Tumor Metastasis Formation Research Articles

Gangliosides as therapeutic targets for cancer.

Gangliosides, sialic acid-containing glycosphingolipids, have engendered great interest for more than 20 years in the search for target molecules of relevance for tumour growth and formation of metastases and as potential targets for immunotherapy. These molecules show large quantitative and structural variability, which is related to cell type and developmental stage. Their potential role in the formation of tumour metastases was suggested from data supporting that they are involved in cell growth regulation and in cell-cell and cell-matrix adhesion. Moreover, gangliosides are expressed on the cell surface and thereby are accessible for antibodies or other ganglioside-binding molecules to induce cell death, inhibit cell growth and/or inhibit formation of tumour metastasis. All tumours exhibit aberrant ganglioside expression. This includes overexpression of normal ganglioside constituents, which appears to be common among various tumours, and expression of gangliosides not found in normal adult tissue but often found during fetal development. The ganglioside composition of melanoma cells has been found to correlate with their metastatic potential and also to be selectively expressed in cells of a tumour mass and invading tumour cells. Passive immunotherapy using murine or murine/human chimeric monoclonal antiganglioside antibodies in their native form or combined with various effector molecules has been investigated. However, the vaccination strategy using native or structurally modified tumour-associated gangliosides in combination with adjuvants is currently the dominant method in clinical trials. The outcomes reported so far vary between type of tumour and treatment strategies. However, we believe that targeting gangliosides is as promising as any other immune therapeutic strategy, and basic research as well as clinical trials utilising new aspects is encouraged.

Regulation of endothelial cell integrin function and angiogenesis by COX-2, cAMP and Protein Kinase A.

Angiogenesis, the development of new blood vessels from preexisting vessels, is a key step in tumor growth, invasion and metastasis formation. Inhibition of tumor angiogenesis is considered as an attractive approach to suppress cancer progression and spreading. Adhesion receptors of the integrin family promote tumor angiogenesis by mediating cell migration, proliferation and survival of angiogenic endothelial cells. Integrins up regulated and highly expressed on neovascular endothelial cells, such as alphaVbeta3 and alpha5beta1, have been considered as relevant targets for anti-angiogenic therapies. Small molecular integrin antagonists or blocking antibodies suppress angiogenesis and tumor progression in many animal models, and some of them are currently being tested in cancer clinical trials as anti-angiogenic agents. COX-2 inhibitors exert anti-cancer effects, at least in part, by inhibiting tumor angiogenesis. We have recently shown that COX-2 inhibitors suppress endothelial cell migration and angiogenesis by preventing alphaVbeta3-mediated and cAMP/PKA-dependent activation of the small GTPases Rac and Cdc42. Here we will review the evidence for the involvement of vascular integrins in mediating angiogenesis and the role of COX-2 metabolites in modulating the cAMP/Protein Kinase A pathway and alphaVbeta3-dependent Rac activation in endothelial cells.

Overview of the clinical efficacy of investigational anticancer drugs.

The purpose of this overview was to make a broad inventory of investigational drugs for medicinal cancer treatment and, specifically, to indicate the evidence of clinical efficacy. Information was retrieved from electronic database searches in Medline and CANCERLIT and relevant published reviews. As the most recent findings are first reported as conference abstracts, an important basis for identification of new drugs and clinical results was a hand search of 13,392 abstracts from five major recent cancer conferences. A total of 209 investigational approaches or drugs were identified and classified into one of eight groups according to proposed mechanism of action. For 28 drugs/approaches survival data were available from randomized controlled trials. Statistically significant benefit was observed for only 12. In earlier phases no or modest anticancer activity was reported. It is speculated that the expanding knowledge in tumour biology might not easily translate into new substantially better anticancer drugs.

Enhancement of integrins by interleukin-1alpha, and their relationship with metastatic and invasive behavior of human pancreatic ductal adenocarcinoma cells.

