Expression of the adenovirus serotype 5 (Ad5) E1A oncogene sensitizes cells to apoptosis by TNF-α and Fas-ligand. Because TNF-related apoptosis-inducing ligand (TRAIL) kills cells in a similar manner as TNF-α and Fas ligand, we asked whether E1A expression might sensitize cells to lysis by TRAIL. To test this hypothesis, we examined TRAIL-induced killing of human melanoma (A2058) or fibrosarcoma (H4) cells that expressed E1A following either infection with Ad5 or stable transfection with Ad5-E1A. E1A-transfected A2058 (A2058-E1A) or H4 (H4-E1A) cells were highly sensitive to TRAIL-induced killing, but Ad5-infected cells expressing equally high levels of E1A protein remained resistant to TRAIL. Infection of A2058-E1A cells with Ad5 reduced their sensitivity to TRAIL-dependent killing. Therefore, viral gene products expressed following infection with Ad5 inhibited the sensitivity to TRAIL-induced killing conferred by transfection with E1A. E1B and E3 gene products have been shown to inhibit TNF-α- and Fas-dependent killing. The effect of these gene products on TRAIL-dependent killing was examined by using Ad5-mutants that did not express either the E3 (H5 dl327 ) or E1B-19K (H5 dl250 ) coding regions. A2058 cells infected with H5 dl327 were susceptible to TRAIL-dependent killing. Furthermore, TRAIL-dependent killing of A2058-E1A cells was not inhibited by infection with H5 dl327 . Infection with H5 dl250 sensitized A2058 cells to TRAIL-induced killing, but considerably less than H5 dl327 -infection. In summary, expression of Ad5-E1A gene products sensitizes cells to TRAIL-dependent killing, whereas E3 gene products, and to a lesser extent E1B-19K, inhibit this effect.
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