The Cancer Stem Cell–Vascular Niche Complex in Brain Tumor Formation

A new perspective on antiangiogenic antibody drug resistance: Biomarkers, mechanisms, and strategies in malignancies

Drug resistance of malignant tumor leads to disease progression be the bottleneck in clinical treatment. Antiangiogenic therapy, which aims to "starve" the tumor by inhibiting angiogenesis, is one of the key strategies in clinical oncology treatments. Recently, dozens of investigational antibody drugs and biosimilars targeting angiogenesis have obtained regulatory approval for the treatment of various malignancies. Moreover, a new generation of bispecific antibodies based on the principle of antiangiogenesis are being advanced for clinical trial to overcome antiangiogenic resistance in tumor treatment or enhance the efficacy of monotherapy. Tumors often develop resistance to antiangiogenesis therapy, presenting as refractory and sometimes even resistant to new therapies, for which there are currently no effective management strategies. Thus, a detailed understanding of the mechanisms mediating resistance to antiangiogenesis antibodies is crucial for improving drug effectiveness and achieving a durable response to antiangiogenic therapy. In this review, we provide a novel perspective on the tumor microenvironment, including antibody structure, tumor stroma, and changes within tumor cells, to analyze the multifactorial reasons underlying resistance to antiangiogenesis antibodies. The review also enumerates biomarkers that indicate resistance and potential strategies for monitoring resistance. Furthermore, based on recent clinical and preclinical studies, we summarize potential strategies and translational clinical trials aimed at overcoming resistance to antiangiogenesis antibodies. This review provides a valuable reference for researchers and clinical practitioners involved in the development of new drugs or therapeutic strategies to overcome antiangiogenesis antibodies resistance.

Novel insights into astrocyte-mediated signaling of proliferation, invasion and tumor immune microenvironment in glioblastoma

Glioblastoma (GBM) continues to be the most aggressive cancer of the brain. The dismal prognosis is largely attributed to the microenvironment surrounding tumor cells. Astrocytes, the main component of the GBM microenvironment, play several fundamental physiological roles in the central nervous system. During the development of GBM, tumor-associated astrocytes (TAAs) directly contact GBM cells, which activate astrocytes to form reactive astrocytes, facilitating tumor progression, proliferation and migration through multiple well-understood signaling pathways. Notably, TAAs also influence GBM cell behaviors via suppressing immune responses and enhancing the chemoradiotherapy resistance of tumor cells. These new activities are closely linked with the treatment and prognosis of GBM. In this review, we discuss recent advances regarding new functions of reactive astrocytes, including TAA-cancer cell interactions, mechanisms involved in immunosuppressive regulation, and chemoradiotherapy resistance. It is expected that these updated experimental or clinical studies of TAAs may provide a promising approach for GBM treatment in the near future.

Spinal Cord Tumor Microenvironment

Intramedullary spinal cord tumors (IMSCT) are rare entities for which there currently exist no standardized treatment paradigms. Consequently, patients usually receive treatment modalities that were established for intracerebral tumors; these approaches, however, typically result in functional impairment, recurrent tumor growth, and short overall survival. There is a distinct lack of promising research efforts in this field, which raises questions about whether spinal cord tumor microenvironment (TME) might promote the development, progression, and treatment resistance of IMSCT. In this review, we aim to examine spinal cord biology, compare spinal cord and brain microenvironments, and discuss mutual interactions between IMSCT and TME. Manipulating these pathways may provide new treatment approaches for future patient groups.

