Elevated cyclin E levels, inactive retinoblastoma protein, and suppression of the p27(KIP1) inhibitor characterize early development of promyeloid cells into macrophages

Cyclin E1 in Murine and Human Liver Cancer: A Promising Target for Therapeutic Intervention during Tumour Progression

Simple Summary

The cell cycle regulator Cyclin E1 is a key mediator and biomarker of liver cancer progression in mice and man independent of its canonical interacting partner Cyclin-dependent kinase 2. Over-expression of Cyclin E1 during hepatocarcinogenesis modulates several distinct biological processes such as proliferation, DNA damage response, stemness, invasion and the tumour microenvironment. Interventional depletion of Cyclin E1 in the course of liver cancer progression significantly reduces tumour burden. In contrast, the expression of Cyclin-dependent kinase 2 is dispensable for the progression of liver cancer in mice and lacked diagnostic or prognostic value in patients. Thus, specific inhibition of Cyclin E1 expression represents a promising strategy for the treatment of liver cancer.

Abstract

Cyclin E1 (CCNE1) is a regulatory subunit of Cyclin-dependent kinase 2 (CDK2) and is thought to control the transition of quiescent cells into cell cycle progression. Recently, we identified CCNE1 and CDK2 as key factors for the initiation of hepatocellular carcinoma (HCC). In the present study, we dissected the contributions of CCNE1 and CDK2 for HCC progression in mice and patients. Therefore, we generated genetically modified mice allowing inducible deletion of Ccne1 or Cdk2. After initiation of HCC, using the hepatocarcinogen diethylnitrosamine (DEN), we deleted Ccne1 or Cdk2 and subsequently analysed HCC progression. The relevance of CCNE1 or CDK2 for human HCC progression was investigated by in silico database analysis. Interventional deletion of Ccne1, but not of Cdk2, substantially reduced the HCC burden in mice. Ccne1-deficient HCCs were characterised by attenuated proliferation, impaired DNA damage response and downregulation of markers for stemness and microinvasion. Additionally, the tumour microenvironment of Ccne1-deficient mice showed a reduction in immune mediators, myeloid cells and cancer-associated fibroblasts. In sharp contrast, Cdk2 was dispensable for HCC progression in mice. In agreement with our mouse data, CCNE1 was overexpressed in HCC patients independent of risk factors, and associated with reduced disease-free survival, a common signature for enhanced chromosomal instability, proliferation, dedifferentiation and invasion. However, CDK2 lacked diagnostic or prognostic value in HCC patients. In summary, CCNE1 drives HCC progression in a CDK2-independent manner in mice and man. Therefore, interventional inactivation of CCNE1 represents a promising strategy the treatment of liver cancer.

RETINOBLASTOMA-RELATED Genes Specifically Control Inner Floral Organ Morphogenesis and Pollen Development in Rice

RETINOBLASTOMA-RELATED (RBR) is an essential gene in plants, but its molecular function outside of its role in cell cycle entry remains poorly understood. We characterized the functions of OsRBR1 and OsRBR2 in plant growth and development in rice using both forward- and reverse-genetics methods. The two genes were coexpressed and performed redundant roles in vegetative organs but exhibited separate functions in flowers. OsRBR1 was highly expressed in the floral meristem and regulated the expression of floral homeotic genes to ensure floral organ formation. Mutation of OsRBR1 caused loss of floral meristem identity, resulting in the replacement of lodicules, stamens, and the pistil with either a panicle-like structure or whorls of lemma-like organs. OsRBR2 was preferentially expressed in stamens and promoted pollen formation. Mutation of OsRBR2 led to deformed anthers without pollen. Similar to the protein interaction between AtRBR and AtMSI1 that is essential for floral development in Arabidopsis, OsMSI1 was identified as an interaction partner of OsRBR1 and OsRBR2. OsMSI1 was ubiquitously expressed and appears to be essential for development in rice (Oryza sativa), as the mutation of OsMSI1 was lethal. These results suggest that OsRBR1 and OsRBR2 function with OsMSI1 in reproductive development in rice. This work characterizes further functions of RBRs and improves current understanding of specific regulatory pathways of floral specification and pollen formation in rice.

Augmentation of Butyrate-induced Differentiation of Human Hepatocyte by Cyclin E Over-expression

In mammalian cells, cellular differentiation into specific cell types is usually preceded by growth arrest. On the other hand, the induced differentiation may also be preceded by an enhanced G1–S transition of the cell cycle prior to the growth arrest. This suggests that an early increase in proliferation is in some way a prerequisite for subsequent differentiation. We therefore attempted to assess whether we could produce human hepatocytes with further differentiated functions by promoting G1-S transition in a butyrate-treated human hepatocyte cell line. A cyclin E-over-expressing cell line was established by transfecting human cyclin E cDNA. Upon butyrate treatment, the cyclin E-over-expressing cells exhibited a significantly increased albumin-secreting and ammonia-detoxifying capacity when compared to the control cells. In particular, the ornithine transcarbamylase activity was increased in these cells. Collectively, these results implicate that the cyclin E over-expression may augment the hepatocyte-specific functions during the butyrate-induced differentiation process of human hepatocytes by enhancing G1-S cell cycle transition.

