Uni-axial stretching regulates intracellular localization of Hic-5 expressed in smooth-muscle cells in vivo

Sorafenib inhibits invasion of multicellular organoids that mimic Lymphangioleiomyomatosis nodules

Lymphangioleiomyomatosis (LAM) is a debilitating, progressive lung disease with few therapeutic options, largely due to a paucity of mechanistic knowledge of disease pathogenesis. Lymphatic endothelial cells (LECs) are known to envelope and invade clusters of LAM-cells, comprising of smooth muscle α-actin and/or HMB-45 positive "smooth muscle-like cells" however the role of LECs in LAM pathogenesis is still unknown. To address this critical knowledge gap, we investigated wether LECs interact with LAM-cells to augment their metastatic behaviour of LAM-cells. We performed in situ spatialomics and identified a core of transcriptomically related cells within the LAM nodules. Pathway analysis highlights wound and pulmonary healing, VEGF signaling, extracellular matrix/actin cytoskeletal regulating and the HOTAIR regulatory pathway enriched in the LAM Core cells. We developed an organoid co-culture model combining primary LAM-cells with LECs and applied this to evaluate invasion, migration, and the impact of Sorafenib, a multi-kinase inhibitor. LAM-LEC organoids had significantly higher extracellular matrix invasion, decreased solidity and a greater perimeter, reflecting increased invasion compared to non-LAM control smooth muscle cells. Sorafenib significantly inhibited this invasion in both LAM spheroids and LAM-LEC organoids compared to their respective controls. We identified TGFβ1ι1, a molecular adapter coordinating protein-protein interactions at the focal adhesion complex and known to regulate VEGF, TGFβ and Wnt signalling, as a Sorafenib-regulated kinase in LAM-cells. In conclusion we have developed a novel 3D co-culture LAM model and have demonstrated the effectiveness of Sorafenib to inhibit LAM-cell invasion, identifying new avenues for therapeutic intervention.

Knockdown of mechanosensitive adaptor Hic-5 ameliorates post-traumatic osteoarthritis in rats through repression of MMP-13

Osteoarthritis (OA) is the most common joint disease associated with articular cartilage destruction. Matrix metalloproteinase-13 (MMP-13) has an essential role in OA pathogenesis by degradation of collagen II, a major component of articular cartilage. Hydrogen peroxide-inducible clone-5 (Hic-5; TGFB1I1), a transforming growth factor-β-inducible mechanosensor, has previously been reported to promote OA pathogenesis by upregulating MMP-13 expression in mouse osteoarthritic lesions. In our current study, immunohistochemical analysis showed that Hic-5 protein expression was increased in human OA cartilage compared with normal cartilage. Functional experiments demonstrated that Hic-5 and MMP-13 expression was increased by mechanical stress, and mechanical stress-induced MMP-13 expression was suppressed by Hic-5 siRNA in human chondrocytes. Moreover, intracellular localization of Hic-5 shifted to the nucleus from focal adhesions in human chondrocytes subjected to mechanical stress, and nuclear Hic-5 increased MMP-13 gene expression. In vivo, intra-articular injection of Hic-5 siRNA decreased the Osteoarthritis Research Society International score and MMP-13 protein expression in articular cartilage of OA rats. Our findings suggest that Hic-5 regulates transcription of MMP-13 in human chondrocytes, and Hic-5 may be a novel therapeutic target for OA because OA progression was suppressed by intra-articular injection of Hic-5 siRNA in rats.

TNS1: Emerging Insights into Its Domain Function, Biological Roles, and Tumors

Tensins are a family of cellular-adhesion constituents that have been extensively studied. They have instrumental roles in the pathogenesis of numerous diseases. The mammalian tensin family comprises four members: tensin1 (TNS1), tensin2, tensin3, and tensin4. Among them, TNS1 has recently received attention from researchers because of its structural properties. TNS1 engages in various biological processes, such as cell adhesion, polarization, migration, invasion, proliferation, apoptosis, and mechano-transduction, by interacting with various partner proteins. Moreover, the abnormal expression of TNS1 in vivo is associated with the development of various diseases, especially tumors. Interestingly, the role of TNS1 in different tumors is still controversial. Here, we systematically summarize three aspects of TNS1: the gene structure, the biological processes underlying its action, and the dual regulatory role of TNS1 in different tumors through different mechanisms, of which we provide the first overview.

Maintenance of Ligament Homeostasis of Spheroid-Colonized Embroidered and Functionalized Scaffolds after 3D Stretch

Anterior cruciate ligament (ACL) ruptures are usually treated with autograft implantation to prevent knee instability. Tissue engineered ACL reconstruction is becoming promising to circumvent autograft limitations. The aim was to evaluate the influence of cyclic stretch on lapine (L) ACL fibroblasts on embroidered scaffolds with respect to adhesion, DNA and sulphated glycosaminoglycan (sGAG) contents, gene expression of ligament-associated extracellular matrix genes, such as type I collagen, decorin, tenascin C, tenomodulin, gap junctional connexin 43 and the transcription factor Mohawk. Control scaffolds and those functionalized by gas phase fluorination and cross-linked collagen foam were either pre-cultured with a suspension or with spheroids of LACL cells before being subjected to cyclic stretch (4%, 0.11 Hz, 3 days). Stretch increased significantly the scaffold area colonized with cells but impaired sGAGs and decorin gene expression (functionalized scaffolds seeded with cell suspension). Stretching increased tenascin C, connexin 43 and Mohawk but decreased decorin gene expression (control scaffolds seeded with cell suspension). Pre-cultivation of functionalized scaffolds with spheroids might be the more suitable method for maintaining ligamentogenesis in 3D scaffolds compared to using a cell suspension due to a significantly higher sGAG content in response to stretching and type I collagen gene expression in functionalized scaffolds.

Stress fiber strain recognition by the LIM protein testin is cryptic and mediated by RhoA

The actin cytoskeleton is a key regulator of mechanical processes in cells. The family of LIM domain proteins have recently emerged as important mechanoresponsive cytoskeletal elements capable of sensing strain in the actin cytoskeleton. The mechanisms regulating this mechanosensitive behavior, however, remain poorly understood. Here we show that the LIM domain protein testin is peculiar in that despite the full-length protein primarily appearing diffuse in the cytoplasm, the C-terminal LIM domains alone recognize focal adhesions and strained actin, while the N-terminal domains alone recognize stress fibers. Phosphorylation mutations in the dimerization regions of testin, however, reveal its mechanosensitivity and cause it to relocate to focal adhesions and sites of strain in the actin cytoskeleton. Finally, we demonstrate that activated RhoA causes testin to adorn stress fibers and become mechanosensitive. Together, our data show that testin’s mechanoresponse is regulated in cells and provide new insights into LIM domain protein recognition of the actin cytoskeleton’s mechanical state.

LIM domain proteins in cell mechanobiology

The actin cytoskeleton is important for maintaining mechanical homeostasis in adherent cells, largely through its regulation of adhesion and cortical tension. The LIM (Lin-11, Isl1, MEC-3) domain-containing proteins are involved in a myriad of cellular mechanosensitive pathways. Recent work has discovered that LIM domains bind to mechanically stressed actin filaments, suggesting a novel and widely conserved mechanism of mechanosensing. This review summarizes the current state of knowledge of LIM protein mechanosensitivity.

