Transient up-regulation of P2Y(2) nucleotide receptor mRNA expression is an immediate early gene response in activated thymocytes

In studies designed to understand the roles of P2 nucleotide receptors in differentiation of T lymphocytes, we observed a transient and protein synthesis-independent enhancement of mRNA expression for the G protein-coupled P2Y(2) receptor in mouse thymocytes after the addition of steroid hormone or T cell receptor (TCR) crosslinking by anti-TCR mAb. Conversely, dexamethasone-induced increases in mRNA expression for the ligand-gated ion channel P2X(1) receptor was detected in rat, but not mouse, thymocytes, raising questions about the previously suggested role of P2X(1) receptors in thymocyte apoptosis. Flow cytometry analysis of thymocyte subsets excluded the possibility that the observed increases in P2Y(2) receptor mRNA expression were due to the enrichment of steroid-treated cells with an P2Y(2) mRNA-rich thymocyte subset. Triggering of TCR-mediated intracellular signaling pathways through crosslinking of TCR or by addition of phorbol ester and Ca2+ ionophore also resulted in the up-regulation of P2Y(2), but not P2X(1), receptor mRNA. It is proposed that the rapid increase of P2Y(2) receptor mRNA expression could be a common early event in responses of T cells to different activating stimuli. Taken together with the recently discovered ability of nucleotide receptor-initiated signaling to antagonize or enhance the effects of TCR crosslinking or steroids on thymocytes, the observed rapid up-regulation of P2Y(2) receptor mRNA expression may reflect an immediate early gene response where newly expressed cell surface nucleotide receptors provide regulatory feedback signaling from extracellular ATP in the T cell differentiation process.