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

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.

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.

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.

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.

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.