Calbindin-D28k is expressed in osteoblastic cells and suppresses their apoptosis by inhibiting caspase-3 activity

The rate of osteoblast apoptosis is a critical determinant of the rate of bone formation. Because the calcium-binding protein calbindin-D-28k has anti-apoptotic properties in neuronal cells and lymphocytes, we searched for the presence of this protein in osteoblastic cells and investigated whether it can modify their response to proapoptotic signals. Calbindin-D-28k was expressed at low levels in several osteoblastic cell lines and at high levels in primary cultures of murine osteoblastic cells. Transient transfection of rat calbindin-D-28k cDNA blocked tumor necrosis factor alpha (TNF alpha)-induced apoptosis in osteoblastic MC3T3-E1 cells, as determined by cell viability and nuclear morphology of cells cotransfected with the green fluorescent protein targeted to the nucleus, whereas transfection of the empty vector had no effect. Calbindin-D-28k levels in several stably transfected MC3T3-E1 lines were directly related to protection from TNF alpha-induced apoptosis, Purified rat calbindin-D-28k markedly reduced the activity of caspase-3, a critical molecule for the degradation phase of apoptosis, in a cell-free assay. In addition, cell extracts from MC3T3-E1 cells expressing high levels of calbindin-D-28k decreased caspase-3 activity, compared with extracts from vector-transfected cells. This effect was apparently unrelated to the calcium binding properties of calbindin, as chelation of calcium by EGTA or addition of other calcium-binding proteins such as calbindin-D-9k, S100, calmodulin, and osteocalcin, did not affect caspase-3 activity. Last, calbindin-D-28k interacts with the active form of caspase-3 as demonstrated by a GST pull-down assay. These results demonstrate that calbindin-D-28k is a biosynthetic product of osteoblasts with a role in the regulation of apoptosis. They also reveal that the antiapoptotic properties of calbindin-D-28k may result not only from calcium buffering but also from the ability of the protein to interact with and to inhibit caspase-3 activity, a property that is independent of its calcium binding capability.