Gemfibrozil significantly lowers cynomolgus monkey plasma lipoprotein[a]-protein and liver apolipoprotein[a] mRNA levels

Lipoprotein(a) metabolism: potential sites for therapeutic targets

Lipoprotein(a) [Lp(a)] resembles low-density lipoprotein (LDL), with an LDL lipid core and apolipoprotein B (apoB), but contains a unique apolipoprotein, apo(a). Elevated Lp(a) is an independent risk factor for coronary and peripheral vascular diseases. The size and concentration of plasma Lp(a) are related to the synthetic rate, not the catabolic rate, and are highly variable with small isoforms associated with high concentrations and pathogenic risk. Apo(a) is synthesized in the liver, although assembly of apo(a) and LDL may occur in the hepatocytes or plasma. While the uptake and clearance site of Lp(a) is poorly delineated, the kidney is the site of apo(a) fragment excretion. The structure of apo(a) has high homology to plasminogen, the zymogen for plasmin and the primary clot lysis enzyme. Apo(a) interferes with plasminogen binding to C-terminal lysines of cell surface and extracellular matrix proteins. Lp(a) and apo(a) inhibit fibrinolysis and accumulate in the vascular wall in atherosclerotic lesions. The pathogenic role of Lp(a) is not known. Small isoforms and high concentrations of Lp(a) are found in healthy octogenarians that suggest Lp(a) may also have a physiological role. Studies of Lp(a) function have been limited since it is not found in commonly studied small mammals. An important aspect of Lp(a) metabolism is the modification of circulating Lp(a), which has the potential to alter the functions of Lp(a). There are no therapeutic drugs that selectively target elevated Lp(a), but a number of possible agents are being considered. Recently, new modifiers of apo(a) synthesis have been identified. This review reports the regulation of Lp(a) metabolism and potential sites for therapeutic targets.

Inhibitors of lipoprotein(a) assembly

Compounds of the general structure A and B were investigated for their activity as lipoprotein(a), [Lp(a)], assembly (coupling) inhibitors. SAR around the amino acid derivatives (structure A) gave compound 14-6 as a potent coupling inhibitor. Oral dosing of compound 14-6 to Lp(a) transgenic mice and cymologous monkeys resulted in a>30% decrease in plasma Lp(a) levels after 1-2 weeks of treatment at 100 mg/kg/day.

Lipoprotein(a): an emerging cardiovascular risk factor

Lipoprotein(a) is a cholesterol-enriched lipoprotein, consisting of a covalent linkage joining the unique and highly polymorphic apolipoprotein(a) to apolipoprotein B100, the main protein moiety of low-density lipoproteins. Although the concentration of lipoprotein(a) in humans is mostly genetically determined, acquired disorders might influence synthesis and catabolism of the particle. Raised concentration of lipoprotein(a) has been acknowledged as a leading inherited risk factor for both premature and advanced atherosclerosis at different vascular sites. The strong structural homologies with plasminogen and low-density lipoproteins suggest that lipoprotein(a) might represent the ideal bridge between the fields of atherosclerosis and thrombosis in the pathogenesis of vascular occlusive disorders. Unfortunately, the exact mechanisms by which lipoprotein(a) promotes, accelerates, and complicates atherosclerosis are only partially understood. In some clinical settings, such as in patients at exceptionally low risk for cardiovascular disease, the potential regenerative and antineoplastic properties of lipoprotein(a) might paradoxically counterbalance its athero-thrombogenicity, as attested by the compatibility between raised plasma lipoprotein(a) levels and longevity.

Alcohol-extracted, but not intact, dietary soy protein lowers lipoprotein(a) markedly

We previously found that dietary soy protein produces higher lipoprotein(a) [Lp(a)] plasma concentrations than does casein. This study tested the hypothesis that soy protein contains Lp(a)-raising alcohol-removable components. Twelve normolipidemic women and men consumed, in a crossover design, liquid-formula diets containing casein, soy protein, or alcohol-extracted soy protein. Dietary periods of 32 days were separated by washout periods on self-selected diets. Fasting lipid and Lp(a) levels were measured throughout. Median Lp(a) concentration was >2-fold greater after 28 to 32 days on a soy protein diet than after an extracted soy protein diet (P<0.001). Lp(a) concentrations after casein and extracted soy protein diets were virtually identical. Women and men responded similarly. When the switch was made from a self-selected to a soy protein diet, median Lp(a) concentration increased 16% after 1 week (P<0.01) and subsequently decreased toward baseline; extracted soy protein and casein diets never exhibited increased median Lp(a) levels, and after 28 to 32 days, these levels were decreased >60% below baseline (P<0.001 and P<0.01, respectively). Low density lipoprotein cholesterol concentrations were not different after the 3 experimental diets. The data indicate that (1) dietary soy protein can increase Lp(a) concentrations, (2) this effect is eliminated after alcohol extraction, and (3) high Lp(a) concentrations may be markedly reduced by diet.

