Mitosis-dependent phosphorylation and activation of LIM-kinase 1

A Haspin-ARHGAP11A axis regulates epithelial morphogenesis through Rho-ROCK dependent modulation of LIMK1-Cofilin

Throughout mitosis, a plethora of processes must be efficiently concerted to ensure cell proliferation and tissue functionality. The mitotic spindle does not only mediate chromosome segregation, but also defines the axis of cellular division, thus determining tissue morphology. Functional spindle orientation relies on precise actin dynamics, shaped in mitosis by the LIMK1-Cofilin axis. The kinase Haspin acts as a guardian of faithful chromosome segregation that ensures amphitelic chromosome attachment and prevents unscheduled cohesin cleavage. Here, we report an unprecedented role for Haspin in the determination of spindle orientation in mitosis. We show that, during mitosis, Haspin regulates Rho-ROCK activity through ARHGAP11A, a poorly characterized GAP, and that ROCK is in turn responsible for the mitotic activation of LIMK1 and stabilization of the actin cytoskeleton, thus supporting a functional spindle orientation. By exploiting 3D cell cultures, we show that this pathway is pivotal for the establishment of a morphologically functional tissue.

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.

LIM Kinases in Osteosarcoma Development

Tumorigenesis is a long-term and multistage process that often leads to the formation of metastases. During this pathological course, two major events appear to be crucial: primary tumour growth and metastatic expansion. In this context, despite research and clinical advances during the past decades, bone cancers remain a leading cause of death worldwide among paediatric cancer patients. Osteosarcomas are the most common malignant bone tumours in children and adolescents. Notwithstanding advances in therapeutic treatments, many patients succumb to these diseases. In particular, less than 30% of patients who demonstrate metastases at diagnosis or are poor responders to chemotherapy survive 5 years after initial diagnosis. LIM kinases (LIMKs), comprising LIMK1 and LIMK2, are common downstream effectors of several signalization pathways, and function as a signalling node that controls cytoskeleton dynamics through the phosphorylation of the cofilin family proteins. In recent decades, several reports have indicated that the functions of LIMKs are mainly implicated in the regulation of actin microfilament and the control of microtubule dynamics. Previous studies have thus identified LIMKs as cancer-promoting regulators in multiple organ cancers, such as breast cancer or prostate cancer. This review updates the current understanding of LIMK involvement in osteosarcoma progression.

Cofilin-1, LIMK1 and SSH1 are differentially expressed in locally advanced colorectal cancer and according to consensus molecular subtypes

Background

Colorectal cancer (CRC) is among the deadliest cancers, wherein early dissemination of tumor cells, and consequently, metastasis formation, are the main causes of mortality and poor prognosis. Cofilin-1 (CFL-1) and its modulators, LIMK1/SSH1, play key roles in mediating the invasiveness by driving actin cytoskeleton reorganization in various cancer types. However, their clinical significance and prognostic value in CRC has not been fully explored. Here, we evaluated the clinical contribution of these actin regulators according to TNM and consensus molecular subtypes (CMSs) classification.

Methods

CFL-1, LIMK1 and SSH1 mRNA/protein levels were assessed by real-time PCR and immunohistochemical analyses using normal adjacent and tumor tissues obtained from a clinical cohort of CRC patients. The expression levels of these proteins were associated with clinicopathological features by using the chi square test. In addition, using RNA-Seq data of CRC patients from The Cancer Genome Atlas (TCGA) database, we determine how these actin regulators are expressed and distributed according to TNM and CMSs classification. Based on gene expression profiling, Kaplan–Meier survival analysis was used to evaluated overall survival.

Results

Bioinformatic analysis revealed that LIMK1 expression was upregulated in all tumor stages. Patients with high levels of LIMK1 demonstrated significantly lower overall survival rates and exhibited greater lymph node metastatic potential in a clinical cohort. In contrast, CFL-1 and SSH1 have expression downregulated in all tumor stages. However, immunohistochemical analyses showed that patients with high protein levels of CFL-1 and SSH1 exhibited greater lymph node metastatic potential and greater depth of local invasion. In addition, using the CMSs classification to evaluate different biological phenotypes of CRC, we observed that LIMK1 and SSH1 genes are upregulated in immune (CMS1) and mesenchymal (CMS4) subtypes. However, patients with high levels of LIMK1 also demonstrated significantly lower overall survival rates in canonical (CMS2), and metabolic (CMS3) subtypes.

