Characterization of protein kinase A-mediated phosphorylation of ezrin in gastric parietal cell activation

Identification of ezrin as a target of gastrin in immature mouse gastric parietal cells

The gastric acid-secreting parietal cell exhibits profound morphological changes on stimulation. Studies in gastrin null (Gas-KO) mice indicate that maturation of parietal cell function depends on the hormone gastrin acting at the G-protein-coupled cholecystokinin 2 receptor. The relevant cellular mechanisms are unknown. The application of differential mRNA display to samples of the gastric corpus of wild-type (C57BL/6) and Gas-KO mice identified the cytoskeletal linker protein, ezrin, as a previously unsuspected target of gastrin. Gastrin administered in vivo or added to gastric glands in vitro increased ezrin abundance in Gas-KO parietal cells. In parietal cells of cultured gastric glands from wild-type mice treated with gastrin, histamine or carbachol, ezrin was localized to vesicular structures resembling secretory canaliculi. In contrast, in cultured parietal cells from Gas-KO mice, ezrin was typically distributed in the cytosol, and this did not change after incubation with gastrin, histamine or carbachol. However, priming with gastrin for approximately 24 h, either in vivo prior to cell culture or by addition to cultured gastric glands, induced the capacity for secretagogue-stimulated localization of ezrin to large vesicular structures in Gas-KO mice. Similarly, in a functional assay based on measurement of intracellular pH, cultured parietal cells from Gas-KO mice were refractory to gastrin unless primed. The priming effect of gastrin was not attributable to the paracrine mediator histamine, but was prevented by inhibitors of protein kinase C and transactivation of the epidermal growth factor receptor. We conclude that in gastrin null mice there is reduced ezrin expression and a defect in ezrin subcellular distribution in gastric parietal cells, and that both can be reversed by priming with gastrin.

Mucolipin 1 channel activity is regulated by protein kinase A-mediated phosphorylation

Mucolipins constitute a family of cation channels with homology with the transient receptor potential family. Mutations in MCOLN1 (mucolipin 1) have been linked to mucolipidosis type IV, a recessive lysosomal storage disease characterized by severe neurological and ophthalmologic abnormalities. At present, little is known about the mechanisms that regulate MCOLN1 activity. In the present paper, we addressed whether MCOLN1 activity is regulated by phosphorylation. We identified two PKA (protein kinase A) consensus motifs in the C-terminal tail of MCOLN1, containing Ser(557) and Ser(559). Ser(557) was the principal phosphorylation site, as mutation of this residue to alanine caused a greater than 75% reduction in the total levels of phosphorylated MCOLN1 C-terminal tail. Activation of PKA with forskolin promoted MCOLN1 phosphorylation, both in vitro and in vivo. In contrast, addition of the PKA inhibitor H89 abolished MCOLN1 phosphorylation. We also found that PKA-mediated phosphorylation regulates MCOLN1 channel activity. Forskolin treatment decreased MCOLN1 channel activity, whereas treatment with H89 increased MCOLN1 channel activity. The stimulatory effect of H89 on MCOLN1 function was not observed when Ser(557) and Ser(559) were mutated to alanine residues, indicating that these two residues are essential for PKA-mediated negative regulation of MCOLN1. This paper presents the first example of regulation of a member of the mucolipin family by phosphorylation.

Molecular dissection of HCl secretion in gastric parietal cells using streptolysin O permeabilization

Histamine-stimulated gastric acid secretion involves a transient elevation of intracellular Ca(2+) and the cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) cascade through phosphorylation, the actions of which ultimately result in the fusion of vesicles containing H,K-ATPase (adenosine triphosphatase) to the apical plasma membrane of parietal cells. To dissect the signaling events underlying gastric acid secretion, we have developed a permeabilized gastric gland model using streptolysin O (SLO). The advantage of this model is its ability to retain cytosolic components that are required for the secretory machinery while granting accessibility for the introduction of macromolecules into the cytoplasm. Our studies showed that acid secretion in SLO-permeabilized glands is a cAMP-dependent process and involves the recruitment of H,K-ATPase-rich tubulovesicles into the apical plasma membrane as judged by biochemical assays. These studies established a functional permeabilized gland model in which the resting-to-secreting transition can be triggered by second messengers, while the manipulation of the cytoplasmic environment can be achieved with ease.

