Involvement of nuclear export in human papillomavirus type 18 E6-mediated ubiquitination and degradation of p53

New insights into the interactions of HPV-16 E6*I and E6*II with p53 isoforms and induction of apoptosis in cancer-derived cell lines

An important characteristic of cancers associated with high-risk human papillomaviruses (HR-HPV) is the inability of p53 to activate apoptosis due to the effect of the oncoprotein E6. However, the effect of HPV-16 E6 splice variant isoforms (namely E6*I and E6*II), their interaction with the existing p53 isoforms, and their influence on apoptosis is unclear. Here, we report the outcome of ectopic expression of HPV-16 E6, E6*I, and E6*II on the relative levels of p53 and p53 isoforms Δ40p53 and Δ133p53 and their interactions with these proteins. Additionally, we evaluated the effect of ectopic expression of p53, Δ40p53, and Δ133p53 on apoptosis in a p53 null pulmonary cell line (H1299) co-transfected with E6 isoforms and p53^+/+ cell lines with HR-HPV (SiHa and HeLa), transfected with p53 isoforms and treated with cisplatin, a conventional drug used to treat cervical cancer. Our results show that E6 and E6*II induced a significant decrease in p53, but only E6 triggered a Δ40p53 decrease and that E6*II interacts with p53 but not with Δ40p53 and Δ133p53. On the other hand, E6*I did not show any effect or interaction with the p53 isoforms. We found that apoptosis was elevated in H1299 cells transfected with p53 (p = 0.0001) and Δ40p53 (p = 0.0001). A weak apoptotic effect was observed when Δ133p53 was ectopically expressed (p = 0.0195). We observed that both p53 (p = 0.0006) and Δ40p53 (p = 0.0014) induced apoptosis in cisplatin-treated SiHa cells; however in cisplatin-treated HeLa cells, only p53 induced apoptosis (p = 0.0029). No significant differences in apoptosis were observed upon ectopic expression of p53, Δ40p53, and Δ133p53 in SiHa and HeLa cells. Our findings suggest a possible therapeutic application for the combining of p53 or Δ40p53 with cisplatin to induce an increased apoptosis of cancer cells expressing E6 isoforms from HPV-16.

Non-human primate papillomavirus E6-mediated p53 degradation reveals ancient evolutionary adaptation of carcinogenic phenotype to host niche

Non-human primates (NHPs) are infected with papillomaviruses (PVs) closely related to their human counterparts, but there are few studies on the carcinogenicity of NHP-PVs. Using an in vitro cell co-transfection assay, we systematically screened the biochemical activity of E6 proteins encoded by macaque PVs for their ability to bind and promote degradation of host p53 proteins. A host species barrier exists between HPV16 and MfPV3 with respect to E6-mediated p53 degradation that is reversed when p53 residue 129 is swapped between human and macaque hosts. Systematic investigation found that E6 proteins encoded by most macaque PV types in the high-risk species α12, but not other Alpha-PV clades or Beta-/Gamma-PV genera, can effectively promote monkey p53 degradation. Interestingly, two macaque PV types (MfPV10 and MmPV1) can simultaneously inhibit the expression of human and monkey p53 proteins, revealing complex cross-host interactions between PV oncogenes and host proteomes. Single point-mutant experiments revealed that E6 residue 47 directly interacts with p53 residue 129 for host-specific degradation. These findings suggest an ancient host niche adaptation toward a carcinogenic phenotype in high-risk primate PV ancestors. Following periods of primate host speciation, a loss-of-function mutation model could be responsible for the formation of a host species barrier to E6-mediated p53 degradation between HPVs and NHP-PVs. Our work lays a genetic and functional basis for PV carcinogenicity, which provides important insights into the origin and evolution of specific pathogens in host pathogenesis.

How SARS-CoV-2 and Other Viruses Build an Invasion Route to Hijack the Host Nucleocytoplasmic Trafficking System

The host nucleocytoplasmic trafficking system is often hijacked by viruses to accomplish their replication and to suppress the host immune response. Viruses encode many factors that interact with the host nuclear transport receptors (NTRs) and the nucleoporins of the nuclear pore complex (NPC) to access the host nucleus. In this review, we discuss the viral factors and the host factors involved in the nuclear import and export of viral components. As nucleocytoplasmic shuttling is vital for the replication of many viruses, we also review several drugs that target the host nuclear transport machinery and discuss their feasibility for use in antiviral treatment.

