Black TiO2 nanoprobe-mediated mild phototherapy reduces intracellular lipid levels in atherosclerotic foam cells via cholesterol regulation pathways instead of apoptosis

Given that apoptosis increases the risk of plaque rupture, strategies that reduce intracellular lipid levels without killing foam cells are warranted for safe and effective treatment of atherosclerosis. In this study, a mild phototherapy strategy is carried out to achieve the hypothesis. Foam cell-targeted nanoprobes that allow photothermal therapy (PTT) and/or photodynamic therapy (PDT) were prepared by loading hyaluronan and porphine onto black TiO2 nanoparticles. The results showed that when temperatures below 45 °C, PTT alone and PTT + PDT significantly reduced the intracellular lipid burden without inducing evidently apoptosis or necrosis. In contrast, the use of PDT alone resulted in only a slight reduction in lipid levels and induced massive apoptosis or necrosis. The protective effect against apoptosis or necrosis after mild-temperature PTT and PTT + PDT was correlated with the upregulation of heat shock protein 27. Further, mild-temperature PTT and PTT + PDT attenuated intracellular cholesterol biosynthesis and excess cholesterol uptake via the SREBP2/LDLR pathway, and also triggered ABCA1-mediated cholesterol efflux, ultimately inhibiting lipid accumulation in foam cells. Our results offer new insights into the mechanism of lipid regulation in foam cells and indicate that the black TiO2 nanoprobes could allow safer and more effective phototherapy of atherosclerosis.