Fig. 6: Scheme summarizing the antioxidant and cell death mechanisms activated in human breast cells by MB-PDT.

Breast cells display different susceptibility to photo-oxidative stress (PhOxS) induced by MB-PDT, being the highest effect observed in MDA-MB-231. In this cell line, no antioxidant response was mounted upon PhOxS. In addition, low levels of GPX4 and CoQ₁₀, combined with high amount of iron (Fe²⁺) and PUFA-phospholipid content (PL-PUFA), resulted in ferroptosis activation by MB-PDT (purple arrows and letters). This cell death was inhibited by Fer-1 pretreatment. Lysosomal damage was observed in all cell lines, evidenced by the release of cathepsin B through lysosomal membrane permeabilization (LMP) (green arrows and letters). Pretreatment with CA-074, a cathepsin B inhibitor, alleviated cell death. In both tumorigenic cells, MDA-MB-231 and MCF-7, necroptosis activation with MLKL plasma membrane pore formation (blue arrows and letters) was observed. Inhibition of RIPK1, RIPK3, or MLKL phosphorylation, by gene silencing or pretreatment with Nec-1 or NSA, rescue tumorigenic cells from death. A possible link between LMP and necroptosis was found in tumorigenic cells (green dotted arrows). As MCF-7 cells lack significant amounts of oxidizable phospholipids, lipid peroxidation was not observed and, therefore, ferroptosis did not contribute to death. However, a complete antioxidant response was not sustained in these cells, making them also highly affected by MB-PDT. The scenario after PhOxS was quite different for MCF-10A cells. Even undergoing LMP and lipid peroxidation, they were significantly more resistant to MB-PDT than the other cells. Neither ferroptosis nor MLKL phosphorylation nor necroptosis were observed.