Adhesion and invasion of tumor cells to extracellular matrix (ECM) proteins play an important role in tumor metastasis formation. We investigated the enhancement of adhesive and invasive behavior to ECM proteins of human pancreatic cancer cells by interleukin-1alpha (IL-1alpha) to examine the mechanism of adhesion and invasion of metastatic human pancreatic cancer cells to ECM proteins. The enhancement of integrin subunits by IL-1alpha was examined by cellular enzyme-linked immunosorbent assay (CELISA) in two metastatic human pancreatic cancer cell lines (BxPC-3 and SW1990) and two nonmetastatic pancreatic cancer cell lines (PaCa-2 and PANC-1). In addition, assays of cancer cell adhesion and invasion to ECM proteins were performed to investigate whether increased integrin expression affected the invasive interaction between cancer cells and the putative integrin ECM ligands. Expression of the alpha6 subunit by metastatic cancer cells was enhanced by IL-1alpha. Metastatic cancer cells also exhibited preferential adherence and invasion to laminin compared with nonmetastatic cancer cells, and this was enhanced by IL-1alpha. The enhancement of alpha6beta1-integrin by Il-1alpha acting through IL-1RI, as well as the expression of alpha6beta1-integrin, plays an important role in metastasis formation in pancreatic cancer

In vivo imaging of metastatic cancer with fluorescent proteins.

In vivo imaging of metastatic cancer with fluorescent proteins.

The Thy-1/Thy-1 Ligand Interaction Is Involved in Binding of Melanoma Cells to Activated Thy-1- Positive Microvascular Endothelial Cells

The expression of adhesion molecules on endothelial cells as well on tumor cells regulates and directs adhesion and transmigration of tumor cells through the endothelial cell barrier as one prerequisite to the formation of metastasis. Thy-1 is an inducible activation-associated cell-adhesion molecule on human dermal microvascular endothelial cells (HDMECs). In this study we investigated whether the Thy-1/Thy-1 ligand interaction may also play a role in adhesion of melanoma cells to endothelial cells. In situ, a strong Thy-1 expression on endothelial cells in melanoma and melanoma metastases was observed. In vitro, Thy-1 expression was stimulated by melanoma-cell-derived soluble factors, reflecting that Thy-1 expression in melanoma is not only due to a nonspecific inflammatory response. TNFα and bFGF were not responsible for this effect. In vitro and in situ a Thy-1 ligand was detected on melanoma cells. In cell-adhesion assays we showed the involvement of the Thy-1/Thy-1 ligand interaction in adhesion of melanoma cells to HDMECs. In summary, the data support that the study of the Thy-1/Thy-1 ligand interaction might give a more detailed insight into the regulation and direction of adhesion of melanoma cells to endothelial cells as one critical step in the formation of tumor metastasis.

Human macrophage metalloelastase worsens the prognosis of pancreatic cancer.

To evaluate the role of human macrophage metalloelastase (HME) in pancreatic cancer. HME, a member of the human matrix metalloproteinase family, possesses elastolytic activity and is critical for the degradation of extracellular matrix proteins. Inasmuch as tumor invasion and metastasis formation require lysis of extracellular matrix, HME plays a critical role in both processes. HME expression was analyzed by Northern blot analysis, reverse transcriptase-polymerase chain reaction, Western blot analysis, and immunohistochemistry in 39 pancreatic cancer tissues and 13 normal controls. The molecular data were related to clinicopathologic parameters and patient survival. In human pancreatic cancer, overexpression of HME mRNA was present in 25 of 39 pancreatic cancer tissues (64%) and in five pancreatic cancer cell lines. In contrast, low levels of HME mRNA expression were present in 13 normal pancreatic tissues samples. By Western blot analysis, high levels of HME were found in pancreatic cancer tissues and in the pancreatic cancer cell lines compared with the normal controls. Fifty-six percent of the cancer samples exhibited HME immunoreactivity in the cancer cells, and 63% in the stromal cells. Analysis of the survival data revealed that patients whose tumors exhibited HME mRNA overexpression lived significantly shorter compared with patients whose tumors did not overexpress HME. No relationship between HME expression and tumor stage, tumor grading, or presence of lymph node metastases was found. These findings indicate that HME participates in pancreatic cancer progression and that its presence worsens the prognosis. These data suggest a benefit of its inhibition in the treatment of pancreatic cancer.

Suppression of tumor angiogenesis through the inhibition of integrin function and signaling in endothelial cells: which side to target?