Paxillin family of focal adhesion adaptor proteins and regulation of cancer cell invasion

The paxillin family of proteins, including paxillin, Hic-5, and leupaxin, are focal adhesion adaptor/scaffolding proteins which localize to cell-matrix adhesions and are important in cell adhesion and migration of both normal and cancer cells. Historically, the role of these proteins in regulating the actin cytoskeleton through focal adhesion-mediated signaling has been well documented. However, studies in recent years have revealed additional functions in modulating the microtubule and intermediate filament cytoskeletons to affect diverse processes including cell polarization, vesicle trafficking and mechanosignaling. Expression of paxillin family proteins in stromal cells is also important in regulating tumor cell migration and invasion through non-cell autonomous effects on the extracellular matrix. Both paxillin and Hic-5 can also influence gene expression through a variety of mechanisms, while their own expression is frequently dysregulated in various cancers. Accordingly, these proteins may serve as valuable targets for novel diagnostic and treatment approaches in cancer.

Alleviation of murine osteoarthritis by deletion of the focal adhesion mechanosensitive adapter, Hic-5

Excessive mechanical stress is a major cause of knee osteoarthritis. However, the mechanism by which the mechanical stress begets osteoarthritis development remains elusive. Hydrogen peroxide-inducible clone-5 (Hic-5; TGFβ1i1), a TGF-β inducible focal adhesion adaptor, has previously been reported as a mediator of mechanotransduction. In this study, we analyzed the in vivo function of Hic-5 in development of osteoarthritis, and found that mice lacking Hic-5 showed a significant reduction in development of osteoarthritis in the knee. Furthermore, we found reduced expression of catabolic genes, such as metalloproteinase-13 and a disintegrin and metalloproteinase with thrombospondin type 1 motif 5 in osteoarthritic lesions in mice lacking Hic-5. During osteoarthritis development, Hic-5 is detected in chondrocytes of articular cartilage. To investigate the role of Hic-5 in chondrocytes, we isolated chondrocytes from articular cartilage of wild type and Hic-5-deficient mice. In these primary cultured chondrocytes, Hic-5 deficiency resulted in suppression of catabolic gene expression induced by osteoarthritis-related cytokines such as tumor necrosis factor α and interleukin 1β. Furthermore, Hic-5 deficiency in chondrocytes suppressed catabolic gene expression induced by mechanical stress. Revealing the regulation of chondrocyte catabolism by Hic-5 contributes to understanding the pathophysiology of osteoarthritis induced by mechanical stress.

Signaling Mechanisms of Myofibroblastic Activation: Outside-in and Inside-Out

Myofibroblasts are central mediators of fibrosis. Typically derived from resident fibroblasts, myofibroblasts represent a heterogeneous population of cells that are principally defined by acquired contractile function and high synthetic ability to produce extracellular matrix (ECM). Current literature sheds new light on the critical role of ECM signaling coupled with mechanotransduction in driving myofibroblastic activation. In particular, transforming growth factor β1 (TGF-β1) and extra domain A containing fibronectin (EDA-FN) are thought to be the primary ECM signaling mediators that form and also induce positive feedback loops. The outside-in and inside-out signaling circuits are transmitted and integrated by TGF-β receptors and integrins at the cell membrane, ultimately perpetuating the abundance and activities of TGF-β1 and EDA-FN in the ECM. In this review, we highlight these conceptual advances in understanding myofibroblastic activation, in hope of revealing its therapeutic anti-fibrotic implications.

Hic-5 regulates fibrillar adhesion formation to control tumor extracellular matrix remodeling through interaction with tensin1

The linearization of the stromal extracellular matrix (ECM) by cancer associated fibroblasts (CAFs) facilitates tumor cell growth and metastasis. However, the mechanism by which the ECM is remodeled is not fully understood. Hic-5 (TGFβ1i1), a focal adhesion scaffold protein, has previously been reported to be crucial for stromal ECM deposition and remodeling in vivo. Herein we show that CAFs lacking Hic-5 exhibit a significant reduction in the ability to form fibrillar adhesions, a specialized form of focal adhesion that promote fibronectin fibrillogenesis. Hic-5 was found to promote fibrillar adhesion formation through a newly characterized interaction with tensin1. Furthermore, Src dependent phosphorylation of Hic-5 facilitated the interaction with tensin1 to prevent β1 integrin internalization and trafficking to the lysosome. The interaction between Hic-5 and tensin1 was mechanosensitive, promoting fibrillar adhesion formation and fibronectin fibrillogenesis in a rigidity dependent fashion. Importantly, this Src dependent mechanism was conserved in three-dimensional (3D) ECM environments. Immunohistochemistry of tensin1 showed enrichment in CAFs in vivo, which was abrogated upon deletion of Hic-5. Interestingly, elevated Hic-5 expression correlates with reduced distant metastasis free survival in patients with basal-like, HER2+ and grade 3 tumors. Thus, we have identified Hic-5 as a crucial regulator of ECM remodeling in CAFs by promoting fibrillar adhesion formation through a novel interaction with tensin1.

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.

CRP2, a new invadopodia actin bundling factor critically promotes breast cancer cell invasion and metastasis

A critical process underlying cancer metastasis is the acquisition by tumor cells of an invasive phenotype. At the subcellular level, invasion is facilitated by actin-rich protrusions termed invadopodia, which direct extracellular matrix (ECM) degradation. Here, we report the identification of a new cytoskeletal component of breast cancer cell invadopodia, namely cysteine-rich protein 2 (CRP2). We found that CRP2 was not or only weakly expressed in epithelial breast cancer cells whereas it was up-regulated in mesenchymal/invasive breast cancer cells. In addition, high expression of the CRP2 encoding gene CSRP2 was associated with significantly increased risk of metastasis in basal-like breast cancer patients. CRP2 knockdown significantly reduced the invasive potential of aggressive breast cancer cells, whereas it did not impair 2D cell migration. In keeping with this, CRP2-depleted breast cancer cells exhibited a reduced capacity to promote ECM degradation, and to secrete and express MMP-9, a matrix metalloproteinase repeatedly associated with cancer progression and metastasis. In turn, ectopic expression of CRP2 in weakly invasive cells was sufficient to stimulate cell invasion. Both GFP-fused and endogenous CRP2 localized to the extended actin core of invadopodia, a structure primarily made of actin bundles. Purified recombinant CRP2 autonomously crosslinked actin filaments into thick bundles, suggesting that CRP2 contributes to the formation/maintenance of the actin core. Finally, CRP2 depletion significantly reduced the incidence of lung metastatic lesions in two xenograft mouse models of breast cancer. Collectively, our data identify CRP2 as a new cytoskeletal component of invadopodia that critically promotes breast cancer cell invasion and metastasis.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Mechanical signals activate p38 MAPK pathway-dependent reinforcement of actin via mechanosensitive HspB1

Mechanical force induces protein phosphorylations, subcellular redistributions, and actin remodeling. We show that mechanical activation of the p38 MAPK pathway leads to phosphorylation of HspB1 (hsp25/27), which redistributes to cytoskeletal structures, and contributes to the actin cytoskeletal remodeling induced by mechanical stimulation.

Despite the importance of a cell’s ability to sense and respond to mechanical force, the molecular mechanisms by which physical cues are converted to cell-instructive chemical information to influence cell behaviors remain to be elucidated. Exposure of cultured fibroblasts to uniaxial cyclic stretch results in an actin stress fiber reinforcement response that stabilizes the actin cytoskeleton. p38 MAPK signaling is activated in response to stretch, and inhibition of p38 MAPK abrogates stretch-induced cytoskeletal reorganization. Here we show that the small heat shock protein HspB1 (hsp25/27) is phosphorylated in stretch-stimulated mouse fibroblasts via a p38 MAPK-dependent mechanism. Phosphorylated HspB1 is recruited to the actin cytoskeleton, displaying prominent accumulation on actin “comet tails” that emanate from focal adhesions in stretch-stimulated cells. Site-directed mutagenesis to block HspB1 phosphorylation inhibits the protein’s cytoskeletal recruitment in response to mechanical stimulation. HspB1-null cells, generated by CRISPR/Cas9 nuclease genome editing, display an abrogated stretch-stimulated actin reinforcement response and increased cell migration. HspB1 is recruited to sites of increased traction force in cells geometrically constrained on micropatterned substrates. Our findings elucidate a molecular pathway by which a mechanical signal is transduced via activation of p38 MAPK to influence actin remodeling and cell migration via a zyxin-independent process.

