Protein phosphatases 1 and 2A maintain endothelial cells in a resting state, limiting the motility that is needed for the morphogenic process of angiogenesis

Apolipoprotein(a), an enigmatic anti-angiogenic glycoprotein in human plasma: A curse or cure?

Angiogenesis is a finely co-ordinated, multi-step developmental process of the new vascular structure. Even though angiogenesis is regularly occurring in physiological events such as embryogenesis, in adults, it is restricted to specific tissue sites where rapid cell-turnover and membrane synthesis occurs. Both excessive and insufficient angiogenesis lead to vascular disorders such as cancer, ocular diseases, diabetic retinopathy, atherosclerosis, intra-uterine growth restriction, ischemic heart disease, stroke etc. Occurrence of altered lipid profile and vascular lipid deposition along with vascular disorders is a hallmark of impaired angiogenesis. Among lipoproteins, lipoprotein(a) needs special attention due to the presence of a multi-kringle protein subunit, apolipoprotein(a) [apo(a)], which is structurally homologous to many naturally occurring anti-angiogenic proteins such as plasminogen and angiostatin. Researchers have constructed different recombinant forms of apo(a) (rhLK68, rhLK8, RHACK2, KV-11, and AU-6) and successfully exploited its potential to inhibit unwanted angiogenesis during tumor metastasis and retinal neovascularization. Similar to naturally occurring anti-angiogenic proteins, apo(a) can directly interfere with angiogenic signaling pathways. Besides this, apo(a) can also exert its anti-angiogenic effect indirectly by inducing endothelial cell apoptosis, by inhibiting endothelial progenitor cell functions or by upregulating nuclear factors in endothelial cells via apo(a)-bound oxPLs. However, the impact of the anti-angiogenic potential of native apo(a) during physiological angiogenesis in embryos and wounded tissues is not yet explored. In this context, we review the studies so far done to demonstrate the anti-angiogenic activity of apo(a) and the recent developments in using apo(a) as a therapeutic agent to treat impaired angiogenesis during vascular disorders, with emphasis on the gaps in the literature.

Role of Protein Phosphatase 1 in Angiogenesis and Odontoblastic Differentiation of Human Dental Pulp Cells

INTRODUCTION

The aims of this study were to examine the immunolocalization of protein phosphatase 1 (PP1) in developing mouse pulp tissue and to explore the role of PP1 in odontoblastic differentiation and in vitro angiogenesis in human dental pulp cells (HDPCs).

METHODS

Immunolocalization of PP1 was assessed in developing mouse pulp tissue. Odontogenic differentiation was examined by alkaline phosphatase activity, alizarin red staining, and reverse transcriptase polymerase chain reaction. Angiogenesis was evaluated by endothelial cell migration and capillary tube formation. Signaling pathways were analyzed by Western blotting and confocal immunofluorescence.

RESULTS

PP1 expression was detected in preodontoblasts, odontoblasts, dental pulp cells, and endothelial cells within pulp tissue during the crown formed, root formation, and root completion stages. PP1 messenger RNA (mRNA) and protein levels were up-regulated at the late mineralization stage during odontogenic differentiation of HDPCs. The PP1 activator C2 ceramide increased alkaline phosphatase activity, mineralized nodule formation, and mRNA expression of dentin matrix protein 1 and dentin sialophosphoprotein. In contrast, knockdown by PP1 small interfering RNA inhibited odontoblastic differentiation. Moreover, PP1 activator up-regulated mRNA expression of angiogenic genes in HDPCs and increased the migration and capillary tube formation of endothelial cells, whereas PP1 small interfering RNA showed opposite effects. C2 ceramide increased levels of bone morphogenetic protein 2, phosphorylation of Smad 1/5/8, and mRNA expression of runt-related transcription factor 2 and osterix.

CONCLUSIONS

This study provides the first evidence that PP1 might be a potent regulator of developing pulp tissue in vivo and odontoblastic differentiation and angiogenesis in HDPCs in vitro and may have clinical implications for pulp/dentin regeneration or reparative dentinogenesis.

