Regulation of cell cycle progression and nuclear affinity of the retinoblastoma protein by protein phosphatases

PP2A Affects Angiogenesis via Its Interaction with a Novel Phosphorylation Site of TSP1

Alterations in angiogenic properties play a pivotal role in the manifestation and onset of various pathologies, including vascular diseases and cancer. Thrombospondin-1 (TSP1) protein is one of the master regulators of angiogenesis. This study unveils a novel aspect of TSP1 regulation through reversible phosphorylation. The silencing of the B55α regulatory subunit of protein phosphatase 2A (PP2A) in endothelial cells led to a significant decrease in TSP1 expression. Direct interaction between TSP1 and PP2A-B55α was confirmed via various methods. Truncated TSP1 constructs were employed to identify the phosphorylation site and the responsible kinase, ultimately pinpointing PKC as the enzyme phosphorylating TSP1 on Ser93. The biological effects of B55α-TSP1 interaction were also analyzed. B55α silencing not only counteracted the increase in TSP1 expression during wound closure but also prolonged wound closure time. Although B55α silenced cells initiated tube-like structures earlier than control cells, their spheroid formation was disrupted, leading to disintegration. Cells transfected with phosphomimic TSP1 S93D exhibited smaller spheroids and reduced effectiveness in tube formation, revealing insights into the effects of TSP1 phosphorylation on angiogenic properties. In this paper, we introduce a new regulatory mechanism of angiogenesis by reversible phosphorylation on TSP1 S93 by PKC and PP2A B55α.

Quantum-based modeling implies that bidentate Arg89-substrate binding enhances serine/threonine protein phosphatase-2A(PPP2R5D/PPP2R1A/PPP2CA)-mediated dephosphorylation

PP2A-serine/threonine protein phosphatases function as heterotrimeric holoenzymes, composed of a common scaffold (A-subunit encoded by PPP2R1A/PPP2R1B), a common catalytic (C-subunit encoded by PPP2CA/PPP2CB), and one of many variable regulatory (B) subunits. The site of phosphoprotein phosphatase (PPP) hydrolysis features a bimetal system (M₁/M₂), an associated bridge hydroxide [W¹(OH^-)], and a highly-conserved core sequence. In the presumptive common mechanism, the phosphoprotein's seryl/threonyl phosphate coordinates the M₁/M₂ system, W¹(OH^-) attacks the central P atom, rupturing the antipodal bond, and simultaneously, a histidine/aspartate tandem protonates the exiting seryl/threonyl alkoxide. Based on studies of PPP5C, a conserved arginine proximal to M₁ is also expected to bind the substrate's phosphate group in a bidentate fashion. However, in PP2A isozymes, the role of the arginine (Arg⁸⁹) in hydrolysis is not clear because two independent structures for PP2A(PPP2R5C) and PP2A(PPP2R5D) show that Arg⁸⁹ engages in a weak salt bridge at the B:C interface. These observations raise the question of whether hydrolysis proceeds with or without direct involvement of Arg⁸⁹. The interaction of Arg⁸⁹ with B:Glu¹⁹⁸ in PP2A(PPP2R5D) is significant because the pathogenic E198K variant of B56δ is associated with irregular protein phosphorylation levels and consequent developmental disorders (Jordan's Syndrome; OMIM #616355). In this study, we perform quantum-based hybrid [ONIOM(UB3LYP/6-31G(d):UPM7)] calculations on 39-residue models of the PP2A(PPP2R5D)/pSer (phosphoserine) system to estimate activation barriers for hydrolysis in the presence of bidentate Arg⁸⁹-substrate binding and when Arg⁸⁹ is otherwise engaged in the salt-bridge interaction. Our solvation-corrected results yield ΔH^‡ ≈ ΔE^‡ = +15.5 kcal/mol for the former case, versus +18.8 kcal/mol for the latter, indicating that bidentate Arg⁸⁹-substrate binding is critical for optimal catalytic function of the enzyme. We speculate that PP2A(PPP2R5D) activity is suppressed by B:Glu¹⁹⁸ sequestration of C:Arg⁸⁹ under native conditions, whereas the PP2A(PPP2R5D)-holoenzyme containing the E198K variant has a positively-charged lysine in this position that alters normal function.

PP2A-B55 and its adapter proteins IER2 and IER5 regulate the activity of RB family proteins and the expression of cell cycle-related genes

The retinoblastoma (RB) tumour suppressor protein regulates cell proliferation, motility, differentiation and apoptosis. The phosphorylation state of RB is modulated by kinases and phosphatases, and RB exhibits phosphorylation-sensitive interactions with E2F family transcription factors. Here, we characterize RB dephosphorylation by protein phosphatase 2A (PP2A). The growth factor-inducible immediate early response (IER) proteins IER2 and IER5 possess an adapter-like function in which IER proteins bind to both PP2A and its target proteins and enhance PP2A activity towards the proteins. IER2 interacts with RB and facilitates dephosphorylation of RB at T821/T826 by PP2A. In IER2 knockdown cells, elevated phosphorylation of RB resulted in reduced binding of RB to the promoters and derepression of cyclin D1 and p21. IER5 binds to both RB and RB-like 1 (p107/RBL1), enhances dephosphorylation of these proteins by PP2A and represses the expression of various cell cycle-related genes. However, IER2-regulated dephosphorylation at T821/T826 is not necessary for the repression function of RB in cell mobility-related gene expression. Our data identify PP2A adapter proteins as critical regulators of RB family proteins and suggest that the phosphorylation status of RB differentially affects gene expression.

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.

Mitochondrial cytochrome c shot towards histone chaperone condensates in the nucleus

Despite mitochondria being key for the control of cell homeostasis and fate, their role in DNA damage response is usually just regarded as an apoptotic trigger. However, growing evidence points to mitochondrial factors modulating nuclear functions. Remarkably, after DNA damage, cytochrome c (Cc) interacts in the cell nucleus with a variety of well-known histone chaperones, whose activity is competitively inhibited by the haem protein. As nuclear Cc inhibits the nucleosome assembly/disassembly activity of histone chaperones, it might indeed affect chromatin dynamics and histone deposition on DNA. Several histone chaperones actually interact with Cc Lys residues through their acidic regions, which are also involved in heterotypic interactions leading to liquid-liquid phase transitions responsible for the assembly of nuclear condensates, including heterochromatin. This relies on dynamic histone-DNA interactions that can be modulated by acetylation of specific histone Lys residues. Thus, Cc may have a major regulatory role in DNA repair by fine-tuning nucleosome assembly activity and likely nuclear condensate formation.

Role of the Holoenzyme PP1-SPN in the Dephosphorylation of the RB Family of Tumor Suppressors During Cell Cycle

Simple Summary

Cell cycle progression is highly regulated by modulating the phosphorylation status of retinoblastoma (RB) family proteins. This process is controlled by a balance in the action of kinases, such as the complexes formed by cyclin-dependent kinases (CDKs) and cyclins, and phosphatases, mainly the protein phosphatase 1 (PP1). However, while the phosphorylation of the RB family has been largely studied, its dephosphorylation is less known. Recently, the PP1-Spinophilin (SPN) holoenzyme has been described as the main phosphatase responsible for the dephosphorylation of RB proteins during the G0/G1 transition and at the end of G1. Here, we describe the regulation of the phosphorylation status of RB family proteins, giving importance not only to their inactivation by phosphorylation but also to their dephosphorylation to restore the cell cycle.

