Molecular targets for selective killing of TRAIL-resistant leukemic cells

INTRODUCTION: TNF-related apoptosis-inducing ligand (TRAIL) is a member of the TNF family of cytokines, and shows promising therapeutic activity against solid tumors and lymphomas, in a variety of Phase I and II clinical trials. In contrast, primary leukemias have shown poor susceptibility to TRAIL-mediated cytotoxicity, suggesting the need for sensitizing TRAIL-resistant leukemic cells, by combining soluble recombinant TRAIL either with chemotherapeutic drugs, or with targeted small molecules. AREAS COVERED: This review discusses potential therapeutic applications of combinations able to restore the sensitivity of leukemic cells to either recombinant TRAIL or anti-TRAIL-receptor agonistic antibodies for the treatment of hematological malignancies. EXPERT OPINION: Up-to-date knowledge of the most innovative anti-leukemic therapies including functional screening of specific-sensitizers, enhancing TRAIL-mediated cytotoxicity. Strategies aimed to enhance TRAIL-mediated apoptosis, include the combination of novel sensitizers, functionally identified from libraries of pharmaceutically active, synthetic or naturally derived compounds. Other approaches aim to employ the administration of stem cells engineered to express TRAIL, in the leukemic stem cell niche, and promise to be a successful treatment with reduced specific toxicity.     

Targeting TRAIL in the treatment of cancer: new developments

Introduction: While apoptosis is critical for maintaining homeostasis in normal cells, defective apoptosis contributes to the survival of cancer cells. TNF-related apoptosis-inducing ligand (TRAIL)-targeted therapy has attracted significant effort for treating cancer, but the clinical results have revealed limitations. The authors review the current status of development of TRAIL-targeted therapy with an outlook towards the future.

Areas covered: Recombinant human proteins, small molecules and agonistic monoclonal antibodies targeting death receptors that trigger TRAIL-mediated apoptosis are covered in this article. The authors review both intrinsic and extrinsic apoptotic pathways, highlighting how the apoptosis serves as a promising therapeutic target. They also review different categories of TRAIL pathway targeting agents and provide a brief overview of clinical trials using these agents. The authors discuss the limitations of conventional approaches for targeting the TRAIL pathway as well as future directions.

Expert opinion: The development of better combination partners for pro-apoptotic TRAIL pathway modulators including novel agents inhibiting anti-apoptotic molecules or targeting alternative resistance pathways may improve the chances for anti-tumor responses in the clinic. Developing predictive biomarkers via circulating tumor cells/DNA, apoptosis signal products, and genetic signatures/protein biomarkers from tumor tissue are also suggested as future directions.     

TRAIL receptor signaling and therapeutics

Importance of the field: TNF-related apoptosis-inducing ligand (TRAIL) is a member of the TNF family of cytokines, which can induce apoptotic cell death in a variety of tumor cells by engaging specific death receptors, TRAIL-R1 and TRAIL-R2, while having low toxicity towards normal cells. There is interest in cancer therapy inducing cell death by activation of the death-receptor-mediated apoptotic pathway while avoiding decoy-receptor-mediated neutralization of the signal. This has led to the development of a number of receptor-specific TRAIL-variants and agonistic antibodies. Some of these soluble recombinant TRAIL and agonist antibodies targeting TRAIL-R1 and/or TRAIL-R2 are progressing in clinical trials. In addition, TRAIL-resistant tumors can be sensitized to TRAIL by a combination of TRAIL or agonistic antibodies with chemotherapeutic agents, targeted small molecules or irradiation.

Areas covered in this review: Recent advances in developing TRAIL or its agonist receptor antibodies in cancer therapy. We also discuss combination therapies in overcoming TRAIL resistance in cancer cells.

What the reader will gain: Knowledge of current clinical trials, the promise and obstacles in the future development of therapies affecting TRAIL signaling pathways.

Take home message: Cancer therapeutics targeting the TRAIL/TRAIL receptor signaling pathway hold great promise for molecularly targeted pro-apoptotic anti-cancer therapy.     

Therapeutic targeting of leukemic stem cells in acute myeloid leukemia – the biological background for possible strategies

Introduction: Acute myeloid leukemia (AML) is an aggressive malignancy, caused by the accumulation of immature leukemic blasts in blood and bone marrow. There is a relatively high risk of chemoresistant relapse even for the younger patients who can receive the most intensive antileukemic treatment. Treatment directed against the remaining leukemic and preleukemic stem cells will most likely reduce the risk of later relapse.