Protein kinase A-mediated phosphorylation of the NF2 tumor suppressor protein merlin at serine 10 affects the actin cytoskeleton

Mutations in the neurofibromatosis 2 tumor suppressor gene (NF2) encoding merlin (moesin-ezrin-radixin like-protein) induce tumors of the nervous system. Merlin localizes to the cell membrane where it links the actin cytoskeleton to membrane proteins. Cell proliferation is regulated by merlin in many cell types, but merlin's tumor suppressor function still remains unclear. Phosphorylation has been suggested to regulate merlin's activity. The C-terminal serine 518 is phosphorylated both by p21-activated kinases (PAKs) and protein kinase A (PKA). In this work, we identify a novel PKA phosphorylation site, serine 10, in the N terminus of merlin. We show that a non-phosphorylatable form of serine 10 (S10A) affects cellular morphology. Regulation of this site also influences actin cytoskeleton organization and dynamics in vivo, as merlin S10A reduces the amount of cellular F-actin and merlin S10D stabilizes F-actin filaments. By using a wound-healing assay and live cell imaging, we demonstrate that dephosphorylation of serine 10 leads to defects in migration, possibly through altered ability of the cells to form lamellipodia. This study suggests a role for merlin in mediating PKA-induced changes of the actin cytoskeleton.

Compartmentalized cAMP signalling in regulated exocytic processes in non-neuronal cells

Cyclic adenosine monophosphate (cAMP) is a central second messenger controlling a plethora of vital functions. Studies of cAMP dynamics in living cells have revealed markedly inhomogeneous concentrations of the second messenger in different compartments. Moreover, cAMP effectors such as cAMP-dependent protein kinase (PKA) and cAMP-activated GTP-exchange factors (Epacs) are tethered to specific cellular sites. Both the tailoring of cAMP concentrations, and the activities of cAMP-dependent signalling systems at specific cellular locations are prerequisites for most, if not all, cAMP-dependent processes. This review focuses on the role of compartmentalized cAMP signalling in exocytic processes in non-neuronal cells. Particularly, the insertion of aquaporin-2 into the plasma membrane of renal principal cells as an example for a cAMP-dependent exocytic process in a non-secretory cell type, renin secretion from juxtaglomerular cells as a cAMP-triggered exocytosis from an endocrine cell, insulin release from pancreatic beta-cells as a Ca2+-mediated and cAMP-potentiated exocytic processes in an endocrine cell, and cAMP- or Ca2+ -triggered H+ secretion from gastric parietal cells as an exocytic process in an exocrine cell are discussed. The selected examples of cAMP-regulated exocytic pathways are reviewed with regard to key proteins involved: adenylyl cyclases, phosphodiesterases, PKA, A kinase anchoring proteins (AKAPs) and Epacs.

Phosphorylation of human Sgo1 by NEK2A is essential for chromosome congression in mitosis

Chromosome segregation in mitosis is orchestrated by the interaction of the kinetochore with spindle microtubules. Our recent study shows that NEK2A interacts with MAD1 at the kinetochore and possibly functions as a novel integrator of spindle checkpoint signaling. However, it is unclear how NEK2A regulates kinetochore-microtubule attachment in mitosis. Here we show that NEK2A phosphorylates human Sgo1 and such phosphorylation is essential for faithful chromosome congression in mitosis. NEK2A binds directly to HsSgo1 in vitro and co-distributes with HsSgo1 to the kinetochore of mitotic cells. Our in vitro phosphorylation experiment demonstrated that HsSgo1 is a substrate of NEK2A and the phosphorylation sites were mapped to Ser(14) and Ser(507) as judged by the incorporation of (32)P. Although such phosphorylation is not required for assembly of HsSgo1 to the kinetochore, expression of non-phosphorylatable mutant HsSgo1 perturbed chromosome congression and resulted in a dramatic increase in microtubule attachment errors, including syntelic and monotelic attachments. These findings reveal a key role for the NEK2A-mediated phosphorylation of HsSgo1 in orchestrating dynamic kinetochore-microtubule interaction. We propose that NEK2A-mediated phosphorylation of human Sgo1 provides a link between centromeric cohesion and spindle microtubule attachment at the kinetochores.

