Phenotype-related differential α-2,6- or α-2,3-sialylation of glycoprotein N-glycans in human chondrocytes

SummaryObjectiveSialic acids frequently occur at the terminal positions of glycoprotein N-glycans present at chondrocyte surfaces or in the cartilage matrix. Sialic acids are transferred to glycoproteins in either α-2,3 or α-2,6 linkage by specific sialyltransferases (SiaTs) and can potentially affect cell functions and cell–matrix interactions. The present study aimed to assess the relationship between the expression of the human chondrocyte phenotype and the sialylation of chondrocyte glycoprotein N-glycans.MethodsThe transcription of 5 SiaT was quantified using real-time Reverse transcription polymerase chain reaction (RT-PCR) assays. N-glycan analysis was performed using LC–ESI-MS. Primary human chondrocytes were cultured in monolayer or alginate beads and compared to the chondrocyte cell lines C-28/I2 and SW1353. In addition, effects of interleukin-1β (IL-1β) or tumour necrosis factor-α (TNF-α) on primary cells were assessed.ResultsPrimary human chondrocytes predominantly express α-2,6-specific SiaTs and accordingly, α-2,6-linked sialic acid residues in glycoprotein N-glycans. In contrast, the preponderance of α-2,3-linked sialyl residues and, correspondingly, reduced levels of α-2,6-specific SiaTs are associated with the altered chondrocyte phenotype of C-28/I2 and SW1353 cells. Importantly, a considerable shift towards α-2,3-linked sialic acids and α-2,3-specific SiaT mRNA levels occurred in primary chondrocytes treated with IL-1β or tumour necrosis factor-alpha (TNF-α).ConclusionThe expression of the differentiated chondrocyte phenotype is linked to the ratio of α-2,6- to α-2,3-linked sialic acids in chondrocyte glycoprotein N-glycans. A shift towards altered sialylation might contribute to impaired cell–matrix interactions in disease conditions.