A novel thyroid stimulating hormone beta-subunit isoform in human pituitary, peripheral blood leukocytes, and thyroid

Unconventional Actions of Glycoprotein Hormone Subunits: A Comprehensive Review

The glycoprotein hormones (GPH) are heterodimers composed of a common α subunit and a specific β subunit. They act by activating specific leucine-rich repeat G protein-coupled receptors. However, individual subunits have been shown to elicit responses in cells devoid of the receptor for the dimeric hormones. The α subunit is involved in prolactin production from different tissues. The human chorionic gonadotropin β subunit (βhCG) plays determinant roles in placentation and in cancer development and metastasis. A truncated form of the thyrotropin (TSH) β subunit is also reported to have biological effects. The GPH α- and β subunits are derived from precursor genes (gpa and gpb, respectively), which are expressed in most invertebrate species and are still represented in vertebrates as GPH subunit paralogs (gpa2 and gpb5, respectively). No specific receptor has been found for the vertebrate GPA2 and GPB5 even if their heterodimeric form is able to activate the TSH receptor in mammals. Interestingly, GPA and GPB are phylogenetically and structurally related to cysteine-knot growth factors (CKGF) and particularly to a group of antagonists that act independently on any receptor. This review article summarizes the observed actions of individual GPH subunits and presents the current hypotheses of how these actions might be induced. New approaches are also proposed in light of the evolutionary relatedness with antagonists of the CKGF family of proteins.

TSH promotes adiposity by inhibiting the browning of white fat

Adiposity is caused by an imbalance between energy intake and consumption. Promotion of the browning of white fat increases energy expenditure and could combat adiposity. Thyroid-stimulating hormone (TSH) has been confirmed to positively correlate with adiposity. However, the putative connection between TSH and white adipose browning has never been explored. In this study, we sought to assess the effect of TSH on white adipose tissue browning and energy metabolism. Subclinical hypothyroidism mice, thyroid-specific Tshr-knockout mice injected with TSH, adipocyte-specific and global Tshr-knockout micewere subjected to morphological, physiological, genetic or protein expression analyses and metabolic cages to determine the role of TSH on the browning of white adipose tissue and metabolism. 3T3-L1 and primary SVF cells were used to verify the effects and mechanism of TSH on the browning of white adipocytes. We show that increased circulation TSH level decreases energy expenditure, promotes adiposity, impairs glucose and lipid metabolism. Knockout of Tshr decreases adiposity, increases energy expenditureand markedly promotes the development of beige adipocytesin both epididymal and inguinal subcutaneous white fat via a mechanism that likely involves AMPK/PRDM16/PGC1α. Our results reveal an important role of TSH in regulating energy balance and adiposity by inhibiting the browning of white fat.

The Human TSHβ Subunit Proteins and Their Binding Sites on the TSH Receptor Using Molecular Dynamics Simulation

To gain further insight into the binding of the normal and variant human TSHβ subunits (TSHβ and TSHβv), we modeled the 2 monomeric proteins and studied their interaction with the TSH receptor ectodomain (TSHR-ECD) using molecular dynamics simulation Furthermore, analyzed their bioactivity in vitro using recombinant proteins to confirm that such binding was physiologically relevant. Examining the interaction of TSHβ and TSHβv with the TSHR-ECD model using molecular dynamic simulation revealed strong binding of these proteins to the receptor ECD. The specificity of TSHβ and TSHβv binding to the TSHR-ECD was examined by analyzing the hydrogen-bonding residues of these subunits to the FSH receptor ECD, indicating the inability of these molecules to bind to the FSH receptors. Furthermore, the modelling suggests that TSHβ and TSHβv proteins clasped the concave surface of the leucine rich region of the TSHR ECD in a similar way to the native TSH using dynamic hydrogen bonding. These mutually exclusive stable interactions between the subunits and ECD residues included some high-affinity contact sites corresponding to binding models of native TSH. Furthermore, we cloned TSHβ and TSHβv proteins using the entire coding ORF and purified the flag-tagged proteins. The expressed TSHβ subunit proteins retained bioactivity both in a coculture system as well as with immune-purified proteins. In summary, we showed that such interactions can result in a functional outcome and may exert physiological or pathophysiological effects in immune cells.

