The osteocyte product sclerostin is emerging as an important paracrine regulator 
of bone mass. It has recently been shown that osteocyte production of receptor 
activator of NF-κB ligand (RANKL) is important in osteoclastic bone resorption, 
and we reported that exogenous treatment of osteocytes with sclerostin can 
increase RANKL-mediated osteoclast activity. There is good evidence that 
osteocytes can themselves liberate mineral from bone in a process known as 
osteocytic osteolysis. In the current study, we investigated 
sclerostin-stimulated mineral dissolution by human primary osteocyte-like cells 
(hOCy) and mouse MLO-Y4 cells. We found that sclerostin upregulated osteocyte 
expression of carbonic anhydrase 2 (CA2/Car2), cathepsin K (CTSK/Ctsk), and 
tartrate-resistant acid phosphatase (ACP5/Acp5). Because acidification of the 
extracellular matrix is a critical step in the release of mineral from bone, we 
further examined the regulation by sclerostin of CA2. Sclerostin stimulated CA2 
mRNA and protein expression in hOCy and in MLO-Y4 cells. Sclerostin induced a 
decrease in intracellular pH (pHi) in both cell types as well as a decrease in 
extracellular pH (pHo) and the release of calcium ions from mineralized 
substrate. These effects were reversed in the co-presence of the carbonic 
anhydrase inhibitor, acetozolamide. Car2-siRNA knockdown in MLO-Y4 cells 
significantly inhibited the ability of sclerostin to both reduce the pHo and 
release calcium from a mineralized substrate. Knockdown in MLO-Y4 cells of each 
of the putative sclerostin receptors, Lrp4, Lrp5 and Lrp6, using siRNA, 
inhibited the sclerostin induction of Car2, Catk and Acp5 mRNA, as well as pHo 
and calcium release. Consistent with this activity of sclerostin resulting in 
osteocytic osteolysis, human trabecular bone samples treated ex vivo with 
recombinant human sclerostin for 7 days exhibited an increased osteocyte lacunar 
area, an effect that was reversed by the co-addition of acetozolamide. These 
findings suggest a new role for sclerostin in the regulation of perilacunar 
mineral by osteocytes.