The capture of extracellular vesicles endogenously released by xenotransplanted tumours induces an inflammatory reaction in the premetastatic niche

The capture of tumour-derived extracellular vesicles (TEVs) by cells in the tumour microenvironment (TME) contributes to metastasis and notably to the formation of the pre-metastatic niche (PMN). However, due to the challenges associated with modelling release of small EVs in vivo, the kinetics of PMN formation in response to endogenously released TEVs have not been examined. Here, we have studied the endogenous release of TEVs in mice orthotopically implanted with metastatic human melanoma (MEL) and neuroblastoma (NB) cells releasing GFP-tagged EVs (GFTEVs) and their capture by host cells to demonstrate the active contribution of TEVs to metastasis. Human GFTEVs captured by mouse macrophages in vitro resulted in transfer of GFP vesicles and the human exosomal miR-1246. Mice orthotopically implanted with MEL or NB cells showed the presence of TEVs in the blood between 5 and 28 days after implantation. Moreover, kinetic analysis of TEV capture by resident cells relative to the arrival and outgrowth of TEV-producing tumour cells in metastatic organs demonstrated that the capture of TEVs by lung and liver cells precedes the homing of metastatic tumour cells, consistent with the critical roles of TEVs in PMN formation. Importantly, TEV capture at future sites of metastasis was associated with the transfer of miR-1246 to lung macrophages, liver macrophages, and stellate cells. This is the first demonstration that the capture of endogenously released TEVs is organotropic as demonstrated by the presence of TEV-capturing cells only in metastatic organs and their absence in non-metastatic organs. The capture of TEVs in the PMN induced dynamic changes in inflammatory gene expression which evolved to a pro-tumorigenic reaction as the niche progressed to the metastatic state. Thus, our work describes a novel approach to TEV tracking in vivo that provides additional insights into their role in the earliest stages of metastatic progression.

Fibroblasts and macrophages cooperate to create a pro-tumorigenic and immune resistant environment via activation of TGF-β/IL-6 pathway in neuroblastoma

Tumor-associated macrophages (TAM) and cancer-associated fibroblasts (CAF) and their precursor mesenchymal stromal cells (MSC) are often detected together in tumors, but how they cooperate is not well understood. Here, we show that TAM and CAF are the most abundant nonmalignant cells and are present together in untreated human neuroblastoma (NB) tumors that are also poorly infiltrated with T and natural killer (NK) cells. We then show that MSC and CAF-MSC harvested from NB tumors protected human monocytes (MN) from spontaneous apoptosis in an interleukin (IL)-6 dependent mechanism. The interactions of MN and MSC with NB cells resulted in a significant induction or increase in the expression of several pro-tumorigenic cytokines/chemokines (TGF-β1, MCP-1, IL-6, IL-8, and IL-4) but not of anti-tumorigenic cytokines (TNF-α, IL-12) by MN or MSC, while also inducing cytokine expression in quiescent NB cells. We then identified a TGF-β1/IL-6 pathway where TGF-β1 stimulated the expression of IL-6 in NB cells and MSC, promoting TAM survival. Evidence for the contribution of TAM and MSC to the activation of this pathway was then provided in xenotransplanted NB tumors and patients with primary tumors by demonstrating a direct correlation between the presence of CAF and p-SMAD2 and p-STAT3. The data highlight a new mechanism of interaction between TAM and CAF supporting their pro-tumorigenic function in cancer.

The Tumor Microenvironment in Neuroblastoma: New Players, New Mechanisms of Interaction and New Perspectives

The contribution of the tumor microenvironment (TME) to cancer progression has been well recognized in recent decades. As cancer therapeutic strategies are increasingly precise and include immunotherapies, knowledge of the nature and function of the TME in a tumor becomes essential. Our understanding of the TME in neuroblastoma (NB), the second most common solid tumor in children, has significantly progressed from an initial focus on its Schwannian component to a better awareness of its complex nature, which includes not only immune but also non-immune cells such as cancer-associated fibroblasts (CAFs), the contribution of which to inflammation and interaction with tumor-associated macrophages (TAMs) is now recognized. Recent studies on the TME landscape of NB tumors also suggest significant differences between MYCN-amplified (MYCN-A) and non-amplified (MYCN-NA) tumors, in their content in stromal and inflammatory cells and their immunosuppressive activity. Extracellular vesicles (EVs) released by cells in the TME and microRNAs (miRs) present in their cargo could play important roles in the communication between NB cells and the TME. This review article discusses these new aspects of the TME in NB and the impact that information on the TME landscape in NB will have in the design of precise, biomarker-integrated clinical trials.

