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.

Effects of increased accumulation of doxorubicin due to emodin on efflux transporter and LRP1 expression in lung adenocarcinoma and colorectal carcinoma cells

Treatment with doxorubicin (dox) and emodin, separately and together, under normoxic and hypoxia-like conditions induced by CoCl₂, led to greater intracellular compound accumulation over 10 h post-addition in the presence of CoCl₂ in lung adenocarcinoma (A549) and colorectal carcinoma (HCT-15) cell lines. Confocal microscopy revealed that emodin, by itself, showed high cytosolic distribution in both cell lines, at 40 min post-addition but had entered the nuclei by 2 h, while dox entered the nuclei by 40 min. Both compounds modulated the expression of the efflux transporters (PgP, ABCG2, or MRP1-4) and the endocytic receptor, low-density lipoprotein receptor-related protein 1 (LRP1), to different extents under the study conditions. Efflux transporter upregulation was linked to lower intracellular compound levels due to greater efflux. Increased dox accumulation was accompanied by unaltered expression or upregulation of LRP1 in A549 cells. In both cell lines, increased accumulation of dox and emodin was observed whenever LRP1 and the efflux transporters known to transport dox and emodin were all up- or downregulated concomitantly. Increased growth inhibition was linked to co-treatment with dox and emodin and with increased ligand accumulation. The results presented in this study raise the hypothesis that higher production of LRP1 protein may be associated with higher endocytosis of upregulated transporter proteins at the cell surface, and hence, increased dox and emodin accumulation and growth inhibition. If so, elevation of LRP1 expression may be a useful target for interventions to promote the efficacy of these and other anticancer drugs.

Microbiome-host systems interactions: protective effects of propionate upon the blood-brain barrier

BACKGROUND

Gut microbiota composition and function are symbiotically linked with host health and altered in metabolic, inflammatory and neurodegenerative disorders. Three recognised mechanisms exist by which the microbiome influences the gut-brain axis: modification of autonomic/sensorimotor connections, immune activation, and neuroendocrine pathway regulation. We hypothesised interactions between circulating gut-derived microbial metabolites, and the blood-brain barrier (BBB) also contribute to the gut-brain axis. Propionate, produced from dietary substrates by colonic bacteria, stimulates intestinal gluconeogenesis and is associated with reduced stress behaviours, but its potential endocrine role has not been addressed.

RESULTS

After demonstrating expression of the propionate receptor FFAR3 on human brain endothelium, we examined the impact of a physiologically relevant propionate concentration (1 μM) on BBB properties in vitro. Propionate inhibited pathways associated with non-specific microbial infections via a CD14-dependent mechanism, suppressed expression of LRP-1 and protected the BBB from oxidative stress via NRF2 (NFE2L2) signalling.

CONCLUSIONS

Together, these results suggest gut-derived microbial metabolites interact with the BBB, representing a fourth facet of the gut-brain axis that warrants further attention.

Decreased Expression of the Low-density Lipoprotein Receptor-related Protein-1 (LRP-1) in Rats with Prostate Cancer

The aim of this work was to evaluate by immunohistochemistry (IHC) the expression of both LRP-1 and urokinase-type plasminogen activator receptor (uPAR) at different developmental stages of rat prostate disease by using a prostate cancer model previously developed in our laboratory. We found that LRP-1 was weakly expressed in normal prostates and in rats with hyperplastic glands. The expression of this receptor increased and correlated with the degree of premalignant lesions (PIN I, II, and III). The IHC for uPAR in normal prostates and in premalignant lesions showed a score of immunostaining that correlated with the expression of LRP-1. On the other hand, in prostates with adenocarcinomas and undifferentiated carcinomas, LRP-1 was undetectable or weakly detected, whereas uPAR showed a significantly higher level of expression. Based on the IHC results in rat prostates with premalignant and malignant lesions and considering that LRP-1, by mediating the internalization of uPAR, is involved in the regulation of extracellular matrix remodeling and cell migration, we conclude that a decreased expression of LRP-1 could be involved with the increasing activation of plasminogen activators shown in cancers.

