Tumors in mice transgenic for the envelope protein of Jaagsiekte sheep retrovirus

Adeno-associated virus vector mediated expression of an oncogenic retroviral envelope protein induces lung adenocarcinomas in immunocompetent mice

Lung cancer is the most common cause of cancer-related death worldwide. A poor overall survival rate of 16% necessitates the need for novel treatment strategies. Mouse models of lung cancer are important tools for analyzing the significance of somatic mutations in the initiation and progression of lung cancer. Of additional importance, however, are animal models of virally induced cancers. JSRV is a simple betaretrovirus that causes contagious lung cancer in sheep known as ovine pulmonary adenocarcinoma and closely resembles human lung adenocarcinoma. Previously we showed that expression of the JSRV envelope (Env) from an AAV vector induced lung tumors in immunodeficient mice, but not in immunocompetent mice. Because of the importance of studying lung cancer in the context of an intact immune system we sought to improve our mouse model. In this report, we employed the use of a strong JSRV enhancer-promoter combination to express Env at high levels and demonstrate for the first time, lung tumor induction in immunocompetent mice. This occurred despite a robust Env-specific antibody-mediated immune response. The PI3K/Akt and MAPK pathways were activated in both immunocompetent and immunodeficient mice, however, differential activation of PTEN, GSKα, p70S6K, p38MAPK, ATF2 and STAT5 was observed. A JSRV Env lung tumor-derived cell line was shown to have a similar signal transduction activation profile as Env-induced lung tumors in C57BL/6 mice. Given the similarities between our model and pulmonary adenocarcinomas in humans, and the ease with which tumors can be induced in any transgenic mouse, this system can be used to uncover novel mechanisms involved lung tumorigenesis.

Jaagsiekte sheep retrovirus infects multiple cell types in the ovine lung

Ovine pulmonary adenocarcinoma (OPA) is a transmissible lung cancer of sheep caused by Jaagsiekte sheep retrovirus (JSRV). The details of early events in the pathogenesis of OPA are not fully understood. For example, the identity of the JSRV target cell in the lung has not yet been determined. Mature OPA tumors express surfactant protein-C (SP-C) or Clara cell-specific protein (CCSP), which are specific markers of type II pneumocytes or Clara cells, respectively. However, it is unclear whether these are the cell types initially infected and transformed by JSRV or whether the virus targets stem cells in the lung that subsequently acquire a differentiated phenotype during tumor growth. To examine this question, JSRV-infected lung tissue from experimentally infected lambs was studied at early time points after infection. Single JSRV-infected cells were detectable 10 days postinfection in bronchiolar and alveolar regions. These infected cells were labeled with anti-SP-C or anti-CCSP antibodies, indicating that differentiated epithelial cells are early targets for JSRV infection in the ovine lung. In addition, undifferentiated cells that expressed neither SP-C nor CCSP were also found to express the JSRV Env protein. These results enhance the understanding of OPA pathogenesis and may have comparative relevance to human lung cancer, for which samples representing early stages of tumor growth are difficult to obtain.

Jaagsiekte sheep retrovirus biology and oncogenesis

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of a lung cancer in sheep known as ovine pulmonary adenocarcinoma (OPA). The disease has been identified around the world in several breeds of sheep and goats, and JSRV infection typically has a serious impact on affected flocks. In addition, studies on OPA are an excellent model for human lung carcinogenesis. A unique feature of JSRV is that its envelope (Env) protein functions as an oncogene. The JSRV Env-induced transformation or oncogenesis has been studied in a variety of cell systems and in animal models. Moreover, JSRV studies have provided insights into retroviral genomic RNA export/expression mechanisms. JSRV encodes a trans-acting factor (Rej) within the env gene necessary for the synthesis of Gag protein from unspliced viral RNA. This review summarizes research pertaining to JSRV-induced pathogenesis, Env transformation, and other aspects of JSRV biology.

Functional interaction between Env oncogene from Jaagsiekte sheep retrovirus and tumor suppressor Sprouty2

Background

Jaagsiekte sheep retrovirus (JSRV) is a type D retrovirus capable of transforming target cells in vitro and in vivo. The Envelope (Env) gene from JSRV and from related retroviruses can induce oncogenic transformation, although the detailed mechanism is yet to be clearly understood. Host cell factors are envisaged to play a critical determining role in the regulation of Env-mediated cell transformation.

Results

JSRV Env-mediated transformation of a lung adenocarcinoma cell line induced rapid proliferation, anchorage-independent growth and tumor formation, but completely abrogated the migration ability. An analysis of the signaling scenario in the transformed cells suggested the involvement of the ERK pathway regulated by Sprouty2 in cell migration, and the PI3K-Akt and STAT3 pathways in proliferation and anchorage-independence. On the other hand, in a normal lung epithelial cell line, Env-mediated transformation only decreased the migration potential while the other functions remained unaltered. We observed that Env induced the expression of a tumor suppressor, Sprouty2, suggesting a correlation between Env-effect and Sprouty2 expression. Overexpression of Sprouty2 per se not only decreased the migratory potential and tumor formation potential of the target cells but also made them resistant to subsequent Env-mediated transformation. On the other hand, over expression of the functional mutants of Sprouty2 had no inhibitory effect, confirming the role of Sprouty2 as a tumor suppressor.

