"Role of thyroid hormones in bone development and maintenance"

Transcription

1 "Role of thyroid hormones in bone development and maintenance" Graham R. Williams Molecular Endocrinology Group Department of Medicine & MRC Clinical Sciences Centre Imperial College London

2 Thyroid hormones and bone development Children Hypothyroidism Growth arrest, delayed bone age, epiphyseal dysgenesis, immature body proportion Thyrotoxicosis Accelerated growth, advanced bone age, short stature, craniosynostosis Normal euthyroid status during childhood is essential to establish peak bone mass Rivkees et al 1988 NEJM 318; Segni & Gorman 2001 J Pediatr Endocrinol Metab 14:

3 Thyroid hormones and bone Bone strength and fracture risk Peak bone mass Euthyroid status is essential during growth Rate of bone loss Thyroid hormones regulate rate of bone turnover Hypothyroidism Associated with increased susceptibility to fracture Thyrotoxicosis High bone turnover osteoporosis Sub-clinical hyperthyroidism Post-menopausal women with suppressed TSH have 4-fold increased risk of fracture 3% of women over the age of 50 take thyroxine and over 20% receive excessive replacement Murphy & Williams 2004 Clin Endocrinol 61: Vestergaard & Mosekilde 2002 Thyroid 12: Vestergaard et al 2005 Calcif Tiss Int 77: Ahmed et al 2006 Osteoporosis Int 17:46-53 Bauer et al 2001 Ann Intern Med 134: Parle et al 1993 Br J Gen Pract 34:

4 HPT axis heritability Variation in thyroid function in individuals is narrow T4, T3 and TSH levels fluctuate by less than 50% of population reference range Heritability UK adult twin registry 39% for ft4, 23% for ft3, 65% for TSH Genome wide scan identified 8 QTL linked to ft3, ft4 or TSH level Danish twin study 65% for ft4, 64% for ft3, 64% for TSH Peak bone mass Heritability between 50-80% depending on skeletal site and age of subjects studied Hypothesis Variations in BMD and fracture susceptibility between normal healthy individuals are associated with differences in their thyroid status Andersen et al 2002 J Clin Endocrinol Metab 87: Panicker et al 2008 Clin Endocrinol 68: Panicker et al 2008 J Clin Endocrinol Metab 93: Hansen et al 2004 J Clin Endocrinol Metab 89: Ralston & de Combrugghe 2006 Genes Dev 20:

5 Hypothalamic-pituitary-thyroid axis and bone Hypothalamus T 4 T 3 TRH Pituitary TSH Thyroid T 4 T 3 Bone TRα

6 Hypothalamic-pituitary-thyroid axis and bone Hypothalamus Pituitary Thyroid T 4 T 3 TRH TSH Controversy OB and OC express TSHR mrna Congenitally hypothyroid TSHR -/- mice display high bone turnover & low bone mass at 6-7 weeks TSH proposed as a negative regulator of bone turnover Inconsistencies TSHR -/- mice require TH at weaning T 4 T 3 TSHR stimulating Ab in Graves disease do not protect against high bone turnover Bone TRα TSHR Children with TSHβ mutations have normal bone mineral density Problem Reciprocal effects of T3 and TSH are difficult to resolve because of HPT axis negative feedback

7 Hypothalamic-pituitary-thyroid axis and bone Hypothalamus Pituitary Thyroid T 4 T 3 TRH TSH Controversy OB and OC express TSHR mrna Congenitally hypothyroid TSHR -/- mice display high bone turnover & low bone mass at 6-7 weeks TSH proposed as a negative regulator of bone turnover Inconsistencies TSHR -/- mice require TH at weaning T 4 T 3 TSHR stimulating Ab in Graves disease do not protect against high bone turnover Bone TRα TSHR Children with TSHβ mutations have normal bone mineral density Problem Reciprocal effects of T3 and TSH are difficult to resolve because of HPT axis negative feedback

8 hyt/hyt and Pax8 -/- mice hyt/hyt TSHR Pro 556 Leu mutation does not bind TSH Hypoplastic thyroid & congenital hypothyroidism ft 4 0.1x, ft x, TSH 1900x Elevated TSH non-functional TSHR Pax8 -/- Thyroid follicular cell agenesis Congenital hypothyroidism ft 4 & ft 3 undetectable, TSH 2300x Elevated TSH active TSHR If TSH is important in vivo these mice must have opposite skeletal phenotypes

9 Congenitally hypothyroid Pax8 -/- and hyt/hyt mice Delayed growth & ossification Impaired endochondral ossification Reduced cortical bone 200μm Bassett et al 2008 Mol Endocrinol 22:

10 TSH does not affect osteoblast differentiation or mineralization Alkaline phosphatase activity Alizarin red camp activity Bassett et al 2008 Mol Endocrinol 22:

11 TSH does not alter osteoclast numbers or TRAP activity Osteoclast numbers Dentine resorption camp activity Bassett et al 2008 Mol Endocrinol 22:

12 Primary osteoblasts and osteoclasts express low levels of TSHR protein Bassett et al 2008 Mol Endocrinol 22:

13 Summary In vivo Pax8 -/- and hyt/hyt mice have similar skeletal phenotypes In vitro Osteoblasts and osteoclasts express very low levels of TSHR protein TSH and TSHR stimulating antibodies do not affect osteoblast and osteoclast function in vitro Skeletal consequences of congenital hypothyroidism are not mediated primarily by TSH Bassett et al 2008 Mol Endocrinol 22:

