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Mantle cell lymphoma (MCL) and diffuse large B-cell lymphoma (DLBCL) are aggressive hematologic malignancies characterized by the accumulation of lymphoid cells defective in cell apoptosis biology and function. The anti-apoptotic B-cell lymphoma 2 (BCL-2) family proteins are pivotal regulators of the mitochondrial apoptotic pathway and genetic aberrations in these genes are associated with lymphomagenesis and chemotherapeutic resistance. Notably, the anti-apoptotic myeloid cell leukemia 1 (MCL-1) protein is recurrently highly expressed in various kinds of non-Hodgkin's B-cell lymphomas and promotes the survival of lymphoma cells by counteracting pro-apoptotic protein activity. Collectively, these data support the hypothesis that MCL-1 plays a central role in B-cell lymphoma progression and drug resistance. Pharmacologically targeting MCL-1, therefore, represents an attractive strategy to combat these lymphomas. However, previous clinical and pre-clinical data suggest that treatment with single agent anti-BCL-2 family member therapy is associated with rapid acquisition of resistance. To this end, there is a great need to develop and apply selective small molecule MCL-1 inhibitors as part of a first-line therapy or upon emergence of tumor resistance characterized by upregulation of MCL-1 for lymphoma therapy. Here, we exploited the MCL-1 dependency in MCL and DLBCL by implementing pharmacogenomic and chemical proteomic approaches to investigate the molecular drug response and resistance mechanism to MCL-1 inhibitors. In anticipation of the evolution of MCL-1 inhibitor resistance, we modeled MCL-1 inhibitor resistance mechanisms by developing S63845 resistant lines with high doses of S63845 treatment for an extended period in MCL, DLBCL and MCL-derived lines. RNA sequencing and chemical proteomics on paired parental and resistant cells demonstrated that transcriptome and kinome reprograming linked to the MEK and ERK pathways contribute to MCL-1 inhibitor resistance via regulation of the BCL-2 family profile (BCL-2 and BIM), and as such, represent a novel targetable vulnerability in MCL-1 inhibitor resistant lymphoma. Additional analyses revealed synergistic activity of MCL-1 inhibitors (S63845, AZD5991) in combinations with inhibitors of MEK (Trametinib), ERK (SCH772984) and BCR (Ibrutinib) in MCL-1 inhibitor resistant MCL/DLBCL lines and primary samples. These results provide a strong rationale for further evaluation of MCL-1 inhibitor in combination with established therapy in the clinical setting and highlight a potential strategy for overcoming MCL-1 inhibitor resistance. Disclosures Shah: NCCN: Vice-Chair, Acute Lymphoblastic Leukemia Working Group: Membership on an entity's Board of Directors or advisory committees; Kite/Gilead, Precision Biosciences, Novartis, AstraZeneca: Other: TRAVEL, ACCOMMODATIONS, EXPENSES; Kite/Gilead, Jazz, Incyte: Research Funding; Moffitt Cancer Center: Current Employment; Kite/Gilead, Celgene/Juno/BMS, Novartis, Pfizer, Amgen, Spectrum/Acrotech, Precision Biosciences, Beigene, AstraZeneca, Pharmacyclics/Jansen, Adaptive: Honoraria. Shain:AbbVie: Research Funding; GlaxoSmithKline: Speakers Bureau; Takeda: Honoraria, Speakers Bureau; Amgen: Speakers Bureau; Sanofi/Genzyme: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Adaptive: Consultancy, Honoraria; Janssen: Honoraria, Speakers Bureau; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Karyopharm: Research Funding, Speakers Bureau.

