Changes in cyclin-dependent kinase 2 and p27kip1 accompany glial cell differentiation of central glia-4 cells

Kisspeptin signaling in astrocytes modulates the reproductive axis

Reproduction is safeguarded by multiple, often cooperative, regulatory networks. Kisspeptin signaling, via KISS1R, plays a fundamental role in reproductive control, primarily by regulation of hypothalamic GnRH neurons. We disclose herein a pathway for direct kisspeptin actions in astrocytes that contributes to central reproductive modulation. Protein-protein interaction and ontology analyses of hypothalamic proteomic profiles after kisspeptin stimulation revealed that glial/astrocyte markers are regulated by kisspeptin in mice. This glial-kisspeptin pathway was validated by the demonstrated expression of Kiss1r in mouse astrocytes in vivo and astrocyte cultures from humans, rats, and mice, where kisspeptin activated canonical intracellular signaling-pathways. Cellular coexpression of Kiss1r with the astrocyte markers GFAP and S100-β occurred in different brain regions, with higher percentage in Kiss1- and GnRH-enriched areas. Conditional ablation of Kiss1r in GFAP-positive cells in the G-KiR-KO mouse altered gene expression of key factors in PGE2 synthesis in astrocytes and perturbed astrocyte-GnRH neuronal appositions, as well as LH responses to kisspeptin and LH pulsatility, as surrogate marker of GnRH secretion. G-KiR-KO mice also displayed changes in reproductive responses to metabolic stress induced by high-fat diet, affecting female pubertal onset, estrous cyclicity, and LH-secretory profiles. Our data unveil a nonneuronal pathway for kisspeptin actions in astrocytes, which cooperates in fine-tuning the reproductive axis and its responses to metabolic stress.

Cellular and Molecular Mechanisms of R/S-Roscovitine and CDKs Related Inhibition under Both Focal and Global Cerebral Ischemia: A Focus on Neurovascular Unit and Immune Cells

Ischemic stroke is the second leading cause of death worldwide. Following ischemic stroke, Neurovascular Unit (NVU) inflammation and peripheral leucocytes infiltration are major contributors to the extension of brain lesions. For a long time restricted to neurons, the 10 past years have shown the emergence of an increasing number of studies focusing on the role of Cyclin-Dependent Kinases (CDKs) on the other cells of NVU, as well as on the leucocytes. The most widely used CDKs inhibitor, (R)-roscovitine, and its (S) isomer both decreased brain lesions in models of global and focal cerebral ischemia. We previously showed that (S)-roscovitine acted, at least, by modulating NVU response to ischemia. Interestingly, roscovitine was shown to decrease leucocytes-mediated inflammation in several inflammatory models. Specific inhibition of roscovitine majors target CDK 1, 2, 5, 7, and 9 showed that these CDKs played key roles in inflammatory processes of NVU cells and leucocytes after brain lesions, including ischemic stroke. The data summarized here support the investigation of roscovitine as a potential therapeutic agent for the treatment of ischemic stroke, and provide an overview of CDK 1, 2, 5, 7, and 9 functions in brain cells and leucocytes during cerebral ischemia.

Low-Field Magnetic Stimulation Accelerates the Differentiation of Oligodendrocyte Precursor Cells via Non-canonical TGF-β Signaling Pathways

Demyelination and oligodendrocyte loss are characteristic changes in demyelinating disorders. Low-field magnetic stimulation (LFMS) is a novel transcranial neuromodulation technology that has shown promising therapeutic potential for a variety of neuropsychiatric conditions. The cellular and molecular mechanisms of magnetic stimulation remain unclear. Previous studies mainly focused on the effects of magnetic stimulation on neuronal cells. Here we aimed to examine the effects of a gamma frequency LFMS on the glial progenitor cells. We used rat central glia-4 (CG4) cell line as an in vitro model. CG4 is a bipotential glial progenitor cell line that can differentiate into either oligodendrocyte or type 2-astrocyte. The cells cultured in a defined differentiation media were exposed to a 40-Hz LFMS 20 min daily for five consecutive days. We found that LFMS transiently elevated the level of TGF-β1 in the culture media in the first 24 h after the treatment. In correlation with the TGF-β1 levels, the percentage of cells possessing complex branches and expressing the late oligodendrocyte progenitor marker O4 was increased, indicating the accelerated differentiation of CG4 cells towards oligodendrocyte in LFMS-treated cultures. LFMS increased phosphorylation of Akt and Erk1/2 proteins, but not SMAD2/3. TGF-β1 receptor I specific inhibitor LY 364947 partially suppressed the effects of LFMS on differentiation and on levels of pAkt and pErk1/2, indicating that LFMS enhances the differentiation of oligodendrocyte progenitor cells via activation of non-canonical TGF-β-Akt and TGF-β-Erk1/2 pathways but not the canonical SMAD pathway. The data from this study reveal a novel mechanism of magnetic stimulation as a potential therapy for demyelination disorders.

MAST1 modulates neuronal differentiation and cell cycle exit via P27 in neuroblastoma cells

Although 19p13.13 microdeletion syndrome has been consistently associated with intellectual disability, overgrowth, and macrocephaly, the underlying mechanisms remain unclear. MAST1, a member of the microtubule‐associated serine/threonine kinase family, has been suggested as a potential candidate gene responsible for neurologic abnormalities in 19p13.13 microdeletion syndrome, but its role in nervous system development remains to be elucidated. Here, we investigated how MAST1 contributes to neuronal development. We report that MAST1 is upregulated during neuronal differentiation of the human neuroblastoma cell line, SH‐SY5Y. Inhibition of MAST1 expression by RNA interference attenuated neuronal differentiation of SH‐SY5Y cells. Cell cycle analyses revealed that MAST1‐depleted cells did not undergo cell cycle arrest after RA treatment. Consistent with this observation, the number of EdU‐positive cells significantly increased in MAST1 knockdown cells. Intriguingly, levels of P27, a cyclin‐dependent kinase inhibitor, were also increased during neuronal differentiation, and MAST1 knockdown reduced the expression of P27. Moreover, reduced neuronal differentiation caused by MAST1 depletion was rescued partially by P27 overexpression in SH‐SY5Y cells. Collectively, these results suggest that MAST1 influences nervous system development by affecting neuronal differentiation through P27.

