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Cytokines and glucocorticoids (GCs) signaling pathways interfere with each other in the regulation of apoptosis and gene expression in the immune system. Interleukin-2 (IL-2), through the Janus kinase/signal transducers and activators of transcription (Jak/STAT) and mitogen-activated protein kinase (MAPK) pathways, activates STAT5 and activated protein-1 (AP-1) transcription factors, respectively, which are known to repress glucocorticoid receptor (GR) activity, at least in part, through protein-protein interactions. In this work, we have analyzed the mechanisms whereby IL-2 down-regulates the GC-induced transactivation of the mouse mammary tumor virus long terminal repeat (MMTV-LTR) in murine CTLL-2 T lymphocytes. Mutagenesis studies revealed that the MMTV-LTR STAT5 binding site (-923/-914) was not required for IL-2-mediated inhibition but identified both glucocorticoid response elements (GREs) and the -104/+1 region as critical elements for this negative response. The DNA binding activities of transcription factors required for GC-mediated activation of the MMTV-LTR promoter and that bind to the -104/+1 region (nuclear factor-1, Oct-1) were not affected by IL-2 treatment. Overexpression of wild-type STAT5B enhanced the effect of IL-2 on MMTV-LTR activity, and a dominant negative form of STAT5B (Y699F) abolished the IL-2-mediated MMTV-LTR inhibition, whereas AP-1 activation had no effect in this system. Direct interaction between liganded GR and STAT5 was observed in CTLL-2 cells in a STAT5 phosphorylation-independent manner. Overexpression of nuclear coactivators CBP (CREB-binding protein) or SRC-1a (steroid receptor coactivator 1a) did not blunt IL-2 inhibitory effects. We suggest that the STAT5-repressive activity on the GC-dependent transcription may involve direct interaction of STAT5 with GR, is dependent on the promoter context and STAT5 activation level, and occurs independently of coactivators levels in T cells.

We have used the gibbon ape leukemia cell line MLA-144 and its corticoid-sensitive subclone MLA-E7T to analyze the mechanisms whereby interleukin-2 (IL-2) can protect T cells against dexamethasone-induced apoptosis. MLA cells are characterized by the constitutive expression of intermediate affinity receptors for IL-2, together with IL-4 receptors. MLA-144 cells secrete IL-2 and are insensitive to dexamethasone, whereas MLA-E7T cells do not constitutively produce significant amounts of IL-2 and undergo apoptotic cell death in the presence of dexamethasone. Exogenous IL-2 was shown to protect MLA-E7T cells against the apoptotic effect of dexamethasone and to increase both the DNA binding and transactivating functions of activator protein-1 (AP-1). The functional relationship between AP-1 and glucocorticoid receptors transcriptional activities was further investigated using transient expression of reporter gene constructs whose transcriptions are regulated by promoters containing TPA-responsive elements or glucocorticoid-responsive elements. The data reported here demonstrate that in MLA-144 cells, IL-2 or PMA stimulation antagonizes the glucocorticoid receptor, whereas in MLA-E7T, synergistic effects are observed between dexamethasone and IL-2 or PMA for transactivation of MMTV-CAT. Taken together with the finding that IL-2 but not PMA protects MLA-E7T from dexamethasone-induced apoptosis, our results indicate that IL-2 does not induce such a protection by repressing the transcriptional activity of the glucocorticoid receptor.