A novel TARP-promoter-based adenovirus against hormone-dependent and hormone-refractory prostate cancer

Oncolyic Virotherapy for Prostate Cancer: Lighting a Fire in Winter

As the most common cancer of the genitourinary system, prostate cancer (PCa) is a global men's health problem whose treatments are an urgent research issue. Treatment options for PCa include active surveillance (AS), surgery, endocrine therapy, chemotherapy, radiation therapy, immunotherapy, etc. However, as the cancer progresses, the effectiveness of treatment options gradually decreases, especially in metastatic castration-resistant prostate cancer (mCRPC), for which there are fewer therapeutic options and which have a shorter survival period and worse prognosis. For this reason, oncolytic viral therapy (PV), with its exceptional properties of selective tumor killing, relatively good safety in humans, and potential for transgenic delivery, has attracted increasing attention as a new form of anti-tumor strategy for PCa. There is growing evidence that OV not only kills tumor cells directly by lysis but can also activate anticancer immunity by acting on the tumor microenvironment (TME), thereby preventing tumor growth. In fact, evidence of the efficacy of this strategy has been observed since the late 19th century. However, subsequently, interest waned. The renewed interest in this therapy was due to advances in biotechnological methods and innovations at the end of the 20th century, which was also the beginning of PCa therapy with OV. Moreover, in combination with chemotherapy, radiotherapy, gene therapy or immunotherapy, OV viruses can have a wide range of applications and can provide an effective therapeutic result in the treatment of PCa.

TARP as antigen in cancer immunotherapy

In recent decades, immunotherapy has become a pivotal element in cancer treatment. A remaining challenge is the identification of cancer-associated antigens suitable as targets for immunotherapeutics with potent on-target and few off-tumor effects. The T-cell receptor gamma (TCRγ) chain alternate reading frame protein (TARP) was first discovered in the human prostate and androgen-sensitive prostate cancer. Thereafter, TARP was also identified in breast and endometrial cancers, salivary gland tumors, and pediatric and adult acute myeloid leukemia. Interestingly, TARP promotes tumor cell proliferation and migration, which is reflected in an association with worse survival. TARP expression in malignant cells, its role in oncogenesis, and its limited expression in normal tissues raised interest in its potential utility as a therapeutic target, and led to development of immunotherapeutic targeting strategies. In this review, we provide an overview of TARP expression, its role in different cancer types, and currently investigated TARP-directed immunotherapeutic options.

Targeting strategies of adenovirus‑mediated gene therapy and virotherapy for prostate cancer (Review)

Prostate cancer (PCa) poses a high risk to older men and it is the second most common type of male malignant tumor in western developed countries. Additionally, there is a lack of effective therapies for PCa at advanced stages. Novel treatment strategies such as adenovirus-mediated gene therapy and virotherapy involve the expression of a specific therapeutic gene to induce death in cancer cells, however, wild-type adenoviruses are also able to infect normal human cells, which leads to undesirable toxicity. Various PCa-targeting strategies in adenovirus-mediated therapy have been developed to improve tumor-targeting effects and human safety. The present review summarizes the relevant knowledge regarding available adenoviruses and PCa-targeting strategies. In addition, future directions in this area are also discussed. In conclusion, although they remain in the early stages of basic research, adenovirus-mediated gene therapy and virotherapy are expected to become important therapies for tumors in the future due to their potential targeting strategies.

Influence of Androgen Deprivation Therapy on the Uptake of PSMA-Targeted Agents: Emerging Opportunities and Challenges

Prostate-specific membrane antigen (PSMA) is an attractive target for both diagnosis and therapy because of its high expression in the vast majority of prostate cancers. Development of small molecules for targeting PSMA is important for molecular imaging and radionuclide therapy of prostate cancer. Recent evidence implies that androgen-deprivation therapy increase PSMA-ligand uptake in some cases. The reported upregulations in PSMA-ligand uptake after exposure to second-generation antiandrogens such as enzalutamide and abiraterone might disturb PSMA-targeted imaging for staging and response monitoring of patients undergoing treatment with antiandrogen-based drugs. On the other hand, second-generation antiandrogens are emerging as potential endoradio-/chemosensitizers. Therefore, the enhancement of the therapeutic efficiency of PSMA-targeted theranostic methods can be listed as a new capability of antiandrogens. In this manuscript, we will present what is currently known about the mechanism of increasing PSMA uptake following exposure to antiandrogens. In addition, we will discuss whether these above-mentioned antiandrogens could play the role of endoradio-/chemosensitizers in combination with the well-established PSMA-targeted methods for pre-targeting of prostate cancer.

