Epigenetics and the plasticity of differentiation in normal and cancer stem cells.

Embryonic stem cells are characterized by their differentiation to all cell types during embryogenesis. In adult life, different tissues also have somatic stem cells, called adult stem cells, which in specific niches can undergo multipotent differentiation. The use of these adult stem cells has considerable therapeutic potential for the regeneration of damaged tissues. In both embryonic and adult stem cells, differentiation is controlled by epigenetic mechanisms, and the plasticity of differentiation in these cells is associated with transcription accessibility for genes expressed in different normal tissues. Abnormalities in genetic and/or epigenetic controls can lead to development of cancer, which is maintained by self-renewing cancer stem cells. Although the genetic abnormalities produce defects in growth and differentiation in cancer stem cells, these cells have not always lost the ability to undergo differentiation through epigenetic changes that by-pass the genomic abnormalities, thus creating the basis for differentiation therapy. Like normal stem cells, cancer stem cells can show plasticity for differentiation. This plasticity of cancer stem cells is also associated with transcription accessibility for genes that are normally expressed in different tissues, including tissues other than those from which the cancers originated. This broad transcription accessibility can also contribute to the behavior of cancer cells by overexpressing genes that promote cell viability, growth and metastasis.

Autoregulation of interleukin 6 and granulocyte-macrophage colony-stimulating factor in the differentiation of myeloid leukemic cells.

Induction of differentiation in one type of clone of mouse myeloid leukemic cells by mouse or human interleukin 6 (IL-6) and in another type of clone by mouse granulocyte-macrophage colony-stimulating factor (GM-CSF) was found to be associated with induction of IL-6 and GM-CSF mRNA and protein. The results indicated that IL-6 and GM-CSF could positively autoregulate their own gene expression during myeloid cell differentiation. It is suggested that this autoregulation may serve to enhance and prolong the signal induced by these proteins in cells transiently exposed to IL-6 or GM-CSF.

Expression of AML1-d, a short human AML1 isoform, in embryonic stem cells suppresses in vivo tumor growth and differentiation.

The human AML1 gene encodes a heterodimeric transcription factor which plays an important role in mammalian hematopoiesis. Several alternatively spliced AML1 mRNA species were identified, some of which encode short protein products that lack the transactivation domain. When transfected into cells these short isoforms dominantly suppress transactivation mediated by the full length AML1 protein. However, their biological function remains obscure. To investigate the role of these short species in cell proliferation and differentiation we generated embryonic stem (ES) cells overexpressing one of the short isoforms, AML1-d, as well as cells expressing the full length isoforms AML1-b and AML2. The in vitro growth rate and differentiation of the transfected ES cells were unchanged. However, overexpression of AML1-d significantly affected the ES cells' ability to form teratocarcinomas in vivo in syngeneic mice, while a similar overexpression of AML1-b and AML2 had no effect on tumor formation. Histological analysis revealed that the AML1-d derived tumors were poorly differentiated and contained numerous apoptotic cells. These data highlight the pleiotropic effects of AML1 gene products and demonstrate for the first time an in vivo growth regulation function for the short isoform AML1-d.

Spatial and temporal expression pattern of Runx3 (Aml2) and Runx1 (Aml1) indicates non-redundant functions during mouse embryogenesis.

The human RUNX3/AML2 gene belongs to the 'runt domain' family of transcription factors that act as gene expression regulators in major developmental pathways. Here, we describe the expression pattern of Runx3 during mouse embryogenesis compared to the expression pattern of Runx1. E10.5 and E14.5-E16.5 embryos were analyzed using both immunohistochemistry and beta-galactosidase activity of targeted Runx3 and Runx1 loci. We found that Runx3 expression overlapped with that of Runx1 in the hematopoietic system, whereas in sensory ganglia, epidermal appendages, and developing skeletal elements, their expression was confined to different compartments. These data provide new insights into the function of Runx3 and Runx1 in organogenesis and support the possibility that cross-regulation between them plays a role in embryogenesis.

Interferon-gamma inhibits apoptosis induced by wild-type p53, cytotoxic anti-cancer agents and viability factor deprivation in myeloid cells.

