Escherichia coliEnterotoxin B Subunit Triggers Apoptosis of CD8+T Cells by Activating Transcription Factor c-Myc

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Abstract
Heat-labile enterotoxin from enterotoxinogenicEscherichia coliis not only an important cause of diarrhea in humans and domestic animals but also possesses potent immunomodulatory properties. Recently, the nontoxic, receptor-binding B subunit of heat-labile enterotoxin (EtxB) was found to induce the selective death of CD8+T cells, suggesting that EtxB may trigger activation of proapoptotic signaling pathways. Here we show that EtxB treatment of CD8+T cells but not of CD4+T cells triggers the specific up-regulation of the transcription factorc-myc, implicated in the control of cell proliferation, differentiation, and death. A concomitant elevation in Myc protein levels was also evident, with peak expression occurring 4 h posttreatment. Preincubation with c-mycantisense oligodeoxynucleotides demonstrated that Myc expression was necessary for EtxB-mediated apoptosis. Myc activation was also associated with an increase of IκBα turnover, suggesting that elevated Myc expression may be dependent on NF-κB. When CD8+T cells were pretreated with inhibitors of IκBα turnover and NF-κB translocation, this resulted in a marked reduction in both EtxB-induced apoptosis and Myc expression. Further, a non-receptor-binding mutant of EtxB, EtxB(G33D), was shown to lack the capacity to activate Myc transcription. These findings provide further evidence that EtxB is a signaling molecule that triggers activation of transcription factors involved in cell survival.