Oncolytic adenovirus-mediated therapy for prostate cancer

Prostate cancer is a leading cause of cancer-related death and morbidity in men in the Western world. Tumor progression is dependent on functioning androgen receptor signaling, and initial administration of antiandrogens and hormone therapy (androgen-deprivation therapy) prevent growth and spread. Tumors frequently develop escape mechanisms to androgen-deprivation therapy and progress to castration-resistant late-stage metastatic disease that, in turn, inevitably leads to resistance to all current therapeutics, including chemotherapy. In spite of the recent development of more effective inhibitors of androgen–androgen receptor signaling such as enzalutamide and abiraterone, patient survival benefits are still limited. Oncolytic adenoviruses have proven efficacy in prostate cancer cells and cause regression of tumors in preclinical models of numerous drug-resistant cancers. Data from clinical trials demonstrate that adenoviral mutants have limited toxicity to normal tissues and are safe when administered to patients with various solid cancers, including prostate cancer. While efficacy in response to adenovirus administration alone is marginal, findings from early-phase trials targeting local-ized and metastatic prostate cancer suggest improved efficacy in combination with cytotoxic drugs and radiation therapy. Here, we review recent progress in the development of multimodal oncolytic adenoviruses as biological therapeutics to improve on tumor elimination in prostate cancer patients. These optimized mutants target cancer cells by several mechanisms including viral lysis and by expression of cytotoxic transgenes and immune-stimulatory factors that activate the host immune system to destroy both infected and noninfected prostate cancer cells. Additional modifications of the viral capsid proteins may support future systemic delivery of oncolytic adenoviruses.

Preclinical safety assessment of Ad[I/PPT-E1A], a novel oncolytic adenovirus for prostate cancer

Prostate cancer is the most common malignancy in the Western world. Patients can be cured only when the tumor has not metastasized outside the prostate. However, treatment with curative intent fails in a significant number of men, often resulting in untreatable progressive disease with a fatal outcome. Oncolytic adenovirus therapy may be a promising adjuvant treatment to reduce local failure or the outgrowth of micrometastatic disease. Within the European gene therapy consortium GIANT, we have developed a novel prostate-specific oncolytic adenovirus: Ad[I/PPT-E1A]. This adenovirus specifically kills prostate cells via prostate-specific replication. This article describes the clinical development of Ad[I/PPT-E1A] with particular reference to the preclinical safety assessment of this novel virus. The preclinical safety assessment involved an efficacy study in a human orthotopic xenograft mouse model, a specificity study in human primary cells, and a toxicity study in normal mice. These studies confirmed that Ad[I/PPT-E1A] efficiently kills prostate tumor cells in vivo, is not harmful to other organs, and is well tolerated in mice after systemic delivery. The safety, as well as the immunological effects of Ad[I/PPT-E1A] as a local adjuvant therapy, will now be studied in a phase I dose-escalating trial in patients with localized prostate cancer who are scheduled for curative radical prostatectomy and can be used as an updated paradigm for similar therapeutic viruses.

Tat-PTD-modified oncolytic adenovirus driven by the SCG3 promoter and ASH1 enhancer for neuroblastoma therapy

Secretogranin III (SGC3) belongs to the granin family and is highly expressed in endocrine and neural tissues. The human SCG3 promoter has not yet been characterized. We identified that a 0.5-kb DNA fragment upstream of the SCG3 gene can selectively drive transgene expression in neuroblastoma cell lines. The strength of transgene expression was further increased, with specificity maintained, by addition of the human achaete-scute complex homolog 1 (ASH1) enhancer. We developed an oncolytic serotype 5-based adenovirus, in which the SCG3 promoter and ASH1 enhancer drive E1A gene expression. The virus was further modified with a cell-penetrating peptide (Tat-PTD) in the viral capsid, which we have previously shown results in increased adenovirus transduction efficiency of many neuroblastoma cell lines. The virus, Ad5PTD(ASH1-SCG3-E1A), shows selective and efficient killing of neuroblastoma cell lines in vitro, including cisplatin-, etoposide-, and doxorubicin-insensitive neuroblastoma cells. Furthermore, it delays tumor growth and thereby prolonged survival for nude mice harboring subcutaneous human neuroblastoma xenograft. In conclusion, we report a novel oncolytic adenovirus with potential use for neuroblastoma therapy.