Different hematopoietic cytokines including colony-stimulating factors and interleukins can inhibit apoptotic cell death induced in myeloid cells by the tumor-suppressor gene wild-type 53 and a variety of cytotoxic anti-cancer agents. In this study we identity interferon-gamma as an anti-apoptotic cytokine for myeloid cells in which apoptosis was induced by wild-type p53, cytotoxic anti-cancer agents or viability factor deprivation. The inhibition of wild-type p53-mediated apoptosis in myeloid leukemic cells by interferon-gamma was not associated with downregulated expression of wild-type p53 or the p53-induced cyclin-dependent kinase inhibitor gene WAF-1, or with upregulated expression of the apoptosis-inhibiting gene bcl-2. Interferon-gamma also inhibited induction of apoptosis by a p53-independent pathway. Interferon-gamma inhibited apoptotic cell death caused by withdrawal of viability factors in normal myeloid precursor cells, the interleukin 3-dependent 32D cell line and differentiating myeloid leukemic cells. Interferon-alpha/beta did not inhibit apoptotic cell death in any of these systems. The results indicate that although interferon-gamma can inhibit cell multiplication and differentiation in myeloid cells, it shares with other hematopoietic cytokines the ability to protect normal and leukemic myeloid cells from induction of apoptosis.

Control of apoptosis in hematopoiesis and leukemia by cytokines, tumor suppressor and oncogenes.

Hematopoietic cells require certain cytokines including colony-stimulating factors and interleukins to maintain viability. Without these cytokines the program of apoptotic cell death is activated. Cells from many myeloid leukemias require cytokines for viability, and apoptosis is also activated in these leukemic cells after cytokine withdrawal resulting in reduced leukemogenicity. The same cytokines protect normal and leukemic cells from induction of apoptosis by irradiation and cytotoxic chemotherapeutic compounds. This suggests that decreasing the levels of viability inducing cytokines may increase the effectiveness of cytotoxic anti-cancer therapy. The susceptibility of normal and cancer cells to induction of apoptosis is also regulated by the balance between apoptosis-inducing genes such as the tumor suppressor wild-type p53, and c-myc and bax, and apoptosis-suppressing genes such as the oncogene mutant p53, and bcl-2 and bcl-XL. Cell susceptibility to induction of apoptosis in leukemic cells could be enhanced by increased expression of apoptosis-inducing genes and/or decreased expression of apoptosis-suppressing genes. Modulation of expression of apoptosis-regulating genes should thus also be useful for improvement of anti-cancer therapy.

Control of in vivo differentiation of myeloid leukemic cells.

The differentiation of leukemic cells in vivo can be a useful approach to therapy. In vivo differentiation of myeloid leukemic cells was studied in intraperitoneally implanted diffusion chambers, containing different soluble antigens. The presence of these antigens in the chambers induced differentiation of myeloid leukemic cells and this was inhibited in immune-deficient mice. Transfer of normal spleen cells enriched for T-lymphocytes or antigen-specific helper T lymphocyte cell lines to mice in which differentiation of leukemic cells was inhibited, restored in vivo differentiation of the leukemic cells. Antigen-specific helper T cells produce myeloid regulatory proteins and can accumulate at a site that contains the specific antigen. It is suggested that migration in response to antigen of helper T cells producing regulatory proteins may play an important role in inducing in vivo differentiation of leukemic cells. We have identified a class of myeloid leukemic cells that can be induced to differentiate in vitro by incubation with pure MGI-1GM (GM-CSF) or IL-3, but not with MGI-1G (G-CSF). Experiments with pure recombinant proteins have shown that MGI-1GM and IL-3, but not MGI-1G, can also induce these myeloid leukemic cells to differentiate in vivo. These results and our previous studies on the myeloid cell differentiation-inducing protein MGI-2, demonstrate the potential use of normal hematopoietic regulatory proteins not only in regulation of normal hematopoiesis, but also in the treatment of myeloid leukemia by in vivo induction of terminal cell differentiation.

