RELEASE OF CERAMIDE AFTER MEMBRANE SPHINGOMYELIN HYDROLYSIS DECREASESTHE BASOLATERAL SECRETION OF TRIACYLGLYCEROL AND APOLIPOPROTEIN-B IN CULTURED HUMAN INTESTINAL-CELLS

The effect of sphingomyelin hydrolysis on triacylglycerol-rich lipopro tein secretion was examined in the human intestinal cell line, CaCo-2. Addition of sphingomyelinase decreased sphingomyelin and phosphatidyl ethanolamine by 60 and 20%, respectively. Sphingomyelin hydrolysis dec reased the basolateral secretion of triacylglycerol mass, newly synthe sized triacylglycerol, and apo B mass. Pulse-chase experiments with [S -35]methionine demonstrated a decrease in apo B synthesis and a marked decrease in apo B100 and apo B48 secretion without altering apo Al se cretion. Sphingomyelin hydrolysis did not change apo B mRNA levels nor apo B turnover. Phosphatidylcholine-specific phospholipase C did not decrease apo B synthesis or its basolateral secretion. Membrane protei n kinase C (PKC) activity was decreased twofold after sphingomyelin hy drolysis. The PKC inhibitor staurosporine decreased apo B mass and new ly synthesized apo B secretion. Sphingomyelinase and staurosporine tog ether caused an additional decrease in apo B secretion suggesting that sphingomyelin hydrolysis decreased apo B secretion independently of i ts effect on PKC activity. Moreover, conditions that increase PKC acti vity did not increase apo B secretion. Cell-permeable analogs of ceram ide decreased immunoreactive apo B secretion. Sphingosine was without effect. The hydrolysis of membrane sphingomyelin by intestinal or panc reatic neutral sphingomyelinase may lead to the accumulation of cellul ar ceramide, which, in turn, could inhibit triacylglycerol-rich lipopr otein secretion.