Conclusions

We demonstrated that CFL-1 and its modulators, LIMK1/SSH1, are differentially expressed and associated with lymph node metastasis in CRC. Finally, this expression profile may be useful to predict patients with aggressive signatures, particularly, the immune and mesenchymal subtypes of CRC.

LIMK1 and LIMK2 regulate cortical development through affecting neural progenitor cell proliferation and migration

LIMK1 and LIMK2 are key downstream targets to mediate the effects of the Rho family small GTPases and p21-activated kinases (PAK) in the regulation of the actin cytoskeleton. LIMKs are also critical for synaptic transmission, plasticity and memory formation. Changes in LIMK signaling are associated with several neurodevelopmental and neurodegenerative diseases, including autism, intellectual disability and Alzheimer's disease. However, the role of LIMK signaling in brain development remains unknown. In this study, we used LIMK1 KO and LIMK2 KO mice to investigate the role of LIMK signaling in the cerebral cortical development. We found that these KO mice are reduced in the number of pyramidal neurons in upper cortical layers and this reduction is accompanied by a smaller pool of neural progenitor cells and impaired neuronal migration. These results are similar to those found in PAK1 KO mice and suggest that LIMK-dependent actin regulation may play a key role in mediating the effects of PAK1 and Rho signaling in the regulation of cortical development.

LIM kinase1 regulates mitotic centrosome integrity via its activity on dynein light intermediate chains

Abnormal centrosome number and function have been implicated in tumour development. LIM kinase1 (LIMK1), a regulator of actin cytoskeleton dynamics, is found to localize at the mitotic centrosome. However, its role at the centrosome is not fully explored. Here, we report that LIMK1 depletion resulted in multi-polar spindles and defocusing of centrosomes, implicating its involvement in the regulation of mitotic centrosome integrity. LIMK1 could influence centrosome integrity by modulating centrosomal protein localization at the spindle pole. Interestingly, dynein light intermediate chains (LICs) are able to rescue the defects observed in LIMK1-depleted cells. We found that LICs are potential novel interacting partners and substrates of LIMK1 and that LIMK1 phosphorylation regulates cytoplasmic dynein function in centrosomal protein transport, which in turn impacts mitotic spindle pole integrity.

MiR-138-5p targeting LIMK1 suppresses breast cancer cell proliferation and motility

Breast cancer is the most frequently diagnosed female cancer.

Overexpression of LIMK1 promotes tumor growth and metastasis in gastric cancer

Gastric cancer is the second-leading cause of cancer death in Asia. Despite improvement of therapies, the outcome in patients remains extremely poor because of metastasis. In the present study, we found that LIMK1 is overexpressed in gastric cancer, and its expression level correlate with tumor size, lymph node metastasis and TNM stage. Knockdown of LIMK1 expression could inhibit cell proliferation, migration and invasion in vitro, as well as suppress the activation of FAK/paxillin pathway. Moreover, knockdown of LIMK1 expression retarded tumor growth and peritoneal ametastasis in vivo. This highlights that LIMK1 might be used as a potential target in the treatment of gastric cancer.

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.

A functional cooperativity between Aurora A kinase and LIM kinase1: implication in the mitotic process

Aurora kinase A (Aur-A), a mitotic kinase, regulates initiation of mitosis through centrosome separation and proper assembly of bipolar spindles. LIM kinase 1 (LIMK1), a modulator of actin and microtubule dynamics, is involved in the mitotic process through inactivating phosphorylation of cofilin. Phosphorylated LIMK1 is recruited to the centrosomes during early prophase, where it colocalizes with γ-tubulin. Here, we report a novel functional cooperativity between Aur-A and LIMK1 through mutual phosphorylation. LIMK1 is recruited to the centrosomes during early prophase and then to the spindle poles, where it colocalizes with Aur-A. Aur-A physically associates with LIMK1 and activates it through phosphorylation, which is important for its centrosomal and spindle pole localization. Aur-A also acts as a substrate of LIMK1, and the function of LIMK1 is important for its specific localization and regulation of spindle morphology. Taken together, the novel molecular interaction between these two kinases and their regulatory roles on one another's function may provide new insight on the role of Aur-A in manipulation of actin and microtubular structures during spindle formation.

