Stem cell repopulation efficiency but not pool size is governed by p27(kip1)

Diverse mechanisms regulate stem cell self-renewal

To what extent are the pathways that regulate self-renewal conserved between stem cells at different stages of development and in different tissues? Some pathways play a strikingly conserved role in regulating the self-renewal of diverse stem cells, whereas other pathways are specific to stem cells in certain tissues or at certain stages of development. Recent studies have highlighted differences between the self-renewal of embryonic, fetal and adult stem cells. By understanding these similarities and differences we may come to a molecular understanding of how stem cells replicate themselves and why aspects of this process differ between stem cells.

A limited role for p16Ink4a and p19Arf in the loss of hematopoietic stem cells during proliferative stress

It has long been known that prolonged culture or serial transplantation leads to the loss of hematopoietic stem cells (HSCs); however, the mechanisms for this loss are not well understood. We hypothesized that expression of p16Ink4a or p19Arf or both may play a role in the loss of HSCs during conditions of enhanced proliferation, either in vitro or in vivo. Arf was not expressed in freshly isolated HSCs from adult mice but was induced in phenotypically primitive cells after 10 to 12 days in culture. When cultured bone marrow cells from either Arf-/- or Ink4a-Arf-/- mice were compared to wild-type cells in a competitive repopulation assay, no significant differences in HSC activity were seen. We then evaluated the role of p19Arf and p16Ink4a in the loss of HSCs during serial transplantation. Bone marrow cells from Ink4a-Arf-/-, but not Arf-/-, mice had a modestly extended life span and, on average, supported reconstitution of one additional recipient compared to wild-type cells. Mice given transplants of Ink4a-Arf-/-cells eventually did die of hematopoietic failure in the next round of transplantation. We conclude that mechanisms independent of the Ink4a-Arf gene locus play a dominant role in HSC loss during conditions of proliferative stress.

Administration of low-dose cytarabine results in immediate S-phase arrest and subsequent activation of cell cycling in murine stem cells

OBJECTIVE

Hematopoietic stem cells (HSC) are considered to display a quiescent state with low turnover rate. We investigated the cell-cycle kinetics of HSC after a single dose of cytarabine (Ara-C).

MATERIALS AND METHODS

We analyzed by flow cytometry the cell-cycle status of lin(low)sca-1(+)c-kit(+) (LSK) stem cells isolated from the bone marrow of C57Bl/6 mice sacrificed at 0, 2, 4, 6, 8, 12, 20, 48, 72, and 96 hours after intraperitoneal injection of Ara-C (100 mg/kg) using 7-aminoactinomycin-D (7-AAD) for DNA staining. In vivo bromodeoxyuridine (BrdU) incorporation and Ki-67 expression in HCS were also measured.

RESULTS

Two hours after administration of Ara-C, LSK cells ceased to incorporate BrdU. At 4 hours, a decrease of S-phase cells from 10% at baseline to 4% was found (p < 0.05), followed by a rapid increase of BrdU and 7-AAD incorporation reaching a maximum of 28% S-phase cells at 20 hours (p < 0.001). Ki-67 expression suggested recruitment of 20% of cells from G0 into cell cycle. The total number of LSK cells increased 2.5-fold within this short time interval. After 72 hours, a recovery of cell cycling to baseline levels was observed.

CONCLUSION

This data shows that a single injection of Ara-C first rapidly induced S-phase arrest in HSC for up to 4 hours. Subsequently, an unexpectedly rapid activation of HCS with recruitment of G0 cells into cell cycle was observed. The mechanism of cell-cycle activation of LSK cells remains unknown, but reduction of the number of differentiated end cell did not appear to be the primary trigger.

Negative cell-cycle regulators cooperatively control self-renewal and differentiation of haematopoietic stem cells

Haematopoietic stem cells (HSCs) are capable of shifting from a state of relative quiescence under homeostatic conditions to rapid proliferation under conditions of stress. The mechanisms that regulate the relative quiescence of stem cells and its association with self-renewal are unclear, as is the contribution of molecular regulators of the cell cycle to these decisions. Understanding the mechanisms that govern these transitions will provide important insights into cell-cycle regulation of HSCs and possible therapeutic approaches to expand HSCs. We have investigated the role of two negative regulators of the cell cycle, p27(Kip1) and MAD1, in controlling this transition. Here we show that Mad1(-/-)p27(Kip1-/-) bone marrow has a 5.7-fold increase in the frequency of stem cells, and surprisingly, an expanded pool of quiescent HSCs. However, Mad1(-/-)p27(Kip1-/-) stem cells exhibit an enhanced proliferative response under conditions of stress, such as cytokine stimulation in vitro and regeneration of the haematopoietic system after ablation in vivo. Together these data demonstrate that the MYC-antagonist MAD1 and cyclin-dependent kinase inhibitor p27(Kip1) cooperate to regulate the self-renewal and differentiation of HSCs in a context-dependent manner.

Disruption of PLZP in mice leads to increased T-lymphocyte proliferation, cytokine production, and altered hematopoietic stem cell homeostasis

Deregulated function of members of the POK (POZ and Kruppel) family of transcriptional repressors, such as promyelocytic leukemia zinc finger (PLZF) and B-cell lymphoma 6 (BCL-6), plays a critical role in the pathogenesis of acute promyelocytic leukemia (APL) and non-Hodgkin's lymphoma, respectively. PLZP, also known as TZFP, FAZF, or ROG, is a novel POK protein that displays strong homology with PLZF and has been implicated in the pathogenesis of the cancer-predisposing syndrome, Fanconi's anemia, and of APL, in view of its ability to heterodimerize with the FANC-C and PLZF proteins, respectively. Here we report the generation and characterization of mice in which we have specifically inactivated the PLZP gene through in-frame insertion of a lacZ reporter and without perturbing the expression of the neighboring MLL2 gene. We show that PLZP-deficient mice display defects in cell cycle control and cytokine production in the T-cell compartment. Importantly, PLZP inactivation perturbs the homeostasis of the hematopoietic stem and/or progenitor cell. On the basis of our data, a deregulation of PLZP function in Fanconi's anemia and APL may affect the biology of the hematopoietic stem cell, in turn contributing to the pathogenesis of these disorders.

