Anti-cancer agent siramesine is a lysosomotropic detergent that induces cytoprotective autophagosome accumulation

Abstract

A σ-2 receptor ligand siramesine induces lysosomal leakage and cathepsin-dependent death of cancer cells in vitro and displays potent anti-cancer activity in vivo. The mechanism by which siramesine destabilizes lysosomes is, however, unknown. Here, we show that siramesine induces a rapid rise in the lysosomal pH that is followed by lysosomal leakage and dysfunction. The rapid accumulation of siramesine into cancer cell lysosomes, its ability to destabilize isolated lysosomes, and its chemical structure as an amphiphilic amine indicate that it is a lysosomotropic detergent. Notably, siramesine triggers also a substantial Atg6-and Atg7-dependent accumulation of autophagosomes that is associated with a rapid and sustained inhibition of mammalian target of rapamycin complex 1 (mTORC1; an inhibitor of autophagy). Siramesine fails, however, to increase the degradation rate of long-lived proteins. Thus, the massive accumulation of autophagosomes is likely to be due to a combined effect of activation of autophagy signaling and decreased autophagosome turnover. Importantly, pharmacological and RNA interference-based inhibition of autophagosome formation further sensitizes cancer cells to siramesine-induced cytotoxicity. These data identify siramesine as a lysosomotropic detergent that triggers cell death via a direct destabilization of lysosomes and cytoprotection by inducing the accumulation of autophagosomes. Threrefore, the combination of siramesine with inhibitors of autophagosome formation appears as a promising approach for future cancer therapy.

Imaging the Sigma-2 Receptor for Diagnosis and Prediction of Therapeutic Response

“…Under physiological conditions, these proteases are found within the lysosomes but are released into the cytoplasm upon exposure to cell damaging agents, thereby triggering a cascade of intracellular events leading to cell death. The σ 2 selective ligand siramesine has been reported to cause lysosomal leakage and induce cell death by caspase-independent mechanisms [41,45]. The localization of fluorescent σ 2 receptor probes in the lysosomes is consistent with the hypothesis that siramesine induces cell death partially by targeting lysosomes to cause lysosomal damage, the release of proteases, and eventually cell death.…”

“…Several studies have shown that σ 2 ligands induced cell death. The proposed mechanisms of cell death include caspase-independent apoptosis [40], lysosomal leakage [41], Ca 2+ release [42,43], oxidative stress [41], ceramide production [44] and autophagy [45]. However, the subcellular localizations where the σ 2 ligands bind and function were not known.…”

BAMLET Activates a Lysosomal Cell Death Program in Cancer Cells

“…The data presented above add BAMLET to the growing list of cytotoxic agents that can induce a nonapoptotic lysosomal cell death pathway (35). In light of these data, combination therapies with agents further sensitizing cells to lysosomal destabilization (such as microtubule disturbing drugs vincristine and paclitaxel) as well as with those inducing lysosomal destabilization (such as siramesine) should be experimentally tested (38,39). Given that many cancers have fundamental defects in both mitochondrial and death receptor signaling pathways, and that these apoptotic defects specifically characterize the most aggressive therapy resistant tumors, BAMLET alone or in combination therapies may provide new treatment options in aggressive cancers.…”

The autophagic paradox in cancer therapy

“…Nevertheless, it is noteworthy that the linkage from autophagy to LMP is not always unidirectional. In this regard, some lysosomotropic detergents, such as siramesine, which trigger cell death via a direct destabilization of lysosomes, have been shown to induce cytoprotective accumulation of autophagosomes (Ostenfeld et al, 2008).…”