Vesicular trafficking is a key determinant of the statin response in acute myeloid leukemia

Cholesterol homeostasis has been proposed as one mechanism contributing to chemoresistance in AML and hence, inclusion of statins in therapeutic regimens as part of clinical trials in AML has shown encouraging results. Chemical screening of primary human AML specimens by our group led to the identification of lipophilic statins as potent inhibitors of AMLs from a wide range of cytogenetic groups. Genetic screening to identify modulators of the statin response uncovered the role of protein geranylgeranylation and of RAB proteins, coordinating various aspect of vesicular trafficking, in mediating the effects of statins on AML cell viability. We further show that statins can inhibit vesicle-mediated transport in primary human specimens, and that statins sensitive samples show expression signatures reminiscent of enhanced vesicular trafficking. Overall, this study sheds light into the mechanism of action of statins in AML and identifies a novel vulnerability for cytogenetically diverse AML.

Mubritinib Targets the Electron Transport Chain Complex I and Reveals the Landscape of OXPHOS Dependency in Acute Myeloid Leukemia

To identify therapeutic targets in acute myeloid leukemia (AML), we chemically interrogated 200 sequenced primary specimens. Mubritinib, a known ERBB2 inhibitor, elicited strong anti-leukemic effects in vitro and in vivo. In the context of AML, mubritinib functions through ubiquinone-dependent inhibition of electron transport chain (ETC) complex I activity. Resistance to mubritinib characterized normal CD34⁺ hematopoietic cells and chemotherapy-sensitive AMLs, which displayed transcriptomic hallmarks of hypoxia. Conversely, sensitivity correlated with mitochondrial function-related gene expression levels and characterized a large subset of chemotherapy-resistant AMLs with oxidative phosphorylation (OXPHOS) hyperactivity. Altogether, our work thus identifies an ETC complex I inhibitor and reveals the genetic landscape of OXPHOS dependency in AML.

Abstract 1509: RUNX1 dosage dictates gene signature and in vitro response to glucocorticoids in acute myeloid leukemia

Background: Poor prognosis subgroups of Acute Myeloid Leukemia (AML), such as RUNX1-mutated (RUNX1mut) AML, would greatly benefit from more efficacious therapies that target leukemic stem cells (LSC) and improve patient outcome. To understand factors that predict sensitivity to drugs, we used a chemogenomic approach to interrogate primary AML specimens.

Methods: We performed RNA sequencing of 415 primary AML specimens comprising various cytogenetic subgroups. Using culture conditions that support LSC activity ex vivo, we carried out a viability screen including 20 primary AML specimens and ~5,100 low molecular weight compounds. RUNX1mut specimens showed increased sensitivity to glucocorticoid compounds (GCs). Validation screens were done in 248 primary AML samples and 32 AML cell lines treated with selected GCs in a dose-response manner. The effect of RUNX1 dosage in the in vitro response to drugs was assessed by shRNA gene knockdown. GCs target, the glucocorticoid receptor (GR), was validated by chemical blockage and shRNA silencing.

Results: RUNX1mut specimens were associated with older age, French-American-British (FAB) M0 morphology, intermediate-risk cytogenetics with abnormal karyotype and poor patient survival. RUNX1mut gene expression signature showed the overexpression of previously described genes such as DNTT, BAALC and CD34, as well as novel genes such as PROM1 and EGFEM1P. Most interestingly, the expression levels of these genes was influenced by the nature of the RUNX1 mutations, with levels progressively increasing with mutations corresponding to decreased levels of functional RUNX1. Chemical screens comprising 33 RUNX1mut specimens confirmed that RUNX1mut are more sensitive to GCs than RUNX1 wild-type samples. In agreement with the RUNX1mut gene signature, the anti-proliferative effect of GCs anti-correlated with levels of functional RUNX1. Specimens harboring loss-of-function and dominant-negative RUNX1 mutations showed increased sensitivity to GCs when compared to samples carrying missense mutations expected to have little impact on RUNX1 function. Mutations in other genes, such as CEBPA and SRSF2, had an additive effect on GC sensitivity when combined with RUNX1 mutations. In accordance with our hypothesis, the downregulation of RUNX1 could reverse GC-resistance in AML cell lines, and the sensitivity to compounds was proportional to knockdown levels. Treatment of cell lines with the GR antagonist RU486 blocked the inhibitory response induced by GCs and GR silencing completely abrogated the anti-proliferative effects of GCs in GC-sensitive cells, confirming that GC operate through the GR in AML.