Problem: Multiple myeloma (MM) is a treatable yet incurable hematologic cancer that lacks predictive biomarkers. Approach: Here we apply a systems biology approach to determine patient-specific mechanisms, as well as signatures of drug resistance in MM. To achieve this goal, we have combined ex vivo drug sensitivity data from 307 MM fresh primary samples tested with 162 drugs and combinations, with paired molecular data (RNAseq and mutational profiling) from a larger overlapping cohort of 606 MM samples from Moffitt's Multiple Myeloma Working Group (MMWG) repository in collaboration with M2Gen/Oncology Research Information Exchange Network (ORIEN). With the purpose of decoupling biological function from intracellular control mechanisms, we have re-constructed a MM-specific transcriptional regulatory network composed of clusters of co-expressing genes. We demonstrate how this gene cluster network regulates biology, and how different biological functions (e.g. Proteasome, Ribosome, Oxidative Phosphorylation) share common regulatory circuits. We have used gene set enrichment analysis (GSEA) to identify gene clusters with transcriptional profiles, and investigated mutations associated with drug resistance. Results: As a preliminary validation of this approach, we have confirmed established mechanisms of resistance (MOR) to targeted therapies, as well as proposed novel MOR to clinically relevant and experimental drugs in MM, as well as putative synergistic drug combinations. In addition, we have identified a list of low frequency mutations ( We have also explored evolution of drug resistance in sequential samples. Consistent with altered transcriptional programming in therapeutic escape, single sample GSEA demonstrated cumulative dysregulation of cancer-related genes with increasing lines of therapy. We have identified 60 MM-specific transcriptional core auto-regulatory circuits (CRC) correlated with ex vivo drug resistance, suggesting that characterization of transcriptional regulatory circuits is a critical approach to infer mechanisms of MM resistance, and providing a novel rationale for combination therapy. We hypothesized that identifying and targeting these transcriptional CRCs could facilitate patient-specific rational combination therapies, with the goal to overcome therapy resistance in the clinic. As proof-of-principle, we have identified a novel transcriptional network consisting of 3 of these CRCs (FOXP1, JUNB and JUN) associated with BCL2 inhibitor (BCL2i) response in MM. Our preliminary data suggests that this transcriptional regulatory circuit is associated to t(11;14) MM through CCND1 up-regulation, but is also present in non-t(11;14) BCL2i-sensitive primary samples, and can be modulated to induce BCL2i sensitivity in non-t(11;14) MM through HDAC inhibitors. Conclusion and next steps: Preliminary results confirm the potential of this combination of unsupervised and supervised, yet functionally testable approach, to infer novel, and patient-specific MOR for MM drugs. Disclosures Dai: M2Gen: Employment. Dalton:MILLENNIUM PHARMACEUTICALS, INC.: Honoraria. Shain:Bristol-Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees; Sanofi Genzyme: Membership on an entity's Board of Directors or advisory committees; AbbVie: Research Funding; Adaptive Biotechnologies: Consultancy; Janssen: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees.

Mantle cell lymphoma (MCL) is an aggressive and incurable B-cell malignancy. Prognosis remains poor due to emergence of drug resistance and MCL progression. Ibrutinib is a novel B-cell receptor (BCR) downstream bruton's tyrosine kinase (BTK) inhibitor with high response rates reported in MCL patients. The initial success of ibrutinib is likely attributed to attenuation of BCR-dependent lymphoma-tumor microenvironment (TME) interactions. Moreover, despite the dramatic responses to ibrutinib, ibrutinib resistance (IR) inevitably develops. Remarkably, once patients relapse after ibrutinib treatment, MCL patients experience disease progression and die within 12 months. Thus, there is an urgent need to define mechanisms of ibrutinib resistance (IR) and to identify novel targets to bring forward novel treatment options with real curative potential for this fatal complication. Currently the mechanisms driving IR are poorly understood and no recurrent driver mutations have been identified in MCL. We have modeled acquired resistance to ibrutinib and implemented chemical proteomics and a cell-based drug screen approaches in IR MCL lines and primary samples, we have shown that MCL cells become resistant to ibrutinib through a kinome-adaptive reprogramming mechanism that lead to constitutive activation of the PI3K/AKT/mTOR pathway with increased levels of Myc and sustained transcription activation. Significantly, the acquired IR MCL cells have increased rates of growth and augmented adhesion to stromal cells. Collectively, our published and preliminary studies indicate that IR MCL relies on global transcriptome remodeling and subsequent kinome reprogramming, leading to molecularly and clinically aggressive phenotype and resistance to ibrutinib therapy. Rather than there being a single mechanism of acquired IR, kinase networks are rewired in a plethora of ways in MCL cells as they become resistant to ibrutinib treatment. Adaptive remodeling of the kinome creates therapeutic challenges, where even combination targeted kinase inhibitor treatments are unlikely to be successful. We reasoned instead that global adaptive kinome as a whole must be blocked for overcoming IR. Sustained transcriptional activation and associated adaptive signaling changes drive the malignant phenotype of IR and, accordingly, that targeting the transcriptional machinery responsible for conferring IR triggers synthetic lethality in MCL. We leveraged powerful genomic and pharmacoproteomic approaches available to our laboratory and IR model to identify the key upstream genetic and epigenetic driver for IR. Ultimately, we designed transcription activation based combination therapy to overcome drug resistance and hinder MCL progression. In addition, performing cell-based drug screen assay on primary MCL cells of MCL patients allowed us to predict drug response and tailor therapeutic drugs for individual patients, thus, creating a personalized therapeutic strategy for MCL patients Disclosures No relevant conflicts of interest to declare.

