# Effects of Epigallocatechin-3-O-Gallate on Bone Health
## 3 Biochemical Effects of EGCG
### Hornetic Properties and Toxicity of EGCG  
EGCG exhibits a biphasic dose-response relationship in cellular systems: at low concentrations, it demonstrates antioxidant, anti-inflammatory, and osteoprotective properties, whereas at higher concentrations, it can exert cytotoxic effects. This dual activity is attributed to the pro-oxidative potential of EGCG under specific cellular and environmental conditions [149].  
In a study conducted on astrocytes, it was shown that EGCG at a high concentration (50 uM) induced mitochondrial membrane depolarisation and activated the opening of the mitochondrial permeability transition pore (mPTP). This led to a sharp decline in the inner mitochondrial membrane potential, increased permeability to ions and small molecules, and mitochondrial swelling, thereby increasing the risk of membrane rupture. Concurrently, pro-apoptotic proteins, such as cytochrome c, were released into the cytoplasm. In comparison, the effect at a concentration of 1 uM was significantly weaker [150]. Similar phenomena were observed in studies on human cancer cell lines and rat hepatocytes [151].  
EGCG itself can generate reactive oxygen species (ROS) via auto-oxidation at high concentrations, particularly through Fenton-like reactions in the presence of elevated levels of iron ions (Fe\({}^{2+}\)), such as in the liver or tumour tissues, and copper ions (Cu\({}^{+}\)), which may accumulate in certain inflammatory conditions [148; 152; 153; 154]. Additionally, EGCG can enhance oxidative stress when combined with cytotoxic agents (doxorubicin and bortezomib), heavy metals (e.g., cadmium), ultraviolet radiation, or H\({}_{2}\)O\({}_{2}\)[155; 156; 157; 158; 159].  
Furthermore, excess EGCG has been shown to induce endoplasmic reticulum (ER) stress and disrupt proper protein folding, leading to the activation of the unfolded protein response (UPR). In mouse-derived MRPE cells, EGCG reduced the expression of chaperones and UPR markers such as GRP78, CHOP, PERK, IRE1\(\alpha\), and caspase-12 [128]. Similarly, in studies on the model organism _Caenorhabditis elegans_, EGCG modulated cadmium-induced ER stress via the PERK-elF2\(\alpha\)-ATF4 signalling pathway [160].  
However, at higher concentrations (\(\geq\)50 uM), the increased expression of UPR markers was observed, indicating an activation of the stress response and the potential induction of cell death [161]. A review by the European Food Safety Authority (EFSA) indicates that a daily oral dose of EGCG \(\geq\)800 mg may lead to hepatotoxicity, highlighting the importance of defining safe dose ranges and confirming the hormtetic properties of this compound. In a mouse model, EGCG at a dose of 1500 mg/kg induced acute liver injury, while doses in the range of 400-800 mg/kg were toxic under conditions of dietary restriction (Figure 4) [162].  
Figure 3: The mechanism of the Fenton reaction.  
Major conclusions are as follows:  
* EGCG reduces osteoclast differentiation and activation: EGCG administration leads to a reduction in osteoclast differentiation and activation by increasing OPG mRNA expression and inhibiting RANK ligand production [19; 36; 42].
* It enhances OPG mRNA expression: EGCG exhibits a stronger capacity to enhance OPG mRNA expression in pluripotent stem cells compared to other green tea catechins, contributing to its role in bone health [42].
* It influences bone growth and osteogenesis: EGCG promotes bone growth by stimulating VEGF synthesis, affecting the SAPK/JNK pathway, and reducing IL-6 synthesis [34; 56; 57; 58; 59].