Paracrine/autocrine growth mechanisms in tumor metastasis.

The successful growth of metastatic tumor cells is due to their responses to local paracrine growth factors and inhibitors and their production and responses to autocrine growth factors. At early stages of metastatic progression, there is a tendency for many common malignancies to metastasize and grow preferentially at particular sites, suggesting that paracrine growth mechanisms may dominate the growth signals affecting metastatic cells. At later stages of metastatic progression, where widespread dissemination to various tissues and organs occurs, autocrine growth mechanisms may dominate the growth signals affecting metastatic cells. The progression of malignant cells to completely autonomous (acrine) states can ultimately occur, and at this stage of metastatic progression cell growth can be completely independent of growth factors or inhibitors. Various strategies have been developed to treat cancer that are based on the responses of malignant cells to growth factor or inhibitor analogs, anti-receptor antibodies, or antibody- or growth factor-toxin conjugates. Since the responses and expression of growth factor receptors can change during malignant progression, the development of cancer treatments using analogs of specific growth inhibitors or antagonists of growth factors, such as monoclonal antibodies or other agents, to block growth signaling mechanisms may only be useful at the early stages of malignant cancer progression before widespread metastasis of acrine cells occurs.     

Trophic factors and central nervous system metastasis

To metastasize to the central nervous system (CNS) malignant cells must attach to brain microvessel endothelial cells, respond to brain endothelial cell-derived motility factors, respond to CNS-derived invasion factors and invade the blood-brain barrier (BBB), and finally, respond to CNS survival and growth factors. Trophic factors such as the neurotrophins play an important role in tumor cell invasion into the CNS and in the survival of small numbers of malignant cells under stress conditions. Trophic factors promote BBB invasion by enhancing the production of basement membrane-degrading enzymes in neurotrophin-responsive cells. The expression of certain neurotrophin receptors on brain-metastasic neuroendrocrine cells occurs in relation to their invasive and survival properties. For example, CNS-metastatic melanoma cells respond to particular neurotrophins (nerve growth factor, neurotrophin-2) that can be secreted by normal cells within the CNS. In addition, a paracrine form of transferrin is important in CNS metastasis, and brain-metastatic cells respond to low levels of transferrin and express high levels of transferrin receptors. CNS-metastatic tumor cells can also produce autocrine factors and inhibitors that influence their growth, invasion and survival in the brain. Synthesis of paracrine factors and cytokines may influence the production of trophic factors by normal brain cells adjacent to tumor cells. Moreover, we found increased amounts of neurotrophins in brain tissue at the invasion front of human melanoma tumors in CNS biopsies. Thus the ability to form metastatic colonies in the CNS is dependent on tumor cell responses to trophic factors as well as autocrine and paracrine growth factors and probably other underdescribed factors.     

Malignant melanoma metastasis to brain: role of degradative enzymes and responses to paracrine growth factors

Mouse and human melanoma cells metastatic to the brain express degradative enzyme activities that are used for invasion of brain basement membrane and parenchyma. Compared to poorly metastatic or lung- or ovary-metastatic murine melanoma lines, the brain-metastatic sublines secreted higher levels of a variety of degradative enzymes. Brain-metastatic murine and human melanoma cells also degraded subendothelial basement membrane and reconstituted basement membrane at rates higher than other metastatic melanoma cells. In some cases these degradative activities in mouse and human melanoma cells can be induced by paracrine factors known to be present in the brain parenchyma, such as nerve growth factor (NGF). NGF stimulates the expression of degradative enzymes, such as the endo--glucuronidase heparanase, that are important in basement membrane penetration but this factor does not stimulate melanoma cell growth. The growth of brain-metastasizing melanoma cells appears to be stimulated by other paracrine growth factors, such as paracrine transferrin. Melanoma cells metastatic to brain express higher numbers of transferrin receptors and respond and proliferate at lower concentrations of transferrin than do melanoma cells metastatic to other sites or poorly metastatic melanoma cells. The results suggest that degradation and invasion of brain basement membrane and responses to paracrine neurotrophins and paracrine transferrins are important properties in brain metastasis of murine and human malignant melanoma cells.     

Differential stimulation of the growth of lung-metastasizing tumor cells by lung (paracrine) growth factors: identification of transferrin-like mitogens in lung tissue-conditioned medium.

