GLUCOCORTICOID EFFECTS IN THE HUMAN PLACENTA - EVIDENCE THAT DEXAMETHASONE-MEDIATED INHIBITION OF FIBRONECTIN EXPRESSION IN CYTOTROPHOBLASTS INVOLVES A PROTEIN INTERMEDIATE

Oncofetal fibronectin is an extracellular matrix protein that is sugge sted to play an important role in regulating adherence at uterine-plac ental interfaces. The purpose of the present study was to elucidate a mechanism through which glucocorticoids (GCs) inhibit the synthesis of FN in human placenta as part of their matrix-suppressive action near parturition. We observed that treatment of cytotrophoblasts isolated f rom human term placentas for 48 h with 10(-7) mol/L dexamethasone (DEX ) down-regulated levels of FN expression to 13-19% of control levels i n immunoprecipitation, Northern blotting, and enzyme-linked immunosorb ent assay experiments. Conversely, GC treatment increased FN expressio n in placental fibroblasts to 164-310% of control levels in Northern b lotting and enzyme-linked immunosorbent assay procedures, suggesting t hat CC-mediated suppression of FN expression is specific to cytotropho blasts. Results indicated that the DEX-mediated suppression of FN expr ession in cytotrophoblasts was not mediated through changes in the sta bility of FN messenger ribonucleic acid (mRNA). Run-on transcription a ssays using isolated nuclei suggested that GC treatment did not marked ly affect transcription of the FN gene in cytotrophoblasts. To test wh ether the CC-mediated suppression of FN expression was mediated throug h a protein intermediate, levels of FN mRNA were examined by Northern blotting in Cells treated for 48 h with and without 10(-7) mol/L DEX a nd cycloheximide (CHX; 125 ng/mL). We observed that CHX treatment incr eased FN expression in DEX-treated cells to 91% of control values. We noted that whereas the presence of 100-300 ng/mL CHX reversed the DEX- mediated inhibition of FN mRNA expression in cytotrophoblasts, it did not alter the overall rates of protein synthesis in DEX-treated and co ntrol cells. These data suggest that suppression of FN mRNA expression by GC in cytotrophoblasts requires de novo protein synthesis and is m ediated through a short lived intermediate, the synthesis of which is inhibited at low concentrations of CHX. Thus, CC-induced protein inter mediates may influence uterine-placental adherence by modulating level s of oncofetal FN at sites of uterine-placental contact.