BAG3 (Bcl-2-Associated Athanogene-3) Coding Variant in Mice Determines Susceptibility to Ischemic Limb Muscle Myopathy by Directing Autophagy

BACKGROUND

Critical limb ischemia is a manifestation of peripheral artery disease that carries significant mortality and morbidity risk in humans, although its genetic determinants remain largely unknown. We previously discovered 2 overlapping quantitative trait loci in mice, Lsq-1 and Civq-1, that affected limb muscle survival and stroke volume after femoral artery or middle cerebral artery ligation, respectively. Here, we report that a Bag3 variant (Ile81Met) segregates with tissue protection from hind-limb ischemia.

METHODS

We treated mice with either adeno-associated viruses encoding a control (green fluorescent protein) or 2 BAG3 (Bcl-2-associated athanogene-3) variants, namely Met81 or Ile81, and subjected the mice to hind-limb ischemia.

RESULTS

We found that the BAG3 Ile81Met variant in the C57BL/6 (BL6) mouse background segregates with protection from tissue necrosis in a shorter congenic fragment of Lsq-1 (C.B6-Lsq1-3). BALB/c mice treated with adeno-associated virus encoding the BL6 BAG3 variant (Ile81; n=25) displayed reduced limb-tissue necrosis and increased limb tissue perfusion compared with Met81- (n=25) or green fluorescent protein- (n=29) expressing animals. BAG3Ile81, but not BAG3Met81, improved ischemic muscle myopathy and muscle precursor cell differentiation and improved muscle regeneration in a separate, toxin-induced model of injury. Systemic injection of adeno-associated virus-BAG3Ile81 (n=9), but not BAG3Met81 (n=10) or green fluorescent protein (n=5), improved ischemic limb blood flow and limb muscle histology and restored muscle function (force production). Compared with BAG3Met81, BAG3Ile81 displayed improved binding to the small heat shock protein (HspB8) in ischemic skeletal muscle cells and enhanced ischemic muscle autophagic flux.

CONCLUSIONS

Taken together, our data demonstrate that genetic variation in BAG3 plays an important role in the prevention of ischemic tissue necrosis. These results highlight a pathway that preserves tissue survival and muscle function in the setting of ischemia.

Interrelationship between Protein Phosphatase-2A and Cytoskeletal Architecture during the Endothelial Cell Response to Soluble Products Produced by Human Head and Neck Cancer

Tumor neovascularization is necessary for the progressive development of all solid tumors, including head and neck squamous cell carcinomas (HNSCCs). The angiogenic process includes increased endothelial cell motility. Our prior studies have shown the importance of protein phos-phatase-2A (PP-2A) in restricting endothelial cell motility. Because motility is regulated by the polymerization/depolymerization of the cellular cytoskeleton, the present study defined the interrelationship between PP-2A and the cytoskeleton during endothelial cell responses to HNSCC-derived angiogenic factors. PP-2A was shown to colocalize with microtubules of unstimulated endothelial cells. However, exposure to HNSCC-derived products resulted in a more diffuse distribution of PP-2A staining and a loss of filamentous tubulin. The feasibility of pharmacologically preventing this cytoskeletal disorganization as a means of blocking tumor-induced angiogenesis was tested. This was accomplished by use of 1α,25-dihydroxyvitamin D3[1,25 (OH)2D3] and all- trans-retinoic acid to indirectly stimulate PP-2A activity through their capacity to elevated intracellular levels of the second messenger ceramide. Pretreatment of endothelial cells with either 1,25(OH)2D3or retinoic acid prevented the cytoskeletal disorganization that otherwise occurs in endothelial cells on exposure to HNSCC-derived products. These studies support the feasibility of using elevation of PP-2A to prevent the mor-phogenic component of the angiogenic process that is stimulated by HNSCC-derived factors.

