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

A novel, nerve growth factor-activated pathway involving nitric oxide, p53, and p21WAF1 regulates neuronal differentiation of PC12 cells

During development, neuronal differentiation is closely coupled with cessation of proliferation. We use nerve growth factor (NGF)-induced differentiation of PC12 pheochromocytoma cells as a model and find a novel signal transduction pathway that blocks cell proliferation. Treatment of PC12 cells with NGF leads to induction of nitric oxide synthase (NOS) (Peunova, N., and Enikolopov, G. (1995) Nature 375, 68-73). The resulting nitric oxide (NO) acts as a second messenger, activating the p21(WAF1) promoter and inducing expression of p21(WAF1) cyclin-dependent kinase inhibitor. NO activates the p21(WAF1) promoter by p53-dependent and p53-independent mechanisms. Blocking production of NO with an inhibitor of NOS reduces accumulation of p53, activation of the p21(WAF1) promoter, expression of neuronal markers, and neurite extension. To determine whether p21(WAF1) is required for neurite extension, we prepared a PC12 line with an inducible p21(WAF1) expression vector. Blocking NOS with an inhibitor decreases neurite extension, but induction of p21(WAF1) with isopropyl-1-thio-beta-D-galactopyranoside restored this response. Levels of p21(WAF1) induced by isopropyl-1-thio-beta-D-galactopyranoside were similar to those induced by NGF. Therefore, we have identified a signal transduction pathway that is activated by NGF; proceeds through NOS, p53, and p21(WAF1) to block cell proliferation; and is required for neuronal differentiation by PC12 cells.

Oligodendrocyte precursor differentiation is perturbed in the absence of the cyclin-dependent kinase inhibitor p27Kip1

During development of the central nervous system, oligodendrocyte progenitor cells (O-2A) undergo an orderly pattern of cell proliferation and differentiation, culminating in the ability of oligodendrocytes to myelinate axons. Here we report that p27(Kip1), a cyclin-dependent kinase inhibitor, is an important component of the decision of O-2A cells to withdraw from the cell cycle. In vitro, accumulation of p27 correlates with differentiation of oligodendrocytes. Furthermore, only a fraction of O-2A cells derived from p27-knockout mice differentiate successfully compared to controls. Inability to differentiate correlates with continued proliferation, suggesting that p27 is an important component of the machinery required for the G1/G0 transition in O-2A cells. In vivo, expansion of O-2A precursors before withdrawal, in part, leads to a greater number of oligodendrocytes. Together these data indicate a role for p27 during the decision to withdraw from the cell cycle in the oligodendrocyte lineage.

PPARgamma induces cell cycle withdrawal: inhibition of E2F/DP DNA-binding activity via down-regulation of PP2A

PPAR gamma is an adipose-selective nuclear hormone receptor that plays a key role in the control of adipocyte differentiation. Previous studies indicated that activation of ectopically expressed PPAR gamma induces differentiation when cells have ceased growth because of confluence. We show here that ligand activation of PPAR gamma is sufficient to induce growth arrest in fibroblasts and SV40 large T-antigen transformed, adipogenic HIB1B cells. Cell cycle withdrawal is accompanied by a decrease in the DNA-binding and transcriptional activity of the E2F/DP complex, which is attributable to an increase in the phosphorylation of these proteins, especially DP-1. This effect is a consequence of decreased expression of the catalytic subunit of the serine-threonine phosphatase PP2A. These data suggest an important role for PP2A in the control of E2F/DP activity and a new mode of cell cycle control in differentiation.

Features of Macrophage Differentiation Induced by p19INK4d, a Specific Inhibitor of Cyclin D–Dependent Kinases

The mitogen-dependent induction of cyclin D–dependent kinase activity is required for cells to enter the DNA synthetic (S) phase of their division cycle. Immature 32Dcl3 myeloid cells (32D) proliferating in the presence of interleukin-3 (IL-3) normally express cyclins D2 and D3, which assemble into binary holoenzyme complexes with their catalytic subunits, CDK4 and CDK6. When 32D cells are switched to medium containing granulocyte colony-stimulating factor (G-CSF ) instead of IL-3, D-type cyclins are degraded and, in the absence of their associated kinase activity, the cells arrest in the first gap phase (G1 ) of the cell cycle and differentiate to neutrophils. We derived 32D cells in which the expression of p19INK4d, a specific polypeptide inhibitor of CDK4 and CDK6, is regulated by the heavy metal-inducible sheep metallothionein promoter. Induction of p19INK4d in response to zinc prolonged cell survival in the absence of growth factor treatment. When maintained in medium containing both IL-3 and zinc, these cells lost cyclin D–dependent kinase activity, underwent G1 phase arrest, and acquired certain morphologic, antigenic, and functional properties of mononuclear phagocytes. Cells induced to express p19INK4d did not synthesize receptors for macrophage colony-stimulating factor (M-CSF/CSF-1) and reverted to an immature myeloid phenotype when shifted back into medium containing IL-3 alone. These cells exhibited accelerated differentiation to neutrophils in response to G-CSF but also gave rise to macrophage-like cells when maintained in medium containing both G-CSF and zinc. Therefore, the acquisition of macrophage properties in response to zinc treatment neither depended upon IL-3 nor upon G1 phase arrest per se and instead reflects some ability of p19INK4d, and presumably cyclin D–dependent kinases, to affect myeloid differentiation.

Features of Macrophage Differentiation Induced by p19INK4d, a Specific Inhibitor of Cyclin D–Dependent Kinases

The mitogen-dependent induction of cyclin D–dependent kinase activity is required for cells to enter the DNA synthetic (S) phase of their division cycle. Immature 32Dcl3 myeloid cells (32D) proliferating in the presence of interleukin-3 (IL-3) normally express cyclins D2 and D3, which assemble into binary holoenzyme complexes with their catalytic subunits, CDK4 and CDK6. When 32D cells are switched to medium containing granulocyte colony-stimulating factor (G-CSF ) instead of IL-3, D-type cyclins are degraded and, in the absence of their associated kinase activity, the cells arrest in the first gap phase (G1 ) of the cell cycle and differentiate to neutrophils. We derived 32D cells in which the expression of p19INK4d, a specific polypeptide inhibitor of CDK4 and CDK6, is regulated by the heavy metal-inducible sheep metallothionein promoter. Induction of p19INK4d in response to zinc prolonged cell survival in the absence of growth factor treatment. When maintained in medium containing both IL-3 and zinc, these cells lost cyclin D–dependent kinase activity, underwent G1 phase arrest, and acquired certain morphologic, antigenic, and functional properties of mononuclear phagocytes. Cells induced to express p19INK4d did not synthesize receptors for macrophage colony-stimulating factor (M-CSF/CSF-1) and reverted to an immature myeloid phenotype when shifted back into medium containing IL-3 alone. These cells exhibited accelerated differentiation to neutrophils in response to G-CSF but also gave rise to macrophage-like cells when maintained in medium containing both G-CSF and zinc. Therefore, the acquisition of macrophage properties in response to zinc treatment neither depended upon IL-3 nor upon G1 phase arrest per se and instead reflects some ability of p19INK4d, and presumably cyclin D–dependent kinases, to affect myeloid differentiation.

