Research ArticleCancer

The kinase activity of the Ser/Thr kinase BUB1 promotes TGF-β signaling

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Sci. Signal.  06 Jan 2015:
Vol. 8, Issue 358, pp. ra1
DOI: 10.1126/scisignal.2005379

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Placing BUB1 in the TGF-β Pathway

The transforming growth factor–β (TGF-β) pathway regulates cell proliferation and migration, processes involved in development, regeneration, and tumorigenesis. The kinase BUB1, which promotes proper chromosome alignment as cells prepare to divide, also regulates cell proliferation. Nyati et al. connected BUB1 to TGF-β signaling. They found that knocking down BUB1 impaired TGF-β–mediated proliferation of tumor cells—but not by acting at chromosomes. Instead, cytoplasmic BUB1 interacted with TGF-β receptor subunits at the cell surface, promoting the interaction between receptor subunits and between the receptor and downstream signaling proteins. Inhibiting the kinase activity of BUB1 suppressed TGF-β pathway activity in cells in culture and in xenografts. The findings suggest a possible point of crosstalk between the mitotic checkpoint and TGF-β signaling.

Abstract

Transforming growth factor–β (TGF-β) signaling regulates cell proliferation and differentiation, which contributes to development and disease. Upon binding TGF-β, the type I receptor (TGFBRI) binds TGFBRII, leading to the activation of the transcription factors SMAD2 and SMAD3. Using an RNA interference screen of the human kinome and a live-cell reporter for TGFBR activity, we identified the kinase BUB1 (budding uninhibited by benzimidazoles-1) as a key mediator of TGF-β signaling. BUB1 interacted with TGFBRI in the presence of TGF-β and promoted the heterodimerization of TGFBRI and TGFBRII. Additionally, BUB1 interacted with TGFBRII, suggesting the formation of a ternary complex. Knocking down BUB1 prevented the recruitment of SMAD3 to the receptor complex, the phosphorylation of SMAD2 and SMAD3 and their interaction with SMAD4, SMAD-dependent transcription, and TGF-β–mediated changes in cellular phenotype including epithelial-mesenchymal transition (EMT), migration, and invasion. Knockdown of BUB1 also impaired noncanonical TGF-β signaling mediated by the kinases AKT and p38 MAPK (mitogen-activated protein kinase). The ability of BUB1 to promote TGF-β signaling depended on the kinase activity of BUB1. A small-molecule inhibitor of the kinase activity of BUB1 (2OH-BNPP1) and a kinase-deficient mutant of BUB1 suppressed TGF-β signaling and formation of the ternary complex in various normal and cancer cell lines. 2OH-BNPP1 administration to mice bearing lung carcinoma xenografts reduced the amount of phosphorylated SMAD2 in tumor tissue. These findings indicated that BUB1 functions as a kinase in the TGF-β pathway in a role beyond its established function in cell cycle regulation and chromosome cohesion.

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