Rapid dephosphorylation of p107 following UV irradiation

Understanding Retinoblastoma Post-Translational Regulation for the Design of Targeted Cancer Therapies

Simple Summary

Rb1 is a regulator of cell cycle progression and genomic stability. This review focuses on post-translational modifications, their effect on Rb1 interactors, and their role in intracellular signaling in the context of cancer development. Finally, we highlight potential approaches to harness these post-translational modifications to design novel effective anticancer therapies.

Abstract

The retinoblastoma protein (Rb1) is a prototypical tumor suppressor protein whose role was described more than 40 years ago. Together with p107 (also known as RBL1) and p130 (also known as RBL2), the Rb1 belongs to a family of structurally and functionally similar proteins that inhibits cell cycle progression. Given the central role of Rb1 in regulating proliferation, its expression or function is altered in most types of cancer. One of the mechanisms underlying Rb-mediated cell cycle inhibition is the binding and repression of E2F transcription factors, and these processes are dependent on Rb1 phosphorylation status. However, recent work shows that Rb1 is a convergent point of many pathways and thus the regulation of its function through post-translational modifications is more complex than initially expected. Moreover, depending on the context, downstream signaling can be both E2F-dependent and -independent. This review seeks to summarize the most recent research on Rb1 function and regulation and discuss potential avenues for the design of novel cancer therapies.

PP2A holoenzymes, substrate specificity driving cellular functions and deregulation in cancer

PP2A is a highly conserved eukaryotic serine/threonine protein phosphatase of the PPP family of phosphatases with fundamental cellular functions. In cells, PP2A targets specific subcellular locations and substrates by forming heterotrimeric holoenzymes, where a core dimer consisting of scaffold (A) and catalytic (C) subunits complexes with one of many B regulatory subunits. PP2A plays a key role in positively and negatively regulating a myriad of cellular processes, as it targets a very sizable fraction of the cellular substrates phosphorylated on Ser/Thr residues. This review focuses on insights made toward the understanding on how the subunit composition and structure of PP2A holoenzymes mediates substrate specificity, the role of substrate modulation in the signaling of cellular division, growth, and differentiation, and its deregulation in cancer.

The Temporal Regulation of S Phase Proteins During G1

Successful DNA replication requires intimate coordination with cell-cycle progression. Prior to DNA replication initiation in S phase, a series of essential preparatory events in G₁ phase ensures timely, complete, and precise genome duplication. Among the essential molecular processes are regulated transcriptional upregulation of genes that encode replication proteins, appropriate post-transcriptional control of replication factor abundance and activity, and assembly of DNA-loaded protein complexes to license replication origins. In this chapter we describe these critical G₁ events necessary for DNA replication and their regulation in the context of both cell-cycle entry and cell-cycle progression.

Methylation of promoter of RBL1 enhances the radioresistance of three dimensional cultured carcinoma cells

Three dimensional (3D) culture in vitro is a new cell culture model that more closely mimics the physiology features of the in vivo environment and is being used widely in the field of medical and biological research. It has been demonstrated that cancer cells cultured in 3D matrices are more radioresistant compared with cells in monolayer (2D). However, the mechanisms causing this difference remain largely unclear. Here we found that the cell cycle distribution and expression of cell cycle regulation genes in 3D A549 cells are different from the 2D. The higher levels of the promotor methylation of cell cycle regulation genes such as RBL1 were observed in 3D A549 cells compared with cells in 2D. The treatments of irradiation or 5-Aza-CdR activated the demethylation of RBL1 promotor and resulted in the increased expression of RBL1 only in 3D A549 cells. Inhibition of RBL1 enhanced the radioresistance and decreased the G2/M phase arrest induced by irradiation in 2D A549 and MCF7 cells. Overexpression of RBL1 sensitized 3D cultured A549 and MCF7 cells to irradiation. Taken together, to our knowledge, it is the first time to revealthat the low expression of RBL1 due to itself promotor methylation in 3D cells enhances the radioresistance. Our finding sheds a new light on understanding the features of the 3D cultured cell model and its application in basic research into cancer radiotherapy and medcine development.

PP2A: more than a reset switch to activate pRB proteins during the cell cycle and in response to signaling cues

In their active hypophosphorylated state, members of the retinoblastoma family of pocket proteins negatively regulate cell cycle progression at least in part by repressing expression of E2F-dependent genes. Mitogen-dependent activation of G1 and G1/S Cyclin Dependent Kinases (CDKs) results in coordinated hyperphosphorylation and inactivation of these proteins, which no longer bind and repress E2Fs. S and G2/M CDKs maintain pocket protein hyperphosphorylated through the end of mitosis. The inactivating action of inducible CDKs is opposed by the Ser/Thr protein phosphatases PP2A and PP1. Various trimeric PP2A holoenzymes have been implicated in dephosphorylation of pocket proteins in response to specific cellular signals and stresses or as part of an equilibrium with CDKs throughout the cell cycle. PP1 has specifically been implicated in dephosphorylation of pRB in late mitosis and early G1. This review is particularly focused on the emerging role of PP2A as a major hub for integration of growth suppressor signals that require rapid inactivation of pocket proteins. Of note, activation of particular PP2A holoenzymes triggers differential activation of pocket proteins in the presence of active CDKs.

