Gamma-interferon induces an irreversible growth arrest in mid-G1 in mammary epithelial cells which correlates with a block in hyperphosphorylation of retinoblastoma.

In this study, we analyze effects of IFN-gamma on the proliferation of normal human mammary epithelial cells (MECs) and several mammary carcinoma cell lines. We demonstrate that IFN-gamma blocks the proliferation of MECs in a time- and concentration-dependent manner. This growth arrest is irreversible and occurs at a specific stage in the G1 phase of the cell cycle. IFN-gamma caused a rapid (within 12-24 h) down-regulation of cyclin A, c-myc, and cdc2 proteins, as well as a disappearance of hyperphosphorylated forms of the retinoblastoma family proteins, Rb and p130. The synthesis of several other growth control proteins, p53, p21/Waf1, and proliferating cell nuclear antigen, was down-regulated between 24 and 48 h. In MECs synchronized by epidermal growth factor deprivation and released for cell cycle traverse by re-addition of epidermal growth factor to the medium, IFN-gamma was able to block DNA synthesis only if added in the first 6 to 7 h after epidermal growth factor. The block in Rb phosphorylation and cyclin A expression was coordinately regulated during the same narrow window of G1. Several mammary carcinoma cell lines demonstrated resistance to the growth-inhibitory effects of IFN-gamma and did not exhibit down-regulation of cdc2 and cyclin A expression or a change in hyperphosphorylation of Rb when treated with IFN-gamma. Initial studies suggest, in some carcinoma cell lines, that resistance to IFN-gamma may be caused by defects in the IFN-gamma signal transduction pathway (measured by expression of the IFN-gamma-responsive gene GBP), while resistance in others may be due to defects in cell cycle regulatory proteins that are the targets of IFN-gamma action.