Tyrosine phosphorylation of annexin II tetramer is stimulated by membrane binding

The ANXA2/S100A10 Complex—Regulation of the Oncogenic Plasminogen Receptor

The generation of the serine protease plasmin is initiated by the binding of its zymogenic precursor, plasminogen, to cell surface receptors. The proteolytic activity of plasmin, generated at the cell surface, plays a crucial role in several physiological processes, including fibrinolysis, angiogenesis, wound healing, and the invasion of cells through both the basement membrane and extracellular matrix. The seminal observation by Albert Fischer that cancer cells, but not normal cells in culture, produce large amounts of plasmin formed the basis of current-day observations that plasmin generation can be hijacked by cancer cells to allow tumor development, progression, and metastasis. Thus, the cell surface plasminogen-binding receptor proteins are critical to generating plasmin proteolytic activity at the cell surface. This review focuses on one of the twelve well-described plasminogen receptors, S100A10, which, when in complex with its regulatory partner, annexin A2 (ANXA2), forms the ANXA2/S100A10 heterotetrameric complex referred to as AIIt. We present the theme that AIIt is the quintessential cellular plasminogen receptor since it regulates the formation and the destruction of plasmin. We also introduce the term oncogenic plasminogen receptor to define those plasminogen receptors directly activated during cancer progression. We then discuss the research establishing AIIt as an oncogenic plasminogen receptor-regulated during EMT and activated by oncogenes such as SRC, RAS, HIF1α, and PML-RAR and epigenetically by DNA methylation. We further discuss the evidence derived from animal models supporting the role of S100A10 in tumor progression and oncogenesis. Lastly, we describe the potential of S100A10 as a biomarker for cancer diagnosis and prognosis.

Serum exosomal-annexin A2 is associated with African-American triple-negative breast cancer and promotes angiogenesis

Background

Limited information is available on biomarker(s) for triple-negative breast cancer (TNBC) that can address the higher incidence and aggressiveness of TNBC in African-American (AA) women. Our previous studies have demonstrated annexin A2 (AnxA2) association with exosomes which promotes angiogenesis and metastasis. Therefore, our goal was to examine the expression and function of exosomal-annexin A2 (exo-AnxA2) derived from the serum samples of breast cancer patients.

Methods

The expression of serum exo-AnxA2 and its association with clinicopathological features of the breast cancer patients were determined. The role of serum exo-AnxA2 to promote angiogenesis was determined by an in vivo Matrigel plug assay.

Results

Our results show that the expression of serum exo-AnxA2 in breast cancer patients (n = 169; 83.33 ± 2.040 ng/mL, P < 0.0001) is high compared to non-cancer females (n = 68; 34.21 ± 2.238 ng/mL). High expression of exo-AnxA2 levels in breast cancer was significantly associated with tumor grade (P < 0.0001), poor overall survival (hazard ratio (HR) 2.802; 95% confidence intervals (CI) = 1.030–7.620; P = 0.0353), and poor disease-free survival (HR 7.934; 95% CI = 1.778–35.398; P = 0.0301). The expression of serum exo-AnxA2 levels was significantly elevated in TNBC (n = 68; 109.1 ± 2.905 ng/mL; P < 0.0001) in comparison to ER⁺ (n = 50; 57.35 ± 1.545 ng/mL), HER2⁺ (n = 59; 78.25 ± 1.146 ng/mL), and non-cancer females (n = 68; 34.21 ± 2.238 ng/mL). Exo-AnxA2 showed diagnostic values with a maximum AUC as 1.000 for TNBC, 0.8304 for ER⁺, and 0.9958 for HER2⁺ compared to non-cancer females. The expression of serum exo-AnxA2 was significantly elevated in AA women with TNBC (n = 29; 118.9 ± 4.086 ng/mL, P < 0.0001) in comparison to Caucasian-American TNBC (n = 27; 97.60 ± 3.298 ng/mL) patients. Our in vivo results suggest a role of serum exo-AnxA2 in angiogenesis and its association with aggressiveness of TNBC in AA women.

Conclusions

Our results demonstrated that the expression of serum exo-AnxA2 is high in AA women with TNBC and promotes angiogenesis. These findings suggest that exo-AnxA2 holds promise as a potential prognosticator of TNBC and may lead to an effective therapeutic option.

The Known and Potential Intersections of Rab-GTPases in Human Papillomavirus Infections

Papillomaviruses (PVs) were the first viruses recognized to cause tumors and cancers in mammalian hosts by Shope, nearly a century ago (Shope and Hurst, 1933). Over 40 years ago, zur Hausen (1976) first proposed that human papillomaviruses (HPVs) played a role in cervical cancer; in 2008, he shared the Nobel Prize in Medicine for his abundant contributions demonstrating the etiology of HPVs in genital cancers. Despite effective vaccines and screening, HPV infection and morbidity remain a significant worldwide burden, with HPV infections and HPV-related cancers expected increase through 2040. Although HPVs have long-recognized roles in tumorigenesis and cancers, our understanding of the molecular mechanisms by which these viruses interact with cells and usurp cellular processes to initiate infections and produce progeny virions is limited. This is due to longstanding challenges in both obtaining well-characterized infectious virus stocks and modeling tissue-based infection and the replicative cycles in vitro. In the last 20 years, the development of methods to produce virus-like particles (VLPs) and pseudovirions (PsV) along with more physiologically relevant cell- and tissue-based models has facilitated progress in this area. However, many questions regarding HPV infection remain difficult to address experimentally and are, thus, unanswered. Although an obligatory cellular uptake receptor has yet to be identified for any PV species, Rab-GTPases contribute to HPV uptake and transport of viral genomes toward the nucleus. Here, we provide a general overview of the current HPV infection paradigm, the epithelial differentiation-dependent HPV replicative cycle, and review the specifics of how HPVs usurp Rab-related functions during infectious entry. We also suggest other potential interactions based on how HPVs alter cellular activities to complete their replicative-cycle in differentiating epithelium. Understanding how HPVs interface with Rab functions during their complex replicative cycle may provide insight for the development of therapeutic interventions, as current viral counter-measures are solely prophylactic and therapies for HPV-positive individuals remain archaic and limited.

