Arresting developments in cell-cycle control

Steroidal regulation of cell cycle progression

Sex steroid hormones and their antagonists have well-defined mitogenic and growth-inhibitory effects on target cells including cancer cells. These effects are mediated by cell cycle phase-specific actions, implying that steroids control rates of cell cycle progression by regulating the expression of key cell cycle regulatory genes. An emerging model of cell cycle control involves transcriptional induction of cyclin genes and consequent activation of cyclin-dependent kinases, which initiate cellular events necessary to complete checkpoints within the cell cycle. Our recent studies have focused on the roles of G1 cyclins, particularly cyclin D1, in the control of cell cycle progression in human breast cancer cells. These studies show that cyclin D1 induction is an early response to mitogenic stimulation by oestrogens and progestins, is rate-limiting for G1 progression and is sufficient for completion of the cell cycle in cells arrested in early G1 phase by serum deprivation. Furthermore, inhibition of cyclin D1 expression is an early response to growth-inhibitory anti-oestrogens. These results suggest that cyclin D1 is a target for regulation of cell cycle progression by sex steroids and their antagonists.

Separation of live cells in different phases of the cell cycle for gene expression analysis

BACKGROUND

Homogeneity of cell populations is a basic requirement for gene expression analyses of the cell cycle, such as those based on microarrays. The most common approach to obtain specific populations is the use of synchronization methods that increase the number of cells representing a certain cell cycle stage. On the one hand, conventional synchronization usually causes undesirable effects. On the other hand, cell separation methods may imply loss of RNA quality, another limiting factor for expression profiling. We describe a new strategy to specifically separate live cells in different phases of the cell cycle (G(1) and G(2)/M) to obtain good quality RNA for gene expression analyses.

METHODS

The experimental design included sorting G(1) and G(2)/M cells with the vital fluorochrome Hoechst 33342, followed by RNA isolation from the sorted cells.

RESULTS

Sorted living G(1) and G(2)/M cells, analyzed by immunocytochemistry and laser scanning cytometry, showed strong enrichment. The quality and specificity of the isolated RNA were demonstrated by northern blot.

CONCLUSIONS

This new approach has many potential applications, such as expression profiling of specific cell populations after eliminating the irrelevant data produced by cells in other stages of the cycle.

Human Speedy: a novel cell cycle regulator that enhances proliferation through activation of Cdk2

The decision for a cell to self-replicate requires passage from G1 to S phase of the cell cycle and initiation of another round of DNA replication. This commitment is a critical one that is tightly regulated by many parallel pathways. Significantly, these pathways converge to result in activation of the cyclin-dependent kinase, cdk2. It is, therefore, important to understand all the mechanisms regulating cdk2 to determine the molecular basis of cell progression. Here we report the identification and characterization of a novel cell cycle gene, designated Speedy (Spy1). Spy1 is 40% homologous to the Xenopus cell cycle gene, X-Spy1. Similar to its Xenopus counterpart, human Speedy is able to induce oocyte maturation, suggesting similar biological characteristics. Spy1 mRNA is expressed in several human tissues and immortalized cell lines and is only expressed during the G1/S phase of the cell cycle. Overexpression of Spy1 protein demonstrates that Spy1 is nuclear and results in enhanced cell proliferation. In addition, flow cytometry profiles of these cells demonstrate a reduction in G1 population. Changes in cell cycle regulation can be attributed to the ability of Spy1 to bind to and prematurely activate cdk2 independent of cyclin binding. We demonstrate that Spy1-enhanced cell proliferation is dependent on cdk2 activation. Furthermore, abrogation of Spy1 expression, through the use of siRNA, demonstrates that Spy1 is an essential component of cell proliferation pathways. Hence, human Speedy is a novel cell cycle protein capable of promoting cell proliferation through the premature activation of cdk2 at the G1/S phase transition.

Differential regulation of Cdc2 and Cdk2 by RINGO and cyclins

Cyclin-dependent kinases (Cdks) are key regulators of the eukaryotic cell division cycle. Cdk1 (Cdc2) and Cdk2 should be bound to regulatory subunits named cyclins as well as phosphorylated on a conserved Thr located in the T-loop for full enzymatic activity. Cdc2- and Cdk2-cyclin complexes can be inactivated by phosphorylation on the catalytic cleft-located Thr-14 and Tyr-15 residues or by association with inhibitory subunits such as p21(Cip1). We have recently identified a novel Cdc2 regulator named RINGO that plays an important role in the meiotic cell cycle of Xenopus oocytes. RINGO can bind and activate Cdc2 but has no sequence homology to cyclins. Here we report that, in contrast with Cdc2- cyclin complexes, the phosphorylation of Thr-161 is not required for full activation of Cdc2 by RINGO. We also show that RINGO can directly stimulate the kinase activity of Cdk2 independently of Thr-160 phosphorylation. Moreover, RINGO-bound Cdc2 and Cdk2 are both less susceptible to inhibition by p21(Cip1), whereas the Thr-14/Tyr-15 kinase Myt1 can negatively regulate the activity of Cdc2-RINGO with reduced efficiency. Our results indicate that Cdk-RINGO complexes may be active under conditions in which cyclin-bound Cdks are inhibited and can therefore play different regulatory roles.

Investigating the hows and whys of DNA endoreduplication

Endoreduplication is a form of nuclear polyploidization that results in multiple, uniform copies of chromosomes. This process is common in plants and animals, especially in tissues with high metabolic activity, and it generally occurs in cells that are terminally differentiated. In plants, endoreduplication is well documented in the endosperm and cotyledons of developing seeds, but it also occurs in many tissues throughout the plant. It is thought that endoreduplication provides a mechanism to increase the level of gene expression, but the function of this process has not been thoroughly investigated. Numerous observations have been made of endoreduplication, or at least extra cycles of S-phase, as a consequence of mutations in genes controlling several aspects of cell cycle regulation. However, until recently there were few studies directed at the molecular mechanisms responsible for this specialized cell cycle. It is suggested that endoreduplication requires nothing more elaborate than a loss of M-phase cyclin-dependent kinase activity and oscillations in the activity of S-phase cyclin-dependent kinase.

