Membrane receptor for thyroid hormone: physiologic and pharmacologic implications

Tetrac Delayed the Onset of Ocular Melanoma in an Orthotopic Mouse Model

Ocular melanoma research, the most common primary intraocular malignancy in adults, is hindered by limited in vivo models. In a series of experiments using melanoma cells injected intraocularly into mouse eyes, we developed a model for ocular melanoma. Inoculation of 5 × 10⁵ B16F10 cells led to rapid tumor growth, extensive lung metastasis, and limited animal survival, while injection of 10² cells was sufficient for intraocular tumors to grow with extended survival. In order to improve tumor visualization, 10² melanoma cells (B16F10 or B16LS9) were inoculated into Balb/C albino mouse eyes. These mice developed intraocular tumors that did not metastasize and exhibited extended survival. Next, we studied the therapeutic potential of inhibitor of the thyroid hormones-αvβ3 integrin signaling pathway in ocular melanoma. By utilizing tetraiodothyroacetic acid (tetrac), a thyroid hormone derivative, a delay in tumor onset in the B16F10 (integrin+) arm was observed, compared to the untreated group, while in the B16LS9 cells (integrin-) a similar rate of tumor onset was noticed in both experimental and control groups. In summary, following an optimization process, the mouse ocular melanoma model was developed. The models exhibited an extended therapeutic window and can be utilized as a platform for investigating various drugs and other treatment modalities.

The induction of myeloma cell death and DNA damage by tetrac, a thyroid hormone derivative

Multiple myeloma (MM) is a plasma cell malignancy in which involvement of the thyroid hormone-integrin αvβ3 pathway was shown, and pharmacologic inhibition of this pathway is a rational approach to disease management. A thyroid hormone derivative, tetraiodothyroacetic acid (tetrac), which inhibits l-thyroxine (T₄) and 3,5,3'-triiodo-l-thyronine (T₃) binding to αvβ3 integrin, was studied in five MM cell lines and primary bone marrow (BM) MM cells. Tetrac inhibited MM cell proliferation (absolute cell number/viability) and induced caspase-dependent apoptosis (annexin-V/PI and cell cycle). Activation of caspase-9 and caspase-3 was further demonstrated. Moreover, DNA damage markers, ataxia-telangiectasia-mutated (ATM) kinase, poly ADP-ribose polymerase (PARP-1) and histone γH2AX were induced by tetrac. The various tetrac-initiated effects were attenuated by Arg-Gly-Asp (RGD) peptide, suggesting integrin involvement. Primary BM mononuclear cells were harvested from MM patients (n = 39) at various disease stages. Tetrac-induced apoptosis (12/17 samples) and sensitized the cytotoxic action of bortezomib (6/9 samples). Lastly, expression of plasma membrane integrin αvβ3 was shown not only in the malignant plasma clone, but also in other cell populations within the BM samples (n = 25). Tetrac is anti-proliferative and pro-apoptotic in MM and cells may offer a therapeutic approach for this disease.

Evolution of thyroid hormone distributor proteins

Thyroid hormones (THs) are evolutionarily old hormones, having effects on metabolism in bacteria, invertebrates and vertebrates. THs bind specific distributor proteins (THDPs) to ensure their efficient distribution through the blood and cerebrospinal fluid in vertebrates. Albumin is a THDP in the blood of all studied species of vertebrates, so may be the original vertebrate THDP. However, albumin has weak affinity for THs. Transthyretin (TTR) has been identified in the blood across different lineages in adults vs juveniles. TTR has intermediate affinity for THs. Thyroxine-binding globulin has only been identified in mammals and has high affinity for THs. Of these THDPs, TTR is the only one known to be synthesised in the brain and is involved in moving THs from the blood into the cerebrospinal fluid. We analysed the rates of evolution of these three THDPs: TTR has been most highly conserved and albumin has had the highest rate of divergence.

Thyroid hormone regulation of intestinal epithelial stem cell biology

The gastrointestinal tract is a well-characterized target of thyroid hormones and thyroid hormone nuclear receptors TRs, as extensively described in the literature. The paradigm is its important remodelling in amphibians during thyroid hormone-dependent metamorphosis. Interestingly, several studies have described the conservation of this hormonal signal during intestinal development in mammals. Additional data suggested that it may also play a role in intestinal homeostasis, stem cell physiology and progenitor commitment as well as in tumour development. It is worth underlining that in the mammalian intestine the functionality of the TRα1 receptor is coordinated and integrated with other signalling pathways, such as Wnt and Notch, specifically at the level of stem/progenitor cell populations. Here, we summarize these data and concepts and discuss this new role for thyroid hormones and the TRα1 receptor in the biology of intestinal epithelial precursor cells.

Neuronal effects of thyroid hormone metabolites

Thyroid hormones and their metabolites are active regulators of gene expression, mitochondrial function and various other physiological actions in different organs and tissues. These actions are mediated by a spatio-temporal regulation of thyroid hormones and metabolites within a target cell. This spatio-temporal resolution as well as classical and non-classical actions of thyroid hormones and metabolites is accomplished and regulated on multiple levels as uptake, local activation and signaling of thyroid hormones. In this review, we will give an overview of the systems involved in regulating the presence and activity of thyroid hormones and their metabolites within the brain, specifically in neurons. While a wealth of data on thyroxin (T₄) and 3,5,3'-triiodothyronine (T₃) in the brain has been generated, research into the presence of action of other thyroid hormone metabolites is still sparse and requires further investigations.

Thyroid hormones derivatives reduce proliferation and induce cell death and DNA damage in ovarian cancer

Ovarian cancer is a highly aggressive disease and novel treatments are required. Thyroid hormones binding to αvβ3 integrin produced growth-promoting activities in ovarian cancer and we hypothesized that natural thyroid hormone derivatives may antagonize these actions. The effect of three antagonists, tetraiodoacetic acid (tetrac), triiodothyroacetic acid (triac) and 3-iodothyronamine (T1AM), on cell proliferation, cell death and DNA damage was studied in two ovarian cancer cell lines (OVCAR3 and A2780), normal hamster ovary control cells (CHOK1) and αvβ3-deficient or transfected HEK293 cells. A differential inhibition of cell proliferation was observed in ovarian cancer cells compared to CHOK1. In OVCAR3, an induction of cell cycle regulators was further shown. Apoptosis was confirmed (annexin-PI, SubG1/cell-cycle, apoptotic genes, caspase-3 and poly ADP ribose polymerase-1 (PARP-1) cleavage) and was reversed by a pan-caspase inhibitor. Induction in apoptosis inducing factor (AIF) was observed, suggesting a parallel caspase-independent mechanism. Integrin-involvement in triac/T1AM apoptotic action was shown in αvβ3-transfected HEK293 cells. Lastly, in ovarian cancer models, key proteins that coordinate recognition of DNA damage, ataxia-telangiectasia mutated (ATM) and PARP-1, were induced. To conclude, the cytotoxic potential of thyroid hormone derivatives, tetrac, triac and T1AM, in ovarian cancer may provide a much-needed novel therapeutic approach.