Interaction of HE4 and ANXA2 exists in various malignant cells-HE4-ANXA2-MMP2 protein complex promotes cell migration

Background

The interaction between human epididymis protein 4 (HE4) and annexin A2 (Annexin A2) has been found in ovarian cancer. However, it is dimness whether the interaction exists in other malignant tumors.

Methods

Real-time PCR, western blotting and immunocytochemistry were used to detect mRNA and proteins expression. Co-immunoprecipitation and double-labeling immunofluorescence were used to detect the interaction among HE4, ANXA2 and MMP2. MTS assay was used to test cell proliferation. Adhesion test was used to test cell adhesion. Flow cytometry was applied to examine cell cycle. The scratch test and Transwell assay was performed to detect the migration and invasion of various malignant cell lines.

Results

Here we show that the overexpression of HE4 and ANXA2 in various malignant cells is a common phenomenon. HE4 and ANXA2 are co-localized in the cytoplasm and membrane of various tumor cells. ES-2 cells which had both high expression of HE4 and ANXA2 were much stronger in proliferation, adhesion, invasion, and migration than other tumor cells. HE4–ANXA2–MMP2 could form a triple protein complex. HE4 could mediate the expression of MMP2 via ANXA2 to promote cell migration progress.

Conclusions

The interaction of HE4 and ANXA2 exists in various types of cancer cells. HE4 and ANXA2 can promote the proliferation, adhesion, invasion, and migration of cancer cells. HE4–ANXA2–MMP2 form a protein complex and ANXA2 plays the role of “bridge”. They performed together to promote cell migration.

Cancer-associated fibroblasts promote epithelial-mesenchymal transition and EGFR-TKI resistance of non-small cell lung cancers via HGF/IGF-1/ANXA2 signaling

The involvement of the tumor stromal cells in acquired resistance of non-small cell lung cancers (NSCLCs) to tyrosine kinase inhibitors (TKIs) has previously been reported, but the precise mechanism remains unclear. In the present study, we investigated the role and mechanism underlying Cancer-associated fibroblasts (CAFs) in TKI resistance of NSCLCs. In vitro and in vivo experiments showed that HCC827 and PC9 cells, non-small cell lung cancer cells with EGFR-activating mutations, became resistant to the EGFR-TKI gefitinib when cultured with CAFs isolated from NSCLC tissues. Moreover, we showed that CAFs could induce epithelial-mesenchymal transition (EMT) phenotype of HCC827 and PC9 cells, with an associated change in the expression of epithelial to mesenchymal transition markers. Using proteomics-based method, we identified that CAFs significantly increased the expression of the Annexin A2 (ANXA2). More importantly, knockdown of ANXA2 completely reversed EMT phenotype and gefitinib resistance induced by CAFs. Furthermore, we found that CAFs increased the expression and phosphorylation of ANXA2 by secretion of growth factors HGF and IGF-1 and by activation of the corresponding receptors c-met and IGF-1R. Dual inhibition of HGF/c-met and IGF-1/IGF-1R pathways could significantly suppress ANXA2, and markedly reduced CAFs-induced EMT and gefitinib resistance. Taken together, these findings indicate that CAFs promote EGFR-TKIs resistance through HGF/IGF-1/ANXA2/EMT signaling and may be an ideal therapeutic target in NSCLCs with EGFR-activating mutations.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Protein phosphorylation and its role in the regulation of Annexin A2 function

BACKGROUND

Annexin A2 (AnxA2) is a multifunctional protein involved in endocytosis, exocytosis, membrane domain organisation, actin remodelling, signal transduction, protein assembly, transcription and mRNA transport, as well as DNA replication and repair.

SCOPE OF REVIEW

The current knowledge of the role of phosphorylation in the functional regulation of AnxA2 is reviewed. To provide a more comprehensive treatment of this topic, we also address in depth the phosphorylation process in general and discuss its possible conformational effects. Furthermore, we discuss the apparent limitations of the methods used to investigate phosphoproteins, as exemplified by the study of AnxA2.

MAJOR CONCLUSIONS

AnxA2 is subjected to complex regulation by post-translational modifications affecting its cellular functions, with Ser11, Ser25 and Tyr23 representing important phosphorylation sites. Thus, Ser phosphorylation of AnxA2 is involved in the recruitment and docking of secretory granules, the regulation of its association with S100A10, and sequestration of perinuclear, translationally inactive mRNP complexes. By contrast, Tyr phosphorylation of AnxA2 regulates its role in actin dynamics and increases its association with endosomal compartments. Modification of its three main phosphorylation sites is not sufficient to discriminate between its numerous functions. Thus, fine-tuning of AnxA2 function is mediated by the joint action of several post-translational modifications.

