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

ATF5 regulates the proliferation and differentiation of oligodendrocytes

The transcription factor ATF5 is expressed in cells of the embryonic and neonatal ventricular zone/subventricular zone (VZ/SVZ), and must be down-regulated for their differentiation into neurons and astrocytes. Here, we show that ATF5 plays a major role in directing oligodendrocyte development. ATF5 is expressed by oligodendrocyte precursors but is absent from mature oligodendroglia. Constitutively expressed ATF5 maintains SVZ cells and O4(+) oligodendrocyte precursors in cycle and inhibits their differentiation into oligodendrocytes in vitro and in vivo. In contrast, ATF5 loss-of-function (LOF; produced by a dominant-negative form of the protein) accelerates oligodendrocyte differentiation of O4(+) cells in vitro and of SVZ cells in vivo. Significantly, the accelerated oligodendrocyte differentiation promoted by ATF5 LOF in vivo results in aberrant migration. Thus, appropriately regulated expression of ATF5 is required for proper expansion of oligodendroglial progenitors as well as for their timely differentiation. Regulation of oligodendrocyte, astrocyte, and neuronal differentiation indicates that ATF5 operates as a general regulator of the timing of differentiation, independent of cell lineage.

S-phase entry of oligodendrocyte lineage cells is associated with increased levels of p21Cip1

The mechanisms regulating the number of myelinating cells in the central nervous system are crucial for both normal development and repair in pathological conditions. Among relevant growth factors involved in this process, fibroblast growth factor-2 (FGF2) induces oligodendrocyte progenitors (OLPs) to proliferate and stimulates mature oligodendrocytes (OLs) to reenter the S-phase of the cell cycle. S-phase entry is modulated by the formation of complexes between cyclins and cyclin-dependent kinases (CDKs), on one hand, and by their interactions with cell cycle inhibitors (e.g., p18INK, p27Kip1, p21Cip1), on the other. Although the roles of cyclin E/CDK2 complexes and the inhibitor p27Kip1 have been extensively investigated relative to proliferation and differentiation in the OL lineage, less is known about the regulation of the formation of cyclin D1/CDK4 complexes and the role of p21Cip1 in these events. In this study, we show that the FGF2-mediated increase in bromodeoxyuridine (BrdU) incorporation into OL progenitors and mature OLs occurs concomitantly with increase in the levels of p21Cip1 and the formation of p21Cip1/cyclin D1/CDK4 ternary complexes. These complexes are functionally active is indicated by the ensuing FGF2-dependent hyperphosphorylation of the downstream target Rb. In untreated mature OLs that do not incorporate BrdU, the levels of p21Cip1 are low, and the level of the inhibitor p18INK is high. Furthermore, p18INK sequesters CDK2 into binary complexes, precluding the formation of p21Cip1/cyclin D1/CDK4 ternary complexes in these cells. Therefore, we propose that p21Cip1 is acting as a positive regulator, rather than an inhibitor, of cell cycle entry by favoring the assembly of active cyclin D1/CDK4 complexes.

Histone modifications affect timing of oligodendrocyte progenitor differentiation in the developing rat brain

Timely differentiation of progenitor cells is critical for development. In this study we asked whether global epigenetic mechanisms regulate timing of progenitor cell differentiation into myelin-forming oligodendrocytes in vivo. Histone deacetylation was essential during a specific temporal window of development and was dependent on the enzymatic activity of histone deacetylases, whose expression was detected in the developing corpus callosum. During the first 10 postnatal days, administration of valproic acid (VPA), the specific inhibitor for histone deacetylase activity, resulted in significant hypomyelination with delayed expression of late differentiation markers and retained expression of progenitor markers. Differentiation resumed in VPA-injected rats if a recovery period was allowed. Administration of VPA after myelination onset had no effect on myelin gene expression and was consistent with changes of nucleosomal histones from reversible deacetylation to more stable methylation and chromatin compaction. Together, these data identify global modifications of nucleosomal histones critical for timing of oligodendrocyte differentiation and myelination in the developing corpus callosum.

Activation of protease-activated receptor-1 triggers astrogliosis after brain injury

We have studied the involvement of the thrombin receptor [protease-activated receptor-1 (PAR-1)] in astrogliosis, because extravasation of PAR-1 activators, such as thrombin, into brain parenchyma can occur after blood-brain barrier breakdown in a number of CNS disorders. PAR1-/- animals show a reduced astrocytic response to cortical stab wound, suggesting that PAR-1 activation plays a key role in astrogliosis associated with glial scar formation after brain injury. This interpretation is supported by the finding that the selective activation of PAR-1 in vivo induces astrogliosis. The mechanisms by which PAR-1 stimulates glial proliferation appear to be related to the ability of PAR-1 receptor signaling to induce sustained extracellular receptor kinase (ERK) activation. In contrast to the transient activation of ERK by cytokines and growth factors, PAR-1 stimulation induces a sustained ERK activation through its coupling to multiple G-protein-linked signaling pathways, including Rho kinase. This sustained ERK activation appears to regulate astrocytic cyclin D1 levels and astrocyte proliferation in vitro and in vivo. We propose that this PAR-1-mediated mechanism underlying astrocyte proliferation will operate whenever there is sufficient injury-induced blood-brain barrier breakdown to allow extravasation of PAR-1 activators.

Stage-dependent fate determination of neural precursor cells in mouse forebrain

Cortical neural precursor cells (NPCs) sequentially undergo expansion, neurogenic and gliogenic phases during development, although the underlying mechanisms are poorly understood. Recent studies have identified a number of extrinsic factors that regulate the fate of NPCs. For example, we have shown that Wnt signaling induces neuronal differentiation of NPCs in an instructive manner. Importantly, Wnt signaling does so in late stage (neurogenic phase) of NPCs but not in early stage (expansion phase) of NPCs. Instead, Wnt signaling promotes proliferation of early NPCs. Likewise, STAT3-activating ligands induce astrocytic differentiation in late (gliogenic phase) but not in early (expansion and neurogenic phases) NPCs. These stage-dependent responses of NPCs might play a central role in determining the timing of differentiation and the size of final population of each differentiated cell type.

Thyroid hormone and remyelination in adult central nervous system: a lesson from an inflammatory-demyelinating disease

Re-myelination in the adult CNS has been demonstrated in different experimental models of demyelinating diseases. However, there is no clear evidence that re-myelination is effective in multiple sclerosis (MS), the most diffuse demyelinating disease. Moreover, chronic disabilities in MS are believed to be due to remyelination failure and consequent neuron damage and degeneration. Due to the presence of numerous oligodendrocyte precursors inside demyelination plaques, reasons for remyelination failure are unknown. In this paper, we reviewed data from embryonic development and in vitro studies supporting the primary role of thyroid hormone in oligodendrocyte maturation. We also reviewed personal data on the possibility of promoting myelination in chronic experimental allergic encephalomyelitis (EAE), a widely used experimental model of MS, by recruiting progenitors and channeling them into oligodendroglial lineage through the administration of thyroid hormone.

Unliganded thyroid hormone receptor beta1 inhibits proliferation of murine fibroblasts by delaying the onset of the G1 cell-cycle signals

Thyroid hormone receptors (TRs) are members of the ligand-inducible transcription factor superfamily. The two major functional TRs (alpha1 and beta1) have different spatial and temporal expression patterns and specific physiological functions for these isoforms are now starting to emerge. By expressing these TR isoforms individually in Swiss 3T3 fibroblasts, we found that TRbeta1 expression, in the absence of hormone, provokes a proliferation arrest in G0/G1, lengthening the cycling time. Upon serum stimulation TRbeta1-expressing cells showed a marked delay in the induction of cyclins D and E, in the phosphorylation of retinoblastoma protein, and in the activation of cyclin-dependent kinase 2, accompanied by increased levels of cyclin-dependent kinase inhibitor p27Kip1. Accordingly, serum-stimulated E2F-1 transcriptional activity was repressed by TRbeta1 in transient transfection experiments. Analysis of the receptor domains required for this effect confirmed that there is no need for a functional ligand-binding domain while the DNA-binding domain is essential. In this work, we demonstrate for the first time that TRbeta1 participates in the molecular mechanisms that control cell proliferation. The unliganded TRbeta1 impairs the normal induction of the G1/S cycle regulators preventing progression into the S phase.

