Protein phosphatase 2A interacts with the Src kinase substrate p130(CAS)

Reduction of protein phosphatase 2A (PP2A) complexity reveals cellular functions and dephosphorylation motifs of the PP2A/B'δ holoenzyme

Protein phosphatase 2A (PP2A) is a large enzyme family responsible for most cellular Ser/Thr dephosphorylation events. PP2A substrate specificity, localization, and regulation by second messengers rely on more than a dozen regulatory subunits (including B/R2, B'/R5, and B″/R3), which form the PP2A heterotrimeric holoenzyme by associating with a dimer comprising scaffolding (A) and catalytic (C) subunits. Because of partial redundancy and high endogenous expression of PP2A holoenzymes, traditional approaches of overexpressing, knocking down, or knocking out PP2A regulatory subunits have yielded only limited insights into their biological roles and substrates. To this end, here we sought to reduce the complexity of cellular PP2A holoenzymes. We used tetracycline-inducible expression of pairs of scaffolding and regulatory subunits with complementary charge-reversal substitutions in their interaction interfaces. For each of the three regulatory subunit families, we engineered A/B charge-swap variants that could bind to one another, but not to endogenous A and B subunits. Because endogenous Aα was targeted by a co-induced shRNA, endogenous B subunits were rapidly degraded, resulting in expression of predominantly a single PP2A heterotrimer composed of the A/B charge-swap pair and the endogenous catalytic subunit. Using B'δ/PPP2R5D, we show that PP2A complexity reduction, but not PP2A overexpression, reveals a role of this holoenzyme in suppression of extracellular signal-regulated kinase signaling and protein kinase A substrate dephosphorylation. When combined with global phosphoproteomics, the PP2A/B'δ reduction approach identified consensus dephosphorylation motifs in its substrates and suggested that residues surrounding the phosphorylation site play roles in PP2A substrate specificity.

Two Distinct Actin Networks Mediate Traction Oscillations to Confer Focal Adhesion Mechanosensing

Focal adhesions (FAs) are integrin-based transmembrane assemblies that connect a cell to its extracellular matrix (ECM). They are mechanosensors through which cells exert actin cytoskeleton-mediated traction forces to sense the ECM stiffness. Interestingly, FAs themselves are dynamic structures that adapt their growth in response to mechanical force. It is unclear how the cell manages the plasticity of the FA structure and the associated traction force to accurately sense ECM stiffness. Strikingly, FA traction forces oscillate in time and space, and govern the cell mechanosensing of ECM stiffness. However, precisely how and why the FA traction oscillates is unknown. We developed a model of FA growth that integrates the contributions of the branched actin network and stress fibers (SFs). Using the model in combination with experimental tests, we show that the retrograde flux of the branched actin network promotes the proximal growth of the FA and contributes to a traction peak near the FA’s distal tip. The resulting traction gradient within the growing FA favors SF formation near the FA’s proximal end. The SF-mediated actomyosin contractility further stabilizes the FA and generates a second traction peak near the center of the FA. Formin-mediated SF elongation negatively feeds back with actomyosin contractility, resulting in central traction peak oscillation. This underpins the observed FA traction oscillation and, importantly, broadens the ECM stiffness range over which FAs can accurately adapt to traction force generation. Actin cytoskeleton-mediated FA growth and maturation thus culminate with FA traction oscillation to drive efficient FA mechanosensing.

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.