TGF-β cascade regulation by PPP1 and its interactors -impact on prostate cancer development and therapy

Protein phosphorylation is a key mechanism by which normal and cancer cells regulate their main transduction pathways. Protein kinases and phosphatases are precisely orchestrated to achieve the (de)phosphorylation of candidate proteins. Indeed, cellular health is dependent on the fine-tune of phosphorylation systems, which when deregulated lead to cancer. Transforming growth factor beta (TGF-β) pathway involvement in the genesis of prostate cancer has long been established. Many of its members were shown to be hypo- or hyperphosphorylated during the process of malignancy. A major phosphatase that is responsible for the vast majority of the serine/threonine dephosphorylation is the phosphoprotein phosphatase 1 (PPP1). PPP1 has been associated with the dephosphorylation of several proteins involved in the TGF-β cascade. This review will discuss the role of PPP1 in the regulation of several TGF-β signalling members and how the subversion of this pathway is related to prostate cancer development. Furthermore, current challenges on the protein phosphatases field as new targets to cancer therapy will be addressed.

Prostate cancer: the need for biomarkers and new therapeutic targets

Prostate cancer (PCa) incidence and mortality have decreased in recent years. Nonetheless, it remains one of the most prevalent cancers in men, being a disquieting cause of men's death worldwide. Changes in many cell signaling pathways have a predominant role in the onset, development, and progression of the disease. These include prominent pathways involved in the growth, apoptosis, and angiogenesis of the normal prostate gland, such as androgen and estrogen signaling, and other growth factor signaling pathways. Understanding the foundations of PCa is leading to the discovery of key molecules that could be used to improve patient management. The ideal scenario would be to have a panel of molecules, preferably detectable in body fluids, that are specific and sensitive biomarkers for PCa. In the early stages, androgen deprivation is the gold standard therapy. However, as the cancer progresses, it eventually becomes independent of androgens, and hormonal therapy fails. For this reason, androgen-independent PCa is still a major therapeutic challenge. By disrupting specific protein interactions or manipulating the expression of some key molecules, it might be possible to regulate tumor growth and metastasis formation, avoiding the systemic side effects of current therapies. Clinical trials are already underway to assess the efficacy of molecules specially designed to target key proteins or protein interactions. In this review, we address that recent progress made towards understanding PCa development and the molecular pathways underlying this pathology. We also discuss relevant molecular markers for the management of PCa and new therapeutic challenges.

PP2A regulatory subunit Bα controls endothelial contractility and vessel lumen integrity via regulation of HDAC7

To supply tissues with nutrients and oxygen, the cardiovascular system forms a seamless, hierarchically branched, network of lumenized tubes. Here, we show that maintenance of patent vessel lumens requires the Bα regulatory subunit of protein phosphatase 2A (PP2A). Deficiency of Bα in zebrafish precludes vascular lumen stabilization resulting in perfusion defects. Similarly, inactivation of PP2A-Bα in cultured ECs induces tubulogenesis failure due to alteration of cytoskeleton dynamics, actomyosin contractility and maturation of cell-extracellular matrix (ECM) contacts. Mechanistically, we show that PP2A-Bα controls the activity of HDAC7, an essential transcriptional regulator of vascular stability. In the absence of PP2A-Bα, transcriptional repression by HDAC7 is abrogated leading to enhanced expression of the cytoskeleton adaptor protein ArgBP2. ArgBP2 hyperactivates RhoA causing inadequate rearrangements of the EC actomyosin cytoskeleton. This study unravels the first specific role for a PP2A holoenzyme in development: the PP2A-Bα/HDAC7/ArgBP2 axis maintains vascular lumens by balancing endothelial cytoskeletal dynamics and cell-matrix adhesion.

