*2.2. Cell Viability, Iron Content, and ROS Measurement*

The cell viability was measured with the CCK-8 assay (Cell Counting Kit-8; BS350B, Biosharp). The iron content was measured using a colorimetric assay. Briefly, cells were counted and then suspended in 5 M HCl. After incubation at 60 ◦C for 4 h, the cells were centrifuged and the supernatant was transferred. The supernatant was added with the freshly prepared detection reagent (0.08% K2S2O8, 8% KSCN, and 3.6% HCl) and incubated at room temperature for 10 min. The absorbance at 490 nm was measured using an absorption reader (BioTek, Winooski, VT, USA). The iron content was determined according to a standard curve generated with FeCl3 solution. The iron content was calculated as micrograms per cell. ROS was measured by the flow cytometer method. In brief, the cells were stained with 2- ,7- -dichlorodihydrofluorescein diacetate (DCFH-DA) using Reactive Oxygen Species Assay Kit (Beyotime, Nantong, China) according to the manufacturer's instructions. ROS changes indicated by fluorescence shift was analyzed on a CytoFLEX LX Flow Cytometer (Beckman, Brea, CA, USA).
