Adenoviral infection of thyroid cells: a rationale for gene therapy for metastatic thyroid carcinoma

Eukaryotic expression vectors and immunoconjugates for cancer therapy

This review considers ways to address specificity to therapeutic targeted anticancer agents. These include transcriptional activation of tissue- and tumor-specific promoters in eukaryotic expression vectors and use of antitumor-directed immunoconjugates. The review deals with analysis of strategies used for selection of targeted promoters and examples of antibody fusion proteins exhibiting antitumor activity. A new direction in antitumor treatment pooling together methods of gene therapy and antibody therapy has appeared. This direction is based on the development of vectors encoding secreted forms of immunoconjugates. After vector introduction into a cell, the latter is capable of synthesizing and secreting antibody fusion protein composed of a therapeutic anticancer agent and antibody specifically targeted to cancer cells.

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.

A tandemly repeated thyroglobulin core promoter has potential to enhance efficacy for tissue-specific gene therapy for thyroid carcinomas

Recombinant adenoviruses, carrying herpes simplex virus thymidine kinase (HSVtk) genes, were developed to evaluate the possibility of tissue-specific gene therapy for thyroid carcinomas. The HSVtk gene was driven by a minimal thyroglobulin (TG) promoter (AdTGtk) and a tandemly repeated minimal TG promoter (Ad2 x TGtk) to obtain thyroid-specific cell killing ability. The transduction of HSVtk genes by infection with Ad2 x TGtk followed by ganciclovir (GCV) treatment showed more powerful cytotoxicity for TG-producing FRTL5 cells, a rat normal thyroid cell line, and FTC-133 cells, a human follicular thyroid carcinoma cell line, than when infected with AdTGtk in vitro. The cell killing ability of Ad2 x TGtk was 10- to 30-fold higher than that of AdTGtk and similar to that of AdCMVtk, which carries HSVtk under the control of CMV promoter. Whereas after treatment with adenovirus/GCV to non-TG-producing cell lines (undifferentiated thyroid carcinoma cell lines and carcinoma cell lines from other tissues), Ad2 x TGtk and AdTGtk needed more than 100-fold concentrated GCV to reach IC(50) compared to AdCMVtk. We confirmed the enhanced efficacy of Ad2 x TGtk for tissue-specific cytotoxicity in vivo. After adenovirus/GCV treatment for FTC-133 tumor-bearing nude mice, Ad2 x TGtk enhanced tumor growth inhibition and survival rates compared to AdTGtk. Tumor growth inhibition and survival rates by Ad2 x TGtk were similar to that by AdCMVtk. Moreover, any toxic effect for rat normal tissues was not revealed after intravenous injections with Ad2 x TGtk and intraperitoneal administrations with GCV in vivo, whereas severe liver damages were observed after treatment with AdCMVtk/GCV. These data indicate a beneficial effect of Ad2 x TGtk for tissue-specific gene therapy for TG-producing thyroid carcinomas without toxicity for normal tissues.

Adenovirus HSV-TK construct with thyroid-specific promoter: enhancement of activity and specificity with histone deacetylase inhibitors and agents modulating the camp pathway

The successful use of tissue- or tumor-selective promoters in targeted gene therapy for cancer depends on high and selective activity. Tg is a thyroid-specific protein that is expressed in the normal thyroid and a majority of thyroid tumors. In the present study, we show, using a luciferase reporter assay, that a construct containing the putative Tg promoter and enhancer is active in 4 thyroid carcinoma cell lines (including 2 anaplastic thyroid carcinoma cell lines) and not in 5 cancer cell lines arising from nonthyroid tissues. Furthermore, both the activity and the specificity of this construct were increased by pretreatment with 8-Br-cAMP and the histone deacetylase inhibitor depsipeptide (FR901228). Expression of thymidine kinase in thyroid cancer cells infected with a recombinant adenovirus (Ad) carrying a Tg enhancer/promoter-thymidine kinase expression cassette (AdTg enhancer/promoter-TK) correlated with the level of Tg enhancer/promoter activity in these cells. Under similar conditions, TK expression was not observed in cancer cell lines arising from nonthyroid tissues. Cells infected with AdTg enhancer/promoter-TK demonstrated preferential GCV sensitivity, with up to a 100,000-fold increase in GCV sensitivity in thyroid cancer cell lines compared to cancer cell lines of nonthyroid origin. The construct described herein can be used to selectively target thyroid cancer cells, and its expression can be modulated to further increase its specificity and selectivity, especially in anaplastic thyroid carcinoma cells, using 8-Br-cAMP and depsipeptide.

