Nonredundant roles of the mPer1 and mPer2 genes in the mammalian circadian clock

Mice carrying a null mutation in the Period 1 (mPer1) gene were generated using embryonic stem cell technology. Homozygous mPer1 mutants display a shorter circadian period with reduced precision and stability. Mice deficient in both mPer1 and mPer2 do not express circadian rhythms. While mPER2 regulates clock gene expression at the transcriptional level, mPER1 is dispensable for the rhythmic RNA expression of mPer1 and mPer2 and may instead regulate mPER2 at a posttranscriptional level. Studies of clock-controlled genes (CCGs) reveal a complex pattern of regulation by mPER1 and mPER2, suggesting independent controls by the two proteins over some output pathways. Genes encoding key enzymes in heme biosynthesis are under circadian control and are regulated by mPER1 and mPER2. Together, our studies show that mPER1 and mPER2 have distinct and complementary roles in the mouse clock mechanism.

Postmortem stability of melatonin receptor binding and clock-relevant mRNAs in mouse suprachiasmatic nucleus

The stability of receptor proteins and mRNAs in brain tissue is variable after death. As a prelude to quantitative studies of melatonin receptor density and clock gene expression in the human brain, the stability of these macromolecules was examined in the mouse brain under simulated postmortem conditions using the model of Spokes and Koch. In the mouse suprachiasmatic nucleus (SCN), melatonin receptor binding was significantly reduced after 18 to 24 h under postmortem conditions. Two mRNAs that are rhythmically expressed in the SCN, mPer1 and prepropressophysin (AVP), also decreased significantly over the interval studied, and mPer1 declined more rapidly than AVP. Both mPer1 and AVP mRNA levels in the SCN declined more rapidly in vivo than under postmortem conditions, suggesting that the degradation of these mRNAs is an active process. The results indicate that quantitative studies of melatonin receptor density on human postmortem material are feasible and that detection of rhythmic gene expression in the human SCN will likely require collection of specimens with a rather short (< 8 h) interval from death to tissue collection. The relative stability of melatonin receptor binding in the SCN also suggests that receptor binding may be a reliable marker for the location of the SCN in studies assessing clock gene expression in postmortem material.

Alterations of the 9p21 and 9q33 chromosomal bands in clinical bladder cancer specimens by fluorescence in situ hybridization

PURPOSE

To better define cytogenetic mechanisms of CDKN2 loss at 9p21 and of DBCCR1 loss at 9q33 in bladder cancer, and to determine correlation with p53 and pRb.

EXPERIMENTAL DESIGN

Two-color fluorescence in situ hybridization (FISH) using a chromosome 9 centromeric probe and locus-specific probes was performed. p53 and pRb were assessed by immunohistochemistry.

RESULTS

Thirty-seven of fifty-five (67%) samples exhibited 9p21 loss, and 32 of 44 (73%) exhibited 9q33 loss. Twelve of 43 informative samples exhibited only 9p21 loss (5 cases) or only 9q33 loss (7 cases). Homozygous deletions were noted at 9p21 and 9q33 in 31 and 14% of cases, respectively, but 9q33 homozygous deletions were generally observed in only a minor clone. There was no correlation of any chromosome 9 loss with stage, but stage did correlate with chromosome 9 ploidy status; aneusomy 9 was observed in 33% of T(a) lesions and 71% of more advanced cases (P = 0.01). Aneusomy 9 was loosely correlated with p53 abnormalities (P = 0.07), but no correlation between any chromosome 9 and pRb abnormalities was discerned.

CONCLUSIONS

This study strengthens the proposition that chromosome 9 losses occur early in bladder oncogenesis and before p53 alterations or development of aneusomy. The correlation of aneusomy 9 with p53 abnormalities is consistent with the presumed role of p53 in maintaining cytogenetic stability. Although the observed homozygous deletions strengthen the hypotheses that CDKN2 and DBCCR1 are important tumor suppressor genes, there is no evidence that either is a more critical or an earlier target for oncogenesis.