Ubiquitin-dependent degradation of CDK2 drives the therapeutic differentiation of AML by targeting PRDX2

A distinct hallmark of acute myeloid leukemia (AML) is the arrest of leukemic myeloblasts at an immature stage of development. Therapies that overcome differentiation arrest have emerged as a powerful strategy for treating AML, but targeting leukemia differentiation remains challenging, mainly because of an incomplete mechanistic understanding of the process. Here, we unveil a new role for cyclin-dependent kinase 2 (CDK2) in blocking myeloid differentiation in AML. We show that among several interphase CDK, only CDK2 undergoes ubiquitin-dependent proteasome degradation, which is accompanied by AML cell differentiation. By using the yeast 2-hybrid system and functional analyses, KLHL6 was identified as a specific E3 ubiquitin ligase regulating the degradation of CDK2. Importantly, inhibiting CDK2, but not other cyclin-dependent kinases CDK1/4/6, effectively induced granulocytic differentiation in AML cell lines and 5 major subtypes of primary patient-derived AML samples. Mechanistically, CDK2 depletion led to the reactivation of differentiation pathway translation, and the differentiation blockade function of CDK2 may be achieved directly by maintaining the activity of PRDX2. Finally, CDK2 depletion arrested tumor growth of AML cells in nude mice and extended survival in both AML cell line and PDX-AML cells derived xenograft mouse models. Thus, our work not only provides experimental evidence for validating CDK2 as a potential therapeutic target for differentiation, but also uncovers the biological function of the CDK2-PRDX2 axis in blocking AML differentiation.

miR-638 regulates differentiation and proliferation in leukemic cells by targeting cyclin-dependent kinase 2

MicroRNAs have been extensively studied as regulators of hematopoiesis and leukemogenesis. We identified miR-638 as a novel regulator in myeloid differentiation and proliferation of leukemic cells. We found that miR-638 was developmentally up-regulated in cells of myeloid but not lymphoid lineage. Furthermore, significant miR-638 down-regulation was observed in primary acute myeloid leukemia (AML) blasts, whereas miR-638 expression was dramatically up-regulated in primary AML blasts and leukemic cell lines undergoing forced myeloid differentiation. These observations suggest that miR-638 might play a role in myeloid differentiation, and its dysregulation may contribute to leukemogenesis. Indeed, ectopic expression of miR-638 promoted phorbol 12-myristate 13-acetate- or all-trans-retinoic acid-induced differentiation of leukemic cell lines and primary AML blasts, whereas miR-638 inhibition caused an opposite phenotype. Consistently, miR-638 overexpression induced G1 cell cycle arrest and reduced colony formation in soft agar. Cyclin-dependent kinase 2 (CDK2) was found to be a target gene of miR-638. CDK2 inhibition phenotypically mimicked the overexpression of miR-638. Moreover, forced expression of CDK2 restored the proliferation and the colony-forming ability inhibited by miR-638. Our data suggest that miR-638 regulates proliferation and myeloid differentiation by targeting CDK2 and may serve as a novel target for leukemia therapy or marker for AML diagnosis and prognosis.

Characterization of schwann cells in self-assembled sheets from thermoresponsive substrates

Schwann cells are the vital glial cells in the development and regeneration of the peripheral nervous system (PNS). Recently, Schwann cell transplantation has emerged as one of the attractive candidates in treating demyelinating diseases resulting from the PNS and central nervous system injuries. Schwann cells are usually injected as cell suspensions or transplanted after being seeded on extracellular matrix proteins or biodegradable polymeric scaffolds. In these approaches, the adherens junctions between Schwann cells present in vivo are not readily replicated as Schwann cells dispersed as individual cells. Here we describe a procedure to grow a large amount of Schwann cells in a sheet architecture that can be either transplanted or injected and provide some insights into the influence of a sheet-like cell organization on the function of Schwann cells, including their viability, proliferation, alignment, and migration. The Schwann cell sheet was successfully generated through coating the culture plate surfaces by layer-by-layer self-assembly of the thermoresponsive polymer poly-(N-isopropylacrylamide) (PNIPAAM). Further characterization of the Schwann cell sheet showed that Schwann cells in sheet were highly viable, but maintained a lower proliferation rate than individual Schwann cells. The levels of nerve growth factor and glial cell-derived neurotrophic factor were also maintained in Schwann cell sheets. The protein level of a cyclin-dependent kinase inhibitor, p27, was upregulated in the Schwann cell sheet. Both alignment with axon-like nanofibers and migration of Schwann cells are not significantly different between Schwann cells in a sheet-like organization and as individual cells. We conclude that Schwann cell sheet engineering presents a promising method for cell-based nerve injury therapy, as well as a model to study the control of Schwann cell proliferation in response to intercellular organization.

The role of redox environment in neurogenic development

The dynamic changes of cellular redox elements during neurogenesis allow the control of specific programs for selective lineage progression. There are many redox couples that influence the cellular redox state. The shift from a reduced to an oxidized state and vice versa may act as a cellular switch mechanism of stem cell mode of action from proliferation to differentiation. The redox homeostasis ensures proper functioning of redox-sensitive signaling pathways through oxidation/reduction of critical cysteine residues on proteins involved in signal transduction. This review presents the current knowledge on the relation between changes in the cellular redox environment and stem cell programming in the course of commitment to a restricted neural lineage, focusing on in vivo neurogenesis and in vitro neuronal differentiation. The first two sections outline the main systems that control the intracellular redox environment and make it more oxidative or reductive. The last section provides the background on redox-sensitive signaling pathways that regulate neurogenesis.

Differential requirement of cyclin-dependent kinase 2 for oligodendrocyte progenitor cell proliferation and differentiation

Cyclin-dependent kinases (Cdks) and their cyclin regulatory subunits control cell growth and division. Cdk2-cyclin E complexes, phosphorylating the retinoblastoma protein, drive cells through the G1/S transition into the S phase of the cell cycle. Despite its fundamental role, Cdk2 was found to be indispensable only in specific cell types due to molecular redundancies in its function. Converging studies highlight involvement of Cdk2 and associated cell cycle regulatory proteins in oligodendrocyte progenitor cell proliferation and differentiation. Giving the contribution of this immature cell type to brain plasticity and repair in the adult, this review will explore the requirement of Cdk2 for oligodendrogenesis, oligodendrocyte progenitor cells proliferation and differentiation during physiological and pathological conditions.

The cyclin-dependent kinase inhibitor p57Kip2 regulates cell cycle exit, differentiation, and migration of embryonic cerebral cortical precursors

Mounting evidence indicates cyclin-dependent kinase (CDK) inhibitors (CKIs) of the Cip/Kip family, including p57(Kip2) and p27(Kip1), control not only cell cycle exit but also corticogenesis. Nevertheless, distinct activities of p57(Kip2) remain poorly defined. Using in vivo and culture approaches, we show p57(Kip2) overexpression at E14.5-15.5 elicits precursor cell cycle exit, promotes transition from proliferation to neuronal differentiation, and enhances process outgrowth, while opposite effects occur in p57(Kip2)-deficient precursors. Studies at later ages indicate p57(Kip2) overexpression also induces precocious glial differentiation, suggesting stage-dependent effects. In embryonic cortex, p57(Kip2) overexpression advances cell radial migration and alters postnatal laminar positioning. While both CKIs induce differentiation, p57(Kip2) was twice as effective as p27(Kip1) in inducing neuronal differentiation and was not permissive to astrogliogenic effects of ciliary neurotrophic factor, suggesting that the CKIs differentially modulate cell fate decisions. At molecular levels, although highly conserved N-terminal regions of both CKIs elicit cycle withdrawal and differentiation, the C-terminal region of p57(Kip2) alone inhibits in vivo migration. Furthermore, p57(Kip2) effects on neurogenesis and gliogenesis require the N-terminal cyclin/CDK binding/inhibitory domains, while previous p27(Kip1) studies report cell cycle-independent functions. These observations suggest p57(Kip2) coordinates multiple stages of corticogenesis and exhibits distinct and common activities compared with related family member p27(Kip1).

Antidepressants and Cdk inhibitors: releasing the brake on neurogenesis?

