title: Signal transducer and activator of transcription-5 mediates neuronal apoptosis induced by inhibition of Rac GTPase activity.
abstract: In several neuronal cell types, the small GTPase Rac is essential for survival. We have shown previously that the Rho family GTPase inhibitor Clostridium difficile toxin B (ToxB) induces apoptosis in primary rat cerebellar granule neurons (CGNs) principally via inhibition of Rac GTPase function. In the present study, incubation with ToxB activated a proapoptotic Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway, and a pan-JAK inhibitor protected CGNs from Rac inhibition. STAT1 expression was induced by ToxB; however, CGNs from STAT1 knock-out mice succumbed to ToxB-induced apoptosis as readily as wild-type CGNs. STAT3 displayed enhanced tyrosine phosphorylation following treatment with ToxB, and a reputed inhibitor of STAT3, cucurbitacin (JSI-124), reduced CGN apoptosis. Unexpectedly, JSI-124 failed to block STAT3 phosphorylation, and CGNs were not protected from ToxB by other known STAT3 inhibitors. In contrast, STAT5A tyrosine phosphorylation induced by ToxB was suppressed by JSI-124. In addition, roscovitine similarly inhibited STAT5A phosphorylation and protected CGNs from ToxB-induced apoptosis. Consistent with these results, adenoviral infection with a dominant negative STAT5 mutant, but not wild-type STAT5, significantly decreased ToxB-induced apoptosis of CGNs. Finally, chromatin immunoprecipitation with a STAT5 antibody revealed increased STAT5 binding to the promoter region of prosurvival Bcl-xL. STAT5 was recruited to the Bcl-xL promoter region in a ToxB-dependent manner, and this DNA binding preceded Bcl-xL down-regulation, suggesting transcriptional repression. These data indicate that a novel JAK/STAT5 proapoptotic pathway significantly contributes to neuronal apoptosis induced by the inhibition of Rac GTPase.
authors.affiliation: Department of Biological Sciences and Eleanor Roosevelt Institute, University of Denver, Denver, Colorado 80208, USA.
authors.forename: Trisha R
authors.initials: TR
authors.lastname: Stankiewicz
authors.forename: F Alexandra
authors.initials: FA
authors.lastname: Loucks
authors.forename: Emily K
authors.initials: EK
authors.lastname: Schroeder
authors.forename: Marja T
authors.initials: MT
authors.lastname: Nevalainen
authors.forename: Kenneth L
authors.initials: KL
authors.lastname: Tyler
authors.forename: Klaus
authors.initials: K
authors.lastname: Aktories
authors.forename: Ron J
authors.initials: RJ
authors.lastname: Bouchard
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Linseman
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D001426:Bacterial Proteins / Q000494:pharmacology*; D001427:Bacterial Toxins / Q000494:pharmacology*; D002478:Cells, Cultured; D002531:Cerebellum / Q000166:cytology / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D009154:Mutation; D009474:Neurons / Q000166:cytology / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D051381:Rats; D050794:STAT1 Transcription Factor / Q000235:genetics / Q000378:metabolism; D050796:STAT3 Transcription Factor / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D050799:STAT5 Transcription Factor / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects* / Q000235:genetics; D014315:Triterpenes / Q000494:pharmacology; D051020:bcl-X Protein / Q000235:genetics / Q000378:metabolism; D020744:rac GTP-Binding Proteins / Q000037:antagonists & inhibitors*
chemical_list: D001426:Bacterial Proteins; D001427:Bacterial Toxins; C494812:Bcl2l1 protein, mouse; C494813:Bcl2l1 protein, rat; D050794:STAT1 Transcription Factor; D050796:STAT3 Transcription Factor; D050799:STAT5 Transcription Factor; C494080:Stat1 protein, mouse; C494081:Stat1 protein, rat; C494087:Stat3 protein, mouse; C494088:Stat3 protein, rat; C494093:Stat5a protein, mouse; C494094:Stat5a protein, rat; D014315:Triterpenes; D051020:bcl-X Protein; C031815:toxB protein, Clostridium difficile; D020744:rac GTP-Binding Proteins; C038106:cucurbitacin I