Role of cell cycle regulatory molecules in retinoic acid- and vitamin D3-induced differentiation of acute myeloid leukaemia cells

The important role of cell cycle regulatory molecules in all trans-retinoic acid (ATRA)- and vitamin D3-induced growth inhibition and differentiation induction has been intensively studied in both acute myeloid leukaemia primary cells and a variety of leukaemia cell lines. Cyclin-dependent kinases (CDK)-activating kinase has been demonstrated to interact with retinoic acid receptor (RAR)α in acute promyelocytic leukaemia cells, and inhibition of CDK-activating kinase by ATRA causes hypophosphorylation of PML-RARα, leading to myeloid differentiation. In many cases, downregulation of CDK activity by ATRA and vitamin D3 is a result of elevated p21- and p27-bound CDKs. Activation of p21 is regulated at the transcriptional level, whereas elevated p27 results from both (indirectly) transcriptional activation and post-translational modifications. CDK inhibitors (CKIs) of the INK family, such as p15, p16 and p18, are mainly involved in inhibition of cell proliferation, whereas CIP/KIP members, such as p21, regulate both growth arrest and induction of differentiation. ATRA and vitamin D3 can also downregulate expression of G1 CDKs, especially CDK2 and CDK6. Inhibition of cyclin E expression has only been observed in ATRA- but not in vitamin D3-treated leukaemic cells. In vitro, not only dephosphorylation of pRb but also elevation of total pRb is required for ATRA and vitamin D3 to suppress growth and trigger their differentiation. Finally, sharp reduction in c-Myc has been observed in several leukaemia cell lines treated with ATRA, which may regulate expression of CDKs and CKIs.

Impaired adult myeloid progenitor CMP and GMP cell function in conditional c-myb-knockout mice

The differentiation of myeloid progenitors to mature, terminally differentiated cells is a highly regulated process. Here, we showed that conditional disruption of the c-myb proto-oncogene in adult mice resulted in dramatic reductions in CMP, GMP and MEP myeloid progenitors, leading to a reduction of neutrophils, basophils, monocytes and platelets in peripheral blood. In addition, c-myb plays a critical role at multiple stages of myeloid development, from multipotent CMP and bipotent GMP to unipotent CFU-G and CFU-M progenitor cells. c-myb controls the differentiation of these cells and is required for the proper commitment, maturation and normal differentiation of CMPs and GMPs. Specifically, c-myb regulates the precise commitment to the megakaryocytic and granulo-monocytic pathways and governs the granulocytic-monocytic lineage choice. c-myb is also required for the commitment along the granulocytic pathway for early myeloid progenitor cells and for the maturation of committed precursor cells along this pathway. On the other hand, disruption of the c-myb gene favors the commitment to the monocytic lineage, although monocytic development was abnormal with cells appearing more mature with atypical CD41 surface markers. These results demonstrate that c-myb plays a pivotal role in the regulation of multiple stages in adult myelogenesis.

Retinoblastoma protein modulates the inverse relationship between cellular proliferation and elastogenesis

The mechanism that leads to the inverse relationship between heightened cellular proliferation and the cessation of elastic fibers production, observed during formation of the arterial occlusions and dermal scars, is not fully understood. Because the retinoblastoma protein (Rb), responsible for cell cycle initiation, has also been implicated in insulin-like growth factor-I-mediated signaling stimulating elastin gene activation, we explored whether differential phosphorylation of Rb by various cyclin·cyclin-dependent kinase complexes would be responsible for promoting either elastogenic or pro-proliferative signals. We first tested cultures of dermal fibroblasts derived from Costello syndrome patients, in which heightened proliferation driven by mutated oncogenic H-Ras coincides with inhibition of elastogenesis. We found that Costello syndrome fibroblasts display elevated level of Rb phosphorylation on serine 780 (Ser(P)-780-Rb) and that pharmacological inhibition of Ras with radicicol, Mek/Erk with PD98059, or cyclin-dependent kinase 4 with PD0332991 not only leads to down-regulation of Ser(P)-780-Rb levels but also enhances Rb phosphorylation on threonine-821 (Thr(P)-821-Rb), which coincides with the recovery of elastin production. Then we demonstrated that treatment of normal skin fibroblasts with the pro-proliferative PDGF BB also up-regulates Ser(P)-780-Rb levels, but treatment with the pro-elastogenic insulin-like growth factor-I activates cyclinE-cdk2 complex to phosphorylate Rb on Thr-821. Importantly, we have established that elevation of Thr(P)-821-Rb promotes Rb binding to the Sp1 transcription factor and that successive binding of the Rb-Sp1 complex to the retinoblastoma control element within the elastin gene promoter stimulates tropoelastin transcription. In summary, we provide novel insight into the role of Rb in mediating the inverse relationship between elastogenesis and cellular proliferation.

