Acid sphingomyelinase activity regulates de novo sphingolipid synthesis in liver through a negative feedback mechanism

The rate of sphingolipid synthesis through the de novo pathway has emerged as a key determinant of the levels of ceramide and complex sphingolipids such as sphingomyelin in eukaryotic cells. Our study provides evidence that in the liver, the flux through this pathway is regulated by a negative feedback mechanism involving acid sphingomyelinase (ASMase), an enzyme catalysing the turnover of sphingomyelin in endosomal//lysosomal subcellular compartment. Inhibition of ASMase activity in HepG2 cells lead to increases in the incorporation of radioactive precursor of the de novo pathway ³H-palmitate into ceramide and sphingomyelin. Moreover, myriocin, a specific inhibitor of the first step in the de novo sphingolipid pathway abrogated the effects of ASMase inhibition, implicating that ASMase activity regulated this biosynthetic pathway. The use of HPLC to separate sphingolipids containing the dihydrosphingoid long chain base, which is intermediate of only the de novo pathway showed elevation in sphinganine and dihydroceramide in livers of ASMase-deficient mice, indicating further that the de novo synthesis of sphingolipids in vivo was stimulated. Together, these observations suggest that the rate of sphingolipid synthesis in hepatocytes is strictly regulated in a manner that seemingly serve to prevent the excessive accumulation of ceramide and sphingomyelin thereby to limit the potentially detrimental effect of these lipids on cellular function.