Integrin VLA-4 enhances sialyl-Lewisx/a-negative melanoma adhesion to and extravasation through the endothelium under low flow conditions

Integrins and the Metastasis-like Dissemination of Acute Lymphoblastic Leukemia to the Central Nervous System

Acute lymphoblastic leukemia (ALL) disseminates with high prevalence to the central nervous system (CNS) in a process resembling aspects of the CNS surveillance of normal immune cells as well as aspects of brain metastasis from solid cancers. Importantly, inside the CNS, the ALL blasts are typically confined within the cerebrospinal fluid (CSF)-filled cavities of the subarachnoid space, which they use as a sanctuary protected from both chemotherapy and immune cells. At present, high cumulative doses of intrathecal chemotherapy are administered to patients, but this is associated with neurotoxicity and CNS relapse still occurs. Thus, it is imperative to identify markers and novel therapy targets specific to CNS ALL. Integrins represent a family of adhesion molecules involved in cell-cell and cell-matrix interactions, implicated in the adhesion and migration of metastatic cancer cells, normal immune cells, and leukemic blasts. The ability of integrins to also facilitate cell-adhesion mediated drug resistance, combined with recent discoveries of integrin-dependent routes of leukemic cells into the CNS, have sparked a renewed interest in integrins as markers and therapeutic targets in CNS leukemia. Here, we review the roles of integrins in CNS surveillance by normal lymphocytes, dissemination to the CNS by ALL cells, and brain metastasis from solid cancers. Furthermore, we discuss whether ALL dissemination to the CNS abides by known hallmarks of metastasis, and the potential roles of integrins in this context.

Tumor cell E-selectin ligands determine partialefficacy of bortezomib on spontaneous lung metastasis formation of solid human tumors in vivo

Extravasation of circulating tumor cells (CTCs) is critical for metastasis and is initiated by adhesive interactions between glycoligands on CTCs and E-selectin on endothelia. Here, we show that the clinically approved proteasome inhibitor bortezomib (BZM; Velcade) counteracts the cytokine-dependent induction of E-selectin in the lung mediated by the primary tumor, thereby impairing endothelial adhesion and thus spontaneous lung metastasis in vivo. However, the efficacy of BZM crucially depends on the tumor cells' E-selectin ligands, which determine distinct adhesion patterns. The canonical ligands sialyl-Lewis A (sLeA) and sLeX mediate particularly high-affinity E-selectin binding so that the incomplete E-selectin-reducing effect of BZM is not sufficient to disrupt adhesion or metastasis. In contrast, tumor cells lacking sLeA/X nevertheless bind E-selectin, but with low affinity, so that adhesion and lung metastasis are significantly diminished. Such low-affinity E-selectin ligands apparently consist of sialylated MGAT5 products on CD44. BZM no longer has anti-metastatic activity after CD44 knockdown in sLeA/X-negative tumor cells or E-selectin knockout in mice. sLeA/X can be determined by immunohistochemistry in cancer samples, which might aid patient stratification. These data suggest that BZM might act as a drug for inhibiting extravasation and thus distant metastasis formation in malignancies expressing low-affinity E-selectin ligands.

Prevention of Melanoma Extravasation as a New Treatment Option Exemplified by p38/MK2 Inhibition

Melanoma releases numerous tumor cells into the circulation; however, only a very small fraction of these cells is able to establish distant metastasis. Intravascular survival of circulating tumor cells is limited through hemodynamic forces and by the lack of matrix interactions. The extravasation step is, thus, of unique importance to establish metastasis. Similar to leukocyte extravasation, this process is under the control of adhesion molecule pairs expressed on melanoma and endothelial cells, and as for leukocytes, ligands need to be adequately presented on cell surfaces. Based on melanoma plasticity, there is considerable heterogeneity even within one tumor and one patient resulting in a mixture of invasive or proliferative cells. The molecular control for this switch is still ill-defined. Recently, the balance between two kinase pathways, p38 and JNK, has been shown to determine growth characteristics of melanoma. While an active JNK pathway induces a proliferative phenotype with reduced invasive features, an active p38/MK2 pathway results in an invasive phenotype and supports the extravasation step via the expression of molecules capable of binding to endothelial integrins. Therapeutic targeting of MK2 to prevent extravasation might reduce metastatic spread.

Myeloid Cells in Metastasis

Metastatic disease is the leading cause of death in patients with solid cancers. The progression to metastasis is a multistep process that involves detachment of tumor cells from their constraining basement membrane at the primary site, migration and intravasation into the circulation, survival in the circulation, extravasation into the secondary organ, and survival and growth at the secondary site. During these steps, tumor and immune cells interact and influence each other both within the tumor microenvironment and systemically. In particular, myeloid cells such as monocytes, macrophages, neutrophils, and myeloid-derived suppressor cells (myeloid regulatory cells) have been shown to play important roles in the metastatic process. These interactions open new avenues for targeting cancer metastasis, especially given the increasing interest in development of cancer immunotherapies. In this review, we describe the currently reported pathways and mechanisms involved in myeloid cell enhancement of the metastatic cascade.

Neutrophils: Homing in on the myeloid mechanisms of metastasis

The metastasis cascade is complex and comprises several stages including local invasion into surrounding tissue, intravasation and survival of tumour cells in the circulation, and extravasation and colonisation of a distant site. It is increasingly clear that these processes are driven not only by signals within the tumour cells, but are also profoundly influenced by stromal cells and signals in the tumour microenvironment. Amongst the many cell types within the tumour microenvironment, immune cells such as lymphocytes, macrophages and neutrophils play a prominent role in tumour development and progression. Neutrophils, however, have only recently emerged as important players, particularly in metastasis. Here we review the current evidence suggesting a multi-faceted role for neutrophils in the metastatic cascade.

Kindlin-1 Promotes Pulmonary Breast Cancer Metastasis

In breast cancer, increased expression of the cytoskeletal adaptor protein Kindlin-1 has been linked to increased risks of lung metastasis, but the functional basis is unknown. Here, we show that in a mouse model of polyomavirus middle T antigen-induced mammary tumorigenesis, loss of Kindlin-1 reduced early pulmonary arrest and later development of lung metastasis. This phenotype relied on the ability of Kindlin-1 to bind and activate β integrin heterodimers. Kindlin-1 loss reduced α4 integrin-mediated adhesion of mammary tumor cells to the adhesion molecule VCAM-1 on endothelial cells. Treating mice with an anti-VCAM-1 blocking antibody prevented early pulmonary arrest. Kindlin-1 loss also resulted in reduced secretion of several factors linked to metastatic spread, including the lung metastasis regulator tenascin-C, showing that Kindlin-1 regulated metastatic dissemination by an additional mechanism in the tumor microenvironment. Overall, our results show that Kindlin-1 contributes functionally to early pulmonary metastasis of breast cancer.Significance: These findings provide a mechanistic proof in mice that Kindin-1, an integrin-binding adaptor protein, is a critical mediator of early lung metastasis of breast cancer. Cancer Res; 78(6); 1484-96. ©2018 AACR.