The retinoblastoma protein: a master regulator of cell cycle, differentiation and apoptosis

The retinoblastoma susceptibility gene is a tumour suppressor and its product retinoblastoma protein (pRb) has been known for 10 years as a repressor of progression towards S phase. Its major activity was supposed to be sequestration or inactivation of the transcription factor E2F which is required for activation of S phase genes. However, within recent years growing evidence has been accumulating for a more general function of pRb at both the transcriptional level and the cellular level. pRb not only regulates the activity of certain protein-encoding genes but also the activity of RNA polymerase pol I and pol III transcription. This protein appears to be the major player in a regulatory circuit in the late G1 phase, the so-called restriction point. Moreover, it is involved in regulating an elusive switch point between cell cycle, differentiation and apoptosis. Here, it seems to cooperate with another major tumour suppressor, p53. Thus, pRb sits at the interface of the most important cell-regulatory processes and therefore deserves close attention by specialists from different fields of research. This review provides an introduction to the complex functions of pRb.

Isolation of a differentiation-defective myoblastic cell line, INC-2, expressing muscle LIM protein under differentiation-inducing conditions

A non-differentiating myoblastic cell line, INC2, and a differentiating cell line, COM3, were established from the mouse myoblastic cell line C2C12. Under differentiation conditions, both COM3 and INC2 cells stopped proliferation in a similar manner. The COM3 cells then differentiated into myotubes during the 4-day differentiation culture. In contrast, almost none of the INC2 cells differentiated into myotubes even in differentiation medium. Reverse transcriptase-polymerase chain reaction (RT-PCR) and immunoblot analyses showed that the levels of myogenin and MyoD proteins were significantly decreased in INC2 cells. The differentiation marker sarcomeric myosin heavy chain (MHC) was expressed in COM3 but not in INC2 cells. In contrast, both INC2 and COM3 cells expressed another myogenic regulatory factor, muscle LIM protein (MLP), in a differentiation condition-dependent manner. These results suggest that MLP gene expression is regulated in a myogenin/MyoD-independent manner. Enforced expression of the myogenin gene induced MHC expression in INC2 cells. Thus, the signaling pathway situated downstream is assumed to be intact in INC2 cells and suppression of myogenin, gene expression may be a primary defect in INC2 cells.

Cyclin D1 overexpression promotes cardiomyocyte DNA synthesis and multinucleation in transgenic mice

D-type cyclin/cyclin-dependent kinase (CDK) complexes regulate transit through the restriction point of the cell cycle, and thus are required for the initiation of DNA synthesis. Transgenic mice which overexpress cyclin D1 in the heart were produced to determine if D-type cyclin deregulation would alter myocardial development. Cyclin D1 overexpression resulted in a concomitant increase in CDK4 levels in the adult myocardium, as well as modest increases in proliferating cell nuclear antigen and CDK2 levels. Flow cytometric and morphologic analyses of dispersed cell preparations indicated that the adult transgenic cardiomyocytes had abnormal patterns of multinucleation. Histochemical analyses confirmed a marked increase in number of cardiomyocyte nuclei in sections prepared from the transgenic mice as compared with those from control animals. Tritiated thymidine incorporation analyses revealed sustained cardiomyocyte DNA synthesis in adult transgenic hearts.

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.

p21WAF1 mutations and human malignancies

During the past few years, several categories of cyclin-dependent kinase inhibitors (CDKIs), which negatively regulate cyclin/cyclin-dependent kinase (CDK) activities, were cloned. The p21WAF1, also known as CIP1 or SDI1, was the first reported CDKI: it's expression is induced by wild-type p53. The p21WAF1 is a potent inhibitor of most cyclin/CDK complexes and also inhibits the ability of the proliferating cell nuclear antigen (PCNA) to activate DNA polymerase d. Alterations of the cell-cycle can cause cellular transformation. We analysed 471 primary samples from 15 types of human malignancies and 36 cell lines for structural alterations of the p21WAF1 gene. No changes were found in the coding region of p21WAF1 gene by polymerase-chain reaction-single-strand conformation polymorphism (PCR-SSCP) analysis. Many of these tumors had a normal p53 gene. Other investigators showed that p21WAF1 knockout mice did not have an increased incidence of cancer, while p53 knock-out mice did. Taken together, the absence of alterations of p21WAF1 in a series of malignancies suggests that p21WAF1 may not have a role in either onset or progression of most human cancers. Furthermore, p53 probably activates additional, critical tumor suppressor pathways.

Cyclin dependent kinase 5, cdk5, is a positive regulator of myogenesis in mouse C2 cells

We have examined the expression, activity and localization of cyclin dependent kinase 5 (cdk5), during myogenesis. Cdk5 protein was found expressed in adult mouse muscle. In murine C2 cells, both the protein level and kinase activity of cdk5 showed a marked increase during early myogenesis with a peak between 36 and 48 hours of differentiation, decreasing as myotubes fuse after 60 to 72 hours. This increase in cdk5 protein level was specific for differentiation and not simply related to cell cycle arrest since it was not observed in fibroblasts grown for 48 hours in low serum medium. Indirect immunofluorescence using monospecific purified anti-cdk5 antibodies showed a low level cytoplasmic staining in proliferative myoblasts, a rapid increase in nuclear staining during the initial 12 hours of differentiation and a predominant nuclear staining in myotubes. Microinjection of plasmids encoding wild-type cdk5 into C2 myoblasts enhanced differentiation as assessed by both myogenin and troponin T expression after 48 hours of differentiation. In contrast, microinjection of plasmids encoding a dominant negative mutant of cdk5 inhibited the onset of differentiation. These data imply a previously unsuspected role for cdk5 protein kinase as a positive modulator of early myogenesis.

