E4 antibodies facilitate detection and type-assignment of active HPV infection in cervical disease.

High-risk human papillomavirus (HPV) infections are the cause of nearly all cases of cervical cancer. Although the detection of HPV DNA has proved useful in cervical diagnosis, it does not necessarily predict disease presence or severity, and cannot conclusively identify the causative type when multiple HPVs are present. Such limitations may be addressed using complementary approaches such as cytology, laser capture microscopy, and/or the use of infection biomarkers. One such infection biomarker is the HPV E4 protein, which is expressed at high level in cells that are supporting (or have supported) viral genome amplification. Its distribution in lesions has suggested a role in disease staging. Here we have examined whether type-specific E4 antibodies may also allow the identification and/or confirmation of causal HPV-type. To do this, type-specific polyclonal and monoclonal antibodies against three E4 proteins (HPV-16, -18, and -58) were generated and validated by ELISA and western blotting, and by immunohistochemistry (IHC) staining of epithelial rafts containing these individual HPV types. Type-specific detection of HPV and its associated disease was subsequently examined using formalin-fixed paraffin-embedded cervical intra-epithelial neoplasias (CIN, (n = 247)) and normal controls (n = 28). All koilocytotic CIN1 lesions showed type-specific E4 expression of their respective HPV types. Differences were noted amongst E4 expression patterns in CIN3. HPV-18 E4 was not detected in any of the 6 HPV-18 DNA-positive CIN3 lesions examined, whereas in HPV-16 and -58 CIN3, 28/37 (76%) and 5/9 (55.6%) expressed E4 respectively, usually in regions of epithelial differentiation. Our results demonstrate that type-specific E4 antibodies can be used to help establish causality, as may be required when multiple HPV types are detected. The unique characteristics of the E4 biomarker suggest a role in diagnosis and patient management particularly when used in combination.

One virus, one lesion--individual components of CIN lesions contain a specific HPV type.

In 20-40% of cervical intra-epithelial neoplasia (CIN) and in 4-8% of cervical carcinoma tissue specimens, multiple HPV genotypes have been detected. Whole tissue section (WTS) PCR does not determine how the individual types relate causally to complex and multiple CIN. Our objective was to determine whether laser capture micro-dissection (LCM) with HPV PCR genotyping (LCM-PCR) could accurately recover type-specific HPV DNA from epithelial cells in individual areas of CIN and normal epithelium, and whether one or more viruses are present in one lesion. For that, histologically selected samples of CIN and normal epithelium were isolated by LCM and analysed by the SPF(10) PCR/LiPA(25) (version 1) HPV genotyping system for 25 HPV genotypes. HPV genotypes detected in 756 areas of CIN (grade 1, 2 or 3) by LCM-PCR were compared with results obtained by WTS-PCR in 60 cases (74 biopsies). We showed that when a single HPV type is detected by WTS-PCR, that type was almost always (94%; 29/31) recovered by LCM-PCR from CIN. When multiple HPV types were present by WTS-PCR, their distribution within histological sections could be mapped by LCM-PCR. Association of a single HPV type with a discrete area of CIN was found for 93% (372/399) of LCM fragments analysed by PCR. We found colliding CIN lesions associated with separate HPV types and only 62% (61/99) of HPV types detected by WTS-PCR were found in CIN by LCM-PCR. Therefore, the LCM-PCR technique was found very accurate for high-resolution HPV genotyping and for assigning an individual HPV type to an area of CIN. At LCM level, in cervical biopsy sections with multiple HPV infections, the relation between HPV types and CIN lesions is often complex. Almost every HPV type found in CIN by LCM-PCR is associated with a biological separate independent CIN lesion-one virus, one lesion.

The protein phosphatase 2A phosphatase activator is a novel peptidyl-prolyl cis/trans-isomerase.

