Human apoB contributes to increased serum total apo(a) level in LPA transgenic mice

Antisense oligonucleotide lowers plasma levels of apolipoprotein (a) and lipoprotein (a) in transgenic mice

OBJECTIVES

This study sought to assess whether an antisense oligonucleotide (ASO) directed to apolipoprotein (a) [apo(a)] reduces apo(a) and lipoprotein (a) [Lp(a)] levels in transgenic mouse models.

BACKGROUND

Elevated Lp(a) is a causal, independent, genetic risk factor for cardiovascular disease and myocardial infarction. Effective therapies to specifically lower plasma Lp(a) levels are lacking.

METHODS

Three transgenic mouse models were utilized: 8K-apo(a) mice expressing 8 kringle IV (KIV) repeats with a single copy of KIV-2; 8K-Lp(a) mice expressing both the 8K apo(a) plus human apolipoprotein B-100; and 12K-apo(a) mice expressing a 12K apo(a) with 3 KIV-2 repeats. The mice were treated intraperitoneally with saline, a control ASO, or ASO 144367 directed to KIV-2 for 4 to 6 weeks. Apo(a), Lp(a), and oxidized phospholipids present on human apoB (OxPL/h-apoB) or apo(a) [OxPL/apo(a)] were measured at baseline and on and off therapy.

RESULTS

ASO 144367 significantly reduced Lp(a) by 24.8% in 8K-Lp(a) mice, and reduced apo(a) levels by 19.2% in 8K-Lp(a) mice, 30.0% in 8K-apo(a) mice, and 86% in 12K-apo(a) mice; ASO 144367 also significantly reduced OxPL/apoB 22.4% in 8K-Lp(a) mice, and OxPL/apo(a) levels by 19.9% in 8K-Lp(a) mice, 22.1% in 8K-apo(a) mice, and 92.5% in 12K-apo(a) mice (p < 0.004, or less, for all). No significant changes occurred in Lp(a), apo(a), OxPL/apoB, or OxPL/apo(a) levels with control ASO or saline.

CONCLUSIONS

This study documents the first specific therapy, to our knowledge, for lowering apo(a)/Lp(a) levels and their associated OxPL. A more potent effect was documented in mice expressing apo(a) with multiple KIV-2 repeats. Targeting liver expression of apo(a) with ASOs directed to KIV-2 repeats may provide an effective approach to lower elevated Lp(a) levels in humans.

Activity of peroxisomal enzymes, and levels of polyamines in LPA-transgenic mice on two different diets

BACKGROUND

In man, elevated levels of plasma plipoprotein (a) (Lp(a)) is a cardiovascular risk factor, and oxidized phospholipids are believed to play a role as modulators of inflammatory processes such as atherosclerosis. Polyamines are potent antioxidants and anti-inflammatory agents. It was therefore of interest to examine polyamines and their metabolism in LPA transgenic mice. Concentration of the polyamines putrescine, spermidine and spermine as well as the activity of peroxisomal polyamine oxidase and two other peroxisomal enzymes, acyl-CoA oxidase and catalase were measured. The mice were fed either a standard diet or a diet high in fat and cholesterol (HFHC). Some of the mice in each feeding group were in addition given aminoguanidine (AG), a specific inhibitor of diamine oxidase, which catalyses degradation of putrescine, and also inhibits non-enzymatic glycosylation of protein which is implicated in the aetiology of atherosclerosis in diabetic patients. Non-transgenic mice were used as controls.

RESULTS

Intestinal peroxisomal polyamine oxidase activity was significantly higher in LPA transgenic mice than in the non-transgenic mice, while intestinal peroxisomal catalase activity was significantly lower. Hepatic beta-oxidation increased in Lp(a) transgenic mice fed the HFHC diet, but not in those on standard diet. Hepatic spermidine concentration was increased in all mice fed the HFHC diet compared to those fed a standard diet, while spermine concentration was decreased. With exception of the group fed only standard diet, transgenic mice showed a lower degree of hepatic steatosis than non-transgenic mice. AG had no significant effect on hepatic steatosis.

CONCLUSION

The present results indicate a connection between peroxisomal enzyme activity and the presence of the human LPA gene in the murine genome. The effect may be a result of changes in oxidative processes in lipid metabolism rather than resulting from a direct effect of the LPA construct on the peroximal gene expression.

