Turnover Rates of the Low-Density Lipoprotein Receptor and PCSK9: Added Dimension to the Cholesterol Homeostasis Model

OBJECTIVE

We measured the turnover rates of the LDLR (low-density lipoprotein receptor) and PCSK9 (proprotein convertase subtilisin/kexin type 9) in mice by metabolic labeling with heavy water and mass spectrometry. Approach and Results: In liver of mice fed high-cholesterol diets, LDLR mRNA levels and synthesis rates were markedly lower with complete suppression of cholesterol synthesis and higher cholesterol content, consistent with the Brown-Goldstein model of tissue cholesterol homeostasis. We observed markedly lower PCSK9 mRNA levels and synthesis rates in liver and lower concentrations and synthesis rates in plasma. Hepatic LDLR half-life (t½) was prolonged, consistent with an effect of reduced PCSK9, and resulted in no reduction in hepatic LDLR content despite reduced mRNA levels and LDLR synthesis rates. These changes in PCSK9 synthesis complement and expand the well-established model of tissue cholesterol homeostasis in mouse liver, in that reduced synthesis and levels of PCSK9 counterbalance lower LDLR synthesis by promoting less LDLR catabolism, thereby maintaining uptake of LDL cholesterol into liver despite high intracellular cholesterol concentrations.

CONCLUSIONS

Lower hepatic synthesis and secretion of PCSK9, an SREBP2 (sterol response element binding protein) target gene, results in longer hepatic LDLR t½ in response to cholesterol feeding in mice in the face of high intracellular cholesterol content. PCSK9 modulation opposes the canonical lowering of LDLR mRNA and synthesis by cholesterol surplus and preserves LDLR levels. The physiological and therapeutic implications of these opposing control mechanisms over liver LDLR are of interest and may reflect subservience of hepatic cholesterol homeostasis to whole body cholesterol needs.

Decreased plasma levels of soluble low density lipoprotein receptor-related protein-1 (sLRP) and the soluble form of the receptor for advanced glycation end products (sRAGE) in the clinical diagnosis of Alzheimer's disease

Soluble low density lipoprotein receptor-related protein-1 (sLRP) and the soluble form of the receptor for advanced glycation end products (sRAGE) may reflect some peripheral plasma features of the pathophysiological process of Alzheimer's disease (AD). Decreased plasma levels of sLRP and sRAGE in patients with AD have been documented. However, whether different levels of these proteins can differentiate AD from other types of dementia has not been described. In the present study we assessed the concentrations of these two proteins in 126 patients with AD, 96 with vascular dementia (VaD), 30 with non-AD neurodegenerative dementias (NND) and 98 cognitively normal controls (NC). Plasma sLRP was significantly lower in the group with AD compared with any of the other three groups (p<0.001). Sensitivity of sLRP was 77.8% for AD, whereas specificity was 93.3% for NND, 85.7% for the NC and 58.3% for those with VaD. Plasma sRAGE showed a significantly lower concentration in the group with AD compared with those in the VaD or NC group, but there were no significant differences between the AD compared to the NND group or the VaD compared to the NND group. Sensitivity of sRAGE was 82.5% for patients with AD, whereas specificity was 53.5% for NND, 73.5% for the NC group and 43.8% for those with VaD. The receiving operator characteristic analysis of combined sLRP and sRAGE showed a higher diagnostic accuracy (area under the curve, 0.88; 95% confidence interval, 0.84-0.93) than that of either sLRP or sRAGE considered singly. The results support the possibility that these two biomarkers may help with the diagnosis of AD.

