Differentiation dependent expression of tensin and cortactin in chicken osteoclasts

Profiling Kinase Activities for Precision Oncology in Diffuse Gastric Cancer

Gastric cancer (GC) remains a leading cause of cancer-related mortality globally. This is due to the fact that majority of the cases of GC are diagnosed at an advanced stage when the treatment options are limited and prognosis is poor. The diffuse subtype of gastric cancer (DGC) under Lauren's classification is more aggressive and usually occurs in younger patients than the intestinal subtype. The concept of personalized medicine is leading to the identification of multiple biomarkers in a large variety of cancers using different combinations of omics technologies. Proteomic changes including post-translational modifications are crucial in oncogenesis. We analyzed the phosphoproteome of DGC by using paired fresh frozen tumor and adjacent normal tissue from five patients diagnosed with DGC. We found proteins involved in the epithelial-to-mesenchymal transition (EMT), c-MYC pathway, and semaphorin pathways to be differentially phosphorylated in DGC tissues. We identified three kinases, namely, bromodomain adjacent to the zinc finger domain 1B (BAZ1B), WNK lysine-deficient protein kinase 1 (WNK1), and myosin light-chain kinase (MLCK) to be hyperphosphorylated, and one kinase, AP2-associated protein kinase 1 (AAK1), to be hypophosphorylated. LMNA hyperphosphorylation at serine 392 (S392) was demonstrated in DGC using immunohistochemistry. Importantly, we have detected heparin-binding growth factor (HDGF), heat shock protein 90 (HSP90), and FTH1 as potential therapeutic targets in DGC, as drugs targeting these proteins are currently under investigation in clinical trials. Although these new findings need to be replicated in larger study samples, they advance our understanding of signaling alterations in DGC, which could lead to potentially novel actionable targets in GC.

Hematopoietic cytoplasmic adaptor protein Hem1 promotes osteoclast fusion and bone resorption in mice

Hem1 (hematopoietic protein 1), a hematopoietic cell-specific member of the Hem family of cytoplasmic adaptor proteins, is essential for lymphopoiesis and innate immunity as well as for the transition of hematopoiesis from the fetal liver to the bone marrow. However, the role of Hem1 in bone cell differentiation and bone remodeling is unknown. Here, we show that deletion of Hem1 resulted in a markedly increase in bone mass because of defective bone resorption in mice of both sexes. Hem1-deficient osteoclast progenitors were able to differentiate into osteoclasts, but the osteoclasts exhibited impaired osteoclast fusion and decreased bone-resorption activity, potentially because of decreased mitogen-activated protein kinase and tyrosine kinase c-Abl activity. Transplantation of bone marrow hematopoietic stem and progenitor cells from wildtype into Hem1 knockout mice increased bone resorption and normalized bone mass. These findings indicate that Hem1 plays a pivotal role in the maintenance of normal bone mass.

FYCO1 regulates migration, invasion, and invadopodia formation in HeLa cells through CDC42/N-WASP/Arp2/3 signaling pathway

FYCO1, an autophagy adaptor, plays an essential role in the trafficking toward the plus-end of microtubules and the fusion of autophagosomes. Autophagic dysfunction is involved in numerous disease states, including cancers. Previous studies have implicated FYCO1 as one of the critical genes involved in the adenoma to carcinoma transition, but the biological function and mechanism of FYCO1 in carcinogenesis remain unclear. This study aims to elucidate the role and mechanism of up- and downregulation of FYCO1 in mediating tumor effects in HeLa cells. Functionally, FYCO1 promotes cellular migration, invasion, epithelial-mesenchymal transition, invadopodia formation, and matrix degradation, which are detected through wound healing, transwell, immunofluorescence, and Western blot approaches. Interestingly, the data show that although FYCO1 does not affect HeLa cell proliferation, cell cycle distribution, nor vessels' formation, FYCO1 can block the apoptotic function. FYCO1 inhibits cleavage of PARP, caspase3, and caspase9 and increases Bcl-2/Bax ratio. Then, we used CK666, an Arp2/3 specific inhibitor, to confirm that FYCO1 may promote the migration and invasion of HeLa cells through the CDC42/N-WASP/Arp2/3 signaling pathway. Taken together, these results provide a new insight that FYCO1, an autophagy adaptor, may also be a new regulator of tumor metastasis.

Baculovirus actin-rearrangement-inducing factor ARIF-1 induces the formation of dynamic invadosome clusters

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), a pathogen of lepidopteran insects, has a striking dependence on the host cell actin cytoskeleton. During the delayed-early stage of infection, AcMNPV was shown to induce the accumulation of actin at the cortex of infected cells. However, the dynamics and molecular mechanism of cortical actin assembly remained unknown. Here, we show that AcMNPV induces dynamic cortical clusters of dot-like actin structures that mediate degradation of the underlying extracellular matrix and therefore function similarly to clusters of invadosomes in mammalian cells. Furthermore, we find that the AcMNPV protein actin-rearrangement-inducing factor-1 (ARIF-1), which was previously shown to be necessary and sufficient for cortical actin assembly and efficient viral infection in insect hosts, is both necessary and sufficient for invadosome formation. We mapped the sequences within the C-terminal cytoplasmic region of ARIF-1 that are required for invadosome formation and identified individual tyrosine and proline residues that are required for organizing these structures. Additionally, we found that ARIF-1 and the invadosome-associated proteins cortactin and the Arp2/3 complex localize to invadosomes and Arp2/3 complex is required for their formation. These ARIF-1-induced invadosomes may be important for the function of ARIF-1 in systemic virus spread.

Immunohistochemical Analysis of the Expression of Adhesion Proteins: TNS1, TNS2 and TNS3 in Correlation with Clinicopathological Parameters in Gastric Cancer

Tensins belong to the group of adhesion proteins that are involved in cell adhesion and migration, actin cytoskeleton maintenance and intercellular communication. TNS1, TNS2 and TNS3 proteins expression was evaluated in 90 patients with gastric cancer by immunohistochemistry method. TNS1 was more frequently present in non-differentiated tumors compared to poorly and moderately differentiated tumors (p = 0.016). TNS1 was also more often observed in metastatic tumors compared to those without distant metastases (p = 0.001). TNS2 was more common in moderately differentiated tumors than in poorly or non-differentiated ones (p = 0.041). TNS2 expression was also more frequently present in tumors with peritumoral inflammation (p = 0.041) and with concomitant H. pylori infection (p = 0.023). In contrast, TNS3 protein was more prevalent in moderately than in poorly and non-differentiated tumors (p = 0.023). No significant relationship was found between tensins' expression and the overall survival rate of patients. TNS1 protein expression is associated with a poor-prognosis type of GC. Higher expression of TNS2 is accompanied by peritumoral inflammation and H. pylori infection, which favor the development of GC of a better prognosis, similarly to higher TNS3 protein expression.

