Targeting Nuclear Receptor Coactivator SRC‐1 Prevents Colorectal Cancer Immune Escape by Reducing Transcription and Protein Stability of PD‐L1

Abstract Programmed death‐ligand 1 (PD‐L1) is overexpressed in multiple cancers and critical for their immune escape. It has previously shown that the nuclear coactivator SRC‐1 promoted colorectal cancer (CRC) progression by enhancing CRC cell viability, yet its role in CRC immune escape is unclear. Here, we demonstrate that SRC‐1 is positively correlated with PD‐L1 in human CRC specimens. SRC‐1 deficiency significantly inhibits PD‐L1 expression in CRC cells and retards murine CRC growth in subcutaneous grafts by enhancing CRC immune escape via increasing tumor infiltration of CD8 + T cells. Genetic ablation of SRC‐1 in mice also decreases PD‐L1 expression in AOM/DSS‐induced murine CRC. These results suggest that tumor‐derived SRC‐1 promotes CRC immune escape by enhancing PD‐L1 expression. Mechanistically, SRC‐1 activated JAK‐STAT signaling by inhibiting SOCS1 expression and coactivated STAT3 and IRF1 to enhance PD‐L1 transcription as well as stabilized PD‐L1 protein by inhibiting proteasome‐dependent degradation mediated by speckle type POZ protein (SPOP). Pharmacological inhibition of SRC‐1 improved the antitumor effect of PD‐L1 antibody in both subcutaneous graft and AOM/DSS‐induced murine CRC models. Taken together, these findings highlight a crucial role of SRC‐1 in regulating PD‐L1 expression and targeting SRC‐1 in combination with PD‐L1 antibody immunotherapy may be an attractive strategy for CRC treatment.