INCREASED SPHINGOMYELIN CONTENT OF PLASMA-LIPOPROTEINS IN APOLIPOPROTEIN-E KNOCKOUT MICE REFLECTS COMBINED PRODUCTION AND CATABOLIC DEFECTSAND ENHANCES REACTIVITY WITH MAMMALIAN SPHINGOMYELINASE

Apolipoprotein E knockout (apoEO) mice accumulate atherogenic remnant lipoproteins in plasma, We now provide evidence that these particles a re enriched in sphingomyelin (SM), and explore the mechanisms and poss ible pathophysiological consequences of this finding, The phosphatidyl choline/sphingomyelin (PC/SM) ratio was reduced in all lipoproteins in apoEO mice compared with wild-type (Wt) mice (2.0+/-0.2 vs, 4.7+/-0.5 ; 2.8+/-0.5 vs, 5.5+/-0.9; 1.9+/-0.5 vs. 4.6+/-0.5 for VLDL, LDL, and HDL), reflecting 400 and 179% increases in plasma pools of SM and PC, respectively, Turnover studies using [C-14]PC/[H-3]SM VLDL or HDL show ed that the fractional catabolic rate (FCR) of VLDL-SM and HDL-SM were markedly reduced in the apoEO mice compared with Wt mice, while the F CRs of VLDL-PC and HDL-PC were similar, By contrast, the FCRs of [H-3] PC ether and [C-14]SM were identical in apoEO and Wt mice, The product ion rates of VLDL-SM and HDL-SM in apoEO mice were much higher than in Wt mice, while the production rates of lipoprotein PC were similar, T o assess the underlying mechanisms, we also measured the PC/SM ratio i n VLDL and LDL of LDL receptor knockout (LDLrO) and hepatic LDL recept or-related protein knockout/LDLrO mice, but found no difference with W t mice, Using S-sphingomyelinase, an enzyme secreted by macrophages an d endothelial cells, we found that VLDL and LDL from apoEO, but not fr om Wt or LDLrO mice, were significantly aggregated, and that aggregati on was not prevented by adding back apoE, We then enriched the apoEO-V LDL and Wt-VLDL with different amounts of SM, and found that VLDL aggr egation was enhanced, Thus, the increased SM content of lipoproteins i n apoEO mice is due to combined synthesis and clearance defects. Impai red SM clearance reflects resistance to intravascular enzymes and dela yed removal by a non-LDLr, non-LDLr related protein pathway. The incre ased SM content in slowly cleared remnant lipoproteins may enhance the ir susceptibility to arterial wall SMase and increase their atherogeni c potential.