Accelerated DNA replication in E2F1- and E2F2-deficient macrophages leads to induction of the DNA damage response and p21(CIP1)-dependent senescence

E2F1-3 proteins appear to have distinct roles in progenitor cells and in differentiating cells undergoing cell cycle exit. However, the function of these proteins in paradigms of terminal differentiation that involve continued cell division has not been examined. Using compound E2F1/E2F2-deficient mice, we have examined the effects of E2F1 and E2F2 loss on the differentiation and simultaneous proliferation of bone-marrow-derived cells toward the macrophage lineage. We show that E2F1/E2F2 deficiency results in accelerated DNA replication and cellular division during the initial cell division cycles of bone-marrow-derived cells, arguing that E2F1/E2F2 are required to restrain proliferation of pro-monocyte progenitors during their differentiation into macrophages, without promoting their cell cycle exit. Accelerated proliferation is accompanied by early expression of DNA replication and cell cycle regulators. Remarkably, rapid proliferation of E2F1/E2F2 compound mutant cultures is temporally followed by induction of a DNA damage response and the implementation of a p21(CIP1)-dependent senescence. We further show that differentiating E2F1/E2F2-knockout macrophages do not trigger a DNA damage response pathway in the absence of DNA replication. These findings underscore the relevance of E2F1 and E2F2 as suppressors of hematopoietic progenitor expansion. Our data indicate that their absence in differentiating macrophages initiates a senescence program that results from enforcement of a DNA damage response triggered by DNA hyper-replication.

Arabidopsis RETINOBLASTOMA-RELATED is required for stem cell maintenance, cell differentiation, and lateral organ production

Several genes involved in the regulation of postembryonic organ initiation and growth have been identified. However, it remains largely unclear how developmental cues connect to the cell cycle. RETINOBLASTOMA RELATED (RBR) is a plant homolog of the tumor suppressor Retinoblastoma (pRb), which is a key regulator of the cell cycle. Using inducible RNA interference (RNAi) against Arabidopsis thaliana RBR (RBRi), we reduced RBR expression levels at different stages of plant development. Conditional reduction or loss of RBR function disrupted cell division patterns, promoted context-dependent cell proliferation, and negatively influenced establishment of cell differentiation. Several lineages of toti- and pluripotent cells, including shoot apical meristem stem cells, meristemoid mother cells, and procambial cells, failed to produce appropriately differentiated cells. Meristem activity was altered, leading to a disruption of the CLAVATA-WUSCHEL feedback loop and inhibition of lateral organ formation. Release of RBR from RNAi downregulation restored meristem activity. Gene profiling analyses soon after RBRi induction revealed that a change in RBR homeostasis is perceived as a stress, even before genes regulated by RBR-E2F become deregulated. The results establish RBR as a key cell cycle regulator required for coordination of cell division, differentiation, and cell homeostasis.

LPS-induced indoleamine 2,3-dioxygenase is regulated in an interferon-gamma-independent manner by a JNK signaling pathway in primary murine microglia

Inflammation-induced activation of the tryptophan catabolizing enzyme indoleamine 2,3-dioxygenase (IDO) causes depressive-like behavior in mice following acute activation of the innate immune system by lipopolysaccharide (LPS). Here we investigated the mechanism of IDO expression induced by LPS in primary cultures of microglia derived from neonatal C57BL/6J mice. LPS (10 ng/ml) induced IDO transcripts that peaked at 8h and enzymatic activity at 24h, resulting in an increase in extracellular kynurenine, the catabolic product of IDO-induced tryptophan catabolism. This IDO induction by LPS was accompanied by synthesis and secretion of the proinflammatory cytokines TNFalpha and IL-6, but without detectable IFNgamma expression. To explore the mechanism of LPS-induced IDO expression, microglia were pretreated with the c-Jun-N-terminal kinase (JNK) inhibitor SP600125 for 30 min before LPS treatment. We found that SP600125 blocked JNK phosphorylation and significantly decreased IDO expression induced by LPS, which was accompanied by a reduction of LPS-induced expression of TNFalpha and IL-6. Collectively, these data extend to microglia the property that LPS induces IDO expression via an IFNgamma-independent mechanism that depends upon activation of JNK. Inhibition of the JNK pathway may provide a new therapy for inflammatory depression.