Impaired lipoprotein receptor-mediated peripheral binding of plasma amyloid-β is an early biomarker for mild cognitive impairment preceding Alzheimer's disease

Soluble circulating low density lipoprotein receptor-related protein-1 (sLRP) provides key plasma binding activity for Alzheimer's disease (AD) amyloid-β peptide (Aβ). sLRP normally binds 70-90% of plasma Aβ preventing free Aβ access to the brain. In AD, Aβ binding to sLRP is compromised by increased levels of oxidized sLRP which does not bind Aβ. Here, we determined plasma oxidized sLRP and Aβ40/42 sLRP-bound, other proteins-bound and free plasma fractions, cerebrospinal fluid (CSF) tau/Aβ42 ratios, and mini-mental state examination (MMSE) scores in patients with mild cognitive impairment (MCI) who progressed to AD (MCI-AD, n = 14), AD (n = 14) and neurologically healthy controls (n = 14) recruited from the Göteborg MCI study. In MCI-AD patients prior to conversion to AD and AD patients, the respective increases in oxidized sLRP and free plasma Aβ40 and Aβ42 levels were 4.9 and 3.7-fold, 1.8, and 1.7-fold and 4.3 and 3.3-fold (p < 0.05, ANOVA with Tuckey post-hoc test). In MCI-AD and AD patients increases in oxidized sLRP and free plasma Aβ40 and Aβ42 correlated with increases in CSF tau/Aβ42 ratios and reductions in MMSE scores (p < 0.05, Pearson analysis). A heterogeneous group of 'stable' MCI patients that was followed over 2-4 years (n = 24) had normal CSF tau/Aβ42 ratios but increased oxidized sLRP levels (p < 0.05, Student's t test). Data suggests that a deficient sLRP-Aβ binding might precede and correlate later in disease with an increase in the tau/Aβ42 CSF ratio and global cognitive decline in MCI individuals converting into AD, and therefore is an early biomarker for AD-type dementia.

Clearance of amyloid-beta by circulating lipoprotein receptors

Low-density lipoprotein receptor-related protein-1 (LRP) on brain capillaries clears amyloid beta-peptide (Abeta) from brain. Here, we show that soluble circulating LRP (sLRP) provides key endogenous peripheral 'sink' activity for Abeta in humans. Recombinant LRP cluster IV (LRP-IV) bound Abeta in plasma in mice and Alzheimer's disease-affected humans with compromised sLRP-mediated Abeta binding, and reduced Abeta-related pathology and dysfunction in a mouse model of Alzheimer disease, suggesting that LRP-IV can effectively replace native sLRP and clear Abeta.

Binding of low density lipoprotein to platelet apolipoprotein E receptor 2' results in phosphorylation of p38MAPK

Binding of low density lipoprotein (LDL) to platelets enhances platelet responsiveness to various aggregation-inducing agents. However, the identity of the platelet surface receptor for LDL is unknown. We have previously reported that binding of the LDL component apolipoprotein B100 to platelets induces rapid phosphorylation of p38 mitogen-activated protein kinase (p38MAPK). Here, we show that LDL-dependent activation of this kinase is inhibited by receptor-associated protein (RAP), an inhibitor of members of the LDL receptor family. Confocal microscopy revealed a high degree of co-localization of LDL and a splice variant of the LDL receptor family member apolipoprotein E receptor-2 (apoER2') at the platelet surface, suggesting that apoER2' may contribute to LDL-induced platelet signaling. Indeed, LDL was unable to induce p38MAPK activation in platelets of apoER2-deficient mice. Furthermore, LDL bound efficiently to soluble apoER2', and the transient LDL-induced activation of p38MAPK was mimicked by an anti-apoER2 antibody. Association of LDL to platelets resulted in tyrosine phosphorylation of apoER2', a process that was inhibited in the presence of PP1, an inhibitor of Src-like tyrosine kinases. Moreover, phosphorylated but not native apoER2' co-precipitated with the Src family member Fgr. This suggests that exposure of platelets to LDL induces association of apoER2' to Fgr, a kinase that is able to activate p38MAPK. In conclusion, our data indicate that apoER2' contributes to LDL-dependent sensitization of platelets.

