Activation of cyclin-dependent kinases by Myc mediates induction of cyclin A, but not apoptosis

Involvement of Myc activity in a G(1)/S-promoting mechanism parallel to the pRb/E2F pathway

The retinoblastoma protein (pRb)/E2F pathway regulates commitment of mammalian cells to replicate DNA. On the other hand, mitogen-stimulated cells deprived of E2F activity can still maintain physiologically relevant levels of cyclin E-dependent kinase activity and gradually enter S phase, suggesting the existence of a DNA synthesis-inducing mechanism parallel to the pRb/E2F axis. Here we show that regulatable ectopic expression of cyclin E or transcriptionally active Myc can rapidly induce DNA synthesis in U2OS-derived cell lines whose E2F activity is blocked by a constitutively active pRb (pRbDeltacdk) mutant. The effect of Myc is associated with Cdc25A phosphatase and cyclin E-CDK2 kinase activation and abolished by antagonizing Myc activity with the dominant-negative (dn) MadMyc chimera. Moreover, while abrogation of either endogenous E2F or Myc activity only delays and lowers DNA synthesis in synchronized U2OS cells or rat diploid fibroblasts, concomitant neutralization of both abolishes it. Whereas ectopic Myc and E2F1 rescue the G(1)/S delay caused by pRbDeltacdk (or dnDP1) and MadMyc, respectively, cyclin E or Cdc25A can restore DNA replication even in cells concomitantly exposed to pRbDeltacdk and MadMyc. However, coexpression of dnCDK2 neutralizes all of these rescuing effects. Finally, proper transcription of cyclin E and Cdc25A at the G(1)/S transition requires both Myc and E2F activities, and subthreshold levels of ectopic cyclin E and Cdc25A synergistically restore DNA synthesis in cells with silenced Myc and E2F activities. These results suggest that Myc controls a G(1)/S-promoting mechanism regulating cyclin E-CDK2 in parallel to the "classical" pRb/E2F pathway.

Abundance of cyclin B1 regulates γ-radiation–induced apoptosis

γ-Radiation is a potent inducer of apoptosis. There are multiple pathways regulating DNA damage-induced apoptosis, and we set out to identify novel mechanisms regulating γ-radiation–induced apoptosis in hematopoietic cells. In this report, we present data implicating the cyclin B1 protein as a regulator of apoptotic fate following DNA damage. Cyclin B1 is the regulatory subunit of the cdc2 serine/threonine kinase, and accumulation of cyclin B1 in late G2 phase of the cell cycle is a prerequisite for mitotic initiation in mammalian cells. We find that abundance of the cyclin B1 protein rapidly increases in several mouse and human hematopoietic cells (Ramos, DP16, HL60, thymocytes) undergoing γ-radiation–induced apoptosis. Cyclin B1 accumulation occurs in all phases of the cell cycle. Antisense inhibition of cyclin B1 accumulation decreases apoptosis, and ectopic cyclin B1 expression is sufficient to induce apoptosis. These observations are consistent with the idea that cyclin B1 is both necessary and sufficient for γ-radiation-induced apoptosis.

Abundance of cyclin B1 regulates γ-radiation–induced apoptosis

γ-Radiation is a potent inducer of apoptosis. There are multiple pathways regulating DNA damage-induced apoptosis, and we set out to identify novel mechanisms regulating γ-radiation–induced apoptosis in hematopoietic cells. In this report, we present data implicating the cyclin B1 protein as a regulator of apoptotic fate following DNA damage. Cyclin B1 is the regulatory subunit of the cdc2 serine/threonine kinase, and accumulation of cyclin B1 in late G2 phase of the cell cycle is a prerequisite for mitotic initiation in mammalian cells. We find that abundance of the cyclin B1 protein rapidly increases in several mouse and human hematopoietic cells (Ramos, DP16, HL60, thymocytes) undergoing γ-radiation–induced apoptosis. Cyclin B1 accumulation occurs in all phases of the cell cycle. Antisense inhibition of cyclin B1 accumulation decreases apoptosis, and ectopic cyclin B1 expression is sufficient to induce apoptosis. These observations are consistent with the idea that cyclin B1 is both necessary and sufficient for γ-radiation-induced apoptosis.

v-Jun overrides the mitogen dependence of S-phase entry by deregulating retinoblastoma protein phosphorylation and E2F-pocket protein interactions as a consequence of enhanced cyclin E-cdk2 catalytic activity

v-Jun accelerates G(1) progression and shares the capacity of the Myc, E2F, and E1A oncoproteins to sustain S-phase entry in the absence of mitogens; however, how it does so is unknown. To gain insight into the mechanism, we investigated how v-Jun affects mitogen-dependent processes which control the G(1)/S transition. We show that v-Jun enables cells to express cyclin A and cyclin A-cdk2 kinase activity in the absence of growth factors and that deregulation of cdk2 is required for S-phase entry. Cyclin A expression is repressed in quiescent cells by E2F acting in conjunction with its pocket protein partners Rb, p107, and p130; however, v-Jun overrides this control, causing phosphorylated Rb and proliferation-specific E2F-p107 complexes to persist after mitogen withdrawal. Dephosphorylation of Rb and destruction of cyclin A nevertheless occur normally at mitosis, indicating that v-Jun enables cells to rephosphorylate Rb and reaccumulate cyclin A without exogenous mitogenic stimulation each time the mitotic "clock" is reset. D-cyclin-cdk activity is required for Rb phosphorylation in v-Jun-transformed cells, since ectopic expression of the cdk4- and cdk6-specific inhibitor p16(INK4A) inhibits both DNA synthesis and cell proliferation. Despite this, v-Jun does not stimulate D-cyclin-cdk activity but does induce a marked deregulation of cyclin E-cdk2. In particular, hormonal activation of a conditional v-Jun-estrogen receptor fusion protein in quiescent, growth factor-deprived cells stimulates cyclin E-cdk2 activity and triggers Rb phosphorylation and DNA synthesis. Thus, v-Jun overrides the mitogen dependence of S-phase entry by deregulating Rb phosphorylation, E2F-pocket protein interactions, and ultimately cyclin A-cdk2 activity. This is the first report, however, that cyclin E-cdk2, rather than D-cyclin-cdk, is likely to be the critical Rb kinase target of v-Jun.

