Prointegrin maturation follows rapid trafficking and processing of MT1-MMP in Furin-Negative Colon Carcinoma LoVo Cells

Combined inhibition of surface CD51 and γ-secretase-mediated CD51 cleavage improves therapeutic efficacy in experimental metastatic hepatocellular carcinoma

BACKGROUND & AIMS

Integrin αv (ITGAV, CD51) is regarded as a key component in multiple stages of tumor progression. However, the clinical failure of cilengitide, a specific inhibitor targeting surface CD51, suggests the importance of yet-unknown mechanisms by which CD51 promotes tumor progression.

METHODS

In this study, we used several hepatocellular carcinoma (HCC) cell lines and murine hepatoma cell lines. To investigate the role of CD51 on HCC progression, we used a 3D invasion assay and in vivo bioluminescence imaging. We used periostin-knockout transgenic mice to uncover the role of the tumor microenvironment on CD51 cleavage. Moreover, we used several clinically relevant HCC models, including patient-derived organoids and patient-derived xenografts, to evaluate the therapeutic efficacy of cilengitide in combination with the γ-secretase inhibitor LY3039478.

RESULTS

We found that CD51 could undergo transmembrane cleavage by γ-secretase to produce a functional intracellular domain (CD51-ICD). The cleaved CD51-ICD facilitated HCC invasion and metastasis by promoting the transcription of oxidative phosphorylation-related genes. Furthermore, we identified cancer-associated fibroblast-derived periostin as the major driver of CD51 cleavage. Lastly, we showed that cilengitide-based therapy led to a dramatic therapeutic effect when supplemented with LY3039478 in both patient-derived organoid and xenograft models.

CONCLUSIONS

In summary, we revealed previously unrecognized mechanisms by which CD51 is involved in HCC progression and uncovered the underlying cause of cilengitide treatment failure, as well as providing evidence supporting the translational prospects of combined CD51-targeted therapy in the clinic.

IMPACT AND IMPLICATIONS

Integrin αv (CD51) is a widely recognized pro-tumoral molecule that plays a crucial role in various stages of tumor progression, making it a promising therapeutic target. However, despite early promising results, cilengitide, a specific antagonist of CD51, failed in a phase III clinical trial. This prompted further investigation into the underlying mechanisms of CD51's effects. This study reveals that the γ-secretase complex directly cleaves CD51 to produce an intracellular domain (CD51-ICD), which functions as a pro-tumoral transcriptional regulator and can bypass the inhibitory effects of cilengitide by entering the nucleus. Furthermore, the localization of CD51 in the nucleus is significantly associated with the prognosis of patients with HCC. These findings provide a theoretical basis for re-evaluating cilengitide in clinical settings and highlight the importance of identifying a more precise patient subpopulation for future clinical trials targeting CD51.

There and back again: Intracellular trafficking, release and recycling of matrix metalloproteinases

Matrix metalloproteinases are a family of zinc-dependent endopeptidases that are involved in a large variety of proteolytic processes in physiological and pathological scenarios, including immune cell surveillance, tissue homeostasis, or tumor cell metastasis. This is based on their ability to cleave a plethora of substrates that include components of the extracellular matrix, but also cell surface-associated and intracellular proteins. Accordingly, a tight regulatory web has evolved that closely regulates spatiotemporal activity of specific MMPs. An often underappreciated mechanism of MMP regulation involves their trafficking to and from specific subcellular sites that require MMP activity only for a certain period. In this review, we focus on the current knowledge of MMP intracellular trafficking, their secretion or surface exposure, as well as their recycling back from the cell surface. We discuss molecular mechanisms that enable these steps, in particular microtubule-dependent motility of vesicles that is driven by molecular motors and directed by vesicle regulatory proteins. Finally, we also point out open questions in the field of MMP motility that may become important in the future.

OUP accepted manuscript

Tissue inhibitors of metalloproteinases (TIMPs) are a conserved family of proteins that were originally identified as endogenous inhibitors of matrixin and adamalysin endopeptidase activity. The matrixins and adamalysins are the major mediators of extracellular matrix (ECM) turnover, thus making TIMPs important regulators of ECM structure and composition. Despite their high sequence identity and relative redundancy in inhibitory profiles, each TIMP possesses unique biological characteristics that are independent of their regulation of metalloproteinase activity. As our understanding of TIMP biology has evolved, distinct roles have been assigned to individual TIMPs in cancer progression. In this respect, data regarding TIMP2's role in cancer have borne conflicting reports of both tumor suppressor and, to a lesser extent, tumor promoter functions. TIMP2 is the most abundant TIMP family member, prevalent in normal and diseased mammalian tissues as a constitutively expressed protein. Despite its apparent stable expression, recent work highlights how TIMP2 is a cell stress-induced gene product and that its biological activity can be dictated by extracellular posttranslational modifications. Hence an understanding of TIMP2 molecular targets, and how its biological functions evolve in the progressing tumor microenvironment may reveal new therapeutic opportunities. In this review, we discuss the continually evolving functions of TIMP proteins, future perspectives in TIMP research, and the therapeutic utility of this family, with a particular focus on TIMP2.

Nucleobindin-1 regulates ECM degradation by promoting intra-Golgi trafficking of MMPs

Matrix metalloproteinases (MMPs) degrade several ECM components and are crucial modulators of cell invasion and tissue organization. Although much has been reported about their function in remodeling ECM in health and disease, their trafficking across the Golgi apparatus remains poorly understood. Here we report that the cis-Golgi protein nucleobindin-1 (NUCB1) is critical for MMP2 and MT1-MMP trafficking along the Golgi apparatus. This process is Ca2+-dependent and is required for invasive MDA-MB-231 cell migration as well as for gelatin degradation in primary human macrophages. Our findings emphasize the importance of NUCB1 as an essential component of MMP transport and its overall impact on ECM remodeling.

GRAF2, WDR44, and MICAL1 mediate Rab8/10/11-dependent export of E-cadherin, MMP14, and CFTR ΔF508

In addition to the classical pathway of secretion, some transmembrane proteins reach the plasma membrane through alternative routes. Several proteins transit through endosomes and are exported in a Rab8-, Rab10-, and/or Rab11-dependent manner. GRAFs are membrane-binding proteins associated with tubules and vesicles. We found extensive colocalization of GRAF1b/2 with Rab8a/b and partial with Rab10. We identified MICAL1 and WDR44 as direct GRAF-binding partners. MICAL1 links GRAF1b/2 to Rab8a/b and Rab10, and WDR44 binds Rab11. Endogenous WDR44 labels a subset of tubular endosomes, which are closely aligned with the ER via binding to VAPA/B. With its BAR domain, GRAF2 can tubulate membranes, and in its absence WDR44 tubules are not observed. We show that GRAF2 and WDR44 are essential for the export of neosynthesized E-cadherin, MMP14, and CFTR ΔF508, three proteins whose exocytosis is sensitive to ER stress. Overexpression of dominant negative mutants of GRAF1/2, WDR44, and MICAL1 also interferes with it, facilitating future studies of Rab8/10/11-dependent exocytic pathways of central importance in biology.

