Development of a hypoxia-inducible cytosine deaminase expression vector for gene-directed prodrug cancer therapy

Hypoxia theranostics of a human prostate cancer xenograft and the resulting effects on the tumor microenvironment

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Developed a hypoxia theranostic imaging strategy to eliminate hypoxic cells.

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Hypoxic cell elimination resulted in fewer cancer associated fibroblasts (CAFs)

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Collagen 1 fiber patterns were altered with hypoxic cell elimination.

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cDNA nanoparticles with HRE driven prodrug enzyme expression can target hypoxia.

Hypoxia is frequently observed in human prostate cancer, and is associated with chemoresistance, radioresistance, metastasis, and castrate-resistance. Our purpose in these studies was to perform hypoxia theranostics by combining in vivo hypoxia imaging and hypoxic cancer cell targeting in a human prostate cancer xenograft. This was achieved by engineering PC3 human prostate cancer cells to express luciferase as well as a prodrug enzyme, yeast cytosine deaminase, under control of hypoxic response elements (HREs). Cancer cells display an adaptive response to hypoxia through the activation of several genes mediated by the binding of hypoxia inducible factors (HIFs) to HRE in the promoter region of target gene that results in their increased transcription. HIFs promote key steps in tumorigenesis, including angiogenesis, metabolism, proliferation, metastasis, and differentiation. HRE-driven luciferase expression allowed us to detect hypoxia in vivo to time the administration of the nontoxic prodrug 5-fluorocytosine that was converted by yeast cytosine deaminase, expressed under HRE regulation, to the chemotherapy agent 5-fluorouracil to target hypoxic cells. Conversion of 5-fluorocytosine to 5-fluorouracil was detected in vivo by ¹⁹F magnetic resonance spectroscopy. Morphological and immunohistochemical staining and molecular analyses were performed to characterize tumor microenvironment changes in cancer-associated fibroblasts, cell viability, collagen 1 fiber patterns, and HIF-1α. These studies expand our understanding of the effects of eliminating hypoxic cancer cells on the tumor microenvironment and in reducing stromal cell populations such as cancer-associated fibroblasts.

Hypoxia-targeted triple suicide gene therapy radiosensitizes human colorectal cancer cells

The hypoxic microenvironment, an important feature of human solid tumors but absent in normal tissue, may provide an opportunity for cancer-specific gene therapy. The purpose of the present study was to investigate whether hypoxia-driven triple suicide gene TK/CD/UPRT expression enhances cytotoxicity to ganciclovir (GCV) and 5-fluorocytosine (5-FC), and sensitizes human colorectal cancer to radiation in vitro and in vivo. Stable transfectant of human colorectal HCT8 cells was established which expressed hypoxia-inducible vectors (HRE-TK/eGFP and HRE-CD/UPRT/mDsRed). Hypoxia-induced expression/function of TK, CD and UPRT was verified by western blot analysis, flow cytometry, fluorescent microscopy and cytotoxicity assay of GCV and 5-FC. Significant radiosensitization effects were detected after 5-FC and GCV treatments under hypoxic conditions. In the tumor xenografts, the distribution of TK/eGFP and CD/UPRT/mDsRed expression visualized with fluorescence microscopy was co-localized with the hypoxia marker pimonidazole positive staining cells. Furthermore, administration of 5-FC and GCV in mice in combination with local irradiation resulted in tumor regression, as compared with prodrug or radiation treatments alone. Our data suggest that the hypoxia-inducible TK/GCV+CDUPRT/5-FC triple suicide gene therapy may have the ability to specifically target hypoxic cancer cells and significantly improve the tumor control in combination with radiotherapy.

Gene expression analysis in prostate cancer: the importance of the endogenous control

BACKGROUND

Aberrant gene expression is a hallmark of cancer. Quantitative reverse-transcription PCR (qRT-PCR) is the gold-standard for quantifying gene expression, and commonly employs a house-keeping gene (HKG) as an endogenous control to normalize results; the choice of which is critical for accurate data interpretation. Many factors, including sample type, pathological state, and oxygen levels influence gene expression including putative HKGs. The aim of this study was to determine the suitability of commonly used HKGs for qRT-PCR in prostate cancer.