The B″ regulatory subunit of protein phosphatase 2A mediates the dephosphorylation of rice retinoblastoma-related protein-1

The phosphorylation of plant retinoblastoma-related (RBR) proteins by cyclin-dependent kinases (CDKs) is well documented, but the counteracting phosphatases have not been identified yet. We report here that rice retinoblastoma-related protein-1 (OsRBR1) interacted with the B″ subunit of rice protein phosphatase 2A (OsPP2A B″) and underwent reversible phosphorylation during the cell division cycle. The OsRBR1-OsPP2A B" association required B domain in OsRBR1 and the C-terminal region of OsPP2A B″. We found by immunoprecipitation that OsPP2A B″, OsPP2A catalytic subunit subtype II, PSTAIRE-type CDK and OsRBR1 were in the same protein complex, indicating a physical association between the phosphatase, the kinase and their common substrate. OsPP2A B″ contains three predicted CDK phosphorylation sites: Ser95, Ser102 and Ser119. The in vitro phosphorylation of Ser95 and Ser119 with PSTAIRE-kinases was verified by mass spectrometry. We generated a series of phosphorylation site mutants to mimic the dephosphorylated or phosphorylated states of OsPP2A B″, and confirmed that all of the three predicted sites can be phosphorylated. Yeast two-hybrid experiments suggested that the phosphorylation of OsPP2A B″ promoted the formation of the OsPP2A holoenzyme. A triple phosphorylation mimicking OsPP2A B″ mutant containing holoenzyme showed higher activity in phosphatase assays. Our data collectively show that the phosphatase activity of OsPP2A against OsRBR1 is regulated by the phosphorylation of its B″ regulatory subunit. However, the analysis of the effect of okadaic acid, a phosphatase inhibitor, in rice cell suspension cultures revealed that the dephosphorylation of OsRBR1 was completely inhibited only by high dose (300 nM) of the okadaic acid during the cell cycle progression. Therefore the role of the protein phosphatase 1 should be considered as an additional post translational regulatory component of RBR protein function in higher plants.

Activation of p107 by fibroblast growth factor, which is essential for chondrocyte cell cycle exit, is mediated by the protein phosphatase 2A/B55α holoenzyme

The phosphorylation state of pocket proteins during the cell cycle is determined at least in part by an equilibrium between inducible cyclin-dependent kinases (CDKs) and serine/threonine protein phosphatase 2A (PP2A). Two trimeric holoenzymes consisting of the core PP2A catalytic/scaffold dimer and either the B55α or PR70 regulatory subunit have been implicated in the activation of p107/p130 and pRB, respectively. While the phosphorylation state of p107 is very sensitive to forced changes of B55α levels in human cell lines, regulation of p107 in response to physiological modulation of PP2A/B55α has not been elucidated. Here we show that fibroblast growth factor 1 (FGF1), which induces maturation and cell cycle exit in chondrocytes, triggers rapid accumulation of p107-PP2A/B55α complexes coinciding with p107 dephosphorylation. Reciprocal solution-based mass spectrometric analysis identified the PP2A/B55α complex as a major component in p107 complexes, which also contain E2F/DPs, DREAM subunits, and/or cyclin/CDK complexes. Of note, p107 is one of the preferred partners of B55α, which also associates with pRB in RCS cells. FGF1-induced dephosphorylation of p107 results in its rapid accumulation in the nucleus and formation of larger complexes containing p107 and enhances its interaction with E2F4 and other p107 partners. Consistent with a key role of B55α in the rapid activation of p107 in chondrocytes, limited ectopic expression of B55α results in marked dephosphorylation of p107 while B55α knockdown results in hyperphosphorylation. More importantly, knockdown of B55α dramatically delays FGF1-induced dephosphorylation of p107 and slows down cell cycle exit. Moreover, dephosphorylation of p107 in response to FGF1 treatment results in early recruitment of p107 to the MYC promoter, an FGF1/E2F-regulated gene. Our results suggest a model in which FGF1 mediates rapid dephosphorylation and activation of p107 independently of the CDK activities that maintain p130 and pRB hyperphosphorylation for several hours after p107 dephosphorylation in maturing chondrocytes.

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.

PP1 and PP2A phosphatases--cooperating partners in modulating retinoblastoma protein activation

The retinoblastoma/pocket protein family is one of the master regulators of the eukaryotic cell cycle. It includes the retinoblastoma protein (Rb) and the related p107 and p130 proteins. The importance of the Rb pathway for homeostasis and tumour suppression is evident from the fact that inactivating mutations in Rb are frequently associated with many cancers. Rbs regulate the cell cycle by controlling the activity of the E2F family of transcription factors. The activity of Rb proteins themselves is modulated by their phosphorylation status at several Ser/Thr residues: phosphorylation by cyclin-dependent kinases inactivates Rb proteins and positively influences the transcription of genes necessary for cell cycle progression. Although the mechanisms of cyclin-dependent kinase-mediated inactivation of Rb proteins are understood in great detail, our knowledge of the process that counteracts Rb phosphorylation is still quite limited. The present review focuses on the Ser/Thr phosphatases that are responsible for the dephosphorylation and thus activation of Rb proteins. Two major scenarios are considered: (a) when pocket proteins are dephosphorylated during regular cell cycle progression and (b) when rapid dephosphorylation is dictated by external stress or growth inhibitory conditions, such as oxidative stress, UV radiation or other DNA-damaging stimuli, and cell differentiation factors. It transpires that protein phosphatase 1 and protein phosphatase 2A can efficiently modulate pocket protein activity in a highly context-dependent manner and both are tightly regulated by the presence of different regulatory subunits or interacting proteins.