Areas covered: Relevant publications were identified through literature searches. The authors searched for original articles and recent reviews describing (i) the characteristics of leukemic/preleukemic stem cells; (ii) the importance of the bone marrow stem cell niches in leukemogenesis; and (iii) possible therapeutic strategies to target the preleukemic/leukemic stem cells.

Expert opinion: Leukemia relapse/progression seems to be derived from residual chemoresistant leukemic or preleukemic stem cells, and a more effective treatment directed against these cells will likely be important to improve survival both for patients receiving intensive treatment and leukemia-stabilizing therapy. Several possible strategies are now considered, including the targeting of the epigenetic regulation of gene expression, proapoptotic intracellular signaling, cell metabolism, telomere activity and the AML-supporting effects by neighboring stromal cells. Due to disease heterogeneity, the most effective stem cell-directed therapy will probably differ between individual patients.     

Pegylated TRAIL retains anti-leukemic cytotoxicity and exhibits improved signal transduction activity with respect to TRAIL

To improve the pharmacokinetic profile of tumor necrosis factor-related apoptosis inducing ligand (TRAIL) an N-terminal specific pegylation was performed to generate pegylated TRAIL (PEG-TRAIL). In in vitro experiments, we found that although PEG-TRAIL was slightly less efficient than recombinant TRAIL in promoting leukemic cell apoptosis, it showed an improved ability to promote migration of bone-marrow mesenchymal stem cells and to elicit the ERK1/2 intracellular signal transduction pathway. Overall, these data suggest that TRAIL pegylation retains, or even enhances, the biological activities of TRAIL relevant for its therapeutic applications.     

Therapeutic targeting of the p53 pathway in cancer stem cells

Introduction: Cancer stem cells (CSCs) are a high profile drug target for cancer therapeutics due to their indispensable role in cancer progression, maintenance and therapeutic resistance. Restoring wild-type (WT) p53 function is an attractive new therapeutic approach for the treatment of cancer due to the well-described powerful tumor suppressor function of p53. As emerging evidence intimately links p53 and stem cell biology, this approach also provides an opportunity to target CSCs.

Areas covered: This review covers the therapeutic approaches to restore the function of WT p53, cancer and normal stem cell biology in relation to p53 and the downstream effects of p53 on CSCs.

Expert opinion: The restoration of WT p53 function by targeting p53 directly, its interacting proteins or its family members holds promise as a new class of cancer therapies. This review examines the impact that such therapies may have on normal and CSCs based on the current evidence linking p53 signaling with these populations.     

Targeting CSCs within the tumor microenvironment for cancer therapy: a potential role of mesenchymal stem cells

Introduction: Mesenchymal stem cells (MSCs) are one subgroup of adult stem cells and possess a proliferative potential and ability to differentiate into various ceells.

Areas covered: Emerging evidence suggests that MSCs can reprogram toward cancer stem cells (CSCs), due to alterations of intrinsic and extrinsic microenvironments, leading to tumorigenesis. The CSC concept has fundamental clinical implications because of its involvement in cell migration/invasion, metastasis, and treatment resistance. Therefore, targeting CSCs provides a novel therapeutic strategy for cancer treatment. However, the origin of CSCs and its molecular connections are not fully understood. Emerging evidence suggests the existence of an inter-relationship between CSCs and epithelial-to-mesenchymal transition (EMT) phenotypic cells, in the context of inflammation and hypoxia, as well as the potential role of miRNAs.

Expert opinion: We suggest that targeting CSC signatures along with EMT, inflammation, and hypoxia will provide a more effective therapeutic approach for the elimination of CSCs. To that end, curcumin especially its synthetic novel analog CDF have been shown to attenuate CSC characteristics along with the deregulation of multiple pathways and miRNAs, leading to the inhibition of human tumor growth in vivo, suggesting the potential role of CDF as an anti-tumor agent for the prevention/treatment of tumor progression.     

Muscle-derived stem cells: important players in peripheral nerve repair

ABSTRACT

Introduction: Stem cell therapy for peripheral nerve repair is a rapidly evolving field in regenerative medicine. Although most studies to date have investigated stem cells originating from bone marrow or adipose, skeletal muscle has recently been recognized as an abundant and easily accessible source of stem cells. Muscle-derived stem cells (MDSCs) are a diverse population of multipotent cells with pronounced antioxidant and regenerative capacity.

Areas covered: The current literature on the various roles MDSCs serve within the micro- and macro-environment of nerve injury. Furthermore, the exciting new research that is establishing MDSC-cellular therapy as an important therapeutic modality to improve peripheral nerve regeneration.