Glioblastoma: Microenvironment and Niche Concept

The niche concept was originally developed to describe the location of normal neural stem cells (NSCs) in the subependymal layer of the sub-ventricular zone. In this paper, its significance has been extended to the location of tumor stem cells in glioblastoma (GB) to discuss the relationship between GB stem cells (GSCs) and endothelial cells (ECs). Their interaction is basically conceived as responsible for tumor growth, invasion and recurrence. Niches are described as the points of utmost expression of the tumor microenvironment (TME), therefore including everything in the tumor except for tumor cells: NSCs, reactive astrocytes, ECs, glioma-associated microglia/macrophages (GAMs), myeloid cells, pericytes, fibroblasts, etc. and all intrinsic and extrinsic signaling pathways. Perivascular (PVNs), perinecrotic (PNNs) and invasive niches were described from the pathological point of view, highlighting the basic significance of the EC/tumor stem cell couple. PNN development was reinterpreted based on the concept that hyperproliferative areas of GB are composed of GSCs/progenitors. TME was depicted in its function as the main regulator of everything that happens in the tumor. A particular emphasis was given to GAMs, pericytes and reactive astrocytes as important elements affecting proliferation, growth, invasion and resistance to therapies of tumor cells.

Thomson T, J. Centelles J, F. Graham S, Cascante M. Untargeted metabolomics reveals distinct metabolic reprogramming in endothelial cells co-cultured with CSC and non-CSC prostate cancer cell subpopulations

Tumour angiogenesis is an important hallmark of cancer and the study of its metabolic adaptations, downstream to any cellular change, can reveal attractive targets for inhibiting cancer growth. In the tumour microenvironment, endothelial cells (ECs) interact with heterogeneous tumour cell types that drive angiogenesis and metastasis. In this study we aim to characterize the metabolic alterations in ECs influenced by the presence of tumour cells with extreme metastatic abilities. Human umbilical vein endothelial cells (HUVECs) were subjected to different microenvironmental conditions, such as the presence of highly metastatic PC-3M and highly invasive PC-3S prostate cancer cell lines, in addition to the angiogenic activator vascular endothelial growth factor (VEGF), under normoxia. Untargeted high resolution liquid chromatography-mass spectrometry (LC-MS) based metabolomics revealed significant metabolite differences among the various conditions and a total of 25 significantly altered metabolites were identified including acetyl L-carnitine, NAD+, hypoxanthine, guanine and oleamide, with profile changes unique to each of the experimental conditions. Biochemical pathway analysis revealed the importance of fatty acid oxidation and nucleotide salvage pathways. These results provide a global metabolic preview that could help in selectively targeting the ECs aiding in either cancer cell invasion or metastasis in the heterogeneous tumour microenvironment.

Stem cells and the role of ETS transcription factors in the differentiation hierarchy of normal and malignant prostate epithelium

Prostate cancer is the most common cancer of men in the UK and accounts for a quarter of all new cases. Although treatment of localised cancer can be successful, there is no cure for patients presenting with invasive prostate cancer and there are less treatment options. They are generally treated with androgen-ablation therapies but eventually the tumours become hormone resistant and patients develop castration-resistant prostate cancer (CRPC) for which there are no further successful or curative treatments. This highlights the need for new treatment strategies. In order to prevent prostate cancer recurrence and treatment resistance, all the cell populations in a heterogeneous prostate tumour must be targeted, including the rare cancer stem cell (CSC) population. The ETS transcription factor family members are now recognised as a common feature in multiple cancers including prostate cancer; with aberrant expression, loss of tumour suppressor function, inactivating mutations and the formation of fusion genes observed. Most notably, the TMPRSS2-ERG gene fusion is present in approximately 50% of prostate cancers and in prostate CSCs. However, the role of other ETS transcription factors in prostate cancer is less well understood. This review will describe the prostate epithelial cell hierarchy and discuss the evidence behind prostate CSCs and their inherent resistance to conventional cancer therapies. The known and proposed roles of the ETS family of transcription factors in prostate epithelial cell differentiation and regulation of the CSC phenotype will be discussed, as well as how they might be targeted for therapy.

Positive feedback loop between cancer stem cells and angiogenesis in hepatocellular carcinoma

Anti-angiogenesis-related therapies have become the standard care for patients with advanced hepatocellular carcinoma (HCC), as HCC is a highly vascularized solid tumor. Unfortunately, only modest and limited efficacies are observed. Emerging evidence have attributed to the limited efficacy to the presence of cancer stem cells (CSCs) in the tumor. CSCs predominantly drives angiogenesis via releasing proangiogenic factors and exosomes. They have the ability to resistant intratumoral hypoxia via autophagy or by directly forming the tubular structure to obtain blood. On the other hand, the vascular niche in tumor microenvironment also releases growth factors via juxtacrine and paracrine mechanisms to support the growth of CSCs and maintain its stemness features. This positive feedback loop between angiogenesis and CSCs exists in liver tumor microenvironment that is responsible for the development and poor prognosis of HCC. In this review, we summarize recent advances in our understanding of the crosstalks between angiogenesis and CSCs, and their interactions in liver tumor microenvironment and their purpose that an effective anti-angiogenic therapy should also target CSCs for HCC treatment.

