Cytochrome C (Cyt C) is a central mediator of intrinsic apoptosis, whereas its heme-free precursor, apocytochrome C (APO-Cyt C), competitively inhibits this process. Sodium aescinate (SA), a natural triterpene saponin, is known to facilitate the endosomal escape of protein drugs. We initially aimed to investigate the effects of enhancing endosomal escape efficiency on protein activity through the combination of SA and Cyt C. However, this study investigates an cytotoxicity observed when APO-Cyt C is combined with SA and aims to elucidate the underlying molecular mechanism. Contrary to its established anti-apoptotic function, APO-Cyt C, when co-administered with a non-toxic concentration of SA, induced potent, caspase-dependent mitochondrial apoptosis in cancer cells. This pro-apoptotic switch was not primarily triggered by BCL-2 family protein modulation, ROS generation, or calcium overload. Instead, the primary mechanism is the induction of excessive and lethal autophagy. SA was found to induce lysosomal membrane damage, evidenced by Galectin-9 recruitment, which initiates lysophagy. The addition of APO-Cyt C significantly amplified the autophagic flux, leading to decreased p62 levels and enhanced LC3-II turnover. Mechanistically, this synergy is driven by a dual impact on the AKT-mTOR-TFEB pathway: APO-Cyt C treatment decreased mTOR phosphorylation, while the combination promoted the nuclear translocation of the autophagy regulator, TFEB. Inhibition of autophagic flux using Bafilomycin A1 or Tetrandrine rescued cells from apoptosis, confirming that excessive autophagy is the direct cause of cell death. This study reveals a novel therapeutic strategy wherein an anti-apoptotic protein is converted into a potent pro-apoptotic agent. The combination of APO-Cyt C and SA triggers apoptosis by overwhelming the cell with excessive autophagic flux, driven by synergistic inhibition of the mTOR-TFEB axis. These findings highlight the therapeutic potential of modulating autophagy and suggest that combining mTOR inhibitors with lysosome-targeting agents like SA could be an effective anti-cancer strategy.
Sun et al. (Wed,) studied this question.