Genetic and functional characteristics of the human in vivo LRP1/A2MR receptor suggested as a risk marker for Alzheimer’s disease and other complex (degenerative) diseases

LDL receptor-related protein/alpha2-macroglobulin receptor (LRP1/A2MR) a multiligand receptor is considered as not only being a possible risk factor of neurodegenerative diseases like Alzheimer's disease but also as determining the progression of other complex diseases like atherosclerosis and cancer. Although a large number of in vitro studies have highlighted its functional importance, as yet not enough is known about the clinical importance of the genetic background of LRP1 in human diseases. The aim of this ex vivo/in vivo study of 448 subjects was to present data on genetic LRP1 variants of healthy European Caucasians from Central Germany. Genotype-dependent LRP1 expression was analyzed in a representative subgroup (gene expression: n = 127, protein expression: n = 44). These data were evaluated in comparison to other published clinical LRP1 studies. For 15 functionally interesting genetic variants the genotype and allele distributions of the German Caucasians were presented in relation to their in vivo LRP1 gene and protein expression. A direct influence of the LRP1 promoter polymorphism c.1-25C>G on the human in vivo LRP1 expression level was demonstrated. In an analysis of 48 further studies genomic and functional results were evaluated. The analysis especially on Alzheimers's disease partly highlighted contradictory results, but suggested that ethnic as well as genomic characteristics determine LRP1 expression and must be considered in clinical investigations on human LRP1.

[Properties and expression of the human activated alpha2-macroglobulin receptor]

A multi-step purification procedure has been used for isolation of alpha 2M/LRP receptor from human placenta. Enzymatic properties of complexes of purified alpha 2M trypsin with alpha 2M/LRP receptor were studied. This complex is characterized by enzymatic activity measured by a kinetic procedure based on the trypsin reaction with synthetic substrate alpha 2N-benzoyl-D,L-arginine nitroparaanilide. Purified triple complexes of alpha 2M trypsin with alpha 2M/LRP had 22-28% residual trypsin activity, while that of trypsin complex with alpha 2-macroglobulin was 52-53%. Patients' blood samples were characterized by a high level of residual activity of the studied double and triple complexes only initially. A decrease in the activities of these complexes was observed in patients with high levels of malonic dialdehyde (by TBA-reactive products) combined with decreased level of superoxidedismutase-like activity.

The relationship among apolipoprotein(a) polymorphisms, the low-density lipoprotein receptor-related protein, and the very low density lipoprotein receptor genes, and plasma lipoprotein(A) concentration in the Czech population

Increased plasma concentration of lipoprotein(a) [Lp(a)] is an established independent risk factor for coronary artery disease (CAD), which is strongly genetically determined. This study was designed to investigate the relationship between the K-IV and (TTTTA)n apolipoprotein(a) [apo(a), protein; APOA, gene] polymorphisms, as well as the C766T low-density lipoprotein receptor-related protein (LRP) and the (CGG)n very low density lipoprotein receptor (VLDLR) polymorphisms on the one hand, and plasma Lp(a) levels in Czech subjects who underwent coronary angiography on the other hand. The lengths of the alleles of the APOA K-IV and (TTTTA)n polymorphisms were strongly inversely correlated with plasma Lp(a) levels in univariate analysis (r = -0.41, p < 10(-4) and r = -0.20, p < 0.01, respectively). Multivariate analysis revealed significant associations between the APOA polymorphisms studied and plasma Lp(a) levels in subjects expressing only one APOA K-IV allele (p < 10(-6) for K-IV and p < 0.001 for TTTTA). In subjects expressing both APOA K-IV alleles, the multivariate analysis revealed that only the APOA K-IV alleles were inversely correlated with plasma Lp(a) levels (p < 0.001). Associations between both the LRP and VLDLR gene polymorphisms and plasma Lp(a) levels were only of borderline significance (p < 0.06 and p < 0.07, respectively) and were not confirmed in multivariate analysis. In conclusion, both APOA length polymorphisms significantly influenced plasma Lp(a) concentration in the Czech population studied, and this circumstance could explain the association in this population observed earlier between APOA (TTTTA)n polymorphism and CAD (Benes et al. 2000). Only a minor role in the regulation of plasma Lp(a) levels is suggested for the C766T LRP and the (CGG)n VLDLR polymorphisms.