Expert opinion: MDSCs are a promising therapeutic agent for the repair of peripheral nerves; MDSCs not only undergo gliogenesis and angiogenesis, but they also orchestrate larger pro-regenerative host responses. However, the isolation, transformation, and in-vivo behavior of MDSCs require further evaluation prior to clinical application.     

Advances in stem cells,induced pluripotent stem cells,and engineered cells: delivery vehicles for anti-glioma therapy

Introduction: A limitation of small molecule inhibitors, nanoparticles (NPs) and therapeutic adenoviruses is their incomplete distribution within the entirety of solid tumors such as malignant gliomas. Currently, cell-based carriers are making their way into the clinical setting as they offer the potential to selectively deliver many types of therapies to cancer cells.

Areas covered: Here, we review the properties of stem cells, induced pluripotent stem cells and engineered cells that possess the tumor-tropic behavior necessary to serve as cell carriers. We also report on the different types of therapeutic agents that have been delivered to tumors by these cell carriers, including: i) therapeutic genes; ii) oncolytic viruses; iii) NPs; and iv) antibodies. The current challenges and future promises of cell-based drug delivery are also discussed.

Expert opinion: While the emergence of stem cell-mediated therapy has resulted in promising preclinical results and a human clinical trial utilizing this approach is currently underway, there is still a need to optimize these delivery platforms. By improving the loading of therapeutic agents into stem cells and enhancing their migratory ability and persistence, significant improvements in targeted cancer therapy may be achieved.     

The key role of adult stem cells: therapeutic perspectives

ABSTRACT

Background: The origin, function and physiology of totipotent embryonic cells are configured to construct organs and create cross-talk between cells for the biological and neurophysiologic development of organisms. Adult stem cells are involved in regenerating tissues for renewal and damage repair.

Findings: Adult stem cells have been isolated from adult tissue, umbilical cord blood and other non-embryonic sources, and can transform into many tissues and cell types in response to pathophysiological stimuli.

Clinical applications of adult stem cells and progenitor cells have potential in the regeneration of blood cells, skin, bone, cartilage and heart muscle, and may have potential in degenerative diseases. Multi-pluripotent adult stem cells can change their phenotype in response to trans-differentiation or fusion and their therapeutic potential could include therapies regulated by pharmacological modulation, for example mobilising endogenous stem cells and directing them within a tissue to stimulate regeneration. Adult stem cells could also provide a vehicle for gene therapy, and genetically-engineered human adult stem cells have shown success in treatment of genetic disease.

Conclusion: Deriving embryonic stem cells from early human embryos raises ethical, legal, religious and political questions. The potential uses of stem cells for generating human tissues are the subject of ongoing public debate. Stem cells must be used in standardised and controlled conditions in order to guarantee the best safety conditions for the patients. One critical point will be to verify the risk of tumourigenicity; this issue may be more relevant to embryonic than adult stem cells.     

Cytokines for the treatment of gastrointestinal cancers: clinical experience and new perspectives

Introduction: Cytokines are key mediators of the immune system and have been proposed as therapeutic agents against cancer, either as recombinant proteins, or as transgenes in gene therapy approaches. Stimulation of immune responses against cancer cells is an appealing method to treat tumors with high risk of relapse and systemic dissemination.

Areas covered: We provide a critical overview of clinical trials involving the use of cytokines for the treatment of liver, colon and pancreatic cancers. Special attention has been paid to advances in the field of gene therapy and oncolytic viruses. The potential of new developments still in a pre-clinical stage is also discussed. We have revised public sources of information (PubMed, US National Institutes of Health clinical trials database) up to January 2013.

Expert opinion: The complexity of the immune system and the unfavorable pharmacokinetic properties of cytokines limit the efficacy of these molecules as single agents for the treatment of cancer. Expression from gene therapy vectors, together with new methods of targeting and stabilization, may overcome these hurdles. We believe cytokines will play a crucial role as part of combined approaches, enhancing the action of adoptive cell immunotherapy, oncolytic viruses or biological therapies.     

