SPAG5 upregulation contributes to enhanced c-MYC transcriptional activity via interaction with c-MYC binding protein in triple-negative breast cancer

Background: Triple-negative breast Cancer (TNBC) is an aggressive breast Cancer subtype that lacks effective therapeutic targets. Sperm-associated antigen 5 (SPAG5) is a mitotic spindle-associated protein that is involved in various biological processes in cervical Cancer and bladder urothelial carcinoma. However, the role of SPAG5 in TNBC remains undefined.

Methods: The expression of SPAG5 was examined in TNBC patients via quantitative real-time polymerase chain reaction (qRT-PCR), western blotting, and immunohistochemistry (IHC). The biological functions of SPAG5 in TNBC and the underlying mechanisms were investigated in vitro and in vivo.

Results: SPAG5 expression was significantly upregulated in TNBC tissues compared with that in paired adjacent noncancerous tissues (ANTs). High SPAG5 expression was associated with increased lymph node metastasis and high risk of local recurrence. SPAG5 protein expression was significantly associated with poor disease-free survival in TNBC. Gene set enrichment analysis of TNBC data from The Cancer Genome Atlas (TCGA) indicated that high SPAG5 expression was significantly associated with cell cycle and the ATR-BRCA pathway. Functional assays demonstrated that SPAG5 expression promoted tumor growth in vitro and in vivo. In addition, SPAG5-silenced cells were more sensitive to the PARP Inhibitor (PARPi) olaparib. Mechanistically, SPAG5 interacted with c-Myc binding protein (MYCBP), thereby increasing MYCBP protein levels and leading to increased c-Myc transcriptional activity, which promoted the expression of the c-Myc target genes: CDC20, CDC25C, BRCA1, BRCA2, and RAD51.Knockdown of MYCBP or c-Myc abolished the SPAG5-induced cell-cycle progression and cell proliferation of TNBC.

Conclusions: Collectively, our results indict that SPAG5 is an efficient prognostic factor in TNBC, and that SPAG5 knockdown increases the sensitivity of TNBC to the PARPi olaparib. SPAG5 promotes tumor growth and DNA repair by increasing c-Myc transcriptional activity via interaction with MYCBP. The SPAG5/MYCBP/c-Myc axis may represent a potential therapeutic target for TNBC treatment.