Regulated expression and phosphorylation of a possible mammalian cell-cycle control protein

Nerve growth factor regulates the expression and activity of p33cdk2 and p34cdc2 kinases in PC12 pheochromocytoma cells

In the absence of serum, nerve growth factor (NGF) promotes the survival and differentiation of the PC12 pheochromocytoma cell line. In the presence of serum, NGF acts primarily as a differentiation factor and negative regulator of cell cycling. To investigate NGF control of cell cycling, we have analyzed the regulation of cyclin dependent kinases during PC12 cell differentiation. NGF treatment leads to a reduction in the steady-state protein levels of p33cdk2 and p34cdc2, two key regulators of cell cycle progression. The decrease in p33cdk2 and p34cdc2 coincides with a decrease in the enzymatic activity of cyclinA-p34cdc2, cyclinB-p34cdc2, cyclinE-p33cdk2, and cyclinA-p33cdk2 kinases. The decline in p33cdk2 and p34cdc2 kinase activity in response to NGF is accelerated in cells that over-express the p140trk NGF receptor, suggesting that the timing of the down- regulation is dependent on the level of p140trk and the strength of the NGF signal. The level of cyclin A, a regulatory subunit of p33cdk2 and p34cdc2, is relatively constant during PC12 differentiation. Nevertheless, the DNA binding activity of the cyclinA-associated transcription factor E2F/DP decreases. Thus, NGF down-regulates the activity of cyclin dependent kinases and cyclin-transcription factor complexes during PC12 differentiation.

Cell cycle analysis of the activity, subcellular localization, and subunit composition of human CAK (CDK-activating kinase)

The activity of cyclin-dependent kinases (cdks) depends on the phosphorylation of a residue corresponding to threonine 161 in human p34cdc2. One enzyme responsible for phosphorylating this critical residue has recently been purified from Xenopus and starfish. It was termed CAK (for cdk-activating kinase), and it was shown to contain p40MO15 as its catalytic subunit. In view of the cardinal role of cdks in cell cycle control, it is important to learn if and how CAK activity is regulated during the somatic cell cycle. Here, we report a molecular characterization of a human p40MO15 homologue and its associated CAK activity. We have cloned and sequenced a cDNA coding for human p40MO15, and raised specific polyclonal and monoclonal antibodies against the corresponding protein expressed in Escherichia coli. These tools were then used to demonstrate that p40MO15 protein expression and CAK activity are constant throughout the somatic cell cycle. Gel filtration suggests that active CAK is a multiprotein complex, and immunoprecipitation experiments identify two polypeptides of 34 and 32 kD as likely complex partners of p40MO15. The association of the three proteins is near stoichiometric and invariant throughout the cell cycle. Immunocytochemistry and biochemical enucleation experiments both demonstrate that p40MO15 is nuclear at all stages of the cell cycle (except for mitosis, when the protein redistributes throughout the cell), although the p34cdc2/cyclin B complex, one of the major purported substrates of CAK, occurs in the cytoplasm until shortly before mitosis. The absence of obvious changes in CAK activity in exponentially growing cells constitutes a surprise. It suggests that the phosphorylation state of threonine 161 in p34cdc2 (and the corresponding residue in other cdks) may be regulated primarily by the availability of the cdk/cyclin substrates, and by phosphatase(s).

c-Myc cooperates with activated Ras to induce the cdc2 promoter

Expression of c-myc with constitutively active mutants of the ras gene results in the cooperative transformation of primary fibroblasts, although the precise mechanism by which these genes cooperate is unknown. Since c-Myc has been shown to function as a transcriptional activator, we have examined the ability of c-Myc and activated Ras (H-RasV-12) to cooperatively induce the promoter activity of cdc2, a gene which is critical for cell cycle progression. Microinjection of expression constructs encoding H-RasV-12 and c-Myc along with a cdc2 promoter-luciferase reporter plasmid into quiescent cells led to an increase in cdc2 promoter activity approximately 30 h after injection, a period which coincides with the S-to-G2/M transition in these cells. Expression of H-RasV-12 alone weakly activated the cdc2 promoter, while expression of c-Myc alone had no effect. Mutants of c-Myc lacking either the leucine zipper dimerization domain or the phosphoacceptor site Ser-62 could not cooperate with H-RasV-12 to induce the cdc2 promoter. These mutants also lacked the ability to cooperate with H-RasV-12 to stimulate DNA synthesis. Deletion analysis identified a distinct region of the cdc2 promoter which was required for c-Myc responsiveness. Taken together, these observations suggest a mechanistic link between the molecular activities of c-Myc and Ras and induction of the cell cycle regulator Cdc2.

c-Myc cooperates with activated Ras to induce the cdc2 promoter

Expression of c-myc with constitutively active mutants of the ras gene results in the cooperative transformation of primary fibroblasts, although the precise mechanism by which these genes cooperate is unknown. Since c-Myc has been shown to function as a transcriptional activator, we have examined the ability of c-Myc and activated Ras (H-RasV-12) to cooperatively induce the promoter activity of cdc2, a gene which is critical for cell cycle progression. Microinjection of expression constructs encoding H-RasV-12 and c-Myc along with a cdc2 promoter-luciferase reporter plasmid into quiescent cells led to an increase in cdc2 promoter activity approximately 30 h after injection, a period which coincides with the S-to-G2/M transition in these cells. Expression of H-RasV-12 alone weakly activated the cdc2 promoter, while expression of c-Myc alone had no effect. Mutants of c-Myc lacking either the leucine zipper dimerization domain or the phosphoacceptor site Ser-62 could not cooperate with H-RasV-12 to induce the cdc2 promoter. These mutants also lacked the ability to cooperate with H-RasV-12 to stimulate DNA synthesis. Deletion analysis identified a distinct region of the cdc2 promoter which was required for c-Myc responsiveness. Taken together, these observations suggest a mechanistic link between the molecular activities of c-Myc and Ras and induction of the cell cycle regulator Cdc2.

