4.1. The Functionalities of TRβ
Thyroid hormone (TH) signaling is a primeval pathway. Though iodine is toxic to pathogens, including viruses, anaerobic bacteria, yeast and fungi, it is a crucial micronutrient [
54]. It is taken up by single-cell species, such as photosynthetic bacteria and algae, as iodate which is converted to reactive iodide anions that have a high affinity for tyrosine [
54]. Iodide coupling creates the highly reactive iodotyrosine signaling molecule. Virtually all organisms use iodine in this manner for use in essential biochemical tasks. The synthesis of THs evolved from iodotyrosines, which are present in all animal genera except arthropods [
54,
55]. THs regulate the growth and development in non-bilaterian animals, such as porifera (sponges) and cnidarians (medusae and polyps), and are present throughout lophotrochozoans (molluscs and worms) onwards [
55]. Lophotrochozoans have a TH signaling system that is TR-mediated. Essentially, THs are important universally to the conduct of normal development. They provide a pro-survival stimulus and support metamorphosis in amphibians and lampreys and life-transitions in higher animals [
56], and their effects are highly time- and dose-specific and depend on tissue type and developmental stage [
57].
THs exert their effect in vetebrates via TRs. The TR-coding cDNA was identified as a homologue to the avian erythroblastosis virus v-erb-A oncogene [
58].TR proteins are much shorter than the AR; TRα has 490 amino acids, and TRβ has 461amino acids. However, a vast variety of isoforms is generated by alternative splicing. TRs are stimulated by thyroxine (T4) and its active derivative T3. They form homodimers or act as heterodimers in conjunction with RXRs [
59]. There are two main TR isoforms: TRα and TRβ. TRα is predominantly expressed in the heart, bone, and brain, whereas TRβ is more abundant in the liver, kidney and thyroid. TRα and TRβ seem to exert distinct tissue-dependent effects which are very likely complementary to each other, as revealed from studies of mutants [
60]. Moreover, TRα and TRβ have opposing roles by differences in their molecular properties and modes of action [
61], based on diverse DNA binding affinities [
62].
Unlike the AR, TRs belong to the class II NRs and reside constantly in the nucleus as a heterodimer with RXR. The canonical TR pathway involves binding of TR/RXR to thyroid response elements (TREs), which are, as mentioned above, direct repeats of two hexamer DNA-motifs spaced by 4 nucleotides (DR4) [
24]. This binding mode is highly fine tuned by the more specific DNA motifs [
63]. Shuttling of T3 to the nucleus and binding to TRs attached to TRE elements in the genome leads to the loss of corepressors and the subsequent activation of gene transcription [
64,
65].
Further extensive analyses of TR DNA binding and potential ways of gene expression regulation have led to indications that there is gene regulation beyond canonical binding modes. TRs can bind to DNA as monomers [
66], and dimeric variants have been described [
19,
67]. TRβ is the more interesting TR and the gene that encodes it is located at chromosome 3. It seems to act in a more versatile manner, and several missense mutations are reported, which, as for the AR, map to the homodimer interface and might lead to altered ligand or coregulator binding.
4.2. THs/TRβ and Prostate Cancer
THs contribute to vertebrate development with links to many endocrine systems, as seen from studies of zebrafish, birds, rodents and dogs [
68,
69,
70,
71,
72]. There are indications that a close interaction between thyroid and gonadal hormone systems controls sex-determination and testicular development [
73]. In particular, THs impinge on sexual differentiation and gonadal development in mammals: they are involved in the maturation and functioning of the female [
74] and male reproductive systems [
71,
75], including the stimulation of androgen release in the testis [
76]. In men, THs regulate testicular development and function [
75]. Thyroid disorders are associated with gonadal dysfunction, hypogonadism and reduced prostate weight [
77,
78,
79].
THs have long been suspected to be involved in the development of malignancies [
80], whereby the maintenance of a hypothyroxinemia was suggested to improve the survival of cancer patients [
81]. Whether THs, including T3 and T4, play a role in supporting PCa growth, progression, and metastasis has remained largely unexplored. Albeit, THs influence the development and physiology of the human prostate [
82] and the incidence of PCa, as seen from studies using a diet-induced regimen [
83]. Epidemiological studies have linked low plasma T3 levels with a low incidence of PCa [
84,
85] and high T4 levels are significantly associated with an increased risk of any solid cancer, particularly PCa, lung, and breast cancer [
84,
86]. The latter finding led to the hypothesis that high TH levels correlate with tumor progression [
87,
88,
89,
90]. Several case reports have reported a correlation between low TH levels and slower cancer growth [
64,
91,
92,
93]. Studies of the availability of T3 for binding to TRβ support a role for T3 and TRβ in PCa. μ-Crystallin (CRYM) sequesters T3 in the cytosol to prevent T3 binding to nuclear TRβ [
94], thereby controlling downstream target activation [
88]. Our study showed that CRYM and TRβ show reciprocal expression in PCa tissue, whereby low CRYM represents a feature of metastatic PCa [
95]. We and others have demonstrated that CRYM expression is deficient in hormone-refractory PCa patients [
96,
97] indicating reciprocal roles for CRYM and TRβ [
98]
The literature is more extensive regarding the specific role of TRβ, perhaps reflecting a more significant role in cancer. TRβ is mainly described as a tumor suppressor [
28] because expression is often reduced in human tumors due to deletions and epigenetic modifications [
99]. A reduction has been linked to a poor prognosis in hepatocellular, renal, thyroid and breast cancers [
100,
101,
102]. TRβ expression is lost upon chromosomal 3p deletions, especially in breast and liver cancers [
64]. In addition to somatic loss-of-function mutations that foster thyroid cancers, there is a high incidence of
de novo inactivating TRα and TRβ mutations in hepatocellular, renal, and thyroid cancers [
99] which may relate to selection pressures during tumor progression [
103]. Loss of TRβ dysregulates several growth control pathways, leading to the exaggerated growth of various cancers [
104].
