The E6 oncoproteins of high-risk papillomaviruses bind to a novel putative GAP protein, E6TP1, and target it for degradation

Molecular Mechanisms Associated with the Development of the Metritis Complex in Dairy Cattle

The metritis complex (MC), a group of post-partum uterine diseases, is associated with increased treatment costs and reduced milk yield and fertility. The goal of this study was to identify genetic variants, genes, or genomic regions that modulate MC disease. A genome-wide association study was performed using a single-locus mixed linear model of 1967 genotypes (624,460 SNPs) and metritis complex records. Then, in-silico functional analyses were performed to detect biological mechanisms and pathways associated with the development of MC. The ATP8A2, COX16, AMN, and TRAF3 genes, located on chromosomes 12, 10, and 21, were associated with MC at p ≤ 0.0001. These genes are involved in the regulation of cholesterol metabolism in the stromal tissue of the uterus, which can be directly associated with the mode of transmission for pathogens causing the metritis complex. The modulation of cholesterol abundance alters the efficiency of virulence factors and may affect the susceptibility of the host to infection. The SIPA1L1, DEPDC5, and RNF122 genes were also significantly associated with MC at p ≤ 0.0001 and are involved in the PI3k-Akt pathway, responsible for activating the autophagic processes. Thus, the dysregulation of these genes allows for unhindered bacterial invasion, replication, and survival within the endometrium.

Ecdysoneless Protein Regulates Viral and Cellular mRNA Splicing to Promote Cervical Oncogenesis

High-risk human papillomaviruses (HPV), exemplified by HPV16/18, are causally linked to human cancers of the anogenital tract, skin, and upper aerodigestive tract. Previously, we identified Ecdysoneless (ECD) protein, the human homolog of the Drosophila ecdysoneless gene, as a novel HPV16 E6-interacting protein. Here, we show that ECD, through its C-terminal region, selectively binds to high-risk but not to low-risk HPV E6 proteins. We demonstrate that ECD is overexpressed in cervical and head and neck squamous cell carcinoma (HNSCC) cell lines as well as in tumor tissues. Using The Cancer Genome Atlas dataset, we show that ECD mRNA overexpression predicts shorter survival in patients with cervical and HNSCC. We demonstrate that ECD knockdown in cervical cancer cell lines led to impaired oncogenic behavior, and ECD co-overexpression with E7 immortalized primary human keratinocytes. RNA-sequencing analyses of SiHa cells upon ECD knockdown showed to aberrations in E6/E7 RNA splicing, as well as RNA splicing of several HPV oncogenesis-linked cellular genes, including splicing of components of mRNA splicing machinery itself. Taken together, our results support a novel role of ECD in viral and cellular mRNA splicing to support HPV-driven oncogenesis. IMPLICATIONS: This study links ECD overexpression to poor prognosis and shorter survival in HNSCC and cervical cancers and identifies a critical role of ECD in cervical oncogenesis through regulation of viral and cellular mRNA splicing.

The human papillomavirus oncoproteins: a review of the host pathways targeted on the road to transformation

Persistent infection with high-risk human papillomaviruses (HR-HPVs) is the causal factor in over 99 % of cervical cancer cases, and a significant proportion of oropharyngeal and anogenital cancers. The key drivers of HPV-mediated transformation are the oncoproteins E5, E6 and E7. Together, they act to prolong cell-cycle progression, delay differentiation and inhibit apoptosis in the host keratinocyte cell in order to generate an environment permissive for viral replication. The oncoproteins also have key roles in mediating evasion of the host immune response, enabling infection to persist. Moreover, prolonged infection within the cellular environment established by the HR-HPV oncoproteins can lead to the acquisition of host genetic mutations, eventually culminating in transformation to malignancy. In this review, we outline the many ways in which the HR-HPV oncoproteins manipulate the host cellular environment, focusing on how these activities can contribute to carcinogenesis.

Is viral E6 oncoprotein a viable target? A critical analysis in the context of cervical cancer

An understanding of the pathology of cervical cancer (CC) mediated by E6/E7 oncoproteins of high-risk human papillomavirus (HPV) was developed by late 80's. But if we look at the present scenario, not a single drug could be developed to inhibit these oncoproteins and in turn, be used specifically for the treatment of CC. The readers are advised not to presume the "viability of E6 protein" as mentioned in the title relates to just druggability of E6. The viability aspect will cover almost everything a researcher should know to develop E6 inhibitors until the preclinical stage. Herein, we have analysed the achievements and shortcomings of the scientific community in the last four decades in targeting HPV E6 against CC. Role of all HPV proteins has been briefly described for better perspective with a little detailed discussion of the role of E6. We have reviewed the articles from 1985 onward, reporting in vitro inhibition of E6. Recently, many computational studies have reported potent E6 inhibitors and these have also been reviewed. Subsequently, a critical analysis has been reported to cover the in vitro assay protocols and in vivo models to develop E6 inhibitors. A paragraph has been devoted to the role of public policy to fight CC employing vaccines and whether the vaccine against HPV has quenched the zeal to develop drugs against it. The review concludes with the challenges and the way forward.

Biomarkers in Cervical Cancer

Cervical cancer, a potentially preventable disease, remains the second most common malignancy in women worldwide. Human papillomavirus (HPV) is the single most important etiological agent in cervical cancer, contributing to neoplastic progression through the action of viral oncoproteins, mainly E6 and E7. Cervical screening programs using Pap smear testing have dramatically improved cervical cancer incidence and reduced deaths, but cervical cancer still remains a global health burden. The biomarker discovery for accurate detection and diagnosis of cervical carcinoma and its malignant precursors (collectively referred to as high-grade cervical disease) represents one of the current challenges in clinical medicine and cytopathology.

