Paxillin modulates squamous cancer cell adhesion and is important in pressure-augmented adhesion

The role of Actopaxin in tumor metastasis

Actopaxin is a newly discovered focal adhesions (FAs) protein, actin-binding protein and pseudopodia-enriched molecule. It can not only bind to a variety of FAs proteins (such as Paxillin, ILK and PINCH) and non-FAs proteins (such as TESK1, CdGAP, β2-adaptin, G3BP2, ADAR1 and CD29), but also participates in multiple signaling pathways. Thus, it plays a crucial role in regulating important processes of tumor metastasis, including matrix degradation, migration, and invasion, etc. This review covers the latest progress in the structure and function of Actopaxin, its interaction with other proteins as well as its involvement in regulating tumor development and metastasis. Additionally, the current limitations for Actopaxin related studies and the possible research directions on it in the future are also discussed. It is hoped that this review can assist relevant researchers to obtain a deep understanding of the role that Actopaxin plays in tumor progression, and also enlighten further research and development of therapeutic approaches for the treatment of tumor metastasis.

ZINC40099027 promotes monolayer circular defect closure by a novel pathway involving cytosolic activation of focal adhesion kinase and downstream paxillin and ERK1/2

ZINC40099027 (ZN27) is a specific focal adhesion kinase (FAK) activator that promotes murine mucosal wound closure after ischemic or NSAID-induced injury. Diverse motogenic pathways involve FAK, but the direct consequences of pure FAK activation have not been studied, and how ZN27-induced FAK activation stimulates wound closure remained unclear. We investigated signaling and focal adhesion (FA) turnover after FAK activation by ZN27 in Caco-2 cells, confirming key results in CCD841 cells. ZN27 increased Caco-2 FAK-Y-397, FAK-Y-576/7, paxillin-Y-118, and ERK 1/2 phosphorylation and decreased FAK-Y-925 phosphorylation, without altering FAK-Y-861, p38, Jnk, or Akt phosphorylation. ZN27 increased FAK-paxillin interaction while decreasing FAK-Grb2 association. ZN27 increased membrane-associated FAK-Y-397 and FAK-Y-576/7 phosphorylation and paxillin-Y-118 and ERK 1/2 phosphorylation but decreased FAK-Y-925 phosphorylation without altering Src or Grb2. Moreover, ZN27 increased the fluorescence intensity of GFP-FAK and pFAK-Y397 in FAs and increased the total number of FAs but reduced their size in GFP-FAK-transfected Caco-2 cells, consistent with increased FA turnover. In contrast, FAK-Y397F transfection prevented ZN27 effects on FAK size and number and FAK and pFAK fluorescent intensity in FAs. We confirmed the proposed FAK/paxillin/ERK pathway using PP2 and U0126 to block Src and MEK1/2 in Caco-2 and CCD841 cells. These results suggest that ZN27 promotes intestinal epithelial monolayer defect closure by stimulating autophosphorylation of FAK in the cytosol, distinct from classical models of FAK activation in the FA. Phosphorylated FAK translocates to the membrane, where its downstream substrates paxillin and ERK are phosphorylated, leading to FA turnover and human intestinal epithelial cell migration.

Gut homeostasis, injury, and healing: New therapeutic targets

The integrity of the gastrointestinal mucosa plays a crucial role in gut homeostasis, which depends upon the balance between mucosal injury by destructive factors and healing via protective factors. The persistence of noxious agents such as acid, pepsin, nonsteroidal anti-inflammatory drugs, or Helicobacter pylori breaks down the mucosal barrier and injury occurs. Depending upon the size and site of the wound, it is healed by complex and overlapping processes involving membrane resealing, cell spreading, purse-string contraction, restitution, differentiation, angiogenesis, and vasculogenesis, each modulated by extracellular regulators. Unfortunately, the gut does not always heal, leading to such pathology as peptic ulcers or inflammatory bowel disease. Currently available therapeutics such as proton pump inhibitors, histamine-2 receptor antagonists, sucralfate, 5-aminosalicylate, antibiotics, corticosteroids, and immunosuppressants all attempt to minimize or reduce injury to the gastrointestinal tract. More recent studies have focused on improving mucosal defense or directly promoting mucosal repair. Many investigations have sought to enhance mucosal defense by stimulating mucus secretion, mucosal blood flow, or tight junction function. Conversely, new attempts to directly promote mucosal repair target proteins that modulate cytoskeleton dynamics such as tubulin, talin, Ehm², filamin-a, gelsolin, and flightless I or that proteins regulate focal adhesions dynamics such as focal adhesion kinase. This article summarizes the pathobiology of gastrointestinal mucosal healing and reviews potential new therapeutic targets.

