Metalloproteinase-mediated release of the ectodomain of L1 adhesion molecule

Therapeutic Potential of Targeting Regulated Intramembrane Proteolysis Mechanisms of Voltage-Gated Ion Channel Subunits and Cell Adhesion Molecules

Several integral membrane proteins undergo regulated intramembrane proteolysis (RIP), a tightly controlled process through which cells transmit information across and between intracellular compartments. RIP generates biologically active peptides by a series of proteolytic cleavage events carried out by two primary groups of enzymes: sheddases and intramembrane-cleaving proteases (iCLiPs). Following RIP, fragments of both pore-forming and non-pore-forming ion channel subunits, as well as immunoglobulin super family (IgSF) members, have been shown to translocate to the nucleus to function in transcriptional regulation. As an example, the voltage-gated sodium channel β1 subunit, which is also an IgSF-cell adhesion molecule (CAM), is a substrate for RIP. β1 RIP results in generation of a soluble intracellular domain, which can regulate gene expression in the nucleus. In this review, we discuss the proposed RIP mechanisms of voltage-gated sodium, potassium, and calcium channel subunits as well as the roles of their generated proteolytic products in the nucleus. We also discuss other RIP substrates that are cleaved by similar sheddases and iCLiPs, such as IgSF macromolecules, including CAMs, whose proteolytically generated fragments function in the nucleus. Importantly, dysfunctional RIP mechanisms are linked to human disease. Thus, we will also review how understanding RIP events and subsequent signaling processes involving ion channel subunits and IgSF proteins may lead to the discovery of novel therapeutic targets. SIGNIFICANCE STATEMENT: Several ion channel subunits and immunoglobulin superfamily molecules have been identified as substrates of regulated intramembrane proteolysis (RIP). This signal transduction mechanism, which generates polypeptide fragments that translocate to the nucleus, is an important regulator of gene transcription. RIP may impact diseases of excitability, including epilepsy, cardiac arrhythmia, and sudden death syndromes. A thorough understanding of the role of RIP in gene regulation is critical as it may reveal novel therapeutic strategies for the treatment of previously intractable diseases.

Functional Diversity of Neuronal Cell Adhesion and Recognition Molecule L1CAM through Proteolytic Cleavage

The neuronal cell adhesion and recognition molecule L1 does not only 'keep cells together' by way of homophilic and heterophilic interactions, but can also promote cell motility when cleaved into fragments by several proteases. It has largely been thought that such fragments are signs of degradation. Now, it is clear that proteolysis contributes to the pronounced functional diversity of L1, which we have reviewed in this work. L1 fragments generated at the plasma membrane are released into the extracellular space, whereas other membrane-bound fragments are internalised and enter the nucleus, thus conveying extracellular signals to the cell interior. Post-translational modifications on L1 determine the sequence of cleavage by proteases and the subcellular localisation of the generated fragments. Inside the neuronal cells, L1 fragments interact with various binding partners to facilitate morphogenic events, as well as regenerative processes. The stimulation of L1 proteolysis via injection of L1 peptides or proteases active on L1 or L1 mimetics is a promising tool for therapy of injured nervous systems. The collective findings gathered over the years not only shed light on the great functional diversity of L1 and its fragments, but also provide novel mechanistic insights into the adhesion molecule proteolysis that is active in the developing and diseased nervous system.

Kallikrein 8: A key sheddase to strengthen and stabilize neural plasticity

Neural networks are modified and reorganized throughout life, even in the matured brain. Synapses in the networks form, change, or disappear dynamically in the plasticity state. The pre- and postsynaptic signaling, transmission, and structural dynamics have been studied considerably well. However, not many studies have shed light on the events in the synaptic cleft and intercellular space. Neural activity-dependent protein shedding is a phenomenon in which (1) presynaptic excitation evokes secretion or activation of sheddases, (2) sheddases are involved not only in cleavage of membrane- or matrix-bound proteins but also in mechanical modulation of cell-to-cell connectivity, and (3) freed activity domains of protein factors play a role in receptor-mediated or non-mediated biological actions. Kallikrein 8/neuropsin (KLK8) is a kallikrein family serine protease rich in the mammalian limbic brain. Accumulated evidence has suggested that KLK8 is an important modulator of neural plasticity and consequently, cognition. Insufficiency, as well as excess of KLK8 may have detrimental effects on limbic functions.

Neuronal post-developmentally acting SAX-7S/L1CAM can function as cleaved fragments to maintain neuronal architecture in C. elegans

Whereas remarkable advances have uncovered mechanisms that drive nervous system assembly, the processes responsible for the lifelong maintenance of nervous system architecture remain poorly understood. Subsequent to its establishment during embryogenesis, neuronal architecture is maintained throughout life in the face of the animal's growth, maturation processes, the addition of new neurons, body movements, and aging. The C. elegans protein SAX-7, homologous to the vertebrate L1 protein family of neural adhesion molecules, is required for maintaining the organization of neuronal ganglia and fascicles after their successful initial embryonic development. To dissect the function of sax-7 in neuronal maintenance, we generated a null allele and sax-7S-isoform-specific alleles. We find that the null sax-7(qv30) is, in some contexts, more severe than previously described mutant alleles, and that the loss of sax-7S largely phenocopies the null, consistent with sax-7S being the key isoform in neuronal maintenance. Using a sfGFP::SAX-7S knock-in, we observe sax-7S to be predominantly expressed across the nervous system, from embryogenesis to adulthood. Yet, its role in maintaining neuronal organization is ensured by post-developmentally acting SAX-7S, as larval transgenic sax-7S(+) expression alone is sufficient to profoundly rescue the null mutants' neuronal maintenance defects. Moreover, the majority of the protein SAX-7 appears to be cleaved, and we show that these cleaved SAX-7S fragments together, not individually, can fully support neuronal maintenance. These findings contribute to our understanding of the role of the conserved protein SAX-7/L1CAM in long-term neuronal maintenance, and may help decipher processes that go awry in some neurodegenerative conditions.

The 5-HT4 receptor interacts with adhesion molecule L1 to modulate morphogenic signaling in neurons

Morphological remodeling of dendritic spines is critically involved in memory formation and depends on adhesion molecules. Serotonin receptors are also implicated in this remodeling, though the underlying mechanisms remain enigmatic. Here, we uncovered a signaling pathway involving the adhesion molecule L1CAM (L1) and serotonin receptor 5-HT₄ (5-HT₄R, encoded by HTR4). Using Förster resonance energy transfer (FRET) imaging, we demonstrated a physical interaction between 5-HT₄R and L1, and found that 5-HT₄R-L1 heterodimerization facilitates mitogen-activated protein kinase activation in a G_s-dependent manner. We also found that 5-HT₄R-L1-mediated signaling is involved in G₁₃-dependent modulation of cofilin-1 activity. In hippocampal neurons in vitro, the 5-HT₄R-L1 pathway triggers maturation of dendritic spines. Thus, the 5-HT₄R-L1 signaling module represents a previously unknown molecular pathway regulating synaptic remodeling.

