Rb-independent induction of apoptosis by bovine papillomavirus type 1 E7 in response to tumor necrosis factor alpha

Bovine papillomavirus E5 and E7 oncoproteins in naturally occurring tumors: are two better than one?

Bovine papillomaviruses (BPVs) are oncogenic DNA viruses, which mainly induce benign lesions of cutaneous and/or mucosal epithelia in cattle. Thirteen (BPV 1-13) different viral genotypes have been characterized so far. BPVs are usually species-specific but BPV 1/2 may also infect equids as well as buffaloes and bison and cause tumors in these species. BPV-induced benign lesions usually regress, however occasionally they develop into cancer particularly in the presence of environmental carcinogenic co-factors. The major transforming protein of BPV is E5, a very short hydrophobic, transmembrane protein with many oncogenic activities. E5 contributes to cell transformation through the activation of the cellular β receptor for the platelet-derived growth factor (PDGFβ-r), it also decreases cell surface expression of major histocompatibility complex class I (MHCI) causing viral escape from immunosurveillance, and plays a role in the inhibition of the intracellular communication by means of aberrant connexin expression. E7 is considered as a weak transforming gene, it synergies with E5 in cell transformation during cancer development. E7 expression correlates in vivo with the over-expression of β1-integrin, which plays a role in the regulation of keratinocytes proliferation and differentiation. Additionally, E7 is involved in cell-mediated immune responses leading to tumour rejection, in anoikis process by direct binding to p600, and in invasion process by upregulation of Matrix metalloproteinase1 (MMP-1) expression. Studies on the role of BPV E5 and E7 oncoproteins in naturally occurring tumours are of scientific value, as they may shed new light on the biological role of these two oncogenes in cell transformation.

Oncogene addiction: setting the stage for molecularly targeted cancer therapy

In pugilistic parlance, the one-two punch is a devastating combination of blows, with the first punch setting the stage and the second delivering the knock-out. This analogy can be extended to molecularly targeted cancer therapies, with oncogene addiction serving to set the stage for tumor cell killing by a targeted therapeutic agent. While in vitro and in vivo examples abound documenting the existence of this phenomenon, the mechanistic underpinnings that govern oncogene addiction are just beginning to emerge. Our current inability to fully exploit this weakness of cancer cells stems from an incomplete understanding of oncogene addiction, which nonetheless represents one of the rare chinks in the formidable armor of cancer cells.

RbAp48 Is a Critical Mediator Controlling the Transforming Activity of Human Papillomavirus Type 16 in Cervical Cancer

Although human papillomavirus (HPV) infections are the primary cause of cervical cancer, the molecular mechanism by which HPV induces cervical cancer remains largely unclear. We used two-dimensional electrophoresis with mass spectrometry to study protein expression profiling between HPV16-positive cervical mucosa epithelial H8 cells and cervical cancer Caski cells to identify 18 differentially expressed proteins. Among them, retinoblastoma-binding protein 4 (RbAp48) was selected, and its differentiation expression was verified with both additional cervical cancer-derived cell lines and human tissues of cervical intraepithelial neoplasia and cervical cancer. Suppression of RbAp48 using small interfering RNA approach in H8 cells significantly stimulated cell proliferation and colony formation and inhibited senescence-like phenotype. Remarkably, H8 cells acquired transforming activity if RpAp48 was suppressed, because H8 cells stably transfected with RbAp48 small interfering RNA led to tumor formation in nude mice. In addition, overexpression of RbAp48 significantly inhibited cell growth and tumor formation. This RbAp48-mediated transformation of HPV16 is probably because of the regulation by RbAp48 of tumor suppressors retinoblastoma and p53, apoptosis-related enzymes caspase-3 and caspase-8, and oncogenic genes, including E6, E7, cyclin D1 (CCND1), and c-MYC. In brief, RbAp48, previously unknown in cervical carcinogenesis, was isolated in a global screen and identified as a critical mediator controlling the transforming activity of HPV16 in cervical cancer.

Rb2/p130 and protein phosphatase 2A: key mediators of ovarian carcinoma cell growth suppression by all-trans retinoic acid

Despite a number of attempts to improve treatment of ovarian cancer, it remains the most common cause of death from gynecological cancers. Thus, it is very important to identify more effective drugs for treatment and prevention of ovarian cancer. All-trans-retinoic acid (ATRA) has been shown to arrest the growth of ovarian carcinoma cells in G0/G1 and to significantly elevate levels of Rb2/p130 protein, a member of the retinoblastoma family of tumor suppressors. As ATRA treatment leads to a significant increase in the amount of Rb2/p130 protein but not mRNA, the elevated levels of Rb2/p130 protein is likely the result of increased stability. In studies to elucidate the mechanism by which ATRA alters Rb2/p130 stability in ovarian cancer cells, it was determined that PP2A, a serine/threonine phosphatase, binds and dephosphorylates Rb2/p130. Dephosphorylated Rb2/p130 exhibits decreased ubiquitination and thus is not degraded by the proteasome. The sites at which PP2A catalytic subunit (PP2Ac) interacts with Rb2/p130 have been localized to the NLS in the C-terminus of Rb2/p130. These sites are also involved in the interaction of Rb/p130 with importin beta and importin alpha, members of the nuclear transport machinery. It is known that importin alpha recognizes a NLS on a target protein and importin beta binds the nuclear pore complex. Moreover, it has been shown that the binding of importin alpha to NLS significantly decreases with phosphorylation of NLS. In ATRA-treated ovarian carcinoma cells, PP2A binds to Rb2/p130 and dephosphorylates the NLS of Rb2/p130 leading to the interaction of importin alpha with Rb2/p130. Importin beta then binds to the importin alpha-Rb2/p130 complex, leading to the translocation of the Rb2/p130 to the nucleus where it acts to arrest ovarian cancer cells in G1 and suppress proliferation.

