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

p57Kip2 Phosphorylation Modulates Its Localization, Stability, and Interactions

p57Kip2 is a member of the cyclin-dependent kinase (CDK) Interacting Protein/Kinase Inhibitory Protein (CIP/Kip) family that also includes p21Cip1/WAF1 and p27Kip1. Different from its siblings, few data are available about the p57Kip2 protein, especially in humans. Structurally, p57Kip2 is an intrinsically unstructured protein, a characteristic that confers functional flexibility with multiple transient interactions influencing the metabolism and roles of the protein. Being an IUP, its localization, stability, and binding to functional partners might be strongly modulated by post-translational modifications, especially phosphorylation. In this work, we investigated by two-dimensional analysis the phosphorylation pattern of p57Kip2 in different cellular models, revealing how the human protein appears to be extensively phosphorylated, compared to p21Cip1/WAF1 and p27Kip1. We further observed clear differences in the phosphoisoforms distributed in the cytosolic and nuclear compartments in asynchronous and synchronized cells. Particularly, the unmodified form is detectable only in the nucleus, while the more acidic forms are present in the cytoplasm. Most importantly, we found that the phosphorylation state of p57Kip2 influences the binding with some p57Kip2 partners, such as CDKs, LIMK1 and CRM1. Thus, it is necessary to completely identify the phosphorylated residues of the protein to fully unravel the roles of this CIP/Kip protein, which are still partially identified.

The role of actin cytoskeleton CFL1 and ADF/cofilin superfamily in inflammatory response

Actin remodeling proteins are important in immune diseases and regulate cell cytoskeletal responses. These responses play a pivotal role in maintaining the delicate balance of biological events, protecting against acute or chronic inflammation in a range of diseases. Cofilin (CFL) and actin depolymerization factor (ADF) are potent actin-binding proteins that cut and depolymerize actin filaments to generate actin cytoskeleton dynamics. Although the molecular mechanism by which actin induces actin cytoskeletal reconstitution has been studied for decades, the regulation of actin in the inflammatory process has only recently become apparent. In this paper, the functions of the actin cytoskeleton and ADF/cofilin superfamily members are briefly introduced, and then focus on the role of CFL1 in inflammatory response.

CIP/KIP and INK4 families as hostages of oncogenic signaling

CIP/KIP and INK4 families of Cyclin-dependent kinase inhibitors (CKIs) are well-established cell cycle regulatory proteins whose canonical function is binding to Cyclin-CDK complexes and altering their function. Initial experiments showed that these proteins negatively regulate cell cycle progression and thus are tumor suppressors in the context of molecular oncology. However, expanded research into the functions of these proteins showed that most of them have non-canonical functions, both cell cycle-dependent and independent, and can even act as tumor enhancers depending on their posttranslational modifications, subcellular localization, and cell state context. This review aims to provide an overview of canonical as well as non-canonical functions of CIP/KIP and INK4 families of CKIs, discuss the potential avenues to promote their tumor suppressor functions instead of tumor enhancing ones, and how they could be utilized to design improved treatment regimens for cancer patients.

LIM Kinases, LIMK1 and LIMK2, Are Crucial Node Actors of the Cell Fate: Molecular to Pathological Features

LIM kinase 1 (LIMK1) and LIM kinase 2 (LIMK2) are serine/threonine and tyrosine kinases and the only two members of the LIM kinase family. They play a crucial role in the regulation of cytoskeleton dynamics by controlling actin filaments and microtubule turnover, especially through the phosphorylation of cofilin, an actin depolymerising factor. Thus, they are involved in many biological processes, such as cell cycle, cell migration, and neuronal differentiation. Consequently, they are also part of numerous pathological mechanisms, especially in cancer, where their involvement has been reported for a few years and has led to the development of a wide range of inhibitors. LIMK1 and LIMK2 are known to be part of the Rho family GTPase signal transduction pathways, but many more partners have been discovered over the decades, and both LIMKs are suspected to be part of an extended and various range of regulation pathways. In this review, we propose to consider the different molecular mechanisms involving LIM kinases and their associated signalling pathways, and to offer a better understanding of their variety of actions within the physiology and physiopathology of the cell.

A Beckwith-Wiedemann-Associated CDKN1C Mutation Allows the Identification of a Novel Nuclear Localization Signal in Human p57Kip2

p57^Kip2 protein is a member of the CIP/Kip family, mainly localized in the nucleus where it exerts its Cyclin/CDKs inhibitory function. In addition, the protein plays key roles in embryogenesis, differentiation, and carcinogenesis depending on its cellular localization and interactors. Mutations of CDKN1C, the gene encoding human p57^Kip2, result in the development of different genetic diseases, including Beckwith–Wiedemann, IMAGe and Silver–Russell syndromes. We investigated a specific Beckwith–Wiedemann associated CDKN1C change (c.946 C>T) that results in the substitution of the C-terminal amino acid (arginine 316) with a tryptophan (R316W-p57^Kip2). We found a clear redistribution of R316W-p57^Kip2, in that while the wild-type p57^Kip2 mostly occurs in the nucleus, the mutant form is also distributed in the cytoplasm. Transfection of two expression constructs encoding the p57^Kip2 N- and C-terminal domain, respectively, allows the mapping of the nuclear localization signal(s) (NLSs) between residues 220–316. Moreover, by removing the basic RKRLR sequence at the protein C-terminus (from 312 to 316 residue), p57^Kip2 was confined in the cytosol, implying that this sequence is absolutely required for nuclear entry. In conclusion, we identified an unreported p57^Kip2 NLS and suggest that its absence or mutation might be of relevance in CDKN1C-associated human diseases determining significant changes of p57^Kip2 localization/regulatory roles.

Functional Versatility of the CDK Inhibitor p57Kip2

The cyclin/CDK inhibitor p57^Kip2 belongs to the Cip/Kip family, with p21^Cip1 and p27^Kip1, and is the least studied member of the family. Unlike the other family members, p57^Kip2 has a unique role during embryogenesis and is the only CDK inhibitor required for embryonic development. p57^Kip2 is encoded by the imprinted gene CDKN1C, which is the gene most frequently silenced or mutated in the genetic disorder Beckwith-Wiedemann syndrome (BWS), characterized by multiple developmental anomalies. Although initially identified as a cell cycle inhibitor based on its homology to other Cip/Kip family proteins, multiple novel functions have been ascribed to p57^Kip2 in recent years that participate in the control of various cellular processes, including apoptosis, migration and transcription. Here, we will review our current knowledge on p57^Kip2 structure, regulation, and its diverse functions during development and homeostasis, as well as its potential implication in the development of various pathologies, including cancer.

