Fibrils of prostatic acid phosphatase fragments boost infections with XMRV (xenotropic murine leukemia virus-related virus), a human retrovirus associated with prostate cancer

Evaluation of cellular determinants required for in vitro xenotropic murine leukemia virus-related virus entry into human prostate cancer and noncancerous cells

The newly identified retrovirus-the xenotropic murine leukemia virus-related virus (XMRV)-has recently been shown to be strongly associated with familial prostate cancer in humans (A. Urisman et al., PLoS Pathog. 2:e25, 2006). While that study showed evidence of XMRV infection exclusively in the prostatic stromal fibroblasts, a recent study found XMRV protein antigens mainly in malignant prostate epithelial cells (R. Schlaberg et al., Proc. Natl. Acad. Sci. U. S. A. 106:16351-16356, 2009). To help elucidate the mechanisms behind XMRV infection, we show that prostatic fibroblast cells express Xpr1, a known receptor of XMRV, but its expression is absent in other cell lines of the prostate (i.e., epithelial and stromal smooth muscle cells). We also show that certain amino acid residues located within the predicted extracellular loop (ECL3 and ECL4) sequences of Xpr1 are required for efficient XMRV entry. Although we found strong evidence to support XMRV infection of prostatic fibroblast cell lines via Xpr1, we learned that XMRV was indeed capable of infecting cells that did not necessarily express Xpr1, such as those of the prostatic epithelial and smooth muscle origins. Further studies suggest that the expression of Xpr1 and certain genotypes of the RNASEL gene, which could restrict XMRV infection, may play important roles in defining XMRV tropisms in certain cell types. Collectively, our data reveal important cellular determinants required for XMRV entry into different human prostate cells in vitro, which may provide important insights into the possible role of XMRV as an etiologic agent in human prostate cancer.

Inhibition of xenotropic murine leukemia virus-related virus by APOBEC3 proteins and antiviral drugs

Xenotropic murine leukemia virus-related virus (XMRV), a gammaretrovirus, has been isolated from human prostate cancer tissue and from activated CD4(+) T cells and B cells of patients with chronic fatigue syndrome, suggesting an association between XMRV infection and these two diseases. Since APOBEC3G (A3G) and APOBEC3F (A3F), which are potent inhibitors of murine leukemia virus and Vif-deficient human immunodeficiency virus type 1 (HIV-1), are expressed in human CD4(+) T cells and B cells, we sought to determine how XMRV evades suppression of replication by APOBEC3 proteins. We found that expression of A3G, A3F, or murine A3 in virus-producing cells resulted in their virion incorporation, inhibition of XMRV replication, and G-to-A hypermutation of the viral DNA with all three APOBEC3 proteins. Quantitation of A3G and A3F mRNAs indicated that, compared to the human T-cell lines CEM and H9, prostate cell lines LNCaP and DU145 exhibited 50% lower A3F mRNA levels, whereas A3G expression in 22Rv1, LNCaP, and DU145 cells was nearly undetectable. XMRV proviral genomes in LNCaP and DU145 cells were hypermutated at low frequency with mutation patterns consistent with A3F activity. XMRV proviral genomes were extensively hypermutated upon replication in A3G/A3F-positive T cells (CEM and H9), but not in A3G/A3F-negative cells (CEM-SS). We also observed that XMRV replication was susceptible to the nucleoside reverse transcriptase (RT) inhibitors zidovudine (AZT) and tenofovir and the integrase inhibitor raltegravir. In summary, the establishment of XMRV infection in patients may be dependent on infection of A3G/A3F-deficient cells, and cells expressing low levels of A3G/A3F, such as prostate cancer cells, may be ideal producers of infectious XMRV. Furthermore, the anti-HIV-1 drugs AZT, tenofovir, and raltegravir may be useful for treatment of XMRV infection.

Host range and cellular tropism of the human exogenous gammaretrovirus XMRV

Recently, the first human infection with an exogenous gammaretrovirus (XMRV) was reported. In its initial description, XMRV was confined to prostate stromal fibroblasts, although subsequent reports demonstrated XMRV protein expression in prostate epithelial cells. Most recently, XMRV has been detected in blood cells of patients with chronic fatigue syndrome. The aim of this study was to elucidate the transmission routes and tissue tropism of XMRV by comparing its host range, receptor usage and LTR functionality with other MLV isolates. We demonstrate using pseudotype experiments that XMRV Env mediates efficient infection of cells from different species. We show that replication competent XMRV infects various human cell types, including hematopoietic cell lines and prostate stromal fibroblasts. XMRV-LTR activity is significantly higher in the prostate cancer cell line LNCaP and in prostate stromal fibroblasts, compared to other cell types tested and could be one factor contributing to efficient viral spread in prostate tissue.

Xenotropic murine leukemia virus-related virus establishes an efficient spreading infection and exhibits enhanced transcriptional activity in prostate carcinoma cells

Xenotropic murine leukemia virus-related virus (XMRV) is a novel human gammaretrovirus discovered in association with human prostate tumors. XMRV was first identified in prostate stromal cells surrounding the tumors of patients carrying a mutation in the HPC1 gene locus. To determine the tropism of XMRV in cell culture, we tested the ability of XMRV to spread and replicate in various prostate and nonprostate cell lines. We found that although the expression of XMRV viral proteins and the spread of infectious virus were minimal in a variety of cell lines, XMRV displayed robust expression and infection in LNCaP prostate tumor cells. The transcriptional activity of the XMRV long terminal repeat (LTR) was found to be higher than the Moloney murine leukemia virus LTRs in both LNCaP and WPMY-1 (simian virus 40-transformed prostate stromal cells). The U3 promoter of XMRV and a glucocorticoid response element (GRE) within the U3 were required for the transcriptional activity in LNCaP cells. Coexpression of the androgen receptor and stimulation with dihydrotestosterone stimulated XMRV-LTR-dependent transcription in 293T cells, and the GRE was required for this activity. These data suggest that XMRV may replicate more efficiently in LNCaP cells in part due to the transcriptional environment in LNCaP cells.