Enhanced TRAIL-mediated apoptosis in prostate cancer cells by the bioactive compounds neobavaisoflavone and psoralidin isolated from Psoralea corylifolia

Numerous compounds detected in medical plants and dietary components or supplements possess chemopreventive, antitumor and immunomodulatory properties. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is an important endogenous anticancer factor that induces apoptosis selectively in cancer cells. However, some tumor cells are resistant to TRAIL-mediated apoptosis. Naturally occurring agents could sensitize TRAIL-resistant cancer cells and augment their apoptotic activity.We examined the cytotoxic and apoptotic effects of neobavaisoflavone and psoralidin in combination with TRAIL on LNCaP prostate cancer cells. The cytotoxicity was evaluated by MTT and LDH assays. The apoptosis was detected using Annexin V-FITC by flow cytometry and fluorescence microscopy. The LNCaP cells were shown to be resistant to TRAIL-induced apoptosis. Our study demonstrated that neobavaisoflavone and psoralidin sensitized TRAIL-resistant cells and markedly augmented TRAIL-mediated apoptosis and cytotoxicity in prostate cancer cells. Cotreatment of LNCaP cells with 100 ng/ml TRAIL and 50 μM neobavaisoflavone or 50 μM psoralidin increased the percentage of the apoptotic cells to 77.5±0.5% or 64.4±0.5%, respectively. The data indicate the potential role of the bioactive compounds isolated from the medicinal plant Psoralea corylifolia (neobavaisoflavone and psoralidin) in prostate cancer chemoprevention through enhancement of TRAIL-mediated apoptosis.     

Intranasal delivery of stem cells to the brain

Introduction: Stem cell-based therapy has proved to be a promising treatment option for neurological disorders. However, there are difficulties in successfully administrating these stem cells. For example, the brain–blood barrier impedes the entrance of stem cells into the CNS after systemic administration. Direct transplantation or injection may result in brain injury, and these strategies are clinically less feasible. Intranasal administration is a non-invasive and effective alternative for the delivery of drugs, vector-encoded viruses or even phages to the CNS. Recent studies have in fact demonstrated that stem cells may enter the CNS after intranasal administration. These results suggest that intranasal delivery may provide an alternative strategy for stem cell-based therapy.

Areas covered: This review summarizes current studies that have applied the intranasal delivery of stem cells into the brain. In addition, the distribution and fate of stem cells in the brain and the potential opportunities as well as challenges of intranasal stem cell delivery are also discussed.

Expert opinion: Intranasal delivery of stem cells is a new method with great potential for the transplantation of stem cells into the brain, and it may provide an extraordinary approach to overcoming the existing barriers of stem cell delivery for the treatment of many neurological disorders. This potential benefit emphasizes the importance of future research into intranasal delivery of stem cells.     

Current and future approaches to treat graft failure after allogeneic hematopoietic stem cell transplantation

Introduction: One significant obstacle to the success of allogeneic hematopoietic stem cell transplantation (HSCT) is represented by graft failure, defined as either lack of initial engraftment of donor cells (primary graft failure) or loss of donor cells after initial engraftment (secondary graft failure). Graft failure mediated by host immune cells attacking donor stem cells is named graft rejection. Factors associated with graft failure include HLA disparity in the donor/recipient pair, underlying disease, viral infections, type of conditioning regimen and stem cell source employed.

Areas covered: In this article, the experts summarize current approaches to treat graft failure/rejection after HSCT, and they discuss new strategies of graft manipulation and immune therapy of particular interest for preventing/treating this complication.

Expert opinion: A limited array of options is available to treat graft failure. The experts believe that re-transplantation from another donor or the same donor (if there is no evidence of immunologically mediated graft failure) is the treatment of choice for patients with primary graft failure or acute graft rejection. The experts think that strategies based on innovative approaches of graft manipulation, new agents or cellular therapies could render in the future graft failure a much less relevant problem for HSCT recipients.     

The cancer stem cell paradigm: a new understanding of tumor development and treatment

Importance of the field: Cancer is the second leading cause of death in the United States, and therefore remains a central focus of modern medical research. Accumulating evidence supports a ‘cancer stem cell’ (CSC) model – where cancer growth and/or recurrence is driven by a small subset of tumor cells that exhibit properties similar to stem cells. This model may provide a conceptual framework for developing more effective cancer therapies that target cells propelling cancer growth.

Areas covered in this review: We review evidence supporting the CSC model and associated implications for understanding cancer biology and developing novel therapeutic strategies. Current controversies and unanswered questions of the CSC model are also discussed.

What the reader will gain: This review aims to describe how the CSC model is key to developing novel treatments and discusses associated shortcomings and unanswered questions.

Take home message: A fresh look at cancer biology and treatment is needed for many incurable cancers to improve clinical prognosis for patients. The CSC model posits a hierarchy in cancer where only a subset of cells drive malignancy, and if features of this model are correct, has implications for development of novel and hopefully more successful approaches to cancer therapy.     