Ceramide-induced cell death is independent of the Fas/Fas ligand pathway and is prevented by Nur77 overexpression in A20 B cells

The role of ceramide in triggering apoptosis is still a matter of debate. While in some experimental systems, ceramide was shown to mediate Fas-induced cell death, in other instances it was claimed to induce the expression of Fas ligand (FasL), killing cells in a caspase-dependent fashion. We found that, in mature A20 B cells, ceramide-induced apoptosis is independent of the caspase pathway, since we observed no ICE-like, CPP32-like and Mch2 activities and no PARP proteolysis. Moreover, we were unable to protect these cells from ceramide-induced apoptosis using caspase inhibitors, while they blocked Fas-induced apoptosis and no FasL induction could be detected following ceramide treatment. These results suggest that ceramide does not induce apoptosis through the Fas/FasL pathway. We also found that overexpression of Nur77, a zinc-finger transcription factor described to upregulate FasL, antagonizes ceramide-induced apoptosis, but not Fas-induced apoptosis. This further supports the hypothesis that Fas and ceramide death pathways are independent in A20 cells. Ceramide-induced cell death was associated with increased c-myc, p53, Bax and p27kip1 levels; in contrast, cells transfected with Nur77 (A20Nur77), resistant to ceramide-induced apoptosis, showed a marked downregulation of p53 after ceramide treatment, with neither Bax nor p27kip1 induction. In conclusion, our results suggest that, in the A20 B cell line, Fas and ceramide trigger two distinct pathways and that Nur77 overexpression confers protection against ceramide-mediated apoptosis which correlates with inhibition of p53, Bax and p27kip1 induction.

Cell proliferation and apoptosis

Cell proliferation and cell death are essential yet opposing cellular processes. Crosstalk between these processes promotes a balance between proliferation and death, and it limits the growth and survival of cells with oncogenic mutations. New insights into the mechanisms by which strong signals to proliferate and activation of cyclin-dependent kinases promote apoptosis have recently been published, and a novel cell cycle regulated caspase inhibitor, Survivin, has been described.

Function of the c-Myc oncogenic transcription factor

The c-myc gene and the expression of the c-Myc protein are frequently altered in human cancers. The c-myc gene encodes the transcription factor c-Myc, which heterodimerizes with a partner protein, termed Max, to regulate gene expression. Max also heterodimerizes with the Mad family of proteins to repress transcription, antagonize c-Myc, and promote cellular differentiation. The constitutive activation of c-myc expression is key to the genesis of many cancers, and hence the understanding of c-Myc function depends on our understanding of its target genes. In this review, we attempt to place the putative target genes of c-Myc in the context of c-Myc-mediated phenotypes. From this perspective, c-Myc emerges as an oncogenic transcription factor that integrates the cell cycle machinery with cell adhesion, cellular metabolism, and the apoptotic pathways.

Altered expression of cell cycle proteins and prolonged duration of cardiac myocyte hyperplasia in p27KIP1 knockout mice

-The precise role of cell cycle-dependent molecules in controlling the switch from cardiac myocyte hyperplasia to hypertrophy remains to be determined. We report that loss of p27(KIP1) in the mouse results in a significant increase in heart size and in the total number of cardiac myocytes. In comparison to p27(KIP1)+/+ myocytes, the percentage of neonatal p27(KIP1)-/- myocytes in S phase was increased significantly, concomitant with a significant decrease in the percentage of G(0)/G(1) cells. The expressions of proliferating cell nuclear antigen, G(1)/S and G(2)/M phase-acting cyclins, and cyclin-dependent kinases (CDKs) were upregulated significantly in ventricular tissue obtained from early neonatal p27(KIP1)-/- mice, concomitant with a substantial decrease in the expressions of G(1) phase-acting cyclins and CDKs. Furthermore, mRNA expressions of the embryonic genes atrial natriuretic factor and alpha-skeletal actin were detectable at significant levels in neonatal and adult p27(KIP1)-/- mouse hearts but were undetectable in p27(KIP1)+/+ hearts. In addition, loss of p27(KIP1) was not compensated for by the upregulation of other CDK inhibitors. Thus, the loss of p27(KIP1) results in prolonged proliferation of the mouse cardiac myocyte and perturbation of myocyte hypertrophy.

p53-independent apoptosis induced by muscle differentiation stimuli in polyomavirus large T-expressing myoblasts