It is now clear that neurogenesis occurs in the brain of adult mammals. Many studies have attempted to establish relationships among neurogenesis, depression and the mechanism of action of antidepressant drugs. Therapeutic effects of antidepressants appear to be linked to increased neurogenesis in the hippocampus. Cdk inhibitors are expressed in multiple brain regions, presumably maintaining quiescence in differentiated neurons. Recently, the abundant expression of p21(Cip1) was found in neuroblasts and in newly developing neurons in the subgranular zone of the hippocampus, a region where adult neurogenesis occurs. Chronic treatment with the tricyclic antidepressant imipramine markedly decreased p21(Cip1) mRNA and protein levels and stimulated neurogenesis in this region. These results suggest that p21(Cip1) restrains neurogenesis in the hippocampus, and antidepressant-induced stimulation of neurogenesis might be a consequence of decreased p21(Cip1) expression, with the subsequent release of neuronal progenitor cells from the blockade of proliferation. These findings suggest the potential for new therapeutic strategies for the treatment of depression that target cell cycle proteins. However, there is a possibility that long-term stimulation of neurogenesis might exhaust the proliferation potentials of neuronal progenitors.

The transcription factor Yin Yang 1 is essential for oligodendrocyte progenitor differentiation

The progression of progenitors to oligodendrocytes requires proliferative arrest and the activation of a transcriptional program of differentiation. While regulation of cell cycle exit has been extensively characterized, the molecular mechanisms responsible for the initiation of differentiation remain ill-defined. Here, we identify the transcription factor Yin Yang 1 (YY1) as a critical regulator of oligodendrocyte progenitor differentiation. Conditional ablation of yy1 in the oligodendrocyte lineage in vivo induces a phenotype characterized by defective myelination, ataxia, and tremor. At the cellular level, lack of yy1 arrests differentiation of oligodendrocyte progenitors after they exit from the cell cycle. At the molecular level, YY1 acts as a lineage-specific repressor of transcriptional inhibitors of myelin gene expression (Tcf4 and Id4), by recruiting histone deacetylase-1 to their promoters during oligodendrocyte differentiation. Thus, we identify YY1 as an essential component of the transcriptional network regulating the transition of oligodendrocyte progenitors from cell cycle exit to differentiation.

Galectin-4 is involved in p27-mediated activation of the myelin basic protein promoter

Our previous studies have found that expression of p27 in oligodendrocytes enhances myelin basic protein (MBP) gene expression through a mechanism that involves the transcription factor Sp1. In this study we show that this activation only requires the N-terminal 45 amino acids of p27 containing a functional cyclin-binding motif. In an effort to identify other cofactors that are involved in the p27-mediated activation of MBP gene expression, a yeast two-hybrid assay was performed using an N-terminal truncated p27 and a mouse embryo cDNA library. Galectin-4 was found to interact with p27 in the yeast two-hybrid assay. This novel interaction was also confirmed using a glutathione-S-transferase interaction assay and immunoprecipitation assays. Expression of galectin-4 in primary oligodendrocytes was confirmed by western blot. Additionally, the MBP promoter could be activated by expression of galectin-4 in CG4 oligodendrocytes, similar to the effects of increased p27 levels. We also show that Sp1 and galectin-4 interact in cells, while a complex of all three proteins could not be found. We conclude that galectin-4 is involved in the p27-mediated activation of the MBP gene, possibly through modulation of the glycosylation status of the transcription factor Sp1.

The presence of active Cdk5 associated with p35 in astrocytes and its important role in process elongation of scratched astrocyte

Cyclin-dependent kinase 5 (Cdk5) is a unique member of the Cdk family; its kinase activity requires association with its activator, p35 or p39. p35 is the strongest and best characterized activator. Previous studies showed that p35 is a neuron-specific protein that restricts Cdk5 activity in neurons. However, a high expression level of Cdk5 is found in astrocytes, which raises the possibility that astrocytic Cdk5 is functional. Here we show the presence of functional Cdk5 associated with p35 in astrocytes and demonstrate its important role in process elongation of scratched astrocytes. We found that p35 and glial fibrillary acidic protein (GFAP) were co-localized in primary cultured and acute isolated brain cells. Cdk5 could form an immunocomplex with p35 and its activity was shown in pure primary cultured astrocytes. p35 was upregulated in astrocytes injured by scratching, concomitantly with upregulation of Cdk5 kinase activity. Pretreatment of the scratched astrocytes with a Cdk5 inhibitor, roscovitine, could delay wound healing by inhibiting the reorganization of tubulin, GFAP, and the extension of hypertrophic processes. Moreover, overexpression of dominant negative Cdk5 could shorten the length of extending protrusion of reactive astrocytes. Thus, our findings demonstrated that functional Cdk5, associated with p35, was expressed in astrocytes and its activity could be upregulated in reactive astrocytes, a new role of Cdk5 that has never been reported in the nervous system. The present study may provide new insight for understanding the multifunctional protein complex Cdk5/p35 in the nervous system.

The Yin and Yang of cell cycle progression and differentiation in the oligodendroglial lineage

In white matter disorders such as leukodystrophies (LD), periventricular leucomalacia (PVL), or multiple sclerosis (MS), the hypomyelination or the remyelination failure by oligodendrocyte progenitor cells involves errors in the sequence of events that normally occur during development when progenitors proliferate, migrate through the white matter, contact the axon, and differentiate into myelin-forming oligodendrocytes. Multiple mechanisms underlie the eventual progressive deterioration that typifies the natural history of developmental demyelination in LD and PVL and of adult-onset demyelination in MS. Over the past few years, pathophysiological studies have mostly focused on seeking abnormalities that impede oligodendroglial maturation at the level of migration, myelination, and survival. In contrast, there has been a strikingly lower interest for early proliferative and differentiation events that are likely to be equally critical for white matter development and myelin repair. This review highlights the Yin and Yang principles of interactions between intrinsic factors that coordinately regulate progenitor cell division and the onset of differentiation, i.e. the initial steps of oligodendrocyte lineage progression that are obviously crucial in health and diseases.

Basic fibroblast growth factor exhibits dual and rapid regulation of cyclin D1 and p27 to stimulate proliferation of rat cerebral cortical precursors

While extracellular signals play a major role in brain neurogenesis, little is known about the cell cycle machinery underlying mitogen stimulation of precursor proliferation. Current models suggest that the D cyclins function as primary sensors of extracellular mitogens. Here we define the mechanisms by which basic fibroblast growth factor (bFGF) stimulates cortical precursors, with particular attention to the responses of cell cycle promitogenic and antimitogenic regulators. bFGF produced a 4-fold increase in DNA synthesis and a 3-fold rise in bromodeoxyuridine labeling, suggesting that the factor promotes the G1/S transition. There was also a 3-fold increase in cyclin-dependent kinase 2 (CDK2) kinase activity, which is critical for S phase entry. CDK2 activation was apparently cyclin E dependent, since only its protein and mRNA levels were elevated at 24 h, whereas CDK2, p27KIP1 and p57KIP2 levels were unaltered. Late G1 phase CDK2/cyclin E activity depends on early G1 D cyclin function. Indeed, cyclin D1, but not cyclin D3, was upregulated selectively at 8 h after bFGF treatment, a time when cyclin E was unchanged. The sequential activation of cyclin D1 and cyclin E supports the idea that cyclin E gene transcription is regulated by cyclin-D/CDK4/6-mediated pRb phosphorylation and subsequent E2F transcription factor release. However, in addition to increased D1 cyclin, we unexpectedly detected a 75% reduction in p27KIP1 protein at 8 h, suggesting that both pro- and antimitogenic regulators are targets of extracellular mitogens during brain development.

