Xinyan Li

Radiotherapy is a fundamental modality in cancer treatment, used in the management of more than half of cancer patients. Beyond direct cytotoxicity, ionizing radiation activates innate immune pathways and inflammatory cytokines required for local tumor control and systemic (abscopal) responses. Yet in human tumors, DNA damage-induced immune activation is attenuated compared with the robust responses observed in mice, suggesting a human-biased checkpoint that constrains radioimmunity. Here, we identify a tumor-intrinsic, human-biased farnesoid X receptor (FXR)-tripartite motif containing 22 (TRIM22)-STAT1 proteostatic checkpoint that suppresses radiation-induced type I interferon signaling. Ionizing radiation stabilizes FXR, a nuclear receptor, through ubiquitin-specific peptidase 7 (USP7)-dependent deubiquitination, enabling FXR to transcriptionally induce the E3 ligase TRIM22. TRIM22 selectively binds STAT1 and catalyzes its K48-linked ubiquitination and degradation, extinguishing interferon-stimulated gene (ISG) programs downstream of cGAS-STING signaling. This regulatory circuit is absent in murine tumor cells, which lack a TRIM22 family member capable of destabilizing STAT1, providing a mechanistic explanation for the stronger radiation-induced immune responses observed in mice. Genetic disruption of FXR or TRIM22 preserves STAT1, amplifies type I interferon signaling, and enhances CD8⁺ T-cell activation and abscopal tumor control in peripheral blood mononuclear cell-humanized mice. Pharmacologic modulation of this pathway with ursodeoxycholic acid, a bile acid used clinically with reported FXR-antagonistic activity, suppresses radiation-induced TRIM22, restores STAT1-ISG signaling, and potentiates radiotherapy alone or with anti-PD-1 without overt toxicity in humanized models. Collectively, these findings define a human-biased checkpoint that limits radiotherapy-induced immunity and is absent in murine tumor cells, and highlight the FXR-TRIM22-STAT1 axis as an actionable target to enhance cancer radioimmunotherapy.

Poly (ADP-ribose) polymerase inhibitors (PARPi) demonstrate effective treatment outcomes in ovarian cancer patients with BRCA1/2 mutations or homologous recombination (HR) repair deficiencies, leveraging the principle of synthetic lethality. However, PARPi resistance and HR proficiency remain significant challenges in the clinical management of PARPi, necessitating the development of novel strategies for PARPi therapy in ovarian cancer. Our previous research identified PAK1's involvement in replication stress-induced cytotoxicity. Nonetheless, whether PAK1 also affects HR repair and PARPi sensitivity in ovarian cancer remains unresolved. In this research, we found that the expression of PAK1 correlated with an unfavorable prognosis in ovarian cancer. Depletion of PAK1, introduction of a kinase-dead mutation, or treatment with the inhibitor IPA-3 could reduce HR repair efficiency and increase ovarian cancer cell sensitivity to the PARP inhibitor olaparib. The combination of olaparib and IPA-3 synergistically increased olaparib-induced DNA replication stress and double-stranded breaks. Using cell line-derived xenograft, patient-derived organoid, and patient-derived xenograft models, we discovered that IPA-3 potentiated the therapeutic efficacy of olaparib both in vivo and ex vivo. Collectively, our findings suggest that targeting PAK1 might offer a new avenue for increasing the sensitivity of olaparib and improving the outcomes of ovarian cancer patients.

Figure S2 shows TGS1 is associated with poor prognosis in a variety of cancers

The clinical information for all patient samples in TMA slices

Figure S9 shows sinefungin treatment does not impair HR repair after TGS1 knockdown

Figure S8 shows sinefungin treatment enhances cellular sensitivity to olaparib in PAAD cells

Figure S1 shows TGS1 is upregulated in PAAD and associated with poor prognosis

Figure S6 shows high TGS1 expression is correlated with olaparib resistance in PAAD patient-derived xenografts

Pancreatic adenocarcinoma remains one of the most lethal malignancies, primarily due to its robust resistance to genotoxic therapies, such as chemotherapy and radiotherapy. Understanding the mechanisms underlying this resistance is essential to improve clinical outcomes. In this study, we identified trimethylguanosine synthase 1 (TGS1), previously known for its role in RNA modification, as a critical mediator of homologous recombination (HR) repair that specifically contributes to resistance in pancreatic adenocarcinoma. TGS1 was significantly overexpressed in pancreatic adenocarcinoma tissues, correlating strongly with advanced disease stages, therapy resistance, and poor patient prognosis. Following DNA damage, ATM kinase phosphorylated TGS1 at serine residues S389 and S531, which mediated its direct interaction with BRCA1 and subsequent recruitment of BRCA1 to DNA damage sites. The phosphorylation-dependent interaction enhanced HR repair efficiency, enabling cancer cells to survive genotoxic stress. Depletion or pharmacologic inhibition of TGS1 induced HR deficiency and markedly enhanced sensitivity to DNA-damaging agents, particularly PARP inhibitors, in pancreatic adenocarcinoma cell lines in vitro and in both cell line– and patient-derived xenograft models in vivo. Collectively, these findings uncover an ATM–TGS1–BRCA1 signaling axis that promotes DNA repair and resistance to genotoxic therapies in pancreatic cancer, positioning TGS1 as a promising predictive biomarker and therapeutic target to enhance treatment efficacy.Significance:The ATM–TGS1–BRCA1 signaling axis is a critical regulator of DNA repair that can be targeted to overcome genotoxic therapy resistance, providing a promising therapeutic strategy that could transform pancreatic cancer treatment.

Figure S3 shows TGS1 promotes BRCA1 recruitment and HR repair independent of methyltransferase activity