HGAL, a germinal center specific protein, decreases lymphoma cell motility by modulation of the RhoA signaling pathway

Whole-genome sequencing reveals complex genomic features underlying anti-CD19 CAR T-cell treatment failures in lymphoma

CD19-directed chimeric antigen receptor (CAR-19) T cells are groundbreaking immunotherapies approved for use against large B-cell lymphomas. Although host inflammatory and tumor microenvironmental markers associate with efficacy and resistance, the tumor-intrinsic alterations underlying these phenomena remain undefined. CD19 mutations associate with resistance but are uncommon, and most patients with relapsed disease retain expression of the wild-type receptor, implicating other genomic mechanisms. We therefore leveraged the comprehensive resolution of whole-genome sequencing to assess 51 tumor samples from 49 patients with CAR-19-treated large B-cell lymphoma. We found that the pretreatment presence of complex structural variants, APOBEC mutational signatures, and genomic damage from reactive oxygen species predict CAR-19 resistance. In addition, the recurrent 3p21.31 chromosomal deletion containing the RHOA tumor suppressor was strongly enriched in patients for whom CAR T-cell therapy failed. Pretreatment reduced expression or monoallelic loss of CD19 did not affect responses, suggesting CAR-19 therapy success and resistance are related to multiple mechanisms. Our study showed that tumor-intrinsic genomic alterations are key among the complex interplay of factors that underlie CAR-19 efficacy and resistance for large B-cell lymphomas.

HGAL inhibits lymphoma dissemination by interacting with multiple Cytoskeletal proteins

In vivo experiments demonstrate that HGAL expression in lymphoma decreases tumor dissemination and prolongs animal survival.

HGAL inhibits cell motility by interacting with multiple cytoskeletal proteins, thereby affecting cell motility by multiple mechanisms.

Human germinal center–associated lymphoma (HGAL) is an adaptor protein specifically expressed in germinal center lymphocytes. High expression of HGAL is a predictor of prolonged survival of diffuse large B-cell lymphoma (DLBCL) and classic Hodgkin lymphoma. Furthermore, HGAL expression is associated with early-stage DLBCL, thus potentially limiting lymphoma dissemination. In our previous studies, we demonstrated that HGAL regulates B-cell receptor signaling and cell motility in vitro and deciphered some molecular mechanisms underlying these effects. By using novel animal models for in vivo DLBCL dispersion, we demonstrate here that HGAL decreases lymphoma dissemination and prolongs survival. Furthermore, by using an unbiased proteomic approach, we demonstrate that HGAL may interact with multiple cytoskeletal proteins thereby implicating a multiplicity of effects in regulating lymphoma motility and spread. Specifically, we show that HGAL interacts with tubulin, and this interaction may also contribute to HGAL effects on cell motility. These findings recapitulate previous observations in humans, establish the role of HGAL in dissemination of lymphoma in vivo, and explain improved survival of patients with HGAL-expressing lymphomas.

Conditional expression of HGAL leads to the development of diffuse large B-cell lymphoma in mice

Diffuse large B-cell lymphomas (DLBCLs) are clinically and genetically heterogeneous tumors. Deregulation of diverse biological processes specific to B cells, such as B-cell receptor (BCR) signaling and motility regulation, contribute to lymphomagenesis. Human germinal center associated lymphoma (HGAL) is a B-cell-specific adaptor protein controlling BCR signaling and B lymphocyte motility. In normal B cells, it is expressed in germinal center (GC) B lymphocytes and promptly downregulated upon further differentiation. The majority of DLBCL tumors, primarily GC B-cell types, but also activated types, express HGAL. To investigate the consequences of constitutive expression of HGAL in vivo, we generated mice that conditionally express human HGAL at different stages of hematopoietic development using 3 restricted Cre-mediated approaches to initiate expression of HGAL in hematopoietic stem cells, pro-B cells, or GC B cells. Following immune stimulation, we observed larger GCs in mice in which HGAL expression was initiated in GC B cells. All 3 mouse strains developed DLBCL at a frequency of 12% to 30% starting at age 13 months, leading to shorter survival. Immunohistochemical studies showed that all analyzed tumors were of the GC B-cell type. Exon sequencing revealed mutations reported in human DLBCL. Our data demonstrate that constitutive enforced expression of HGAL leads to DLBCL development.