Certain metastatic tumor cells successfully form metastases at particular organ sites, and their organ colonization properties cannot be explained by mechanical or anatomic factors. These tumor cells possess the ability to colonize such sites through preferential adhesion to organ microvessel endothelial cells, preferential organ invasion by expression of particular degradative enzymes and response to organ motility factors, and preferential organ growth by response to growth factors present at relatively higher concentrations in the target organ. The likelihood that target organ-associated growth factors exist and are important in metastatic colonization has been approached by studying the mitogenic effects of target organ extracts, fragments, or conditioned media on poorly and highly metastatic tumor cells that show organ preference of metastasis. We previously described the isolation of a major organ-derived (paracrine) growth factor from lung tissue-conditioned medium. Characterization of this mitogen has demonstrated that it is a transferrin or a transferrin-like glycoprotein, and antibodies to transferrin can remove significant growth activity from lung tissue-conditioned medium. Further demonstration of the existence and characterization of metastasis-associated organ (paracrine) growth factors and their receptors will be helpful in understanding the organ preference of metastasis.     

Growth factor independence and growth regulatory pathways in human melanoma development

Summary This review concentrates on growth autonomy of tumor cells in relation to tumor progression. Human malignant melanoma serves as an example for progressive growth factor independence at subsequent stages of tumor progression. Mechanisms by which malignant cells acquire growth factor independence are discussed. In melanoma, deregulation of growth regulatory pathways has been described on four levels: 1) aberrant production of autocrine growth factors that substitute for exogenous growth factors (basic fibroblast growth factor [bFGF]); 2) alterations in the response to negative autocrine growth factors (interleukin [IL]-6 and transforming growth factor [TGF]-); 3) overexpression of epidermal growth factor receptors (EGF-R); and 4) alterations of cellular protooncogenes involved in signal transduction (RAS, MYB) and growth suppression (p53). In addition to bFGF and IL-6, multiple other growth factor genes are activated in malignant melanoma cells but not normal melanocytes. These include both chains of platelet-derived growth factor (PDGF), TGF-, IL-1, IL-8, and tumor necrosis factor (TNF)-. Of these, PDGF-B has been investigated in more detail. Melanoma-derived PDGF clearly does not act in a direct autocrine mode, but has important paracrine effects on normal tissue constituents, notably fibroblasts and endothelial cells, that are essential for tumor developmentin vivo. It is speculated that other melanoma-derived growth factors with as yet undefined functions similarly exert such paracrine or indirect autocrine effects that cannot be sufficiently addressed in studies on cultured cells.     

Paracrine growth regulation of human colon carcinoma organ-specific metastasis

Summary The process of cancer metastasis consists of a series of steps resulting in the spread of malignant cells beyond the site of origin and formation of metastases in distant organs. The outcome of this nonrandom process depends, in part, on the interaction of unique tumor cells with a compatible organ microenvironment. The molecular basis of the intrinsic capacity of distinct malignant cells to colonize specific organs and the degree to which host factors influence this process is under intense investigation. Biological analyses of human colon carcinoma tumors obtained from surgical specimens and implanted orthotopically into athymic nude mice revealed that these tumors are heterogeneous for metastatic properties. Moreover, recent evidence using this model suggest that whereas nonmetastatic and highly metastatic cells can grow at local sites, growth in the secondary liver-specific site was associated only with highly metastatic HCC cells. These cells also respond to mitogenic signals produced by damaged normal tissues, suggesting that physiological signals can be utilized by neoplastic cells. Molecular characterization of highly metastatic HCC cells selected in the nude mouse model as well asin situ mRNA hybridization of archival HCC surgical specimens for specific growth factor receptors correlated with the malignant cell's ability to respond to organ-specific growth factors. This article will focus on biological and molecular evidence supporting the hypothesis that organ-derived, paracrine growth factors regulate the site-specific growth of receptive malignant cells that possess the appropriate receptors.     