Comprehensive Structural and Molecular Comparison of Spike Proteins of SARS-CoV-2, SARS-CoV and MERS-CoV, and Their Interactions with ACE2

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has recently emerged in China and caused a disease called coronavirus disease 2019 (COVID-19). The virus quickly spread around the world, causing a sustained global outbreak. Although SARS-CoV-2, and other coronaviruses, SARS-CoV and Middle East respiratory syndrome CoV (MERS-CoV) are highly similar genetically and at the protein production level, there are significant differences between them. Research has shown that the structural spike (S) protein plays an important role in the evolution and transmission of SARS-CoV-2. So far, studies have shown that various genes encoding primarily for elements of S protein undergo frequent mutation. We have performed an in-depth review of the literature covering the structural and mutational aspects of S protein in the context of SARS-CoV-2, and compared them with those of SARS-CoV and MERS-CoV. Our analytical approach consisted in an initial genome and transcriptome analysis, followed by primary, secondary and tertiary protein structure analysis. Additionally, we investigated the potential effects of these differences on the S protein binding and interactions to angiotensin-converting enzyme 2 (ACE2), and we established, after extensive analysis of previous research articles, that SARS-CoV-2 and SARS-CoV use different ends/regions in S protein receptor-binding motif (RBM) and different types of interactions for their chief binding with ACE2. These differences may have significant implications on pathogenesis, entry and ability to infect intermediate hosts for these coronaviruses. This review comprehensively addresses in detail the variations in S protein, its receptor-binding characteristics and detailed structural interactions, the process of cleavage involved in priming, as well as other differences between coronaviruses.

Recent insights into natural product inhibitors of matrix metalloproteinases

Members of the matrix metalloproteinase (MMP) family have biological functions that are central to human health and disease, and MMP inhibitors have been investigated for the treatment of cardiovascular disease, cancer and neurodegenerative disorders. The outcomes of initial clinical trials with the first generation of MMP inhibitors proved disappointing. However, our growing understanding of the complexities of the MMP function in disease, and an increased understanding of MMP protein architecture and control of activity now provide new opportunities and avenues to develop MMP-focused therapies. Natural products that affect MMP activities have been of strong interest as templates for drug discovery, and for their use as chemical tools to help delineate the roles of MMPs that still remain to be defined. Herein, we highlight the most recent discoveries of structurally diverse natural product inhibitors to these proteases.

Targeting the MMP-14/MMP-2/integrin αvβ3 axis with multispecific N-TIMP2-based antagonists for cancer therapy

The pathophysiological functions of the signaling molecules matrix metalloproteinase-14 (MMP-14) and integrin α_vβ₃ in various types of cancer are believed to derive from their collaborative activity in promoting invasion, metastasis, and angiogenesis, as shown in vitro and in vivo The two effectors act in concert in a cell-specific manner through the localization of pro-MMP-2 to the cell surface, where it is processed to intermediate and matured MMP-2. The matured MMP-2 product is localized to the cell surface via its binding to integrin α_vβ₃ The MMP-14/MMP-2/integrin α_vβ₃ axis thus constitutes an attractive putative target for therapeutic interventions, but the development of inhibitors that target this axis remains an unfulfilled task. To address the lack of such multitarget inhibitors, we have established a combinatorial approach that is based on flow cytometry screening of a yeast-displayed N-TIMP2 (N-terminal domain variant of tissue inhibitor of metalloproteinase-2) mutant library. On the basis of this screening, we generated protein monomers and a heterodimer that contain monovalent and bivalent binding epitopes to MMP-14 and integrin α_vβ₃ Among these proteins, the bi-specific heterodimer, which bound strongly to both MMP-14 and integrin α_vβ₃, exhibited superior ability to inhibit MMP-2 activation and displayed the highest inhibitory activity in cell-based models of a MMP-14-, MMP-2-, and integrin α_vβ₃-dependent glioblastoma and of endothelial cell invasiveness and endothelial capillary tube formation. These assays enabled us to show the superiority of the combined target effects of the inhibitors and to investigate separately the role each of the three signaling molecules in various malignant processes.

Matrix metalloproteinases - From the cleavage data to the prediction tools and beyond

Understanding the physiological role of any protease requires identification of both its cleavage substrates and their relative cleavage efficacy as compared with other substrates and other proteinases. Our review manuscript is focused on the cleavage preferences of the individual matrix metalloproteinases (MMPs) and the cleavage similarity and distinction that exist in the human MMP family. The recent in-depth analysis of MMPs by us and many others greatly increased knowledge of the MMP biology and structural-functional relationships among this protease family members. A better knowledge of cleavage preferences of MMPs has led us to the development of the prediction tools that are now capable of the high throughput reliable prediction and ranking the MMP cleavage sites in the peptide sequences in silico. Our software unifies and consolidates volumes of the pre-existing data. Now this prediction-ranking in silico tool is ready to be used by others. The software we developed may facilitate both the identification of the novel proteolytic regulatory pathways and the discovery of the previously uncharacterized substrates of the individual MMPs. Because now the MMP research may be based on the mathematical probability parameters rather than on either random luck or common sense alone, the researchers armed with this novel in silico tool will be better equipped to fine-tune or, at least, to sharply focus their wet chemistry experiments. This article is part of a Special Issue entitled: Matrix Metalloproteinases edited by Rafael Fridman.

Metalloproteinases ADAM12 and MMP-14 are associated with cavernous sinus invasion in pituitary adenomas

Invasion of tumor cells critically depends on cell-cell or cell-extracellular matrix interactions. Enzymes capable of modulating these interactions belong to the proteinase families of ADAM (a disintegrin and metalloprotease) and MMP (matrix metalloprotease) proteins. Our objective is to examine their expression levels and evaluate the relationship between expression levels and cavernous sinus invasion in pituitary adenomas. Tissue samples from 35 patients with pituitary adenomas were analyzed. Quantitative real-time polymerase chain reaction (qPCR) was employed to assess mRNA expression levels for ADAM and MMP genes. Protein levels were examined using immunohistochemistry and Western Blot. Correlation analyses between expression levels and clinical parameters were performed. By silencing ADAM12 and MMP-14 with siRNA in a mouse pituitary adenoma cell line (TtT/GF), their cellular effects were investigated. In our study, nine women and 26 men were included, with a mean age of 53.1 years (range 15-84 years) at the time of surgery. There were 19 cases with cavernous sinus invasion. The proteins ADAM12 and MMP-14 were significantly up-regulated in invasive adenomas compared to noninvasive adenomas. Both human isoforms of ADAM12 (ADAM12L and ADAM12s) were involved in tumor invasion; moreover, ADAM12L was found to correlate positively with Ki-67 proliferation index in pituitary adenomas. In TtT/GF pituitary adenoma cells, silencing of ADAM12 and MMP-14 significantly inhibited cell invasion and migration, respectively, whereas only silencing of ADAM12 suppressed cell proliferation. We conclude that ADAM12 and MMP-14 are associated with cavernous sinus invasion in pituitary adenomas, which qualifies these proteins in diagnosis and therapy.