Extracellular Vesicle and Particle Biomarkers Define Multiple Human Cancers

There is an unmet clinical need for improved tissue and liquid biopsy tools for cancer detection. We investigated the proteomic profile of extracellular vesicles and particles (EVPs) in 426 human samples from tissue explants (TEs), plasma, and other bodily fluids. Among traditional exosome markers, CD9, HSPA8, ALIX, and HSP90AB1 represent pan-EVP markers, while ACTB, MSN, and RAP1B are novel pan-EVP markers. To confirm that EVPs are ideal diagnostic tools, we analyzed proteomes of TE- (n = 151) and plasma-derived (n = 120) EVPs. Comparison of TE EVPs identified proteins (e.g., VCAN, TNC, and THBS2) that distinguish tumors from normal tissues with 90% sensitivity/94% specificity. Machine-learning classification of plasma-derived EVP cargo, including immunoglobulins, revealed 95% sensitivity/90% specificity in detecting cancer. Finally, we defined a panel of tumor-type-specific EVP proteins in TEs and plasma, which can classify tumors of unknown primary origin. Thus, EVP proteins can serve as reliable biomarkers for cancer detection and determining cancer type.

Anti-CD105 Antibody Eliminates Tumor Microenvironment Cells and Enhances Anti-GD2 Antibody Immunotherapy of Neuroblastoma with Activated Natural Killer Cells

PURPOSE

We determined whether elimination of CD105⁺ cells in the tumor microenvironment (TME) with anti-CD105 antibodies enhanced anti-disialoganglioside (GD2) antibody dinutuximab therapy of neuroblastoma when combined with activated natural killer (aNK) cells.

EXPERIMENTAL DESIGN

The effect of MSCs and monocytes on antibody-dependent cellular cytotoxicity (ADCC) mediated by dinutuximab with aNK cells against neuroblastoma cells was determined in vitro. ADCC with anti-CD105 mAb TRC105 and aNK cells against MSCs, monocytes, and endothelial cells, which express CD105, was evaluated. Anti-neuroblastoma activity in immunodeficient NSG mice of dinutuximab with aNK cells without or with anti-CD105 mAbs was determined using neuroblastoma cell lines and a patient-derived xenograft.

RESULTS

ADCC mediated by dinutuximab with aNK cells against neuroblastoma cells in vitro was suppressed by addition of MSCs and monocytes, and dinutuximab with aNK cells was less effective against neuroblastomas formed with coinjected MSCs and monocytes in NSG mice than against those formed by tumor cells alone. Anti-CD105 antibody TRC105 with aNK cells mediated ADCC against MSCs, monocytes, and endothelial cells. Neuroblastomas formed in NSG mice by two neuroblastoma cell lines or a patient-derived xenograft coinjected with MSCs and monocytes were most effectively treated with dinutuximab and aNK cells when anti-human (TRC105) and anti-mouse (M1043) CD105 antibodies were added, which depleted human MSCs and murine endothelial cells and macrophages from the TME.

CONCLUSIONS

Immunotherapy of neuroblastoma with anti-GD2 antibody dinutuximab and aNK cells is suppressed by CD105⁺ cells in the TME, but suppression is overcome by adding anti-CD105 antibodies to eliminate CD105⁺ cells.

Cancer-Associated Fibroblasts Share Characteristics and Protumorigenic Activity with Mesenchymal Stromal Cells

Cancer-associated fibroblasts (CAF) have been suggested to originate from mesenchymal stromal cells (MSC), but their relationship with MSCs is not clear. Here, we have isolated from primary human neuroblastoma tumors a population of αFAP- and FSP-1-expressing CAFs that share phenotypic and functional characteristics with bone marrow-derived MSCs (BM-MSC). Analysis of human neuroblastoma tumors also confirmed the presence of αFAP- and FSP-1-positive cells in the tumor stroma, and their presence correlated with that of M2 tumor-associated macrophages. These cells (designated CAF-MSCs) enhanced in vitro neuroblastoma cell proliferation, survival, and resistance to chemotherapy and stimulated neuroblastoma tumor engraftment and growth in immunodeficient mice, indicating an effect independent of the immune system. The protumorigenic activity of MSCs in vitro and in xenografted mice was dependent on the coactivation of JAK2/STAT3 and MEK/ERK1/2 in neuroblastoma cells. In a mouse model of orthotopically implanted neuroblastoma cells, inhibition of JAK2/STAT3 and MEK/ERK/1/2 by ruxolitinib and trametinib potentiated tumor response to etoposide and increased overall survival. These data point to a new type of protumorigenic CAF in the tumor microenvironment of neuroblastoma and to STAT3 and ERK1/2 as mediators of their activity. Cancer Res; 77(18); 5142-57. ©2017 AACR.

