Rel/NF-kappaB transcription factors and IkappaB inhibitors are substrates for the cell-death protease caspase-3

Apoptosis is a form of programmed cell death by which an organism rids itself of unwanted or unneeded cells. This cell suicide is carried out in most cells by a death program that involves the activation of a family of proteases known as caspases. Misregulation of the apoptotic program can contribute to cancer and other diseases.; The Rel/NF-κB family of transcription factors is induced in response to many signals that lead to cell growth, differentiation, inflammatory responses, the regulation of apoptosis, and neoplastic transformation. Rel/NF-κB transcription factors are regulated by interaction with a family of inhibitors, the IκB proteins. Because of the role of the Rel/NF-κB signal transduction pathway in regulating apoptosis, one goal of this thesis was to determine whether factors in this pathway are in turn substrates for apoptotic caspases.; It is shown herein that chicken IκBα is a substrate for caspase-3 in vitro and in vivo. The caspase-3 cleavage site in chicken IκBα is at Asp-35. Cleavage of IκBα by caspase-3 creates a form that is predicted to constitutively inhibit NF-κB, and is distinct from signal-induced proteolysis of IκBα.; v-Rel is the oncoprotein encoded by the avian Rev-T retrovirus, which can malignantly transform avian lymphoid cells in vivo and in vitro. v-Rel is a truncated and mutated version of the proto-oncoprotein c-Rel. Three caspase-3 cleavage sites have been identified in c-Rel. These cleavage sites are mutated or missing in v-Rel, thus rendering v-Rel resistant to caspase-3 cleavage. To determine the significance of these mutations in v-Rel, reciprocal mutations were created in v-Rel and c-Rel that make them sensitive or resistant, respectively, to cleavage by caspase-3 in vitro and in vivo. Mutations at these caspase-3 sites do not appreciably affect activities such as DNA binding or transcriptional activation of v-Rel or c-Rel. In addition, in vitro and in vivo oncogenicity assays revealed that these mutations do not appear to affect the oncogenic potential of Rel proteins. Therefore, although caspase-3 cleavage site mutations in v-Rel may have been selected during the oncogenic progression of v-Rel, their effect on the activity of v-Rel is not yet clear.; In summary, Rel/NF-κB family members and their inhibitors, IκBs, are substrates for cell-death caspases. Future work will be aimed at determining the role of caspase-3 cleavage of these proteins in apoptosis, and the possible therapeutic role of alterations in the caspase sensitivity of these molecules.