The Molecular Pathobiology of Metastasis to the Brain: A Review

Tumor microenvironment and exosomes in brain metastasis: Molecular mechanisms and clinical application

Metastasis is one of the important biological features of malignant tumors and one of the main factors responsible for poor prognosis. Although the widespread application of newer clinical technologies and their continuous development have significantly improved survival in patients with brain metastases, there is no uniform standard of care. More effective therapeutic measures are therefore needed to improve prognosis. Understanding the mechanisms of tumor cell colonization, growth, and invasion in the central nervous system is of particular importance for the prevention and treatment of brain metastases. This process can be plausibly explained by the “seed and soil” hypothesis, which essentially states that tumor cells can interact with various components of the central nervous system microenvironment to produce adaptive changes; it is this interaction that determines the development of brain metastases. As a novel form of intercellular communication, exosomes play a key role in the brain metastasis microenvironment and carry various bioactive molecules that regulate receptor cell activity. In this paper, we review the roles and prospects of brain metastatic tumor cells, the brain metastatic tumor microenvironment, and exosomes in the development and clinical management of brain metastases.

Defibrotide suppresses brain metastasis by activating the adenosine A2A receptors

Brain metastasis is a devastating clinical condition globally as one of the most common central nervous system malignancies. The current study aimed to assess the effect of defibrotide, an Food and Drug Administration-approved drug, against brain metastasis and the underlying molecular mechanisms. Two tumor cell lines with high brain metastasis potential, PC-9 and 231-BR, were subjected to defibrotide treatment of increasing dosage. The metastasis capacity of the tumor cells was evaluated by cell invasion and migration assays. Western blotting was employed to determine the levels of tight junction proteins in the blood-brain barrier (BBB) including Occludin, Zo-1, and Claudin-5, as well as metastasis-related proteins including CXCR4, MMP-2, and MMP-9. The in-vitro observations were further verified in nude mice, by monitoring the growth of xenograft tumors, mouse survival and brain metastasis foci following defibrotide treatment. Defibrotide inhibited proliferation, migration, invasion, and promotes lactate dehydrogenase release of brain metastatic tumor cells, elevated the levels of BBB tight junction proteins and metastasis-related proteins. Such beneficial role of defibrotide was mediated by its inhibitory action on the SDF-1/CXCR4 signaling axis both in vitro and in vivo, as CXCR4 agonist SDF1α negated the anti-tumoral effect of defibrotide on mouse xenograft tumor growth, mouse survival and brain metastasis. Defibrotide inhibits brain metastasis through activating the adenosine A2A receptors, which in turn inhibits the SDF-1/CXCR4 signaling axis. Our study hereby proposes defibrotide as a new and promising candidate drug against brain metastasis of multiple organ origins.

Identification of potential genes related to breast cancer brain metastasis in breast cancer patients

Brain metastases (BMs) usually develop in breast cancer (BC) patients. Thus, the molecular mechanisms of breast cancer brain metastasis (BCBM) are of great importance in designing therapeutic strategies to treat or prevent BCBM. The present study attempted to identify novel diagnostic and prognostic biomarkers of BCBM. Two datasets (GSE125989 and GSE100534) were obtained from the Gene Expression Omnibus (GEO) database to find differentially expressed genes (DEGs) in cases of BC with and without brain metastasis (BM). A total of 146 overlapping DEGs, including 103 up-regulated and 43 down-regulated genes, were identified. Functional enrichment analysis showed that these DEGs were mainly enriched for functions including extracellular matrix (ECM) organization and collagen catabolic fibril organization. Using protein-protein interaction (PPI) and principal component analysis (PCA) analysis, we identified ten key genes, including LAMA4, COL1A1, COL5A2, COL3A1, COL4A1, COL5A1, COL5A3, COL6A3, COL6A2, and COL6A1. Additionally, COL5A1, COL4A1, COL1A1, COL6A1, COL6A2, and COL6A3 were significantly associated with the overall survival of BC patients. Furthermore, COL6A3, COL5A1, and COL4A1 were potentially correlated with BCBM in human epidermal growth factor 2 (HER2) expression. Additionally, the miR-29 family might participate in the process of metastasis by modulating the cancer microenvironment. Based on datasets in the GEO database, several DEGs have been identified as playing potentially important roles in BCBM in BC patients.