Molecular characterization of the feline T-cell receptor γ alternate reading frame protein (TARP) ortholog

T-cell receptor γ alternate reading frame protein (TARP) is expressed by human prostate epithelial, prostate cancer, and mammary cancer cells, but is not found in normal mammary tissue. To date, this protein has only been described in humans. Additionally, no animal model has been established to investigate the potential merits of TARP as tumor marker or a target for adoptive tumor immunotherapy. In this study conducted to characterize feline T-cell receptor γ sequences, constructs very similar to human TARP transcripts were obtained by RACE from the spleen and prostate gland of cats. Transcription of TARP in normal, hyperplastic, and neoplastic feline mammary tissues was evaluated by conventional RT-PCR. In felines similarly to the situation reported in humans, a C-region encoding two open reading frames is spliced to a J-region gene. In contrast to humans, the feline J-region gene was found to be a pseudogene containing a deletion within its recombination signal sequence. Our findings demonstrated that the feline TARP ortholog is transcribed in the prostate gland and mammary tumors but not normal mammary tissues as is the case with human TARP.

Islet Engraftment and Revascularization in Clinical and Experimental Transplantation

Proper revascularization after transplantation is assumed to be crucial for appropriate islet graft function. We developed a novel noninvasive imaging method, based on adenoviral transduction of islets with a hypoxia responsive reporter gene, for continuous in vivo monitoring of hypoxia in islet grafts in a mouse model. In addition, morphological data were obtained from a deceased patient previously subject to intraportal transplantation. We detected only transient hypoxia in a minority of the animals transplanted. Importantly, a clear response to hypoxia was observed in vitro after removal of the islet grafts on day 28 after transplantation. Also, the morphological data from the deceased patient demonstrated an extensive revascularization of the transplanted islets. In fact, no differences could be seen between native islets, in pancreas biopsies taken prior to islet isolation, and transplanted islets regarding the number, distribution, and shape of the blood vessels. However, fewer small islets (diameter <39 μm) were found in the liver compared to those found in native pancreases. Notably, an absolute majority of the transplanted islets were found remaining within the venous lumen, in direct contact with the vessel wall. In conclusion, the results presented show less pronounced islet graft hypoxia after subcapsular transplantation than previously reported using more invasive methods. Also, formation of an extensive intraislet capillary network, similar to that seen in native islets in the pancreas, was seen after clinical islet transplantation.

Baculoviruses as gene therapy vectors for human prostate cancer

Prostate cancer is the most commonly diagnosed cancer in ageing men in the western world. While the primary cancers can be treated with androgen ablation, radiotherapy and surgery, recurrent castration resistant cancers have an extremely poor prognosis, hence promoting research that could lead to a better treatment. Targeted therapeutic gene therapy may provide an attractive option for these patients. By exploiting the natural ability of viruses to target and transfer their genes into cancer cells, either naturally or after genetic manipulation, new generations of biological control can be developed. In this review we present the advantages and practicalities of using baculovirus as a vector for prostate cancer gene therapy and provide evidence for the potential of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) as a safer alternative vehicle for targeting cancer cells. Strategies to target baculovirus binding specifically to prostate cell surfaces are also presented. The large insertion capacity of baculoviruses also permits restricted, prostate-specific gene expression of therapeutic genes by cloning extended human transcriptional control sequences into the baculovirus genome.

Adenovirus with hexon Tat-protein transduction domain modification exhibits increased therapeutic effect in experimental neuroblastoma and neuroendocrine tumors

