Accumulation of RXRα During Activation of Cycling Human T Lymphocytes: Modulation of RXRE Transactivation Function by Mitogen-Activated Protein Kinase Pathways

Abstract

We have previously reported that the activation of resting human immature peripheral blood T (PBT) lymphocytes is associated with the loss of retinoid X receptor α (RXRα) expression. In the present study, we have demonstrated that, unlike resting cells, activation of cycling human mature PBT lymphocytes, and T lymphocyte leukemia cell lines is accompanied by the accumulation of RXRα mRNA and protein. Interestingly, cyclosporin A further augmented RXRα expression, indicating the involvement of calcineurin pathways in the process. 9-cis retinoic acid inhibited the accumulation, suggesting that retinoids can regulate the synthesis of their own receptors during T cell activation. Transfection analysis in Jurkat cells, using RXRE-dependent reporter assays, showed that RXRα accumulated during T cell activation was transcriptionally inactive. To investigate the mechanism of such inhibition, the role of two mitogen-activated protein kinase pathways, c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK), in modulating RXRE-dependent transcription, was explored. The expression of constitutively active MAP/ERK kinase kinase 1 (MEKK1) inhibited RXRE-dependent transcription, whereas dominant negative MEKK1 increased the transcription, indicating the involvement of JNK signaling pathways in the process. In contrast, expression of constitutively active MEK1, which activates ERK pathway, enhanced RXRE-dependent activation. When both were activated simultaneously, JNK pathway was dominant over ERK pathway and resulted in inhibition of RXRE-mediated transcription. These data demonstrate a dual regulatory control of RXRα expression during the activation of resting and cycling T lymphocytes and indicate a dynamic balance between JNK and ERK pathways in modulating RXRE-mediated transactivation.