Multiple paxillin binding sites regulate FAK function

FAK inhibitors in cancer, a patent review - an update on progress

INTRODUCTION

Focal adhesion kinase (FAK) is a cytoplasmic non-receptor tyrosine kinase over-expressed in various malignancies which is related to various cellular functions such as adhesion, metastasis and proliferation.

AREAS COVERED

There is growing evidence that FAK is a promising therapeutic target for designing inhibitors by regulating the downstream pathways of FAK. Some potential FAK inhibitors have entered clinical phase research.

EXPERT OPINION

FAK could be an effective target in medicinal chemistry research and there were a variety of FAKIs have been patented recently. Here, we updated an overview of design, synthesis and structure-activity relationship of chemotherapeutic FAK inhibitors (FAKIs) from 2017 until now based on our previous work. We hope our efforts can broaden the understanding of FAKIs and provide new ideas and insights for future cancer treatment from medicinal chemistry point of view.

Targeting focal adhesion kinase (FAK) for cancer therapy: FAK inhibitors, FAK-based dual-target inhibitors and PROTAC degraders

Focal adhesion kinase (FAK), a non-receptor tyrosine kinase, plays an essential role in regulating cell proliferation, migration and invasion through both kinase-dependent enzymatic function and kinase-independent scaffolding function. The overexpression and activation of FAK is commonly observed in various cancers and some drug-resistant settings. Therefore, targeted disruption of FAK has been identified as an attractive strategy for cancer treatment. To date, numerous structurally diverse inhibitors targeting distinct domains of FAK have been developed, encompassing kinase domain inhibitors, FERM domain inhibitors, and FAT domain inhibitors, with several FAK inhibitors advanced to clinical trials. Moreover, given the critical role of FAK scaffolding function in signal transduction, FAK-targeted PROTACs have also been developed. Although no current FAK-targeted therapeutics have been approved for the market, the combination of FAK inhibitors with other anticancer drugs has shown considerable promise in the clinic. This review provides an overview of current drug discovery strategies targeting FAK, including the development of FAK inhibitors, FAK-based dual-target inhibitors and proteolysis-targeting chimeras (PROTACs) in both literature and patent applications. Accordingly, their design and optimization process, mechanisms of action and biological activities are discussed to offer insights into future directions of FAK-targeting drug discovery in cancer therapy.

Evaluation of Paxillin Expression in Epithelial Dysplasia, Oral Squamous Cell Carcinoma, Lichen Planus with and without Dysplasia, and Hyperkeratosis: A Retrospective Cross-Sectional Study

BACKGROUND

Paxillin is a cytoskeletal protein involved in the pathogenesis of several types of cancers. However, the roles of paxillin in epithelial dysplasia (ED), oral squamous cell carcinoma (OSCC), oral lichen planus with dysplasia (OLPD), hyperkeratosis (HK), and oral lichen planus (OLP) have remained unnoticed in the literature. This study aimed to evaluate its attainable functions in the pathogenesis and malignant transformation of potentially malignant oral epithelium and benign lesions.

METHODS

In this retrospective cross-sectional study, paxillin expression was investigated in 99 tissue samples, including 18 cases of OSCC, 21 ED, 23 OLP, 21 OLPD, and 16 cases of HK. The tissue sections also underwent immunohistochemical paxillin staining using 3,3-diaminobenzidine (DAB) chromogen. The intensity, location, and percentage of staining were examined across all groups. Data were analyzed using the Shapiro-Wilk test, ANOVA, Pearson chi-square, Kruskal-Wallis, and Dunn's post hoc test.

RESULTS

The cytoplasmic percentage and intensity staining of Paxillin expression were evident in the central/suprabasal and basal/peripheral layers of all the obtained samples. The final staining score was significantly higher in OSCC and dysplasia compared to HK and OLP (p = 0.004). It was found that paxillin expression is associated with the grade of dysplastic samples (p < 0.001).

CONCLUSION

The present study provides evidence that paxillin may be involved in the pathogenesis of OSCC and the development and progression of dysplastic tissue, since the paxillin expression was higher than that of HK and OLP.

Paxillin regulates Rab5-mediated vesicle motility through modulating microtubule acetylation

Rab GTPase-mediated vesicle trafficking of cell surface proteins, including integrins, through endocytic and recycling pathways is important in controlling cell-extracellular matrix interactions during cell migration. The focal adhesion adaptor protein, paxillin, plays a central role in regulating adhesion dynamics and was previously shown to promote anterograde vesicle trafficking through modulation of microtubule acetylation via its inhibition of the deacetylase HDAC6. The role of paxillin in retrograde trafficking is unknown. Herein, we identified a role for paxillin in the modulation of the Rab5 GTPase, which is necessary for regulating early endosome dynamics and focal adhesion turnover. Using MDA-MB-231 breast cancer cells and paxillin (-/-) fibroblasts, paxillin was shown to impact Rab5-associated vesicle size and distribution, as well as Rab5 GTPase activity, through its modulation of HDAC6. Using a combination of real-time imaging and particle tracking analysis, paxillin was shown to promote Rab5-associated vesicle motility through inhibition of HDAC6-mediated micro-tubule deacetylation, along with the localization of active integrin to focal adhesions.

Rutin and Quercetin Counter Doxorubicin-Induced Liver Toxicity in Wistar Rats via Their Modulatory Effects on Inflammation, Oxidative Stress, Apoptosis, and Nrf2

The presented study was performed to verify whether rutin and/or quercetin can inhibit liver injury induced by doxorubicin (DXR) in male Wistar rats. In this study, male Wistar rats were treated via the oral route with rutin and quercetin (50 mg/kg) either alone or in combination every other day for five weeks concomitant with receiving intraperitoneal DXR (2 mg/kg) two times a week for five successive weeks. Quercetin, rutin, and their combination significantly improved the deteriorated serum AST, ALT, and ALP activities and total bilirubin level, as well as albumin, AFP, and CA 19.9 levels in DXR-injected rats. Treatments of the DXR-injected group with quercetin and rutin prevented the elevation in liver lipid peroxidation and the reduction in superoxide dismutase, glutathione-S-transferase and glutathione peroxidase activities, and glutathione content. Treatments with quercetin and rutin significantly repressed the elevated expression of liver p53 and TNF-α and enhanced Nrf2 expression. Furthermore, the treatments significantly reduced DXR-induced liver histological changes. In conclusion, rutin and quercetin either alone or in combination may have potential preventive effects against DXR-induced hepatotoxicity through inhibiting oxidative stress, inflammation, and apoptosis as well as modulating the Nrf2 expression.

Synergistic phase separation of two pathways promotes integrin clustering and nascent adhesion formation

Integrin adhesion complexes (IACs) are integrin-based plasma-membrane-associated compartments where cells sense environmental cues. The physical mechanisms and molecular interactions that mediate initial IAC formation are unclear. We found that both p130Cas ('Cas') and Focal adhesion kinase ('FAK') undergo liquid-liquid phase separation in vitro under physiologic conditions. Cas- and FAK- driven phase separation is sufficient to reconstitute kindlin-dependent integrin clustering in vitro with recombinant mammalian proteins. In vitro condensates and IACs in mouse embryonic fibroblasts (MEFs) exhibit similar sensitivities to environmental perturbations including changes in temperature and pH. Furthermore, mutations that inhibit or enhance phase separation in vitro reduce or increase the number of IACs in MEFs, respectively. Finally, we find that the Cas and FAK pathways act synergistically to promote phase separation, integrin clustering, IAC formation and partitioning of key components in vitro and in cells. We propose that Cas- and FAK-driven phase separation provides an intracellular trigger for integrin clustering and nascent IAC formation.

