The low-density lipoprotein receptor-related protein associates with calnexin, calreticulin, and protein disulfide isomerase in receptor-associated-protein-deficient fibroblasts

Regulation of LRP-1 expression: make the point

The low-density lipoprotein receptor-related protein-1 (LRP-1) is a membrane receptor displaying both scavenging and signaling functions. The wide variety of extracellular ligands and of cytoplasmic scaffolding and signaling proteins interacting with LRP-1 gives it a major role not only in physiological processes, such as embryogenesis and development, but also in critical pathological situations, including cancer and neurological disorders. In this review, we describe the molecular mechanisms involved at distinct levels in the regulation of LRP-1, from its expression to the proper location and stability at the cell surface.

Deficiency of receptor-associated protein attenuates angiotensin II-induced atherosclerosis in hypercholesterolemic mice without influencing abdominal aortic aneurysms

OBJECTIVE

Receptor-associated protein (RAP) was initially described as a regulator of low density lipoprotein receptor-related protein 1 (LRP1), but is now known to regulate many proteins. Since the direct effects of RAP on vascular pathologies have not been studied, this study determined whether RAP deficiency influenced angiotensin II (AngII)-induced atherosclerosis and abdominal aortic aneurysms (AAAs) in hypercholesterolemic mice.

METHODS AND RESULTS

Male LDL receptor -/- mice that were either RAP +/+ or -/- were infused with AngII (500 ng/kg/min) for 4 weeks while consuming a saturated fat-enriched diet. RAP deficiency had no effects on body weight or AngII-induced increases of systolic blood pressure. Despite increased plasma cholesterol concentrations, RAP deficiency reduced atherosclerotic lesion size in aortic arches, while having no effect on AngII-induced AAAs. RAP deficiency profoundly reduced LRP1 protein abundance in macrophages, but did not change its abundance in aortic smooth muscle cells. Also, RAP deficiency had no effects on mRNA abundance of LRP1 or lipoprotein lipase in macrophages. To determine whether RAP deficiency in leukocytes influenced AngII-induced atherosclerosis, irradiated male LDL receptor -/- mice were repopulated with bone marrow-derived cells from either RAP +/+ or -/- male mice. The chimeric mice were infused with AngII (500 ng/kg/min) for 4 weeks while fed the saturated fat-enriched diet. RAP deficiency in bone marrow-derived cells did not influence either plasma cholesterol concentrations or atherosclerotic lesion size.

CONCLUSIONS

Whole body RAP deficiency attenuated atherosclerosis without influencing AAAs in hypercholesterolemic mice infused with AngII. The anti-atherogenic effect was not attributable to RAP deficiency in bone marrow-derived cells.

Protein folding and quality control in the ER

The endoplasmic reticulum (ER) uses an elaborate surveillance system called the ER quality control (ERQC) system. The ERQC facilitates folding and modification of secretory and membrane proteins and eliminates terminally misfolded polypeptides through ER-associated degradation (ERAD) or autophagic degradation. This mechanism of ER protein surveillance is closely linked to redox and calcium homeostasis in the ER, whose balance is presumed to be regulated by a specific cellular compartment. The potential to modulate proteostasis and metabolism with chemical compounds or targeted siRNAs may offer an ideal option for the treatment of disease.

Functional analysis of recombinant calreticulin fragment 39-272: implications for immunobiological activities of calreticulin in health and disease

Although calreticulin (CRT) is a major Ca(2+)-binding luminal resident protein, it can also appear on the surface of various types of cells and it functions as an immunopotentiating molecule. However, molecular mechanisms underlying the potent immunobiological activity of cell surface CRT are still unclear. In the present study, a recombinant fragment (rCRT/39-272) covering the lectin-like N domain and partial P domain of murine CRT has been expressed in Escherichia coli. The affinity-purified rCRT/39-272 assembles into homodimers and oligomers in solution and exhibits high binding affinity to various glycans, including carrageenan, alginic acids, and hyaluronic acids. Functionally, rCRT/39-272 is capable of driving the activation and maturation of B cells and cytokine production by macrophages in a TLR-4-dependent manner in vitro. It specifically binds recombinant mouse CD14, but not BAFFR and CD40. It is also able to trigger Ig class switching by B cells in the absence of T cell help both in vitro and in vivo. Furthermore, this fragment of CRT exhibits strong adjuvanticity when conjugated to polysaccharides or expressed as part of a fusion protein. Soluble CRT can be detected in the sera of patients with rheumatoid arthritis or systemic lupus erythematosus, but not in healthy subjects. We argue that CRT, either on the membrane surface of cells or in soluble form, is a potent stimulatory molecule to B cells and macrophages via the TLR-4/CD14 pathway and plays important roles in the pathogenesis of autoimmune diseases.