In silico modeling of the cryptic E2∼ubiquitin-binding site of E6-associated protein (E6AP)/UBE3A reveals the mechanism of polyubiquitin chain assembly

To understand the mechanism for assembly of Lys⁴⁸-linked polyubiquitin degradation signals, we previously demonstrated that the E6AP/UBE3A ligase harbors two functionally distinct E2∼ubiquitin-binding sites: a high-affinity Site 1 required for E6AP Cys⁸²⁰∼ubiquitin thioester formation and a canonical Site 2 responsible for subsequent chain elongation. Ordered binding to Sites 1 and 2 is here revealed by observation of UbcH7∼ubiquitin-dependent substrate inhibition of chain formation at micromolar concentrations. To understand substrate inhibition, we exploited the PatchDock algorithm to model in silico UbcH7∼ubiquitin bound to Site 1, validated by chain assembly kinetics of selected point mutants. The predicted structure buries an extensive solvent-excluded surface bringing the UbcH7∼ubiquitin thioester bond within 6 Å of the Cys⁸²⁰ nucleophile. Modeling onto the active E6AP trimer suggests that substrate inhibition arises from steric hindrance between Sites 1 and 2 of adjacent subunits. Confirmation that Sites 1 and 2 function in trans was demonstrated by examining the effect of E6APC820A on wild-type activity and single-turnover pulse-chase kinetics. A cyclic proximal indexation model proposes that Sites 1 and 2 function in tandem to assemble thioester-linked polyubiquitin chains from the proximal end attached to Cys⁸²⁰ before stochastic en bloc transfer to the target protein. Non-reducing SDS-PAGE confirms assembly of the predicted Cys⁸²⁰-linked ¹²⁵I-polyubiquitin thioester intermediate. Other studies suggest that Glu⁵⁵⁰ serves as a general base to generate the Cys⁸²⁰ thiolate within the low dielectric binding interface and Arg⁵⁰⁶ functions to orient Glu⁵⁵⁰ and to stabilize the incipient anionic transition state during thioester exchange.

Mammalian ECD Protein Is a Novel Negative Regulator of the PERK Arm of the Unfolded Protein Response

Mammalian Ecdysoneless (ECD) is a highly conserved ortholog of the DrosophilaEcd gene product whose mutations impair the synthesis of Ecdysone and produce cell-autonomous survival defects, but the mechanisms by which ECD functions are largely unknown. Here we present evidence that ECD regulates the endoplasmic reticulum (ER) stress response. ER stress induction led to a reduced ECD protein level, but this effect was not seen in PKR-like ER kinase knockout (PERK-KO) or phosphodeficient eukaryotic translation initiation factor 2α (eIF2α) mouse embryonic fibroblasts (MEFs); moreover, ECD mRNA levels were increased, suggesting impaired ECD translation as the mechanism for reduced protein levels. ECD colocalizes and coimmunoprecipitates with PERK and GRP78. ECD depletion increased the levels of both phospho-PERK (p-PERK) and p-eIF2α, and these effects were enhanced upon ER stress induction. Reciprocally, overexpression of ECD led to marked decreases in p-PERK, p-eIF2α, and ATF4 levels but robust increases in GRP78 protein levels. However, GRP78 mRNA levels were unchanged, suggesting a posttranscriptional event. Knockdown of GRP78 reversed the attenuating effect of ECD overexpression on PERK signaling. Significantly, overexpression of ECD provided a survival advantage to cells upon ER stress induction. Taken together, our data demonstrate that ECD promotes survival upon ER stress by increasing GRP78 protein levels to enhance the adaptive folding protein in the ER to attenuate PERK signaling.

Roles of Rap1 signaling in tumor cell migration and invasion

Ras-associated protein-1 (Rap1), a small GTPase in the Ras-related protein family, is an important regulator of basic cellular functions (e.g., formation and control of cell adhesions and junctions), cellular migration, and polarization. Through its interaction with other proteins, Rap1 plays many roles during cell invasion and metastasis in different cancers. The basic function of Rap1 is straightforward; it acts as a switch during cellular signaling transduction and regulated by its binding to either guanosine triphosphate (GTP) or guanosine diphosphate (GDP). However, its remarkably diverse function is rendered by its interplay with a large number of distinct Rap guanine nucleotide exchange factors and Rap GTPase activating proteins. This review summarizes the mechanisms by which Rap1 signaling can regulate cell invasion and metastasis, focusing on its roles in integrin and cadherin regulation, Rho GTPase control, and matrix metalloproteinase expression.

Comparative expression study of sipa family members during early Xenopus laevis development

The signal-induced proliferation-associated (SIPA) protein family belongs to the RapGAP protein superfamily. Previous studies mainly focused on the expression and function of SIPA genes in vertebrate neuronal tissue. Only limited data about the embryonic expression pattern of the genes are currently available. Our study provides the first expression analysis of sipa1, sipa1l1, sipa1l2, and sipa1l3 during early development of the vertebrate organism Xenopus laevis. In silico, analysis revealed that all genes are highly conserved across species. Semi-quantitative RT-PCR experiments demonstrated that the RNA of all genes was maternally supplied. By whole mount in situ hybridization approaches, we showed that sipa1 is mainly expressed in various sensory organs, the respiratory and blood system, heart, neural tube, and eye. In contrast, sipa1l1 showed a broad expression during development in particular within the brain, somites, eye, and heart. Sipa1l2 was detected in the branchial arches, glomerulus, and the developing eye. In contrast, sipa1l3 revealed a tissue specific expression within the olfactory and otic vesicles, the cranial placodes and ganglia, neural tube, pronephros, retina, and lens. In summary, all sipa gene family members are expressed throughout the whole developing Xenopus organism and might play an important role during vertebrate early embryogenesis.

HPV16 E6 upregulates Aurora A expression

Overexpression of Aurora A kinase occurs in certain types of cancer, and therefore results in chromosome instability and phosphorylation-mediated ubiquitylation and degradation of p53 for tumorigenesis. The high-risk subtype human papillomavirus (HPV)16 early oncoprotein E6 is a major contributor inducing host cell immortalization and transformation through interaction with a number of cellular factors. In the present study, co-immunoprecipitation, glutathione S-transferase pull-down and immunostaining were used to show that HPV16 E6 and Aurora A bind to each other in vivo and in vitro. Western blotting and reverse transcription-polymerase chain reaction were used to reveal that HPV16 E6 inhibited cell apoptosis by stabilizing Aurora A expression. The present study may report a new mechanism for the involvement of HPV16 E6 in carcinogenesis, as HPV16 E6 elevates Aurora A expression and the latter may be a common target for oncogenic viruses that result in cell carcinogenesis.