Cell migration: cooperation between myosin II isoforms in durotaxis

A new study reveals that non-muscle myosin II plays a central role in the durotaxis of mesenchymal stem cells, with the two major isoforms, II-A and II-B, being cooperatively required for this cell movement, and serine phosphorylation of the II-A isoform playing a negative role.

p130Cas: a key signalling node in health and disease

p130Cas/breast cancer anti-oestrogen resistance 1 (BCAR1) is a member of the Cas (Crk-associated substrate) family of adaptor proteins, which have emerged as key signalling nodes capable of interactions with multiple proteins, with important regulatory roles in normal and pathological cell function. The Cas family of proteins is characterised by the presence of multiple conserved motifs for protein-protein interactions, and by extensive tyrosine and serine phosphorylations. Recent studies show that p130Cas contributes to migration, cell cycle control and apoptosis. p130Cas is essential during early embryogenesis, with a critical role in cardiovascular development. Furthermore, p130Cas has been reported to be involved in the development and progression of several human cancers. p130Cas is able to perform roles in multiple processes due to its capacity to regulate a diverse array of signalling pathways, transducing signals from growth factor receptor tyrosine kinases, non-receptor tyrosine kinases, and integrins. In this review we summarise the current understanding of the structure, function, and regulation of p130Cas, and discuss the importance of p130Cas in both physiological and pathophysiological settings, with a focus on the cardiovascular system and cancer.

Semaphorin 3A elevates endothelial cell permeability through PP2A inactivation

VE-cadherin-mediated cell-cell junction weakening increases paracellular permeability in response to both angiogenic and inflammatory stimuli. Although Semaphorin 3A has emerged as one of the few known anti-angiogenic factors to exhibit pro-permeability activity, little is known about how it triggers vascular leakage. Here we report that Semaphorin 3A induced VE-cadherin serine phosphorylation and internalization, cell-cell junction destabilization, and loss of barrier integrity in brain endothelial cells. In addition, high-grade glioma-isolated tumour initiating cells were found to secrete Semaphorin 3A, which promoted brain endothelial monolayer permeability. From a mechanistic standpoint, Semaphorin 3A impinged upon the basal activity of the serine phosphatase PP2A and disrupted PP2A interaction with VE-cadherin, leading to cell-cell junction disorganization and increased permeability. Accordingly, both pharmacological inhibition and siRNA-based knockdown of PP2A mimicked Semaphorin 3A effects on VE-cadherin. Hence, local Semaphorin 3A production impacts on the PP2A/VE-cadherin equilibrium and contributes to elevated vascular permeability.

PP2A phosphatase suppresses function of the mesenchymal invasion regulator NEDD9

The mesenchymal mode of cancer cell invasion characterized by active adhesion turnover and a polarized actin cytoskeleton, is critically regulated by the adaptor protein NEDD9/HEF1/Cas-L. While it is known that NEDD9 is subject to extensive phosphorylation modification, the molecules that determine NEDD9 phosphorylation to stimulate adhesion turnover and mesenchymal cell morphologies are currently unknown. Earlier studies have suggested that the serine/threonine phosphatase PP2A regulates interconversion between a low molecular mass NEDD9 phosphoform and higher molecular mass phosphoforms. However, previous studies have used chemical inhibitors to block PP2A activity. In the present study we therefore aimed to specifically inhibit PP2A activity via siRNA and dominant negative approaches to investigate the effect of PP2A on interconversion between 115 kDa and 105 kDa NEDD9 and determine the functional consequence of PP2A activity for NEDD9 function. Strikingly, we find that while the phosphatase inhibitor Calyculin A indeed abrogates detachment-induced dephosphorylation of the 115 kDa NEDD9 phosphoform, PP2A depletion does not inhibit 115 kDa to 105 kDa interconversion. Our data suggest instead that PP2A targets discrete NEDD9 phosphorylation modifications separate to the events that mediate interconversion between the two forms. Functionally, PP2A depletion increases NEDD9 mediated cell spreading and mutation of S369 in the serine-rich region of NEDD9 to aspartate mimics this effect. Importantly, mutation of S369 to alanine abrogates the ability of dominant negative PP2A to increase NEDD9-mediated cell spreading. Collectively, our data reveal that the tumour suppressor PP2A may act via S369 to regulated NEDD9-mediated cell spreading.

