Thyroid hormone receptors/THR genes in human cancer

Abstract

Thyroid hormone (triiodothyronine, T3) is a pleiotropic regulator of growth, differentiation and tissue homeostasis in higher organisms that acts through the control of target gene expression. Most, if not all, major T3 actions are mediated by specific high affinity nuclear receptors (TR) which are encoded by two genes, THRA and THRB. Several TRα and TRβ receptor isoforms are expressed. Abundant and contradictory literature exists on the relationship between circulating thyroid hormone levels, thyroid diseases and human cancer. In 1986, a connection between TR and cancer became evident when the chicken TRα1 was characterized as the c-erbA proto-oncogene, the cellular counterpart of the retroviral v-erbA oncogene. V-erbA causes erythroleukemias and sarcomas in birds, and hepatocellular carcinomas in transgenic mice. In recent years, many studies have analyzed the presence of quantitative (abnormal levels) or qualitative (mutations) alterations in the expression of THR genes in different types of human neoplasias. While their role in tumor generation or progression is currently unclear, both gross chromosomal and minor mutations (deletions, aberrant splicing, point mutations) and changes in the level of expression of THRA and THRB genes have been found. Together with other in vitro data indicating connections between TR and p53, Rb, cyclin D and other cell cycle regulators and oncogenes, these results suggest that THRA and THRB may be involved in human cancer.

Introduction

Thyroid hormone (triiodothyronine or T3) is an important regulator of growth, development and differentiation in vertebrates. T3 binds to specific high affinity receptors (thyroid receptors, TR) which belong to the superfamily of nuclear receptors [1]. They function as ligand-modulated transcription factors by binding to sequences known as thyroid hormone response elements (TRE) usually located in the promoter regions of target genes [2], [3]. Multiple studies have shown that gene regulation by T3 is modulated by the formation of heterodimers between TR and other members of the receptor superfamily, mainly the retinoic X receptors (RXR), and by subsequent interaction with transcriptional co-regulators (co-activators and co-repressors) and protein complexes [4], [5]. Both positive and negative effects of T3 on gene transcription have been described [6], [7]. In addition, ligand-activated TRs control gene transcription indirectly by interfering with the function of other transcription factors such as activating protein-1 (AP-1), formed by members of the c-Jun and c-Fos proto-oncogenic proteins [8], [9]. A third, less known mechanism of gene regulation by TR is the control of various post-transcriptional processes including splicing, mRNA stability and translation, and protein turn-over [10], [11], [12], probably by regulating gene transcription [13]. Finally, non-genomic actions of T3 through rapid mechanisms that remain uncharacterized have been also proposed [14].

The human TRs are encoded by the TRα (THRA) and TRβ (THRB) genes, located on human chromosomes 17 and 3, respectively. By alternative splicing and differential promoter usage, these two genes yield several polypeptides [2], [3] (Fig. 1). TRα1, TRβ1 and TRβ2 are well characterized and encode functional products. Another isoform named TRβ3 has been cloned from a rat osteosarcoma line and its function is still unknown [15]. Other isoforms such as TRα2 do not bind T3 or dimerize but they can still bind to TREs and inhibit functional receptors in transiently transfected cells [16], [17]. In vivo, however, this inhibitory action of TRα2 is not clear as cells expressing high TRα2 levels respond well to T3 [18] and effects of deleting TRα2 expression in mice have been observed [19]. The physiological roles of the non-binding polypeptides TRα2, TRΔβ3, and two additional truncated TRΔα1 and TRΔα2 polypeptides [20] remain elusive.

TRs affect gene transcription in four ways: (a) induction by hormone-bound TR or (b) inhibition by unliganded TR at positive TREs; and (c) inhibition by hormone-bound TR or (d) activation by unliganded TR at negative TREs. At positive TREs repression of basal transcription by unliganded TR (silencing) is mediated by interaction with co-repressors. TRβ2 seems to be the exception, activating transcription even in the absence of hormone [21]. Hormone binding induces a conformational change in TR leading to co-repressors release and subsequent co-activator binding that leads to modification of chromatin structure and transcriptional activation [5], [22], [23]. The mechanism of transcriptional inhibition at negative TREs is less well understood.

TR are modular proteins (Fig. 2). Their most characteristic domains are the highly conserved DNA-binding domain (DBD) which is also involved in dimerization, and the carboxy-terminal ligand-binding domain (LBD) responsible for T3 binding, dimerization and interaction with co-regulators [1]. Interaction with the cell basal transcription apparatus depends on the amino-terminal region (activating function 1, AF-1), while hormone-dependent transcription regulation (co-repressor and co-activator binding) relies on the extreme carboxy-terminal helix 12 sequence (activating function-2, AF-2). However, some isoforms such as rat TRβ3 [15] or the chicken TRβ0 [24] and TRαp40 (translated from an internal AUG codon; [25]) lack the AF-1 region.

In 1986, TRα1 was found to be the cellular counterpart of the retroviral v-erbA oncogene that contributes to the appearance of erythroleukemia and sarcomas in birds [26], [27]. v-erbA encodes a highly mutated chicken TRα1 protein that does not bind T3 and acts as a constitutive repressor of T3-regulated genes [28]. V-erbA has weak oncogenic potential itself but potentiates the activity of oncoproteins derived from tyrosine kinases and downstream signal transducers (see reviews [29], [30]). It enhances the transformed phenotype of cultured erythroblasts and fibroblasts by arresting cellular differentiation and favoring proliferation (see reviews [29], [30]). Importantly, v-erbA transgenic mice develop hepatocellular carcinomas [31].

In recent years, increasing evidence has suggested that aberrant expression and mutations in THR genes could be associated with carcinogenesis in humans. The focus of this review will be to provide an overview of what we know about the status and expression of THR genes in human cancers.