The hematopoietic oncoprotein FOXP1 promotes tumor cell survival in diffuse large B-cell lymphoma by repressing S1PR2 signaling

Abstract

Key Points• The sphingosine-1-phosphate receptor 2 (S1PR2) is a novel tumor suppressor and survival prognosticator in the ABC subtype of DLBCL.• S1PR2 is a direct, repressed FOXP1 target; ectopic S1PR2 expression induces apoptosis in DLBCL cells in vitro and prevents tumor growth.Aberrant expression of the oncogenic transcription factor forkhead box protein 1 (FOXP1) is a common feature of diffuse large B-cell lymphoma (DLBCL). We have combined chromatin immunoprecipitation and gene expression profiling after FOXP1 depletion with functional screening to identify targets of FOXP1 contributing to tumor cell survival. We find that the sphingosine-1-phosphate receptor 2 (S1PR2) is repressed by FOXP1 in activated B-cell (ABC) and germinal center B-cell (GCB) DLBCL cell lines with aberrantly high FOXP1 levels; S1PR2 expression is further inversely correlated with FOXP1 expression in 3 patient cohorts. Ectopic expression of wild-type S1PR2, but not a point mutant incapable of activating downstream signaling pathways, induces apoptosis in DLBCL cells and restricts tumor growth in subcutaneous and orthotopic models of the disease. The proapoptotic effects of S1PR2 are phenocopied by ectopic expression of the small G protein Ga13 but are independent of AKT signaling. We further show that low S1PR2 expression is a strong negative prognosticator of patient survival, alone and especially in combination with high FOXP1 expression. The S1PR2 locus has previously been demonstrated to be recurrently mutated in GCB DLBCL; the transcriptional silencing of S1PR2 by FOXP1 represents an alternative mechanism leading to inactivation of this important hematopoietic tumor suppressor. (Blood. 2016;127(11):1438-1448 IntroductionThe forkhead box protein 1 (FOXP1) transcription factor is aberrantly expressed in the activated B-cell-like (ABC) subtype of diffuse large B-cell lymphoma (DLBCL) and represents a widely accepted biomarker for survival prognostication in DLBCL. The FOXP1 genomic locus is recurrently targeted by genomic rearrangements in ABC DLBCL as well as in marginal zone lymphoma of mucosa-associated lymphoid tissue and in chronic lymphocytic leukemia, which correlates with a poor prognosis in each disease entity. 1-3 Primary FOXP1 translocations predominantly involve the immunoglobulin heavy chain locus, leading to overexpression of the full-length protein 3 ; in contrast, the rare nonimmunoglobulin rearrangements of FOXP1 generate N-truncated isoforms that are believed to drive disease progression rather than initiation. 4 Most FOXP1-expressing lymphomas exhibit no apparent structural aberrations of the gene 5 ; the short, putatively oncogenic isoforms in particular are highly expressed from the wild-type locus in ABC DLBCL as a consequence of "normal" B-cell activation. 6 We have shown earlier that aberrant FOXP1 expression in ABC DLBCL may alternatively also result from dysregulated posttranscriptional regulation. 7,8 FOXP1 protein levels are regulated by the microRNA miR-34a, which itself is either transcriptionally or ep...

N-terminally truncated FOXP1 protein expression and alternate internal FOXP1 promoter usage in normal and malignant B cells

“…Interestingly, an siRNA that preferentially silenced FOXP1 L was recently reported to be less effective at inducing cell death than total FOXP1 targeting in multiple DLBCL cell lines. 45 While the same study generally observed stronger regulation of gene expression on silencing all FOXP1 isoforms, there were some exceptions. 47 Our isoform-biased depletion studies presented here indicate that, in accordance with our previous findings (where the FOXP1 target gene HIP1R was preferentially regulated in ABC-DLBCL), 22 and complimentary to some common functions, 47 FOXP1 L and FOXP1 S have at least some distinct functional properties, including CD19 regulation.…”

“…25,32,33,45 In addition, Dekker et al have recently shown that total FOXP1 activates nearly all BCR-dependent genes; however, as FOXP1 silencing did not decrease BCR clustering, they concluded that it contributed to chronic B-cell receptor signaling (CABS) but only minimally to CABS-directed NF-kB activation. 46 Our findings, showing no decrease in CD19 expression upon total FOXP1 silencing, are consistent with these data.…”

Gene expression changes in bioceramic paste-treated human dental pulp cells

“…These results suggest that vascular permeability, angiogenesis, and inflammation responses related to ANGPTL4 and glucose homeostasis related to TCF7L2 may play an important role in the wound healing of the dental pulp tissue after early exposure to iRoot BP. FOXP1 has been reported to function as either an oncogene or tumorsuppressor in various cancers and also promote cell survival (22,23). In the present study, FOXP1 transcript levels were found to be lower than that of the control group after 24 h of incubation, suggesting that FOXP1 may affect cell survival on iRoot BP-treated hDPCs.…”

S1PR2 deficiency in DLBCL: a FOXy connection

“…In this issue of Blood, Flori et al have now identified S1PR2 as a tumor suppressor that is transcriptionally silenced by forkhead box protein 1 (FOXP1) in the aggressive, activated B-cell (ABC-DLBCL) subtype. 1 T he lipid metabolite, S1P, has potently emerged in recent years as a central component of the complex microenvironmental cues that control lymphocyte trafficking at the systemic and local levels. This involves a tight interplay with chemokines, promoting the egress of lymphocytes that have entered secondary lymphoid organs (SLOs) from the bloodstream to return to the lymph and orchestrating their movements within lymphoid tissues to reach functionally relevant localizations.…”

“…7 Using chromatin immunoprecipitation and gene expression profiling in initially FOXP1 high DLBCL cell lines then depleted of FOXP1, Flori et al identified several genes either upregulated or downregulated by FOXP1. 1 Data mining on 2 publicly accessible gene expression databases of DLBCL patients showed that, among these FOXP1 targets, only S1PR2 had a strong inverse correlation with FOXP1 expression. Importantly, when the analysis was carried out in relation to patient survival, high S1PR2 expression was found to be a positive prognostic factor, especially in combination with low FOXP1 expression.…”