Paxillin Promotes Breast Tumor Collective Cell Invasion through Maintenance of Adherens Junction Integrity

Distant organ metastasis is linked to poor prognosis during cancer progression. The expression level of the focal adhesion adapter protein paxillin varies among different human cancers, but its role in tumor progression is unclear. Herein, we utilize a newly generated PyMT mammary tumor mouse model with conditional paxillin ablation in breast tumor epithelial cells, combined with in vitro 3D tumor organoids invasion analysis and 2D calcium switch assays, to assess the roles for paxillin in breast tumor cell invasion. Paxillin had little effect on primary tumor initiation and growth but is critical for the formation of distant lung metastasis. In paxillin-depleted 3D tumor organoids, collective cell invasion was substantially perturbed. Two-dimensional cell culture revealed paxillin-dependent stabilization of adherens junctions (AJ). Mechanistically, paxillin is required for AJ assembly through facilitating E-cadherin endocytosis and recycling and HDAC6-mediated microtubule acetylation. Furthermore, Rho GTPase activity analysis and rescue experiments with a RhoA activator or Rac1 inhibitor suggest paxillin is potentially regulating the E-cadherin-dependent junction integrity and contractility through control of the balance of RhoA and Rac1 activities. Together, these data highlight new roles for paxillin in the regulation of cell-cell adhesion and collective tumor cell migration to promote the formation of distance organ metastases. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text].

Drug Discovery Targeting Focal Adhesion Kinase (FAK) as a Promising Cancer Therapy

FAK is a nonreceptor intracellular tyrosine kinase which plays an important biological function. Many studies have found that FAK is overexpressed in many human cancer cell lines, which promotes tumor cell growth by controlling cell adhesion, migration, proliferation, and survival. Therefore, targeting FAK is considered to be a promising cancer therapy with small molecules. Many FAK inhibitors have been reported as anticancer agents with various mechanisms. Currently, six FAK inhibitors, including GSK-2256098 (Phase I), VS-6063 (Phase II), CEP-37440 (Phase I), VS-6062 (Phase I), VS-4718 (Phase I), and BI-853520 (Phase I) are undergoing clinical trials in different phases. Up to now, there have been many novel FAK inhibitors with anticancer activity reported by different research groups. In addition, FAK degraders have been successfully developed through “proteolysis targeting chimera” (PROTAC) technology, opening up a new way for FAK-targeted therapy. In this paper, the structure and biological function of FAK are reviewed, and we summarize the design, chemical types, and activity of FAK inhibitors according to the development of FAK drugs, which provided the reference for the discovery of new anticancer agents.

FAK displacement from focal adhesions: a promising strategy to target processes implicated in cancer progression and metastasis

BACKGROUND

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that is overexpressed or activated in several advanced-stage solid cancers. It is known to play both kinase-dependent and -independent roles in promoting tumor progression and metastasis. Numerous inhibitors, targeting either the enzymatic or scaffolding activities of FAK have been generated, with varying degree of success. Here, we describe a novel approach to site-specifically target both kinase-dependent and -independent FAK functions at focal adhesions (FAs), the primary sites at which the kinase exerts its activity.

METHODS

We took advantage of the well-characterized interactions between the paxillin LD motifs and the FAK FAT domain and generated a polypeptide (LD2-LD3-LD4) expected to compete with interactions with paxillin. Co-immunoprecipitation experiments were performed to examine the interaction between the LD2-LD3-LD4 polypeptide and FAK. The effects of LD2-LD3-LD4 in the localization and functions of FAK, as well as FA composition, were evaluated using quantitative immunofluorescence, cell fractionation, FA isolation and Western Blot analysis. Live cell imaging, as well as 2-D migration and cell invasion assays were used to examine the effects on FA turnover and tumor cell migration and invasion.

RESULTS

Expression of the LD2-LD3-LD4 polypeptide prevents FAK localization at FAs, in a controlled and dose-dependent manner, by competing with endogenous paxillin for FAK binding. Importantly, the LD2-LD3-LD4 peptide did not otherwise affect FA composition or integrin activation. LD2-LD3-LD4 inhibited FAK-dependent downstream integrin signaling and, unlike existing inhibitors, also blocked FAK's scaffolding functions. We further show that LD2-LD3-LD4 expression markedly reduces FA turnover and inhibits tumor cell migration and invasion. Finally, we show that dimers of a single motif, linked through a flexible linker of the proper size, are sufficient for the displacement of FAK from FAs and for inhibition of tumor cell migration. This work raises the possibility of using a synthetic peptide as an antimetastatic agent, given that effective displacement of FAK from FAs only requires dimers of a single LD motif linked by a short flexible linker.

CONCLUSION

In conclusion, these results suggest that FAK displacement from FAs is a promising new strategy to target critical processes implicated in cancer progression and metastasis. Video abstract.

Progress in the Development of Small Molecular Inhibitors of Focal Adhesion Kinase (FAK)

Focal adhesion kinase (FAK) is a nonreceptor intracellular tyrosine kinase that plays an essential role in cancer cell adhesion, survival, proliferation, and migration through both its enzymatic activities and scaffolding functions. Overexpression of FAK has been found in many human cancer cells from different origins, which promotes tumor progression and influences clinical outcomes in different classes of human tumors. Therefore, FAK has been considered as a promising target for small molecule anticancer drug development. Many FAK inhibitors targeting different domains of FAK with various mechanisms of functions have been reported, including kinase domain inhibitors, FERM domain inhibitors, and FAT domain inhibitors. In addition, FAK-targeting PROTACs, which can induce the degradation of FAK, have also been developed. In this Perspective, we summarized the progress in the development of small molecular FAK inhibitors and proposed the perspectives for the future development of agents targeting FAK.

Recognition of LD motifs by the focal adhesion targeting domains of focal adhesion kinase and proline-rich tyrosine kinase 2-beta: Insights from molecular dynamics simulations

The focal adhesion kinase (FAK) and the proline-rich tyrosine kinase 2-beta (PYK2) are implicated in cancer progression and metastasis and represent promising biomarkers and targets for cancer therapy. FAK and PYK2 are recruited to focal adhesions (FAs) via interactions between their FA targeting (FAT) domains and conserved segments (LD motifs) on the proteins Paxillin, Leupaxin, and Hic-5. A promising new approach for the inhibition of FAK and PYK2 targets interactions of the FAK domains with proteins that promote localization at FAs. Advances toward this goal include the development of surface plasmon resonance, heteronuclear single quantum coherence nuclear magnetic resonance (HSQC-NMR) and fluorescence polarization assays for the identification of fragments or compounds interfering with the FAK-Paxillin interaction. We have recently validated this strategy, showing that Paxillin mimicking polypeptides with 2 to 3 LD motifs displace FAK from FAs and block kinase-dependent and independent functions of FAK, including downstream integrin signaling and FA localization of the protein p130Cas. In the present work we study by all-atom molecular dynamics simulations the recognition of peptides with the Paxillin and Leupaxin LD motifs by the FAK-FAT and PYK2-FAT domains. Our simulations and free-energy analysis interpret experimental data on binding of Paxillin and Leupaxin LD motifs at FAK-FAT and PYK2-FAT binding sites, and assess the roles of consensus LD regions and flanking residues. Our results can assist in the design of effective inhibitory peptides of the FAK-FAT: Paxillin and PYK2-FAT:Leupaxin complexes and the construction of pharmacophore models for the discovery of potential small-molecule inhibitors of the FAK-FAT and PYK2-FAT focal adhesion based functions.