METHODS

Prostate cancer (LNCaP, 22Rv1, PC3, and DU145) and normal (PWR1E and RWPE1) cell lines were cultured in air and hypoxia. The performance of 16 HKGs was assessed using Normfinder and coefficient of variation. In silico promoter analysis was performed to identify putative hypoxia response elements (HREs). The impact of the endogenous control on expression levels of HIF1A and GSTP1 was investigated by qRT-PCR in cell lines and tissue specimens respectively.

RESULTS

Hypoxia altered expression of several HKGs: IPO8, B2M, and PGK1. The most stably expressed HKGs were ACTB, PPIA, and UBC. Both UBC and ACTB showed constitutive expression of HIF1A in air and hypoxia, while PGK1 falsely implied a sixfold hypoxia-induced down-regulation. In prostate tumors, UBC and PGK1 both revealed down-regulation of GSTP1 relative to matched benign, whereas ACTB showed variability.

CONCLUSIONS

This study demonstrates that no universal endogenous control exists for gene expression studies, even within one disease type. It highlights the importance of validating expression of intended HKGs between different sample types and environmental exposures.

Local bystander effect induces dormancy in human medullary thyroid carcinoma model in vivo

The extent of local bystander effect induced by fusion yeast cytosine deaminase::uracil phosphoribosyltransferase (yCD) in combination with 5-fluorocytosine (5FC) was evaluated in xenogeneic model of human medullary thyroid carcinoma (MTC). This approach to gene-directed enzyme/prodrug therapy (GDEPT) induces strong bystander cytotoxicity. Effector yCD-TT mixed with target EGFP-TT cells in a ratio 2:9 could achieve significant tumor regression and 14-fold decrease in serum marker calcitonin upon 5FC administration. Histopathological analysis unraveled that antitumor effect resulted in tumor dormancy and proliferation arrest of remaining tumor cell clusters in vivo. yCD/5FC combination represents another GDEPT approach to achieve tumor growth control in MTC.

Long-term and stable correction of uremic anemia by intramuscular injection of plasmids containing hypoxia-regulated system of erythropoietin expression

Relative deficiency in production of glycoprotein hormone erythropoietin (Epo) is a major cause of renal anemia. This study planned to investigate whether the hypoxia-regulated system of Epo expression, constructed by fusing Epo gene to the chimeric phosphoglycerate kinase (PGK) hypoxia response elements (HRE) in combination with cytomegalovirus immediate-early (CMV IE) basal gene promoter and delivered by plasmid intramuscular injection, might provide a long-term physiologically regulated Epo secretion expression to correct the anemia in adenine-induced uremic rats. Plasmid vectors (pHRE-Epo) were synthesized by fusing human Epo cDNA to the HRE/CMV promoter. Hypoxia-inducible activity of this promoter was evaluated first in vitro and then in vivo in healthy and uremic rats (n = 30 per group). The vectors (pCMV-Epo) in which Epo expression was directed by a constitutive CMV gene promoter served as control. ANOVA and Student's t-test were used to analyze between-group differences. A high-level expression of Epo was induced by hypoxia in vitro and in vivo. Though both pHRE-Epo and pCMV-Epo corrected anemia, the hematocrit of the pCMV-Epo-treated rats exceeded the normal (P < 0.05), but that of the pHRE-Epo-treated rats didn't. Hypoxia-regulated system of Epo gene expression constructed by fusing Epo to the HRE/CMV promoter and delivered by plasmid intramuscular injection may provide a long-term and stable Epo expression and secretion in vivo to correct the anemia in adenine-induced uremic rats.

Hypoxia targeted bifunctional suicide gene expression enhances radiotherapy in vitro and in vivo

PURPOSE

To investigate whether hypoxia targeted bifunctional suicide gene expression-cytosine deaminase (CD) and uracil phosphoribosyltransferase (UPRT) with 5-FC treatments can enhance radiotherapy.