Interaction between bone marrow stromal cells and neuroblastoma cells leads to a VEGFA-mediated osteoblastogenesis

The potential role of osteoblasts in bone and bone marrow (BM) metastases in neuroblastoma (NBL) remains unclear. In this study, we examined the effect of NBL cells on the osteoblastic differentiation of BM-derived mesenchymal stromal cells (BMMSC). We show that the presence of NBL cells enhanced the osteoblastic differentiation of BMMSC driven by bone morphogenetic protein (BMP)-4, in the absence of any effect on NBL cell proliferation. Expression profiles of BMMSC driven toward osteoblastic differentiation revealed an increase in vascular endothelial growth factor A (Vegfa) expression in the presence of NBL cells. We demonstrated that NBL cells increased BMMSC-derived VEGFA mRNA and protein and that this was enhanced by BMP-4. However, in similar conditions, neither the addition of an mVEGFA blocking antibody nor exogenous recombinant (r) mVEGFA affected osteoblastic differentiation. In contrast, siRNA- mediated knock-down of VEGFA in BMMSC prevented osteoblastic differentiation in BMP-4-treated cocultures, an effect that was not reversed in the presence of rmVEGFA. An analysis of murine bones injected with hNBL cells revealed an increase of mVEGFA producing cells near tumor cells concomitantly with an increase in Vegfa and Runx2 mRNA. This coincided with an increase in osteoclasts, in Rankl/Opg mRNA ratio and with the formation of osteolytic lesions. Thus NBL cells promote osteoblastogenesis in the BM by increasing VEGFA expression in BMMSC. Our study provides a new insight into the role of VEGFA in NBL metastases by pointing to the role of stroma-derived intracrine VEGFA in osteoblastogenesis.

Bone marrow-derived mesenchymal stromal cells promote survival and drug resistance in tumor cells

Bone marrow mesenchymal stromal cells (BMMSC) have antitumorigenic activities. Here, we hypothesized that circulating BMMSC are incorporated into tumors and protect tumor cells from therapy-induced apoptosis. Adherent cells harvested from murine bone marrow and expressing phenotypic and functional characteristics of BMMSC were tested for their antitumor activity against murine 4T1 mammary adenocarcinoma and LL/2 Lewis lung carcinoma cells. BMMSC but not NIH3T3 or murine skin fibroblasts stimulated the expansion of 4T1 cells in three-dimensional (3D) cocultures, and conditioned medium (CM) from these cells increased the viability of 4T1 and LL/2 cells in two-dimensional (2D) cultures. 4T1 cells exposed to BMMSC CM exhibited a 2-fold reduction in apoptosis under low serum concentrations (0.5% to 1%). Furthermore, exposure of 4T1 and LL/2 cells to BMMSC CM increased their viability in the presence of paclitaxel or doxorubicin at therapeutic concentrations. This effect was accompanied by reductions in caspase-3 activity and Annexin V expression. When coinjected with 4T1 cells in the mammary fat pad of mice subsequently treated with doxorubicin, BMMSC (and not fibroblasts) also inhibited drug-induced apoptosis in tumor cells by 44%. We demonstrated that BMMSC were attracted by 4T1 and LL/2 cells but not by NIH3T3 cells in vitro and that when injected intravenously in 4T1 tumor-bearing mice, these cells (and not NIH3T3) were specifically detected in tumors within 12 to 18 days in which they preferentially localized at the invasive front. Overall, our data identify BMMSC as an important mediator of tumor cell survival and treatment resistance in primary tumors.

Abstract 2601: Interaction between bone marrow mesenchymal stem cells and neuroblastoma cells leads to osteoblastogenesis and VEGFA expression

Bone metastasis in patients with neuroblastoma often heralds a poor prognosis despite intensive therapy. We previously reported that neuroblastoma cells interact with osteoclasts and bone marrow mesenchymal stem cells (BMSC), and that the nature of this interaction is both osteolytic and pro-tumorigenic. In many cancers, the suppression of osteoblastogenesis plays a part in increased osteolysis. However, little is known about the interaction between neuroblastoma cells and osteoblasts within the bone marrow microenvironment. We have examined this interaction within the context of BMSC differentiation to osteoblasts by bone morphogenetic protein 4 (BMP4).