Development of a Fragment-Based Screening Assay for the Focal Adhesion Targeting Domain Using SPR and NMR

The Focal Adhesion Targeting (FAT) domain of Focal Adhesion Kinase (FAK) is a promising drug target since FAK is overexpressed in many malignancies and promotes cancer cell metastasis. The FAT domain serves as a scaffolding protein, and its interaction with the protein paxillin localizes FAK to focal adhesions. Various studies have highlighted the importance of FAT-paxillin binding in tumor growth, cell invasion, and metastasis. Targeting this interaction through high-throughput screening (HTS) provides a challenge due to the large and complex binding interface. In this report, we describe a novel approach to targeting FAT through fragment-based drug discovery (FBDD). We developed two fragment-based screening assays-a primary SPR assay and a secondary heteronuclear single quantum coherence nuclear magnetic resonance (HSQC-NMR) assay. For SPR, we designed an AviTag construct, optimized SPR buffer conditions, and created mutant controls. For NMR, resonance backbone assignments of the human FAT domain were obtained for the HSQC assay. A 189-compound fragment library from Enamine was screened through our primary SPR assay to demonstrate the feasibility of a FAT-FBDD pipeline, with 19 initial hit compounds. A final total of 11 validated hits were identified after secondary screening on NMR. This screening pipeline is the first FBDD screen of the FAT domain reported and represents a valid method for further drug discovery efforts on this difficult target.

Development of a High-Throughput Fluorescence Polarization Assay to Detect Inhibitors of the FAK-Paxillin Interaction

Focal adhesion kinase (FAK) is a promising cancer drug target due to its massive overexpression in multiple solid tumors and its critical role in the integration of signals that control proliferation, invasion, apoptosis, and metastasis. Previous FAK drug discovery and high-throughput screening have exclusively focused on the identification of inhibitors that target the kinase domain of FAK. Because FAK is both a kinase and scaffolding protein, the development of novel screening assays that detect inhibitors of FAK protein-protein interactions remains a critical need. In this report, we describe the development of a high-throughput fluorescence polarization (FP) screening assay that measures the interactions between FAK and paxillin, a focal adhesion-associated protein. We designed a tetramethylrhodamine (TAMRA)-tagged paxillin peptide based on the paxillin LD2 motif that binds to the focal adhesion targeting (FAT) domain with significant dynamic range, specificity, variability, stability, and a Z'-factor suitable for high-throughput screening. In addition, we performed a pilot screen of 1593 compounds using this FP assay, showing its feasibility for high-throughput drug screening. Finally, we identified three compounds that show dose-dependent competition of FAT-paxillin binding. This assay represents the first described high-throughput screening assay for FAK scaffold inhibitors and can accelerate drug discovery efforts for this promising drug target.

Regulation of Transmembrane Signaling by Phase Separation

Cell surface transmembrane receptors often form nanometer- to micrometer-scale clusters to initiate signal transduction in response to environmental cues. Extracellular ligand oligomerization, domain-domain interactions, and binding to multivalent proteins all contribute to cluster formation. Here we review the current understanding of mechanisms driving cluster formation in a series of representative receptor systems: glycosylated receptors, immune receptors, cell adhesion receptors, Wnt receptors, and receptor tyrosine kinases. We suggest that these clusters share properties of systems that undergo liquid-liquid phase separation and could be investigated in this light.

Targeting Focal Adhesion Kinase Using Inhibitors of Protein-Protein Interactions

Focal adhesion kinase (FAK) is a cytoplasmic non-receptor protein tyrosine kinase that is overexpressed and activated in many human cancers. FAK transmits signals to a wide range of targets through both kinase-dependant and independent mechanism thereby playing essential roles in cell survival, proliferation, migration and invasion. In the past years, small molecules that inhibit FAK kinase function have been developed and show reduced cancer progression and metastasis in several preclinical models. Clinical trials have been conducted and these molecules display limited adverse effect in patients. FAK contain multiple functional domains and thus exhibit both important scaffolding functions. In this review, we describe the major FAK interactions relevant in cancer signalling and discuss how such knowledge provide rational for the development of Protein-Protein Interactions (PPI) inhibitors.

Cell Adhesion and Matrix Stiffness: Coordinating Cancer Cell Invasion and Metastasis

Metastasis is a multistep process in which tumor extracellular matrix (ECM) and cancer cell cytoskeleton interactions are pivotal. ECM is connected, through integrins, to the cell’s adhesome at cell–ECM adhesion sites and through them to the actin cytoskeleton and various downstream signaling pathways that enable the cell to respond to external stimuli in a coordinated manner. Cues from cell-adhesion proteins are fundamental for defining the invasive potential of cancer cells, and many of these proteins have been proposed as potent targets for inhibiting cancer cell invasion and thus, metastasis. In addition, ECM accumulation is quite frequent within the tumor microenvironment leading in many cases to an intense fibrotic response, known as desmoplasia, and tumor stiffening. Stiffening is not only required for the tumor to be able to displace the host tissue and grow in size but also contributes to cell–ECM interactions and can promote cancer cell invasion to surrounding tissues. Here, we review the role of cell adhesion and matrix stiffness in cancer cell invasion and metastasis.

Paxillin regulates cell polarization and anterograde vesicle trafficking during cell migration

Cell polarization and directed migration play pivotal roles in diverse physiological and pathological processes. Herein, we identify new roles for paxillin-mediated HDAC6 inhibition in regulating key aspects of cell polarization in both two-dimensional and one-dimensional matrix environments. Paxillin, by modulating microtubule acetylation through HDAC6 regulation, was shown to control centrosome and Golgi reorientation toward the leading edge, a hallmark of cell polarization to ensure directed trafficking of promigratory factors. Paxillin was also required for pericentrosomal Golgi localization and centrosome cohesion, independent of its localization to, and role in, focal adhesion signaling. In addition, we provide evidence of an accumulation of paxillin at the centrosome that is dependent on focal adhesion kinase (FAK) and identify an important collaboration between paxillin and FAK signaling in the modulation of microtubule acetylation, as well as centrosome and Golgi organization and polarization. Finally, paxillin was also shown to be required for optimal anterograde vesicular trafficking to the plasma membrane.

Addressing the Functional Determinants of FAK during Ciliogenesis in Multiciliated Cells

We previously identified focal adhesion kinase (FAK) as an important regulator of ciliogenesis in multiciliated cells. FAK and other focal adhesion (FA) proteins associate with the basal bodies and their striated rootlets and form complexes named ciliary adhesions (CAs). CAs display similarities with FAs but are established in an integrin independent fashion and are responsible for anchoring basal bodies to the actin cytoskeleton during ciliogenesis as well as in mature multiciliated cells. FAK down-regulation leads to aberrant ciliogenesis due to impaired association between the basal bodies and the actin cytoskeleton, suggesting that FAK is an important regulator of the CA complex. However, the mechanism through which FAK functions in the complex is not clear, and in this study we examined the role of this protein in both ciliogenesis and ciliary function. We show that localization of FAK at CAs depends on interactions taking place at the amino-terminal (FERM) and carboxyl-terminal (FAT) domains and that both domains are required for proper ciliogenesis and ciliary function. Furthermore, we show that an interaction with another CA protein, paxillin, is essential for correct localization of FAK in multiciliated cells. This interaction is indispensable for both ciliogenesis and ciliary function. Finally, we provide evidence that despite the fact that FAK is in the active, open conformation at CAs, its kinase activity is dispensable for ciliogenesis and ciliary function revealing that FAK plays a scaffolding role in multiciliated cells. Overall these data show that the role of FAK at CAs displays similarities but also important differences compared with its role at FAs.

Nuclear transport of paxillin depends on focal adhesion dynamics and FAT domains

The nuclear transport of paxillin appears to be crucial for paxillin function but the mechanism of transport remains unclear. Here, we show that the nuclear transport of paxillin is regulated by focal adhesion turnover and the presence of FAT domains. Focal adhesion turnover was controlled using triangular or circular fibronectin islands. Circular islands caused higher focal adhesion turnover and increased the nuclear transport of paxillin relative to triangular islands. Mutating several residues of paxillin had no effect on its nuclear transport, suggesting that the process is controlled by multiple domains. Knocking out FAK (also known as PTK2) and vinculin caused an increase in nuclear paxillin. This could be reversed by rescue with wild-type FAK but not by FAK with a mutated FAT domain, which inhibits paxillin binding. Expressing just the FAT domain of FAK not only brought down nuclear levels of paxillin but also caused a large immobile fraction of paxillin to be present at focal adhesions, as demonstrated by fluorescence recovery after photobleaching (FRAP) studies. Taken together, focal adhesion turnover and FAT domains regulate the nuclear localization of paxillin, suggesting a possible role for transcriptional control, through paxillin, by focal adhesions.