SECuring the perimeter

Exquisitely regulated trafficking and fusion of vesicles is crucial for proper cell function. The molecules that regulate vesicle fusion are highly conserved among eukaryotes, but they have also undergone expansion and specialization within single genomes. With diversity comes the potential for functions in unique cell processes, and recent work in Arabidopsis reveals how a member of the SEC1 family, KEULE, functions in plant cell cytokinesis.

Atm knock-in mice harboring an in-frame deletion corresponding to the human ATM 7636del9 common mutation exhibit a variant phenotype

ATM, the gene mutated in the human immunodeficiency disorder ataxia-telangiectasia (A-T), plays a central role in recognizing ionizing radiation damage in DNA and in controlling several cell cycle checkpoints. We describe here a murine model in which a nine-nucleotide in-frame deletion has been introduced into the Atm gene by homologous recombination followed by removal of the selectable marker cassette by Cre-loxP site-specific, recombination-mediated excision. This mouse, Atm-DeltaSRI, was designed as a model of one of the most common deletion mutations (7636del9) found in A-T patients. The murine Atm deletion results in the loss of three amino acid residues (SRI; 2556-2558) but produces near full-length detectable Atm protein that lacks protein kinase activity. Radiosensitivity was observed in Atm-DeltaSRI mice, whereas the immunological profile of these mice showed greater heterogeneity of T-cell subsets than observed in Atm(-/-) mice. The life span of Atm-DeltaSRI mice was significantly longer than that of Atm(-/-) mice when maintained under nonspecific pathogen-free conditions. This can be accounted for by a lower incidence of thymic lymphomas in Atm-DeltaSRI mice up to 40 weeks, after which time the animals died of other causes. The thymic lymphomas in Atm-DeltaSRI mice were characterized by extensive apoptosis, which appears to be attributable to an increased number of cells expressing Fas ligand. A variety of other tumors including B-cell lymphomas, sarcomas, and carcinomas not seen in Atm(-/-) mice were observed in older Atm-DeltaSRI animals. Thus, expression of mutant protein in Atm-DeltaSRI knock-in mice gives rise to a discernibly different phenotype to Atm(-/-) mice, which may account for the heterogeneity seen in A-T patients with different mutations.

PTEN induces G(1) cell cycle arrest and decreases cyclin D3 levels in endometrial carcinoma cells

Inactivating mutations in the PTEN tumor suppressor gene occur in approximately 30-50% of endometrial carcinomas. PTEN is a phosphatase that negatively regulates the phosphoinositide 3-kinase signaling pathway, including the downstream effector AKT. To evaluate the role of PTEN in endometrial growth regulation, we expressed wild-type or mutant PTEN in endometrial carcinoma cell lines. As expected, expression of exogenous PTEN decreased levels of activated AKT in all cell lines examined. However, PTEN induced a G(1) cell cycle arrest specifically in endometrial carcinoma cells that lack endogenous wild-type PTEN. Growth of cells containing wild-type PTEN was unaffected by exogenous PTEN expression. Growth arrest required a functional phosphatase domain but not the PDZ interaction motif of PTEN. Overall levels of CIP/KIP and INK4 family members, the known inhibitory regulators of the G(1) phase of the cell cycle, were unchanged. However, PTEN induced a specific reduction of cyclin D3 levels and an associated increase in the amount of the inhibitor p27(KIP1) complexed with CDK2. Enforced expression of cyclin D3 abrogated the PTEN-induced cell cycle arrest. Although PTEN signaling directly regulates p27(KIP1) levels in some settings, in endometrial carcinoma cells, PTEN expression indirectly regulated p27(KIP1) activity by modulating levels of cyclin D3. These data support multiple mechanisms of PTEN-induced cell cycle arrest.