Thyroid stimulating hormone β-subunit splice variant is expressed in all fractional subsets of bone marrow hematopoietic cells and peripheral blood leukocytes and is modulated during bacterial infection

Thyroid stimulating hormone (TSH), a hormone produced in the anterior pituitary, is used to regulate thyroid hormone secretion. It has been known for over three decades that TSH is made by the cells of the immune system; however, the functional role of immune system TSH is unclear. We previously demonstrated that an alternatively-spliced isoform of TSHβ, referred to as the TSHβ splice variant (TSHβv), is the primary form of TSHβ made by hematopoietic cells in mice and humans. Most studies have linked TSHβv expression to myeloid cells of the immune system; however, it has recently been demonstrated that plasma cells in patients with Hashimoto's thyroiditis may be a source of immune system TSHβv. Here, we demonstrate that TSHβv is expressed in bone marrow precursors of lymphoid cells, monocytes, and granulocytes, as well as in mesenteric lymph node (MLN) cells. Plasma cells generated by in vitro culture with bacterial lipopolysaccharide (LPS), and MLN cells from mice infected with L. monocytogenes expressed TSHβv. There was an increase in the intensity of intracellular TSHβv expression in MLN cells following exposure to LPS, and in the proportion of TSHβv⁺ CD138⁺ MLN cells following L. monocytogenes infection. The number of TSHβv⁺ cells increased in MLN cells, particularly among CD138⁺ cells, following bacterial infection. This was confirmed by an increase in gene expression of BLIMP-1, the transcription factor for CD138, following infection. Levels of circulating thyroxine dropped significantly in mice 24 hrs post-infection. These findings suggest that immune system TSHβv may contribute to the host immune response during bacterial infection.

HPT axis‑independent TSHβ splice variant regulates the synthesis of thyroid hormone in mice

Thyroid stimulating hormone (TSH) consists of an α‑subunit and a unique β‑subunit. The first in‑frame TSHβ splice variant produced by the cells of immune system was identified in 2009. The TSHβ splice variant and native TSHβ exhibit different expression profiles, and research has been conducted to elucidate the role of the TSHβ splice variant in different diseases. However, understanding of the fundamental physiological characteristics of the TSHβ splice variant is currently limited. To verify whether the TSHβ splice variant has the potential to induce thyroid follicular cells to synthesize thyroid hormone, in vivo and in vitro stimulation experiments were conducted in the present study. A total of 60 C57BL/6 mice were divided into control‑, 5 and 10 µg TSHβ splice variant‑treated groups at random. Mice were sacrificed at 0.5, 1 and 4 h after intraperitoneal injection, and serum levels of tri‑iodothyronine (T3) and thyroxine (T4) were determined using a radioimmunoassay. Thyroid follicular cells were isolated from the thyroids of mice, and stimulated with 2 µg/ml TSHβ splice variant. Supernatants were collected, and the levels of T3 and T4 were detected. The protein expression levels of the sodium‑iodide symporter, thyroperoxidase and thyroglobulin in thyroid follicular cells were quantified using western blot analysis. To verify whether the TSHβ splice variant expression was regulated by the hypothalamus‑pituitary‑thyroid (HPT) axis, similar to native TSHβ, a total of 60 C57BL/6 mice were equally divided into control, 2 mg/kg T3 intraperitoneal injection and 0.05 mg/kg thyroid‑releasing hormone intraperitoneal injection groups at random. Mice were sacrificed at 1 and 4 h after injection. Alterations in the expression of the TSHβ splice variant in the pituitary, thyroid, peripheral blood leukocytes and spleen tissues were detected using western blot analysis. The present study demonstrated that the TSHβ splice variant is not regulated by the HPT axis and may affect thyroid hormone synthesis. Modifications in the expression of the TSHβ splice variant may occur in a uniquely regulated manner to provide peripheral immunological compartments with a source of activated cells, particularly under immune stress.

