Molecular cloning of human paxillin, a focal adhesion protein phosphorylated by P210BCR/ABL

A comparative analysis of paxillin and Hic-5 proximity interactomes

Focal adhesions serve as structural and signaling hubs, facilitating bidirectional communication at the cell-extracellular matrix interface. Paxillin and the related Hic-5 (TGFβ1i1) are adaptor/scaffold proteins that recruit numerous structural and regulatory proteins to focal adhesions, where they perform both overlapping and discrete functions. In this study, paxillin and Hic-5 were expressed in U2OS osteosarcoma cells as biotin ligase (BioID2) fusion proteins and used as bait proteins for proximity-dependent biotinylation in order to directly compare their respective interactomes. The fusion proteins localized to both focal adhesions and the centrosome, resulting in biotinylation of components of each of these structures. Biotinylated proteins were purified and analyzed by mass spectrometry. The list of proximity interactors for paxillin and Hic-5 comprised numerous shared core focal adhesion proteins that likely contribute to their similar functions in cell adhesion and migration, as well as proteins unique to paxillin and Hic-5 that have been previously localized to focal adhesions, the centrosome, or the nucleus. Western blotting confirmed biotinylation and enrichment of FAK and vinculin, known interactors of Hic-5 and paxillin, as well as several potentially unique proximity interactors of Hic-5 and paxillin, including septin 7 and ponsin, respectively. Further investigation into the functional relationship between the unique interactors and Hic-5 or paxillin may yield novel insights into their distinct roles in cell migration.

TATA-box-binding protein promotes hepatocellular carcinoma metastasis through epithelial-mesenchymal transition

BACKGROUND

HCC characterizes malignant metastasis with high incidence and recurrence. Thus, it is pivotal to discover the mechanisms of HCC metastasis. TATA-box-binding protein (TBP), a general transcriptional factor (TF), couples with activators and chromatin remodelers to sustain the transcriptional activity of target genes. Here, we investigate the key role of TBP in HCC metastasis.

METHODS

TBP expression was measured by PCR, western blot, and immunohistochemistry. RNA-sequencing was performed to identify downstream proteins. Functional assays of TBP and downstream targets were identified in HCC cell lines and xenograft models. Luciferase reporter and chromatin immunoprecipitation assays were used to demonstrate the mechanism mediated by TBP.

RESULTS

HCC patients showed high expression of TBP, which correlated with poor prognosis. Upregulation of TBP increased HCC metastasis in vivo and in vitro, and muscleblind-like-3 (MBNL3) was the effective factor of TBP, positively related to TBP expression. Mechanically, TBP transactivated and enhanced MBNL3 expression to stimulate exon inclusion of lncRNA-paxillin (PXN)-alternative splicing (AS1) and, thus, activated epithelial-mesenchymal transition for HCC progression through upregulation of PXN.

CONCLUSIONS

Our data revealed that TBP upregulation is an HCC enhancer mechanism that increases PXN expression to drive epithelial-mesenchymal transition.

A Systems Biology Approach for Addressing Cisplatin Resistance in Non-Small Cell Lung Cancer

Translational research in medicine, defined as the transfer of knowledge and discovery from the basic sciences to the clinic, is typically achieved through interactions between members across scientific disciplines to overcome the traditional silos within the community. Thus, translational medicine underscores 'Team Medicine', the partnership between basic science researchers and clinicians focused on addressing a specific goal in medicine. Here, we highlight this concept from a City of Hope perspective. Using cisplatin resistance in non-small cell lung cancer (NSCLC) as a paradigm, we describe how basic research scientists, clinical research scientists, and medical oncologists, in true 'Team Science' spirit, addressed cisplatin resistance in NSCLC and identified a previously approved compound that is able to alleviate cisplatin resistance in NSCLC. Furthermore, we discuss how a 'Team Medicine' approach can help to elucidate the mechanisms of innate and acquired resistance in NSCLC and develop alternative strategies to overcome drug resistance.

The explorations of dynamic interactions of paxillin at the focal adhesions

Paxillin is one of the most important adapters in integrin-mediated adhesions that performs numerous crucial functions relying on its dynamic interactions. Its structural behavior serves different purposes, providing a base for several activities. The various domains of paxillin display different functions in the whole process of cell movements and have a significant role in cell adhesion, migration, signal transmission, and protein-protein interactions. On the other hand, some paxillin-associated proteins provide a unique spatiotemporal mechanism for regulating its dynamic characteristics in the tissue homeostasis and make it a more complex and decisive protein at the focal adhesions. This review briefly describes the structural adaptations and molecular mechanisms of recruitment of paxillin into adhesions, explains paxillin's binding dynamics and impact on adhesion stability and turnover, and reveals a variety of paxillin-associated regulatory mechanisms and how paxillin is embedded into the signaling networks.

CK2β Is a Gatekeeper of Focal Adhesions Regulating Cell Spreading

CK2 is a hetero-tetrameric serine/threonine protein kinase made up of two CK2α/αʹ catalytic subunits and two CK2β regulatory subunits. The free CK2α subunit and the tetrameric holoenzyme have distinct substrate specificity profiles, suggesting that the spatiotemporal organization of the individual CK2 subunits observed in living cells is crucial in the control of the many cellular processes that are governed by this pleiotropic kinase. Indeed, previous studies reported that the unbalanced expression of CK2 subunits is sufficient to drive epithelial to mesenchymal transition (EMT), a process involved in cancer invasion and metastasis. Moreover, sub-stoichiometric expression of CK2β compared to CK2α in a subset of breast cancer tumors was correlated with the induction of EMT markers and increased epithelial cell plasticity in breast carcinoma progression. Phenotypic changes of epithelial cells are often associated with the activation of phosphotyrosine signaling. Herein, using phosphotyrosine enrichment coupled with affinity capture and proteomic analysis, we show that decreased expression of CK2β in MCF10A mammary epithelial cells triggers the phosphorylation of a number of proteins on tyrosine residues and promotes the striking activation of the FAK1-Src-PAX1 signaling pathway. Moreover, morphometric analyses also reveal that CK2β loss increases the number and the spatial distribution of focal adhesion signaling complexes that coordinate the adhesive and migratory processes. Together, our findings allow positioning CK2β as a gatekeeper for cell spreading by restraining focal adhesion formation and invasion of mammary epithelial cells.

