p21Cip1 and p27Kip1 Induce Distinct Cell Cycle Effects and Differentiation Programs in Myeloid Leukemia Cells

Cytoplasmic p27Kip1 counteracts the pro-apoptotic function of the open conformation of PTEN by retention and destabilization of PTEN outside of the nucleus

The tumor suppressor activity of p27Kip1 takes place in the cell nucleus by inhibitory binding to cyclin/CDK complexes. p27Kip1 can also be localized in the cytoplasm, where it has been proposed to have oncogenic properties. Here, we describe a novel role for cytoplasmic p27Kip1 which could account for its activity as an oncoprotein by negative regulation of the PTEN tumor suppressor. p27Kip1 physically interacted with the open conformation of PTEN, which is competent to enter the nucleus. In mammalian cells, cytoplasmic p27Kip1 retained to nuclear-targeted PTEN in the cytoplasm. This retention was exerted by the C-terminal p27Kip1 region, and was independent of cyclin/CDK-binding. The nuclear accumulation of PTEN triggered by pro-apoptotic TNFα treatment was abolished by cytoplasmic p27Kip1. Furthermore, conformationally-open PTEN displayed diminished protein stability and pro-apoptotic activity in the presence of cytoplasmic p27Kip1. Our results support a conformationally-dependent model of cytoplasmic retention and negative regulation of the activity of nuclear PTEN by oncogenic cytoplasmic p27Kip1, and suggest the existence of reciprocal mechanisms to regulate the levels of both p27Kip1 and PTEN.

Forced expression of cyclin-dependent kinase 6 confers resistance of pro-B acute lymphocytic leukemia to Gleevec treatment

The gene encoding c-ABL, a nonreceptor protein tyrosine kinase, is involved in a chromosomal translocation resulting in expression of a BCR-Abl fusion protein that causes most chronic myelogenous and some acute lymphocytic leukemias (CML and ALL) in humans. The Abelson murine leukemia virus (A-MuLV) expresses an alternative form of c-Abl, v-Abl, that transforms murine pro-B cells, resulting in acute leukemia and providing an experimental model for human disease. Gleevec (STI571) inhibits the Abl kinase and has shown great utility against CML and ALL in humans, although its usefulness is limited by acquired resistance. Since STI571 is active against A-MuLV-transformed cells in vitro, we performed a retroviral cDNA library screen for genes that confer resistance to apoptosis induced by STI571. We found that forced expression of Cdk6 promotes continued cell division and decreased apoptosis of leukemic cells. We then determined that the transcription factor E2A negatively regulates Cdk6 transcription in leukemic pro-B cells and that the v-Abl kinase stimulates Cdk6 expression via an extracellular signal-regulated kinase 1-dependent pathway. Finally, we show that the cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor PD0332991 can act synergistically with STI571 to enhance leukemic cell death, suggesting a potential role for CDK6 inhibitors in the treatment of STI571-resistant CML or ALL.

MYC in chronic myeloid leukemia: induction of aberrant DNA synthesis and association with poor response to imatinib

Untreated chronic myeloid leukemia (CML) progresses from chronic phase to blastic crisis (BC). Increased genomic instability, deregulated proliferation, and loss of differentiation appear associated to BC, but the molecular alterations underlying the progression of CML are poorly characterized. MYC oncogene is frequently deregulated in human cancer, often associated with tumor progression. Genomic instability and induction of aberrant DNA replication are described as effects of MYC. In this report, we studied MYC activities in CML cell lines with conditional MYC expression with and without exposure to imatinib, the front-line drug in CML therapy. In cells with conditional MYC expression, MYC did not rescue the proliferation arrest mediated by imatinib but provoked aberrant DNA synthesis and accumulation of cells with 4C content. We studied MYC mRNA expression in 66 CML patients at different phases of the disease, and we found that MYC expression was higher in CML patients at diagnosis than control bone marrows or in patients responding to imatinib. Further, high MYC levels at diagnosis correlated with a poor response to imatinib. MYC expression did not directly correlate with BCR-ABL levels in patients treated with imatinib. Overall our study suggests that, as in other tumor models, MYC-induced aberrant DNA synthesis in CML cells is consistent with MYC overexpression in untreated CML patients and nonresponding patients and supports a role for MYC in CML progression, possibly through promotion of genomic instability.

