Inhibition of myogenic differentiation in proliferating myoblasts by cyclin D1-dependent kinase

Tumor cell complementation groups based on myogenic potential: evidence for inactivation of loci required for basic helix-loop-helix protein activity

Basic helix-loop-helix (bHLH) proteins mediate terminal differentiation in many lineages. By using the bHLH protein MyoD, which can dominantly activate the myogenic differentiation program in numerous cell types, we demonstrated that recessive defects in bHLH protein function are present in human tumor lines. In contrast to prior work with primary cell cultures, MyoD did not activate the myogenic program in six of the eight tumor lines we tested. Cell fusions between the MyoD-defective lines and fibroblasts restored MyoD activity, indicating that the deficiency of a gene or factor prevents bHLH protein function in the tumor lines. Fusions between certain pairings of the MyoD-defective lines also restored MyoD activity, allowing the tumor lines to be assigned to complementation groups on the basis of their ability to execute the myogenic program and indicating that multiple mechanisms exist for abrogation of bHLH protein activity. These groups provide a basis for identifying genes critical for bHLH-mediated differentiation and tumor progression by using genetic complementation.

Cell cycle regulation by the retinoblastoma family of growth inhibitory proteins

The retinoblastoma family of growth-inhibitory proteins act by binding and inhibiting several proteins with growth-stimulatory activity, the most prominent of which is the cellular transcription factor E2F. In higher organisms, progression through the cell division cycle is accompanied by the cyclical activation of a number of protein kinases, the cyclin-dependent kinases. Phosphorylation of retinoblastoma family proteins by these cyclin-dependent kinases leads to release of the associated growth-stimulatory proteins which in turn mediate progression through the cell division cycle.

p16ink4a gene and hematological malignancies

Progression of eukaryotic cells through major cell cycle transitions is mediated by sequential assembly and activation of regulators, the cyclin-dependent CDKkinases (CDKs). Recent studies have identified different CDK inhibitory genes (CDKis), and two of them, p16ink4a/MTS1/CDKN2 and p15ink4b/MTS2 are both mapped to chromosome 9p21 and inhibit cyclin D-CDK4 and -CDK6 complexes. A feedback regulatory loop involving pRb, p16ink4a, and CDKs seems to regulate G1/S phases transition. p16ink4a and p15ink4b are deleted in high frequency in human cell lines and in some fresh solid tumors. Point mutations of p16ink4a have also been sequenced, especially in familial melanomas and digestive cancers but preferential mechanism of p16ink4a/p15ink4b inactivation seems to be biallelic deletion. In hematological malignancies, homozygous deletions of p16ink4a and p15ink4b occur frequently in acute lymphoblastic leukemia (ALL) (14-40%), lymphoid type blast crisis of chronic myeloid leukemia (CML), and adult T cell leukemia (ATL), but p16ink4a deletions are more frequent than p15ink4b deletions, and hemizygous deletions of either p16ink4a and p15ink4b are rare. In ALL an association of homozygous deletions of p16ink4a and p15ink4b, and T-lineage, 9p abnormalities, and prognostic factors was found in some but not all reports. This review presents recent data on p16ink4a and p15ink4b functions and analyses their implications in hematological malignancies.

Control of root growth and development by cyclin expression

Root development is plastic, with post-embryonic organogenesis being mediated by meristems. Although cell division is intrinsic to meristem initiation, maintenance and proliferative growth, the role of the cell cycle in regulating growth and development is unclear. To address this question, we examined the expression of cdc2 and cye genes, which encode the catalytic and regulatory subunits, respectively, of cyclin-dependent protein kinases that control progression through the cell cycle. Unlike cdc2, which is expressed not only in apical meristems but also before lateral root initiation in quiescent, pericycle cells arrested in the G2 phase of the cell cycle, cyc1At transcripts accumulate specifically in dividing cells immediately before cytokinesis. Ectopic expression of cyc1At under the control of the cdc2aAt promoter in Arabidopsis plants markedly accelerates growth without altering the pattern of lateral root development or inducing neoplasia. Thus cyclin expression is a limiting factor for growth, which in turn drives indeterminate development of the root system.