Phospho-regulation of HsCdc14A By Polo-like kinase 1 is essential for mitotic progression

Chromosome segregation in mitosis is orchestrated by dynamic interactions between spindle microtubules and centromeres, which in turn are governed by protein kinase- and phosphatase-signaling cascades. Previous studies showed that overexpression of human phosphatase HsCdc14A, an antagonist of cyclin-dependent kinase 1, affects several aspects of cell division. However, the molecular mechanism underlying HsCdc14A regulation in mitosis has remained elusive. Here we show that HsCdc14A activity is regulated by an auto-inhibitory mechanism via its intra-molecular association. Our biochemical study demonstrated that Polo-like kinase 1 (PLK1) interacts with and phosphorylates HsCdc14A. This phosphorylation partially releases the auto-inhibition of HsCdc14A judged by its phosphatase activity in vitro. To examine the functional relevance of such phospho-regulation of HsCdc14A in vivo, a phospho-mimicking mutant of HsCdc14A was expressed in HeLa cells. Importantly, overexpression of the phospho-mimicking mutants caused aberrant chromosome alignment with a prometaphase delay, suggesting the temporal regulation of HsCdc14A activity is critical for orchestrating mitotic events. Given the fact that HsCdc14A forms an intra-molecular association and PLK1-mediated phospho-regulation promotes HsCdc14A phosphatase activity, we propose that PLK1-HsCdc14A interaction provides a temporal regulation of HsCdc14A in chromosome segregation during mitosis.

Single-molecule detection of phosphorylation-induced plasticity changes during ezrin activation

Ezrin-radixin-moesin protein family provides a regulated link between the cortical actin cytoskeleton and the plasma membrane. Phosphorylation of ezrin has been functionally linked to membrane dynamics and plasticity. Our recent study demonstrated that phosphorylation of the conserved T567 residue of ezrin alters the physiology of gastric parietal cells. However, the molecular mechanism of phosphorylation-induced ezrin activation has remained elusive. Here we use atomic force microscopy (AFM) to probe phosphorylation-mediated activation of ezrin in single molecules. The phospho-mimicking and non-phosphorylatable mutant ezrin proteins were generated and purified to homogeneity. Comparative analyses of two ezrin mutants by AFM demonstrate the unfolding of the N- and C-terminal domains upon the phospho-activation. To measure the physical force underlying the inter-domain contact during mechanical unfolding, we probed the defined region of ezrin using the N-terminal ezrin coated onto the AFM tip. Comparative force measurements indicate that T567 phosphorylation-induced unfolding of ezrin favors the inter-molecular association. Taken together, these results provide molecular illustration of phosphorylation elicited functional activation of ERM proteins and indicate that stimulus-induced protein conformational change can be used as a signaling mechanism orchestrating cellular dynamics.

High turnover of ezrin T567 phosphorylation: conformation, activity, and cellular function

In its dormant state, the membrane cytoskeletal linker protein ezrin takes on a NH(2) terminal-to-COOH terminal (N-C) binding conformation. In vitro evidence suggests that eliminating the N-C binding conformation by Thr(567) phosphorylation leads to ezrin activation. Here, we found for resting gastric parietal cells that the levels of ezrin phosphorylation on Thr(567) are low and can be increased to a small extent ( approximately 40%) by stimulating secretion via the cAMP pathway. Treatment of cells with protein phosphatase inhibitors led to a rapid, dramatic increase in Thr(567) phosphorylation by 400% over resting levels, prompting the hypothesis that ezrin activity is regulated by turnover of phosphorylation on Thr(567). In vitro and in vivo fluorescence resonance energy transfer analysis demonstrated that Thr(567) phosphorylation opens the N-C interaction. However, even in the closed conformation, ezrin localizes to membranes by an exposed NH(2) terminal binding site. Importantly, the opened phosphorylated form of ezrin more readily cosediments with F-actin and binds more tightly to membrane than the closed forms. Furthermore, fluorescence recovery after photobleaching analysis in live cells showed that the Thr567Asp mutant had longer recovery times than the wild type or the Thr567Ala mutant, indicating the Thr(567)-phosphorylated form of ezrin is tightly associated with F-actin and the membrane, restricting normal activity. These data demonstrate and emphasize the functional importance of reversible phosphorylation of ezrin on F-actin binding. A novel model is proposed whereby ezrin and closely associated kinase and phosphatase proteins represent a motor complex to maintain a dynamic relationship between the varying membrane surface area and filamentous actin length.

cAMP synthesis and degradation by phagosomes regulate actin assembly and fusion events: consequences for mycobacteria

