Ceramide metabolism and the regulation of PD-L1 signaling to control metastasis and resistance to immunotherapy in TNBC

SUMMARY Metastasis is a significant cause of mortality for patients with triple-negative breast cancer (TNBC), with a median overall survival of less than one year. The introduction of immunotherapy has revolutionized the systemic treatment of metastatic cancer. However, there are underlying resistance mechanisms that limit response to immunotherapy in TNBCs. Increased PD-L1 cell surface expression is associated with improved response to α- PD-L1 or α-PD-1 therapies. One potential mechanism by which tumor cells acquire resistance to immunotherapy is by reducing PD-L1 expression in the cell membrane. Furthermore, recent reports have demonstrated that internalized (non-membranous) PD-L1 participates in oncogenic/pro-metastatic signaling within cancer cells without much mechanistic understanding. It is known that lipid metabolism and signaling alterations play a role in cancer cell migration/invasion and tumor metastasis, including reductions of bioactive sphingolipid ceramide that mediates pro-apoptotic and anti-proliferative signaling. We recently showed that reductions in ceramide synthase 4 (CerS4)-generated long-chain C18-C20-ceramides induce TNBC migration and metastasis by activating the TGF-β/Sonic hedgehog (Shh) signaling axis. However, the regulatory components of this mechanism remain unknown. Based on our published and unpublished preliminary data, this application is designed to test a novel overall hypothesis that the reduction of CerS4-generated ceramide signaling enhances PD-L1 internalization, induces pro-metastatic signaling and facilitates resistance to immunotherapy in TNBC. There are two Specific Aims proposed: Aim 1 is designed to define the mechanism by which reduced CerS4/ceramide signaling regulates PD-L1 internalization and its intracellular metastatic signaling. Aim 2 is designed to determine how CerS4/ceramide signaling regulates the PD-L1/Caprin-1 complex to control TNBC metastasis and resistance to immunotherapy. Overall, this application describes a novel resistance mechanism to immunotherapy and intracellular PD-L1-dependent pro-metastatic signaling driven by lipid/ceramide metabolism alterations. Combination therapies targeting this signaling network could reduce the metastatic burden and improve metastatic TNBC response to immunotherapy, collectively addressing clinically unmet needs in this application.