ALBERT

Description: Myeloid cell leukaemia-1 (Mcl-1) is an anti- apoptotic member of the Bcl-2 family commonly expressed in multiple myeloma (MM). Drugs (e.g. bortezomib) can induce a 28kD Mcl-1 fragment (Mcl-1Δ128–350) in a caspase- dependent manner, which induces inhibition of MM cell proliferation and apoptosis. Here we sought to delineate molecular sequelae downstream of Mcl-1Δ128–350 which mediate its anti- proliferative and pro-apoptotic effects in MM and other malignant cells. Our results demonstrate that exogenous Mcl-1Δ128–350 induces upregulation and nuclear accumulation of c-Jun, as well as generation of a pro-apoptotic 60kD c-Abl fragment (c-Abl Δ). Bortezomib treatment triggered c-Jun upregulation in Mcl-1wt/wt, but not Mcl-1 null, murine embryonic fibroblasts (MEFs), and neither transfection with exogenous c-Jun nor with exogenous 60kD c-Abl Δ triggered the generation of Mcl-1Δ128–350. Moreover, drug-induced generation of Mcl-1Δ128–350 was not abrogated by specific transient knockdown of c-Jun or c-Abl by small interfering RNA, further supporting the requirement of Mcl-1Δ128–350 for c-Jun upregulation. Our studies also identified mechanisms downstream of upregulated c-Jun which trigger inhibition of MM cell proliferation and apoptosis. Interestingly and similar to c-Jun and c-Abl Δ, Mcl-1Δ128–350 accumulates within the nuclear fraction. Indeed, interaction of Mcl-1Δ128–350 with c-Jun, as well as subsequent enhanced AP-1 reporter activity, demonstrate a direct regulatory role of Mcl-1Δ128–350 in c-Jun- dependent gene transcription. Finally, gene profiles in MM cells transfected with either Mcl-1wt or Mcl-1Δ128–350 identify differentially expressed genes associated with MM cell proliferation, survival and drug resistance. Taken together, these data both delineate the role of Mcl-1 in MM pathogenesis and further support targeting Mcl-1 in novel MM treatment strategies.

Description: Agents targeting not only myeloma cells directly but also bone marrow stromal cells (BMSCs), endothelial cells, and osteoclasts (OCLs) that cause enhancement of tumor cell growth, angiogenesis, and MM bone disease, respectively, are promising new treatment modalities for multiple myeloma. Here we investigated the novel, orally available multi-kinase inhibitor BAY 73-4506 (BAY), currently in phase II clinical trials, for its therapeutic effect in MM. BAY is a potent inhibitor of angiogenic (VEGFR 1–3, PDGFR-β), as well as oncogenic, (cKIT, RET, FGFR, Raf) kinases We first tested the ability of BAY to suppress proliferation and survival in a wide array of MM cell lines, including those resistant to conventional chemotherapeutics. Our data show that BAY, in a low micromolar range that is well below concentrations achieved in patient plasma during the first clinical trial in solid tumors, induces apoptosis by caspase-9 and caspase-3 activation in all cell lines tested. Importantly, BAY also overcomes the growth advantage conferred in a BMSC-MM, as well as an endothelial cell-MM, co-culture system. BAY treatment abrogates growth factor-stimulated MEK, ERK and AKT phosphorylation at sub-micromolar concentrations. Since the VEGF signaling pathway is a potent inducer of angiogenesis and BAY targets VEGFR 1–3, we examined its anti-angiogenic properties. BAY inhibits endothelial cell growth and endothelial cell tubule formation in vitro at concentrations less than 1μM; moreover, it also markedly inhibited VEGF-induced cell migration on fibronectin. Activation of MAP kinase is a critical event during OCL differentiation, activation, and survival; and importantly, BAY also inhibits osteoclastogenesis, evidenced by blockade of M-CSF/RANKL-triggered differentiation of mononuclear cells to TRAP-positive osteoclasts. Finally, BAY significantly delays tumor growth and abrogates blood vessel formation in vivo in a xenograft mouse model of human MM. These in vitro and in vivo results provide the basis for further clinical evaluation of BAY to improve patient outcome in MM.

