Mitotic dissociation of IQGAP1 from Rac-bound beta1-integrin is mediated by protein phosphatase 2A

Connections between the cell cycle, cell adhesion and the cytoskeleton

Cell division, the purpose of which is to enable cell replication, and in particular to distribute complete, accurate copies of genetic material to daughter cells, is essential for the propagation of life. At a morphological level, division not only necessitates duplication of cellular structures, but it also relies on polar segregation of this material followed by physical scission of the parent cell. For these fundamental changes in cell shape and positioning to be achieved, mechanisms are required to link the cell cycle to the modulation of cytoarchitecture. Outside of mitosis, the three main cytoskeletal networks not only endow cells with a physical cytoplasmic skeleton, but they also provide a mechanism for spatio-temporal sensing via integrin-associated adhesion complexes and site-directed delivery of cargoes. During mitosis, some interphase functions are retained, but the architecture of the cytoskeleton changes dramatically, and there is a need to generate a mitotic spindle for chromosome segregation. An economical solution is to re-use existing cytoskeletal molecules: transcellular actin stress fibres remodel to create a rigid cortex and a cytokinetic furrow, while unipolar radial microtubules become the primary components of the bipolar spindle. This remodelling implies the existence of specific mechanisms that link the cell-cycle machinery to the control of adhesion and the cytoskeleton. In this article, we review the intimate three-way connection between microenvironmental sensing, adhesion signalling and cell proliferation, particularly in the contexts of normal growth control and aberrant tumour progression. As the morphological changes that occur during mitosis are ancient, the mechanisms linking the cell cycle to the cytoskeleton/adhesion signalling network are likely to be primordial in nature and we discuss recent advances that have elucidated elements of this link. A particular focus is the connection between CDK1 and cell adhesion. This article is part of a discussion meeting issue 'Forces in cancer: interdisciplinary approaches in tumour mechanobiology'.

Chemotherapy induced PRL3 expression promotes cancer growth via plasma membrane remodeling and specific alterations of caveolae-associated signaling

BACKGROUND

The outcome of cancer therapy is greatly defined by the ability of a tumor cell to evade treatment and re-establish its bulk mass after medical interventions. Consequently, there is an urgent need for the characterization of molecules affecting tumor reoccurrence. The phosphatase of regenerating liver 3 (PRL3) protein was recently emerged among the targets that could affect such a phenomenon.

METHODS

The expression induction of PRL3 in melanoma cells treated with chemotherapeutic agents was assessed by western blotting. The effect of PRL3 expression on cancer growth was investigated both in vitro and in vivo. The association of PRL3 with the caveolae structures of the plasma membrane was analyzed by detergent free raft purification. The effect of PRL3 expression on the membrane organization was assayed by electron microscopy and by membrane biophysical measurements. Purification of the plasma membrane fraction and co-immunoprecipitation were used to evaluate the altered protein composition of the plasma membrane upon PRL3 expression.

RESULTS

Here, we identified PRL3 as a genotoxic stress-induced oncogene whose expression is significantly increased by the presence of classical antitumor therapeutics. Furthermore, we successfully connected the presence of this oncogene with increased tumor growth, which implies that tumor cells can utilize PRL3 effects as a survival strategy. We further demonstrated the molecular mechanism that is connected with the pro-growth action of PRL3, which is closely associated with its localization to the caveolae-type lipid raft compartment of the plasma membrane. In our study, PRL3 was associated with distinct changes in the plasma membrane structure and in the caveolar proteome, such as the dephosphorylation of integrin β1 at Thr788/Thr789 and the increased partitioning of Rac1 to the plasma membrane. These alterations at the plasma membrane were further associated with the elevation of cyclin D1 in the nucleus.

CONCLUSIONS

This study identifies PRL3 as an oncogene upregulated in cancer cells upon exposure to anticancer therapeutics. Furthermore, this work contributes to the existing knowledge on PRL3 function by characterizing its association with the caveolae-like domains of the plasma membrane and their resident proteins.