We showed recently that actin assembly by phagosomal membranes facilitates fusion with late endocytic organelles in macrophages. Moreover, lipids that induced phagosomal actin also stimulated this fusion process. In macrophages infected with pathogenic mycobacteria actin-stimulatory lipids led to an increase in pathogen destruction, whereas inhibitors facilitated their growth. A model was proposed whereby phagosomal membrane actin assembly provides tracks for lysosomes to move towards phagosomes, thereby facilitating fusion. Here, we investigated how cAMP affected phagosomal actin assembly in vitro, and phagosomal actin, acidification and late fusion events in J774 macrophages. Latex bead phagosomes are shown to possess adenylyl cyclase activity, which synthesizes cAMP, and phosphodiesterase activity, which degrades cAMP. The system is regulated by protein kinase A (PKA). Increasing cAMP levels inhibited, whereas decreasing cAMP levels stimulated, actin assembly in vitro and within cells. Increasing cAMP levels also inhibited phagosome-lysosome fusion and acidification in cells, whereas reducing cAMP had the opposite effect. High cAMP levels induced an increase in intraphagosomal growth in macrophages of both the non-pathogenic Mycobacterium smegmatis and the pathogenic Mycobacterium tuberculosis, whereas low cAMP levels or inhibition of PKA correlated with increased bacterial destruction. We argue that the phagosome cAMP-PKA system behaves as a molecular switch that regulates phagosome actin and maturation in macrophages.

ERM proteins in epithelial cell organization and functions

ERM (Ezrin, Radixin, Moesin) proteins are membrane-cytoskeleton linkers that regulate the structure and the function of specific domains of the plasma membrane. ERM proteins are expressed in all metazoan analyzed so far. Genetic analysis of ERM protein functions has recently been performed simultaneously in three different organisms, mouse, Drosophila melanogaster and C. elegans. These studies have revealed a remarkable conservation of the protein functions through evolution. Moreover they have shed light on the crucial role these proteins play in various physiological processes that occur in epithelial cells.

Proteomic identification and functional characterization of a novel ARF6 GTPase-activating protein, ACAP4

ARF6 GTPase is a conserved regulator of membrane trafficking and actin-based cytoskeleton dynamics at the leading edge of migrating cells. A key determinant of ARF6 function is the lifetime of the GTP-bound active state, which is orchestrated by GTPase-activating protein (GAP) and GTP-GDP exchanging factor. However, very little is known about the molecular mechanisms underlying ARF6-mediated cell migration. To systematically analyze proteins that regulate ARF6 activity during cell migration, we performed a proteomic analysis of proteins selectively bound to active ARF6 using mass spectrometry and identified a novel ARF6-specific GAP, ACAP4. ACAP4 encodes 903 amino acids and contains two coiled coils, one pleckstrin homology domain, one GAP motif, and two ankyrin repeats. Our biochemical characterization demonstrated that ACAP4 has a phosphatidylinositol 4,5-bisphosphate-dependent GAP activity specific for ARF6. The co-localization of ACAP4 with ARF6 occurred in ruffling membranes formed upon AIF(4) and epidermal growth factor stimulation. ACAP4 overexpression limited the recruitment of ARF6 to the membrane ruffles in the absence of epidermal growth factor stimulation. Expression of GTP hydrolysis-resistant ARF6(Q67L) resulted in accumulations of ACAP4 and ARF6 in the cytoplasmic membrane, suggesting that GTP hydrolysis is required for the ARF6-dependent membrane remodeling. Significantly the depletion of ACAP4 by small interfering RNA or inhibition of ARF6 GTP hydrolysis by overexpressing GAP-deficient ACAP4 suppressed ARF6-dependent cell migration in wound healing, demonstrating the importance of ACAP4 in cell migration. Thus, our study sheds new light on the biological function of ARF6-mediated cell migration.

PKA-mediated protein phosphorylation regulates ezrin–WWOX interaction

The ezrin-radixin-moesin proteins provide a regulated linkage between membrane proteins and the cortical cytoskeleton, and also participate in signal-transduction pathways. Ezrin is localized to the apical membrane of parietal cells and couples the cAMP-dependent protein kinase activation cascade to the regulated HCl secretion in gastric parietal cells. Our recent studies have mapped the PKA-mediated phosphorylation site to Ser(66) and established its functional role in parietal cell activation [R. Zhou et al., Characterization of protein kinase A-mediated phosphorylation of ezrin in gastric parietal cell activation, J. Biol. Chem. 278 (2003) 35651-35659], but the underlying basis for this regulation is not known. Here, we provide the first evidence that PKA-mediated phosphorylation of Ser(66)regulates the interaction of ezrin with WWOX, a WW domain-containing protein. Our biochemical study reveals that ezrin directly binds to the first WW domain of WWOX via its C-terminal tyrosine-containing polyproline sequence (470)PPPPPPVY(477). Mutational analyses further demonstrate that tyrosine(477) is essential for the ezrin-WWOX interaction. In addition, our study shows that PKA-mediated phosphorylation of ezrin is essential and sufficient for the apical localization of WWOX protein as disruption of ezrin-WWOX interaction eliminated the apical localization of WWOX. Finally, our study demonstrates the essential role of ezrin-WWOX interaction in the apical membrane remodeling associated with H,K-ATPase recruitment. Taken together, these results define a novel molecular mechanism underlying phospho-regulation of ezrin function by PKA in parietal cell activation.