L1 Cell Adhesion Molecule Overexpression Down Regulates Phosphacan and Up Regulates Structural Plasticity-Related Genes Rostral and Caudal to the Complete Spinal Cord Transection

L1 cell adhesion molecule (L1CAM) supports spinal cord cellular milieu after contusion and compression lesions, contributing to neuroprotection, promoting axonal outgrowth, and reducing outgrowth-inhibitory molecules in lesion proximity. We extended investigations into L1CAM molecular targets and explored long-distance effects of L1CAM rostral and caudal to complete spinal cord transection (SCT) in adult rats. L1CAM overexpression in neurons and glia after Th10/Th11 SCT was achieved using adeno-associated viral vector serotype 5 (AAV5) injected into an L1-lumbar segment immediately after transection. At 5 weeks, a L1CAM mRNA profound decrease detected rostral and caudal to the transection site was alleviated by AAV5-L1CAM treatment, with increased endogenous L1CAM rostral to the SCT. Transected corticospinal tract fibers showed attenuated retraction after treatment, accompanied by a multi-segmental increase of lesion-reduced expression of adenylate cyclase 1 (Adcy1), synaptophysin, growth-associated protein 43, and myelin basic protein genes caudal to transection, and Adcy1 rostral to transection. In parallel, chondroitin sulfate proteoglycan phosphacan elevated after SCT was downregulated after treatment. Low-molecular L1CAM isoforms generated after spinalization indicated the involvement of sheddases in L1CAM processing and long-distance effects. A disintegrin and metalloproteinase (ADAM)10 sheddase immunoreactivity, stronger in AAV5-L1CAM than AAV5- enhanced green fluorescent protein (EGFP)-transduced motoneurons indicated local ADAM10 upregulation by L1CAM. The results suggest that increased L1CAM availability and penetration of diffusible L1CAM fragments post-lesion induce both local and long-distance neuronal and glial responses toward better neuronal maintenance, neurite growth, and myelination. Despite the fact that intervention promoted beneficial molecular changes, kinematic analysis of hindlimb movements showed minor improvement, indicating that spinalized rats require longer L1CAM treatment to regain locomotor functions.

L1cam-mediated developmental processes of the nervous system are differentially regulated by proteolytic processing

Normal brain development depends on tight temporal and spatial regulation of connections between cells. Mutations in L1cam, a member of the immunoglobulin (Ig) superfamily that mediate cell-cell contacts through homo- and heterophilic interactions, are associated with several developmental abnormalities of the nervous system, including mental retardation, limb spasticity, hydrocephalus, and corpus callosum aplasia. L1cam has been reported to be shed from the cell surface, but the significance of this during different phases of brain development is unknown. We here show that ADAM10-mediated shedding of L1cam is regulated by its fibronectin type III (FNIII) domains. Specifically, the third FNIII domain is important for maintaining a conformation where access to a membrane proximal cleavage site is restricted. To define the role of ADAM10/17/BACE1-mediated shedding of L1cam during brain development, we used a zebrafish model system. Knockdown of the zebrafish, l1camb, caused hydrocephalus, defects in axonal outgrowth, and myelination abnormalities. Rescue experiments with proteinase-resistant and soluble L1cam variants showed that proteolytic cleavage is not required for normal axonal outgrowth and development of the ventricular system. In contrast, metalloproteinase-mediated shedding is required for efficient myelination, and only specific fragments are able to mediate this stimulatory function of the shedded L1cam.

The functional role of integrins during intra- and extravasation within the metastatic cascade

Formation of distant metastases is by far the most common cause of cancer-related deaths. The process of metastasis formation is complex, and within this complex process the formation of migratory cells, the so called epithelial mesenchymal transition (EMT), which enables cancer cells to break loose from the primary tumor mass and to enter the bloodstream, is of particular importance. To break loose from the primary cancer, cancer cells have to down-regulate the cell-to-cell adhesion molecuIes (CAMs) which keep them attached to neighboring cancer cells. In contrast to this downregulation of CAMS in the primary tumor, cancer cells up-regulate other types of CAMs, that enable them to attach to the endothelium in the organ of the future metastasis. During EMT, the expression of cell-to-cell and cell-to-matrix adhesion molecules and their down- and upregulation is therefore critical for metastasis formation. Tumor cells mimic leukocytes to enable transmigration of the endothelial barrier at the metastatic site. The attachment of leukocytes/cancer cells to the endothelium are mediated by several CAMs different from those at the site of the primary tumor. These CAMs and their ligands are organized in a sequential row, the leukocyte adhesion cascade. In this adhesion process, integrins and their ligands are centrally involved in the molecular interactions governing the transmigration. This review discusses the integrin expression patterns found on primary tumor cells and studies whether their expression correlates with tumor progression, metastatic capacity and prognosis. Simultaneously, further possible, but so far unclearly characterized, alternative adhesion molecules and/or ligands, will be considered and emerging therapeutic possibilities reviewed.

Expression of neural cell adhesion molecule L1 (CD171) in neuroectodermal and other tumors: An immunohistochemical study of 5155 tumors and critical evaluation of CD171 prognostic value in gastrointestinal stromal tumors

The neural cell adhesion molecule L1 (CD171) is a multidomain type 1 membrane glycoprotein of the immunoglobulin superfamily important in the nervous system development, kidney morphogenesis, and maintenance of the immune system. Recent studies reported CD171 expression being associated with adverse clinical outcome in different types of cancer and there has been a growing interest in targeting this cell membrane molecule on neoplastic cells by chimeric antigen receptor redirected T lymphocytes or specific antibodies. Nevertheless, conflicting results regarding the prognostic value of CD171 expression in renal cell carcinomas and gastrointestinal stromal tumors were published. In this study, CD171 expression was immunohistochemically analyzed in 5155 epithelial, mesenchymal, melanocytic, and lymphohematopoietic tumors to assess its utility in diagnostic pathology and to pinpoint potential targets for CD171-targeting therapy. A newly developed anti-CD171 rabbit monoclonal antibody, clone 014, was selected from the panel of commercially available CD171 antibodies. Immunohistochemistry was performed using Leica Bond Max automation and multitumor blocks containing up to 60 tumor samples. CD171 was constitutively and strongly expressed in neuroectodermal tumors such as schwannoma, neuroblastoma, and paraganglioma, whereas other mesenchymal tumors including schwannoma mimics showed only rarely CD171 positivity. Frequent CD171-expression was also detected in ovarian serous carcinoma, malignant mesothelioma, and testicular embryonal carcinoma. CD171 immunohistochemistry may have some role in immunophenotypic differential diagnosis of neurogenic tumors and pinpointing potential candidates for anti-CD171 therapy. Though, because of its rare expression and lack of predictive value, CD171 is neither a diagnostic nor prognostic marker for gastrointestinal stromal tumors.

Clinical impact of L1CAM expression measured on the transcriptome level in ovarian cancer

Background

High expression of L1 cell adhesion molecules (L1CAM) has been repeatedly shown to be associated with aggressive disease behavior, which translates in poor clinical outcome in various cancer entities. However, in ovarian cancer results based either on immunohistochemistry or cytosolic protein quantifications remained conflicting regarding clinical behavior. In the present work we assessed L1CAM expression on the transcriptome level with the highly sensitive quantitative real-time PCR (qRT-PCR) to define its relevance in ovarian cancer biology.

Results

There was a significant difference in L1CAM high and low mRNA expressing cancers with regard to disease-free (p=0.002) and overall survival (p=0.008). L1CAM proofed to be an independent predictor for disease progression (HR 1.8, p=0.01) and overall survival (HR 1.6, p=0.04). Furthermore, a significant positive correlation between the level of L1CAM and the grade of tumor differentiation (p=0.04), the FIGO stage (p=0.025) as well as the histological subtype (p= 0.002) was found.

Methods

This study included fresh frozen tissue samples of 138 patients with FIGO I-IV stage ovarian cancer. L1CAM mRNA expression was determined using qRT-PCR. In the calculations special attention was put on the various histological subtypes. In survival analysis median L1CAM mRNA expression obtained in the entire cohort of ovarian cancer samples was used as a cut-off to distinguish between high and low L1CAM mRNA expression.

Conclusion

L1CAM mRNA expression appears to play a substantial role in the pathophysiology of ovarian cancer that is translated into poor clinical outcome. Additionally humanized L1CAM antibodies, which can serve as potential future treatment options are under testing.