Polypyrimidine tract-binding protein enhances the internal ribosomal entry site-dependent translation of p27Kip1 mRNA and modulates transition from G1 to S phase

The p27(Kip1) protein plays a critical role in the regulation of cell proliferation through the inhibition of cyclin-dependent kinase activity. Translation of p27(Kip1) is directed by an internal ribosomal entry site (IRES) in the 5' nontranslated region of p27(Kip1) mRNA. Here, we report that polypyrimidine tract-binding protein (PTB) specifically enhances the IRES activity of p27(Kip1) mRNA through an interaction with the IRES element. We found that addition of PTB to an in vitro translation system and overexpression of PTB in 293T cells augmented the IRES activity of p27(Kip1) mRNA but that knockdown of PTB by introduction of PTB-specific small interfering RNAs (siRNAs) diminished the IRES activity of p27(Kip1) mRNA. Moreover, the G(1) phase in the cell cycle (which is maintained in part by p27(Kip1)) was shortened in cells depleted of PTB by siRNA knockdown. 12-O-Tetradecanoylphorbol-13-acetate (TPA)-induced differentiation in HL60 cells was used to examine PTB-induced modulation of p27(Kip1) protein synthesis during differentiation. The IRES activity of p27(Kip1) mRNA in HL60 cells was increased by TPA treatment (with a concomitant increase in PTB protein levels), but the levels of p27(Kip1) mRNA remained unchanged. Together, these data suggest that PTB modulates cell cycle and differentiation, at least in part, by enhancing the IRES activity of p27(Kip1) mRNA.

Apotransferrin induces cAMP/CREB pathway and cell cycle exit in immature oligodendroglial cells

We have demonstrated previously that a single intracranial injection of apotransferrin (aTf) in neonatal rats increases myelination and accelerates differentiation of oligodendroglial cells (OLGc). In addition, we have shown through in vitro experiments that OLGc isolated from 4-day-old rats (OLGc-4) treated with aTf were more differentiated than were controls although aTf had no effect upon OLGc isolated from 10-day-old animals (OLGc-10). In the present work, we analyzed the role of second messengers in the effect of aTf upon the maturation of OLGc at different stages of development. We isolated OLGc-4 and OLGc-10 from rat brain using a Percoll density gradient and briefly treated the cells with a pulse of aTf or kept them in culture during 2 days in the presence or absence of aTf. In OLGc-4, after a short pulse of aTf, there was an increase in the levels of cyclic AMP (cAMP), in the phosphorylation of cAMP response element-binding protein (CREB) and in the DNA-binding capacity of cAMP-responsive transcription factors. Treatment of OLGc-4 with aTf diminished bromodeoxyuridine (BrdU) incorporation and changed levels of p27 and cyclin D1. This glycoprotein seemed to act on OLGc through the cAMP pathway only at early stages of development and on a certain sensitive cell population, accelerating their differentiation, probably as a consequence of premature withdrawal from the cell cycle.

Shaker-type potassium channel subunits differentially control oligodendrocyte progenitor proliferation

Oligodendrocyte precursor (OP) cells are exposed to multiple extrinsic signals that control their proliferation and differentiation. Previous cell proliferation studies and electrophysiological analysis in cultured cells and in brain slices have suggested that outward potassium channels, particularly Kv1 subunits, may have a prominent role in OP cell proliferation. In the present study, we assessed to what extent overexpression of Kv1.3, Kv1.4, Kv1.5, and Kv1.6 can affect OP cell proliferation and differentiation in culture. We observed that overexpression of Kv1.3 or Kv1.4 increased OP cell proliferation in the absence of mitogens, whereas Kv1.6 overexpression inhibited mitogen-induced OP cell cycle progression. Interestingly, Kv1.3, Kv1.4, Kv1.5, and Kv1.6 overexpression did not interfere with the kinetics of oligodendrocyte differentiation. This study represents the first demonstration that the activity of potassium channels containing distinct Kv1 subunit proteins directly controls oligodendroglial proliferation in the presence of mitogens, as well as in growth factor-free conditions.

Overexpression of p27 Kip 1, probability of cell cycle exit, and laminar destination of neocortical neurons

Neocortical projection neurons arise from a pseudostratified ventricular epithelium (PVE) from embryonic day 11 (E11) to E17 in mice. The sequence of neuron origin is systematically related to mechanisms that specify neuronal class properties including laminar fate destination. Thus, the neurons to be assembled into the deeper layers are the earliest generated, while those to be assembled into superficial layers are the later generated neurons. The sequence of neuron origin also correlates with the probability of cell cycle exit (Q) and the duration of G1-phase of the cell cycle (T(G1)) in the PVE. Both Q and T(G1) increase as neuronogenesis proceeds. We test the hypothesis that mechanisms regulating specification of neuronal laminar destination, Q and T(G1) are coordinately regulated. We find that overexpression of p27(Kip1) in the PVE from E12 to E14 increases Q but not T(G1) and that the increased Q is associated with a commensurate increase in the proportion of exiting cells that is directed to superficial layers. We conclude that mechanisms that govern specification of neocortical neuronal laminar destination are coordinately regulated with mechanisms that regulate Q and are independent of mechanisms regulatory to cell cycle duration. Moreover, they operate prior to postproliferative mechanisms necessary to neocortical laminar assembly.

Protection of p27Kip1 mRNA by quaking RNA binding proteins promotes oligodendrocyte differentiation

The quaking (Qk) locus expresses a family of RNA binding proteins, and the expression of several alternatively spliced isoforms coincides with the development of oligodendrocytes and the onset of myelination. Quaking viable (Qk(v)) mice harboring an autosomal recessive mutation in this locus have uncompacted myelin in the central nervous system owing to the inability of oligodendrocytes to properly mature. Here we show that the expression of two QKI isoforms, absent from oligodendrocytes of Qk(v) mice, induces cell cycle arrest of primary rat oligodendrocyte progenitor cells and differentiation into oligodendrocytes. Injection of retroviruses expressing QKI into the telencephalon of mouse embryos induced differentiation and migration of multipotential neural progenitor cells into mature oligodendrocytes localized in the corpus callosum. The mRNA encoding the cyclin-dependent kinase (CDK)-inhibitor p27(Kip1) was bound and stabilized by QKI, leading to an increased accumulation of p27(Kip1) protein in oligodendrocytes. Our findings demonstrate that QKI is upstream of p27(Kip1) during oligodendrocyte differentiation.

Thyroid hormone administration enhances remyelination in chronic demyelinating inflammatory disease

Chronic disabilities in multiple sclerosis are believed to be due to neuron damage and degeneration, which follow remyelination failure. Due to the presence of numerous oligodendrocyte precursors inside demyelination plaques, one reason for demyelination failure could be the inability of oligodendrocyte precursor cells to turn into myelinating oligodendrocytes. In this study, we show that thyroid hormone enhances and accelerates remyelination in an experimental model of chronic demyelination, i.e., experimental allergic encephalomyelitis in congenic female Dark Agouti rats immunized with complete guinea pig spinal cord. Thyroid hormone, when administered during the acute phase of the disease, increases expression of platelet-derived growth factor alpha receptor, restores normal levels of myelin basic protein mRNA and protein, and allows an early and morphologically competent reassembly of myelin sheaths. Moreover, thyroid hormone exerts a neuroprotective effect with respect to axonal pathology.

