Human B-lymphocytes express 5-lipoxygenase (5-LOX) and 5-LOX activating protein (FLAP) and can convert arachidonic acid to leukotriene B₄. Mantle cell lymphoma (MCL) cells contain similar amounts of 5-LOX as human neutrophils but the function and mechanism of activation of 5-LOX in MCL cells, and in normal B-lymphocytes, are unclear. Here we show that the intrinsic 5-LOX pathway in the MCL cell line JeKo-1 has an essential role in migration and adherence of the cells, which are important pathophysiological characteristics of B-cell lymphoma. Incubation of JeKo-1 with the FLAP inhibitor GSK2190915 or the 5-LOX inhibitor zileuton, at a concentration below 1 μM, prior to stimulation with the chemotactic agent CXCL12, led to a significant reduction of migration. CRISPR/Cas9 mediated deletion of ALOX5 gene in JeKo-1 cells also led to a significantly decreased migration of the cells. Furthermore, 5-LOX and FLAP inhibitors markedly decreased the adherence of JeKo-1 cells to stromal cells. In comparison, these drugs had a similar effect on adherence of JeKo-1 cells as the Bruton tyrosine kinase inhibitor ibrutinib, which has a proven anti-tumour effect. These results indicate that inhibition of 5-LOX may be a novel treatment for MCL and certain other B-cell lymphomas.

Sterile alpha motif and histidine-aspartate (HD) domain–containing protein 1 (SAMHD1) is a deoxynucleoside triphosphate triphosphohydrolase with ara-CTPase activity that confers cytarabine (ara-C) resistance in several hematological malignancies. Targeting SAMHD1’s ara-CTPase activity has recently been demonstrated to enhance ara-C efficacy in acute myeloid leukemia. Here, we identify the transcription factor SRY-related HMG-box containing protein 11 (SOX11) as a novel direct binding partner and first known endogenous inhibitor of SAMHD1. SOX11 is aberrantly expressed not only in mantle cell lymphoma (MCL), but also in some Burkitt lymphomas. Coimmunoprecipitation of SOX11 followed by mass spectrometry in MCL cell lines identified SAMHD1 as the top SOX11 interaction partner, which was validated by proximity ligation assay. In vitro, SAMHD1 bound to the HMG box of SOX11 with low-micromolar affinity. In situ crosslinking studies further indicated that SOX11-SAMHD1 binding resulted in a reduced tetramerization of SAMHD1. Functionally, expression of SOX11 inhibited SAMHD1 ara-CTPase activity in a dose-dependent manner resulting in ara-C sensitization in cell lines and in a SOX11-inducible mouse model of MCL. In SOX11-negative MCL, SOX11-mediated ara-CTPase inhibition could be mimicked by adding the recently identified SAMHD1 inhibitor hydroxyurea. Taken together, our results identify SOX11 as a novel SAMHD1 interaction partner and its first known endogenous inhibitor with potentially important implications for clinical therapy stratification.

Interactions between lymphoma cells and stromal cells play a key role in promoting tumor survival and development of drug resistance. We identified differences in key signaling pathways between the JeKo-1 and REC-1 mantle cell lymphoma (MCL) cell lines, displaying different patterns of stromal cell adhesion and chemotaxis towards stroma-conditioned medium. The identified adhesion-regulated genes reciprocated important aspects of microenvironment-mediated gene modulation in MCL patients. Five-hundred and ninety genes were differently regulated between the cell lines upon adhesion to stromal cells, while 32 genes were similarly regulated in both cell lines. Regulation of B-cell Receptor (BCR) signature genes in adherent cells was specific for JeKo-1. Inhibition of BCR using siRNA or clinically approved inhibitors, Ibrutinib and Acalabrutinib, decreased adhesion of JeKo-1, but not REC-1 cells. Cell surface levels of chemokine receptor CXCR4 were higher in JeKo-1, facilitating migration and adhesion of JeKo-1 but not REC-1 cells. Surface levels of ICAM1 adhesion protein differ for REC-1 and JeKo-1. While ICAM1 played a positive role in adherence of both cell lines to stromal cells, S1PR1 had an inhibitory effect. Our results provide a model framework for further investigation of mechanistic differences in patient-response to new pathway-specific drugs.