Role of thyroid hormones in the neoplastic process: an overview

Thyroid hormones (TH) are critical regulators of several physiological processes, which include development, differentiation and growth in virtually all tissues. In past decades, several studies have shown that changes in TH levels caused by thyroid dysfunction, disruption of deiodinases and/or thyroid hormone receptor (TR) expression in tumor cells, influence cell proliferation, differentiation, survival and invasion in a variety of neoplasms in a cell type-specific manner. The function of THs and TRs in neoplastic cell proliferation involves complex mechanisms that seem to be cell specific, exerting effects via genomic and nongenomic pathways, repressing or stimulating transcription factors, influencing angiogenesis and promoting invasiveness. Taken together, these observations indicate an important role of TH status in the pathogenesis and/or development of human neoplasia. Here, we aim to present an updated and comprehensive picture of the accumulated knowledge and the current understanding of the potential role of TH status on the different hallmarks of the neoplastic process.

The Heart of the Matter: Cardiac Manifestations of Endocrine Disease

Endocrine disorders manifest as a disturbance in the milieu of multiple organ systems. The cardiovascular system may be directly affected or alter its function to maintain the state of homeostasis. In this article, we aim to review the pathophysiology, diagnosis, clinical features and management of cardiac manifestations of various endocrine disorders.

Novel aspects of T3 actions on GH and TSH synthesis and secretion: physiological implications

Thyroid hormones (THs) classically regulate the gene expression by transcriptional mechanisms. In pituitary, the encoding genes for growth hormone (GH) and thyroid-stimulating hormone (TSH) are examples of genes regulated by triiodothyronine (T₃) in a positive and negative way, respectively. Recent studies have shown a rapid adjustment of GH and TSH synthesis/secretion induced by T₃ posttranscriptional actions. In somatotrophs, T₃ promotes an increase in Gh mRNA content, poly(A) tail length and binding to the ribosome, associated with a rearrangement of actin cytoskeleton. In thyrotrophs, T₃ reduces Tshb mRNA content, poly(A) tail length and its association with the ribosome. In parallel, it promotes a redistribution of TSH secretory granules to more distal regions of the cell periphery, indicating a rapid effect of T₃ inhibition of TSH secretion. T₃ was shown to affect the content of tubulin and the polymerization of actin and tubulin cytoskeletons in the whole anterior pituitary gland, and to increase intracellular alpha (CGA) content. This review summarizes genomic and non-genomic/posttranscriptional actions of TH on the regulation of several steps of GH and TSH synthesis and secretion. These distinct mechanisms induced by T₃ can occur simultaneously, even though non-genomic effects are promptly elicited and precede the genomic actions, coexisting in a functional network within the cells.

Molecular insights into the transcriptional regulatory role of thyroid hormones in ovarian cancer

The regulation of cancer-relevant genes by the thyroid hormones, 3, 5, 3'-Triiodo-L-thyronine (T3) and L-thyroxine (T4), was recently acknowledged. However, limited data exists on the hormonal effects on gene expression in ovarian cancer, a gynecological malignancy associated with a low cure rate. The expression of fifteen genes involved in DNA repair, cell cycle, apoptosis, and tumor suppression was evaluated in OVCAR-3 and A2780 cell lines, using real-time PCR following short incubation with T3 (1 nM) or T4 (100 nM). The thyroid hormones downregulated the expression of the majority of genes examined. Support for the involvement of the MAPK and PI3K in thyroid hormone-mediated gene expression was shown for a set of genes. FAS expression was inhibited in A2780 cells, while an unexpected induction was demonstrated in OVCAR-3 cells. An analogous effect on the protein levels of FAS receptor and its soluble form was demonstrated by Western blotting. We further established, using primer sets that discriminate between the different RNA isoforms, that the hormones increase the mRNA levels of both coding and non-coding FAS mRNAs. The prevalence of these isoforms, using The Cancer Genome Atlas (TCGA) analysis, was significantly more abundant in 17 cancer types, including ovarian cancer, compared to normal tissues. Our results highlight the role of thyroid hormones in the expression of cancer-relevant-genes in ovarian cancer and provide an important insight into the pathways by which mitogenic and anti-apoptotic effects are exerted.

Lower TSH and higher free thyroxine predict incidence of prostate but not breast, colorectal or lung cancer

CONTEXT

Thyroid hormones modulate proliferative, metabolic and angiogenic pathways. However few studies have examined associations of thyroid hormones with cancer risk.

OBJECTIVES

To explore associations of thyrotropin (TSH), free thyroxine (FT4) and anti-thyroperoxidase antibodies (TPOAb) with the incidence of all (non-skin) cancers and specific common cancers.

DESIGN AND SETTING

A prospective cohort study of a community-dwelling population aged 25-84 years in Western Australia.

MAIN OUTCOME MEASURES

Archived sera from 3649 participants in the 1994/1995 Busselton Health Survey were assayed for TSH, FT4 and TPOAb. Cancer outcomes until 30 June 2014 were ascertained using data linkage. Longitudinal analyses were performed using Cox proportional hazards regression.

RESULTS

During 20-year follow-up, 600 participants were diagnosed with non-skin cancer, including 126, 100, 103 and 41 prostate, breast, colorectal and lung cancers respectively. Higher TSH was associated with a lower risk of prostate cancer after adjusting for potential confounders, with a 30% lower risk for every 1 IU/L increase in TSH (adjusted hazard ratio (HR): 0.70, 95% confidence interval (CI): 0.55-0.90, P = 0.005). Similarly, higher FT4 was associated with an increased risk of prostate cancer (adjusted HR: 1.11 per 1 pmol/L increase, 95% CI: 1.03-1.19, P = 0.009). There were no associations of TSH, FT4 or TPOAb with all non-skin cancer events combined, or with breast, colorectal or lung cancer.

CONCLUSION

In a community-dwelling population, lower TSH and higher FT4 were associated with an increased risk of prostate cancer. Further studies are required to assess if thyroid function is a biomarker or risk factor for prostate cancer.

