The glycine 90 to aspartate alteration in the Abeta subunit of PP2A (PPP2R1B) associates with breast cancer and causes a deficit in protein function

PPP2R1B is modulated by ubiquitination and is essential for spermatogenesis

The serine-threonine protein phosphatase 2A (PP2A) is a heterotrimeric enzyme complex that regulates many fundamental cellular processes. PP2A is involved in tumorigenesis because mutations in the scaffold subunit, PPP2R1B, were found in several types of cancers. However, the biological function of PPP2R1B remains largely unknown. We report here that homozygous deletion of Ppp2r1b in Mus musculus impairs meiotic recombination and causes meiotic arrest in spermatocytes. Consistently, male mice lacking Ppp2r1b are characterized with infertility. Furthermore, heterozygous missense mutations in the Homo sapiens PPP2R1B gene, which encodes PPP2R1B, are identified in azoospermia patients with meiotic arrest. We found that PPP2R1B mutants are susceptible to degradation by an E3 ligase CRL4A^DCAF6 , and resistant to de-polyubiquitylation by ubiquitin-specific protease 5 (USP5). In addition, heterozygous mutations in PPP2R1B reduce stability of the wild-type PPP2R1B. Our results demonstrate an essential role of PPP2R1B in spermatogenesis and identify upstream regulators of PPP2R1B.

Protein phosphatase 2A (PP2A): a key phosphatase in the progression of chronic obstructive pulmonary disease (COPD) to lung cancer

Lung cancer (LC) has the highest relative risk of development as a comorbidity of chronic obstructive pulmonary disease (COPD). The molecular mechanisms that mediate chronic inflammation and lung function impairment in COPD have been identified in LC. This suggests the two diseases are more linked than once thought. Emerging data in relation to a key phosphatase, protein phosphatase 2A (PP2A), and its regulatory role in inflammatory and tumour suppression in both disease settings suggests that it may be critical in the progression of COPD to LC. In this review, we uncover the importance of the functional and active PP2A holoenzyme in the context of both diseases. We describe PP2A inactivation via direct and indirect means and explore the actions of two key PP2A endogenous inhibitors, cancerous inhibitor of PP2A (CIP2A) and inhibitor 2 of PP2A (SET), and the role they play in COPD and LC. We explain how dysregulation of PP2A in COPD creates a favourable inflammatory micro-environment and promotes the initiation and progression of tumour pathogenesis. Finally, we highlight PP2A as a druggable target in the treatment of COPD and LC and demonstrate the potential of PP2A re-activation as a strategy to halt COPD disease progression to LC. Although further studies are required to elucidate if PP2A activity in COPD is a causal link for LC progression, studies focused on the potential of PP2A reactivating agents to reduce the risk of LC formation in COPD patients will be pivotal in improving clinical outcomes for both COPD and LC patients in the future.

Genome-wide association and transcriptome studies identify target genes and risk loci for breast cancer

Genome-wide association studies (GWAS) have identified more than 170 breast cancer susceptibility loci. Here we hypothesize that some risk-associated variants might act in non-breast tissues, specifically adipose tissue and immune cells from blood and spleen. Using expression quantitative trait loci (eQTL) reported in these tissues, we identify 26 previously unreported, likely target genes of overall breast cancer risk variants, and 17 for estrogen receptor (ER)-negative breast cancer, several with a known immune function. We determine the directional effect of gene expression on disease risk measured based on single and multiple eQTL. In addition, using a gene-based test of association that considers eQTL from multiple tissues, we identify seven (and four) regions with variants associated with overall (and ER-negative) breast cancer risk, which were not reported in previous GWAS. Further investigation of the function of the implicated genes in breast and immune cells may provide insights into the etiology of breast cancer.

The B56γ3 regulatory subunit-containing protein phosphatase 2A outcompetes Akt to regulate p27KIP1 subcellular localization by selectively dephosphorylating phospho-Thr157 of p27KIP1

The B56γ-containing protein phosphatase 2A (PP2A-B56γ) has been postulated to have tumor suppressive functions. Here, we report regulation of p27KIP1 subcellular localization by PP2A-B56γ3. B56γ3 overexpression enhanced nuclear localization of p27KIP1, whereas knockdown of B56γ3 decreased p27KIP1 nuclear localization. B56γ3 overexpression decreased phosphorylation at Thr157 (phospho-Thr157), whose phosphorylation promotes cytoplasmic localization of p27KIP1, whereas B56γ3 knockdown significantly increased the level of phospho-Thr157. In vitro, PP2A-B56γ3 catalyzed dephosphorylation of phospho-Thr157 in a dose-dependent and okadaic acid-sensitive manner. B56γ3 did not increase p27KIP1 nuclear localization by down-regulating the upstream kinase Akt activity and outcompeted a myristoylated constitutively active Akt (Aktca) in regulating Thr157 phosphorylation and subcellular localization of p27KIP1. In addition, results of interaction domain mapping revealed that both the N-terminal and C-terminal domains of p27 and a domain at the C-terminus of B56γ3 are required for interaction between p27 and B56γ3. Furthermore, we demonstrated that p27KIP1 levels are positively correlated with B56γ levels in both non-tumor and tumor parts of a set of human colon tissue specimens. However, positive correlation between nuclear p27KIP1 levels and B56γ levels was found only in the non-tumor parts, but not in tumor parts of these tissues, implicating a dysregulation in PP2A-B56γ3-regulated p27KIP1 nuclear localization in these tumor tissues. Altogether, this study provides a new mechanism by which the PP2A-B56γ3 holoenzyme plays its tumor suppressor role.