HPV-mediated nuclear export of HP1γ drives cervical tumorigenesis by downregulation of p53

E6 oncoprotein derived from high-risk human papillomavirus (HPV) drives the development of cervical cancer through p53 degradation. Because cervical cancer therapies to inactivate HPV or E6 protein are not available, alternative strategies are required. Here, we show that HPV-mediated nuclear export of human heterochromatin protein 1γ (HP1γ) reduces the stability of p53 through UBE2L3-mediated p53 polyubiquitination during cervical cancer progression. In general, HP1 plays a key role in heterochromatin formation and transcription in the nucleus. However, our immunostaining data showed that the majority of HP1γ is localized in the cytoplasm in HPV-mediated cervical cancer. We found that HPV E6 protein drives unusual nuclear export of HP1γ through the interaction between the NES sequence of HP1γ and exportin-1. The mutation of the NES sequence in HP1γ led to nuclear retention of HP1γ and reduced cervical cancer cell growth and tumor generation. We further discovered that HP1γ directly suppresses the expression of UBE2L3 which drives E6-mediated proteasomal degradation of p53 in cervical cancer. Downregulation of UBE2L3 by overexpression of HP1γ suppressed UBE2L3-dependent p53 degradation-promoting apoptosis of cervical cancer cells. Our findings propose a useful strategy to overcome p53 degradation in cervical cancer through the blockage of nuclear export of HP1γ.

HP1γ Sensitizes Cervical Cancer Cells to Cisplatin through the Suppression of UBE2L3

Cisplatin is the most frequently used agent for chemotherapy against cervical cancer. However, recurrent use of cisplatin induces resistance, representing a major hurdle in the treatment of cervical cancer. Our previous study revealed that HP1γ suppresses UBE2L3, an E2 ubiquitin conjugating enzyme, thereby enhancing the stability of tumor suppressor p53 specifically in cervical cancer cells. As a follow-up study of our previous findings, here we have identified that the pharmacological substances, leptomycin B and doxorubicin, can improve the sensitivity of cervical cancer cells to cisplatin inducing HP1γ-mediated elevation of p53. Leptomycin B, which inhibits the nuclear export of HP1γ, increased cisplatin-dependent apoptosis induction by promoting the activation of p53 signaling. We also found that doxorubicin, which induces the DNA damage response, promotes HP1γ-mediated silencing of UBE2L3 and increases p53 stability. These effects resulted from the nuclear translocation and binding of HP1γ on the UBE2L3 promoter. Doxorubicin sensitized the cisplatin-resistant cervical cancer cells, enhancing their p53 levels and rate of apoptosis when administered together with cisplatin. Our findings reveal a therapeutic strategy to target a specific molecular pathway that contributes to p53 degradation for the treatment of patients with cervical cancer, particularly with cisplatin resistance.

Delineation of the HPV11E6 and HPV18E6 Pathways in Initiating Cellular Transformation

Although high-risk human papillomaviruses (HPVs) are the major risk factors for cervical cancer they have been associated with several other cancers, such as head and neck and oral cancers. Since integration of low-risk HPV11 DNA has been demonstrated in esophageal tumor genomes, this study compared the effects of low-risk HPV11E6 and high-risk HPV18E6 on cellular gene expression. The HPV11E6 and HPV18E6 genes were cloned into an adenoviral vector and expressed in human keratinocytes (HaCaT) in order to investigate early events and to eliminate possible artifacts introduced by selective survival of fast growing cells in stable transfection experiments. HPV11E6 had very little effect on p21 and p53 gene expression, while HPV18E6 resulted in a marked reduction in both these proteins. Both HPV11E6 and HPV18E6 enabled growth of colonies in soft agar, but the level of colony formation was higher in HPV18E6 infected cells. DNA microarray analysis identified significantly differentially regulated genes involved in the cellular transformation signaling pathways. These findings suggest that HPV11E6 and HPV18E6 are important in initiating cellular transformation via deregulation of signaling pathways such as PI3K/AKT and pathways that are directly involved in DNA damage repair, cell survival, and cell proliferation. This study shows that the low-risk HPV11E6 may have similar effects as the high-risk HPV18E6 during the initial stages of infection, but at a much reduced level.