Pisosterol induces monocytic differentiation in HL-60 cells

The aim of this study was to determine whether the antiproliferative effects observed for pisosterol, a cytotoxic triterpene isolated from Pisolithus tinctorius, are related to cell differentiation induction using HL-60 cell line as a model. Also, the effects of pisosterol on normal human cells were examined in peripheral blood mononuclear cells (PBMC). The effects on cell viability and morphological changes were the first indications showing that pisosterol induces HL-60 differentiation. The demonstration of blue tetrazolium reduction in HL-60 cells exposed to pisosterol demonstrated differentiation in a dose- and time-dependent manner, reaching a maximum effect after 72 h incubation at 5 microg/mL. Assays for alpha-naphthyl acetate esterase activity indicated that pisosterol triggers differentiation towards a monocytic cell-like pathway. The antiproliferative effect of pisosterol was determined by inhibition of DNA synthesis based on BrdU incorporation into HL-60 proliferating cells. It appears that pisosterol-treated cells, despite displaying a differentiated phenotype, continued to proliferate at all doses tested after 72 h, with a slightly decrease at 5 microg/mL. Apoptosis was observed in pisosterol-treated cells in a dose-dependent way. Nevertheless, after the same period of incubation, no cytotoxicity was detected in PBMC in the presence of pisosterol even at 25 microg/mL, providing some evidence that pisosterol may be selective for tumor cells. The mechanisms underlying the effect of pisosterol in leukemia cells indicates the induction of a monocytic cell-like differentiation, suggesting that this compound could be used in the development of new pharmacological tools with potential therapeutic value in the management of leukemia with fewer side effects.

Atheroma development in apolipoprotein E-null mice is not regulated by phosphorylation of p27(Kip1) on threonine 187

Excessive cellular proliferation is thought to contribute to neointimal lesion development during atherosclerosis and restenosis after angioplasty. Inhibition of cyclin-dependent kinase (CDK) activity by p27 inhibits mammalian cell growth. Mounting evidence indicates that p27 negatively regulates neointimal thickening in animal models of restenosis and atherosclerosis, and its expression in human neointimal lesions is consistent with such a protective role. Cell cycle progression is facilitated by cyclinE/CDK2-dependent phosphorylation of p27 on threonine 187 (T187) during late G1. The purpose of this study was to assess whether this phosphorylation event plays a role during atherosclerosis. To this end, we generated apolipoprotein E-null mice with both p27 alleles replaced by a mutated form non-phosphorylatable at T187 (apoE-/-p27T187A mice) and investigated the kinetics of atheroma development in these animals compared to apoE-/- controls with an intact p27 gene. Fat feeding resulted in comparable level of hypercholesterolemia in both groups of mice. Surprisingly, aortic p27 expression was not increased in fat-fed apoE-/-p27T187A mice compared with apoE-/- controls. Moreover, atheroma size, lesion cellularity, proliferation, and apoptotic rates were undistinguishable in both groups of fat-fed mice. Thus, in contrast to previous studies that highlight the importance of p27 phosphorylation at T187 on the control of p27 expression and function in different tissues and pathophysiological scenarios, our findings demonstrate that this phosphorylation event is not implicated in the control of aortic p27 expression and atheroma progression in hypercholesterolemic mice.

The 1,25(OH)2D3-Regulated Transcription Factor MN1 Stimulates Vitamin D Receptor-Mediated Transcription and Inhibits Osteoblastic Cell Proliferation

The vitamin D endocrine system is essential for maintaining mineral ion homeostasis and preserving bone density. The most bioactive form of vitamin D, 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] elicits its effects by binding to the vitamin D receptor (VDR) and regulating the transcription of target genes. In osteoblasts, the bone-forming cells of the skeleton, 1,25-(OH)2D3 regulates cell proliferation, differentiation, and mineralization of the extracellular matrix. Despite these well-characterized biological functions, relatively few 1,25-(OH)2D3 target genes have been described in osteoblasts. In this study, we characterize the regulation and function of MN1, a novel 1,25-(OH)2D3-induced gene in osteoblastic cells. MN1 is a nuclear protein first identified as a gene disrupted in some meningiomas and leukemias. Our studies demonstrate that MN1 preferentially stimulates VDR-mediated transcription through its ligand-binding domain and synergizes with the steroid receptor coactivator family of coactivators. Furthermore, forced expression of MN1 in osteoblastic cells results in a profound decrease in cell proliferation by slowing S-phase entry, suggesting that MN1 is an antiproliferative factor that may mediate 1,25-(OH)2D3-dependent inhibition of cell growth. Collectively, these data indicate that MN1 is a 1,25-(OH)2D3-induced VDR coactivator that also may have critical roles in modulating osteoblast proliferation.