PP2A targeting by viral proteins: a widespread biological strategy from DNA/RNA tumor viruses to HIV-1

Protein phosphatase 2A (PP2A) is a large family of holoenzymes that comprises 1% of total cellular proteins and accounts for the majority of Ser/Thr phosphatase activity in eukaryotic cells. Although initially viewed as constitutive housekeeping enzymes, it is now well established that PP2A proteins represent a family of highly and sophistically regulated phosphatases. The past decade, multiple complementary studies have improved our knowledge about structural and functional regulation of PP2A holoenzymes. In this regard, after summarizing major cellular regulation, this review will mainly focus on discussing a particulate biological strategy, used by various viruses, which is based on the targeting of PP2A enzymes by viral proteins in order to specifically deregulate, for their own benefit, cellular pathways of their hosts. The impact of such PP2A targeting for research in human diseases, and in further therapeutic developments, is also discussed.

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.

CAS proteins in normal and pathological cell growth control

Proteins of the CAS (Crk-associated substrate) family (BCAR1/p130Cas, NEDD9/HEF1/Cas-L, EFS/SIN and CASS4/HEPL) are integral players in normal and pathological cell biology. CAS proteins act as scaffolds to regulate protein complexes controlling migration and chemotaxis, apoptosis, cell cycle, and differentiation, and have more recently been linked to a role in progenitor cell function. Reflecting these complex functions, over-expression of CAS proteins has now been strongly linked to poor prognosis and increased metastasis in cancer, as well as resistance to first-line chemotherapeutics in multiple tumor types including breast and lung cancers, glioblastoma, and melanoma. Further, CAS proteins have also been linked to additional pathological conditions including inflammatory disorders, Alzheimer's and Parkinson's disease, as well as developmental defects. This review will explore the roles of the CAS proteins in normal and pathological states in the context of the many mechanistic insights into CAS protein function that have emerged in the past decade.

Phosphorylation of human enhancer of filamentation (HEF1) on serine 369 induces its proteasomal degradation

Human enhancer of filamentation 1 (HEF1) is a multi-domain docking protein of the p130 Cas family. HEF1 is present at focal adhesions and is involved in integrin signalling mediating cytoskeleton reorganization associated with cell migration, adhesion or apoptosis. HEF1 functions are regulated in part by phosphorylation on tyrosine residues. HEF1 is also phosphorylated on serines/threonines leading to two isoforms refered to as p105 and p115. In most cases, the serine/threonine kinase(s) responsible for HEF1 phosphorylation have not been identified. In the present study, we have investigated HEF1 ser/thr phosphorylation. In the HCT-116 cell line transiently overexpressing Flag-HEF1 we showed that Hesperadin, a synthetic indolinone displaying antiproliferative effect and described as an inhibitor of various kinases including Aurora-B, prevented HEF1 phosphorylation induced by the ser/thr phosphatase PP2A inhibitor: okadaic acid (OA). In addition we showed that conversion of endogenous HEF1 p105 to p115 in HaCaT cells was prevented upon treatment with Hesperadin, resulting in accumulation of p105HEF1. We also identified serine 369 as the target site of phosphorylation by this Hesperadin-inhibited kinase in HCT-116. Finally, we provide evidence that phosphorylation on serine 369 but not phosphorylation on serine 296, triggers HEF1 degradation by the proteasomal machinery. These data suggest that conversion of p105 to p115 results from a ser-369-dependent phosphorylation mediated by an Hesperadin-sensitive kinase and regulates the half-life of HEF1.

Protein phosphatase 2A regulatory subunits and cancer

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

Overlapping Binding Sites in Protein Phosphatase 2A for Association with Regulatory A and α-4 (mTap42) Subunits