GENERAL SIGNIFICANCE

AnxA2 participates in malignant cell transformation, and its overexpression and/or phosphorylation is associated with cancer progression and metastasis. Thus, tight regulation of AnxA2 function is an integral aspect of cellular homeostasis. The presence of AnxA2 in cancer cell-derived exosomes, as well as the potential regulation of exosomal AnxA2 by phosphorylation or other PTMs, are topics of great interest.

Regulation of the Equilibrium between Closed and Open Conformations of Annexin A2 by N-Terminal Phosphorylation and S100A4-Binding

Annexin A2 (ANXA2) has a versatile role in membrane-associated functions including membrane aggregation, endo- and exocytosis, and it is regulated by post-translational modifications and protein-protein interactions through the unstructured N-terminal domain (NTD). Our sequence analysis revealed a short motif responsible for clamping the NTD to the C-terminal core domain (CTD). Structural studies indicated that the flexibility of the NTD and CTD are interrelated and oppositely regulated by Tyr24 phosphorylation and Ser26Glu phosphomimicking mutation. The crystal structure of the ANXA2-S100A4 complex showed that asymmetric binding of S100A4 induces dislocation of the NTD from the CTD and, similar to the Ser26Glu mutation, unmasks the concave side of ANXA2. In contrast, pTyr24 anchors the NTD to the CTD and hampers the membrane-bridging function. This inhibition can be restored by S100A4 and S100A10 binding. Based on our results we provide a structural model for regulation of ANXA2-mediated membrane aggregation by NTD phosphorylation and S100 binding.

Heterogeneous Stromal Signaling within the Tumor Microenvironment Controls the Metastasis of Pancreatic Cancer

Understanding how stromal signals regulate the development of pancreatic ductal adenocarcinoma (PDAC) may suggest novel therapeutic interventions in this disease. In this study, we assessed the metastatic role of stromal signals suggested to be important in the PDAC microenvironment. Src and IGF-1R phosphorylated the prometastatic molecule Annexin A2 (AnxA2) at Y23 and Y333 in response to stromal signals HGF and IGF-1, respectively, and IGF-1 expression was regulated by the Sonic Hedgehog (Shh) pathway. Both Shh and HGF were heterogeneously expressed in PDAC stroma, and only dual inhibition of these pathways could significantly suppress AnxA2 phosphorylation, PDAC growth, and metastasis. Taken together, our results illuminate tumor-stromal interactions, which drive metastasis, and provide a mechanism-based rationale for a stroma-directed therapy for PDAC. Cancer Res; 77(1); 41-52. ©2016 AACR.

WOMEN IN CANCER THEMATIC REVIEW: Ovarian cancer-peritoneal cell interactions promote extracellular matrix processing

Ovarian cancer has a distinct tendency for metastasising via shedding of cancerous cells into the peritoneal cavity and implanting onto the peritoneum that lines the pelvic organs. Once ovarian cancer cells adhere to the peritoneal cells, they migrate through the peritoneal layer and invade the local organs. Alterations in the extracellular environment are critical for tumour initiation, progression and intra-peritoneal dissemination. To increase our understanding of the molecular mechanisms involved in ovarian cancer metastasis and to identify novel therapeutic targets, we recently studied the interaction of ovarian cancer and peritoneal cells using a proteomic approach. We identified several extracellular matrix (ECM) proteins including, fibronectin, TGFBI, periostin, annexin A2 and PAI-1 that were processed as a result of the ovarian cancer-peritoneal cell interaction. This review focuses on the functional role of these proteins in ovarian cancer metastasis. Our findings together with published literature support the notion that ECM processing via the plasminogen-plasmin pathway promotes the colonisation and attachment of ovarian cancer cells to the peritoneum and actively contributes to the early steps of ovarian cancer metastasis.

Reactive oxygen species exert opposite effects on Tyr23 phosphorylation of the nuclear and cortical pools of annexin A2

Annexin A2 (AnxA2) is a multi-functional and -compartmental protein whose subcellular localisation and functions are tightly regulated by its post-translational modifications. AnxA2 and its Tyr23-phosphorylated form (pTyr23AnxA2) are involved in malignant cell transformation, metastasis and angiogenesis. Here, we show that H₂O₂ exerts rapid, simultaneous and opposite effects on the Tyr23 phosphorylation status of AnxA2 in two distinct compartments of rat pheochromocytoma (PC12) cells. Reactive oxygen species induce dephosphorylation of pTyr23AnxA2 located in the PML bodies of the nucleus, whereas AnxA2 associated with F-actin at the cell cortex is Tyr23 phosphorylated. The H₂O₂-induced responses in both compartments are transient and the pTyr23AnxA2 accumulating at the cell cortex is subsequently incorporated into vesicles and then released to the extracellular space. Blocking nuclear export by leptomycin B does not affect the nuclear pool of pTyr23AnxA2, but increases the amount of total AnxA2 in this compartment, indicating that the protein might have several functions in the nucleus. These results suggest that Tyr23 phosphorylation can regulate the function of AnxA2 at distinct subcellular sites.

Summary: Reactive oxygen species cause two opposite and transient Tyr23-based modifications of annexin A2; its dephosphorylation in the nucleus and phosphorylation at the cell cortex, resulting in release of the protein in exosomes.