Conclusion: Altogether, these findings highlight the impact of RUNX1 dosage on gene expression and GCs sensitivity in AML cells in vitro. Further studies should investigate the benefits of repositioning GCs in the treatment of RUNX1mut AML patients.

Citation Format: Laura Simon, Vincent-Philippe Lavallée, Marie-Eve Bordeleau, Jana Krosl, Irène Baccelli, Geneviève Boucher, Bernhard Lehnertz, Tara MacRae, Réjean Ruel, Sébastien Lemieux, Anne Marinier, Josée Hébert, Guy Sauvageau. RUNX1 dosage dictates gene signature and in vitro response to glucocorticoids in acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1509. doi:10.1158/1538-7445.AM2017-1509

A novel approach for the identification of efficient combination therapies in primary human acute myeloid leukemia specimens

Appropriate culture methods for the interrogation of primary leukemic samples were hitherto lacking and current assays for compound screening are not adapted for large-scale investigation of synergistic combinations. In this study, we report a novel approach that efficiently distills synthetic lethal interactions between small molecules active on primary human acute myeloid leukemia (AML) specimens. In single-dose experiments and under culture conditions preserving leukemia stem cell activity, our strategy considerably reduces the number of tests needed for the identification of promising compound combinations. Initially conducted with a selected library of 5000 small molecules and 20 primary AML specimens, it reveals 5 broad classes of sensitized therapeutic target pathways along with their synergistic patient-specific fingerprints. This novel method opens new avenues for the development of AML personalized therapeutics and may be generalized to other tumor types, for which in vitro cancer stem cell cultures have been developed.

High-throughput screening in niche-based assay identifies compounds to target preleukemic stem cells

Current chemotherapies for T cell acute lymphoblastic leukemia (T-ALL) efficiently reduce tumor mass. Nonetheless, disease relapse attributed to survival of preleukemic stem cells (pre-LSCs) is associated with poor prognosis. Herein, we provide direct evidence that pre-LSCs are much less chemosensitive to existing chemotherapy drugs than leukemic blasts because of a distinctive lower proliferative state. Improving therapies for T-ALL requires the development of strategies to target pre-LSCs that are absolutely dependent on their microenvironment. Therefore, we designed a robust protocol for high-throughput screening of compounds that target primary pre-LSCs maintained in a niche-like environment, on stromal cells that were engineered for optimal NOTCH1 activation. The multiparametric readout takes into account the intrinsic complexity of primary cells in order to specifically monitor pre-LSCs, which were induced here by the SCL/TAL1 and LMO1 oncogenes. We screened a targeted library of compounds and determined that the estrogen derivative 2-methoxyestradiol (2-ME2) disrupted both cell-autonomous and non-cell-autonomous pathways. Specifically, 2-ME2 abrogated pre-LSC viability and self-renewal activity in vivo by inhibiting translation of MYC, a downstream effector of NOTCH1, and preventing SCL/TAL1 activity. In contrast, normal hematopoietic stem/progenitor cells remained functional. These results illustrate how recapitulating tissue-like properties of primary cells in high-throughput screening is a promising avenue for innovation in cancer chemotherapy.

The methyltransferase G9a regulates HoxA9-dependent transcription in AML

Lehnertz et al. identify the histone methyltransferase G9a as a selective regulator of fast proliferating myeloid progenitors with no discernible function in hematopoietic stem cells. Loss of G9a significantly delays disease progression and reduces leukemia stem cell frequency in mouse models of acute myeloid leukemia. G9a interacts with the leukemogenic transcription factor HoxA9 and regulates HoxA9-dependent transcription. These results highlight G9a inhibition as a means to counteract the proliferation and self-renewal of AML cells.

Chromatin modulators are emerging as attractive drug targets, given their widespread implication in human cancers and susceptibility to pharmacological inhibition. Here we establish the histone methyltransferase G9a/EHMT2 as a selective regulator of fast proliferating myeloid progenitors with no discernible function in hematopoietic stem cells (HSCs). In mouse models of acute myeloid leukemia (AML), loss of G9a significantly delays disease progression and reduces leukemia stem cell (LSC) frequency. We connect this function of G9a to its methyltransferase activity and its interaction with the leukemogenic transcription factor HoxA9 and provide evidence that primary human AML cells are sensitive to G9A inhibition. Our results highlight a clinical potential of G9A inhibition as a means to counteract the proliferation and self-renewal of AML cells by attenuating HoxA9-dependent transcription.

