Phosphorylation of keratin and vimentin polypeptides in normal and transformed mitotic human epithelial amnion cells: behavior of keratin and vimentin filaments during mitosis

F-Actin Interactome Reveals Vimentin as a Key Regulator of Actin Organization and Cell Mechanics in Mitosis

Most metazoan cells entering mitosis undergo characteristic rounding, which is important for accurate spindle positioning and chromosome separation. Rounding is driven by contractile tension generated by myosin motors in the sub-membranous actin cortex. Recent studies highlight that alongside myosin activity, cortical actin organization is a key regulator of cortex tension. Yet, how mitotic actin organization is controlled remains poorly understood. To address this, we characterized the F-actin interactome in spread interphase and round mitotic cells. Using super-resolution microscopy, we then screened for regulators of cortex architecture and identified the intermediate filament vimentin and the actin-vimentin linker plectin as unexpected candidates. We found that vimentin is recruited to the mitotic cortex in a plectin-dependent manner. We then showed that cortical vimentin controls actin network organization and mechanics in mitosis and is required for successful cell division in confinement. Together, our study highlights crucial interactions between cytoskeletal networks during cell division.

•

Comparison of the F-actin interactome in spread interphase and round mitotic cells

•

Proteomics identifies vimentin and plectin as key regulators of the mitotic cortex

•

Vimentin intermediate filaments localize under the actin cortex in mitosis

•

Sub-cortical vimentin regulates actin cortex organization and mechanics in mitosis

Intermediate filaments and IF-associated proteins: from cell architecture to cell proliferation

Intermediate filaments (IFs), in coordination with microfilaments and microtubules, form the structural framework of the cytoskeleton and nucleus, thereby providing mechanical support against cellular stresses and anchoring intracellular organelles in place. The assembly and disassembly of IFs are mainly regulated by the phosphorylation of IF proteins. These phosphorylation states can be tracked using antibodies raised against phosphopeptides in the target proteins. IFs exert their functions through interactions with not only structural proteins, but also non-structural proteins involved in cell signaling, such as stress responses, apoptosis, and cell proliferation. This review highlights findings related to how IFs regulate cell division through phosphorylation cascades and how trichoplein, a centriolar protein originally identified as a keratin-associated protein, regulates the cell cycle through primary cilium formation.

Unconventional actin conformations localize on intermediate filaments in mitosis

Different structural conformations of actin have been identified in cells and shown to reside in distinct subcellular locations of cells. In this report, we describe the localization of actin on a cage-like structure in metaphase HEK 293T cells. Actin was detected with the anti-actin antibodies 1C7 and 2G2, but not with the anti-actin antibody C4. Actin contained in this structure is independent of microtubules and actin filaments, and colocalizes with vimentin. Taking advantage of intermediate filament collapse into a perinuclear dense mass of cables when microtubules are depolymerized, we were able to relocalize actin to such structures. We hypothesize that phosphorylation of intermediate filaments at mitosis entry triggers the recruitment of different actin conformations to mitotic intermediate filaments. Storage and partition of the nuclear actin and antiparallel "lower dimer" actin conformations between daughter cells possibly contribute to gene transcription and transient actin filament dynamics at G1 entry.

Mesothelial proteins are expressed in the human cornea

The goal of our study was to determine whether proteins typical of the human mesothelial cell phenotype, such as mesothelin, HBME-1 (Hector Battifora mesothelial cell-1) protein and calbindin 2, are expressed in the human cornea, especially in endothelial cells. Cryosections and endothelial and epithelial imprints of sixteen human cadaverous corneoscleral discs were used. The presence of proteins was examined using immunohistochemistry and Western blotting, while mRNA levels were determined by qRT-PCR. A strong signal for mesothelin was present in the corneal epithelium, while less intense staining was visible in the endothelium. Similarly, higher and lower mRNA levels were detected using qRT-PCR in the corneal epithelium and endothelium, respectively. HBME-1 antibody strongly stained the corneal endothelium and stromal keratocytes. Marked positivity was present in the corneal stromal extracellular matrix, while no staining was present in the sclera. Calbindin 2 was detected using immunohistochemistry and Western blotting in the corneal epithelium, endothelium and stroma. qRT-PCR confirmed its expression in epithelial and endothelial cells. Three proteins expressed constitutively in mesothelial cells were detected in the human cornea. The possible function of mesothelin in cell-cell contact on the ocular surface is discussed. The presence of HBME-1 protein in the endothelial layer may indicate a still unknown function that could be shared with mesothelial cells of the pleura and peritoneum. The much more pronounced occurrence of calbindin 2 in the corneal epithelium compared to fewer positive endothelial cells explains the higher turnover of epithelial cells compared to the proliferatively inactive endothelium.

Head and rod 1 interactions in vimentin: identification of contact sites, structure, and changes with phosphorylation using site-directed spin labeling and electron paramagnetic resonance

We have used site-directed spin labeling (SDSL) and electron paramagnetic resonance (EPR) to identify residues 17 and 137 as sites of interaction between the head domain and rod domain 1A of the intermediate filament protein vimentin. This interaction was maximal when compared with the spin labels placed at up- and downstream positions in both head and rod regions, indicating that residues 17 and 137 were the closest point of interaction in this region. SDSL EPR characterization of residues 120-145, which includes the site of head contact with rod 1A, reveals that this region exhibits the heptad repeat pattern indicative of alpha-helical coiled-coil structure, but that this heptad repeat pattern begins to decay near residue 139, suggesting a transition out of coiled-coil structure. By monitoring the spectra of spin labels placed at the 17 and 137 residues during in vitro assembly, we show that 17-137 interaction occurs early in the assembly process. We also explored the effect of phosphorylation on the 17-137 interaction and found that phosphorylation-induced changes affected the head-head interaction (17-17) in the dimer, without significantly influencing the rod-rod (137-137) and head-rod (17-137) interactions in the dimer. These data provide the first direct evidence for, and location of, head-rod interactions in assembled intermediate filaments, as well as direct evidence of coiled-coil structure in rod 1A. Finally, the data identify changes in the structure in this region following in vitro phosphorylation.

