Serum Apolipoprotein H Alters Cellular Lipid Composition to Inhibit Ferroptosis

Ferroptosis is an iron-dependent regulated cell death caused by the accumulation of lipid peroxidation for the uncontrolled metabolism. Serum, as the major medium for the cultured cells, resembles the contents of the extracellular fluid in vivo and provides biomolecules for cellular metabolism. The efficiency of ferroptosis induction is influenced by several factors including the extracellular environment. However, the effect of serum on ferroptosis remains largely unclear. We found that cells cultured in different serums have varying efficiencies in ferroptosis induction. By purifying and identifying active serum components, we discovered that serum protein apolipoprotein H (APOH) play essential role in inhibiting ferroptosis. Moreover, APOH activates the phosphoinositide 3-kinase (PI3K)/AKT-Sterol regulatory element-binding proteins (SREBPs) pathway. SREBPs upregulate the stearoyl-CoA desaturase (SCD) increasing cellular monounsaturated fatty acid-containing phospholipids (MUFA-PLs), leading to ferroptosis inhibition. Our findings indicate that APOH, as an extracellular protein, plays an important role in cellular lipid metabolism and inhibition of ferroptosis, thus may having therapeutic applications in cancer treatment and ferroptosis-related diseases. Overall design: Given the impact of serum on cellular sensitivity to ferroptosis, we aimed to elucidate the mechanisms by which R-FBS (resistance FBS to ferroptosis)mediates cellular resistance to ferroptosis. We analyzed the transcriptome of MEFs cultured in Rcm(R-FBS complete medium), Scm (sensitive to ferroptosis, S-FBS, the S-FBS complete), and exchanged medium for 12h (referred to as R-Scm-12h or S-Rcm-12h, respectively) and 24h (referred to as R-Scm-24h or S-Rcm-24h, respectively) by RNA sequencing (RNA-seq)