Using the Dot Assay to Analyze Migration of Cell Sheets

Although complex organisms appear static, their tissues are under a continuous turnover. As cells age, die, and are replaced by new cells, cells move ...

National Institutes of Health

We used 2D-cocultures employing fibroblasts of different genetic backgrounds and MCF10A-derived human breast epithelial cells of increasingly malignant potential to investigate tumor-stroma interactions in breast cancer and to identify possible signaling pathways involved. Tumor cells induced expression of matrix-metalloproteinase 9 (MMP-9) in fibroblasts in a pattern dependent on the degree of their malignancy. In-situ zymography localized the main gelatinolytic activity around stromal cells in cocultures and xenografted tumors. Use of Smad3 knockout fibroblasts, small molecule inhibitors, and neutralizing antibodies showed that MMP-9 expression was induced by tumor cell-derived TNF-alpha and TGF-beta, dependent on Smad-, Ras-, and PI3-kinase-signaling, and likewise modulated by subsequent HGF- and EGF-signaling. Together, our results indicate that MMP-9 levels in tumor fibroblasts are regulated by a complex tumor-stroma cross-talk, involving multiple ligands and cellular signaling pathways.

Breast cancer cells induce stromal fibroblasts to express MMP-9 via secretion of TNF-alpha and TGF-beta.

Extracellular matrix glycoproteins and proteoglycans bind a variety of growth factors and cytokines thereby regulating matrix assembly as well as bone formation. However, little is known about the mechanisms by which extracellular matrix molecules modulate osteogenic stem cells and bone formation. Using mice deficient in two members of the small leucine-rich proteoglycans, biglycan and decorin, we uncovered a role for these two extracellular matrix proteoglycans in modulating bone formation from bone marrow stromal cells. Our studies showed that the absence of the critical transforming growth factor-beta (TGF-beta)-binding proteoglycans, biglycan and decorin, prevents TGF-beta from proper sequestration within the extracellular matrix. The excess TGF-beta directly binds to its receptors on bone marrow stromal cells and overactivates its signaling transduction pathway. Overall, the predominant effect of the increased TGF-beta signaling in bgn/dcn-deficient bone marrow stromal cells is a "switch in fate" from growth to apoptosis, leading to decreased numbers of osteoprogenitor cells and subsequently reduced bone formation. Thus, biglycan and decorin appear to be essential for maintaining an appropriate number of mature osteoblasts by modulating the proliferation and survival of bone marrow stromal cells. These findings underscore the importance of the micro-environment in controlling the fate of adult stem cells and reveal a novel cellular and molecular basis for the physiological and pathological control of bone mass.

Extracellular matrix proteoglycans control the fate of bone marrow stromal cells.

Although transforming growth factor beta (TGF-beta) typically inhibits proliferation of epithelial cells, consistent with a tumor suppressor activity, it paradoxically also exhibits pro-metastatic activity in the later stages of carcinogenesis. Since tumors often display altered TGF-beta signaling, particularly involving the Smad-pathway, we investigated the role of Smad4-expression in breast cancer.

Smad4-expression is decreased in breast cancer tissues: a retrospective study.

Canonical TGF-beta is involved in cell differentiation, tissue maintenance, and wound healing, but also plays a central role in the pathogenesis of diseases such as cancer Here we describe a lentivirus-based reporter vector system expressing green fluorescent protein (GFP) or red fluorescent protein (RFP) under the control of a Smad3-responsive element (CAGA)12 that allows observation of the temporospatial pattern of endogeneous Smad3-mediated signaling on a cellular level. Use of this method will be valuable to identify cells with active Smad3 signaling and investigate the role of endogenous Smad3 signaling in complex systems such as co-cultures in vitro, or in tumors during tumor cell invasion and metastasis in vivo.

Lentiviral reporter constructs for fluorescence tracking of the temporospatial pattern of Smad3 signaling.

We investigated the influence of acute wounding on tumor growth in a syngeneic mouse breast cancer model. Metastatic mouse breast cancer cells (4T1) were orthotopically injected into the mammary fat pads of BALB/c mice, and animals were wounded locally by full thickness dermal incisions above the mammary fat pads or remotely above the scapula 9 days later. Local, but not remote, wounding increased tumor size when compared with sham treatment. Injection of wound fluid close to the tumor site increased tumor growth, whereas in vitro wound fluid compared with serum increased the proliferation rate of 4T1 cells. Our results show that wound stroma can unfavorably influence growth of nearby tumors. This effect is T cell-dependent, as local wounding had no effect on tumor growth in nu/nu mice. The effect of wounding on tumor growth can be mimicked by acellular wound fluid, suggesting that T cells secrete or mediate secretion of cytokines or growth factors that then accelerate tumor growth. Here, we define an experimental model of wound-promoted tumor growth that will enable us to identify mechanisms and therapeutic targets to reduce the negative effect of tissue repair on residual tumors.

Acute wounds accelerate tumorigenesis by a T cell-dependent mechanism.