MATERIALS AND METHODS

Stable transfectants of R3327-AT cells were established which express a triple-fusion-gene: CD, UPRT and monomoric DsRed (mDsRed) controlled by a hypoxia inducible promoter. Hypoxia-induced expression/function of CDUPRTmDsRed was verified by western blot, flow cytometry, fluorescent microscopy, and cytotoxicity assay of 5-FU and 5-FC. Tumor-bearing mice were treated with 5-FC and local radiation. Tumor volume was monitored and compared with those treated with 5-FC or radiation alone. In addition, the CDUPRTmDsRed distribution in hypoxic regions of tumor sections was visualized with fluorescent microscopy.

RESULTS

Hypoxic induction of CDUPRTmDsRed protein correlated with increased sensitivity to 5-FC and 5-FU. Significant radiosensitization effects were detected after 5-FC treatments under hypoxic conditions. In the tumor xenografts, the distribution of CDUPRTmDsRed expression visualized with fluorescence microscopy was co-localized with the hypoxia marker pimonidazole positive staining cells. Furthermore, administration of 5-FC to mice in combination with local irradiation resulted in significant tumor regression, as in comparison with 5-FC or radiation treatments alone.

CONCLUSIONS

Our data suggest that the hypoxia-inducible CDUPRT/5-FC gene therapy strategy has the ability to specifically target hypoxic cancer cells and significantly improve the tumor control in combination with radiotherapy.

Use of macrophages to target therapeutic adenovirus to human prostate tumors

New therapies are required to target hypoxic areas of tumors as these sites are highly resistant to conventional cancer therapies. Monocytes continuously extravasate from the bloodstream into tumors where they differentiate into macrophages and accumulate in hypoxic areas, thereby opening up the possibility of using these cells as vehicles to deliver gene therapy to these otherwise inaccessible sites. We describe a new cell-based method that selectively targets an oncolytic adenovirus to hypoxic areas of prostate tumors. In this approach, macrophages were cotransduced with a hypoxia-regulated E1A/B construct and an E1A-dependent oncolytic adenovirus, whose proliferation is restricted to prostate tumor cells using prostate-specific promoter elements from the TARP, PSA, and PMSA genes. When such cotransduced cells reach an area of extreme hypoxia, the E1A/B proteins are expressed, thereby activating replication of the adenovirus. The virus is subsequently released by the host macrophage and infects neighboring tumor cells. Following systemic injection into mice bearing subcutaneous or orthotopic prostate tumors, cotransduced macrophages migrated into hypoxic tumor areas, upregulated E1A protein, and released multiple copies of adenovirus. The virus then infected neighboring cells but only proliferated and was cytotoxic in prostate tumor cells, resulting in the marked inhibition of tumor growth and reduction of pulmonary metastases. This novel delivery system employs 3 levels of tumor specificity: the natural "homing" of macrophages to hypoxic tumor areas, hypoxia-induced proliferation of the therapeutic adenovirus in host macrophages, and targeted replication of oncolytic virus in prostate tumor cells.

Hypoxia and hypoxia-inducible factors: master regulators of metastasis

Hypoxia is a common condition found in a wide range of solid tumors and is often associated with poor prognosis. Hypoxia increases tumor glycolysis, angiogenesis, and other survival responses, as well as invasion and metastasis by activating relevant gene expressions through hypoxia-inducible factors (HIF). HIF-1α and HIF-2α undergo oxygen-dependent regulation, and their overexpression is frequently associated with metastasis and poor clinical outcomes. Recent studies show that each step of the metastasis process, from the initial epithelial-mesenchymal transition to the ultimate organotropic colonization, can potentially be regulated by hypoxia, suggesting a master regulator role of hypoxia and HIFs in metastasis. Furthermore, modulation of cancer stem cell self-renewal by HIFs may also contribute to the hypoxia-regulated metastasis program. The hypoxia-induced metastatic phenotype may be one of the reasons for the modest efficacy of antiangiogenic therapies and may well explain the recent provocative findings that antiangiogenic therapy increased metastasis in preclinical models. Multiple approaches to targeting hypoxia and HIFs, including HIF inhibitors, hypoxia-activated bioreductive prodrugs, and gene therapies may become effective treatments to prevent or reduce metastasis.