Induction of DNA synthesis and apoptosis in cardiac myocytes by E1A oncoprotein

Beginning during the second half of gestation, increasing numbers of cardiac myocytes withdraw from the cell cycle such that DNA synthesis is no longer detectable in these cells by neonatal day 17 in vivo. The mechanisms that exclude these and other terminally differentiated cells from the cell division cycle are poorly understood. To begin to explore the molecular basis of the barrier to G1/S progression in cardiac myocytes, we used adenoviruses to express wild-type and mutant E1A proteins in primary cultures from embryonic day 20 rats. While most of these cardiac myocytes are ordinarily refractory to DNA synthesis, even in the presence of serum growth factors, expression of wild-type E1A stimulates DNA synthesis in up to 94% or almost all successfully transduced cells. Rather than complete the cell cycle, however, these cells undergo apoptosis. Apoptosis is limited to those cells that engage in DNA synthesis, and the kinetics of the two processes suggest that DNA synthesis precedes apoptosis. Mutations in E1A that disable it from binding Rb and related pocket proteins have little effect on its ability to stimulate DNA synthesis in cardiac myocytes. In contrast, mutants that are defective in binding the cellular protein p300 stimulate DNA synthesis 2.4-4.1-fold less efficiently, even in the context of retained E1A pocket protein binding. In the absence of ElA pocket protein binding, the usual situation in the cell, loss of p300 binding severely decreases the ability of ElA to stimulate DNA synthesis. These results suggest that the barrier to G1/S progression in cardiac myocytes is mediated. at least in part, by the same molecules that gate the G1/S transition in actively cycling cells, and that p300 or related family members play an important role in this process.

Analysis of wild-type and mutant p21WAF-1 gene activities

The p21WAF-1 gene is positively regulated by the wild-type p53 protein. p21WAF-1 has been shown to interact with several cyclin-dependent kinase complexes and block the activity of G1 cyclin-dependent kinases (cdks). Mutational analysis with the p21WAF-1 gene localized a site, at amino acid residues 21 and 24 in the amino terminus of the protein, for p21WAF-1 binding to cyclins D and E. This region of the protein is conserved (residues 21 to 26) in other p21WAF-1 family members, p27kip-1 and p57kip-2. The same p21WAF-121,24 mutant also fails to bind to cyclin D1-cdk 4 or cyclin E-cdk 2 complexes in vitro, suggesting that amino acid residues 21 and 24 are important for p21WAF-1-cdk-cyclin trimeric complex interactions. The p21WAF-1 wild-type protein will suppress tumor cell growth in culture while p21WAF-1 mutant proteins with defects in residues 21 and 24 fail to suppress tumor cell growth. The overexpression of cyclin D or E in these cells will partially overcome the growth suppression of wild-type p21WAF-1 protein in cells. These results provide evidence that p21WAF-1 acts through cyclin D1-cdk4 and cyclin E-cdk2 complexes in vivo to induce the growth suppression. The p21WAF-1 binding sites for cyclins (residues 21 to 26), cdk2 (residues 49 to 71), and proliferating-cell nuclear antigen (residues 124 to 164) have all been mapped to discrete sites on the protein.

The cyclin-dependent kinase inhibitor p21 (WAF1) is required for survival of differentiating neuroblastoma cells

We are employing recent advances in the understanding of the cell cycle to study the inverse relationship between proliferation and neuronal differentiation. Nerve growth factor and aphidicolin, an inhibitor of DNA polymerases, synergistically induce neuronal differentiation of SH-SY5Y neuroblastoma cells and the expression of p21WAF1, an inhibitor of cyclin-dependent kinases. The differentiated cells continue to express p21WAF1, even after removal of aphidicolin from the culture medium. The p21WAF1 protein coimmunoprecipitates with cyclin E and inhibits cyclin E-associated protein kinase activity. Each of three antisense oligonucleotides complementary to p21WAF1 mRNA partially blocks expression of p21WAF1 and promotes programmed cell death. These data indicate that p21WAF1 expression is required for survival of these differentiating neuroblastoma cells. Thus, the problem of neuronal differentiation can now be understood in the context of negative regulators of the cell cycle.