From Cancer Immunoediting to New Strategies in Cancer Immunotherapy: The Roles of Immune Cells and Mechanics in Oncology

For the last three decades, the concept of immunoediting has evolved to characterize our increasing understanding of the interactions between cells from the immune system and cancer development. Elucidating the role of immune cells in the progression of cancer has been very challenging due to their dual role; the immune system can either suppress tumor formation by killing cancer cells, or it can also promote tumor growth. Revealing how immune cells are hampered by the tumor microenvironment and how they aid tumor progression has signaled strategies to reverse these effects and control cancer cell growth; this has been the advent of immunotherapy design. More recently, the role of physical forces in the process of immunoediting has been highlighted by multiple studies focusing on understanding how force changes in the stiffness of the extracellular matrix and fluid flow shear stress contribute to tumor development. Using models in vitro that incorporate biomechanical components, it has been shown that these physical aspects are not only important during the formation and growth of primary tumors, but in the metastatic process as well. In this way, we have also gained insight into the interactions occurring within the vascular system, which are highly affected by the dynamics of physical collisions between cells and by shear forces. Here, we review the concept of cancer immunoediting with an emphasis on biomechanics and conclude with a summary on current immunotherapies and potential new strategies.

Nanoscale Tuning of VCAM-1 Determines VLA-4-Dependent Melanoma Cell Plasticity on RGD Motifs

The biophysical fine-tuning of cancer cell plasticity is crucial for tumor progression but remains largely enigmatic. Although vascular cell adhesion molecule-1 (VCAM-1/CD106) has been implicated in melanoma progression, here its presentation on endothelial cells was associated with diminished melanoma cell spreading. Using a specific nanoscale modulation of VCAM-1 (tunable from 70 to 670 ligands/μm²) next to integrin ligands (RGD motifs) in a bifunctional system, reciprocal regulation of integrin α4 (ITGA4/VLA-4/CD49d)-dependent adhesion and spreading of melanoma cells was found. As the VCAM-1/VLA-4 receptor pair facilitated adhesion, while at the same time antagonizing RGD-mediated spreading, melanoma cell morphogenesis on these bifunctional matrices was directly regulated by VCAM-1 in a dichotomic and density-dependent fashion. This was accompanied by concordant regulation of F-actin cytoskeleton remodeling, Rac1-expression, and paxillin-related adhesion formation. The novel function of VCAM-1 was corroborated in vivo using two murine models of pulmonary metastasis. The regulation of melanoma cell plasticity by VCAM-1 highlights the complex regulation of tumor-matrix interactions.Implications: Nanotechnology has revealed a novel dichotomic function of the VCAM-1/VLA-4 interaction on melanoma cell plasticity, as nanoscale tuning of this interaction reciprocally determines adhesion and spreading in a ligand density-dependent manner. Mol Cancer Res; 16(3); 528-42. ©2017 AACR.

VE-Cadherin Disassembly and Cell Contractility in the Endothelium are Necessary for Barrier Disruption Induced by Tumor Cells

During metastasis, breakdown of the endothelial barrier is critical for tumor cell extravasation through blood vessel walls and is mediated by a combination of tumor secreted soluble factors and receptor-ligand interactions. However, a complete mechanism governing tumor cell transendothelial migration remains unclear. Here, we investigate the roles of tumor-associated signals in regulating endothelial cell contractility and adherens junction disassembly leading to endothelial barrier breakdown. We show that Src mediates VE-cadherin disassembly in response to metastatic melanoma cells. Through the use of pharmacological inhibitors of cytoskeletal contractility we find that endothelial cell contractility is responsive to interactions with metastatic cancer cells and that reducing endothelial cell contractility abrogates migration of melanoma cells across endothelial monolayers. Furthermore, we find that a combination of tumor secreted soluble factors and receptor-ligand interactions mediate activation of Src within endothelial cells that is necessary for phosphorylation of VE-cadherin and for breakdown of the endothelial barrier. Together, these results provide insight into how tumor cell signals act in concert to modulate cytoskeletal contractility and adherens junctions disassembly during extravasation and may aid in identification of therapeutic targets to block metastasis.

The Anticancer Activity of Organotelluranes: Potential Role in Integrin Inactivation

Organic Te(IV) compounds (organotelluranes) differing in their labile ligands exhibited anti-integrin activities in vitro and anti-metastatic properties in vivo. They underwent ligand substitution with l-cysteine, as a thiol model compound. Unlike inorganic Te(IV) compounds, the organotelluranes did not form a stable complex with cysteine, but rather immediately oxidized it. The organotelluranes inhibited integrin functions, such as adhesion, migration, and metalloproteinase secretion mediation in B16F10 murine melanoma cells. In comparison, a reduced derivative with no labile ligand inhibited adhesion of B16F10 cells to a significantly lower extent, thus pointing to the importance of the labile ligands of the Te(IV) atom. One of the organotelluranes inhibited circulating cancer cells in vivo, possibly by integrin inhibition. Our results extend the current knowledge on the reactivity and mechanism of organotelluranes with different labile ligands and highlight their clinical potential.

PPARγ activation reduces ischemia/reperfusion-induced metastasis in a murine model of hepatocellular carcinoma

Ischemia/reperfusion (I/R) injury during liver resection or transplantation for the treatment of hepatocellular carcinoma (HCC) may increase the risk of metastasis. Peroxisome proliferator-activated receptor-γ (PPARγ) activation has been observed to exert a protective effect against hepatic I/R injury. However, whether PPARγ activation exerts a protective effect against I/R-associated liver metastasis remains unknown. Therefore, the present study aimed to investigate the effects of the PPAR agonist rosiglitazone and the specific PPARγ antagonist GW9662 on tumor metastasis following hepatic I/R. An experimental mouse model of hepatic I/R-induced HCC metastasis was designed in order to determine the effects of I/R on tumor metastasis in the liver. Four groups were established: Sham, control (I/R), rosiglitazone (Ro) and rosiglitazone with GW9662 (Ro + GW) groups. In the latter two groups, the treatments were administered intravenously 1 h prior to the induction of ischemia. Tumor load was measured 12 days after the procedure. Furthermore, tissue analyses were conducted to determine the expression levels of alanine aminotransferase, myeloperoxidase (MPO), matrix metalloproteinase (MMP)-9, vascular cell adhesion molecule (VCAM)-1, nuclear factor (NF)-κB and PPARγ. Rosiglitazone pretreatment appeared to significantly mitigate hepatic I/R injury, as indicated by serological and histological analysis. The levels of VCAM-1, MPO and MMP-9 expression in the Ro group were significantly reduced at 8 h following ischemia compared with those in the control and Ro + GW groups. In addition, rosiglitazone inhibited the I/R-induced activation of NF-κB, and GW9662 attenuated the inhibitory effect. To the best of our knowledge, the present study is the first to report on the expression and the functional roles of PPARγ in I/R-associated metastasis. Short-term treatment of mice with rosiglitazone, a potent PPARγ agonist, confers protective effects against hepatic I/R-associated metastasis. Thus, PPARγ may be a potential therapeutic target for the protection of the liver against I/R-associated metastasis.

