Lisakovska O. The role of vitamin D3 in the regulation of NF-κB-dependent signaling pathways in prednisolone-induced osteoporosis

This PhD thesis is dedicated to studying the role of the NF-κB-dependent signaling pathway RANKL/RANK/OPG in the mechanisms of disturbances in bone tissue remodeling under glucocorticoid(GC)-induced osteoporosis and the evaluation of the possible corrective effect of vitamin D3 (cholecalciferol). It was shown for the first time that along with a decrease in the level of serum 25OHD and reduced VDR tissue content, GC-induced tissue-specific abnormalities in the CYP27B1 level were found: an inhibition of the protein CYP27B1 synthesis in the bone marrow and an increase in mRNA expression in the bone tissue after prednisolone administration. It was found that oxidative-nitrosative stress is one of the mechanisms of prednisolone-induced hepatotoxicity. NF-κB activation, induced by the enhanced production of reactive oxygen species, increased level of NO production and protein nitration were associated with the increase in both necrotic and apoptotic cell death among hepatocytes after prednisolone action. The effect of cholecalciferol on hepatocytes, based on the reduction of the NF-κB phosphorylation and the raise in the IκB level, can be proposed as the key mechanism of vitamin D involvement in preventing the development of oxidative-nitrosative stress and cell death under the long-term GC action. NF-κB/IκB system was shown to be the key molecular switch in the interaction of RANKL/RANK/OPG, VDR- and GR-mediated signaling pathways in different tissues. Along with the reduced GR and OPG levels in the bone marrow there was an increase in the RANKL content and an elevated RANKL binding to RANK. A reduced OPG/RANKL ratio in bone tissue was found, suggesting impairments in bone remodeling. Prednisolone administration caused NF-κB activation and its translocation to the nuclei in bone marrow cells, while in the bone tissue NF-κB inhibition due to a significant increase in IκB content was observed. An elevated number of RANK-positive osteoclast precursors after prednisolone action in the bone marrow, peripheral blood, and spleen was shown for the first time. Vitamin D3 administration exerted a corrective effect on GC-induced impairments in the expression of the components of RANKL/RANK/OPG and NF-κB/IκB systems in rat bone marrow and bone tissue. Based on the ability of vitamin D3 to partially normalize the pool of circulating osteoclast precursors and to modulate the state of the NF-κB-associated signaling pathways in different tissues, we can recommend to use cholecalciferol in order to correct prednisolone-induced disturbances of bone remodeling and liver functioning.