Selective regulation by hydrocortisone of induction of in vivo differentiation of myeloid leukemic cells with granulocyte-macrophage colony-stimulating factor, interleukin 6 and interleukin 1 alpha.

Clones of myeloid leukemic cells can differ in their ability to be induced to differentiate in vitro by different cytokines. Using such leukemic clones, we studied the regulation by hydrocortisone of induction of in vivo differentiation by injection of recombinant interleukin 6 (IL-6), interleukin 1 alpha (IL-1 alpha), and granulocyte-macrophage colony-stimulating factor (GM-CSF). Injection of IL-6 and IL-1 alpha induced in vivo differentiation of leukemic cells that were induced to differentiate by these cytokines in vitro, but not of leukemic cells that were not susceptible to these cytokines in vitro. In contrast, injection of GM-CSF induced in vivo differentiation both in leukemic cells that were susceptible or not susceptible to GM-CSF in vitro. The effect of GM-CSF, but not of IL-6 or IL-1 alpha, on inducing differentiation in vivo was inhibited by pretreatment with hydrocortisone. In leukemic cells that were not induced to differentiate with GM-CSF in vitro, this inhibition of differentiation by pretreatment with hydrocortisone was greater than inhibition of differentiation obtained by pretreatment with cyclophosphamide or irradiation or the use of nude mice. After hydrocortisone pretreatment, the number of peritoneal cells and their ability to produce GM-CSF and IL-6 were suppressed. It is suggested that hydrocortisone can inhibit the effect of an injected cytokine such as GM-CSF on induction of in vivo differentiation of leukemic cells by inhibiting the ability of host cells to produce cytokines to which the leukemic cells are susceptible.

Protection of human myeloid leukemic cells against doxorubicin-induced apoptosis by granulocyte-macrophage colony-stimulating factor and interleukin 3.

Hematopoietic cells require certain cytokines to maintain viability by preventing apoptotic cell death. These cytokines can also protect leukemic cell lines against induction of apoptosis by cytotoxic anticancer compounds. We now show that the cytokines granulocyte-macrophage colony-stimulating factor and interleukin 3 can protect primary human myeloid leukemic cells against doxorubicin-induced apoptosis. Protection was detected in cells from 72% of the myeloid leukemic patients tested. The results indicate that these, and perhaps other, hematopoietic cytokines can decrease the effectiveness of cytotoxic anticancer therapy in some human myeloid leukemias. Leukemic cell sensitization to cytotoxic therapy may, therefore, require decreasing the availability of certain cytokines.

Induction of genes for transcription factors by normal hematopoietic regulatory proteins in the differentiation of myeloid leukemic cells.

Induction of differentiation to macrophages in two different clones of myeloid leukemic cells by the hematopoietic regulatory proteins interleukin-6 (IL-6), or by granulocyte-macrophage colony-stimulating factor (GM-CSF) or interleukin-3 (IL-3), is shown to be associated with sustained accumulation of c-jun, jun-B, and c-fos mRNA that code for proteins that form complexes that are transcription factors (AP-1). In one but not in the other of these leukemic clones, differentiation is also associated with sustained accumulation of mRNA for the putative transcription factor zif/268. The results indicate that differentiation of myeloid cells by normal hematopoietic regulatory proteins is associated with induction of sustained elevated levels of mRNA for transcription factors that can regulate and maintain gene expression in the differentiation program, and that zif/268 gene expression is not essential for differentiation to macrophages.

Regulation of the genes for interleukin-6 and granulocyte-macrophage colony stimulating factor by different inducers of differentiation in myeloid leukemic cells.