Abundant existence of 40kD-cdc2-related protein in rat thyroid cells

The cdc2 kinase family is known to be one of the important factors for cell proliferation in both yeast and mammalian cells. By using polyclonal antibodies against PSTAIRE region of cdc2HS, we studied the amount of cdc2-related kinases in the rat thyroid cell line, FRTL-5, during the cell cycle. The immunoreactive protein with molecular weight 34kD was hardly detectable, instead, 40kD protein exists constantly in amount through G0 phase to S phase. Further, it did not decrease when the cells were cultured in serum- and hormone-free medium. Moreover, we observed an increase of this protein in the nuclear fraction as the cells enter S phase. On the contrary, we could not detect any immunoreactive 40kD protein in primary cultures of rat thyroid cells. These results may indicate abundant existence of this protein might concern cell cycle regulation of FRTL-5 or its immortal feature.

Cell cycle: regulatory events in G1-->S transition of mammalian cells

A cell divides into two daughter cells by progressing serially through the precisely controlled G1, S, G2, and M phases of the cell cycle. The crossing of the G1/S border, which is marked by the initiation of DNA synthesis, represents commitment to division into two complete cells. Beyond this critical point no further external signals are required. We now have more comprehensive knowledge of the temporal sequence of systems at this key transition from G1 to S--growth factor responses, a cascade of kinase reactions, activation of cyclins and their associated kinases, and oncogene and tumor suppressor gene products. Furthermore, we know that the absolute requirement for calcium and the timing of events associated with calmodulin and the 68 kDa calmodulin-binding protein are consistent with overall Ca++/calmodulin control of all steps from the response to growth factors in G1 to DNA replication in S phase. We now have to sort out the inter-relationships of myriad control proteins and their relation to the Ca++/calmodulin-dependent controls--Which are causes? Which are effects? And which are parallel processes? The answers will be important, as they represent both a much deeper understanding of this key process of life and an important opportunity for improving therapeutic medicine.

Growth factor, steroid, and steroid antagonist regulation of cyclin gene expression associated with changes in T-47D human breast cancer cell cycle progression

Cyclins and proto-oncogenes including c-myc have been implicated in eukaryotic cell cycle control. The role of cyclins in steroidal regulation of cell proliferation is unknown, but a role for c-myc has been suggested. This study investigated the relationship between regulation of T-47D breast cancer cell cycle progression, particularly by steroids and their antagonists, and changes in the levels of expression of these genes. Sequential induction of cyclins D1 (early G1 phase), D3, E, A (late G1-early S phase), and B1 (G2 phase) was observed following insulin stimulation of cell cycle progression in serum-free medium. Transient acceleration of G1-phase cells by progestin was also accompanied by rapid induction of cyclin D1, apparent within 2 h. This early induction of cyclin D1 and the ability of delayed administration of antiprogestin to antagonize progestin-induced increases in both cyclin D1 mRNA and the proportion of cells in S phase support a central role for cyclin D1 in mediating the mitogenic response in T-47D cells. Compatible with this hypothesis, antiestrogen treatment reduced the expression of cyclin D1 approximately 8 h before changes in cell cycle phase distribution accompanying growth inhibition. In the absence of progestin, antiprogestin treatment inhibited T-47D cell cycle progression but in contrast did not decrease cyclin D1 expression. Thus, changes in cyclin D1 gene expression are often, but not invariably, associated with changes in the rate of T-47D breast cancer cell cycle progression. However, both antiestrogen and antiprogestin depleted c-myc mRNA by > 80% within 2 h. These data suggest the involvement of both cyclin D1 and c-myc in the steroidal control of breast cancer cell cycle progression.

Growth factor, steroid, and steroid antagonist regulation of cyclin gene expression associated with changes in T-47D human breast cancer cell cycle progression

Cyclins and proto-oncogenes including c-myc have been implicated in eukaryotic cell cycle control. The role of cyclins in steroidal regulation of cell proliferation is unknown, but a role for c-myc has been suggested. This study investigated the relationship between regulation of T-47D breast cancer cell cycle progression, particularly by steroids and their antagonists, and changes in the levels of expression of these genes. Sequential induction of cyclins D1 (early G1 phase), D3, E, A (late G1-early S phase), and B1 (G2 phase) was observed following insulin stimulation of cell cycle progression in serum-free medium. Transient acceleration of G1-phase cells by progestin was also accompanied by rapid induction of cyclin D1, apparent within 2 h. This early induction of cyclin D1 and the ability of delayed administration of antiprogestin to antagonize progestin-induced increases in both cyclin D1 mRNA and the proportion of cells in S phase support a central role for cyclin D1 in mediating the mitogenic response in T-47D cells. Compatible with this hypothesis, antiestrogen treatment reduced the expression of cyclin D1 approximately 8 h before changes in cell cycle phase distribution accompanying growth inhibition. In the absence of progestin, antiprogestin treatment inhibited T-47D cell cycle progression but in contrast did not decrease cyclin D1 expression. Thus, changes in cyclin D1 gene expression are often, but not invariably, associated with changes in the rate of T-47D breast cancer cell cycle progression. However, both antiestrogen and antiprogestin depleted c-myc mRNA by > 80% within 2 h. These data suggest the involvement of both cyclin D1 and c-myc in the steroidal control of breast cancer cell cycle progression.

Analysis of 5' flanking sequences from the Schizosaccharomyces pombe cdc2 gene

Previously, the sequence of the Schizosaccaromyces pombe cdc2 gene was reported to begin at a HindIII site, 141 nucleotides (nt) upstream from the ATG start codon [Hindley and Phear, Gene 31 (1984) 129-134]. We have extended the sequence of the 5' untranslated region of the gene to a PsI site at -822 nt. We demonstrate by primer extension analysis that transcription of the gene initiates at one major point 180 nt upstream from the ATG start codon. Since the 822-nt fragment extending from the PstI site to the start codon has been used in many studies as the promoter for cdc2 [Booher and Beach, Mol. Cell. Biol. 6 (1986) 3523-3530; Carr et al., Mol. Gen. Genet 218 (1989) 41-49; Gould and Nurse, Nature 342 (1989) 39-45], we investigated the strength of this promoter element relative to the SV40 early promoter, a promoter known to work very well in S. pombe [Jones et al., Cell 53 (1988) 659-667]. We confirm that the cdc2 gene fragment has significant promoting activity, albeit 20- to 60-fold less than the SV40 early promoter, when assayed in S. pombe.

