Molecular biology of cell division

The antitumorigenic roles of BRCA1-BARD1 in DNA repair and replication

The tumour suppressor breast cancer type 1 susceptibility protein (BRCA1) promotes DNA double-strand break (DSB) repair by homologous recombination and protects DNA replication forks from attrition. BRCA1 partners with BRCA1-associated RING domain protein 1 (BARD1) and other tumour suppressor proteins to mediate the initial nucleolytic resection of DNA lesions and the recruitment and regulation of the recombinase RAD51. The discovery of the opposing functions of BRCA1 and the p53-binding protein 1 (53BP1)-associated complex in DNA resection sheds light on how BRCA1 influences the choice of homologous recombination over non-homologous end joining and potentially other mutagenic pathways of DSB repair. Understanding the functional crosstalk between BRCA1-BARD1 and its cofactors and antagonists will illuminate the molecular basis of cancers that arise from a deficiency or misregulation of chromosome damage repair and replication fork maintenance. Such knowledge will also be valuable for understanding acquired tumour resistance to poly(ADP-ribose) polymerase (PARP) inhibitors and other therapeutics and for the development of new treatments. In this Review, we discuss recent advances in elucidating the mechanisms by which BRCA1-BARD1 functions in DNA repair, replication fork maintenance and tumour suppression, and its therapeutic relevance.

Hepatocyte growth factor/scatter factor-induced intracellular signalling

Hepatocyte growth factor (HGF) identical to scatter factor (SF) is a glycoprotein involved in the development of a number of cellular phenotypes, including proliferation, mitogenesis, formation of branching tubules and, in the case of tumour cells, invasion and metastasis. This fascinating cytokine transduces its activities via its receptor encoded by the c-met oncogene, coupled to a number of transducers integrating the HGF/SF signal to the cytosol and the nucleus. The downstream transducers coupled to HGF/MET, most of which participate in overlapping pathways, determine the development of the cell's phenotype, which in most cell types is dual.

Multivariate flow cytometry of epidermal regeneration provoked by a skin irritant and a tumor promoter

The DNA content and the changes in cellular and nuclear size of isolated regenerating mouse epidermal basal cells were studied after topical application of the skin irritant cantharidin and the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) to the back skin of hairless mice. The DNA and protein contents of isolated basal cells were stained with propidium iodide and fluorescein isothiocyanate, respectively, and analysed by flow cytometry using the total protein fluorescence as an estimate of cell size and the DNA fluorescence pulse width as an estimate of nuclear size. Transmission electron microscopy was used to identify cells sorted from regions in the bivariate DNA/protein distributions. The results showed that both chemicals induced an increase in cellular as well as nuclear size of the basal cells. The increase in size was higher in TPA treated than in cantharidin treated animals, and the bivariate DNA/protein distributions of TPA treated cells differed from those of cantharidin treated cells in that two subpopulations of cycling keratinocytes could be identified. These deviations are probably related to the higher proliferative response observed after TPA treatment and the possibility that proliferative subpopulations in epidermis respond differently to TPA. It may reflect mechanisms providing for a growth advantage of initiated cells, important in tumor promotion. About 8% of the cells in the suspensions from treated animals were non-cycling non-keratinocytes, probably infiltrating leukocytes. The results indicate a strong correlation between rapid regenerative cell cycle progression, i.e., reduced G1 transit time and increased cellular and nuclear size.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between DNA methylation and cell proliferation in Trypanosoma cruzi

5-Azacytidine treatment of T. cruzi epimastigotes in culture induces active cell proliferation. This effect was detected as an increase in the cell number and [3H-methyl]thymidine incorporation into DNA. 5-Azacytidine does not alter other metabolic parameters. We have previously demonstrated that 5-azacytidine induces DNA hypomethylation in T. cruzi. Accordingly, we suggest that this chemical modification may be related to the control of T. cruzi cell division.

Serum inhibition of proliferation of serum-free mouse embryo cells

Serum-free mouse embryo (SFME) cells, derived in medium supplemented with insulin, transferrin, high density lipoprotein, epidermal growth factor, and fibronectin, do not undergo crisis, maintain a predominantly diploid karyotype with no detectable chromosomal abnormalities for well over 100 population doublings in vitro, and are growth inhibited by concentrations of serum that are growth-stimulatory for most cell lines in culture. Serum inhibition of SFME cell proliferation was reversible and was not prevented by addition of the supplements of the serum-free medium, even when added repeatedly during the culture period. The serum effect on SFME cell proliferation could be detected after incubation in serum-containing medium for as little as 8 h. SFME cells in serum-containing medium were arrested in the G1 phase of the cell cycle with a greatly reduced rate of incorporation of precursors into DNA and thymidine kinase activity, while a reduction in rate of incorporation of amino acids into protein was not observed. SFME cultures maintained for extended periods in serum-containing medium underwent a crisis-like period followed by the appearance of variant cells capable of growing in serum-supplemented medium. These cells exhibited abnormal karyotype and were resistant to several inhibitors of proliferation active on the parent SFME cell type.