Sipa1l1 is an early biomarker of liver fibrosis in CCl4-treated rats

At present, several procedures are used for staging liver fibrosis. However, these methods may involve clinical complications and/or present diagnostic uncertainty mainly in the early stages of the disease. Thus, this study was designed to unveil new non-invasive biomarkers of liver fibrosis in an in vivo model of fibrosis/cirrhosis induction by CCl₄ inhalation by using a label-free quantitative LC-MS/MS approach. We analyzed 94 serum samples from adult Wistar rats with different degrees of liver fibrosis and 36 control rats. Firstly, serum samples from 18 CCl₄-treated rats were clustered into three different groups according to the severity of hepatic and the serum proteome was characterized by label-free LC-MS/MS. Furthermore, three different pooled serum samples obtained from 16 control Wistar rats were also analyzed. Based on the proteomic data obtained, we performed a multivariate analysis which displayed three main cell signaling pathways altered in fibrosis. In cirrhosis, more biological imbalances were detected as well as multi-organ alterations. In addition, hemopexin and signal-induced proliferation-associated 1 like 1 (SIPA1L1) were selected as potential serum markers of liver fibrogenesis among all the analyzed proteins. The results were validated by ELISA in an independent group of 76 fibrotic/cirrhotic rats and 20 controls which confirmed SIPA1L1 as a potential non-invasive biomarker of liver fibrosis. In particular, SIPA1L1 showed a clear diminution in serum samples from fibrotic/cirrhotic rats and a great accuracy at identifying early fibrotic stages. In conclusion, the proteomic analysis of serum samples from CCl₄-treated rats has enabled the identification of SIPA1L1 as a non-invasive marker of early liver fibrosis.

Summary: Proteomic analysis of serum samples from fibrotic and cirrhotic rats revealed a reduction in SIPA1L1 concentration with potential use as a marker for early liver fibrosis diagnosis.

Amnion as a surrogate tissue reporter of the effects of maternal preeclampsia on the fetus

BACKGROUND

Preeclampsia, traditionally characterized by high blood pressure and proteinuria, is a common pregnancy complication, which affects 2-8 % of all pregnancies. Although children born to women with preeclampsia have a higher risk of hypertension in later life, the mechanism of this increased risk is unknown. DNA methylation is an epigenetic modification that has been studied as a mediator of cellular memory of adverse exposures in utero. Since each cell type in the body has a unique DNA profile, cell subtype composition is a major confounding factor in studies of tissues with heterogeneous cell types. The best way to avoid this confounding effect is by using purified cell types. However, using purified cell types in large cohort translational studies is difficult. The amnion, the inner layer of the fetal membranes of the placenta, is derived from the epiblast and consists of two cell types, which are easy to isolate from the delivered placenta. In this study, we demonstrate the value of using amnion samples for DNA methylation studies, revealing distinctive patterns between fetuses exposed to proteinuria or hypertension and fetuses from normal pregnancies.

RESULTS

We performed a genome-wide DNA methylation analysis, HpaII tiny fragment Enrichment by Ligation-mediated PCR (HELP)-tagging, on 62 amnion samples from the placentas of uncomplicated, normal pregnancies and from those with complications of preeclampsia or hypertension. Using a regression model approach, we found 123, 85, and 99 loci with high-confidence hypertension-associated, proteinuria-associated, and hypertension- and proteinuria-associated DNA methylation changes, respectively. A gene ontology analysis showed DNA methylation changes to be selecting genes with different biological processes in exposure status. We also found that these differentially methylated regions overlap loci previously reported as differentially methylated regions in preeclampsia.

CONCLUSIONS

Our findings support prior observations that preeclampsia is associated with changes of DNA methylation near genes that have previously been found to be dysregulated in preeclampsia. We propose that amniotic membranes represent a valuable surrogate fetal tissue on which to perform epigenome-wide association studies of adverse intrauterine conditions.

Testing for Ancient Selection Using Cross-population Allele Frequency Differentiation

A powerful way to detect selection in a population is by modeling local allele frequency changes in a particular region of the genome under scenarios of selection and neutrality and finding which model is most compatible with the data. A previous method based on a cross-population composite likelihood ratio (XP-CLR) uses an outgroup population to detect departures from neutrality that could be compatible with hard or soft sweeps, at linked sites near a beneficial allele. However, this method is most sensitive to recent selection and may miss selective events that happened a long time ago. To overcome this, we developed an extension of XP-CLR that jointly models the behavior of a selected allele in a three-population tree. Our method - called "3-population composite likelihood ratio" (3P-CLR) - outperforms XP-CLR when testing for selection that occurred before two populations split from each other and can distinguish between those events and events that occurred specifically in each of the populations after the split. We applied our new test to population genomic data from the 1000 Genomes Project, to search for selective sweeps that occurred before the split of Yoruba and Eurasians, but after their split from Neanderthals, and that could have led to the spread of modern-human-specific phenotypes. We also searched for sweep events that occurred in East Asians, Europeans, and the ancestors of both populations, after their split from Yoruba. In both cases, we are able to confirm a number of regions identified by previous methods and find several new candidates for selection in recent and ancient times. For some of these, we also find suggestive functional mutations that may have driven the selective events.

Novel Functions of the Human Papillomavirus E6 Oncoproteins

Human papillomaviruses (HPVs) infect the epidermis as well as mucous membranes of humans. They are the causative agents of anogenital tract and some oropharyngeal cancers. Infections begin in the basal epithelia, where the viral genome replicates slowly along with its host cell. As infected cells begin to differentiate and progress toward the periphery, the virus drives proliferation in cells that would otherwise be quiescent. To uncouple differentiation from continued cellular propagation, HPVs express two oncoproteins, HPV E6 and E7. This review focuses on high-risk α-HPV E6, which in addition to supporting viral replication has transforming properties. HPV E6 promotes p53 degradation and activates telomerase, but the multifaceted oncoprotein has numerous other functions that are highlighted here.

Viral Mimicry to Usurp Ubiquitin and SUMO Host Pathways

Posttranslational modifications (PTMs) of proteins include enzymatic changes by covalent addition of cellular regulatory determinants such as ubiquitin (Ub) and small ubiquitin-like modifier (SUMO) moieties. These modifications are widely used by eukaryotic cells to control the functional repertoire of proteins. Over the last decade, it became apparent that the repertoire of ubiquitiylation and SUMOylation regulating various biological functions is not restricted to eukaryotic cells, but is also a feature of human virus families, used to extensively exploit complex host-cell networks and homeostasis. Intriguingly, besides binding to host SUMO/Ub control proteins and interfering with the respective enzymatic cascade, many viral proteins mimic key regulatory factors to usurp this host machinery and promote efficient viral outcomes. Advanced detection methods and functional studies of ubiquitiylation and SUMOylation during virus-host interplay have revealed that human viruses have evolved a large arsenal of strategies to exploit these specific PTM processes. In this review, we highlight the known viral analogs orchestrating ubiquitin and SUMO conjugation events to subvert and utilize basic enzymatic pathways.

