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

Abstract

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 μM) 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.

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