Cell surface translocation of annexin A2 facilitates glutamate-induced extracellular proteolysis

Glutamate-induced elevation in intracellular Ca(2+) has been implicated in excitotoxic cell death. Neurons respond to increased glutamate levels by activating an extracellular proteolytic cascade involving the components of the plasmin-plasminogen system. AnxA2 is a Ca(2+)-dependent phospholipid binding protein and serves as an extracellular proteolytic center by recruiting the tissue plasminogen activator and plasminogen and mediating the localized generation of plasmin. Ratiometric Ca(2+) imaging and time-lapse confocal microscopy demonstrated glutamate-induced Ca(2+) influx. We showed that glutamate translocated both endogenous and AnxA2-GFP to the cell surface in a process dependent on the activity of the NMDA receptor. Glutamate-induced translocation of AnxA2 is dependent on the phosphorylation of tyrosine 23 at the N terminus, and mutation of tyrosine 23 to a non-phosphomimetic variant inhibits the translocation process. The cell surface-translocated AnxA2 forms an active plasmin-generating complex, and this activity can be neutralized by a hexapeptide directed against the N terminus. These results suggest an involvement of AnxA2 in potentiating glutamate-induced cell death processes.

Annexin A2 and S100A10 regulate human papillomavirus type 16 entry and intracellular trafficking in human keratinocytes

Human papillomaviruses (HPVs) cause benign and malignant tumors of the mucosal and cutaneous epithelium. The initial events regulating HPV infection impact the establishment of viral persistence, which is requisite for malignant progression of HPV-infected lesions. However, the precise mechanisms involved in HPV entry into host cells, including the cellular factors regulating virus uptake, are not clearly defined. We show that HPV16 exposure to human keratinocytes initiates epidermal growth factor receptor (EGFR)-dependent Src protein kinase activation that results in phosphorylation and extracellular translocation of annexin A2 (AnxA2). HPV16 particles interact with AnxA2 in association with S100A10 as a heterotetramer at the cell surface in a Ca(2+)-dependent manner, and the interaction appears to involve heparan-sulfonated proteoglycans. We show multiple lines of evidence that this interaction promotes virus uptake into host cells. An antibody to AnxA2 prevents HPV16 internalization, whereas an antibody to S100A10 blocks infection at a late endosomal/lysosomal site. These results suggest that AnxA2 and S100A10 have separate roles during HPV16 binding, entry, and trafficking. Our data additionally imply that AnxA2 and S100A10 may be involved in regulating the intracellular trafficking of virus particles prior to nuclear delivery of the viral genome.

Autoantibodies and resident renal cells in the pathogenesis of lupus nephritis: getting to know the unknown

Systemic lupus erythematosus is characterized by a breakdown of self-tolerance and production of autoantibodies. Kidney involvement (i.e., lupus nephritis) is both common and severe and can result in permanent damage within the glomerular, vascular, and tubulo-interstitial compartments of the kidney, leading to acute or chronic renal failure. Accumulating evidence shows that anti-dsDNA antibodies play a critical role in the pathogenesis of lupus nephritis through their binding to cell surface proteins of resident kidney cells, thereby triggering the downstream activation of signaling pathways and the release of mediators of inflammation and fibrosis. This paper describes the mechanisms through which autoantibodies interact with resident renal cells and how this interaction plays a part in disease pathogenesis that ultimately leads to structural and functional alterations in lupus nephritis.

Clathrin mediates endocytosis of progastrin and activates MAPKs: role of cell surface annexin A2

Cell-surface-associated annexin A2 (CS-ANXA2) is a nonconventional "receptor" for progastrin; expression levels of both are elevated in colon cancers, and downregulation of either reduces tumorigenic potential of cells. We recently reported internalization of progastrin in target cells. Here, mechanisms mediating internalization of progastrin were examined. Initially, we confirmed that cell-surface ANXA2 mediates binding and internalization of progastrin in intestinal cells. Progastrin, covalently linked to sepharose beads, failed to activate p38MAPK/ERKs, suggesting internalization of progastrin was required for eliciting biological effects; importantly annexin A2 expression and availability of CS-ANXA2 were required for internalization of progastrin. Clathrin expression and formation of clathrin-coated pits were critically required for endocytotic internalization of progastrin; in the absence of clathrin, progastrin failed to activate p38MAPK/ERKs. Downregulation of caveolin had no effect on binding or internalization of progastrin. We therefore demonstrate for the first time that progastrin binds CS-ANXA2 and is rapidly internalized via clathrin-mediated endocytotic pathway, resulting in activation of MAPKinases. Targeting clathrin-mediated endocytosis of progastrin may thus inhibit previously reported co-carcinogenic/tumorigenic effects of progastrin on intestinal cells.

The role of annexin A2 in tumorigenesis and cancer progression

Annexin A2 is a calcium-dependent, phospholipid-binding protein found on various cell types. It is up-regulated in various tumor types and plays multiple roles in regulating cellular functions, including angiogenesis, proliferation, apoptosis, cell migration, invasion and adhesion. Annexin A2 binds with plasminogen and tissue plasminogen activator on the cell surface, which leads to the conversion of plasminogen to plasmin. Plasmin is a serine protease which plays a key role in the activation of metalloproteinases and degradation of extracellular matrix components essential for metastatic progression. We have recently found that both annexin A2 and plasmin are increased in conditioned media of co cultured ovarian cancer and peritoneal cells. Our studies suggest that annexin A2 is part of a tumor-host signal pathway between ovarian cancer and peritoneal cells which promotes ovarian cancer metastasis. Accumulating evidence suggest that interactions between annexin A2 and its binding proteins play an important role in the tumor microenvironment and act together to enhance cancer metastasis. This article reviews the current knowledge on the biological role of annexin A2 and its binding proteins in solid malignancies including ovarian cancer.