Pan-cancer analysis reveals an immunological role and prognostic potential of PXN in human cancer

Paxillin (PXN) is a protein involved in numerous physiological processes, and its presence is closely related to the occurrence and development of many types of tumors. However, no studies have analyzed PXN from a pan-cancer perspective. We analyzed PXN expression, immune cell infiltration, prognosis, and biological function across different types of tumors included in The Cancer Genome Atlas and Gene Expression Omnibus datasets. The results showed that expression of PXN varies in different tumors. Expression of PXN strongly correlated with prognosis in patients with tumors; higher PXN expression usually was linked to poor overall and disease-free survival. Expression of PXN in breast invasive carcinoma and lymphoid neoplasm diffuse large B-cell lymphoma was related to the degree of CD8+ T-cell infiltration, and infiltration of cancer-associated fibroblasts, such as kidney renal papillary cell carcinoma and brain lower-grade glioma, was also observed in other tumors. The results of pan-cancer analysis showed that abnormal PXN expression was related to poor prognosis, immune infiltration, and protein phosphorylation in different tumor types. Therefore, the PXN gene may become a potential biomarker of clinical tumor prognosis.

A Non-genetic Mechanism Involving the Integrin β4/Paxillin Axis Contributes to Chemoresistance in Lung Cancer

Tumor heterogeneity and cisplatin resistance are major causes of tumor relapse and poor survival. Here, we show that in lung cancer, interaction between paxillin (PXN) and integrin β4 (ITGB4), components of the focal adhesion (FA) complex, contributes to cisplatin resistance. Knocking down PXN and ITGB4 attenuated cell growth and improved cisplatin sensitivity, both in 2D and 3D cultures. PXN and ITGB4 independently regulated expression of several genes. In addition, they also regulated expression of common genes including USP1 and VDAC1, which are required for maintaining genomic stability and mitochondrial function, respectively. Mathematical modeling suggested that bistability could lead to stochastic phenotypic switching between cisplatin-sensitive and resistant states in these cells. Consistently, purified subpopulations of sensitive and resistant cells re-created the mixed parental population when cultured separately. Altogether, these data point to an unexpected role of the FA complex in cisplatin resistance and highlight a novel non-genetic mechanism.

The Scaffold Proteins Paxillin B and α-Actinin Regulate Septation in Aspergillus nidulans via Control of Actin Ring Contraction

Cytokinesis, as the final step of cell division, plays an important role in fungal growth and proliferation. In the filamentous fungus Aspergillus nidulans, defective cytokinesis is able to induce abnormal multinuclear or nonnucleated cells and then result in reduced hyphal growth and abolished sporulation. Previous studies have reported that a conserved contractile actin ring (CAR) protein complex and the septation initiation network (SIN) signaling kinase cascade are required for cytokinesis and septation; however, little is known about the role(s) of scaffold proteins involved in these two important cellular processes. In this study, we show that a septum-localized scaffold protein paxillin B (PaxB) is essential for cytokinesis/septation in A. nidulans The septation defects observed in a paxB deletion strain resemble those caused by the absence of another identified scaffold protein, α-actinin (AcnA). Deletion of α-actinin (AcnA) leads to undetectable PaxB at the septation site, whereas deletion of paxB does not affect the localization of α-actinin at septa. However, deletion of either α-actinin (acnA) or paxB causes the actin ring to disappear at septation sites during cytokinesis. Notably, overexpression of α-actinin acnA partially rescues the septum defects of the paxB mutant but not vice versa, suggesting AcnA may play a dominant role over that of PaxB for cytokinesis and septation. In addition, PaxB and α-actinin affect the septal dynamic localization of MobA, a conserved component of the SIN pathway, suggesting they may affect the SIN protein complex function at septa. Protein pull-down assays combined with liquid chromatography-mass spectrometry identification indicate that α-actinin AcnA and PaxB likely do not directly interact, but presumably belong to an actin cytoskeleton protein network that is required for the assembly and contraction of the CAR. Taken together, findings in this study provide novel insights into the roles of conserved scaffold proteins during fungal septation in A. nidulans.

Comparison between curcumin and all-trans retinoic acid in the osteogenic differentiation of mouse bone marrow mesenchymal stem cells

The use of bone marrow mesenchymal stem cells (BMSCs) has great potential in cell therapy, particularly in the orthopedic field. BMSCs represent a valuable renewable cell source that have been successfully utilized to treat damaged skeletal tissue and bone defects. BMSCs can be induced to differentiate into osteogenic lineages via the addition of inducers to the growth medium. The present study examined the effects of all-trans retinoic acid (ATRA) and curcumin on the osteogenic differentiation of mouse BMSCs. Morphological changes, the expression levels of the bone-associated gene markers bone morphogenetic protein 2, runt-related transcription factor and osterix during differentiation, an in vitro mineralization assay, and changes in osteocalcin expression revealed that curcumin supplementation promoted the osteogenic differentiation of BMSCs. By contrast, the application of ATRA increased osteogenic differentiation during the early stages, but during the later stages, it decreased the mineralization of differentiated cells. In addition, to the best of our knowledge, the present study is the first to examine the effect of curcumin on the osteogenic potency of mouse embryonic fibroblasts (MEFs) after reprogramming with human lim mineralization protein (hLMP-3), which is a positive osteogenic regulator. The results revealed that curcumin-supplemented culture medium increased hLMP-3 osteogenic potency compared with that of MEFs cultured in the non-supplemented medium. The present results demonstrate that enrichment of the osteogenic culture medium with curcumin, a natural osteogenic inducer, increased the osteogenic differentiation capacity of BMSCs as well as that of MEFs reprogrammed with hLMP-3.

Direct conversion of mouse embryonic fibroblast to osteoblast cells using hLMP-3 with Yamanaka factors

Large bone defects and bone loss after fractures remain significant challenges for orthopedic surgeons. Our study aims to find an available, applicable and biological treatment for bone regeneration overcoming the limitations in ESC/iPSC technology. We directly reprogrammed the mouse embryonic fibroblast (MEF) into osteoblast cells using different combinations of Yamanaka factors with human lim mineralization protein-3 (hLMP-3). LMP is an intracellular LIM-domain protein acting as an effective positive regulator of the osteoblast differentiation. After transduction, cells were cultured in osteogenic medium, and then examined for osteoblast formation. The expression of osteogenic markers (BMP2, Runx2 and Osterix) during reprogramming and in vitro mineralization assay revealed that the best reprogramming cocktail was (c-Myc - Oct4) with hLMP-3. In addition, both immunofluorescent staining and western blot analysis confirmed that osteocalcin (OCN) expression increased in the cells treated with the c-Myc/Oct4/hLMP3 cocktail than using hLMP-3 alone. Furthermore, this reprogramming cocktail showed efficient healing in an induced femoral bone defect in rat animal model one month after transplantation. In the present study, we reported for the first time the effect of combining Yamanaka factors with hLMP-3 to induce osteoblast cells from MEF both in vitro and in vivo.