MicroRNA signature in classical Hodgkin lymphoma

Classical Hodgkin lymphoma (cHL) is one of the most prevalent lymphomas with a unique cell composition compared to other lymphoma entities. Rare, malignant Hodgkin and Reed-Sternberg (HRS) cells embedded with an extensive but ineffective immune infiltration were previously characterized by a large number of genetic and epigenetic alterations. Recently, microRNA profiling studies highlighted the importance of small non-coding RNA in cHL. This review summarizes available literature data and provides a detailed comparison of four studies where cHL cell lines and microdissected HRS cells were used. Several microRNAs were found to be consistently up- (let-7-f, mir-9, mir-21, mir-23a, mir-27a, mir-155, and mir-196a) or downregulated (mir-138 and mir-150) in cHL. These deregulated microRNAs are involved in the processes crucial for cHL pathogenesis, such as impaired B cell development (mir-9, mir-150, and mir-155), NFκB hyperactivation (mir-155 and mir-196a), and immune evasion (mir-138). Therefore, the deregulation of microRNA expression can be considered a complementary mechanism to genetic alterations promoting lymphomagenesis. Moreover, the expression of let-7f, mir-9 and mir-27a is specific for cHL and can serve as a biomarker to distinguish this lymphoma from other B cell lymphomas. However, additional in-depth and high throughput analysis of microRNA expression in HRS cells is necessary to decipher the complete picture of microRNA in cHL.

Interplay between HGAL and Grb2 proteins regulates B-cell receptor signaling

Human germinal center (GC)-associated lymphoma (HGAL) is an adaptor protein expressed in GC B cells. HGAL regulates cell motility and B-cell receptor (BCR) signaling, processes that are central for the successful completion of the GC reaction. Herein, we demonstrate phosphorylation of HGAL by Syk and Lyn kinases at tyrosines Y80, Y86, Y106Y107, Y128, and Y148. The HGAL YEN motif (amino acids 107-109) is similar to the phosphopeptide motif pYXN used as a binding site to the growth factor receptor-bound protein 2 (Grb2). We demonstrate by biochemical and molecular methodologies that HGAL directly interacts with Grb2. Concordantly, microscopy studies demonstrate HGAL-Grb2 colocalization in the membrane central supramolecular activation clusters (cSMAC) following BCR activation. Mutation of the HGAL putative binding site to Grb2 abrogates the interaction between these proteins. Further, this HGAL mutant localizes exclusively in the peripheral SMAC and decreases the rate and intensity of BCR accumulation in the cSMAC. Furthermore, we demonstrate that Grb2, HGAL, and Syk interact in the same complex, but Grb2 does not modulate the effects of HGAL on Syk kinase activity. Overall, the interplay between the HGAL and Grb2 regulates the magnitude of BCR signaling and synapse formation.

Serine-threonine kinase ROCK2 regulates germinal center B cell positioning and cholesterol biosynthesis

Germinal center (GC) responses require B cells to respond to a dynamic set of intercellular and microenvironmental signals that instruct B cell positioning, differentiation, and metabolic reprogramming. RHO-associated coiled-coil-containing protein kinase 2 (ROCK2), a serine-threonine kinase that can be therapeutically targeted by ROCK inhibitors or statins, is a key downstream effector of RHOA GTPases. Although RHOA-mediated pathways are emerging as critical regulators of GC responses, the role of ROCK2 in B cells is unknown. Here, we found that ROCK2 was activated in response to key T cell signals like CD40 and IL-21 and that it regulated GC formation and maintenance. RNA-Seq analyses revealed that ROCK2 controlled a unique transcriptional program in GC B cells that promoted optimal GC polarization and cholesterol biosynthesis. ROCK2 regulated this program by restraining AKT activation and subsequently enhancing FOXO1 activity. ATAC-Seq (assay for transposase-accessible chromatin with high-throughput sequencing) and biochemical analyses revealed that the effects of ROCK2 on cholesterol biosynthesis were instead mediated via a novel mechanism. ROCK2 directly phosphorylated interferon regulatory factor 8 (IRF8), a crucial mediator of GC responses, and promoted its interaction with sterol regulatory element-binding transcription factor 2 (SREBP2) at key regulatory regions controlling the expression of cholesterol biosynthetic enzymes, resulting in optimal recruitment of SREBP2 at these sites. These findings thus uncover ROCK2 as a multifaceted and therapeutically targetable regulator of GC responses.

