A recombinant endogenous retrovirus amplified in a mouse neuroblastoma is involved in tumor growth in vivo

Abstract

The theory of immunoediting postulates that tumor cells exhibit a reduced immunogenicity to escape eradication by the host immune system. It has been proposed that endogenous retroviruses-provided that they are active-could play a role in this process, via the immunosuppressive domain carried by their envelope protein. Here, we demonstrate that the Neuro-2a tumor cell line-originating from a spontaneous A/J mouse neuroblastomaproduces an infectious retrovirus that most probably results from a recombination event between 2 mouse endogenous retroviral elements. This Neuro-2a-associated recombinant retrovirus derives from the unique ecotropic provirus located at the Emv-1 locus, but with a gag sequence conferring B-tropism, thus allowing its highlevel amplification in Neuro-2a cells. We show that knocking down -by RNA interference-this endogenous retrovirus in Neuro-2a cells has no effect on the transformed phenotype of the cells, but results in delayed tumor growth and prolonged animal survival, following engraftment of the cells into immunocompetent mice. Recombination between endogenous retroviruses, amplification of the resulting element and high-level expression of its immunosuppressive activity are therefore likely steps of an immunoediting process, leading to an invading tumor. ' 2006 Wiley-Liss, Inc.Key words: endogenous retrovirus; neuroblastoma; tumor; immunoediting; RNA interference Although several evidences support the notion of cancer immunosurveillance in mice and humans, 1,2 some individuals with an intact immune system still develop cancer. This can be explained by a failure of the host immunity to eradicate specifically the transformed cells. The term ''cancer immunoediting'' has been coined to emphasize the interplay between the host immune system and the tumor, that sometimes lead to ''tumor escape.'' 2,3 The molecular and cellular basis of this process is not yet elucidated. The high incidence of spontaneous tumors in some inbred mouse strains has been related to the sustained expression of endogenous murine leukemia viruses (MLVs) early in life. 4 It is now well established that these elements can play a role in the transformation process via insertional mutagenesis.5 They could also be involved not in the transformation process per se but in tumor progression in vivo, by subverting the immune response. Indeed, the envelope protein of retroviruses-including the MLVs-possesses an immunosuppressive activity, which can be revealed by an in vivo assay.6-8 Furthermore, we have recently shown that an endogenous retrovirus specifically produced by a murine melanoma is absolutely required for tumor growth in immunocompetent mice, precisely because its immunosuppressive activity subverts host immunosurveillance. Indeed, in this melanoma tumor model, knocking down-by RNA interference-the endogenous retrovirus resulted in the rejection of the engrafted tumor cells, without any alteration in the transformed phenotype of the cells. In this article, we analyze another class of spontaneous mouse tumors, i.e...

Mobility and integration sites of a murine C57BL/6 melanoma endogenous retrovirus involved in tumor progression in vivo

“…A similar switch in the tropism of an endogenous retrovirus, followed by its high-level amplification, has been observed in another class of murine tumor, namely a neuroblastoma. 37 A second interesting outcome of the present investigation is the identification of new MelARV insertions in the highly metastatic B16F10 subline. This subline had been positively selected for enhanced metastatic activity, and it is of interest that the 2 identified insertions are precisely within genes: in one case within the itga2 gene, with the insertion resulting in a 5-to 6-fold decrease in transcript level, and in the other case within the dok5 gene, resulting in a 29-fold increase in transcript level.…”

Molecular functions of human endogenous retroviruses in health and disease

“…Indeed, overexpression of Moloney MLV transmembrane subunit in murine tumor cell lines led to tumor growth in recipient mice that would otherwise immunologically reject them [98,214]. Consistently, knockdown of env transcripts in melanoma and neuroblastoma cell lines, both of which produce infectious MLVs derived from endogenous retroviral precursors, rendered them susceptible to immune rejection in vivo [215,216]. In human DNA, in addition to Syncytins, the Env genes of HERV-E (ERVE) [217] and HERV-H (ERVH) [218] were shown to possess immunosuppressive potential [98].…”

Endogenous retroviruses

“…Likewise, a cell line (Neuro-2a) derived from a spontaneous A/J mouse neuroblastoma produced an infectious ERV, dubbed Neuro-2a-associated retrovirus (NeRV). Southern blot analyses showed that NeRV was strongly amplified in Neuro-2a cells, in accord with retrotransposition or reinfection of NeRV in Neuro-2a cells [32]. Interestingly, both MelARV and NeRV were most likely generated by recombination events between N-tropic endogenous MLV proviruses and parts of gag sequences from B-tropic endogenous MLV.…”

“…Interestingly, both MelARV and NeRV were most likely generated by recombination events between N-tropic endogenous MLV proviruses and parts of gag sequences from B-tropic endogenous MLV. Since A/J and C57BL/6 mice are only permissive for retroviruses with B-tropic, but not with N-tropic Gag proteins, these recombination events likely conferred the ability to replicate in A/J and C57BL/6 mice cells [32,33]. Evidence suggesting that MelARV and NeRV may also promote tumor growth at the level of host immunosuppression will be discussed below.…”

“…Likewise, expression of endogenous MMTVrelated proviruses in lactating mammary glands may lead to the release of infectious virus, followed by reinfection, novel proviral insertions, and activation of cellular proto-oncogenes [4]. Two more recent studies using either a murine melanoma or a murine neuroblastoma model have provided additional evidence for mobility and amplification of ERVs in murine tumor cells [32,33]. Murine B16 melanomas from C57BL/6 mice spontaneously produce an ERV, termed melanoma-associated retrovirus (MelARV).…”