Current Drug Targets - Immune, Endocrine & Metabolic Disorders - Volume 4, Issue 3, 2004

The sodium-iodide symporter (NIS) is an intrinsic plasma membrane protein that mediates active transport of iodide in the thyroid gland and several other extra-thyroidal tissues. This activity has been utilized for many years for imaging the thyroid gland and for treatment of thyroid disease both benign and malignant. Cloning and characterization of NIS has more recently allowed research into its use in nonthyroidal cancers through gene transfer for both diagnosis and treatment.

Thyroid cancer is a relatively common malignancy with an estimated prevalence of 250, 000 in the U.S. A minority of patients have poorly differentiated thyroid carcinoma that is unresponsive to radioiodine therapy. Redifferentiation agents that ‘reprogram’ these tumors to concentrate radioiodine would be of great value in treating patients with advanced thyroid cancer. The retinoid isotretinoin is the most extensively studied of these agents. It appears that 20-40% of patients respond to isotretinoin treatment by concentration of radioiodine in metastatic tumors, but the clinical utility of this redifferentiation is still unclear. In vitro studies suggest that the retinoid receptors (RARβ and RXRγ) are required for this effect. Abnormal DNA methylation may be an early event in thyroid tumorigenesis and methylation of the sodium iodide symporter (NIS) may play a role in the loss of iodine concentration in these tumors. Inhibitors of methylation (5- azacytidine, phenylacetate and sodium butyrate) have been shown to increase NIS expression and iodine uptake in cell culture models, but published trials in humans are not yet available. Histone acetylation is required for efficient transcription of genes necessary for differentiated function. Proteins that cause histone deacetylation inhibit gene transcription and differentiated function. Inhibitors of histone deacetylation (depsipeptide, trichostatin A) have been shown to increase NIS expression and iodine uptake in poorly differentiated and undifferentiated cell lines. Phase II human trials are currently underway for depsipeptide. Finally, commonly used agents such as thiazolidinediones (diabetes) and HMG-CoA reductase inhibitors (hypercholesterolemia) have shown promise in preliminary in vitro studies in advanced thyroid cancer cell lines. Development of these and other novel agents for the treatment of advanced thyroid cancer is critical for us to treat an uncommon progression of a common malignancy.

Thyroid cancer is a heterogeneous disorder characterized by gene mutations that activate signaling pathways, and also by abnormalities in tumor suppressor genes and cell cycle proteins. Activation of the Akt / PKB signaling pathway appears to be an important event in thyroid tumorigenesis and, perhaps, in tumor progression too. Akt is activated in Cowden's syndrome through inactivation of PTEN, a negative regulator of Akt. Cowden's syndrome is an autosomal dominant multiorgan hamartoma syndrome characterized by benign and malignant thyroid tumors, breast cancers, and colon cancers. In addition, the Akt pathway appears to be activated in a significant proportion of sporadic thyroid cancers through activation of growth factor pathways by thyroid oncogenes and / or receptor overexpression. Disruption of PI3-kinase activity pharmacologically or disruption of Akt signaling using dominant negative cDNA expression have demonstrated salutary effects on several cancer models in vitro. Therefore, Akt represents an attractive target for pharmaceutical development for a variety of malignancies, including thyroid cancer.

Apoptosis or programmed cell death occurs in both normal and pathological conditions, including cancer. Dysregulation of apoptosis allows transformed cells to continually and uninhibitedly enter the cell cycle, thus perpetuating the sequence of mutation, genomic instability and, finally, oncogenesis. The cell death machinery includes cell surface receptors, adaptor molecules, proteolytic enzymes, such as caspases, and a variety of mitochondrial proteins, which interact with each other in a complex fashion. In addition, extensive “cross-talk” exists between the apoptotic pathways and several other signaling systems that govern growth and differentiation. Recent advances in molecular techniques have shed light upon elements of the above pathways in assorted malignancies, including non-medullary thyroid carcinoma (ThyrCa). A subgroup of ThyrCa patients is (or becomes over time) refractory to standard treatment modalities and eventually succumbs to their disease. For such patients with clinically aggressive ThyrCa, novel therapeutic agents are urgently needed. Changes in the sensitivity of cells to apoptosis have clear implications for the treatment of any malignancy. In this review, we outline the main molecular targets that play a role in apoptosis in ThyrCa cells, and discuss various options for promoting apoptosis, either by pharmacologic or gene transfer therapeutic interventions.

Over the past two decades significant progress has been made in elucidating the pathogenesis of thyroid cancer. The ongoing identification of mutations in cellular signaling pathways has revolutionized the field of thyroid cancer biology and has led to the development of novel new therapeutic agents. One of the signaling cascades implicated in the oncogenic process is the ERK pathway that normally functions to transmit mitogenic signals from the cell membrane to the nucleus. Genetic alterations of key components of this cascade, namely RET, Ras and Raf, are thought to result in constitutive activation of the pathway and subsequent thyroid tumorigenesis. Targeting of these components with pharmaceutical agents holds the potential of providing newer and more effective treatment modalities for thyroid cancer. Several such drugs are currently being developed to inhibit RET, Ras, Raf, as well as other factors impacted by the ERK pathway. These include a vast array of agents such as antisense compounds, small molecule inhibitors as well as inhibitors of farnesyl transferase, heat shock proteins, matrix metalloproteinases and histone deacetylases. Some of these drugs have already entered preclinical and clinical testing with promising anti-tumor effects. These as well as even newer agents may offer exciting possibilities for the future treatment of thyroid cancer.

