Intracellular early and late modifications of human apolipoprotein A-II. Effect of glutamine- +1 to leucine substitution

Overexpression of human apolipoprotein A-II in mice induces hypertriglyceridemia due to defective very low density lipoprotein hydrolysis

Two lines of transgenic mice, hAIItg-delta and hAIItg-lambda, expressing human apolipoprotein (apo)A-II at 2 and 4 times the normal concentration, respectively, displayed on standard chow postprandial chylomicronemia, large quantities of very low density lipoprotein (VLDL) and low density lipoprotein (LDL) but greatly reduced high density lipoprotein (HDL). Hypertriglyceridemia may result from increased VLDL production, decreased VLDL catabolism, or both. Post-Triton VLDL production was comparable in transgenic and control mice. Postheparin lipoprotein lipase (LPL) and hepatic lipase activities decreased at most by 30% in transgenic mice, whereas adipose tissue and muscle LPL activities were unaffected, indicating normal LPL synthesis. However, VLDL-triglyceride hydrolysis by exogenous LPL was considerably slower in transgenic compared with control mice, with the apparent Vmax of the reaction decreasing proportionately to human apoA-II expression. Human apoA-II was present in appreciable amounts in the VLDL of transgenic mice, which also carried apoC-II. The addition of purified apoA-II in postheparin plasma from control mice induced a dose-dependent decrease in LPL and hepatic lipase activities. In conclusion, overexpression of human apoA-II in transgenic mice induced the proatherogenic lipoprotein profile of low plasma HDL and postprandial hypertriglyceridemia because of decreased VLDL catabolism by LPL.

Illegitimate expression of apolipoprotein A-II in Caco-2 cells is due to chromatin organization

Transcriptional activity of the human apolipoprotein (apo) A-II promoter has been reported in transiently transfected Caco-2 cells, but not in the intestine in vivo. In the present study we established that the transcription of a stably transfected reporter gene under the control of the -911/+29 human apo A-II, decreases with the onset of the differentiation process. This decrease paralleled that of the expression of the endogenous apo A-II gene. The decrease in apo A-II expression is also followed by a marked increase in the expression of the intestine-specific apo A-IV gene, analyzed here as a marker of enterocytic differentiation. Using clonal glucose metabolic variants of Caco-2 cells we have also observed that the lowest levels of apo A-II mRNA are associated with the lowest rates of glucose consumption. The illegitimate apo A-II transcriptional activity observed in Caco-2 cells is linked to the presence of DNase-I hypersensitive sites within the enhancer. This reflects a chromatin organization which allows, in Caco-2 cells as in the liver, the communication between the apo A-II enhancer and the proximal promoter, unlike what is observed in intestinal epithelial cells.

Regulation of the metabolism of plasma lipoproteins by apolipoprotein A-II

Mouse apolipoprotein (apo) A-II has three variants (type A, B, and C) among inbred strains. To clarify the role of ApoA-II in the metabolism of high density lipoproteins (HDL), we constructed a new congenic mouse strain (P1.R1-Apoa2b) with type B ApoA-II of the SAMR1 strain on the genetic background of the SAMP1 strain, and examined it together with another ApoA-II congenic strain (R1.P1-Apoa2c) containing type C ApoA-II of the SAMPI strain on the SAMR1 strain and the parental SAMP1 and SAMR1 strains. Genetic characterization of the congenic strains indicated that only small regions surrounding the ApoA-II gene of the parental strains had been transferred. The strains with Apoa2c had lower plasma concentrations of HDL and ApoA-II, and a smaller HDL particle size than strains with Apoa2b. We detected no significant differences in the mRNA levels of ApoA-II or in the in vitro translational efficiency of the ApoA-II mRNA among the four strains. These findings suggested that the differences in the post-translational modification or efficiency of secretion between the Apoa2b and Apoa2c protein regulates the ApoA-II concentration which in turn determines the concentration and size of HDL in mice.

Purification and characterization of recombinant human apolipoprotein A-II expressed in Escherichia coli

We have expressed recombinant human apolipoprotein A-II (apoA-II) in Escherichia coli, as a fusion protein with Schistosoma japonicum glutathione-S-transferase (GST). The GST-AII fusion protein was recovered by affinity chromatography using glutathione as a ligand. After thrombin cleavage and removal of the GST carrier, recombinant apoA-II was obtained in a highly purified form and was exclusively composed of dimeric apoA-II. Kinetics of association to dimyristoylglycerophosphocholine (Myr2GroPCho) vesicles showed that recombinant apoA-II exhibited the same pattern of association as human plasma apoA-II. Electron microscopic analysis of the complexes showed a typical pattern of rouleaux, characteristic of stacked discs, with a diameter similar to that determined by gradient-gel electrophoresis. Circular dichroism measurements showed that the alpha-helical content of both plasma and recombinant apoA-II increased similarly when the proteins associated with Myr2GroPCho vesicles, at the expense of a random-coil structure. Lipid-bound apoA-II consisted of 70-72% alpha helices, suggesting the presence of three 18-residue alpha helices/apoA-II monomer. Cross-linking experiments indicated that Myr2GroPCho complexes contained two molecules dimeric apoA-II/vesicle. Recombinant apoA-II was as efficient as plasma apoA-II in associating with HDL subclasses, and in displacing apoA-I from dipalmitoylglycerophosphocholine/cholesterol/apoA-I complexes, most likely due to its highly ordered secondary structure when associated with Myr2GroPCho vesicles. These findings demonstrate that recombinant apoA-II exhibits the same structural and functional properties as human plasma apoA-II. Thus, the expression system utilized is appropriate to produce mutagenized forms to further structure/function analysis.