The Management of Brain Metastases-Systematic Review of Neurosurgical Aspects

Simple Summary

In this comprehensive review, we focused on the neurosurgical treatment as an integrative part of the challenging multidisciplinary management of cerebral metastases, a neuro-oncologic entity, which has been observed to have an increased incidence over the last years. In selected cases, the surgical removal of the space-occupying mass reduces the intracranial pressure, normalizes the metabolic environment, reduces the symptom burden, and allows for the intensification of local and systemic adjuvant treatment. In detail, we discuss the incidence of brain metastases, the role of surgical resection, as well as the evolution of current neurosurgical techniques, the surgical morbidity and mortality of single and multiple lesions, and we enlighten the role of surgery for recurrent tumors.

Abstract

The multidisciplinary management of patients with brain metastases (BM) consists of surgical resection, different radiation treatment modalities, cytotoxic chemotherapy, and targeted molecular treatment. This review presents the current state of neurosurgical technology applied to achieve maximal resection with minimal morbidity as a treatment paradigm in patients with BM. In addition, we discuss the contribution of neurosurgical resection on functional outcome, advanced systemic treatment strategies, and enhanced understanding of the tumor biology.

Adenosine A2A receptor activation reduces brain metastasis via SDF-1/CXCR4 axis and protecting blood-brain barrier

Brain metastasis is a leading cause of death worldwide, but the mechanism involved remains unclear. Stromal cell-derived factor-1 (SDF-1)/C-X-C motif chemokine receptor 4 (CXCR4) signaling has been reported to induce the directed metastasis of cancers, and adenosine A2A receptor activation suppresses the SDF-1/CXCR4 interaction. However, whether A2A receptor activation implicates the SDF-1/CXCR4 signaling pathway and thus modulates brain metastasis remains unclear. In this study, Western blot was performed to evaluate the protein levels. Cell invasion and migration assays were used to estimate the metastasis ability of PC-9 cells. The viability of cells was demonstrated by lactate dehydrogenase and cell proliferation assays. And the findings in vitro were further identified in nude mice. Notably, adenosine A2A receptor activation inhibited the proliferation and viability of PC-9 cells and thus suppressed the brain metastasis. A2A receptor stimulation protected the function of blood-brain barrier (BBB). The suppression of brain metastasis and the protection of BBB by A2A receptor relied on SDF-1/CXCR4 signaling, and treatment using A2A receptor agonist and CXCR4 antagonist protected the nude mice from malignancy metastasis in vivo. Adenosine A2A receptor activation suppressed the brain metastasis by implicating the SDF-1/CXCR4 axis and protecting the BBB.

Mechanisms and Therapy for Cancer Metastasis to the Brain

Advances in chemotherapy and targeted therapies have improved survival in cancer patients with an increase of the incidence of newly diagnosed brain metastases (BMs). Intracranial metastases are symptomatic in 60–70% of patients. Magnetic resonance imaging (MRI) with gadolinium is more sensitive than computed tomography and advanced neuroimaging techniques have been increasingly used in the detection, treatment planning, and follow-up of BM. Apart from the morphological analysis, the most effective tool for characterizing BM is immunohistochemistry. Molecular alterations not always reflect those of the primary tumor. More sophisticated methods of tumor analysis detecting circulating biomarkers in fluids (liquid biopsy), including circulating DNA, circulating tumor cells, and extracellular vesicles, containing tumor DNA and macromolecules (microRNA), have shown promise regarding tumor treatment response and progression. The choice of therapeutic approaches is guided by prognostic scores (Recursive Partitioning Analysis and diagnostic-specific Graded Prognostic Assessment-DS-GPA). The survival benefit of surgical resection seems limited to the subgroup of patients with controlled systemic disease and good performance status. Leptomeningeal disease (LMD) can be a complication, especially in posterior fossa metastases undergoing a “piecemeal” resection. Radiosurgery of the resection cavity may offer comparable survival and local control as postoperative whole-brain radiotherapy (WBRT). WBRT alone is now the treatment of choice only for patients with single or multiple BMs not amenable to surgery or radiosurgery, or with poor prognostic factors. To reduce the neurocognitive sequelae of WBRT intensity modulated radiotherapy with hippocampal sparing, and pharmacological approaches (memantine and donepezil) have been investigated. In the last decade, a multitude of molecular abnormalities have been discovered. Approximately 33% of patients with non-small cell lung cancer (NSCLC) tumors and epidermal growth factor receptor mutations develop BMs, which are targetable with different generations of tyrosine kinase inhibitors (TKIs: gefitinib, erlotinib, afatinib, icotinib, and osimertinib). Other “druggable” alterations seen in up to 5% of NSCLC patients are the rearrangements of the “anaplastic lymphoma kinase” gene TKI (crizotinib, ceritinib, alectinib, brigatinib, and lorlatinib). In human epidermal growth factor receptor 2-positive, breast cancer targeted therapies have been widely used (trastuzumab, trastuzumab-emtansine, lapatinib-capecitabine, and neratinib). Novel targeted and immunotherapeutic agents have also revolutionized the systemic management of melanoma (ipilimumab, nivolumab, pembrolizumab, and BRAF inhibitors dabrafenib and vemurafenib).