Glucocorticoid inhibition of fibroblast proliferation and regulation of the cyclin kinase inhibitor p21Cip1

Glucocorticoids inhibit the proliferation of fibroblastic cells in vivo and in culture; however, the molecular mechanism that accounts for this effect has remained obscure. We have undertaken to elucidate the mechanism whereby glucocorticoids decrease the rate of proliferation of mouse L929 fibroblastic cells. Addition of dexamethasone to mid-log phase fibroblasts prolongs G1 phase. This increase in the G1 interval is associated with, and probably due to, inhibition of phosphorylation of the product of the Rb-1 tumor suppressor gene, pRb. Inhibition of pRb phosphorylation by cyclin D-dependent kinases can be demonstrated in vitro. Nevertheless, there is no detectable change in the expression of cyclin D1, cyclin D2, or cyclin D3. Cyclin-dependent kinase-4 (Cdk4) and Cdk6 are not down-regulated in L929 cells after addition of glucocorticoids, and the abundance of cyclin D/Cdk4 complexes does not change. Inhibition of pRb kinase activity is associated with an increase in the abundance of one of the Cdk inhibitors, p21Cip1. The abundance of another cyclin kinase inhibitor, p27Kip1, remains constant. The amount of Cdk4 that is bound to p21Cip1 increases rapidly after addition of dexamethasone, and the activity of Cdk4-pRb kinase decreases in parallel. These results indicate that glucocorticoid inhibition of fibroblast proliferation is due to induction of p21Cip1, which binds to and inactivates cyclinD/Cdk4 complexes. The abundance of p21 mRNA increases about 5-fold within 2 h after addition of dexamethasone. This effect does not obtain in L929 mutants that are null for the glucocorticoid receptor, and a variant that expresses the glucocorticoid receptor from a tetracycline-repressible expression vector demonstrates induction of p21 mRNA only in the absence of tetracycline. Cycloheximide does not block induction of p21 mRNA, and dexamethasone has no detectable effect on the apparent rate of degradation of p21 mRNA. Nuclear run-on transcription of the Cip1 gene increases within 2 h after addition of dexamethasone. This effect can be blocked by tetracycline-mediated repression of the glucocorticoid receptor.

Transforming growth factor beta stabilizes p15INK4B protein, increases p15INK4B-cdk4 complexes, and inhibits cyclin D1-cdk4 association in human mammary epithelial cells

The effects of transforming growth factor beta (TGF-beta) were studied in closely related human mammary epithelial cells (HMEC), both finite-life-span 184 cells and immortal derivatives, 184A1S, and 184A1L5R, which differ in their cell cycle responses to TGF-beta but express type I and type II TGF-beta receptors and retain TGF-beta induction of extracellular matrix. The arrest-resistant phenotype was not due to loss of cyclin-dependent kinase (cdk) inhibitors. TGF-beta was shown to regulate p15INK4B expression at at least two levels: mRNA accumulation and protein stability. In TGF-beta-arrested HMEC, there was not only an increase in p15 mRNA but also a major increase in p5INK4B protein stability. As cdk4- and cdk6-associated p15INK4B increased during TGF-beta arrest of sensitive cells, there was a loss of cyclin D1, p21Cip1, and p27Kip1 from these kinase complexes, and cyclin E-cdk2-associated p27Kip1 increased. In HMEC, p15INK4B complexes did not contain detectable cyclin. p15INK4B from both sensitive and resistant cells could displace in vitro cyclin D1, p21Cip1, and p27Kip1 from cdk4 isolated from sensitive cells. Cyclin D1 could not be displaced from cdk4 in the resistant 184A1L5R cell lysates. Thus, in TGF-beta arrest, p15INK4B may displace already associated cyclin D1 from cdks and prevent new cyclin D1-cdk complexes from forming. Furthermore, p27Kip1 binding shifts from cdk4 to cyclin E-cdk2 during TGF-beta-mediated arrest. The importance of posttranslational regulation of p15INK4B by TGF-beta is underlined by the observation that in TGF-beta-resistant 184A1L5R, although the p15 transcript increased, p15INK4B protein was not stabilized and did not accumulate, and cyclin D1-cdk association and kinase activation were not inhibited.

Amphibian limb regeneration: rebuilding a complex structure

The ability to regenerate complex structures is widespread in metazoan phylogeny, but among vertebrates the urodele amphibians are exceptional. Adult urodeles can regenerate their limbs by local formation of a mesenchymal growth zone or blastema. The generation of blastemal cells depends not only on the local extracellular environment after amputation or wounding but also on the ability to reenter the cell cycle from the differentiated state. The blastema replaces structures appropriate to its proximodistal position. Axial identity is probably encoded as a graded property that controls cellular growth and movement through local cell interactions. The molecular basis is not understood, but proximodistal identity in newt blastemal cells may be respecified by signaling through a retinoic acid receptor isoform. The possibility of inducing a blastema on a mammalian limb cannot be discounted, although the molecular constraints are becoming clearer as we understand more about the mechanisms of urodele regeneration.

Cyclins, cyclin-dependent kinases and differentiation

Cyclin-dependent kinases and their regulatory subunits, the cyclins, are known to regulate progression through the cell cycle. Yet these same proteins are often expressed in non-cycling, differentiated cells. This review surveys the available information about cyclins and cyclin-dependent kinases in differentiated cells and explores the possibility that these proteins may have important functions that are independent of cell cycle regulation.

Enhanced ribosomal association of p27(Kip1) mRNA is a mechanism contributing to accumulation during growth arrest

p27(Kip1) regulates the decision to enter into S-phase or withdraw from the cell cycle by establishing an inhibitory threshold above which G1 cyclin-dependent kinases accumulate before activation. We have used the HL-60 cell line to study regulation of p27 as cells withdraw from the cell cycle following treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA). We found that the amount of p27 is maximal in G0 cells, lower in G1 cells, and undetectable in S-phase cells. In contrast to the protein, the amount of p27 mRNA was the same in these populations, suggesting that accumulation of p27 during the cell cycle and as cells withdraw from the cell cycle is controlled by post-transcriptional mechanisms. In S-phase cells, the degradation of p27 appears to predominate as a regulatory mechanism. In G0 cells, there was an increase in the synthesis rate of p27. Our data demonstrate that, in G0 cells, accumulation of p27 is due to an increase in the amount of p27 mRNA in polyribosomes.

Identification of functional domains in the neuronal Cdk5 activator protein

Cyclin-dependent kinase 5 (Cdk5) is activated by the neuronal-specific activator protein, p35. In contrast to the activation of typical CDKs by cyclin subunits, p35.Cdk5 was not further activated by the CDK-activating kinase (CAK) and was neither phosphorylated nor inhibited by the Tyr-15-specific Wee1 kinase. The previously identified proteolytic active fragment of p35, p25 (residues 91-307) as well as the slightly smaller fragment containing residues 109-291, was found to be sufficient to bind and activate Cdk5. Other CDKs, including Cdk2, associated weakly with p25. However, their kinase activity was only activated to the low level observed for cyclin A.Cdk2 without Thr-160 phosphorylation, and phosphorylation of Thr-160 in Cdk2 did not activate the p25.Cdk2 complex further. We have identified distinct regions in p35 required for binding to Cdk5 or activation of Cdk5. Residues approximately 150-200 of p35 were sufficient for binding to Cdk5, but residues approximately 279-291 were needed in addition for activation of Cdk5 in vitro.

