Apolipoprotein M is required for preβ-HDL formation and cholesterol efflux to HDL and protects against atherosclerosis

High-density lipoproteins (HDLs) are considered antiatherogenic because they mediate reverse cholesterol transport from the periphery to the liver for excretion and degradation. Here we show that mice deficient in apolipoprotein M ( apoM), a component of the HDL particle, accumulated cholesterol in large HDL particles (HDL1) while the conversion of HDL to pre beta-HDL was impaired. Accordingly, apoM-deficient mice lacked pre beta-HDL, a subclass of lipid-poor apolipoproteins that serves as a key acceptor of peripheral cellular cholesterol. This deficiency led to a markedly reduced cholesterol efflux from macrophages to apoM-deficient HDL compared to normal HDL in vitro. Overexpression of apoM in Ldlr(-/-) mice protected against atherosclerosis when the mice were challenged with a cholesterol-enriched diet, showing that apoM is important for the formation of pre-HDL and cholesterol efflux to HDL, and thereby inhibits formation of atherosclerotic lesions.