Biological Mechanisms by Which Antiproliferative Actions of Resveratrol Are Minimized

Preclinical and clinical studies have offered evidence for protective effects of various polyphenol-rich foods against cardiovascular diseases, neurodegenerative diseases, and cancers. Resveratrol is among the most widely studied polyphenols. However, the preventive and treatment effectiveness of resveratrol in cancer remain controversial because of certain limitations in existing studies. For example, studies of the activity of resveratrol against cancer cell lines in vitro have often been conducted at concentrations in the low μM to mM range, whereas dietary resveratrol or resveratrol-containing wine rarely achieve nM concentrations in the clinic. While the mechanisms underlying the failure of resveratrol to inhibit cancer growth in the intact organism are not fully understood, the interference by thyroid hormones with the anticancer activity of resveratrol have been well documented in both in vitro and xenograft studies. Thus, endogenous thyroid hormones may explain the failure of anticancer actions of resveratrol in intact animals, or in the clinic. In this review, mechanisms involved in resveratrol-induced antiproliferation and effects of thyroid hormones on these mechanisms are discussed.

Triiodothyroacetic acid in health and disease

Thyroid hormone (TH) is crucial for development and metabolism of many tissues. The physiological relevance and therapeutic potential of TH analogs have gained attention in the field for many years. In particular, the relevance and use of 3,3',5-triiodothyroacetic acid (Triac, TA₃) has been explored over the last decades. Although TA₃ closely resembles the bioactive hormone T₃, differences in transmembrane transport and receptor isoform-specific transcriptional activation potency exist. For these reasons, the application of TA₃ as a treatment for resistance to TH (RTH) syndromes, especially MCT8 deficiency, is topic of ongoing research. This review is a summary of all currently available literature about the formation, metabolism, action and therapeutic applications of TA₃.

Mechanisms of action of nonpeptide hormones on resveratrol-induced antiproliferation of cancer cells

Nonpeptide hormones, such as thyroid hormone, dihydrotestosterone, and estrogen, have been shown to stimulate cancer proliferation via different mechanisms. Aside from their cytosolic or membrane-bound receptors, there are receptors on integrin α_v β₃ for nonpeptide hormones. Interaction between hormones and integrin α_v β₃ can induce signal transduction and eventually stimulate cancer cell proliferation. Resveratrol induces inducible COX-2-dependent antiproliferation via integrin α_v β₃ . Resveratrol and hormone-induced signals are both transduced by activated extracellular-regulated kinases 1 and 2 (ERK1/2); however, hormones promote cell proliferation, while resveratrol induces antiproliferation in cancer cells. Hormones inhibit resveratrol-stimulated phosphorylation of p53 on Ser15, resveratrol-induced nuclear COX-2 accumulation, and formation of p53-COX-2 nuclear complexes. Subsequently, hormones impair resveratrol-induced COX-2-/p53-dependent gene expression. The inhibitory effects of hormones on resveratrol action can be blocked by different antagonists of specific nonpeptide hormone receptors but not integrin α_v β₃ blockers. Results suggest that nonpeptide hormones inhibit resveratrol-induced antiproliferation in cancer cells downstream of the interaction between ligand and receptor and ERK1/2 activation to interfere with nuclear COX-2 accumulation. Thus, the surface receptor sites for resveratrol and nonpeptide hormones are distinct and can induce discrete ERK1/2-dependent downstream antiproliferation biological activities. It also indicates the complex pathways by which antiproliferation is induced by resveratrol in various physiological hormonal environments. .

Molecular Basis of Nongenomic Actions of Thyroid Hormone

Nongenomic actions of thyroid hormone are initiated by the hormone at receptors in the plasma membrane, in cytoplasm, or in mitochondria and do not require the interaction of nuclear thyroid hormone receptors (TRs) with their primary ligand, 3,5,3'-triiodo-l-thyronine (T₃). Receptors involved in nongenomic actions may or may not have structural homologies with TRs. Certain nongenomic actions that originate at the plasma membrane may modify the state and function of intranuclear TRs. Reviewed here are nongenomic effects of the hormone-T₃ or, in some cases, l-thyroxine (T₄)-that are initiated at (a) truncated TRα isoforms, e.g., p30 TRα1, (b) cytoplasmic proteins, or (c) plasma membrane integrin αvβ3. p30 TRα1 is not transcriptionally competent, binds T₃ at the cell surface, and consequently expresses a number of important functions in bone cells. Nongenomic hormonal control of mitochondrial respiration involves a TRα isoform, and another truncated TRα isoform nongenomically regulates the state of cellular actin. Cytoplasmic hormone-binding proteins involved in nongenomic actions of thyroid hormone include ketimine reductase, pyruvate kinase, and TRβ that shuttle among intracellular compartments. Functions of the receptor for T₄ on integrin αvβ3 include stimulation of proliferation of cancer and endothelial cells (angiogenesis) and regulation of transcription of cancer cell survival pathway genes. T₄ serves as a prohormone for T₃ in genomic actions of thyroid hormone, but T₄ is a hormone at αvβ3 and more important to cancer cell function than is T₃. Thus, characterization of nongenomic actions of the hormone has served to broaden our understanding of the cellular roles of T₃ and T₄.

Thyroid hormone receptor alpha (TRa) tissue expression in ductal invasive breast cancer: A study combining quantitative immunohistochemistry with digital slide image analysis

BACKGROUND

In breast cancer, hormonal receptors hold promise for developing novel targeted therapies. The thyroid exerts its actions via the thyroid hormone receptors alpha and beta. The clinical significance of the expression of thyroid hormone receptors in breast cancer is unclear.

MATERIAL AND METHODS

We studied thyroid hormone receptor alpha (TRa) expression in 82 samples from 41 women with ductal invasive breast cancer and no thyroid disease. We performed quantitative immunohistochemistry with digital image analysis and correlated TRa expression with clinicopathological parameters.

RESULTS

TRa was expressed in both normal breast epithelium and breast cancer, but expression in breast cancer was significantly lower. TRa was expressed significantly less in larger and grade III tumors. Conversely, breast cancers with lymphovascular invasion showed increased TRa expression compared to cancers without lymphovascular invasion. TRa expression was not significantly different between node-positive and node-negative breast cancers, or among different hormonal profiles and intrinsic subtypes.

DISCUSSION

This is the first-in-human study to combine quantitative immunohistochemistry with image analysis to study TRa expression in women with ductal invasive breast cancer and no clinical or biochemical evidence of thyroid dysfunction. We confirm that TRa is expressed in both normal and malignant breast epithelium and suggest that TRa expression is downregulated during breast carcinogenesis. Larger and higher grade breast cancers demonstrate partial loss in TRa expression. Alterations in TRa expression take place even in the absence of clinical or biochemical thyroid disease. The underlying mechanism of these findings and their potential significance in survival and relapse mandate further research.

