Tetrandrine enhances the ubiquitination and degradation of Syk through an AhR-c-src-c-Cbl pathway and consequently inhibits osteoclastogenesis and bone destruction in arthritis

Recently, we reported that tetrandrine, a natural alkaloid, could inhibit the osteoclastogenesis and bone erosion through enhancing the ubiquitination and degradation of spleen tyrosine kinase (Syk). Herein, we addressed whether and how Aryl Hydrocarbon Receptor (AhR) mediate the effect of tetrandrine. In vitro, tetrandrine was shown to repress RANKL-induced osteoclastogenesis and the expression of osteoclast-related marker genes, which was almost completely reversed by either AhR antagonist CH223191 or siRNA. In pre-osteoclasts, tetrandrine enhanced the ubiquitination and degradation of Syk through the AhR/c-src/c-Cbl signaling pathway, downregulated the expression of phospho-Syk and phospho-PLCγ2, and inhibited the nuclear translocation of NFATc1, a master transcription factor for osteoclastogenesis. Notably, tetrandrine acted through the non-genomic pathway of the ligand-activated AhR, as evidenced by the fact that the effect of tetrandrine did not change in the absence of AhR nuclear translocator. In collagen-induced arthritis rats, oral administration of tetrandrine decreased the number of phospho-Syk-positive cells and osteoclasts, and reduced the bone erosion in the areas of the proximal tibial epiphysis excluding the cortical bone. A combined use with CH223191 almost abolished the effect of tetrandrine. These findings revealed that tetrandrine enhanced the ubiquitination and degradation of Syk and consequently repressed the osteoclastogenesis and bone destruction through the AhR-c-src-c-Cbl pathway.