Cardenolides; calotropin and gomphogenin from Calotropis procera (Aiton) mitigate bone turnover in ovariectomized osteoporotic rats: Targeting RANKL/OPG axis and estrogen receptor-alpha

The present study aimed to examine the effect of Calotropis procera (Aiton) and its major cardenolides; calotropin and gomphogenin on ovariectomy-induced osteoporosis in rats. Osteoporotic rats were orally treated with C. procera alcoholic extract (100 mg/kg), calotropin (CLT; 100 μg/kg) and gomphogenin (GPG; 100 μg/kg) for 14 consecutive days. Bone resorption/formation biomarkers; bone specific alkaline phosphatase (BALP), osteoprotegerin (OPG) and nuclear factor-κβ ligand (RANKL) as well as serum calcium and phosphorus were assessed 24 h after last doses of treatments. Serum levels of estradiol (E2) and catalase were also measured.

Oral treatment with C. procera extract, CLT and GPG caused E2 restoration to normal level with a marked regulation in the RANKL/OPG axis. Serum phosphorus and calcium were up-leveled whereas BALP was downregulated. Histopathological examination, bone histomorphometric analysis and immunohistochemical staining for osteopontin (OPN) inspection further emphasized the aforementioned outcomes. The results revealed the superiority of CLT and to a lesser extent GPG osteoporotic effect over C. procera extract. Molecular docking of the two compounds on ER-α and RANKL/OPG complex showed noteworthy binding affinities which also confirmed the supremacy of CLT due to the additional hydrogen bonding of the hydroxyl groups of the sugar moiety with RANKL/OPG complex. Finally, it is concluded that CLT and GPG from C. procera hinder bone turnover by decreasing osteoclastic bone cells activity and increasing calcium mineralization thus suppressing bone remodeling and preventing bone infirmity in OVX osteoporotic rats directly via binding to RANKL/OPG complex and ER-α and indirectly through elevating level of E2.