Intestinal Immune System and Amplification of Mouse Mammary Tumor Virus

Mouse mammary tumor virus (MMTV) is a virus that induces breast cancer in mice. During lactation, MMTV can transmit from mother to offspring through milk, and Peyer’s patches (PPs) in mouse intestine are the first and specific target organ. MMTV can be transported into PPs by microfold cells and then activate antigen-presenting cells (APCs) by directly binding with Toll-like receptors (TLRs) whereas infect them through mouse transferrin receptor 1 (mTfR1). After being endocytosed, MMTV is reversely transcribed and the cDNA inserts into the host genome. Superantigen (SAg) expressed by provirus is presented by APCs to cognate CD4+ T cells via MHCII molecules to induce SAg response, which leads to substantial proliferation and recruitment of related immune cells. Both APCs and T cells can be infected by MMTV and these extensively proliferated lymphocytes and recruited dendritic cells act as hotbeds for viral replication and amplification. In this case, intestinal lymphatic tissues can actually become the source of infection for the transmission of MMTV in vivo, which results in mammary gland infection by MMTV and eventually lead to the occurrence of breast cancer.

A Retrotranslocation Assay That Predicts Defective VCP/p97-Mediated Trafficking of a Retroviral Signal Peptide

Endoplasmic reticulum-associated degradation (ERAD) is a form of cellular protein quality control that is manipulated by viruses, including the betaretrovirus, mouse mammary tumor virus (MMTV). MMTV-encoded signal peptide (SP) has been shown to interact with an essential ERAD factor, VCP/p97 ATPase, to mediate its extraction from the ER membrane, also known as retrotranslocation, for RNA binding and nuclear function.

The mouse mammary tumor virus intasome exhibits distinct dynamics on target DNA

Retroviral intasomes are complexes assembled from purified integrase (IN) and oligonucleotides mimicking viral DNA ends (vDNA). Recombinant intasomes faithfully recapitulate integration of vDNA into a target DNA. Structural studies of retroviral intasomes have revealed an array of IN oligomer forms, which appear to share a conserved intasome core coordinating the vDNA ends for strand transfer into the target DNA. Here we have explored the biochemical and dynamic properties of the mouse mammary tumor virus (MMTV) octameric intasome. We show that the MMTV intasome is remarkably stable compared to the prototype foamy virus (PFV) tetrameric intasome. MMTV integration activity peaks within the range of physiological ionic strength and is more active in the presence of manganese compared to magnesium. Single-molecule images demonstrate that the target DNA search by MMTV intasomes appears rate-limiting, similar to PFV intasomes. The time between strand transfer of the two MMTV vDNA ends into the target DNA is ~3 fold slower than PFV intasomes. MMTV intasomes can form extremely stable, largely immobile filaments on a target DNA that are comprised of multiple intasomes. This unusual property suggests that MMTV intasomes may readily form higher order oligomers that might underpin their increased stability.

Wingless-type mouse mammary tumor virus integration site (WNT) regulation of bovine theca cells

Ovarian paracrine mediation by components of the wingless-type mouse mammary tumor virus integration site ligands (WNT1 to 11) and their receptors, Frizzled family members (FZD1 to 10), has been proposed. Secreted truncated forms of FZD proteins (e.g., SFRP4) block the action of WNT ligands. Dickkopf-1 (DKK1) is another WNT antagonist, and R-spondin-1 (RSPO1) is one of a group of four secreted proteins that enhance WNT/β-catenin signaling. Our hypothesis was that granulosa cells signal theca cells (TC) via SFRP4, DKK1, RSPO1 and WNT secretion to regulate TC differentiation and proliferation. Therefore, in vitro experiments were conducted to study the effects of WNT family member 3A (WNT3A), WNT5A, RSPO1, DKK1, insulin-like growth factor 1 (IGF1), bone morphogenetic protein 7 (BMP7), Indian hedgehog (IHH), and fibroblast growth factor-9 (FGF9) on bovine TC proliferation and steroidogenesis. Theca cells of large (8 to 20 mm) and small (3 to 6 mm) follicles were collected from bovine ovaries, TC monolayers were established in vitro, and treated with various doses of recombinant human WNT3A, WNT5A, RSPO1, DKK1, IGF1, FGF9, BMP7, IHH and/or ovine LH in serum-free medium for 48 h. In experiment 1 using LH-treated TC, IGF1, IHH and WNT3A increased (P < 0.05) cell numbers and androstenedione production, whereas WNT3A and BMP7 inhibited (P < 0.05) progesterone production. In experiment 2, FGF9 blocked (P < 0.05) the WNT3A-induced increase in androstenedione production in LH plus IGF1-treated TC. In experiment 3, RSPO1 further increased (P < 0.05) LH plus IGF1-induced progesterone and androstenedione production. In experiment 4, SFRP4 and DKK1 alone had no significant effect on TC proliferation or progesterone production of large-follicle TC, but both blocked the inhibitory effect of WNT5A on androstenedione production. In contrast, DKK1 alone inhibited (P < 0.05) small-follicle TC androstenedione production whereas SFRP4 was without effect. We conclude that the ovarian TC WNT system is functional in cattle, with WNT3A increasing proliferation and androstenedione production of TC.

