Background: Mucin-type O-glycosylation is one of the most abundant types of O-glycosylation and plays important roles in various human carcinomas, including breast cancer. A large family of polypeptide N-acetyl-alpha-galactosaminyltransferases (GALNTs) initiate and define sites of mucin-type O-glycosylation. However, the specific mechanisms underlying GALNT8 expression and its roles in tumorigenesis remain poorly characterized. Methods: GALNT8 expression was assessed in 140 breast cancer patients. Immunofluorescence, immunoprecipitation, lectin blot and quantitative real-time PCR were used to investigate the expression of GALNT8 and its role in regulating estrogen receptor alpha (ER alpha) via bone morphogenetic protein (BMP) signaling. Results: The expression of GALNT8 was associated with breast cancer patient survival. GALNT8 downregulation was associated with a reduction in ER alpha levels, while GALNT8 overexpression elevated the transcription and protein levels of ER alpha and suppressed colony formation, suggesting an important role of GALNT8 in cancer cell proliferation. Conversely, GALNT8 knockdown led to the inhibition of BMP/SMAD/RUNX2 axis, which decreased ER alpha transcription. Further analysis suggested that BMP receptor 1A (BMPR1A) was O-GalNAcylated. Sites mutation of BMPR1A indicated that Thr137 and Ser37/Ser39/Ser44/Thr49 of BMPR1A were the main O-glycosylation sites. Although we cannot exclude the indirect effect of GALNT8, our results demonstrated that the expression of GALNT8 and O-glycosylation of BMPR1A play key roles in regulating the activity of BMP/SMAD/ RUNX2 signaling and ER alpha expression. Conclusion: These findings suggest that GALNT8 expression and abnormal O-GalNAcylation of BMPR1A increase ER alpha expression and suppress breast cancer cell proliferation by modulating the BMP signaling pathway. General significance: Our results identify the involvement of GALNT8 in regulating ER alpha expression.