Characteristics of antitumor activity of mutant type p27Kip1 gene in an oral cancer cell line

It is well known that loss of the cyclin-dependent kinase inhibitor p27Kip1 protein correlates with the poor prognosis of various cancers including oral squamous cell carcinoma (SCC). Posttranslational degradation of p27Kip1 protein is mediated by phosphorylation of Thr-187 of p27Kip1 protein, which follows ubiquitination. In this study, we constructed an expression vector expressing mutant type p27Kip1 gene (pcDNA3.1-p27Kip1 mt), with mutation of Thr-187/Pro-188 (ACGCCC) to Met-187/Ile-188 (ATGATC), which is not influenced by ubiquitin-mediated degradation. We transfected mutant and wild type p27Kip1 genes into an oral SCC cell line, B88 to up-regulate the expression of mutant or wild p27Kip1 gene in each transfectant. B88-p27Kip1 mt showed significant growth inhibition than B88-p27Kip1 wt or B88-neo in vitro (p < 0.01). Also, both types of B88-p27Kip1 showed G1 arrest and apoptosis, however, B88-p27Kip1 mt showed remarkable G1 arrest. In addition, B88-p27Kip1 mt and B88-p27Kip1 wt showed markedly inhibition of the migration and out-growth of cancer cells than B88-p27Kip1 wt or B88-neo. Moreover, B88-p27Kip1 mt also showed remarkable suppression of tumor growth and cervical lymph metastasis than B88-p27Kip1 wt or B88-neo in vivo (p < 0.01). In short, the mutant type p27Kip1 gene could show more potent antitumor effects than wild type p27Kip1 gene in B88 cells. These findings suggest that mutant type p27Kip1 gene has the potential to become a novel and powerful gene therapy tool for patients with oral cancers.

Thyroid hormone regulates oligodendrocyte accumulation in developing rat brain white matter tracts

Thyroid hormone (TH) is necessary for normal axonal myelination. Myelin basic protein (MBP) is a structural protein essential for myelin function. In this study, we demonstrate that perinatal hypothyroidism regulates MBP mRNA levels via indirect mechanisms. We observed decreased MBP mRNA accumulation in the hypothyroid rat brain at postnatal (PN) d 10 and 50. Acute TH replacement did not rescue hypothyroid MBP mRNA levels at PN5, 10, or 50. TH is necessary for normal intrahemispheric commissure development including the anterior commissure (AC) and the corpus callosum (CC). We determined that perinatal hypothyroidism decreases AC area and cellularity in the developing rat brain by PN10 and 50. In the developing CC, hypothyroidism initially increases area and cellularity by PN5, but then ultimately decreases area and cellularity by PN50. MBP-expressing oligodendrocytes are a recognized target of TH and are responsible for myelination within intrahemispheric commissures. We found that hypothyroidism reduces the number of mature oligodendrocytes within both the AC and CC. This reduction is noted at PN5, 10, and 50 in the AC and by PN10 and 50 in the CC. Together, these data suggest that TH regulates MBP mRNA levels through indirect mechanisms. These data demonstrate the complex mechanisms whereby TH regulates myelination in the developing brain.

Prognostic significance of p27(kip-1) expression in colorectal adenocarcinomas is associated with tumor stage

PURPOSE

Although the decreased expression of p27(kip-1), a cyclin-dependent kinase inhibitor, has been correlated with advanced tumor stage and short survival of patients with colorectal adenocarcinomas (CRCs), its prognostic value based on the tumor site, tumor stage, and patient ethnicity was not assessed. Therefore, in this study, we investigated whether the prognostic value of p27(kip-1) expression varies with the tumor site, tumor stage and patient ethnicity.

EXPERIMENTAL DESIGN

We evaluated 206 (85 African Americans and 121 Caucasians) archival tissue specimens of first primary CRCs for immunohistochemical expression of p27(kip-1), and its prognostic significance was analyzed using univariate Kaplan-Meier and multivariate Cox regression survival methods.

RESULTS

Although, similar proportion of CRCs with decreased p27(kip-1) expression was observed in all stages (range, 26-36%), the decreased p27(kip-1) expression has been shown as a marker of poor prognosis only for patients with stage III tumors both in univariate (log-rank test, P = 0.014) and multivariate (hazard ratio = 3.2, 95% confidence interval = 1.3-7.7; P = 0.01) survival analyses. The decreased expression of p27(kip-1) was associated with a high histologic grade (P = 0.016) in stage II CRCs, and with distal tumors (P = 0.001), tumor invasion (P = 0.044), and with local recurrence (P = 0.008) in stage III CRCs.

CONCLUSIONS

No prognostic significance was found for p27(kip-1) expression in stages I, II, or IV CRCs, and its prognostic value was not associated with either ethnicity or tumor location. These studies suggest that decreased expression of p27(kip-1) is an indicator of poor prognosis and aids in identifying a subset of patients with aggressive forms of stage III CRCs.

Reduced expression of p27 is a novel mechanism of docetaxel resistance in breast cancer cells

Introduction

Docetaxel is one of the most effective chemotherapeutic agents in the treatment of breast cancer. Breast cancers can have an inherent or acquired resistance to docetaxel but the causes of this resistance remain unclear. However, apoptosis and cell cycle regulation are key mechanisms by which most chemotherapeutic agents exert their cytotoxic effects.

Methods

We created two docetaxel-resistant human breast cancer cell lines (MCF-7 and MDA-MB-231) and performed cDNA microarray analysis to identify candidate genes associated with docetaxel resistance. Gene expression changes were validated at the RNA and protein levels by reverse transcription PCR and western analysis, respectively.

Results

Gene expression cDNA microarray analysis demonstrated reduced p27 expression in docetaxel-resistant breast cancer cells. Although p27 mRNA expression was found to be reduced only in MCF-7 docetaxel-resistant sublines (2.47-fold), reduced expression of p27 protein was noted in both MCF-7 and MDA-MB-231 docetaxel-resistant breast cancer cells (2.83-fold and 3.80-fold, respectively).

Conclusions

This study demonstrates that reduced expression of p27 is associated with acquired resistance to docetaxel in breast cancer cells. An understanding of the genes that are involved in resistance to chemotherapy may allow further development in modulating drug resistance, and may permit selection of those patients who are most likely to benefit from such therapies.

Prospero maintains the mitotic potential of glial precursors enabling them to respond to neurons

During central nervous system development, glial cells need to be in the correct number and location, at the correct time, to enable axon guidance and neuropile formation. Repair of the injured or diseased central nervous system will require the manipulation of glial precursors, so that the number of glial cells is adjusted to that of neurons, enabling axonal tracts to be rebuilt, remyelinated and functional. Unfortunately, the molecular mechanisms controlling glial precursor proliferative potential are unknown. We show here that glial proliferation is regulated by interactions with axons and that the Drosophila gene prospero is required to maintain the mitotic potential of glia. During growth cone guidance, Prospero positively regulates cycE promoting cell proliferation. Neuronal Vein activates the MAPKinase signalling pathway in the glia with highest Prospero levels, coupling axon extension with glial proliferation. Later on, Prospero maintains glial precursors in an undifferentiated state by activating Notch and antagonising the p27/p21 homologue Dacapo. This enables prospero-expressing cells alone to divide further upon elimination of neurons and to adjust glial number to axons during development.

