Richard E. B. Seftor

Acidic pH enhances the invasive behavior of human melanoma cells

As a consequence of poor perfusion and elevated acid production, the extracellular pH (pHex) of tumors is generally acidic. Despite this, most in vitro experiments are still performed at the relatively alkaline pHex of 7.4. This is significant, because slight changes in pHex can have profound effects on cell phenotype. In this study we examined the effects of mildly acidic conditions on the in vitro invasive potential of two human melanoma cell lines: the highly invasive C8161, and poorly invasive A375P. We observed that culturing of either cell line at acidic pH (6.8) caused dramatic increases in both migration and invasion, as measured with the Membrane Invasion Culture System (MICS). This was not due to a direct effect of pH on the invasive machinery, since cells cultured at normal pH (7.4) and tested at acidic pH did not exhibit increased invasive potential. Similarly, cells cultured at acidic pH were more aggressive than control cells when tested at the same medium pH. These data indicate that culturing of cells at mildly acidic pH induces them to become more invasive. Since acid pH will affect the intracellular pH (pHin) and intracellular calcium ([Ca2+]in), we examined the effect of these parameters on invasion. While changes in [Ca2+]inwere not consistent with invasive potential, the changes in pHin were. While these conditions decrease the overall amount of gelatinases A and B secreted by these cells, there is a consistent and significant increase in the proportion of the activated form of gelatinase B.

Angiogenesis: Vasculogenic mimicry and tumour-cell plasticity: lessons from melanoma

The gene-expression profile of aggressive cutaneous and uveal melanoma cells resembles that of an undifferentiated, embryonic-like cell. The plasticity of certain types of cancer cell could explain their ability to mimic the activities of endothelial cells and to participate in processes such as neovascularization and the formation of a fluid-conducting, matrix-rich meshwork. This ability has been termed vasculogenic mimicry. How does vasculogenic mimicry contribute to tumour progression, and can it be targeted by therapeutic agents?

A SIMPLE QUANTITATIVE ASSAY FOR STUDYING THE INVASIVE POTENTIAL OF HIGH AND LOW HUMAN METASTATIC VARIANTS

This paper presents a more reliable model for studying the extent of tumor cell migration and invasion in vitro. Polycarbonate filters were uniformly coated with a reconstituted basement membrane material and allowed to dry; each filter measured 0.035 mm in thickness when hydrated with media. Subsequently, the membrane-coated filters were suspended in Membrane Invasion Culture System (MICS) chambers, and high (A375M) and low (A375P) metastatic variants of human melanoma cells were seeded onto the membranes and allowed to incubate for 3 days. At the end of this period, cells were examined morphologically, and the invasive cells of both metastatic variants were collected, stained and counted microscopically. The tumor cells could be seen attached to the reconstituted basement membrane, buried within it, and forming colony-like aggregates in the matrix. It was determined that approximately twice as many A375M cells invaded the artificial biological matrix compared with the A375P cells (P less than 0.0005). Substantially more cells from each variant invaded uncoated polycarbonate filters, thus indicating the selective barrier imposed by the Matrigel. The utilization of such a reconstituted matrix for in vitro invasion studies allows one the opportunity to examine tumor cell attachment, migration and invasion of a uniform matrix over an extended period of time.

Role of intermediate filaments in migration, invasion and metastasis

SummaryThe expression of intermediate filament proteins is remarkably tissue-specific which suggests that the intermediate filament (IF) type(s) present in cells is somehow related to their biological function. However, in some cancers-particularly malignant melanoma and breast carcinoma, there is a strong indication that vimentin and keratin IFs are coexpressed, thus presenting as a dedifferentiated or interconverted (between epithelial and mesenchymal) phenotype. In this review, twoin vitro models are presented which recapitulate the interconverted phenotype in human melanoma and breast carcinoma, and allow, for the first time, unique observations to be made with respect to the role of IFs in cancer progression.These studies have provided direct evidence linking overexpression of keratin IFs in human melanoma with increased migratory and invasive activityin vitro, which can be down-regulated by substituting dominant-negative keratin mutants. Overexpression of vimentin IFs in the breast carcinoma model leads to augmentation of motility and invasivenessin vitro, which can be transiently down-regulated by treatment with antisense oligonucleotides to vimentin. Additional experimental evidence suggests that the mechanism(s) responsible for the differential expression of metastatic properties associated with the interconverted phenotype rest(s) in the unique interaction, either direct or indirect, of IFs with specific integrins interacting with the extracellular matrix.In this review, we discuss the observations derived from the human melanoma and breast carcinoma models to address the hypothesis that the ability to coexpress vimentin and keratins confers a selective advantage to tumor cells in their interpretation of and response to signaling cues from the extracellular matrix. The ramifications of these observations are discussed with respect to the patholophysiology of the respectivein situ tumors.