Abstract

Cell cycle progression is highly regulated by modulating the phosphorylation status of the retinoblastoma protein (pRB) and the other two members of the RB family, p107 and p130. This process is controlled by a balance in the action of kinases, such as the complexes formed by cyclin-dependent kinases (CDKs) and cyclins, and phosphatases, mainly the protein phosphatase 1 (PP1). However, while the phosphorylation of the RB family has been largely studied, its dephosphorylation is less known. Phosphatases are holoenzymes formed by a catalytic subunit and a regulatory protein with substrate specificity. Recently, the PP1-Spinophilin (SPN) holoenzyme has been described as the main phosphatase responsible for the dephosphorylation of RB proteins during the G0/G1 transition and at the end of G1. Moreover, SPN has been described as a tumor suppressor dependent on PP1 in lung and breast tumors, where it promotes tumorigenesis by increasing the cancer stem cell pool. Therefore, a connection between the cell cycle and stem cell biology has also been proposed via SPN/PP1/RB proteins.

Cyclin G2 upregulation impairs migration, invasion, and network formation through RNF123/Dvl2/JNK signaling in the trophoblast cell line HTR8/SVneo, a possible role in preeclampsia

Disruption of extravillous trophoblast (EVT) migration and invasion is considered to be responsible for pathological placentation in preeclampsia (PE). Cyclin G2 (CCNG2) is an atypical cyclin that inhibits cell cycle progression. However, its biological function and underlying molecular mechanism in PE are poorly understood. In this study, clinical data demonstrated that CCNG2 was significantly upregulated in PE placenta and associated with invasive EVT dysfunction. Additionally, Ccng2 knockout led to an attenuation of PE-like symptoms in the PE mouse model produced via treatment with NG-nitro-L-arginine methyl ester (L-NAME). In vitro, CCNG2 inhibited the migration, invasion, and endothelial-like network formation of human trophoblast cell line HTR8/SVneo. Mechanically, CCNG2 suppressed JNK-dependent Wnt/PCP signaling and its downstream indicators including epithelial-to-mesenchymal transition (EMT) markers and matrix metalloproteinases (MMPs) via promoting the polyubiquitination degradation of dishevelled 2 (Dvl2) protein in HTR8/SVneo cells. We also discovered that the E3 ligase Ring finger protein 123 (RNF123), as a novel CCNG2 target among HTR8/SVneo cells, interacted with Dvl2 and participated in CCNG2-induced polyubiquitination degradation of Dvl2. Moreover, we verified that the treatment of HTR8/SVneo cells with RNF123-specific siRNA improved polyubiquitination-induced degradation of Dvl2 and the activity of Wnt/PCP-JNK signaling mediated by CCNG2. Taken together, our results reveal that the CCNG2/RNF123/Dvl2/JNK axis may be involved in the pathogenesis and progression of PE through trophoblastic cell function modulation, thus probably providing us with new therapeutic strategies for PE treatment.

Human polyomaviruses and cancer: an overview

The name of the family Polyomaviridae, derives from the early observation that cells infected with murine polyomavirus induced multiple (poly) tumors (omas) in immunocompromised mice. Subsequent studies showed that many members of this family exhibit the capacity of mediating cell transformation and tumorigenesis in different experimental models. The transformation process mediated by these viruses is driven by viral pleiotropic regulatory proteins called T (tumor) antigens. Similar to other viral oncoproteins T antigens target cellular regulatory factors to favor cell proliferation, immune evasion and downregulation of apoptosis. The first two human polyomaviruses were isolated over 45 years ago. However, recent advances in the DNA sequencing technologies led to the rapid identification of additional twelve new polyomaviruses in different human samples. Many of these viruses establish chronic infections and have been associated with conditions in immunosuppressed individuals, particularly in organ transplant recipients. This has been associated to viral reactivation due to the immunosuppressant therapy applied to these patients. Four polyomaviruses namely, Merkel cell polyomavirus (MCPyV), Trichodysplasia spinulosa polyomavirus (TSPyV), John Cunningham Polyomavirus (JCPyV) and BK polyomavirus (BKPyV) have been associated with the development of specific malignant tumors. However, present evidence only supports the role of MCPyV as a carcinogen to humans. In the present review we present a summarized discussion on the current knowledge concerning the role of MCPyV, TSPyV, JCPyV and BKPyV in human cancers.

PP2A as the Main Node of Therapeutic Strategies and Resistance Reversal in Triple-Negative Breast Cancer

Triple negative breast cancer (TNBC), is defined as a type of tumor lacking the expression of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2). The ER, PR and HER2 are usually the molecular therapeutic targets for breast cancers, but they are ineffective for TNBC because of their negative expressions, so chemotherapy is currently the main treatment strategy in TNBC. However, drug resistance remains a major impediment to TNBC chemotherapeutic treatment. Recently, the protein phosphatase 2A (PP2A) has been found to regulate the phosphorylation of some substrates involved in the relevant target of TNBC, such as cell cycle control, DNA damage responses, epidermal growth factor receptor, immune modulation and cell death resistance, which may be the effective therapeutic strategies or influence drug sensitivity to TNBCs. Furthermore, PP2A has also been found that could induce ER re-expression in ER-negative breast cancer cells, and which suggests PP2A could promote the sensitivity of tamoxifen to TNBCs as a resistance reversal agent. In this review, we will summarize the potential therapeutic value of PP2A as the main node in developing targeting agents, disrupting resistance or restoring drug sensitivity in TNBC.

Coordinated downregulation of Spinophilin and the catalytic subunits of PP1, PPP1CA/B/C, contributes to a worse prognosis in lung cancer

The scaffold protein Spinophilin (Spinophilin, PPP1R9B) is one of the regulatory subunits of phosphatase-1 (PP1), directing it to distinct subcellular locations and targets. The loss of Spinophilin reduces PP1 targeting to pRb, thereby maintaining higher levels of phosphorylated pRb. Spinophilin is absent or reduced in approximately 40% of human lung tumors, correlating with the malignant grade. However, little is known about the relevance of the coordinated activity or presence of Spinophilin and its reported catalytic partners in the prognosis of lung cancer. In the present work, we show that the downregulation of Spinophilin, either by protein or mRNA, is related to a worse prognosis in lung tumors. This effect is more relevant in squamous cell carcinoma, SCC, than in adenocarcinoma. Downregulation of Spinophilin is related to a decrease in the levels of its partners PPP1CA/B/C, the catalytic subunits of PP1. A decrease in these subunits is also related to prognosis in SCC and, in combination with a decrease in Spinophilin, are markers of a poor prognosis in these tumors. The analysis of the genes that correlate to Spinophilin in lung tumors showed clear enrichment in ATP biosynthesis and protein degradation GO pathways. The analysis of the response to several common and pathway-related drugs indicates a direct correlation between the Spinophilin/PPP1Cs ratio and the response to oxaliplatin and bortezomib. This finding indicates that this ratio may be a good predictive biomarker for the activity of the drugs in these tumors with a poor prognosis.

The role of the NORE1A tumor suppressor in Oncogene-Induced Senescence

The Ras genes are the most frequently mutated oncogenes in human cancer. However, Ras biology is quite complex. While Ras promotes tumorigenesis by regulating numerous growth promoting pathways, activated Ras can paradoxically also lead to cell cycle arrest, death, and Oncogene-Induced Senescence (OIS). OIS is thought to be a critical pathway that serves to protect cells against aberrant Ras signaling. Multiple reports have highlighted the importance of the p53 and Rb tumor suppressors in Ras mediated OIS. However, until recently, the molecular mechanisms connecting Ras to these proteins remained unknown. The RASSF family of tumor suppressors has recently been identified as direct effectors of Ras. One of these members, NORE1A (RASSF5), may be the missing link between Ras-induced senescence and the regulation of p53 and Rb. This occurs both quantitatively, by promoting protein stability, as well as qualitatively via promoting critical pro-senescent post-translational modifications. Here we review the mechanisms by which NORE1A can activate OIS as a barrier against Ras-mediated transformation, and how this could lead to improved therapeutic strategies against cancers having lost NORE1A expression.

