TRIM28 promotes tumor growth and metastasis in breast cancer by targeting the BRD7 protein for ubiquitination and degradation.

Purpose: Bromodomain-containing protein 7 (BRD7) is downregulated and functions as a tumor suppressor in many types of cancers including breast cancer, and the dysregulation of BRD7 expression is closely related to the development and progression of breast cancer. Whereas little attention has been focused on the regulation of BRD7 protein levels in breast cancer, which needs to be further elucidated.

Methods: The protein stability of BRD7 in breast cancer cells and BRD7 protein level in breast cancer tissues was examined by Western Blotting. The potential E3 ubiquitin ligase proteins that interact with the BRD7 was screened by coimmunoprecipitation combined with mass spectrometry analysis in MDA-MB-231 cells. We proved the interaction between BRD7 and tripartite motif containing 28 (TRIM28) through Co-Immunoprecipitation (Co-IP) and immunofluorescence assays. Co-IP and ubiquitination assay were used to explore the specific binding domain between BRD7 and TRIM28 and the ubiquitination site of BRD7. The effects of TRIM28 on the BRD7 protein stability and ubiquitination level was investigated by qPCR, Western Blot and Co-IP assay. CCK-8 and clone formation assays were carried out to assess the effect of TRIM28 on proliferation ability of breast cancer ells. Transwell assay and wound healing assay were used to investigate the effect of TRIM28 on breast cancer cell invasion and migration. Flow cytometry was used to detect the effect of TRIM28 on cell cycle and apoptosis of breast cancer cells. In addition, we confirmed effect of TRIM28 on tumor growth and metastasis by xenograft and metastatic mouse models. We designed some recovery assays to explore the role of recovery BRD7 in TRIM28-mediated promotion of malignant progression of breast cancer in vivo and in vitro. Finally, the clinical significance of TRIM28 and BRD7 was proved by immunohistochemistry.

Results: In this study, we demonstrated that BRD7 was an unstable protein and might be regulated by ubiquitination in breast cancer; furthermore, we found that the Coiled-Coil region of TRIM28 could directly bind to N-terminal of BRD7, and TRIM28 mediates BRD7 ubiquitination and degradation dependent on K21 by acting as a potential E3 ubiquitin ligase. Moreover, TRIM28 promoted cell proliferation, migration, invasion, xenograft tumor growth and metastasis, thus playing an oncogenic role in breast cancer. Furthermore, the restoration of BRD7 expression in breast cancer significantly reversed the promotional effects of TRIM28 on malignant progression both in vitro and in vivo. In addition, TRIM28 was highly expressed in the biopsy tissues of breast cancer, and its expression was negatively correlated with BRD7 expression and positively correlated with TNM stage and poor prognosis of BC patients.

Conclusions: Our findings provide a novel mechanism by which TRIM28 significantly facilitates BRD7 ubiquitination and degradation, thus promoting breast cancer malignant progression. Targeting the TRIM28/BRD7 axis might be a novel potential strategy for the clinical diagnosis and treatment of breast cancer.