Novel Splicing of Immune System Thyroid Stimulating Hormone β-Subunit-Genetic Regulation and Biological Importance

Thyroid stimulating hormone (TSH), a glycoprotein hormone produced by the anterior pituitary, controls the production of thyroxine (T₄) and triiodothyronine (T₃) in the thyroid. TSH is also known to be produced by the cells of the immune system; however, the physiological importance of that to the organism is unclear. We identified an alternatively-spliced form of TSHβ that is present in both humans and mice. The TSHβ splice variant (TSHβv), although produced at low levels by the pituitary, is the primary form made by hematopoietic cells in the bone marrow, and by peripheral leukocytes. Recent studies have linked TSHβv functionally to a number of health-related conditions, including enhanced host responses to infection and protection against osteoporosis. However, TSHβv also has been associated with autoimmune thyroiditis in humans. Yet to be identified is the process by which the TSHβv isoform is produced. Here, a set of genetic steps is laid out through which human TSHβv is generated using splicing events that result in a novel transcript in which exon 2 is deleted, exon 3 is retained, and the 3' end of intron 2 codes for a signal peptide of the TSHβv polypeptide.

Actions of pituitary hormones beyond traditional targets

Studies over the past decade have challenged the long-held belief that pituitary hormones have singular functions in regulating specific target tissues, including master hormone secretion. Our discovery of the action of thyroid-stimulating hormone (TSH) on bone provided the first glimpse into the non-traditional functions of pituitary hormones. Here we discuss evolving experimental and clinical evidence that growth hormone (GH), follicle-stimulating hormone (FSH), adrenocorticotrophic hormone (ACTH), prolactin, oxytocin and arginine vasopressin (AVP) regulate bone and other target tissues, such as fat. Notably, genetic and pharmacologic FSH suppression increases bone mass and reduces body fat, laying the framework for targeting the FSH axis for treating obesity and osteoporosis simultaneously with a single agent. Certain 'pituitary' hormones, such as TSH and oxytocin, are also expressed in bone cells, providing local paracrine and autocrine networks for the regulation of bone mass. Overall, the continuing identification of new roles for pituitary hormones in biology provides an entirely new layer of physiologic circuitry, while unmasking new therapeutic targets.

Expanding the Role of Thyroid-Stimulating Hormone in Skeletal Physiology

The dogma that thyroid-stimulating hormone (TSH) solely regulates the production of thyroid hormone from the thyroid gland has hampered research on its wider physiological roles. The action of pituitary TSH on the skeleton has now been well described; in particular, its action on osteoblasts and osteoclasts. It has also been recently discovered that the bone marrow microenvironment acts as an endocrine circuit with bone marrow-resident macrophages capable of producing a novel TSH-β subunit variant (TSH-βv), which may modulate skeletal physiology. Interestingly, the production of this TSH-βv is positively regulated by T3 accentuating such modulation in the presence of thyroid overactivity. Furthermore, a number of small molecule ligands acting as TSH agonists, which allosterically modulate the TSH receptor have been identified and may have similar modulatory influences on bone cells suggesting therapeutic potential. This review summarizes our current understanding of the role of TSH, TSH-β, TSH-βv, and small molecule agonists in bone physiology.

T3 Regulates a Human Macrophage-Derived TSH-β Splice Variant: Implications for Human Bone Biology

TSH and thyroid hormones (T3 and T4) are intimately involved in bone biology. We have previously reported the presence of a murine TSH-β splice variant (TSH-βv) expressed specifically in bone marrow-derived macrophages and that exerted an osteoprotective effect by inducing osteoblastogenesis. To extend this observation and its relevance to human bone biology, we set out to identify and characterize a TSH-β variant in human macrophages. Real-time PCR analyses using human TSH-β-specific primers identified a 364-bp product in macrophages, bone marrow, and peripheral blood mononuclear cells that was sequence verified and was homologous to a human TSH-βv previously reported. We then examined TSH-βv regulation using the THP-1 human monocyte cell line matured into macrophages. After 4 days, 46.1% of the THP-1 cells expressed the macrophage markers CD-14 and macrophage colony-stimulating factor and exhibited typical morphological characteristics of macrophages. Real-time PCR analyses of these cells treated in a dose-dependent manner with T3 showed a 14-fold induction of human TSH-βv mRNA and variant protein. Furthermore, these human TSH-βv-positive cells, induced by T3 exposure, had categorized into both M1 and M2 macrophage phenotypes as evidenced by the expression of macrophage colony-stimulating factor for M1 and CCL-22 for M2. These data indicate that in hyperthyroidism, bone marrow resident macrophages have the potential to exert enhanced osteoprotective effects by oversecreting human TSH-βv, which may exert its local osteoprotective role via osteoblast and osteoclast TSH receptors.