Abnormal proliferation signals, driven by cellular or viral oncogenes, can result in the induction of apoptosis under sub-optimal cell growth conditions. The tumor suppressor p53 plays a central role in mediating oncogene-induced apoptosis, therefore transformed cells lacking p53 are generally resistant to apoptosis-promoting treatments. In a previous work we have reported that the expression of polyomavirus large T antigen causes apoptosis in differentiating myoblasts and that this phenomenon is dependent on the onset of muscle differentiation in the absence of a correct cell cycle arrest. Here we report that polyomavirus large T increases the levels and activity of p53, but these alterations are not involved in the apoptotic mechanism. Apoptosis in polyomavirus large T-expressing myoblasts is not prevented by the expression of a p53 dominant-negative mutant nor it is increased by p53 over-expression. Moreover, forced differentiation induced through the over-expression of the muscle regulatory factor MyoD, leads to apoptosis without altering p53 function and, more significantly, even in a p53-null background. Our results indicate that apoptosis induced by the activation of muscle differentiation pathways in oncogene-expressing cells can occur in a p53-independent manner.

c-Myc regulates cyclin D-Cdk4 and -Cdk6 activity but affects cell cycle progression at multiple independent points

c-myc is a cellular proto-oncogene associated with a variety of human cancers and is strongly implicated in the control of cellular proliferation, programmed cell death, and differentiation. We have previously reported the first isolation of a c-myc-null cell line. Loss of c-Myc causes a profound growth defect manifested by the lengthening of both the G1 and G2 phases of the cell cycle. To gain a clearer understanding of the role of c-Myc in cellular proliferation, we have performed a comprehensive analysis of the components that regulate cell cycle progression. The largest defect observed in c-myc-/- cells is a 12-fold reduction in the activity of cyclin D1-Cdk4 and -Cdk6 complexes during the G0-to-S transition. Downstream events, such as activation of cyclin E-Cdk2 and cyclin A-Cdk2 complexes, are delayed and reduced in magnitude. However, it is clear that c-Myc affects the cell cycle at multiple independent points, because restoration of the Cdk4 and -6 defect does not significantly increase growth rate. In exponentially cycling cells the absence of c-Myc reduces coordinately the activities of all cyclin-cyclin-dependent kinase complexes. An analysis of cyclin-dependent kinase complex regulators revealed increased expression of p27(KIP1) and decreased expression of Cdk7 in c-myc-/- cells. We propose that c-Myc functions as a crucial link in the coordinate adjustment of growth rate to environmental conditions.

c-Myc and Mxi1 immunoreactivities in the calcifying areas of the epiphyseal-plate cartilage matrix of growing rats

We looked for the protooncogene protein, c-Myc, its dimerization partner, Max, and the repressors of its transactivation activity, Mad1 and Mxi1, in the epiphyseal-plate cartilage matrix of growing rats by immunocytochemistry in the electron microscope. c-Myc and Mxi1 immunoreactivities were found in the calcifying areas of the cartilage matrix only. There was no immunolabeling in response to anti-Max or anti-Mad1 antibodies. Mxi1 immunoreactivity was mainly in the early calcifying areas, in the calcification front and ahead of it, whereas c-Myc immunoreactivity was essentially in the incompletely calcified regions of the matrix. The two immunolabelings occurred mainly over the large type II collagen fibrils of the cartilage matrix and over the thin filaments connecting them. c-Myc and Mxi1 immunoreactivities were rarely found along the dark cristallites. There was no immunolabeling associated with the matrix vesicles, or in their immediate surroundings. The data suggest that the protooncogene proteins, c-Myc and Mxi1, could be implicated in the calcification involving type II collagen fibrils of the epiphyseal-plate cartilage. The absence of Max immunoreactivity from the calcifying cartilage matrix raises the question of whether there are other c-Myc- and Mxi1-dimerization partners.

Mechanisms of apoptosis by c-Myc

Much recent research on c-Myc has focused on how it drives apoptosis. c-Myc is widely known as a crucial regulator of cell proliferation in normal and neoplastic cells, but until relatively recently its apoptotic properties, which appear to be intrinsic, were not fully appreciated. Its death-dealing aspects have gained wide attention in part because of their potential therapeutic utility in advanced malignancy, where c-Myc is frequently deregulated and where novel modalities are badly needed. Although its exact function remains obscure, c-Myc is a transcription factor and advances have been made in characterizing target genes which may mediate its apoptotic properties. Candidate regulators and effectors are also emerging. Among recent findings are connections to the CD95/Fas and TNF pathways and roles for the tumor suppressor p19ARF and the c-Myc-interacting adaptor protein Binl in mediating cell death. In this review I summarize the data establishing a role for c-Myc in apoptosis in diverse settings and present a modified dual signal model for c-Myc function. It is proposed that c-Myc induces apoptosis through separate 'death priming' and 'death triggering' mechanisms in which 'death priming' and mitogenic signals are coordinated. Investigation of the mechanisms that underlie the triggering steps may offer new therapeutic opportunities.

Mysterious liaisons: the relationship between c-Myc and the cell cycle

A large body of physiological evidence shows that either upregulation or downregulation of intracellular c-Myc activity has profound consequences on cell cycle progression. Recent work suggests that c-Myc may stimulate the activity of cyclin E/cyclin-dependent kinase 2 (Cdk2) complexes and antagonize the action of the Cdk inhibitor p27KIP1. Cyclin D/Cdk4/6 complexes have also been implicated as targets of c-Myc activity. However, in spite of considerable effort, the mechanisms by which c-Myc interacts with the intrinsic cyclin/Cdk cell cycle machinery remain undefined.

cdc25A is necessary but not sufficient for optimal c-myc-induced apoptosis and cell proliferation of vascular smooth muscle cells