RAP1 GTPase overexpression is associated with cervical intraepithelial neoplasia

RAP1 (RAS proximate 1), a small GTP-binding protein of the RAS superfamily, is a putative oncogene that is highly expressed in several malignant cell lines and types of cancers, including some types of squamous cell carcinoma. However, the participation of RAP1 in cervical carcinogenesis is unknown. We conducted a cross-sectional study of paraffin-embedded cervical biopsies to determine the association of RAP1 with cervical intraepithelial neoplasia (CIN). Standard and quantitative immunohistochemistry assessment of RAP1 expression in fixed tissue was performed on 183 paraffin-embedded cervical biopsies that were classified as normal or non-dysplastic mucosa (NDM) (n = 33); CIN grade 1 (n = 84) and CIN grade 2/3 (n = 66). A gradual increase in RAP1 expression in NDM < CIN 1 < CIN 2/3 (p<0.001) specimens was observed and was in agreement with the histopathologic diagnosis. A progressive increase in the RAP1 expression levels increased the risk of CIN 1 [odds ratio (OR) = 3.50; 95% confidence interval (CI) 1.30-10.64] 3.5 fold and the risk of CIN 2/3 (OR = 19.86, 95% CI 6.40-70.79) nearly 20 fold when compared to NDM. In addition, stereotype ordinal regression analysis showed that this progressive increase in RAP1 expression more strongly impacted CIN 2/3 than CIN 1. Our findings suggest that RAP1 may be a useful biomarker for the diagnosis of CIN.

Oncogenes and RNA splicing of human tumor viruses

Approximately 10.8% of human cancers are associated with infection by an oncogenic virus. These viruses include human papillomavirus (HPV), Epstein–Barr virus (EBV), Merkel cell polyomavirus (MCV), human T-cell leukemia virus 1 (HTLV-1), Kaposi's sarcoma-associated herpesvirus (KSHV), hepatitis C virus (HCV) and hepatitis B virus (HBV). These oncogenic viruses, with the exception of HCV, require the host RNA splicing machinery in order to exercise their oncogenic activities, a strategy that allows the viruses to efficiently export and stabilize viral RNA and to produce spliced RNA isoforms from a bicistronic or polycistronic RNA transcript for efficient protein translation. Infection with a tumor virus affects the expression of host genes, including host RNA splicing factors, which play a key role in regulating viral RNA splicing of oncogene transcripts. A current prospective focus is to explore how alternative RNA splicing and the expression of viral oncogenes take place in a cell- or tissue-specific manner in virus-induced human carcinogenesis.

Prediction of intrinsic disorder in proteins using MFDp2

Intrinsically disordered proteins (IDPs) are either entirely disordered or contain disordered regions in their native state. IDPs were found to be abundant across all kingdoms of life, particularly in eukaryotes, and are implicated in numerous cellular processes. Experimental annotation of disorder lags behind the rapidly growing sizes of the protein databases and thus computational methods are used to close this gap and to investigate the disorder. MFDp2 is a novel webserver for accurate sequence-based prediction of protein disorder which also outputs well-described sequence-derived information that allows profiling the predicted disorder. We conveniently visualize sequence conservation, predicted secondary structure, relative solvent accessibility, and alignments to chains with annotated disorder. The webserver allows predictions for multiple proteins at the same time, includes help pages and tutorial, and the results can be downloaded as text-based (parsable) file. MFDp2 is freely available at http://biomine.ece.ualberta.ca/MFDp2/.

Target-based molecular signature characteristics of cervical adenocarcinoma and squamous cell carcinoma

There is an urgent need for molecular marker studies of adenocarcinoma (AC) and squamous cell carcinoma (SCC) of the uterine cervix. This study utilized oligomicroarray and pathway analyses to characterize a transcriptomic signature with molecular networks associated with AC and SCC. A 10K oligomicroarray was used to identify potential transcripts that were differentially expressed in cervical cancers from 28 patients and common reference RNAs from 17 different normal cervixes. Molecular networks were correlated using genomics tools to globally explore cellular pathways. Gene expression levels of 46 transcripts separated cancer samples into AC and SCC groups. Genes including: KRT17, IGFBP2, CALCA and VIPR1 were differentially expressed in AC and SCC. In addition, we identified a transcriptomic signature that predicted tumor classification and progression based upon its cellular processes. The downregulated signatures for SCC were cell death of pheochromocytoma cells (P=0.0037), apoptosis of neurons (P=0.009) and damage to DNA (P=0.0038). By contrast, the upregulated molecular signatures in AC were immunological disorder (P=0.006), splenomegaly (P=0.0053) and hepatic system disorder (P=0.006). The G2/M DNA damage checkpoint regulation pathway (P=0.05) was found to be significantly linked to IGF1R as a new regulatory component of a putative cytoplasmic signaling cascade in SCC. By contrast, the antigen presenting canonical pathway (P=0.038) appeared to be linked to PPARγ in AC. Taken together, these experiments provide important new information regarding the role of molecular networks in mediating SCC and AC, possibly through two independent pathways, and contribute to provide new targets for the prevention and treatment of cervical cancer.

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.

Research resource: identification of novel coregulators specific for thyroid hormone receptor-β2

Thyroid hormone receptors (TRs) are expressed as a series of interrelated isoforms that perform distinct biological roles. The TRβ2 isoform is found predominantly in the hypothalamus, pituitary, retina, and cochlea and displays unique transcriptional properties relative to the other TR isoforms. To more fully understand the isoform-specific biological and molecular properties of TRβ2, we have identified a series of previously unrecognized proteins that selectively interact with TRβ2 compared with the more widely expressed TRβ1. Several of these proteins preferentially enhance the transcriptional activity of TRβ2 when coexpressed in cells and are likely to represent novel, isoform-specific coactivators. Additional proteins were also identified in our screen that bind equally to TRβ1 and TRβ2 and may function as isoform-independent auxiliary proteins for these and/or other nuclear receptors. We propose that a combination of isoform-specific recruitment and tissue-specific expression of these newly identified coregulator candidates serves to customize TR function for different biological purposes in different cell types.