Background: c-MYC is a transcription factor that promotes oncogenesis by activating and repressing its target genes that control cell growth, metabolism, and proliferation. MYC is deregulated in a large proportion of aggressive B-cell lymphomas. A typical example is the Double-Hit Lymphoma (DHL) and Double-Expression Lymphoma (DEL) which present with a rapidly progressing clinical course, refractory to treatment, poor clinical outcome, and currently considered incurable. Nevertheless, MYC is considered as an "undruggable" target since it has no "active site" amenable to binding by conventional small molecule inhibitors. Moreover, MYC has a broad spectrum of functions in cell proliferation, survival, metabolism, and others, so direct inhibition would likely cause severe side effects. Besides direct inhibition, another practical strategy is to target druggable proteins that are essential for the viability of MYC-driven tumors, inducing MYC-dependent "synthetic lethality". The advantage of such approach is a capability of killing tumor cells discriminately, while leaving non-tumor cells intact or less influenced. This study is designed to identify such targets and explore practical novel strategies to treat MYC-driven lymphomas, especially DHL/DEL. Methods and Results: By integrating activity-based proteomic profiling and drug screens in isogenic MYC on/off lymphoma cells, we identified polo-like kinase-1 (PLK1) as an essential regulator of the MYC-dependent kinome in DHL/DEL. Notably, PLK1 was expressed at high levels in DHL, correlated with MYC expression and connoted poor outcome. Further, PLK1 is directly activated by MYC on transcriptional level and in turn, PLK1 signaling augmented MYC protein stability by promoting its phosphorylation and suppressing its degradation. Thus, MYC and PLK1 form a feed-forward circuit in lymphoma cells. Finally, both in vitro and in vivo studies demonstrated that inhibition of PLK1 triggered degradation of MYC and of the anti-apoptotic protein MCL1, and PLK1 inhibitors showed synergy with BCL-2 antagonists in blocking DHL/DEL cell growth, survival, and tumorigenicity. These data support that PLK1 is a promising therapeutic target in MYC-driven lymphomas. Brief summary: Functional pharmacoproteomics identified PLK1 as a therapeutic vulnerability for MYC-driven lymphoma, which was a synthetic lethal for DHL/DEL when targeted with BCL-2 inhibitors. Disclosures Vose: Roche: Honoraria; Merck Sharp & Dohme Corp.: Research Funding; Acerta Pharma: Research Funding; Seattle Genetics, Inc.: Research Funding; Novartis: Honoraria, Research Funding; Kite Pharma: Research Funding; Bristol Myers Squibb: Research Funding; Epizyme: Honoraria; Legend Pharmaceuticals: Honoraria; Abbvie: Honoraria; Celgene: Research Funding; Incyte Corp.: Research Funding.