IGF-I and FGF-2 coordinately enhance cyclin D1 and cyclin E-cdk2 association and activity to promote G1 progression in oligodendrocyte progenitor cells

A critical question in developmental neurobiology is how stem and progenitor cells interpret multiple signals to decide whether to proliferate or exit the cell cycle. Insulin-like growth factor (IGF)-I and fibroblast growth factor (FGF)-2 have known functions individually in development of neural stem cells as well as more restricted neuronal and glial progenitor cells. The goal of this study was to elucidate how IGF-I and FGF-2 coordinately regulate the cell cycle machinery in primary oligodendrocyte progenitors (OPs). IGF-I/FGF-2 synergistically increased the numbers of OP cells recruited into S phase. IGF-I enhanced FGF-2 induction of cyclin D1, activation of G(1) cyclin-cyclin-dependent kinase (cdk) complexes, and hyperphosphorylation of retinoblastoma protein (pRb). Moreover, IGF-I was required for G(2)/M progression. In contrast, FGF-2 decreased levels of the cdk inhibitor p27(Kip1) associated with cyclin E-cdk2. These studies provide a mechanistic basis for coordinate regulation of cell cycle progression in progenitor cells by multiple growth factors.

Emerging roles for bone morphogenetic proteins in central nervous system glial biology

Bone morphogenetic proteins, members of the TGFbeta superfamily have been implicated in a variety of roles in the developing and mature nervous system. These divergent functions are a reflection of the closely defined spatial and temporal expression of BMPs in the CNS, and the potential interactions of the BMP signaling pathway with the STAT and MAP kinase pathways. In this review we discuss the roles of BMPs in early patterning of the CNS, determination of neural cell fate, and regulation of oligodendrocyte maturation during CNS development. Additional functions for members of the TGFbeta superfamily in CNS injury responses are emerging suggesting these molecules represent useful targets for manipulating neural responses to CNS insults.

Roles for p53 and p73 during oligodendrocyte development

Oligodendrocytes make myelin in the vertebrate central nervous system (CNS). They develop from oligodendrocyte precursor cells (OPCs), most of which divide a limited number of times before they stop and differentiate. OPCs can be purified from the developing rat optic nerve and stimulated to proliferate in serum-free culture by PDGF. They can be induced to differentiate in vitro by either thyroid hormone (TH) or PDGF withdrawal. It was shown previously that a dominant-negative form of p53 could inhibit OPC differentiation induced by TH but not by PDGF withdrawal, suggesting that the p53 family of proteins might play a part in TH-induced differentiation. As the dominant-negative p53 used inhibited all three known p53 family members - p53, p63 and p73 - it was uncertain which family members are important for this process. Here, we provide evidence that both p53 and p73, but not p63, are involved in TH-induced OPC differentiation and that p73 also plays a crucial part in PDGF-withdrawal-induced differentiation. This is the first evidence for a role of p73 in the differentiation of a normal mammalian cell.

The nuclear orphan receptor COUP-TFI is important for differentiation of oligodendrocytes

We report here that a member of the nuclear hormone receptor superfamily, chicken ovalbumin upstream promoter-transcription factor 1 (COUP-TFI), plays a critical role in glial cell development and subsequent central nervous system myelination. We demonstrate that COUP-TF1 is expressed in cells of oligodendrocyte lineage. Furthermore, we demonstrate that COUP-TFI null mutant mice exhibit delayed axon myelination and increased dysmyelination in the central nervous system. Using in vitro differentiation assays, we show that these myelination defects are due to delays in oligodendrocyte differentiation. Finally, in situ hybridization and transfection analysis suggests that COUP-TFI acts as an upstream regulator of SCIP/Oct-6/Tst-1, a transcription factor involved in axon myelination. Taken together, these results suggest that COUP-TFI is an important regulator of oligodendrocyte differentiation.

Molecular mechanisms underlying IGF-I-induced attenuation of the growth-inhibitory activity of trastuzumab (Herceptin) on SKBR3 breast cancer cells

The clinical usefulness of trastuzumab (Herceptin; Genentech, San Francisco, CA) in breast cancer treatment is limited by the rapid development of resistance. We previously reported that IGF-I signaling confers resistance to the growth-inhibitory actions of trastuzumab in a model system, but the underlying molecular mechanism remains unknown. We used SKBR3/neo cells (expressing few IGF-I receptors) and SKBR3/IGF-IR cells (overexpressing IGF-I receptor) as our experimental model. IGF-I antagonized the trastuzumab-induced increase in the level of the Cdk inhibitor p27(Kip1). This resulted in decreased association of p27(Kip1) with Cdk2, restoration of Cdk2 activity and attenuation of cell-cycle arrest in G(1) phase, all of which had been induced by trastuzumab treatment in SKBR3/IGF-IR cells. We also found that the decrease in p27(Kip1) induced by IGF-I was accompanied by an increase in expression of Skp2, which is a ubiquitin ligase for p27(Kip1), and by increased Skp2 association with p27(Kip1). A specific proteasome inhibitor (LLnL) completely blocked the ability of IGF-I to reduce the p27(Kip1) protein level, while IGF-I increased p27(Kip1) ubiquitination. This suggests that the action of IGF-I in conferring resistance to trastuzumab involves targeting of p27(Kip1) to the ubiquitin/proteasome degradation machinery. Finally, specific inhibitors of MAPK and PI3K suggest that the IGF-I-mediated reduction in p27(Kip1) protein level by increased degradation predominantly involves the PI3K pathway. Our results provide an example of resistance to an antineoplastic therapy that targets one tyrosine kinase receptor by increased signal transduction through an alternative pathway in a complex regulatory network.

p27 deregulation in breast cancer: prognostic significance and implications for therapy

p27 is a key regulator of G1-to-S phase progression. It prevents premature activation of cyclin E-cdk2 in G1 and promotes the assembly and activation of D-type cyclin-cdks. While the p27 gene is rarely mutated in human cancers, the action of p27 is impaired in breast and other human cancers through accelerated p27 proteolysis, sequestration by cyclin D-cdks, and by p27 mislocalization in tumor cell cytoplasm. Reduced p27 protein is strongly associated with high histopathologic tumor grade, reflecting a lack of tumor differentiation. Loss of p27 is also an indicator of poor patient outcome in a majority of breast cancer studies, including node negative disease. The broad application of p27 in the clinical evaluation of breast cancer prognosis will require a consensus on methods of tumor fixation, staining, and scoring. This review will focus on mechanisms of p27 regulation in normal cells and how deregulation of p27 may arise in breast and other human cancers. The prognostic significance of p27 in human breast cancer and the possible therapeutic implications of these findings will also be reviewed.

Increased P27KIP1 protein expression in the dentate gyrus of chronically stressed rats indicates G1 arrest involvement

Various chronic stress paradigms decrease new cell proliferation in the hippocampal dentate gyrus, yet the exact underlying mechanism is still unclear. In the first gap (G1) phase of the cell cycle, both stimulatory and inhibitory signals derived from the extracellular environment converge. Corticosteroids, which increase during stress and are well-known anti-mitotics, cause cells in vitro to arrest in the G1 phase. Following 3 weeks of unpredictable stress, we therefore expected a change in protein expression of various important G1 cell cycle regulators in the adult rat subgranular zone. Using quantitative immunocytochemistry, we show that particularly cyclin-dependent kinase inhibitor p27Kip1 expression is significantly increased. In addition, 3 weeks of recovery after stress normalized the numbers of p27Kip1-expressing cells, consistent with the recovered adult cell proliferation in these animals. P27Kip1-positive cells do not overlap with GFAP-staining and only to a limited extent with Ki-67-expressing cells. Numbers of cyclin E- and cyclin D1-expressing cells did not change after chronic stress. These results indicate that chronic stress causes cycling cells in the adult hippocampus to arrest in G1, thereby providing more mechanistic insight in the stress-induced decrease in cell proliferation.