Aspirin reduces apolipoprotein(a) (apo(a)) production in human hepatocytes by suppression of apo(a) gene transcription

High serum lipoprotein(a) (Lp(a)) is a risk factor for vascular disorders. Our preliminary observations suggest that, in some patients with coronary heart disease with high serum Lp(a) levels, administration of aspirin reduced Lp(a) levels. Therefore, we aimed to analyze the effects of aspirin on the production of apo(a), the expression of apolipoprotein(a) (apo(a)) mRNA and the transcriptional activity of apo(a) gene promoter. Aspirin (5 mM) reduced the apo(a) levels in culture medium of human hepatocytes and suppressed apo(a) mRNA expression to 73% and 85% of the controls, respectively. Aspirin also reduced the transcriptional activity of apo(a) gene transfected into HepG2 hepatoma cells in a dose-dependent manner, with a maximal effect at 5 mM (44.3 +/- 1.5% of the control). Sodium salicylate (5 mM) also reduced apo(a) gene transcription, whereas indomethacin (10 microM) had no effect. Deletion analysis of apo(a) gene promoter showed that promoter region extending from -30 to +138 is critical for the effect of aspirin. Furthermore, enhanced production, mRNA expression, and gene transcription of apo(a) by interleukin-6 were also inhibited by aspirin. These results demonstrate that aspirin reduces apo(a) production from hepatocytes via reduction of the transcriptional activity of apo(a) gene with suppression of apo(a) mRNA expression. The suppression of apo(a) production by aspirin may at least in part play a role in the anti-atherogenic effect of aspirin in vascular disorders.

Inhibition of apolipoprotein(a) synthesis in cynomolgus monkey hepatocytes by retinoids via involvement of the retinoic acid receptor

We have shown previously that retinoids induce apolipoprotein (apo) A-I gene expression in cultured cynomolgus hepatocytes and do not have an effect on apo B-100 synthesis. In the present study, the effect of retinoids on apolipoprotein(a) (apo(a)) synthesis in cultured hepatocytes was investigated. The addition of all-trans retinoic acid (at-RA) to the medium of the hepatocytes resulted in a dose- and time-dependent decrease in apo(a) synthesis. Maximal inhibition was 54% after 72 hr of incubation with 10 micromol/L at-RA. Apo B-100 synthesis remained constant, while apo A-I synthesis was increased by 112% after treatment with 10 micromol/L at-RA for 72 hr, indicating that at-RA does not have a general effect on apolipoprotein synthesis in hepatocytes. 9-cis-RA (-36%) and 13-cis-RA (-20%) also inhibited apo(a) synthesis, whereas retinol was not active. To investigate which retinoid receptors are involved in the inhibition of apo(a) synthesis, specific retinoid X receptor (RXR) and retinoic acid receptor (RAR) ligands were used. 4-[1-(3,5,5,8,8-Pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)-ethenyl] benzoic acid (3-methyl-TTNEB), a specific RXR agonist, did not have an effect on apo(a) synthesis, whereas incubation with (E)-4-[2-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-2-naphthalenyl)-1-prope nyl] benzoic acid (TTNPB), a specific RAR agonist, resulted in a decrease of 34%. Steady-state apo(a) mRNA levels were decreased by 42% and 33% after the cells were incubated for 48 hr with 10 micromol/L at-RA and TTNPB, respectively, indicating that the decreased synthesis is regulated at the (post)transcriptional level. We conclude that retinoids down-regulate apo(a) synthesis and mRNA via involvement of RAR and not the RXR homodimer in cynomolgus hepatocytes.