Regulation of the cell cycle by the cdk2 protein kinase in cultured human fibroblasts

In mammalian cells inhibition of the cdc2 function results in arrest in the G2-phase of the cell cycle. Several cdc2-related gene products have been identified recently and it has been hypothesized that they control earlier cell cycle events. Here we have studied the relationship between activation of one of these cdc2 homologs, the cdk2 protein kinase, and the progression through the cell cycle in cultured human fibroblasts. We found that cdk2 was activated and specifically localized to the nucleus during S phase and G2. Microinjection of affinity-purified anti-cdk2 antibodies but not of affinity-purified anti-cdc2 antibodies, during G1, inhibited entry into S phase. The specificity of these effects was demonstrated by the fact that a plasmid-driven cdk2 overexpression counteracted the inhibition. These results demonstrate that the cdk2 protein kinase is involved in the activation of DNA synthesis.

Cell cycle dependent regulation of cdc2 mRNA in mouse fibroblasts: requirement of protein synthesis and of continued mitogenic stimulation

In the chemically transformed mouse fibroblasts (BP-A31) placed in a serum-free medium, the cdc2 mRNA content decreases in parallel with the cessation of [3H]thymidine incorporation. Extinction of the cdc2 gene expression is also observed in BP-A31 cells overexpressing the human c-myc oncogene. At quiescence, the cdc2 gene expression can be reinduced with serum or with other mitogens such as insulin or 12-O-tetradecanoyl phorbol 13-acetate (TPA). The kinetics of induction is characterized by a lag period which differs according to the mitogen used and reflects the length of the G1 phase (4-6 h with insulin or serum, 9-12 h with TPA). The cdc2 mRNA accumulation is prevented when protein synthesis is blocked with cycloheximide, also if the drug is added at a time when the synthesis of cdc2 mRNA is already under way. Similarly, removal of the mitogen leads to a cessation of the cdc2 mRNA accumulation. These results suggest that the increased expression of the cdc2 gene is mediated by (a) short-lived, growth factor-regulated protein(s).

Activators of protein kinase C induce p34cdc2 histone H1 kinase stimulation in Swiss 3T3 fibroblasts

Phorbol-12,13-dibutyrate and 1,2-dioctanoylglycerol, activators of protein kinase C (PKC) that stimulate DNA synthesis in serum-deprived Swiss 3T3 fibroblasts, induce histone H1 kinase activity associated with anti-cdc2 immunoprecipitates after a lag period of 15h, a time point close to G1/S boundary of the cell cycle in these cells. Downregulation of PKC does not affect the basal cdc2 kinase activity, but potently inhibits both phorbol dibutyrate- and dioctanoylglycerol-induced cdc2 kinase activation. Phorbol dibutyrate induces a dramatic increase in the p34cdc2 protein level as well as the appearance of p35-p36 forms of cdc2 on Western blot. In PKC-downregulated cells, the p34 form of cdc2 remains elevated but p35-p36 forms do not appear upon phorbol dibutyrate stimulation. These results demonstrate that PKC activation leads to cdc2 kinase activation in mitogenically responsive Swiss 3T3 cells, and strongly suggest that both expression of p34cdc2 protein and its posttranslational modification(s) are involved in this process. Western blot analysis of PKC isozymes suggests that either PKC alpha, PKC delta or PKC epsilon may be involved in p34cdc2 kinase activation and mitogenesis.

A growth-dependent post-translational modification of annexin VI

Annexin VI (p68, 67-kDa calelectrin) is a member of a family of Ca2+/phospholipid-binding proteins, that includes p35 (annexin I) and p36 (annexin II), the major cellular substrates for phosphorylation by the epidermal growth factor receptor and pp60v-src tyrosine kinase activities, respectively. We report here that like annexins I and II, annexin VI is phosphorylated in vivo, but that in contrast, annexin VI phosphorylation is associated with cell growth. In both Swiss 3T3 fibroblasts and human T-lymphoblasts the pattern of phosphorylation followed an almost identical profile. In particular, annexin VI was not phosphorylated in quiescent cells, but was phosphorylated on serine and to a lesser extent threonine, several hours following cell stimulation. Furthermore, annexin VI also incorporated phosphate in a growth-dependent manner, in a form other than a phosphoamino-acid. The phosphate was visualised following acid hydrolysis of immunoprecipitated annexin VI, as part of a complex having high mobility on 2-D thin-layer electrophoresis. The identity of this complex is not known. The results suggest that a post-translational modification other than direct protein phosphorylation may influence the activity of annexin VI and provide evidence linking cell growth with regulation of annexin VI function.

