Three mutations in v-Rel render it resistant to cleavage by cell-death protease caspase-3

The retroviral oncoprotein v-Rel is a transcriptional activator in the Rel/ NF-kappaB family. v-Rel causes rapidly fatal lymphomas in young chickens, a nd transforms and immortalizes chicken lymphoid cells in vitro. Several mut ations that have enhanced the oncogenicity of v-Rel have been selected duri ng in vitro and in vivo passage of v-Rel-containing retroviruses. In this r eport, we show that the C-terminal deletion and two point mutations (Asp -- > Gly at residue 91 and Asp --> Asn at residue 437) in v-Rel make it resist ant to cleavage by the cell-death protease caspase-3. In contrast, c-Rel, w hich has Asp residues at these sites. can be cleaved by caspase-3 in vitro as well as in vivo in cells induced to undergo apoptosis. We have character ized activities of v-Rel mutants with recreated single caspase-3 cleavage s ites, two cleavage sites. or an introduced artificial cleavage site. All of these mutant v-Rel proteins are sensitive to caspase-3 cleavage in vitro, and show wildtype activity in terms of nuclear localization in chicken fibr oblasts and DNA binding in vitro. Moreover, all caspase-3-sensitive v-Rel m utants transform chicken spleen cells in vitro and induce fatal lymphoid tu mors in vivo to approximately the same extent as wild-type v-Rel. As with v -Rel mutants, caspase-3-resistant c- Rel mutants behave similarly to caspas e-3-sensitive wild-type c-Rel in terms of DNA binding, transcriptional acti vation. in vitro transformation, and tumorigenicity. Mammalian c-Rel protei ns can also be cleaved by caspase-3 in vitro, and a c-Rel mutant from a hum an pre-T lymphoma cell line is less sensitive than wild-type human c-Rel to cleavage by caspase-3. Taken together, these results demonstrate that spec ific mutations render oncogenic forms of Rel proteins resistant to cleavage by a cell-death caspase; however. the biological relevance of this resista nce remains unclear. Nevertheless, to our knowledge, this is the first demo nstration of mutations in caspase-3 recognition sites occurring during the evolution of an oncogenic protein. (C) 2001 Elsevier Science B.V. All right s reserved.