Ras signaling through RASSF proteins

There are six core RASSF family proteins that contain conserved Ras Association domains and may serve as Ras effectors. They lack intrinsic enzymatic activity and appear to function as scaffolding and localization molecules. While initially being associated with pro-apoptotic signaling pathways such as Bax and Hippo, it is now clear that they can also connect Ras to a surprisingly broad range of signaling pathways that control senescence, inflammation, autophagy, DNA repair, ubiquitination and protein acetylation. Moreover, they may be able to impact the activation status of pro-mitogenic Ras effector pathways, such as the Raf pathway. The frequent epigenetic inactivation of RASSF genes in human tumors disconnects Ras from pro-death signaling systems, enhancing Ras driven transformation and metastasis. The best characterized members are RASSF1A and RASSF5 (NORE1A).

Phosphoprotein Phosphatase 1 Is Required for Extracellular Calcium-Induced Keratinocyte Differentiation

Extracellular calcium is a major regulator of keratinocyte differentiation in vitro and appears to play that role in vivo, but the mechanism is unclear. We have previously demonstrated that, following calcium stimulation, PIP5K1α is recruited by the E-cadherin-β-catenin complex to the plasma membrane where it provides the substrate PIP2 for both PI3K and PLC-γ1. This signaling pathway is critical for calcium-induced generation of second messengers including IP3 and intracellular calcium and keratinocyte differentiation. In this study, we explored the upstream regulatory mechanism by which calcium activates PIP5K1α and the role of this activation in calcium-induced keratinocyte differentiation. We found that treatment of human keratinocytes in culture with calcium resulted in an increase in serine dephosphorylation and PIP5K1α activation. PP1 knockdown blocked extracellular calcium-induced increase in serine dephosphorylation and activity of PIP5K1α and induction of keratinocyte differentiation markers. Knockdown of PLC-γ1, the downstream effector of PIP5K1α, blocked upstream dephosphorylation and PIP5K1α activation induced by calcium. Coimmunoprecipitation revealed calcium induced recruitment of PP1 to the E-cadherin-catenin-PIP5K1α complex in the plasma membrane. These results indicate that PP1 is recruited to the extracellular calcium-dependent E-cadherin-catenin-PIP5K1α complex in the plasma membrane to activate PIP5K1α, which is required for PLC-γ1 activation leading to keratinocyte differentiation.

PP2A as a master regulator of the cell cycle

Protein phosphatase 2A (PP2A) plays a critical multi-faceted role in the regulation of the cell cycle. It is known to dephosphorylate over 300 substrates involved in the cell cycle, regulating almost all major pathways and cell cycle checkpoints. PP2A is involved in such diverse processes by the formation of structurally distinct families of holoenzymes, which are regulated spatially and temporally by specific regulators. Here, we review the involvement of PP2A in the regulation of three cell signaling pathways: wnt, mTOR and MAP kinase, as well as the G1→S transition, DNA synthesis and mitotic initiation. These processes are all crucial for proper cell survival and proliferation and are often deregulated in cancer and other diseases.

Ras Regulates Rb via NORE1A

Mutations in the Ras oncogene are one of the most frequent events in human cancer. Although Ras regulates numerous growth-promoting pathways to drive transformation, it can paradoxically promote an irreversible cell cycle arrest known as oncogene-induced senescence. Although senescence has clearly been implicated as a major defense mechanism against tumorigenesis, the mechanisms by which Ras can promote such a senescent phenotype remain poorly defined. We have shown recently that the Ras death effector NORE1A plays a critical role in promoting Ras-induced senescence and connects Ras to the regulation of the p53 tumor suppressor. We now show that NORE1A also connects Ras to the regulation of a second major prosenescent tumor suppressor, the retinoblastoma (Rb) protein. We show that Ras induces the formation of a complex between NORE1A and the phosphatase PP1A, promoting the activation of the Rb tumor suppressor by dephosphorylation. Furthermore, suppression of Rb reduces NORE1A senescence activity. These results, together with our previous findings, suggest that NORE1A acts as a critical tumor suppressor node, linking Ras to both the p53 and the Rb pathways to drive senescence.

Restricted protein phosphatase 2A targeting by Merkel cell polyomavirus small T antigen

Merkel cell polyomavirus (MCV) is a newly discovered human cancer virus encoding a small T (sT) oncoprotein. We performed MCV sT FLAG-affinity purification followed by mass spectroscopy (MS) analysis, which identified several protein phosphatases (PP), including PP2A A and C subunits and PP4C, as potential cellular interacting proteins. PP2A targeting is critical for the transforming properties of nonhuman polyomaviruses, such as simian virus 40 (SV40), but is not required for MCV sT-induced rodent cell transformation. We compared similarities and differences in PP2A binding between MCV and SV40 sT. While SV40 sT coimmunopurified with subunits PP2A Aα and PP2A C, MCV sT coimmunopurified with PP2A Aα, PP2A Aβ, and PP2A C. Scanning alanine mutagenesis at 29 sites across the MCV sT protein revealed that PP2A-binding domains lie on the opposite molecular surface from a previously described large T stabilization domain (LSD) loop that binds E3 ligases, such as Fbw7. MCV sT-PP2A interactions can be functionally distinguished by mutagenesis from MCV sT LSD-dependent 4E-BP1 hyperphosphorylation and viral DNA replication enhancement. MCV sT has a restricted range for PP2A B subunit substitution, inhibiting only the assembly of B56α into the phosphatase holoenzyme. In contrast, SV40 sT inhibits the assembly of B55α, B56α and B56ε into PP2A. We conclude that MCV sT is required for Merkel cell carcinoma growth, but its in vitro transforming activity depends on LSD interactions rather than PP2A targeting.

IMPORTANCE

Merkel cell polyomavirus is a newly discovered human cancer virus that promotes cancer, in part, through expression of its small T (sT) oncoprotein. Animal polyomavirus sT oncoproteins have been found to cause experimental tumors by blocking the activities of a group of phosphatases called protein phosphatase 2A (PP2A). Our structural analysis reveals that MCV sT also displaces the B subunit of PP2A to inhibit PP2A activity. MCV sT, however, only displaces a restricted subset of PP2A B subunits, which is insufficient to cause tumor cell formation in vitro. MCV sT instead transforms tumor cells through another region called the large T stabilization domain. The PP2A targeting and transforming activities lie on opposite faces of the MCV sT molecule and can be genetically separated from each other.

The same, only different - DNA damage checkpoints and their reversal throughout the cell cycle

Cell cycle checkpoints activated by DNA double-strand breaks (DSBs) are essential for the maintenance of the genomic integrity of proliferating cells. Following DNA damage, cells must detect the break and either transiently block cell cycle progression, to allow time for repair, or exit the cell cycle. Reversal of a DNA-damage-induced checkpoint not only requires the repair of these lesions, but a cell must also prevent permanent exit from the cell cycle and actively terminate checkpoint signalling to allow cell cycle progression to resume. It is becoming increasingly clear that despite the shared mechanisms of DNA damage detection throughout the cell cycle, the checkpoint and its reversal are precisely tuned to each cell cycle phase. Furthermore, recent findings challenge the dogmatic view that complete repair is a precondition for cell cycle resumption. In this Commentary, we highlight cell-cycle-dependent differences in checkpoint signalling and recovery after a DNA DSB, and summarise the molecular mechanisms that underlie the reversal of DNA damage checkpoints, before discussing when and how cell fate decisions after a DSB are made.

