Induction of p21Waf1/Cip1 by TNFalpha requires NF-kappaB activity and antagonizes apoptosis in Ewing tumor cells.

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-kappaB is a survival factor for Ewing tumor-derived cells. In fact, inhibition of NF-kappaB activation as a consequence of the overexpression of a degradation-resistant form of IkappaBalpha, IkappaBalpha (A32/36), sensitized these cells to TNFalpha-induced killing. Although treatment with TNFalpha did not modify the cellular expression of Bcl-2, c-IAP1, c-IAP2, p53 and EWS - FLI-1 proteins, it increased p21Waf1/Cip1 levels. This induction required NF-kappaB activation since it was not observed in the IkappaBalpha (A32/36) expressing cells. Moreover, overexpression of p21Waf1/Cip1 in these IkappaBalpha (A32/36)-expressing cells, in which NF-kappaB and consequently p21Waf1/Cip1 are no longer inducible by TNFalpha, decreased their susceptibility to TNFalpha-induced killing. Our results therefore identify p21Waf1/Cip1 as a mediator of the antiapoptotic effect of TNFalpha-induced NF-kappaB in Ewing tumor cells.