Monocytically differentiating HL60 cells proliferate rapidly before they mature

Regulation of FOXP3 expression in myeloid cells in response to all-trans-retinoic acid, interleukin 2 and transforming growth factor β

FoxP3 is a transcription factor essential for differentiation and function of T regulatory cells (Tregs). There are two major subsets of Tregs: natural Tregs (nTregs) generated in thymus and inducible Tregs (iTregs) produced in peripheral immune system. It has been documented that iTreg development is dependent on soluble mediators including interleukin 2 (IL2), transforming growth factor β (TGFβ) and all-trans-retinoic acid (ATRA). In our experiments we performed a gene expression array, followed by Real-time PCR experiments to study expression of genes regulated by ATRA in cells of myeloid origin. Our experiments revealed that ATRA alone, but also a cocktail of mediators consisting of IL2, TGFβ and ATRA, upregulate expression of FOXP3 gene in normal and leukemic myeloid cells. Our results indicate that signaling pathways which are used at the late steps of T cell differentiation, are also active in the cells of myeloid lineage.

Developmental Vitamin D Availability Impacts Hematopoietic Stem Cell Production

Vitamin D insufficiency is a worldwide epidemic affecting billions of individuals, including pregnant women and children. Despite its high incidence, the impact of active vitamin D3 (1,25(OH)D3) on embryonic development beyond osteo-regulation remains largely undefined. Here, we demonstrate that 1,25(OH)D3 availability modulates zebrafish hematopoietic stem and progenitor cell (HSPC) production. Loss of Cyp27b1-mediated biosynthesis or vitamin D receptor (VDR) function by gene knockdown resulted in significantly reduced runx1 expression and Flk1+cMyb+ HSPC numbers. Selective modulation in vivo and in vitro in zebrafish indicated that vitamin D3 acts directly on HSPCs, independent of calcium regulation, to increase proliferation. Notably, ex vivo treatment of human HSPCs with 1,25(OH)D3 also enhanced hematopoietic colony numbers, illustrating conservation across species. Finally, gene expression and epistasis analysis indicated that CXCL8 (IL-8) was a functional target of vitamin D3-mediated HSPC regulation. Together, these findings highlight the relevance of developmental 1,25(OH)D3 availability for definitive hematopoiesis and suggest potential therapeutic utility in HSPC expansion.

The Use of 1α,25-Dihydroxyvitamin D₃ as an Anticancer Agent

The notion that vitamin D can influence the incidence of cancer arose from epidemiological studies. The major source of vitamin D in the organism is skin production upon exposure to ultra violet-B. The very first observation of an inverse correlation between exposure of individuals to the sun and the likelihood of cancer was reported as early as 1941. In 1980, Garland and Garland hypothesised, from findings from epidemiological studies of patients in the US with colon cancer, that vitamin D produced in response to sun exposure is protective against cancer as opposed to sunlight per se. Later studies revealed inverse correlations between sun exposure and the occurrence of prostate and breast cancers. These observations prompted laboratory investigation of whether or not vitamin D had an effect on cancer cells. Vitamin D is not active against cancer cells, but the most active metabolite 1α,25-dihydroxyvitamin D₃ (1,25D) has profound biological effects. Here, we review the anticancer action of 1,25D, clinical trials of 1,25D to date and the prospects of the future therapeutic use of new and low calcaemic analogues.

Low serum vitamin D levels are associated with shorter survival after first-line azacitidine treatment in patients with myelodysplastic syndrome and secondary oligoblastic acute myeloid leukemia

BACKGROUND & AIMS

Azacitidine (AZA) therapy has become the recommended first-line treatment for patients with high-risk myelodysplastic syndromes (MDS) and oligoblastic (<30% bone marrow blasts) acute myeloid leukemia (AML). However, improvement of the efficacy of AZA treatment remains a challenge. We retrospectively tested the hypothesis that VitD levels (25-hydroxyvitamin D3) prior to start of first-line AZA therapy are predictive of overall survival (OS) in patients diagnosed with MDS and secondary oligoblastic AML. Furthermore, the antiproliferative effects of AZA in combination with 25-hydroxyvitamin D3 and 1α,25-dihydroxyvitamin D3 were investigated in vitro.