PKA-mediated protein phosphorylation protects ezrin from calpain I cleavage

Ezrin is localized to the apical membrane of parietal cells and couples the cAMP-dependent protein kinase (PKA) activation cascade to the regulated HCl secretion in gastric parietal cells. Our recent studies demonstrate the functional relevance of PKA-mediated phosphorylation of ezrin in parietal cell secretion [R. Zhou, X. Cao, C. Watson, Y. Miao, Z. Guo, J.G. Forte, X. Yao, Characterization of protein kinase A-mediated phosphorylation of ezrin in gastric parietal cell activation, J. Biol. Chem. 278 (2003) 35651]. Here we show that activation of PKA protects ezrin from calpain I-mediated proteolysis without alteration of calpain I activation and fodrin breakdown. To determine whether phosphorylation of Ser66 by PKA affects the insensitivity to the calpain I-mediated cleavage, recombinant proteins of ezrin, both wild type and S66A/D mutants, were incubated with the purified calpain I. Indeed, phosphorylation-like S66D mutant ezrin is resistant to calpain I-mediated proteolysis while wild type and S66A mutant were sensitive. In fact, expression of phosphorylation-like S66D, but not S66A, mutant in parietal cells confers its resistance to calpain I-mediated proteolysis. Taken together, these results indicate that phosphorylation of ezrin by PKA modulates its sensitivity to calpain I cleavage.

Phosphorylation of ezrin on threonine 567 produces a change in secretory phenotype and repolarizes the gastric parietal cell

Phosphorylation of the membrane-cytoskeleton linker protein ezrin has been functionally linked to acid secretion and vesicle recruitment to the apical secretory membrane in gastric parietal cells. Phosphorylation of the conserved T567 residue of ezrin has been shown to alter the N/C oligomerization of ezrin and promote the formation of actin-rich surface projections in other cells. To test the importance of T567 as a regulatory site for ezrin in parietal cell activation, we incorporated wild-type (WT) and mutant forms of ezrin, including the nonphosphorylatable T567A mutation and a mutant mimicking permanent phosphorylation, T567D. All ezrin constructs included C-terminal cyan-fluorescent protein (CFP) and were incorporated into adenoviral constructs for efficient introduction into cultured parietal cells from rabbit stomach. Fluorescence microscopy was used to localize CFP-ezrin and monitor morphological responses. Accumulation of a weak base (aminopyrine) was used to monitor receptor-mediated acid secretory response of the cultured cells. Similar to endogenous ezrin, WT and T567A CFP-ezrin localized heavily to apical membrane vacuoles with considerably lower levels associated with the surrounding basolateral membrane. Interestingly, H,K-ATPase within cytoplasmic tubulovesicles was incorporated into the apical vacuoles along with WT and T567A mutant ezrin. In these parietal cells secretagogue stimulation produced a striking vacuolar expansion associated with HCl secretion and the secretory phenotype. Expression of T567D CFP-ezrin was quite different, being rarely associated with apical vacuoles. T567D was more typically localized to the basolateral membrane, often associated with long spikes and fingerlike projections. Moreover, the cells did not display secretagogue-dependent morphological changes and, to our surprise, H,K-ATPase was recruited to the T567D CFP-ezrin-enriched basolateral projections. We conclude that T567 phosphorylation, which is probably regulated through Rho signaling pathway, may direct ezrin to membrane-cytoskeletal activity at the basolateral membrane and away from apical secretory activity. The large basolateral expansion is predicted to recruit membranes from sources not normally targeted to that surface.

