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

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 (apoMQ22A) introduces a functional signal peptidase cleavage site. Expression of apoMQ22A in ABCA1-expressing HEK293 cells resulted in the formation of smaller nascent HDL particles compared with wild type apoM (apoMWT). When apoMQ22A was expressed in vivo, using recombinant adenoviruses, smaller plasma HDL particles and decreased plasma S1P and apoM were observed relative to expression of apoMWT. Hepatocytes isolated from both apoMWT- and apoMQ22A-expressing mice displayed an equivalent increase in cellular levels of S1P, relative to LacZ controls; however, relative to apoMWT, apoMQ22A hepatocytes displayed more rapid apoM and S1P secretion but minimal apoMQ22A bound to nascent lipoproteins. Pharmacologic inhibition of ceramide synthesis increased cellular sphingosine and S1P but not medium S1P in both apoMWT and apoMQ22A 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. Background: ApoM overexpression generates larger nascent and plasma HDLs. Results: ApoMQ22A overexpression generates smaller nascent and plasma HDL and enhances apoM and S1P secretion as compared with apoMWT. 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.