Stem and progenitor cell-mediated tumor selective gene therapy

Novel treatment strategies for malignant gliomas using neural stem cells

Recent studies in stem cell biology have refined our understanding of the origin and progression of cancer. Identification and characterization of endogenous neural stem cells (NSCs), especially those in the adult human brain, have inspired new ideas for selectively targeting and destroying malignant gliomas. Gliomas consist of a heterogeneous population of cells, and some of these cells have characteristics of cancer stem cells. These brain tumor stem cells (BTSCs) share certain characteristics with normal NSCs. It is still unclear, however, whether malignant gliomas in human patients originate from these aberrant BTSCs. Nonetheless, the cellular and molecular similarities between BTSCs and normal NSCs suggest a common research landscape underlying both normal and cancer stem cell biology, wherein findings of one field are relevant to the other. Furthermore, the natural tropism of NSCs to gliomas has generated the idea that modified NSCs can deliver modified genes to selectively destroy malignant brain tumor cells, and even BTSCs, while leaving healthy surrounding neurons intact. These studies and others on the basic biology of both BTSCs and NSCs will be crucial to expanding our treatment strategies for malignant gliomas.

Adenoviral virotherapy for malignant brain tumors

Glioblastoma multiforme is the most common form of primary brain cancer. In the past decade, virotherapy of tumors has gained credence, particularly in glioma management, as these tumors are not completely resectable and tend to micro-metastasize. Adenoviral vectors have an advantage over other viral vectors in that they are relatively non-toxic and do not integrate in the genome. However, the lack of coxsackie and adenovirus receptors on surface of gliomas provides for inefficient transduction of wild-type adenoviral vectors in these tumors. By targeting receptors that are overexpressed in gliomas, modified adenoviral constructs have been shown to efficiently infect glioma cells. In addition, by taking advantage of tumor-specific promoter elements, oncolytic adenoviral vectors offer the promise of selective tumor-specific replication. This dual targeting strategy has enabled specificity in both laboratory and pre-clinical settings. This review examines current trends in adenoviral virotherapy of gliomas, with an emphasis on targeting modalities and future clinical applications.

Applications of neural and mesenchymal stem cells in the treatment of gliomas

In addition to stem cells providing a better understanding about the biology and origins of gliomas, new therapeutic approaches have been developed based on the use of stem cells as delivery vehicles. The unique ability of stem cells to track down tumor cells makes them a very appealing therapeutic modality. This review introduces neural and mesenchymal stem cells, discusses the advances that have been made in the utilization of these stem cells as therapies and in diagnostic imaging (to track the advancement of the stem cells towards the tumor cells), and concludes by addressing various challenges and concerns regarding these therapies.

Cell migration in the normal and pathological postnatal mammalian brain

In the developing brain, cell migration is a crucial process for structural organization, and is therefore highly regulated to allow the correct formation of complex networks, wiring neurons, and glia. In the early postnatal brain, late developmental processes such as the production and migration of astrocyte and oligodendrocyte progenitors still occur. Although the brain is completely formed and structured few weeks after birth, it maintains a degree of plasticity throughout life, including axonal remodeling, synaptogenesis, but also neural cell birth, migration and integration. The subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampus are the two main neurogenic niches in the adult brain. Neural stem cells reside in these structures and produce progenitors that migrate toward their ultimate location: the olfactory bulb and granular cell layer of the DG respectively. The aim of this review is to synthesize the increasing information concerning the organization, regulation and function of cell migration in a mature brain. In a normal brain, proteins involved in cell-cell or cell-matrix interactions together with secreted proteins acting as chemoattractant or chemorepellant play key roles in the regulation of neural progenitor cell migration. In addition, recent data suggest that gliomas arise from the transformation of neural stem cells or progenitor cells and that glioma cell infiltration recapitulates key aspects of glial progenitor migration. Thus, we will consider glioma migration in the context of progenitor migration. Finally, many observations show that brain lesions and neurological diseases trigger neural stem/progenitor cell activation and migration toward altered structures. The factors involved in such cell migration/recruitment are just beginning to be understood. Inflammation which has long been considered as thoroughly disastrous for brain repair is now known to produce some positive effects on stem/progenitor cell recruitment via the regulation of growth factor signaling and the secretion of a number of chemoattractant cytokines. This knowledge is crucial for the development of new therapeutic strategies. One of these strategies could consist in increasing the mobilization of endogenous progenitor cells that could replace lost cells and improve functional recovery.