Nanofiber for cardiovascular tissue engineering

Introduction: Organ/tissue replacement therapy is inherently difficult for application in the tissue engineering field due to immune rejection that limits the long-term efficacy of implanted devices. As the application of tissue engineering in the biomedical field has steadily expanded, stem cells have emerged as a viable option to promote the immune acceptance of implantable devices and to expedite alleviation of the pathological conditions. With various novel scaffolds being introduced, nanofibers which have a three-dimensional architecture can be considered as an efficient carrier for stem cells.

Areas covered: This article reviews the novel tissue engineering processes involved with nanofiber and stem cells. Topics such as the fabrication of nanofiber via electrospinning techniques, the interaction between nanofiber scaffold and specific cell and advanced techniques to enhance the stability of stem cells are delineated in detail. In addition, cardiovascular applications of nanofiber scaffolds loaded with stem cells are examined from a clinical perspective.

Expert opinion: Electrospun nanofibers have been intensively explored as a tool for the architecture control of cardiovascular tissue engineering due to their tunable physicochemical properties. The modification of nanofiber with biological cues, which provide rapid differentiation of stem cells into a specific lineage and protect stem cells under the harsh conditions (i.e., hypoxia), will significantly enhance therapeutic efficacies of transplanted cells. A combination of nanofiber carriers and stem cell therapy for tissue regeneration seems to pose enormous potential for the treatment of cardiac diseases including atherosclerosis and myocardial infarction.     

Ex vivo fucosylation of stem cells to improve engraftment: WO2004094619

Background: The application is in the field of haematopoietic stem cell (HSC) and umbilical cord blood (CB) transplantation.

Objective: It aims at determining an enzymatic procedure to improve the engraftment of CB stem cells to the bone marrow (BM) as well as the rate of success of CB transplantation.

Methods: Human CB stem cells were treated with α1 – 3 fucosyltransferase VI (FTVI) for 30 min ex vivo prior to transplantation. Human CB stem cells were administered intravenously in irradiated nonobese diabetic/severe combined immune deficiency (NOD/SCID) mice.

Results: Treatment of CB stem cells with FTVI improves their engraftment in the BM of NOD/SCID mice.

Conclusion: Treatment of CB stem cells with FTVI, ex vivo prior to transplantation, may reduce the delay of engraftment of CB stem cells to the BM and the risk of early infections associated with CB transplants, particularly in adults. It may improve the outcome and rate of success of CB transplantation. The application claims the use of fucosyltransferase for improving the engraftment of CB HSCs to the BM for the treatment of haematologic diseases. The procedure may be used to improve the engraftment of HSCs from other sources and other types of stem cells, on transplantation, particularly when administered intravenously.     

Targeting DOT1L action and interactions in leukemia: the role of DOT1L in transformation and development

Importance of the field: The establishment and maintenance of specialized chromatin is crucial for correct gene expression and chromosome stability in mammalian cells. Therefore, epigenetic insults are frequently observed in cancer. Several chromatin modifying enzymes have been implicated in leukemia, and are attractive candidates for the development of therapeutic agents.

Areas covered in this review: The histone methyltransferase DOT1L is responsible for methylation of histone H3 at lysine 79 and is involved in the pathobiology of several leukemias, the majority of which are characterized by chromosomal translocations involving the mixed lineage leukemia (MLL) gene. Leukemic translocations yield fusion proteins involving MLL and other proteins that physically interact with DOT1L. These oncogenic fusion proteins recruit DOT1L to ectopic loci (including HOX gene clusters), whose mis-expression contributes to the transformed phenotype. Studies from stem cells and certain leukemias suggest a second mechanism of leukemogenesis, in which reduced or mistargeted DOT1L activity yields altered centromeric chromatin and consequent chromosomal instability. Targeting DOT1L enzymatic activity as well as interactions with leukemogenic fusion proteins is discussed as possible leads in therapeutic interventions.

What the reader will gain: In this review, we discuss the normal functions of DOT1L, its mechanistic roles in leukemogenesis, and possible strategies for targeting DOT1L in leukemia. DOT1L is an atypical histone lysine methyltransferase in that it does not contain an enzymatic domain common to all other lysine methyltranferases. This attribute makes DOT1L a unique and specifically targetable enzyme. An emerging role for DOT1L under normal cellular conditions as well as transformed conditions is emerging and shedding light on the biology and mechanisms of some translocation-induced leukemias.

Take home message: DOT1L is critical in development, as shown in studies in mouse embryos and embryonic stem cells. DOT1L enzymatic activity is also required for the leukemic transformation capabilities of a number of oncogenic fusion proteins. In addition, interactions between DOT1L and oncogenic fusion proteins are necessary for the transformation process. Therefore, it may be possible to specifically target DOT1L enzymatic activity or DOT1L interactions with leukemogenic fusion proteins.