Malignant peripheral nerve sheath tumor: a comparison of grade, immunophenotype, and cell cycle/growth activation marker expression in sporadic and neurofibromatosis 1-related lesions

This study investigates differences in expression of the cell cycle/growth activation markers p53, p16, and p27, and their relationship with nerve sheath cell and proliferation markers among plexiform neurofibromas (PNF), NF1-related and non-NF1 MPNSTs of different histologic grades and between benign-appearing and malignant areas in the MPNSTs associated with PNFs. Formalin-fixed, paraffin-embedded archival tissue from PNFs and MPNSTs were immunostained using the avidin-biotin-complex method with antibodies to S-100 protein (S-100), Leu7 (CD57), CD34, p16, p27, p53, Mib-1, and topoisomerase II-alpha (TopoIIalpha), with appropriate controls. All PNFs and most low-grade MPNSTs displayed diffuse or focal reactivity for S-100, Leu7, CD34, p16, and p27 and negative reactivity for p53, Mib-1, and TopoIIalpha. Most high-grade MPNSTs displayed decreased or negative reactivity to S-100, Leu7, CD34, p16, and p27 but increased reactivity to p53 (59%), Mib-1 (72%), and TopoIIalpha (72%). In addition, combined nuclear and cytoplasmic (nucleocytoplasmic) p27 staining, which was not seen in the PNF or low-grade MPNST, was observed in 33% of high-grade MPNSTs. These findings suggest that p53, p16, and p27 may be involved in tumor progression in the PNF-MPNST sequence. However, alterations in p53, p16, and p27 do not distinguish between low-grade MPNST and PNF, including PNF adjacent to high-grade MPNST. Although p53, p16, and p27 are unlikely to be reliable markers for early detection of tumor progression in MPNST, p53 reactivity was more frequent in NF1-associated high-grade MPNST and appeared to be a marker for high tumor grade. Combining immunohistochemical stains with histologic grading with careful examination of mitotic activity may provide insight into the progression of peripheral nerve sheath tumors.

The inhibition of cyclin-dependent kinases induces differentiation of supernumerary hair cells and Deiters' cells in the developing organ of Corti

In the embryonic day 19 organs of Corti, we showed that roscovitine, a chemical inhibitor of cyclin-dependent kinases (CDKs), significantly increased the number of hair cells (HCs) and corresponding supporting cells (SCs) by triggering differentiation of precursor cells without interacting with cell proliferation. The effect of roscovitine was mimicked by other CDK1, 2, 5, and 7 inhibitors but not by CDK4/6 and mitogen-activated protein kinase pathway antagonists. Immunohistochemical analysis indicated that roscovitine-specific intracellular targets, CDK1, 2, 5, and 7, were expressed in the organ of Corti and especially in Hensen's cells. Affinity chromatography studies showed a tight correlation between the protein levels of CDK1/2 and 5 and the rate of roscovitine-induced supernumerary cells in the organ of Corti. In addition, we demonstrated that basal CDK activity was higher and more roscovitine-sensitive at developmental stages that are selectively permissive for the emergence of supernumerary cells. These results suggest that CDKs are involved in the normal development of the organ of Corti and that, at least in E19 embryos, inhibition of CDKs is sufficient to trigger the differentiation of HCs and corresponding SCs, presumably from the Hensen's cell progenitors and/or from progenitors located in the greater epithelial ridge area.

Control of thyroid hormone action in the developing rat brain

Thyroid hormones play important roles in brain development. The physiologic function of thyroid hormones in the developing brain is to provide a timing signal that leads to the induction of differentiation and maturation programs during precise stages of development. Inappropriate initiation of these timing events leads to asynchrony in developmental processes and a deleterious outcome. The developing brain is protected from premature thyroid hormone signaling through a variety of measures. Firstly, local brain levels of both thyroxine and triiodothyronine are controlled by ontogenically regulated patterns of production and metabolism. Secondly, developmentally regulated expression of nuclear proteins involved with the nuclear TH response apparatus control the temporal response of brain genes to thyroid hormone. Finally, developmental regulation of TH action modulating transcription factor expression also controls TH action in the developing brain. Together these molecular mechanisms cooperatively act to temporally control TH action during brain development. A description of these controlling mechanisms is the subject of this review.

Identification of the molecular requirements for an RAR alpha-mediated cell cycle arrest during granulocytic differentiation

Retinoids are potent inducers of cell cycle arrest and differentiation of numerous cell types, notably granulocytes. However the mechanisms by which retinoids mediate cell cycle arrest during differentiation remain unclear. We have used myeloid differentiation to characterize the molecular pathways that couple cell cycle withdrawal to terminal differentiation. Using primary cells from mice deficient for either the cyclin-dependent kinase inhibitor (CDKi) p27(Kip1), the Myc antagonist Mad1, or both Mad1 and p27(Kip1), we observed that signals mediated through retinoic acid receptor alpha (RAR alpha), but not RAR beta or gamma, required both Mad1 and p27(Kip1) to induce cell cycle arrest and to accelerate terminal differentiation of granulocytes. Although RAR alpha did not directly regulate Mad1 or p27(Kip1), the RAR alpha target gene C/EBP epsilon directly regulated transcription of Mad1. Induction of C/EBP epsilon activity in granulocytic cells led to rapid induction of Mad1 protein and transcript, with direct binding of C/EBP epsilon to the Mad1 promoter demonstrated through chromatin immunoprecipitation assay. These data demonstrate that cell cycle arrest in response to RAR alpha specifically requires Mad1 and p27(Kip1) and that Mad1 is transcriptionally activated by CCAAT/enhancer-binding protein epsilon (C/EBP epsilon). Moreover, these data demonstrate selectivity among the RARs for cell cycle arrest pathways and provide a direct mechanism to link differentiation induction and regulation of the Myc antagonist Mad1.

p57Kip2 regulates actin dynamics by binding and translocating LIM-kinase 1 to the nucleus

p57Kip2 is the only cyclin-dependent kinase (Cdk) inhibitor shown to be essential for mouse embryogenesis. The fact suggests that p57 has a specific role that cannot be compensated by other Cdk inhibitors. LIM-kinase 1 (LIMK-1) is a downstream effector of the Rho family of GTPases that phosphorylates and inactivates an actin depolymerization factor, cofilin, to induce the formation of actin fiber. Here we demonstrate that p57 regulates actin dynamics by binding and translocating LIMK-1 from the cytoplasm into the nucleus, which in turn results in a reorganization of actin fiber. The central region of p57, a unique feature among the Cdk inhibitors, and the N-terminal region of LIMK-1, which contains the LIM domains were essential for the interaction. Expression of p57, but not p27Kip1 or a p57 mutant, with a deletion in the central region was shown to induce marked reorganization of actin filament and a translocation of LIMK-1. Our findings indicate p57 may act as a key regulator in embryogenesis by bearing two distinct functions, the regulation of cell cycle through binding to Cdks and the regulation of actin dynamics through binding to LIMK-1, both of which should be important in developmental procedure.

Spy1 interacts with p27Kip1 to allow G1/S progression

Progression through the G1/S transition commits cells to synthesize DNA. Cyclin dependent kinase 2 (CDK2) is the major kinase that allows progression through G1/S phase and subsequent replication events. p27 is a CDK inhibitor (CKI) that binds to CDK2 to prevent premature activation of this kinase. Speedy (Spy1), a novel cell cycle regulatory protein, has been found to prematurely activate CDK2 when microinjected into Xenopus oocytes and when expressed in mammalian cells. To determine the mechanism underlying Spy1-induced proliferation in mammalian cell cycle regulation, we used human Spy1 as bait in a yeast two-hybrid screen to identify interacting proteins. One of the proteins isolated was p27; this novel interaction was confirmed both in vitro, using bacterially expressed and in vitro translated proteins, and in vivo, through the examination of endogenous and transfected proteins in mammalian cells. We demonstrate that Spy1 expression can overcome a p27-induced cell cycle arrest to allow for DNA synthesis and CDK2 histone H1 kinase activity. In addition, we utilized p27-null cells to demonstrate that the proliferative effect of Spy1 depends on the presence of endogenous p27. Our data suggest that Spy1 associates with p27 to promote cell cycle progression through the G1/S transition.

