Nuclear localization and apoptotic regulation of an amino-terminal domain focal adhesion kinase fragment in endothelial cells

Cooperative involvement of zipper-interacting protein kinase (ZIPK) and the dual-specificity cell-division cycle 14A phosphatase (CDC14A) in vascular smooth muscle cell migration

Zipper-interacting protein kinase (ZIPK) is a Ser/Thr protein kinase with regulatory involvement in vascular smooth muscle cell (VSMC) actin polymerization and focal adhesion assembly dynamics. ZIPK silencing can induce cytoskeletal remodeling with disassembly of actin stress fiber networks and coincident loss of focal adhesion kinase (FAK)-pY397 phosphorylation. The link between ZIPK inhibition and FAK phosphorylation is unknown, and critical interactor(s) and regulator(s) are not yet defined. In this study, we further analyzed the ZIPK-FAK relationship in VSMCs. The application of HS38, a selective ZIPK inhibitor, to coronary artery vascular smooth muscle cells (CASMCs) suppressed cell migration, myosin light chain phosphorylation (pT18&pS19) and FAK-pY397 phosphorylation as well. This was associated with the translocation of cytoplasmic FAK to the nucleus. ZIPK inhibition with HS38 was consistently found to suppress the activation of FAK and attenuate the phosphorylation of other focal adhesion protein components (i.e., pCas130, paxillin, ERK). In addition, our study showed a decrease in human cell-division cycle 14A phosphatase (CDC14A) levels with ZIPK-siRNA treatment and increased CDC14A with transient transfection of ZIPK. Proximity ligation assays (PLA) revealed CDC14A localized with ZIPK and FAK. Silencing CDC14A showed an increase of FAK-pY397 phosphorylation. Ultimately, the data presented herein strongly support a regulatory mechanism of FAK in CASMCs by a ZIPK-CDC14A partnership; ZIPK may act as a key signal integrator to control CDC14A and FAK during VSMC migration.

Antioxidant and Biological Properties of Mesenchymal Cells Used for Therapy in Retinitis Pigmentosa

Both tissue repair and regeneration are a priority in regenerative medicine. Retinitis pigmentosa (RP), a complex retinal disease characterized by the progressive loss of impaired photoreceptors, is currently lacking effective therapies: this represents one of the greatest challenges in the field of ophthalmological research. Although this inherited retinal dystrophy is still an incurable genetic disease, the oxidative damage is an important pathogenetic element that may represent a viable target of therapy. In this review, we summarize the current neuroscientific evidence regarding the effectiveness of cell therapies in RP, especially those based on mesenchymal cells, and we focus on their therapeutic action: limitation of both oxidative stress and apoptotic processes triggered by the disease and promotion of cell survival. Cell therapy could therefore represent a feasible therapeutic option in RP.

The roles of nuclear focal adhesion kinase (FAK) on Cancer: a focused review

FAK is a tyrosine kinase overexpressed in cancer cells and plays an important role in the progression of tumors to a malignant phenotype. Except for its typical role as a cytoplasmic kinase downstream of integrin and growth factor receptor signaling, related studies have shown new aspects of the roles of FAK in the nucleus. FAK can promote p53 degradation through ubiquitination, leading to cancer cell growth and proliferation. FAK can also regulate GATA4 and IL-33 expression, resulting in reduced inflammatory responses and immune escape. These findings establish a new model of FAK from the cytoplasm to the nucleus. Activated FAK binds to transcription factors and regulates gene expression. Inactive FAK synergizes with different E3 ligases to promote the turnover of transcription factors by enhancing ubiquitination. In the tumor microenvironment, nuclear FAK can regulate the formation of new blood vessels, affecting the tumor blood supply. This article reviews the roles of nuclear FAK in regulating gene expression. In addition, the use of FAK inhibitors to target nuclear FAK functions will also be emphasized.

Adhesion protein networks reveal functions proximal and distal to cell-matrix contacts

Cell adhesion to the extracellular matrix is generally mediated by integrin receptors, which bind to intracellular adhesion proteins that form multi-molecular scaffolding and signalling complexes. The networks of proteins, and their interactions, are dynamic, mechanosensitive and extremely complex. Recent efforts to characterise adhesions using a variety of technologies, including imaging, proteomics and bioinformatics, have provided new insights into their composition, organisation and how they are regulated, and have also begun to reveal unexpected roles for so-called adhesion proteins in other cellular compartments (for example, the nucleus or centrosomes) in diseases such as cancer. We believe this is opening a new chapter on understanding the wider functions of adhesion proteins, both proximal and distal to cell-matrix contacts.

Paracrine mechanisms of mesenchymal stem cell-based therapy: current status and perspectives

Mesenchymal stem cells (MSCs) are one of a few stem cell types to be applied in clinical practice as therapeutic agents for immunomodulation and ischemic tissue repair. In addition to their multipotent differentiation potential, a strong paracrine capacity has been proposed as the principal mechanism that contributes to tissue repair. Apart from cytokine/chemokine secretion, MSCs also display a strong capacity for mitochondrial transfer and microvesicle (exosomes) secretion in response to injury with subsequent promotion of tissue regeneration. These unique properties of MSCs make them an invaluable cell type to repair damaged tissues/organs. Although MSCs offer great promise in the treatment of degenerative diseases and inflammatory disorders, there are still many challenges to overcome prior to their widespread clinical application. Particularly, their in-depth paracrine mechanisms remain a matter for debate and exploration. This review will highlight the discovery of the paracrine mechanism of MSCs, regulation of the paracrine biology of MSCs, important paracrine factors of MSCs in modulation of tissue repair, exosome and mitochondrial transfer for tissue repair, and the future perspective for MSC-based therapy.