ZINC40099027 activates human focal adhesion kinase by accelerating the enzymatic activity of the FAK kinase domain

Focal adhesion kinase (FAK) regulates gastrointestinal epithelial restitution and healing. ZINC40099027 (Zn27) activates cellular FAK and promotes intestinal epithelial wound closure in vitro and in mice. However, whether Zn27 activates FAK directly or indirectly remains unknown. We evaluated Zn27 potential modulation of the key phosphatases, PTP-PEST, PTP1B, and SHP2, that inactivate FAK, and performed in vitro kinase assays with purified FAK to assess direct Zn27-FAK interaction. In human Caco-2 cells, Zn27-stimulated FAK-Tyr-397 phosphorylation despite PTP-PEST inhibition and did not affect PTP1B-FAK interaction or SHP2 activity. Conversely, in vitro kinase assays demonstrated that Zn27 directly activates both full-length 125 kDa FAK and its 35 kDa kinase domain. The ATP-competitive FAK inhibitor PF573228 reduced basal and ZN27-stimulated FAK phosphorylation in Caco-2 cells, but Zn27 increased FAK phosphorylation even in cells treated with PF573228. Increasing PF573228 concentrations completely prevented activation of 35 kDa FAK in vitro by a normally effective Zn27 concentration. Conversely, increasing Zn27 concentrations dose-dependently activated kinase activity and overcame PF573228 inhibition of FAK, suggesting the direct interactions of Zn27 with FAK may be competitive. Zn27 increased the maximal activity (Vmax ) of FAK. The apparent Km of the substrate also increased under laboratory conditions less relevant to intracellular ATP concentrations. These results suggest that Zn27 is highly potent and enhances FAK activity via allosteric interaction with the FAK kinase domain to increase the Vmax of FAK for ATP. Understanding Zn27 enhancement of FAK activity will be important to redesign and develop a clinical drug that can promote mucosal wound healing.

Small molecule FAK activator promotes human intestinal epithelial monolayer wound closure and mouse ulcer healing

GI mucosal healing requires epithelial sheet migration. The non-receptor tyrosine kinase focal adhesion kinase (FAK) stimulates epithelial motility. A virtual screen identified the small drug-like FAK mimic ZINC40099027, which activates FAK. We assessed whether ZINC40099027 promotes FAK-Tyr-397 phosphorylation and wound healing in Caco-2 monolayers and two mouse intestinal injury models. Murine small bowel ulcers were generated by topical serosal acetic acid or subcutaneous indomethacin in C57BL/6J mice. One day later, we began treatment with ZINC40099027 or DMSO, staining the mucosa for phosphorylated FAK and Ki-67 and measuring mucosal ulcer area, serum creatinine, ALT, and body weight at day 4. ZINC40099027 (10–1000 nM) dose-dependently activated FAK phosphorylation, without activating Pyk2-Tyr-402 or Src-Tyr-419. ZINC40099027 did not stimulate proliferation, and stimulated wound closure independently of proliferation. The FAK inhibitor PF-573228 prevented ZINC40099027-stimulated wound closure. In both mouse ulcer models, ZINC40099027accelerated mucosal wound healing. FAK phosphorylation was increased in jejunal epithelium at the ulcer edge, and Ki-67 staining was unchanged in jejunal mucosa. ZINC40099027 serum concentration at sacrifice resembled the effective concentration in vitro. Weight, creatinine and ALT did not differ between groups. Small molecule FAK activators can specifically promote epithelial restitution and mucosal healing and may be useful to treat gut mucosal injury.

ZINC4085554 inhibits cancer cell adhesion by interfering with the interaction of Akt1 and FAK

Perioperative or circulatory forces enhance disseminated cancer cell adhesiveness by modulating focal adhesion kinase (FAK)-Akt1 interaction. Selectively blocking FAK-Akt1 interaction by a peptide derived from the FAK-Four-point-one, ezrin, radixin, moesin (FERM) domain reduces colon cancer cell adhesion in vitro and in mice. A preliminary in silico screening identified two small molecules resembling a peptide that may inhibit pressure-stimulated SW620 cancer cell adhesion to collagen I. The present study selected ZINC4085554 for further study to validate its proposed mechanism of action, using human SW620 colon cancer cells as a model system. At 25 and 50 µM, ZINC4085554 inhibited the pressure-stimulated adhesion of SW620 colon cancer cells to collagen I. This molecule prevented pressure-stimulated FAK-Tyr-397 phosphorylation; however, it did not affect Akt1-Ser-473 phosphorylation, indicating that ZINC4085554 acts downstream of Akt1, while Akt-Thr-308 remains unchanged in the presence of pressure and or ZINC4085554. Indeed, ZINC4085554 inhibited FAK-Akt1 interaction in response to increased extracellular pressure, consistent with the proposed mechanism. ZINC4085554 did not inhibit FAK-Tyr-397 phosphorylation in response to cell adhesion to collagen I, indicating the specificity of the inhibitory effects towards force-stimulated pathways. Finally, the present study confirmed that ZINC4085554 at 50 µM prevented pressure-activation of adhesion to surgical wounds in vivo in parallel to its ablation of intracellular signaling. In summary, ZINC4085554 is a small molecule mimicking part of the structure of FAK that reduces cancer cell adhesion by impairing pressure-stimulated FAK-Akt1 interaction and its downstream consequences. ZINC4085554 does not inhibit conventional outside-in FAK signaling and may be less toxic than global FAK inhibitors, and ZINC4085554 may be an important step towards the inhibition of metastasis.