Stem cell niches in glioblastoma: a neuropathological view

Glioblastoma (GBM) stem cells (GSCs), responsible for tumor growth, recurrence, and resistance to therapies, are considered the real therapeutic target, if they had no molecular mechanisms of resistance, in comparison with the mass of more differentiated cells which are insensitive to therapies just because of being differentiated and nonproliferating. GSCs occur in tumor niches where both stemness status and angiogenesis are conditioned by the microenvironment. In both perivascular and perinecrotic niches, hypoxia plays a fundamental role. Fifteen glioblastomas have been studied by immunohistochemistry and immunofluorescence for stemness and differentiation antigens. It has been found that circumscribed necroses develop inside hyperproliferating areas that are characterized by high expression of stemness antigens. Necrosis developed inside them because of the imbalance between the proliferation of tumor cells and endothelial cells; it reduces the number of GSCs to a thin ring around the former hyperproliferating area. The perinecrotic GSCs are nothing else that the survivors remnants of those populating hyperproliferating areas. In the tumor, GSCs coincide with malignant areas so that the need to detect where they are located is not so urgent.

Emerging roles of microRNA in modulating cell-death processes in malignant glioma

MicroRNAs (miRNAs) are small noncoding RNA molecules that regulate protein expression by cleaving or repressing the translation of target mRNAs. In mammals, their function mainly represses the mRNA transcripts via imperfect complementary sequences in the 3'UTR of target mRNAs. Several miRNAs have been recently reported to be involved in modulation of different genes in tumors, including glioblastoma, the most frequent brain tumor in adults. Despite the improvements in treatments, survival of patients remains poor, and glioblastoma is one of the most lethal form of human cancer. To define novel strategies against this tumor, emerging research investigated miRNAs involvement in glioblastoma. In particular, this review is focused on miRNAs involved on the two principal programmed cell-death, apoptosis and autophagy, recently described from the literature. Moreover, the discovery of miRNAs role in glioma cell-death pathways has also revealed a new category of therapeutic targets, fundamental for this kind of tumor.

Graphene nanoflakes as an efficient ionizing matrix for MALDI-MS based lipidomics of cancer cells and cancer stem cells

This study demonstrates that graphene nanoflakes can be efficiently used as a successful, interference free matrix for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS).

Three-dimensional vasculature reconstruction of tumour microenvironment via local clustering and classification

The vasculature inside breast cancers is one important component of the tumour microenvironment. The investigation of its spatial morphology, distribution and interactions with cancer cells, including cancer stem cells, is essential for elucidating mechanisms of tumour development and treatment response. Using confocal microscopy and fluorescent markers, we have acquired three-dimensional images of vasculature within mammary tumours and normal mammary gland of mouse models. However, it is difficult to segment and reconstruct complex vasculature accurately from the in vivo three-dimensional images owing to the existence of uneven intensity and regions with low signal-to-noise ratios (SNR). To overcome these challenges, we have developed a novel three-dimensional vasculature segmentation method based on local clustering and classification. First, images of vasculature are clustered into local regions, whose boundaries well delineate vasculature even in low SNR and uneven intensity regions. Then local regions belonging to vasculature are identified by applying a semi-supervised classification method based on three informative features of the local regions. Comparison of results using simulated and real vasculature images, from mouse mammary tumours and normal mammary gland, shows that the new method outperforms existing methods, and can be used for three-dimensional images with uneven background and low SNR to achieve accurate vasculature reconstruction.

