Generation of breast cancer stem cells through epithelial-mesenchymal transition

Recently, two novel concepts have emerged in cancer biology: the role of so-called “cancer stem cells” in tumor initiation, and the involvement of an epithelial-mesenchymal transition (EMT) in the metastatic dissemination of epithelial cancer cells. Using a mammary tumor progression model, we show that cells possessing both stem and tumorigenic characteristics of “cancer stem cells” can be derived from human mammary epithelial cells following the activation of the Ras-MAPK pathway. The acquisition of these stem and tumorigenic characters is driven by EMT induction.

Stable suppression of tumorigenicity by virus-mediated RNA interference

Most human tumors harbor multiple genetic alterations, including dominant mutant oncogenes. It is often not clear which of these oncogenes are continuously required and which, when inactivated, may inhibit tumorigenesis. Recently, we developed a vector that mediates suppression of gene expression through RNA interference. Here, we use a retroviral version of this vector to specifically and stably inhibit expression of only the oncogenic K-RAS(V12) allele in human tumor cells. Loss of expression of K-RAS(V12) leads to loss of anchorage-independent growth and tumorigenicity. These results indicate that viral delivery of small interfering RNAs can be used for tumor-specific gene therapy to reverse the oncogenic phenotype of cancer cells.

Woodchuck hepatitis virus posttranscriptional regulatory element enhances expression of transgenes delivered by retroviral vectors

The expression of genes delivered by retroviral vectors is often inefficient, a potential obstacle for their widespread use in human gene therapy. Here, we explored the possibility that the posttranscriptional regulatory element of woodchuck hepatitis virus (WPRE) might help resolve this problem. Insertion of the WPRE in the 3' untranslated region of coding sequences carried by either oncoretroviral or lentiviral vectors substantially increased their levels of expression in a transgene-, promoter- and vector-independent manner. The WPRE thus increased either luciferase or green fluorescent protein production five- to eightfold, and effects of a comparable magnitude were observed with either the immediate-early cytomegalovirus or the herpesvirus thymidine kinase promoter and with both human immunodeficiency virus- and murine leukemia virus-based vectors. The WPRE exerted this influence only when placed in the sense orientation, consistent with its predicted posttranscriptional mechanism of action. These results demonstrate that the WPRE significantly improves the performance of retroviral vectors and emphasize that posttranscriptional regulation of gene expression should be taken into account in the design of gene delivery systems.

Reconstitution of telomerase activity in normal human cells leads to elongation of telomeres and extended replicative life span

Normal somatic cells have a finite life span [1] and lose telomeric DNA, present at the ends of chromosomes, each time they divide as a function of age in vivo or in culture [2-4]. In contrast, many cancer cells and cell lines established from tumours maintain their telomere length by activation of an RNA-protein complex called telomerase, an enzyme originally discovered in Tetrahymena [5], that synthesizes telomeric repeats [6-8]. These findings have led to the formation of the 'telomere hypothesis', which proposes that critical shortening of telomeric DNA due to the end-replication problem [9] is the signal for the initiation of cellular senescence [10,11]. In yeast, the EST2 gene product, the catalytic subunit of telomerase, is essential for telomere maintenance in vivo [12-14]. The recent cloning of the cDNA encoding the catalytic subunit of human telomerase (hTERT) [15,16] makes it possible to test the telomere hypothesis. In this study, we expressed hTERT in normal human diploid fibroblasts, which lack telomerase activity, to determine whether telomerase activity could be reconstituted leading to extension of replicative life span. Our results show that retroviral-mediated expression of hTERT resulted in functional telomerase activity in normal aging human cells. Moreover, reconstitution of telomerase activity in vivo led to an increase in the length of telomeric DNA and to extension of cellular life span. These findings provide direct evidence in support of the telomere hypothesis, indicating that telomere length is one factor that can determine the replicative life span of human cells.