Regulation of ENaC trafficking in rat kidney

The epithelial Na channel (ENaC) forms a pathway for Na(+) reabsorption in the distal nephron, and regulation of these channels is essential for salt homeostasis. In the rat kidney, ENaC subunits reached the plasma membrane in both immature and fully processed forms, the latter defined by either endoglycosidase H-insensitive glycosylation or proteolytic cleavage. Animals adapted to a low-salt diet have increased ENaC surface expression that is specific for the mature forms of the subunit proteins and is similar (three- to fourfold) for α, β, and γENaC. Kidney membranes were fractionated using differential centrifugation, sucrose-gradient separation, and immunoabsorption. Endoplasmic reticulum membranes, isolated using an antibody against calnexin, expressed immature γENaC, and the content decreased with Na depletion. Golgi membranes, isolated with an antibody against the cis-Golgi protein GM130, expressed both immature and processed γENaC; Na depletion increased the content of processed γENaC in this fraction by 3.8-fold. An endosomal compartment isolated using an antibody against Rab11 contained both immature and processed γENaC; the content of processed subunit increased 2.4-fold with Na depletion. Finally, we assessed the content of γENaC in the late endocytic compartments indirectly using urinary exosomes. All of the γENaC in these exosomes was in the fully cleaved form, and its content increased by 4.5-fold with Na depletion. These results imply that stimulation of ENaC surface expression results at least in part from increased rates of formation of fully processed subunits in the Golgi and subsequent trafficking to the apical membrane.

A novel role of endothelium in activation of latent pro-membrane type 1 MMP and pro-MMP-2 in rat aorta

AIMS

Aortic stiffness is an independent risk factor for progression of cardiovascular diseases. Degradation of elastic fibres in aorta due to angiotensin II (ANGII)-stimulated overactivation of latent membrane type 1 matrix metalloproteinase (MT1MMP) and matrix metalloproteinase-2 (MMP2) is regarded to represent an important cause of aortic stiffness. Therefore, clarification of the causal mechanisms triggering the overactivation of these MMPs is of utmost importance. This study addresses the endothelium as a novel key activator of latent pro-MT1MMP and pro-MMP2 in rat aorta.

METHODS AND RESULTS

Using a co-culture model of rat aortic endothelial cells (ECs) and smooth muscle cells (SMCs), we found that ANGII stimulation resulted in activation of latent pro-MT1MMP and pro-MMP2 in SMCs exclusively when co-cultured with ECs (assessed with western blot and gelatin zymography, respectively). EC-specific AT1 receptor stimulation triggered endothelin-1 release and paracrine action on SMCs. Endothelin-1 increased expression and activity of pro-protein convertase furin in SMCs via endothelin receptor type A (assessed with qPCR and furin activity assay, respectively). Consequently, furin acted in two ways. First, it increased the activation of latent pro-MT1MMP and, second, it activated pro-αvβ3 integrin. Both pathways led to overactivation of latent pro-MMP2. In vitro findings in the co-culture model were fully consistent with the ex vivo findings obtained in isolated rat aorta.

CONCLUSIONS

We propose that the endothelium under ANGII stimulation acts as a novel and key activator of latent pro-MT1MMP and pro-MMP2 in SMCs of rat aorta. Therefore, endothelium may critically contribute to pathophysiology of aortic stiffness.

High-resolution analysis and functional mapping of cleavage sites and substrate proteins of furin in the human proteome

Background

There is a growing appreciation of the role of proteolytic processes in human health and disease, but tools for analysis of such processes on a proteome-wide scale are limited. Furin is a ubiquitous proprotein convertase that cleaves after basic residues and transforms secretory proproteins into biologically active proteins. Despite this important role, many furin substrates remain unknown in the human proteome.

Methodology/Principal Findings

We devised an approach for proteinase target identification that combines an in silico discovery pipeline with highly multiplexed proteinase activity assays. We performed in silico analysis of the human proteome and identified over 1,050 secretory proteins as potential furin substrates. We then used a multiplexed protease assay to validate these tentative targets. The assay was carried out on over 3,260 overlapping peptides designed to represent P7-P1’ and P4-P4’ positions of furin cleavage sites in the candidate proteins. The obtained results greatly increased our knowledge of the unique cleavage preferences of furin, revealed the importance of both short-range (P4-P1) and long-range (P7-P6) interactions in defining furin cleavage specificity, demonstrated that the R-X-R/K/X-R↓ motif alone is insufficient for predicting furin proteolysis of the substrate, and identified ∼490 potential protein substrates of furin in the human proteome.

Conclusions/Significance

The assignment of these substrates to cellular pathways suggests an important role of furin in development, including axonal guidance, cardiogenesis, and maintenance of stem cell pluripotency. The novel approach proposed in this study can be readily applied to other proteinases.

ADAM12 redistributes and activates MMP-14, resulting in gelatin degradation, reduced apoptosis, and increased tumor growth

Matrix metalloproteases (MMPs), in particular MMP-2, -9, and -14, play a key role in various aspects of cancer pathology. Likewise, ADAMs (A Disintegrin And Metalloproteases), including ADAM12, are upregulated in malignant tumors and contribute to the pathology of cancers. Here we showed a positive correlation between MMP-14 and ADAM12 expression in human breast cancer. We demonstrated that in 293-VnR and human breast cancer cells expressing ADAM12 at the cell surface, endogenous MMP-14 was recruited to the cell surface, resulting in its activation. Subsequent to this activation, gelatin degradation was stimulated and tumor-cell apoptosis was decreased, with reduced expression of the pro-apoptotic proteins BCL2L11 and BIK. The effect on gelatin degradation was abrogated by inhibition of the MMP-14 activity and appeared to be dependent on cell-surface αVβ3 integrin localization, but neither the catalytic activity of ADAM12 nor the cytoplasmic tail of ADAM12 were required. The significance of ADAM12-induced activation of MMP-14 was underscored by a reduction in MMP-14–mediated gelatin degradation and abolition of apoptosis-protective effects by specific monoclonal antibodies against ADAM12. Furthermore, orthotopic implantation of ADAM12-expressing MCF7 cells in nude mice produced tumors with increased levels of activated MMP-14 and confirmed that ADAM12 protects tumor cells against apoptosis, leading to increased tumor progression. In conclusion, our data suggest that a ternary protein complex composed of ADAM12, αVβ3 integrin, and MMP-14 at the tumor cell surface regulates MMP-14 functions. This interaction may point to a novel concept for the development of MMP-14–targeting drugs in treating cancer.