To examine the effect of secreted factors from neuroblastoma cells on the BMP4 induced differentiation of murine BMSC, we cultured them in the presence or absence of neuroblastoma cells using culture inserts. We found that the mRNA encoding the osteoblast marker alkaline phosphatase (ALP) was increased by 10 fold and ALP protein level and activity were 1.5 fold higher in the presence versus absence of neuroblastoma cells. The mRNAs encoding the osteoblast master regulator Runx2 and its target osteocalcin were increased by 4 and 1.5 fold (p<0.05), respectively, when BMP4 treated BMSC were co-cultured with tumor cells. Further, neuroblastoma cells stimulated the proliferation of BMSC in co-culture by >2 fold. Together, these data indicate that neuroblastoma cells synergise with BMP4 to promote osteoblast growth and differentiation from BMSC.

To understand the mechanism by which neuroblastoma cells enhance osteoblastogenesis, we compared the mRNA profile of murine MSC driven to differentiate into osteoblasts by doxycycline-induced Runx2 expression (ST2Rx2dox) in the presence or absence of neuroblastoma cells. Triplicate RNA samples were labeled and hybridized to Illumina BeadChips containing probes for >19,100 genes. In silico analysis identified 9 genes upregulated when co-cultured with neuroblastoma cells. Of these, two (Spondin2 and VEGFA) encode proteins with established roles in osteoblastogenesis. To validate these findings in primary murine BMSC, we showed by RT-qPCR a 2 fold increase in VEGFA mRNA in BMSC treated with BMP4 in the presence versus absence of neuroblastoma cells. ELISA analysis demonstrated a parallel 2 fold increase in intra- and extracellular VEGFA protein levels. We then found that the treatment of BMSC with BMP4 and VEGFA together led to a 1.5 fold increase (p<0.05) in BMSC viability compared to VEGFA alone or no treatment, suggesting that VEGFA may provide a BMP4 dependent growth advantage.

Our data suggest that in contrast to many cancers that suppress osteoblastogenesis, neuroblastoma may promote osteoblastogenesis in part by enhancing the expression of VEGFA in BMSC. We are currently investigating the consequence of this osteoblastogenic activity on neuroblastoma bone metastasis.

Citation Format: Josephine H. HaDuong, Laurence Blavier, Sanjeev K. Baniwal, Baruch Frenkel, Yves A. DeClerck. Interaction between bone marrow mesenchymal stem cells and neuroblastoma cells leads to osteoblastogenesis and VEGFA expression. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2601. doi:10.1158/1538-7445.AM2013-2601

Abstract 1478: Bone marrow mesenchymal stem cells suppress tumor cell apoptosis and are selectively recruited to early-stage tumors

Bone marrow mesenchymal stem cells (BMMSC) are recruited to primary tumors and have been reported to display pro-tumorigenic as well as anti-tumorigenic activities. However, the mechanisms behind their effect on tumor development are still unclear. We have isolated murine bone marrow aspirates and plated them on fibronectin/collagen I, which allows expansion of an adherent stromal cell population in 3-6 weeks. These cells express the CD44 hyaluronic acid receptor in the absence of hematopoietic (CD34), myeloid (CD45), and endothelial (CD31/VEGFR2) markers and display both spontaneous and inducible adipogenesis/ osteoblastogenesis in culture. 3D matrigel cultures of murine 4T1 mammary carcinoma cells revealed enhanced generation of tumor spheroids in the presence of BMMSC or their conditioned media. Conditioned media from BMMSC also increased 4T1 cell expansion in 2D monolayer cultures, with loss of this phenomenon after heat inactivation suggesting the presence of protein-based signaling. Cell cycle analysis of 4T1 cells treated with BMMSC conditioned media revealed no statistically significant change in mitotic potential in either growth (serum present) or growth-arrest (serum depleted) phase, as determined by PI staining and BrdU uptake. However, analysis of apoptosis in 4T1 cells treated with BMMSC conditioned media showed increased percentages of live cells by PI uptake and AnnexinV staining, as well as a reduction in caspase-3 and caspase-9 activation by colorimetric assay, as the cells entered growth arrest phase. Furthermore, treatment with BMMSC conditioned media preserved viability in 4T1 cells exposed to paclitaxel under no-serum conditions. Complementary studies of BMMSC tumor-directed migration indicate that BMMSC show preferential in vitro recruitment by murine tumor cell lines which generate reactive stroma (4T1, LL/2 lung carcinoma) vs. no stroma (B16 melanoma). In vivo recruitment of BMMSC by early-stage/slow-growing subcutaneous 4T1 tumors was also visualized by bioluminescent imaging, red fluorescence, and magnetic resonance imaging of BMMSC labeled with iron-nanoparticles. These results were confirmed by immunohistochemical evaluation of iron deposits by Prussian blue stain and by immunofluorescent detection of PKH26-labeled BMMSC. Overall, our data identify BMMSC as an important mediator of tumor cell survival/treatment resistance and reveal an enhanced tropism of BMMSC toward early 4T1 tumors.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1478. doi:1538-7445.AM2012-1478