Role of the microenvironment in ovarian cancer stem cell maintenance

Despite recent progresses in cancer therapy and increased knowledge in cancer biology, ovarian cancer remains a challenging condition. Among the latest concepts developed in cancer biology, cancer stem cells and the role of microenvironment in tumor progression seem to be related. Indeed, cancer stem cells have been described in several solid tumors including ovarian cancers. These particular cells have the ability to self-renew and reconstitute a heterogeneous tumor. They are characterized by specific surface markers and display resistance to therapeutic regimens. During development, specific molecular cues from the tumor microenvironment can play a role in maintaining and expanding stemness of cancer cells. The tumor stroma contains several compartments: cellular component, cytokine network, and extracellular matrix. These different compartments interact to form a permissive niche for the cancer stem cells. Understanding the molecular cues underlying this crosstalk will allow the design of new therapeutic regimens targeting the niche. In this paper, we will discuss the mechanisms implicated in the interaction between ovarian cancer stem cells and their microenvironment.

Large hemangiopericytoma associated with arteriovenous malformations and dural arteriovenous fistulae

BACKGROUND

Hemangiopericytomas are rare vascular tumors of the central nervous system. Although hemangiopericytomas have been associated with other vascular malformations, there is no report of an intracranial hemangiopericytoma found in association with multiple arteriovenous malformations and dural arteriovenous fistulae.

CASE DESCRIPTION

We present the case of an otherwise healthy 25-year-old woman who presented with a large hemangiopericytoma involving the superior sagittal sinus. The highly vascular nature of the lesion, the total occlusion of the sinus anterior to the tumor, and the presence of multiple arteriovenous malformations and dural arteriovenous fistulae complicated resection of this tumor.

CONCLUSION

To our knowledge, no prior report has described a tumor of this magnitude in association with multiple AVMs and dAVFs.

Tumor microenvironment in the brain

In addition to malignant cancer cells, tumors contain a variety of different stromal cells that constitute the tumor microenvironment. Some of these cell types provide crucial support for tumor growth, while others have been suggested to actually inhibit tumor progression. The composition of tumor microenvironment varies depending on the tumor site. The brain in particular consists of numerous specialized cell types such as microglia, astrocytes, and brain endothelial cells. In addition to these brain-resident cells, primary and metastatic brain tumors have also been shown to be infiltrated by different populations of bone marrow-derived cells. The role of different cell types that constitute tumor microenvironment in the progression of brain malignancies is only poorly understood. Tumor microenvironment has been shown to be a promising therapeutic target and diagnostic marker in extracranial malignancies. A better understanding of tumor microenvironment in the brain would therefore be expected to contribute to the development of improved therapies for brain tumors that are urgently required due to a poor availability of treatments for these malignancies. This review summarizes some of the known interactions between brain tumors and different stromal cells, and also discusses potential therapeutic approaches within this context.

Tumor heterogeneity: mechanisms and bases for a reliable application of molecular marker design

Tumor heterogeneity is a confusing finding in the assessment of neoplasms, potentially resulting in inaccurate diagnostic, prognostic and predictive tests. This tumor heterogeneity is not always a random and unpredictable phenomenon, whose knowledge helps designing better tests. The biologic reasons for this intratumoral heterogeneity would then be important to understand both the natural history of neoplasms and the selection of test samples for reliable analysis. The main factors contributing to intratumoral heterogeneity inducing gene abnormalities or modifying its expression include: the gradient ischemic level within neoplasms, the action of tumor microenvironment (bidirectional interaction between tumor cells and stroma), mechanisms of intercellular transference of genetic information (exosomes), and differential mechanisms of sequence-independent modifications of genetic material and proteins. The intratumoral heterogeneity is at the origin of tumor progression and it is also the byproduct of the selection process during progression. Any analysis of heterogeneity mechanisms must be integrated within the process of segregation of genetic changes in tumor cells during the clonal expansion and progression of neoplasms. The evaluation of these mechanisms must also consider the redundancy and pleiotropism of molecular pathways, for which appropriate surrogate markers would support the presence or not of heterogeneous genetics and the main mechanisms responsible. This knowledge would constitute a solid scientific background for future therapeutic planning.