Adenovirus serotype 5 (Ad5) is widely used as an oncolytic agent for cancer therapy. However, its infectivity is highly dependent on the expression level of coxsackievirus-adenovirus receptor (CAR) on the surfaces of tumor cells. Furthermore, infected cells overproduce adenovirus fiber proteins, which are released prior to cell lysis. The released fibers block CAR on noninfected neighboring cells, thereby preventing progeny virus entry. Our aim was to add a CAR-independent infection route to Ad5 to increase the infectivity of tumor cells with low CAR expression and prevent the fiber-masking problem. We constructed Ad5 viruses that encode the protein transduction domain (PTD) of the HIV-1 Tat protein (Tat-PTD) in hypervariable region 5 (HVR5) of the hexon protein. Tat-PTD functions as a cell-penetrating peptide, and Tat-PTD-modified Ad5 showed a dramatic increased transduction of CAR-negative cell lines compared to unmodified vector. Moreover, while tumor cell infectivity was severely reduced for Ad5 in the presence of fiber proteins, it was only marginally reduced for Tat-PTD-modified Ad5. Furthermore, because of the sequence alteration in the hexon HVR, coagulation factor X-mediated virus uptake was significantly reduced. Mice harboring human neuroblastoma and neuroendocrine tumors show suppressed tumor growths and prolonged survival when treated with Tat-PTD-modified oncolytic viruses. Our data suggest that modification of Ad5 with Tat-PTD in HVR5 expands its utility as an oncolytic agent.

In vitro primary cell culture as a physiologically relevant method for preclinical testing of human oncolytic adenovirus

Ad[I/PPT-E1A] is an oncolytic adenovirus that specifically kills prostate cells via restricted replication by a prostate-specific regulatory element. Off-target replication of oncolytic adenoviruses would have serious clinical consequences. As a proposed ex vivo test, we describe the assessment of the specificity of Ad[I/PPT-E1A] viral cytotoxicity and replication in human nonprostate primary cells. Four primary nonprostate cell types were selected to mimic the effects of potential in vivo exposure to Ad[I/PPT-E1A] virus: bronchial epithelial cells, urothelial cells, vascular endothelial cells, and hepatocytes. Primary cells were analyzed for Ad[I/PPT-E1A] viral cytotoxicity in MTS assays, and viral replication was determined by hexon titer immunostaining assays to quantify viral hexon protein. The results revealed that at an extreme multiplicity of infection of 500, unlikely to be achieved in vivo, Ad[I/PPT-E1A] virus showed no significant cytotoxic effects in the nonprostate primary cell types apart from the hepatocytes. Transmission electron microscopy studies revealed high levels of Ad[I/PPT-E1A] sequestered in the cytoplasm of these cells. Adenoviral green fluorescent protein reporter studies showed no evidence for nuclear localization, suggesting that the cytotoxic effects of Ad[I/PPT-E1A] in human primary hepatocytes are related to viral sequestration. Also, hepatocytes had increased amounts of coxsackie adenovirus receptor surface protein. Active viral replication was only observed in the permissive primary prostate cells and LNCaP prostate cell line, and was not evident in any of the other nonprostate cells types tested, confirming the specificity of Ad[I/PPT-E1A]. Thus, using a relevant panel of primary human cells provides a convenient and alternative preclinical assay for examining the specificity of conditionally replicating oncolytic adenoviruses in vivo.

Experimental models to study lymphatic and blood vascular metastasis

As a model system for the understanding of human cancer, the mouse has proved immensely valuable. Indeed, studies of mouse models have helped to define the nature of cancer as a genetic disease and demonstrated the causal role of genetic events found in tumors. As an experimental platform, they have provided critical insight into the process of tumor metastasis in the lymphovascular system. Once viewed with skepticism, mouse models are now an integral arm of basic and clinical cancer research. The use of a genetically tractable organism that shares organ systems and an immense degree of genetic similarity to humans provides a means to examine multiple features of human disease. Mouse models enable development and testing of new approaches to disease prevention and treatment, identification of early diagnostic markers and novel therapeutic targets, and an understanding of the in vivo biology and genetics of tumor initiation, promotion, progression, and metastasis. This review summarizes recent mouse models for lymphangiogenesis and the process of lymphovascular metastasis, focusing on the use of the cornea as an experimental platform for lymphangiogenesis in inflammation and immunity, and on the use of molecular and viral vector mediated imaging and to identify and monitor lymph node metastases of prostate cancer.

Oncolytic adenovirus modified with somatostatin motifs for selective infection of neuroendocrine tumor cells

We have previously described the oncolytic adenovirus, Ad(CgA-E1A-miR122), herein denoted Ad5(CgA-E1A-miR122) that selectively replicates in and kills neuroendocrine cells, including freshly isolated midgut carcinoid cells from liver metastases. Ad5(CgA-E1A-miR122) is based on human adenovirus serotype 5 (Ad5) and infects target cells by binding to the coxsackie-adenovirus receptor (CAR) and integrins on the cell surface. Some neuroendocrine tumor (NET) and neuroblastoma cells express low levels of CAR and are therefore poorly transduced by Ad5. However, they often express high levels of somatostatin receptors (SSTRs). Therefore, we introduced cyclic peptides, which contain four amino acids (FWKT) and mimic the binding site for SSTRs in the virus fiber knob. We show that FWKT-modified Ad5 binds to SSTR₂ on NET cells and transduces midgut carcinoid cells from liver metastases about 3-4 times better than non-modified Ad5. Moreover, FWKT-modified Ad5 overcomes neutralization in an ex vivo human blood loop model to greater extent than Ad5, indicating that fiber knob modification may prolong the systemic circulation time. We conclude that modification of adenovirus with the FWKT motif may be beneficial for NET therapy.