Identification of small molecules that support human leukemia stem cell activity ex vivo

Leukemic stem cells (LSCs) are considered a major cause of relapse in acute myeloid leukemia (AML). Defining pathways that control LSC self-renewal is crucial for a better understanding of underlying mechanisms and for the development of targeted therapies. However, currently available culture conditions do not prevent spontaneous differentiation of LSCs, which greatly limits the feasibility of cell-based assays. To overcome these constraints we conducted a high-throughput chemical screen and identified small molecules that inhibit differentiation and support LSC activity in vitro. Similar to reports with cord blood stem cells, several of these compounds suppressed the aryl-hydrocarbon receptor (AhR) pathway, which we show to be inactive in vivo and rapidly activated ex vivo in AML cells. We also identified a compound, UM729, that collaborates with AhR suppressors in preventing AML cell differentiation. Together, these findings provide newly defined culture conditions for improved ex vivo culture of primary human AML cells.

RNAi screen identifies Jarid1b as a major regulator of mouse HSC activity

Histone methylation is a dynamic and reversible process proposed to directly impact on stem cell fate. The Jumonji (JmjC) domain-containing family of demethylases comprises 27 members that target mono-, di-, and trimethylated lysine residues of histone (or nonhistone) proteins. To evaluate their role in regulation of hematopoietic stem cell (HSC) behavior, we performed an in vivo RNAi-based functional screen and demonstrated that Jarid1b and Jhdm1f play opposing roles in regulation of HSC activity. Decrease in Jarid1b levels correlated with an in vitro expansion of HSCs with preserved long-term in vivo lymphomyeloid differentiation potential. Through RNA sequencing analysis, Jarid1b knockdown was associated with increased expression levels of several HSC regulators (Hoxa7, Hoxa9, Hoxa10, Hes1, Gata2) and reduced levels of differentiation-associated genes. shRNA against Jhdmlf, in contrast, impaired hematopoietic reconstitution of bone marrow cells. Together, our studies identified Jarid1b as a negative regulator of HSC activity and Jhdmlf as a positive regulator of HSC activity.

A key role for EZH2 and associated genes in mouse and human adult T-cell acute leukemia

In this study, we show the high frequency of spontaneous γδ T-cell leukemia (T-ALL) occurrence in mice with biallelic deletion of enhancer of zeste homolog 2 (Ezh2). Tumor cells show little residual H3K27 trimethylation marks compared with controls. EZH2 is a component of the PRC2 Polycomb group protein complex, which is associated with DNA methyltransferases. Using next-generation sequencing, we identify alteration in gene expression levels of EZH2 and acquired mutations in PRC2-associated genes (DNMT3A and JARID2) in human adult T-ALL. Together, these studies document that deregulation of EZH2 and associated genes leads to the development of mouse, and likely human, T-ALL.

Sustained in vitro trigger of self-renewal divisions in Hoxb4hiPbx1(10) hematopoietic stem cells

Factors that trigger and sustain self-renewal divisions in tissue stem cells remain poorly characterized. By modulating the levels of Hoxb4 and its co-factor Pbxl in primary hematopoietic cells (Hoxb4hiPbxl(10) cells), we report an in vitro expansion of mouse hematopoietic stem cells (HSCs) by 105-fold over 2 weeks, with subsequent preservation of HSC properties. Clonal analyses of the hematopoietic system in recipients of expanded HSCs indicate that up to 70% of Hoxb4hiPbxl(10) stem cells present at initiation of culture underwent self-renewal in vitro. In this setting, Hoxb4 and its co-factor did not promote an increase in DNA synthesis, or a decrease in doubling time of Scal+Lin- cells when compared to controls. Q-PCR analyses further revealed a downregulation of Cdknlb (p27Kipl) and Mxdl (MadI) transcript levels in Hoxb4hiPbxl(l0) primitive cells, accompanied by a more subtle increase in c-myc and reduction in Ccnd3 (Cyclin D3). We thus put forward this strategy as an efficient in vitro HSC expansion tool, enabling a further step into the avenue of self-renewal molecular effectors.