Inhibition of cancer cell proliferation by PPARγ is mediated by a metabolic switch that increases reactive oxygen species levels

The nuclear receptor peroxisome-proliferation-activated receptor gamma (PPARγ), a transcriptional master regulator of glucose and lipid metabolism, inhibits the growth of several common cancers, including lung cancer. In this study, we show that the mechanism by which activation of PPARγ inhibits proliferation of lung cancer cells is based on metabolic changes. We found that treatment with the PPARγ agonist pioglitazone triggers a metabolic switch that inhibits pyruvate oxidation and reduces glutathione levels. These PPARγ-induced metabolic changes result in a marked increase of reactive oxygen species (ROS) levels that lead to rapid hypophosphorylation of retinoblastoma protein (RB) and cell-cycle arrest. The antiproliferative effect of PPARγ activation can be prevented by suppressing pyruvate dehydrogenase kinase 4 (PDK4) or β-oxidation of fatty acids in vitro and in vivo. Our proposed mechanism also suggests that metabolic changes can rapidly and directly inhibit cell-cycle progression of cancer cells by altering ROS levels.

Small-molecule intramimics of formin autoinhibition: a new strategy to target the cytoskeletal remodeling machinery in cancer cells

Although the cancer cell cytoskeleton is a clinically validated target, few new strategies have emerged for selectively targeting cell division by modulating the cytoskeletal structure, particularly ways that could avoid the cardiotoxic and neurotoxic effects of current agents such as taxanes. We address this gap by describing a novel class of small-molecule agonists of the mammalian Diaphanous (mDia)-related formins, which act downstream of Rho GTPases to assemble actin filaments, and their organization with microfilaments to establish and maintain cell polarity during migration and asymmetric division. GTP-bound Rho activates mDia family members by disrupting the interaction between the DID and DAD autoregulatory domains, which releases the FH2 domain to modulate actin and microtubule dynamics. In screening for DID-DAD disruptors that activate mDia, we identified two molecules called intramimics (IMM-01 and -02) that were sufficient to trigger actin assembly and microtubule stabilization, serum response factor-mediated gene expression, cell-cycle arrest, and apoptosis. In vivo analysis of IMM-01 and -02 established their ability to slow tumor growth in a mouse xenograft model of colon cancer. Taken together, our work establishes the use of intramimics and mDia-related formins as a new general strategy for therapeutic targeting of the cytoskeletal remodeling machinery of cancer cells.

PPP1R42, a PP1 binding protein, regulates centrosome dynamics in ARPE-19 cells

BACKGROUND

The centrosome is the primary site for microtubule nucleation in cells and orchestrates reorganisation of the microtubule cytoskeleton during the cell cycle. The activities of the centrosome must be closely aligned with progression of the cell cycle; misregulation of centrosome separation and duplication is a hallmark of cancer. In a subset of cells, including the developing spermatid, the centrosome becomes specialised to form the basal body thereby supporting growth of the axoneme in morphogenesis of cilia and flagella, structures critical for signalling and motility. Mammalian spermatogenesis is an excellent model system to investigate the transformations in cellular architecture that accompany these changes including formation of the flagellum. We have previously identified a leucine-rich repeat protein (PPP1R42) that contains a protein phosphatase-1 binding site and translocates from the apical nucleus to the centrosome at the base of the flagellum during spermiogenesis. In this manuscript, we examine localisation and function of PPP1R42 in a ciliated epithelial cell model as a first step in understanding the role of this protein in centrosome function and flagellar formation.

RESULTS

We demonstrate that PPP1R42 localises to the basal body in ARPE-19 retinal epithelial cells. Co-localisation and co-immunoprecipitation experiments further show that PPP1R42 interacts with γ-tubulin. Inhibition of PPP1R42 with small interfering RNAs causes accumulation of centrosomes indicating premature centrosome separation. Importantly, the activity of two signalling molecules that regulate centrosome separation, PP1 phosphatase and NEK2 kinase, changes when PPP1R42 is inhibited: PP1 activity is reduced with a corresponding increase in NEK2 activity.

CONCLUSIONS

We have identified a role for the PP1-binding protein, PPP1R42, in centrosome separation in ciliated ARPE-19 cells. Our finding that inhibition of PPP1R42 expression increases the number of centrosomes per cell is consistent with our model that PPP1R42 is a positive regulator of PP1. PPP1R42 depletion reduces the activity of PP1 leading to activation of NEK2, the kinase responsible for phosphorylation of centrosomal linker proteins promoting centrosome separation. This work identifies a new molecule localised to the centrosome and basal body with a role in the complex signalling network responsible for controlling centrosome activities.

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.

Response to DNA damage: why do we need to focus on protein phosphatases?

Eukaryotic cells are continuously threatened by unavoidable errors during normal DNA replication or various sources of genotoxic stresses that cause DNA damage or stalled replication. To maintain genomic integrity, cells have developed a coordinated signaling network, known as the DNA damage response (DDR). Following DNA damage, sensor molecules detect the presence of DNA damage and transmit signals to downstream transducer molecules. This in turn conveys the signals to numerous effectors, which initiate a large number of specific biological responses, including transient cell cycle arrest mediated by checkpoints, DNA repair, and apoptosis. It is recently becoming clear that dephosphorylation events are involved in keeping DDR factors inactive during normal cell growth. Moreover, dephosphorylation is required to shut off checkpoint arrest following DNA damage and has been implicated in the activation of the DDR. Spatial and temporal regulation of phosphorylation events is essential for the DDR, and fine-tuning of phosphorylation is partly mediated by protein phosphatases. While the role of kinases in the DDR has been well documented, the complex roles of protein dephosphorylation have only recently begun to be investigated. Therefore, it is important to focus on the role of phosphatases and to determine how their activity is regulated upon DNA damage. In this work, we summarize current knowledge on the involvement of serine/threonine phosphatases, especially the protein phosphatase 1, protein phosphatase 2A, and protein phosphatase Mg²⁺/Mn²⁺-dependent families, in the DDR.

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.

Tumour suppressive microRNA-874 regulates novel cancer networks in maxillary sinus squamous cell carcinoma

BACKGROUND

On the basis of the microRNA (miRNA) expression signature of maxillary sinus squamous cell carcinoma (MSSCC), we found that miR-874 was significantly reduced in cancer cells. We focused on the functional significance of miR-874 in cancer cells and identification of miR-874-regulated novel cancer networks in MSSCC.

METHODS

We used PCR-based methods to investigate the downregulated miRNAs in clinical specimens of MSSCC. Our signature analyses identified 23 miRNAs that were significantly reduced in cancer cells, such as miR-874, miR-133a, miR-375, miR-204, and miR-1. We focused on miR-874 as the most downregulated novel miRNA in our analysis.

RESULTS

We found potential tumour suppressive functions such as inhibition of cancer cell proliferation and invasion. A molecular target search of miR-874 revealed that PPP1CA was directly regulated by miR-874. Overexpression of PPP1CA was observed in MSSCC clinical specimens. Silencing of the PPP1CA gene significantly inhibited cancer cell proliferation and invasion.