FAK signaling in human cancer as a target for therapeutics

Focal adhesion kinase (FAK) is a key regulator of growth factor receptor- and integrin-mediated signals, governing fundamental processes in normal and cancer cells through its kinase activity and scaffolding function. Increased FAK expression and activity occurs in primary and metastatic cancers of many tissue origins, and is often associated with poor clinical outcome, highlighting FAK as a potential determinant of tumor development and metastasis. Indeed, data from cell culture and animal models of cancer provide strong lines of evidence that FAK promotes malignancy by regulating tumorigenic and metastatic potential through highly-coordinated signaling networks that orchestrate a diverse range of cellular processes, such as cell survival, proliferation, migration, invasion, epithelial-mesenchymal transition, angiogenesis and regulation of cancer stem cell activities. Such an integral role in governing malignant characteristics indicates that FAK represents a potential target for cancer therapeutics. While pharmacologic targeting of FAK scaffold function is still at an early stage of development, a number of small molecule-based FAK tyrosine kinase inhibitors are currently undergoing pre-clinical and clinical testing. In particular, PF-00562271, VS-4718 and VS-6063 show promising clinical activities in patients with selected solid cancers. Clinical testing of rationally designed FAK-targeting agents with implementation of predictive response biomarkers, such as merlin deficiency for VS-4718 in mesothelioma, may help improve clinical outcome for cancer patients. In this article, we have reviewed the current knowledge regarding FAK signaling in human cancer, and recent developments in the generation and clinical application of FAK-targeting pharmacologic agents.

FAK dimerization controls its kinase-dependent functions at focal adhesions

Focal adhesion kinase (FAK) controls adhesion-dependent cell motility, survival, and proliferation. FAK has kinase-dependent and kinase-independent functions, both of which play major roles in embryogenesis and tumor invasiveness. The precise mechanisms of FAK activation are not known. Using x-ray crystallography, small angle x-ray scattering, and biochemical and functional analyses, we show that the key step for activation of FAK's kinase-dependent functions--autophosphorylation of tyrosine-397--requires site-specific dimerization of FAK. The dimers form via the association of the N-terminal FERM domain of FAK and are stabilized by an interaction between FERM and the C-terminal FAT domain. FAT binds to a basic motif on FERM that regulates co-activation and nuclear localization. FAK dimerization requires local enrichment, which occurs specifically at focal adhesions. Paxillin plays a dual role, by recruiting FAK to focal adhesions and by reinforcing the FAT:FERM interaction. Our results provide a structural and mechanistic framework to explain how FAK combines multiple stimuli into a site-specific function. The dimer interfaces we describe are promising targets for blocking FAK activation.

FERM domain promotes resveratrol-induced apoptosis in endothelial cells via inhibition of NO production

Focal adhesion kinase (FAK) consists of an N-terminal band 4.1; ezrin, radixin, moesin (FERM) domain; tyrosine kinase domain; and C-terminal FA targeting domain. Here we show that ectopically expressed FERM is largely located in the cytosolic fraction under quiescent conditions. We further found that this ectopically expressed FERM domain aggravates endothelial cell apoptosis triggered by 100 μM resveratrol, whereas FERM had no effect on apoptosis induced by TNF-α. We determined that resveratrol at low doses (<20 μM) promotes phosphorylation (S1177) of eNOS via an AMPK-dependent pathway. The presence of the FERM domain blocked this resveratrol-stimulated eNOS phosphorylation and NO production. Thus, the pro-apoptotic activity of cytosolic FERM domain is at least partially mediated by down-regulation of NO, a critical cell survival factor. Consistently, we found that the apoptosis induced by cytosolic FERM in the presence of resveratrol was reversed by an NO donor, SNAP. In conclusion, FERM located in the cytosolic fraction plays a pivotal role in aggravating cell apoptosis through diminishing NO production.

Targeting the p53 pathway

This article summarizes data on translational studies to target the p53 pathway in cancer. It describes the functions of the p53 and Mdm-2 signaling pathways, and discusses current therapeutic approaches to target p53 pathways, including reactivation of p53. In addition, direct interaction and colocalization of the p53 and focal adhesion kinase proteins in cancer cells have been demonstrated, and different approaches to target this interaction are reviewed. This is a broad review of p53 function as it relates to the diagnosis and treatment of a wide range of cancers.

FAK and p53 protein interactions

Focal Adhesion Kinase plays a major role in cell adhesion, motility, survival, proliferation, metastasis, angiogenesis and lymphangiogenesis. In 2004, we have cloned the promoter sequence of FAK and found that p53 inhibits its activity (BBA, v. 1678, 2004). In 2005, we were the first group to show that FAK and p53 proteins directly interact in the cells (JBC, v. 280, 2005). We have shown that FAK and p53 proteins interact in the cytoplasm and in the nucleus by immunoprecipitation, pull-down and confocal microscopy assays. We have shown that FAK inhibited activity of p53 with the transcriptional targets: p21, Bax and Mdm-2 through protein-protein interactions. We identified the 7 amino-acid site in p53 that is involved in interaction with FAK protein. The present review will discuss the interaction of FAK and p53 proteins and discuss the mechanism of FAK-p53 loop regulation: inhibition of FAK promoter activity by p53 protein and also inhibition of p53 transcriptional activity by FAK protein.

The caspase 6 derived N-terminal fragment of DJ-1 promotes apoptosis via increased ROS production

In pathological conditions, the amount of DJ-1 determines whether a cell can survive or engage a cell death program. This is exemplified in epithelial cancers, in which DJ-1 expression is increased, while autosomal recessive early onset Parkinson's disease mutations of DJ-1 generally lead to decreased stability and expression of the protein. We have shown previously that DJ-1 is cleaved by caspase-6 during induction of apoptosis. We demonstrate here that the N-terminal cleaved fragment of DJ-1 (DJ-1 Nt) is specifically expressed in the nucleus and promotes apoptosis in SH-SY5Y neuroblastoma cell lines. In addition, overexpression of DJ-1 Nt in different cell lines leads to a loss of clonogenic potential and sensitizes to staurosporin and 1-methyl-4-phenylpyridinium (MPP+)-mediated caspase activation and apoptosis. Importantly, inhibition of endogenous DJ-1 expression with sh-RNA or DJ-1 deficiency mimics the effect of DJ-1 Nt on cell growth and apoptosis. Moreover, overexpression of DJ-1 Nt increases reactive oxygen species (ROS) production, and sensitizes to MPP+-mediated apoptosis and DJ-1 oxidation. Finally, specific exclusion of DJ-1 Nt from the nucleus abrogates its pro-apoptotic effect. Taken together, our findings identify an original pathway by which generation of a nuclear fragment of DJ-1 through caspase 6-mediated cleavage induces ROS-dependent amplification of apoptosis.