MECHANISMS IN ENDOCRINOLOGY: Impact of isolated TSH levels in and out of normal range on different tissues

Routine treatment of thyroid cancer (TC) includes long-term suppression of TSH. The necessity of this treatment in low- and intermediate-risk patients as well as the extent of TSH suppression is currently under discussion. A literature search was performed to illustrate the role of TSH in extrathyroidal cells and to identify potential reasons for different effects of exogenously suppressed and endogenously low TSH levels. Although adverse effects of subnormal and supranormal TSH blood levels on heart and brain have not been consistently found, studies show a clear negative effect of suppressed TSH levels on bone mineral density. Experimental data also support an important role of TSH in the immune system. The ability of levothyroxine (l-T4) to regulate TSH levels and triiodothyronine levels in a physiological manner is limited. Reduction of circadian changes in TSH levels, decrease of thyroid hormone-binding proteins, prevention of potential compensatory increases of TSH levels (e.g., in old age), and unresponsiveness of TSH-producing cells to TRH on l-T4 treatment might cause adverse effects of suppressed TSH levels. In view of the adverse effects of aggressive TSH suppression, achieving the suggested levels of TSH between 0.9 and 1 mU/l in the treatment of low-to-intermediate risk TC patients appears justified.

Splenic Leukocytes Traffic to the Thyroid and Produce a Novel TSHβ Isoform during Acute Listeria monocytogenes Infection in Mice

The thyroid stimulating hormone beta-subunit (TSHβ) with TSHα form a glycoprotein hormone that is produced by the anterior pituitary in the hypothalamus-pituitary-thyroid (HPT) axis. Although TSHβ has been known for many years to be made by cells of the immune system, the role of immune system TSH has remained unclear. Recent studies demonstrated that cells of the immune system produce a novel splice variant isoform of TSHβ (TSHβv), but little if any native TSHβ. Here, we show that within three days of systemic infection of mice with Listeria monocytogenes, splenic leukocytes synthesized elevated levels of TSHβv. This was accompanied by an influx of CD14⁺, Ly6C⁺, Ly6G⁺ cells into the thyroid of infected mice, and increased levels of intrathyroidal TSHβv gene expression. Adoptive transfer of carboxyfluorescein succinimidyl ester (CFSE)-labeled splenic leukocytes from infected mice into non-infected mice migrated into the thyroid as early as forty-eight hours post-cell transfer, whereas CFSE-labeled cells from non-infected mice failed to traffic to the thyroid. These findings demonstrate for the first time that during bacterial infection peripheral leukocytes produce elevated levels of TSHβv, and that spleen cells traffic to the thyroid where they produce TSHβv intrathyroidally.

Functional human TSHβ splice variant produced by plasma cell may be involved in the immunologic injury of thyroid in the patient with Hashimoto's thyroiditis

Hashimoto's Thyroiditis (HT) is the most common cause of hypothyroidism in areas of the world where iodine levels are sufficient. However, the pathogenesis of HT has not been completely elucidated. The first functional human TSHβ splice variant was supposed to be involved in the pathology of Hashimoto's thyroiditis. The question remains as to which kind of intrathyroid cells expresses functional TSHβ splice variant and whether there are expression variations of functional TSHβ splice variant in the injured thyroid of patient with HT. For the answer to this question, immune-injured thyroids were obtained from 30 patients with HT. Localization study of functional TSHβ splice variant in injured thyroid was done by immunofluorescence double staining. Transcription and translation level of functional TSHβ splice variant were detected by using qRT-PCR and semi-quantitative immunohistochemistry method, respectively. The correlation between expression level of functional TSHβ splice variant and degree of thyroid follicles damage was assessed. It was firstly identified that functional TSHβ splice variant was predominately expressed by plasma cells infiltrated around follicles and germinal center in injured thyroid of patient with HT. Of particular interest, the TSHβ splice variant was expressed at significantly higher levels in the thyroid tissues of patients with HT than that in the normal thyroid tissues, furthermore, expression level of TSHβ splice variant was positive related with the degree of follicles damage in thyroid of patient with HT. These findings defined the immune-derived functional TSHβ splice variant that resided in the thyroid of patient with HT, which exerted the unique effects on the pathogenesis of HT, meanwhile, we considered these findings to have significant implications for understanding immune-endocrine interactions in a number of ways.

