P127 Triptolide inhibits virus-induced interferon antiviral response and enhances vesicular stomatitis virus oncolysis

Abstract

Introduction Rapid induction of type I interferon (IFN) expression is a central event in establishing the innate antiviral response. It requires pathogen-inducible activation of transcription factors that function in a synergistic fashion to induce gene expression. Vesicular stomatitis virus (VSV) is highly sensitive to the antiviral effects of type I IFN, as well as other innate immune effectors that strongly inhibit VSV in non-malignant cells and tissues; malignant cells acquire diminished responsiveness to IFN action and are specifically infected and killed by VSV. However, some cancer types, including androgen-independent prostate cancer, appear to be resistant to VSV-induced oncolysis. To overcome this resistance, experimental strategies are now combining oncolytic viruses with different cytotoxic compounds to enhance tumor cell killing. Triptolide (TPL) is a key biologically active component purified from the Chinese herb Tripterygium wilfordii Hook F . This small molecule mediates a variety of biological activities including immune suppression and anti-inflammatory effects. Interestingly, TPL has been well characterized as a proapoptotic and antiproliferative agent in vitro and in vivo . TPL and its derivatives have entered human clinical trials for several diseases, including cancer. In the present study, we investigate the role of TPL in relation to VSV oncolysis in the human prostate cancer cell line PC-3. Methods PC-3 prostate cancer cells were pretreated with TPL then infected with VSV. The induction of various ISGs expression following VSV infection was monitored by qPCR and Western blot. VSV-induced apoptosis and viral replication were assessed by flow cytometry and plaque assay, respectively. For in vivo studies, PC-3 cells were injected subcutaneously in athymic nude mice and VSV-infected mice received intraperitoneal injection of TPL or control vehicle. Tumor size was measured using a caliper, and tumor volume was calculated as length X width 2 /2. Results We demonstrate that TPL potently inhibits induction of the type I IFN innate antiviral response. Additionally, we also observe that TPL increases VSV replication in PC-3 cells in a dose-dependent manner. Furthermore, we tested the effect of known RNA polymerase II inhibitors (flavopiridol and α-amanitin) in combination with VSV and observed a similar effect on interferon-stimulated gene (ISG) expression and viral replication as with TPL. Thus, as previously reported, TPL inhibition of ISG induction appears to be mediated via the inhibition of RNA polymerase II activity. TPL enhanced VSV-induced apoptosis in a dose- and time-dependent manner in PC3, as measured by PI/annexin V staining. All together, these results indicate that TPL enhances VSV replication and induces cell death following VSV infection by inhibiting the induction of IFN signaling. In vivo, using a PC-3 xenograft model in athymic male nude mice, TPL inhibited tumor growth, indicating that TPL is a promising candidate to enhance VSV oncolytic activity in vivo . Conclusion This study reveals for the first time the role of TPL in the regulation of IFN antiviral activity and enhancement of VSV-induced apoptosis.