Description: In multiple myeloma (MM) protein kinase C (PKC) signaling pathways have been implicated in cell proliferation, survival, and migration. Here we investigated the novel, orally available PKC-inhibitor enzastaurin for its anti-MM activity. Enzastaurin specifically inhibits phorbol ester–induced activation of PKC isoforms, as well as phosphorylation of downstream signaling molecules MARCKS and PKCμ. Importantly, it also inhibits PKC activation triggered by growth factors and cytokines secreted by bone marrow stromal cells (BMSCs), costimulation with fibronectin, vascular endothelial growth factor (VEGF), or interleukin-6 (IL-6), as well as MM patient serum. Consequently, enzastaurin inhibits proliferation, survival, and migration of MM cell lines and MM cells isolated from multidrug-resistant patients and overcomes MM-cell growth triggered by binding to BMSCs and endothelial cells. Importantly, strong synergistic cytotoxicity is observed when enzastaurin is combined with bortezomib and moderate synergistic or additive effects when combined with melphalan or lenalidomide. Finally, tumor growth, survival, and angiogenesis are abrogated by enzastaurin in an in vivo xenograft model of human MM. Our results therefore demonstrate in vitro and in vivo efficacy of the orally available PKC inhibitor enzastaurin in MM and strongly support its clinical evaluation, alone or in combination therapies, to improve outcome in patients with MM.

Description: Both monoallelic and biallelic oncogenic NRAS mutations are identified in human leukemias, suggesting a dose-dependent role of oncogenic NRAS in leukemogenesis. Here, we use a hypomorphic oncogenic Nras allele and a normal oncogenic Nras allele (Nras G12Dhypo and Nras G12D, respectively) to create a gene dose gradient ranging from 25% to 200% of endogenous Nras G12D/+. Mice expressing Nras G12Dhypo/G12Dhypo develop normally and are tumor-free, whereas early embryonic expression of Nras G12D/+ is lethal. Somatic expression of Nras G12D/G12D but not Nras G12D/+ leads to hyperactivation of ERK, excessive proliferation of myeloid progenitors, and consequently an acute myeloproliferative disease. Using a bone marrow transplant model, we previously showed that ∼ 95% of animals receiving Nras G12D/+ bone marrow cells develop chronic myelomonocytic leukemia (CMML), while ∼ 8% of recipients develop acute T-cell lymphoblastic leukemia/lymphoma [TALL] (TALL-het). Here we demonstrate that 100% of recipients transplanted with Nras G12D/G12D bone marrow cells develop TALL (TALL-homo). Although both TALL-het and -homo tumors acquire Notch1 mutations and are sensitive to a γ-secretase inhibitor, endogenous Nras G12D/+ signaling promotes TALL through distinct genetic mechanism(s) from Nras G12D/G12D. Our data indicate that the tumor transformation potential of endogenous oncogenic Nras is both dose- and cell type-dependent.