High αv Integrin Level of Cancer Cells Is Associated with Development of Brain Metastasis in Athymic Rats

BACKGROUND/AIM

Brain metastases commonly occur in patients with malignant skin, lung and breast cancers resulting in high morbidity and poor prognosis. Integrins containing an αv subunit are cell adhesion proteins that contribute to cancer cell migration and cancer progression. We hypothesized that high expression of αv integrin cell adhesion protein promoted metastatic phenotypes in cancer cells.

MATERIALS AND METHODS

Cancer cells from different origins were used and studied regarding their metastatic ability and intetumumab, anti-αv integrin mAb, sensitivity using in vitro cell migration assay and in vivo brain metastases animal models.

RESULTS

The number of brain metastases and the rate of occurrence were positively correlated with cancer cell αv integrin levels. High αv integrin-expressing cancer cells showed significantly faster cell migration rate in vitro than low αv integrin-expressing cells. Intetumumab significantly inhibited cancer cell migration in vitro regardless of αv integrin expression level. Overexpression of αv integrin in cancer cells with low αv integrin level accelerated cell migration in vitro and increased the occurrence of brain metastases in vivo.

CONCLUSION

αv integrin promotes brain metastases in cancer cells and may mediate early steps in the metastatic cascade, such as adhesion to brain vasculature. Targeting αv integrin with intetumumab could provide clinical benefit in treating cancer patients who develop metastases.

[Tumor Cells and Micro-environment in Brain Metastases]

近年来，随着早期诊断的方法的出现及精准治疗的应用，肺癌患者的生存及生活质量都得到很大改善。然而，对于肺癌的脑转移病灶，目前仍缺乏一个理想的治疗方案，严重影响了该部分患者生存状态。了解肿瘤细胞如何在中枢神经系统定植、生长和侵袭等相关生物学行为及其产生机制对预防及治疗肿瘤细胞脑转移病灶具有重大的意义。“种子-土壤”这一假说可以很好的解释这一过程，这一假说的关键即肿瘤细胞可与中枢神经系统微环境各组成之间产生相互适应性变化，正是这种相互作用决定了脑转移病灶的发生发展。本文就脑转移肿瘤细胞、脑转移肿瘤微环境及他们之间的相互作用进行综述，旨在为脑转移病灶的治疗提供新的思路。

Diagnostic Value of CYFRA 21-1 in the Cerebrospinal Fluid for Leptomeningeal Metastasis

Cerebrospinal fluid (CSF) cytology has low sensitivity for leptomeningeal metastasis (LM); thus, new markers are needed to improve the diagnostic accuracy of LM. We measured carcinoembryonic antigen (CEA) and cytokeratin 19 fragments (CYFRA 21-1) in paired samples of CSF and serum from patients with LM and patients with nonmalignant neurological diseases (NMNDs) as controls. Receiver operating curve analysis was performed to assess their diagnostic accuracy for LM. In patients with NMNDs, CEA and CYFRA 21-1 levels in the CSF were significantly lower than the serum levels. In patients with LM, there was no significant difference between the CSF and serum CEA levels, whereas the CYFRA 21-1 levels were significantly higher in the CSF than the serum. CSF/serum quotients of CYFRA 21-1 were higher than those of CEA in patients with LM and patients with NMNDs. CSF CYFRA 21-1 and CSF/serum quotient of CYFRA 21-1 had high accuracy for differentiating LM from NMNDs that was similar to CSF CEA and CSF/serum quotient of CYFRA 21-1, whereas serum CYFRA 21-1 is of poor diagnostic value. Measurement of CSF CYFRA 21-1 should not be overlooked in patients with suspected LM, even if the serum CYFRA 21-1 level is within normal limits.

