Release of Ceramide after Membrane Sphingomyelin Hydrolysis Decreases the Basolateral Secretion of Triacylglycerol and Apolipoprotein B in Cultured Human Intestinal Cells

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The effect of sphingomyelin hydrolysis on triacylglycerol-rich lipoprotein secretion was examined in the human intestinal cell line, CaCo-2. Addition of sphingomyelinase decreased sphingomyelin and phosphatidylethanolamine by 60 and 20%, respectively. Sphingomyelin hydrolysis decreased the basolateral secretion of triacylglycerol mass, newly synthesized triacylglycerol, and apo B mass. Pulse-chase experiments with (Sulfur-35)methionine demonstrated a decrease in apo B synthesis and a marked decrease in apo B100 and apo B48 secretion without altering apo A1 secretion. 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 protein kinase C (PKC) activity was decreased twofold after sphingomyelin hydrolysis. The PKC inhibitor staurosporine decreased apo B mass and newly synthesized apo B secretion. Sphingomyelinase and staurosporine together caused an additional decrease in apo B secretion suggesting that sphingomyelin hydrolysis decreased apo B secretion independently of its effect on PKC activity. Moreover, conditions that increase PKC activity did not increase apo B secretion. Cell-permeable analogs of ceramide decreased immunoreactive apo B secretion. Sphingosine was without effect. The hydrolysis of membrane sphingomyelin by intestinal or pancreatic neutral sphingomyelinase may lead to the accumulation of cellular ceramide, which, in turn, could inhibit triacylglycerol-rich lipoprotein secretion. (J. Clin. Invest. 1993. 92:2609-2619) Key words: sphingomyelin. sphingosine. protein kinase C. apo B. CaCo-2 cells

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