Optimization of Protein-Protein Interaction Measurements for Drug Discovery Using AFM Force Spectroscopy

Increasingly targeted in drug discovery, protein-protein interactions challenge current high throughput screening technologies in the pharmaceutical industry. Developing an effective and efficient method for screening small molecules or compounds is critical to accelerate the discovery of ligands for enzymes, receptors and other pharmaceutical targets. Here, we report developments of methods to increase the signal-to-noise ratio (SNR) for screening protein-protein interactions using atomic force microscopy (AFM) force spectroscopy. We have demonstrated the effectiveness of these developments on detecting the binding process between focal adhesion kinases (FAK) with protein kinase B (Akt1), which is a target for potential cancer drugs. These developments include optimized probe and substrate functionalization processes and redesigned probe-substrate contact regimes. Furthermore, a statistical-based data processing method was developed to enhance the contrast of the experimental data. Collectively, these results demonstrate the potential of the AFM force spectroscopy in automating drug screening with high throughput.

Inhibition of pressure-activated cancer cell adhesion by FAK-derived peptides

Forces within the surgical milieu or circulation activate cancer cell adhesion and potentiate metastasis through signaling requiring FAK-Akt1 interaction. Impeding FAK-Akt1 interaction might inhibit perioperative tumor dissemination, facilitating curative cancer surgery without global FAK or AKT inhibitor toxicity. Serial truncation and structurally designed mutants of FAK identified a seven amino acid, short helical structure within FAK that effectively competes with Akt1-FAK interaction. Adenoviral overexpression of this FAK-derived peptide inhibited pressure-induced FAK phosphorylation and AKT-FAK coimmunoprecipitation in human SW620 colon cancer cells briefly exposed to 15mmHg increased pressure, consistent with laparoscopic or post-surgical pressures. Adenoviral FAK-derived peptide expression prevented pressure-activation of SW620 adhesion not only to collagen-I-coated plates but also to murine surgical wounds. A scrambled peptide did not. Finally, we modeled operative shedding of tumor cells before irrigation and closure by transient cancer cell adhesion to murine surgical wounds before irrigation and closure. Thirty minute preincubation of SW620 cells at 15mmHg increased pressure impaired subsequent tumor free survival in mice exposed to cells expressing the scrambled peptide. The FAK-derived sequence prevented this. These results suggest that blocking FAK-Akt1 interaction may prevent perioperative tumor dissemination and that analogs or mimics of this 7 amino acid FAK-derived peptide could impair metastasis.

Paxillin promotes tumor progression and predicts survival and relapse in oral cavity squamous cell carcinoma by microRNA-218 targeting

High-risk human papillomavirus (HPV) 16-infected oral cavity squamous cell carcinoma (OCSCC) differs significantly from non-HPV-infected OCSCC. However, the molecular pathogenesis of HPV-infected OCSCC remains unclear. Paxillin (PXN) has been reported to promote lung tumor progression by miR-218 targeting. In addition, expression of miR-218 has been shown to be reduced by HPV16 E6 in cervical cancer. We thus asked whether PXN can promote tumor progression by E6-reduced miR-218 in OCSCC, especially in HPV-infected OCSCC. Mechanistic studies demonstrated that PXN expression increased markedly upon E6-mediated reductions in miR-218, resulting in increased colony formation and invasion capabilities in HPV-infected OCSCC cells. Among tumor specimens, HPV16/18 infection was negatively associated with miR-218 expression and positively associated with PXN expression. Kaplan-Meier and Cox regression models demonstrated that patients with low-miR-218 tumors or high-PXN tumors exhibited shorter overall survival (OS) and relapse-free survival (RFS) than those with high-miR-218 tumors or low-PXN tumors. Interestingly, HPV-infected patients with low-miR-218, high-PXN tumors and both combinations exhibited the worst OS and RFS compared with patients in their counterparts. These observations in patients were consistent with the findings from the cell model. Therefore, we suggest that PXN might be targeted to suppress tumor progression and consequently to improve outcomes in OCSCC, especially in HPV-infected OCSCC.