Metastasis in Adult Brain Tumors

Metastatic cancer to the central nervous system is primarily deposited by hematogenous spread in various anatomically distinct regions: calvarial, pachymeningeal, leptomeningeal, and brain parenchyma. A patient's overall clinical status and the information needed to make treatment decisions are the primary considerations in initial imaging modality selection. Contrast-enhanced MR imaging is the preferred imaging modality. Morphologic MR imaging is limited to delineating anatomic deraignment of tissues. Dynamic susceptibility contrast-enhanced perfusion and diffusion-weighted physiology-based MR imaging sequences have been developed that complement morphologic MR imaging by providing additional diagnostic information.

Interactions between αv-Integrin and HER2 and Their Role in the Invasive Phenotype of Breast Cancer Cells In Vitro and in Rat Brain

Background

We tested the hypothesis that αv-integrin and the human epidermal growth factor receptor type 2 (HER2) interact with each other in brain trophic metastatic breast cancer cells and influence their invasive phenotype.

Methods

Clones of MDA-MB231BR human breast cancer cells with stable knock down of αv-integrin in combination with high or low levels of HER2 were created. The interactions of these two proteins and their combined effect on cell migration and invasion were investigated in vitro and in vivo.

Results

Knockdown of αv-integrin in MDA-MB231BR clones altered the actin cytoskeleton and cell morphology. HER2 co-precipitated with αv-integrin in three breast cancer cell lines in vitro, suggesting they complex in cells. Knockdown of αv-integrin altered HER2 localization from its normal membrane position to a predominantly lysosomal localization. When αv-integrin expression was decreased by 69–93% in HER2-expressing cells, cellular motility was significantly reduced. Deficiency of both αv-integrin and HER2 decreased cellular migration and invasion by almost 90% compared to cells expressing both proteins (P<0.01). After intracerebral inoculation, cells expressing high levels of both αv-integrin and HER2 showed a diffusely infiltrative tumor phenotype, while cells deficient in αv-integrin and/or HER2 showed a compact tumor growth phenotype. In the αv-integrin positive/HER2 positive tumors, infiltrative growth was 57.2 ± 19% of tumor volume, compared to only 5.8 ± 6.1% infiltration in the double deficient tumor cells.

Conclusions

αv-integrin interacts with HER2 in breast cancer cells and may regulate HER2 localization. The combined impacts of αv-integrin and HER2 influence the invasive phenotype of breast cancer cells. Targeting αv-integrin in HER2-positive breast cancer may slow growth and decrease infiltration in the normal brain.

Molecular biology of brain metastasis

Metastasis to the central nervous system (CNS) remains a major cause of morbidity and mortality in patients with systemic cancer. As the length of survival in patients with systemic cancer improves, thanks to multimodality therapies, focusing on metastases to the CNS becomes of paramount importance. Unique interactions between the brain’s micro-environment, blood-brain barrier, and tumor cells are hypothesized to promote distinct molecular features in CNS metastases that may require tailored therapeutic approaches. This review will focus on the pathophysiology, epigenetics, and immunobiology of brain metastases in order to understand the metastatic cascade. Cancer cells escape the primary tumor, intravasate into blood vessels, survive the hematogenous dissemination to the CNS, arrest in brain capillaries, extravasate, proliferate, and develop angiogenic abilities to establish metastases. Molecular biology, genetics, and epigenetics are rapidly expanding, enabling us to advance our knowledge of the underlying mechanisms involved. Research approaches using cell lines that preferentially metastasize in vivo to the brain and in vitro tissue-based studies unfold new molecular leads into the disease. It is important to identify and understand the molecular pathways of the metastatic cascade in order to target the investigation and development of more effective therapies and research directions.