Regulation of p27Kip1 and p57Kip2 Functions by Natural Polyphenols

In numerous instances, the fate of a single cell not only represents its peculiar outcome but also contributes to the overall status of an organism. In turn, the cell division cycle and its control strongly influence cell destiny, playing a critical role in targeting it towards a specific phenotype. Several factors participate in the control of growth, and among them, p27^Kip1 and p57^Kip2, two proteins modulating various transitions of the cell cycle, appear to play key functions. In this review, the major features of p27 and p57 will be described, focusing, in particular, on their recently identified roles not directly correlated with cell cycle modulation. Then, their possible roles as molecular effectors of polyphenols’ activities will be discussed. Polyphenols represent a large family of natural bioactive molecules that have been demonstrated to exhibit promising protective activities against several human diseases. Their use has also been proposed in association with classical therapies for improving their clinical effects and for diminishing their negative side activities. The importance of p27^Kip1 and p57^Kip2 in polyphenols’ cellular effects will be discussed with the aim of identifying novel therapeutic strategies for the treatment of important human diseases, such as cancers, characterized by an altered control of growth.

The CDK inhibitor p57Kip2 enhances the activity of the transcriptional coactivator FHL2

The eukaryotic cell cycle is negatively regulated by cyclin-dependent kinase inhibitors (CKIs). p57^Kip2 is a member of the Cip/Kip family of CKIs and frequently inactivated by genomic mutations associated with human overgrowth disorders. There is increasing evidence for p57 to control cellular processes in addition to cell cycle and CDK regulation including transcription, apoptosis, migration or development. In order to obtain molecular insights to unknown functions of p57, we performed a protein interaction screen. We identified the transcription regulator four-and-a-half LIM-only protein 2 (FHL2) as a novel p57-binding protein. Co-immunoprecipitation and reporter gene assays were used to elucidate the physiological and functional relevance of p57/FHL2 interaction. We found in cancer cells that endogenous p57 and FHL2 are in a complex. We observed a substantial induction of established FHL2-regulated gene promoters by p57 in reporter gene experiments and detected strong induction of the intrinsic transactivation activity of FHL2. Treatment of cells with histone deacetylase (HDAC) inhibitors and binding of exogenous FHL2 to HDACs indicated repression of FHL2 transcription activity by HDACs. In the presence of the HDAC inhibitor sodium butyrate activation of FHL2 by p57 is abrogated suggesting that p57 shares a common pathway with HDAC inhibitors. p57 competes with HDACs for FHL2 binding which might partly explain the mechanism of FHL2 activation by p57. These results suggest a novel function of p57 in transcription regulation.

P57-mediated autophagy promotes the efficacy of EGFR inhibitors in hepatocellular carcinoma

BACKGROUND & AIMS

Resistance to EGFR-targeted therapy is a major obstacle in hepatocellular carcinoma (HCC) treatment, but its underlying mechanism remains unclear. Autophagy plays a vital role in antitumour treatment. Our previous study suggested that p57 is associated with autophagy and cisplatin resistance. The present study aimed to investigate whether p57 can enhance the sensitivity of HCC cells to Erlotinib (Er)/Cetuximab(C-225) and further explore the potential mechanisms of Er/C-225 resistance.

METHODS

HCC cells were transfected with pIRES2-EGFP-p57 and pIRES2-EGFP-nc, accompanied by Er/C-225 treatment. Cell viability was detected by an Annexin apoptosis kit and MTT assay. Xenograft experiments were performed to study the function of p57 in the treatment of Er/C-225 in vivo. The level of autophagy was determined by analysis of the appearance of autophagic vacuoles. Western blotting was used to investigate the potential pathways involved.

RESULTS

Up-regulation of p57 decreased the level of Er/C-225-induced autophagy and enhanced the decrease in Er/C-225-induced cell viability. P57 overexpression combined with CQ treatment further enhanced the therapeutic efficiency of Er/C-225. The xenograft experiment verified that p57 up-regulation sensitizes HCC cells to Er/C-225. Moreover, a mechanistic investigation demonstrated that the up-regulation of p57 resulted in a decrease of LC3B-II and beclin-1, and an increase in p-PI3K, p-AKT and p-mTOR protein expressions.

CONCLUSIONS

Through activating the PI3K/AKT/mTOR signalling pathway, p57 can reverse Er/C-225-induced autophagy, and thereby increase the therapeutic efficiency of Er/C-225 treatment. Given these results, p57 up-regulation may be applicable as a therapeutic strategy to improve EGFR-targeted therapy in HCC.

Mapping Interactions between p27 and RhoA that Stimulate Cell Migration

p27 mediates cell cycle arrest by binding to and inhibiting cyclin-dependent kinase/cyclin complexes, but p27 can also contribute to pro-oncogenic signaling upon mislocalization to the cytoplasm. Cytoplasmic p27 stimulates cell migration by associating with RhoA and interfering with the exchange of GDP from RhoA stimulated by guanine nucleotide exchange factors. We used biophysical methods to show that the N-terminus of p27 directly interacts with RhoA in vitro. The affinity of p27 for RhoA is low, with an equilibrium dissociation constant of hundreds of micromolar; however, at high concentrations, p27 interfered with guanine nucleotide exchange factor-mediated nucleotide exchange from RhoA. We also show that promotion of cell migration in scratch wound cell healing assays requires full-length p27 despite the C-terminus being dispensable for the direct interaction between p27 and RhoA in vitro. These results suggest that there may be an unidentified factor(s) that associates with the C-terminus of p27 to enhance its interactions with RhoA and promote cell migration.

Molecular Mechanisms Involved in Schwann Cell Plasticity

Schwann cell incredible plasticity is a hallmark of the utmost importance following nerve damage or in demyelinating neuropathies. After injury, Schwann cells undergo dedifferentiation before redifferentiating to promote nerve regeneration and complete functional recovery. This review updates and discusses the molecular mechanisms involved in the negative regulation of myelination as well as in the reprogramming of Schwann cells taking place early following nerve lesion to support repair. Significant advance has been made on signaling pathways and molecular components that regulate SC regenerative properties. These include for instance transcriptional regulators such as c-Jun or Notch, the MAPK and the Nrg1/ErbB2/3 pathways. This comprehensive overview ends with some therapeutical applications targeting factors that control Schwann cell plasticity and highlights the need to carefully modulate and balance this capacity to drive nerve repair.

