Induction of p21Waf1/Cip1 by TNFα requires NF-κB activity and antagonizes apoptosis in Ewing tumor cells

Abstract

The Ewing family of tumors is characterized by recurrent reciprocal translocations that generate chimeric proteins, either EWS–FLI-1 or EWS–ERG. These proteins are potent transcriptional activators and are responsible for maintaining the oncogenic properties of tumor cells. Since apoptosis appears to be the main mechanism whereby chemotherapy and radiation kill tumor cells, identification of events that can antagonize apoptosis in Ewing tumors is essential for improving their response to conventional therapies. Here, we report that the transcriptional factor NF-κB is a survival factor for Ewing tumor-derived cells. In fact, inhibition of NF-κB activation as a consequence of the overexpression of a degradation-resistant form of IκBα, IκBα (A32/36), sensitized these cells to TNFα-induced killing. Although treatment with TNFα did not modify the cellular expression of Bcl-2, c-IAP1, c-IAP2, p53 and EWS–FLI-1 proteins, it increased p21^Waf1/Cip1 levels. This induction required NF-κB activation since it was not observed in the IκBα (A32/36) expressing cells. Moreover, overexpression of p21^Waf1/Cip1 in these IκBα (A32/36)-expressing cells, in which NF-κB and consequently p21^Waf1/Cip1 are no longer inducible by TNFα, decreased their susceptibility to TNFα-induced killing. Our results therefore identify p21^Waf1/Cip1 as a mediator of the antiapoptotic effect of TNFα-induced NF-κB in Ewing tumor cells.