The importance of estrogen for bone protection in experimental hyperthyroidism in human osteoblasts

Abstract Triiodothyronine (T 3 ) and estrogen (E 2 ) play important roles in the bone remodeling process and signaling of receptor activator of the nuclear factor-kappa β (RANKL) and osteoprotegerin (OPG) expressed by osteoblasts. However, little is known of the molecular action of these hormones in conditions of hyperthyroidism and associated E 2 in human cells. AIMS: This study evaluated the effects of the physiological concentration of E 2 (10 nM), alone or in association with physiological (1 nM) and supraphysiological (10 nM) concentrations of T 3 , on RANKL and OPG gene expression in human osteoblasts. MAIN METHODS: Alkaline phosphatase and osteocalcin assays were performed to verify the presence of mature osteoblasts. After mimicking the experimental hyperthyroidism in osteoblasts untreated or treated with E 2 , RANKL and OPG gene expression was analyzed by real-time PCR and protein expression by western Blot and ELISA. Alizarin Red staining analyzed the amount of bone matrix after hormonal treatments. KEY FINDINGS: E 2 enhanced the gene expression of OPG when associated with 1 nM and 10 nM T 3 . E 2 was able to restore the bone matrix after an initial decrease using 1 nM and 10 nM T 3 . The protective effect of E 2 on the RANKL and OPG signaling pathway was demonstrated. E2 restored the bone matrix induced by experimental hyperthyroidism. SIGNIFICANCE: The data highlight the importance of E 2 to maintain OPG expression and osteoblast activity against possible loss of bone mass, especially in conditions where T 3 is in excess.