Proanthocyanidins enhance antitumor immunity by promoting ubiquitin-proteasomal PD-L1 degradation via stabilization of LKB1 and SYVN1

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Abstract

Programmed cell death 1 ligand 1–targeted (PD-L1–targeted) immune checkpoint inhibitors are revolutionizing cancer therapy. However, strategies to induce endogenous PD-L1 degradation represent an emerging therapeutic paradigm. Here, we identified proanthocyanidins (PC) as a potent inducer of PD-L1 degradation through an endoplasmic reticulum–associated degradation (ERAD) mechanism. Mechanistically, PC exerted dual effects: First, it targeted and stabilized LKB1 to activate AMPK in tumor cells, subsequently inducing the phosphorylation of PD-L1 at Ser195 — a disruption that in turn impaired glycosylation of PD-L1 and promoted its retention in the ER. Second, PC directly bound to the E3 ubiquitin ligase SYVN1 to increase its protein stability, which strengthened PD-L1–SYVN1 binding, thereby accelerating K48-linked ubiquitination and proteasomal degradation of ER-retained PD-L1. This cascade culminated in the activation of CD8+ T cell–dominated antitumor immune responses, accompanied by suppression of myeloid-derived suppressor cells and regulatory T cells. In preclinical models of lung and colorectal cancer, PC exhibited synergistic antitumor efficacy when combined with anti–cytotoxic T lymphocyte antigen 4 (anti–CTLA-4) antibodies. Notably, PC also potently inhibited the progression of azoxymethane/dextran sodium sulfate–induced orthotopic colorectal cancer in mice. Collectively, our findings unveil an antitumor mechanism of PC, establishing this small-molecule compound as an ERAD pathway–exploiting immune checkpoint modulator with promising translational potential for cancer therapy.

Authors

Mengting Xu, Xuwen Lin, Hanchi Xu, Hongmei Hu, Xinying Xue, Qing Zhang, Dianping Yu, Saisai Tian, Mei Xie, Linyang Li, Xiaoyu Tao, Xinru Li, Simeng Li, Shize Xie, Yating Tian, Xia Liu, Hanchen Xu, Qun Wang, Weidong Zhang, Sanhong Liu