G1 control in yeast and animal cells

In budding yeast, Saccharomyces cerevisiae, the cell cycle is controlled at the G1/S phase transition by regulating the activity of the CDC28 protein kinase. This is the budding yeast homologue of the cdc2 protein kinase associated in most organisms with control of mitosis. In budding yeast CDC28 controls both the G1/S phase transition and the G2/M phase transition by being differentially activated by two distinct classes of positive regulatory subunits known as G1 cyclins or CLNs and B-type cyclins or CLBs, respectively. To establish whether a similar dual role for Cdc2-related kinases exists in animal cells, we and others have sought human homologues of yeast G1 cyclins. Of several candidates, cyclin E is the most promising in that it accumulates prior to S phase and is associated with a pre-S phase protein kinase activity. The kinetics of accumulation of cyclin E-associated protein kinase activity is consistent with a role at the mammalian cell cycle restriction point.

Id3 is a novel regulator of p27kip1 mRNA in early G1 phase and is required for cell-cycle progression

P27kip is a key inhibitory protein of the cell-cycle progression, which is rapidly downregulated in early G1 phase by a post-translational mechanism involving the proteosomal degradation. In this study, using a wounding model that induces cell-cycle entry of human dermal fibroblasts, we demonstrate that p27mRNA is downregulated when cells progress into the G1 phase, and then it returns to its basal level when cells approach the S phase. By using a quantitative polymerase chain reaction screening we identified inhibitors of differentiation (Id3), a bHLH transcriptional repressor, as a candidate mediator accounting for p27 mRNA decrease. Id3 silencing, using an small interfering RNA approach, reversed the injury mediated p27 downregulation demonstrating that Id3 is involved in the transcriptional repression of p27. Reporter gene experiments and a chromatin immunoprecipitation assay showed that Id3 likely exerts its repressive action through ELK1 inhibition. By inhibiting early p27 downregulation, Id3 depletion blocked (i) the G1-phase progression as assessed by the inhibition of pRb phosphorylation and p130 degradation and (ii) the G1/S transition as observed by the inhibition of cyclin A induction, demonstrating that p27 mRNA decrease is required for cell proliferation. Apart from its effect on the early p27 diminution, Id3 appears also involved in the control of the steady-state level of p27 at the G1/S boundary. In conclusion, this study identifies a novel mechanism of p27 regulation which besides p27 protein degradation also implicates a transcriptional mechanism mediated by Id3.

[When chromosomal dynamics control cell division]

In most tumor cells a chromosomal instability leads to an abnormal chromosome number (aneuploidy). The mitotic checkpoint is essential for ensuring accurate chromosome segregation by allowing mitotic delay in response to a spindle defect. This checkpoint delays the onset of anaphase until all the chromosomes are correctly aligned on the mitotic spindle. When unattached kinetochores are present, the metaphase/anaphase transition is not allowed and the time available for chromosome-microtubule capture increases. Genes required for this delay were first identified in Saccharomyces cerevisiae (the MAD, BUB and MPS1 genes) and subsequently, homologs have been identified in higher eucaryotes showing that the spindle checkpoint pathway is highly conserved. The checkpoint functions by preventing an ubiquitin ligase called the anaphase-promoting complex/cyclosome (APC) from ubiquitinylating proteins whose destruction is required for anaphase onset.

Expression and DNA-binding activity of C/EBPalpha and C/EBPbeta in human liver and differentiated primary hepatocytes

BACKGROUND/AIMS

Limited information is available on the expression and role of C/EBP factors in human liver and hepatocytes. We investigated the expression and DNA-binding activity of C/EBPalpha and C/EBPbeta in human liver needle biopsies, surgical lobectomies and differentiated cultured hepatocytes derived from lobectomies.

METHODS

RNA and protein extracts were analyzed by RNAse protection, immunoblot and gel shift assays.

RESULTS

C/EBP mRNAs, isoforms and DNA-binding activities were low/undetectable in lobectomies. In contrast, several C/EBPalpha (47, 45, 35 and 33 kDa) and C/EBPbeta isoforms (47, 43, 40, 35 and 21 kDa) were observed in needle biopsies. In cultured hepatocytes, the C/EBP expression pattern dramatically changed with time. C/EBPalpha mRNA and the 45 kDa isoform increased in parallel, reaching a maximum after 3-4 weeks coincident with weak DNA-binding activity. C/EBPbeta mRNA and isoform expression increased rapidly reaching a plateau within 1-2 weeks; all C/EBPbeta isoforms were phosphorylated. C/EBPbeta exhibited greater DNA-binding activity than C/EBPalpha, and this activity paralleled C/EBPbeta isoform expression.

