Interferon-gamma modulates retinoblastoma gene mRNA in monocytoid cells

To study the effect of interferon gamma (IFN-gamma) on the expression of the retinoblastoma (RB) susceptibility gene, we performed Northern-blot analysis on RNA extracted from Wish, HEL and monocytoid cell lines U-937 and THP-1 treated with 1,000 IU/ml of recombinant IFN-gamma. In U-937 and THP-1 cells, IFN-gamma increased the abundance of RB mRNA. In Wish and HEL cells, co-treatment with cycloheximide was required for IFN-gamma to increase the level of RB mRNA. Pre-treatment of THP-1 cells with cycloheximide prior to IFN-gamma treatment augmented the effects of IFN-gamma on RB gene expression. The effect of IFN-gamma in THP-1 cells was observed after 3 hr of treatment, being more pronounced after 6 hr and persisting until at least 18 hr, although at a lower level. These results suggest that IFN-gamma regulates the level of RB mRNA by different mechanisms in the different cell types. This cytokine increases the abundance of RB mRNA in monocytoid cell lines, reinforced by prior treatment with cycloheximide. Inhibition of protein synthesis is required in Wish and HEL cell lines before IFN-gamma has an effect on RB gene expression.