The broken "Off" switch in cancer signaling: PP2A as a regulator of tumorigenesis, drug resistance, and immune surveillance

Aberrant activation of signal transduction pathways can transform a normal cell to a malignant one and can impart survival properties that render cancer cells resistant to therapy. A diverse set of cascades have been implicated in various cancers including those mediated by serine/threonine kinases such RAS, PI3K/AKT, and PKC. Signal transduction is a dynamic process involving both "On" and "Off" switches. Activating mutations of RAS or PI3K can be viewed as the switch being stuck in the "On" position resulting in continued signaling by a survival and/or proliferation pathway. On the other hand, inactivation of protein phosphatases such as the PP2A family can be seen as the defective "Off" switch that similarly can activate these pathways. A problem for therapeutic targeting of PP2A is that the enzyme is a hetero-trimer and thus drug targeting involves complex structures. More importantly, since PP2A isoforms generally act as tumor suppressors one would want to activate these enzymes rather than suppress them. The elucidation of the role of cellular inhibitors like SET and CIP2A in cancer suggests that targeting these proteins can have therapeutic efficacy by mechanisms involving PP2A activation. Furthermore, drugs such as FTY-720 can activate PP2A isoforms directly. This review will cover the current state of knowledge of PP2A role as a tumor suppressor in cancer cells and as a mediator of processes that can impact drug resistance and immune surveillance.

Potential anti-tumor effects of FTY720 associated with PP2A activation: a brief review

FTY720 (Fingolimod, Gilenya (†) ) is an FDA-approved immunosuppressant currently used in the treatment of multiple sclerosis. However, a large number of studies over the last few years have shown that FTY720 shows potent antitumor properties that suggest its potential usefulness as a novel anticancer agent. Interestingly, the restoration of protein phosphatase 2A (PP2A) activity mediated by FTY720 could play a key role in its antitumor effects. Taking into account that PP2A inactivation is a common event that determines poor outcome in several tumor types, FTY720 could serve as an alternative therapeutic strategy for cancer patients with such alterations.

Protein Phosphatase 2A as a Therapeutic Target in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a heterogeneous malignant disorder of hematopoietic progenitor cells in which several genetic and epigenetic aberrations have been described. Despite progressive advances in our understanding of the molecular biology of this disease, the outcome for most patients is poor. It is, therefore, necessary to develop more effective treatment strategies. Genetic aberrations affecting kinases have been widely studied in AML; however, the role of phosphatases remains underexplored. Inactivation of the tumor-suppressor protein phosphatase 2A (PP2A) is frequent in AML patients, making it a promising target for therapy. There are several PP2A inactivating mechanisms reported in this disease. Deregulation or specific post-translational modifications of PP2A subunits have been identified as a cause of PP2A malfunction, which lead to deregulation of proliferation or apoptosis pathways, depending on the subunit affected. Likewise, overexpression of either SET or cancerous inhibitor of protein phosphatase 2A, endogenous inhibitors of PP2A, is a recurrent event in AML that impairs PP2A activity, contributing to leukemogenesis progression. Interestingly, the anticancer activity of several PP2A-activating drugs (PADs) depends on interaction/sequestration of SET. Preclinical studies show that pharmacological restoration of PP2A activity by PADs effectively antagonizes leukemogenesis, and that these drugs have synergistic cytotoxic effects with conventional chemotherapy and kinase inhibitors, opening new possibilities for personalized treatment in AML patients, especially in cases with SET-dependent inactivation of PP2A. Here, we review the role of PP2A as a druggable tumor suppressor in AML.

All roads lead to PP2A: exploiting the therapeutic potential of this phosphatase

Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase involved in the regulation of many cellular processes. A confirmed tumor suppressor protein, PP2A is genetically altered or functionally inactivated in many cancers highlighting a need for its therapeutic reactivation. In this review we discuss recent literature on PP2A: the elucidation of its structure and the functions of its subunits, and the identification of molecular lesions and post-translational modifications leading to its dysregulation in cancer. A final section will discuss the proteins and small molecules that modulate PP2A and how these might be used to target dysregulated forms of PP2A to treat cancers and other diseases.

