Interactions of a hemidesmosome component and actinin family members

Native autoantigen complex detects pemphigoid autoantibodies

Pemphigoid diseases are a group of autoimmune disorders characterized by subepidermal blistering in the skin and mucosa. Among them, mucous membrane pemphigoid (MMP) autoantibodies are characterized by targeting multiple molecules in the hemidesmosomes, including collagen XVII, laminin-332, and integrin a6/β4. Traditionally, recombinant proteins of the autoantigens have been employed to identify circulating autoantibodies by immune assays. However, developing an efficient detection system for MMP autoantibodies has been challenging because the autoantibodies have heterogeneous profiles and the antibody titers are typically low. In this study, we introduce an ELISA that takes advantage of a native autoantigen complex rather than simple recombinant proteins. We generated HaCaT keratinocytes with a DDDDK-tag knocked in at the COL17A1 locus by CRISPR/Cas9-mediated gene editing. Immunoprecipitation using the DDDDK-tag isolated a native complex that contained full-length and processed collagen XVII and integrin α6/β4. Then, we used the complex proteins to prepare an ELISA system and enrolled 55 MMP cases to validate its diagnostic performance. The sensitivity and specificity of the ELISA for detecting MMP autoantibodies were 70.9% and 86.7%, respectively, far superior to those of conventional assays. In autoimmune diseases such as MMP, in which autoantibodies target various molecules, isolating the antigen-protein complexes can help establish a diagnostic system.

BP180/Collagen XVII: A Molecular View

BP180 is a type II collagenous transmembrane protein and is best known as the major autoantigen in the blistering skin disease bullous pemphigoid (BP). The BP180 trimer is a central component in type I hemidesmosomes (HD), which cause the adhesion between epidermal keratinocytes and the basal lamina, but BP180 is also expressed in several non-HD locations, where its functions are poorly characterized. The immunological roles of intact and proteolytically processed BP180, relevant in BP, have been subject to intensive research, but novel functions in cell proliferation, differentiation, and aging have also recently been described. To better understand the multiple physiological functions of BP180, the focus should return to the protein itself. Here, we comprehensively review the properties of the BP180 molecule, present new data on the biochemical features of its intracellular domain, and discuss their significance with regard to BP180 folding and protein-protein interactions.

Metalloproteomics analysis in human mammary cell lines treated with inorganic mercury

The interest in inorganic Hg toxicity and carcinogenicity has been pointed to target organs such as kidney, brain or placenta, but only a few studies have focused on the mammary gland. In this work, analytical combination techniques (SDS-PAGE followed by CV-AFS, and nanoUPLC-ESI-MS/MS) were used to determine proteins that could bind Hg in three human mammary cell lines. Two of them were tumorigenic (MCF-7 and MDA-MB-231) and the other one was the non-tumorigenic cell line (MCF-10A). There are no studies that provide this kind of information in breast cell lines with IHg treatment. Previously, we described the viability, uptake and the subcellular distribution of Hg in human breast cells and analysis of RNA-seq about the genes that encode proteins which are related to cytotoxicity of Hg. This work provides important protein candidates for further studies of Hg toxicity in the mammary gland, thus expanding our understanding of how environmental contaminants might affect tumor progression and contribute with future therapeutic methods.

Life before and beyond blistering: The role of collagen XVII in epidermal physiology

Type XVII collagen (COL17) is a transmembranous protein that is mainly expressed in the epidermal basal keratinocytes. Epidermal-dermal attachment requires COL17 expression at the hemidesmosomes of the epidermal basement membrane zone because congenital COL17 deficiency leads to junctional epidermolysis bullosa and acquired autoimmunity to COL17 induces bullous pemphigoid. Recently, in addition to facilitating epidermal-dermal attachment, COL17 has been reported to serve as a niche for hair follicle stem cells, to regulate proliferation in the interfollicular epidermis and to be present along the non-hemidesmosomal plasma membrane of epidermal basal keratinocytes. This review focuses on the physiological properties of COL17 in the epidermis, its role in maintaining stem cells and its association with signalling pathways. We propose possible solutions to unanswered questions in this field.

Hemidesmosomes and Focal Adhesions Treadmill as Separate but Linked Entities during Keratinocyte Migration

Hemidesmosomes anchor the epidermal keratin filament cytoskeleton to the extracellular matrix. They are crucial for the mechanical integrity of skin. Their role in keratinocyte migration, however, remains unclear. Examining migrating primary human keratinocytes, we find that hemidesmosomes cluster as ordered arrays consisting of multiple chevrons that are flanked by actin-associated focal adhesions. These hemidesmosomal arrays with intercalated focal adhesions extend from the cell rear to the cell front. New hemidesmosomal chevrons form subsequent to focal adhesion assembly at the cell's leading front, whereas chevrons and associated focal adhesions disassemble at the cell rear in reverse order. The bulk of the hemidesmosome-focal adhesion composite, however, remains attached to the substratum during cell translocation. Similar hemidesmosome-focal adhesion patterns emerge on X-shaped fibronectin-coated micropatterns, during cell spreading and in leader cells during collective cell migration. We further find that hemidesmosomes and focal adhesions affect each other's distribution. We propose that both junctions are separate but linked entities, which treadmill coordinately to support efficient directed cell migration and cooperate to coordinate the dynamic interplay between the keratin and actin cytoskeleton.