Breast cancer subtypes: two decades of journey from cell culture to patients

Recent molecular profiling has identified six major subtypes of breast cancers that exhibit different survival outcomes for patients. To address the origin of different subtypes of breast cancers, we have now identified, isolated, and immortalized (using hTERT) mammary stem/progenitor cells which maintain their stem/progenitor properties even after immortalization. Our decade long research has shown that these stem/progenitor cells are highly susceptible to oncogenesis. Given the emerging evidence that stem/progenitor cells are precursors of cancers and that distinct subtypes of breast cancer have different survival outcome, these cellular models provide novel tools to understand the oncogenic process leading to various subtypes of breast cancers and for future development of novel therapeutic strategies to treat different subtypes of breast cancers.

Cellular transformation by human papillomaviruses: lessons learned by comparing high- and low-risk viruses

The oncogenic potential of papillomaviruses (PVs) has been appreciated since the 1930s yet the mechanisms of virally-mediated cellular transformation are still being revealed. Reasons for this include: a) the oncoproteins are multifunctional, b) there is an ever-growing list of cellular interacting proteins, c) more than one cellular protein may bind to a given region of the oncoprotein, and d) there is only limited information on the proteins encoded by the corresponding non-oncogenic PVs. The perspective of this review will be to contrast the activities of the viral E6 and E7 proteins encoded by the oncogenic human PVs (termed high-risk HPVs) to those encoded by their non-oncogenic counterparts (termed low-risk HPVs) in an attempt to sort out viral life cycle-related functions from oncogenic functions. The review will emphasize lessons learned from the cell culture studies of the HPVs causing mucosal/genital tract cancers.

Regulating Rap small G-proteins in time and space

Signaling by the small G-protein Rap is under tight regulation by its GEFs and GAPs. These are multi-domain proteins that are themselves controlled by distinct upstream pathways, and thus couple different extra- and intracellular cues to Rap. The individual RapGEFs and RapGAPs are, in addition, targeted to specific cellular locations by numerous anchoring mechanisms and, consequently, may control different pools of Rap. Here, we review the various activating signals and targeting mechanisms of these proteins and discuss their contribution to the spatiotemporal regulation and biological functions of the Rap proteins.

Histone deacetylase inhibitors upregulate Rap1GAP and inhibit Rap activity in thyroid tumor cells

Increases in Rap activity have been associated with tumor progression. Although activating mutations in Rap have not been described, downregulation of Rap1GAP is frequent in human tumors including thyroid carcinomas. In this study, we explored whether endogenous Rap1GAP expression could be restored to thyroid tumor cells. The effects of deacetylase inhibitors and a demethylating agent, individually and in combination, were examined in four differentiated and six anaplastic thyroid carcinoma (ATC) cell lines. Treatment with the structurally distinct histone deacetylase (HDAC) inhibitors, sodium butyrate and trichostatin A, increased Rap1GAP expression in all the differentiated thyroid carcinoma cell lines and in four of the six ATC cell lines. The demethylating agent, 5-aza-deoxycytidine, restored Rap1GAP expression in one anaplastic cell line and enhanced the effects of HDAC inhibitors in a second anaplastic cell line. Western blotting indicated that Rap2 was highly expressed in human thyroid cancer cells. Importantly, treatment with HDAC inhibitors impaired Rap2 activity in both differentiated and anaplastic tumor cell lines. The mechanism through which Rap activity is repressed appears to entail effects on the expression of multiple Rap regulators, including RapGEFs and RapGAPs. These results suggest that HDAC inhibitors may provide a tractable approach to impair Rap activity in human tumor cells.

Viral oncogenes, noncoding RNAs, and RNA splicing in human tumor viruses

Viral oncogenes are responsible for oncogenesis resulting from persistent virus infection. Although different human tumor viruses express different viral oncogenes and induce different tumors, their oncoproteins often target similar sets of cellular tumor suppressors or signal pathways to immortalize and/or transform infected cells. Expression of the viral E6 and E7 oncogenes in papillomavirus, E1A and E1B oncogenes in adenovirus, large T and small t antigen in polyomavirus, and Tax oncogene in HTLV-1 are regulated by alternative RNA splicing. However, this regulation is only partially understood. DNA tumor viruses also encode noncoding RNAs, including viral microRNAs, that disturb normal cell functions. Among the determined viral microRNA precursors, EBV encodes 25 from two major clusters (BART and BHRF1), KSHV encodes 12 from a latent region, human polyomavirus MCV produce only one microRNA from the late region antisense to early transcripts, but HPVs appears to produce no viral microRNAs.

The human papillomavirus type 16 E6 oncoprotein activates mTORC1 signaling and increases protein synthesis

The mammalian target of rapamycin (mTOR) kinase acts as a cellular rheostat that integrates signals from a variety of cellular signal transduction pathways that sense growth factor and nutrient availability as well as intracellular energy status. It was previously reported that the human papillomavirus type 16 (HPV16) E6 oncoprotein may activate the S6 protein kinase (S6K) through binding and E6AP-mediated degradation of the mTOR inhibitor tuberous sclerosis complex 2 (TSC2) (Z. Lu, X. Hu, Y. Li, L. Zheng, Y. Zhou, H. Jiang, T. Ning, Z. Basang, C. Zhang, and Y. Ke, J. Biol. Chem. 279:35664-35670, 2004; L. Zheng, H. Ding, Z. Lu, Y. Li, Y. Pan, T. Ning, and Y. Ke, Genes Cells 13:285-294, 2008). Our results confirmed that HPV16 E6 expression causes an increase in mTORC1 activity through enhanced phosphorylation of mTOR and activation of downstream signaling pathways S6K and eukaryotic initiation factor binding protein 1 (4E-BP1). However, we did not detect a decrease in TSC2 levels in HPV16 E6-expressing cells. We discovered, however, that HPV16 E6 expression causes AKT activation through the upstream kinases PDK1 and mTORC2 under conditions of nutrient deprivation. We show that HPV16 E6 expression causes an increase in protein synthesis by enhancing translation initiation complex assembly at the 5' mRNA cap and an increase in cap-dependent translation. The increase in cap-dependent translation likely results from HPV16 E6-induced AKT/mTORC1 activation, as the assembly of the translation initiation complex and cap-dependent translation are rapamycin sensitive. Lastly, coexpression of the HPV16 E6 and E7 oncoproteins does not affect HPV16 E6-induced activation of mTORC1 and cap-dependent translation. HPV16 E6-mediated activation of mTORC1 signaling and cap-dependent translation may be a mechanism to promote viral replication under conditions of limited nutrient supply in differentiated, HPV oncoprotein-expressing proliferating cells.