CONCLUSION

The downregulation of miR-874 was a frequent event in MSSCC, which suggests that miR-874 functions as a tumour suppressive miRNA, directly regulating PPP1CA that has a potential role of an oncogene. The identification of novel miR-874-regulated cancer pathways could provide new insights into potential molecular mechanisms of MSSCC oncogenesis.

PP2A interaction with Rb2/p130 mediates translocation of Rb2/p130 into the nucleus in all-trans retinoic acid-treated ovarian carcinoma cells

One of the mechanisms by which all-trans retinoic acid (ATRA) has been shown to suppress the growth of CAOV3 ovarian carcinoma cells involves an increase in the accumulation of Rb2/p130 protein, a member of the retinoblastoma family of tumor suppressors. This increase in accumulation of RB2/p130 by ATRA results from increased stability of Rb2/p130 protein as a result of an increase in dephosphorylation of the protein by the serine/threonine phosphatase PP2A. We show that upon ATRA treatment, PP2A interacts with the Rb2/p130 C-terminus and specifically dephosphorylates two residues (S1080 and T1097) adjacent to NLS1 and NLS2 of Rb2/p130. Moreover, co-immunoprecipitation studies reveal that Rb2/p130 can form a complex with the nuclear transport proteins, importin α and importin β, binding to the same dephosphorylated NLS1 and NLS2 sites. Finally, mutation of S1080 and T1097 results in retension of Rb2/p130 in the cytoplasm. Our studies suggest that one mechanism by which ATRA treatment of CAOV3 cells induces G0/G1 arrest involves the recruitment of PP2A to the C-terminus of Rb2/p130, resulting in the dephosphorylation of the S1080 and T1097 adjacent to the NLS and the subsequent interaction of Rb2/p130 with importins leading to transport of the Rb2/p130 to the nucleus where it inhibits cell-cycle progression.

Regulation of the Chlamydomonas cell cycle by a stable, chromatin-associated retinoblastoma tumor suppressor complex

We examined the cell cycle dynamics of the retinoblastoma (RB) protein complex in the unicellular alga Chlamydomonas reinhardtii that has single homologs for each subunit-RB, E2F, and DP. We found that Chlamydomonas RB (encoded by MAT3) is a cell cycle-regulated phosphoprotein, that E2F1-DP1 can bind to a consensus E2F site, and that all three proteins interact in vivo to form a complex that can be quantitatively immunopurified. Yeast two-hybrid assays revealed the formation of a ternary complex between MAT3, DP1, and E2F1 that requires a C-terminal motif in E2F1 analogous to the RB binding domain of plant and animal E2Fs. We examined the abundance of MAT3/RB and E2F1-DP1 in highly synchronous cultures and found that they are synthesized and remain stably associated throughout the cell cycle with no detectable fraction of free E2F1-DP1. Consistent with their stable association, MAT3/RB and DP1 are constitutively nuclear, and MAT3/RB does not require DP1-E2F1 for nuclear localization. In the nucleus, MAT3/RB remains bound to chromatin throughout the cell cycle, and its chromatin binding is mediated through E2F1-DP1. Together, our data show that E2F-DP complexes can regulate the cell cycle without dissociation of their RB-related subunit and that other changes may be sufficient to convert RB-E2F-DP from a cell cycle repressor to an activator.

The tumor suppressor Pml regulates cell fate in the developing neocortex

The control of cell fate in neural progenitor cells is critical for nervous system development. Nevertheless, the processes involved are only partially known. We found that the expression of the tumor suppressor Pml was restricted to neural progenitor cells (NPCs) in the developing neocortex of the mouse. Notably, in Pml(-/-) cortices, the overall number of proliferating NPCs was increased and transition between the two major progenitor types, radial glial cells and basal progenitors, was impaired. This in turn resulted in reduced differentiation and an overall decrease in the thickness of the cortex wall. In NPCs, Pml regulated the subcellular distribution of the retinoblastoma protein (pRb) and the protein phosphatase 1alpha, triggering pRb dephosphorylation. Together, these findings reveal an unexpected role of Pml in controlling the function of NPCs in the CNS.

Characterization of the p53 response to oncogene-induced senescence

BACKGROUND

P53 activation can trigger various outcomes, among them reversible growth arrest or cellular senescence. It is a live debate whether these outcomes are influenced by quantitative or qualitative mechanisms. Furthermore, the relative contribution of p53 to Ras-induced senescence is also matter of controversy.

METHODOLOGY/PRINCIPAL FINDINGS

This study compared situations in which different signals drove senescence with increasing levels of p53 activation. The study revealed that the levels of p53 activation do not determine the outcome of the response. This is further confirmed by the clustering of transcriptional patterns into two broad groups: p53-activated or p53-inactivated, i.e., growth and cellular arrest/senescence. Furthermore, while p53-dependent transcription decreases after 24 hrs in the presence of active p53, senescence continues. Maintaining cells in the arrested state for long periods does not switch reversible arrest to cellular senescence. Together, these data suggest that a Ras-dependent, p53-independent, second signal is necessary to induce senescence. This study tested whether PPP1CA (the catalytic subunit of PP1alpha), recently identified as contributing to Ras-induced senescence, might be this second signal. PPP1CA is induced by Ras; its inactivation inhibits Ras-induced senescence, presumably by inhibiting pRb dephosphorylation. Finally, PPP1CA seems to strongly co-localize with pRb only during senescence.

CONCLUSIONS

The levels of p53 activation do not determine the outcome of the response. Rather, p53 activity seems to act as a necessary but not sufficient condition for senescence to arise. Maintaining cells in the arrested state for long periods does not switch reversible arrest to cellular senescence. PPP1CA is induced by Ras; its inactivation inhibits Ras-induced senescence, presumably by inhibiting pRb dephosphorylation. Finally, PPP1CA seems to strongly co-localize with pRb only during senescence, suggesting that PP1alpha activation during senescence may be the second signal contributing to the irreversibility of the senescent phenotype.

Use of protein phosphatase inhibitors

Reversible protein phosphorylation is recognized as a major mechanism regulating the physiology of plant and animal cells. Virtually every biochemical process within eukaryotic cells is controlled by the covalent modification of key regulatory proteins. This in turn dictates the cellular response to a variety of physiological and environmental stimuli; errors in signals transduced by phosphoproteins contribute to many human diseases. Thus, defining protein phosphorylation events, and specifically, the phosphoproteins involved, is crucial for obtaining a better understanding of the physiological events that distinguish normal and diseased states. Protein phosphatase inhibitors are useful when deciphering physiological events regulated by reversible protein phosphorylation but the hormonal stimuli or signaling pathways involved are not known. They are also useful in analyzing the impact of hormones and other physiological stimuli on the function of a specific phosphoprotein. This unit describes protocols for inhibiting the cellular PP1/PP2A activity with okadaic acid, microcystin-LR, and PP2B/calcineurin and a widely utilized strategy for inhibiting protein tyrosine phosphatases.

Use of protein phosphatase inhibitors

Reversible protein phosphorylation is recognized as a major mechanism regulating the physiology of plant and animal cells. Virtually every biochemical process within eukaryotic cells is controlled by the covalent modification of key regulatory proteins. This in turn dictates the cellular response to a variety of physiological and environmental stimuli; errors in signals transduced by phosphoproteins contribute to many human diseases. Thus, defining protein phosphorylation events, and specifically, the phosphoproteins involved, is crucial for obtaining a better understanding of the physiological events that distinguish normal and diseased states. Protein phosphatase inhibitors are useful when deciphering physiological events regulated by reversible protein phosphorylation but the hormonal stimuli or signaling pathways involved are not known. They are also useful in analyzing the impact of hormones and other physiological stimuli on the function of a specific phosphoprotein. This unit describes protocols for inhibiting cellular phosphorylation activity with okadaic acid, microcystin-LR, and PP2B/calcineurin and a widely utilized strategy for inhibiting protein tyrosine phosphatases.

