Dihydroartemisinin Induces Apoptosis by a Bak-Dependent Intrinsic Pathway

Abstract

The sesquiterpene lactone dihydroartemisinin (DHA), a semisynthetic derivative of the herbal antimalaria drug artemisinin, is cytotoxic to human tumor cells. Treatment of Jurkat T-lymphoma cells with DHA induced a breakdown of the mitochondrial transmembrane potential, release of cytochrome c, activation of caspases, and DNA fragmentation indicative of apoptosis induction. Although the absence of FADD or caspase-8 did not alter apoptosis rates in Jurkat cells, overexpression of dominant-negative caspase-9 or of antiapoptotic Bcl-xL or Bcl-2 largely decreased the cytotoxicity of DHA, demonstrating a role of the intrinsic death pathway. The proapoptotic Bcl-2 effector protein Bak and the Bcl-2 homology domain 3-only protein NOXA turned out to be important mediators of DHA-induced apoptosis in Jurkat cells. DHA treatment triggered the expression of NOXA and the activation of Bak. Furthermore, DHA-induced apoptosis was completely abrogated by loss of Bak and largely reduced in cells with siRNA-mediated downregulation of Bak or NOXA. Proapoptotic signaling of DHA also involved the formation of reactive oxygen species and membrane oxidation. Pretreatment with the lipophilic radical scavenger vitamin E or the hydrophilic radical scavengers glutathione and N-acetylcysteine reduced DHA-induced membrane oxidation and apoptosis, respectively. Oxidative changes also occurred in cells with disruption of the mitochondrial death pathway, suggesting a role of reactive oxygen species and oxidative membrane changes in death signaling upstream of the mitochondria. Interestingly, DHA increased the cytotoxic action of ionizing radiation and of the death receptor agonist tumor necrosis factor-related apoptosis-inducing ligand in Jurkat cells, suggesting a potential benefit of DHA in combined treatment strategies.