Oncogenes, growth, and the cell cycle: an overview

In spite of the complexity of the network of regulatory factors which control the balance between the cell cycle and quiescence, a picture is emerging, if only in outline. Several dozens of protooncogenes participate in growth signal transduction and integration, and, when expressed inappropriately, generate growth signals that may override other cellular controls. Some of these controls are provided by the negatively regulating growth factors, and when these are lost (e.g. by chromosomal deletion), or inactivated (e.g. by binding to an inactive analogue or a DNA viral oncoprotein), cell cycle activity is favoured over quiescence. Embryonic tissues are rapidly growing, so their cells are actively cycling and expression of proto-oncogenes is usually observed (Schuuring et al., 1989). As embryonic and stem cells in adult tissues mature, expression of the active proto-oncogenes is generally lost, but other proto-oncogenes may now be expressed (e.g. Muller et al., 1982). These changes in proto-oncogene expression are not achieved by modulation of transcriptional rates alone; transcriptional attenuation, message processing and stability, and post-translational protein modifications are all known to be important for the regulation of proto-oncogene expression during the transition from growth to the differentiated state. When quiescent cells re-enter the cell cycle approximately 60 genes become up-regulated, including proto-oncogene c-fos, the jun family, and c-myc (Zipfel et al., 1989). Evidence is strong that fos and jun proteins are transcriptional regulators. Terminal differentiation, on the other hand, is sometimes accompanied by the up-regulation of the ras gene family, as well as of several other proto-oncogenes. Proto-oncogene function is essential to the cell cycle traverse, but the genes involved are different in various cell types, and the precise order of oncogene expression may not turn out to be important. This is because cell cycle traverse appears to be more dependent on a critical threshold of growth signals propagated by parallel pathways, rather than on a strict order of predetermined steps. The participation of proto-oncogenes in growth signal transduction offers opportunities for errors, and abnormal growth may result from aberrant oncogene products generating a persistent or excessive growth signal, which shifts the balance of input to the integrating genes from quiescence to an active cell cycle. Thus, cancer may result from an entirely normal processing of growth signals that are abnormal.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell cycle dependent genes inducible by different mitogens in cells from different species

A number of genes and cDNA sequences (including at least four oncogenes) are known to be expressed in a cell cycle-dependent manner, i.e. the levels of specific mRNAs vary with the phases of the cell cycle. In order to explore the significance of some of these sequences in the mitogenic response, we have investigated the expression of 8 cell cycle-dependent sequences (plus two control sequences, not expressed in a cell cycle-dependent manner) under a variety of conditions. These conditions included cells of different types, from different species, stimulated to proliferate by different mitogens. The genes (or sequences) studied included five cDNA clones whose sequences are preferentially expressed in early G1, i.e. two cDNA clones inducible by platelet-derived growth factor (JE-3 and KC-1), and three cDNA clones inducible by serum (2A9, 2F1, 4F1); and three oncogenes (c-myc, c-rasHa and p53) whose expression is known to be cycle-dependent. All of the tested genes, except 2A9, c-rasHa and the control genes, are expressed in a cell cycle-dependent manner in human peripheral blood mononuclear cells stimulated by phytohemagglutinin and in serum-stimulated mouse and Syrian hamster fibroblasts. The inducibility of these genes by different mitogens in cells of different types and from different species strongly suggests that these genes play a role in cell cycle progression. This conclusion is further supported by the known structural and functional similarities between cell-cycle dependent genes, oncogenes and genes coding for cell-cycle related molecules.

Immediate effects of serum depletion on dissociation between growth in size and cell division in proliferating 3T3 cells

Proliferating nonconfluent 3T3 cells become committed to proceed through the cell cycle or to enter G0 during the first post-mitotic part of G1 (G1pm). The decision to proceed through G1pm is dependent on the presence of serum growth factors in the culture medium. Cells that have passed this particular growth-factor-dependent cell cycle stage are independent of serum growth factors and undergo mitosis on schedule. We report here that G1ps, S, and G2 cells cease to increase in size when serum is withdrawn. As a result the mitotic cell size after 8 hours serum starvation is reduced to approximately 60% of the normal mitotic cell. This reduced growth in cell size is due to a rapid decrease in protein synthesis and some increase in protein degradation. This dissociation between growth in size and cell-cycle progression within a single cell cycle provides a new approach to study the two processes separately.

