[The anti-diabetic drug troglitazone sensitizes colon cancer cells to TRAIL-induced apoptosis by down-regulating FLIP].

Aims: Induction of apoptosis by the death ligand TRAIL might be a promising therapeutic approach in colorectal cancer therapy. However, some colon cancer cells are resistant to TRAIL because of the expression of anti-apoptotic proteins, such as FLIP. We studied the role of FLIP for apoptosis resistance in colon cancer and developed an approach to overcome the resistance to TRAIL.

Methods: The mechanisms of TRAIL-induced cell death in colon cancer cells were studied by Western blot analysis, apoptosis assays, transient and stable transfections, siRNA-mediated knockdown, and FACS analysis.

Results: The anti-apoptotic protein FLIP is expressed in the majority of colon carcinoma. Stable over-expression of FLIP renders colon carcinoma cells resistant to the death ligand, TRAIL. siRNA-mediated down-regulation of FLIP sensitizes the cells to TRAIL-induced apoptosis. FLIP-expressing colon cancer cells can be sensitized to TRAIL-induced apoptosis by the anti-diabetic drug troglitazone. Troglitazone induces a pronounced reduction in protein expression levels of FLIP. The troglitazone-dependent down-regulation of FLIP occurs on a post-translational level and involves the accelerated FLIP degradation by the proteasome. Moreover, troglitazone suppresses the expression of the anti-apoptotic protein, survivin, and induces the cell surface expression of the TRAIL receptor 2.

Conclusions: The anti-apoptotic FLIP protein plays an important role in apoptosis resistance of colon carcinoma cells. Troglitazone down-regulates FLIP and sensitizes the cells to TRAIL-induced apoptosis. A combined treatment with troglitazone and TRAIL might be a promising experimental therapy for some forms of colorectal cancer because troglitazone sensitizes tumor cells to TRAIL-induced apoptosis via various mechanisms, thereby minimizing the risk of acquired tumor cell resistance.