Cytoskeleton structure and total methylation of mouse cardiac and lung tissue during space flight

The purpose of this work was to evaluate the protein and mRNA expression levels of multiple cytoskeletal proteins in the cardiac and lung tissue of mice that were euthanized onboard the United States Orbital Segment of the International Space Station 37 days after the start of the SpaceX-4 mission (September 2014, USA). The results showed no changes in the cytoskeletal protein content in the cardiac and lung tissue of the mice, but there were significant changes in the mRNA expression levels of the associated genes, which may be due to an increase in total genome methylation. The mRNA expression levels of DNA methylases, the cytosine demethylases Tet1 and Tet3, histone acetylase and histone deacetylase did not change, and the mRNA expression level of cytosine demethylase Tet2 was significantly decreased.

Candidate biomarkers in the cervical vaginal fluid for the (self-)diagnosis of cervical precancer

Purpose

Despite improvement in vaccines against human papilloma virus (HPV), the causative agent of cervical cancer, screening women for cervical precancer will remain indispensable in the coming 30–40 years. A simple test that could be performed at home or at a doctor’s practice and that informs the woman whether she is at risk would significantly help make a broader group of patients who aware that they need medical treatment. Cervical vaginal fluid (CVF) is a body fluid that is very well suited for such a test.

Methods

Narrative review of cervical (pre)cancer candidate biomarkers from cervicovaginal fluid, is based on a detailed review of the literature. We will also discuss the possibilities that these biomarkers create for the development of a self-test or point-of-care test for cervical (pre)cancer.

Results

Several DNA, DNA methylation, miRNA, and protein biomarkers were identified in the cervical vaginal fluid; however, not all of these biomarkers are suited for development of a simple diagnostic assay.

Conclusions

Proteins, especially alpha-actinin-4, are most suited for development of a simple assay for cervical (pre)cancer. Accuracy of the test could further be improved by combination of several proteins or by combination with a new type of biomarker, e.g., originating from the cervicovaginal microbiome or metabolome.

A hemidesmosomal protein regulates actin dynamics and traction forces in motile keratinocytes

During wound healing of the skin, keratinocytes disassemble hemidesmosomes and reorganize their actin cytoskeletons in order to exert traction forces on and move directionally over the dermis. Nonetheless, the transmembrane hemidesmosome component collagen XVII (ColXVII) is found in actin-rich lamella, situated behind the lamellipodium. A set of actin bundles, along which ColXVII colocalizes with actinin4, is present at each lamella. Knockdown of either ColXVII or actinin4 not only inhibits directed migration of keratinocytes but also relieves constraints on actin bundle retrograde movement at the site of lamella, such that actin bundle movement is enhanced more than 5-fold. Moreover, whereas control keratinocytes move in a stepwise fashion over a substrate by generating alternating traction forces, of up to 1.4 kPa, at each flank of the lamellipodium, ColXVII knockdown keratinocytes fail to do so. In summary, our data indicate that ColXVII-actinin4 complexes at the lamella of a moving keratinocyte regulate actin dynamics, thereby determining the direction of cell movement.-Hiroyasu, S., Colburn, Z. T., Jones, J. C. R. A hemidesmosomal protein regulates actin dynamics and traction forces in motile keratinocytes.

Tandem phosphorylation within an intrinsically disordered region regulates ACTN4 function

Phosphorylated residues occur preferentially in the intrinsically disordered regions of eukaryotic proteins. In the disordered amino-terminal region of human α-actinin-4 (ACTN4), Tyr(4) and Tyr(31) are phosphorylated in cells stimulated with epidermal growth factor (EGF), and a mutant with phosphorylation-mimicking mutations of both tyrosines exhibits reduced interaction with actin in vitro. Cleavage of ACTN4 by m-calpain, a protease that in motile cells is predominantly activated at the rear, removes the Tyr(4) site. We found that introducing a phosphomimetic mutation at only Tyr(31) was sufficient to inhibit the interaction with actin in vitro. However, molecular dynamics simulations predicted that Tyr(31) is mostly buried and that phosphorylation of Tyr(4) would increase the solvent exposure and thus kinase accessibility of Tyr(31). In fibroblast cells, EGF stimulation increased tyrosine phosphorylation of a mutant form of ACTN4 with a phosphorylation-mimicking residue at Tyr(4), whereas a truncated mutant representing the product of m-calpain cleavage exhibited EGF-stimulated tyrosine phosphorylation at a background amount similar to that observed for a double phosphomimetic mutant of Tyr(4) and Tyr(31). We also found that inhibition of the receptor tyrosine kinases of the TAM family, such as AXL, blocked EGF-stimulated tyrosine phosphorylation of ACTN4. Mathematical modeling predicted that the kinetics of phosphorylation at Tyr(31) can be dictated by the kinase affinity for Tyr(4). This study suggests that tandem-site phosphorylation within intrinsically disordered regions provides a mechanism for a site to function as a switch to reveal a nearby function-regulating site.

Structure of the cortical cytoskeleton in fibers of postural muscles and cardiomyocytes of mice after 30-day 2-g centrifugation

Altered external mechanical loading during spaceflights causes negative effects on muscular and cardiovascular systems. The aim of the study was estimation of the cortical cytoskeleton statement of the skeletal muscle cells and cardiomyocytes. The state of the cortical cytoskeleton in C57BL6J mice soleus, tibialis anterior muscle fibers, and left ventricle cardiomyocytes was investigated after 30-day 2-g centrifugation ("2-g" group) and within 12 h after its completion ("2-g + 12-h" group). We used atomic force microscopy for estimating cell's transverse stiffness, Western blotting for measuring protein content, and RT-PCR for estimating their expression level. The transverse stiffness significantly decreased in cardiomyocytes (by 16%) and increased in skeletal muscles fibers (by 35% for soleus and by 29% for tibialis anterior muscle fibers) in animals of the 2-g group (compared with the control group). For cardiomyocytes, we found that, in the 2-g + 12-h group, α-actinin-1 content decreased in the membranous fraction (by 27%) and increased in cytoplasmic fraction (by 28%) of proteins (compared with the levels in the 2-g group). But for skeletal muscle fibers, similar changes were noted for α-actinin-4, but not for α-actinin-1. In conclusion, we showed that the different isoforms of α-actinins dissociate from cortical cytoskeleton under increased/decreased of mechanical load.

