Oncogene (2007) 26, 789–801 & 2007 Nature Publishing Group All rights reserved 0950-9232/07 $30.00 www.nature.com/onc REVIEW Oncogenic transformation by the jaagsiekte sheep retrovirus envelope protein S-L Liu1 and AD Miller2 1Department of Microbiology and Immunology, McGill University, Montreal, Canada and 2Division of Human Biology, Fred Hutchinson Cancer Research Center, Seattle, WA, USA Retroviruses have played profound roles in our under- sarcoma virus, both of which cause tumors in chickens standing of the genetic and molecular basis of cancer. (for a comprehensive review see Rosenberg and Jaagsiekte sheep retrovirus (JSRV) is a simple retrovirus Jolicoeur (1997)). Oncogenic retroviruses are historically that causes contagious lung tumors in sheep, known as classified into acute transforming retroviruses and ovine pulmonary adenocarcinoma (OPA). Intriguingly, nonacute retroviruses. Acute transforming retroviruses OPA resembles pulmonary adenocarcinoma in humans, induce tumors through acquisition and expression of and may provide a model for this frequent human cancer. cellular proto-oncogenes – a process referred to as Distinct from the classical mechanisms of retroviral oncogene capture. These retroviruses induce tumors oncogenesis by insertional activation of or virus capture rapidly, the tumors are of polyclonal origin, and the of host oncogenes, the native envelope (Env) structural viral oncogenes in these viruses are not essential for protein of JSRV is itself the active oncogene. A major virus replication. In fact, acute transforming retro- pathway for Env transformation involves interaction of viruses are often replication-defective (Rous sarcoma the Env cytoplasmic tail with as yet unidentified cellular virus is an exception) because of the loss or disruption of adaptor(s), leading to the activation of PI3K/Akt and essential viral genes during the oncogene capture MAPK signaling cascades. Another potential mechanism process. Nonacute retroviruses, on the other hand, do involves the cell-entry receptor for JSRV, Hyaluronidase not carry oncogenes and induce tumors slowly. This 2 (Hyal2), and the RON receptor tyrosine kinase, but the type of retrovirus causes tumors by activating cellular exact roles of these proteins in JSRV Env transformation proto-oncogenes close to the proviral DNA integration remain to be better understood. Recently, a mouse model sites – a process termed insertional activation. Nume- of lung cancer induced by JSRV Env has been developed, rous replication-competent retroviruses cause clonal and the tumors in mice resemble those seen in sheep tumors by proviral insertional activation. In addition infected with JSRV and in humans. In this review, we to oncogene capture and insertional activation, a few summarize recent progress in our understanding the replication-competent retroviruses induce tumors by molecular mechanisms of oncogenic transformation by expression of viral proteins. For example, human T-cells JSRV Env protein, and discuss the relevance to human leukemia virus types 1 and 2 (HTLV-1 and HTLV-2, lung cancer. respectively) induce adult T-cell immortalization and Oncogene (2007) 26, 789–801. doi:10.1038/sj.onc.1209850; leukemia in humans by expression of viral Tax proteins published online 14 August 2006 that transactivate several cytokines and cytokine recep- tors. It is worth noting that HTLV-1 and HTLV-2 are Keywords: Jaagsiekte sheep retrovirus; envelope pro- the only retroviruses identified so far, that directly cause tein; transformation; lung cancer; signaling pathways; human cancer. mouse model Jaagsiekte sheep retrovirus (JSRV) is simple retro- virus encoding Gag, Pro, Pol, and Env proteins (York et al., 1992; Palmarini et al., 1999; DeMartini et al., 2001), and is now classified as an ovine betaretrovirus. Sheep retroviral oncogenesis: a historical perspective This family also includes a closely related retrovirus of sheep and goats, enzootic nasal tumor virus (ENTV) For almost a hundred years, retroviruses have been (De las Heras et al., 2003). JSRV transforms peripheral known to induce tumors. The field started with the lung epithelial cells leading to ovine pulmonary adeno- discovery of avian erythroblastosis virus and Rous carcinoma (OPA) (Palmarini et al., 1999; DeMartini et al., 2001), while ENTV transforms nasal epithelial cells resulting in enzootic nasal tumor (ENT) (De las Correspondence: Professor S-L Liu, Department of Microbiology and Heras et al., 2003). OPA is a significant veterinary and Immunology, McGill University, 3775University Street, Lyman Duff economic problem in many countries, and in some Building Room 608, Montreal, QC Canada H3A 2B4. E-mail: [email protected] countries such as Britain and South Africa, the incidence Received 12 May 