Number of oligodendrocyte progenitors recruited to the lesioned spinal cord is modulated by the levels of the cell cycle regulatory protein p27Kip-1

Remyelination is a critical step for recovery of function after demyelination and defines the ability to generate new myelin. This repair process is dependent on the presence of resident oligodendrocyte progenitors (OLPs) that have been shown to remyelinate axons after demyelination. We have previously shown that the levels of the cell cycle inhibitor p27Kip-1 modulate the number of neonatal cortical OLPs. We now asked whether this cell cycle molecule plays also a role in regulating the number of adult OLP in the spinal cord after demyelination induced by lysolecithin injection. The proliferative response of OLP in the spinal cord of injected wild-type (wt) and p27Kip-1 null mice was evaluated 3 days after lesion. In vivo labeling with bromodeoxyuridine (BrdU) was used to identify cells in S phase. Double immunofluorescence for the OLP marker NG2, and for BrdU was used to count the number of proliferating progenitors. Consistent with a role of p27Kip-1 in regulating the number of adult OLP in the injured spinal cord, a larger number of proliferating OLPs was observed in p27Kip-1null mice compared with wild-type controls. These cells were able to differentiate as assessed by the presence of MBP+ cells in the spinal cord 14 days after injury. We conclude that the cellular levels of the cell cycle inhibitor p27Kip-1 modulate the repair response of OLPs to injury in the adult spinal cord.

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.

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.

End-stage differentiation of neutrophil granulocytes in vivo is accompanied by up-regulation of p27kip1 and down-regulation of CDK2, CDK4, and CDK6

The in vivo expression profiles of cell-cycle proteins regulating G1-to-S-phase transition were determined in three neutrophil precursor populations from human bone marrow: myeloblasts (MBs) and promyelocytes (PMs); myelocytes (MCs) and metamyelocytes (MMs); and band cells (BCs) and segmented neutrophil cells (SCs) and in mature polymorphonuclear neutrophils (PMNs) from peripheral blood. Complete cell-cycle arrest was observed in BCs/SCs and PMNs. Cyclins D1, D2, and D3 were found to be down-regulated during granulopoiesis, whereas a slight increase of cyclin E was seen. In contrast, cyclin-dependent kinase (CDK)2, -4, and -6 were down-regulated from the MC/MM stages and onward. The transcript levels of CDK2, -4, and -6 were concurrently down-regulated. As the only CDK inhibitor, p27kip1 protein and mRNA expression were up-regulated in MCs/MMs and reached peak levels in PMNs. Protein expression of retinoblastoma protein and the related pocket proteins p107 and p130 was down-regulated from the MC/MM stages and onward. This is the first report to describe expression levels of cell-cycle proteins during granulopoiesis in vivo, and it strongly contrasts the observations made in cell-culture systems in vitro.

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 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.

Astrocyte differentiation states and glioma formation

Gliomas are the most common primary malignancy in human central nervous system. Many similarities in cell morphology and expression of markers exist between cancerous cells and normal undifferentiated progenitor cells. At the molecular level, many important gene products are causally implicated in both the glial differentiation process and glial neoplasm formation. These observations raise the question of to what degree cell differentiation state influences glioma formation. In this review, we discuss new insights into the parallels between glial differentiation and glioma formation as well as the potential application of differentiation-inducing therapy.

Low level of p27(Kip1) protein expression in gastric adenocarcinoma is associated with disease progression and poor outcome

BACKGROUND AND OBJECTIVES

Low tumor expression of the p27(Kip1) protein, which is involved in cell cycle control and apoptosis, is considered a negative prognostic factor in different types of cancer. The aim of this study was to evaluate the clinical and pathological significance of low p27(Kip1) protein expression in patients who had undergone resection for gastric adenocarcinoma.

METHODS

p27(Kip1) protein was studied by immunohistochemistry in formalin-fixed tumor sections from 95 patients who underwent resection for gastric adenocarcinoma between 1991 and 1996. Based on the median value of protein expression, p27(Kip1) protein expression was classified as low or high.

RESULTS

Low p27(Kip1) protein expression was significantly associated with tumor de-differentiation, increased penetration through the gastric wall, lymph node metastasis, and advanced tumor stage. In the group of 84 patients who underwent curative surgery, 5-year survival was 74% in cases with high p27(Kip1) protein expression and 38% in those with low p27(Kip1) protein expression (P < 0.001). At multivariate analysis, low p27(Kip1) protein expression was an independent negative prognostic factor for survival (RR = 3.671; P = 0.004).

CONCLUSIONS

In gastric adenocarcinoma, low p27(Kip1) protein expression is associated with poorly differentiated and advanced tumors and is a negative prognostic factor of potential clinical value.

Posttranscriptional regulation of p18 and p27 Cdk inhibitor proteins and the timing of oligodendrocyte differentiation

A cell-intrinsic timer helps control when rodent oligodendrocyte precursor cells (OPCs) exit the cell cycle and terminally differentiate when cultured in platelet-derived growth factor (PDGF) and thyroid hormone (TH). There is evidence that the cyclin-dependent kinase inhibitor (CKI) p27/Kip1 (p27) is a component of this TH-regulated timer, as it increases as OPCs proliferate and is required for the timer to operate accurately. Here, we provide evidence that another CKI, p18/INK (p18), may also be a component of the timer: it increases as OPCs proliferate, and its overexpression in OPCs accelerates the timer, causing the cells to differentiate prematurely. We also show that the overexpression of p27 accelerates the timer and that the increases in both p27 and p18 that occur in proliferating OPCs are controlled posttranscriptionally. By contrast, we show that the overexpression of either p18 or p27 in OPCs proliferating in PDGF and the absence of TH greatly slows the cell cycle but fails to accelerate the spontaneous differentiation that normally occurs independently of TH.

Regulation of the cell cycle in normal and pathological glia

Precise regulation of the glial cell cycle is essential during nervous system development and in response to injury, whereas disruption of cell cycle control is associated with malignant glial tumors and other nervous system diseases. The Ras signaling pathway plays a central role in regulating the mammalian cell cycle, and uncontrolled Ras signaling has been implicated in a wide range of human cancers, including malignant gliomas. Recent studies in glia have demonstrated that activation of Ras can either induce or inhibit proliferation through complex interactions among downstream signaling pathways impinging on cell cycle regulatory proteins. Studies in Schwann cells have begun to delineate the pathways by which Ras regulates the cell cycle in normal and pathological glia, and have identified promising targets for therapeutic intervention in the treatment of PNS and CNS malignant glial tumors.

