Unraveling the mechanism of cell death induced by chemical fibrils.

We previously discovered a small-molecule inducer of cell death, named 1541, that noncovalently self-assembles into chemical fibrils ('chemi-fibrils') and activates procaspase-3 in vitro. We report here that 1541-induced cell death is caused by the fibrillar rather than the soluble form of the drug. A short hairpin RNA screen reveals that knockdown of genes involved in endocytosis, vesicle trafficking and lysosomal acidification causes partial 1541 resistance. We confirm the role of these pathways using pharmacological inhibitors. Microscopy shows that the fluorescent chemi-fibrils accumulate in punctae inside cells that partially colocalize with lysosomes. Notably, the chemi-fibrils bind and induce liposome leakage in vitro, suggesting they may do the same in cells. The chemi-fibrils induce extensive proteolysis including caspase substrates, yet modulatory profiling reveals that chemi-fibrils form a distinct class from existing inducers of cell death. The chemi-fibrils share similarities with proteinaceous fibrils and may provide insight into their mechanism of cellular toxicity.