Cancer stem cells hypothesis and stem cells in head and neck cancers

There is increasing evidence that the growth and spread of cancer is driven by a small subpopulation of cancer cells, defined as cancer stem cells (CSCs). Recent data indicate that the initiation, growth, recurrence and metastasis of cancers are related to the behavior of a small population of malignant cells with properties of stem cells, and information about them are potentially helpful in identifying the target for the tumor's therapeutic elimination. The presence of subpopulation cells with phenotypic and behavioral characteristics corresponding to both normal epithelial stem cells and to cells capable of initiating tumors has been also reported in head and neck squamous cell carcinomas (HNSCCs).

Antigenic and Genotypic Similarity between Primary Glioblastomas and Their Derived Neurospheres

Formation of neurospheres (NS) in cultures of glioblastomas (GBMs), with self-renewal, clonogenic capacities, and tumorigenicity following transplantation into immunodeficient mice, may denounce the existence of brain tumor stem cells (BTSCs) in vivo. In sixteen cell lines from resected primary glioblastomas, NS showed the same genetic alterations as primary tumors and the expression of stemness antigens. Adherent cells (AC), after adding 10% of fetal bovine serum (FBS) to the culture, were genetically different from NS and prevailingly expressed differentiation antigens. NS developed from a highly malignant tumor phenotype with proliferation, circumscribed necrosis, and high vessel density. Beside originating from transformed neural stem cells (NSCs), BTSCs may be contained within or correspond to dedifferentiated cells after mutation accumulation, which reacquire the expression of stemness antigens.

Tumor-initiating cells and tumor vascularization

Tumor-initiating cells (TICs) with stem-like cell properties initiate and sustain progressive growth, resulting in a heterogeneous tumor mass. The survival and growth of tumors rely on the development of a vasculature to provide nutrients and oxygen. Crosstalk between TICs and vascularization may be one of the central players in the initiation, long-term maintenance, and progression of tumors. This review surveys current evidence concerning the crosstalk that occurs in tumor/stromal interactions, including genetic change, vascular niche, hypoxia, and dormancy of tumors. A better understanding of this crosstalk might help provide the basis for developing more effective therapeutic drug targets.

CD133+ niches and single cells in glioblastoma have different phenotypes

Putative CD133(+) brain tumor stem cells have been shown to be located in niches and as single cells. This is the first study providing insight into the different phenotypes of CD133(+) cells in glioblastoma according to localization. Paraffin sections were stained by double immunofluorescence with CD133 and the candidate stem cell markers Sox2, Bmi-1, EGFR, podoplanin and nestin, the proliferation marker Ki67 and the endothelial cell markers CD31, CD34, and VWF. Cell counting showed that the CD133(+) cells in the niches had a significantly higher expression of Sox2, EGFR and nestin compared to CD133(+) single cells, but only a 3% Ki67 labeling index versus 14% found for CD133(+) single cells. Only low endothelial cell marker expression was found in the niches or the CD133(-) tumor areas, while 43% CD133(+)/CD31(+) and 25% CD133(+)/CD34(+) single cells were found. CD133(+) blood vessels within CD133(+) niches were less proliferative and more often Bmi-1(+) than CD133(+) blood vessels outside niches. In conclusion, different CD133(+) cell phenotypes exist according to the in situ localization, and also the phenotype of CD133(+) blood vessels vary according to the localization. CD133(+) niches contain stem-like cells with a lower proliferation index than CD133(+) single cells, which have an endothelial differentiation profile suggesting a role in angiogenesis.

The perivascular niche microenvironment in brain tumor progression

Glioblastoma, the most frequent and aggressive malignant brain tumor, has a very poor prognosis of approximately 1-year. The associated aggressive phenotype and therapeutic resistance of glioblastoma is postulated to be due to putative brain tumor stem-like cells (BTSC). The best hope for improved therapy lies in the ability to understand the molecular biology that controls BTSC behavior. The tumor vascular microenvironment of brain tumors has emerged as important regulators of BTSC behavior. Emerging data have identified the vascular microenvironment as home to a multitude of cell types engaged in various signaling that work collectively to foster a supportive environment for BTSCs. Characterization of the signaling pathways and intercellular communication between resident cell types in the microvascular niche of brain tumors is critical to the identification of potential BTSC-specific targets for therapy.

