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IG20, a MADD Splice Variant, Increases Cell Susceptibility to -Irradiation and Induces Soluble Mediators That Suppress Tumor Cell Growth

¹ Department of Microbiology and Immunology, University of Illinois at Chicago, Chicago, Illinois, and
² LUMINEX Core Facility, University of Pittsburgh Cancer Institute, Pittsburgh, Pennsylvania

The IG20 gene encodes at least four splice variants, including DENN-SV and IG20. DENN-SV is constitutively expressed at higher levels in tumor tissues. Cells transfected with the DENN-SV cDNA show increased resistance to tumor necrosis factor (TNF), TNF-related apoptosis-inducing ligand (TRAIL), etoposide, and vinblastine treatment, whereas overexpression of IG20 enhanced susceptibility to both intrinsic (drugs) and extrinsic (e.g., TNF and TRAIL) death signals. In this study, we investigated whether expression of the IG20 can render cells susceptible to -irradiation. Consistent with previous results, overexpression of DENN-SV and IG20 in HeLa cells conferred resistance and susceptibility, respectively, to the effects of -irradiation. HeLa IG20 cell susceptibility was attributable to enhanced apoptosis and reduced cell growth. This growth suppression was mediated by secreted soluble factors. Although HeLa DENN-SV cells grew more rapidly than control cells, replenishment with conditioned media from HeLa IG20 cells suppressed their growth. In addition, the conditioned media from HeLa IG20 cells stopped the growth of ovarian PA-1 cancer cells in the G₁-G₀ cell cycle stage. Among an array of cytokines tested, interleukin 6 (IL-6) was found at the highest levels in HeLa IG20 culture supernatants, and IL-6 neutralization showed that it was, in part, responsible for the cell growth suppression. HeLa IG20 cells had elevated basal nuclear factor B levels, a known regulator of IL-6 transcription. Finally, IG20 overexpression enhanced the combined apoptotic effects of TRAIL and -irradiation on HeLa cells. These results suggest that understanding further the mechanism of action of the IG20 splice variant may help in the advancement of cancer therapies.

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