The Genetic/Non-genetic Duality of Drug 'Resistance' in Cancer

Drug resistance is a serious impediment to the treatment of cancer. However, the mechanisms involved remain poorly understood. While it is widely held that the phenomenon is genetic in nature, emerging evidence suggests that non-genetic mechanisms may also be important. Furthermore, at least in some cases, refractoriness to treatment can be reversed by epigenetic reprogramming, and combination and intermittent therapies, as opposed to sustained monotherapy, appear more effective in attenuating it. Here we iterate the confusion in understanding the phenomenon by which cancer cells evade drug response and underscore the need to recognize the genetic/non-genetic duality of drug resistance in cancer. We discuss how ecological and evolutionary principles may help to reconcile the duality and may even offer new treatment strategies.

FAK and paxillin, two potential targets in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is a devastating cancer in large part due to late diagnosis and a lack of effective screening tests. In spite of recent progress in imaging, surgery and new therapeutic options for pancreatic cancer, the overall five-year survival still remains unacceptably low. Numerous studies have shown that focal adhesion kinase (FAK) is activated in many cancers including PDAC and promotes cancer progression and metastasis. Paxillin, an intracellular adaptor protein that plays a key role in cytoskeletal organization, connects integrins to FAK and plays a key role in assembly and disassembly of focal adhesions. Here, we have reviewed evidence in support of FAK as a potential therapeutic target and summarized related combinatorial therapies.

C-reactive protein binds to integrin α2 and Fcγ receptor I, leading to breast cell adhesion and breast cancer progression

C-reactive protein (CRP) is an acute phase protein synthesized upon the inflammatory responses, associated with breast cancer. The process of tumor cell invasion and metastasis involves the adherence of cells to the extracellular matrix via integrin as a receptor for matrix molecules. The present study investigated the role of CRP in the adhesive phenotype of breast cells and the underlying mechanisms. Here, we first showed that CRP induces adhesion of MCF10A human breast epithelial cells through the activation of integrin α2 signaling. Expression of integrin α2 was induced by CRP in which transcription factors c-fos and SP1 may be involved. Binding of CRP with integrin α2 leads to the activation of focal adhesion kinase (FAK), paxillin and ERKs. CRP also binds to an Fcγ receptor Fcγ receptor I (FcγRI), and induces activation of paxillin, FAK and ERKs. Integrin α2 and FAK have crucial roles in the adhesive and invasive phenotypes as well as MMP-9 upregulation induced by CRP in MCF10A cells. Treatment with an inflammatory lipid sphingosine-1-phosphate induced CRP, which may be secreted and exert an autocrine effect by binding to FcγRI and integrin α2. Involvement of CRP in adhesion, invasion, anchorage-independent growth and upregulation of integrin α2, paxillin and FAK was observed in MDA-MB-231 triple-negative human breast cancer (TNBC) cells. Using an in vivo invasion model and an orthotopic mouse tumor model with MDA-MB-231 cells, we showed that CRP has an important role in intravasation and tumor growth in vivo, demonstrating the in vivo relevance of our in vitro results. The present study elucidates a critical molecular basis between CRP, integrin α2 and FcγRI pathways in MCF10A breast cells and MDA-MB-231 TNBC cells, thereby providing useful information on CRP-induced aggressiveness of breast cells in the inflammatory microenvironment.

Paxillin: a crossroad in pathological cell migration

Paxilllin is a multifunctional and multidomain focal adhesion adapter protein which serves an important scaffolding role at focal adhesions by recruiting structural and signaling molecules involved in cell movement and migration, when phosphorylated on specific Tyr and Ser residues. Upon integrin engagement with extracellular matrix, paxillin is phosphorylated at Tyr31, Tyr118, Ser188, and Ser190, activating numerous signaling cascades which promote cell migration, indicating that the regulation of adhesion dynamics is under the control of a complex display of signaling mechanisms. Among them, paxillin disassembly from focal adhesions induced by extracellular regulated kinase (ERK)-mediated phosphorylation of serines 106, 231, and 290 as well as the binding of the phosphatase PEST to paxillin have been shown to play a key role in cell migration. Paxillin also coordinates the spatiotemporal activation of signaling molecules, including Cdc42, Rac1, and RhoA GTPases, by recruiting GEFs, GAPs, and GITs to focal adhesions. As a major participant in the regulation of cell movement, paxillin plays distinct roles in specific tissues and developmental stages and is involved in immune response, epithelial morphogenesis, and embryonic development. Importantly, paxillin is also an essential player in pathological conditions including oxidative stress, inflammation, endothelial cell barrier dysfunction, and cancer development and metastasis.

Paxillin functions as an oncogene in human gliomas by promoting cell migration and invasion

Background

Paxillin is implicated in tumorigenesis, progression and aggressive phenotypes of various malignancies, highlighting its functions in cellular adhesion, migration and survival. However, the roles of paxillin in human gliomas remain unclear. The aim of this study was to evaluate the clinical implication of paxillin expression in patients with gliomas and its biological function in glioma cells.

Patients and methods

Expression levels of paxillin gene and protein, respectively, were detected by quantitative real-time reverse transcription polymerase chain reaction, Western blot and immunohistochemistry analyses in 120 pairs of glioma and matched nontumorous brain tissues. The associations between paxillin expression and various histopathological features of glioma patients were also statistically evaluated. Then, the functions of paxillin in cell migration and invasion of glioma cell lines were determined by transwell assays in vitro.

Results

The expression levels of both paxillin gene and protein in glioma tissues were markedly higher than those in matched nontumorous brain tissues. Notably, paxillin overexpression was significantly associated with the grade of malignancy (P<0.05). Moreover, the enforced expression of paxillin promoted the migration and invasion of glioma cells, while the loss of paxillin expression efficiently suppressed cell migration and invasion of glioma cell lines.

Conclusion

Our data suggest that paxillin may function as an oncogene and its overexpression may be closely correlated with tumor progression of human gliomas by modulating tumor cell motility, implying the potential of paxillin as a new therapeutic target for glioma intervention.

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.

