Robust prostate-specific expression for targeted gene therapy based on the human kallikrein 2 promoter

Applications of tissue-specific and cancer-selective gene promoters for cancer diagnosis and therapy

Current treatment of solid tumors with standard of care chemotherapies, radiation therapy and/or immunotherapies are often limited by severe adverse toxic effects, resulting in a narrow therapeutic index. Cancer gene therapy represents a targeted approach that in principle could significantly reduce undesirable side effects in normal tissues while significantly inhibiting tumor growth and progression. To be effective, this strategy requires a clear understanding of the molecular biology of cancer development and evolution and developing biological vectors that can serve as vehicles to target cancer cells. The advent and fine tuning of omics technologies that permit the collective and spatial recognition of genes (genomics), mRNAs (transcriptomics), proteins (proteomics), metabolites (metabolomics), epiomics (epigenomics, epitranscriptomics, and epiproteomics), and their interactomics in defined complex biological samples provide a roadmap for identifying crucial targets of relevance to the cancer paradigm. Combining these strategies with identified genetic elements that control target gene expression uncovers significant opportunities for developing guided gene-based therapeutics for cancer. The purpose of this review is to overview the current state and potential limitations in developing gene promoter-directed targeted expression of key genes and highlights their potential applications in cancer gene therapy.

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.

Targeted BikDD expression kills androgen-dependent and castration-resistant prostate cancer cells

Targeted gene therapy is a promising approach for treating prostate cancer after the discovery of prostate cancer-specific promoters such as prostate-specific antigen, rat probasin, and human glandular kallikrein. However, these promoters are androgen dependent, and after castration or androgen ablation therapy, they become much less active or sometimes inactive. Importantly, the disease will inevitably progress from androgen-dependent (ADPC) to castration-resistant prostate cancer (CRPC), at which treatments fail and high mortality ensues. Therefore, it is critical to develop a targeted gene therapy strategy that is effective in both ADPC and CRPC to eradicate recurrent prostate tumors. The human telomerase reverse transcriptase-VP16-Gal4-WPRE integrated systemic amplifier composite (T-VISA) vector we previously developed, which targets transgene expression in ovarian and breast cancer, is also active in prostate cancer. To further improve its effectiveness based on androgen response in ADPC progression, the ARR2 element (two copies of androgen response region from rat probasin promoter) was incorporated into T-VISA to produce AT-VISA. Under androgen analog (R1881) stimulation, the activity of AT-VISA was increased to a level greater than or comparable to the cytomegalovirus promoter in ADPC and CRPC cells, respectively. Importantly, AT-VISA demonstrated little or no expression in normal cells. Systemic administration of AT-VISA-BikDD encapsulated in liposomes repressed prostate tumor growth and prolonged mouse survival in orthotopic animal models as well as in the transgenic adenocarcinoma mouse prostate model, indicating that AT-VISA-BikDD has therapeutic potential to treat ADPC and CRPC safely and effectively in preclinical setting.

Targeted expression of Escherichia coli purine nucleoside phosphorylase and Fludara® for prostate cancer therapy

BACKGROUND

Previous studies have shown that Herpes Simplex Virus thymidine kinase (HSV-tk)/ganciclovir (GCV) comprised the most commonly used suicide gene therapy for prostate cancer, with modest results being obtained. However, novel suicide genes, such as Escherichia coli purine nucleoside phosphorylase (PNP), have been utilized to demonstrate more potent tumor killing and an enhanced bystander effect on local, non-expressing cells compared to HSV-tk.

METHODS

PNP/fludarabine (Fludara®; fludarabine phosphate; Berlex Labs, Richmond, CA, USA) was deliveried by prostate-specific, rat probasin-based promoter, ARR2PB. After infection of various cell lines with ADV.ARR(2) PB-PNP and administration of androgen analog, R1881, expression of PNP mRNA was detected; in vivo, the antitumor effect of the ARR(2) PB-PNP/Fludara system was monitored and analyzed, as well as animal survival.

RESULTS

After in vitro infection with ADV.ARR(2) PB-PNP (multiplicity of infection  =  10), LNCaP cells were more sensitive to a lower concentration Fludara (LD(50) , approximately 0.1 µg/ml) in the presence of R1881. Furthermore, robust bystander effects after R1881/Fludara treatment were observed in LNCaP cells after infection with bicistronic vector ADV.ARR2PB/PNP-IRES-EGFP in contrast to a much weaker effect in cells treated with ADV.CMV-HSV-tk/GCV. In vivo, tumor size in the ADV.ARR2PB-PNP/Fludara treatment group was dramatically smaller than in the control groups, and the mice treated with our system had a significantly prolonged survival, with three of eight mice surviving up to the 160-day termination point, as well as no systemic toxicity.