Alpha actinin-1 regulates cell-matrix adhesion organization in keratinocytes: consequences for skin cell motility

The migration of keratinocytes in wound healing requires coordinated activities of the motility machinery of a cell, the cytoskeleton and matrix adhesions. In this study we assessed the role of alpha actinin-1 (ACTN1), one of the two alpha actinin isoforms expressed in keratinocytes, in skin cell migration via an shRNA-mediated knockdown approach. Keratinocytes deficient in ACTN1 exhibit changes in their actin cytoskeleton organization, a loss in front-rear polarity and impaired lamellipodial dynamics. They also display aberrant directed motility and move slower than their wild-type counterparts. Moreover, they have abnormally arranged matrix adhesion sites. Specifically, the focal adhesions in ACTN1 knockdown keratinocytes are not organized as distinct entities. Rather, focal adhesion proteins are arranged in a circle subjacent to cortical fibers of actin. In the same cells, hemidesmosome proteins arrange in cat paw patterns, more typical of confluent, stationary cells and β4 integrin dynamics are reduced in knockdown cells compared with control keratinocytes. In summary, our data suggest a mechanism by which ACTN1 determines the motility of keratinocytes by regulating the organization of the actin cytoskeleton, focal adhesion and hemidesmosome proteins complexes, thereby modulating cell speed, lamellipodial dynamics and directed migration.

Possible role of non-muscle alpha-actinins in muscle cell mechanosensitivity

The main hypothesis suggested that changes in the external mechanical load would lead to different deformations of the submembranous cytoskeleton and, as a result, dissociation of different proteins from its structure (induced by increased/decreased mechanical stress). The study subjects were fibers of the soleus muscle and cardiomyocytes of Wistar rats. Changes in external mechanical conditions were reconstructed by means of antiorthostatic suspension of the animals by their tails for 6, 12, 18, 24 and 72 hours. Transversal stiffness was measured by atomic force microscopy imaging; beta-, gamma-actin, alpha-actinin 1 and alpha-actinin 4 levels in membranous and cytoplasmic fractions were quantified by Western blot analysis; expression rates of the corresponding genes were studied using RT-PCR.

RESULTS

In 6 hours, alpha-actinin 1 and alpha-actinin 4 levels decreased in the membranous fraction of proteins of cardiomyocytes and soleus muscle fibers, respectively, but increased in the cytoplasmic fraction of the abovementioned cells. After 6-12 hours of suspension, the expression rates of beta-, gamma-actin, alpha-actinin 1 and alpha-actinin 4 were elevated in the soleus muscle fibers, but the alpha-actinin 1 expression rate returned to the reference level in 72 hours. After 18-24 hours, the expression rates of beta-actin and alpha-actinin 4 increased in cardiomyocytes, while the alpha-actinin 1 expression rate decreased in soleus muscle fibers. After 12 hours, the beta- and gamma-actin content dropped in the membranous fraction and increased in the cytoplasmic protein fractions from both cardiomyocytes and soleus muscle fibers. The stiffness of both cell types decreased after the same period of time. Further, during the unloading period the concentration of nonmuscle actin and different isoforms of alpha-actinins increased in the membranous fraction from cardiomyocytes. At the same time, the concentration of the abovementioned proteins decreased in the soleus muscle fibers.

Structure of cortical cytoskeleton in fibers of mouse muscle cells after being exposed to a 30-day space flight on board the BION-M1 biosatellite

The aim of the work was to analyze changes in the organization of the cortical cytoskeleton in fibers of the mouse soleus muscle, tibialis anterior muscle and left ventricular cardiomyocytes after completion of a 30-day space flight on board the BION-M1 biosatellite (Russia, 2013). The transversal stiffness of the cortical cytoskeleton of the cardiomyocytes and fibers of the skeletal muscles did not differ significantly within the study groups compared with the vivarium control group. The content of beta- and gamma-actin in the membranous fraction of proteins in the left ventricular cardiomyocytes did not differ significantly within all study groups and correlated with the transversal stiffness. A similar situation was revealed in fibers of the soleus muscle and tibialis anterior muscle. At the same time, the content of beta-actin in the cytoplasmic fraction of proteins was found to be decreased in all types of studied tissues compared with the control levels in the postflight group, with lowered beta-actin gene expression rates in the postflight group. After completion of the space flight, the content of alpha-actinin-4 was found to be reduced in the membranous fraction of proteins from the mouse cardiomyocytes, while its content in the cytoplasmic fraction of proteins did not change significantly. Furthermore, gene expression rates of this protein were decreased at the time of dissection (it was started after 13 h after landing). At the same time, the content of alpha-actinin-1 decreased in the membranous fraction and increased in the cytoplasmic fraction of proteins from the soleus muscle fibers.

Actin-binding protein alpha-actinin 4 (ACTN4) is a transcriptional co-activator of RelA/p65 sub-unit of NF-kB

ACTN4 is an actin-binding protein that participates in cytoskeleton organisation. It resides both in the cytoplasm and nucleus and physically associates with various transcription factors. Here, we describe an effect of ACTN4 expression on transcriptional activity of the RelA/p65 subunit of NF-kB. We demonstrate that ACTN4 enhances RelA/p65-dependant expression of c-fos, MMP-3 and MMP-1 genes, but it does not affect TNC, ICAM1 and FN1 expression. Importantly, actin-binding domains of ACTN4 are not critical for the nuclear translocation and co-activation of RelA/p65-dependent transcription. Collectively, our data suggest that in the nucleus, ACTN4 functions as a selective transcriptional co-activator of RelA/p65.