Opposing effects of bovine papillomavirus type 1 E6 and E7 genes on Fas-mediated apoptosis

Programmed cell death (PCD), best exemplified by apoptosis, is a genetically programmed process of cellular destruction that is indispensable for normal development and homeostasis of multicellular organisms. Tumor necrosis factor alpha (TNF) and related cytokines are employed by host defenses to eliminate virally infected cells through induction of apoptosis. Many viruses have evolved specific gene products to modulate this process. We have recently shown that the bovine papillomavirus type 1 (BPV-1) E6 and E7 genes independently sensitize mouse cells to TNF-induced apoptosis. In this report, we investigated the effect of E6 and E7 expression on Fas-mediated apoptosis. In contrast to TNF-mediated apoptosis, E6 and E7 demonstrated opposite effects: while E7 potentiated apoptosis triggered by an agonistic Fas antibody, E6 attenuated the effect. The mitochondrial pathway leading to the activation of caspases appears to be involved in Fas-mediated apoptosis in C127 cells. To further explore the mechanisms by which E6 and E7 modulate Fas-mediated apoptosis, we examined the surface expression of Fas in cells expressing E6 and E7. Significantly, levels of surface Fas expression correlated with the opposing effects of E6 and E7 on Fas-mediated apoptosis. Specifically, while E7 increased the surface expression of Fas, E6 reduced surface Fas expression. Mutational analysis demonstrated a correlation of E6's ability to downregulate surface Fas expression and apoptosis. Since the tumor suppressor p53 can be targeted for degradation by human papillomavirus and has been shown to induce apoptosis by upregulating surface Fas expression, we investigated the role of p53 in BPV-1 E6 and E7 modulation of Fas-mediated apoptosis. Our results demonstrated that the modulatory effects by E6 and E7 could occur in the absence of p53. Interestingly, the reduced Fas protein level on the cell surface is not accompanied by a decrease in total Fas levels in E6-expressing cells. Instead, considerably more Fas protein is found in the cytoplasm of cells expressing E6. These results highlight a novel activity of E6 and E7 that may be involved in viral pathogenesis.

The HPV-16 E7 Oncogene Sensitizes Malignant Cells to IFN-α-Induced Apoptosis

Interferons (IFNs) exert antitumor effects in several human malignancies, but their mechanism of action is unclear. There is a great variability in sensitivity to IFN treatment depending on both tumor type and the individual patient. The reason for this variable sensitivity is not known. The fact that several IFN-induced anticellular effects are exerted through modulation of proto-oncogenes and tumor suppressor genes may indicate that the malignant genotype may be decisive in the cell's sensitivity to IFN. To determine if a deregulated oncogene could alter the cellular response to IFN, a mouse lymphoma cell line (J3D) was stably transfected with the viral human papillomavirus-16 (HPV-16) E7 oncogene. The E7-transfected cells and their respective mock-transfected sister clones were treated with IFN-alpha and examined for possible IFN-induced anticellular effects. We found that the E7-transfected clones were greatly sensitized to IFN-alpha-induced apoptosis compared with their mock-transfected counterparts. Induction of apoptosis in the transfected cells correlated with the ability of IFN to activate parts of the proapoptotic machinery specifically in these cells, including activation of caspases and the proapoptotic protein Bak. In summary, our data suggest that transfection of malignant cells with the E7 oncogene can sensitize them to IFN-alpha-induced apoptosis. This demonstrates that an oncogenic event may alter the cellular sensitivity to IFN and might also have implications for treatment of HPV-related diseases with IFN.

Macrophages kill human papillomavirus type 16 E6-expressing tumor cells by tumor necrosis factor alpha- and nitric oxide-dependent mechanisms