Signaling regulation of fetoplacental angiogenesis

During normal pregnancy, dramatically increased placental blood flow is critical for fetal growth and survival as well as neonatal birth weights and survivability. This increased blood flow results from angiogenesis, vasodilatation, and vascular remodeling. Locally produced growth factors including fibroblast growth factor 2 (FGF2) and vascular endothelial growth factor A (VEGFA) are key regulators of placental endothelial functions including cell proliferation, migration, and vasodilatation. However, the precise signaling mechanisms underlying such regulation in fetoplacental endothelium are less well defined, specifically with regard to the interactions amongst protein kinases (PKs), protein phosphatase, and nitric oxide (NO). Recently, we and other researchers have obtained solid evidence showing that different signaling mechanisms participate in FGF2- and VEGFA-regulated fetoplacental endothelial cell proliferation and migration as well as NO production. This review will briefly summarize currently available data on signaling mediating fetoplacental angiogenesis with a specific emphasis on PKs, ERK1/2, AKT1, and p38 MAPK and protein phosphatases, PPP2 and PPP3.

Suppression of protein phosphatase 2 differentially modulates VEGF- and FGF2-induced signaling in ovine fetoplacental artery endothelial cells

Vascular endothelial growth factor (VEGF) and fibroblast growth factor 2 (FGF2) elicit cellular responses via activation of protein kinases and phosphatases. We have reported that the MEK1/2/ERK1/2 and PI3K/AKT1 pathways are critical for VEGF- and FGF2-stimulated ovine fetoplacental artery endothelial (OFPAE) cell proliferation. We have also shown that protein phosphatase 3 (PPP3) differentially modulates VEGF- and FGF2-stimulated cell proliferation and activation of ERK1/2 and AKT1 in OFPAE cells. Herein, we investigated if protein phosphatase 2 (PPP2) modulated VEGF- and FGF2-induced ERK1/2, AKT1, and p38 MAPK activation and VEGF- and FGF2-stimulated cell proliferation in OFPAE cells. Small interfering RNA (siRNA) specifically targeting human PPP2CA catalytic subunit alpha (PPP2CA) was used to suppress PPP2CA expression in OFPAE cells. When compared with scrambled siRNA, PPP2CA siRNA decreased (p<0.05) PPP2CA protein levels (approximately 70%) and activity (approximately 50%) without altering protein levels of PPP3 catalytic subunit alpha (PPP3CA), nitric oxide synthase 3 (NOS3), ERK1/2, AKT1, and p38 MAPK. FGF2, but not VEGF rapidly (< or =5 min) induced p38 MAPK phosphorylation. Suppression of PPP2CA enhanced (p<0.05) VEGF-induced AKT1, but not ERK1/2 phosphorylation, whereas inhibited (p<0.05) FGF2-induced ERK1/2 and p38 MAPK and slightly attenuated FGF2-induced AKT1 phosphorylation. Suppression of PPP2CA did not significantly affect VEGF- and FGF2-stimulated OFPAE cell proliferation. Thus, suppression of PPP2CA alone differentially modulated VEGF- and FGF2-induced ERK1/2, AKT1, and p38 MAPK activation, without altering VEGF- and FGF2-stimulated cell proliferation in OFPAE cells. These data also suggest that signaling molecules other than ERK1/2, AKT1, and p38 MAPK are important mediators for VEGF- and FGF2-stimulated OFPAE cell proliferation after PPP2CA suppression.

AMP-activated protein kinase activation as a strategy for protecting vascular endothelial function

1. AMP-activated protein kinase (AMPK) is a serine/threonine protein kinase involved in the regulation of cellular and organismal metabolism. AMPK has a heterotrimeric structure, consisting of a catalytic alpha-subunit and regulatory beta- and gamma-subunits, each of which has two or more isoforms that are differentially expressed in various tissues and that arise from distinct genes. The AMPK system acts as a sensor of cellular energy status that is conserved in all eukaryotic cells. In addition, AMPK is activated by physiological stimuli and oxidants. 2. The importance of AMPK in cardiovascular functions is best demonstrated by recent studies showing that widely used drugs, including statins, metformin and rosiglitazone, execute cardiovascular protective effects at least partly through the activation of AMPK. As a consequence, AMPK has been proposed as a candidate target for therapeutic intervention in the treatment of both Type 2 diabetes and metabolic syndrome owing to its central role in the regulation of energy balance; it may also have a role in weight control. 3. In the present brief review, we summarize the recent progress of AMPK signalling and regulation focusing on vascular endothelial cells. We further hypothesize that AMPK is a dual sensor for energy and redox status within a cell and AMPK may be a therapeutic target for protecting vascular endothelial function.

