Submitted on September 22, 2006
Revised on December 5, 2006
Accepted on December 5, 2006
Sphingolipids and inflammation: Modulation of the activity and the arachidonate selectivity of group X secretory phospholipase A2 by sphingomyelin and ceramide
Dev K. Singh and Papasani V. Subbaiah
Medicine, University of Illinois, Chicago, IL 60612
Corresponding Author: psubbaia{at}uic.edu
To investigate the role of sphingomyelin (SM) in the regulation of inflammatory reactions, we studied its effect on the activity and fatty acid specificity of group X secretory phospholipase A2 (sPLA2X). Compared to other phospholipases, recombinant sPLA2X released more arachidonate from HDL. Pre-treatment of HDL with SMase C activated the sPLA2X activity, but the release of arachidonate was stimulated less than that of linoleate. In liposomes containing synthetic phosphatidylcholines (PCs), sPLA2X showed no clear selectivity among the various sn-2 unsaturated fatty acids. However, when SM was incorporated into liposomes at 30 mol%, the enzyme exhibited strong preference for arachidonate, although its overall activity was inhibited. Degradation of liposomal SM by SMase C resulted in sPLA2X activation, and loss of its arachidonate preference. Incorporation of ceramide into HDL or PC liposomes activated the enzyme activity, the release of arachidonate being stimulated more than that of linoleate. SM-deficient cells released more arachidonate than normal cells in response to exogenous sPLA2X. SMase pre-treatment of normal cells stimulated the release of arachidonate by the exogenous sPLA2X. These results show that SM not only inhibits sPLA2X activity, but also contributes to its selectivity for arachidonate, whereas ceramide stimulates the hydrolysis of arachidonate-containing PCs.
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