Dietary compound gossypetin inhibits bone resorption through down-regulating lysosomal cathepsin K activity and autophagy-related protein induction in actin ring-bearing osteoclasts

• The anti-osteoclastogenic activity of gossypetin is demonstrated for the first time.
• Gossypetin inhibited acidification of osteoclast resorption lacunae via proton secretion.
• This compound retards ruffled border formation through fusion of acid hydrolase-containing lysosomes.
• Gossypetin blunts LC lipidation responsible for lysosomal cathepsin K secretion into osteoclast resorption lacunae.

Gossypetin, usually isolated from the flowers and the calyx of Hibiscus sabdariffa, possesses anti-microbial and anti-atherosclerotic effects. However, anti-osteoporotic effects of gossypetin have not been elucidated. Gossypetin attenuated RANKL-induced multinucleated osteoclast formation with enhanced TRAP activity and blunted bone resorption active in osteoclasts. Gossypetin inhibited the actin ring formation and αvβ3 integrin induction for sealing zones. This compound suppressed the induction of CAII, V-ATPase, ClC-7 and Ae2, all required for secretion of proton and chloride ions into resorption lacunae. Furthermore, gossypetin reduced lysosomal cathepsin K transcription and MMP-9 activity, blunting accumulation of lysosomes in osteoclasts displaying an actin ring. The presence of gossypetin deterred the induction of Rab7, and Atg12–Atg5 conjugate and Atg7 involved in LC3 lipidation, all prerequisites to osteoclast ruffled border formation. These observations demonstrate for the first time that gossypetin was effective in retarding ruffled border formation and acidification in a sealed microenvironment of osteoclast resorption lacunae.