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

Abstract

Triiodothyronine (T₃) and estrogen (E₂) 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 E₂ in human cells.

Aims: This study evaluated the effects of the physiological concentration of E₂ (10 nM), alone or in association with physiological (1 nM) and supraphysiological (10 nM) concentrations of T₃, 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 E₂, 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: E₂ enhanced the gene expression of OPG when associated with 1 nM and 10 nM T₃. E₂ was able to restore the bone matrix after an initial decrease using 1 nM and 10 nM T₃. The protective effect of E₂ 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 E₂ to maintain OPG expression and osteoblast activity against possible loss of bone mass, especially in conditions where T₃ is in excess.

Keywords: Estrogen; Osteoblast; RANKL and OPG; Stem cells; Triiodothyronine.