Achlorhydria by ezrin knockdown: defects in the formation/expansion of apical canaliculi in gastric parietal cells

Loss of gastric acid secretion is pathologically known as achlorhydria. Acid-secreting parietal cells are characterized by abundant expression of ezrin (Vil2), one of ezrin/radixin/moesin proteins, which generally cross-link actin filaments with plasma membrane proteins. Here, we show the direct in vivo involvement of ezrin in gastric acid secretion. Ezrin knockout (Vil2^−/−) mice did not survive >1.5 wk after birth, making difficult to examine gastric acid secretion. We then generated ezrin knockdown (Vil2^kd/kd) mice by introducing a neomycin resistance cassette between exons 2 and 3. Vil2^kd/kd mice born at the expected Mendelian ratio exhibited growth retardation and a high mortality. Approximately 7% of Vil2^kd/kd mice survived to adulthood. Ezrin protein levels in Vil2^kd/kd stomachs decreased to <5% of the wild-type levels without compensatory up-regulation of radixin or moesin. Adult Vil2^kd/kd mice suffered from severe achlorhydria. Immunofluorescence and electron microscopy revealed that this achlorhydria was caused by defects in the formation/expansion of canalicular apical membranes in gastric parietal cells.

Ezrin oligomers are the membrane-bound dormant form in gastric parietal cells

Ezrin is a member of ezrin, radixin, moesin (ERM) protein family that links F-actin to membranes. The NH(2)- and COOH-terminal association domains of ERM proteins, known respectively as N-ERMAD and C-ERMAD, participate in interactions with membrane proteins and F-actin, and intramolecular and intermolecular interactions within and among ERM proteins. In gastric parietal cells, ezrin is heavily represented on the apical membrane and is associated with cell activation. Ezrin-ezrin interactions are presumably involved in functional regulation of ezrin and thus became a subject of our study. Fluorescence resonance energy transfer (FRET) was examined with cyan fluorescent protein (CFP)- and yellow fluorescent protein (YFP)-tagged ezrin incorporated into HeLa cells and primary cultures of parietal cells. Constructs included YFP at the NH(2) terminus of ezrin (YFP-Ez), CFP at the COOH terminus of ezrin (Ez-CFP), and double-labeled ezrin (N-YFP-ezrin-CFP-C). FRET was probed using fluorescence microscopy and spectrofluorometry. Evidence of ezrin oligomer formation was found using FRET in cells coexpressing Ez-CFP and YFP-Ez and by performing coimmunoprecipitation of endogenous ezrin with fluorescent protein-tagged ezrin. Thus intermolecular NH(2)- and COOH-terminal association domain (N-C) binding in vivo is consistent with the findings of earlier in vitro studies. After the ezrin oligomers were separated from monomers, FRET was observed in both forms, indicating intramolecular and intermolecular N-C binding. When the distribution of native ezrin as oligomers vs. monomers was examined in resting and maximally stimulated parietal cells, a shift of ezrin oligomers to the monomeric form was correlated with stimulation, suggesting that ezrin oligomers are the membrane-bound dormant form in gastric parietal cells.

Conditional ROCK activation in vivo induces tumor cell dissemination and angiogenesis

Progression of tumors to invasive and metastatic forms requires that tumor cells undergo dramatic morphologic changes, a process regulated by Rho GTPases. Elevated expression of RhoA and RhoC, as well as the Rho effector proteins ROCK I and ROCK II, are commonly observed in human cancers and are often associated with more invasive and metastatic phenotypes. To examine how ROCK contributes to the progression of solid tumors, we established a conditionally activated form of ROCK II by fusing the kinase domain to the estrogen receptor hormone-binding domain (ROCK:ER). ROCK:ER-expressing colon carcinoma cells grown as tumors in immunocompromised nude mice organized into discrete clusters surrounding blood vessels. However, ROCK:ER activation resulted in the aggressive dissemination of tumor cells into the surrounding stroma, indicating that increased ROCK signaling is sufficient to promote invasion from solid tumors. In addition, tumors in which ROCK:ER was activated were more highly vascularized, indicating that ROCK contributes to tumor angiogenesis. ROCK:ER activation resulted in changes to epithelial morphology and organization that facilitated motility in vitro, likely by inducing the redistribution of proteins such as ezrin, as well as adherens junction and extracellular matrix-binding proteins. These results suggest that ROCK inhibitors would be useful antimetastatic and antiangiogenic chemotherapeutic agents in tumors associated with elevated RhoA, RhoC, ROCK I, or ROCK II expression.