Cross talk between focal adhesion kinase and cadherins: role in regulating endothelial barrier function

A layer of endothelial cells attached to their underlying matrices by complex transmembrane structures termed focal adhesion (FA) proteins maintains the barrier property of microvascular endothelium. FAs sense the physical properties of the extracellular matrix (ECM) and organize the cytoskeleton accordingly. The close association of adherens junction (AJ) protein, cadherin, with the cytoskeleton is known to be essential in coordinating the appropriate mechanical properties to cell-cell contacts. Recently, it has become clear that a crosstalk exists between focal adhesion kinase (FAK) and cadherin that regulates signaling at intercellular endothelial junctions. This review discusses recent advances in our understanding of the dynamic regulation of the molecular connections between FAK and the cadherin complex and cadherin-catenin-actin interaction-dependent changes as well as the role of small GTPases in endothelial barrier regulation. This review also discusses how a signaling network regulates a range of cellular processes important for barrier function and diseases.

Focal adhesion kinase functions as an akt downstream target in migration of colorectal cancer cells

Migration is a complex process that, besides its various physiological functions in embryogenesis and adult tissues, plays a crucial role in cancer cell invasion and metastasis. The focus of this study is the involvement and collaboration of Akt, focal adhesion kinase (FAK), and Src kinases in migration and invasiveness of colorectal cancer cells. We show that all three kinases can be found in one protein complex; nevertheless, the interaction between Akt and Src is indirect and mediated by FAK. Interestingly, induced Akt signaling causes an increase in tyrosine phosphorylation of FAK, but this increase is attenuated by the Src inhibitor SU6656. We also show that active Akt strongly stimulates cell migration, but this phenomenon is fully blocked by FAK knockdown or partly by inhibition of Src kinase. In addition, we found that all three kinases were indispensable for the successful invasion of colorectal cancer cells. Altogether, the presented data bring new insights into the mechanism how the phosphatidylinositol-3-kinase (PI3-K)/Akt pathway can influence migration of colorectal adenocarcinoma cells. Because FAK is indispensable for cell movements and functions downstream of Akt, our results imply FAK kinase as a potential key molecule during progression of tumors with active PI3-K/Akt signaling.

Cellular functions of FAK kinases: insight into molecular mechanisms and novel functions

Focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (Pyk2) are related tyrosine kinases that have important cellular functions, primarily through regulation of the cytoskeleton. Recent studies have identified multiple molecular mechanisms that regulate cytoskeletal responses, and have provided important and exciting insights into how FAK and Pyk2 control cellular processes such as cell migration. Equally exciting are reports of novel and originally unanticipated functions of these kinases, providing the groundwork for future avenues of investigation. This Commentary summarizes some of these recent discoveries that are relevant to the control of biological responses of the cell.

NCAM-induced neurite outgrowth depends on binding of calmodulin to NCAM and on nuclear import of NCAM and fak fragments

The neural cell adhesion molecule NCAM plays important functional roles not only during nervous system development, but also in the adult after injury and in synaptic plasticity. Homophilic binding of NCAM triggers intracellular signaling events resulting in cellular responses such as neurite outgrowth that require NCAM palmitoylation-dependent raft localization and activation of the nonreceptor tyrosine kinases fyn and fak. In this study, we show that stimulation of NCAM by a function-triggering NCAM antibody results in proteolytic processing of NCAM and fak. The C-terminal fragment of NCAM, consisting of the intracellular domain, the transmembrane domain, and a stub of the extracellular domain, and the N-terminal fragment of fak are imported into the nucleus. NCAM-stimulated fak activation, generation, and nuclear import of NCAM and fak fragments as well as neurite outgrowth are abolished by mutation of the calmodulin binding motif in the intracellular domain of NCAM that is responsible for the calcium-dependent binding of calmodulin to NCAM. This mutation interferes neither with NCAM cell surface expression, palmitoylation, and raft localization nor with fyn activation. The way by which the transmembrane NCAM fragment reaches the nucleus in a calmodulin- and calcium-dependent manner is by endocytotic transport via the endoplasmic reticulum and the cytoplasm. The generation and nuclear import of NCAM and phosphorylated fak fragments resulting from NCAM stimulation may represent a signal pathway activating cellular responses in parallel or in association with classical kinase- and phosphorylation-dependent signaling cascades.

Focal adhesion kinase and p53 signal transduction pathways in cancer

Human cancer is characterized by a process of tumor cell motility, invasion, and metastasis. One of the critical tyrosine kinases that is linked to these processes of tumor invasion and survival is the Focal Adhesion Kinase (FAK). Our laboratory was the first to isolate FAK from human tumors, and we had demonstrated that FAK mRNA was up-regulated in invasive and metastatic human breast and colon cancer samples. We have cloned FAK promoter and have found that FAK promoter contains p53 binding sites, and that p53 inhibits FAK transcription and regulates its expression in tumor samples. In addition, we have found a high correlation between FAK overexpression and p53 mutations in 600 population-based series of breast cancer patients. found that N-myc binds FAK promoter and induces FAK transcription in neuroblastoma cells. Thus, this review will be focused on FAK and p53 signal transduction pathways in cancer.