The role of focal adhesion kinase in the regulation of cellular mechanical properties

The regulation of mechanical properties is necessary for cell invasion into connective tissue or intra- and extravasation through the endothelium of blood or lymph vessels. Cell invasion is important for the regulation of many healthy processes such as immune response reactions and wound healing. In addition, cell invasion plays a role in disease-related processes such as tumor metastasis and autoimmune responses. Until now the role of focal adhesion kinase (FAK) in regulating mechanical properties of cells and its impact on cell invasion efficiency is still not well known. Thus, this review focuses on mechanical properties regulated by FAK in comparison to the mechano-regulating protein vinculin. Moreover, it points out the connection between cancer cell invasion and metastasis and FAK by showing that FAK regulates cellular mechanical properties required for cellular motility. Furthermore, it sheds light on the indirect interaction of FAK with vinculin by binding to paxillin, which then impairs the binding of paxillin to vinculin. In addition, this review emphasizes whether FAK fulfills regulatory functions similar to vinculin. In particular, it discusses the differences and the similarities between FAK and vinculin in regulating the biomechanical properties of cells. Finally, this paper highlights that both focal adhesion proteins, vinculin and FAK, synergize their functions to regulate the mechanical properties of cells such as stiffness and contractile forces. Subsequently, these mechanical properties determine cellular invasiveness into tissues and provide a source sink for future drug developments to inhibit excessive cell invasion and hence, metastases formation.

Fascin-1, ezrin and paxillin contribute to the malignant progression and are predictors of clinical prognosis in laryngeal squamous cell carcinoma

Aims

Fascin-1, ezrin and paxillin, cytoskeleton-associated proteins, have been implicated in several human cancers, but their role in laryngeal squamous cell carcinoma (LSCC) is unknown. We investigated the association of their expression and clinicopathologic factors and their prognostic value in LSCC.

Materials and Methods

Quantitative RT-PCR and western blot analyses were used to examine mRNA and protein levels in 10 fresh LSCC specimens and 10 corresponding adjacent normal margin (ANM) tissues from patients undergoing surgery in 2012. We used immunohistochemistry to retrospectively study 216 paraffin blocks of LSCC samples from patients (193 men) who had undergone surgery between 2000 and 2006 and had not received special treatment before the diagnosis. Univariate analysis of patient survival involved the Kaplan–Meier method. Multivariate analyses involved the Cox proportional hazards model.

Results

The relative mRNA and protein levels of fascin-1, ezrin and paxillin were significantly greater in LSCC than ANM tissue (P<0.05). The high expression of fascin-1, ezrin or paxillin was positively correlated with poor tumor differentiation, cervical lymph node metastasis (N+), and advanced clinical stage (III+IV) (P<0.05) but not sex or metastasis. In addition, a high expression of fascin-1 (P = 0.007) or ezrin (P = 0.047) was associated with advanced tumor stage (T3+T4). The expression of fascin-1 was higher in smokers than non-smokers (P = 0.019). A high expression of fascin-1, ezrin or paxillin was associated with poor prognosis.

Conclusions

Fascin-1, ezrin and paxillin may be prognostic of poor outcome with LSCC after surgery. Our study may lead to establishing new molecular therapeutic targets and/or prognostic biomarkers in LSCC.

Extracellular pressure stimulates adhesion of sarcoma cells via activation of focal adhesion kinase and Akt

BACKGROUND

The effect of extracellular pressure on adhesion and adhesiogenic focal adhesion kinase (FAK) and Akt signaling in sarcomas was investigated.

METHODS

Human sarcoma cells (HT-1080 fibrosarcoma, KHOS-240S osteosarcoma, and A-673 rhabdomyosarcoma) were subjected to increased pressure followed by adhesion assay. Two cell lines were pretreated with the FAK inhibitor 1,2,4,5-benzenetetraamine tetrahydrochloride (Y15) or Akt IV inhibitor, followed by Western analysis for activated FAK and Akt. Parallel studies were conducted in cells from a resected human fibrous histiosarcoma.

RESULTS

Pressure increased adhesion in all 3 sarcoma lines and primary histosarcoma cells by 7% to 18% (n = 6; P < .01 each). Pressure activated FAK and Akt (n = 5; P < .01). Inhibiting FAK or Akt inhibited FAK or Akt phosphorylation and the stimulation of adhesion by increased pressure (n = 5 each; P < .01 each).

CONCLUSIONS

Pressure increases sarcoma cell adhesiveness via Akt and FAK. Perioperative manipulation or forces in lymphatic or circulatory systems may potentiate local recurrence or distant metastasis.