Regulation of T-lymphocyte motility, adhesion and de-adhesion by a cell surface mechanism directed by low density lipoprotein receptor-related protein 1 and endogenous thrombospondin-1

T lymphocytes are highly motile and constantly reposition themselves between a free-floating vascular state, transient adhesion and migration in tissues. The regulation behind this unique dynamic behaviour remains unclear. Here we show that T cells have a cell surface mechanism for integrated regulation of motility and adhesion and that integrin ligands and CXCL12/SDF-1 influence motility and adhesion through this mechanism. Targeting cell surface-expressed low-density lipoprotein receptor-related protein 1 (LRP1) with an antibody, or blocking transport of LRP1 to the cell surface, perturbed the cell surface distribution of endogenous thrombospondin-1 (TSP-1) while inhibiting motility and potentiating cytoplasmic spreading on intercellular adhesion molecule 1 (ICAM-1) and fibronectin. Integrin ligands and CXCL12 stimulated motility and enhanced cell surface expression of LRP1, intact TSP-1 and a 130,000 MW TSP-1 fragment while preventing formation of a de-adhesion-coupled 110 000 MW TSP-1 fragment. The appearance of the 130 000 MW TSP-1 fragment was inhibited by the antibody that targeted LRP1 expression, inhibited motility and enhanced spreading. The TSP-1 binding site in the LRP1-associated protein, calreticulin, stimulated adhesion to ICAM-1 through intact TSP-1 and CD47. Shear flow enhanced cell surface expression of intact TSP-1. Hence, chemokines and integrin ligands up-regulate a dominant motogenic pathway through LRP1 and TSP-1 cleavage and activate an associated adhesion pathway through the LRP1-calreticulin complex, intact TSP-1 and CD47. This regulation of T-cell motility and adhesion makes pro-adhesive stimuli favour motile responses, which may explain why T cells prioritize movement before permanent adhesion.

Lipopolysaccharide downregulates CD91/low-density lipoprotein receptor-related protein 1 expression through SREBP-1 overexpression in human macrophages

Sterol regulatory element-binding proteins (SREBPs) negatively modulate the expression of the CD91/low-density lipoprotein receptor-related protein (LRP1), a carrier and signaling receptor that mediates the endocytosis of more than 40 structurally and functionally distinct ligands. The aim of this work was to analyze whether lipopolysaccharide (LPS) can regulate LRP1 expression through SREBPs in human monocyte-derived macrophages (HMDM). LPS led to LRP1 mRNA and protein inhibition in a dose- and time-dependent manner. Concomitantly, a strong upregulation of SREBP-1 mRNA and SREBP-1 nuclear protein levels was observed in LPS-treated HMDM. The specific silencing of SREBP-1 efficiently prevented LRP1 reduction caused by LPS. SREBP-1 mRNA and nuclear protein levels remained high in HMDM treated with LPS unexposed or exposed to LDL. Native (nLDL) or aggregated LDL (agLDL) per se downregulated SREBP-2 expression levels and increased LRP1 expression. However, lipoproteins did not significantly alter the effect of LPS on SREBP-1 and LRP1 expression. Collectively, these data support that lipoproteins and LPS exert their modulatory effect on LRP1 expression through different SREBP isoforms, SREBP-2 and SREBP-1, respectively. These results highlight a crucial role of SREBP-1 as a mediator of the downregulatory effects of LPS on LRP1 expression in human macrophages, independently of the absence or presence of modified lipoproteins.