The HDAC inhibitor FK228 enhances adenoviral transgene expression by a transduction-independent mechanism but does not increase adenovirus replication

The histone deacetylase inhibitor FK228 has previously been shown to enhance adenoviral transgene expression when cells are pre-incubated with the drug. Upregulation of the coxsackie adenovirus receptor (CAR), leading to increased viral transduction, has been proposed as the main mechanism. In the present study, we found that the highest increase in transgene expression was achieved when non-toxic concentrations of FK228 were added immediately after transduction, demonstrating that the main effect by which FK228 enhances transgene expression is transduction-independent. FK228 had positive effects both on Ad5 and Ad5/f35 vectors with a variety of transgenes and promoters, indicating that FK228 works mainly by increasing transgene expression at the transcriptional level. In some cases, the effects were dramatic, as demonstrated by an increase in CD40L expression by FK228 from 0.3% to 62% when the murine prostate cancer cell line TRAMP-C2 was transduced with Ad[CD40L]. One unexpected finding was that FK228 decreased the transgene expression of an adenoviral vector with the prostate cell-specific PPT promoter in the human prostate adenocarcinoma cell lines LNCaP and PC-346C. This is probably a consequence of alteration of the adenocarcinoma cell lines towards a neuroendocrine differentiation after FK228 treatment. The observations in this study indicate that FK228 enhances adenoviral therapy by a transduction-independent mechanism. Furthermore, since histone deacetylase inhibitors may affect the differentiation of cells, it is important to keep in mind that the activity and specificity of tissue- and tumor-specific promoters may also be affected.

Double-detargeted oncolytic adenovirus shows replication arrest in liver cells and retains neuroendocrine cell killing ability

Background

We have previously developed an oncolytic serotype 5 adenovirus (Ad5) with chromogranin-A (CgA) promoter-controlled E1A expression, Ad[CgA-E1A], with the intention to treat neuroendocrine tumors, including carcinoids. Since carcinoids tend to metastasize to the liver it is important to fully repress viral replication in hepatocytes to avoid adenovirus-related liver toxicity. Herein, we explore miRNA-based regulation of E1A expression as a complementary mechanism to promoter-based transcriptional control.

Methodology/Principal Findings

Ad[CgA-E1A-miR122], where E1A expression is further controlled by six tandem repeats of the target sequence for the liver-specific miR122, was constructed and compared to Ad[CgA-E1A]. We observed E1A suppression and replication arrest of the miR122-detargeted adenovirus in normal hepatocytes, while the two viruses killed carcinoid cells to the same degree. Repeated intravenous injections of Ad[CgA-E1A] induced liver toxicity in mice while Ad[CgA-E1A-miR122] injections did not. Furthermore, a miR122-detargeted adenovirus with the wild-type E1A promoter showed reduced replication in hepatic cells compared to wild-type Ad5 but not to the same extent as the miR122-detargeted adenovirus with the neuroendocrine-selective CgA promoter.

Conclusions/Significance

A combination of transcriptional (promoter) and post-transcriptional (miRNA target) regulation to control virus replication may allow for the use of higher doses of adenovirus for efficient tumors treatment without liver toxicity.