Increasing evidence indicates that the control of cell proliferation and apoptosis are linked. The c-myc proto-oncogene is induced early after cell-cycle entry in vascular smooth muscle cells (VSMCs) in vitro and after arterial injury and regulates both cell proliferation and apoptosis. Although both proliferation and apoptosis are likely to be mediated via transcriptional activation of target genes, few c-myc targets have been identified. Therefore, the recent identification that cdc25A, a cell-cycle phosphatase involved in G1 progression, is transcriptionally activated by c-myc and regulates c-myc-induced apoptosis has suggested that cdc25A may be the principal mediator of c-myc in VSMCs. We examined cdc25A regulation of c-myc-induced proliferation and apoptosis by expressing cdc25A or antisense cdc25A in primary rat VSMCs or in VSMCs expressing deregulated c-myc or adenovirus E1A. Ectopic c-myc increased cdc25A expression, but cdc25A was still responsive to serum components, which indicated that c-myc alone is not the main determinant of cdc25A expression. Antisense cdc25A inhibited c-myc-induced proliferation and apoptosis; however, drug and metabolic blocks indicated that this effect was limited to G1. Ectopic cdc25A augmented the proproliferative and proapoptotic action of c-myc but did not increase cell proliferation or apoptosis in the absence of ectopic c-myc. In contrast, E1A/E2F-induced apoptosis was independent of cdc25A. We conclude that cdc25A expression modulates the ability of c-myc to induce apoptosis in G1. However, cdc25A alone does not induce apoptosis and cannot substitute for c-myc in VSMCs. Additional targets of c-myc are therefore involved in apoptosis of both G1 and post-G1 VSMCs.

Different regulation of c-Myc- and E2F-1-induced apoptosis during the ongoing cell cycle

The transcription factors c-Myc and E2F-1 have been shown to harbour both mitogenic and apoptotic properties. Both factors have been implicated in the regulation of the transition from the G1 phase to the S phase in the mammalian cell cycle. However, whether cell death triggered by these molecules is dependent on the cell's position in the ongoing cell cycle remained elusive. Using centrifugal elutriation we here show for the first time that c-Myc induces apoptosis in G1 and in G2 phase, whereas E2F-1-induced apoptosis specifically occurs in G1. S phase cells are resistant to cell death triggered by these factors. We demonstrate that this is not a general phenomenon, since S phase cells are susceptible to apoptosis induced by treatment with actinomycin D and to the anti-apoptotic activity of Bcl-2. Our data indicate that S phase cells harbour specific protective activities against c-Myc- and E2F-1-induced apoptosis. Our results demonstrate that these transcription factors, although probably sharing specific apoptotic pathways, also take distinct routes to induce cell death and that apoptosis can occur at different phases of the cell cycle depending on the apoptotic stimulus. In this report we present the usefulness of a new approach to determine the regulation of apoptosis in the ongoing unperturbated cell cycle. This approach has clear implications for the identification of target genes involved in the regulation of cell death.

Analysis of cell cycle arrest in adipocyte differentiation

Confluent 3T3-L1 preadipocytes differentiate to adipocytes in the presence of insulin, dexamethasone, and isobutylmethylxanthine (IDI). A transient increase of DNA synthesis is induced in 3T3-L1 cells 18 h after addition of IDI, followed by an arrest in the G1 phase of the cell cycle. Growth arrested cells express the proto-oncogene c-myc and the gene for the CCAAT/enhancer binding protein (C/EBPalpha) between day 2 and 5. While c-Myc is strongly implicated in cell proliferation, C/EBPalpha: is a differentiation-specific transcription factor with antiproliferative activity. Here we have characterized the cell cycle arrest in differentiating 3T3-L1 cells. Arrested cells express the Cdk inhibitors p21 and p27, but, at the same time, show hyperphosphorylation of Rb and expression of the E2F-regulated thymidine kinase gene. The addition of new serum to arrested cells resulted in cyclin A expression and Cdk2 activity, but not in DNA synthesis. Simian virus 40 large tumor antigen (LTAg) is a potent mitogen. The mutant LTAg-K1, deficient in binding of pocket proteins and unable to induce DNA synthesis in serum-starved 3T3-L1 cells, efficiently induced DNA synthesis in differentiating 3T3-L1 cells. This indicates that pocket proteins are probably not involved in the control of the cell cycle arrest during 3T3-L1 cell differentiation. Our data suggest that the differentiation-specific cell cycle block in 3T3-L1 cells is resistant to high levels of c-Myc, inactivation of pocket proteins, upregulation of cyclin A levels, and Cdk2 activation, but can be abolished by a function of LTAg that is independent of binding to pocket proteins.

Growth arrest failure, G1 restriction point override, and S phase death of sensory precursor cells in the absence of neurotrophin-3

More than half of the dorsal root ganglion (DRG) neurons are lost by excessive cell death coinciding with precursor proliferation and cell cycle exit in neurotrophin-3 null mutant (NT-3-/-) mice. We find that in the absence of NT-3, sensory precursor cells fail to arrest the cell cycle, override the G1 phase restriction point, and die by apoptosis in S phase, which can be prevented in vivo by a cell cycle blocker. Uncoordinated cell cycle reentry is preceded by a failure of nuclear N-myc downregulation and is paralleled by the activation of the full repertoire of G1 and S phase cell cycle proteins required for cell cycle entry. Our results provide evidence for novel activity of neurotrophins in cell cycle control and point toward an N-myc sensitization to cell death in the nervous system that is under the control of NT-3.