Description: The generation of hematopoietic stem cells (HSCs) via endothelial-to-hematopoietic transition within the aorta-gonad-mesonephros (AGM) region of the mammalian embryo is crucial for development of the adult hematopoietic system. Many questions remain unanswered regarding the molecular program in hemogenic endothelium that promotes the budding of hematopoietic cell clusters containing HSCs. Previously, we described a deletion of a Gata2 cis-element (+9.5) that depletes fetal liver HSCs, is lethal at E13-14 of embryogenesis, and is mutated in an immunodeficiency that progresses to myelodysplasia (MDS)/leukemia. In contrast to Gata2 knockout mice, which die around E10.5 because of anemia, the prolonged embryonic development of +9.5 site knockout mice provides a unique model system to investigate the potential roles for GATA-2 in HSC production, migration and function, and more specifically, the requirement for the +9.5 element to regulate Gata2 expression during these processes. Using an ex vivo system involving culturing intact AGM, or AGM dissociated into single cells and then reaggregated into an organoid, we demonstrated that the +9.5 deletion reduced Gata2 expression in uncultured AGM (1.4 fold, p

Description: Abstract 4183 The oncogenic NRAS mutations are frequently identified in myeloid diseases but rare in lymphoid diseases. They occur in 4% of acute T-cell lymphoblastic leukemia/lymphoma (T-ALL) patients and 22% of human T-ALL cell lines. Its differential roles in myeloid versus lymphoid disease development remain unclear. Here we examine the tumorigenic potential of oncogenic Nras in T-cells using two conditional Nras G12D murine knock-in models that either hypomorphically (NrasG12D Hypo) or normally (NrasG12D Norm) expresses oncogenic Nras G12D from its endogenous locus. Mice expressing monoallelic or biallelic NrasG12D Hypo develop normally and are tumor free. However, NrasG12D Norm leads to acute T-cell leukemia/lymphoma (TAL/L) in a bone marrow transplantation model, with a low incidence (∼8%) when expressing one allele (TAL/L-het) and a complete penetrance when expressing two alleles (TAL/L-homo). TAL/L-het tumors are associated with spontaneous up-regulation of oncogenic Nras in ∼67% of animals, and tumor cells are TdT positive, suggesting that they are transformed at an immature stage. In contrast, TAL/L-homo tumors express comparable levels of Nras to control thymocytes, and tumor cells are TdT negative, suggesting that they are transformed at a more mature stage. Both TAL/L-het and TAL/L-homo tumors are oligoclonal or polyclonal. Above 70% of these tumors contain clonal Notch1 mutations and are sensitive to gamma-secretase inhibitor. These data indicate that Notch1 mutations are acquired at an early stage and play an important role in the development of TAL/L-het and TAL/L-homo tumors. Together, our results show that engdogenous oncogenic Nras mutation leads to TAL/L in a dose-dependent manner, and thus explain the low incidence of oncogenic NRAS mutations in human T-cell diseases. Disclosures: No relevant conflicts of interest to declare.

Description: Small GTPases regulate multiple signaling pathways and individual Ras member can have distinct biological function. To overcome embryonic lethality of Kras-deficient mice, we generated and examined mice with hematopoietic- and T cell-specific deletion of Kras. In VavCreKrasfl/fl mice with hematopoietic deletion of Kras, thymic T-cell development was normal based on the presence of normal populations of total, CD4- CD8-, CD4+ CD8+, CD4+ and CD8+ thymocytes. The populations of splenic CD4+ and CD8+ T cells were also comparable between VavCreKrasfl/fl relative to control mice. In addition, no consistent defects in the 3 H-thymidine incorporation rate of Kras-deficient splenic CD4+ or CD8+ T cells in response to anti-CD3 or anti-CD3 plus IL-2 was detected. Nonetheless, we studied the effect of Kras deficiency on CD8 T-cell immune response to acute infection of the Armstrong strain of lymphocytic choriomeningitis virus (LCMV). Sub-lethally irradiated Rag1-deficient mice transplanted with bone marrow (BM) cells from VavCreKrasfl/fl or control mice were subjected to LCMV infection. Infection-induced expansion of CD8 T cells and generation of LCMV epitope gp33-specific CD8 T cells were markedly reduced in the recipients that received the BM from VavCreKrasfl/fl relative to control mice. Following in vitro stimulation with the LCMV epitope gp33, the induction of IFNg-expressing CD8 T cells from LCMV-infected recipients that received the