Immunohistochemical actinin-4 expression in infiltrating gliomas: association with WHO grade and differentiation

Actinin-4 is an isoform of nonmuscular α-actinin and actin-bundling protein that plays an important role in cancer invasion and metastasis by enhancing cellular motility. Recent studies have revealed an association between several clinicopathological profiles and actinin-4 overexpression in human cancers. In this study, we investigated the immunohistochemical actinin-4 expression in 39 infiltrating gliomas. The specimens included three diffuse astrocytomas, three oligodendrogliomas, one oligoastrocytoma, two anaplastic astrocytomas, four anaplastic oligodendrogliomas, three anaplastic oligoastrocytomas, 17 glioblastomas, four gliosarcomas, and two glioblastomas with oligodendroglial component. All seven World Health Organization (WHO) grade II tumors were negative for actinin-4, whereas 20 of 22 tumors with strong actinin-4 expression were WHO grade IV. Actinin-4 expression was significantly associated with histological grade (P < 0.0001) and proliferative activity measured by Ki-67 staining (P = 0.0045). Notably, actinin-4 expression was more pronounced in high-grade astrocytic tumors than oligodendroglial tumors (P < 0.0001). Additionally, pseudopalisading cells in glioblastoma exhibited stronger actinin-4 expression than the rest, likely reflecting enhanced cellular motility in pseudopalisades. This study is the first to demonstrate significant correlation between actinin-4 expression and tumor grade using clinical glioma samples. Although diagnostic utility of this marker awaits future exploration, actinin-4 may help distinguish between astrocytic and oligodendroglial lines of differentiation.

Actinin-4 in keratinocytes regulates motility via an effect on lamellipodia stability and matrix adhesions

During wound repair, epidermal cells at the edge of an injury establish front-rear polarity through orchestrated changes in their cytoskeleton and adhesion structures. The polarity and directed migration of such cells is determined by the assembly, extension, and stabilization of a lamellipodium. Actinin-4 associates with lamellipodia and has been implicated in regulating lamellipodial structure, function and assembly. To study the functions of actinin-4 in human keratinocytes, we used shRNA to generate knockdown cells and compared their motility behavior and matrix adhesion assembly to scrambled shRNA treated control keratinocytes. Actinin-4 knockdown keratinocytes lack polarity, assemble multiple lamellipodia with a 2× increased area over controls, display reduced activity of the actin remodeling protein cofilin, and fail to migrate in a directional manner. This motility defect is rescued by plating knockdown cells on preformed laminin-332 matrix. In actinin-4-knockdown keratinocytes, focal contact area is increased by 25%, and hemidesmosome proteins are mislocalized. Specifically, α6β4 integrin localizes to large lamellipodial extensions, displays reduced dynamics, and fails to recruit its bullous pemphigoid antigen binding partners. Together, our data indicate a role for actinin-4 in regulating the steering mechanism of keratinocytes via profound effects on their matrix adhesion sites.

HAMLET: functional properties and therapeutic potential

Human α-lactalbumin made lethal to tumor cells (HAMLET) is the first member in a new family of protein–lipid complexes that kills tumor cells with high selectivity. The protein component of HAMLET is α-lactalbumin, which in its native state acts as a substrate specifier in the lactose synthase complex, thereby defining a function essential for the survival of lactating mammals. In addition, α-lactalbumin acquires tumoricidal activity after partial unfolding and binding to oleic acid. The lipid cofactor serves the dual role as a stabilizer of the altered fold of the protein and a coactivator of specific steps in tumor cell death. HAMLET is broadly tumoricidal, suggesting that the complex identifies conserved death pathways suitable for targeting by novel therapies. Sensitivity to HAMLET is defined by oncogene expression including Ras and c-Myc and by glycolytic enzymes. Cellular targets are located in the cytoplasmic membrane, cytoskeleton, mitochondria, proteasomes, lysosomes and nuclei, and specific signaling pathways are rapidly activated, first by interactions of HAMLET with the cell membrane and subsequently after HAMLET internalization. Therapeutic effects of HAMLET have been demonstrated in human skin papillomas and bladder cancers, and HAMLET limits the progression of human glioblastomas, with no evidence of toxicity for normal brain or bladder tissue. These findings open up new avenues for cancer therapy and the understanding of conserved death responses in tumor cells.

Transmembrane collagen XVII is a novel component of the glomerular filtration barrier

The kidney filtration barrier consists of the capillary endothelium, the glomerular basement membrane and the slit diaphragm localized between foot processes of neighbouring podocytes. We report that collagen XVII, a transmembrane molecule known to be required for epithelial adhesion, is expressed in podocytes of normal human and mouse kidneys and in endothelial cells of the glomerular filtration barrier. Immunoelectron microscopy has revealed that collagen XVII is localized in foot processes of podocytes and in the glomerular basement membrane. Its role in kidney has been analysed in knockout mice, which survive to birth but have high neonatal mortality and skin blistering and structural abnormalities in their glomeruli. Morphometric analysis has shown increases in glomerular volume fraction and surface densities of knockout kidneys, indicating an increased glomerular amount in the cortex. Collagen XVII deficiency causes effacement of podocyte foot processes; however, major slit diaphragm disruptions have not been detected. The glomerular basement membrane is split in areas in which glomerular and endothelial basement membranes meet. Differences in the expression of collagen IV, integrins α3 or β1, laminin α5 and nephrin have not been observed in mutant mice compared with controls. We propose that collagen XVII has a function in the attachment of podocyte foot processes to the glomerular basement membrane. It probably contributes to podocyte maturation and might have a role in glomerular filtration.