CONCLUSIONS

The ARR(2) PB-PNP/Fludara system induced massive tumor cell death and a prolonged life span without systemic cytotoxicity; therefore, it might be a more attractive strategy for suicide gene therapy of prostate cancer.

Molecular and traditional chemotherapy: a united front against prostate cancer

Castrate resistant prostate cancer (CRPC) is essentially incurable. Recently though, chemotherapy demonstrated a survival benefit ( approximately 2months) in the treatment of CRPC. While this was a landmark finding, suboptimal efficacy and systemic toxicities at the therapeutic doses warranted further development. Smart combination therapies, acting through multiple mechanisms to target the heterogeneous cell populations of PC and with potential for reduction in individual dosing, need to be developed. In that, targeted molecular chemotherapy has generated significant interest with the potential for localized treatment to generate systemic efficacy. This can be further enhanced through the use of oncolytic conditionally replicative adenoviruses (CRAds) to deliver molecular chemotherapy. The prospects of chemotherapy and molecular-chemotherapy as single and as components of combination therapies are discussed.

Targeting of prostate cancer cells by a cytotoxic lentiviral vector containing a prostate stem cell antigen (PSCA) promoter

BACKGROUND

The efficacy of prostate cancer gene therapy is limited by the inefficiency of prostate-specific promoters as compared to ubiquitous viral promoters. The purpose of this investigation was to evaluate the specificity and efficacy of a lentiviral vector driven by a PSCA promoter.

METHODS

Prostate cancer (LNCap, C42-B, and LAPC-4) and non-prostate cancer (HeLa, MB231, and MCF-7) cells were transduced with a lentiviral vector expressing either the luciferase or the HSV-TK suicide gene and driven by a short PSCA promoter. Specificity and efficacy were evaluated in vitro and in vivo.

RESULTS

Luciferase expression was only detected in prostate cancer cells and was comparable to the universal CMV promoter. Luciferase expression in prostate cancer cells cultured with androgen was higher than that in cells cultured without androgen. In subsequent cytotoxicity experiments in which the luciferase marker gene was replaced with the HSV-TK gene, the lentiviral vector harboring the PSCA promoter induced cytotoxicity in prostate cancer cell lines while demonstrating a minimal effect on non-prostate cells. Cellular toxicity was correlated to increasing concentrations of the prodrug ganciclovir. Androgen had a positive effect on the cytotoxicity of this lentiviral construct. Intratumoral injection of prostate cancer xenografts with the lentiviral construct induced tumor growth inhibition versus saline controls.

CONCLUSION

Our results indicate that a lentiviral gene therapy vector driven by a short PSCA promoter can induce prostate-specific cellular toxicity in vivo and in vitro and may provide a strategy to selectively treat local and advanced metastatic prostate cancer. Prostate 69: 1422-1434, 2009. (c) 2009 Wiley-Liss, Inc.

A novel hTERT promoter-driven E1A therapeutic for ovarian cancer

Currently, an effective gene therapy strategy, which not only retains cancer-specific expression but also limits toxicity, has yet to be developed for ovarian cancer. Mounting reports over the years have shown that human telomerase activity is significantly elevated in cancer cells compared with normal cells. In this study, we evaluated the human telomerase reverse transcriptase (hTERT; T) promoter and showed that it can direct target gene expression preferentially in ovarian cancer cells. However, its promoter (T) activity is much lower than that of cytomegalovirus (CMV), a commonly used nonspecific promoter. To overcome this problem, we have integrated the T promoter into our recently developed VP16-Gal4-WPRE integrated systemic amplifier (VISA) system and dramatically enhanced transgene expression. In addition, to further develop this cancer-specific promoter gene expression system into an applicable therapeutic vector, we expressed E1A (an adenoviral type 5 transcription factor that possesses anticancer properties) through this novel VISA platform. We showed that the T-VISA system specifically targeted the expression of E1A to ovarian cancer cells at a level greater than or comparable with the commonly used CMV promoter, yet remained nearly silent in normal cells, thus making this a suitable gene therapy construct. By using this cancer-specific promoter that limits target gene expression in normal cells/tissues, potential toxicity induced by the CMV promoter would be prevented. More importantly, we showed significant antitumor activity with much less toxicity in animal models through i.v. delivery of T-VISA-E1A:liposomal nanoparticles, suggesting a promising role of T-VISA-E1A for ovarian cancer treatment under a gene therapy setting.