Recent BCR stimulation induces a negative autoregulatory loop via FBXO10 mediated degradation of HGAL

Regulating B-cell receptor (BCR) signaling after antigenic stimulation is essential to properly control immune responses. Currently known mechanisms of inhibiting BCR signaling are via co-receptor stimulation and downstream immunoreceptor tyrosine-based inhibition motif (ITIM) phosphorylation. Herein we demonstrate that BCR stimulation induces rapid and reversible palmitoylation of the SCF-FBXO10 ubiquitin E3 ligase. This results in FBXO10 relocation to the cell membrane, where it targets the human germinal center-associated lymphoma (HGAL) protein for ubiquitylation and degradation, leading to decreases in both BCR-induced calcium influx and phosphorylation of proximal BCR effectors. Importantly, FBXO10 recognition and degradation of HGAL is phosphorylation independent and instead relies on a single evolutionarily conserved HGAL amino acid residue (H91) and FBXO10 relocalization to the cytoplasmic membrane. Together our findings demonstrate the first evidence of negative BCR signaling regulation from direct BCR stimulation and define the temporospatial functions of the FBXO10-HGAL axis. FBXO10 is infrequently mutated in DLBCL but some of these mutations deregulate BCR signaling. These observations may have important implications on lymphomagenesis and other immune processes.

The transcriptional response of mouse spleen B cells to IL-4: Comparison to the response of human peripheral blood B cells

The Th2 cytokine IL-4 triggers a signaling cascade which activates transcription by STAT6. The goals of the present study are to define the transcriptomic response of mouse spleen B cells (mSBC) to IL-4 used as single stimulus, its specificity compared to human peripheral blood B cells (hPBBC) and to mouse spleen T cells (mSTC), and the pathways affected. Oligonucleotide-based microarrays were performed using two references, the untreated sample and the cells cultured without IL-4, an experimental design which reduces the potential confounding effect of cellular stress during culture. Specificity was addressed by comparing the response of mSBC and our previously published study on hPBBC, of similar design, and a study by other authors on mSTC. We detected an mSBC-specific response (including novel genes, e.g., Sertad4, Lifr, Pmepa1, Epcam, Tbxas1; and common genes, e.g., Usp2, Cst7, Grtp1, and Casp6), an hPBBC-specific response (e.g., CCL17, MTCL1, GCSAM, HOMER2, IL2RA), and a common mSBC/hPBBC response (e.g., CISH, NFIL3, SOCS1, VDR, CDH1). In contrast, the mSBC and mSTC responses were largely divergent. Gene set enrichment analysis (GSEA) was applied for the first time to identify the pathways affected. Both in mSBC and hPBBC, IL-4 activated Myc, the transcriptional machinery itself, cell cycle, mitochondria and respiratory chain, ribosome, proteasome and antigen presentation, and Wnt signaling, and inhibited GPCR signaling. However, significant differences were found in histone demethylation, Nod signaling, and Rho signaling, which were downregulated in mSBC, and in chromatin condensation, which was downregulated in hPBBC. These findings may have therapeutic implications for the treatment of allergic diseases and parasitic infections.