Molecular dissection of Meis1 reveals 2 domains required for leukemia induction and a key role for Hoxa gene activation

The Hoxa9 and Meis1 genes represent important oncogenic collaborators activated in a significant proportion of human leukemias with genetic alterations in the MLL gene. In this study, we show that the transforming property of Meis1 is modulated by 3 conserved domains, namely the Pbx interaction motif (PIM), the homeodomain, and the C-terminal region recently described to possess transactivating properties. Meis1 and Pbx1 interaction domain-swapping mutants are dysfunctional separately, but restore the full oncogenic activity of Meis1 when cotransduced in primary cells engineered to overexpress Hoxa9, thus implying a modular nature for PIM in Meis1-accelerated transformation. Moreover, we show that the transactivating domain of VP16 can restore, and even enhance, the oncogenic potential of the Meis1 mutant lacking the C-terminal 49 amino acids. In contrast to Meis1, the fusion VP16-Meis1 is spontaneously oncogenic, and all leukemias harbor genetic activation of endogenous Hoxa9 and/or Hoxa7, suggesting that Hoxa gene activation represents a key event required for the oncogenic activity of VP16-Meis1.

Analysis of HSC activity and compensatory Hox gene expression profile in Hoxb cluster mutant fetal liver cells

Overexpression of Hoxb4 in bone marrow cells promotes expansion of hematopoietic stem cell (HSC) populations in vivo and in vitro, indicating that this homeoprotein can activate the genetic program that determines self-renewal. However, this function cannot be solely attributed to Hoxb4 because Hoxb4(-/-) mice are viable and have an apparently normal HSC number. Quantitative polymerase chain reaction analysis showed that Hoxb4(-/-) c-Kit+ fetal liver cells expressed moderately higher levels of several Hoxb cluster genes than control cells, raising the possibility that normal HSC activity in Hoxb4(-/-) mice is due to a compensatory up-regulation of other Hoxb genes. In this study, we investigated the competitive repopulation potential of HSCs lacking Hoxb4 alone, or in conjunction with 8 other Hoxb genes. Our results show that Hoxb4(-/-) and Hoxb1-b9 (-/-) fetal liver cells retain full competitive repopulation potential and the ability to regenerate all myeloid and lymphoid lineages. Quantitative Hox gene expression profiling in purified c-Kit+ Hoxb1-b9(-/-) fetal liver cells revealed an interaction between the Hoxa, b, and c clusters with variation in expression levels of Hoxa4,-a11, and -c4.Together, these studies show a complex network of genetic interactions between several Hox genes in primitive hematopoietic cells and demonstrate that HSCs lacking up to 30% of the active Hox genes remain fully competent.

Molecular interactions involved in HOXB4-induced activation of HSC self-renewal

HOXB4 overexpression induces unique in vivo and in vitro expansion of hemopoietic stem cells (HSCs) without causing leukemia. Very little is known about the molecular basis underlying HOXB4-induced HSC self-renewal. We now report the in vitro proliferation and in vivo expansion capacity of primary bone marrow (BM) cells engineered to overexpress selected HOXB4 point mutants lacking either the capacity to directly bind DNA (HOXB4(A)), or to cooperate with members of the PBX family (HOXB4(W→G)) in DNA binding. The DNA binding–incompetent HOXB4 mutant failed to enhance the proliferation activity of transduced BM populations in vitro and HSC expansion in vivo. In contrast, the HOXB4(W→G) mutant conferred a pronounced in vitro proliferation advantage to the transduced BM populations, and dramatically enhanced their in vivo regenerative potential. We also demonstrate a correlation between HOXB4 protein levels and in vitro proliferative capacity of primary BM cells. Our observations thus suggest that the capacity of HOXB4 to induce HSC expansions is DNA-binding dependent and does not require direct HOX/PBX interaction, and sets the stage for identifying HOXB4-dependent targets involved in HSC expansion.

In vitro expansion of hematopoietic stem cells by recombinant TAT-HOXB4 protein

Hematopoietic stem cells (HSCs) can self-renew extensively after transplantation. The conditions supporting their in vitro expansion are still being defined. Retroviral overexpression of the human homeobox B4 (HOXB4) gene in mouse bone marrow cells enables over 40-fold expansion of HSCs in vitro. To circumvent the requirement for retroviral infection, we used recombinant human TAT-HOXB4 protein carrying the protein transduction domain of the HIV transactivating protein (TAT) as a potential growth factor for stem cells. HSCs exposed to TAT-HOXB4 for 4 d expanded by about four- to sixfold and were 8-20 times more numerous than HSCs in control cultures, indicating that HSC expansion induced by TAT-HOXB4 was comparable to that induced by the human HOXB4 retrovirus during a similar period of observation. Our results also show that TAT-HOXB4-expanded HSC populations retain their normal in vivo potential for differentiation and long-term repopulation. It is thus feasible to exploit recombinant HOXB4 protein for rapid and significant ex vivo expansion of normal HSCs.

