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 differentiation of HT-29 cells, Furthermore, it was shown that undifferentiated HT-29 cells display an important autophagic sequestration, controlled by a heterotrimeric G(i3) protein. In order to correlate the metabolism of sphingo(glyco)lipids with the extent of autophagic sequestration we have incubated undifferentiated and differentiated HT-29 cells with tritium-labelled GM1 ganglioside and sphingosine in the absence and presence of pertussis toxin (an inhibitor of autophagic sequestration) or asparagine (an inhibitor of autophagic vacuole maturation). In addition, undifferentiated HT-29 cells transfected with a cDNA encoding the G alpha(i3) protein (cells expressing an amplified autophagic pathway) were labelled with both GM1 and sphingosine. The results show that the catabolism of sphingo(glyco)lipids is dramatically enhanced in parallel with the increase of the autophagic pathway while at the same time their biosynthesis is reduced. The inhibition of autophagy in both undifferentiated cells and alpha(i3)-overexpressing cells restores sphingo(glyco)lipid metabolism, as normally expressed in differentiated ferentiated cells, as well as in other mammalian cell types. We conclude that autophagy plays an important role in governing the metabolic fate of sphingo(glyco)lipids in HT-29 cells. Since autophagy regulates the N-Linked glycoprotein metabolism in this cell line, our results corroborate the idea that glycolipid and glycoprotein metabolisms are controlled by similar mechanisms.