The TGFβ-induced lncRNA TBILA promotes non-small cell lung cancer progression in vitro and in vivo via cis-regulating HGAL and activating S100A7/JAB1 signaling

Long non-coding RNAs (lncRNAs) play critical roles in multiple cellular processes in non-small cell lung cancer (NSCLC); however, the involvement of lncRNAs in the transforming growth factor-beta (TGFβ) signaling pathway, the critical tumor cell epithelial-mesenchymal transition (EMT) and metastasis pathway, remains poorly understood. To address this issue, we compared the lncRNAs expression patterns of NSCLC cells treated with and without TGFβ1 treatment. We observed that one of the most prominent hits, TGFβ-induced lncRNA (TBILA), promoted NSCLC progression and was upregulated in tumor tissues. Upregulated TBILA promotes human germinal center-associated lymphoma (HGAL) expression by binding to the Smad transcription factor complex, thereby enhancing RhoA activation. In addition, TBILA induces the S100A7-c-Jun activation domain-binding protein 1 (JAB1) pathway by binding to nuclear S100A7 and enhances pro-survival pathways in NSCLC. These findings have provided us with a new perspective regarding the regulation of the TGFβ signaling pathway in NSCLC and suggest that the lncRNA TBILA can serve as a target for anticancer therapies.

Virus-encoded microRNA contributes to the molecular profile of EBV-positive Burkitt lymphomas

Burkitt lymphoma (BL) is an aggressive neoplasm characterized by consistent morphology and phenotype, typical clinical behavior and distinctive molecular profile. The latter is mostly driven by the MYC over-expression associated with the characteristic translocation (8;14) (q24; q32) or with variant lesions. Additional genetic events can contribute to Burkitt Lymphoma pathobiology and retain clinical significance. A pathogenetic role for Epstein-Barr virus infection in Burkitt lymphomagenesis has been suggested; however, the exact function of the virus is largely unknown.

In this study, we investigated the molecular profiles (genes and microRNAs) of Epstein-Barr virus-positive and -negative BL, to identify specific patterns relying on the differential expression and role of Epstein-Barr virus-encoded microRNAs.

First, we found significant differences in the expression of viral microRNAs and in selected target genes. Among others, we identified LIN28B, CGNL1, GCET2, MRAS, PLCD4, SEL1L, SXX1, and the tyrosine kinases encoding STK10/STK33, all provided with potential pathogenetic significance. GCET2, also validated by immunohistochemistry, appeared to be a useful marker for distinguishing EBV-positive and EBV-negative cases. Further, we provided solid evidences that the EBV-encoded microRNAs (e.g. BART6) significantly mold the transcriptional landscape of Burkitt Lymphoma clones.

In conclusion, our data indicated significant differences in the transcriptional profiles of EBV-positive and EBV-negative BL and highlight the role of virus encoded miRNA.

The RhoA-ROCK pathway in the regulation of T and B cell responses

Effective immune responses require the precise regulation of dynamic interactions between hematopoietic and non-hematopoietic cells. The Rho subfamily of GTPases, which includes RhoA, is rapidly activated downstream of a diverse array of biochemical and biomechanical signals, and is emerging as an important mediator of this cross-talk. Key downstream effectors of RhoA are the Rho kinases, or ROCKs. The ROCKs are two serine-threonine kinases that can act as global coordinators of a tissue’s response to stress and injury because of their ability to regulate a wide range of biological processes. Although the RhoA-ROCK pathway has been extensively investigated in the non-hematopoietic compartment, its role in the immune system is just now becoming appreciated. In this commentary, we provide a brief overview of recent findings that highlight the contribution of this pathway to lymphocyte development and activation, and the impact that dysregulation in the activation of RhoA and/or the ROCKs may exert on a growing list of autoimmune and lymphoproliferative disorders.

Isolated Follicles Enriched for Centroblasts and Lacking t(14;18)/BCL2 in Lymphoid Tissue: Diagnostic and Clinical Implications

We sought to address the significance of isolated follicles that exhibit atypical morphologic features that may be mistaken for lymphoma in a background of reactive lymphoid tissue. Seven cases that demonstrated centroblast-predominant isolated follicles and absent BCL2 staining in otherwise-normal lymph nodes were studied. Four of seven cases showed clonal B-cell proliferations amid a polyclonal B cell background; all cases lacked the IGH-BCL2 translocation and BCL2 protein expression. Although three patients had invasive breast carcinoma at other sites, none were associated with systemic lymphoma up to 44 months after diagnosis. The immunoarchitectural features of these highly unusual cases raise the question of whether a predominance of centroblasts and/or absence of BCL2 expression could represent a precursor lesion or atypical reactive phenomenon. Differentiating such cases from follicular lymphoma or another mimic is critical, lest patients with indolent proliferations be exposed to unnecessarily aggressive treatment.