Molecular biology of the cell cycle

Genes and cDNA clones have been identified in animal cells that are cell cycle-regulated, i.e. they are preferentially expressed in a phase of the cell cycle. Some of these genes, including four oncogenes, are induced when G0 cells are stimulated to proliferate. Four approaches are described to identify the genes that regulate the transition of cells from a resting to a growing stage. The interrelationship among cell cycle-regulated genes, oncogenes, growth factors and receptors for growth factors points the way to a genetic dissection of cell cycle progression.

Could the loss of regulation of genetic expression in cancer cells be used to cause their necrosis?

In cancer cells the control over the genetic message involved in the induction of mitosis is irreversibly lost. This fact is indicated by certain phenomena displayed by cancer cells under restricted nutritional conditions. Cells transformed by DNA viruses (which stabilize "p53") keep on cycling and die. In starving cells at the inside of tumors the synthesis of pre-rRNA still proceeds while all other anabolic processes are already at a standstill. The reason is that glutamine, glycine and aspartate are channelled into the enzymatic pathways for the synthesis of nucleosides: thus, protein synthesis is denied those aminoacids. Such situations might be imitated through the administration of excess nucleosides and (within limits) the simultaneous restriction of some selected aminoacids. DNA replication depends on the stabilization of p53, but an accumulation of pre-rRNA might occur, which ultimately might be harmful for cancer cells. Several ways to improve this rationale might be tested on cultured cells and on research animals. They include the destruction of methionine with bacterial enzymes, or the addition of ornithine, a precursor of putrescine, which is an important factor of DNA and pre-rRNA synthesis.

The effect of cycloheximide on the expression of cell cycle dependent genes

We have investigated the inducibility of several cell cycle-dependent genes (plus control sequences, not expressed in a cell cycle-dependent manner) in the presence of cycloheximide, an inhibitor of protein synthesis. The genes studied include: 1) five cDNA clones that are preferentially expressed in the G1 phase of the cell cycle: KC-1, JE-3, 2F1, 4F1 and 2A9; 2) one gene preferentially expressed in late G1/S phase: histone H3; and 3) the cell cycle-dependent oncogene p53. All the genes studied are induced by serum even in the presence of cycloheximide. Previous results in the literature have shown that 2 other oncogenes, c-myc and c-fos, can be induced by growth factors in the presence of cycloheximide. Together with our results, these findings indicate that protein synthesis is not required for the induction of at least nine cell cycle genes by growth factors.

A study of mitochondrial and nuclear transcription with cloned cDNA probes. Changes in the relative abundance of mitochondrial transcripts after stimulation of quiescent mouse fibroblasts

From a cDNA library constructed in pBR322 we have isolated and studied a set of clones corresponding to mRNAs whose abundance changes when serum-deprived murine fibroblasts are stimulated to enter the cell cycle. A subset of these clones was derived from mRNA species whose abundance decreased during the G1 period following serum stimulation; all but one of these clones turned out to be clones of mitochondrial poly(A)mRNAs. There was no detectable change in the rate of transcription of the mitochondrial genome compared with the nuclear genome, and the lengths of the poly(A) segments on both mRNA species did not change significantly after serum stimulation. We conclude that the apparent decline in the relative abundance of the mitochondrial mRNAs is the result of a relative increase in the processing and/or transport of nuclear mRNA.

Partial characterization of the mitogenic action of pp60v-src, the oncogenic protein product of the src gene of avian sarcoma virus

NRK cells infected with a temperature-sensitive, transformation-defective mutant of avian sarcoma virus (ASV), tsLA23, are transformed at 36 degrees C, but at 40 degrees C they behave as nontransformed cells because of the inactivation of the abnormally thermolabile pp60v-src product of the virus' transforming src gene. At 40 degrees C, these tsLA23-NRK cells were arrested in G1/G0 by severe serum deprivation. They were induced to enter G1, initiate DNA synthesis 7 or 10 hours later, and then divide as (1) nontransformed cells by adding serum or platelet-derived growth factor (PDGF) at 40 degrees C, or (2) transformed cells by lowering the temperature to a pp60v-src-activating 36 degrees C without adding exogenous growth factor(s). The level of pp60v-src kinase activity rose dramatically in these serum-deprived cells within 30 minutes of lowering the temperature to the permissive 36 degrees C, and it fell just as rapidly when the cells were returned to the restrictive 40 degrees C. As little as a 2-hour exposure to 36 degrees C, with an attendant 2-hour burst of pp60v-src kinase activity, was enough to stimulate serum-deprived tsLA23-NRK cells to transit G1 and initiate DNA replication, but not to divide. Much more prolonged pp60v-src activity was needed for these serum-deprived cells to complete their cycle and divide. The prereplicative development of quiescent tsLA23-NRK cells stimulated by serum or PDGF was accompanied by greatly increased protein synthesis and slightly decreased protein degradation, but the pp60v-src-stimulated cells progressed through G1 and initiated DNA replication without appreciably affecting the protein synthetic machinery of the cell. The cells stimulated by the mitogenic action of pp60v-src, like the cells stimulated by serum, needed to activate early prereplicative genes in order to initiate DNA replication. The needed RNA transcripts induced by serum and pp60v-src were produced with comparable efficiency, although it took longer for pp60v-src-stimulated cells to translate these transcripts and to initiate DNA replication, probably because of their unstimulated protein-synthetic machinery.