Tumor angiogenesis is an essential step in tumor progression and metastasis formation. Suppression of tumor angiogenesis results in the inhibition of tumor growth. Recent evidence indicates that vascular integrins, in particular alpha V beta 3, are important regulators of angiogenesis, including tumor angiogenesis. Integrin alpha V beta 3 antagonists, such as blocking antibodies or peptides, suppress tumor angiogenesis and tumor progression in many preclinical tumor models. The potential therapeutic efficacy of extracellular integrin antagonists in human cancer is currently being tested in clinical trials. Selective disruption of the tumor vasculature by high doses of tumor necrosis factor (TNF) and interferon gamma (IFN-gamma), and the antiangiogenic activity of nonsteroidal anti-inflammatory drugs are associated with the suppression of integrin alpha V beta 3 function and signaling in endothelial cells. Furthermore, expression of isolated integrin cytoplasmic domains disrupts integrin-dependent adhesion, resulting in endothelial cell detachment and apoptosis. These results confirm the critical role of vascular integrins in promoting endothelial cell survival and angiogenesis and suggest that intracellular targeting of integrin function and signaling may be an alternative strategy to extracellular integrin antagonists for the therapeutic inhibition of tumor angiogenesis.

Influences of the lysosomal associated membrane proteins (Lamp-1, Lamp-2) and Mac-2 binding protein (Mac-2-BP) on the prognosis of pancreatic carcinoma.

Lamps and Mac-2-BP are ligands of galectin-3, and they were suggested to influence tumor proliferation and metastasis formation. The authors studied the expression of Lamp-1, Lamp-2, and Mac-2-BP in pancreatic carcinoma and evaluated their influence on patient prognosis. Northern blot analysis, in situ hybridization, and immunohistochemistry were performed in 12 normal and 28 pancreatic carcinoma tissue samples and in pancreatic carcinoma cell lines. The molecular findings in the tumor samples were correlated with the prognosis and histopathologic tumor characteristics. In addition, in Lamp-1 transfected CAPAN-1 pancreatic carcinoma cells, cell proliferation was analyzed. Lamp-1, Lamp-2, and Mac-2-BP were overexpressed in 61% (1.6-fold increase, not significant), 71% (3.0-fold increase, P < 0.01), and 93% (5.6-fold increase, P < 0.01) of the pancreatic carcinoma samples. Lamp-1 and Lamp-2 immunoreactivity was present at the luminal side of the ductal carcinoma cells whereas Mac-2-BP immunoreactivity was diffusely spread over the whole cytoplasm and the nucleolus of ductal carcinoma cells. Correlation of the molecular data with clinical patient parameters revealed that patients whose tumors exhibited high Lamp-1 mRNA expression lived significantly longer (median, 17 months) after tumor resection than patients whose tumors exhibited low to moderate Lamp-1 mRNA levels (median, 8 months; P < 0.02). No relation between Lamp-2 and Mac-2-BP mRNA expression and any of the histopathologic parameters was found. Lamp-1 transfected CAPAN-1 cells showed decreased cell growth compared with the nontransfected cells. Lamp-1 might influence local tumor progression rather than the formation of tumor metastasis in pancreatic carcinoma, whereas Mac-2-BP and Lamp-2 seem to have little influence on these parameters in pancreatic carcinoma.

No effect of thromboxane A2 on the attachment of tumor cell lines MDA MB 231, DU145, and U937 to the basement membrane in an in-vitro model.

A broad body of evidence indicates the involvement of P450 TxA2 (thromboxane A2 synthetase) in tumor metastasis formation. A distinct function of the enzyme in this multistep process, however, is still unknown. Therefore the effect of TxA2 (thromboxane A2) on tumor cell adhesion to the basement membrane, a key event in metastasis formation, was investigated. A wide variety of compounds designed in our work group and identified as P450 TxA2 inhibitors were applied to several P450 TxA2-positive tumor cell lines to test their influence on tumor cell adhesion. For this purpose an in-vitro basement membrane adhesion model with the matrix gel preparation Matrigel was used. Most of the P450 TxA2 inhibitors tested had no effect on cell adhesion. Although two compounds significantly reduced tumor cell adhesion in a concentration-dependent manner, this was not related to P450 TxA2 inhibition. These data indicate that TxA2 might not be involved in the attachment of tumor cell lines to the basement membrane.

Incidence of chromosomal imbalances in advanced colorectal carcinomas and their metastases.