Mouse Mammary Tumor Virus (MMTV)-Like env Sequence in Brazilian Breast Cancer Samples: Implications in Clinicopathological Parameters in Molecular Subtypes

Background: Breast cancer (BC) is a complex disease in which susceptibility and clinical course depend on multiple factors. Evidence suggests that a mouse mammary tumor virus (MMTV)-homolog may be present in human BCs; however, little is known about its clinical implications. Methods: MMTV-like env nucleotide-sequence was searched in tumor and tumor-adjacent tissues from 217 Brazilian BC patients through nested-PCR and confirmed through PCR-sequencing. Blood samples were also tested for patients with MMTV-like env gene-positive tumors. Correlations with clinicopathological parameters were evaluated. Results: MMTV-like env sequence was detected in tumor and tumor-adjacent tissue samples from 41/217 and 30/196 patients, respectively. In blood, MMTV-like was detected in 17/32 patients. In Luminal-B tumors, MMTV-like in tumor tissue was negatively correlated with tumor size and disease stage, whereas in HER2 tumors it anti-correlated with lymph node metastasis (LNM) and disease stage. Considering blood, MMTV-like env gene positivity negatively correlated with age in general BC, while in Luminal-A tumors it positively correlated with Ki67 but negatively correlated with age and LNM. The associations with decreased LNM frequency were independent of other prognostic factors. Conclusion: MMTV-like env positivity is associated with better prognostic parameters in BC subtypes, which might be explainable by its anti-metastatic potential and by putative activation of immune milieu.

Unconventional ER to Endosomal Trafficking by a Retroviral Protein

ABSTRACTMouse mammary tumor virus (MMTV) encodes a Rem precursor protein that specifies both regulatory and accessory functions. Rem is cleaved at the ER membrane into a functional N-terminal signal peptide (SP) and the C-terminus (Rem-CT). Rem-CT lacks a membrane-spanning domain and a known ER retention signal, yet was not detectably secreted into cell supernatants. Inhibition of intracellular trafficking by the drug Brefeldin A (BFA), which interferes with the ER to Golgi secretory pathway, resulted in dramatically reduced intracellular Rem-CT levels. A Rem mutant lacking glycosylation sites was cleaved into SP and Rem-CT, but was insensitive to BFA, suggesting that unglycosylated Rem-CT does not exit the ER or reach a degradative compartment. BFA reduction of Rem-CT levels was not rescued by proteasome or lysosomal inhibitors. Rem-CT has simple glycans, which are necessary for Rem-CT stability and trafficking, but indicate that Rem-CT does not traffic through the Golgi. Analysis of wild-type Rem-CT and its glycosylation mutant by confocal microscopy revealed that both were primarily localized to the ER lumen. A small fraction of wild-type Rem-CT, but not the unglycosylated mutant, were co-localized with Rab5+ endosomes. Expression of a dominant-negative (DN) form of ADP ribosylation factor 1 (Arf1) (T31N) mimicked the effects of BFA by reducing Rem-CT levels, suggesting that Arf1 prevents Rem-CT localization to a degradative compartment. A DN form of the AAA ATPase, p97/VCP, rescued Rem-CT in the presence of BFA or DN Arf1. Thus, Rem-CT uses an unconventional trafficking scheme, perhaps to thwart innate immunity to MMTV infection.IMPORTANCEMouse mammary tumor virus is a complex retrovirus that encodes a regulatory/accessory protein, Rem. Rem is a precursor protein that is processed at the endoplasmic reticulum (ER) membrane by signal peptidase. The N-terminal SP eludes ER-associated degradation to traffic to the nucleus and serve a human immunodeficiency virus Rev-like function. In contrast, the function of the C-terminal glycosylated cleavage product (Rem-CT) is unknown. Since localization is critical for protein function, we used multiple methods to localize Rem-CT. Surprisingly, Rem-CT, which lacks a transmembrane domain or an ER retention signal, was detected primarily within the ER and required glycosylation for trafficking to endosomes. Blocking of retrograde trafficking through Arf1 reduced Rem-CT levels, but was not restored by lysosomal or proteasomal inhibitors. The unique trafficking of Rem-CT suggests a novel intracellular trafficking pathway, potentially impacting host anti-viral immunity.