HOXA5 inhibits metastasis via regulating cytoskeletal remodelling and associates with prolonged survival in non-small-cell lung carcinoma

Homeobox genes comprise a family of regulatory genes that contain a common homeobox domain and act as transcription factors. Recent studies indicate that homeobox A5 (HOXA5) may serve as a tumour suppressor gene in breast cancers. However, the precise role and the underlying mechanism of HOXA5 in lung cancer remain unclear. Oligonucleotide microarrays and an invasion/metastasis lung adenocarcinoma cell line model were used to determine the correlation between HOXA5 expression and cancer cell invasion ability. We found that ectopic expression of HOXA5 in highly invasive cancer cells suppressed cell migration, invasion, and filopodia formation in vitro and inhibited metastatic potential in vivo. Knockdown of HOXA5 promoted the invasiveness of lung cancer cells. In addition, HOXA5 expression was associated with better clinical outcome in non-small cell lung cancer patients with wild-type EGFR. Furthermore, genome-wide transcriptomic and pathway analyses were performed to identify the potential molecular mechanisms. Our data showed that HOXA5 may bind to the promoters of the cytoskeleton-related genes and downregulate their mRNA and protein expression levels. Our studies provide new insights into how HOXA5 may contribute to the suppression of metastasis in lung cancer via cytoskeleton remodelling regulation. Therefore, targeted induction of HOXA5 may represent a promising approach for non-small-cell lung cancer therapy.

Spatially defined EGF receptor activation reveals an F-actin-dependent phospho-Erk signaling complex

We investigated the association of signaling proteins with epidermal growth factor (EGF) receptors (EGFR) using biotinylated EGF bound to streptavidin that is covalently coupled in an ordered array of micron-sized features on silicon surfaces. Using NIH-3T3 cells stably expressing EGFR, we observe concentration of fluorescently labeled receptors and stimulated tyrosine phosphorylation that are spatially confined to the regions of immobilized EGF and quantified by cross-correlation analysis. We observe recruitment of phosphorylated paxillin to activated EGFR at these patterned features, as well as β1-containing integrins that preferentially localize to more peripheral EGF features, as quantified by radial fluorescence analysis. In addition, we detect recruitment of EGFP-Ras, MEK, and phosphorylated Erk to patterned EGF in a process that depends on F-actin and phosphoinositides. These studies reveal and quantify the coformation of multiprotein EGFR signaling complexes at the plasma membrane in response to micropatterned growth factors.

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.

Abnormal expression of paxillin correlates with tumor progression and poor survival in patients with gastric cancer

Background

Paxillin (PXN) has been found to be aberrantly regulated in various malignancies and involved in tumor growth and invasion. The clinicopathological and prognostic significance of PXN in gastric cancer is still unclear.

Methods

The expression of PXN was determined in paired gastric cancer tissues and adjacent normal tissues by Western blotting and real-time PCR. Immunohistochemistry was performed to detect the expression of PXN in 239 gastric cancer patients. Statistical analysis was applied to investigate the correlation between PXN expression and clinicopathological characteristics and prognosis in patients. Additionally, the effects of PXN on gastric cancer cell proliferation and migration were also evaluated.

Results

PXN was up-regulated in gastric cancer tissues and cell lines as compared with adjacent normal tissues and normal gastric epithelial cell line GES-1. Overexpression of PXN was correlated with distant metastasis (P = 0.001) and advanced tumor stage (P = 0.021) in gastric cancer patients. Patients with high PXN expression tended to have poor prognosis compared with patients with low PXN expression (P < 0.001). Multivariate analysis demonstrated that PXN expression was an independent prognostic factor (P = 0.020). Moreover, ectopic expression of PXN promotes cell proliferation and migration in AGS cells whereas knockdown of PXN inhibits cell proliferation and migration in SGC7901 cells.

Conclusions

PXN plays an important role in tumor progression and may be used as a potential prognostic indicator in gastric cancer.

Paxillin mutations affect focal adhesions and lead to altered mitochondrial dynamics: relevance to lung cancer

Cytoskeletal and focal adhesion abnormalities are observed in several types of cancer, including lung cancer. We have previously reported that paxillin (PXN) was mutated, amplified, and overexpressed in a significant number of lung cancer patient samples, that PXN protein was upregulated in more advanced stages of lung cancer compared with lower stages, and that the PXN gene was also amplified in some pre-neoplastic lung lesions. Among the mutations investigated, we previously found that PXN variant A127T in lung cancer cells enhanced cell proliferation and focal adhesion formation and colocalized with the anti-apoptotic protein B Cell Lymphoma 2 (BCL-2), which is known to localize to the mitochondria, among other sites. To further explore the effects of activating mutations of PXN on mitochondrial function, we cloned and expressed wild-type PXN and variants containing the most commonly occurring PXN mutations (P46S, P52L, G105D, A127T, P233L, T255I, D399N, E423K, P487L, and K506R) in a GFP-tagged vector using HEK-293 human embryonic kidney cells. Utilizing live-cell imaging to systematically study the effects of wild-type PXN vs. mutants, we created a model that recapitulates the salient features of the measured dynamics and conclude that compared with wild-type, some mutant clones confer enhanced focal adhesion and lamellipodia formation (A127T, P233L, and P487L) and some confer increased association with BCL-2, Dynamin-related Protein-1 (DRP-1), and Mitofusion-2 (MFN-2) proteins (P233L and D399N). Further, PXN mutants, through their interactions with BCL-2 and DRP-1, could regulate cisplatin drug resistance in human lung cancer cells. The data reported herein suggest that mutant PXN variants play a prominent role in mitochondrial dynamics with direct implications on lung cancer progression and hence, deserve further exploration as therapeutic targets.

C3G forms complexes with Bcr-Abl and p38α MAPK at the focal adhesions in chronic myeloid leukemia cells: implication in the regulation of leukemic cell adhesion

Background

Previous studies by our group and others have shown that C3G interacts with Bcr-Abl through its SH3-b domain.

Results

In this work we show that C3G and Bcr-Abl form complexes with the focal adhesion (FA) proteins CrkL, p130Cas, Cbl and Abi1 through SH3/SH3-b interactions. The association between C3G and Bcr-Abl decreased upon Abi1 or p130Cas knock-down in K562 cells, which suggests that Abi1 and p130Cas are essential partners in this interaction. On the other hand, C3G, Abi1 or Cbl knock-down impaired adhesion to fibronectin, while p130Cas silencing enhanced it. C3G, Cbl and p130Cas-SH3-b domains interact directly with common proteins involved in the regulation of cell adhesion and migration. Immunoprecipitation and immunofluorescence studies revealed that C3G form complexes with the FA proteins paxillin and FAK and their phosphorylated forms. Additionally, C3G, Abi1, Cbl and p130Cas regulate the expression and phosphorylation of paxillin and FAK. p38α MAPK also participates in the regulation of adhesion in chronic myeloid leukemia cells. It interacts with C3G, CrkL, FAK and paxillin and regulates the expression of paxillin, CrkL and α5 integrin, as well as paxillin phosphorylation. Moreover, double knock-down of C3G/p38α decreased adhesion to fibronectin, similarly to the single silencing of one of these genes, either C3G or p38α. These suggest that C3G and p38α MAPK are acting through a common pathway to regulate cell adhesion in K562 cells, as previously described for the regulation of apoptosis.

