title: Proteasomal degradation of caspase-6 in 17beta-estradiol-treated neurons.
abstract: In primary cultures of human neurons, 17beta-estradiol (17beta-E2) prevents caspase-6-mediated cell death and induces a caspase inhibitory factor (CIF) inhibiting active caspase-6 (Csp-6) in vitro. Here, we show that treatment of neurons with 17beta-E2 results in a proteasomal-dependent but ubiquitin-independent degradation of endogenous and exogenous active Csp-6 in live neurons and in cell free assays, respectively. We further show that the proteasomal-dependent degradation of Csp-6 is not required for its inhibition. Using several protease inhibitors, we find that leupeptin, E-64, and ALLN prevent inhibition of recombinant active Csp-6 (R-Csp-6) in 17beta-E2-treated neuronal protein extracts. Because all three protease inhibitors have the ability to inhibit cysteine proteases, we believe that a cysteinyl protease activity may be required for 17beta-E2-mediated inhibition of active Csp-6. However, we exclude caspases, calpains, and cathepsins as potential cysteinyl proteases involved in the 17beta-E2-mediated Csp-6 inhibition. The results suggest that a proteolytic activity inhibited by leupeptin, E-64, and ALLN is needed to inhibit Csp-6 and that the inhibited Csp-6 is subsequently degraded by the proteasome. The mechanism of 17beta-E2-mediated inhibition of Csp-6 is different from the ubiquitin-dependent proteasomal degradation of Csp-3 and Csp-7 by XIAP and cIAP2 but consistent with the mechanism of Baculovirus p35 inhibition of caspases.
authors.affiliation: Department of Neurology and Neurosurgery, McGill University, Montreal, Quebec, Canada.
authors.forename: Omar
authors.initials: O
authors.lastname: Tounekti
authors.forename: Yan
authors.initials: Y
authors.lastname: Zhang
authors.forename: Guy
authors.initials: G
authors.lastname: Klaiman
authors.forename: Cynthia G
authors.initials: CG
authors.lastname: Goodyer
authors.forename: Andréa
authors.initials: A
authors.lastname: LeBlanc
mesh_terms: D002154:Calpain / Q000037:antagonists & inhibitors; D053178:Caspase 6; D061945:Caspase Inhibitors; D020169:Caspases / Q000378:metabolism*; D002403:Cathepsins / Q000037:antagonists & inhibitors; D002458:Cell Fractionation; D002478:Cells, Cultured; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004958:Estradiol / Q000494:pharmacology*; D005333:Fetus / Q000166:cytology; D006801:Humans; D015151:Immunoblotting; D009097:Multienzyme Complexes / Q000037:antagonists & inhibitors / Q000378:metabolism*; D009474:Neurons / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D021122:Protein Subunits / Q000037:antagonists & inhibitors / Q000378:metabolism
chemical_list: D061945:Caspase Inhibitors; D004791:Enzyme Inhibitors; D009097:Multienzyme Complexes; D021122:Protein Subunits; D004958:Estradiol; D002403:Cathepsins; C505514:CASP6 protein, human; D002154:Calpain; D053178:Caspase 6; D020169:Caspases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex