Tetrandrine blocks autophagic flux and induces apoptosis via energetic impairment in cancer cells

Lysosomes are acidic organelles that have a crucial role in degrading intracellular macromolecules and organelles during the final stage of autophagy. Tetrandrine (Tet), a bisbenzylisoquinoline alkaloid, was reported as an autophagy activator. Here, in contrast with previous studies, we show that Tet is a potent lysosomal deacidification agent and is able to block autophagic flux in the degradation stage. Single-agent Tet induces significant apoptosis both in vitro and in xenograft models. In the presence of Tet, apoptosis was preceded by a robust accumulation of autophagosomes and an increased level of microtubule-associated protein 1 light chain 3, type II (LC3-II). However, Tet increased the level of sequestosome 1 and decreased the turnover of LC3, indicating the blockade of autophagic flux in the degradation stage. As blockade of autophagic flux decreases the recycling of cellular fuels, Tet reduces the uptake of glucose in cancer cells. These effects lead to insufficient substrates for tricarboxylic acid (TCA) cycle and impaired oxidative phosphorylation. Blunting autophagosome formation using 3-methyladenine or genetic knockdown of Beclin-1 failed to rescue cells upon Tet treatment. By contrast, addition of methyl pyruvate to supplement TCA substrates protected Tet-treated tumor cells. These results demonstrate that energetic impairment is required in Tet-induced apoptosis. Tet, as a potent lysosomal inhibitor, is translatable to the treatment of malignant tumor patients.