BM from VavCreKrasfl/fl mice was dramatically reduced. Further, BM chimeric mice with CD8 T cell-specific deficiency generated by transplantation of lethally irradiated CD8 T cell-depleted CD45.1 congenic mice with a mixture of BM cells from VavCreKrasfl/fl mice and BM cells from CD8 T cell-deficient mice exhibited an impaired CD8 T-cell immune response to LCMV infection. Lastly, we examined the role of Kras in TCR signaling. The level of total TCR-activated Ras (Ras-GTP) was markedly reduced in Kras-deficient relative to control CD8 T cells. Importantly, TCR-induced ERK1/2 activation was impaired in Kras-deficient relative to control CD8 T cells. Consistently, TCR-induced activation of Raf-1 and MEK1/2 was markedly reduced in mutant CD8 T cells. However, TCR-induced JNK and p38 activation as well as Ca2+ flux were normal in Kras-deficient CD8 T cells. Of note, TCR-induced activation of Ca2+ flux, JNK and p38 as well as ERK1/2, MEK1/2 and Raf1 was normal in Kras-deficient relative to control CD4 cells. Taken together, these data demonstrate that Kras is dispensable for T cell development or TCR-induced proliferation of CD4 or CD8 T cells in vitro, but regulates TCR-induced activation of the Raf-1/MEK/ERK pathway in CD8 but not CD4 T cells and intrinsically controls CD8 T-cell immune response to viral infection. Disclosures No relevant conflicts of interest to declare.

Description: As members of small GTPase super family, the functional output of Ras proteins depends on their GTP binding status, which is regulated by the interactions with guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs). Activating mutations in NRAS and KRAS isoforms are identified in various types of hematopoietic malignancies. Interestingly, the same oncogenic mutation (G12D) at the endogenous Kras locus displays much more potent leukemogenic activity than that at the endogenous Nras locus in vivo. Moreover, combined inhibition of MEK and ERK provides long-term disease-free survival in NrasG12D/G12D mice but had much less effect in KrasG12D/+ mice. During our investigation to understand the potent leukemogenic activity of oncogenic Kras, we found that in total bone marrow cells, oncogenic Kras, but not oncogenic Nras, induces hyperactivation of wild-type (WT) Hras and Nras. We hypothesize that the hyperactivated WT Ras significantly contributes to oncogenic Kras-mediated leukemogenesis and inhibition of this process might improve the sensitivity of oncogenic Kras cells towards combined therapy. Because Sos1, a RAS GEF, has been implicated in oncogenic Ras-mediated activation of WT Ras in human cancer cell lines, we investigated whether Sos1 plays an essential role in this process in vivo. We find that Sos1 is overexpressed in KrasG12D/+ bone marrow cells. Genetic deletion of Sos1 indeed significantly decreases the GTP-bound active form of WT Nras and Hras without affecting the activation status of oncogenic Kras. Consequently, Sos1 deficiency-mediated downregulation of ERK activation rescues oncogenic Kras mediated depletion of hematopoietic stem cells (HSCs). HSCs, multipotent progenitors (MPPs) and LSKs (Lin-Sca-1+c-Kit+) in KrasG12D/+;Sos1-/- mice are much more quiescent than those in KrasG12D/+ mice. Moreover, Sos1 deficiency significantly inhibits granulocyte-macrophage colony stimulating factor (GM-CSF) evoked ERK signaling in KrasG12D/+ myeloid progenitor and precursor cells. Consistent with these biochemical data, we show that myeloproliferative neoplasm (MPN) phenotypes are significantly alleviated in KrasG12D/+;Sos1-/- mice and these animals survived significantly longer than KrasG12D/+ mice. However, we find that in differentiated myeloid cells (e.g. neutrophils), loss of Sos1 does not affect GM-CSF-evoked ERK activation. This result is consistent with our previous finding that Ras-mediated ERK activation in differentiated myeloid cells is predominantly through Kras but not Hras or Nras. Together, our results demonstrate that Sos1 mediates oncogenic Kras-induced hyperactivation of WT Ras. Inhibition of Sos1 thus blocks this process and attenuates the leukemogenic activity of oncogenic Kras. In contrast, Sos1 deficiency does not affect the unique signaling mediated by oncogenic Kras itself. Therefore, we hypothesize that targeting Sos1 alone will not effectively treat KrasG12D-associated leukemias but it might increase the sensitivity of KrasG12D cells to other therapies, such as combined inhibition of MEK and JAK. We are currently testing this hypothesis in vivo. Disclosures No relevant conflicts of interest to declare.