SKP2 oncogene is a direct MYC target gene and MYC down-regulates p27(KIP1) through SKP2 in human leukemia cells

SKP2 is the ubiquitin ligase subunit that targets p27(KIP1) (p27) for degradation. SKP2 is induced in the G(1)-S transit of the cell cycle, is frequently overexpressed in human cancer, and displays transformation activity in experimental models. Here we show that MYC induces SKP2 expression at the mRNA and protein levels in human myeloid leukemia K562 cells with conditional MYC expression. Importantly, in these systems, induction of MYC did not activate cell proliferation, ruling out SKP2 up-regulation as a consequence of cell cycle entry. MYC-dependent SKP2 expression was also detected in other cell types such as lymphoid, fibroblastic, and epithelial cell lines. MYC induced SKP2 mRNA expression in the absence of protein synthesis and activated the SKP2 promoter in luciferase reporter assays. With chromatin immunoprecipitation assays, MYC was detected bound to a region of human SKP2 gene promoter that includes E-boxes. The K562 cell line derives from human chronic myeloid leukemia. In a cohort of chronic myeloid leukemia bone marrow samples, we found a correlation between MYC and SKP2 mRNA levels. Analysis of cancer expression databases also indicated a correlation between MYC and SKP2 expression in lymphoma. Finally, MYC-induced SKP2 expression resulted in a decrease in p27 protein in K562 cells. Moreover, silencing of SKP2 abrogated the MYC-mediated down-regulation of p27. Our data show that SKP2 is a direct MYC target gene and that MYC-mediated SKP2 induction leads to reduced p27 levels. The results suggest the induction of SKP2 oncogene as a new mechanism for MYC-dependent transformation.

p21(Cip1) confers resistance to imatinib in human chronic myeloid leukemia cells

Imatinib is a Bcr-Abl inhibitor used as first-line therapy of chronic myeloid leukemia (CML). p21(Cip1), initially described as a cell cycle inhibitor, also protects from apoptosis in some models. We describe that imatinib down-regulates p21(Cip1) expression in CML cells. Using K562 cells with inducible p21 expression and transient transfections we found that p21 confers partial resistance to imatinib-induced apoptosis. This protection is not related to the G2-arrest provoked by p21, a decrease in the imatinib activity against Bcr-Abl or a cytoplasmic localization of p21. The results suggest an involvement of p21(Cip1) in the response to imatinib in CML.

SKP2 siRNA inhibits the degradation of P27kip1 and down-regulates the expression of MRP in HL-60/A cells

S-phase kinase-associated protein 2 (SKP2) gene is a tumor suppressor gene, and is involved in the ubiquitin-mediated degradation of P27kip1. SKP2 and P27kip1 affect the proceeding and prognosis of leukemia through regulating the proliferation, apoptosis and differentiation of leukemia cells. In this study, we explored the mechanism of reversing of HL-60/A drug resistance through SKP2 down-regulation. HL-60/A cells were nucleofected by Amaxa Nucleofector System with SKP2 siRNA. The gene and protein expression levels of Skp2, P27kip1, and multi-drug resistance associated protein (MRP) were determined by reverse transcription-polymerase chain reaction and western blot analysis, respectively. The cell cycle was analyzed by flow cytometry. The 50% inhibitory concentration value was calculated using cytotoxic analysis according to the death rate of these two kinds of cells under different concentrations of chemotherapeutics to compare the sensitivity of the cells. HL-60/A cells showed multi-drug resistance phenotype characteristic by cross-resistance to adriamycin, daunorubicin, and arabinosylcytosine, due to the expression of MRP. We found that the expression of SKP2 was higher in HL-60/A cells than in HL-60 cells, but the expression of P27kip1 was lower. The expression of SKP2 in HL-60/A cells nucleofected by SKP2 siRNA was down-regulated whereas the protein level of P27kip1 was up-regulated. Compared with the MRP expression level in the control group (nucleofected by control siRNA), the mRNA and protein expression levels of MRP in HL-60/A cells nucleofected by SKP2 siRNA were lower, and the latter cells were more sensitive to adriamycin, daunorubicin, and arabinosylcytosine. Down-regulating the SKP2 expression and arresting cells in the G0/G1 phase improve drug sensitivity of leukemia cells with down-regulated MRP expression.

