Model systems for rapid and slow induction of apoptosis obtained by inducible expression of pro-apoptotic proteins

Killing tumor cells is a central goal of non-surgical cancer therapy and induction of apoptosis in the malignant cells is the major strategy used to achieve it. However, this may have serious drawbacks, since apoptotic cells reportedly induce immunological tumor tolerance and activate a tumor repopulation program. The debate on which type of cell death has the most beneficial therapeutic effects in cancer treatment is intense and controversial. Stringently regulated, doxycycline-inducible transgene expression systems trigger cell death in a defined manner, which might help us find answers to these problems. A conditional suicide switch established transiently in Jurkat T-cells was used to test a set of pro-apoptotic proteins for their potency to induce cell death. The activated forms of caspase-3 (revCasp-3) and Bid (tBid) were very effective and, therefore, analyzed after stable integration into human leukemia and murine melanoma cell lines. Expression of either protein resulted in more than 95% cell death, but with strongly different kinetics. 85% cell death was observed if tBid and revCasp-3 were expressed for 4-6 hours and 18-24 hours, respectively. The human cell lines expressing these suicide switches can serve as cancer models in xeno-transplanted mice, the corresponding murine cell lines allow syngeneic and allogeneic murine cancer models to be established.