UBR4 Mediates Integrated Stress Response in Renal Tubular Epithelial Cells During AKI

Background: The integrated stress response (ISR) is a conserved biological mechanism that enables cells to adapt to internal and external stressors by modulating protein synthesis. While ISR activation promotes cellular adaptation to injury, excessive ISR activation triggers apoptosis, leading to tissue damage. UBR4, an E3 ubiquitin ligase, recognizes and mediates the ubiquitination and degradation of target proteins containing specific residues. This study investigates the dynamic changes, functional roles, and mechanisms of UBR4 and ISR in acute AKI, aiming to identify potential therapeutic targets for AKI. Methods: Evaluated UBR4 expression and distribution in CIS-induced AKI mouse kidneys and renal biopsy specimens from AKI patients. Constructed AAV9 vectors carrying shCTRL or shUBR4. Mice were divided into four groups: shCTRL+PBS, shCTRL+CIS，shUBR4+PBS, and shUBR4+CIS. Assessments included: H&E staining for renal tubular injury. SCr and BUN measurements. Western blot and RT-qPCR of UBR4, KIM-1, NGAL, HRI, ATF4, and CHOP IF for ATF4 and CHOP nuclear translocation. Cultured HK-2 cells, transfected with UBR4-siRNA, and treated with CIS . Analyzed UBR4, KIM-1, NGAL, HRI, ATF4, and CHOP expression, nuclear translocation via immunofluorescence, and apoptosis via flow cytometry. Evaluated the therapeutic effect of the HRI inhibitor ISRIB in cisplatin-induced AKI, both in vitro and in vivo. Results: UBR4 was weakly expressed in normal kidneys but significantly upregulated in CIS-induced AKI mice. The shUBR4+CIS group exhibited aggravated renal dysfunction, tubular injury, and elevated KIM-1, NGAL, HRI, ATF4, and CHOP levels compared to the shCTRL+CIS group. IF demonstrated enhanced nuclear translocation of ATF4 and CHOP in the shUBR4+CIS group. In vitro, CIS treatment increased UBR4, KIM-1, NGAL, HRI, ATF4, and CHOP expression in HK-2 cells, alongside enhanced nuclear translocation and apoptosis. UBR4 silencing further amplified these effects. ISRIB partially alleviated UBR4 deficiency-induced cellular damage and ISR overactivation. Conclusion: UBR4 is markedly upregulated in renal tubular epithelial cells during AKI. Its deficiency exacerbates renal injury by impairing HRI degradation, leading to excessive activation of the ATF4-CHOP pathway and apoptosis. UBR4 exerts a protective role in AKI by suppressing ISR overactivation, highlighting its potential as a therapeutic target.