Cell cycle, cytoskeleton dynamics and beyond: the many functions of cyclins and CDK inhibitors

While targeting experiments carried out on the genes encoding many cell cycle regulators have challenged our views of cell cycle control, they also suggest that redundancy might not be the only explanation for the observed perplexing phenotypes. Indeed, several observations hint at functions of cyclins and CDK inhibitors that cannot be accounted for by their sole role as kinase regulators. They are found involved in many cellular transactions, depending or not on CDKs that are not directly linked to cell cycle control, but participating to general mechanisms such as transcription, DNA repair or cytoskeleton dynamics. In this review we discuss the roles that these alternative functions might have in cancer cell proliferation and migration that sometime even challenge their definition as proliferation markers.

Cellular Response upon Stress: p57 Contribution to the Final Outcome

Progression through the cell cycle is one of the most important decisions during the life of a cell and several kinds of stress are able to influence this choice. p57 is a cyclin-dependent kinase inhibitor belonging to the CIP/KIP family and is a well-known regulator of the cell cycle during embryogenesis and tissue differentiation. p57 loss has been reported in a variety of cancers and great effort has been spent during the past years studying the mechanisms of p57 regulation and the effects of p57 reexpression on tumor growth. Recently, growing amount of evidence points out that p57 has a specific function in cell cycle regulation upon cellular stress that is only partially shared by the other CIP/KIP inhibitors p21 and p27. Furthermore, it is nowadays emerging that p57 plays a role in the induction of apoptosis and senescence after cellular stress independently of its cell cycle related functions. This review focuses on the contribution that p57 holds in regulating cell cycle arrest, apoptosis, and senescence after cellular stress with particular attention to the response of cancer cells.

Intracellular Protein Shuttling: A Mechanism Relevant for Myelin Repair in Multiple Sclerosis?

A prominent feature of demyelinating diseases such as multiple sclerosis (MS) is the degeneration and loss of previously established functional myelin sheaths, which results in impaired signal propagation and axonal damage. However, at least in early disease stages, partial replacement of lost oligodendrocytes and thus remyelination occur as a result of resident oligodendroglial precursor cell (OPC) activation. These cells represent a widespread cell population within the adult central nervous system (CNS) that can differentiate into functional myelinating glial cells to restore axonal functions. Nevertheless, the spontaneous remyelination capacity in the adult CNS is inefficient because OPCs often fail to generate new oligodendrocytes due to the lack of stimulatory cues and the presence of inhibitory factors. Recent studies have provided evidence that regulated intracellular protein shuttling is functionally involved in oligodendroglial differentiation and remyelination activities. In this review we shed light on the role of the subcellular localization of differentiation-associated factors within oligodendroglial cells and show that regulation of intracellular localization of regulatory factors represents a crucial process to modulate oligodendroglial maturation and myelin repair in the CNS.

Oligodendroglial maturation is dependent on intracellular protein shuttling

Multiple sclerosis is an autoimmune disease of the CNS resulting in degeneration of myelin sheaths and loss of oligodendrocytes, which means that protection and electrical insulation of axons and rapid signal propagation are impaired, leading to axonal damage and permanent disabilities. Partial replacement of lost oligodendrocytes and remyelination can occur as a result of activation and recruitment of resident oligodendroglial precursor cells. However, the overall remyelination capacity remains inefficient because precursor cells often fail to generate new oligodendrocytes. Increasing evidence points to the existence of several molecular inhibitors that act on these cells and interfere with their cellular maturation. The p57kip2 gene encodes one such potent inhibitor of oligodendroglial differentiation and this study sheds light on the underlying mode of action. We found that subcellular distribution of the p57kip2 protein changed during differentiation of rat, mouse, and human oligodendroglial cells both in vivo and in vitro. Nuclear export of p57kip2 was correlated with promoted myelin expression, higher morphological phenotypes, and enhanced myelination in vitro. In contrast, nuclear accumulation of p57kip2 resulted in blocked oligodendroglial differentiation. Experimental evidence suggests that the inhibitory role of p57kip2 depends on specific interactions with binding proteins such as LIMK-1, CDK2, Mash1, and Hes5 either by controlling their site of action or their activity. Because functional restoration in demyelinating diseases critically depends on the successful generation of oligodendroglial cells, a therapeutic need that is currently unmet, the regulatory mechanism described here might be of particular interest for identifying suitable drug targets and devising novel therapeutic approaches.

miR-25 promotes glioma cell proliferation by targeting CDKN1C

MicroRNAs (miRNA) have oncogenic or tumor-suppressive roles in the development and growth of human glioma. Glioma development is also associated with alteration in the activities and expression of cell cycle regulators, and miRNAs are emerging as important regulators of cell cycle progression. Here, we show that miR-25 is overexpressed in 91% of examined human glioma tissues and 4 out of 6 human glioma cell lines. MiR-25 increases cell proliferation in two independent glioma cell lines. Ectopic expression of miR-25 was found to reduce CDKN1C protein levels by directly targeting its 3'-untranslated region (UTR). Notably, ablation of endogenous miR-25 rescued CDKN1C expression and significantly decreased glioma cell proliferation by facilitating normal cell cycle progression. Our clinical investigation found CDKN1C and miR-25 levels were inversely correlated. Lastly, downregulation of CDKN1 by siRNA blocked the activity of miR-25 on promoting glioma cell proliferation. Overall, our results for the first time show an oncogenic role of miR-25 in human glioma by targeting CDKN1C and that miR-25 could potentially be a therapeutic target for glioma intervention.

The actin depolymerizing factor (ADF)/cofilin signaling pathway and DNA damage responses in cancer

The actin depolymerizing factor (ADF)/cofilin protein family is essential for actin dynamics, cell division, chemotaxis and tumor metastasis. Cofilin-1 (CFL-1) is a primary non-muscle isoform of the ADF/cofilin protein family accelerating the actin filamental turnover in vitro and in vivo. In response to environmental stimulation, CFL-1 enters the nucleus to regulate the actin dynamics. Although the purpose of this cytoplasm-nucleus transition remains unclear, it is speculated that the interaction between CFL-1 and DNA may influence various biological responses, including DNA damage repair. In this review, we will discuss the possible involvement of CFL-1 in DNA damage responses (DDR) induced by ionizing radiation (IR), and the implications for cancer radiotherapy.

