Regulation of the Nur77 Orphan Steroid Receptor in Activation-Induced Apoptosis

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Abstract
T-cell receptor (TCR)-mediated apoptosis in immature thymocytes and T-cell hybridomas is calcium dependent and can be inhibited by cyclosporin A (CsA). Induction of the orphan steroid receptor Nur77 (NGFI-B) is required for activation-induced apoptosis. Here, we examined the regulation of Nur77 expression, in response to apoptotic TCR signals, which consists of kinase C and calcium pathways. We show that the major control of Nur77 induction is mediated by the calcium signaling pathway. In contrast, protein kinase C signals induce only a low level of Nur77 activity. Nur77 promoter activity parallels its protein levels. CsA decreases both Nur77 protein levels and promoter activity, and the kinetics of CsA inhibition of apoptosis correlates with a decrease in Nur77 protein levels. TCR signals and kinase C signals result in a similar level of Nur77 protein phosphorylation but mediate differential transactivation activity of Nur77. In addition, Nur77 promoter deletion analysis revealed two RSRF (related to serum-responsive factor) binding sites, which can confer calcium and CsA sensitivity on a heterologous promoter. Taken together, our data suggest that the levels of transcriptional induction of Nur77 play an important role during activation-induced apoptosis and that calcium signals regulate a novel CsA-sensitive nuclear factor required for Nur77 transcription in T cells.