Myc inhibits p27-induced erythroid differentiation of leukemia cells by repressing erythroid master genes without reversing p27-mediated cell cycle arrest

Inhibition of differentiation has been proposed as an important mechanism for Myc-induced tumorigenesis, but the mechanisms involved are unclear. We have established a genetically defined differentiation model in human leukemia K562 cells by conditional expression of the cyclin-dependent kinase (Cdk) inhibitor p27 (inducible by Zn(2+)) and Myc (activatable by 4-hydroxy-tamoxifen). Induction of p27 resulted in erythroid differentiation, accompanied by Cdk inhibition and G(1) arrest. Interestingly, activation of Myc inhibited p27-mediated erythroid differentiation without affecting p27-mediated proliferation arrest. Microarray-based gene expression indicated that, in the presence of p27, Myc blocked the upregulation of several erythroid-cell-specific genes, including NFE2, JUNB, and GATA1 (transcription factors with a pivotal role in erythropoiesis). Moreover, Myc also blocked the upregulation of Mad1, a transcriptional antagonist of Myc that is able to induce erythroid differentiation. Cotransfection experiments demonstrated that Myc-mediated inhibition of differentiation is partly dependent on the repression of Mad1 and GATA1. In conclusion, this model demonstrates that Myc-mediated inhibition of differentiation depends on the regulation of a specific gene program, whereas it is independent of p27-mediated cell cycle arrest. Our results support the hypothesis that differentiation inhibition is an important Myc tumorigenic mechanism that is independent of cell proliferation.

Sodium butyrate-induced upregulation of p18( INK4C ) gene affects K562 cell G (0)/G (1) arrest and differentiation

Histone deacetylase inhibitor sodium butyrate (NaBu) can induce G(0)/G(1) arrest and erythroid differentiation in K562 cells, but the molecular mechanisms underlying this process are unclear. Here we show that both p18( INK4C ) mRNA and protein levels were upregulated during K562 cell erythroid differentiation induced by NaBu. Moreover, the NaBu activation of p18( INK4C ) was dependent on the integrity of Sp1 clusters in the promoter. NaBu caused hyperacetylation of histones H3 and H4 on endogenous p18( INK4C ) promoter and enhanced binding of transcription factor Sp1 in vivo. Also, overexpression of p18( INK4C ) in K562 cells resulted in G(0)/G(1) arrest and partial erythroid differentiation. Our results suggested that NaBu-mediated p18( INK4C ) regulation played a role in cell cycle arrest and erythroid differentiation in K562 cells.

Induction of differentiation of myeloid leukemia cells in primary culture in response to lithocholic acid acetate, a bile acid derivative, and cooperative effects with another differentiation inducer, cotylenin A

Lithocholic acid (LCA) acetate induced the differentiation of human leukemia cells. Treatment with a combination of LCA acetate and cotylenin A, an inducer of the differentiation of leukemia cells, was more effective than that with LCA acetate or cotylenin A alone at inducing monocytic differentiation. LCA acetate activated mitogen-activated protein kinase (MAPK) before inducing differentiation. Cotylenin A did not activate MAPK, suggesting that cotylenin A has a different mode of action. The cooperative effects of LCA acetate and cotylenin A on inducing differentiation were, at least partly, due to the enhancement of LCA acetate-induced MAPK activation by cotylenin A.

alpha-CaMKII controls the growth of human osteosarcoma by regulating cell cycle progression

Osteosarcoma is the most frequent type of primary bone cancer in children and adolescents. These malignant osteoid forming tumors are characterized by their uncontrolled hyperproliferation. Here, we investigate the role of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in the growth of human osteosarcoma. We show that alpha-CaMKII is expressed in human osteosarcoma cell lines and in primary osteosarcoma tissue derived from patients. The pharmacologic inhibition of CaMKII in MG-63 and 143B human osteosarcoma cells by KN-93 resulted in an 80 and 70% decrease in proliferation, respectively, and induced cell cycle arrest in the G(0)/G(1) phase. The in vivo administration of KN-93 to mice xenografted with human osteosarcoma cells significantly decreased intratibial and subcutaneous tumor growth. Mechanistically, KN-93 and alpha-CaMKII siRNA increased p21((CIP/KIP)) gene expression, protein levels, and decreased the phosphorylation of retinoblastoma protein and E2F transactivation. Furthermore, the inhibition of CaMKII decreased membrane-bound Tiam1 and GTP-bound Rac1, which are known to be involved in p21 expression and tumor growth in a variety of solid malignant neoplasms. Our results suggest that CaMKII plays a critical role in the growth of osteosarcoma, and its inhibition could be an attractive therapeutic target to combat conventional high-grade osteosarcoma in children.