RapGAPs in brain: multipurpose players in neuronal Rap signalling

Small Rap guanosine-tri-phosphate (GTP)ases are crucially involved in many cellular processes, including cell proliferation, differentiation, survival, adhesion and movement. In line, it has been shown that Rap signalling is involved in various aspects of neuronal differentiation, like the establishment of neuronal polarity or axonal growth cone movement. Rap GTPases can be activated by a wide variety of external stimuli, and this is mediated by specific guanine nucleotide exchange factors (RapGEFs). Inactivation of RapGTP can be achieved with the aid of specific GTPase-activating proteins (RapGAPs). In the brain, the most prominent RapGAPs are Rap1GAP and those of the spine-associated RapGAP (SPAR) family. This latter family consists of three members (SPAR1-3), from which two of them, namely SPAR1 and 2, have been investigated in more detail. As such, the localization of RapGAPs is crucially important in regulating Rap signalling at various sites in the cell and, for both SPAR1 and 2, enrichment at synaptic sites has been demonstrated. In recent years particularly the role of SPAR1 in shaping dendritic spine morphology has attracted considerable interest. In this review we will summarize the described actions of different RapGAPs expressed in the brain, and we will focus in particular on the SPAR family members.

Downregulation of Rap1GAP in human tumor cells alters cell/matrix and cell/cell adhesion

Rap1GAP expression is decreased in human tumors. The significance of its downregulation is unknown. We show that Rap1GAP expression is decreased in primary colorectal carcinomas. To elucidate the advantages conferred on tumor cells by loss of Rap1GAP, Rap1GAP expression was silenced in human colon carcinoma cells. Suppressing Rap1GAP induced profound alterations in cell adhesion. Rap1GAP-depleted cells exhibited defects in cell/cell adhesion that included an aberrant distribution of adherens junction proteins. Depletion of Rap1GAP enhanced adhesion and spreading on collagen. Silencing of Rap expression normalized spreading and restored E-cadherin, beta-catenin, and p120-catenin to cell/cell contacts, indicating that unrestrained Rap activity underlies the alterations in cell adhesion. The defects in adherens junction protein distribution required integrin signaling as E-cadherin and p120-catenin were restored at cell/cell contacts when cells were plated on poly-l-lysine. Unexpectedly, Src activity was increased in Rap1GAP-depleted cells. Inhibition of Src impaired spreading and restored E-cadherin at cell/cell contacts. These findings provide the first evidence that Rap1GAP contributes to cell/cell adhesion and highlight a role for Rap1GAP in regulating cell/matrix and cell/cell adhesion. The frequent downregulation of Rap1GAP in epithelial tumors where alterations in cell/cell and cell/matrix adhesion are early steps in tumor dissemination supports a role for Rap1GAP depletion in tumor progression.

Rap1, a mercenary among the Ras-like GTPases

The small Ras-like GTPase Rap1 is an evolutionary conserved protein that originally gained interest because of its capacity to revert the morphological phenotype of Ras-transformed fibroblasts. Rap1 is regulated by a large number of stimuli that include growth factors and cytokines, but also physical force and osmotic stress. Downstream of Rap1, a plethora of effector molecules has been proposed on the basis of biochemical studies. Here, we present an overview of genetic studies on Rap1 in various model organisms and relate the observed phenotypes to in vitro studies. The picture that emerges is one in which Rap1 is a versatile regulator of morphogenesis, by regulating diverse processes that include establishment of cellular polarity, cell-matrix interactions and cell-cell adhesion. Surprisingly, genetic experiments indicate that in the various model organisms, Rap1 uses distinct effector molecules that impinge upon the actin cytoskeleton and adhesion molecules.

Evolution of human papillomavirus carcinogenicity

Members of the Alphapapillomavirus genus are the causative agent for virtually all cases of cervical cancer. However, strains (commonly referred to as types) within this genus span the entire range of pathogenicity from highly carcinogenic (e.g., HPV16, odds ratio = 281.9, responsible for 50% of all cervical cancers), moderately carcinogenic (e.g., HPV31) to not carcinogenic (e.g., HPV71). The persistent expression of the viral oncoproteins (E6 and E7) from HPV16 has been shown to be necessary and sufficient to transform primary human keratinocytes in vitro. A plethora of functions have been described for both oncoproteins, and through functional comparisons between HPV16 and HPV6, a subset of these functions have been suggested to be oncogenic. However, extrapolating functional differences from these comparisons is unlikely to tease apart the fine details. In this review, we argue that a thorough understanding of the molecular mechanisms differentiating oncogenic from nononcogenic types should be obtained by performing functional assays in an evolutionary and epidemiological framework. We continue by interpreting some recent results using this paradigm and end by suggesting directions for future inquiries.

The ubiquitin-specific peptidase USP15 regulates human papillomavirus type 16 E6 protein stability

Proteomic identification of human papillomavirus type 16 (HPV16) E6-interacting proteins revealed several proteins involved in ubiquitin-mediated proteolysis. In addition to the well-characterized E6AP ubiquitin-protein ligase, a second HECT domain protein (HERC2) and a deubiquitylating enzyme (USP15) were identified by tandem affinity purification of HPV16 E6-associated proteins. This study focuses on the functional consequences of the interaction of E6 with USP15. Overexpression of USP15 resulted in increased levels of the E6 protein, and the small interfering RNA-mediated knockdown of USP15 decreased E6 protein levels. These results implicate USP15 directly in the regulation of E6 protein stability and suggest that ubiquitylated E6 could be a substrate for USP15 ubiquitin peptidase activity. It remains possible that E6 could affect the activity of USP15 on specific cellular substrates, a hypothesis that can be tested as more is learned about the substrates and pathways controlled by USP15.