Description: Recent studies have demonstrated a role for the master regulator of hematopoiesis GATA-2 in MonoMAC Syndrome, a human immunodeficiency disorder associated with myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). Though GATA2 coding region and cis-regulatory element mutations underlie MonoMAC syndrome, many questions remain unanswered regarding how GATA-2 is controlled physiologically and how it is dysregulated in pathological contexts. We dissected how a T354M mutation in the GATA-2 DNA binding zinc finger, which is frequently detected in MonoMAC syndrome and familial MDS/AML, alters GATA-2 activity. The T354M mutation reduced GATA-2 chromatin occupancy, induced GATA-2 hyperphosphorylation, and disrupted GATA-2 subnuclear localization. These molecular phenotypes also characterized an additional familial MDS/AML-associated GATA-2 mutant (Δ355T). T354M hyperphosphorylation and ectopic subnuclear localization were detected in hematopoietic and non-hematopoietic cell lines. We developed a new model system in mouse aortic endothelial (MAE) cells to quantitate GATA-2 activity to regulate endogenous target genes. T354M exhibited significantly reduced activity in this assay (GATA-2: 200-fold activation; T354M: 7.7-fold activation). Mass spectrometric analysis of the phosphorylation states of GATA-2 and T354M revealed that the T354M mutation enhanced phosphorylation at several GATA-2 residues. Analysis of single phosphorylation site mutants indicated that only mutation of S192 (S192A) abolished T354M-induced hyperphosphorylation. The S192A mutation attenuated phosphorylation of sites within wild-type GATA-2 and reduced transactivation activity (50% decrease, p 〈 0.01). A distinct 60 amino acid (aa) region within the GATA-2 N-terminus was required for T354M hyperphosphorylation and ectopic subnuclear localization. Deletion of this sequence decreased GATA-2 transactivation activity (60 aa deletion: 85% decrease, p 〈 0.01; 10 aa deletion: 45% decrease, p 〈 0.05). GATA-1 lacks an analogous subnuclear targeting sequence, and accordingly, a GATA-1(T263M) mutant, which corresponds to the GATA-2(T354M) mutant, localized normally and was not hyperphosphorylated. However, a GATA-1 chimera containing the GATA-2 subnuclear targeting sequence localized to ectopic subnuclear foci in a T263M-dependent manner. The GATA-2 N-terminus endowed GATA-1 with the capacity to induce GATA-2 target genes. By contrast, a GATA-2 chimera containing the GATA-1 N-terminus exhibited normal subnuclear localization. Thus, the leukemogenic T354M mutation utilizes the GATA-2-specific subnuclear targeting sequence to disrupt the normal subnuclear localization pattern, and this disruption is associated with S192-dependent hyperphosphorylation. In addition to its involvement in AML, GATA-2 interfaces with RAS signaling to promote the development of non-small cell lung cancer. We discovered that RAS signaling promotes S192-dependent GATA-2 hyperphosphorylation and ectopic subnuclear localization and propose that GATA-2 is an important component in oncogenic RAS-dependent leukemogenesis, which is being formally tested using innovative mouse models. In summary, dissecting the mechanistic deficits of a leukemogenic GATA-2 mutant revealed unexpected insights into mechanisms underlying physiological GATA-2 function and GATA-2-dependent hematologic pathologies. Disclosures: No relevant conflicts of interest to declare.