Cell cycle effects of microinjected antisense oligodeoxynucleotides to p34cdc2 kinase

In this study the effect of antisense oligomers targeted against the mRNA transcripts of p34cdc2 kinase on G1 progression into S-phase was examined. For this purpose, antisense, sense, or nonsense oligomers were introduced directly into the cytoplasm of T98G cells grown in monolayer cultures by glass-capillary microinjection. The microinjection of antisense oligomers (but not sense or nonsense oligomers) into growth-arrested cells before serum stimulation inhibited G1 progression into S-phase. This inhibition was correlated with a reduction in the steady-state levels of nuclear p34cdc2 protein. Microinjection of antisense oligomers into cells at 2 and 6 hours after serum stimulation also resulted in a marked inhibition in the ability of cells to enter S-phase. The inhibitory effect decreased when cells were microinjected at 12 hours after serum stimulation. When cells were microinjected at 18 and 24 hours after serum stimulation, only a slight inhibition was observed. As the antisense oligomers were introduced directly into the cytoplasm of cells at each of the time points examined, the observed differences in the inhibitory effects of the antisense oligomers at later times after serum stimulation cannot be explained by differences in uptake. An alternative explanation is that after a certain threshold level of nuclear p34cdc2 protein is reached in late G1 phase; no further increase is necessary, because the cells become committed to enter S-phase. In yeast, p34cdc2 appears to play an important role in the G1/S-phase transition at a control point in late G1 phase called START (reviewed by Lewin). In mammalian cells a control point that could be equivalent to START is the "restriction point" which is defined as the time after which inhibition of protein synthesis fails to block entry into S-phase (reviewed by Pardee). The effects observed with antisense oligomers to p34cdc2 kinase are strikingly similar to what is observed when low concentrations of the drug cycloheximide are added to these cells at different times after serum stimulation; entry into S-phase is significantly inhibited when cycloheximide is added up to 12 hours postimulation. Thus, the results reported in this study are in agreement with the idea that p34cdc2 kinase plays a role in the G1/S phase transition in mammalian cells. Finally, introduction of antisense oligomers directly into the cytoplasm of cells grown in monolayer cultures by glass-capillary microinjection appears to be a viable alternative to simply adding the oligomers to the culture medium.(ABSTRACT TRUNCATED AT 400 WORDS)

Oncogenic ras triggers the activation of 42-kDa mitogen-activated protein kinase in extracts of quiescent Xenopus oocytes

Quiescent, full-grown Xenopus oocytes, which are arrested at the G2/M border of meiosis, contain an inactive 42-kDa mitogen-activated protein kinase (p42MAPK) that is activated when oocytes are stimulated to resume the meiotic cell cycle. We have made extracts from these oocytes that respond to four cell cycle activators: oncogenic [Val12]Ras protein, clam cyclins A delta 60 and B delta 97, and the phosphatase inhibitor okadaic acid. All four induce the tyrosine phosphorylation and activation of p42MAPK. Both cyclins and okadaic acid, but not [Val12]Ras, also lead to activation of the endogenous cyclin B/cdc2 kinase complexes in extracts of quiescent oocytes. Using extracts prepared from cycloheximide-arrested interphase cells, we show that although p42MAPK activation can occur in response to cyclin-activated cdc2, the Ras-induced activation of p42MAPK occurs without intervening cdc2 activation. Neither the nononcogenic [Gly12]Ras nor [Val12,Arg186]Ras, a mutant that lacks the C-terminal consensus sequence directing prenylation and subsequent membrane association, is an effective activator of p42MAPK in vitro.

Identification of an early-growth-response gene encoding a novel putative protein kinase

Early-growth-response genes, also known as immediate-early genes, play important roles in regulating cell proliferation. We have identified a new type of early-growth-response gene product, a 77,811-Da putative serine/threonine kinase, which is highly inducible by serum and phorbol ester. mRNA encoding this putative kinase is markedly elevated within 1 h after treatment with mitogen, and this induction is synergistically increased by cycloheximide. Dexamethasone blocks serum induction of the kinase mRNA, as does transformation by v-Ki-ras. The kinase mRNA was detected in mouse brain, lung, and heart. This new putative kinase, which we term Snk, for serum-inducible kinase, showed similarity in its proposed catalytic domain to many other protein kinases; however, no other kinase showed enough sequence similarity with Snk to suggest the existence of a common function. Hence, Snk represents a new type of protein kinase involved in the early mitogenic response whose activity is transcriptionally and posttranscriptionally regulated.

Identification of an early-growth-response gene encoding a novel putative protein kinase

Early-growth-response genes, also known as immediate-early genes, play important roles in regulating cell proliferation. We have identified a new type of early-growth-response gene product, a 77,811-Da putative serine/threonine kinase, which is highly inducible by serum and phorbol ester. mRNA encoding this putative kinase is markedly elevated within 1 h after treatment with mitogen, and this induction is synergistically increased by cycloheximide. Dexamethasone blocks serum induction of the kinase mRNA, as does transformation by v-Ki-ras. The kinase mRNA was detected in mouse brain, lung, and heart. This new putative kinase, which we term Snk, for serum-inducible kinase, showed similarity in its proposed catalytic domain to many other protein kinases; however, no other kinase showed enough sequence similarity with Snk to suggest the existence of a common function. Hence, Snk represents a new type of protein kinase involved in the early mitogenic response whose activity is transcriptionally and posttranscriptionally regulated.

Control of the cell cycle

Cell division is arguably the most fundamental developmental process for single-celled and multicellular organisms alike. The pathway from one cell division to the next is known as the cell cycle. A conserved biochemical regulatory network controls progress along this pathway in plants, animals, and yeasts. This review is intended to serve as a primer on the current state of the eukaryotic cell cycle regulatory model, an introduction to the special roles of cell division and its control in plant development, and a review of recent progress in applying the universal mitotic control paradigm to higher plant systems.

Isolation and characterization of a cdc 2 cDNA from Dictyostelium discoideum

A cdc2 homologous sequence was amplified from Dictyostelium discoideum by the polymerase chain reaction and used to isolate several cDNA clones. The amino acid sequence encoded by these cDNAs exhibited approx. 60% identity to the Cdc2 proteins of other species. A cDNA containing the entire coding sequence complemented the temperature sensitive cdc28 mutant of Saccharomyces cerevisiae, although growth of the transformants was slow and limited. Southern blot analysis of restriction digests under high stringency conditions provided evidence that Dictyostelium contains a single cdc2 gene, although at lower stringency multiple fragments were detected, suggesting the existence of a cdc2 gene family. Northern blot analysis of RNA from different stages of Dictyostelium development showed that cdc2 mRNA levels increased during aggregation and then decreased to low levels by the pseudoplasmodial stage of development. By contrast, cdc2 mRNA levels remained relatively constant as cells passed from exponential growth to the stationary phase.

