Killing of sarcoma cells by proapoptotic Bcl-XS:: Role of the BH3 domain and regulation by Bcl-XL

Kaposi's sarcoma (KS) is the most common tumor affecting AIDS patients with over 20% of these patients afflicted by this disease. Previous studies have demonstrated that KS tumor cells predominantly express the prosurvival protein Bcl-X-L compared with Bcl-2. In the current study, we have used an adenoviral vector that expresses Bcl-X-S, a functional inhibitor of Bcl-X-L, to study the significance of Bcl-X-L expression in the KS cell line (SLK) or KS primary cultures. The results demonstrate that 75% to 80% of SLK or KS primary cells were killed by the Bcl-X-S containing adenovirus whereas KS cells infected with control adenovirus showed no significant cell death or growth inhibition. Overexpression of Bcl-XL, but not BcI-2, in SILK cells attenuated apoptosis induced by adenovirus Bcl-X-S. Immunoprecipitation experiments revealed that adenoviral Bcl-X-S associated with Bcl-XL, but not with Bcl-2. Mutational analysis showed that the alpha2 helical region of Bcl-X-S containing the BH3 motif was critical for killing activity and interaction with Bcl-X-L. These results suggest that Bcl-X-S is a direct killer and Bcl-XL may act by interacting with and sequestering Bcl-X-S. These studies also suggest that targeting Bcl-X-L may be of therapeutic benefit for the treatment of tumors that are characterized by inappropriate expression of Bcl-X-L.