Analyses of movement and contact of two nucleated cells using a gas-driven micropipette aspiration technique

Adhesion between two nucleated cells undergoes specific significances in immune responses and tumor metastasis since cellular adhesive molecules usually express on two apposed cell membranes. However, quantification of the interactions between two nucleated cells is still challenging in microvasculature. Here distinct cell systems were used, including three types of human cells (Jurkat cell or PMN vs. MDA-MB-231 cell) and two kinds of murine native cells (PMN vs. liver sinusoidal endothelial cell). Cell movement, compression to, and relaxation from the counterpart cell were quantified using an in-house developed gas-driven micropipette aspiration technique (GDMAT). This assay is robust to quantify this process since cell movement and contact inside a pipette are independent of the repeated test cycles. Measured approaching or retraction velocity follows well a normal distribution, which is independent on the cycle period. Contact area or duration also fits a Gaussian distribution and moreover contact duration is linearly correlated with the cycle period. Cell movement is positively related to gas flux but negatively associated to medium viscosity. Cell adhesion tends to reach an equilibrium state with increase of cycle period or contact duration. These results further the understanding in the dynamics of cell movement and contact in microvasculature.

Mutant B-Raf(V600E) Promotes Melanoma Paracellular Transmigration by Inducing Thrombin-mediated Endothelial Junction Breakdown

Tumor invasiveness depends on the ability of tumor cells to breach endothelial barriers. In this study, we investigated the mechanism by which the adhesion of melanoma cells to endothelium regulates adherens junction integrity and modulates tumor transendothelial migration (TEM) by initiating thrombin generation. We found that the B-Raf(V600E) mutation in metastatic melanoma cells up-regulated tissue factor (TF) expression on cell membranes and promoted thrombin production. Co-culture of endothelial monolayers with metastatic melanoma cells mediated the opening of inter-endothelial spaces near melanoma cell contact sites in the presence of platelet-free plasma (PFP). By using small interfering RNA (siRNA), we demonstrated that B-Raf(V600E) and TF silencing attenuated the focal disassembly of adherens junction induced by tumor contact. Vascular endothelial-cadherin (VE-cadherin) disassembly was dependent on phosphorylation of p120-catenin on Ser-879 and VE-cadherin on Tyr-658, Tyr-685, and Tyr-731, which can be prevented by treatment with the thrombin inhibitor, hirudin, or by silencing the thrombin receptor, protease-activated receptor-1, in endothelial cells. We also provided strong evidence that tumor-derived thrombin enhanced melanoma TEM by inducing ubiquitination-coupled VE-cadherin internalization, focal adhesion formation, and actin assembly in endothelium. Confocal microscopic analysis of tumor TEM revealed that junctions transiently opened and resealed as tumor cells accomplished TEM. In addition, in the presence of PFP, tumor cells preferentially transmigrated via paracellular routes. PFP supported melanoma transmigration under shear conditions via a B-Raf(V600E)-thrombin-dependent mechanism. We concluded that the activation of thrombin generation by cancer cells in plasma is an important process regulating melanoma extravasation by disrupting endothelial junction integrity.

C6-ceramide nanoliposome suppresses tumor metastasis by eliciting PI3K and PKCζ tumor-suppressive activities and regulating integrin affinity modulation

Nanoliposomal formulation of C₆-ceramide, a proapoptotic sphingolipid metabolite, presents an effective way to treat malignant tumor. Here, we provide evidence that acute treatment (30 min) of melanoma and breast cancer cells with nanoliposomal C₆-ceramide (NaL-C₆) may suppress cell migration without inducing cell death. By employing a novel flow migration assay, we demonstrated that NaL-C₆ decreased tumor extravasation under shear conditions. Compared with ghost nanoliposome, NaL-C₆ triggered phosphorylation of PI3K and PKCζ and dephosphorylation of PKCα. Concomitantly, activated PKCζ translocated into cell membrane. siRNA knockdown or pharmacological inhibition of PKCζ or PI3K rescued NaL-C₆-mediated suppression of tumor migration. By inducing dephosphorylation of paxillin, PKCζ was responsible for NaL-C₆-mediated stress fiber depolymerization and focal adhesion disassembly in the metastatic tumor cells. PKCζ and PI3K regulated cell shear-resistant adhesion in a way that required integrin α_vβ₃ affinity modulation. In conclusion, we identified a novel role of acute nanoliposomal ceramide treatment in reducing integrin affinity and inhibiting melanoma metastasis by conferring PI3K and PKCζ tumor-suppressive activities.

CD44 variant, but not standard CD44 isoforms, mediate disassembly of endothelial VE-cadherin junction on metastatic melanoma cells

Loss of endothelial adherens junctions is involved in tumor metastasis. Here, we demonstrate that, in the metastatic Lu1205 melanoma cells, expression of the CD44 variant CD44v8-v10 induced junction disassembly and vascular endothelial (VE)-cadherin phosphorylation at Y658 and Y731. Short interfering RNA (siRNA)-mediated CD44 knockdown or sialic acid cleavage reversed these effects. Moreover, microspheres coated with recombinant CD44v8-v10 promoted endothelial junction disruption. Overexpression of CD44v8-v10 but not of standard CD44 (CD44s) promoted gap formation in the non-metastatic WM35 melanoma cells, whereas CD44 knockdown or neuraminidase treatment dramatically diminished melanoma transendothelial migration. Endothelial cells transfected with the phosphomimetic VE-cadherin mutant Y658E supported transmigration of CD44-silenced Lu1205 cells. Our findings imply that CD44 variant isoform (CD44v) but not CD44s regulates endothelial junction loss, promoting melanoma extravasation.

Cancer cells remodel themselves and vasculature to overcome the endothelial barrier

Metastasis refers to the spread of cancer cells from a primary tumor to distant organs mostly via the bloodstream. During the metastatic process, cancer cells invade blood vessels to enter circulation, and later exit the vasculature at a distant site. Endothelial cells that line blood vessels normally serve as a barrier to the movement of cells into or out of the blood. It is thus critical to understand how metastatic cancer cells overcome the endothelial barrier. Epithelial cancer cells acquire increased motility and invasiveness through epithelial-to-mesenchymal transition (EMT), which enables them to move toward vasculature. Cancer cells also express a variety of adhesion molecules that allow them to attach to vascular endothelium. Finally, cancer cells secrete or induce growth factors and cytokines to actively prompt vascular hyperpermeability that compromises endothelial barrier function and facilitates transmigration of cancer cells through the vascular wall. Elucidation of the mechanisms underlying metastatic dissemination may help develop new anti-metastasis therapeutics.