LIM domain kinases as potential therapeutic targets for neurofibromatosis type 2

Neurofibromatosis type 2 (NF2) is caused by mutations in the NF2 gene that encodes a tumor-suppressor protein called merlin. NF2 is characterized by formation of multiple schwannomas, meningiomas and ependymomas. Merlin loss-of-function is associated with increased activity of Rac and p21-activated kinases (PAKs) and deregulation of cytoskeletal organization. LIM domain kinases (LIMK1 and 2) are substrate for Cdc42/Rac-PAK and modulate actin dynamics by phosphorylating cofilin at serine-3. This modification inactivates the actin severing and depolymerizing activity of cofilin. LIMKs also translocate into the nucleus and regulate cell cycle progression. Significantly, LIMKs are overexpressed in several tumor types, including skin, breast, lung, liver and prostate. Here we report that mouse Schwann cells (MSCs) in which merlin function is lost as a result of Nf2 exon2 deletion (Nf2(ΔEx2)) exhibited increased levels of LIMK1, LIMK2 and active phospho-Thr508/505-LIMK1/2, as well as phospho-Ser3-cofilin, compared with wild-type normal MSCs. Similarly, levels of LIMK1 and 2 total protein and active phosphorylated forms were elevated in human vestibular schwannomas compared with normal human Schwann cells (SCs). Reintroduction of wild-type NF2 into Nf2(ΔEx2) MSC reduced LIMK1 and LIMK2 levels. We show that pharmacological inhibition of LIMK with BMS-5 decreased the viability of Nf2(ΔEx2) MSCs in a dose-dependent manner, but did not affect viability of control MSCs. Similarly, LIMK knockdown decreased viability of Nf2(ΔEx2) MSCs. The decreased viability of Nf2(ΔEx2) MSCs was not due to caspase-dependent or -independent apoptosis, but rather due to inhibition of cell cycle progression as evidenced by accumulation of cells in G2/M phase. Inhibition of LIMKs arrests cells in early mitosis by decreasing aurora A activation. Our results suggest that LIMKs are potential drug targets for NF2 and tumors associated with merlin deficiency.

LIM kinase 1 deficient mice have reduced bone mass

The cytoskeleton determines cell shape and is involved in cell motility. It also plays a role in differentiation and in modulating specialized cellular functions. LIM kinase 1 (LIMK1) participates in cytoskeletal remodeling by phosphorylating and inactivating the actin-severing protein, cofilin. Severing F-actin to release G-actin monomers is required for actin cytoskeletal remodeling. Although less well established, LIMK1 may also influence the cell cycle and modulate metalloproteinase activity. Since the role of LIMK1 in bone cell biology has not been reported, the skeletal phenotype of LIMK1(-/-) mice was examined. LIMK1(-/-) mice had significantly reduced trabecular bone mass when analyzed by microCT (p<0.01). Histomorphometric analyses demonstrated a 31% reduction in the number of osteoblasts (p=0.0003) and a 23% reduction in osteoid surface (p=0.0005). The number of osteoclasts was no different in control and knock out animals. Consistent with the in vivo findings in osteoblasts, the number of osteoblast colony forming units in LIMK1(-/-) bone marrow was reduced by nearly 50%. Further, osteoblasts isolated from LIMK1(-/-) mice showed significantly reduced rates of mineralization in vitro. Osteoclasts from LIMK1(-/-) mice evidenced more rapid cytoskeletal remodeling in response to treatment with CSF1. In keeping with this latter finding, basal levels of phospho-cofilin were reduced in LIMK1(-/-) osteoclasts. LIMK1(-/-) osteoclasts also resorbed dentine slices to a greater extent in vitro and were more active in a pit assay. These data support the hypothesis that LIMK1 is required for normal osteoblast differentiation. In addition, its absence leads to increased cytoskeletal remodeling and bone resorption in osteoclasts.

Phospholipase C-η2 is required for retinoic acid-stimulated neurite growth

Phospholipase C-η2 is a recently identified phospholipase C (PLC) implicated in the regulation of neuronal differentiation/maturation. PLCη2 activity is triggered by intracellular calcium mobilization and likely serves to amplify Ca²⁺ signals by stimulating further Ca²⁺ release from Ins(1,4,5)P₃-sensitive stores. The role of PLCη2 in neuritogenesis was assessed during retinoic acid (RA)-induced Neuro2A cell differentiation. PLCη2 expression increased two-fold during a 4-day differentiation period. Stable expression of PLCη2-targetted shRNA led to a decrease in the number of differentiated cells and total length of neurites following RA-treatment. Furthermore, RA response element activation was perturbed by PLCη2 knockdown. Using a bacterial two-hybrid screen, we identified LIM domain kinase 1 (LIMK1) as a putative interaction partner of PLCη2. Immunostaining of PLCη2 revealed significant co-localization with LIMK1 in the nucleus and growing neurites in Neuro2A cells. RA-induced phosphorylation of LIMK1 and cAMP-responsive element-binding protein was reduced in PLCη2 knock-down cells. The phosphoinositide-binding properties of the PLCη2 PH domain, assessed using a FRET-based assay, revealed this domain to possess a high affinity toward PtdIns(3,4,5)P₃. Immunostaining of PLCη2 together with PtdIns(3,4,5)P₃ in the Neuro2A cells revealed a high degree of co-localization, indicating that PtdIns(3,4,5)P₃ levels in cellular compartments are likely to be important for the spatial control of PLCη2 signaling.

Signaling mechanisms and functional roles of cofilin phosphorylation and dephosphorylation

Cofilin and actin-depolymerizing factor (ADF) are actin-binding proteins that play an essential role in regulating actin filament dynamics and reorganization by stimulating the severance and depolymerization of actin filaments. Cofilin/ADF are inactivated by phosphorylation at the serine residue at position 3 by LIM-kinases (LIMKs) and testicular protein kinases (TESKs) and are reactivated by dephosphorylation by the slingshot (SSH) family of protein phosphatases and chronophin. This review describes recent advances in our understanding of the signaling mechanisms regulating LIMKs and SSHs and the functional roles of cofilin phospho-regulation in cell migration, tumor invasion, mitosis, neuronal development, and synaptic plasticity. Accumulating evidence demonstrates that the phospho-regulation of cofilin/ADF is a key convergence point of cell signaling networks that link extracellular stimuli to actin cytoskeletal dynamics and that spatiotemporal control of cofilin/ADF activity by LIMKs and SSHs plays a crucial role in a diverse array of cellular and physiological processes. Perturbations in the normal control of cofilin/ADF activity underlie many pathological conditions, including cancer metastasis and neurological and cardiovascular disorders.

