PIAS4 inhibition induces cell cycle arrest and exhibits a synergistic effect in combination with CDK4/6 inhibitor in breast cancer treatment.

Protein post-translational modifications are crucial in driving cancer development and progression. The SUMO E3 ligase PIAS4 regulates various cellular processes, thereby promoting tumour advancement. However, the role of PIAS4 in breast cancer remains unclear. This study aimed to examine the protein levels of PIAS4 in clinical specimens, explore PIAS4 regulates cell cycle mechanism and investigate the synergistic inhibitory effects of PIAS4 and CDK6 inhibition on breast cancer progression. Our findings revealed that PIAS4 was highly expressed in breast cancer and negatively correlated with prognosis. Abrogation of PIAS4 inhibited breast cancer cell proliferation and induced G1 phase cell cycle delay. The primary mechanism involved PIAS4-mediated SUMOylation of CDK6, which enhanced retinoblastoma 1 (RB1) phosphorylation and the transcription of downstream cell cycle genes, facilitating cell cycle G1 phase progression. We found that CDK6 underwent SUMO1 and SUMO2/3 modifications and identified the main SUMO2/3 modification sites. Mutations at these sites inhibit CDK6-mediated RB1 phosphorylation, consequently blocking cell cycle progression in the G1 phase. CDK6 kinase activity is reduced following PIAS4 knockdown, which may be associated with decreased binding of Cyclin D1 to CDK6. Moreover, in mouse xenograft models, combining PIAS4 and CDK6 inhibition enhanced therapeutic efficacy against breast cancer. Therefore, targeting PIAS4 to impede cell cycle progression may be a novel strategy for breast cancer treatment.