METHODS

A total of 58 patients treated at our center between 2006 and 2014 were analyzed. Serum levels of VitD were quantified using a standard, commercially available 25-hydroxyvitamin D3 chemiluminescent immunoassay. Effects on cell proliferation were assessed using tetrazolium-based MTT assays.

RESULTS

Median serum VitD level prior to AZA treatment was 32.8 nM (range 11.0-101.5 nM). Patient, disease and treatment characteristics did not differ significantly between the low (≤32.8 nM; n = 29) and high (>32.8 nM; n = 29) VitD group. Estimated probability of 2-year OS in the low versus high VitD group was 14% versus 40% (P < 0.05). In multivariable analysis with OS as endpoint, adverse cytogenetics (HR 2.66, P = 0.03) and VitD (per 10 nM decrease, HR 1.68, P = 0.02) were independent predictors of worse survival. In-vitro treatment of myeloid cell lines with AZA in combination with VitD produced synergistic and additive antiproliferative effects. Addition of nanomolar VitD concentrations to AZA resulted in potentiation of AZA activity. Conversely, combination with the VitD antagonist TEI-9647 resulted in inhibition of AZA activity.

CONCLUSIONS

Our study suggests that higher VitD levels were associated with a survival advantage following first-line AZA therapy. Enhanced cytotoxic effects upon combination treatment may contribute to the observed clinical effects. VitD repletion/supplementation during AZA treatment should be explored.

Natural compounds and pharmaceuticals reprogram leukemia cell differentiation pathways

In addition to apoptosis resistance and cell proliferation capacities, the undifferentiated state also characterizes most cancer cells, especially leukemia cells. Cell differentiation is a multifaceted process that depends on complex regulatory networks that involve transcriptional, post-transcriptional and epigenetic regulation of gene expression. The time- and spatially-dependent expression of lineage-specific genes and genes that control cell growth and cell death is implicated in the process of maturation. The induction of cancer cell differentiation is considered an alternative approach to elicit cell death and proliferation arrest. Differentiation therapy has mainly been developed to treat acute myeloid leukemia, notably with all-trans retinoic acid (ATRA). Numerous molecules from diverse natural or synthetic origins are effective alone or in association with ATRA in both in vitro and in vivo experiments. During the last two decades, pharmaceuticals and natural compounds with various chemical structures, including alkaloids, flavonoids and polyphenols, were identified as potential differentiating agents of hematopoietic pathways and osteogenesis.

Perspectives of differentiation therapies of acute myeloid leukemia: the search for the molecular basis of patients' variable responses to 1,25-dihydroxyvitamin d and vitamin d analogs

The concept of differentiation therapy of cancer is ~40 years old. Despite many encouraging results obtained in laboratories, both in vitro and in vivo studies, the only really successful clinical application of differentiation therapy was all-trans-retinoic acid (ATRA)-based therapy of acute promyelocytic leukemia (APL). ATRA, which induces granulocytic differentiation of APL leukemic blasts, has revolutionized the therapy of this disease by converting it from a fatal to a curable one. However, ATRA does not work for other acute myeloid leukemias (AMLs). Since 1,25-dihydroxyvitamin D₃ (1,25D) is capable of inducing monocytic differentiation of leukemic cells, the idea of treating other AMLs with vitamin D analogs (VDAs) was widely accepted. Also, some types of solid cancers responded to in vitro applied VDAs, and hence it was postulated that VDAs can be used in many clinical applications. However, early clinical trials in which cancer patients were treated either with 1,25D or with VDAs, did not lead to conclusive results. In order to search for a molecular basis of such unpredictable responses of AML patients toward VDAs, we performed ex vivo experiments using patient’s blast cells. Experiments were also performed using 1,25D-responsive and 1,25D-non-responsive cell lines, to study their mechanisms of resistance toward 1,25D-induced differentiation. We found that one of the possible reasons might be due to a very low expression level of vitamin D receptor (VDR) mRNA in resistant cells, which can be increased by exposing the cells to ATRA. Our considerations concerning the molecular mechanism behind the low VDR expression and its regulation by ATRA are reported in this paper.