Nanoparticulate Tetrac Inhibits Growth and Vascularity of Glioblastoma Xenografts

Thyroid hormone as L-thyroxine (T₄) stimulates proliferation of glioma cells in vitro and medical induction of hypothyroidism slows clinical growth of glioblastoma multiforme (GBM). The proliferative action of T₄ on glioma cells is initiated nongenomically at a cell surface receptor for thyroid hormone on the extracellular domain of integrin αvβ3. Tetraiodothyroacetic acid (tetrac) is a thyroid hormone derivative that blocks T₄ action at αvβ3 and has anticancer and anti-angiogenic activity. Tetrac has been covalently bonded via a linker to a nanoparticle (Nanotetrac, Nano-diamino-tetrac, NDAT) that increases the potency of tetrac and broadens the anticancer properties of the drug. In the present studies of human GBM xenografts in immunodeficient mice, NDAT administered daily for 10 days subcutaneously as 1 mg tetrac equivalent/kg reduced tumor xenograft weight at animal sacrifice by 50%, compared to untreated control lesions (p < 0.01). Histopathological analysis of tumors revealed a 95% loss of the vascularity of treated tumors compared to controls at 10 days (p < 0.001), without intratumoral hemorrhage. Up to 80% of tumor cells were necrotic in various microscopic fields (p < 0.001 vs. control tumors), an effect attributable to devascularization. There was substantial evidence of apoptosis in other fields (p < 0.001 vs. control tumors). Induction of apoptosis in cancer cells is a well-described quality of NDAT. In summary, systemic NDAT has been shown to be effective by multiple mechanisms in treatment of GBM xenografts.

Targeted delivery of paclitaxel and doxorubicin to cancer xenografts via the nanoparticle of nano-diamino-tetrac

The tetraiodothyroacetic acid (tetrac) component of nano-diamino-tetrac (NDAT) is chemically bonded via a linker to a poly(lactic-co-glycolic acid) nanoparticle that can encapsulate anticancer drugs. Tetrac targets the plasma membrane of cancer cells at a receptor on the extracellular domain of integrin αvβ3. In this study, we evaluate the efficiency of NDAT delivery of paclitaxel and doxorubicin to, respectively, pancreatic and breast cancer orthotopic nude mouse xenografts. Intra-tumoral drug concentrations were 5-fold (paclitaxel; P<0.001) and 2.3-fold (doxorubicin; P<0.01) higher than with conventional systemic drug administration. Tumor volume reductions reflected enhanced xenograft drug uptake. Cell viability was estimated by bioluminescent signaling in pancreatic tumors and confirmed an increased paclitaxel effect with drug delivery by NDAT. NDAT delivery of chemotherapy increases drug delivery to cancers and increases drug efficacy.

Targeted delivery of cisplatin to tumor xenografts via the nanoparticle component of nano-diamino-tetrac

Aim: Nano-diamino-tetrac (NDAT) targets a receptor on integrin αvβ3; αvβ3 is generously expressed by cancer cells and dividing endothelial cells and to a small extent by nonmalignant cells. The tetrac (tetraiodothyroacetic acid) of NDAT is covalently bound to a poly(lactic-co-glycolic acid) nanoparticle that encapsulates anticancer drugs. We report NDAT delivery efficiency of cisplatin to agent-susceptible urinary bladder cancer xenografts. Materials & methods: Cisplatin-loaded NDAT (NDAT-cisplatin) was administered to xenograft-bearing nude mice. Tumor size response and drug content were measured. Results: Intratumoral drug concentration was up to fivefold higher (p < 0.001) in NDAT-cisplatin-exposed lesions than with conventional systemic administration. Tumor volume reduction achieved was NDAT-cisplatin > NDAT without cisplatin > cisplatin alone. Conclusion: NDAT markedly enhances cisplatin delivery to urinary bladder cancer xenografts and increases drug efficacy.

In vitro effects of tetraiodothyroacetic acid combined with X-irradiation on basal cell carcinoma cells

We investigated radiosensitization in an untreated basal cell carcinoma (TE.354.T) cell line and post-pretreatment with tetraiodothyroacetic acid (tetrac) X 1 h at 37°C, 0.2 and 2.0 µM tetrac. Radioresistant TE.354.T cells were grown in modified medium containing fibroblast growth factor-2, stem cell factor-1 and a reduced calcium level. We also added reproductively inactivated (30 Gy) “feeder cells” to the medium. The in vitro doubling time was 34.1 h, and the colony forming efficiency was 5.09 percent. These results were therefore suitable for clonogenic radiation survival assessment. The 250 kVp X-ray survival curve of control TE.354.T cells showed linear-quadratic survival parameters of α_X-ray = 0.201 Gy⁻¹ and β_X-ray = 0.125 Gy⁻². Tetrac concentrations of either 0.2 or 2.0 µM produced α_X-ray and β_X-ray parameters of 2.010 and 0.282 Gy⁻¹ and 2.050 and 0.837 Gy⁻², respectively. The surviving fraction at 2 Gy (SF₂) for control cells was 0.581, while values for 0.2 and 2.0 µM tetrac were 0.281 and 0.024. The SF₂ data show that tetrac concentrations of 0.2 and 2.0 µM sensitize otherwise radioresistant TE.354.T cells by factors of 2.1 and 24.0, respectively. Thus, radioresistant basal cell carcinoma cells may be radiosensitized pharmacologically by exposure to tetrac.

Thyroid Hormone Controls Breast Cancer Cell Movement via Integrin αV/β3/SRC/FAK/PI3-Kinases

Thyroid hormones (TH) play a fundamental role in diverse processes, including cellular movement. Cell migration requires the integration of events that induce changes in cell structure towards the direction of migration. These actions are driven by actin remodeling and stabilized by the development of adhesion sites to extracellular matrix via transmembrane receptors linked to the actin cytoskeleton. Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that promotes cell migration and invasion through the control of focal adhesion turnover. In this work, we demonstrate that the thyroid hormone triiodothyronine (T3) regulates actin remodeling and cell movement in breast cancer T-47D cells through the recruitment of FAK. T3 controls FAK phosphorylation and translocation at sites where focal adhesion complexes are assembled. This process is triggered via rapid signaling to integrin αV/β3, Src, phosphatidylinositol 3-OH kinase (PI3K), and FAK. In addition, we established a cellular model with different concentration of T3 levels: normal, absence, and excess in T-47D breast cancer cells. We found that the expression of Src, FAK, and PI3K remained at normal levels in the excess of T3 model, while it was significantly reduced in the absence model. In conclusion, these results suggest a novel role for T3 as an important modulator of cell migration, providing a starting point for the development of new therapeutic strategies for breast cancer treatment.