Endothelial cell-initiated signaling promotes the survival and self-renewal of cancer stem cells

Recent studies have demonstrated that cancer stem cells play an important role in the pathobiology of head and neck squamous cell carcinomas (HNSCC). However, little is known about functional interactions between head and neck cancer stem-like cells (CSC) and surrounding stromal cells. Here, we used aldehyde dehydrogenase activity and CD44 expression to sort putative stem cells from primary human HNSCC. Implantation of 1,000 CSC (ALDH+CD44+Lin-) led to tumors in 13 (out of 15) mice, whereas 10,000 noncancer stem cells (ALDH-CD44-Lin-) resulted in 2 tumors in 15 mice. These data demonstrated that ALDH and CD44 select a subpopulation of cells that are highly tumorigenic. The ability to self-renew was confirmed by the observation that ALDH+CD44+Lin- cells sorted from human HNSCC formed more spheroids (orospheres) in 3-D agarose matrices or ultra-low attachment plates than controls and were serially passaged in vivo. We observed that approximately 80% of the CSC were located in close proximity (within 100-μm radius) of blood vessels in human tumors, suggesting the existence of perivascular niches in HNSCC. In vitro studies demonstrated that endothelial cell-secreted factors promoted self-renewal of CSC, as demonstrated by the upregulation of Bmi-1 expression and the increase in the number of orospheres as compared with controls. Notably, selective ablation of tumor-associated endothelial cells stably transduced with a caspase-based artificial death switch (iCaspase-9) caused a marked reduction in the fraction of CSC in xenograft tumors. Collectively, these findings indicate that endothelial cell-initiated signaling can enhance the survival and self-renewal of head and neck CSC.

Identification of internalizing human single-chain antibodies targeting brain tumor sphere cells

Glioblastoma multiforme (GBM) is the most common and aggressive form of primary brain tumor for which there is no curative treatment to date. Resistance to conventional therapies and tumor recurrence pose major challenges to treatment and management of this disease, and therefore new therapeutic strategies need to be developed. Previous studies by other investigators have shown that a subpopulation of GBM cells can grow as neurosphere-like cells when cultured in restrictive medium and exhibits enhanced tumor-initiating ability and resistance to therapy. We report here the identification of internalizing human single-chain antibodies (scFv) targeting GBM tumor sphere cells. We selected a large naive phage antibody display library on the glycosylation-dependent CD133 epitope-positive subpopulation of GBM cells grown as tumor spheres and identified internalizing scFvs that target tumor sphere cells broadly, as well as scFvs that target the CD133-positive subpopulation. These scFvs were found to be efficiently internalized by GBM tumor sphere cells. One scFv GC4 inhibited self-renewal of GBM tumor sphere cells in vitro. We have further developed a full-length human IgG1 based on this scFv, and found that it potently inhibits proliferation of GBM tumor sphere cells and GBM cells grown in regular nonselective medium. Taken together, these results show that internalizing human scFvs targeting brain tumor sphere cells can be readily identified from a phage antibody display library, which could be useful for further development of novel therapies that target subpopulations of GBM cells to combat recurrence and resistance to treatment.

Beta1,4-galactosyltransferase V regulates self-renewal of glioma-initiating cell

Glioma results from unregulated expansion of a self-renewing glioma-initiating cell population. The regulatory pathways which are essential for sustaining the self-renewal of glioma-initiating cells remain largely unknown. Cell surface N-linked oligosaccharides play functional roles in determining cell fate and are associated with glioma malignancy. Previously, we have reported that beta1,4-galactosyltransferase V (beta1,4GalT V) effectively galactosylates the GlcNAcbeta1-->6Man arm of the highly branched N-glycans and positively regulates glioma cell growth. Here, we show that decreasing the expression of beta1,4GalT V by RNA interference in glioma cells attenuated the formation of polylactosamine and inhibited the ability of tumor formation in vivo. Down-regulation of beta1,4GalT V depleted CD133-positive cells in glioma xenograft, and inhibited the self-renewal capacity and the tumorigenic potential of glioma-initiating cells. These data reveal a critical role of beta1,4GalT V in the self-renewal and tumorigenicity of glioma-initiating cells, and indicate that manipulating beta1,4GalT V expression may have therapeutic potential for the treatment of malignant glioma.