Thyrostimulin Regulates Osteoblastic Bone Formation During Early Skeletal Development

The ancestral glycoprotein hormone thyrostimulin is a heterodimer of unique glycoprotein hormone subunit alpha (GPA)2 and glycoprotein hormone subunit beta (GPB)5 subunits with high affinity for the TSH receptor. Transgenic overexpression of GPB5 in mice results in cranial abnormalities, but the role of thyrostimulin in bone remains unknown. We hypothesized that thyrostimulin exerts paracrine actions in bone and determined: 1) GPA2 and GPB5 expression in osteoblasts and osteoclasts, 2) the skeletal consequences of thyrostimulin deficiency in GPB5 knockout (KO) mice, and 3) osteoblast and osteoclast responses to thyrostimulin treatment. Gpa2 and Gpb5 expression was identified in the newborn skeleton but declined rapidly thereafter. GPA2 and GPB5 mRNAs were also expressed in primary osteoblasts and osteoclasts at varying concentrations. Juvenile thyrostimulin-deficient mice had increased bone volume and mineralization as a result of increased osteoblastic bone formation. However, thyrostimulin failed to induce a canonical cAMP response or activate the noncanonical Akt, ERK, or mitogen-activated protein kinase (P38) signaling pathways in primary calvarial or bone marrow stromal cell-derived osteoblasts. Furthermore, thyrostimulin did not directly inhibit osteoblast proliferation, differentiation or mineralization in vitro. These studies identify thyrostimulin as a negative but indirect regulator of osteoblastic bone formation during skeletal development.

Biological Impact of the TSHβ Splice Variant in Health and Disease

Thyroid stimulating hormone (TSH), a glycoprotein hormone composed of α and β chains, is produced by thyrotrope cells of the anterior pituitary. Within the conventional endocrine loop, pituitary-derived TSH binds to receptors in the thyroid, resulting in the release of the thyroid hormones thyroxine (T₄) and triiodothyronine (T₃). T₄ and T₃ in turn regulate nearly every aspect of mammalian physiology, including basal metabolism, growth and development, and mood and cognition. Although TSHβ has been known for years to be produced by cells of the immune system, the significance of that has remained largely unclear. Recently, a splice variant of TSHβ (TSHβv), which consists of a truncated but biologically functional portion of the native form of TSHβ, was shown to be produced by bone marrow cells and peripheral blood leukocytes, particularly cells of the myeloid/monocyte lineage. In contrast, full-length native TSHβ is minimally produced by cells of the immune system. The present article will describe the discovery of the TSHβv and will discuss its potential role in immunity and autoimmunity, inflammation, and bone remodeling.

A newly identified TSHβ splice variant is involved in the pathology of Hashimoto's thyroiditis