CONCLUSIONS

C/EBP isoforms exhibit markedly different expression patterns in lobectomies, needle biopsies and cultured hepatocytes. Stress stimuli during and/or after surgery for lobectomy resections may account for this difference. The pattern of C/EBP isoform expression in long-term highly differentiated cultured hepatocytes is close to that observed in needle biopsies.

The Ets2 transcription factor inhibits apoptosis induced by colony-stimulating factor 1 deprivation of macrophages through a Bcl-xL-dependent mechanism

Bcl-xL, a member of the Bcl-2 family, inhibits apoptosis, and its expression is regulated at the transcriptional level, yet nothing is known about the transcription factors specifically activating this promoter. The bcl-x promoter contains potential Ets binding sites, and we show that the transcription factor, Ets2, first identified by its sequence identity to v-ets of the E26 retrovirus, can transactivate the bcl-x promoter. Transient expression of Ets2 results in the upregulation of Bcl-xL but not of Bcl-xS, an alternatively spliced gene product which induces apoptosis. Ets2 is ubiquitously expressed at low levels in a variety of cell types and tissues but is specifically induced to abundant levels during macrophage differentiation. Since Bcl-xL is also upregulated during macrophage differentiation, we asked whether the bcl-x could be a direct downstream target gene of Ets2 in macrophages. BAC1.2F5 macrophages, which are dependent on macrophage colony-stimulating factor 1 (CSF-1) for their growth and survival, were used in these studies. We show that CSF-1 stimulation of BAC1.2F5 macrophages results in the upregulation of expression of ets2 and bcl-xL with similar kinetics of induction. In the absence of CSF-1, these macrophages undergo cell death by apoptosis, whereas constitutive expression of Ets2 rescues these cells from cell death, and bcl-xL is upregulated. These results strongly suggest a novel role of Ets2 in affecting apoptosis through its regulation of Bcl-xL transcription.

Early G1 growth arrest of hybridoma B cells by DMSO involves cyclin D2 inhibition and p21[CIP1] induction

Dimethylsulfoxide (DMSO) was shown to inhibit the proliferation of several B cell lines including Raji, Daudi, and SKW6-CL4 but the mechanisms involved in this growth arrest are still unclear. We show that in 7TD1 mouse hybridoma cells a DMSO-induced reversible G1 arrest involves inactivation of Rb kinases, cyclin D2/CDK4 and cyclin E/CDK2. This occurs by at least three distinct mechanisms. Inhibition of cyclin D2 neosynthesis leads to a dramatic decrease of cyclinD2/CDK4 complexes. This in turn enables the redistribution of p27[KIP1] from cyclin D2/CDK4 to cyclin E/CDK2 complexes. In addition, the simultaneous accumulation of p21[CIP1] entails increasing association with cyclin D3/CDK4 and cyclin E/CDK2. Thus, p21[CIP1] and p27[KIP1], act in concert to inhibit cyclin E/CDK2 activity which, together with CDK4 inactivation, confers a G1-phase arrest.

A cell cycle-regulated inhibitor of cyclin-dependent kinases

Cyclin-dependent kinases (Cdks) previously have been shown to drive the major cell cycle transitions in eukaryotic organisms ranging from yeast to humans. We report here the identification of a 28-kDa protein, p28Ick (inhibitor of cyclin-dependent kinase), that binds to and inhibits the kinase activity of preformed Cdk/cyclin complexes from human cells. p28 inhibitory activity fluctuates during the cell cycle with maximal levels in G1 and accumulates in G1- and G0-arrested cells. These results suggest that control of the G1/S transition may be influenced by a family of Cdk inhibitors that include p28Ick and the recently described inhibitors p21Cip1/Waf1/Cap20 and p16Ink4.

G1 control in mammalian cells

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.