Inhibition of antiviral drug cidofovir on proliferation of human papillomavirus-infected cervical cancer cells

In order to evaluate the potential application value of cidofovir (CDV) in the prevention of human papillomavirus (HPV) infection and treatment of cervical cancer, the inhibitory effect of CDV on the proliferation of HPV 18-positive HeLa cells in cervical cancer was preliminarily investigated, using cisplatin (DDP) as a positive control. An MTT assay was used to analyze the effects of CDV and DDP on HeLa cell proliferation. In addition, clone formation assay and Giemsa staining were used to examine the extent of HeLa cell apoptosis caused by CDV and DDP. Flow cytometry was also used to detect the shape and size of apoptotic cells following propidium iodide staining, while western blot analysis identified the expression levels of of E6 and p53 proteins in HeLa cells. A cell climbing immunofluorescence technique was used to locate the subcellular position of p53 in HeLa cells. The results demonstrated that CDV and DDP inhibited the proliferation of HeLa cells in a concentration- and time-dependent manner. Flow cytometry showed that CDV and DDP treatments resulted in cell arrest in the S-phase, and triggered programmed cell death. Furthermore, western blot analysis revealed that CDV and DDP inhibited E6 protein expression and activated p53 expression in HeLa cells. Finally, the immunofluorescence results indicated that CDV and DDP inhibited the nuclear export of p53 by E6 protein, which is required for degradation of endogenous p53 by MDM2 and human papilloma virus E6. In conclusion, CDV and DDP inhibited HeLa cell proliferation in a concentration- and time-dependent manner, reduced the expression of E6 protein, and reinstated p53 protein activity. Thus, CDV regulates cell cycle arrest and apoptosis, and may be a potential cervical cancer therapeutic strategy.

Inhibition of the nuclear transporter, Kpnβ1, results in prolonged mitotic arrest and activation of the intrinsic apoptotic pathway in cervical cancer cells

The karyopherin β proteins are involved in nuclear-cytoplasmic trafficking and are crucial for protein and RNA subcellular localization. We previously showed that Kpnβ1, a nuclear importin protein, is overexpressed in cervical cancer and is critical for cervical cancer cell survival and proliferation, whereas non-cancer cells are less dependent on its expression. This study aimed to identify the mechanisms by which inhibition of Kpnβ1 results in cervical cancer cell death. We show that the inhibition of Kpnβ1 results in the induction of apoptosis and a prolonged mitotic arrest, accompanied by distinct mitotic defects in cervical cancer cells but not non-cancer cells. In cervical cancer cells, Kpnβ1 downregulation results in sustained degradation of the antiapoptotic protein, Mcl-1, and elevated Noxa expression, as well as mitochondrial membrane permeabilization resulting in the release of cytochrome C and activation of associated caspases. Although p53 becomes stabilized in Kpnβ1 knockdown cervical cancer cells, apoptosis occurs in a p53-independent manner. These results demonstrate that blocking Kpnβ1 has potential as an anticancer therapeutic approach.

Papillomavirus E6 oncoproteins

Papillomaviruses induce benign and malignant epithelial tumors, and the viral E6 oncoprotein is essential for full transformation. E6 contributes to transformation by associating with cellular proteins, docking on specific acidic LXXLL peptide motifs found on these proteins. This review examines insights from recent studies of human and animal E6 proteins that determine the three-dimensional structure of E6 when bound to acidic LXXLL peptides. The structure of E6 is related to recent advances in the purification and identification of E6 associated protein complexes. These E6 protein-complexes, together with other proteins that bind to E6, alter a broad array of biological outcomes including modulation of cell survival, cellular transcription, host cell differentiation, growth factor dependence, DNA damage responses, and cell cycle progression.