Brain tumor hypoxia: tumorigenesis, angiogenesis, imaging, pseudoprogression, and as a therapeutic target

Hypoxia is implicated in many aspects of tumor development, angiogenesis, and growth in many different tumors. Brain tumors, particularly the highly aggressive glioblastoma multiforme (GBM) with its necrotic tissues, are likely affected similarly by hypoxia, although this involvement has not been closely studied. Invasion, apoptosis, chemoresistance, resistance to antiangiogenic therapy, and radiation resistance may all have hypoxic mechanisms. The extent of the influence of hypoxia in these processes makes it an attractive therapeutic target for GBM. Because of their relationship to glioma and meningioma growth and angiogenesis, hypoxia-regulated molecules, including hypoxia inducible factor-1, carbonic anhydrase IX, glucose transporter 1, and vascular endothelial growth factor, may be suitable subjects for therapies. Furthermore, other novel hypoxia-regulated molecules that may play a role in GBM may provide further options. Emerging imaging techniques may allow for improved determination of hypoxia in human brain tumors to better focus therapeutic treatments; however, tumor pseudoprogression, which may be prompted by hypoxia, poses further challenges. An understanding of the role of hypoxia in tumor development and growth is important for physicians involved in the care of patients with brain tumors.

Gene therapy approaches to enhance bioreductive drug treatment

Hypoxia, or a lack of oxygen, occurs in 50-60% of solid human tumours. Clinical studies have shown that the presence and extent of hypoxia in a tumour cannot be predicted by size or histopathological stage but it is predictive of a poor outcome following radiotherapy, chemotherapy and surgery. However, as a physiological feature of tumours, it can be exploited and researchers have developed many hypoxia-selective chemotherapies or bioreductive drugs that are in varying stages of clinical development. These agents are prodrugs that have two key requirements for their biological activation: they require the reductive environment of a hypoxic tumour cell and the appropriate complement of cellular reductase enzymes. To overcome tumour heterogeneity in reductase enzyme levels and enhance bioreductive drug metabolism a gene therapy strategy can be employed. We have reviewed this field and also present our own pre-clinical research using gene therapy to enhance bioreductive drug treatment for the treatment of cancer. We have specifically focused on studies enhancing lead clinical bioreductive drugs. We consider the metabolic requirements for their activation and we highlight the key in vivo studies supporting the future clinical development of hypoxia-targeted gene-directed enzyme prodrug therapy.

Cytosine deaminase/5-fluorocytosine exposure induces bystander and radiosensitization effects in hypoxic glioblastoma cells in vitro

PURPOSE

Treatment of glioblastoma (GBM) is limited by therapeutic ratio; therefore, successful therapy must be specifically cytotoxic to cancer cells. Hypoxic cells are ubiquitous in GBM, and resistant to radiation and chemotherapy, and, thus, are logical targets for gene therapy. In this study, we investigated whether cytosine deaminase (CD)/5-fluorocytosine (5-FC) enzyme/prodrug treatment induced a bystander effect (BE) and/or radiosensitization in hypoxic GBM cells.

METHODS AND MATERIALS

We stably transfected cells with a gene construct consisting of the SV40 minimal promoter, nine copies of a hypoxia-responsive element, and the yeast CD gene. During hypoxia, a hypoxia-responsive element regulates expression of the CD gene and facilitates the conversion of 5-FC to 5-fluorouracil, a highly toxic antimetabolite. We used colony-forming efficiency (CFE) and immunofluorescence assays to assess for BE in co-cultures of CD-expressing clone cells and parent, pNeo- or green fluorescent protein-stably transfected GBM cells. We also investigated the radiosensitivity of CD clone cells treated with 5-FC under hypoxic conditions, and we used flow cytometry to investigate treatment-induced cell cycle changes.

RESULTS

Both a large BE and radiosensitization occurred in GBM cells under hypoxic conditions. The magnitude of the BE depended on the number of transfected cells producing CD, the functionality of the CD, the administered concentration of 5-FC, and the sensitivity of cell type to 5-fluorouracil.

CONCLUSION

Hypoxia-inducible CD/5-FC therapy in combination with radiation therapy shows both a pronounced BE and a radiosensitizing effect under hypoxic conditions.

