In vivo conversion of recombinant human proapolipoprotein AI (rh-Met-proapo AI) to apolipoprotein AI in rabbits

Characterization of woodchuck apolipoprotein A-I: a new tool for drug delivery and identification of altered isoforms in the woodchuck chronic hepatitis model

Apolipoprotein A-I (ApoA-I) is the major protein component of high density lipoprotein (HDL) particles in serum, and participates in the reverse transport of cholesterol from tissues to the liver for excretion. The natural HDL tropism to the liver and cancer cells has been used extensively to target encapsulated drugs. The alteration of the plasmatic isoforms of ApoA-I is a hallmark of chronic hepatitis and hepatocarcinoma in mice and humans. Woodchucks infected with the woodchuck hepatitis virus (WHV) represent the best animal model for the study of chronic viral hepatitis B and viral induced hepatocarcinoma (HCC). WHV-infected woodchuck represents a clinically relevant animal model under which new treatment strategies can be evaluated and optimized. Therapeutic efficacy in this model is likely to be translated into a successful therapy for patients infected with HBV. The present study describes, for the first time, the cloning and characterization of woodchuck ApoA-I. The open reading frame (ORF) of the woodchuck ApoA-I is 795 bp long, coding for 264 amino acids. Unexpectedly, phylogenetic analysis revealed that the closest sequences are those of human and macaque. Woodchuck HDLs were isolated successfully from sera by density gradient ultracentrifugation. A commercial antibody that recognized the woodchuck ApoA-I was also identified. Finally, taking advantage of the techniques and tools developed in this study, two potential applications of woodchuck HDLs are illustrated: drug delivery to a woodchuck hepatocarcinoma cell line and the use of isoelectrofocusing to identify ApoA-I isoforms.

Identification of proteomic biomarkers of preeclampsia in amniotic fluid using SELDI-TOF mass spectrometry

OBJECTIVE

To identify proteomic biomarkers in amniotic fluid (AF) that can distinguish preeclampsia (PE) from chronic hypertension (CHTN) and normotensive controls (CTR).

METHODS

AF from women with PE, CHTN, and CTR were subjected to proteomic analysis by surface-enhanced laser desorption/ionization time-of-flight mass spectrometry.

RESULTS

Proteomic profiling of AF identified 2 biomarkers: peak X (17399.11 Da), which distinguished PE from CTR, and peak Y (28023.34 Da), which distinguished PE and CHTN from CTR. High performance liquid chromatography fractions containing the biomarkers were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and in-gel tryptic digestion. The biomarkers were matched to proapolipoprotein A-I (peak Y) and a functionally obscure peptide, SBBI42 (peak X). Western blot analysis confirmed that AF from PE and CHTN had higher proapolipoprotein A-I levels than CTR.

CONCLUSION

Proteomic analysis of AF can distinguish PE from CHTN and CTR. The discriminatory proteins were identified as proapolipoprotein A-I and SBBI42.

Inhibition of cholesteryl ester transfer protein increases serum apolipoprotein (apo) A-I levels by increasing the synthesis of apo A-I in rabbits

BACKGROUND

Inhibition of cholesteryl ester transfer protein (CETP) is an effective way to increase HDL levels in animals and humans. The effects of a CETP inhibitor, JTT-705, on the in vivo kinetics of apolipoprotein (apo) A-I and apo A-I gene expression in the liver and intestine were investigated.

METHODS

Japanese White rabbits were randomly fed normal rabbit chow LRC-4 (n=10, control) or a food admixture of LRC-4 and 0.75% JTT-705 (n=10, treated) for 7 months. An in vivo kinetics study of apo A-I was performed by injecting rabbit 125I-apo A-I, and apo A-I mRNA levels were quantified by RT-PCR.

RESULTS

JTT-705 significantly inhibited CETP activities, increased serum levels of HDL-cholesterol (C), HDL2-C, HDL-phospholipid, and apo A-I, and decreased HDL-triglyceride levels. The synthetic rate of apo A-I was higher in the treated rabbits than in control rabbits (13.7 +/- 2.6 versus 9.5 +/- 1.3 mg/kg per day, P < 0.05), while the fractional catabolic rate was similar in the two groups. JTT-705 increased apo A-I mRNA levels in the liver without affecting those in the intestine.

