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Articles by K. Sun
Total Records ( 2 ) for K. Sun
  X. O Zhu , Z Yang , C. M Guo , X. T Ni , J. N Li , Y. C Ge , L Myatt and K. Sun
 

Human amnion fibroblasts produce abundant prostaglandins toward the end of gestation, which is one of the major events leading to parturition. In marked contrast to its well-described antiinflammatory effect, glucocorticoids have been shown to up-regulate cyclooxygenase-2 (COX-2) expression in human amnion fibroblasts. The mechanisms underlying this paradoxical induction of COX-2 by glucocorticoids have not been resolved. Using cultured human amnion fibroblasts, we found that the induction of COX-2 mRNA expression by cortisol was a glucocorticoid receptor (GR)-dependent process requiring ongoing transcription. Upon transfection of a COX-2 promoter-driven reporter gene into the amnion fibroblasts, cortisol stimulated the COX-2 promoter activity. This was abolished by mutagenesis of a cAMP response element (CRE) at –53 to approximately –59bp as well as by cotransfection of a plasmid expressing dominant-negative CRE-binding protein (CREB). The phosphorylation level of CREB-1 was significantly increased by cortisol treatment of the amnion fibroblasts, whereas the effect was attenuated either by the protein kinase A inhibitor H89 or the p38 -MAPK inhibitor SB203580. The induction of the COX-2 promoter activity and the phosphorylation of CREB-1 were also blocked by the GR antagonist RU486. Chromatin immunoprecipitation (ChIP) assay revealed that the binding of CREB-1 to the CRE of the COX-2 promoter was increased by cortisol treatment of the amnion fibroblasts. In conclusion, cortisol, via binding to GR, stimulated COX-2 expression by increasing phosphorylated CREB-1 binding to the CRE of the COX-2 gene. Cortisol may phosphorylate CREB-1 by activating either protein kinase A or p38-MAPK in the amnion fibroblasts.

  C Guo , J Li , L Myatt , X Zhu and K. Sun
 

Cytosolic phospholipase A (cPLA2) catalyzes the formation of arachidonic acid in prostaglandin synthesis. In contrast to the well-described down-regulation of cPLA2, up-regulation of cPLA2 by glucocorticoids has been reported in human amnion fibroblasts, which may play a key role in parturition. The mechanisms underlying this paradoxical induction of cPLA2 by glucocorticoids remain largely unknown. Using cultured human amnion fibroblasts, we found that the induction of cPLA2 by cortisol required ongoing transcription and synthesis of at least one other protein. The induction of cPLA2 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 cPLA2 expression by cortisol in human fetal lung fibroblasts (HFL-1). Cortisol increased Gs expression in amnion fibroblasts but not in HFL-1 cells. Inhibition of Gs with NF449 attenuated the phosphorylation of cAMP response element-binding protein-1 (CREB-1) and the induction of cPLA2 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 cPLA2 by cortisol was blocked by GR antagonist RU486 or protein kinase A inhibitor H89 or dominant-negative CREB-1. In conclusion, cortisol activates the cAMP/protein kinase A/CREB-1 pathway via Gs induction, and the phosphorylated CREB-1 interacts with GR at the GRE to promote cPLA2 expression in amnion fibroblasts.

 
 
 
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