The antitumor effects of interferon-alpha are maintained in mice challenged with a STAT1-deficient murine melanoma cell line

Introduction. Interferon-alpha (IFN-α) is currently administered to patients with metastatic malignant melanoma and those who are at risk for recurrence following surgery for high-risk lesions. Signal transducer and activator of transcription 1 (STAT1) is a transcription factor that is activated by IFN-α and is thought to mediate the majority of its antitumor effects. Loss of STAT1 has been found in IFN-resistant melanoma cells. We developed a murine melanoma cell line in a STAT1-deficient mouse. We also transfected B16 melanoma cells with a wild-type form of STAT1 to induce its overexpression. Using the resulting cell lines and STAT1-deficient mice, we tested whether IFN-α could exert an antitumor effect on melanoma cells in the absence of STAT1-mediated signal transduction. Materials and methods. A melanoma tumor was induced in STAT1-deficient mice via the application of DMBA (tumor initiator) followed by croton oil (tumor promoter). Immunohistochemical analysis confirmed that the resulting tumor was a malignant melanoma. Immunoblot analysis, intracellular flow cytometry, and gel-shift analysis were used to confirm the lack of STAT1 in the derivative cell line (AGS-1). In addition, the STAT1 protein was overexpressed in B16 melanoma cells by stable transfection with a plasmid construct encoding wild-type STAT1. The effects of IFN-α on these cell lines were studied in vitro and in vivo. Results. STAT1 was not expressed in the AGS-1 murine melanoma cell line. Treatment with IFN-α did not lead to activation of STAT1. Cell proliferation assays revealed that while IFN-α did not exert an antiproliferative effect on this cell line, it was capable of prolonging the survival of STAT1-competent C57BL/6 mice bearing 1 × 10⁶ AGS-1 tumor cells in the intraperitoneal position (n = 20, P < 0.05), as compared to PBS-treated controls. Also, the survival of IFN-α-treated mice (as compared to PBS-treated controls) was not affected by the overexpression of STAT1 in B16 tumor cells. Conclusions. This data suggests that IFN-α can enhance survival in an animal model where STAT1-mediated signal transduction and gene regulation is absent within the tumor but is present within the host. This data also indicates that the overexpression of STAT1 within the tumor does not significantly enhance the effects of exogenously administered IFN-α in this model. These findings indicate that the bulk of the antitumor actions of IFN-α may be derived from its effects on host tissues.