Growth inhibition of human glioma cells by transfection-induced P21 and its effects on telomerase activity

The aim of this study is to investigate the effect of the p21 gene transfection on the growth of cultured human glioma cell lines, and analyze the telomerase activity, and detection of telomerase components in p21 transfectant. The p21 gene was transfected into human glioma cell lines, U251MG and T98G with our novel liposome. The cell growth was assessed by counting the number of trypan blue-excluding cells in a hemocytometer and flow cytometry analysis. The expression of P21 protein and its mRNA were examined by Western and Northern blot analysis. The telomerase activity was assayed by TRAP (telomerase repeat amplification protocol)/TRAP-HPA (hybridization protection assay) method qualitatively and quantitatively. The length of telomere was measured by Southern blot analysis. The expression of telomerase components (hTERT, hTERC and TEP1) were examined by RT-PCR (reverse transcriptase-polymerase chain reaction). The p21 transfectant demonstrated the expression of P21 protein and its mRNA. The p21 transfection of human glioma cells results in growth inhibition and G0/G1 arrest. The p21 transfectant revealed a decrease of telomerase activity and hTERT expression as compared with control cells. These results suggest that p21 transfection induces G0/G1 arrest in human glioma cells which associates with the reduction in the telomerase activity and hTERT expression.

The abundance of cell cycle regulatory protein Cdc4p is controlled by interactions between its F box and Skp1p

Posttranslational modification of a protein by ubiquitin usually results in rapid degradation of the ubiquitinated protein by the proteasome. The transfer of ubiquitin to substrate is a multistep process. Cdc4p is a component of a ubiquitin ligase that tethers the ubiquitin-conjugating enzyme Cdc34p to its substrates. Among the domains of Cdc4p that are crucial for function are the F-box, which links Cdc4p to Cdc53p through Skp1p, and the WD-40 repeats, which are required for binding the substrate for Cdc34p. In addition to Cdc4p, other F-box proteins, including Grr1p and Met30p, may similarly act together with Cdc53p and Skp1p to function as ubiquitin ligase complexes. Because the relative abundance of these complexes, known collectively as SCFs, is important for cell viability, we have sought evidence of mechanisms that modulate F-box protein regulation. Here we demonstrate that the abundance of Cdc4p is subject to control by a peptide segment that we term the R-motif (for "reduced abundance"). Furthermore, we show that binding of Skp1p to the F-box of Cdc4p inhibits R-motif-dependent degradation of Cdc4p. These results suggest a general model for control of SCF activities.

Differential regulation of p27kip1 levels and CDK activities by hypertrophic and hyperplastic agents in vascular smooth muscle cells

To understand the molecular mechanisms that determine the fate of a cell to undergo either hypertrophy or hyperplasia, we studied the effects of angiotensin II (Ang II) and platelet-derived growth factor (PDGF)-BB, hypertrophic and hyperplastic agents, respectively, on the modulation of G1/S transition molecules in smooth muscle cells. Ang II increased protein synthesis while PDGF-BB induced both DNA and protein synthesis. Ang II had no significant effect on the steady-state levels of cyclin-dependent kinase (CDK) inhibitor (CDKI), p27kip1, and on the activities of CDK2 and CDK4, although it caused a modest increase in cyclin E levels. In contrast, PDGF-BB induced depletion of p27kip1 and increased cyclins D1 and E levels and CDK2 and CDK4 activities. Reflecting its lack of effect on CDK activities, Ang II failed to phosphorylate tumor suppressor retinoblastoma protein, Rb. PDGF-BB, on the other hand, induced phosphorylation of Rb, consistent with its ability to activate CDKs. Together, these findings suggest that Ang II-induced hypertrophy may be due to its failure to activate cellular signaling events required for G1/S transition.

Functional domains in cyclin D1: pRb-kinase activity is not essential for transformation

Although cyclin D1 plays a major role during cell cycle progression and is involved in human tumourigenesis, its domain structure is still poorly understood. In the present study, we have generated a series of cyclin D1 N- and C-terminal deletion constructs. These mutants were used to define the domains required for transformation of rat embryonal fibroblasts (REF) in cooperation with activated Ha-ras and and to establish correlations with defined biochemical properties of cyclin D1. Protein binding and REF assays showed that the region of the cyclin box required for the interaction with CDK4 as well as C-terminal sequences determining protein stability were crucial for transformation. Surprisingly, however, the N-terminal deletion of 20 amino acids which impaired pRb kinase activity did not affect the transforming ability of cyclin D1. Likewise, no effect on transformation was observed with mutants defective in p21CIP interaction. These observations argue against a crucial role of pRb inactivation or p21CIP squelching in cyclin D1-mediated transformation.

Stage-specific activity of the Leishmania major CRK3 kinase and functional rescue of a Schizosaccharomyces pombe cdc2 mutant

Cell cycle control by cdc2-related kinases (CRKs) is essential to the regulation of cell proliferation and developmental processes in many organisms. Alternating phases of growth, arrest, and differentiation are characteristics of the infectious cycle of many trypanosomatid parasites, raising the possibility that members of the trypanosomatid CRK gene family participate in the regulation of these essential processes. Here we describe properties of the CRK3 gene from Leishmania major, which encodes a 36 kDa protein kinase showing 60% amino acid sequence identity with human CDK2, including several conserved sites implicated in regulation of kinase activity. CRK3 mRNA was constitutively expressed throughout the parasite life cycle, but histone H1 kinase activity of an epitope tagged CRK3 protein was greater in log-phase than in stationary-phase promastigotes. When integrated into the genome and expressed at the optimal level, CRK3 was able to rescue the growth defect of a Schizosaccharomyces pombe cdc2 mutant (cdc2-33(ts)), indicating that CRK3 is a functional homolog of cdc2. Mutants of CRK3 at several key regulatory residues showed the expected dominant negative effects on the S. pombe mutant. This is the first example of functional expression of a trypanosomatid CRK in yeast, opening the way for further genetic studies within this amenable organism.

cAMP inhibits linoleic acid-induced growth by antagonizing p27(kip1) depletion, but not interfering with the extracellular signal-regulated kinase or AP-1 activities

To understand the underlying signaling events of polyunsaturated fatty acid-induced growth, we studied the effect of cAMP on early and delayed growth response events induced by linoleic acid in smooth muscle cells (SMC). cAMP significantly inhibited both basal and linoleic acid-induced DNA synthesis. Linoleic acid-induced early growth response events, such as activation of ERKs, induction of expression of c-fos and jun-B and stimulation of AP-1 activity, however, were not affected by cAMP. In contrast, linoleic acid-induced c-jun expression was blocked by cAMP. cAMP alone stimulated ERKs activation, c-fos and jun-B expression and increased AP-1 activity. Linoleic acid induced depletion of p27kip1 and increased CDK2 activity, events required for G1/S transition. In contrast to early growth response events, linoleic acid-induced G1/S transition signals were significantly inhibited by cAMP. These findings suggest that in addition to inducing immediate early growth response events, linoleic acid mimics growth factors in activating cell cycle events that are associated with G1/S transition in SMC and the negative regulation of linoleic acid-induced growth by cAMP is apparently due to its antagonism with linoleic acid-induced p27kip1 depletion and CDK2 activation.