Molecular mechanisms underlying nucleocytoplasmic shuttling of actinin-4

In addition to its well-known role as a crosslinker of actin filaments at focal-adhesion sites, actinin-4 is known to be localized to the nucleus. In this study, we reveal the molecular mechanism underlying nuclear localization of actinin-4 and its novel interactions with transcriptional regulators. We found that actinin-4 is imported into the nucleus through the nuclear pore complex in an importin-independent manner and is exported by the chromosome region maintenance-1 (CRM1)-dependent pathway. Nuclear actinin-4 levels were significantly increased in the late G2 phase of the cell cycle and were decreased in the G1 phase, suggesting that active release from the actin cytoskeleton was responsible for increased nuclear actinin-4 in late G2. Nuclear actinin-4 was found to interact with the INO80 chromatin-remodeling complex. It also directs the expression of a subset of cell-cycle-related genes and interacts with the upstream-binding factor (UBF)-dependent rRNA transcriptional machinery in the M phase. These findings provide molecular mechanisms for both nucleocytoplasmic shuttling of proteins that do not contain a nuclear-localization signal and cell-cycle-dependent gene regulation that reflects morphological changes in the cytoskeleton.

FAK nuclear export signal sequences

Ubiquitously expressed focal adhesion kinase (FAK), a critical component in transducing signals from sites of cell contacts with extracellular matrix, was named after its typical localization in focal adhesions. A nuclear localization of FAK has been also reported and its scaffolding role in nucleus and requirement for p53 ubiquitination were only recently described. Whereas FAK nuclear localization signal (NLS) was found in F2 lobe of FERM domain, nuclear export signal (NES) sequences have not been yet determined. Here we demonstrate that FAK has two NES sequences, NES1 in F1 lobe of FERM domain and NES2 in kinase domain. Although, both NES1 and NES2 are evolutionary conserved, and present as well in FAK-related protein kinase Pyk2, only NES2 demonstrates full biological nuclear export activity.

FERM control of FAK function: implications for cancer therapy

Integrins are transmembrane receptors that bind to extracellular matrix proteins and convey anchorage-dependent signals regulating normal cell proliferation. Integrin signals within the tumor micro-environment also impact cancer cell survival and invasion during tumor progression. These integrin-associated signaling events are transduced in part through the activation of non-receptor protein-tyrosine kinases. Focal adhesion kinase (FAK) is activated by beta-subunit integrins in both normal and transformed cells. As genetic inactivation of beta1 integrin or FAK yield early embryonic lethal phenotypes associated with decreased cell proliferation, and dominant-negative inhibition of FAK can cause increased cell apoptosis, there is a concern that FAK inhibition may have cytotoxic effects on cell growth or survival. However, FAK-specific small molecule inhibitors do not directly impact cell growth in culture, but yet show potent anti-tumor growth effects in vivo. Additionally, recent studies have shed new insight into the FAK kinase-independent regulation of cell proliferation and survival mediated by the FAK N-terminal FERM (band 4.1, ezrin, radixin, moesin homology) domain. Herein, we review the role of the FAK FERM domain in both the intrinsic regulation of FAK kinase activity and how FERM-mediated nuclear localization of FAK promotes enhanced cell survival through the inhibition of tumor suppressor p53 activation during development and under conditions of cellular stress. As we find that FAK FERM-mediated regulation of p53 occurs in human carcinoma cells, elevated FAK expression in tumors may promote both kinase-dependent and -independent survival mechanisms. We discuss how the pharmacological inhibition of FAK kinase activity may impact tumor progression through combined effects of blocking both tumor- and stromal-associated signaling regulating neo-vascularization.

Focal adhesion kinase versus p53: apoptosis or survival?

Focal adhesion kinase (FAK) is a tyrosine kinase that interacts with a multitude of signaling partners and helps cells to survive in the face of various proapoptotic signals. One of the most important interactions for FAK is with the tumor suppressor protein p53. p53 binds not only to the amino-terminal domain of FAK but also to the FAK promoter to inhibit its transcription. A study now reports the biological implications of the kinase-independent interaction of FAK with p53, which opens up future perspectives in cell signaling and cancer research. We focus on FAK and p53 signaling, which link signal transduction pathways from the extracellular matrix and cytoplasm to the nucleus, in human and mouse cells. FAK is proposed to be a critical scaffold protein that sequesters proapoptotic proteins, such as p53, to mediate cell survival.

Focal adhesion kinase and p53 signaling in cancer cells

The progression of human cancer is characterized by a process of tumor cell motility, invasion, and metastasis to distant sites, requiring the cancer cells to be able to survive the apoptotic pressures of anchorage-independent conditions. One of the critical tyrosine kinases linked to these processes of tumor invasion and survival is the focal adhesion kinase (FAK). FAK was first isolated from human tumors, and FAK mRNA was found to be upregulated in invasive and metastatic human breast and colon cancer samples. Recently, the FAK promoter was cloned, and it has been found to contain p53-binding sites. p53 inhibits FAK transcription, and recent data show direct binding of FAK and p53 proteins in vitro and in vivo. The structure of FAK and p53, proteins interacting with FAK, and the role of FAK in tumorigenesis and FAK-p53-related therapy are reviewed. This review focuses on FAK signal transduction pathways, particularly on FAK and p53 signaling, revealing a new paradigm in cell biology, linking signaling from the extracellular matrix to the nucleus.

Extracellular matrix and integrin signaling in lens development and cataract

During development of the vertebrate lens there are dynamic interactions between the extracellular matrix (ECM) of the lens capsule and lens cells. Disruption of the ECM causes perturbation of lens development and cataract. Similarly, changes in cell signaling can result in abnormal ECM and cataract. Integrins are key mediators of ECM signals and recent studies have documented distinct repertoires of integrin expression during lens development, and in anterior subcapsular cataract (ASC) and posterior caspsule opacification (PCO). Increasingly, studies are being directed to investigating the signaling pathways that integrins modulate and have identified Src, focal adhesion kinase (FAK) and integrin-linked kinase (ILK) as downstream kinases that mediate proliferation, differentiation and morphological changes in the lens during development and cataract formation.