TGF beta1 induces multiple independent signals to regulate human trophoblastic differentiation: mechanistic insights

Transforming growth factor-beta 1 (TGF beta1) plays a crucial role in controlling trophoblast growth and invasion. Loss of this key regulatory function provides the pathophysiological basis for several tumors, which are characterized by uncontrolled telomerase activity. We have shown earlier that telomerase activity is negatively regulated during terminal differentiation of human trophoblasts, and that TGF beta1 may be an important factor governing the transcription of human telomerase reverse transcriptase (hTERT) (the catalytic subunit of the telomerase complex) during this process. In the present study, we extend these observations to identify possible functional effectors of TGF beta1-induced loss in telomerase activity during human trophoblastic differentiation. We show that this regulation may involve the suppression of c-Myc and an increased production of Mad1. We also observed a simultaneous increase in the expression of cyclin-dependent-kinase inhibitors, p21, p27, p15 and p16, associated with a loss in expression of Cyclin-A2 and Cyclin-E. Thus, TGF beta1 may induce multiple independent signals to check the proliferative potential of human trophoblastic cells and allow their functional differentiation.

Expression of cyclins, cyclin-dependent kinases and cyclin-dependent kinase inhibitors in oligodendrogliomas in humans

Cyclins are regulatory proteins of the cell cycle which bind and activate kinases. In gliomas, contrary to many malignancies, cyclin D1 is rarely amplified, but together with other cyclins, it increases with anaplasia. In a series of 23 surgical biopsies of grade II and III oligodendroglioma, cyclin D1, E, A, B1, CDK4-6, CDK2, Cdc2 and p27/Kip.1 have been studied by immunohistochemistry and Western blot. Cyclin D1 and A increased with anaplasia, showing a linear correlation with MIB.1 labeling index and an inverse correlation with p27/Kip.1 expression. Cyclin E and B1 and kinases were almost only expressed in grade III tumors. Normal oligodendrocytes and microglia cells of the cortex and white matter showed a clear positivity for cyclin D1, but not for other cyclins or kinases.

The role of ErbB2 signaling in the onset of terminal differentiation of oligodendrocytes in vivo

The knock-out analyses of neuregulin and its receptors have indicated that they play essential roles in Schwann cell development. However, the role they play in oligodendrocyte development in vivo has remained unclear, because such knock-out animals die before CNS myelination begins. We examined the role of neuregulin signaling in the CNS by generating transgenic mice that express a dominant-negative mutant of the ErbB2 receptor among oligodendrocytes, using an MBP promoter. The transgenic mice exhibited widespread hypomyelination, resulting from a reduction in oligodendrocyte differentiation. The number of progenitors was conversely increased in the transgenic mice. We report that a reduction in oligodendrocyte differentiation is attributed in part to apoptosis of oligodendrocyte progenitors as they exit the cell cycle. A significant reduction in the number of p27+ oligodendrocyte precursors in the transgenic mice supports this conclusion. Taken together, these data suggest that for oligodendrocyte progenitors, ErbB2 signaling plays a role in governing a properly timed exit from the cell cycle during development into myelinating oligodendrocytes.

Cell cycle regulation and neural differentiation

The general mechanisms that control the cell cycle in mammalian cells have been studied in depth and several proteins that are involved in the tight regulation of cell cycle progression have been identified. However, the analysis of which molecules participate in cell cycle exit of specific cell lineages is not exhaustive yet. Moreover, the strict relation between cell cycle exit and induction of differentiation has not been fully understood and seems to depend on the cell type. Several in vivo and in vitro studies have been performed in the last few years to address these issues in cells of the nervous system. In this review, we focus our attention on cyclin-cyclin-dependent kinase complexes, cyclin kinase inhibitors, genes of the retinoblastoma family, p53 and N-Myc, and we aim to summarize the latest evidence indicating their involvement in the control of the cell cycle and induction of differentiation in different cell types of the peripheral and central nervous systems. Studies on nervous system tumors and a possible contributory role in tumorigenesis of polyomavirus T antigen are reported to point out the critical contribution of some cell cycle regulators to normal neural and glial development.

The role of ErbB2 signaling in the onset of terminal differentiation of oligodendrocytes in vivo

The knock-out analyses of neuregulin and its receptors have indicated that they play essential roles in Schwann cell development. However, the role they play in oligodendrocyte development in vivo has remained unclear, because such knock-out animals die before CNS myelination begins. We examined the role of neuregulin signaling in the CNS by generating transgenic mice that express a dominant-negative mutant of the ErbB2 receptor among oligodendrocytes, using an MBP promoter. The transgenic mice exhibited widespread hypomyelination, resulting from a reduction in oligodendrocyte differentiation. The number of progenitors was conversely increased in the transgenic mice. We report that a reduction in oligodendrocyte differentiation is attributed in part to apoptosis of oligodendrocyte progenitors as they exit the cell cycle. A significant reduction in the number of p27+ oligodendrocyte precursors in the transgenic mice supports this conclusion. Taken together, these data suggest that for oligodendrocyte progenitors, ErbB2 signaling plays a role in governing a properly timed exit from the cell cycle during development into myelinating oligodendrocytes.

The Sp1 family of transcription factors is involved in p27(Kip1)-mediated activation of myelin basic protein gene expression

p27(Kip1) levels increase in many cells as they leave the cell cycle and begin to differentiate. The increase in p27(Kip1) levels generally precedes the expression of differentiation-specific genes. Previous studies from our laboratory showed that the overexpression of p27(Kip1) enhances myelin basic protein (MBP) promoter activity. This activation is specific to p27(Kip1). Additionally, inhibition of cyclin-dependent kinase activity alone is not sufficient to increase MBP expression. In this study, we focused on understanding how p27(Kip1) can activate gene transcription by using the MBP gene in oligodendrocytes as a model. We show that the enhancement of MBP promoter activity by p27(Kip1) is mediated by a proximal region of the MBP promoter that contains a conserved GC box binding sequence. This sequence binds transcription factors Sp1 and Sp3. Increased expression of p27(Kip1) increases the level of Sp1 promoter binding to the GC box but does not change the level of Sp3 binding. The binding of Sp1 to this element activates the MBP promoter. p27(Kip1) leads to increased Sp1 binding through a decrease in Sp1 protein turnover. Enhancement of MBP promoter activity by an increase in the level of p27(Kip1) involves a novel mechanism that is mediated through the stabilization and binding of transcription factor Sp1.