Tumor hypoxia and targeted gene therapy

Hypoxia is an integral characteristic of the tumor microenvironment, primarily due to the microvascular defects that accompany the accelerated neoplastic growth. The presence of tumor hypoxic areas correlates with negative outcome after radiotherapy, chemotherapy, and surgery, as hypoxia not only provides an environment directly facilitating chemo- and radio-resistance, but also encourages the evolution of phenotypic changes inducing permanent resistance to treatment and metastatic spread. Therefore, successful treatment of hypoxic cells has the potential to not only improve local control but also impact overall patient survival. Specific and selective targeting of hypoxic tumor areas can be achieved at all three steps of a gene therapy treatment: delivery of the therapeutic gene to the tumor, regulation of gene expression, and therapeutic efficacy. In this review the latest developments and innovations in hypoxia-targeted gene therapy are discussed. In particular, approaches such as hypoxia-conditionally replicating viruses, cellular vehicles, and gene therapy means to disrupt the hypoxia-inducible factor (HIF) signaling are outlined.

Hypoxic-response elements in the oncolytic parvovirus Minute virus of mice do not allow for increased vector production at low oxygen concentration

Vectors derived from the autonomous parvovirus Minute virus of mice, MVM(p), are promising tools for the gene therapy of cancer. The validation of their in vivo anti-tumour effect is, however, hampered by the difficulty to produce high-titre stocks. In an attempt to increase vector titres, host cells were subjected to low oxygen tension (hypoxia). It has been shown that a number of viruses are produced at higher titres under these conditions. This is the case, among others, for another member of the family Parvoviridae, the erythrovirus B19 virus. Hypoxia stabilizes a hypoxia-inducible transcription factor (HIF-1alpha) that interacts with a 'hypoxia-responsive element' (HRE), the consensus sequence of which ((A)/(G)CGTG) is present in the B19 and MVM promoters. Whilst the native P4 promoter was induced weakly in hypoxia, vector production was reduced dramatically, and adding HRE elements to the P4 promoter of the vector did not alleviate this reduction. Hypoxia has many effects on cell metabolism. Therefore, even if the P4 promoter is activated, the cellular factors that are required for the completion of the parvoviral life cycle may not be expressed.

Hypoxia- and radiation-inducible, breast cell-specific targeting of retroviral vectors

To facilitate a more efficient radiation and chemotherapy of mammary tumours, synthetic enhancer elements responsive to hypoxia and ionizing radiation were coupled to the mammary-specific minimal promoter of the murine whey acidic protein (WAP) encoding gene. The modified WAP promoter was introduced into a retroviral promoter conversion (ProCon) vector. Expression of a transduced reporter gene in response to hypoxia and radiation was analysed in stably infected mammary cancer cell lines and an up to 9-fold increase in gene expression demonstrated in comparison to the respective basic vector. Expression analyses in vitro, moreover, demonstrated a widely preserved mammary cell-specific promoter activity. For in vivo analyses, xenograft tumours consisting of infected human mammary adenocarcinoma cells were established in SCID/beige mice. Immunohistochemical analyses demonstrated a hypoxia-specific, markedly increased WAP promoter-driven expression in these tumours. Thus, this retroviral vector will facilitate a targeted gene therapeutic approach exploiting the unique environmental condition in solid tumours.

Pharmacologic transgene control systems for gene therapy

Pharmacologic transgene-expression dosing is considered essential for future gene therapy scenarios. Genetic interventions require precise transcription or translation fine-tuning of therapeutic transgenes to enable their titration into the therapeutic window, to adapt them to daily changing dosing regimes of the patient, to integrate them seamlessly into the patient's transcriptome orchestra, and to terminate their expression after successful therapy. In recent years, decisive progress has been achieved in designing high-precision trigger-inducible mammalian transgene control modalities responsive to clinically licensed and inert heterologous molecules or to endogenous physiologic signals. Availability of a portfolio of compatible transcription control systems has enabled assembly of higher-order control circuitries providing simultaneous or independent control of several transgenes and the design of (semi-)synthetic gene networks, which emulate digital expression switches, regulatory transcription cascades, epigenetic expression imprinting, and cellular transcription memories. This review provides an overview of cutting-edge developments in transgene control systems, of the design of synthetic gene networks, and of the delivery of such systems for the prototype treatment of prominent human disease phenotypes.