Thyrotropin (TSH) is a protein that plays a key role in the control of thyroid function. TSH consists of a common α-subunit and a unique β-subunit; the latter is responsible for hormone specificity. A novel splice variant of human TSHβ was identified in 2009. To date, only the tissue distribution of the human TSHβ splice variant mRNA has been studied. Therefore, we aimed to characterize the protein translated from this splice variant. Salting-out, dialysis and concentration of serum proteins were followed by immunoprecipitation to identify the hTSHβ splice variant in serum. Stable CHO cell lines expressing the hTSHβ splice variant and V5-hTSHα were generated. After co-culture, co-immunoprecipitation was used to determine if the hTSHβ splice variant can dimerise with TSHα. We showed for the first time that the hTSHβ splice variant exists in human serum and dimerises with TSHα. To explore the relationship between the TSHβ splice variant and the pathogenesis of autoimmune thyroiditis, we assessed variations in the mRNA expression of the TSHβ splice variant in the peripheral blood leukocytes (PBLs) of Hashimoto's thyroiditis (HT) patients using quantitative RT-PCR. We found that the mRNA expression levels of the TSHβ splice variant were higher in the PBLs of HT patients who were not undergoing prednisone therapy (n = 10, P < 0.0001) and in the PBLs of HT patients with a longer duration of illness (>18 months) who were undergoing prednisone therapy (n = 5, P = 0.023) than in those of the control group. This pattern was reversed in the PBLs of HT patients with a shorter duration of illness (<9 months) who were undergoing prednisone therapy (n = 8, P < 0.0001). Dexamethasone inhibition of the TSHβ splice variant mRNA expression occurred in a dose- and time-dependent manner. These results demonstrated that the TSHβ splice variant may participate in the pathogenesis of HT.

Extrapituitary production of anterior pituitary hormones: an overview

Protein hormones from the anterior pituitary gland have well-established endocrine roles in their peripheral target glands. It is, however, now known that these proteins are also produced within many of their target tissues, in which they act as local autocrine or paracrine factors, with physiological and/or pathophysiological significance. This emerging concept is the focus of this brief review.

Immunological regulation of metabolism--a novel quintessential role for the immune system in health and disease

The hypothalamus-pituitary-thyroid (HPT) axis is an integrated hormone network that is essential for maintaining metabolic homeostasis. It has long been known that thyroid stimulating hormone (TSH), a central component of the HPT axis, can be made by cells of the immune system; however, the role of immune system TSH remains enigmatic and most studies have viewed it as a cytokine used to regulate immune function. Recent studies now indicate that immune system-derived TSH, in particular, a splice variant of TSHβ that is preferentially made by cells of the immune system, is produced by a subset of hematopoietic cells that traffic to the thyroid. On the basis of these and other findings, we propose the novel hypothesis that the immune system is an active participant in the regulation of basal metabolism. We further speculate that this process plays a critical role during acute and chronic infections and that it contributes to a wide range of chronic inflammatory conditions with links to thyroid dysregulation. This hypothesis, which is amenable to empirical analysis, defines a previously unknown role for the immune system in health and disease, and it provides a dynamic connection between immune-endocrine interactions at the organismic level.

Splicing variants impact in thyroid normal physiology and pathological conditions

RNA splicing is an essential, precisely regulated process that occurs after gene transcription and before mRNA translation, in which introns may be removed and exons, retained. Variability in splicing patterns is a major source of protein diversity from the genome and function to generate a tremendously diverse proteome from a relatively small number of genes. Changes in splice site choice can determine different effects on the encoded protein. Small changes in peptide sequence can alter ligand binding, enzymatic activity, allosteric regulation, or protein localization. Errors in splicing regulation have been implicated in a number of different disease states. This study reviewed the mechanisms of splicing and their repercussion in endocrinology, emphasizing its importance in some thyroid physiological and pathological conditions.

Alternative splicing in development and function of chordate endocrine systems: a focus on Pax genes

Genome sequencing has facilitated an understanding of gene networks but has also shown that they are only a small part of the answer to the question of how genes translate into a functional organism. Much of the answer lies in epigenetics-heritable traits not directly encoded by the genome. One such phenomenon is alternative splicing, which affects over 75% of protein coding genes and greatly amplifies the number of proteins. Although it was postulated that alternative splicing and gene duplication are inversely proportional and, therefore, have similar effects on the size of the proteome, for ancient duplications such as occurred in the Pax family of transcription factors, that is not necessarily so. The importance of alternative splicing in development and physiology is only just coming to light. However, several techniques for studying isoform functions both in vitro and in vivo have been recently developed. As examples of what is known and what is yet to be discovered, this review focuses on the evolution and roles of the Pax family of transcription factors in development and on alternative splicing of endocrine genes and the factors that regulate them.