The regulation and functions of cdk7

cdk7 started its life rather anonymously as a kinase called MO15, identified during a search for cDNA's which encode protein kinases related to cdc2. For several years its function remained obscure, but during the last 18 months MO15 has revealed itself as the catalytic subunit of cdk activating kinase, associating with at least two other subunits including a new cyclin, cyclin H. MO15(cdk7) has therefore been established paradoxically as both a new member and a regulator of the cyclin dependent kinase family. New evidence now suggests that cdk7 is also involved in the processes of transcription initiation and DNA repair, associating with the general transcription factor TFIIH. The engima of cdk7 is likely to remain for a while yet, and perhaps even more surprises are in store.

The two phases of regulated exocytosis in permeabilized pancreatic acini are modulated differently by heterotrimeric G-proteins

In this study we examined the influence on AlF4- and GTP gamma S on amylase secretion from alpha toxin permeabilized pancreatic acini. AlF4- only activates heterotrimeric G-proteins, whereas GTP gamma S activates both small ras-like GTP-binding proteins and heterotrimeric G-proteins (Kahn, R. A., J. Biol. Chem., 266, 15595-15597, 1991). GTP gamma S, but not AlF4-, significantly stimulated Ca2(+)-independent amylase secretion, suggesting that a small GTP-binding protein controls regulated exocytosis distal to the site of action of Ca2+. In contrast, both AlF4- and GTP gamma S modulated Ca(2+)-dependent amylase secretion. AlF4- and GTP gamma S stimulated the initial rapid, ATP-independent, phase of Ca(2+)-dependent secretion but inhibited the second slower sustained, ATP-dependent, phase of release. There were significant differences in the GTP gamma S requirements for the stimulation and inhibition of Ca(2+)-dependent amylase secretion, consistent with GTP gamma S activating separate heterotrimeric G-proteins to modulate each phase of the Ca(2+)-dependent secretory response. Our studies also indicated that neither G-protein is a member of the Gi/o class of heterotrimeric G-proteins.

An essential domain within Cdc34p is required for binding to a complex containing Cdc4p and Cdc53p in Saccharomyces cerevisiae

The CDC34 gene of the yeast Saccharomyces cerevisiae encodes a ubiquitin-conjugating protein that transfers ubiquitin onto substrates to signal rapid degradation via the proteasome. Cdc34p has been implicated in signaling the destruction of a variety of substrates including the cyclin-dependent kinase inhibitor, Sic1p, which must be degraded for cells to enter S-phase. Mutants lacking CDC34 activity fail to degrade Sic1p and fail to enter S-phase, a phenotype that is also shared with cells lacking CDC4 and CDC53 activity. Here we demonstrate that Cdc4p, Cdc34p, and Cdc53p interact in vivo. We have mapped a Cdc4p/Cdc53p-binding region on Cdc34p; this region is essential for S-phase entry and thus the association of these three proteins is required for Sic1p degradation. All three proteins migrate in gel filtration to sizes that greatly exceed their actual size suggesting that they form stable associations with other proteins and we observe Cdc4p, Cdc34p, and Cdc53p fractionating into overlapping families of high molecular weight complexes. Finally, we demonstrate that Cdc4p, Cdc34p, and Cdc53p are stable throughout the cell cycle and that Cdc34p permanently resides as part of a complex throughout the cell cycle. This suggests that all Cdc34p substrates are ubiquitinated by a similar high molecular weight complex.

Differential phosphorylation of T-47D human breast cancer cell substrates by D1-, D3-, E-, and A-type cyclin-CDK complexes

The cyclin-dependent kinases (CDKs) promote cell cycle transitions in mammalian cells by phosphorylation of key substrates. To characterize substrates of the G1 and S phase cyclin-CDK complexes, including cyclin D1-CDK4, cyclin D3-CDK4, cyclin D3-CDK6, cyclin E-CDK2, and cyclin A-CDK2, which are largely undefined, we phosphorylated T-47D breast cancer cell nuclear lysates partially purified by ion-exchange chromatography with purified baculovirus expressed cyclin-CDK complexes. A comparison of the substrates that were phosphorylated by the different cyclin D-CDKs revealed some common as well as specific substrates. Hence, cyclin D1-CDK4 specifically phosphorylated a 38-kDa protein while cyclin D3-CDK4 specifically phosphorylated proteins of 105, 102, and 42 kDa. A 24-kDa protein was phosphorylated by both complexes. Cyclin D3-CDK6 exhibited similar substrate preferences to cyclin D3-CDK4, phosphorylating the 105- and 102-kDa proteins but not the 24-kDa protein. Hence, both the cyclin D1 and D3 as well as CDK4 and CDK6 subunits can confer substrate specificity on the overall cyclin D-CDK complex. Cyclin E-CDK2 and cyclin A-CDK2 phosphorylated a greater number of substrates than the cyclin D-CDKs, ranging in size from 10 kDa to over 200 kDa. Twenty-two substrates were common to both complexes, while six were specific for cyclin A-CDK2 and only one protein of 34 kDa was specific for cyclin E-CDK2. These studies indicate that cyclins E and A modulate the specificity of CDK2 and have demonstrated substrates that may be important for the specific roles of these cyclin-CDKs during G1 and S phase progression. Protein sequencing of one of the cyclin-CDK substrates characterized in this study identified this protein as nucleolin, a previously characterized CDC2 (CDK1) substrate, thus indicating the utility of this approach in identifying cyclin-CDK targets. These results show that both the cyclin and CDK subunits can regulate the substrate specificity of the overall cyclin-CDK complex and have demonstrated numerous substrates of D-, E-, and A-type cyclin-CDK complexes potentially involved in regulating transit through the G1 and S phases of the cell cycle.

