Silencing FLIP(L) modifies TNF-induced apoptotic protein expression.

Death-receptor induced apoptosis is regulated by FLIP [FLICE (Fas-associated protein with death domain-like IL-1β-converting enzyme)-inhibitory protein] via modification of caspase-8 activation. As an important modulator of apoptosis, the long isoform, FLIPL, regulates life and death in many various types of normal and tumor cells and tissues to render resistance to death receptor-mediated apoptosis. In addition, FLIPL has been shown to be involved in regulation of intrinsic (mitochondrial) pathways of apoptosis as well as regulating other proteins involved in cytoprotection and cell cycle progression. Therefore, understanding the role of FLIPL in complex regulatory networks of cell survival/death mechanisms is vital for future developments to control diseases such as cancer. Here, we shown that silencing FLIPL in HEK 293 cells changed the expression levels of proteins that are involved in both extrinsic and intrinsic apoptosis, as well as regulating tumor necrosis factor-α (TNF)-mediated apoptotic patterns. We also show that FLIPL-silenced cells have a lower rate of proliferation and cell cycle progression when compared to control cells. Moreover, treatment with TNF restored proliferation rates in FLIPL-silenced cells back to more normal levels when compared to control cells. These results suggest that cells have evolved complex compensatory mechanisms to overcome the absence of a key apoptotic regulatory proteins.