Gonadoblastoma locus and the TSPY gene on the human Y chromosome

The gonadoblastoma (GBY) locus is the only oncogenic locus on the human Y chromosome. It is postulated to serve a normal function in the testis, but could exert oncogenic effects in dysgenetic gonads of individuals with intersex and/or dysfunctional testicular phenotypes. Recent studies establish the testis-specific protein Y-encoded (TSPY) gene to be the putative gene for GBY. TSPY serves normal functions in male stem germ cell proliferation and differentiation, but is ectopically expressed in early and late stages of gonadoblastomas, testicular carcinoma in situ (the premalignant precursor for all testicular germ cell tumors), seminomas, and selected nonseminomas. Aberrant TSPY expression stimulates protein synthetic activities, accelerates cell proliferation, and promotes tumorigenicity in athymic mice. TSPY binds to type B cyclins, enhances an activated cyclin B-CDK1 kinase activity, and propels a rapid G(2)/M transition in the cell cycle. TSPY also counteracts the normal functions of its X-homologue, TSPX, which also binds to cyclin B and modulates the cyclin B-CDK1 activity to insure a proper G(2)/M transition in the cell cycle. Hence, ectopic expression and actions of the Y-located TSPY gene in incompatible germ cells, such as those in dysgenetic or ovarian environments and dysfunctional testis, disrupt the normal cell cycle regulation and predispose the host cells to tumorigenesis. The contrasting properties of TSPY and TSPX suggest that somatic cancers, such as intracranial germ cell tumors, melanoma, and hepatocellular carcinoma, with detectable TSPY expression could exhibit sexual dimorphisms in the initiation and/or progression of the respective oncogenesis.

Targeted expression of BikDD eradicates pancreatic tumors in noninvasive imaging models

Pancreatic cancer is an aggressive malignancy with morbidity rates almost equal to mortality rates because of the current lack of effective treatment options. Here, we describe a targeted approach to treating pancreatic cancer with effective therapeutic efficacy and safety in noninvasive imaging models. We developed a versatile expression vector "VISA" (VP16-GAL4-WPRE integrated systemic amplifier) and a CCKAR (cholecystokinin type A receptor) gene-based, pancreatic-cancer-specific promoter VISA (CCKAR-VISA) composite to target transgene expression in pancreatic tumors in vivo. Targeted expression of BikDD, a potent proapoptotic gene driven by CCKAR-VISA, exhibited significant antitumor effects on pancreatic cancer and prolonged survival in multiple xenograft and syngeneic orthotopic mouse models of pancreatic tumors with virtually no toxicity.

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.

Enhancement of gene transactivation activity of androgen receptor by hepatitis B virus X protein

Hepatitis B virus (HBV) X protein (HBx) is a regulatory protein that is required for efficient replication of HBV in its natural host. In this report, we demonstrate by co-immunoprecipitation experiments that HBx can physically bind to the androgen receptor (AR), which is a nuclear hormone receptor that is expressed in many different tissues including the liver. This observation is further supported by confocal microscopy, which reveals that HBx can alter the subcellular localization of the AR both in the presence and in the absence of dihydrotestosterone (DHT). Further studies indicate that HBx can enhance the gene transactivation activity of AR by enhancing its DNA binding activity in a DHT-dependent manner. However, HBx does not remain associated with AR on the DNA. As AR can regulate the expression of a number of cellular genes, our results raise the possibility that HBV pathogenesis may be mediated in part via the interaction between HBx and AR.

EZC-Prostate Models Offer High Sensitivity and Specificity for Noninvasive Imaging of Prostate Cancer Progression and Androgen Receptor Action