The effect of anti-CD44 monoclonal antibodies on differentiation and proliferation of human acute myeloid leukemia cells

Acute myeloid leukemia (AML) is a clonal malignant disease characterized by an increasing number of immature myeloid cells arrested at various stages of granulocytic and monocytic differentiation. The stage of the blockage defines distinct AML subtypes (AML1 to AML5 are the most frequent ones). There is increasing evidence that the malignant clone is maintained by rare AML stem cells endowed with self-renewal capacity, which through extensive proliferation coupled to partial differentiation, generate leukemic progenitors and blasts, of which the vast majority have limited proliferative capacity. Contrarily to chemotherapy alone, which is still unable to cure most AML patients, the differentiation therapy, which consists in releasing the differentiation blockage of leukemic blasts, has succeeded, when it is combined with chemotherapy, to greatly improve the survival of AML3 patients, using retinoic acid as differentiating agent. However, this molecule is ineffective in other AML subtypes, which are the most frequent. We have shown that specific monoclonal antibodies (mAbs, H90 and A3D8) directed to the CD44 cell surface antigen, that is strongly expressed on human AML blasts, are capable of triggering terminal differentiation of leukemic blasts in AML1 to AML5 subtypes. These results have raised the perspective of developing a CD44-targeted differentiation therapy in most AML cases. Interestingly, these anti-CD44 mAbs can also induce the differentiation of AML cell lines, inhibit their proliferation and, in some cases, induce their apoptotic death. These results suggest that H90 and/or A3D8 mAbs may be capable to inhibit the proliferation of leukemic progenitors, to promote the differentiation of the leukemic stem cells at the expense of their self-renewal, and, perhaps, to induce their apoptotic death, thereby contributing to decrease the size of the leukemic clone. The challenges of an anti-CD44 based differentiation therapy in AML, and its importance in relation to the new other therapies developed in this malignancy, are discussed in this review.

Clinical strategies for expansion of haematopoietic stem cells

Haematopoietic stem cells (HSCs) give rise to all blood and immune cells and are used in clinical transplantation protocols to treat a wide variety of diseases. The ability to increase the number of HSCs either in vivo or in vitro would provide new treatment options, but the amplification of HSCs has been difficult to achieve. Recent insights into the mechanisms of HSC self-renewal now make the amplification of HSCs a plausible clinical goal. This article reviews the molecular mechanisms that control HSC numbers and discusses how these can be modulated to increase the number of HSCs. Clinical applications of HSC expansion are then discussed for their potential to address the current limitations of HSC transplantation.

ASB2 is an Elongin BC-interacting protein that can assemble with Cullin 5 and Rbx1 to reconstitute an E3 ubiquitin ligase complex

The ankyrin repeat-containing protein with a suppressor of cytokine signaling box-2 (ASB2) gene was identified as a retinoic acid-response gene and a target of the promyelocytic leukemia-retinoic acid receptor-alpha oncogenic protein characteristic of acute promyelocytic leukemia. Expression of ASB2 in myeloid leukemia cells inhibits growth and promotes commitment, recapitulating an early step known to be critical for differentiation. Here we show that ASB2, by interacting with the Elongin BC complex, can assemble with Cullin5.Rbx1 to form an E3 ubiquitin ligase complex that stimulates polyubiquitination by the E2 ubiquitin-conjugating enzyme Ubc5. This is a first indication that a member of the ASB protein family, ASB2, is a subunit of an ECS (Elongin C-Cullin-SOCS box)-type E3 ubiquitin ligase complex. Altogether, our results strongly suggest that ASB2 targets specific proteins to destruction by the proteasome in leukemia cells that have been induced to differentiate.

Regulation of hematopoietic stem cell growth

Hematopoietic stem cells (HSC) must balance self-renewal and differentiation to provide sufficient primitive cells to sustain hematopoiesis, while generating more mature cells with specialized capabilities. The enhanced self-renewal capacity of primitive HSCs enables their ability to sustain hematopoiesis throughout decades of life and their ability to repopulate a host when used therapeutically in bone marrow transplantation. However, hematopoietic cell perturbations resulting in unchecked self-renewal participate in leukemogenesis. While mechanisms governing self-renewal are still being uncovered, they are thought to bear relationship to the malignant process in a variety of tumor types and may therefore provide useful therapeutic targets in putative cancer stem cells. This review discusses molecular mechanisms recently defined to participate in HSC governance and highlights features of stem cell interactions with the microenvironment that may help guide therapies directed at HSCs.

Molecular pathways regulating the self-renewal of hematopoietic stem cells

Hematopoietic stem cells (HSC) self-renew throughout life, but the molecular mechanisms by which this process occurs and is regulated are imprecisely understood. We review the published data from overexpression and knockout studies describing genes that influence stem cell self-renewal, including transcription factors, cell cycle regulators, and genes that influence chromosome structure. One model suggesting how some of these disparate classes of molecular regulators might be integrated is presented, focusing on the role of G1/S progression in the developmental switch toward stem cell self-renewal vs differentiation. Experimental exploration of this model and other related hypotheses will hopefully lead to a more complete description of HSC self-renewal and its regulation, both in normal physiology and in applied therapeutics.

Evaluation of peptide-mediated transduction in human CD34+ cells

Protein transduction domains (PTDs) have been used increasingly to deliver biologically active agents to a variety of cell types in vitro and in vivo. To define the most effective PTDs for transducing hematopoietic cells, we have screened a panel of PTD peptides in human CD34(+) cells for delivery of a 60-kd marker protein and assessed its impact on phenotypic maintenence in vitro. Compared to the HIV-TAT peptide, most peptide complexes displayed high efficiency in transducing the CD34(+) cells, except for those based on shorter peptides (4R, 4K, and 5RQ). In particular, the arginine homopolymers including 8R, 10R, and 12R, were internalized by the cells to a greater extent than the other PTDs. Transduction was significantly potentiated by preincubation of cells with dextran sulfate. Importantly, colony forming ability and CD34(+) CD38(-) primitive phenotype were not significantly altered in the presence of these peptides during a short-term liquid culture. Together, these data suggest the potential usefulness of arginine homopolymers in hematopoietic stem and progenitor cell manipulations.

Cell cycle inhibitors in normal and tumor stem cells

Emerging data suggest that stem cells may be one of the key elements in normal tissue regeneration and cancer development, although they are not necessarily the same entity in both scenarios. As extensively demonstrated in the hematopoietic system, stem cell repopulation is hierarchically organized and is intrinsically limited by the intracellular cell cycle inhibitors. Their inhibitory effects appear to be highly associated with the differentiation stage in stem/progenitor pools. While this negative regulation is important for maintaining homeostasis, especially at the stem cell level under physiological cues or pathological insults, it constrains the therapeutic use of adult stem cells in vitro and restricts endogenous tissue repair after injury. On the other hand, disruption of cell cycle inhibition may contribute to the formation of the so-called 'tumor stem cells' (TSCs) that are currently hypothesized to be partially responsible for tumorigenesis and recurrence of cancer after conventional therapies. Therefore, understanding how cell cycle inhibitors control stem cells may offer new strategies not only for therapeutic manipulations of normal stem cells but also for novel therapies selectively targeting TSCs.

Transforming growth factor beta-induced cell cycle arrest of human hematopoietic cells requires p57KIP2 up-regulation

Transforming growth factor beta (TGFbeta) is one of few known negative regulators of hematopoiesis, yet the mechanisms by which it affects cell cycle arrest and stem cell quiescence are poorly understood. Induction of the cyclin-dependent kinase inhibitors, p15INK4b (p15) and p21WAF1 (p21) is important for TGFbeta-mediated cytostasis in epithelial cells but not in hematopoietic cells. Using primary human hematopoietic cells and microarray analysis, we identified p57KIP2 (p57) as the only cyclin-dependent kinase inhibitor induced by TGFbeta. Up-regulation of p57 mRNA and protein occurs before TGFbeta-induced G1 cell cycle arrest, requires transcription, and is mediated via a highly conserved region of the proximal p57 promoter. The up-regulation of p57 is essential for TGFbeta-induced cell cycle arrest in these cells, because two different small interfering RNAs that prevent p57 up-regulation block the cytostatic effects of TGFbeta on human hematopoietic cells. Reduction of basal p57 expression by this approach also allows hematopoietic cells to proliferate more readily in the absence of TGFbeta. p57 is a putative tumor suppressor gene whose expression is frequently silenced by promoter hypermethylation in hematologic malignancies. Our studies identify a molecular pathway by which TGFbeta mediates its cytostatic effects on human hematopoietic cells and suggests an explanation for the frequent silencing of p57 expression.