Gene therapy for immunodeficiency due to adenosine deaminase deficiency

BACKGROUND

We investigated the long-term outcome of gene therapy for severe combined immunodeficiency (SCID) due to the lack of adenosine deaminase (ADA), a fatal disorder of purine metabolism and immunodeficiency.

METHODS

We infused autologous CD34+ bone marrow cells transduced with a retroviral vector containing the ADA gene into 10 children with SCID due to ADA deficiency who lacked an HLA-identical sibling donor, after nonmyeloablative conditioning with busulfan. Enzyme-replacement therapy was not given after infusion of the cells.

RESULTS

All patients are alive after a median follow-up of 4.0 years (range, 1.8 to 8.0). Transduced hematopoietic stem cells have stably engrafted and differentiated into myeloid cells containing ADA (mean range at 1 year in bone marrow lineages, 3.5 to 8.9%) and lymphoid cells (mean range in peripheral blood, 52.4 to 88.0%). Eight patients do not require enzyme-replacement therapy, their blood cells continue to express ADA, and they have no signs of defective detoxification of purine metabolites. Nine patients had immune reconstitution with increases in T-cell counts (median count at 3 years, 1.07x10(9) per liter) and normalization of T-cell function. In the five patients in whom intravenous immune globulin replacement was discontinued, antigen-specific antibody responses were elicited after exposure to vaccines or viral antigens. Effective protection against infections and improvement in physical development made a normal lifestyle possible. Serious adverse events included prolonged neutropenia (in two patients), hypertension (in one), central-venous-catheter-related infections (in two), Epstein-Barr virus reactivation (in one), and autoimmune hepatitis (in one).

CONCLUSIONS

Gene therapy, combined with reduced-intensity conditioning, is a safe and effective treatment for SCID in patients with ADA deficiency. (ClinicalTrials.gov numbers, NCT00598481 and NCT00599781.)

Functional and physical interactions of the ARF tumor suppressor with p53 and Mdm2

The INK4a-ARF locus encodes two proteins, p16(INK4a) and p19(ARF), that restrain cell growth by affecting the functions of the retinoblastoma protein and p53, respectively. Disruption of this locus by deletions or point mutations is a common event in human cancer, perhaps second only to the loss of p53. Using insect cells infected with baculovirus vectors and NIH 3T3 fibroblasts infected with ARF retrovirus, we determined that mouse p19(ARF) can interact directly with p53, as well as with the p53 regulator mdm2. ARF can bind p53-DNA complexes, and it depends upon functional p53 to transcriptionally induce mdm2 and the cyclin-dependent kinase inhibitor p21(Cip1), and to arrest cell proliferation. Binding of p19(ARF) to p53 requires the ARF N-terminal domain (amino acids 1-62) that is necessary and sufficient to induce cell cycle arrest. Overexpression of p19(ARF) in wild type or ARF-null mouse embryo fibroblasts increases the half-life of p53 from 15 to approximately 75 min, correlating with an increased p53-dependent transcriptional response and growth arrest. Surprisingly, when overexpressed at supra-physiologic levels after introduction into ARF-null NIH 3T3 cells or mouse embryo fibroblasts, the p53 protein is handicapped in inducing this checkpoint response. In this setting, reintroduction of p19(ARF) restores p53's ability to induce p21(Cip1) and mdm2, implying that, in addition to stabilizing p53, ARF modulates p53-dependent function through an additional mechanism.

Identification of primitive human hematopoietic cells capable of repopulating NOD/SCID mouse bone marrow: implications for gene therapy

The development of stem-cell gene therapy is hindered by the absence of repopulation assays for primitive human hematopoietic cells. Current methods of gene transfer rely on in vitro colony-forming cell (CFC) and long-term culture-initiating cell (LTC-IC) assays, as well as inference from other mammalian species. We have identified a novel human hematopoietic cell, the SCID-repopulating cell (SRC), a cell more primitive than most LTC-ICs and CFCs. The SRC, exclusively present in the CD4+CD8- fraction, is capable of multilineage repopulation of the bone marrow of nonobese diabetic mice with severe combined immunodeficiency disease (NOD/SCID mice). SRCs were rarely transduced with retroviruses, distinguishing them from most CFCs and LTC-ICs. This observation is consistent with the low level of gene marking seen in human gene therapy trials. An SRC assay may aid in the characterization of hematopoiesis, as well as the improvement of transduction methods.