Summary: We find that nuclear translocation of paxillin is tuned by focal adhesion maturation. This could provide a method for mechanosensing signals to be used by cells to control transcription.

Structural Basis for the Interaction between Pyk2-FAT Domain and Leupaxin LD Repeats

Proline-rich tyrosine kinase 2 (Pyk2) is a nonreceptor tyrosine kinase and belongs to the focal adhesion kinase (FAK) family. Like FAK, the C-terminal focal adhesion-targeting (FAT) domain of Pyk2 binds to paxillin, a scaffold protein in focal adhesions; however, the interaction between the FAT domain of Pyk2 and paxillin is dynamic and unstable. Leupaxin is another member in the paxillin family and was suggested to be the native binding partner of Pyk2; Pyk2 gene expression is strongly correlated with that of leupaxin in many tissues including primary breast cancer. Here, we report that leupaxin interacts with Pyk2-FAT. Leupaxin has four leucine–aspartate (LD) motifs. The first and third LD motifs of leupaxin preferably target the two LD-binding sites on the Pyk2-FAT domain, respectively. Moreover, the full-length leupaxin binds to Pyk2-FAT as a stable one-to-one complex. Together, we propose that there is an underlying selectivity between leupaxin and paxillin for Pyk2, which may influence the differing behavior of the two proteins at focal adhesion sites.

Interaction with the Paxillin LD1 Motif Relieves MEKK2 Auto-inhibition

The cell signaling molecule MEK kinase 2 (MEKK2) is a key upstream regulator of MAPK activity that regulates numerous cellular functions, but the mechanisms that control MEKK2 activity are not well understood. Recently, we reported that MEKK2 both binds and promotes ubiquitylation of the scaffold protein paxillin, and thereby modulates the composition of adhesion complexes. In this study, we have extended our examination of this interaction and report that recombinant paxillin is sufficient to induce MEKK2 auto-phosphorylation. Furthermore, we utilize siRNA-mediated paxillin expression knockdown to reveal that MEKK2 activity is reduced in paxillin-deficient cells. Finally, we show that the paxillin leucine-rich motif 1 (LD1) is sufficient to bind to the MEKK2 amino terminal region and activate MEKK2. Taken together, our results show for the first time that paxillin association promotes MEKK2 activation and reveal the existence of a novel bi-directional regulatory relationship between MEKK2 and paxillin.

Engineering Synthetic Antibody Inhibitors Specific for LD2 or LD4 Motifs of Paxillin

Focal adhesion protein paxillin links integrin and growth factor signaling to actin cytoskeleton. Most of paxillin signaling activity is regulated via leucine-rich LD motifs (LD1-LD5) located at the N-terminus. Here, we demonstrate a method to engineer highly selective synthetic antibodies (sABs) against LD2 and LD4 that are binding sites for focal adhesion kinase (FAK) and other proteins. Phage display selections against peptides were used to generate sABs recognizing each LD motif. In the obtained X-ray crystal structures of the LD-sAB complexes, the LD motifs are helical and bind sABs through a hydrophobic side, similarly as in the structures with natural paxillin partners. The sABs are capable of pulling down endogenous paxillin in complex with FAK and can visualize paxillin in focal adhesions in cells. They were also used as selective inhibitors to effectively compete with focal adhesion targeting domain of FAK for the binding to LD2 and LD4. The sABs are tools for investigation of paxillin LD binding "platforms" and are capable of inhibiting paxillin interactions, thereby useful as potential therapeutics in the future.

c-Abl mediated tyrosine phosphorylation of paxillin regulates LPS-induced endothelial dysfunction and lung injury

Paxillin is phosphorylated at multiple residues; however, the role of tyrosine phosphorylation of paxillin in endothelial barrier dysfunction and acute lung injury (ALI) remains unclear. We used siRNA and site-specific nonphosphorylable mutants of paxillin to abrogate the function of paxillin to determine its role in lung endothelial permeability and ALI. In vitro, lipopolysaccharide (LPS) challenge of human lung microvascular endothelial cells (HLMVECs) resulted in enhanced tyrosine phosphorylation of paxillin at Y31 and Y118 with no significant change in Y181 and significant barrier dysfunction. Knockdown of paxillin with siRNA attenuated LPS-induced endothelial barrier dysfunction and destabilization of VE-cadherin. LPS-induced paxillin phosphorylation at Y31 and Y118 was mediated by c-Abl tyrosine kinase, but not by Src and focal adhesion kinase. c-Abl siRNA significantly reduced LPS-induced endothelial barrier dysfunction. Transfection of HLMVECs with paxillin Y31F, Y118F, and Y31/118F double mutants mitigated LPS-induced barrier dysfunction and VE-cadherin destabilization. In vivo, the c-Abl inhibitor AG957 attenuated LPS-induced pulmonary permeability in mice. Together, these results suggest that c-Abl mediated tyrosine phosphorylation of paxillin at Y31 and Y118 regulates LPS-mediated pulmonary vascular permeability and injury.

Metformin Treatment for the Prevention and/or Treatment of Breast/Mammary Tumorigenesis

There is increasing interest in metformin's effects on the development, treatment and/or progression of breast cancer. This emerges from observational studies that diabetic women treated with metformin in comparison to other antidiabetic compounds had lower breast cancer incidence and/or mortality rates. The mechanism of action is considered to be activation of hepatic AMPK resulting in reduced gluconeogenesis. Calorie restriction, which consistently reduces mammary tumorigenesis in rodents, is also thought to act through this pathway leading to the hypothesis that metformin's anticancer effects are mediated in a similar fashion. Here we review the literature evaluating metformin's anticancer effects in relation to breast/mammary tumorigenesis. We include clinical observations, as well as studies utilizing rodent models and mammary cell lines. In addition to the anticancer effect of metformin mediated through the AMPK pathway, additional mechanisms of action that directly target tissues have been identified including effects on stem cells, apoptosis, STAT3 and HER2.

Optimal ligand descriptor for pocket recognition based on the Beta-shape

Structure-based virtual screening is one of the most important and common computational methods for the identification of predicted hit at the beginning of drug discovery. Pocket recognition and definition is frequently a prerequisite of structure-based virtual screening, reducing the search space of the predicted protein-ligand complex. In this paper, we present an optimal ligand shape descriptor for a pocket recognition algorithm based on the beta-shape, which is a derivative structure of the Voronoi diagram of atoms. We investigate six candidates for a shape descriptor for a ligand using statistical analysis: the minimum enclosing sphere, three measures from the principal component analysis of atoms, the van der Waals volume, and the beta-shape volume. Among them, the van der Waals volume of a ligand is the optimal shape descriptor for pocket recognition and best tunes the pocket recognition algorithm based on the beta-shape for efficient virtual screening. The performance of the proposed algorithm is verified by a benchmark test.

RhoGEFs in cell motility: novel links between Rgnef and focal adhesion kinase

Rho guanine exchange factors (GEFs) are a large, diverse family of proteins defined by their ability to catalyze the exchange of GDP for GTP on small GTPase proteins such as Rho family members. GEFs act as integrators from varied intra- and extracellular sources to promote spatiotemporal activity of Rho GTPases that control signaling pathways regulating cell proliferation and movement. Here we review recent studies elucidating roles of RhoGEF proteins in cell motility. Emphasis is placed on Dbl-family GEFs and connections to development, integrin signaling to Rho GTPases regulating cell adhesion and movement, and how these signals may enhance tumor progression. Moreover, RhoGEFs have additional domains that confer distinctive functions or specificity. We will focus on a unique interaction between Rgnef (also termed Arhgef28 or p190RhoGEF) and focal adhesion kinase (FAK), a non-receptor tyrosine kinase that controls migration properties of normal and tumor cells. This Rgnef-FAK interaction activates canonical GEF-dependent RhoA GTPase activity to govern contractility and also functions as a scaffold in a GEF-independent manner to enhance FAK activation. Recent studies have also brought to light the importance of specific regions within the Rgnef pleckstrin homology (PH) domain for targeting the membrane. As revealed by ongoing Rgnef-FAK investigations, exploring GEF roles in cancer will yield fundamental new information on the molecular mechanisms promoting tumor spread and metastasis.

