MUC13 protects colorectal cancer cells from death by activating the NF-κB pathway and is a potential therapeutic target

MUC13 is a transmembrane mucin glycoprotein that is over produced by many cancers, although its functions are not fully understood. Nuclear factor-κB (NF-κB) is a key transcription factor promoting cancer cell survival, but therapeutically targeting this pathway has proved difficult because NF-κB has pleiotropic functions. Here, we report that MUC13 prevents colorectal cancer cell death by promoting two distinct pathways of NF-kB activation, consequently upregulating BCL-X_L. MUC13 promoted tumor necrosis factor (TNF)-induced NF-κB activation by interacting with TNFR1 and the E3 ligase, cIAP1, to increase ubiquitination of RIPK1. MUC13 also promoted genotoxin-induced NF-κB activation by increasing phosphorylation of ATM and SUMOylation of NF-κB essential modulator. Moreover, elevated expression of cytoplasmic MUC13 and NF-κB correlated with colorectal cancer progression and metastases. Our demonstration that MUC13 enhances NF-κB signaling in response to both TNF and DNA-damaging agents provides a new molecular target for specific inhibition of NF-κB activation. As proof of principle, silencing MUC13 sensitized colorectal cancer cells to killing by cytotoxic drugs and inflammatory signals and abolished chemotherapy-induced enrichment of CD133⁺ CD44⁺ cancer stem cells, slowed xenograft growth in mice, and synergized with 5-fluourouracil to induce tumor regression. Therefore, these data indicate that combining chemotherapy and MUC13 antagonism could improve the treatment of metastatic cancers.

Human papillomavirus type 6b virus-like particles are able to activate the Ras-MAP kinase pathway and induce cell proliferation

The initial step in viral infection is the attachment of the virus to the host cell via an interaction with its receptor. We have previously shown that a receptor for human papillomavirus is the alpha6 integrin. The alpha6 integrin is involved in the attachment of epithelial cells with the basement membrane, but recent evidence suggests that ligation of many integrins results in intracellular signaling events that influence cell proliferation. Here we present evidence that exposure of A431 human epithelial cells to human papillomavirus type 6b L1 virus-like particles (VLPs) results in a dose-dependent increase in cell proliferation, as measured by bromodeoxyuridine incorporation. This proliferation is lost if VLPs are first denatured or incubated with a monoclonal antibody against L1 protein. The MEK1 inhibitor PB98059 inhibits the VLP-mediated increase in cell proliferation, suggesting involvement of the Ras-MAP kinase pathway. Indeed, VLP binding results in rapid phosphorylation of the beta4 integrin upon tyrosine residues and subsequent recruitment of the adapter protein Shc to beta4. Within 30 min, the activation of Ras, Raf, and Erk2 was observed. Finally, the upregulation of c-myc mRNA was observed at 60 min. These data indicate that human papillomavirus type 6b is able to signal cells via the Ras-MAP kinase pathway to induce cell proliferation. We hypothesize that such a mechanism would allow papillomaviruses to infect hosts more successfully by increasing the potential pool of cells they are able to infect via the initiation of proliferation in resting keratinocyte stem and suprabasal cells.

The human papillomavirus E7 protein is able to inhibit the antiviral and anti-growth functions of interferon-alpha

It is now well recognized that cervical cancer is caused by infection with certain human papillomavirus (HPV) subtypes and while interferon-alpha (IFN-alpha) is used to treat HPV-infected lesions, HPV appears to have developed a means to avoid the effects of IFN-alpha. Clinically, resistance appears to be associated with the expression of the E7 oncoprotein. Here we investigated the effects of expression in cells of the E7 protein from high- and low-risk papillomavirus subtypes on a range of responses to IFN-alpha. 2fTGH, a cell line dependent on IFN-alpha for growth in selection medium, grew significantly less well in the presence of E7, and the antiproliferative effects of IFN-alpha upon epithelial cells was lost upon E7 expression. The antiviral effects of IFN-alpha were abrogated in E7-expressing cells. Loss of response to IFN-alpha was found to occur in both high- and low-risk papillomaviruses. Finally, deletion of amino acids 21-24 of HPV type 16 E7 protein partially reversed repression. We conclude that E7 inhibits the functional effects of IFN-alpha and that this property is shared by all HPV subtypes tested.