Stem cells and cancer; the polycomb connection

Proteins from the Polycomb group (PcG) are epigenetic chromatin modifiers involved in cancer development and also in the maintenance of embryonic and adult stem cells. The therapeutic potential of stem cells and the growing conviction that tumors contain stem cells highlights the importance of understanding the extrinsic and intrinsic circuitry controlling stem cell fate and their connections to cancer.

Regenerative response in ischemic brain restricted by p21cip1/waf1

Neural precursor cells from adults have exceptional proliferative and differentiative capability in vitro yet respond minimally to in vivo brain injury due to constraining mechanisms that are poorly defined. We assessed whether cell cycle inhibitors that restrict stem cell populations in other tissues may participate in limiting neural stem cell reactivity in vivo. The cyclin-dependent kinase inhibitor, p21^cip1/waf1 (p21), maintains hematopoietic stem cell quiescence, and we evaluated its role in the regenerative response of neural tissue after ischemic injury using the mice deficient in p21. Although steady-state conditions revealed no increase in primitive cell proliferation in p21-null mice, a significantly larger fraction of quiescent neural precursors was activated in the hippocampus and subventricular zone after brain ischemia. The hippocampal precursors migrated and differentiated into a higher number of neurons after injury. Therefore, p21 is an intrinsic suppressor to neural regeneration after brain injury and may serve as a common molecular regulator restricting proliferation among stem cell pools from distinct tissue types.

Bone marrow-derived immune cells regulate vascular disease through a p27(Kip1)-dependent mechanism

The cyclin-dependent kinase inhibitors are key regulators of cell cycle progression. Although implicated in carcinogenesis, they inhibit the proliferation of a variety of normal cell types, and their role in diverse human diseases is not fully understood. Here, we report that p27(Kip1) plays a major role in cardiovascular disease through its effects on the proliferation of bone marrow-derived (BM-derived) immune cells that migrate into vascular lesions. Lesion formation after mechanical arterial injury was markedly increased in mice with homozygous deletion of p27(Kip1), characterized by prominent vascular infiltration by immune and inflammatory cells. Vascular occlusion was substantially increased when BM-derived cells from p27(-/-) mice repopulated vascular lesions induced by mechanical injury in p27(+/+) recipients, in contrast to p27(+/+) BM donors. To determine the contribution of immune cells to vascular injury, transplantation was performed with BM derived from RAG(-/-) and RAG(+/+) mice. RAG(+/+) BM markedly exacerbated vascular proliferative lesions compared with what was found in RAG(-/-) donors. Taken together, these findings suggest that vascular repair and regeneration is regulated by the proliferation of BM-derived hematopoietic and nonhematopoietic cells through a p27(Kip1)-dependent mechanism and that immune cells largely mediate these effects.

Polycomb group gene mel-18 modulates the self-renewal activity and cell cycle status of hematopoietic stem cells

OBJECTIVE

Mel-18 is a member of the mammalian Polycomb group (PcG) genes. This family of genes regulates global gene expression in many biologic processes, including hematopoiesis and anterior-posterior axis formation by manipulating specific target genes, including members of the Hox family. Here, we demonstrate that mel-18 negatively regulates the self-renewal activity of hematopoietic stem cells (HSCs).

MATERIALS AND METHODS

Long-term reconstitution activity was evaluated by competitive repopulating unit (CRU) and mean activity of the stem cells (MAS) assays in vivo in bone marrow cells (BMCs) derived from mel-18(-/-) and mel-18 tg mice. The expression levels of mel-18 and Hoxb4 were measured by quantitative real-time reverse transcription polymerase chain reaction.

RESULTS

The Hoxb4 gene was highly expressed in HSCs derived from mel-18(-/-) mice. The observed CRUs were 3.21, 4.77, 3.32, and 1.64 CRU per 10(5) BMCs in mel-18(+/+), mel-18(-/-), C57BL/6, and mel-18 tg, respectively. MAS was 0.58, 0.18, 0.41, and 5.89 in mel-18(+/+), mel-18(-/-), C57BL/6, and mel-18 tg, respectively. The percentage in G0 phase HSCs (lin(-)flk2(-)c-Kit(+)Sca1+ cells) was increased in mel-18(-/-) mice and decreased in mel-18 tg mice.

CONCLUSION

Loss or knockdown of mel-18 leads to the expression of Hoxb4, an increase in the proportion of HSCs in G0 phase, and the subsequent promotion of HSC self-renewal. These findings will enable us to develop new approaches for controlling HSC activity for hematopoietic transplantations based on ex vivo expansion of HSCs.

Nitric Oxide Is a Regulator of Hematopoietic Stem Cell Activity

Hematopoietic stem cells give rise to various multipotent progenitor populations, which expand in response to cytokines and which ultimately generate all of the elements of the blood. Here we show that it is possible to increase the number of stem and progenitor cells in the bone marrow (BM) by suppressing the activity of NO synthases (NOS). Exposure of mice to NOS inhibitors, either directly or after irradiation and BM transplantation, increases the number of stem cells in the BM. In the transplantation model, this increase is followed by a transient increase in the number of neutrophils in the peripheral blood. Thus, our results indicate that NO is important for the control of hematopoietic stem cells in the BM. They further suggest that suppression of NO synthase activity may allow expansion of the number of hematopoietic stem and progenitor cells or neutrophils for therapeutic purposes.

SDF-1 involvement in endothelial phenotype and ischemia-induced recruitment of bone marrow progenitor cells

Chemokine stromal derived factor 1 (SDF-1) is involved in trafficking of hematopoietic stem cells (HSCs) from the bone marrow (BM) to peripheral blood (PB) and has been found to enhance postischemia angiogenesis. This study was aimed at investigating whether SDF-1 plays a role in differentiation of BM-derived c-kit(+) stem cells into endothelial progenitor cells (EPCs) and in ischemia-induced trafficking of stem cells from PB to ischemic tissues. We found that SDF-1 enhanced EPC number by promoting alpha(2), alpha(4), and alpha(5) integrin-mediated adhesion to fibronectin and collagen I. EPC differentiation was reduced in mitogen-stimulated c-kit(+) cells, while cytokine withdrawal or the overexpression of the cyclin-dependent kinase (CDK) inhibitor p16(INK4) restored such differentiation, suggesting a link between control of cell cycle and EPC differentiation. We also analyzed the time course of SDF-1 expression in a mouse model of hind-limb ischemia. Shortly after femoral artery dissection, plasma SDF-1 levels were up-regulated, while SDF-1 expression in the bone marrow was down-regulated in a timely fashion with the increase in the percentage of PB progenitor cells. An increase in ischemic tissue expression of SDF-1 at RNA and protein level was also observed. Finally, using an in vivo assay such as injection of matrigel plugs, we found that SDF-1 improves formation of tubulelike structures by coinjected c-kit(+) cells. Our findings unravel a function for SDF-1 in increase of EPC number and formation of vascular structures by bone marrow progenitor cells.