Myogenin expression, cell cycle withdrawal, and phenotypic differentiation are temporally separable events that precede cell fusion upon myogenesis

During terminal differentiation of skeletal myoblasts, cells fuse to form postmitotic multinucleated myotubes that cannot reinitiate DNA synthesis. Here we investigated the temporal relationships among these events during in vitro differentiation of C2C12 myoblasts. Cells expressing myogenin, a marker for the entry of myoblasts into the differentiation pathway, were detected first during myogenesis, followed by the appearance of mononucleated cells expressing both myogenin and the cell cycle inhibitor p21. Although expression of both proteins was sustained in mitogen-restimulated myocytes, 5-bromodeoxyuridine incorporation experiments in serum-starved cultures revealed that myogenin-positive cells remained capable of replicating DNA. In contrast, subsequent expression of p21 in differentiating myoblasts correlated with the establishment of the postmitotic state. Later during myogenesis, postmitotic (p21-positive) mononucleated myoblasts activated the expression of the muscle structural protein myosin heavy chain, and then fused to form multinucleated myotubes. Thus, despite the asynchrony in the commitment to differentiation, skeletal myogenesis is a highly ordered process of temporally separable events that begins with myogenin expression, followed by p21 induction and cell cycle arrest, then phenotypic differentiation, and finally, cell fusion.

Cdk inhibitors in development and cancer

Activation of cyclin-dependent kinases is the primary control point of cell proliferation. Recent advances in the understanding of cell-cycle regulation have uncovered two families of proteins that bind and inhibit these kinases. These proteins are likely mediators of development and differentiation, and may provide molecular connections between the pathways of development and tumorigenesis.

An intestine-specific homeobox gene regulates proliferation and differentiation

Precise regulation of cellular proliferation, differentiation, and senescence results in the continuous renewal of the intestinal epithelium with maintenance of a highly ordered tissue architecture. Here we show that an intestine-specific homeobox gene, Cdx2, is a transcription factor that regulates both proliferation and differentiation in intestinal epithelial cells. Conditional expression of Cdx2 in IEC-6 cells, an undifferentiated intestinal cell line, led to arrest of proliferation for several days followed by a period of growth resulting in multicellular structures containing a well-formed columnar layer of cells. The columnar cells had multiple morphological characteristics of intestinal epithelial cells. Enterocyte-like cells were polarized with tight junctions, lateral membrane interdigitations, and well-organized microvilli with associated glycocalyx located at the apical pole. Remarkably, there were also cells with a goblet cell-like ultrastructure, suggesting that two of the four intestinal epithelial cell lineages may arise from IEC-6 cells. Molecular evidence for differentiation was shown by demonstrating that cells expressing high levels of Cdx2 expressed sucrase-isomaltase, an enterocyte-specific gene which is a well-defined target for the Cdx2 protein. Taken together, our data suggest that Cdx2 may play a role in directing early processes in intestinal cell morphogenesis and in the maintenance of the differentiated phenotype by supporting transcription of differentiated gene products. We propose that Cdx2 is part of a regulatory network that orchestrates a developmental program of proliferation, morphogenesis, and gene expression in the intestinal epithelium.

E2F-1 blocks terminal differentiation and causes proliferation in transgenic megakaryocytes

The transcription factor E2F-1 plays a central role in the cell cycle through its ability to activate genes involved in cell division. E2F-1 activity is regulated by a number of proteins, including the retinoblastoma susceptibility gene product, cyclin-dependent kinases, and their inhibitors, proteins that have been implicated in the control of certain developmental processes. To investigate a potential role of E2F-1 in differentiation, we assayed the ability of megakaryocytes to form platelets in an in vivo transgenic model. E2F-1 expression in megakaryocytes blocked differentiation during maturation, resulting in severe thrombocytopenia. Ultrastructural analysis of megakaryocytes revealed abnormal development characterized by hyperdemarcation of cytoplasmic membranes and reduced numbers of alpha granules. Administration of megakaryocyte growth and development factor or interleukin 6 could not overcome the differentiation block. Additionally, E2F-1 caused massive megakaryocyte accumulation in both normal and ectopic sites, first evident in E15 embryonic liver. Furthermore, significant apoptosis was observed in transgenic megakaryocytes. These data indicate that E2F-1 can prevent terminal differentiation, probably through its cell cycle-stimulatory activity.