BPV1 E2 protein enhances packaging of full-length plasmid DNA in BPV1 pseudovirions

We studied determinants of efficient encapsidation of circular DNA, incorporating a PV early region DNA sequence (nt 584-1978) previously shown to enhance packaging of DNA within papillomavirus (PV)-like particles (VLPs). Insect coelomic cells (Sf-9) and cultured monkey kidney cells (Cos-1) were transfected with an 8-kb reporter plasmid incorporating the putative BPV packaging sequence and infected with BPV1 L1 and L2 recombinant baculovirus or vaccinia virus. Heavy (1.34 g/ml) and light (1.30 g/ml) VLPs were produced, and each packaged some of the input plasmid. In light VLPs, truncated plasmids, which nevertheless incorporated the PV-derived DNA packaging sequence, were more common than full-length plasmids. Packaging efficiency of the plasmid was estimated at 1 plasmid per 10(4) VLPs in both Cos-1 and Sf-9 cells. In each cell type, expression of the BPV1 early region protein E2 in trans doubled the quantity of heavy but not light VLPs and also increased the packaging efficiency of full-length circular plasmids by threefold in heavy VLPs. The resultant pseudovirions incorporated significant amounts of E2 protein. Pseudovirions, comprising plasmids packaged within heavy VLPs, mediated the delivery of packaged plasmid into Cos-1 cells, whereby "infectivity" was blocked by antisera to BPV1 L1, but not antisera to BPV1 E4. We conclude that (a) packaging of DNA within PV L1+L2 pseudovirions is enhanced by BPV1 E2 acting in trans, (b) E2 may be packaged with the pseudovirion, and (c) E2-mediated enhancement of packaging favors 8-kb plasmid incorporation over incorporation of shorter DNA sequences.

Expression of the alpha6 integrin confers papillomavirus binding upon receptor-negative B-cells

Papillomaviruses (PV) bind to a wide range of cell lines in a specific and saturable manner. We have recently identified a candidate receptor for papillomavirus as the alpha6 integrin (Evander et al., J. Virol. 71, 2449-2456, 1997). We have further investigated the role the alpha6 integrin plays in PV binding. Here we show that the cells expressing the alpha6 integrin, partnered with either the beta4 integrin or the beta1 integrin, are equally able to bind PV HPV6b L1 virus-like particles, indicating that the beta partner does not play a major role in virus binding. In order to provide definitive evidence that the alpha6 integrin is required for PV binding we undertook to genetically complement the receptor-negative B-cell line DG75 by expressing the human alpha6A gene. The transduction of the alpha6 integrin gene into DG75 cells results in the cell surface expression of the alpha6 protein and this expression confers upon DG75 cells the ability to bind laminin, a normal ligand for alpha6 integrin. Furthermore, the alpha6 protein is partnered with the beta1 integrin in DG75 cells. Finally, we show that the DG75-alpha6 cells were able to bind papillomavirus VLPs and this binding was inhibited by a functionally blocking anti-alpha6 antibody. Together these data indicate that the alpha6 integrin is a primary cell receptor for papillomaviruses and is both necessary and sufficient for PV binding.

The human papillomavirus E7 oncoprotein abrogates signaling mediated by interferon-alpha

Greater than 95% of all cervical carcinomas have been found to be associated with "high-risk" human papillomavirus (mainly types 16 and 18) infections, with the viral E6 and E7 oncoproteins essential for neoplastic development and maintenance. Interferon-alpha (IFNalpha) is used in the treatment of HPV infections yet both in vivo and in vitro data suggest that the virus has developed mechanisms to avoid the effects of interferon. Here we show that the HPV16 E7 oncoprotein is able to inhibit the induction of IFNalpha-inducible genes but has no effect of IFNgamma-inducible genes. Expression of E7 correlates with the loss of formation of the interferon-stimulated gene factor 3 (ISGF3) transcription complex. Moreover, in the presence of E7, p48, the DNA-binding component of ISGF3, was unable to translocate to the nucleus upon IFNalpha stimulation. A direct protein-protein interaction was identified between E7 and p48 with the site of interaction within E7 defined as the region between amino acids 17-37, a domain that includes the binding site for the retinoblastoma protein, pRb. These results suggest that HPV, via E7, targets p48, resulting in the loss of IFNalpha-mediated signal transduction and may provide a means by which HPV can avoid the innate immune system.