In vivo self-renewing divisions of haematopoietic stem cells are increased in the absence of the early G1-phase inhibitor, p18INK4C

Self-renewal of stem cells is critical for tissue repair and maintenance of organ integrity in most mammalian systems. The relative asymmetry between self-renewal and differentiation in balance with apoptosis determines the size and durability of a stem-cell pool. Regulation of the cell cycle is one of the fundamental mechanisms underlying determination of cell fate. Absence of p21(Cip1/Waf1), a late G1-phase cyclin-dependent kinase inhibitor (CKI), has previously been shown to enable cell-cycle entry of haematopoietic stem cells, but leads to premature exhaustion of the stem cells under conditions of stress. We show here that deletion of an early G1-phase CKI, p18(INK4C), results in strikingly improved long-term engraftment, largely by increasing self-renewing divisions of the primitive cells in murine transplant models. Therefore, different CKIs have highly distinct effects on the kinetics of stem cells, possibly because of their active position in the cell cycle, and p18(INK4C) appears to be a strong inhibitor limiting the potential of stem-cell self-renewal in vivo.

Cdk-inhibitors and exit from quiescence in primitive haematopoietic cell subsets

Prolonged quiescence of haematopoietic stem cells has been proposed to support durable haematopoiesis through clonal succession. Genetic experiments in mice have implicated the cyclin-dependent kinase inhibitor (cdki) p21Waf1 in sustaining stem cell quiescence, and the cdki p27Kip1 in inhibiting the expansion of more mature progenitor cells. The expression of these inhibitory proteins in human haematopoietic stem cell candidates has not hitherto been studied. We describe a rare subpopulation (3 x 10-7 umbilical cord mononuclear cells) of lineage-negative cells that exhibited sustained resistance over months to cytokine-induced cycling, and characterized the expression of p21Waf1 and p27Kip1 proteins in these cells. Whereas p27Kip1 was uniformly expressed in these cells, the expression of p21Waf1 in this population and in lineage-negative CD34+ cells was variable. For this rare subset of cells exhibiting prolonged quiescence, p21Waf1 may be dispensable and p27Kip1 necessary for growth arrest.

Huntingtin Interacting Protein 1 mutations lead to abnormal hematopoiesis, spinal defects and cataracts

Huntingtin Interacting Protein 1 (HIP1) binds clathrin and AP2, is overexpressed in multiple human tumors, and transforms fibroblasts. The function of HIP1 is unknown although it is thought to play a fundamental role in clathrin trafficking. Gene-targeted Hip1-/- mice develop premature testicular degeneration and severe spinal deformities. Yet, although HIP1 is expressed in many tissues including the spleen and bone marrow and was part of a leukemogenic translocation, its role in hematopoiesis has not been examined. In this study we report that three different mutations of murine Hip1 lead to hematopoietic abnormalities reflected by diminished early progenitor frequencies and resistance to 5-FU-induced bone marrow toxicity. Two of the Hip1 mutant lines also display the previously described spinal defects. These observations indicate that, in addition to being required for the survival/proliferation of cancer cells and germline progenitors, HIP1 is also required for the survival/proliferation of diverse types of somatic cells, including hematopoietic progenitors.

The biology of CML blast crisis

Chronic myelogenous leukemia (CML) evolves from a chronic phase characterized by the Philadelphia chromosome as the sole genetic abnormality into blast crisis, which is often associated with additional chromosomal and molecular secondary changes. Although the pathogenic effects of most CML blast crisis secondary changes are still poorly understood, ample evidence suggests that the phenotype of CML blast crisis cells (enhanced proliferation and survival, differentiation arrest) depends on cooperation of BCR/ABL with genes dysregulated during disease progression. Most genetic abnormalities of CML blast crisis have a direct or indirect effect on p53 or Rb (or both) gene activity, which are primarily required for cell proliferation and survival, but not differentiation. Thus, the differentiation arrest of CML blast crisis cells is a secondary consequence of these abnormalities or is caused by dysregulation of differentiation-regulatory genes (ie, C/EBPalpha). Validation of the critical role of certain secondary changes (ie, loss of p53 or C/EBPalpha function) in murine models of CML blast crisis and in in vitro assays of BCR/ABL transformation of human hematopoietic progenitors might lead to the development of novel therapies based on targeting BCR/ABL and inhibiting or restoring the gene activity gained or lost during disease progression (ie, p53 or C/EBPalpha).

Different in vivo repopulating activities of purified hematopoietic stem cells before and after being stimulated to divide in vitro with the same kinetics

OBJECTIVE

The Hoechst 33342-effluxing side population (SP) of adult mouse bone marrow (BM) contains most of the hematopoietic stem cells (HSCs). Here we measured the HSC content of specific subsets of SP cells and then used a highly HSC-enriched fraction to investigate the effect of different growth factors on the initial rate of HSC proliferation in vitro and the accompanying maintenance (or loss) of HSCs in the first-division progeny.

MATERIALS AND METHODS

Staining with Rhodamine-123 (Rho) was used to subfractionate lineage marker-negative (lin-) SP cells. Cells were assayed for HSCs by examining their ability to generate sustained (>4 months) multi-lineage lympho-myeloid clones in irradiated hosts. Cultures of single lin- Rho- SP cells were used to monitor growth factor effects on HSC proliferation and function.

RESULTS

More than 40% of mice injected with single lin- Rho- SP cells showed long-term lympho-myeloid reconstitution. Some clones peaked within 8 weeks but others developed more slowly apparently unrelated to the pattern of lineage representation. 3/3 clones tested repopulated secondary mice. Either Steel factor+interleukin-11 (+/- flt3-ligand) or Steel factor+thrombopoietin stimulated at least 75% of single lin- Rho- SP cells to divide in vitro with the same synchronous kinetics. However, in the first cocktail, the frequency of HSCs among the first-division doublets was preserved but in the latter it was greatly diminished.

CONCLUSION

Exogenous growth factors can differentially affect the ability of HSCs to execute a self-renewal division within a single cell cycle even when the kinetics of proliferation are the same.