Molecular characterization of the cyclin-dependent kinase inhibitor p27 promoter

p27Kip1 is a member of the family of cyclin-dependent kinase inhibitors (CKIs), which play critical roles in the regulation of cell cycle. To study the transcriptional regulation that controls the expression of p27, we have isolated the p27 promoter, defined its transcription initiation site, and performed various analyses for sequences upstream to 3 kb. Transient transfection assays using fusion reporters containing progressively truncated p27 promoter fragments showed that a region of 170 bp upstream of the start site is sufficient for maximal transcription activity. Detailed sequence analysis of this 170 bp region identified several GC-rich segments, putative sites of the transcription factor Sp1. Footprinting experiments revealed two Sp1-protected boxes, named BoxI and BoxII, which are located at positions -133 to -117 and -87 to -72, respectively. Binding of Sp1 to the two boxes was further demonstrated by gel mobility shift assays and supershift assays. Co-transfection studies in Drosophila Schneider line 2 cells showed that Sp1 indeed activates the p27 promoter constructs that harbor one or both of the GC-rich sequences. Furthermore, the GC-rich sequences could confer Sp1-dependent transactivation to a heterologous prolactin minimal promoter. Mutations in the GC-rich sequences abolished both binding and transactivation by Sp1. Taken together, our data strongly show that the p27 promoter is activated by the ubiquitously expressed transcription factor Sp1, which may provide a molecular mechanism for the constitutive nature of p27 transcription.

Structure and function of the low Mr phosphotyrosine protein phosphatases

Phosphotyrosine protein phosphatases (PTPases) catalyse the hydrolysis of phosphotyrosine residues in proteins and are hence implicated in the complex mechanism of the control of cell proliferation and differentiation. The low Mr PTPases are a group of soluble PTPases displaying a reduced molecular mass; in addition, a group of low molecular mass dual specificity (ds)PTPases which hydrolyse phosphotyrosine and phosphoserine/threonine residues in proteins are known. The enzymes belonging to the two groups are unrelated to each other and to other PTPase classes except for the presence of a CXXXXXRS/T sequence motif containing some of the catalytic residues (active site signature) and for the common catalytic mechanism, clearly indicating convergent evolution. The low Mr PTPases have a long evolutionary history since microbial (prokaryotic and eukaryotic) counterparts of both tyrosine-specific and dsPTPases have been described. Despite the relevant number of data reported on the structural and catalytic features of a number of low Mr PTPases, only limited information is presently available on the substrate specificity and the true biological roles of these enzymes, in prokaryotic, yeast and eukaryotic cells.

Dynamics of cell cycle regulators: artifact-free analysis by recultivation of cells synchronized by centrifugal elutriation

Studies on the molecular properties of cell cycle regulators in animal cells require cell preparations highly enriched in particular cell cycle phases. Centrifugal elutriation is frequently used to synchronize cells because this technique was thought to cause only minimal distortions in protein expression or metabolic functions. However, in primary chicken erythroblasts, we consistently observed artefacts in mitotic cyclin mRNA expression and p70 S6 kinase activity, which were clearly caused by the elutriation procedure. Therefore, we modified the standard protocol by reseeding various elutriated fractions into preconditioned medium, a process termed recultivation, and harvesting after an appropriate amount of time. This avoided the pleiotropic effects caused by stress and lack of growth factor supply during elutriation. Using this recultivation procedure, highly synchronous progression starting from any given cell cycle phase could be achieved for a variety of cell types, including primary, factor-dependent cells of hematopoietic origin. Mitotic cyclin expression and S6 kinase activity was found to be normal again in recultivated cultures, as opposed to elutriated ones. Finally, monitoring of mitosis-specific cyclin A degradation in recultivated G2 phase cells showed that recultivation provided an excellent tool to follow cells through M phase into G1 without the requirement for a chemical cell cycle block.

Cyclin ET, a new splice variant of human cyclin E with a unique expression pattern during cell cycle progression and differentiation

Cyclin E is the regulatory subunit of the cdc2-related protein kinase cdk2 and is a rate limiting factor for the entry into S phase. To date, cyclin E is the only cyclin for which alternative splicing has been described. We report here the isolation of a new splice variant of cyclin E, termed cyclin ET, which has an internal deletion of 45 amino acids compared with the full-length cyclin E protein. Even though cyclin ETcontains an intact cyclin box, it is unable to complement a triple cln mutant strain of Saccharomyces cerevisiae or to interfere with rescue by cyclin E, indicating that an intact cyclin box is functionally insufficient. The expression pattern of cyclin ET during cell cycle entry, progression and differentiation differs from that of cyclin E. Thus, ET expression precedes that of the other isoforms during the G0-->S progression; it shows a sharp peak in early G1 in cells released from a mitotic block and is strongly down-regulated in terminally differentiated myeloid cells. These observations point to different functions for cyclin ET and E and show for the first time that the alternative splicing of cyclin E is a regulated mechanism governed by the cell cycle and differentiation.

c-ABL tyrosine kinase activity is regulated by association with a novel SH3-domain-binding protein

The c-ABL tyrosine kinase is activated following either the loss or mutation of its Src homology domain 3 (SH3), resulting in both increased autophosphorylation and phosphorylation of cellular substrates and cellular transformation. This suggests that the SH3 domain negatively regulates c-ABL kinase activity. For several reasons this regulation is thought to involve a cellular protein that binds to the SH3 domain. Hyperexpression of c-ABL results in an activation of its kinase, the kinase activity of purified c-ABL protein in the absence of cellular proteins is independent of either the presence or absence of a SH3 domain, and point mutations and deletions within the SH3 domain are sufficient to activate c-ABL transforming ability. To identify proteins that interact with the c-ABL SH3 domain, we screened a cDNA library by the yeast two-hybrid system, using the c-ABL SH3SH2 domains as bait. We identified a novel protein, AAP1 (ABL-associated protein 1), that associates with these c-ABL domains and fails to bind to the SH3 domain in the activated oncoprotein BCRABL. Kinase experiments demonstrated that in the presence of AAP1, the ability of c-ABL to phosphorylate either glutathione S-transferase-CRK or enolase was inhibited. In contrast, AAP1 had little effect on the phosphorylation of glutathione S-transferase-CRK by the activated ABL oncoproteins v-ABL and BCRABL. We conclude that AAP1 inhibits c-ABL tyrosine kinase activity but has little effect on the tyrosine kinase activities of oncogenic BCRABL or v-ABL protein and propose that AAP1 functions as a trans regulator of c-ABL kinase. Our data also indicate that loss of susceptibility to AAP1 regulation correlates with oncogenicity of the activated forms of c-ABL.

