Interdependence between muscle differentiation and cell-cycle control

Functional impact and targetability of PI3KCA, GNAS, and PTEN mutations in a spindle cell rhabdomyosarcoma with MYOD1 L122R mutation

Spindle cell/sclerosing rhabdomyosarcoma (ssRMS) is a rare subtype of rhabdomyosarcoma, commonly harboring a gain-of-function L122R mutation in the muscle-specific master transcription factor MYOD1. MYOD1-mutated ssRMS is almost invariably fatal, and development of novel therapeutic approaches based on the biology of the disease is urgently needed. MYOD1 L122R affects the DNA-binding domain and is believed to confer MYC-like properties to MYOD1, driving oncogenesis. Moreover, the majority of the MYOD1-mutated ssRMS harbor additional alterations activating the PI3K/AKT pathway. It is postulated that the PI3K/AKT pathway cooperates with MYOD1 L122R. To address this biological entity, we established and characterized a new patient-derived ssRMS cell line OHSU-SARC001, harboring MYOD1 L122R as well as alterations in PTEN, PIK3CA, and GNAS. We explored the functional impact of these aberrations on oncogenic signaling with gain-of-function experiments in C2C12 murine muscle lineage cells. These data reveal that PIK3CA^I459_T462del, the novel PIK3CA variant discovered in this patient specimen, is a constitutively active kinase, albeit to a lesser extent than PI3KCA^E545K, a hotspot oncogenic mutation. Furthermore, we examined the effectiveness of molecularly targeted PI3K/AKT/mTOR and RAS/MAPK inhibitors to block oncogenic signaling and suppress the growth of OHSU-SARC001 cells. Dual PI3K/mTOR (LY3023414, bimiralisib) and AKT inhibitors (ipatasertib, afuresertib) induced dose-dependent reductions in cell growth. However, mTOR-selective inhibitors (everolimus, rapamycin) alone did not exert cytotoxic effects. The MEK1/2 inhibitor trametinib did not impact proliferation even at the highest doses tested. Our data suggest that molecularly targeted strategies may be effective in PI3K/AKT/mTOR-activated ssRMS. Taken together, these data highlight the importance of utilizing patient-derived models to assess molecularly targetable treatments and their potential as future treatment options.

Hnf4a Is Required for the Development of Cdh6-Expressing Progenitors into Proximal Tubules in the Mouse Kidney

BACKGROUND

Hepatocyte NF 4α (Hnf4a) is a major regulator of renal proximal tubule (PT) development. In humans, a mutation in HNF4A impairs PT functions and is associated with Fanconi renotubular syndrome (FRTS). In mice, mosaic deletion of Hnf4a in the developing kidney reduces the population of PT cells, leading to FRTS-like symptoms. The molecular mechanisms underlying the role of Hnf4a in PT development remain unclear.

METHODS

The gene deletion tool Osr2Cre removed Hnf4a in developing nephrons in mice, generating a novel model for FRTS. Immunofluorescence analysis characterized the mutant phenotype, and lineage analysis tested whether Cadherin-6 (Cdh6)-expressing cells are PT progenitors. Genome-wide mapping of Hnf4a binding sites and differential gene analysis of Hnf4a mutant kidneys identified direct target genes of Hnf4a.

RESULTS

Deletion of Hnf4a with Osr2Cre led to the complete loss of mature PT cells, lethal to the Hnf4a mutant mice. Cdh6^high, lotus tetragonolobus lectin-low (LTL^low) cells serve as PT progenitors and demonstrate higher proliferation than Cdh6^low, LTL^high differentiated PT cells. Additionally, Hnf4a is required for PT progenitors to differentiate into mature PT cells. Genomic analyses revealed that Hnf4a directly regulates the expression of genes involved in transmembrane transport and metabolism.

CONCLUSIONS

Hnf4a promotes the differentiation of PT progenitors into mature PT cells by regulating the expression of genes associated with reabsorption, the major function of PT cells.

TAp63gamma is required for the late stages of myogenesis

p53 family members, p63 and p73, play a role in controlling early stage of myogenic differentiation. We demonstrated that TAp63gamma, unlike the other p53 family members, is markedly up-regulated during myogenic differentiation in murine C2C7 cell line. We also found that myotubes formation was inhibited upon TAp63gamma knock-down, as also indicated by atrophyic myotubes and reduction of myoblasts fusion index. Analysis of TAp63gamma-dependend transcripts identified several target genes involved in skeletal muscle contractility energy metabolism, myogenesis and skeletal muscle autocrine signaling. These results indicate that TAp63gamma is a late marker of myogenic differentiation and, by controlling different sub-sets of target genes, it possibly contributes to muscle growth, remodeling, functional differentiation and tissue homeostasis.