The role of peroxiredoxin III on late stage of proerythrocyte differentiation

Peroxiredoxin III (Prdx III), the mitochondrial peroxidase, was preferentially expressed in murine erythroleukemia (MEL) cells. However, the mechanisms by which Prdx III regulates erythroid differentiation are unknown. In this study, K562 cells were differentiated by Ara-C treatment, and Prdx III was dramatically increased until day 5. We also investigated Prdx III expression pattern on in vitro erythropoiesis of human CD34(+) cells. When human CD34(+) cells became proerythrocyte on day 7, Prdx III was diminished, and then augmented on day 12. We established the stable sublines of Prdx III overexpression (O/E), and dominant-negative (D/N). The intracellular ROS level of Prdx III O/E cell line was lower than D/N stable cell lines. Moreover, Prdx III O/E cell line was placed in G1-arrest, but not D/N cell lines. Finally, the expression level of beta-globin and GATA-1 was dramatically increased in Prdx III O/E cell line.

Uterine Temporal Response to Acute Exposure to 17α-Ethinyl Estradiol in the Immature Rat

The rat uterus responds to acute estrogen treatment with a series of well-characterized physiological responses; however, the gene expression changes required to elicit these responses have not been fully characterized. In order to understand early events induced by estrogen exposure in vivo, we evaluated the temporal gene expression in the uterus of the immature rat after a single dose of 17 alpha-ethinyl estradiol (EE) by microarray analysis, evaluating the expression of 15,923 genes. Immature 20-day-old rats were exposed to a single dose of EE (10 microg/kg), and the effects on uterine histology, weight, and gene expression were determined after 1, 2, 8, 24, 48, 72, and 96 h. EE induced changes in the expression of 3867 genes, at least at one time point (p < or = 0.0001), and at least 1.5-fold (up- or downregulated). Specifically, the expression of 8, 116, 3030, 2076, 381, 445, and 125 genes was modified at 1, 2, 8, 24, 48, 72, or 96 h after exposure to EE, respectively (p < or = 0.0001, t-test). At the tissue and organ level, a clear uterotrophic response was elicited by EE after only 8 h, reaching a maximum after 24 h and remaining detectable even after 96 h of exposure. The uterine phenotypic changes were induced by sequential changes in the transcriptional status of a large number of genes, in a program that involves multiple molecular pathways. Using the Gene Ontology to better understand the temporal response to estrogen exposure, we determined that the earliest changes were in the expression of genes whose products are involved in transcriptional regulation and signal transduction, followed by genes implicated in protein synthesis, energy utilization, solute transport, cell proliferation and differentiation, tissue remodeling, and immunological responses among other pathways. The compendium of genes here presented represents a comprehensive compilation of estrogen-responsive genes involved in the uterotrophic response.

The functional haplotype of peptidylarginine deiminase IV (S55G, A82V and A112G) associated with susceptibility to rheumatoid arthritis dominates apoptosis of acute T leukemia Jurkat cells

Peptidylarginine deiminase IV (PADI4) posttranslationally converts peptidylarginine to citrulline. It plays an essential role in immune cell differentiation and apoptosis. A haplotype of single-nucleotide polymorphisms (SNPs) in PADI4 is functionally relevant as a rheumatoid arthritis (RA) gene. It could increase enzyme activity leading to raised levels of citrullinated protein and stimulating autoantibody. Previously, our study showed that inducible PADI4 causes haematopoietic cell death. Herein, we further investigate whether RA risk PADI4 haplotype (SNP PADI4; S55G, A82V and A112G) and the increase of its enzymatic activity induce apoptosis. In the tetracycline (Tet)-On Jurkat T cells, ionomycin (Ion) only treatment didn't induce apoptosis however it promoted inducible PADI4-decreased cell viability and -enhanced apoptosis. Through in vitro and in vivo PADI enzyme activity assay, we demonstrated that PADI4 enzyme activity of SNP PADI4 was higher than RA non-risk PADI4 haplotype (WT PADI4). The effect of SNP PADI4-induced apoptosis was superior to WT PADI4. In addition, both Ion and SNP PADI4 synergistically provoked apoptosis were compared with both Ion and WT PADI4. Concurrently, in the conditionally inducible SNP PADI4 cells of Ion treatment-induced apoptosis, not only the expression of Bcl-xL was down-regulated and Bax up-regulated, but also cytochrome c was released from mitochondria to cytoplasm in significant amounts. Western blotting data showed the increase in apoptosomal caspase activation during programmed cell death in the inducible SNP PADI4 cells subsequent to Ion treatment. These data demonstrated that both SNP PADI4 increasing their enzyme activity could enhance apoptosis through the mitochondrial pathway and further provide a conceivable explanation in the pathogenesis of RA following the upregulation of PADI4 activity in its SNPs.

