Application and mechanisms of targeting BRD4 in osteosarcoma.

OBJECTIVES: Metastasis is the primary cause of death in osteosarcoma, and current clinical treatments remain limited. BRD4, a key epigenetic regulator, has shown therapeutic promise in various cancers through its inhibition. However, the mechanistic role of BRD4 in osteosarcoma remains poorly understood. This study aims to elucidate the molecular mechanisms by which BRD4 regulate osteosarcoma progression and to explore novel therapeutic strategies.

METHODS: Immunofluorescence was used to assess BRD4 expression levels in a tissue microarray containing 80 osteosarcoma samples from different patients. The Gene Expression Omnibus (GEO) dataset (GSE42352, containing survival data from 88 osteosarcoma patients) was downloaded to perform Kaplan-Meier survival analysis based on BRD4 gene expression levels. In vivo, an orthotopic intramedullary osteosarcoma model was established using HOS cells in C57 mice, followed by treatment with varying doses of the BRD4 inhibitor (+)-JQ1. Micro-CT, 3D reconstruction of bone tissue, and HE staining were employed to evaluate pathological changes in bone and intestinal lymph nodes. In vitro, cell viability was measured using the methyl thiazolyl tetrazolium (MTT) assay, while colony formation and Transwell assays assessed proliferative and invasive capacities. Chromatin-bound BRD4 was analyzed via co-immunoprecipitation combined with mass spectrometry (Co-IP/MS), and O-GlcNAc glycosylation sites and glycan chains of BRD4 were identified using Co-IP with Nano-LC MS/MS. Real-time PCR and Western blotting were used to analyze the relative mRNA and protein expression levels of target genes, respectively.

RESULTS: BRD4 was positively expressed in 61.25% (49/80) of osteosarcoma tissues. Patients with high BRD4 expression exhibited significantly shorter survival times (P<0.05). In the orthotopic mouse model, intervention with (+)-JQ1, a potent and commonly used BETi, significantly inhibited tumor growth in vivo and reduced bone destruction (P<0.05). (+)-JQ1 treatment significantly suppressed the proliferation (P<0.001), invasion (P<0.001), and migration (P<0.05) of HOS cells. In osteosarcoma cells, BRD4 exhibited O-GlcNAc modifications at both N- and C- C-termini, particularly at Thr73, which is essential for protein stability. This modification also contributed to the activation of the EGFR tyrosine kinase inhibitor resistance pathway (KEGG Pathway: hsa01521). (+)-JQ1 treatment displaced BRD4 from enhancers and downregulated the transcription of pathway-related genes, such as EGFR and PDGFC, thereby suppressing the malignant behavior of osteosarcoma cells.

CONCLUSIONS: BRD4 promotes osteosarcoma progression via O-GlcNAc modification at Thr73 and plays a crucial role in tumor growth and metastasis.