Identification of the alpha6 integrin as a candidate receptor for papillomaviruses

Papillomaviruses (PVs) bind in a specific and saturable fashion to a range of epithelial and other cell lines. Treatment of cells with trypsin markedly reduces their ability to bind virus particles, suggesting that binding is mediated via a cell membrane protein. We have investigated the interaction of human PV type 6b L1 virus-like particles (VLPs) with two epithelial cell lines, CV-1 and HaCaT, which bind VLPs, and a B-cell line (DG75) previously shown not to bind VLPs. Immunoprecipitation of a mixture of PV VLPs with [35S]methionine-labeled cell extracts and with biotin-labeled cell surface proteins identified four proteins from CV-1 and HaCaT cells of 220, 120, 87, and 35 kDa that reacted with VLPs and were not present in DG75 cells. The alpha6beta4 integrin complex has subunits corresponding to the VLP precipitated proteins, and the tissue distribution of this complex suggested that it was a candidate human PV receptor. Monoclonal antibodies (MAbs) to the alpha6 or beta4 integrin subunits precipitated VLPs from a mixture of CV-1 cell proteins and VLPs, whereas MAbs to other integrin subunits did not. An alpha6 integrin-specific MAb (GoH3) inhibited VLP binding to CV-1 and HaCaT cells, whereas an anti-beta4 integrin MAb and a range of integrin-specific and other MAbs did not. Furthermore, human laminin, the natural ligand for the alpha6beta4 integrin, was able to block VLP binding. By use of sections of monkey esophagus, the distribution of alpha6 integrin expression in the basal epithelium was shown to coincide with the distribution of bound VLPs. Taken together, these data suggest that VLPs bind specifically to the alpha6 integrin subunit and that integrin complexes containing alpha6 integrin complexed with either beta1 or beta4 integrins may act as a receptor for PV binding and entry into epithelial cells.

HIV-1 Tat directly interacts with the interferon-induced, double-stranded RNA-dependent kinase, PKR

We present evidence that the HIV-1 Tat protein and the RNA-dependent cellular protein kinase, PKR, interact with each other both in vitro and in vivo. Using GST fusion chromatography, we demonstrate that PKR, interacts directly with the HIV-1 Tat protein. The region in Tat sufficient for binding PKR maps within amino acids 20 to 72. In in vitro assays, the two-exon form of Tat (Tat 86) was phosphorylated by PKR, while the one exon form of Tat (Tat 72) inhibited PKR autophosphorylation and substrate phosphorylation. The ability of Tat to interact with PKR was demonstrated in both yeast and mammalian cells. Expression of PKR in yeast results in a growth suppressor phenotype which was reversed by coexpression of a one exon form of Tat. Expression of Tat 72 in HeLa cells resulted in direct interaction with PKR as detected by coimmunprecipitation with a Tat antibody. Tat and PKR also form a coimmunoprecipitable complex in cell-free extracts prepared from productively infected T lymphocytes. The interaction of Tat with PKR provides a potential mechanism by which HIV could suppress the interferon system.

Mutational analysis of the double-stranded RNA (dsRNA) binding domain of the dsRNA-activated protein kinase, PKR

The interferon-induced, double-stranded RNA (dsRNA)-dependent protein kinase, PKR, is an inhibitor of translation and has antiviral, antiproliferative, and antitumor properties. Previously, the dsRNA binding domain had been located within the N-terminal region of PKR and subsequently shown to include two nearly identical domains comprising residues 55-75 and 145-166. We have undertaken both random and site-directed, alanine-scanning mutagenesis in order to investigate the contribution of individual amino acids within these domains to dsRNA binding. Here we identify 2 residues that were absolutely required for dsRNA binding, glycine 57 and lysine 60. Mutation of 2 other residues within the domain (lysine 64 and leucine 75) resulted in less than 10% binding (compared to wild type). We have also identified a number of other residues that influence dsRNA binding to varying degrees. Mutants that were unable to bind dsRNA were not active in vitro and possessed no antiproliferative activity in vivo. However, dsRNA binding mutants were partially transdominant over wild type PKR in mammalian cells, suggesting that binding of dsRNA activator is not the mechanism responsible for the phenotype of PKR mutants.