Identification and implantation stage-specific expression of an interferon-alpha-regulated gene in human and rat endometrium

Implantation of the developing blastocyst is regulated by multiple effectors, such as steroid hormones, growth factors, and cytokines. To understand how these diverse signaling pathways interact to modulate uterine gene expression, we employed a gene expression screen technique to identify the molecules that are induced in the periimplantation rat uterus. Here we report the isolation of a complementary DNA representing a novel gene, interferon-regulated gene 1 (IRG1). This gene exhibits significant homology to interferon (IFN)-alpha/beta-inducible human genes p27 and 6-16, indicating that these genes may belong to the same family. Consistent with this finding, expression of IRG1 messenger RNA (mRNA) in rat uterus increased about 20-fold in response to IFNalpha. Uterine expression of IRG1 was also stimulated by estrogen and was partially inhibited by an antiestrogen, ICI 182,780. In pregnant rats, IRG1 expression was high on day 1, but declined on days 2 and 3. The level of IRG1 mRNA again rose transiently on day 4 immediately preceding implantation. In situ hybridization analysis localized the IRG1 mRNA expression in the endometrial epithelium and the surrounding stroma. Interestingly, the expression of p27, which shows high homology to IRG1, was strongly enhanced in human endometrium during the midsecretory phase of the menstrual cycle, overlapping the putative window of implantation. Both IRG1 and p27 mRNAs are therefore induced in the endometrium in an implantation stage-specific manner. We also observed a synergistic interaction between IFNalpha and estrogen receptor signaling pathways that led to maximal induction of p27 mRNA in Ishikawa cells. Although the functional roles of IRG1 and p27 remain unclear, we describe for the first time, identification of a gene family regulated by IFNalpha in both rodent and human uteri. More importantly, our studies reveal that a complex interplay between the steroid hormone and IFN pathways regulates the expression of these genes in the endometrium at the time of implantation.

Foci on fanconi

The protein network protecting the stability of the genome is defective in Fanconi anemia (FA). The newest in a series of FA proteins is involved in DNA damage response, but the mechanism is still unclear. Clues may come from yeast two-hybrid experiments, an extraordinarily successful tool for determining molecular function.

Ataxia telangiectasia: new neurons and ATM

Despite the rarity of the human autosomal recessive disease ataxia telangiectasia (A-T) (affecting approximately 1/40000-1/100000), interest in the function of the mutated gene product (ATM) in this syndrome is intense. Mutation of this single gene can lead to a diverse array of features, including cancer, immune defects, infertility and radiosensitivity. However, it is the pronounced and debilitating neurodegeneration that is the hallmark of this disease. Thus, from a clinical perspective, it is ATM function in the nervous system that, arguably, is the most important to understand. Although the case for DNA damage as a causative factor for neurodegeneration in A-T is compelling, new data point to a possible link to defects in neurogenesis. Thus, whereas ATM is important for nervous system development, it could also be important for adult neurogenesis.

Genetic alterations of candidate tumor suppressor ING1 in human esophageal squamous cell cancer

Overexpression of ING1, a candidate tumor suppressor gene, efficiently blocks cell growth or induces apoptosis in different experimental systems. ING1 maps to chromosome 13q33-34, and because loss of the terminal region of chromosome 13q has been implicated in esophageal squamous cell cancer (ESCC), we examined ESCC for genetic alterations of ING1. Among 31 informative cases of ESCC, 58.9% of the tumors showed allelic loss at chromosome 13q33-34, and we detected four tumor-specific missense nucleotide changes. These alterations were found within the PHD finger domain and nuclear localization motif of the ING1 and may be functionally involved in the development of ESCC. Because immunohistochemical study revealed that all of the ESCC samples showed loss of ING1 protein expression, genetic or epigenetic alterations that abrogate the normal function of ING1 may contribute to esophageal squamous cell carcinogenesis.