Actinomyosin contraction, phosphorylation of VE-cadherin, and actin remodeling enable melanoma-induced endothelial cell-cell junction disassembly

During melanoma cell extravasation through the vascular endothelium, melanoma cells interact with endothelial cells through secretion of cytokines and by adhesion between proteins displayed on opposing cell surfaces. How these tumor cell associated signals together regulate the dynamics of intracellular signaling pathways within endothelial cells leading to endothelial cell-cell junction disruption is not well understood. Here, we used a combination of experimental and computational approaches to examine the individual and combined effects of activation of the vascular cell adhesion molecule (VCAM)-1, interleukin (IL)-8, and IL-1β signaling pathways on the integrity of vascular junctions. Our simulations predict a multifaceted interplay of signaling resulting from individual activation of VCAM-1, IL-8 and IL-1β pathways that is neither synergistic nor additive compared to all inputs turned on simultaneously. Furthermore, we show that the levels of phosphorylated proteins associated with actinomyosin contractility and junction disassembly peak prior to those related to actin remodeling. The results of this work provide insight into the dynamics of tumor-mediated endothelial junction disassembly and suggest that targeting proteins downstream of several interaction pathways may be the most effective therapeutic approach to reduce melanoma extravasation.

Melanoma upregulates ICAM-1 expression on endothelial cells through engagement of tumor CD44 with endothelial E-selectin and activation of a PKCα-p38-SP-1 pathway

Cancer metastasis involves multistep adhesive interactions between tumor cells (TCs) and endothelial cells (ECs), but the molecular mechanisms of intercellular communication in the tumor microenvironment remain elusive. Using static and flow coculture systems in conjunction with flow cytometry, we discovered that certain receptors on the ECs are upregulated on melanoma cell adhesion. Direct contact but not separate coculture between human umbilical endothelial cells (HUVECs) and a human melanoma cell line (Lu1205) increased intercellular adhesion molecule 1 (ICAM-1) and E-selectin expression on HUVECs by 3- and 1.5-fold, respectively, compared with HUVECs alone. The nonmetastatic cell line WM35 failed to promote ICAM-1 expression changes in HUVECs on contact. Enzyme-linked immunosorbent assay (ELISA) revealed that EC-TC contact has a synergistic effect on the expression of the cytokines interleukin (IL)-8, IL-6, and growth-related oncogene α (Gro-α). By using E-selectin cross-linking and beads coated with CD44 immunopurified from Lu1205 cells, we showed that CD44/selectin ligation was responsible for the ICAM-1 up-regulation on HUVECs. Protein kinase Cα (PKC-α) activation was found to be the downstream target of the CD44/selectin-initiated signaling, as ICAM-1 elevation was inhibited by siRNA targeting PKCα or a dominant negative form of PKCα (PKCα DN). Western blot analysis and electrophoretic mobility shift assays (EMSAs) showed that TC-EC contact mediated p38 phosphorylation and binding of the transcription factor SP-1 to its regulation site. In conclusion, CD44/selectin binding signals ICAM-1 up-regulation on the EC surface through a PKCα-p38-SP-1 pathway, which further enhances melanoma cell adhesion to ECs during metastasis.

Lipid-polymer nanoparticles encapsulating curcumin for modulating the vascular deposition of breast cancer cells

Vascular adhesion and endothelial transmigration are critical steps in the establishment of distant metastasis by circulating tumor cells (CTCs). Also, vascular inflammation plays a pivotal role in steering CTCs out of the blood stream. Here, long circulating lipid-polymer nanoparticles encapsulating curcumin (NANOCurc) are proposed for modulating the vascular deposition of CTCs. Upon treatment with NANOCurc, the adhesion propensity of highly metastatic breast cancer cells (MDA-MB-231) onto TNF-α stimulated endothelial cells (HUVECs) reduces by ~70%, in a capillary flow. Remarkably, the CTCs vascular deposition already reduces up to ~50% by treating solely the inflamed HUVECs. The CTCs arrest is mediated by the interaction between ICAM-1 on HUVECs and MUC-1 on cancer cells, and moderate doses of curcumin down-regulate the expression of both molecules. This suggests that NANOCurc could prevent metastasis and limit the progression of the disease by modulating vascular inflammation and impairing the CTCs arrest.

FROM THE CLINICAL EDITOR

In this novel study, lipid nanoparticles encapsulating curcumin were able to prevent metastasis formation and limited the progression of the disease by modulating vascular inflammation and impairing the circulating tumor cells' arrest as a result of down-regulation of ICAM1 and MUC1 in a highly metastatic breast cancer cell line model.

Melanoma stem cells and metastasis: mimicking hematopoietic cell trafficking?

Malignant melanoma is a highly metastatic cancer that bears responsibility for the majority of skin cancer-related deaths. Amidst the research efforts to better understand melanoma progression, there has been increasing evidence that hints at a role for a subpopulation of virulent cancer cells, termed malignant melanoma stem or initiating cells (MMICs), in metastasis formation. MMICs are characterized by their preferential ability to initiate and propagate tumor growth and their selective capacity for self-renewal and differentiation into less tumorigenic melanoma cells. The frequency of MMICs has been shown to correlate with poor clinical prognosis in melanoma. In addition, MMICs are enriched among circulating tumor cells in the peripheral blood of cancer patients, suggesting that MMICs may be a critical factor in the metastatic cascade. Although these links exist between MMICs and metastatic disease, the mechanisms by which MMICs may advance metastatic progression are only beginning to be elucidated. Recent studies have shown that MMICs express molecules critical for hematopoietic cell maintenance and trafficking, providing a possible explanation for how circulating MMICs could drive melanoma dissemination. We therefore propose that MMICs might fuel melanoma metastasis by exploiting homing mechanisms commonly utilized by hematopoietic cells. Here we review the biological properties of MMICs and the existing literature on their metastatic potential. We will discuss possible mechanisms by which MMICs might initiate metastases in the context of established knowledge of cancer stem cells in other cancers and of hematopoietic homing molecules, with a particular focus on selectins, integrins, chemokines and chemokine receptors known to be expressed by melanoma cells. Biological understanding of how these molecules might be utilized by MMICs to propel the metastatic cascade could critically impact the development of more effective therapies for advanced disease.