Regulation of vascular endothelial cell barrier function and cytoskeleton structure by protein phosphatases of the PPP family

Reversible phosphorylation of cytoskeletal and cytoskeleton-associated proteins is a significant element of endothelial barrier function regulation. Therefore, understanding the mechanisms of phosphorylation/dephosphorylation of endothelial cell cytoskeletal proteins is vital to the treatment of severe lung disorders such as high permeability pulmonary edema. In vivo, there is a controlled balance between the activities of protein kinases and phosphatases. Due to various external or internal signals, this balance may be shifted. The actual balances at a given time alter the phosphorylation level of certain proteins with appropriate physiological consequences. The latest information about the structure and regulation of different types of Ser/Thr protein phosphatases participating in the regulation of endothelial cytoskeletal organization and barrier function will be reviewed here.

VCAM-1 activation of endothelial cell protein tyrosine phosphatase 1B

Lymphocytes migrate from the blood into tissue by binding to and migrating across endothelial cells. One of the endothelial cell adhesion molecules that mediate lymphocyte binding is VCAM-1. We have reported that binding to VCAM-1 activates endothelial cell NADPH oxidase for the generation of reactive oxygen species (ROS). The ROS oxidize and stimulate an increase in protein kinase C (PKC)alpha activity. Furthermore, these signals are required for VCAM-1-dependent lymphocyte migration. In this report, we identify a role for protein tyrosine phosphatase 1B (PTP1B) in the VCAM-1 signaling pathway. In primary cultures of endothelial cells and endothelial cell lines, Ab cross-linking of VCAM-1 stimulated an increase in serine phosphorylation of PTP1B, the active form of PTP1B. Ab cross-linking of VCAM-1 also increased activity of PTP1B. This activation of PTP1B was downstream of NADPH oxidase and PKCalpha in the VCAM-1 signaling pathway as determined with pharmacological inhibitors and antisense approaches. In addition, during VCAM-1 signaling, ROS did not oxidize endothelial cell PTP1B. Instead PTP1B was activated by serine phosphorylation. Importantly, inhibition of PTP1B activity blocked VCAM-1-dependent lymphocyte migration across endothelial cells. In summary, VCAM-1 activates endothelial cell NADPH oxidase to generate ROS, resulting in oxidative activation of PKCalpha and then serine phosphorylation of PTP1B. This PTP1B activity is necessary for VCAM-1-dependent transendothelial lymphocyte migration. These data show, for the first time, a function for PTP1B in VCAM-1-dependent lymphocyte migration.

VCAM-1 signals activate endothelial cell protein kinase Calpha via oxidation

Lymphocyte binding to VCAM-1 activates endothelial cell NADPH oxidase, resulting in the generation of 1 muM H(2)O(2). This is required for VCAM-1-dependent lymphocyte migration. In this study, we identified a role for protein kinase Calpha (PKCalpha) in VCAM-1 signal transduction in human and mouse endothelial cells. VCAM-1-dependent spleen cell migration under 2 dynes/cm(2) laminar flow was blocked by pretreatment of endothelial cells with dominant-negative PKCalpha or the PKCalpha inhibitors, Rö-32-0432 or Gö-6976. Phosphorylation of PKCalpha(Thr638), an autophosphorylation site indicating enzyme activity, was increased by Ab cross-linking of VCAM-1 on endothelial cells or by the exogenous addition of 1 muM H(2)O(2). The anti-VCAM-1-stimulated phosphorylation of PKCalpha(Thr638) was blocked by scavenging of H(2)O(2) and by inhibition of NADPH oxidase. Furthermore, anti-VCAM-1 signaling induced the oxidation of endothelial cell PKCalpha. Oxidized PKCalpha is a transiently active form of PKCalpha that is diacylglycerol independent. This oxidation was blocked by inhibition of NADPH oxidase. In summary, VCAM-1 activation of endothelial cell NADPH oxidase induces transient PKCalpha activation that is necessary for VCAM-1-dependent transendothelial cell migration.