Hic-5 remodeling of the stromal matrix promotes breast tumor progression

The remodeling of the stromal extracellular matrix (ECM) plays a crucial, but incompletely understood role during tumor progression and metastasis. Hic-5, a focal adhesion scaffold protein, has previously been implicated in tumor cell invasion, proliferation and metastasis. To investigate the role of Hic-5 in breast tumor progression in vivo, Hic-5^−/− mice were generated and crossed with the Mouse Mammary Tumor Virus-Polyoma Middle T Antigen (MMTV-PyMT) mouse. Tumors from the Hic-5^−/−;PyMT mice exhibited increased latency and reduced growth, with fewer lung metastases, as compared to Hic-5^+/−;PyMT mice. Immunohistochemical analysis showed that Hic-5 is primarily expressed in the cancer associated fibroblasts (CAFs). Further analysis revealed that the Hic-5^−/−;PyMT tumor stroma contains fewer CAFs and exhibits reduced ECM deposition. The remodeling of the stromal matrix by CAFs has been shown to increase tumor rigidity to indirectly regulate FAK Y397 phosphorylation in tumor cells to promote their growth and invasion. Accordingly, the Hic-5^−/−;PyMT tumor cells exhibited a reduction in FAK Y397 phosphorylation. Isolated Hic-5^−/−;PyMT CAFs were defective in stress fiber organization and exhibited reduced contractility. These cells also failed to efficiently deposit and organize the ECM in two and three dimensions. This, in turn, impacted three dimensional MDA-MB-231 tumor cell migration behavior. Thus, using a new knockout mouse model, we have identified Hic-5 expression in CAFs as a key requirement for deposition and remodeling of the stromal ECM to promote non-cell autonomous breast tumor progression.

Matrix mechanics controls FHL2 movement to the nucleus to activate p21 expression

Substrate rigidity affects many physiological processes through mechanochemical signals from focal adhesion (FA) complexes that subsequently modulate gene expression. We find that shuttling of the LIM domain (domain discovered in the proteins, Lin11, Isl-1, and Mec-3) protein four-and-a-half LIM domains 2 (FHL2) between FAs and the nucleus depends on matrix mechanics. In particular, on soft surfaces or after the loss of force, FHL2 moves from FAs into the nucleus and concentrates at RNA polymerase (Pol) II sites, where it acts as a transcriptional cofactor, causing an increase in p21 gene expression that will inhibit growth on soft surfaces. At the molecular level, shuttling requires a specific tyrosine in FHL2, as well as phosphorylation by active FA kinase (FAK). Thus, we suggest that FHL2 phosphorylation by FAK is a critical, mechanically dependent step in signaling from soft matrices to the nucleus to inhibit cell proliferation by increasing p21 expression.

Hic-5 mediates the initiation of endothelial sprouting by regulating a key surface metalloproteinase

During angiogenesis, endothelial cells must coordinate matrix proteolysis with migration. Here, we tested whether the focal adhesion scaffold protein Hic-5 (also known as TGFB1I1) regulated endothelial sprouting in three dimensions. Hic-5 silencing reduced endothelial sprouting and lumen formation, and sprouting defects were rescued by the return of Hic-5 expression. Pro-angiogenic factors enhanced colocalization and complex formation between membrane type-1 matrix metalloproteinase (MT1-MMP, also known as MMP14) and Hic-5, but not between paxillin and MT1-MMP. The LIM2 and LIM3 domains of Hic-5 were necessary and sufficient for Hic-5 to form a complex with MT1-MMP. The degree of interaction between MT1-MMP and Hic-5 and the localization of the complex within detergent-resistant membrane fractions were enhanced during endothelial sprouting, and Hic-5 depletion lowered the surface levels of MT1-MMP. In addition, we observed that loss of Hic-5 partially reduced complex formation between MT1-MMP and focal adhesion kinase (FAK, also known as PTK2), suggesting that Hic-5 bridges MT1-MMP and FAK. Finally, Hic-5 LIM2-LIM3 deletion mutants reduced sprout initiation. Hic-5, MT1-MMP and FAK colocalized in angiogenic vessels during porcine pregnancy, supporting that this complex assembles during angiogenesis in vivo. Collectively, Hic-5 appears to enhance complex formation between MT1-MMP and FAK in activated endothelial cells, which likely coordinates matrix proteolysis and cell motility.

Hic-5 is required for myofibroblast differentiation by regulating mechanically dependent MRTF-A nuclear accumulation

How mechanical cues from the extracellular environment are translated biochemically to modulate the effects of TGF-β on myofibroblast differentiation remains a crucial area of investigation. We report here that the focal adhesion protein, Hic-5 (also known as TGFB1I1), is required for the mechanically dependent generation of stress fibers in response to TGF-β. Successful generation of stress fibers promotes the nuclear localization of the transcriptional co-factor MRTF-A (also known as MKL1), and this correlates with the mechanically dependent induction of α smooth muscle actin (α-SMA) and Hic-5 in response to TGF-β. As a consequence of regulating stress fiber assembly, Hic-5 is required for the nuclear accumulation of MRTF-A and the induction of α-SMA as well as cellular contractility, suggesting a crucial role for Hic-5 in myofibroblast differentiation. Indeed, the expression of Hic-5 was transient in acute wounds and persistent in pathogenic scars, and Hic-5 colocalized with α-SMA expression in vivo. Taken together, these data suggest that a mechanically dependent feed-forward loop, elaborated by the reciprocal regulation of MRTF-A localization by Hic-5 and Hic-5 expression by MRTF-A, plays a crucial role in myofibroblast differentiation in response to TGF-β.

Cytoskeletal signaling in TGFβ-induced epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is a physiological process that plays an important role in embryonic development and wound healing and is appropriated during pathological conditions including fibrosis and cancer metastasis. EMT can be initiated by a variety of factors, including transforming growth factor (TGF)-β, and is characterized by loss of epithelial features including cell-cell contacts and apicobasal polarity and acquisition of a motile, mesenchymal phenotype. A key feature of EMT is reorganization of the cytoskeleton and recent studies have elucidated regulation mechanisms governing this process. This review describes changes in gene expression patterns of cytoskeletal associated proteins during TGFβ-induced EMT. It further reports TGFβ-induced intracellular signaling cascades that regulate cytoskeletal reorganization during EMT. Finally, it highlights how changes in cytoskeletal architecture during EMT can regulate gene expression, thus further promoting EMT progression.

Involvement of Mechanical Stress in Androgenetic Alopecia

Context:

Androgenetic alopecia (AGA) is a frequent disorder characterized by progressive hair miniaturization in a very similar pattern among all affected men. The pathogenesis is related to androgen-inducible overexpression of transforming growth factor β-1 from balding dermal papilla cells, which is involved in epithelial inhibition and perifollicular fibrosis. Recent research shows that hair follicle androgen sensitivity is regulated by Hic-5, an androgen receptor co-activator which may be activated by the mechanical stimulation. Moreover, the dermis of scalp susceptible to be affected by AGA is firmly bounded to the galea aponeurotica, so the physical force exerted by the occipitofrontalis muscle is transmitted to the scalp skin.