The multiple roles of the cyclin-dependent kinase inhibitory protein p57(KIP2) in cerebral cortical neurogenesis

The members of the CIP/KIP family of cyclin-dependent kinase (CDK) inhibitory proteins (CKIs), including p57(KIP2), p27(KIP1), and p21(CIP1), block the progression of the cell cycle by binding and inhibiting cyclin/CDK complexes of the G1 phase. In addition to this well-characterized function, p57(KIP2) and p27(KIP1) have been shown to participate in an increasing number of other important cellular processes including cell fate and differentiation, cell motility and migration, and cell death/survival, both in peripheral and central nervous systems. Increasing evidence over the past few years has characterized the functions of the newest CIP/KIP member p57(KIP2) in orchestrating cell proliferation, differentiation, and migration during neurogenesis. Here, we focus our discussion on the multiple roles played by p57(KIP2) during cortical development, making comparisons to p27(KIP1) as well as the INK4 family of CKIs.

Cyclin A2, Rho GTPases and EMT

Cell cycle regulators, such as cyclins, are often upregulated in many proliferative disorders, and Cyclin A2 is generally considered as a marker of aggressive cancers. Our recent work, which revealed decreased expression of Cyclin A2 upon metastasis of colorectal cancer, suggests a more complicated situation. Consistent with this, we identified a role for Cyclin A2, via RhoA, in regulation of the actin cytoskeleton and the control of cell invasion. Cyclin A2 also regulates spindle orientation which, when misoriented, could disrupt cell polarity and favor cancer cell detachment from the tumor as part of a transforming process, such as epithelial to mesenchymal transition (EMT). During EMT, cells undergo morphological and molecular changes toward a mesenchymal phenotype. Upregulation, or increased activity of some Rho GTPases, such as Cdc42, Rac1 or RhoC, increases the invasive potential of these cells. This correlates with the inverse relationship between RhoA and RhoC activities we observed in an epithelial cell type. Altogether, these observations raise the possibility that Cyclin A2 is instrumental in preventing EMT and therefore cancers of epithelial tissues.

Rho GTPase regulation by miRNAs and covalent modifications

To date, most studies of Rho GTPase regulation have focused on the classic GTPase cycle - GTP binding and hydrolysis - controlled by guanine nucleotide exchange factors (GEFs), GTPase-activating proteins (GAPs) and GDP-dissociation inhibitors (GDIs). Recent investigations have unveiled important additional regulatory mechanisms: microRNA (miRNA) regulating post-transcriptional processing of Rho GTPase-encoding mRNAs; palmitoylation and nuclear targeting affecting intracellular distribution; post-translational phosphorylation, transglutamination and AMPylation impacting Rho GTPase signaling; and ubiquitination controlling Rho GTPase protein stability and turnover. These modes of regulation add to the complexity of the Rho GTPase signaling network and allow precise spatiotemporal control of individual Rho GTPases. This review discusses these 'unconventional' modes of regulation and their contribution to cellular function, focusing on post-transcriptional and post-translational events beyond the classic GTPase cycle regulatory model.

The placental imprintome and imprinted gene function in the trophoblast glycogen cell lineage

Imprinted genes represent a unique class of autosomal genes expressed from only one of the parental alleles during development. The choice of the expressed allele is not random but rather is determined by the parental origin of the allele. Consequently, the mouse genome contains more than 100 genes expressed preferentially or exclusively from the maternally or the paternally inherited allele. Current research efforts are focused on understanding the molecular mechanism of this epigenetic phenomenon as well as the biological functions of the genes under its regulation. Both theoretical considerations and experimental results support a role for genomic imprinting in the regulation of embryonic growth and placental biology. In this review, recent efforts to establish the complete set of genes showing imprinted expression in the mouse placenta are first discussed. Then, the evidence suggesting that imprinted genes might be implicated in the emergence, maintenance and function of trophoblast glycogen cells is presented. Although the origin and functions of this trophoblast cell lineage are currently unknown, the analysis of mutations in imprinted genes in the mouse are providing new insights into these issues. The implications of this work for placental pathologies in human are also discussed.

The Wnt pathway destabilizes adherens junctions and promotes cell migration via β-catenin and its target gene cyclin D1

The Wnt pathway regulates cell proliferation, mobility and differentiation. Among the many Wnt target genes is CCND1 which codes for cyclin D1. Cyclin D1, in complex with cdk4 and cdk6, regulates G1/S phase transition during cell cycle. Independently of CDK, cyclin D1 also regulates the migration of macrophages. Here we analyzed the effects of cyclin D1 on the migration of cancer cell lines using the transwell migration and scratch assays. We also tested the effect of cyclin D1 and β-catenin on E-cadherin-mediated cell–cell contacts. Our results show that the Wnt pathway promotes cellular migration via its target gene cyclin D1. Moreover we show that cyclin D1 influences the actin cytoskeleton and destabilizes adherens junctions.

Downregulation of p57kip² promotes cell invasion via LIMK/cofilin pathway in human nasopharyngeal carcinoma cells

The members of Rho family are well known for their regulation of actin cytoskeleton to control cell migration. The Cip/kip members of cyclin-dependent (CDK) inhibitors have shown to implicate in cell migration and cytoskeletal dynamics. p57(kip2) , a CDK inhibitor, is frequently down-regulated in several malignancy tumors. However, its biological roles in human nasopharyngeal carcinoma (NPC) cells remained to be investigated. Here, we found p57(kip2) has nuclear and cytoplasm distributions and depletion of endogenous p57(kip2) did not change the cell-cycle progression. Inhibition of cell proliferation by mitomycin C promoted FBS-mediated cell migration and accompanied with the downregulation of ΔNp63α and p57(kip2), but did not change the level of p27(kip1) , another CDK inhibitor. By using siRNA transfection and cell migration/invasion assays, we found that knockdown of p57(kip2) , but not ΔNp63α, involved in promotion of NPC cell migration and invasion via decrease of phospho-cofilin (p-cofilin). Treatment with Y-27632, a specific ROCK inhibitor, we found that dysregulation of ROCK/cofilin pathway decreased p-cofilin expression and induced cell migration. This change of p-cofilin induced actin remodeling and pronounced increase of membrane protrusions. Further, silence of p57(kip2) not only decreased the interaction between p57(kip2) and LIMK-1 assayed by immunoprecipitation but also reduced the level of phospho-LIMK1/2. Therefore, this study indicated that dysregulation of p57(kip2) promoted cell migration and invasion through modulation of LIMK/cofilin signaling and suggested this induction of inappropriate cell motility might contribute to promoting tumor cell for metastasis.

