Retroviral transfer of human MDR1 gene to hematopoietic cells: effects of drug selection and of transcript splicing on expression of encoded P-glycoprotein

Multiple actions of doxorubicin on the sphingolipid network revealed by flux analysis

Sphingolipids (SLs) have been implicated in numerous important cellular biologies; however, their study has been hindered by the complexities of SL metabolism. Furthermore, enzymes of SL metabolism represent a dynamic and interconnected network in which one metabolite can be transformed into other bioactive SLs through further metabolism, resulting in diverse cellular responses. Here we explore the effects of both lethal and sublethal doses of doxorubicin (Dox) in MCF-7 cells. The two concentrations of Dox resulted in the regulation of SLs, including accumulations in sphingosine, sphingosine-1-phosphate, dihydroceramide, and ceramide, as well as reduced levels of hexosylceramide. To further define the effects of Dox on SLs, metabolic flux experiments utilizing a d17 dihydrosphingosine probe were conducted. Results indicated the regulation of ceramidases and sphingomyelin synthase components specifically in response to the cytostatic dose. The results also unexpectedly demonstrated dose-dependent inhibition of dihydroceramide desaturase and glucosylceramide synthase in response to Dox. Taken together, this study uncovers novel targets in the SL network for the action of Dox, and the results reveal the significant complexity of SL response to even a single agent. This approach helps to define the role of specific SL enzymes, their metabolic products, and the resulting biologies in response to chemotherapeutics and other stimuli.

Chemoprotection by transfer of resistance genes

Dose-limiting toxicity of chemotherapeutic agents, i.e., myelosuppression, can limit their effectiveness. The transfer and expression of drug-resistance genes might decrease the risks associated with acute hematopoietic toxicity. Protection of hematopoietic stem/progenitor cells by transfer of drug-resistance genes provides the possibility of intensification or escalation of antitumor drug doses and consequently an improved therapeutic index. This chapter reviews drug-resistance gene transfer strategies for either myeloprotection or therapeutic gene selection. Selecting candidate drug-resistance gene(s), gene transfer methodology, evaluating the safety and the efficiency of the treatment strategy, relevant in vivo models, and oncoretroviral transduction of human hematopoietic stem/progenitor cells under clinically applicable conditions are described.

An approach to achieve long-term expression in skin gene therapy

For gene therapy purposes, the skin is an attractive organ to target for systemic delivery of therapeutic proteins to treat systemic diseases, skin diseases, or skin cancer. To achieve long-term stable expression of a therapeutic gene in keratinocytes (KC), we have developed an approach using a bicistronic retroviral vector expressing the desired therapeutic gene linked to a selectable marker (multidrug resistant gene, MDR) that is then introduced into KC and fibroblasts (FB) to create genetically modified human skin equivalent (HSE). After grafting the HSE onto immunocompromised mice, topical colchicine treatment is used to select and enrich for genetically modified keratinocyte stem cells (KSC) that express MDR and are resistant to colchicine's antimitotic effects. Both the apparatus for topical colchicine delivery and the colchicine doses have been optimized for application to human skin. This approach can be validated by systemic delivery of therapeutic factors such as erythropoietin and the antihypertensive atrial natriuretic peptide.

Hematopoietic stem cell gene therapy with drug resistance genes: an update

Transfer of drug resistance genes into hematopoietic stem cells (HSCs) has promise for the treatment of a variety of inherited, that is, X-linked severe combined immune deficiency, adenosine deaminase deficiency, thalassemia, and acquired disorders, that is, breast cancer, lymphomas, brain tumors, and testicular cancer. Drug resistance genes are transferred into HSCs either for providing myeloprotection against chemotherapy-induced myelosuppression or for selecting HSCs that are concomitantly transduced with another gene for correction of an inherited disorder. In this review, we describe ongoing experimental approaches, observations from clinical trials, and safety concerns related to the drug resistance gene transfer.

Hematoprotection by transfer of drug-resistance genes

Myelosuppression represents a major side effect of cytotoxic anti-cancer agents. Infection due to granulocytopenia and the risk of bleeding due to thrombocytopenia compromise the potential of curative and palliative chemotherapy. Considering the many chemotherapeutic agents for which drug resistance genes have been described, and the recent improvements in vector and transduction technology, it seems conceivable that drug resistance gene transfer into a patient's autologous hematopoietic stem or progenitor cells will be able to reduce or abolish chemotherapy-induced myelosuppression.