Endometrial effects of RU486 in primates--antiproliferative action despite signs of estrogen action and increased cyclin-B expression

Continuous antiprogestin administration to hormone replaced, castrate monkeys inhibits estrogen-induced endometrial proliferation through mechanisms which remains unclear. To elucidate the molecular mechanisms of RU486-induced endometrial suppression, we treated six intact female cynomolgus monkeys on cycle days 2-22 sequentially with placebo, RU486 (1 mg/kg/day) and levonorgestrel (LNG) (2 microg/kg/day) intramuscularly (i.m.), with uterine wedge sections and endometrial biopsies collected on day 22 of each cycle. The uterine sections were evaluated for morphology, mitosis and proliferating cell nuclear antigen (PCNA) immunohistochemistry. Changes in the mRNA levels of ER, PR, cyclin-B and tumour suppressor gene p21 were assessed using co-amplification with beta-actin by reverse transcriptase-polymerase chain reaction (RT-PCR). Administration of RU486 uniformly resulted in characteristic suppression of endometrium with few mitosis, dense stroma and simple glands, whereas the effects of LNG were less uniform. Following RU486 administration, the levels of endometrial ER and PR mRNA were comparable to proliferative phase endometrium, and significantly higher than those seen in the secretory endometrium, indicating that some of the biological actions of E2 were not inhibited during RU486 treatment. Despite scarce mitosis, PCNA was readily detectable in all samples. Curiously, in comparison to secretory phase controls, the levels of cyclin-B, but not p21, mRNA were markedly increased following RU486. The effects of LNG on the levels of these mRNA species varied, with mean levels falling between those of the secretory phase controls, and RU486-treated specimens. The increase in cyclin-B mRNA and lack of mitosis suggests that anti-proliferative actions of RU486 in the primate endometrium might be associated with a cell-cycle block at the G2-M interphase. Whether mechanisms similar to these are associated with the beneficial clinical effects of RU486 seen in the treatment of various hormone dependent maladies remains to be determined.

Cytometry of cyclin proteins

Cyclins are key components of the cell cycle progression machinery. They activate their partner cyclin-dependent kinases (CDKs) and possibly target them to respective substrate proteins within the cell. CDK-mediated phosphorylation of specific sets of proteins drives the cell through particular phases or checkpoints of the cell cycle. During unperturbed growth of normal cells, the timing of expression of several cyclins is discontinuous, occurring at discrete and well-defined periods of the cell cycle. Immunocytochemical detection of cyclins in relation to cell cycle position (DNA content) by multiparameter flow cytometry has provided a new approach to cell cycle studies. This approach, like no other method, can be used to detect the unscheduled expression of cyclins, namely, the presentation of G1 cyclins by cells in G2/M and of G2/M cyclins by G1 cells, without the need for cell synchronization. Such unscheduled expression of cyclins B1 and A was seen when cell cycle progression was halted, e.g., after synchronization at the G1/S boundary by inhibitors of DNA replication. The unscheduled expression of cyclins B1 or E, but not of A, was also observed in some tumor cell lines even when their growth was unperturbed. Likewise, whereas the expression of cyclins D1 or D3 in nontumor cells was restricted to an early section of G1, the presentation of these proteins in many tumor cell lines also was seen during S and G2/M. This suggests that the partner kinase CDK4 (which upon activation by D-type cyclins phosphorylates pRB committing the cell to enter S) is perpetually active throughout the cell cycle in these tumor lines. Expression of cyclin D also may serve to discriminate G0 vs. G1 cells and, as an activation marker, to identify the mitogenically stimulated cells entering the cell cycle. Differences in cyclin expression make it possible to discriminate between cells having the same DNA content but residing at different phases such as in G2 vs. M or G2/M of a lower DNA ploidy vs. G1 cells of a higher ploidy. The expression of cyclins D, E, A and B1 provides new cell cycle landmarks that can be used to subdivide the cell cycle into several distinct subcompartments. The point of cell cycle arrest by many antitumor agents can be estimated with better accuracy in relation to these compartments compared to the traditional subdivision into four cell cycle phases. The latter applications, however, pertain only to normal cells or to tumor cells whose phenotype is characterized by scheduled expression of cyclins. As sensitive and specific indicators of the cell's proliferative potential, the cyclins, in particular D-type cyclins, are expected to be key prognostic markers in neoplasia.

The insulin-like growth factor and epidermal growth factor families in mammary cell growth in ruminants: action and interaction with hormones

Selective breeding and improved management have had major effects in increasing peak milk yields but relatively little effect on lactation persistency. In ruminants, cell loss appears to be largely responsible for the decline in milk yield. Little is known about the longevity of individual cells, but, in lactating dairy cows, few epithelial cells are in the S phase (DNA synthesis) of the cell cycle. The IGF and epidermal growth factor families are direct mitogens, stimulating DNA synthesis in cultures of ruminant mammary epithelial cells. Receptors that mediate the effects of these growth factors, the type 1 IGF receptor and the epidermal growth factor receptor, respectively, are present at similar levels in membranes prepared from the mammary glands of nonpregnant and pregnant sheep. Binding capacity falls by parturition and remains low during lactation. These findings suggest that the drive to mammary development in pregnancy comes from control of growth factors, and, in the case of IGF, modulating binding proteins, a control exerted by hormones, which, in general, are not themselves mitogens. A paracrine or autocrine mode of action and, therefore, local growth factor synthesis, are more likely to be important than systemic concentrations of growth factor. Stimulatory growth factors produced locally by the mammary gland include IGF-I, IGF-II, transforming growth factor-alpha, and amphiregulin. More information is needed on the control of stimulatory and inhibitory growth factors and on how growth factors control the cell cycle. Knowledge of these processes could result in strategies to improve lactation persistency by increasing secretory cell renewal or reducing cell loss during lactation.

Prospects for the development of validated screening tests that measure developmental toxicity potential: view of one skeptic

Humans are exposed to a variety of potential developmental toxicants. This fact, combined with the knowledge that human development can be disrupted by "environmental" agents, has led to the development of methods designed to identify potential developmental toxicants. Currently, the principal method used to screen drugs and chemicals that are potential human developmental toxicants is the segment II study (i.e., a study in which prospective drugs and chemicals are tested in pregnant animals). Because of the cost and time involved in such studies and the pressure to reduce the number of animals used in such testing, alternative methods for developmental toxicity testing have been sought. This has resulted in a number of in vitro tests whose aim is to screen large numbers of agents quickly and inexpensively. Although numerous in vitro tests of developmental toxicity have been developed during the last 15 years, no one system or combination of tests have been validated for the purpose intended. Nonetheless, two systems--the limb bud/CNS micromass, and the chick embryo neural retina cell culture (CERC)--continue to be advanced as viable in vitro developmental toxicology tests. The purpose of this commentary is to evaluate the prospects for the development of an in vitro test system(s) that can screen the universe of drugs and chemicals and reliably identify those that require further study and those that do not. The conclusion of this investigator is that the prospects for validating such in vitro tests are not promising. This conclusion is based primarily on the lack of basic knowledge regarding the relevance of end points assayed in the micromass and CERC test systems to those end points known or thought to be critical for normal development.