Cell-specific viral gene therapy of a Hurthle cell tumor

We evaluated the effectiveness of a replication-defective adenovirus-transducing thymidine kinase (TK) gene under the control of the rat Tg (rTg) promoter (AdrTgtk) in therapy of a human Hurthle cancer (XTC-1 cell) in vitro and in vivo. The ganciclovir (GCV) sensitivity of infected XTC-1 cells was assessed in vitro by H(3)-thymidine incorporation assay and Trypan-blue exclusion, and by an in vivo tumor development assay. Proliferation was strongly inhibited by adding GCV into the culture medium of infected cells, but not uninfected cells, proving cell infection and expression of TK in the XTC-1 cells. AdrTgtk, and also viruses that have the noncell-specific cytomegalovirus (CMV) promoter-directing expression of TK (AdCMVtk), or luciferase (AdCMVLuc), were used to transduce XTC-1 cells to evaluate killing effects. After infection with AdCMVtk or AdrTgtk, followed by GCV treatment, 70% of infected cells were killed in the presence of GCV, compared with less than 20% of cells infected by AdCMVLuc and treated with GCV. In vivo toxicity was studied in BALB/c mice. When adenovirus is given iv, liver is the major organ infected. No significant changes of the serum transaminase levels and no histological abnormalities were found in animals treated with AdrTgtk/GCV given iv, compared with control animals. High levels of serum transaminases, lymphocyte infiltration, some Kupffer's cell prominence, and extensive single-cell hepatocyte death were found in AdCMVtk/GCV-treated animals, indicating severe liver damage induced, as expected, by the noncell-specific CMV promoter. XTL-1 cells (2 x 10(6)) were injected sc into BALB/c-severe combined immunodeficient mice (BALB/c-SCID), and the mice developed tumors after 3 wk. After intratumoral injection of AdrTgtk and treatment with GCV, tumors stabilized in 15 of 17 mice within 3 wk, 9 tumors remained stabilized after 5 wk of treatment, and 2 disappeared during observation. In AdCMVLuc/GCV-treated control mice, almost all tumors grew continuously. The average tumor size in AdrTgtk-treated mice was significantly smaller than that of control animals after 2 wk of treatment. Our data confirm the effectiveness and specificity of an adenovirus using rTg promoter to express TK, and support its future application to thyroid cancer gene therapy in humans.

Feline hyperthyroidism: advances towards novel molecular therapeutics

Feline hyperthyroidism is the most common endocrine disorder of the elderly cat. Traditionally, the disease is treated by surgical thyroidectomy, medical management with antithyroid drugs or radiation therapy using iodine-131. However, none of these treatments is ideal and molecular therapeutics may offer novel methods of treating the disease. This article reviews the background of, and preliminary investigations into, the development of a transcriptionally targeted somatic gene therapy strategy for the treatment of this feline condition.

Adenoviral-mediated gene therapy for thyroid carcinoma using thymidine kinase controlled by thyroglobulin promoter demonstrates high specificity and low toxicity

