DEXAMETHASONE ALTERS RAPIDLY ACTIN POLYMERIZATION DYNAMICS IN HUMAN ENDOMETRIAL CELLS - EVIDENCE FOR NONGENOMIC ACTIONS INVOLVING CAMP TURNOVER

Glucocorticoids, in addition to their well characterized effects on th e genome, may affect cell function in a manner not involving genomic p athways. The mechanisms by which the latter is achieved are not yet cl ear. A possible means for this action may involve the actin cytoskelet on, since the dynamic equilibrium of actin polymerization changes rapi dly following exposure to several stimuli, including hormones. The aim of the present work was to find out if glucocorticoids exert rapid, n ongenomic effects on actin polymerization in Ishikawa human endometria l cells, which represent a well characterized in vitro cell model expr essing functional glucocorticoid receptors. Short term exposure of the cells to the synthetic glucocorticoid dexamethasone resulted in an ov erall decrease of the G/total-actin ratio in a time- and dose-dependen t manner. Specifically, in untreated Ishikawa cells the G/total-actin ratio was 0.48 +/- 0.01 (n = 26). It became 0.35 +/- 0.01 (n = 13, P < 0.01) following exposure to 10(-7)M dexamethasone for 15 min. This wa s induced by a significant decrease of the cellular G-actin level, wit hout affecting the total actin content, indicating a rapid actin polym erization. This conclusion was fully confirmed by direct fluorimetry m easurements, that showed a significant increase of the F-actin content by 44% (n = 6, P < 0.001) in cells treated with dexamethasone (10(-7) M, 15 min). The rapid dexamethasone-induced alterations of the state o f actin polymerization were further supported by fluorescence microsco py. The latter studies showed that the microfilaments of cells pretrea ted with 10(-7)M dexamethasone for 15 min were more resistant to vario us concentrations of the antimicrofilament drug cytochalasin B, compar ed to untreated cells, implying microfilament stabilization. The actio n of dexamethasone on actin polymerization seems to be mediated via sp ecific glucocorticoid binding sites, since the addition of the glucoco rticoid antagonist RU486 completely abolished its effect. Moreover, it appears to act via non-transcriptional pathways, since actinomycin D did not block the dexamethasone-induced actin polymerization. In addit ion, cell treatment with 10(-7)M dexamethasone for 15 min fully revers ed the forskolin-, but not the 8-bromo-cAMP-induced actin depolymeriza tion. In line with these findings, the cAMP content of Ishikawa cells was decreased by 29.2% after a 15 min treatment with 10(-7)M dexametha sone (n = 4, P < 0.01). In conclusion, our results showed that dexamet hasone induces rapid, time-, and dose-dependent changes in actin polym erization dynamics in Ishikawa cells. This action seems to be mediated via cAMP, involving probably nongenomic pathways. The above findings offer new perspectives for the understanding of the early cellular res ponses to glucocorticoids. (C) 1996 Wiley-Liss, Inc.