Functional redundancy of the Nur77 and Nor-1 orphan steroid receptors in T-cell apoptosis

The transcription factor Nur77 (NGFI-B), a member of the steroid nuclear receptor superfamily, is induced to a high level during T-cell receptor (TCR)-mediated apoptosis, A transgenic dominant-negative Nur77 protein can inhibit the apoptotic process accompanying negative selection in thymocytes, while constitutive expression of Nur77 leads to massive cell death, Nur77-deficient mice, however, have no phenotype, suggesting the possible existence of a protein with redundant function to Nur77, To explore this possibility, we have characterized the role of two Nur77 family members, Nurr1 and Nor-1, in TCR-induced apoptosis, We found that Nor-1 and Nurr1 can transactivate through the same DNA element as Nur77, and that their transactivation activities can be blocked by a Nur77 dominant-negative protein, In thymocytes, Nor-1 protein is induced to a very high level upon TCR stimulation and has similar kinetics to Nur77, In contrast, Nurr1 is undetectable in stimulated thymocytes, Furthermore, constitutive expression of Nor-1 in thymocytes leads to massive apoptosis and up-regulation of CD25, suggesting a functional redundancy between Nur77 and Nor-1 gene products, As in the case of our Nur77-FL mice, Fast is not detectable in the thymocytes of Nor-1 transgenic mice, Constitutive expression of Nur77 in gld/gld mice rescues the lymphoproliferative phenotype of the Fast mutant mice, Thus, Nor-1 and Nur77 demonstrate functional redundancy in an apparently Fas-independent apoptosis.