Furin Functions as a Nonproteolytic Chaperone for Matrix Metalloproteinase-28: MMP-28 Propeptide Sequence Requirement

Although MMP-28 is involved in numerous important physiologic and pathologic conditions, the mechanisms of action of this secreted proteinase is not well understood. We now have demonstrated that furin serves as an intermolecular chaperone for MMP-28 secretion by interacting with the propeptide domain of MMP-28. Employing COS-1 cells transfected with MMP-28 cDNA, protein levels of MMP-28 were quite low in conditioned media as compared to cell lysates. Coexpression of MMP-28 with furin cDNA resulted in markedly enhanced MMP-28 secretion. Contrary to expectation, cleavage of MMP-28 at the furin consensus sequence did not occur and proteolytic inactive furin was equally effective in enhancing MMP-28 secretion. Furin and MMP-28 coimmunoprecipitated and were partially coimmunolocalized in the cytoplasm of transfected cells. Cotransfection with furin cDNA also enhanced MMP-28 induced cell migration. In conclusion, our data provide a novel mechanism for MMP-28 function in cells in which furin serves as an intermolecular chaperone.

The Wnt/planar cell polarity protein-tyrosine kinase-7 (PTK7) is a highly efficient proteolytic target of membrane type-1 matrix metalloproteinase: implications in cancer and embryogenesis

PTK7 is an essential component of the Wnt/planar cell polarity (PCP) pathway. We provide evidence that the Wnt/PCP pathway converges with pericellular proteolysis in both normal development and cancer. Here, we demonstrate that membrane type-1 matrix metalloproteinase (MT1-MMP), a key proinvasive proteinase, functions as a principal sheddase of PTK7. MT1-MMP directly cleaves the exposed PKP(621)↓LI sequence of the seventh Ig-like domain of the full-length membrane PTK7 and generates, as a result, an N-terminal, soluble PTK7 fragment (sPTK7). The enforced expression of membrane PTK7 in cancer cells leads to the actin cytoskeleton reorganization and the inhibition of cell invasion. MT1-MMP silencing and the analysis of the uncleavable L622D PTK7 mutant confirm the significance of MT1-MMP proteolysis of PTK7 in cell functions. Our data also demonstrate that a fine balance between the metalloproteinase activity and PTK7 levels is required for normal development of zebrafish (Danio rerio). Aberration of this balance by the proteinase inhibition or PTK7 silencing results in the PCP-dependent convergent extension defects in the zebrafish. Overall, our data suggest that the MT1-MMP-PTK7 axis plays an important role in both cancer cell invasion and normal embryogenesis in vertebrates. Further insight into these novel mechanisms may promote understanding of directional cell motility and lead to the identification of therapeutics to treat PCP-related developmental disorders and malignancy.

VAMP3, syntaxin-13 and SNAP23 are involved in secretion of matrix metalloproteinases, degradation of the extracellular matrix and cell invasion

Cellular remodeling of the extracellular matrix (ECM), an essential component of many physiological and pathological processes, is dependent on the trafficking and secretion of matrix metalloproteinases (MMPs). Soluble NSF attachment protein receptor (SNARE)-mediated membrane traffic has documented roles in cell-ECM interactions and the present study specifically examines SNARE function in the trafficking of MMPs during ECM degradation. Using the invasive human fibrosarcoma cell line HT-1080, we demonstrate that a plasma membrane SNARE, SNAP23, and an endosomal v-SNARE, VAMP3 (also known as cellubrevin), partly colocalize with MMP2 and MMP9, and that inhibition of these SNAREs using dominant-negative SNARE mutants impaired secretion of the MMPs. Inhibition of VAMP3, SNAP23 or syntaxin-13 using dominant-negative SNARES, RNA interference or tetanus toxin impaired trafficking of membrane type 1 MMP to the cell surface. Consistent with these observations, we found that blocking the function of these SNAREs reduced the ability of HT-1080 cells to degrade a gelatin substrate in situ and impaired invasion of HT-1080 cells in vitro. The results reveal the importance of VAMP3, syntaxin-13 and SNAP23 in the trafficking of MMP during degradation of ECM substrates and subsequent cellular invasion.

Internal cleavages of the autoinhibitory prodomain are required for membrane type 1 matrix metalloproteinase activation, although furin cleavage alone generates inactive proteinase

The functional activity of invasion-promoting membrane type 1 matrix metalloproteinase (MT1-MMP) is elevated in cancer. This elevated activity promotes cancer cell migration, invasion, and metastasis. MT1-MMP is synthesized as a zymogen, the latency of which is maintained by its prodomain. Excision by furin was considered sufficient for the prodomain release and MT1-MMP activation. We determined, however, that the full-length intact prodomain released by furin alone is a potent autoinhibitor of MT1-MMP. Additional MMP cleavages within the prodomain sequence are required to release the MT1-MMP enzyme activity. Using mutagenesis of the prodomain sequence and mass spectrometry analysis of the prodomain fragments, we demonstrated that the intradomain cleavage of the PGD/L(50) site initiates the MT1-MMP activation, whereas the (108)RRKR(111)/Y(112) cleavage by furin completes the removal and the degradation of the autoinhibitory prodomain and the liberation of the functional activity of the emerging enzyme of MT1-MMP.

Golgi reassembly stacking protein 55 interacts with membrane-type (MT) 1-matrix metalloprotease (MMP) and furin and plays a role in the activation of the MT1-MMP zymogen

Membrane-type 1 matrix metalloproteinase (MT1-MMP) is a proteinase involved in the remodelling of extracellular matrix and the cleavage of a number of substrates. MT1-MMP is synthesized as a zymogen that requires intracellular post-translational cleavage to gain biological activity. Furin, a member of the pro-protein convertase family, has been implicated in the proteolytic removal of the MT1-MMP prodomain sequence. In the present study, we demonstrate a role for the peripheral Golgi matrix protein GRASP55 in the furin-dependent activation of MT1-MMP. MT1-MMP and furin were found to co-localize with Golgi reassembly stacking protein 55 (GRASP55). Further analysis revealed that GRASP55 associated with the cytoplasmic domain of both proteases and that the LLY(573) motif in the MT1-MMP intracellular domain was crucial for the interaction with GRASP55. Overexpression of GRASP55 was found to enhance the formation of a complex between MT1-MMP and furin. Finally, we report that disruption of the interaction between GRASP55 and furin led to a reduction in pro-MT1-MMP activation. Taken together, these data suggest that GRASP55 may function as an adaptor protein coupling MT1-MMP with furin, thus leading to the activation of the zymogen.

