Crosstalk pathway for inhibition of glucocorticoid-induced apoptosis by T cell receptor signaling.

Activation of the glucocorticoid receptor (GR) triggers apoptosis in T cells. However, activation of the T cell antigen receptor (TCR) blocks glucocorticoid-induced apoptosis, implying functional crosstalk between these two distinct signaling systems. By reconstructing or selectively blocking TCR-stimulated signaling pathways, we show here that TCR activation of the mitogen-activated protein kinase kinase/extracellular signal regulated kinase (MEK/ERK) cascade via Ras is necessary and sufficient to inhibit GR-mediated death in immortalized T and thymocyte cell lines and in primary T cells. Moreover, we found that activation of various pathway components (TCR, Ras, MEK1) altered the transcriptional regulatory activity of GR. In contrast, phosphatidylinositol 3-kinase and Akt, which down-regulate other lymphocyte apoptosis pathways, did not inhibit glucocorticoid-induced apoptosis. Our findings, which link signaling from the TCR cell surface receptor to that from the GR intracellular receptor, demonstrate the importance of the integration of signal transduction pathways in defining regulatory circuits. Because the TCR/Ras/MEK pathway has been shown previously to be essential for positive selection of thymocytes, the TCR/Ras/MEK signaling to GR crosstalk described herein may affect T cell development and homeostasis.