Aims:

To know whether mechanical stress supported by hair follicles is involved in AGA phenomenon.

Materials and Methods:

It is performed with a finite element analysis of a galea model and a schematic representation of AGA progression according to Hamilton–Norwood scale in order to establish the correlation between elastic deformation in scalp and clinical progression of male pattern baldness.

Results:

The result was a highly significant correlation (r: −0.885, P < 0.001) that clearly identifies a mechanical factor in AGA development.

Conclusions:

All these data suggest that mechanical stress determines AGA patterning and a stretch-induced and androgen-mediated mechanotransduction in dermal papilla cells could be the primary mechanism in AGA pathogenesis.

Hic-5 Regulates Actin Cytoskeletal Reorganization and Expression of Fibrogenic Markers and Myocilin in Trabecular Meshwork Cells

PURPOSE

To explore the role of inducible focal adhesion (FA) protein Hic-5 in actin cytoskeletal reorganization, FA formation, fibrogenic activity, and expression of myocilin in trabecular meshwork (TM) cells.

METHODS

Using primary cultures of human TM (HTM) cells, the effects of various external factors on Hic-5 protein levels, as well as the effects of recombinant Hic-5 and Hic-5 small interfering RNA (siRNA) on actin cytoskeleton, FAs, myocilin, α-smooth muscle actin (αSMA), and collagen-1 were determined by immunofluorescence and immunoblot analyses.

RESULTS

Hic-5 distributes discretely to the FAs in HTM cells and throughout the TM and Schlemm's canal of the human aqueous humor (AH) outflow pathway. Transforming growth factor-β2 (TGF-β2), endothelin-1, lysophosphatidic acid, hydrogen peroxide, and RhoA significantly increased Hic-5 protein levels in HTM cells in association with reorganization of actin cytoskeleton and FAs. While recombinant Hic-5 induced actin stress fibers, FAs, αv integrin redistribution to the FAs, increased levels of αSMA, collagen-1, and myocilin, Hic-5 siRNA suppressed most of these responses in HTM cells. Hic-5 siRNA also suppressed TGF-β2-induced fibrogenic activity and dexamethasone-induced myocilin expression in HTM cells.

CONCLUSIONS

Taken together, these results reveal that Hic-5, whose levels were increased by various external factors implicated in elevated intraocular pressure, induces actin cytoskeletal reorganization, FAs, expression of fibrogenic markers, and myocilin in HTM cells. These characteristics of Hic-5 in TM cells indicate its importance in regulation of AH outflow through the TM in both normal and glaucomatous eyes.

Emerging properties of adhesion complexes: what are they and what do they do?

The regulation of cell adhesion machinery is central to a wide variety of developmental and pathological processes and occurs primarily within integrin-associated adhesion complexes. Here, we review recent advances that have furthered our understanding of the composition, organisation, and dynamics of these complexes, and provide an updated view on their emerging functions. Key findings are that adhesion complexes contain both core and non-canonical components. As a result of the dramatic increase in the range of components observed in adhesion complexes by proteomics, we comment on newly emerging functions for adhesion signalling. We conclude that, from a cellular or tissue systems perspective, adhesion signalling should be viewed as an emergent property of both the core and non-canonical adhesion complex components.

Hic-5 Mediates TGFβ-Induced Adhesion in Vascular Smooth Muscle Cells by a Nox4-Dependent Mechanism

OBJECTIVE

Focal adhesions (FAs) link the cytoskeleton to the extracellular matrix and as such play important roles in growth, migration, and contractile properties of vascular smooth muscle cells. Recently, it has been shown that downregulation of Nox4, a transforming growth factor (TGF) β-inducible, hydrogen peroxide (H2O2)-producing enzyme, affects the number of FAs. However, the effectors downstream of Nox4 that mediate FA regulation are unknown. The FA resident protein H2O2-inducible clone (Hic)-5 is H2O2 and TGFβ inducible, and a binding partner of the heat shock protein (Hsp) 27. The objective of this study was to elucidate the mechanism, by which Hic-5 and Hsp27 participate in TGFβ-induced, Nox4-mediated vascular smooth muscle cell adhesion and migration.

APPROACH AND RESULTS

Through a combination of molecular biology and biochemistry techniques, we found that TGFβ, by a Nox4-dependent mechanism, induces the expression and interaction of Hic-5 and Hsp27, which is essential for Hic-5 localization to FAs. Importantly, we found that Hic-5 expression is required for the TGFβ-mediated increase in FA number, adhesive forces and migration. Mechanistically, Nox4 downregulation impedes Smad (small body size and mothers against decapentaplegic) signaling by TGFβ, and Hsp27 and Hic-5 upregulation by TGFβ is blocked in small body size and mothers against decapentaplegic 4-deficient cells.

CONCLUSIONS

Hic-5 and Hsp27 are effectors of Nox4 required for TGFβ-stimulated FA formation, adhesion strength and migration in vascular smooth muscle cell.

Non-receptor tyrosine kinases and the actin cytoskeleton in contractile vascular smooth muscle

The contractility of vascular smooth muscle cells within the walls of arteries is regulated by mechanical stresses and vasoactive signals. Transduction of these diverse stimuli into a cellular response occurs through many different mechanisms, one being reorganisation of the actin cytoskeleton. In addition to a structural role in maintaining cellular architecture it is now clear that the actin cytoskeleton of contractile vascular smooth muscle cells is a dynamic structure reacting to changes in the cellular environment. Equally clear is that disrupting the cytoskeleton or interfering with its rearrangement, has profound effects on artery contractility. The actin cytoskeleton associates with dense plaques, also called focal adhesions, at the plasma membrane of smooth muscle cells. Vasoconstrictors and mechanical stress induce remodelling of the focal adhesions, concomitant with cytoskeletal reorganisation. Recent work has shown that non-receptor tyrosine kinases and tyrosine phosphorylation of focal adhesion proteins such as paxillin and Hic-5 are important for actin cytoskeleton and focal adhesion remodelling and contraction.

LIM proteins in actin cytoskeleton mechanoresponse

The actin cytoskeleton assembles into branched networks or bundles to generate mechanical force for critical cellular processes such as establishment of polarity, adhesion, and migration. Stress fibers (SFs) are contractile actomyosin structures that physically couple to the extracellular matrix through integrin-based focal adhesions (FAs), thereby transmitting force into and across the cell. Recently, LIN-11, Isl1, and MEC-3 (LIM) domain proteins have been implicated in mediating this cytoskeletal mechanotransduction. Among the more well-studied LIM domain adapter proteins is zyxin, a dynamic component of both FAs and SFs. Here we discuss recent research detailing the mechanisms by which SFs adjust their structure and composition to balance mechanical forces and suggest ways that zyxin and other LIM domain proteins mediate mechanoresponse.

LIM domains target actin regulators paxillin and zyxin to sites of stress fiber strain

Contractile actomyosin stress fibers are critical for maintaining the force balance between the interior of the cell and its environment. Consequently, the actin cytoskeleton undergoes dynamic mechanical loading. This results in spontaneous, stochastic, highly localized strain events, characterized by thinning and elongation within a discrete region of stress fiber. Previous work showed the LIM-domain adaptor protein, zyxin, is essential for repair and stabilization of these sites. Using live imaging, we show paxillin, another LIM-domain adaptor protein, is also recruited to stress fiber strain sites. Paxillin recruitment to stress fiber strain sites precedes zyxin recruitment. Zyxin and paxillin are each recruited independently of the other. In cells lacking paxillin, actin recovery is abrogated, resulting in slowed actin recovery and increased incidence of catastrophic stress fiber breaks. For both paxillin and zyxin, the LIM domains are necessary and sufficient for recruitment. This work provides further evidence of the critical role of LIM-domain proteins in responding to mechanical stress in the actin cytoskeleton.