Antizyme, a natural ornithine decarboxylase inhibitor, induces apoptosis of haematopoietic cells through mitochondrial membrane depolarization and caspases' cascade

Antizymes delicately regulate ornithine decarboxylase (ODC) enzyme activity and polyamine transportation. One member of the family, antizyme-1, plays vital roles in molecular and cellular functions, including developmental regulation, cell cycle, proliferation, cell death, differentiation and tumorigenesis. However, the question of how does it participate in the cell apoptotic mechanism is still unsolved. To elucidate the contribution of human antizyme-1 in haematopoietic cell death, we examine whether inducible overexpression of antizyme enhances apoptotic cell death. Antizyme reduced the viability in a dose- and time-dependent manner of human leukemia HL-60 cells, acute T leukemia Jurkat cells and mouse macrophage RAW 264.7 cells. The apoptosis-inducing activities were determined by nuclear condensation, DNA fragmentation, sub-G(1) appearance, loss of mitochondrial membrane potential (Deltapsi( m )), release of mitochondrial cytochrome c into cytoplasm and proteolytic activation of caspase 9 and 3. Following conditional antizyme overexpression, all protein levels of cyclin-dependent kinases (Cdks) and cyclins are not significantly reduced, except cyclin D, before their entrance into apoptotic cell death. However, introduced cyclin D1 into Jurkat T tetracycline (Tet)-On cell system still couldn't rescue cells from apoptosis. Antizyme doesn't influence the expression of tumor suppressor p53 and its downstream p21, but it interferes in the expressions of Bcl-2 family. Inducible antizyme largely enters mitochondria resulting in cytochrome c release from mitochondria to cytosol following Bcl-xL decrease and Bax increase. According to these data, we suggest that antizyme induces apoptosis mainly through mitochondria-mediated and cell cycle-independent pathway. Furthermore, antizyme induces apoptosis not only by Bax accumulation reducing the function of the Bcl-2 family, destroying the Deltapsi( m ), and releasing cytochrome c to cytoplasm but also by the activation of apoptosomal caspase cascade.

Changes in motility, gene expression and actin dynamics: Cdk6-induced cytoskeletal changes associated with differentiation in mouse astrocytes

Cyclin dependent kinase (cdk) 4 and cdk6 have historically been understood to be D-cyclin kinases that phosphorylate pRb in the nucleus to regulate G1 phase of the cell cycle. In conflict with this understood redundancy are several studies that have demonstrated a novel role for cdk6 in differentiation. Cdk6 expression must be reduced to allow proper osteoblast and osteoclast differentiation, enforced cdk6 expression blocked differentiation of mouse embryo fibroblasts, and cdk6 expression in primary astrocytes favored the expression of progenitor cell markers (Ericson et al. [2003] Mol Cancer Res 1:654-664; Matushansky et al. [2003] Oncogene 22:4143-4149; Ogasawara et al. [2004a] J Bone Miner Res 19:1128-1136; Ogasawara et al. [2004b] Mol Cell Biol 24:6560-6568). Experiments shown here investigate novel cytoplasmic and nuclear functions of cdk6. These data demonstrate that cdk6 expression in mouse astrocytes results in changes in patterns of gene expression, changes in the actin cytoskeleton including loss of stress fibers, and enhanced motility. These changes in cdk6-infected cells are associated with the process of cellular differentiation.

Upregulation of p27 and its inhibition of CDK2/cyclin E activity following DNA damage by a novel platinum agent are dependent on the expression of p21

The cisplatin analogue 1R,2R-diaminocyclohexane(trans-diacetato)(dichloro)platinum^IV (DAP) is a DNA-damaging agent that will be entering clinical trials for its potent cytotoxic effects against cisplatin-resistant tumour cells. This cytotoxicity may reside in its ability to selectively activate G1-phase checkpoint response by inhibiting CDKs via the p53/p21 pathway. We have now evaluated the role of another CDK inhibitor p27 as a contributor to DAP-mediated inhibition of G1-phase CDK2 activity. Our studies in ovarian A2780 tumour cells demonstrate that p27 levels induced by DAP are comparable to or greater than those seen for p21. The induction of p27 is not through a transcriptional mechanism, but rather is due to a four-fold increase in protein stabilisation through a mechanism dependent on p21. Moreover, DAP-induced p21 promoted the selective increase of p27 in the CDK2 complex, but not in CDK4 complex, and this selective increase contributed to inhibition of the CDK2 kinase activity. The inhibited complex contained either p27 or p21, but not both, with the relative levels of cyclin E associated with p27 and p21 indicating that about 25% of the inhibition of CDK2 activity was due to p27 and 75% due to p21. This study provides the first evidence that p27 upregulation is directly attributable to activation of the p53/p21 pathway by a DNA-damaging agent, and promulgates p53/p21/p27 axis as a significant component of checkpoint response.

