ACS DIVISION OF BIOLOGICAL CHEMISTRY

October 9th, 2014 by Gutierrez, E., Richardson, D. R., Jansson, P. J.

Autophagy functions as a survival mechanism during cellular stress and contributes to resistance against anti-cancer agents. The selective anti-tumor and anti-metastatic chelator, di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT), causes lysosomal membrane permeabilization and cell death. Considering the integral role of lysosomes in autophagy and cell death, it was important to assess the effect of Dp44mT on autophagy to further understand its mechanism of action. Notably, Dp44mT affected autophagy by two mechanisms. First, concurrent with its anti-proliferative activity, Dp44mT increased the expression of the classical autophagic marker, LC3-II, as a result of induced autophagosome synthesis. Second, this effect was supplemented by a reduction in autophagosome degradation, as shown by the accumulation of the autophagic substrate and receptor, p62. On the other hand, the classical iron chelator, desferrioxamine, induced autophagosome accumulation only by inhibiting autophagosome degradation. The formation of redox active iron or copper Dp44mT complexes was critical for its dual effect on autophagy. The cytoprotective anti-oxidant, N-acetylcysteine, inhibited Dp44mT-induced autophagosome synthesis and p62 accumulation. Importantly, Dp44mT inhibited autophagosome degradation via lysosomal disruption. This effect prevented the fusion of lysosomes with autophagosomes to form autolysosomes, which is crucial for the completion of the autophagic process. The anti-proliferative activity of Dp44mT was suppressed by Beclin1 and ATG5 silencing, indicating the role of persistent autophagosome synthesis in Dp44mT-induced cell death. These studies demonstrate that Dp44mT can overcome the pro-survival activity of autophagy in cancer cells by utilizing this process to potentiate cell death.