Early growth response gene 1 stimulates development of hematopoietic progenitor cells along the macrophage lineage at the expense of the granulocyte and erythroid lineages.

Using a variety of differentiation-inducible myeloid cell lines, we previously showed that the zinc-finger transcription factor early growth response gene 1 (Egr-1) is a positive modulator of macrophage differentiation and negatively regulates granulocytic differentiation. In this study, high-efficiency retroviral transduction was used to ectopically express Egr-1 in myeloid-enriched or stem cell-enriched bone marrow cultures to explore its effect on the development of hematopoietic progenitors in vitro and in lethally irradiated mice. It was found that ectopic Egr-1 expression in normal hematopoietic progenitors stimulates development along the macrophage lineage at the expense of development along the granulocyte or erythroid lineages, regardless of the cytokine used. Moreover, Egr-1 accelerated macrophage development by suppressing the proliferative phase of the growth-to-macrophage developmental program. The remarkable ability of Egr-1 to dictate macrophage development at the expense of development along other lineages resulted in failure of Egr-1-infected hematopoietic progenitors to repopulate the bone marrow and spleen, and thereby prevent death, in lethally irradiated mice. These observations further highlight the role Egr-1 plays in monocytic differentiation and growth suppression.

CR6: A third member in the MyD118 and Gadd45 gene family which functions in negative growth control.

MyD118 and Gadd45 are two related genes which encode for proteins that play important roles in negative growth control, including both growth suppression and apoptosis. A strategy was employed to clone new members of the MyD118 and Gadd45 family of genes. Based on alignment of the deduced amino acid sequences, one cDNA clone was found to encode for the murine homologue of human CR6, originally cloned as an IL-2 immediate-early response gene. The murine and human CR6 proteins were observed to be 97% identical, indicating that CR6 is an evolutionarily conserved protein. Analysis of CR6 expression during hematopoietic cell development associated with growth arrest and apoptotic cell death, upon exposure of hematopoietic cells to a variety of growth arrest and apoptotic stimuli, and in a variety of murine tissues, has revealed that CR6 expression differs significantly from the expression of the related MyD118 and Gadd45 genes. Nevertheless, CR6, like MyD118 and Gadd45, suppressed colony formation of human lung carcinoma H1299 cells. These data suggest that CR6 plays similar, but not identical, roles to MyD118 and Gadd45 in negative control of cell growth.

The zinc finger transcription factor Egr-1 activates macrophage differentiation in M1 myeloblastic leukemia cells.

We previously have shown that the zinc finger transcription factor Egr-1 blocked granulocytic differentiation of HL-60 cells, restricting differentiation along the monocytic lineage. Egr-1 also was observed to block granulocyte colony-stimulating factor (G-CSF)-induced differentiation of interleukin-3 (IL-3)-dependent 32Dcl3 hematopoietic precursor cells, endowing the cells with the ability to be induced by granulocyte-macrophage colony-stimulating factor (GM-CSF) for terminal differentiation along the macrophage lineage. To better understand the function of Egr-1 as a positive modulator of monocytic differentiation, in this work we have studied the effect of ectopic expression of Egr-1 on the murine myeloblastic leukemic cell line M1, which is induced for differentiation by the physiological inducer IL-6. It is shown that, unlike in HL-60 and 32Dcl3 cells, ectopic expression of Egr-1 in M1 cells resulted in activation of the macrophage differentiation program in the absence of differentiation inducer. This included the appearance of morphologically differentiated cells, decreased growth rate in mass culture, and cloning efficiency in soft agar, and expression of endogenous c-myb and c-myc mRNAs was markedly downregulated. Untreated M1Egr-1 cells also exhibited cell adherence, expression of Fc and C3 receptors, and upregulation of the myeloid differentiation primary response genes c-Jun, junD, and junB and the late genetic markers ferritin light-chain and lysozyme. Ectopic expression of Egr-1 in M1 cells also dramatically increased the sensitivity of the cells for IL-6-induced differentiation, allowed a higher proportion of M1 cells to become terminally differentiated under conditions of optimal stimulation for differentiation, and decreased M1 leukemogenicity in vivo. These findings demonstrate that the functions of Egr-1 as a positive modulator of macrophage differentiation vary, depending on the state of lineage commitment for differentiation of the hematopoietic cell type.

Lineage-specific regulation of hematopoiesis by HOX-B8 (HOX-2.4): inhibition of granulocytic differentiation and potentiation of monocytic differentiation.