Tumor stem cells or cancer initiating cells (CICs) are single tumor cells that can regenerate a tumor or a metastasis. The identification and isolation of CICs remain challenging, and a variety of putative CIC markers have been described. We hypothesized that cell lines of the NCI60 panel contain CICs and express putative CIC markers. We investigated expression of putative CIC surface markers (CD15, CD24, CD44, CD133, CD166, CD326, PgP) and the activity of aldehyde dehydrogenase in the NCI60 panel singly and in combination by six-color fluorescence-activated cell sorting analysis. All investigated markers were expressed in cell lines of the NCI60 panel. Expression levels of individual markers varied widely across the 60 cell lines, and neither single marker expression nor simple combinations nor co-expression patterns correlated with the colony-formation capacity of cell lines. Rather, marker expression patterns correlated with tumor types in multidimensional analysis. Whereas some expression patterns correlated with tumor entities such as basal breast cancer, other expression patterns occurred across different tumor types and largely related to expression of a more mesenchymal phenotype in individual breast, lung, renal, and melanoma cell lines. Our data for the first time demonstrate that tumor cell lines display CIC markers in a complex pattern that relates to the tumor type. The complexity and tumor type specificity of marker display creates challenges for the application of cell sorting and other approaches to isolation of putative tumor stem cell populations and suggests that therapeutic targeting strategies will need to take this into account.

Complex display of putative tumor stem cell markers in the NCI60 tumor cell line panel.

Carcinoma are complex societies of mutually interacting cells in which there is a progressive failure of normal homeostatic mechanisms, causing the parenchymal component to expand inappropriately and ultimately to disseminate to distant sites. When a cancer cell metastasizes, it first will be exposed to cancer associated fibroblasts in the immediate tumor microenvironment and then to normal fibroblasts as it traverses the underlying connective tissue towards the bloodstream. The interaction of tumor cells with stromal fibroblasts influences tumor biology by mechanisms that are not yet fully understood. Here, we report a role for normal stroma fibroblasts in the progression of invasive tumors to metastatic tumors. Using a coculture system of human metastatic breast cancer cells (MCF10CA1a) and normal murine dermal fibroblasts, we found that medium conditioned by cocultures of the two cell types (CoCM) increased migration and scattering of MCF10CA1a cells in vitro, whereas medium conditioned by homotypic cultures had little effect. Transient treatment of MCF10CA1a cells with CoCM in vitro accelerated tumor growth at orthotopic sites in vivo, and resulted in an expanded pattern of metastatic engraftment. The effects of CoCM on MCF10CA1a cells were dependent on small amounts of active TGF-beta1 secreted by fibroblasts under the influence of the tumor cells, and required intact ALK5-, p38-, and JNK signaling in the tumor cells. In conclusion, these results demonstrate that transient interactions between tumor cells and normal fibroblasts can modify the acellular component of the local microenvironment such that it induces long-lasting increases in tumorigenicity and alters the metastatic pattern of the cancer cells in vivo. TGF-beta appears to be a key player in this process, providing further rationale for the development of anti-cancer therapeutics that target the TGF-beta pathway.

Transient tumor-fibroblast interactions increase tumor cell malignancy by a TGF-Beta mediated mechanism in a mouse xenograft model of breast cancer.

Cancer cells alter their migratory properties during tumor progression to invade surrounding tissues and metastasize to distant sites. However, it remains unclear how migratory behaviors differ between tumor cells of different malignancy and whether these migratory behaviors can be utilized to assess the malignant potential of tumor cells. Here, we analyzed the migratory behaviors of cell lines representing different stages of breast cancer progression using conventional migration assays or time-lapse imaging and particle image velocimetry (PIV) to capture migration dynamics. We find that the number of migrating cells in transwell assays, and the distance and speed of migration in unconstrained 2D assays, show no correlation with malignant potential. However, the directionality of cell motion during 2D migration nicely distinguishes benign and tumorigenic cell lines, with tumorigenic cell lines harboring less directed, more random motion. Furthermore, the migratory behaviors of epithelial sheets observed under basal conditions and in response to stimulation with epidermal growth factor (EGF) or lysophosphatitic acid (LPA) are distinct for each cell line with regard to cell speed, directionality, and spatiotemporal motion patterns. Surprisingly, treatment with LPA promotes a more cohesive, directional sheet movement in lung colony forming MCF10CA1a cells compared to basal conditions or EGF stimulation, implying that the LPA signaling pathway may alter the invasive potential of MCF10CA1a cells. Together, our findings identify cell directionality as a promising indicator for assessing the tumorigenic potential of breast cancer cell lines and show that LPA induces more cohesive motility in a subset of metastatic breast cancer cells.

Real-time motion analysis reveals cell directionality as an indicator of breast cancer progression.

