Role of Rho/ROCK signaling in the interaction of melanoma cells with the blood-brain barrier

Immune escape and metastasis mechanisms in melanoma: breaking down the dichotomy

Melanoma is one of the most lethal neoplasms of the skin. Despite the revolutionary introduction of immune checkpoint inhibitors, metastatic spread, and recurrence remain critical problems in resistant cases. Melanoma employs a multitude of mechanisms to subvert the immune system and successfully metastasize to distant organs. Concerningly, recent research also shows that tumor cells can disseminate early during melanoma progression and enter dormant states, eventually leading to metastases at a future time. Immune escape and metastasis have previously been viewed as separate phenomena; however, accumulating evidence is breaking down this dichotomy. Recent research into the progressive mechanisms of melanoma provides evidence that dedifferentiation similar to classical epithelial to mesenchymal transition (EMT), genes involved in neural crest stem cell maintenance, and hypoxia/acidosis, are important factors simultaneously involved in immune escape and metastasis. The likeness between EMT and early dissemination, and differences, also become apparent in these contexts. Detailed knowledge of the mechanisms behind "dual drivers" simultaneously promoting metastatically inclined and immunosuppressive environments can yield novel strategies effective in disabling multiple facets of melanoma progression. Furthermore, understanding progression through these drivers may provide insight towards novel treatments capable of preventing recurrence arising from dormant dissemination or improving immunotherapy outcomes.

The origin of brain malignancies at the blood-brain barrier

Despite improvements in extracranial therapy, survival rate for patients suffering from brain metastases remains very poor. This is coupled with the incidence of brain metastases continuing to rise. In this review, we focus on core contributions of the blood-brain barrier to the origin of brain metastases. We first provide an overview of the structure and function of the blood-brain barrier under physiological conditions. Next, we discuss the emerging idea of a pre-metastatic niche, namely that secreted factors and extracellular vesicles from a primary tumor site are able to travel through the circulation and prime the neurovasculature for metastatic invasion. We then consider the neurotropic mechanisms that circulating tumor cells possess or develop that facilitate disruption of the blood-brain barrier and survival in the brain's parenchyma. Finally, we compare and contrast brain metastases at the blood-brain barrier to the primary brain tumor, glioma, examining the process of vessel co-option that favors the survival and outgrowth of brain malignancies.

Rho GTPase signalling networks in cancer cell transendothelial migration

Rho GTPases are small signalling G-proteins that are central regulators of cytoskeleton dynamics, and thereby regulate many cellular processes, including the shape, adhesion and migration of cells. As such, Rho GTPases are also essential for the invasive behaviour of cancer cells, and thus involved in several steps of the metastatic cascade, including the extravasation of cancer cells. Extravasation, the process by which cancer cells leave the circulation by transmigrating through the endothelium that lines capillary walls, is an essential step for metastasis towards distant organs. During extravasation, Rho GTPase signalling networks not only regulate the transmigration of cancer cells but also regulate the interactions between cancer and endothelial cells and are involved in the disruption of the endothelial barrier function, ultimately allowing cancer cells to extravasate into the underlying tissue and potentially form metastases. Thus, targeting Rho GTPase signalling networks in cancer may be an effective approach to inhibit extravasation and metastasis. In this review, the complex process of cancer cell extravasation will be discussed in detail. Additionally, the roles and regulation of Rho GTPase signalling networks during cancer cell extravasation will be discussed, both from a cancer cell and endothelial cell point of view.

Picturing Breast Cancer Brain Metastasis Development to Unravel Molecular Players and Cellular Crosstalk

Simple Summary

Breast cancer is a devastating disorder affecting millions of women worldwide. With improved therapeutics for the primary tumor, the appearance of metastasis has been increasing. Breast cancer frequently metastasizes to the brain, constituting a major hurdle without cure and with a poor survival. It is imperative to better understand the mechanisms involved in malignant cell transposition of the brain microvasculature and parenchymal colonization by deciphering the alterations occurring in the tumor and microvascular cells, as well as the occurrence of intercellular communication during the process. We aimed to profile the process of the formation of breast cancer brain metastasis and the timeline of events governing it. We used a specific mouse model of the disease to perform extensive microscopic analyses. We identified phenotypic changes and the activation of relevant molecular players in tumorigenesis, together with vascular alterations, and the occurrence of crosstalk. Our findings unravel putative therapeutic targets to tackle breast cancer brain metastasis.