Multiple degradation pathways regulate versatile CIP/KIP CDK inhibitors

The mammalian CIP/KIP family of cyclin-dependent kinase (CDK) inhibitors (CKIs) comprises three proteins--p21(Cip1/WAF1), p27(Kip1), and p57(Kip2)--that bind and inhibit cyclin-CDK complexes, which are key regulators of the cell cycle. CIP/KIP CKIs have additional independent functions in regulating transcription, apoptosis and actin cytoskeletal dynamics. These divergent functions are performed in distinct cellular compartments and contribute to the seemingly contradictory observation that the CKIs can both suppress and promote cancer. Multiple ubiquitin ligases (E3s) direct the proteasome-mediated degradation of p21, p27 and p57. This review analyzes recent data highlighting our current understanding of how distinct E3 pathways regulate subpopulations of the CKIs to control their diverse functions.

Downregulation of p57 accelerates the growth and invasion of hepatocellular carcinoma

p57 is a multifunctional protein involved in the regulation of tumor formation and development; however, the biological role of p57 in the pathogenesis of hepatocellular carcinoma (HCC) is poorly understood. To explore the role of p57 in the development of HCC, we examined p57 messenger RNA (mRNA) and protein levels in HCC tissues and adjacent non-cancerous tissues by immunohistochemistry, real-time polymerase chain reaction and western blot analysis. Moreover, we generated stable p57 knockdown HCC cell lines to investigate the impact of p57 downregulation on the growth and invasion of HCC in vitro and in vivo. Our results showed that p57 mRNA and protein levels were significantly decreased in human HCC tissues. In addition, this reduction in p57 expression was associated with increased tumor size, more advanced TNM stages, the presence of capsule invasion and extrahepatic metastasis and decreased overall survival time. In human HCC cell lines, p57 downregulation increased the expression of cyclin D1 and CDK2 and enhanced the activities of CDK4/cyclin D1 and CDK2/cyclin E complexes, resulting in increased cellular proliferation and growth of xenografts. Furthermore, p57 downregulation accelerated the invasion of HCC cells in vitro and in vivo by controlling the activity of LIMK1. In conclusion, the downregulation of p57 accelerates the growth and invasion of HCC, indicating that p57 is an important tumor suppressor in HCC. Based on these findings, p57 may be a potential target for HCC prevention and therapy.

p57(Kip2) and cancer: time for a critical appraisal

p57(Kip2) is a cyclin-dependent kinase inhibitor belonging to the Cip/Kip family, which also includes p21(Cip1) and p27(Kip1). So far, p57(Kip2) is the least-studied Cip/Kip protein, and for a long time its relevance has been related mainly to its unique role in embryogenesis. Moreover, genetic and molecular studies on animal models and patients with Beckwith-Wiedemann syndrome have shown that alterations in CDKN1C (the p57(Kip2) encoding gene) have functional relevance in the pathogenesis of this disease. Recently, a number of investigations have identified and characterized heretofore unexpected roles for p57(Kip2). The protein appears to be critically involved in initial steps of cell and tissue differentiation, and particularly in neuronal development and erythropoiesis. Intriguingly, p27(Kip1), the Cip/Kip member that is most homologous to p57(Kip2), is primarily involved in the process of cell cycle exit. p57(Kip2) also plays a critical role in controlling cytoskeletal organization and cell migration through its interaction with LIMK-1. Furthermore, p57(Kip2) appears to modulate genome expression. Finally, accumulating evidence indicates that p57(Kip2) protein is frequently downregulated in different types of human epithelial and nonepithelial cancers as a consequence of genetic and epigenetic events. In summary, the emerging picture is that several aspects of p57(Kip2)'s functions are only poorly clarified. This review represents an appraisal of the data available on the p57(Kip2) gene and protein structure, and its role in human physiology and pathology. We particularly focus our attention on p57(Kip2) changes in cancers and pharmacological approaches for modulating p57(Kip2) levels.

Fetal overgrowth in the Cdkn1c mouse model of Beckwith-Wiedemann syndrome

Mutations in the imprinted CDKN1C gene are associated with the childhood developmental disorder Beckwith-Wiedemann syndrome (BWS). Multiple mouse models with deficiency of Cdkn1c recapitulate some aspects of BWS but do not exhibit overgrowth of the newborn, a cardinal feature of patients with BWS. In this study, we found that Cdkn1c mutants attained a 20% increase in weight during gestation but experienced a rapid reversal of this positive growth trajectory very late in gestation. We observed a marked effect on placental development concurrently with this loss of growth potential, with the appearance of large thrombotic lesions in the labyrinth zone. The trilaminar trophoblast layer that separates the maternal blood sinusoids from fetal capillaries was disordered with a loss of sinusoidal giant cells, suggesting a role for Cdkn1c in maintaining the integrity of the maternal-fetal interface. Furthermore, the overgrowth of mutant pups decreased in the face of increasing intrauterine competition, identifying a role for Cdkn1c in the allocation of the maternal resources via the placenta. This work explains one difficulty in precisely replicating BWS in this animal model: the differences in reproductive strategies between the multiparous mouse, in which intrauterine competition is high, and humans, in which singleton pregnancies are more common.

The hallmarks of CDKN1C (p57, KIP2) in cancer

Cyclin-dependent kinase inhibitor 1C CDKN1C (p57(KIP2)) regulates several hallmarks of cancer, including apoptosis, cell invasion and metastasis, tumor differentiation and angiogenesis. p57(KIP2) is generally not mutated in cancer, but its expression is downregulated through epigenetic changes such as DNA methylation and repressive histone marks at the promoter. This opens up possibilities for therapeutic intervention through reactivation of p57(KIP2) gene expression. Furthermore, p57(KIP2) has been tested as a prognostic factor for many types of cancer, even differentiating between early and late stage cancer. In this review, the multifunctional tumor suppressor capabilities of p57(KIP2), the mechanisms of p57(KIP2) transcriptional repression in cancer, and the therapeutic potential of reactivation of p57(KIP2) protein expression will be discussed.

The small GTPase RhoA localizes to the nucleus and is activated by Net1 and DNA damage signals

Background

Rho GTPases control many cellular processes, including cell survival, gene expression and migration. Rho proteins reside mainly in the cytosol and are targeted to the plasma membrane (PM) upon specific activation by guanine nucleotide exchange factors (GEFs). Accordingly, most GEFs are also cytosolic or associated with the PM. However, Net1, a RhoA-specific GEF predominantly localizes to the cell nucleus at steady-state. Nuclear localization for Net1 has been seen as a mechanism for sequestering the GEF away from RhoA, effectively rendering the protein inactive. However, considering the prominence of nuclear Net1 and the fact that a biological stimulus that promotes Net1 translocation out the nucleus to the cytosol has yet to be discovered, we hypothesized that Net1 might have a previously unidentified function in the nucleus of cells.