GNA13 loss in germinal center B cells leads to impaired apoptosis and promotes lymphoma in vivo

GNA13 is the most frequently mutated gene in germinal center (GC)-derived B-cell lymphomas, including nearly a quarter of Burkitt lymphoma and GC-derived diffuse large B-cell lymphoma. These mutations occur in a pattern consistent with loss of function. We have modeled the GNA13-deficient state exclusively in GC B cells by crossing the Gna13 conditional knockout mouse strain with the GC-specific AID-Cre transgenic strain. AID-Cre(+) GNA13-deficient mice demonstrate disordered GC architecture and dark zone/light zone distribution in vivo, and demonstrate altered migration behavior, decreased levels of filamentous actin, and attenuated RhoA activity in vitro. We also found that GNA13-deficient mice have increased numbers of GC B cells that display impaired caspase-mediated cell death and increased frequency of somatic hypermutation in the immunoglobulin VH locus. Lastly, GNA13 deficiency, combined with conditional MYC transgene expression in mouse GC B cells, promotes lymphomagenesis. Thus, GNA13 loss is associated with GC B-cell persistence, in which impaired apoptosis and ongoing somatic hypermutation may lead to an increased risk of lymphoma development.

Analysis of human upstream open reading frames and impact on gene expression

The upstream open reading frame (uORF) is a post-transcriptional regulatory element in the 5' untranslated region (5'UTR), which modulates the translation levels of main open reading frame (mORF). Earlier studies showed that disturbed uORF-mediated translation control can result in drastic changes in translation levels of mORF, leading to genetic disorders. To date, there has been no systematic investigation into the relationship between variations in patients and uORF status. Here, taking the advantage of several datasets, including gene ontology (GO) annotations and sequence feature analysis, we have examined uORF impacts in human transcripts. GO annotations indicate that uORF-containing genes are enriched in certain features such as oncogenes and transcription factors. Sequence feature analysis reveals that uORF is a factor for determination of the translation initiation site (TIS) in human transcripts. We show that genes with uORFs have lower protein expression levels than genes without uORFs in multiple human tissues. Moreover, by examining three disease variation databases, we identified uORF-altering mutations from a total of 3,740,225 variations, which are highly suspected to be associated with changed levels of gene expression. For an experimental validation, we found four mutations with significant effects on protein expression but with only modest changes in transcription levels. These findings will provide researchers on related diseases with new insights into the importance of known mutations.

HGAL localization to cell membrane regulates B-cell receptor signaling

Human germinal center-associated lymphoma (HGAL) is specifically expressed only in germinal center (GC) B lymphocytes and GC-derived lymphomas. HGAL protein decreases lymphocyte motility by inhibiting the ability of myosin to translocate actin via direct interaction with F-actin and myosin II and by activating RhoA signaling via direct interactions with RhoA-specific guanine nucleotide exchange factors. HGAL protein also regulates B-cell receptor (BCR) signaling by directly binding to and enhancing Syk kinase activity and activation of its downstream effectors. Herein we demonstrate that HGAL protein can be myristoylated and palmitoylated and that these modifications localize HGAL to cellular membrane raft microdomains with distinct consequences for BCR signaling and chemoattractant-induced cell mobility. In BCR signaling, raft localization of HGAL facilitates interaction with Syk and modulation of the BCR activation and signaling, which induces HGAL phosphorylation and redistribution from lipid raft to bulk membrane and cytoplasm, followed by degradation. In contrast, HGAL myristoylation and palmitoylation avert its inhibitory effects on chemoattractant-induced cell motility. These findings further elucidate the growing and complex role of HGAL in B-cell biology and suggest that membrane-bound and cytoplasmic HGAL protein differently regulates distinct biological processes.