A replication defective adenovirus transducing thymidine kinase (TK) gene under the control of the rat thyroglobulin (rTg) promoter (AdrTgtk) was developed to evaluate its cell-specific killing activity in gene therapy. We also developed adenoviruses containing the TK gene driven by the cytomegalovirus (CMV) promoter (AdCMVtk), and luciferase (Luc) gene driven by the rTg or CMV promoter (AdrTgLuc or AdCMVLuc). Luc activity in FRTL-5, HepG2, COS1, rMTC, hMTC, Hela, GH3, T98G, and CA77 cells was measured after infection with AdrTgLuc or AdCMVLuc. FRTL-5 cells produce thyroglobulin (Tg), whereas all other cells are non-Tg-producing cell lines. Transduction by AdCMVLuc caused high Luc activity in all cell lines. However, infection with AdrTgLuc induced Luc activity only in FRTL-5 cells. AdCMVtk or AdrTgtk was used to transduce various cell lines to evaluate the different killing effect. After infection with AdCMVtk vector followed by ganciclovir (GCV) treatment, cell growth was strongly suppressed in all cell lines compared both to noninfected cells and to cells infected by AdCMVLuc in the presence of GCV. When FRTL-5 cells were infected with AdrTgtk followed by GCV treatment, more than 90% were killed, but only a minimal effect was observed in other cell lines, indicating that the Tg promoter transduced TK expression only in Tg-producing cells. When adenovirus is given intravenously, liver and spleen are the major organs infected. A high Luc activity was found in liver and spleen of AdCMVLuc treated animals. No Luc activity was found in liver and spleen of AdrTgLuc-treated animals, indicating that rTg does not transduce Luc expression in non-Tg-producing tissues in vivo. No significant changes of the serum transaminase levels and histologic abnormalities were found in animals treated with AdrTgtk/GCV compared with control animals. High levels of serum transaminases, lymphocyte infiltration, some Kupffer's cell prominence, and extensive single cell hypatocyte death were found in AdCMVtk/GCV-treated animals, indicating severe liver damage induced, as expected, by a noncell-specific promoter. These results indicate that transfer of TK gene driven by the rTg promoter has thyroid cell-specific killing ability in the presence of GCV, little in vivo toxicity, and should be useful in the future for treating thyroid Tg-producing cancers.

Effective formation of major histocompatibility complex class II-peptide complexes from endogenous antigen by thyroid epithelial cells

In autoimmune thyroid disease, thyroid epithelial cells (TEC) express major histocompatibility complex (MHC) class II molecules, potentially enabling them to present thyroid self-antigens to CD4-positive T cells. However, despite this, TEC may fail to present endogenous antigen as a result of limited processing or MHC class II loading capacity, or inadequate MHC class II levels. We addressed these issues using the cloned rat TEC line, Fischer rat thyroid cell line (FRTL5), which was transfected using an adenoviral expression vector that expressed ovalbumin (OVA) as an integral membrane protein. OVA-expressing FRTL5 cells very efficiently activated a panel of OVA-specific, class II-restricted T-cell hybridomas. This response was dependent on induction of MHC class II molecules by interferon-gamma (IFN-gamma) and was blocked by anti-MHC class II antibodies. Poor responses were seen to exogenously added OVA or OVA peptides. These results provide the most direct evidence to date that TEC can form MHC class II-peptide complexes derived from self-antigen in sufficient quantities to activate T cells.

Overexpression of TTF-1 and PAX-8 restores thyroglobulin gene promoter activity in ARO and WRO cell lines

BACKGROUND

In anticipation of developing gene therapy against thyroid carcinoma we created an expression vector using the thyroglobulin (Tg) gene promoter. The inhibition of both Tg and thyroid-specific transcription factor (TTF-1 and PAX-8) gene expression, however, has been well documented in thyroid carcinomas. We therefore examined the effects of overexpression of TTF-1 and PAX-8 on Tg gene promoter activity in the human thyroid carcinoma cell lines, ARO (anaplastic) and WRO (follicular).

METHODS

ARO, WRO, and nonthyroid cells were transfected with an expression vector in which beta-galactosidase (beta-gal) is driven by the Tg gene promoter (beta-gal). Tg, TTF-1, and PAX-8 gene expression were also examined by reverse transcriptase-polymerase chain reaction (RT-PCR).

RESULTS

ARO and WRO exhibited decreased gene expression of Tg, TTF-1, and PAX-8. Transfection with TG--gal alone exhibited minimal beta-gal expression, whereas cotransfection with TTF-1 and PAX-8 resulted in markedly increased expression. There was no evidence of beta-gal expression with or without TTF-1 and PAX-8 in nonthyroid cells.