Cytoskeleton in motion: the dynamics of keratin intermediate filaments in epithelia

Epithelia are exposed to multiple forms of stress. Keratin intermediate filaments are abundant in epithelia and form cytoskeletal networks that contribute to cell type–specific functions, such as adhesion, migration, and metabolism. A perpetual keratin filament turnover cycle supports these functions. This multistep process keeps the cytoskeleton in motion, facilitating rapid and protein biosynthesis–independent network remodeling while maintaining an intact network. The current challenge is to unravel the molecular mechanisms underlying the regulation of the keratin cycle in relation to actin and microtubule networks and in the context of epithelial tissue function.

Type XVII collagen regulates lamellipod stability, cell motility, and signaling to Rac1 by targeting bullous pemphigoid antigen 1e to alpha6beta4 integrin

Rac1 activity, polarity, lamellipodial dynamics, and directed motility are defective in keratinocytes exhibiting deficiency in β4 integrin or knockdown of the plakin protein Bullous Pemphigoid Antigen 1e (BPAG1e). The activity of Rac, formation of stable lamellipodia, and directed migration are restored in β4 integrin-deficient cells by inducing expression of a truncated form of β4 integrin, which lacks binding sites for BPAG1e and plectin. In these same cells, BPAG1e, the truncated β4 integrin, and type XVII collagen (Col XVII), a transmembrane BPAG1e-binding protein, but not plectin, colocalize along the substratum-attached surface. This finding suggested to us that Col XVII mediates the association of BPAG1e and α6β4 integrin containing the truncated β4 subunit and supports directed migration. To test these possibilities, we knocked down Col XVII expression in keratinocytes expressing both full-length and truncated β4 integrin proteins. Col XVII-knockdown keratinocytes exhibit a loss in BPAG1e-α6β4 integrin interaction, a reduction in lamellipodial stability, an impairment in directional motility, and a decrease in Rac1 activity. These defects are rescued by a mutant Col XVII protein truncated at its carboxyl terminus. In summary, our results suggest that in motile cells Col XVII recruits BPAG1e to α6β4 integrin and is necessary for activation of signaling pathways, motile behavior, and lamellipodial stability.

HAMLET binding to α-actinin facilitates tumor cell detachment

Cell adhesion is tightly regulated by specific molecular interactions and detachment from the extracellular matrix modifies proliferation and survival. HAMLET (Human Alpha-lactalbumin Made LEthal to Tumor cells) is a protein-lipid complex with tumoricidal activity that also triggers tumor cell detachment in vitro and in vivo, suggesting that molecular interactions defining detachment are perturbed in cancer cells. To identify such interactions, cell membrane extracts were used in Far-western blots and HAMLET was shown to bind α-actinins; major F-actin cross-linking proteins and focal adhesion constituents. Synthetic peptide mapping revealed that HAMLET binds to the N-terminal actin-binding domain as well as the integrin-binding domain of α-actinin-4. By co-immunoprecipitation of extracts from HAMLET-treated cancer cells, an interaction with α-actinin-1 and -4 was observed. Inhibition of α-actinin-1 and α-actinin-4 expression by siRNA transfection increased detachment, while α-actinin-4-GFP over-expression significantly delayed rounding up and detachment of tumor cells in response to HAMLET. In response to HAMLET, adherent tumor cells rounded up and detached, suggesting a loss of the actin cytoskeletal organization. These changes were accompanied by a reduction in β1 integrin staining and a decrease in FAK and ERK1/2 phosphorylation, consistent with a disruption of integrin-dependent cell adhesion signaling. Detachment per se did not increase cell death during the 22 hour experimental period, regardless of α-actinin-4 and α-actinin-1 expression levels but adherent cells with low α-actinin levels showed increased death in response to HAMLET. The results suggest that the interaction between HAMLET and α-actinins promotes tumor cell detachment. As α-actinins also associate with signaling molecules, cytoplasmic domains of transmembrane receptors and ion channels, additional α-actinin-dependent mechanisms are discussed.

Actinin-4 gene amplification in ovarian cancer: a candidate oncogene associated with poor patient prognosis and tumor chemoresistance

Actinin-4, an isoform of non-muscular alpha-actinin, enhances cell motility by bundling the actin cytoskeleton. We previously reported a prognostic implication of high immunohistochemical expression of actinin-4 protein in ovarian cancers. Chromosomal gain or amplification of the 19q12-q13 region has been reported in ovarian cancer. We hypothesized that the actinin-4 (ACTN4) gene might be a target of the 19q12-q13 amplicon and play an essential role of ovarian cancer progression. In total, 136 advanced-stage ovarian cancers were investigated for the copy number of the ACTN4 gene on chromosome 19q13, using fluorescence in situ hybridization, and the correlation of the ACTN4 copy number with actinin-4 protein immunoreactivity and major clinicopathological factors was investigated. A higher copy number (> or =4 copies) of the ACTN4 gene was detected in 29 (21%) cases and was highly associated with the intensity of actinin-4 immunoreactivity (P<0.0001), a high histological tumor grade (P=0.030), a clear-cell adenocarcinoma histology (P=0.012), resistance to first-line chemotherapies (P=0.028), and poor patient outcome (P=0.0011). Univariate analyses using the Cox regression model showed that a higher ACTN4 gene copy number was able to predict patient outcome more accurately than high actinin-4 immunoreactivity (relative risk: 2.48 vs 1.55). Multivariate analysis showed that a higher copy number of the ACTN4 gene and the degree of residual disease were independent prognostic factors for overall patient survival. The actinin-4 gene may be a target of the 19q amplicon, acting as a candidate oncogene, and serve as a predictor of poor outcome and tumor chemoresistance in patients with advanced-stage ovarian cancers.