Diverse functions of protein Ser/Thr phosphatases depend on the distribution of the catalytic subunits among multiple regulatory subunits. In cells protein phosphatase 2A catalytic subunit (PP2Ac) mostly binds to a scaffold subunit (A subunit or PR65); however, PP2Ac alternatively binds to alpha-4, a subunit related to yeast Tap42 protein, which also associates with phosphatases PP4 or PP6. We mapped alpha-4 binding to PP2Ac to the helical domain, residues 19-165. We mutated selected residues and transiently expressed epitope-tagged PP2Ac to assay for association with A and alpha-4 subunits by co-precipitation. The disabling H118N mutation at the active site or the presence of the active site inhibitor microcystin-LR did not interfere with binding of PP2Ac to either the A subunit or alpha-4, showing that these are allosteric regulators. Positively charged side chains Lys(41), Arg(49), and Lys(74) on the back surface of PP2Ac are unique to PP2Ac, compared with phosphatases PP4, PP6, and PP1. Substitution of one, two, or three of these residues with Ala produced a progressive loss of binding to the A subunit, with a corresponding increase in binding to alpha-4. Conversely, mutation of Glu(42) in PP2Ac essentially eliminated PP2Ac binding to alpha-4, with an increase in binding to the A subunit. Reciprocal changes in binding because of mutations indicate competitive distribution of PP2Ac between these regulatory subunits and demonstrate that the mutated catalytic subunits retained a native conformation. Furthermore, neither the Lys(41)-Arg(49)-Lys(74) nor Glu(42) mutations affected the phosphatase-specific activity or binding to microcystin-agarose. Binding of PP2Ac to microcystin and to alpha-4 increased with temperature, consistent with an activation energy barrier for these interactions. Our results reveal that the A subunit and alpha-4 (mTap42) require charged residues in separate but overlapping surface regions to associate with the back side of PP2Ac and modulate phosphatase activity.

Determinants for the interaction between Janus kinase 2 and protein phosphatase 2A

We have shown that the serine/threonine protein phosphatase 2A (PP2A) associates with the Jak2 tyrosine kinase in a myeloid progenitor line. In this study, we characterized the regions of Jak2 and PP2A responsible for association and evaluated the functional consequences of association. We demonstrate that PP2A interacts with truncated forms of Jak2 containing the JH1 catalytic domain. Using GST fusion proteins, we show that the isolated JH1 and JH3 domains of Jak2 bind directly to PP2A. Jak2 contains putative PP2A binding sequences (LXXLL) in the JH1 domain (residues 1078-1082) and in the JH3 domain (residues 474-478). Mutation of the LXXLL sequence in the JH1 domain decreased PP2A binding in vitro, while mutation of the similar JH3 sequence did not affect PP2A binding. We analyzed full-length Jak2 bearing the LXXLL mutation in Cos-7 cells for association with PP2A. The JH1 mutation impaired Jak2 activity and had a modest effect on PP2A binding. Finally, we show that a mutant form of the PP2A catalytic subunit lacking a site for phosphorylation (Y307F) binds more tightly to Jak2 than wild-type PP2A, consistent with a model where phosphorylation disrupts the Jak2-PP2A interaction.

Protein phosphatase-2A restricts migration of Lewis lung carcinoma cells by modulating the phosphorylation of focal adhesion proteins

Compared to metastatic Lewis lung carcinoma (LLC) cells, nonmetastatic LLC cells have increased levels of activity of the protein phosphatase PP-2A, which functions to limit their migration through transwell chambers. Inhibition of PP-2A in nonmetastatic LLC stimulates their transmigration to levels similar to those of metastatic LLC cells. Studies to define the signaling pathways intermediate between diminished PP-2A activity and stimulated migration showed that inhibiting PP-2A activity resulted in paxillin serine hyperphosphorylation and tyrosine dephosphorylation. Paxillin was important for the stimulated migration because the increased transmigration in response to PP-2A inhibition was dampened by expression of mutant paxillin at the LIM3 S457 and S481 residues. Inhibition of PP-2A also led to the dissolution of FAK/Src/paxillin focal adhesion complexes, which was also dependent on paxillin S457 and S481 residues. In addition, inhibition of PP-2A resulted in dephosphorylation of Src inhibitory Y527 residue, suggesting increased Src activity. The stimulated transmigration of cells with diminished PP-2A was in part dependent on this Src activity. These studies show the importance of PP-2A in limiting tumor cell migration through its modulation of proteins of the focal adhesions.