Spatial pattern of cdc2 expression in relation to meristem activity and cell proliferation during plant development

The p34 protein kinase encoded by the cdc2 gene is a key component of the eukaryotic cell cycle required for the G1- to S-phase transition and entry into mitosis. To study the regulation of plant meristem activity and cell proliferation, we have examined the tissue-specific accumulation of cdc2 transcripts in Arabidopsis thaliana and the related crucifer radish (Raphanus sativus) by in situ hybridization using A. thaliana cdc2 cDNA sequences as a probe. cdc2 transcripts accumulated in leaf primordia and within the vegetative shoot apical meristem. During flower development, high levels of expression were observed in meristems, in the basal regions of developing organs, in the developing vasculature, and associated with rib meristems elaborated late in the development of some floral organs. In root tips, cdc2 transcripts accumulated in the meristematic region and adjacent daughter cells but were not detected in the quiescent center. There was strong hybridization throughout the pericycle, and a further localized accumulation of cdc2 transcripts was observed in the initial stages of the activation of a new meristem at sites of lateral root development. We conclude that cdc2 expression is a critical factor in the regulation of meristem activity and establishment of proliferative competence.

Transforming growth factor beta 1 (TGF beta 1) reduces cellular levels of p34cdc2, and this effect is abrogated by adenovirus independently of the E1A-associated pRB binding activity

We have used E1A probes to study the roles of the p34cdc2 kinase and the retinoblastoma tumor susceptibility gene product (pRB) in transforming growth factor beta 1 (TGF beta 1)-mediated growth suppression in mink lung epithelial (Mv1Lu) cells. In agreement with previous reports, we see a decline in p34cdc2 kinase activity and a loss of pRB phosphorylation after TGF beta 1 treatment. We report here that TGF beta 1 induces not only a change in p34cdc2 kinase activity but a strong repression of p34cdc2 synthesis. Loss of p34cdc2 kinase activity is not seen until the steady-state level of p34cdc2 declines, suggesting that the intra-cellular signals induced by TGF beta 1 affect p34cdc2 at the level of expression, rather than by altering the posttranslational modifications of p34cdc2 that regulate its kinase activity. Infection with adenovirus expressing either wild-type E1A or a mutant E1A (pm928) defective for pRB binding alleviated TGF beta 1-mediated suppression of DNA synthesis, indicating that E1A does not need to bind pRB physically to keep cell growth-suppressing functions from being activated by TGF beta 1. The E1A.928 mutant virus is able to maintain p34cdc2 expression and kinase activity, as well as pRB phosphorylation in the presence of TGF beta 1, which may account for its ability to maintain cell cycle activity without directly sequestering pRB. Overall our results suggest that TGF beta 1 acts by signaling changes at the level of control of G1 gene expression, not at the level of posttranslational modification of p34cdc2 or its substrates.

Coordinated synthesis and degradation of cdc2 in the mammalian cell cycle

The product of the cdc2 gene (cdc2 or p34cdc2), the catalytic subunit of M phase-promoting factor (MPF), is held at a constant steady-state level throughout the cell cycle. In this report, we show that the constant concentration is maintained by a coordinated regulation of protein synthesis and degradation. At the end of each mitosis, cdc2 transcription is shut off, and the mRNA is rapidly degraded. A 12-fold activation of cdc2 gene transcription occurs every round of the cell cycle at the G1/S transition, in a growth factor-dependent manner. The increase in mRNA correlates with the accumulation of newly synthesized cdc2 during S and G2 phases. At the onset of mitosis, the translation of cdc2 mRNA is shut off. During G1 phase, the cdc2 protein has a relatively long half-life of 18 hr, so cdc2 made in the previous cell cycle is maintained. Once synthesis is activated at G1/S, a concurrent mechanism of degradation is activated, and the protein half-life is reduced to 7.5 hr. By the end of interphase, new cdc2 accounts for 75-85% of the total cdc2 pool. In addition, we show that greater than 75% of the new cdc2 complexes with cyclin, suggesting that a majority of the new cdc2 functions as MPF.

Cell cycle regulation of the human cdc2 gene

Transcription of the human cdc2 gene is cell cycle regulated and restricted to proliferating cells. Nuclear run-on assays show that cdc2 transcription is high in S and G2 phases of the cell cycle but low in G1. To investigate transcriptional control further, genomic clones of the human cdc2 gene containing 5' flanking sequences were isolated and shown to function as a growth regulated promoter in vivo when fused to a CAT reporter gene. In primary human fibroblasts, the human cdc2 promoter is negatively regulated by arrest of cell growth in a similar fashion to the endogenous gene. This requires specific 5' flanking upstream negative control (UNC) sequences which mediate repression. The retinoblastoma susceptibility gene product (Rb) specifically represses cdc2 transcription in cycling cells via 136 bp of 5' flanking sequence located between -245 and -109 within the UNC region. E2F binding sites in this region were shown to be essential for optimal repression. A model is proposed where Rb negatively regulates the cdc2 promoter in non-cycling and cycling G1 cells.

Cloning of a D-type cyclin from murine erythroleukemia cells

We report the complete coding sequence of a cDNA, designated CYL2, derived from a murine erythroleukemia cell library. CYL2 is considered to encode a D-type cyclin because (i) there is cross hybridization with CYL1 (a murine homolog of human cyclin D1) and the encoded protein has 64% amino acid sequence identity with CYL1 and (ii) murine erythroleukemia cell-derived CYL2 contains an amino acid sequence identical to that previously reported for the C-terminal portion of a partially sequenced CYL2. Transcripts of murine erythroleukemia cell CYL2 undergo alternative polyadenylylation like that of human cyclin D1. A major 6.5-kilobase CYL2 transcript changes its expression during the cell cycle with a broad peak through G1 and S phases and a decrease in G2/M phases. The present findings suggest that CYL2 plays a role in the G1 to S phase progression.