Comparative genomic hybridization (CGH) was used to screen 54 advanced colon carcinomas. i.e., 24 primary tumors and 30 metastases, for chromosomal alterations. Using a sensitive statistical method for the determination of DNA imbalances and histograms for analysis of the incidence of changes, we identified the DNA over-representation of chromosome 20q as the most common alteration being present in 100% of cases. High incidence deletions were observed on 18q21-18q23 (96%), 4q27-4q28 (96%), 4p14 (87%), 5q21 (81%), 1p21-1p22 (72%), 21q21 (74%), 6q16 (72%), 3p12 (66%), 8p24-8p21 (66%), 9p21 (64%), 11q22 (64%), and 14q13-14q21 (64%). Further frequent over-representation was found on 7q12-7q11.2 (75%), 16p11-16p12 (70%), 19p13 (70%), 9q34 (67%), 19q13 (67%), 13q34 (64%), 13q13 (64%), 17q21 (59%), 22q11 (61%), 8q24 (57%), and 1q21 (57%). Pronounced DNA gains and losses being defined as regions in which the ratio profiles exceeded the values of 1.5 and 0.5, respectively, frequently colocalized with peaks of incidence curve. The use of difference histograms for the comparison of tumor subgroups as well as case-by-case histogram for the analysis of 15 paired tumor samples identified several of the above alterations as relevant for tumor progression and metastasis formation. The study identified additional loci and delineates more precisely those that have been previously reported. For comparative purposes, we have made our primary data (ratio profiles, clinicopathological parameters, histograms) available at the interactive web site http://amba.charite.de/cgh, where the incidence of changes can be determined at individual loci and additional parameters can be applied for the analysis of our CGH results.

Function and spatial distribution of ion channels and transporters in cell migration

Cell migration plays a central role in many physiological and pathophysiological processes, such as embryogenesis, immune defense, wound healing, or the formation of tumor metastases. Detailed models have been developed that describe cytoskeletal mechanisms of cell migration. However, evidence is emerging that ion channels and transporters also play an important role in cell migration. The purpose of this review is to examine the function and subcellular distribution of ion channels and transporters in cell migration. Topics covered will be a brief overview of cytoskeletal mechanisms of migration, the role of ion channels and transporters involved in cell migration, and ways by which a polarized distribution of ion channels and transporters can be achieved in migrating cells. Moreover, a model is proposed that combines ion transport with cytoskeletal mechanisms of migration.

The Role of Matrix Metalloproteinases in Tumor Invasion, Metastasis, and Angiogenesis

Tumor invasion and metastasis formation are the hallmarks of malignant cancer. Metastatic spread of cancer cells is a result of a complex cascade of cellular events. This article discusses the matrix metalloproteinase family, the regulation of matrix metalloproteinase activity, and the functions of matrix metalloproteinases in tumor progression.

Anti-PSCA mAbs inhibit tumor growth and metastasis formation and prolong the survival of mice bearing human prostate cancer xenografts.

Prostate stem-cell antigen (PSCA) is a cell-surface antigen expressed in normal prostate and overexpressed in prostate cancer tissues. PSCA expression is detected in over 80% of patients with local disease, and elevated levels of PSCA are correlated with increased tumor stage, grade, and androgen independence, including high expression in bone metastases. We evaluated the therapeutic efficacy of anti-PSCA mAbs in human prostate cancer xenograft mouse models by using the androgen-dependent LAPC-9 xenograft and the androgen-independent recombinant cell line PC3-PSCA. Two different anti-PSCA mAbs, 1G8 (IgG1kappa) and 3C5 (IgG2akappa), inhibited formation of s.c. and orthotopic xenograft tumors in a dose-dependent manner. Furthermore, administration of anti-PSCA mAbs led to retardation of established orthotopic tumor growth and inhibition of metastasis to distant sites, resulting in a significant prolongation in the survival of tumor-bearing mice. These studies suggest PSCA as an attractive target for immunotherapy and demonstrate the therapeutic potential of anti-PSCA mAbs for the treatment of local and metastatic prostate cancer.

Ion channels and transporters on the move.

Cell migration plays a crucial role in a variety of (patho)physiological processes such as immune defense, wound healing, and formation of tumor metastases. Detailed models have been developed to describe cytoskeletal mechanisms of migration. However, evidence is accumulating that the activity of ion channels and transporters is also required for optimal cell locomotion.