2006; revised 22 June 2006; accepted 24 June 2006; of OPA can be as high as 30% (Sharp and Angus, 1990). published online 14 August 2006 The clinical signs of OPA are associated with respiratory Oncogenesis by the JSRV envelope protein S-L Liu and AD Miller 790 dysfunction and include exaggerated breathing after JSRV and ENTV are members of a small family of exercise. Late stages of disease are often accompanied acutely transforming retroviruses in which an Env by the secretion of copious lung fluid that contains protein acts as an oncogene. infectious virus. Intriguingly, lung tumors induced by JSRV in sheep JSRV can induce lung tumors in newborn lambs in as morphologically resemble a subtype of human pulmonary little as 10 days following experimental inoculation adenocarcinomas previously referred to bronchiolo- (Verwoerd et al., 1980), showing that JSRV is an acutely alveolar carcinoma or BAC (Bonne, 1939). For exam- transforming retrovirus. However, JSRV is a simple ple, both OPA and BAC tumor cells express markers of retrovirus with no apparent oncogene present in its type II pneumocytes and Clara cells, and tumors are genome (Figure 1a). An alternate open reading frame, generally multifocal and localized to the periphery of the orf-X, which is present within the pol region of JSRV, lungs (Mornex et al., 2003). However, given the new was initially suspected to be an oncogenic factor but was definition of BAC by WHO (Travis et al., 1999), later excluded. Orf-X bears no resemblance to any important distinctions exist between OPA and BAC. known cellular oncogenes except sharing a weak BAC is now defined as a peripheral, well differentiated similarity with the adenosine A3 receptor (Bai et al., adenocarcinoma with pure bronchio-alveolar growth 1999). Disruption of orf-X has no effect on cell and no evidence of stromal, vascular and pleural transformation by JSRV (Maeda et al., 2001). Remark- invasion (Travis et al., 1999); while OPA is a mixed ably, expression of native JSRV Env protein alone is peripheral adenocarcinoma with acinar, papillary and sufficient to induce cell transformation in culture bronchio-alveolar growth (Palmarini and Fan, 2001). (Maeda et al., 2001; Rai et al., 2001; Allen et al., 2002; While the true frequency of pure BAC remains Danilkovitch-Miagkova et al., 2003; Liu and Miller, uncertain, tumors with histologically mixed BAC and 2005), indicating that Env is an active oncogene and is adenocarcinoma account for >20% all non-small cell likely to be the key factor in sheep oncogenesis. The only lung carcinomas (NSCLC) (Raz et al., 2006), and are other examples of retroviruses with oncogenic Env increasing in frequency (Barsky et al., 1994). proteins are ENTV (Alberti et al., 2002; Dirks et al., Similarities between OPA and human lung adenocar- 2002), a close relative of JSRV; avian hemangioma cinoma have led to the hypothesis that JSRV or a virus, which expresses an Env that induces proliferation related virus might be involved in human cancer. In one of monkey epithelial cells and NIH 3T3 cells (Alian study, antisera raised against the JSRV capsid protein et al., 2000); and spleen focus-forming virus, a replica- recognized B30% of human BAC and lung adenocar- tion-defective virus that expresses a recombinant non- cinoma samples but in general did not recognize other functional Env protein that induces cell proliferation in tumor types or normal lung tissue (De las Heras et al., cultured cells and in animals (Ruscetti, 1999). Therefore, 2000). In this case, PCR studies were not carried out to a JSRV Gag Pro Orf-X Env LTR Pol LTR 1 kb b JSRV Env SU TM SP MCT 0.3 kb c Env cytoplasmic tail (CT) 590 JSRV7 RGMVRDFLKMRVEMLHMKYRNMLQHQHLMELLKNKERGDAGD--D-P JSRV21 ............................................... JSRVSA ..L.................T....R............A...--.P. ENTV ..LIK...Q..I.LI.....Y...Y.K..DFV.KR.GSCG.QPAEG- ESRV .SI.KE..H...-LI.---K..................A...--.-. Figure 1 Schematic representations of JSRV genome, Env structure and sequence alignment of Env cytoplasmic tail. (a) JSRV is a simple retrovirus encoding Gag, Pol, Pro and Env that are flanked by the long term repeats (LTRs). An additional open reading frame X (Orf-X) is present within the Pol region with unknown function. (b) JSRV Env is type I transmembrane protein, with B620 amino acids in length. It is composed of two subunits, surface (SU) and transmembrane (TM). SP is signal peptide. M is membrane-spanning domain. CT is cytoplasmic tail. (c) Sequence alignment of JSRV Env cytoplasmic tail and in comparison with that of ENTV (Y16627) and endogenous sheep retrovirus. Note that a YXXM motif, the putative binding site for PI3K/p85, is conserved among all transforming JSRV and ENTV Env, but is absent in that of endogenous sheep retrovirus.