DEXAMETHASONE INDUCES RAPID ACTIN ASSEMBLY IN HUMAN ENDOMETRIAL CELLSWITHOUT AFFECTING ITS SYNTHESIS

Dexamethasone exerts a stimulatory effect of rapid-onset on the polyme rization of actin. This has been documented in human endometrial adeno carcinoma Ishikawa cells, resulting in an acute, dose-dependent decrea se in the G/total-actin ratio. in the present study we completely char acterized this fast and apparently nongenomic effect of dexamethasone on actin assembly. We followed the morphological alterations of actin cytoskeleton and measured the time-dependent dynamics of actin polymer ization both by ruling out any changes of total actin in the cells and by measuring its transcript. Rapid changes in actin polymerization we re accurately measured using a highly sensitive and quantitative rhoda mine-phalloidin fluorimetric assay. Ishikawa cells. exposed to 0.1 mu M dexamethasone for Various time periods up to 24 h, showed a highly s ignificant, rapid. and transient increase in the polymerization of act in starting within 15 min of dexamethasone exposure and lasting h. Tre ated cells showed a significant (1.79-fold) enhancement of the fluores cent signal compared to untreated cells at 15 min. This value decrease d continuously in a time-dependent manner, reaching control levels aft er 120 min and remained so for the next 24 h. Confocal laser scanning microscopy: studies confirmed these findings. intensive coloration of microfilaments over several scanning sections suggested an enhanced de gree of actin polymerization in cells preincubated for 15 min with 0.1 mu M dexamethasone. Moreover, actin filaments we Ere more resistant t o cytochalasin B. Additionally, quantitative immunoblot analysis skewe d that the content of total cellular actin remained the same during th is period, suggesting that the biosynthesis of actin was unaffected. N orthern blot analysis showed that the concentration of the actin trans cript was also unaffected. Our data suggest thai glucocorticoids induc e a fast and self-limited polymerization of actin in human endometrial cells without affecting its synthesis. These findings strengthen the hypothesis that glucocorticoids exert rapid, nongenomic cellular effec ts and that the actin-based cytoskeleton is an integral part of this p athway, playing an essential role in receiving and mediating steroid s ignals for tile modulation of Cellular responses. J. Cell. Biochem. 65 :492-500. (C) 1997 Wiley-Liss, Inc.