Curcumin suppresses RANKL-induced osteoclast precursor autophagy in osteoclastogenesis by inhibiting RANK signaling and downstream JNK-BCL2-Beclin1 pathway

Abstract

Background: Curcumin ameliorates bone loss by inhibiting osteoclastogenesis. Curcumin inhibits RANKL-promoted autophagy in osteoclast precursors (OCPs), which mediates its anti-osteoclastogenic effect. But the role of RANKL signaling in curcumin-regulated OCP autophagy is unknown. This study aimed to explore the relationship between curcumin, RANKL signaling, and OCP autophagy during osteoclastogenesis.

Methods: We investigated the role of curcumin in RANKL-related molecular signaling in OCPs, and identified the significance of RANK-TRAF6 signaling in curcumin-treated osteoclastogenesis and OCP autophagy using flow sorting and lentiviral transduction. Tg-hRANKL mice were used to observe the in vivo effects of curcumin on RANKL-regulated bone loss, osteoclastogenesis, and OCP autophagy. The significance of JNK-BCL2-Beclin1 pathway in curcumin-regulated OCP autophagy with RANKL was explored via rescue assays and BCL2 phosphorylation detection.

Results: Curcumin inhibited RANKL-related molecular signaling in OCPs, and repressed osteoclast differentiation and autophagy in sorted RANK⁺ OCPs but did not affect those of RANK^- OCPs. Curcumin-inhibited osteoclast differentiation and OCP autophagy were recovered by TRAF6 overexpression. But curcumin lost these effects under TRAF6 knockdown. Furthermore, curcumin prevented the decrease in bone mass and the increase in trabecular osteoclast formation and autophagy in RANK⁺ OCPs in Tg-hRANKL mice. Additionally, curcumin-inhibited OCP autophagy with RANKL was reversed by JNK activator anisomycin and TAT-Beclin1 overexpressing Beclin1. Curcumin inhibited BCL2 phosphorylation at Ser70 and enhanced protein interaction between BCL2 and Beclin1 in OCPs.

Conclusions: Curcumin suppresses RANKL-promoted OCP autophagy by inhibiting signaling pathway downstream of RANKL, contributing to its anti-osteoclastogenic effect. Moreover, JNK-BCL2-Beclin1 pathway plays an important role in curcumin-regulated OCP autophagy.

Keywords: Autophagy; BCL2; Beclin1; Curcumin; Osteoclast; RANKL.