In contrast to a tumor suppressor role for TRβ, TRα/TRβ1 double knockout mice develop fewer skin tumors [
105], and enhanced TRβ expression has been detected in colon and head and neck cancers [
106,
107]. A recent study showed that low cytosolic and enhanced nuclear TRβ levels are indicators of a poor outcome in breast cancer [
108], highlighting the importance of distinguishing between cytosolic and nuclear TRβ abundance, as recognised previously from studies of HeLa liver cancer cells [
109]. Intriguingly, RXRs seem to have an anti-oncogenic role in PCa [
110], providing tentative support to a tumor-promoting action of activated TRβ.
Literature regarding the influence of THs on cancer often presents a view of the clinical implications/morbidity relating to hypothyroidism [
84,
111,
112]. THs stimulate the growth of many cancers, including breast, ovarian, pancreatic, hepatocellular, and renal cancer cells [
113,
114,
115,
116]. From in vitro studies, T3 supported the growth of PCa, lung, breast, ovarian and squamous cell cancer cells [
64,
88,
117], and THs have a direct stimulatory effect on some key oncogenic signaling pathways, including the phosphatidylinositol-3-kinase (PI3K)- and extracellular signal-regulated kinase (ERK1/2)-mediated pathways. In vitro studies using the androgen-sensitive LNCaP cells revealed that nmolar T3 stimulated cell growth and the expression of prostate specific antigen [
118]. Longterm incubation led to an increased androgen binding capacity in the nucleus of LNCaP cells, suggesting that T3 might induce AR expression ([
119,
120]. It is known that T3 and androgens act cooperatively to affect prostate specific antigen (PSA) expression [
95,
118]. However, whether there is a direct interaction between AR and TRβ is unknown.
T3/TRβ driven gene regulation has been analyzed in hepatocellular cancer to identify the proteins involved in tumor progression. In particular, T3-mediated upregulation of expression of the protease furin was seen for hepatoma cell lines, and the investigators concluded that this might enhance tumor metastasis [
115,
121,
122,
123]. TRβ has been shown to regulate cell metabolism and tumor-relevant genes. They include the hypoxia-inducible factor 1 subunit α [
124], the CD44 stemness factor in the brain [
125], cathepsin H (
CTSH gene) which is a tumor invasive factor in HepG2 [
123], and extracellular matrix proteins ([
115]. TRβ has been shown to effect mitochondrial respiration directly [
126], which probably contributes to senescence, DNA damage and oxidative stress.
Regarding in vivo studies, upregulation of mRNAs for TRβ together with other NRs and ONRs was observed for a PCa cell xenograft model [
127]. Administering 6-n-propyl-2-thiouracil (PTU), a thyreostatic agent, reduced the growth of DU145 and PC-3 xenografts in mice [
117] and reduced their growth in hypothyroid mice although enhanced aggressive behaviour of hepatocellular and breast cancer cell xenografts was observed [
99].
4.3. Crosstalk between TRβ and Other NRs/Transcription Factors
There is evidence to support the view that there is cross-regulation between THs and the androgen axis and that this occurs in cancer [
128] (
Figure 3). Exposure of the testes of the Western clawed frog (
Silurana tropicalis) to T3 led to increased DHT production, and a low level of androgens in the serum of PCa patients correlated with decreased T3/T4 levels [
129]. An androgen/TH interplay was recently observed in the PCa microenvironment to exert a cooperative tumor-promoting effect [
130]. However, the above hormonal cross-regulation raises the question of whether the downstream effector NRs – in this case, TRβ and AR – control the same or overlapping gene sets and processes [
27]. TRβ/AR crosstalk was shown in silico by detailed promoter analysis of TH- and androgen-dependent genes, which revealed binding sites for both receptors (TRE and ARE sites) in the promoters of AR- and TR-regulated genes, indicating mutual or collaborative gene regulation [
131]. Accordingly, TRβ-mediated gene expression is stimulated by both T3 and DHT robustly [
132]. Other workers showed that THs increased the expression of AR and androgen synthesis enzymes [
133]. These findings were corroborated by a recent transcription factor binding site study that detected an overlap of NRs binding to the same regulatory elements in PCa [
134], opening the possibility of a mutual or collaborative TR/AR DNA-binding mode.