Targeting the cytoskeleton against metastatic dissemination

Cancer is a pathology characterized by a loss or a perturbation of a number of typical features of normal cell behaviour. Indeed, the acquisition of an inappropriate migratory and invasive phenotype has been reported to be one of the hallmarks of cancer. The cytoskeleton is a complex dynamic network of highly ordered interlinking filaments playing a key role in the control of fundamental cellular processes, like cell shape maintenance, motility, division and intracellular transport. Moreover, deregulation of this complex machinery contributes to cancer progression and malignancy, enabling cells to acquire an invasive and metastatic phenotype. Metastasis accounts for 90% of death from patients affected by solid tumours, while an efficient prevention and suppression of metastatic disease still remains elusive. This results in the lack of effective therapeutic options currently available for patients with advanced disease. In this context, the cytoskeleton with its regulatory and structural proteins emerges as a novel and highly effective target to be exploited for a substantial therapeutic effort toward the development of specific anti-metastatic drugs. Here we provide an overview of the role of cytoskeleton components and interacting proteins in cancer metastasis with a special focus on small molecule compounds interfering with the actin cytoskeleton organization and function. The emerging involvement of microtubules and intermediate filaments in cancer metastasis is also reviewed.

Evaluation of lumican effects on morphology of invading breast cancer cells, expression of integrins and downstream signaling

The small leucine-rich proteoglycan lumican regulates estrogen receptors (ERs)-associated functional properties of breast cancer cells, expression of matrix macromolecules, and epithelial-to-mesenchymal transition. However, it is not known whether the ER-dependent lumican effects on breast cancer cells are related to the expression of integrins and their intracellular signaling pathways. Here, we analyzed the effects of lumican in three breast cancer cell lines: the highly metastatic ERβ-positive MDA-MB-231, cells with the respective ERβ-suppressed (shERβMDA-MB-231), and lowly invasive ERα-positive MCF-7/c breast cancer cells. Scanning electron microscopy, confocal microscopy, real-time PCR, western blot, and cell adhesion assays were performed. Lumican effects on breast cancer cell morphology were also investigated in 3-dimensional collagen cultures. Lumican treatment induced cell-cell contacts and cell grouping and inhibited microvesicles and microvilli formation. The expression of the cell surface adhesion receptor CD44, its isoform and variants, hyaluronan (HA), and HA synthases was also investigated. Lumican inhibited the expression of CD44 and HA synthases, and its effect on cell adhesion revealed a major role of α1, α2, α3, αVβ3, and αVβ5 integrins in MDA-MB-231 cells, but not in MCF-7/c cells. Lumican upregulated the expression of α2 and β1 integrin subunits both in MDA-MB-231 and in shERβMDA-MB-231 as compared to MCF-7/c cells. Downstream signaling pathways for integrins, such as FAK, ERK 1/2 MAPK 42/44, and Akt, were found to be downregulated by lumican. Our data shed light to the molecular mechanisms responsible for the anticancer activity of lumican in invasive breast cancer.

Epithelial-to-mesenchymal transition and invadopodia markers in breast cancer: Lumican a key regulator

A great hallmark of breast cancer is the absence or presence of estrogen receptors ERα and ERβ, with a dominant role in cell proliferation, differentiation and cancer progression. Both receptors are related with Epithelial-to-Mesenchymal Transition (EMT) since there is a relation between ERs and extracellular matrix (ECM) macromolecules expression, and therefore, cell-cell and cell-ECM interactions. The endocrine resistance of ERα endows epithelial cells with increased aggressiveness and induces cell proliferation, resulting into a mesenchymal phenotype and an EMT status. ERα signaling may affect the transcriptional factors which govern EMT. Knockdown or silencing of ERα and ERβ in MCF-7 and MDA-MB-231 breast cancer cells respectively, provoked pivotal changes in phenotype, cellular functions, mRNA and protein levels of EMT markers, and consequently the EMT status. Mesenchymal cells owe their migratory and invasive properties to invadopodia, while in epithelial cells, lamellipodia and filopodia are mostly observed. Invadopodia, are actin-rich protrusions of plasma membrane, promoting proteolytic degradation of ECM and tumor invasion. Cortactin and MMP-14 govern the formation and principal functions of invadopodia. In vitro experiments proved that lumican inhibits cortactin and MMP-14 expression, alters the formation of lamellipodia and transforms mesenchymal cells into epithelial-like. Conclusively, lumican may inhibit or even reverse the several metastatic features that EMT endows in breast cancer cells. Therefore, a lumican-based anti-cancer therapy which will pharmacologically target and inhibit EMT might be interesting to be developed.

Specialized Roles for Actin in Osteoclasts: Unanswered Questions and Therapeutic Opportunities

Osteoclasts are cells of the hematopoietic lineage that are specialized to resorb bone. In osteoclasts, the actin cytoskeleton engages in at least two unusual activities that are required for resorption. First, microfilaments form a dynamic and structurally elaborate actin ring. Second, microfilaments bind vacuolar H⁺-ATPase (V-ATPase) and are involved in forming the V-ATPase-rich ruffled plasma membrane. The current review examines these two specialized functions with emphasis on the identification of new therapeutic opportunities. The actin ring is composed of substructures called podosomes that are interwoven to form a cohesive superstructure. Studies examining the regulation of the formation of actin rings and its constituent proteins are reviewed. Areas where there are gaps in the knowledge are highlighted. Microfilaments directly interact with the V-ATPase through an actin binding site in the B2-subunit of V-ATPase. This binding interaction is required for ruffled membrane formation. Recent studies show that an inhibitor of the interaction blocks bone resorption in pre-clinical animal models, including a model of post-menopausal osteoporosis. Because the unusual actin-based resorption complex is unique to osteoclasts and essential for bone resorption, it is likely that deeper understanding of its underlying mechanisms will lead to new approaches to treat bone disease.

Tensin 3 is a new partner of Dock5 that controls osteoclast podosome organization and activity

Bone resorption by osteoclasts is mediated by a typical adhesion structure called the sealing zone or actin ring, whose architecture is based on a belt of podosomes. The molecular mechanisms driving podosome organization into superstructures remain poorly understood to date, in particular at the osteoclast podosome belt. We performed proteomic analyses in osteoclasts and found that the adaptor protein tensin 3 is a partner of Dock5, a Rac exchange factor necessary for podosome belt formation and bone resorption. Expression of tensin 3 and Dock5 concomitantly increase during osteoclast differentiation. These proteins associate with the osteoclast podosome belt but not with individual podosomes, in contrast to vinculin. Super-resolution microscopy revealed that, even if they colocalize in the x-y plane of the podosome belt, Dock5 and tensin 3 differentially localize relative to vinculin in the z-axis. Tensin 3 increases Dock5 exchange activity towards Rac, and suppression of tensin 3 in osteoclasts destabilizes podosome organization, leading to delocalization of Dock5 and a severe reduction in osteoclast activity. Our results suggest that Dock5 and tensin 3 cooperate for osteoclast activity, to ensure the correct organization of podosomes.