A mouse cdc25 homolog is differentially and developmentally expressed

The timing and activation of the p34cdc2 kinase in mammals is associated with dephosphorylation of phosphotyrosine and phosphothreonine residues on the p34cdc2 kinase. For fission yeast, the timing of mitosis is regulated by cyclic accumulation of cdc25, which promotes dephosphorylation of p34cdc2 and concomitant protein kinase activation. We report the identification and characterization of a structural and functional mouse homolog, Cdc25M2, of the cdc25 phosphatase. Cdc25M2 shows high sequence identity to the previously reported human homolog cdc25Hu2. Cdc25M2 can functionally complement for a Schizosaccharomyces pombe cdc25ts mutation, and when expressed in Escherichia coli and purified, Cdc25M2 is an active phosphatase. cdc25M2 mRNA shows variation in expression in different tissues in the mouse embryo and is expressed in a developmental and cell-cycle-dependent fashion. We suggest that the expression and accumulation of the cdc25 mitotic inducer may play a critical role in the regulation of mouse development.

Treatment of myeloid leukemic cells with the phosphatase inhibitor okadaic acid induces cell cycle arrest at either G1/S or G2/M depending on dose

The phosphatase inhibitor okadaic acid was found to induce cell cycle arrest of human myeloid leukemic cell lines HL-60 and U937 in a concentration- and time-dependent manner. Exposure to low concentrations of okadaic acid (2-8nM) for 24-48 hr caused greater than 70% of cells to arrest at G2/M, with up to 40% of the cells arrested in early mitosis. Cell viability decreased rapidly after 48 hr of treatment, and morphological and DNA structure analysis indicated that this was primarily due to the induction of apoptosis. The cells arrested in mitosis by 8 nM okadaic acid could be highly enriched by density gradient centrifugation and underwent apoptosis when further cultured either with or without okadaic acid, indicating that the effects of okadaic acid were irreversible. In contrast to the effects of low concentrations of okadaic acid, high concentrations (500 nM), inhibited proliferation in less than 3 hr. Remarkably, the majority of cells also entered a mitosis-like state characterized by dissolution of the nuclear membrane and condensation and partial separation of chromosomes. However, these cells had a diploid content of DNA, indicating that the cell cycle arrest occurred at G1/S with premature chromosome condensation (PCC), rather than at G2/M. If cells were first blocked at G1/S with hydroxyurea and then treated with okadaic acid, greater than 90% developed PCC in less than 3 hr without replicating their DNA. Caffeine was not able to induce PCC in these cells, either with or without prior inhibition of DNA synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell cycle control in normal and neoplastic cells

Recent studies of cell cycle control suggest that cyclin-dependent protein kinases play a central role in the cell's commitment to a new division cycle in late G1. The regulation of these kinases in normal and neoplastic growth is becoming clear.

A novel nuclear inhibitor I-92 regulates DNA binding activity of octamer binding protein p92 during the cell cycle

Nuclear DNA binding protein p92 is a sequence specific octamer binding protein with identical molecular weight as the ubiquitous octamer binding protein Oct-1. It binds to octamer related sequences from the enhancer of human papillomavirus type 18. The activity and intracellular distribution of p92 is regulated by extracellular signals. In serum starved Hela-fibroblast hybrid cells p92 is localized to the cytosol. Serum stimulation leads to nuclear import of p92. In fractions of asynchronously growing cells, which were separated according to cell cycle phases into G1, S, and G2 populations by centrifugal elutriation, p92 DNA binding is confined to S phase. In binding site blots however, p92 DNA binding activity is also present in G1 and G2. In G1 and G2 DNA binding activity of p92 is masked by a novel nuclear inhibitor I-92. The cyclic association of p92 with its inhibitor I-92 provides a new mechanism of regulating S phase dependent activity of a sequence specific DNA binding protein.

Cloning of a human cDNA encoding a CDC2-related kinase by complementation of a budding yeast cdc28 mutation

We have cloned two different human cDNAs that can complement cdc28 mutations of budding yeast Saccharomyces cerevisiae. One corresponds to a gene encoding human p34CDC2 kinase, and the other to a gene (CDK2; cell division kinase) that has not been characterized previously. The CDK2 protein is highly homologous to p34CDC2 kinase (65% identical) and more significantly is homologous to Xenopus Eg1 kinase (89% identical), suggesting that CDK2 is the human homolog of Eg1. The human CDC2 and CDK2 genes were both able to complement the inviability of a null allele of S. cerevisiae CDC28. This result indicates that the CDK2 protein has a biological activity closely related to the CDC28 and p34CDC2 kinases. However, CDK2 was unable to complement cdc2 mutants in fission yeast Schizosaccharomyces pombe under the condition where the human CDC2 gene could complement them. CDK2 mRNA appeared late in G1 or in early S phase, slightly before CDC2 mRNA, after growth stimulation in normal human fibroblast cells. These results suggest that in human cells, two different CDC2-like kinases may regulate the cell cycle at distinct stages.

Human cyclin E, a new cyclin that interacts with two members of the CDC2 gene family

A new human cyclin, named cyclin E, was isolated by complementation of a triple cln deletion in S. cerevisiae. Cyclin E showed genetic interactions with the CDC28 gene, suggesting that it functioned at START by interacting with the CDC28 protein. Two human genes were identified that could interact with cyclin E to perform START in yeast containing a cdc28 mutation. One was CDC2-HS, and the second was the human homolog of Xenopus CDK2. Cyclin E produced in E. coli bound and activated the CDC2 protein in extracts from human G1 cells, and antibodies against cyclin E immunoprecipitated a histone H1 kinase from HeLa cells. The interactions between cyclin E and CDC2, or CDK2, may be important at the G1 to S transition in human cells.

