Selective Killing

Abstract

One of the main goals when developing anticancer therapies is to find a compound that will target a tumor cell but not adversely affect a normal cell. Yagoda et al . discovered one such compound, erastin, in a screen of 24,000 compounds. The compounds were screened for the ability to cause cell death (as determined by Trypan blue exclusion) in an engineered human tumor cell line with oncogenic v-Ha-ras Harvey rat sarcoma viral oncogene homolog (H-Ras) V12 but not in a nontumorigenic cell line that did not contain oncogenic Ras. Ras is a small guanosine triphosphatase that activates the mitogen-activated protein kinase (MAPK) pathway in response to signaling of receptor tyrosine kinases. Three mammalian ras genes are oncogenic: H - ras , K-ras , and N-ras . Light and electron microscopic analyses showed that treatment with erastin did not result in changes in the nucleus that are characteristic of apoptotic cell death, but instead resulted in loss of the structural integrity of mitochondria. The authors performed another screen (for suppressor compounds) and found that antioxidants blocked erastin-induced cell death. Erastin treatment of the H-Ras-expressing cell line, but not the nontumorigenic cell line, resulted in the production of oxidative species, but this did not lead to activation of apoptosis, as the authors were unable to detect markers of apoptotic cell death, including cytochrome c release from the mitochondria or cleavage of pro-caspase-3. A lung carcinoma cell line (which expresses oncogenic K-Ras) was sensitive to erastin-induced cell death, which was blocked by lentiviral-mediated expression of K-Ras-specific short hairpin RNAs (shRNAs). Three inhibitors of mitogen-activated or extracellular signal-regulated protein kinase kinase (MAPKK) blocked erastin-induced cell death in the H-Ras-expressing cell line. Affinity chromatography with a death-inducing analog of erastin (erastin A6) and an inactive analog (erastin B2) led to the isolation of the mitochondrial voltage-dependent anion channel (VDAC) as a specific target of erastin A6. The abundance of VDAC was greater in the H-Ras-expressing cell line than in the nontumorigenic cell line, and after 10 hours of treatment of H-Ras-expressing cells with erastin, VDAC2 and VDAC3 proteins were no longer detectable by Western blotting. Whereas knockdown of VDAC3 by shRNA in an N-Ras-positive cell line resulted in resistance to erastin treatment, increased expression of VDAC3 in another cell line did not confer sensitivity to erastin, showing that VDAC3 is necessary but not sufficient to mediate sensitivity to erastin. Together, these data show that the interaction between erastin and VDAC isoforms results in oxidative, nonapoptotic cell death and that this process is selective for cells that have increased Ras-mediated activation of the MAPK pathway. N. Yagoda, M. von Rechenberg, E. Zaganjor, A. J. Bauer, W. S. Yang, D. J. Fridman, A. J. Wolpaw, I. Smukste, J. M. Peltier, J. J. Boniface, R. Smith, S. L. Lessnick, S. Sahasrabudhe, B. R. Stockwell, RAS-RAF-MEK-dependent oxidative cell death involving voltage-dependent anion channels. Nature 447 , 864-868 (2007). [PubMed]