Regulation of monocyte differentiation by specific signaling modules and associated transcription factor networks

Monocyte/macrophages are important players in orchestrating the immune response as well as connecting innate and adaptive immunity. Myelopoiesis and monopoiesis are characterized by the interplay between expansion of stem/progenitor cells and progression towards further developed (myelo)monocytic phenotypes. In response to a variety of differentiation-inducing stimuli, various prominent signaling pathways are activated. Subsequently, specific transcription factors are induced, regulating cell proliferation and maturation. This review article focuses on the integration of signaling modules and transcriptional networks involved in the determination of monocytic differentiation.

Effect of 25-hydroxyvitamin D3 and 1 α,25 dihydroxyvitamin D3 on differentiation and apoptosis of human osteosarcoma cell lines

Osteosarcoma (OS) is a malignant bone tumor predominantly affecting children and adolescents. OS has a 60% survival rate with current treatments; hence, there is a need to identify novel adjuncts to chemotherapeutic regimens. In this pilot study, we investigated the dose-response to 1α,25-dihdroxyvitamin D(3) (1,α 25(OH)(2) D(3)) and 25-hydroxyvitamin D(3) (25(OH)D(3)) by human OS cell lines, SaOS-2, and 143B. We hypothesized that 1,α 25(OH)(2) D(3) and 25(OH)D(3) would stimulate differentiation and induce apoptosis in OS cells in a dose-dependent manner. Human OS cell lines, SaOS-2, and 143B, were treated with 1,α 25(OH)(2)D(3) or 25(OH)D(3) or an ethanol control, respectively, at concentrations ranging from 1 to 1,000 nM. Ki67 (a marker of cellular proliferation) immunocytochemistry revealed no significant changes in the expression of Ki-67 or MIB-1 in 1α,25(OH)(2)D(3) or 25(OH)D(3) treated SaOS-2 or 143B cells. Both control and 1α,25(OH)(2) D(3) treated SaOS-2 and 143B cells expressed vitamin D receptor (VDR). Markers of osteoblastic differentiation in 143B cells and SaOS-2 cells were induced by both 25(OH)D(3) and 1α,25(OH)(2) D, and evident by increases in alkaline phosphatase (ALP) activity, osteocalcin (OCN) mRNA expression, and mineralization of extra-cellular matrix (ECM) by alizarin red staining. An increasing trend in apoptosis in response to 25(OH)D(3), in both SaOS-2 and 143B cells was detected by terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL) staining. With 1α,25(OH)(2)D(3) treatment, apoptosis was evident at higher concentrations only. These preliminary findings suggest that OS cells express VDR and respond to 25(OH)D(3) and 1α,25(OH)(2)D(3) by undergoing differentiation and apoptosis.

Vitamin D insufficiency in myeloproliferative neoplasms and myelodysplastic syndromes: clinical correlates and prognostic studies

Vitamin D insufficiency is commonly observed in the general population; observational studies have suggested an association with increased risk of cancer development. We examined the clinical and prognostic relevance of low plasma levels of 25-hydroxyvitamin D (25[OH]D) in myeloproliferative neoplasms (MPN) and myelodysplastic syndromes (MDS). A total of 409 patients were studied: 247 (60%) with primary myelofibrosis (PMF), 74 (18%) with de novo MDS, 63 (15%) with polycythemia vera (PV), and 25 (6%) with essential thrombocythemia (ET). Plasma 25(OH)D levels were measured by liquid chromatography-tandem mass spectrometry; a level lower than 25 ng/mL indicated vitamin D insufficiency and a level lower than 10 ng/mL indicated severe deficiency. The proportion of patients with 25(OH)D insufficiency was significantly greater in PMF (48%) and PV (43%) when compared with ET (28%) and MDS (28%) (P = 0.01). Severe 25(OH)D deficiency was significantly more frequent in ET (12%) and PMF (9%), compared with PV (3%) and MDS (1%) (P = 0.05). There were no significant correlations between 25(OH)D insufficiency, or severe deficiency, and a variety of clinical or laboratory variables in PMF, MDS, or PV. Furthermore, Vitamin D insufficiency did not influence either overall or leukemia-free survival in PMF, MDS, or PV (P > 0.05). We conclude that while hypovitaminosis D is relatively common in MPN and MDS, its clinical relevance for prognosis is limited.