Molecular Determinants of Ovarian Cancer Plasticity

During development, the formation and remodeling of primary vascular networks occurs by vasculogenesis and angiogenesis. Recently, the term vasculogenic mimicry has been used by our laboratory and collaborators to reflect the embryonic-like ability of aggressive, but not nonaggressive, melanoma tumor cells to form a pattern of matrix-rich networks (containing channels) surrounding spheroids of tumor cells in three-dimensional culture, concomitant with their expression of vascular cell markers. Ovarian cancer is usually diagnosed as advanced stage disease in most patients when widespread metastases have already been established within the peritoneal cavity. In this study, we explored whether invasive ovarian carcinoma cells could engage in molecular vasculogenic mimicry reflected by their plasticity, compared with their normal cell counterparts. The data revealed that the invasive ovarian cancer cells, but not normal ovarian surface epithelial cells, formed patterned networks containing solid and hollow matrix channels when grown in three-dimensional cultures containing Matrigel or type I collagen, in the absence of endothelial cells or fibroblasts. Immunohistochemical analysis showed that matrix metalloproteinases (MMP)-1, -2, and -9, and MT1-MMP were discretely localized to these networks, and the formation of the networks was inhibited by treatment with MMP inhibitors. Furthermore, the RNase protection assay revealed the expression of multiple vascular cell-associated markers by the invasive ovarian cancer cells. In patient tumor sections from high-stage, high-grade ovarian cancers, 7 to 10% of channels containing red blood cells were lined by tumor cells. By comparison, all vascular areas in benign tumors and low-stage cancers were endothelial lined. These results may offer new insights and molecular markers for consideration in ovarian cancer diagnosis and treatment strategies based on molecular vascular mimicry by aggressive tumor cells.

Molecular plasticity of human melanoma cells

The molecular analysis of tumors, such as melanoma, has benefited significantly from microarray technology that can facilitate the classification of tumors based on the differential expression of genes. The data summarized in this review describe the molecular profile of aggressive cutaneous and uveal melanoma cells as that of multiple phenotypes similar to a pluripotent, embryonic-like stem cell. A noteworthy example of the plasticity of the aggressive melanoma cell phenotype is demonstrated by the ability of these tumor cells to engage in vasculogenic mimicry and neovascularization. A review of the current evidence demonstrating important cellular and molecular determinants of melanoma vasculogenic mimicry is presented. In addition, novel signaling pathways are discussed, involving VE-cadherin, EphA2, FAK, and PI 3-kinase, which promote cell migration, invasion, and matrix remodeling. The observations summarized in this review describe some of the key molecular events that regulate the process of melanoma vasculogenic mimicry and identify new signal transduction pathways that can serve as putative targets for therapeutic intervention.

Isolation and culture of human trabecular meshwork cells by extracellular matrix digestion

Like corneal endothelial cells, human trabecular meshwork cells are believed to be of neural crest origin, but demonstrate physiological properties and an antithrombogenic surface similar to vascular endothelial cells. One current method for isolating trabecular meshwork cells utilizes the motile nature of these cells to migrate away from a trabecular meshwork explant in culture to more distal regions of the culture dish. This outgrowth technique is limited in practice by the relatively small number of cells that migrate per explant per unit time, thus hindering the ability to gather sufficient numbers of cells for comprehensive experimentation. For this reason, we have modified an extracellular matrix digestion technique in current use for the isolation of microvascular endothelial cells to isolate human trabecular meshwork cells. This procedure is both efficient and rapid for isolating large numbers of trabecular meshwork cells and results in the availability of trabecular meshwork cells in sufficient quantities for subsequent experimentation.

