# Effects of Epigallocatechin-3-O-Gallate on Bone Health
## 3 Biochemical Effects of EGCG
### RANK/RANKL/OPG Path Regulation  
RANKL, the receptor activator of nuclear factor-\(\upkappa\)B (RANK), and OPG belong to the TNFs and their receptor superfamilies. As a TNF superfamily molecule, RANKL forms a homotrim and binds to its receptors. RANK and OPG act as monomers and homodimers, respectively. The RANKL/RANK/OPG is a crucial system for bone resorption [61]. RANK/RANKL/OPG form the essential cytokine system that is capable of regulating all ranges of osteoclast functions (proliferation, differentiation, fusion, activation, and apoptosis). The balance in bone resorption depends on the local ratio of RANKL to OPG [62]. Figure 2: The EGCG impact on biochemical effects in bone health. The central regulation of osteoclast proliferation and differentiation is controlled by OPG, RANKL, and the macrophage colony-stimulating factor (M-CSF) [63]. RANK is found in osteoclasts and their precursors. RANKL is produced in both soluble and membrane-bound forms by osteoblasts and stromal cells in the bone marrow and has a pivotal role in osteoclastogenesis, differentiation, activation, and the apoptosis of osteoclasts. OPG is a soluble product of stromal cells and osteoblasts, and it plays the role of a decoy receptor of the RANK ligand. The function of OPG is to block the RANK-RANKL interactions, which are required for the differentiation and activation of osteoclasts [64; 65]. The inhibition of the RANKL-RANK signalling pathway in bone can increase bone mass by preventing osteocl bone resorption [66]. The activities of RANKL are mediated by its binding to RANK. The interaction between RANKL and RANK is the preliminary key step in the osteoclastogenesis pathway. The inhibition of RANKL binding with RANK by OPG can therefore suppress bone resorption and favour bone formation [65; 67]. The connection of RANKL to RANK is followed by the recruitment of the TNF receptor-associated factor (TRAF). Although several of these proteins (TRAFs 1, 2, 3, 5, and 6) are involved in the RANK signalling pathway, TRAF-6 seems to be the crucial protein for RANKL signalling in osteoclasts [61; 65; 68]. TRAF proteins are believed to serve as second messengers that activate multiple downstream signalling pathways, including those involving transcription factors like NF-\(\kappa\)B and \(c\)-Jun. When activated, NF-\(\kappa\)B increases the expression of \(c\)-Fos [69; 70], which subsequently combines with the NFAT-\(c\)1 to stimulate the transcription of genes involved in osteoclast activity [65]. The RANKL/RANK/OPG system, therefore, acts as a common final pathway for several cytokines and hormonal factors involved in bone resorption [61; 65]. The inhibition of RANKL synthesis caused by EGCG treatment was also noted by [PERSON] et al. [54] on NRG cells (a mouse marrow-derived fibroblast-like cell line with osteoblast-like characteristics) and was accompanied by a reduction in the production of IL-6, which resulted in the inhibition of inflammation in the treatment of osteomyelitis. Green tea catechins (EC, EGC, ECG, EGCG) can enhance the expression of OPG mRNA in pluripotent mesenchymal cells, but of these, EGCG has the strongest effect, which leads to the inhibition of osteoclast differentiation and activity [42]. OPG production is stimulated by bone morphogenetic protein 4 (BMP-4) [71]. By controlling this protein expression in osteoblasts, [PERSON] et al. [43] examined the effects of EGCG and coffee polyphenol chlorogenic acid (CGA) on OPG synthesis in cloned osteoblast-like MC3T3-E1 cells derived from newborn mouse calvaria. The findings demonstrated that EGCG considerably inhibits osteoclast differentiation and activation through increasing the release of OPG triggered by BMP-4, whereas CGA has no discernible impact. [PERSON] et al.