Follicular dendritic cells (FDCs), an essential component of the lymph node microenvironment, regulate and support B-lymphocyte differentiation, survival, and lymphoma progression. Here, we demonstrate that adhesion of mantle cell lymphoma and other non-Hodgkin lymphoma cells to FDCs reduces cell apoptosis and is associated with decreased levels of the proapoptotic protein, Bim. Bim down-regulation is posttranscriptionally regulated via up-regulation of microRNA-181a (miR-181a). miR-181a overexpression decreases, whereas miR-181a inhibition increases Bim levels by directly targeting Bim. Furthermore, we found that cell adhesion–up-regulated miR-181a contributes to FDC-mediated cell survival through Bim down-regulation, implicating miR-181a as an upstream effector of the Bim-apoptosis signaling pathway. miR-181a inhibition and Bim upregulation significantly suppressed FDC-mediated protection against apoptosis in lymphoma cell lines and primary lymphoma cells. Thus, FDCs protect B-cell lymphoma cells against apoptosis, in part through activation of a miR-181a–dependent mechanism involving down-regulation of Bim expression. We demonstrate, for the first time, that cell-cell contact controls tumor cell survival and apoptosis via microRNA in mantle cell and other non-Hodgkin lymphomas. Regulation of microRNAs by B-cell–FDC interaction may support B-cell survival, representing a novel molecular mechanism for cell adhesion–mediated drug resistance and a potential therapeutic target in B-cell lymphomas.

Abstract 5094 A dynamic interaction occurs between the lymphoma cell and its microenvironment, with each profoundly influencing the behavior of the other. Here, we demonstrate that adhesion of mantle cell lymphoma (MCL) and other non-Hodgkin lymphoma cells to lymph node stromal cells, follicular dendritic cells (FDCs), confers drug resistance, enhances lymphoma cell clonogenicity, and is associated with induction of histone deacetylase 6 (HDAC6). Furthermore, stroma down-regulated miR-548m contributes to HDAC6 up-regulation, and HDAC6 is a key determinant for FDC-mediated lymphoma cell survival and colony formation. We further showed that stroma-mediated drug resistance and clonogenic growth are reversed by enforced expression of miR-548m and inhibition of HDAC6. The HDAC6-selective inhibitor tubastatin significantly enhances cell death, abolishes cell adhesion-mediated drug resistance, and suppresses clonogenicity and lymphoma growth suppression ex vivo and in vivo. Together, these data suggest that the lymphoma–stroma interaction in lymph node microenvironment directly impacts the biology of lymphoma through epigenetic regulation of miRNA and HDAC, with HDAC6 as a potential therapeutic target to overcome drug resistance in MCL and other B-cell lymphomas. Disclosures: No relevant conflicts of interest to declare.

Abstract 4618 c-Myc (hereafter Myc) overexpression has been recognized in aggressive B-cell lymphomas and linked to adverse prognosis. Myc activation results in widespread repression of miRNA expression and lymphoma aggressive progression. Our recent study identified a Myc-miRNAs-EZH2 feed-forward loop linking over-expression of Myc, EZH2, and miRNA repression. Here, using a novel small-molecule bromodomain inhibitor, JQ1, to silence Myc and the EZH2 inhibitor DZNep, we demonstrated that combined inhibition of Myc and EZH2 cooperatively disrupted Myc activation, resulting in restoration of miR-26a expression, which subsequently suppressed lymphoma growth and clonogenicity in aggressive lymphoma cells. Furthermore, Myc and EZH2 cooperatively regulated miR-26a expression in aggressive lymphoma cells. MiR-26a functioned as a tumor suppressor miRNA and mediated the combinatorial effect of JQ1 and DZNep. These findings represent a promising, novel approach for silencing Myc with JQ1 for combinatorial therapy of aggressive B-cell lymphomas. Disclosures: No relevant conflicts of interest to declare.

Abstract 4918 Despite intensive efforts in developing new therapies, B-cell lymphoma remains essentially incurable with standard therapeutic approaches. Dynamic interactions between the lymphoma cell and its microenvironment play a critical role in lymphoma development and response to therapy. There is accumulating evidence pointing to an essential role of BCR signaling in B-cell lymphomas. Recently, inhibitors of BCR signaling have become an area of substantial clinical interest. We therefore studied the role of BCR signal pathways in stroma-mediated lymphoma cell survival. We demonstrated that adhesion of the various B-cell lymphoma cell lines to lymph node stromal cells enhanced activation of BCR signaling pathways: activation of phosphatidylinositol 3-kinase (PI3K), Bruton's tyrosine kinase (Btk) and extracellular signal-regulated kinase (ERK) pathways. Inhibition of Btk by PCI 32765 or PI3K by CAL101 significantly blocked BCR signal pathways, triggered lymphoma cell apoptosis and overcame stroma-mediated drug resistance. Furthermore, targeting BCR signal pathway disrupted microenvironmental stroma-lymphoma interaction through regulation of CXCR4 expression. Collectively, these data support that BCR activation not only controls intrinsic survival pathway related to B lymphoma cell but also regulates stroma-mediated extrinsic lymphoma cell survival as well as lymphoma cell homing and interplay with its microenvironment. As a result, this study suggests that targeting BCR signal pathway molecules is a promising therapeutic strategy to lymphoma therapy. Disclosures: No relevant conflicts of interest to declare.