Constitutive c-Met signaling through a nonautocrine mechanism promotes metastasis in a transgenic transplantation model

Normal cells are dependent on exogenous, receptor-mediated growth stimulation for cell cycle entry and progression, providing a critical homeostatic mechanism regulating cellular proliferation. In contrast, tumor cells acquire some degree of growth signal autonomy, often through their ability to produce growth factors as well as their receptors (autocrine signaling). Recently, data have begun to emerge implicating heterotypic signaling between diverse cell types within a tumor in the genesis and progression of cancer; however, current experimental approaches in vivo have not adequately addressed this critical relationship. Here we used transgenic mice overexpressing hepatocyte growth factor/scatter factor (HGF/SF), or its growth antagonist NK2, as genetically modified hosts for transplantation of tumor cells expressing their receptor, c-Met, to directly assess the contribution of heterotypic signaling to metastatic colonization. We demonstrate that metastatic potential under nonautocrine signaling conditions (i.e., where tumor cells expressing c-Met are transplanted into transgenic hosts producing HGF/SF) rivaled that observed under conditions of autocrine signaling (i.e., where tumor cells expressing both HGF/SF and c-Met are transplanted into wild-type hosts). HGF/SF and NK2 were not functionally equivalent in vivo. Attenuation of NK2-associated growth inhibition by the presence of an HGF/SF-Met autocrine loop uncovered a shift in metastatic site preference from lung to liver only in NK2-transgenic hosts, a qualitative behavioral alteration likely detectable only through the genetic approach used here. Our data demonstrate that growth factors not intrinsic to malignant cells can have profound effects on metastatic efficiency in vivo and provide experimental support of a role for heterotypic signaling in tumor progression.     

Tumor and host molecules important in the organ preference of metastasis

Many cancers metastasize nonrandomly to particular distant sites, and their colonization properties cannot be explained by mechanical considerations, such as arrest of tumor cells in the first microcirculatory network encountered. Metastatic cells that show a high propensity to metastasize to certain organs adhere at higher rates to microvessel endothelial cells isolated from these target sites, invade into target tissue at higher rates and respond better to paracrine growth factors from the target site. These properties depend on multiple tumor cell, host cell, and stromal molecules that are differentially expressed by particular tumor and organ cells and by the extracellular matrix. For example, some of the adhesion molecules involved in tumor cell-endothelial cell adhesion have been identified on both tumor and host cells. Among them are: integrins, endogenous lectins, annexins, and other molecules. The invasive properties of particular tumor cells are dependent on the production of degradative enzymes, such as metalloproteinases, heparanase and cys and ser proteinases, and on responses to organ-derived paracrine and autocrine motility factors. The subsequent growth of particular tumor cells at certain organ sites is determined, in part, by their responses to organ paracrine growth factors and the organ extracellular matrix. Collectively these factors appear to determine the organ colonization properties of blood-borne metastatic cells.     

Neurotensin (NTS) and its receptor (NTSR1) causes EGFR,HER2 and HER3 over-expression and their autocrine/paracrine activation in lung tumors,confirming responsiveness to erlotinib

Alterations in the signaling pathways of epidermal growth factor receptors (HERs) are associated with tumor aggressiveness. Neurotensin (NTS) and its high affinity receptor (NTSR1) are up regulated in 60% of lung cancers. In a previous clinical study, NTSR1 overexpression was shown to predict a poor prognosis for 5 year overall survival in a selected population of stage I lung adenocarcinomas treated by surgery alone. In a second study, shown here, the frequent and high expression of NTSR1 was correlated with a pejorative prognosis in 389 patients with stage I to III lung adenocarcinoma, and was an independent prognosis marker.Interactions between NTS and NTSR1 induce pro-oncogenic biological effects associated with neoplastic processes and tumor progression. Here we highlight the cellular mechanisms activated by Neurotensin (NTS) and its high affinity receptor (NTSR1) contributing to lung cancer cell aggressiveness. We show that the NTS autocrine and/or paracrine regulation causes EGFR, HER2, and HER3 over-expression and activation in lung tumor cells. The EGFR and HER3 autocrine activation is mediated by MMP1 activation and EGF “like” ligands (HB-EGF, Neuregulin 1) release. By establishing autocrine and/or paracrine NTS regulation, we show that tumor growth is modulated according to NTS expression, with a low growth rate in those tumors that do not express NTS. Accordingly, xenografted tumors expressing NTS and NTSR1 showed a positive response to erlotinib, whereas tumors void of NTSR1 expression had no detectable response. This is consistent with the presence of a NTS autocrine loop, leading to the sustained activation of EGFR and responsible for cancer aggressiveness.We propose the use of NTS/NTSR1 tumor expression, as a biomarker for the use of EGFR tyrosine kinase inhibitors in patients lacking EGFR mutation.     