Flightless I regulates hemidesmosome formation and integrin-mediated cellular adhesion and migration during wound repair

Flightless I (Flii), a highly conserved member of the gelsolin family of actin-remodelling proteins associates with actin structures and is involved in cellular motility and adhesion. Our previous studies have shown that Flii is an important negative regulator of wound repair. Here, we show that Flii affects hemidesmosome formation and integrin-mediated keratinocyte adhesion and migration. Impaired hemidesmosome formation and sparse arrangements of keratin cytoskeleton tonofilaments and actin cytoskeleton anchoring fibrils were observed in Flii(Tg/+) and Flii(Tg/Tg) mice with their skin being significantly more fragile than Flii(+/-) and WT mice. Flii(+/-) primary keratinocytes showed increased adhesion on laminin and collagen I than WT and Flii(Tg/Tg) primary keratinocytes. Decreased expression of CD151 and laminin-binding integrins alpha3, beta1, alpha6 and beta4 were observed in Flii overexpressing wounds, which could contribute to the impaired wound re-epithelialization observed in these mice. Flii interacts with proteins directly linked to the cytoplasmic domain of integrin receptors suggesting that it may be a mechanical link between ligand-bound integrin receptors and the actin cytoskeleton driving adhesion-signaling pathways. Therefore Flii may regulate wound repair through its effect on hemidesmosome formation and integrin-mediated cellular adhesion and migration.

Motility-related actinin alpha-4 is associated with advanced and metastatic ovarian carcinoma

Advanced and metastatic ovarian cancer is a leading cause of death from gynecologic malignancies. A more detailed understanding of the factors controlling invasion and metastasis may lead to novel anti-metastatic therapies. To model cellular interactions that occur during intraperitoneal metastasis, comparative cDNA microarray analysis and confirmatory real-time reverse transcription PCR (RT-PCR) were employed to uncover changes in gene expression that may occur in late stage ovarian cancer in response to microenvironmental cues, particularly native three-dimensional collagen I. Gene expression in human ovarian carcinoma tissues was evaluated on the RNA and protein level using real-time RT-PCR and immunohistochemistry. Cell invasion and migration were evaluated in a collagen invasion assay and a scratch wound assay. Three-dimensional collagen I culture led to differential expression of several genes. The role of actinin alpha-4 (ACTN4), a cytoskeleton-associated protein implicated in the regulation of cell motility, was examined in detail. ACTN4 RNA and protein expression were associated with advanced and metastatic human ovarian carcinoma. This report demonstrates that a cytoskeletal-associated protein ACTN4 is upregulated by three-dimensional collagen culture conditions, leading to increased invasion and motility of ovarian cancer cells. Expression of ACTN4 in human ovarian tumors was found to be associated with advanced-stage disease and peritoneal metastases.

Alpha-actinin is required for tightly regulated remodeling of the actin cortical network during cytokinesis

Localization of the actin crosslinking protein, alpha-actinin, to the cleavage furrow has been previously reported. However, its functions during cytokinesis remain poorly understood. We have analyzed the functions of alpha-actinin during cytokinesis by a combination of molecular manipulations and imaging-based techniques. alpha-actinin gradually dissipated from the cleavage furrow as cytokinesis progressed. Overexpression of alpha-actinin caused increased accumulation of actin filaments because of inhibition of actin turnover, leading to cytokinesis failure. Global depletion of alpha-actinin by siRNA caused a decrease in the density of actin filaments throughout the cell cortex, surprisingly inducing accelerated cytokinesis and ectopic furrows. Local ablation of alpha-actinin induced accelerated cytokinesis specifically at the site of irradiation. Neither overexpression nor depletion of alpha-actinin had an apparent effect on myosin II organization. We conclude that cytokinesis in mammalian cells requires tightly regulated remodeling of the cortical actin network mediated by alpha-actinin in coordination with actomyosin-based cortical contractions.

Actinin-4 expression in ovarian cancer: a novel prognostic indicator independent of clinical stage and histological type

Actinin-4 is an isoform of non-muscular alpha-actinin and actin-bundling protein. By enhancing cell motility, actinin-4 shows different biological properties from another isoform of non-muscular actinin 'actinin-1' and variable clinicopathological implications of actinin-4 have been demonstrated in some human malignancies such as breast cancers, lung cancers, and colorectal cancers. We herein described the clinicopathological and prognostic significance of actinin-4 expression in ovarian cancers. Actinin-4 expression was analyzed immunohistochemically in 265 primary ovarian carcinomas: 116 serous, 71 clear cell, 43 endometrioid, and 35 mucinous adenocarcinomas. With reference to endothelial immunoreactivity, cytoplasmic expression of actinin-4 was classified as either low (including negative) or high. Then, various parameters such as patients' characteristics, histopathological findings including E-cadherin and beta-catenin immunoreactivity, and clinical outcome, were compared between groups showing differences in the intensity or intracellular distribution of actinin-4 immunoreactivity. High expression of actinin-4 was demonstrated in 137 (57%) cases and was associated with serous histology (P=0.0075), high histological grade (P<0.0001), an advanced disease stage (P=0.036), a high degree of residual disease after initial surgery (P=0.0047), poor patient outcome (5-year survival: 52.4% in the high-expression group vs 71.9% in the low expression group, P=0.0043 by log-rank test), and also with reduced E-cadherin and preserved beta-catenin expressions (P=0.0097 and 0.017, respectively). Nuclear immunoreactivity for actinin-4 was detected in 20 (7.5%) cases and was associated with low histological grade (P=0.0079) but not with other variables. Multivariate analysis showed that high actinin-4 expression was an independent prognostic factor for overall survival, as well as a high degree of residual disease and clear-cell histology. Accumulation of actinin-4 in the cytoplasm may be related to a higher propensity for tumor invasiveness and metastasis, probably by enhancing cell motility, and could be a novel prognostic indicator for patients with ovarian carcinomas.