Pituitary adenylate cyclase activating polypeptide anti-mitogenic signaling in cerebral cortical progenitors is regulated by p57Kip2-dependent CDK2 activity

Generation of distinct cell types and numbers in developing cerebral cortex is subject to regulation by extracellular factors that positively or negatively control precursor proliferation. Although signals stimulating proliferation are well described, factors halting cell cycle progression are less well defined. At the molecular level, production and association of cyclins, cyclin-dependent kinases (CDKs), and CDK inhibitors (CKIs) regulate cycle progression. We now report that the endogenous peptide, pituitary adenylate cyclase activating polypeptide (PACAP), negatively regulates the cell cycle by inhibiting p57Kip2-dependent CDK2 activity in embryonic cortex. Protein levels of CDK2 and members of the CIP/KIP family of CKIs (p27Kip1, p57Kip2) were detected in developing rat cortex from embryonic day 13.5 through postnatal day 2. With advancing development, CDK2 protein levels decreased, whereas CKI expression increased, suggesting that stimulatory and inhibitory cycle proteins control cell cycle exit. Using a well defined, nonsynchronized, 8 hr precursor culture, PACAP decreased the fraction of cells crossing the G1/S boundary, inhibiting DNA synthesis by 35%. CDK2 kinase activity was inhibited 75% by PACAP, whereas kinase protein and its regulatory cyclin E subunit were unaffected. Moreover, decreased kinase activity was accompanied by a twofold increase in levels of p57Kip2 protein, but not p21Cip1 or p27Kip1, suggesting that p57Kip2 mediates PACAP anti-mitogenic effects. Indeed, immunoprecipitation of CDK2 complex revealed increased p57Kip2 association with the kinase and concomitant reduction in free inhibitor after PACAP exposure, suggesting that p57Kip2 interactions directly regulate CDK2 activity. These observations establish a mechanism whereby anti-mitogenic signals actively induce cell cycle withdrawal in developing cortex.

p27(Kip1) enhances myelin basic protein gene promoter activity

The process of oligodendrocyte differentiation is a complex event that requires cell cycle withdrawal, followed by the activation of a specific transcriptional program responsible for the synthesis of myelin genes. Because growth arrest precedes differentiation, we sought to investigate the role of cell cycle molecules in the activation of myelin gene promoters. We hypothesized that the cell cycle inhibitor p27(Kip1), which is primarily responsible for arresting proliferating oligodendrocyte progenitors, may be involved in the transcriptional regulation of myelin genes. In agreement with this hypothesis, overexpression of p27(Kip1) in the CG4 cell line, but not in 3T3 fibroblasts, enhances the expression of luciferase driven by the myelin basic protein (MBP) promoter. Interestingly, this effect is specific for p27(Kip1); overexpression of other cell cycle inhibitors had no effect. Additionally, this effect is independent of halting the cell cycle; treatment with the cell cycle blocker roscovitine did not affect MBP promoter usage. We conclude that p27(Kip1) contributes to oligodendrocyte differentiation by regulating transcription of the MBP gene.

Inhibition of cyclin E-cyclin-dependent kinase 2 complex formation and activity is associated with cell cycle arrest and withdrawal in oligodendrocyte progenitor cells

Stimulatory and inhibitory signals regulate cell proliferation through the activity of specific enzymes that operate in distinct phases of the cell cycle. We have studied cell cycle progression, arrest, and withdrawal in the oligodendrocyte progenitor (OP) cell model system, focusing on the G(1) phase and G(1)-S transition. Not only were proliferating OPs found to display higher protein levels of cyclin E and D and cyclin-dependent kinases (cdk) 2, 4, and 6 than cells that had permanently withdrawn from the cycle, but the kinase activities of both cyclin D-cdk4/6 and cyclin E-cdk2 were also higher in dividing OPs. This was associated with a decrease in the formation of the cyclin E-cdk2 and cyclin D-cdk4/cyclin D-cdk6 complexes in differentiated oligodendrocytes that had permanently withdrawn from the cell cycle. Reversible cell cycle arrest in G(1) induced by glutamatergic and beta-adrenergic receptor activation or cell depolarization, however, did not modify cyclin E and cdk2 protein expression compared with proliferating OPs. Instead, these agents caused a selective decrease in cdk2 activity and an impairment of cyclin E-cdk2 complex formation. Although cyclin D protein levels were higher than in proliferating cells, cyclin D-associated kinase activity was not modified in G(1)-arrested OPs. Analysis in corpus callosum in vivo showed that cyclin E-cdk2 activity increased between postnatal days 3 and 15 and decreased between postnatal days 15 and 30. Our results indicate that the cyclin E-cdk2 complex is a major regulator of OP cell cycle progression and that the cdks involved in reversible cell cycle arrest are distinct from those implicated in permanent cell cycle withdrawal.

Cyclin-dependent kinase inhibitors, p21(waf1/cip1) and p27(kip1), are expressed site- and hair cycle-dependently in rat hair follicles

In order to investigate the role of cyclin-dependent kinase (CDK) inhibitors in hair growth, we analyzed the expressions of p21(waf1/cip1) and p27(kip1) during the synchronized hair cycle of rat coat. The mRNAs of both p21(waf1/cip1) and p27(kip1) were detected in anagen hair follicles by reverse transcription and polymerase chain reaction and their localization was clearly demonstrated in the upper half portion of the hair bulb and the cortex by in situ hybridization. The dermal tissue containing hair follicles was then excised from the anterior dorsal skin of the 5-12-week-old rats at 0.5 week intervals and the expressions of p21(waf1/cip1) and p27(kip1) were analyzed by northern blot hybridization. The mRNA of both CDK inhibitors was expressed at relatively high levels during anagen than during telogen, a fact which correlated with the mRNA expression levels of hair differentiation markers, type I hair keratin (Ha3) and high sulfur protein B2. These results imply that CDK inhibitors, p21(waf1/cip1) and p27(kip1), are involved in the differentiation of follicular epithelial cells.

Cell cycle exit during terminal erythroid differentiation is associated with accumulation of p27Kip1 and inactivation of cdk2 kinase

Progression through the mammalian cell cycle is regulated by cyclins, cyclin- dependent kinases (CDKs), and cyclin-dependent kinase inhibitors (CKIs). The function of these proteins in the irreversible growth arrest associated with terminally differentiated cells is largely unknown. The function of Cip/Kip proteins p21Cip1and p27Kip1 during erythropoietin-induced terminal differentiation of primary erythroblasts isolated from the spleens of mice infected with the anemia-inducing strain of Friend virus was investigated. Both p21Cip1 and p27Kip1 proteins were induced during erythroid differentiation, but only p27Kip1 associated with the principal G1CDKs—cdk4, cdk6, and cdk2. The kinetics of binding of p27Kip1 to CDK complexes was distinct in that p27Kip1 associated primarily with cdk4 (and, to a lesser extent, cdk6) early in differentiation, followed by subsequent association with cdk2. Binding of p27Kip1 to cdk4 had no apparent inhibitory effect on cdk4 kinase activity, whereas inhibition of cdk2 kinase activity was associated with p27Kip1binding, accumulation of hypo-phosphorylated retinoblastoma protein, and G1 growth arrest. Inhibition of cdk4 kinase activity late in differentiation resulted from events other than p27Kip1 binding or loss of cyclin D from the complex. The data demonstrate that p27Kip1 differentially regulates the activity of cdk4 and cdk2 during terminal erythroid differentiation and suggests a switching mechanism whereby cdk4 functions to sequester p27Kip1 until a specified time in differentiation when cdk2 kinase activity is targeted by p27Kip1 to elicit G1 growth arrest. Further, the data imply that p21Cip1 may have a function independent of growth arrest during erythroid differentiation.