Tensin Is Expressed in Glomerular Mesangial Cells and Is Related to Their Attachment to Surrounding Extracellular Matrix

Glomerular expression of tensin was immunohistochemically studied in normal and diseased rat kidneys to determine whether tensin might be related to specific binding in individual glomerular cells. Normal rat kidneys displayed an intense immunofluorescence reaction for tensin along the basal aspects of proximal and distal tubule cells and parietal epithelial cells of Bowman's capsules. In glomeruli, a positive reaction for tensin was detected only in the mesangial areas. Immunoelectron microscopy revealed a positive reaction in the mesangial cell (MC) processes. RT-PCR and immunoprecipitation demonstrated mRNA and protein levels of tensin in cultured rat MCs. Mesangial tensin expression was decreased when the mesangium was injured by Habu snake venom. During the regenerative process after mesangiolysis, tensin expression was not detected in early-phase proliferating MCs that did not have extracellular matrix (ECM). The expression of tensin recovered in late-phase proliferating MCs, which became attached to regenerated ECM. It appears that tensin is related to MC attachment to surrounding ECM, which suggests that signal transduction regulated by tensin may be related to a specific mechanism of MC matrix regeneration. Furthermore, tensin can act as a marker for rat MCs because the expression of tensin was detected only in MCs in glomeruli.

The matricellular protein CCN5 regulates podosome function via interaction with integrin αvβ 3

CCN proteins play crucial roles in cell motility, matrix turnover, and proliferation. In particular, CCN5 plays a role in cell motility and proliferation in several cell types; however, no functional binding proteins for CCN5 have been identified. In this study we report that CCN5 binds to the cell surface receptor integrin αvβ3 in vascular smooth muscle cells. Furthermore, this interaction takes place in podosomes, organelles known to degrade matrix and mediate motility. We show that CCN5 regulates the ability of podosomes to degrade matrix, but does not affect podosome formation. The level of CCN5 present in a podosome negatively correlates with its ability to degrade matrix. Conversely, knockdown of CCN5 greatly enhances the matrix-degrading ability of podosomes. These findings suggest that the antimotility effects of CCN5 may be mediated through the direct interaction of CCN5 and integrin αvβ3 in podosomes and the concomitant suppression of matrix degradation that is required for cell migration.

Microtubule dynamic instability controls podosome patterning in osteoclasts through EB1, cortactin, and Src

In osteoclasts (OCs) podosomes are organized in a belt, a feature critical for bone resorption. Although microtubules (MTs) promote the formation and stability of the belt, the MT and/or podosome molecules that mediate the interaction of the two systems are not identified. Because the growing "plus" ends of MTs point toward the podosome belt, plus-end tracking proteins (+TIPs) might regulate podosome patterning. Among the +TIPs, EB1 increased as OCs matured and was enriched in the podosome belt, and EB1-positive MTs targeted podosomes. Suppression of MT dynamic instability, displacement of EB1 from MT ends, or EB1 depletion resulted in the loss of the podosome belt. We identified cortactin as an Src-dependent interacting partner of EB1. Cortactin-deficient OCs presented a defective MT targeting to, and patterning of, podosomes and reduced bone resorption. Suppression of MT dynamic instability or EB1 depletion increased cortactin phosphorylation, decreasing its acetylation and affecting its interaction with EB1. Thus, dynamic MTs and podosomes interact to control bone resorption.

Spatiotemporal regulation of Src and its substrates at invadosomes

In the past decade, substantial progress has been made in understanding how Src family kinases regulate the formation and function of invadosomes. Invadosomes are organized actin-rich structures that contain an F-actin core surrounded by an adhesive ring and mediate invasive migration. Src kinases orchestrate, either directly or indirectly, each phase of the invadosome life cycle including invadosome assembly, maturation and matrix degradation and disassembly. Complex arrays of Src effector proteins are involved at different stages of invadosome maturation and their spatiotemporal activity must be tightly regulated to achieve effective invasive migration. In this review, we highlight some recent progress and the challenges of understanding how Src is regulated temporally and spatially to orchestrate the dynamics of invadosomes and mediate cell invasion.

Degrading devices: invadosomes in proteolytic cell invasion

Podosomes and invadopodia, collectively known as invadosomes, are cell-matrix contacts in a variety of cell types, such as monocytic cells or cancer cells, that have to cross tissue barriers. Both structures share an actin-rich core, which distinguishes them from other matrix contacts, and are regulated by a multitude of signaling pathways including RhoGTPases, kinases, actin-associated proteins, and microtubule-dependent transport. Invadosomes recruit and secrete proteinases and are thus able to lyse extracellular matrix components. They are therefore considered to be potential key structures in proteolytic cell invasion in both physiological and pathological settings. This review provides an overview of the field, with special focus on current developments such as intracellular transport processes, ultrastructural analysis, the possible involvement of invadosomes in disease, and the tentative identification of invadosomes in 3D environments and in vivo.

Subversion of the actin cytoskeleton during viral infection

Viral infection converts the normal functions of a cell to optimize viral replication and virion production. One striking observation of this conversion is the reconfiguration and reorganization of cellular actin, affecting every stage of the viral life cycle, from entry through assembly to egress. The extent and degree of cytoskeletal reorganization varies among different viral infections, suggesting the evolution of myriad viral strategies. In this Review, we describe how the interaction of viral proteins with the cell modulates the structure and function of the actin cytoskeleton to initiate, sustain and spread infections. The molecular biology of such interactions continues to engage virologists in their quest to understand viral replication and informs cell biologists about the role of the cytoskeleton in the uninfected cell.

Anterior segment dysgenesis and early-onset glaucoma in nee mice with mutation of Sh3pxd2b

PURPOSE

Mutations in SH3PXD2B cause Frank-Ter Haar syndrome, a rare condition characterized by congenital glaucoma, as well as craniofacial, skeletal, and cardiac anomalies. The nee strain of mice carries a spontaneously arising mutation in Sh3pxd2b. The purpose of this study was to test whether nee mice develop glaucoma.

METHODS

Eyes of nee mutants and strain-matched controls were comparatively analyzed at multiple ages by slit lamp examination, intraocular pressure recording, and histologic analysis. Cross sections of the optic nerve were analyzed to confirm glaucomatous progression.

RESULTS

Slit lamp examination showed that, from an early age, nee mice uniformly exhibited severe iridocorneal adhesions around the entire circumference of the eye. Presumably as a consequence of aqueous humor outflow blockage, they rapidly developed multiple indices of glaucoma. By 3 to 4 months of age, they exhibited high intraocular pressure (30.8 ± 12.5 mm Hg; mean ± SD), corneal opacity, and enlarged anterior chambers. Although histologic analyses at P17 did not reveal any indices of damage, similar analysis at 3 to 4 months of age revealed a course of progressive retinal ganglion cell loss, optic nerve head excavation, and axon loss.

