Lipid Metabolism Alterations Induce PD-L1 Internalization and Signaling to Regulate Breast Cancer Metastasis and Response to Immunotherapy [EXOSC10 RNA-IP-seq]
Programmed death-ligand 1, PD-L1 (CD274), is expressed by cancer cells to facilitate immune evasion. PD-L1 also exerts pro-survival functions in cancer cells, and mechanisms that regulate intracellular PD-L1 signaling remain largely unknown. Here, we report a new mechanism whereby internalization of PD-L1 in response to alterations of bioactive lipid/ceramide metabolism by ceramide synthase 4 (CerS4) induces sonic-hedgehog (Shh) and TGF-b-receptor signaling to enhance tumor metastasis in triple-negative breast cancers (TNBC), which are highly resistant to immunotherapy. Mechanistically, we show here that internalized PD-L1, mediated by its cytoplasmic domain, including the S278/S279 residues, in response to CerS4/C18/C20-ceramide downregulation or genetic loss, interacts with an RNA-binding protein Caprin-1 in various metastatic breast cancer models, including 4T1 orthotopic breast allografts and mammary tumors developed in MMTV-PyMT/CerS4-/- compared to MMTV-PyMT/CerS4+/+ mice, consistent with Shh/TGFBR1/Wnt/b-catenin activation in-vivo. Genetic loss or molecular knockdown of CerS4 in breast tumors also resulted in increased infiltration of FoxP3+ T regs that are known to suppress anti-tumor immunity. Targeting the CerS4/Shh/PD-L1 axis using Sonidegib and anti-PD-L1 antibody in mice containing metastatic breast tumors in which CerS4 is downregulated vastly decreased tumor growth and metastasis, leading to inhibition of PD-L1 internalization and Shh/Wnt signaling, restoring anti-tumor immune response. These data, validated in clinical samples and multiple patient datasets, demonstrate a mechanism to define intracellular metastatic signaling by PD-L1 internalization in response to alterations of ceramide metabolism, providing a new therapeutic strategy to improve immunotherapy responses in metastatic TNBCs. Overall design: To investigate the role of the PD-L1/Caprin-1 complex on  mRNA degradation in the context of altered lipid metabolsim, RNA-immunprecipatation using anti-EXOSC10 antibodies was performed on LM2-4175 cells stably expressing a dox-inducible CerS4 construct. IgG was used as a control. The primary comparison was between mRNA profiles bound to EXOSC10 in untreated (no dox) versus dox-treated cells.