Diacylglycerol signaling and metabolism

Diacylglycerol is a key intracellular messenger in many physiological responses. It is both an activator of protein kinase C and a precursor in the synthesis of complex lipids. The dysregulated production of diacylglycerol has also been demonstrated in pathological responses. Clostridium perfringens type A, the etiologic agent of gas gangrene, produces an exotoxin with phospholipase C activity, alpha toxin, that degrades phosphatidylcholine to diacyglycerol and phosphorylcholine. Alpha toxin treated endothelial cells contained elevated levels of diacylglycerol and produced platelet-activating factor and prostacyclin. Furthermore, the stimulated endothelial cells became adhesive to neutrophils by a mechanism that was mediated by P-selectin and platelet-activating factor. These endothelial cell responses appeared dysregulated and may contribute to both the localized and systemic manifestations of gas gangrene. Diacylglycerol lipase catalyzes the deacylation of diacylglycerol at the sn-1 position and is the first step in the release of arachidonic acid from arachidonate containing diacylglycerols. The primary diacylglycerol lipase from human platelets was purified and demonstrated to be identical with the human lysosomal acid lipase. Additionally, the lysosomal acid lipase exhibited diacylglycerol transacylase activity. Wolman's fibroblasts which lack lysosomal acid lipase had undetectable levels of diacylglycerol lipase and diacylglycerol transacylase activity. Furthermore, these cells contained normal levels of endogenous diacylglycerol and metabolized exogenous diacylglycerol similar to control cells. This study represents the first complete purification of a diacylglycerol lipase or diacylglycerol transacylase activity from mammalian cells. Diacylglycerol can also be phosphorylated by diacylglycerol kinase in an ATP dependent reaction. A novel 3.5 kilobase cDNA that encodes the human diacylglycerol kinase$/zeta$ was isolated from an endothelial cell library. The corresponding mRNA is highly expressed in brain, skeletal muscle, and heart. Diacylglycerol kinase$/zeta$ contains two zinc finger motifs, a sequence that is similar to the phosphorylation site domain of the myristoylated, alanine-rich C-kinase substrate, an ATP binding consensus site, and four carboxy-terminal ankyrin repeats. The kinase activity displayed stereoselectivity for 1,2 diacylglycerol but did not exhibit any specificity for molecular species of long chain diacylglycerols.