Protection of the endothelium is provided by circulating sphingosine-1-phosphate (S1P), which maintains vascular integrity. We show that HDL-associated S1P is bound specifically to both human and ...murine apolipoprotein M (apoM). Thus, isolated human ApoM⺠HDL contained S1P, whereas ApoMâ» HDL did not. Moreover, HDL in Apomâ»/â» mice contains no S1P, whereas HDL in transgenic mice overexpressing human apoM has an increased S1P content. The 1.7-à structure of the S1P-human apoM complex reveals that S1P interacts specifically with an amphiphilic pocket in the lipocalin fold of apoM. Human ApoM⺠HDL induced S1Pâ receptor internalization, downstream MAPK and Akt activation, endothelial cell migration, and formation of endothelial adherens junctions, whereas apoMâ» HDL did not. Importantly, lack of S1P in the HDL fraction of Apomâ»/â» mice decreased basal endothelial barrier function in lung tissue. Our results demonstrate that apoM, by delivering S1P to the S1Pâ receptor on endothelial cells, is a vasculoprotective constituent of HDL.
Lipoproteins consist of lipids solubilized by apolipoproteins. The lipid-binding structural motifs of apolipoproteins include amphipathic α-helixes and β-sheets. Plasma apolipoprotein (apo) M lacks ...an external amphipathic motif but, nevertheless, is exclusively associated with lipoproteins (mainly high density lipoprotein). Uniquely, however, apoM is secreted to plasma without cleavage of its hydrophobic NH2-terminal signal peptide. To test whether the signal peptide serves as a lipoprotein anchor for apoM in plasma, we generated mice expressing a mutated apoMQ22A cDNA in the liver (apoMQ22A-Tg mice (transgenic mice)) and compared them with mice expressing wild-type human apoM (apoM-Tg mice). The substitution of the amino acid glutamine 22 with alanine in apoMQ22A results in secretion of human apoM without a signal peptide. The human apoM mRNA level in liver and the amount of human apoM protein secretion from hepatocytes were similar in apoM-Tg and apoMQ22A-Tg mice. Nevertheless, human apoM was not detectable in plasma of apoMQ22A-Tg mice, whereas it was easily measured in the apoM-Tg mice. To examine the plasma metabolism, recombinant apoM lacking the signal peptide was produced in Escherichia coli and injected into wild-type mice. The apoM without signal peptide did not associate with lipoproteins and was rapidly cleared in the kidney. Accordingly, ligation of the kidney arteries in apoMQ22A-Tg mice resulted in rapid accumulation of human apoM in plasma. The data suggest that hydrophobic signal peptide sequences, if preserved upon secretion, can anchor plasma proteins in lipoproteins. In the case of apoM, this mechanism prevents rapid loss by filtration in the kidney.
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