The protein phosphatase 2A (PP2A) phosphatase activator (PTPA) is an essential protein involved in the regulation of PP2A and the PP2A-like enzymes. In this study we demonstrate that PTPA and its yeast homologues Ypa1 and Ypa2 can induce a conformational change in some model substrates. Using these model substrates in different assays with and without helper proteases, this isomerase activity is similar to the isomerase activity of FKBP12, the human cyclophilin A, and one of its yeast homologs Cpr7 but dissimilar to the isomerase activity of Pin1. However, neither FKBP12 nor Cpr7 can reactivate the inactive form of PP2A. Therefore, PTPA belongs to a novel peptidyl-prolyl cis/trans-isomerase (PPIase) family. The PPIase activity of PTPA correlates with its activating activity since both are stimulated by the presence of Mg2+ATP, and a PTPA mutant (Delta208-213) with 400-fold less activity in the activation reaction of PP2A also showed almost no PPIase activity. The point mutant Asp205 --> Gly (in Ypa1) identified this amino acid as essential for both activities. Moreover, PTPA dissociates the inactive form from the complex with the PP2A methylesterase. Finally, Pro190 in the catalytic subunit of PP2A (PP2AC) could be identified as the target Pro isomerized by PTPA/Mg2+ATP since among the 14 Pro residues present in 12 synthesized peptides representing the microenvironments of these prolines in PP2AC, only Pro190 could be isomerized by PTPA/Mg2+ATP. This Pro190 is present in a predicted loop structure near the catalytic center of PP2AC and, if mutated into a Phe, the phosphatase is inactive and can no longer be activated by PTPA/Mg2+ATP.

PP2A: the expected tumor suppressor.

PP2A is one of the few serine/threonine-specific phosphatases in the cell, and its complex structure and regulation guarantees its many different functions. Some viruses have chosen to target this enzyme system in order to manage the host cell machinery for their own profit and to program cells into a malignant state. Suppression of PR61/B'gamma, a specific third regulatory subunit of PP2A, can substitute for the viral SV40 protein small t antigen in causing tumorigenic transformation of several human cell lines -- provided that telomerase, SV40 large T antigen and oncogenic Ras are also present. Accumulation of c-Myc seems to be the common denominator.

Specific interactions of PP2A and PP2A-like phosphatases with the yeast PTPA homologues, Ypa1 and Ypa2.

To elucidate the specific biological role of the yeast homologues of PTPA (phosphatase 2A phosphatase activator), Ypa1 and Ypa2 (where Ypa stands for yeast phosphatase activator), in the regulation of PP2A (protein phosphatase 2A), we investigated the physical interaction of both Ypa proteins with the catalytic subunit of the different yeast PP2A-like phosphatases. Ypa1 interacts specifically with Pph3, Sit4 and Ppg1, whereas Ypa2 binds to Pph21 and Pph22. The Ypa1 and Ypa2 proteins do not compete with Tap42 (PP2A associating protein) for binding to PP2A family members. The interaction of the Ypa proteins with the catalytic subunit of PP2A-like phosphatases is direct and independent of other regulatory subunits, implicating a specific function for the different PP2A-Ypa complexes. Strikingly, the interaction of Ypa2 with yeast PP2A is promoted by the presence of Ypa1, suggesting a positive role of Ypa1 in the regulation of PP2A association with other interacting proteins. As in the mammalian system, all yeast PP2A-like enzymes associate as an inactive complex with Yme (yeast methyl esterase). Ypa1 as well as Ypa2 can reactivate all these inactive complexes, except Pph22-Yme. Ypa1 is the most potent activator of PP2A activity, suggesting that there is no direct correlation between activation potential and binding capacity.

Genomic organisation, chromosomal localisation tissue distribution and developmental regulation of the PR61/B' regulatory subunits of protein phosphatase 2A in mice.