PP2A inhibition determines poor outcome and doxorubicin resistance in early breast cancer and its activation shows promising therapeutic effects

The protein phosphatase 2A (PP2A) is a key tumor suppressor which has emerged as a novel molecular target in some human cancers. Here, we show that PP2A inhibition is a common event in breast cancer and identified PP2A phosphorylation and deregulation SET and CIP2A as molecular contributing mechanisms to inactivate PP2A. Interestingly, restoration of PP2A activity after FTY720 treatment reduced cell growth, induced apoptosis and decreased AKT and ERK activation. Moreover, FTY720 led to PP2A activation then enhancing doxorubicin-induced antitumor effects both in vitro and in vivo. PP2A inhibition (CPscore: PP2A phosphorylation and/or CIP2A overexpression) was detected in 27% of cases (62/230), and associated with grade (p = 0.017), relapse (p < 0.001), negative estrogen (p < 0.001) and progesterone receptor expression (p < 0.001), HER2-positive tumors (p = 0.049), Ki-67 expression (p < 0.001), and higher AKT (p < 0.001) and ERK (p < 0.001) phosphorylation. Moreover, PP2A inhibition determined shorter overall (p = 0.006) and event-free survival (p = 0.003), and multivariate analysis confirmed its independent prognostic impact. Altogether, our results indicate that PP2A is frequently inactivated in breast cancer and determines worse outcome, and its restoration using PP2A activators represents an alternative therapeutic strategy in this disease.

TNF-α mutation affects the gene expression profiles of patients with multiple trauma

Multiple trauma can induce sepsis and organ failure, even threaten people’s lives. To further study the mechanisms of multiple trauma, we analyzed microarray of GSE5760. GSE5760 was downloaded from the Gene Expression Omnibus including a total of 58 peripheral blood transcriptome from patients without (WT, n = 30) and carrying (MUT, n = 28) the tumor necrosis factor (TNF) rs1800629 A variant. The differentially expressed genes (DEGs) were screened using the limma package in R and the Benjamin and Hochberg method in a multi-test package. Then, functional enrichment analysis of DEGs was performed. Also, transcription factors significantly related to DEGs were searched using WebGestalt and interaction network of transcription factors and DEGs were constructed using STRING online software. Furthermore, pathway enrichment analysis for the DEGs in the interaction network was conducted using KO-Based Annotation System (KOBAS). We screened 39 DEGs including 27 upregulated and 12 downregulated genes. The enriched functions were associated with biological process (BP) (such as response to hypoxia, P value = 0.039803), cell components (CC) (such as mitochondrial part, P value = 0.043857), and molecular function (MF) (such as structural constituent of ribosome, P value = 0.008735). Besides, RPS7 and RPL17 were associated with ribosome and participated in ribosome pathway. PPP2R2B was related to mitochondrion. KCNMA1, ALAS2 and SOCS3 were associated with hypoxia. Moreover, transcription factors of LEF1, CHX10, ELK1, SP1, and MAZ were significantly related to DEGs. RPS7, RPL17, PPP2R2B, KCNMA1, ALAS2, and SOCS3 might relate to multiple trauma. And TNF-α mutation could cause sepsis in patients with multiple trauma by changing the expression of these genes.

The Basic Biology of PP2A in Hematologic Cells and Malignancies

Reversible protein phosphorylation plays a crucial role in regulating cell signaling. In normal cells, phosphoregulation is tightly controlled by a network of protein kinases counterbalanced by several protein phosphatases. Deregulation of this delicate balance is widely recognized as a central mechanism by which cells escape external and internal self-limiting signals, eventually resulting in malignant transformation. A large fraction of hematologic malignancies is characterized by constitutive or unrestrained activation of oncogenic kinases. This is in part achieved by activating mutations, chromosomal rearrangements, or constitutive activation of upstream kinase regulators, in part by inactivation of their anti-oncogenic phosphatase counterparts. Protein phosphatase 2A (PP2A) represents a large family of cellular serine/threonine phosphatases with suspected tumor suppressive functions. In this review, we highlight our current knowledge about the complex structure and biology of these phosphatases in hematologic cells, thereby providing the rationale behind their diverse signaling functions. Eventually, this basic knowledge is a key to truly understand the tumor suppressive role of PP2A in leukemogenesis and to allow further rational development of therapeutic strategies targeting PP2A.