The T cell leukemia oncoprotein SCL/tal-1 is essential for development of all hematopoietic lineages

The T cell leukemia oncoprotein SCL/tal-1, a basic-helix-loop-helix transcription factor, is required for production of embryonic red blood cells in the mouse yolk sac. To define roles in other lineages, we studied the hematopoietic potential of homozygous mutant SCL/tal-1 -/- embryonic stem cells upon in vitro differentiation and in vivo in chimeric mice. Here we show that in the absence of SCL/tal-1, hematopoiesis, Including the generation of red cells, myeloid cells, megakaryocytes, mast cells, and both T and B lymphoid cells, is undetectable. These findings suggest that SCL/tal-1 functions very early in hematopoietic development, either in specification of ventral mesoderm to a blood cell fate, or in formation or maintenance of immature progenitors.

Hoxa9 transforms primary bone marrow cells through specific collaboration with Meis1a but not Pbx1b

Hoxa9, Meis1 and Pbx1 encode homeodomaincontaining proteins implicated in leukemic transformation in both mice and humans. Hoxa9, Meis1 and Pbx1 proteins have been shown to physically interact with each other, as Hoxa9 cooperatively binds consensus DNA sequences with Meis1 and with Pbx1, while Meis1 and Pbx1 form heterodimers in both the presence and absence of DNA. In this study, we sought to determine if Hoxa9 could transform hemopoietic cells in collaboration with either Pbx1 or Meis1. Primary bone marrow cells, retrovirally engineered to overexpress Hoxa9 and Meis1a simultaneously, induced growth factor-dependent oligoclonal acute myeloid leukemia in <3 months when transplanted into syngenic mice. In contrast, overexpression of Hoxa9, Meis1a or Pbx1b alone, or the combination of Hoxa9 and Pbx1b failed to transform these cells acutely within 6 months post-transplantation. Similar results were obtained when FDC-P1 cells, engineered to overexpress these genes, were transplanted to syngenic recipients. Thus, these studies demonstrate a selective collaboration between a member of the Hox family and one of its DNA-binding partners in transformation of hemopoietic cells.

Lineage-independent determination of cell type in the embryonic mouse retina

We previously used a retroviral vector to mark clones in the postnatal rodent retina and showed that at least two types of neurons and Müller glia can arise from a common progenitor. Here we describe the use of exo utero surgery to introduce a marker retrovirus into the proliferative zone of the retinas of embryonic day 13 and 14 mice. Analysis of marked clones in the resulting adult retinas shows that almost all progenitor cells that continued mitosis were multipotential and that a single progenitor can generate most retinal cell types. The size of marked clones indicates that retinal cells do not employ a stem cell mode of division, but instead, both daughter cells of a progenitor can continue to divide. These results suggest that cell type determination in the rodent retina is independent of lineage. We propose a model for the generation of retinal cell types in which the cessation of mitosis and cell type determination are independent events, controlled by environmental interactions.

Similar MLL-associated leukemias arising from self-renewing stem cells and short-lived myeloid progenitors

We have used the hematopoietic system as a model to investigate whether acute myeloid leukemia arises exclusively from self-renewing stem cells or also from short-lived myeloid progenitors. When transduced with a leukemogenic MLL fusion gene, prospectively isolated stem cells and myeloid progenitor populations with granulocyte/macrophage differentiation potential are efficiently immortalized in vitro and result in the rapid onset of acute myeloid leukemia with similar latencies following transplantation in vivo. Regardless of initiating cell, leukemias displayed immunophenotypes and gene expression profiles characteristic of maturation arrest at an identical late stage of myelomonocytic differentiation, putatively a monopotent monocytic progenitor stage. Our findings unequivocally establish the ability of transient repopulating progenitors to initiate myeloid leukemias in response to an MLL oncogene, and support the existence of cancer stem cells that do not necessarily overlap with multipotent stem cells of the tissue of origin.