Role of protein phosphatase 2A in the regulation of endothelial cell cytoskeleton structure

Our recently published data suggested the involvement of protein phosphatase 2A (PP2A) in endothelial cell (EC) barrier regulation (Tar et al. [2004] J Cell Biochem 92:534-546). In order to further elucidate the role of PP2A in the regulation of EC cytoskeleton and permeability, PP2A catalytic (PP2Ac) and A regulatory (PP2Aa) subunits were cloned and human pulmonary arterial EC (HPAEC) were transfected with PP2A mammalian expression constructs or infected with PP2A recombinant adenoviruses. Immunostaining of PP2Ac or of PP2Aa + c overexpressing HPAEC indicated actin cytoskeleton rearrangement. PP2A overexpression hindered or at least dramatically reduced thrombin- or nocodazole-induced F-actin stress fiber formation and microtubule (MT) dissolution. Accordingly, it also attenuated thrombin- or nocodazole-induced decrease in transendothelial electrical resistance indicative of barrier protection. Inhibition of PP2A by okadaic acid abolished its effect on agonist-induced changes in EC cytoskeleton; this indicates a critical role of PP2A activity in EC cytoskeletal maintenance. The overexpression of PP2A significantly attenuated thrombin- or nocodazole-induced phosphorylation of HSP27 and tau, two cytoskeletal proteins, which potentially could be involved in agonist-induced cytoskeletal rearrangement and in the increase of permeability. PP2A-mediated dephosphorylation of HSP27 and tau correlated with PP2A-induced preservation of EC cytoskeleton and barrier maintenance. Collectively, our observations clearly demonstrate the crucial role of PP2A in EC barrier protection.

Smad7 and protein phosphatase 1alpha are critical determinants in the duration of TGF-beta/ALK1 signaling in endothelial cells

BACKGROUND

In endothelial cells (EC), transforming growth factor-beta (TGF-beta) can bind to and transduce signals through ALK1 and ALK5. The TGF-beta/ALK5 and TGF-beta/ALK1 pathways have opposite effects on EC behaviour. Besides differential receptor binding, the duration of TGF-beta signaling is an important specificity determinant for signaling responses. TGF-beta/ALK1-induced Smad1/5 phosphorylation in ECs occurs transiently.

RESULTS

The temporal activation of TGF-beta-induced Smad1/5 phosphorylation in ECs was found to be affected by de novo protein synthesis, and ALK1 and Smad5 expression levels determined signal strength of TGF-beta/ALK1 signaling pathway. Smad7 and protein phosphatase 1alpha (PP1alpha) mRNA expression levels were found to be specifically upregulated by TGF-beta/ALK1. Ectopic expression of Smad7 or PP1alpha potently inhibited TGF-beta/ALK1-induced Smad1/5 phosphorylation in ECs. Conversely, siRNA-mediated knockdown of Smad7 or PP1alpha enhanced TGF-beta/ALK1-induced signaling responses. PP1alpha interacted with ALK1 and this association was further potentiated by Smad7. Dephosphorylation of the ALK1, immunoprecipitated from cell lysates, was attenuated by a specific PP1 inhibitor.

CONCLUSION

Our results suggest that upon its induction by the TGF-beta/ALK1 pathway, Smad7 may recruit PP1alpha to ALK1, and thereby control TGF-beta/ALK1-induced Smad1/5 phosphorylation.