Focal adhesion kinase protein levels in gut epithelial motility

Mucosal healing requires migration and proliferation. Most studies of focal adhesion kinase (FAK), a protein that regulates motility, proliferation, and apoptosis, have focused on rapid phosphorylation. We reported lower FAK protein levels in motile Caco-2 colon cancer cells and postulated that this reduction in FAK available for activation might impact cell migration and mucosal healing. Therefore, total and active FAK (FAK(397)) immunoreactivity was assessed at the migrating fronts of human Caco-2 and rat IEC-6 intestinal epithelial cells. Caco-2 and IEC-6 motility, quantitated as migration into linear or circular wounds, was examined following FAK protein inhibition by small interfering RNA (siRNA). FAK protein stability and mRNA expression were ascertained by cycloheximide decay, RT-PCR, and in situ hybridization in static and migrating Caco-2 cells. Cells at the migrating front of Caco-2 and IEC-6 monolayers exhibited lower immunostaining for both total and activated FAK than cells immediately behind the front. Western blot analysis also demonstrated diminished FAK protein levels in motile cells by >/=30% in both the differential density seeding and multiple scrape models. siRNA FAK protein inhibition enhanced motility in both the linear scrape (20% in Caco-2) and circular wound (16% in Caco-2 and 19% in IEC-6 cells) models. FAK protein degradation did not differ in motile and static Caco-2 cells and was unaffected by FAK(397) phosphorylation, but FAK mRNA was lower in migrating Caco-2 cells. Thus FAK protein abundance appears regulated at the mRNA level during gut epithelial cell motility and may influence epithelial cell migration coordinately with signals that modify FAK phosphorylation.

Direct Interaction of the N-terminal Domain of Focal Adhesion Kinase with the N-terminal Transactivation Domain of p53

Focal adhesion kinase (FAK) is a nonreceptor kinase that is overexpressed in many types of tumors and associates with multiple cell surface receptors and intracellular signaling proteins through which it can play an important role in survival signaling. A link between FAK and p53 in survival signaling has been reported, although the molecular basis of these events has not been described. In the present study, we report that FAK physically and specifically interacts with p53 as demonstrated by pull-down, immunoprecipitation, and co-localization analyses. Using different constructs of N-terminal, central, and C-terminal fragments of FAK and p53 proteins, we determined that the N-terminal fragment of FAK directly interacts with the N-terminal transactivation domain of p53. Inhibition of p53 with small interfering p53 RNA resulted in a decreased complex of FAK and p53 proteins in 293 cells, and induction of p53 with doxorubicin in normal human fibroblasts caused an increase of FAK and p53 interaction. Introduction of the FAK plasmid into p53-null SAOS-2 cells was able to rescue these cells from apoptosis induced by expression of wild type p53. In HCT 116 colon cancer cells, co-transfection of FAK plasmid with p21, MDM-2, and BAX luciferase plasmids resulted in significant inhibition of p53-responsive luciferase activities, demonstrating that FAK can reduce transcriptional activity of p53. The results of the FAK and p53 interaction study strongly support the conclusion that FAK can suppress p53-mediated apoptosis and inhibit transcriptional activity of p53. This provides a novel mechanism for FAK-p53-mediated survival/apoptotic signaling.

Spectrin repeat proteins in the nucleus

Spectrin repeat sequences are among the more common repeat elements identified in proteins, typically occurring in large structural proteins. Examples of spectrin repeat-containing proteins include dystrophin, alpha-actinin and spectrin itself--all proteins with well-demonstrated roles of establishing and maintaining cell structure. Over the past decade, it has become clear that, although these proteins display a cytoplasmic and plasma membrane distribution, several are also found both at the nuclear envelope, and within the intranuclear space. In this review, we provide an overview of recent work regarding various spectrin repeat-containing structural proteins in the nucleus. As well, we hypothesize about the regulation of their nuclear localization and possible nuclear functions based on domain architecture, known interacting proteins and evolutionary relationships. Given their large size, and their potential for interacting with multiple proteins and with chromatin, spectrin repeat-containing proteins represent strong candidates for important organizational proteins within the nucleus. Supplementary material for this article can be found on the BioEssays website (http://www.interscience.wiley.com/jpages/0265-9247/suppmat/index.html).

Involvement of c-Src kinase in the regulation of TGF-beta1-induced apoptosis

Transforming growth factor-beta1 (TGF-beta1) is a potent inducer of apoptosis in normal hepatocytes, and acquiring resistance to TGF-beta1 may be a critical step in the development of hepatocellular carcinoma (HCC). In this study, we investigated the possible involvement of c-Src in the regulation of TGF-beta1-induced apoptosis. TGF-beta1 induced transient activation of c-Src and its subsequent caspase-mediated degradation concomitant with cell death in FaO hepatoma cells, which are sensitive to TGF-beta1. In response to TGF-beta1, activated c-Src was translocated into the cytoplasmic membrane, then relocated to the nuclei of apoptotic cells during its cleavage. In TGF-beta1-induced apoptotic cells, c-Src maintained its tight association with p85 FAK fragment cleaved by caspases, possibly contributing to focal adhesion disassembly. TGF-beta1-induced apoptosis was enhanced by either inhibition of c-Src activity using PP1 or PP2, or by overexpression of dominant-negative c-Src. In contrast, overexpression of constitutively active c-Src inhibited apoptosis suppressing TGF-beta1-induced activation of p38, JNK and caspases. In many HCC cell lines resistant to TGF-beta1, enhanced c-Src activity was detected. We hypothesize that activated c-Src in HCC may contribute to resistance against the apoptotic and/ or antiproliferative properties of TGF-beta1.