Conclusions

Our results indicate that C3G-p38αMAPK pathway regulates K562 cell adhesion through the interaction with FA proteins and Bcr-Abl, modulating the formation of different protein complexes at FA.

Diverse roles for the paxillin family of proteins in cancer

The paxillin family of intracellular scaffold proteins includes paxillin, Hic-5, and leupaxin, and all have been identified as key regulators of the cellular migration machinery in both 2- and 3-dimensional microenvironments. Herein, we provide insight into the roles of these proteins during tumorigenesis and metastasis, highlighting their functions in cancer initiation as well as tumor cell dissemination and survival. Furthermore, we speculate on the potential of paxillin family proteins as both future prognostic and therapeutic targets.

Iron chelators for the treatment of cancer

The study of iron chelators as anti-tumor agents is still in its infancy. Iron is important for cellular proliferation and this is demonstrated by observations that iron-depletion results in cell cycle arrest and also apoptosis. In addition, many iron chelators are known to inhibit ribonucleotide reductase, the iron-containing enzyme that is the rate-limiting step for DNA synthesis. Desferrioxamine is a well known chelator used for the treatment of iron-overload disease, but it has also been shown to possess anti-cancer activity. Another class of chelators, namely the thiosemicarbazones, have been shown to possess anti-cancer activity since the 1950's, although their mechanism(s) of action have only recently been more comprehensively elucidated. In fact, the redox activity of thiosemicarbazone iron complexes is thought to be important in mediating their potent cytotoxicity. Moreover, unlike typical iron chelators which simply act to deplete tumors of iron, several thiosemicarbazones (i.e., Bp44mT and Dp44mT) do not induce this effect, their anti-cancer efficacy being due to other mechanisms e.g., redox activity. Other reports have also shown that some thiosemicarbazones inhibit topoisomerase IIα, demonstrating that this class of agents have multiple molecular targets and act by various mechanisms. The most well characterized thiosemicarbazone iron chelator in terms of its assessment in humans is 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP). Observations from these clinical trials highlight the less than optimal activity of this ligand and several side effects related to its use, including myelo-suppression, hypoxia and methemoglobinemia. The mechanisms responsible for these latter effects must be elucidated and the design of the ligand altered to minimize these problems and increase efficacy. This review discusses the development of chelators as unique agents for cancer treatment.

Specific expression of Kcna10, Pxn and Odf2 in the organ of Corti

The development of the organ of Corti and the highly specialized cells required for hearing involves a multitude of genes, many of which remain unknown. Here we describe the expression pattern of three genes not previously studied in the inner ear in mice at a range of ages both embryonic and early postnatal. Kcna10, a tetrameric Shaker-like potassium channel, is expressed strongly in the hair cells themselves. Odf2, as its centriolar isoform Cenexin, marks the dendrites extending to and contacting hair cells, and Pxn, a focal adhesion scaffold protein, is most strongly expressed in pillar cells during the ages studied. The roles of these genes are yet to be elucidated, but their specific expression patterns imply potential functional significance in the inner ear.

Actomyosin-generated tension controls the molecular kinetics of focal adhesions

Focal adhesions (FAs) have key roles in the interaction of cells with the extracellular matrix (ECM) and in adhesion-mediated signaling. These dynamic, multi-protein structures sense the ECM both chemically and physically, and respond to external and internal forces by changing their size and signaling activity. However, this mechanosensitivity is still poorly understood at the molecular level. Here, we present direct evidence that actomyosin contractility regulates the molecular kinetics of FAs. We show that the molecular turnover of proteins within FAs is primarily regulated by their dissociation rate constant (k(off)), which is sensitive to changes in forces applied to the FA. We measured the early changes in k(off) values for three FA proteins (vinculin, paxillin and zyxin) upon inhibition of actomyosin-generated forces using two methods - high temporal resolution FRAP and direct measurement of FA protein dissociation in permeabilized cells. When myosin II contractility was inhibited, the k(off) values for all three proteins changed rapidly, in a highly protein-specific manner: dissociation of vinculin from FAs was facilitated, whereas dissociation of paxillin and zyxin was attenuated. We hypothesize that these early kinetic changes initiate FA disassembly by affecting the molecular turnover of FAs and altering their composition.

Significance of talin in cancer progression and metastasis

Upon detachment from the extracellular matrix, tumor epithelial cells and tumor-associated endothelial cells are capable of overcoming anoikis, gain survival benefits, and hence contribute to the process of metastasis. The focal-adhesion complex formation recruits the association of key adaptor proteins such as FAK (focal-adhesion kinase). Vimentin, paxillin, and talin are responsible for mediating the interaction between the actin cytoskeleton and integrins. Talin is an early-recruited focal-adhesion player that is of structural and functional significance in mediating interactions with integrin cytoplasmic tails leading to destabilization of the transmembrane complex and resulting in rearrangements in the extracellular integrin compartments that mediate integrin activation. Talin-mediated integrin activation plays a definitive role in integrin-mediated signaling and induction of downstream survival pathways leading to protection from anoikis and consequently resulting in cancer progression to metastasis. We recently reported that talin expression is significantly increased in prostate cancer compared with benign and normal prostate tissue and that this overexpression correlates with progression to metastatic disease implicating a prognostic value for talin during tumor progression. At the molecular level, talin is functionally associated with enhanced survival and proliferation pathways and confers anoikis resistance and metastatic spread of primary tumor cells via activation of the Akt survival pathway. In this review, we discuss the growing evidence surrounding the value of talin as a prognostic marker of cancer progression to metastasis and as therapeutic target in advanced prostate cancer, as well as the current understanding of mechanisms regulating its signaling activity in cancer.