Activation of phosphatidylinositol 3-kinase is sufficient for cell cycle entry and promotes cellular changes characteristic of oncogenic transformation

Using a new inducible form of phosphatidylinositol 3-kinase (PI 3-kinase) we have found that PI 3-kinase activation has the following effects on cell growth and proliferation. (i) Activation of PI 3-kinase was sufficient to promote entry into S phase of the cell cycle within several hours. This was shown by activation of cyclin-dependent kinase 4 (Cdk4) and Cdk2 and by the induction of DNA synthesis. (ii) PI 3-kinase activation alone was not, however, sufficient to provide for progression through the entire cell cycle. Instead, prolonged activation of PI 3-kinase in the absence of serum stimulation resulted in apoptosis. It is possible that the cells undergo apoptosis because the PI 3-kinase-induced entry into the cell cycle is abnormal. For example, we found that the cyclin E-Cdk2 complex, which normally disappears after entry into S phase of the cell cycle, fails to be downregulated following induction by PI 3-kinase. (iii) Finally, we found that prolonged activation of PI 3-kinase in the presence of serum resulted in cellular changes that resemble those associated with oncogenic transformation. The cells reached high densities, were irregular and refractile in appearance, and formed colonies in soft agar. In contrast, neither PI 3-kinase nor serum stimulation alone could induce these changes. Our results suggest that activation of PI 3-kinase promotes anchorage-independent cell growth and entry into the cell cycle but does not abrogate the growth factor requirement for cell proliferation.

Id2 promotes apoptosis by a novel mechanism independent of dimerization to basic helix-loop-helix factors

Members of the helix-loop-helix (HLH) family of Id proteins have demonstrated roles in the regulation of differentiation and cell proliferation. Id proteins inhibit differentiation by HLH-mediated heterodimerization with basic HLH transcription factors. This blocks their sequence-specific binding to DNA and activation of target genes that are often expressed in a tissue-specific manner. Id proteins can also act as positive regulators of cell proliferation. The different mechanisms proposed for Id-mediated promotion of entry into S phase also involve HLH-mediated interactions affecting regulators of the G1/S transition. We have found that Id2 augments apoptosis in both interleukin-3 (IL-3)-dependent 32D.3 myeloid progenitors and U2OS osteosarcoma cells. We could not detect a similar activity for Id3. In contrast to the effects of Id2 on differentiation and cell proliferation, Id2-mediated apoptosis is independent of HLH-mediated dimerization. The ability of Id2 to promote cell death resides in its N-terminal region and is associated with the enhanced expression of a known component of the programmed cell death pathway, the proapoptotic gene BAX.

Deregulated expression of CDK2- or CDK3-associated kinase activities enhances c-Myc-induced apoptosis

Activation of high ectopic levels of c-Myc in serum-deprived Rat1-MycER cells by 4-hydroxytamoxifen induces both proliferation and apoptosis. To further elucidate the role of G1 cyclin-dependent kinases (CDKs) in the process of Myc-induced apoptosis, we generated Rat1-MycER cells stably overexpressing CDK2 or CDK3. Ectopic expression of these CDKs in Myc-overexpressing cells was accompanied by upregulation of the specific kinase activities. Whereas neither high ectopic CDK2 nor CDK3 alone induced apoptosis in serum-deprived Rat1 cells, both CDKs markedly elevated the incidence of Myc-induced apoptosis. It was shown earlier that in Rat1-MycER cells, which are resistant to tumor necrosis factor-alpha (TNF) when grown in high serum concentrations, the addition of TNF with the concomitant activation of Myc resulted in apoptotic cell death. Here, we show that neither CDK2 nor CDK3 induces susceptibility to the cytotoxic action of TNF in Rat1 cells. However, both molecules heavily elevated the incidence of apoptosis induced by TNF together with Myc. It has earlier been reported that Myc-induced apoptosis in serum-deprived Rat1 fibroblasts is inhibited by specific cytokines, such as platelet-derived growth factor (PDGF). Here, we demonstrate that PDGF-mediated protection from Myc-induced apoptosis is almost lost in Rat1 cells overexpressing CDK2 or CDK3. These apoptotic effects of CDK2 or CDK3 are not accompanied by alterations of proliferation parameters, such as DNA distribution, time the cells spend in each phase of the cell cycle, thymidine incorporation into DNA, or cell size analyzed during Myc-induced apoptosis. However, we found CDK3 to deregulate E2F-dependent transcription. In this report, we provide evidence for a not yet described property of CDK2 or CDK3 besides their activity in promoting proliferation: these G1-CDKs can promote apoptosis by interfering with the cell's response to survival factors.

A matter of life and cell death

In multicellular organisms, mutations in somatic cells affecting critical genes that regulate cell proliferation and survival cause fatal cancers. Repair of the damage is one obvious option, although the relative inconsequence of individual cells in metazoans means that it is often a "safer" strategy to ablate the offending cell. Not surprisingly, corruption of the machinery that senses or implements DNA damage greatly predisposes to cancer. Nonetheless, even when oncogenic mutations do occur, there exist potent mechanisms that limit the expansion of affected cells by suppressing their proliferation or triggering their suicide. Growing understanding of these innate mechanisms is suggesting novel therapeutic strategies for cancer.