In vivo imaging advances have greatly expanded the use of animal cancer models. Herein, we describe two new models that permit prostate imaging ex vivo, in vivo, and in utero. Further, we show the use of these models for detecting small metastasis and testing reagents that modulate the androgen receptor (AR) axis. A luciferase reporter gene was directed to the prostate epithelium using three composite promoters called human kallikrein 2 (hK2)-E3/P, PSA-E2/P, and ARR2PB, derived from hK2, PSA, and rat probasin regulatory elements, to generate the EZC1, EZC2, and EZC3-prostate mice, respectively. EZC2 and EZC3-prostate display robust expression in the prostate with only minimal detectable expression in other organs, including testes and epididymis. Luciferase expression was detected as early as embryonic day 13 (E13) in the urogenital track. To image prostate cancer progression, lines of EZC mice were bred with prostate cancer models TRAMP and JOCK1, and imaged longitudinally. When crossed with prostate cancer models, EZC3 facilitated detection of metastatic lesions although total prostate luciferase expression was static or reduced due to weakening of AR-regulated promoters. Castration reduced luciferase expression by 90% and 97% in EZC2 and EZC3 mice, respectively, and use of GnRH antagonist also led to extensive inhibition of reporter activity. The EZC-prostate model permits prostate imaging in vivo and should be useful for imaging prostate development, growth, metastasis, and response to treatment noninvasively and longitudinally. These models also provide powerful new reagents for developing improved drugs that inhibit the AR axis.

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.

The androgen receptor directly targets the cellular Fas/FasL-associated death domain protein-like inhibitory protein gene to promote the androgen-independent growth of prostate cancer cells

Androgens provide survival signals to prostate epithelial cells, and androgen ablation induces apoptosis in the prostate gland. However, the molecular mechanisms of actions of the androgen-signaling pathway in these processes are not fully understood. Here, we report that androgens induced expression of the cellular Fas/FasL-associated death domain protein-like inhibitory protein (c-FLIP) gene, which is a potent inhibitor of Fas/FasL-mediated apoptosis. The androgen receptor was recruited to the promoter of the c-FLIP gene in the presence of androgens. We found that c-FLIP promoter contained multiple functional androgen response elements. In addition, we show that c-FLIP overexpression accelerated progression to androgen independence by inhibiting apoptosis in LNCaP prostate tumors implanted in nude mice. Our results suggest that the androgen receptor affects survival and apoptosis of prostate cells through regulation of the c-FLIP gene in response to androgens.

IDENTIFYING THE COMBINATION OF THE TRANSCRIPTIONAL REGULATORY SEQUENCES ON PROSTATE SPECIFIC ANTIGEN AND HUMAN GLANDULAR KALLIKREIN GENES

PURPOSE

The combination of prostate specific antigen (PSA) and human glandular kallikrein (KLK2) promoters and/or enhancers was used to establish a new model to determine the feasibility of tissue specific expression for prostate cancer.

MATERIALS AND METHODS

In vitro studies used the construction of PSA and KLK2 promoters/enhancers vectors to elucidate the link between the promoter/enhancer of PSA and KLK2. Reverse transcriptase-polymerase chain reaction assays were used to determine cell specific expression. Therefore, an attractive tissue specific expression vector for PSA and KLK2 gene was identified.

RESULTS

The reporter vectors driven by KLK2 promoter had much lower luciferase activities than those of the reporter vectors driven by PSA promoter in LNCaP cells. Furthermore, the most efficient and cell specific reporter activity after 5alpha-androstan-17beta-ol-3-one treatment among the reporter vectors constructed in this study was that of pKLK2EPSABHE, which was driven by KLK2 enhancer and PSA promoter/enhancer. The pKLK2EPSABHE reporter vector could induce 800-fold higher than the KLK2 basic promoter and its reporter activity was 16 times that of the enhancer/promoter element of KLK2 following induction by androgen.

CONCLUSIONS

The results verify that the PSA promoter/enhancer must be combined with KLK2 to ensure the full activity and cell specificity of the gene. These expressions coupled with mechanic target validation yield valuable clues regarding the model of action of complex mixtures. This model is a potentially useful tool in gene therapy for metastatic prostate cancer.

The EZC-Prostate Model: Noninvasive Prostate Imaging in Living Mice

Recently, progress in the development of prostate-specific promoters and high resolution imaging techniques has made real-time monitoring of transgenic expression possible, opening a vista of potentially important in vivo models of prostate disease. Herein, we describe a novel prostate reporter model, called the EZC-prostate model that permits both ex vivo and in vivo imaging of the prostate using a sensitive charge-coupled device. Firefly luciferase and enhanced green fluorescent protein were targeted to the prostate epithelium using the composite human kallikrein 2 (hK2)-based promoter, hK2-E3/P. In EZC-prostate mice, the ventral and dorsal/lateral prostate lobes were brilliant green under fluorescence microscopy, with expression localized to the secretory epithelium. In contrast, enhanced green fluorescent protein was undetectable in the anterior lobes of prostate, seminal vesicles, testes, liver, lung, and brain. The kinetics of luciferase activity in intact and castrated living mice monitored with the IVIS charge-coupled device-based imaging system confirmed that firefly luciferase expression was largely prostate restricted, increased with age up to 24 wk, and was androgen dependent. Decreases in reporter expression after 24 wk may reflect well known, age-related decreases in androgen signaling with age in humans. Ex vivo imaging of microdissected animals further confirmed that the luminescence detected in living mice emanated predominately from the prostate, with minor signals originating from the testes and cecum. These data demonstrate that the hK2-E3/P promoter directs strong prostate-specific expression in a transgenic mouse model. Multigenic models, generated by crosses with various hyperplastic and neoplastic prostate disease models, could potentially provide powerful new tools in longitudinal monitoring of changes in prostate size, androgen signaling, metastases, or response to novel therapies without sacrificing large cohorts of animals.