Monocytes mediate metastatic breast tumor cell adhesion to endothelium under flow

Endothelial adhesion is necessary for the hematogenous dissemination of tumor cells. However, the metastatic breast tumor cell MDA-MB-231 does not bind to the endothelium under physiological flow conditions, suggesting alternate mechanisms of adhesion. Since monocytes are highly represented in the tumor microenvironment, and also bind to endothelium during inflammation, we hypothesized that the monocytes assist in the arrest of MDA-MB-231 on the endothelium. Using in vitro models of the dynamic shear environment of the vasculature, we show that TNF-α-activated THP1/primary human monocytes and MDA-MB-231 cells form stable aggregates, and that the monocytes in these aggregates mediate the adhesion of otherwise nonadherent MDA-MB-231 cells to inflamed endothelium under flow (55±2.4 vs. 1.7±0.82 at a shear stress of 0.5 dyn/cm(2), P<0.01). We also show that the hydrodynamic forces determine the size and orientation of aggregates adhered to the endothelium, and strongly favor the attachment of small aggregates with tumor cells downstream of flow (74-86% doublets at 0.5-2 dyn/cm(2), P<0.01). The 5-fold up-regulation of ICAM-1 on TNF-α-activated MDA-MB-231 cells through the Nf-κB pathway was found to be critical in MDA-MB-231-monocyte aggregation and endothelial adhesion. Our results demonstrate that, under inflammatory conditions, monocytes may serve to disseminate tumor cells through circulation, and the tumor-monocyte-endothelial axis may represent a new therapeutic target to reduce cancer metastasis.

Gene therapy for advanced melanoma: selective targeting and therapeutic nucleic acids

Despite recent advances, the treatment of malignant melanoma still results in the relapse of the disease, and second line treatment mostly fails due to the occurrence of resistance. A wide range of mutations are known to prevent effective treatment with chemotherapeutic drugs. Hence, approaches with biopharmaceuticals including proteins, like antibodies or cytokines, are applied. As an alternative, regimens with therapeutically active nucleic acids offer the possibility for highly selective cancer treatment whilst avoiding unwanted and toxic side effects. This paper gives a brief introduction into the mechanism of this devastating disease, discusses the shortcoming of current therapy approaches, and pinpoints anchor points which could be harnessed for therapeutic intervention with nucleic acids. We bring the delivery of nucleic acid nanopharmaceutics into perspective as a novel antimelanoma therapeutic approach and discuss the possibilities for melanoma specific targeting. The latest reports on preclinical and already clinical application of nucleic acids in melanoma are discussed.

Expression of E-selectin ligands on circulating tumor cells: cross-regulation with cancer stem cell regulatory pathways?

Although significant progress has been made in the fight against cancer, successful treatment strategies have yet to be developed to combat those tumors that have metastasized to distant organs. Poor characterization of the molecular mechanisms of cancer spread is a major impediment to designing predictive diagnostics and effective clinical interventions against late stage disease. In hematogenous metastasis, it is widely suspected that circulating tumor cells (CTCs) express specific adhesion molecules that actively initiate contact with the vascular endothelium lining the vessel walls of the target organ. This "tethering" is mediated by ligands expressed by CTCs that bind to E-selectin expressed by endothelial cells. However, it is currently unknown whether expression of functional E-selectin ligands on CTCs is related to cancer stem cell regulatory or maintenance pathways, particularly epithelial-to-mesenchymal transition and the reverse, mesenchymal-to-epithelial transition. In this hypothesis and theory article, we explore the potential roles of these mechanisms on the dynamic regulation of selectin ligands mediating CTC trafficking during metastasis.

Tumor necrosis factor-α enhanced fusions between oral squamous cell carcinoma cells and endothelial cells via VCAM-1/VLA-4 pathway

Fusion between cancer cells and host cells, including endothelial cells, may strongly modulate the biological behavior of tumors. However, no one is sure about the driving factors and underlying mechanism involved in such fusion. We hypothesized in this study that inflammation, one of the main characteristics in tumor microenvironment, serves as a prominent catalyst for fusion events. Our results showed that oral cancer cells can fuse spontaneously with endothelial cells in co-culture and inflammatory cytokine tumor necrosis factor-α (TNF-α) increased fusion of human umbilical vein endothelium cells and oral cancer cells by up to 3-fold in vitro. Additionally, human oral squamous cell carcinoma cell lines and 35 out of 50 (70%) oral squamous carcinoma specimens express VLA-4, an integrin, previously implicated in fusions between human peripheral blood CD34-positive cells and murine cardiomyocytes. Expression of VCAM-1, a ligand for VLA-4, was evident on vascular endothelium of oral squamous cell carcinoma. Moreover, immunocytochemistry and flow cytometry analysis revealed that expression of VCAM-1 increased obviously in TNF-α-stimulated endothelial cells. Anti-VLA-4 or anti-VCAM-1 treatment can decrease significantly cancer-endothelial adhesion and block such fusion. Collectively, our results suggested that TNF-α could enhance cancer-endothelial cell adhesion and fusion through VCAM-1/VLA-4 pathway. This study provides insights into regulatory mechanism of cancer-endothelial cell fusion, and has important implications for the development of novel therapeutic strategies for prevention of metastasis.

Cancer cell adhesion and metastasis: selectins, integrins, and the inhibitory potential of heparins

Cell adhesion molecules play a significant role in cancer progression and metastasis. Cell-cell interactions of cancer cells with endothelium determine the metastatic spread. In addition, direct tumor cell interactions with platelets, leukocytes, and soluble components significantly contribute to cancer cell adhesion, extravasation, and the establishment of metastatic lesions. Clinical evidence indicates that heparin, commonly used for treatment of thromboembolic events in cancer patients, is beneficial for their survival. Preclinical studies confirm that heparin possesses antimetastatic activities that lead to attenuation of metastasis in various animal models. Heparin contains several biological activities that may affect several steps in metastatic cascade. Here we focus on the role of cellular adhesion receptors in the metastatic cascade and discuss evidence for heparin as an inhibitor of cell adhesion. While P- and L-selectin facilitation of cellular contacts during hematogenous metastasis is being accepted as a potential target of heparin, here we propose that heparin may also interfere with integrin activity and thereby affect cancer progression. This review summarizes recent findings about potential mechanisms of tumor cell interactions in the vasculature and antimetastatic activities of heparin.