Ontology based molecular signatures for immune cell types via gene expression analysis

Background

New technologies are focusing on characterizing cell types to better understand their heterogeneity. With large volumes of cellular data being generated, innovative methods are needed to structure the resulting data analyses. Here, we describe an ‘Ontologically BAsed Molecular Signature’ (OBAMS) method that identifies novel cellular biomarkers and infers biological functions as characteristics of particular cell types. This method finds molecular signatures for immune cell types based on mapping biological samples to the Cell Ontology (CL) and navigating the space of all possible pairwise comparisons between cell types to find genes whose expression is core to a particular cell type’s identity.

Results

We illustrate this ontological approach by evaluating expression data available from the Immunological Genome project (IGP) to identify unique biomarkers of mature B cell subtypes. We find that using OBAMS, candidate biomarkers can be identified at every strata of cellular identity from broad classifications to very granular. Furthermore, we show that Gene Ontology can be used to cluster cell types by shared biological processes in order to find candidate genes responsible for somatic hypermutation in germinal center B cells. Moreover, through in silico experiments based on this approach, we have identified genes sets that represent genes overexpressed in germinal center B cells and identify genes uniquely expressed in these B cells compared to other B cell types.

Conclusions

This work demonstrates the utility of incorporating structured ontological knowledge into biological data analysis – providing a new method for defining novel biomarkers and providing an opportunity for new biological insights.

Germinal centre protein HGAL promotes lymphoid hyperplasia and amyloidosis via BCR-mediated Syk activation

The human germinal centre associated lymphoma (HGAL) gene is specifically expressed in germinal centre B-lymphocytes and germinal centre-derived B-cell lymphomas, but its function is largely unknown. Here we demonstrate that HGAL directly binds Syk in B-cells, increases its kinase activity upon B-cell receptor stimulation and leads to enhanced activation of Syk downstream effectors. To further investigate these findings in vivo, HGAL transgenic mice were generated. Starting from 12 months of age these mice developed polyclonal B-cell lymphoid hyperplasia, hypergammaglobulinemia and systemic reactive AA amyloidosis, leading to shortened survival. The lymphoid hyperplasia in the HGAL transgenic mice are likely attributable to enhanced B-cell receptor signalling as shown by increased Syk phosphorylation, ex vivo B-cell proliferation and increased RhoA activation. Overall, our study shows for the first time that the germinal centre protein HGAL regulates B-cell receptor signalling in B-lymphocytes which, without appropriate control, may lead to B-cell lymphoproliferation.

MicroRNAs and Glucocorticoid-Induced Apoptosis in Lymphoid Malignancies

The initial response of lymphoid malignancies to glucocorticoids (GCs) is a critical parameter predicting successful treatment. Although being known as a strong inducer of apoptosis in lymphoid cells for almost a century, the signaling pathways regulating the susceptibility of the cells to GCs are only partly revealed. There is still a need to develop clinical tests that can predict the outcome of GC therapy. In this paper, I discuss important parameters modulating the pro-apoptotic effects of GCs, with a specific emphasis on the microRNA world comprised of small players with big impacts. The journey through the multifaceted complexity of GC-induced apoptosis brings forth explanations for the differential treatment response and raises potential strategies for overcoming drug resistance.

Identification of LMO2 transcriptome and interactome in diffuse large B-cell lymphoma

LMO2 regulates gene expression by facilitating the formation of multipartite DNA-binding complexes. In B cells, LMO2 is specifically up-regulated in the germinal center (GC) and is expressed in GC-derived non-Hodgkin lymphomas. LMO2 is one of the most powerful prognostic indicators in diffuse large B-cell (DLBCL) patients. However, its function in GC B cells and DLBCL is currently unknown. In this study, we characterized the LMO2 transcriptome and transcriptional complex in DLBCL cells. LMO2 regulates genes implicated in kinetochore function, chromosome assembly, and mitosis. Overexpression of LMO2 in DLBCL cell lines results in centrosome amplification. In DLBCL, the LMO2 complex contains some of the traditional partners, such as LDB1, E2A, HEB, Lyl1, ETO2, and SP1, but not TAL1 or GATA proteins. Furthermore, we identified novel LMO2 interacting partners: ELK1, nuclear factor of activated T-cells (NFATc1), and lymphoid enhancer-binding factor1 (LEF1) proteins. Reporter assays revealed that LMO2 increases transcriptional activity of NFATc1 and decreases transcriptional activity of LEF1 proteins. Overall, our studies identified a novel LMO2 transcriptome and interactome in DLBCL and provides a platform for future elucidation of LMO2 function in GC B cells and DLBCL pathogenesis.