The competitive nature of HOXB4-transduced HSC is limited by PBX1: the generation of ultra-competitive stem cells retaining full differentiation potential

We previously showed that HOXB4 is a potent stimulator of hematopoietic stem cell (HSC) proliferation in vivo and ex vivo. As a result, HOXB4 overexpressing HSCs are 20- to 50-times more competitive than untransduced cells when transplanted into mice. By knocking down the expression of PBX1 (PBX1(K.D.)) in HOXB4 overexpressing cells, we now present the possibility of generating HSCs that are >20-times more competitive than those that overexpress HOXB4. The differentiation activity of these cells appears intact, since they competitively contributed to the reconstitution of normal myeloid and lymphoid compartments in vivo. We also show that the in vivo expansion of HOXB4-PBX1(K.D.)-expressing HSCs regenerated normal stem cell pools and did not lead to HSC levels above those detected in unmanipulated mice. The vigorous competitive nature of these cells in vivo compared to HOXB4-transduced HSCs suggests the existence of a distinct, non-cell autonomous mechanism that limits the expansion of HOXB4-transduced hemopoietic stem cells in mice.

AP-1 complex is effector of Hox-induced cellular proliferation and transformation

Hox gene products, initially characterized as master regulators of embryonic patterning, are also required for proper functioning of adult tissues. There is also a growing body of evidence that links Hox proteins to regulation of cellular proliferation/transformation. However, the underlying molecular mechanisms of Hox-associated transformation and tissue growth have yet to be elucidated. Using a well established model system for studying changes in cellular proliferation induced by Hoxb4, we show that AP-1 activity is markedly increased in Hoxb4-transduced cells due to significant upregulation of Jun-B and Fra-1 protein levels. Furthermore, we also show that the specific changes in AP-1 protein expression are necessary for the proliferation effects induced by Hoxb4, and that these changes converge to increase levels of cyclin D1, a known integrator of proliferation signals. Our observations thus link Hox gene products with key elements of the cell cycle machinery.

The oncoprotein E2A-Pbx1a collaborates with Hoxa9 to acutely transform primary bone marrow cells

A recurrent translocation between chromosome 1 (Pbx1) and 19 (E2A) leading to the expression of the E2A-Pbx1 fusion oncoprotein occurs in approximately 5 to 10% of acute leukemias in humans. It has been proposed that some of the oncogenic potential of E2A-Pbx1 could be mediated through heterocomplex formation with Hox proteins, which are also involved in human and mouse leukemias. To directly test this possibility, mouse bone marrow cells were engineered by retroviral gene transfer to overexpress E2A-Pbx1a together with Hoxa9. The results obtained demonstrated a strong synergistic interaction between E2A-Pbx1a and Hoxa9 in inducing growth factor-independent proliferation of transduced bone marrow cells in vitro and leukemic growth in vivo in only 39 +/- 2 days. The leukemic blasts which coexpress E2A-Pbx1a and Hoxa9 showed little differentiation and produced cytokines such as interleukin-3, granulocyte colony-stimulating factor, and Steel. Together, these studies demonstrate that the Hoxa9 and E2A-Pbx1a gene products collaborate to produce a highly aggressive acute leukemic disease.

The hyperresponsiveness of cells expressing truncated erythropoietin receptors is contingent on insulin-like growth factor-1 in fetal calf serum

We demonstrate herein that the well documented hyperresponsiveness to erythropoietin (Epo) of Ba/F3 cells expressing C-terminal truncated erythropoietin receptors (EpoRs) is contingent on these cells being in fetal calf serum (FCS). In the absence of FCS, their Epo-induced proliferation is far poorer than Ba/F3 cells expressing wild-type (WT) EpoRs. This hyporesponsiveness in the absence of serum is also seen in DA-3 cells expressing these truncated EpoRs. In fact, long-term proliferation studies performed in the absence of serum show that even at saturating concentrations of Epo, Ba/F3 cells expressing these truncated receptors die via apoptosis, while cells bearing WT EpoRs do not, and this programmed cell death correlates with an inability of Epo-stimulated Ba/F3 cells expressing truncated EpoRs to induce the tyrosine phosphorylation of MAPK and the activation of p70(S6K). Using neutralizing antibodies to insulin-like growth factor (IGF)-1, we show that a major non-Epo factor in FCS that contributes to the hyperresponsive phenotype of Ba/F3 cells expressing truncated EpoRs is IGF-1. Our results suggest that the Epo-hypersensitivity of truncated EpoR expressing Ba/F3 cells is due to the combined effects of these EpoRs not possessing a binding site for the negative regulator, SHP-1, and the triggering of proliferation-inducing/apoptosis-inhibiting cascades, lost through EpoR truncation, by IGF-1.