Selection of drug-resistant transduced cells with cytosine nucleoside analogs using the human cytidine deaminase gene

Hematopoietic toxicity produced by most anticancer drugs limits their potential for curative therapy. We have shown previously that the human cytidine deaminase (CD) gene can confer drug resistance in murine bone marrow cells (BMCs) to the nucleoside analog, cytosine arabinoside (ARA-C). In the present study, as the first objective we showed that the CD gene can also render drug resistance in BMCs to related analogs, 2',2'-difluorodeoxycytidine (dFdC) and 5-azadeoxycytidine (5-AZA-CdR). As a second objective, we investigated the potential of ex vivo selection with cytosine nucleoside analogs of CD-transduced BMC. The goal of this approach was to enrich the fraction of CD-transduced BMCs so as to increase the transgene expression and level of drug resistance before transplantation. This strategy may have the potential to circumvent the problem in clinical gene therapy of low level of gene transfer and adequate long-term gene expression. Using a bicistronic retroviral vector containing the CD and the green fluorescent protein (CDiGFP), we transduced murine L1210 leukemic cells. All three analogs, ARA-C, dFdC, and 5-AZA-CdR were demonstrated in vitro to enrich (>95%) the population of leukemic cells expressing the GFP transgene. However, with CD-transduced primary murine BMCs cultivated at high cell density we observed that in vitro selection with ARA-C was not possible due to release of CD into the culture medium at amounts that were sufficient to inactivate the analog. The CD-containing medium produced a chemoprotective effect on mock BMCs as shown by lack of significant growth inhibition in the presence of ARA-C. However, at low cell density in a cell mixture containing CD-transduced cells, the mock BMCs showed marked drug sensitivity to ARA-C as determined by clonogenic assay. Selection with ARA-C was shown to significantly increase the CD enzyme activity in transduced BMC. These results suggest that CD gene has the potential to be a good selectable marker and a possible tool for chemoprotection in cancer gene therapy.

The FBMD-1 stroma cell line secretes a unique moiety which can increase retroviral transduction of lineage-committed and primitive human peripheral blood progenitor cells

Peripheral blood progenitor cells are a prime target for gene therapy approaches. As recent data point to the relevance of soluble stroma factors for the efficient transduction of progenitor cells, we tested the stroma-conditioned medium (SCM) of the two cell lines FBMD-1 and L88/5 as well as desulfated and O-sulfated heparin (HS dS and HS OS) for their effect on transduction of peripheral blood progenitor cells. We transduced CD34+ cells of nine tumor patients with the retroviral SF-MDR vector containing the human multidrug resistance 1 (MDR1) gene under serum-free conditions on the fibronectin fragment CH-296 with or without SCM. Provirus-specific polymerase chain reaction showed a median 1.6-fold higher integration rate of the transgene into committed progenitor cells for the group with added FBMD-1 SCM (P=.008). This was maintained after 2 (P=.02) and, as a trend, after 5 weeks of stroma-dependent long-term culture. We found a median 1.5-fold increase in rhodamine-123 (Rh-123) exclusion in myeloid lineage-committed progeny cells following transduction in the presence of FBMD-1 SCM (P=.0004). After 2 or 5 weeks of long-term culture, a significantly higher proportion of Rh-123(dull) cells could still be detected in the FBMD-1 SCM transduction group (P=.003 and P=.04, respectively). L88/5 SCM or HS OS or HS dS was not effective as supplement for improving gene transfer. The FBMD-1 stroma cell line appears to secrete a unique moiety, which can increase retroviral transduction of lineage-committed and primitive progenitor cells. The FBMD-1 stroma activity is not attributable to heparan sulfate.

Efficient retrovirus-mediated transduction of primitive human peripheral blood progenitor cells in stroma-free suspension culture

Retroviral transduction of hematopoietic cells has resulted in unsatisfactory gene marking in clinical studies. Since cytokine-stimulated stem cells have engrafted poorly in animal models, we investigated phenotypic changes during culture of peripheral blood progenitor cells (PBPC). Human CD34(+) HLA-DR(low) cells, immunomagnetically separated from PBPC collections, were found to extrude rhodamine-123, which is characteristic for primitive hematopoietic cells. Cells were grown in suspension cultures supplemented with cytokines. While interleukin-3-containing factor combinations promoted cell proliferation they caused loss of rhodamine-123 extrusion and reduced the frequencies of cobblestone area-forming cells (CAFC). Several other cytokines failed to stimulate cell divisions, which are required for retroviral transduction. A combination including Flt-3 ligand (FL), interleukin-6 and stem cell factor (SCF) preserved an immature phenotype for 5 to 6 days and stimulated cell divisions, which was improved upon addition of leukemia inhibitory factor and interleukin-11. Furthermore, the CAFC frequency among cells treated with these cytokines was increased as compared with widely used cocktails containing interleukin-3, interleukin-6 and SCF. Rhodamine-123 appeared to be a particularly sensitive indicator for differentiation of PBPC. For analysis of gene transfer, amphotropic retroviruses conferring an MDR1 cDNA were added repeatedly for 6 days to cytokine-treated PBPC stroma-free cultures. Proviral cDNA was detected by polymerase chain reaction in 68% of cobblestone areas derived from CD34(+)HLA-DR(low) cells that had been exposed to Flt-3 ligand, interleukin-6 and SCF. In summary, conditions were identified that facilitate efficient transduction of early PBPC with amphotropic retroviruses while preserving a primitive phenotype for extended periods.

