IL-6 inhibits apoptosis and retains oxidative DNA lesions in human gastric cancer AGS cells through up-regulation of anti-apoptotic gene mcl-1

Apoptosis plays a critical role in maintaining genomic integrity by selectively removing the most heavily damaged cells from the population. Reactive oxygen species (ROS) and certain inflammatory cytokines are always elevated during the human carcinogenic process. However, the biological significance of the interplay between ROS and inflammatory cytokine remains elusive. This study demonstrates that interleukin-6 (IL-6) effectively protects gastric cancer cells from the apoptosis induced by hydrogen peroxide (H2O2). The cell death signaling JNK pathway elicited by H2O2 is also inhibited by IL-6. We further found that Mcl-1, but not other Bcl-2 family members, was up-regulated by IL-6, by a substantial level over 24 h. We further transfected a mcl-1 expression vector, pCMV-mcl-1, into the AGS cells, and successfully obtained several mcl-1-overexpressing clones. Flow cytometric analysis shows that these mcl-1-overexpressing AGS cells are more resistant to the apoptosis induced by H2O2 when compared with the neo control AGS cells. Consistently, the activation of the JNK pathway induced by H2O2 is also blocked in mcl-1-overexpressed cells. These results indicate that the anti-apoptotic effect of IL-6 is, at least in part, due to the up-regulation of mcl-1. To our surprise, either IL-6 exposure or mcl-1 overexpression fails to reduce the level of intracellular peroxides in the AGS cells triggered by H2O2. This study also determined the level of 8-hydroxydeoxyguanosine (8-OH-dGua), an indicator for oxidative DNA lesions in IL-6-treated or mcl-1-overexpressed AGS cells after treatment with H2O2. Notably, our results indicate that a majority of the 8-OH-dGua is efficiently removed in the AGS cells without IL-6 treatment, whereas only similar to50% of the 8-OH-dGua was repaired in the IL-6-treated AGS cells after 24 h. Similarly, similar to60-70% of the 8-OH-dGua also failed to repair and was retained in the genomic DNA of the mcl-1 transfectants. Results in this study provide a novel mechanism by which up-regulation of the Mcl-1 protein by IL-6 may enhance the susceptibility to H2O2-induced oxidative DNA lesions by overriding apoptosis.