Nuclear import of N-terminal FAK by activation of the FcepsilonRI receptor in RBL-2H3 cells

As FAK integrates membrane receptor signalling, yet is also found in the nucleus, we investigated whether nuclear FAK is regulated by membrane receptor activation. Activation of the mast cell FcepsilonRI receptor leads to the release and synthesis of inflammatory mediators as well as increased proliferation and survival. Using RBL-2H3 basophilic leukaemia cells, FAK and the FcepsilonRI receptor were co-localised following cross-linking of IgE with antigen. This also resulted in a significant increase in the nucleus of several N-terminal FAK fragments, the largest of which included the kinase domain but not the focal adhesion targeting domain. This was confirmed using cells that stably expressed recombinant EGFP-FAK. Furthermore, treatment of EGFP-FAK expressing cells with Leptomycin B, an inhibitor of nuclear export, resulted in increased nuclear localisation of EGFP-FAK. Therefore, FAK can shuttle between the nuclear and cytoplasmic compartments and the cellular distribution of N-terminal FAK is regulated by membrane receptor activation.

Decreased expression of ?1-integrin and focal adhesion kinase in epithelial cells may initiate involution of mammary glands

The mechanisms regulating involution of mammary glands after weaning are not clear, but engorgement with milk is a key trigger. Many cell types require to be anchored to an extracellular matrix (ECM) as a prerequisite for survival and this is achieved via intregrins binding to specific motifs and signalling their attachment, intracellularly, via focal adhesion kinase (FAK). We sought to determine firstly, if expression of beta1-integrin and FAK is reduced during the first stage of involution. Expression of beta1-integrin and FAK was significantly reduced at 6 h after sealing teats and this was accompanied with a decreased abundance of cytochrome C in mitochondria. Secondly, we sought to determine if expression of beta1-integrin and FAK was restored during the first, partially reversible stage of involution (at 24 h), but not during the second irreversible stage, which occurs after 72 h. Re-suckling restored full expression of the 80 kDa fragment of FAK, but not of the 125 kDa protein or beta1-integrin at 24 h after weaning. Re-suckling did not restore expression of either peptide after 72 h. Changes in expression of cytochrome C and pro-caspase-3 (apoptotic markers) were similar to that of the 80 kDa fragment of FAK. These data suggest that epithelial cells can restore partial contact with their basement membrane during the first, reversible stage, but not during the second irreversible stage of involution. We speculate that decreased contact between epithelial cells and their basement membrane initiates apoptosis in mammary glands at weaning. This process begins within 6 h of pup withdrawal.

Expression of focal adhesion kinase and phosphorylated focal adhesion kinase in squamous cell carcinoma of the larynx

OBJECTIVES

Focal adhesion kinase (FAK) is overexpressed in a variety of human cancers including those derived from the oral cavity. The purpose of this work is to determine the expression patterns of FAK and its activated form, FAK pY397, in squamous cell carcinoma of the larynx and to correlate FAK expression with tumor differentiation and clinical parameters.

STUDY DESIGN

A retrospective study using archival tissue.

METHODS

Thirty-five paraffin embedded tissue specimens of laryngeal carcinoma were obtained from the Department of Pathology at the University of Florida College of Medicine. Immunohistochemical staining of the specimens for FAK and activated phospho-FAK (FAK pY397) was performed. Intensity of staining, distribution of staining, and percentage of cells stained was determined by one pathologist.

RESULTS

There was a statistically significant correlation between FAK staining intensity and tumor differentiation. Poorly differentiated tumors stained more intensely than moderately differentiated tumors (P <.001). There was no correlation between FAK pY397 staining and differentiation (P =.163). However, FAK pY397 staining was unexpectedly found in the nuclei of many specimens. FAK was present in the basal layer of cells within nontransformed squamous mucosa derived from tonsillectomy specimens and in blood vessels. A small amount of FAK pY397 was also localized to blood vessels in nontransformed squamous mucosa.

CONCLUSION

FAK and phospho-FAK are overexpressed in squamous cell carcinoma of the larynx. FAK expression correlates with differentiation. Future investigations will examine the potential of FAK and FAK pY397 expression both as a prognostic indicator and a point of therapeutic inhibition.

Focal adhesion kinase N-terminus in breast carcinoma cells induces rounding, detachment and apoptosis

Focal adhesion kinase (FAK) has a central role in adhesion-mediated cell signalling. The N-terminus of FAK is thought to function as a docking site for a number of proteins, including the Src-family tyrosine kinases. In the present study, we disrupted FAK signalling by expressing the N-terminal domain of FAK (FAK-NT) in human breast carcinoma cells, BT474 and MCF-7 lines, and non-malignant epithelial cells, MCF-10A line. Expression of FAK-NT led to rounding, detachment and apoptosis in human breast cancer cells. Apoptosis was accompanied by dephosphorylation of FAK Tyr(397), degradation of the endogenous FAK protein and activation of caspase-3. Over-expression of FAK rescued FAK-NT-mediated cellular rounding. Expression of FAK-NT in non-malignant breast epithelial cells did not lead to rounding, loss of FAK phosphorylation or apoptosis. Thus FAK-NT contributes to cellular adhesion and survival pathways in breast cancer cells which are not required for survival in non-malignant breast epithelial cells.

