Shift in sphingolipid metabolism leads to an accumulation of ceramide in senescence

Ceramide mediates the effects of several agonists leading to differentiation, apoptosis or senescence. We previously showed that ceramide becomes elevated in senescent fibroblasts. In the present study, senescent cultures of Wi-38 fibroblasts and human umbilical-vein endothelial cells were compared to low-passage cultures in order to identify which of the several pathways is predominantly responsible for the increased ceramide. We found that senescent cells take up the ceramide precursor [3H]palmitic acid and convert it to ceramide at essentially equivalent rates to their low-passage counterparts, suggesting that, as a whole, the inherent steps are unaltered. Analysis of subsequent steps, however, revealed changes in ceramide metabolism. The rate of ceramide conversion to sphingomyelin was reduced while glucosylceramide synthesis differed between the cell lines, while the rate of the reverse reactions tended to be increased in senescent cells. We also found a decrease in acidic but not alkaline ceramidase. The data show an overall change in favor increased ceramide levels. Of all of the pathways, neutral sphingomyelinase appears to be the most likely source of the senescence-associated ceramide. The relevance to mitosis and apoptosis are discussed.