Recombinant adenovirus vector mediated expression of lipoprotein (a) [Lp(a)] in rabbit plasma

Lipoprotein (a) [Lp(a)] is a heterodimer of apolipoprotein (a) [apo(a)] and apolipoprotein B-100 (apoB-100) of low density lipoprotein linked by a disulfide bond. Apo(a) and apoB-100 are synthesized by the liver and covalently associate or couple to form Lp(a) extracellularly. Elevated plasma Lp(a) is an independent risk factor for vascular injury disorders such as restenosis after balloon angioplasty and accelerated graft atherosclerosis following heart transplantation. Lp(a) is not expressed in laboratory animals making studies of its pathophysiology difficult. To overcome this problem, we explored the possibility of generating Lp(a) in rabbit plasma using replication-deficient adenovirus vector mediated gene delivery. Rabbits were chosen because of their large vessels and unlike mouse or rat, rabbit apoB-100 could interact with apo(a) to generate Lp(a). The recombinant (r) adenovirus vector construct used encoded a 200 kDa apo(a) [Ad-apo(a)]. Ad-apo(a) injection into the rabbit marginal vein caused the appearance of plasma rLp(a). Injection of a r adenovirus vector expressing the bacterial LacZ gene (Ad-LacZ) or PBS (vehicle) did not result in detectable plasma rLp(a). These are the first results to demonstrate plasma expression of rLp(a) in rabbits using adenovirus vector mediated gene transfer. Therefore, this system may be suitable for investigating Lp(a)'s role in the development of vascular injury diseases in a rabbit model.

Lack of predictability of classical animal models for hypolipidemic activity: a good time for mice?

Hypolipidemic drugs that are efficacious in man are not always active in classical animal models of dyslipidemia. Inhibitors of HMG-CoA reductase (statins) do not lower plasma cholesterol in rats, but yet this species was alone in providing activity for fibrate-type drugs. Nicotinic acid possesses many desirable features with regard to clinical use, but most of these actions are lacking in rats and monkeys. The metabolism of low density lipoproteins in hamsters is widely thought to be similar to that in humans, yet neither statins or fibrates lower plasma lipids in these species. With the advent of mouse models expressing specific human genes (or disruption of genes) it is now possible to re-examine the effect of established drugs and to characterize new hypolipidemic compounds with respect to site and mechanism of action. Drug responses observed in humans are now being seen in such mouse models (e.g. HDL elevation with fenofibrate in mice with the human apo A-I gene). Moreover, mice are now being screened for compounds that lower plasma (human) Lp(a), or lower plasma cholesterol in the absence of LDL receptors. It is proposed that these new genetic mouse models may afford a more focused examination of drug action and provide, for new compounds, better prediction of the human response.

Bacterial expression and characterization of human recombinant apolipoprotein(a) kringle IV type 9

Elevated plasma lipoprotein(a) [Lp(a)] is an independent risk factor for several vascular diseases. Lp(a) particles are generated through the formation of a disulfide bond between Cys4057 of kringle IV type 9, (KIVt9), of the multikringle apolipoprotein(a) [apo(a)] and a cysteine in apoB-100 low-density lipoprotein (LDL). To better understand this interaction, we have expressed and purified KIVt9 from Escherichia coli as a His-Tag fusionprotein. Dithiothreitol (DTT)-treated purified KIVt9 migrated as a single approximately 17. 3-kDa band on SDS-PAGE gels. Without DTT, an additional band twice the molecular weight of KIVt9 was observed. The double-size band presumably resulted from dimerization of individual kringles, through their unpaired cysteine residues, since a mutation Cys4057 --> Ser ([Ser4057]KIVt9) abolished dimer formation. Using a gel-shift assay, we showed that KIVt9 could couple to 14-amino-acid apoB-100 synthetic peptides (apoB3732-3745 and apoB4319-4332) containing Cys3734 or Cys4326. Both of these apoB-100 cysteines have been reported to associate with apo(a) to generate Lp(a). In the presence of either apoB-100 peptide, KIVt9 was shifted to a higher molecular weight that was consistent with the covalent addition of a 1.2-kDa apoB-100 peptide. Identical apoB-100 peptides in which the cysteine residues were replaced by alanine ([Ala3734]apoB3732-3745 and [Ala4326]apoB4319-4332) had no effect in the gel-shift assay. Furthermore, [Ser4057]KIVt9 did not covalently interact with apoB3732-3745 or apoB4319-4332. These results indicated that KIVt9 couples to the Cys-apoB-100 peptides through a disulfide linkage. This system may be suitable for further investigating the apo(a)/apoB-100 coupling reaction and the structure of KIVt9 through X-ray crystallographic studies.

