Loss of IκBα-Mediated Control over Nuclear Import and DNA Binding Enables Oncogenic Activation of c-Rel

ABSTRACT

The IκBα protein is able both to inhibit nuclear import of Rel/NF-κB proteins and to mediate the export of Rel/NF-κB proteins from the nucleus. We now demonstrate that the c-Rel–IκBα complex is stably retained in the cytoplasm in the presence of leptomycin B, a specific inhibitor of Crm1-mediated nuclear export. In contrast, leptomycin B treatment results in the rapid and complete relocalization of the v-Rel–IκBα complex from the cytoplasm to the nucleus. IκBα also mediates the rapid nuclear shuttling of v-Rel in an interspecies heterokaryon assay. Thus, continuous nuclear export is required for cytoplasmic retention of the v-Rel–IκBα complex. Furthermore, although IκBα is able to mask the c-Rel-derived nuclear localization sequence (NLS), IκBα is unable to mask the v-Rel-derived NLS in the context of the v-Rel–IκBα complex. Taken together, our results demonstrate that IκBα is unable to inhibit nuclear import of v-Rel. We have identified two amino acid differences between c-Rel and v-Rel (Y286S and L302P) which link the failure of IκBα to inhibit nuclear import and DNA binding of a mutant c-Rel protein to oncogenesis. Our results support a model in which loss of IκBα-mediated control over c-Rel leads to oncogenic activation of c-Rel.

ACKNOWLEDGMENTS

We thank Andrew Chappell and Michelle Wald for technical assistance and David J. Pintel and Peter Wilden for critical review of the manuscript. We thank Henry R. Bose, Jr., for his gift of the HY87 and 3C1 anti-Rel monoclonal antibodies, Dan Donoghue for his gift of the anti-LBD serum, and Minoru Yoshida for his generous gift of leptomycin B.

This work was supported by American Cancer Society grant RPG-98-097-01-MGO, by Public Health Service grant CA-55027 from the National Cancer Institute, by USDA NRICGP award 95-04073, by University of Missouri Research Board grant RB97-175, and by the University of Missouri Molecular Biology Program.