The role of the asparagine-linked oligosaccharides of the α-subunit in human thyrotropin bioactivity

TSH and the gonadotropins (FSH, LH, and hCG) are a family of heterodimeric proteins that share a common α-subunit and differ in their hormone-specific β-subunit. The asparagine-linked (N-linked) oligosaccharides on these hormones are important in signal transduction. The N-linked oligosaccharides on the α-subunit have no effect on hCG and hFSH receptor binding, but are critical for their biological activity. Here, we analyzed the role of α- subunit N-linked oligosaccharides in human TSH (hTSH) bioactivity by site- directed mutagenesis and gene transfer. This was achieved by mutating the asparagine (Asn) residue in the N-linked glycosylation consensus sequence (Asn-X-Thr/Ser) to aspartic acid. The wild-type bTSH and its variants were expressed in Chinese hamster ovary cells. Wild-type α-subunit and its mutants (α₁, α₂, and α(₁₊₂)) were efficiently combined with TSHβ- subunit and secreted as dimers. The bioactivity of TSH glycosylation variants was determined by measuring their abilities to stimulate cAMP formation and T₃ secretion using a serum-free culture system of human thyroid follicles. Using this system, wild-type hTSH was significantly effective in the stimulation of cAMP formation and T₃ secretion. Deletion of the oligosaccharide units from either site 1 (α₁) or site 2 (α₂) of the α- subunit increased the biological activity of the dimer by about 30%. However, deletion of carbohydrate units from both sites of hTSH α-subunit (α(₁₊₂)) resulted in a significant reduction in cAMP formation (by ~70%) and T₃ secretion (by ~40%) compared to that with wild-type hTSH. These findings emphasize the importance of the α-subunit N-linked oligosaccharide chains on hTSH bioactivity.