Cyclin D2controls B cell progenitor numbers

Cyclin D2 affects B cell proliferation and differentiation in vivo. It is rate-limiting for B cell receptor (BCR)-dependent proliferation of B cells, and cyclin D2-/- mice lack CD5+(B1) B lymphocytes. We show here that the bone marrow (BM) of cyclin D2-/- mice contains half the numbers of Sca1+B220+ B cell progenitors but normal levels of Sca1+ progenitor cells of other lineages. In addition, clonal analysis of BM from the cyclin D2-/- and cyclin D2+/+ mice confirmed that there were fewer B cell progenitors (B220+) in the cyclin D2-/- mice. In addition, the colonies from cyclin D2-/- mice were less mature (CD19lo) than those from cyclin D2+/+ mice (CD19Hi). The number of mature B2 B cells in vivo is the same in cyclin D2-/- and cyclin D2+/+ animals. Lack of cyclin D2 protein may be compensated by cyclin D3, as cyclin-dependent kinase (cdk)6 coimmunoprecipitates with cyclin D3 but not cyclin D1 from BM mononuclear cells of cyclin D2-/- mice. It is active, as endogenous retinoblastoma protein is phosphorylated at the cdk6/4-cyclin D-specific sites, S807/811. We conclude that cyclin D2 is rate-limiting for the production of B lymphoid progenitor cells whose proliferation does not depend on BCR signaling.

Functional and kinetic characterization of granulocyte colony-stimulating factor-primed CD34− human stem cells

We assessed the functional properties and the kinetic status in vitro, and the engraftment potential in vivo of human haematopoietic stem cells according to the expression of CD34 antigen. Lin-CD34- and Lin-CD34+ cells were isolated from granulocyte colony-stimulating factor-primed peripheral blood (PB) cells of healthy donors. The CD34- cell fraction did not contain either clonogenic cells in semisolid culture or long-term culture initiating cells (LTC-IC). However, stroma-dependent liquid cultures and cytokines induced CD34 expression on a minority of stem cells, acquisition of clonogenic capacity and generation of LTC-IC. Significantly higher percentages of quiescent G0 cells and lower percentages of cycling G1 cells were found in Lin-CD34- cells when compared with Lin-CD34+ cells. Kinetic quiescence of Lin-CD34- cells was associated with a significantly higher expression of the negative regulators of the cell cycle, p27Kip1 and p21(cip1/waf1). Cytokine-mediated induction of CD34, in vitro, resulted in cycling of stem cells and downregulation of p27. There was a higher rate of human long-term engraftment in immunocompromised non-obese diabetic (NOD)/recombination activating gene 1null and NOD/severe combined immunodeficient-beta2microglobulin(null) mice injected with CD34+ cells. Thus, our study indicated that CD34 expression on human PB stem cells was associated with haematopoietic activity, cell-cycle recruitment and downregulation of p27Kip1 in vitro and higher engraftment capacity in vivo.

CD44: a new means to inhibit acute myeloid leukemia cell proliferation via p27Kip1

Acute myeloid leukemia (AML) is sustained by the extensive proliferation of leukemic stem and progenitor cells, which give rise to the population of leukemic blasts with defective differentiation and low proliferative capacity. We have recently shown that ligation of CD44, a cell surface molecule present on AML cells, with specific monoclonal antibodies (mAbs) inhibits their proliferation. However, its mechanism has not been investigated yet. Here, using the NB4 cell line as a model of proliferating human AML cells, and the A3D8 mAb to ligate CD44, we show for the first time that CD44 ligation stabilizes the cyclin-dependent kinase inhibitor p27(Kip1) (p27) protein, resulting in increased association with cyclin E/Cdk2 complexes and inhibition of their kinase activity. Moreover, using a p27 antisense vector, we provide direct evidence that p27 is the main mediator of cell growth arrest by CD44. CD44 ligation also leads to p27 accumulation in THP-1, KG1a, and HL60 cell lines and in primary leukemic cells, suggesting that this process is general in AML. Taken together, our present results suggest that CD44 is a new and efficient means to increase the expression of p27 in AML cells. Considering that elevated expression of p27 is a factor of good prognosis in AML, these results provide a new basis for developing CD44-targeted therapy in AML.

Selective inactivation of p27(Kip1) in hematopoietic progenitor cells increases neointimal macrophage proliferation and accelerates atherosclerosis

Excessive proliferation of immune cells and vascular smooth myocytes (VSMCs) contributes to atherosclerosis. We have previously shown that whole-body inactivation of the growth suppressor p27 exacerbates atherosclerosis in apolipoprotein E-null mice (apoE-/-), and this correlated with increased proliferation of arterial macrophages and VSMCs. In the present study, we postulated that targeted disruption of bone marrow (BM) p27 is sufficient to enhance arterial macrophage proliferation and atherosclerosis. To test this hypothesis, sublethally irradiated apoE-/- mice with an intact p27 gene received a BM transplant from either apoE-/- or p27-/-apoE-/- doubly deficient donor mice and challenged with a high-cholesterol diet. Compared with mice that received an apoE-/- BM transplant, reconstitution with p27-/-apoE-/- doubly deficient marrow increased the expression of proliferating cell nuclear antigen in neointimal macrophages and accelerated aortic atherosclerosis, and this correlated with augmented aortic expression of the inflammatory cytokines CCL2/MCP-1 (monocyte chemoattractant protein 1) and CCL5/RANTES (regulated on activation, normal T-cell expressed and secreted). Overall, these findings provide evidence that p27 deficiency in hematopoietic progenitor cells enhances the inflammatory/proliferative response induced by dietary cholesterol and accelerates atherosclerosis.

Fanconi anemia type C-deficient hematopoietic stem/progenitor cells exhibit aberrant cell cycle control

The pathogenesis of bone marrow failure in Fanconi anemia is poorly understood. Suggested mechanisms include enhanced apoptosis secondary to DNA damage and altered inhibitory cytokine signaling. Recent data determined that disrupted cell cycle control of hematopoietic stem and/or progenitor cells disrupts normal hematopoiesis with increased hematopoietic stem cell cycling resulting in diminished function and increased sensitivity to cell cycle-specific apoptotic stimuli. Here, we used Fanconi anemia complementation type C-deficient (Fancc-/-) mice to demonstrate that Fancc-/- phenotypically defined cell populations enriched for hematopoietic stem and progenitor cells exhibit increased cycling. In addition, we established that the defect in cell cycle regulation is not a compensatory mechanism from enhanced apoptosis occurring in vivo. Collectively, these data provide a previously unrecognized phenotype in Fancc-/- hematopoietic stem/progenitor cells, which may contribute to the progressive bone marrow failure in Fanconi anemia.

Repopulating defect of mismatch repair-deficient hematopoietic stem cells

Mismatch repair deficiency is associated with carcinogenesis, increased spontaneous and induced mutagenesis, and resistance to methylating agents. In humans, leukemias and lymphomas arise in the background of mismatch repair deficiency, raising the possibility that hematopoiesis is abnormal as well. To address hematopoiesis in MSH2-/- mice, we collected marrow and performed serial transplantations of these cells, alone or mixed with wild-type cells, into lethally irradiated healthy mice. Transplant recipients were observed or treated with the methylating agent, temozolomide (TMZ). Methylating agent tolerance was evident by the competitive survival advantage of MSH2-/- marrow progenitors compared with wild-type cells after each TMZ exposure. However, serial repopulation by MSH2-/- cells was deficient compared with wild-type cells. In recipients of mixed populations, the MSH 2-/- cells were lost from the marrow, and mice receiving MSH2-/- cells plus TMZ could not be reconstituted in the third passage, whereas all wild-type cell recipients survived. No differences in telomere length, cell cycle distribution, or homing were observed, but an increase in microsatellite instability was seen in the MSH2-/- early progenitor colony-forming unit (CFU) and Sca+Kit+lin--derived clones. Thus, mismatch repair deficiency is associated with a hematopoietic repopulation defect and stem cell exhaustion because of accumulation of genomic instability.

