Modulation of the alpha 2 macroglobulin receptor/low density lipoprotein receptor related protein by interferon-gamma in human astroglial cells

S100B protects LAN-5 neuroblastoma cells against Abeta amyloid-induced neurotoxicity via RAGE engagement at low doses but increases Abeta amyloid neurotoxicity at high doses

At the concentrations normally found in the brain extracellular space the glial-derived protein, S100B, protects neurons against neurotoxic agents by interacting with the receptor for advanced glycation end products (RAGE). It is known that at relatively high concentrations S100B is neurotoxic causing neuronal death via excessive stimulation of RAGE. S100B is detected within senile plaques in Alzheimer's disease, where its role is unknown. The present study was undertaken to evaluate a putative neuroprotective role of S100B against Abeta amyloid-induced neurotoxicity. We treated LAN-5 neuroblastoma cultures with toxic amounts of Abeta25-35 amyloid peptide. Our results show that at nanomolar concentrations S100B protects cells against Abeta-mediated cytotoxicity, as assessed by 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and terminal deoxynucleotidyl transferase-mediated dUTP-fluorescein isothiocyanate nick end-labeling (TUNEL) experiments, by countering the Abeta-mediated decrease in the expression of the anti-apoptotic factor Bcl-2. This effect depends on S100B binding to RAGE because S100B is unable to contrast Abeta-mediated neurotoxicity in neurons overexpressing a signaling-deficient RAGE mutant lacking the cytosolic and transducing domain. Our data suggest that at nanomolar doses S100B counteracts Abeta peptide neurotoxicity in a RAGE-mediated manner. However, at micromolar doses S100B is toxic to LAN-5 cells and its toxicity adds to that of the Abeta peptide, suggesting that additional molecular mechanisms may be involved in the neurotoxic process.

Expression of placental low-density lipoprotein receptor-related protein and scavenger receptors AI/AII transcripts in the baboon

OBJECTIVE

This study was undertaken to assess potential secondary lipoprotein-dependent mechanisms, scavenger receptor AI and AII (SR-AI and SR-AII), and low-density lipoprotein (LDL) receptor-related protein (LRP) in the baboon (Papio sp) placenta.

STUDY DESIGN

Baboon placental villous tissue and enriched fractions of syncytiotrophoblast cells were collected at early (approximately day 60, n = 3), mid (approximately day 100, n = 4), and late (approximately day 160, n = 4) pregnancy (term = approximately 184 days). The abundance of messenger RNA (mRNA) transcripts for SR-AI, SR-AII, and LRP were determined relative to constitutively expressed, glyceraldehyde-3-phosphate-dehydrogenase (GAPDH).

RESULTS

No quantitative differences (P >.05) in SR-AI, SR-AII, or LRP mRNAs were noted in respect to stage of gestation. However, SR-AII/GAPDH mRNA ratios in syncytiotrophoblast cells (0.38 +/- 0.09, n = 11) were significantly (P <.05) higher than in villous tissue (0.15 +/- 0.04, n = 11), whereas LRP/GAPDH mRNA ratios were higher in villous tissue (0.43 +/- 0.10, n = 11) than in syncytiotrophoblast cells (0.18 +/- 0.02, n = 11).

CONCLUSION

SR-AII mRNA in baboon placenta appears to be expressed, in greatest abundance, in the endocrinologically active syncytiotrophoblast.

Adipocyte low density lipoprotein receptor-related protein gene expression and function is regulated by peroxisome proliferator-activated receptor gamma

The alpha(2)-macroglobulin receptor/low density lipoprotein receptor-related protein (LRP) is a large multifunctional receptor that interacts with a variety of molecules. It is implicated in biologically important processes such as lipoprotein metabolism, neurological function, tissue remodeling, protease complex clearance, and cell signal transduction. However, the regulation of LRP gene expression remains largely unknown. In this study, we have analyzed 2 kb of the 5'-flanking region of the LRP gene and identified a predicted peroxisome proliferator response element (PPRE) from -1185 to -1173. Peroxisome proliferator-activated receptor gamma (PPARgamma) ligands such as fatty acids and rosiglitazone increased functional cell surface LRP by 1.5-2.0-fold in primary human adipocytes and in the SW872 human liposarcoma cell line as assessed by activated alpha(2)-macroglobulin binding and degradation. These agents were found to increase LRP transcription. Gel shift analysis of the putative PPRE demonstrated direct binding of PPARgamma/retinoid X receptor alpha heterodimers to the PPRE in the LRP gene. Furthermore, these heterodimers could no longer interact with a mutated PPRE probe. The isolated promoter was functional in SW872 cells, and its activity was increased by 1.5-fold with the addition of rosiglitazone. Furthermore, the isolated response element was similarly responsive to rosiglitazone when placed upstream of an ideal promoter. Mutagenesis of the predicted PPRE abolished the ability of this construct to respond to rosiglitazone. These data demonstrate that fatty acids and rosiglitazone directly stimulate transcription of the LRP gene through activation of PPARgamma and increase functional LRP expression.