Vitamin D3-driven signals for myeloid cell differentiation--implications for differentiation therapy

Primitive myeloid leukemic cell lines can be driven to differentiate to monocyte-like cells by 1alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), and, therefore, 1,25(OH)(2)D(3) may be useful in differentiation therapy of myeloid leukemia and myelodysplastic syndromes (MDS). Recent studies have provided important insights into the mechanism of 1,25(OH)(2)D(3)-stimulated differentiation. For myeloid progenitors to complete monocytic differentiation a complex network of intracellular signals has to be activated and/or inactivated in a precise temporal and spatial pattern. 1,25(OH)(2)D(3) achieves this change to the 'signaling landscape' by (i) direct genomic modulation of the level of expression of key regulators of cell signaling and differentiation pathways, and (ii) activation of intracellular signaling pathways. An improved understanding of the mode of action of 1,25(OH)(2)D(3) is facilitating the development of new therapeutic regimens.

The LINC-less granulocyte nucleus

The major blood granulocyte (neutrophil) is rapidly recruited to sites of bacterial and fungal infections. It is a highly malleable cell, allowing it to squeeze out of blood vessels and migrate through tight tissue spaces. The human granulocyte nucleus is lobulated and exhibits a paucity of nuclear lamins, increasing its capability for deformation. The present study examined the existence of protein connections between the nuclear envelope and cytoskeletal elements (the LINC complex) in differentiated cell states (i.e. granulocytic, monocytic and macrophage) of the human leukemic cell line HL-60, as well as in human blood leukocytes. HL-60 granulocytes exhibited a deficiency of several LINC complex proteins (i.e. nesprin 1 giant, nesprin 2 giant, SUN1, plectin and vimentin); whereas, the macrophage state revealed nesprin 1 giant, plectin and vimentin. Both states possessed SUN2 in the nuclear envelope. Parallel differences were observed with some of the LINC complex proteins in isolated human blood leukocytes, including macrophage cells derived from blood monocytes. The present study documenting the paucity of LINC complex proteins in granulocytic forms, in combination with previous data on granulocyte nuclear shape and nuclear envelope composition, suggest the hypothesis that these adaptations evolved to facilitate granulocyte cellular malleability.

Isolation of specific and biologically active peptides that bind cells from patients with acute myeloid leukemia (AML)

Purpose

In a departure from conventional strategies to improve treatment outcome for myeloid malignancies, we report the isolation of leukemia-specific peptides using a phage display library screened with freshly obtained human myeloid leukemia cells.

Results

A phage display library was screened by 5 rounds of biopanning with freshly isolated human AML cells. Individual colonies were randomly picked and after purification, biologic activity (growth and differentiation) on fresh AML cells was profiled. Ten peptides were synthesized for further biological studies. Multiple peptides were found to selectively bind to acute myeloid leukemia (AML) cells. The peptides bound to leukemia cells, were internalized and could induce proliferation and/or differentiation in the target patient cells. Two of the peptides, HP-A2 and HP-G7, appeared to have a novel mechanism of inducing differentiation since they did not cause G1 arrest in cycling cells even as the expression of the differentiation marker CD11b increased.

Conclusion

Peptide induced differentiation of leukemia cells offers a novel treatment strategy for myeloid malignancies, whereas their ability to induce proliferation could be harnessed to make cells more sensitive to chemotherapy. Conceptually, these leukemia specific peptides can also be used to refine diagnosis, document minimal residual disease, and selectively deliver toxins to malignant cells.

The WD-repeat protein GRWD1: potential roles in myeloid differentiation and ribosome biogenesis

A cDNA fragment originally identified in U-937 cells as a vitamin D(3)-regulated gene is here designated the glutamate-rich WD-repeat (GRWD1) gene. WD-repeat proteins are a class of functionally divergent molecules that cooperate with other proteins to regulate cellular processes. GRWD1 encodes a 446-amino-acid protein containing a glutamate-rich region followed by four WD repeats. The yeast homologue of GRWD1, Rrb1, has been shown to be an essential protein involved in ribosome biogenesis. Northern analysis of GRWD1 message levels in the myeloid cell line HL-60 undergoing differentiation induced by vitamin D(3) or retinoic acid demonstrate downregulation coincident with slowing of cellular proliferation. A siRNA designed to downregulate GRWD1 similarly results in a decrease in cellular proliferation within 293 cells. Metabolic labeling of cells expressing the siRNA to GRWD1 shows a decrease in global protein synthesis. Finally, nuclear fractionation studies show cosedimentation of GRWD1 with preribosomal complexes, as well as the WD-repeat-containing protein Bop1, which has previously been implicated in ribosome biogenesis. These studies suggest that within mammalian cells GRWD1 plays a role in ribosome biogenesis and during myeloid differentiation its levels are regulated.