Protein phosphatase 2A (PP2A) is a major serine/threonine-specific phosphatase playing central roles in development, cell growth and transformation. Regulation is largely accomplished by the regulatory B subunits, which determine substrate specificity, subcellular localisation and catalytic activity. The B' family, also known as the PR61 family, is the most diverse, consisting of five genes (alpha,beta,gamma,delta and epsilon) that give rise to a number of splice variants. We deduced the sequences of the different PR61 proteins in mice and found evidence for the expression of PR61alpha, beta1, gamma1, gamma2, gamma3, delta1 and epsilon. We report the genomic organisation and localisation of the murine PR61 genes (Ppp2r5a-Ppp2r5e). This information will be useful for the future realisation of PR61 knockouts. Using Northern blotting, we examined the expression of the five PR61 isoforms in different tissues. A brain-specific function can be expected for the PR611beta protein based on the high expression levels observed in murine brain. In situ hybridisation analysis of the adult brain revealed a distinct and partially overlapping pattern of mRNA expression of the various PR61 isoforms. The PR61 mRNA expression during embryonic development was examined by Northern blotting. The PR61 transcripts were differentially expressed, suggesting a specific function for each of the PR61 proteins during embryonic development and/or adult life.

PP2A fulfills its promises as tumor suppressor: which subunits are important?

Reversible phosphorylation of proteins, catalyzed by kinases and phosphatases, is a key regulatory mechanism in the control of multiple cellular signal transduction pathways. Uncontrolled regulation by the altered phosphorylation state of the components of these pathways often leads to increased cell proliferation and cell transformation. Many viruses encode oncogenic proteins, required for their efficient viral replication, which deregulate the activity of host cell proteins. This might program cells to a malignant state, underlying the molecular mechanism of tumor formation and cancer development. Recent studies reveal a role for a specific form of protein phosphatase 2A (PP2A) in viral-induced cell transformation by interaction with the small t antigen (ST) of the DNA tumor simian virus 40 (SV40).

An inactive protein phosphatase 2A population is associated with methylesterase and can be re-activated by the phosphotyrosyl phosphatase activator.

We have described recently the purification and cloning of PP2A (protein phosphatase 2A) leucine carboxylmethyltransferase. We studied the purification of a PP2A-specific methylesterase that co-purifies with PP2A and found that it is tightly associated with an inactive dimeric or trimeric form of PP2A. These inactive enzyme forms could be reactivated as Ser/Thr phosphatase by PTPA (phosphotyrosyl phosphatase activator of PP2A). PTPA was described previously by our group as a protein that stimulates the in vitro phosphotyrosyl phosphatase activity of PP2A; however, PP2A-specific methyltransferase could not bring about the activation. The PTPA activation could be distinguished from the Mn2+ stimulation observed with some inactive forms of PP2A, also found associated with PME-1 (phosphatase methylesterase 1). We discuss a potential new function for PME-1 as an enzyme that stabilizes an inactivated pool of PP2A.

Phosphatases in apoptosis: to be or not to be, PP2A is in the heart of the question.

Protein phosphatase type 2A (PP2A) is a major Ser/Thr phosphatase involved in several cellular signal transduction pathways. In this review, we will focus on recent progress concerning the role of PP2A in apoptotic signalling. Since PP2A activates pro-apoptotic and inhibits anti-apoptotic proteins of the Bcl-2 family, we conclude that PP2A has a positive regulatory function in apoptosis. However, in Drosophila, a specific subset of the PP2A holoenzyme family, containing B'/PR61 as third regulatory subunit, is inhibitory for apoptosis, suggesting different regulatory mechanisms and substrates in different species. Moreover, PP2A acts not only upstream as a regulator of the apoptotic signal transduction pathway but also downstream as a substrate of effector caspases. Hence, PP2A is involved in the regulation as well as in the cellular response of apoptosis. Probably, various PP2A holoenzymes with distinct regulatory subunits specifically target different apoptotic substrates. This could explain the implication of PP2A at several levels of the apoptotic signal transduction pathway. Finally, some viral proteins such as adenovirus E4orf4 and simian virus small t target PP2A to alter its activity, resulting in induction of apoptosis as a regulatory mechanism to enhance virus spread.

Identification and characterization of B"-subunits of protein phosphatase 2 A in Xenopus laevis oocytes and adult tissues.