The annexin A2 system and vascular homeostasis

Optimal fibrin balance requires precisely controlled plasmin generation on the surface of endothelial cells, which line the blood vessel wall. As a co-receptor for plasminogen and tissue plasminogen activator (tPA), which are key factors in plasmin generation, the annexin A2 (A2) complex promotes vascular fibrinolysis. The intracellular A2 complex is a heterotetramer of two A2 monomers and two copies of the associated protein, p11. In response to endothelial cell activation, A2 is phosphorylated by src-kinase, and translocated to the cell surface in a highly regulated manner. Over-expression of A2 is seen in acute promyelocytic leukemia during the early hemorrhagic phase, while high titer antibodies to A2, as in antiphospholipid syndrome or cerebral venous thrombosis, are associated with thrombosis. In experimental hyperhomocysteinemia, moreover, derivatization of A2 by homocysteine leads to intravascular fibrin accumulation and dysangiogenesis, features that phenocopy the Anxa2(-/-) mouse. Exogenous A2 may also offer a novel therapeutic approach to ischemic thrombotic stroke, as administration of A2 in conjunction with conventional tPA-based thrombolytic therapy improved outcome in an animal model. Here, we discuss the role of the A2 system in vascular homeostasis, the molecular interactions that regulate its profibrinolytic activity, and its potential role in the pathogenesis and treatment of vascular disease.

Annexin A2 regulates phagocytosis of photoreceptor outer segments in the mouse retina

The daily phagocytosis of shed photoreceptor outer segments by pigment epithelial cells is critical for the maintenance of the retina. In a subtractive polymerase chain reaction analysis, we found that functional differentiation of human ARPE19 retinal pigment epithelial (RPE) cells is accompanied by up-regulation of annexin (anx) A2, a major Src substrate and regulator of membrane-cytoskeleton dynamics. Here, we show that anx A2 is recruited to the nascent phagocytic cup in vitro and in vivo and that it fully dissociates once the phagosome is internalized. In ARPE19 cells depleted of anx A2 by using small interfering RNA and in ANX A2(-/-) mice the phagocytosis of outer segments was impaired, and in ANX A2(-/-) mice there was an accumulation of phagocytosed outer segments in the RPE apical processes, indicative of retarded phagosome transport. We show that anx A2 is tyrosine phosphorylated at the onset of phagocytosis and that the synchronized activation of focal adhesion kinase and c-Src is abnormal in ANX A2(-/-) mice. These findings reveal that anx A2 is involved in the circadian regulation of outer segment phagocytosis, and they provide new insight into the protein machinery that regulates phagocytic function in RPE cells.

Annexin A2: biology and relevance to the antiphospholipid syndrome

Antiphospholipid antibodies (aPL), the majority of which are directed against beta(2)-glycoprotein I (beta(2)GPI), are associated with an increased incidence of venous and arterial thrombosis. The pathogenesis of antiphospholipid/anti-beta(2)GPI-associated thrombosis has not been defined, and is likely multifactorial. However, accumulating evidence suggests an important role for endothelial cell activation with the acquisition of a procoagulant phenotype by the activated endothelial cell. Previous work demonstrated that endothelial activation by antiphospholipid/anti-beta(2)GPI antibodies is beta(2)GPI-dependent. We extended these observations by defining annexin A2 as an endothelial beta(2)GPI binding site. We also observed that annexin A2 plays a critical role in endothelial cell activation induced by anti-beta(2)GPI antibodies, and others have described direct endothelial activation by anti-annexin A2 antibodies in patients with aPL . Similar findings have been reported using human monocytes, which also express annexin A2. Because annexin A2 is not a transmembrane protein, how binding of beta(2)GPI/anti-beta(2)GPI antibodies, or anti-annexin A2 antibodies, to endothelial annexin A2 causes cellular activation is unknown. Recent studies, however, suggest an important role for the Toll-like receptor family, particularly TLR4. In this article, we review the role of these interactions in the activation of endothelial cells by aPL . The influence of these antibodies on the ability of annexin A2 to enhance t-PA-mediated plasminogen activation is also discussed.

Annexin A2 binding to endosomes and functions in endosomal transport are regulated by tyrosine 23 phosphorylation

The phospholipid-binding annexin A2 (AnxA2) is known to play a role in the regulation of membrane and actin dynamics, in particular in the endocytic pathway. The protein is present on early endosomes, where it regulates membrane traffic, including the biogenesis of multivesicular transport intermediates destined for late endosomes. AnxA2 membrane association depends on the protein N terminus and membrane cholesterol but does not involve the AnxA2 ligand p11/S100A10. However, the precise mechanisms that control AnxA2 membrane association and function are not clear. In the present study, we have investigated the role of AnxA2 N-terminal phosphorylation in controlling association to endosomal membranes and functions. We found that endosomal AnxA2 was partially tyrosine-phosphorylated and that mutation of Tyr-23 to Ala (AnxA2Y23A), but not of Ser-25 to Ala, impaired AnxA2 endosome association. We then found that the AnxA2Y23A mutant was unable to bind endosomes in vivo, whereas a phospho-mimicking AnxA2 mutant (Y23D) showed efficient endosome binding capacity. Similarly, we found that AnxA2Y23D interacted more efficiently with liposomes in vitro when compared with AnxA2Y23A. To investigate the role of Tyr-23 in vivo, AnxA2 was knocked down with small interfering RNAs, and then cells were recomplemented with RNA interference-resistant forms of the protein. Using this strategy, we could show that AnxA2Y23D, but not AnxA2Y23A, could restore early-to-late endosome transport after AnxA2 depletion. We conclude that phosphorylation of Tyr-23 is essential for proper endosomal association and function of AnxA2, perhaps because it stabilizes membrane-associated protein via a conformational change.