Tumor necrosis factor alpha inhibits insulin-like growth factor I-induced hematopoietic cell survival and proliferation

Proinflammatory cytokines, such as TNFalpha and IL-1beta, are both cytostatic and cytotoxic. In contrast, IGF-I promotes proliferation and survival of hematopoietic progenitor cells. In this report, we establish that both the cytostatic and cytotoxic activity of TNFalpha on murine myeloid progenitor cells is only evident in the presence of IGF-I. We first confirmed that IGF-I (100 ng/ml) increases DNA synthesis and reduces apoptosis in murine myeloid progenitor cells induced to die by growth factor withdrawal. TNFalpha inhibits, in a dose-dependent fashion from 0.1 to 10 ng/ml, both activities of IGF-I. TNFalpha activity was not detected in the absence of IGF-I. Another proinflammatory cytokine, IL-1beta, did not inhibit IGF-I-induced activity in murine factor-dependent cell progenitor-1/Mac-1 cells. However, the ability of TNFalpha to impair IGF-I-induced DNA synthesis in human promyeloid cells extends to IL-1beta. Statistically significant inhibition of all these events occurs at very low concentrations of 1 ng/ml or less. These results support the general concept that proinflammatory cytokines impair the actions of hormones on hematopoietic cells, leading to IGF-I receptor resistance.

IL-1β Suppresses Prolonged Akt Activation and Expression of E2F-1 and Cyclin A in Breast Cancer Cells

Cell cycle aberrations occurring at the G(1)/S checkpoint often lead to uncontrolled cell proliferation and tumor growth. We recently demonstrated that IL-1beta inhibits insulin-like growth factor (IGF)-I-induced cell proliferation by preventing cells from entering the S phase of the cell cycle, leading to G(0)/G(1) arrest. Notably, IL-1beta suppresses the ability of the IGF-I receptor tyrosine kinase to phosphorylate its major docking protein, insulin receptor substrate-1, in MCF-7 breast carcinoma cells. In this study, we extend this juxtamembrane cross-talk between cytokine and growth factor receptors to downstream cell cycle machinery. IL-1beta reduces the ability of IGF-I to activate Cdk2 and to induce E2F-1, cyclin A, and cyclin A-dependent phosphorylation of a retinoblastoma tumor suppressor substrate. Long-term activation of the phosphatidylinositol 3-kinase/Akt signaling pathway, but not the mammalian target of rapamycin or mitogen-activated protein kinase pathways, is required for IGF-I to hyperphosphorylate retinoblastoma and to cause accumulation of E2F-1 and cyclin A. In the absence of IGF-I to induce Akt activation and cell cycle progression, IL-1beta has no effect. IL-1beta induces p21(Cip1/Waf1), which may contribute to its inhibition of IGF-I-activated Cdk2. Collectively, these data establish a novel mechanism by which prolonged Akt phosphorylation serves as a convergent target for both IGF-I and IL-1beta; stimulation by growth factors such as IGF-I promotes G(1)-S phase progression, whereas IL-1beta antagonizes IGF-I-induced Akt phosphorylation to induce cytostasis. In this manner, Akt serves as a critical bridge that links proximal receptor signaling events to more distal cell cycle machinery.

From hormones to immunity: the physiology of immunology

Discoveries in the physiology of immunology have increased at an increasing rate during the past two decades. It is now recognized that the immune system is just another physiological system that regulates, and is regulated by, other physiological systems such as the brain. These advances make it clear that recent findings in genomic biology must be interpreted in the context of the environment in which animals and humans live. Lack of a strong genetic basis for significant human mental health disorders, such as major depression, points to the critical importance of interactions. Several examples of environmental x genetic x disease interactions are presented. Regulation of cells of the hematopoietic lineage by two genes that control over 80% of postnatal growth, growth hormone and IGF-I, are then highlighted. The reciprocal relationship of how proinflammatory cytokines from the immune system regulate the growth hormone/IGF-I axis is also summarized. Particular emphasis is placed upon TNFalpha-induced IGF-I resistance in neurons, muscle cells and epithelial cells. This cytokine regulation of hormone action may ultimately be more important for human and animal health than direct effects of growth hormone and IGF-I on hematopoietic cells. Wasting of AIDS patients is given as an important clinical example of how TNFalpha from an activated immune system reduces IGF-I sensitivity in multiple physiologic systems, including muscle, nervous and hematopoietic tissues.