Stromelysin-1 (MMP-3) is a target and a regulator of Wnt1-induced epithelial-mesenchymal transition (EMT)

Matrix metalloproteinases (MMPs) play a well-defined role in later stages of tumor progression. However, there has been evidence that they also contribute to earlier stages of malignant transformation. The Wnt signaling transduction pathway plays a critical role in development and in the pathogenesis of many epithelial cancers. Here we have used Wnt1-induced epithelial-mesenchymal transition (EMT) in C57MG murine mammary epithelial cells to study the role of MMPs in this early step of malignant progression. Overexpression of Wnt1 in C57MG cells promoted EMT, the translocation of β-catenin from the cell membrane to the nucleus and its transcriptional activity, cell proliferation and cell motility. Simultaneously, we observed an increased expression of stromelysin-1 (MMP-3) and a 5.5-fold increase in MMP-3 promoter activity in C57MG cells expressing Wnt1 compared with C57MG cells. Treatment of Wnt-overexpressing cells with MMP inhibitor AG3340 decreased MMP-3 expression. We also found evidence that MMP-3 and Wnt3a cooperate in enhancing the transcriptional activity of β-catenin in C57MG cells. Consistently, the effects of Wnt1 on EMT, proliferation and migration were inhibited by MMP inhibitors, or upon downregulation of MMP-3 by siRNA. These results suggest that MMP-3 is both a direct transcriptional target and a necessary contributor of the Wnt/β-catenin signaling pathway.

Modifying the soil to affect the seed: role of stromal-derived matrix metalloproteinases in cancer progression

In the 1980's, as the importance of matrix metalloproteinases (MMPs) in cancer progression was discovered, it was recognized that in most tumors these proteases were abundantly and sometimes exclusively expressed not by tumor cells, but by normal host-derived cells like fibroblasts, vascular endothelial cells, myofibroblasts, pericytes or inflammatory cells that contribute to the tumor microenvironment. Later experiments in mice deficient in specific MMPs revealed that host-derived MMPs play a critical role not only in tumor cell invasion, but also in carcinogenesis, angiogenesis, vasculogenesis and metastasis. Tumor cells secrete many factors, cytokines and chemokines that directly or indirectly increase the expression of these MMPs in the tumor microenvironment where they exert extracellular matrix (ECM) degrading and sheddase activities. The knowledge of the complex role that stromal-derived MMPs play in the interaction between tumor cells and stromal cells should allow us to consider specific windows in cancer treatment when MMP inhibition could have a valuable therapeutic effect.

Matrix metalloproteinases play an active role in Wnt1-induced mammary tumorigenesis

The Wnt signaling transduction pathway plays a critical role in the pathogenesis of several murine and human epithelial cancers. Here, we have used mouse mammary tumor virus (MMTV)-Wnt1 transgenic mice, which develop spontaneous mammary adenocarcinoma, to examine whether matrix metalloproteinases (MMPs)--a family of extracellular proteases implicated in multiple steps of cancer progression--contributed to Wnt1-induced tumorigenesis. An analysis of the expression of several MMPs by RT-PCR and in situ hybridization revealed an increase in the expression of MMP-2, MMP-3, MMP-9, MMP-13, and MT1-MMP (MMP-14) in hyperplastic glands and in mammary tumors of MMTV-Wnt1 transgenic mice. Interestingly, whereas MMP-2, MMP-3, and MMP-9 were exclusively expressed by stromal cells in mammary tumors, MMP-13 and MT1-MMP were expressed by transformed epithelial cells in addition to the tumor stroma. To determine whether these MMPs contributed to tumorigenesis, MMTV-Wnt1 mice were crossed with transgenic mice overexpressing tissue inhibitor of metalloproteinase-2-a natural MMP inhibitor-in the mammary gland. In the double MMTV-Wnt1/tissue inhibitor of metalloproteinases-2 transgenic mice, we observed an increase in tumor latency and a 26.3% reduction in tumor formation. Furthermore, these tumors grew at a slower rate, exhibited an 18% decrease in proliferative rate, and a 12.2% increase in apoptotic rate of the tumor cells in association with a deficit in angiogenesis when compared with tumors from MMTV-Wnt1 mice. Thus, for the first time, the data provides evidence for the active role of MMPs in Wnt1-induced mammary tumorigenesis.