Non-proteolytic neurotrophic effects of tissue plasminogen activator on cultured mouse cerebrocortical neurons

Most biological effects of tissue plasminogen activator (tPA), such as fibrinolysis, are mediated by its protease activity. Recent studies, however, have demonstrated that tPA also has several protease-independent effects such as: neuroprotection, microglial activation, and promoting LTP formation. In order to gain a better understanding of how tPA affects neurons, we examined neurite outgrowth and cell survival in low density cerebrocortical neuronal culture in the presence of tPA. tPA enhanced neurite elongation and neuronal survival. tPA protease inhibitors, PAI-1 or PMSF, did not alter either effect. Consistent with neurotrophic effects, tPA activated Raf-K/ERK, PKC and PI3-K/Akt, 5-60 min after treatment. In addition, specific inhibitors of these kinases reduced tPA-induced neurite outgrowth. Interestingly, survival-promoting effect of tPA was attenuated only by PI3-K inhibitors. Activation of signaling kinases suggests that tPA activates an upstream membrane receptor. Thus far, three membrane proteins, low density lipoprotein receptor-related protein (LRP), mannose receptor (MR), and annexin-II (AII), have been identified to bind tPA. While inhibiting LRP or MR did not change tPA-induced neurite outgrowth and cell survival, inhibiting AII blocked neurotrophic effects of tPA. Taken together, our results indicate that tPA has novel, non-proteolytic neurotrophic effects on cultured cortical neurons, which are likely mediated by AII.

An Annexin 2 Phosphorylation Switch Mediates p11-dependent Translocation of Annexin 2 to the Cell Surface

Annexin 2 is a profibrinolytic co-receptor for plasminogen and tissue plasminogen activator that stimulates activation of the major fibrinolysin, plasmin, at cell surfaces. In human subjects, overexpression of annexin 2 in acute promyelocytic leukemia leads to a bleeding diathesis reflective of excessive cell surface annexin 2-dependent generation of plasmin (Menell, J. S., Cesarman, G. M., Jacovina, A. T., McLaughlin, M. A., Lev, E. A., and Hajjar, K. A. (1999) N. Engl. J. Med. 340, 994-1004). In addition, mice completely deficient in annexin 2 display fibrin accumulation within blood vessels and impaired clearance of injury-induced thrombi (Ling Q., Jacovina, A.T., Deora, A.B., Febbraio, M., Simantov, R., Silverstein, R. L., Hempstead, B. L., Mark, W., and Hajjar, K. A. (2004) J. Clin. Investig. 113, 38-48). Here, we show that endothelial cell annexin 2, a protein that lacks a typical signal peptide, translocates from the cytoplasm to the extracytoplasmic plasma membrane in response to brief temperature stress both in vitro and in vivo in the absence of cell death or cell lysis. This regulated response is independent of new protein or mRNA synthesis and does not require the classical endoplasmic reticulum-Golgi pathway. Temperature stress-induced annexin 2 translocation is dependent on both expression of protein p11 (S100A10) and tyrosine phosphorylation of annexin 2 because annexin 2 release is completely eliminated on depletion of p11, inactivation of tyrosine kinase, or mutation of tyrosine 23. Translocation of annexin 2 to the cell surface dramatically increases tissue plasminogen activator-dependent plasminogen activation potential and may represent a novel stress-induced protein secretion pathway.

Evidence that annexin II Is not a putative membrane receptor for 1?,25(OH)2-vitamin D3

The seco-steroid hormone 1alpha,25(OH)(2)-vitamin D(3) (1,25-D(3)) is known to generate biological responses via both genomic and non-genomic rapid signal transduction pathways. The calcium regulated annexin II/p11 heterotetramer (AII(2)/p11(2)] was proposed by Baran and co-authors to be the membrane receptor responsible for mediating non-genomic, rapid actions of 1,25-D(3), based on ligand affinity labeling, competition, and saturation analysis experiments. Given the cytosolic presence of both the monomeric and heterotetrameric form of AII and their functional regulation by intracellular calcium concentrations, which are known to be affected by 1,25-D(3) rapid, non-genomic activities, we investigated in vitro the affinity of [(3)H]1,25-D(3) for the AII monomer and AII(2)/p11(2) in the absence and presence of calcium using saturation analysis and gel-filtration chromatography. Using two different techniques for separating bound from free ligand (perchlorate and hydroxylapatite (HAP)) over a series of 30 experiments, no evidence for specific binding of [(3)H]1,25-D(3) was obtained with or without the presence of 700 nM exogenous calcium, using either the AII monomer or AII(2)/p11(2). However saturable binding of [(3)H]1,25-D(3) to the lipid raft/caveolae enriched rat intestinal fraction was consistently observed (K(d) = 3.0 nM; B(max) = 45 fmols/mg total protein). AII was detected in lipid raft/caveolae enriched fractions from rat and mouse intestine and ROS 17/2.8 and NB4 cells by Western blot, but incubation in the presence of exogenous calcium did not ablate 1,25-D(3) binding as reported by Baran et al. Our results suggest that AII does not bind 1,25-D(3) in a physiologically relevant manner; however, recent studies linking AII(2)/p11(2) phosphorylation to vesicle fusion and its calcium regulated localization may make AII a possible down-stream substrate for 1,25-D(3) induced rapid cellular effects.