Abstract 3853 B-cell differentiation is tightly regulated by synchronized suppression and/or induction of specific transcription factors. Among them, B-cell lymphoma 6 (BCL6) and PRDM1 are considered to be master regulators for germinal center formation and terminal B-cell differentiation. Dysregulation of BCL6 and PRDM1 also have been associated with lymphomagenesis. Their regulation still need further study especially at the posttranscriptional level. Here, by using co-culture system and whole genomic microRNA microarray profiling, we show for the first time that direct B lymphoma cell-stroma cell contact between follicular dendritic cells and B-lymphocytes could induce upregulation of miR-30 family and downregulation of miR-9 and let-7 family. In silico analysis showed that miR-30s can target genes BCL6 and miR-9/let7 can target PRDM1 with direct binding sites in 3`UTR region of their mRNAs. The microarray data can be proved by microRNA specific Q-RT-PCR. Specifically, by both gain of function and loss of function studies, we functionally verified that FDCs Regulate Expression of BCL6 and PRDM1 via Cell-Cell Direct Contact induced correlated microRNA dysregulation. To further validate the direct interaction between BCL6 and miR-30, we constructed luciferase reporters containing the BCL6 3`-UTR that included miR-30 binding sites and a mutant 3`-UTR harboring mutations in the “seed pairing” sequences of the miR-30 binding site. Co-transfection of miR-30 and reporter construct into cells significantly decreased luciferase activity in wild-type but not in mutant BCL6-3`-UTR transfected cells, supporting the role of miR-30 family in the regulation of BCL6 expression. BCL6 and PRDM1 and their regulation miRNAs, let-7 and miR-30, also can be validated in primary normal B-lymphocytes and lymphoma cells by using our co-culture system. Dysregulation of BCL6 and PRDM1 is often associated with lymphomagenesis. We firstly identified that BCL6 is the direct target of miR-30 family and also verified PRDM1 is the target of miR-9, and let-7 in our system. Our studies provide a novel mechanism of post-transcriptional regulation of BCL6 and PRDM1 by several microRNAs. In the context of micro-environment, it provides a clue for germinal center B-cell differentiation as well as B-cell lymphomas progression regulated by lymphocyte cell-stroma cell contact through microRNAs. Disclosures: No relevant conflicts of interest to declare.

Abstract 3122 Adult T-cell leukemia/lymphoma (ATL) is an aggressive, fatal malignancy of CD4+ T cells that is etiologically associated with infection by human T-cell leukemia virus-type. ATL has been classified into four main subtypes, indolent smoldering, chronic forms, acute and lymphomatous forms. Most individuals in the chronic stage of ATL eventually undergo progression to a highly aggressive acute stage. Compelling evidence supports a pivotal role for altered tumor suppressor genes involved in leukemogenesis and lymphomagenesis of ATL as well as progression of the disease from the chronic/smoldering to the acute/lymphomatous type. Further investigations are needed as to how the alteration of cell cycle regulatory genes are involved in the aberrant growth of HTLV-infected T cells. The emerging role of microRNAs in tumor development and viral infection has prompted investigations on the role of microRNA in ATL pathogenesis and progression. We therefore performed microRNA microarrays on 16 lymphomatous tissues of ATL patients and normal CD4 lymphocytes of 5 healthy donors. After normalization, we identified a set of aberrantly expressed miRNAs in ATL lymphoma samples when compared with control CD4 lymphocytes including 49 upregulated microRNAs and 89 downregulated microRNAs. MiR-150, miR-24, miR-221, miR-222, miR-142-5p, miR-92b, miR-93, miR-181a, miR-103, miR-107 stands out as highly elevated miRNAs in ATL samples while miR-30b, miR-34a, miR-20b, miR-196b, miR-361-5p, miR-520b/e are found downregulated in ATLL samples. Using gain-and-loss experimentations, we demonstrated miR-181a overexpression decreases, whereas miR-181a inhibition increases a proapoptotic protein, Bim, levels by directly targeting Bim. Furthermore computational analysis, and previous studies revealed that that miR-214, miR-22, miR-93 increased AKT activity by repressing PTEN expression, and miR-221/222 and miR-93 promoted cell proliferation through inhibition of CDKN1B (p27). Overall, this study demonstrated a distinct set of miRNAs that are known to be critical in cellular transformation, representing a novel molecular mechanism and potential therapeutic targets in ATL lymphoma. Disclosures: No relevant conflicts of interest to declare.