The CRM1 nuclear export protein in normal development and disease

CRM1 (Chromosomal Maintenance 1, also known as Exportin 1) is the major mammalian export protein that facilitates the transport of large macromolecules including RNA and protein across the nuclear membrane to the cytoplasm. The gene encoding CRM1 was originally identified in yeast as required to maintain higher order chromosome structure. In mammalian cells, CRM1 was found to bind several nuclear pore proteins hence its role in nuclear-cytosolic transport was discovered. In addition to nuclear-cytosolic transport, CRM1 also plays a role in centrosome duplication and spindle assembly, especially in response to DNA damage. The crystal structure of CRM1 suggests a complex protein that binds the Ran protein bound to GTP, allowing for a conformational change that facilitates binding to different cargo proteins through a nuclear export signal (NES). Included in the cadre of cargo are multiple tumor suppressor and oncoproteins as p53, BRCA1, Survivin, NPM, and APC, which function in the nucleus to regulate transcription or aid in chromosomal assembly and movement. An imbalance in the cytosolic level of these proteins has been observed in cancer cells, resulting in either inactivation (tumor suppressor) or an excess of anti-apoptotic activity (oncoprotein). Thus, the concept of inhibiting CRM1 has been explored as a potential therapeutic intervention. Indeed, inhibition of CRM1 by a variety of small molecules that interfere with cargo-NES binding results in cancer cell death. Whether all of these proteins together are responsible for this phenotype or whether specific proteins are required for this effect is unclear at this time.

p53 degradation activity, expression, and subcellular localization of E6 proteins from 29 human papillomavirus genotypes

Human papillomaviruses (HPVs) are the etiological agents of cervical cancer and other human malignancies. HPVs are classified into high- and low-risk genotypes according to their association with cancer. Host cell transformation by high-risk HPVs relies in part on the ability of the viral E6 protein to induce the degradation of p53. We report the development of a cellular assay that accurately quantifies the p53 degradation activity of E6 in vivo, based on the fusion of p53 to Renilla luciferase (RLuc-p53). This assay was used to measure the p53 degradation activities of E6 proteins from 29 prevalent HPV types and variants of HPV type 16 (HPV16) and HPV33 by determining the amount of E6 expression vector required to reduce by half the levels of RLuc-p53 (50% effective concentration [EC₅₀]). These studies revealed an unexpected variability in the p53 degradation activities of different E6 proteins, even among active types whose EC₅₀s span more than 2 log units. Differences in activity were greater between types than between variants and did not correlate with differences in the intracellular localization of E6, with most being predominantly nuclear. Protein and mRNA expression of the 29 E6 proteins was also examined. For 16 high-risk types, spliced transcripts that encode shorter E6*I proteins of variable sizes and abundances were detected. Mutation of the splice donor site in five different E6 proteins increased their p53 degradation activity, suggesting that mRNA splicing can limit the activity of some high-risk E6 types. The quantification of p53 degradation in vivo represents a novel tool to systematically compare the oncogenic potentials of E6 proteins from different HPV types and variants.

Sts1 plays a key role in targeting proteasomes to the nucleus

The evidence that nuclear proteins can be degraded by cytosolic proteasomes has received considerable experimental support. However, the presence of proteasome subunits in the nucleus also suggests that protein degradation could occur within this organelle. We determined that Sts1 can target proteasomes to the nucleus and facilitate the degradation of a nuclear protein. Specific sts1 mutants showed reduced nuclear proteasomes at the nonpermissive temperature. In contrast, high expression of Sts1 increased the levels of nuclear proteasomes. Sts1 targets proteasomes to the nucleus by interacting with Srp1, a nuclear import factor that binds nuclear localization signals. Deletion of the NLS in Sts1 prevented its interaction with Srp1 and caused proteasome mislocalization. In agreement with this observation, a mutation in Srp1 that weakened its interaction with Sts1 also reduced nuclear targeting of proteasomes. We reported that Sts1 could suppress growth and proteolytic defects of rad23Δ rpn10Δ. We show here that Sts1 suppresses a previously undetected proteasome localization defect in this mutant. Taken together, these findings explain the suppression of rad23Δ rpn10Δ by Sts1 and suggest that the degradation of nuclear substrates requires efficient proteasome localization.