To survive chemotherapy, lymphoma cells can relocate to protective niches where they receive support from the non-malignant cells. The biolipid 2-arachidonoylglycerol (2-AG), an agonist for the cannabinoid receptors CB1 and CB2, is released by stromal cells in the bone marrow. To investigate the role of 2-AG in lymphoma, we analyzed the chemotactic response of primary B-cell lymphoma cells enriched from peripheral blood of twenty-two chronic lymphocytic leukemia (CLL) and five mantle cell lymphoma (MCL) patients towards 2-AG alone and/or to the chemokine CXCL12. The expression of cannabinoid receptors was quantified using qPCR and the protein levels visualized by immunofluorescence and Western blot. Surface expression of CXCR4, the main cognate receptor to CXCL12, was analyzed by flow cytometry. Phosphorylation of key downstream signaling pathways activated by 2-AG and CXCL12 were measured by Western blot in three MCL cell lines and two primary CLL samples. We report that 2-AG induces chemotaxis in 80% of the primary samples, as well as 2/3 MCL cell lines. 2-AG induced in a dose-dependent manner, the migration of JeKo-1 cell line via CB1 and CB2. 2-AG affected the CXCL12-mediated chemotaxis without impacting the expression or internalization of CXCR4. We further show that 2-AG modulated p38 and p44/42 MAPK activation. Our results suggest that 2-AG has a previously unrecognized role in the mobilization of lymphoma cells by effecting the CXCL12-induced migration and the CXCR4 signaling pathways, however, with different effects in MCL compared to CLL.

Chronic lymphocytic leukemia (CLL) cells depend on microenvironment niches for proliferation and survival. The adhesion of tumor cells to stromal cells in such niches triggers the activation of signaling pathways crucial for their survival, including B-cell receptor (BCR) signaling. While inhibitors of Bruton’s tyrosine kinase (BTKi) have shown efficacy in patients with CLL by disrupting these interactions, acquired resistance and toxicity remain a challenge during long-term therapy. Thus, identifying additional therapeutic modalities is important. Previously, we demonstrated that 5-lipoxygenase (5-LOX) pathway inhibitors reduced mantle cell lymphoma (MCL) cell adhesion to stromal cells, motivating us to investigate their potential in the context of CLL. We employed an ex vivo co-culture model to study CLL cell adhesion to stromal cells in the absence and presence of 5-LOX pathway inhibitors (zileuton and MK886) as well as the BTKi ibrutinib that was included for comparative purposes. Our findings demonstrated that different CLL samples adhere to stromal cells differentially. We observed a variable decrease in CLL cell adhesion to stromal cells following the inhibition of the 5-LOX pathway across a spectrum of patient samples that was distinct to the spectrum for ibrutinib. Positive and negative correlations were shown between the clinical and genetic features of the CLL samples and their level of adherence to stromal cells in both the absence and presence of the tested inhibitors. These results suggest the 5-LOX pathway as a candidate for assessment as a new therapeutic target in CLL.