Fibrin serves as a divalent ligand that regulates neutrophil-mediated melanoma cells adhesion to endothelium under shear conditions

Elevated soluble fibrin (sFn) levels are characteristic of melanoma hematogeneous dissemination, where tumor cells interact intimately with host cells. Melanoma adhesion to the blood vessel wall is promoted by immune cell arrests and tumor-derived thrombin, a serine protease that converts soluble fibrinogen (sFg) into sFn. However, the molecular requirement for sFn-mediated melanoma-polymorphonuclear neutrophils (PMNs) and melanoma-endothelial interactions under physiological flow conditions remain elusive. To understand this process, we studied the relative binding capacities of sFg and sFn receptors e.g., α(v)β(3) integrin and intercellular adhesion molecule-1 (ICAM-1) expressed on melanoma cells, ICAM-1 on endothelial cells (EC), and CD11b/CD18 (Mac-1) on PMNs. Using a parallel-plate flow chamber, highly metastatic melanoma cells (1205Lu and A375M) and human PMNs were perfused over an EC monolayer expressing ICAM-1 in the presence of sFg or sFn. It was found that both the frequency and lifetime of direct melanoma adhesion or PMN-facilitated melanoma adhesion to the EC in a shear flow were increased by the presence of sFn in a concentration-dependent manner. In addition, sFn fragment D and plasmin-treated sFn failed to increase melanoma adhesion, implying that sFn-bridged cell adhesion requires dimer-mediated receptor-receptor cross-linking. Finally, analysis of the respective kinetics of sFn binding to Mac-1, ICAM-1, and α(v)β(3) by single bond cell tethering assays suggested that ICAM-1 and α(v)β(3) are responsible for initial capture and firm adhesion of melanoma cells. These results provide evidence that sFn enhances melanoma adhesion directly to ICAM-1 on the EC, while prolonged shear-resistant melanoma adhesion requires interactions with PMNs.

The inhibition of the integrin VLA-4 in MV3 melanoma cell binding by non-anticoagulant heparin derivatives

INTRODUCTION

The integrin VLA-4-mediated binding is important for the metastatic dissemination of melanoma cells. Recently we found that heparin possesses a binding capacity to VLA-4. This could contribute to the heparin function to attenuate metastasis in a selectin-dependent manner. Aiming to a purposive, anti-adhesive heparin application, structural requirements of heparin for VLA-4 recognition have to be elucidated.

MATERIALS AND METHODS

A series of non-anticoagulant heparin derivatives were investigated concerning their inhibitory capacities for VLA-4 mediated binding of human melanoma MV3 cells to VCAM-1 under physiological flow conditions in vitro. A surface acoustic wave biosensor was applied to detect kinetic constants of selected derivatives binding to both, VLA-4 or P- and L-selectin.

RESULTS

Experimental metastasis of MV3 cells in mice confirmed the relevance of VLA-4 for metastatic dissemination. LMWHs (enoxaparin, tinzaparin) efficiently blocked VLA-4 cell binding, dominantly via the integrin`s α-chain. Desulfation at 2-O-position, N-acetylation or a size smaller than tetradecasaccharide disfavoured VLA-4 inhibition. Glycol-splitting of heparin and thus higher chain flexibility is a tolerable parameter. A derivative with 50% 6-O-desulfation appeared promising and exceeded tinzaparin in VLA-4 inhibition, both compounds displayed binding affinities to VLA-4 in the low micromolar range.

CONCLUSIONS

These findings provide structure-activity relationships for heparin VLA-4 binding, which partly differ from P- and L-selectin requirements. The data confirm that anti-coagulative and anti-adhesive function of heparin can be distinguished favouring applications of non-anticoagulant heparins in antimetastatic approaches without the risk of bleeding complications. The 50% 6-O-desulfated heparin-derivative appears promising to further evaluate the interference with selectin and VLA-4 binding functions in vivo.

Determining beta2-integrin and intercellular adhesion molecule 1 binding kinetics in tumor cell adhesion to leukocytes and endothelial cells by a gas-driven micropipette assay

Interactions between polymorphonuclear neutrophils (PMNs) and tumor cells have been reported to facilitate the adhesion and subsequent extravasation of tumor cells through the endothelium under blood flow, both of which are mediated by binding β(2)-integrin to intercellular adhesion molecule 1 (ICAM-1). Here the adhesions between human WM9 metastatic melanoma cells, PMNs, and human pulmonary microvascular endothelial cells (HPMECs) were quantified by a gas-driven micropipette aspiration technique (GDMAT). Our data indicated that the cellular binding affinity of PMN-WM9 pair was 3.9-fold higher than that of the PMN-HPMEC pair. However, the effective binding affinities per molecular pair were comparable between the two cell pairs no matter whether WM9 cells or HPMECs were quiescent or cytokine-activated, indicating that the stronger adhesion between PMN-WM9 pair is mainly attributed to the high expression of ICAM-1 on WM9 cells. These results proposed an alternative mechanism, where WM9 melanoma cells adhere first with PMNs near vessel-wall regions and then bind to endothelial cells via PMNs under blood flow. In contrast, the adhesions between human MDA-MB-231 metastatic breast carcinoma cells and PMNs showed a comparable cellular binding affinity to PMN-HPMEC pair because the ICAM-1 expressions on MDA-MB-231 cells and HPMECs are similar. Furthermore, differences were observed in the intrinsic forward and reverse rates of the β(2)-integrin-ICAM-1 bond between PMN-TC and PMN-EC pairs. This GDMAT assay enables us to quantify the binding kinetics of cell adhesion molecules physiologically expressed on nucleated cells. The findings also further the understanding of leukocyte-facilitated tumor cell adhesion from the viewpoint of molecular binding kinetics.

A novel function of junctional adhesion molecule-C in mediating melanoma cell metastasis

Hematogenous dissemination of melanoma is a life-threatening complication of this malignant tumor. Here, we identified junctional adhesion molecule-C (JAM-C) as a novel player in melanoma metastasis to the lung. JAM-C expression was identified in human and murine melanoma cell lines, in human malignant melanoma, as well as in metastatic melanoma including melanoma lung metastasis. JAM-C expressed on both murine B16 melanoma cells as well as on endothelial cells promoted the transendothelial migration of the melanoma cells. We generated mice with inactivation of JAM-C. JAM-C(-/-) mice as well as endothelial-specific JAM-C-deficient mice displayed significantly decreased B16 melanoma cell metastasis to the lung, whereas treatment of mice with soluble JAM-C prevented melanoma lung metastasis. Together, JAM-C represents a novel therapeutic target for melanoma metastasis.

A miniature Couette to generate shear for flow cytometry: studying real-time modulation of intracellular calcium in monocytic cells

Extracellular hydrodynamic forces may be transmitted to the interior of cells through the alteration of integrin conformation and affinity. Integrin activation regulates leukocyte recruitment, cell activation, and transmigration. The cellular and molecular mechanisms for integrin activation are not precisely known, although intracellular calcium signaling is involved. Flow cytometry offers a versatile way to study intracellular calcium signaling in real-time. We report a novel method to generate defined shear by using a miniature Couette. Testing involved measuring shear-induced intracellular calcium signals of human monoblastoid U937 cells in suspension. The Couette was connected externally to a flow cytometer and pressurized at 6 PSI (4.1 N/m(2) ). Cells were subjected to a well-defined shear between 0 and 1,000 s(-1) and delivered continuously within 10 s to a FACScan at 1 μl/s. Intracellular calcium levels and the percentage of cells activated increased as shear increased in duration and intensity.