Cyclin D1 (PRAD1) alternative transcript b: full-length cDNA cloning and expression in breast cancers

The cyclin D1/PRAD1 protooncogene is a key regulator of the G1 phase of the cell cycle and has been incriminated in the pathogenesis of a variety of primary human tumors. Recently, part of a novel alternatively spliced cyclin D1 transcript, called transcript b, has been identified. This variant transcript showed a failure of splicing at the 3' end of exon 4 and as a result, the expected protein product is altered at its C-terminus. Because of similar transcript sizes, previous Northern analyses would not have been expected to distinguish the two variants, and the relative levels of the two cyclin D1 transcripts in human tumors is unknown. To elucidate the role of cyclin D1 transcript b, we have isolated cDNA clones of this variant transcript from human breast cancer cell lines and report the sequence of the entire coding region of the cDNA. The protein predicted from the cDNA sequence consists of 274 amino acid residues and lacks a PEST sequence in its C-terminus. Examination of the levels of the two alternative cyclin D1 transcripts in primary breast cancers and breast cancer cell lines by Northern blot analysis and reverse transcription-polymerase chain reaction (RT-PCR) assays showed that the variant transcript b is indeed expressed in primary breast cancers and breast cancer cell lines, but the level of transcript b is dramatically lower than that of the originally reported transcript a of the cyclin D1 gene. In breast cancers, oncogenic overexpression of cyclin D1 mRNA appears to consist overwhelmingly of transcript a, and the role of transcript b, if any, in oncogenesis remains to be established. Science Ireland Ltd.

MyoD functions as a transcriptional repressor in proliferating myoblasts

The myogenic basic helix-loop-helix (myo-bHLH) proteins are a family of transcriptional regulators expressed in myoblasts and differentiated skeletal muscle. Ectopic expression of myo-bHLH regulators transdetermines some fibroblast cell lines into myoblasts, which exit the cell cycle and differentiate into skeletal muscle when cultured in low mitogen medium. While members of the myo-bHLH family have been shown to function as transcriptional activators in differentiating muscle, the molecular basis of their function in proliferating myoblasts has not been elucidated. In this report, we present evidence that MyoD functions as a transcriptional repressor in myoblasts. We show that transcription from a cyclin B1 promoter construct is repressed in proliferating myoblasts and that repression is mediated by a pair of MyoD binding sites. We also show that transcription from the cyclin B1 promoter is repressed in proliferating C3H10T1/2 cells by ectopic expression of MyoD. These results demonstrate that MyoD can repress transcription of specific genes in proliferating cells, a novel function that may be important to maintenance of the myogenic phenotype and to cell cycle regulation in myoblasts.

Angiotensin II and serum differentially regulate expression of cyclins, activity of cyclin-dependent kinases, and phosphorylation of retinoblastoma gene product in neonatal cardiac myocytes

The hypertrophic response in cardiac myocytes and the mitogenic response in other cell types share various early cellular responses. However, how the subsequent cell growth response, such as cell cycle machinery, is regulated in cardiac hypertrophy is not understood. Using cultured neonatal rat cardiac myocytes, we examined the effect of angiotensin II (Ang II), a hypertrophic stimulus, on mRNA and protein expression of cyclins and cyclin-dependent protein kinases (cdks), activity of cdks, and phosphorylation of retinoblastoma gene product (pRb). The effect of FCS, a stimulus that was previously reported to initiate both protein and DNA synthesis in cardiac myocytes, was also examined for comparison. Ang II activated cdk4 and caused phosphorylation of pRb, peaking at 12 hours, but subsequently downregulated cyclin D1, D3, and A expression and cdk2 activity. FCS increased the expression of G1-S cyclins, caused activation of cdk4, cdk2, and cdc2, and strongly phosphorylated pRb but failed to significantly stimulate DNA synthesis in neonatal cardiac myocytes. These results suggest that Ang II transiently activates but subsequently downregulates cell cycle regulators. Induction of G1 and G1-S cyclins and activation of cdks by FCS are not sufficient to drive cardiac myocytes into S phase. The functional role of pRb phosphorylation by Ang II and serum stimulation and, by inference, the subsequent liberation of E2F in terminally differentiated myocytes remain to be elucidated.

Molecular mechanisms of myogenic coactivation by p300: direct interaction with the activation domain of MyoD and with the MADS box of MEF2C

By searching for molecules that assist MyoD in converting fibroblasts to muscle cells, we have found that p300 and CBP, two related molecules that act as transcriptional adapters, coactivate the myogenic basic-helix-loop-helix (bHLH) proteins. Coactivation by p300 involves novel physical interactions between p300 and the amino-terminal activation domain of MyoD. In particular, disruption of the FYD domain, a group of three amino acids conserved in the activation domains of other myogenic bHLH proteins, drastically diminishes the transactivation potential of MyoD and abolishes both p300-mediated coactivation and the physical interaction between MyoD and p300. Two domains of p300, at its amino and carboxy terminals, independently function to both mediate coactivation and physically interact with MyoD. A truncated segment of p300, unable to bind MyoD, acts as a dominant negative mutation and abrogates both myogenic conversion and transactivation by MyoD, suggesting that endogenous p300 is a required coactivator for MyoD function. The p300 dominant negative peptide forms multimers with intact p300. p300 and CBP serve as coactivators of another class of transcriptional activators critical for myogenesis, myocyte enhancer factor 2 (MEF2). In fact, transactivation mediated by the MEF2C protein is potentiated by the two coactivators, and this phenomenon is associated with the ability of p300 to interact with the MADS domain of MEF2C. Our results suggest that p300 and CBP may positively influence myogenesis by reinforcing the transcriptional autoregulatory loop established between the myogenic bHLH and the MEF2 factors.