Requirement of LIM domains for the transient accumulation of paxillin at damaged stress fibres

Cells recognize and respond to changes in intra- and extracellular mechanical conditions to maintain their mechanical homeostasis. Linear contractile bundles of actin filaments and myosin II known as stress fibres (SFs) mediate mechanical signals. Mechanical cues such as excessive stress driven by myosin II and/or external force may damage SFs and induce the local transient accumulation of SF-repair complexes (zyxin and VASP) at the damaged sites. Using an atomic force microscope mounted on a fluorescence microscope, we applied mechanical damage to cells expressing fluorescently tagged cytoskeletal proteins and recorded the subsequent mobilization of SF-repair complexes. We found that a LIM protein, paxillin, transiently accumulated at the damaged sites earlier than zyxin, while paxillin knockdown did not affect the kinetics of zyxin translocation. The C-terminal half of paxillin, comprising four-tandem LIM domains, can still translocate to damaged sites on SFs, suggesting that the LIM domain is essential for the mechanosensory function of paxillin. Our findings demonstrate a crucial role of the LIM domain in mechanosensing LIM proteins.

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.

A HIC-5- and KLF4-dependent mechanism transactivates p21(Cip1) in response to anchorage loss

Anchorage loss elicits a set of responses in cells, such as transcriptional changes, in order to prevent inappropriate cell growth in ectopic environments. However, the mechanisms underlying these responses are poorly understood. In this study, we investigated the transcriptional up-regulation of cyclin-dependent kinase inhibitor p21(Cip1) during anchorage loss, which is important for cell cycle arrest of nonadherent cells in the G1 phase. Up-regulation was mediated by an upstream element, designated as the detachment-responsive element (DRE), that contained Kruppel-like factor 4 (KLF4) and runt-related transcription factor 1 (RUNX1) recognition sites; both of these together were necessary for transactivation, as individually they were insufficient. RNAi experiments revealed that KLF4 and a multidomain adaptor protein, hydrogen peroxide-inducible clone 5 (HIC-5), were critically involved in DRE transactivation. The role of HIC-5 in this mechanism was to tether KLF4 to DNA sites in response to cellular detachment. In addition, further analysis suggested that oligomerization and subsequent nuclear matrix localization of HIC-5, which was accelerated spontaneously in cells during anchorage loss, was assumed to potentiate the scaffolding function of HIC-5 in the nucleus and consequently regulate p21(Cip1) transcription in a manner responding to anchorage loss. At the RUNX1 site, a LIM-only protein, CRP2, imposed negative regulation on transcription, which appeared to be removed by anchorage loss and contributed to increased transcriptional activity of DRE together with regulation at the KLF4 sites. In conclusion, this study revealed a novel transcriptional mechanism that regulated gene expression in a detachment-dependent manner, thereby contributing to anchorage-dependent cell growth.

Identification of Hic-5 as a novel regulatory factor for integrin αIIbβ3 activation and platelet aggregation in mice

BACKGROUND

Integrin αIIbβ3 plays key roles in platelet aggregation and subsequent thrombus formation. Hydrogen peroxide-inducible clone-5 (Hic-5), a member of the paxillin family, serves as a focal adhesion adaptor protein associated with αIIbβ3 at its cytoplasmic strand.

OBJECTIVES

Hic-5 function in αIIbβ3 activation and subsequent platelet aggregation remains unknown. To address this question, platelets from Hic-5(-/-) mice were analyzed.

METHODS AND RESULTS

Hic-5(-/-) mice displayed a significant hemostatic defect and resistance to thromboembolism, which were explained in part by weaker thrombin-induced aggregation in Hic-5(-/-) platelets. Mechanistically, Hic-5(-/-) platelets showed limited activation of αIIbβ3 upon thrombin treatment. Morphological alteration in Hic-5(-/-) platelets after thrombin stimulation on fibrinogen plates was also limited. As a direct consequence, the quantity of actin co-immunoprecipitating with the activated αIIbβ3 was smaller in Hic-5(-/-) platelets than in wild-type platelets.

CONCLUSION

We identified Hic-5 as a novel and specific regulatory factor for thrombin-induced αIIbβ3 activation and subsequent platelet aggregation in mice.

Bundling actin filaments from membranes: some novel players

Progress in live-cell imaging of the cytoskeleton has significantly extended our knowledge about the organization and dynamics of actin filaments near the plasma membrane of plant cells. Noticeably, two populations of filamentous structures can be distinguished. On the one hand, fine actin filaments which exhibit an extremely dynamic behavior basically characterized by fast polymerization and prolific severing events, a process referred to as actin stochastic dynamics. On the other hand, thick actin bundles which are composed of several filaments and which are comparatively more stable although they constantly remodel as well. There is evidence that the actin cytoskeleton plays critical roles in trafficking and signaling at both the cell cortex and organelle periphery but the exact contribution of actin bundles remains unclear. A common view is that actin bundles provide the long-distance tracks used by myosin motors to deliver their cargo to growing regions and accordingly play a particularly important role in cell polarization. However, several studies support that actin bundles are more than simple passive highways and display multiple and dynamic roles in the regulation of many processes, such as cell elongation, polar auxin transport, stomatal and chloroplast movement, and defense against pathogens. The list of identified plant actin-bundling proteins is ever expanding, supporting that plant cells shape structurally and functionally different actin bundles. Here I review the most recently characterized actin-bundling proteins, with a particular focus on those potentially relevant to membrane trafficking and/or signaling.

Paxillin and Hic-5 interaction with vinculin is differentially regulated by Rac1 and RhoA

Cell migration is of paramount importance to organism development and maintenance as well as multiple pathological processes, including cancer metastasis. The RhoGTPases Rac1 and RhoA are indispensable for cell migration as they regulate cell protrusion, cell-extracellular matrix (ECM) interactions and force transduction. However, the consequences of their activity at a molecular level within the cell remain undetermined. Using a combination of FRET, FRAP and biochemical analyses we show that the interactions between the focal adhesion proteins vinculin and paxillin, as well as the closely related family member Hic-5 are spatially and reciprocally regulated by the activity of Rac1 and RhoA. Vinculin in its active conformation interacts with either paxillin or Hic-5 in adhesions in response to Rac1 and RhoA activation respectively, while inactive vinculin interacts with paxillin in the membrane following Rac1 inhibition. Additionally, Rac1 specifically regulates the dynamics of paxillin as well as its binding partner and F-actin interacting protein actopaxin (α-parvin) in adhesions. Furthermore, FRET analysis of protein:protein interactions within cell adhesions formed in 3D matrices revealed that, in contrast to 2D systems vinculin interacts preferentially with Hic-5. This study provides new insight into the complexity of cell-ECM adhesions in both 2D and 3D matrices by providing the first description of RhoGTPase-coordinated protein:protein interactions in a cellular microenvironment. These data identify discrete roles for paxillin and Hic-5 in Rac1 and RhoA-dependent cell adhesion formation and maturation; processes essential for productive cell migration.

