Estrogen reduces the depth of resorption pits by disturbing the organic bone matrix degradation activity of mature osteoclasts

Decreased E2 levels after menopause cause bone loss through increased penet rative resorption. The reversal effect of E2 substitution therapy is well d ocumented in vivo, although the detailed mechanism of action is not fully u nderstood. To study the effects of E2 on bone resorption, we developed a no vel in vitro bone resorption assay in which degradation of inorganic and or ganic matrix could be measured separately. E2 treatment significantly decre ased the depth of resorption pits, although the area resorbed was not chang ed. Electron microscopy further revealed that the resorption pits were fill ed with nondegraded collagen, suggesting that E2 disturbed the organic matr ix degradation. Two major groups of proteinases, matrix metalloproteinases (MMPs) and cysteine proteinases, have been suggested to participate in orga nic matrix degradation by osteoclasts. We show here that MMP-9 released a c ross-linked carboxyl-terminal telopeptide of type I collagen from bone coll agen, and cathepsin K released another C-terminal fragment, the C-terminal cross-linked peptide of type I collagen. E2 significantly inhibited the rel ease of the C-terminal cross-linked peptide of type I collagen into the cul ture medium without affecting the release of cross-linked carboxyl-terminal telopeptide of type I collagen in osteoclast cultures. These results sugge st that organic matrix degradation is initiated by MMPs and continued by cy steine proteases; the latter event is regulated by E2.