UVB-irradiation regulates VLA-4-mediated melanoma cell adhesion to endothelial VCAM-1 under flow conditions

The major aspect contributing to the mortality of melanoma is its ability to spread, or metastasize. Ultraviolet B light (UVB) is considered an indirect cause of melanoma formation. However, little is known about the potential effects of UVB to melanoma metastasis. Integrins, a large family of cell adhesion molecules (CAMs) expressed on the melanoma cell surface, are important for cell signaling, growth, and migration during metastasis. Most critically, tumor cell tissue invasion is dependent on the initial interaction of tumor cells with vascular endothelium at the target organ, and there is increasing evidence for a prominent role of melanoma very late antigen-4 (VLA-4) integrin binding to its endothelial ligand vascular cell adhesion molecule-1 (VCAM-1) in this process. This research focuses on the quantitative modulation of VLA-4 integrin expression and function on melanoma cells after UVB irradiation. The present data show that at 3, 12, and 18 h post-UVB irradiation, VLA-4 expression was unchanged relative to untreated cells, but adhesion to VCAM-1 decreased significantly. Immunofluorescence studies implied that the spatial organization of VLA-4 on the melanoma cell surface contributed to the changes in avidity for VCAM-1 upon UVB irradiation. With increased understanding of the molecular mechanisms underlying melanoma-endothelial interactions upon UVB irradiation, clinical advances for melanoma may be developed.

Adhesion of bio-functionalized ultrasound microbubbles to endothelial cells by targeting to vascular cell adhesion molecule-1 under shear flow

The expression of certain endothelial cell adhesion molecules is increased during endothelial dysfunction or inflammatory activation. This has led to the concept of using microbubbles for targeted molecular imaging or drug delivery. In this approach, microbubbles with a specific ligand to receptors expressed at the site of specific diseases are constructed. The present study aimed to engineer a novel type of bio-functionalized microbubbles (vascular cell adhesion molecule 1 [VCAM-1]-targeted microbubbles), and determine whether VCAM-1-targeted microbubbles exhibit specific adhesion to lipopolysaccharide (LPS)-activated endothelial cells. Our data showed that VCAM-1 expression was significantly upregulated in both LPS-activated endothelial cells in vitro and endothelium in a rat atherosclerosis model in vivo. Targeted microbubbles were designed by conjugating anti-VCAM-1 monoclonal antibodies to the shell of microbubbles using biotin-avidin bridging chemistry methods. Microbubble adhesion to endothelial cells was assessed in a flow chamber at two shear stress conditions (6.3 and 10.4 dynes/cm²). Our data showed that microbubble adhesion depends on both the surface anti-VCAM-1 antibody densities and the exposed shear stresses. Adhesion of VCAM-1-targeted microbubbles onto LPS-activated endothelial cells increased with the surface antibody densities, and decreased with the exposed shear stresses. These findings showed that the specific ligand-carrying microbubbles have considerable potential in targeted ultrasound molecular imaging or ultrasound-assisted drug/gene delivery applications.

Sequential binding of αVβ3 and ICAM-1 determines fibrin-mediated melanoma capture and stable adhesion to CD11b/CD18 on neutrophils

Fibrin (Fn) deposition defines several type 1 immune responses, including delayed-type hypersensitivity and autoimmunity in which polymorphonuclear leukocytes (PMNs) are involved. Fn monomer and fibrinogen are multivalent ligands for a variety of cell receptors during cell adhesion. These cell receptors provide critical linkage among thrombosis, inflammation, and cancer metastasis under venous flow conditions. However, the mechanisms of Fn-mediated interactions among immune cells and circulating tumor cells remain elusive. By using a cone-plate viscometer shear assay and dual-color flow cytometry, we demonstrated that soluble fibrinogen and Fn had different abilities to enhance heterotypic aggregation between PMNs and Lu1205 melanoma cells in a shear flow, regulated by thrombin levels. In addition, the involvement of integrin α(v)β(3), ICAM-1, and CD11b/CD18 (Mac-1) in fibrin(ogen)-mediated melanoma-PMN aggregations was explored. Kinetic studies provided evidence that ICAM-1 mediated initial capture of melanoma cells by PMNs, whereas α(v)β(3) played a role in sustained adhesion of the two cell types at a shear rate of 62.5 s(-1). Quantitative analysis of the melanoma-PMN interactions conducted by a parallel-plate flow chamber assay further revealed that at a shear rate of 20 s(-1), α(v)β(3) had enough contact time to form bonds with Mac-1 via Fn, which could not otherwise occur at a shear rate higher than 62.5 s(-1). Our studies have captured a novel finding that leukocytes could be recruited to tumor cells via thrombin-mediated Fn formation within a tumor microenvironment, and α(v)β(3) and ICAM-1 may participate in multistep fibrin(ogen)-mediated melanoma cell adhesion within the circulation.

The roles of Akt and NOSs in regulation of VLA-4-mediated melanoma cell adhesion to endothelial VCAM-1 after UVB-irradiation

UVB-reduced avidity between M624 melanoma and HUVEC cells is dependent on the interaction of VLA-4 with its endothelial ligand VCAM-1. Our previous studies suggested that a spatial organization of α4 integrin, one of the two subunits of VLA-4, on the melanoma cell surface contributed to the changes in avidity for VCAM-1 upon UVB-irradiation. In this study, we demonstrate that Akt plays an important role in regulation of the expression and surface level of α4 integrin on melanoma cells upon UVB-irradiation. While the cell surface level of α4 integrin is not significantly affected by UVB-irradiation or Akt inhibitor alone, it is dynamically altered after UVB-irradiation when Akt is inhibited. Inhibition of Akt also reverses the reduction of avidity of cells after the irradiation. Our data also shows that UVB reduces the level of Akt. The inhibition of Akt activity correlates with a reduced amount of coupled cNOS and reduced amount of iNOS after UVB-irradiation. However, the effect of NOSs on melanoma cell adhesion appears due to their roles in regulation of apoptosis after UVB-irradiation. Base on these results, we propose that the UVB-induced reduction of avidity of melanoma cells is coordinatively regulated by NOSs and Akt through two differential mechanisms.