NMR structure in a membrane environment reveals putative amyloidogenic regions of the SEVI precursor peptide PAP(248-286)

Semen is the main vector for HIV transmission worldwide. Recently, a peptide fragment (PAP(248-286)) has been isolated from seminal fluid that dramatically enhances HIV infectivity by up to 4-5 orders of magnitude. PAP(248-286) appears to enhance HIV infection by forming amyloid fibers known as SEVI, which are believed to enhance the attachment of the virus by bridging interactions between virion and host-cell membranes. We have solved the atomic-level resolution structure of the SEVI precursor PAP(248-286) using NMR spectroscopy in SDS micelles, which serve as a model membrane system. PAP(248-286), which does not disrupt membranes like most amyloid proteins, binds superficially to the surface of the micelle, in contrast to other membrane-disruptive amyloid peptides that generally penetrate into the core of the membrane. The structure of PAP(248-286) is unlike most amyloid peptides in that PAP(248-286) is mostly disordered when bound to the surface of the micelle, as opposed to the alpha-helical structures typically found of most amyloid proteins. The highly disordered nature of the SEVI peptide may explain the unique ability of SEVI amyloid fibers to enhance HIV infection as partially disordered amyloid fibers will have a greater capture radius for the virus than compact amyloid fibers. Two regions of nascent structure (an alpha-helix from V262-H270 and a dynamic alpha/3(10) helix from S279-L283) match the prediction of highly amyloidogenic sequences and may serve as nuclei for aggregation and amyloid fibril formation. The structure presented here can be used for the rational design of mutagenesis studies on SEVI amyloid formation and viral infection enhancement.

Xenotropic murine leukemia virus-related virus is susceptible to AZT

The xenotropic murine leukemia virus-related virus (XMRV) is a human retrovirus, recently isolated from tissues of prostate cancer patients with impaired RNase L activity. In this study, we evaluated 10 licensed anti-HIV-1 compounds for their activity against XMRV, including protease inhibitors (PI), nucleoside reverse transcriptase (RT) inhibitors (NRTI), non-nucleoside RT inhibitors (NNRTI) and an integrase inhibitor. No PI affected XMRV production; even high concentrations of Ritonavir failed to inhibit the maturation of XMRV Gag polyproteins. Among the NRTI, NNRTI and integrase inhibitors used in this study, only AZT blocked XMRV infection and replication through inhibition of viral reverse transcription. This sensitivity of XMRV to AZT may be explained by the modest homology in the motif D sequences of HIV-1 and XMRV reverse transcriptases. If XMRV becomes established as an etiological agent for prostate cancer or other diseases, AZT may be useful for preventing or treating XMRV infections in humans.

Aminoquinoline surfen inhibits the action of SEVI (semen-derived enhancer of viral infection)

In semen, proteolytic peptide fragments from prostatic acid phosphatase can form amyloid fibrils termed SEVI (semen-derived enhancer of viral infection). These fibrils greatly enhance human immunodeficiency virus (HIV) infectivity by increasing the attachment of virions to target cells. Therefore, SEVI may have a significant impact on whether HIV is successfully transmitted during sexual contact. Here, we demonstrate that surfen, a small molecule heparan sulfate proteoglycan antagonist, inhibits both SEVI- and semen-mediated enhancement of HIV type 1 infection. Surfen interferes with the binding of SEVI to both target cells and HIV type 1 virions but does not deaggregate SEVI fibrils. Because SEVI can increase HIV infectivity by several orders of magnitude, supplementing current HIV microbicide candidates with SEVI inhibitors, such as surfen, might greatly increase their potency.

Virology. A new virus for old diseases?

A retrovirus associated with cancer is linked to chronic fatigue syndrome.

Lack of evidence for xenotropic murine leukemia virus-related virus(XMRV) in German prostate cancer patients

Background

A novel gammaretrovirus named xenotropic murine leukemia virus-related virus (XMRV) has been recently identified and found to have a prevalence of 40% in prostate tumor samples from American patients carrying a homozygous R462Q mutation in the RNaseL gene. This mutation impairs the function of the innate antiviral type I interferon pathway and is a known susceptibility factor for prostate cancer. Here, we attempt to measure the prevalence of XMRV in prostate cancer cases in Germany and determine whether an analogous association with the R462Q polymorphism exists.

Results

589 prostate tumor samples were genotyped by real-time PCR with regard to the RNaseL mutation. DNA and RNA samples from these patients were screened for the presence of XMRV-specific gag sequences using a highly sensitive nested PCR and RT-PCR approach. Furthermore, 146 sera samples from prostate tumor patients were tested for XMRV Gag and Env antibodies using a newly developed ELISA assay. In agreement with earlier data, 12.9% (76 samples) were shown to be of the QQ genotype. However, XMRV specific sequences were detected at neither the DNA nor the RNA level. Consistent with this result, none of the sera analyzed from prostate cancer patients contained XMRV-specific antibodies.

Conclusion

Our results indicate a much lower prevalence (or even complete absence) of XMRV in prostate tumor patients in Germany. One possible reason for this could be a geographically restricted incidence of XMRV infections.