Targeting PRMT3 Impairs Methylation and Oligomerization of HSP60 to Boost Anti-Tumor Immunity by Activating cGAS/STING Signaling

Hepatocellular carcinoma (HCC) is a highly lethal cancer for which current available treatment options have limited efficacy. Immunotherapy has emerged as a promising therapeutic option for HCC, yet resistance to immunotherapy is a major challenge. Here, we uncover protein arginine methyltransferase 3 (PRMT3) as a novel driver of immunotherapy resistance in HCC. We show that PRMT3 expression is induced by activated T cells in response to immune checkpoint blockade (ICB) via an interferon-gamma (IFNgamma)-STAT1 signaling pathway, and that high PRMT3 expression inversely correlates with tumor-infiltrating CD8 + T cells and predicts poor response to ICB in HCC patients. We demonstrate that genetic depletion and pharmacological inhibition of PRMT3 induce a profound influx of T cells into tumors and activate anti-tumor immunity to suppress HCC progression. Mechanistically, we demonstrate that PRMT3 methylates HSP60, a mitochondrial chaperone protein, at R446, and that this novel post-translational modification is required for HSP60 oligomerization and maintaining mitochondrial homeostasis. We reveal that targeting PRMT3-dependent HSP60-R446 methylation boosts anti-tumor immunity by disrupting mitochondrial function, increasing mitochondrial DNA (mtDNA) leakage, and activating the cGAS/STING pathway, a key innate immune sensor of cytosolic DNA. Importantly, we provide genetic and pharmacologic evidence that targeting PRMT3 enhances anti-PD1 efficacy and anti-tumor immunity in HCC mouse models. Our study identifies PRMT3 as a potential biomarker and therapeutic target to overcome immunotherapy resistance in HCC.