Different clones of myeloid leukemic cells can be induced to differentiate to mature macrophages and/or granulocytes by hematopoietic regulatory proteins and by other compounds. We now show that induction of differentiation in different clones of myeloid leukemic cells with the normal hematopoietic proteins granulocyte-colony stimulating factor (G-CSF), granulocyte-macrophage colony stimulating factor (GM-CSF), or interleukin 3 and by compounds such as dexamethasone or cytosine arabinoside (ara C) induces the expression of genes for the myeloid differentiation inducing protein MGI-2 that we have shown is interleukin 6 (IL-6) and for GM-CSF. We have previously shown that induction of differentiation with interleukin-1, IL-6, or bacterial lipopolysaccharide (LPS) also induces IL-6 and GM-CSF gene expression. Treatment of these leukemic clones with hematopoietic proteins that do not induce differentiation did not induce IL-6 or GM-CSF gene expression. The results indicate that induction of IL-6 and GM-CSF gene expression is part of the normal differentiation program in myeloid cells and support our previous evidence that there is transregulation of gene expression between different hematopoietic regulatory proteins.

Clonal variation in susceptibility to differentiation by different protein inducers in the myeloid leukemia cell line M1.

Differentiation-competent clones of myeloid leukemic cells, independently isolated from the M1 cell line in Rehovot, Israel, and in Saitama, Japan, can be induced to differentiate to mature cells by the protein which we called macrophage and granulocyte differentiation-inducing protein-2 (MGI-2) that we have shown is interleukin 6 (IL-6). We now show that our MGI-2/IL-6-susceptible clones of M1 cells were not induced to differentiate with the differentiation-inducing protein called D-factor/leukemia inhibitory factor (LIF) which has also been called human interleukin for DA cells (HILDA), whereas this protein induced differentiation to macrophages in the M1 clone isolated in Saitama which was also used in Melbourne, Australia, The D-factor/LIF susceptible clone also showed a 4-fold lower sensitivity to MGI-2/IL-6 than the D-factor/LIF resistant clone. Both types of clones differentiated with interleukin-1 alpha (IL-1 alpha) and dexamethasone, whereas the D-factor/LIF resistant clone, but not the D-factor/LIF susceptible clone, was induced by bacterial lipopolysaccharide (LPS) to differentiate to mature macrophages. The present results show that clonal differences in susceptibility to differentiation-inducing proteins in the M1 cell line can explain the isolation of different differentiation-inducing proteins in M1 leukemic cells in different laboratories.

The RUNX3 gene--sequence, structure and regulated expression.

The RUNX3 gene belongs to the runt domain family of transcription factors that act as master regulators of gene expression in major developmental pathways. In mammals the family includes three genes, RUNX1, RUNX2 and RUNX3. Here, we describe a comparative analysis of the human chromosome 1p36.1 encoded RUNX3 and mouse chromosome 4 encoded Runx3 genomic regions. The analysis revealed high similarities between the two genes in the overall size and organization and showed that RUNX3/Runx3 is the smallest in the family, but nevertheless exhibits all the structural elements characterizing the RUNX family. It also revealed that RUNX3/Runx3 bears a high content of the ancient mammalian repeat MIR. Together, these data delineate RUNX3/Runx3 as the evolutionary founder of the mammalian RUNX family. Detailed sequence analysis placed the two genes at a GC-rich H3 isochore with a sharp transition of GC content between the gene sequence and the downstream intergenic region. Two large conserved CpG islands were found within both genes, one around exon 2 and the other at the beginning of exon 6. RUNX1, RUNX2 and RUNX3 gene products bind to the same DNA motif, hence their temporal and spatial expression during development should be tightly regulated. Structure/function analysis showed that two promoter regions, designated P1 and P2, regulate RUNX3 expression in a cell type-specific manner. Transfection experiments demonstrated that both promoters were highly active in the GM1500 B-cell line, which endogenously expresses RUNX3, but were inactive in the K562 myeloid cell line, which does not express RUNX3.

Control of in-vivo differentiation of myeloid leukemic cells--V. Regulation by response to antigen.