Genomic era diagnosis and management of hereditary and sporadic colon cancer

The morbidity and mortality attributable to heritable and sporadic carcinomas of the colon are substantial and affect children and adults alike. Despite current colonoscopy screening recommendations colorectal adenocarcinoma (CRC) still accounts for almost 140000 cancer cases yearly. Familial adenomatous polyposis (FAP) is a colon cancer predisposition due to alterations in the adenomatous polyposis coli gene, which is mutated in most CRC. Since the beginning of the genomic era next-generation sequencing analyses of CRC continue to improve our understanding of the genetics of tumorigenesis and promise to expand our ability to identify and treat this disease. Advances in genome sequence analysis have facilitated the molecular diagnosis of individuals with FAP, which enables initiation of appropriate monitoring and timely intervention. Genome sequencing also has potential clinical impact for individuals with sporadic forms of CRC, providing means for molecular diagnosis of CRC tumor type, data guiding selection of tumor targeted therapies, and pharmacogenomic profiles specifying patient specific drug tolerances. There is even a potential role for genomic sequencing in surveillance for recurrence, and early detection, of CRC. We review strategies for diagnostic assessment and management of FAP and sporadic CRC in the current genomic era, with emphasis on the current, and potential for future, impact of genome sequencing on the clinical care of these conditions.

Targeting c-MYC by antagonizing PP2A inhibitors in breast cancer

The transcription factor c-MYC is stabilized and activated by phosphorylation at serine 62 (S62) in breast cancer. Protein phosphatase 2A (PP2A) is a critical negative regulator of c-MYC through its ability to dephosphorylate S62. By inactivating c-MYC and other key signaling pathways, PP2A plays an important tumor suppressor function. Two endogenous inhibitors of PP2A, I2PP2A, Inhibitor-2 of PP2A (SET oncoprotein) and cancerous inhibitor of PP2A (CIP2A), inactivate PP2A and are overexpressed in several tumor types. Here we show that SET is overexpressed in about 50-60% and CIP2A in about 90% of breast cancers. Knockdown of SET or CIP2A reduces the tumorigenic potential of breast cancer cell lines both in vitro and in vivo. Treatment of breast cancer cells in vitro or in vivo with OP449, a novel SET antagonist, also decreases the tumorigenic potential of breast cancer cells and induces apoptosis. We show that this is, at least in part, due to decreased S62 phosphorylation of c-MYC and reduced c-MYC activity and target gene expression. Because of the ubiquitous expression and tumor suppressor activity of PP2A in cells, as well as the critical role of c-MYC in human cancer, we propose that activation of PP2A (here accomplished through antagonizing endogenous inhibitors) could be a novel antitumor strategy to posttranslationally target c-MYC in breast cancer.

Risk classification of cancer survival using ANN with gene expression data from multiple laboratories

Numerous cancer studies have combined gene expression experiments and clinical survival data to predict the prognosis of patients of specific gene types. However, most results of these studies were data dependent and were not suitable for other data sets. This study performed cross-laboratory validations for the cancer patient data from 4 hospitals. We investigated the feasibility of survival risk predictions using high-throughput gene expression data and clinical data. We analyzed multiple data sets for prognostic applications in lung cancer diagnosis. After building tens of thousands of various ANN architectures using the training data, five survival-time correlated genes were identified from 4 microarray gene expression data sets by examining the correlation between gene signatures and patient survival time. The experimental results showed that gene expression data can be used for valid predictions of cancer patient survival classification with an overall accuracy of 83.0% based on survival time trusted data. The results show the prediction model yielded excellent predictions given that patients in the high-risk group obtained a lower median overall survival compared with low-risk patients (log-rank test P-value<0.00001). This study provides a foundation for further clinical studies and research into other types of cancer. We hope these findings will improve the prognostic methods of cancer patients.

Protein phosphatase 2A isoforms utilizing Aβ scaffolds regulate differentiation through control of Akt protein

Protein phosphatase 2A (PP2A) regulates almost all cell signaling pathways. It consists of a scaffolding A subunit to which a catalytic C subunit and one of many regulatory B subunits bind. Of the more than 80 PP2A isoforms, 10% use Aβ as a scaffold. This study demonstrates the isoform-specific function of the A scaffold subunits. Polyomaviruses have shown the importance of phosphotyrosine, PI3K, and p53 in transformation. Comparisons of polyoma and SV40 small T antigens implicate Aβ in the control of differentiation. Knockdown of Aβ enhanced differentiation. Akt signaling regulated differentiation; its activation or inhibition promoted or blocked it, respectively. Aβ bound Akt. Enhancement of PP2A Aβ/Akt interaction by polyoma small T antigen increased turnover of Akt Ser-473 phosphorylation. Conversely, knockdown of Aβ promoted Akt activity and reduced turnover of phosphate at Ser-473 of Akt. These data provide new insight into the regulation of Akt, a protein of extreme importance in cancer. Furthermore, our results suggest that the role for Aβ in differentiation and perhaps tumor suppression may lie partly in its ability to negatively regulate Akt.

