Spider and bacterial sphingomyelinases D target cellular lysophosphatidic acid receptors by hydrolyzing lysophosphatidylcholine

Bites by Loxosceles spiders can produce severe clinical symptoms, including dermonecrosis, thrombosis, vascular leakage, hemolysis, and persistent inflammation. The causative factor is a sphingomyelinase D (SMaseD) that cleaves sphingomyelin into choline and ceramide 1-phosphate. A similar enzyme, showing comparable bioactivity, is secreted by certain pathogenic corynebacteria and acts as a potent virulence factor. However, the molecular basis for SMaseD toxicity is not well understood, which hampers effective therapy. Here we show that the spider and bacterial SMases D hydrolyze albumin-bound lysophosphatidylcholine (LPC), but not sphingosylphosphorylcholine, with K(m) values ( approximately 20-40 microm) well below the normal LPC levels in blood. Thus, toxic SMases D have intrinsic lysophospholipase D activity toward LPC. LPC hydrolysis yields the lipid mediator lysophosphatidic acid (LPA), a known inducer of platelet aggregation, endothelial hyperpermeability, and pro-inflammatory responses. Introduction of LPA(1) receptor cDNA into LPA receptor-negative cells renders non-susceptible cells susceptible to SmaseD, but only in LPC-containing media. Degradation of circulating LPC to LPA with consequent activation of LPA receptors may have a previously unappreciated role in the pathophysiology of secreted SMases D.