Proteomic analysis of protein phosphorylations in heat shock response and thermotolerance

Heat shock (HS) induces a wide variety of biological processes, including inhibition of protein synthesis, elevated expression of heat shock proteins, induction of thermotolerance, and apoptotic cell death in a dose-dependent manner. We compared phosphorylated proteins in heat-shocked and thermotolerant cells using proteome analysis. After HS treatment of control RIF-1 and their thermotolerant derivatives, TR-RIF-1 cells, cellular proteins were separated by two-dimensional gel electrophoresis and the phosphorylated proteins were detected with the anti-phosphotyrosine antibodies. We found that 93 proteins showed significant changes in phosphorylation between control and thermotolerant cells as a function of recovery time after HS; we identified 81 of these proteins with peptide mass fingerprinting using MALDI-TOF MS after in-gel trypsin digestion. These phosphorylated proteins exhibit various cellular functions, including chaperones, ion channels, signaling molecules, in transcription and translation processes, in amino acid biosynthesis, oxidoreduction, energy metabolism, and cell motility or structure, suggesting that HS turns on the various signaling pathways by activating protein-tyrosine kinases (PTKs). Of these, 20 proteins were previously identified phosphorylated proteins and 64 were newly identified. These proteins can be grouped into three families: 1) proteins highly phosphorylated in TR-RIF-1 cells at basal level and phosphorylated more significantly by HS in RIF-1 than TR-RIF-1; 2) proteins highly phosphorylated in control RIF-1 cells at basal level and phosphorylated more easily by HS in TR-RIF-1 than in RIF-1 cells; and 3) proteins with a similar basal phosphorylation level in both RIF-1 and TR-RIF-1 cells and responding to HS similarly in both cells. Most of the phosphorylated proteins are presumably involved in HS signaling in different ways, with the first and second families of proteins influencing thermotolerance. The possible tyrosine phosphorylation sites, the possible PTKs phosphorylating these proteins, and the proteins binding to these phosphorylated sites were predicted by the Netphos, ScanProsite, and Scansite programs. These results suggest that HS can activate various PTKs and HS responses can be regulated by phosphorylations of proteins having various functions.

Annexins: from structure to function

Annexins are Ca2+ and phospholipid binding proteins forming an evolutionary conserved multigene family with members of the family being expressed throughout animal and plant kingdoms. Structurally, annexins are characterized by a highly alpha-helical and tightly packed protein core domain considered to represent a Ca2+-regulated membrane binding module. Many of the annexin cores have been crystallized, and their molecular structures reveal interesting features that include the architecture of the annexin-type Ca2+ binding sites and a central hydrophilic pore proposed to function as a Ca2+ channel. In addition to the conserved core, all annexins contain a second principal domain. This domain, which NH2-terminally precedes the core, is unique for a given member of the family and most likely specifies individual annexin properties in vivo. Cellular and animal knock-out models as well as dominant-negative mutants have recently been established for a number of annexins, and the effects of such manipulations are strikingly different for different members of the family. At least for some annexins, it appears that they participate in the regulation of membrane organization and membrane traffic and the regulation of ion (Ca2+) currents across membranes or Ca2+ concentrations within cells. Although annexins lack signal sequences for secretion, some members of the family have also been identified extracellularly where they can act as receptors for serum proteases on the endothelium as well as inhibitors of neutrophil migration and blood coagulation. Finally, deregulations in annexin expression and activity have been correlated with human diseases, e.g., in acute promyelocytic leukemia and the antiphospholipid antibody syndrome, and the term annexinopathies has been coined.

Control of the nuclear-cytoplasmic partitioning of annexin II by a nuclear export signal and by p11 binding

This study investigated mechanisms controlling the nuclear-cytoplasmic partitioning of annexin II (AnxII). AnxII and its ligand, p11, were localized by immunofluorescence to the cytoplasmic compartment of U1242MG cells, with minimal AnxII or p11 detected within nuclei. Similarly, GFP-AnxII and GFP-p11 chimeras localized to the endogenous proteins. Likewise, GFP-AnxII(1-22) was excluded from nuclei, whereas GFP-AnxII(23-338) and GFP alone were distributed throughout the cells. Immunoprecipitation and biochemical studies showed that GFP-AnxII did not form heteromeric complexes with endogenous p11 and AnxII. Thus, the AnxII N-tail is necessary and sufficient to cause nuclear exclusion of the GFP fusion protein but this does not involve p11 binding. A nuclear export signal consensus sequence was found in the AnxII 3-12 region. The consensus mutant GFP-AnxII(L10A/L12A) confirmed that these residues are necessary for nuclear exclusion. The nuclear exclusion of GFP-AnxII(1-22) was temperature-dependent and reversible, and the nuclear export inhibitor leptomycin B (LmB) caused GFP-AnxII or overexpressed AnxII monomer to accumulate in nuclei. Therefore, AnxII monomer can enter the nucleus and is actively exported. However, LmB had little effect on the localization of AnxII/p11 complex in U1242MG cells, indicating that the complex is sequestered in the cytoplasm. By contrast, LmB treatment of v-src-transformed fibroblasts caused endogenous AnxII to accumulate in nuclei. The LmB-induced nuclear accumulation of AnxII was accelerated by pervanadate and inhibited by genistein, suggesting that phosphorylation promotes nuclear entry of AnxII. Thus, nuclear exclusion of AnxII results from nuclear export of the monomer and sequestration of AnxII/p11 complex, and may be modulated by phosphorylation.

