Induction of Gα_s contributes to the paradoxical stimulation of cytosolic phospholipase A_2α expression by cortisol in human amnion fibroblasts

Cytosolic phospholipase A (cPLA_2α) catalyzes the formation of arachidonic acid in prostaglandin synthesis. In contrast to the well-described down-regulation of cPLA_2α, up-regulation of cPLA_2α by glucocorticoids has been reported in human amnion fibroblasts, which may play a key role in parturition. The mechanisms underlying this paradoxical induction of cPLA_2α by glucocorticoids remain largely unknown. Using cultured human amnion fibroblasts, we found that the induction of cPLA_2α by cortisol required ongoing transcription and synthesis of at least one other protein. The induction of cPLA _2α by cortisol was abolished by mutagenesis of a glucocorticoid response element (GRE) in the promoter. The same GRE was found mediating the classical inhibition of cPLA_2α expression by cortisol in human fetal lung fibroblasts (HFL-1). Cortisol increased Gα_s expression in amnion fibroblasts but not in HFL-1 cells. Inhibition of Gα_s with NF449 attenuated the phosphorylation of cAMP response element-binding protein-1 (CREB-1) and the induction of cPLA_2α by cortisol in amnion fibroblasts. Both glucocorticoid receptor (GR) and CREB-1 were found bound to the GRE upon cortisol stimulation of amnion fibroblasts. The induction of cPLA_2α by cortisol was blocked by GR antagonist RU486 or protein kinase A inhibitor H89 or dominantnegative CREB-1. In conclusion, cortisol activates the cAMP/protein kinase A/CREB-1 pathway via Gα_s induction, and the phosphorylated CREB-1 interacts with GR at the GRE to promote cPLA_2α expression in amnion fibroblasts.