Tissue elongation requires oscillating contractions of a basal actomyosin network

Understanding how molecular dynamics leads to cellular behaviours that ultimately sculpt organs and tissues is a major challenge not only in basic developmental biology but also in tissue engineering and regenerative medicine. Here we use live imaging to show that the basal surfaces of Drosophila follicle cells undergo a series of directional, oscillating contractions driven by periodic myosin accumulation on a polarized actin network. Inhibition of the actomyosin contractions or their coupling to extracellular matrix (ECM) blocked elongation of the whole tissue, whereas enhancement of the contractions exaggerated it. Myosin accumulated in a periodic manner before each contraction and was regulated by the small GTPase Rho, its downstream kinase, ROCK, and cytosolic calcium. Disrupting the link between the actin cytoskeleton and the ECM decreased the amplitude and period of the contractions, whereas enhancing cell-ECM adhesion increased them. In contrast, disrupting cell-cell adhesions resulted in loss of the actin network. Our findings reveal a mechanism controlling organ shape and an experimental model for the study of the effects of oscillatory actomyosin activity within a coherent cell sheet.

The effects of mechanical forces on intestinal physiology and pathology

The epithelial and non-epithelial cells of the intestinal wall experience a myriad of physical forces including strain, shear, and villous motility during normal gut function. Pathologic conditions alter these forces, leading to changes in the biology of these cells. The responses of intestinal epithelial cells to forces vary with both the applied force and the extracellular matrix proteins with which the cells interact, with differing effects on proliferation, differentiation, and motility, and the regulation of these effects involves similar but distinctly different signal transduction mechanisms. Although normal epithelial cells respond to mechanical forces, malignant gastrointestinal epithelial cells also respond to forces, most notably by increased cell adhesion, a critical step in tumor metastasis. This review will focus on the phenomenon of mechanical forces influencing cell biology and the mechanisms by which the gut responds these forces in both the normal as well as pathophysiologic states when forces are altered. Although more is known about epithelial responses to force, information regarding mechanosensitivity of vascular, neural, and endocrine cells within the gut wall will also be discussed, as will, the mechanism by which forces can regulate epithelial tumor cell adhesion.

Biological impact of mechanical stimuli on tumor metastasis

Increasing evidence suggests tumor cell exposure to mechanical stimuli during the perioperative period as well as throughout the normal disease process may have a discernable impact on tumor metastasis and patient outcome. In vitro studies have demonstrated that transient exposure to increased extracellular pressure and shear forces modulates integrin binding affinity and stimulates cancer cell adhesion through a cytoskeleton- and focal adhesion complex-dependent signaling mechanism. More prolonged exposure to elevated pressures stimulates tumor cell proliferation by a distinct signaling pathway. Whether pressure effects on cell adhesion and proliferation pose biological ramifications in vivo remained unknown. We recently reported that pressure activation of malignant cells does indeed have a biological impact on surgical wound implantation, tumor development and tumor-free survival in a murine colon tumor model. Moreover, this effect can be disrupted by preoperative administration of colchicine. Taken together with previous work from our laboratory and others, these findings suggest that further elucidation of the mechanical signaling pathways governing pressure-stimulated tumor cell adhesion and proliferation may identify novel therapeutic targets for the treatment and prevention of tumor metastasis.

Melanoma and the tumor microenvironment

Melanoma arises through the accrual of mutations in genes critical for proliferation and survival. Although melanoma had been traditionally conceptualized as a cell-autonomous event, increasing evidence supports the notion that these tumors are not isolated entities but rather depend, interact with, and react to the adjacent microenvironment. Melanoma is composed of not only the malignant cells but also the supporting stroma, which includes fibroblasts, endothelial cells, immune cells, soluble molecules, and the extracellular matrix (ECM). Tumor cells actively interact with the microenvironment in a bidirectional manner through molecular signals that modulate the malignant phenotype. This article briefly reviews the molecular basis of melanomagenesis as well as the interplay of melanoma with other cells of the tumor microenvironment and components of the ECM. It also discusses the influence of the microenvironment on therapeutic targeting of melanoma, highlighting recent studies that propose novel strategies to target tumor-microenvironment interactions.