Bone morphogenetic protein-2 inhibits terminal differentiation of myogenic cells by suppressing the transcriptional activity of MyoD and myogenin

Bone morphogenetic protein (BMP) is a family of cytokines that induce ectopic bone formation when implanted into muscular tissues. We reported that BMP-2 inhibits the terminal differentiation of C2C12 myoblasts and converts them into osteoblast lineage cells (Katagiri, T., Yamaguchi, A., Komaki, M., Abe, E., Takahashi, N., Ikeda, T., Rosen, V., Wozney, J. M., Fujisawa-Sehara, A., and Suda, T. (1994) J. Cell Biol. 127, 1755-1766). In the present study, we examined the molecular mechanism of the inhibitory effect of BMP-2 on terminal differentiation of myogenic cells. When either MyoD or myogenin cDNA was introduced into C3H10T1/2 (10T1/2) cells with a muscle-specific CAT reporter containing four copies of the right E-box of muscle creatine kinase (MCK) enhancer, the CAT activity was dose-dependently suppressed by BMP-2. Furthermore, BMP-2 inhibited the terminal differentiation of these subclonal 10T1/2 cells that stably expressed MyoD or myogenin into mature myotubes that expressed myosin heavy chain and troponin T. The differentiation of a subclone of the MyoD-transfected NIH3T3 cells into mature muscle cells was also inhibited by BMP-2. BMP-2 induced alkaline phosphatase activity in 10T1/2-derived, but not in NIH3T3-derived MyoD-transfected cells. These cells constitutively expressed exogenous MyoD and myogenin, which were localized exclusively in the nuclei irrespective of the presence and the absence of BMP-2. However, these cells failed to express the mRNAs of endogenous myogenic factors and MCK when cultured with BMP-2. In the electrophoresis mobility shift assay using nuclear extracts of the myogenic cells, MyoD and myogenin bound to the right E-box in the enhancer region of the MCK gene even in the presence of BMP-2. These results suggest that BMP-2 inhibits the terminal differentiation of myogenic cells by suppressing the transcriptional activity of the myogenic factors.

Accumulation of the cyclin-dependent kinase inhibitor p27/Kip1 and the timing of oligodendrocyte differentiation

Many types of vertebrate precursor cells divide a limited number of times before they stop and terminally differentiate. In no case is it known what causes them to stop dividing. We have been studying this problem in the proliferating precursor cells that give rise to postmitotic oligodendrocytes, the cells that make myelin in the central nervous system. We show here that two components of the cell cycle control system, cyclin D1 and the Cdc2 kinase, are present in the proliferating precursor cells but not in differentiated oligodendrocytes, suggesting that the control system is dismantled in the oligodendrocytes. More importantly, we show that the cyclin-dependent kinase (Cdk) inhibitor p27 progressively accumulates in the precursor cells as they proliferate and is present at high levels in oligodendrocytes. Our findings are consistent with the possibility that the accumulation of p27 is part of both the intrinsic counting mechanism that determines when precursor cell proliferation stops and differentiation begins and the effector mechanism that arrests the cell cycle when the counting mechanism indicates it is time. The recent findings of others that p27-deficient mice have an increased number of cells in all of the organs examined suggest that this function of p27 is not restricted to the oligodendrocyte cell lineage.

Inhibition of myogenesis by multiple cyclin-Cdk complexes. Coordinate regulation of myogenesis and cell cycle activity at the level of E2F

During skeletal myogenesis, cell cycle withdrawal accompanies the expression of the contractile phenotype. Here we show that ectopic expression of each D-type cyclin is sufficient to inhibit the transcriptional activation of the muscle-specific creatine kinase (MCK) gene. In contrast, ectopic expression of cyclin A or cyclin E inhibits MCK expression only when they are co-expressed with their catalytic partner cyclin-dependent kinase 2 (Cdk2). For each of these conditions, myogenic transcriptional inhibition is reversed by the ectopic co-expression of the general Cdk inhibitor p21. Inhibition of MCK expression by cyclins or cyclin-Cdk combinations correlates with E2F activation, suggesting that the inhibition is mediated by the overall Rb-kinase activities of the Cdk complexes. In support of this hypothesis, a hyperactive mutant of Rb was found to partially reverse the inhibition of MCK expression by cyclin D1 and by the combination of cyclin A and Cdk2. These data demonstrate that the inhibition of myogenic transcriptional activity is a general feature of overall Cdk activity which is mediated, at least in part, by an pocket protein/E2F-dependent pathway. MCK promoter activity is also inhibited by ectopic E2F1 expression, but this inhibition is not reversed by the co-expression of p21. Analyses of a series of E2F1 mutants revealed that the transcriptional activation, leucine zipper, basic, and cyclin A/Cdk2-binding domains are dispensable, but the helix-loop-helix region is essential for myogenic inhibition. These data demonstrate that myocyte proliferation and differentiation are coordinated at the level of E2F and that these opposing activities are regulated by different E2F domains.

Influence of PDGF-BB on proliferation and transition through the MyoD-myogenin-MEF2A expression program during myogenesis in mouse C2 myoblasts

We have previously demonstrated that PDGF-BB enhances proliferation of C2 myoblasts. This has led us to examine whether the mitogenic influence of PDGF-BB in the C2 model correlates with modulation of specific steps associated with myogenic differentiation. C2 myoblasts transiting through these differentiation specific steps were monitored via immunocytochemistry. We show that the influence of PDGF on enhancing cell proliferation correlates with a delay in the emergence of cells positive for sarcomeric myosin. We further monitored the influence of PDGF-BB on differentiation steps preceding the emergence of myosin+ cells. We demonstrate that mononucleated C2 cells first express MyoD (MyoD+/myogenin- cells) and subsequently, myogenin. Cells negative for both MyoD and myogenin (the phenotype preceding the MyoD+ state) were present at all times in culture and comprised the majority, if not all, of the cells which responded mitogenically to PDGF. Additionally, the frequency of the MyoD+/myogenin+ cell phenotype was reduced in cultures receiving PDGF, suggesting that PDGF can modulate the transition of the cells into the myogenin+ state. We determined that many of the myogenin+ cells subsequently become MEF2A+ and this phenomenon is not influenced by PDGF-BB. FGF-2 also enhanced the proliferation of C2 myoblasts and suppressed the appearance of the myogenin+ cells, but did not influence the subsequent transition into the MEF2A+ state. The study raises the possibility that PDGF-BB and FGF-2 might delay the transition of the C2 cells into the MyoD+/myogenin+ state by depressing a paracrine signal that enhances differentiation.

Decreased expression and rare somatic mutation of the CIP1/WAF1 gene in human hepatocellular carcinoma

CIP1/WAF1, a critical downstream effector of tumor suppressor p53, encodes a cyclin-dependent kinase inhibitor. By Northern blot analysis, the CIP1/WAF1 mRNA level in the tumor was significantly lower than that in the corresponding normal liver from 19 Japanese patients with hepatocellular carcinoma (P < 0.05). In the tumor from only one out of 19 patients (5%), somatic mutations of the CIP1/WAF1 as well as that of p53 gene were identified by RT-PCR/SSCP analysis. These results suggest that the decreased CIP1/WAF1 expression is involved in the carcinogenesis or the progression of hepatocellular carcinoma.

