Showing papers on "Cyclin-dependent kinase 8 published in 1995"

Notch signaling : Signal transduction

Abstract: The Notch/Lin-12/Glp-1 receptor family mediates the specification of numerous cell fates during development in Drosophila and Caenorhabditis elegans. Studies on the expression, mutant phenotypes, and developmental consequences of unregulated receptor activation have implicated these proteins in a general mechanism of local cell signaling, which includes interactions between equivalent cells and between different cell types. Genetic approaches in flies and worms have identified putative components of the signaling cascade, including a conserved family of extracellular ligands and two cellular factors that may associate with the Notch Intracellular domain. One factor, the Drosophila Suppressor of Hairless protein, is a DNA-binding protein, which suggests that Notch signaling may involve relatively direct signal transmission from the cell surface to the nucleus. Several vertebrate Notch receptors have also been discovered recently and play important roles in normal development and tumorigenesis

p57KIP2, a structurally distinct member of the p21CIP1 Cdk inhibitor family, is a candidate tumor suppressor gene.

Abstract: Cyclin-dependent kinases (Cdks) are positive regulators of cell proliferation, whereas Cdk inhibitors (CKIs) inhibit proliferation. We describe a new CKI, p57KIP2, which is related to p21CIP1 and p27KIP1. p57KIP2 is a potent, tight-binding inhibitor of several G1 cyclin/Cdk complexes, and its binding is cyclin dependent. Unlike CIP1, KIP2 is not regulated by p53. Overexpression of p57KIP2 arrests cells in G1. p57KIP2 proteins have a complex structure. Mouse p57KIP2 consists of four structurally distinct domains: an amino-terminal Cdk inhibitory domain, a proline-rich domain, an acidic-repeat region, and a carboxy-terminal domain conserved with p27KIP1. Human p57KIP2 appears to have conserved the amino- and carboxy-terminal domains but has replaced the internal regions with sequences containing proline-alanine repeats. In situ hybridization during mouse embryogenesis revealed that KIP2 mRNA displays a striking pattern of expression during development, showing high level expression in skeletal muscle, brain, heart, lungs, and eye. Most of the KIP2-expressing cells are terminally differentiated, suggesting that p57KIP2 is involved in decisions to exit the cell cycle during development and differentiation. Human KIP2 is located at 11p15.5, a region implicated in both sporadic cancers and Beckwith-Wiedemann syndrome, a familial cancer syndrome, marking it as a candidate tumor suppressor. The discovery of a new member of the p21CIP1 inhibitor family with novel structural features and expression patterns suggests a complex role for these proteins in cell cycle control and development.

A kinase–cyclin pair in the RNA polymerase II holoenzyme

Abstract: The RNA polymerase II holoenzyme consists of RNA polymerase II, a subset of general transcription factors, and regulatory proteins known as SRB proteins. The genes encoding SRB proteins were isolated as suppressors of mutations in the RNA polymerase II carboxy-terminal domain (CTD). The CTD and SRB proteins have been implicated in the response to transcriptional regulators. We report here the isolation of two new SRB genes, SRB10 and SRB11, which encode kinase- and cyclin-like proteins, respectively. Genetic and biochemical evidence indicates that the SRB10 and SRB11 proteins form a kinase-cyclin pair in the holoenzyme. The SRB10/11 kinase is essential for a normal transcriptional response to galactose induction in vivo. Holoenzymes lacking SRB10/11 kinase function are strikingly deficient in CTD phosphorylation. Although defects in the kinase substantially affect transcription in vivo, purified holoenzymes lacking SRB10/11 kinase function do not show defects in defined in vitro transcription systems, suggesting that the factors necessary to elicit the regulatory role of the SRB10/11 kinase are missing in these systems. These results indicate that the SRB10/11 kinase is involved in CTD phosphorylation and suggest that this modification has a role in the response to transcriptional regulators in vivo.

WT1 suppresses synthesis of the epidermal growth factor receptor and induces apoptosis.