FAK transduces extracellular forces that orient the mitotic spindle and control tissue morphogenesis

Spindle orientation is critical for proper morphogenesis of organs and tissues as well as for the maintenance of tissue morphology. Although significant progress has been made in understanding the mechanisms linking the cell cortex to the spindle and the well-documented role that extracellular forces play in spindle orientation, how such forces are transduced to the cortex remains poorly understood. Here we report that focal adhesion kinase (FAK) is necessary for correct spindle orientation and as a result, indispensable for proper epithelial morphogenesis in the vertebrate embryo. We show that FAK's role in spindle orientation is dependent on its ability to localize at focal adhesions and its interaction with paxillin, but is kinase activity independent. Finally, we present evidence that FAK is required for external force-induced spindle reorientation, suggesting that FAK's involvement in this process stems from a role in the transduction of external forces to the cell cortex.

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.

The Chlamydia effector TarP mimics the mammalian leucine-aspartic acid motif of paxillin to subvert the focal adhesion kinase during invasion

Host cell signal transduction pathways are often targets of bacterial pathogens, especially during the process of invasion when robust actin remodeling is required. We demonstrate that the host cell focal adhesion kinase (FAK) was necessary for the invasion by the obligate intracellular pathogen Chlamydia caviae. Bacterial adhesion triggered the transient recruitment of FAK to the plasma membrane to mediate a Cdc42- and Arp2/3-dependent actin assembly. FAK recruitment was via binding to a domain within the virulence factor TarP that mimicked the LD2 motif of the FAK binding partner paxillin. Importantly, bacterial two-hybrid and quantitative imaging assays revealed a similar level of interaction between paxillin-LD2 and TarP-LD. The conserved leucine residues within the L(D/E)XLLXXL motif were essential to the recruitment of FAK, Cdc42, p34^Arc, and actin to the plasma membrane. In the absence of FAK, TarP-LD-mediated F-actin assembly was reduced, highlighting the functional relevance of this interaction. Together, the data indicate that a prokaryotic version of the paxillin LD2 domain targets the FAK signaling pathway, with TarP representing the first example of an LD-containing Type III virulence effector.

Structural and mechanistic insights into the interaction between Pyk2 and paxillin LD motifs

Proline-rich tyrosine kinase 2 (Pyk2) is a member of the focal adhesion kinase (FAK) subfamily of cytoplasmic tyrosine kinases. The C-terminal Pyk2-focal adhesion targeting (FAT) domain binds to paxillin, an adhesion molecule. Paxillin has five leucine-aspartate (LD) motifs (LD1-LD5). Here, we show that the second LD motif of paxillin, LD2, interacts with Pyk2-FAT, similar to the known Pyk2-FAT/LD4 interaction. Both LD motifs can target two ligand binding sites on Pyk2-FAT. Interestingly, they also share similar binding affinity for Pyk2-FAT with preferential association to one site relative to the other. Nevertheless, the LD2-LD4 region of paxillin (paxillin(133-290)) binds to Pyk2-FAT as a 1:1 complex. However, our data suggest that the Pyk2-FAT and paxillin complex is dynamic and it appears to be a mixture of two distinct conformations of paxillin that almost equally compete for Pyk2-FAT binding. These studies provide insight into the underlying selectivity of paxillin for Pyk2 and FAK that may influence the differing behavior of these two closely related kinases in focal adhesion sites.

How to find a leucine in a haystack? Structure, ligand recognition and regulation of leucine-aspartic acid (LD) motifs

LD motifs (leucine-aspartic acid motifs) are short helical protein-protein interaction motifs that have emerged as key players in connecting cell adhesion with cell motility and survival. LD motifs are required for embryogenesis, wound healing and the evolution of multicellularity. LD motifs also play roles in disease, such as in cancer metastasis or viral infection. First described in the paxillin family of scaffolding proteins, LD motifs and similar acidic LXXLL interaction motifs have been discovered in several other proteins, whereas 16 proteins have been reported to contain LDBDs (LD motif-binding domains). Collectively, structural and functional analyses have revealed a surprising multivalency in LD motif interactions and a wide diversity in LDBD architectures. In the present review, we summarize the molecular basis for function, regulation and selectivity of LD motif interactions that has emerged from more than a decade of research. This overview highlights the intricate multi-level regulation and the inherently noisy and heterogeneous nature of signalling through short protein-protein interaction motifs.

Targeting the C-terminal focal adhesion kinase scaffold in pancreatic cancer

Preliminary studies in our laboratory have demonstrated the importance of both the NH2 and COOH terminus scaffolding functions of focal adhesion kinase (FAK). Here, we describe a new small molecule inhibitor, C10, that targets the FAK C-terminus scaffold. C10 showed marked selectivity for cells overexpressing VEGFR3 when tested in isogenic cell lines, MCF7 and MCF7-VEGFR3. C10 preferentially inhibited pancreatic tumor growth in vivo in cells with high FAK-Y925 and VEGFR3 expression. Treatment with C10 led to a significant inhibition in endothelial cell proliferation and tumor endothelial and lymphatic vessel density and decrease in interstitial fluid pressure. These results highlight the underlying importance of targeting the FAK scaffold to treat human cancers.

Focal adhesion kinase (FAK) perspectives in mechanobiology: implications for cell behaviour

Mechanobiology is a scientific interface discipline emerging from engineering and biology. With regard to tissue-regenerative cell-based strategies, mechanobiological concepts, including biomechanics as a target for cell and human mesenchymal stem cell behaviour, are on the march. Based on the periodontium as a paradigm, this mini-review discusses the key role of focal-adhesion kinase (FAK) in mechanobiology, since it is involved in mediating the transformation of environmental biomechanical signals into cell behavioural responses via mechanotransducing signalling cascades. These processes enable cells to adjust quickly to environmental cues, whereas adjustment itself relies on the specific intramolecular phosphorylation of FAK tyrosine residues and the multiple interactions of FAK with distinct partners. Furthermore, interaction-triggered mechanotransducing pathways govern the dynamics of focal adhesion sites and cell behaviour. Facets of behaviour not only include cell spreading and motility, but also proliferation, differentiation and apoptosis. In translational terms, identified and characterized biomechanical parameters can be incorporated into innovative concepts of cell- and tissue-tailored clinically applied biomaterials controlling cell behaviour as desired.

GxE interactions between FOXO genotypes and drinking tea are significantly associated with prevention of cognitive decline in advanced age in China

Logistic regression analysis based on data from 822 Han Chinese oldest old aged 92+ demonstrated that interactions between carrying FOXO1A-266 or FOXO3-310 or FOXO3-292 and tea drinking at around age 60 or at present time were significantly associated with lower risk of cognitive disability at advanced ages. Associations between tea drinking and reduced cognitive disability were much stronger among carriers of the genotypes of FOXO1A-266 or FOXO3-310 or FOXO3-292 compared with noncarriers, and it was reconfirmed by analysis of three-way interactions across FOXO genotypes, tea drinking at around age 60, and at present time. Based on prior findings from animal and human cell models, we postulate that intake of tea compounds may activate FOXO gene expression, which in turn may positively affect cognitive function in the oldest old population. Our empirical findings imply that the health benefits of particular nutritional interventions, including tea drinking, may, in part, depend upon individual genetic profiles.