Capturing changes in the brain microenvironment during initial steps of breast cancer brain metastasis

Brain metastases are difficult to treat and mostly develop late during progressive metastatic disease. Patients at risk would benefit from the development of prevention and improved treatments. This requires knowledge of the initial events that lead to brain metastasis. The present study reveals cellular events during the initiation of brain metastasis by breast cancer cells and documents the earliest host responses to incoming cancer cells after carotid artery injection in immunodeficient and immunocompetent mouse models. Our findings capture and characterize heterogeneous astrocytic and microglial reactions to the arrest and extravasation of cancer cells in the brain, showing immediate and drastic changes in the brain microenvironment on arrival of individual cancer cells. We identified reactive astrocytes as the most active host cell population that immediately localizes to individual invading tumor cells and continuously associates with growing metastatic lesions. Up-regulation of matrix metalloproteinase-9 associated with astrocyte activation in the immediate vicinity of extravasating cancer cells might support their progression. Early involvement of different host cell types indicates environmental clues that might codetermine whether a single cancer cell progresses to macrometastasis or remains dormant. Thus, information on the initial interplay between brain homing tumor cells and reactive host cells may help develop strategies for prevention and treatment of symptomatic breast cancer brain metastases.

Differentiation of dermal multipotent cells into odontogenic lineage induced by embryonic and neonatal tooth germ cell-conditioned medium

Stem cell-based therapy represents a novel and more advantageous modality of treatment for tooth defect or loss. However, this strategy is challenged in the circumstances where tooth-derived stem cells are not readily accessible. In present study we sought to explore the possibility of utilizing dermal multipotent cells (DMCs) easily available from skin tissue for odontogenic induction. Using the limiting dilution technique, colony-forming cell population was isolated and characterized by proliferative activity and multilineage differentiation potential. By exposure to conditioned medium of embryonic and neonatal tooth germ cells in culture, the proliferation and mineralization activity of DMCs was elevated, while the embryonic tooth germ cell-conditioned medium (ETGC-CM) produced more significant effects. Meanwhile, ETGC-CM-treated DMCs phenocopied the odontoblasts in vitro as indicated by specific lineage markers. Following in vivo transplantation as cell pellet, ETGC-CM-treated DMCs were capable of producing blocks of mineralized tissues, which resembled those of dental pulp stem cell (DPSC) explants in the same subcutaneous pockets environment. These observations suggest that although more sufficient and continuous inductive microenvironment may be needed for undifferentiated DMCs to perform as odontoblasts, ETGC-CM-treated DMCs indeed acquire properties as those of DPSCs. Our work highlights the potential utility of DMCs as an alternative candidate cell source in hopes of developing more practical strategy of tooth regeneration research and offering promising opportunities for therapeutic approach.

The many roles of microRNAs in brain tumor biology

MicroRNAs (miRNAs) are now recognized as the primary RNAs involved in the purposeful silencing of the cell's own message. In addition to the established role of miRNAs as developmental regulators of normal cellular function, they have recently been shown to be important players in pathological states such as cancer. The authors review the literature on the role of miRNAs in the formation and propagation of gliomas and medulloblastomas, highlighting the potential of these molecules and their inhibitors as therapeutics.

Simultaneous choroid plexus carcinoma and pilocytic astrocytoma in a pediatric patient

Simultaneous primary brain tumors in pediatric patients without prior chemotherapy or radiotherapy, phacomatosis, or known familial history are a rare occurrence. The authors report the case of a 4-year-old boy with simultaneous choroid plexus carcinoma and pilocytic astrocytoma with features of oligodendroglioma. Magnetic resonance imaging studies revealed diffuse heterogeneously enhancing left intraventricular and posterior fossa tumors initially believed most consistent with multicentric choroid plexus carcinomas. A multiple staged resection was carried out for each tumor and gross-total resection was achieved. Upon gross inspection intraoperatively as well as postoperative histological analysis, 2 distinct simultaneous tumors were identified: choroid plexus carcinoma and pilocytic astrocytoma. To the authors' knowledge this is the first case report published identifying 2 distinct tumor types with similar radiological appearances in a pediatric patient with no prior history of radiotherapy, chemotherapy, or phacomatosis.