Principal Findings

Using an affinity precipitation method to pulldown the active form of Rho GEFs from different cellular fractions, we show here that nuclear Net1 does in fact exist in an active form, contrary to previous expectations. We further demonstrate that a fraction of RhoA resides in the nucleus, and can also be found in a GTP-bound active form and that Net1 plays a role in the activation of nuclear RhoA. In addition, we show that ionizing radiation (IR) specifically promotes the activation of the nuclear pool of RhoA in a Net1-dependent manner, while the cytoplasmic activity remains unchanged. Surprisingly, irradiating isolated nuclei alone also increases nuclear RhoA activity via Net1, suggesting that all the signals required for IR-induced nuclear RhoA signaling are contained within the nucleus.

Conclusions/Significance

These results demonstrate the existence of a functional Net1/RhoA signaling pathway within the nucleus of the cell and implicate them in the DNA damage response.

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).

LIM kinases are attractive targets with many macromolecular partners and only a few small molecule regulators

The LIM kinases 1 and 2 (LIMK1 and LIMK2) are dual specificity (serine/threonine and tyrosine) kinases. Although they show significant structural similarity, LIMK1 and LIMK2 show different expression, subcellular localization, and functions. They are involved in many cellular functions, such as migration, cycle, and neuronal differentiation and also have a role in pathological processes, such as cancer cell invasion and metastatis, as well as in neurodevelopmental disorders (namely, the William's syndrome). LIM kinases have a relevant number of known partners that are able to induce or limit the ability of LIMK1 and LIMK2 to phosphorylate and inactivate their major substrate, cofilin. On the contrary, only a limited number of small molecules that interact with the two proteins to modulate their kinase activity have been identified. In this review, the most important partners of LIM kinases and their modulating activity toward LIMKs are described. The small compounds identified as LIMK1 and LIMK2 modulators are also reported, as well as their role as possible therapeutic agents for LIMK-induced diseases.

Transcriptome analysis of embryonic and adult sensory axons reveals changes in mRNA repertoire localization

mRNAs are transported, localized, and translated in axons of sensory neurons. However, little is known about the full repertoire of transcripts present in embryonic and adult sensory axons and how this pool of mRNAs dynamically changes during development. Here, we used a compartmentalized chamber to isolate mRNA from pure embryonic and adult sensory axons devoid of non-neuronal or cell body contamination. Genome-wide microarray analysis reveals that a previously unappreciated number of transcripts are localized in sensory axons and that this repertoire changes during development toward adulthood. Embryonic axons are enriched in transcripts encoding cytoskeletal-related proteins with a role in axonal outgrowth. Surprisingly, adult axons are enriched in mRNAs encoding immune molecules with a role in nociception. Additionally, we show Tubulin-beta3 (Tubb3) mRNA is present only in embryonic axons, with Tubb3 locally synthesized in axons of embryonic, but not adult neurons where it is transported, thus validating our experimental approach. In summary, we provide the first complete catalog of embryonic and adult sensory axonal mRNAs. In addition we show that this pool of axonal mRNAs dynamically changes during development. These data provide an important resource for studies on the role of local protein synthesis in axon regeneration and nociception during neuronal development.

CRL2(LRR-1) targets a CDK inhibitor for cell cycle control in C. elegans and actin-based motility regulation in human cells

The Cip/Kip CDK inhibitor (CKI) p21(Cip1/WAF1) has a critical role in the nucleus to limit cell proliferation by inhibiting CDK-cyclin complexes. In contrast, cytoplasmic p21 regulates cell survival and the actin cytoskeleton. These divergent functions for p21 in different cellular compartments suggest the necessity for complex regulation. In this study, we identify the CRL2(LRR-1) ubiquitin ligase as a conserved regulator of Cip/Kip CKIs that promotes the degradation of C. elegans CKI-1 and human p21. The nematode CRL2(LRR-1) complex negatively regulates nuclear CKI-1 levels to ensure G1-phase cell cycle progression in germ cells. In contrast, human CRL2(LRR1) targets cytoplasmic p21, acting as a critical regulator of cell motility that promotes a nonmotile stationary cell state by preventing p21 from inhibiting the Rho/ROCK/LIMK pathway. Inactivation of human CRL2(LRR1) leads to the activation of the actin-depolymerizing protein cofilin, dramatic reorganization of the actin cytoskeleton, and increased cell motility.

The role of cell cycle in retinal development: cyclin-dependent kinase inhibitors co-ordinate cell-cycle inhibition, cell-fate determination and differentiation in the developing retina

The mature retina is formed through multi-step developmental processes, including eye field specification, optic vesicle evagination, and cell-fate determination. Co-ordination of these developmental events with cell-proliferative activity is essential to achieve formation of proper retinal structure and function. In particular, the molecular and cellular dynamics of the final cell cycle significantly influence the identity that a cell acquires, since cell fate is largely determined at the final cell cycle for the production of postmitotic cells. This review summarizes our current understanding of the cellular mechanisms that underlie the co-ordination of cell-cycle and cell-fate determination, and also describes a molecular role of cyclin-dependent kinase inhibitors (CDKIs) as co-ordinators of cell-cycle arrest, cell-fate determination and differentiation.

p27(Kip1), a double-edged sword in Shh-mediated medulloblastoma: Tumor accelerator and suppressor

Medulloblastoma, a brain tumor arising in the cerebellum, is the most common solid childhood malignancy. the current standard of care for medulloblastoma leaves survivors with life-long side effects. Gaining insight into mechanisms regulating transformation of medulloblastoma cells-of-origin may lead to development of better treatments for these tumors. Cerebellar granule neuron precursors (CGNps) are proposed cells-of-origin for certain classes of medulloblastoma, specifically those marked by aberrant Sonic hedgehog (Shh) signaling pathway activation. CGNps require signaling by Shh for proliferation during brain development. In mitogen-stimulated cells, nuclear localized cyclin dependent kinase (cdk) inhibitor p27 (Kip1) functions as a checkpoint control at the G1- to S-phase transition by inhibiting cdk2. Recent studies have suggested cytoplasmically localized p27(Kip1) acquires oncogenic functions. Here, we show that p27(Kip1) is cytoplasmically localized in CGNps and mouse Shh-mediated medulloblastomas. transgenic mice bearing an activating mutation in the Shh pathway and lacking one or both p27(Kip1) alleles have accelerated tumor incidence compared to mice bearing both p27(Kip1) alleles. Interestingly, mice heterozygous for p27(Kip1) have decreased survival latency compared to p27(Kip1)-null animals. our data indicate that this may reflect the requirement for at least one copy of p27(Kip1) for recruiting cyclin D/cdk4/6 to promote cell cycle progression yet insufficient expression in the heterozygous or null state to inhibit cyclin E/cdk2. Finally, we find that mis-localized p27(Kip1) may play a positive role in motility in medulloblastoma cells. Together, our data indicate that the dosage of p27(Kip1) plays a role in cell cycle progression and tumor suppression in Shh-mediated medulloblastoma expansion.