Regulation of in-vivo differentiation of myeloid leukemic cells by response to antigen was analysed with different clones of mouse myeloid leukemic cells and human myeloid leukemic cells (HL-60). Differentiation was studied in diffusion chambers implanted into the peritoneal cavity of mice and the antigens used were bovine serum albumin and chicken ovalbumin. It is shown that the presence of either of these antigens in the diffusion chambers can induce differentiation in MGI+D+ mouse and human myeloid leukemic cells, and that pre-immunization with antigen enhanced this in-vivo differentiation. This enhancement showed immunological specificity and was transferred from immunized to non-immunized mice by spleen cells enriched for T lymphocytes. In contrast to these results with MGI+D+ clones of myeloid leukemic cells, clones of WEHI-3B myeloid leukemic cells were induced to differentiate in vivo to the same extent either in the presence or absence of antigen. The results indicate: that in-vivo differentiation of MGI+D+ clones of myeloid leukemic cells can be induced by response to antigen and that in-vivo differentiation of different clones of myeloid leukemic cells can be regulated in different ways.

Indirect induction of differentiation of normal and leukemic myeloid cells by recombinant interleukin 1.

Different clones of myeloid leukemic cells can be induced to differentiate to mature macrophages or granulocytes by different normal hematopoietic regulatory proteins. The present experiments with recombinant IL-1 alpha and recombinant IL-1 beta show that, (a) that there are clones of myeloid leukemic cells which can be induced to differentiate to mature cells by the myeloid cell differentiation-inducing protein MGI-2 and can also be induced to differentiate to mature macrophages and granulocytes by both types of IL-1; (b) this IL-1-induced differentiation is mediated by endogenous production of differentiation-inducing protein MGI-2; (c) IL-1 and MGI-2 induce production of GM-CSF in these leukemic cells; and (d) IL-1 also induces cell differentiation and production of MGI-2 and GM-CSF in normal myeloid precursor cells. The results indicate that IL-1 induces differentiation indirectly.

Screening for induction of differentiation and toxicity to blast cells by chemotherapeutic compounds in human myeloid leukemia.

Bone marrow cells from 9 patients with acute myeloid leukemia and 1 patient with a blast crisis of chronic myeloid leukemia were cultured to determine their ability to be induced to differentiate by different chemotherapeutic compounds. Five of these 10 patients showed differentiation to granulocytic and/or monocytic cells by culture with medium containing the myeloid cell differentiation-inducing protein MGI-2. Actinomycin D induced differentiation in cells from 2 of the patients who did not show differentiation with MGI-2 containing medium. In these 7 patients there was an increase in the ratio of differentiated myeloid cells to blasts. None of these 10 patients showed induction of differentiation by cytosine arabinoside, adriamycin, or daunomycin, but treatment with these compounds showed in some patients an increase in the ratio of differentiated myeloid cells to blasts. The results indicate that this ratio can be increased by differentiation and also in some patients by toxicity to blast cells. With dexamethasone or vinblastine there was no induction of differentiation and no increase in this ratio in any of the 10 patients tested. After in vivo chemotherapy with low dose cytosine arabinoside, cells from one patient showed a similar response in culture to actinomycin D as cells before chemotherapy, whereas in another patient the cells had acquired the ability to respond to actinomycin D. In contrast, after high-dose in vivo chemotherapy with cytosine arabinoside and daunomycin, cells from a third patient seemed to have lost the ability to differentiate in vitro by MGI-2 containing medium or actinomycin D. The results indicate that pre-screening for differentiation-inducing compounds and compounds that show toxicity to blast cells should be useful to select the appropriate compounds to be used for therapy, and that it is advisable to screen the cells both before and after initiation of therapy.

Cell differentiation and therapeutic effect of low doses of cytosine arabinoside in human myeloid leukemia.

Bone marrow cells from 2 patients over 60 years of age with acute myeloblastic (AML) or monoblastic (AMoL) leukemia were cultured in the presence of a low dose of cytosine arabinoside. In the cells from the AML patient this treatment induced differentiation to metamyelocytes and a decrease in the number of blasts, so that there was an 11-fold increase in the ratio of differentiated myeloid cells to blasts. In the patient with AMoL there was differentiation to monocytes and macrophages and only a 3-fold increase in the ratio of differentiated myeloid cells to blasts. In the latter patient actinomycin D was a more potent inducer of differentiation than cytosine arabinoside, daunomycin was similar to cytosine arabinoside and adriamycin showed the lowest response. Four courses of low dose treatment with cytosine arabinoside produced remission in the patient with AML and in another patient with AMoL whose cells were not tested in culture. No remission was induced by this low dose treatment in the patient with AMoL whose cells showed only a small decrease in blast cells in culture with cytosine arabinoside. It is suggested that prescreening for effective compounds in patients with myeloid leukemias and the use of low dose therapy can be of help in obtaining remission without serious side effects. This could be especially useful in patients where there may be severe toxic effects after high dose chemotherapy.