Hydrogen peroxide-inducible clone 5 (Hic-5) as a potential therapeutic target for vascular and other disorders

Hydrogen peroxide-inducible clone-5 (Hic-5) is a focal adhesion scaffold protein primarily expressed in vascular and visceral smooth muscle cells. We recently generated mice lacking Hic-5, which grew with no apparent abnormality (Kim-Kaneyama J, et al. J Mol Cell Cardiol. 2011;50(1):77-86). However, we discovered that recovery of arterial media following vascular injury is delayed significantly in Hic-5 knockout mice consequent to enhanced apoptosis of cultured vascular smooth muscle cells after mechanical stress; thus, Hic-5 is regarded as a novel factor in vascular remodeling. The Hic-5 gene is also induced by transforming growth factor-β, a well-known accelerator in fibrosis. Hic-5 involvement in various fibrotic disorders, e.g., scar formation, keloid formation and glomerulosclerosis, has been proposed. siRNA silencing of Hic-5 in a breast cancer cell line reduces its invasiveness; moreover, Hic-5 serves as a steroid hormone co-activator and likely participates in endometriosis and prostate cancer. Thus, functional characterization of Hic-5 in various pathophysiological conditions may afford novel mechanistic insights into a wide variety of diseases.

Expression analysis of LIM gene family in poplar, toward an updated phylogenetic classification

BACKGROUND

Plant LIM domain proteins may act as transcriptional activators of lignin biosynthesis and/or as actin binding and bundling proteins. Plant LIM genes have evolved in phylogenetic subgroups differing in their expression profiles: in the whole plant or specifically in pollen. However, several poplar PtLIM genes belong to uncharacterized monophyletic subgroups and the expression patterns of the LIM gene family in a woody plant have not been studied.

FINDINGS

In this work, the expression pattern of the twelve duplicated poplar PtLIM genes has been investigated by semi quantitative RT-PCR in different vegetative and reproductive tissues. As in other plant species, poplar PtLIM genes were widely expressed in the tree or in particular tissues. Especially, PtXLIM1a, PtXLIM1b and PtWLIM1b genes were preferentially expressed in the secondary xylem, suggesting a specific function in wood formation. Moreover, the expression of these genes and of the PtPLIM2a gene was increased in tension wood. Western-blot analysis confirmed the preferential expression of PtXLIM1a protein during xylem differentiation and tension wood formation. Genes classified within the pollen specific PLIM2 and PLIM2-like subgroups were all strongly expressed in pollen but also in cottony hairs. Interestingly, pairs of duplicated PtLIM genes exhibited different expression patterns indicating subfunctionalisations in specific tissues.

CONCLUSIONS

The strong expression of several LIM genes in cottony hairs and germinating pollen, as well as in xylem fibers suggests an involvement of plant LIM domain proteins in the control of cell expansion. Comparisons of expression profiles of poplar LIM genes with the published functions of closely related plant LIM genes suggest conserved functions in the areas of lignin biosynthesis, pollen tube growth and mechanical stress response. Based on these results, we propose a novel nomenclature of poplar LIM domain proteins.

HIC-5: A Mobile Molecular Scaffold Regulating the Anchorage Dependence of Cell Growth

HIC-5 is a multidomain LIM protein homologous to paxillin that serves as a molecular scaffold at focal adhesions and in the nucleus. It forms mobile molecular units with LIM-only proteins, PINCH, and CRP2 and translocates in and out of the nucleus via a nuclear export signal (NES). Of note, NES of HIC-5 is distinctive in its sensitivity to the cellular redox state. Recently, the mobile units of HIC-5 have been suggested to be involved in the regulation of the anchorage dependence of cell growth. On loss of adhesion, an increase in reactive oxygen species in the cells modifies NES and stops shuttling, which leads to cell-cycle control. More specifically, the system circumvents nuclear localization of cyclin D1 and transactivates p21^Cip1 in detached cells, thereby avoiding anchorage-independent cell growth. Thus, the HIC-5-LIM only protein complex has emerged as a fail-safe system for regulating the anchorage dependence of cell growth.

Transforming growth factor-β1-induced transcript 1 protein, a novel marker for smooth muscle contractile phenotype, is regulated by serum response factor/myocardin protein

Serum response factor (SRF) plays a central role in regulating expression of smooth muscle-specific genes partly by associating with the potent tissue-specific cofactor myocardin. Previous studies have shown that transforming growth factor-β1-induced transcript 1 (TGFB1I1, also known as Hic-5) is a TGF-β-responsive gene and is involved in the cellular response to vascular injury, but the regulation of TGFB1I1 expression remains elusive. In this report, we demonstrated that TGFB1I1 is a novel marker for the smooth muscle contractile phenotype and is regulated by SRF/myocardin. We found that TGFB1I1 is specifically expressed in smooth muscle cells (SMCs) and in smooth muscle-rich tissues. Furthermore, TGFB1I1 expression is significantly down-regulated in a variety of models for smooth muscle phenotypic modulation. The TGFB1I1 promoter contains an evolutionarily conserved CArG element, and this element is indispensible for myocardin-induced transactivation of TGFB1I1 promoter. By oligonucleotide pulldown and chromatin immunoprecipitation assays, we found that SRF binds to this CArG element in vitro and in vivo. Ectopic expression of myocardin is sufficient to induce endogenous TGFB1I1 expression in multiple cell lines whereas knocking-down myocardin or SRF significantly attenuated TGFB1I1 expression in SMCs. Furthermore, our data demonstrated that SRF is essential for TGF-β-mediated induction of TGFB1I1. Finally, silencing of TGFB1I1 expression significantly promotes SMC proliferation. Collectively, this study provides the first evidence that TGFB1I1 is not only an SRF/myocardin-regulated smooth muscle marker but also critical for maintaining smooth muscle contractile phenotype by inhibiting smooth muscle proliferation.

Distinct roles for paxillin and Hic-5 in regulating breast cancer cell morphology, invasion, and metastasis

This study reveals novel roles for the focal adhesion proteins paxillin and Hic-5 in regulating breast cancer invasion strategies and metastasis. Depletion of paxillin promotes a hypermesenchymal phenotype while dysregulating 3D adhesion dynamics. In contrast, RNAi of Hic-5 induces a hyperamoeboid phenotype with dysregulated RhoA/pMLC signaling.

Individual metastatic tumor cells exhibit two interconvertible modes of cell motility during tissue invasion that are classified as either mesenchymal or amoeboid. The molecular mechanisms by which invasive breast cancer cells regulate this migratory plasticity have yet to be fully elucidated. Herein we show that the focal adhesion adaptor protein, paxillin, and the closely related Hic-5 have distinct and unique roles in the regulation of breast cancer cell lung metastasis by modulating cell morphology and cell invasion through three-dimensional extracellular matrices (3D ECMs). Cells depleted of paxillin by RNA interference displayed a highly elongated mesenchymal morphology, whereas Hic-5 knockdown induced an amoeboid phenotype with both cell populations exhibiting reduced plasticity, migration persistence, and velocity through 3D ECM environments. In evaluating associated signaling pathways, we determined that Rac1 activity was increased in cells devoid of paxillin whereas Hic-5 silencing resulted in elevated RhoA activity and associated Rho kinase–induced nonmuscle myosin II activity. Hic-5 was essential for adhesion formation in 3D ECMs, and analysis of adhesion dynamics and lifetime identified paxillin as a key regulator of 3D adhesion assembly, stabilization, and disassembly.