CONCLUSION

Inhibition of CETP activity by JTT-705 increases HDL levels by increasing the synthesis of apo A-I, suggesting that it could be a promising therapeutic approach for atherosclerosis.

Evaluation of apolipoprotein A-I kinetics in rabbits in vivo using in situ and exogenous radioiodination methods

The kinetics of in vivo clearance of apolipoprotein (apo) A-I radioiodinated by the iodine monochloride (ICI) method of McFarlane [McFarlane, A.S. (1958) Efficient Trace-Labelling of Proteins with Iodine, Nature 182, 53] as modified by Bilheimer and co-workers [Bilheimer, D.W., Eisenberg, S., and Levy, R.I. (1972) The Metabolism of Very Low Density Lipoprotein Proteins. I. Preliminary in vitro and in vivo Observations, Biochim. Biophys. Acta 260, 212-221] and by using the IODO Beads Iodination Reagent were evaluated in rabbits. Both human apoA-I and rabbit HDL radioiodinated by the IODO Beads Iodination Reagent were cleared faster from plasma of rabbits than those radiolabeled by the ICI method. However, the different radiolabeling procedures in the ICI method, i.e., apoA-I radiolabeled either exogenously or in situ as a part of intact HDL, were not associated with a significant difference in the in vivo kinetics of apoA-I in rabbits if apoA-I was prepared by the guanidine HCI method and used fresh. 125I-ApoA-I subjected to delipidation and lyophilization was cleared only slightly faster from the plasma of rabbits than fresh 125I-apoA-I. We also found that apoA-I separated by the guanidine HCI method and used fresh was cleared faster from the plasma of rabbits when it was injected as free apoA-I without adding serum albumin or after in vitro incubation with rabbit HDL than when injected after reassociation with rabbit plasma. We conclude that the ICI method is a more appropriate radioiodination method for studying the in vivo kinetics of HDL than the IODO Beads Iodination Reagent and that the in vitro incubation conditions before injection are important factors that affect the in vivo kinetics of apo A-I.

In vivo kinetics of human apolipoprotein A-I variants in rabbits

BACKGROUND

Genetic variants of human apolipoprotein (apo) A-I, the major protein component of high-density lipoprotein (HDL), with a single amino acid substitution have been reported, and some of these result in very low plasma HDL-cholesterol (C) levels. Examining the kinetics of radiolabelled apolipoprotein is a straightforward technique for determining its metabolism in vivo. In this study, we investigated the in vivo kinetics of several human apo A-I variants, which we had identified previously, in rabbits.

MATERIALS AND METHODS

Apo A-I variants from heterozygous carriers of Lys-107-->0, Lys-107-->Met, Pro-3-->Arg, Pro-4-->Arg, Pro-165-->Arg and Glu-198-->Lys and the corresponding normal apo A-I were purified and then radioiodinated with 131I and 125I. A kinetic study of apo A-I variants was performed in normolipidaemic rabbits after simultaneous injection of the two isotopes that had been incorporated into HDL. The fractional catabolic rate (FCR) was calculated from the radioactive decay curve.

RESULTS

Acidic mature (negatively charged) apo A-I variants caused by a single amino acid substitution (Lys-107-->0, and Lys-107-->Met) were catabolized faster (FCR, 1.931 +/- 0.539 per day vs. 1.636 +/- 0.460 per day, P </= 0.01 using the Wilcoxon signed-rank test) and basic mature (positively charged) apo A-I variants (Pro-3-->Arg, Pro-4-->Arg, Pro-165-->Arg and Glu-198-->Lys) were catabolized more slowly (FCR 1.470 +/- 0.380 per day vs. 1.654 +/- 0.430 per day, P </= 0.01) than the corresponding normal mature apo A-I in vivo in rabbits. In addition, an inverse linear relationship was observed between the deviation in the FCR of variant human apo A-I from that of normal human apo A-I and the number of electric charges that the apo A-I variant carried (r = -0. 90, k = -0.188, P = 0.0003), as assessed by a linear regression analysis, suggesting that the electric charge of apo A-I variants may determine, at least in part, its in vivo kinetics in rabbits.