The diverse biofunctions of LIM domain proteins: determined by subcellular localization and protein-protein interaction

The LIM domain is a cysteine- and histidine-rich motif that has been proposed to direct protein-protein interactions. A diverse group of proteins containing LIM domains have been identified, which display various functions including gene regulation and cell fate determination, tumour formation and cytoskeleton organization. LIM domain proteins are distributed in both the nucleus and the cytoplasm, and they exert their functions through interactions with various protein partners.

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.

Nucleocytoplasmic functions of the PDZ-LIM protein family: new insights into organ development

Recent work on the PDZ-LIM protein family has revealed that it has important activities at the cellular level, mediating signals between the nucleus and the cytoskeleton, with significant impact on organ development. We review and integrate current knowledge about the PDZ-LIM protein family and propose a new functional role, sequestering nuclear factors in the cytoplasm. Characterized by their PDZ and LIM domains, the PDZ-LIM family is comprised of evolutionarily conserved proteins found throughout the animal kingdom, from worms to humans. Combining two functional domains in one protein, PDZ-LIM proteins have wide-ranging and multi-compartmental cell functions during development and homeostasis. In contrast, misregulation can lead to cancer formation and progression. New emerging roles include interactions with integrins, T-box transcription factors, and receptor tyrosine kinases. Facilitating the assembly of protein complexes, PDZ-LIM proteins can act as signal modulators, influence actin dynamics, regulate cell architecture, and control gene transcription.

Evaluation of mammalian cell-free systems of nuclear disassembly and assembly

Mammalian cell-free systems are very useful for the biochemical and structural study of nuclear disassembly and assembly. Through experimental manipulations, the role of specific proteins in these processes can be studied. Recently, we intended to examine the involvement of integral and peripheral inner nuclear membrane proteins in nuclear disassembly and assembly. However, we could not achieve proper disassembly when isolated interphase HeLa nuclei were exposed to mitotic soluble extracts obtained from the same cell line and containing cyclin B1. Homogenates of synchronized mitotic HeLa cells left to reassemble their nuclei generated incomplete nuclear envelopes on chromatin masses. Digitonin-permeabilized mitotic cells also assembled incomplete nuclei, generating a lot of cytoplasmic inclusions of inner nuclear membrane proteins as an intermediate. These results were therefore used as a basis for a critical evaluation of mammalian cell-free systems. We present here evidence that cell synchronization itself can interfere with the progress of nuclear assembly, possibly by causing aberrant nuclear disassembly and/or by inducing the formation of an abnormal number of mitotic spindles.

Parkin interacts with LIM Kinase 1 and reduces its cofilin-phosphorylation activity via ubiquitination

Mutations in the PARKIN (PARK2) gene have been found in the majority of early-onset familial Parkinson's disease (PD) patients with autosomal recessive juvenile parkinsonism (ARJP). Parkin protein functions as an ubiquitin (E3) ligase that targets specific proteins for degradation in the 26S proteasome. Here, based on a mass spectrometry analysis of the human dopaminergic neuroblastoma-derived cell line SH-SY5Y that over-expresses parkin, we found that parkin may suppress cofilin phosphorylation. LIM Kinase 1 (LIMK1) is the upstream protein that phosphorylates cofilin, an actin depolymerizing protein. Thus, we postulated a possible connection between parkin and LIMK1. Our studies in other cell lines, using co-transfection assays, demonstrated that LIMK1 and parkin bind each other. LIMK1 also interacted with previously known parkin interactors Hsp70 and CHIP. Parkin enhanced LIMK1-ubiquitination in the human neuroblastoma-derived BE(2)-M17 cell line, but not in the human embryonic kidney-derived HEK293 cell line. In fact, parkin-over-expression reduced the level of LIMK1-induced phosphocofilin in the BE(2)-M17 cells but not in the HEK293 cells. Additionally, in simian kidney-derived COS-7 cells, parkin-over-expression reduced LIMK1-induced actin filament accumulation. LIMK1 in cultured cells regulates parkin reversibly: LIMK1 did not phosphorylate parkin but LIMK1 overexpression reduced parkin self-ubiquitination in vitro and in HEK293 cells. Furthermore, in the cells co-transfected with parkin and p38, LIMK1 significantly decreased p38-ubiquitination by parkin. These findings demonstrate a cell-type dependent functional interaction between parkin and LIMK1 and provide new evidence that links parkin and LIMK1 in the pathogenesis of familial PD.