Interaction of viral oncoproteins with cellular target molecules: infection with high-risk vs low-risk human papillomaviruses

Persistent infection by a subgroup of so-called high-risk human papillomaviruses (HPVs) that have a tropism for mucosal epithelia has been defined as the cause of more than 98% of cervical carcinomas as well as a high proportion of other cancers of the anogenital region. Infection of squamous epithelial tissues in the head and neck region by these same high-risk HPVs is also associated with a subset of cancers. Despite the general conservation of genetic structure amongst all HPV types, infection by the low-risk types, whether in genital or head and neck sites, carries a negligible risk of malignant progression, and infections have a markedly different pathology. In this review, we will examine and discuss the interactions that the principal viral oncoproteins of the high-risk mucosotrophic HPVs and their counterparts from the low-risk group make with cellular target proteins, with a view to explaining the differences in their respective pathology.

p53-based cancer therapy

Inactivation of p53 functions is an almost universal feature of human cancer cells. This has spurred a tremendous effort to develop p53 based cancer therapies. Gene therapy using wild-type p53, delivered by adenovirus vectors, is now in widespread use in China. Other biologic approaches include the development of oncolytic viruses designed to replicate and kill only p53 defective cells and also the development of siRNA and antisense RNA's that activate p53 by inhibiting the function of the negative regulators Mdm2, MdmX, and HPV E6. The altered processing of p53 that occurs in tumor cells can elicit T-cell and B-cell responses to p53 that could be effective in eliminating cancer cells and p53 based vaccines are now in clinical trial. A number of small molecules that directly or indirectly activate the p53 response have also reached the clinic, of which the most advanced are the p53 mdm2 interaction inhibitors. Increased understanding of the p53 response is also allowing the development of powerful drug combinations that may increase the selectivity and safety of chemotherapy, by selective protection of normal cells and tissues.

The location of endogenous wild-type p53 protein in 293T and HEK293 cells expressing low-risk HPV-6E6 fusion protein with GFP

The mechanism underlining human papillomaviruses (HPVs) causing cancer has been studied extensively, and it was concluded that the high-risk HPVs' E6 targeted and degraded tumor suppressor protein p53, leading to infected cells malignant transformation. In contrast, the low-risk HPVs only cause proliferative but non-invasive lesions of infected epithelia. Therefore, we hypothesized that low-risk HPVs' E6 might interact with p53 in a different pattern. We used a mammalian green fluorescent protein (GFP) expression system to express HPV-18E6 and HPV-6E6 fusion proteins in wild-type (wt) p53 cell lines, 293T and HEK293 cells, to investigate the traffic and location of E6s and p53. The results indicated GFP-18E6 was mainly expressed in nucleus, whereas GFP-6E6 was expressed exclusively in cytoplasm. Endogenous wt p53 was shown to be localized in the nuclei of cells transfected with GFP- 18E6. Interestingly, for the first time, we observed that p53 was trapped in the cytoplasm and never translocated into the cell nuclei transfected with GFP-6E6. In conclusion, HPV-6E6 was responsible for the cytoplasmic localization of p53. Therefore, our experiments provide a new insight into the pathogenesis of HPV.

Aberrant splice variants of HAS1 (Hyaluronan Synthase 1) multimerize with and modulate normally spliced HAS1 protein: a potential mechanism promoting human cancer

Most human genes undergo alternative splicing, but aberrant splice forms are hallmarks of many cancers, usually resulting from mutations initiating abnormal exon skipping, intron retention, or the introduction of a new splice sites. We have identified a family of aberrant splice variants of HAS1 (the hyaluronan synthase 1 gene) in some B lineage cancers, characterized by exon skipping and/or partial intron retention events that occur either together or independently in different variants, apparently due to accumulation of inherited and acquired mutations. Cellular, biochemical, and oncogenic properties of full-length HAS1 (HAS1-FL) and HAS1 splice variants Va, Vb, and Vc (HAS1-Vs) are compared and characterized. When co-expressed, the properties of HAS1-Vs are dominant over those of HAS1-FL. HAS1-FL appears to be diffusely expressed in the cell, but HAS1-Vs are concentrated in the cytoplasm and/or Golgi apparatus. HAS1-Vs synthesize detectable de novo HA intracellularly. Each of the HAS1-Vs is able to relocalize HAS1-FL protein from diffuse cytoskeleton-anchored locations to deeper cytoplasmic spaces. This HAS1-Vs-mediated relocalization occurs through strong molecular interactions, which also serve to protect HAS1-FL from its otherwise high turnover kinetics. In co-transfected cells, HAS1-FL and HAS1-Vs interact with themselves and with each other to form heteromeric multiprotein assemblies. HAS1-Vc was found to be transforming in vitro and tumorigenic in vivo when introduced as a single oncogene to untransformed cells. The altered distribution and half-life of HAS1-FL, coupled with the characteristics of the HAS1-Vs suggest possible mechanisms whereby the aberrant splicing observed in human cancer may contribute to oncogenesis and disease progression.