Phosphatase 2A is involved in endothelial cell microtubule remodeling and barrier regulation

We have recently shown that microtubule (MT) inhibitor, nocodazole (2-5 microM) significantly increases endothelial cells (EC) actomyosin contraction and permeability indicating the importance of MT in maintaining the EC barrier (Verin et al. [2001]: Cell Mol Physiol 281:L565-L574). Okadaic acid (OA, 2-5 nM), a powerful inhibitor of protein phosphatase 2A (PP2A), significantly potentiates the effect of submaximal concentrations of nocodazole (50-200 nM) on transendothelial electrical resistance (TER) suggesting the involvement of PP2A activity in the MT-mediated EC barrier regulation. Immunofluorescent staining of EC revealed that in control cells PP2A distributes in a pattern similar to MT. Consistent with these results, we demonstrated that significant amounts of PP2A were present in MT-enriched EC fractions indicating tight association of PP2A with MT in endothelium. Treatment of EC with OA leads to disappearance of MT-like PP2A staining suggesting dissociation of PP2A from the MT network. Next, we examined the effect of PP2A inhibition on phosphorylation status of MT-associated protein tau, which in its unphosphorylated form promotes MT assembly. OA caused significant increases in tau phosphorylation confirming that tau is a substrate for PP2A in endothelium. Immunofluorescent experiments demonstrated that the OA-induced increases in tau phosphorylation strongly correlated with translocation of phospho-tau to cell periphery and disassembly of peripheral MT. These results suggest the involvement of PP2A-mediated tau dephosphorylation in alteration of EC MT structure and highlight the potential importance of PP2A in the regulation of EC the MT cytoskeleton and barrier function.

A Truncated Kringle Domain of Human Apolipoprotein(a) Inhibits the Activation of Extracellular Signal-regulated Kinase 1 and 2 through a Tyrosine Phosphatase-dependent Pathway

Most proangiogenic factors exert their biological effects primarily by activating extracellular signal-regulated kinase (ERK) and phosphoinositide 3-kinase (PI3-K)/Akt signaling pathways. These pathways appear to play a critical role in endothelial cell migration, because selective inhibition of either ERK or PI3-K/Akt signaling almost completely prevented endothelial cell migration. Recently, we demonstrated that a truncated kringle domain of human apolipoprotein(a), termed rhLK68, inhibits endothelial cell migration in vitro. However, its mechanism of action was not well defined. In this study, we determined the effects of rhLK68 on ERK1/2 and PI3-K/Akt signaling pathways to explore the molecular mechanism of rhLK68-mediated inhibition of endothelial cell migration. Treatment with rhLK68 inhibited ERK1/2 phosphorylation but did not influence Akt activation. Interestingly, an inhibitor of protein-tyrosine phosphatase, sodium orthovanadate, dose-dependently reversed both rhLK68-induced dephosphorylation of ERK1/2 and decreased migration of endothelial cells, whereas rhLK68 showed no significant effects on MEKs phosphorylation. In conclusion, these results indicate that inhibition of endothelial cell migration by rhLK68 may be achieved by interfering with ERK1/2 activation via a protein-tyrosine phosphatase-dependent pathway.

PP2A activation by beta2-adrenergic receptor agonists: novel regulatory mechanism of keratinocyte migration

Understanding the mechanisms that regulate cell migration is important for devising novel therapies to control metastasis or enhance wound healing. Previously, we demonstrated that beta2-adrenergic receptor (beta2-AR) activation in keratinocytes inhibited their migration by decreasing the phosphorylation of a critical promigratory signaling component, the extracellular signal-related kinase (ERK). Here we demonstrate that beta2-AR-induced inhibition of migration is mediated by the activation of the serine/threonine phosphatase PP2A. Pretreating human keratinocytes with the PP2A inhibitor, okadaic acid, prevented the beta2-AR-induced inhibition of migration, either as isolated cells or as a confluent sheet of cells repairing an in vitro "wound" and also prevented the beta2-AR-induced reduction in ERK phosphorylation. Similar results were obtained with human corneal epithelial cells. In keratinocytes, immunoprecipitation studies revealed that beta2-AR activation resulted in the rapid association of beta2-AR with PP2A as well as a 37% increase in association of PP2A with ERK2. Finally, beta2-AR activation resulted in a rapid and transient 2-fold increase in PP2A activity. Thus, we provide the first evidence that beta2-AR activation in keratinocytes modulates migration via a novel pathway utilizing PP2A to alter the promigratory signaling cascade. Exploiting this pathway may result in novel therapeutic approaches for control of epithelial cell migration.