CD44 is the Principal Mediator of Hyaluronic-Acid-Induced Melanoma Cell Proliferation

Interactions of the extracellular matrix component hyaluronic acid and its cellular receptors CD44 and RHAMM/IHABP have been linked to tumor progression and metastasis formation. We investigated the expression and hyaluronic-acid-dependent functions of CD44 and RHAMM/IHABP in human melanoma. Immunohistochemistry of tumor specimens at different stages of melanoma progression revealed an increased expression of CD44 and RHAMM/IHABP. High mRNA expression of CD44 was found in three highly tumorigenic melanoma cell lines compared with less tumorigenic melanoma cells or nontransformed melanocytes. RHAMM/IHABP expression was upregulated in all cell lines analyzed but not in melanocytes. In contrast to the cell surface localization of CD44, RHAMM/IHABP was detected exclusively within the cytoplasm of melanoma cells. Binding and adhesion of melanoma cells to hyaluronic acid is mainly CD44 dependent as it was inhibited to 60%--80% by an anti-CD44 monoclonal antibody whereas anti-RHAMM/IHABP sera had no effect. Culture of melanoma cells in the presence of hyaluronic acid resulted in a dose-dependent, CD44-mediated increase of melanoma cell proliferation and enhanced release of basic fibroblast growth factor and transforming growth factor beta 1. We conclude that (i) the expression of CD44 and RHAMM/IHABP is increased during melanoma progression, (ii) CD44 is the principal hyaluronic acid surface receptor on melanoma cells, and (iii) the hyaluronic-acid-induced increase of the proliferative capacity of melanoma cells is mainly dependent on CD44--hyaluronic acid interactions.

On the role of cell surface carbohydrates and their binding proteins (lectins) in tumor metastasis.

This review focuses on the recent advances in investigations of the role of cell surface carbohydrates in tumor metastasis. It also summarizes the results of extensive studies of endogenous lectins, their structure, carbohydrate specificity and biological functions with the major emphasis on the significance of lectin-cell surface carbohydrate interactions in a metastatic process. Numerous data demonstrate that malignant transformation is associated with various and complex alterations in the glycosylation process. Some of these changes might provide a selective advantage for tumor cells during their progression to more invasive and metastatic phenotype. Cell glycosylation depends on the expression and function of various glycosyltransferases and glycosidases. Recently, transfection of genes encoding various glysosyltransferases gene in sense and antisense orientation helped to bring direct evidence that changes in cell surface carbohydrates are important for the metastatic behavior of tumor cells. Cell surface carbohydrates affect tumor cell interactions with normal cells or with the extracellular matrix during metastatic spread and growth. These interactions can be mediated via tumor cell carbohydrates and their binding proteins known as endogenous lectins. The family of the discovered endogenous lectins is rapidly expanding. The number of C-type lectins has reached 50 and at least 10 galectins have been identified. The biological significance of the endogenous lectins and their possible role in tumor growth and metastasis formation has started to unravel. Some lectins recognize the 'foreign' patterns of cell surface carbohydrates expressed by microorganisms and tumor cells, and play a role in innate and adaptive immunity. It was shown that lectins affect tumor cell survival, adhesion to the endothelium or extracellular matrix, as well as tumor vascularization and other processes that are crucial for metastatic spread and growth.

Coexpression of Integrin αvβ3 and Matrix Metalloproteinase-2 (MMP-2) Coincides with MMP-2 Activation: Correlation with Melanoma Progression