Expression of L1CAM in curettage or high L1CAM level in preoperative blood samples predicts lymph node metastases and poor outcome in endometrial cancer patients

BACKGROUND

Several studies have identified L1 cell adhesion molecule (L1CAM) as a strong prognostic marker in endometrial cancer. To further underline the clinical usefulness of this biomarker, we investigated L1CAM as a predictive marker for lymph node metastases and its prognostic impact in curettage specimens and preoperative plasma samples. In addition, we aimed to validate the prognostic value of L1CAM in hysterectomy specimen.

METHODS

Immunohistochemical staining of L1CAM was performed for 795 hysterectomy and 1134 curettage specimen from endometrial cancer patients. The L1CAM level in preoperative blood samples from 372 patients was determined using ELISA.

RESULTS

Expression of L1CAM in curettage specimen was significantly correlated to L1CAM level in corresponding hysterectomy specimen (P<0.001). Both in curettage and preoperative plasma samples L1CAM upregulation was significantly associated with features of aggressive disease and poor outcome (P<0.001). The L1CAM was an independent predictor of lymph node metastases, after correction for curettage histology, both in curettage specimen (P=0.002) and plasma samples (P=0.048). In the hysterectomy samples L1CAM was significantly associated with poor outcome (P<0.001).

CONCLUSIONS

We demonstrate that preoperative evaluation of L1CAM levels, both in curettage or plasma samples, predicts lymph node metastases and adds valuable information on patient prognosis.

A point mutation in the extracellular domain of L1 blocks its capacity to confer metastasis in colon cancer cells via CD10

The neural L1 transmembrane cell adhesion receptor of the immunoglobulin-like family is a target gene of Wnt-β-catenin signaling in human colorectal cancer (CRC) cells and is expressed at the invasive edge of the tumor tissue. L1 overexpression in cultured CRC cells confers enhanced proliferation, motility and liver metastasis. We have analyzed the mechanisms of L1-mediated signaling in CRC cells by using various point mutations in the L1 ectodomain that are known to cause severe genetically inherited mental retardation disorders in patients. We found that all such L1 ectodomain mutations abolish the ability of L1 to confer metastatic properties in CRC cells. Using gene array analysis, we identified L1-mutation-specific gene expression signatures for the L1/H210Q and L1/D598N mutations. We identified CD10, a metalloprotease (neprilysin, neutral endopeptidase) and a gene that is specifically induced in CRC cells by L1 in an L1/H210Q mutation-specific manner. CD10 expression was required for the L1-mediated induction of cell proliferation, motility and metastasis, as suppression of CD10 levels in L1-expressing CRC cells abolished the L1 effects on CRC progression. The signaling from L1 to CD10 was mediated through the L1-ezrin-NF-κB pathway. In human CRC tissue L1 and CD10 were localized in partially overlapping regions in the more invasive areas of the tumor tissue. The results suggest that CD10 is a necessary component conferring the L1 effects in CRC cells. The identification of gene expression patterns of L1-domain-specific point mutations may provide novel markers and targets for interfering with L1-mediated CRC progression.

Cell Adhesion Molecules and Ubiquitination-Functions and Significance

Cell adhesion molecules of the immunoglobulin (Ig) superfamily represent the biggest group of cell adhesion molecules. They have been analyzed since approximately 40 years ago and most of them have been shown to play a role in tumor progression and in the nervous system. All members of the Ig superfamily are intensively posttranslationally modified. However, many aspects of their cellular functions are not yet known. Since a few years ago it is known that some of the Ig superfamily members are modified by ubiquitin. Ubiquitination has classically been described as a proteasomal degradation signal but during the last years it became obvious that it can regulate many other processes including internalization of cell surface molecules and lysosomal sorting. The purpose of this review is to summarize the current knowledge about the ubiquitination of cell adhesion molecules of the Ig superfamily and to discuss its potential physiological roles in tumorigenesis and in the nervous system.

MMP16 Mediates a Proteolytic Switch to Promote Cell-Cell Adhesion, Collagen Alignment, and Lymphatic Invasion in Melanoma

Lymphatic invasion and accumulation of continuous collagen bundles around tumor cells are associated with poor melanoma prognosis, but the underlying mechanisms and molecular determinants have remained unclear. We show here that a copy-number gain or overexpression of the membrane-type matrix metalloproteinase MMP16 (MT3-MMP) is associated with poor clinical outcome, collagen bundle assembly around tumor cell nests, and lymphatic invasion. In cultured WM852 melanoma cells derived from human melanoma metastasis, silencing of MMP16 resulted in cell-surface accumulation of the MMP16 substrate MMP14 (MT1-MMP) as well as L1CAM cell adhesion molecule, identified here as a novel MMP16 substrate. When limiting the activities of these trans-membrane protein substrates toward pericellular collagen degradation, cell junction disassembly, and blood endothelial transmigration, MMP16 supported nodular-type growth of adhesive collagen-surrounded melanoma cell nests, coincidentally steering cell collectives into lymphatic vessels. These results uncover a novel mechanism in melanoma pathogenesis, whereby restricted collagen infiltration and limited mesenchymal invasion are unexpectedly associated with the properties of the most aggressive tumors, revealing MMP16 as a putative indicator of adverse melanoma prognosis.

L1CAM: a major driver for tumor cell invasion and motility

The L1 cell adhesion molecule (L1CAM) plays a major role in the development of the nervous system and in the malignancy of human tumors. In terms of biological function, L1CAM comes along in two different flavors: (1) a static function as a cell adhesion molecule that acts as a glue between cells; (2) a motility promoting function that drives cell migration during neural development and supports metastasis of human cancers. Important factors that contribute to the switch in the functional mode of L1CAM are: (1) the cleavage from the cell surface by membrane proximal proteolysis and (2) the ability to change binding partners and engage in L1CAM-integrin binding. Recent studies have shown that the cleavage of L1CAM by metalloproteinases and the binding of L1CAM to integrins via its RGD-motif in the sixth Ig-domain activate signaling pathways distinct from the ones elicited by homophilic binding. Here we highlight important features of L1CAM proteolysis and the signaling of L1CAM via integrin engagement. The novel insights into L1CAM downstream signaling and its regulation during tumor progression and epithelial-mesenchymal transition (EMT) will lead to a better understanding of the dualistic role of L1CAM as a cell adhesion and/or motility promoting cell surface molecule.

The neural cell adhesion molecules L1 and CHL1 are cleaved by BACE1 protease in vivo

The β-site amyloid precursor protein-cleaving enzyme BACE1 is a prime drug target for Alzheimer disease. However, the function and the physiological substrates of BACE1 remain largely unknown. In this work, we took a quantitative proteomic approach to analyze the secretome of primary neurons after acute BACE1 inhibition, and we identified several novel substrate candidates for BACE1. Many of these molecules are involved in neuronal network formation in the developing nervous system. We selected the adhesion molecules L1 and CHL1, which are crucial for axonal guidance and maintenance of neural circuits, for further validation as BACE1 substrates. Using both genetic BACE1 knock-out and acute pharmacological BACE1 inhibition in mice and cell cultures, we show that L1 and CHL1 are cleaved by BACE1 under physiological conditions. The BACE1 cleavage sites at the membrane-proximal regions of L1 (between Tyr(1086) and Glu(1087)) and CHL1 (between Gln(1061) and Asp(1062)) were determined by mass spectrometry. This work provides molecular insights into the function and the pathways in which BACE1 is involved, and it will help to predict or interpret possible side effects of BACE1 inhibitor drugs in current clinical trials.

Prognostic significance of L1CAM in ovarian cancer and its role in constitutive NF-κB activation

BACKGROUND

Overexpression of L1-cell adhesion molecule (L1CAM) has been observed for various carcinomas and correlates with poor prognosis and late-stage disease. In vitro, L1CAM enhances proliferation, cell migration, adhesion and chemoresistance. We tested L1CAM and interleukin-1 beta (IL-1β) expression in tumor samples and ascitic fluid from ovarian carcinoma patients to examine its role as a prognostic marker.