Abstract

With breast cancer (BC) therapy improvements, the appearance of brain metastases has been increasing, representing a life-threatening condition. Brain metastasis formation involves BC cell (BCC) extravasation across the blood–brain barrier (BBB) and brain colonization by unclear mechanisms. We aimed to disclose the actors involved in BC brain metastasis formation, focusing on BCCs’ phenotype, growth factor expression, and signaling pathway activation, correlating with BBB alterations and intercellular communication. Hippocampi of female mice inoculated with 4T1 BCCs were examined over time by hematoxylin-eosin, immunohistochemistry and immunofluorescence. Well-established metastases were observed at seven days, increasing thereafter. BCCs entering brain parenchyma presented mesenchymal, migratory, and proliferative features; however, with time, they increasingly expressed epithelial markers, reflecting a mesenchymal–epithelial transition. BCCs also expressed platelet-derived growth factor-B, β₄ integrin, and focal adhesion kinase, suggesting autocrine and/or paracrine regulation with adhesion signaling activation, while balance between Rac1 and RhoA was associated with the motility status. Intercellular communication via gap junctions was clear among BCCs, and between BCCs and endothelial cells. Thrombin accumulation, junctional protein impairment, and vesicular proteins increase reflect BBB alterations related with extravasation. Expression of plasmalemma vesicle-associated protein was increased in BCCs, along with augmented vascularization, whereas pericyte contraction indicated mural cells’ activation. Our results provide further understanding of BC brain metastasis formation, disclosing potential therapeutic targets.

Site-specific metastasis: A cooperation between cancer cells and the metastatic microenvironment

The conclusion derived from the information provided in this review is that disseminating tumor cells (DTC) collaborate with the microenvironment of a future metastatic organ site in the establishment of organ‐specific metastasis. We review the basic principles of site‐specific metastasis and the contribution of the cross talk between DTC and the microenvironment of metastatic sites (metastatic microenvironment [MME]) to the establishment of the organ‐specific premetastatic niche; the targeted migration of DTC to the endothelium of the future organ‐specific metastasis; the transmigration of DTC to this site and the seeding and colonization of DTC in their future MME. We also discuss the role played by DTC‐MME interactions on tumor dormancy and on the differential response of tumor cells residing in different MMEs to antitumor therapy. Finally, we summarize some studies dealing with the effects of the MME on a unique site‐specific metastasis—brain metastasis.

Response of the neurovascular unit to brain metastatic breast cancer cells

Therapeutic resistance of cerebral secondary tumours largely depends on unique aspects linked to the neurovascular unit, especially cerebral endothelial cells and astrocytes. By using advanced microscopy techniques, here we explored novel mechanisms related to the neurovascular unit during extravasation and proliferation of triple negative breast cancer cells in the brain. Metastatic mammary carcinoma cells arrested and elongated within one hour in cerebral microvessels, but their number decreased by almost 80% in the first two days. Interestingly, malignant cells induced vasoconstriction and development of intraluminal endothelial plugs, which isolated invading cells from the circulation. During diapedesis - which usually took place on day four and five after inoculation of the tumour cells - continuity of cerebral endothelial tight junctions remained intact, indicating migration of cancer cells through the transcellular pathway. In addition, metastatic cells induced formation of multiluminal vessels and claudin-5-positive endothelial blebs. However, even severe endothelial blebbing could be reversed and the vessel morphology was restored shortly after the tumour cells completed transendothelial migration. Similar to neuro-inflammatory leukocytes, tumour cells migrated not only through the endothelial layer, but through the glia limitans perivascularis as well. Nevertheless, along with the growth of metastatic lesions by co-option of pre-existing capillaries, astrocytes and astrocyte end-feet were gradually expelled from the vessels to the border of the tumour. Taken together, we identified previously unknown mechanisms involved in the reaction of brain resident cells to invading breast cancer cells. Our results contribute to a better understanding of the complex cross-talk between tumour cells and host cells in the brain, which is essential for the identification of new therapeutic targets in this devastating disease.