Correction of junctional epidermolysis bullosa by transplantation of genetically modified epidermal stem cells

The continuous renewal of human epidermis is sustained by stem cells contained in the epidermal basal layer and in hair follicles. Cultured keratinocyte stem cells, known as holoclones, generate sheets of epithelium used to restore severe skin, mucosal and corneal defects. Mutations in genes encoding the basement membrane component laminin 5 (LAM5) cause junctional epidermolysis bullosa (JEB), a devastating and often fatal skin adhesion disorder. Epidermal stem cells from an adult patient affected by LAM5-beta3-deficient JEB were transduced with a retroviral vector expressing LAMB3 cDNA (encoding LAM5-beta3), and used to prepare genetically corrected cultured epidermal grafts. Nine grafts were transplanted onto surgically prepared regions of the patient's legs. Engraftment was complete after 8 d. Synthesis and proper assembly of normal levels of functional LAM5 were observed, together with the development of a firmly adherent epidermis that remained stable for the duration of the follow-up (1 year) in the absence of blisters, infections, inflammation or immune response. Retroviral integration site analysis indicated that the regenerated epidermis is maintained by a defined repertoire of transduced stem cells. These data show that ex vivo gene therapy of JEB is feasible and leads to full functional correction of the disease.

Induction of AIDS-like disease in macaque monkeys with T-cell tropic retrovirus STLV-III

The T-cell tropic retrovirus of macaque monkeys STLV-III has morphologic, growth, and antigenic properties indicating that it is related to HTLV-III/LAV, the etiologic agent of the acquired immune deficiency syndrome (AIDS) in humans. Four of six rhesus monkeys died within 160 days of STLV-III inoculation with a wasting syndrome, opportunistic infections, a primary retroviral encephalitis, and immunologic abnormalities including a decrease in T4+ peripheral blood lymphocytes. These data show that an immunodeficiency syndrome can be produced experimentally in a nonhuman primate by an agent from the HTLV-III/LAV group of retroviruses. The STLV-III-macaque system will thus provide a useful model for the study of antiviral agents and vaccine development for human AIDS.

Positive feedback regulation of type I IFN genes by the IFN-inducible transcription factor IRF-7

The interferon regulatory factor (IRF) family of transcription factors regulate the interferon (IFN) system, among which IRF-3 is involved in the virus-induced IFN-beta gene expression. Here we show that another member IRF-7 is critical for the IFN-alpha gene induction. Unlike the IRF-3 gene, the IRF-7 gene is induced by IFNs through activation of the ISGF3 transcription factor, and IRF-7 undergoes virus-induced nuclear translocation. In cells lacking p48, an essential component of IFN stimulated gene factor 3 (ISGF3), ectopic expression of IRF-7 but not IRF-3 can rescue the deficiency to induce IFN-alpha genes. These results indicate that IRF-7 is a key factor in the positive feedback regulation of IFN-alpha/beta production.

Gene therapy: trials and tribulations

The art and science of gene therapy has received much attention of late. The tragic death of 18-year-old Jesse Gelsinger, a volunteer in a Phase I clinical trial, has overshadowed the successful treatment of three children suffering from a rare but fatal immunological disease. In the light of the success and tragedy, it is timely to consider the challenges faced by gene therapy--a novel form of molecular medicine that may be poised to have an important impact on human health in the new millennium.