Proteolytic and non-proteolytic roles of membrane type-1 matrix metalloproteinase in malignancy

This manuscript provides an overview of the dynamic interactions which play an important role in regulating cancer cell functions. We describe and discuss, primarily, those interactions which involve membrane type-1 matrix metalloproteinase (MT1-MMP), its physiological inhibitor tissue inhibitor of metalloproteinases-2 (TIMP-2), furin-like proprotein convertases and the low density lipoprotein-related protein 1 (LRP1) signaling scavenger receptor. The interaction among these cellular proteins controls the efficiency of the activation of MT1-MMP and the unorthodox intracellular signaling which is generated by the catalytically inert complex of MT1-MMP with TIMP-2 and which plays a potentially important role in the migration of cancer cells. Our in-depth understanding of these cellular mechanisms may provide the key to solving the puzzling TIMP-2 paradox. This unsolved paradox arises from the fact that TIMP-2 is a powerful inhibitor of MMPs including MT1-MMP, but at the same time high levels of TIMP-2 positively correlate with an unfavorable prognosis in cancer patients. Solving the TIMP-2 paradox may lead to solving a similar PAI-1 paradox and produce a clearer understanding of the biochemical mechanisms which control the functionality of the urokinase-type plasminogen activator*urokinase receptor*plasminogen activator inhibitor type-1 (uPAR*uPA*PAI-1) system in cancer.

Matrix metalloproteinases, T cell homing and beta-cell mass in type 1 diabetes

The pathogenesis of type 1 diabetes begins with the activation of autoimmune T killer cells and is followed by their homing into the pancreatic islets. After penetrating the pancreatic islets, T cells directly contact and destroy insulin-producing beta cells. This review provides an overview of the dynamic interactions which link T cell membrane type-1 matrix metalloproteinase (MT1-MMP) and the signaling adhesion CD44 receptor with T cell transendothelial migration and the subsequent homing of the transmigrated cells to the pancreatic islets. MT1-MMP regulates the functionality of CD44 in diabetogenic T cells. By regulating the functionality of T cell CD44, MT1-MMP mediates the transition of T cell adhesion to endothelial cells to the transendothelial migration of T cells, thus, controlling the rate at which T cells home into the pancreatic islets. As a result, the T cell MT1-MMP-CD44 axis controls the severity of the disease. Inhibition of MT1-MMP proteolysis of CD44 using highly specific and potent synthetic inhibitors, which have been clinically tested in cancer patients, reduces the rate of transendothelial migration and the homing of T cells. Result is a decrease in the net diabetogenic efficiency of T cells and a restoration of beta cell mass and insulin production in NOD mice. The latter is a reliable and widely used model of type I diabetes in humans. Overall, existing experimental evidence suggests that there is a sound mechanistic rationale for clinical trials of the inhibitors of T cell MT1-MMP in human type 1 diabetes patients.

Vesicular trafficking and secretion of matrix metalloproteinases-2, -9 and tissue inhibitor of metalloproteinases-1 in neuronal cells

Matrix metalloproteinases (MMPs) are endopeptidases that cleave matrix, soluble and membrane-bound proteins and are regulated by their endogenous inhibitors the tissue inhibitors of MMPs (TIMPs). Nothing is known about MMP/TIMP trafficking and secretion in neuronal cells. We focussed our attention on the gelatinases MMP-2 and MMP-9, and their inhibitor TIMP-1. MMPs and TIMP-1 fused to GFP were expressed in N2a neuroblastoma and primary neuronal cells to study trafficking and secretion using real time video-microscopy, imaging, electron microscopy and biochemical approaches. We show that MMPs and TIMP-1 are secreted in 160-200 nm vesicles in a Golgi-dependent pathway. These vesicles distribute along microtubules and microfilaments, co-localise differentially with the molecular motors kinesin and myosin Va and undergo both anterograde and retrograde trafficking. MMP-9 retrograde transport involves the dynein/dynactin molecular motor. In hippocampal neurons, MMP-2 and MMP-9 vesicles are preferentially distributed in the somato-dendritic compartment and are found in dendritic spines. Non-transfected hippocampal neurons also demonstrate vesicular secretion of MMP-2 in both its pro- and active forms and gelatinolytic activity localised within dendritic spines. Our results show differential trafficking of MMP and TIMP-1-containing vesicles in neuronal cells and suggest that these vesicles could play a role in neuronal and synaptic plasticity.

Metaloproteinases 1 e 7 e câncer colorretal

A metaloproteinase-1 (MMP-1) e a metaloproteinase-7 (MMP-7) são proteinases da matriz extracelular (MEC), zinco-dependentes, envolvidas no processo inicial da carcinogênese por permitirem a invasão tumoral na célula e promover o processo de metastatização. O polimorfismo dessas proteinases tem sido estudado recentemente com o objetivo de validar susa expressão e/ou atividade como marcador prognóstico. Evidências cumulativas revelam importante papel das MMP's 1 e 7 em diferentes fases da carcinogênese. A MMP-1 tem ação direta sobre a principal proteína da MEC, que é o colágeno do tecido intersticial conectivo. Sua expressão aumentada neste tecido pode indicar alto potencial de disseminação tumoral em diferentes tipos de câncer, incluindo o colorretal. A associação deste aumento da expressão também parece ser verdadeira para a MMP-7.

Changes in gene-expression profiles of colon carcinoma cells induced by wild type K-ras2

AIM: To further elucidate the possible molecular biological activity of wild type K-ras2 gene by detecting changes in wild type K-ras2 gene-induced gene-expression profiles of colon carcinoma cells using cDNA microarray techniques.

METHODS: Total RNA was isolated from peripheral blood of health volunteers. Reverse transcription of RNA and polymerase chain reaction were used to synthesize wild type K-ras2 cDNA. K-ras2 cDNA fragment was cloned into a T easy vector and sequenced. A eukaryotic expression vector pCI-neo-K-ras2 was constructed and transfected to Caco2 cell line using the liposome method. Finally, mRNA was isolated, reverse-transcribed to cDNA from pCI-neo-K-ras2 or pCI-neo blank vector-transfected Caco cells, and analyzed by cDNA microarray assay.

RESULTS: Restriction enzyme analysis and DNA sequencing verified that the constructed expression vector was accurate. High-quality RNA was extracted and reverse transcribed to cDNA for microarray assay. Among the 135 genes, the expression was up-regulated in 24 and down-regulated in 121. All these differentially expressed genes were related to cell proliferation, differentiation, apoptosis and signal transduction.

CONCLUSION: Differentially expressed genes can be successfully screened from wild type K-ras2-transfected colon carcinoma cells using microarray techniques. The results of our study suggest that wild type K-ras2 is related to the negative regulation of cell proliferation, metabolism and transcriptional control, and provide new clues to the further elucidation of its possible biological activity.

