cGAS suppresses hepatocellular carcinoma independent of its cGAMP synthase activity.

Cyclic GMP-AMP synthase (cGAS) is a key innate immune sensor that recognizes cytosolic DNA to induce immune responses against invading pathogens. The role of cGAS is conventionally recognized as a nucleotidyltransferase to catalyze the synthesis of cGAMP upon recognition of cytosolic DNA, which leads to the activation of STING and production of type I/III interferon to fight against the pathogen. However, given that hepatocytes are lack of functional STING expression, it is intriguing to define the role of cGAS in hepatocellular carcinoma (HCC), the liver parenchymal cells derived malignancy. In this study, we revealed that cGAS was significantly downregulated in clinical HCC tissues, and its dysregulation contributed to the progression of HCC. We further identified cGAS as an immune tyrosine inhibitory motif (ITIM) containing protein, and demonstrated that cGAS inhibited the progression of HCC and increased the response of HCC to sorafenib treatment by suppressing PI3K/AKT/mTORC1 pathway in cellular and animal models. Mechanistically, cGAS recruits SH2-containing tyrosine phosphatase 1 (SHP1) via ITIM, and dephosphorylates p85 in phosphatidylinositol 3-kinase (PI3K), which leads to the suppression of AKT-mTORC1 pathway. Thus, cGAS is identified as a novel tumor suppressor in HCC via its function independent of its conventional role as cGAMP synthase, which indicates a novel therapeutic strategy for advanced HCC by modulating cGAS signaling.

Inhibiting Src-mediated PARP1 tyrosine phosphorylation confers synthetic lethality to PARP1 inhibition in HCC.

Hepatocellular carcinoma (HCC), a heterogeneous cancer with high mortality, is resistant to single targeted therapy; thus, combination therapy based on synthetic lethality is a promising therapeutic strategy for HCC. Poly (adenosine diphosphate [ADP]-ribose) polymerase 1 (PARP1) is the most recognized target for synthetic lethality; however, the therapeutic effect of PARP1 inhibition on HCC is disappointing. Therefore, exploring new synthetic lethal partners for the efficient manipulation of HCC is urgently required. In this study, we identified Src and PARP1 as novel synthetic lethal partners, and the combination therapy produced significant anti-tumor effects without causing obvious side effects. Mechanistically, Src interacted with PARP1 and phosphorylated PARP1 at the Y992 residue, which further mediated resistance to PARP1 inhibition. Overall, this study revealed that Src-mediated PARP1 phosphorylation induced HCC resistance to PARP1 inhibitors and indicated a therapeutic window of the Y992 phosphorylation of PARP1 for HCC patients. Moreover, synthetic lethal therapy by co-targeting PARP1 and Src have the potential to broaden the strategies for HCC and might benefit HCC patients with high Src activation and resistance to PARP1 inhibitors alone.

FGFR3 Destabilizes PD-L1 via NEDD4 to Control T-cell-Mediated Bladder Cancer Immune Surveillance.

Fibroblast growth factor receptor 3 (FGFR3) is frequently activated by mutation or overexpression, and it is a validated therapeutic target in urothelial carcinoma (UC) of the bladder. However, the role and detailed molecular mechanism of FGFR3 in the immune microenvironment of bladder cancer remain largely unknown. Here, we demonstrate that inhibition of FGFR3 in FGFR3-activated bladder cancer elevates PD-L1 protein levels by affecting its ubiquitination, thereby inhibiting the antitumor activity of CD8+ T cells. Tissue microarray analysis in human UC showed an inverse correlation between FGFR3 and PD-L1. Furthermore, NEDD4, an E3 ubiquitin ligase of the NEDD4 family of proteins, was phosphorylated by FGFR3 activation and served as a regulator of PD-L1 ubiquitination. Mechanistically, NEDD4 interacted with PD-L1 and catalyzed Lys48 (K48)-linked polyubiquitination of PD-L1. In mice bearing NEDD4 knockout bladder cancer, CD8+ T-cell infiltration and antitumor activity were significantly inhibited due to PD-L1 upregulation in bladder cancer cells. Furthermore, multiple FGFR3-activated tumor-bearing mouse models suggested that attenuated CD8+ T-cell-mediated antitumor efficacy following FGFR3-targeted therapy could be rescued by a combination with anti-PD-1 immunotherapy, which leads to effective tumor suppression. This study establishes a key molecular link between targeted therapy and immune surveillance and identifies NEDD4 as a crucial E3 ubiquitin ligase that targets PD-L1 for degradation in FGFR3-activated bladder cancer. These findings may potentially be exploited for combination therapies in UC of the bladder and possibly other malignancies with activated FGFR3. SIGNIFICANCE: NEDD4 links two important molecules associated with targeted therapy and immune surveillance, providing mechanistic rationale and preclinical support for immuno-targeted combination therapy for FGFR3-activated bladder cancer.