Boundaries in vertebrate genomes: different solutions to adequately insulate gene expression domains

Gene expression domains are normally not arranged in vertebrate genomes according to their expression patterns. Instead, it is not unusual to find genes expressed in different cell types, or in different developmental stages, sharing a particular region of a chromosome. Therefore, the existence of boundaries, or insulators, as non-coding gene regulatory elements, is instrumental for the adequate organization and function of vertebrate genomes. Through the evolution and natural selection at the molecular level, and according to available DNA sequences surrounding a locus, previously existing or recently mobilized, different elements have been recruited to serve as boundaries, depending on their suitability to properly insulate gene expression domains. In this regard, several gene regulatory elements, including scaffold/matrix-attachment regions, members of families of DNA repetitive elements (such as LINEs or SINEs), target sites for the zinc-finger multipurpose nuclear factor CTCF, enhancers and locus control regions, have been reported to show functional activities as insulators. In this review, we will address how such a variety of apparently different genomic sequences converge in a similar function, namely, to adequately insulate a gene expression domain, thereby allowing the locus to be expressed according to their own gene regulatory elements without interfering itself and being interfered by surrounding loci. The identification and characterization of genomic boundaries is not only interesting as a theoretical exercise for better understanding how vertebrate genomes are organized, but also allows devising new and improved gene transfer strategies to ensure the expression of heterologous DNA constructs in ectopic genomic locations.

Increased therapeutic efficacy of the prostate-specific oncolytic adenovirus Ad[I/PPT-E1A] by reduction of the insulator size and introduction of the full-length E3 region

Conditionally replicating adenoviruses are developing as a complement to traditional cancer therapies. Ad[I/PPT-E1A] is an E1B/E3-deleted virus that replicates exclusively in prostate cells, since the expression of E1A is controlled by the recombinant 1.4 kb prostate-specific PPT promoter. The transcriptional integrity of PPT is maintained by the 3.0 kb mouse H19 insulator that was introduced directly upstream of the PPT sequence. In order to increase the cloning capacity to be able to reintroduce E3 sequences in the 35.7 kb Ad[I/PPT-E1A] genome, various shorter insulators were examined in a luciferase reporter gene assay. It was found that the 1.6 kb core H19 insulator (i) improves the activity of PPT, compared to the 3.0 kb full-length insulator, while still maintaining prostate cell specificity and releasing 1.4 kb of space for insertion of additional sequences. To improve the ability of the virus to efficiently lyse infected cells and persist in vivo, we inserted the adenovirus death protein (ADP) or the full-length adenovirus E3 region. The oncolytic activity of PPT-E1A-based viruses was studied using MTS, crystal violet and replication assays. The virus with the reintroduced full-length E3-region (Ad[i/PPT-E1A, E3]) showed the highest cytopathic effects in vitro. Furthermore, this virus suppressed the growth of aggressively growing prostate tumors in vivo. Therefore, we conclude that Ad[i/PPT-E1A, E3] is a prostate-specific oncolytic adenovirus with a high potential for treating localized prostate cancer.

A small chimeric promoter for high prostate-specific transgene expression from adenoviral vectors

BACKGROUND

Specificity of transgene expression is important for safety during gene therapeutical applications. For prostate cancer, transcriptional targeting has been applied but was hampered by loss of specificity and low activity. We constructed a small chimeric promoter for high and prostate-specific transgene expression from adenoviral vectors.

METHODS

A chimeric promoter, composed of the prostate-specific antigen (PSA) enhancer and the rat probasin promoter, was cloned into an adenoviral vector and its activity was compared to vectors containing conventional prostate-specific promoters and the constitutive Cytomegalovirus (CMV) promoter in in vitro and in vivo prostate cancer models.

RESULTS

The chimeric PSA-probasin promoter was the most active prostate-specific promoter reaching up to 20% of CMV promoter activity while maintaining prostate-specificity.

CONCLUSIONS

The chimeric PSA-probasin promoter is a small promoter that can be utilized in viral vectors for high prostate-specific transgene expression.

Advances in preclinical investigation of prostate cancer gene therapy

Treating recurrent prostate cancer poses a great challenge to clinicians. Research efforts in the last decade have shown that adenoviral vector-based gene therapy is a promising approach that could expand the arsenal against prostate cancer. This maturing field is at the stage of being able to translate many preclinical discoveries into clinical practices. At this juncture, it is important to highlight the promising strategies including prostate-targeted gene expression, the use of oncolytic vectors, therapy coupled to reporter gene imaging, and combined treatment modalities. In fact, the early stages of clinical investigation employing combined, multimodal gene therapy focused on loco-regional tumor eradication and showed promising results. Clinicians and scientists should seize the momentum of progress to push forward to improve the therapeutic outcome for the patients.