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.

Protein phosphatase-2A regulates endothelial cell motility and both the phosphorylation and the stability of focal adhesion complexes

Solid cancers must stimulate expansion of the vascular network for continued growth. The process of angiogenesis involves endothelial cell migration so as to reorganize into vessel structures. The extent of cellular motility is regulated in part by the balance between serine/threonine kinases and protein phosphatases. In the present study, we show a decline in the activity of the serine/threonine phosphatase PP-2A in endothelial cells whose motility is stimulated by exposure to medium conditioned by either murine LLC cells or human HNSCC cells. Inhibition of endothelial cell PP-2A pharmacologically by treatment with okadaic acid also stimulated endothelial cell motility. Identification of mechanisms by which PP-2A inhibition might stimulate endothelial cell motility focused on proteins of the focal adhesions. Inhibition of PP-2A caused hyperphosphorylation of the paxillin serine residues and dephosphorylation of its tyrosine residues, dissolution of FAK/Src/paxillin complexes and decreased phosphorylation of the inhibitory Y529 residue of Src, suggesting increased Src activity. Inhibition of Src activity prevented the stimulation of PP-2A-inhibited cell motility. Our results suggest an interrelationship between tumor inhibition of PP-2A, dissolution of focal adhesion complexes and stimulated motility of endothelial cells.

Protein phosphatase 2A: the Trojan Horse of cellular signaling

Dynamic phosphorylation and dephosphorylation of proteins are fundamental mechanisms utilized by cells to transduce signals. Whereas transduction by protein kinases has been a major focus of studies in the last decade, protein phosphatase 2A (PP2A) enzymes emerge in this millenium as the most fashionable players in cellular signaling. Viral proteins target specific PP2A enzymes in order to deregulate chosen cellular pathways in the host and promote viral progeny. The observation that a variety of viruses utilize PP2A to alienate cellular behavior emphasizes the fundamental importance of PP2A in signal transduction. This review will primarily focus on discussing the uniqueness of PP2A regulation and uncovering the critical role played by protein-protein interactions in the modulation of PP2A signaling. Moreover, the place of PP2A in signaling pathways and its functional significance for human diseases will be discussed.

Interrelationship between protein phosphatase-2A and cytoskeletal architecture during the endothelial cell response to soluble products produced by human head and neck cancer

Tumor neovascularization is necessary for the progressive development of all solid tumors, including head and neck squamous cell carcinomas (HNSCCs). The angiogenic process includes increased endothelial cell motility. Our prior studies have shown the importance of protein phosphatase-2A (PP-2A) in restricting endothelial cell motility. Because motility is regulated by the polymerization/depolymerization of the cellular cytoskeleton, the present study defined the interrelationship between PP-2A and the cytoskeleton during endothelial cell responses to HNSCC-derived angiogenic factors. PP-2A was shown to colocalize with microtubules of unstimulated endothelial cells. However, exposure to HNSCC-derived products resulted in a more diffuse distribution of PP-2A staining and a loss of filamentous tubulin. The feasibility of pharmacologically preventing this cytoskeletal disorganization as a means of blocking tumor-induced angiogenesis was tested. This was accomplished by use of 1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and all-trans -retinoic acid to indirectly stimulate PP-2A activity through their capacity to elevated intracellular levels of the second messenger ceramide. Pretreatment of endothelial cells with either 1,25(OH)(2)D(3) or retinoic acid prevented the cytoskeletal disorganization that otherwise occurs in endothelial cells on exposure to HNSCC-derived products. These studies support the feasibility of using elevation of PP-2A to prevent the morphogenic component of the angiogenic process that is stimulated by HNSCC-derived factors.