PATIENTS AND METHODS

We investigated tumor samples and ascitic fluid from 232 serous ovarian carcinoma patients for L1CAM by enzyme-linked immunosorbent assay. L1CAM expression was correlated with pathoclinical parameters and patients' outcome. IL-1β levels were measured in tumor cell lysates. Ovarian cancer cell lines were analyzed for the contribution of L1CAM to IL-1β production and nuclear factor 'kappa-light-chain-enhancer' of activated B-cells (NF-κB) activation.

RESULTS

We observed that L1CAM-expressing tumors show a highly invasive phenotype associated with restricted tumor resectability at primary debulking surgery and increased lymphogenic spread. Soluble L1CAM proved to be a marker for poor progression-free survival and chemoresistance. In ovarian carcinoma cell lines, the specific knock-down of L1CAM reduces IL-1β expression and NF-κB activity.

CONCLUSIONS

L1CAM expression contributes to the invasive and metastatic phenotype of serous ovarian carcinoma. L1CAM expression and shedding in the tumor microenvironment could contribute to enhanced invasion and tumor progression through increased IL-1β production and NF-κB activation.

Loss of EpCAM expression in breast cancer derived serum exosomes: role of proteolytic cleavage

OBJECTIVE

Cancer cells in the body release soluble and membranous factors that manipulate the tumor environment to facilitate growth and survival. Recent years have provided evidence that small microvesicles that are termed exosomes may play a pivotal role in this process. Exosomes are membrane vesicles with a size of 40-100 nm that are released by both tumor and normal cells and can be found in various body fluids. Tumor-derived exosomes carry functional proteins, mRNAs, and miRNAs and could serve as novel platform for tumor diagnosis and prognosis. However, marker proteins that allow enrichment of tumor-derived exosomes over normal exosomes are less well defined.

METHODS

We used Western blot analysis and antibody coupled magnetic beads to characterize CD24 and EpCAM as markers for exosomes. We investigated ovarian carcinoma ascites, pleural effusions and serum of breast carcinoma patients. As non-tumor derived control we used exosomes from ascites of liver cirrhosis patients.

RESULTS

Exosomes could be isolated from all body fluids and contained marker proteins as well as miRNAs. We observed that CD24 and EpCAM were selectively present on ascites exosomes of tumor patients and copurified together on anti-EpCAM or anti-CD24 magnetic beads. In breast cancer patients CD24 was present but EpCAM was absent from serum exosomes. Instead, the intact EpCAM ectodomain was recovered in a soluble form. We provide evidence that EpCAM can be cleaved from exosomes via serum metalloproteinase(s).

CONCLUSION

Loss of EpCAM on serum exosomes may hamper enrichment by immune-affinity isolation. We suggest that CD24 could be an additional marker for the enrichment of tumor-derived exosomes from blood.

Molecular and cellular mechanisms of ectodomain shedding

The extracellular domain of several membrane-anchored proteins is released from the cell surface as soluble proteins through a regulated proteolytic mechanism called ectodomain shedding. Cells use ectodomain shedding to actively regulate the expression and function of surface molecules, and modulate a wide variety of cellular and physiological processes. Ectodomain shedding rapidly converts membrane-associated proteins into soluble effectors and, at the same time, rapidly reduces the level of cell surface expression. For some proteins, ectodomain shedding is also a prerequisite for intramembrane proteolysis, which liberates the cytoplasmic domain of the affected molecule and associated signaling factors to regulate transcription. Ectodomain shedding is a process that is highly regulated by specific agonists, antagonists, and intracellular signaling pathways. Moreover, only about 2% of cell surface proteins are released from the surface by ectodomain shedding, indicating that cells selectively shed their protein ectodomains. This review will describe the molecular and cellular mechanisms of ectodomain shedding, and discuss its major functions in lung development and disease.

Modification of the L1-CAM carboxy-terminus in pancreatic adenocarcinoma cells

The neural cell adhesion molecule L1 has recently been shown to be expressed in pancreatic adenocarcinoma (PDAC) cells. In this report, we demonstrate that L1 is expressed by moderately- to poorly-differentiated PDAC cells in situ, and that L1 expression is a predictor of poor patient survival. In vitro, reduced reactivity of an anti-L1 carboxy-terminus-specific antibody was observed in the more poorly differentiated fast-growing (FG) variant of the COLO357 population, versus its well-differentiated slow-growing (SG) counterpart, even though they express equivalent total L1. The carboxy-terminus of L1 mediates binding to the MAP kinase-regulating protein RanBPM and mutation of T1247/S1248 within this region attenuates the expression of malignancy associated proteins and L1-induced tumorigenicity in mice. Therefore, we reasoned that the differential epitope exposure observed might be indicative of modifications responsible for regulating these events. However, epitope mapping demonstrated that the major determinant of binding was actually N1251; mutation of T1247 and S1248, alone or together, had little effect on C20 binding. Moreover, cluster assays using CD25 ectodomain/L1 cytoplasmic domain chimeras demonstrated the N1251-dependent, RanBPM-independent stimulation of erk phosphorylation in these cells. Reactivity of this antibody also reflects the differential exposure of extracellular epitopes in these COLO357 sublines, consistent with the previous demonstration of L1 ectodomain conformation modulation by intracellular modifications. These data further support a central role for L1 in PDAC, and define a specific role for carboxy-terminal residues including N1251 in the regulation of L1 activity in PDAC cells.

L1CAM malfunction in the nervous system and human carcinomas

Research over the last 25 years on the cell adhesion molecule L1 has revealed its pivotal role in nervous system function. Mutations of the human L1CAM gene have been shown to cause neurodevelopmental disorders such as X-linked hydrocephalus, spastic paraplegia and mental retardation. Impaired L1 function has been also implicated in the aetiology of fetal alcohol spectrum disorders, defective enteric nervous system development and malformations of the renal system. Importantly, aberrant expression of L1 has emerged as a critical factor in the development of human carcinomas, where it enhances cell proliferation, motility and chemoresistance. This discovery promoted collaborative work between tumour biologists and neurobiologists, which has led to a substantial expansion of the basic knowledge about L1 function and regulation. Here we provide an overview of the pathological conditions caused by L1 malfunction. We further discuss how the available data on gene regulation, molecular interactions and posttranslational processing of L1 may contribute to a better understanding of associated neurological and cancerous diseases.

Inside-out regulation of L1 conformation, integrin binding, proteolysis, and concomitant cell migration

The ectodomain structure and function of the neural cell adhesion molecule L1 is shown to be regulated by the intracellular phosphorylation of a novel threonine, T1172. In pancreatic cancer cells, T1172 exhibits steady-state saturated phosphorylation, an event regulated by CKII and PKC, and which further regulates cell migration.

Previous reports on the expression of the cell adhesion molecule L1 in pancreatic ductal adenocarcinoma (PDAC) cells range from absent to high. Our data demonstrate that L1 is expressed in poorly differentiated PDAC cells in situ and that threonine-1172 (T1172) in the L1 cytoplasmic domain exhibits steady-state saturated phosphorylation in PDAC cells in vitro and in situ. In vitro studies support roles for casein kinase II and PKC in this modification, consistent with our prior studies using recombinant proteins. Importantly, T1172 phosphorylation drives, or is associated with, a change in the extracellular structure of L1, consistent with a potential role in regulating the shift between the closed conformation and the open, multimerized conformation of L1. We further demonstrate that these distinct conformations exhibit differential binding to integrins αvβ3 and αvβ5 and that T1172 regulates cell migration in a matrix-specific manner and is required for a disintegrin and metalloproteinase-mediated shedding of the L1 ectodomain that has been shown to regulate cell migration. These data define a specific role for T1172 of L1 in regulating aspects of pancreatic adenocarcinoma cell phenotype and suggest the need for further studies to elucidate the specific ramifications of L1 expression and T1172 phosphorylation in the pathobiology of pancreatic cancer.

