Lactacystin, a proteasome inhibitor, potentiates the apoptotic effect of parthenolide, an inhibitor of NFkappaB activation, on drug-resistant mouse leukemia L1210 cells

An L1210 cell line (Y8) selected for resistance to deoxyadenosine does not express p53 mRNA or protein but expresses WAF1/p21 even under basal conditions. The Y8 cell line had been previously shown to have an increased apoptotic response to a variety of agents that included DNA damaging agents, kinase inhibitors and drugs directed at NFkappa B activation. In this study we show that lactacystin (LC, an inhibitor of Proteasome activity) in combination with parthenolide (PA) caused a synergistic increase in the apoptotic fraction of the Y8 cells. LC (2.5 microM) alone and PA (5.0 microM) caused less than 20% of the Y8 cells to undergo Apoptosis. However, the combination of LC (2.5 microM) plus PA (5.0 microM) caused 60% of the Y8 cells to undergo Apoptosis. The combination of drugs had no effects on the parental wild-type L1210 cells. Pretreatment of the intact Y8 cells with the Caspase-3 inhibitor, Ac-DEVD-CHO, resulted in a marked decrease in the Apoptosis caused by the LC plus PA combination. Cell-free extracts prepared from the LC plus PA combination-treated cells had activated Caspase activities in the Caspase cascade: Caspase-3 >> Caspase-8 > caspase-6 and Caspase-10. These results suggest that there are interacting pathways involving aspects of NFkappa B activation and Proteasome activity that could be exploited in therapy directed at p53-deficient tumor cells that would lead to Caspase-3 activation and Apoptosis bypassing the p53-dependent chemotherapy insensitivity.