Uncleaved ApoM Signal Peptide Is Required for Formation of Large ApoM/Sphingosine 1-Phosphate (S1P)-enriched HDL Particles

Abstract

Background: ApoM overexpression generates larger nascent and plasma HDLs. Results: ApoM Q22A overexpression generates smaller nascent and plasma HDL and enhances apoM and S1P secretion as compared with apoM WT . Conclusion: ApoM secretion regulates hepatocyte S1P secretion, and its uncleaved signal peptide delays apoM secretion, promoting larger nascent and plasma HDL particle formation. Significance: ApoM signal peptide is required for large apoM/S1P-enriched HDL formation.Apolipoprotein M (apoM), a plasma sphingosine 1-phosphate (S1P) carrier, associates with plasma HDL via its uncleaved signal peptide. Hepatocyte-specific apoM overexpression in mice stimulates formation of both larger nascent HDL in hepatocytes and larger mature apoM/S1P-enriched HDL particles in plasma by enhancing hepatic S1P synthesis and secretion. Mutagenesis of apoM glutamine 22 to alanine (apoM Q22A ) introduces a functional signal peptidase cleavage site. Expression of apoM Q22A in ABCA1-expressing HEK293 cells resulted in the formation of smaller nascent HDL particles compared with wild type apoM (apoM WT ). When apoM Q22A was expressed in vivo, using recombinant adenoviruses, smaller plasma HDL particles and decreased plasma S1P and apoM were observed relative to expression of apoM WT . Hepatocytes isolated from both apoM WT -and apoM Q22A -expressing mice displayed an equivalent increase in cellular levels of S1P, relative to LacZ controls; however, relative to apoM WT , apoM Q22A hepatocytes displayed more rapid apoM and S1P secretion but minimal apoM Q22A bound to nascent lipoproteins. Pharmacologic inhibition of ceramide synthesis increased cellular sphingosine and S1P but not medium S1P in both apoM WT and apoM Q22A hepatocytes. We conclude that apoM secretion is rate-limiting for hepatocyte S1P secretion and that its uncleaved signal peptide delays apoM trafficking out of the cell, promoting formation of larger nascent apoM-and S1P-enriched HDL particles that are probably precursors of larger apoM/S1P-enriched plasma HDL.HDL cholesterol concentrations are inversely correlated with coronary heart disease (1, 2). The atheroprotective nature of HDL is probably due to its ability to promote reverse cholesterol transport (3), inhibit inflammation (4) and oxidation (5), and transport cardioprotective molecules. One likely atheroprotective HDL molecule is apolipoprotein M (apoM), 2 which is present on ϳ5% of plasma HDL particles (6). ApoM is proposed to be atheroprotective by stimulating pre-␤ HDL formation (7-9), promoting macrophage cholesterol efflux (9), increasing antioxidative activity of HDL (10), and transporting sphingosine 1-phosphate (S1P) on HDL (11).ApoM is secreted with an uncleaved signal peptide due to the lack of a functional signal peptidase cleavage site (6). The retained signal peptide acts as an anchor for membrane and lipoprotein binding (12,13). The substitution of glutamine 22 for alanine (Q22A) creates a signal peptidase cleavage site in apoM (apoM Q22A ), leading to signal peptide processing, poor binding to...

The roles of bile acids and sphingosine-1-phosphate signaling in the hepatobiliary diseases

“…Moreover, it has been demonstrated that hepatocyte-specific apoM overexpression facilitates formation of large apoM/S1P-enriched HDL by promoting formation of large nascent HDL and stimulating sphingolipid synthesis and S1P secretion. These results suggest that there is coordination between sphingolipid and cholesterol metabolism (51,52). Taken together, it is possible that the liver regulates not only plasma bile acid levels, but also plasma S1P levels by regulating its uptake and secretion.…”

Sphingosine-1-phosphate metabolism: A structural perspective

“…Additional interactions are provided by a complex water-mediated hydrogen-bond network (Figure 3C). Recently, it was found that the Q22A mutation in ApoM resulted in increased secretion of S1P and dhS1P from hepatocytes (Liu et al ., 2015). The Q22A mutation introduces a cleavable signal peptide which, once cleaved, increases the secretion rate of ApoM Q22A .…”

High-Density Lipoprotein–Associated Apolipoprotein M Limits Endothelial Inflammation by Delivering Sphingosine-1-Phosphate to the Sphingosine-1-Phosphate Receptor 1

“…7,8 Five different membrane-bound G-protein-coupled S1P receptors (S1PRs) are known, and binding of S1P to the receptors activates multiple receptor-specific downstream signaling pathways. In this way, S1P is able to regulate several biological processes, such as immune cell trafficking, angiogenesis, endothelial cell migration, and endothelial barrier function.…”