THE METABOLISM OF SPHINGO(GLYCO)LIPIDS IS CORRELATED WITH THE DIFFERENTIATION-DEPENDENT AUTOPHAGIC PATHWAY IN HT-29 CELLS

Recently it was demonstrated that the metabolism of both glycoproteins and sphingo(glyco)lipids is dependent upon the state of enterocytic d ifferentiation of HT-29 cells, Furthermore, it was shown that undiffer entiated HT-29 cells display an important autophagic sequestration, co ntrolled by a heterotrimeric G(i3) protein. In order to correlate the metabolism of sphingo(glyco)lipids with the extent of autophagic seque stration we have incubated undifferentiated and differentiated HT-29 c ells with tritium-labelled GM1 ganglioside and sphingosine in the abse nce and presence of pertussis toxin (an inhibitor of autophagic seques tration) or asparagine (an inhibitor of autophagic vacuole maturation) . In addition, undifferentiated HT-29 cells transfected with a cDNA en coding the G alpha(i3) protein (cells expressing an amplified autophag ic pathway) were labelled with both GM1 and sphingosine. The results s how that the catabolism of sphingo(glyco)lipids is dramatically enhanc ed in parallel with the increase of the autophagic pathway while at th e same time their biosynthesis is reduced. The inhibition of autophagy in both undifferentiated cells and alpha(i3)-overexpressing cells res tores sphingo(glyco)lipid metabolism, as normally expressed in differe ntiated ferentiated cells, as well as in other mammalian cell types. W e conclude that autophagy plays an important role in governing the met abolic fate of sphingo(glyco)lipids in HT-29 cells. Since autophagy re gulates the N-Linked glycoprotein metabolism in this cell line, our re sults corroborate the idea that glycolipid and glycoprotein metabolism s are controlled by similar mechanisms.