Mass Spectrometry Analysis of the Native Protein Complex Containing Actinin-4 in Prostate Cancer Cells

Actinin-4 was originally identified as an actin-binding protein associated with cell motility and cancer invasion and metastasis. However, actinin-4 forms complexes with a large number of different partner proteins and is speculated to have several distinct functions depending on its partner. The level of actinin-4 expression was found to be significantly lower in prostate cancer cells than in non-cancerous basal cells, and restoration of actinin-4 expression inhibited cell proliferation by prostate cancer cell line 22RV1. Immunoprecipitation and mass spectrometry analysis revealed that actinin-4 forms native complexes with several partner proteins in 22RV1 cells, including with beta/gamma-actin, calmodulin, the clathrin heavy chain, non-muscular myosin heavy chain, heterogeneous nuclear ribonucleoprotein A1, and Ras-GTPase-activating protein SH3 domain-binding protein. Clathrin is a coat protein that covers the internalized membrane pit that forms during early endocytosis. We found that other clathrin-related and unrelated cargo proteins, including dynamin, adaptin-delta, beta subunit of neuronal adaptin-like protein, and p47A, also interact with actinin-4. Immunofluorescence microscopy revealed that dynamin and clathrin co-localized with actinin-4 at the sites of membrane ruffling, and transfection of actinin-4 cDNA facilitated the transport of transferrin into perinuclear endosomes. Endocytosis terminates signaling evoked by cell surface receptors and regulates the recycling of receptors and ligands. We identified a panel of proteins whose expression and/or subcellular localization was regulated by actinin-4 by performing organelle fractionation and ICAT-LC-MS/MS. The decreased expression of actinin-4 protein in prostate cancer cells may cause aberrations in the intracellular trafficking of various cell surface molecules and contribute to carcinogenesis.

Caspase proteolysis of the integrin beta4 subunit disrupts hemidesmosome assembly, promotes apoptosis, and inhibits cell migration

Caspases are a conserved family of cell death proteases that cleave intracellular substrates at Asp residues to modify their function and promote apoptosis. In this report we identify the integrin beta4 subunit as a novel caspase substrate using an expression cloning strategy. Together with its alpha6 partner, alpha6beta4 integrin anchors epithelial cells to the basement membrane at specialized adhesive structures known as hemidesmosomes and plays a critical role in diverse epithelial cell functions including cell survival and migration. We show that integrin beta4 is cleaved by caspase-3 and -7 at a conserved Asp residue (Asp(1109)) in vitro and in epithelial cells undergoing apoptosis, resulting in the removal of most of its cytoplasmic tail. Caspase cleavage of integrin beta4 produces two products, 1) a carboxyl-terminal product that is unstable and rapidly degraded by the proteasome and 2) an amino-terminal cleavage product (amino acids 1-1109) that is unable to assemble into mature hemidesmosomes. We also demonstrate that caspase cleavage of integrin beta4 sensitizes epithelial cells to apoptosis and inhibits cell migration. Taken together, we have identified a previously unrecognized proteolytic truncation of integrin beta4 generated by caspases that disrupts key structural and functional properties of epithelial cells and promotes apoptosis.

Interaction of zonula occludens-1 (ZO-1) with alpha-actinin-4: application of functional proteomics for identification of PDZ domain-associated proteins

The use of recombinant "bait" proteins to capture protein-binding partners, followed by identification of protein interaction networks by mass spectrometry (MS), has gained popularity and widespread acceptance. We have developed an approach using recombinant PDZ protein interaction modules of the membrane-associated guanylate kinase (MAGUK) protein zonula occludens-1 (ZO-1) to pull-down and screen for proteins that interact with these modules via their PDZ domain binding motifs. Identification of proteins by MS of pull-down material was achieved using a vacuum-based chromatography sample preparation device designed for matrix-assisted laser desorption/ionization (MALDI) MS. MS analysis of tryptic fragments in pull-down material revealed a number of potential ZO-1 interacting candidates, including the presence of peptides corresponding to the cortical membrane scaffolding protein alpha-actinin-4. Interaction of alpha-actinin-4 with ZO-1 was confirmed by coimmunoprecipitation of these two proteins from cultured cells, as well as from brain, liver, and heart, and by immunoblot detection of alpha-actinin-4 after pull-down with the first PDZ domain of ZO-1. In contrast, the highly homologous alpha-actinin family member, alpha-actinin-1, displayed no association with ZO-1. Immunofluorescence showed colocalization of alpha-actinin-4 with ZO-1 in cultured HeLa and C6 glioma cells, as well as in a variety of tissues in vivo, including brain, heart, liver, and lung. This study demonstrates the utility of MS-based functional proteomics for identifying cellular components of the ZO-1 scaffolding network. Our finding of the interaction of ZO-1 with alpha-actinin-4 provides a mechanism for linking the known protein recruitment and signaling activities of ZO-1 with alpha-actinin-4-associated plasma membrane proteins that have regulatory activities at cell-cell and cell-extracellular matrix contacts.