IQGAP1 is associated with nuclear envelope reformation and completion of abscission

The final stage of mitosis is cytokinesis, which results in 2 independent daughter cells. Cytokinesis has 2 phases: membrane ingression followed by membrane abscission. IQGAP1 is a scaffold protein that interacts with proteins implicated in mitosis, including F-actin, myosin and CaM. IQGAP1 in yeast recruits actin and myosin II filaments to the contractile ring for membrane ingression. In contrast, we show that mammalian IQGAP1 is not required for ingression, but coordinates nuclear pore complex (NPC) reassembly and completion of abscission. Depletion of IQGAP1 disrupts Nup98 and mAb414 nuclear envelope localization and delays abscission timing. IQGAP1 phosphorylation increases 15-fold upon mitotic entry at S86, S330 and T1434, with the latter site being targeted by CDK2/Cyclin A and CDK1/Cyclin A/B in vitro. Expressing the phospho-deficient mutant IQGAP1-S330A impairs NPC reassembly in cells undergoing abscission. Thus, mammalian IQGAP1 functions later in mitosis than its yeast counterpart to regulate nuclear pore assembly in a S330 phosphorylation-dependent manner during the abscission phase of cytokinesis.

The biology of IQGAP proteins: beyond the cytoskeleton

IQGAP scaffold proteins are evolutionarily conserved in eukaryotes and facilitate the formation of complexes that regulate cytoskeletal dynamics, intracellular signaling, and intercellular interactions. Fungal and mammalian IQGAPs are implicated in cytokinesis. IQGAP1, IQGAP2, and IQGAP3 have diverse roles in vertebrate physiology, operating in the kidney, nervous system, cardio-vascular system, pancreas, and lung. The functions of IQGAPs can be corrupted during oncogenesis and are usurped by microbial pathogens. Therefore, IQGAPs represent intriguing candidates for novel therapeutic agents. While modulation of the cytoskeletal architecture was initially thought to be the primary function of IQGAPs, it is now clear that they have roles beyond the cytoskeleton. This review describes contributions of IQGAPs to physiology at the organism level.

Rac1 is deactivated at integrin activation sites through an IQGAP1-filamin-A-RacGAP1 pathway

Cell migration makes a fundamental contribution to both normal physiology and disease pathogenesis. Integrin engagement with extracellular ligands spatially controls, via the cyclical activation and deactivation of the small GTPase Rac1, the dynamic membrane protrusion and cytoskeletal reorganization events that are required for directional migration. Although the pathways that control integrin-mediated Rac1 activation are reasonably well defined, the mechanisms that are responsible for switching off activity are poorly understood. Here, proteomic analysis of activated integrin-associated complexes suggests filamin-A and IQ-motif-containing GTPase-activating protein 1 (IQGAP1) as candidates that link β1 integrin to Rac1. siRNA-mediated knockdown of either filamin-A or IQGAP1 induced high, dysregulated Rac1 activity during cell spreading on fibronectin. Using immunoprecipitation and immunocytochemistry, filamin-A and IQGAP1 were shown to be part of a complex that is recruited to active β1 integrin. Mass spectrometric analysis of individual filamin-A, IQGAP1 and Rac1 pull-downs and biochemical analysis, identified RacGAP1 as a novel IQGAP1 binding partner. Further immunoprecipitation and immunocytochemistry analyses demonstrated that RacGAP1 is recruited to IQGAP1 and active β1 integrin, and that suppression of RacGAP1 expression triggered elevated Rac1 activity during spreading on fibronectin. Consistent with these findings, reduced expression of filamin-A, IQGAP1 or RacGAP1 triggered unconstrained membrane protrusion and disrupted directional cell migration on fibrillar extracellular matrices. These findings suggest a model whereby integrin engagement, followed by filamin-A, IQGAP1 and RacGAP1 recruitment, deactivates Rac1 to constrain its activity spatially and thereby coordinate directional cell migration.

IQGAP1: a regulator of intracellular spacetime relativity

Activating and inhibiting receptors of lymphocytes collect valuable information about their mikròs kósmos. This information is essential to initiate or to turn off complex signaling pathways. Irrespective of these advances, our knowledge on how these intracellular activation cascades are coordinated in a spatiotemporal manner is far from complete. Among multiple explanations, the scaffolding proteins have emerged as a critical piece of this evolutionary tangram. Among many, IQGAP1 is one of the essential scaffolding proteins that coordinate multiple signaling pathways. IQGAP1 possesses multiple protein interaction motifs to achieve its scaffolding functions. Using these domains, IQGAP1 has been shown to regulate a number of essential cellular events. This includes actin polymerization, tubulin multimerization, microtubule organizing center formation, calcium/calmodulin signaling, Pak/Raf/Mek1/2-mediated Erk1/2 activation, formation of maestrosome, E-cadherin, and CD44-mediated signaling and glycogen synthase kinase-3/adenomatous polyposis coli-mediated β-catenin activation. In this review, we summarize the recent developments and exciting new findings of cellular functions of IQGAP1.