Immunophenotypic changes induced on human HL60 leukaemia cells by 1alpha,25-dihydroxyvitamin D3 and 12-O-tetradecanoyl phorbol-13-acetate

1alpha,25-Dihydroxyvitamin D3 (1,25D3) induces HL60 cells to acquire a monocyte-like phenotype, while cells treated with 12-O-tetradecanoyl phorbol-13-acetate (TPA) resemble macrophages. Using a microarray of 82 CD antibodies, 24 cluster of differentiation (CD) antigens were detected on HL60 cells. 1,25D3 induced the following antigens in decreasing order of the change: CD14, CD11c, CD11b, CD54, CD86, CD38 and CD66c, with repression of CD117, CD71, CD95, CD45 and CD64. TPA induced the following antigens in decreasing order of the change: CD11c, CD9, CD11b, CD54, CD38, CD45RO and CD66c, with repression of CD4, CD117, CD95, CD71 and CD64. The results presented provide a basis for monitoring differentiation therapy of myeloid leukaemias in patients.

Polypyrimidine tract-binding protein enhances the internal ribosomal entry site-dependent translation of p27Kip1 mRNA and modulates transition from G1 to S phase

The p27(Kip1) protein plays a critical role in the regulation of cell proliferation through the inhibition of cyclin-dependent kinase activity. Translation of p27(Kip1) is directed by an internal ribosomal entry site (IRES) in the 5' nontranslated region of p27(Kip1) mRNA. Here, we report that polypyrimidine tract-binding protein (PTB) specifically enhances the IRES activity of p27(Kip1) mRNA through an interaction with the IRES element. We found that addition of PTB to an in vitro translation system and overexpression of PTB in 293T cells augmented the IRES activity of p27(Kip1) mRNA but that knockdown of PTB by introduction of PTB-specific small interfering RNAs (siRNAs) diminished the IRES activity of p27(Kip1) mRNA. Moreover, the G(1) phase in the cell cycle (which is maintained in part by p27(Kip1)) was shortened in cells depleted of PTB by siRNA knockdown. 12-O-Tetradecanoylphorbol-13-acetate (TPA)-induced differentiation in HL60 cells was used to examine PTB-induced modulation of p27(Kip1) protein synthesis during differentiation. The IRES activity of p27(Kip1) mRNA in HL60 cells was increased by TPA treatment (with a concomitant increase in PTB protein levels), but the levels of p27(Kip1) mRNA remained unchanged. Together, these data suggest that PTB modulates cell cycle and differentiation, at least in part, by enhancing the IRES activity of p27(Kip1) mRNA.

Complex changes in cellular inositol phosphate complement accompany transit through the cell cycle

Inositol polyphosphates other than Ins(1,4,5)P3 are involved in several aspects of cell regulation. For example, recent evidence has implicated InsP6, Ins(1,3,4,5,6)P5 and their close metabolic relatives, which are amongst the more abundant intracellular inositol polyphosphates, in chromatin organization, DNA maintenance, gene transcription, nuclear mRNA transport, membrane trafficking and control of cell proliferation. However, little is known of how the intracellular concentrations of inositol polyphosphates change through the cell cycle. Here we show that the concentrations of several inositol polyphosphates fluctuate in synchrony with the cell cycle in proliferating WRK-1 cells. InsP6, Ins(1,3,4,5,6)P5 and their metabolic relatives behave similarly: concentrations are high during G1-phase, fall to much lower levels during S-phase and rise again late in the cycle. The Ins(1,2,3)P3 concentration shows especially large fluctuations, and PP-InsP5 fluctuations are also very marked. Remarkably, Ins(1,2,3)P3 turns over fastest during S-phase, when its concentration is lowest. These results establish that several fairly abundant intracellular inositol polyphosphates, for which important biological roles are emerging, display dynamic behaviour that is synchronized with cell-cycle progression.