Paxillin and focal adhesion kinase colocalise in human skeletal muscle and its associated microvasculature

Focal adhesion kinase (FAK) and paxillin are functionally linked hormonal- and mechano-sensitive proteins. We aimed to describe paxillin's subcellular distribution using widefield and confocal immunofluorescence microscopy and test the hypothesis that FAK and paxillin colocalise in human skeletal muscle and its associated microvasculature. Percutaneous muscle biopsies were collected from the m. vastus lateralis of seven healthy males, and 5-μm cryosections were stained with anti-paxillin co-incubated with anti-dystrophin to identify the sarcolemma, anti-myosin heavy chain type I for fibre-type differentiation, anti-dihydropyridine receptor to identify T-tubules, lectin UEA-I to identify the endothelium of microvessels and anti-α-smooth muscle actin to identify vascular smooth muscle cells (VSMC). Colocalisation of anti-paxillin with anti-dystrophin or anti-FAK was quantified using Pearson's correlation coefficient on confocal microscopy images. Paxillin was primarily present in (sub)sarcolemmal regions of skeletal muscle fibres where it colocalised with dystrophin (r = 0.414 ± 0.026). The (sub)sarcolemmal paxillin immunofluorescence intensity was ~2.4-fold higher than in sarcoplasmic regions (P < 0.001) with sarcoplasmic paxillin immunofluorescence intensity ~10 % higher in type I than in type II fibres (P < 0.01). In some longitudinally orientated fibres, paxillin formed striations that corresponded to the I-band region. Paxillin immunostaining was highest in endothelial and VSMC and distributed heterogeneously in both cell types. FAK and paxillin colocalised at (sub)sarcolemmal regions and within the microvasculature (r = 0.367 ± 0.036). The first images of paxillin in human skeletal muscle suggest paxillin is present in (sub)sarcolemmal and I-band regions of muscle fibres and within the microvascular endothelium and VSMC. Colocalisation of FAK and paxillin supports their suggested role in hormonal and mechano-sensitive signalling.

Altering FAK-paxillin interactions reduces adhesion, migration and invasion processes

Focal adhesion kinase (FAK) plays an important role in signal transduction pathways initiated at sites of integrin-mediated cell adhesion to the extracellular matrix. Thus, FAK is involved in many aspects of the metastatic process including adhesion, migration and invasion. Recently, several small molecule inhibitors which target FAK catalytic activity have been developed by pharmaceutical companies. The current study was aimed at addressing whether inhibiting FAK targeting to focal adhesions (FA) represents an efficient alternative strategy to inhibit FAK downstream pathways. Using a mutagenesis approach to alter the targeting domain of FAK, we constructed a FAK mutant that fails to bind paxillin. Inhibiting FAK-paxillin interactions led to a complete loss of FAK localization at FAs together with reduced phosphorylation of FAK and FAK targets such as paxillin and p130Cas. This in turn resulted in altered FA dynamics and inhibition of cell adhesion, migration and invasion. Moreover, the migration properties of cells expressing the FAK mutant were reduced as compared to FAK^-/- cells. This was correlated with a decrease in both phospho-Src and phospho-p130Cas levels at FAs. We conclude that targeting FAK-paxillin interactions is an efficient strategy to reduce FAK signalling and thus may represent a target for the development of new FAK inhibitors.

Grb2 promotes integrin-induced focal adhesion kinase (FAK) autophosphorylation and directs the phosphorylation of protein tyrosine phosphatase α by the Src-FAK kinase complex

The integrin-activated Src-focal adhesion kinase (FAK) kinase complex phosphorylates PTPα at Tyr789, initiating PTPα-mediated signaling that promotes cell migration. Recruitment of the BCAR3-Cas complex by PTPα-phospho-Tyr789 at focal adhesions is one mechanism of PTPα signaling. The adaptor protein Grb2 is also recruited by PTPα-phospho-Tyr789, although the role of the PTPα-Grb2 complex in integrin signaling is unknown. We show that silencing Grb2 expression in fibroblasts abolishes PTPα-Tyr789 phosphorylation and that this is due to two unexpected actions of Grb2. First, Grb2 promotes integrin-induced autophosphorylation of FAK-Tyr397. This is impaired in Grb2-depleted cells and prohibits FAK activation and formation of the Src-FAK complex. Grb2-depleted cells contain less paxillin, and paxillin overexpression rescues FAK-Tyr397 phosphorylation, suggesting that the FAK-activating action of Grb2 involves paxillin. A second distinct role for Grb2 in PTPα-Tyr789 phosphorylation involves Grb2-mediated coupling of Src-FAK and PTPα. This requires two phosphosites, FAK-Tyr925 and PTPα-Tyr789, for Grb2-Src homology 2 (SH2) binding. We propose that a Grb2 dimer links FAK and PTPα, and this positions active Src-FAK in proximity with other, perhaps integrin-clustered, molecules of PTPα to enable maximal PTPα-Tyr789 phosphorylation. These findings identify Grb2 as a new FAK activator and reveal its essential role in coordinating PTPα tyrosine phosphorylation to enable downstream integrin signaling and migration.

Integrins on eggs: focal adhesion kinase is activated at fertilization, forms a complex with integrins, and is necessary for cortex formation and cell cycle initiation

We investigate the proposal that integrins and focal adhesion kinase (FAK) form a complex that has structural and signaling functions in eggs. FAK protein is present in eggs and is phosphorylated at fertilization. pY(397)FAK localizes to the membrane 30 min after fertilization, which correlates with the expression of βC integrins and egg cortex development. The βC integrin and pY(397)FAK coimmunoprecipitate from egg cortex lysates. PF573 228 and Y11, inhibitors of FAK, interfere with pronuclear fusion and reduce the abundance of pY(397)FAK and cortical actin without affecting microvillar actin. Cyclin E normally accumulates in the nucleus 15 min after fertilization, then returns to background levels. PF573 228- or Y11-treated eggs accumulate cyclin E in the nucleus; however, levels remain high. In addition, PF573 228 interferes with the accumulation of pERK1/2 in the nucleus and in eggs initiating mitosis. Injection of eggs with a fusion protein consisting of the focal adhesion-targeting domain of FAK fused to green fluorescent protein interferes with cortex formation and produces abnormal nuclei. These data indicate that an integrin-FAK adhesion complex forms at the egg surface that functions in formation of actin arrays in the egg cortex and provides signaling inputs for cell cycle initiation.

Dynamics of focal adhesions and reorganization of F-actin in VEGF-stimulated NSCs under varying differentiation states

Precise migration of neural stem/progenitor cells (NSCs) is crucially important for neurogenesis and repair in the nervous system. However, the detailed mechanisms are not clear. Our previous results showed that NSCs in varying differentiation states possess different migratory ability to vascular endothelial growth factor (VEGF). In this study, we demonstrate the different dynamics of focal adhesions (FAs) and reorganization of F-actin in NSCs during spreading and migration stimulated by VEGF. We found that the migrating NSCs of 0.5 and 1 day differentiation possess more FAs at leading edge than cells of other states. Moreover, the phosphorylation of focal adhesion kinase (FAK) and paxillin in NSCs correlates closely with their differentiation states. VEGF promotes FA formation with broad lamellipodium generation at the leading edge in chemotaxing cells of 0, 0.5, and 1 day differentiation, but not in cells of 3 days differentiation. Furthermore, cells of 1 day differentiation show a maximal asymmetry of FAs between lamella and cell rear, orchestrating cell polarization and directional migration. Time-lapse video analysis shows that the disassembly of FAs and the cell tail detachment in NSCs of 1 day differentiation are more rapid, along with the concurrent enlarged size of FAs at the leading edge, leading to the most effective chemotactic response to VEGF. Collectively, these results indicate that the dynamics of FAs and reorganization of F-actin in NSCs that undergo directional migration correlate closely with their differentiation states, contributing to the different chemotactic responses of these cells to VEGF.

