Effects of dexamethasone and phorbol ester on P2 receptor-coupled Ca2+ signalling and lipocortin 1 presentation in U937 cells.

1. Cell surface bound lipocortin 1 (LC1) is a putative mediator of the antiproliferative and anti-inflammatory effects of glucocorticoids. This 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 P2-receptor mediated intracellular Ca2+ accumulation in U937 cells as an appropriate model. 2. Addition of ATP (1-100 microM) to cells resulted in a transient 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 was potentiated in these cells. For cells treated with dex-p for 24 h, cell surface levels of LC1 were significantly reduced by 63%. 3. Differentiation 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 was unaffected by long-term PMA treatment, and incubating cells with LC1 did not alter Tg-induced Ca2+ mobilization and influx, or the ATP-mediated Ca2+ response. 4. Data from this study suggest that: (1) dex-p does not inhibit P2-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 linked to an increase in the cell surface level of LC1, and (3) differentiation of U937 cells with PMA downregulates the ATP-induced Ca2+ response, but does not affect the thapsigargin-sensitive Ca2+ pool or store-operated Ca2+ influx of these cells.