miR-155 regulates HGAL expression and increases lymphoma cell motility

HGAL, a prognostic biomarker in patients with diffuse large B-cell lymphoma and classic Hodgkin lymphoma, inhibits lymphocyte and lymphoma cell motility by activating the RhoA signaling cascade and interacting with actin and myosin proteins. Although HGAL expression is limited to germinal center (GC) lymphocytes and GC-derived lymphomas, little is known about its regulation. miR-155 is implicated in control of GC reaction and lymphomagenesis. We demonstrate that miR-155 directly down-regulates HGAL expression by binding to its 3'-untranslated region, leading to decreased RhoA activation and increased spontaneous and chemoattractant-induced lymphoma cell motility. The effects of miR-155 on RhoA activation and cell motility can be rescued by transfection of HGAL lacking the miR-155 binding site. This inhibitory effect of miR-155 suggests that it may have a key role in the loss of HGAL expression on differentiation of human GC B cells to plasma cell. Furthermore, this effect may contribute to lymphoma cell dissemination and aggressiveness, characteristic of activated B cell-like diffuse large B-cell lymphoma typically expressing high levels of miR-155 and lacking HGAL expression.

PRDM1/Blimp1 downregulates expression of germinal center genes LMO2 and HGAL

Human germinal center-associated lymphoma (HGAL) and LIM domain only-2 (LMO2) are proteins highly expressed in germinal center (GC) B lymphocytes. HGAL and LMO2 are also expressed in GC-derived lymphomas and distinguish biologically distinct subgroups of diffuse large B-cell lymphomas (DLBCL) associated with improved survival. However, little is known about their regulation. PRDM1/Blimp1 is a master regulator of terminal B cell differentiation and may also function as a tumor suppressor in the pathogenesis of DLBCL, where it is frequently inactivated by mutations and deletions. We now demonstrate that both HGAL and LMO2 are directly regulated by the transcription repressor PRDM1. In vivo studies demonstrate that PRDM1 directly binds to the recognition sites within the upstream promoters of both HGAL and LMO2. PRDM1 binding suppresses endogenous protein and mRNA levels of HGAL and LMO2. In addition, promoter analysis reveals that site-specific binding of PRDM1 to the promoters is capable of repressing transcriptional activity. This inhibitory effect of PRDM1 suggests that it has a key role in the loss of HGAL and LMO2 expression upon differentiation of GC B cells to plasma cells and may also contribute to absence of HGAL and LMO2 expression in post-GC lymphoid tumors.

Germinal center-specific protein human germinal center associated lymphoma directly interacts with both myosin and actin and increases the binding of myosin to actin

Human germinal center associated lymphoma (HGAL) is a germinal center-specific gene whose expression correlates with a favorable prognosis in patients with diffuse large B-cell and classic Hodgkin lymphomas. HGAL is involved in negative regulation of lymphocyte motility. The movement of lymphocytes is directly driven by actin polymerization and actin-myosin interactions. We demonstrate that HGAL interacts directly and independently with both actin and myosin and delineate the HGAL and myosin domains responsible for the interaction. Furthermore, we show that HGAL increases the binding of myosin to F-actin and inhibits the ability of myosin to translocate actin by reducing the maximal velocity of myosin head/actin movement. No effects of HGAL on actomyosin ATPase activity and the rate of actin polymerization from G-actin to F-actin were observed. These findings reveal a new mechanism underlying the inhibitory effects of germinal center-specific HGAL protein on lymphocyte and lymphoma cell motility.