Tissue inhibitor of metalloproteinase-2 (TIMP-2) regulates myogenesis and beta1 integrin expression in vitro

Myogenesis in vitro involves myoblast cell cycle arrest, migration, and fusion to form multinucleated myotubes. Extracellular matrix (ECM) integrity during these processes is maintained by the opposing actions of matrix metalloproteinase (MMP) proteases and their inhibitors, the tissue inhibitor of metalloproteinases (TIMPs). Here, we report that TIMP-2, MMP-2, and MT1-MMP are differentially expressed during mouse myoblast differentiation in vitro. A specific role for TIMP-2 in myogenesis is demonstrated by altered TIMP-2(-/-) myotube formation. When differentiated in horse serum-containing medium, TIMP-2(-/-) myotubes are larger than wild-type myotubes. In contrast, when serum-free medium is used, TIMP-2(-/-) myotubes are smaller than wild-type myotubes. Regardless of culture condition, myotube size is directly correlated with MMP activity and inversely correlated with beta1 integrin expression. Treatment with recombinant TIMP-2 rescues reduced TIMP-2(-/-) myotube size and induces increased MMP-9 activation and decreased beta1 integrin expression. Treatment with either MMP-2 or MMP-9 similarly rescues reduced myotube size, but has no effect on beta1 integrin expression. These data suggest a specific regulatory relationship between TIMP-2 and beta1 integrin during myogenesis. Elucidating the role of TIMP-2 in myogenesis in vitro may lead to new therapeutic options for the use of TIMP-2 in myopathies and muscular dystrophies in vivo.

Kurozu moromimatsu inhibits tumor growth of Lovo cells in a mouse model in vivo

OBJECTIVE

In Japan, rice vinegar that has been matured and fermented for years in earthenware jars is considered a health food with anticolon cancer action. It is divided into the liquid component (Kurozu) and the sediment (Kurozu moromimatsu), which contains large amounts of organic materials and minerals. The effect of Kurozu moromimatsu (Kurozu-M) on cancer has not yet been examined. In this study, we examined the activity of Kurozu-M on colon cancer and investigated the mechanisms involved, focusing on active oxygen generation, apoptosis, and metalloproteinases (MMPs).

METHODS

We used Lovo cells transplanted into nude mice as an experimental model. We measured the tumor volume and MMP levels and conducted hematoxylin-eosin staining (for polymorphonuclear leukocytes), terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling staining (for apoptosis), and immunostaining for nitrotyrosine (a marker of active oxygen generation) in control, Kurozu-treated, and Kurozu-M--treated groups.

RESULTS

The tumor volume was the same in the control group (231 +/- 36 mm(3)) and Kurozu group (238 +/- 52 mm(3)), but was significantly reduced in the Kurozu-M group (152 +/- 28 mm(3), P < 0.001 versus control). Apoptosis of tumor cells and accumulation of polymorphonuclear leukocytes were not observed. Nitrotyrosine production, total MMP levels, and MMP activation were significantly reduced in the Kurozu-M group.

CONCLUSION

The administration of Kurozu-M prolonged the lifespan of cancer cell-transplanted mice, inhibited tumor progression, and reduced nitrotyrosine production and MMP activation, but did not induce apoptosis.

Mislocalization and unconventional functions of cellular MMPs in cancer

MMPs are multifunctional enzymes capable of targeting the extracellular matrix, growth factors, cytokines and cell surface-associated adhesion and signaling receptors. The cellular localization and the activity of MMPs are tightly controlled at both the transcriptional and the post-transcriptional levels. Mislocalization and presentation in unconventional cellular compartments provide MMPs with an opportunity to cleave previously unidentified proteins. This review is focused on two, entirely different MMPs, one of which is membrane-tethered and another of which is soluble (MT1-MMP and MMP-26, respectively) from twenty four known human MMPs. Our recent studies determined that both of these enzymes functioned at unexpected cellular compartments and it was resulted in the identification of novel proteolytic pathways, whose significance we only partially comprehend as of this writing. It is reasonable, however, to hypothesize from these data that many individual MMPs perform in a similar manner and display a much broader range of functions compared to what we earlier thought.

Screening of genes differentially expressed in Caco2 cells transfected with wild type K-ras2 by cDNA microarray

AIM: To investigate the genes differentially expressed in human colon carcinoma cell line Caco2 transfected with wild type K-ras2-expressing plasmid and further elucidate the potential molecular biological function of wild type K-ras2.

METHODS: Sequence specific primers were designed and synthesized, and the wild type K-ras2 DNA fragment was amplified with polymerase chain reaction (PCR) technique. The expressive vector of pCI-neo-K-ras2 was constructed by routine molecular biological methods. cDNA microarray technique was employed to detect the mRNA expression in Caco2 cells transfected with pCI-neo-K-ras2 and pCI-neo, respectively, using lipofectamine.

RESULTS: The expressive vector was constructed and confirmed by restriction enzyme digestion and DNA sequencing analysis. High quality RNA and cDNA were prepared and successful microarray screening was conducted. The scanning results indicated that among 8568 genes which were obtained from gene expression profile analysis, there were 135 different ones of which 121 were down-regulated and 24 were up-regulated in the wild type K-ras2-expressing Caco2 cells. These genes differentially regulated by wild type K-ras2 included human genes encoding proteins involved in cell signal transduction, cell apoptosis, cell proliferation and differentiation.

CONCLUSION: cDNA microarray is successfully used to screen the genes differentially expressed in wild type K-ras2-expressing Caco2 cells, and the alteration of expression profile induced by the wide-type K-ras2 suggested the negatively regulatory function of wild type K-ras2 for cell signal transduction, apoptosis, cell proliferation and differentiation.

Inhibition of insulin receptor isoform-A signalling restores sensitivity to gefitinib in previously de novo resistant colon cancer cells

Resistance to antiepidermal growth factor (EGFR) strategies is an emerging clinical problem. Using human colorectal cancer (CRC) cells, we evaluated the involvement of the insulin receptor isoform-A (InsR-A) in de novo resistance to gefitinib, an EGFR tyrosine kinase inhibitor. Challenging the EGFR positive LoVo cells with gefitinib (1 microM) resulted in a small ( approximately 18%) inhibition of cell growth and although a modest reduction in phospho (p)EGFR Tyr845 was seen, pEGFR at residues -Tyr1068 and -Tyr1173 were unchanged. LoVo cells produced unprocessed pro-IGF-1R protein, substantial levels of IGF-II mRNA and mature InsR protein, consisting mainly of the InsR-A isoform. Insulin and IGF-II promoted cell growth and pEGFR Tyr845, Tyr1068 and Tyr1173 activity and conversely, the insulin-like growth factor-1 receptor (IGF-1R)/InsR inhibitor ABDP (1 muM) inhibited growth and reduced pEGFR activity at all three tyrosine residues. pInsR and pAkt levels were increased after gefitinib treatment. Blocking of pInsR with ABDP enabled gefitinib to markedly reduce pEGFR Tyr845, Tyr1068 and Tyr1173. Short-term gefitinib/ABDP dual treatment was more effective than either agent alone and chronic exposure to this combination resulted in total cell loss after 9 weeks, preventing acquisition of resistance to ABDP. LoVo cells with acquired resistance to ABDP were acutely sensitive to gefitinib. We concluded that InsR-A reduces sensitivity to gefitinib in LoVo CRC cells, thus its co-targeting alongside EGFR can improve the anti-tumour effect of gefitinib.