Transcriptional activation of the Cdk inhibitor p21 by vitamin D3 leads to the induced differentiation of the myelomonocytic cell line U937

The hormonal form of vitamin D, 1,25-dihydroxyvitamin D3, acting through its cognate nuclear receptor (vitamin D3 receptor, VDR) will induce myeloid leukemic cell lines to terminally differentiate into monocytes/macrophages. Because VDR acts by transcriptionally regulating responsive genes in a ligand-dependent manner, we sought target genes of the receptor that initiate, the differentiation process in response to ligand. We screened a cDNA library prepared from the myelomonocytic U937 cell line with probes generated from either 1,25-dihydroxyvitamin D3-treated or untreated cells. We report here that a candidate clone that hybridized differentially is the Cdk inhibitor p21WAF1, CIP1. Furthermore, we show that p21 is transcriptionally induced by 1,25-dihydroxyvitamin D3 in a VDR-dependent, but not p53-dependent, manner, and we identify a functional vitamin D response element in the p21 promoter. Transient overexpression of p21 and/or the related Cdk inhibitor p27 in U937 cells in the absence of 1,25-dihydroxyvitamin D3 results in the cell-surface expression of monocyte/macrophage-specific markers, suggesting that ligand-modulated transcriptional induction of the p21 gene facilitates the induced differentiation of this monoblastic cell line. We believe that this is the first report demonstrating that the ectopic overexpression of a Cdk inhibitor such as p21 or p27 directly leads to a terminal differentiation program.

Induced differentiation, the cell cycle, and the treatment of cancer

Hybrid polar compounds, of which hexamethylene bisacetamide (HMBA) is the prototype, have been shown to be potent inducers of differentiation of many types of transformed cells. With virus-transformed murine erythroleukemia cells as a model, HMBA was shown to cause these cells to arrest in G1 phase and express globin genes. HMBA action involves modulation of factors regulating G1 to S phase progression, including a decrease in the G1 cyclin-dependent kinase 4 accumulation of underphosphorylated retinoblastoma protein, and an increase in the level of both retinoblastoma protein and the related protein, p107. In turn, p107 complexes with transcription factors such as E2F and, presumably, inhibits transcriptional activity of these factors for genes whose products are required for DNA synthesis. This provides a possible mechanism for HMBA-induced terminal cell division of transformed cells. Evidence that hybrid polar compounds have therapeutic potential for cancer treatment is also reviewed.

Differential CDK-inhibitor gene expression in aging human diploid fibroblasts

Cellular aging is accompanied by a reduction in proliferative activity and changes in gene expression. To further elucidate the mRNA phenotype of aging fibroblasts we have monitored the expression of an array of genes implicated in regulating cell-cycle progression. Fourteen genes, including 3 cyclin-dependent kinase (CDK) inhibitors (p16INK4, p21SDI/CIP/WAF and p27KIP), 5 cyclins, 4 CDKs, Cdi-1, and PCNA were tested in four primary fibroblast strains. Relative mRNA expression levels were assessed using a rapid and sensitive Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) assay called the "Primer-dropping" method. p16INK4, a specific inhibitor of the cyclin D-associated kinases CDK4 and CDK6, was, in addition to p21 and cyclin D1, overexpressed in higher passage cells, while the abundance of the D-type kinase mRNAs remained relatively constant. Levels of cyclin H, a component of the CDK-activating kinase (CAK) were markedly reduced in all strains examined, suggesting that the activity of target cyclin/CDK complexes may not be activated in aging cells. These results corroborate and extend previous observations demonstrating elevated expression of specific cell cycle genes in higher passage cells and suggest that overexpression of the CDK-inhibitors p16INK4 and p21SDI/CIP/WAF, but not p27KIP, may contribute to lower proliferative activity of senescing primary fibroblasts.

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.

