The apoptogenic response of human myeloid leukaemia cell lines and of normal and malignant haematopoietic progenitor cells to the proteasome inhibitor PSI

Degradation of several intracellular proteins involved in cell cycle contro l and tumour growth is regulated by the ubiquitin-dependent multicatalytic protease complex (proteasome). We report that proteasome inhibitor Z-Ile-Gl u(OtBu)-Ala-Leucinal (PSI) was cytotoxic on most human myeloid leukaemia ce ll lines at IC50 doses ranging from 5 to 25 nmol/l. Additionally, PSI pretr eatment enhanced cytotoxicity by taxol and cisplatinum. PSI was more active on leukaemic than on normal CD34(+) bone marrow progenitors because the 50 % growth inhibition of colony-forming unit granulocyte macrophage (CFU-GM) from cases of chronic myelogenous leukaemia (CML) and normal subjects was a chieved by 15 nmol/l and 50 nmol/l PSI respectively. PSI killed cells by ap optosis as revealed by ultrastructural changes, nuclear DNA fragmentation, cleavage of poly (ADP-ribose) polymerase (PARP) and of beta -catenin, and w as antagonized by ectopic expression of Bcl-2 but not by inactivating mutat ions of p53. This event was associated with a slight accumulation of Bcl-2, a decrease of Bax but no changes in Bcl-X-L protein expression at any time point. In Ph+ cell lines BCR-ABL protein was only down-regulated after 48 h of treatment with 10 nmol/l PSI. Altogether, these results indicate that PSI, alone or in association with other cytotoxic agents, has anti-tumour a ctivity against myeloid malignancies and is more effective on leukaemic tha n on normal haematopoietic progenitor cells.