Physiological inhibitors of protein kinase C

Increasing numbers of proteins that have the capacity of interacting with protein kinase C isozymes in vitro and inhibiting their enzymatic activity in a noncompetitive manner have been purified. While these proteins can be hypothesized to be part of a tight regulatory system for protein kinase C enzymatic activity, critical examinations of the roles of these proteins in the context of whole cells have not yet been performed. Interesting new data suggest that some of the classes of protein kinase C inhibitors may have a much broader role of interacting with multiple types of kinases and proto-oncogene products. cDNAs encoding a number of these inhibitor proteins have been isolated, which will allow the design and implementation of experiments on their cell biology and help address their function outside of the context of their operational definitions.

A possible role for human papillomaviruses in head and neck cancer

Human papillomaviruses (HPVs) cause benign tumors in the respiratory tract. Mounting evidence suggests that they also play a role in the etiology of a subset of head and neck cancers. Carcinomas in patients with a history of recurrent respiratory papillomatosis clearly are caused by persisting HPV interacting with one of more carcinogenic agents. Verrucous carcinomas of the oral cavity, tonsillar and tongue carcinomas are strongly linked with HPVs, based on molecular epidemiologic data. Tonsillar cancer have been shown to express HPV RNA, presumed necessary to induce and maintain a carcinoma, supporting a viral etiology. This paper reviews the molecular and cellular basis for considering HPVs as causative agents of cancer, and reviews the literature that considers the possible role of HPVs in head and neck cancer.

An inhibitor of p34cdc2/cyclin B that regulates the G2/M transition in Xenopus extracts

The activity of maturation-promoting factor (MPF), a protein kinase complex composed of p34cdc2 and cyclin B, is undetectable during interphase but rises abruptly at the G2/M transition to induce mitosis. After the synthesis of cyclin B, the suppression of MPF activity before mitosis has been attributed to the phosphorylation of p34cdc2 on sites (threonine-14 and tyrosine-15) that inhibit its catalytic activity. We previously showed that the activity of the mitotic p34cdc2/cyclin B complex is rapidly suppressed when added to interphase Xenopus extracts that lack endogenous cyclin B. Here we show that a mutant of p34cdc2 that cannot be inhibited by phosphorylation (threonine-14-->alanine, tyrosine-15-->phenylalanine) is also susceptible to inactivation, demonstrating that inhibitory mechanisms independent of threonine-14 and tyrosine-15 phosphorylation must exist. We have partially characterized this inhibitory pathway as one involving a reversible binding inhibitor of p34cdc2/cyclin B that is tightly associated with cell membranes. Kinetic analysis suggests that this inhibitor, in conjunction with the kinases that mediate the inhibitory phosphorylations on p34cdc2, maintains the interphase state in Xenopus; it may play an important role in the exact timing of the G2/M transition.

A model for a bistable biochemical trigger of mitosis

The activation of maturation promoting factor (MPF, cyclin B/Cdc2), which starts mitosis, is modeled as a bistable biochemical switch or trigger. A small, slow parameter change can cause an abrupt transition by a saddle-node bifurcation from a stable steady state of low activity to one of high activity. The switch is not reversed if the parameter change is reversed (hysteresis). The dynamical features necessary for this triggering action are the presence of two stable steady states (low-activity and high-activity), and one unstable steady state. The key biochemical kinetic features of the model are (1) mutual activation by MPF and Cdc25, which makes the activation of MPF effectively autocatalytic, and (2) binding of MPF by Suc1, which inhibits MPF autocatalysis and stabilizes the low-activity steady state until the amount of MPF begins to approach or exceed stoichiometrically the amount of Suc1, then allows strong autocatalysis and full activation. The special virtues of bistable triggering, and the general types of biochemical mechanism which can produce it, are discussed.

The role of DNA damage in cellular aging: is it time for a reassessment?

There is now evidence that the immediate cause of the loss of proliferative capacity in senescent cells is mediated by a specific inhibitor. If this tentative interpretation is correct, the next hurdle will be to determine mechanism(s) that regulate this putative senescence cell inhibitor that would, in effect, be the determinant of proliferative life span. One previously proposed hypothesis predicts that the decline of replicative activity is analogous to a checkpoint response to accumulated chromosomal damage (Rosenberger et al., 1991). Advances in our basic understanding of the nature of DNA damage, DNA repair mechanisms, and the response of eukaryotic cells to accumulated DNA damage provide a solid rationale for a reassessment of the causal role of the accumulation of chromosomal damage in cell senescence in vitro.

p53 protein expression in putative precursor lesions of epithelial ovarian cancer

p53 protein expression has been studied in epithelial inclusion cysts adjacent to and contralateral to serous carcinoma of the ovary and compared to epithelial inclusion cysts associated with borderline tumours and in normal ovaries. Atypia was found in epithelial inclusion cysts in eight of the thirteen advanced (stage III) serous ovarian carcinomas. Of these eight with atypical epithelial inclusion cysts, five showed immunoreactivity for p53. In the borderline tumours and normal ovaries no atypia in such cysts was found. p53 expression was also seen more frequently in surface epithelium associated with ovarian serous adenocarcinoma (ten of 13) than in normal ovaries (one of 13). We postulate that focal areas of atypia (ovarian intra-epithelial neoplasia) in epithelial inclusion cysts are the precursors of ovarian malignancy. In some cases, at least, p53 protein expression may precede overt cytological abnormalities.

Sulindac sulfide, an aspirin-like compound, inhibits proliferation, causes cell cycle quiescence, and induces apoptosis in HT-29 colon adenocarcinoma cells

Nonsteroidal antiinflammatory drugs (NSAIDs), have cancer preventive and tumor regressive effects in the human colon. They lower the incidence of and mortality from colorectal cancer and sulindac reduces the number and size of polyps in patients with familial adenomatous polyposis. We studied the effect of sulindac, and its metabolite sulindac sulfide, on the proliferation of HT-29 colon adenocarcinoma cells. Both compounds reduced the proliferation rate of these cells, changed their morphology, and caused them to accumulate in the G0/G1 phase of the cell cycle. These responses were time- and concentration-dependent and reversible. In addition, these compounds reduced the level and activity of several cyclin-dependent kinases (cdks), which regulate cell cycle progression. Sulindac and sulindac sulfide also induced apoptosis in these cells at concentrations that affected their proliferation, morphology, and cell cycle phase distribution. Sulindac sulfide was approximately sixfold more potent than sulindac in inducing these cellular responses. Our results indicate that inhibition of cell cycle progression and induction of apoptotic cell death contribute to the anti-proliferative effects of sulindac and sulindac sulfide in HT-29 cells. These findings may be relevant to the cancer preventive and tumor regressive effects of these compounds in humans.

