Abstract 1570: Lipid metabolism alterations promote TNBC metastasis and immunotherapy resistance via PD-L1 internalization and signaling

Abstract Programmed death-ligand 1, PD-L1 (CD274), is an immune checkpoint protein upregulated by cancer cells to facilitate immune evasion. PD-L1 also performs oncogenic functions within cancer cells, but the underlying mechanisms of PD-L1 internalization and signaling remain largely unknown. Here, we report a new mechanism whereby altered ceramide metabolism via ceramide synthase 4 (CerS4) drives PD-L1 internalization to potentiate a TGFβ/Shh/β-catenin signaling axis to drive triple-negative breast cancer (TNBC) migration and metastasis. In response to altered ceramide metabolism, we observed that PD-L1 is internalized via clathrin-mediated endocytosis and depends on S278/S279 in its cytoplasmic domain. Once internalized, PD-L1 directly interacts with Caprin-1, an RNA-binding protein previously unassociated with PD-L1 signaling. Through our analysis of clinical specimens, we established a positive association between PD-L1/Caprin-1 interaction and metastatic progression in human TNBC. In terms of mechanism, we determined that the PD-L1/Caprin-1 complex plays a pivotal role in mediating CerS4-dependent cell migration. This complex selectively binds and stabilizes mRNA, resulting in heightened TGFβ/Shh/β-catenin signaling. Conversely, CerS4 restoration in metastatic TNBC cells impaired cellular migration and the interaction between PD-L1 and Caprin-1. Genetic depletion of CerS4 enhanced lung metastasis derived from 4T1 orthotopic allografts and transgenic MMTV-PyMT breast tumors concomitant with PD-L1/Caprin-1 interaction and TGFβ/Shh/β-catenin signaling. Bioinformatics analysis of clinical datasets was used to validate the impact of the CerS4/PD-L1/Caprin-1 axis on metastatic progression in human TNBC. In alignment with enhanced tumor progression, CerS4 depletion in tumors promoted regulatory T-cell infiltration, suggesting an immunosuppressive tumor microenvironment (TME). Treatment with sonidegib, an FDA-approved Hedgehog (Hh) inhibitor, effectively prevented PD-L1 internalization and its interaction with Caprin-1. We hypothesized that sonidegib could synergize with immune checkpoint blockade to sensitize TNBC to immunotherapy, which typically has low response rates. Sonidegib, in combination with αPD-L1 therapy, was significantly more effective in controlling TNBC primary and metastatic tumor growth, especially when Cers4 was downregulated, compared to αPD-L1 monotherapy. Our data also suggested that this potent anti-tumor effect was due to immunotherapy sensitization after combined flow cytometry, multiplexed immunofluorescence, and bulk RNA-sequencing analyses. Altogether, these findings demonstrate a mechanism to define intracellular PD-L1 signaling in response to altered ceramide metabolism, providing a new therapeutic strategy to target TNBC metastasis and improve response to immunotherapy. Citation Format: Wyatt O. Wofford, Jisun Kim, Dosung Kim, Alhaji H. Janneh, Han Gyul Lee, F. Cansu Atilgan, Natalia Oleinik, Elizabeth Hill, Ozge Saatci, Salih Gencer, Paramita Chakraborty, Shikhar Mehrotra, Ozgur Sahin, Besim Ogretmen. Lipid metabolism alterations promote TNBC metastasis and immunotherapy resistance via PD-L1 internalization and signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1570.