Role of the p53 protein in cell proliferation as studied by microinjection of monoclonal antibodies

Two monoclonal antibodies against the p53 protein, PAb 122 and 200-47, were microinjected into mammalian cells as a probe to determine the role of the p53 protein in cell proliferation. PAb 122 recognizes the p53 proteins of mouse and human cells but not of hamster cells, whereas 200-47 recognizes the p53 proteins of mouse and hamster cells but not of human cells. The ability of these antibodies to inhibit serum-stimulated DNA synthesis of cells in culture correlates with their ability to recognize the species-specific antigenic determinants. More important, however, is the observation that microinjected PAb 122 inhibits the transition of Swiss 3T3 cells from G0 to S phase, but has no effect on the progression of these cells from mitosis to the S phase.

Control of protein degradation and growth phase in normal and neoplastic cells

Cells have to double their protein mass in order to divide. Whether this is achieved through increased synthesis (PS), decreased degradation (PD), or a combination of both is still debated. Likewise open are other basic questions: whether, beyond differences relating to growth phase (GP) or rate, reduced PD rates are a general characteristic of neoplastic versus normal cells, conferring to them a definite growth advantage; which mechanisms are operating the PD regulation, if any, during GP transitions, and which ones may be defective in neoplastic cells. Growing liver under conditions of regeneration or development is known to achieve a net protein accumulation thanks to increased PS, and particularly, to decreased PD rates, as compared with the adult, steady-state tissue; the level of lysosomal proteinase (LP) activities is reduced; in the regenerating liver this reduction has been located in cycling hepatocytes. AH-130 Yoshida ascites hepatoma cells effect the transition from log to stationary GP by concurrently reducing PS and accelerating PD (slow turnover protein pool); while PD is virtually not affected by lysosomal inhibitors (LI) in growing cells, the extra PD in resting cells is all inhibitable; there is no regulation of LP levels over this GP transition, which is due to depletion of oxygen and nutrients. GP transitions in normal 3T3 cells are also coupled with regulations of both PS and PD, the extra PD in quiescent cells being all suppressible by LI. Quiescence of 3T3 cells, due to depletion of growth factors, is associated with a marked elevation of some LP activities.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the p53 protein in cell proliferation as studied by microinjection of monoclonal antibodies

Two monoclonal antibodies against the p53 protein, PAb 122 and 200-47, were microinjected into mammalian cells as a probe to determine the role of the p53 protein in cell proliferation. PAb 122 recognizes the p53 proteins of mouse and human cells but not of hamster cells, whereas 200-47 recognizes the p53 proteins of mouse and hamster cells but not of human cells. The ability of these antibodies to inhibit serum-stimulated DNA synthesis of cells in culture correlates with their ability to recognize the species-specific antigenic determinants. More important, however, is the observation that microinjected PAb 122 inhibits the transition of Swiss 3T3 cells from G0 to S phase, but has no effect on the progression of these cells from mitosis to the S phase.

Cellular DNA replication is independent of the synthesis or accumulation of ribosomal RNA

We have used an antibody against RNA polymerase I to investigate the role of rRNA synthesis and/or accumulation in the control of cell proliferation. The antibody was microinjected directly into the nuclei of quiescent Swiss 3T3 cells that were subsequently stimulated with serum. Under the experimental conditions used, the microinjection of the antibody against RNA polymerase I (RNA pol I) caused a 50-70% decrease in nucleolar RNA synthesis that lasted for at least 17 h, a greater than 90% inhibition in the accumulation of nucleolar RNA, and a 70% inhibition in the accumulation of total cellular RNA. A control IgG, similarly microinjected into Swiss 3T3 cells had no inhibitory effect on either the synthesis or accumulation of nucleolar and cellular RNA. Despite the dramatic effect on the synthesis and accumulation of ribosomal RNA (rRNA) the antibody against RNA (rRNA) the antibody against RNA pol I was totally ineffective in inhibiting the entry into S phase of serum-stimulated Swiss 3T3 cells. Cells depleted of cellular RNA by metaphase arrest also entered S phase with subnormal amounts of cellular RNA. The results of these experiments clearly indicate that a normal rate of nucleolar RNA synthesis, and a normal rate of accumulation of total cellular RNA are not a prerequisite for the entry of cells into S phase.