Analysis of CD44-containing lipid rafts: Recruitment of annexin II and stabilization by the actin cytoskeleton

CD44, the major cell surface receptor for hyaluronic acid (HA), was shown to localize to detergent-resistant cholesterol-rich microdomains, called lipid rafts, in fibroblasts and blood cells. Here, we have investigated the molecular environment of CD44 within the plane of the basolateral membrane of polarized mammary epithelial cells. We show that CD44 partitions into lipid rafts that contain annexin II at their cytoplasmic face. Both CD44 and annexin II were released from these lipid rafts by sequestration of plasma membrane cholesterol. Partition of annexin II and CD44 to the same type of lipid rafts was demonstrated by cross-linking experiments in living cells. First, when CD44 was clustered at the cell surface by anti-CD44 antibodies, annexin II was recruited into the cytoplasmic leaflet of CD44 clusters. Second, the formation of intracellular, submembranous annexin II-p11 aggregates caused by expression of a trans-dominant mutant of annexin II resulted in coclustering of CD44. Moreover, a frequent redirection of actin bundles to these clusters was observed. These basolateral CD44/annexin II-lipid raft complexes were stabilized by addition of GTPgammaS or phalloidin in a semipermeabilized and cholesterol-depleted cell system. The low lateral mobility of CD44 in the plasma membrane, as assessed with fluorescent recovery after photobleaching (FRAP), was dependent on the presence of plasma membrane cholesterol and an intact actin cytoskeleton. Disruption of the actin cytoskeleton dramatically increased the fraction of CD44 which could be recovered from the light detergent-insoluble membrane fraction. Taken together, our data indicate that in mammary epithelial cells the vast majority of CD44 interacts with annexin II in lipid rafts in a cholesterol-dependent manner. These CD44-containing lipid microdomains interact with the underlying actin cytoskeleton.

Design of allele-specific inhibitors to probe protein kinase signaling

BACKGROUND

Deconvoluting protein kinase signaling pathways using conventional genetic and biochemical approaches has been difficult because of the overwhelming number of closely related kinases. If cell-permeable inhibitors of individual kinases could be designed, the role of each kinase could be systematically assessed.

RESULTS

We have devised an approach combining chemistry and genetics to develop the first highly specific cell-permeable inhibitor of the oncogenic tyrosine kinase v-Src. A functionally silent active-site mutation was made in v-Src to distinguish it from all other cellular kinases. A tight-binding cell-permeable inhibitor of this mutant kinase that does not inhibit wild-type kinases was designed and synthesized. In vitro and whole-cell assays established the unique specificity of the mutant v-Src-inhibitor pair. The inhibitor reversed cell transformation by the engineered but not the 'wild type' v-Src, establishing that changes in cellular signaling can be attributed to specific inhibition of the engineered kinase. The generality of the method was tested by engineering another tyrosine kinase, Fyn, to contain the corresponding active-site mutation to the one in v-Src. The same compound that inhibited mutant v-Src could also potently inhibit the engineered Fyn kinase.

CONCLUSIONS

Allele-specific cell-permeable inhibitors of individual Src family kinases can be rapidly developed in an approach that should be applicable to all kinases. This approach will be useful for the deconvolution of kinase-mediated cellular pathways and for validating novel kinases as good targets for drug discovery both in vitro and in vivo.

The role of annexin II tetramer in the activation of plasminogen

Annexin II tetramer (AIIt) is a major Ca2+-binding protein of endothelial cells which has been shown to exist on both the intracellular and extracellular surfaces of the plasma membrane. In this report, we demonstrate that AIIt stimulates the activation of plasminogen by facilitating the tissue plasminogen activator (t-PA)-dependent conversion of plasminogen to plasmin. Fluid-phase AIIt stimulated the rate of activation of [Glu]plasminogen about 341-fold compared with an approximate 6-fold stimulation by annexin II. AIIt bound to [Glu]plasminogen(S741C-fluorescein) with a Kd of 1. 26 +/- 0.04 microM (mean +/- S.D., n = 3) and this interaction resulted in a large conformational change in [Glu]plasminogen. Kinetic analysis established that AIIt produces a large increase of about 190-fold in the kcat, app and a small increase in the Km,app which resulted in a 90-fold increase in the catalytic efficiency (kcat/Km) of t-PA for [Glu]plasminogen. AIIt also stimulated the t-PA-dependent activation of [Lys]plasminogen about 28-fold. Furthermore, other annexins such as annexin I, V, or VI did not produce comparable activation of t-PA-dependent conversion of [Glu]plasminogen to plasmin. The stimulation of the activation of [Glu]plasminogen by AIIt was Ca2+-independent and inhibited by epsilon-aminocaproic acid. AIIt bound to human 293 cells potentiated t-PA-dependent plasminogen activation. AIIt that was bound to phospholipid vesicles or heparin also stimulated the activation of [Glu]plasminogen 5- or 11-fold, respectively. Furthermore, immunofluorescence labeling of nonpermeabilized HUVEC revealed a punctated distribution of AIIt subunits on the cell surface. These results therefore identify AIIt as a potent in vitro activator of plasminogen.