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The murine haemopexin receptor. Evidence that the haemopexin-binding site resides on a 20 kDa subunit and that receptor recycling is regulated by protein kinase C.

ABSTRACT: Haemopexin receptors from mouse hepatoma (Hepa) cells were affinity-labelled by cross-linking to haem-125I-haemopexin complexes using two homo-[disuccinimidyl suberate (DSS) and 3,3'-dithiobis(succinimidyl propionate) (DTSSP)] and one hetero-[sulphosuccinimidyl 4-(p-maleimidophenyl)butyrate (sulpho-SMPB)] bifunctional cross-linking agents. Analysis of the cross-linked products by SDS/PAGE in the absence of reducing agents revealed that 125I-haemopexin was cross-linked specifically to a protein of apparent molecular mass 85-90 kDa. Upon reduction, haemopexin remained cross-linked to a protein of 20 kDa, suggesting that the murine haemopexin receptor has a subunit structure. Two subunits were identified: alpha (p65) and beta (p20). Furthermore, because haemopexin was cross-linked by all three agents to p20, the shortest cross-linker arm being 1.1 nm (11 A), we propose that the haem-haemopexin-binding site resides on this subunit. In addition, a cysteine residue of p20 is located near the haemopexin-binding site, since haemopexin, which has no free thiol groups, is cross-linked to this subunit by the hetero-bifunctional agent sulpho-SMPB. Exposure of Hepa cells to the tumour-promoting phorbol ester 4 alpha-phorbol 12-myristate 13-acetate (PMA) causes a rapid redistribution of haemopexin receptors from the cell surface to the cell interior. Within 2-4 min of incubation with 100 nM-PMA, there was an approx. 50% decrease in cell-surface haemopexin receptors, as judged by ligand binding at 0 degrees C and affinity labelling of the receptor. This time- and dose-dependent down-regulation was fully reversible within 60-90 min after removal of PMA, and the affinity of the remaining receptors was unaltered by PMA. The specificity of PMA was demonstrated by comparison with the non-tumour-promoter 4 alpha-phorbol, which did not affect any of the parameters examined. The amine H-7, a specific inhibitor of protein kinase C, antagonised the receptor redistribution effect of PMA, suggesting that the down-regulation of haemopexin receptors on the cell surface was a consequence of protein kinase C activation. The PMA-induced decrease in surface haemopexin receptors was due to a 2-fold increase in the rate of internalization (from 0.73 min-1 to 1.32 min-1), whereas the rate of exocytosis (0.6 min-1) was unchanged. PMA treatment, like binding of the natural ligand, haem-haemopexin, results in a lower steady-state level of surface haemopexin receptors independent of receptor synthesis, and the receptors were not degraded but were recycled back to the cell surface.

SUBMITTER: Smith A

PROVIDER: S-EPMC1151108 | biostudies-other | 1991 Jun

REPOSITORIES: biostudies-other

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Project description:BACKGROUND: Anthrax is caused by Bacillus anthracis that produce two exotoxins, lethal toxin and edema toxin. The lethal toxin is composed of the lethal factor (LF) complexed with the cell binding protective antigen (PA83, 83 kDa). Likewise, the edema factor (EF) binds to the PA83 to form the edema toxin. Once PA83 is bound to the host cell surface, a furin-like protease cleaves the full-length, inactive protein into 63 kDa and 20 kDa antigens (PA63 and PA20). PA63 forms a heptamer and is internalized via receptor mediated endocytosis forming a protease-stable pore, which allows EF and LF to enter the cell and exert their toxic effects.Both proteolytically cleaved protective antigens (PA63 and PA20 fragments) are found in the blood of infected animals. The 63 kDa protective antigen PA63 fragment has been thoroughly studied while little is known about the PA20. METHODS: In this study we examined the role of PA20 using high throughput gene expression analysis of human peripheral blood mononuclear cells (PBMC) exposed to the PA20. We constructed a PA mutant in which a Factor Xa proteolytic recognition site was genetically engineered into the protective antigen PA83 to obtain PA20 using limited digestion of this recombinant PA83 with trypsin. RESULTS: Global gene expression response studies indicated modulation of various immune functions and showed gene patterns indicative of apoptosis via the Fas pathway in a subset of the lymphoid cells. This finding was extended to include observations of increased Caspase-3 enzymatic activity and the identification of increases in the population of apoptotic, but not necrotic cells, based on differential staining methods. We identified a list of approximately 40 inflammatory mediators and heat-shock proteins that were altered similarly upon exposure of PBMC to either rPA20 or B. anthracis spores/vegetative cells. CONCLUSION: This study shows that the PA20 has an effect on human peripheral blood leukocytes and can induce apoptosis in the absence of other PA components.

Project description:Considerable clinical evidence supports that increased blood-brain barrier (BBB) permeability is linked to immune extravasation of CNS parenchyma during neuroinflammation. Although BBB permeability and immune extravasation are known to be provoked by vascular endothelial growth factor-A (i.e., VEGF-A) and C-X-C motif chemokine ligand 12 (CXCL12), respectively, the mechanisms that link both processes are still elusive. The interleukin-20 (i.e., IL-20) cytokine signaling pathway was previously implicated in VEGF-mediated angiogenesis and is known to induce cellular response by way of signaling through IL-20 receptor subunit β (i.e., IL-20RB). Dysregulated IL-20 signaling is implicated in many inflammatory pathologies, but it's contribution to neuroinflammation has yet to be reported. We hypothesize that the IL-20 cytokine, and the IL cytokine subfamily more broadly, play a key role in CNS neuroinflammation by signaling through IL-20RB, induce VEGF activity, and enhance both BBB-permeability and CXCL12-mediated immune extravasation. To address this hypothesis, we actively immunized IL-20RB-/- mice and wild-type mice to induce experimental autoimmune encephalomyelitis (EAE) and found that IL-20RB-/- mice showed amelioration of disease progression compared to wild-type mice. Similarly, we passively immunized IL-20RB-/- mice and wild-type mice with myelin-reactive Th1 cells from either IL-20RB-/- and wild-type genotype. Host IL-20RB-/- mice showed lesser disease progression than wild-type mice, regardless of the myelin-reactive Th1 cells genotype. Using multianalyte bead-based immunoassay and ELISA, we found distinctive changes in levels of pro-inflammatory cytokines between IL-20RB-/- mice and wild-type mice at peak of EAE. We also found detectable levels of all cytokines of the IL-20 subfamily within CNS tissues and specific alteration to IL-20 subfamily cytokines IL-19, IL-20, and IL-24, expression levels. Immunolabeling of CNS region-specific microvessels confirmed IL-20RB protein at the spinal cord microvasculature and upregulation during EAE. Microvessels isolated from macaques CNS tissues also expressed IL-20RB. Moreover, we identified the expression of all IL-20 receptor subunits: IL-22 receptor subunit α-1 (IL-22RA1), IL-20RB, and IL-20 receptor subunit α (IL-20RA) in human CNS microvessels. Notably, human cerebral microvasculature endothelial cells (HCMEC/D3) treated with IL-1β showed augmented expression of the IL-20 receptor. Lastly, IL-20-treated HCMEC/D3 showed alterations on CXCL12 apicobasal polarity consistent with a neuroinflammatory status. This evidence suggests that IL-20 subfamily cytokines may signal at the BBB via IL-20RB, triggering neuroinflammation.

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The murine haemopexin receptor. Evidence that the haemopexin-binding site resides on a 20 kDa subunit and that receptor recycling is regulated by protein kinase C.

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