Increased growth of residual tumors in the proximity of acute surgical wounds has been reported; however, the mechanisms of wound-promoted tumor growth remain unknown. Here, we used a syngeneic, orthotopic mouse model of breast cancer to study mechanisms of wound-promoted tumor growth. Our results demonstrate that exposure of metastatic mouse breast cancer cells (4T1) to SDF-1α, which is increased in wound fluid, results in increased tumor growth. Both, wounding and exposure of 4T1 cells to SDF-1α not only increased tumor growth, but also tumor cell proliferation rate and stromal collagen deposition. Conversely, systemic inhibition of SDF-1α signaling with the small molecule AMD 3100 abolished the effect of wounding, and decreased cell proliferation, collagen deposition, and neoangiogenesis to the levels observed in control animals. Furthermore, using different mouse strains we could demonstrate that the effect of wounding on tumor growth and SDF-1α levels is host dependent and varies between mouse strains. Our results show that wound-promoted tumor growth is mediated by elevated SDF-1α levels and indicate that the effect of acute wounds on tumor growth depends on the predetermined wound response of the host background and its predetermined wound response.

SDF-1α mediates wound-promoted tumor growth in a syngeneic orthotopic mouse model of breast cancer.

Migration of cells along gradients of effector molecules, i.e., chemotaxis, is necessary in immune response and is involved in development and cancer metastasis. The experimental assessment of chemotaxis thus is of high interest. The agarose spot assay is a simple tissue culture system used to analyze chemotaxis. Although direction sensing requires gradients to be sufficiently steep, how the chemical gradients developed in this assay change over time, and thus, under what conditions chemotaxis is plausible, has not yet been determined. Here, we use numerical solution of the diffusion equation to determine the chemoattractant gradient produced in the assay. Our analysis shows that, for the usual spot size, the lifetime of the assay is optimized if the chemoattractant concentration in the spot is initially 30 times the dissociation constant of the chemoattractant-receptor bond. This result holds regardless of the properties of the chemoattractant. With this initial concentration, the chemoattractant gradient falls to the minimum threshold for directional sensing at the same time that the concentration drops to the optimal level for detecting gradient direction. If a higher initial chemoattractant concentration is used, the useful lifetime of the assay is likely to be shortened because receptor saturation may decrease the cells' sensitivity to the gradient; lower initial concentrations would result in too little chemoattractant for the cells to detect. Moreover, chemoattractants with higher diffusion coefficients would sustain gradients for less time. Based on previous measurements of the diffusion coefficients of the chemoattractants EGF and CXCL12, we estimate that the assay will produce gradients that cells can sense for a duration of 10 h for EGF and 5 h for CXCL12. These gradient durations are comparable to what can be achieved with the Boyden chamber assay. The analysis presented in this work facilitates determination of suitable parameters for the assay, and can be used to assess whether observed cell motility is likely due to chemotaxis or chemokinesis.

Improving the design of the agarose spot assay for eukaryotic cell chemotaxis.

Many cell lines currently used in medical research, such as cancer cells or stem cells, grow in confluent sheets or colonies. The biology of individual cells provide valuable information, thus the separation of touching cells in these microscopy images is critical for counting, identification and measurement of individual cells. Over-segmentation of single cells continues to be a major problem for methods based on morphological watershed due to the high level of noise in microscopy cell images. There is a need for a new segmentation method that is robust over a wide variety of biological images and can accurately separate individual cells even in challenging datasets such as confluent sheets or colonies.

FogBank: a single cell segmentation across multiple cell lines and image modalities.

Tumorigenesis is driven by genetic and physiological alterations of tumor cells as well as by the host microenvironment. In a co-culture of breast cancer cells and fibroblasts, short term interactions between tumor cells and stromal fibroblasts increase levels of active, fibroblast derived TGF-β in the extracellular medium, which in turn induces an expanded metastatic pattern of MCF10CA1a cells. These findings suggest that the effects of stromal TGF-β on tumor cell phenotype can be modeled as a dynamical system rather than a continuous linear system. In such a model, small changes of certain parameters of a system that is at a critical point can cause sudden changes of the system, explaining why experimentally and clinically observed small changes in the tumor environment can cause dramatic changes in cell phenotype or disease outcome.

How Little Is Too Much? - How Transient Tumor-Stromal Crosstalk Can Control Tumor Progression.

Migrating cells can influence the direction of their own migration by metabolizing chemoattractants present in their environment. This is illustrated by the dispersal of melanoma cells, which break down lysophosphatidic acid and generate a gradient with increasing concentrations of lysophosphatidic acid distant from the tumor. Melanoma cells can then disperse away from the tumor as they migrate in the self-generated lysophosphatidic acid gradient. Thus, dispersal of tumor cells during invasion of the surrounding stroma might be driven by chemotaxis of cells along self-generated chemoattractant gradients.

Christina H. Stuelten

Laboratory of Cellular and Molecular Biology,
National Cancer Institute,
Laboratory of Cellular and Molecular Biology, National Cancer Institute

National Institutes of Health

Using the Dot Assay to Analyze Migration of Cell Sheets

Christina H. Stuelten

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Laboratory of Cellular and Molecular Biology,National Cancer Institute,Laboratory of Cellular and Molecular Biology, National Cancer Institute

National Institutes of Health

Using the Dot Assay to Analyze Migration of Cell Sheets

Laboratory of Cellular and Molecular Biology,
National Cancer Institute,
Laboratory of Cellular and Molecular Biology, National Cancer Institute