LRP and senile plaques in Alzheimer's disease: colocalization with apolipoprotein E and with activated astrocytes

The low density lipoprotein receptor-related protein (LRP) is a multifunctional receptor which is present on senile plaques in Alzheimer's disease (AD). It is suggested to play an important role in the balance between amyloid beta (Abeta) synthesis and clearance mechanisms. One of its ligands, apolipoprotein E (apoE), is also present on senile plaques and has been implicated as a risk factor for AD, potentially affecting the deposition, fibrillogenesis and clearance of Abeta. Using immunohistochemistry we show that LRP was present only on cored, apoE-containing senile plaques, in both PDAPP transgenic mice and human AD brains. We detected strong LRP staining in neurons and in reactive astrocytes, and immunostaining of membrane-bound LRP showed colocalization with fine astrocytic processes surrounding senile plaques. LRP was not present in plaques in young transgenic mice or in plaques of APOE-knockout mice. As LRP ligands associated with Abeta deposits in AD brain may play an important role in inducing levels of LRP in both neurons and astrocytes, our findings support the idea that apoE might be involved in upregulation of LRP (present in fine astrocytic processes) and act as a local scaffolding protein for LRP and Abeta. The upregulation of LRP would allow increased clearance of LRP ligands as well as clearance of Abeta/ApoE complexes.

Role of alpha2-macroglobulin in regulating amyloid beta-protein neurotoxicity: protective or detrimental factor?

alpha2-Macroglobulin (alpha2M) has been identified as a carrier protein for beta-amyloid (Abeta) decreasing fibril formation and affecting the neurotoxicity of this peptide. The alpha2-macroglobulin receptor/low density lipoprotein receptor related protein (LRP) is involved in the internalization and degradation of the alpha2M/Abeta complexes and its impairment has been reported to occur in Alzheimer's disease. Previous studies have shown alpha2M to determine an enhancement or a reduction of Abeta toxicity in different culture systems. In order to clarify the role of alpha2M in Abeta neurotoxicity, we challenged human neuroblastoma cell lines with activated alpha2M in combination with Abeta. Our results show that in neuroblastoma cells expressing high levels of LRP, the administration of activated alpha2M protects the cells from Abeta neurotoxicity. Conversely, when this receptor is not present alpha2M determines an increase in Abeta toxicity as evaluated by MTT and TUNEL assays. In LRP-negative cells transfected with the full-length human LRP, the addition of activated alpha2M resulted to be protective against Abeta-induced neurotoxicity. By means of recombinant proteins we ascribed the neurotoxic activity of alpha2M to its FP3 fragment which has been previously shown to bind and neutralize transforming growth factor-beta. These studies provide evidence for both a neuroprotective and neurotoxic role of alpha2M regulated by the expression of its receptor LRP.

Alpha 2-macroglobulin-mediated degradation of amyloid beta 1--42: a mechanism to enhance amyloid beta catabolism

Peptides derived from proteolytic degradation of the amyloid precursor protein, e.g., amyloid beta (A beta), are considered to be central to the pathology of Alzheimer's disease (AD). Soluble A beta is present in measurable concentrations in cerebrospinal fluid and blood. There are indications that soluble A beta present in circulation can cross the blood-brain barrier via transcytosis mediated by brain capillary endothelial cells. It implies that A beta originating from circulation may contribute to vascular and parenchymal A beta deposition in AD. Enhancing of A beta catabolism mediated by proteolytic degradation or receptor-mediated endocytosis could be a key mechanism to maintain low concentrations of soluble A beta. To launch A beta clearance we have exploited the A beta-degrading activity of diverse alpha 2-macroglobulin (alpha 2-M)-proteinase complexes. Complexes with trypsin, alpha-chymotrypsin, and bromelain strongly degrade (125)I-A beta 1--42 whereas complexes with endogenous proteinases, e.g., plasmin and prostate-specific antigen, were not effective. A beta degradation by the complexes was not inhibited by alpha 1-antichymotrypsin and soybean trypsin inhibitor which normally would inactivate the free serine proteinases. A prerequisite for A beta degradation is its binding to specific binding sites in alpha 2-M that may direct A beta to the active site of the caged proteinase. Ex vivo, enhanced degradation of (125)I-A beta 1--42 in blood could be achieved upon oral administration of high doses of proteinases to volunteers. These results suggest that up-regulation of A beta catabolism could probably reduce the risk of developing AD by preventing A beta accumulation in brain and vasculature.