Colchicine inhibits pressure-induced tumor cell implantation within surgical wounds and enhances tumor-free survival in mice

Iatrogenic tumor cell implantation within surgical wounds can compromise curative cancer surgery. Adhesion of cancer cells, in particular colon cancer cells, is stimulated by exposure to increased extracellular pressure through a cytoskeleton-dependent signaling mechanism requiring FAK, Src, Akt, and paxillin. Mechanical stimuli during tumor resection may therefore negatively impact patient outcome. We hypothesized that perioperative administration of colchicine, which prevents microtubule polymerization, could disrupt pressure-stimulated tumor cell adhesion to surgical wounds and enhance tumor-free survival. Ex vivo treatment of Co26 and Co51 colon cancer cells with colchicine inhibited pressure-stimulated cell adhesion to murine surgical wounds and blocked pressure-induced FAK and Akt phosphorylation. Surgical wound contamination with pressure-activated Co26 and Co51 cells significantly reduced tumor-free survival compared with contamination with tumor cells under ambient pressure. Mice treated with pressure-activated Co26 and Co51 cells from tumors preoperatively treated with colchicine in vivo displayed reduced surgical site implantation and significantly increased tumor-free survival compared with mice exposed to pressure-activated cells from tumors not pretreated with colchicine. Our data suggest that pressure activation of malignant cells promotes tumor development and impairs tumor-free survival and that perioperative colchicine administration or similar interventions may inhibit this effect.

Evolving management of colorectal cancer

This article reviews recent advances in surgical techniques and adjuvant therapies for colorectal cancer, including total mesorectal excision, the resection of liver and lung metastasis and advances in chemoradiation and foreshadows some interventions that may lie just beyond the frontier. In particular, little is known about the intracellular and extracellular cascades that may influence colorectal cancer cell adhesion and metastasis. Although the phosphorylation of focal adhesion kinases and focal adhesion associated proteins in response to integrin-mediated cell matrix binding (”outside in integrin signaling”) is well described, the stimulation of cell adhesion by intracellular signals activated by pressure prior to adhesion represents a different signal paradigm. However, several studies have suggested that increased pressure and shear stress activate cancer cell adhesion. Further studies of the pathways that regulate integrin-driven cancer cell adhesion may identify ways to disrupt these signals or block integrin-mediated adhesion so that adhesion and eventual metastasis can be prevented in the future.

Reduced paxillin expression contributes to the antimetastatic effect of 4-hydroxycoumarin on B16-F10 melanoma cells

Background

4-Hydroxycoumarin (4-HC) is a coumarin that lacks anticoagulant activity. 4-HC affects the cytoskeletal stability and decreases cell adhesion and motility of the melanoma cell line B16-F10. Together with integrins and other cytoskeletal proteins, paxillin participates in the regulation of cell adhesion and motility, acting as an adapter protein at focal adhesions. The present study determined the participation of paxillin in the reported effects of 4-HC and analyzed the role of paxillin in the formation of melanoma metastases.

Results

4-HC decreased protein and mRNA levels of α- and β-paxillin isoforms in B16-F10 cells. Paxillin downregulation correlated with an inadequate translocation of paxillin to focal adhesions and a reduced phosphotyr¹¹⁸-paxillin pool. Consequently, 4-HC altered paxillin-mediated signaling, decreasing the phosphorylation of FAK and the level of GTP-bound Rac-1. These results partially explain the mechanism of the previously reported effects of 4-HC. Additionally, we studied the effect of 4-HC on metastatic potential of B16-F10 cells through experimental metastasis assays. In vitro treatment of cells with 4-HC inhibited their capability to originate pulmonary metastases. 4-HC did not affect cell proliferation or survival, demonstrating that its antimetastatic effect is unrelated to changes on cell viability. We also studied the importance of paxillin in metastasis by transfecting melanoma cells with paxillin-siRNA. Transfection produced a modest reduction on metastatic potential, indicating that: i) paxillin plays a role as inducer of melanoma metastasis; and ii) paxillin downregulation is not sufficient to explain the antimetastatic effect of 4-HC. Therefore, we evaluated other changes in gene expression by differential display RT-PCR analysis. Treatment with 4-HC produced a downregulation of Adhesion Regulating Molecule-1 (ARM-1), which correlated with a decreased adhesion of melanoma cells to lung slides.

Conclusion

This study shows that reduced paxillin expression is associated with the impaired cell adhesion and motility seen in 4-HC-treated cells and partially contributes to the antimetastatic effect of 4-HC. In contrast, the role of ARM-1 reduced expression in the effects of 4-HC is still to be clarified. The antimetastatic effect of 4-HC suggests that this compound, or others with similar mode of action, might be useful for the development of adjuvant therapies for melanoma.