The contribution of bone marrow-derived cells to the tumor vasculature in neuroblastoma is matrix metalloproteinase-9 dependent

The contribution of the tumor stroma to cancer progression has been increasingly recognized. We had previously shown that in human neuroblastoma tumors orthotopically implanted in immunodeficient mice, stromal-derived matrix metalloproteinase-9 (MMP-9) contributes to the formation of a mature vasculature by promoting pericyte recruitment along endothelial cells. Here we show that MMP-9 is predominantly expressed by bone marrow-derived CD45-positive leukocytes. Using a series of bone marrow transplantation experiments in MMP-9(+/+) and MMP-9(-/-) mice xenotransplanted with human neuroblastoma tumors, we show that bone marrow-derived MMP-9 is critical for the recruitment of leukocytes from bone marrow into the tumor stroma and for the integration of bone marrow-derived endothelial cells into the tumor vasculature. Expression of MMP-9 by bone marrow-derived cells in the tumor stroma is also critical for the formation of a mature vasculature and coverage of endothelial cells with pericytes. Furthermore, in primary human neuroblastoma tumor specimens of unfavorable histology, we observed a higher level of tumor infiltration with MMP-9 expressing phagocytic cells and a higher degree of coverage of endothelial cells by pericytes when compared with tumor specimens with a favorable histology. Taken together, the data show that in neuroblastoma, MMP-9 plays a critical role in the recruitment of bone marrow-derived cells to the tumor microenvironment where they positively contribute to angiogenesis and tumor progression.

Considering the critical interface between tumor cells and stromal cells in the search for targets for anticancer therapy

In this issue of Cancer Cell, a paper by Lynch et al. demonstrates how the careful study of changes that occur at the interface between tumor cells and stromal cells led to the discovery of a new function for matrix metalloproteinase-7 (MMP-7) in the formation of osteolytic lesions in prostate cancer. The data suggest that MMP-7 inhibition could be a therapeutic target in prostate cancer.

Matrix metalloproteinases (MMP) and cathepsin K contribute differently to osteoclastic activities

The best established proteolytic event of osteoclasts is bone matrix solubilization by the cysteine proteinase cathepsin K. Here, however, we draw the attention on osteoclastic activities depending on matrix metalloproteinases (MMPs). We discuss the observations supporting that MMPs contribute significantly to bone matrix solubilization in specific areas of the skeleton and in some developmental and pathological situations. Our discussion takes into account (1) the characteristics of the bone remodeling persisting in the absence of cathepsin K, (2) the ultrastructure of the resorption zone in response to inactivation of MMPs and of cathepsin K in different bone types, (3) bone resorption levels in MMP knockout mice compared to wild-type mice, (4) the identification of MMPs in osteoclasts and surrounding cells, and (5) the effect of different bone pathologies on the serum concentrations of specific collagen fragments believed to discriminate between cathepsin K and MMP cleavage. Next, we provide evidence that MMPs are very critical for osteoclast migration, thereby controlling also the cell-matrix interactions required for cell attachment/detachment. The evidence supporting this role is based on a model of osteoclast recruitment in primitive long bones, an assay of osteoclast invasion through collagen gel, and the effect of proteinase inhibitors/knockouts in these models. Furthermore, we mention observations indicating a role of MMPs in initiation of bone resorption. Finally, we emphasize the many distinct ways MMPs may alter focally the extracellular environment thereby regulating the osteoclast behavior. Although the understanding of MMPs in osteoclast biology is rapidly expanding, it is suspected that important roles remain to be discovered.