PP2A Counterbalances Phosphorylation of pRB and Mitotic Proteins by Multiple CDKs: Potential Implications for PP2A Disruption in Cancer

Protein Phosphatase 2A (PP2A) consists of a collection of heterotrimeric serine/threonine phosphatase holoenzymes that play multiple roles in cell signaling via dephosphorylation of numerous substrates of a large family of serine/threonine kinases. PP2A substrate specificity is mediated by B regulatory subunits of four different families, which selectively recognize diverse substrates by mechanisms that are not well understood. Among the many signaling pathways with critical PP2A functions are several deregulated in cancer cells, and PP2A is a know tumor suppressor. However, the precise composition of the heterotrimeric PP2A complexes with tumor supressor activity is not well understood. This review is centered on the emerging role of the B regulatory subunit B55α and related subfamilly members in the modulation of the phosphorylation state of pocket proteins and mitotic CDK substrates, as well as the implications of PP2A function disruption in cancer in the context of these activities.

Impaired expression of protein phosphatase 2A subunits enhances metastatic potential of human prostate cancer cells through activation of AKT pathway

Background:

Protein phosphatase 2A (PP2A) is a dephosphorylating enzyme, loss of which can contribute to prostate cancer (PCa) pathogenesis. The aim of this study was to analyse the transcriptional and translational expression patterns of individual subunits of the PP2A holoenzyme during PCa progression.

Methods:

Immunohistochemistry (IHC), western blot, and real-time PCR was performed on androgen-dependent (AD) and androgen-independent (AI) PCa cells, and benign and malignant prostate tissues for all the three PP2A (scaffold, regulatory, and catalytic) subunits. Mechanistic and functional studies were performed using various biochemical and cellular techniques.

Results:

Through immunohistochemical analysis we observed significantly reduced levels of PP2A-A and -B′γ subunits (P<0.001 and P=0.0002) in PCa specimens compared with benign prostate. Contemporarily, there was no significant difference in PP2A-C subunit expression between benign and malignant tissues. Similar to the expression pattern observed in tissues, the endogenous levels of PP2A-A and B′γ subunits were abrogated from the low metastatic to high metastatic and AD to AI cell line models, without any change in the catalytic subunit expression. Furthermore, using in vitro studies we demonstrated that PP2A-Aα scaffold subunit has a role in dampening AKT, β-catenin, and FAK (focal adhesion kinase) signalling.

Conclusion:

We conclude that loss of expression of scaffold and regulatory subunits of PP2A is responsible for its altered function during PCa pathogenesis.

Role of a novel functional variant in the PPP2R1A promoter on the regulation of PP2A-Aalpha and the risk of hepatocellular carcinoma

Previously, we identified the genetic variant −241 (−/G) (rs11453459) in the PP2A-Aα gene (PPP2R1A) promoter and demonstrated that this variant influences the DNA-binding affinity of nuclear factor-kappa B (NF-κB). In this study, we further confirmed that the transcriptional activity of PPP2R1A may be regulated by NF-κB through the functional genetic variant −241 (−/G). Moreover, we also demonstrated that the methylation status of CpG islands in the promoter of PPP2R1A influences the activity of this gene promoter. Few studies have examined the role of this −241 (−/G) variant in genetic or epigenetic regulation in hepatocellular carcinoma (HCC). To investigate whether this functional variant in the PPP2R1A promoter is associated with the risk of HCC and confirm the function of the −241 (−/G) variant in the HCC population, we conducted a case-control study involving 251 HCC cases and 252 cancer-free controls from a Han population in southern China. Compared with the −241 (−−) homozygote, the heterozygous −241 (−G) genotype (adjusted OR  = 0.32, 95% confidence interval (CI)  = 0.17–0.58, P<0.001) and the −241 (−G)/(GG) genotypes (adjusted OR  = 0.38, 95% CI  = 0.22–0.67, P  = 0.001) were both significantly associated with a reduced risk of HCC. Stratification analysis indicated that the protective role of −241 (−G) was more pronounced in individuals who were ≤ 40 years of age, female and HBV-negative. Our data suggest that the transcriptional activity of PPP2R1A is regulated by NF-κB through the −241 (−/G) variant and by the methylation of the promoter region. Moreover, the functional −241 (−/G) variant in the PPP2R1A promoter contributes to the decreased risk of HCC. These findings contribute novel information regarding the gene transcription of PPP2R1A regulated by the polymorphism and methylation in the promoter region through genetic and epigenetic mechanisms in hepatocarcinogenesis.