Hepatocyte growth factor/MET in cancer progression and biomarker discovery

Signaling driven by hepatocyte growth factor (HGF) and MET receptor facilitates conspicuous biological responses such as epithelial cell migration, 3‐D morphogenesis, and survival. The dynamic migration and promotion of cell survival induced by MET activation are bases for invasion–metastasis and resistance, respectively, against targeted drugs in cancers. Recent studies indicated that MET in tumor‐derived exosomes facilitates metastatic niche formation and metastasis in malignant melanoma. In lung cancer, gene amplification‐induced MET activation and ligand‐dependent MET activation in an autocrine/paracrine manner are causes for resistance to epidermal growth factor receptor tyrosine kinase inhibitors and anaplastic lymphoma kinase inhibitors. Hepatocyte growth factor secreted in the tumor microenvironment contributes to the innate and acquired resistance to RAF inhibitors. Changes in serum/plasma HGF, soluble MET (sMET), and phospho‐MET have been confirmed to be associated with disease progression, metastasis, therapy response, and survival. Higher serum/plasma HGF levels are associated with therapy resistance and/or metastasis, while lower HGF levels are associated with progression‐free survival and overall survival after treatment with targeted drugs in lung cancer, gastric cancer, colon cancer, and malignant melanoma. Urinary sMET levels in patients with bladder cancer are higher than those in patients without bladder cancer and associated with disease progression. Some of the multi‐kinase inhibitors that target MET have received regulatory approval, whereas none of the selective HGF‐MET inhibitors have shown efficacy in phase III clinical trials. Validation of the HGF‐MET pathway as a critical driver in cancer development/progression and utilization of appropriate biomarkers are key to development and approval of HGF‐MET inhibitors for clinical use.     

Autocrine and paracrine growth inhibitors of breast cancer cells

Breast epithelial cells produce both mitogens and growth inhibitors which are involved in the control of mammary gland development through autocrine and paracrine pathways. While the mechanisms of action of several growth factors have been well established and related strategies proposed for breast cancer therapy, little is known concerning growth inhibitors. In this review, we present an overview of current information about major autocrine and paracrine growth inhibitors of breast epithelial cells, and we discuss their potential functions in the control of breast cancer development.     

Fibroblast growth factor overexpressing breast carcinoma cells as models of angiogenesis and metastasis

Summary Progression of breast cancer from an estrogen-dependent, slowly growing tumor amenable to tamoxifen treatment to an aggressive, metastatic, estrogen-independent phenotype has been mimicked by the transfection of MCF-7 breast carcinoma cells with fibroblast growth factors 1 or 4. FGF-transfected cells are aggressively tumorigenic in ovariectomized or tamoxifen-treated nude mice, conditions under which the parental cells would not produce tumors. When detection of metastasis was enhanced bylacZ transfection, the FGF-transfected MCF-7 cells were reliably metastatic to lymph nodes and frequently metastatic to lungs, in further contrast to parental cells. An antiangiogenic drug, AGM-1470, given to mice bearing tumors produced by FGF-transfected MCF-7 cells, produced a decrease in tumor size. The decreased tumor size was not as marked as that produced by treatment with pentosan polysulfate, an agent which would abrogate all autocrine or paracrine effects of the transfected FGF. Thus, increased angiogenesis may be a component of the phenotypic change produced by the FGF transfection, but other autocrine or paracrine effects may also be important.Since a clonal FGF-4 andlacZ doubly-transfected cell line, MKL-4, progressively lost expression of the transfectedlacZ gene in individual cells, we performed successive rounds of fluorescence-activated cell sorting to select high-expressing cells. High-expressing cell populations thus obtained rapidly lost expression of ß-gal activity in continued culture. High ß-gal expressing clonal cell lines of MKL-4 cells established by either one or two rounds of low-density cloning also lostlacZ expression with continued culture. Southern analysis of DNA fromlacZ transfected cell lines showed the transfected sequences to be present and grossly intact in both high and low expressing populations. However, Northern analysis revealed that high-expressing populations of MKL-4 cells contained the mostlacZ mRNA, implying that in the unstable MKL-4 cell line, individual cells are down-regulating mRNA levels oflacZ. StablelacZ expression has been obtained in other FGF-transfected and parental MCF-7 cell lines using the same expression vector. Thus, the MKL-4 cell line is down-regulating mRNA encoding the transfected gene through a mechanism not dependent on the CMV promotor utilized in the expression vector. This evidence suggests thatlacZ expression is not a benign modification in certain cells.     

