Induction of apoptosis by esculetin in human leukemia U937 cells: Roles of Bcl-2 and extracellular-regulated kinase signaling

Abstract

In the present study, we reported that apoptosis induced by esculetin, a phenolic compound with apop- totic activity in cancer cells, was markedly blocked by Bcl-2-overexpression, but restored by HA14-1, a small-molecule Bcl-2 inhibitor, in human leukemic U937 cells. The combined use of esculetin and HA14-1 effectively induced Bid cleavage and loss of mitochondrial membrane potential (MMP, Dwm) leading to the activation of caspases and cleavage of poly(ADP-ribose) polymerase (PARP) in Bcl-2-over- expressing (U937/Bcl-2) cells. Combined treatment with esculetin and HA14-1 upregulated the expres- sion of death receptor 4 (DR4), and activation of extracellular-regulated kinase (ERK) in a time- dependent manner. In addition, esculetin and HA14-1-mediated apoptosis was reduced by ERK inhibitors through inhibition of DR4 expression, suggesting that the synergistic effect was at least partially medi- ated through ERK-dependent induction of DR4 expression. The results indicate that HA14-1-induced reversal of the anti-apoptotic effect of Bcl-2 confers apoptosis sensitivity to esculetin by a mitochondrial amplification step and through the ERK-dependent induction of DR4 expression in U937/Bcl-2 cells. Thus, HA14-1 reversal of Bcl-2-mediated esculetin resistance suggests a novel strategy for increasing esculetin sensitivity in Bcl-2-overexpressing leukemia cells. 2009 Elsevier Ltd. All rights reserved. Keywords: Esculetinl Apoptosis Bcl-2 HA14-1 ERK