Phosphatase: PP2A structural importance, regulation and its aberrant expression in cancer

Protein Phosphatase 2A (PP2A) is an important and ubiquitously expressed serine threonine phosphatase and regulates the function by dephosphorylating many critical cellular molecules like Akt, p53, c-Myc and β-catenin. It plays a critical role in cellular processes, such as cell proliferation, signal transduction and apoptosis. Structurally, it is multifarious as it is composed of catalytic, scaffold and regulatory subunits. The catalytic and scaffold subunits have two isoforms and the regulatory subunit has four different families containing different isoforms. The regulatory subunit is the most diverse with temporal and spatial specificity. PP2A undergoes post-translational modifications (i.e. phosphorylation and methylation), which in turn, regulates its enzymatic activity. Aberrant expression, mutations and somatic alterations of the PP2A scaffold and regulatory subunits have been observed in various human malignancies, including lung, breast, skin and colon cancer, highlighting its role as a 'tumor suppressor'. This review is focused on the structural complexity of serine/threonine phosphatase PP2A and summarizes its expression pattern in cancer. Additionally, the PP2A interacting and regulatory proteins and substrates are also discussed. Finally, the mouse models developed to understand the biological role of PP2A subunits in an in vivo model system are also reviewed in this article.

A genetic variant in a PP2A regulatory subunit encoded by the PPP2R2B gene associates with altered breast cancer risk and recurrence

A recent candidate gene association study identified a single nucleotide polymorphism (SNP) in the PPP2R2B gene (rs319217, A/G) that manifests allelic differences in the cellular responses to treatment with chemotherapeutic agents (Vazquez et al., Nat Rev Drug Discov 2008;7:979-87). This gene encodes a regulatory subunit of protein phosphatase 2A (PP2A), one of the major Ser/Thr phosphatases implicated in the negative control of cell growth and division. Given the tumor suppressor activities of PP2A, here we evaluate whether this genetic variant associates with the age of diagnosis and recurrence of breast cancer in women. To investigate the linkage disequilibrium in the vicinity of this SNP, PPP2R2B haplotypes were analyzed using HapMap data for 90 Caucasians. It is found that the A variant of rs319217 tags a haplotype that appears tobe under positive selection in the Caucasian population, implying that this SNP is functional. Subsequently, associations with cellular responses were investigated using data reported by the NCI anticancer drug screen and associations with breast cancer clinical variables were analyzed in a cohort of 819 Caucasian women. The A allele associates with a better response of tumor derived cell lines, lower risk of breast cancer recurrence, later time to recurrence, and later age of diagnosis of breast cancer in Caucasian women. Taken together these results indicate that the A variant of the rs319217 SNP is a marker of better prognosis in breast cancer.

PP2A impaired activity is a common event in acute myeloid leukemia and its activation by forskolin has a potent anti-leukemic effect

Protein phosphatase 2A (PP2A) is a human tumor suppressor that inhibits cellular transformation by regulating the activity of several signaling proteins critical for malignant cell behavior. PP2A has been described as a potential therapeutic target in chronic myeloid leukemia, Philadelphia chromosome-positive acute lymphoblastic leukemia and B-cell chronic lymphocytic leukemia. Here, we show that PP2A inactivation is a recurrent event in acute myeloid leukemia (AML), and that restoration of PP2A phosphatase activity by treatment with forskolin in AML cells blocks proliferation, induces caspase-dependent apoptosis and affects AKT and ERK1/2 activity. Moreover, treatment with forskolin had an additive effect with Idarubicin and Ara-c, drugs used in standard induction therapy in AML patients. Analysis at protein level of the PP2A activation status in a series of patients with AML at diagnosis showed PP2A hyperphosphorylation in 78% of cases (29/37). In addition, we found that either deregulated expression of the endogenous PP2A inhibitors SET or CIP2A, overexpression of SETBP1, or downregulation of some PP2A subunits, might be contributing to PP2A inhibition in AML. In conclusion, our results show that PP2A inhibition is a common event in AML cells and that PP2A activators, such as forskolin or FTY720, could represent potential novel therapeutic targets in AML.

Allele-specific copy number analysis of tumors

We present an allele-specific copy number analysis of the in vivo breast cancer genome. We describe a unique bioinformatics approach, ASCAT (allele-specific copy number analysis of tumors), to accurately dissect the allele-specific copy number of solid tumors, simultaneously estimating and adjusting for both tumor ploidy and nonaberrant cell admixture. This allows calculation of "ASCAT profiles" (genome-wide allele-specific copy-number profiles) from which gains, losses, copy number-neutral events, and loss of heterozygosity (LOH) can accurately be determined. In an early-stage breast carcinoma series, we observe aneuploidy (>2.7n) in 45% of the cases and an average nonaberrant cell admixture of 49%. By aggregation of ASCAT profiles across our series, we obtain genomic frequency distributions of gains and losses, as well as genome-wide views of LOH and copy number-neutral events in breast cancer. In addition, the ASCAT profiles reveal differences in aberrant tumor cell fraction, ploidy, gains, losses, LOH, and copy number-neutral events between the five previously identified molecular breast cancer subtypes. Basal-like breast carcinomas have a significantly higher frequency of LOH compared with other subtypes, and their ASCAT profiles show large-scale loss of genomic material during tumor development, followed by a whole-genome duplication, resulting in near-triploid genomes. Finally, from the ASCAT profiles, we construct a genome-wide map of allelic skewness in breast cancer, indicating loci where one allele is preferentially lost, whereas the other allele is preferentially gained. We hypothesize that these alternative alleles have a different influence on breast carcinoma development.