Genetic and clonal dissection of osteosarcoma progression and lung metastasis

Osteosarcoma is a primary malignant bone tumor that has a high potential to metastasize to lungs. Little is known about the mechanisms underlying the dissemination of OS cancer cells to lungs. We performed whole exome sequencing of 13 OS primary tumors, with matched lung metastases and normal tissues. Phylogenetic analyses revealed that lung metastatic tumors often harbor clones that are nonexistent or rare in the matched primary OS tumors. Spatially and temporally separated lung metastases were from parallel seeding events with a polyphyletic pattern. Loss of TP53 or RB1 is among the early events during OS tumorigenesis, while loss of PTEN is involved at the later stages associated with lung metastases. Finally, KEAP1 was identified as a novel biomarker for increased metastatic risk. Patients whose primary tumors harbored KEAP1 amplification have significantly poorer lung‐metastasis free survival. This finding was validated in two independent datasets. Further, in vitro experiments exhibited that KEAP1 depletion suppressed the invasion of OS cells. Our findings uncover the patterns of clonal evolution during OS progression and highlight KEAP1 as a novel candidate associated with the risk of lung metastasis in OS patients.     

Autocrine factors that sustain glioma invasion and paracrine biology in the brain microenvironment

Invasion is a defining hallmark of glioblastoma multiforme, just as metastasis characterizes other high-grade tumors. Glial tumors invariably recur due to the regrowth of invasive cells, which are unaffected by standard treatment modalities. Drivers of glioma invasion include autocrine signals propagated by secreted factors that signal through receptors on the tumor. These secreted factors are able to diffuse through the peritumoral stroma, thereby influencing parenchymal cells that surround the tumor mass. Here we describe various autocrine motility factors that are expressed by invasive glioma cells and explore the effects that they may have on normal cells present in the path of invasion. Conversely, normal brain parenchymal cells secrete ligands that can stimulate receptors on invasive glioma cells and potentially facilitate glioma invasion or create a permissive microenvironment for malignant progression. Parallel observations have been made for solid tumors of epithelial origin, in which parenchymal and stromal cells either support or suppress tumor invasion. Most autocrine and paracrine interactions involved in glioma invasion constitute known signaling systems in stages of central nervous system development that involve the migration of precursor cells that populate the developing brain. Key paracrine interactions between glioma cells and the brain microenvironment can influence glioma pathobiology and therefore contribute to its poor prognosis. Current therapies for glioma that could have an impact on paracrine communication between tumors and normal cells are discussed. We suggest that cells in the normal brain parenchyma be considered as potential targets for adjuvant therapies to control glioma growth because such cells are less likely to develop resistance than glioma cells.     

EMMPRIN regulates tumor growth and metastasis by recruiting bone marrow-derived cells through paracrine signaling of SDF-1 and VEGF

EMMPRIN, a cell adhesion molecule highly expressed in a variety of tumors, is associated with poor prognosis in cancer patients. Mechanistically, EMMPRIN has been characterized to contribute to tumor development and progression by controlling the expression of MMPs and VEGF. In the present study, by using fluorescently labeled bone marrow-derived cells (BMDCs), we found that the down-regulation of EMMPRIN expression in cancer cells reduces tumor growth and metastasis, and is associated with the reduced recruitment of BMDCs. Further protein profiling studies suggest that EMMPRIN controls BMDC recruitment through regulating the secretion of soluble factors, notably, VEGF and SDF-1. We demonstrate that the expression and secretion of SDF-1 in tumor cells are regulated by EMMPRIN. This study reveals a novel mechanism by which EMMPRIN promotes tumor growth and metastasis by recruitment of BMDCs through controlling secretion and paracrine signaling of SDF-1 and VEGF.     