Brain metastasis-initiating cells: survival of the fittest

Brain metastases (BMs) are the most common brain tumor in adults, developing in about 10% of adult cancer patients. It is not the incidence of BM that is alarming, but the poor patient prognosis. Even with aggressive treatments, median patient survival is only months. Despite the high rate of BM-associated mortality, very little research is conducted in this area. Lack of research and staggeringly low patient survival is indicative that a novel approach to BMs and their treatment is needed. The ability of a small subset of primary tumor cells to produce macrometastases is reminiscent of brain tumor-initiating cells (BTICs) or cancer stem cells (CSCs) hypothesized to form primary brain tumors. BTICs are considered stem cell-like due to their self-renewal and differentiation properties. Similar to the subset of cells forming metastases, BTICs are most often a rare subpopulation. Based on the functional definition of a TIC, cells capable of forming a BM could be considered to be brain metastasis-initiating cells (BMICs). These putative BMICs would not only have the ability to initiate tumor growth in a secondary niche, but also the machinery to escape the primary tumor, migrate through the circulation, and invade the neural niche.

Automated seeding-based nuclei segmentation in nonlinear optical microscopy

Nonlinear optical (NLO) microscopy based, e.g., on coherent anti-Stokes Raman scattering (CARS) or two-photon-excited fluorescence (TPEF) is a fast label-free imaging technique, with a great potential for biomedical applications. However, NLO microscopy as a diagnostic tool is still in its infancy; there is a lack of robust and durable nuclei segmentation methods capable of accurate image processing in cases of variable image contrast, nuclear density, and type of investigated tissue. Nonetheless, such algorithms specifically adapted to NLO microscopy present one prerequisite for the technology to be routinely used, e.g., in pathology or intraoperatively for surgical guidance. In this paper, we compare the applicability of different seeding and boundary detection methods to NLO microscopic images in order to develop an optimal seeding-based approach capable of accurate segmentation of both TPEF and CARS images. Among different methods, the Laplacian of Gaussian filter showed the best accuracy for the seeding of the image, while a modified seeded watershed segmentation was the most accurate in the task of boundary detection. The resulting combination of these methods followed by the verification of the detected nuclei performs high average sensitivity and specificity when applied to various types of NLO microscopy images.

Update on bevacizumab and other angiogenesis inhibitors for brain cancer

INTRODUCTION

Primary and metastatic brain tumors remain a major challenge. The most common primary adult malignant brain tumor, glioblastoma (GBM), confers a dismal prognosis as does the development of CNS metastases for most systemic malignancies. Anti-angiogenic therapy has been a major clinical research focus in neuro-oncology over the past 5 years.

AREAS COVERED

Culmination of this work includes US FDA accelerated approval of bevacizumab for recurrent GBM and the completion of two placebo-controlled Phase III studies of bevacizumab for newly diagnosed GBM. A multitude of anti-angiogenics are in evaluation for neuro-oncology patients but none has thus far surpassed the therapeutic benefit of bevacizumab.

EXPERT OPINION

These agents demonstrate adequate safety and the majority of GBM patients derive benefit. Furthermore, their anti-permeability effect can substantially decrease tumor-associated edema leading to stable or improved neurologic function and quality of life. In particular, anti-angiogenics significantly prolong progression-free survival - a noteworthy achievement in the context of infiltrative and destructive brain tumors like GBM; however, in a manner analogous to other cancers, their impact on overall survival for GBM patients is modest at best. Despite substantial clinical research efforts, many fundamental questions regarding anti-angiogenic agents in brain tumor patients remain unanswered.

The molecular biology of brain metastasis

Metastasis to the central nervous system (CNS) remains a major cause of morbidity and mortality in patients with systemic cancers. Various crucial interactions between the brain environment and tumor cells take place during the development of the cancer at its new location. The rapid expansion in molecular biology and genetics has advanced our knowledge of the underlying mechanisms involved, from invasion to final colonization of new organ tissues. Understanding the various events occurring at each stage should enable targeted drug delivery and individualized treatments for patients, with better outcomes and fewer side effects. This paper summarizes the principal molecular and genetic mechanisms that underlie the development of brain metastasis (BrM).