Thyroid hormone receptors are ligand-dependent transcription factors that mediate the biological activities of the thyroid hormone (T3) in development, growth, differentiation, and metabolism. Two human TR genes, THRA and THRB, located on different chromosomes, encode thyroid hormone (T3) binding TR isoforms (TRα1, β1, β2, and β3). These TR isoforms share extensive sequence homology in the DNA and ligand-binding domains, but differ in the length and amino acid sequence at the amino terminal A/B domain. These TR isoforms are expressed in a tissue- and development-dependent manner. Besides sharing common functions, TRβ and TRα also mediate isoform-dependent actions [1
]. In the past decades, significant progress has been made in understanding the molecular mechanisms by which TR functions to maintain normal physiological T3-mediated homeostasis. However, the roles of TR in human cancers are less well understood.
Early evidence to suggest that mutated TR could be involved in carcinogenesis came from the discovery that TRα1 is the cellular counterpart of the retroviral v-ERBA oncoprotein involved in the neoplastic transformation leading to acute erythroleukemia and sarcomas [2
]. v-ERBA oncoprotein is a highly mutated chicken TRα1 that does not bind T3 and loses the ability to activate gene transcription. That mutated TRs could be involved in human cancers was supported by the findings that somatic mutations of TRs have been found in human hepatocellular carcinoma [4
], renal clear cell carcinoma [5
], breast cancer [7
], pituitary tumor [8
], and thyroid cancer [10
]. Many of these TR mutants have lost T3 binding activity and transcription capacity, and some exhibit dominantly negative activity [5
The proposal that the loss of TR functions could be involved in the development of human cancers gained further support by association studies. Loss in the expression of the THRB gene because of the truncation/deletion of chromosome 3p where the gene is located was reported in many malignancies including lung, melanoma, breast, head and neck, renal cell, uterine cervical, ovarian, testicular and gastrointestinal tumors [11
]. Moreover, decreased expression due to silencing of the THRB gene by promoter hypermethylation has been found in human cancers including breast, lung, and thyroid carcinoma [17
]. These association studies raised the possibility that TRs could function as tumor suppressors in human cancers. This possibility has gained additional support from recent studies showing that the loss of normal TRβ functions by mutations leads to spontaneous development of follicular thyroid cancer [21
]. However, the molecular mechanisms by which TRβ acts to suppress tumor development and progression are largely unknown.
In the present study, we aimed to understand how TRβ could act as a tumor suppressor in the carcinogenesis of human thyroid epithelial cells mediated by Simian virus-40 (designated as HTori cells)[23
]. HTori cells were derived from transfection of human primary thyroid follicular epithelial cells with a plasmid containing an origin-defective SV40 genome (SVori-)[23
]. Sequences of the SV40 virus have been found in human tumors such as pleural mesotheliomas, ependymomas, choroid plexus tumors, and other brain tumors [24
]. Particularly, evidence has been presented to show the association of SV40 virus with human papillary thyroid carcinomas [28
]. Moreover, transgenic mice in which the expression of SV40Tag was driven by the thyroglobulin gene promoter developed thyroid hyperplasia and adenocarcinomas [31
]. These observations suggested that SV40 virus could be involved in the development of thyroid cancer. Thus, HTori cells presented an opportunity to elucidate the role of TRβ in thyroid carcinogenesis.
We therefore stably expressed TRβ in HTori cells and assessed how TRβ affected SV40-Tag induced thyroid tumorigenesis. We found that TRβ suppressed tumorigenesis by physical interaction with SV40Tag, leading to inactivation of the oncogenic actions of SV40Tag by blocking its recruitment of the retinoblastoma protein (Rb) and p53 tumor suppressors. The re-actavited Rb and p53 decreased cell proliferation and activated apoptosis, thereby inhibiting tumorigenesis. The present study uncovered a novel action of TRβ as a tumor suppressor via interfering with the recruitment of Rb and p53 by SV40Tag oncoprotein.