Relationship of p15 and p16 gene alterations to elevated dihydrofolate reductase in childhood acute lymphoblastic leukaemia

The downstream effects of p15 and p16 gene deletions and loss of transcripts on dihydrofolate reductase (DHFR) were examined in 63 B-precursor (BP) acute lymphoblastic leukaemia (ALL) samples. p15 and/or p16 gene deletions were seen in 6% and 8%, respectively, of BP-ALL samples; however, losses of p15 and/or p16 transcripts were seen in 26 out of 63 (41%) samples. Loss of p15 transcripts (36.5%) exceeded that for p16 (17.5%). For the 26 BP-ALLs that lacked p15 and/or p16 transcripts, only six (23%) exhibited low levels of DHFR by flow cytometry assay with Pt430, a fluorescent anti-folate. Conversely, 18 out of 37 (49%) BP-ALL samples with intact p15 and/or p16 genes and transcripts showed low levels of DHFR (P = 0.04). In p15- and p16-null K562 cells transfected with a tetracycline-inducible p15 cDNA construct, induction of p15 transcripts and protein was accompanied by decreased growth rates, decreased S-phase fraction, decreased retinoblastoma protein phosphorylation, and markedly reduced levels of DHFR transcripts and protein. Collectively, our results suggest that losses of p15 and/or p16 gene expression result in elevated levels of DHFR in BP-ALL in children. However, additional downstream factors undoubtedly also contribute to elevated levels of this enzyme target.

Altered expression of nuclear matrix proteins in etoposide induced apoptosis in HL-60 cells

The events of cell death and the expression of nuclear matrix protein (NMP) have been investigated in a promyelocytic leukemic cell line HL-60 induced with etoposide. By means of TUNEL assay, the nuclei displayed a characteristic morphology change, and the amount of apoptotic cells increased early and reached maximun about 39% after treatment with etoposide for 2 h. Nucleosomal DNA fragmentation was observed after treatment for 4 h. The morphological change of HL-60 cells, thus, occurred earlier than the appearance of DNA ladder. Total nuclear matrix proteins were analyzed by 2-dimensional gel electrophoresis. Differential expression of 59 nuclear matrix proteins was found in 4 h etoposide treated cells. Western blotting was then performed on three nuclear matrix acssociated proteins, PML, HSC70 and NuMA. The expression of the suppressor PML protein and heat shock protein HSC70 were significantly upregulated after etoposide treatment, while NuMA, a nuclear mitotic apparatus protein, was down regulated. These results demonstrate that significant biochemical alterations in nuclear matrix proteins take place during the apoptotic process.

Immunohistochemical detection of cell growth fraction in formalin-fixed and paraffin-embedded murine tissue

Monoclonal antibody MIB-1 is a reliable tool for determining proliferating cells in human tissues, but does not react with the homologous mouse antigen and is therefore useless in experimental pathology using mice as model systems. Standard method for assessment of cellular proliferation in formalin-fixed, paraffin-embedded murine tissues is immunohistochemical detection of DNA synthesis using antibodies against exogenously injected 5-bromodeoxyuridine (BrdU), which is a tedious procedure and not useful for routine investigations. We tested monoclonal antibody MIB-5 and monoclonal and polyclonal anti-MCM3 antibodies as immunohistochemical proliferation markers for paraffin-embedded nonneoplastic and neoplastic tissues of wild-type and transgenic mice, compared to anti-BrdU immunostaining. Percentage of proliferating cells was determined with continuously decreasing antibody dilutions. Percentages of MIB-5 and anti-BrdU immunostained cells correlated strongly, as well as percentage of MIB-5-decorated cells and frequency of mitotic figures. Anti-MCM3 antibodies labeled significantly higher percentages of cells than anti-BrdU or MIB-5, and showed a linear decrease with increasing antibody dilutions. We conclude that MIB-5 detects reliably the cell growth fraction in formalin fixed, paraffin-embedded murine tissues, bypassing methodological drawbacks of BrdU. Anti-MCM3 antibodies are less useful for determination of proliferating cells although they might detect the fraction of cells remaining competent for proliferation.

Topological regulation of cell division in E. coli. spatiotemporal oscillation of MinD requires stimulation of its ATPase by MinE and phospholipid

Topological regulation of cell division in E. coli requires positioning a cell division inhibitor, MinC, at the poles of the cell, thus restricting the potential for division to midcell. This positioning is achieved through a rapid oscillation of MinC from pole to pole, a process requiring MinD and MinE. However, the mechanistic basis for this oscillation is not known. Here we report that MinE stimulates MinD ATPase activity, but only in the presence of phospholipid vesicles. Analysis of MinE mutants demonstrates that this stimulation is required for MinD oscillation and suggests that the level of stimulation determines the period of the oscillation. A model is presented in which the requirements for the MinD ATPase contribute spatial and temporal inputs that provide the mechanistic basis for the oscillation.

