Naturally occurring asteriscunolide A induces apoptosis and activation of mitogen-activated protein kinase pathway in human tumor cell lines

Abstract

Sesquiterpene lactones have attracted much attention because they display a wide range of biological activities, including antitumor properties. Here, we show the effects of the naturally occurring sesquiterpene lactone asteriscunolide A (AS) on viability of human melanoma, leukemia and cells that overexpress antiapoptotic proteins, namely Bcl-2 and Bcl-x(L). All cell lines were sensitive to this compound, with IC50 values of similar to 5 mu M. The cytotoxic effects of AS were accompanied by a G(2)-M phase arrest of the cell cycle and a concentration- and time-dependent appearance of apoptosis as determined by DNA fragmentation, translocation of phosphatidylserine to the cell surface and sub-G(1) ratio. Apoptosis was associated with caspase-3 activity and poly(ADP-ribose) polymerase cleavage and was prevented by the nonspecific caspase inhibitor z-VAD-fmk, indicating that caspases are essential components in this pathway. The apoptotic effect of AS was also associated with (i) the release of cytochrome c from mitochondria which was accompanied by dissipation of the mitochondrial membrane potential (Delta Psi(m)) and (ii) the activation of the mitogen-activated protein kinases (MAPKs) pathway. AS-induced cell death was potentiated by inhibition of extracellular signal-regulated kinases (ERK) 1/2 signaling with U0126 and PD98059. Intracellular reactive oxygen species (ROS) seem to play a pivotal role in this process since high levels of ROS were produced early (1 h) and apoptosis was completely blocked by the free radical scavenger N-acetyl-L-cysteine (NAC). The present study demonstrates that AS-induced cell death is mediated by an intrinsic-dependent apoptotic event involving mitochondria and MAPKs, and through a mechanism dependent on ROS generation.