Journal of cell science. Supplement最新文献

The fission yeast cut5+ (identical to rad4+) gene is essential for S phase. Its temperature-sensitive (ts) mutation causes mitosis while S phase is inhibited: dependence of mitosis upon the completion of S phase is abolished. If DNA is damaged in mutant cells, however, cell division is arrested. Thus the checkpoint control system for DNA damage is functional, while that for DNA synthesis inhibition is not in the cut5 mutants. Transcription of the cut5+ gene is not under the direct control of cdc10+, which encodes a transcription factor for the START of cell cycle. The transcript level does not change during the cell cycle. The protein product has four distinct domains and is enriched in the nucleus. Its level does not alter during the cell cycle. The N-domain is important for cut5 protein function: it is essential for complementation of ts cut5 mutations and its overexpression blocks cell division. Furthermore, it resembles the N-terminal repeat domain of proto-oncoprotein Ect2, which, in the C-domain, contains a regulator-like sequence for small G proteins. We discuss a hypothesis that the cut5 protein is an essential component of the checkpoint control system for the completion of DNA synthesis. The restraint of mitosis until the completion of S phase is mediated by the cut5 protein, which can sense the state of chromosome duplication and negatively interacts with M phase regulators such as cdc25 and cdc2.

The D-type cyclins are expressed during the progression from G0/G1 to S phase in the mammalian cell cycle. There is considerable evidence that they contribute to the development of specific cancers, both in humans and in mouse models. For example, cyclin D1 can be activated by chromosomal translocation, DNA amplification and retroviral integration. Cyclins D1, D2 and D3 preferentially associate with two closely related members of the cyclin-dependent kinase family, Cdk4 and Cdk6 and the various complexes are each capable of phosphorylating the retinoblastoma gene product (pRb), at least in vitro. This suggests that the growth promoting effects of the D-cyclins may be manifest via their interactions with tumour suppressor genes.

The bcl-2 gene provides a window on the basic cellular machinery of apoptosis or programmed cell death, a process involved in virtually all biologic events in multicellular organisms, but particularly relevant to neoplasia and development. bcl-2 gene function supports cell survival and appears to lie at a nodal point in pathways leading to activation or execution of apoptosis. Carcinogenesis may involve several steps at which cell death programs are normally activated and are bypassed in cancer cells, including apoptotic pathways activated by several oncogenes. Functional redundancy and the complexity of the regulation of cell survival are demonstrated by the less than expected phenotype of bcl-2 knockout mice and the cloning of several bcl-2 related genes, some of which promote cell death. The molecular function for bcl-2 is unknown, but several lines of evidence support a role in protection from oxidative stress. These studies suggest that many environmental perturbations and genetic pathways converge to disrupt a metabolic balance between oxidant generation and anti-oxidant defenses.

Cyclin-dependent kinases (Cdks) control the major cell cycle transitions in eukaryotic cells. On the basis of a variety of experiments where cyclin function either is impaired or enhanced, D-type cyclins as well as cyclins E and A have been linked to G1 and G1/S phase roles in mammalian cells. We therefore sought to determine if agents that block the G1/S phase transition do so at the level of regulating the Cdk activities associated with these cyclins. A variety of conditions that lead to G1 arrest were found to correlate with accumulation of G1-specific Cdk inhibitors, including treatment of fibroblasts with ionizing radiation, treatment of epithelial cells with TGF-beta, treatment of HeLa cells with the drug lovastatin, and removal of essential growth factors from a variety of different cell types. Mechanistically, inhibition of Cdks was found to involve the stoichiometric binding of Cdk inhibitor proteins. p21Waf1/Cip1 was associated with DNA damage induced arrest while p27Kip1/p28Ick1 accumulated under a variety of antiproliferative conditions.

Major checkpoints that gate progression through the cell cycle function at the G1/S transition, entry into mitosis and exit from mitosis. Cells use feedback mechanisms to inhibit passage through these checkpoints in response to growth control signals, incomplete DNA replication or spindle assembly. In many organisms, transition points seem to involve regulation of the activity of cyclin-dependent kinases (cdks) not only through their interactions with various cyclins, but also by phosphorylation-dephosphorylation cycles acting on the kinase activity of the cdks. These phosphorylation cycles are modulated by the regulation of the opposing kinases and phosphatases that act on cdks and form feedback loops. In this article, we discuss the role of positive and negative feedback loops in cell cycle timing and checkpoints, focusing more specifically on the regulation of the dual specificity cdc25 phosphatase.