HIV-1 TAR RNA has an intrinsic ability to activate interferon-inducible enzymes

The TAR sequence at the 5'-termini of all HIV-1 mRNA species forms a stable structure that is responsible for both transcriptional and translational regulation of HIV-1. Previously we and others reported that purified TAR RNA synthesized by in vitro transcription could activate two interferon-induced enzymes, the protein kinase (PKR) and 2-5A-synthetase. Because the PKR- and 2-5A-systems block protein synthesis initiation and induce RNA decay, respectively, these findings suggested mechanisms for the control of HIV-1 replication by the interferon system. To determine if contaminating dsRNA from in vitro transcription reactions was responsible for this effect, as suggested by Gunnery et al. 1990, (Proc., Natl. Acad. Sci. USA 87, 8687), we have reexamined these findings using chemically synthesized TAR (nucleotides +1 to +57). TAR RNA is shown here to have an intrinsic ability to activate PKR and 2-5A-synthetase. In contrast, a mutant form of TAR designed to have a disrupted secondary structure did not stimulate either enzyme. Chemically synthesized TAR mimicked other dsRNA species in its ability to activate and inhibit PKR at low and high RNA concentrations, respectively. HIV-1 TAT protein inhibited activation of PKR by HIV-1 TAR RNA suggesting an escape mechanism for the virus.

RNA transcript mapping of the Wiseana iridescent virus genome

The replication of Wiseana iridescent virus (WIV) was studied in Lymantria dispar tissue culture cells. Using a combination of [35S]methionine pulse-labeling and Northern blotting with WIV DNA probes, a transcriptional map of the genome was constructed. WIV has a wide dispersal of immediate-early genes with seven different regions identified. WIV has been reported to have extensive repetitive DNA sequences but no early transcription was observed in these regions. Although fine-mapping is required, some early regions (Bam L and Eco O) have been identified which are transcriptionally active at 6- and 12-h but are shut down by 24 h. These regions could provide probes for early genes and the hypothesized switch from nuclear to cytoplasmic replication for iridoviruses.

Localization of the human interferon-induced, ds-RNA activated p68 kinase gene (PRKR) to chromosome 2p21-p22

The interferon-induced dsRNA-activated protein kinase (PRKR) belongs to a subclass of serine/threonine kinases, involved in the regulation of protein synthesis by phosphorylation of the alpha subunit of initiation factor eIF2. Somatic cell hybrids segregating human chromosomes were used to assign this kinase to human chromosome 2. Fluorescence in situ hybridization confirmed this assignment and further localized the gene (PRKR) to the boundary region of bands p21 and 22.

A comparison of the structural polypeptides of three iridescent viruses (types 6, 9, and 16) and the mapping of the DNA region coding for their major capsid polypeptides

The iridoviruses from Wiseana cervinata (WIV, type 9), Costelytra zealandica (CzIV, type 16) and Chilo suppressalis (CIV, type 6) were compared by SDS-PAGE and Western protein blotting for antigenic determinants. The major capsid proteins were isolated and oligonucleotide probes were synthesized from the partial amino acid sequences. The DNA regions coding for the major capsid proteins of WIV (VP52), CzIV (VP53) and CIV (VP50) were located by hybridization of the oligonucleotide probes to blots of the viral DNA. The major capsid protein was used as the zero point for the proposed linearized maps of these viruses. Using antibody and 125I-labelling, several proteins were identified as being on the surface of the virion. It was also shown that CIV was not as antigenically distinct from these two viruses as previously reported.

Comparison of the genomes of two sympatric iridescent viruses (types 9 and 16)

A map of the sites in the genome of Costelytra zealandica iridescent virus (CzIV), using the restriction enzymes BamHI, KpnI, and PstI, showed the genome size to be 170.2 kbp in length. It was found that the genome was cyclically permuted and that 39% of the genome of CzIV contained repetitive sequence elements. The genome was found to hybridize with the genome of another iridescent virus, type 9 (WIV), in DNA-DNA hybridization experiments. A region of the WIV DNA genome (23.4 kbp) did not hybridize with CzIV DNA and this region is similar in size to the total genomic size difference between CzIV and WIV (22.4 kbp). A unique repeat sequence from iridescent virus type 6 (CIV) was shown to be present in the genome of WIV but not that of CzIV. Finally, the positions of the major capsid protein genes, VP53 and VP52, in the restriction enzyme maps for type 16 and type 9 respectively were determined.