A B56gamma mutation in lung cancer disrupts the p53-dependent tumor-suppressor function of protein phosphatase 2A

Earlier studies have shown both p53-dependent and -independent tumor-suppressive functions of B56gamma-specific protein phosphatase 2A (B56gamma-PP2A). In the absence of p53, B56gamma-PP2A can inhibit cell proliferation and cell transformation by an unknown mechanism. In the presence of p53, on DNA damage, a complex including B56gamma-PP2A and p53 is formed, which leads to Thr55 dephosphorylation of p53, induction of the p53 transcriptional target p21 and inhibition of cell proliferation. In spite of its significance in inhibition of cell proliferation, no B56gamma mutations have been linked to human cancer to date. In this study, we first differentiate between the p53-dependent and -independent functions of B56gamma-PP2A by identifying a domain of the B56gamma protein required for interaction with p53. Within this region, we identify a B56gamma mutation, F395C, in lung cancer that disrupts the B56gamma-p53 interaction. More importantly, we show that F395C is unable to promote p53 Thr55 dephosphorylation, transcriptional activation of p21 and the p53-dependent tumor-suppressive function of PP2A. This finding provides a mechanistic basis for the p53-dependent and -independent functions of B56gamma-PP2A and establishes a critical link between B56gamma-PP2A p53-dependent tumor-suppressive function and tumorigenesis.

Tumor Suppressor RARRES1 Regulates DLG2, PP2A, VCP, EB1, and Ankrd26

Retinoic Acid Receptor Responder (RARRES1) initially identified as a novel retinoic acid receptor regulated gene in the skin is a putative tumor suppressor of unknown function. RARRES1 was knocked down in immortalized human prostatic epithelial cell line PWR-1E cells and differential protein expression was identified using differential in-gel electrophoresis (DIGE) followed by matrix-assisted laser desorption ionization (MALDI) mass spectrometry and western Blot analysis excluding highly abundant proteins routinely identified in almost all proteomics projects. Knock-down of RARRES1: 1- down-regulates PP2A, an enzyme involved in the negative regulation of the growth hormone-stimulated signal transduction pathways; 2- down-regulates Valosin-containing protein causing impaired autophagy; 3- up-regulates the tumor suppressor disks large 2; 4- up-regulates Ankrd26 that belongs to the POTE family of genes that are highly expressed in cancer patients with poor outcome; and 5- down-regulates EB1, a protein that is involved in spindle dynamics and chromosome alignment during mitosis.

Protein phosphatase 2A subunit gene haplotypes and proliferative breast disease modify breast cancer risk

BACKGROUND

Protein phosphatase 2A (PP2A) is a major cellular phosphatase and plays key regulatory roles in growth, differentiation, and apoptosis. Women who are diagnosed with benign proliferative breast disease are at increased risk for the subsequent development of breast cancer.

METHODS

The authors evaluated genetic variation of PP2A holoenzyme subunits for their potential contribution to breast cancer risk. A nested case-control investigation was performed on a cohort of women who had a history of benign breast disease. The women were followed for an average of 18 years, and DNA prepared from the original archival benign breast biopsy (1954-1995) was available for 450 women who were diagnosed with breast cancer on follow-up and for 890 of 900 women in a control group who were matched on race, age, and year of entry biopsy.

RESULTS

Single allele-based and haplotype-based tests of association were conducted with assessment of significance by permutation testing. Significant risk and protective haplotypes of the PP2A structural/regulatory subunit A alpha isoform (PPP2R1A) were identified and had odds ratios of 1.63 (95% confidence interval [CI], 1.3-2.1) and 0.55 (95% CI, 0.41-0.76), respectively. These odds ratios remained significant after the analysis was adjusted for multiple comparisons. Women who had both the PPP2R1A risk haplotype and a history of proliferative breast disease had an odds ratio of 2.44 (95% CI, 1.7-3.5) for the subsequent development of breast cancer. The effects of haplotypes for 2 PP2A regulatory subunit genes, PP2 regulatory subunit B alpha isoform (PPP2R2A) and PP2A regulatory subunit B' epsilon isoform (PPP2R5E) on breast cancer risk were nominally significant but did not remain significant after the analysis was adjusted for multiple comparisons.