CONCLUSIONS

Genetic variants of apo A-I with a single amino acid substitution show abnormal kinetics, and the electric charge of a apo A-I variant could contribute to determining its kinetics in vivo in this xenologous model.

Recombinant apolipoproteins for the treatment of vascular diseases

The protein components of human lipoproteins, apolipoproteins, allow the redistribution of cholesterol from the arterial wall to other tissues and exert beneficial effects on systems involved in the development of arterial lesions, like inflammation and hemostasis. Because of these properties, the antiatherogenic apolipoproteins, particularly apo A-I and apo E, may provide an innovative approach to the management of vascular diseases. The recent availability of extractive or biosynthetic molecules is allowing a detailed overview of their therapeutic potential in a number of animal models of arterial disease. Infusions of apo E, or more dramatically, of apo A-I, both recombinant or extractive, cause a direct reduction of the atherosclerotic burden in experimental animals. Naturally, as the apo A-I(Milano) (apo A-I(M)) dimer, or engineered recombinant apolipoproteins with prolonged permanence in plasma and improved function may offer an even better approach to the therapeutic handling of arterial disease. This progress will go on in parallel with innovations in the technologies for direct, non invasive assessments of human atherosclerosis, thus allowing closer monitoring of this potential new approach to therapy.

In vivo kinetics as a sensitive method for testing physiologically intact human recombinant apolipoprotein A-I: comparison of three different expression systems

In order to assess the structural and functional integrity of recombinant human apoA-I, we expressed apoA-I using three different expression systems: Baculovirus transfected Spodoptera frugiperda (Sf9) cells, stably transfected Chinese hamster ovary (CHO) cells, and transformed Escherichia coli (E. coli). Purified apoA-I from the three expression systems was radioiodinated and their catabolism was compared in normolipemic rabbits. The kinetic turnover studies of radiolabelled apoA-I in normolipemic rabbits revealed that highly purified recombinant apoA-I had an identical decay curve compared to native apoA-I, regardless whether it was purified from Sf9 cells, CHO cells, or E. coli. We also determined the association of the three recombinant apoA-I forms with both rabbit and human HDL. All three recombinant apoA-I forms were associated with HDL2 and HDL3 after injection into the rabbits and after incubation with human serum using both a Superose 6 column separation system and density gradient ultracentrifugation. The addition of the pro-segment or the addition of methionine at the amino-terminal end of apoA-I did not alter its metabolism and association to HDL. In conclusion, all studied expression systems are capable of producing high levels of physiologically intact recombinant human apoA-I. The aminoterminal addition of the prosegment of apoA-I or methionine did not alter the in vivo metabolism of apoA-I or its association to HDL.

Production of mature human apolipoprotein A-I in a baculovirus-insect cell system: propeptide is not essential for intracellular processing but may assist rapid secretion

To achieve expression of human mature apolipoprotein A-I (apoA-I) in the baculovirus-insect cell expression system, the propeptide encoding region of full-length preproapoA-I was deleted using polymerase chain reaction and the resulting cDNA was cloned into BacPak8 plasmid. After transfection into Sf21 insect cells and plaque purification, mature human apoA-I was secreted by the infected cells into the medium as determined by immunoblotting, amino-terminal sequencing, and molecular weight determination. In both monolayer cell cultures, and in suspension cell culture, maximum expression was achieved by the fifth day. For the first 4 days, 50 to 70% of the synthesized apoA-I was retained in the cells. This intracellular apoA-I was represented by mature apoA-I as shown by immunoblotting and amino-terminal sequencing. Further incubation resulted in a sharp decrease in the cell apoA-I content without a corresponding increase in protein in the medium and most likely represents intracellular degradation of the protein. We conclude that the deletion of the propeptide, while not preventing the correct cleavage of prepeptide during intracellular processing, results in reduced secretion of mature apoA-I. The baculovirus-insect cell expression system described in this study provides a useful method for producing recombinant mature apoA-I and is a potential tool for understanding the function of propeptide in intracellular transport and secretion of apoA-I from cells.