Redox state as a central modulator of precursor cell function

In our attempts to understand how the balance between self-renewal and differentiation is regulated in dividing precursor cells, we have discovered that intracellular redox state appears to be a critical modulator of this balance in oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells. The intracellular redox state of freshly isolated progenitor cells allows prospective isolation of cells with different self-renewal characteristics, which can be further modulated in opposite directions by prooxidants and antioxidants. Redox state is itself modulated by cell-extrinsic signaling molecules that alter the balance between self-renewal and differentiation: growth factors that promote self-renewal cause progenitors to become more reduced, while exposure to signaling molecules that promote differentiation causes progenitors to become more oxidized. Moreover, pharmacological antagonists of the redox effects of these cell-extrinsic signaling molecules antagonize their effects on self-renewal and differentiation, further suggesting that cell-extrinsic signaling molecules that modulate this balance converge on redox modulation as a critical component of their effector mechanism. A further example of the potential relevance of intracellular redox state to development processes emerges from our attempts to understand why different central nervous system (CNS) regions exhibit different temporal patterns of oligodendrocyte generation and myelinogenesis. Characterization of O-2A progenitor cells (O-2A/OPCs) isolated from different regions indicates that these developmental patterns are consistent with properties of the specific O-2A/OPCs resident in each region. Marked differences were seen in self-renewal and differentiation characteristics of O-2A/OPCs isolated from cortex, optic nerve, and optic chiasm. In conditions where optic nerve-derived O-2A/OPCs generated oligodendrocytes within 2 days, oligodendrocytes arose from chiasm-derived cells after 5 days and from cortical O-2A/OPCs after only 7-10 days. These differences, which appear to be cell intrinsic, were manifested both in reduced percentages of clones producing oligodendrocytes and in a lesser representation of oligodendrocytes in individual clones. In addition, responsiveness of optic nerve-, chiasm-, and cortex-derived O-2A/OPCs to thyroid hormone (TH) and ciliary neurotrophic factor (CNTF), well-characterized inducers of oligodendrocyte generation, was inversely related to the extent of self-renewal observed in basal division conditions. These results demonstrate hitherto unrecognized complexities among the precursor cells thought to be the immediate ancestors of oligodendrocytes and suggest that the properties of these different populations may contribute to the diverse time courses of myelination in different CNS regions. Strikingly, O-2A/OPCs isolated from cortex and analyzed immediately upon isolation were more reduced in their redox state than were optic nerve-derived cells, precisely as would be predicted from our analysis of the role of redox state in modulating the balance between self-renewal and differentiation. Chiasm-derived cells, which exhibited self-renewal properties intermediate between cortex- and optic nerve-derived cells, were more reduced than optic nerve cells but more oxidized that cortical O-2A/OPCs.

Retinoic acid decreases targeting of p27 for degradation via an N-myc-dependent decrease in p27 phosphorylation and an N-myc-independent decrease in Skp2

Poor prognosis neuroblastoma (NB) tumors are marked by amplification and overexpression of N-myc. Retinoic acid (RA) decreases N-myc levels and induces cell cycle arrest in vitro and increases event-free survival in advanced stage NB patients. In this study, we investigated the mechanism(s) by which RA regulates cell cycle and how N-myc affects NB cell cycle progression. Constitutive N-myc overexpression stimulates increases in cyclin E-dependent kinase activity and decreases in p27 resulting in increased DNA synthesis. N-myc regulates p27 levels through an increase in targeting of p27 to the proteasome via cyclin E kinase-dependent phosphorylation of p27 and its ubiquitination. N-myc also stimulates an increase in proteasome activity. In RA-treated cells in which N-myc levels decline as p27 levels increase, degradation of p27 is also decreased. However, RA does not affect the activity of proteasome. The decrease in the degradation of p27 in RA-treated cells is due in part to a decrease in the N-myc stimulated phosphorylation of p27. However, RA also decreases Skp2 levels thus impairing the ability of p27 to be ubiquitinated. Thus, RA induces both N-myc-dependent and -independent mechanisms to minimize the degradation of p27 and arrest NB cell growth.

Triiodothyronine is a survival factor for developing oligodendrocytes

Thyroid hormone plays an important role in oligodendrocyte development. The studies presented here suggest that thyroid hormone is required for oligodendrocyte survival during development. Oligodendrocyte precursor cells, astrocytes and microglia were cultured in a defined media. Oligodendrocyte precursor cell differentiation was induced by growth factor removal. Time course studies revealed that oligodendrocytes cultured in the presence or absence of triiodothyronine (T3) develop similarly during the first 3 days of development. Oligodendrocytes cultured in the absence of T3, however, die after developmental day 3. TdT-Mediated dUDP Nick End Labeling assay and Hoechst staining indicate that T3 rescues developing oligodendrocytes from death by apoptosis. Apoptosis is likely induced by the presence of the cytokines TNFalpha and IL-1beta. However, expression of these cytokines is not altered by thyroid hormone administration. Thus, thyroid hormone has been demonstrated to effect proliferation, myelin gene expression and now the survival of developing oligodendrocytes.

PAX3-FKHR transformation increases 26 S proteasome-dependent degradation of p27Kip1, a potential role for elevated Skp2 expression

PAX3-FKHR is an oncogenic form of the developmental regulator Pax3 transcription factor. PAX3-FKHR results from a t(2,13) chromosomal translocation, a unique genetic marker of alveolar rhabdomyosarcoma. In this study, we showed that ectopic expression of PAX3-FKHR, but not Pax3, in fibroblasts altered cell cycle control and accelerated G(0)/G(1) to S cell cycle transition. PAX3-FKHR-expressing cells had reduced expression of p27(Kip1) protein, a key cell cycle regulator. The reduction in p27(Kip1) levels by PAX3-FKHR resulted from destabilization of p27(Kip1) as shown by cycloheximide treatment and in vivo pulse-chase labeling experiments. The reduced p27(Kip1) protein level in PAX3-FKHR-expressing cells was restored to the level of control cells by treatment with chemical inhibitors that specifically blocked 26 S proteasome activity. Along with the reduction in p27(Kip1) protein, PAX3-FKHR-expressing cells exhibited elevated expression of F-box Skp2 protein, a substrate-specific component of SCF (Skp1-Cullin-F box protein) ligase involved in the cell cycle-dependent control of p27(Kip1) ubiquitination and 26 S proteasome dependent degradation. Finally, we showed that ectopic expression of p27(Kip1) in PAX3-FKHR-expressing cells significantly reduced the proliferation and colony-forming potential of these cells, implicating that down-regulation of p27(Kip1) protein played an active role in the PAX3-FKHR-directed cell transformation.

Alterations in cyclin A, B, and D1 in mouse dentate gyrus following TMT-induced hippocampal damage

The interactions of glia and neurons during injury and subsequent neurodegeneration are a subject of interest both in disease and chemical-induced brain injury. One such model is the prototypical hippocampal toxicant trimethyltin (TMT). An acute injection of TMT (2.0 mg/kg, i.p.) to postnatal day 21 CD-1 male mice produced neuronal necrosis and loss of dentate granule cells, astrocyte hypertrophy, and microglia activation in the hippocampus within 24 hrs. Neuronal necrosis and microglia differentiation to a phagocytic phenotype is temporally correlated with peak elevations in TNF-alpha, cyclin A2, cyclin B1 and cyclin D1 at 72 h post-TMT. TNF-alpha mRNA levels were significantly elevated in the hippocampus by 12 h and remained elevated for 72 h. mRNA levels for cyclin A2 and cyclin B1 were elevated by approximately 2-fold at 72 h. Immunohistochemistry suggested a cellular localization of cyclin A to microglia in the region of neuronal necrosis in the dentate, cyclin B in glial cells in juxtaposition to neurons in the hilus of the hippocampus and cyclin D1 to non-glial cells in the dentate. mRNA levels for cyclin D1 were elevated approximately 1.5-fold by 72 h as determined by RNase protection assay. No changes were seen in mRNA levels for cyclins E, F, G1, G2, H or I nor cyclin dependent kinases. These elevations are not associated with proliferation of microglia as determined by BrdU incorporation and Ki-67 immunohistochemistry. Upregulation of cell cycle genes was associated with cellular processes other than proliferation and may contribute to the differentiation of microglia to a phagocytic phenotype. These data suggest an integrated role for cell cycle regulation of neural cells in the manifestation of hippocampal pathophysiology.

