Retroviral insertional mutagenesis in murine mammary cancer

Transposon integration enhances expression of stress response genes

Transposable elements possess specific patterns of integration. The biological impact of these integration profiles is not well understood. Tf1, a long-terminal repeat retrotransposon in Schizosaccharomyces pombe, integrates into promoters with a preference for the promoters of stress response genes. To determine the biological significance of Tf1 integration, we took advantage of saturated maps of insertion activity and studied how integration at hot spots affected the expression of the adjacent genes. Our study revealed that Tf1 integration did not reduce gene expression. Importantly, the insertions activated the expression of 6 of 32 genes tested. We found that Tf1 increased gene expression by inserting enhancer activity. Interestingly, the enhancer activity of Tf1 could be limited by Abp1, a host surveillance factor that sequesters transposon sequences into structures containing histone deacetylases. We found the Tf1 promoter was activated by heat treatment and, remarkably, only genes that themselves were induced by heat could be activated by Tf1 integration, suggesting a synergy of Tf1 enhancer sequence with the stress response elements of target promoters. We propose that the integration preference of Tf1 for the promoters of stress response genes and the ability of Tf1 to enhance the expression of these genes co-evolved to promote the survival of cells under stress.

The Wnt5A/protein kinase C pathway mediates motility in melanoma cells via the inhibition of metastasis suppressors and initiation of an epithelial to mesenchymal transition

We have shown that Wnt5A increases the motility of melanoma cells. To explore cellular pathways involving Wnt5A, we compared gain-of-function (WNT5A stable transfectants) versus loss-of-function (siRNA knockdown) of WNT5A by microarray analysis. Increasing WNT5A suppressed the expression of several genes, which were re-expressed after small interference RNA-mediated knockdown of WNT5A. Genes affected by WNT5A include KISS-1, a metastasis suppressor, and CD44, involved in tumor cell homing during metastasis. This could be validated at the protein level using both small interference RNA and recombinant Wnt5A (rWnt5A). Among the genes up-regulated by WNT5A was the gene vimentin, associated with an epithelial to mesenchymal transition (EMT), which involves decreases in E-cadherin, due to up-regulation of the transcriptional repressor, Snail. rWnt5A treatment increases Snail and vimentin expression, and decreases E-cadherin, even in the presence of dominant-negativeTCF4, suggesting that this activation is independent of Wnt/beta-catenin signaling. Because Wnt5A can signal via protein kinase C (PKC), the role of PKC in Wnt5A-mediated motility and EMT was also assessed using PKC inhibition and activation studies. Treating cells expressing low levels of Wnt5A with phorbol ester increased Snail expression inhibiting PKC in cells expressing high levels of Wnt5A decreased Snail. Furthermore, inhibition of PKC before Wnt5A treatment blocked Snail expression, implying that Wnt5A can potentiate melanoma metastasis via the induction of EMT in a PKC-dependent manner.

Using gene delivery to protect HIV-susceptible CNS cells: inhibiting HIV replication in microglia

Antiretroviral chemotherapy penetrates the CNS poorly. CNS HIV, thus sheltered, may injure the brain and complicate control of systemic HIV infection. Microglial cells play a major role in HIV persistence in the CNS but are rarely targeted for gene delivery. Because recombinant SV40 vectors (rSV40s) transduce other phagocytic cells efficiently, we tested rSV40 delivery of anti-HIV genetic therapy to microglial cells. Microglia prepared as enriched cultures from human fetal brain, were transduced with marker vectors, SV(RFP) and SV(Nef/FLAG), respectively, carrying DsRed and HIV-1 Nef bearing a FLAG epitope. By immunostaining and FACS, 95% of unselected cells expressed the transgenes, without detectable toxicity. Microglia were transduced with SV(AT), carrying human alpha1-antitrypsin (alpha1AT), which blocks Env and Gag processing. SV(AT)-treated microglia strongly resisted challenge with HIV-1BaL, even when microglia were transduced with SV(AT) following HIV challenge. Thus, rSV40s effectively transduce microglia and protect them from HIV.

Human, rhesus macaque, and feline sequences highly similar to mouse mammary tumor virus sequences

Sequences highly similar (>95%) to the mouse mammary tumor virus (MMTV) env gene have been amplified from human DNA samples, including DNA samples from patients with breast cancer (BC) and persons who did not have BC. The sequences from human DNA were distinct from the MMTV sequences used as controls in these PCR reactions, indicating that these results are not simply due to contamination. In addition to both, mouse and human-related sequences were also amplified from some monkey and cat genomic DNA samples. These products were shown to be distinct from, but highly related to, the MMTV env gene, whereas, testing of other sources (lambda phage, snake, cockroach, sea urchin, chicken, or dog) demonstrated no specific amplification. A sequence 90% similar to the MMTV group antigen gene (gag) was amplified from cat DNA. These results indicate that DNA from vertebrate species other than rodents, including some but not all humans, monkeys, and cats, can contain sequences closely related to MMTV.

