RABBIT AORTA AND HUMAN ATHEROSCLEROTIC LESIONS HYDROLYZE THE SPHINGOMYELIN OF RETAINED LOW-DENSITY-LIPOPROTEIN - PROPOSED ROLE FOR ARTERIAL-WALL SPHINGOMYELINASE IN SUBENDOTHELIAL RETENTION AND AGGREGATION OF ATHEROGENIC LIPOPROTEINS

Aggregation and retention of LDL in the arterial wall are key events i n atherogenesis, but the mechanisms in vivo are not known. Previous wo rk from our laboratories has shown that exposure of LDL to bacterial s phingomyelinase (SMase) in vitro leads to the formation of LDL aggrega tes that can be retained by extracellular matrix and that are able to stimulate macrophage foam cell formation. We now provide evidence that retained LDL is hydrolyzed by an arterial-wall SMase activity. First, we demonstrated that SMase-induced aggregation is caused by an increa se in particle ceramide content, even in the presence of excess sphing omyelin (SM). This finding is compatible with previous data showing th at lesional LDL is enriched in SM, though its ceramide content has not previously been reported. To address this critical compositional issu e, the ceramide content of lesional LDL was assayed and, remarkably, f ound to be 10-50-fold enriched compared with plasma LDL ceramide. Furt hermore, the ceramide was found exclusively in lesional LDL that was a ggregated; unaggregated lesional LDL, which accounted for 20-25% of th e lesional material, remained ceramide poor. When [H-3]SM-LDL was incu bated with strips of rabbit aorta ex vivo, a portion of the LDL was re tained, and the [H-3]SM of this portion, but not that of unretained LD L, was hydrolyzed to [H-3]ceramide by a nonlysosomal arterial hydrolas e. In summary, LDL retained in atherosclerotic lesions is acted upon b y an arterial-wall SMase, which may participate in LDL aggregation and possibly other SMase-mediated processes during atherogenesis.