Hic-5 deficiency enhances mechanosensitive apoptosis and modulates vascular remodeling

Forces associated with blood flow are crucial not only for blood vessel development but also for regulation of vascular pathology. Although there have been many studies characterizing the responses to mechanical stimuli, molecular mechanisms linking biological responses to mechanical forces remain unclear. Hic-5 (hydrogen peroxide-inducible clone-5) is a focal adhesion adaptor protein proposed as a candidate for a mediator of mechanotransduction. In the present study, we generated Hic-5-deficient mice by targeted mutation. Mice lacking Hic-5 are viable and fertile, and show no obvious histological abnormalities including vasculature. However, after wire injury of the femoral artery in Hic-5 deficient mice, histological recovery of arterial media was delayed due to enhanced apoptosis of vascular wall cells, whereas neointima formation was enhanced. Stretch-induced apoptosis was enhanced in cultured vascular smooth muscle cells (vascular SMCs) from Hic-5 deficient mice. Mechanical stress also induced the alteration of intracellular distribution of vinculin from focal adhesions to the whole cytoplasm in SMCs. Immunoelectron microscopic study of vascular SMCs from a wire-injured artery demonstrated that vinculin was dispersed in the nucleus and the cytoplasm in Hic-5-deficient mice whereas vinculin was localized mainly in the sub-plasma membrane region in wild type mice. Our findings indicate that Hic-5 may serve as a key regulator in mechanosensitive vascular remodeling.

Arabidopsis LIM proteins: a family of actin bundlers with distinct expression patterns and modes of regulation

Recently, a number of two LIM-domain containing proteins (LIMs) have been reported to trigger the formation of actin bundles, a major higher-order cytoskeletal assembly. Here, we analyzed the six Arabidopsis thaliana LIM proteins. Promoter-β-glucuronidase reporter studies revealed that WLIM1, WLIM2a, and WLIM2b are widely expressed, whereas PLIM2a, PLIM2b, and PLIM2c are predominantly expressed in pollen. LIM-green fluorescent protein (GFP) fusions all decorated the actin cytoskeleton and increased actin bundle thickness in transgenic plants and in vitro, although with different affinities and efficiencies. Remarkably, the activities of WLIMs were calcium and pH independent, whereas those of PLIMs were inhibited by high pH and, in the case of PLIM2c, by high [Ca(2+)]. Domain analysis showed that the C-terminal domain is key for the responsiveness of PLIM2c to pH and calcium. Regulation of LIM by pH was further analyzed in vivo by tracking GFP-WLIM1 and GFP-PLIM2c during intracellular pH modifications. Cytoplasmic alkalinization specifically promoted release of GFP-PLIM2c but not GFP-WLIM1, from filamentous actin. Consistent with these data, GFP-PLIM2c decorated long actin bundles in the pollen tube shank, a region of relatively low pH. Together, our data support a prominent role of Arabidopsis LIM proteins in the regulation of actin cytoskeleton organization and dynamics in sporophytic tissues and pollen.

Ultraviolet B radiation regulates cysteine-rich protein 1 in human keratinocytes

BACKGROUND

Cysteine-rich protein 1 (CRP1) is a growth-inhibitory cytoskeletal protein that is induced by ultraviolet (UV) C radiation radiation in fibroblasts. Our aim was to investigate the effects of UV radiation on CRP1 in keratinocytes, the main cell type subjected to UV radiation in the human body.

METHODS

The effects of physiologically relevant doses of UVB radiation on CRP1 protein levels were studied in cultured primary keratinocytes and transformed cell lines (HaCaT, A-431) by immunoblotting. UVB-induced keratinocyte apoptosis was assessed by flow cytometry and monitoring caspase activity. Expression of CRP1 in human skin in vivo was studied by immunohistochemistry in samples of normal skin, actinic keratosis (AK) representing UV-damaged skin and squamous cell carcinoma (SCC), a UV-induced skin cancer.

RESULTS

CRP1 expression increased by UVB radiation in primary but not in immortalized keratinocytes. Upon high, apoptosis-inducing doses of UV radiation, CRP1 was cleaved in a caspase-dependent manner. In normal skin, CRP1 was expressed in smooth muscle cells, vasculature, sweat glands, sebaceous glands and hair root sheath, but very little CRP1 was present in keratinocytes. CRP1 expression was elevated in basal cells in AK but not in SCC.

CONCLUSION

CRP1 expression is regulated by UVB in human keratinocytes, suggesting a role for CRP1 in the phototoxic responses of human skin.

[The cytoskeletal protein zyxin--a universal regulator of cell adhesion and gene expression]

The attachment of a cell to an extracellular matrix or the surface of another cells affects not only the cell motility, but also gene expression. In view of this, an important problem is to establish the molecular mechanisms of signal transduction from the receptors of cell adhesion to the nucleus, in particular, to identify and investigate the protein transducers of these signals. One of these transducers, the LIM domain protein zyxin, is predominantly localized at the sites of cell adhesion, where it participates in the assembly of actin filaments. Owing to its location near the inner surface of the membrane, zyxin can interact with the transmembrane receptors of some signaling cascades and affect the signal transduction from the extracellular ligands of these receptors. Furthermore, under particular conditions, zyxin moves from the sites of cell contacts to the nucleus, where it directly participates in the regulation of gene expression. Of particular interest is the function of zyxin as a possible coordinator of gene expression and morphogenetic movements in embryogenesis. The published data discussed in the present review indicate the important role of zyxin in transmitting information from the regions of cell contacts to the genetic apparatus of the cell.

Hic-5 as a regulator of endothelial cell morphology and connective tissue growth factor gene expression

The functional role of the LIM-domain protein Hic-5 was investigated in microvascular endothelial cells using a siRNA approach. Knock down of Hic-5 reduced endothelial cell spreading and impaired structural organization of the cells on basement membrane extracts. Furthermore, Hic-5 was involved in the regulation of the multifunctional protein connective tissue growth factor (CTGF, CCN2). Upon Hic-5 down-regulation, induction of CTGF by lysophosphatidic acid or colchicine was reduced. Inhibition of CTGF expression was even more pronounced in cells treated with transforming growth factor beta and inhibitors of histone deacetylases. Treatment of endothelial cells with Hic-5 siRNA reduced CTGF promoter activity. Mutation analyses of the promoter revealed transcription factors binding to the basic control element as part of the proposed Hic-5-modulated transcription complex. Further analyses showed down-regulation of Hic-5 protein upon overnight treatment with inhibitors of histone deacetylases. These data suggest that the reduced expression of Hic-5 may contribute to the anti-angiogenic effects of histone deacetylase inhibitors.

Mechanosensing in actin stress fibers revealed by a close correlation between force and protein localization

The mechanics of the actin cytoskeleton have a central role in the regulation of cells and tissues, but the details of how molecular sensors recognize deformations and forces are elusive. By performing cytoskeleton laser nanosurgery in cultured epithelial cells and fibroblasts, we show that the retraction of stress fibers (SFs) is restricted to the proximity of the cut and that new adhesions form at the retracting end. This suggests that SFs are attached to the substrate. A new computational model for SFs confirms this hypothesis and predicts the distribution and propagation of contractile forces along the SF. We then analyzed the dynamics of zyxin, a focal adhesion protein present in SFs. Fluorescent redistribution after laser nanosurgery and drug treatment shows a high correlation between the experimentally measured localization of zyxin and the computed localization of forces along SFs. Correlative electron microscopy reveals that zyxin is recruited very fast to intermediate substrate anchor points that are highly tensed upon SF release. A similar acute localization response is found if SFs are mechanically perturbed with the cantilever of an atomic force microscope. If actin bundles are cut by nanosurgery in living Drosophila egg chambers, we also find that zyxin redistribution dynamics correlate to force propagation and that zyxin relocates at tensed SF anchor points, demonstrating that these processes also occur in living organisms. In summary, our quantitative analysis shows that force and protein localization are closely correlated in stress fibers, suggesting a very direct force-sensing mechanism along actin bundles.

