Induction of apoptotic cell death in mouse lymphoma and human leukemia cell lines by a calcium-binding protein complex, calprotectin, derived from inflammatory peritoneal exudate cells.

We have previously shown that the calcium-binding protein complex, calprotectin, purified from rat inflammatory peritoneal cells exerts marked cytotoxic activity against rat, mouse, and human tumor cells. We studied here whether the cytotoxicity is caused by induction of apoptosis, using mouse EL-4 lymphoma and human MOLT-4 leukemia lines as targets. The rat calprotectin sample inhibited [3H]thymidine incorporation into these cells by partially 24 h and almost completely in 48 h of culture at concentrations of 100-200 micrograms/ml. Morphological changes, that is, loss of cell volume and nuclear condensation and/or fragmentation, appeared in both cell types cultured with calprotectin from 20 h, and such apoptotic cells subsequently increased in number to compose the great majority of the cells at 40 h. Cell death, measured by stainability with trypan blue, lagged behind the emergence of the apoptotic morphology by about 2 and 10 h in EL-4 and MOLT-4 cells, respectively. DNA fragmentation was observed in EL-4 cells cultured with calprotectin, whereas it was not observed in MOLT-4 cells, consistent with results of flow cytometry showing that loss of cell DNA content caused by the factor was greater in EL-4 cells. The data indicate that calprotectin induces the apoptosis of certain tumor cells but that the occurrence of DNA fragmentation is dependent on cell type. Finally, the apoptosis-inducing activity of the calprotectin sample was abrogated by the presence of 10 microM zinc, whereas it was not affected by 5 mM calcium or magnesium.