PP2A holoenzymes negatively and positively regulate cell cycle progression by dephosphorylating pocket proteins and multiple CDK substrates

Cell cycle progression is negatively regulated by the retinoblastoma family of pocket proteins and CDK inhibitors (CKIs). In contrast, CDKs promote progression through multiple phases of the cell cycle. One prominent way by which CDKs promote cell cycle progression is by inactivation of pocket proteins via hyperphosphorylation. Reactivation of pocket proteins to halt cell cycle progression requires dephosphorylation of multiple CDK-phosphorylated sites and is accomplished by PP2A and PP1 serine/threonine protein phosphatases. The same phosphatases are also implicated in dephosphorylation of multiple CDK substrates as cells exit mitosis and reenter the G1 phase of the cell cycle. This review is primarily focused on the role of PP2A and PP1 in the activation of pocket proteins during the cell cycle and in response to signaling cues that trigger cell cycle exit. Other functions of PP2A during the cell cycle will be discussed in brief, as comprehensive reviews on this topic have been published recently (De Wulf et al., 2009; Wurzenberger and Gerlich, 2011).

The substrate of Greatwall kinase, Arpp19, controls mitosis by inhibiting protein phosphatase 2A

Initiation and maintenance of mitosis require the activation of protein kinase cyclin B-Cdc2 and the inhibition of protein phosphatase 2A (PP2A), which, respectively, phosphorylate and dephosphorylate mitotic substrates. The protein kinase Greatwall (Gwl) is required to maintain mitosis through PP2A inhibition. We describe how Gwl activation results in PP2A inhibition. We identified cyclic adenosine monophosphate-regulated phosphoprotein 19 (Arpp19) and α-Endosulfine as two substrates of Gwl that, when phosphorylated by this kinase, associate with and inhibit PP2A, thus promoting mitotic entry. Conversely, in the absence of Gwl activity, Arpp19 and α-Endosulfine are dephosphorylated and lose their capacity to bind and inhibit PP2A. Although both proteins can inhibit PP2A, endogenous Arpp19, but not α-Endosulfine, is responsible for PP2A inhibition at mitotic entry in Xenopus egg extracts.

B55alpha PP2A holoenzymes modulate the phosphorylation status of the retinoblastoma-related protein p107 and its activation

Pocket proteins negatively regulate transcription of E2F-dependent genes and progression through the G(0)/G(1) transition and the cell cycle restriction point in G(1). Pocket protein repressor activities are inactivated via phosphorylation at multiple Pro-directed Ser/Thr sites by the coordinated action of G(1) and G(1)/S cyclin-dependent kinases. These phosphorylations are reversed by the action of two families of Ser/Thr phosphatases: PP1, which has been implicated in abrupt dephosphorylation of retinoblastoma protein (pRB) in mitosis, and PP2A, which plays a role in an equilibrium that counteracts cyclin-dependent kinase (CDK) action throughout the cell cycle. However, the identity of the trimeric PP2A holoenzyme(s) functioning in this process is unknown. Here we report the identification of a PP2A trimeric holoenzyme containing B55α, which plays a major role in restricting the phosphorylation state of p107 and inducing its activation in human cells. Our data also suggest targeted selectivity in the interaction of pocket proteins with distinct PP2A holoenzymes, which is likely necessary for simultaneous pocket protein activation.

p107 in the public eye: an Rb understudy and more

p107 and its related family members Rb and p130 are critical regulators of cellular proliferation and tumorigenesis. Due to the extent of functional overlap within the Rb family, it has been difficult to assess which functions are exclusive to individual members and which are shared. Like its family members, p107 can bind a variety of cellular proteins to affect the expression of many target genes during cell cycle progression. Unlike Rb and p130, p107 is most highly expressed during the G1 to S phase transition of the cell cycle in actively dividing cells and accumulating evidence suggests a role for p107 during DNA replication. The specific roles for p107 during differentiation and development are less clear, although emerging studies suggest that it can cooperate with other Rb family members to control differentiation in multiple cell lineages. As a tumor suppressor, p107 is not as potent as Rb, yet studies in knockout mice have revealed some tumor suppressor functions in mice, depending on the context. In this review, we identify the unique and overlapping functions of p107 during the cell cycle, differentiation, and tumorigenesis.

Repression of RAD51 gene expression by E2F4/p130 complexes in hypoxia

We and others have shown that the dysregulation of DNA repair pathways can contribute to the phenomenon of hypoxia-induced genetic instability within the tumor microenvironment. Several studies have revealed that the recombinational repair genes, RAD51 and BRCA1, and the DNA mismatch repair genes, MLH1 and MSH2, are decreased in expression in response to hypoxic stress, prompting interest in elucidating the mechanistic basis for these responses. Here we report that the downregulation of RAD51 by hypoxia is specifically mediated by repressive E2F4/p130 complexes that bind to a single E2F site in the proximal promoter of the gene. Intriguingly, this E2F site is conserved in the promoter of the BRCA1 gene, which is also regulated by a similar mechanism in hypoxia. Mechanistically, we have found that hypoxia induces substantial p130 dephosphorylation and nuclear accumulation, leading to the formation of E2F4/p130 complexes and increased occupancy of E2F4 and p130 at the RAD51 and BRCA1 promoters. These findings reveal a coordinated transcriptional program mediated by the formation of repressive E2F4/p130 complexes that represents an integral response to hypoxic stress. In addition, this co-regulation of key factors within the homology-dependent DNA repair pathway provides a further basis for understanding genetic instability in tumors and may guide the design of new therapeutic strategies for cancer.

