Role and Mechanisms of Actions of Thyroid Hormone on the Skeletal Development

Bone Research (2013) 2: 146-161. www.boneresearch.org REVIEW Role and Mechanisms of Actions of Thyroid Hormone on the Skeletal Development Ha-Young Kim1,2,3, Subburaman Mohan1,2* 1Musculoskeletal Disease Center, Loma Linda VA HealthCare System, Loma Linda, CA 92357, USA; 2Departments of Medicine, Loma Linda University, Loma Linda, CA 92354, USA; 3Division of Endocrinology, Department of Internal Medicine, Wonkwang University Sanbon Hospital, Gunpo, Gyeonggi, Korea The importance of the thyroid hormone axis in the regulation of skeletal growth and maintenance has been well established from clinical studies involving patients with mutations in proteins that regulate synthesis and/or actions of thyroid hormone. Data from genetic mouse models involving disruption and overexpression of components of the thyroid hormone axis also provide direct support for a key role for thyroid hormone in the regulation of bone metabolism. Thyroid hormone regulates proliferation and/or differentiated actions of multiple cell types in bone including chondrocytes, osteoblasts and osteoclasts. Thyroid hormone effects on the target cells are mediated via ligand-inducible nuclear receptors/transcription factors, thyroid hormone receptor (TR) α and β, of which TRα seems to be critically important in regulating bone cell functions. In terms of mechanisms for thyroid hormone action, studies suggest that thyroid hormone regulates a number of key growth factor signaling pathways including insulin-like growth factor-I, parathyroid hormone related protein, fibroblast growth factor, Indian hedgehog and Wnt to influence skeletal growth. In this review we describe findings from various genetic mouse models and clinical mutations of thyroid hormone signaling related mutations in humans that pertain to the role and mechanism of action of thyroid hormone in the regulation of skeletal growth and maintenance. Keywords: thyroid hormone; bone; cartilage; growth factors; bone cells Bone Research (2013) 2: 146-161. doi: 10.4248/BR201302004 Introduction Thyroid hormone (TH) plays an important role in normal endochondral ossification and is essential for skeletal development, linear growth, maintenance of bone mass, and efficient fracture healing (1). Juvenile hypothyroidism causes growth arrest with delayed bone formation and mineralization, and T4 replacement induces rapid catch-up growth (2). By contrast, childhood thyrotoxicosis accelerates bone formation with premature closure of the growth plates and skull sutures, leading to short stature and craniosynostosis (3). Although there is *Correspondence: Subburaman Mohan E-mail: Tel: 909-825-7084 (ext 2932); Fax: 909-796-1680 Received 13 March 2013; Accepted 28 April 2013 considerable evidence regarding the importance of TH in skeletal development, the molecular mechanisms of TH action in bone are poorly understood. In this chapter, we discuss regulation and mechanisms of action of TH during skeletal development with particular emphasis on areas in which recent advances have been made. Physiology of TH: Regulation, metabolism and TH receptor Regulation Systemic TH levels are maintained by the classical negative feedback loop involving the hypothalamus-pituitary-thyroid (HPT) axis (Figure 1). Thyrotropin releasing hormone (TRH) is synthesized in the paraventricular nucleus (PVN) of the hypothalamus and stimulates synthesis and secretion of thyroid stimulating hormone (TSH) from Ha-Young Kim et al. thyrotroph cells in the anterior pituitary gland. TSH subsequently acts via the TSH receptor (TSHR) on thyroid follicular cells to stimulate synthesis and release of 3,5,3',5'-L-tetraiodothyronine (thyroxine, T4) and 3,5,3'-Ltriiodothyronine (T3). The circulating T4 and T3 are predominantly bound to carrier proteins including thyroxine binding globulin, transthyretin (previously known as thyroxine binding pre-albumin) and albumin, with only approximately 0.2% of the total T3 and 0.02% of the total T4 available as free unbound hormones (fT3, fT4) in plasma. Figure 1 TRH-TSH-T3 feedback loop. The hypothalamic neurons secrete thyrotropin releasing hormone (TRH) which is carried down to the adenohypophysis of the pituitary by the hypothalamic portal vein where it releases thyroid stimulating hormone (TSH). The released TSH reaches thyroid glands via blood stream to bind to TSH receptor (TSHR) to stimulate production and release of thyroxin (T4) and T3. T3 exerts its actions on bone mainly by binding to TRα. TSH can also act directly on bone cells by binding to TSHR. Increased levels of T3 can act by negative feedback loop via TRβ to inhibit release of TRH and TSH, thereby preventing hyperparathyroidism. TH is known to act via the nuclear TH receptor β (TRβ) in the hypothalamus and pituitary to inhibit TRH and TSH production and secretion (4-5) and thus complete a negative feedback loop that maintains systemic thyroid status within a normal reference range. This negative feedback loop maintains a physiological inverse relationship between TSH and circulating T3 and T4 levels that www.boneresearch.org | Bone Research defines the HPT axis set-point (6-7). Metabolism The predominant circulating TH is the pro-hormone T4, which can be converted to the biologically more potent hormone, T3. TH metabolism is mediated by three iodothyronine deiodinases. The type 1 and type 2 enzymes (D1 and D2) convert T4 to T3 by catalyzing removal of a 5'-iodine atom. By contrast, the type 3 enzyme (D3) irreversibly removes a 5-iodine atom from either T4 or T3 to generate the inactive metabolites 3,3',5'-L-triiodothyronine (reverse T3, rT3) and 3,3'-diiodothyronine (T2), respectively (8-9). D1 is not expressed in skeletal cells (10-11), indicating D1 does not influence T3 action on bone directly. D2 is restricted to mature primary osteoblasts but is undetectable in chondrocytes and osteoclasts (12). The cellular influx as well as efflux of iodothyronines is known to be mediated by several specific membrane transporter proteins including the monocarboxylate transporters 8 and 10 (MCT8 and MCT10), sodiumdependent organic anion co-transporting polypeptide 1 (OATP1), the sodium taurocholate co-transporting polypeptide (NTCP) and the L-type amino acid transporter 1 (LAT1) and LAT2 (13-15). A study by Capelo et al revealed that MCT8, LAT1 and LAT2 are expressed in the skeletal tissues of mice as well as in osteoblastic MC3T3E1 cells (16). Thus, the intra-cellular levels of the active hormone, T3, and its availability to nuclear TH receptors (TRs) are determined by the relative activities of D2 and D3 as well as expression levels of TH transport proteins. TH receptor/ TH action The major action of TH is exerted through nuclear TH receptors (TRs), which are ligand-inducible transcription factors. Based on chromosomal localization and amino acid homology, two classes of TRs, α and β, have been identified. Due to differential splicing of these two genes, multiple TRs are generated as α1, α2, α3, β (...truncated)