VEGF-C-VEGFR3/Flt4 axis regulates mammary tumor growth and metastasis in an autocrine manner

Purpose: Lymphangiogenic factors, such as vascular endothelial growth factor-C (VEGF-C) and VEGFC-D, and their receptor, VEGF receptor-3 (VEGFR3), play a pivotal role in the promotion of metastasis to regional lymph nodes. In the present study we explored the role of VEGF-C as an autocrine growth factor for breast cancer cells. Methods: We examined the expression of VEGF-C and VEGFR3 in mammary tumor cells lines and examined whether blocking the VEGF-C-VEGFR3/Flt4 pathway using a VEGFR3 antagonist would inhibit proliferation of mammary tumor cells resulting in a decrease in tumor growth and metastasis. Results: We report expression of VEGF-C and its receptor VEGFR3 by mammary tumor cells, and their association with aggressiveness. Inhibition of VEGF-C-VEGFR3/Flt4 in mammary tumor cells decreased their proliferation and survival. Mammary tumor bearing mice treated with a VEGFR3 antagonist showed a significant decrease in tumor growth and the extent of spontaneous and experimental lung metastases. Conclusion: These findings demonstrate the VEGF-C-VEGFR3/Flt4 autocrine signaling pathway regulates mammary tumor cell survival and proliferation and that neutralization of VEGFR3 signaling might lead to development of a novel therapeutic approach for malignant breast cancer.     

巨噬细胞与肿瘤转移

临床及基础研究已经发现,巨噬细胞能够诱发肿瘤并促进其恶化进程。巨噬细胞的聚集提示肿瘤预后不佳;在肿瘤起始阶段,巨噬细胞创造炎症环境,致细胞突变并促进其生长。随着肿瘤的发展,巨噬细胞在肿瘤即将转移至的部位,为肿瘤细胞的到来提前制造转移前生态位;继而,巨噬细胞又在肿瘤转移后的初始增殖中发挥重要的营养作用。目前,随着对巨噬细胞相关研究的不断进展,相信必然能为肿瘤治疗开辟新的方向。     

Molecular mechanisms of cancer metastasis: tumor and host properties and the role of oncogenes and suppressor genes

The natural progression of benign tumors to malignancy and metastasis is characterized by their ability to circumvent microenvironmental controls on cellular proliferation, diversity, and positioning, and evolve into heterogeneous phenotypes, a process that probably involves qualitative and quantitative changes in gene expression. The oncogenes and suppressor genes that can effect tumor progression may control important points in the regulation of sets of genes that are ultimately responsible for the cellular alterations seen in highly metastatic cells. Because many cancers metastasize nonrandomly to particular distant sites, their colonization properties cannot be explained by mechanical considerations, such as arrest of tumor cells in the first microcirculatory network encountered. Metastatic cells that show a high propensity to metastasize to certain organs adhere at higher rates to microvessel endothelial cells isolated from their target sites, invade into target tissue at higher rates, and respond better to paracrine growth factors from the target site. These properties depend on multiple tumor cell, host cell, and stromal molecules that are differentially expressed by particular tumor and organ cells and by the organ extracellular matrix. Together these tumor and host factors appear to determine the organ metastatic properties of malignant cells.     

Overexpression of lysosomal-type sialidase leads to suppression of metastasis associated with reversion of malignant phenotype in murine B16 melanoma cells

Increased sialylation in cell surface glycoproteins is one characteristic feature of cancer cells, particularly related to their metastatic potential and invasiveness. Expression of lysosomal-type sialidase, which plays a major role in hydrolysis of such sialo-glycoproteins, is therefore considered to have a great influence on malignant properties of cancer cells. To investigate whether the sialidase expression level is linked to the malignant phenotype, we transfected B16-BL6 murine melanoma cells, a highly invasive and metastatic line, with an expression vector harboring a rat lysosomal sialidase cDNA; then clones were isolated and examined for changes in biological character. Sialidase-overexpressing cells showed suppression of experimental pulmonary metastasis and tumor progression. The transfectants exhibited diminished cell growth, anchorage-independent growth and increased sensitivity to apoptosis induced by suspension culture or serum depletion in vitro, but no significant alterations in invasiveness, cell motility and cell attachment to fibronectin, collagen IV and laminin. Flow cytometric analysis with either peanut agglutinin (PNA) or Ricinus communis agglutinin (RCA) lectin revealed that desialylated forms of glycoproteins on the cell surfaces were increased. In particular, a desialylated form of a cell surface glycoprotein of 83 kDa was prominent in the transfectants, as determined by galactose oxidase labeling. These observations indicate that sialidase expression is inversely associated with metastatic potential and tumor growth in cancer cells, probably through a regulation mechanism that suppresses cell growth and anchorage-independent growth and promotes apoptosis with deprivation of cell anchorage.