The Karyopherin proteins, Crm1 and Karyopherin beta1, are overexpressed in cervical cancer and are critical for cancer cell survival and proliferation

The Karyopherin proteins are involved in nucleo-cytoplasmic trafficking and are critical for protein and RNA subcellular localization. Recent studies suggest they are important in nuclear envelope component assembly, mitosis and replication. Since these are all critical cellular functions, alterations in the expression of the Karyopherins may have an impact on the biology of cancer cells. In this study, we examined the expression of the Karyopherins, Crm1, Karyopherin beta1 (Kpnbeta1) and Karyopherin alpha2 (Kpnalpha2), in cervical tissue and cell lines. The functional significance of these proteins to cancer cells was investigated using individual siRNAs to inhibit their expression. Microarrays, quantitative RT-PCR and immunofluorescence revealed significantly higher expression of Crm1, Kpnbeta1 and Kpnalpha2 in cervical cancer compared to normal tissue. Expression levels were similarly elevated in cervical cancer cell lines compared to normal cells, and in transformed epithelial and fibroblast cells. Inhibition of Crm1 and Kpnbeta1 in cancer cells significantly reduced cell proliferation, while Kpnalpha2 inhibition had no effect. Noncancer cells were unaffected by the inhibition of Crm1 and Kpnbeta1. The reduction in proliferation of cancer cells was associated with an increase in a subG1 population by cell cycle analysis and Caspase-3/7 assays revealed increased apoptosis. Crm1 and Kpnbeta1 siRNA-induced apoptosis was accompanied by an increase in the levels of growth inhibitory proteins, p53, p27, p21 and p18. Our results demonstrate that Crm1, Kpnbeta1 and Kpnalpha2 are overexpressed in cervical cancer and that inhibiting the expression of Crm1 and Kpnbeta1, not Kpnalpha2, induces cancer cell death, making Crm1 and Kpnbeta1 promising candidates as both biomarkers and potential anticancer therapeutic targets.

Viral ubiquitin ligase WSSV222 is required for efficient white spot syndrome virus replication in shrimp

The E3 ligase WSSV222 of white spot syndrome virus (WSSV) is involved in anti-apoptosis regulation by ubiquitin-mediated degradation of tumour suppressor-like protein (TSL), a shrimp tumour suppressor. In the present study, WSSV222 gene expression was silenced by using specific small interfering RNA (siRNA) in Sf9 and BHK cells. Based on the results of the in vitro silencing, WSSV-challenged shrimp were treated with anti-WSSV222 siRNA to knock down WSSV222 protein expression. The survival rate of shrimp and the efficiency of WSSV replication were assessed to evaluate the efficacy of anti-WSSV222 siRNA in regulating WSSV infection in shrimp. The anti-WSSV222 siRNA reduced the cumulative mortality in shrimp challenged with 10(3) copies of WSSV and delayed the mean time to death in shrimp challenged with the higher dose of 10(6) copies. The results of real-time quantitative PCR showed that virus replication was delayed and reduced in WSSV-challenged shrimp treated with anti-WSSV222 siRNA in comparison with challenged shrimp treated with random-control siRNA. Co-immunoprecipitation assays revealed that WSSV222 silencing inhibited the degradation of TSL in WSSV-challenged shrimp, indicating the requirement for WSSV222 for efficient replication of WSSV in shrimp.

