The novel triterpenoid 2-cyano-3,12-dioxoolean-1,9-dien-28-oic acid induces apoptosis of human myeloid leukemia cells by a caspase-8-dependent mechanism

The oleanane triterpenoid 2-cyano-3,12-dioxoolean-1,9-dien-28-oic acid (CDD O) is a multifunctional molecule that induces growth inhibition and differe ntiation of human myeloid leukemia cells. The present studies demonstrate t hat CDDO treatment results in apoptosis of U-937 and HL-60 myeloid leukemia cells. Similar to 1-beta-D-arabinofuranosylcytosine (ara-C), another agent that inhibits growth and induces apoptosis of these cells, CDDO induced th e release of mitochondrial cytochrome c and activation of caspase-3. Overex pression of Bcl-x(L) blocked cytochrome c release, caspase-3 activation, an d apoptosis in ara-C-treated cells. By contrast, CDDO-induced release of cy tochrome c, and activation of caspase-3 were diminished only in part by Bcl -x(L). In concert with these findings, we demonstrate that CDDO, but not ar a-C, activates caspase-8 and thereby caspase-3 by a cytochrome c-independen t mechanism. The results also show that CDDO-induced cytochrome c release i s mediated by caspase-8-dependent cleavage of Bid, These findings demonstra te that CDDO induces apoptosis of myeloid leukemia cells and that this nove l agent activates an apoptotic signaling cascade distinct from that induced by the cytotoxic agent ara-C.