Peptide-based ionophore perturbed overall ion homeostasis, thereby triggering endoplasmic reticulum stress-mediated apoptosis.

The disturbance of ion homeostasis is a novel strategy for the acceleration of cell malfunctions. The disturbed ion homeostasis typically elicits apoptotic activity by ion transportation. Considering the general apoptotic mechanism, ROS are overproduced, inducing oxidative stress in cells. The severe oxidative conditions inflict damage to mitochondria and then activate mitochondria-dependent apoptosis pathways. Although ionophore-based apoptosis inducers have been recently investigated, the apoptotic mechanisms are not fully understood. Moreover, the induction of apoptosis by synthetic ionophores has not been verified in vivo due to the lack of bio-applicability.

In this study, DaeYong Lee (Prof. Yeun-Chun Kim's group) introduced a crown ether moiety in the polypeptide to impart water solubility and potassium transport characteristics. In addition, the tetramethylammonium group and hydrocarbon chain allowed for interactions between lipid plasma membranes and the polypeptide. Because of their structural features, AIPs possess potassium ion transport activity. It was hypothesized that when tested in a cellular environment, AIPs would perturb potassium homeostasis, inducing ER stress-mediated apoptosis. As expected, AIPs perturbed potassium homeostasis and accelerated calcium influx into the cytosol. ER stress-related proteins were detected to confirm the stimulation of ER stress by disturbed potassium homeostasis. The results indicated that perturbed ion homeostasis induces the unfolded protein responses in the ER and then highly stimulates the ER stress-related signaling cascade for apoptosis. Furthermore, ER stress-related proteins were detected under caspase-blocking conditions to confirm whether ER stress pathways were governed by the caspase cascade. Overall, ER pathways were not affected by caspase signaling, indicating that ER stress is generated prior to apoptosis activation. They also assessed the pathways of ER stress-mediated apoptosis by immunoblot and apoptosis-related assays. It was found that stressed ER exerted oxidative stress in the mitochondria and then stimulated mitochondria-dependent apoptosis. Finally, they evaluated the apoptotic activity in vivo to demonstrate its practicability as a therapeutic. All the in vivo results implied that AIP2 strongly inhibited tumor growth by ER stress-mediated apoptosis without any side effects and might be used as a potential anticancer agent.

In conclusion, Prof. Kim's lab developed a new potassium ionophore that induces ER stress-mediated apoptosis and revealed detailed apoptotic mechanisms. This study presented the first evidence that the perturbation of potassium homeostasis strongly activated ER stress-induced apoptosis. Moreover, the strategy for disturbing potassium homeostasis can be harnessed for future anticancer drugs.