Cell cycle exit during terminal erythroid differentiation is associated with accumulation of p27Kip1 and inactivation of cdk2 kinase

Progression through the mammalian cell cycle is regulated by cyclins, cyclin- dependent kinases (CDKs), and cyclin-dependent kinase inhibitors (CKIs). The function of these proteins in the irreversible growth arrest associated with terminally differentiated cells is largely unknown. The function of Cip/Kip proteins p21Cip1and p27Kip1 during erythropoietin-induced terminal differentiation of primary erythroblasts isolated from the spleens of mice infected with the anemia-inducing strain of Friend virus was investigated. Both p21Cip1 and p27Kip1 proteins were induced during erythroid differentiation, but only p27Kip1 associated with the principal G1CDKs—cdk4, cdk6, and cdk2. The kinetics of binding of p27Kip1 to CDK complexes was distinct in that p27Kip1 associated primarily with cdk4 (and, to a lesser extent, cdk6) early in differentiation, followed by subsequent association with cdk2. Binding of p27Kip1 to cdk4 had no apparent inhibitory effect on cdk4 kinase activity, whereas inhibition of cdk2 kinase activity was associated with p27Kip1binding, accumulation of hypo-phosphorylated retinoblastoma protein, and G1 growth arrest. Inhibition of cdk4 kinase activity late in differentiation resulted from events other than p27Kip1 binding or loss of cyclin D from the complex. The data demonstrate that p27Kip1 differentially regulates the activity of cdk4 and cdk2 during terminal erythroid differentiation and suggests a switching mechanism whereby cdk4 functions to sequester p27Kip1 until a specified time in differentiation when cdk2 kinase activity is targeted by p27Kip1 to elicit G1 growth arrest. Further, the data imply that p21Cip1 may have a function independent of growth arrest during erythroid differentiation.

Regulation of cell cycle proteins by TNF-alpha and TGF-beta in cells of oligodendroglial lineage

Proliferation and apoptosis are two dynamic, interrelated processes that are regulated by growth factors and cytokines. We investigated the effects of tumor necrosis factor-alpha (TNF-alpha) and transforming growth factor-beta (TGF-beta) on apoptosis and regulation of cell cycle proteins in OLG lineage cells. We found that: (1) both cytokines enhanced apoptosis in neonatal pre-OLGs but only TNFalpha-mediated apoptosis persisted in the presence of a mitogen, fibroblast growth factor (FGF); (2) cell cycle proteins such as p21(waf1/cip1), p27(kip1), cyclin D1 and PCNA were differentially regulated by TNF-alpha and TGF-beta. We conclude that differential modulation of cell cycle proteins by TNF-alpha and TGF-beta contributes to the diversity of their biological effects in OLG lineage cells.

Cell cycle control of Schwann cell proliferation: role of cyclin-dependent kinase-2

Schwann cell proliferation is regulated by multiple growth factors and axonal signals. However, the molecules that control growth arrest of Schwann cells are not well defined. Here we describe regulation of the cyclin-dependent kinase-2 (CDK2) protein, an enzyme that is necessary for the transition from G1 to S phase. Levels of CDK2 protein were elevated in proliferating Schwann cells cultured in serum and forskolin. However, when cells were grown with either serum-free media or at high densities, CDK2 levels declined to low levels. The decrease in CDK2 levels was associated with growth arrest of Schwann cells. The modulation of CDK2 appears to be regulated at the transcriptional level, because CDK2 mRNA levels and its promoter activity both decline during cell cycle arrest. Furthermore, analysis of the CDK2 promoter suggests that Sp1 DNA binding sites are essential for maximal activation in Schwann cells. Together, these data suggest that CDK2 may represent a significant target of developmental signals that regulate Schwann cell proliferation and that this regulation is mediated, in part, through regulation of Sp1 transcriptional activity.

Multiple functions of p27(Kip1) and its alterations in tumor cells: a review

Cyclin-dependent kinases (CDKs), together with cyclins, their regulatory subunits, govern cell-cycle progression in eukaryotic cells. p27(Kip1) is a member of a family of CDK inhibitors (CDIs) that bind to cyclin/CDK complexes and arrest cell division. There is considerable evidence that p27(Kip1) plays an important role in multiple fundamental cellular processes, including cell proliferation, cell differentiation, and apoptosis. Moreover, p27(Kip1) is a putative tumor-suppressor gene that appears to play a critical role in the pathogenesis of several human malignancies and its reduced expression has been shown to correlate with poor prognosis in cancer patients. This study reviews current information on the functions of p27(Kip1), its abnormalities found in human tumors, and the possible clinical implications of these findings with respect to the management of cancer patients.

p27(Kip1) regulates cell cycle withdrawal of late multipotent progenitor cells in the mammalian retina

The cyclin-dependent kinase inhibitor protein, p27(Kip1), is necessary for the timing of cell cycle withdrawal that precedes terminal differentiation in oligodendrocytes of the optic nerve. Although p27(Kip1) is widely expressed in the developing central nervous system, it is not known whether this protein has a similar role in neuronal differentiation. To address this issue, we have examined the expression and function of p27(Kip1) in the developing retina, a well-characterized part of the central nervous system. p27(Kip1) is expressed in a pattern coincident with the onset of differentiation of most retinal cell types. In vitro analyses show that p27(Kip1) accumulation in retinal cells correlates with cell cycle withdrawal and differentiation, and when overexpressed, p27(Kip1) inhibits proliferation of the progenitor cells. Furthermore, the histogenesis of photoreceptors and Müller glia is extended in the retina of p27(Kip1)-deficient mice. Finally, we examined the adult retinal dysplasia in p27(Kip1)-deficient mice with cell-type-specific markers. Contrary to previous suggestions that the dysplasia is caused by excess production of photoreceptors, we suggest that the dysplasia is due to the displacement of reactive Müller glia into the layer of photoreceptor outer segments. These results demonstrate that p27(Kip1) is part of the molecular mechanism that controls the decision of multipotent central nervous system progenitors to withdraw from the cell cycle. Second, postmitotic Müller glia have a novel and intrinsic requirement for p27(Kip1) in maintaining their differentiated state.

