FOXP1 directly represses transcription of proapoptotic genes and cooperates with NF-κB to promote survival of human B cells

Abstract

Key Points• FOXP1 directly represses multiple proapoptotic genes in primary mature human B cells and DLBCL cell lines.• FOXP1 cooperates with NF-kB signaling to promote expansion of primary mature human B cells by inhibition of caspase-dependent apoptosis.The forkhead transcription factor FOXP1 is involved in B-cell development and function and is generally regarded as an oncogene in activated B-cell-like subtype of diffuse large B-cell lymphoma (DLBCL) and mucosa-associated lymphoid tissue lymphoma, lymphomas relying on constitutive nuclear factor kB (NF-kB) activity for survival. However, the mechanism underlying its putative oncogenic activity has not been established. By gene expression microarray, upon overexpression or silencing of FOXP1 in primary human B cells and DLBCL cell lines, combined with chromatin immunoprecipitation followed by next-generation sequencing, we established that FOXP1 directly represses a set of 7 proapoptotic genes. Low expression of these genes, encoding the BH3-only proteins BIK and Harakiri, the p53-regulatory proteins TP63, RASSF6, and TP53INP1, and AIM2 and EAF2, is associated with poor survival in DLBCL patients. In line with these findings, we demonstrated that FOXP1 promotes the expansion of primary mature human B cells by inhibiting caspase-dependent apoptosis, without affecting B-cell proliferation. Furthermore, FOXP1 is dependent upon, and cooperates with, NF-kB signaling to promote B-cell expansion and survival. Taken together, our data indicate that, through direct repression of proapoptotic genes, (aberrant) expression of FOXP1 complements (constitutive) NF-kB activity to promote B-cell survival and can thereby contribute to B-cell homeostasis and lymphomagenesis. (Blood. 2014;124(23):3431-3440) IntroductionThe forkhead transcription factor FOXP1 plays an important role in a wide diversity of biological processes, including T-cell development and differentiation 1,2 and B-cell development and function. [3][4][5] Furthermore, FOXP1 has long been recognized as a potential oncogene in various types of B-cell non-Hodgkin lymphomas; however, its mode of oncogenic action is largely unknown. 6,7 In diffuse large B-cell lymphoma (DLBCL) and mucosaassociated lymphoid tissue (MALT) lymphoma, aberrantly high expression of FOXP1 is associated with poor prognosis and FOXP1-positive MALT lymphomas were shown to be at risk of transforming into aggressive DLBCLs. 8,9 This overexpression of FOXP1 can be caused by a t(3;14)(p14;q32) translocation, involving FOXP1 and IgH loci, which has recurrently been observed in MALT lymphoma and activated B-cell-like (ABC) DLBCL.10-13 FOXP1 expression is also frequently upregulated in ABC-DLBCL as a result of trisomy 3 or more restricted focal amplifications, 14 whereas aberrant Myc expression in transformed gastric MALT lymphomas leads to upregulation of FOXP1 due to repression of the FOXP1 targeting miRNA 34a. 15 Furthermore, expression levels of FOXP1 can be used as a discriminator between the ABC and germinal center (GC) subtypes of DLBCL, ...

The forkhead transcription factor FOXP1 represses human plasma cell differentiation

“…MBCs were isolated as previously described. 25 IgA 2 population, using antibodies against CD19, CD27, and PEconjugated antibodies against IgM and IgA (Southern Biotech). Tonsils were obtained from children undergoing routine tonsillectomy as previously described.…”

“…Tonsils were obtained from children undergoing routine tonsillectomy as previously described. 25 Tonsillar B-cell subsets were isolated by FACSAria sorting using an PE-conjugated antibody against CD19 (DAKO), an FITC-conjugated antibody against IgD (Southern Biotech), and an APC-conjugated antibody against CD38 (BD).…”

“…[21][22][23][24] In accordance, we recently demonstrated that FOXP1 overexpression in primary human B cells cooperates with nuclear factor kB pathway activity to promote B-cell survival. 14,25 Furthermore, a recent study by Sagardoy et al 26 showed that FOXP1 expression is temporarily repressed at the GC stage, which is needed for appropriate GC B-cell function. 26 However, potential functions of FOXP1 in differentiation of post-GC B cells have not yet been assessed.…”

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

“…18 Another study demonstrated that FOXP1 suppresses apoptosis of DLBCL cells by transcriptional repression of a set of proapoptotic target genes, which includes the BH3-only protein BIK and several p53 regulatory proteins. 19 Here, we have combined a genome-wide search for direct FOXP1 targets using chromatin immunoprecipitation (ChIP) followed by next-generation sequencing and RNA interference with functional screening for biologically relevant target genes to identify novel tumor suppressor proteins in FOXP1-positive DLBCL. We provide evidence that the repressed FOXP1 target sphingosine-1-phosphate receptor 2 (S1PR2) has robust tumor suppressive activity in DLBCL cells in vitro and in vivo and represents an excellent prognostic biomarker that, either alone or in combination with FOXP1 expression, accurately predicts survival of ABC DLBCL patients.…”

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

“…32 Direct FOXP1 target genes, including PAX5, PRDM1, and POU2F1, support a functional role in the GC reaction. 32 Recent studies have provided insight into FOXP1 function in DLBCL cells, indicating contributions to proliferation, 33 inhibition of apoptosis to compliment NF-kB-dependent regulation of proliferation, 34 and Wnt signaling. 25 However, further understanding of FOXP1 isoform expression and functionality is required to pursue emerging evidence indicating that FOXP1 L and FOXP1 S proteins have distinct functions.…”