The serine protease inhibitor neuroserpin regulates the growth and maturation of hippocampal neurons through a non-inhibitory mechanism

Neuroserpin is a brain-specific serine protease inhibitor that is expressed in the developing and adult nervous system. Its expression profile led to suggestions that it played roles in neuronal growth and connectivity. In this study, we provide direct evidence to support a role for neuroserpin in axon and dendritic growth. We report that axon growth is enhanced while axon and dendrite diameter are reduced following neuroserpin treatment of hippocampal neurons. More complex effects are seen on dendritic growth and branching with neuroserpin-stimulating dendritic growth and branching in young neurons but switching to an inhibitory response in older neurons. The protease inhibitory activity of neuroserpin is not required to activate changes in neuronal morphology and a proportion of responses are modulated by an antagonist to the LRP1 receptor. Collectively, these findings support a key role for neuroserpin as a regulator of neuronal development through a non-inhibitory mechanism and suggest a basis for neuroserpin's effects on complex emotional behaviours and recent link to schizophrenia.

Up-regulation of hepatic low-density lipoprotein receptor-related protein 1: a possible novel mechanism of antiatherogenic activity of hydroxymethylglutaryl-coenzyme A reductase inhibitor Atorvastatin and hepatic LRP1 expression

Low-density lipoprotein receptor-related protein 1 (LRP1) binds to apolipoprotein E and serves as a receptor for remnant lipoproteins in the liver, thus playing an important role in clearing these atherogenic particles. In this study, we investigated the effect of atorvastatin, a hydroxymethylglutaryl-coenzyme A reductase inhibitor, on hepatic LRP1 expression. We used HepG2 and Hep3B cells for in vitro study, and Otsuka Long-Evans Tokushima fatty and Sprague-Dawley rats for in vivo study. We used relatively high pharmacologic dose of atorvastatin in this study (in vitro, 0.5 μmol/L in culture media, for 48 hours; in vivo, 20 mg/[kg d], for 6 weeks). Atorvastatin increased LRP1 and low-density lipoprotein (LDL) receptor expression in HepG2 and Hep3B cells and induced hepatic LRP1 and LDL receptor expression in chow diet-fed Sprague-Dawley rats and high-fat diet-fed Otsuka Long-Evans Tokushima fatty rats. Atorvastatin decreased intracellular sterol level and increased the amount of the nuclear form of sterol response element-binding protein-2 (SREBP-2) in both HepG2 and Hep3B cells as well as in two animal models. Treatment of HepG2 cells with LDL increased intracellular sterol level and reduced LRP1, LDL receptor, and SREBP-2. When SREBP-2 in HepG2 cells was knocked down by small interfering RNA, the induction of LRP1 expression by atorvastatin did not take place. In conclusion, up-regulation of hepatic LRP1 might be a novel mechanism by which statin treatment decreases remnant lipoproteins. In addition, SREBP-2 acts as a mediator of atorvastatin-induced up-regulation of hepatic LRP1. Future studies using standard doses of atorvastatin in humans are needed to elucidate clinical relevance of these findings.

A unique function for LRP-1: a component of a two-receptor system mediating specific endocytosis of plasma-derived factor V by megakaryocytes

BACKGROUND

Factor V is endocytosed by megakaryocytes from plasma via a specific, receptor-mediated, clathrin-dependent mechanism to form the unique platelet-derived FV pool.

OBJECTIVE

The role of low-density lipoprotein (LDL) receptor-related protein-1 (LRP-1), or a related family member, in FV endocytosis by megakaryocytes was examined because of its known interactions with other proteins involved in hemostasis.

METHODS

LRP-1 expression by megakaryocytes and its functional role in FV endocytosis was confirmed using reverse transcription polymerase chain reaction (RT-PCR) and specific antibodies. FV binding to megakaryocytes was performed under Ca(2+)-free conditions to quantify binding in the absence of endocytosis.