A non-canonical role for Rgnef in promoting integrin-stimulated focal adhesion kinase activation

Rgnef (also known as p190RhoGEF or ARHGEF28) is a Rho guanine-nucleotide-exchange factor (GEF) that binds focal adhesion kinase (FAK). FAK is recruited to adhesions and activated by integrin receptors binding to matrix proteins, such as fibronectin (FN). Canonical models place Rgnef downstream of integrin-FAK signaling in regulating Rho GTPase activity and cell movement. Herein, we establish a new, upstream role for Rgnef in enhancing FAK localization to early peripheral adhesions and promoting FAK activation upon FN binding. Rgnef-null mouse embryo fibroblasts (MEFs) exhibit defects in adhesion formation, levels of FAK phosphotyrosine (pY)-397 and FAK localization to peripheral adhesions upon re-plating on FN. Rgnef re-expression rescues these defects, but requires Rgnef-FAK binding. A mutation in the Rgnef pleckstrin homology (PH) domain inhibits adhesion formation, FAK localization, and FAK-Y397 and paxillin-Y118 phosphorylation without disrupting the Rgnef-FAK interaction. A GEF-inactive Rgnef mutant rescues FAK-Y397 phosphorylation and early adhesion localization, but not paxillin-Y118 phosphorylation. This suggests that, downstream of FN binding, paxillin-pY118 requires Rgnef GEF activity through a mechanism distinct from adhesion formation and FAK activation. These results support a scaffolding role for Rgnef in FAK localization and activation at early adhesions in a PH-domain-dependent but GEF-activity-independent manner.

LKB1 represses focal adhesion kinase (FAK) signaling via a FAK-LKB1 complex to regulate FAK site maturation and directional persistence

Liver kinase β1 (LKB1, also known as STK11) is a serine/threonine kinase that has multiple cellular functions including the regulation of cell polarity and motility. Murine proteomic studies show that LKB1 loss causes aberrant adhesion signaling; however, the mechanistic underpinnings of this relationship are unknown. We show that cells stably depleted of LKB1 or its co-activator STRADα have increased phosphorylation of focal adhesion kinase (FAK) at Tyr(397)/Tyr(861) and enhanced adhesion to fibronectin. LKB1 associates in a complex with FAK and LKB1 accumulation at the cellular leading edge is mutually excluded from regions of activated Tyr(397)-FAK. LKB1-compromised cells lack directional persistence compared with wild-type cells, but this is restored through subsequent pharmacological FAK inhibition or depletion, showing that cell directionality is mediated through LKB1-FAK signaling. Live cell confocal imaging reveals that LKB1-compromised cells lack normal FAK site maturation and turnover, suggesting that defects in adhesion and directional persistence are caused by aberrant adhesion dynamics. Furthermore, re-expression of full-length wild-type or the LKB1 N-terminal domain repressed FAK activity, whereas the kinase domain or C-terminal domain alone did not, indicating that FAK suppression is potentially regulated through the LKB1 N-terminal domain. Based upon these results, we conclude that LKB1 serves as a FAK repressor to stabilize focal adhesion sites, and when LKB1 function is compromised, aberrant FAK signaling ensues, resulting in rapid FAK site maturation and poor directional persistence.

SiRNA-mediated silencing of paxillin down-regulates ERK1/2 signaling and alters cell ultrastructure in colorectal carcinoma cell line SW480

AIM: To study the effect of silencing of paxillin overexpression on cell signaling and ultrastructure in colorectal carcinoma cell line SW480.

METHODS: Using empty plasmid as a negative control, two siRNA fragments were transfected into a colorectal carcinoma cell line SW480 which overexpresses paxillin. Stably transfected cells were screened and three new cell lines NC, SW545 and SW782 were obtained, which carried the negative control, the siRNA targeting the site 545-565, and the siRNA targeting the site 782-802, respectively. The expression and site-specific phosphorylation of paxillin, FAK, ERK1/2 and AKT1/2/3 were examined in the four cell lines by Western blot. Specimens were prepared with cultured carcinoma cells to observe cell ultrastructure by transmission electron microscopy.

RESULTS: Paxillin overexpression in SW545 cells was not silenced at all, whereas silenced paxillin overexpression and remarkably reduced phosphorylation of paxillin (Tyr118) were observed in SW782 cells. Expression of AKT1/2/3 and FAK as well as their site-specific phosphorylation were substantially the same in the four cell lines. Although expression of ERK1/2 was substantially the same in the four cell lines, significantly reduced phosphorylation of ERK1/2 (Thr202/Tyr204) was observed in SW782 cells. There was no distinct ultrastructural difference between NC cells and SW480 cells, whereas dramatic ultrastructural changes were observed in SW782 cells, such as much more microvilli, microfilament and microtubule bundles, lysosomes and much less mitochondria.

CONCLUSION: Paxillin overexpression may play an important role in the malignant transformation of colorectal carcinoma cells, which is characterized by dramatic ultrastructural changes that can be reversed by silencing paxillin overexpression. Activation of ERK1/2 signaling downstream of paxillin is indispensable for the malignant transformation of colorectal carcinoma cells.

Visualizing and manipulating focal adhesion kinase regulation in live cells

Focal Adhesion Kinase (FAK) is essential for cell migration and plays an important role in tumor metastasis. However, the complex intermolecular and intramolecular interactions that regulate FAK activity at the focal adhesion remain unresolved. We have engineered a toolbox of FRET sensors that retain all of the individual FAK domains but modulate a key intramolecular regulatory interaction between the band 4.1/ezrin/radixin/moesin (FERM) and kinase domains of FAK. We demonstrate systematic control and quantitative measurement of the FERM-kinase interaction at focal adhesions, which in turn allows us to control cell migration. Using these sensors, we find that Tyr-397 phosphorylation, rather than kinase activity of FAK, is the key determinant of cell migration. Our sensors directly demonstrate, for the first time, a pH-dependent change in a protein-protein interaction at a macromolecular structure in live cells. The FERM-kinase interaction at focal adhesions is enhanced at acidic pH, with a concomitant decrease in Tyr-397 phosphorylation, providing a potential mechanism for enhanced migration of cancer cells.

Expression of claudin, paxillin and FRA-1 in non-nodular breast lesions in association with microcalcifications

CONTEXT AND OBJECTIVE

The possible role of adhesion molecules in early breast carcinogenesis has been shown in the literature. We aimed to analyze early adhesion imbalances in non-nodular breast lesions and their association with precursor lesions, in order to ascertain whether these alterations exist and contribute towards early carcinogenesis.

DESIGN AND SETTING

Retrospective cross-sectional study based on medical records at a private radiological clinic in São Paulo, Brazil.

METHODS

We retrospectively reviewed the medical records of all consecutive women attended between August 2006 and July 2007 who presented mammographic evidence of breast microcalcifications classified as Breast Imaging Reporting and Data System Atlas (BI-RADS) type 4. These women underwent stereotaxic biopsy. Clinical, radiological and pathological data were collected, and immunohistochemical assays searched for claudin, paxillin, FRA-1 and HER-2.

RESULTS

Over this period, 127 patients were evaluated. Previous BI-RADS diagnoses showed that 69 cases were in category 4A, 47 in 4B and 11 in 4C. Morphological assessment showed benign entities in 86.5%. Most of the benign lesions showed preserved claudin expression, associated with paxillin (P < 0.001). Paxillin and HER-2 expressions were correlated. FRA-1 expression was also strongly associated with HER-2 expression (P < 0.001).

CONCLUSIONS

Although already present in smaller amounts, imbalance of adhesion molecules is not necessarily prevalent in non-nodular breast lesions. Since FRA-1 expression reached statistically significant correlations with radiological and morphological diagnoses and HER-2 status, it may have a predictive role in this setting.