Of mice, cats, and men: Is human breast cancer a Zoonosis?

Mouse mammary tumor virus (MMTV), a member of the betaretroviridae, is the most common cause of breast cancer (BC) in mice. MMTV is transmitted in mice both in the germline as endogenous proviruses and exogenously as infectious virions. Here, we review a variety of evidence accumulated for six decades that has suggested that a human homologue of MMTV may exist. The findings include recent studies from several independent laboratories that have detected sequences very closely related to MMTV in DNA isolated from human BC tumors. Other laboratories, however, have failed to detect the MMTV-related sequences in human DNA samples, and conclusive evidence for a human mammary tumor virus has been elusive. We also reviewed additional studies, suggesting that betaretroviruses are present in a much wider range of species than previously known, including rodents, felines, and primates. The observation that a subset of cats may be infected with a close homologue of MMTV may be of epidemiological significance for human BC. Cats may become infected by MMTV from mice, and in turn may transmit the virus to humans, possibly after selection for variants with an expanded host range.

Analysis of chicken Wnt-13 expression demonstrates coincidence with cell division in the developing eye and is consistent with a role in induction

We used a degenerate polymerase chain reaction (PCR) strategy to search for Wnt RNA in developing ocular tissues. We isolated a Macaca monkey Wnt-13 PCR fragment, orthologous to the human and murine Wnt-13 and Xenopus Wnt-2b, and a chick Wnt13 cDNA. Wnt-13 is a member of the Wnt-1 class of transforming Wnt molecules. In situ RNA hybridization revealed a dynamic Wnt-13 expression pattern in numerous developing tissues. Within the eye, Wnt-13 is expressed in the proliferative epithelium of the lens and both pigmented and non-pigmented layers of the ciliary margin. In vitro BrdU incorporation studies coupled with in situ hybridization showed that cWnt-13 expression domains in the lens were coincident with cell division. In addition to the eye, cWnt-13 was expressed in head ectoderm, prospective forelimb mesenchyme, lung bud, pharyngeal arches, the brain, as well as the otic vesicle. Our data are consistent with previous observations linking transforming Wnts with cell division and implicate a cascade of events involving cWnt-13 first in dorsoventral patterning and later in cell proliferation regulation associated with lens development. Dev Dyn 1999;215:215-224.

Insertional mutations in mammals and mammalian cells

The retroposon sequences, their mechanisms of transposition and the occurrence of insertional mutation in the mammalian genome are reviewed. Insertional mutations fall into two broad categories: those due to the disruption of a gene following the physical integration of a foreign DNA sequence result in loss of gene product and would be expected to be associated with a recessive mutation. A second class of insertional mutation is well documented in which upon integration the promoter/enhancer activities inherent in the retroposon genome exert their influence on neighboring genes. This promoter/enhancer activity of integrated retroposons may have effects over relatively long distances and thus limit the possibilities of establishing an association between retroposon integration and mutation. It is emphasized that a systematic search for insertional mutations in the mammalian genome involves an extensive two-dimensional array of possible retroposon sequences and mutant alleles. Present results represent only a small portion of the total array. Future studies promise to be fruitful in efforts to isolate genes through insertional tagging, to characterize the mechanisms of retroposon transposition, as well as to study the stability of the mammalian genome.

Prolactin effects on the dietary regulation of mouse mammary tumor virus proviral DNA expression

Chronic energy-intake restriction inhibits mouse mammary tumor virus (MMTV)-induced mammary tumors in C3H/Ou mice by greater than 90%. We have shown that associated with suppression of mammary tumorigenesis there is a reduction or inhibition of circulating prolactin, MMTV particles expressed, and MMTV mRNA transcription in mammary glands (and in most organs tested). To understand the concerted action of prolactin, energy-consumption level, and MMTV on inducing mammary tumors, experiments were designed to control prolactin and energy levels in order to evaluate their effects on MMTV mRNA expression. Mice on restricted diets were grafted with adenohypophyses, and mice fed ad libitum were treated with the dopaminomimetic agent octahydrobenzo [g]quinoline. Adenohypophyseal grafting significantly increased prolactin in dietary (energy)-restricted mice, and this effect was associated with an increase in MMTV mRNA expression within the mammary gland; a linear correlation between prolactin levels and MMTV mRNA expression in the mammary gland was found. Conversely, elimination of the nocturnal peak of circulating prolactin by i.p. injection of dopaminomimetic octahydrobenzo [g]quinoline to mice fed ad libitum delayed (by 8 weeks) and reduced (even as long as 25 weeks) mammary gland MMTV mRNA expression. These findings associate prolactin influences with MMTV mRNA production in mice and help explain the link between chronic energy-intake restriction and reduced MMTV gene expression.