The expression of adenovirus serotype 2 or 5 (Ad2/5) E1A sensitizes cells to killing by NK cells and activated macrophages, a property that correlates with the ability of E1A to bind the transcriptional coadaptor proteins p300-CBP. The E6 oncoproteins derived from the high-risk human papillomaviruses (HPV) interact with p300 and can complement mutant forms of E1A that cannot interact with p300 to induce cellular immortalization. Therefore, we determined if HPV type 16 (HPV16) E6 could sensitize cells to killing by macrophages and NK cells. HPV16 E6 expression sensitized human (H4 and C33A) and murine (MCA-102) cell lines to lysis by macrophages but not by NK cells. The lysis of cells that expressed E6 by macrophages was p53 independent but dependent on the production of tumor necrosis factor alpha (TNF-alpha) or nitric oxide (NO) by macrophages. Unlike cytolysis assays with macrophages, E6 expression did not significantly sensitize cells to lysis by the direct addition of NO or TNF-alpha. Like E1A, E6 has been reported to sensitize cells to lysis by TNF-alpha by inhibiting the TNF-alpha-induced activation of NF-kappaB. We found that E1A, but not E6, blocked the TNF-alpha-induced activation of NF-kappaB, an activity that correlated with E1A-p300 binding. In summary, Ad5 E1A and HPV16 E6 sensitized cells to lysis by macrophages. Unlike E1A, E6 did not block the ability of TNF-alpha to activate NF-kappaB or sensitize cells to lysis by NK cells, TNF-alpha, or NO. Thus, there appears to be a spectrum of common and unique biological activities that result as a consequence of the interaction of E6 or E1A with p300-CBP.

Activation of c-Myc contributes to bovine papillomavirus type 1 E7-induced cell proliferation

Inactivation of the tumor suppressor pRB by the human papillomavirus (HPV) oncoprotein E7 is a mechanism by which HPV promotes cell growth. The bovine papillomavirus type 1 (BPV-1) E7 does not bind pRB efficiently yet is required for full transformation of murine cells by BPV-1. In the present study, we investigated the mechanism of BPV-1 E7-induced cell proliferation. Our studies indicate that expression of BPV-1 E7 induces DNA synthesis and stimulates cells to enter S phase in quiescent cells. The induction of cell proliferation by BPV-1 E7 can occur in the retinoblastoma gene (Rb)-null cells, suggesting an Rb-independent mechanism. Consistent with this observation, BPV-1 E7 does not efficiently activate the transcription of the E2F family of transcription factors (E2F)-responsive promoters. Notably, c-Myc is able to induce cells to enter S phase in quiescent cells through an Rb/E2F-independent pathway. Significantly, c-Myc levels are increased in BPV-1 E7-expressing cells. Moreover, expression of a dominant negative c-Myc mutant inhibited BPV-1 E7-induced DNA synthesis. Consistent with the notion that c-Myc could down-regulate p27 and activate Cdk2, p27 level is decreased while both cyclin A and cyclin E-associated kinase activities are up-regulated in BPV-1 E7-expressing cells. These studies indicate an important role for c-Myc in BPV-1 E7-induced cell proliferation.

Role of activin A in murine mast cells: modulation of cell growth, differentiation, and migration

Activins, members of the transforming growth factor-beta (TGF-beta) superfamily, are potent growth and differentiation factors. Our previous studies revealed that activin A, a homodimer of inhibin/activin beta(A), was induced in mast cells and peritoneal macrophages in response to their activation. In the present study, we examined the roles of activin A in murine bone marrow-derived, cultured mast cell progenitors (BMCMCs), which expressed gene transcripts for molecules involved in activin signaling, suggesting that BMCMCs could be target cells of activin A. Treatment of activin A inhibited 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide uptake into BMCMCs in a dose-dependent manner. The IC(50) concentration was 2.1 nM, which was less potent than 185 pM TGF-beta(1). Activin A treatment caused morphological changes toward the differentiated cells at 2 nM and up-regulated mRNA of mouse mast cell protease-1 (mMCP-1), a marker enzyme of mature mucosal mast cells, at 1 nM. Activin A also showed activity in inducing migration of BMCMCs; the optimal concentration for maximal migration was 10 pM, which was much lower than the concentrations to inhibit cell growth and to activate the mMCP-1 gene. Taking the present results together with our previous results, it is suggested that activin A secreted from activated immune cells recruits mast cell progenitors to sites of inflammation and that with increasing activin A concentration, the progenitors differentiate into mature mast cells. Thus, activin A may positively regulate the functions of mast cells as effector cells of the immune system.

Bovine papillomavirus type 1 E6-induced sensitization to apoptosis is distinct from its transforming activity

The bovine papillomavirus type 1 (BPV-1) E6 oncoprotein induces tumorigenic transformation of murine C127 cells and stimulates transcription when targeted to a promoter. We have previously shown that C127 cells expressing BPV-1 E6 exhibited increased tumor necrosis factor alpha (TNF)-mediated apoptosis. To understand the mechanisms by which BPV-1 E6 sensitizes cells to apoptosis and to investigate the relevance of E6-enhanced apoptosis to its other biological activities, we analyzed a BPV-1 E6 mutant (491, with four amino acids deleted at the C-terminus) for its ability to sensitize C127 cells to apoptosis. The result was then compared with the E6 mutant's ability to transform cells, to activate transcription, and to associate with known cellular binding proteins. Our data indicated that the transcriptional activation function of BPV-1 E6 correlated with sensitization of cells to TNF-mediated apoptosis. Moreover, functions required for BPV-1 E6-mediated sensitization of cells to apoptosis are distinct from those required for transformation. A potential role of paxillin in E6 sensitization of cells to apoptosis is implicated. These results thus indicate that sensitization of cells to TNF-induced apoptosis represents a novel function of BPV-1 E6.