Phosphospecific site tyrosine phosphorylation of p125FAK and proline-rich kinase 2 is differentially regulated by cholecystokinin receptor type A activation in pancreatic acini

The focal adhesion kinases, p125FAK and proline-rich kinase 2 (PYK2), are involved in numerous processes as adhesion, cytoskeletal changes, and growth. These kinases have 45% homology and share three tyrosine phosphorylation (TyrP) sites. Little information exists on the ability of stimulants to cause TyrP of each kinase site and the cellular mechanism involved. We explored the ability of the neurotransmitter/hormone, CCK, to stimulate TyrP at each site. In rat pancreatic acini, CCK stimulated TyrP at each site in both kinases. TyrP was rapid except for pY397FAK. The magnitude of TyrP differed with the different FAK and PYK2 sites. The CCK dose-response curve for TyrP for sites in each kinase was similar. CCK-JMV, an agonist of the high affinity receptor state and antagonist of the low affinity receptor state, was less efficacious than CCK at each FAK/PYK2 site and inhibited CCK maximal stimulation. Thapsigargin decreased CCK-stimulated TyrP of pY402PYK2 and pY925FAK but not the other sites. GF109203X reduced TyrP of only the PYK2 sites, pY402 and pY580. GF109203X with thapsigargin decreased TyrP of pY402PYK2 and the three FAK sites more than either inhibitor alone. Basal TyrP of pY397FAK was greater than other sites. These results demonstrate that CCK stimulates tyrosine phosphorylation of each of the three homologous phosphorylation sites in FAK and PYK2. However, CCK-stimulated TyrP at these sites differs in kinetics, magnitude, and participation of the high/low affinity receptor states and by protein kinase C and [Ca2+]i. These results show that phosphorylation of these different sites is differentially regulated and involves different intracellular mechanisms in the same cell.

Caspase-dependent cleavage of c-Abl contributes to apoptosis

The nonreceptor tyrosine kinase c-Abl may contribute to the regulation of apoptosis. c-Abl activity is induced in the nucleus upon DNA damage, and its activation is required for execution of the apoptotic program. Recently, activation of nuclear c-Abl during death receptor-induced apoptosis has been reported; however, the mechanism remains largely obscure. Here we show that c-Abl is cleaved by caspases during tumor necrosis factor- and Fas receptor-induced apoptosis. Cleavage at the very C-terminal region of c-Abl occurs mainly in the cytoplasmic compartment and generates a 120-kDa fragment that lacks the nuclear export signal and the actin-binding region but retains the intact kinase domain, the three nuclear localization signals, and the DNA-binding domain. Upon caspase cleavage, the 120-kDa fragment accumulates in the nucleus. Transient-transfection experiments show that cleavage of c-Abl may affect the efficiency of Fas-induced cell death. These data reveal a novel mechanism by which caspases can recruit c-Abl to the nuclear compartment and to the mammalian apoptotic program.

Mast cell chymase induces smooth muscle cell apoptosis by a mechanism involving fibronectin degradation and disruption of focal adhesions

OBJECTIVE

Chymase released from activated mast cells has been shown to induce apoptosis of vascular smooth muscle cells (SMCs) in vitro. The proteolytic activity of chymase is essential for the proapoptotic effect, but the mechanism of chymase-induced apoptosis has remained unknown.

METHODS AND RESULTS

Here we show by means of FACS analysis, immunohistochemistry, and Western blotting that mast cell-derived chymase induces SMC apoptosis by a mechanism involving degradation of an extracellular matrix component, fibronectin (FN), with subsequent disruption of focal adhesions. The FN degradation products induced SMC apoptosis of similar magnitude and with similar changes in outside-in signaling, as did chymase. Sodium orthovanadate, an inhibitor of tyrosine phosphatases, inhibited the chymase-induced SMC apoptosis. Focal adhesion kinase (FAK), one of the key mediators of integrin-extracellular matrix interactions and cell survival, was rapidly degraded in the presence of chymase or FN degradation products. Loss of phosphorylated FAK (p-FAK) resulted in a rapid dephosphorylation of the p-FAK-dependent downstream mediator Akt.

CONCLUSIONS

The results suggest that chymase-secreting mast cells can mediate apoptosis of neighboring SMCs through a mechanism involving degradation of pericellular FN and disruption of the p-FAK-dependent cell-survival signaling cascade.

The focal adhesion kinase amino-terminal domain localises to nuclei and intercellular junctions in HEK 293 and MDCK cells independently of tyrosine 397 and the carboxy-terminal domain

The function and intracellular localisation of the non-catalytic NH(2)-terminal region of focal adhesion kinase (FAK) are unclear. We investigated the targetting of the FAK NH(2)-terminal domain in HEK 293 and epithelial MDCK cells. Exogenous expression of a variety of GFP-fused and epitope-tagged NH(2) terminal domain constructs either including or lacking the major Tyr 397 autophosphorylation and Src-binding site targeted to nuclei and cell-cell junctions in HEK 293 cells and co-localised at junctions with occludin, and beta1 integrin subunits at junctions. Mutation of Tyr 397 also had no effect on localisation of the NH(2)-terminal domain. In contrast, constructs encoding either the kinase or focal adhesion targeting (FAT) domains but lacking the NH(2)-terminal region failed to localise to intercellular junctions or nuclei. The NH(2)-terminal domain was not associated with beta1 integrin subunits as indicated by co-immunoprecipitation experiments, but did co-localise with cortical actin filaments. The NH(2)-terminal domain also targetted to nuclei and intercellular junctions in MDCK cells, whereas full-length FAK localised only to focal adhesions in these cells. These results indicate that the FAK NH(2)-terminal domain targets to epithelial intercellular junctions and nuclei and suggest novel functions for FAK NH(2)-terminal domain fragments independent of Y397, kinase, and FAT domains.