Mechanisms simultaneously regulate smooth muscle proliferation and differentiation

Vascular smooth muscle cell (VSMC) differentiation and proliferation are two important physiological processes during vascular development. The phenotypic alteration from differentiated to proliferative VSMC contributes to the development of several major cardiovascular diseases including atherosclerosis, hypertension, restenosis after angioplasty or bypass, diabetic vascular complications, and transplantation arteriopathy. Since the VSMC phenotype in these pathological conditions resembles that of developing VSMC during embryonic development, understanding of the molecular mechanisms that control VSMC differentiation will provide fundamental insights into the pathological processes of these cardiovascular diseases. Although VSMC differentiation is usually accompanied by an irreversible cell cycle exit, VSMC proliferation and differentiation occur concurrently during embryonic development. The molecular mechanisms simultaneously regulating these two processes, however, remain largely unknown. Our recent study demonstrates that cell division cycle 7, a key regulator of cell cycle, promotes both VSMC differentiation and proliferation through different mechanisms during the initial phase of VSMC differentiation. Conversely, Krüppel-like factor 4 appears to be a repressor for both VSMC differentiation and proliferation. This review attempts to highlight the novel role of cell division cycle 7 in TGF-β-induced VSMC differentiation and proliferation. The role of Krüppel-like factor 4 in suppressing these two processes will also be discussed.

MyoD regulates p57kip2 expression by interacting with a distant cis-element and modifying a higher order chromatin structure

The bHLH transcription factor MyoD, the prototypical master regulator of differentiation, directs a complex program of gene expression during skeletal myogenesis. The up-regulation of the cdk inhibitor p57^kip2 plays a critical role in coordinating differentiation and growth arrest during muscle development, as well as in other tissues. p57^kip2 displays a highly specific expression pattern and is subject to a complex epigenetic control driving the imprinting of the paternal allele. However, the regulatory mechanisms governing its expression during development are still poorly understood. We have identified an unexpected mechanism by which MyoD regulates p57^kip2 transcription in differentiating muscle cells. We show that the induction of p57^kip2 requires MyoD binding to a long-distance element located within the imprinting control region KvDMR1 and the consequent release of a chromatin loop involving p57^kip2 promoter. We also show that differentiation-dependent regulation of p57^kip2, while involving a region implicated in the imprinting process, is distinct and hierarchically subordinated to the imprinting control. These findings highlight a novel mechanism, involving the modification of higher order chromatin structures, by which MyoD regulates gene expression. Our results also suggest that chromatin folding mediated by KvDMR1 could account for the highly restricted expression of p57^kip2 during development and, possibly, for its aberrant silencing in some pathologies.

Direct laser machining-induced topographic pattern promotes up-regulation of myogenic markers in human mesenchymal stem cells

The engineering of tissue is preferably done with stem cells, which can be differentiated into the tissue of interest using biochemical or physical cues. While much effort has been focused on using biological factors to regulate stem cell differentiation, recently interest in the contribution of physical factors has increased. In this work, three-dimensional (3-D) microchannels with topographic micropatterns were fabricated by femtosecond laser machining on a biodegradable polymer (poly(L-lactide-co-ε-caprolactone)) substrate. Two substrates with narrow and wide channels respectively were created. Human mesenchymal stem cells (hMSCs) were cultured on the scaffolds for cell proliferation and cellular organization. Gene expression and the immunostaining of myogenic and neurogenic markers were studied. Both scaffolds improved the cell alignment along the channels as compared to the control group. Microfilaments within hMSCs were more significantly aligned and elongated on the narrower microchannels. The gene expression study revealed significant up-regulation of several hallmark markers associated with myogenesis for hMSCs cultured on the scaffold with narrow microchannels, while osteogenic and neurogenic markers were down-regulated or remained similar to the control at day 14. Immunostaining of myogen- and neurogen-specific differentiation markers were used to further confirm the specific differentiation towards a myogenic lineage. This study demonstrates that femtosecond laser machining is a versatile tool for generating controllable 3-D microchannels with topographic features that can be used to induce specific myogenic differentiation of hMSCs in vitro, even in the absence of biological factors.