Two-step amplification of the human PPT sequence provides specific gene expression in an immunocompetent murine prostate cancer model

The recombinant prostate-specific PPT sequence comprises a prostate-specific antigen enhancer, a PSMA enhancer and a TARP promoter. It is transcriptionally active in human prostate cancer cells both in the presence and absence of testosterone. However, in experimental murine prostate cancer, it has no detectable transcriptional activity. Herein, we describe that the PPT sequence in combination with a two-step transcriptional amplification (TSTA) system becomes active also in murine prostate cancer cells. An adenovirus with TSTA-amplified PPT-controlled expression of the luciferase reporter gene, Ad[PPT/TSTA-Luc], has up to 100-fold higher prostate-specific transcriptional activity than a non-amplified PPT-based adenovirus, Ad[PPT-Luc], in human cells. In addition, Ad[PPT/TSTA-Luc] confers prostate-specific transgene expression in murine cells, with an activity that is approximately 23% of Ad[CMV-Luc] in the transgenic adenocarcinoma of the mouse prostate (TRAMP)-C2 cells. Moreover, to visualize luciferase expression in living mice a charge-coupled device camera was used. Ad[PPT/TSTA-Luc] yielded approximately 30-fold higher transgene expression than Ad[PPT-Luc] in LNCaP tumor xenografts. Importantly, Ad[PPT/TSTA-Luc] also showed activity in murine TRAMP-C2 tumors, whereas Ad[PPT-Luc] activity was undetectable. These results highlight that the recombinant PPT sequence is active in murine prostate cancer cells when augmented by a TSTA system. This finding opens up for preclinical studies with prostate-specific therapeutic gene expression in immunocompetent mice.

Gene expression and gene therapy imaging

The fast growing field of molecular imaging has achieved major advances in imaging gene expression, an important element of gene therapy. Gene expression imaging is based on specific probes or contrast agents that allow either direct or indirect spatio-temporal evaluation of gene expression. Direct evaluation is possible with, for example, contrast agents that bind directly to a specific target (e.g., receptor). Indirect evaluation may be achieved by using specific substrate probes for a target enzyme. The use of marker genes, also called reporter genes, is an essential element of MI approaches for gene expression in gene therapy. The marker gene may not have a therapeutic role itself, but by coupling the marker gene to a therapeutic gene, expression of the marker gene reports on the expression of the therapeutic gene. Nuclear medicine and optical approaches are highly sensitive (detection of probes in the picomolar range), whereas MRI and ultrasound imaging are less sensitive and require amplification techniques and/or accumulation of contrast agents in enlarged contrast particles. Recently developed MI techniques are particularly relevant for gene therapy. Amongst these are the possibility to track gene therapy vectors such as stem cells, and the techniques that allow spatiotemporal control of gene expression by non-invasive heating (with MRI guided focused ultrasound) and the use of temperature sensitive promoters.

Use of the PSA enhancer core element to modulate the expression of prostate- and non-prostate-specific basal promoters in a lentiviral vector context

Composite promoters combining the prostate-specific antigen (PSA) enhancer core element with promoter elements derived from gene coding for human prostate-specific transglutaminase gene, prostate-specific membrane antigen gene, prostate-specific antigen, rat probasin or phosphoglycerate kinase were characterized for their ability to specifically express the enhanced green fluorescent protein (EGFP) gene in prostate versus non-prostate cancer cell lines when transferred with a human immunodeficiency virus-1-based lentiviral vector. By themselves minimal proximal promoter elements were found to inefficiently promote relevant tissue-specific expression; in all the vectors tested, addition of the PSA enhancer core element markedly improved EGFP expression in LnCaP, a cancer prostate cell line used as a model for prostate cancer. The composite promoter was inactive in HuH7, a hepatocarcinoma cell line used as a model of neighboring non-prostate cancer cells. Among the promoters tested, the combination of the PSA enhancer and the rat probasin promoter showed both high specificity and a strong EGFP expression. Neither a high viral input nor the presence of the cPPT/CTS sequence affected composite promoter behavior. Our data suggest that composite prostate-specific promoters constructed by combining key elements from various promoters can improve and/or confer tissue specific expression in a lentiviral vector context.