Abstract: The Wilms tumor suppressor gene WT1 encodes a developmentally regulated transcription factor that is mutated in a subset of embryonal tumors. To test its functional properties, we developed osteosarcoma cell lines expressing WT1 under an inducible tetracycline-regulated promoter. Induction of WT1 resulted in programmed cell death. This effect, which was differentially mediated by the alternative splicing variants of WT1, was independent of p53. WT1-mediated apoptosis was associated with reduced synthesis of the epidermal growth factor receptor (EGFR), but not of other postulated WT1-target genes, and it was abrogated by constitutive expression of EGFR. WT1 repressed transcription from the EGFR promoter, binding to two TC-rich repeat sequences. In the developing kidney, EGFR expression in renal precursor cells declined with the onset of WT1 expression. Repression of EGFR and induction of apoptosis by mechanism that may contribute to its critical role in normal kidney development and to the immortalization of tumor cells with inactivated WT1 alleles.

Drosophila warts--tumor suppressor and member of the myotonic dystrophy protein kinase family

Abstract: Tumor suppressor genes represent a broad class of genes that normally function in the negative regulation of cell proliferation. Loss-of-function mutations in these genes lead to unrestrained cell proliferation and tumor formation. A fundamental understanding of how tumor suppressor genes regulate cell proliferation and differentiation should reveal important aspects of signalling pathways and cell cycle control. A recent report describing the Drosophila tumor suppressor gene warts has implications in the study of the human myotonic dystrophy gene. These genes encode members of a cyclic AMP-dependent protein kinase subfamily that includes other plant and animal orthologues.

The P53 Tumor Suppressor Protein

Abstract: In response to damaged DNA, mammalian cell growth is arrested at cell cycle checkpoints in Gl, near the border of S phase, or in G2, before mitosis (Murray, 1992; Hunter, 1993; Weinert and Lydall, 1993). In some circumstances, DNA damage initiates apoptosis, a program that results in cell death. Recent studies have shown that the p53 tumor suppressor protein is an essential component of the G1 checkpoint pathway (Kastan et al., 1991); it also modulates the initiation of apoptosis (Oren, 1994). The arrest of cell cycle progression provides time for DNA damage to be repaired, whereas apoptosis may insure the death of more severely damaged cells that are at risk of loss of growth control through genome rearrangements. Thus, these functions account, at least in part, for the importance of p53 in suppressing or eliminating preneoplastic or neoplastic cells in the human and other vertebrate species. In turn, p53 function is mediated through its physical characteristics, and these may be modulated by post-translational mechanisms (Ullrich et al., 1992; Meek, 1994). Thus, biophysical studies of p53 and its functional domains are fundamental to an understanding of those properties that are important for normal p53 function.

The first intron of human h-ras is regulated by p53 - mediation of specific activation by a p53-binding element.

Abstract: We conducted transient reporter gene expression assays to show that the wild-type but not the mutant-type p53 protein positively regulates the expression of the human H-ras gene. This activation of expression is mediated through a specific p53-binding DNA element located in the first intron of the H-ras gene. This element is similar to a previously identified p53-binding element. Without this element, wild-type p53 represses the H-ras promoter, as do several p53 mutants. The repression function is lost when the N-terminal 81 amino acids of the p53 protein are deleted, suggesting that the N-terminus is required for repression. Therefore, p53 seems to regulate the H-ras through both positive and negative mechanisms.

Antibody-induced inhibition of growth of egfr overexpressing tumors occurs in the absence of receptor down-regulation.

Abstract: Using two antibodies which bind to distinct epitopes on the extracellular domain of the EGF receptor (EGFR) we have developed a novel method for monitoring EGFR expression and the behaviour of monoclonal antibody (mAb) bound to the receptor. We have used this method to investigate the fate of the rat mAb ICR80 following binding to the EGF receptor on tumour cells. Antibody ICR80, which was raised against the external domain of the EGF receptor on a human brain tumour (A172) cell line and was employed in this study, has the following properties. It (a) blocks the binding of EGF, TGF alpha and HB-EGF to the EGFR, (b) prevents the EGF, TGF alpha and HB-EGF induced tyrosine phosphorylation of the EGFR, and (c) inhibits the growth in vitro of the head and neck tumour (HN5) cell line overexpressing the EGF receptor. Our results presented herein also show that EGF receptor blockade by antibody ICR80 is not accompanied by detectable loss of antibody from the cell surface or down-regulation of the receptor. On the basis of these results we conclude that the long-lasting blockade of the EGF receptor on tumour cells by antibody may be an important factor in preventing the binding of growth factors which are essential for their continued proliferation.