Cell division cycle 7 is a novel regulator of transforming growth factor-β-induced smooth muscle cell differentiation

Smooth muscle cell (SMC) differentiation and proliferation occur simultaneously during embryonic development. The underlying mechanisms especially common factors regulating both processes, however, remain largely unknown. The present study has identified cell division cycle 7 (Cdc7) as one of the factors mediating both the proliferation and SMC differentiation. TGF-β induces Cdc7 expression and phosphorylation in the initial phase of SMC differentiation of pluripotent mesenchymal C3H10T1/2 cells. Cdc7 specific inhibitor or shRNA knockdown suppresses TGF-β-induced expression of SMC early markers including α-SMA, SM22α, and calponin. Cdc7 overexpression, on the other hand, enhances SMC marker expression. Cdc7 function in inducing SMC differentiation is independent of Dumbbell former 4 or Dbf4, the catalytic subunit of Cdc7 critical for cell proliferation, suggesting that Cdc7 mediates SMC differentiation through a mechanism distinct from cell proliferation. Cdc7 regulates SMC differentiation via activating SMC marker gene transcription. Knockdown of Cdc7 by shRNA inhibits SMC marker gene promoter activities. Mechanistically, Cdc7 interacts with Smad3 to induce SMC differentiation. Smad3 is required for Cdc7 function in inducing SMC promoter activities and marker gene expression. Likewise, Cdc7 enhances Smad3 binding to SMC marker promoter via supporting Smad3 nuclear retention and physically interacting with Smad3. Taken together, our studies have demonstrated a novel role of Cdc7 in SMC differentiation.

Role of heat shock proteins in stem cell behavior

Stress response is well appreciated to induce the expression of heat shock proteins (Hsps) in the cell. Numerous studies have demonstrated that Hsps function as molecular chaperones in the stabilization of intracellular proteins, repairing damaged proteins, and assisting in protein translocation. Various kinds of stem cells (embryonic stem cells, adult stem cells, or induced pluripotent stem cells) have to maintain their stemness and, under certain circumstances, undergo stress. Therefore, Hsps should have an important influence on stem cells. Actually, numerous studies have indicated that some Hsps physically interact with a number of transcription factors as well as intrinsic and extrinsic signaling pathways. Importantly, alterations in Hsp expression have been demonstrated to affect stem cell behavior including self-renewal, differentiation, sensitivity to environmental stress, and aging. This chapter summarizes recent findings related to (1) the roles of Hsps in maintenance of stem cell dormancy, proliferation, and differentiation; (2) the expression signature of Hsps in embryonic/adult stem cells and differentiated stem cells; (3) the protective roles of Hsps in transplanted stem cells; and (4) the possible roles of Hsps in stem cell aging.

Regulation of erythroid differentiation by miR-376a and its targets

Lineage differentiation is a continuous process during which fated progenitor cells execute specific programs to produce mature counterparts. This lineage-restricted pathway can be controlled by particular regulators, which are usually exclusively expressed in certain cell types or at specific differentiation stages. Here we report that miR-376a participates in the regulation of the early stages of human erythropoiesis by targeting cyclin-dependent kinase 2 (CDK2) and Argonaute 2 (Ago2). Among various human leukemia cell lines, miR-376a was only detected in K562 cells which originated from a progenitor common to the erythroid and megakaryotic lineages. Enforced expression of miR-376a or silencing of CDK2 and Ago2 by RNAi inhibits erythroid differentiation of K562 cells. Hematopoietic progenitor cells transduced with miR-376a showed a significant reduction of their erythroid clonogenic capacity. MiR-376a is relatively abundant in erythroid progenitor cells, where it reduces expression of CDK2 and maintains a low level of differentiation due to cell cycle arrest and decreased cell growth. Following erythroid induction, miR-376a is significantly down-regulated and CDK2 is released from miR-376a inhibition, thereby facilitating the escape of progenitor cells from the quiescent state into erythroid differentiation. Moreover, our results establish a functional link between miR-376a and Ago2, a key factor in miRNA biogenesis and silencing pathways with novel roles in human hematopoiesis.

pRb-dependent cyclin D3 protein stabilization is required for myogenic differentiation