The ability of GAP1IP4BP to function as a Rap1 GTPase-activating protein (GAP) requires its Ras GAP-related domain and an arginine finger rather than an asparagine thumb

GAP1(IP4BP) is a member of the GAP1 family of Ras GTPase-activating proteins (GAPs) that includes GAP1(m), CAPRI, and RASAL. Composed of a central Ras GAP-related domain (RasGRD), surrounded by amino-terminal C2 domains and a carboxy-terminal PH/Btk domain, these proteins, with the notable exception of GAP1(m), possess an unexpected arginine finger-dependent GAP activity on the Ras-related protein Rap1 (S. Kupzig, D. Deaconescu, D. Bouyoucef, S. A. Walker, Q. Liu, C. L. Polte, O. Daumke, T. Ishizaki, P. J. Lockyer, A. Wittinghofer, and P. J. Cullen, J. Biol. Chem. 281:9891-9900, 2006). Here, we have examined the mechanism through which GAP1(IP4BP) can function as a Rap1 GAP. We show that deletion of domains on either side of the RasGRD, while not affecting Ras GAP activity, do dramatically perturb Rap1 GAP activity. By utilizing GAP1(IP4BP)/GAP1(m) chimeras, we establish that although the C2 and PH/Btk domains are required to stabilize the RasGRD, it is this domain which contains the catalytic machinery required for Rap1 GAP activity. Finally, a key residue in Rap1-specific GAPs is a catalytic asparagine, the so-called asparagine thumb. By generating a molecular model describing the predicted Rap1-binding site in the RasGRD of GAP1(IP4BP), we show that mutagenesis of individual asparagine or glutamine residues that lie in close proximity to the predicted binding site has no detectable effect on the in vivo Rap1 GAP activity of GAP1(IP4BP). In contrast, we present evidence consistent with a model in which the RasGRD of GAP1(IP4BP) functions to stabilize the switch II region of Rap1, allowing stabilization of the transition state during GTP hydrolysis initiated by the arginine finger.

HPV16 E6-induced and E6AP-dependent inhibition of the transcriptional coactivator hADA3 in human cervical carcinoma cells

To determine whether there exists an additional pathway of E6 that is independent of direct P53 degradation and whether hADA3, a transcriptional coactivator, is involved in this process. We investigated the association between E6 and hADA3, as well as E6-associated protein (E6AP) and hADA3, in SiHa cells via RNA interference technique. Our results showed that the expression of hADA3 protein was significantly increased, cellular proliferation was decreased and apoptotic rate was increased in SiHa cells treated by E6 siRNA and E6AP siRNA respectively. Our results suggested that oncoprotein E6 inhibits hADA3 in cervical cancer cells and this process is E6AP-dependent.

Loss of Rap1GAP in papillary thyroid cancer

CONTEXT

Rap1 GTPase-activating protein (GAP) regulates the activity of Rap1, a putative oncogene. We previously reported Rap1GAP was highly expressed in normal human thyroid cells and decreased in five papillary thyroid carcinomas (PTCs).

OBJECTIVES

To confirm the significance of these findings, we analyzed Rap1GAP expression in a larger set of benign tumors (adenomas and hyperplastic nodules) and PTCs. We determined whether the presence of the BRAF(V600E) mutation or allelic loss of Rap1GAP related to changes in Rap1GAP protein expression. To determine the consequences of Rap1GAP loss, we targeted Rap1GAP in culture using small interfering RNA.

DESIGN, PATIENTS, AND METHODS

A highly specific Rap1GAP antibody was applied to sections of 55 human thyroid tissues. Genomic DNA was analyzed for the presence of the BRAF(V600E) mutation, and loss of Rap1GAP. Rap1GAP expression in rat thyroid cells was abolished using small interfering RNA.

RESULTS

We observed that down-regulation of Rap1GAP in benign lesions and PTCs was common. Rap1GAP expression was more severely decreased in PTCs. Loss of Rap1GAP expression was observed in multiple histological variants of PTCs. Approximately 20% of PTCs and adenomas exhibited allelic loss of Rap1GAP. Loss of Rap1GAP was not associated with the presence of the BRAF(V600E) mutation. In vitro, loss of Rap1GAP was sufficient to increase Rap1 activity in thyroid cells.

CONCLUSIONS

These data indicate that loss of Rap1GAP is a frequent event in PTC. The more frequent and greater down-regulation of Rap1GAP in PTCs compared with adenomas suggests a role for Rap1GAP depletion in the progression of human thyroid tumors, possibly through unrestrained Rap activity.

Papillomavirus E6 proteins

The papillomaviruses are small DNA viruses that encode approximately eight genes, and require the host cell DNA replication machinery for their viral DNA replication. Thus papillomaviruses have evolved strategies to induce host cell DNA synthesis balanced with strategies to protect the cell from unscheduled replication. While the papillomavirus E1 and E2 genes are directly involved in viral replication by binding to and unwinding the origin of replication, the E6 and E7 proteins have auxillary functions that promote proliferation. As a consequence of disrupting the normal checkpoints that regulate cell cycle entry and progression, the E6 and E7 proteins play a key role in the oncogenic properties of human papillomaviruses with a high risk of causing anogenital cancers (HR HPVs). As a consequence, E6 and E7 of HR HPVs are invariably expressed in cervical cancers. This article will focus on the E6 protein and its numerous activities including inactivating p53, blocking apoptosis, activating telomerase, disrupting cell adhesion, polarity and epithelial differentiation, altering transcription and reducing immune recognition.

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.

Human papillomavirus and molecular considerations for cancer risk

Human papillomaviruses (HPVs) are a major cause of cancer globally, including cervical cancer. The HPV 'early' proteins, E6 and E7, are the chief oncoproteins involved in cancer progression. These oncoproteins are more highly expressed in high-grade dysplasias and invasive cancer coincident with reduced viral DNA replication and reduced production of infective progeny virions. The E6 and E7 oncoproteins interact with several cellular proteins-classically TP53 and RB1, respectively-leading to the degradation of several of these proteins, although all interactions do not necessarily result in the degradation of a cellular protein. HPV infection is also associated with viral and host DNA methylation changes, many of which also occur in cancer types not associated with HPV infection. The E6 and E7 interactions with cellular proteins and DNA methylation changes are associated with changes in the integrity of key cellular pathways that regulate genomic integrity, cell adhesion, the immune response, apoptosis, and cell cycle control. The alterations in key cellular pathways may provide useful biomarkers to improve the sensitivity of current cancer screening methods, such as the Papanicolaou test. This review provides a detailed summary of the interactions of E6 and E7 with cellular proteins and alterations in cellular DNA methylation associated with HPV infection. The importance of molecular biomarkers to the clinical setting, underserved populations, and general public health is discussed.

Human papilloma virus (HPV) and host cellular interactions

Viral-induced carcinogenesis has been attributed to the ability of viral oncoproteins to target and interact with the host cellular proteins. It is generally accepted that Human papilloma virus (HPV) E6 and E7 function as the dominant oncoproteins of 'high-risk' HPVs by altering the function of critical cellular proteins. Initially it was shown that HPV E6 enhances the degradation of p53, while HPV E7 inactivates the function of the retinoblastoma tumor suppressor protein Rb. However, recent studies during the last decade have identified a number of additional host cellular targets of both HPV E6 and E7 that may also play an important role in malignant cellular transformation. In this review we present the interactions of HPV E6 and E7 with the host cellular target proteins. We also present the role of DNA integration in the malignant transformation of the epithelial cell.

