Proteolytic loss of bcl-x_L in FL5.12 cells undergoing apoptosis induced by MK886

Apoptosis induced in the IL3-dependent murine pro-B lymphocytic (FL5.12) cell line by the 5-lipoxygenase activating protein inhibitor MK886 is accompanied by the rapid loss of the anti-apoptotic bcl-x_L and bcl-2, but not the proapoptotic bax proteins (Datta et al., J. Biol. Chem. 273, 28163-28169, 1998). Since several reports indicate important roles for noncaspase proteases in apoptosis, the participation of lysosomes, as well as serine, cysteine, or aspartic acid proteases, in the effects of MK886 were investigated. Consistent with the involvement of various proteases, lysosomal degranulation was evident, as observed by a decrease in acridine orange fluorescence at 2 h and an increase in cytosolic β-hexosaminidase activity at 4 h after treating FL5.12 cells with 10 μm MK886. The disappearance of bcl-x_L from FLS.12 cells upon MK886 treatment was prevented in a dose-dependent manner by pretreatment with leupeptin, pepstatin, phenylmethylsulfonyl fluoride, or the broad-spectrum caspase inhibitor Boc-D-FMK. Each of the noncaspase protease inhibitors partially inhibited MK886-induced apoptosis as measured by phosphatidylserine externalization and DNA fragmentation. The noncaspase inhibitors also blocked about half of the increase in caspase-3-like activity. Boc-D-FMK completely inhibited this enzyme and prevented apoptosis. None of the inhibitors were able to directly inhibit activated caspase-3 in cell lysates, suggesting their effects were upstream of caspase activation. These observations suggest the involvement of various proteases, possibly originating from lysosomes, upstream of active caspase-3, in the loss of bcl-x_L protein and in the signaling pathway of MK886-induced apoptosis in FL5.12 cells. This pathway may be unique to MK886 since these same protease inhibitors had only minimal effects on etoposide-induced apoptosis and the accompanying moderate loss of bcl-x_L in FL5.12 cells.