Interaction of glucocorticoids and ATP on externalisation of annexin 1 and calcium transients in folliculostellate cells
John Morris, Howell Williams & Helen Christian
Annexin 1 (ANXA1) externalised from folliculostellate cells is known to be an essential intermediate in the early delayed (30 min- 3 h) negative feedback of glucocorticoids on the release of ACTH and other pituitary hormones. We have used a well-characterised folliculostellate cell line (TtT/GF cells) to investigate the mechanism by which ANXA1, a cytoplasmic protein, is externalised by cells. Externalisation is stimulated by dexamethasone (Dex) and inhibited by inhibitors of the ATP-binding cassette transporter ABCA1. However, depolarisation of TtT/GF cells with potassium also causes ANXA1 externalisation. Sulphonylurea-sensitive K-ATP channels are present on folliculostellate (TtT/GF) cells as are receptors for ATP and adenosine. We have therefore investigated whether exposure of TtT/GF cells to ATP causes externalisation of ANXA1 and the effects of ATP and Dex on calcium transients in TtT/GF cells. ATP (10 μM) causes an immediate, robust increase in intracellular calcium and when applied for 2 h, externalisation of ANXA1 measured by quantitative Western blotting. Despite causing externalisation of ANXA1 depolarisation with 56 mM potassium did not cause calcium transients in the cells. Application of Dex (0.1 μM, 2 h) caused the expected externalisation of ANXA1, but did not evoke calcium transients. When TtT/GF cells were exposed to Dex (0.1 μM) for 10 min prior to application with ATP (10 μM) the calcium transients were inhibited. These findings suggest that Dex blocks the effect of ATP on calcium handling by the folliculostellate cells and that changes in free intracellular calcium play little part in the mechanism by which ANXA1 is externalised from folliculostellate cells.