Abstract 3121 MicroRNAs (miR)-15a and miR-16-1 are known to act as tumor suppressors. Expression of these miRNAs inhibits cell proliferation, promotes apoptosis of cancer cells, and suppresses tumorigenicity by targeting multiple oncogenes. Loss or down-regulation of these miRNAs has been reported in a variety of hematopoietic and solid tumors. Our recent study demonstrated that miR-15a/16-1 are down-regulated in the majority of patients with mantle cell lymphoma (MCL, Zhao et al., Blood. 2010 1;115(13):2630-9). However, the mechanism responsible for miR-15a/16 suppression was unknown. Here, we have investigated mechanism of miR-15a/16-1 repression and its transcriptional and epigenetic regulation by Myc and histone deacetylase (HDAC) in MCL. Over-expression of Myc protein was detected in all MCL cell lines, and miR-15a/16 and DLEU-2 mRNA were significantly up-regulated in Mino and Jeko-1 cells when Myc was depleted by c-Myc siRNA. Treatment with pan HDAC inhibitor, SAHA and specific HDAC-3 shRNA resulted in increase of miR15a/16 expression. Co-immunoprecipitation study showed that c-Myc interacted with HDAC3. Moreover, chromatin immunoprecipitation (ChIP)demonstrated that both Myc and HDAC3 co-localized to the two promoters of miR-15a/16 cluster gene, DLEU-2. Luciferase reporter assay confirmed that both c-Myc and HDAC3 inhibited transcriptional activity of two E-box (Myc-binding)-specific DLEU-2 promoters. Computational analysis and mutagenesis study further supported direct association of Myc with miR-15a/16 promoter resulting in HDAC3-mediated transcriptional repression. These findings highlight an important role for HDAC-3 in Myc-driven lymphoma cell proliferation and reveal novel transcriptional and posttranscriptional mechanisms in Myc-mediated malignant transformation in MCL. Disclosures: No relevant conflicts of interest to declare.

Abstract 3118 Follicular dendritic cells (FDCs), essential components of the lymph node microenvironment, regulate and support B lymphocyte differentiation, survival, and lymphoma progression. Here, we demonstrated that adhesion of mantle cell and other non-Hodgkin lymphoma cells to FDCs reduced cell apoptosis, associated with decreased levels of Bim, a pro-apoptotic protein. Bim down-regulation was epigenetically regulated via up-regulation of microRNA-181a (miR-181a). miR-181a overexpression decreased, whereas miR-181a inhibition increased Bim levels by directly targeting Bim. Furthermore, we found that cell adhesion-up-regulated miR-181a contributes to FDC-mediated cell survival through Bim down-regulation, implicating miR-181a as an upstream effector of the Bim-apoptosis signaling pathway. miR-181a inhibition and Bim up-regulation significantly suppressed FDC-mediated protection against apoptosis in lymphoma cell lines and primary lymphoma cells. Thus, FDCs protect B-cell lymphoma cells against apoptosis, in part through activation of a miR-181a-dependent mechanism involving down-regulation of Bim expression. We demonstrate, for the first time, that cell-cell contact controls tumor cell survival and apoptosis via microRNA in mantle cell and other non-Hodgkin lymphomas. Regulation of microRNAs by B-cell-FDC interaction may support B-cell survival, representing a novel molecular mechanism for cell-adhesion mediated drug resistance (CAM-DR) and a potential therapeutic target in B-cell lymphomas. Disclosures: No relevant conflicts of interest to declare.