Ex vivo targeting of p21Cip1/Waf1 permits relative expansion of human hematopoietic stem cells

Relative quiescence is a defining characteristic of hematopoietic stem cells. Reasoning that inhibitory tone dominates control of stem cell cycling, we previously showed that mice engineered to be deficient in the cyclin-dependent kinase inhibitor, p21Cip1/Waf1 (p21), have an increased stem cell pool under homeostatic conditions. Since p21 was necessary to maintain stem cell quiescence and its absence sufficient to permit increased murine stem cell cycling, we tested whether reduction of p21 alone in human adult-derived stem cells could affect stem cell proliferation. We demonstrate here that interrupting p21 expression ex vivo resulted in expanded stem cell number and in vivo stem cell function compared with control, manipulated cells. Further, we demonstrate full multilineage reconstitution capability in cells where p21 expression was knocked down. Therefore, lifting the brake on cell proliferation by altering cell cycle checkpoints provides an alternative paradigm for increasing hematopoietic stem cell numbers. This approach may be useful for relative ex vivo human stem cell expansion.

Chronic myelogenous leukemia as a paradigm of early cancer and possible curative strategies

The chronological history of the important discoveries leading to our present understanding of the essential clinical, biological, biochemical, and molecular features of chronic myelogenous leukemia (CML) are first reviewed, focusing in particular on abnormalities that are responsible for the massive myeloid expansion. CML is an excellent target for the development of selective treatment because of its highly consistent genetic abnormality and qualitatively different fusion gene product, p210(bcr-abl). It is likely that the multiple signaling pathways dysregulated by p210(bcr-abl) are sufficient to explain all the initial manifestations of the chronic phase of the disease, although understanding of the circuitry is still very incomplete. Evidence is presented that the signaling pathways that are constitutively activated in CML stem cells and primitive progenitors cooperate with cytokines to increase the proportion of stem cells that are activated and thereby increase recruitment into the committed progenitor cell pool, and that this increased activation is probably the primary cause of the massive myeloid expansion in CML. The cooperative interactions between Bcr-Abl and cytokine-activated pathways interfere with the synergistic interactions between multiple cytokines that are normally required for the activation of stem cells, while at the same time causing numerous subtle biochemical and functional abnormalities in the later progenitors and precursor cells. The committed CML progenitors have discordant maturation and reduced proliferative capacity compared to normal committed progenitors, and like them, are destined to die after a limited number of divisions. Thus, the primary goal of any curative strategy must be to eliminate all Philadelphia positive (Ph+) primitive cells that are capable of symmetric division and thereby able to expand the Ph+ stem cell pool and recreate the disease. Several highly potent and moderately selective inhibitors of Bcr-Abl kinase have recently been discovered that are capable of killing the majority of actively proliferating early CML progenitors with minimal effects on normal progenitors. However, like their normal counterparts, most of the CML primitive stem cells are quiescent at any given time and are relatively invulnerable to the Bcr-Abl kinase inhibitors as well as other drugs. We propose that survival of dormant Ph+ stem cells may be the most important reason for the inability to cure the disease during initial treatment, while resistance to the inhibitors and other drugs becomes increasingly important later. An outline of a possible curative strategy is presented that attempts to take advantage of the subtle differences in the proliferative behavior of normal and Ph+ stem cells and the newly discovered selective inhibitors of Bcr-Abl. Leukemia (2003) 17, 1211-1262. doi:10.1038/sj.leu.2402912

Polycomb group genes as epigenetic regulators of normal and leukemic hemopoiesis

Epigenetic modification of chromatin structure underlies the differentiation of pluripotent hemopoietic stem cells (HSCs) into their committed/differentiated progeny. Compelling evidence indicates that Polycomb group (PcG) genes play a key role in normal and leukemic hemopoiesis through epigenetic regulation of HSC self-renewal/proliferation and commitment. The PcG proteins are constituents of evolutionary highly conserved molecular pathways regulating cell fate in several other tissues through diverse mechanisms, including 1) regulation of self-renewal/proliferation, 2) regulation of senescence/immortalization, 3) interaction with the initiation transcription machinery, 4) interaction with chromatin-condensation proteins, 5) modification of histones, 6) inactivation of paternal X chromosome, and 7) regulation of cell death. It is therefore not surprising that PcG genes lead to pleiotropic phenotypes when mutated and have been associated with malignancies in several systems in both mice and humans. Although much remains to be learned regarding the PcG mechanism(s) of action, advances in identifying the functional domains and enzymatic activities of these multimeric protein complexes have provided insights into how PcG proteins accomplish such processes. Some of the new insights into a role for the PcG cellular memory system in regulating normal and leukemic hemopoiesis are reviewed here, with special emphasis on their potential involvement in epigenetic regulation of gene expression through modification of chromatin structure.

P2Y-like receptor, GPR105 (P2Y14), identifies and mediates chemotaxis of bone-marrow hematopoietic stem cells

Hematopoiesis in mammals undergoes a developmental shift in location from fetal liver to bone marrow accompanied by a gradual transition from highly proliferative to deeply quiescent stem cell populations. P2Y receptors are G-protein-coupled nucleotide receptors participating in vascular and immune responses to injury. We identified a P2Y-like receptor for UDP-conjugated sugars, GPR105 (P2Y14), with restricted expression on primitive cells in the hematopoietic lineage. Anti-GPR105 antibody selectively isolated a subset of hematopoietic cells within the fetal bone marrow, but not in the fetal liver, that was enriched for G0 cell cycle status and for in vitro stem-cell-like multipotential long-term culture capability. Conditioned media from bone marrow stroma induced receptor activation and chemotaxis that was sensitive to G alpha i and anti-receptor antibody inhibition. GPR105 is a G-protein-coupled receptor identifying a quiescent, primitive population of hematopoietic cells restricted to bone marrow. It mediates primitive cell responses to specific hematopoietic microenvironments and extends the known immune system functions of P2Y receptors to the stem cell level. These data suggest a new class of receptors participating in the regulation of the stem cell compartment.

p27(Kip1) (Cdkn1b)-deficient mice are susceptible to chemical carcinogenesis and may be a useful model for carcinogen screening

The two-year rodent bioassay is one of several tests that are widely used by governmental regulatory agencies as well as pharmaceutical and chemical companies to determine the carcinogenic potential of chemicals or environmental agents where human exposure is anticipated. That this assay has remained relatively unchanged for the last 25 years is a testament to the power of this approach to identify carcinogens and thus to minimize human exposure. However, there has long been controversy over the specificity and relevance of the rodent bioassay as well as its high cost in terms of time, expense, and numbers of mice. Much discussion has been generated in recent years over how to improve the 2-year rodent bioassay for more accurate and faster detection of potential human carcinogens. Here, we argue that the use of p27(Kip1) (Cdkn1b) knockout mouse for carcinogen screening may solve several shortcomings inherent in the conventional bioassay while preserving its best quality, that is, protecting public health by providing reliable in vivo information on the potential of chemicals to cause cancer.