Anti-sense oligonucleotide of p21(waf1/cip1) prevents interleukin 4-mediated elevation of p27(kip1) in low grade astrocytoma cells

Elevation of the cyclin-dependent kinase (cdk) inhibitor, p27(kip1) is necessary for Interleukin (IL)-4-mediated growth arrest of human low grade astrocytoma (RTLGA) cells and occurs at 24 h of treatment. Pathways involved in IL4 alteration of p27(kip1) are unknown, however. Here we investigated whether other cdk inhibitors contributed to the actions of IL-4 on RTLGA cells. By 12 h of IL-4 treatment, both cdk4 and cdk2 kinase activities against the retinoblastoma protein (pRb) were reduced and nuclear entry of pRb was prohibited. Twelve-hour cdk complexes contained elevated p21(waf1/cip1) but not p27(kip1), p15(ink4B) or p16(ink4A). IL-4 increased p21(waf1/cip1) but not p27(kip1) mRNA levels, and stimulated luciferase activity of a p21(waf1/cip1) promoter-luciferase reporter. In p53-mutant WITG3 cells, IL-4 did not alter p21(waf1/cip1) mRNA and promoter-luciferase activity or p27(kipl) protein, suggesting a need for functional p53. STAT6 phosphorylation by IL-4, however, occurred in both p53-mutant WITG3 and p53-functional RTLGA cells. Pre-treatment of RTLGA with anti-sense but not missense p21(waf1/cip1) oligonucleotide prior to IL-4: (a) restored cdk activities; (b) reduced cdk4-associated p21(waf1/cip1) levels; (c) prevented p27(kipl) elevation; and (d) reversed growth arrest. These results are the first to suggest that p21(waf1/cip1) is essential for IL-4-mediated elevation of p27(kip) and growth arrest of astrocytoma cells.

p27Xic1, a Cdk inhibitor, promotes the determination of glial cells in Xenopus retina

p27Xic1, a member of the Cip/Kip family of Cdk inhibitors, besides its known function of inhibiting cell division, induces Müller glia from retinoblasts. This novel gliogenic function of p27Xic1 is mediated by part of the N-terminal domain near but distinct from the region that inhibits cyclin-dependent kinases. Cotransfections with dominant-negative and constitutively active Delta and Notch constructs indicate that the gliogenic effects of p27Xic1 work within the context of an active Notch pathway. The gradual increase of p27Xic1 in the developing retina thus not only limits the number of retinal cells but also increasingly favors the fate of the last cell type to be born in the retina, the Müller glia.

Anomalous p27kip1 expression in a subset of malignant gliomas

p27Kip (p27) expression was immunohistochemically investigated in 28 astrocytic tumors, and compared with the cell proliferation index (MIB-1 staining index). Normal rat brains and surgical specimens from human nonneoplastic brain lesions were used as controls. In the rat brains, the astrocytes were exclusively p27-positive. The reactive astrocytes in various disease processes sometimes lacked p27 expression. The distribution of p27-positive cells was uniform in low-grade astrocytomas and heterogeneous in high-grade tumors. Double staining of p27 and MIB-1 showed a reciprocal pattern in most cases. The frequency of p27 expression was inversely correlated with MIB-1 staining index and tumor grade. However, several malignant gliomas showed high p27 expression in spite of high MIB-1 staining indices. In such cases, MIB-1-positive cells were occasionally p27-positive. In this paper we discuss the etiology of the anomalous p27 expression in a subset of malignant gliomas.

Tumor suppression by p27Kip1 and p21Cip1 during chemically induced skin carcinogenesis

p27Kip1 and p21Cip1 are cyclin dependent kinase inhibitors which can arrest cell proliferation and p27 is a tumor suppressor gene. To address the mechanism of tumor suppression by p27 and to determine if p21 has a tumor suppressor phenotype, we utilized the two stage skin carcinogenesis model on p27 and p21 knockout mice. In this model, initiation, which involves mutation of H-ras induced by DMBA, can be distinguished from promotion induced by TPA, and progression to carcinoma. The mean number of papillomas did not differ between p27-/- and control littermates, but papilloma growth rate was increased and carcinomas developed earlier. Thus, p27 deficiency did not enhance initiation, but resulted in more rapid clonal expansion of initiated cells during promotion. TPA treatment reduced p27 expression in keratinocytes also supporting a role for p27 during promotion. Tumors from p27-/- mice contained mutant H-ras indicating that p27 deficiency did not substitute for mutant ras and further, that during ras driven tumor growth, p27 is partially antagonistic since its removal led to faster growth. The treated p27-/- mice also developed intestinal adenomas. p21-/- mice did not display a significant increase in tumor numbers, growth rate or progression to carcinomas and these tumors also had mutated H-ras. Carcinomas from p21-/- mice were more poorly differentiated with a high frequency of anaplastic spindle cell carcinomas. Thus p21 deficiency mainly resulted in higher grade undifferentiated tumors.

Differential expression of the cyclin-dependent kinase inhibitor P27 in primary hepatocytes in early-mid G1 and G1/S transitions

P27, an inhibitor of cyclin-dependent kinases, plays an important role in the control of cell adhesion and contact inhibition-dependent cell cycle regulation. Hepatocytes, maintained in primary culture, offer a model of synchronized primary epithelial cells which retain a differentiated profile while stimulated to proliferate. We therefore investigated the pattern of endogenous p27 expression in cyclin rat hepatocytes isolated by collagenase perfusion followed by mitogenic stimulation. P27 was expressed in whole normal liver and freshly isolated hepatocytes. We then observed a sharp decrease in p27 levels, concomitant with the progression in early-mid G1, followed by reaccumulation in late G1 and the G1/S transition. Immunochemistry and BrdU labelling demonstrated nuclear localization of p27 and its expression in cells engaged in both G1 and S phase. P27 was detected in late G1 in complexes containing cyclins D1, E and A. Cyclin E- and A-associated kinase activities, however, were detected at the G1/S transition and depletion experiments confirmed that most active complexes were free of p27. Phosphorylated forms of p27 were detected in unstimulated and stimulated hepatocytes in both early-mid G1 and G1/S. Finally, two-dimensional gel electrophoresis showed evidence for several forms of p27 with a distinct profile of distribution in quiescent and stimulated hepatocytes. Collectively, our data offer a model in which p27 shows a biphasic profile of accumulation, with the early decrease possibly involved in the progression through early and mid G1. In contrast with most cell types tested so far, the late G1 accumulation did not impair formation of active cyclin E- and A associated kinases, and thus G1/S transition.

Differentiation-related changes in the cell cycle traverse

This review examines recent developments relating to the interface between cell proliferation and differentiation. It is suggested that the mechanism responsible for this transition is more akin to a "dimmer" than to a "switch," that it is more useful to refer to early and late stages of differentiation rather than to "terminal" differentiation, and examples of the reversibility of differentiation are provided. An outline of the established paradigm of cell cycle regulation is followed by summaries of recent studies that suggest that this paradigm is overly simplified and should be interpreted in the context of different cell types. The role of inhibitors of cyclin-dependent kinases in differentiation is discussed, but the data are still inconclusive. An increasing interest in the changes in G2/M transition during differentiation is illustrated by examples of polyploidization during differentiation, such as megakaryocyte maturation. Although the retinoblastoma protein is currently maintaining its prominent role in control of proliferation and differentiation, it is anticipated that equally important regulators will be discovered and provide an explanation at the molecular level for the gradual transition from proliferation to differentiation.

