The N-glycan processing in HT-29 cells is a function of their state of enterocytic differentiation. Evidence for an atypical traffic associated with change in polypeptide stability in undifferentiated HT-29 cells.
Résumé
When the human colon cancer cells HT-29 undergo enterocytic differentiation, they correctly process their N-glycans, whereas their undifferentiated counterpart are unable to process Man9-8-GlcNAc2 species, the natural substrate of alpha-mannosidase I. As this enzyme is fully active in both HT-29 cell populations, we hypothesize that N-glycoproteins are unable to reach the cis Golgi, the site where alpha-mannosidase I has been localized. We have demonstrated this point by using 1-deoxymannojirimycin, leupeptin, and monensin. In the presence of 1-deoxymannojirimycin, a specific inhibitor of alpha-mannosidase I, differentiated HT-29 cells, as expected, accumulate Man9-8-GlcNAc2 species, whereas in undifferentiated HT-29 cells these compounds continue to be rapidly degraded. In contrast, the use of leupeptin, a specific inhibitor of thiol and serine proteases, leads to the accumulation of these oligosaccharides in undifferentiated HT-29 cells. Monensin, a carboxylic ionophore that perturbs distal Golgi functions, is unable to stabilize these compounds. Therefore, we conclude that N-linked glycoproteins in undifferentiated HT-29 cells rapidly egress from the exocytic pathway to a leupeptin-sensitive degradative compartment without entering a monensin-sensitive compartment. These results favor the hypothesis that a direct pathway should exist between the rough endoplasmic reticulum and a leupeptin-sensitive degradative compartment in undifferentiated HT-29 cells. The emergence of this new pathway could explain why protein stability and N-glycan processing may vary as a function of the state of cell differentiation.