There is evidence to support the view that TRs interact with RARs. In 1993, Rosen and colleagues characterized a homologous 20-amino acid region conserved in TRβ, RAR and VDR essential for heterodimerization [
135]. They postulated the existence of various heterodimeric combinations to allow for enhanced variability and sensitivity of gene expression regulation. Other early studies also claimed that TRs and RAR form heterodimers, supported by overexpression and subsequent cell-free DNA-binding (EMSAs) and luciferase reporter assays in vitro [
59,
136,
137,
138]. Where RARs form heterodimers with TR, they have affinities for consensus and natural HREs like those for TR/RXR heterodimers. Therefore, TR/RAR heterodimers can regulate T3-mediated gene expression [
138]. For amphibian limb blastemal and COS transfected cells, ATRA was observed to mediate both RAR- and T3-mediated effects [
139]. Stromelysin 3, a vital tissue remodeling protease, was found to be regulated by THs and ATRA due to the presence of both TREs and RAREs in the gene promoter [
140]. For rat GH3 pituitary gland cells, ATRA has been shown to antagonize T3 action, presumably at a receptor level [
141]. In addition to this complex hormonal crosstalk, T3 stimulation has been shown to regulate the expression of the ATRA synthesizing enzymes within mouse brain cells ([
142], underscoring the importance of hormonal crosstalk.
An important recent finding regarding PCa relates to the nuclear corepressor 2/silencing mediator (NCOR2) for RAR and TR. The expression of this corepressor is frequently altered in PCa and other cancers [
143,
144]. Reduced expression of NCOR2 has been shown to accelerate the failure of androgen deprivation. The investigators used the CWR22 xenograft model to show that a stable reduction of expression of NCOR2 accelerated the recurrence of disease post-ADT [[
145].
In parallel to the early TR/RAR interaction studies, Schräder and colleagues showed that VDR might directly interact with TRβ, and that polarity directs gene expression [
146,
147]. Other workers identified interference by 1,25D3 and ATRA regarding TH regulation of glucocorticoid hormone expression, concluding that VDR-TR and VDR-RAR heterodimers act as competitors of TR-RXR and RAR-RXR complexes [
148]. In contrast, others have failed to provide evidence for VDR-TR heterodimerization but showed that both VDR and TR compete for RXR binding [
149], still highlighting a crosstalk between both receptors. Support to this view by a study that showed that VDR heterodimerizes with RXR, but not with TRβ [
150], and that a delicate balance of ligand availability and the limiting amounts of RXR, which would shift between TR and VDR, would affect the repression versus activation of various genes. Beyond the above findings, reports regarding crosstalk between TR and VDR have been limited. However, a very interesting crosstalk was described for adipocyte [
151] whereby TRβ expression was controlled by 1,25D3, and vice versa, and that T3 regulates VDR gene expression in mouse photoreceptors has been reported [
152].
There is a crosstalk between TRβ and other NRs. TRβ interacts with ER isoforms, resulting in flexible regulation of the consensus estrogen response element [
153], and TRβ interaction with ERα has been shown to regulate mitochondrial activity ([
154]. TRβ interacts with PPARγ [
155] and is then oncogenic [
156]. Crosstalk between TRβ and liver X receptor (LXR) has been identified for lipid metabolism-related genes and other physiological systems such as the central nervous system [
157]. Regarding other TFs, TRβ interaction with the Wnt pathway is important based on direct interaction between TRβ and β catenin [
158]. Overall, TRβ interaction with the Wnt pathway for several organisms and tissues indicates the promotion of a stem cell phenotype [
159]. Other crosstalk includes regarding jun and fos oncogene activities, based on the abilities of TRs to inhibit AP-1 binding to DNA [
160]. TRβ mediates repression of STAT5 activity [
161], and overactivation of STAT5 signaling has been linked to an oncogenic TRβ variant in breast cancer [
162].
Diverse findings bring to attention the complex role that TRβ plays in regulating the physiological status of cells. TRβ interacts with intracellular signalling pathways and with PI3K to play a role in the maturation of mouse hippocampal synapses [
163]. There is TH regulation of steroid hormone-associated genes, for example, regulation of the expression of kidney androgen-regulated protein in the developing kidney ([
164], and T3 regulation of the expression of androgen receptor-associate protein 70 (ARA70) and sex hormone binding globulin (SHBG) in HepG2 cells [
122,
165]. TRβ modulates the function of the tumor suppressor p53, leading to differential regulation of p53-regulated genes [
153]. Early studies revealed a connection between TR-mediated gene expression and changes to the expression of cell cycle regulators [
156,
166]. Overexpression of TRβ in thyroid cancer cells led to activation of the RhoB signaling pathway and p21-induced cell-cycle arrest [
167].