Cyclin dependent kinase regulation

Cyclin-dependent kinases (CDKs) are key regulators of the cell cycle and their activities are consequently tightly regulated. Recent developments in the field of CDK regulation have included the discovery and characterization of CDK inhibitors. These developments have had an impact on our understanding of how other signalling pathways may be linked to the cell cycle machinery.

Diversification of cell cycle controls in developing embryos

During embryogenesis, a genetic program coordinates cell proliferation with morphogenesis and cell differentiation. Recent studies using Drosophila have shown how, as development proceeds, this program directs different cell types to acquire unique modes of cell cycle regulation. As maternal cell cycle factors are exhausted and replaced by differentially expressed zygotic factors, an increasing repertoire of gene products become potential regulators of the cycle. Cyclin B, Cdc25, and Cyclin E each act as limiting regulators in Drosophila in specific cell types at particular developmental stages. The genes encoding these and many other candidate regulators have been cloned from mice, but their roles in vivo have yet to be understood.

FAR1 is required for oriented polarization of yeast cells in response to mating pheromones

Cell polarization involves specifying an area on the cell surface and organizing the cytoskeleton towards that landmark. The mechanisms by which external signals are translated into internal landmarks for polarization are poorly understood. The yeast Saccharomyces cerevisiae exhibits polarized growth during mating: the actin cytoskeleton of each cell polarizes towards its partner, presumably to allow efficient cell fusion. The external signal which determines the landmark for polarization is thought to be a gradient of peptide pheromone released by the mating partner. Here we described mutants that exhibit random polarization. Using two assays, including a direct microscope assay for orientation (Segall, J. 1993. Proc. Natl. Acad. Sci. USA. 90:8332-8337), we show that these mutants cannot locate the source of a pheromone gradient although they are able to organize their cytoskeleton. These mutants appear to be defective in mating because they are unable to locate the mating partner. They carry mutations of the FAR1 gene, denoted far1-s, and identify a new function for the Far1 protein. Its other known function is to promote cell cycle arrest during mating by inhibiting a cyclin-dependent kinase (Peter, M., and I. Herskowitz. 1994. Science (Wash. DC). 265:1228-1232). The far1-s mutants exhibit normal cell cycle arrest in response to pheromone, which suggests that Far1 protein plays two distinct roles in mating: one in cell cycle arrest and the other in orientation towards the mating partner.

Opposing early and late effects of insulin-like growth factor I on differentiation and the cell cycle regulatory retinoblastoma protein in skeletal myoblasts

The mechanisms by which insulin-like growth factors (IGFs) can be both mitogenic and differentiation-promoting in skeletal myoblasts are unclear because these two processes are believed to be mutually exclusive in this tissue. The phosphorylation state of the ubiquitous nuclear retinoblastoma protein (Rb) plays an important role in determining whether myoblasts proliferate or differentiate: Phosphorylated Rb promotes mitogenesis, whereas un- (or hypo-) phosphorylated Rb promotes cell cycle exit and differentiation. We hypothesized that IGFs might affect the fate of myoblasts by regulating the phosphorylation of Rb. Although long-term IGF treatment is known to stimulate differentiation, we find that IGFs act initially to inhibit differentiation and are exclusively mitogenic. These early effects of IGFs are associated with maintenance of Rb phosphorylation typical of proliferating cells; upregulation of the gene expression of cyclin-dependent kinase 4 and cyclin D1, components of a holoenzyme that plays a principal role in mediating Rb phosphorylation; and marked inhibition of the gene expression of myogenin, a member of the MyoD family of skeletal muscle-specific transcription factors that is essential in muscle differentiation. We also find that IGF-induced inhibition of differentiation occurs through a process that is independent of its mitogenic effects. We demonstrate, thus, that IGFs regulate Rb phosphorylation and cyclin D1 and cyclin-dependent kinase 4 gene expression; together with their biphasic effects on myogenin expression, these results suggest a mechanism by which IGFs are initially mitogenic and subsequently differentiation-promoting in skeletal muscle.

