Lipoprotein association of human apolipoprotein E/A-I chimeras. Expression in transfected hepatoma cells

The specific amino acid sequence between helices 7 and 8 influences the binding specificity of human apolipoprotein A-I for high density lipoprotein (HDL) subclasses: a potential for HDL preferential generation

Humans have two major high density lipoprotein (HDL) sub-fractions, HDL(2) and HDL(3), whereas mice have a monodisperse HDL profile. Epidemiological evidence has suggested that HDL(2) is more atheroprotective; however, currently there is no direct experimental evidence to support this postulate. The amino acid sequence of apoA-I is a primary determinant of HDL subclass formation. The majority of the alpha-helical repeats in human apoA-I are proline-punctuated. A notable exception is the boundary between helices 7 and 8, which is located in the transitional segment between the stable N-terminal domain and the C-terminal hydrophobic domain. In this study we ask whether the substitution of a proline-containing sequence (PCS) separating other helices in human apoA-I for the non-proline-containing sequence (NPCS) between helices 7 and 8 (residues 184-190) influences HDL subclass association. The human apoA-I mutant with PCS2 replacing NPCS preferentially bound to HDL(2). In contrast, the mutant where PCS3 replaced NPCS preferentially associated with HDL(3). Thus, the specific amino acid sequence between helices 7 and 8 influences HDL subclass association. The wild-type and mutant proteins exhibited similar physicochemical properties except that the two mutants displayed greater lipid-associated stability versus wild-type human apoA-I. These results focus new attention on the influence of the boundary between helices 7 and 8 on the properties of apoA-I. The expression of these mutants in mice may result in the preferential generation of HDL(2) or HDL(3) and allow us to examine experimentally the anti-atherogenicity of the HDL subclasses.

Apolipoprotein A-I alpha -helices 7 and 8 modulate high density lipoprotein subclass distribution

Mice have a monodisperse high density lipoprotein (HDL) profile, whereas humans have two major subfractions designated HDL(2) and HDL(3). Human apoA-I transgenic mice exhibit a human-like HDL profile, indicating that the amino acid sequence of apoA-I is a determinant of the HDL profile. Comparison of the primary sequence of mouse and human apoA-I and the previously designated "hinge" domain of apoA-I led us to hypothesize that alpha-helices 7 and 8 (7/8) are determinants of HDL subclass distribution. The following proteins were expressed in Escherichia coli: human apoA-I, T7-hAI; mouse apoA-I, T7-mAI; chimeric human apoA-I containing murine helices 7/8 in place of human helices 7/8, T7-hAI(m7/8); and the reciprocal chimera, T7-mAI(h7/8). The recombinant proteins were examined for their association with human plasma HDL subclasses. The results demonstrated that T7-hAI bound HDL(2) and HDL(3) equally well, whereas T7-mAI bound to HDL(2) preferentially. T7-hAI(m7/8) behaved like T7-mAI, and T7-mAI(h7/8) behaved like T7-hAI. Thus, alpha-helices 7/8 are strong contributors to the pattern of HDL subclass association. Self-association, alpha-helicity, cholesterol efflux, and lecithin-cholesterol acyltransferase activity of the recombinant proteins were also assessed. Human apoA-I self-associates more and activates human lecithin-cholesterol acyltransferase better than mouse apoA-I. These differential characteristics of human and mouse apoA-I are not dependent on helices 7/8.

Evidence that apolipoprotein A-IMilano has reduced capacity, compared with wild-type apolipoprotein A-I, to recruit membrane cholesterol

Human carriers of apolipoprotein (apo) A-IMilano are heterozygous for an Arg173-->Cys substitution in the apoA-I primary sequence; despite severe reductions in HDL cholesterol concentrations, affected individuals do not develop coronary heart disease, suggesting that apoA-IMilano may possess antiatherogenic properties. As the beneficial effects of wild-type apoA-I are linked to its role in HDL cholesterol transport, we examined the capacity of apoA-IMilano to recruit cell cholesterol and activate lecithin:cholesterol acyltransferase (LCAT) (two key events in the antiatherogenic reverse cholesterol transport pathway). ApoA-IMilano and wild-type apoA-I were expressed in Chinese hamster ovary cells, and their ability to recruit membrane phospholipid and cholesterol for the assembly of nascent HDL was compared. Both clonal cell lines exhibited similar levels of apolipoprotein accumulation in serum-free medium (approximately 2 micrograms/mg cell protein per 24 hours), and 15% of each apolipoprotein was associated with membrane lipids to form nascent HDL (d = 1.063 to 1.21 g/mL). SDS-PAGE showed that a majority (66 +/- 12%) of the lipidated apoA-IMilano was in the homodimer form. Compositional analyses revealed that apoA-IMilano nascent HDL had a significantly lower (P < .001) unesterified cholesterol/phospholipid mole ratio (0.47 +/- 0.10) than wild-type apoA-I complexes (1.29 +/- 0.14), indicating that apoA-IMilano had a reduced capacity to recruit cell cholesterol. In addition to the reduced unesterified cholesterol/phospholipid ratio, apoA-IMilano nascent HDL consisted mostly of small 7.4-nm particles compared with wild-type apoA-I, in which 11- and 9-nm particles predominated. Despite these changes in nascent HDL particle size and composition, apoA-IMilano activated LCAT normally. We conclude that, even though apoA-IMilano is a normal activator of LCAT, it is less efficient that wild-type apoA-I in recruiting cell cholesterol, suggesting that the putative antiatherogenic properties attributed to apoA-IMilano may be unrelated to the initial stages of reverse cholesterol transport.

Sterol carrier protein-2 overexpression enhances sterol cycling and inhibits cholesterol ester synthesis and high density lipoprotein cholesterol secretion

Recent data indicate that sterol carrier protein-2 (SCP-2) functions in the rapid movement of newly synthesized cholesterol to the plasma membrane (Puglielli, L., Rigotti, A., Greco, A. V., Santos, M. J., and Nervi, F. (1995) J. Biol. Chem. 270, 18723-18726). In order to further characterize the cellular function of SCP-2, we transfected McA-RH7777 rat hepatoma cells with a pre-SCP-2 cDNA expression construct. In stable transfectants, pre-SCP-2 processing resulted in an 8-fold increase in peroxisomal levels of SCP-2. SCP-2 overexpression increased the rates of newly synthesized cholesterol transfer to the plasma membrane and plasma membrane cholesterol internalization by 4-fold. There was no effect of SCP-2 overexpression on the microsomal levels of acyl-CoA:cholesterol acyltransferase and neutral cholesterol ester (CE) hydrolase; however, in the intact cell, CE synthesis and mass were reduced by 50%. SCP-2 overexpression also reduced high density lipoprotein-cholesterol secretion and apoA-I gene expression by 70% and doubled the rate of plasma membrane desmosterol conversion to cholesterol. We conclude that SCP-2 overexpression enhances the rate of cholesterol cycling, which reduces the availability of cholesterol for CE synthesis and alters the activity of a cellular cholesterol pool involved in regulating apoA-I-mediated high density lipoprotein cholesterol secretion. The net result of these changes in cholesterol metabolism is a 46% increase in plasma membrane cholesterol content, the implications of which are discussed.