TGF-β induces apoptosis in human B cells by transcriptional regulation of BIK and BCL-XL

Abstract

TGF-β potently induces apoptosis in Burkitt’s Lymphoma (BL) cell lines and in explanted primary human B lymphocytes. The physiological relevance and mechanism of TGF-β-mediated apoptosis induction in these cells remains to be determined. Here we demonstrate the requirement for TGF-β-mediated regulation of BIK and BCL-XL to activate an intrinsic apoptotic pathway in centroblastic BL cells. TGF-β directly induced transcription of BIK and a consensus Smad binding element identified in the BIK promoter recruits TGF-β-activated Smad transcription factor complexes in vivo. TGF-β also transcriptionally repressed expression of the apoptosis inhibitor BCL-XL. Inhibition of BCL-XL sensitised BL cells to TGF-β-induced apoptosis while overexpression of BCL-XL or suppression of BIK by shRNA, diminished TGF-β-induced apoptosis. BIK and BCL-XL were also identified as TGF-β target genes in purified normal human centroblast B cells and immunohistochemical analyses of tonsil tissue revealed widespread TGF-β receptor-regulated Smad activation and a focal pattern of BIK expression. Furthermore, using a selective inhibitor of the TGF-β receptor we provide evidence that autocrine TGF-β signaling through ALK5 contributes to the default apoptotic program in normal human centroblasts undergoing spontaneous apoptosis. Our data suggests that TGF-β may act as a physiological mediator of human germinal centre homeostasis via regulation of BIK and BCL-XL.

Kaposi's Sarcoma-Associated Herpesvirus-Encoded MicroRNA miR-K12-11 Attenuates Transforming Growth Factor Beta Signaling through Suppression of SMAD5

“…In order to study whether KSHVencoded miR-K12-11 is involved in modulation of TGF-␤ signaling, we transduced TGF-␤-sensitive Ramos cells, as described previously (2,12,40), with a lentiviral vector containing doxycycline-inducible miR-K12-11 or scrambled control miRNA and then selected with puromycin and blasticidin. In this inducible system, miR-K12-11 expression was induced over a period of 96 h after Dox treatment, which facilitated the determination of the effects of increased levels of miR-K12-11 on TGF-␤ signaling.…”

Repression of the Proapoptotic CellularBIK/NBKGene by Epstein-Barr Virus Antagonizes Transforming Growth Factor β1-Induced B-Cell Apoptosis

“…BIK is upregulated in B cells following antigen receptor stimulation (40,41) and is critical to the apoptotic selection of mature B lymphocytes. More recently, the mechanism of action of TGF-␤ in GC-derived centroblasts and BL-derived cell lines has been shown to involve BIK upregulation (22). We report here for the first time that BIK is a negative transcriptional target of EBV and is repressed by the EBNA2-driven Lat III program, independently of c-MYC.…”

“…In the GC environment, only those B cells that express the highest-affinity immunoglobulins are rescued from stringent proapoptotic pathways that signal through transforming growth factor ␤ (TGF-␤) (22,23), FAS (24,25), and B-cell receptors (26). Bcl-2 proteins are critical for setting the threshold of resistance to apoptosis and initiating the apoptotic cascade, and members are grouped primarily by reference to distinct Bcl-2 homology (BH) domains (for a review, see reference 27).…”

BCR-signaling-induced cell death demonstrates dependency on multiple BH3-only proteins in a murine model of B-cell lymphoma

“…64,65 However, in recent years, Bik has been implicated in B-cell homeostatic control in both malignant and non-neoplastic B-lymphocytes, via its involvement in TGF-β-induced cell death. 66 In a malignant setting, we and others demonstrate conserved upregulation of Bik transcripts downstream of BCR stimulation in multiple models (Figure 3a and Supplementary Figures 7 and 8), 55 and here for the first time, we show a reduction in BCR-induced killing of Eμ-Myc lymphoma cells following loss of Bik. It is therefore likely that BCR-signaling-induced cell death has different BH3-only requirements, or at least different kinetics of BH3-only induction, in alternate cellular contexts.…”