Role of bone marrow stromal antigen 2 (BST-2) in viral pathogenesis and breast cancer progression

Bone marrow stromal antigen 2 (BST-2/tetherin) is a type II transmembrane protein that plays various roles, including protective and detrimental roles in the host. Cellular responses to BST-2 expression or the lack thereof, may be cell type and context-dependent and may vary with time. When protective, BST-2 functions as an antiviral factor, renowned for its ability to tether budding enveloped viruses to the membrane of infected cells. Tethering of budding virions prevents their release into the extracellular milieu limiting infection of naïve cells. The antiviral role of BST-2 has been predominantly studied using cultured cells. Insight into the role of BST-2 in inhibition of viral infection in vivo came from our study of the alphavirus Chikungunya virus (CHIKV) and the retrovirus mouse mammary tumor virus, (MMTV). BST-2 prevents the release of CHIKV and MMTV virions from infected cells and limits the replication of both viruses in mice. In the context of CHIKV infection, BST-2 protects the host in a tissue-type dependent manner. In lymphoid and most non-lymphoid tissues, expression of BST-2 limits CHIKV replication. In addition, BST-2 regulates CHIKV-induced inflammatory responses in mice, an indication that BST-2 may function to initiate and amplify innate immune responses. Host response to MMTV infection depends on the stage of the infection and disease sequela. Acute infection of immune cells with MMTV results in an initial increase in BST-2 expression followed by a sharp decline. In contrast, in MMTV-induced mammary tumors, BST-2 mRNA and protein are elevated, so is the viral load. This is an indication that the antiviral role of BST-2 is not operative once mammary tumors have developed. These data provided the initial evidence that BST-2 may promote breast cancer progression. Indeed, data from two mouse models of breast cancer show that expression of BST-2 is necessary for cell to cell and cell to extracellular matrix interactions. Thus, BST-2 expression in breast cancer cells enhances cancer cell adhesion, anchorage-independency, migration, and invasion, culminating in increased tumor mass, increased metastases, and reduced host survival. Structurally, BST-2 homodimerization is important for its cancer-promoting role as dimers of BST-2 regulate anchorage-independency, resistance to anoikis, and enhanced adhesion between cancer cells and components (proteins and cells) of the tumor microenvironment. How BST-2 is enriched in breast cancer cells was elusive until our in silico analyses of a large human breast cancer dataset that revealed the involvement of epigenetic regulation of BST-2 in breast tumors. In highly aggressive breast cancers, specific CpG sites in and at close proximity to the BST-2 promoter are hypomethylated. This is in sharp contrast to non-aggressive luminal cancers and normal breast epithelial cells. These data suggest that a progressive loss of methylation on the BST-2 gene may contribute to constitutive overexpression of BST-2 in tumors. Overall, these findings show that 1) BST-2 contributes to the emergence and progression of breast malignancies and may be used as a therapeutic target or as a biomarker for aggressive breast cancers; and, 2) BST-2 acts as a viral sensor to initiate antiviral inflammatory responses and could be exploited therapeutically to treat viral infections. We highlight the need for additional research on the antiviral and cancer-promoting roles of BST-2 to reconcile both functions for the purpose of therapeutics.