Torpor: The Rise and Fall of 3-Monoiodothyronamine from Brain to Gut-From Gut to Brain?

3-Monoiodothyronamine (T1AM), first isolated from rat brain, is reported to be an endogenous, rapidly acting metabolite of thyroxine. One of its numerous effects is the induction of a "torpor-like" state in experimental animals. A critical analysis of T1AM, to serve as an endogenous cryogen, is given. The proposed biosynthetic pathway for formation of T1AM, which includes deiodinases and ornithine decarboxylase in the upper intestinum, is an unusual one. To reach the brain via systemic circulation, enterohepatic recycling and passage through the liver may occur. The possible role of gut microbiota is discussed. T1AM concentrations in human serum, measured by a specific monoclonal assay are up to three orders of magnitude higher compared to values obtained by MS/MS technology. The difference is explained by the presence of a high-affinity binder for T1AM (Apolipoprotein B-100) in serum, which permits the immunoassay to measure the total concentration of the analyte but limits MS/MS technology to detect only the unbound (free) analyte, a view, which is contested here.

Thyroid hormones and cardiovascular disease

Myocardial and vascular endothelial tissues have receptors for thyroid hormones and are sensitive to changes in the concentrations of circulating thyroid hormones. The importance of thyroid hormones in maintaining cardiovascular homeostasis can be deduced from clinical and experimental data showing that even subtle changes in thyroid hormone concentrations - such as those observed in subclinical hypothyroidism or hyperthyroidism, and low triiodothyronine syndrome - adversely influence the cardiovascular system. Some potential mechanisms linking the two conditions are dyslipidaemia, endothelial dysfunction, blood pressure changes, and direct effects of thyroid hormones on the myocardium. Several interventional trials showed that treatment of subclinical thyroid diseases improves cardiovascular risk factors, which implies potential benefits for reducing cardiovascular events. Over the past 2 decades, accumulating evidence supports the association between abnormal thyroid function at the time of an acute myocardial infarction (MI) and subsequent adverse cardiovascular outcomes. Furthermore, experimental studies showed that thyroid hormones can have an important therapeutic role in reducing infarct size and improving myocardial function after acute MI. In this Review, we summarize the literature on thyroid function in cardiovascular diseases, both as a risk factor as well as in the setting of cardiovascular diseases such as heart failure or acute MI, and outline the effect of thyroid hormone replacement therapy for reducing the risk of cardiovascular disease.

Actions of l-thyroxine and Nano-diamino-tetrac (Nanotetrac) on PD-L1 in cancer cells

The PD-1 (programmed death-1)/PD-L1 (PD-ligand 1) checkpoint is a critical regulator of activated T cell-cancer cell interactions, defending tumor cells against immune destruction. Nano-diamino-tetrac (NDAT; Nanotetrac) is an anticancer/anti-angiogenic agent targeted to the thyroid hormone-tetrac receptor on the extracellular domain of integrin αvβ3. NDAT inhibits the cancer cell PI3-K and MAPK signal transduction pathways that are critical to PD-L1 gene expression. We examined actions in vitro of thyroid hormone (l-thyroxine, T₄) and NDAT on PD-L1 mRNA abundance (qPCR) and PD-L1 protein content in human breast cancer (MDA-MB-231) cells and colon carcinoma (HCT116 and HT-29) cells. In MDA-MB-231 cells, a physiological concentration of T₄ (10^-7M total; 10^-10M free hormone) stimulated PD-L1 gene expression by 38% and increased PD-L1 protein by 2.7-fold (p<0.05, all changes). NDAT (10^-7M) reduced PD-L1 in T₄-exposed cells by 21% (mRNA) and 39% (protein) (p<0.05, all changes). In HCT116 cells, T₄ enhanced PD-L1 gene expression by 17% and protein content by 24% (p<0.05). NDAT reduced basal PD-L1 mRNA by 35% and protein by 31% and in T₄-treated cells lowered mRNA by 33% and protein by 66%. In HT-29 cells, T₄ increased PD-L1 mRNA by 62% and protein by 27%. NDAT lowered basal and T₄-stimulated responses in PD-L1 mRNA and protein by 35-40% (p<0.05). Activation of ERK1/2 was involved in T₄-induced PD-L1 accumulation. We propose that, by a nongenomic mechanism, endogenous T₄ may clinically support activity of the defensive PD-1/PD-L1 checkpoint in tumor cells. NDAT non-immunologically suppresses basal and T₄-induced PD-L1 gene expression and protein accumulation in cancer cells.

Actions of Thyroid Hormone Analogues on Chemokines

The extracellular domain of plasma membrane integrin αvβ3 contains a receptor for thyroid hormone (L-thyroxine, T₄; 3,5,3′-triiodo-L-thyronine, T₃); this receptor also binds tetraiodothyroacetic acid (tetrac), a derivative of T₄. Tetrac inhibits the binding of T₄ and T₃ to the integrin. Fractalkine (CX3CL1) is a chemokine relevant to inflammatory processes in the CNS that are microglia-dependent but also important to normal brain development. Expression of the CX3CL1 gene is downregulated by tetrac, suggesting that T₄ and T₃ may stimulate fractalkine expression. Independently of its specific receptor (CX3CR1), fractalkine binds to αvβ3 at a site proximal to the thyroid hormone-tetrac receptor and changes the physical state of the integrin. Tetrac also affects expression of the genes for other CNS-relevant chemokines, including CCL20, CCL26, CXCL2, CXCL3, and CXCL10. The chemokine products of these genes are important to vascularity of the brain, particularly of the choroid plexus, to inflammatory processes in the CNS and, in certain cases, to neuroprotection. Thyroid hormones are known to contribute to regulation of each of these CNS functions. We propose that actions of thyroid hormone and hormone analogues on chemokine gene expression contribute to regulation of inflammatory processes in brain and of brain blood vessel formation and maintenance.