Description: Notch signaling pathway contributes to the pathogenesis of a wide spectrum of human cancers, including hematopoietic malignancies. Its functions highly depend on the specific cellular context. Gain-of-function NOTCH1 mutations are prevalent in human T cell leukemia, while loss of Notch signaling is reported in myeloid leukemias. Here, we report a novel oncogenic function of Notch signaling in oncogenic Kras-induced myeloproliferative neoplasm (MPN). We used genetic approaches to down-regulate Notch signaling in KrasG12D/+mice, which develop both T-ALL and MPN. Down-regulation of Notch signaling in hematopoietic cells is achieved through Mx1-Cre-mediated conditional expression of Rosa26-GFP-dnMAML1, which inhibits canonical Notch signaling, or conditional knockout of Pofut1, which catalyzes O-fucosylation of Notch receptors and modulates Notch receptor ligand interactions. To determine whether down-regulating Notch signaling prevents T-ALL and/or promotes MPN in a cell-autonomous manner, we transplanted the same number of KrasG12D/+, KrasG12D/+; Rosa26GFP-dnMAML1/+, or KrasG12D/+; Pofut1-/- bone marrow cells (CD45.2+) along with congeneic competitor cells (CD45.1+) into lethally irradiated mice (CD45.1+). As expected, inhibiting Notch signaling significantly blocked T-cell development and completely prevented T-ALL development in recipients; T-ALL that developed in a fraction of recipient mice were derived from rare donor cells that expressed oncogenic Kras and preserved intact Notch signaling. Surprisingly, we found that the percentage of donor-derived myeloid cells was significantly lower in recipients transplanted with KrasG12D/+; Rosa26GFP-dnMAML1/+ or KrasG12D/+; Pofut1-/- bone marrow cells and consequently none of them developed donor-derived MPN-like disease. In contrast, ~20% of the recipient mice transplanted with KrasG12D/+cells developed a lethal, donor-derived MPN (P=0.02). Because the hematopoietic stem cell (HSC) frequency was significantly lower in KrasG12D/+; Rosa26GFP-dnMAML1/+bone marrow than that in KrasG12D/+ bone marrow, we investigated whether the absence of MPN was due to the reduced HSC reconstitution in recipients. To normalize for HSC numbers, we transplanted lethally irradiated mice with same number of KrasG12D/+ or KrasG12D/+; Rosa26GFP-dnMAML1/+ splenocytes, which contained similar numbers of HSCs mobilized from the bone marrow. Consistent with our previous observation, only 1 out of 21 recipient mice with KrasG12D/+; Rosa26GFP-dnMAML1/+ cells developed a donor-derived (KrasG12D/+; Rosa26GFP-dnMAML1/+) MPN disease, while 6 out of 12 recipient mice with KrasG12D/+cells died with donor-derived MPN (P=0.002). Of note, in 3 T-ALL free recipients with KrasG12D/+; Rosa26GFP-dnMAML1/+cells, although mutant cells were dominant in hematopoietic tissues, none of them displayed MPN phenotypes over the time. Together, our results indicate that blocking Notch signaling inhibits both T-ALL and MPN development in a cell-autonomous manner. Mechanistically, analysis of donor-derived hematopoiesis revealed that loss of Notch signaling significantly reduced myeloid compartment in Kras recipients with significant decrease of proliferation and cytokine-evoked ERK1/2 activation in Kras myeloid progenitor (MP) cells. RNA-Seq analysis of control and mutant MPs suggest that overexpression of DUSP1, a dual phosphatase that inactivates ERK, might lead to reduced ERK signaling in KrasG12D/+; Rosa26GFP-dnMAML1/+ MPs. Moreover, Kras MPs exhibited enhanced oxidative phosphorylation and mitochondria respiration, and this aberrant gene expression pattern was largely restored by DNMAML expression. These metabolic changes were further validated in functional assays. Our results demonstrate an essential role of Notch in oncogenic Kras-induced MPN through modulating ERK signaling and mitochondrial metabolism. Disclosures No relevant conflicts of interest to declare.