Tumor cell invasion and metastasis formation depend on both adhesive and proteolytic mechanisms. Previous studies have shown that expression of matrix metalloproteinase-2 and integrin alphavbeta3 correlate with melanoma progression. Recently, direct binding of matrix metalloproteinase-2 to alpha(v)beta3 was implicated in presenting activated matrix metalloproteinase-2 on the cell surface of invasive cells. In this study we investigated this, using the highly metastatic, alpha(v)beta3-negative melanoma cell lines MV3 and BLM, their beta3-transfected alpha(v)beta3 expressing counterparts, xenografts derived from these cell lines, and fresh human cutaneous melanoma lesions comprising all stages of melanoma progression. Expression and activation status of matrix metalloproteinase-2 were studied by reverse transcription-polymerase chain reaction, immunohistochemistry, western blotting, and zymographic analysis, respectively. Matrix metalloproteinase-2 protein expression in vitro was similar in both alpha(v)beta3-negative and alpha(v)beta3-positive cell lines Remarkable differences, however, exist in the localization of inactive and active matrix metalloproteinase-2. Soluble active matrix metalloproteinase-2 was detectable only in the conditioned medium of alpha(v)beta3-negative cell lines and undetectable in the alpha(v)beta3-positive cell lines. Conversely, active matrix metalloproteinase-2 was present exclusively on the cell surface of the alpha(v)beta3 expressing transfectants. Western blot analysis of other components that are involved in matrix metalloproteinase-2 activation showed that processing of proMT1-matrix metalloproteinase to the activated form was enhanced in beta3 transfectants, whereas secretion of tissue inhibitor of metalloproteinase-2 was decreased. In vivo, the presence of functionally active matrix metalloproteinase-2 was significantly higher in xenografts derived from the alpha(v)beta3 expressing MV3 and BLM cell lines. In human cutaneous melanoma lesions, neither matrix metalloproteinase-2 nor integrin alpha(v)beta3 is detectable in melanoma in situ as determined by immunohistochemistry. In contrast, the number of matrix metalloproteinase-2-positive and alphavbeta3-positive tumor cells was clearly increased in primary melanomas, and melanoma metastases. Double staining experiments and confocal laser microscopy demonstrated that the percentage of cells coexpressing matrix metalloproteinase-2 and alpha(v)beta3 increased in advanced primary melanomas and melanoma metastases. In addition, zymography showed that functionally active matrix metalloproteinase-2 was frequently present in melanoma metastases. In these lesions a high proportion of matrix metalloproteinase-2- and alphavbeta3-double-positive melanoma cells were detectable. Our study demonstrates that the presence of activated matrix metalloproteinase-2 correlates with expression of alpha(v)beta3 in human melanoma cells both in vitro and in vivo, and also in fresh human melanoma lesions. These findings strongly suggest that co-ordinated expression of both factors may be required for melanoma cell invasion and metastasis formation.

The Role of Vascular Endothelial Growth Factor (VEGF) in Tumor Angiogenesis and Early Clinical Development of VEGFReceptor Kinase Inhibitors

Angiogenesis, or the formation of new blood vessels from preexisting vasculature, plays a major role in tumor growth and metastasis formation. Therefore, inhibiting tumor angiogenesis may be a promising therapeutic strategy. Paracrine stimuli from tumor cells are the main promoters of angiogenesis. They activate endothelial cells to proliferate and migrate, subsequently resulting in new tube formation and blood flow. This complex process involves numerous biological activities. Vascular endothelial growth factor (VEGF) is a potent and specific angiogenic factor. Originally identified for its ability to induce vascular permeability and stimulate endothelial cell growth, VEGF is now known to be a key requirement for tumor growth. Currently, three highaffinity tyrosine kinase receptors for VEGF have been identified, of which VEGF receptor (VEGFR)-Flk-1/KDR (VEGFR-2) is exclusively expressed in vascular endothelial cells. Because the VEGFR-2 system is a dominant signal-transduction pathway in regulating tumor angiogenesis, specific inhibitors of this pathway inhibit metastases, microvessel formation, and tumor-cell proliferation. Induction of apoptosis in tumor cells and endothelial cells has also been observed. The clinical importance of VEGF for tumor growth is supported by the fact that most tumors produce VEGF and that the inhibition of VEGF-induced angiogenesis significantly inhibits tumor growth in vivo. In this review, we discuss the biologic role of VEGF and the therapeutic options for inhibiting VEGF in cancer patients.

The Syk tyrosine kinase suppresses malignant growth of human breast cancer cells.

Syk is a protein tyrosine kinase that is widely expressed in haematopoietic cells. It is involved in coupling activated immunoreceptors to downstream signalling events that mediate diverse cellular responses including proliferation, differentiation and phagocytosis. Syk expression has been reported in cell lines of epithelial origin, but its function in these cells remains unknown. Here we show that Syk is commonly expressed in normal human breast tissue, benign breast lesions and low-tumorigenic breast cancer cell lines. Syk messenger RNA and protein, however, are low or undetectable in invasive breast carcinoma tissue and cell lines. Transfection of wild-type Syk into a Syk-negative breast cancer cell line markedly inhibited its tumour growth and metastasis formation in athymic mice. Conversely, overexpression of a kinase-deficient Syk in a Syk-positive breast cancer cell line significantly increased its tumour incidence and growth. Suppression of tumour growth by the reintroduction of Syk appeared to be the result of aberrant mitosis and cytokinesis. We propose that Syk is a potent modulator of epithelial cell growth and a potential tumour suppressor in human breast carcinomas.