Deubiquitinating enzyme USP39 promotes the growth and metastasis of gastric cancer cells by modulating the degradation of RNA-binding protein RBM39

It has been revealed recently that the RNA-binding motif protein RBM39 is highly expressed in several cancers, which results in poor patient survival. However, how RBM39 is regulated in gastric cancer cells is unknown. Here, affinity purification-mass spectrometry and a biochemical screening are employed to identify the RBM39-interacting proteins and the deubiquitinating enzymes that regulate the RBM39 protein level. Integration of the data obtained from these two approaches uncovers USP39 as the potential deubiquitinating enzyme that regulates RBM39 stability. Bioinformatic analysis discloses that USP39 is increased in gastric cancer tissues and its elevation shortens the duration of overall survival for gastric cancer patients. Biochemical experiments verify that USP39 and RBM39 interact with each other and highly colocalize in the nucleus. Expression of USP39 elevates while USP39 knockdown attenuates the RBM39 protein level and their interaction regulates this modulation and their colocalization. Mechanistic studies reveal that USP39 reduces the K48-linked polyubiquitin chains on RBM39, thus enhancing its stability and increasing the protein level by preventing its proteasomal degradation. USP39 overexpression promotes while its knockdown attenuates the growth, colony formation, migration, and invasion of gastric cancer cells. Interestingly, overexpression of RBM39 partially restores the impact of USP39 depletion, while RBM39 knockdown partially abolishes the effect of USP39 overexpression on the growth, colony formation, migration, and invasion of gastric cancer cells. Collectively, this work identifies the first DUB for RBM39 and elucidates the regulatory functions and the underlying mechanism, providing a possible alternative approach to suppressing RBM39 by inhibiting USP39 in cancer therapy.