Identification of an ionomycin/cyclosporin A-responsive element within the human T cell receptor gamma enhancer

Activation of the mouse TCRgamma enhancers by STAT5

The IL-7R controls local accessibility of joining (J) gamma gene segments in the mouse TCRgamma locus by recruiting signal transducers and activators of transcription (STAT) 5 and transcriptional coactivators to the Jgamma germ line promoters and inducing histone acetylation and germ line transcription. Because STAT consensus motifs are conserved not only in the Jgamma promoters but also in the TCRgamma 3' enhancer (Egamma) elements, it is possible that STAT5 interacts with and activates Egamma. To address this question, we first showed that the lysine 4 residue of histone H3 is substantially methylated at Egamma1 and Egamma4 elements in wild-type early thymocytes and that the levels of the methylation are reduced in IL-7R alpha chain-deficient mice. We also showed that STAT5 has potential to elevate histone acetylation of the Egamma elements in a cytokine-dependent cell line by cytokine stimulation. Next, we demonstrated that STAT5 is recruited to the STAT consensus motifs in the Egamma elements after cytokine stimulation and that transcription factors Runt-related (Runx) and c-Myb are constitutively recruited to Egamma. Furthermore, we showed that STAT5 augments basal Egamma activity controlled by Runx and c-Myb. These results suggest that STAT5 is recruited to the consensus motifs in the Egamma elements by cytokine stimulation and augments basal Egamma activity independent of Runx and c-Myb. Therefore, this study implies that the Egamma elements might be activated in two successive steps, first by Runx and c-Myb and next by STAT5.

Requirement of a second signal via protein kinase C or protein kinase A for maximal expression of CD40 ligand. Involvement of transcriptional and posttranscriptional mechanisms

High levels of CD40 ligand (CD40L) protein expression are induced on native T cells by increasing the intracellular Ca2+ concentration. In the present study we have shown that ionomycin induces CD40L gene transcription leading to mRNA accumulation which translates to high levels of protein expression. Conversely, agents which increase the intracellular levels of cyclic AMP (cAMP), such as prostaglandin E2 (PGE2) or dibutyryl cyclic AMP (dbcAMP), were unable to induce CD40L expression on T lymphocytes. Cell activation by phorbol 12-myristate 13-acetate (PMA) treatment had a slight effect on increasing CD40L mRNA and protein levels. However, PMA and dbcAMP synergized with ionomycin to significantly increase and to prolong the CD40L expression. Nuclear run-on assays revealed that PMA, but not dbcAMP, increased threefold the CD40L gene transcription rate induced by ionomycin. This effect was independent of de novo protein synthesis. In addition, at a posttranscriptional level, both reagents synergized with the Ca2+ ionophore to prolong the CD40L mRNA half-life by a mechanism which was also independent of de novo protein synthesis. Moreover, when transcription was blocked with actinomycin D, an increment of the CD40L transcript levels induced by PMA or dbcAMP on ionomycin-treated cells was observed in the presence of cycloheximide. This probably means that newly synthesized protein may contribute to the CD40L mRNA destabilization. In summary, these data show that PMA and dbcAMP synergized with ionomycin to increase the CD40L mRNA and protein levels. The up-regulatory effect of PMA was accomplished at a transcriptional and posttranscriptional level, whereas dbcAMP exerted its synergistic effect exclusively at a posttranscriptional level.

Localization of putative calcium-responsive regions in the rat BDNF gene

Brain-derived neurotrophic factor (BDNF) has potent trophic and protective actions on CNS neurons, including mesencephalic dopaminergic neurons, ventral forebrain cholinergic neurons and spinal motor neurons. To evaluate the effects of calcium and other second messengers on BDNF gene transcription, C6 glioma cells were treated for 4 h with the calcium ionophore A23187, forskolin + isobutyl-methyl-xanthine (IBMX), or the phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate. Semi-quantitative RT-PCR analysis revealed that A23187 treatment increased BDNF transcripts containing the protein coding exon by 4.4-6.4-fold. Alternate BDNF transcripts were elevated to varying degrees after treatment with this ionophore and a subset of these transcripts was elevated following forskolin + IBMX treatment. When co-incubated with the RNA polymerase inhibitor, actinomycin D, A23187-induced increases were reduced or abolished, suggesting that calcium-mediated regulation of BDNF mRNA expression occurs at transcriptional as well as post-transcriptional levels. Transient transfection experiments employing reporter constructs containing serial 5' deletions of alternate BDNF promoters suggested that A23187-induced elevations in BDNF exon 1b, 1d and 1e containing transcripts are mediated by putative calcium-responsive regions flanking all three of these exons.