Small DNA tumor viruses, such as adenovirus, encode proteins that deregulate the cell cycle. These proteins are potent transforming agents when tested in standard oncogenic assays. For adenovirus the best characterized viral oncoproteins are the early region 1A (E1A) products. Mutational studies have shown that E1A's oncogenic ability is determined primarily by its ability to bind to certain cellular proteins and interfere with their function. One of these cellular targets for E1A is the product of the retinoblastoma tumor suppressor gene, pRB. pRB is a negative regulator of cell proliferation, and its inactivation has been shown to be an important oncogenic step in the development of many human cancers. In adenovirusmediated transformation, E1A binds to pRB and inactivates it, thus functionally mimicking the loss of pRB often seen in human tumors. There is now compelling evidence to suggest that pRB regulates transcription at specific phases of the cell cycle by physically associating with key transcription factors. The best characterized target of pRB is the transcription factor E2F. The interaction of pRB and E2F leads to the inhibition of E2F-mediated transactivation. Most of the genes that are known to be controlled by E2F have key roles in the regulation of cell proliferation. During cell cycle progression, phosphorylation of pRB appears to change its conformation and E2F is released. In pathogenic settings E2F transactivation is not regulated by pRB binding. In human tumors with mutations in the retinoblastoma gene, functional pRB is absent and hence can no longer inhibit E2F activity.(ABSTRACT TRUNCATED AT 250 WORDS)

The Pax gene family consists of nine members encoding nuclear transcription factors. Their temporally and spatially restricted expression pattern during embryogenesis suggests that they may play a key role during embryogenesis. Direct evidence for the important role of the Pax genes during embryonic development has been demonstrated by the correlation of mouse developmental mutants and human syndromes with mutations in some Pax genes. To date three Pax genes have been shown to be mutated in undulated, Splotch and small eye, respectively. In man, Pax-3 is mutated in the Waardenburg syndrome, while in aniridia Pax-6 is mutated.

Studies on the attachment and spreading of cells in culture have provided valuable insights into the mechanisms by which cells transmit information from the outside to the inside of the cell. This brief review considers recent information on the role of focal adhesion-associated protein tyrosine kinases in integrin-regulated cell signalling.

The macrophage-specific colony-stimulating factor 1 (CSF-1 or M-CSF) is required throughout the G1 phase of the cell cycle to regulate both immediate and delayed early responses necessary for cell proliferation. These are triggered by the binding of the growth factor to the colony-stimulating factor 1 receptor and the activation of its intrinsic tyrosine-specific protein kinase. Phosphorylation of the colony-stimulating factor 1 receptor on specific tyrosine residues enables it to bind directly to cytoplasmic effector proteins, which in turn relay receptor-induced signals through multiple-signal transduction pathways. The activity of p21ras as well as transcription factors of the ets gene family appears to be required for colony-stimulating factor 1 to induce the c-myc gene, and the latter response is essential to ensure cell proliferation. Genes within the fos/jun or activator protein 1 family are targeted via a parallel and independently regulated signal transduction pathway. The continuous requirement for colony-stimulating factor 1 after the immediate early response is initiated indicates that expression of additional delayed early response genes, although contingent on previously induced gene products, might also depend on colony-stimulating factor 1-induced signals. Among the growth factor-regulated delayed early response genes are D-type G1 cyclins, which play an important role in cell-cycle progression.

In order for cells to respond to their environment, a series of regulated molecular events has to take place. External signalling molecules bind to cellular receptors and thereby trigger the activation of multiple intracellular pathways, which modify cellular phenotypes. The cell-surface receptors for a wide range of polypeptide hormones possess protein tyrosine kinase activity, which is induced by binding of the appropriate extracellular ligand. Tyrosine phosphorylation can act as a molecular switch, by initiating the recruitment of cytoplasmic effector molecules containing Src homology (SH) 2 domains, to activated receptors. These SH2-containing proteins, in turn, regulate intracellular signalling pathways. Here, we discuss the role of tyrosine phosphorylation in triggering signalling pathways, as well as the functions of SH2 domains, which mediate these events through phosphotyrosine-dependent protein-protein interactions.