Expression of alpha(2)-macroglobulin receptor/low density lipoprotein receptor-related protein (LRP) in rat microglial cells

Low density lipoprotein receptor-related protein (LRP) participates in the uptake and degradation of several ligands implicated in neuronal pathophysiology including apolipoprotein E (apoE), activated alpha(2) -macroglobulin (alpha(2)M*) and beta-amyloid precursor protein (APP). The receptor is expressed in a variety of tissues. In the brain LRP is present in pyramidal-type neurons in cortical and hippocampal regions and in astrocytes that are activated as a result of injury or neoplasmic transformation. As LRP is expressed in the monocyte/macrophage cell system, we were interested in examining whether LRP is expressed in microglia. We isolated glial cells from the brain of neonatal rats and LRP was immunodetected both in microglial cells and in astrocytes expressing glial fibrillar acidic protein (GFAP). Microglial cells were able to bind and internalize LRP-specific ligand, alpha(2)M*. The internalization was inhibitable by RAP, with a Kd of 1.7 nM. The expression of LRP was up-regulated by dexamethasone, and down-regulated by lipopolysaccharide (LPS), gamma interferon (IFN-gamma) or a combination of both. LRP was less sensitive to dexamethasone in activated astrocytes than in microglia. We provided the first analysis of LRP expression and regulation in microglia. Our results open the possibility that microglial cells could be related to the participation of LRP and its ligands in different pathophysiological states in brain.

Activated alpha2macroglobulin increases beta-amyloid (25-35)-induced toxicity in LAN5 human neuroblastoma cells

The presence of the alpha2macroglobulin receptor/low density lipoprotein receptor-related protein (alpha2Mr/LRP) and its ligands alpha2macroglobulin (alpha2M), apoliprotein E, and plasminogen activators was detected in senile plaques of Alzheimer's disease (AD). To explore a possible role of alpha2M in neurodegenerative processes occurring in AD, we analyzed the effect of alpha2M on Abeta 25-35-induced neurotoxicity. Treatment of LAN5 human neuroblastoma cells with 10 microM beta-amyloid peptide fragment 25-35 (Abeta 25-35) for 72 h resulted in a 50% decrease in cell viability as determined by MTT incorporation and cell counts. The addition of alpha2M to the culture medium of these cells did not determine any effect, but when the activated form alpha2M* was used a dose-dependent decrease in cell viability was observed, the maximum effect being reached at 140 and 280 nM. Moreover, treatment of LAN5 cells with alpha2M* in combination with Abeta 25-35 increased the neurotoxicity of the amyloid peptide by 25%. This neurotoxic effect of alpha2M* seems to be related to its capability to bind and inactivate TGFbeta in the culture medium, since it was mimicked by a TGFbeta neutralizing antibody. A possible involvement of receptor-mediated endocytosis was ruled out, since alpha2M receptor is not present on LAN5, as revealed by RT-PCR and Western blotting experiments. The presence of alpha2M* in amyloid deposits of Alzheimer's disease has been recently reported and a possible impairment of LRP internalization processes has been hypothesized. Our data suggest that the local accumulation of alpha2M* in AD plaques may increase Abeta 25-35-induced neurotoxicity by neutralizing TGFbeta-mediated neuroprotective mechanisms.

Importance of immunological and inflammatory processes in the pathogenesis and therapy of Alzheimer's disease

The contribution of autoimmune processes or inflammatory components in the etiology and pathogenesis of Alzheimer's disease (AD) has been suspected for many years. The presence of antigen-presenting, HLA-DR-positive and other immunoregulatory cells, components of complement, inflammatory cytokines and acute phase reactants have been established in tissue of AD neuropathology. Although these data do not confirm the immune response as a primary cause of AD, they indicate involvement of immune processes at least as a secondary or tertiary reaction to the preexisting pathogen and point out its driving-force role in AD pathogenesis. These processes may contribute to systemic immune response. Thus, experimental and clinical studies indicate impairments in both humoral and cellular immunity in an animal model of AD as well as in AD patients. On the other hand, anti-inflammatory drugs applied for the treatment of some chronic inflammatory diseases have been shown to reduce risk of AD in these patients. Therefore, it seems that anti-inflammatory drugs and other substances which can control the activity of immunocompetent cells and the level of endogenous immune response can be valuable in the treatment of AD patients.

The expression of the LDL receptor-related protein (LRP) correlates with the differentiation of human neuroblastoma cells

The low density lipoprotein receptor-related protein (LRP) has been localized in human brain at the level of neurons, astrocytes and along capillary membranes. It is a multifunctional receptor responsible for binding and internalization of lipoproteins enriched with apoliprotein E, lipoprotein lipase, protease-alpha 2 macroglobulin complexes and plasminogen activator-inhibitor complexes. LRP expression is observed in cells involved in Alzheimer's disease, neoplastic transformation and tissue repair. Moreover, its synthesis is modulated during brain development. In this study we used the SK-N-AS human neuroblastoma cell line as a model system to study LRP expression during cellular differentiation induced by phorbol esters, retinoic acid and interferon gamma. Since LRP plays a major role in the regulation of lipid metabolism, the decreased levels of LRP measured by immunofluorescence, western blot and PCR on differentiated neuroblastoma cells may be the consequence of the lower requirements of cholesterol and lipids of differentiated cells in relation to their reduced mitotic index.