Pocket protein function in melanocyte homeostasis and neoplasia

Melanoma is the most lethal of human skin cancers and its incidence is increasing worldwide [L.K. Dennis (1999). Arch. Dermatol. 135, 275; C. Garbe et al. (2000). Cancer 89, 1269]. Melanomas often metastasize early during the course of the disease and are then highly intractable to current therapeutic regimens [M.F. Demierre and G. Merlino (2004). Curr. Oncol. Rep. 6, 406]. Consequently, understanding the factors that maintain melanocyte homeostasis and prevent their neoplastic transformation into melanoma is of utmost interest from the perspective of therapeutic interdiction. This review will focus on the role of the pocket proteins (PPs), Rb1 (retinoblastoma protein), retinoblastoma-like 1 (Rbl1 also known as p107) and retinoblastoma-like 2 (Rbl2 also known as p130), in melanocyte homeostasis, with particular emphasis on their functions in the cell cycle and the DNA damage repair response. The potential mechanisms of PP deregulation in melanoma and the possibility of PP-independent pathways to melanoma development will also be considered. Finally, the role of the PP family in ultraviolet radiation (UVR)-induced melanoma and the precise contribution that each PP family member makes to melanocyte homeostasis will be discussed in the context of a number of genetically engineered mouse models.

The retinoblastoma gene family in cell cycle regulation and suppression of tumorigenesis

Since its discovery in 1986, as the first tumor suppressor gene, the retinoblastoma gene (Rb) has been extensively studied. Numerous biochemical and genetic studies have elucidated in great detail the function of the Rb gene and placed it at the heart of the molecular machinery controlling the cell cycle. As more insight was gained into the genetic events required for oncogenic transformation, it became clear that the retinoblastoma gene is connected to biochemical pathways that are dysfunctional in virtually all tumor types. Besides regulating the E2F transcription factors, pRb is involved in numerous biological processes such as apoptosis, DNA repair, chromatin modification, and differentiation. Further complexity was added to the system with the discovery of p107 and p130, two close homologs of Rb. Although the three family members share similar functions, it is becoming clear that these proteins also have unique functions in differentiation and regulation of transcription. In contrast to Rb, p107 and p130 are rarely found inactivated in human tumors. Yet, evidence is accumulating that these proteins are part of a "tumor-surveillance" mechanism and can suppress tumorigenesis. Here we provide an overview of the knowledge obtained from studies involving the retinoblastoma gene family with particular focus on its role in suppressing tumorigenesis.

Mechanisms underlying differential responses to FGF signaling

Fibroblast growth factors (FGFs) are key regulators of several developmental processes in which cell fate and differentiation to various tissue lineages are determined. The importance of the proper spatial and temporal regulation of FGF signals is evident from human and mouse genetic studies which show that mutations leading to the dysregulation of FGF signals cause a variety of developmental disorders including dominant skeletal diseases and cancer. The FGF ligands signal via a family of receptor tyrosine kinases and, depending on the cell type or stage of maturation, produce diverse biological responses that include proliferation, growth arrest, differentiation or apoptosis. A central issue in FGF biology is to understand how these diverse cellular responses are determined and how similar signaling inputs can generate distinct patterns of gene expression that govern the specificity of the cellular response. In this review we draw upon studies from the past fifteen years and attempt to construct a molecular picture of the different levels of regulation by which such specific cellular responses could be achieved by FGF signals. We discuss whether specificity could lie in the nature of the ligand, the particular receptor, the signal transduction pathways utilized, or the transcriptional regulation of specific genes. Finally, we also discuss how the interplay of FGF signals with other signaling systems could contribute to the cellular response. In particular we focus on the interaction with the Wnt pathway since FGF/Wnt cross-talk is emerging as an important nexus in regulating a variety of biological processes.

SV40 large T antigen promotes dephosphorylation of p130

SV40 large T antigen (Ag) binds to all members of the retinoblastoma (RB) tumor suppressor family including pRb, p107, and p130. Although the LXCXE motif of T Ag binds directly to the RB proteins, it is not sufficient to fully inactivate their function. The N-terminal DNA J domain of T Ag cooperates with the LXCXE motif to override RB-mediated repression of E2F-dependent transcription. In addition, T Ag can reduce the overall phosphorylation state of p107 and p130 that is dependent on an intact J domain and LXCXE motif. However, the mechanism of this activity has not been described. Here we describe the use of a cell-free system to characterize the effect of T Ag on p130 phosphorylation. When incubated in extracts prepared from S phase cells, p130 undergoes specific phosphorylation. Addition of T Ag to S phase extracts leads to a reduction of p130 phosphorylation in vitro. The ability of T Ag to reduce the phosphorylation of p130 in vitro is dependent on an intact DNA J domain and can be inhibited by okadaic acid and PP2A-specific inhibitors. These results suggest that T Ag recruits a phosphatase activity in a DNA J domain-dependent manner to reduce the phosphorylation of p130.

Active localization of the retinoblastoma protein in chromatin and its response to S phase DNA damage

The Rb protein suppresses development of an abnormal state of endoreduplication arising after S phase DNA damage. In diploid, S phase cells, the activity of protein phosphatase 2A (PP2A) licenses the stable association of un(der)phosphorylated Rb with chromatin. After damage, chromatin-associated pRb is attracted to certain chromosomal replication initiation sites in the order in which they normally fire. Like S phase DNA damage in Rb(-/-) cells, specific interruption of PP2A function in irradiated, S phase wt cells also elicited a state of endoreduplication. Thus, PP2A normally licenses the recruitment of Rb to chromatin sites in S phase from which, after DNA damage, it relocalizes to selected replication control sites and suppresses abnormal, postdamage rereplicative activity.