Overexpression of p27(Kip1) induces growth arrest and apoptosis in an oral cancer cell line

p27(Kip1) is a cyclin-dependent kinase inhibitor which regulates progression of cells from G1 into S phase in a cell cycle. Loss of p27(Kip1) has been associated with disease progression and an unfavorable outcome in several malignancies. In the present study, we conducted to examine whether up-regulation or down-regulation of p27(KiP1) can affect the growth of oral cancer cell (B88 cell) in vitro and in vivo. We constructed an expression vector containing sense- or antisense-oriented human p27(Kip1) cDNA with pcDNA3.1(Invitrogen). We transfected B88 cells with the sense or antisense expression vector to regulate the expression of p27(Kip1) gene in each transfectant. The expression of p27(Kip1) protein was up-regulated in the sense transfectants and down-regulated in the antisense transfectants. Moreover, up-regulation of p27(Kip1) protein exerted the growth inhibitory effect, and down-regulation of p27(Kip1) protein enhanced the growth of B88 cells in vitro and in vivo. Furthermore, we detected the G1 arrest and sub-G1 peak in the sense transfectants by flow cytometry analysis. These results suggest that up-regulation of p27(Kip1) protein may exert the growth inhibitory effects through induction of G1 arrest and apoptosis on oral cancer cell line.

TGF-beta induces cell death in the oligodendroglial cell line OLI-neu

We have shown that TGF-beta plays an important role during the period of developmental cell death in the nervous system. Immunoneutralization of TGF-beta prevents ontogenetic neuron death in vivo. Like neurons, oligodendrocytes are generated in excess and eliminated by apoptosis. It has been shown that oligodendrocyte progenitors and newly formed oligodendrocytes are especially susceptible to apoptosis. We choose the oligodendrocyte precursor cell line OLI-neu to address the question if TGF-beta could play a role for the control of oligodendrocyte proliferation and cell death. Flow cytometric analysis revealed that OLI-neu cells arrested in the G1 phase of the cell cycle underwent apoptosis in response to TGF-beta. TUNEL assays, apoptosis ELISA, and caspase assays substantiated the finding that OLI-neu cells died after TGF-beta treatment. Cell death could be inhibited by application of pan-caspase or caspase 8 and 9 inhibitors, whereas the inhibition of calpain was unaffected. Furthermore, we found a reduction of bcl-X(L) at the protein as well as at the mRNA level, while p27 was upregulated. The Smad cascade was activated while TGF-beta reduced the activity of the p42/p44 MAP kinase pathway. Together, these data show that TGF-beta induced apoptotic cell death in cells of oligodendroglial origin, whereby the signaling cascade involved the downregulation of antiapoptotic signaling such as bcl-X(L) leading to the activation of caspases.

Expression of cyclin-dependent kinase inhibitor p27/Kip1 and AP-1 coactivator p38/Jab1 correlates with differentiation of embryonal rhabdomyosarcoma

Cyclin-dependent kinase (CDK) inhibitor p27/Kip1 (p27) is a diagnostic and prognostic marker of various malignancies. Low expression of p27 reflects poor differentiation and poor prognosis, and an inverse correlation between the expression of p27 and degree of tumor malignancy has been reported. Because p27 mutation is extremely rare in human tumors, it is important to study the expression of p27 and its inactivator, p38/Jab1 (JAB1). Here we analyzed the expression of p27 and JAB1 by immunohistochemistry in embryonal rhabdomyosarcoma (E-RMS). We first confirmed the expression of p27 and JAB1 in normal human tonsillar epithelium, and observed a coordinated expression pattern depending on cell differentiation. Subsequently, specimens of eight poorly- and three well-differentiated E-RMS were examined for the expression of p27 and JAB1. The analyses revealed that four out of eight poorly-differentiated E-RMS were negative for p27, with positivity for nuclear JAB (NJAB) (- / + for p27/NJAB) in three and negativity for any JAB-1 expression ( - / -) in one. The remaining four poorly-differentiated E-RMS expressed p27 in the nuclei, together with predominant NJAB (+ / +). In three well-differentiated E-RMS, only one expressed nuclear p27 and all of these three expressed no NJAB (+ / - for p27/NJAB), but expressed predominant cytoplasmic JAB1 (CJAB). These findings suggest that JAB1 may play an important role in determining the differentiation stage of rhabdomyosarcoma cells by modulating the activity of CDK inhibitor p27.

Why does remyelination fail in multiple sclerosis?

Multiple sclerosis is a common cause of neurological disability in young adults. The disease is complex -- its aetiology is multifactorial and largely unknown; its pathology is heterogeneous; and, clinically, it is difficult to diagnose, manage and treat. However, perhaps its most frustrating aspect is the inadequacy of the healing response of remyelination. This regenerative process generally occurs with great efficiency in experimental models, and sometimes proceeds to completion in multiple sclerosis. But as the disease progresses, the numbers of lesions in which demyelination persists increases, significantly contributing to clinical deterioration. Understanding why remyelination fails is crucial for devising effective methods by which to enhance it.

Role and function of the 26S proteasome in proliferation and apoptosis

The 26S proteasome constitutes the central proteolytic machinery of the highly conserved ubiquitin/proteasome system, the cell's major tool for extralysosomal protein degradation. Recently, a plethora of cell proteins implicated in the regulation of basic cellular processes, such as proliferation, differentiation, cell cycling, and apoptosis have been discovered to undergo processing and functional limitation by entering the ubiquitin/proteasome pathway with the final destination to be proteolytically degraded by the 26S proteasome. Because both negative and positive regulators of proliferation and apoptosis undergo proteasomal degradation in a tightly regulated and temporally controlled fashion, the 26S proteasome can play opposite roles in the regulation of proliferation and apoptosis. These roles are apparently defined by the cell's environment and proliferative state. Finally, proteasomal protein degradation is deregulated in a number of human diseases, including cancer and neurodegenerative and myodegenerative diseases, which all exhibit an imbalance of proliferation and apoptosis. An improved understanding of the modes of proteasomal action should lead to the development of beneficial therapeutic and diagnostic strategies in the future.

The control of neural stem cells by morphogenic signals

A complex orchestration of stem-cell specification, expansion and differentiation is required for the proper development of the nervous system. Although progress has been made on the role of individual genes in each of these processes, there are still unresolved questions about how gene function translates to the dynamic assembly of cells into tissues. Recently, stem-cell biology has emerged as a bridge between the traditional fields of cell biology and developmental genetics. In addition to their potential therapeutic role, stem cells are being exploited as experimental 'logic chips' that integrate information and exhibit self-organizing properties. Recent studies provide new insights on how morphogenic signals coordinate major stem cell decisions to regulate the size, shape and cellular diversity of the nervous system.

Influence of cell density and thyroid hormone on glial cell development in primary cultures of embryonic rat cerebral hemisphere

The influence of cell density and thyroid hormone (TH) on the development of astrocytes and oligodendrocytes was investigated in primary cultures prepared from rat cerebral hemisphere on embryonic day (E)18. At the beginning of the culture, most of the cells were microtubule-associated protein 2 (MAP2)-positive neurons, whereas O1-positive oligodendrocytes and glial fibrillary acidic protein (GFAP)-positive astrocytes were rarely observed. After the cells were maintained in serum-free defined medium, astrocytes developed at high cell density but rarely at a low one. When leukemia inhibitory factor (LIF) was supplemented in low-density cultures, the levels of GFAP expression markedly increased to almost the same extent as in high-density culture without TH. This suggests that, in low-density cultures, astrocyte progenitors could not differentiate because of insufficient astrocyte-inducing factors. Interestingly, the addition of TH increased GFAP expression levels only at high density. The number of oligodendrocytes increased with TH addition at both cell densities, although the effects were more remarkable at high density. These results suggest that cell density and TH are pivotal factors in the development of both astrocytes and oligodendrocytes. It is also suggested that the effects of TH on glial cell development could be accelerated via cell-cell communications.