Differentiation commitment in normal hemopoiesis and leukemic transformation

Investigating the genetic control of stem cell behavior

The precise control of hematopoietic stem cell fate decisions involves molecules that are differentially expressed in the stem cell versus its non-stem progeny. The construction of representative and high quality cDNA libraries containing stem cell-specific sequences is a first step in elucidating stem cell control mechanisms. Automated bioinformatics in conjunction with high-throughput random sequencing and high-density parallel array hybridization studies make it possible to dissect stem cell molecular pathways and networks. It is the differential analysis of these interacting pathways which will provide the greatest insights into the biological differences underlying stem cell behavior.

Guanosine-rich oligodeoxynucleotides induce proliferation of macrophage progenitors in cultures of murine bone marrow cells

Widely used to specifically inhibit gene expression, synthetic oligodeoxynucleotides (ODN) can exert a plethora of non-antisense effects. Immunostimulation by CpG-ODN has attracted particular attention. ODN rich in the nucleotide guanosine (G-rich ODN) constitute another type of sequences displaying non-antisense-mediated effects. We have examined the effects of CpG- and G-rich ODN on primary mouse bone marrow cells (BMC) in vitro. CpG-ODN induced rapid proliferation of B cells and production of IL-6 and IL-12p40. However, when tested in agar colony assays, CpG-ODN failed to promote the formation of colonies. In marked contrast, G-rich non-CpG-ODN led to sustained proliferation of macrophage-like cells without inducing cytokines or hemopoietic growth factors. Unlike CpG-ODN, G-rich ODN effectively induced the formation of macrophage colonies in agar assays, indicating a direct action on progenitor cells. Electrophoretic mobility shift assays revealed specific binding of G-rich ODN to a non-nuclear protein. The ability of a panel of ODN to compete for binding correlated with their potential to induce proliferation of macrophage-like cells from primary mouse BMC. As such, these data reveal a so far unrecognized potential of G-rich ODN to signal directly outgrowth of macrophage progenitors from BMC.

Optimal Proliferation of a Hematopoietic Progenitor Cell Line Requires Either Costimulation With Stem Cell Factor or Increase of Receptor Expression That Can Be Replaced by Overexpression of Bcl-2

In vitro proliferation of hematopoietic stem cells requires costimulation by multiple regulatory factors whereas expansion of lineage-committed progenitor cells generated by stem cells usually requires only a single factor. The distinct requirement of factors for proliferation coincides with the differential temporal expression of the subunits of cytokine receptors during early stem cell differentiation. In this study, we explored the underlying mechanism of the requirement of costimulation in a hematopoietic progenitor cell line TF-1. We found that granulocyte-macrophage colony-stimulating factor (GM-CSF) optimally activated proliferation of TF-1 cells regardless of the presence or absence of stem cell factor (SCF). However, interleukin-5 (IL-5) alone sustained survival of TF-1 cells and required costimulation of SCF for optimal proliferation. The synergistic effect of SCF was partly due to its anti-apoptosis activity. Overexpression of the IL-5 receptor  subunit (IL5R) in TF-1 cells by genetic selection or retroviral infection also resumed optimal proliferation due to correction of the defect in apoptosis suppression. Exogenous expression of an oncogenic anti-apoptosis protein, Bcl-2, conferred on TF-1 cells an IL-5–dependent phenotype. In summary, our data suggested SCF costimulation is only necessary when the expression level of IL5R is low and apoptosis suppression is defective in the signal transduction of IL-5. Expression of Bcl-2 proteins released the growth restriction of the progenitor cells and may be implicated in leukemia formation.

Optimal Proliferation of a Hematopoietic Progenitor Cell Line Requires Either Costimulation With Stem Cell Factor or Increase of Receptor Expression That Can Be Replaced by Overexpression of Bcl-2

In vitro proliferation of hematopoietic stem cells requires costimulation by multiple regulatory factors whereas expansion of lineage-committed progenitor cells generated by stem cells usually requires only a single factor. The distinct requirement of factors for proliferation coincides with the differential temporal expression of the subunits of cytokine receptors during early stem cell differentiation. In this study, we explored the underlying mechanism of the requirement of costimulation in a hematopoietic progenitor cell line TF-1. We found that granulocyte-macrophage colony-stimulating factor (GM-CSF) optimally activated proliferation of TF-1 cells regardless of the presence or absence of stem cell factor (SCF). However, interleukin-5 (IL-5) alone sustained survival of TF-1 cells and required costimulation of SCF for optimal proliferation. The synergistic effect of SCF was partly due to its anti-apoptosis activity. Overexpression of the IL-5 receptor  subunit (IL5R) in TF-1 cells by genetic selection or retroviral infection also resumed optimal proliferation due to correction of the defect in apoptosis suppression. Exogenous expression of an oncogenic anti-apoptosis protein, Bcl-2, conferred on TF-1 cells an IL-5–dependent phenotype. In summary, our data suggested SCF costimulation is only necessary when the expression level of IL5R is low and apoptosis suppression is defective in the signal transduction of IL-5. Expression of Bcl-2 proteins released the growth restriction of the progenitor cells and may be implicated in leukemia formation.