The expression of retinoblastoma (pRb) and cyclin D3 proteins is highly induced during the process of skeletal myoblast differentiation. We have previously shown that cyclin D3 is nearly totally associated with hypophosphorylated pRb in differentiated myotubes, whereas Rb-/- myocytes fail to accumulate the cyclin D3 protein despite normal induction of cyclin D3 mRNA. Here we report that pRb promotes cyclin D3 protein accumulation in differentiating myoblasts by preventing cyclin D3 degradation. We show that cyclin D3 displays rapid turnover in proliferating myoblasts, which is positively regulated through glycogen synthase kinase 3beta (GSK-3beta)-mediated phosphorylation of cyclin D3 on Thr-283. We describe a novel interaction between pRb and cyclin D3 that maps to the C terminus of pRb and to a region of cyclin D3 proximal to the Thr-283 residue and provide evidence that the pRb-cyclin D3 complex formation in terminally differentiated myotubes hinders the access of GSK-3beta to cyclin D3, thus inhibiting Thr-283 phosphorylation. Interestingly, we observed that the ectopic expression of a stabilized cyclin D3 mutant in C2 myoblasts enhances muscle-specific gene expression; conversely, cyclin D3-null embryonic fibroblasts display impaired MyoD-induced myogenic differentiation. These results indicate that the pRb-dependent accumulation of cyclin D3 is functionally relevant to the process of skeletal muscle cell differentiation.

p27Kip1 acts downstream of N-cadherin-mediated cell adhesion to promote myogenesis beyond cell cycle regulation

It is widely acknowledged that cultured myoblasts can not differentiate at very low density. Here we analyzed the mechanism through which cell density influences myogenic differentiation in vitro. By comparing the behavior of C2C12 myoblasts at opposite cell densities, we found that, when cells are sparse, failure to undergo terminal differentiation is independent from cell cycle control and reflects the lack of p27Kip1 and MyoD in proliferating myoblasts. We show that inhibition of p27Kip1 expression impairs C2C12 cell differentiation at high density, while exogenous p27Kip1 allows low-density cultured C2C12 cells to enter the differentiative program by regulating MyoD levels in undifferentiated myoblasts. We also demonstrate that the early induction of p27Kip1 is a critical step of the N-cadherin-dependent signaling involved in myogenesis. Overall, our data support an active role of p27Kip1 in the decision of myoblasts to commit to terminal differentiation, distinct from the regulation of cell proliferation, and identify a pathway that, reasonably, operates in vivo during myogenesis and might be part of the phenomenon known as "community effect".

v-Src inhibits myogenic differentiation by interfering with the regulatory network of muscle-specific transcriptional activators at multiple levels

The conversion of skeletal myoblasts to terminally differentiated myocytes is negatively controlled by several growth factors and oncoproteins. In this study, we have investigated the molecular mechanisms by which v-Src, a prototypic tyrosine kinase, perturbs myogenesis in primary avian myoblasts and in established murine C2C12 satellite cells. We determined the expression levels of the cell cycle regulators pRb, cyclin D1 and D3 and cyclin-dependent kinase inhibitors p21 and p27 in v-Src-transformed myoblasts and found that, in contrast to myogenin, they are normally modulated by differentiative cues, implying that v-Src affects myogenesis independent of cell proliferation. We then examined the levels of expression, DNA-binding ability and transcription-activation potentials of myogenic regulatory factors in transformed myoblasts and in myotubes after reactivation of a temperature-sensitive allele of v-Src. Our results reveal two distinct potential modes of repression targeted to myogenic factors. On the one hand, we show that v-Src reversibly inhibits the expression of MyoD and myogenin in C2C12 cells and of myogenin in quail myoblasts. Remarkably, these loci become resistant to activation of the kinase in the postmitotic compartment. On the other hand, we demonstrate that v-Src efficiently inhibits muscle gene expression by repressing the transcriptional activity of myogenic factors without affecting MyoD DNA-binding activity. Indeed, forced expression of MyoD and myogenin allows terminal differentiation of transformed myoblasts. Finally, we found that ectopic expression of the coactivator p300 restores transcription from extrachromosomal muscle-specific promoters.

MyoD stimulates RB promoter activity via the CREB/p300 nuclear transduction pathway

The induction of RB gene transcription by MyoD is a key event in the process of skeletal muscle differentiation, because elevated levels of the retinoblastoma protein are essential for myoblast cell cycle arrest as well as for the terminal differentiation and survival of postmitotic myocytes. We previously showed that MyoD stimulates transcription from the RB promoter independently of direct binding to promoter sequences. Here we demonstrate that stimulation by MyoD requires a cyclic AMP-responsive element (CRE) in the RB promoter, bound by the transcription factor CREB in differentiating myoblasts. We also show that both the CREB protein level and the level of phosphorylation of the CREB protein at Ser-133 rapidly increase at the onset of muscle differentiation and that both remain high throughout the myogenic process. Biochemical and functional evidence indicates that in differentiating myoblasts, MyoD becomes associated with CREB and is targeted to the RB promoter CRE in a complex also containing the p300 transcriptional coactivator. The resulting multiprotein complex stimulates transcription from the RB promoter. These and other observations strongly suggest that MyoD functions by promoting the efficient recruitment of p300 by promoter-bound, phosphorylated CREB.