Cyclin A2 and c-myc mRNA expression in ethinyl estradiol induced liver proliferation

The time-course of c-myc and cyclin A2 mRNA expression was determined in the liver of male Sprague-Dawley rats during transient liver cell proliferation induced by a single dose of ethinyl estradiol (EE), and was compared to that during liver regeneration following two-thirds hepatectomy (PH). Cell proliferation was assessed in terms of 5'-bromodeoxyuridine (BrdU) labeling. EE administration and PH both increased BrdU labeling between 18 and 48 h, with peak values at 18 and 24 h. An early (2 h) increase in BrdU labeling was observed after EE but not PH. Maximal increases in cyclin A2 mRNA levels and BrdU labeling coincided after both EE and PH, and cyclin A2 mRNA expression was proportional to the intensity of the proliferative response. In contrast, the degree of c-myc mRNA expression was similar after EE administration and PH, but the time course was different: c-myc gene expression rose concomitantly with DNA replication after EE, while after PH it increased during the prereplicative phase. This indicates that the pattern of c-myc gene expression in the liver is strongly related to the type of proliferative response.

c-Myc or cyclin D1 mimics estrogen effects on cyclin E-Cdk2 activation and cell cycle reentry

Estrogen-induced progression through G1 phase of the cell cycle is preceded by increased expression of the G1-phase regulatory proteins c-Myc and cyclin D1. To investigate the potential contribution of these proteins to estrogen action, we derived clonal MCF-7 breast cancer cell lines in which c-Myc or cyclin D1 was expressed under the control of the metal-inducible metallothionein promoter. Inducible expression of either c-Myc or cyclin D1 was sufficient for S-phase entry in cells previously arrested in G1 phase by pretreatment with ICI 182780, a potent estrogen antagonist. c-Myc expression was not accompanied by increased cyclin D1 expression or Cdk4 activation, nor was cyclin D1 induction accompanied by increases in c-Myc. Expression of c-Myc or cyclin D1 was sufficient to activate cyclin E-Cdk2 by promoting the formation of high-molecular-weight complexes lacking the cyclin-dependent kinase inhibitor p21, as has been described, following estrogen treatment. Interestingly, this was accompanied by an association between active cyclin E-Cdk2 complexes and hyperphosphorylated p130, identifying a previously undefined role for p130 in estrogen action. These data provide evidence for distinct c-Myc and cyclin D1 pathways in estrogen-induced mitogenesis which converge on or prior to the formation of active cyclin E-Cdk2-p130 complexes and loss of inactive cyclin E-Cdk2-p21 complexes, indicating a physiologically relevant role for the cyclin E binding motifs shared by p130 and p21.

Control of cell proliferation by Myc proteins

Taken together, the available data appear to be consistent with a model in which Myc proteins function downstream of D-type cyclins and synergize with E2F proteins in the activation of the cyclin E/cdk2 kinase. This view of Myc proteins appears strikingly similar to established models for the E2F/DP family of proteins. However, it should be noted that there are clear differences and several predictions of such a model that have been critically tested for E2F proteins are still untested for Myc in this model. First, it appears that at least some target genes of Myc implicated in this process are still unknown; second, clear data from knockout cells that link p107 to Myc function are missing; and third, we are not aware of studies of tumour samples that clarify whether mutations in myc genes relieve the requirement for mutations in the cyclin D/p16 pathway.

A critical role for cyclin C in promotion of the hematopoietic cell cycle by cooperation with c-Myc

Cyclin C, a putative G1 cyclin, was originally isolated through its ability to complement a Saccharomyces cerevisiae strain lacking the G1 cyclin gene CLN1-3. Unlike cyclins D1 and E, the other two G1 cyclins obtained by the same approach and subsequently shown to play important roles during the G1/S transition, there is thus far no evidence to support the hypothesis that cyclin C is indeed critical for the promotion of cell cycle progression. In BAF-B03 cells, an interleukin 3 (IL-3)-dependent murine pro-B-cell line, cyclin C gene mRNA was induced at the G1/S phase upon IL-3 stimulation and reached a maximal level in the S phase. Enforced expression of exogenous cyclin C in this cell line failed to alter its growth properties. In the present study, we examined whether cyclin C is capable of cooperating with the cytokine-responsive immediate-early gene products c-Myc and c-Fos in the promotion of cell proliferation. We found that cyclin C is able to cooperate functionally with c-Myc, but not c-Fos, to induce both BAF-B03 cell proliferation in a cytokine-independent fashion and the formation of cell clusters. Furthermore, cyclin C was primarily responsible for the induction of cdc2 gene expression. Our data define a novel role for cyclin C in the regulation of both the G1/S and G2/M phases of the cell cycle, and this effect appears to be independent of the activity of CDK8 in the control of transcription.

Control of cell proliferation by Myc

Myc proteins are key regulators of mammalian cell proliferation. They are transcription factors that activate genes as part of a heterodimeric complex with the protein Max. This review summarizes recent progress in understanding how Myc stimulates cell proliferation and how this might contribute to cellular transformation and tumorigenesis.

