title: X-linked inhibitor of apoptosis regulates lung fibroblast resistance to Fas-mediated apoptosis.
abstract: The accumulation of apoptosis-resistant fibroblasts within fibroblastic foci is a characteristic feature of idiopathic pulmonary fibrosis (IPF), but the mechanisms underlying apoptosis resistance remain unclear. A role for the inhibitor of apoptosis (IAP) protein family member X-linked inhibitor of apoptosis (XIAP) has been suggested by prior studies showing that (1) XIAP is localized to fibroblastic foci in IPF tissue and (2) prostaglandin E₂ suppresses XIAP expression while increasing fibroblast susceptibility to apoptosis. Based on these observations, we hypothesized that XIAP would be regulated by the profibrotic mediators transforming growth factor (TGF)β-1 and endothelin (ET)-1 and that increased XIAP would contribute to apoptosis resistance in IPF fibroblasts. To address these hypotheses, we examined XIAP expression in normal and IPF fibroblasts at baseline and in normal fibroblasts after treatment with TGF-β1 or ET-1. The role of XIAP in the regulation of fibroblast susceptibility to Fas-mediated apoptosis was examined using functional XIAP antagonists and siRNA silencing. In concordance with prior reports, fibroblasts from IPF lung tissue had increased resistance to apoptosis compared with normal lung fibroblasts. Compared with normal fibroblasts, IPF fibroblasts had significantly but heterogeneously increased basal XIAP expression. Additionally, TGF-β1 and ET-1 induced XIAP protein expression in normal fibroblasts. Inhibition or silencing of XIAP enhanced the sensitivity of lung fibroblasts to Fas-mediated apoptosis without causing apoptosis in the absence of Fas activation. Collectively, these findings support a mechanistic role for XIAP in the apoptosis-resistant phenotype of IPF fibroblasts.
authors.affiliation: Division of Pulmonary and Critical Care Medicine, Department of Medicine, University of Michigan Medical Center, Ann Arbor, MI, USA.
authors.forename: Iyabode O
authors.initials: IO
authors.lastname: Ajayi
authors.forename: Thomas H
authors.initials: TH
authors.lastname: Sisson
authors.forename: Peter D R
authors.initials: PD
authors.lastname: Higgins
authors.forename: Adam J
authors.initials: AJ
authors.lastname: Booth
authors.forename: Rommel L
authors.initials: RL
authors.lastname: Sagana
authors.forename: Steven K
authors.initials: SK
authors.lastname: Huang
authors.forename: Eric S
authors.initials: ES
authors.lastname: White
authors.forename: Jessie E
authors.initials: JE
authors.lastname: King
authors.forename: Bethany B
authors.initials: BB
authors.lastname: Moore
authors.forename: Jeffrey C
authors.initials: JC
authors.lastname: Horowitz
mesh_terms: D017209:Apoptosis; D002460:Cell Line; D015232:Dinoprostone / Q000378:metabolism; D019332:Endothelin-1 / Q000494:pharmacology; D005347:Fibroblasts / Q000187:drug effects* / Q000378:metabolism; D005786:Gene Expression Regulation; D006801:Humans; D054990:Idiopathic Pulmonary Fibrosis / Q000473:pathology; D008168:Lung / Q000378:metabolism / Q000473:pathology; D061251:Primary Cell Culture; D034741:RNA, Small Interfering / Q000235:genetics / Q000378:metabolism; D014162:Transfection; D053773:Transforming Growth Factor beta1 / Q000494:pharmacology; D051636:X-Linked Inhibitor of Apoptosis Protein / Q000037:antagonists & inhibitors / Q000378:metabolism*; D019014:fas Receptor / Q000235:genetics / Q000378:metabolism*
chemical_list: D019332:Endothelin-1; C491901:FAS protein, human; D034741:RNA, Small Interfering; D053773:Transforming Growth Factor beta1; D051636:X-Linked Inhibitor of Apoptosis Protein; C496866:XIAP protein, human; D019014:fas Receptor; D015232:Dinoprostone