PALS1 specifies the localization of ezrin to the apical membrane of gastric parietal cells

The ERM (ezrin/radixin/moesin) proteins provide a regulated linkage between membrane proteins and the cortical cytoskeleton and also participate in signal transduction pathways. Ezrin is localized to the apical membrane of parietal cells and couples the protein kinase A activation cascade to regulated HCl secretion in gastric parietal cells. Here, we show that the integrity of ezrin is essential for parietal cell activation and provide the first evidence that ezrin interacts with PALS1, an evolutionarily conserved PDZ and SH3 domain-containing protein. Our biochemical study verifies that ezrin binds to PALS1 via its N terminus and is co-localized with PALS1 to the apical membrane of gastric parietal cells. Furthermore, our study shows that PALS1 is essential for the apical localization of ezrin, as either suppression of PALS1 protein accumulation or deletion of the PALS1-binding domain of ezrin eliminated the apical localization of ezrin. Finally, our study demonstrates the essential role of ezrin-PALS1 interaction in the apical membrane remodeling associated with parietal cell secretion. Taken together, these results define a novel molecular mechanism linking ezrin to the conserved apical polarity complexes and their roles in polarized epithelial secretion of gastric parietal cells.

Src-dependent ezrin phosphorylation in adhesion-mediated signaling

In addition to providing a regulated linkage between the membrane and the actin cytoskeleton, ezrin participates in signal transduction pathways. Here we describe that expression of the ezrin Y145F mutant delays epithelial cell spreading on fibronectin by inhibiting events leading to FAK activation. The defect in spreading was rescued by the overexpression of catalytically functional Src. We demonstrate that ezrin Y145 is phosphorylated in A431 cells stimulated with epidermal growth factor (EGF) and in v-Src-transformed cells. Moreover in cells devoid of Src, SYF-/- fibroblasts, ezrin Y145 phosphorylation could only be detected upon the introduction of an active form of Src. The phosphorylation of ezrin at Y145 required prior binding of the Src SH2 domain to ezrin. Our results further show that Src activity influences its binding to ezrin and a positive feedback mechanism for Src-mediated Y145 phosphorylation is implied. Interestingly, cells expressing ezrin Y145F did not proliferate when cultured in a 3D collagen gel. Collectively, our results demonstrate a key signaling input of Src-dependent ezrin phosphorylation in adhesion-mediated events in epithelial cells.

Effect of proton pump inhibitor pretreatment on resistance of solid tumors to cytotoxic drugs

BACKGROUND

Resistance to antitumor agents is a major cause of treatment failure in patients with cancer. Some mechanisms of tumor resistance to cytotoxic drugs may involve increased acidification of extracellular compartments. We investigated whether proton pump inhibitors (PPIs), currently used in the anti-acid treatment of peptic disease, could inhibit the acidification of the tumor microenvironment and increase the sensitivity of tumor cells to cytotoxic agents.

METHODS

We pretreated cell lines derived from human melanomas, adenocarcinomas, and lymphomas with the PPIs omeprazole, esomeprazole, or pantoprazole and tested their response to cytotoxic drugs in cell death assays. We also evaluated extracellular and intracellular pH and vacuolar-H+-ATPase (V-H+-ATPase) expression, distribution, and activity in PPI-pretreated cells by using western blot analyses, immunocytochemistry, laser scanning confocal analysis, and bioluminescence assays. Finally, we evaluated human melanoma growth and cisplatin sensitivity with or without omeprazole pretreatment in xenografted SCID/SCID mice.

RESULTS

PPI pretreatment sensitized tumor cell lines to the effects of cisplatin, 5-fluorouracil, and vinblastine, with an IC50 value reduction up to 2 logs. PPI pretreatment was associated with the inhibition of V-H+-ATPase activity and increases in both extracellular pH and the pH of lysosomal organelles. PPI pretreatment induced a marked increase in the cytoplasmic retention of the cytotoxic drugs, with clear targeting to the nucleus in the case of doxorubicin. In in vivo experiments, oral pretreatment with omeprazole was able to induce sensitivity of human solid tumors to cisplatin.

CONCLUSION

Our results open new possibilities for the treatment of drug-resistant tumors through combination strategies based on the use of well-tolerated pH modulators such as PPIs.

Nek2A kinase regulates the localization of numatrin to centrosome in mitosis

Chromosome segregation in mitosis is orchestrated by the kinetochore and spindle microtubules stemming from two centrosomes. Our recent studies demonstrated the importance of Nek2A in faithful chromosome segregation during mitosis. Here, we report that Nek2A regulates the function of numatrin in mitosis. The biochemical interaction between Nek2A and numatrin in mitotic cells was revealed by a set of reciprocal immunoprecipitation experiments using Nek2A and numatrin antibodies, respectively. The interaction is validated by a pull-down assay using recombinant Nek2A and numatrin proteins. Moreover, our immunofluorescence studies demonstrate that numatrin becomes centrosome-associated as the cell enters into mitosis and depart from the centrosome after sister chromatid separation in anaphase. The co-localization of numatrin and Nek2A to the centrosome suggests their interaction with and involvement in centrosome function. Indeed, elimination of Nek2A kinase by siRNA diminished its association with the centrosome. Furthermore, we show that numatrin is phosphorylated by wild type but not kinase-death Nek2A. Our studies suggest that the Nek2A kinase cascade is essential for the localization of numatrin to the centrosome.