Hic-5, an adaptor protein expressed in vascular smooth muscle cells, modulates the arterial response to injury in vivo

Focal adhesion components are targets for biochemical and mechanical stimuli that evoke crucial injury. Hic-5 (hydrogen peroxide-inducible clone 5) is a multidomain adaptor protein which is implicated in the regulation of integrin signaling in focal adhesion. The aim of this research was to test the hypothesis that Hic-5, a focal adhesion LIM protein expressed in smooth muscle cells, is involved in dynamic processes by pathological stimuli in the vessel wall. Here, we describe the analysis of the function of Hic-5 using a mouse model of vascular injury that may mimic balloon angioplasty. At 4 days after vascular injury, marked down-regulation of the Hic-5 expression was observed in the smooth muscle layer, and local delivery of the Hic-5 using adenovirus vectors repressed injury-induced neointimal expansion. In addition, Hic-5 reduced cells migration into three-dimensional collagen gels, and the forced expression of Hic-5 in cells embedded in the collagen gel matrix repressed the expression of uPA that participates in smooth muscle cell migration. These results suggest that Hic-5 modulates cellular responses to pathological stimuli in the vessel wall.

Regulation of contractility by Hsp27 and Hic-5 in rat mesenteric small arteries

The regulation of small artery contractility by vasoconstrictors is important for vascular function, and actin cytoskeleton remodeling is required for contraction. p38 MAPK and tyrosine kinases are implicated in actin polymerization and contraction through heat shock protein 27 (Hsp27) and the cytoskeletal protein paxillin, respectively. We evaluated the roles of downstream targets of p38 MAPK and tyrosine kinases in cytoskeletal reorganization and contraction and whether the two signaling pathways regulate contraction independent of each other. We identified the expression of the paxillin homologue hydrogen peroxide-inducible clone-5 (Hic-5) and showed its activation by norepinephrine (NE) in a Src-dependent manner. Furthermore, we demonstrated a NE-induced interaction of proline-rich tyrosine kinase-2 (PYK2) but not Src or p125 focal adhesion kinase with Hic-5. This interaction was Src dependent, suggesting that Hic-5 was a substrate for PYK2 downstream from Src. The activation of Hic-5 induced its relocalization to the cytosol. The parallel activation of Hsp27 by NE was p38 MAPK dependent and led to its dissociation from actin filaments and translocation from membrane to cytosol and increased actin polymerization. Both Hsp27 and Hic-5 activation resulted in their association within the same time frame as NE-induced contraction, and the inhibition of either p38 MAPK or Src inhibited the interaction between Hsp27 and Hic-5 and the contractile response. Furthermore, combined p38 MAPK and Src inhibition had no greater effect on contraction than individual inhibition, suggesting that the two pathways act through a common mechanism. These data show that NE-induced activation and the association of Hsp27 and Hic-5 are required for the reorganization of the actin cytoskeleton and force development in small arteries.

Cytoskeleton-interacting LIM-domain protein CRP1 suppresses cell proliferation and protects from stress-induced cell death

Members of the cysteine-rich protein (CRP) family are actin cytoskeleton-interacting LIM-domain proteins known to act in muscle cell differentiation. We have earlier found that CRP1, a founding member of this family, is transcriptionally induced by UV radiation in human diploid fibroblasts [M. Gentile, L. Latonen, M. Laiho, Cell cycle arrest and apoptosis provoked by UV radiation-induced DNA damage are transcriptionally highly divergent responses, Nucleic Acids Res. 31 (2003) 4779-4790]. Here we show that CRP1 is induced by growth-inhibitory signals, such as increased cellular density, and cytotoxic stress induced by UV radiation or staurosporine. We found that high levels of CRP1 correlate with differentiation-associated morphology towards the myofibroblast lineage and that expression of ectopic CRP1 suppresses cell proliferation. Following UV- and staurosporine-induced stresses, expression of CRP1 provides a survival advantage evidenced by decreased cellular death and increased cellular metabolic activity and attachment. Our studies identify that CRP1 is a novel stress response factor, and provide evidence for its growth-inhibitory and cytoprotective functions.

The focal adhesion protein Hic-5 is highly expressed in the rat myometrium during late pregnancy and labour and co-localizes with FAK

BACKGROUND

Myometrial growth and remodeling of the cytoskeleton and focal adhesions during late pregnancy may be critical aspects of myometrial activation and thus labour. Yet our understanding of these aspects is inhibited by the paucity of information concerning the components of focal adhesions in the myometrium. The focal adhesion protein hydrogen peroxide-inducible clone-5 (Hic-5) has recently been found in mononuclear smooth muscle but was not examined in the myometrium during pregnancy. Thus, the goal of this study was to characterize Hic-5 mRNA and protein expression in the rat myometrium during pregnancy and labour.

METHODS

Rat myometrium samples were obtained from non-pregnant animals, pregnant animals on days (d) 6, 12, 15, 17, 19, 21, 22, 23 (active labour) and 1 day postpartum (PP). In addition, myometrium samples were collected from rats within a progesterone-delayed labour paradigm. Hic-5 mRNA expression was analyzed by Northern blot analysis while Hic-5 protein expression was examined by immunoblot and immunofluorescence analysis.

RESULTS

Hic-5 mRNA expression on d15, d19 and d21 was found to be significantly elevated compared to d6 and d12 of pregnancy and expression on d23 was significantly elevated over d6 (p < 0.05). Immunofluorescence analysis demonstrated that detection of Hic-5 protein in the circular muscle layer appeared to increase from d17 onwards, except PP, and Hic-5 was detectable in the cell cytoplasm and more continuously associated with myometrial cell membranes. In the longitudinal muscle layer Hic-5 was readily detectable by d15 and thereafter and primarily associated at myometrial cell membranes. Co-immunofluorescence analysis of potential Hic-5 and focal adhesion kinase (FAK) association in situ demonstrated a limited level of co-localization on d19, d23 and PP in the circular muscle layer while in the longitudinal muscle layer Hic-5 and FAK were readily co-localized at myometrial cell membranes.

CONCLUSION

Hic-5 is highly expressed in the rat myometrium during late pregnancy and labour and co-localizes with FAK in situ. Our results are consistent with a potential role for Hic-5 in focal adhesion remodeling in the rat myometrium during late pregnancy.

Hic-5 contributes to epithelial-mesenchymal transformation through a RhoA/ROCK-dependent pathway

Epithelial-mesenchymal transformation (EMT) in response to TGFbeta1 is a coordinated process of tissue morphogenesis that occurs during embryonic development as well as during certain pathologic events including kidney tubulointerstitial fibrosis. It is characterized by the disassembly of cell-cell junctions and dramatic alterations in the actin cytoskeleton that facilitates cell-matrix adhesion and stimulates migration. The focal adhesion adapter protein, Hic-5, has previously been reported to be upregulated during TGFbeta1-induced EMT in mouse mammary epithelial cells and the current study recapitulates this result in both mouse kidney proximal tubule epithelial, MCT, cells and human mammary epithelial, MCF10A, cells. To evaluate a causative role for Hic-5 in EMT, Hic-5 RNA interference (siRNA) was used to prevent Hic-5 expression in response to TGFbeta1 stimulation and was shown to suppress cell migration and actin stress fiber formation. It also resulted in the retention of a robust epithelial cell morphology characterized by elevated E-cadherin protein expression and well-organized adherens junctions. In addition, Hic-5 siRNA treatment led to the suppression of TGFbeta1 induction of RhoA activation. In contrast, forced expression of Hic-5 led to the formation of ROCK-dependent actin stress fibers. Furthermore, the induction of Hic-5 expression in response to TGFbeta1 was shown to be a RhoA/ROCK I-dependent process. Together, these data implicate Hic-5 as a key regulator of EMT and suggest that RhoA stimulated Hic-5 expression in response to TGFbeta1 may be functioning in a feed forward mechanism whereby Hic-5 maintains the mesenchymal phenotype through sustained RhoA activation and signaling.