CONCLUSIONS

Eyes of nee mice exhibit anterior segment dysgenesis and early-onset glaucoma. Because SH3PXD2B is predicted to be a podosome adaptor protein, these findings implicate podosomes in normal development of the iridocorneal angle and the genes influencing podosomes as candidates in glaucoma. Because of the early-onset, high-penetrance glaucoma, nee mice offer many potential advantages as a new mouse model of the disease.

Cortactin: a multifunctional regulator of cellular invasiveness

Branched actin assembly is critical for a variety of cellular processes that underlie cell motility and invasion, including cellular protrusion formation and membrane trafficking. Activation of branched actin assembly occurs at various subcellular locations via site-specific activation of distinct WASp family proteins and the Arp2/3 complex. A key branched actin regulator that promotes cell motility and links signaling, cytoskeletal and membrane trafficking proteins is the Src kinase substrate and Arp2/3 binding protein cortactin. Due to its frequent overexpression in advanced, invasive cancers and its general role in regulating branched actin assembly at multiple cellular locations, cortactin has been the subject of intense study. Recent studies suggest that cortactin has a complex role in cellular migration and invasion, promoting both on-site actin polymerization and modulation of autocrine secretion. Diverse cellular activities may derive from the interaction of cortactin with site-specific binding partners.

Analysis of the signaling pathways regulating Src-dependent remodeling of the actin cytoskeleton

Cell adhesion to the extracellular matrix is mediated by adhesion receptors, mainly integrins, which upon interaction with the extracellular matrix, bind to the actin cytoskeleton via their cytoplasmic domains. This association is mediated by a variety of scaffold and signaling proteins, which control the mechanical and signaling activities of the adhesion site. Upon transformation of fibroblasts with active forms of Src (e.g., v-Src), focal adhesions are disrupted, and transformed into dot-like contacts known as podosomes, and consisting of a central actin core surrounded by an adhesion ring. To clarify the mechanism underlying Src-dependent modulation of the adhesive phenotype, and its influence on podosome organization, we screened for the effect of siRNA-mediated knockdown of tyrosine kinases, MAP kinases and phosphatases on the reorganization of the adhesion-cytoskeleton complex, induced by a constitutively active Src mutant (SrcY527F). In this screen, we discovered several genes that are involved in Src-induced remodeling of the actin cytoskeleton. We further showed that knockdown of Src in osteoclasts abolishes the formation of the podosome-based rings and impairs cell spreading, without inducing stress fiber development. Our work points to several genes that are involved in this process, and sheds new light on the molecular plasticity of integrin adhesions.

Podosomes and Invadopodia: Related structures with Common Protein Components that May Promote Breast Cancer Cellular Invasion

A rate-limiting step in breast cancer progression is acquisition of the invasive phenotype, which can precede metastasis. Expression of cell-surface proteases at the leading edge of a migrating cell provides cells with a mechanism to cross tissue barriers. A newly appreciated mechanism that may be relevant for breast cancer cell invasion is the formation of invadopodia, well-defined structures that project from the ventral membrane and promote degradation of the extracellular matrix, allowing the cell to cross a tissue barrier. Recently, there has been some controversy and discussion as to whether invadopodia, which are associated with carcinoma cells, are related to a similar structure called podosomes, which are associated with normal cells. Invadopodia and podosomes share many common characteristics, including a similar size, shape, subcellular localization and an ability to promote invasion. These two structures also share many common protein components, which we outline herein. It has been speculated that podosomes may be precursors to invadopodia and by extension both structures may be relevant to cancer cell invasion. Here, we compare and contrast the protein components of invadopodia and podosomes and discuss a potential role for these proteins and the evidence that supports a role for invadopodia and podosomes in breast cancer invasion.

A proteomic analysis of protein variations during differentiation of v-myb-transformed monoblasts

v-myb oncogene of avian myeloblastosis virus (AMV) transforms myelomonocytic cells in vitro and induces acute monoblastic leukemia in vivo. The transforming effect of the v-myb can be suppressed using phorbol ester (TPA) or histone deacetylase inhibitor trichostatin A (TSA), the inducers of cell differentiation that are in clinical trials. In this study, we used proteomics-based approach to identify proteins with variable expression in differentiated BM2 cells. Proteome variations induced by TPA and TSA were compared to examine the mechanism of differentiation-promoting effects of these drugs. We found that expression of several proteins participating in cell cytoskeleton rearrangement, heat shock response, proteosynthesis and cell signaling was altered in TPA- or TSA-treated cells. We present here the first comparative proteome analysis of v-myb-transformed monoblasts BM2 focused on identification of proteins involved in their terminal differentiation.

WIP regulates the stability and localization of WASP to podosomes in migrating dendritic cells

The Wiskott-Aldrich Syndrome protein (WASP) is an adaptor protein that is essential for podosome formation in hematopoietic cells [1]. Given that 80% of identified Wiskott-Aldrich Syndrome patients result from mutations in the binding site for WASP-interacting-protein (WIP) [2], we examined the possible role of WIP in the regulation of podosome architecture and cell motility in dendritic cells (DCs). Our results show that WIP is essential both for the formation of actin cores containing WASP and cortactin and for the organization of integrin and integrin-associated proteins in circular arrays, specific characteristics of podosome structure. We also found that WIP is essential for the maintenance of the high turnover of adhesions and polarity in DCs. WIP exerts these functions by regulating calpain-mediated cleavage of WASP and by facilitating the localization of WASP to sites of actin polymerization at podosomes. Taken together, our results indicate that WIP is critical for the regulation of both the stability and localization of WASP in migrating DCs and suggest that WASP and WIP operate as a functional unit to control DC motility in response to changes in the extracellular environment.

Integrins in Cell Migration

Integrins are cell-surface adhesion receptors that play a central role in regulating cell migration by mediating interactions between the extracellular matrix and the actin cytoskeleton. Substantial progress has been made in understanding the mechanisms by which the formation and breakdown of adhesions are regulated. Here we describe general methods used to study integrin-mediated cell migration. Furthermore, we outline detailed procedures to examine focal adhesion assembly and disassembly using time-lapse fluorescent microscopy. Finally, we provide methods for the analysis of podosomes, which are highly dynamic adhesive structures that form in immune cells and invasive cancer cells.