Inhibition of cyclin-dependent kinase activity triggers neuronal differentiation of mouse neuroblastoma cells

Studies on the molecular mechanisms underlying neuronal differentiation are frequently performed using cell lines established from neuroblastomas. In this study we have used mouse N1E-115 neuroblastoma cells that undergo neuronal differentiation in response to DMSO. During differentiation, cyclin-dependent kinase (cdk) activities decline and phosphorylation of the retinoblastoma gene product (pRb) is lost, leading to the appearance of a pRb-containing E2F DNA-binding complex. The loss of cdk2 activity is due to a decrease in cdk2 abundance whereas loss of cdk4 activity is caused by strong association with the cdk inhibitor (CKI) p27KIP1 and concurrent loss of cdk4 phosphorylation. Moreover, neuronal differentiation can be induced by overexpression of p27KIP1 or pRb, suggesting that inhibition of cdk activity leading to loss of pRb phosphorylation, is the major determinant for neuronal differentiation.

Ras p21Val inhibits myogenesis without altering the DNA binding or transcriptional activities of the myogenic basic helix-loop-helix factors

MRF4, MyoD, myogenin, and Myf-5 are muscle-specific basic helix-loop-helix transcription factors that share the ability to activate the expression of skeletal muscle genes such as those encoding alpha-actin, myosin heavy chain, and the acetylcholine receptor subunits. The muscle regulatory factors (MRFs) also exhibit the unique capacity to initiate the myogenic program when ectopically expressed in a variety of nonmuscle cell types, most notably C3H10T1/2 fibroblasts (10T1/2 cells). The commitment of myoblasts to terminal differentiation, although positively regulated by the MRFs, also is controlled negatively by a variety of agents, including several growth factors and oncoproteins such as fibroblast growth factor (FGF-2), transforming growth factor beta 1 (TGF-beta 1), and Ras p21Val. The molecular mechanisms by which these varied agents alter myogenic terminal differentiation events remain unclear. In an effort to establish whether Ras p21Val represses MRF activity by directly targeting the MRF proteins, we examined the DNA binding and transcription activation potentials of MRF4 and MyoD when expressed in 10T1/2 cells or in 10T1/2 cells expressing Ras p21Val. Our results demonstrate that Ras p21Val inhibits terminal differentiation events by targeting the basic domain of the MRFs, and yet the mechanism underlying this inhibition does not involve altering the DNA binding or the inherent transcriptional activity of these regulatory factors. In contrast, FGF-2 and TGF-beta 1 block terminal differentiation by repressing the transcriptional activity of the MRFs. We conclude that the Ras p21Val block in differentiation operates via an intracellular signaling pathway that is distinct from the FGF-2 and TGF-beta 1 pathways.

Redistribution of the CDK inhibitor p27 between different cyclin.CDK complexes in the mouse fibroblast cell cycle and in cells arrested with lovastatin or ultraviolet irradiation

The cyclin-dependent kinase (CDK) inhibitor p27 binds and inhibits the kinase activity of several CDKs. Here we report an analysis of the behavior and partners of p27 in Swiss 3T3 mouse fibroblasts during normal mitotic cell cycle progression, as well as in cells arrested at different stages in the cycle by growth factor deprivation, lovastatin treatment, or ultraviolet (UV) irradiation. We found that the level of p27 is elevated in cells arrested in G0 by growth factor deprivation or contact inhibition. In G0, p27 was predominantly monomeric, although some portion was associated with residual cyclin A.Cdk2. During G1, all of p27 was associated with cyclin D1.Cdk4 and was then redistributed to cyclin A.Cdk2 as cells entered S phase. The loss of the monomeric p27 pool as cyclins accumulate in G1 is consistent with the in vivo and in vitro data showing that p27 binds better to cyclin.CDK complexes than to monomeric CDKs. In growing cells, the majority of p27 was associated with cyclin D1 and the level of p27 was significantly lower than the level of cyclin D1. In cells arrested in G1 with lovastatin, cyclin D1 was degraded and p27 was redistributed to cyclin A.Cdk2. In contrast to p21 (which is a p27-related CDK inhibitor and is induced by UV irradiation), the level of p27 was reduced after UV irradiation, but because cyclin D1 was degraded more rapidly than p27, there was a transient increase in binding of p27 to cyclin A.Cdk2. These data suggest that cyclin D1.Cdk4 acts as a reservoir for p27, and p27 is redistributed from cyclin D1.Cdk4 to cyclin A.Cdk2 complexes during S phase, or when cells are arrested by growth factor deprivation, lovastatin treatment, or UV irradiation. It is likely that a similar principle of redistribution of p27 is used by the cell in other instances of cell cycle arrest.