Assessment of therapeutic efficacy and fate of engineered human mesenchymal stem cells for cancer therapy

The poor prognosis of patients with aggressive and invasive cancers combined with toxic effects and short half-life of currently available treatments necessitate development of more effective tumor selective therapies. Mesenchymal stem cells (MSCs) are emerging as novel cell-based delivery agents; however, a thorough investigation addressing their therapeutic potential and fate in different cancer models is lacking. In this study, we explored the engineering potential, fate, and therapeutic efficacy of human MSCs in a highly malignant and invasive model of glioblastoma. We show that engineered MSC retain their "stem-like" properties, survive longer in mice with gliomas than in the normal brain, and migrate extensively toward gliomas. We also show that MSCs are resistant to the cytokine tumor necrosis factor apoptosis ligand (TRAIL) and, when engineered to express secreted recombinant TRAIL, induce caspase-mediated apoptosis in established glioma cell lines as well as CD133-positive primary glioma cells in vitro. Using highly malignant and invasive human glioma models and employing real-time imaging with correlative neuropathology, we demonstrate that MSC-delivered recombinant TRAIL has profound anti-tumor effects in vivo. This study demonstrates the efficacy of diagnostic and therapeutic MSC in preclinical glioma models and forms the basis for developing stem cell-based therapies for different cancers.

Rat umbilical cord stem cells completely abolish rat mammary carcinomas with no evidence of metastasis or recurrence 100 days post-tumor cell inoculation

Genetically engineered stem cells efficiently deliver therapeutic proteins to cancer and other sites of inflammation. However, a major advantage would be realized if tumor-trafficking stem cells that have not been genetically modified exhibit an inherent antitumor effect, thus circumventing the necessity of the expression of exogenous genes by the cells. We transplanted Fisher 344 rat-derived mammary adenocarcinoma cells (Mat B III) orthotopically into syngeneic F344 rats with an intact immune system. Rat umbilical cord matrix stem (rUCMS) cells derived from Wharton's jelly were then administered intratumoral (i.t) or i.v. 4 days later. The tumor attenuation effect was significantly evident starting from day 14 in i.v. and i.t. rUCMS cell-transplanted rats compared with sham-transplanted rats. In addition, unmodified rUCMS cell-transplanted rats showed complete regression of tumors to undetectable levels by 34 to 38 days with no evidence of metastasis or recurrence 100 days post-tumor cell inoculation. Dye-loaded rUCMS cells were identified within tumors only 4 days after their i.v. transplantation. In vitro colony assays with rUCMS cells as feeder layers markedly reduced Mat B III colony size and number. Growth attenuation of Mat B III cells exposed to either rUCMS cells directly or to the conditioned medium derived from rUCMS cells was associated with apoptosis indicators, including increased activated caspase-3. In addition, rUCMS cells cocultured with Mat B III cells had a dose-dependent antiproliferative effect on Mat B III cells. These findings suggest that unmodified human UCMS cells could be used for targeted cytotherapy for breast cancer.

Targeted therapy in the treatment of malignant gliomas

Malignant gliomas are invasive tumors with the potential to progress through current available therapies. These tumors are characterized by a number of abnormalities in molecular signaling that play roles in tumorigenesis, spread, and survival. These pathways are being actively investigated in both the pre-clinical and clinical settings as potential targets in the treatment of malignant gliomas. We will review many of the therapies that target the cancer cell, including the epidermal growth factor receptor, mammalian target of rapamycin, histone deacetylase, and farnesyl transferase. In addition, we will discuss strategies that target the extracellular matrix in which these cells reside as well as angiogenesis, a process emerging as central to tumor development and growth. Finally, we will briefly touch on the role of neural stem cells as both potential targets as well as delivery vectors for other therapies. Interdependence between these varied pathways, both in maintaining health and in causing disease, is clear. Thus, attempts to easily classify some targeted therapies are problematic.