Reduced LPA expression after peroxisome proliferator-activated receptor alpha (PPARα) activation in LPA-YAC transgenic mice

BACKGROUND:: Apolipoprotein(a) (apo(a)), which is part of the atherogenic lipoprotein Lp(a), shares structural homology with plasminogen (plg). Genes coding for plasminogen (PLG) and apo(a) (LPA) are linked and situated 40kb apart in the telomeric region of the long arm of chromosome 6. LPA is naturally expressed only in primates and hedgehogs. Thus, access to knowledge regarding the mechanism by which LPA expression is regulated is limited due to shortage of appropriate animal models. However, mice transgenic for the human LPA gene have been produced. Lp(a) levels in man are genetically determined and not altered significantly by dietary changes. In contrast, mice transgenic for LPA-yeast artificial chromosome (LPA-YAC) have markedly reduced apo(a) levels after maintenance on a high-fat diet. LPA-YAC carries the 40kb LPA-PLG intergenic region, which includes a putative binding site for peroxisome proliferator-activated receptor alpha (PPARalpha). Therefore, we examined if fibrates, which exert their effect via PPARalpha, could alter LPA expression in transgenic mice. METHODS:: Two LPA transgenic mouse lines with or without the LPA-PLG intergenic region we fed either PPARalpha agonist fenofibrate (FF) or 4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid (WY 14643) containing diets for 3 weeks. For the study of serum apo(a) levels, blood were sampled prior the experiment and when the animals were sacrificed. For the study of gene expression pattern pieces of livers were collected and submerged in RNAlater buffer and stored at -70 degrees C until analysis by quantitative PCR. RESULTS AND CONCLUSIONS:: The results showed that fibrates reduce LPA expression in LPA-YAC transgenic mice, but have no impact on hepatic apo(a) mRNA or serum apo(a) protein levels in LPA-cDNA transgenic mice, which lack the LPA-PLG intergenic region. This suggests that the effect of fibrates on LPA expression is mediated upstream of the LPA gene. However, on the basis of current data it is not possible to conclude that PPARalpha is the primary factor that represses LPA expression in LPA-YAC transgenic mice. Negative correlation between FXR and apo(a) mRNA levels, in addition to putative FXR DNA binding sequence in LPA-PLG intergenic region, suggest that it is equally likely that reduced expression of LPA could be a secondary consequence of PPARalpha activation on other genes, such as FXR.

Atherogenesis and vascular calcification in mice expressing the human LPA gene

Background: Lp(a) lipoprotein (Lp(a)) contains polymorphic glycoprotein, apolipoprotein(a) (apo(a)) and low density lipoprotein (LDL). The extensive homology between apo(a) and plasminogen is believed to contribute to the pathogenicity of apo(a), but the precise mechanisms by which Lp(a) participates in atherogenesis is still unknown. We used LPA-yeast artificial chromosome (LPA-YAC) transgenic mice with or without the human APOB (hAPOB) gene to study pathogenicity of apo(a)/Lp(a) and illucidate its role in regulation of serum lipid levels. Methods: Middle-aged (1-year-old) mice were fed a control (AIN-76), a high-cholesterol (HC) or a high-cholesterol/high-fat (HCHF) diet for 7 weeks. For the study of serum total apo(a) and lipid levels, mice were sampled prior to the experiment, at 2 weeks and at 7 weeks when the animals were sacrificed. Hearts with ascending aorta were fixed in formalin, embedded in gelatine and prepared for sections on a cryostat. Livers were washed in ice cold saline and submerged in RNAlater trade mark buffer and stored at -70 degrees C until mRNA analysis. Results: Wild type mice fed the control diet did not develop aortic lesions. Presence of the LPA gene was sufficient to induce development of aortic lesions, but neither coexpression of the hAPOB gene nor feeding the HC diet or the HCHF diet augmented the development of aortic lesions in LPA-YAC transgenic mice. On the control diet transgenic females had larger aortic lesion size than transgenic males. Furthermore, aortic lesions in transgenic females were associated with calcification more often than in transgenic males. Serum total cholesterol levels were higher both in wild type and LPA-YAC transgenic males than in females mainly because of higher serum high-density lipoprotein cholesterol levels. HC and HCHF feeding had more pronounced effect on total cholesterol levels in LPA-YAC/hAPOB transgenic mice than in either wild type or LPA-YAC transgenic mice, due to increased low density lipoprotein cholesterol levels. Furthermore, these diets reduced serum total apo(a) levels in both transgenic mouse lines. Conclusion: Expression of the human LPA gene in mice is sufficient to trigger development of aortic lesions. Similar frequency of calcified lesions in LPA-YAC transgenic mice with or without hAPOB gene may suggest that apo(a) is the part of the Lp(a) molecule that causes aortic calcification. The basis for reduced serum total apo(a) level in response to cholesterol feeding is not clear, but interplay between LPA and factors involved in cholesterol or bile acid homeostasis is worth of future studies.