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

Thyroid hormone and anti-apoptosis in tumor cells

The principal secretory product of the thyroid gland, L-thyroxine (T₄), is anti-apoptotic at physiological concentrations in a number of cancer cell lines. Among the mechanisms of anti-apoptosis activated by the hormone are interference with the Ser-15 phosphorylation (activation) of p53 and with TNFα/Fas-induced apoptosis. The hormone also decreases cellular abundance and activation of proteolytic caspases and of BAX and causes increased expression of X-linked inhibitor of apoptosis (XIAP). The anti-apoptotic effects of thyroid hormone largely are initiated at a cell surface thyroid hormone receptor on the extracellular domain of integrin αvβ3 that is amply expressed and activated in cancer cells. Tetraiodothyroacetic acid (tetrac) is a T₄ derivative that, in a model of resveratrol-induced p53-dependent apoptosis in glioma cells, blocks the anti-apoptotic action of thyroid hormone, permitting specific serine phosphorylation of p53 and apoptosis to proceed. In a nanoparticulate formulation limiting its action to αvβ3, tetrac modulates integrin-dependent effects on gene expression in human cancer cell lines that include increased expression of a panel of pro-apoptotic genes and decreased transcription of defensive anti-apoptotic XIAP and MCL1 genes. By a variety of mechanisms, thyroid hormone (T₄) is an endogenous anti-apoptotic factor that may oppose chemotherapy-induced apoptosis in αvβ3-expressing cancer cells. It is possible to decrease this anti-apoptotic activity pharmacologically by reducing circulating levels of T₄ or by blocking effects of T₄ that are initiated at αvβ3.

Low thyroid hormone levels improve survival in murine model for ocular melanoma

Uveal melanoma is highly metastatic, prognosis is poor and there are no effective treatments to extend survival. Accumulating evidence suggests that thyroid hormones have a mitogenic effect via binding to αvβ3 integrin. We aimed to examine the impact of thyroid status on survival in a murine B16F10 model for ocular melanoma, highly expressing the integrin. In two independent experiments oral propylthiouracil (PTU) was used to induce hypothyroidism (n=9), thyroxine to induce hyperthyroidism (n=11) and mice given plain water served as control (n=8). At day 21, the subretinal space was inoculated with 10(2) B16F10 cells. In non-inoculated mice (n=6 of each group) serum free T4 (FT4) levels were measured and additional non-inoculated mice (3 given PTU and 4 given thyroxine or water) served as internal control to demonstrate the impact of the dissolved substance. The PTU-inoculated mice showed clinical evidence of intraocular tumor growth significantly later than the thyroxine mice (P=0.003) and survival time was significantly longer (P<0.001). FT4 levels differed significantly between groups (P<0.001) and with no signs of illness in the internal control group. Our findings suggest that hyperthyroidism shortens survival, whereas relative hypothyroidism may have a protective role in metastatic ocular melanoma.

Thyroid hormones and their membrane receptors as therapeutic targets for T cell lymphomas

Thyroid hormones (THs) are important regulators of metabolism, differentiation and cell proliferation. They can modify the physiology of human and murine T cell lymphomas (TCL). These effects involve genomic mechanisms, mediated by specific nuclear receptors (TR), as well as nongenomic mechanisms, that lead to the activation of different signaling pathways through the activation of a membrane receptor, the integrin αvβ3. Therefore, THs are able to induce the survival and growth of TCL. Specifically, the signaling induced by THs through the integrin αvβ3 activates proliferative and angiogenic programs, mediated by the regulation of the vascular endothelial growth factor (VEGF). The genomic or pharmacologic inhibition of integrin αvβ3 reduces the production of VEGF and induces cell death both in vitro and in xenograft models of human TCL. Here we review the mechanisms involved in the modulation of the physiology of TCL induced by THs, the analysis of the interaction between genomic and nongenomic actions of THs and their contribution to T cell lymphomagenesis. These actions of THs suggest a novel mechanism for the endocrine modulation of the physiopathology of TCL and they provide a potential molecular target for its treatment.

Nongenomic actions of thyroid hormone

The nongenomic actions of thyroid hormone begin at receptors in the plasma membrane, mitochondria or cytoplasm. These receptors can share structural homologies with nuclear thyroid hormone receptors (TRs) that mediate transcriptional actions of T3, or have no homologies with TR, such as the plasma membrane receptor on integrin αvβ3. Nongenomic actions initiated at the plasma membrane by T4 via integrin αvβ3 can induce gene expression that affects angiogenesis and cell proliferation, therefore, both nongenomic and genomic effects can overlap in the nucleus. In the cytoplasm, a truncated TRα isoform mediates T4-dependent regulation of intracellular microfilament organization, contributing to cell and tissue structure. p30 TRα1 is another shortened TR isoform found at the plasma membrane that binds T3 and mediates nongenomic hormonal effects in bone cells. T3 and 3,5-diiodo-L-thyronine are important to the complex nongenomic regulation of cellular respiration in mitochondria. Thus, nongenomic actions expand the repertoire of cellular events controlled by thyroid hormone and can modulate TR-dependent nuclear events. Here, we review the experimental approaches required to define nongenomic actions of the hormone, enumerate the known nongenomic effects of the hormone and their molecular basis, and discuss the possible physiological or pathophysiological consequences of these actions.

Thyroid hormone regulates adhesion, migration and matrix metalloproteinase 9 activity via αvβ3 integrin in myeloma cells

Thyroid hormone (3,5,3′-triiodothyronine, T3; L-thyroxine, T4) enhances cancer cell proliferation, invasion and angiogenesis via a discrete receptor located near the RGD recognition site on αvβ3 integrin. Tetraiodothyroacetic acid (tetrac) and its nanoparticulate formulation interfere with binding of T3/T4 to the integrin. This integrin is overexpressed in multiple myeloma (MM) and other cancers. MM cells interact with αvβ3 integrin to support growth and invasion. Matrix metalloproteinases (MMPs) are a family of enzymes active in tissue remodeling and cancer. The association between integrins and MMPs secretion and action is well established. In the current study, we examined the effects of thyroid hormone on myeloma cell adhesion, migration and MMP activity.

We show that T3 and T4 increased myeloma adhesion to fibronectin and induced αvβ3 clustering. In addition, the hormones induced MMP-9 expression and activation via αvβ3 and MAPK induction. Bortezomib, a standard myeloma treatment, caused a decrease in activity/quantity of MMPs and thyroid hormone opposed this effect. RGD peptide and tetrac impaired the production of MMP-9 in cell lines and in primary BM cells from myeloma patients.

In conclusion, thyroid hormone-dependent regulation via αvβ3 of myeloma cell adhesion and MMP-9 production may play a role in myeloma migration and progression.