WAVE2 Protein Complex Coupled to Membrane and Microtubules

E-cadherin is one of the key molecules in the formation of cell-cell adhesion and interacts intracellularly with a group of proteins collectively named catenins, through which the E-cadherin-catenin complex is anchored to actin-based cytoskeletal components. Although cell-cell adhesion is often disrupted in cancer cells by either genetic or epigenetic alterations in cell adhesion molecules, disruption of cell-cell adhesion alone seems to be insufficient for the induction of cancer cell migration and invasion. A small GTP-binding protein, Rac1, induces the specific cellular protrusions lamellipodia via WAVE2, a member of WASP/WAVE family of the actin cytoskeletal regulatory proteins. Biochemical and pharmacological investigations have revealed that WAVE2 interacts with many proteins that regulate microtubule growth, actin assembly, and membrane targeting of proteins, all of which are necessary for directional cell migration through lamellipodia formation. These findings might have important implications for the development of effective therapeutic agents against cancer cell migration and invasion.

The substrates and binding partners of protein kinase Cepsilon

The epsilon isoform of protein kinase C (PKCepsilon) has important roles in the function of the cardiac, immune and nervous systems. As a result of its diverse actions, PKCepsilon is the target of active drug-discovery programmes. A major research focus is to identify signalling cascades that include PKCepsilon and the substrates that PKCepsilon regulates. In the present review, we identify and discuss those proteins that have been conclusively shown to be direct substrates of PKCepsilon by the best currently available means. We will also describe binding partners that anchor PKCepsilon near its substrates. We review the consequences of substrate phosphorylation and discuss cellular mechanisms by which target specificity is achieved. We begin with a brief overview of the biology of PKCepsilon and methods for substrate identification, and proceed with a discussion of substrate categories to identify common themes that emerge and how these may be used to guide future studies.

Regulation of protein phosphatase 2A-mediated recruitment of IQGAP1 to beta1 integrin by EGF through activation of Ca2+/calmodulin-dependent protein kinase II

Maintenance of beta1 integrin-mediated cell adhesion in quiescent human mammary epithelial (HME) cells requires protein phosphatase (PP) 2A for not only dephosphorylation of beta1 integrin but also recruitment of IQGAP1 to Rac-bound beta1 integrin. However, how PP2A-dependent regulatory machinery of cell adhesion responds to EGF remains to be elucidated. We report here that phosphorylated Ca2+/calmodulin-dependent protein kinase II (CaMKII) at threonine 286 was involved in the beta1 integrin complex that consisted of PP2A, Rac, and IQGAP1 in quiescent HME cells. Stimulation of the cells with EGF concomitantly induced an increase in intracellular Ca2+, activation of CaMKII, and dissociation of PP2A-IQGAP1-CaMKII from beta1 integrin-Rac. Because the activation of CaMKII and dissociation of PP2A-IQGAP1-CaMKII were blocked by either Ca2+-chelator or CaMKII inhibitor, we therefore propose that EGF has the ability to abrogate the PP2A function in the maintenance of beta1 integrin-mediated cell adhesion by dissociation of PP2A-IQGAP1-CaMKII from beta1 integrin-Rac through activation of CaMKII.

Negative regulation of Akt activity by p38alpha MAP kinase in cardiomyocytes involves membrane localization of PP2A through interaction with caveolin-1

Cardiomyocyte-derived cell lines deficient in p38alpha are more resistant to apoptosis owing to lower expression of the pro-apoptotic proteins Bax and Fas and upregulation of the ERK survival pathway. Here, we show that increased Akt activity also contributes to the enhanced survival of p38alpha-deficient cardiomyocytes. We found that the serine/threonine phosphatase PP2A can be targeted to caveolae through interaction with caveolin-1 in a p38alpha-dependent manner. In agreement with this, PP2A activity associated with caveolin-1 was higher in wild type than in p38alpha-deficient cells. Akt was also present in caveolae and incubation of wild-type cells with the PP2A inhibitor okadaic acid increases the levels of Akt activity. Thus, p38alpha-induced re-localization of PP2A to caveolae can lead to dephosphorylation and inhibition of Akt, which in turn would contribute to the decreased survival observed in wild type cells. However, cell detachment impairs the formation of the PP2A/caveolin-1 complex and, as a consequence, phospho-Akt levels and survival are no longer regulated by p38alpha in detached wild type cardiomyocytes. Our results suggest that p38alpha can negatively modulate Akt activity, independently of PI3K, by regulating the interaction between caveolin-1 and PP2A through a mechanism dependent on cell attachment.