Neuronal polarity selection by topography-induced focal adhesion control

Interaction between differentiating neurons and the extracellular environment guides the establishment of cell polarity during nervous system development. Developing neurons read the physical properties of the local substrate in a contact-dependent manner and retrieve essential guidance cues. In previous works we demonstrated that PC12 cell interaction with nanogratings (alternating lines of ridges and grooves of submicron size) promotes bipolarity and alignment to the substrate topography. Here, we investigate the role of focal adhesions, cell contractility, and actin dynamics in this process. Exploiting nanoimprint lithography techniques and a cyclic olefin copolymer, we engineered biocompatible nanostructured substrates designed for high-resolution live-cell microscopy. Our results reveal that neuronal polarization and contact guidance are based on a geometrical constraint of focal adhesions resulting in an angular modulation of their maturation and persistence. We report on ROCK1/2-myosin-II pathway activity and demonstrate that ROCK-mediated contractility contributes to polarity selection during neuronal differentiation. Importantly, the selection process confined the generation of actin-supported membrane protrusions and the initiation of new neurites at the poles. Maintenance of the established polarity was independent from NGF stimulation. Altogether our results imply that focal adhesions and cell contractility stably link the topographical configuration of the extracellular environment to a corresponding neuronal polarity state.

Focal adhesion plaque associated cytoskeletons are involved in the invasion and metastasis of human colorectal carcinoma

The protein and mRNA expression of focal adhesion plaque associated cytoskeletons, including talin, vinculin, paxillin, and tensin, was studied using immunofluorescence in combination with confocal laser scanning microscopy and fluorescent quantitative polymerase chain reaction in 41 matched samples of human normal colorectal mucosae, primary colorectal adenocarcinomas, and 19 separate lymph node metastases. All specimens showed expression. The results showed talin, vinculin, tensin, and paxillin expression were correlated with carcinogenesis, invasion, and metastasis of colorectal carcinoma (CRC). Talin, vinculin, and tensin underwent downregulation while paxillin went up. So these cytoskeletons may play bidirectional regulating roles during the progression of CRC.

LIM domain-containing adaptor, leupaxin, localizes in focal adhesion and suppresses the integrin-induced tyrosine phosphorylation of paxillin

Focal adhesion (FA) consists of multiple cellular proteins including paxillin and serves as a center for adhesion-mediated signaling. The assembly and disassembly of FAs is regulated by locally produced intracellular signals, and tyrosine phosphorylation of paxillin has been implicated in this process. A Lin-11 Isl-1 Mec-3 (LIM) domain-containing adaptor protein, leupaxin, a member of the paxillin family, is expressed in leukocytes as well as in certain cancer cells, and shares overall structural characteristics with paxillin. However, it remains unknown whether leupaxin and paxillin cooperate with or antagonize each other in integrin signaling. Here we show that leupaxin potently represses the tyrosine phosphorylation of paxillin. When expressed in mouse thymoma BW5147 cells bound to ICAM-1, leupaxin accumulated in FA-like patches in the cell periphery. When expressed in NIH3T3 and HEK293T cells, leupaxin localized to FAs upon cell adhesion to fibronectin and strongly suppressed the integrin-induced tyrosine phosphorylation of paxillin. In integrin-stimulated HEK293T cells, leupaxin's LIM3 domain appeared essential for selective FA localization and the suppression of paxillin tyrosine phosphorylation. Leupaxin's LD3 motif, which is critical for stable association with FAK, was dispensable for leupaxin's suppressive ability. In addition, leupaxin reduced the spreading of NIH3T3 cells on fibronectin, which required both the LD3 motif and LIM3 domain. When expressed in human leukocytic K562 cells, leupaxin significantly suppressed integrin alpha5beta1-mediated cell adhesion to fibronectin and the tyrosine phosphorylation of paxillin. These findings indicate that leupaxin functions as a paxillin counterpart that potently suppresses the tyrosine phosphorylation of paxillin during integrin signaling.

A BCR-ABL mutant lacking direct binding sites for the GRB2, CBL and CRKL adapter proteins fails to induce leukemia in mice

The BCR-ABL tyrosine kinase is the defining feature of chronic myeloid leukemia (CML) and its kinase activity is required for induction of this disease. Current thinking holds that BCR-ABL forms a multi-protein complex that incorporates several substrates and adaptor proteins and is stabilized by multiple direct and indirect interactions. Signaling output from this highly redundant network leads to cellular transformation. Proteins known to be associated with BCR-ABL in this complex include: GRB2, c-CBL, p62(DOK), and CRKL. These proteins in turn, link BCR-ABL to various signaling pathways indicated in cellular transformation. In this study we show that a triple mutant of BCR-ABL with mutations of the direct binding sites for GRB2, CBL, p62(DOK) and CRKL, is defective for transformation of primary hematopoietic cells in vitro and in a murine CML model, while it retains the capacity to induce IL-3 independence in 32D cells. Compared to BCR-ABL, the triple mutant's ability to activate the MAP kinase and PI3-kinase pathways is severely compromised, while STAT5 phosphorylation is maintained, suggesting that the former are crucial for the transformation of primary cells, but dispensable for transformation of factor dependent cell lines. Our data suggest that inhibition of BCR-ABL-induced leukemia by disrupting protein interactions could be possible, but would require blocking of multiple sites.

Crk and CrkL adaptor proteins: networks for physiological and pathological signaling

The Crk adaptor proteins (Crk and CrkL) constitute an integral part of a network of essential signal transduction pathways in humans and other organisms that act as major convergence points in tyrosine kinase signaling. Crk proteins integrate signals from a wide variety of sources, including growth factors, extracellular matrix molecules, bacterial pathogens, and apoptotic cells. Mounting evidence indicates that dysregulation of Crk proteins is associated with human diseases, including cancer and susceptibility to pathogen infections. Recent structural work has identified new and unusual insights into the regulation of Crk proteins, providing a rationale for how Crk can sense diverse signals and produce a myriad of biological responses.

Role of c-Met in cancer: emphasis on lung cancer

Lung cancer remains the leading cause of cancer death. It is often diagnosed at late stages and is treated systemically with cytotoxic chemotherapy, which is generally ineffective. Research efforts have focused on developing therapies targeted to growth factor receptor pathways, such as epidermal growth factor receptor (EGFR), but the results from clinical trials overall show very small improvements in survival. Research on signaling pathways dysregulated in lung cancer is ongoing, including investigation of the hepatocyte growth factor receptor (HGFR) or c-Met. Protein tyrosine kinases, such as EGFR and c-Met, are a family of oncogenes that regulate important cellular processes, such as differentiation, proliferation, cell cycle, motility, and apoptosis. Hepatocyte growth factor (HGF), a ligand for c-Met, is secreted by mesodermal cells during development. It produces multiple effects upon binding to its receptor (HGFR/c-Met) and regulates proliferation, motility, mitogenesis, and morphogenesis. Studies in cell lines isolated from various tumors show that several intracellular pathways participate in c-Met signaling, including growth factor receptor-bound protein 2 (Grb2), mitogen-activated protein (MAP) kinase, phosphoinositol 3-kinase (PI3K), and phospholipase C-gamma (PLC-gamma). c-Met is overexpressed in many tumors. However, overexpression may not be sufficient to cause increased activity; the receptor needs to be activated. In some cases, the kinases are constitutively active due to mutations in the gene. The cytoskeletal protein paxillin also appears to be activated along with c-Met. Correlative studies from patient tissue samples and cell lines have rendered the same information, indicating that the signaling pathways dysregulated are complex and interdependent. Mutations in human c-Met have been exogenously expressed in Caenorhabditis elegans, which can serve as a model for determining the role of gene mutations in a whole organism. Several inhibitors of c-Met/HGF binding are in development, including some in phase I trials. Their effectiveness in improving cancer outcomes will be determined in the near future.