Tumor necrosis factor alpha inhibits cyclin A expression and retinoblastoma hyperphosphorylation triggered by insulin-like growth factor-I induction of new E2F-1 synthesis

Cyclin A is required for cell cycle S phase entry, and its overexpression contributes to tumorigenesis. Release of pre-existing E2Fs from inactive complexes of E2F and hypophosphorylated retinoblastoma (RB) is the prevailing dogma for E2F transcriptional activation of target genes such as cyclin A. Here we explored the hypothesis that new synthesis of E2F-1 is required for insulin-like growth factor-I (IGF-I) to induce cyclin A accumulation and RB hyperphosphorylation, events that are targeted by tumor necrosis factor alpha (TNFalpha) to arrest cell cycle progression. We first established that IGF-I increases expression of cyclin A, causes hyperphosphorylation of RB, and augments the mass of E2F-1 in a time-dependent manner. As expected, E2F-1 small interfering RNA blocks the ability of IGF-I to increase synthesis of E2F-1. Most important, this E2F-1 small interfering RNA also blocks the ability of IGF-I to increase cyclin A accumulation and to hyperphosphorylate RB. We next established that TNFalpha dose-dependently inhibits IGF-I-induced phosphorylation of both RB and histone H1 by cyclin A-dependent cyclin-dependent kinases. Cyclin-dependent kinase 2 (Cdk2) mediates this suppression because co-immunoprecipitation experiments revealed that TNFalpha reduces the amount of IGF-I-induced cyclin A that binds Cdk2, leading to a reduction in Cdk2 enzymatic activity. TNFalpha antagonizes the ability of IGF-I to increase mass of both E2F-1 and cyclin A but not cyclin E or D1. The cytostatic property of TNFalpha is also shown by its ability to block IGF-I-stimulated luciferase activity of a cyclin A promoter reporter. Deletion of an E2F recognition site from this reporter eliminates the regulatory effects of both IGF-I and TNFalpha on cyclin A transcription, indicating the essential role of E2F-1 in mediating their cross-talk. Collectively, these results establish that TNFalpha targets IGF-I-induced E2F-1 synthesis, leading to inhibition of the subsequent accumulation in cyclin A, formation of cyclin A-Cdk2 complexes, hyperphosphorylation of RB, and cell cycle arrest.

Retinoic acid-induced cell cycle arrest of human myeloid cell lines is associated with sequential down-regulation of c-Myc and cyclin E and posttranscriptional up-regulation of p27(Kip1)

All-trans retinoic acid (ATRA) is a potential therapeutic agent for the treatment of hematopoietic malignancies, because of its function as an inducer of terminal differentiation of leukemic blasts. Although the efficacy of ATRA as an anticancer drug has been demonstrated by the successful treatment of acute promyelocytic leukemia (APL), the molecular mechanisms of ATRA-induced cell cycle arrest of myeloid cells have not been fully investigated. In this study, we show that the onset of ATRA-induced G(0)/G(1) arrest of human monoblastic U-937 cells is linked to a sharp down-regulation of c-Myc and cyclin E levels and an increase in p21(WAF1/CIP1) expression. This is followed by an increase in p27(Kip1) protein expression due to enhanced protein stability. The importance of an early decrease in Myc expression for these events was demonstrated by the failure of a U-937 subline with constitutive exogenous expression of v-Myc to cell cycle arrest and regulate cyclin E and p27(Kip1) in response to ATRA. Preceding the initiation of G(1) arrest, a transient rise in retinoblastoma protein (pRb), p107, and cyclin A levels was detected. Later, a rapid fall in the levels of cyclins A and B and a coordinate dephosphorylation of pRb at Ser780, Ser795, and Ser807/811 coincided with the accumulation of cells in G(1). These results thus identify a decrease in c-Myc and cyclin E levels and a posttranscriptional up-regulation of p27(Kip1) as important early changes, and position them in the complex chain of events regulating ATRA-induced cell cycle arrest of myeloid cells.

The insulin-like growth factor system in hematopoietic cells

The insulin-like growth factor (IGF) system regulates proliferation and differentiation of hematopoietic cells. IGFs exert their effects through specific receptors on growing and differentiating blood cells as they emerge from their small pool of ancestral stem cells. The IGF system is complex as both stimulating and inhibiting effects occur by interaction of IGFs and IGF-binding proteins (IGFBPs). IGFs stimulate erythrocytes and lymphocytes but also promote leukemic hematopoietic cell proliferation. IGF-I appears to be correlated with hemoglobin levels in anemia and could also be of benefit for patients with bone marrow aplasia after transplantation. Hypersensitivity to IGF-I has been implicated as an underlying cause of polycythemia vera. Loss of imprinting of IGF-II is found in acute myeloid leukemia and myelodysplastic syndrome. Apoptosis of hematopoietic cells is significantly reduced by IGF-I involving an intriguing signal transduction pathway. IGFs could therefore, although not classical hematopoietic growth factors, be of benefit for patients with diverse hematopoietic disorders.