Cytokines in the differentiation therapy of leukemia: from laboratory investigations to clinical applications

Differentiation therapy of leukemia is the treatment of leukemia cells with biological or chemical agents that induce the terminal differentiation of the cancer cells. It is regarded as a novel and targeted approach to leukemia treatment, based on our better understanding of the hematopoietic process and the mechanisms of its deregulation during leukemogenesis. Clinically, differentiation therapy has been most successful in acute promyelocytic leukemia using all-trans-retinoic acid as the inducer, either alone or in combination with chemotherapy. This review presents evidence that a number of hematopoietic cytokines play important roles in both normal and aberrant hematopoietic processes. In vitro laboratory investigations in the past two decades using well-characterized myeloid leukemic cell lines and primary blast cells from leukemia patients have revealed that many hematopoietic cytokines can trigger lineage-specific differentiation of leukemia cells, which may have important implications in the clinical setting. Moreover, our current understanding of cytokine interactions and the molecular mechanisms of cytokine-induced leukemic cell differentiation will be discussed in the light of recent findings. Finally, ways in which laboratory research on cytokines in the differentiation therapy of leukemia can lead to the improved design of protocols for future clinical applications to leukemia therapy will also be addressed.

Overexpression of peptidylarginine deiminase IV features in apoptosis of haematopoietic cells

Peptidylarginine deiminases (PADIs) convert peptidylarginine into citrulline via posttranslational modification. One member of the family, PADI4, plays an important role in immune cell differentiation and cell death. To elucidate the participation of PADI4 in haematopoietic cell death, we examine whether inducible overexpression of PADI4 enhances the apoptotic cell death. PADI4 reduced the viability in a dose- and time-dependent manner of human leukemia HL-60 cells and human acute T leukemia Jurkat cells. The apoptosis-inducing activities were determined by nuclear condensation, DNA fragmentation, sub-G1 appearance, loss of mitochondrial membrane potential (delta psi(m)), release of mitochondrial cytochrome c into cytoplasm and proteolytic activation of caspase 9 and 3. Following PADI4 overexpression, cells arrest in G1 phase significantly before their entrance into apoptotic cell death. PADI4 increases tumor suppressor p53 and its downstream p21 to control cell cycle. In the detections of protein expression and kinase activity, all protein levels of cyclin-dependent kinases (CDKs) and cyclins are not reduced except cyclin D, however, CDK2 (G1 entry S phase) and CDK1 (G2 entry M phase) enzyme activities are inhibited by conditionally inducible PADI4. p53 also expands its other downstream Bax to induce cytochrome c release from mitochondria. According to these data, we suggest that PADI4 induces apoptosis mainly through cell cycle arrest and mitochondria-mediated pathway. Furthermore, p53 features in PADI4-induced apoptosis by increasing intracellular p21 to control cell cycle and by Bax accumulation to decline Bcl-2 function, destroy delta psi(m), release cytochrome c to cytoplasm and activate the caspase cascade.

Antileukemic roles of human phospholipid scramblase 1 gene, evidence from inducible PLSCR1-expressing leukemic cells

Phospholipid scramblase 1 (PLSCR1) is a multiply palmitoylated protein which is localized in either the cell membrane or nucleus depending on its palmitoylated state. The increasing evidence showed the biological roles of PLSCR1 in cell signaling, maturation and apoptosis. To investigate the functions of PLSCR1 in leukemic cells, we generated an inducible PLSCR1-expressing cell line using myeloid leukemic U937 cells. In this cell line, PLSCR1 was tightly regulated and induced upon tetracycline withdrawal. Our results showed that inducible PLSCR1 expression arrested the proliferation of U937 cells at G1 phase. Meanwhile, PLSCR1-overexpressing U937 cells also underwent granulocyte-like differentiation with increased sensitivity to etoposide-induced apoptosis. Furthermore, we also found that PLSCR1 induction increased cyclin-dependent kinase inhibitors p27(Kip1) and p21(Cip1) proteins, together with downregulation of S phase kinase-associated protein 2 (SKP2), an F-box subunit of the ubiquitin-ligase complex that targets proteins for degradation. Additionally, PLSCR1 induction significantly decreased c-Myc protein and antiapoptotic Bcl-2 protein. Although the exact mechanism by which PLSCR1 regulates these cellular events and gene expression remains unresolved, our results suggest that PLSCR1 plays the antagonistic role regarding leukemia development. These data will shed new insights into understanding the biochemical and biological functions of PLSCR1 protein.