TTK kinase is essential for the centrosomal localization of TACC2

Chromosome segregation in mitosis is orchestrated by dynamic interaction between spindle microtubule and the kinetochore. Our recent ultrastructural studies demonstrated a dynamic distribution of TTK, from the kinetochore to the centrosome, as cell enters into anaphase. Here, we show that a centrosomal protein TACC2 is phosphorylated in mitosis by TTK signaling pathway. TACC2 was pulled down by wild type TTK but not kinase death mutant, suggesting the potential phosphorylation-mediated interaction between these two proteins. Our immunofluorescence studies revealed that both TTK and TACC2 are located to the centrosome. Interestingly, expression of kinase death mutant of TTK eliminated the centrosomal localization of TACC2 but not other centrosomal proteins such as gamma-tubulin and NuMA, a phenotype seen in TTK-depleted cells. In these centrosomal TACC2-liberated cells, chromosomes were lagging and mis-aligned. In addition, the distance between two centrosomes was markedly reduced, suggesting that centrosomal TACC2 is required for mitotic spindle maintenance. The inter-relationship between TTK and TACC2 established here provides new avenue to study centrosome and spindle dynamics underlying cell divisional control.

Cyclic AMP-dependent protein kinase phosphorylates merlin at serine 518 independently of p21-activated kinase and promotes merlin-ezrin heterodimerization

Mutations in the NF2 tumor suppressor gene encoding merlin induce the development of tumors of the nervous system. Merlin is highly homologous to the ERM (ezrin-radixin-moesin) family of membrane/cytoskeleton linker proteins. However, the mechanism for the tumor suppressing activity of merlin is not well understood. Previously, we characterized a novel role for merlin as a protein kinase A (PKA)-anchoring protein, which links merlin to the cAMP/PKA signaling pathway. In this study we show that merlin is also a target for PKA-induced phosphorylation. In vitro [gamma-(33)P]ATP labeling revealed that both the merlin N and C termini are phosphorylated by PKA. Furthermore, both in vitro and in vivo phosphorylation studies of the wild-type and mutated C termini demonstrated that PKA can phosphorylate merlin at serine 518, a site that is phosphorylated also by p21-activated kinases (PAKs). Merlin was phosphorylated by PKA in cells in which PAK activity was suppressed, indicating that the two kinases function independently. Both in vitro and in vivo interaction studies indicated that phosphorylation of serine 518 promotes heterodimerization between merlin and ezrin, an event suggested to convert merlin from a growth-suppressive to a growth-permissive state. This study provides further evidence on the connection between merlin and cAMP/PKA signaling and suggests a role for merlin in the cAMP/PKA transduction pathway.

NEK2A interacts with MAD1 and possibly functions as a novel integrator of the spindle checkpoint signaling

Chromosome segregation in mitosis is orchestrated by protein kinase signaling cascades. A biochemical cascade named spindle checkpoint ensures the spatial and temporal order of chromosome segregation during mitosis. Here we report that spindle checkpoint protein MAD1 interacts with NEK2A, a human orthologue of the Aspergillus nidulans NIMA kinase. MAD1 interacts with NEK2A in vitro and in vivo via a leucine zipper-containing domain located at the C terminus of MAD1. Like MAD1, NEK2A is localized to HeLa cell kinetochore of mitotic cells. Elimination of NEK2A by small interfering RNA does not arrest cells in mitosis but causes aberrant premature chromosome segregation. NEK2A is required for MAD2 but not MAD1, BUB1, and HEC1 to associate with kinetochores. These NEK2A-eliminated or -suppressed cells display a chromosome bridge phenotype with sister chromatid inter-connected. Moreover, loss of NEK2A impairs mitotic checkpoint signaling in response to spindle damage by nocodazole, which affected mitotic escape and led to generation of cells with multiple nuclei. Our data demonstrate that NEK2A is a kinetochore-associated protein kinase essential for faithful chromosome segregation. We hypothesize that NEK2A links MAD2 molecular dynamics to spindle checkpoint signaling.