Age-associated loss of bone marrow hematopoietic cells is reversed by GH and accompanies thymic reconstitution

Deterioration of the thymus gland during aging is accompanied by a reduction in plasma GH. Here we report gross and microscopic results from 24-month-old Wistar-Furth rats treated with rat GH derived from syngeneic GH3 cells or with recombinant human GH. Histological evaluation of aged rats treated with either rat or human GH displayed clear morphologic evidence of thymic regeneration, reconstitution of hematopoietic cells in the bone marrow, and multiorgan extramedullary hematopoiesis. Quantitative evaluation of formalin-fixed, hematoxylin and eosin-stained sections of bone marrow from aged rats revealed at least a 50% reduction in the number hematopoietic bone marrow cells, compared with that of young 3-month-old rats. This age-associated decline in bone marrow leukocytes, as well as the increase in bone marrow adipocytes, was significantly reversed by in vivo treatment with GH. Restoration of bone marrow cellularity was caused primarily by erythrocytic and granulocytic cells, but all cell lineages were represented and their proportions were similar to those in aged control rats. On a per-cell basis, GH treatment in vivo significantly increased the number of in vitro myeloid colony forming units in both bone marrow and spleen. Morphological evidence of enhanced extramedullary hematopoiesis was observed in the spleen, liver, and adrenal glands from animals treated with GH. These results confirm that GH prevents thymic aging. Furthermore, these data significantly extend earlier findings by establishing that GH dramatically promotes reconstitution of another primary hematopoietic tissue by reversing the accumulation of bone marrow adipocytes and by restoring the number of bone marrow myeloid cells of both the erythrocytic and granulocytic lineages.

Cell cycle control genes and hematopoietic cell differentiation

To maintain the fidelity and integrity of blood formation, the cell cycle is under strict regulation during hematopoietic cell differentiation. This review summarizes recent studies, including our own, on the expression of cell cycle control genes in hematopoietic stem cells and its changes during differentiation. In our study, mRNA expression of cyclin-dependent kinases (cdks) and cyclins, except cdk4, was found to be generally suppressed in CD34+ cells isolated from the bone marrow of healthy volunteers. Among four major cdk inhibitors, p16 was expressed higher in CD34+ cells than in CD34 bone marrow mononuclear cells, whereas the amounts of p21 and p27 transcripts increased in the CD34 population. The behavior of cell cycle control genes during hematopoietic differentiation was classified into four patterns: (i) universal up-regulation (cdc2, cdk2, cyclin A, cyclin B, p21); (ii) up-regulation in specific lineages (cyclin D1, cyclin D3, and p5); (iii) no induction or stable expression (cdk4, cyclin D2, cyclin E, and p27); and (iv) universal down-regulation (p16). Lineage-specific changes include a sustained elevation of cdc2 and cyclin A during erythroid differentiation, cyclin D1 and p15 induction in myeloid lineage cells, and selective up-regulation of cyclin D3 during megakaryocyte development. These results suggest that the expression of cell cycle control genes is distinctively regulated in a lineage-dependent manner, reflecting the cell cycle characteristics of each lineage. Additional data from other laboratories are summarized and their significance is discussed in comparison with our findings.

The growth suppressor p27(Kip1) protects against diet-induced atherosclerosis

The molecular basis of atherosclerosis is associated with excessive proliferation of vascular cells. Previous studies have suggested an inverse correlation between the expression of the growth suppressor p27(Kip1) (p27) and cellular proliferation within human atherosclerotic tissue. However, no causal link between diminished p27 expression and atherogenesis has been established. We investigated the effect of p27 inactivation on diet-induced atherogenesis. We find that p27-deficient mice challenged with a high-fat diet for 1 month remain normocholesterolemic and have essentially no visible atheromas. However, when generated in an apolipoprotein E-null genetic background that leads to severe hypercholesterolemia in response to the atherogenic diet, deletion of p27 enhances arterial cell proliferation (approximately fourfold) and accelerates atherogenesis (approximately sixfold) compared with apolipoprotein E-deficient mice with an intact p27 gene. Analysis of apolipoprotein E-null mice bearing only one p27 allele inactivated reveals that a moderate decrease in p27 protein expression in the setting of hypercholesterolemia is sufficient to predispose to atherogenesis. Thus, our study establishes a molecular link between decreased p27 protein expression and atherogenesis in hypercholesterolemic animals.