Soluble adhesion molecules in human cancers: sources and fates

Adhesion molecules endow tumor cells with the necessary cell-cell contacts and cell-matrix interactions. As such, adhesion molecules are involved in cell signalling, proliferation and tumor growth. Rearrangements in the adhesion repertoire allow tumor cells to migrate, invade and form metastases. Besides these membrane-bound adhesion molecules several soluble adhesion molecules are detected in the supernatant of tumor cell lines and patient body fluids. Truncated soluble adhesion molecules can be generated by several conventional mechanisms, including alternative splicing of mRNA transcripts, chromosomal translocation, and extracellular proteolytic ectodomain shedding. Secretion of vesicles (ectosomes and exosomes) is an alternative mechanism mediating the release of full-length adhesion molecules. Soluble adhesion molecules function as modulators of cell adhesion, induce proteolytic activity and facilitate cell signalling. Additionally, adhesion molecules present on secreted vesicles might be involved in the vesicle-target cell interaction. Based on currently available data, released soluble adhesion molecules contribute to cancer progression and therefore should not be regarded as unrelated and non-functional side products of tumor progression.

Tyrosine and serine phosphorylation regulate the conformation and subsequent threonine phosphorylation of the L1 cytoplasmic domain

Previously we identified threonine-1172 (T1172) in the cytoplasmic domain of the cell adhesion molecule L1 as phosphorylated in pancreatic cancer cells. Although both CKII- and PKC-blockade suppressed this modification, only CKII was capable of phosphorylating T1172 of a recombinant L1 cytoplasmic domain, suggesting the requirement for additional events to facilitate availability of T1172 to PKC. In this study, we demonstrate that the region around T1172 exists in distinct conformations based on both T1172 phosphorylation and the integrity of surrounding residues. We further demonstrate the role of membrane-proximal and membrane-distal residues in regulating cytoplasmic domain conformation, and that modification of 3 of the 4 tyrosines in the L1 cytoplasmic domain promote conformational changes that facilitate other events. In particular, phenylalanine-substitution of tyrosine-1151 or tyrosine-1229 promote opening up of the cytoplasmic domain in a manner that facilitates phosphorylation of the other 3 tyrosines, as well as phosphorylation of T1172 by PKCalpha. Importantly, we show that phosphorylation of serine-1181 is required for T1172 phosphorylation by CKII. These data define a specific role for secondary structure in regulating the availability of T1172 that facilitates phosphorylation by PKC.

The soluble form of the cancer-associated L1 cell adhesion molecule is a pro-angiogenic factor

A soluble form of the L1 cell adhesion molecule (sL1) is released from various tumor cells and can be found in serum and ascites fluid of uterine and ovarian carcinoma patients. sL1 is a ligand for several Arg-Gly-Asp (RGD)-binding integrins and can be deposited in the extracellular matrix. In this study we describe a novel function of this physiologically relevant form of L1 as a pro-angiogenic factor. We demonstrated that the anti-L1 monoclonal antibody (mAb) chCE7 binds near or to the sixth Ig-like domain of human L1 which contains a single RGD sequence. mAb chCE7 inhibited the RGD-dependent adhesion of ovarian carcinoma cells to sL1 and reversed the sL1-induced proliferation, matrigel invasion and tube formation of bovine aortic endothelial (BAE) cells. A combination of sL1 with vascular endothelial growth factor-A (VEGF-A(165)), which is an important angiogenic inducer in tumors, strongly potentiated VEGF receptor-2 tyrosine phosphorylation in BAE cells. Chick chorioallantoic membrane (CAM) assays revealed the pro-angiogenic potency of sL1 in vivo which could be abolished by chCE7. These results indicate an important role of released L1 in tumor angiogenesis and represent a novel function of antibody chCE7 in tumor therapy.

The Tumor Microenvironment: Key to Early Detection

The tumor microenvironment plays an important role equal to the tumor cell population in the progression of cancer. Consisting of stromal fibroblasts, inflammatory cells, components of the vasculature, normal epithelia, and extracellular matrix, the surrounding environment interacts or "cross-talks" with tumor cells through the release of growth factors, cytokines, proteases, and other bioactive molecules. Tumor growth, formation of new vascular networks, evasion of the host immune system, and invasion and metastasis are processes that co-evolve and become finely optimized and regulated within the tumor microenvironment. However, relatively recent reports on three areas of study have come together to add new levels of complexity to the tumor microenvironment. These include ectodomain shedding of proteins, shedding of membrane-derived vesicles, and novel roles for phospholipids. These dynamic changes that take place in the tumor microenvironment provide new avenues for study and for the early detection of cancer, whereas proteomic technologies provide the means to detect these unique proteins and lipids. Here we review the evolving concepts of the tumor microenvironment that, together with advances in proteomic technologies, hold the promise to facilitate the detection of early-stage cancer.

The biochemical, biological, and pathological kaleidoscope of cell surface substrates processed by matrix metalloproteinases

Matrix metalloproteinases (MMPs) constitute a family of more than 20 endopeptidases. Identification of specific matrix and non-matrix components as MMP substrates showed that, aside from their initial role as extracellular matrix modifiers, MMPs play significant roles in highly complex processes such as the regulation of cell behavior, cell-cell communication, and tumor progression. Thanks to the comprehensive examination of the expanded MMP action radius, the initial view of proteases acting in the soluble phase has evolved into a kaleidoscope of proteolytic reactions connected to the cell surface. Important classes of cell surface molecules include adhesion molecules, mediators of apoptosis, receptors, chemokines, cytokines, growth factors, proteases, intercellular junction proteins, and structural molecules. Proteolysis of cell surface proteins by MMPs may have extremely diverse biological implications, ranging from maturation and activation, to inactivation or degradation of substrates. In this way, modification of membrane-associated proteins by MMPs is crucial for communication between cells and the extracellular milieu, and determines cell fate and the integrity of tissues. Hence, insights into the processing of cell surface proteins by MMPs and the concomitant effects on physiological processes as well as on disease onset and evolution, leads the way to innovative therapeutic approaches for cancer, as well as degenerative and inflammatory diseases.

Breaking up the tie: Disintegrin-like metalloproteinases as regulators of cell migration in inflammation and invasion

Cell adhesion and cell migration are essential for a variety of important events in both embryonic development and in the adult organism. Cell adhesion molecules (CAM) like selectins, immunoglobulin superfamily members, integrins, and cadherins undergo diverse mechanisms of regulation. Dysregulation of adhesion can lead to pathological processes, including inflammatory diseases or tumor metastasis either by disrupting the normal anchorage, thereby altering cell movement and regulatory signalling, or by promoting inappropriate temporal and spatial adhesion. An increasing body of evidence has emerged showing that members of the a disintegrin and metalloproteinase (ADAM) family critically contribute to the regulation of CAM functions. While the disintegrin domain can interact with integrins and mediate adhesion, the metalloproteinase domain can mediate anti-adhesive functions by cleaving the membrane bound adhesion molecules. This "shedding" process leads to the release of often still functional soluble ectodomains and can additionally influence intracellular cell signalling pathways. Several soluble CAMs have been detected in vitro and in vivo. Some of them are strongly increased in inflammatory diseases or in the serum of cancer patients. Therefore the level of soluble CAMs but also the expression of the metalloproteinases responsible for their release might provide prognostic information. It could also be useful for monitoring malignant disease stages and for evaluating the effectiveness of various therapeutic approaches. Moreover, metalloproteases of the ADAM family are emerging as promising targets for new therapeutic options.