Roles for p21waf1/cip1 and p27kip1 during the adaptation response to massive intestinal resection

The magnitude of gut adaptation is a decisive factor in determining whether patients are able to live independent of parenteral nutrition after massive small bowel loss. We previously established that the cyclin-dependent kinase inhibitor (CDKI) p21(waf1/cip1) is necessary for enterocyte proliferation and a normal adaptation response. In the present study, we have further elucidated the role of this CDKI in the context of p27(kip1), another member of the Cip/Kip CDKI family. Small bowel resections (SBRs) or sham operations were performed in control (C57/BL6), p21(waf1/cip1)-null, p27(kip1)-null, and p21(waf1/cip1)/p27(kip1) double-null mice. Morphological (villus height/crypt depth) alterations in the mucosa, the kinetics of enterocyte turnover (rates of enterocyte proliferation and apoptosis), and the protein expression of various cell cycle-regulatory proteins were recorded at various postoperative times. Enterocyte compartment-specific mRNA expression was investigated using laser capture microdissection. Resection-induced adaptation in control mice coincided with increased protein expression of p21(waf1/cip1) and decreased p27(kip1) within 3 days postoperatively. Identical changes in mRNA expression were detected in crypt but not in villus enterocytes. Adaptation occurred normally in control and p27(kip1)-null mice; however, mice deficient in both p21(waf1/cip1) and p27(kip1) failed to increase baseline rates of enterocyte proliferation and adaptation. The expression of p21(waf1/cip1) protein and mRNA in the proliferative crypt compartment is necessary for resection-induced enterocyte proliferation and adaptation. The finding that deficient expression of p27(kip1) does not affect adaptation suggests that these similar CDKI family members display distinctive cellular functions during the complex process of intestinal adaptation.

Targeting of CTCF to the nucleolus inhibits nucleolar transcription through a poly(ADP-ribosyl)ation-dependent mechanism

Multiple functions have been reported for the transcription factor and candidate tumour suppressor, CTCF. Among others, they include regulation of cell growth, differentiation and apoptosis, enhancer-blocking activity and control of imprinted genes. CTCF is usually localized in the nucleus and its subcellular distribution during the cell cycle is dynamic; CTCF was found associated with mitotic chromosomes and the midbody, suggesting different roles for CTCF at different stages of the cell cycle. Here we report the nucleolar localization of CTCF in several experimental model systems. Translocation of CTCF from nucleoplasm to the nucleolus was observed after differentiation of K562 myeloid cells and induction of apoptosis in MCF7 breast cancer cells. CTCF was also found in the nucleoli in terminally differentiated rat trigeminal ganglion neurons. Thus our data show that nucleolar localization of CTCF is associated with growth arrest. Interestingly, the 180 kDa poly(ADP-ribosyl)ated isoform of CTCF was predominantly found in the nucleoli fractions. By transfecting different CTCF deletion constructs into cell lines of different origin we demonstrate that the central zinc-finger domain of CTCF is the region responsible for nucleolar targeting. Analysis of subnucleolar localization of CTCF revealed that it is distributed homogeneously in both dense fibrillar and granular components of the nucleolus, but is not associated with fibrillar centres. RNA polymerase I transcription and protein synthesis were required to sustain nucleolar localization of CTCF. Notably, the labelling of active transcription sites by in situ run-on assays demonstrated that CTCF inhibits nucleolar transcription through a poly(ADP-ribosyl)ation-dependent mechanism.

Beyond the cell cycle: a new role for Cdk6 in differentiation

Over 10 years ago, cdk6 was identified as a new member in a family of vertebrate cdc-2 related kinases. This novel kinase was found to partner with the D-type cyclins and to possess pRb kinase activity in vitro and has since been understood to function solely as a pRb kinase in the regulation of the G(1) phase of the cell cycle. In the past 2 years, several independent studies in multiple cell types have indicated a novel role for cdk6 in differentiation. For example, cdk6 expression must be reduced to allow proper osteoblast and osteoclast differentiation, forced cdk6 expression blocked differentiation of mouse erythroid leukemia cells and cdk6 expression in primary astrocytes favors the expression of progenitor cell markers. Since exit from the cell cycle is a necessary step in terminal differentiation, down-regulation of a mitogenic factor may be expected in this process, however it is surprising that this association has not been previously uncovered and that it is apparently not shared with cdk4, long understood to be a functional homolog of cdk6. The mechanism of cdk6 function in differentiation is not understood, but it may extend beyond the established role of cdk6 as a pRb kinase. As this story unfolds it will be important to discover if the function of cdk6 in differentiation is pRb-dependent or pRb-independent, since pRb has long been established as a key factor in initiating and maintaining cell cycle exit during differentiation.

