Control of AC133/CD133 and impact on human hematopoietic progenitor cells through nucleolin

AC133 is a prominent surface marker of CD34+ and CD34- hematopoietic stem/progenitor cell (HSPC) subsets. AC133+ HSPCs contain high progenitor cell activity and are capable of hematopoietic reconstitution. Furthermore, AC133 is used for prospective isolation of tumor-initiating cells in several hematological malignancies. Nucleolin is a multifunctional factor of growing and cancer cells, which is aberrantly active in certain hematological neoplasms, and serves as a candidate molecular target for cancer therapy. Nucleolin is involved in gene transcription and RNA metabolism and is prevalently expressed in HSPCs, as opposed to differentiated hematopoietic tissue. The present study dissects nucleolin-mediated activation of surface AC133 and its cognate gene CD133, via specific interaction of nucleolin with the tissue-dependent CD133 promoter P1, as a mechanism that crucially contributes to AC133 expression in CD34+ HSPCs. In mobilized peripheral blood (MPB)-derived HSPCs, nucleolin elevates colony-forming unit (CFU) frequencies and enriches granulocyte-macrophage CFUs. Furthermore, nucleolin amplifies long-term culture-initiating cells and also promotes long-term, cytokine-dependent maintenance of hematopoietic progenitor cells. Active β-catenin, active Akt and Bcl-2 levels in MPB-derived HSPCs are nucleolin-dependent, and effects of nucleolin on these cells partially rely on β-catenin activity. The study provides new insights into molecular network relevant to stem/progenitor cells in normal and malignant hematopoiesis.

Rb and nucleolin antagonize in controlling human CD34 gene expression

Retinoblastoma protein (Rb) controls cell proliferation, differentiation, survival and gene expression and it has a central role in the signaling network that provides a cell cycle checkpoint in the G1 phase of the cell cycle. Studies in mice have shown that Rb regulates interactions between hematopoietic stem cells and their bone marrow microenvironment and it acts as a critical regulator of hematopoietic stem and progenitor cells under stress. In human hematopoiesis, the CD34 protein is expressed on a subset of progenitor cells capable of self-renewal, multilineage differentiation, and hematopoietic reconstitution, and CD34 has a role in the differentiation of hematopoietic cells. Here we find that, in CD34-positive hematopoietic cells, Rb controls the human CD34 promoter region by antagonizing the CD34 promoter factor nucleolin to provide a mechanism that links expression of endogenous CD34 to cell cycle progression. Our study suggests a direct involvement of Rb in the transcriptional program of human CD34-positive hematopoietic stem/progenitor cells, thus providing further insights into the molecular network relevant to the features of these cells.

Daxx-beta and Daxx-gamma, two novel splice variants of the transcriptional co-repressor Daxx

Daxx is involved in transcriptional control and apoptosis. It comprises several domains, including a regulatory C terminus that is responsible for the interaction with numerous proteins such as p53, promyelocytic leukemia protein (PML), and Hsp27. Here, we describe the identification and characterization of two novel variants of Daxx termed Daxx-β and Daxx-γ, which are generated by alternative splicing. Alternative splicing results in a truncated regulatory C terminus in both proteins. As a consequence, Daxx-β and Daxx-γ show a markedly decreased affinity to PML, which in turn is associated with a different subnuclear localization of these proteins compared with Daxx. Although Daxx is localized mainly in PML-oncogenic domains (PODs) Daxx-β and Daxx-γ display a distinct distribution pattern. Furthermore, Daxx-β and Daxx-γ show a decreased affinity to p53 also due to the truncated C terminus. We provide evidence that the p53 recruitment into PODs is Daxx isoform-dependent. The decreased affinity of Daxx-β/-γ to p53 and PML results in a diffuse localization of p53 throughout the nucleus. In contrast to Daxx, Daxx-β and Daxx-γ are unable to repress p53-mediated transcription. Therefore, alternative splicing of Daxx might indicate an additional level in the cellular apoptosis network.

Regulation of p53 isoform expression in renal cell carcinoma

Differential expression of p53 isoforms might participate in the marked resistance towards conventional chemotherapy of renal cell carcinomas (RCCs). Therefore, we analysed their differential expression and regulation in RCCs. RCCs expressed a more p53 activating isoform pattern during tumor initiation and progression, in vivo. In vitro, two cell lines exhibiting a similar sensitivity towards Topotecan-induced cell death revealed a similar induction of p53 target genes but strongly differed in their extent of apoptosis. Furthermore, they strongly differed in their basal expression patterns and differential regulation of the isoforms. In conclusion, our study examined for the first time the differential expression and regulation of all p53 isoforms in a tumor in vivo. Furthermore, novel results in our in vitro studies show that p53 isoforms are strongly differentially regulated by chemotherapy in RCCs and that expression and regulation of so-called "p53-target genes" are obviously at least in part regulated by other transcription factors. In addition, our original findings show that p53 isoform expression in RCC cell lines is of minor importance for sensitivity towards chemotherapy.