MyoD induces apoptosis in the absence of RB function through a p21(WAF1)-dependent re-localization of cyclin/cdk complexes to the nucleus

During differentiation of skeletal myoblasts, MyoD promotes growth arrest through the induction of the cdk inhibitor p21 and the accumulation of hypophosphorylated RB protein. Myoblasts lacking RB function fail to accomplish full differentiation and undergo apoptosis. Here we show that exogenous MyoD induces apoptosis in several cell backgrounds sharing RB inactivation. This process is associated with increased levels of cell cycle-driving proteins and aberrant cell cycle progression. The inability of MyoD to induce apoptosis in a p21-null background, highlights a requirement of p21 in RB-regulated apoptosis during myogenesis. This pro-apoptotic function of p21 cannot be exerted by simple p21 over-expression, but requires the co-operation of MyoD. We also suggest that the essential aspect of p21 activity involved in such a process is related to its ability to induce the nuclear accumulation and aberrant activity of cyclin/cdk complexes. These results establish a novel link between MyoD, p21 and RB during myogenesis, providing new insights into the antagonism between muscle differentiation and loss of RB function.

The alpha-like RNA polymerase II core subunit 3 (RPB3) is involved in tissue-specific transcription and muscle differentiation via interaction with the myogenic factor myogenin

RNA polymerase II core subunit 3 (RPB3) is an a-like core subunit of RNA polymerase II (pol II). It is selectively down-regulated upon treatment with doxorubicin (dox). Due to the failure of skeletal muscle cells to differentiate when exposed to dox, we hypothesized that RPB3 is involved in muscle differentiation. To this end, we have isolated human muscle RPB3-interacting proteins by using yeast two-hybrid screening. It is of interest that an interaction between RPB3 and the myogenic transcription factor myogenin was identified. This interaction involves a specific region of RPB3 protein that is not homologous to the prokaryotic a subunit. Although RPB3 contacts the basic helix-loop-helix (HLH) region of myogenin, it does not bind other HLH myogenic factors such as MyoD, Myf5, and MRF4. Coimmunoprecipitation experiments indicate that myogenin contacts the pol II complex and that the RPB3 subunit is responsible for this interaction. We show that RPB3 expression is regulated during muscle differentiation. Exogenous expression of RPB3 slightly promotes myogenin transactivation activity and muscle differentiation, whereas the region of RPB3 that contacts myogenin, when used as a dominant negative molecule (Sud), counteracts these effects. These results indicate for the first time that the RPB3 pol II subunit is involved in the regulation of tissue-specific transcription.

Selective repression of myoD transcription by v-Myc prevents terminal differentiation of quail embryo myoblasts transformed by the MC29 strain of avian myelocytomatosis virus

We have investigated the mechanism by which expression of the v-myc oncogene interferes with the competence of primary quail myoblasts to undergo terminal differentiation. Previous studies have established that quail myoblasts transformed by myc oncogenes are severely impaired in the accumulation of mRNAs encoding the myogenic transcription factors Myf-5, MyoD and Myogenin. However, the mechanism responsible for such a repression remains largely unknown. Here we present evidence that v-Myc selectively interferes with quail myoD expression at the transcriptional level. Cis-regulatory elements involved in the auto-activation of qmyoD are specifically targeted in this unique example of transrepression by v-Myc, without the apparent participation of Myc-specific E-boxes or InR sequences. Transiently expressed v-Myc efficiently interfered with MyoD-dependent transactivation of the qmyoD regulatory elements, while the myogenin promoter was unaffected. Finally, we show that forced expression of MyoD in v-myc-transformed quail myoblasts restored myogenin expression and promoted extensive terminal differentiation. These data suggest that transcriptional repression of qmyoD is a major and rate-limiting step in the molecular pathway by which v-Myc severely inhibits terminal differentiation in myogenic cells.