Expression of LIM kinase 1 is associated with reversible G1/S phase arrest, chromosomal instability and prostate cancer

Background

LIM kinase 1 (LIMK1), a LIM domain containing serine/threonine kinase, modulates actin dynamics through inactivation of the actin depolymerizing protein cofilin. Recent studies have indicated an important role of LIMK1 in growth and invasion of prostate and breast cancer cells; however, the molecular mechanism whereby LIMK1 induces tumor progression is unknown. In this study, we investigated the effects of ectopic expression of LIMK1 on cellular morphology, cell cycle progression and expression profile of LIMK1 in prostate tumors.

Results

Ectopic expression of LIMK1 in benign prostatic hyperplasia cells (BPH), which naturally express low levels of LIMK1, resulted in appearance of abnormal mitotic spindles, multiple centrosomes and smaller chromosomal masses. Furthermore, a transient G1/S phase arrest and delayed G2/M progression was observed in BPH cells expressing LIMK1. When treated with chemotherapeutic agent Taxol, no metaphase arrest was noted in these cells. We have also noted increased nuclear staining of LIMK1 in tumors with higher Gleason Scores and incidence of metastasis.

Conclusion

Our results show that increased expression of LIMK1 results in chromosomal abnormalities, aberrant cell cycle progression and alteration of normal cellular response to microtubule stabilizing agent Taxol; and that LIMK1 expression may be associated with cancerous phenotype of the prostate.

LIM kinases: function, regulation and association with human disease

The LIM kinase family consists of just two members: LIM kinase 1 (LIMK1) and LIM kinase 2 (LIMK2). With uniquely organised signalling domains, LIM kinases are regulated by several upstream signalling pathways, principally acting downstream of Rho GTPases to influence the architecture of the actin cytoskeleton by regulating the activity of the cofilin family proteins cofilin1, cofilin2 and destrin. Although the LIM kinases are very homologous, particularly when comparing kinase domains, there is emerging evidence that each may be subject to different regulatory pathways and may contribute to both distinct and overlapping cellular and developmental functions. Normal central nervous system development is reliant upon the presence of LIMK1, and its deletion has been implicated in the development of the human genetic disorder Williams syndrome. Normal testis development, on the other hand, is disrupted by the deletion of LIMK2. In addition, the possible involvement of each kinase in cardiovascular disorders as well as cancer has recently emerged. The LIM kinases have been proposed to play an important role in tumour-cell invasion and metastasis; fine-tuning the balance between phosphorylated and non-phosphorylated cofilin may be a significant determinant of tumour-cell metastatic potential. In this review, we outline the structure, regulation and function of LIM kinases and their functions at cellular and organismal levels, as well as their possible contributions to human disease.

Lim kinases, regulators of actin dynamics

The members of the LIM kinase (LIMK) family, which include LIMK 1 and 2, are serine protein kinases involved in the regulation of actin polymerisation and microtubule disassembly. Their activity is regulated by phosphorylation of a threonine residue within the activation loop of the kinase by p21-activated kinases 1 and 4 and by Rho kinase. LIMKs phosphorylate and inactivate the actin depolymerising factors ADF/cofilin resulting in net increase in the cellular filamentous actin. Hsp90 regulates the levels of the LIM kinase proteins by promoting their homo-dimerisation and trans-phosphorylation. Rnf6 is an E3 ubiquitin ligase responsible for LIMK degradation in neurons. The activity of LIMK1 is also required for microtubule disassembly in endothelial cells. While LIMK1 localizes mainly at focal adhesions, LIMK2 is found in cytoplasmic punctae, suggesting that they may have different cellular functions. LIMK1 was shown to be involved in cancer metastasis, while LIMK2 activation promotes cells cycle progression.