14 Hypothalamic-pituitary-thyroid axis Hypothalamus T 4 T 3 TRH Pituitary Thyroid TSH T 4 T 3 If T3 mediates the major effects of the HPT axis in bone, which TR is involved? -TRα - - Both TRα and Bone TRα

15 Thyroid status of TRα 0/0 and -/- mice T4 (μg/dl) T3 (ng/ml) TSH (mu/l) WT 3.8± ±0.3 25±3.0 Euthyroid α 0/0 0.9x 1.2x 0.9x Euthyroid β -/- 4x 6x 12x RTH TRα mutants are euthyroid mutants have pituitary resistance with elevated TH levels Gauthier C et al 2001 Mol Cell Biol 21:

16 Deletion of TRα or affects growth TRα 0/0 Transient growth delay -/- Persistent short stature Bassett et al 2007 Mol Endocrinol 21:

17 Deletion of TRα or affects ossification TRα 0/0 Delayed endochondral ossification -/- Advanced endochondral ossification Bassett et al 2007 Mol Endocrinol 21:

18 Deletion of TRα or affects bone mass WT TRα 0/0 -/- Osteosclerosis Osteoporosis Bassett et al 2007 Mol Endocrinol 21:

19 Opposite phenotypes in TRα and knockout mice TRα 0/0 -/- Delayed ossification, reduced calcified bone and growth retardation Increased adult bone mass (reduced bone resorption) Advanced ossification, increased calcified bone, accelerated early growth but persistent short stature Reduced adult bone mass and mineralization (increased bone resorption)

20 TRα is the predominant TR isoform in bone 14 * Relative mrna expression TRα1 O Shea et al 2003 Mol Endocrinol 17:

21 Relationship between TRα and Hypothalamus Pituitary T 4 T 3 TRH TSH Forrest et al 1996 EMBO J 15: Abel et al 2001 J Clin Invest 107: Thyroid T 4 T 3 Bone TRα Bassett et al 2007 Mol Endocrinol 21: Bassett et al 2007 Mol Endocrinol 21:

22 Pituitary Circulation Bone TRα 0/0 -/- TRα TRE Euthyroid tissue predominant - TSH expression normal TRα TRE Hypothyroid tissue knockout - TSH repression impaired T3, T4 Normal T3, T4 TSH High O Shea et al 2006 Nuc Rec Signaling 4:e011

23 Pituitary Circulation Bone TRα 0/0 TRα T3, T4 Normal TRα TRα TRα TRα TRα TRα TRα TRα TRE Euthyroid tissue predominant - TSH expression normal TRE Hypothyroid tissue TRα knockout -/- TRα T3, T4 TSH High TRα TRα TRα TRα TRα TRα TRα TRα TRE Hypothyroid tissue knockout - TSH repression impaired TRE Thyrotoxic tissue TRα over activated O Shea et al 2006 Nuc Rec Signaling 4:e011

24 FGFR expression in TRα 0/0 bone FGFR1 Cortical bone WT TRα 0/0 Cortical bone Decreased FGFR1 expression in TRα 0/0 osteoblasts FGFR2 Cortical bone Cortical bone Unchanged FGFR2 expression in TRα 0/0 osteoblasts FGFR3 Growth plate Growth plate Decreased FGFR3 expression in TRα 0/0 chondrocytes TRα 0/0 skeleton is hypothyroid Stevens DA et al 2003 Mol Endocrinol 17:

25 FGFR expression in -/- bone FGFR1 Cortical bone WT -/- Cortical bone Increased FGFR1 expression in -/- osteoblasts FGFR1 Growth plate Growth plate Increased FGFR1 expression in -/- chondrocytes FGFR3 Growth plate Growth plate Increased FGFR3 expression in -/- chondrocytes -/- skeleton is thyrotoxic

26 T3 action in bone is mediated via TRα TRα 0/0 Results in skeletal hypothyroidism despite normal thyroid status because T3 action in bone is disrupted Delays ossification and growth Reduces bone turnover resulting in osteosclerosis -/- Affects bone indirectly because elevated circulating thyroid hormones act in bone cells via intact TRα Accelerates ossification and growth Increases bone turnover resulting in osteoporosis T3 action in the skeleton in vivo is mediated via TRα

27 Conclusions TSH does not have important physiological actions in bone Pax8 -/- and hyt/hyt mice T3 action in bone is mediated by TRα TRα and null mice TRα is a potential drug target

28 Acknowledgements Molecular Endocrinology Marta Archanco Duncan Bassett Nicholas Bernstein Moira Cheung Thomas Galliford Apostolos Gogakos Rebecca Hernandez Yan Lu Elaine Murphy Jonathan Nicholls Sofia Rashid Collaborators Richard Eastell (Sheffield) David Reid (Aberdeen) Dieter Felsenberg (Berlin) Claus Gluer (Kiel) Christian Roux (Paris) Sam Refetoff & Roy Weiss (Chicago) Gilbert Vassart & Sabine Costagliola (Brussels) Jacques Samarut (Lyon) Alan Boyde (QMUL) Peter Howell (UCL)