Coupling of cell growth control and apoptosis functions of Id proteins

The Id family of helix-loop-helix proteins function as negative regulators of cell differentiation and as positive regulators of G1 cell cycle control. We report here that enforced overexpression of the Id3 gene suppresses the colony-forming efficiency of primary rat embryo fibroblasts. Cotransfection with the antiapoptotic Bcl2 or BclXL gene alleviates this suppression and leads to cell immortalization. Consistent with this, enforced expression of Id genes in isolation was found to be a strong inducer of apoptosis in serum-deprived fibroblast cells. Id3-induced apoptosis was mediated at least in part through p53-independent mechanisms and could be efficiently rescued by Bcl2, BclXL, and the basic helix-loop-helix protein E47, which is known to oppose the functions of Id3 in vivo through the formation of stable heterodimers. Enforced overexpression of Id proteins has previously been shown to promote the cell cycle S phase in serum-deprived embryo fibroblasts (R. W. Deed, E. Hara, G. Atherton, G. Peters, and J. D. Norton, Mol. Cell. Biol. 17:6815-6821, 1997). The extent of apoptosis induced by loss- and gain-of-function Id3 mutants and by wild-type Id3 either alone or in combination with the Bcl2, BClXL, and E47 genes was invariably correlated with the relative magnitude of cell cycle S phase promotion. In addition, Id3-transfected cell populations displaying apoptosis and those in S phase were largely coincident in different experiments. These findings highlight the close coupling between the G1 progression and apoptosis functions of Id proteins and hint at a common mechanism for this family of transcriptional regulators in cell determination.

Estrogen regulation of cell cycle progression in breast cancer cells

Estrogens are potent mitogens in a number of target tissues including the mammary gland where they play a pivotal role in the development and progression of mammary carcinoma. The demonstration that estrogen-induced mitogenesis is associated with the recruitment of non-cycling, G0, cells into the cell cycle and an increased rate of progression through G1 phase, has focused attention on the estrogenic regulation of molecules with a known role in the control of G1-S phase progression. These experiments provide compelling evidence that estrogens regulate the expression and function of c-Myc and cyclin D1 and activate cyclin E-Cdk2 complexes, all of which are rate limiting for progression from G1 to S phase. Furthermore, these studies reveal a novel mechanism of activation of cyclin E-Cdk2 complexes whereby estrogens promote the formation of high molecular weight complexes lacking the CDK inhibitor p21. Inducible expression of either c-Myc or cyclin D1 can mimic the effects of estrogen in activating the cyclin E-Cdk2 complexes and promoting S phase entry, providing evidence for distinct c-Myc and cyclin D1 pathways in estrogen-induced mitogenesis which converge on the activation of cyclin E-Cdk2. These data provide new mechanistic insights into the known mitogenic effects of estrogens and identify potential downstream targets that contribute to their role in oncogenesis.

The MDM2 C-terminal region binds to TAFII250 and is required for MDM2 regulation of the cyclin A promoter

MDM2 proto-oncogene expression is aberrant in many human tumors. Its normal role is to modulate the functions of p53. The N terminus of MDM2 interacts with p53, whereas the properties of the rest of the molecule are poorly understood. We show that MDM2 binds to the general transcription factor TFIID in vivo. The C-terminal Ring finger interacts with TAFII250/CCG1, and the central acidic domain interacts with TBP. Expression of MDM2 activates the cyclin A gene promoter but not c-fos, showing that the effects of MDM2 are specific. Deletion of the C-terminal region of MDM2 abolishes activation, showing that the C-terminal domain of MDM2 is functionally important. We found that increasing MDM2 expression to higher levels inhibits the cyclin A promoter. Inhibition appears to result from titration of general transcription factors because MDM2 overexpression inhibits c-fos as well as other promoters in vivo and basal transcription in vitro. The mechanisms of repression of the cyclin A and fos promoters appear to be different. Cyclin A repression is lost by deleting the C terminus, whereas that of c-fos is lost by removal of the acidic domain. These results reinforce the conclusion that the C terminus of MDM2 mediates effects on the cyclin A promoter. MDM2 transformed cells contain elevated levels of cyclin A mRNA, showing that activation occurs under physiological conditions. There is a positive correlation between MDM2 binding to TAFII250 and MDM2 activation of the cyclin A promoter. The C-terminal region of MDM2, which contains the Ring finger, interacts with TAFII250 and is required for regulation of the cyclin A promoter by MDM2. Our results link the activity of MDM2, a transforming protein implicated in many human tumors, with cyclin A, a regulator of the cell cycle.

Role of Oncogenic Transcription Factor c-Myc in Cell Cycle Regulation, Apoptosis and Metabolism

The myc gene was initially discovered as a prototypical retrovirally transduced oncogene. Over the decades, abundant evidence has emerged to support a causal role for the activated cellular gene, c-myc, in animal and human tumors. The gene encodes an oncogenic helix-loop-helix leucine zipper transcription factor that acts as a heterodimer with its partner protein, Max, to activate genes regulating the cell cycle machinery as well as critical metabolic enzymes. The additional ability of c-Myc to repress transcription of differentiation-related genes suggest that c-Myc is a central and key molecular integrator of cell proliferation, differentiation and metabolism.

Induction of DNA synthesis and apoptosis are separable functions of E2F-1

The family of E2F transcription factors have an essential role in mediating cell cycle progression, and recently, one of the E2F protein family, E2F-1, has been shown to participate in the induction of apoptosis. Cooperation between E2F and the p53 tumor suppressor protein in this apoptotic response had led to the suggestion that cell cycle progression induced by E2F-1 expression provides an apoptotic signal when placed in conflict with an arrest to cell cycle progression, such as provided by p53. We show here that although apoptosis is clearly enhanced by p53, E2F-1 can induce significant apoptosis in the absence of p53. Furthermore, this apoptotic function of E2F-1 is separable from the ability to accelerate entry into DNA synthesis. Analysis of E2F-1 mutants indicates that although DNA-binding is required, transcriptional transactivation is not necessary for the induction of apoptosis by E2F-1, suggesting that it may be mediated through alleviation of E2F-dependent transcriptional repression. These results indicate that E2F-1 can show independent cell cycle progression and apoptotic functions, consistent with its putative role as a tumor suppressor.