Follicular thyroid carcinoma (FTC) accounts for approximately 20% of all thyroid cancers, and up to 40% of the deaths associated with this disease. Current treatment approaches include surgery, followed by radioactive iodine therapy. However, a significant proportion of locally advanced and metastatic FTC fails to concentrate iodine. Because traditional chemotherapeutic agents have not been shown to alter outcomes in this disease, novel therapeutic strategies are needed for advanced disease. Recently, a genomic rearrangement has been identified in up to 50% of FTC, involving a translocation event between chromosome regions 3p25 and 2q13. This translocation fuses the thyroid-specific transcription factor PAX8 gene with the PPARγ gene, a ubiquitously expressed transcription factor. We have confirmed that this Pax8 / PPARγ fusion gene (designated PPFP) is an oncogene, which accelerates cell growth, reduces rates of apoptosis and permits anchorage independent and contact uninhibited growth of a thyroid cell line. The action of PPFP arises, at least in part, through its activity as a dominant-negative inhibitor of the wild-type PPARγ transcription factor. Although the mechanism by which PPFP impairs PPARγ activity remains unknown at this time, it is likely to be mediated by competition for the genomic PPARγ response elements, the endogenous ligand, or various cofactors, including the Retinoid X Receptor (RXR). Consequently, modulation of PPFP activity might be possible through the use of PPARγ agonists, RXR-agonists, or specific modulators of PPFP itself. Alternatively, modulation of several down-stream regulatory pathways may become possible, as the consequences of PPARγ inhibition become better known. PPFP represents a potential novel target for the management of advanced FTC.

Thyroid cancers are of special interest in gene therapy, since it is possible to direct gene expression specifically to the thyroid derived cells by using promoters with limited expression, and secondly, because destruction of the normal tissue by introduction of a toxic gene would have no important adverse effect. A variety of methods for gene delivery are available. Adenovirus is a well studied and widely used vector and is useful for targeting genes because it infects many cell types, including differentiated thyroid cancer and medullary thyroid cancer cells. Strategies that have been employed successfully in animal models include adenoviral mediated expression of thymidine kinase under control of a thyroglobulin promoter, similarly expression of the cytokine IL-2, and perhaps most effectively, expression of IL-12. Combinations of vectors expressing thymidine kinase and IL-12 under control of a strong but non-tissue specific CMV promoter effectively destroy a model anaplastic thyroid tumor in Wistar rats. Replicating adenoviruses, in contrast to the non-replicating form commonly used, have also been used to infect tumor cells and express P-53 protein, leading to apoptosis of tumor cells. Medullary thyroid cancer provides a target much like differentiated thyroid cancer because it is possible to address gene expression specifically to the medullary thyroid cells by the use of a modified calcitonin promoter. Animal models of this tumor are available in a mouse and Wag / Rij rat model. In the latter system, treatment with adenoviruses expressing genes under control of the modified calcitonin promoter and expressing thymidine kinase or IL-12 leads to destruction of growing medullary thyroid cancer tumors, destroy distant tumors after injection in one tumor, and cause induction of long lasting immunity to subsequent tumor development in the animals. There are many ongoing studies of gene therapy in humans using various genes such as thymidine kinase, IL-2, and now IL-12. Although none of these trials to date shows complete eradication of metastatic tumors in humans, there are reports showing distinctly that the viral mediated gene therapy approach can effectively destroy human tumors after in vivo administration. Tumors that have been treated include melanomas, glioblastomas, breast tumors, and prostate carcinomas. In the latter studies, it has been possible to show objective responses documented by a fall in serum PSA levels of 50% or more that are sustained for prolonged periods. Gene therapy using the adenoviral vectors appears to be safe in studies reported so far. A problem is prior or induced immunity to adenoviral proteins, but direct injection of the vector into a tumor nodule largely circumvents this problem. New genes and new vectors under development will certainly lead to the established use of these methods in the therapy of human thyroid carcinomas in the near future.

A new approach for anti-tumor immunotherapy is to use dendritic cells (DCs) as adjuvants in order to actively immunize cancer patients with antigens specifically expressed in tumor cells. DCs possess a unique capacity to effectively activate CD4+ T helper cells and CD8+ cytotoxic T cells. During the last years, several clinical trials in various malignancies demonstrated that immunizations with tumor antigen pulsed DCs could break the tolerance of the immune system against antigens expressed by the tumor cells resulting in partial or complete remission in some cases. This review describes the most important findings on the interaction between DCs and T cells as well as natural killer cells and summarizes recent data on DC vaccination of endocrine and non-endocrine malignancies. The results from current pilot studies suggest that DC vaccination may represent a promising strategy for the development of an anti-cancer vaccine to treat chemotherapy and radioresistant endocrine malignancies.

Telomerase is known to be activated in almost all cancer cells and is quiescent in almost all normal cells. Therefore, it follows that therapeutic strategies directed against cancer would include the targeting of telomerase, as well as the use of telomerase. Several approaches have been used both in vitro and in vivo and include the following: 1) antisense; 2) immunotherapy directed against hTERT; and 3) the use of telomerase promoter to direct cytotoxic therapy. Herein we review these approaches and discuss their potential applicability against thyroid cancer.