Cellular proliferation and transformation induced by HOXB4 and HOXB3 proteins involves cooperation with PBX1

The products of PBX homeobox genes, which were initially discovered in reciprocal translocations occurring in human leukemias, have been shown to cooperate in the in vitro DNA binding with HOX proteins. Despite the growing body of data implicating Hox genes in the development of various cancers, little is known about the role of HOX-PBX interactions in the regulation of proliferation and induction of transformation of mammalian cells. We build on the existing model of Hox-induced transformation of Rat-1 cells to show that both cellular transformation and proliferation induced by Hoxb4 and Hoxb3 are greatly modulated by the levels of available PBX1 present in these cells. Furthermore, we show that the transforming capacity of these two HOX proteins depends on their conserved tetrapeptide and homeodomain regions which mediate binding to PBX and DNA, respectively. Taken together, results of this study demonstrate that cooperation between HOX and PBX proteins modulates cellular proliferation and strongly suggest that cooperative DNA binding by these two groups of proteins represent the basis for Hox-induced cellular transformation.

Hoxa9 transforms primary bone marrow cells through specific collaboration with Meis1a but not Pbx1b

Hoxa9, Meis1 and Pbx1 encode homeodomaincontaining proteins implicated in leukemic transformation in both mice and humans. Hoxa9, Meis1 and Pbx1 proteins have been shown to physically interact with each other, as Hoxa9 cooperatively binds consensus DNA sequences with Meis1 and with Pbx1, while Meis1 and Pbx1 form heterodimers in both the presence and absence of DNA. In this study, we sought to determine if Hoxa9 could transform hemopoietic cells in collaboration with either Pbx1 or Meis1. Primary bone marrow cells, retrovirally engineered to overexpress Hoxa9 and Meis1a simultaneously, induced growth factor-dependent oligoclonal acute myeloid leukemia in <3 months when transplanted into syngenic mice. In contrast, overexpression of Hoxa9, Meis1a or Pbx1b alone, or the combination of Hoxa9 and Pbx1b failed to transform these cells acutely within 6 months post-transplantation. Similar results were obtained when FDC-P1 cells, engineered to overexpress these genes, were transplanted to syngenic recipients. Thus, these studies demonstrate a selective collaboration between a member of the Hox family and one of its DNA-binding partners in transformation of hemopoietic cells.

The role of erythropoietin receptor tyrosine phosphorylation in erythropoietin-induced proliferation

Although studies with truncated erythropoietin receptors (EpoRs) have suggested the tyrosine phosphorylation (Yphos) of the EpoR may not play a significant role in Epo-induced proliferation, we found, using a full length EpoR mutant designed Null, in which all 8 of the intracellular tyrosines (Ys) were substituted with phenylalanines (Fs), that Null cells required 5-10 fold more Epo than wild type (WT) EpoR containing cells in order to proliferate as well. Moreover, a comparison of Epo-induced proliferation with Epo-induced Yphos patterns, using DA-3 cells expressing WT, Null and various Y to F EpoR point mutants revealed that Stat5 Yphos and activation correlated directly with proliferation and was mediated primarily throuhg the most membrane proximal Y, i.e., Y343, although other tyrosines (most likely Y401 and Y431) within the EpoR could activate Stat5 in its absence. We also found that EpoR Yphos was essential for the Yphos of Shc and for the Yphos and association of a 145 kDa protein with Shc. We purified and cloned this Shc-associated 145 kDa protein and found that it was a unique SH2 containing inositol polyphosphate-5-phosphatase. This novel enzyme, which we have called SHIP for SH2-containing inositol-phosphatase, may modulate both Ras and inositol signaling pathways.