Mutations in the agouti (ASIP), the extension (MC1R), and the brown (TYRP1) loci and their association to coat color phenotypes in horses (Equus caballus)

Coat color genetics, when successfully adapted and applied to different mammalian species, provides a good demonstration of the powerful concept of comparative genetics. Using cross-species techniques, we have cloned, sequenced, and characterized equine melanocortin-1-receptor (MC1R) and agouti-signaling-protein (ASIP), and completed a partial sequence of tyrosinase-related protein 1 (TYRP1). The coding sequences and parts of the flanking regions of those genes were systematically analyzed in 40 horses and mutations typed in a total of 120 horses. Our panel represented 22 different horse breeds, including 11 different coat colors of Equus caballus. The comparison of a 1721-bp genomic fragment of MC1R among the 11 coat color phenotypes revealed no sequence difference apart from the known chestnut allele (C901T). In particular, no dominant black (ED) mutation was found. In a 4994-bp genomic fragment covering the three putative exons, two introns and parts of the 5'- and 3'-UTRs of ASIP, two intronic base substitutions (SNP-A845G and C2374A), a point mutation in the 3'-UTRs (A4734G), and an 11-bp deletion in exon 2 (ADEx2) were detected. The deletion was found to be homozygous and completely associated with horse recessive black coat color (Aa/Aa) in 24 black horses out of 9 different breeds from our panel. The frameshift initiated by ADEx2 is believed to alter the regular coding sequence, acting as a loss-of-function ASIP mutation. In TYRP1 a base substitution was detected in exon 2 (C189T), causing a threonine to methionine change of yet unknown function, and an SNP (A1188G) was found in intron 2.

Persistent low-level engraftment of rhesus peripheral blood progenitor cells transduced with the fanconi anemia C gene after conditioning with low-dose irradiation

The hematopoietic stem cell has long been considered an ideal target for the introduction of therapeutic genes to treat human disorders such as Fanconi anemia (FA). Although recent progress in large animal models is encouraging, application to nonmalignant conditions is limited by the perceived necessity of myeloablative conditioning. We and others have shown that very low irradiation doses are sufficient to allow significant hematopoietic engraftment in murine hosts even after the introduction of xenogeneic genes. To determine the degree of engraftment of genetically modified cells attainable with very low irradiation doses in larger animals, we employed the rhesus macaque competitive repopulation model. Four animals underwent mobilization with stem cell factor (SCF) and granulocyte colony-stimulating factor (G-CSF) followed by apheresis. The apheresis product was enriched for the CD34-positive fraction by immunomagnetic selection and split equally for transduction with either G1FC26, a retroviral vector carrying the Fanconi anemia complementation group C gene, or PLII, a nonexpression control retroviral vector carrying both neomycin and beta-galactosidase gene sequences modified to prevent translation. Transductions were performed daily in the presence of fresh IL-3, IL-6, SCF, and Flt-3 ligand on fibronectin-coated plates over 96 h. Animals were conditioned with a single dose of either 100 (n = 2) or 200 (n = 2) cGy and received the combined products of transduction on the following day. None of the animals experienced clinically significant neutropenia nor required the use of central line placement, transfusional support with blood products, or intravenous antibiotics. Using real-time PCR, circulating levels of genetically modified cells as high as 1% were initially detected. Stable, albeit, significantly lower levels from both vector-transduced aliquots (<0.1%) persisted beyond 12 months posttransplant in all four animals. Although not sufficient to correct the phenotype in many human disorders, stable low-level engraftment by genetically modified cells following low-intensity conditioning may prove adequate in disorders such as FA due to the selective advantage conferred upon corrected cells.