Insulin-like growth factor I controls a mutually exclusive association of RACK1 with protein phosphatase 2A and beta1 integrin to promote cell migration

The WD repeat scaffolding protein RACK1 can mediate integration of the insulin-like growth factor I receptor (IGF-IR) and integrin signaling in transformed cells. To address the mechanism of RACK1 function, we searched for regulatory proteins that associate with RACK1 in an IGF-I-dependent manner. The serine threonine phosphatase protein phosphatase 2A (PP2A) was found associated with RACK1 in serum-starved cells, and it dissociated immediately upon stimulation with IGF-I. This dissociation of PP2A from RACK1 and an IGF-I-mediated decrease in cellular PP2A activity did not occur in cells expressing either the serine 1248 or tyrosine 1250/1251 mutants of the IGF-IR that do not interact with RACK1. Recombinant RACK1 could bind to PP2A in vitro and restore phosphatase activity to PP2A from IGF-I-stimulated cells. Ligation of integrins with fibronectin or Matrigel was sufficient to facilitate IGF-I-mediated dissociation of PP2A from RACK1 and also to recruit beta1 integrin as PP2A dissociated. By using TAT-fused N-terminal and C-terminal deletion mutants of RACK1, we determined that both PP2A and beta1 integrin interact in the C terminus of RACK1 within WD repeats 4 to 7. This suggests that integrin ligation displaces PP2A from RACK1. MCF-7 cells overexpressing RACK1 exhibited enhanced motility, which could be reversed by the PP2A inhibitor okadaic acid. Small interfering RNA-mediated suppression of RACK1 also decreased the migratory capacity of DU145 cells. Taken together, our findings indicate that RACK1 enhances IGF-I-mediated cell migration through its ability to exclusively associate with either beta1 integrin or PP2A in a complex at the IGF-IR.

Involvement of protein phosphatase 2A in the maintenance of E-cadherin-mediated cell-cell adhesion through recruitment of IQGAP1

Serine/threonine protein phosphatase (PP) 2A regulates many biological processes, however it remains unclear whether PP2A participates in cadherin-mediated cell-cell adhesion. We show here that the core enzyme of PP2A (PP2A-AC) is localized in the cell-cell adhesion sites between adjacent cells and associated with the E-cadherin-catenins complex in non-malignant human mammary epithelial (HME) cells at confluence. Treatment of the cells with either okadaic acid (OA), an inhibitor of PP2A, or siRNA for the regulatory subunit A of PP2A (PP2A-A) caused disruption of cell-cell adhesion and F-actin assembly, without affecting the complex formation of E-cadherin with beta- and alpha-catenins. While a small GTPase Rac and its effector IQGAP1 were associated with the E-cadherin-catenins complex, either OA or PP2A-A siRNA concomitantly induced the dissociation of IQGAP1, but not Rac, from the complex and the internalization of E-cadherin from the cell surface. We therefore propose that PP2A plays a crucial role in the maintenance of cell-cell adhesion through recruitment of IQGAP1 to the Rac-bound E-cadherin-catenins complex.

Cell adhesion regulates Ser/Thr phosphorylation and proteasomal degradation of HEF1

Human enhancer of filamentation 1 (HEF1), a multifunctional docking protein of the Cas family, participates in integrin and growth factor signaling pathways that regulate global cellular processes including growth, motility and apoptosis. HEF1 consists of two isoforms, p105 and p115, the larger molecular weight form resulting from Ser/Thr phosphorylation of p105HEF1. The molecular mechanisms that regulate the interconversion of the two HEF1 species as well as the function of HEF1 Ser/Thr phosphorylation are unknown. Our study reveals that cell adhesion and detachment regulate the interconversion of the two HEF1 isoforms. Experiments using various inhibitors of cytoskeletal organization indicated that disruption of actin microfilaments but not intermediate filaments or microtubules resulted in a complete conversion of p115HEF1 to p105HEF1. The conversion of p115HEF1 to p105HEF1 was prevented by inhibition of protein phosphatase 2A (PP2A), suggesting that cytoskeletal regulation of PP2A activity controlled the dephosphorylation of p115HEF1. Degradation of endogenous HEF1 was dependent on proteasomes with the p115 species of HEF1 being preferentially targeted for turnover. Dephosphorylation of HEF1 by suspending cells or disrupting actin filaments protected HEF1 from degradation. These results suggest that the adhesion-dependent actin organization regulates proteasomal turnover of HEF1 through the activity of PP2A.