Expression and purification of recombinant human apolipoprotein A-I in Chinese hamster ovary cells

The expression of apolipoprotein A-I (apoA-I) has been shown to be very difficult due to its amphiphilic character, autoaggregation, and degradation. We have expressed apoA-I using CHO cells, Baculovirus, and Escherichia coli [Schmidt et al., J. Biol. Chem. (1995) 270, 469-475]. Here we report about optimized conditions for the expression of proapoA-I in CHO cells, testing various serum-free media. We were able to yield apoA-I expression up to 80 micrograms/ml, by far the highest ever reported. However, immunoblot analysis revealed degraded apoA-I. The best apoA-I expression testing various conditions was about 20-30 micrograms/ml without any evidence of degradation. Interestingly, the apoA-I expression resulted in reproducible apoA-I fragments of 26 and 14 kDa. These fragments are consistent with already reported in vivo findings, in which carboxy-terminal proteolysis was suggested. The use of the protease inhibitors pepstatin and chymostatin, both carboxy-peptidase inhibitors, did result in contrast to other studied protease inhibitors in increased apoA-I yield. Therefore, limited carboxy-terminal proteolysis contributes to the degradation of CHO cell-secreted apoA-I. In addition, we evaluated various purification methods for the preparative isolation of recombinant apoA-I. In our hands we obtained the best recovery and no degradation with reversed-phase chromatography using a FPLC system.

A new case of apoA-I deficiency showing codon 8 nonsense mutation of the apoA-I gene without evidence of coronary heart disease

We report a 39-year-old Japanese man with HDL and apoA-I deficiency as well as data from members of his family. Corneal opacity and a stomatocyte were found but not tonsillar hypertrophy, xanthomas, or splenomegaly. His serum HDL cholesterol, apoA-I, apoA-II, and LDL cholesterol levels were t mg/dL, < 3 mg/dL, 6 mg/dL, and 175 mg/dL, respectively. Plasma triglyceride, phospholipid, apoB, apoC-III, and apoE levels were all within normal limits. Lecithin:cholesterol acyltransferase activity was half of normal, while lipoprotein lipase and hepatic triglyceride lipase activities were within normal limits. ApoA-I deficiency was confirmed by combined isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by an immunoblotting method. We surveyed the apoA-I gene of the patient and five of his family members by direct sequencing after amplification by polymerase chain reaction and found a codon 8 nonsense mutation (TGG --> TAG, Trp --> stop) in exon 3 of the apoA-I gene. The results of a pedigree analysis by DNA sequencing and restricted fragment length polymorphism (Sty I) were consistent with an autosomal codominant trait. Coronary angiography was performed to evaluate coronary atherosclerosis, but no significant luminal narrowing was detected. An intracoronary ultrasound study showed mild intimal hyperplasia in segment 6. In summary, this is a case of apoA-I deficiency without evidence of coronary heart disease.

Carboxyl-terminal domain truncation alters apolipoprotein A-I in vivo catabolism

Apolipoprotein A-I (apoA-I), the major protein of high density lipoproteins, facilitates reverse cholesterol transport from peripheral tissue to liver. To determine the structural motifs important for modulating the in vivo catabolism of human apoA-I (h-apoA-I), we generated carboxyl-terminal truncation mutants at residues 201 (apoA-I201), 217 (apoA-I217), and 226 (apoA-I226) by site-directed mutagenesis. ApoA-I was expressed in Escherichia coli as a fusion protein with the maltose binding protein, which was removed by factor Xa cleavage. The in vivo kinetic analysis of the radioiodinated apoA-I in normolipemic rabbits revealed a markedly increased rate of catabolism for the truncated forms of apoA-I. The fractional catabolic rates (FCR) of 9.10 +/- 1.28/day (+/- S.D.) for apoA-I201, 6.34 +/- 0.81/day for apoA-I217, and 4.42 +/- 0.51/day for apoA-I226 were much faster than the FCR of recombinant intact apoA-I (r-apoA-I, 0.93 +/- 0.07/day) and h-apoA-I (0.91 +/- 0.34/day). All the truncated forms of apoA-I were associated with very high density lipoproteins, whereas the intact recombinant apoA-I (r-apoA-I) and h-apoA-I associated with HDL2 and HDL3. Gel filtration chromatography revealed that in contrast to r-apoA-I, the mutant apoA-I201 associated with a phospholipid-rich rabbit apoA-I containing particle. Analysis by agarose gel electrophoresis demonstrated that the same mutant migrated in the pre-beta position, but not within the alpha position as did r-apoA-I. These results indicate that the carboxyl-terminal region (residue 227-243) of apoA-I is critical in modulating the association of apoA-I with lipoproteins and in vivo metabolism of apoA-I.

