Abstract
There has been accumulating evidence for the clinical benefit of chemoradiation therapy (CRT), whereas mechanisms in CRT-recurrent clones derived from the primary tumor are still elusive. Herein, we identified an aberrant BUB1B/BUBR1 expression in CRT-recurrent clones in bladder cancer (BC) by comprehensive proteomic analysis. CRT-recurrent BC cells exhibited a cell-cycle-independent upregulation of BUB1B/BUBR1 expression rendering an enhanced DNA repair activity in response to DNA double-strand breaks (DSBs). With DNA repair analyses employing the CRISPR/cas9 system, we revealed that cells with aberrant BUB1B/BUBR1 expression dominantly exploit mutagenic nonhomologous end joining (NHEJ). We further found that phosphorylated ATM interacts with BUB1B/BUBR1 after ionizing radiation (IR) treatment, and the resistance to DSBs by increased BUB1B/BUBR1 depends on the functional ATM. In vivo, tumor growth of CRT-resistant T24R cells was abrogated by ATM inhibition using AZD0156. A dataset analysis identified FOXM1 as a putative BUB1B/BUBR1-targeting transcription factor causing its increased expression. These data collectively suggest a redundant role of BUB1B/BUBR1 underlying mutagenic NHEJ in an ATM-dependent manner, aside from the canonical activity of BUB1B/BUBR1 on the G2/M checkpoint, and offer novel clues to overcome CRT resistance.
Highlights
Emerging evidence for the administration of chemoradiation therapy (CRT) with curative intent has been increasingly reported in the treatment of various types of cancers [1]
We found BUB1B/BUBR1 as the most upregulated protein in CRT-recurrent tumor as compared with the primary treatment-naïve tumor (Fig. 1b) and immunoblotting from the corresponding samples confirmed the upregulation of BUB1B/BUBR1 in CRT-recurrent tumor
BUB1B/BUBR1 constitutes the mitotic-checkpoint complex (MCC) with Bub3, Mad2, and Cdc20, which inhibits the anaphasepromoting complex/cyclosome (APC/C) and in turns controls mitotic phases [37]. In addition to those canonical functions, the results from the current study suggest that a constitutive upregulation of BUB1B/BUBR1 throughout the cell-cycle phases in CRT-recurrent tumor offers a redundant function to repair double-strand breaks (DSBs), which dominantly exploits mutagenic nonhomologous end joining (NHEJ) rather than precise NHEJ or homologous recombination (HR), leading to the CRT-resistant clones harboring accumulated mutations
Summary
Emerging evidence for the administration of chemoradiation therapy (CRT) with curative intent has been increasingly reported in the treatment of various types of cancers [1]. It is clear that NHEJ is classified into at least two models, referred to as classical nonhomologous end joining (C-NHEJ) and alternative nonhomologous end joining (A-NHEJ). It has been well-documented that C-NHEJ utilizes the core factors, including KU (KU70/80), DNAPKcs, and DNA ligase IV (LIG4) [3], and recent studies have indicated that the consequence of C-NHEJ at DSBs is mostly error-free [4, 5]. A-NHEJ has been considered error-prone and mutagenic with various factors contributing to this repair mechanism, the molecular basis of this is still poorly understood
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