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1.  MicroRNA Let-7 Downregulates STAT3 Phosphorylation in Pancreatic Cancer Cells by Increasing SOCS3 Expression 
Cancer letters  2014;347(1):54-64.
Although dispensable for normal pancreatic function, STAT3 signaling is frequently activated in pancreatic cancers. Consistent downregulation of expression of microRNA let-7 is also characteristic of pancreatic ductal adenocarcinoma (PDAC) biopsy specimens. We demonstrate in this study that re-expression of let-7 in poorly-differentiated PDAC cell lines reduced phosphorylation/activation of STAT3 and its downstream signaling events and reduced the growth and migration of PDAC cells. Let-7 re-expression did not repress expression of STAT3 protein or its activator cytokine interleukin 6 (IL-6). However, let-7 re-expression enhanced cytoplasmic expression of suppressor of cytokine signaling 3 (SOCS3), which blocks STAT3 activation by JAK2. Our study thus identified a mechanism by which STAT3 signaling can be inhibited in pancreatic cancer cells by modifying let-7 expression.
PMCID: PMC3972339  PMID: 24491408
Pancreatic cancer; STAT3; let-7; SOCS3; EMT
Virology  2005;345(2):10.1016/j.virol.2005.10.003.
The long terminal repeat (LTR) region of leukemia viruses plays a critical role in tissue tropism and pathogenic potential of the viruses. We have previously reported that U3-LTR from Moloney murine and feline leukemia viruses (Mo-MuLV and FeLV) upregulate specific cellular genes in trans in an integration-independent way. The U3-LTR region necessary for this action does not encode a protein but instead makes a specific RNA transcript. Because several cellular genes transactivated by the U3-LTR can also be activated by NFκB, and because the antiapoptotic and growth promoting activities of NFκB have been implicated in leukemogenesis, we investigated whether FeLV U3-LTR can activate NFκB signaling. Here we demonstrate that FeLV U3-LTR indeed upregulates NFκB signaling pathway via activation of Ras-Raf-IκB kinase (IKK) and degradation of IκB. LTR-mediated transcriptional activation of genes did not require new protein synthesis suggesting an active role of the LTR transcript in the process. Using Toll-like receptor (TLR) deficient HEK293 cells and PKR−/− mouse embryo fibroblasts we further demonstrate that although dsRNA activated protein kinase R (PKR) is not necessary, TLR3 is required for the activation of NFκB by the LTR. Our study thus demonstrates involvement of a TLR3 dependent but PKR independent dsRNA mediated signaling pathway for NFκB activation and thus provides a new mechanistic explanation of LTR-mediated cellular gene transactivation.
PMCID: PMC3808874  PMID: 16289658
Leukemia virus; LTR; transactivation; NFκB; TLR3
3.  Identification of LTR-specific small non-coding RNA in FeLV infected cells 
FEBS letters  2009;583(8):1386-1390.
The U3-LTR region of leukemia viruses transactivates cancer-related signaling pathways through the production of a non-coding RNA transcript although the role of this transcript in virus infection remains unknown. In this study we demonstrate for the first time that an LTR-specific small non-coding RNA is produced from a FeLV-infected feline cell line. RNA cloning identified this as a 104 base transcript that originates from the U3-LTR region. We also demonstrate that in in vitro assays this LTR RNA transcript activates NFκB signaling. Taken together, our findings suggest a possible role for this LTR transcript in FeLV pathogenesis.
PMCID: PMC3774839  PMID: 19336234
FeLV; LTR; non-coding RNA
4.  Advances in Virus-Directed Therapeutics against Epstein-Barr Virus-Associated Malignancies 
Advances in Virology  2012;2012:509296.
Epstein-Barr virus (EBV) is the causal agent in the etiology of Burkitt's lymphoma and nasopharyngeal carcinoma and is also associated with multiple human malignancies, including Hodgkin's and non-Hodgkin's lymphoma, and posttransplantation lymphoproliferative disease, as well as sporadic cancers of other tissues. A causal relationship of EBV to these latter malignancies remains controversial, although the episomic EBV genome in most of these cancers is clonal, suggesting infection very early in the development of the tumor and a possible role for EBV in the genesis of these diseases. Furthermore, the prognosis of these tumors is invariably poor when EBV is present, compared to their EBV-negative counterparts. The physical presence of EBV in these tumors represents a potential “tumor-specific” target for therapeutic approaches. While treatment options for other types of herpesvirus infections have evolved and improved over the last two decades, however, therapies directed at EBV have lagged. A major constraint to pharmacological intervention is the shift from lytic infection to a latent pattern of gene expression, which persists in those tumors associated with the virus. In this paper we provide a brief account of new virus-targeted therapeutic approaches against EBV-associated malignancies.
