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  • 1
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Two cDNA clones encoding distinct members of the corticotropin-releasing factor (CRF) receptor family have been isolated from Xenopus laevis with PCR-based approaches. The first full-length cDNA amplified from Xenopus brain encoded a 415-amino acid protein with ∼80% identity to mammalian CRF receptor type 1 (CRF-R1). The second full-length cDNA isolated from Xenopus brain and heart encoded a 413-amino acid protein with ∼81% identity to the α-variant of mammalian CRF receptor, type 2 (CRF-R2). No evidence could be obtained that the β-variant of CRF-R2 existed in Xenopus laevis. Binding studies using human embryonic kidney 293 (HEK 293) cells stably transfected with xenopus CRF-R2 showed that the CRF analogues urotensin I, urocortin, and sauvagine were bound with higher affinities than human/rat CRF, xenopus CRF, and ovine CRF. In contrast to human CRF-R1, xenopus CRF-R1 (xCRF-R1) was very selective for different CRF ligands. Urotensin I, urocortin, human/rat CRF, and xenopus CRF were bound with significantly (10–22-fold) higher affinities than ovine CRF (KD = 31.7 nM) and sauvagine (KD = 51.4 nM). In agreement with these binding data, EC50 values of 39.7 and 1.1 nM were found for sauvagine and for human/rat CRF or xenopus CRF, respectively, when the cyclic AMP production in HEK 293 cells stably transfected with xCRF-R1 was determined.
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  • 2
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The physiological role of the corticotropin-releasing factor (CRF) family of peptides has recently been extended by emerging evidence of their cytoprotective effects. To determine whether CRF-mediated cytoprotection is linked to caspase-dependent apoptosis, the effect of CRF on the activation of caspases was investigated in detail in Y79 human retinoblastoma cells. The results presented here demonstrate that the cytoprotective effect of CRF against the actions of camptothecin (CT) was mediated by CRF receptor subtype 1, but not subtype 2. The observed CRF-mediated cytoprotection involved rapid and pronounced suppression of proteolytic processing and activation of procaspase-3, exerted even when CRF was added hours after the application of the cytotoxic agent. Surprisingly, activation of procaspase-3 preceded activation of the initiator procaspases 2, 8, 9 and 10 during CT-induced apoptosis of Y79 cells. The mechanism of the effect of CRF was examined using inhibitors of signalling pathways such as Wortmannin (Akt), cyclic AMP-dependent protein kinase (PKA), extracellular signal-regulated kinase (ERK), protein kinase c (PKC), p38 mitogen-activated protein kinase (p38 MAPK), phospholipase c (PLC), nuclear factor-κB (NF-κΒ) and c-jun N-terminal kinase (JNK). The involvement of PKA in the mediation of the anti-apoptotic effect of CRF has been established. Taken together, these results demonstrate for the first time that the cytoprotective effect of CRF involved suppression of pro-apoptotic pathways at a site upstream of activation of procaspase-3.
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  • 3
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Homologous receptor desensitization is an important regulatory response to continuous activation by agonist that involves the uncoupling of a receptor from its G protein. When human retinoblastoma Y-79 cells expressing corticotropin-releasing factor (CRF) receptors were preincubated with CRF for 10 min-4 h, a time-dependent reduction in both the peak and sensitivity of CRF-stimulated intracellular cyclic AMP (cAMP) accumulation developed with a t1/2 of 38 min and an EC50 of 6–7 nM CRF. CRF receptor desensitization was slowly reversible after a 4-h CRF preincubation with a t1/2 of 13 h and a full restoration of cAMP responsiveness to CRF at 24 h following the removal of 10 nM CRF. Because the ability of vasoactive intestinal peptide, forskolin, or (−)-isoproterenol to stimulate cAMP accumulation was not diminished in Y-79 cells desensitized with 10 nM CRF, the observed desensitization was considered to be a specific homologous action of CRF. CRF receptor desensitization was markedly attenuated by CRF receptor antagonists, which alone did not produce any appreciable reduction in CRF-stimulated cAMP accumulation. Although recent reports have demonstrated a rapid decline in steady-state levels of CRF receptor type 1 (CRF-R1) mRNA in anterior pituitary cells during several hours of exposure to CRF, there was no observed reduction in CRF-R1 mRNA levels when Y-79 cells were preincubated with 10 nM CRF for 10 min-24 h despite a rapid time- and concentration-dependent loss of CRF receptors from the retinoblastoma cell surface.
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  • 4
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The subcellular location of the secretases processing the β-amyloid precursor protein (APP) is not established yet. We analyzed the generation of the β-amyloid peptide (Aβ) in human embryonic kidney 293 cell lines stably expressing wild-type and noninternalizing mutants of human APP. APP lacking the entire cytoplasmic domain or with both tyrosine residues of the motif GYENPTY mutated to alanine showed at least fivefold reduced endocytosis. In these cell lines, the production of Aβ1-40 was substantially reduced, but accompanied by the appearance of two prominent alternative Aβ peptides differing at the amino-termini. Based on antibody reactivity and mobility in high-resolution gels in comparison with defined Aβ fragments, these peptides were identified as Aβ3-40 and Aβ5-40. Notably, these alternative Aβ peptides were not generated when the APP mutants were retained in the early secretory pathway by treatment with brefeldin A. These results indicate that the alternative processing is the result of APP accumulation at the plasma membrane and provide evidence of distinct β-secretase activities. Cleavage amino-terminal to position 1 of Aβ occurs predominantly in endosomes, whereas the processing at positions 3 or 5 takes place at the plasma membrane.
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