Molecular Role(s) for Integrins in Human Melanoma Invasion

There are fundamental issues regarding the role of integrins in human disease which remain to be elucidated. Human cutaneous melanoma is an attractive model for studying integrin involvement in tumor progression because it generally follows a sequential series of definable stages. Furthermore, the most specific marker for the transition of cells from the more benign, non-metastatic radial growth phase stage to the more malignant, metastatically competent vertical growth phase stage is associated with the onset of αvβ3 integrin expression and function. This same pattern, however, does not hold true for human ocular/uveal melanomas which do not progress through these stages, but preferentially metastasize to the liver by dissemination of the cells via a direct hematogenous pathway. It is also unclear whether the αvβ3 integrin is functionally involved in uveal melanoma metastasis or not. Our results show that perturbation of the αvβ3 integrin on moderately invasive A375M human cutaneous melanoma cells with either specific antibodies or ligands results in an increase in the cells ability to invade in vitro coincident with an increase in the cells expression and extracellular levels of matrix metalloproteinase-2 (MMP-2, gelatinase A). The highly invasive C8161 human cutaneous melanoma cells express little-to-no αvβ3 integrin, but are more invasive and express higher levels of MMPs after perturbation of their α5β1 integrin. This augmented invasiveness could subsequently be abrogated with a function-blocking anti-MMP-2 antibody. Primary uveal melanoma cells and cells derived from uveal metastases appear to grow in either a spindle or epithelioid morphology. The less invasive uveal melanoma cells are spindle shaped and express higher levels of the αvβ3 integrin, while the more invasive cell lines are epithelioid shaped and express reduced levels of the αvβ3 integrin. The apparent conflict between these results and the current model for cutaneous melanoma progression may be addressed as follows: The expression and function of the αvβ3 integrin plays an important role(s) during the transition of cells from the radial growth phase stage to the vertical growth phase stage. However, further progression leading to metastases may require changes in the cells integrins that would facilitate their ability to leave the primary tumor, and aid in their ability to invade and ultimately form metastases. It is also conceivable that the αvβ3 integrin is reexpressed during various stages of metastatic dissemination, and, in particular, during tumor reestablishment.

Expression of multiple molecular phenotypes by aggressive melanoma tumor cells : role in vasculogenic mimicry

Cutaneous melanoma has been increasing at an alarming rate over the past two decades, however, there are no acceptable histopathological markers that classify various stages of melanoma progression. Recently, the molecular analysis of cancer has contributed significantly to our understanding of the cellular and molecular underpinnings of tumor progression. The data summarized in this review describe the molecular signature of aggressive cutaneous melanoma cells as that of multiple phenotypes which may be similar to a pluripotent, embryonic-like phenotype. An example of the plasticity of this phenotype is demonstrated by the ability of aggressive melanoma cells to engage in vasculogenic mimicry and neovascularization. A review of the current data demonstrating important cellular and molecular determinants of human melanoma vasculogenic mimicry is presented. These findings should stimulate additional studies to address the biological relevance of the multiple molecular phenotypes expressed by aggressive melanoma cells which may lead to the development of new diagnostic markers and therapeutic targets for clinical intervention.

Remodeling of the Microenvironment by Aggressive Melanoma Tumor Cells

Abstract: A necessity for development and tumor progression is a blood supply formed by vasculogenic and/or angiogenic events, involving the cooperative interactions of cells with their microenvironment. Based on the recent characterization of vasculogenic mimicry by aggressive melanoma cells, particularly their ability to express VE (vascular endothelial)‐cadherin, TIE‐1, and EphA2, current studies have focused on the molecular signals deposited by these cells as they remodel their microenvironment. The experimental approach utilizes unique three‐dimensional collagen matrices preconditioned by metastatic melanoma cells, which contain laminin 5 γ2 chain‐enriched tracks with promigratory cleavage fragments produced by cooperative interactions with specific matrix metalloproteinases (MMPs). The results demonstrate that the collagen matrices preconditioned by the metastatic cells induce poorly aggressive melanoma cells to express, for the first time, key angiogenic/vasculogenic/matrix‐remodeling genes. Treatment of aggressive melanoma cells with an MMP inhibitor resulted in the inhibition of vasculogenic mimicry‐associated genes in these tumor cells and abrogation of the inductive effects of the preconditioned matrix on poorly aggressive melanoma cells. These observations illustrate the remarkable influence of the microenvironment on the phenotype of melanoma cells and may provide new perspectives on tumor cell plasticity and unique treatment strategies.