Fas-induced apoptosis of renal cell carcinoma is mediated by apoptosis signal-regulating kinase 1 via mitochondrial damage-dependent caspase-8 activation

Renal cell carcinoma (RCC) is a prototype of a chemo refractory tumour. It remains the most lethal of the common urologic cancers and is highly resistant to conventional therapy. Here, we confirmed the efficiency of anti-Fas monoclonal antibody (CH11) as alternative therapeutic approach for the treatment of RCC and investigated the molecular mechanism(s), whereby CH11 induces apoptosis of RCC cells. The present study shows an essential role for apoptosis signal-regulating kinase 1 (ASK1), together with both c-jun-N-terminal kinase (JNK) and p38 pathways, and caspase-8 in this process. Furthermore, CH11-dependent induction of the ASK1-JNK/p38 pathways was found to activate the transcription factors AP-1 and ATF-2, and FADD-caspase-8-Bid signalling, resulting in the translocation of both Bax and Bak proteins, and subsequently mitochondrial dysregulation that is characterized by the loss of mitochondrial membrane potential (DeltaPsim), cytochrome c release and cleavage of caspase-9, caspase-3 and PARP. Thus, the described molecular mechanisms of CH11-induced apoptosis suggest the reliability of Fas activation as an alternative therapeutic approach for the treatment of patients with advanced renal cell carcinoma.

Stem cell divisions controlled by the proto-oncogene BMI-1

Divisions of somatic stem cells are required for the maintenance and regeneration of normal tissues, while divisions of cancerous stem cells likely underlie the existence of certain malignant diseases. Studies of recent years suggest that molecular mechanisms governing stem cell self-renewal can be subverted in tumorigenesis to maintain cancerous growth. This is exemplified by the proto-oncogene BMI-1 that is involved in the maintenance of somatic stem cells and in carcinogenesis within the same tissues. BMI-1 interferes with the central cellular tumor suppressor pathways linked to retinoblastoma protein (Rb) and p53. These signaling pathways control the cell cycle, cell differentiation, cellular senescence and cell death. While the roles of the pathways associated with Rb and p53 in cancer are broadly established, further elucidation thereof in stem cells might have implications in cancer research, stem cell biology and regenerative medicine.

Cellular signaling in normal and cancerous stem cells

Self-renewing divisions of normal and cancerous stem cells are responsible for the initiation and maintenance of normal and certain cancerous tissues, respectively. Recent findings suggest that tumor surveillance mechanisms can reduce regenerative capacity and frequency of normal stem cells, thereby contributing to tissue aging. Signaling pathways promoting self-renewal of stem cells can also drive proliferation in cancer. The BMI-1 proto-oncogene is required for the maintenance of tissue-specific stem cells and is involved in carcinogenesis within the same tissues. BMI-1 promotes self-renewal of stem cells largely by interfering with two central cellular tumor suppressor pathways, p16(Ink4a)/retinoblastoma protein (Rb) and ARF/p53, whose disruption is a hallmark of cancer. Nucleolin, an Rb-associated protein, is abundant in proliferating cancerous cells and likely contributes to the maintenance of human CD34-positive stem/progenitor cells of hematopoiesis. Elucidation of the involvement of proto-oncogenes and tumor suppressors in the maintenance of stem cells might have therapeutic implications.

Nucleolin regulates gene expression in CD34-positive hematopoietic cells

CD34 glycoprotein in human hematopoiesis is expressed on a subset of progenitor cells capable of self-renewal, multilineage differentiation, and hematopoietic reconstitution. Nucleolin is an abundant multifunctional phosphoprotein of growing eukaryotic cells, involved in regulation of gene transcription, chromatin remodeling, and RNA metabolism, whose transcripts are enriched in murine hematopoietic stem cells, as opposed to differentiated tissue. Here we show that, in human CD34-positive hematopoietic cells, nucleolin activates endogenous CD34 and Bcl-2 gene expression, and cell surface CD34 protein expression is thereby enhanced by nucleolin. Nucleolin-mediated activation of CD34 gene transcription results from direct sequence-specific interactions with the CD34 promoter region. Nucleolin expression prevails in CD34-positive cells mobilized into peripheral blood (PB), as opposed to CD34-negative peripheral blood mononuclear cells (PBMCs). Therefore, in intact CD34-positive mobilized PB cells, a recruitment of nucleolin to the CD34 promoter region takes place, accompanied by nucleosomal determinants of gene activity, which are absent from the CD34 promoter region in CD34-negative PBMCs. Our data show that nucleolin acts as a component of the gene regulation program of CD34-positive hematopoietic cells and provide further insights into processes by which human CD34-positive hematopoietic stem/progenitor cells are maintained.