Phosphorylation-dependent regulation of unique nuclear and nucleolar localization signals of LIM kinase 2 in endothelial cells

LIM kinases (LIMKs) regulate actin dynamics through cofilin phosphorylation and also have a function in the nucleus. Recently we have shown that LIMK2 shuttles between cytoplasm and nucleus in endothelial cells and that nuclear import is inhibited by protein kinase C-mediated phosphorylation of Ser-283. Here we aimed to identify the structural features of LIMK2 responsible for nuclear import. We found that the kinase domain of LIMK2 is localized exclusively in the nucleus and, in contrast to the kinase domain of LIMK1, it accumulated in the nucleolus. Through site-directed mutagenesis, we identified the basic amino acid-rich motif KKRTLRKNDRKKR (amino acids 491-503) as the functional nuclear and nucleolar localization signal of LIMK2. After fusing this motif to enhanced green fluorescent protein, the fusion protein localized exclusively in the nucleus and nucleolus. Mutagenesis studies showed that phosphorylation of Thr-494, a putative protein kinase C phosphorylation site identified within the nuclear localization signal, inhibits nuclear import of the enhanced green fluorescent protein-PDZ kinase domain of LIMK2. After inhibiting nuclear export with leptomycin B, phosphorylation of either Ser-283 or Thr-494 reduced the nuclear import of LIMK2. Phosphorylation of both Ser-283 and Thr-494 sites inhibited nuclear import completely. Our findings identify a unique basic amino acid-rich motif (amino acids 491-503) in LIMK2 which is not present in LIMK1 that serves to target the protein not only to the nucleus but also to the nucleolus. Phosphorylation of Thr-494 within this motif negatively regulates nuclear import of LIMK2.

LIM kinase 1 increases tumor metastasis of human breast cancer cells via regulation of the urokinase-type plasminogen activator system

Mammalian LIM kinase 1 (LIMK1) phosphorylates and inactivates the actin-binding and -depolymerizing factor cofilin and induces actin cytoskeletal changes. LIMK1 is reported to play an important role in cell motility, but the mechanism of induction of cell motility and the role of LIMK1 in tumor growth, angiogenesis and invasion are poorly understood. Here we show that expression of LIMK1 in MDA-MB-435 human breast cancer cells enhanced cell proliferation and cell invasiveness and promoted in vitro angiogenesis. Since tumor metastasis requires degradation of the extracellular matrix by the serine protease urokinase type plasminogen activator (uPA), we examined the role of LIMK1 in the regulation of uPA/uPAR system. LIMK1 overexpression in breast cancer cells upregulated the uPA system, increased uPA promoter activity, induced uPA and uPAR mRNA and protein expression and induced uPA secretion. In contrast, cells transfected with the catalytically inactive LIMK mutant D460N-LIMK1 did not exhibit these phenotypic changes. Blocking antibodies against uPA and uPAR suppressed LIMK1-induced cell invasiveness. In addition, LIMK1 overexpression increased tumor growth in female athymic nude mice, promoted tumor angiogenesis and induced metastasis to livers and lungs, possibly by increasing uPA expression in the tumors. Finally, LIMK1 and uPAR were coordinately overexpressed in human breast tumors. These results suggested an important role for LIMK1 signaling in breast cancer tumor growth, angiogenesis and invasion and a regulatory connection between LIMK1 and the uPA system.