Dominant negative effect of TGF-beta1 and TNF-alpha on basal and IL-6-induced lipoprotein(a) and apolipoprotein(a) mRNA expression in primary monkey hepatocyte cultures

Lipoprotein(a) [Lp(a)] consists of apolipoprotein(a) [apo(a)] disulfide linked to apolipoprotein B-100 of LDL. Elevated plasma Lp(a) is an independent risk factor for a variety of vascular diseases. Lp(a) has been reported to be an acute-phase reactant, suggesting that cytokines may regulate its levels. To determine whether Lp(a) expression was subject to modulation by cytokines, primary monkey hepatocytes that endogenously express Lp(a) were used. Hepatocytes were treated with interleukin (IL)-6, the major mediator of the acute-phase response, and several other cytokines. IL-6 treatment (0.3 to 10 ng/mL) resulted in a marked, dose-dependent, 2- to 4-fold enhancement of Lp(a) accumulation in the hepatocyte culture media that was highly correlated with changes in apo(a) mRNA levels (r>0.9). Several other cytokines, such as IL-2, IL-8, and hepatocyte growth factor, had no significant effect on Lp(a) levels; however, transforming growth factor-beta1 (TGF-beta1) and tumor necrosis factor-alpha (TNF-alpha) were very active in inhibiting Lp(a) accumulation in the culture media, with IC50s of approximately 0.3 and 1 ng/mL, respectively. Both TGF-beta1 and TNF-alpha also decreased the apo(a) transcript. Mixing experiments, in which hepatocytes were treated with 10 ng/mL of IL-6 and 0.3 to 10 ng/mL of TGF-beta1 or TNF-alpha, demonstrated that the IL-6-mediated induction of Lp(a) and apo(a) mRNA was ablated with very low levels of either inhibitory cytokine, suggesting a dominant negative effect of TGF-beta1 and TNF-alpha. These results show that Lp(a) and apo(a) mRNA expression in primary monkey hepatocytes is subject to both positive (IL-6) and negative (TGF-beta1 and TNF-alpha) regulation by physiological levels of cytokines. Thus, in vivo Lp(a) levels may be dependent on the balance between stimulatory and inhibitory cytokines.

Atorvastatin and gemfibrozil metabolites, but not the parent drugs, are potent antioxidants against lipoprotein oxidation

Increased atherosclerosis risk in hyperlipidemic patients may be a result of the enhanced oxidizability of their plasma lipoproteins. We have previously shown that hypocholesterolemic drug therapy, including the 3-hydroxy-3-methyl-glutaryl CoenzymeA (HMG-CoA) reductase inhibitors, and the hypotriglyceridemic drug bezafibrate, significantly reduced the enhanced susceptibility to oxidation of low density lipoprotein (LDL) isolated from hyperlipidemic patients. Although this antioxidative effect could not be obtained in vitro with all of these drugs, the active drug metabolites, which are formed in vivo, could affect lipoprotein oxidizability. We thus sought to analyze the effect of atorvastatin and gemfibrozil, as well as specific hydroxylated metabolites, on the susceptibility of LDL, very low density lipoprotein (VLDL), and high density lipoprotein (HDL) to oxidation. LDL oxidation induced by either copper ions (10 microM CuSO4), by the free radical generator system 2'-2'-azobis 2-amidino propane hydrochloride (5 mM AAPH), or by the J-774A.1 macrophage-like cell line, was not inhibited by the parent forms of atorvastatin or gemfibrozil, but was substantially inhibited (57-97%), in a concentration-dependent manner, by pharmacological concentrations of the o-hydroxy and the p-hydroxy metabolites of atorvastatin, as well as by the p-hydroxy metabolite (metabolite I) of gemfibrozil. On using the atorvastatin o-hydroxy metabolite and gemfibrozil metabolite I in combination an additive inhibitory effect on LDL oxidizability was found. Similar inhibitory effects (37-96%) of the above metabolites were obtained for the susceptibility of VLDL and HDL to oxidation in the oxidation systems outlined above. The inhibitory effects of these metabolites on LDL, VLDL, and HDL oxidation could be related to their free radical scavenging activity, as well as (mainly for the gemfibrozil metabolite I) to their metal ion chelation capacities. In addition, inhibition of HDL oxidation was associated with the preservation of HDL-associated paraoxonase activity. We conclude that atorvastatin hydroxy metabolites, and gemfibrozil metabolite I possess potent antioxidative potential, and as a result protect LDL, VLDL, and HDL from oxidation. We hypothesize that in addition to their beneficial lipid regulating activity, specific metabolites of both drugs may also reduce the atherogenic potential of lipoproteins through their antioxidant properties.