Gene therapy for BCR-ABL+ human CML with dual phosphorylation resistant p27Kip1 and stable RNA interference using an EBV vector

BACKGROUND

BCR-ABL-mediated chronic myelogenous leukemia (CML) CD34(+) cell proliferation mostly depends on the nucleo-cytoplasmic ratio of the cyclin-dependent kinase inhibitor p27. The ubiquitin-ligase SCF(Skp2) promotes degradation of phosphorylated p27 at T187 in the nucleus, resulting in G1/S progression of the cells. On the other hand, phosphatidylinositol-3-kinase (PI3K)-directed T157 nuclear localization signal (NLS) phosphorylation results in cytoplasmic sequestration of p27, leading to abnormal integrin-mediated proliferation of CD34(+) CML cells.

METHODS

We demonstrate the generation of an engineered Epstein-Barr virus (EBV) vector with a BAC backbone that has the unique capacity to carry doubly modified (DM) p27 (i.e. T187A, T157A p27) along with the BCR-ABL siRNA expression construct. The HSV-tk suicide gene has also been incorporated in the same vector, which promotes apoptosis in a BCR-ABL-independent pathway.

RESULTS

Expression of DM p27 markedly inhibits proliferation of BCR-ABL(+) primary human CML cells. Moreover, DM p27 strongly inhibits the growth of imatinib-resistant CML cells, compared to the T157A p27 (SM p27). The CML growth inhibition is found to be the result of significant G1/S arrest with concomitant increase in hypophosphorylated retinoblastoma (Rb). Moreover, the EBV vector mediated stable RNA interference induces apoptosis in K562 cells and reduces myeloid colony forming units.

CONCLUSIONS

We therefore propose a multi-gene delivery strategy for BCR-ABL(+) CML cells by targeting not only the fusion transcript, but also the downstream signaling, to overcome drug resistance in the acute phase of CML.

Effects of overexpression of HBP1 upon growth and differentiation of leukemic myeloid cells

HMG-box containing protein 1 (HBP1) is a member of the high mobility group (HMG) of chromosomal proteins. Since HBP1 exhibits tumor-suppressor activity in nonmyeloid tissues, we examined the effects of ectopic overexpression of HBP1 upon the growth and differentiation of myeloid cells. We prepared transient and stable transfectants of the myeloblast cell line K562, which overexpress HBP1 mRNA and protein. HBP1 transfectants displayed slower growth in cell culture and reduced colony formation in soft agar, retardation of S-phase progression, reduced expression of cyclin D1 and D3 mRNAs and increased expression of p21 mRNA. HBP1 transfectants also underwent increased apoptosis, as demonstrated by morphology and binding of Annexin V. Fas ligand mRNA levels were increased in HBP1 transfectants, suggesting involvement of the Fas/Fas ligand pathway. HBP1 overexpression enhanced differentiation of K562 cells towards erythroid and megakaryocyte lineages, as evidenced by increased hemoglobin and CD41a expression. Overexpression of HBP1 modulated mRNA levels for myeloid-specific transcription factors C/EBPalpha, c-Myb, c-Myc, and JunB, as well as lineage-specific transcription factors PU.1, GATA-1, and RUNX1. These findings suggest that in myeloid cells HBP1 may serve as a tumor suppressor and a general differentiation inducer and may synergize with chemical differentiating agents to enhance lineage-specific differentiation.

CTCF regulates growth and erythroid differentiation of human myeloid leukemia cells

CTCF is a transcription factor and a candidate tumor suppressor that contains a DNA-binding domain composed of 11 zinc fingers. We reported previously that CTCF is differentially regulated during differentiation of human myeloid leukemia cells. In this study we aimed to investigate the role of CTCF in myeloid cell differentiation. A human cell line, K562, that can be chemically induced to differentiate into various hematopoietic lineages was chosen as a model system for this study. Several K562 cell lines with constitutive and conditional expression of CTCF have been generated. By using these model systems we demonstrated that: (i) ectopic expression of CTCF in K562 cells led to growth retardation and promotion of differentiation into the erythroid lineage; (ii) CTCF knock-down significantly inhibited differentiation of K562 cells into erythroid lineage; (iii) differentiation of K562 into the megakaryocytic lineage was not significantly affected; and (iv) down-regulation of MYC has been identified as one of the mechanisms by which CTCF promotes erythroid differentiation. Taken together our results demonstrate that CTCF is involved in the control of myeloid cell growth and differentiation.