Normal regulation of Rbf1/E2f1 target genes in Drosophila type 1 protein phosphatase mutants

G1 Cyclin/Cdk complexes phosphorylate and inactivate the pRb tumor suppressor by preventing its ability to bind and repress E2F transcription factors. Current molecular and biochemical evidence suggests that type 1 protein phosphatases (PP1) dephosphorylate and thereby activate pRb, but the functional significance of this has not been addressed in the context of animal development. Here, we use genetic analyses to determine the role of PP1 in the regulation of Rbf1 activity during Drosophila development. While Rbf1 is required for E2f1 inhibition and G1 arrest in the embryonic epidermis and for the periodic expression of E2f1 target genes during endocycle S phase in the embryonic midgut and larval salivary gland, PP1 is not. PP1 regulates periodic cyclin E protein accumulation in ovarian nurse cells independently of Rbf1, which is dispensable for endocycle regulation in this tissue. We conclude that PP1 is not a major regulator of the Rbf1/E2F1 pathway in Drosophila.

Protein phosphatase 2A subunit PR70 interacts with pRb and mediates its dephosphorylation

The retinoblastoma tumor suppressor protein (pRb) regulates cell proliferation and differentiation via phosphorylation-sensitive interactions with specific targets. While the role of cyclin/cyclin-dependent kinase complexes in the modulation of pRb phosphorylation has been extensively studied, relatively little is known about the molecular mechanisms regulating phosphate removal by phosphatases. Protein phosphatase 2A (PP2A) is constituted by a core dimer bearing catalytic activity and one variable B regulatory subunit conferring target specificity and subcellular localization. We previously demonstrated that PP2A core dimer binds pRb and dephosphorylates pRb upon oxidative stress. In the present study, we identified a specific PP2A-B subunit, PR70, that was associated with pRb both in vitro and in vivo. PR70 overexpression caused pRb dephosphorylation; conversely, PR70 knockdown prevented both pRb dephosphorylation and DNA synthesis inhibition induced by oxidative stress. Moreover, we found that intracellular Ca(2+) mobilization was necessary and sufficient to trigger pRb dephosphorylation and PP2A phosphatase activity of PR70 was Ca(2+) induced. These data underline the importance of PR70-Ca(2+) interaction in the signal transduction mechanisms triggered by redox imbalance and leading to pRb dephosphorylation.

NOM1 targets protein phosphatase I to the nucleolus

Protein phosphatase I (PP1) is an essential eukaryotic serine/threonine phosphatase required for many cellular processes, including cell division, signaling, and metabolism. In mammalian cells there are three major isoforms of the PP1 catalytic subunit (PP1alpha, PP1beta, and PP1gamma) that are over 90% identical. Despite this high degree of identity, the PP1 catalytic subunits show distinct localization patterns in interphase cells; PP1alpha is primarily nuclear and largely excluded from nucleoli, whereas PP1gamma and to a lesser extent PP1beta concentrate in the nucleoli. The subcellular localization and the substrate specificity of PP1 catalytic subunits are determined by their interaction with targeting subunits, most of which bind PP1 through a so-called "RVXF" sequence. Although PP1 targeting subunits have been identified that direct PP1 to a number of subcellular locations and/or substrates, no targeting subunit has been identified that localizes PP1 to the nucleolus. Identification of nucleolar PP1 targeting subunit(s) is important because all three PP1 isoforms are included in the nucleolar proteome, enzymatically active PP1 is present in nucleoli, and PP1gamma is highly concentrated in nucleoli of interphase cells. In this study, we identify NOM1 (nucleolar protein with MIF4G domain 1) as a PP1-interacting protein and further identify the NOM1 RVXF motif required for its binding to PP1. We also define the NOM1 nucleolar localization sequence. Finally, we demonstrate that NOM1 can target PP1 to the nucleolus and show that a specific NOM1 RVXF motif and the NOM1 nucleolar localization sequence are required for this targeting activity. We therefore conclude that NOM1 is a PP1 nucleolar targeting subunit, the first identified in eukaryotic cells.

Phosphorylation of pRB at Ser612 by Chk1/2 leads to a complex between pRB and E2F-1 after DNA damage

The retinoblastoma tumor suppressor protein (pRB) plays a critical role in the control of cell proliferation and in the DNA damage checkpoints. pRB inhibits cell cycle progression through interactions with the E2F family of transcription factors. Here, we report that DNA damage induced not only the dephosphorylation of pRB at Cdk phosphorylation sites and the binding of pRB to E2F-1, but also the phosphorylation of pRB at Ser612. Phosphorylation of pRB at Ser612 enhanced the formation of a complex between pRB and E2F-1. Substitution of Ser612 with Ala decreased pRB-E2F-1 binding and the transcriptional repression activity. Until now, Ser612 of pRB has been thought to be phosphorylated by Cdk2. However, the phosphorylation of pRB at Ser612 was conducted by Chk1/2 after DNA damage, and inhibition of ATM-Chk1/2 activity suppressed the phosphorylation of Ser612 and the binding of pRB to E2F-1. These results suggest that Ser612 is phosphorylated by Chk1/2 after DNA damage, leading to the formation of pRB-E2F-1. This is the first report that pRB is phosphorylated in vivo by a kinase other than Cdk.

Protein phosphatase 1alpha activity prevents oncogenic transformation

Cyclin-dependent kinase 2 (Cdk2) phosphorylates Thr320 of protein phosphatase 1alpha (PP1alpha) in late G(1), thereby inhibiting its activity. Phosphorylation-resistant PP1alphaT320A, acting as a constitutively active (CA) mutant, causes a late G(1) arrest by preventing the phosphorylation and inactivation of the retinoblastoma protein (pRb). Both PP1alpha-mediated G(1) arrest and PP1alpha phosphorylation in late G(1) require the presence of pRb, indicating that PP1alpha is a crucial regulator of the pRb pathway, which is almost invariably mutated in human cancer. These findings prompted us to investigate whether PP1alpha interferes with oncogenic transformation. The ability of NIH 3T3 cells to form foci after transformation with ras/cyclin D1 was significantly inhibited by co-transfection with PP1alphaT320A, but not PP1alpha. Likewise, cells expressing PP1alphaT320A or PP1alphaT320A fused to green fluorescent protein (GFP) were unable to form colonies in soft agar, regardless of whether PP1alpha constructs were co-transfected with ras/cyclin D1 or transfected into stably transformed cells. Overexpressed wild-type (Wt) PP1alpha and GFP-PP1alpha were phosphorylated in Thr320, most likely explaining its lack of effect. Expression of GFP-PP1alphaT320A was associated with caspase-cleaved pRb in Western blots (WB) and morphological signs of cell death. These findings demonstrate that PP1alpha activity can override oncogenic signaling by causing cell-cycle arrest and/or apoptosis rather than restoring contact inhibition or anchorage dependence.

