PARG inhibition suppresses breast cancer progression and potentiates immunoresponse through MTDH degradation mediated by E3 ligase ITCH-dependent ubiquitination.

PARG plays an important role in cancer progression and immunoresponse, but its role in breast cancer is not yet fully elucidated. The advanced breast cancer is still a persistent challenge in clinical oncology. Despite the encouraging progress made in cancer immunotherapy, its efficacy is still limited by tumor malignancy and the immunosuppressive microenvironment. Through comprehensive proteomic analysis employing liquid chromatography-tandem mass spectrometry, we identified ITCH-a HECT-family E3 ubiquitin ligase-as a novel interaction partner of PARG. Co-immunoprecipitation and ubiquitination assays revealed that the PARG inhibition markedly enhances the PARylation of the E3 ubiquitin ligase ITCH. This triggers its autoubiquitination-dependent activation, after which activated ITCH catalyzes the ubiquitination of the oncoprotein metadherin (MTDH). This ultimately leads to MTDH's proteasomal degradation and the consequent suppression of tumor growth. Importantly, RNA immunoprecipitation analysis further demonstrated that PARGi-induced ITCH/MTDH potentiates the MHC-I antigen presentation machinery in tumor cells, which correlates with increased infiltration of cytotoxic T lymphocytes within the tumor microenvironment. In invivo validation, combinatorial therapy using the selective PARG inhibitor COH34 with anti-PD-L1 immune checkpoint blockade demonstrated synergistic therapeutic efficacy in a murine breast cancer model. To the best of our knowledge, PARG inhibition plays an important role in limiting cancer progression and potentiating immune response, depending on its regulatory function in PARylation and ubiquitination in breast cancer.