Regulatory role of p27kip1 in the mouse and human testis

p27kip1 is a cyclin-dependent kinase inhibitor that regulates the G1/S transition of the cell cycle. Immunohistochemical analysis showed that during mouse testicular development p27kip1 is induced when the fetal germ cells, gonocytes, become quiescent on day 16 postcoitum, suggesting that p27kip1 is an important factor for the G1/G0 arrest in gonocytes. In the adult mouse and human testis, in general, spermatogonia are proliferating actively, except for undifferentiated spermatogonia that also go through a long G1/G0 arrest. However, none of the different types of germ cells immunohistochemically stained for p27kip1. During development, Sertoli cells are proliferating actively and only occasionally were lightly p27kip1 stained Sertoli cells observed. In contrast, in the adult testis the terminally differentiated Sertoli cells heavily stain for p27kip1. Twenty to 30% of both fetal and adult type Leydig cells lightly stained for p27kip1, possibly indicating the proportion of terminally differentiated cells in the Leydig cell population. In p27kip1 knockout mice, aberrations in the spermatogenic process were observed. First, an increase in the numbers ofA spermatogonia was found, and second, abnormal (pre)leptotene spermatocytes were observed, some of which seemingly tried to enter a mitotic division instead of entering the meiotic prophase. These observations indicate that p27kip1 has a role in the regulation of spermatogonial proliferation, or apoptosis, and the onset of the meiotic prophase in preleptotene spermatocytes. However, as p27kip1 is only expressed in Sertoli cells, the role of p27kip1 in both spermatogonia and preleptotene spermatocytes must be indirect. Hence, part of the supportive and/or regulatory role of Sertoli cells in the spermatogenic process depends on the expression of p27kip1 in these cells. Finally, we show that the expression of p27kip1 transiently increases by a factor of 3 after x-irradiation in whole testicular lysates. Hence, p27kip1 seems to be involved in the cellular response after DNA damage.

Neurotransmitter receptor activation triggers p27(Kip1)and p21(CIP1) accumulation and G1 cell cycle arrest in oligodendrocyte progenitors

We examined the pathways that link neurotransmitter receptor activation and cell cycle arrest in oligodendrocyte progenitors. We had previously demonstrated that glutamate receptor activation inhibits oligodendrocyte progenitor proliferation and lineage progression. Here, using purified oligodendrocyte progenitors and cerebellar slice cultures, we show that norepinephrine and the beta-adrenergic receptor agonist isoproterenol also inhibited the proliferation, but in contrast to glutamate, isoproterenol stimulated progenitor lineage progression, as determined by O4 and O1 antibody staining. This antiproliferative effect was specifically attributable to a beta-adrenoceptor-mediated increase in cyclic adenosine monophosphate, since analogs of this cyclic nucleotide mimicked the effects of isoproterenol on oligodendrocyte progenitor proliferation, while alpha-adrenoceptor agonists were ineffective. Despite the opposite effects on lineage progression, both isoproterenol and the glutamate receptor agonist kainate caused accumulation of the cyclin-dependent kinase inhibitors p27(Kip1)and p21(CIP1), and G1 arrest. Studies with oligodendrocyte progenitor cells from INK4a−/− mice indicated that the G1 cyclin kinase inhibitor p16(INK4a) as well as p19(ARF)were not required for agonist-stimulated proliferation arrest. Our results demonstrate that beta-adrenergic and glutamatergic receptor activation inhibit oligodendrocyte progenitor proliferation through a mechanism that may involve p27(Kip1) and p21(CIP1); but while neurotransmitter-induced accumulation of p27(Kip1) is associated with cell cycle arrest, it does not by itself promote oligodendrocyte progenitor differentiation.

Caco-2 intestinal cell differentiation is associated with G1 arrest and suppression of CDK2 and CDK4

The cellular mechanisms regulating intestinal proliferation and differentiation remain largely undefined. Previously, we showed an early induction of the cyclin-dependent kinase (CDK) inhibitor p21(Waf1/Cip1) in Caco-2 cells, a human colon cancer line that spontaneously differentiates into a small bowel phenotype. The purpose of our present study was to assess the timing of cell cycle arrest in relation to differentiation in Caco-2 cells and to examine the mechanisms responsible for CDK inactivation. Caco-2 cells undergo a relative G1/S block and cease to proliferate at day 3 postconfluency; an increase in the activity of terminally differentiated brush-border enzymes (sucrase and alkaline phosphatase) was noted at day 6 postconfluency. Cell cycle block was associated with suppression of both CDK2 and CDK4 activities, which are important for G1/S progression. Treatment of the CDK immune complexes with the detergent deoxycholate (DOC) resulted in restoration of CDK2, but not CDK4, activity at day 3 postconfluency, suggesting the presence of inhibitory protein(s) binding to the cyclin/CDK2 complex at this time point. An increased binding of p21(Waf1/Cip1) to CDK2 complexes at day 3 postconfluency was noted, suggesting a potential role for p21(Waf1/Cip1) in CDK2 inactivation; however, immunodepletion of p21(Waf1/Cip1) from Caco-2 protein extracts demonstrated that p21(Waf1/Cip1) is only partially responsible for CDK2 suppression at day 3 postconfluency. A decrease in the cyclin E/CDK2 complex appears to contribute to the CDK2 inactivation noted at days 6 and 12 postconfluency. Taken together, our results suggest that multiple mechanisms contribute to CDK suppression during Caco-2 cell differentiation. Inhibition of CDK2 and CDK4 leads to G1 arrest and inhibition of proliferation that precede Caco-2 cell differentiation.

Purification and characterization of adult oligodendrocyte precursor cells from the rat optic nerve

Oligodendrocyte precursor cells (OPCs) persist in substantial numbers in the adult brain in a quiescent state suggesting that they may provide a source of new oligodendrocytes after injury. To determine whether adult OPCs have the capacity to divide rapidly, we have developed a method to highly purify OPCs from adult optic nerve and have directly compared their properties with their perinatal counterparts. When cultured in platelet-derived growth factor (PDGF), an astrocyte-derived mitogen, perinatal OPCs divided approximately once per day, whereas adult OPCs divided only once every 3 or 4 d. The proliferation rate of adult OPCs was not increased by addition of fibroblast growth factor (FGF) or of the neuregulin glial growth factor 2 (GGF2), two mitogens that are normally produced by retinal ganglion cells. cAMP elevation has been shown previously to be essential for Schwann cells to survive and divide in response to GGF2 and other mitogens. Similarly we found that when cAMP levels were elevated, GGF2 alone was sufficient to induce perinatal OPCs to divide slowly, approximately once every 4 d, but adult OPCs still did not divide. When PDGF was combined with GGF2 and cAMP elevation, however, the adult OPCs began to divide rapidly. These findings indicate that adult OPCs are intrinsically different than perinatal OPCs. They are not senescent cells, however, because they retain the capacity to divide rapidly. Thus, after demyelinating injuries, enhanced axonal release of GGF2 or a related neuregulin might collaborate with astrocyte-derived PDGF to induce rapid division of adult OPCs.

The role of cyclin E in cell proliferation, development and cancer

Normal cell proliferation is under strict regulation governed by checkpoints located at distinct points in the cell cycle. The deregulation of these checkpoint events and the molecules associated with them may transform a normal cell into a cancer cell. One of these checkpoints whose deregulation results in transformation occurs at the Restriction point, near the G1/S boundary. The periodic appearance of one of the recently identified regulatory cyclins, cyclin E, coincides precisely with the timing of the Restriction point. The deregulation in the expression and activity of cyclin E has been associated with a number of cancers and is thought to be involved in the process of oncogenesis. In this chapter, we summarise the current knowledge on the regulation and apparent function of cyclin E in normal proliferating cells and in developing tissue and alterations of these processes in cancer.