Protein phosphatase 2A is a phosphoserine/threonine phosphatase implicated in many cellular processes. The core enzyme comprises a catalytic and a PR65/A-subunit. The substrate specificity and subcellular localization are determined by a third regulatory B-subunit (PR55/B, PR61/B' and PR72/130/B"). To identify the proteins of the B" family in Xenopus laevis oocytes, a prophase Xenopus oocyte cDNA library was screened using human PR130 cDNA as a probe. Three different classes of cDNAs were isolated. One class is very similar to human PR130 and is probably the Xenopus orthologue of PR130 (XPR130). A second class of clones (XN73) is identical to the N-terminal part of XPR130 but ends a few amino acids downstream of the putative splicing site of PR130. To investigate how this occurs, the genomic structure of the human PR130 gene was determined. This novel protein does not act as a PP2A subunit but might compete with the function of PR130. The third set of clones (XPR70) is very similar to human PR48 but has an N-terminal extension. Further analysis of the human EST-database and the human PR48 gene structure, revealed that the human PR48 clone published is incomplete. The Xenopus orthologue of PR48 encodes a protein of 70 kDa which like the XPR130, interacts with the A-subunit in GST pull-down assays. XPR70 is ubiquitously expressed in adult tissues and oocytes whereas expression of XPR130 is very low in brain and oocytes. Expression of XN73 mainly parallels XPR130 with the exception of the brain.

Identification and functional analysis of two Ca2+-binding EF-hand motifs in the B"/PR72 subunit of protein phosphatase 2A.

Protein phosphatase 2A (PP2A) is a multifunctional serine/threonine phosphatase that is critical to many cellular processes including cell cycle regulation and signal transduction. PP2A is a heterotrimer containing a structural (A) and catalytic (C) subunit, associated with one variable regulatory or targeting B-type subunit, of which three families have been described to date (B/PR55, B'/PR61, and B"/PR72). We identified two functional and highly conserved Ca(2+)-binding EF-hand motifs in human B"/PR72 (denoted EF1 and EF2), demonstrating for the first time the ability of Ca(2+) to interact directly with and regulate PP2A. EF1 and EF2 apparently bind Ca(2+) with different affinities. Ca(2+) induces a significant conformational change, which is dependent on the integrity of the motifs. We have further evaluated the effects of Ca(2+) on subunit composition, subcellular targeting, catalytic activity, and function during the cell cycle of a PR72-containing PP2A trimer (PP2A(T72)) by site-directed mutagenesis of either or both motifs. The results suggest that integrity of EF2 is required for A/PR65 subunit interaction and proper nuclear targeting of PR72, whereas EF1 might mediate the effects of Ca(2+) on PP2A(T72) activity in vitro and is at least partially required for the ability of PR72 to alter cell cycle progression upon forced expression.

Protein phosphatase 2A on track for nutrient-induced signalling in yeast.

Early studies identified two bona fide protein phosphatase 2A (PP2A)-encoding genes in Saccharomyces cerevisiae, designated PPH21 and PPH22. In addition, three PP2A-related phosphatases, encoded by PPH3, SIT4 and PPG1, have been identified. All share as much as 86% sequence similarity at the amino acid level. This review will focus primarily on Pph21 and Pph22, but some aspects of Sit4 regulation will also be discussed. Whereas a role for PP2A in yeast morphology and cell cycle has been readily recognized, uncovering its function in yeast signal transduction is a more recent breakthrough. Via their interaction with phosphorylated Tap42, PP2A and Sit4 play a pivotal role in target of rapamycin (TOR) signalling. PPH22 overexpression mimics overactive cAMP-PKA (protein kinase A) signalling and PP2A and Sit4 might represent ceramide signalling targets. The methylation of its catalytic subunit stabilizes the heterotrimeric form of PP2A and might counteract TOR signalling. We will show how these new elements could lead us to understand the role and regulation of PP2A in nutrient-induced signalling in baker's yeast.