The role of host lymphoid populations in the response of mouse EMT6 tumor to photodynamic therapy

Photodynamic therapy (PDT) treatment of murine EMT6 mammary sarcoma using Photofrin (10 mg/kg) and light (110 J/cm2) cured all these lesions growing in syngeneic BALB/c mice. In contrast, the same treatment produced initial ablation but no long-term cures of EMT6 tumors growing in either scid or nude mice, the immunodeficient strains sharing the same genetic background with BALB/c mice. No difference was detected in either the level of Photofrin accumulated per g of tumor tissue or the extent of tumor cell killing during the first 24 h after PDT of EMT6 tumors growing in BALB/c or scid mice. The assumption that the difference in tumor cures could be ascribed to the absence of functional lymphoid cells in scid and nude mice was supported by the results of experiments involving the adoptive T-cell transfer into scid mice or bone marrow transfer between BALB/c and scid mice. The adoptive transfer of splenic virgin T lymphocytes from BALB/c mice into scid mice performed 9 days before PDT of EMT6 tumors growing in the recipients was successful in delaying the recurrence of treated tumors. Adoptive transfer done immediately after PDT or 7 days after PDT had no obvious benefit. Even better improvement and a high cure rate of PDT-treated tumors was obtained with scid mice reconstituted with BALB/c bone marrow. In contrast, a marked drop in tumor cure rate was observed with BALB/c mice reconstituted with scid bone marrow. These results suggest that the activity of host lymphoid populations was essential for preventing the recurrence of EMT6 tumors following the PDT treatment used in this study. The contribution of PDT-induced immune reaction may, therefore, be of critical importance for the cure with at least some tumors.

Interleukin-3 (IL-3) inhibits erythropoietin-induced differentiation in Ba/F3 cells via the IL-3 receptor alpha subunit

Introduction of erythropoietin receptors (EpoRs) into the interleukin-3 (IL-3)-dependent murine hemopoietic cell line, Ba/F3, enables these cells to not only proliferate, after an initial lag in G1, but also to increase beta-globin mRNA levels in response to erythropoietin (Epo). With IL-3 and Epo costimulation, IL-3-induced signaling appears to be dominant since no increase in beta-globin mRNA occurs. Differentiation and proliferation signals may be uncoupled since EpoRs lacking all eight intracellular tyrosines were compromised in proliferative signaling but retained erythroid differentiation ability. Intriguingly, a chimeric receptor of the extracellular domain of the EpoR and the transmembrane and intracellular domains of IL-3RbetaIL-3 chain (EpoR/IL-3RbetaIL-3) was capable of Epo-induced proliferative and differentiating signaling, suggesting either the existence of a second EpoR subunit responsible for differentiation or that the alpha subunit of the IL-3 receptor (IL-3R) prevents it. Arguing against the former, a truncated EpoR lacking an intracellular domain was incapable of promoting proliferation or differentiation. An EpoR/IL-3Ralpha chimera, in contrast, was capable of transmitting a weak Epo-induced proliferative signal but failed to stimulate accumulation of beta-globin mRNA. Most significantly, coexpression of the EpoR/IL-3Ralpha chimera with either EpoR/IL-3Rbeta or wild-type EpoRs suppressed Epo-induced beta-globin mRNA accumulation. Taken together, these results suggest an active role for the IL-3Ralpha subunit in inhibiting EpoR-specific differentiating signals.

Potentiation of photodynamic therapy-elicited antitumor response by localized treatment with granulocyte-macrophage colony-stimulating factor

Murine squamous cell carcinoma (SCCVII) cells were genetically engineered to produce marine granulocyte-macrophage colony-stimulating factor (GM-CSF). GM-CSF immunotherapy, based on the peritumoral injection of lethally irradiated GM-CSF-producing SCCVII cells, was examined as adjuvant to photodynamic therapy (PDT) treatment of this tumor. The GM-CSF immunotherapy administered three times in 48-h intervals, starting 2 days before the light treatment, substantially improved the curative effect of Photofrin-mediated PDT. A comparable effect of GM-CSF immunotherapy was observed in the combination with benzoporphyrin derivative-mediated PDT. The tumor-localized GM-CSF immunotherapy alone had no obvious effect on the growth of parental SCCVII tumors. This treatment did not significantly alter the differential peripheral WBC count and appeared not to affect tumor leukocyte infiltration. However, GM-CSF treatment did increase the cytotoxic activity of tumor-associated macrophages against SCCVII tumor cells. It appears, therefore, that tumor-localized immune stimulation by GM-CSF amplifies a PDT-induced antitumor immune reaction, which has a potentiating effect on tumor control.