Thyroid Hormone, Hormone Analogs, and Angiogenesis

Modulation by thyroid hormone and hormone analogs of angiogenesis in the heart after experimental infarction, and in other organs, has been appreciated for decades. Description of a plasma membrane receptor for thyroid hormone on the extracellular domain of integrin αvβ3 on endothelial cells has revealed the complexity of the nongenomic regulation of angiogenesis by the hormone. From αvβ3, the hormone directs transcription of specific vascular growth factor genes, regulates growth factor receptor/growth factor interactions and stimulates endothelial cell migration to a vitronectin cue; these actions are implicated experimentally in tumor-relevant angiogenesis and angioproliferative pulmonary hypertension. Derived from L-thyroxine (T4), tetraiodothyroacetic acid (tetrac) can be covalently bound to a polymer and as Nanotetrac acts exclusively at the hormone receptor on αvβ3 to block actions of T4 and 3,5,3'-triiodo-L-thyronine (T3) on angiogenesis. Other antiangiogenic actions of Nanotetrac include disruption of crosstalk between integrin αvβ3 and adjacent cell surface vascular growth factor receptors, resulting in disordered vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF; FGF2) actions at their respective plasma membrane receptors. From αvβ3, Nanotetrac also downregulates expression of VEGFA and epidermal growth factor receptor (EGFR) genes, upregulates transcription of the angiogenesis suppressor gene, thrombospondin 1 (THBS1; TSP1) and decreases cellular abundance of Ang-2 protein and matrix metalloproteinase-9. Existence of this receptor provides new insights into the multiple mechanisms by which thyroid hormone and hormone analogs may regulate angiogenesis at the molecular level. The receptor also offers pharmacological opportunities for interruption of pathological angiogenesis via integrin αvβ3.

Thyroid hormones and tetrac: new regulators of tumour stroma formation via integrin αvβ3

To improve our understanding of non-genomic, integrin αvβ3-mediated thyroid hormone action in tumour stroma formation, we examined the effects of triiodo-l-thyronine (T3), l-thyroxine (T4) and integrin-specific inhibitor tetrac on differentiation, migration and invasion of mesenchymal stem cells (MSCs) that are an integral part of the tumour's fibrovascular network. Primary human bone marrow-derived MSCs were treated with T3 or T4 in the presence of hepatocellular carcinoma (HCC) cell-conditioned medium (CM), which resulted in stimulation of the expression of genes associated with cancer-associated fibroblast-like differentiation as determined by qPCR and ELISA. In addition, T3 and T4 increased migration of MSCs towards HCC cell-CM and invasion into the centre of three-dimensional HCC cell spheroids. All these effects were tetrac-dependent and therefore integrin αvβ3-mediated. In a subcutaneous HCC xenograft model, MSCs showed significantly increased recruitment and invasion into tumours of hyperthyroid mice compared to euthyroid and, in particular, hypothyroid mice, while treatment with tetrac almost completely eliminated MSC recruitment. These studies significantly improve our understanding of the anti-tumour activity of tetrac, as well as the mechanisms that regulate MSC differentiation and recruitment in the context of tumour stroma formation, as an important prerequisite for the utilisation of MSCs as gene delivery vehicles.

Type 2 Iodothyronine Deiodinase Activity Is Required for Rapid Stimulation of PI3K by Thyroxine in Human Umbilical Vein Endothelial Cells

Thyroid hormones (THs) exert a number of physiological effects on the cardiovascular system. Some of the nongenomic actions of T3 are achieved by cross coupling the TH receptor (TR) with the phosphatidylinositol 3-kinase (PI3K)/protein kinase Akt (Akt) pathway. We observed that both T3 and T4 rapidly stimulated Akt phosphorylation and Ras-related C3 botulinum toxin substrate 1 (Rac1) activation, which resulted in cell migration, in a PI3K-dependent manner in human umbilical vein endothelial cells (HUVECs). We identified the expression of type 2 iodothyronine deiodinase (D2), which converts T4 to T3, and TRα1 in HUVECs. D2 activity was significantly stimulated by (Bu)2cAMP in HUVECs. The blockade of D2 activity through transfection of small interfering RNA (siRNA) specific to D2 as well as by addition of iopanoic acid, a potent D2 inhibitor, abolished Akt phosphorylation, Rac activation, and cell migration induced by T4 but not by T3. The inhibition of TRα1 expression by the transfection of siRNA for TRα1 canceled Akt phosphorylation, Rac activation, and cell migration induced by T3 and T4. These findings suggest that conversion of T4 to T3 by D2 is required for TRα1/PI3K-mediated nongenomic actions of T4 in HUVECs, including stimulation of Akt phosphorylation and Rac activation, which result in cell migration.

Overlapping nongenomic and genomic actions of thyroid hormone and steroids

The genomic actions of thyroid hormone and steroids depend upon primary interactions of the hormones with their specific nuclear receptor proteins. Formation of nuclear co-activator or co-repressor complexes involving the liganded receptors subsequently result in transcriptional events-either activation or suppression-at genes that are specific targets of thyroid hormone or steroids. Nongenomic actions of thyroid hormone and steroids are in contrast initiated at binding sites on the plasma membrane or in cytoplasm or organelles and do not primarily require formation of intranuclear receptor protein-hormone complexes. Importantly, hormonal actions that begin nongenomically outside the nucleus often culminate in changes in nuclear transcriptional events that are regulated by both traditional intranuclear receptors as well as other nuclear transcription factors. In the case of thyroid hormone, the extranuclear receptor can be the classical "nuclear" thyroid receptor (TR), a TR isoform, or integrin αvβ3. In the case of steroid hormones, the membrane receptor is usually, but not always, the classical "nuclear" steroid receptor. This concept defines the paradigm of overlapping nongenomic and genomic hormone mechanisms of action. Here we review some examples of how extranuclear signaling by thyroid hormone and by estrogens and androgens modulates intranuclear hormone signaling to regulate a number of vital biological processes both in normal physiology and in cancer progression. We also point out that nongenomic actions of thyroid hormone may mimic effects of estrogen in certain tumors.

