The number of N-glycans (n) is a distinct feature of each glycoprotein sequence and cooperates with the physical properties of the Golgi N-glycan-branching pathway to regulate surface glycoprotein levels. The Golgi pathway is ultrasensitive to hexosamine flux for the production of tri- and tetra-antennary N-glycans, which bind to galectins and form a molecular lattice that opposes glycoprotein endocytosis. Glycoproteins with few N-glycans (e.g., TbetaR, CTLA-4, and GLUT4) exhibit enhanced cell-surface expression with switch-like responses to increasing hexosamine concentration, whereas glycoproteins with high numbers of N-glycans (e.g., EGFR, IGFR, FGFR, and PDGFR) exhibit hyperbolic responses. Computational and experimental data reveal that these features allow nutrient flux stimulated by growth-promoting high-n receptors to drive arrest/differentiation programs by increasing surface levels of low-n glycoproteins. We have identified a mechanism for metabolic regulation of cellular transition between growth and arrest in mammals arising from apparent coevolution of N-glycan number and branching.

The Golgi enzyme beta1,6 N-acetylglucosaminyltransferase V (Mgat5) is up-regulated in carcinomas and promotes the substitution of N-glycan with poly N-acetyllactosamine, the preferred ligand for galectin-3 (Gal-3). Here, we report that expression of Mgat5 sensitized mouse cells to multiple cytokines. Gal-3 cross-linked Mgat5-modified N-glycans on epidermal growth factor and transforming growth factor-beta receptors at the cell surface and delayed their removal by constitutive endocytosis. Mgat5 expression in mammary carcinoma was rate limiting for cytokine signaling and consequently for epithelial-mesenchymal transition, cell motility, and tumor metastasis. Mgat5 also promoted cytokine-mediated leukocyte signaling, phagocytosis, and extravasation in vivo. Thus, conditional regulation of N-glycan processing drives synchronous modification of cytokine receptors, which balances their surface retention against loss via endocytosis.