Endocrine aspects of cancer gene therapy

The field of cancer gene therapy is in continuous expansion, and technology is quickly moving ahead as far as gene targeting and regulation of gene expression are concerned. This review focuses on the endocrine aspects of gene therapy, including the possibility to exploit hormone and hormone receptor functions for regulating therapeutic gene expression, the use of endocrine-specific genes as new therapeutic tools, the effects of viral vector delivery and transgene expression on the endocrine system, and the endocrine response to viral vector delivery. Present ethical concerns of gene therapy and the risk of germ cell transduction are also discussed, along with potential lines of innovation to improve cell and gene targeting.

Transcriptional Targeting in Cancer Gene Therapy

Cancer gene therapy has been one of the most exciting areas of therapeutic research in the past decade. In this review, we discuss strategies to restrict transcription of transgenes to tumour cells. A range of promoters which are tissue-specific, tumour-specific, or inducible by exogenous agents are presented. Transcriptional targeting should prevent normal tissue toxicities associated with other cancer treatments, such as radiation and chemotherapy. In addition, the specificity of these strategies should provide improved targeting of metastatic tumours following systemic gene delivery. Rapid progress in the ability to specifically control transgenes will allow systemic gene delivery for cancer therapy to become a real possibility in the near future.

Gene Therapy Applications to Cancer Treatment

Over the past ten years significant advances have been made in the fields of gene therapy and tumour immunology, such that there now exists a considerable body of evidence validating the proof in the principle of gene therapy based cancer vaccines. While clinical benefit has so far been marginal, data from preclinical and early clinical trials of gene therapy combined with standard therapies are strongly suggestive of additional benefit. Many reasons have been proposed to explain the paucity of clinical responses to single agent vaccination strategies including the poor antigenicity of tumour cells and the development of tolerance through down-regulation of MHC, costimulatory, signal transduction, and other molecules essential for the generation of strong immune responses. In addition, there is now evidence from animal models that the growing tumour may actively inhibit the host immune response. Removal of the primary tumour prior to T cell transfer from the spleen of cancer bearing animals, led to effective tumour cell line specific immunity in the recipient mouse suggesting that there is an ongoing tumour-host interaction. This model also illustrates the potential difficulties of clinical vaccine trials in patients with advanced stage disease.

Baculoviruses as Vectors for Gene Therapy against Human Prostate Cancer

Current curative strategies for prostate cancer are restricted to the primary tumour, and the effect of treatments to control metastatic disease is not sustained. Therefore, the application of gene therapy to prostate cancer is an attractive alternative. Baculoviruses are highly restricted insect viruses, which can enter, but not replicate in mammalian cells. Baculoviruses can incorporate large amounts of extra genetic material, and will express transgenes in mammalian cells when under the control of a mammalian or strong viral promoter. Successful gene delivery has been achieved both in vitro and in vivo and into both dividing and nondividing cells, which is important since prostate cancers divide relatively slowly. In addition, the envelope protein gp64 is sufficiently mutable to allow targeted transduction of particular cell types. In this review, the advantages of using baculoviruses for prostate cancer gene therapy are explored, and the mechanisms of viral entry and transgene expression are described.

Human kallikrein 8: immunoassay development and identification in tissue extracts and biological fluids

BACKGROUND

The serine protease human kallikrein 8 (hK8; neuropsin), a new member of the human kallikrein family, was predicted to be secreted; thus, it is expected to be present in biological fluids. The aim of this study was to develop a sensitive and specific immunoassay for hK8 (hK8-ELISA) and establish the distribution of hK8 in tissue extracts and biological fluids.