Manipulation of the ubiquitin-proteasome pathway by small DNA tumor viruses

Viruses have evolved to use cellular pathways to their advantage, including the ubiquitin-proteasome pathway of protein degradation. In several cases, viruses produce proteins that highjack cellular E3 ligases to modify their substrate specificity in order to eliminate unwanted cellular proteins, in particular inhibitors of the cell cycle. They can also inhibit E3 ligase to prevent specific protein degradation or even use the system to control the level of expression of their own proteins. In this review we explore the specific ways that small DNA tumor viruses exploit the ubiquitin-proteasome pathway for their own benefit.

Comparison of p53 and the PDZ domain containing protein MAGI-3 regulation by the E6 protein from high-risk human papillomaviruses

Central to cellular transformation caused by human papillomaviruses (HPVs) is the ability of E6 proteins to target cellular p53 and proteins containing PDZ domains, including MAGI-3, for degradation. The aim of this study was to compare E6-mediated degradation of p53 and MAGI-3 under parallel experimental conditions and further with respect to the involvement of proteasomes and ubiquitination. We also compared the degradation of p53 and MAGI-3 by E6 from several HPV types including different variants from HPV-33. All of the E6 genes from different HPV types displayed similar abilities to mediate the degradation of both p53 and MAGI-3 although there may be subtle differences observed with the different 33E6 variants. There were however differences in E6 mediated degradation of p53 and MAGI-3. Proteasome inhibition assays partially protected p53 from E6 mediated degradation, but did not protect MAGI-3. In addition, under conditions where p53 was ubiquitinated by E6 and MDM2 in vivo, ubiquitination of MAGI-3 was not detected. These results imply that although both p53 and MAGI-3 represent effective targets for oncogenic E6, the mechanisms by which E6 mediates p53 and MAGI-3 degradation are distinct with respect to the involvement of ubiquitination prior to proteasomal degradation.

Selective induction of apoptosis by leptomycin B in keratinocytes expressing HPV oncogenes

Human papillomavirus (HPV) infection is strongly associated with the development of anogenital neoplasia, particularly cervical cancer. It has been estimated that 99.7% of all cervical carcinomas are attributable to infection with HPV, and types 16 and 18 account for the vast majority of such cases. Both of these 'high risk' HPV types encode the oncoproteins E6 and E7, which exert multiple effects on many proteins involved in cell-cycle regulation, including p53. The nuclear export protein inhibitor leptomycin B (LMB) has been shown to cause the nuclear sequestration of p53 in cervical carcinoma cells. We demonstrate that LMB induces apoptosis selectively at nanomolar concentrations in primary human keratinocytes (PHKs) expressing HPV oncogenes. Both monolayer and organotypic raft cultures of transduced PHKs were highly susceptible to treatment with LMB. By contrast, although LMB stimulated p53 accumulation in normal PHKs, no significant induction of apoptosis was detected on Western blots or immunostained monolayer/raft cells, or following pulsed exposure to the drug. Furthermore, topical application of microM concentrations of LMB to mouse skin was non-toxic. These data suggest that the topical application of LMB to HPV-infected intra-epithelial lesions may represent a specific and effective therapeutic strategy against HPV-associated anogenital neoplasia.

Ubiquitin-independent degradation of p53 mediated by high-risk human papillomavirus protein E6

In vitro, high-risk human papillomavirus E6 proteins have been shown, in conjunction with E6-associated protein (E6AP), to mediate ubiquitination of p53 and its degradation by the 26S proteasome by a pathway that is thought to be analogous to Mdm2-mediated p53 degradation. However, differences in the requirements of E6/E6AP and Mdm2 to promote the degradation of p53, both in vivo and in vitro, suggest that these two E3 ligases may promote p53 degradation by distinct pathways. Using tools that disrupt ubiquitination and degradation, clear differences between E6- and Mdm2-mediated p53 degradation are presented. The consistent failure to fully protect p53 protein from E6-mediated degradation by disrupting the ubiquitin-degradation pathway provides the first evidence of an E6-dependent, ubiquitin-independent, p53 degradation pathway in vivo.