Evidence of a Functional Role for Interaction between ICAM-1 and Nonmuscle α-Actinins in Leukocyte Diapedesis

ICAM-1 is involved in both adhesion and extravasation of leukocytes to endothelium during inflammation. It has been shown that the ICAM-1 cytoplasmic domain is important for transendothelial migration of leukocytes but the precise molecular mechanisms involving the intracytoplasmic portion of ICAM-1 is not known. To characterize precisely the molecular scaffolding associated with ICAM-1, we have used the yeast two-hybrid system, and we have identified six different proteins interacting with the ICAM-1 cytoplasmic domain. In this study, we report that the two forms of nonmuscle alpha-actinin (i.e., alpha-actinin 1 and alpha-actinin 4) associate with ICAM-1, and that these interactions are essential for leukocyte extravasation. These interactions were further confirmed by coimmunoprecipitation and immunofluorescence in endothelial cells and in ICAM-1-transfected Chinese hamster ovary cells. The function of these interactions was analyzed by point mutation of charged amino acids located on ICAM-1 cytoplasmic domain. We have identified three charged amino acids (arginine 480, lysine 481, and arginine 486) which are essential in the binding of alpha-actinins to the ICAM-1 cytoplasmic tail. Mutation of these amino acids completely inhibited ICAM-1-mediated diapedesis. Experiments with siRNA inhibiting specifically alpha-actinin 1 or alpha-actinin 4 on endothelial cells indicated that alpha-actinin 4 had a major role in this phenomenon. Thus, our data demonstrate that ICAM-1 directly interacts with cytoplasmic alpha-actinin 1 and 4 and that this interaction is required for leukocyte extravasation.

E-cadherin regulates the association between beta-catenin and actinin-4

The E-cadherin/catenin system acts as an invasion suppressor of epithelial malignancies. This invasion suppressive activity seems be mediated not only by the cell adhesive activity of E-cadherin but by other undetermined signaling pathways elicited by beta-catenin. In fact, cancer cells that have infiltrated the stroma reduce the expression of E-cadherin and accumulate beta-catenin. We attempted to identify the alternative partner proteins that make complexes with beta-catenin in the absence of E-cadherin. An approximately 100-kDa protein was constantly coimmunoprecipitated with beta-catenin from SW480 colorectal cancer cells, which lack the expression of E-cadherin, and was identified as actinin-4 by mass spectrometry. Transfection of E-cadherin cDNA suppressed the association between beta-catenin and actinin-4. Inhibition of E-cadherin by RNA interference transferred the beta-catenin and actinin-4 proteins into the membrane protrusions of DLD-1 cells. Immunofluorescence histochemistry of clinical colorectal cancer specimens showed that the beta-catenin and actinin-4 proteins were colocalized in colorectal cancer cells infiltrating the stroma. We reported previously that overexpression of actinin-4 induces cell motility and specifically promotes lymph node metastasis by colorectal cancer. The association between beta-catenin and actinin-4 and its regulation by E-cadherin may represent a novel molecular link connecting cell adhesion and motility. Shutting down the signals mediating this association may be worth considering as a therapeutic approach to cancer invasion and metastasis.

Ligation of the beta4 integrin triggers adhesion behavior of human keratinocytes by an "inside-out" mechanism

Carcinogenesis is considered as a multistep process involving functional changes in the hemidesmosomal organization. In normal skin keratinocytes, expression of the alpha(6)beta(4) integrin is restricted to the proliferative basal layer and mediates stable adhesion to the underlying basement membrane. Observations in carcinoma cells show a functional and spatial dissociation of the alpha(6)beta(4) integrin from the hemidesmosomal complex, which stimulates cell migration and, therefore, may contribute to carcinoma invasion. We now have evaluated the adhesion behavior of epithelial cells at different stages of transformation in response to activation of the beta(4) integrin. It is demonstrated that ligation of the beta(4) integrin augmented adhesion of carcinoma and pre-carcinoma cells to non-modified plastic. In contrast, adhesion behavior of normal human keratinocytes was not influenced by ligation of the beta(4) integrin. In order to explain the mechanism of beta(4)-mediated adhesion, the hypothesis of an "inside-out" activation of integrins was tested. Evidence is given that for cells expressing the alpha(6)beta(4) integrin, ligation of the beta(4) integrin increased beta(1) integrin-mediated adhesion. Furthermore, ligation of the beta(4) integrin led to phosphorylation of PKB/Akt at both phosphorylation sites. Functional blocking of PKB/Akt by dominant-negative overexpression decreased cell adhesion in response to beta(4) integrin ligation. Taken together, the present data establish a link between the ligation of the beta(4) integrin and beta(1) integrin-mediated cell adhesion in carcinoma and pre-carcinoma cells. Hence, these findings provide further insight into the conversion processes during carcinogenesis and show the beta(4) integrin to be a key regulator of cellular adhesion.

Mutant α-actinin-4 promotes tumorigenicity and regulates cell motility of a human lung carcinoma

The precise role of alpha-actinin-4 encoding gene (ACTN4) is not very well understood. It has been reported to elicit tumor suppressor activity and to regulate cellular motility. To further assess the function of human ACTN4, we studied a lung carcinoma cell line expressing a mutated alpha-actinin-4, which is recognized as a tumor antigen by autologous CD8(+) cytotoxic T lymphocytes (CTL). Confocal immunofluorescence microscopy indicated that, while wild-type (WT) alpha-actinin-4 stains into actin cytoskeleton and cell surface ruffles, the mutated protein is only dispersed in the cytoplasm of the lung carcinoma cells. This loss of association with the cell surface did not appear to correlate with a decrease in in vitro alpha-actinin-4 crosslinking to filamentous (F)-actin. Interestingly, experiments using cell lines stably expressing ACTN4 demonstrated that as opposed to WT gene, mutant ACTN4 was unable to inhibit tumor cell growth in vitro and in vivo. Moreover, the expression of mutant alpha-actinin-4 resulted in the loss of tumor cell capacity to migrate. The identification of an inactivating mutation in ACTN4 emphasizes its role as a tumor suppressor gene and underlines the involvement of cytoskeleton alteration in tumor development and metastasis.