RESULTS AND CONCLUSION

Cell surface expression of LRP-1 by CD34+ ex vivo-derived megakaryocytes and the megakaryocyte-like cell line CMK was confirmed using anti-LRP-1 antibodies and was consistent with the detection of LRP-1 message in these cells. All cells capable of endocytosing FV expressed LRP-1. Anti-LRP-1 antibodies and receptor-associated protein (RAP), a known antagonist of LDL receptor family members, displaced only 50% of the [(125)I]FV bound to megakaryocytes. FV binding to megakaryocytes showed positive cooperativity (Hill coefficient = 1.92 +/- 0.18) that was substantially reduced in the presence of RAP (1.47 +/- 0.26). As FV endocytosis is specific to this cofactor, a model is hypothesized where FV binding to a specific receptor facilitates binding and endocytosis of a second FV molecule by LRP-1, or a related family member. These combined observations describe a unique role for LRP-1 in endocytosis of a coagulation protein trafficked to alpha-granules and not destined for lysosomal degradation.

Uptake of triacylglycerol-rich lipoproteins of differing triacylglycerol molecular species and unsaponifiable content by liver cells

The fatty acid composition of dietary oils can modulate the incorporation of triacylglycerol-rich lipoproteins (TRL) into hepatocytes, thus affecting the atherogenicity of these particles. However, nothing is known about the effect of the unsaponifiable fraction of the oils. In the present study, we evaluated the influence of these components on the uptake of TRL by rat primary hepatocytes. TRL were isolated from human serum after the intake of meals enriched in high-oleic sunflower oil (HOSO), virgin olive oil (VOO) or VOO enriched in its own unsaponifiable fraction (EVO). HOSO and HOSO-TRL differed from VOO and EVO and their corresponding TRL in the composition of triacylglycerol molecular species and of the unsaponifiable fraction. Furthermore, the increase in the unsaponifiable fraction of VOO led to changes in the triacylglycerol molecular species in the EVO-TRL. On incubation with hepatocytes, HOSO-TRL were taken up at a faster rate than VOO-TRL or EVO-TRL. In addition, in comparison to VOO-TRL, HOSO-TRL increased the expression of mRNA for the LDL receptor-related protein receptor, which plays an important role in the internalisation of remnant lipoproteins. EVO-TRL also increased LDL receptor-related protein mRNA expression in comparison with VOO-TRL, but this change was not accompanied by a rise in the uptake rate, suggesting that the unsaponifiable fraction of VOO may inhibit LDL receptor-related protein expression or activity post-transcriptionally. In conclusion, TRL from dietary oils with differing triacylglycerol molecular species and unsaponifiable fraction content are taken up by liver cells at different rates, and this may be important in the atherogenicity of these particles.

The expression of low density lipoprotein receptor-related protein in colorectal carcinoma

Low density lipoprotein receptor-related protein (LRP) is a multifunctional cell surface receptor binding many different ligands including proteinases and their inhibitors, some of which are known to be involved in tumor biology. We studied the expression of LRP and its putative role in colorectal carcinoma. Tissue samples were obtained from 50 patients with colorectal carcinoma and fixed in formalin and embedded in paraffin. Immunohistochemical staining was performed using antibodies directed against LRP, cathepsin B and urokinase-type plasminogen activator (u-PA). The expression of LRP was further studied by polymerase chain reaction. The TNM stage was determined according to UICC guide lines and was based upon histological analysis. LRP was primarily expressed in stroma cells [36 patients (72%)] and less frequently in tumor cells [6 patients (12%)]. In 22% of all cases LRP was prominent at the invasion front. Cathepsin B was found both in the tumor stroma [50 (100%)] and in the tumor cells [46 (92%)]. u-PA was present in the tumor stroma [44 (88%)] and in the tumor cells [44 (88%)]. In stromal cells the expression of LRP correlated significantly with the expression of u-PA (p=0.043). Furthermore, the expression of LRP and of u-PA in tumor cells correlated with the tumor stage according to UICC (p=0.038 and 0.018, respectively). We provide evidence that LRP is expressed in colorectal cancer. As LRP forms complexes with u-PA and its inhibitor, we suspect that LRP can influence the known effects of u-PA on tumor biology.