Identification of PP1alpha as a caspase-9 regulator in IL-2 deprivation-induced apoptosis

One of the mechanisms that regulate cell death is the reversible phosphorylation of proteins. ERK/MAPK phosphorylates caspase-9 at Thr(125), and this phosphorylation is crucial for caspase-9 inhibition. Until now, the phosphatase responsible for Thr(125) dephosphorylation has not been described. Here, we demonstrate that in IL-2-proliferating cells, phosphorylated serine/threonine phosphatase type 1alpha (PP1alpha) associates with phosphorylated caspase-9. IL-2 deprivation induces PP1alpha dephosphorylation, which leads to its activation and, as a consequence, dephosphorylation and activation of caspase-9 and subsequent dissociation of both molecules. In cell-free systems supplemented with ATP caspase-9 activation is induced by addition of cytochrome c and we show that in this process PP1alpha is indispensable for triggering caspase-9 as well as caspase-3 cleavage and activation. Moreover, PP1alpha associates with caspase-9 in vitro and in vivo, suggesting that it is the phosphatase responsible for caspase-9 dephosphorylation and activation. Finally, we describe two novel phosphatase-binding sites different from the previously described PP1alpha consensus motifs, and we demonstrate that these novel sites mediate the interaction of PP1alpha with caspase-9.

ATM regulates ionizing radiation-induced disruption of HDAC1:PP1:Rb complexes

Ionizing radiation elicits signaling events that coordinate DNA repair and interruption of cell cycle progression. We previously demonstrated that ionizing radiation (IR) of cells activates nuclear protein phosphatase-1 (PP1) by promoting dephosphorylation of Thr320, an inhibitory site in the enzyme and that the ATM kinase is required for this response. We sought to identify potential targets of IR-activated PP1. Untreated and IR-treated Jurkat cells were labeled with (32)P orthophosphate, and nuclear extracts were subjected to microcystin affinity chromatography to recover phosphatase complexes that were analyzed by 2D-PAGE and mass spectrometry. Several proteins associated with protein phosphatases demonstrated a significant decrease in (32)P intensity following IR, and one of these was identified as HDAC1. Co-immunoprecipitation revealed complexes containing PP1 with HDAC1 and Rb in cell extracts. In response to IR, there was an ATM-dependent activation of PP1, dephosphorylation of HDAC1, dissociation of HDAC1-PP1-Rb complexes and increased HDAC1 activity. These results suggest that IR regulates HDAC1 phosphorylation and activity through ATM-dependent activation of PP1.

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.

Downregulation of Fer induces PP1 activation and cell-cycle arrest in malignant cells

Fer is a nuclear and cytoplasmic intracellular tyrosine kinase. Herein we show that Fer is required for cell-cycle progression in malignant cells. Decreasing the level of Fer using the RNA interference (RNAi) approach impeded the proliferation of prostate and breast carcinoma cells and led to their arrest at the G0/G1 phase. At the molecular level, knockdown of Fer resulted in the activation of the retinoblastoma protein (pRB), and this was reflected by profound hypo-phosphorylation of pRB on both cyclin-dependent kinase CDK4 and CDK2 phosphorylation sites. Dephosphorylation of pRB was not seen upon the direct targeting of either CDK4 or CDK2 expression, and was only partially achieved by the simultaneous depletion of these two kinases. Amino-acid sequence analysis revealed two protein phosphatase 1 (PP1) binding motifs in the kinase domain of Fer and the association of Fer with the pRB phosphatase PP1alpha was verified using co-immunoprecipitation analysis. Downregulation of Fer potentiated the activation of PP1alpha and overexpression of Fer decreased the enzymatic activity of that phosphatase. Our findings portray Fer as a regulator of cell-cycle progression in malignant cells and as a potential target for cancer intervention.

Gene amplification and overexpression of protein phosphatase 1alpha in oral squamous cell carcinoma cell lines

Gene amplification of chromosomal band 11q13 is observed frequently in oral squamous cell carcinomas (OSCC). Several genes have been identified in the 11q13 amplicon, including FGF3, FGF4, CCND1, EMS1 and TAOS1. Some of these genes show good correlation between gene copy number and gene expression, and are thought to play a role in driving 11q13 amplification. The PPP1CA gene, which encodes the catalytic subunit of serine/threonine protein phosphatase protein phosphatase 1alpha (PP1alpha), is also located in 11q13. Protein phosphatase 1alpha, one of the isoforms of PP1, regulates critical cellular events, such as cell cycle progression, and apoptosis. We sought to explore the possibility that PPP1CA was amplified and overexpressed in OSCC cells. Indeed, some OSCC cell lines had PPP1CA gene amplification, as analysed by fluorescence in situ hybridization. We have also demonstrated that PPP1CA gene copy number is increased in 21% of the OSCC cell lines determined by quantitative microsatellite analysis. PP1alpha RNA expression determined by quantitative reverse transcription-polymerase chain reaction was significantly higher in OSCC cell lines with 11q13 amplification compared to those without 11q13 amplification (P=0.011). The difference was even more significant between cell lines with at least three copies of the PPP1CA gene and those with less than three copies of the gene (P=0.00045). Relative PP1alpha protein levels were also significantly associated with PPP1CA gene copy number (P=0.014). Furthermore, knockdown of PP1alpha and/or cyclin D1 by small interfering RNA suppressed OSCC cell growth, at least in part by modulating pRB phosphorylation, resulting in G0 growth arrest. These data suggest that like the cyclin D1 gene, CCND1, amplification and overexpression of the PP1alpha gene, PPP1CA, may be involved in OSCC tumorigenesis and/or progression.

S-phase arrest by reactive nitrogen species is bypassed by okadaic acid, an inhibitor of protein phosphatases PP1/PP2A

In mammalian cells DNA damage activates a checkpoint that halts progression through S phase. To determine the ability of nitrating agents to induce S-phase arrest, mouse C10 cells synchronized in S phase were treated with nitrogen dioxide (NO(2)) or SIN-1, a generator of reactive nitrogen species (RNS). SIN-1 or NO(2) induced S-phase arrest in a dose- and time-dependent manner. As for the positive controls adozelesin and cisplatin, arrest was accompanied by phosphorylation of ATM kinase; dephosphorylation of pRB; decreases in RF-C, cyclin D1, Cdc25A, and Cdc6; and increases in p21. Comet assays indicated that RNS induce minimal DNA damage. Moreover, in a cell-free replication system, nuclei from cells treated with RNS were able to support control levels of DNA synthesis when incubated in cytosolic extracts from untreated cells, whereas nuclei from cells treated with cisplatin were not. Induction of phosphatase activity may represent one mechanism of RNS-induced arrest, for the PP1/PP2A phosphatase inhibitor okadaic acid inhibited dephosphorylation of pRB; prevented decreases in the levels of RF-C, cyclin D1, Cdc6, and Cdc25A; and bypassed arrest by SIN-1 or NO(2), but not cisplatin or adozelesin. Our studies suggest that RNS may induce S-phase arrest through mechanisms that differ from those elicited by classical DNA-damaging agents.

Localization of myosin phosphatase target subunit and its mutants

Transient transfection of NIH3T3 cells with various constructs of myosin phosphatase target subunit (MYPT1) and GFP showed distinct cellular localizations. Constructs containing the N-terminal nuclear localization signals (NLS), i.e. full-length MYPT1 and N-terminal MYPT1 fragments, were concentrated in the nucleus. Full-length chicken and human MYPT1-GFP showed discrete nuclear foci. Deletion of the N-terminal NLS or use of central or C-terminal MYPT1 fragments did not show unique nuclear distributions (C-terminal NLS are present). Transient transfection of NIH3T3 cells (in the presence of serum) with full-length MYPT1-GFP caused a marked decrease in number of attached cells, an apparent block in the cell cycle prior to M phase and signs of increased apoptosis. Under conditions of serum starvation the unique nuclear localization of MYPT1-GFP was not found and there was no marked decrease in the number of attached cells (after 48 h). Stable transfection of HEK 293 cells with GFP-MYPT1 was obtained. MYPT1 and its N-terminal mutants bound to retinoblastoma protein (Rb), raising the possibility that Rb is implicated in the effects caused by overexpression of MYPT1.