SiRNA-mediated reduction of alpha-actinin-1 inhibits pressure-induced murine tumor cell wound implantation and enhances tumor-free survival

Viable cancer cells can commonly be recovered from surgical sites and venous blood during tumor resection. The adhesion of these cells to surrounding tissues may impact patient outcomes. Iatrogenic exposure to increased extracellular pressure modulates integrin binding affinity and stimulates colon cancer cell adhesion in vitro through an alpha-actinin-1-dependent signaling pathway. We hypothesized that preoperative small interfering RNA-mediated silencing of alpha-actinin-1 in tumor tissue could disrupt pressure-stimulated cancer cell adhesion to murine surgical wounds and thereby enhance subsequent tumor-free survival. Reducing alpha-actinin-1 in CT26 murine adenocarcinoma cells blocked cell adhesion to collagen in vitro and similarly inhibited pressure-induced CT26 implantation in murine surgical wounds in vivo. Surgical wound contamination with pressure-activated CT26 cells significantly reduced tumor-free survival compared to contamination with tumor cells maintained under ambient pressure. However, mice treated with pressure-activated CT26 cells preoperatively transfected with alpha-actinin-1-specific small interfering RNA displayed reduced surgical site implantation and increased tumor-free survival compared to mice exposed to pressure-activated cells expressing normal levels of alpha-actinin-1 protein. These results suggest that pressure activation of malignant cells promotes tumor development and impairs tumor-free survival. alpha-Actinin-1 may be an effective therapeutic target to inhibit perioperative pressure-stimulated tumor cell implantation.

Pressure activates colon cancer cell adhesion via paxillin phosphorylation, Crk, Cas, and Rac1

Physical forces can activate colon cancer cell adhesion, critical for metastasis. Paxillin is phosphorylated by FAK and required for pressure-stimulated adhesion. However, whether paxillin acts as an inert scaffolding protein or whether paxillin phosphorylation is required is unknown. Transfection with paxillin point-phosphorylation mutants demonstrated that phosphorylation at tyrosines 31 and 118 together is necessary for pressure-stimulated adhesion. We further evaluated potential paxillin partners. Reducing the adaptor protein Crk or the focal adhesion protein p130Cas blocked pressure-stimulated adhesion. Furthermore, Crk and p130Cas both displayed increased co-immunoprecipitation with paxillin in response to increased pressure, except in cells transfected with a Y31Y118 paxillin mutant. Inhibiting the small GTPase Rac1 also abolished pressure-stimulated adhesion, and reducing paxillin by siRNA blocked Rac1 phosphorylation by pressure. Thus, paxillin phosphorylation at tyrosines 31 and 118 together is necessary for pressure-induced adhesion. Paxillin, Crk and Cas form a trimeric complex that activates Rac1 and mediates this effect.

An intracellular signal pathway that regulates cancer cell adhesion in response to extracellular forces

Increasing evidence suggests that tumor cells can regulate their own adhesion via intracellular signals that modulate integrin binding affinity. Although the full pathway has not yet been elucidated, the effects of pressure seem likely to require cytoskeletal mechanosensing, Src, phosphatidylinositol 3-kinase, focal adhesion kinase, and Akt-1 activation. Ultimately, activated focal adhesion kinase accumulates at the membrane in association with beta(1)-integrin heterodimers and may modulate integrin binding affinity. This pathway may be a promising target for manipulation to inhibit metastatic cancer cell adhesion.

AFAP-110 is required for actin stress fiber formation and cell adhesion in MDA-MB-231 breast cancer cells

Regulation of actin organization and dynamics is a highly complex process that involves a number of actin-binding proteins, including capping, branching, severing, sequestering, and cross-linking proteins. The actin-binding and cross-linking protein AFAP-110 is expressed in normal myoepithelial cells. Screening of different breast epithelial cell lines revealed high expression levels of AFAP-110 in the human breast cancer cell lines MDA-MB-231 and MDA-MB-435. Knockdown of AFAP-110 expression in MDA-MB-231 cells does not result in any changes in cell proliferation but did result in a loss of actin stress fiber cross-linking and decreased adhesion to fibronectin. An inducible knockdown approach confirms that MDA-MB-231 breast cancer cells require AFAP-110 expression for stress fiber formation and adhesion. Thus, AFAP-110 may provide cytoskeletal tension through stress fiber formation, which is required for focal adhesion formation. Indeed, we could not detect any focal contacts or focal adhesions in AFAP-110 knockdown cells after adhesion to fibronectin. Although expression levels of crucial focal adhesion components were not influenced by AFAP-110 expression levels, treatment of AFAP-110 knockdown cells with LPA did not result in induction of actin stress fibers and focal adhesions. In summary, AFAP-110 plays an important role in MDA-MB-231 breast cancer cell adhesion possibly by regulating stress filament cross-linking which would promote focal adhesion formation.

Cytoskeletal signaling by way of alpha-actinin-1 mediates ERK1/2 activation by repetitive deformation in human Caco2 intestinal epithelial cells

BACKGROUND

Repetitive deformation stimulates proliferation in human Caco2 intestinal epithelial cells by way of an ERK1/2-dependent pathway. We examined the effects of cytoskeletal perturbation on deformation-induced signaling in Caco2 cells.