Foe or friend? Janus-faces of the neurovascular unit in the formation of brain metastases

Despite the potential obstacle represented by the blood-brain barrier for extravasating malignant cells, metastases are more frequent than primary tumors in the central nervous system. Not only tightly interconnected endothelial cells can hinder metastasis formation, other cells of the brain microenvironment (like astrocytes and microglia) can also be very hostile, destroying the large majority of metastatic cells. However, malignant cells that are able to overcome these harmful mechanisms may benefit from the shielding and even support provided by cerebral endothelial cells, astrocytes and microglia, rendering the brain a sanctuary site against anti-tumor strategies. Thus, cells of the neurovascular unit have a Janus-faced attitude towards brain metastatic cells, being both destructive and protective. In this review, we present the main mechanisms of brain metastasis formation, including those involved in extravasation through the brain vasculature and survival in the cerebral environment.

Mangiferin inhibits cell migration and invasion through Rac1/WAVE2 signalling in breast cancer

Breast tumour progression results from the advancement of the disease to a metastatic phenotype. Rac1 and Cdc42 belong to the Rho family of genes that, together with their downstream effectors, Wiskott-Aldrich Syndrome protein-family verprolin-homologous protein 2 (WAVE2) and Arp2/3, assume a vital part in cytoskeletal rearrangement and the arrangement of film projections that advance malignant cell relocation and invasion. Mangiferin is a characteristic polyphenolic compound from Mangifera indica L. (Anacardiaceae), ordinarily referred to as mango, that is consumed worldwide as a natural product, including culinary and seasoning applications. Mangiferin delays breast malignancy development and progression by inhibiting different signalling pathways required in mitogenic signalling and metastatic progression. Studies were performed to analyse the impact of mangiferin on Rac1/WAVE2 flagging, relocation and invasion in highly metastatic human MDA-MB-231 mammary cells. Additional studies led to the observation that comparative treatment with mangiferin caused marked reduction in tumour cell movement and invasion. Taken together, these discoveries demonstrate that mangiferin treatment adequately hinders Rac1/WAVE2 flagging and diminishes metastatic phenotypic expression in malignant mammary cells, indicating that mangiferin may provide a benefit as a novel restorative approach in the treatment of metastatic breast cancer.

The Rho-associated kinase inhibitors Y27632 and fasudil promote microglial migration in the spinal cord via the ERK signaling pathway

Rho-associated kinase (ROCK) is a key regulatory protein involved in inflammatory secretion in microglia in the central nervous system. Our previous studies showed that ROCK inhibition enhances phagocytic activity in microglia through the extracellular signal-regulated kinase (ERK) signaling pathway, but its effect on microglial migration was unknown. Therefore, in this study, we investigated the effects of the ROCK inhibitors Y27632 and fasudil on the migratory activity of primary cultured microglia isolated from the spinal cord, and we examined the underlying mechanisms. The microglia were treated with Y27632, fasudil and/or the ERK inhibitor U0126. Cellular morphology was observed by immunofluorescence. Transwell chambers were used to assess cell migration. ERK levels were measured by in-cell western blot assay. Y27632 and fasudil increased microglial migration, and the microglia were irregularly shaped and had many small processes. These inhibitors also upregulated the levels of phosphorylated ERK protein. The ERK inhibitor U0126 suppressed these effects of Y27632 and fasudil. These findings suggest that the ROCK inhibitors Y27632 and fasudil promote microglial migration in the spinal cord through the ERK signaling pathway.

Brain metastasization of breast cancer

Central nervous system metastases have been reported in 15-25% of breast cancer patients, and the incidence is increasing. Moreover, the survival of these patients is generally poor, with reports of a 1-year survival rate of 20%. Therefore, a better knowledge about the determinants of brain metastasization is essential for the improvement of the clinical outcomes. Here, we summarize the current data about the metastatic cascade, ranging from the output of cancer cells from the primary tumour to their colonization in the brain, which involves the epithelial-mesenchymal transition, invasion of mammary tissue, intravasation into circulation, and homing into and extravasation towards the brain. The phenotypic change in malignant cells, and the importance of the microenvironment in the formation of brain metastases are also inspected. Finally, the importance of genetic and epigenetic changes, and the recently disclosed effects of microRNAs in brain metastasization of breast cancer are highlighted.