Neural stem cell tropism to glioma: critical role of tumor hypoxia

Hypoxia is a critical aspect of the microenvironment in glioma and generally signifies unfavorable clinical outcome. Effective targeting of hypoxic areas in gliomas remains a significant therapeutic challenge. New therapeutic platforms using neural stem cells (NSC) for tumor-targeted drug delivery show promise in treatment of cancers that are refractory to traditional therapies. However, the molecular mechanisms of NSC targeting to hypoxic tumor areas are not well understood. Therefore, we investigated the role of hypoxia in directed migration of NSCs to glioma and identified the specific signaling molecules involved. Our data showed that hypoxia caused increased migration of human HB1.F3 NSCs to U251 human glioma-conditioned medium in vitro. In HB1.F3 NSCs, hypoxia led to up-regulation of CXCR4, urokinase-type plasminogen activator receptor (uPAR), vascular endothelial growth factor receptor 2 (VEGFR2), and c-Met receptors. Function-inhibiting antibodies to these receptors inhibited the migration of HB1.F3 cells to glioma-conditioned medium. Small interfering RNA knockdown of hypoxia-inducible factor-1alpha in glioma cells blocked the hypoxia-induced migration of NSCs, which was due to decreased expression of stromal cell-derived factor-1 (SDF-1), uPA, and VEGF in glioma cells. Our in vivo data provided direct evidence that NSCs preferentially distributed to hypoxic areas inside intracranial glioma xenografts, as detected by pimonidazole hypoxia probe, as well as to the tumor edge, and that both areas displayed high SDF-1 expression. These observations indicate that hypoxia is a key factor in determining NSC tropism to glioma and that SDF-1/CXCR4, uPA/uPAR, VEGF/VEGFR2, and hepatocyte growth factor/c-Met signaling pathways mediate increased NSC-to-glioma tropism under hypoxia. These results have significant implications for development of stem cell-mediated tumor-selective gene therapies.

Neural stem cells target intracranial glioma to deliver an oncolytic adenovirus in vivo

Adenoviral oncolytic virotherapy represents an attractive treatment modality for central nervous system (CNS) neoplasms. However, successful application of virotherapy in clinical trials has been hampered by inadequate distribution of oncolytic vectors. Neural stem cells (NSCs) have been shown as suitable vehicles for gene delivery because they track tumor foci. In this study, we evaluated the capability of NSCs to deliver a conditionally replicating adenovirus (CRAd) to glioma. We examined NSC specificity with respect to viral transduction, migration and capacity to deliver a CRAd to tumor cells. Fluorescence-activated cell sorter (FACS) analysis of NSC shows that these cells express a variety of surface receptors that make them amenable to entry by recombinant adenoviruses. Luciferase assays with replication-deficient vectors possessing a variety of transductional modifications targeted to these receptors confirm these results. Real-time PCR analysis of the replication profiles of different CRAds in NSCs and a representative glioma cell line, U87MG, identified the CRAd-Survivin (S)-pk7 virus as optimal vector for further delivery studies. Using in vitro and in vivo migration studies, we show that NSCs infected with CRAd-S-pk7 virus migrate and preferentially deliver CRAd to U87MG glioma. These results suggest that NSCs mediate an enhanced intratumoral distribution of an oncolytic vector in malignant glioma when compared with virus injection alone.

Tumor-targeted interferon-alpha delivery by Tie2-expressing monocytes inhibits tumor growth and metastasis

The use of type I interferons (IFNs) in cancer therapy has been limited by ineffective dosing and significant toxicity. Here, we exploited the tumor-homing ability of proangiogenic Tie2-expressing monocytes (TEMs) to deliver IFN-alpha to tumors. By transplanting hematopoietic progenitors transduced with a Tie2 promoter/enhancer-driven Ifna1 gene, we turned TEMs into IFN-alpha cell vehicles that efficiently targeted the IFN response to orthotopic human gliomas and spontaneous mouse mammary carcinomas and obtained significant antitumor responses and near complete abrogation of metastasis. TEM-mediated IFN-alpha delivery inhibited tumor angiogenesis and activated innate and adaptive immune cells but did not impair myelopoiesis and wound healing detectably. These results illustrate the therapeutic potential of gene- and cell-based IFN-alpha delivery and should allow the development of IFN treatments that more effectively treat cancer.

