(-)-Epigallocatechin gallate inhibits thyroid hormone-stimulated osteocalcin synthesis in osteoblasts

It is recognized that catechin possesses beneficial properties for bone metabolism. We previously revealed that triiodothyronine (T₃)-activated p38 mitogen-activated protein (MAP) kinase, but not p44/p42 MAP kinase, is involved in the synthesis of osteocalcin in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the effect of (-)-epigallocatechin gallate (EGCG), the predominant green tea polyphenol, on the synthesis of osteocalcin in MC3T3-E1 cells. EGCG significantly suppressed T₃-stimulated osteocalcin synthesis. The inhibitory effect of EGCG was dose-dependent in the range of 3 to 30 µM. On the other hand, T₃-induced phosphorylation of p38 MAP kinase was not affected by EGCG. EGCG profoundly inhibited T₃-stimulated transcriptional activity. These results strongly suggest that EGCG suppresses T3-stimulated osteocalcin synthesis upstream of the transcriptional level in osteoblast-like MC3T3-E1 cells.