Nuclear envelope and chromatin compositional differences comparing undifferentiated and retinoic acid- and phorbol ester-treated HL-60 cells

The human leukemic cell line (HL-60) can be induced to differentiate in vitro to granulocytic form with retinoic acid (RA), or to monocytic/macrophage form with phorbol ester (TPA). The granulocytic form acquires nuclear lobulation, nuclear envelope-limited chromatin sheets (ELCS), and cytoskeletal polarization, none of which are acquired following treatment with TPA. Immunoblotting analyses and capillary zone electrophoresis demonstrated that following RA treatment: lamins A/C and B1, and vimentin decreased to negligible amounts; LAP2 beta, lamin B2 and emerin remained essentially unchanged; lamin B receptor (LBR) increased markedly; histone subtypes H1.4 and 1.5 exhibited dephosphorylation. Following TPA treatment: lamins A/C and B1, B2 and vimentin increased in amount; LAP2 beta and emerin remained essentially unchanged; LBR increased markedly; histone subtypes H1.4 and 1.5 exhibited dephosphorylation. Emerin, which was cytoplasmic in undifferentiated or granulocytic cells, localized into the nuclear envelope following TPA. Normal human granulocytes revealed compositional differences compared to granulocytic forms of HL-60, namely increased vimentin and appearance of histone subtype H1.3. A working hypothesis for nuclear lobulation postulates a combination of: increased nuclear envelope deformability due to lamins A/C and B1 deficiency; an increase in nuclear surface area/volume; an increase in chromatin-nuclear envelope interactions.

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.

Up-regulation of steroid sulphatase activity in HL60 promyelocytic cells by retinoids and 1alpha,25-dihydroxyvitamin D3

HL60 promyeloid cells express both classes of oestrogen receptor (ERalpha and ERbeta). We show that hydrolysis of oestrone sulphate by steroid sulphatase is a major source of oestrone in HL60 cells, and that most of the released oestrone is not metabolized further to 17beta-oestradiol. Treatment of HL60 cells with retinoids or 1alpha,25-dihydroxyvitamin D3 increased steroid sulphatase mRNA and activity in parallel with the induction of CD11b, an early marker of myeloid differentiation that is expressed before the differentiating cells stop proliferating. Use of agonists and antagonists against retinoid receptor-alpha and retinoid receptor-X revealed that both classes of retinoid receptor can drive steroid sulphatase up-regulation. Steroid sulphatase activity fluctuates during the cell cycle, being highest around the transition from G1 to S phase. During the differentiation of HL60 cells induced by all-trans-retinoic acid or 1alpha,25-dihydroxyvitamin D3, there is increased conversion of 17beta-oestradiol into oestrone by an oxidative 17beta-hydroxysteroid dehydrogenase. Treatment of Caco-2 colon adenocarcinoma cells with all-trans-retinoic acid or 1alpha,25-dihydroxyvitamin D3 also increases 17beta-oestradiol oxidation to oestrone. An increase in local oestrone production therefore occurs in multiple cell types following treatment with retinoids and 1alpha,25-dihydroxyvitamin D3. The possible involvement of locally produced oestrogenic steroids in regulating the proliferation and differentiation of myeloid cells is discussed.

Lineage-specific regulation of cell cycle control gene expression during haematopoietic cell differentiation

To maintain the fidelity and integrity of blood formation, the cell cycle is under strict regulation during haematopoietic cell differentiation. To elucidate the molecular mechanisms of cell cycle regulation during haematopoiesis, we examined cell cycle control gene expression during lineage-specific differentiation from CD34+ progenitor cells. Expression of cyclin-dependent kinases (cdks) and cyclins, except cdk4, was generally suppressed in CD34+ cells freshly isolated from the bone marrow of healthy volunteers. Among four major cdk inhibitors, p16 was expressed more highly in CD34+ cells than in CD34-negative bone marrow mononuclear cells, whereas the amounts of p21 and p27 transcripts increased in the CD34- population. The behaviour of cell cycle control genes during haematopoietic differentiation was classified into four patterns: (i) universal upregulation (cdc2, cdk2, cyclin A, cyclin B and p21); (ii) upregulation in specific lineages (cyclin D1, cyclin D3 and p15); (iii) no induction or stable expression (cdk4, cyclin D2, cyclin E and p27); and (iv) universal downregulation (p16). Lineage-specific changes included the sustained elevation of cdc2 and cyclin A during erythroid differentiation, cyclin D1 and p15 induction in myeloid lineage and selective upregulation of cyclin D3 in megakaryocytes. Blocking induction of cyclin D3 resulted in the inhibition of megakaryocytic differentiation. These results suggest that the expression of cell cycle control genes is distinctively regulated in a lineage-dependent manner, reflecting the cell cycle characteristics of each lineage. Some of these genes play an essential role in the process of differentiation itself.