The mesenchymal stromal cell contribution to homeostasis

Adult mesenchymal stromal cells (MSCs) are undifferentiated multi-potent cells predominantly residing in the bone marrow (BM), but also present with similar but not identical features in many other tissues such as blood, placenta, dental pulp, and adipose tissue. MSCs have the potential to differentiate into multiple skeletal phenotypes like osteoblasts, chondrocytes, adipocytes, stromal cells, fibroblasts, and possibly tendons. MSCs differentiation potential, ex vivo expansion capacity, nurturing and immunomodulatory proficiencies oriented these versatile cells in several areas of ongoing clinical applications. However, the absence of MSC-specific markers for isolation and characterization together with the lack of a comprehensive view of the molecular pathways governing their particular biological properties, remains a primary obstacle to their research and application. In this review we discuss some areas of growing interest in MSCs biology: their contribution to the hematopoietic stem cell (HSC) niche, to regenerative medicine, their role in cancer and in therapy as delivery tools and their micro-RNA (miRNA) signatures. Despite rapid progress in the MSC field, it is generally thought that only a fraction of their full potential has been realized thus far.

Modifications of human carboxylesterase for improved prodrug activation

BACKGROUND

Carboxylesterases (CEs) are ubiquitous enzymes responsible for the hydrolysis of numerous clinically useful drugs. As ester moieties are frequently included in molecules to improve their water solubility and bioavailability, de facto they become substrates for CEs.

OBJECTIVE

In this review, we describe the properties of human CEs with regard to their ability to activate anticancer prodrugs and demonstrate how structure-based design can be used to modulate substrate specificity and to increase efficiency of hydrolysis.

METHODS

A specific example using CPT-11 and a human liver CE is discussed. However, these techniques can be applied to other enzymes and their associated prodrugs.

RESULTS

Structure-guided mutagenesis of CEs can be employed to alter substrate specificity and generate novel enzymes that are efficacious at anticancer prodrug activation.

Presence of pluripotent CD133+ cells correlates with malignancy of gliomas

BACKGROUND

Presence of CD133(+) cancer stem cells has been demonstrated within glioblastoma multiforme (GBM), the most malignant phenotype of gliomas (WHO grade IV). Since GBM frequently develops from low grade gliomas (WHO grade II) we assessed a possible qualitative or quantitative correlation of CD133(+) cells and glioma grade to get new insights in gliomagenesis.

RESULTS

The amount of CD133(+) cells within the bulk tumor mass, analyzed by immunostaining and Western blotting, showed a clear quantitative correlation with glioma grade (WHO degrees II, III and IV). Most of CD133(+) cells were arranged in clusters frequently associated to tumor vessels. Protein analysis revealed high cellular coexpression of CD133 with Musashi-I but not CD34 indicating a neural, i.e. local origin of these cells. In vitro, no differences in stem cell properties concerning self-renewal and multi-lineage differentiation have been found for CD133(+) cells isolated from gliomas of different grades.

CONCLUSIONS

These findings indicate a solely quantitative correlation of glioma grade with the presence of neural CD133(+) cells within tumors supporting the concept of a CD133(+) stem cell dependent gliomagenesis.

Stem cell-like glioma cells promote tumor angiogenesis through vascular endothelial growth factor

Malignant gliomas are highly lethal cancers dependent on angiogenesis. Critical tumor subpopulations within gliomas share characteristics with neural stem cells. We examined the potential of stem cell-like glioma cells (SCLGC) to support tumor angiogenesis. SCLGC isolated from human glioblastoma biopsy specimens and xenografts potently generated tumors when implanted into the brains of immunocompromised mice, whereas non-SCLGC tumor cells isolated from only a few tumors formed secondary tumors when xenotransplanted. Tumors derived from SCLGC were morphologically distinguishable from non-SCLGC tumor populations by widespread tumor angiogenesis, necrosis, and hemorrhage. To determine a potential molecular mechanism for SCLGC in angiogenesis, we measured the expression of a panel of angiogenic factors secreted by SCLGC. In comparison with matched non-SCLGC populations, SCLGC consistently secreted markedly elevated levels of vascular endothelial growth factor (VEGF), which were further induced by hypoxia. In an in vitro model of angiogenesis, SCLGC-conditioned medium significantly increased endothelial cell migration and tube formation compared with non-SCLGC tumor cell-conditioned medium. The proangiogenic effects of glioma SCLGC on endothelial cells were specifically abolished by the anti-VEGF neutralizing antibody bevacizumab, which is in clinical use for cancer therapy. Furthermore, bevacizumab displayed potent antiangiogenic efficacy in vivo and suppressed growth of xenografts derived from SCLGC but limited efficacy against xenografts derived from a matched non-SCLGC population. Together these data indicate that stem cell-like tumor cells can be a crucial source of key angiogenic factors in cancers and that targeting proangiogenic factors from stem cell-like tumor populations may be critical for patient therapy.