Vitamin D analogues up-regulate p21 and p27 during growth inhibition of pancreatic cancer cell lines

To obtain information regarding the growth-inhibitory effect of 1,25-dihydroxyvitamin D3 and its non-calcaemic analogue 22-oxa-1,25-dihydroxyvitamin D3 on pancreatic cancer cell lines, differences in the effects of G1-phase cell cycle-regulating factors were studied in vitamin D-responsive and non-responsive cell lines. Levels of expression of cyclins (D1, E and A), cyclin-dependent kinases (2 and 4) and cyclin-dependent kinase inhibitors (p21 and p27) were analysed by Western blotting after treatment with these compounds. In the responsive cells (BxPC-3, Hs 700T and SUP-1), our observations were: (1) marked up-regulation of p21 and p27 after 24 h treatment with 10(-7) mol l(-1) 1,25-dihydroxyvitamin D3 and 22-oxa-1,25-dihydroxyvitamin D3; and (2) marked down-regulation of cyclins, cyclin-dependent kinases and cyclin-dependent kinase inhibitors after 7 days' treatment. In non-responsive cells (Hs 766T and Capan-1), no such changes were observed. In conclusion, vitamin D analogues up-regulate p21 and p27 as an early event, which in turn could block the G1/S transition and induce growth inhibition in responsive cells.

MyoD protein is differentially accumulated in fast and slow skeletal muscle fibres and required for normal fibre type balance in rodents

MyoD is a muscle-specific transcription factor involved in commitment of cells to myogenesis. MyoD mRNA levels differ between fast and slow muscles, suggesting that MyoD may regulate aspects of fibre type. Here we show that detectable MyoD protein becomes restricted during development to the nuclei of the fastest classes of fibres in fast muscles. myoDm1 mice, in which the myoD gene has been disrupted, show subtle shifts in fibre type of fast muscles toward a slower character, suggesting that MyoD is involved in the maintenance of the fast IIB/IIX fibre type. In contrast, slow muscle shifts to a faster phenotype in myoDm1. Moreover, MD6.0-lacZ transgenic mice with the myoD promoter driving lacZ, show highest beta-galactosidase activity in the fastest fibres of fast muscles, but also express low levels in slow fibres of slow, but not fast, muscles, suggesting distinct regulation of gene expression in slow fibres of fast and slow muscles.

Dacapo, a cyclin-dependent kinase inhibitor, stops cell proliferation during Drosophila development

Most cell types in multicellular eukaryotes exit from the mitotic cell cycle before terminal differentiation. We show that the dacapo gene is required to arrest the epidermal cell proliferation at the correct developmental stage during Drosophila embryogenesis. dacapo encodes an inhibitor of cyclin E/cdk2 complexes with similarity to the vertebrate Cip/Kip inhibitors. dacapo is transiently expressed beginning late in the G2 phase preceding the terminal division (mitosis 16). Mutants unable to express the inhibitor fail to arrest cell proliferation after mitosis 16 and progress through an extra division cycle. Conversely, premature dacapo expression in transgenic embryos results in a precocious G1 arrest.

Cyclin D1 overexpression in non-small cell lung carcinoma: correlation with Ki67 labelling index and poor cytoplasmic differentiation

Cyclin D1 is part of the molecular system regulating the cell cycle G1 to S transition point. Its overexpression, a common finding in carcinomas of the breast, oesophagus, and head and neck, has also been demonstrated in a high percentage of non-small cell lung carcinomas (NSCLCs). The role of cyclin D1 in NSCLC has been studied by correlating its immunoreactivity with the Ki67 labelling index in paraffin-embedded, autoclaved surgical samples of 56 NSCLC cases. In addition, flow cytometric determination of ploidy and cell cycle status was carried out on 172 fresh tumour samples from the same cases. Twenty-four (42.8 per cent) NSCLCs showed positive cyclin D1 immunostaining, a finding which showed no relationship to ploidy pattern, cell cycle phase, histological subtype, or lymph node metastasis, but was significantly associated with the Ki67 labelling index (P = 0.03) and with poor cytoplasmic differentiation (P = 0.01). Cyclin D1-positive nuclei were abundant in poorly differentiated zones and absent in the best differentiated areas, particularly in heavily keratinized fields. These data indicate that in NSCLC, cyclin D1 overexpression is not only associated with a high cell proliferation rate, but also seems to play a role in the process of tumour differentiation.

Cyclin-mediated inhibition of muscle gene expression via a mechanism that is independent of pRB hyperphosphorylation

It was recently demonstrated that ectopic expression of cyclin D1 inhibits skeletal muscle differentiation and, conversely, that expression of cyclin-dependent kinase (cdk) inhibitors facilitates activation of this differentiation program (S. S. Rao, C. Chu, and D. S. Kohtz, Mol. Cell. Biol. 14:5259-5267, 1994; S. S. Rao and D. S. Kohtz, J. Biol. Chem. 270:4093-4100, 1995; S. X. Skapek, J. Rhee, D. B. Spicer, and A. B. Lassar, Science 267:1022-1024, 1995). Here we demonstrate that cyclin D1 inhibits muscle gene expression without affecting MyoD DNA binding activity. Ectopic expression of cyclin D1 inhibits muscle gene activation by both MyoD and myogenin, including a mutated form of myogenin in which two potential inhibitory cdk phosphorylation sites are absent. Because the retinoblastoma gene product, pRB, is a known target for cyclin D1-cdk phosphorylation, we determined whether cyclin D1-mediated inhibition of myogenesis was due to hyperphosphorylation of pRB. In pRB-deficient fibroblasts, the ability of MyoD to activate the expression of muscle-specific genes requires coexpression of ectopic pRB (B. G. Novitch, G. J. Mulligan, T. Jacks, and A. B. Lassar, J. Cell Biol., 135:441-456, 1996). In these cells, the expression of cyclins A and E can lead to pRB hyperphosphorylation and can inhibit muscle gene expression. The negative effects of cyclins A or E on muscle gene expression are, however, reversed by the presence of a mutated form of pRB which cannot be hyperphosphorylated. In contrast, cyclin D1 can inhibit muscle gene expression in the presence of the nonhyperphosphorylatable form of pRB. On the basis of these results we propose that G1 cyclin-cdk activity blocks the initiation of skeletal muscle differentiation by two distinct mechanisms: one that is dependent on pRB hyperphosphorylation and one that is independent of pRB hyperphosphorylation.

pRb controls proliferation, differentiation, and death of skeletal muscle cells and other lineages during embryogenesis

Mice deficient for the RB gene (RB-/-), prior to death at embryonic day 14.5, show increased cell death in all tissues that normally express RB1: the nervous system, liver, lens, and skeletal muscle precursor cells. We have generated transgenic mice (RBlox) that express low levels of pRb, driven by an RB1 minigene. RBlox/RB-/- mutant fetuses die at birth with specific skeletal muscle defects, including increased cell death prior to myoblast fusion, shorter myotubes with fewer myofibrils, reduced muscle fibers, accumulation of elongated nuclei that actively synthesized DNA within the myotubes, and reduction in expression of the late muscle-specific genes MCK and MRF4. Thus, insufficient pRb results in failure of myogenesis in vivo, manifest in two ways. First, the massive apoptosis of myoblasts implicates a role of pRb in cell survival. Second, surviving myotubes failed to develop normally and accumulated large polyploid nuclei, implicating pRb in permanent withdrawal from the cell cycle. These results demonstrate a role for pRb during terminal differentiation of skeletal muscles in vivo and place pRb at a nodal point that controls cell proliferation, differentiation, and death.

