Abstract
Ubiquitin modification of TGFβ pathway components is emerging as a key mechanism of TGFβ pathway regulation. All together, 6 different deubiquitinating enzymes have been identified to regulate the TGFβ pathway with 4 different deubiquitinating enzymes targeting the TGFβ receptor alone. We therefore sought to determine the individual roles of these deubiquitinating enzymes in TGFβ receptor kinetics. We find that while USP4, USP11, and UCH37 appear to directly deubiquitinate the TGFβ receptor, USP15 directly targets the E3 ligase SMURF2 resulting in decreased TGFβ receptor ubiquitination. USP15 deubiquitinates SMURF2 in vivo and in vitro resulting in reduced SMURF2 activity and TGFβ receptor ubiquitination. It has recently been shown that USP15 plays a dual role in the TGFβ pathway. First, USP15 opposes R-SMAD monoubiquitylation and SMURF mediated polyubiquitination, permitting SMAD promoter recognition. Secondly, USP15 binds to the SMAD7-SMURF2- TGFβ receptor complex regulating TGFβ receptor ubiquitination and stability. Our results suggest that SMURF2 is a critical target of USP15 and may explain how USP15 targets multiple nodes in the TGFβ pathway. Furthermore, the USP15 gene is found amplified in glioblastoma, breast cancer and ovarian cancer, and in glioblastoma high expression of USP15 correlates with high TGFβ activity. Moreover, depletion of USP15 decreases the oncogenic capacity of patient-derived glioma-initiating cells mainly due to the repression of TGFβ signaling. Our results show that USP15 regulates the TGFβ pathway and is a critical factor in glioblastoma pathogenesis.