Cyclin D1 provides a link between development and oncogenesis in the retina and breast

Mice lacking cyclin D1 have been generated by gene targeting in embryonic stem cells. Cyclin D1-deficient animals develop to term but show reduced body size, reduced viability, and symptoms of neurological impairment. Their retinas display a striking reduction in cell number due to proliferative failure during embryonic development. In situ hybridization studies of normal mouse embryos revealed an extremely high level of cyclin D1 in the retina, suggesting a special dependence of this tissue on cyclin D1. In adult mutant females, the breast epithelial compartment fails to undergo the massive proliferative changes associated with pregnancy despite normal levels of ovarian steroid hormones. Thus, steroid-induced proliferation of mammary epithelium during pregnancy may be driven through cyclin D1.

Cyclin-dependent kinases and pRb: regulators of the proliferation-differentiation switch

The retinoblastoma susceptibility gene (RB1) is essential for normal embryonic development. Loss of RB1 leads to uncontrolled proliferation of a number of cell types but may also prevent proper terminal differentiation. The growth-suppressive and differentiation-inducing properties of pRb are impaired by cyclin-dependent kinase (cdk)-mediated phosphorylation. Hence, inhibition of cdk activity is probably a prerequisite for terminal differentiation. Indeed, forced cyclin or cdk expression can prevent terminal differentiation in various cell types, probably through inhibition of pRb and, possibly, differentiation-specific transcription factors.

Suppression of mammary epithelial cell differentiation by the helix-loop-helix protein Id-1

Cell proliferation and differentiation are precisely coordinated during the development and maturation of the mammary gland, and this balance invariably is disrupted during carcinogenesis. Little is known about the cell-specific transcription factors that regulate these processes in the mammary gland. The mouse mammary epithelial cell line SCp2 grows well under standard culture conditions but arrests growth, forms alveolus-like structures, and expresses beta-casein, a differentiation marker, 4 to 5 days after exposure to basement membrane and lactogenic hormones (differentiation signals). We show that this differentiation entails a marked decline in the expression of Id-1, a helix-loop-helix (HLH) protein that inactivates basic HLH transcription factors in other cell types. SCp2 cells stably transfected with an Id-1 expression vector grew more rapidly than control cells under standard conditions, but in response to differentiation signals, they arrested growth and formed three-dimensional structures similar to those of control cells. Id-1-expressing cells did not, however, express beta-casein. Moreover, 8 to 10 days after receiving differentiation signals, they lost three-dimensional organization, invaded the basement membrane, and then resumed growth. SCp2 cells expressing an Id-1 antisense vector grew more slowly than controls; in response to differentiation signals, they remained stably growth arrested and fully differentiated, as did control cells. We suggest that Id-1 renders cells refractory to differentiation signals and receptive to growth signals by inactivating one or more basic HLH proteins that coordinate growth and differentiation in the mammary epithelium.

Inhibitors of mammalian G1 cyclin-dependent kinases

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Correlation of terminal cell cycle arrest of skeletal muscle with induction of p21 by MyoD

Skeletal muscle differentiation entails the coordination of muscle-specific gene expression and terminal withdrawal from the cell cycle. This cell cycle arrest in the G0 phase requires the retinoblastoma tumor suppressor protein (Rb). The function of Rb is negatively regulated by cyclin-dependent kinases (Cdks), which are controlled by Cdk inhibitors. Expression of MyoD, a skeletal muscle-specific transcriptional regulator, activated the expression of the Cdk inhibitor p21 during differentiation of murine myocytes and in nonmyogenic cells. MyoD-mediated induction of p21 did not require the tumor suppressor protein p53 and correlated with cell cycle withdrawal. Thus, MyoD may induce terminal cell cycle arrest during skeletal muscle differentiation by increasing the expression of p21.