Muscle differentiation during repair of myocardial necrosis in rats via gene transfer with MyoD

Myocardial infarcts heal by scar formation because there are no stem cells in myocardium, and because adult myocytes cannot divide and repopulate the wound. We sought to redirect the heart to form skeletal muscle instead of scar by transferring the myogenic determination gene, MyoD, into cardiac granulation (wound repair) tissue. A replication-defective adenovirus was constructed containing MyoD under transcriptional control of the Rous sarcoma virus long terminal repeat. The virus converted cultured cardiac fibroblasts to skeletal muscle, indicated by expression of myogenin and skeletal myosin heavy chains (MHCs). To determine if MyoD could induce muscle differentiation in vivo, we injected 2 x 10(9) or 10(10) pfu of either the MyoD or a control beta-galactosidase adenovirus into healing rat hearts, injured 1 wk previously by freeze-thaw. After receiving the lower viral dose, cardiac granulation tissue expressed MyoD mRNA and protein, but did not express myogenin or skeletal MHC. When the higher dose of virus was administered, double immunostaining showed that cells in reparative tissue expressed both myogenin and embryonic skeletal MHC. No muscle differentiation occurred after beta-galactosidase transfection. Thus, MyoD gene transfer can induce skeletal muscle differentiation in healing heart lesions. Modifications of this strategy might eventually provide new contractile tissue to repair myocardial infarcts.

The adaptor protein Shc couples a class of integrins to the control of cell cycle progression

We provide evidence that a class of integrins combines with the adaptor Shc and thereby with Grb2. Coimmunoprecipitation and mutagenesis experiments indicate that the recruitment of Shc is specified by the extracellular or transmembrane domain of integrin alpha subunit and suggest that this process is mediated by caveolin. Mutagenesis and dominant-negative inhibition studies reveal that Shc is necessary and sufficient for activation of the MAP kinase pathway in response to integrin ligation. Mitogens and Shc-activating integrins cooperate to promote transcription from the Fos serum response element and transit through G1. In contrast, adhesion mediated by integrins not linked to Shc results in cell cycle arrest and apoptosis even in presence of mitogens. These findings indicate that the association of specific integrins with Shc regulates cell survival and cell cycle progression.

Growth and differentiation of C2 myogenic cells are dependent on serum response factor

In order to study to what extent and at which stage serum response factor (SRF) is indispensable for myogenesis, we stably transfected C2 myogenic cells with, successively, a glucocorticoid receptor expression vector and a construct allowing for the expression of an SRF antisense RNA under the direction of the mouse mammary tumor virus long terminal repeat. In the clones obtained, SRF synthesis is reversibly down-regulated by induction of SRF antisense RNA expression by dexamethasone, whose effect is antagonized by the anti-hormone RU486. Two kinds of proliferation and differentiation patterns have been obtained in the resulting clones. Some clones with a high level of constitutive SRF antisense RNA expression are unable to differentiate into myotubes; their growth can be blocked by further induction of SRF antisense RNA expression by dexamethasone. Other clones are able to differentiate and are able to synthesize SRF, MyoD, myogenin, and myosin heavy chain at confluency. When SRF antisense RNA expression is induced in proliferating myoblasts by dexamethasone treatment, cell growth is blocked and cyclin A concentration drops. When SRF antisense RNA synthesis is induced in arrested confluent myoblasts cultured in a differentiation medium, cell fusion is blocked and synthesis of not only SRF but also MyoD, myogenin, and myosin heavy chain is inhibited. Our results show, therefore, that SRF synthesis is indispensable for both myoblast proliferation and myogenic differentiation.

Skeletal muscle cells lacking the retinoblastoma protein display defects in muscle gene expression and accumulate in S and G2 phases of the cell cycle

Viral oncoproteins that inactivate the retinoblastoma tumor suppressor protein (pRb) family both block skeletal muscle differentiation and promote cell cycle progression. To clarify the dependence of terminal differentiation on the presence of the different pRb-related proteins, we have studied myogenesis using isogenic primary fibroblasts derived from mouse embryos individually deficient for pRb, p107, or p130. When ectopically expressed in fibroblasts lacking pRb, MyoD induces an aberrant skeletal muscle differentiation program characterized by normal expression of early differentiation markers such as myogenin and p21, but attenuated expression of late differentiation markers such as myosin heavy chain (MHC). Similar defects in MHC expression were not observed in cells lacking either p107 or p130, indicating that the defect is specific to the loss of pRb. In contrast to wild-type, p107-deficient, or p130-deficient differentiated myocytes that are permanently withdrawn from the cell cycle, differentiated myocytes lacking pRb accumulate in S and G2 phases and express extremely high levels of cyclins A and B, cyclin-dependent kinase (Cdk2), and Cdc2, but fail to readily proceed to mitosis. Administration of caffeine, an agent that removes inhibitory phosphorylations on inactive Cdc2/cyclin B complexes, specifically induced mitotic catastrophe in pRb-deficient myocytes, consistent with the observation that the majority of pRb-deficient myocytes arrest in S and G2. Together, these findings indicate that pRb is required for the expression of late skeletal muscle differentiation markers and for the inhibition of DNA synthesis, but that a pRb-independent mechanism restricts entry of differentiated myocytes into mitosis.

Proliferation precedes differentiation in IGF-I-stimulated myogenesis

The insulin-like growth factors (IGFs) have dramatic and complex effects on the growth of many tissues and have been implicated in both the proliferation and differentiation of skeletal muscle cells. A detailed analysis of gene expression was performed in L6E9 myoblast cultures treated with IGF-I to dissect the early events leading to the stimulation of myogenic differentiation by this growth factor. A time course of transcript accumulation in confluent L6E9 myoblasts treated with defined media containing IGF-I revealed an initial transient decrease in myogenic factors, accompanied by an increase in cell cycle markers and cell proliferation. This pattern was reversed at later time points, when the subsequent activation of myogenic factors resulted in a net increase in structural gene expression and larger myotubes. The data presented here support the hypothesis that IGF-I activates proliferation first, and subsequently stimulates events leading to the expression of muscle-specific genes in myogenic cell cultures.

