LASS5 is the predominant ceramide synthase isoform involved in de novo sphingolipid synthesis in lung epithelia

Ceramide is a key bioactive mediator that inhibits surfactant phosphatidylcholine (PtdCho) synthesis in lung epithelia. Ceramide availability is governed by sphingomy- elin (SM) hydrolysis, but less is known regarding its de novo synthesis. In this study, we observed that ceramide synthesis within murine lung epithelia was associated with high-level ceramide synthase (dihydroceramide synthase) activity. Lon- gevity assurance homolog 5 (LASS5) was the predominant cer- amide synthase isoform detected in lung epithelia, whereas relatively lower level expression was detected for the other five mammalian homologs. Pulmonary LASS5 was develop- mentally regulated, but its expression was spatially and gender nonspecific. Exogenously expressed LASS5 in lung epithelia was membrane-associated, triggering increased ceramide syn- thesis, whereas knockdown studies using fumonisin B 1 or LASS5 small, interfering RNA reduced ceramide synthase ac- tivity by 78% or 45%, respectively. Overexpression of LASS5 also reduced PtdCho synthesis, but maximal inhibition was achieved when LASS5 was coexpressed with a plasmid en- coding a neutral sphingomyelinase involved in SM hydroly- sis. These results demonstrate that LASS5 is the major ceramide synthase gene product involved in sphingolipid production that may also regulate PtdCho metabolism in pul- monary epithelia. —Xu, Z., J. Zhou, D. M. McCoy, and R. K. Mallampalli. LASS5 is the predominant ceramide synthase isoform involved in de novo sphingolipid synthesis in lung epithelia. J. Lipid Res. 2005. 46: 1229-1238.