Investigation of glucocorticoid and dissociated glucocorticoid activity in hepatoma cell lines with specific reference to regulation of the corticosteroid binding globulin (CBG) proximal promoter'

[摘要] ENGLISH ABSTRACT:This study investigated the effect of several hormones on the rat corticosteroid bindingglobulin proximal promotor and for the first time showed that modulation occurs at thepromotor level and can be correlated with changes in corticosteroid binding globulinmRNA and protein levels. The effect of various physical and psychological stressors onrat liver corticosteroid binding globulin mRNA levels was also tested and it was shownthat voluntary running had no effect on rat corticosteroid binding globulin levels butthat involuntary swimming and immobilization decreased rat corticosteroid bindingglobulin mRNA levels. Glucocorticoid responsiveness of the corticosteroid bindingglobulin promoter was investigated further by using truncated contructs of thecorticosteroid binding globulin proximal promoter. Glucocorticoid responsiveness wasdelineated to between -296 and -145bp from the transcription start site an area thatcontains putative binding sites for D-site binding protein, hepatic nuclear factor-3 andCAAT/enhancer binding protein suggesting that these transcription factors may beinvolved in glucocorticoid responsiveness of the corticosteroid binding globulinproximal promoter.The dissociative glucocorticoid activity of medroxyprogesterone acetate and CompoundA, both putative dissociated glucocorticoids, was compared to standard glucocorticoidsby examining transactivation of glucocorticoid response element-containing reporterconstructs and transrepression of corticosteroid binding globulin gene expression inhepatic cell lines. Results showed that medroxyprogesterone acetate, but not CompoundA, trans activates only in the presence, but not in the absence, of co-transfectedglucocorticoid receptor. Medroxyprogesterone acetate down modulated dexamethasonetransactivation while the modulatory effect of Compound A depends on the order of addition of Compound A. If added together Compound A has no effect ondexamethasone transactivation, however, if Compound A was added beforedexamethasone, Compound A significantly decreased dexamethasone transactivation.Both medroxyprogesterone acetate and Compound A, like glucocorticoids,transrepressed the rat corticosteroid binding globulin proximal promoter. The potencyof repression was similar but Compound A repressed with a higher efficacy thanmedroxyprogesterone acetate. We conclude that Compound A is a completelydissociated glucocorticoid in contrast to medroxyprogesterone acetate that displays onlypartial dissociation, which is dependent on glucocorticoid receptor levels.

[发布机构] Stellenbosch University