Multiple Acquired Renal Carcinoma Tumor Capabilities Abolished upon Silencing of ADAM17

Malignancy is a manifestation of acquired defects in regulatory circuits that direct normal cell proliferation and homeostasis. Most of these circuits operate through cell autonomous pathways, whereas others potentially involve the neighboring microenvironment. We report that the metalloprotease ADAM17 plays a pivotal role in several acquired tumor cell capabilities by mediating the availability of soluble transforming growth factor-α, an epidermal growth factor receptor (EGFR) ligand, and thus the establishment of a key autocrine signaling pathway. Silencing of ADAM17 in human renal carcinoma cell lines corrects critical features associated with cancer cells, including growth autonomy, tumor inflammation, and tissue invasion. Highly malignant renal carcinoma cancer cells fail to form in vivo tumors in the absence of ADAM17, confirming the essential function of this molecule in tumorigenesis. These data show that ligand shedding is a crucial step in endogenous EGFR activation and endorse prospective therapeutic strategies targeting ADAM17 in human cancer. (Cancer Res 2006; 66(16): 8083-90)

(Make) stick and cut loose--disintegrin metalloproteases in development and disease

"A disintegrin and metalloprotease" (ADAM) proteases form a still growing family of about 40 type 1 transmembrane proteins. They are defined by a common modular ectodomain architecture that combines cell deadhesion/adhesion and fusion motifs (disintegrin and cysteine-rich domains), with a Zn-protease domain capped by a large prodomain. Their ectodomain thus strikingly resembles snake venom disintegrin proteases, which by combined integrin blocking and extracellular proteolysis, can cause extensive tissue damage after snake bites. A surprisingly large proportion (13 ADAMs) is exclusively expressed in the male gonads, and only a minority can be found throughout all tissues. As predicted by their amino acid sequence, a major proportion of this family has not maintained a functional protease domain, most probably rendering them into pure adhesion and/or fusion proteins. For most ADAMs, the respective key function has remained elusive. Despite their overall conserved ectodomain structure, ADAMs appear to be subdivided into those with a predominant role in direct adhesion (e.g., ADAMs 1, 2, and 3) and those mainly acting as proteases (e.g., ADAMs 10 and 17). Only for a few of them are functions of more than one domain documented (e.g., ADAM9 in cell fusion and proteolysis). Several ADAMs exist in both membrane-resident and secreted isoforms; the functional significance of this dichotomy is in most cases still unclear. Knockout phenotypes have been informative only in a few cases (ADAMs 1, 2, 10, 12, 15, 17, and 19) and are mainly related to their protease function. A common denominator of ADAM-mediated proteolysis is the ectodomain shedding of a broad spectrum of substrates, including paracrine growth factors like epidermal growth factor receptor (EGFR) ligands, cell adhesion molecules like CD44 or cadherins, and the initiation of regulated intramembrane proteolysis (RIP), whereby the transmembrane fragment of the respective substrate is further cleaved by an intramembrane cleaving protease to release an intracellular domain acting as a nuclear transcription regulator. Most ADAMs feature a significant overlap of substrate specificities, explaining why an inactivation of individual ADAMs only rarely causes major phenotypes.

The cell adhesion molecule l1 is required for chain migration of neural crest cells in the developing mouse gut

BACKGROUND & AIMS

During development, the enteric nervous system is derived from neural crest cells that emigrate from the hindbrain, enter the foregut, and colonize the gut. Defects in neural crest migration can result in intestinal aganglionosis. Hirschsprung's disease (congenital aganglionosis) is a human condition in which enteric neurons are absent from the distal bowel. A number of clinical studies have implicated the cell adhesion molecule L1 in Hirschsprung's disease. We examined the role of L1 in the migration of neural crest cells through the developing mouse gut.

METHODS

A variety of in vitro and in vivo assays were used to examine: (1) the effect of L1 blocking antibodies or exogenous soluble L1 protein known to compromise L1 function on the rate of crest cell migration, (2) the effect of blocking L1 activity on the dynamic behavior of crest cells using time-lapse microscopy, and (3) whether the colonization of the gut by crest cells in L1-deficient mice differs from control mice.

RESULTS

We show that L1 is expressed by neural crest cells as they colonize the gut. Perturbation studies show that disrupting L1 activity retards neural crest migration and increases the number of solitary neural crest cells. L1-deficient mice show a small but significant reduction in neural crest cell migration at early developmental stages, but the entire gastrointestinal tract is colonized.

CONCLUSIONS

L1 is important for the migration of neural crest cells through the developing gut and is likely to be involved in the etiology of Hirschsprung's disease.

The shed ectodomain of Nr-CAM stimulates cell proliferation and motility, and confers cell transformation

Nr-CAM, a cell-cell adhesion molecule of the immunoglobulin-like cell adhesion molecule family, known for its function in neuronal outgrowth and guidance, was recently identified as a target gene of beta-catenin signaling in human melanoma and colon carcinoma cells and tissue. Retrovirally mediated transduction of Nr-CAM into fibroblasts induces cell motility and tumorigenesis. We investigated the mechanisms by which Nr-CAM can confer properties related to tumor cell behavior and found that Nr-CAM expression in NIH3T3 cells protects cells from apoptosis in the absence of serum by constitutively activating the extracellular signal-regulated kinase and AKT signaling pathways. We detected a metalloprotease-mediated shedding of Nr-CAM into the culture medium of cells transfected with Nr-CAM, and of endogenous Nr-CAM in B16 melanoma cells. Conditioned medium and purified Nr-CAM-Fc fusion protein both enhanced cell motility, proliferation, and extracellular signal-regulated kinase and AKT activation. Moreover, Nr-CAM was found in complex with alpha4beta1 integrins in melanoma cells, indicating that it can mediate, in addition to homophilic cell-cell adhesion, heterophilic adhesion with extracellular matrix receptors. Suppression of Nr-CAM levels by small interfering RNA in B16 melanoma inhibited the adhesive and tumorigenic capacities of these cells. Stable expression of the Nr-CAM ectodomain in NIH3T3 cells conferred cell transformation and tumorigenesis in mice, suggesting that the metalloprotease-mediated shedding of Nr-CAM is a principal route for promoting oncogenesis by Nr-CAM.

Metalloprotease-induced ectodomain shedding of neural cell adhesion molecule (NCAM)

Transmembrane forms of neural cell adhesion molecule (NCAM140, NCAM180(1)) are key regulators of neuronal development. The extracellular domain of NCAM can occur as a soluble protein in normal brain, and its levels are elevated in neuropsychiatric disorders, such as schizophrenia; however the mechanism of ectodomain release is obscure. Ectodomain shedding of NCAM140, releasing a fragment of 115 kD, was found to be induced in NCAM-transfected L-fibroblasts by the tyrosine phosphatase inhibitor pervanadate, but not phorbol esters. Pervanadate-induced shedding was mediated by a disintegrin metalloprotease (ADAM), regulated by ERK1/2 MAP kinase. In primary cortical neurons, NCAM was shed at high levels, and the metalloprotease inhibitor GM6001 significantly increased NCAM-dependent neurite branching and outgrowth. Moreover, NCAM-dependent neurite outgrowth and branching were inhibited in neurons isolated from a transgenic mouse model of NCAM shedding. These results suggest that regulated metalloprotease-induced ectodomain shedding of NCAM down-regulates neurite branching and neurite outgrowth. Thus, increased levels of soluble NCAM in schizophrenic brain have the potential to impair neuronal connectivity.