Cyclin E/cdk2 and cyclin A/cdk2 kinases associate with p107 and E2F in a temporally distinct manner

Cyclin E is classified as a putative G1 cyclin on the basis of its cyclic pattern of mRNA expression, with maximal levels being detected near the G1/S boundary. We report here that cyclin E is found associated with the transcription factor E2F in a temporally regulated fashion. E2F is known to be a critical transcription factor for the expression of some S phase-specific proteins and is thought to be important for a series of others. Antisera specific for cyclin E were raised and used to demonstrate an association between cyclin E and E2F. This cyclin E/E2F complex was seen in a variety of human cell lines from various tissues, but its appearance was detected primarily during the G1 phase of the cell cycle. The cyclin E/E2F association decreased as cells entered S phase, just as the association of E2F with cyclin A became detectable. We characterized the cyclin E-E2F complex further to show that both the cyclin-dependent kinase-2 (cdk2) and p107 were present. Therefore, the p107/E2F complex is associated with two different cdk2 kinase complexes--one containing cyclin A and the other containing cyclin E--and the appearance of these complexes is temporally regulated during the cell cycle. The presence of cyclin E/E2F complexes in the G1 phase suggests a role for cyclin E in the control of genes required for the G1-to-S transition.

Regulation of retinoblastoma protein functions by ectopic expression of human cyclins

The retinoblastoma susceptibility gene (RB) product, the retinoblastoma protein (pRb), functions as a regulator of cell proliferation. Introduction of the RB gene into SAOS-2 osteosarcoma cells, which lack functional pRb, prevents cell cycle progression. Such growth-suppressive functions can be modulated by phosphorylation of pRb, which occurs via cell cycle-regulated kinases. We show that constitutively expressed cyclins A and E can overcome pRb-mediated suppression of proliferation. pRb becomes hyperphosphorylated in cells overexpressing these cyclins, and this phosphorylation is essential for cyclin A- and cyclin E-mediated rescue of pRb-blocked cells. This suggests that G1 and S phase cyclins can act as regulators of pRb function in the cell cycle by promoting pRb phosphorylation.

Induction of cyclin mRNA and cyclin-associated histone H1 kinase during liver regeneration

Cyclins and cyclin-associated cdc kinases are key regulators of oocyte maturation (Maller, J. L. (1990) in The Biology and Medicine of Signal Transduction (Nishizuka, Y., Endo, M., and Tanaka, C., eds) pp. 323-328, Raven Press, New York), yeast cell cycles (Nurse, P. (1990) Nature 344, 503-508), DNA replication in cell-free systems (D'Urso, F., Marraccino, R. L., Marshak, R. R., and Roberts, J. M. (1990) Science 250, 786-791), and amphibian cell proliferative transitions (Hunt, T. (1991) Nature 350, 462-463). The extent to which these regulatory molecules participate in the growth control of differentiated epithelial cells like hepatocytes is unknown. Therefore, we investigated the expression of "G1" (E, C, and D) and "G2/M" (A, B1, and B2) cyclin mRNAs, the relative levels of cyclin A- and B1-associated histone H1-kinase activity, and the appearance of cyclin-associated kinases (p32/p33cdk2 and p33/p34cdc2) in regenerating rat liver and in control tissues from sham hepatectomized rats. To do this, we exploited a battery of human cyclin cDNAs and cyclin antisera that recognize rat molecules. The results suggest an apparent sequence of regeneration-specific changes: 1) elevated and induced expression of cyclins E (2.1 kilobases (kb)) and C (4 kb), and D mRNAs (4 kb), within 12 h, respectively; 2) induction of cyclins A (3.4 and 1.8 kb), B1 (2.5 and 1.8 kb), and B2 (1.9 kb) mRNAs at 24 h; 3) induction of cyclin A- and B1-associated nuclear histone H1 kinase at 24 h; and 4) enhanced levels of PSTAIRE-containing proteins of Mr approximately 32-33 and 33-34 kDa in nuclear extracts from 24-h regenerating liver that co-immunoprecipitate with cyclin A and B1 antisera, respectively. These observations provide an intellectual framework that unifies the biology of hepatocyte mitogenesis, proto-oncogene expression, and the machinery of the cell cycle.

