Differential inhibition of radiation-induced apoptosis.

The most common mechanism by which radiation kills cells is the induction of DNA double-strand breaks that results in the loss of cell proliferation. Even though apoptosis is increasingly identified in experimental systems in vitro and in vivo, it is still generally regarded as a rare mode of radiation-induced cell kill with minor relevance for the clinical effects of radiation. This review will focus on pro- and antiapoptotic signaling that affects the apoptotic outcome in irradiated mammalian cells. In particular, we will concentrate on the sphingomyelin/ceramide signal transduction pathway which is involved in initiation of stress-induced apoptosis in a variety of normal and neoplastic cells. We will also discuss the crosstalk between the sphingomyelin/ceramide pathway and the protein kinase C pathway which constitutes an antiapoptotic pathway, and the potential for pharmacological modulation to increase the fraction of apoptotic cells undergoing apoptosis after radiation exposure.