SummaryBRD4 belongs to the bromodomain and extraterminal (BET) family of chromatin reader proteins that bind acetylated histones and regulate gene expression. Pharmacological inhibition of BRD4 by BET inhibitors (BETi) has indicated antitumor activity against multiple cancer types. We show that BRD4 is essential for the repair of DNA double-strand breaks (DSBs) and mediates the formation of oncogenic gene rearrangements by engaging the non-homologous end joining (NHEJ) pathway. Mechanistically, genome-wide DNA breaks are associated with enhanced acetylation of histone H4, leading to BRD4 recruitment, and stable establishment of the DNA repair complex. In support of this, we also show that, in clinical tumor samples, BRD4 protein levels are negatively associated with outcome after prostate cancer (PCa) radiation therapy. Thus, in addition to regulating gene expression, BRD4 is also a central player in the repair of DNA DSBs, with significant implications for cancer therapy.
Approximately 50% of prostate cancers are associated with gene fusions of the androgen-regulated gene, TMPRSS2, to the oncogenic ETS transcription factor, ERG. The three-dimensional proximity of TMPRSS2 and ERG genes, in combination with DNA breaks facilitate the formation of TMPRSS2-ERG gene fusions. However, the origins of DNA breaks that underlie gene fusion formation in prostate cancers are far from clear. We demonstrate a role for inflammation induced oxidative stress in the formation of DNA breaks leading to recurrent TMPRSS2-ERG gene fusions. The transcriptional status and epigenetic features of the target genes influence this effect. Importantly, inflammation induced de novo genomic rearrangements are blocked by homologous recombination (HR) and promoted by non-homologous end-joining (NHEJ) pathways. In conjunction with the association of proliferative inflammatory atrophy (PIA) with human prostate cancer, our results support a working model in which recurrent genomic rearrangements induced by inflammatory stimuli leads to the development of prostate cancer.
Recently, the combination of androgen deprivation therapy (ADT) and radiation therapy (RT) has been shown to block the androgen receptor (AR)-driven DNA damage response (DDR) and enhance RT-mediated cell kill of prostate cancer (PCa). Since ADT may induce expression of AR splice variants (ARVs) we hypothesized that ARVs can drive DDR and mediate resistance to combined ADT and RT. Herein, we demonstrate that ARVs can increase the clonogenic survival of PCa cells following RT in an ADT-independent manner. RT induces the interaction between ARVs and a DDR driver, the DNA-dependent protein kinase catalytic subunit (DNA-PKc). Pharmacological inhibition of DNA-PKc blocks its interaction with ARVs and results in persistence of DNA damage, increased tumor cell kill and improved PCa cell survival following RT. These results indicate that combinatorial targeting of DNA-PKc with ADT and RT may be an effective strategy for overcoming radioresistance when treating clinically localized PCa.
Cisplatin and its analogues are the most commonly used agents in the treatment of head and neck squamous cell carcinoma. In this study, we investigated a possible role of epidermal growth factor (EGF) receptor (EGFR) phosphorylation and degradation in cisplatin-induced cytotoxicity. Cisplatin treatment led to an increase in initial EGFR phosphorylation at Y1045, the binding site of ubiquitin ligase, Casitas B-lineage lymphoma (c-Cbl), followed by ubiquitination in the relatively cisplatin-sensitive cell lines. However, cisplatin-resistant cell lines underwent minimal EGFR phosphorylation at the Y1045 site and minimal ubiquitination. We found that EGFR degradation in response to cisplatin was highly correlated with cytotoxicity in seven head and neck cancer cell lines. Pretreatment with EGF enhanced cisplatin-induced EGFR degradation and cytotoxicity, whereas erlotinib pretreatment blocked EGFR phosphorylation, degradation, and cisplatin-induced cytotoxicity. Expression of a mutant Y1045F EGFR, which is relatively resistant to c-Cbl-mediated degradation, in Chinese hamster ovary cells and the UMSCC11B human head and neck cancer cell line protected EGFR from cisplatin-induced degradation and enhanced cell survival compared with wild-type (WT) EGFR. Transfection of WT c-Cbl enhanced EGFR degradation and cisplatin-induced cytotoxicity compared with control vector. These results show that cisplatin-induced EGFR phosphorylation and subsequent ubiquitination and degradation is an important determinant of cisplatin sensitivity. Our findings suggest that treatment with an EGFR inhibitor before cisplatin would be antagonistic, as EGFR inhibition would protect EGFR from cisplatin-mediated phosphorylation and subsequent ubiquitination and degradation, which may explain the negative results of several recent clinical trials. Furthermore, they suggest that EGFR degradation is worth exploring as an early biomarker of response and as a target to improve outcome.
The epidermal growth factor receptor (EGFR) has been targeted for inhibition using tyrosine kinase inhibitors and monoclonal antibodies, with improvement in outcome in subsets of patients with head and neck, lung, and colorectal carcinomas. We have previously found that EGFR stability plays a key role in cell survival after chemotherapy and radiotherapy. Heat shock protein 90 (HSP90) is known to stabilize mutant EGFR and ErbB2, but its role in cancers with wild-type (WT) WT-EGFR is unclear. In this report, we demonstrate that fully mature, membrane-bound WT-EGFR interacts with HSP90 independent of ErbB2. Further, the HSP90 inhibitors geldanamycin (GA) and AT13387 cause a decrease in WT-EGFR in cultured head and neck cancer cells. This decrease results from a significantly reduced half-life of WT-EGFR. WT-EGFR was also lost in head and neck xenograft specimens after treatment with AT13387 under conditions that inhibited tumor growth and prolonged survival of the mice. Our findings demonstrate that WT-EGFR is a client protein of HSP90 and that their interaction is critical for maintaining both the stability of the receptor as well as the growth of EGFR-dependent cancers. Furthermore, these findings support the search for specific agents that disrupt HSP90's ability to act as an EGFR chaperone.
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