Intracellular calcium-induced ROS generation promotes squaraine phototoxicity

Photodynamic therapy (PDT) is a minimally invasive therapeutic modality approved by the Food and Drug Administration (FDA) for the treatment of several pathological conditions, including cancer. Developing new photosensitizers (PSs) for PDT is of great interest to increase the treatment efficacy while minimizing side effects. In this regard, a better understanding of the signal transduction triggered by PS photo-activation may help to optimize PS efficacy. In the present work, we synthesized a series of squaraines (SQs) featuring different indolenine ring modifications. Our results show that unsubstituted SQ has significant phototoxic activity, further increased by the introduction of a bromine in the indolenine ring (Br-SQ-C4) as well as the replacement of the squaryl oxygen atoms with sulfur atoms (SQ-S-C4). Phototoxicity positively correlates with higher photo-induced cytoplasmic Ca²⁺ signals as well as reactive oxygen species (ROS) generation. However, the different substituents strongly affect the signaling pathway triggered. The bromine substituent strengthens the localization of the dye in the endoplasmic reticulum (ER), while the sulfur substituent shifts its preferential localization to the mitochondria. Consistently, photo-activation of Br-SQ-C4 induces a larger ER Ca²⁺ release followed by SOCE, which fuels secondary ROS generation able to sustain a remarkable mitochondrial Ca²⁺ uptake and subsequent mitochondrial ROS generation. On the other hand, SQ-S-C4 can induce, already at the basal level, a greater perturbation of the Ca²⁺/ROS dynamics. Overall, our results contribute to a deeper understanding of the intracellular signaling triggered by SQs, paving the way for the development of novel strategies aimed at increasing PDT efficacy.