Endogenous Retrovirus Expression Is Required for Murine Melanoma Tumor Growth In vivo

Abstract

Tumor development is a multistep process in which both genetic and epigenetic events cooperate for the emergence of a malignant clone. The possibility that endogenous retroviruses promote the expansion of a neoplastic clone by subverting immune surveillance has been proposed, but remained elusive. Here we show that knocking down-by RNA interference-an endogenous retrovirus spontaneously induced in the B16 murine melanoma results in the rejection of the tumor cells in immunocompetent mice, under conditions where control melanoma cells grow into lethal tumors. The knockdown does not modify the transformed phenotype of the cells, as measured both in vitro by a soft agar assay and in vivo by tumor cell proliferation in immunoincompetent (Xirradiated and severe combined immunodeficiency) mice. Tumor rejection can be reverted upon adoptive transfer of regulatory T cells from control melanoma-engrafted mice, as well as upon reexpression of the sole envelope gene of the endogenous retrovirus in the knocked down cells. These results show that endogenous retroviruses can be essential for a regulatory T-cell-mediated subversion of immune surveillance and could be relevant to human tumors where such elements-and especially their envelope gene-are induced.

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

“…Although it is surprising that such a high activity is maintained in the highly metastatic B16F10 cells-one would expect that mutagenic capacity is no more required for a fully transformed tumor cell-it is most likely that this activity is maintained for another function of the MelARV proviruses, namely the immunosuppressive activity of their envelope protein gene, that we have previously unravelled and demonstrated to be required for subversion of the mouse immune system. 10 Altogether, the present analysis reveals some of the multiple roles that ERVs can play in tumorigenesis in vivo and the complex selection pressure that has been exerted on these sequences to generate (via a change from N-to B-tropism) proviral elements able to mediate insertional mutagenesis, resulting in turn in alterations of the level of expression of the targeted genes and high level expression of the proviruses themselves, inducing immunosuppression.…”

“…Following transfer, membranes were hybridized overnight at 65°C in 7% SDS, 1 mmol of EDTA per liter, and 0.5 mmol of Na 2 HPO4 (pH 7) per liter, using a 32 P-labeled env fragment (nt 6,007-6,515) specific for ecotropic retroviruses, as described in Refs. 14, 15 and obtained by PCR amplification using pDFGMelARVenv 10 as a template and primers 6007-CTCTGTATGTTGGCCCTCC-6025 and 6515-GAGACGTATAGGCGCAATC-6497. The membranes were then washed twice with 23 SSC plus 0.1% SDS at 65°C for 15 min.…”

“…7,8 Indeed, in the B16 spontaneous melanoma of C57BL/6 mice, an ecotropic ERV-the melanoma-associated retrovirus (MelARV)-is induced, and has been demonstrated to be involved in tumorigenesis. 9, 10 Mangeney et al 10 have recently shown that MelARV expression is absolutely required for tumor growth in vivo, via an unanticipated function associated with immunosurveillance: knocking down, by RNA interference, this ERV resulted in the rejection of the tumor cells in immunocompetent mice, without any alteration of their transformed phenotype. MelARV induction has also been shown to result in the production of viral particles in the supernatant of B16 cell cultures, and such particles were found to be infectious.…”

Distinct roles for LINE-1 and HERV-K retroelements in cell proliferation, differentiation and tumor progression

“…In our view, the reduced tumorigenicity of HERV-K stably interfered cells (pS-H-Ki) might well reflect the immunosuppressive effect exerted by env expression in growing tumors. It has been clearly shown that expression of env, either encoded from infectious retroviruses (Mangeney and Heidmann, 1998;Blaise et al, 2001) or from endogenous retroviral elements (Mangeney et al, , 2005, can subvert the immune surveillance system in mice, thereby eluding the rejection of tumors in immunocompetent animal models. The reduced tumorigenic ability of cells in which HERV-K expression is stably inactivated could be easily explained by envisaging that HERV-K-encoded env protein plays a similar immunosuppressive role, including in nude mice, which do in fact maintain a low level of immunosuppressive activity (Hasui et al, 1989;Farag and Caligiuri, 2006).…”

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

“…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. 9 In this article, we analyze another class of spontaneous mouse tumors, i.e. neuroblastoma, using the Neuro-2a cell line as a model, 10 to determine whether endogenous retroviruses are similarly involved in the tumor process.…”