oncogenes is a consequence of PI(3)K signaling. In addition, our data reveal that elimination of c-FLIP promotes the survival of ECM-detached cells and that c-FLIP overexpression in cancer cells restricts the viability of cancer cells grown in anchorage-independent conditions. Taken together, our study reveals an unexpected role for c-FLIP in constraining the viability of cancer cells during ECM-detachment and raises the idea that c-FLIP may have context-dependent pro- and anti-cell death roles during tumorigenesis.