METHODS

Recombinant hK8 was produced in a baculovirus expression system and purified with a three-step chromatographic procedure. Purified hK8 was injected into mice and rabbits for antibody generation. A highly specific and sensitive sandwich-type immunoassay (ELISA) was developed using the rabbit and mouse antisera to hK8. The hK8-ELISA was then used to study the distribution of hK8 in various biological fluids and tissue extracts.

RESULTS

The dynamic range of the hK8-ELISA was 0.2 (detection limit) to 20 micro g/L, and imprecision (CV) was <10% within this range. This hK8-ELISA was specific for hK8 and had no detectable cross-reactivity with other members of the human kallikrein family. With this assay, hK8 was detected in tissue extracts of esophagus (highest concentrations), skin, testis, tonsil, kidney, breast, and salivary gland and in the biological fluids breast milk (highest concentrations), amniotic fluid, seminal plasma, and serum. Furthermore, in some cancer cell lines, the concentration of hK8 was regulated by steroid hormones.

CONCLUSIONS

We report for the first time production of recombinant hK8 protein, generation of antibodies, and development of a highly sensitive and specific immunoassay for quantification of hK8 in tissue extracts and biological fluids. This assay can be used to explore the potential of hK8 as a marker of cancer or other conditions.

Identification of an androgen-dependent enhancer within the prostate stem cell antigen gene

Prostate stem cell antigen (PSCA) is emerging as an important diagnostic marker and therapeutic target in prostate cancer. Previous studies indicated that PSCA was directly regulated by androgens, but the mechanism has not been elucidated. Here we describe the identification of a compact cell-specific and androgen-responsive enhancer between 2.7 and 3 kb upstream of the transcription start site. The enhancer functions autonomously when positioned immediately adjacent to a minimal promoter. Deoxyribonuclease I footprinting analysis with recombinant androgen receptor (AR) reveals that the enhancer contains two AR binding sites at one end. Mutational analysis of the AR binding sites revealed the importance of the higher affinity one. The dissociation constant of the high affinity binding site (androgen response element I) was determined to be approximately 87 nM. The remainder of the enhancer contains elements that function synergistically with the AR. We discuss the structural organization of the PSCA enhancer and compare it with that found in other AR-regulated genes.

Molecular engineering of a two-step transcription amplification (TSTA) system for transgene delivery in prostate cancer

Gene therapy is founded on the concept that tissue-specific promoters can express heterologous genes for molecular imaging or therapeutic applications. The engineering of cell-specific enhancers to improve potency and the development of two-step transcriptional activation (TSTA) approaches have significantly improved the efficacy of transgene expression. Here we combine these technologies to create a robust, titratable, androgen-responsive system targeted to prostate cancer cells. Our "chimeric" TSTA system uses a duplicated variant of the prostate-specific antigen (PSA) gene enhancer to express GAL4 derivatives fused to one, two, or four VP16 activation domains. We targeted the resulting activators to cells with reporter templates bearing one, two, or five GAL4 binding sites upstream of firefly luciferase. We monitored activity via firefly luciferase assays in transfected cell extracts and in live nude mice using a cooled charge-coupled device (CCD) imaging system. In this system, we found that firefly luciferase expression in prostate cancer cells can be varied over an 800-fold range. We also found that a single plasmid bearing the optimized enhancer, GAL4-VP16 derivative, and reporter expressed firefly luciferase at 20-fold higher levels than the cytomegalovirus enhancer. We discuss the implications of this strategy and its application to molecular imaging and therapy.

Prostate cancer gene therapy and the role of radiation

Even though prostate cancer is detected earlier than in the pre-PSA era, prostate cancer is the second leading cause of cancer mortality in the American male. Prostate cancer therapy is not ideal, especially for high-risk localized and metastatic cancer; therefore, investigators have sought new therapeutic modalities such as angiogenesis inhibitors, inhibitors of the cell signaling pathway, vaccines, and gene therapy. Gene therapy has emerged as potential therapy for both localized and systemic prostate cancer. Gene therapy has been shown to work supra-additively with radiation in controlling prostate cancer in vivo. With further technological advances in radiation therapy, gene therapy, and the understanding of prostate cancer biology, gene therapy will potentially have an important role in prostate cancer therapy.

Cancer gene therapy: fringe or cutting edge?

Direct targeting of cancer cells with gene therapy has the potential to treat cancer on the basis of its molecular characteristics. But although laboratory results have been extremely encouraging, many practical obstacles need to be overcome before gene therapy can fulfil its goals in the clinic. These issues are not trivial, but seem less formidable than the challenge of killing cancers selectively and rationally--a challenge that has been successfully addressed.