Dictyostelium discoideum paxillin regulates actin-based processes

Paxillin is a key player in integrating the actin cytoskeleton with adhesion, and thus is essential to numerous cellular processes, including proliferation, differentiation, and migration in animal cells. PaxB, the Dictyostelium discoideum orthologue of paxillin, has been shown to be important for adhesion and development, much like its mammalian counterpart. Here, we use the overproduction of PaxB to gain better insight into its role in regulating the actin cytoskeleton and adhesion. We find that PaxB-overexpressing (PaxBOE) cells can aggregate and form mounds normally, but are blocked in subsequent development. This arrest can be rescued by addition of wild-type cells, indicating a non-cell autonomous role for PaxB. PaxBOE cells also have alterations in several actin-based processes, including adhesion, endocytosis, motility, and chemotaxis. PaxBOE cells exhibit an EDTA-sensitive increase in cell-cell cohesion, suggesting that PaxB-mediated adhesion is Ca(2+) or Mg(2+) dependent. Interestingly, cells overexpressing paxB are less adhesive to the substratum. In addition, PaxBOE cells display decreased motility under starved conditions, decreased endocytosis, and are unable to efficiently chemotax up a folate gradient. Taken together, the data suggest that proper expression of PaxB is vital for the regulation of development and actin-dependent processes.

Enhanced interaction between focal adhesion and adherens junction proteins: involvement in sphingosine 1-phosphate-induced endothelial barrier enhancement

Sphingosine 1-phosphate (S1P) is an important vascular barrier regulatory agonist which enhances the junctional integrity of human lung endothelial cell monolayers. We have now demonstrated that S1P induced cortical actin ring formation and redistribution of focal adhesion kinase (FAK) and paxillin to the cell periphery suggesting the critical role of cell-cell adhesion in endothelial barrier enhancement. Co-immunoprecipitation studies revealed increased association of VE-cadherin with FAK and paxillin in S1P-challenged human pulmonary artery endothelial cell (HPAEC) monolayers. Furthermore, S1P-induced enhancement of VE-cadherin interaction with alpha-catenin and beta-catenin was associated with the increased formation of FAK-beta-catenin protein complexes. Depletion of beta-catenin (siRNA) resulted in loss of S1P-mediated VE-cadherin association with FAK and paxillin rearrangement. Furthermore, transendothelial electrical resistance (an index of barrier function) demonstrated that beta-catenin siRNA significantly attenuated S1P-induced barrier enhancement. These results demonstrate a mechanism of S1P-induced endothelial barrier enhancement via beta-catenin-linked adherens junction and focal adhesion interaction.

LIM domain protein TES changes its conformational states in different cellular compartments

The human TESTIN (TES) is a putative tumor suppressor and localizes to the cytoplasm as a component of focal adhesions and cell contacts. TES contains a PET domain in the NH(2)-terminus and three tandem LIM domains in the COOH-terminus. It has been hypothesized that interactions between two termini of TES might lead to a "closed" conformational state of the protein. Here, we provide evidence for different conformational states of TES. We confirmed that the NH(2)-terminus of TES can interact with its third LIM domain in the COOH-terminus by GST pull-down assays. In addition, antisera against the full-length or two truncations of TES were prepared to examine the relationship between the conformation and cellular distribution of the protein. We found that these antisera recognize different regions of TES and showed that TES is co-localised with the marker protein B23 in nucleolus, in addition to its localization in endoplasmic reticulum (ER). Furthermore, our co-immunoprecipitation (co-IP) analysis of TES and B23 demonstrated their co-existence in the same complex. Taken together, our results suggest that TES has different conformational states in different cellular compartments, and a "closed" conformational state of TES may be involved in nucleolar localization.

Paxillin is a target for somatic mutations in lung cancer: implications for cell growth and invasion

Lung cancer is characterized by abnormal cell growth and invasion, and the actin cytoskeleton plays a major role in these processes. The focal adhesion protein paxillin is a target of a number of oncogenes involved in key signal transduction and important in cell motility and migration. In lung cancer tissues, we have found that paxillin was highly expressed (compared with normal lung), amplified (12.1%, 8 of 66) and correlated with increased MET and epidermal growth factor receptor (EGFR) gene copy numbers, or mutated (somatic mutation rate of 9.4%, 18 of 191). Paxillin mutations (19 of 21) were clustered between LD motifs 1 and 2 and the LIM domains. The most frequent point mutation (A127T) enhanced lung cancer cell growth, colony formation, focal adhesion formation, and colocalized with Bcl-2 in vitro. Gene silencing from RNA interference of mutant paxillin led to reduction of cell viability. A murine in vivo xenograft model of A127T paxillin showed an increase in tumor growth, cell proliferation, and invasion. These results establish an important role for paxillin in lung cancer.

Vinculin controls focal adhesion formation by direct interactions with talin and actin

Focal adhesions (FAs) regulate cell migration. Vinculin, with its many potential binding partners, can interconnect signals in FAs. Despite the well-characterized structure of vinculin, the molecular mechanisms underlying its action have remained unclear. Here, using vinculin mutants, we separate the vinculin head and tail regions into distinct functional domains. We show that the vinculin head regulates integrin dynamics and clustering and the tail regulates the link to the mechanotransduction force machinery. The expression of vinculin constructs with unmasked binding sites in the head and tail regions induces dramatic FA growth, which is mediated by their direct interaction with talin. This interaction leads to clustering of activated integrin and an increase in integrin residency time in FAs. Surprisingly, paxillin recruitment, induced by active vinculin constructs, occurs independently of its potential binding site in the vinculin tail. The vinculin tail, however, is responsible for the functional link of FAs to the actin cytoskeleton. We propose a new model that explains how vinculin orchestrates FAs.