BTK mediated apoptosis, a possible mechanism for failure to generate high titer retroviral producer clones

BACKGROUND

It has been shown previously that mutations in the cytoplasmic protein kinase, Bruton's tyrosine kinase (BTK) lead to X-linked agammaglobulinemia, an inherited primary immunodeficiency, thus making it a potential candidate for gene therapy.

METHODS

Producer cell lines using retroviral BTK constructs were generated and retroviral titers determined. Southern blot analysis was performed to check for pro-viral integrity in the respective clones. Furthermore, cotransfection of green fluorescent protein (GFP) with BTK expression plasmids was used in HeLa cells to establish and characterize the role of BTK in apoptosis.

RESULTS

Following the attempt to generate retroviral producer clones by conventional methods, we observed that the BTK gene is deleted from neomycin-resistant high titer clones. We show that BTK mediated apoptosis in GP#E86 and HeLa cells. Furthermore, membrane targeting and kinase activity are required for this effect. In addition, BTK induced apoptosis could be inhibited by using a specific inhibitor for p38 mitogen-activated protein kinase (MAPK), SB203580.

CONCLUSION

Failure to generate retroviral producer clones may be caused by the induction of apoptosis mediated by the therapeutic gene product.

Chemoprotection of hematopoietic cells by a mutant P-glycoprotein resistant to a potent chemosensitizer of multidrug-resistant cancers

Cancers are frequently chemoresistant because of overexpression of P-glycoprotein. Two different approaches to improve cancer treatment are currently being investigated in clinical trials: inhibition of P-glycoprotein function by reversing agents, and alleviation of leukocytopenia by MDR1 gene transfer to normal bone marrow of patients. We report here that retroviral vectors encoding a mutant P-glycoprotein (MDR1-F983A) protect hematopoietic cells from anticancer drugs even in the presence of trans-(E)-flupentixol, an inhibitor of P-glycoprotein. Transfer of either mutant or wild-type MDR1 to K562 erythroleukemia cells or primary murine bone marrow resulted in reduced accumulation of daunomycin and vinblastine because of increased drug efflux.trans-(E)-Flupentixol at concentrations up to 10 microM failed to reverse drug efflux mediated by the product of the mutant MDR1 while wild-type P-glycoprotein was inhibited. In the presence of 2 microM trans-(E)-flupentixol chemoresistance to daunomycin was circumvented only in K562 cells transduced with wild-type, but not with mutant, MDR1. Moreover, drug resistance of KB-8-5 epidermoid cancer cells, which express the wild-type MDR1 gene at levels comparable to clinical specimens from multidrug-resistant cancers, was fully overcome in the presence of trans-(E)-flupentixol. Vectors expressing mutant P-glycoprotein may help improve chemotherapy by allowing safe dose intensification under conditions in which multidrug-resistant cancers are rendered drug sensitive by reversing agents.

Drug selection of MDR1-transduced hematopoietic cells ex vivo increases transgene expression and chemoresistance in reconstituted bone marrow in mice

The MDR1 (multidrug resistance) gene, transferred to hematopoietic cells, is expected to protect them from anticancer chemotherapy and may serve as a selectable marker, restoring gene expression in vivo. Appropriate selection strategies, however, need to be established. To investigate whether preselection ex vivo affects chemoresistance, murine bone marrow cells were retrovirally transduced with high-titer or, as a model for suboptimal gene expression, low-titer retroviruses and exposed to daunomycin or colchicine for 48-96 h. Selection significantly increased chemoresistance of clonogenic progenitor cells. In tissue culture, the entire target population was rendered highly drug resistant after MDR1 transfer with high-titer viruses. If transduction was performed under suboptimal conditions, drug selection increased the frequency of chemoresistant colonies up to 40% over the number of unselected cells. Colchicine and daunomycin were equally efficient in increasing drug resistance ex vivo, but colchicine-preselected cells rescued lethally irradiated mice under conditions where daunomycin-selected bone marrow cells failed to do so. Hence, while hematopoietic cells can be protected by MDR1, the selection strategy is critical for repopulation of bone marrow with transduced cells. Preselection in culture before transplantation significantly increased P-gp expression and chemoresistance in vivo in mice reconstituted with transduced bone marrow cells. This study may help to facilitate the use of MDR1 as a selectable marker in gene therapy of the hematopoietic system. Gene Therapy (2000) 7, 348-358.