STAT2 in the regulation of tumor growth and antitumor effects of type I interferons in a mouse model of melanoma

Abstract

Signal Transducer and Activator of Transcription (STAT) 2 is one of seven members of the STAT family of transcription factors with dual roles in signal transduction and gene activation. STAT2 is a central transcription factor that regulates the antiviral, apoptotic and cell growth inhibitory effects of type I interferons (IFN-α/β), a small family of secreted glycoproteins induced by the host after sensing the presence of tumor cells and pathogens. The creation of Stat2-/- mice established the pivotal role of STAT2 in type I IFN signaling and in antiviral immunity. In vitro studies conducted in STAT2 deficient tumor cell lines suggested a role in suppressing tumor cell growth in response to IFN treatment. Based on these properties STAT2 is presumed to have tumor suppressor functions but data to support this notion in animal models of cancer are limited. To address the role of STAT2 in cancer, I used the murine B16-F1 tumor transplantation model of human melanoma. The B16-F1 melanoma cell line was established from a spontaneous tumor that arose in mice. I discovered that tumor cells transplanted subcutaneously in Stat2-/- mice grew more aggressively than in the counterpart wild type mice. Closer examination of B16-F1 tumors harvested from wild type and Stat2-/- mice revealed an unexpected dramatic similar reduction of STAT2 and STAT1 proteins. Yet soluble factors secreted by B16-F1 tumors established in Stat2-/- mice alone were sufficient to enhance proliferation of B16-F1 tumor cells. I further showed that tumor-bearing wild type mice treated with IFN-β developed smaller tumors compared to Stat2-/- mice, whose tumors continued to grow and hence were unresponsive to IFN intervention. Lastly, to elucidate a mechanism that leads to enhanced tumor growth in Stat2-/- mice, I questioned the involvement of the host immune response in restricting tumor growth. I found that tumor specific T cell priming by Stat2-/- dendritic cells (DCs) was defective since generated cytotoxic T cells (CD8+ T lymphocytes) produced low levels of IFN-γ and IL-2 and adoptive transfer of these B16-F1 tumor specific CD8+ T cells in B16-F1 bearing Stat2-/- mice did not cause tumor regression with IFN-β intervention. Collectively, my findings reveal that host STAT2 restricts the establishment of melanoma tumors. More importantly, type I IFN/STAT2 signaling on DCs plays a pivotal role in tumor antigen cross-presentation to CD8+ T cells and in the development of a protective antitumor response resulting in tumor rejection. To now address whether STAT2 expression in cancer cells could influence tumor establishment and the antitumor effects of type I IFNs, STAT2 expression was silenced in B16-F1 tumor cells. Contrary to my expectation, silencing STAT2 augmented the growth inhibitory effects of IFN-β both in vitro and in vivo. However, loss of STAT2 expression in the tumor did not cause B16-F1 tumor cells to grow more aggressively compared to control B16-F1 cells. Furthermore, compared to B16-F1 control cells, STAT2-silenced B16-F1 cells showed an initial delay but later persistent STAT activation and formation of the ISGF3 transcriptional complex (consisting of STAT1, STAT2 and IRF9). This observation paralleled with an initial delay and then later an increase in the expression of IFN regulated genes. In addition, reduced activation of STAT5 induced by IFN-β was observed in STAT2-silenced B16-F1 cells. This may partially explain the enhanced growth inhibitory effects of type I IFNs. Together these results shed light on the unexpected role of tumor STAT2 expression in diminishing the efficacy of type I IFN treatment of melanoma.