Silencing of the interferon-inducible gene Ifi204/p204 induces resistance to interferon-γ-mediated cell growth arrest of tumor cells

Many tumor cells escape from cancer immunosurveillance and resist treatment with interferons (IFNs). Although the mechanism underlying IFN resistance is mostly attributed to a deficiency of components of the IFN-signaling pathway, some types of tumor cells resist IFN-mediated cell growth arrest despite the presence of an intact JAK/STAT signaling pathway. However, the molecular mechanisms underlying the unresponsiveness to IFNs independent of the defective JAK/STAT pathway remain to be clarified. To elucidate the mechanisms underlying IFN gamma resistance, we examined the anti-proliferative effect of IFN gamma on mouse tumor cell lines. Mouse squamous cell carcinoma (SCCVII) cells were resistant to IFN gamma-mediated cell growth arrest despite the presence of the IFN gamma-induced STAT1-dependent signaling pathway, whereas IFN gamma inhibited cell growth of B16/F1 cells, a well-known IFN gamma-sensitive mouse melanoma cell line, at the G1 phase of the cell cycle. Treatment of SCCVII cells with IFN gamma neither downregulated the expression of cyclin D1, cyclin A2, and cyclin E1 nor induced a hypo-phosphorylated, active form of retinoblastoma protein (pRb). Interestingly, the hyper-phosphorylated, inactive form of pRb was exclusively localized in the cytoplasm in SCCVII cells. The IFN-inducible 204 gene (Ifi204), whose gene product, p204, binds to pRb and exerts an anti-proliferative effect, was repressed in SCCVII cells. p204 overexpression in SCCVII significantly inhibited cell growth, and mutation of a pRb-binding LXCXE motif decreased the anti-proliferative effect. These results suggest that silencing of Ifi204/p204 induces resistance to IFN gamma-mediated cell growth arrest in SCCVII cells.