A role for histone deacetylase HDAC1 in modulating the transcriptional activity of MyoD: inhibition of the myogenic program

The molecular mechanism(s) that are responsible for suppressing MyoD's transcriptional activities in undifferentiated skeletal muscle cells have not yet been determined. We now show that MyoD associates with a histone deacetylase-1 (HDAC1) in these cells and that this interaction is responsible for silencing MyoD-dependent transcription of endogenous p21 as well as muscle-specific genes. Specifically, we present evidence that HDAC1 can bind directly to MyoD and use an acetylated MyoD as a substrate in vitro, whereas a mutant version of HDAC1 (H141A) can not. Further more, this mutant also fails to repress MyoD-mediated transcription in vivo, and unlike wild-type HDAC1 it can not inhibit myogenic conversion, as judged by confocal microscopy. Finally, we show that an endogenous MyoD can be acetylated upon its conversion to a hypophosphorylated state and only when the cells have been induced to differentiate. These results provide for a model which postulates that MyoD may be co-dependent on HDAC1 and P/CAF for temporally controlling its transcriptional activity before and after the differentiation of muscle cells.

Distinct effects of Rac1 on differentiation of primary avian myoblasts

Rho family GTPases have been implicated in the regulation of the actin cytoskeleton in response to extracellular cues and in the transduction of signals from the membrane to the nucleus. Their role in development and cell differentiation, however, is little understood. Here we show that the transient expression of constitutively active Rac1 and Cdc42 in unestablished avian myoblasts is sufficient to cause inhibition of myogenin expression and block of the transition to the myocyte compartment, whereas activated RhoA affects myogenic differentiation only marginally. Activation of c-Jun N-terminal kinase (JNK) appears not to be essential for block of differentiation because, although Rac1 and Cdc42 GTPases modestly activate JNK in quail myoblasts, a Rac1 mutant defective for JNK activation can still inhibit myogenic differentiation. Stable expression of active Rac1, attained by infection with a recombinant retrovirus, is permissive for terminal differentiation, but the resulting myotubes accumulate severely reduced levels of muscle-specific proteins. This inhibition is the consequence of posttranscriptional events and suggests the presence of a novel level of regulation of myogenesis. We also show that myotubes expressing constitutively active Rac1 fail to assemble ordered sarcomeres. Conversely, a dominant-negative Rac1 variant accelerates sarcomere maturation and inhibits v-Src-induced selective disassembly of I-Z-I complexes. Collectively, our findings provide a role for Rac1 during skeletal muscle differentiation and strongly suggest that Rac1 is required downstream of v-Src in the signaling pathways responsible for the dismantling of tissue-specific supramolecular structures.

Rhabdomyosarcoma--working out the pathways

Rhabdomyosarcomas constitute a collection of childhood malignancies thought to arise as a consequence of regulatory disruption of skeletal muscle progenitor cell growth and differentiation. Our understanding of the pathogenesis of this neoplasm has recently benefited from the study of normal and malignant myogenic cells in vitro, facilitating the identification of diagnostic cytogenetic markers and the elucidation of mechanisms by which myogenesis is regulated. It is now appreciated that the delicate balance between proliferation and differentiation, mutually exclusive yet intimately associated processes, is normally controlled in large part through the action of a multitude of growth factors, whose signals are interpreted by members of the MyoD family of helix - loop - helix proteins, and key regulatory cell cycle factors. The latter have proven to be frequent targets of mutational events that subvert myogenesis and promote the development of rhabdomyosarcoma. Although significant progress has been made in the treatment of rhabdomyosarcoma, patients presenting with metastatic disease or certain high risk features are still faced with a dismal prognosis. Only now are genetically engineered mouse models becoming available that are certain to provide fresh insights into the molecular/genetic pathways by which rhabdomyosarcomas arise and progress, and to suggest novel avenues of therapeutic opportunity.

Critical role played by cyclin D3 in the MyoD-mediated arrest of cell cycle during myoblast differentiation

During the terminal differentiation of skeletal myoblasts, the activities of myogenic factors regulate not only tissue-specific gene expressions but also the exit from the cell cycle. The induction of cell cycle inhibitors such as p21 and pRb has been shown to play a prominent role in the growth arrest of differentiating myoblasts. Here we report that, at the onset of differentiation, activation by MyoD of the Rb, p21, and cyclin D3 genes occurs in the absence of new protein synthesis and with the requirement of the p300 transcriptional coactivator. In differentiated myocytes, cyclin D3 also becomes stabilized and is found nearly totally complexed with unphosphorylated pRb. The detection of complexes containing cyclin D3, cdk4, p21, and PCNA suggests that cdk4, along with PCNA, may get sequestered into high-order structures held together by pRb and cyclin D3. Cyclin D3 up-regulation and stabilization is inhibited by adenovirus E1A, and this correlates with the ability of E1A to promote pRb phosphorylation; conversely, the overexpression of cyclin D3 in differentiated myotubes counteracts the E1A-mediated reactivation of DNA synthesis. These results indicate that cyclin D3 critically contributes to the irreversible exit of differentiating myoblasts from the cell cycle.

