Apolipoprotein E effectively inhibits lipoprotein lipase-mediated lipolysis of chylomicron-like triglyceride-rich lipid emulsions in vitro and in vivo

Apolipoprotein E (apoE) is an important determinant for the liver uptake of triglyceride rich lipoproteins and emulsions by the remnant receptor, In the current study, we assessed an additional role of apoE as modulator of the metabolism of triglyceride-rich lipoproteins in vitro and in vivo. Glycerol tri[H-3]oleate [C-14]cholesteryl oleate double-labeled triglyceride-rich emulsions were injected into fasted rats, The serum half-life of glycerol tri[SH]oleate was 3-fold faster (5.4 min) than that of [C-14]cholesteryl oleate (16.7 min), confirming lipoprotein lipase (LPL) mediated processing. To establish a specific effect of apoE on emulsion lipolysis rather than liver uptake, rats were functionally hepatectomized, and hypo(apo)lipoproteinemia was induced by 17 alpha-ethinyl estradiol treatment. An apoE concentration-dependent inhibition of emulsion-triglyceride hydrolysis was observed, reaching a 14.8-fold increased half-life of glycerol tri[H-3]oleate as compared with that in the absence of exogenous apoE. The mechanism and specificity of the effect of apoE on emulsion lipolysis by purified LPL was assessed in vitro. Addition of apoE to glycerol tri[H-3]oleate-labeled emulsions led to a concentration-dependent inhibition of [H-3]oleate release (9.5% residual LPL activity at 60 mu g/ml apoE), while apoA-I was ineffective. The inhibitory effect of apoE was not abolished by reductive methylation of lysine residues, whereas selective modification of arginine residues by 1,2-cyclohexadione completely cancelled the inhibitory effect of apoE. It is concluded that apoE can specifically inhibit the LPL-mediated hydrolysis of emulsion triglycerides both in vitro and in vivo, and that arginine residues in apoE are essential for this effect. We suggest that in addition to its role in receptor recognition, apoE also modulates the LPL-mediated processing of triglyceride rich lipoproteins.