Both Sphingomyelin Synthases SMS1 and SMS2 Are Required for Sphingomyelin Homeostasis and Growth in Human HeLa Cells

Abstract

Sphingomyelin (SM) is a vital component of cellular membranes in organisms ranging from mammals to protozoa. Its production involves the transfer of phosphocholine from phosphatidylcholine to ceramide, yielding diacylglycerol in the process. The mammalian genome encodes two known SM synthase (SMS) isoforms, SMS1 and SMS2. However, the relative contributions of these enzymes to SM production in mammalian cells remained to be established. Here we show that SMS1 and SMS2 are co-expressed in a variety of cell types and function as the key Golgi-and plasma membrane-associated SM synthases in human cervical carcinoma HeLa cells, respectively. RNA interference-mediated depletion of either SMS1 or SMS2 caused a substantial decrease in SM production levels, an accumulation of ceramides, and a block in cell growth. Although SMS-depleted cells displayed a reduced SM content, external addition of SM did not restore growth. These results indicate that the biological role of SM synthases goes beyond formation of SM.

Tales and Mysteries of the Enigmatic Sphingomyelin Synthase Family

“…In CHO cells over-expression of SMS1 or SMS2 induced a significant accumulation of total DAG. 77 On the other hand, in HeLa cells down-regulation of either SMS1 or SMS2 did not alter total DAG levels 66,74 and in Huh7, SMSs down-regulation caused a tendency to decrease that was not significant. 72 Therefore, also in the case of DAG as for ceramide, cells react differently to modulation of DAG levels by SMS activity and that may have to do with the bioactivity of these lipids.…”

“…Moreover, SMS1 and a pool of SMS2 are both localized to the Golgi. 58,66,72,74,76 Why is this organelle equipped with two different SMS enzymes? What is the relevance of the reverse activity of SMS enzymes?…”

“…77 An increase of ceramide levels was also observed upon down-regulation of either SMS1 or SMS2 in HeLa and Huh cells, 66,72,74 suggesting that changes in ceramide levels due to modulation of SMSs might be differently regulated depending on the specific cellular context.…”

“…Whereas the levels of PC do not seem to change following modulation of SMSs, 66,72,74,77 altering the expression of SMS1 or SMS2 exerts diverse responses on DAG levels. In CHO cells over-expression of SMS1 or SMS2 induced a significant accumulation of total DAG.…”

“…58,66 In addition to SMS1 and SMS2 genes, the mammalian genome contains a third, SMS-related (SMSr) gene that is highly conserved, from humans and worms to the fruit fly, Drosophila melanogaster. Interestingly, Drosophila lacks SMS1 and SMS2 homologues and does not synthesize SM.…”