TORC2 regulates germinal center repression of the TCL1 oncoprotein to promote B cell development and inhibit transformation

Abstract

Aberrant expression of the TCL1 oncoprotein promotes malignant transformation of germinal center (GC) B cells. Repression of TCL1in GC B cells facilitates FAS-mediated apoptosis and prevents lymphoma formation. However, the mechanism for this repression is unknown. Here we show that the CREB coactivator TORC2 directly regulates TCL1 expression independent of CREB Ser-133 phosphorylation and CBP/p300 recruitment. GC signaling through CD40 or the BCR, which activates pCREB-dependent genes, caused TORC2 phosphorylation, cytosolic emigration, and TCL1 repression. Signaling via cAMP-inducible pathways inhibited TCL1 repression and reduced apoptosis, consistent with a prosurvival role for TCL1 before GC selection and supporting an initiating role for aberrant TCL1 expression during GC lymphomagenesis. Our data indicate that a novel CREB/TORC2 regulatory mode controls the normal program of GC gene activation and repression that promotes B cell development and circumvents oncogenic progression. Our results also reconcile a paradox in which signals that activate pCREB/CBP/ p300 genes concurrently repress TCL1 to initiate its silencing.gene regulation ͉ lymphomagenesis ͉ signal transduction T he germinal center (GC) is home to T cell-dependent antigen-driven B cell maturation, memory B and plasma cell production, and the site of origin for most human B cell lymphomas (1-3). GC B cells undergo critical changes in gene expression that are required for proper development and protection from oncogenesis (4, 5). The generation of an appropriate humoral response is insured by negative selection, primarily mediated by FAS-induced apoptosis (6-8). Escape from apoptosis results in autoimmunity and B cell transformation (8).TCL1 functions as a coactivator of the cell survival kinase AKT (9, 10). In mature T cell tumors, TCL1 expression is aberrantly elevated by rearrangements into T cell receptor loci (11). Physiologic TCL1 expression is largely limited to B lineage cells and is robust from pre-B cells through peripheral naïve B cells, followed by a critical 40-60% repression in GC B cells and complete silencing in post-GC memory B and plasma cells (12,13). Most B cell lymphomas that arise by transformation of GC-experienced B cells exhibit elevated TCL1 expression by escape from GC mechanism(s) of TCL1 repression (12,14,15). Interestingly, transgenic mice with TCL1 expression levels stabilized in GC lymphocytes develop cancers that resemble a spectrum of human GC B cell lymphomas (16).Unrepressed TCL1 expression inhibits FAS-induced B cell apoptosis independent of activation by BCR survival signaling (17, 18). Impaired apoptosis from failed TCL1 repression in GC B cells connects TCL1 dysregulation in patient lymphomas (12, 14) to a mechanism for increased transformation (16,18). Factors so far suggested to regulate TCL1, including Nur77 (19,20), miRNA-29 and miRNA-181 (21), Sp1 (22), and EBV infection (23), fail to adequately explain TCL1 repression in GC B cells and its continued aberrant expression in lymphomas (11).Its role in ...

TCL1A expression delineates biological and clinical variability in B-cell lymphoma

“…31 This is consistent with the interpretation that the increased expression of TCL1A observed in multiple B-cell lymphoma types is the consequence of the escape from GC mechanisms of TCL1A repression. 13 ARACNE generates a putative transcriptional network to predict broader functional relationships and could help in the understanding the biological role of TCL1A. One of the important findings of network analysis was probable regulatory interaction of TCL1A and CD27.…”

“…9,12 Aberrant TCL1A expression also promotes malignant transformation of germinal center B cells. 13 A role of TCL1A oncogene in the pathogenesis of other B-cell nonHodgkin's lymphoma malignancies has already been proposed, 4 but the exact mechanisms by which TCL1A regulates tumor development in non-Hodgkin's lymphoma is not yet fully understood.…”

High TCL1 levels are a marker of B-cell receptor pathway responsiveness and adverse outcome in chronic lymphocytic leukemia

“…Overall, these data suggest that TCL1 levels may represent the best signature yet identified to predict the BCR-responsive subset of CLL. 12,[14][15][16][17][42][43][44] The factors driving high-level expression of TCL1 in CLL are still under investigation, but they probably include epigenetic regulation resulting from microRNA down-regulation 45 and transcriptional mechanisms resulting from cytokine-responsive TCL1 promoter elements, [46][47][48] but not chromosome translocation or TCL1 genomic amplification. 49 We have previously reported in CLL and other B-cell tumors that TCL1 levels often show striking intratumoral variations, which are correlated with proliferative state.…”

Expression of sprouty2 inhibits B-cell proliferation and is epigenetically silenced in mouse and human B-cell lymphomas

“…Nalm-6 cells stably expressing SPRY2, CA-MEK1 (constitutively active MEK1, MEK1 D218,D222 ), or an empty control vector were generated using spin transfection with retroviruses expressing MSCV-SPRY2-IRES-PURO, MSCV-CA-MEK1-IRES-PURO, or MSCV-IRES-PURO as previously described. 38 CA-MEK1 was cloned from the Addgene plasmid 15 268, pBabe-Puro-MEK-DD. 39 5-aza-2Ј-deoxycytidine (5-aza-dC) alone or in combination with trichostatin A (TSA) treatment of human and mouse B-cell lines was performed as previously described.…”