Abstract 1957 Poster Board I-980 Mantle cell lymphoma (MCL) is now recognized as a distinct clinicopathologic subtype of B-cell lymphomas and carries the poorest prognosis among NHL subtypes with a median survival is 3 to 4 years. Clearly better understanding of MCL oncogenesis and novel therapeutic approaches to treating MCL are needed. Mounting evidence now shows that small noncoding microRNAs (miRNAs) can play roles as oncogenes or tumor suppressor genes, suggesting their contribution to cancer development and progression. Here, we for the first time, demonstrated that that miR-29a-c is frequently deleted or down-regulated and IGF-1R is often over-expressed in primary mantle cell lymphoma cells. Expression of miR-29 inversely correlates with the expression of IGF-1R. Consistent with these results, 1) overexpression of miR-29-c down-regulated IGF-R1, 2) overexpression of miR-29c inhibited cell growth, colony formation and induced cell cycle arrest in MCL cells, 3) depletion of miR-29-c by anti-miR29-c increased the threshold for apoptosis, 4) enforced expression of miR-29c suppressed MCL growth and tumor progression in NOD/SCID mouse xenograft models, 5) miR-29 directly regulates IGF-1R expression . Furthermore cell cycle arrest induced by these miRNAs depends on the expression of IGF-1R. Our results indicate that miR-29a-c are implicated in MCL cell cycle control, survival and likely contribute to the tumorigenesis of MCL. Disclosures: No relevant conflicts of interest to declare.

Abstract 4588 B-cell differentiation process is tightly regulated by suppression or induction of specific transcription factors. Among various transcriptional regulators, BCL6 and PRDM-1 are master regulators for germinal center (GC) formation and terminal B-cell differentiation. Dysregulation of BCL6 and PRDM-1 have been associated with lymphomagenesis. However how these transcription factors are regulated and what determines their expression are unclear. Given that follicular dendritic cells (FDC) closely interact with B cells within the GC, provide survival signal to protect B cells from apoptosis and are essential for the differentiation of GC B cells, we used an in vitro FDC-B-cell co-culture model to explore the role of FDC-B cell interaction and FDC-induced miRNA in the regulation of BCL6 and PRDM-1 expression. In this study 1) we revealed that follicular dendritic cells (FDCs, HK) regulate expression of transcription factor (BCL6, and PRDM1) via cell-cell contact, 2) we showed that FDCs regulate expression of B-cell survival and differentiation-related microRNAs, 3) we demonstrated that microRNAs regulate expression of transcription factors BCl6 and PRDM1 and 4) we documented that follicular dendritic cells regulate expression of transcription factor (BCL6, and PRDM1) through microRNAs and plays an important role in B-differentiation. These studies establish new molecular mechanisms for regulation of BCL6 and PRDM-1. FDC-induce miRNA mediated down- and up-regulation of transcriptional factors may contribute to the phenotype maintenance of GC, and pathogenesis of non-Hodgkin's lymphoma (NHL) by interfering with normal B-cell terminal differentiation. Disclosures: No relevant conflicts of interest to declare.

Patients with mantle cell lymphoma (MCL) are typically older adults with a male predominance and usually present with stage IV disease. Response to chemotherapy usually results in a tumor response but remissions are short. It carries the poorest prognosis among non-Hodgkin’s lymphoma subtypes and the median survival is 3 to 4 years. However, recent clinical observations have identified variable clinical courses of MCL patients: some patients succumb rapidly to the disease, while others have a more chronic course and may not require treatment for prolonged periods of time. Given this clinical heterogeneity, we performed microRNA expression profiling on 34 mantle cell lymphoma (MCL) biopsy specimens to elucidate its pathogenesis and develop prognostic markers for survival of MCL patients. We identified downregulation of 18 miRNAs and upregulation of 22 miRNAs in MCL when compared with normal peripheral blood B lymphocytes, which is closely associated with molecular abnormalities. Furthermore, we found that expression levels of miR-29 family (miR-29a, miR-29b and miR-29c) were inversely correlated with MCL patient survival. A strong correlation between miR-29a, miR-29b and miR-29c values and newly described prognostic index for MCL patients, MIPI is identified. This first microRNA profling in MCL demonstrated that the miR-29a-c could be useful biomarkers for MCL prognosis and aberrant miRNA expression could contribute molecular MCL oncogenesis.