Betulinic acid enhances 1alpha,25-dihydroxyvitamin D3-induced differentiation in human HL-60 promyelocytic leukemia cells

Betulinic acid (BA) is a pentacyclic triterpene found in a number of medicinal plants and has been shown to cause apoptosis in a number of cell lines. We report here that BA may also have an effect on HL-60 cell differentiation. BA was cytotoxic to HL-60 cells with an IC50 of 5.7 microM after a 72-h treatment. Flow cytometry analysis showed that after exposure to 1-12 microM of BA for 72 h, approximately 10% of viable cells were in the sub-G1, presumably apoptotic, phase. At the same time differentiation was induced in approximately 10% (at 1 microM BA) to a maximum of 20% (at 6 microM BA) of cells as judged by the NBT-reduction test, and the expression of membrane markers CD11b and CD14. On the other hand, at 1 and 5 nM, 1alpha,25-dihydroxyvitamin D3 (DHD3) induced differentiation in approximately 10 and 70% of cells, respectively. At 1 nM DHD3, the addition of 1 microM BA increased differentiated cells from 10 to 43% and with 3 microM BA the increase was to 80%. BA also enhanced the effects of DHD3 in the expansion of the G1 cell population with a concomitant decrease of S phase cells. The effects of DHD3 and BA on CD11b and CD14 expression were inhibited by PD98059, a MEK inhibitor. Our results suggest that BA may enhance the effect of DHD3 in inducing mitogen-activated protein kinase kinase/extracellular signal-regulated protein kinase-mediated HL-60 cell differentiation.

Retinoblastoma protein and CCAAT/enhancer-binding protein beta are required for 1,25-dihydroxyvitamin D3-induced monocytic differentiation of HL60 cells

Derivatives of vitamin D (deltanoids) are well known to have the ability to induce differentiation of a variety of malignant cells, including human leukemia cells, but the signaling pathways that lead to such an outcome are unclear. In this study we investigated the role of the retinoblastoma protein (pRb) and the CCAAT/enhancer-binding protein (C/EBP) beta in 1,25-dihydroxyvitamin D(3) (1,25D(3))-induced monocytic differentiation of human leukemia HL60 cells. It was found that in this system, pRb is up-regulated within 12 h of exposure to the inducer, and the kinetics of its increase parallel the appearance of the early markers of differentiation, CD14 and monocyte-specific esterase. The increase in pRb expression was accompanied by a similar increase in C/EBPbeta protein, and these two proteins coimmunoprecipitated, suggesting formation of a complex. Oligonucleotides antisense to pRb or C/EBPbeta (but not to C/EBPalpha) or containing the C/EBP-binding sequence ("decoys"), all inhibited 1,25D(3)-induced differentiation. Inhibition of signaling by vitamin D receptor or by mitogen-activated protein kinase (MAPK) extracellular signal-regulated kinase and c-Jun-NH(2)-terminal kinase pathways using pharmacological inhibitors ZK159222, PD98059, or SP600125, respectively, inhibited pRb and C/EBPbeta expression and differentiation in a coordinate manner. In contrast, inhibition of the p38MAPK pathway by SB202190 potentiated differentiation and the up-regulation of pRb and C/EBPbeta. We suggest that 1,25D(3) may signal monocytic differentiation of HL60 cells in a vitamin D receptor-dependent manner that includes activation of extracellular signal-regulated kinase and c-Jun-NH(2)-terminal kinase MAPK pathways, which then up-regulate pRb and C/EBPbeta expression and in turn initiate the differentiation process.

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.

Regulation of Id gene expression by type I insulin-like growth factor: roles of Stat3 and the tyrosine 950 residue of the receptor

Id proteins are known to play important roles in the proliferation and differentiation of many cell types. The type 1 insulin-like growth factor receptor (IGF-IR), activated by its ligand, induces the differentiation of 32D IGF-IR cells, a murine hematopoietic cell line, expressing a human IGF-IR. Expression in 32D IGF-IR cells of a dominant negative mutant of Stat3 (DNStat3) inhibits IGF-I-mediated differentiation. DNStat3 causes a dramatic increase in Id2 gene expression. This increase, however, is IGF-I dependent and is abrogated by a mutation at tyrosine 950 of the IGF-IR. These results indicate that in 32D cells, the IGF-IR regulates the expression of the Id2 gene and that this regulation is modulated by both positive and negative signals. Our results also suggest that in this model, Id2 proteins influence the differentiation program of cells but are not sufficient for the full stimulation of their proliferation program.

