Resistance to apoptosis, which plays an important role in tumors that are refractory to chemotherapy, is regulated by the ratio of antiapoptotic to proapoptotic proteins. By manipulating levels of these proteins, cells can become sensitized to undergo apoptosis in response to chemotherapeutic agents. Alternative splicing of the bcl-x gene gives rise to two proteins with antagonistic functions: Bcl-xL, a well-characterized antiapoptotic protein, and Bcl-xS, a proapoptotic protein. We show here that altering the ratio of Bcl-xL to Bcl-xS in the cell using an antisense oligonucleotide permitted cells to be sensitized to undergo apoptosis in response to ultraviolet B radiation and chemotherapeutic drug treatment. These results demonstrate the ability of a chemically modified oligonucleotide to alter splice site selection in an endogenous gene and illustrate a powerful tool to regulate cell survival.

The epidermis is continually exposed to harmful mutagens that have the potential to cause DNA damage. To protect the skin from accumulating mutated cells, keratinocytes have developed a highly regulated mechanism of eliminating damaged cells through apoptosis. Bclx L is a well-described cell survival protein that when overexpressed in skin can protect keratinocytes from UV radiation-induced apoptosis. To begin to unravel the complex mechanisms that keratinocytes use to survive, we wanted to characterize the role of endogenous Bcl-x L in protecting cells from death. In this study, we describe the development and characterization of an antisense inhibitor to Bcl-x L . We show that this inhibitor reduces Bcl-x L RNA and protein in a concentration-dependent, sequence-speci®c manner. Furthermore, treatment of keratinocytes and epithelial cells with this inhibitor sensitizes these cells to UV-B radiation and cisplatinum treatment-induced apoptosis. Thus, these results o er direct evidence that Bcl-x L is critical in the protection of skin and epithelial cells from apoptosis and provide a basis for the role of Bcl-x L in keratinocyte and epithelial cell survival.