L1 is sequentially processed by two differently activated metalloproteases and presenilin/gamma-secretase and regulates neural cell adhesion, cell migration, and neurite outgrowth

The immunoglobulin superfamily recognition molecule L1 plays important functional roles in the developing and adult nervous system. Metalloprotease-mediated cleavage of this adhesion molecule has been shown to stimulate cellular migration and neurite outgrowth. We demonstrate here that L1 cleavage is mediated by two distinct members of the disintegrin and metalloprotease family, ADAM10 and ADAM17. This cleavage is differently regulated and leads to the generation of a membrane bound C-terminal fragment, which is further processed through gamma-secretase activity. Pharmacological approaches with two hydroxamate-based inhibitors with different preferences in blocking ADAM10 and ADAM17, as well as loss of function and gain of function studies in murine embryonic fibroblasts, showed that constitutive shedding of L1 is mediated by ADAM10 while phorbol ester stimulation or cholesterol depletion led to ADAM17-mediated L1 cleavage. In contrast, N-methyl-d-aspartate treatment of primary neurons stimulated ADAM10-mediated L1 shedding. Both proteases were able to affect L1-mediated adhesion and haptotactic migration of neuronal cells. In particular, both proteases were involved in L1-dependent neurite outgrowth of cerebellar neurons. Thus, our data identify ADAM10 and ADAM17 as differentially regulated L1 membrane sheddases, both critically affecting the physiological functions of this adhesion protein.

Heterophilic interactions between cell adhesion molecule L1 and alphavbeta3-integrin induce HUVEC process extension in vitro and angiogenesis in vivo

Cell adhesion molecule L1 was implicated in angiogenic processes, tumor formation and metastasis. Here, we provide evidence that the sixth Ig-like domain of L1 (L1Ig6) interacts with alpha(v)beta3 to induce process extension of human umbilical vein endothelial cells (HUVECs) in vitro and angiogenesis in vivo. HUVECs formed network-like structures on full-length L1 or L1Ig6 substrates comparable to structures found on matrigel. In the presence of mab alpha(v)beta3 or cyclic RGD, apoptosis was induced. In fibrin matrices where L1Ig6 was covalently incorporated, HUVECs formed multicellular and hollow processes through interactions between cell-surface alpha(v)beta3 and RGD-sites of matrix-immobilized L1Ig6. No such processes were induced by L1Ig6 having non-functional RDG-sites, or in the presence of mab alpha(v)beta3 or cyclic RGD. In those matrices, increased apoptosis was found. Co-immunoprecipitation of L1 or L1Ig6 with alpha(v)beta3 suggests close interactions. Furthermore, L1Ig6 stimulated HUVECs showed increased tyrosine phosphorylation of alpha(v)beta3 and phosphorylation of MAP kinases (ERK1 and ERK2) but not AKT indicating specific activation of alpha(v) and alpha(v)beta3 followed by activation of downstream kinases. Application of L1Ig6-modified fibrin matrices on CAMs induced 50-60% increased alpha(v) and alpha(v)beta3 protein expression and in vivo angiogenesis indicated by approximately 50% increased mean vascular length density. The results demonstrate angiogenic potential of L1Ig6 involving ligation and activation of alpha(v)beta3.

Cleavage of L1 in exosomes and apoptotic membrane vesicles released from ovarian carcinoma cells

PURPOSE

The L1 adhesion molecule (CD171) is overexpressed in human ovarian and endometrial carcinomas and is associated with bad prognosis. Although expressed as a transmembrane molecule, L1 is released from carcinoma cells in a soluble form. Soluble L1 is present in serum and ascites of ovarian carcinoma patients. We investigated the mode of L1 cleavage and the function of soluble L1.

EXPERIMENTAL DESIGN

We used ovarian carcinoma cell lines and ascites from ovarian carcinoma patients to analyze soluble L1 and L1 cleavage by Western blot analysis and ELISA.

RESULTS

We find that in ovarian carcinoma cells the constitutive cleavage of L1 proceeds in secretory vesicles. We show that apoptotic stimuli like C2-ceramide, staurosporine, UV irradiation, and hypoxic conditions enhance L1-vesicle release resulting in elevated levels of soluble L1. Constitutive cleavage of L1 is mediated by a disintegrin and metalloproteinase 10, but under apoptotic conditions multiple metalloproteinases are involved. L1 cleavage occurs in two types of vesicles with distinct density features: constitutively released vesicles with similarity to exosomes and apoptotic vesicles. Both types of L1-containing vesicles are present in the ascites fluids of ovarian carcinoma patients. Soluble L1 from ascites is a potent inducer of cell migration and can trigger extracellular signal-regulated kinase phosphorylation.

CONCLUSIONS

We suggest that tumor-derived vesicles may be an important source for soluble L1 that could regulate tumor cell function in an autocrine/paracrine fashion.

The L1 Cell Adhesion Molecule Is Induced in Renal Cancer Cells and Correlates with Metastasis in Clear Cell Carcinomas

Purpose: The L1 cell adhesion molecule is overexpressed in many human carcinomas. The objectives of the study were to provide a comprehensive description of L1 distribution in human kidney and to establish the prognostic relevance of L1 expression in renal cell carcinomas (RCC).

Experimental Design: Using two antibodies to the extracellular part and the cytoplasmic domain, respectively, we first compared L1 expression in normal kidney and renal tumors of diverse histopathologic origin, then we studied L1 expression together with tumor stage, grade, molecular prognostic biomarkers, and metastatic behavior.

Results: In normal kidney, L1 immunoreactive with both antibodies was expressed in all epithelial cells originating from the ureteric bud except for intercalated cells. In renal tumors, L1 was mainly detected in those originating from cells that do not express L1 in the normal kidney [i.e., 33 of 72 clear cell RCC (ccRCC) and 25 of 88 papillary RCC (papRCC)]. Both in ccRCC and papRCC, L1 reacted only with the antibody to the extracellular domain, suggesting that the protein was truncated. In these carcinomas, L1 expression was strongly correlated with Ki-67 proliferation index (ccRCC, P = 0.0059; papRCC, P = 0.0039), but only in ccRCC, the presence of L1 was associated with the risk of metastasis (P = 0.0121). This risk was higher if cyclin D1 was concurrently absent in tumor cells (P &lt; 0.0001). The L1+/cyclin D1− profile was an independent prognostic factor of metastasis occurrence in multivariate analysis (P = 0.0023).

Conclusion: We have found a combination of markers that can serve to identify a subgroup of high-risk patients with ccRCC that may require more aggressive therapies.

Integrin clipping: a novel adhesion switch?

During human prostate cancer progression, the majority of normally expressed integrins are suppressed with the exception of the alpha6, alpha3, and beta1 integrins. We have shown that in prostate cancer, the alpha6 integrin is found paired with the beta1 integrin and that a novel form of the alpha6 integrin that lacks a large portion of the extracellular domain (alpha6p) exists. The alpha6pbeta1 integrin is found in human prostate cancer tissue specimens as well as tissue culture cell lines and is formed on the cell surface. This review discusses the mechanism of alpha6pbeta1 production and the potential functions of this integrin variant. Our current working model predicts that the alpha6pbeta1 integrin maintains the intracellular cytoskeletal connections associated with the heterodimer while allowing for an alteration in cell adhesion. The mechanism provides a selective advantage for cancer cell metastasis.

Hepatocyte growth factor-induced ectodomain shedding of cell adhesion molecule L1: role of the L1 cytoplasmic domain

The L1 cell adhesion molecule and its soluble form are tumor-associated proteins and potential markers for tumor staging as well as targets for therapeutic intervention. Soluble L1 is produced by metalloprotease-mediated ectodomain shedding of L1. We investigated effects of hepatocyte growth factor (HGF), a growth factor shown to increase invasiveness of renal carcinoma cells, on ectodomain shedding of L1 from these cells. All of the tested L1-positive renal carcinoma cell lines released a 180-kDa form of L1 into the medium. In the presence of serum, addition of HGF led to a dose-dependent increase in L1 shedding with a maximum reached at 5 ng/ml. In contrast, L1 shedding was inhibited by glial cell line-derived neurotrophic factor (GDNF). The tyrosine kinase inhibitor Genistein reduced basal and HGF-stimulated L1 shedding, indicating that protein phosphorylation is involved. To investigate the role of the L1 intracellular domain, two mutants of the L1 cytoplasmic part were constructed. L1trun lacking the complete intracellular domain showed enhanced basal shedding. In a L1YH mutant, containing the mutation tyrosine 1229 to histidine that deletes the ankyrin binding motif of L1, basal shedding was reduced. Disruption of actin assembly by cytochalasin D also reduced shedding of L1. These results indicate that the cytoplasmic domain regulates basal shedding of L1, and association with the cytoskeleton through the L1 ankyrin binding site is involved. HGF stimulated L1 shedding in both mutants, indicating that receptor-mediated phosphorylation in the L1 cytoplasmic domain is not required for HGF-stimulated shedding.

