Epidermal growth factor receptor stabilizes programmed death ligand 1 by glycosylation in colorectal cancer with microstatellite instability status

Programmed death ligand-1 (PD-L1) is involved in inhibiting of T lymphocyte proliferation, producing cytokine, cytolytic activity, and suppressing of the immune response. Genes with molecular alterations involved in DNA mismatch repair promote cancer initiation and tumor progression. Clinical studies show that colorectal cancer (CRC) patients harboring microsatellite instability (MSI) have a higher anti-programmed cell death protein 1/PD-L1 immunotherapy response ratio compared with microsatellite stable subgroup patients. The underlying mechanism has however remained unclear. Here, we found that compared with microsatellite stable samples, PD-L1 was glycosylated and highly expressed both in MSI CRC cell lines and tissue samples. Specifically, PD-L1 was N-glycosylated at its N35, N192, N200, and N219 sites, and the four glycosylation sites were all responsible for PD-L1 degradation. Additionally, non-glycosylated PD-L1 underwent rapid degradation compared with glycosylated PD-L1 through the 26S proteasome pathway. The faster degradation of the non-glycosylated PD-L1 was ascribed to its binding to glycogen synthase kinase 3β via ubiquitination. This degradation phenotype was, however, not observed for glycosylated PD-L1. Significantly, glycosylated PD-L1 was up-regulated by activated epidermal growth factor receptor in MSI CRC cells. Together, our results indicate that epidermal growth factor receptor stabilized PD-L1 via glycosylation in MSI CRC cells, uncovering a novel role of PD-L1 in MSI CRC immunosuppression and disease progression. The study was approved by the Clinical Ethics Review Committee at the Six Affiliated Hospital of Sun Yat-sen University, China (Approval No. 2019ZSLYEC-005). Key words: epidermal growth factor receptor; glycogen synthase kinase 3β; glycosylation; microsatellite instability; microsatellite stability; programmed death ligand-1