Wnt 3a promotes proliferation and suppresses osteogenic differentiation of adult human mesenchymal stem cells

Multipotential adult mesenchymal stem cells (MSCs) are able to differentiate along several known lineages, and lineage commitment is tightly regulated through specific cellular mediators and interactions. Recent observations of a low/high bone-mass phenotype in patients expressing a loss-/gain-of-function mutation in LRP5, a coreceptor of the Wnt family of signaling molecules, suggest the importance of Wnt signaling in bone formation, possibly involving MSCs. To analyze the role of Wnt signaling in mesenchymal osteogenesis, we have profiled the expression of WNTs and their receptors, FRIZZLEDs (FZDs), and several secreted Wnt inhibitors, such as SFRPs, and examined the effect of Wnt 3a, as a representative canonical Wnt member, during MSC osteogenesis in vitro. WNT11, FZD6, SFRP2, and SFRP3 are upregulated during MSC osteogenesis, while WNT9A and FZD7 are downregulated. MSCs also respond to exogenous Wnt 3a, based on increased beta-catenin nuclearization and activation of a Wnt-responsive promoter, and the magnitude of this response depends on the MSC differentiation state. Wnt 3a exposure inhibits MSC osteogenic differentiation, with decreased matrix mineralization and reduced alkaline phosphatase mRNA and activity. Wnt 3a treatment of fully osteogenically differentiated MSCs also suppresses osteoblastic marker gene expression. The Wnt 3a effect is accompanied by increased cell number, resulting from both increased proliferation and decreased apoptosis, particularly during expansion of undifferentiated MSCs. The osteo-suppressive effects of Wnt 3a are fully reversible, i.e., treatment prior to osteogenic induction does not compromise subsequent MSC osteogenesis. The results also showed that sFRP3 treatment attenuates some of the observed Wnt 3a effects on MSCs, and that inhibition of canonical Wnt signaling using a dominant negative TCF1 enhances MSC osteogenesis. Interestingly, expression of Wnt 5a, a non-canonical Wnt member, appeared to promote osteogenesis. Taken together, these findings suggest that canonical Wnt signaling functions in maintaining an undifferentiated, proliferating progenitor MSC population, whereas non-canonical Wnts facilitate osteogenic differentiation. Release from canonical Wnt regulation is a prerequisite for MSC differentiation. Thus, loss-/gain-of-function mutations of LRP5 would perturb Wnt signaling and depress/promote bone formation by affecting the progenitor cell pool. Elucidating Wnt regulation of MSC differentiation is important for their potential application in tissue regeneration.

Akt activation by growth factors is a multiple-step process: the role of the PH domain

The protein kinase encoded by the Akt proto-oncogene is activated by phospholipid binding, membrane translocation and phosphorylation. To address the relative roles of these mechanisms of Akt activation, we have employed a combination of genetic and pharmacological approaches. Transient transfection of NIH3T3 cells with wild-type Akt, pleckstrin homology (PH) domain mutants, generated on the basis of a PH domain structural model, and phosphorylation site Akt mutants provided evidence for a model of Akt activation consisting of three sequential steps: (1) a PH domain-dependent, growth factor-independent step, marked by constitutive phosphorylation of threonine 450 (T450) and perhaps serine 124 (S124), that renders the protein responsive to subsequent activation events; (2) a growth factor-induced, PI3-K-dependent membrane-translocation step; and (3) a PI3-K-dependent step, characterized by phosphorylation at T308 and S473, that occurs in the cell membrane and is required for activation. When forced to translocate to the membrane, wild-type Akt and PH domain Akt mutants that are defective in the first step become constitutively active, suggesting that the purpose of this step is to prepare the protein for membrane translocation. Both growth factor stimulation and forced membrane translocation, however, failed to activate a T308A mutant. This, combined with the finding that T308D/S473D double mutant is constitutively active, suggests that the purpose of the three-step process of Akt activation is the phosphorylation of the protein at T308 and S473. The proposed model provides a framework for a comprehensive understanding of the temporal and spatial requirements for Akt activation by growth factors.

