Downregulation of FLIP by cycloheximide sensitizes human fat cells to CD95-induced apoptosis.

Adipocyte apoptosis is an important regulator of adipocyte number in fat depots. We have previously shown that an inhibition of protein synthesis sensitizes human adipocytes for apoptosis. In vivo, dramatic changes in the fat cell's protein expression should be anticipated under special conditions such as calorie restriction. Here, we studied the underlying mechanism by which human preadipocytes and adipocytes are sensitized for death receptor induced apoptosis in vitro. The protein synthesis blocker cycloheximide (CHX) sensitized human fat cells for CD95-induced apoptosis in a caspase-dependent manner. Treatment with CHX differentially changed expression of pro- and anti-apoptotic proteins. Most noticeably, FLICE-like inhibitory protein (FLIP) expression rapidly decreased during CHX treatment. Reduction of FLIP levels resulted in undetectable amounts of FLIP at the CD95 death-inducing signaling complex (DISC) upon CD95 stimulation, thereby enhancing recruitment and activation at caspase-8. Down-regulation of FLIP by shRNA sensitized preadipocytes for CD95-induced apoptosis. In mice, adipose tissue mRNA levels of Flip were down-regulated upon fasting. In conclusion, we identify FLIP as an important regulator of apoptosis sensitivity in fat cells. Modulating adipocyte homeostasis by apoptosis might provide a new therapeutic concept to get rid of excess adipose tissue, and FLIP might be a possible target molecule.