Vomitoxin (deoxynivalenol)-mediated inhibition of nuclear protein binding to NRE-A, an IL-2 promoter negative regulatory element, in EL-4 cells

Interleukin-2 (IL-2) gene expression is superinduced by the trichothecene mycotoxin vomitoxin (VT, deoxynivalenol) in primary and cloned murine T cells-an activity that relates to this toxin's capacity to inhibit protein synthesis. Binding of the transcription factor ZEB (NIL-2-a) to the negative regulatory element NRE-A plays a critical role in silencing IL-2 expression in the EL-4 cell line, a murine Th1-like lymphoma. The objective of this study was to test the hypothesis that VT impairs NRE-A-binding activity in the EL-4 T cells model. Electrophoretic mobility shift assay (EMSA) in control EL-4 cells revealed a slower migrating band, previously identified as ZEB, and a faster migrating band. VT inhibited NRE-A binding activity in unstimulated EL-4 cells as evidenced by the concentration-dependent reduction of both bands with as little as 50 ng/ml of the toxin being inhibitory. Specificity of NRE-A binding for the two bands was verified by demonstrating (1) competition with excess unlabeled NRE-A probe and absence of competition with mutant NRE-A or unrelated (NF-kappaB) probes and (2) EMSA supershift using antibody specific for ZEB. NRE-A binding activity was also reduced when cells were treated with phorbol 12-myristate 13-acetate (PMA) plus ionomycin (ION). Cotreatment of VT potentiated PMA+ION-mediated reduction of the NRE-A binding activity in concentration-dependent fashion. VT-mediated reduction of NRE-A binding was observed in PMA+ION-stimulated cells as early as 2 h and was still detectable after 24 h. Competitive RT-PCR analysis of mRNA indicated that VT did not reduce ZEB transcript levels. Western analysis of nuclear extracts revealed that, although VT exposure decreased ZEB protein expression, the effects were minimal, required high VT concentrations (500-1000 ng/ml) and were not qualitatively consistent with large concomitant decreases in NRE-A binding activity. Furthermore, VT at levels up to 1000 ng/ml had no effect on ZEB expression in whole cell lysates, suggesting that VT did not selectively inhibit translation of this negative transcription factor. Taken together, these data indicate that the capacity of VT to up-regulate IL-2 expression may relate, in part, to its capacity to decrease ZEB binding to the negative regulatory element within this cytokine's promoter. The inability to associate decreased NRE-A binding with impaired ZEB transcription or translation suggests that post-translational modification of this negative transcription factor may be requisite for VT's down-regulatory effects.