Human THP-1 monocytic leukemic cells induced to undergo monocytic differentiation by bryostatin 1 are refractory to proteasome inhibitor-induced apoptosis

The ubiquitin-proteasome pathway is the principal mechanism for the degrada tion of short-lived proteins in eukaryotic cells, We demonstrated that trea tment of THP-1 human monocytic leukemia cells with Z-LLL-CHO, a reversible proteasome inhibitor, induced cell death through an apoptotic pathway. Apop tosis in THP-I cells induced by Z-LLL-CHO involved a cytochrome c-dependent pathway, which included the release of mitochondrial cytochrome c, activat ion of caspase-9 and -3, and cleavage of Bcl-2 into a shortened 22-kDa frag ment. Induction of apoptosis by protease inhibitor also was detected in U93 7 and TF-1 leukemia cell lines and cells obtained from acute myelogenous le ukemia patients but not in normal human blood monocytes, Treatment of human blood monocytes with Z-LLL-CHO did not induce apoptosis or Bcl-2 cleavage in these cells that rarely proliferate. Interestingly, when THP-1 cells wer e induced to undergo monocytic differentiation by bryostatin 1, a naturally occurring protein kinase C activator, they were no longer susceptible to a poptosis induced by Z-LLL-CHO, Bryostatin 1-induced differentiation of THP- 1 cells was associated with growth arrest, acquisition of adherent capacity , and expression of membrane markers characteristic of blood monocytes. Lik ewise, differentiated THP-1 cells were refractory to Z-LLL-CHO-induced cyto chrome c release, caspase activation, and Bcl-2 cleavage. Resistance to Z-L LL-CHO-induced apoptosis in differentiated THP-1 cells was not due to cell cycle arrest, These findings show that the action of proteasome inhibitors is mediated primarily through a cytochrome c-dependent pathway and induces apoptosis in leukemic cells that are not differentiated.