Regulation of in-vivo differentiation of myeloid leukemic cells by antigen-specific helper T lymphocytes.

There are clones of myeloid leukemic cells that can be induced to differentiate in vitro and in vivo by normal macrophage and granulocyte differentiation-inducing protein MGI-2 (= DF). The differentiation of these myeloid leukemic cells in vivo is regulated by a cell mediated immune response which requires T lymphocytes. We now show that differentiation of myeloid cells in vivo can be induced by antigen-specific helper T lymphocytes and that this is associated with the ability of the helper T cells to produce myeloid cell differentiation-inducing protein MGI-2. Antigen specific helper T cells can accumulate at a site that contains the antigen. It is suggested that migration in response to antigen of helper T cells producing differentiation factors may play an important role in inducing in vivo differentiation of leukemic cells.

A large variety of alternatively spliced and differentially expressed mRNAs are encoded by the human acute myeloid leukemia gene AML1.

The human chromosome 21 acute myeloid leukemia gene AML1 is frequently rearranged in the leukemia-associated translocations t(8;21) and t(3;21), generating fused proteins containing the amino-terminal part of AML1. In normal blood cells, five size classes (2-8 kb) of AML1 mRNAs have been previously observed. We isolated seven cDNAs corresponding to various AML1 mRNAs. Sequencing revealed that their size differences were mainly due to alternatively spliced 5' and 3' untranslated regions, some of which were vast, exceeding 1.5 kb (5') and 4.3 kb (3'). These untranslated regions contain sequences known to control mRNA translation and stability and seem to modulate AML1 mRNA stability. Further heterogeneity was found in the coding region due to the presence of alternatively spliced stop codon-containing exons. The latter led to production of polypeptides that were smaller than the full-length AML1 protein; they lacked the trans-activation domains but maintained DNA binding and heterodimerization ability. The size of these truncated products was similar to the AML1 segment in the fused t(8;21) and t(3;21) proteins. In thymus, only one mRNA species of 6 kb was detected. Using in situ hybridization, we showed that its expression was confined to the cortical region of the organ. The 6-kb mRNA was also prominent in cultured peripheral blood T cells, and its expression was markedly reduced upon mitogenic activation by phorbol myristate acetate (TPA) plus concanavalin A (ConA). These results and the presence of multiple coding regions flanked by long complex untranslated regions, suggest that AML1 expression is regulated at different levels by several control mechanisms generating the large variety of mRNAs and protein products.

The molecular regulators of macrophage and granulocyte development. Role of MGI-2/IL-6.

The development of a cell culture system for the in vitro cloning and clonal differentiation of normal hematopoietic cells made it possible to identify the proteins that regulate growth and differentiation of different hematopoietic cell lineages and the change in normal controls that produce leukemia. A model system with myeloid cells has identified different myeloid cell colony-inducing proteins, which we called MGI-1 (= CSF, including IL-3). There is another protein that we first described in 1976 and called MGI-2 in 1980 that induces differentiation of myeloid cells to macrophages or granulocytes without inducing the clonal growth of myeloid cells. The four CSF proteins and IL-1 induce the production of MGI-2 in myeloid cells and MGI-2 induces the production of GM-CSF. This shows the participation of MGI-2 in the network of interactions with different myeloid regulatory proteins. Using a monoclonal antibody to MGI-2, amino acid sequencing, and recombinant protein, we have shown in collaboration with the Genetics Institute that the major form of MGI-2 (MGI-2A) is IL-6. This shows that IL-6 is a myeloid cell differentiation inducing protein. The results also suggest new clinical potentials for MGI-2/IL-6.