Inhibition of cellular Cdk2 activity blocks human cytomegalovirus replication

Human cytomegalovirus is a herpesvirus that induces numerous cellular processes upon infection. Among these are activation of cyclin-dependent kinase 2, which regulates cell cycle progression in G1 and S phase. We report here that inhibition of cellular Cdk2 activity blocks HCMV replication. Inhibition of Cdk2 activity by roscovitine inhibits HCMV DNA synthesis, production of infectious progeny, and late antigen expression in infected cells in a dose-dependent manner. HCMV replication is also inhibited by the expression of a Cdk2 dominant negative mutant, whereas expression of wild-type Cdk2 has no effect on viral replication. These data indicate that activation of cellular Cdk2 is necessary for HCMV replication.

Defining a role for c-Myc in breast tumorigenesis

The proto-oncogene c-myc is commonly amplified and overexpressed in human breast tumors, and the tumorigenic potential of c-myc overexpression in mammary tissue has been confirmed by both in vitro and in vivo models of breast cancer. However, the mechanisms by which Myc promotes tumorigenesis are not well understood. Recent evidence indicates that Myc can promote cell proliferation as well as cell death via apoptosis. These studies provide new insight and impetus in defining a role for c-Myc in breast tumorigenesis and may point toward novel targets for breast cancer therapy.

Cooperating oncogenes converge to regulate cyclin/cdk complexes

The cooperation of oncogenes in the transformation of primary rat Schwann cells is a strikingly synergistic process. We have explored the molecular mechanisms involved. Activation of an inducible Raf kinase results in morphologically transformed cells that are arrested in G1 via the induction of p21(CiP1) and subsequent inhibition of cyclin/cdk activity. In contrast, coexpression of SV40 large T (LT) or a dominant-negative mutant of p53 abolishes p21(CiP1) induction and alleviates the growth arrest. Moreover in this scenario, Raf activation results in an increase in the specific activity of cyclin/cdk complexes with Raf and LT cooperating to superinduce cyclin A/cdk2 activity and stimulate proliferation in the absence of mitogens. Thus, signaling by Raf and its cooperating partners converges at the regulation of cyclin/cdk complexes, with the cellular responses to Raf modulated by p53.

Biochemical and cellular effects of roscovitine, a potent and selective inhibitor of the cyclin-dependent kinases cdc2, cdk2 and cdk5

Cyclin-dependent kinases (cdk) play an essential role in the intracellular control of the cell division cycle (cdc). These kinases and their regulators are frequently deregulated in human tumours. Enzymatic screening has recently led to the discovery of specific inhibitors of cyclin-dependent kinases, such as butyrolactone I, flavopiridol and the purine olomoucine. Among a series of C2, N6, N9-substituted adenines tested on purified cdc2/cyclin B, 2-(1-ethyl-2-hydroxyethylamino)-6-benzylamino-9-isopropylpurine (roscovitine) displays high efficiency and high selectivity towards some cyclin-dependent kinases. The kinase specificity of roscovitine was investigated with 25 highly purified kinases (including protein kinase A, G and C isoforms, myosin light-chain kinase, casein kinase 2, insulin receptor tyrosine kinase, c-src, v-abl). Most kinases are not significantly inhibited by roscovitine. cdc2/cyclin B, cdk2/cyclin A, cdk2/cyclin E and cdk5/p35 only are substantially inhibited (IC50 values of 0.65, 0.7, 0.7 and 0.2 microM, respectively). cdk4/cyclin D1 and cdk6/cyclin D2 are very poorly inhibited by roscovitine (IC50 > 100 microM). Extracellular regulated kinases erk1 and erk2 are inhibited with an IC50 of 34 microM and 14 microM, respectively. Roscovitine reversibly arrests starfish oocytes and sea urchin embryos in late prophase. Roscovitine inhibits in vitro M-phase-promoting factor activity and in vitro DNA synthesis in Xenopus egg extracts. It blocks progesterone-induced oocyte maturation of Xenopus oocytes and in vivo phosphorylation of the elongation factor eEF-1. Roscovitine inhibits the proliferation of mammalian cell lines with an average IC50 of 16 microM. In the presence of roscovitine L1210 cells arrest in G1 and accumulate in G2. In vivo phosphorylation of vimentin on Ser55 by cdc2/cyclin B is inhibited by roscovitine. Through its unique selectivity for some cyclin-dependent kinases, roscovitine provides a useful antimitotic reagent for cell cycle studies and may prove interesting to control cells with deregulated cdc2, cdk2 or cdk5 kinase activities.

Myc: a single gene controls both proliferation and apoptosis in mammalian cells

c-myc was discovered as the cellular homologue of the transduced oncogene of several avian retroviruses. The gene encodes a transcription factor, which forms a heteromeric protein complex with a partner protein termed Max. In mammalian cells, Myc is a central regulator of cell proliferation and links external signals to the cell cycle machinery. Myc also induces cells to undergo apoptosis, unless specific signals provided either by cytokines or by oncogenes block the apoptotic pathway. Recent progress sheds light both on the factors regulating the function and expression of Myc and on the downstream targets in the cell cycle. Together, these findings suggest the existence of a novel signal transduction pathway regulating both apoptosis and proliferation.

Cell cycle: on target with Myc

Recent evidence indicates that the c-Myc proto-oncogene activates transcription of cdc25A. The Cdc25A protein phosphatase is required both for progression through mitosis and for Myc-induced apoptosis, making cdc25A the most attractive Myc target gene identified so far.