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Endocrine journal

Regulation of osteoblast differentiation by interleukin-11 via AP-1 and Smad signaling.


PMID 21931225

Abstract

Mechanical stress and parathyroid hormone (PTH) are major stimulators, and aging and glucocorticoids excess are important suppressors of osteoblast differentiation. Mechanical stress and PTH stimulate interleukin (IL)-11 expression in cells of osteoblast lineage by enhancing transcription of IL-11 gene via an increase in intracellular Ca²⁺. The elevated Ca²⁺ activates extracellular signal-regulated kinase (ERK) to enhance phosphorylation of cyclic AMP response element-binding protein (CREB), which binds to the fosB gene promoter and enhances ΔFosB expression. ΔFosB dimerizes with JunD on the IL-11 gene promoter to enhance its transcription. Both mechanical stress and PTH also stimulate phosphorylation of Smad1 via an activation of protein kinase Cδ (PKCδ). Phosphorylated Smad1 binds to the IL-11 gene promoter and forms complex with ΔFosB/JunD to further enhance IL-11 gene transcription. The increased IL-11 then suppresses expression of Wnt inhibitors, including Dickkopf 1 (Dkk1) and 2, and enhances Wnt signaling to stimulate osteoblast differentiation and inhibit adipocyte differentiation. The suppression of osteoblast differentiation by aging involves a decrease in IL-11 gene transcription by a reduction in JunD binding to the activator protein (AP)-1 site of the IL-11 gene promoter. Glucocorticoids inhibit transcriptional activation of IL-11 gene by an interaction of glucocorticoid-glucocorticoid receptor (GR) complex with ΔFosB/JunD heterodimer. Thus, factors that enhance osteoblast differentiation stimulate, and those which suppress osteoblast differentiation inhibit IL-11 gene transcription, and IL-11 enhances Wnt signaling by suppressing expression of its inhibitors. These observations are consistent with the notion that IL-11 mediates stimulatory and inhibitory signals of osteoblast differentiation by affecting Wnt signaling.