Expansion of hematopoietic stem cell phenotype and activity in Trp53-null mice

OBJECTIVES

To study the effects of transformation-related protein 53 (Trp53) and other genes on hematopoiesis and hematopoietic stem cells (HSCs).

METHODS

Frequencies of murine bone marrow cells (BMCs) with the Lin(-)Sca-1(+)c-kit(+)CD34- phenotype were analyzed by flow cytometry, and were increased in mice with germ-line deletion of the Trp53 (Trp53(-/-)) gene but not in 25 other deletions of genes involved in cell cycling, development, cancer, or hematopoiesis. Therefore, Trp53(-/-) and wild-type Trp53(+/+) mice were compared using the following assays: complete blood counts, day-9 colony-forming unit spleen (CFU-S), and competitive repopulation. In the latter assay, donor repopulating ability was analyzed at one, three, and five months, while recipient survival and recipient blood and bone marrow cell composition were analyzed at five months, after transplantation.

RESULTS

In comparison to wild-type controls, Trp53(-/-) mice had normal blood and bone marrow cell counts, increased CD11b(+), and decreased CD45R(+) cell proportions in blood and bone marrow, twice as many Lin(-)Sca-1(+)c-kit(+)CD34(-) BMCs, and 37% more day-9 CFU-S. In the competitive repopulation assay, Trp53(-/-) BMCs engrafted lethally irradiated recipients two to four times better than Trp53(+/+) BMCs. The Trp53(-/-) engraftment advantage increased with time in the recipients. Recipients of Trp53(-/-) donors had two to three times more Lin(-)Sca-1(+)c-kit(+)CD34(-) BMCs than recipients of Trp53(+/+) donors at five months after transplantation. However, only 44% of recipients of Trp53(-/-) donors survived five months after trans-plantation, compared with 92% of recipients of Trp53(+/+) donors.

CONCLUSION

The Trp53-null allele expands bone marrow Lin(-)Sca-1(+)c-kit(+)CD34(-) cells and the overall activity of HSCs; however, it increases recipient mortality.

Defective gene expression, S phase progression, and maturation during hematopoiesis in E2F1/E2F2 mutant mice

E2F plays critical roles in cell cycle progression by regulating the expression of genes involved in nucleotide synthesis, DNA replication, and cell cycle control. We show that the combined loss of E2F1 and E2F2 in mice leads to profound cell-autonomous defects in the hematopoietic development of multiple cell lineages. E2F2 mutant mice show erythroid maturation defects that are comparable with those observed in patients with megaloblastic anemia. Importantly, hematopoietic defects observed in E2F1/E2F2 double-knockout (DKO) mice appear to result from impeded S phase progression in hematopoietic progenitor cells. During DKO B-cell maturation, differentiation beyond the large pre-BII-cell stage is defective, presumably due to failed cell cycle exit, and the cells undergo apoptosis. However, apoptosis appears to be the consequence of failed maturation, not the cause. Despite the accumulation of hematopoietic progenitor cells in S phase, the combined loss of E2F1 and E2F2 results in significantly decreased expression and activities of several E2F target genes including cyclin A2. Our results indicate specific roles for E2F1 and E2F2 in the induction of E2F target genes, which contribute to efficient expansion and maturation of hematopoietic progenitor cells. Thus, E2F1 and E2F2 play essential and redundant roles in the proper coordination of cell cycle progression with differentiation which is necessary for efficient hematopoiesis.

Hematopoietic progenitor cells derived from embryonic stem cells: analysis of gene expression

Rhesus monkey embryonic stem (ES) cells undergo differentiation in vitro to generate hematopoietic progenitor cells. Our previous studies demonstrated a high degree of similarity in the expression of genes associated with hematopoietic differentiation, homing, and engraftment in CD34(+) and CD34(+)CD38(-) cells from rhesus monkey ES cells and from fresh or cultured bone marrow (BM). In the present study, we compared the expression patterns of cyclins, cyclin-dependent kinases (CDKs) and CDK inhibitors (CDIs) in these cells. The expression of genes for cyclins, CDKs, and CDIs was similar among the hematopoietic progenitor cells of different origins, with only minor differences. Differentially expressed genes were also analyzed in CD34(+) lineage-negative cells derived from mouse ES cells and from BM. No difference or totally divergent results were obtained with the latter system, suggesting that this variation may be species specific. We observed, however, that CD34(+) and CD34(+)CD38(-) cells derived from ES cells expressed embryonic epsilon and zeta as well as alpha, beta, and gamma globin genes, whereas no expression of embryonic globins could be detected in the cell preparations from BM. Moreover, erythroblast-enriched CD34(-) cells derived from 4- or 5-week ES cell differentiation cultures also expressed embryonic, fetal, and adult globin genes, with greater beta gene expression, but otherwise were identical to those of the more primitive CD34(+) cells derived from 2-week ES cultures. These latter observations may reflect the presence of heterogeneous cell populations within the cell fractions that were compared, or they may represent variability among ES-cell-derived hematopoietic stem cells.

P21 functions to maintain quiescence of p27-deficient hepatocytes

Hepatocytes rarely proliferate in the healthy adult liver. We explored the roles of the cyclin kinase inhibitors p21 and p27 in maintaining hepatocyte quiescence. p27 is expressed throughout the wild-type liver, but the related protein p21 was not detected. However, p21 was detected in livers of p27-deficient mice. Increased p21 protein levels did not result from an increase in p21 mRNA expression, indicating that p21 expression is regulated post-transcriptionally. p21 protein levels increased in cultured primary hepatocytes treated with the proteasome inhibitor MG132 and cycloheximide, indicating that p21 expression is regulated at the level of protein stability in liver cells. Although increased expression of cyclin-dependent kinase (Cdk) 4, Cdk2, and proliferating cell nuclear antigen was detected in p27-deficient livers, increased hepatocyte proliferation was detected only in livers of mice deficient for both p21 and p27. In p27-deficient livers, p21 was found in complexes with Cdk2 and CdK4 and can compensate for the absence of p27. Our data indicate that cyclin kinase inhibitor activity is important for maintaining hepatocyte quiescence in the adult liver. Significant increases in p21 were detected in multiple tissues of mature p27-deficient mice compared with wild-type mice, suggesting that the ability of p21 to functionally substitute for p27 is not liver-specific.

CDKs and CKIs: molecular targets for tissue remodelling

Effective tissue remodelling is essential to the survival of adult organs. Many of the signalling pathways that control these cellular decisions are regulated by nuclear interactions of cell-cycle proteins. Molecules that target cyclin-dependent kinases (CDKs) or CDK inhibitors (CKIs) represent a new class of therapeutic agents that influence tissue remodelling in several organ systems. An understanding of their cell-specific functions is leading to the development of exciting and bold approaches to the treatment cancer, cardiovascular disease and other diseases.