Cell cycle-controlled interaction of nucleolin with the retinoblastoma protein and cancerous cell transformation

Retinoblastoma protein (Rb) is a multifunctional tumor suppressor, frequently inactivated in certain types of human cancer. Nucleolin is an abundant multifunctional phosphoprotein of proliferating and cancerous cells, recently identified as cell cycle-regulated transcription activator, controlling expression of human papillomavirus type 18 (HPV18) oncogenes in cervical cancer. Here we find that nucleolin is associated with Rb in intact cells in the G1 phase of the cell cycle, and the complex formation is mediated by the growth-inhibitory domain of Rb. Association with Rb inhibits the DNA binding function of nucleolin and in consequence the interaction of nucleolin with the HPV18 enhancer, resulting in Rb-mediated repression of the HPV18 oncogenes. The intracellular distribution of nucleolin in epithelial cells is Rb-dependent, and an altered nucleolin localization in human cancerous tissues results from a loss of Rb. Our findings suggest that deregulated nucleolin activity due to a loss of Rb contributes to tumor development in malignant diseases, thus providing further insights into the molecular network for the Rb-mediated tumor suppression.

Nucleolin as activator of human papillomavirus type 18 oncogene transcription in cervical cancer

High risk human papillomaviruses (HPVs) are central to the development of cervical cancer and the deregulated expression of high risk HPV oncogenes is a critical event in this process. Here, we find that the cell protein nucleolin binds in a sequence-specific manner to the HPV18 enhancer. The DNA binding activity of nucleolin is primarily S phase specific, much like the transcription of the E6 and E7 oncoproteins of HPV18 in cervical cancer cells. Antisense inactivation of nucleolin blocks E6 and E7 oncogene transcription and selectively decreases HPV18(+) cervical cancer cell growth. Furthermore, nucleolin controls the chromatin structure of the HPV18 enhancer. In contrast, HPV16 oncogene transcription and proliferation rates of HPV16(+) SiHa cervical cancer cells are independent of nucleolin activity. Moreover, nucleolin expression is altered in HPV18(+) precancerous and cancerous tissue from the cervix uteri. Whereas nucleolin was homogeneously distributed in the nuclei of normal epithelial cells, it showed a speckled nuclear phenotype in HPV18(+) carcinomas. Thus, the host cell protein nucleolin is directly linked to HPV18-induced cervical carcinogenesis.

Sp1 as G1 cell cycle phase specific transcription factor in epithelial cells

Sp1 binding sites have been identified in enhancer/promoter regions of several growth and cell cycle regulated genes, and it has been shown that Sp1 is increasingly phosphorylated in G1 phase of the cell cycle. Interactions of Sp1 with proteins involved in control of cell cycle and tumor formation have been reported. Here we show that expression of Sp1 protein predominates in the G1 phase of the cell cycle in epithelial cells. This is achieved by proteasome-dependent degradation. Inhibition of endogeneous Sp1 activity by a dominant-negative Sp1 mutant was associated with a cell cycle arrest in G1 phase, a strongly reduced expression of cyclin D1, the EGF-receptor and increased levels of p27Kip1. We have thus identified Sp1 as an important regulator of the cell cycle in G1 phase.

Suppression of tumorigenicity in breast cancer cells by the microfilament protein profilin 1

Differential display screening was used to reveal differential gene expression between the tumorigenic breast cancer cell line CAL51 and nontumorigenic microcell hybrids obtained after transfer of human chromosome 17 into CAL51. The human profilin 1 (PFN1) gene was found overexpressed in the microcell hybrid clones compared with the parental line, which displayed a low profilin 1 level. A comparison between several different tumorigenic breast cancer cell lines with nontumorigenic lines showed consistently lower profilin 1 levels in the tumor cells. Transfection of PFN1 cDNA into CAL51 cells raised the profilin 1 level, had a prominent effect on cell growth, cytoskeletal organization and spreading, and suppressed tumorigenicity of the stable, PFN1-overexpressing cell clones in nude mice. Immunohistochemical analysis revealed intermediate and low levels of profilin 1 in different human breast cancers. These results suggest profilin 1 as a suppressor of the tumorigenic phenotype of breast cancer cells.