Prolonged passive stretch of rat soleus muscle provokes an increase in the mRNA levels of the muscle regulatory factors distributed along the entire length of the fibers

The mRNA levels of the adult and the neonatal sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPases (SERCA1a and SERCA1b, respectively) and those of the muscle regulatory factors (MRFs: myoD, myf-5, myogenin, MRF4) have been assessed by RT PCR in rat soleus muscles immobilized for 3 days in an extended position (passive stretch). The transcript level of the fast type SERCA1a Ca(2+)-transport ATPase decreased to half of its normal value, whereas that of neonatal SERCA1b isoform increased 5-fold above control in stretched muscles. Immunostaining of muscle cross sections showed that the fraction of fibers expressing the SERCA1a protein was decreased evenly along the length of the stretched muscles indicating that a transformation occurred of fast fibers to slow ones. The mRNA levels of MRFs were elevated 3- to 6-fold above the normal level and were distributed evenly along the length of the stretched muscles. However in the controls these transcripts were more abundant at both ends of the muscle. The stretch increased the level of myoD and immunocytochemistry showed the expression of myoD protein in a number of nuclei of the stretched muscles whereas it was practically undetectable by this method in the control muscles. Western blotting did not indicate a significant stretch-induced increase in the level of the myogenin protein, in spite of the fact that immunocytochemistry tended to show more myogenin-positive nuclei in stretched muscles as compared to the controls. These data indicate that after 3 days of passive stretch the central and the terminal parts of the soleus muscle adapt similarly by increasing the levels of the MRFs, by decreasing the overall levels of the fast SERCA1-type of ATPase and by partially re-establishing a neonatal mode of alternative SERCA1 transcript splicing resulting in an increased SERCA1b/1a ratio.

Regulation of differentiation by HBP1, a target of the retinoblastoma protein

Differentiation is a coordinated process of irreversible cell cycle exit and tissue-specific gene expression. To probe the functions of the retinoblastoma protein (RB) family in cell differentiation, we isolated HBP1 as a specific target of RB and p130. Our previous work showed that HBP1 was a transcriptional repressor and a cell cycle inhibitor. The induction of HBP1, RB, and p130 upon differentiation in the muscle C2C12 cells suggested a coordinated role. Here we report that the expression of HBP1 unexpectedly blocked muscle cell differentiation without interfering with cell cycle exit. Moreover, the expression of MyoD and myogenin, but not Myf5, was inhibited in HBP1-expressing cells. HBP1 inhibited transcriptional activation by the MyoD family members. The inhibition of MyoD family function by HBP1 required binding to RB and/or p130. Since Myf5 might function upstream of MyoD, our data suggested that HBP1 probably blocked differentiation by disrupting Myf5 function, thus preventing expression of MyoD and myogenin. Consistent with this, the expression of each MyoD family member could reverse the inhibition of differentiation by HBP1. Further investigation implicated the relative ratio of RB to HBP1 as a determinant of whether cell cycle exit or full differentiation occurred. At a low RB/HBP1 ratio cell cycle exit occurred but there was no tissue-specific gene expression. At elevated RB/HBP1 ratios full differentiation occurred. Similar changes in the RB/HBP1 ratio have been observed in normal C2 differentiation. Thus, we postulate that the relative ratio of RB to HBP1 may be one signal for activation of the MyoD family. We propose a model in which a checkpoint of positive and negative regulation may coordinate cell cycle exit with MyoD family activation to give fidelity and progression in differentiation.

The activity of differentiation factors induces apoptosis in polyomavirus large T-expressing myoblasts