CONCLUSIONS

Weak Tg gene promoter activity in ARO and WRO is associated with decreased expression of transcription factors TTF-1 and PAX-8 but can be restored with their overexpression. This model may serve as a template on which to further develop cell-specific gene therapy against thyroid carcinoma.

Herpesvirus thymidine kinase transgenes that do not cause male sterility are aberrantly transcribed and translated in the testis

Mice that carry the wild-type herpes simplex virus type 1 (HSV1) thymidine kinase (tk) gene coupled to the bovine thyroglobulin (bTG) promoter (bTG-tk1 mice) express viral TK at a high level in the thyroid gland, and at an equally high level, ectopically, in the testis, which renders the males sterile. When the bTG promoter was coupled either to a variant of HSV1-tk (differing from the wild type in 2 nucleotides) (bTG-tk1alpha mice) or to the herpes simplex virus type 2 (HSV2) tk gene (bTG-tk2 mice) viral TK was expressed at high levels in the thyroid gland, and much lower levels in the testis, which causes a reduction in male fecundity rather than sterility. Here, we compare the expression of the three transgenes in the two tissues. Thyroids of all mice exhibited a 1.3 kb RNA initiated at or near the bTG cap site. Testes of all mice exhibited mainly 5'-end-shortened RNAs (bTG-tk1 and bTG-tk1alpha mice, approx. 1.2 kb and 0.9 kb; bTG-tk2 mice, approx. 1.2 kb) initiated from cryptic initiation sites in the HSV1-tk and HSV2-tk coding regions. Also, less abundant RNAs initiated near the bTG cap site were expressed from all three transgenes. Thyroids of bTG-tk1 and bTG-tk1alpha mice contained the full-length HSV-TK protein and a truncated variant previously shown to originate at a non-ATG start codon. Testes of these mice exhibited both proteins but relatively less of the full-length protein. We attribute the high level of viral TK in the testes of bTG-tk1 mice to the expression of a predominant protein of Mr 39000 that originates from ATG-2. Thyroid and testis of bTG-tk2 mice contained only the full-length HSV2-TK protein.

Retrovirus-mediated herpes simplex virus thymidine kinase gene transduction renders human thyroid carcinoma cell lines sensitive to ganciclovir and radiation in vitro and in vivo

In an attempt to develop gene therapy for thyroid carcinomas, the present studies were undertaken to evaluate in vitro and in vivo therapeutic efficacy and toxicity of herpes simplex virus thymidine kinase (HSV-tk) gene and ganciclovir (GCV) treatment, a widely used prodrug/suicide gene therapy, in human thyroid carcinoma cell lines, FRO and WRO cells, using a means of retrovirus-mediated gene transduction. In vitro experiments demonstrated dose- and time-dependent cell killing by transduction of the HSV-tk gene followed by GCV treatment. The IC50 (the concentration required to elicit 50% growth inhibition) shifted from 250 to 0.5 mg/liter in FRO cells, and from 3,000 to 0.09 mg/liter in WRO cells with therapeutic indexes of 500 and 33,000, respectively. Treatment with 30 mg/liter GCV for 4 days led to complete cell death in HSV-tk tumor cells. Nontransduced cells mixed with transduced cells were also effectively killed by GCV (bystander effect). Low concentrations of GCV, which alone showed little cytotoxicity, enhanced radiation-induced cytotoxicity (radiosensitization). In vivo sc FRO-tk tumor models in nude mice also showed dose- and time-dependent tumor regression. The IC50 was less than 2 mg/kg, and treatment with 100 mg/kg GCV for 2 weeks completely eradicated all tumors. The bystander effect and radiosensitization were also obtained in vivo. These results suggest that the HSV-tk/GCV approach to human thyroid carcinoma cells appears to be very efficacious, with a wide therapeutic range, and exerts a bystander effect and radiosensitization both in vitro and in vivo. Thus, HSV-tk/GCV system, alone or in combination with radiotherapy, may be a promising suicide gene therapy for thyroid carcinomas.