Tyrosine 343 in the erythropoietin receptor positively regulates erythropoietin-induced cell proliferation and Stat5 activation

While previous studies with truncated erythropoietin receptors (EpRs) have suggested that the tyrosine phosphorylation of the EpR does not play a role in Ep-induced proliferation, we have found, using a more subtle, full length EpR mutant, designated Null, in which all eight of the intracellular tyrosines have been substituted with phenylalanine residues, that Null cells require substantially more Ep than wild-type cells in order to proliferate as efficiently. A comparison of Ep-induced proliferation with Ep-induced tyrosine phosphorylation patterns, using wild-type and Null EpR-expressing cells, revealed that Stat5 tyrosine phosphorylation and activation correlated directly with proliferation. Moreover, studies with a Y343F EpR point mutant and various EpR deletion mutants revealed that both Ep-induced proliferation and Stat5 activation were mediated primarily through Y343, but that other tyrosines within the EpR could activate Stat5 in its absence.

Erythropoietin and interleukin-3 induce distinct events in erythropoietin receptor-expressing BA/F3 cells

To compare the signal transduction pathways used by erythropoietin (Ep) and interleukin-3 (IL-3), the cDNA for the murine erythropoietin receptor (EpR) was introduced into the IL-3-responsive cell lines Ba/F3 and DA-3 using retrovirally mediated gene transfer. After selection in G418 and IL-3, clones expressing comparable levels of cell surface EpR were identified using biotinylated Ep and flow cytometry. A comparison of the effects of Ep and IL-3 on these cells showed that most EpR+ Ba/F3 clones, when first exposed to Ep, dramatically increased their levels of beta-globin mRNA. The kinetics of appearance of this message after exposure to Ep varied considerably from clone to clone, with some clones showing a marked increase in beta-globin mRNA within 1 hour, while others required several days before an increase was observed. Interestingly, not only was this increase not seen with IL-3, but IL-3 prevented the Ep-induced appearance of beta-globin message. On the other hand, none of the EpR+ DA-3 cell clones tested increased their levels of beta-globin mRNA in response to Ep. While the EpR+ DA-3 clones showed identical proliferative responses to IL-3 and Ep, most EpR+ Ba/F3 clones displayed a marked, albeit transient, proliferative lag when first exposed to Ep. This was manifested as both an increased doubling time in liquid culture and a decreased colony size in methylcellulose. Plating efficiencies of EpR+ Ba/F3 cells in methylcellulose, however, were identical in response to IL-3 and Ep, suggesting that the Ep-induced lag in proliferation reflected a growth delay of the entire population of cells to Ep rather than a selection of an Ep-responsive subpopulation. Flow cytometric analysis established that this growth delay was due to a lengthening of the first G1 period after exposure to Ep. Interestingly, this Ep-induced delay in entry into the S phase was not detected in cells stimulated with both Ep and IL-3 nor in EpR+ Ba/F3 cell clones that did not show an increase in beta-globin mRNA in response to Ep. Thymidine-induced growth arrest, however, showed that delaying entry into S phase alone was not sufficient to stimulate beta-globin mRNA in the absence of Ep. Further studies established that the Ep-induced increase in beta-globin mRNA could be inhibited by the tyrosine kinase inhibitor genistein and the protein kinase C inhibitor Compound 3.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolonged cultivation of rat uterine cells with preserved estrogen responsiveness

A rat uterine cell culture was prepared as an experimental system for investigation of mechanisms of steroid hormone actions. Cells frequently supplemented with fresh medium were successfully cultured for 4 weeks through 2 successive passages. Studies of estrogen responsiveness in the primary culture as well as in it's first subculture were performed by a small scale uptake assay for determination of specific steroid binding. Scatchard analysis of specific ovarian hormone binding confirmed that cultured uterine cells preserve both estradiol and progesterone receptors. Characteristics of specific [3H]estradiol binding detected in cells of the first subculture were comparable to those obtained in the initial primary culture. The number of specific estradiol binding sites was diminished to one third of the initial values only in cells of the second subculture, 22 days after isolation of cells from tissue. In the primary culture and in it's first subculture the cells responded to estradiol with a 2-3-fold increase in progesterone receptor level. The subcellular distribution of steroid receptors was also studied; estradiol receptor complexes were detected predominantly in the nuclei whereas progesterone receptors were nearly equally distributed between nuclei and cytosol.