Conclusions

Our studies demonstrate that Env and Sprouty2 have a functional relationship, probably through shared signaling network. Sprouty2 functions as a tumor suppressor regulating oncogenic transformation of cells, and it therefore has the potential to be exploited as a therapeutic anti-cancer agent.

Receptor binding and low pH coactivate oncogenic retrovirus envelope-mediated fusion

Fusion of enveloped viruses with host cells is triggered by either receptor binding or low pH but rarely requires both except for avian sarcoma leukosis virus (ASLV). We recently reported that membrane fusion mediated by an oncogenic Jaagsiekte sheep retrovirus (JSRV) envelope (Env) requires an acidic pH, yet receptor overexpression is required for this process to occur. Here we show that a soluble form of the JSRV receptor, sHyal2, promoted JSRV Env-mediated fusion at a low pH in normally fusion-negative cells and that this effect was blocked by a synthetic peptide analogous to the C-terminal heptad repeat of JSRV Env. In contrast to the receptor of ASLV, sHyal2 induced pronounced shedding of the JSRV surface subunit, as well as unstable conformational rearrangement of its transmembrane (TM) subunit, yet full activation of JSRV Env fusogenicity, associated with strong TM oligomerization, required both sHyal2 and low pH. Consistently, sHyal2 enabled transduction of nonpermissive cells by JSRV Env pseudovirions, with low efficiency, but substantially blocked viral entry into permissive cells at both binding and postbinding steps, indicating that sHyal2 prematurely activates JSRV Env-mediated fusion. Altogether, our study supports a model that receptor priming promotes fusion activation of JSRV Env at a low pH, and that the underlying mechanism is likely to be different from that of ASLV. Thus, JSRV may provide a useful alternate model for the better understanding of virus fusion and cell entry.

Fusogenicity of Jaagsiekte sheep retrovirus envelope protein is dependent on low pH and is enhanced by cytoplasmic tail truncations

Jaagsiekte sheep retrovirus (JSRV) envelope (Env) is an active oncogene responsible for neoplastic transformation in animals and cultured cells. In this study, we used syncytium induction and fluorescence-based cell fusion assays to investigate JSRV Env fusion and its modulation by the cytoplasmic tail (CT). We found that JSRV Env induced syncytia in cells overexpressing the receptor for JSRV and that a low pH was required for this process to occur. Fusion kinetics studies revealed that cell-cell fusion by JSRV Env at neutral pH was poor, taking up to a day, in sharp contrast to fusion at low pH, which peaked within 2 min following a low-pH trigger. Deletion of the C-terminal 7 or 16 amino acids of the JSRV Env CT had no or little effect on fusion, yet additional truncation toward the membrane-spanning domain, resulting in mutants retaining as little as 1 amino acid of the CT, led to progressively increased syncytium formation at neutral pH that was further enhanced by low-pH treatment. Notably, the severely truncated mutants showed elevated levels of surface subunits in culture medium, suggesting that the CT truncations resulted in conformational changes in the ectodomain of Env that impaired surface subunit associations. Taken together, this study reveals for the first time that the fusion activity of the JSRV Env protein is dependent on a low pH and is modulated by the CT, whose truncation overcomes, at least partially, the low-pH requirement for fusion and enhances Env fusion activity and kinetics.

Specific in vivo expression in type II pneumocytes of the Jaagsiekte sheep retrovirus long terminal repeat in transgenic mice

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma, a transmissible lung cancer in sheep. Previous experiments in differentiated murine tissue culture cell lines suggested that the disease specificity of JSRV for secretory lung epithelial cells (type II pneumocytes an Clara cells) reflects transcriptional specificity of the viral long terminal repeat (LTR) for these cells. To test this in vivo, transgenic mice carrying the bacterial beta-galactosidase (beta-Gal) gene driven by the JSRV LTR were generated. Two transgenic lines showed beta-Gal expression in the lungs but not other tissues of F1 animals, although transgene silencing in subsequent generations was a major problem. The cells expressing the transgene were identified by two- and three-color immunofluorescence for marker proteins of type II pneumocytes (surfactant protein C [SPC]) and Clara cells (CC10) as well as for a T7 gene 10 epitope present in the beta-Gal reporter. F1 animals from both lines showed transgene expression in type II pneumocytes, but somewhat surprisingly not in Clara cells. Expression was not detected in bronchiolo-alveolar stem cells (BASCs) either. These results indicate that the JSRV LTR is specifically active in type II pneumocytes in the mouse lung, which is consistent with the fact that JSRV-induced OPA tumors in sheep largely have phenotypic markers of type II pneumocytes.