Background: Mantle cell lymphoma (MCL) is an aggressive B-cell lymphoma with a high rate of relapses after therapy. Chronic lymphocytic leukemia (CLL) is a heterogeneous disease with varied outcome. For both diseases there is a need for new therapies. Cannabinoid receptors (CBs), which are overexpressed in most cases of MCL and CLL compared to normal B cells (Islam et al., 2003; Gustafsson et al., 2008; Freund et al., 2016) are promising novel therapeutic targets. CBs are membrane-bound receptors that convey signals from the microenvironment to the cells. There are two types of CBs: CB1 and CB2. CB1 is suggested to be involved in retention and/or egress of MCL cells from the tissue to the blood circulation (Wasik et al., 2014). CB2 is expressed by normal B-cells where it regulates positioning and retention of cells in tissue (Pereira et al., 2009; Basu et al., 2011; Muppidi et al., 2011) and in pre-B-cell acute lymphoblastic leukemia, involved in the energy metabolism (Chan et al., 2017). The retention/egress of the B-cell lymphoma cells is mainly regulated by chemokine receptors and adhesion molecules. The chemokine receptor CXCR4 is one of the most highly expressed chemokine receptors in MCL and CLL. 2-arachidonoylglycerol (2-AG, CB1/CB2 endogenous ligand) and CXCL12 (CXCR4 ligand) are synthetized and secreted by stromal cells in the bone marrow (Kose et al., 2018; Burger and Gribben, 2014). The endocannabinoids levels in cancer are suggested to have a role in cancer progression (Sailler et al., 2014) while CXCL12 is already a candidate target for therapy using a CXCR4 inhibitor AMD3100. Aim: To investigate a possible crosstalk between CBs and CXCR4 in MCL and CLL cells. Methods: Patients with newly diagnosed MCL (n=8) or CLL (n=25) gave informed consent to participate in the study. Lymphoma cells were enriched by negative selection. Fifteen primary lymphoma samples and the JeKo MCL cell line were subjected to chemotaxis towards CXCL12 and/or 2-AG. CXCR4 membrane expression was assessed by flow cytometry. Selective CB1 and CB2 antagonists were used to investigate the underlying mechanisms. CB1, CB2 and CXCR4 encoding genes levels were measured by qPCR and normalized to B cells from tonsil. Results and Conclusion: 2-AG induced chemotaxis in 11/15 MCL and CLL samples. In JeKo, 2-AG-induced migration was blocked by a CB2 antagonist, suggesting that signaling via CB2 is involved. When the primary cells were subjected to migration towards CXCL12, two patterns of chemotaxis were observed. The first pattern was seen in 7/15 samples that migrated towards CXCL12. In these samples, the migration was inhibited when 2-AG was combined with CXCL12. The second type of response was observed in 8/15 samples, those samples did not migrate towards CXCL12 but chemotaxis was enhanced by combining 2-AG and CXCL12. MCL and CLL samples expressed variable mRNA levels of CB1 (RFI range: 0.0-204) and CB2 (RFI range: 0.8-14.3) and all expressed CXCR4 at mRNA (RFI range: 0.1-215.8) and protein (MFI range: 1278-19301) levels that did not differ neither between the two diseases nor between the two migratory groups. When all 15 samples were combined, CB1 mRNA levels, but not CB2 mRNA, correlated to the chemotaxis towards CXCL12 (Spearman correlation coefficient = 0.626; p=0.01). In contrast, CB2 mRNA levels, but not CB1, correlated to chemotaxis towards 2-AG (Spearman correlation coefficient = 0.532; p=0.04), which is in agreement with the effects observed in JeKo. Furthermore, CB1 and CB2 mRNA levels correlated to chemotaxis towards the combination of CXCL12 and 2-AG both (for CB1 mRNA: Spearman correlation coefficient= 0.588; p=0.02 and for CB2 mRNA: 0.589; p=0.02). Neither CXCL12-induced CXCR4 receptor internalization, nor recycling was influenced by 2-AG incubation. Our findings indicate a novel pathway regulating chemotaxis of MCL and CLL implicating a cross-talk between CBs and CXCR4. The fact that the capacity to internalize CXCR4 remained intact after incubation with 2-AG suggests that the reduced CXCL12-mediated migration when 2-AG was combined could be due to an impaired downstream signaling in lymphoma cells. Importance: Lymphoma cells residing in the tissue receive pro-survival stimuli and are protected from chemotherapy by signals from the microenvironment. A better understanding of how lymphoma cell migration and tissue retention are regulated can be a step towards more efficient therapies. Disclosures: Wahlin: Gilead: Consultancy, Honoraria, Research Funding; Roche: Research Funding.