Loss of ectodomain shedding due to mutations in the metalloprotease and cysteine-rich/disintegrin domains of the tumor necrosis factor-alpha converting enzyme (TACE)

Tumor necrosis factor-alpha converting enzyme (TACE), a multidomain protease essential for development and disease, releases the ectodomains from many transmembrane proteins in a regulated fashion. To understand the mechanism underlying the regulation of TACE activity, we sought to identify the cause of ectodomain shedding deficiencies in two mutated CHO sublines designated M1 and M2. Transfection of expression vectors for human and mouse TACE restored ectodomain shedding of TNF-alpha and TGF-alpha, suggesting that defects in the TACE gene contribute to the phenotype of M1 and M2 cells. The overall levels of endogenous TACE forms in M1 cells were significantly lower than those found in their parental cells, whereas only TACE zymogen, but not its mature form, was detectable in M2 cells. Molecular analyses suggested that M1 cells contained only one expressible TACE allele encoding an M435I point mutation in the catalytic center of the protease, and M2 cells produced two TACE variants with distinct point mutations in the catalytic domain (C225Y) and the cysteinerich/disintegrin domain (C600Y). Overexpression of the C225Y and C600Y TACE by transient transfection largely compensated for maturation defects in the variants but failed to restore TNF-alpha and TGF-alpha release in the shedding-defective CHO cell lines and fibroblasts derived from TACE-null mouse embryo. Further mutagenesis and functional analyses demonstrated that Cys(600) was absolutely essential for ectodomain shedding, suggesting that Cys(600), similar to Cys(225), participates in disulfide bonding, which is critical for both the processing and catalysis of TACE.

The microvesicle as a vehicle for EMMPRIN in tumor-stromal interactions

EMMPRIN is a transmembrane glycoprotein expressed at high levels by tumor cells. It has been identified as a tumor-derived factor that can stimulate matrix metalloproteinase expression in fibroblasts and hence facilitate tumor invasion and metastasis. Recent studies have shown that full-length EMMPRIN is released by tumor cells, but the mechanism of release remains unclear. Here, we show that EMMPRIN is released from the surface of NCI-H460 cells via microvesicle shedding. However, these vesicles are unstable and rapidly break down to release bioactive EMMPRIN. Although microvesicle shedding has been considered a constitutive process in tumor cells, our data show that it can be amplified upon cell exposure to PMA, elucidating at least one signalling cascade responsible for EMMPRIN release. This pathway is dependent on protein kinase C, calcium mobilization and mitogen-activated protein kinase kinase (MEK 1/2). Thus, the results outline a novel form of tumor-stromal interaction in which extracellular matrix degradation by fibroblasts is controlled through the microvesicular release of EMMPRIN from tumor cells.

L1 expression as a predictor of progression and survival in patients with uterine and ovarian carcinomas

BACKGROUND

Ovarian and uterine carcinomas are the most common cause of cancer-related deaths in gynecological malignant diseases. We aimed to assess whether the L1 adhesion molecule, an important mediator for cell migration for neural and tumour cells, is expressed in these carcinomas.

METHODS

We investigated L1 expression by immunohistochemistry, RT-PCR, and Western blot analysis of tumour samples. Soluble L1 in the serum was detected by ELISA and immunoprecipitation.

FINDINGS

We detected the L1 adhesion molecule in ovarian and uterine tumours in a stage-dependent manner. In a retrospective study L1 was found in 46 of 58 ovarian carcinomas and 20 of 72 uterine adenocarcinomas. L1 expression was an excellent predictor of poor outlook (p<0.00001). Patients with L1 positive uterine tumours were at high risk for progression even in the endometrioid-type tumours, which usually have a favourable prognosis. In uterine tumours, expression of L1 in curettage samples enabled us to identify aggressive tumours before the operation. Soluble L1 was specifically detected in serum samples from patients with ovarian and uterine tumours. ADAM10, which was implicated in previous studies as L1 sheddase, was expressed in tumours in which soluble L1 was present in the serum.

INTERPRETATION

L1 is overexpressed in ovarian and uterine carcinomas and is associated with short survival. L1 can serve as a new marker for prediction of clinical outcome and could be helpful to identify patients with uterine tumours who are at high risk for recurrent disease. L1 expression and cleavage could promote dissemination of tumours by facilitating cell migration.

TLR4-dependent lipopolysaccharide-induced shedding of tumor necrosis factor receptors in mouse bone marrow granulocytes

We reported previously that bone marrow granulocytes respond to small amounts of enterobacterial lipopolysaccharide (LPS) via a CD14-independent and TLR4-mediated mechanism by de novo expression of an inducible receptor (CD14) and by down-modulation of a constitutive receptor (L-selectin). In this report we address another effect of LPS: the down-regulation of receptors for tumor necrosis factor-alpha. In mouse bone marrow cells (BMC), this down-regulation is detectable soon (20 min) after exposure of the cells to low levels (0.5 ng/ml) of LPS. This temperature-dependent effect is rather selective for LPS and requires the presence of a conventional lipid A structure in the LPS molecule and a functional TLR4 molecule in the cells. The down-modulation, due to a shedding of the receptors, is blocked by p38 MAPK inhibitors, by a furin inhibitor, and by three metalloproteinase inhibitors (BB-3103, TIMP-2, and TIMP-3). In contrast, inhibitors of MEK, protein kinase C, cAMP-dependent protein kinase, and kinases of the Src family do not block the shedding. Analysis of BMC from mice lacking tumor necrosis factor receptor-1 (CD120a-/-) or tumor necrosis factor receptor-2 (CD120b-/-) indicates that the LPS-induced shedding is specific for CD120b. Thus, exposure of BMC to LPS triggers a rapid shedding of CD120b via a protein kinase C- and Src-independent pathway mediated by p38 MAPK, furin, and metalloproteinase. The additive effects of furin and metalloproteinase inhibitors suggest that these enzymes are involved in parallel shedding pathways.

The proprotein convertase PC5A and a metalloprotease are involved in the proteolytic processing of the neural adhesion molecule L1

The transmembrane and multidomain neural adhesion molecule L1 plays important functional roles in the developing and adult nervous system. L1 is proteolytically processed at two distinct sites within the extracellular domain, leading to the generation of different fragments. In this report, we present evidence that the proprotein convertase PC5A is the protease that cleaves L1 in the third fibronectin type III domain, whereas the proprotein convertases furin, PC1, PC2, PACE4, and PC7 are not effective in cleaving L1. Analysis of mutations revealed Arg(845) to be the site of cleavage generating the N-terminal 140-kDa fragment. This fragment was present in the hippocampus, which expresses PC5A, but was not detectable in the cerebellum, which does not express PC5A. The 140-kDa L1 fragment was found to be tightly associated with the full-length 200-kDa L1 molecule. The complex dissociated from the membrane upon cleavage by a protease acting at a more membrane-proximal site of full-length L1. This proteolytic cleavage was inhibited by the metalloprotease inhibitor GM 6001 and enhanced by a calmodulin inhibitor. L1-dependent neurite outgrowth of cerebellar neurons was inhibited by GM 6001, suggesting that proteolytic processing of L1 by a metalloprotease is involved in neurite outgrowth.