[Expression of connective tissue growth factor and low density lipoprotein receptor related protein induced by transforming growth factor beta 1 in human pulmonary fibroblasts-1]

OBJECTIVE

To investigate the tendency and correlation between connective tissue growth factor (CTGF) and low density lipoprotein receptor-related protein (LRP) in human pulmonary fibroblasts-1 (HPF-1) induced by transforming growth factor beta1 (TGF-beta1) at different times.

METHODS

After the HPF-1 cells were stimulated with TGF-beta1 (5 microg/L) at different times (0 h, 3 h, 6 h,12 h and 24 h),CTGF and LRP mRNA expressions were analyzed by RT-PCR. The same preparation steps of cell culture were repeated, then, the protein expressions were determined by Western Blot, co-immunoprecipitation Western Blot and immunocytochemistry.

RESULTS

The expressions of CTGF and LRP mRNA had similar tendency, and LRP (224.87 +/- 7.00) correlated with CTGF (131.53 +/- 2.86) positively (r = 0.8402, P < 0.05). Furthermore, the protein expression levels had significant difference between 0 h (190.85 +/- 2.86) and 3 h (222.45 +/- 3.66) groups (F = 18.06, P < 0.01),and by immunoprecipitation, it was discovered that the quantity of CTGF bounded to LRP changes, which was similar to the result of LRP western blot. The analysis of immunocytochemistry disclosed the like results as well (3 h: 88.66 +/- 15.72, 0 h: 27.56 +/- 6.72, F = 244.36, P < 0.01).

CONCLUSION

The expression of LRP coincides with that of CTGF at the same stimulated time point of TGF-beta1. It suggests that LRP may be involved in the biological process of CTGF. And further research about LRP may provide a new direction to pulmonary fibrosis therapy.

[High glucose regulates the expression of connective tissue growth factor and its receptor (low density lipoprotein receptor-related protein) in cultured podocytes]

OBJECTIVE

To observe the expression of connective tissue growth factor (CTGF) and its receptor-low density lipoprotein receptor-related protein (LRP), and the relevant signaling pathway for the regulation by long-term high glucose exposure in cultured podocytes.

METHODS

The effects of high glucose on the expression of CTGF and its receptor LRP were analyzed by western blotting. The activation of mitogen activated protein kinase (MAPKS) signaling pathway by high glucose was also examined.

RESULTS

Basal levels of CTGF were observed in cultured mouse podocytes, the levels of CTGF protein were increased by high glucose medium groups on the 2nd day, reached the peak on the 4th day (P< 0.05), began to decline on the 6th day, returned to the basal level on the 8th day (P>0.05). The levels of CTGF expression in normal glucose and mannitol glucose groups did not change markly. High glucose medium induced phosphorylation of ERK1/2 at as early as minute 30, reached the peak at hour 6; maintained the activity at hours 12 and 24, and declined to the basal level at hour 48. However, phosphorylation of ERK1/2 was not detected in normal glucose and mannitol glucose groups. Blockade of phosphorylation of ERK1/2 with PD98059, a specific ERK1/2 activation inhibitor, did decrease the high glucose-triggered expression of CTGF protein in 4 days. High glucose had no effect on the expression of LRP protein at each time point.

CONCLUSION

Acute high glucose (2-4 days) stimulated the expression of CTGF protein via ERK1/2-dependent signaling pathway in cultured podocytes, while cultured in high glucose for 6-8 days, the podocytes did not increase its CTGF level. Long-term high glucose had no effect on the expression of LRP in podocytes.

Loss of apolipoprotein E receptor LR11 in Alzheimer disease

BACKGROUND

Genetic, epidemiologic, and biochemical evidence suggests that apolipoprotein E, low-density lipoprotein receptors, and lipid metabolism play important roles in sporadic Alzheimer disease (AD).

OBJECTIVE

To identify novel candidate genes associated with sporadic AD.

DESIGN

We performed an unbiased microarray screen for genes differentially expressed in lymphoblasts of patients with sporadic AD and prioritized 1 gene product for further characterization in AD brain.

