Michelolide Enhances the Anticancer Efficacy of Radiation by Downregulating PD-L1 Protein Levels in Tumor Cells

Breast cancer is a type of cancer with the highest incidence and mortality rates among women. PD-L1 suppresses the proliferation and activation of T cells, thereby enabling cancer cells to evade immune surveillance and facilitating tumor progression. Micheliolide (MCL) is a guaianolide-type sesquiterpene lactone with broad biological activities. Our results revealed that radiation upregulates PD-L1 expression in breast cancer cells, while MCL pretreatment can inhibit this effect. Bioinformatics analysis combined with shRNA interference experiments confirmed that radiation upregulates PD-L1 by activating the IRF1-STAT1 signaling pathway, while MCL represses PD-L1 transcription by suppressing this pathway. In addition, MCL also downregulates PD-L1 protein level through accelerating proteasomal degradation of PD-L1. In vivo experiments demonstrated that MCL combined with radiotherapy significantly inhibits the growth of syngeneic tumors and increases intratumoral CD8+ T cell infiltration and the frequencies of granzyme B-positive cells. Taken together, our results indicate that MCL enhances T-cell-mediated antitumor immunity and improves radiotherapy efficacy through inhibiting IRF1-STAT1 signaling pathway-driven PD-L1 transcription and promoting PD-L1 protein degradation. This study provides a theoretical basis for the clinical application of MCL as an immunomodulator and radiosensitizer.