Nuclear localization of Survivin renders HeLa tumor cells more sensitive to apoptosis by induction of p53 and Bax

Clinical studies have shown that nuclear expression of the inhibitor of apoptosis protein Survivin in tumor cells predicted a favorable prognosis whereas cytosolic-localized protein caused a decreased overall survival. Therefore Survivin's subcellular localization may be important for its anti-apoptotic capacity. To address this question, we investigated localization and function of Survivin in normal human lung fibroblasts (NHLFs) and HeLa tumor cells. NHLFs of early passages expressed Survivin in the nucleus and were highly sensitive to C2 ceramide, which induces the mitochondrial apoptotic pathway. In contrast, NHLFs at higher passages relocated Survivin to the cytosol and became more resistant to C2 ceramide. Blocking nuclear export of Survivin by leptomycin B in HeLa cells increased susceptibility to C2 ceramide. In addition, transduction of HeLa cells with Survivin fused to a nuclear localization signal augmented basal expression levels of p53 and Bax and enhanced sensitivity for intrinsic apoptosis. Those findings suggest that a predominant nuclear localization of Survivin increases the sensitivity for pro-apoptotic stimuli, whereas nuclear export enables Survivin to fulfill its inhibitor of apoptosis function. A therapeutic intervention which holds Survivin in the nucleus of tumor cells might improve cancer therapy.

Comparative Analysis of 19 Genital Human Papillomavirus Types with Regard to p53 Degradation, Immortalization, Phylogeny, and Epidemiologic Risk Classification

We have analyzed E6 proteins of 19 papillomaviruses able to infect genital tissue with regard to their ability to degrade p53 and the thus far unknown immortalization potential of the genomes of human papillomaviruses (HPV) 53, 56, 58, 61, 66, and 82 in primary human keratinocytes. E6 proteins of HPV types 16, 18, 33, 35, 39, 45, 51, 52, 56, 58, and 66, defined as high-risk types, were able to induce p53 degradation in vitro, and HPV18-, HPV56-, and HPV58-immortalized keratinocytes revealed markedly reduced levels of p53. In contrast, the E6 proteins of HPV6 and 11 and HPV44, 54, and 61, regarded as possible carcinogenic or low-risk HPV types, respectively, did not degrade p53. Interestingly, the E6 proteins of HPV 53, 70, and 82 inconsistently risk classified in the literature were also found to induce p53 degradation. The genomes of HPV53 and 82 immortalized primary human keratinocytes that revealed almost absent nuclear levels of p53. These data suggest a strict correlation between the biological properties of certain HPV types with conserved nucleotide sequence (phylogeny), which is largely coherent with epidemiologic risk classification. HPV types 16, 18, 33, 35, 39, 45, 51, 52, 56, 58, and 66, generally accepted as high-risk types, behaved in our assays biologically different from HPV types 6, 11, 44, 54, and 61. In contrast, HPV70, regarded as low-risk type, and HPV53 or HPV82, with inconsistent described risk status, were indistinguishable with respect to p53 degradation and immortalization from prototype high-risk HPV types. This could imply that other important functional differences exist between phylogenetically highly related viruses displaying similar biological properties in tissue culture that may affect their carcinogenicity in vivo.

Mechanisms for human cytomegalovirus-induced cytoplasmic p53 sequestration in endothelial cells

Human cytomegalovirus (HCMV) infection results in endothelial dysfunction, typically known as dysregulated apoptosis, and aberrant expression and sub-cellular localization of p53, a tumor suppressor that accumulates at the late stage of infection. In this study, we examined three hypotheses that could be responsible for HCMV-induced cytoplasmic p53 accumulation at the later stage of infection: hyperactive nuclear export, cytoplasmic p53 tethering and delayed p53 degradation. Leptomycin B treatment, a nuclear export inhibitor, was unable to reduce cytoplasmic p53, thereby eliminating the hyperactive nuclear export mechanism. The findings that nascent p53 still entered nuclei after the nuclear export inhibition indicated that cytoplasmic tethering may play a minor role. Cytoplasmic p53 was still observed after the translation activities were blocked by cycloheximide. There was more than an eight-fold increase in the cytoplasmic p53 half-life with abnormal p53 ubiquitination. Taken together, these results suggest that delayed degradation could be responsible for the cytoplasmic p53 accumulation. The general slow-down of the proteasomal activity and the dysregulated p53 ubiquitination process at the later stage of infection could contribute to the reduced cytoplasmic p53 degradation and might be relevant to dysregulated endothelial apoptosis. The HCMV-induced changes in p53 dynamics could contribute to endothelial dysfunction.