Different activity regulation and subcellular localization of LIMK1 and LIMK2 during cell cycle transition

LIM kinases (LIMK1 and LIMK2) regulate actin cytoskeletal reorganization through phosphorylating and inactivating cofilin, an actin-depolymerizing factor of actin filaments. Here, we describe a detailed analysis of the cell-cycle-dependent activity of LIMK2, and a subcellular localization of LIMK1 and LIMK2. The activity of LIMK2, distinct from LIMK1, toward cofilin phosphorylation did not change in the normal cell division cycle. In contrast, LIMK2 was hyperphosphorylated and its activity was markedly increased when HeLa cells were synchronized at mitosis with nocodazole treatment. Immunofluorescence analysis showed that LIMK1 was localized at cell-cell adhesion sites in interphase and prophase, redistributed to the spindle poles during prometaphase to anaphase, and accumulated at the cleavage furrow in telophase. In contrast, LIMK2 was diffusely localized in the cytoplasm during interphase, redistributed to the mitotic spindle, and finally to the spindle midzone during anaphase to telophase. These findings suggest that LIMK2 is activated in response to microtubule disruption, and that LIMK1 and LIMK2 may play different roles in regulating for the mitotic spindle organization, chromosome segregation, and cytokinesis during the cell division cycle.

Studying the effects of actin cytoskeletal destabilization on cell cycle by cofilin overexpression

The significance of actin cytoskeleton on cell growth was historically studied using toxic drugs, such as cytochalasin. However, it is possible that unpredictable effects of these agents may have influenced the reported observations. In our study, we have established a drug-free system using cofilin overexpression to investigate the relationship between actin filaments and cell cycle progression. Cofilin is a member of the actin depolymerization factor (ADF)/cofilin family, cofilin cDNA was cloned to a tetracycline-inducible gene expression vector and stably transfected to human lung cancer H1299 epithelial cells. Destabilization of actin filaments and morphological change was detected in cofilin overexpressing cells by actin analysis and microscopy, respectively. Measurements of growth rates showed that cell proliferation was retarded in cells with overexpressed cofilin. Also, cell cycle analysis showed that approx 90% of cofilin overexpressing cells were arrested in G1 phase, which is consistent with previous reports that drug-mediated disruption of actin filaments can cause G1 phase arrest. Taken together, cofilin overexpression cell model provides evidence that the effects of actin cytoskeletal destabilization on cell cycle progression can be studied using molecular approach instead of drug.

Limkain b1, a novel human autoantigen localized to a subset of ABCD3 and PXF marked peroxisomes

Detection of self-reactive antibodies has an established role in the diagnosis and monitoring of many human autoimmune diseases. Autoantibodies with restricted reactivity to cytoplasmic compartments and structures are an occasional incidental finding following routine examination of serum for antinuclear antibody reactivity. A prerequisite for rational exploitation of self-reactive antibodies, in either clinical or research settings, is the establishment of the molecular identity of the target autoantigen(s). Here we report on the identification of a novel autoantigen that co-localizes with a subset of cytoplasmic microbodies marked by ABCD3 (PMP-70) and/or PXF (PEX19). Immunoscreening a HeLa cell cDNA expression library with a human autoimmune serum identified two clones that encode fragments of limkain b1 (LKAP). We demonstrate that mouse polyclonal antibodies raised against a bacterially expressed fragment of limkain b1 mark the same cytoplasmic structures as human serum, as does an EGFP:LKAPCT429 fusion protein expressed in HeLa cells. An immunoblot screen against a bacterially expressed MBP:LKAPCT429 fusion protein substrate, using a cohort of 16 additional human sera that display Hep 2 cell cytoplasmic staining patterns similar to the prototype serum, identified three additional sera reactive to limkain b1. This is the first report establishing the molecular identity of a peroxisomal autoantigen. Preliminary results suggest that limkain b1 may be a relatively common target of human autoantibodies reactive to cytoplasmic vesicle-like structures.