Cell cycle entry of hematopoietic stem and progenitor cells controlled by distinct cyclin-dependent kinase inhibitors

The therapeutic promise of hematopoietic stem cells in medicine has been expanded as broader differentiation potential of the cells has gained experimental support. However, hurdles for stem cell manipulation in vitro and tissue regeneration in vivo remain because of lack of the molecular biology of the stem cells. In particular, elucidating the molecular control of cell cycle entry is necessary for rational stem cell expansion strategies. Understanding how the stem and progenitor cell populations are controlled by negative regulators of cell cycle entry may provide one basis for manipulating these cells. In this mini-review, we focus on the rationale of targeting the cyclin-dependent kinase inhibitors (CKIs) in stem cell biology. Two CKI members, p21(Cip1/Waf1) (p21) and p27kip1 (p27), have been shown to govern the pool sizes of hematopoietic stem and progenitor cells, respectively. Of note, their inhibitory roles in primitive hematopoietic cells are distinct from the action of the inhibitory cytokine, transforming growth factor-beta1 (TGF-beta1). Therefore, the distinct roles of p21, p27, and TGF-beta1 in hematopoietic cells offer attractive targets for specific manipulation of the stem or progenitor cell populations in therapeutic strategies.

Cell cycle regulators and hematopoiesis

The cell cycle behavior of hematopoietic cells varies from extended quiescence to spectacular proliferation. Cell cycle regulators choreograph these transitions through variation in the makeup of cyclin-dependent kinase (cdk)-containing complexes and through alteration in protein expression levels and subcellular localization. The mechanisms through which cell cycle regulators couple proliferation, differentiation and survival is coming into sharper focus. Cdk-inhibitors, once thought of solely in terms of a checkpoint function on cycling, are now known to interact directly with proteins and pathways central to differentiation and apoptosis. By shuttling between binding partners committed to discrete functional pathways, cell cycle regulators may directly coordinate proliferation with differentiation, migration and apoptosis.

Hematopoietic cytokines, transcription factors and lineage commitment

The past two decades have witnessed significant advances in our understanding of the cellular physiology and molecular regulation of hematopoiesis. At the heart of stem cell self-renewal and lineage commitment decisions lies the relative expression levels of lineage-specific transcription factors. The expression of these transcription factors in early stem cells may be promiscuous and fluctuate, but ultimately comes under the influence of extracellular regulatory signals in the form of hematopoietic cytokines. In this review, we first summarize our current understanding of the phenotypic characterization of hematopoietic stem cells. Next, we describe key known transcription factors which govern stem cell self-renewal and lineage commitment decisions. Finally, we review data concerning the role of specific cytokines in influencing these decisions. From this review, a picture emerges in which stem cell fate decisions are governed by the integrated effects of intrinsic transcription factors and external signaling pathways initiated by regulatory cytokines.

Vitamin D hormone inhibits osteoclastogenesis in vivo by decreasing the pool of osteoclast precursors in bone marrow

Previous observations that vitamin D hormone induces the expression of the receptor activator of nuclear factor kappaB (NF-kappaB) ligand (RANKL), thereby stimulating osteoclastogenesis in vitro, led to the widespread belief that 1alpha,25-dihydroxyvitamin D3 [1a,25(OH)2D3] is a bone-resorbing hormone. Here, we show that alfacalcidol, a prodrug metabolized to 1alpha,25(OH)2D3, suppresses bone resorption at pharmacologic doses that maintain normocalcemia in an ovariectomized (OVX) mouse model of osteoporosis. Treatment of OVX mice with pharmacologic doses of alfacalcidol does not increase RANKL expression, whereas toxic doses that cause hypercalcemia markedly reduce the expression of RANKL. When bone marrow (BM) cells from OVX mice were cultured with sufficient amounts of macrophage colony-stimulating factor (M-CSF) and RANKL, osteoclastogenic activity was higher than in sham mice. Marrow cultures from alfacalcidol- or estrogen-treated OVX mice showed significantly less osteoclastogenic potential compared with those from vehicle-treated OVX mice, suggesting that the pool of osteoclast progenitors in the marrow of vitamin D-treated mice as well as estrogen-treated mice was decreased. Frequency analysis showed that the number of osteoclast progenitors in bone marrow was increased by OVX and decreased by in vivo treatment with alfacalcidol or estrogen. We conclude that the pharmacologic action of active vitamin D in vivo is to decrease the pool of osteoclast progenitors in BM, thereby inhibiting bone resorption. Because of its unusual activity of maintaining bone formation while suppressing bone resorption, in contrast to estrogens that depress both processes, vitamin D hormone and its bone-selective analogs may be useful for the management of osteoporosis.

A genetic determinant that specifically regulates the frequency of hematopoietic stem cells

The regulation of hematopoietic stem cell (HSC) homeostasis is not well understood. We screened for genetic polymorphisms that were linked to differences between mouse strains in the numbers of long-term reconstituting HSCs or restricted progenitors in the bone marrow. AKR/J mice had significantly higher frequencies and numbers of both HSCs and restricted progenitors in their bone marrow than C57BL/Ka-Thy-1.1 mice. The C57BL/Ka-Thy-1.1 alleles were partially dominant. A locus on chromosome 17, including the H-2 complex, was significantly linked to the frequency of long-term self-renewing HSCs but showed no evidence of linkage to the frequency of restricted progenitors. Conversely, a chromosome 1 locus exhibited suggestive linkage to restricted progenitor frequencies but was not linked to HSC frequency. This demonstrates that there are distinct genetic determinants of the frequencies of HSCs and restricted progenitors in vivo. The AKR/J chromosome 17 locus was not sufficient to increase HSC frequencies when bred onto a C57BL background. This suggests that to affect HSC frequencies, the product(s) of this locus likely depend on interactions with unlinked modifying loci.

Survival and cell cycle control in early hematopoiesis: role of bcl-2, and the cyclin dependent kinase inhibitors P27 and P21

Homeostasis of the hematopoietic system is maintained by proliferation and differentiation of a small number of long-term surviving, self-renewing stem cells, which give rise to the fully mature elements. The fine interplay between differentiation, proliferation and death by apoptosis determines the equilibrium of this system. Thus, genes involved in the control of these processes are very important in the regulation and development of hematopoietic cells especially in the initial stages. The interactions among cyclins, their specific cyclin-dependent kinases (CDKs) and, a number of cyclin-dependent kinase inhibitors (CDKIs) such as p27 and p21, exert a direct control on the cell cycle but can also produce other independent effects on hematopoietic differentiation. Proteins of the Bcl-2 family are also crucial in regulating the balance between entry into apoptosis and survival capacity and their roles change in the course of differentiation. In addition, a number of autocrine and paracrine soluble factors (such as TGF-beta1) modulate the behavior and differentiation potential of hematopoietic elements. Studies on a few in vitro systems of early hematopoietic differentiation have stressed the importance of Bcl-2 and of the CDKIs p27 and p21 at this stage, have confirmed cell-cycle independent effects and have demonstrated how the modulation and the effects in response to different stimuli is mostly dependent on the differentiation stage of the target cells.