Interleukin-2-triggered Raf-1 expression, phosphorylation, and associated kinase activity increase through G1 and S in CD3-stimulated primary human T cells

To gain further insight into the role of Raf-1 in normal cell growth, c-raf-1 mRNA expression, Raf-1 protein production, and Raf-1-associated kinase activity in normal human T cells were analyzed. In contrast to the constitutive expression of Raf-1 in continuously proliferating cell lines, c-raf-1 mRNA and Raf-1 protein levels were barely detectable in freshly isolated G0 T lymphocytes. Previous work with fibroblasts has suggested that Raf-1 plays a signaling role in the G0-G1 phase transition. In T cells, triggering via the T-cell antigen receptor (TCR)-CD3 complex (TCR/CD3) resulted in an approximately fourfold increase in c-raf-1 mRNA. In addition, the promotion of G1 progression by interleukin 2 (IL-2) was associated with a 5- to 10-fold immediate/early induction of c-raf-1 mRNA, resulting in up to a 12-fold increase in Raf-1 protein expression. TCR/CD3 activation did not alter the phosphorylation state of Raf-1, whereas interleukin 2 receptor stimulation resulted in a rapid increase in the phosphorylation state of a subpopulation of Raf-1 molecules progressively increasing throughout G1. These findings were complemented by assays for Raf-1-associated kinase activity which revealed a gradual accumulation of serine and threonine autokinase activity in Raf-1 immunoprecipitates during G1, which remained elevated throughout DNA replication.

Colony-stimulating factor 1 regulates novel cyclins during the G1 phase of the cell cycle

Three mouse cyclin-like (CYL) genes were isolated, two of which are regulated by colony-stimulating factor 1 (CSF-1) during the G1 phase of the macrophage cell cycle. CSF-1 deprivation during G1 leads to rapid degradation of CYL proteins (p36CYL) and correlates with failure to initiate DNA synthesis. However, after entering S phase, macrophages no longer require CSF-1 and can complete cell division without expressing CYL genes. During G1, p36CYL is phosphorylated and associates with a polypeptide antigenically related to p34cdc2. The timing of p36CYL expression, its rapid turnover in the absence of CSF-1, and its phosphorylation and transient binding to a cdc2-related polypeptide suggest that CYL genes may function during S phase commitment.

Interleukin-2-triggered Raf-1 expression, phosphorylation, and associated kinase activity increase through G1 and S in CD3-stimulated primary human T cells

To gain further insight into the role of Raf-1 in normal cell growth, c-raf-1 mRNA expression, Raf-1 protein production, and Raf-1-associated kinase activity in normal human T cells were analyzed. In contrast to the constitutive expression of Raf-1 in continuously proliferating cell lines, c-raf-1 mRNA and Raf-1 protein levels were barely detectable in freshly isolated G0 T lymphocytes. Previous work with fibroblasts has suggested that Raf-1 plays a signaling role in the G0-G1 phase transition. In T cells, triggering via the T-cell antigen receptor (TCR)-CD3 complex (TCR/CD3) resulted in an approximately fourfold increase in c-raf-1 mRNA. In addition, the promotion of G1 progression by interleukin 2 (IL-2) was associated with a 5- to 10-fold immediate/early induction of c-raf-1 mRNA, resulting in up to a 12-fold increase in Raf-1 protein expression. TCR/CD3 activation did not alter the phosphorylation state of Raf-1, whereas interleukin 2 receptor stimulation resulted in a rapid increase in the phosphorylation state of a subpopulation of Raf-1 molecules progressively increasing throughout G1. These findings were complemented by assays for Raf-1-associated kinase activity which revealed a gradual accumulation of serine and threonine autokinase activity in Raf-1 immunoprecipitates during G1, which remained elevated throughout DNA replication.

Activation of muscarinic acetylcholine receptors inhibits cell cycle progression of small cell lung carcinoma

We previously reported that activation of muscarinic acetylcholine receptors (mAChR) of M3 subtype causes hydrolysis of phosphoinositides and inhibits voltage-gated Ca2+ channel activity in small cell lung carcinoma (SCLC) cells. We now report that mAChR activation causes exponentially growing SCLC cells to arrest in S and G2/M phases of the cell cycle, concomitant with a decrease in DNA synthesis. Cell cycle progression and DNA synthesis resume when mAChR are down-regulated. In serum-starved SCLC cells, mAChR activation inhibits DNA synthesis induced by serum, bombesin, insulin, or insulin-like growth factor-I. The finding that DNA synthesis is inhibited even when mAChR are activated after exposure of cells to growth factors indicates that decreased signal transduction by growth factor receptors is not the mechanism of mAChR-mediated growth inhibition. Our data suggest that mAChR activation disrupts a common event that is induced by different growth factors and is fundamental for cell cycle progression.

Isolation and characterization of cDNA clones encoding a functional p34cdc2 homologue from Zea mays

We describe the isolation of cDNA clones encoding a p34cdc2 homologue from a higher plant, Zea mays (maize). A full-length cDNA clone, cdc2ZmA, was isolated, sequenced, and shown to complement a cdc28 mutation in Saccharomyces cerevisiae. Comparison of the deduced amino acid sequence of the maize p34cdc2 protein with other homologues showed that it was 64% identical to human p34cdc2 and 63% identical to Schizosaccharomyces pombe and S. cerevisiae p34cdc2 proteins. Studies of expression of the maize cdc2 gene(s) by Northern blot analysis indicated a correlation between the abundance of cdc2 mRNA and the proliferative state of the tissue. Southern blot analysis, as well as isolation of another cDNA clone, cdc2ZmB, which is 96% identical to cdc2ZmA, indicates that maize has multiple cdc2 genes.