O-Glycosylation Regulates Autolysis of Cellular Membrane Type-1 Matrix Metalloproteinase (MT1-MMP)

MT1-MMP is a key enzyme in cancer cell invasion and metastasis. The activity of cellular MT1-MMP is regulated by furin-like proprotein convertases, TIMPs, shedding, autoproteolysis, dimerization, exocytosis, endocytosis, and recycling. Our data demonstrate that, in addition to these already known mechanisms, MT1-MMP is regulated by O-glycosylation of its hinge region. Insignificant autolytic degradation is characteristic for naturally expressed, glycosylated, MT1-MMP. In turn, extensive autolytic degradation, which leads to the inactivation of the protease and the generation of its C-terminal membrane-tethered degraded species, is a feature of overexpressed MT1-MMP. We have determined that incomplete glycosylation stimulates extensive autocatalytic degradation and self-inactivation of MT1-MMP. Self-proteolysis commences during the secretory process of MT1-MMP through the cell compartment to the plasma membrane. The strongly negatively charged sialic acid is the most important functional moiety of the glycopart of MT1-MMP. We hypothesize that sialic acid of the O-glycosylation cassette restricts the access of the catalytic domain to the hinge region and to the autolytic cleavage site and protects MT1-MMP from autolysis. Overall, our results point out that there is a delicate balance between glycosylation and self-proteolysis of MT1-MMP in cancer cells and that when this balance is upset the catalytically potent MT1-MMP pool is self-proteolyzed.

Furin regulates the intracellular activation and the uptake rate of cell surface-associated MT1-MMP

Invasion-promoting membrane type-1 matrix metalloproteinase (MT1-MMP) functions in cancer cells as an oncogene and as a mediator of proteolytic events on the cell surface. To exert its functional activity, MT1-MMP requires proteolytic removal of the prodomain sequence. There are two potential furin cleavage motifs, R(89)-R-P-R-C(93) and R(108)-R-K-R-Y(112), in the prodomain sequence of MT1-MMP. Our data suggest an important role of furin and related proprotein convertases (PCs) in mediating both the activation of MT1-MMP and the levels of functionally active MT1-MMP at the surface of cancer cells. We have determined that the peptide sequence that spans the first cleavage site is susceptible to furin and PC5/6, whereas the second sequence is susceptible to furin and also to PC5/6, PC7 and PACE4. In the structure of the MT1-MMP proenzyme, the R(89)-R-P-R-C(93) site, however, is inaccessible to PCs. Our studies also demonstrated a direct functional link between the activation and the uptake rate of the proenzyme and the enzyme of MT1-MMP. Thus, the uptake rate of the latent MT1-MMP proenzyme noticeably exceeded that of the active enzyme. We conclude that furin and related PCs are the essential components of the specialized cellular machinery that controls the levels of the functionally active, mature, MT1-MMP enzyme on the cell surface to continually support the potency of pericellular proteolysis.

Rapid ATP-induced release of matrix metalloproteinase 9 is mediated by the P2X7 receptor

Matrix metalloproteinase-9 (MMP-9) activity is required for inflammatory response, leukocyte recruitment, and tumor invasion. There is increasing evidence suggesting that the P2X(7) receptor of mononuclear cells, which is activated by extracellular adenosine triphosphate (ATP), is involved in inflammatory responses. In this study, ATP caused a rapid release of MMP-9 and a moderate decrease in tissue inhibitor of metalloproteinase 1 (TIMP-1) release from human peripheral-blood mononuclear cells (PBMCs) over a 30-minute time course. The release was time- and dose-dependent and dissociated from ATP-induced cell death. BzATP, which is the most potent agonist for the P2X(7) receptor, also caused a similar effect at a lower dosage. ATP-induced MMP-9 release was inhibited by the P2X(7) receptor antagonists periodate oxidized ATP and KN-62, or by calcium chelators, as well as by a loss-of-function polymorphism in the P2X(7) receptor, but not by brefeldin A, monensin, or cycloheximide, or by anti-tumor necrosis factor-alpha (TNF-alpha) or anti-interleukin-1beta (IL-1beta) monoclonal antibodies. Results from purified subsets of PBMCs showed monocytes were the major source for MMP-9 and TIMP-1 release, and ATP remained effective in purified monocyte and T-cell populations. These observations suggest a novel role for P2X(7) as a pro-inflammatory receptor involved in rapid MMP-9 release and leukocyte recruitment.

The transmembrane domain is essential for the microtubular trafficking of membrane type-1 matrix metalloproteinase (MT1-MMP)

Membrane type-1 matrix metalloproteinase (MT1-MMP) degrades the extracellular matrix, initiates the activation pathway of soluble MMPs and regulates the functionality of cell adhesion signaling receptors, thus playing an important role in many cell functions. Intracellular transport mechanisms, currently incompletely understood, regulate the presentation of MT1-MMP at the cell surface. We have focused our efforts on identifying these mechanisms. To understand the transport of MT1-MMP across the cell, we used substitution and deletion mutants, the trafficking of which was examined using antibody uptake and Chariot delivery experiments. Our experiments have demonstrated that the microtubulin cytoskeleton and the centrosomes (the microtubulin cytoskeleton-organizing centers) are essential for the trafficking and the internalization of MT1-MMP. We determined that after reaching the plasma membrane, MT1-MMP is internalized in the Rab-4-positive recycling endosomes and the Rab-11-positive pericentrosomal recycling endosomes. The microtubular trafficking causes the protease to accumulate in the pericentrosomal region of the cell. We believe that the presence of the transmembrane domain is required for the microtubular vesicular trafficking of MT1-MMP because the soluble mutants are not presented at the cell surface and they are not delivered to the centrosomes. The observed transport mechanisms provide a vehicle for the intracellular targets and, accordingly, for an intracellular cleavage function of MT1-MMP in malignant cells, which routinely overexpress this protease.