Direct mapping of melanoma cell - endothelial cell interactions

The most life-threatening aspect of cancer is metastasis; cancer patient mortality is mainly due to metastasis. Among all metastases, presence of brain metastasis is one with the poorest prognosis; the median survival time can be counted in months. Therefore, prevention or decreasing their incidence would be highly desired both by patients and physicians. Metastatic cells invading the brain must breach the cerebral vasculature, primarily the blood-brain barrier. The key step in this process is the establishment of firm adhesion between the cancer cell and the cerebral endothelial layer. Using the atomic force microscope, a high-resolution force spectrograph, our aim was to explore the connections among the cell morphology, cellular mechanics, and biological function in the process of transendothelial migration of metastatic cancer cells. By immobilization of a melanoma cell to an atomic force microscope's cantilever, intercellular adhesion was directly measured at quasi-physiological conditions. Hereby, we present our latest results by using this melanoma-decorated probe. Binding characteristics to a confluent layer of brain endothelial cells was directly measured by means of single-cell force spectroscopy. Adhesion dynamics and strength were characterized, and we present data about spatial distribution of elasticity and detachment strength. These results highlight the importance of cellular mechanics in brain metastasis formation and emphasize the enormous potential toward exploration of intercellular dynamic-related processes.

STAT3 pathway regulates lung-derived brain metastasis initiating cell capacity through miR-21 activation

Brain metastases (BM) represent the most common tumor to affect the adult central nervous system. Despite the increasing incidence of BM, likely due to consistently improving treatment of primary cancers, BM remain severely understudied. In this study, we utilized patient-derived stem cell lines from lung-to-brain metastases to examine the regulatory role of STAT3 in brain metastasis initiating cells (BMICs). Annotation of our previously described BMIC regulatory genes with protein-protein interaction network mapping identified STAT3 as a novel protein interactor. STAT3 knockdown showed a reduction in BMIC self-renewal and migration, and decreased tumor size in vivo. Screening of BMIC lines with a library of STAT3 inhibitors identified one inhibitor to significantly reduce tumor formation. Meta-analysis identified the oncomir microRNA-21 (miR-21) as a target of STAT3 activity. Inhibition of miR-21 displayed similar reductions in BMIC self-renewal and migration as STAT3 knockdown. Knockdown of STAT3 also reduced expression of known downstream targets of miR-21. Our studies have thus identified STAT3 and miR-21 as cooperative regulators of stemness, migration and tumor initiation in lung-derived BM. Therefore, STAT3 represents a potential therapeutic target in the treatment of lung-to-brain metastases.

Elasto-mechanical properties of living cells

The possibility to directly measure the elasticity of living cell has emerged only in the last few decades. In the present study the elastic properties of two cell lines were followed. Both types are widely used as cell barrier models (e.g. blood-brain barrier). During time resolved measurement of the living cell elasticity a continuous quasi-periodic oscillation of the elastic modulus was observed. Fast Fourier transformation of the signals revealed that a very limited number of three to five Fourier terms fitted the signal in the case of human cerebral endothelial cells. In the case of canine kidney epithelial cells more than 8 Fourier terms did not result a good fit. Calculating the correlation between nucleus and periphery of the signals revealed a higher correlation factor for the endothelial cells compared to the epithelial cells.

Transmigration characteristics of breast cancer and melanoma cells through the brain endothelium: Role of Rac and PI3K

Brain metastases are common and devastating complications of both breast cancer and melanoma. Although mammary carcinoma brain metastases are more frequent than those originating from melanoma, this latter has the highest tropism to the brain. Using static and dynamic in vitro approaches, here we show that melanoma cells have increased adhesion to the brain endothelium in comparison to breast cancer cells. Moreover, melanoma cells can transmigrate more rapidly and in a higher number through brain endothelial monolayers than breast cancer cells. In addition, melanoma cells have increased ability to impair tight junctions of cerebral endothelial cells. We also show that inhibition of Rac or PI3K impedes adhesion of breast cancer cells and melanoma cells to the brain endothelium. In addition, inhibition of Rac or PI3K inhibits the late phase of transmigration of breast cancer cells and the early phase of transmigration of melanoma cells. On the other hand, the Rac inhibitor EHT1864 impairs the junctional integrity of the brain endothelium, while the PI3K inhibitor LY294002 has no damaging effect on interendothelial junctions. We suggest that targeting the PI3K/Akt pathway may represent a novel opportunity in preventing the formation of brain metastases of melanoma and breast cancer.

