<scp>USP</scp> 4, Transcriptionally Activated by <scp>MEF</scp> 2 <scp>A</scp> , Protects Multiple Myeloma Cells From Endoplasmic Reticulum Stress‐Mediated Apoptosis via Repressing <scp>NR</scp> 4 <scp>A</scp> 1 Ubiquitination

Multiple myeloma (MM) remains an uncurable hematologic cancer due to neoplastic proliferation of plasma cells. The role and detailed mechanisms of Ubiquitin-Specific Protease 4 (USP4) in MM remain to be clarified. Cell viability and apoptosis were evaluated by CCK-8 and TUNEL. Molecular expression was determined by RT-qPCR, Western blotting, and immunohistochemistry. The morphology of endoplasmic reticulum (ER) was observed by ER tracker-red. The protein interaction and ubiquitination level were evaluated by Co-IP. The binding of myocyte enhancer factor 2A (MEF2A) to the USP4 promoter was confirmed by ChIP and dual luciferase reporter assay. The in vivo growth of MM was monitored by subcutaneous xenograft experiments. USP4 knockdown suppressed MM cell proliferation and triggered ER stress and apoptosis, whereas USP4 overexpression resulted in the opposite effect. Mechanistically, USP4 enhanced nuclear receptor subfamily 4 group A member 1 (NR4A1) protein stability by inhibiting its ubiquitination. MEF2A bound to the USP4 promoter to activate its transcription and expression. The promotive effect of sh-USP4 or sh-MEF2A on ER stress and apoptosis of MM cells was counteracted by NR4A1 or USP4 overexpression. MEF2A deficiency restrained in vivo MM growth by activating ER stress and apoptosis through modulation of the USP4/NR4A1 pathway. MEF2A activates USP4 to repress ER stress and apoptosis of MM cells via deubiquitination of NR4A1, thus favoring MM progression. Our findings identify USP4 as an attractive intervention target for MM therapy.