Ceramide

Ceramides are a family of waxy lipid molecules. A ceramide is composed of sphingosine and a fatty acid. Ceramides are found in high concentrations within the cell membrane of cells, since they are component lipids that make up sphingomyelin, one of the major lipids in the lipid bilayer. Contrary to previous assumptions that ceramides and other sphingolipids found in cell membrane were purely supporting structural elements, ceramide can participate in a variety of cellular signaling: examples include regulating differentiation, proliferation, and programmed cell death (PCD) of cells.

The word ceramide comes from the Latin cera (wax) and amide. Ceramide is a component of vernix caseosa, the waxy or cheese-like white substance found coating the skin of newborn human infants.

There are three major pathways of ceramide generation. The sphingomyelinase pathway uses an enzyme to break down sphingomyelin in the cell membrane and release ceramide. The de novo pathway creates ceramide from less complex molecules. Ceramide generation can also occur through breakdown of complex sphingolipids that are ultimately broken down into sphingosine, which is then reused by reacylation to form ceramide. This latter pathway is termed the Salvage pathway.

Hydrolysis of sphingomyelin is catalyzed by the enzyme sphingomyelinase. Because sphingomyelin is one of the four common phospholipids found in the plasma membrane of cells, the implications of this method of generating ceramide is that the cellular membrane is the target of extracellular signals leading to programmed cell death. There has been research suggesting that when ionizing radiation causes apoptosis in some cells, the radiation leads to the activation of sphingomyelinase in the cell membrane and ultimately, to ceramide generation.

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