SLC22A3 regulates ferroptosis in the mesenchymal subtype of triple-negative breast cancer by modulating histone H3K4 serotonylation.

Triple-negative breast cancer (TNBC) remains a challenging and clinically aggressive subtype due to its heterogeneity and high mortality rate. Recent molecular subtyping has identified distinct TNBC subgroups with varying therapeutic responses, highlighting the need for targeted therapeutic strategies. The mesenchymal (MES) subtype is characterized by low immune cell infiltration, cancer stem cell-like features, and resistance to multiple drugs. Ferroptosis, a form of iron-dependent cell death, has emerged as a promising therapeutic strategy in TNBC due to the abundance of iron and lipids in tumor cells. However, ferroptosis sensitivity varies across different TNBC subtypes. Notably, the MES subtype exhibits resistance to ferroptosis despite elevated iron levels, due to impaired ferroptosis-executing mechanisms. This study investigates the role of SLC22A3, an organic cation transporter, which is enriched in MES tumors and positively correlates with markers of tumor stem cells. High SLC22A3 expression in MES-TNBC cells modulates serotonin uptake and metabolism, conferring ferroptosis resistance through two pathways. First, 5-HT(serotonin) acts as a radical-trapping antioxidant, eliminating lipid peroxides and inhibiting ferroptosis. Second, 5-HT induces histone serotonylation, which enhances histone methylation and facilitates the recognition of methylated histones by transcriptional initiation factors. This process activates SIRT1 transcription, inhibiting MAOA transcription mediated by FOXO1, thereby reducing 5-HT degradation and promoting ferroptosis resistance. Moreover, we identified potential SLC22A3 inhibitors and their synergistic combinations with ferroptosis inducers or cisplatin, which suppress tumor growth in both MES-subtype TNBC patient-derived organoids and in vivo, offering a promising strategy for personalized therapy. These findings suggest that targeting SLC22A3, along with ferroptosis inducers, may offer a promising therapeutic strategy for patients with MES-subtype TNBC. SLC22A3 affects serotonin uptake and metabolism in MES-TNBC cells, providing ferroptosis resistance through two mechanisms. 5-HT acts as a radical-trapping antioxidant, eliminating lipid peroxides and inhibiting ferroptosis. In addition, 5-HT induces histone serotonylation, which enhances histone methylation and facilitates the recognition of methylated histones by transcription initiation factors. This process activates SIRT1 transcription, inhibiting MAOA transcription mediated by FOXO1, thereby reducing 5-HT degradation and promoting ferroptosis resistance. TKIs served as SLC22A3 inhibitors and their combinations with RSL3 or cisplatin, which inhibit tumor growth in vivo, offering new personalized treatment options for MES-TNBC patients.