RNA interference-mediated silencing of ppGalNAc-T1 and ppGalNAc-T2 inhibits invasion and increases chemosensitivity potentially by reducing terminal α2,3 sialylation and MMP14 expression in triple-negative breast cancer cells

Glycopeptide-preferring polypeptide N-ace tylgalactosamine transferase (ppGalNAc-T) is a key enzyme that initiates the formation of the first GalNAc monosaccharide to polypeptides at Thr/Ser residues by O-linked glycosylation. In order to investigate the effects of ppGalNAc-T1 and ppGalNAc-T2 on the initiation of O-glycosylation, siRNA-ppGalNAc-T1 (si-T1) and siRNA-ppGalNAc-T2 (si-T2) were transfected into highly-invasive estrogen receptor-negative MDA-MB-231 cells to inhibit O-glycosylation. Downregulation of ppGalNAc-T1 demonstrated a significant reduction in the number of terminal alpha 2,3 sialic acids, when compared to cells transfected with si-T2 or si-T1/T2. This downregulation led to a decrease in the invasion capabilities of the breast carcinoma cells, as well as enhanced chemosensitivity, which was the result antineoplastic drug effects. In addition, immunoprecipitation assays demonstrated that downregulation of ppGalNAc-T1 led to a reduction in the number of terminal alpha 2,3 sialic acids on O-linked glycans of the matrix metalloproteinase-14 (MMP14) glycoprotein. Furthermore, MMP14 and vascular endothelial growth factor were downregulated in the si-T1 groups when compared with the si-T2 and si-T1/T2 groups. In conclusion, the results of the present study suggest that ppGalNAc-T1 may serve a pivotal role in the initiation of O-glycosylation, which may lead to a low density of alpha 2,3 sialic acids on O-linked glycans of MMP14 when downregulated. Glycosylation serves a significant role in regulating the sensitivity of MMP14 to self-proteolysis, which ultimately decreases the invasion capabilities of breast cancer cells. The results of the present study may be useful in establishing the function of ppGalNAc-T1 during breast cancer invasion and metastasis.