Cellular binding partners of the human papillomavirus E6 protein

The high-risk strains of human papillomavirus (HR-HPV) are known to be causative agents of cervical cancer and have recently also been implicated in cancers of the oropharynx. E6 is a potent oncogene of HR-HPVs, and its role in the progression to malignancy has been and continues to be explored. E6 is known to interact with and subsequently inactivate numerous cellular proteins pivotal in the mediation of apoptosis, transcription of tumor suppressor genes, maintenance of epithelial organization, and control of cell proliferation. Binding of E6 to these proteins cumulatively contributes to the oncogenic potential of HPV. This paper provides an overview of these cellular protein partners of HR-E6, the motifs known to mediate oncoprotein binding, and the agents that have the potential to interfere with E6 expression and activity and thus prevent the subsequent progression to oncogenesis.

The EphA4 receptor regulates neuronal morphology through SPAR-mediated inactivation of Rap GTPases

Eph receptors play critical roles in the establishment and remodeling of neuronal connections, but the signaling pathways involved are not fully understood. We have identified a novel interaction between the C terminus of the EphA4 receptor and the PDZ domain of the GTPase-activating protein spine-associated RapGAP (SPAR). In neuronal cells, this binding mediates EphA4-dependent inactivation of the closely related GTPases Rap1 and Rap2, which have recently been implicated in the regulation of dendritic spine morphology and synaptic plasticity. We show that SPAR-mediated inactivation of Rap1, but not Rap2, is critical for ephrin-A-dependent growth cone collapse in hippocampal neurons and decreased integrin-mediated adhesion in neuronal cells. Distinctive effects of constitutively active Rap1 and Rap2 on the morphology of growth cones and dendritic spines support the idea that these two GTPases have different functions in neurons. Together, our data implicate SPAR as an important signaling intermediate that links the EphA4 receptor with Rap GTPase function in the regulation of neuronal morphology.

The differential staurosporine-mediated G1 arrest in normal versus tumor cells is dependent on the retinoblastoma protein

Previously, we reported that breast cancer cells with retinoblastoma (pRb) pathway-defective checkpoints can be specifically targeted with chemotherapeutic agents, following staurosporine-mediated reversible growth inhibition in normal cells. Here we set out to determine if the kinetics of staurosporine-mediated growth inhibition is specifically targeted to the G(1) phase of cells, and if such G(1) arrest requires the activity of wild-type pRb. Normal human mammary epithelial and immortalized cells with intact pRb treated with low concentrations of staurosporine arrested in the G(1) phase of the cell cycle, whereas pRb-defective cells showed no response. The duration of G(1) and transition from G(1) to S phase entry were modulated by staurosporine in Rb-intact cells. In pRb(+) cells, but not in Rb(-) cells, low concentrations of staurosporine also resulted in a significant decrease in cyclin-dependent kinase 4 (CDK4) expression and activity. To directly assess the role of pRb in staurosporine-mediated G(1) arrest, we subjected wild-type (Rb(+/+)) and pRb(-/-) mouse embryo fibroblasts (MEFs) to staurosporine treatments. Our results show that whereas Rb(+/+) MEFs were particularly sensitive to G(1) arrest mediated by staurosporine, pRb(-/-) cells were refractory to such treatment. Additionally, CDK4 expression was also inhibited in response to staurosporine only in Rb(+/+) MEFs. These results were recapitulated in breast cancer cells treated with siRNA to pRb to down-regulate the pRb expression. Collectively, our data suggest that treatment of cells with nanomolar concentrations of staurosporine resulted in down-regulation of CDK4, which ultimately leads to G(1) arrest in normal human mammary epithelial and immortalized cells with an intact pRb pathway, but not in pRb-null/defective cells.

Downregulation of Rap1GAP contributes to Ras transformation

Although abundant in well-differentiated rat thyroid cells, Rap1GAP expression was extinguished in a subset of human thyroid tumor-derived cell lines. Intriguingly, Rap1GAP was downregulated selectively in tumor cell lines that had acquired a mesenchymal morphology. Restoring Rap1GAP expression to these cells inhibited cell migration and invasion, effects that were correlated with the inhibition of Rap1 and Rac1 activity. The reexpression of Rap1GAP also inhibited DNA synthesis and anchorage-independent proliferation. Conversely, eliminating Rap1GAP expression in rat thyroid cells induced a transient increase in cell number. Strikingly, Rap1GAP expression was abolished by Ras transformation. The downregulation of Rap1GAP by Ras required the activation of the Raf/MEK/extracellular signal-regulated kinase cascade and was correlated with the induction of mesenchymal morphology and migratory behavior. Remarkably, the acute expression of oncogenic Ras was sufficient to downregulate Rap1GAP expression in rat thyroid cells, identifying Rap1GAP as a novel target of oncogenic Ras. Collectively, these data implicate Rap1GAP as a putative tumor/invasion suppressor in the thyroid. In support of that notion, Rap1GAP was highly expressed in normal human thyroid cells and downregulated in primary thyroid tumors.

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.

A Wnt-CKIvarepsilon-Rap1 pathway regulates gastrulation by modulating SIPA1L1, a Rap GTPase activating protein

Noncanonical Wnt signals control morphogenetic movements during vertebrate gastrulation. Casein kinase I epsilon (CKIvarepsilon) is a Wnt-regulated kinase that regulates Wnt/beta-catenin signaling and has a beta-catenin-independent role(s) in morphogenesis that is poorly understood. Here we report the identification of a CKIvarepsilon binding partner, SIPA1L1/E6TP1, a GAP (GTPase activating protein) of the Rap small GTPase family. We show that CKIvarepsilon phosphorylates SIPA1L1 to reduce its stability and thereby increase Rap1 activation. Wnt-8, which activates CKIvarepsilon, enhances the CKIvarepsilon-dependent phosphorylation and degradation of SIPA1L1. In early Xenopus or zebrafish development, inactivation of the Rap1 pathway results in abnormal gastrulation and a shortened anterior-posterior axis. Although CKIvarepsilon also transduces Wnt/beta-catenin signaling, inhibition of Rap1 does not alter beta-catenin-regulated gene expression. Our data demonstrate a role for CKIvarepsilon in noncanonical Wnt signaling and indicate that Wnt regulates morphogenesis in part through CKIvarepsilon-mediated control of Rap1 signaling.