Phosphorylation differences among proteins of bloodstream developmental stages of Trypanosoma brucei brucei

Early in an infection the bloodstream forms of the African trypanosome Trypanosoma brucei brucei are long, slender and rapidly dividing. Later, non-dividing, short, stumpy forms may be found. In this report we described biochemical differences between the two parasite populations in the phosphorylation of their proteins in vitro. Compared with the slender populations, the non-dividing stumpy forms of the parasites exhibit decreased phosphorylation of an 80 kDa protein and enhanced phosphorylation of 37 kDa and 42 kDa proteins (pp37 and pp42). These changes occurred regardless of whether the stumpy trypanosomes were generated naturally during the course of the infection or induced by difluoromethylornithine treatment. The phosphorylation of pp37 and pp42 occurs on serine and threonine residues and is totally dependent upon the presence of Mn2+ or Mg2+. However, excess Mn2+ or Mg2+ inhibits phosphorylation. Maximal phosphorylation of pp42 occurs with 1 mm-Mn2+ or 10 mm-Mg2+, whereas that of pp37 occurs with 50 mM-Mn2+ or greater than 100 mm-Mg2+. The phosphorylation of pp37 is greatly enhanced by KCl, whereas that of pp42 is only slightly increased by this salt. Ca2+, calmodulin, phospholipids and cyclic AMP have no discernible effect upon the phosphorylation of pp42 or pp37 in vitro, whereas heparin, suramin, polylysine, polyarginine and polyamines all inhibit phosphorylation. Thus the enzymes that phosphorylate pp42 and pp37 have properties similar to, but distinct from, those of mammalian casein kinase II. Since the casein-kinase-like activity is higher in the slender than in the stumpy forms, the enhanced phosphorylation of pp42 and pp37 in the non-dividing parasites is probably a result of the enhanced synthesis of these acidic proteins.

Cellular aging and senescence

Differentiated eukaryotic cells have only a finite capacity for cell division. This limitation is thought to be a cellular manifestation of organismal aging, and a restraint to tumor progression. The molecular basis for cellular senescence is not known, but a molecular framework for understanding this phenomenon has recently been established.

Transforming growth factor beta 1 inhibition of p34cdc2 phosphorylation and histone H1 kinase activity is associated with G1/S-phase growth arrest

Transforming growth factor beta 1 (TGF beta 1) is a potent inhibitor of epithelial cell proliferation. We present data which indicate that epithelial cell proliferation is inhibited when TGF beta 1 is added throughout the prereplicative G1 phase. Cultures become reversibly blocked in late G1 at the G1/S-phase boundary. The inhibitory effects of TGF beta 1 on cell growth occur in the presence of the RNA synthesis inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole. Associated with this inhibitory effect is a decrease in the phosphorylation and histone H1 kinase activity of the p34cdc2 protein kinase. These data suggest that TGF beta 1 growth inhibition in epithelial cells involves the regulation of p34cdc2 activity at the G1/S transition.

Transforming growth factor beta 1 inhibition of p34cdc2 phosphorylation and histone H1 kinase activity is associated with G1/S-phase growth arrest

Transforming growth factor beta 1 (TGF beta 1) is a potent inhibitor of epithelial cell proliferation. We present data which indicate that epithelial cell proliferation is inhibited when TGF beta 1 is added throughout the prereplicative G1 phase. Cultures become reversibly blocked in late G1 at the G1/S-phase boundary. The inhibitory effects of TGF beta 1 on cell growth occur in the presence of the RNA synthesis inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole. Associated with this inhibitory effect is a decrease in the phosphorylation and histone H1 kinase activity of the p34cdc2 protein kinase. These data suggest that TGF beta 1 growth inhibition in epithelial cells involves the regulation of p34cdc2 activity at the G1/S transition.

Complementation of a yeast cell cycle mutant by an alfalfa cDNA encoding a protein kinase homologous to p34cdc2

The cdc2 protein kinase plays a central role in control of the eukaryotic cell cycle of animals and yeasts. We have isolated a cDNA clone (cdc2Ms) from alfalfa (Medicago sativa L.) that is homologous to the yeast cdc2/CDC28 genes. The encoded protein is 64% identical to the yeast and mammalian counterparts and shows all the prominent structural features known from these organisms. Antibody raised against a 16-amino acid synthetic peptide with crossreactivity against p34 proteins recognized a 34-kilodalton protein in extracts of alfalfa cells. When transferred into a fission yeast, the plant cdc2 homolog can complement a temperature-sensitive cdc2 mutant. Northern analysis revealed higher transcript levels in shoots and suspension cultures than in roots. In addition to the dominant transcript of 1.4 kilobases detected in the poly(A)+fraction, 2.5- and 1.2-kilobase transcripts were detected in total RNA preparations from shoots or somatic embryos. Suspension cultures that were induced to form somatic embryos by an auxin (2,4-dichlorophenoxyacetic acid) showed fluctuations in transcription pattern during the induction period and embryogenesis.

A new class of reversible cell cycle inhibitors

The effects of three compounds on the cell cycle of HL-60 promyeloid leukemia cells has been examined. Ciclopirox olamine, an antifungal agent, and the compound Hoechst 768159 reversibly block the cell cycle at a point occurring roughly 1 h before the arrest mediated by aphidicolin, an inhibitor of DNA polymerase alpha activity, which acts in early S phase. Similar results are also obtained with the compound mimosine, a plant amino acid. Based on these data, it is concluded that all three agents inhibit cell cycle traverse at or very near the G1/S phase boundary and identify a previously undefined reversible cell cycle arrest point.

cdc2 gene expression at the G1 to S transition in human T lymphocytes

The product of the cdc2 gene, designated p34cdc2, is a serine-threonine protein kinase that controls entry of eukaryotic cells into mitosis. Freshly isolated human T lymphocytes (G0 phase) were found to have very low amounts of p34cdc2 and cdc2 messenger RNA. Expression of cdc2 increased 18 to 24 hours after exposure of T cells to phytohemagglutinin, coincident with the G1 to S transition. Antisense oligodeoxynucleotides could reduce the increase in cdc2 expression and inhibited DNA synthesis, but had no effect on several early and mid-G1 events, including blastogenesis and expression of interleukin-2 receptors, transferrin receptors, c-myb, and c-myc. Induction of cdc2 required prior induction of c-myb and c-myc. These results suggest that cdc2 induction is part of an orderly sequence of events that occurs at the G1 to S transition in T cells.