Secreted Tartrate-Resistant Acid Phosphatase 5b is a Marker of Osteoclast Number in Human Osteoclast Cultures and the Rat Ovariectomy Model

To study the effects of estrogen withdrawal on osteoclast number and osteoclast activity in the rat ovariectomy (OVX) model.

We first cultured human CD34+ osteoclast precursor cells on bovine bone slices, allowing them to differentiate into mature resorbing osteoclasts. Secreted tartrate-resistant acid phosphatase 5b (TRACP 5b) and C-terminal cross-linked telopeptides of type I collagen (CTX) were determined from the culture medium. TRACP 5b correlated strongly with osteoclast number and CTX with osteoclast activity, facilitating their subsequent use in the rat OVX model. An 8 week OVX study was then performed including sham-operated rats receiving vehicle, OVX rats receiving vehicle, and OVX rats receiving 10 μg/kg/day 17β-estradiol (E2). Trabecular bone parameters were determined from the tibial metaphysis using peripheral quantitative computed tomography and histomorphometry. Osteoclast number was normalized with bone perimeter (N.Oc/B.Pm) and tissue area (N.Oc/T.Ar, indicating absolute number of osteoclasts). TRACP 5b and CTX were determined from fasting serum samples.

Trabecular bone parameters indicated substantial bone loss after OVX that was prevented by E2. N.Oc/B.Pm increased after OVX, while N.Oc/T.Ar and TRACP 5b decreased, and TRACP 5b correlated strongly with N.Oc/T.Ar. However, CTX values increased after OVX, and the “resorption index” CTX/TRACP 5b showed more substantial changes than either CTX or TRACP 5b alone.

These results show that TRACP 5b is a reliable marker of osteoclast number, and the index CTX/TRACP 5b is a useful parameter in rat OVX model. The high elevation of CTX/TRACP 5b values by OVX demonstrates that estrogen withdrawal generates high activity of osteoclasts in the rat OVX model.