Abstract
BackgroundTriple negative breast cancer (TNBC) is an aggressive subtype of breast cancer with high proliferative activity. TNBC tumors exhibit elevated MYC expression and altered expression of MYC regulatory genes, which are associated with tumor progression and poor prognosis; however, the underlying mechanisms by which MYC retains its high expression and mediates TNBC tumorigenesis require further exploration.MethodsACTL6A regulation of MYC and its target gene, CDK2, was defined using Co-IP, mass spectrometry and ChIP assays. To study the role of ACTL6A in TNBC, we performed soft-agar, colony formation, flow cytometry and tumor formation in nude mice. CDK2 inhibitor and paclitaxel were used in testing combination therapy in vitro and in vivo.ResultsACTL6A bound MYC to suppress glycogen synthase kinase 3 beta (GSK3β)-induced phosphorylation on MYC T58, which inhibited ubiquitination of MYC and stabilized it. Moreover, ACTL6A promoted the recruitment of MYC and histone acetyltransferase KAT5 on CDK2 promoters, leading to hyperactivation of CDK2 transcription. ACTL6A overexpression promoted, while silencing ACTL6A suppressed cell proliferation and tumor growth in TNBC cells in vitro and in vivo, which was dependent on MYC signaling. Furthermore, co-therapy with paclitaxel and CDK2 inhibitor showed synergistic effects in tumor suppression. Notably, ACTL6A/MYC/CDK2 axis was specifically up-regulated in TNBC and high expression of ACTL6A was correlated to shorter survival in patients with TNBC.ConclusionsThese findings reveal a novel mechanism by which ACTL6A prolongs the retention of MYC in TNBC and suggest that pharmacological targeting ACTL6A/MYC/CDK2 axis might have therapeutic potential in patients with TNBC.
Highlights
Triple negative breast cancer (TNBC) is an aggressive subtype of breast cancer with high proliferative activity
Actin-like protein 6A (ACTL6A) interacts with and stabilizes MYC via inhibiting GSK3β-induced MYC degradation We detected the MYC half-life in TNBC and non-TNBC cell lines and found that the half-life was prolonged in four TNBC cell lines (BT-549, MDA-MB-231, MDAMB-468 and SUM159PT), compared with that in nonTNBC cells (MCF-7, BT-474, ZR-75-1 and SKBR-3), suggesting that the protein stability of MYC was upregulated in TNBC (Supplementary Figure 1A)
Immunoprecipitated proteins with anti-HA antibody were detected by MYC antibody and whole cell lysates were immunoblotted as loading controls. h The truncations of MYC and ACTL6A were based on known functional domain. i, j Detailed interactions between MYC and ACTL6A were analyzed by IP assays. k IP assay showed that overexpressing ACTL6A substantially decreased the interaction between MYC and GSK3β. l Transfected with wild-type MYC and different mutant MYC into SUM159PT cells and were analyzed by western blotting to detect the phosphorylated levels of MYC
Summary
Triple negative breast cancer (TNBC) is an aggressive subtype of breast cancer with high proliferative activity. TNBC shows highly malignant features such as rapid recurrence, early onset age, greatly metastatic potential, and the most noticeable one is high proliferative potential [7,8,9]. This prominent biological characteristic means that a majority of patients with TNBC suffer rapid recurrence within 1 or 2 years after surgery or chemotherapy, which results in treatment failure and a short 5-year overall survival [2, 5, 10, 11]. The mechanisms by which MYC is upregulated in TNBC, and how MYC promotes the malignant progress of TNBC remain unclear
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