Relevance of osteoclast-specific enzyme activities in cell-based in vitro resorption assays

Cell-based in vitro resorption assays are an important tool to simulate the in vivo biodegradation of resorbable bone graft materials and to predict their clinical performance. The present study analyses the activity of osteoclast-specific enzymes as potential surrogate measures for classical pit assay, which is not applicable on irregular structured materials. Osteoclasts derived from human peripheral blood mononuclear cells were cultivated on different surfaces: calcium phosphate bone cements (CPC), dentin discs, osteoblast-derived extracellular matrix (ECM) and tissue culture polystyrene as control. Pit formation on the resorbable materials was investigated and correlated with the activity of tartrate resistant acid phosphatase (TRAP), carbonic anhydrase II (CAII) and cathepsin K (CTSK). Furthermore, the relation between intra- and extracellular enzyme activities was examined for TRAP and CTSK during resorption of the different materials. Resorbed area of CPC correlated with intracellular TRAP activity and intracellular CAII activity. Highest resorption was detected at around pH 7.2. Resorbed area on dentin correlated with the extracellular CTSK activity and extracellular TRAP activity and was maximal at around pH 6.8. Osteoclasts cultivated on cell-derived mineralised ECM showed a good correlation between both extracellular TRAP and CTSK activity and the release of calcium ions. Based on these data a different regulation of TRAP and CTSK secretion is hypothesised for the resorption of inorganic calcium phosphate compared to the resorption of collagenous mineralised matrix.