Prognostic significance of co-expression of nm23 and p57 protein in hepatocellular carcinoma

AIM

  To investigate the unbalance of proliferation and apoptosis and the functions of cell-cycle proteins and apoptotic factor in metastasis of hepatocellular carcinoma (HCC) and their effect in prognosis.

METHODS

  Proliferation index and apoptosis index, as well as seven relatively molecular markers, namely p15, p34, p53, p57, p73, survivin and nm23, were evaluated by immunohistochemistry and TUNEL in HCC tissues and compared to adjacent non-cancerous tissues and normal liver tissues. Furthermore, the prognostic significance by follow-up and mutual relationships for each clinicopathologic factor and molecular marker were analysed.

RESULTS

  The dysregulation between proliferation and apoptosis and the abnormal expression of seven molecular markers were observed in HCC tissues. The unbalance of proliferation and apoptosis and abnormal expressions of p15, p34, p57 and nm23 were correlated with TNM stage and extrahepatic metastasis. In particular, the abnormal co-expression of nm23/p57 correlated with advanced TNM stage and bigger tumor size and was an independent prognostic factor of HCC.

CONCLUSION

  The unbalance of proliferation and apoptosis and abnormal expression of cell-cycle proteins promote metastasis of HCC. Moreover, the abnormal co-expression of nm23/p57 may be a useful molecular marker for metastasis and unfavourable prognosis for HCC.

p57KIP2: "Kip"ing the cell under control

p57(KIP2) is an imprinted gene located at the chromosomal locus 11p15.5. It is a cyclin-dependent kinase inhibitor belonging to the CIP/KIP family, which includes additionally p21(CIP1/WAF1) and p27(KIP1). It is the least studied CIP/KIP member and has a unique role in embryogenesis. p57(KIP2) regulates the cell cycle, although novel functions have been attributed to this protein including cytoskeletal organization. Molecular analysis of animal models and patients with Beckwith-Wiedemann Syndrome have shown its nodal implication in the pathogenesis of this syndrome. p57(KIP2) is frequently down-regulated in many common human malignancies through several mechanisms, denoting its anti-oncogenic function. This review is a thorough analysis of data available on p57(KIP2), in relation to p21(CIP1/WAF1) and p27(KIP1), on gene and protein structure, its transcriptional and translational regulation, and its role in human physiology and pathology, focusing on cancer development.

Identification of novel regulators of hematopoietic stem cell development through refinement of stem cell localization and expression profiling

The first adult-repopulating hematopoietic stem cells (HSCs) are detected starting at day 10.5 of gestation in the aorta-gonads-mesonephros (AGM) region of the mouse embryo. Despite the importance of the AGM in initiating HSC production, very little is currently known about the regulators that control HSC emergence in this region. We have therefore further defined the location of HSCs in the AGM and incorporated this information into a spatial and temporal comparative gene expression analysis of the AGM. The comparisons included gene expression profiling (1) in the newly identified HSC-containing region compared with the region devoid of HSCs, (2) before and after HSC emergence in the AGM microenvironment, and (3) on populations enriched for HSCs and their putative precursors. Two genes found to be up-regulated at the time and place where HSCs are first detected, the cyclin-dependent kinase inhibitor p57Kip2/Cdkn1c and the insulin-like growth factor 2, were chosen for further analysis. We demonstrate here that they play a novel role in AGM hematopoiesis. Interestingly, many genes involved in the development of the tissues surrounding the dorsal aorta are also up-regulated during HSC emergence, suggesting that the regulation of HSC generation occurs in coordination with the development of other organs.

The Cdk inhibitor p57(Kip2) controls LIM-kinase 1 activity and regulates actin cytoskeleton dynamics

The cyclin-dependent kinase inhibitor p57(Kip2) gene has been suggested to be a tumor suppressor gene, being inactivated in various cancer types, linked to tumor progression and poor patient outcome. Here, we report that p57(Kip2) interacts with the actin cytoskeleton modifying enzyme, LIM-kinase 1 (LIMK-1) but not LIMK-2. This interaction enhances activity of LIMK-1, independently of its activator Rho-associated kinase. This resulted in an increased phosphorylation and consequent inactivation of the actin depolymerization factor, cofilin. In accordance, selective p57(Kip2) expression promotes actin stress fiber formation in cancer cells. Fluorescence recovery after photobleaching analysis of fluorescent-labeled actin further demonstrated that p57(Kip2) expression results in reduction of actin protein mobile fraction, which affects its turnover rate in cell. Finally, we present evidence that the p57(Kip2) control of LIMK-1 ultimately affects cell mobility negatively. Thus, in addition to its established function in control of proliferation and cell death, these results indicate that p57(Kip2) is critical in the regulation of actin cytoskeleton dynamic and by this means migration ability of cancer cells.

p57Kip2 is a repressor of Mash1 activity and neuronal differentiation in neural stem cells

Mammalian central nervous system (CNS) development is a highly organized process involving the precise and coordinated timing of cell-cycle exit, differentiation, survival, and migration. These events require proper expression of pro-neuronal genes but also repression of alternative cell fates and restriction of cell-type-specific gene expression. Here, we show that the cyclin-dependent kinase (CDK) inhibitor p57Kip2 interacted with pro-neuronal basic helix-loop-helix (bHLH) factors such as Mash1, NeuroD, and Nex/Math2. Increased levels of p57Kip2 inhibited Mash1 transcriptional activity independently of CDK interactions and acted as a direct repressor in transcriptional assays. Proliferating telencephalic neural progenitors co-expressed basal levels of Mash1 and p57Kip2, and endogenous p57Kip2 accumulated transiently in the nuclei of neural stem cells (NSCs) during early stages of astrocyte differentiation mediated by ciliary neurotrophic factor (CNTF), independent of cell-cycle exit and at times when Mash1 expression was still prominent. In accordance with these observations, gain- and loss-of-function studies showed that p57Kip2 repressed neuronal differentiation after mitogen withdrawal, but exerted little or no effect on CNTF-mediated astroglial differentiation of NSCs. Our data suggest a novel role for p57Kip2 as a context-dependent repressor of neurogenic transcription factors and telencephalic neuronal differentiation.