Tumor microvasculature supports proliferation and expansion of glioma-propagating cells

Glioblastoma multiforme (GBM) is the most common and aggressive primary brain tumor. The identification of 'cancer stem cells' (CSC) has shed new light on the potential mechanism of therapy resistance of these tumors. Because these cells appear to be more resistant to conventional treatments, they are thought to drive tumor regrowth after therapy. Therefore, novel therapeutic approaches that target these cells are needed. Tumor cells interact with their microenvironment. It has been reported that close contact between CSCs and tumor microvascular endothelium in GBM is important for CSCs to preserve their undifferentiated state and self-renewal ability. However, our understanding of this interaction is still rudimentary. This is in part due to a lack of suitable in vitro models that accurately represent the in vivo situation. Therefore, we set up a co-culture system consisting of primary brain tumor microvascular endothelial cells (tMVECs) and glioma propagating cells (GPCs) derived from biopsies of GBM patients. We found that tMVECs support the growth of GPCs resulting in higher proliferation rates comparing to GPCs cultured alone. This effect was dependent on direct contact between the 2 cell types. In contrast to GPCs, the FCS-cultured cell line U87 was stimulated by culturing on tMVEC-derived ECM alone, suggesting that both cell types interact different with their microenvironment. Together, these results demonstrate the feasibility and utility of our system to model the interaction of GPCs with their microenvironment. Identification of molecules that mediate this interaction could provide novel targets for directed therapy for GBM.

The paracrine role of Tie-2-expressing monocytes in tumor angiogenesis

Literature data have demonstrated that tumor neovascularization is regulated in part by myelomonocytic cells. Luigi Naldini's group has reported the identification in human peripheral blood of a novel subset of Tie-2-expressing monocytes (TEMs) that promote angiogenesis in paracrine manner. Although recruited to tumors in lower numbers than tumor-associated macrophages (TAMs), TEMs are a more potent source of proangiogenic signals, suggesting that they significantly contribute to tumor angiogenesis. Moreover, TEMs, while stimulating angiogenesis, do not actively incorporate into blood vessels and this subpopulation of Tie-2+ cells, rather than bone marrow-derived endothelial progenitor cells (EPCs), which are incorporated in new-forming blood vessels, promote tumor neovascularization through the release of proangiogenic factors.

The tumor microenvironment and its contribution to tumor evolution toward metastasis

Cancer cells acquire cell-autonomous capacities to undergo limitless proliferation and survival through the activation of oncogenes and inactivation of tumor suppressor genes. Nevertheless, the formation of a clinically relevant tumor requires support from the surrounding normal stroma, also referred to as the tumor microenvironment. Carcinoma-associated fibroblasts, leukocytes, bone marrow-derived cells, blood and lymphatic vascular endothelial cells present within the tumor microenvironment contribute to tumor progression. Recent evidence indicates that the microenvironment provides essential cues to the maintenance of cancer stem cells/cancer initiating cells and to promote the seeding of cancer cells at metastatic sites. Furthermore, inflammatory cells and immunomodulatory mediators present in the tumor microenvironment polarize host immune response toward specific phenotypes impacting tumor progression. A growing number of studies demonstrate a positive correlation between angiogenesis, carcinoma-associated fibroblasts, and inflammatory infiltrating cells and poor outcome, thereby emphasizing the clinical relevance of the tumor microenvironment to aggressive tumor progression. Thus, the dynamic and reciprocal interactions between tumor cells and cells of the tumor microenvironment orchestrate events critical to tumor evolution toward metastasis, and many cellular and molecular elements of the microenvironment are emerging as attractive targets for therapeutic strategies.