The role of the cell cycle in genitourinary carcinoma

The understanding of the cell cycle and the control of cellular proliferation has increased dramatically over the past years. Cancer is most likely caused by multiple genetic aberrations, therefore it seems likely that one or more genes involved in tumorigenesis will regulate a portion of the cell cycle. Cell cycle related genes, such as p53 are becoming important in determining prognosis. In addition, gene replacement therapy for a deleted gene may hold promise in treating malignancies. Further understanding of cell proliferation should bring marked improvements in the diagnosis and treatment of genitourinary malignancies.

c-Myc inactivation by mutant Max alters growth and morphology of NCI-H-630 colon cancer cells

The myc gene family has been implicated in multiple cell processes including proliferation, differentiation, tumorigenesis, and apoptosis. For its cellular growth promoting function, Myc must heterodimerize with Max. To study the effect of Myc inactivation on the growth and differentiation properties of epithelial tumor cells, we transfected the H-630 human colon cancer cell line with bm-max, a mutant Max protein in which DNA-binding activity has been abolished. Cells expressing high levels of bm-Max grow poorly, and the morphology of both colonies and single cells is altered. Moreover, increased bm-Max expression results in a prolonged G alpha/G1 phase accompanied by induced expression of p21 (WAF1/CIP1), elevated levels of alkaline phosphatase (ALP) activity, and accumulation of large fat granuli within the cells. These distinctive cell characteristics are associated with differentiation processes in numerous malignant cell lines. The results of this study support a model in which sequestering of endogenous Myc and Max proteins into "basic mutant" dimers lacking DNA-binding activity is sufficient both to inhibit proliferation and to induce changes in cell behavior consistent with differentiation.

Transcriptional repression by the orphan steroid receptor RVR/Rev-erb beta is dependent on the signature motif and helix 5 in the E region: functional evidence for a biological role of RVR in myogenesis

RVR/Rev-erb beta/BD73 is an orphan steroid receptor that has no known ligand in the "classical' sense. RVR binds as a monomer to an element which consists of an A/T-rich sequence upstream of the consensus hexameric half-site. However, RVR does not activate transcription and blocks transactivation of this element by ROR/RZR. The mechanism of RVR action remains obscure, hence we used the GAL4 hybrid system to identify and characterize an active transcriptional silencer in the ligand binding domain (LBD) of RVR. Rigorous deletion and mutational analysis demonstrated that this repressor domain is encoded by amino acids 416-449 of RVR. Furthermore, we demonstrated that efficient repression is dependent on the so-called LBD-specific signature motif, (F/W)AKxxxxFxxLxxxDQxxLL (which spans loop3-4 and helix 4) and helix 5 (H5; identified in the crystal structures of the steroid receptor LBDs). Although RVR is expressed in many adult tissues, including skeletal muscle, and during embryogenesis, its physiological function in differentiation and mammalian development remains unknown. Since other 'orphans', e.g. COUP-TF II and Rev-erbA alpha, have been demonstrated to regulate muscle and adipocyte differentiation, we investigated the expression and functional role of RVR during mouse myogenesis. In C2C12 myogenic cells, RVR mRNA was detected in proliferating myoblasts and was suppressed when the cells were induced to differentiate into post-mitotic, multinucleated myotubes by serum withdrawal. This decrease in RVR mRNA correlated with the appearance of muscle-specific markers (e.g. myogenin mRNA). RVR 'loss of function' studies by constitutive over-expression of a dominant negative RVR delta E resulted in increased levels of p21Cip1/Waf1 and myogenin mRNAs after serum withdrawal. Time course studies indicated that expression of RVR delta E mRNA results in the precocious induction and accumulation of myogenin and p21 mRNAs after serum withdrawal. In addition, we demonstrated that over-expression of the COUP-TF II and Rev-erbA alpha receptors in C2C12 cells completely blocked induction of p21 mRNA after serum withdrawal. In conclusion, our studies identified a potent transcriptional repression domain in RVR, characterized critical amino acids within the silencing region and provide evidence for the physiological role of RVR during myogenesis.

E2F1 inhibition of transcription activation by myogenic basic helix-loop-helix regulators

Cellular transcription factor E2F1 is thought to regulate the expression of genes important for cell cycle progression and cell proliferation. Deregulated E2F1 expression induces S-phase entry in quiescent cells and inhibits myogenic differentiation. We show here that E2F1 inhibits the activation of gene transcription by myogenic basic helix-loop-helix proteins myoD and myogenin. Transfection assay using different deletion constructs indicates that both the DNA binding and the transactivation domains of E2F1 are required for its inhibition of myoD transcription activation. However, the retinoblastoma protein (RB) binding domain is not required. Furthermore, co-transfection with the RB, which inhibits the transcription activity of E2F1, can also repress E2F1 inhibition of myoD transactivation. These results suggest an essential role of E2F1-mediated transcription in its inhibition of myogenesis.

Amplification of MDM2 inhibits MyoD-mediated myogenesis

One obvious phenotype of tumor cells is the lack of terminal differentiation. We previously classified rhabdomyosarcoma cell lines as having either a recessive or a dominant nondifferentiating phenotype. To study the genetic basis of the dominant nondifferentiating phenotype, we utilized microcell fusion to transfer chromosomes from rhabdomyosarcoma cells into C2C12 myoblasts. Transfer of a derivative chromosome 14 inhibits differentiation. The derivative chromosome 14 contains a DNA amplification. MDM2 is amplified and overexpressed in these nondifferentiating hybrids and in the parental rhabdomyosarcoma. Forced expression of MDM2 inhibits MyoD-dependent transcription. Expression of antisense MDM2 restores MyoD-dependent transcriptional activity. We conclude that amplification and overexpression of MDM2 inhibit MyoD function, resulting in a dominant nondifferentiating phenotype.

Defining the regulatory networks for muscle development

The formation of skeletal muscle during vertebrate embryogenesis requires commitment of mesodermal precursor cells to the skeletal muscle lineage, withdrawal of myoblasts from the cell cycle, and transcriptional activation of dozens of muscle structural genes. The myogenic basic helix-loop-helix (bHLH) factors - MyoD, myogenin, Myf5, and MRF4 - act at multiple points in the myogenic lineage to establish myoblast identity and to control terminal differentiation. Recent studies have begun to define the inductive mechanisms that regulate myogenic bHLH gene expression and muscle cell determination in the embryo. Myogenic bHLH factors interact with components of the cell cycle machinery to control withdrawal from the cell cycle and act combinatorially with other transcription factors to induce skeletal muscle transcription. Elucidation of these aspects of the myogenic program is leading to a detailed understanding of the regulatory circuits controlling muscle development.