Mutational and structural analyses of the hinge region of membrane type 1-matrix metalloproteinase and enzyme processing

Membrane type 1 (MT1)-matrix metalloproteinase (MMP) is a major mediator of collagen degradation in the pericellular space in both physiological and pathological conditions. Previous evidence has shown that on the cell surface, active MT1-MMP undergoes autocatalytic processing to a major membrane-tethered 44-kDa product lacking the catalytic domain and displaying Gly285 at its N terminus, which is at the beginning of the hinge domain. However, the importance of this site and the hinge region in MT1-MMP processing is unknown. In the current study, we generated mutations and deletions in the hinge of MT1-MMP and followed their effect on processing. These studies established Gly284-Gly285 as the main cleavage site involved in the formation of the 44-kDa species. However, alterations at this site did not prevent processing. Instead, they forced downstream cleavages within the stretch of residues flanked by Gln296 and Ser304 in the hinge region, as determined by the processing profile of various hinge deletion mutants. Also, replacement of the hinge of MT1-MMP with the longer MT3-MMP hinge did not prevent processing of MT1-MMP. Molecular dynamic studies using a computational model of MT1-MMP revealed that the hinge region is a highly motile element that undergoes significant motion in the highly exposed loop formed by Pro295-Arg302 consistent with being a prime target for proteolysis, in agreement with the mutational data. These studies suggest that the hinge of MT1-MMP evolved to facilitate processing, a promiscuous but compulsory event in the destiny of MT1-MMP, which may play a key role in the control of pericellular proteolysis.

Membrane type-1 matrix metalloproteinase (MT1-MMP) exhibits an important intracellular cleavage function and causes chromosome instability

Elevated expression of membrane type-1 matrix metalloproteinase (MT1-MMP) is closely associated with malignancies. There is a consensus among scientists that cell surface-associated MT1-MMP is a key player in pericellular proteolytic events. Now we have identified an intracellular, hitherto unknown, function of MT1-MMP. We demonstrated that MT1-MMP is trafficked along the tubulin cytoskeleton. A fraction of cellular MT1-MMP accumulates in the centrosomal compartment. MT1-MMP targets an integral centrosomal protein, pericentrin. Pericentrin is known to be essential to the normal functioning of centrosomes and to mitotic spindle formation. Expression of MT1-MMP stimulates mitotic spindle aberrations and aneuploidy in non-malignant cells. Volumes of data indicate that chromosome instability is an early event of carcinogenesis. In agreement, the presence of MT1-MMP activity correlates with degraded pericentrin in tumor biopsies, whereas normal tissues exhibit intact pericentrin. We believe that our data show a novel proteolytic pathway to chromatin instability and elucidate the close association of MT1-MMP with malignant transformation.

Furin Directly Cleaves proMMP-2 in the trans-Golgi Network Resulting in a Nonfunctioning Proteinase

Proprotein convertases play an important role in tumorigenesis and invasiveness. Here, we report that a dibasic amino acid convertase, furin, directly cleaves proMMP-2 within the trans-Golgi network leading to an inactive form of matrix metalloproteinase-2 (MMP-2). Co-transfection of COS-1 cells with both proMMP-2 and furin cDNAs resulted in the cleavage of the N-terminal propeptide of proMMP-2. The molecular mass of cleaved MMP-2 (63 kDa), detected in both cell lysates and conditioned medium, is between the intermediate and fully activated forms of MMP-2 induced by membrane type 1-MMP. Furin-cleaved MMP-2 does not possess proteolytic activity as examined in a cell-free assay. Treatment of transfected cells with a furin inhibitor resulted in a dose-dependent inhibition of proMMP-2 cleavage; recombinant tissue inhibitor of metalloproteinase-2, which binds to the active site of membrane type 1-MMP, had no inhibitory effect. Site-directed mutagenesis of amino acids in the furin consensus recognition motif of proMMP-2(R69KPR72) prevented propeptide cleavage, thereby identifying the scissile bond and characterizing the basic amino acids required for cleavage. Other experimental observations were consistent with intracellular furin cleavage of proMMP-2 in the trans-Golgi network. The furin cleavage site in other proMMPs was examined. MMP-3, which contains the RXXR furin consensus sequence, was cleaved in furin co-transfected cells, whereas MMP-1, which lacks an RXXR consensus sequence, was not cleaved. In conclusion, we report the novel observation that furin can directly cleave the RXXR amino acid sequence in the propeptide domain of proMMP-2 leading to inactivation of the enzyme.

Distinct Pools of Epithelial Sodium Channels Are Expressed at the Plasma Membrane

The epithelial Na+ channel (ENaC) is assembled in the endoplasmic reticulum from three structurally related subunits (alpha, beta, and gamma). Channel maturation within the biosynthetic pathway involves cleavage of the alpha and gamma subunits by furin and processing of N-linked glycans on alpha, beta, and gamma to complex type. Both mature and immature subunits have been observed at the surface of stably transfected Madin-Darby canine kidney cells. We have examined whether channel maturation is an all-or-none event or whether heterogeneous processing of channel subunits occurs within an individual channel complex. Using an immobilized lectin to isolate proteins with complex type N-glycans, we found that individual channel complexes with mature subunits lack immature subunits. Furthermore, terminal processing of N-glycans on ENaC subunits was not dependent on cleavage of ENaC subunits, and proteolysis of channel subunits was not dependent on prior processing of N-glycans. Our results suggest that processing of subunits within an individual channel complex is an all-or-none event such that channels present on the cell surface contain either all mature or all immature subunits. The presence of immature channel complexes at the plasma membrane provides epithelial cells with a reserve of poorly functional channels that can be activated by proteases in post-Golgi compartments.

Non-proteolytic, receptor/ligand interactions associate cellular membrane type-1 matrix metalloproteinase with the complement component C1q

Membrane type-1 matrix metalloproteinase (MT1-MMP), a prototypic member of the membrane-tethered MMP family, is an essential component of a cellular proteolysis apparatus. Recognition of protein cleavage targets followed by proteolysis is a main function of MT1-MMP. For the first time, however, we present evidence that MT1-MMP and other structurally related membrane MMPs bind C1q, the recognition unit of the first component of complement C1 that initiates activation of the classical pathway of complement. These interactions involve the catalytic domain of MT1-MMP and the C1q globular domain. In silico modeling followed by mutagenesis and the in vitro and cell-based binding studies showed that the His(171)-Glu-Lys-Gln-Ala-Asp(176) and Val(223)-Arg-Asn(224) peptide sequences of MT1-MMP are directly involved in the binding with C1q. These sequence regions are spatially distant from the active site of the protease. As a result, the catalytically active and the catalytically latent forms of cellular MT1-MMP are both efficient in binding with C1q. In agreement, despite the MT1-MMP/C1q interactions, C1q is totally resistant to MT1-MMP proteolysis. The discovery of the unconventional, receptor/ligand-like interactions of MT1-MMP with C1q, an essential component of immunity, is a significant step toward a more complete understanding of the role of this membrane-tethered protease in cancer.