Homeobox proteins comprise a major class of transcription factors, which have been implicated in normal hematopoiesis and leukemogenesis. Notable in this context is the homeobox gene HOX-B8 (formerly known as HOX-2.4), which was shown to cooperate with hematokines to induce leukemia, and to enhance self-renewal of immature myeloid progenitors when expressed alone. How HOX-B8 may affect lineage specific development of hematopoietic progenitor cells is unknown. Here it is shown that ectopic expression of HOX-B8 specifically inhibited dimethyl sulfoxide (DMSO)-induced granulocytic differentiation of autonomously proliferating HL-60 myeloid progenitor cells. HOX-B8 also inhibited the granulocyte colony-stimulating factor (G-CSF)-induced granulocytic developmental program of factor dependent 32Dcl3 hematopoietic progenitors, including survival, proliferation, and differentiation, as evident by rapid apoptosis of the cells following removal of interleukin-3 (IL-3) and addition of G-CSF. In sharp contrast, HOX-B8 had no effect on macrophage differentiation of M1 and HL-60 cells induced by IL-6 and phorbol-12-myristate-13-acetate, respectively. Moreover, HOX-B8 expression endowed the 32Dcl3 cells with the ability to be induced by granulocyte-macrophage colony-stimulating factor (GM-CSF) for terminal differentiation exclusively along the macrophage lineage; this effect was at least partially mediated via expression of the zinc finger transcription factor Egr-1. Thus, ectopic expression of HOX-B8 in hematopoietic progenitor cells appears to differentially affect lineage specific development, negatively regulating granulocyte development and positively regulating macrophage development.

The zinc finger transcription factor Egr-1 potentiates macrophage differentiation of hematopoietic cells.

Previously we have shown that the zinc finger transcription factor Egr-1 is essential for and restricts differentiation of hematopoietic cells along the macrophage lineage, raising the possibility that Egr-1 actually plays a deterministic role in governing the development of hematopoietic precursor cells along the monocytic lineage. To test this hypothesis, we have taken advantage of interleukin-3-dependent 32Dcl3 hematopoietic precursor cells which, in addition to undergoing granulocytic differentiation in response to granulocyte colony-stimulating factor, were found to be induced for limited proliferation, but not differentiation, by granulocyte-macrophage colony-stimulating factor. It was shown that ectopic expression of Egr-1 blocked granulocyte colony-stimulating factor-induced terminal granulocytic differentiation, consistent with previous findings. In addition, ectopic expression of Egr-1 endowed 32Dcl3 cells with ability to be induced by granulocyte-macrophage colony-stimulating factor for terminal differentiation exclusively along the macrophage lineage. Thus, evidence that Egr-1 potentiates terminal macrophage differentiation has been obtained, suggesting that Egr-1 plays a deterministic role in governing the development of hematopoietic cells along the macrophage lineage.

p53 involvement in control of G2 exit of the cell cycle: role in DNA damage-induced apoptosis.

DNA damage in proliferating mammalian cells induces a complex cellular response comprising perturbation of the cell cycle and programmed cell death. The relationship between p53-dependent and p53-independent apoptotic cell death, as well as the cell cycle checkpoints induced by DNA damaging agents were explored in hematopoietic cells, using M1 myeloblastic leukemia cells, which are null for p53 expression, genetically engineered M1 variants, expressing p53ts and bcl-2 transgenes, as well as myeloblast enriched bone-marrow cells obtained from wild type p53 (wt p53) and p53-deficient mice. It is shown that gamma-irradiation of M1p53ts cells activated a function of the temperature sensitive mutant transgene p53 (p53ts), promoting increased apoptosis relative to parental, null p53 M1 cells. It is also shown that the kinetics of apoptotic cell death induced by gamma-irradiation correlated with the rapidity of exit from gamma-ray-induced G2 arrest for all the different hematopoietic cell types indicated above. Finally, data has been obtained to demonstrate that, in addition to a role in apoptosis and G1 arrest, wild-type p53 positively modulated the exit from the gamma-ray-induced G2 checkpoint. Taken together, these findings indicate that this new function for p53 is a component of the physiological pathway by which p53 exerts its role in apoptosis.

Role of outer membrane proteins in immunity against murine salmonellosis--1. Antibody response to crude outer membrane proteins of Salmonella typhimurium.

The humoral immune response to crude outer membrane proteins (comp) of S. typhimurium in mice has been characterized. Maximal and quicker antibody response was observed when 50 micrograms of comp was injected intraperitoneally. The comp of smooth C5 strain of S. typhimurium evoked antibody response to both lipopolysaccharide (LPS) and proteins. Absorption of these sera with LPS coated erythrocytes eliminated the antibodies to LPS completely, while the antibody level to protein was left unaltered. The comp from rough mutant (lacking O-specific chain of LPS) of S. typhimurium elicited antibodies to proteins but not to LPS. These results indicate the concomitant production of antibodies to Salmonella outer membrane proteins also. The significance of such antibodies in protection and diagnosis has been discussed.