Inhibition of nuclear import of LIMK2 in endothelial cells by protein kinase C-dependent phosphorylation at Ser-283

LIM kinases (LIMKs) are mainly in the cytoplasm and regulate actin dynamics through cofilin phosphorylation. Recently, it has been reported that nuclear localization of LIMKs can mediate suppression of cyclin D1 expression. Using immunofluorescence monitoring of enhanced green fluorescent protein-tagged LIMK2 in combination with photobleaching techniques and leptomycin B treatment, we demonstrate that LIMK2 shuttles between the cytoplasm and the nucleus in endothelial cells. Sequence analysis predicted two PKC phosphorylation sites in LIMK2 but not in LIMK1. One site at Ser-283 is present between the PDZ and the kinase domain, and the other site at Thr-494 is within the kinase domain. Activation of PKC by phorbol ester treatment of endothelial cells stimulated LIMK2 phosphorylation at Ser-283 and inhibited nuclear import of LIMK2 and the PDZ kinase construct of LIMK2 (amino acids 142-638) but not of LIMK1. The PKC-delta isoform phosphorylated LIMK2 at Ser-283 in vitro. Mutational analysis indicated that LIMK2 phosphorylation at Ser-283 but not Thr-494 was functional. Serum stimulation of endothelial cells also inhibited nuclear import of PDZK-LIMK2 by protein kinase C-dependent phosphorylation of Ser-283. Our study shows that phorbol ester and serum stimulation of endothelial cells inhibit nuclear import of LIMK2 but not LIMK1. This effect was dependent on PKC-delta-mediated phosphorylation of Ser-283. Since phorbol ester enhanced cyclin D1 expression and subsequent G1-to-S-phase transition of endothelial cells, we suggest that the PKC-mediated exclusion of LIMK2 from the nucleus might be a mechanism to relieve suppression of cyclin D1 expression by LIMK2.

Interplay between components of a novel LIM kinase-slingshot phosphatase complex regulates cofilin

Slingshot (SSH) phosphatases and LIM kinases (LIMK) regulate actin dynamics via a reversible phosphorylation (inactivation) of serine 3 in actin-depolymerizing factor (ADF) and cofilin. Here we demonstrate that a multi-protein complex consisting of SSH-1L, LIMK1, actin, and the scaffolding protein, 14-3-3zeta, is involved, along with the kinase, PAK4, in the regulation of ADF/cofilin activity. Endogenous LIMK1 and SSH-1L interact in vitro and co-localize in vivo, and this interaction results in dephosphorylation and downregulation of LIMK1 activity. We also show that the phosphatase activity of purified SSH-1L is F-actin dependent and is negatively regulated via phosphorylation by PAK4. 14-3-3zeta binds to phosphorylated slingshot, decreases the amount of slingshot that co-sediments with F-actin, but does not alter slingshot activity. Here we define a novel ADF/cofilin phosphoregulatory complex and suggest a new mechanism for the regulation of ADF/cofilin activity in mediating changes to the actin cytoskeleton.

A Lim protein involved in the progression of cytokinesis and regulation of the mitotic spindle

DdLimE regulates cell motility and cytokinesis in Dictyostelium. To specify its function, we generated knock-out mutants and analyzed mitosis by marking the mitotic apparatus with GFP-alpha-tubulin. Characteristic of DdLimE-null cells is a late reversal of cytokinesis caused by backward movement of the incipient daughter cells. This process of "retro-cytokinesis" is accompanied by a delay in disassembly of the mitotic spindle. The length of interphase microtubules is increased and their depolymerization at prophase is impaired. These data indicate that DdLimE links the cortical actin network, where it is located, to the microtubule system, whose dynamics it regulates.

LIM-kinase as a regulator of actin dynamics in spermatogenesis

We have identified LIM-kinase (LIMK1 and LIMK2), the only known catalytic protein among LIM-family molecules. Both LIMK1 and LIMK2 phosphorylate (inactivate) cofilin, an actin depolymerizing factor, and induce actin cytoskeleton reorganization. We as well as others concurrently demonstrated that LIMK activation was regulated by the Rho family of GTPases, and LIMK1 and LIMK2 share the distinct functional portion in Rho GTPases-mediated actin dynamics. Although evidence for their physiological significance is tenuous, several studies have focused on understanding LIMK functions, particularly in meiosis and mitosis. This review addresses recent LIMK findings, with emphasis on spermatogenesis studies, upon which future studies can shed light on the role of the LIMK/actin system in meiosis and mitosis.

