E3 Ubiquitin Ligase Mindbomb1 Is a Novel Negative Regulator of Tumor-Suppressive BMP Signaling

Mindbomb1 (MIB1), an E3 ubiquitin ligase required for NOTCH activation, has been identified as a strongly selective cancer dependency in genome-wide loss-of-function screens in over 1000 cancer cell lines. However, MIB1-correlating dependencies in these screens unexpectedly linked MIB1 not to Notch but rather to the family of Transforming Growth Factor-β (TGF-β) and Bone Morphogenic Protein (BMP) signaling molecules. Here, using genetic and pharmacologic methods, we show that MIB1 loss phenocopies BMP tumor suppressive function. MIB1-dependent cancer cell lines were sensitive to BMP ligands, and MIB1 loss selectively enhanced BMP signaling, as evidenced by SMAD1/5/9 phosphorylation and transcriptional responses that correlated with increased BMP receptor protein levels. Growth inhibition and enhanced signaling were both reversed by inhibitors of BMP type 1 receptors, demonstrating that these MIB1 effects directly reflect a function for MIB1 as a negative regulator of BMP signaling. Unlike wild-type MIB1, re-expression of RING-domain-deleted MIB1 failed to rescue growth inhibition in MIB1-depleted cells, suggesting that negative regulation depends on E3 ligase activity. Supporting this regulatory role, we found that MIB1 and the BMP type II receptor BMPR2 physically associate in both cancer and endothelial cells. By revealing this previously unknown role for MIB1 as a Notch-independent negative regulator of BMP signaling, our study expands the significance of MIB1 in cancer biology, with implications for targeting BMP signaling in cancer and other diseases.