Caveolin-1 drives ferroptosis in MDSCs via PKA-DRP1-mediated ER‒mitochondria crosstalk to shape breast cancer immunosuppression.

[BACKGROUND] Caveolin-1 (Cav-1) has been implicated in breast cancer progression and tumor-microenvironment remodeling. We investigated whether and how tumor-derived Cav-1 regulates myeloid-derived suppressor cell (MDSC) fate and function to shape the immunosuppressive tumor immune microenvironment (TIME).

[METHODS] Using murine 4T1 breast cancer models and complementary cell culture systems, Cav-1 was manipulated by overexpression and CRISPR/Cas9 knockout. MDSCs were isolated from tumors or bone marrow and analyzed by flow cytometry, Western blotting, immunofluorescence, transmission electron microscopy and ELISA. Ferroptosis is modulated pharmacologically (RSL3, ferrostatin-1), and reactive oxygen species (ROS) are inhibited by N-acetylcysteine. ER stress (ERS), endoplasmic reticulum-mitochondrial contacts and PKA-DRP1 signaling were investigated via biochemical assays, immunoprecipitation and transcriptome/bioinformatic analyses. Statistical comparisons were performed via Student's t-test and ANOVA.

[RESULTS] High Cav-1 expression is correlated with immune checkpoint markers; enhanced recruitment of MDSCs with an immunosuppressive phenotype; and increased secretion of IL-6, IL-10 and CXCL1. Cav-1 promoted ER stress (increased p-PERK, IRE1α, and GRP78), strengthened ER-mitochondrial contacts and upregulated MAM proteins (GRP75, IP3R, and VDAC1) via PKA-dependent DRP1 phosphorylation. These changes increased intracellular ROS, iron accumulation and lipid peroxidation; downregulated SLC7A11 and GPX4; and increased HMOX1 and oxidized phosphatidylcholine (OxPC) release, which is consistent with ferroptosis in MDSCs. Ferroptosis or ROS inhibition (ferrostatin-1, NAC) attenuated OxPC release, restored T-cell proliferation and function in coculture and limited tumor growth in vivo.

[CONCLUSIONS] Tumor-derived Cav-1 promotes ROS-dependent ferroptosis in MDSCs via PKA-DRP1-mediated ER-mitochondrial crosstalk, causing the release of oxidized phosphatidylcholines that suppress T-cell function and promote an immunosuppressive TME. Targeting this axis may improve the response of breast cancer patients to immunotherapy.