Nuclear Accumulation of Globular Actin as a Cellular Senescence Marker

We evaluated the nuclear actin accumulation as a new marker of cellular senescence, using human diploid fibroblast (HDF), chondrocyte primary cultures, Mv1Lu epithelial cells, and Huh7 cancer cells. Nuclear accumulation of globular actin (G-actin) and dephosphorylated cofilin was highly significant in the senescent HDF cells, accompanied with inhibition of LIM kinase (LIMK) -1 activity. When nuclear export of the actin was induced by 12-O-tetradecanoylphorbol-13-acetate, DNA synthesis of the senescent cells increased significantly, accompanied with changes of morphologic and biochemical profiles, such as increased RB protein phosphorylation and decreased expressions of p21(WAF1), cytoplasmic p-extracellular signal-regulated kinase 1/2, and caveolins 1 and 2. Significance of these findings was strengthened additionally by the fact that nuclear actin export of young HDF cells was inhibited by the treatment with leptomycin B and mutant cofilin transfection, whose LIMK-1 phosphorylation site was lost, and the old cell phenotypes were duplicated with nuclear actin accumulation, suggesting that nuclear actin accumulation was accompanied with G1 arrest during cellular senescence. The aforementioned changes were observed not only in the replicative senescence but also in the senescence induced by treatment of HDF cells, Mv1Lu, primary culture of human chondrocytes, or Huh7 cells with H-ras virus infection, hydroxyurea, deferoxamine, or H(2)O(2). Nuclear actin accumulation was much more sensitive and an earlier event than the well-known, senescence-associated beta-galactosidase activity.

LIM kinase 1 is essential for the invasive growth of prostate epithelial cells: implications in prostate cancer

Mammalian LIM kinase 1 (LIMK1) is involved in reorganization of actin cytoskeleton through inactivating phosphorylation of the ADF family protein cofilin, which depolymerizes actin filaments. Maintenance of the actin dynamics in an ordered fashion is essential for stabilization of cell shape or promotion of cell motility depending on the cell type. These are the two key phenomena that may become altered during acquisition of the metastatic phenotype by cancer cells. Here we show that LIMK1 is overexpressed in prostate tumors and in prostate cancer cell lines, that the concentration of phosphorylated cofilin is higher in metastatic prostate cancer cells, and that a partial reduction of LIMK1 altered cell proliferation by arresting cells at G2/M, changed cell shape, and abolished the invasiveness of metastatic prostate cancer cells. We also show that the ectopic expression of LIMK1 promotes acquisition of invasive phenotype by the benign prostate epithelial cells. Our data provide evidence of a novel role of LIMK1 in regulating cell division and invasive property of prostate cancer cells and indicate that the effect is not mediated by phosphorylation of cofilin. Our study correlates with the recent observations showing a metastasis-associated chromosomal gain on 7q11.2 in prostate cancer, suggesting a possible gain in LIMK1 DNA (7q11.23).

Cell cycle-associated changes in Slingshot phosphatase activity and roles in cytokinesis in animal cells

During cytokinesis the actomyosin-based contractile ring is formed at the equator, constricted, and then disassembled prior to cell abscission. Cofilin stimulates actin filament disassembly and is implicated in the regulation of contractile ring dynamics. However, little is known about the mechanism controlling cofilin activity during cytokinesis. Cofilin is inactivated by phosphorylation on Ser-3 by LIM-kinase-1 (LIMK1) and is reactivated by a protein phosphatase Slingshot-1 (SSH1). Here we show that the phosphatase activity of SSH1 decreases in the early stages of mitosis and is elevated in telophase and cytokinesis in HeLa cells, a time course correlating with that of cofilin dephosphorylation. SSH1 co-localizes with F-actin and accumulates onto the cleavage furrow and the midbody. Expression of a phosphatase-inactive SSH1 induces aberrant accumulation of F-actin and phospho-cofilin near the midbody in the final stage of cytokinesis and frequently leads to the regression of the cleavage furrow and the formation of multinucleate cells. Co-expression of cofilin rescued the inhibitory effect of phosphatase-inactive SSH1 on cytokinesis. These results suggest that SSH1 plays a critical role in cytokinesis by dephosphorylating and reactivating cofilin in later stages of mitosis.