IGF-I receptor signalling in transformation and differentiation

The type 1 insulin-like growth factor receptor (IGF-IR) sends several signals, some of which are contradictory. When the concentrations of insulin receptor substrate 1 (IRS-1), a major substrate of the IGF-IR, are high, the signal is mitogenic, anti-apoptotic, and can even cause malignant transformation. However, in the absence of IRS-1, the IGF-IR sends a differentiation signal, which leads to granulocytic differentiation in haemopoietic cells. The mitogenic signal of the IGF-IR/IRS-1 combination depends largely, but not exclusively, on the activation of the phosphatidylinositol-3 kinase (PI3K).

Cell proliferation and CD11b expression are controlled independently during HL60 cell differentiation initiated by 1,25 alpha-dihydroxyvitamin D(3) or all-trans-retinoic acid

When 1 alpha,25-dihydroxyvitamin D(3) (D(3)) induces HL60 cells to differentiate to monocytes, a burst of approximately three shortened cell cycles ("maturation divisions") precedes exit from cell cycle and completion of maturation. Here we show that similar maturation divisions occur during neutrophil differentiation induced by all-trans-retinoic acid (ATRA), but without shortening of the cell cycle. Both ATRA and D(3) initiate these maturation divisions as cells pass through a "window of sensitivity" during early G1. We also investigated whether the initiation of maturation divisions and of the expression of CD11b, an early-expressed maturation marker, are linked. Cells treated with D(3) or ATRA start to express CD11b after 9--14 h, before completing the first maturation division. Elutriation was used to isolate small HL60 cells (almost all in G1) and larger cells (in G1 and S phases) from unsynchronized populations. When these were cultured with D(3) or ATRA, most reentered cycle synchronously, multiplied, and differentiated. Following D(3) treatment, the G1-enriched small cells expressed CD11b slightly faster than unsynchronized cultures or fractions dominated by late G1 cells and/or S phase cells. D(3)-induced CD11b expression occurred at a similar rate even in G1 cells that were held at the G1/S boundary by thymidine. In conclusion, changes in the control of the cell cycle that characterize the onset of monocytic and neutrophil differentiation are only triggered in early G1, but CD11b expression can be initiated from most points in the cell cycle. Differentiating agents must therefore regulate the proliferation and the maturation of differentiating myeloid cells by mechanisms that are at least partly independent.

Regulation of Id2 gene expression by the insulin-like growth factor I receptor requires signaling by phosphatidylinositol 3-kinase

The Id proteins play an important role in proliferation, differentiation, and tumor development. We report here that Id gene expression can be regulated by the insulin-like growth factor I receptor (IGF-IR), a receptor that also participates in the regulation of cellular proliferation and differentiation. Specifically, we found that the IGF-IR activated by its ligand was a strong inducer of Id2 gene expression in 32D murine hemopoietic cells. This activation was not simply the result of cellular proliferation, as Id2 gene expression was higher in 32D cells stimulated by IGF-I than in cells exponentially growing in interleukin-3. The up-regulation of Id2 gene expression was largely dependent on the presence of insulin receptor substrate-1, a major substrate of the IGF-IR and a potent activator of the phosphatidylinositol 3-kinase (PI3K) pathway. The role of PI3K activity in the up-regulation of Id2 gene expression by the IGF-IR was confirmed by different methods and in different cell types. In 32D cells, the up-regulation of Id2 gene expression by the PI3K pathway correlated with interleukin-3 independence and inhibition of differentiation.

The contradictions of the insulin-like growth factor 1 receptor

In recent years, the type 1 insulin-like growth factor receptor (IGF-IR) has emerged as a receptor that plays a very important role in the growth of cells, both in vivo and in vitro. The ability of the IGF-IR to induce mitogenesis and to promote survival of cells against a variety of apoptotic agents is well documented. Somewhat less known are other functions of the IGF-IR, like its ability to induce differentiation, to regulate cell size and to affect the organization of the cytoskeleton of cells. This review will focus on these lesser known functions of the IGF-IR. At the same time, we will emphasize how the IGF-IR can send contradictory signals, which depend on different domains of the receptor and the availability of downstream transducing molecules.