Involvement of the Src-cortactin pathway in podosome formation and turnover during polarization of cultured osteoclasts

Osteoclasts are large, multinucleated cells that adhere to bone via podosomes, and degrade it. During osteoclast polarization, podosomes undergo reorganization from a scattered distribution, through the formation of clusters and ring super-structures, to the assembly of a sealing zone at the cell periphery. In the present study, we demonstrate that the levels of podosome-associated actin, and its reorganization in cultured osteoclasts, radically increase upon formation of podosome rings. At the peripheral ring, actin levels and dynamic reorganization were high, whereas paxillin, associated with the same adhesion super-structure, remained relatively stable. These dynamic changes were regulated by the tyrosine kinase pp60c-Src, whose scaffolding activity supported the assembly of immature stationary podosomes; its catalytic activity was essential for podosome maturation and turnover. The enhanced dynamic reorganization of podosomes during osteoclast polarization was inversely related to the local levels of tyrosine phosphorylation of the Src substrate, cortactin. Furthermore, overexpression of cortactin, mutated at its major Src phosphorylation sites, enhanced actin turnover, suggesting that podosome dynamics in polarizing osteoclasts are attributable to the downregulation of cortactin activity by its Src-dependent phosphorylation.

The ARP2/3 complex: an actin nucleator comes of age

The cellular functions of the actin cytoskeleton require precise regulation of both the initiation of actin polymerization and the organization of the resulting filaments. The actin-related protein-2/3 (ARP2/3) complex is a central player in this regulation. A decade of study has begun to shed light on the molecular mechanisms by which this powerful machine controls the polymerization, organization and recycling of actin-filament networks, both in vitro and in the living cell.

Molecular regulation of osteoclast activity

Osteoclasts are multinucleated cells derived from hematopoietic precursors that are primarily responsible for the degradation of mineralized bone during bone development, homeostasis and repair. In various skeletal disorders such as osteoporosis, hypercalcemia of malignancy, tumor metastases and Paget's disease, bone resorption by osteoclasts exceeds bone formation by osteoblasts leading to decreased bone mass, skeletal fragility and bone fracture. The overall rate of osteoclastic bone resorption is regulated either at the level of differentiation of osteoclasts from their monocytic/macrophage precursor pool or through the regulation of key functional proteins whose specific activities in the mature osteoclast control its attachment, migration and resorption. Thus, reducing osteoclast numbers and/or decreasing the bone resorbing activity of osteoclasts are two common therapeutic approaches for the treatment of hyper-resorptive skeletal diseases. In this review, several of the key functional players involved in the regulation of osteoclast activity will be discussed.

Osteoclastic differentiation and function regulated by old and new pathways

The osteoclast is a specialized multinucleated variant of the macrophage family. It degrades mineralized tissue, and is required for modeling and remodeling of bone. The osteoclast has long been known to require vitamin D for its differentiation and to be regulated by parathyroid hormone via circulating Ca(2+) levels. Two local signals important in osteoclast survival and differentiation, CSF-1 and RANKL, were characterized by the mid-1990 s. A basic framework of specialized cell attachment and resorption molecules was also clear by that time, including the alpha(v)beta(3) integrin, the key adhesion molecule of the mature osteoclast, the highly expressed vacuolar-type H(+)-ATPase that drives acid secretion to dissolve mineral, and cathepsin K, the predominant acid proteinase for collagenolysis. Recently, additional detail has been added to this framework, showing that the osteoclast has more complex regulation than was previously believed. These include the findings that one component of the V-H(+)-ATPase is unique to the osteoclast, that chloride transport and probably Cl(-)/H(+) exchange are also required for mineral degradation, and that additional receptors besides RANK and Fms regulate osteoclast formation and survival. Additional receptors include estrogen receptor-alpha, TNF-family receptors other than RANK, and, at least in some cases, glycoprotein hormone receptors including the TSH-R and the FSH-R. Challenges in understanding osteoclast biology include how the signalling mechanisms function cooperatively. Recent findings suggest that there is a network of cytoplasmic adapters, including Gab-2 and BCAR1, which are modified by multiple signalling mechanisms and which serve to integrate the signalling pathways.

Cortactin has an essential and specific role in osteoclast actin assembly

Osteoclasts are essential for bone dynamics and calcium homeostasis. The cells form a tight seal on the bone surface, onto which they secrete acid and proteases to resorb bone. The seal is associated with a ring of actin filaments. Cortactin, a c-Src substrate known to promote Arp2/3-mediated actin assembly in vitro, is expressed in osteoclasts and localizes to the sealing ring. To address the role of cortactin and actin assembly in osteoclasts, we depleted cortactin by RNA interference. Cortactin-depleted osteoclasts displayed a complete loss of bone resorption with no formation of sealing zones. On nonosteoid surfaces, osteoclasts flatten with a dynamic, actin-rich peripheral edge that contains podosomes, filopodia, and lamellipodia. Cortactin depletion led to a specific loss of podosomes, revealing a tight spatial compartmentalization of actin assembly. Podosome formation was restored in cortactin-depleted cells by expression of wild-type cortactin or a Src homology 3 point mutant of cortactin. In contrast, expression of a cortactin mutant lacking tyrosine residues phosphorylated by Src did not restore podosome formation. Cortactin was found to be an early component of the nascent podosome belt, along with dynamin, supporting a role for cortactin in actin assembly.

The molecular dynamics of osteoclast adhesions

Podosomes are specialized adhesive structures that play a central role in bone resorption. In this article we address the molecular diversity and dynamics of podosomes at different states of organization, ranging from scattered distribution over the entire ventral membrane of non-polarized cells, via formation of podosome clusters and developing rings to the assembly of a peripheral belt, resembling the sealing zone of polarized, bone-resorbing osteoclasts. Based on published data and on our own results, we describe here the spatial relationships between key podosome-associated proteins. Using quantitative microscopy, we show here a dramatic increase in the local levels of F-actin, vinculin, paxillin, and alpha-actinin, which occurs upon the transformation of clustered podosomes into rings and sealing zone-like structures. This change is accompanied by a marked decrease in phosphotyrosine levels in the same region. Therefore, our data suggest that a major change in the molecular composition of podosomes is taking place during osteoclast polarization, a change that may be related to adhesion "reinforcement", associated with the assembly of the bone-resorbing apparatus. Studying the nature of the proteins that undergo de-phosphorylation is critical for the understanding of the mechanisms regulating the processes described above.

Cortactin affects cell migration by regulating intercellular adhesion and cell spreading

Cortactin is an F-actin binding protein that stabilizes F-actin networks and promotes actin polymerization by activating the Arp2/3 complex. Overexpression of cortactin, as observed in several human cancers, stimulates cell migration, invasion, and experimental metastasis; however, the underlying mechanism is not understood. To investigate the importance of cortactin in cell migration, we downregulated its expression using RNA interference (RNAi). Stable downregulation of cortactin in HBL100 breast epithelial cells resulted in (i) decreased cell migration and invasion, (ii) enhanced cell-cell adhesion, and (iii) accelerated cell spreading. These phenotypic changes were reversed by expression of RNAi-resistant mouse cortactin. Cortactin colocalized with cadherin and beta-catenin in adherens junctions, consistent with its role in intercellular adhesion. Remarkably, cortactin deficiency did not affect lamellipodia formation. Instead, downregulation of cortactin in human squamous carcinoma cells that overexpress cortactin changed the cytoskeletal organization. We conclude that increased levels of cortactin, as found in human carcinomas, promote cell migration and invasion by reducing cell spreading and intercellular adhesive strength.