Activation of the ERK1/2 and p38 mitogen-activated protein kinase pathways mediates fibroblast growth factor-induced growth arrest of chondrocytes

Fibroblast growth factors (FGFs) regulate long bone development by affecting the proliferation and differentiation of chondrocytes. FGF treatment inhibits the proliferation of chondrocytes both in vitro and in vivo, but the signaling pathways involved have not been clearly identified. In this report we show that both the MEK-ERK1/2 and p38 MAPK pathways, but not phospholipase C gamma or phosphatidylinositol 3-kinase, play a role in FGF-mediated growth arrest of chondrocytes. Chemical inhibitors of the MEK1/2 or the p38 MAPK pathways applied to rat chondrosarcoma (RCS) chondrocytes significantly prevented FGF-induced growth arrest. The retinoblastoma family members p107 and p130 were previously shown to be essential effectors of FGF-induced growth arrest in chondrocytes. The dephosphorylation of p107, one of the earliest events in RCS growth arrest, was significantly blocked by MEK1/2 inhibitors but not by the p38 MAPK inhibitors, whereas that of p130, which occurs later, was partially prevented both by the MEK and p38 inhibitors. Furthermore, by expressing the nerve growth factor (NGF) receptor, TrkA, and the epidermal growth factor (EGF) receptor, ErbB1, in RCS cells we show that NGF treatment of the transfected cells caused growth inhibition, whereas EGF did not. FGF- and NGF-induced growth inhibition is accompanied by a strong and sustained activation of ERK1/2 and p38 MAPK and a decrease of AKT phosphorylation, whereas EGF induces a much more transient activation of p38 and ERK1/2 and increases AKT phosphorylation. These results indicate that inhibition of chondrocyte proliferation by FGF requires both ERK1/2 and p38 MAPK signaling and also suggest that sustained activation of these pathways is required to achieve growth inhibition.

Oxidative stress induces protein phosphatase 2A-dependent dephosphorylation of the pocket proteins pRb, p107, and p130

Oxidative stress induces cell death and growth arrest. In this study, the regulation and the functional role of the retinoblastoma family proteins pRb, p107, and p130 in the cellular response to oxidative stress were investigated. Treatment of endothelial cells with H2O2 induced rapid hypophosphorylation of the retinoblastoma family proteins. This event did not require p53 or p21Waf1/Cip1/Sdi1 and was not associated with cyclin/cyclin-dependent kinase down-modulation. Four lines of evidence indicate that H2O2-induced hypophosphorylation of pRb, p107, and p130 was because of the activity of protein phosphatase 2A (PP2A). First, cell treatment with two phosphatase inhibitors, okadaic acid and calyculin A, prevented the hypophosphorylation of the retinoblastoma family proteins, at concentrations that specifically inhibit PP2A. Second, SV40 small t, which binds and inhibits PP2A, when overexpressed prevented H2O2-induced dephosphorylation of the retinoblastoma family proteins, whereas a SV40 small t mutant unable to bind PP2A was totally inert. Third, PP2A core enzyme physically interacted with pRb and p107, both in H2O2-treated and untreated cells. Fourth, a PP2A phosphatase activity was co-immunoprecipitated with pRb, and the activity of pRb-associated PP2A was positively modulated by cell treatment with H2O2. Because DNA damaging agents inhibit DNA synthesis in a pRb-dependent manner, it was determined whether the PP2A-mediated dephosphorylation of the retinoblastoma family proteins played a role in this S-phase response. Indeed, it was found that inhibition of PP2A by SV40 small t over-expression prevented DNA synthesis inhibition induced by H2O2.

An E2F-binding site mediates the activation of the proliferative isoform of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase by phosphatidylinositol 3-kinase

In the present study, we demonstrate that E2F is implicated in the regulation of the glycolytic enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF2K/Fru-2,6-BPase) during cell division. The expression of this enzyme is induced during the G(1)/S transition of the cell cycle. We identified and monitored the E2F-pocket protein complexes that bind to the E2F site of the F-type promoter during cell-cycle entry, and we analysed their contribution to the phosphatidylinositol 3-kinase (PI 3-kinase)-mediated regulation of the promoter. We found that the predominant E2F complex bound to the F-type promoter in unstimulated/quiescent cells contains E2F4, DP1 and p130 proteins. In serum-stimulated (S-phase) cells, the composition of the complex switched to E2F1/4, DP1 and p107, together with cyclin A and cyclin-dependent kinase 2. Treatment with the PI 3-kinase specific inhibitor LY 294002 prevented the formation of the S-phase complex, suggesting that activation of the PI 3-kinase pathway is essential for the formation of this complex. Further supporting this idea, we obtained results showing that treatment of cycling NIH 3T3 cells with either wortmannin or LY 294002 induces the accumulation of the transcriptionally repressive p130-E2F4-DP1 complex. Using the Rat-1 ER-E2F1 cell line where E2F1 activity can be conditionally induced, we demonstrated that E2F activity is involved in the in vivo transcriptional regulation of the F-type 6PF2K/Fru-2, 6-BPase gene. Taken together, our results show that the F-type 6PF2K/Fru-2, 6-BPase is a genuine E2F-regulated gene, and that its regulation by the PI 3-kinase pathway is at least partially mediated through the E2F transcription factor.