TGF-beta3-induced palatogenesis requires matrix metalloproteinases

Cleft lip and palate syndromes are among the most common congenital malformations in humans. Mammalian palatogenesis is a complex process involving highly regulated interactions between epithelial and mesenchymal cells of the palate to permit correct positioning of the palatal shelves, the remodeling of the extracellular matrix (ECM), and subsequent fusion of the palatal shelves. Here we show that several matrix metalloproteinases (MMPs), including a cell membrane-associated MMP (MT1-MMP) and tissue inhibitor of metalloproteinase-2 (TIMP-2) were highly expressed by the medial edge epithelium (MEE). MMP-13 was expressed both in MEE and in adjacent mesenchyme, whereas gelatinase A (MMP-2) was expressed by mesenchymal cells neighboring the MEE. Transforming growth factor (TGF)-beta3-deficient mice, which suffer from clefting of the secondary palate, showed complete absence of TIMP-2 in the midline and expressed significantly lower levels of MMP-13 and slightly reduced levels of MMP-2. In concordance with these findings, MMP-13 expression was strongly induced by TGF-beta3 in palatal fibroblasts. Finally, palatal shelves from prefusion wild-type mouse embryos cultured in the presence of a synthetic inhibitor of MMPs or excess of TIMP-2 failed to fuse and MEE cells did not transdifferentiate, phenocopying the defect of the TGF-beta3-deficient mice. Our observations indicate for the first time that the proteolytic degradation of the ECM by MMPs is a necessary step for palatal fusion.

Tissue inhibitors of matrix metalloproteinases in cancer

Tissue inhibitors of metalloproteinases (TIMPs) play a key regulatory role in the homeostasis of the extracellular matrix (ECM) by controlling the activity of matrix metalloproteinases (MMPs). Some TIMPs have a second function as well, unrelated to their antiMMP activity, which affects cell proliferation and survival. The role of these inhibitors in cancer has been the subject of extensive investigations that have examined their biological activity in tumor growth, invasion, metastasis and angiogenesis, as well as their potential use in the diagnosis and treatment of human cancer.

Tissue inhibitors of metalloproteinases (TIMP) in invasion and proliferation

Tissue inhibitors of matrix metalloproteinases (TIMPs) are a family of natural inhibitors that control the activity of matrix metalloproteinases (MMPs) in the extracellular matrix (ECM). Four members of this family have been so far characterized in a variety of species. These inhibitors share a similar structural feature characterized by the presence of 12 cysteine residues involved in disulfide bonds and a similar function by their ability to form inhibitory complexes with MMPs. The role of TIMPs in cancer has been the subject of conflicting reports with an antitumor activity reported by some investigators and a growth stimulation activity reported by others. Here we will discuss a series of data obtained in our laboratory supporting a role of TIMPs not only as inhibitors of invasion but also as regulators of cell growth. Using placental development as an example of a regulated invasive process, we have observed that the levels of TIMP-2 and TIMP-3 steadily increase between day 14.5 and 17.5 post-coitus. TIMPs are selectively expressed by spongiotrophoblastic cells that separate the labyrinthine zone, rich in fetal blood vessels and maternal blood sinuses, from the zone of giant cells forming the border between fetal and maternal tissues. TIMPs are also potent inhibitors of tumor growth in vivo. In melanoma cells, we have previously reported that over-expression of TIMP-2 inhibits the growth of tumors implanted in the skin of scid mice. This growth inhibition seems independent of angiogenesis but dependent on the collagen matrix. We observed that in the presence of fibrillar type I collagen, melanoma cells undergo a growth arrest at the G1 to S interphase transition of the cell cycle. This arrest is specific to the fibrillar structure of collagen because it is not observed in the presence of non-fibrillar collagen or other ECM components. It is associated with a specific upregulation of the cyclin inhibitor p27KIP1. The data therefore indicate that anchorage independent cells remain sensitive to growth regulatory signals that originate from the ECM and that these signals can specifically block tumor cell cycle. Thus our concept of the role of protease inhibitors such as TIMPs in cancer has substantially changed from an initial focus on inhibition of tumor invasion and metastasis to a broader focus on being molecules that--via their function as regulators of the ECM homeostasis--can control tumor cell growth.

