Autophagy is a lysosome-dependent degradation process crucial for maintaining cellular homeostasis. While typically protective, excessive autophagy can lead to a distinct form of cell death known as autosis. Triple Negative Breast Cancer (TNBC), a highly aggressive subtype of breast cancer, is characterized by defects in autophagy regulation. In previous studies, we developed stapled peptides that target Beclin 1, a key autophagy protein, to enhance autophagy and promote endolysosomal trafficking. Among these, Tat-SP4 emerged as a lead candidate. In TNBC cells, Tat-SP4 induced a moderate increase in autophagy but exhibited a strong anti-proliferative effect that was not reversed by inhibitors of apoptosis or other programmed cell death pathways.
The cell death triggered by Tat-SP4 displayed hallmark features of autosis, including persistent adherence to the substrate, plasma membrane rupture, and rescue by digoxin, a cardioglycoside known to inhibit Na⁺/K⁺ ATPase. Furthermore, Tat-SP4 induced significant mitochondrial dysfunction, as evidenced by a loss of mitochondrial membrane potential, elevated mitochondrial reactive oxygen species (ROS), and reduced oxidative phosphorylation. These findings were corroborated in vivo using a TNBC xenograft model, where Tat-SP4 significantly suppressed tumor growth.
Our study reveals three critical insights into autosis: (1) autosis can be initiated by a moderate increase in autophagy that surpasses the cell’s endogenous capacity; (2) mitochondrial dysfunction may be a central mechanism linking dysregulated autophagy to autosis; and (3) TNBC cells, due to their intrinsic autophagy deficiency and low mitochondrial bioenergetic activity, are particularly susceptible to autosis. These vulnerabilities present a unique therapeutic opportunity. Tat-SP4 and similar peptides may serve as promising candidates for targeted TNBC treatment by exploiting the autosis pathway, offering a novel strategy against this challenging cancer subtype.