Betulinic acid, a naturally occurring
triterpene found in the bark of the white birch tree, has been demonstrated to induce programmed cell death with
melanoma and certain
neuroectodermal tumor cells. We demonstrate currently that treatment of cultured UISO-Mel-1 (human
melanoma cells) with
betulinic acid leads to the activation of p38 and stress activated
protein kinase/c-Jun NH(2)-terminal
kinase [widely accepted proapoptotic
mitogen-activated protein kinases (MAPKs)] with no change in the phosphorylation of
extracellular signal-regulated kinases (antiapoptotic MAPK). Moreover, these results support a link between the MAPKs and
reactive oxygen species (ROS). As demonstrated previously, cells treated with
betulinic acid generate ROS. Preincubation of cells with
antioxidants blocks the process of programmed cell death, and prevents the phosphorylation of p38 and stress activated
protein kinase/c-Jun NH(2)-terminal
kinase. These data suggest that ROS act upstream of the MAPKs in the signaling pathway of
betulinic acid. In addition to mediating these responses, treatment of cells with
betulinic acid resulted in a gradual depolarization of mitochondrial membrane potential, a phenomenon established to contribute to the induction of programmed cell death. Interestingly, p38 was capable of partially modulating this perturbation, and investigations of mitochondria-associated apoptotic events indicate no involvement of known
caspases. These data provide additional insight in regard to the mechanism by which
betulinic acid induces programmed cell death in cultured human
melanoma cells, and it likely that similar responses contribute to the antitumor effect mediated with human
melanoma carried in athymic mice.