Molecular cloning of the cDNA encoding the carboxy-terminal domain of chimpanzee apolipoprotein(a): an Asp57 --> Asn mutation in kringle IV-10 is associated with poor fibrin binding

Insight into the structural features of human lipoprotein(a) [Lp(a)] which underlie its functional implication in fibrinolysis may be gained from comparative studies of apo(a). Indeed, cloning of rhesus monkey apo(a) has shown that a Trp72 --> Arg mutation in the lysine-binding site (LBS) of KIV-10 leads to loss of lysine-binding properties of the rhesus Lp(a) particle. Consequently, comparative studies of apo(a) sequences in different Old World monkey species should further our understanding of the molecular role of Lp(a) in the fibrinolytic process. In contrast to other Old World monkeys, including rhesus monkey, cynomolgus, and baboon, the chimpanzee exhibits an elevated level of Lp(a) and a distinct isoform distribution as compared to humans [Doucet et al. J. Lipid Res. (1994) 35, 263-270]. Clearly then, the chimpanzee is an interesting animal model for study of the structure, function, and potential pathophysiological roles of Lp(a). We have cloned and sequenced the region of chimpanzee apo(a) cDNA spanning KIV-3 to the stop codon. The global organization of this region is similar to that of human apo(a) with the presence of KV, which is absent in rhesus monkey apo(a). Nucleotide sequence comparison indicates a variation of 1.4% between chimpanzee and man and 5.1% between chimpanzee and rhesus monkey. The differences concerned single base changes. An Asp57 --> Asn mutation was detected in KIV-10; this residue is critical to the LBS of KIV-10 in human apo(a). To verify that the Asp57 --> Asn substitution was specific to apo(a), we have also cloned the cDNA-encoding plasminogen, which exhibited an Asp at the corresponding position in kringle IV. Using an in vitro binding assay, we have demonstrated that chimpanzee Lp(a) exhibits poor lysine-specific interaction with both intact and plasmin-degraded fibrin as compared to its human counterpart. We propose that the Asn57 substitution in KIV-10 of chimpanzee apo(a) is responsible for this property. Chimpanzee Lp(a) therefore represents an appropriate particle with which to explore the potential effects of Lp(a) on the fibrinolytic system, such as the inhibition of plasminogen activation or inhibition of t-PA activity.

No effect of fibrates on synthesis of apolipoprotein(a) in primary cultures of cynomolgus monkey and human hepatocytes: apolipoprotein A-I synthesis increased

Fibrates have been shown to decrease plasma levels of triglyceride-rich lipoproteins and LDL and to increase HDL. Data on the effect of fibrates on lipoprotein(a) levels in man are not consistent. Because lp(a) levels in vivo are mainly regulated at synthesis level, we studied the effect of fibrates on the synthesis of apolipoprotein(a) (apo(a)) in primary cultures of cynomolgus monkey and human hepatocytes. Furthermore, we assessed the effect of fibrates on apolipoprotein A-I (apo A-I) synthesis and investigated whether different fibrates have different effects on the apo(a) and apo A-I synthesis. The addition of gemfibrozil to cultures of monkey and human hepatocytes had no effect on apo(a) synthesis, but resulted in a dose- and time-dependent increase of apo A-I synthesis and mRNA. In simian hepatocytes maximal stimulation was 2.5-fold after incubation for 72 h with 1.0 mM gemfibrozil, whereas apo A-I synthesis was induced 1.8- and 2.0-fold by using 0.1 mM and 0.3 mM, respectively. Similar results were obtained by using human hepatocytes; apo(a) synthesis remained unchanged, while apo A-I secretion was 2.0-fold increased at 1 mM gemfibrozil. Other fibrates like bezafibrate, clofibrate and clofibric acid did not change apo(a) synthesis either. In contrast, they enhanced the synthesis of apo A-I (1.5-, 1.8- and 1.8-fold, respectively), although less potently than gemfibrozil. We conclude that fibrates have no effect on apolipoprotein(a) synthesis in monkey and human hepatocytes and that these drugs induce apo A-I synthesis.

Standardization and clinical management of lipoprotein(a) measurements

The present article proposes personal suggestions to improve determinations and clinical interpretation of results of lipoprotein(a) assays. Methods and procedures for sampling and quantification of the various isoforms of lipoprotein(a) in serum, plasma and urine are reviewed with the aim of improving the reliability and reproducibility of results and reinforcing the clinical utility of lipoprotein(a) measurements.