Overexpression of the protein phosphatase 2A regulatory subunit Bgamma promotes neuronal differentiation by activating the MAP kinase (MAPK) cascade

Protein serine/threonine phosphatase 2A (PP2A) is a multifunctional regulator of cellular signaling. Variable regulatory subunits associate with a core dimer of scaffolding and catalytic subunits and are postulated to dictate substrate specificity and subcellular location of the heterotrimeric PP2A holoenzyme. The role of brain-specific regulatory subunits in neuronal differentiation and signaling was investigated in the PC6-3 subline of PC12 cells. Endogenous Bbeta, Bgamma, and B'beta protein expression was induced during nerve growth factor (NGF)-mediated neuronal differentiation. Transient expression of Bgamma, but not other PP2A regulatory subunits, facilitated neurite outgrowth in the absence and presence of NGF. Tetracycline-inducible expression of Bgamma caused growth arrest and neurofilament expression, further evidence that PP2A/Bgamma can promote differentiation. In PC6-3 cells, but not non-neuronal cell lines, Bgamma specifically promoted long lasting activation of the mitogen-activated protein (MAP) kinase cascade, a key mediator of neuronal differentiation. Pharmacological and dominant-negative inhibition and kinase assays indicate that Bgamma promotes neuritogenesis by stimulating the MAP kinase cascade downstream of the TrkA NGF receptor but upstream or at the level of the B-Raf kinase. Mutational analyses demonstrate that the divergent N terminus is critical for Bgamma activity. These studies implicate PP2A/Bgamma as a positive regulator of MAP kinase signaling in neurons.

FGF signaling targets the pRb-related p107 and p130 proteins to induce chondrocyte growth arrest

Unregulated FGF signaling affects endochondral ossification and long bone growth, causing several genetic forms of human dwarfism. One major mechanism by which FGFs regulate endochondral bone growth is through their inhibitory effect on chondrocyte proliferation. Because mice with targeted mutations of the retinoblastoma (Rb)-related proteins p107 and p130 present severe endochondral bone defects with excessive chondrocyte proliferation, we have investigated the role of the Rb family of cell cycle regulators in the FGF response. Using a chondrocyte cell line, we found that FGF induced a rapid dephosphorylation of all three proteins of the Rb family (pRb, p107, and p130) and a blockade of the cells in the G1 phase of the cell cycle. This cell cycle block was reversed by inactivation of Rb proteins with viral oncoproteins such as polyoma large T (PyLT) antigen and Adenovirus E1A. Expression of a PyLT mutant that efficiently binds pRb, but not p107 and p130, allowed the cells to be growth inhibited by FGF, suggesting that pRb itself is not involved in the FGF response. To investigate more precisely the role of the individual Rb family proteins in FGF-mediated growth inhibition, we used chondrocyte micromass culture of limb bud cells isolated from mice lacking Rb proteins individually or in combination. Although wild-type as well as Rb-/- chondrocytes were similarly growth inhibited by FGF, chondrocytes null for p107 and p130 did not respond to FGF. Furthermore, FGF treatment of metatarsal bone rudiments obtained from p107-/-;p130-/- embryos failed to inhibit proliferation of growth plate chondrocytes, whereas rudiments from p107-null or p130-null embryos showed only a slight inhibition of growth. Our findings indicate that p107 and p130, but not pRb, are critical effectors of FGF-mediated growth inhibition in chondrocytes.

The zinc finger domain of NEMO is selectively required for NF-kappa B activation by UV radiation and topoisomerase inhibitors

Exposure of mammalian cells to UV radiation was proposed to stimulate the transcription factor NF-kappa B by a unique mechanism. Typically, rapid and strong inducers of NF-kappa B, such as tumor necrosis factor alpha (TNF-alpha) and bacterial lipopolysaccharide (LPS), lead to rapid phosphorylation and proteasomal degradation of its inhibitory protein, I kappa B alpha. In contrast, UV, a relatively slower and weaker inducer of NF-kappa B, was suggested not to require phosphorylation of I kappa B alpha for its targeted degradation by the proteasome. We now provide evidence to account for this peculiar degradation process of I kappa B alpha. The phospho-I kappa B alpha generated by UV is only detectable by expressing a Delta F-box mutant of the ubiquitin ligase beta-TrCP, which serves as a specific substrate trap for serine 32 and 36 phosphorylated I kappa B alpha. In agreement with this finding, we also find that the I kappa B kinase (IKK) phospho-acceptor sites on I kappa B alpha, core components of the IKK signalsome, and IKK catalytic activity are all required for UV signaling. Furthermore, deletion and point mutation analyses reveal that both the amino-terminal IKK-binding and the carboxy-terminal putative zinc finger domains of NEMO (IKK gamma) are critical for UV-induced NF-kappa B activation. Interestingly, the zinc finger domain is also required for NF-kappa B activation by two other slow and weak inducers, camptothecin and etoposide. In contrast, the zinc finger module is largely dispensable for NF-kappa B activation by the rapid and strong inducers LPS and TNF-alpha. Thus, we suggest that the zinc finger domain of NEMO likely represents a point of convergence for signaling pathways initiated by slow and weak NF-kappa B-activating conditions.