G1 control in yeast and animal cells

In yeast G1, cyclins control the Cdc28 protein kinase in order to regulate the primary cell cycle gating event known as START. Environmental and internal signals that control the cell cycle do so, apparently, by controlling the synthesis and/or stability of G1 cyclins, hence controlling the activity of the Cdc28 kinase. The substrates of the Cdc28 kinase that are critical for passage through START are not known. One simple hypothesis is that the G1 kinase phosphorylates and thus activates a transcription factor required for the initiation of S phase. The synthesis of an origin of replication-binding factor might be regulated in this fashion. Recent evidence suggests that Cdc28 protein kinase activity directly regulates the transcription of a family of genes whose products are required for DNA replication (N. Marini and S. Reed, in prep.). However, it is not yet known whether this transcriptional activation constitutes the execution of START. The situation in animal cells is more complex. A number of new cyclins and p34s have been identified. It is not clear yet which of these, if any, have functions in G1 and if they do, what functions these might be. If G1 cyclins and p34 kinases do have critical G1 roles, by analogy with yeast, they may couple signals mediated by both positive and negative growth factors to cell cycle progression. Candidates for the critical G1 substrates of these putative G1 protein kinases are the tumor suppressors such as the RB (retinoblastoma) gene product (p105RB).(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of aging of rat muscle glyceraldehyde-3-phosphate dehydrogenase studied by selective enzyme-oxidation

Controlled oxidation of rat muscle glyceraldehyde-3-phosphate dehydrogenase (GPDH) was carried out in an attempt to stimulate age-related effects observed in enzyme samples purified from old animals. A comparative study of the "simulated aged" and of native young and old GPDH forms was done using fluorescence techniques. The present work is based on our previous findings that the locus of the age-related modifications in GPDH is in the nicotinamide-binding site, where the catalytically active Cys-149 residue is located, and that an increase in oxidation potential occurs in old animal tissues which may enable various oxidizing agents to play a significant role in the inactivation of certain enzymes. Thus it has been suggested that the loss of specific activity observed in old GPDH may be due to subtle and irreversible conformational changes caused by reaction of Cys-149 with these agents. The circularly polarized luminescence (CPL) spectrum emitted by the fluorescent sulfhydryl reagent I-AEDANS covalently bound to GPDH through Cys-149 at the nicotinamide binding site, revealed a significant difference in conformation between these sites in young and old GPDH forms. Large differences were also observed between corresponding spectra when the binding sites were saturated with NAD+, reflecting the development of marked conformational changes in both young and old GPDH species upon coenzyme binding. The oxidizing reagents employed in the current study (hydrogen peroxide, superoxide radical and atmospheric dioxygen) are all expected to be more commonly encountered in the less reducing environment of old animal tissues. All of them, though to a different extent, caused a significant inactivation of the enzyme dependent on the initial oxidant concentration. Although the original enzymatic activity could be partially restored by incubation with a reducing agent, the prior oxidation was found to induce some irreversible structural changes as expressed in a decrease in the number of fast reacting SH groups. The extent of irreversible inactivation was a function of both oxidant concentration and the duration of exposure to the oxidant. The affinity of the oxidized GPDH species (termed "aged") toward coenzyme, as monitored by fluorometric titrations, was markedly lower than that observed for both the native young and old GPDHs. In addition, the CPL spectra of the "aged" enzymes were different from those obtained for both native forms.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of the Fc epsilon receptor in calcium channel opening in rat basophilic leukemia cells

The role of the Fc epsilon, receptor (Fc epsilon R), isolated from rat basophilic leukemia cells (line RBL-2H3) in antigen induced Ca++ channel opening has been studied by following ion conductance in reconstituted model membranes. Planar bilayers were constructed from lipid vesicles containing the purified Fc epsilon R alone, or together with the cromolyn binding protein (CBP). Changes in conductivity of these bilayers were measured as a monitor for channel activity, following specific aggregation of Fc epsilon R. Antigen-induced, Fc epsilon R mediated channel activity could only be elicited in membranes containing both proteins. This conductance was abrogated upon disaggregating the complexes with a monovalent hapten (epsilon-N-DNP-L-lysine). No channel activity was observed following antigen-induced aggregation of Fc epsilon R if CBP was not present in the bilayer. The single channels recorded were of approximately equal to 2 pS conductance. The open-time values varied significantly with individual experiments and depended on the protein composition of the membrane and the nature of the aggregating agent. These observations strongly indicate that the Fc epsilon R isolated from RBL cells does not form cation (Ca++) channels by itself. Furthermore, in line with earlier reports, the present data suggest that the CBP is responsible for this activity, and that it interacts directly with Fc epsilon R to open channels upon aggregation.