Oncofetal fibronectin is an extracellular matrix protein that is suggested to play an important role in regulating adherence at uterine-placental 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 from 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 in immunoprecipitation, Northern blotting, and enzyme-linked immunosorbent assay experiments. Conversely, GC treatment increased FN expression in placental fibroblasts to 164-310% of control levels in Northern blotting and enzyme-linked immunosorbent assay procedures, suggesting that GC-mediated suppression of FN expression is specific to cytotrophoblasts. Results indicated that the DEX-mediated suppression of FN expression in cytotrophoblasts was not mediated through changes in the stability of FN messenger ribonucleic acid (mRNA). Run-on transcription assays using isolated nuclei suggested that GC treatment did not markedly affect transcription of the FN gene in cytotrophoblasts. To test whether the GC-mediated suppression of FN expression was mediated through 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 and cycloheximide (CHX; 125 ng/mL). We observed that CHX treatment increased 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 control cells. These data suggest that suppression of FN mRNA expression by GC in cytotrophoblasts requires de novo protein synthesis and is mediated through a short lived intermediate, the synthesis of which is inhibited at low concentrations of CHX. Thus, GC-induced protein intermediates may influence uterine-placental adherence by modulating levels of oncofetal FN at sites of uterine-placental contact.