SLK-mediated phosphorylation of paxillin is required for focal adhesion turnover and cell migration

Focal adhesion turnover is a complex process required for cell migration. We have previously shown that the Ste20-like kinase (SLK) is required for cell migration and efficient focal adhesion (FA) turnover in a FA kinase (FAK)-dependent manner. However, the role of SLK in this process remains unclear. Using a candidate substrate approach, we show that SLK phosphorylates the adhesion adapter protein paxillin on serine 250. Serine 250 phosphorylation is required for paxillin redistribution and cell motility. Mutation of paxillin serine 250 prevents its phosphorylation by SLK in vitro and results in impaired migration in vivo as evidenced by an accumulation of phospho-FAK-Tyr397 and altered FA turnover rates. Together, our data suggest that SLK phosphorylation of paxillin on serine 250 is required for FAK-dependent FA dynamics.

Structural basis for paxillin binding and focal adhesion targeting of β-parvin

β-Parvin is a cytoplasmic adaptor protein that localizes to focal adhesions where it interacts with integrin-linked kinase and is involved in linking integrin receptors to the cytoskeleton. It has been reported that despite high sequence similarity to α-parvin, β-parvin does not bind paxillin, suggesting distinct interactions and cellular functions for these two closely related parvins. Here, we reveal that β-parvin binds directly and specifically to leucine-aspartic acid repeat (LD) motifs in paxillin via its C-terminal calponin homology (CH2) domain. We present the co-crystal structure of β-parvin CH2 domain in complex with paxillin LD1 motif to 2.9 Å resolution and find that the interaction is similar to that previously observed between α-parvin and paxillin LD1. We also present crystal structures of unbound β-parvin CH2 domain at 2.1 Å and 2.0 Å resolution that show significant conformational flexibility in the N-terminal α-helix, suggesting an induced fit upon paxillin binding. We find that β-parvin has specificity for the LD1, LD2, and LD4 motifs of paxillin, with K(D) values determined to 27, 42, and 73 μM, respectively, by surface plasmon resonance. Furthermore, we show that proper localization of β-parvin to focal adhesions requires both the paxillin and integrin-linked kinase binding sites and that paxillin is important for early targeting of β-parvin. These studies provide the first molecular details of β-parvin binding to paxillin and help define the requirements for β-parvin localization to focal adhesions.

Integrin adhesions: who's on first? What's on second? Connections between FAK and talin

Cell migration requires the coordination of adhesion site assembly and turnover. Canonical models for nascent adhesion formation postulate that integrin binding to extracellular matrix (ECM) proteins results in the rapid recruitment of cytoskeletal proteins such as talin and paxillin to integrin cytoplasmic domains. It is thought that integrin-talin clusters recruit and activate tyrosine kinases such as focal adhesion kinase (FAK). However, the molecular connections of this linkage remain unresolved. Our recent findings support an alternative model whereby FAK recruits talin to new sites of β1 integrin-mediated adhesion in mouse embryonic fibroblasts and human ovarian carcinoma cells. This is dependent on a direct binding interaction between FAK and talin and occurs independently of direct talin binding to β1 integrin. Herein, we discuss differences between nascent and mature adhesions, interactions between FAK, talin and paxillin, possible mechanisms of FAK activation and how this FAK-talin complex may function to promote cell motility through increased adhesion turnover.

Multiplexed High Content Screening Reveals That Cigarette Smoke Condensate-Altered Cell Signaling Pathways Are Accentuated Through FAK Inhibition in Human Bronchial Cells

The mechanisms by which cigarette smoke condensate (CSC) disrupts F-actin and decreases cell motility in human bronchial (BEAS-2B) cells were assessed. The hypothesis that CSC activated focal adhesion kinase (FAK), mitogen-activated protein kinases (MAPKs), and paxillin in BEAS-2B cells was tested. When BEAS-2B cells were treated with 20 to 100 μg/mL CSC for 1 hour, FAK increased. The CSC caused F-actin disruption, while FAK inhibition alone caused actin aggregates to collapse to the cell periphery, but FAK inhibition combined with CSC caused actin aggregates to distribute throughout the cells. The CSC treatment of BEAS-2B cells showed a dose-dependent increase in the activation of the MAPKs, c-Jun, JNK, ERK, p38, and heat shock protein 27 (Hsp27) and paxillin. Focal adhesion kinase phosphorylation inhibition combined with CSC treatment increased p38 and ERK at 1 hour and 24 hours along with decreased cell number and motility compared with CSC treatment alone. CSC exerts changes in BEAS-2B cells by altering morphology and activating MAPK pathways.

FAK promotes recruitment of talin to nascent adhesions to control cell motility

An alternative linkage is shown whereby FAK brings talin to nascent adhesions independent of talin binding to β1 integrins.

Cell migration is a dynamic process that involves the continuous formation, maturation, and turnover of matrix–cell adhesion sites. New (nascent) adhesions form at the protruding cell edge in a tension-independent manner and are comprised of integrin receptors, signaling, and cytoskeletal-associated proteins. Integrins recruit focal adhesion kinase (FAK) and the cytoskeletal protein talin to nascent adhesions. Canonical models support a role for talin in mediating FAK localization and activation at adhesions. Here, alternatively, we show that FAK promotes talin recruitment to nascent adhesions occurring independently of talin binding to β1 integrins. The direct binding site for talin on FAK was identified, and a point mutation in FAK (E1015A) prevented talin association and talin localization to nascent adhesions but did not alter integrin-mediated FAK recruitment and activation at adhesions. Moreover, FAK E1015A inhibited cell motility and proteolytic talin cleavage needed for efficient adhesion dynamics. These results support an alternative linkage for FAK–talin interactions within nascent adhesions essential for the control of cell migration.

Focal adhesion kinase-related nonkinase inhibits vascular smooth muscle cell invasion by focal adhesion targeting, tyrosine 168 phosphorylation, and competition for p130(Cas) binding

OBJECTIVE

Focal adhesion kinase-related nonkinase (FRNK), the C-terminal domain of focal adhesion kinase (FAK), is a tyrosine-phosphorylated, vascular smooth muscle cell (VSMC)-specific inhibitor of cell migration. FRNK inhibits both FAK and proline-rich tyrosine kinase 2 (PYK2) in cultured VSMCs, and both kinases may be involved in VSMC invasion during vascular remodeling.

METHODS AND RESULTS

Adenovirally mediated gene transfer of green fluorescent protein-tagged, wild-type (wt) FRNK into balloon-injured rat carotid arteries confirmed that FRNK overexpression inhibited both FAK and PYK2 phosphorylation and downstream signaling in vivo. To identify which kinase was involved in regulating VSMC invasion, adenovirally mediated expression of specific short hairpin RNAs was used to knock down FAK versus PYK2 in cultured VSMCs, but only FAK short hairpin RNA was effective in reducing VSMC invasion. The role of FRNK tyrosine phosphorylation was then examined using adenoviruses expressing nonphosphorylatable (Tyr168Phe-, Tyr232Phe-, and Tyr168,232Phe-) green fluorescent protein-FRNK mutants. wtFRNK and all FRNK mutants localized to FAs, but only Tyr168 phosphorylation was required for FRNK to inhibit invasion. Preventing Tyr168 phosphorylation also increased FRNK-paxillin interaction, as determined by coimmunoprecipitation, total internal reflection fluorescence microscopy, and fluorescence recovery after photobleaching. Furthermore, wtFRNK competed with FAK for binding to p130(Cas) (a critically important regulator of cell migration) and prevented its phosphorylation. However, Tyr168Phe-FRNK was unable to bind p130(Cas).

CONCLUSION

We propose a 3-stage mechanism for FRNK inhibition: focal adhesion targeting, Tyr168 phosphorylation, and competition with FAK for p130 binding and phosphorylation, which are all required for FRNK to inhibit VSMC invasion.