Cortactin signalling and dynamic actin networks

Cortactin was first identified over a decade ago, and its initial characterization as both an F-actin binding protein and v-Src substrate suggested that it was likely to be a key regulator of actin rearrangements in response to tyrosine kinase signalling. The recent discovery that cortactin binds and activates the actin related protein (Arp)2/3 complex, and thus regulates the formation of branched actin networks, together with the identification of multiple protein targets of the cortactin SH3 domain, have revealed diverse cellular roles for this protein. This article reviews current knowledge regarding the role of cortactin in signalling to the actin cytoskeleton in the context of these developments.

Comparative genome analysis of cortactin and HS1: the significance of the F-actin binding repeat domain

BACKGROUND

In human carcinomas, overexpression of cortactin correlates with poor prognosis. Cortactin is an F-actin-binding protein involved in cytoskeletal rearrangements and cell migration by promoting actin-related protein (Arp)2/3 mediated actin polymerization. It shares a high amino acid sequence and structural similarity to hematopoietic lineage cell-specific protein 1 (HS1) although their functions differ considerable. In this manuscript we describe the genomic organization of these two genes in a variety of species by a combination of cloning and database searches. Based on our analysis, we predict the genesis of the actin-binding repeat domain during evolution.

RESULTS

Cortactin homologues exist in sponges, worms, shrimps, insects, urochordates, fishes, amphibians, birds and mammalians, whereas HS1 exists in vertebrates only, suggesting that both genes have been derived from an ancestor cortactin gene by duplication. In agreement with this, comparative genome analysis revealed very similar exon-intron structures and sequence homologies, especially over the regions that encode the characteristic highly conserved F-actin-binding repeat domain. Cortactin splice variants affecting this F-actin-binding domain were identified not only in mammalians, but also in amphibians, fishes and birds. In mammalians, cortactin is ubiquitously expressed except in hematopoietic cells, whereas HS1 is mainly expressed in hematopoietic cells. In accordance with their distinct tissue specificity, the putative promoter region of cortactin is different from HS1.

CONCLUSIONS

Comparative analysis of the genomic organization and amino acid sequences of cortactin and HS1 provides inside into their origin and evolution. Our analysis shows that both genes originated from a gene duplication event and subsequently HS1 lost two repeats, whereas cortactin gained one repeat. Our analysis genetically underscores the significance of the F-actin binding domain in cytoskeletal remodeling, which is of importance for the major role of HS1 in apoptosis and for cortactin in cell migration.

Actin-related protein 2/3 complex is required for actin ring formation

Actin rings are vital for osteoclastic bone resorption, and actin-related protein 2/3 complex is a pivotal regulator of actin polymerization. Actin-related protein 2/3 complex was found in the podosomes of actin rings. A short interfering RNA knocked down expression of actin-related protein 2 in osteoclasts and disrupted actin rings, suggesting that the complex is crucial to actin ring formation.

INTRODUCTION

To resorb bone, osteoclasts form an extracellular acidic compartment segregated by a sealing zone. This is dependent on an actin ring that is composed of filamentous actin organized into dynamic structures called podosomes. The actin-related protein 2/3 (Arp2/3) complex is a vital regulator of actin polymerization. We tested whether the Arp2/3 complex is a component of actin rings and is important for actin ring formation.

MATERIALS AND METHODS

Western blot analysis was used to determine levels of Arp2 and Arp3, two components of the Arp2/3 complex in osteoclast-like cells. Confocal microscopy studies using antibodies for immunocytochemistry demonstrated localization of Arp2/3 complex in osteoclasts. Short interfering RNA oligonucleotides (siRNAs) were made against Arp2 and used to knock down its expression.

RESULTS

A 3-fold increase in Arp2 and Arp3 was detected during RANKL-induced differentiation of RAW 264.7 cells into osteoclast-like cells. Arp2/3 complex was concentrated in actin rings and enriched near the sealing zone. Arp2/3 complex co-localized with cortactin, a component of podosomes, but not vinculin, which surrounds podosomes. siRNA against Arp2, transfected into RAW 264.7 cells 5 days after stimulation with RANKL, reduced Arp2 protein levels 70% compared with cells transfected with ineffective siRNAs. Cytochemical characterization of RAW 264.7 osteoclast-like cells and marrow osteoclasts in which Arp2 was knocked down revealed fewer podosomes and no actin rings, although many cells remained well spread.

CONCLUSIONS

These data show that Arp2/3 complex is a component of actin rings and that the presence of Arp2/3 complex is vital to the formation of actin rings. In addition, the results show the use of siRNAs for the study of RAW 264.7 osteoclast-like cells.

Actin filament organization of foot processes in rat podocytes

The foot processes of podocytes possess abundant microfilaments and modulate glomerular filtration. We investigated the actin filament organization of foot processes in adult rat podocytes and the formation of the actin cytoskeletal system of immature podocytes during glomerulogenesis. Electron microscopy revealed two populations of actin cytoskeletons in foot processes of adult podocytes. One is the actin bundle running above the level of slit diaphragms and the other is the cortical actin network located beneath the plasmalemma. Immunogold labeling for actin-binding proteins demonstrated that alpha-actinin and synaptopodin were localized in the actin bundle, whereas cortactin was in the cortical actin network. Immunofluorescence labeling for actin-binding proteins in immature podocyte showed that alpha-actinin was localized at the level of the junctional complex, whereas cortactin was distributed beneath the entire plasmalemma. Synaptopodin was first observed along the basal plasmalemma from the advanced S-shaped body to the capillary loop stage. We conclude that foot processes have specialized actin filamentous organization and that its establishment is associated with the expression and redistribution of actin-binding proteins during development.

Regulation of Cortical Actin Networks in Cell Migration

The actin cytoskeleton is a primary determinant of cell shape and motility. Studies on actin regulatory proteins are now coupled with studies of the signal transduction that directs actin cytoskeleton reorganization, and we have gained insights into how external stimuli such as chemoattractants drive changes in actin cytoskeleton. Chemoattractants regulate actin regulatory proteins such as the Arp2/3 complex through WASP family proteins, ADF/cofilin downstream of LIM-kinase, and various other phosphoinositide-dependent or -independent pathways. Through branching of actin filaments, Arp2/3 complex-dependent actin polymerization is suffcient to generate the force necessary for protrusion.

v-Src's hold over actin and cell adhesions

The oncoprotein v-Src and its cellular homologue (c-Src) are tyrosine kinases that modulate the actin cytoskeleton and cell adhesions. Through the concerted action of their protein-interaction and kinase domains, they are targeted to cell matrix integrin adhesions or cadherin-dependent junctions between epithelial cells, where they phosphorylate substrates that induce adhesion turnover and actin re-modelling. Recent experiments have defined some of the key targets and effector pathways that mediate the pleiotropic oncogenic effects of v-Src.

