The interferon-inducible protein viperin controls cancer metabolic reprogramming to enhance cancer progression.

Metabolic reprogramming is an important cancer hallmark. However, the mechanisms driving metabolic phenotypes of cancer cells are unclear. Here, we show that the interferon-inducible (IFN-inducible) protein viperin drove metabolic alteration in cancer cells. Viperin expression was observed in various types of cancer and was inversely correlated with the survival rates of patients with gastric, lung, breast, renal, pancreatic, or brain cancer. By generating viperin knockdown or stably expressing cancer cells, we showed that viperin, but not a mutant lacking its iron-sulfur cluster-binding motif, increased lipogenesis and glycolysis via inhibition of fatty acid ß-oxidation in cancer cells. In the tumor microenvironment, deficiency of fatty acids and oxygen as well as production of IFNs upregulated viperin expression via the PI3K/AKT/mTOR/HIF-1α and JAK/STAT pathways. Moreover, viperin was primarily expressed in cancer stem-like cells (CSCs) and functioned to promote metabolic reprogramming and enhance CSC properties, thereby facilitating tumor growth in xenograft mouse models. Collectively, our data indicate that viperin-mediated metabolic alteration drives the metabolic phenotype and progression of cancer.

Viperin Differentially Induces Interferon-Stimulated Genes in Distinct Cell Types.

Viperin is an IFN-stimulated gene (ISG)-encoded protein that was identified in human primary macrophages treated with IFN-γ and in human primary fibroblasts infected with cytomegalovirus (CMV). This protein plays multiple roles in various cell types. It inhibits viral replication, mediates signaling pathways, and regulates cellular metabolism. Recent studies have shown that viperin inhibits IFN expression in macrophages, while it enhances TLR7 and TLR9-mediated IFN production in plasmacytoid dendritic cells, suggesting that viperin can play different roles in activation of the same pathway in different cell types. Viperin also controls induction of ISGs in macrophages. However, the effect of viperin on induction of ISGs in cell types other than macrophages is unknown. Here, we show that viperin differentially induces ISGs in 2 distinct cell types, macrophages and fibroblasts isolated from wild type and viperin knockout mice. Unlike in bone marrow-derived macrophages (BMDMs), viperin downregulates the expression levels of ISGs such as bone marrow stromal cell antigen-2, Isg15, Isg54, myxovirus resistance dynamin like GTPase 2, and guanylate binding protein 2 in murine embryonic fibroblasts (MEFs) treated with type I or II IFN. However, viperin upregulates expression of these ISGs in both BMDMs and MEFs stimulated with polyinosinic-polycytidylic acid or CpG DNA and infected with murine CMV. The efficiency of viral entry is inversely proportional to the expression levels of ISGs in both cell types. The data indicate that viperin differentially regulates induction of ISGs in a cell type-dependent manner, which might provide different innate immune responses in distinct cell types against infections.

Intrinsic expression of viperin regulates thermogenesis in adipose tissues.

Viperin is an interferon (IFN)-inducible multifunctional protein. Recent evidence from high-throughput analyses indicates that most IFN-inducible proteins, including viperin, are intrinsically expressed in specific tissues; however, the respective intrinsic functions are unknown. Here we show that the intrinsic expression of viperin regulates adipose tissue thermogenesis, which is known to counter metabolic disease and contribute to the febrile response to pathogen invasion. Viperin knockout mice exhibit increased heat production, resulting in a reduction of fat mass, improvement of high-fat diet (HFD)-induced glucose tolerance, and enhancement of cold tolerance. These thermogenic phenotypes are attributed to an adipocyte-autonomous mechanism that regulates fatty acid ß-oxidation. Under an HFD, viperin expression is increased, and its function is enhanced. Our findings reveal the intrinsic function of viperin as a novel mechanism regulating thermogenesis in adipose tissues, suggesting that viperin represents a molecular target for thermoregulation in clinical contexts.

Viperin Deficiency Promotes Polarization of Macrophages and Secretion of M1 and M2 Cytokines.

Viperin is a multifunctional protein that was first identified in human primary macrophages treated with interferon-γ and in human fibroblasts infected with human cytomegalovirus. This protein plays a role as an anti-viral protein and a regulator of cell signaling pathways or cellular metabolism when induced in a variety of cells such as fibroblasts, hepatocytes and immune cells including T cells and dendritic cells. However, the role of viperin in macrophages is unknown. Here, we show that viperin is basally expressed in murine bone marrow cells including monocytes. Its expression is maintained in bone marrow monocyte-derived macrophages (BMDMs) depending on macrophage colony-stimulating factor (M-CSF) treatment but not on granulocyte-macrophage colony-stimulating factor (GM-CSF) treatment. In wild type (WT) and viperin knockout (KO) BMDMs differentiated with M-CSF or G-MCSF, there are little differences at the gene expression levels of M1 and M2 macrophage markers such as inducible nitric oxide synthase (iNOS) and arginase-1, and cytokines such as IL-6 and IL-10, indicating that viperin expression in BMDMs does not affect the basal gene expression of macrophage markers and cytokines. However, when BMDMs are completely polarized, the levels of expression of macrophage markers and secretion of cytokines in viperin KO M1 and M2 macrophages are significantly higher than those in WT M1 and M2 macrophages. The data suggest that viperin plays a role as a regulator in polarization of macrophages and secretion of M1 and M2 cytokines.