A general method for the generation of high-titer, pantropic retroviral vectors: highly efficient infection of primary hepatocytes

Retroviral vectors have been central components in many studies leading to human gene therapy. However, the generally low titers and inefficient infectivity of retroviral vectors in human cells have limited their use. We previously reported that the G protein of vesicular stomatitis virus can serve as the exclusive envelope protein component for one specific retroviral vector, LGRNL, that expresses vesicular stomatitis virus G. We now report a more useful general transient transfection scheme for producing very high-titer vesicular stomatitis virus G-enveloped pseudotypes from any Moloney murine leukemia-based retroviral vector without having to rely on the expression of the cytotoxic G protein from the retroviral vector itself. We also demonstrate very high efficiency of infection with a pseudotyped lacZ vector in primary mouse hepatocytes. We suggest that pseudotyped retroviral vectors carrying reporter genes will permit genetic studies in many previously inaccessible vertebrate and invertebrate systems. Furthermore, because these vectors represent retroviral vectors of sufficiently high titer to allow efficient direct retroviral-mediated in vivo gene transfer, we also suggest that pseudotyped vectors carrying potentially therapeutic genes will become useful to test the potential for in vivo gene therapy.

Regulation of the NF-kappaB-inducing kinase by tumor necrosis factor receptor-associated factor 3-induced degradation

The NF-kappaB family of transcription factors plays a pivotal role in regulation of diverse biological processes, including immune responses, cell growth, and apoptosis. Activation of NF-kappaB is mediated by both canonical and noncanonical signaling pathways. Although the canonical pathway has been extensively studied, the mechanism mediating the noncanonical pathway is still poorly understood. Recent studies have identified the NF-kappaB-inducing kinase (NIK) as a key component of the noncanonical pathway of NF-kappaB activation; however, how the signaling function of NIK is regulated remains unknown. We report here that one important mechanism of NIK regulation is through its dynamic interaction with the tumor necrosis factor receptor-associated factor 3 (TRAF3). TRAF3 physically associates with NIK via a specific sequence motif located in the N-terminal region of NIK; this molecular interaction appears to target NIK for degradation by the proteasome. Interestingly, induction of noncanonical NF-kappaB signaling by extracellular signals involves degradation of TRAF3 and the concomitant enhancement of NIK expression. These results suggest that induction of noncanonical NF-kappaB signaling may involve the rescue of NIK from TRAF3-mediated negative regulation.

Uncoupling IL-2 signals that regulate T cell proliferation, survival, and Fas-mediated activation-induced cell death

IL-2 is an important growth and survival factor for T lymphocytes but also sensitizes these cells to Fas-mediated activation-induced cell death (AICD). The molecular basis of these different effects of IL-2 was studied by introducing wild-type and mutant forms of the IL-2 receptor beta (IL-2Rbeta) chain that lacked specific signaling capacities into receptor-deficient T cells by retroviral gene transfer. Activation of Stat5 by IL-2 was found to be involved in T cell proliferation and promoted Fas ligand (FasL) expression and AICD. T cell survival was dependent on a receptor region that activated Akt and the expression of Bcl-2. Thus, distinct IL-2Rbeta chain signaling modules regulate T cell fate by stimulating growth and survival or by promoting apoptosis.

Recombinant gene expression in vivo within endothelial cells of the arterial wall

A technique for the transfer of endothelial cells and expression of recombinant genes in vivo could allow the introduction of proteins of therapeutic value in the management of cardiovascular diseases. Porcine endothelial cells expressing recombinant beta-galactosidase from a murine amphotropic retroviral vector were introduced with a catheter into denuded iliofemoral arteries of syngeneic animals. Arterial segments explanted 2 to 4 weeks later contained endothelial cells expressing beta-galactosidase, an indication that they were successfully implanted on the vessel wall.

Embryonic lethal mutation in mice induced by retrovirus insertion into the alpha 1(I) collagen gene

Experimental insertion of a retrovirus into the germ line of mice has resulted in an embryonic recessive lethal mutation. Integration of the proviral genome occurred at the 5' end of the alpha 1(I) collagen gene, leading to complete transcriptional block. Developmental arrest of embryos homozygous at the mutated allele coincides with high expression of the gene in normal embryogenesis. Insertion mutagenesis by retroviruses may offer a general approach to the identification and isolation of genes which are transcriptionally active during mammalian development.