Activation of Arp2/3 complex-mediated actin polymerization by cortactin

Cortactin, a filamentous actin (F-actin)-associated protein and prominent substrate of Src, is implicated in progression of breast tumours through gene amplification at chromosome 11q13. However, the function of cortactin remains obscure. Here we show that cortactin co-localizes with the Arp2/3 complex, a de novo actin nucleator, at dynamic particulate structures enriched with actin filaments. Cortactin binds directly to the Arp2/3 complex and activates it to promote nucleation of actin filaments. The interaction of cortactin with the Arp2/3 complex occurs at an amino-terminal domain that is rich in acidic amino acids. Mutations in a conserved amino-acid sequence of DDW abolish both the interaction with the Arp2/3 complex and complex activation. The N-terminal domain is not only essential but also sufficient to target cortactin to actin-enriched patches within cells. Interestingly, the ability of cortactin to activate the Arp2/3 complex depends on an activity for F-actin binding, which is almost 20-fold higher than that of the Arp2/3 complex. Our data indicate a new mechanism for activation of actin polymerization involving an enhanced interaction between the Arp2/3 complex and actin filaments.

Actin assembly at membranes controlled by ARF6

The small GTPase, ADP-ribosylation factor-6 (ARF6), has been implicated in regulating membrane traffic and remodeling cortical F-actin. Using real-time video analysis of actin assembly in living cells, we investigated the function and mechanism of ARF6 in control of actin assembly. Expression of an activated form of ARF6 that mimicks the GTP-bound form of the GTPase induced actin assembly resulting in the movement of vesicle-like particles, some of which contain markers for pinosomes. Activated ARF6 also stimulated actin assembly at foci on the ventral surface of the cell and stimulated fluid phase pinocytosis. Particle motility induced by ARF6 involved Arp2/3 complex, tyrosine kinase activity, phospholipase D (PLD) and D3-phosphoinositides, but not phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). We conclude that ARF6 regulates actin assembly for pinosome motility and at foci on the ventral cell surface.

A functional link between dynamin and the actin cytoskeleton at podosomes

Cell transformation by Rous sarcoma virus results in a dramatic change of adhesion structures with the substratum. Adhesion plaques are replaced by dot-like attachment sites called podosomes. Podosomes are also found constitutively in motile nontransformed cells such as leukocytes, macrophages, and osteoclasts. They are represented by columnar arrays of actin which are perpendicular to the substratum and contain tubular invaginations of the plasma membrane. Given the similarity of these tubules to those generated by dynamin around a variety of membrane templates, we investigated whether dynamin is present at podosomes. Immunoreactivities for dynamin 2 and for the dynamin 2-binding protein endophilin 2 (SH3P8) were detected at podosomes of transformed cells and osteoclasts. Furthermore, GFP wild-type dynamin 2aa was targeted to podosomes. As shown by fluorescence recovery after photobleaching, GFP-dynamin 2aa and GFP-actin had a very rapid and similar turnover at podosomes. Expression of the GFP-dynamin 2aa(G273D) abolished podosomes while GFP-dynamin(K44A) was targeted to podosomes but delayed actin turnover. These data demonstrate a functional link between a member of the dynamin family and actin at attachment sites between cells and the substratum.

Fimbrin in podosomes of monocyte-derived osteoclasts

Fimbrin, an actin-bundling protein, is a component of the osteoclast adhesion complexes called podosomes. In this study, we (1) determined the localization of fimbrin in the mature rabbit osteoclast as well as in differentiating osteoclasts using the avian monocyte-derived osteoclast differentiation model, (2) characterized the distribution and accumulation of three fimbrin isotypes (T, L, and I) in avian monocytes as they fused to form multinucleate osteoclast-like cells, and (3) report for the first time, a close spatial relationship between podosomes and microtubules using fimbrin as a marker of the podosome. Immunofluorescence using anti-T-fimbrin, anti-L-fimbrin, and pan-isotype-anti-fimbrin antibodies, showed that fimbrin is an integral component of the podosome core in the mature rabbit osteoclast and in the monocyte-derived osteoclast throughout differentiation. Anti-I-fimbrin, however, did not show immunoreactivity in these cultures. These studies also show that in the avian model of monocyte-derived osteoclast differentiation, day 2 cells (D2) are predominantly mononucleate and have few podosomes. By days 4 and 6 in culture (D4 and D6), many cells have fused and punctate rows of podosomes are commonly observed at cell margins. Analysis by Western blot of protein accumulation showed that after an initial small rise from D2 to D4, L-fimbrin levels remained relatively constant from D4 to D6. However, T-fimbrin protein levels increase steadily from D2 to D6, suggesting that it may be related to the increase in podosome formation as monocytes fuse to form osteoclasts. Finally, we examined the distribution of podosomes relative to other cytoskeletal elements such as microtubules and intermediate filaments. Double immunofluorescence labeling using anti-fimbrin and anti-tubulin showed podosomes lying adjacent to microtubules at cell margins. When osteoclasts were treated with nocodazole (1 X 10(-6) M) to disrupt microtubules, the distribution of podosomes became more random and was no longer confined to the cell periphery. These results suggest that microtubule-podosome interactions may play a role in osteoclast adhesion.

Wiskott-Aldrich syndrome protein regulates podosomes in primary human macrophages

Wiskott-Aldrich syndrome protein (WASp) is a hematopoietic-specific, multidomain protein whose mutation is responsible for the immunodeficiency disorder Wiskott-Aldrich syndrome. WASp contains a binding motif for the Rho GTPase CDC42Hs as well as verprolin/cofilin-like actin-regulatory domains, but no specific actin structure regulated by CDC42Hs-WASp has been identified. We found that WASp colocalizes with CDC42Hs and actin in the core of podosomes, a highly dynamic adhesion structure of human blood-derived macrophages. Microinjection of constitutively active V12CDC42Hs or a constitutively active WASp fragment consisting of the verprolin/cofilin-like domains led to the disassemly of podosomes. Conversely, macrophages from patients expressing truncated forms of WASp completely lacked podosomes. These findings indicate that WASp controls podosome assembly and, in cooperation with CDC42Hs, podosome disassembly in primary human macrophages.