SETTING

Emory University, Atlanta, Ga.

SUBJECTS

Cell lines were used from 14 patients with AD and 9 normal human control subjects.

RESULTS

Six genes were differentially expressed in lymphoblasts of 2 independent groups of patients with probable AD and autopsy-proven AD. We hypothesized that 1 of the genes, termed low-density lipoprotein receptor relative with 11 binding repeats (LR11) (reduced 1.8- and 2.5-fold in AD lymphoblasts vs controls), might be associated with sporadic AD on the basis of its function as neuronal apolipoprotein E receptor. We found dramatic and consistent loss of immunocytochemical staining for LR11 in histologically normal-appearing neurons in AD brains. This reduction of LR11 protein was confirmed by quantitative Western blotting (P =.01).

CONCLUSIONS

There is loss of the microarray-derived candidate, LR11, in neurons of AD brains. This study shows that microarray analysis of widely available lymphoblasts derived from patients with AD holds promise as a primary screen for candidate genes associated with AD.

Serum deprivation increases the expression of low density lipoprotein receptor-related protein in primary cultured rat astrocytes

The low density lipoprotein receptor (LDLR)-related protein (LRP) is a multifunctional receptor which mediates the endocytic uptake of several ligands implicated in Alzheimer's disease pathophysiology. Although LRP, as a member of the LDLR family, is likely to be regulated in response to various cellular stresses, this regulation has not been fully understood yet. In the present study we studied the regulation of LRP expression in primary cultured rat astrocytes in response to serum deprivation as a general cellular stress. A significant increase in LRP expression was detected after serum deprivation and this increase was blocked by treatment of U0126, an inhibitor of MAP kinase. This serum deprivation action was partially reversed by either serum or D-glucose supplementation, but further augmented by glutamine. This result contrasted with a finding that glutamine suppressed gadd153 protein induced by serum deprivation. Taken together, the present data suggest that serum deprivation induces dramatically LRP expression in astrocytes partly by MAPK signaling pathways and by signaling pathways apparently distinct from gadd153 induction.

Evidence that glycoprotein 96 (B2), a stress protein, functions as a Th2-specific costimulatory molecule

After the engagement of Ag receptor, most of the Th cells for their optimal activation require a second (costimulatory) signal provided by the APCs. We demonstrate the isolation and characterization of a 99- to 105-kDa protein (B2), from LPS-activated B cell surface, and its function as a Th2-specific costimulatory molecule. Appearance of B2 as a single entity on two-dimensional gel electrophoresis and as a distinct peak in reverse-phase HPLC ascertains the fact that B2 is homogeneous in preparation. Electron microscopy as well as competitive binding studies reveal that (125)I-labeled B2 specifically binds anti-CD3-activated T cell surface and also competes with its unlabeled form. Internal amino acid sequences of B2 are found to be identical with stress protein gp96. The identity of B2 as gp96 is also revealed by immunological characterization and by confocal microscopic colocalization studies of B2 and gp96 on LPS-activated B cells. Confocal imaging studies also demonstrate that gp96 can be induced on B cell surface without association of MHC molecules. Furthermore, the novel role of gp96 in Th cell proliferation skewing its differentiation toward Th2 phenotype has also been established. Ab-mediated blocking of gp96-induced signaling not only abrogates in vitro proliferation of CD4(+) T cells, but also diminishes the secretion of Th2-specific cytokines. Notably, the expression of CD91 (receptor of gp96/B2) is up-regulated on anti-CD3-activated Th cells and also found to be present on Th1 and Th2 subsets.

