The role of apoptosis in 2 ',2 '-difluoro-2 '-deoxycytidirne (gemcitabine)-mediated radiosensitization

The nucleoside analogue Gemcitabine [2',2'-difluoro-2'-deoxycytidine (dFdCy d)] is active against a wide variety of solid tumors and is a potent radiat ion sensitizer. Because apoptosis has been shown to be an important mechani sm of cell death for many cancers, we wished to investigate the role of apo ptosis in dFdCyd-mediated radiosensitization. We evaluated HT29 colon cance r cells, UMSCC-6 head and neck cancer cells, and A549 lung cancer cells, wh ich differ substantially in the ability to undergo radiation-induced apopto sis, We hypothesized that if dFdCyd produced radiosensitization by potentia ting preexisting death pathways, then only the apoptotic-prone HT29 cells w ould show a substantial increase in apoptosis when treated with the combina tion of dFdCyd and radiation and that UMSCC-6 cells and A549 cells would be radiosensitized through nonapoptotic mechanisms. We found that the radiose nsitization of HT29 cells (enhancement ratio, 1.81 +/- 0.16) was accompanie d by an increase in apoptosis and by caspase activation and that inhibition of this activation by the caspase inhibitor Z-Asp-Glu-Val-Asp-fluoromethyl ketone (DEVD) significantly decreased radiosensitization (to 1.36 +/- 0.24; P < 0.05). In contrast, UMSCC-6 cells and A549 cells were modestly radiose nsitized (enhancement ratio, 1.47 +/- 0.24 and 1.31 +/- 0.04, respectively) via a nonapoptotic mechanism. These findings suggest that although apoptos is can contribute significantly to dFdCyd-mediated radiosensitization, the role of apoptosis in dFdCyd-mediated radiosensitization depends on the cell line rather than representing a general property of the drug.