In vitro reverse cholesterol transport from THP-1-derived macrophage-like cells with synthetic HDL particles consisting of proapolipoprotein A1 or apolipoprotein A1 and phosphatidylcholine

The human monocytic leukaemia cell line THP-1 was induced to differentiate to macrophage-like cells by the addition of phorbol myristoyl acetate (PMA). Subsequently, the cells were enriched in cholesterol and these cholesterol laden cells were used to study the capability of reconstituted discoidal complexes (RDCs), consisting of either human apolipoprotein A1 (apo A1) or recombinant human proapolipoprotein A1 (proapo A1) and phosphatidylcholine (PC), to promote cholesterol efflux. RDCs containing apo A1 and proapo A1 were both effective in the mobilization of intracellular cholesterol, whether this was measured by intracellular cholesterol mass or by the appearance of radiolabelled cholesterol in the supernatant. Using the radiolabelling technique, the activity was saturable and followed Michaelis-Menten kinetics. For both types of complexes and for native HDL the maximum rate of cholesterol removed was approximately 0.5 nmol h-1 per 10(6) cells. For RDCs of proapo A1 and apo A1 and for native HDL the Km values were 3.7, 2.9 and 64.8 micrograms ml-1 respectively. A significant in vitro cholesterol efflux could only be achieved with protein-lipid complexes; no significant export was observed with either free proapo A1 or multilamellar PC liposomes without apolipoprotein. Both RDCs were found to be more active in the mobilization of intracellular cholesterol than HDL isolated from human plasma. The combined results demonstrate that synthetic complexes consisting either of apo A1 or proapo A1 and PC are both active in the in vitro reverse transport of cholesterol.

Deletion of the propeptide of apolipoprotein A-I impairs exit of nascent apolipoprotein A-I from the endoplasmic reticulum

Human apolipoprotein (apo) A-I is secreted as a proprotein of 249 amino acids and is processed extracellularly to the mature form (243 amino acids) by removal of a six-residue propeptide segment. We have examined the role of the apoA-I propeptide in intracellular transport and secretion using transfected baby hamster kidney cells that secreted either proapoA-I (from the wild-type cDNA, A-Iwt) or mature-form apoA-I (from A-I delta pro, a cDNA in which the propeptide sequence was deleted). Deletion of the propeptide from the apoA-I sequence did not affect the rate of apoA-I synthesis, nor did it affect the fidelity of proteolytic removal of the prepeptide. However, the propeptide deletion caused mature-form apoA-I to accumulate within the cells as determined by pulse-chase experiments; the intracellular retention times for the mature-form apoA-I in which the propeptide was prematurely removed was three times longer than that of proapoA-I (t1/2 > 3 h compared with approximately 50 min). There was no detectable degradation of either form of newly synthesized apoA-I. Immunofluorescence microscopy revealed that, whereas the proapoA-I was located predominantly in the Golgi apparatus, large quantities of the mature-form apoA-I were detected in the endoplasmic reticulum and very little was in the Golgi apparatus of A-I delta pro-transfected cells. These findings suggest that the propeptide sequence may be involved in the intracellular transport of apoA-I from the endoplasmic reticulum to the Golgi apparatus. We propose that the function of the propeptide sequence is to facilitate efficient transport of apoA-I through the secretory pathway.