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

Objective: Sialic 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. Methods: The 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. Results: Primary 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). Conclusion: The 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.