Enhanced hepatocyte colony growth in soft agar after in vivo treatment with a genotoxic carcinogen: a potential assay for hepatocarcinogens?

We have shown previously that approximately 1 in 10,000 primary hepatocytes isolated from untreated rats undergo clonal growth in soft agar in vitro in response to the synergistic action of nafenopin, a peroxisome proliferator (PP) and epidermal growth factor (EGF), a naturally occurring liver growth regulator. Here, we demonstrate that prior treatment of the animals with the genotoxic hepatocarcinogen diethylnitrosamine (DEN) caused a dose-dependent increase in soft agar colony numbers formed in vitro. These data suggest that the colony assay may offer a method of detecting in vitro hepatocytes transformed in vivo by DEN. It is known that rats treated with DEN develop enzyme altered foci prior to the development of tumours. The majority of these foci express high levels of gamma-glutamyl transpeptidase (GGT). However, foci promoted by PPs do not show this increased enzyme activity. In the present study, the colonies we have generated in vitro mimicked this pattern since the majority (approximately 80%) of the spontaneous colonies expressed GGT whereas colonies promoted by the synergistic action of nafenopin and EGF were mainly (75%) GGT negative. The proportion of colonies positive for GGT were similar using either hepatocytes isolated from control or from DEN-initiated rats. Further studies are required to assess if the hepatocytes selected for clonal expansion by this EGF/nafenopin regime reflect the presumed pre-neoplastic cells induced by genotoxin in vivo and associated with an increased propensity to cancer.

Cyclins and cyclin-dependent kinases: a biochemical view

Images

Human dual specificity phosphatase VHR activates maturation promotion factor and triggers meiotic maturation in Xenopus oocytes

Bacterially expressed, dual specificity phosphatase VHR protein induced germinal vesicle breakdown (GVBD) when microinjected into Xenopus oocytes, albeit with slower kinetics than that observed in progesterone- or insulin-induced maturation. A mutant VHR protein missing an essential cysteine residue for its in vitro phosphatase activity completely lacked activity in injected oocytes. VHR injection done in conjunction with progesterone or insulin treatment resulted in highly synergized GVBD responses showing much faster kinetics than that produced by VHR or either hormone alone. The delayed kinetics of VHR-induced GVBD and the synergistic responses obtained in the presence of hormones suggested that this protein may be promoting G2/M transition by weakly mimicking the action of cdc25, the dual specificity phosphatase that physiologically activates the maturation promotion factor. Various experimental observations are consistent with such a role for the injected VHR in oocytes: 1) as opposed to hormone-treated oocytes, histone H1 kinase activation is not preceded by MAPK activation in the process of GVBD in VHR-injected oocytes; 2) incubation of purified VHR with highly concentrated cell-free extracts of untreated oocytes resulted in activation of histone H1 kinase activity in the lysates; 3) coinjection of VHR with activated Ras proteins resulted in synergized responses, faster than those produced by either protein alone; 4) coinjection of VHR with the purified amino-terminal SH2 domain of the p85 subunit of phosphatidylinositol 3-kinase (which blocks insulin-induced GVBD) does not affect VHR-induced maturation. The biological actions of VHR in oocytes clearly distinguish it from other dual specificity phosphatases, which have shown inhibitory effects when tested in oocytes. We speculate that VHR may represent a dual specificity phosphatase responsible for activation of cdk-cyclin complex(es) at a still undetermined stage of the cell cycle.

Apoptosis in antitumor strategies: modulation of cell cycle or differentiation

There is a strong evidence that administration of antitumor drugs triggers apoptotic death of target cells. A characteristic feature of apoptosis is active participation of the affected cell in its demise. Attempts have been made, therefore, to potentiate the cytotoxicity of a variety of agents by modulating the propensity of cells to respond by apoptosis. Several strategies to enhance apoptosis that involve modulation of the cell cycle or differentiation are discussed. Loss of control of the G1 checkpoint in tumor cells allows one to design treatments that arrest normal cells at the checkpoint and attempt to selectively kill tumor cells with S phase specific drugs. The possibility of a restoration of the apoptosis triggering function of the tumor suppressor gene p53 when the G1 checkpoint function is abolished is expected to increase tumor cells' sensitivity to S phase poisons. Because induction of apoptosis by many antitumor drugs is cell cycle phase specific, drug combinations that preferentially trigger apoptosis at different phases of the cycle, or recruitment of cells to the sensitive phase, offer another antitumor strategy. There is also evidence that apoptosis is potentiated when cell differentiation is triggered following DNA damage. This observation suggests that strategies which combine DNA damaging and differentiating drugs, under conditions where the latter are administered following DNA damage caused by the former, may be successful.

Threshold expression of cyclin E but not D type cyclins characterizes normal and tumour cells entering S phase

Complexes of cyclin-dependent kinases (cdk) and their partner cyclins drive the cell through the cell cycle, each such complex phosphorylating a distinct set of proteins at a particular check-point or phase of the cycle. Immunocytochemical detection of cyclins combined with measurement of cellular DNA content by flow cytometry makes it possible to relate expression of each of these proteins with the actual cell cycle position, without the necessity of cell synchronization. In the present study, we have investigated expression of E and D type cyclins in G1 cells and in cells entering S phase, in eight different human hematopoietic and solid tumour cell lines (two leukaemias, a lymphoma, three breast carcinomas, a colon carcinoma and a bladder transitional cell carcinoma) during their exponential phase of growth, as well as in normal mitogen stimulated lymphocytes. In all the cell types studied, the average level of D type cyclin expression was invariable throughout the cell cycle. A great intercellular variability, in particular of the G1 cell subpopulations, and the presence of a large fraction of G1, S and G2 + M cells that were cyclin D negative (20-40% in tumour cell lines and about 80% among lymphocytes), were other characteristic features of D type cyclin expression. In contrast to D type cyclins, the expression of cyclin E was discontinuous during the cycle, peaking at the time of cell entrance to S. Also, a well defined threshold in expression of cyclin E characterized cells that were entering S phase, and virtually no cyclin E negative cells were seen during the early portion of S phase. The data indicate that while cell entrance to S phase is unrelated to expression of D type cyclins (at the time of entrance), accumulation of cyclin E up to critical level is a prerequisite for initiation of DNA replication. The great intercellular variability in expression of D type cyclins and their invariant average level across the cell cycle suggest that these cyclins, in addition to their acknowledged function in promoting cell progression through mid- to late-G1 may have other role(s), related or unrelated to the cell cycle progression. The presence of a large number of D type cyclin negative cells in all phases of the cycle suggests that during exponential growth the cells may not express this protein and yet may traverse the cycle, including G1 phase.