Endothelial-mesenchymal transition of brain endothelial cells: possible role during metastatic extravasation

Cancer progression towards metastasis follows a defined sequence of events described as the metastatic cascade. For extravasation and transendothelial migration metastatic cells interact first with endothelial cells. Yet the role of endothelial cells during the process of metastasis formation and extravasation is still unclear, and the interaction between metastatic and endothelial cells during transendothelial migration is poorly understood. Since tumor cells are well known to express TGF-β, and the compact endothelial layer undergoes a series of changes during metastatic extravasation (cell contact disruption, cytoskeletal reorganization, enhanced contractility), we hypothesized that an EndMT may be necessary for metastatic extravasation. We demonstrate that primary cultured rat brain endothelial cells (BEC) undergo EndMT upon TGF-β1 treatment, characterized by the loss of tight and adherens junction proteins, expression of fibronectin, β1-integrin, calponin and α-smooth muscle actin (SMA). B16/F10 cell line conditioned and activated medium (ACM) had similar effects: claudin-5 down-regulation, fibronectin and SMA expression. Inhibition of TGF-β signaling during B16/F10 ACM stimulation using SB-431542 maintained claudin-5 levels and mitigated fibronectin and SMA expression. B16/F10 ACM stimulation of BECs led to phosphorylation of Smad2 and Smad3. SB-431542 prevented SMA up-regulation upon stimulation of BECs with A2058, MCF-7 and MDA-MB231 ACM as well. Moreover, B16/F10 ACM caused a reduction in transendothelial electrical resistance, enhanced the number of melanoma cells adhering to and transmigrating through the endothelial layer, in a TGF-β-dependent manner. These effects were not confined to BECs: HUVECs showed TGF-β-dependent SMA expression when stimulated with breast cancer cell line ACM. Our results indicate that an EndMT may be necessary for metastatic transendothelial migration, and this transition may be one of the potential mechanisms occurring during the complex phenomenon known as metastatic extravasation.

Value of oncogenes for the prediction of brain metastases at initial diagnosis: a review of published data

Identifying cancer patients who are at high risk of developing brain metastases at initial diagnosis and applying effective intervention or monitoring strategies is of vital importance. Recent advances in the biology of brain metastases revealed that some oncogenes from primary tumors may be potential markers for identifying cancer patients likely to metastasize to the brain. We here summarize data on the mechanisms of brain metastases supporting the involvement of oncogene changes in the brain metastatic evolution. We also review the available evidence on clinical studies of oncogenes in the prediction of cancer patients with high incidence of brain metastases.

CB2 receptor activation inhibits melanoma cell transmigration through the blood-brain barrier

During parenchymal brain metastasis formation tumor cells need to migrate through cerebral endothelial cells, which form the morphological basis of the blood-brain barrier (BBB). The mechanisms of extravasation of tumor cells are highly uncharacterized, but in some aspects recapitulate the diapedesis of leukocytes. Extravasation of leukocytes through the BBB is decreased by the activation of type 2 cannabinoid receptors (CB2); therefore, in the present study we sought to investigate the role of CB2 receptors in the interaction of melanoma cells with the brain endothelium. First, we identified the presence of CB1, CB2(A), GPR18 (transcriptional variant 1) and GPR55 receptors in brain endothelial cells, while melanoma cells expressed CB1, CB2(A), GPR18 (transcriptional variants 1 and 2), GPR55 and GPR119. We observed that activation of CB2 receptors with JWH-133 reduced the adhesion of melanoma cells to the layer of brain endothelial cells. JWH-133 decreased the transendothelial migration rate of melanoma cells as well. Our results suggest that changes induced in endothelial cells are critical in the mediation of the effect of CB2 agonists. Our data identify CB2 as a potential target in reducing the number of brain metastastes originating from melanoma.