TGF-beta induces apoptosis in human B cells by transcriptional regulation of BIK and BCL-XL.
Transforming growth factor-beta (TGF-beta) potently induces apoptosis in Burkitt's lymphoma (BL) cell lines and in explanted primary human B lymphocytes. The physiological relevance and mechanism of TGF-beta-mediated apoptosis induction in these cells remains to be determined. Here we demonstrate the requirement for TGF-beta-mediated regulation of BIK and BCL-X(L) to activate an intrinsic apoptotic pathway in centroblastic BL cells. TGF-beta directly induced transcription of BIK and a consensus Smad-binding element identified in the BIK promoter recruits TGF-beta-activated Smad transcription factor complexes in vivo. TGF-beta also transcriptionally repressed expression of the apoptosis inhibitor BCL-X(L). Inhibition of BCL-X(L) sensitised BL cells to TGF-beta-induced apoptosis whereas overexpression of BCL-X(L) or suppression of BIK by shRNA, diminished TGF-beta-induced apoptosis. BIK and BCL-X(L) were also identified as TGF-beta target genes in purified normal human centroblast B cells and immunohistochemical analyses of tonsil tissue revealed widespread TGF-beta receptor-regulated Smad activation and a focal pattern of BIK expression. Furthermore, using a selective inhibitor of the TGF-beta receptor we provide evidence that autocrine TGF-beta signalling through ALK5 contributes to the default apoptotic programme in normal human centroblasts undergoing spontaneous apoptosis. Our data suggests that TGF-beta may act as a physiological mediator of human germinal centre homoeostasis by regulation of BIK and BCL-X(L).
Spender
L C
LC
Growth Factor Signalling Laboratory, The Beatson Institute for Cancer Research, Garscube Estate, Switchback Road, Bearsden, Glasgow, UK. l.spender@beatson.gla.ac.uk
O'Brien
D I
DI
Simpson
D
D
Dutt
D
D
Gregory
C D
CD
Allday
M J
MJ
Clark
L J
LJ
Inman
G J
GJ
D017209:Apoptosis / Q000187:drug effects*; D051017:Apoptosis Regulatory Proteins / Q000235:genetics / Q000378:metabolism*; D001402:B-Lymphocytes / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D045744:Cell Line, Tumor; D002478:Cells, Cultured; D024202:Electrophoretic Mobility Shift Assay; D005434:Flow Cytometry; D005786:Gene Expression Regulation / Q000187:drug effects*; D006801:Humans; D015151:Immunoblotting; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D024101:Mitochondrial Proteins; D020133:Reverse Transcriptase Polymerase Chain Reaction; D016212:Transforming Growth Factor beta / Q000494:pharmacology*; D051020:bcl-X Protein / Q000235:genetics / Q000378:metabolism*
D051017:Apoptosis Regulatory Proteins; C096820:BIK protein, human; D008565:Membrane Proteins; D024101:Mitochondrial Proteins; D016212:Transforming Growth Factor beta; D051020:bcl-X Protein