Norethindrone scatters silver-stained nucleolar organizer regions of Ishikawa cells.

We investigated the effects of sex steroids on silver-stained nucleolar organizer regions (Ag-NORs) and DNA/RNA kinetics in Ishikawa cells. Norethindrone and its isomer norethynodrel exclusively caused Ag-NORs to scatter in the nuclear matrix, from the nucleolus. No such effect occurred with the other sex steroids tested, including progestational, androgenic, and estrogenic compounds. Nuclear argyrophilic substances induced by norethindrone, as well as nucleolar ones, were neither DNA nor RNA but protein. Electron microscopy showed that norethindrone caused nucleolar segregation, in which the fibrillar components disappeared, and it produced islets, consisting of dense fibrillar materials, in the nucleoplasm. Ag-NORs observed on the fibrillar components in control nucleoli were translocated onto the dense fibrillar materials in the nucleoplasm. Although scattering was preferentially found in the cells synthesizing DNA, the scintillation assay of DNA/RNA kinetics suggested that scattering was related to the inhibition of RNA synthesis. These results imply that norethindrone preferentially interacts with intranucleolar DNA when its duplication is occurring and then interferes with rRNA synthesis. Scattering of Ag-NORs might not be caused by the hormonal activity of these agents but by a pharmacological effect derived from their molecular structures.