It is commonly accepted that pathways that regulate proliferation/differentiation processes, if altered in their normal interplay, can lead to the induction of programmed cell death. In a previous work we reported that Polyoma virus Large Tumor antigen (PyLT) interferes with in vitro terminal differentiation of skeletal myoblasts by binding and inactivating the retinoblastoma antioncogene product. This inhibition occurs after the activation of some early steps of the myogenic program. In the present work we report that myoblasts expressing wild-type PyLT, when subjected to differentiation stimuli, undergo cell death and that this cell death can be defined as apoptosis. Apoptosis in PyLT-expressing myoblasts starts after growth factors removal, is promoted by cell confluence, and is temporally correlated with the expression of early markers of myogenic differentiation. The block of the initial events of myogenesis by transforming growth factor beta or basic fibroblast growth factor prevents PyLT-induced apoptosis, while the acceleration of this process by the overexpression of the muscle-regulatory factor MyoD further increases cell death in this system. MyoD can induce PyLT-expressing myoblasts to accumulate RB, p21, and muscle- specific genes but is unable to induce G0(0) arrest. Several markers of different phases of the cell cycle, such as cyclin A, cdk-2, and cdc-2, fail to be down-regulated, indicating the occurrence of cell cycle progression. It has been frequently suggested that apoptosis can result from an unbalanced cell cycle progression in the presence of a contrasting signal, such as growth factor deprivation. Our data involve differentiation pathways, as a further contrasting signal, in the generation of this conflict during myoblast cell apoptosis.

Coupled transcriptional and translational control of cyclin-dependent kinase inhibitor p18INK4c expression during myogenesis

Terminal differentiation of many cell types involves permanent withdrawal from the cell division cycle. The p18INK4c protein, a member of the p16/INK4 cyclin-dependent kinase (CDK) inhibitor family, is induced more than 50-fold during myogenic differentiation of mouse C2C12 myoblasts to become the predominant CDK inhibitor complexed with CDK4 and CDK6 in terminally differentiated myotubes. We have found that the p18INK4c gene expresses two mRNA transcripts--a 2.4-kb transcript, p18(L), and a 1.2-kb transcript, p18(S). In proliferating C2C12 myoblasts, only the larger p18(L) transcript is expressed from an upstream promoter. As C2C12 cells are induced to differentiate into permanently arrested myotubes, the abundance of the p18(L) transcript decreases. The smaller p18(S) transcript expressed from a downstream promoter becomes detectable by 12 h postinduction and is the predominant transcript expressed in terminally differentiated myotubes. Both transcripts contain coding exons 2 and 3, but p18(L) uniquely contains an additional noncoding 1.2-kb exon, exon 1, corresponding exclusively to the 5' untranslated region (5' UTR). The expression pattern of the shorter p18(S) transcript, but not that of the longer p18(L) transcript, correlates with terminal differentiation of muscle, lung, liver, thymus, and eye lens cells during mouse embryo development. The presence of the long 5' UTR in exon 1 attenuated the translation of p18(L) transcript, while its absence from the shorter p18(S) transcript resulted in significantly more efficient translation of the p18 protein. Our results demonstrate that during terminal muscle cell differentiation, induction of the p18 protein is regulated by promoter switching coupled with translational control.

A natural hepatocyte growth factor/scatter factor autocrine loop in myoblast cells and the effect of the constitutive Met kinase activation on myogenic differentiation

As a rule, hepatocyte growth factor/scatter factor (HGF/SF) is produced by mesenchymal cells, while its receptor, the tyrosine kinase encoded by the met proto-oncogene, is expressed by the neighboring epithelial cells in a canonical paracrine fashion. In the present work we show that both HGF/SF and met are coexpressed by undifferentiated C2 mouse myoblasts. In growing cells, the autocrine loop is active as the receptor exhibits a constitutive phosphorylation on tyrosine that can be abrogated by exogenously added anti-HGF/SF neutralizing antibodies. The transcription of HGF/SF and met genes is downregulated when myoblasts stop proliferating and differentiate. The coexpression of HGF/SF and met genes is not exclusive to C2 cells since it has been assessed also in other myogenic cell lines and in mouse primary satellite cells, suggesting that HGF/SF could play a role in muscle development through an autocrine way. To analyze the biological effects of HGF/SF receptor activation, we stably expressed the constitutively activated receptor catalytic domain (p65(tpr-met)) in C2 cells. This active kinase determined profound changes in cell shape and inhibited myogenesis at both morphological and biochemical levels. Notably, a complete absence of muscle regulatory markers such as MyoD and myogenin was observed in p65(tpr-met) highly expressing C2 clones. We also studied the effects of the ectopic expression of human isoforms of met receptor (h-met) and of HGF/SF (h-HGF/SF) in stable transfected C2 cells. Single constitutive expression of h-met or h-HGF/SF does not alter substantially the growth and differentiation properties of the myoblast cells, probably because of a species-specific ligand-receptor interaction. A C2 clone expressing simultaneously both h-met and h-HGF/SF is able to grow in soft agar and shows a decrease in myogenic potential comparable to that promoted by p65(tpr-met) kinase. These data indicate that a met kinase signal released from differentiation-dependent control provides a negative stimulus for the onset of myogenic differentiation.