Characterization of the EMS1 gene and its product, human Cortactin

We have identified a novel gene, EMS1, that is consistently amplified and overexpressed in human carcinomas with an amplification of the chromosome 11q13 region. Comparisons of the EMS1 sequences with those present in the GenBank databases revealed a high identity with chicken cortactin. Southern and western blot analyses confirm the high sequence conservation during evolution. An antiserum specific for human cortactin, showed in gene transfer experiments that both human p80 and p85 isoforms are encoded by the EMS1 cDNA. Further comparisons demonstrated an high sequence and structural homology with HS1 that is implicated in signal transduction in lymphoid cells only. Expression of EMS1/cortactin mRNA was restricted to tumor cell lines derived from non-lymphoid origin. Cortactin contains (i) a filamentous actin binding tandem repeat domain, (ii) a proline-rich SH3-binding and (iii) a SH3 domain that is common in proteins involved in signal transduction. Our data suggest that human EMS1/cortactin has a function in signal transmission between cell-matrix contact sites and the cytoskeleton and, as such, its overexpression due to 11q13 amplification might effect adhesive properties of human carcinomas.

Cortactin associates with the cell-cell junction protein ZO-1 in both Drosophila and mouse

Cortactin is an actin filament-binding protein localizing at cortical regions of cells and a prominent substrate for Src family protein-tyrosine kinases in response to multiple extracellular stimuli. Human cortactin has been identified as a protein product of a putative oncogene, EMS1. In this report, we describe the identification of a Drosophila homolog of cortactin as a molecule that interacts with Drosophila ZO-1 using yeast two-hybrid screening. Drosophila cortactin is a 559-amino acid protein highly expressed in embryos, larvae, and pupae but relatively underexpressed in adult flies. Deletion and substitution mutant analyses revealed that the SH3 domain of Drosophila cortactin binds to a PXXP motif in the proline-rich domain of Drosophila ZO-1. Colocalization of these proteins at cell-cell junction sites was evident under a confocal laser-scanning microscope. In vivo association was confirmed by coimmunoprecipitation of cortactin and ZO-1 from Drosophila embryo lysates. We also demonstrate an association for each of the murine homologs by immunoprecipitation analyses of mouse tissue lysates. Our previous work has demonstrated the involvement of ZO-1 in a signaling pathway that regulates expression of the emc gene in Drosophila. The potential roles of the cortactin.ZO-1 complex in cell adhesion and cell signaling are discussed.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

A peptidomimetic antagonist of the alpha(v)beta3 integrin inhibits bone resorption in vitro and prevents osteoporosis in vivo

Osteoclastic bone degradation requires intimacy between the matrix and the resorptive cell. While the precise role the integrin alpha(v)beta3 plays in the process is not yet understood, occupancy of the heterodimer by soluble ligand or by blocking antibody effectively inhibits bone resorption in vitro and in vivo, suggesting that alpha(v)beta3 blockade may prevent postmenopausal osteoporosis. Thus, we identified a synthetic chemical peptide mimetic, beta-[2-[[5-[(aminoiminomethyl)amino]-1-oxopentyl]amino]-1-+ ++oxoethyl]amino-3-pyridinepropanoic acid, bistrifluoroacetate (SC56631) based upon the alpha(v)beta3 ligand, Arg-Gly-Asp (RGD), which recognizes the isolated integrin, and its relative, alpha(v)beta5, as effectively as does the natural peptide. The mimetic dampens osteoclastic bone resorption in vitro and in vivo. Most importantly, intravenous administration of the mimetic prevents the 55% loss of trabecular bone sustained by rats within 6 wk of oophorectomy. Histological examination of bones taken from SC56631-treated, oophorectomized animals also demonstrates the compound's bone sparing properties and its capacity to decrease osteoclast number. Thus, an RGD mimetic prevents the rapid bone loss that accompanies estrogen withdrawal.

The redistribution of cortactin into cell-matrix contact sites in human carcinoma cells with 11q13 amplification is associated with both overexpression and post-translational modification

The EMS1 gene, located at the chromosome 11q13 region, is the human homologue of p80/p85 cortactin, a chicken pp60(src) tyrosine kinase substrate. In cells derived from breast carcinomas and squamous carcinomas of the head and neck, DNA amplification of this region results in overexpression of cortactin. Overexpression is accompanied by a partial redistribution of cortactin from the cytoplasm into cell-matrix contact sites. To investigate whether overexpression only is sufficient for this redistribution, we performed biochemical analysis of human cortactin derived from carcinoma cell lines with either normal levels (UMSCC8) or with excessive levels of cortactin due to chromosome 11q13 amplification (UMSCC2). Pulse-chase experiments performed with UMSCC2 cells revealed that p85 originated from p80 by post-translational modifications. However, the conversion of p80 into p85 was hardly observed in UMSCC8 cells, indicating a different processing of the two isoforms in cells with a normal expression level of cortactin. Western blot analysis showed that treatment of UMSCC2 cells with cycloheximide, serum, epidermal growth factor, or vanadate resulted in the disappearance of the p80 form and conversion into p85. Conversion of p80 into p85 was accompanied by a redistribution of cortactin from cytoplasm to cell-matrix contact sites. In UMSCC8 cells, these treatments had no effect on the p80/p85 ratio, and cortactin remained in the cytoplasm. Conversion into p85 therefore is correlated with a relocalization of cortactin to the cell periphery. In addition, p85 from epidermal growth factor- or vanadate-treated UMSCC2 cells showed a significant enhancement in phosphorylation compared with p85 in UMSCC8 cells. Our findings demonstrate that in carcinoma cells with 11q13 amplification not only overexpression but also post-translational modifications of cortactin coincides with the redistribution from the cytoplasm into cell-matrix contact sites.

Retinoic acid stimulates expression of the functional osteoclast integrin alpha v beta 3: transcriptional activation of the beta 3 but not the alpha v gene

The capacity of osteoclasts to resorb bone depends, in part, on the surface expression of the integrin alpha v beta 3. We have investigated whether the steroid hormone retinoic acid, known to stimulate bone resorption, regulates the appearance of the alpha v beta 3 complex in avian osteoclast presursors. Using surface labeling, followed by immunoprecipitation with a alpha v beta 3-specific antibody, we show that retinoic acid increases surface expression of the heterodimer in a dose- and time-dependent manner. Northern analysis reveals that the high basal steady-state levels of alpha v mRNA do not change, while those for beta 3 rise significantly from their initially low levels. Nuclear run-on studies confirm that steroid treatment stimulates transcription of the beta 3, but not the alpha v gene. Osteoclast precursors treated with retinoic acid exhibit increased multinucleation and expression of the osteoclast marker enzyme tartrate-resistant acid phosphatase. However, the fused cells do not have an increased capacity to resorb bone. In summary, multinucleated cells generated in this study do not represent fully differentiated bone-resorbing polykaryons. These results suggest that treatment of osteoclast precursors with retinoic acid is necessary, but insufficient, for expression of the mature osteoclast phenotype.