The thyroid hormone-αvβ3 integrin axis in ovarian cancer: regulation of gene transcription and MAPK-dependent proliferation

Ovarian carcinoma is the fifth common cause of cancer death in women, despite advanced therapeutic approaches. αvβ3 integrin, a plasma membrane receptor, binds thyroid hormones (L-thyroxine, T4; 3,5,3'-triiodo-L-thyronine, T3) and is overexpressed in ovarian cancer. We have demonstrated selective binding of fluorescently labeled hormones to αvβ3-positive ovarian cancer cells but not to integrin-negative cells. Physiologically relevant T3 (1 nM) and T4 (100 nM) concentrations in OVCAR-3 (high αvβ3) and A2780 (low αvβ3) cells promoted αv and β3 transcription in association with basal integrin levels. This transcription was effectively blocked by RGD (Arg-Gly-Asp) peptide and neutralizing αvβ3 antibodies, excluding T3-induced β3 messenger RNA, suggesting subspecialization of T3 and T4 binding to the integrin receptor pocket. We have provided support for extracellular regulated kinase (ERK)-mediated transcriptional regulation of the αv monomer by T3 and of β3 monomer by both hormones and documented a rapid (30-120 min) and dose-dependent (0.1-1000 nM) ERK activation. OVCAR-3 cells and αvβ3-deficient HEK293 cells treated with αvβ3 blockers confirmed the requirement for an intact thyroid hormone-integrin interaction in ERK activation. In addition, novel data indicated that T4, but not T3, controls integrin's outside-in signaling by phosphorylating tyrosine 759 in the β3 subunit. Both hormones induced cell proliferation (cell counts), survival (Annexin-PI), viability (WST-1) and significantly reduced the expression of genes that inhibit cell cycle (p21, p16), promote mitochondrial apoptosis (Nix, PUMA) and tumor suppression (GDF-15, IGFBP-6), particularly in cells with high integrin expression. At last, we have confirmed that hypothyroid environment attenuated ovarian cancer growth using a novel experimental platform that exploited paired euthyroid and severe hypothyroid serum samples from human subjects. To conclude, our data define a critical role for thyroid hormones as potent αvβ3-ligands, driving ovarian cancer cell proliferation and suggest that disruption of this axis may present a novel treatment strategy in this aggressive disease.

Thyroid hormone and P-glycoprotein in tumor cells

P-glycoprotein (P-gp; multidrug resistance pump 1, MDR1; ABCB1) is a plasma membrane efflux pump that when activated in cancer cells exports chemotherapeutic agents. Transcription of the P-gp gene (MDR1) and activity of the P-gp protein are known to be affected by thyroid hormone. A cell surface receptor for thyroid hormone on integrin αvβ3 also binds tetraiodothyroacetic acid (tetrac), a derivative of L-thyroxine (T₄) that blocks nongenomic actions of T₄ and of 3,5,3′-triiodo-L-thyronine (T₃) at αvβ3. Covalently bound to a nanoparticle, tetrac as nanotetrac acts at the integrin to increase intracellular residence time of chemotherapeutic agents such as doxorubicin and etoposide that are substrates of P-gp. This action chemosensitizes cancer cells. In this review, we examine possible molecular mechanisms for the inhibitory effect of nanotetrac on P-gp activity. Mechanisms for consideration include cancer cell acidification via action of tetrac/nanotetrac on the Na⁺/H⁺ exchanger (NHE1) and hormone analogue effects on calmodulin-dependent processes and on interactions of P-gp with epidermal growth factor (EGF) and osteopontin (OPN), apparently via αvβ3. Intracellular acidification and decreased H⁺ efflux induced by tetrac/nanotetrac via NHE1 is the most attractive explanation for the actions on P-gp and consequent increase in cancer cell retention of chemotherapeutic agent-ligands of MDR1 protein.

Transport of thyroid hormones via the choroid plexus into the brain: the roles of transthyretin and thyroid hormone transmembrane transporters

Thyroid hormones are key players in regulating brain development. Thus, transfer of appropriate quantities of thyroid hormones from the blood into the brain at specific stages of development is critical. The choroid plexus forms the blood-cerebrospinal fluid barrier. In reptiles, birds and mammals, the main protein synthesized and secreted by the choroid plexus is a thyroid hormone distributor protein: transthyretin. This transthyretin is secreted into the cerebrospinal fluid and moves thyroid hormones from the blood into the cerebrospinal fluid. Maximal transthyretin synthesis in the choroid plexus occurs just prior to the period of rapid brain growth, suggesting that choroid plexus-derived transthyretin moves thyroid hormones from blood into cerebrospinal fluid just prior to when thyroid hormones are required for rapid brain growth. The structure of transthyretin has been highly conserved, implying strong selection pressure and an important function. In mammals, transthyretin binds T4 (precursor form of thyroid hormone) with higher affinity than T3 (active form of thyroid hormone). In all other vertebrates, transthyretin binds T3 with higher affinity than T4. As mammals are the exception, we should not base our thinking about the role of transthyretin in the choroid plexus solely on mammalian data. Thyroid hormone transmembrane transporters are involved in moving thyroid hormones into and out of cells and have been identified in many tissues, including the choroid plexus. Thyroid hormones enter the choroid plexus via thyroid hormone transmembrane transporters and leave the choroid plexus to enter the cerebrospinal fluid via either thyroid hormone transmembrane transporters or via choroid plexus-derived transthyretin secreted into the cerebrospinal fluid. The quantitative contribution of each route during development remains to be elucidated. This is part of a review series on ontogeny and phylogeny of brain barrier mechanisms.

Medically induced euthyroid hypothyroxinemia may extend survival in compassionate need cancer patients: an observational study

BACKGROUND

Clinical studies have shown that interventional lowering of serum free thyroxine (FT4) may be associated with extended survival in patients with some terminal cancers. The report of success with this approach in glioblastoma multiforme caused involvement of the author (A.H.) in the prospective consultative management of 23 end-stage solid tumor patients in whom hypothyroxinemia was induced to prolong life.

PATIENTS AND METHODS

Patients were self-referred or recommended by attending physicians to the author (A.H.) and had advanced cancers of the brain, ovary, lung, pancreas, salivary gland, and breast or had mesothelioma or soft-tissue sarcoma. Hypothyroxinemia was achieved in euthyroid patients by using methimazole, with the addition of 3,3',5-triiodo-L-thyronine (L-T3) to prevent hypothyroidism and suppress endogenous thyrotropin (TSH). In patients with pre-existent primary hypothyroidism, T3 administration was substituted for T4 replacement. Serum FT4 and TSH concentrations were serially monitored to enable adjustments to drug therapy and prevent clinical hypothyroidism. Survival was measured from the date of hypothyroxinemia induction with T3 or methimazole plus T3. Outcomes were compared with the odds of death based on the Surveillance Epidemiology and End Results and American Joint Committee on Cancer databases and literature reports.

RESULTS

The survival time of 83% (19 of 23) of patients exceeded the 20% expected 1-year survival for this hypothyroxinemic, end-stage cancer group. The difference between actual and expected survival was significant.

CONCLUSION

Although this is an uncontrolled observational experience with frank limitations, compassionate medical induction of hypothyroxinemia should be considered for patients with advanced cancers to whom other avenues of treatment are closed.