Exit from G0 and entry into the cell cycle of cells expressing p21Sdi1 antisense RNA

p21Sdi1 (also known as Cip1 and Waf1), an inhibitor of DNA synthesis cloned from senescent human fibroblasts, is an inhibitor of G1 cyclin-dependent kinases (Cdks) in vitro and is transcriptionally regulated by wild-type p53. In addition, p21Sdi1 has been found to inhibit DNA replication by direct interaction with proliferating cell nuclear antigen. In this study we analyzed normal human fibroblast cells arrested in G0 and determined that an excess of p21Sdi1 was present after immunodepletion of various cyclins and Cdks, in contrast to mitogen-stimulated cells in early S phase. Expression of antisense p21Sdi1 RNA in G0-arrested cells resulted in induction of DNA synthesis as well as entry into mitosis. These results suggest that p21Sdi1 functions in G0 and early G1 and that decreased expression of the gene is necessary for cell cycle progression.

Is the cell division cycle gated by a circadian clock? The case of Chlamydomonas reinhardtii

Circadian oscillators are known to regulate the timing of cell division in many organisms. In the case of Chlamydomonas reinhardtii, however, this conclusion has been challenged by several investigators. We have reexamined this issue and find that the division behavior of Chlamydomonas meets all the criteria for circadian rhythmicity: persistence of a cell division rhythm (a) with a period of approximately 24 h under free-running conditions, (b) that is temperature compensated, and (c) which can entrain to light/dark signals. In addition, a mutation that lengthens the circadian period of the phototactic rhythm similarly affects the cell division rhythm. We conclude that a circadian mechanism determines the timing of cell division in Chlamydomonas reinhardtii.

Transcriptional regulation during the mammalian cell cycle

Progression of the cell cycle in mammalian cells, as in all other organisms, is associated with the phase-specific transcription of defined sets of genes. Such periodically expressed genes frequently encode proteins that either directly control cell-cycle progression or function in metabolic processes linked to the cell cycle, such as nucleotide and DNA biosynthesis. Here, I summarize our current knowledge and views of the mechanisms governing the coupling of cell-cycle control mechanisms to transcriptional regulation, with particular emphasis on the transcription factor E2F and its connections with cyclin-dependent kinases and the retinoblastoma gene family.

Okadaic acid, cAMP, and selected nutrients inhibit hepatocyte proliferation at different stages in G1: modulation of the cAMP effect by phosphatase inhibitors and nutrients

The protein phosphatase inhibitor okadaic acid (> 100 nM) caused an abrupt and complete cessation of primary rat hepatocyte cell cycle progression at the restriction point in late G1. A decline in the G1/S transition rate was observed in response to elevated cAMP, excess selected nutrients, and okadaic acid (< 100 nM). Excess nutrients (40 mM glucose +/- 5 mM dihydroxyacetone) acted by imposing an incomplete block in early G1. The cAMP action was potentiated by the phosphatase inhibitor microcystin, which in itself did not affect DNA replication. This suggests that cAMP acted by phosphorylating substrate(s) that is dephosphorylated by a microcystin-sensitive phosphatase. The additive effects of submaximal concentrations of okadaic acid and cAMP analogs indicated that okadaic acid and cAMP acted via different pathways. In conclusion, okadaic acid, cAMP, and excess nutrients, acting through distinct pathways, inhibited hepatocytes in different parts of the G1 phase.

The proliferative response and anti-oncogene expression in old 2BS cells after growth factor stimulation

The limited replicative lifespan of diploid human cells in vitro (cellular senescence) serves as a cellular model of aging. We examined the proliferative response of 2BS cells of different population doubling levels to fibroblast growth factor (FGF). DNA synthesis was measured by thymidine incorporation. As the cells aged, there was a significant decrease in the stimulation of DNA synthesis by FGF addition (P < 0.01). The effective concentration of FGF and the latent period prior to DNA synthesis did not change. Expression of Rb and p53 mRNA after growth factor stimulation was also examined. Young and old cells had similar Rb mRNA levels, whereas the p53 mRNA level was significantly reduced in old cells. After both cells were treated by FGF or epidermal growth factor (EGF), Rb expression increased 210-275% in young cells and 50-60% in old ones. However, no significant change was found in p53 gene transcriptions after FGF addition. The results further suggest that cell aging is associated with a progressive loss of the ability of cells to respond to growth factors.

The initiation of senescence and its relationship to embryonic cell differentiation

Mouse embryonic stem cells have an unlimited lifespan in cultures if they are prevented from differentiating. After differentiating, they produce cells which divide only a limited number of times. These changes seen in cultures parallel events that occur in the developing embryo, where immortal embryonic cells differentiate and produce mortal somatic ones. The data strongly suggest that differentiation initiates senescence, but this view entails additional assumptions in order to explain how the highly differentiated sexual gametes manage to remain potentially immortal. Cells differentiate by blocking expression from large parts of their genome and it is suggested that losses or gains of genetic totipotency determine cellular lifespans. Cells destined to be somatic do not regain totipotency and senesce, while germ-line cells regain complete genome expression and immortality after meiosis and gamete fusions. Losses of genetic totipotency could induce senescence by lowering the levels of repair and maintenance enzymes.

Signal transduction by the antigen receptors of B and T lymphocytes

B and T lymphocytes of the immune system recognize and destroy invading microorganisms but are tolerant to the cells and tissues of one's own body. The basis for this self/non-self-discrimination is the clonal nature of the B and T cell antigen receptors. Each lymphocyte has antigen receptors with a single unique antigen specificity. Multiple mechanisms ensure that self-reactive lymphocytes are eliminated or silenced whereas lymphocytes directed against foreign antigens are activated only when the appropriate antigen is present. The key element in these processes is the ability of the antigen receptors to transmit signals to the interior of the lymphocyte when they bind the antigen for which they are specific. Whether these signals lead to activation, tolerance, or cell death is dependent on the maturation state of the lymphocytes as well as on signals from other receptors. We review the role of antigen receptor signaling in the development and activation of B and T lymphocytes and also describe the biochemical signaling mechanisms employed by these receptors. In addition, we discuss how signal transduction pathways activated by the antigen receptors may alter gene expression, regulate the cell cycle, and induce or prevent programmed cell death.