The LDL receptor-related protein (LRP1/A2MR) and coronary atherosclerosis--novel genomic variants and functional consequences

The LDL receptor-related protein/alpha 2-macroglobulin receptor (LRP1/A2MR) is a multifunctional cell-surface glycoprotein that endocytoses several structurally and functionally distinct ligands. In clinical studies different genomic variants of the LRP1/A2MR and its role in the development of degenerative diseases like atherosclerosis or Alzheimer's disease were studied. We screened for novel genomic variants of LRP1/A2MR and investigated the importance of these variants in 214 coronary patients suffering from myocardial infarction as well as in 224 healthy controls. We detected a novel C>G polymorphism at position -25 in the functionally important promoter region of LRP1/A2MR. This polymorphism (c.1-25C>G) leads to the creation of a new GC-box, recognized by the constitutively expressed SP 1 transcription factor. Investigating the LRP1/A2MR gene expression with respect to this polymorphism, carriers of the mutant G-allele were found to have a higher mRNA expression level. A novel polymorphism in exon 22 (c.4012C>T), and two novel polymorphisms in intron 24 (IVS24+123C>A and IVS24+690G>A) associated with a previously described polymorphism in exon 61 (c.10249G>A), were related to the development of myocardial infarction. Two novel rare genetic variants of exon 88 (c.13933C>T) and intron 88 (IVS88+15G>A) were identified in four patients with severe coronary symptoms. However, the LRP1/A2MR gene expression was found to be independent of all identified novel genomic variants as well as other previously described changes (A217V, A775P, D2080N, D2632E, G4379S) except the promoter polymorphism.

Inducible expression of the alpha2-macroglobulin signaling receptor in response to antigenic stimulation: a study of second messenger generation

Thioglycollate (TG)-elicited murine, peritoneal macrophages express two receptors for activated forms of the proteinase inhibitor alpha2-macroglobulin (alpha2M*)--namely, the low density lipoprotein receptor-related protein (LRP) and the alpha2M signaling receptor (alpha2MSR). We now report that resident peritoneal macrophages express only 400+/-50 alpha2MSR receptors/cell compared to 5000+/-500 receptor/TG-elicited macrophage. By contrast, LRP expression is only 2-2.5-fold greater on elicited cells. The low level of alpha2MSR expression by resident cells is insufficient to trigger signal transduction in contrast to TG-elicited cells which when exposed to alpha2M* demonstrate a rapid rise in inositol 1,4,5-trisphosphate and a concomitant increase in cytosolic free Ca2+. We then studied a variety of preparations injected subcutaneously for their ability to upregulate alpha2MSR. Macroaggregated bovine serum albumin (macroBSA) injection upregulated alpha2MSR and triggered signaling responses by splenic macrophages. Nonaggregated BSA injection alone or in the presence of alum, by contrast, did not alter alpha2MSR expression. Recombivax (hepatitis B antigen adsorbed to alum) injection also upregulated alpha2MSR on splenic macrophages while the alum carrier had no effect. We conclude that macrophage alpha2M* receptors are inducible and their expression may be regulated, in part, by potential antigens.

IL-1beta down-regulates tissue-type plasminogen activator by up-regulating low-density lipoprotein receptor-related protein in AML 12 cells

Interleukin-1 (IL-1) regulation of tPA in hepatocytes was studied in mouse hepatocyte line AML12. IL-1 induced transient accumulation of tPA mRNA as high as threefold by 2 h after the start of treatment. The cytokine also induced the mRNA for serum amyloid A, a typical acute-phase protein in mice, with more sustained kinetics in a time-dependent manner. In contrast to the induction of mRNA, tPA activity and protein levels in the harvested medium were dramatically diminished by IL-1. IL-1 stimulated the uptake of (125)I-tPA by AML 12. This uptake was inhibited by 39-kDa receptor-associated protein (RAP), but not by the sugar mannan. These results revealed that low-density lipoprotein receptor-related protein (LRP), which is known to be a receptor for tPA and to be blocked by RAP, was up-regulated by IL-1. We also demonstrated, for the first time, that IL-1 transiently increased the mRNA level of LRP threefold by 30 min after the start of IL-1 treatment of AML 12. The receptor-mediated endocytosis of tPA by hepatocytes may thus play a crucial role in the down-regulation of fibrinolysis during the acute-phase response.