[Induction of Apoptosis in Leukemic Cells by Inhibiting the Ubiquitin-Proteasome Pathway and Its Possible Mechanism].

The ubiquitin-proteasome pathway is the principal mechanism for the degradation of short-lived proteins in eukaryotic cells. Recently, proteasome inhibitors have been shown to induce apoptosis in many kinds of human malignant cells. In this study, the mechanism of apoptosis induced by proteasome inhibitor in leukemic cells was examined. Evaluated by MTT assay, treatment of leukemic cells with Z-LLL-CHO, a reversible proteasome inhibitor, induced cell death in a dose-dependent manner. Appearance of the sub G(0)/G(1) fraction of cell cycle observed in flow cytometry assay suggested the induction of apoptosis, which was further proved by typical DNA ladder and morphological study. Western blot displayed the cleavage of bcl-2 into a shortened 22 kD fragment and the decrease in the levels of caspase-3 precursor. A highly sensitive colorimetric assay was employed and the elevation of caspase-3 activity was detected in both cell lines after treatment with Z-LLL-CHO. By comparison, these results showed that the leukemic cell line M-07e and KG-1a, which both express bcl-2 at a relative high level, had different susceptibility to undergo apoptosis induced by Z-LLL-CHO, which possibly due to their different levels of expression and activation of caspase-3 precursor, as well as their different degree of bcl-2 cleavage after treated by Z-LLL-CHO.