Increased glia-specific transgene expression with glial fibrillary acidic protein promoters containing multiple enhancer elements

The ability to direct transgene expression to astrocytes has become increasingly important as the roles for these cells continue to expand. Promoters consisting of the 5'-flanking region of the human or mouse glial fibrillary acidic protein (GFAP) gene have generally proved satisfactory. However, a more powerful promoter would be advantageous for several applications, such as expression of dominant negative RNAs or proteins, or for gene therapy. We investigated the possibility of increasing the transcriptional activity of the human GFAP promoter by inserting into it one or three additional copies of putative GFAP enhancer regions. The promoters enhanced with three additional copies gave 75-fold higher LacZ expression levels upon plasmid transfection into GFAP-expressing U251 cells than the parental gfa2 promoter. Surprisingly, in a transgenic mouse model, the enhanced promoters resulted in no or only very low expression of marker genes, probably caused by toxicity. When various cell lines were infected with replication-deficient adenoviral vectors, the enhanced promoters gave LacZ expression levels that were approximately 10-fold higher than those with the parental gfa2 promoter, while retaining specificity for GFAP-expressing cells. Injection of the adenoviral vectors carrying the enhanced promoters into nude mouse brain showed that LacZ expression was limited to GFAP-positive cells. We conclude that gfa2 enhanced promoters are useful for production of short-term, glia-specific, high expression levels of genes in an adenoviral context. Adenoviral vectors containing these enhanced promoters may be useful in glioma gene therapy.

Specific targeting of gene therapy to prostate cancer using a two-step transcriptional amplification system

Significant advances in gene therapy have been made as a result of the improvement of gene delivery systems, discovery of new therapeutic genes, better understanding of mechanisms of disease progression, exploration and improvement of tissue-specific gene regulatory sequences, and development of better prodrug/enzyme systems. This review discusses adenoviral-based and prostate-specific cancer gene therapy--emphasizing tissue-specific promoter choices to increase gene therapy safety and specificity--and the development of prostate-targeted vectors, with a focus on the two-step transactivation system for amplifying gene expression, specifically in prostate cancer cells. Several examples will be discussed for the scientific basis and therapeutic applications. In addition, prostate cancer gene therapy clinical trials and future directions in this field will also be described briefly.

An oncolytic conditionally replicating adenovirus for hormone-dependent and hormone-independent prostate cancer

The use of conditionally replicating adenoviruses offers an attractive complementary treatment strategy for localized prostate cancer. We have produced a replicating adenovirus, Ad[I/PPT-E1A], where E1A gene expression is controlled by a recombinant regulatory sequence designated PPT. The PPT sequence comprises a PSA enhancer, a PSMA enhancer and a T-cell receptor gamma-chain alternate reading frame protein promoter, and it is shielded from transcriptional interference from adenoviral backbone sequences by an H19 insulator. Ad[I/PPT-E1A] yields prostate-specific E1A protein expression, viral replication and cytolysis in vitro. Furthermore, Ad[I/PPT-E1A] considerably regresses the growth of subcutaneous LNCaP prostate cancer tumors in nude mice. Importantly, the viral replication and cytolytic effect of Ad[I/PPT-E1A] are independent of the testosterone levels in the prostate cancer cells. This may be beneficial in a clinical setting since many prostate cancer patients are treated with androgen withdrawal. In conclusion, Ad[I/PPT-E1A] may prove to be useful in the treatment of localized prostate cancer.

Construction of tumor-specific toxins using ubiquitin fusion technique

The use of cytotoxic agents to eliminate cancer cells is limited because of their nonselective toxicity and unwanted side effects. One of the strategies to overcome these limitations is to use latent prodrugs that become toxic in situ after being enzymatically activated in target cells. In this work we describe a method for producing tumor-specific toxins by using a ubiquitin fusion technique. The method is illustrated by the production of recombinant toxins by in-frame fusion of ubiquitin to saporin, a toxin from the plant Saponaria officinalis. Ubiquitin-fused toxins were rapidly degraded via the ubiquitin-proteasome system, significantly reducing their nonspecific toxicity. The insertion of the protease-cleavage sequence between ubiquitin and saporin led to the removal of ubiquitin by the protease and resulted in protease-dependent stabilization of the toxin. We engineered toxins that can be stabilized by specific proteases such as deubiquitinating enzymes and prostate-specific antigen (PSA). Both constructs were activated in vitro and in cultured cells by the appropriate enzyme. Processing by the protease resulted in a greater than 10-fold increase in the toxicity of these constructs. Importantly, the PSA-cleavable toxin was able to kill specifically the PSA-producing prostate cancer cells. The ubiquitin fusion technique is thus a versatile and reliable method for obtaining selective cytotoxic agents and can easily be adapted for different kinds of toxins and activating proteases.