Cancer dissemination--lessons from leukocytes

Cancer cells can move through tissues in a variety of different ways. In some cases, an epithelial-to-mesenchymal transition enables cancer cells to acquire fibroblast-like migratory properties. However, it is also becoming apparent that some cancer cells move in an amoeboid way similar to leukocytes. This theme will be the focus of the review, where we will discuss the similarities and differences between the mechanisms used by cancer cells and leukocytes to cross parenchymal basement membranes, move through interstitial tissue, and enter and exit the vasculature. Further, we propose that the ability to switch between different migratory mechanisms is critical for cells to relocate from one tissue to another.

p38 MAP kinase is necessary for melanoma-mediated regulation of VE-cadherin disassembly

Vascular endothelial (VE)-cadherin is localized to the endothelial borders and the adherens junctions, which are regulated by changes in mitogen-activated protein (MAP) kinases, GTPases, and intracellular calcium. We previously showed that melanoma cells induce VE-cadherin disassembly through contact with human umbilical vein endothelial cells in coculture. However, the exact mechanism by which melanoma cells signal endothelial cells to induce VE-cadherin junction disassembly is not well understood. In this study, VE-cadherin junction disassembly was further examined under fluorescence microscopy. We found that melanoma-induced VE-cadherin junction disassembly and upregulation of p38 MAP kinase in endothelial cells is regulated by both soluble factors from melanomas, particularly interleukin (IL)-8, IL-6, and IL-1beta, and through vascular cell adhesion molecule-1. Neutralizing melanoma-secreted soluble factors reduced endothelial gap formation. Endothelial cells transfected with MAP kinase kinase 6, a direct activator of p38 MAP kinase, increased VE-cadherin-mediated gap formation, facilitating melanoma transendothelial migration. In contrast, endothelial cells transfected with small-interfering RNA against p38 MAP kinase expression largely prevented melanoma transendothelial migration in Boyden chamber experiments. These findings indicate that p38 MAP kinase proteins regulate VE-cadherin junction disassembly, facilitating melanoma migration across endothelial cells.

Immunoediting of leukocyte functions within the tumor microenvironment promotes cancer metastasis development

Attachment of tumor cells to the endothelium (EC) under flow conditions is critical for migration of tumor cells out of the vascular system to establish metastases. We found that neutrophils (PMN) increased melanoma cell extravasation. Endogenous IL-8 liberated from melanoma cells or from PMN induced by melanoma cells contributed to PMN-facilitated melanoma cell arrest on the EC in the microcirculation. Functional blocking of IL-8 receptors on PMN or neutralizing soluble IL-8 in the tumor circulation decreased the level of CD11b/CD18 up-regulation on PMN and subsequently reduced melanoma cell extravasation. We also found that targeting mutant V600EB-Raf interrupted melanoma cell extravasation in vitro and subsequent lung metastasis development in vivo. B-Raf encodes a RAS-regulated kinase that mediates cell growth and malignant transformation kinase pathway activation. Results showed that inhibition of V600EB-Raf reduced IL-8 secretion from melanoma cells and reduced the capacity of IL-8 production from the tumor microenvironment involving PMN. Furthermore, reduction in intercellular adhesion molecule-1 (ICAM-1) expression on melanoma cells was found after V600EB-Raf knockdown. These results provide new evidence for the complex role of secreted chemokine and PMN-melanoma adhesion in the recruitment of metastatic cancer cells to the EC, which are significant in fostering new approaches to cancer treatment through anti-inflammatory therapeutics.

Dynamic regulation of ROCK in tumor cells controls CXCR4-driven adhesion events

CXCR4 is a chemokine receptor often found aberrantly expressed on metastatic tumor cells. To investigate CXCR4 signaling in tumor cell adhesion, we stably overexpressed CXCR4 in MCF7 breast tumor cells. Cell attachment assays demonstrate that stimulation of the receptor with its ligand, CXCL12, promotes adhesion of MCF7-CXCR4 cells to both extracellular matrix and endothelial ligands. To more closely mimic the conditions experienced by a circulating tumor cell, we performed the attachment assays under shear stress conditions. We found that CXCL12-induced tumor cell attachment is much more pronounced under flow. ROCK is a serine/threonine kinase associated with adhesion and metastasis, which is regulated by CXCR4 signaling. Thus, we investigated the contribution of ROCK activity during CXC12-induced adhesion events. Our results demonstrate a biphasic regulation of ROCK in response to adhesion. During the initial attachment, inhibition of ROCK activity is required. Subsequently, re-activation of ROCK activity is required for maturation of adhesion complexes and enhanced tumor cell migration. Interestingly, CXCL12 partially reduces the level of ROCK activity generated by attachment, which supports a model in which stimulation with CXCL12 regulates tumor cell adhesion events by providing an optimal level of ROCK activity for effective migration.

Cancer cells in transit: the vascular interactions of tumor cells

Metastasis is a highly regulated, multistep process in which cancerous cells shed from the primary tumor and enter the circulatory system, where they interact extensively with host cells before they lodge and colonize the target organ. The adhesive interactions of circulating tumor cells with platelets, leukocytes, and endothelial cells facilitate their survival and extravasation from the vasculature, thus representing critical kick-off events for the colonization of distant organs. This review presents our current mechanistic knowledge on vascular interactions of tumor cells, and it discusses biochemical and cell and molecular biology techniques used for the identification of novel receptor-ligand pairs mediating these interactions. This review brings together diverse observations about the contributions of key molecular constituents, including selectins, fibrin(ogen), and CD44, in one mechanistic interpretation. Understanding the molecular underpinnings of adhesive interactions between tumor cells and host cells may provide guidelines for developing promising antimetastatic therapies when initiated early in the course of disease progression.

Expression of adhesion molecules and ligands for activating and costimulatory receptors involved in cell-mediated cytotoxicity in a large panel of human melanoma cell lines

Knowledge of the interactions between MHC-unrestricted cytotoxic effector cells and solid tumour cells is essential for introducing more effective NK cell-based immunotherapy protocols into clinical practise. Here, to begin to obtain an overview of the possible universe of molecules that could be involved in the interactions between immune effector cells and melanoma, we analyse the surface expression of adhesion and costimulatory molecules and of ligands for NK-activating receptors on a large panel of cell lines from the "European Searchable Tumour Cell Line and Data Bank" (ESTDAB, http://www.ebi.ac.uk/ipd/estdab/ ) and discuss their potential role in the immune response against this tumour. We show that most melanoma cell lines express not only adhesion molecules that are likely to favour their interaction with cells of the immune system, but also their interaction with endothelial cells potentially increasing their invasiveness and metastatic capacity. A high percentage of melanoma cell lines also express ligands for the NK-activating receptor NKG2D; whereas, the majority express MICA/B molecules, ULBP expression, however, was rarely found. In addition to these molecules, we also found that CD155 (poliovirus receptor, PVR) is expressed by the majority of melanoma cell lines, whereas CD112 (Nectin-2) expression was rare. These molecules are DNAM-1 ligands, a costimulatory molecule involved in NK cell-mediated cytotoxicity and cytokine production that also mediates costimulatory signals for triggering naïve T cell differentiation. The phenotypical characterisation of adhesion molecules and ligands for receptors involved in cell cytotoxicity on a large series of melanoma cell lines will contribute to the identification of markers useful for the development of new immunotherapy strategies.