Signaling by protein phosphatases in the nucleus

The nucleus contains a large variety of protein phosphatases, which function in key processes such as cell-cycle progression, replication, transcription and RNA processing. Here, we review the pleiotropic action of nuclear protein phosphatases and focus in particular on the underlying signaling strategies. It appears that nuclear protein phosphatases can both mediate and antagonize signaling by protein kinases, sometimes as part of feedback loops. Some protein phosphatases shuttle between the cytoplasm and the nucleus, which enables them to act as signal transducers between both compartments. An emerging theme is the contribution of protein phosphatases to cycles of protein phosphorylation and dephosphorylation that steer the assembly and firing of molecular machines in the nucleus.

ATM-dependent dissociation of B55 regulatory subunit from nuclear PP2A in response to ionizing radiation

Ionizing radiation (IR) is known to activate multiple cell cycle checkpoints that are thought to enhance the ability of cells to respond to DNA damage. Protein phosphatase 2A (PP2A) has been implicated in IR-induced activation of checkpoints; therefore, Jurkat cells were exposed to an activating dose of IR or sham treatment as control, and nuclear extracts were analyzed for PP2A by Mono Q anion exchange chromatography and microcystin affinity chromatography. PP2A exists in eukaryotic cells both as a heterodimer consisting of a 65-kDa scaffolding subunit (A) plus a 36-kDa catalytic subunit (C) and as ABC heterotrimers, containing one of a variety of regulatory (B) subunits. Here we show that IR produces a transient and reversible reduction in the amount of nuclear AB55C heterotrimer without affecting the AB'C heterotrimer or AC heterodimer. In ataxia telangiectasia-mutated (ATM)-deficient cells the amount of nuclear PP2A heterotrimer relative to heterodimer was not reduced by radiation, but the radiation response was restored by transfection of these cells with plasmids encoding ATM. Wortmannin, an inhibitor of kinases such as phosphatidylinositol 3-kinase, also prevented the IR-induced reduction in nuclear PP2A heterotrimer. The changes in nuclear PP2A occurred without a noticeable difference in the carboxyl-terminal methylation of the C subunit, which is known to influence association with B subunits. We conclude a novel ATM-dependent mechanism is regulating association of B55 subunits with nuclear PP2A in response to IR.

Involvement of pRB-related p107 protein in the inhibition of S phase progression in response to genotoxic stress

pRB family pocket proteins consisting of pRB, p107, and p130 are thought to act as a set of growth regulators that inhibit the cell cycle transition from G1 to S phases by virtue of their interaction with E2F transcription factors. When cells are committed to progressing through the cell cycle at the late G1 restriction point, they are hyperphosphorylated by G1 cyclin-cyclin-dependent kinase and are functionally inactivated. Consistent with such a G1 regulatory role, pRB and p130 are abundantly expressed in quiescent cells. In contrast, p107 is present at low levels in the hypophosphorylated form in quiescent cells. As cells progress toward late G1 to S phases, the levels of p107 increase, and the majority become hyperphosphorylated, suggesting a possible role of p107 in post-G1 cell cycle regulation. In this study, we have demonstrated that a nonphosphorylatable and thus constitutively active p107 has the potential to inhibit S phase progression. The levels of the phosphorylation-resistant p107 required for the S phase inhibition are significantly less than those of endogenous p107. We further show herein that the exposure of cells to the DNA-damaging agent, cisplatin, provokes S phase arrest, which is concomitantly associated with the accumulation of hypophosphorylated p107. Furthermore, the S phase inhibitory response to cisplatin is augmented by the ectopic expression of wild type p107, although it is diminished by the adenovirus E1A oncoprotein, which counteracts the pocket protein functions. Because p107 is a major pRB family protein expressed in S phase cells, our results indicate that p107 participates in an inhibition of cell cycle progression in response to DNA damage in S phase cells.

The gene PC3(TIS21/BTG2), prototype member of the PC3/BTG/TOB family: regulator in control of cell growth, differentiation, and DNA repair?

PC3(TIS21/BTG2) is the founding member of a family of genes endowed with antiproliferative properties, namely BTG1, ANA/BTG3, PC3B, TOB, and TOB2. PC3 was originally isolated as a gene induced by nerve growth factor during neuronal differentiation of rat PC12 cells, or by TPA in NIH3T3 cells (named TIS21), and is a marker for neuronal birth in vivo. This and other findings suggested its implication in the process of neurogenesis as mediator of the growth arrest before differentiation. Remarkably, its human homolog, named BTG2, was shown to be p53-inducible, in conditions of genotoxic damage. PC3(TIS21/BTG2) impairs G(1)-S progression, either by a Rb-dependent pathway through inhibition of cyclin D1 transcription, or in a Rb-independent fashion by cyclin E downregulation. PC3(TIS21/BTG2) might also control the G(2) checkpoint. Furthermore, PC3(TIS21/BTG2) interacts with carbon catabolite repressor protein-associated factor 1 (CAF-1), a molecule that associates to the yeast transcriptional complex CCR4 and might influence cell cycle, with the transcription factor Hoxb9, and with the protein-arginine methyltransferase 1, that might control transcription through histone methylation. Current evidence suggests a physiological role of PC3(TIS21/BTG2) in the control of cell cycle arrest following DNA damage and other types of cellular stress, or before differentiation of the neuron and other cell types. The molecular function of PC3(TIS21/BTG2) is still unknown, but its ability to modulate cyclin D1 transcription, or to synergize with the transcription factor Hoxb9, suggests that it behaves as a transcriptional co-regulator.