Cutaneous side-effects in cancer patients treated with the antiepidermal growth factor receptor antibody C225

BACKGROUND

C225 is an antibody to the epidermal growth factor receptor (EGFR), and inhibits growth of various tumour cells. The antibody is currently being used as a therapeutic agent in several clinical trials of patients with carcinomas. Objectives To determine and investigate the cutaneous side-effects in cancer patients treated with C225. Methods We clinically examined 10 patients treated with C225, and performed immunohistochemical and in situ hybridization studies on skin biopsies. Results The most common cutaneous reaction to C225 therapy was the development of an acneiform follicular eruption, which was most pronounced on the face, chest and upper back and typically manifested a week after the onset of treatment. The consistency of the morphology and timing of the clinical findings in 10 different patients following monotherapy with C225 strongly suggested a direct biological effect of the antibody. Additional dermatological side-effects included focal areas of tender paronychial inflammation of toes and fingers and small aphthous ulcers of the oral mucosa. Serial punch biopsies of chest skin before and after treatment (at 8 days) revealed two main reaction patterns: a superficial dermal inflammatory cell infiltrate surrounding hyperkeratotic and ectatic follicular infundibula, and a suppurative superficial folliculitis. In two biopsies focal intraepidermal acantholysis was found. Microbiological cultures failed to reveal an infectious aetiology. Immunohistochemical and in situ hybridization studies on a subset of the biopsies showed an increase in the expression of p27Kip1 in epidermal keratinocytes after treatment with C225. Conclusions Our findings support the concept that p27Kip1 plays a part in the in vivo regulation of follicular and epidermal homeostasis by EGFR.

Genomic structure of the human MAD2 gene and mutation analysis in human lung and breast cancers

Some of the many human cancers that exhibit chromosomal instability also carry mutations in mitotic checkpoint genes and/or reveal reduced expression of some of those genes, such as hMAD2. To facilitate investigation of alterations of hMAD2, we determined its genomic structure and intronic primers designed to amplify the entire coding region. Since general impairment of the mitotic checkpoint is frequently reported in lung cancers, and reduced expression of hMAD2 has been reported in breast cancers as well, we searched for mutations throughout the coding sequence of this gene in the genomic DNA of 30 primary lung tumors, 30 lung-cancer cell lines and 48 primary breast cancers. Our approach, which involved polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis and direct sequencing, revealed nucleotide variants in only two of the 108 specimens. One was a cytosine-to-adenine substitution 3 bp upstream of exon 4 that occurred in one lung cancer cell line and one primary breast tumor, a change that did not alter transcriptional sequence. The other was an adenine-to-guanine substitution within exon 4, of the same lung cell line; this change already had been reported as a polymorphism. The results suggested that the hMAD2 gene is not commonly mutated in either lung nor breast cancers. Further studies should focus on other mechanisms that might account for reduced expression of the hMAD2 gene, and/or pursue analyses of other mitotic checkpoint genes for mutations in human cancer. Nevertheless, the genomic structure, the intronic primer sequences, and polymorphisms of the hMAD2 gene presented here will facilitate future studies to determine the full spectrum and frequency of the genetic events that can affect expression of the hMAD2 gene in human tumors.

Aging, oxidative responses, and proliferative capacity in cultured mouse aortic smooth muscle cells

The cellular mechanisms that contribute to the acceleration of atherosclerosis in aging populations are poorly understood, although it is hypothesized that changes in the proliferative capacity of vascular smooth muscle cells is contributory. We addressed the relationship among aging, generation of reactive oxygen species (ROS), and proliferation in primary culture smooth muscle cells (SMC) derived from the aortas of young (4 mo old) and aged (16 mo old) mice to understand the phenotypic modulation of these cells as aging occurs. SMC from aged mice had decreased proliferative capacity in response to α-thrombin stimulation, yet generated higher levels of ROS and had constitutively increased mitogen-activated protein kinase activity, in comparison with cells from younger mice. These effects may be explained by dysregulation of cell cycle-associated proteins such as cyclin D1 and p27Kip1 in SMC from aged mice. Increased ROS generation was associated with decreased endogenous antioxidant activity, increased lipid peroxidation, and mitochondrial DNA damage. Accrual of oxidant-induced damage and decreased proliferative capacity in SMC may explain, in part, the age-associated transition to plaque instability in humans with atherosclerosis.