Stromal cells are an essential component of the bone marrow microenvironment that regulate or supports tumor survival. The bone marrow stroma has long been recognized as a "sanctuary site" for lymphoma cells during traditional chemotherapy. In this study, we demonstrated that adhesion of the B-cell lymphoma cell lines SUDH-4 and 10 to bone marrow stroma inhibited mitoxantrone-induced apoptosis. This adhesion-dependent inhibition of mitoxantrone-induced apoptosis correlated with decreased activation of caspases- 8 and 9, and cleavage of caspase 3 and PARP. EMSA analysis demonstrated significantly increased NF-kB binding activity in cells adhered to stroma cells compared to lymphoma cells in suspension. This DNA binding activity could be attributed to cell adhesion-mediated proteolysis of p100 NF-kappaB2 resulted in generation of active p52, which translocates to the nucleus in complex with p65 and RelB. Furthermore, co-culture with stromal cells also induced expression of the NF-kB-regulated anti-apoptotic molecules, XIAP, cIAP1 and cIAP2. In addition, because of the knowledge that BAFF shown to be critical for maintenance of normal B cell homeostasis as well as the survival of malignant B cells, we characterized the functional significance of BAFF in BMS-mediated drug resistance and survival. BAFF was detected on BMS cell line HS-5 and BMS cells derived from lymphoma patients by flow cytometry. Concentrations of BAFF were 3- to 25-fold higher in bone marrow aspirate than in peripheral blood. Lymphoma cell adhesion to HS-5 cells significantly increased BAFF secretion evidenced by both ELISA and immunoblotting. Addition of BAFF counteracted mitoxantrone-induced apoptosis, and elicited a reduction in spontaneous apoptosis in primary lymphomas via activation of NF-kB activation. Finally neutralization of BAFF by TACI enhanced lymphoma cell response to chemotherapy and overcame cell-adhesion mediated drug resistance, suggesting that lymphoma cells utilize BAFF as survival factor. These data indicate that stromal cells prevent apoptosis of lymphoma cells by upregulation of BAFF and induction of NF-kB-regulated anti-apoptotic proteins. Bone marrow-derived BAFF plays a key role in lymphoma cell survival and drug resistance in bone marrow microenvironment.

Mounting evidence now suggests that dynamic interactions between the tumor cell and its local and systemic microenvironment play a critical role in tumor development and that all of the clinical properties of a tumor, including cell cycle progression and response to therapy, depend heavily on the tumor stroma. However, the molecular mechanisms of the interaction between stroma and tumor cells are not well defined. In this study we used mantle cell lymphoma (MCL) as a model to characterize the mechanisms by which stroma regulate cell-cycle progression. We demonstrated that adhesion of MCL and other non-Hodgkin lymphoma (NHL) cells to bone marrow stroma cell (HS-5) resulted in a reversible G1 arrest associated with elevated p27Kip1 protein level. The adhesion mediated p27Kip1 increase was posttranslational and regulated through down-regulating Skp2, a substrate-recognition subunit of SCFSkp2 ubiquitin ligase. Overexpression of wild-type Skp2 in MCL decreases p27Kip1, whereas inhibition of Skp2 expression by siRNA increases p27Kip1 level. Furthermore, we found that cell adhesion up-regulated Cdh1, an activating subunit of anaphase-promoting complex (APC) ubiquitin ligase, and reducing Cdh1 by siRNA induced Skp2 accumulation and hence p27Kip1 degradation, implicating Cdh1 as an upstream effector of Skp2/p27Kip1 signaling pathway. Overall, this is a first report demonstrating that the bone marrow stroma controls tumor cell cycle via ubiquitin-proteasome proteolytic pathways in MCL and other NHL. The understanding of this novel molecular pathway may prove valuable in designing new therapeutic approaches modifying tumor cell growth and response to therapy.