CI-1011 lowers lipoprotein(a) and plasma cholesterol concentrations in chow-fed cynomolgus monkeys

Lipoprotein(a) (Lp(a)), which is generated through the covalent association of apolipoprotein(a) (apo(a)) and apo B-100-LDL, is an independent risk factor for several vascular diseases. Therefore, there is interest in developing therapies for lowering Lp(a). This investigation was carried out to determine the effect of CI-1011, a potent lipid regulator in rodents, on Lp(a) and other lipid parameters in cynomolgus monkeys (Macaca fascicularis). Nine healthy male monkeys on a normal chow diet were orally treated with CI-1011 at 30 mg/kg per day for 3 weeks. Lp(a) and total cholesterol levels were significantly decreased after 1 week and maximally reduced to 68 and 73% of control levels, respectively, after 3 treatment weeks. The decreases in total cholesterol were mainly due to changes in low density lipoprotein (LDL). The LDL:HDL ratio decreased by 30%. Triglycerides were unaffected by treatment. Lp(a) and total cholesterol levels returned to pretreatment values after stopping treatment suggesting a direct effect of the compound on their inhibition. Further studies demonstrated that CI-1011 was effective at a low dose of 3 mg/kg per day after 1 week of administration. CI-1011 also decreased apo B-100 to 80% of control levels, but this change was not sufficient to account for the Lp(a) lowering. There was also no correlation between the changes in Lp(a) and apo B-100 levels. Treatment of cynomolgus monkey primary hepatocyte cultures with CI-1011 resulted in a dose-dependent inhibition of Lp(a) levels suggesting a direct hepatic effect of the compound. Western blot analysis of the samples showed that changes in Lp(a) were associated mainly with decreased apo(a) (47%), but not apo B-100 (17%). These results demonstrate that CI-1011 effectively decreases Lp(a) levels both in vivo and in vitro.

Characterization of multiple enhancer regions upstream of the apolipoprotein(a) gene

Plasma concentrations of the atherogenic lipoprotein(a) (Lp(a)) are predominantly determined by inherited sequences within or closely linked to the apolipoprotein(a) gene locus. Much of the interindividual variability in Lp(a) levels is likely to originate at the level of apo(a) gene transcription. However, the liver-specific apo(a) basal promoter is extremely weak and does not exhibit common functional variations that affect plasma Lp(a) concentrations. In a search for additional apo(a) gene control elements, we have identified two fragments with enhancer activity within the 40-kilobase pair apo(a)-plasminogen intergenic region that coincide with DNase I-hypersensitive sites (DHII and DHIII) observed in liver chromatin of mice expressing a human apo(a) transgene. Neither enhancer exhibits tissue specificity. DHIII activity was mapped to a 600-base pair fragment containing nine DNase I-protected elements (footprints) that stimulates luciferase expression from the apo(a) promoter 10-15-fold in HepG2 cells. Binding of the ubiquitous transcription factor Sp1 plays a major role in the function of this enhancer, but no single site was indispensable for activity. DHIII comprises part of the regulatory region of an inactive long interspersed nucleotide element 1 retrotransposon, raising the possibility that retrotransposon insertion can influence the regulation of adjacent genes. DHII enhancer activity was localized to a 180-base pair fragment that stimulates transcription from the apo(a) promoter 4-8-fold in HepG2 cells. Mutations within an Sp1 site or either of two elements composed of direct repeats of the nuclear hormone receptor half-site AGGTCA in this sequence completely abolished enhancer function. Both nuclear hormone receptor elements were shown to bind peroxisome proliferator-activated receptors and other members of the nuclear receptor family, suggesting that this enhancer may mediate drug and hormone responsiveness.

Retinoids inhibit primary cynomolgus monkey hepatocyte lipoprotein(a) levels

Elevated plasma lipoprotein(a) [Lp(a)] independently contributes to a variety of vascular diseases; consequentially, factors that modulate its levels are of interest. Since Lp(a) is produced by a disulfide linkage between apolipoprotein(a) [apo(a)] and apolipoproteinB-100 (apoB-100) of low density lipoprotein (LDL) on the hepatocyte surface, modulation of either particle may be useful in lowering Lp(a). Using primary cynomolgus monkey hepatocyte cultures that endogenously express apo(a) and apoB-100, we showed that all-trans (retinol, retinal, retinoic acid) and 9-cis (retinal, retinoic acid) retinoids lower Lp(a) accumulation in the cell media, with the 9-cis derivatives being > 10-fold more potent than the all-trans stereoisomers. Lp(a) Towering was related to decreases in apo(a) and its cognate transcript, but not to apoB-100. These results demonstrate that retinoids lower Lp(a) levels by decreasing apo(a) through its cognate mRNA.