A DNA damage response pathway controlled by Tel1 and the Mre11 complex

We define a DNA damage checkpoint pathway in S. cerevisiae governed by the ATM homolog Tel1 and the Mre11 complex. In mitotic cells, the Tel1-Mre11 complex pathway triggers Rad53 activation and its interaction with Rad9, whereas in meiosis it acts via Rad9 and the Rad53 paralog Mre4/Mek1. Activation of the Tel1-Mre11 complex pathway checkpoint functions appears to depend upon the Mre11 complex as a damage sensor and, at least in meiotic cells, to depend on unprocessed DNA double-strand breaks (DSBs). The DSB repair functions of the Mre11 complex are enhanced by the pathway, suggesting that the complex both initiates and is regulated by the Tel1-dependent DSB signal. These findings demonstrate that the diverse functions of the Mre11 complex in the cellular DNA damage response are conserved in mammals and yeast.

c-Myc is a critical target for c/EBPalpha in granulopoiesis

CCAAT/enhancer binding protein alpha (C/EBPalpha) is an integral factor in the granulocytic developmental pathway, as myeloblasts from C/EBPalpha-null mice exhibit an early block in differentiation. Since mice deficient for known C/EBPalpha target genes do not exhibit the same block in granulocyte maturation, we sought to identify additional C/EBPalpha target genes essential for myeloid cell development. To identify such genes, we used both representational difference analysis and oligonucleotide array analysis with RNA derived from a C/EBPalpha-inducible myeloid cell line. From each of these independent screens, we identified c-Myc as a C/EBPalpha negatively regulated gene. We mapped an E2F binding site in the c-Myc promoter as the cis-acting element critical for C/EBPalpha negative regulation. The identification of c-Myc as a C/EBPalpha target gene is intriguing, as it has been previously shown that down-regulation of c-Myc can induce myeloid differentiation. Here we show that stable expression of c-Myc from an exogenous promoter not responsive to C/EBPalpha-mediated down-regulation forces myeloblasts to remain in an undifferentiated state. Therefore, C/EBPalpha negative regulation of c-Myc is critical for allowing early myeloid precursors to enter a differentiation pathway. This is the first report to demonstrate that C/EBPalpha directly affects the level of c-Myc expression and, thus, the decision of myeloid blasts to enter into the granulocytic differentiation pathway.

Requirement for p27(KIP1) in retinoblastoma protein-mediated senescence

In vivo and in vitro evidence indicate that cells do not divide indefinitely but instead stop growing and undergo a process termed cellular proliferative senescence. Very little is known about how senescence occurs, but there are several indications that the retinoblastoma protein (pRb) is involved, the most striking being that reintroduction of RB into RB(-/-) tumor cell lines induces senescence. In investigating the mechanism by which pRb induces senescence, we have found that pRb causes a posttranscriptional accumulation of the cyclin-dependent kinase inhibitor p27(KIP1) that is accompanied by an increase in p27(KIP1) specifically bound to cyclin E and a concomitant decrease in cyclin E-associated kinase activity. In contrast, pRb-related proteins p107 and p130, which also decrease cyclin E-kinase activity, do not cause an accumulation of p27(KIP1) and induce senescence poorly. In addition, the use of pRb proteins mutated in the pocket domain demonstrates that pRb upregulation of p27(KIP1) and senescence induction do not require the interaction of pRb with E2F. Furthermore, ectopic expression of p21(CIP1) or p27(KIP1) induces senescence but not the morphology change associated with pRb-mediated senescence, uncoupling senescence from the morphological transformation. Finally, the ability of pRb to maintain cell cycle arrest and induce senescence is reversibly abrogated by ablation of p27(KIP1) expression. These findings suggest that prolonged cell cycle arrest through the persistent and specific inhibition of cdk2 activity by p27(KIP1) is critical for pRb-induced senescence.