A novel glycosylation signal regulates transforming growth factor beta receptors as evidenced by endo-beta-galactosidase C expression in rodent cells.

The αGal (Galα1-3Gal) epitope is a xenoantigen that is responsible for hyperacute rejection in xenotransplantation. This epitope is expressed on the cell surface in the cells of all mammals except humans and Old World monkeys. It can be digested by the enzyme endo-β-galactosidase C (EndoGalC), which is derived from Clostridium perfringens. Previously, we produced EndoGalC transgenic mice to identify the phenotypes that would be induced following EndoGalC overexpression. The mice lacked the αGal epitope in all tissues and exhibited abnormal phenotypes such as postnatal death, growth retardation, skin lesion and abnormal behavior. Interestingly, skin lesions caused by increased proliferation of keratinocytes suggest the role of a glycan structure [in which the αGal epitope has been removed or the N-acetylglucosamine (GlcNAc) residue is newly exposed] as a regulator of signal transduction. To verify this hypothesis, we introduced an EndoGalC expression vector into cultured mouse NIH3T3 cells and obtained several EndoGalC-expressing transfectants. These cells lacked αGal epitope expression and exhibited 1.8-fold higher proliferation than untransfected parental cells. We then used several cytokine receptor inhibitors to assess the signal transduction cascades that were affected. Only SB431542 and LY364947, both of which are transforming growth factor β (TGFβ) receptor type-I (TβR-I) inhibitors, were found to successfully reverse the enhanced cell proliferation rate of EndoGalC transfectants, indicating that the glycan structure is a regulator of TβRs. Biochemical analysis demonstrated that the glycan altered association between TβR-I and TβR-II in the absence of ligands.