PMCID: PMC3303631  PMID: 22500168
5.  Short, Discontinuous Exposure to Butyrate Effectively Sensitizes Latently EBV-Infected Lymphoma Cells to Nucleoside Analogue Antiviral Agents 
Antiviral drugs alone have been unsuccessful in the treatment of Epstein Barr virus (EBV)-associated malignancies because the virus maintains a latent state of replication in these tumors. In recent years, novel therapeutic approaches are being investigated wherein lytic replication of the virus is induced prior to the use of cytotoxic antiviral drugs. The choice of suitable agents to induce lytic replication has been a critical step in this novel approach. We have previously demonstrated that butyrate derivatives induce a lytic pattern of EBV gene expression in patient-derived EBV-positive lymphoblastoid cell lines and, together with nucleoside analog ganciclovir, effectively reduce or eliminate tumor growth in humans. Butyrate has drawbacks as a therapeutic agent, however, as constant intravenous infusion is required to achieve detectable plasma levels of this drug. In this study, we investigated whether discontinuous exposure to butyrate is capable of initiating lytic-phase gene expression and thymidine kinase induction, and sensitizing EBV-positive lymphoma cells to ganciclovir-mediated cell growth arrest and apoptosis. We demonstrate that multiple daily 6 hr exposures of the EBV-positive Burkitt’s lymphoma cell line P3HR1 to butyrate induced sustained expression of the EBV lytic-phase protein BMRF. Viral thymidine kinase was also induced by intermittent exposure, although to a lower level than with continuous exposure treatment. However, discontinuous exposure to butyrate in combination with ganciclovir induced a similar level of tumor cell growth inhibition as did continuous treatment, as measured by serial enumeration of viable cells, MTT cell proliferation assays, and measurement of cellular DNA content. We further demonstrated that those cells which survived initial exposure to butyrate plus ganciclovir remained susceptible to further cycles of combination treatment. These findings suggests that continuous infusion of butyrate may not be necessary for maintaining viral thymidine kinase gene expression and sensitization to anti-viral agents in EBV-associated tumors, and that therapeutic regimens which employ more convenient, discontinuous exposure to butyrate may also be effective clinically.
PMCID: PMC1829174  PMID: 17161633
Epstein-Barr Virus; Butyrate; Thymidine Kinase; Ganciclovir; Growth inhibition; Apoptosis
6.  Long Terminal Repeat Regions from Exogenous but Not Endogenous Feline Leukemia Viruses Transactivate Cellular Gene Expression 
Journal of Virology  2000;74(20):9742-9748.
We have previously reported that the long terminal repeat (LTR) region of feline leukemia viruses (FeLVs) can enhance expression of certain cellular genes such as the collagenase IV gene and MCP-1 in trans (S. K. Ghosh and D. V. Faller, J. Virol. 73:4931–4940, 1999). Genomic DNA of all healthy feline species also contains LTR-like sequences that are related to exogenous FeLV LTRs. In this study, we evaluated the cellular gene transactivational potential of these endogenous FeLV LTR sequences. Unlike their exogenous FeLV counterparts, neither nearly full-length endogenous FeLV molecular clones (CFE-6 and CFE-16) nor their isolated LTRs were able to activate collagenase IV gene or MCP-1 expression in transient transfection assays. We had also demonstrated previously that production of an RNA transcript from exogenous FeLV LTRs correlates with their transactivational activity. In the present study, we demonstrate that the endogenous FeLV LTRs do not generate LTR-specific RNA transcripts in the feline embryo fibroblast cell line AH927. Furthermore, infection of AH927 cells by an exogenous FeLV subgroup A virus did not induce production of such LTR-specific transcripts from the endogenous proviral genomes, although the LTR-specific transcripts from the exogenous virus were readily detected. Finally, LTR-specific transcripts were not generated in BALB/3T3 cells transiently transfected with isolated CFE-6 LTR, in contrast to transfections with LTRs from exogenous viruses. Our data thus suggest that the inability of endogenous FeLV LTRs in gene transactivation is not due to cell line specificity or presence of any upstream inhibitory cis-acting element. Endogenous, nonleukemogenic FeLV LTRs, therefore, do not transactivate cellular gene expression, and this property appears to be specific to exogenous, leukemogenic FeLVs.
PMCID: PMC112408  PMID: 11000248
7.  Feline Leukemia Virus Long Terminal Repeat Activates Collagenase IV Gene Expression through AP-1 
Journal of Virology  1999;73(6):4931-4940.
Leukemia and lymphoma induced by feline leukemia viruses (FeLVs) are the commonest forms of illness in domestic cats. These viruses do not contain oncogenes, and the source of their pathogenic activity is not clearly understood. Mechanisms involving proto-oncogene activation subsequent to proviral integration and/or development of recombinant viruses with enhanced replication properties are thought to play an important role in their disease pathogenesis. In addition, the long terminal repeat (LTR) regions of these viruses have been shown to be important determinants for pathogenicity and tissue specificity, by virtue of their ability to interact with various transcription factors. Previously, we have shown that, in the case of Moloney murine leukemia virus, the U3 region of the LTR independently induces transcriptional activation of specific cellular genes through an LTR-generated RNA transcript (S. Y. Choi and D. V. Faller, J. Biol. Chem. 269:19691–19694, 1994; S.-Y. Choi and D. V. Faller, J. Virol. 69:7054–7060, 1995). In this report, we show that the U3 region of exogenous FeLV LTRs can induce transcription from collagenase IV (matrix metalloproteinase 9) and monocyte chemotactic protein 1 (MCP-1) promoters up to 12-fold. We also show that AP-1 DNA-binding activity and transcriptional activity are strongly induced in cells expressing FeLV LTRs and that LTR-specific RNA transcripts are generated in those cells. Activation of mitogen-activated protein kinase kinases 1 and 2 (MEK1 and -2) by the LTR is an intermediate step in the FeLV LTR-mediated induction of AP-1 activity. These findings thus suggest that the LTRs of FeLVs can independently activate transcription of specific cellular genes. This LTR-mediated cellular gene transactivation may play an important role in tumorigenesis or preleukemic states and may be a generalizable activity of leukemia-inducing retroviruses.
PMCID: PMC112537  PMID: 10233955

Results 1-7 (7)