Staurosporine induces endothelial cell apoptosis via focal adhesion kinase dephosphorylation and focal adhesion disassembly independent of focal adhesion kinase proteolysis

The survival of endothelial cells is dependent on interactions between the matrix and integrins mediated through focal adhesions. Focal adhesion kinase (FAK) is thought to play a key role in maintaining focal adhesion function and cell survival, whereas caspase-mediated FAK proteolysis is implicated in focal adhesion disassembly during apoptosis. We examined the relationship between changes in FAK phosphorylation and proteolysis during apoptosis of primary porcine aortic endothelial cells (PAEC) induced by staurosporine, a widely used apoptogenic agent in diverse cell types. Staurosporine-induced PAEC apoptosis was detected after 1 h and was preceded by disruption and loss of FAK localization to focal adhesions within a few minutes, whereas staurosporine-induced cleavage of FAK occurred only after 8-24 h. Staurosporine induced a very rapid dephosphorylation of FAK at Tyr(861) and Tyr(397) and caused dissociation of phosphorylated FAK from focal adhesions as early as 30 s. The effect of staurosporine was very potent with striking inhibition of Tyr(861) and Tyr(397) phosphorylation and focal adhesion disruption occurring in the range 10-100 nM. Selective inhibition of a known target of staurosporine, protein kinase C, using GF109203X, and of phosphoinositide 3'-kinase using wortmannin, did not reduce FAK tyrosine phosphorylation at Tyr(861) and Tyr(397), or cause disruption of focal adhesions. Cycloheximide, the protein synthesis inhibitor, induced PAEC apoptosis more slowly than staurosporine, but did not induce FAK dephosphorylation or rapid focal adhesion disruption, and instead caused a slower loss of focal adhesions and a marked increase in FAK proteolysis. These studies show that FAK dephosphorylation and focal adhesion disassembly are very early events mediating the onset of staurosporine-induced endothelial cell apoptosis and are dissociated from FAK proteolysis. Cycloheximide induces apoptosis through a pathway involving FAK proteolysis without dephosphorylation.

Src mediates stimulation by vascular endothelial growth factor of the phosphorylation of focal adhesion kinase at tyrosine 861, and migration and anti-apoptosis in endothelial cells

Vascular endothelial growth factor (VEGF) stimulates the tyrosine phosphorylation of focal adhesion kinase (FAK), increases focal adhesion formation and is chemotactic for human umbilical-vein endothelial cells (HUVECs). In the present study we identified the major sites of VEGF-induced FAK tyrosine phosphorylation and investigated the mechanism mediating this pathway in the action of VEGF. VEGF increased the focal adhesion localization of FAK phosphorylated at Tyr-397 (Y397) and Y861 but stimulated a marked increase in phosphorylation at Y861 without significantly affecting the total level of phospho-Y397 FAK. Inhibition of Src with the specific inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) completely blocked VEGF-induced Y861 phosphorylation without decreasing the level of phospho-Y397 FAK. We also examined the role of Src in mediating endothelial functions of VEGF in which FAK has been implicated as having a role. PP2 markedly inhibited VEGF-induced chemotaxis and wound-healing cell migration. The Src inhibitor also decreased the anti-apoptotic effect of VEGF determined by surface staining of annexin V but did not increase FAK proteolysis or prevent the VEGF-dependent inhibition of FAK proteolysis. In contrast, the specific PtdIns 3-kinase inhibitor LY294002 induced apoptosis and markedly decreased p125(FAK) expression and increased FAK proteolysis but had little effect on Y861 phosphorylation. These findings identify Src-dependent FAK phosphorylation at Y861 as a novel VEGF-induced signalling pathway in endothelial cells and suggest that this pathway might be involved in the mechanisms mediating VEGF-induced endothelial cell migration and anti-apoptosis.

Src mediates stimulation by vascular endothelial growth factor of the phosphorylation of focal adhesion kinase at tyrosine 861, and migration and anti-apoptosis in endothelial cells

Vascular endothelial growth factor (VEGF) stimulates the tyrosine phosphorylation of focal adhesion kinase (FAK), increases focal adhesion formation and is chemotactic for human umbilical-vein endothelial cells (HUVECs). In the present study we identified the major sites of VEGF-induced FAK tyrosine phosphorylation and investigated the mechanism mediating this pathway in the action of VEGF. VEGF increased the focal adhesion localization of FAK phosphorylated at Tyr-397 (Y397) and Y861 but stimulated a marked increase in phosphorylation at Y861 without significantly affecting the total level of phospho-Y397 FAK. Inhibition of Src with the specific inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) completely blocked VEGF-induced Y861 phosphorylation without decreasing the level of phospho-Y397 FAK. We also examined the role of Src in mediating endothelial functions of VEGF in which FAK has been implicated as having a role. PP2 markedly inhibited VEGF-induced chemotaxis and wound-healing cell migration. The Src inhibitor also decreased the anti-apoptotic effect of VEGF determined by surface staining of annexin V but did not increase FAK proteolysis or prevent the VEGF-dependent inhibition of FAK proteolysis. In contrast, the specific PtdIns 3-kinase inhibitor LY294002 induced apoptosis and markedly decreased p125(FAK) expression and increased FAK proteolysis but had little effect on Y861 phosphorylation. These findings identify Src-dependent FAK phosphorylation at Y861 as a novel VEGF-induced signalling pathway in endothelial cells and suggest that this pathway might be involved in the mechanisms mediating VEGF-induced endothelial cell migration and anti-apoptosis.

Signaling mechanisms mediating vascular protective actions of vascular endothelial growth factor

Vascular endothelial growth factor (VEGF) is essential for angiogenesis in health and pathophysiology, and it is currently a major focus for drug targeting in the development of treatments for diverse human diseases. Recently, we proposed that VEGF could also play a role as a vascular protective factor in the adult vasculature and in disease. In this model, vascular protection is defined as a VEGF-induced enhancement of endothelial functions that mediate the inhibition of vascular smooth muscle cell proliferation, enhanced endothelial cell survival, suppression of thrombosis, and anti-inflammatory effects. A feature of this model is that protective effects of VEGF are essentially independent of angiogenesis or endothelial cell proliferation. VEGF-dependent cell survival and VEGF-induced synthesis of nitric oxide and prostacyclin are likely to be key mediators of a vascular protective effect. Vascular protection should help to improve insight into the underlying mechanisms of cardiovascular actions of VEGF and prove valuable for developing novel therapeutic approaches to cardiovascular disease.