GAMMA-INTERFERON INDUCES AN IRREVERSIBLE GROWTH ARREST IN MID-G(1) INMAMMARY 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 carc inoma cell lines. We demonstrate that IFN-gamma blocks the proliferati on of MECs in a time- and concentration-dependent manner. This growth arrest is irreversible and occurs at a specific stage in the G(1) phas e of the cell cycle, 1FM-gamma caused a rapid (within 12-24 h) down-re gulation of cyclin A, c-myc, and cdc2 proteins, as well as a disappear ance of hyperphosphorylated forms of the retinoblastoma family protein s, Rb and p130. The synthesis of several other growth control proteins , p53, p21/Waf1, and proliferating cell nuclear antigen, was down-regu lated between 24 and 48 h. In MECs synchronized by epidermal growth fa ctor deprivation and released for cell cycle traverse by re-addition o f epidermal growth factor to the medium, lFN-gamma was able to block D NA synthesis only if added in the first 6 to 7 h after epidermal growt h factor. The block in Rb phosphorylation and cyclin A expression was coordinately regulated during the same narrow window of G(1). Several mammary carcinoma cell lines demonstrated resistance to the growth-inh ibitory effects of lFN-gamma and did not exhibit down-regulation of cd c2 and cyclin A expression or a change in hyperphosphorylation of Rb w hen treated with IFN-gamma. Initial studies suggest, in some carcinoma cell lines, that resistance to lFN-gamma may be caused by defects in the IFN-gamma signal transduction pathway (measured by expression of t he IFN-gamma-responsive gene GBP), while resistance in others may be d ue to defects in cell cycle regulatory proteins that are the targets o f lFN-gamma action.