Do oncogene and tumor suppressor gene abnormalities vary with type of carcinoma of the pancreas

Abstract: Carcinomas of the pancreas continue to provide a challenge regarding early diagnosis, treatment, and strategies for prevention. All histologic subtypes of carcinoma of the exocrine pancreas are included as a single category in most population data sets. This pooling of exocrine carcinomas sometimes carries over into epidemiologic and clinical studies. However, 12 or more histologically distinct exocrine carcinomas are listed in some classifications, 1 suggesting origin from different cell types in the pancreas or by different histogenetic pathways. The age of peak incidence, sex distribution, and prognosis varies widely, and in general, predictably, for various types of carcinoma. Thus, pancreatic exocrine neoplasms can be regarded as several different cancers, although most are ductal adenocarcinomas with varying degrees of differentiation. Several of the subtypes, such as pleomorphic, mucinous, and adenosquamous carcinomas, are regarded as variants of ductal adenocarcinoma and occur in patients with similar demographic characteristics. Observations in humans and in experimental models of carcinogenesis in the pancreas of animals have suggested several cellular pathways for the origin of carcinomas of the pancreas, a Treatment of hamsters with several related nitrosamines yields carcinomas that are nearly always of ductal phenotype. Focal proliferative changes of ductal and ductular cells

The correlation between p53 tumor suppressor gene and proliferating cell nuclear antigen expressions in colorectal adenocarcinoma

Abstract: The tumor suppressor gene p53, 16-20 kb of DNA located on the short arm of chromosome 17, plays an important role in the regulation of cell proliferation and it has been suggested that the loss of normal p53 function is associated with cell transformation and development of neoplasms.’.2 Its level in normal cells is low, with a short half-life. However, in tumor cell lines high levels are found, with an extended half-life. Increased proliferating cell nuclear antigen (PCNA) expression might also be associated with overexpression of p53 protein in t~mors.’~~” More than 50% of colorectal carcinomas have been shown to express p53 protein and a relationship between p53 mutations and prognosis has been s~ggested?~.~~’ PCNA is a 36 kDa nuclear protein that accumulates from stages G, to M of the cell cycle and is also needed for the synthesis of DNA after damaged portions are removed by the cell’s nucleotide excision repair system? Recently, a relationship between PCNA and the malignant potential of colorectal carcinomas has been proposed? Smith et a/. showed that after genotoxic stress, p53functioned as a transcription factor and transactivated effector genes such as Gadd45 and p27; and then p27 inhibited the ability of PCNA to activate DNA replication and Gadd45 stimulated DNA repair by a redistribution of PCNA from sites of DNA replication to sites of DNA damage? As a result, PCNA is a p53 regulated protein that is needed for DNA repair as well as replication. From these data, we suggest that there might be

Modulation of urokinase-type plasminogen-activator expression by the wild-type and mutated p53 tumor-suppressor gene

Abstract: The urokinase-type plasminogen activator contributes to the invasive phenotype of tumor cells by facilitating extracellular matrix hydrolysis. However, the molecular mechanism(s) responsible for urokinase overexpression remains unclear. The current study was undertaken to determine the role of the p53 tumor suppressor gene in the regulation of urokinase expression. Transient cob transfection of a urokinase-producing cell line with a wildtype p53 expression vector and a CAT reporter driven by the urokinase promoter led to a dramatic reduction in CAT activity. Conversely, urokinase promoter activity was increased in cells transiently transfected with a p53 expression vector mutated at codon 175. To determine if the endogenous urokinase gene was modulated by p53, cells were stably transfected with a mutated p53-bearing expression vector. An increased level of urokinase mRNA was apparent in pooled mutant p53-overexpressing clones. These studies argue for a role of the p53 tumor suppressor gene in the regulation of urokinase expression.