Uvomorulin, LAMP-1, and Laminin Are Substrates for Cell Surface β-1,4-Galactosyltransferase on F9 Embryonal Carcinoma Cells: Comparisons between Wild-Type and Mutant 5.51 att- Cells

Abstract

Intercellular adhesions in F9 embryonal carcinoma cells are primarily dependent upon two types of cell adhesion molecules: uvomorulin (Um or E-cadherin), which facilitates intercellular adhesion by homophilic binding, and cell surface β-1,4-galactosyltransferase (GalTase), which binds terminal N-acetylglucosamine residues on consociate glycoprotein substrates on adjacent cell surfaces. The variant F9 cell line, 5.51 att-, undergoes initial cell aggregation, but fails to form the mature intercellular adhesions characteristic of wild-type F9 cells. The 5.51 att- cells show reduced Um expression; however, previous studies have shown that reduced levels of Um are not responsible for the att- mutant phenotype. Therefore, in this study we determined whether the att- mutant phenotype was the result of altered expression of surface GalTase or of its consociate glycoprotein substrates. Surface GalTase was found to be expressed normally on 5.51 att- cells. Furthermore, the initial intercellular adhesions characteristic of reaggregating 5.51 att- cells were dependent upon surface GalTase, since anti-GalTase antibodies inhibited 5.51 intercellular adhesion. Preliminary studies showed that the interaction of surface GalTase with its consociate glycoprotein substrates was abnormal on 5.51 att- cells. To define the biochemical basis for this observation, specific glycoprotein substrates for cell surface GalTase were identified and their expression and utilization were compared between wild-type and 5.51 att- cells. In wild-type cells, uvomorulin (Um), lysosome-associated membrane protein-1 (LAMP-1), and laminin (Lm) were shown to be substrates for cell surface GalTase, suggesting that they may participate in GalTase-specific adhesions. Moreover, the interaction between GalTase and these glycoproteins exhibited characteristic changes during retinoic acid-induced F9 cell differentiation. In contrast to that seen on wild-type cells, surface GalTase interaction with Um, LAMP-1, and Lm was atypical on 5.51 att- cells, resulting, in part, from increased synthesis of oligosaccharide substrates for surface GalTase, as determined by size exclusion and lectin affinity chromatography. N -acetylglucosaminyltransferase activity was elevated in 5.51 att- cells and likely responsible for the increased expression of GalTase oligosaccharide substrates. These results suggest that mutant 5.51 att- cells are able to maintain residual intercellular adhesions because of increased expression of oligosaccharide substrates for surface GalTase. The impact of aberrant GalTase-specific adhesions on the ability of 5.51 att- cells to form mature intercellular adhesions and to differentiate in response to retinoic acid is discussed.