CONCLUSIONS

The current findings supported the previously hypothesized role of PP2A as a tumor suppressor gene in breast cancer.

Negative regulation of CHK2 activity by protein phosphatase 2A is modulated by DNA damage

Checkpoint kinase 2 (CHK2) is a major effector of the DNA damage response pathway and although its mechanism of activation has been well studied, the attenuation of its activity following DNA damage has not been explored. Here, we identify the B'alpha subunit of protein phosphatase 2A (PP 2A) as a CHK2 binding partner and show that their interaction is modulated by DNA damage. B'alpha binds to the SQ/TQ repeat region of CHK2, which is a target of ATM phosphorylation. The induction of DNA double-strand breaks by gamma irradiation as well as treatment with doxorubicin causes dissociation of the B'alpha and CHK2 proteins. This dissociation correlates with an increase in the ATM-dependent phosphorylation of CHK2 at serines 33 and 35 in the SQ/TQ region. Indeed, mutating these sites to mimic phosphorylation increases the dissociation after irradiation. PP 2A negatively regulates CHK2 phosphorylation at multiple sites, as well as its kinase activity. These data reveal a novel mechanism for PP 2A to keep CHK2 inactive under normal conditions while also allowing for a rapid release from this regulation immediately following DNA damage. This is followed by a subsequent reconstitution of the PP 2A/CHK2 complex in later time points after damage, which may help to attenuate the signal.

Protein phosphatase 2A (PP2A), a drugable tumor suppressor in Ph1(+) leukemias

Protein phosphatase-2A (PP2A) is one of the major cellular serine-threonine phosphatases and is involved in the regulation of cell homeostasis through the negative regulation of signaling pathways initiated by protein kinases. As several cancers are characterized by the aberrant activity of oncogenic kinases, it was not surprising that a phosphatase like PP2A has progressively been considered as a potential tumor suppressor. Indeed, multiple solid tumors (e.g. melanomas, colorectal carcinomas, lung and breast cancers) present with genetic and/or functional inactivation of different PP2A subunits and, therefore, loss of PP2A phosphatase activity towards certain substrates. Likewise, impaired PP2A phosphatase activity has been linked to B-cell chronic lymphocytic leukemia, Philadelphia-chromosome positive acute lymphoblastic leukemia and blast crisis chronic myelogenous leukemia. Remarkably, drugs such as forskolin, 1,9-dideoxy-forskolin and FTY720 which lead to PP2A activation effectively antagonize leukemogenesis in both in vitro and in vivo models of these cancers. Thus, PP2A is now in the spotlight as a highly promising drugable target for the development of a new series of anticancer agents potentially capable of overcoming drug-resistance induced in patients by continuous exposure to kinase inhibitor monotherapy. Herein, we review current knowledge of PP2A biology and function with particular emphasis on its tumor suppressor activity and possible therapeutic implications in cancer.

Protein phosphatase 2A regulatory subunits and cancer

The serine/threonine protein phosphatase (PP2A) is a trimeric holoenzyme that plays an integral role in the regulation of a number of major signaling pathways whose deregulation can contribute to cancer. The specificity and activity of PP2A are highly regulated through the interaction of a family of regulatory B subunits with the substrates. Accumulating evidence indicates that PP2A acts as a tumor suppressor. In this review we summarize the known effects of specific PP2A holoenzymes and their roles in cancer relevant pathways. In particular we highlight PP2A function in the regulation of MAPK and Wnt signaling.

Mutation analysis of five candidate genes in familial breast cancer

Most of the known breast cancer susceptibility genes (BRCA1, BRCA2, CHEK2 and ATM) are involved in the damage response pathway. Other members of this pathway are therefore good candidates for additional breast cancer susceptibility genes. ATR, along with ATM, plays a central role in DNA damage recognition and Chk1 relays checkpoint signals from both ATR and ATM. PPP2R1B and PPP2R5B code for subunits of protein phosphatase 2A (PP2A), which regulates autophosphorylation of ATM. In addition, EIF2S6/Int-6, which was originally identified as a common integration site for the mouse mammary tumour virus in virally induced mouse mammary tumours, is a candidate breast cancer susceptibility gene because of its putative role in maintaining chromosome stability. To investigate the role of ATR, CHK1, PPP2R1B, PPP2R5B and EIF2S6/Int-6, we carried out mutation analysis of these genes in the index cases from non-BRCA1/BRCA2 breast cancer families. We also screened sporadic breast tumours for somatic mutations in PPP2R1B and PPP2R5B. Although we identified many novel variants, we found no evidence that highly penetrant germline mutations in these five genes contribute to familial breast cancer susceptibility.