Constitutive nuclear factor-kappaB-RelA activation is required for proliferation and survival of Hodgkin's disease tumor cells

The pathogenesis and etiology of Hodgkin's disease, a common human malignant lymphoma, is still unresolved. As a unique characteristic, we have identified constitutive activation of the transcription factor nuclear factor (NF)-kappaB p50-RelA in Hodgkin/Reed-Sternberg (H/RS) cells, which discriminates these neoplastic cells from most cell types studied to date. In contrast to other lymphoid and nonlymphoid cell lines tested, proliferation of H/RS cells depended on activated NF-kappaB. Furthermore, constitutive NF-kappaB p50-RelA prevented Hodgkin's lymphoma cells from undergoing apoptosis under stress conditions. Consistent with this dual function, Hodgkin's lymphoma cells depleted of constitutive nuclear NF-kappaB revealed strongly impaired tumor growth in severe combined immunodeficient mice. Our findings identify NF-kappaB as an important component for understanding the pathogenesis of Hodgkin's disease and for developing new therapeutic strategies against it.

Cell cycle regulation of nuclear factor p32 DNA-binding activity by novel phase-specific inhibitors

The nuclear factor p92, originally discovered by its interaction with the human papillomavirus type 18 enhancer, is a cellular protein whose activity is restricted to S phase in human primary fibroblasts. The human papillomavirus type 18 p92 binding sequence confers enhancer activity on a heterologous promoter, suggesting that p92 acts as a transcription factor. We have identified a class of nuclear inhibitory proteins, I-92s, which noncovalently associate with p92 but not with other transcription factors such as AP1, E2F, or NF-kappaB. Different I-92s occur in G1, G2, and G0, while no I-92 is detectable in S phase. Phase-specific inhibitors, therefore, are responsible for the cell cycle dependence of p92 activity and provide a novel mechanism linking transcription factor regulation with the cell cycle.

Blocking the transcription factor E2F/DP by dominant-negative mutants in a normal breast epithelial cell line efficiently inhibits apoptosis and induces tumor growth in SCID mice

The transcription factor E2F is regulated during the cell cycle through interactions with the product of the retinoblastoma susceptibility gene and related proteins. It is thought that E2F-mediated gene regulation at the G1/S boundary and during S phase may be one of the rate-limiting steps in cell proliferation. It was reported that in vivo overexpression of E2F-1 in fibroblasts induces S phase entry and leads to apoptosis. This observation suggests that E2F plays a role in both cell cycle regulation and apoptosis. To further understand the role of E2F in cell cycle progression, cell death, and tumor development, we have blocked endogenous E2F activity in HBL-100 cells, derived from nonmalignant human breast epithelium, using dominant-negative mutants under the control of a tetracycline-dependent expression system. We have shown here that induction of dominant-negative mutants led to strong downregulation of transiently transfected E2F-dependent chloramphenicol acetyl transferase reporter constructs and of endogenous c-myc, which has been described as a target gene of the transcription factor E2F/DP. In addition, we have shown that blocking of E2F could efficiently protect from apoptosis induced by serum starvation within a period of 10 d, whereas control cells started to die after 24 h. Surprisingly, blocking of E2F did not alter the rate of proliferation or of DNA synthesis of these cells; this finding indicates that cell-cycle progression could be driven in an E2F-independent manner. In addition, we have been able to show that blocking of endogenous E2F in HBL-100 cells led to rapid induction of tumor growth in severe combined immunodeficiency mice. No tumor growth could be observed in mice that received mock-transfected clones or tetracycline to block expression of the E2F mutant constructs in vivo. Thus, it appears that E2F has a potential tumor-suppressive function under certain circumstances. Furthermore, we provide evidence that dysregulation of apoptosis may be an important step in tumorigenesis.

Multiple octamer-binding proteins are targets for the cell cycle-regulated nuclear inhibitor I-92

p92 is a novel sequence-specific octamer-binding factor interacting with the enhancer of human papillomavirus type 18. The nuclear inhibitor I-92 regulates the DNA binding activity of p92 during the cell cycle such that p92 DNA binding is restricted to S-phase. The sequence motif ++ 5'-AATTGCTTGCATAA, consisting of two partially overlapping octamer-related sequences, represents a recognition site for p92. It was the aim of this study to characterize the complexity of proteins interacting with the 5'-AATTGCTTGCATAA motif and to determine their regulation by I-92. UV cross-linking experiments showed that, besides p92, multiple novel proteins interact with the 5'-AATTGCTGCATAA motif. These novel proteins p84, p75, p73, p69, p61, p57, p49, and p46 specifically bind to this motif, although with different affinities. The inhibitor I-92 regulates, besides p92, the DNA-binding activities of p84, p75, p73, p69, and p57 but not of p61, p49, and p46. The association of I-92 with p92, p84, p75, p73, p69, and p57 was completely reversible after treatment with the detergent deoxycholate (DOC). Finally, we analyzed I-92 specificity and found that I-92 selectively inhibited DNA binding activities of partially purified octamer-binding proteins p84 and p92 whereas DNA binding of the POU factor Oct-1 was not regulated by I-92. Our results show that I-92 regulates multiple octamer-binding proteins and these findings provide an example how gene regulation could be linked to cell cycle regulation.