EFFECTS OF DEXAMETHASONE AND PHORBOL ESTER ON P-2 RECEPTOR-COUPLED CA2-1 PRESENTATION IN U937 CELLS( SIGNALING AND LIPOCORTIN)

1 Cell surface bound lipocortin 1 (LC1) is a putative mediator of the antiproliferative and antiinflammatory effects of glucocorticoids. Thi s study assessed the hypothesis that the glucocorticoid, dexamethasone -phosphate (dex-p), might exert the above effects via an LC1-mediated downregulation of receptor-coupled Ca2+ signalling, using P-2-receptor mediated intracellular Ca2+ accumulation in U937 cells as an appropri ate model. 2 Addition of ATP (1-100 mu M) to cells resulted in a trans ient increase in cytosolic Ca2+ ([Ca2+](i)). Prior treatment of cells with dex-p (3-24 h) increased the magnitude of this Ca2+ transient at high, but not. low concentrations of ATP, and increased thapsigargin ( Tg)-induced Ca2+ influx, indicating that store-operated Ca2+ influx wa s potentiated in these cells. For cells treated with dex-p for 24 h, c ell surface levels of LC1 were significantly reduced by 63%. 3 Differe ntiation of cells with 1 nM phorbol ester (PMA) for 24 h resulted in a 2.4 fold increase in the cell surface level of LC1 and inhibition of the ATP-induced Ca2+ response. However, the Tg-induced Ca2+ response w as unaffected by long-term PMA treatment, and incubating cells with LC 1 did not alter Tg-induced Ca2+ mobilization and influx, or the ATP-me diated Ca2+ response. 4 Data from this study suggest that: (1) dex-p d oes not inhibit P-2-receptor-coupled Ca2+ signalling in this cell line , (2) the observed modulation of the ATP-induced increase in [Ca2+](i) by dex-p and PMA, and store-operated Ca2+ influx by dex-p, is not lin ked to an increase in the cell surface level of LC1, and (3) different iation of U937 cells with PMA downregulates the ATP-induced Ca2+ respo nse, but does not affect the thapsigargin-sensitive Ca2+ pool or store -operated Ca2+ influx of these cells.