Tissue inhibitor of metalloproteinases-2 is expressed in the interstitial matrix in adult mouse organs and during embryonic development

Tissue inhibitor of metalloproteinases-2 (TIMP-2) is a member of a family of inhibitors of matrix-degrading metalloproteinases. A better insight into the role of this inhibitor during development and in organ function was obtained by examining the temporospatial expression of TIMP-2 in mice. Northern blot analysis indicated high levels of TIMP-2 mRNA in the lung, skin, reproductive organs, and brain. Lower levels of expression were found in all other organs with the exception of the liver and gastrointestinal tissue, which were negative of these tissues with complete absence of TIMP-2 mRNA in the epithelium. In the testis, TIMP-2 was present in the Leydig cells, and in the brain, it was expressed in pia matter and in neuronal tissues. TIMP-2 expression in the placenta increased during late gestation and was particularly abundant in spongiotrophoblasts In mouse embryo (day 10.5-18.5), TIMP-2 mRNA was abundant in mesenchymal tissues that surrounded developing epithelia and maturing skeleton. The pattern of expression significantly differs from that observed with TIMP-1 and TIMP-3, therefore, suggesting specific roles for each inhibitor during tissue remodeling and development.

Overexpression of tissue inhibitor of metalloproteinases-2 retroviral-mediated gene transfer in vivo inhibits tumor growth and invasion

We have demonstrated previously that overexpression of tissue inhibitor of metalloproteinases-2 (TIMP-2), an inhibitor of matrix-degrading metalloproteinases, not only inhibits the invasive and metastatic behavior of tumor cells but also significantly decreases tumor growth in vivo (Y. A. DeClerck et at, Cancer Res., 52: 701-708, 1992). This latter effect was found to be dependent on the ability of TIMP-2 to prevent the degradation of the collagen matrix (A. M. Montgomery et al., Cancer Res., 54: 5467-5473, 1994). In this report, we have overexpressed TIMP-2 in tumor tissue by retroviral-mediated gene transfer into tumor cells by co-injecting s.c. in nude mice tumorigenic c-Ha-ras-transfected rat embryo fibroblasts with irradiated packaging cells producing high titer retroviral vectors containing the human TIMP-2 cDNA. The growth rate of tumors derived from cells co-injected with the TIMP-2 vector producer cells was significantly slower than the growth rate of tumors derived from cells co-injected with packaging cells producing a retrovirus containing the Escherichia coli beta-galactosidase gene. The transduction efficiency was estimated at 13%, and the production of a functional human TIMP-2 in tumor cells transduced with the TIMP-2-containing vector was documented. Furthermore, histological analysis of tumors derived from tumor cells co-injected with the TIMP-2 vector producer cells revealed the presence of a thick connective tissue capsule and a lack of local invasion. The data indicate that retroviral-mediated transduction of TIMP-2 cDNA into a limited population of tumor cells in vivo is sufficient to increase the accumulation of connective tissue proteins in tumor tissue, to inhibit growth, and to prevent local invasion.

Matrix metalloproteinases are obligatory for the migration of preosteoclasts to the developing marrow cavity of primitive long bones

A key event in bone resorption is the recruitment of osteoclasts to future resorption sites. We follow here the migration of preosteoclasts from the periosteum to the developing marrow cavity of fetal mouse metatarsals in culture, and investigate the role of proteinases and demineralization in this migration. Our approach consisted in testing inhibitors of proteinases and demineralization on the migration kinetics. Migration was monitored by histomorphometry and the (pre)osteoclasts were identified by their tartrate resistant acid phosphatase (TRAP) activity. At the time of explantation, TRAP+ cells (all mononucleated) are detected only in the periosteum, and the core of the diaphysis (future marrow cavity) consist of calcified cartilage. Upon culture, TRAP+ cells (differentiating progressively into multinucleated osteoclasts) migrate through a seam of osteoid and a very thin and discontinuous layer of mineral, invade the calcified cartilage and transform it into a “marrow' cavity; despite the passage of maturing osteoclasts, the osteoid develops into a bone collar. The migration of TRAP+ cells is completely prevented by matrix metalloproteinase (MMP) inhibitors, but not by a cysteine proteinase inhibitor, an inhibitor of carbonic anhydrase, or a bisphosphonate. The latter three drugs inhibit, however, the resorptive activity of mature osteoclasts at least as efficiently as do the MMP inhibitors, as assessed in cultures of calvariae and radii. Furthermore, in situ hybridizations reveal the expression of 2 MMPs, gelatinase B (MMP-9 or 92 kDa type IV collagenase) in (pre)osteoclasts, and interstitial collagenase (MMP-13) in hypertrophic chondrocytes. It is concluded that only MMPs appear obligatory for the migration of (pre)osteoclasts, and that this role is distinct from the one MMPs may play in the subosteoclastic resorption compartment. We propose that this new role of MMPs is a major component of the mechanism that determines where and when the osteoclasts will attack the bone.