Paxillin phosphorylation controls invadopodia/podosomes spatiotemporal organization

In Rous sarcoma virus (RSV)-transformed baby hamster kidney (BHK) cells, invadopodia can self-organize into rings and belts, similarly to podosome distribution during osteoclast differentiation. The composition of individual invadopodia is spatiotemporally regulated and depends on invadopodia localization along the ring section: the actin core assembly precedes the recruitment of surrounding integrins and integrin-linked proteins, whereas the loss of the actin core was a prerequisite to invadopodia disassembly. We have shown that invadopodia ring expansion is controlled by paxillin phosphorylations on tyrosine 31 and 118, which allows invadopodia disassembly. In BHK-RSV cells, ectopic expression of the paxillin mutant Y31F-Y118F induces a delay in invadopodia disassembly and impairs their self-organization. A similar mechanism is unraveled in osteoclasts by using paxillin knockdown. Lack of paxillin phosphorylation, calpain or extracellular signal-regulated kinase inhibition, resulted in similar phenotype, suggesting that these proteins belong to the same regulatory pathways. Indeed, we have shown that paxillin phosphorylation promotes Erk activation that in turn activates calpain. Finally, we observed that invadopodia/podosomes ring expansion is required for efficient extracellular matrix degradation both in BHK-RSV cells and primary osteoclasts, and for transmigration through a cell monolayer.

Genomic implications of H(2)O (2) for cell proliferation and growth of Caco-2 cells

Evidence indicates that oxidative stress inhibits cell proliferation in several cell systems. To determine whether the proliferation of Caco-2 cells is inhibited by oxidative stress and to identify any novel key regulatory factors involved in protecting or damaging the intestine from oxidative stress, Caco-2 cells were treated with an oxidizing agent and analyzed by transcriptomic oligonucleotide microarrays. Results indicated that expression of genes involved in cell proliferation and growth, including genes involved in lipid synthesis, cell cycle progression and cell division, angiogenesis, RNA processing and translation, cAMP metabolism, cytoskeleton and cell to cell adhesion, receptor tyrosine kinases, and intracellular and extracellular signaling, were repressed. If an oxidant-induced inhibition in cell proliferation is involved in the pathogenesis of intestinal disease, information gained could help explain the mechanisms contributing to the causes and consequences of intestinal disease and could aid in the elucidation of mechanisms by which intestinal cells protect against oxidative stress.

Active Rap1, a small GTPase that induces malignant transformation of hematopoietic progenitors, localizes in the nucleus and regulates protein expression

Rap1, a member of the Ras superfamily, regulates cytoskeletal changes in lower eukaryots and integrin-mediated adhesion in hematopoietic cells. Sustained activation of Rap1 in mouse hematopoietic stem cells causes expansion of hematopoietic progenitors, followed by a myeloproliferative disorder mimicking chronic myeloid leukemia. Moreover, these mice develop a B-cell lymphoproliferative disorder resembling chronic lymphocytic leukemia. Here, we used HEK 293 cells as a tool to examine the molecular effects of Rap1. We observed that a constitutively active Rap1 mutant localized predominantly in the nucleus. Nuclear localization of endogenous Rap1-GTP was also detected upon physiologic activation. A potential consequence of nuclear localization of Rap1-GTP is the regulation of gene expression. We used a high throughput proteomic approach to identify gene products potentially modulated by Rap1-GTP. Out of 1000 proteins examined, 64 proteins were upregulated and 66 proteins were downregulated. The differentially expressed gene products belong to cytoskeletal regulator proteins, signaling molecules, transcription factors, viability regulators, and protein transporters. This analysis provides the first fingerprint of gene product expression regulated by Rap1 and may contribute to our understanding of malignant transformation mechanisms regulated by this small GTPase.

Reassessing the role of phosphocaveolin-1 in cell adhesion and migration

Although phosphorylation on tyrosine 14 was identified early in the discovery of caveolin-1, the functional significance of this modification still remains elusive. Recent evidence points to a role of caveolin-1 tyrosine 14 phosphorylation in cell adhesion and migration. These results are based on a variety of tools, including a widely used mouse monoclonal anti-phosphocaveolin-1 antibody, which labels, in cultured cells, a protein localized at or near focal adhesions. We here report results from three independent laboratories, showing that this antibody recognizes phosphocaveolin-1 amongst other proteins in immunoblot analyses and that the signal obtained with this antibody in immunostaining experiments is in part due to labeling of paxillin. Published data need to be interpreted keeping in mind that images of phosphocaveolin-1 cellular localization obtained using this antibody are not valid. We re-evaluate the current knowledge about the role of caveolin-1 in cell adhesion and migration in view of this new information.

Paxillin and ponsin interact in nascent costameres of muscle cells

Muscle differentiation requires the transition from motile myoblasts to sessile myotubes and the assembly of a highly regular contractile apparatus. This striking cytoskeletal remodelling is coordinated with a transformation of focal adhesion-like cell-matrix contacts into costameres. To assess mechanisms underlying this differentiation process, we searched for muscle specific-binding partners of paxillin. We identified an interaction of paxillin with the vinexin adaptor protein family member ponsin in nascent costameres during muscle differentiation, which is mediated by an interaction of the second src homology domain 3 (SH3) domain of ponsin with the proline-rich region of paxillin. To understand the molecular basis of this interaction, we determined the structure of this SH3 domain at 0.83 A resolution, as well as its complex with the paxillin binding peptide at 1.63 A resolution. Upon binding, the paxillin peptide adopts a polyproline-II helix conformation in the complex. Contrary to the charged SH3 binding interface, the peptide contains only non-polar residues and for the first time such an interaction was observed structurally in SH3 domains. Fluorescence titration confirmed the ponsin/paxillin interaction, characterising it further by a weak binding affinity. Transfection experiments revealed further characteristics of ponsin functions in muscle cells: All three SH3 domains in the C terminus of ponsin appeared to synergise in targeting the protein to force-transducing structures. The overexpression of ponsin resulted in altered muscle cell-matrix contact morphology, suggesting its involvement in the establishment of mature costameres. Further evidence for the role of ponsin in the maintenance of mature mechanotransduction sites in cardiomyocytes comes from the observation that ponsin expression was down-regulated in end-stage failing hearts, and that this effect was reverted upon mechanical unloading. These results provide new insights in how low affinity protein-protein interactions may contribute to a fine tuning of cytoskeletal remodelling processes during muscle differentiation and in adult cardiomyocytes.