Receptor-associated protein (RAP) has two high-affinity binding sites for the low-density lipoprotein receptor-related protein (LRP): consequences for the chaperone functions of RAP

RAP (receptor-associated protein) is a three domain 38 kDa ER (endoplasmic reticulum)-resident protein that is a chaperone for the LRP (low-density lipoprotein receptor-related protein). Whereas RAP is known to compete for binding of all known LRP ligands, neither the location, the number of binding sites on LRP, nor the domains of RAP involved in binding is known with certainty. We have systematically examined the binding of each of the three RAP domains (D1, D2 and D3) to tandem and triple CRs (complement-like repeats) that span the principal ligand-binding region, cluster II, of LRP. We found that D3 binds with low nanomolar affinity to all (CR)₂ species examined. Addition of a third CR domain increases the affinity for D3 slightly. A pH change from 7.4 to 5.5 gave only a 6-fold increase in K_d for D3 at 37 °C, whereas temperature change from 22 °C to 37 °C has a similar small effect on affinity, raising questions about the recently proposed D3-destabilization mechanism of RAP release from LRP. Surprisingly, and in contrast to literature suggestions, D1 and D2 also bind to most (CR)₂ and (CR)₃ constructs with nanomolar affinity. Although this suggested that there might be three high-affinity binding sites in RAP for LRP, studies with intact RAP showed that only two binding sites are available in the intact chaperone. These findings suggest a new model for RAP to function as a folding chaperone and also for the involvement of YWTD domains in RAP release from LRP in the Golgi.

Dissection of RAP-LRP interactions: Binding of RAP and RAP fragments to complement-like repeats 7 and 8 from ligand binding cluster II of LRP

The receptor associated protein (RAP) is a three domain 38kDa ER-resident chaperone that helps folding of LRP and other LDL receptor family members and prevents premature binding of protein ligands. It competes strongly with all known LRP ligands. To further understanding of the specificity of RAP-LRP interactions, the binding of RAP and RAP fragments to two domains (CR7-CR8) from one of the main ligand-binding regions of LRP has been examined by 2D HSQC NMR spectroscopy and isothermal titration calorimetry. We found that RAP contains two binding sites for CR7-CR8, with the higher affinity site (K(d) approximately 1microM) located in the C-terminal two-thirds and the weaker site (K(d) approximately 5microM) in the N-terminal third of RAP. Residues from both CR7 and CR8 are involved in binding at each RAP site. The presence of more than one binding site on RAP for CR domains from LRP, together with the previous demonstration by others that RAP can bind to CR5-CR6 with comparably low affinities suggest an explanation for the dual roles of RAP as a folding chaperone and a tight competitive inhibitor of ligand binding.

Proteomic analysis of oxidatively modified proteins induced by the mitochondrial toxin 3-nitropropionic acid in human astrocytes expressing the HIV protein tat

The human immunodeficiency virus (HIV)-Tat protein has been implicated in the neuropathogenesis of HIV infection. However, its role in modulating astroglial function is poorly understood. Astrocyte infection with HIV has been associated with rapid progression of dementia. Intracellularly expressed Tat is not toxic to astrocytes. In fact, intracellularly expressed Tat offers protection against oxidative stress-related toxins such as the mitochondrial toxin 3-nitroproprionic acid (3-NP). In the current study, human astrocytes expressing Tat (SVGA-Tat) and vector controls (SVGA-pcDNA) were each treated with the irreversible mitochondrial complex II inhibitor 3-NP. Proteomics analysis was utilized to identify changes in protein expression levels. By coupling 2D fingerprinting and identification of proteins by mass spectrometry, actin, heat shock protein 90, and mitochondrial single-stranded DNA binding protein were identified as proteins with increased expression, while lactate dehydrogenase had decreased protein expression levels in SVGA-Tat cells treated with 3-NP compared to SVGA-pcDNA cells treated with 3-NP. Oxidative damage can lead to several events including loss in specific protein function, abnormal protein clearance, depletion of the cellular redox-balance and interference with the cell cycle, ultimately leading to neuronal death. Identification of specific proteins protected from oxidation is a crucial step in understanding the interaction of Tat with astrocytes. In the current study, proteomics also was used to identify proteins that were specifically oxidized in SVGA-pcDNA cells treated with 3-NP compared to SVGA-Tat cells treated with 3-NP. We found beta-actin, calreticulin precursor protein, and synovial sarcoma X breakpoint 5 isoform A to have increased oxidation in control SVGA-pcDNA cells treated with 3-NP compared to SVGA-Tat cells treated with 3-NP. These results are discussed with reference to potential involvement of these proteins in HIV dementia and protection of astrocytes against oxidative stress by the HIV virus, a prerequisite for survival of a viral host cell.