Protein phosphatase 2A: a highly regulated family of serine/threonine phosphatases implicated in cell growth and signalling

Protein phosphatase 2A (PP2A) comprises a family of serine/threonine phosphatases, minimally containing a well conserved catalytic subunit, the activity of which is highly regulated. Regulation is accomplished mainly by members of a family of regulatory subunits, which determine the substrate specificity, (sub)cellular localization and catalytic activity of the PP2A holoenzymes. Moreover, the catalytic subunit is subject to two types of post-translational modification, phosphorylation and methylation, which are also thought to be important regulatory devices. The regulatory ability of PTPA (PTPase activator), originally identified as a protein stimulating the phosphotyrosine phosphatase activity of PP2A, will also be discussed, alongside the other regulatory inputs. The use of specific PP2A inhibitors and molecular genetics in yeast, Drosophila and mice has revealed roles for PP2A in cell cycle regulation, cell morphology and development. PP2A also plays a prominent role in the regulation of specific signal transduction cascades, as witnessed by its presence in a number of macromolecular signalling modules, where it is often found in association with other phosphatases and kinases. Additionally, PP2A interacts with a substantial number of other cellular and viral proteins, which are PP2A substrates, target PP2A to different subcellular compartments or affect enzyme activity. Finally, the de-regulation of PP2A in some specific pathologies will be touched upon.

N-acetyl-L-cysteine protects SHSY5Y neuroblastoma cells from oxidative stress and cell cytotoxicity: effects on beta-amyloid secretion and tau phosphorylation

Redox changes within neurones are increasingly being implicated as an important causative agent in brain ageing and neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and Alzheimer's disease (AD). Cells have developed a number of defensive mechanisms to maintain intracellular redox homeostasis, including the glutathione (GSH) system and antioxidant enzymes. Here we examine the effects of N-acetyl-L-cysteine (NAC) on beta-amyloid (A beta) secretion and tau phosphorylation in SHSY5Y neuroblastoma cells after exposure to oxidative stress inducing/cytotoxic compounds (H(2)O(2), UV light and toxic A beta peptides). A beta and tau protein are hallmark molecules in the pathology of AD while the stress factors are implicated in the aetiology of AD. The results show that H(2)O(2), UV light, A beta 1-42 and toxic A beta 25-35, but not the inactive A beta 35-25, produce a significant induction of oxidative stress and cell cytotoxicity. The effects are reversed when cells are pre-treated with 30 mM NAC. Cells exposed to H(2)O(2), UV light and A beta 25-35, but not A beta 35-25, secrete significantly higher amounts of A beta 1-40 and A beta 1-42 into the culture medium. NAC pre-treatment increased the release of A beta 1-40 compared with controls and potentiated the release of both A beta 1-40 and A beta 1-42 in A beta 25-35-treated cells. Tau phosphorylation was markedly reduced by H(2)O(2) and UV light but increased by A beta 25-35. NAC strongly lowered phospho-tau levels in the presence or absence of stress treatment.

RB-dependent S-phase response to DNA damage

The retinoblastoma tumor suppressor protein (RB) is a potent inhibitor of cell proliferation. RB is expressed throughout the cell cycle, but its antiproliferative activity is neutralized by phosphorylation during the G(1)/S transition. RB plays an essential role in the G(1) arrest induced by a variety of growth inhibitory signals. In this report, RB is shown to also be required for an intra-S-phase response to DNA damage. Treatment with cisplatin, etoposide, or mitomycin C inhibited S-phase progression in Rb(+/+) but not in Rb(-/-) mouse embryo fibroblasts. Dephosphorylation of RB in S-phase cells temporally preceded the inhibition of DNA synthesis. This S-phase dephosphorylation of RB and subsequent inhibition of DNA replication was observed in p21(Cip1)-deficient cells. The induction of the RB-dependent intra-S-phase arrest persisted for days and correlated with a protection against DNA damage-induced cell death. These results demonstrate that RB plays a protective role in response to genotoxic stress by inhibiting cell cycle progression in G(1) and in S phase.

Inhibition of S-phase progression by adeno-associated virus Rep78 protein is mediated by hypophosphorylated pRb

Adeno-associated virus (AAV) has an antiproliferative action on cells. We investigated the effect of the AAV replication proteins (Rep) on the cell division cycle using retroviral vectors. Rep78 and Rep68 inhibited the growth of primary, immortalized and transformed cells, while Rep52 and Rep40 did not. Rep68 induced cell cycle arrest in phases G(1) and G(2), with elevated CDK inhibitor p21 and reduced cyclin E-, A- and B1-associated kinase activity. Rep78-expressing cells were also impaired in S-phase progression and accumu lated almost exclusively with hypophosphorylated retinoblastoma protein (pRb). The differences between Rep78 and Rep68 were mapped to the C-terminal zinc finger domain of Rep78. Rep78-induced S-phase arrest could be bypassed by adenoviral E1A or papillomaviral E7 proteins but not by E1A or E7 mutants unable to bind pRb. Rb(-/-) primary mouse embryonic fibroblasts displayed a strongly reduced S-phase arrest when challenged with Rep78, compared with matched Rb(+/+) controls. These results suggest that physiological levels of active pRb can interfere with S-phase progression. We propose that the AAV Rep78 protein arrests cells within S-phase by a novel mechanism involving the ectopic accumulation of active pRb.

Protein phosphatase 2A: a panoply of enzymes

Protein phosphatase 2A describes an extended family of intracellular protein serine/threonine phosphatases sharing a common catalytic subunit that regulates a variety of processes by means of diverse regulatory subunits. During the past year, studies have shown that protein phosphatase 2A influences events ranging from the initiation of DNA replication to vertebrate axis formation to apoptosis.