METHODS

The Caco2 cell cytoskeleton was disrupted with either cytochalasin D, phalloidin, colchicine, or paclitaxel. Levels of alpha-actinin-1 and -4 and paxillin were reduced by specific small interfering RNA. Cells on collagen I-precoated membranes were subjected to 10% repetitive deformation at 10 cycles/min. After 1 hour, cells were lysed for Western blot analysis.

RESULTS

Strain-activated ERK1/2, focal adhesion kinase, and Src phosphorylation in dimethyl sulfoxide- and/or nontargeting small interfering RNA-treated control cell populations. Cytochalasin D and paclitaxel, but not phalloidin and colchicine, blocked ERK1/2 phosphorylation. A decrease in alpha-actinin-1, but not in alpha-actinin-4 or paxillin, inhibited ERK1/2 and focal adhesion kinase phosphorylation, whereas Src activation appears to be independent of these effects.

CONCLUSIONS

The intestinal epithelial cell cytoskeleton may transduce mechanical signals by way of alpha-actinin-1 into the focal adhesion complex, culminating in ERK1/2 activation and proliferation.

Alpha-actinin-1 phosphorylation modulates pressure-induced colon cancer cell adhesion through regulation of focal adhesion kinase-Src interaction

Physical forces including pressure, strain, and shear can be converted into intracellular signals that regulate diverse aspects of cell biology. Exposure to increased extracellular pressure stimulates colon cancer cell adhesion by a beta(1)-integrin-dependent mechanism that requires an intact cytoskeleton and activation of focal adhesion kinase (FAK) and Src. alpha-Actinin facilitates focal adhesion formation and physically links integrin-associated focal adhesion complexes with the cytoskeleton. We therefore hypothesized that alpha-actinin may be necessary for the mechanical response pathway that mediates pressure-stimulated cell adhesion. We reduced alpha-actinin-1 and alpha-actinin-4 expression with isoform-specific small interfering (si)RNA. Silencing of alpha-actinin-1, but not alpha-actinin-4, blocked pressure-stimulated cell adhesion in human SW620, HT-29, and Caco-2 colon cancer cell lines. Cell exposure to increased extracellular pressure stimulated alpha-actinin-1 tyrosine phosphorylation and alpha-actinin-1 interaction with FAK and/or Src, and enhanced FAK phosphorylation at residues Y397 and Y576. The requirement for alpha-actinin-1 phosphorylation in the pressure response was investigated by expressing the alpha-actinin-1 tyrosine phosphorylation mutant Y12F in the colon cancer cells. Expression of Y12F blocked pressure-mediated adhesion and inhibited the pressure-induced association of alpha-actinin-1 with FAK and Src, as well as FAK activation. Furthermore, siRNA-mediated reduction of alpha-actinin-1 eliminated the pressure-induced association of alpha-actinin-1 and Src with beta(1)-integrin receptor, as well as FAK-Src complex formation. These results suggest that alpha-actinin-1 phosphorylation at Y12 plays a crucial role in pressure-activated cell adhesion and mechanotransduction by facilitating Src recruitment to beta(1)-integrin, and consequently the association of FAK with Src, to enhance FAK phosphorylation.

FAK association with multiple signal proteins mediates pressure-induced colon cancer cell adhesion via a Src-dependent PI3K/Akt pathway

Cancer cell adhesion is traditionally viewed as random, occurring if the cell's receptors match the substrate. Cancer cells are subjected to pressure and shear during growth against a constraining stroma, surgical manipulation, and passage through the venous and lymphatic system. Cells shed into a cavity such as the abdomen postoperatively also experience increased pressure from postoperative edema. Increased extracellular pressure stimulates integrin-mediated cancer cell adhesion via FAK and Src. PI 3-kinase (PI3K) inhibitors (LY294002 or wortmannin), Akt inhibitors, or Akt1 siRNA blocked adhesion stimulated by 15 mmHg pressure in SW620 or primary human malignant colonocytes. Pressure activated PI3K, tyrosine-phosphorylated and membrane-translocated the p85 subunit, and phosphorylated Akt. PI3K inhibitor (LY294002) prevented pressure-stimulated Akt Ser473 and FAK Tyr397, but not FAK576 or Src416 phosphorylation. PP2 inhibited PI3K activity and Akt phosphorylation. FAK siRNA did not affect pressure-induced PI3K activation but blocked Akt phosphorylation. Pressure also stimulated FAK or FAKY397F mutant translocation to the membrane. Akt inhibitor IV blocked pressure-induced Akt and FAK translocation. Pressure activated Src- and PI3K-dependently induced p85 interaction with FAK, and FAK with beta1 integrin. These results delineate a novel force-activated inside-out Src/PI3K/FAK/Akt pathway by which cancer cells regulate their own adhesion. These signals may be potential targets for inhibition of metastatic adhesion.