Role of Macrophage Lysosomal Enzymes in the Degradation of Nucleosomes of Apoptotic Cells

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Abstract
Although apoptotic cells are recognized and engulfed by macrophages via a number of membrane receptors, little is known about the fate of apoptotic cells after the engulfment. We observed in this study that nucleosomal DNA fragments of apoptotic cells disappeared when they were engulfed by the macrophage cell line J774.1 at 37oC. Pretreatment of J774.1 cells with chloroquine inhibited intensive DNA degradation, indicating that the cleavage of nucleosomal DNA fragments of apoptotic cells may take place in the lysosomes of J774.1. When apoptotic cells were exposed to a lysosome-rich fraction derived from J774.1 cells under an acidic condition, nucleosomal DNA fragments of apoptotic cells were no longer detectable by agarose gel electrophoresis. Additionally, we found that the lysosome-rich fraction of J774.1 cells contained an acid DNase that is similar to DNase II with respect to its m.w., optimal pH, and sensitivity to the inhibitors of DNase II. By exposure of apoptotic cells to the lysosomal-rich fraction, nucleosomal core histones of apoptotic cells were hydrolyzed along with degradation of nucleosomal DNA fragments. Addition of pepstatin A to the reaction buffer resulted in accumulation of ∼180-bp DNA fragments and inhibition of hydrolysis of nucleosomal core histones. Leupeptin or CA-074 partially inhibited the degradation of nucleosomal DNA fragments and core histones. These findings suggest that lysosomal enzymes of macrophages, e.g., DNase II-like acid DNase and cathepsins, are responsible for the degradation of nucleosomes of apoptotic cells.