Dehydroepiandrosterone inhibits the proliferation of human umbilical vein endothelial cells by enhancing the expression of p53 and p21, restricting the phosphorylation of retinoblastoma protein, and is androgen- and estrogen-receptor independent

Dehydroepiandrosterone (DHEA), a steroid hormone, modified the proliferation of human umbilical vein endothelial cells in a dose-dependent manner. Its inactive sulfate ester (DHEA-S) and two of its metabolites -- estradiol and testosterone -- had no inhibitory effect at physiological concentrations. Antiproliferation was associated with arrest in the G1 phase of the cell cycle, but not with cell death, as evaluated by cleavage of poly(ADP-ribose) polymerase and exposure of phosphatidylserine. The effect was not blocked by inhibitors of androgen or estrogen receptors. DHEA diminished the levels of phosphorylated retinoblastoma protein and increased the expression of p53 and p21 mRNAs. These results show that DHEA inhibits endothelial cell proliferation by regulating cell cycle relevant proteins through a cytoplasmic steroid hormone-independent pathway.

Protein phosphatase 1alpha is required for murine lung growth and morphogenesis

Protein phosphatase 1 (PP1) plays important roles in cell cycle control and apoptosis, two processes that impinge on morphogenesis and differentiation. Following the precedent set by other molecules regulating the cell cycle and apoptosis, we hypothesized that PP1 may have context-specific roles in development. Therefore, we have studied the spatial and temporal expression of PP1alpha during murine lung development and determined the consequences of loss of PP1alpha function on branching morphogenesis. By using an immunohistochemical approach, we show here that PP1alpha was expressed throughout the epithelium and mesenchyme upon the emergence of the lung primordium on embryonic day 10, with immunostaining exclusively extranuclear. During the late pseudoglandular stage, PP1alpha was predominantly expressed in the distal lung epithelium, whereas the mesenchyme contained very little or no PP1alpha protein. Peri- and postnatally, PP1alpha immunostaining was mostly nuclear in apparently differentiated cells, as judged by colocalization with well-known markers for lung differentiation. Exposure of fetal lung explants to antisense oligodeoxynucleotides against PP1alpha, resulted in decreased overall size of the cultured lung, a defect in forming new airways, lack of expression of surfactant protein C, and histologic signs of poor differentiation. These data suggest that PP1alpha is required for branching morphogenesis and differentiation.

Expression of AIE-75 PDZ-domain protein induces G2/M cell cycle arrest in human colorectal adenocarcinoma SW480 cells

AIE-75 has been known as a 75-kDa autoantigen detected in the serum of autoimmune enteropathy (AIE) and as a colon cancer-related antigen, and now designated as a gene causative of Usher syndrome type 1C hereditary syndromic hearing loss. It binds to a novel putative tumor suppressor MCC2 that is homologous to MCC (mutated in colon cancer) through a PSD-95/Dlg/ZO-1 (PDZ) domain. To clarify the functional role in colon cancer cells, we transfected AIE-75 gene into SW480 colon cancer cells which do not express AIE-75. Expression of AIE-75 suppressed growth of SW480 cells in vitro in correlation with the expression levels. It was due mainly to G2/M phase cell cycle arrest associated with mitotic slippage, resulting in emergence of hyperploid giant-nucleated or multi-nucleated cells. Screening of proteins that bound to PDZ domains of AIE-75 by a yeast two hybrid system showed that three serine/threonine phosphatase catalytic subunits (PP2AC-alpha, PP2AC-beta, and PPP6C) could bind to AIE-75. Since PP2AC is known to regulate G2/M checkpoint, we suggest that AIE-75 interacts with PP2AC and prevent cells to transit mitotic phase.

Differential gene expression in skeletal muscle after induction of heart failure: impact of cytokines on protein phosphatase 2A expression

Different intrinsic alterations of skeletal muscle metabolism and gene expression have been described in chronic heart failure (CHF). As proposed skeletal muscle alterations in CHF may contribute to exercise intolerance and early muscular fatigue. However the exact molecular changes occurring in the skeletal muscle are still unclear. The aim of this study was to characterize the pattern of differential gene expression in an animal model of CHF and to study the regulation of one selected gene. Rats were subjected to LAD ligation or sham operation. mRNA was isolated from musculus quadriceps of both groups and differential gene expression was determined by subtractive hybridization. Quantitative RT-PCR and cell culture experiments were performed to further characterize the changed expression of protein phosphatase 2A (PP2A) in human skeletal muscle biopsies as well as the cytokine dependent regulation of PP2A expression. Out of 800 picked clones differential expression of 24 distinct genes could be identified by sequencing and reverse Northern blotting. PP2A expression demonstrated a significant upregulation in skeletal muscle biopsies from patients with CHF as compared to healthy controls (9.7 +/- 1.9 vs. 4.2 +/- 0.7 arbitrary units; p<0.05). Incubation of rat skeletal muscle myoblasts with a combination of TNF-alpha, IL-1beta, and gamma-IFN caused a 3-fold upregulation of PP2A expression vs. untreated cells. These results suggest that CHF is accompanied by changes in expression of genes involved in energy metabolism, contractility, and apoptosis in the skeletal muscle. The upregulation of PP2A, an important regulator in intracellular signaling and apoptosis, may be due to an increase of inflammatory cytokines.

Inducible expression of catalytically active type 1 serine/threonine protein phosphatase in a human carcinoma cell line

BACKGROUND: One of the major cellular serine/threonine protein phosphatases is protein phosphatase type 1 (PP1). Studies employing many eukaryotic systems all point to a crucial role for PP1 activity in controlling cell cycle progression. One physiological substrate for PP1 appears to be the product of the retinoblastoma susceptibility gene (pRB), a demonstrated tumor suppressor. The growth suppressive activity of pRB is regulated by its phosphorylation state. Of critical importance is the question of the in vivo effect of PP1 activity on pRB and growth regulation. As a first step towards addressing this question, we developed an inducible PP1 expression system to investigate the regulation of PP1 activity. RESULTS: We have established a cell line for inducing protein expression of the type 1, alpha-isotype, serine/threonine protein phosphatase (PP1alpha). A plasmid encoding a fusion protein of the catalytic subunit of PP1alpha with a 6-histidine peptide (6His) and a peptide from hemagluttinin (HA) was transfected into the UMUC3 transitional cell carcinoma cell line, previously transfected with the reverse tetracycline transactivator plasmid pUHD172-1neo. A stable cell line designated LLWO2F was established by selection with hygromycin B. 6His-HA-PP1alpha protein appeared in cell lysates within two hours following addition of doxycycline to the culture medium. This protein localizes to the nucleus as does endogenous PP1alpha, and was shown to associate with PNUTS, a PP1-nuclear targeting subunit. Like endogenous PP1alpha, immunocomplexed 6His-HA-PP1alpha is active toward phosphorylase a and the product of the retinoblastoma susceptibility gene, pRB. When forcibly overexpressing 6His-HA-PP1alpha, there is a concomitant decrease in endogenous PP1alpha levels. CONCLUSIONS: These data suggest the existence of an autoregulatory mechanism by which PP1alpha protein levels and activity remain relatively constant. RT-PCR analyses of isolated polysome fractions support the notion that this putative autoregulatory mechanism is exerted, at least in part, at the translational level. Implications of these findings for the study of PP1alpha function in vivo are discussed.