Cell cycle specific expression and nucleolar localization of human J-domain containing co-chaperone Mrj

J-domain containing co-chaperone Mrj (mammalian relative to DnaJ) has been implicated in diverse cellular functions including placental development and inhibition of Huntingtin mediated cytotoxicity. It has also been shown to interact with keratin intermediate filaments. Since keratins undergo extensive reorganization during cell division, its interactor Mrj might also play an important role in the regulation of cell cycle. In support of this hypothesis, we report the up-regulation of Mrj protein in M-phase of HeLa cells implicating its role in mitosis related activities. The protein is dispersed throughout the cell during late mitosis and is localized in nucleolus during interphase, confirming that the activity of Mrj is regulated by its cell cycle specific expression together with its differential subcellular localization.

Identification of phosphorylation-induced changes in vimentin intermediate filaments by site-directed spin labeling and electron paramagnetic resonance

Phosphorylation drives the disassembly of the vimentin intermediate filament (IF) cytoskeleton at mitosis. Chromatographic analysis has suggested that phosphorylation produces a soluble vimentin tetramer, but little has been determined about the structural changes that are caused by phosphorylation or the structure of the resulting tetramer. In this study, site-directed spin labeling and electron paramagnetic resonance (SDSL-EPR) were used to examine the structural changes resulting from protein kinase A phosphorylation of vimentin IFs in vitro. EPR spectra suggest that the tetrameric species resulting from phosphorylation is the A11 configuration. EPR spectra also establish that the greatest degree of structural change was found in the linker 2 and the C-terminal half of the rod domain, despite the fact that most phosphorylation occurs in the N-terminal head domain. The phosphorylation-induced changes notably affected the proposed "trigger sequences" located in the linker 2 region, which have been hypothesized to mediate the induction of coiled-coil formation. These data are the first to document specific changes in IF structure resulting from a physiologic regulatory mechanism and provide further evidence, also generated by SDSL-EPR, that the linker regions play a key role in IF structure and regulation of assembly/disassembly.

Regulatory mechanisms and functions of intermediate filaments: a study using site- and phosphorylation state-specific antibodies

Intermediate filaments (IF) form the structural framework of the cytoskeleton. Although histopathological detection of IF proteins is utilized for examining cancer specimens as reliable markers, the molecular mechanisms by which IF are involved in the biology of cancer cells are still unclear. We found that site-specific phosphorylation of IF proteins induces the disassembly of filament structures. To further dissect the in vivo spatiotemporal dynamics of IF phosphorylation, we developed site- and phosphorylation state-specific antibodies. Using these antibodies, we detected kinase activities that specifically phosphorylate type III IF, including vimentin, glial fibrillary acidic protein and desmin, during mitosis. Cdk1 phosphorylates vimentin-Ser55 from prometaphase to metaphase, leading to the recruitment of Polo-like kinase 1 (Plk1) to vimentin. Upon binding to Phospho-Ser55 of vimentin, Plk1 is activated, and then phosphorylates vimentin-Ser82. During cytokinesis, Rho-kinase and Aurora-B specifically phosphorylate IF at the cleavage furrow. IF phosphorylation by Cdk1, Plk1, Rho-kinase and Aurora-B plays an important role in the local IF breakdown, and is essential for the efficient segregation of IF networks into daughter cells. As another part of our research on IF, we have set out to find the binding partners with simple epithelial keratin 8/18. We identified tumor necrosis factor receptor type 1-associated death domain protein (TRADD) as a keratin 18-binding protein. Together with data from other laboratories, it is proposed that simple epithelial keratins may play a role in modulating the response to some apoptotic signals. Elucidation of the precise molecular functions of IF is expected to improve our understanding of tumor development, invasion and metastasis.

Nestin promotes the phosphorylation-dependent disassembly of vimentin intermediate filaments during mitosis

The expression of the intermediate filament (IF) protein nestin is closely associated with rapidly proliferating progenitor cells during neurogenesis and myogenesis, but little is known about its function. In this study, we examine the effects of nestin expression on the assembly state of vimentin IFs in nestin-free cells. Nestin is introduced by transient transfection and is positively correlated with the disassembly of vimentin IFs into nonfilamentous aggregates or particles in mitotic but not interphase cells. This nestin-mediated disassembly of IFs is dependent on the phosphorylation of vimentin by the maturation/M-phase-promoting factor at ser-55 in the amino-terminal head domain. In addition, the disassembly of vimentin IFs during mitosis appears to be a unique feature of nestin-expressing cell types. Furthermore, when the expression of nestin is downregulated by the nestin-specific small interfering RNA in nestin-expressing cells, vimentin IFs remain assembled throughout all stages of mitosis. Previous studies suggest that nonfilamentous vimentin particles are IF precursors and can be transported rapidly between different cytoplasmic compartments along microtubule tracks. On the basis of these observations, we speculate that nestin may play a role in the trafficking and distribution of IF proteins and potentially other cellular factors to daughter cells during progenitor cell division.

Aurora-B regulates the cleavage furrow-specific vimentin phosphorylation in the cytokinetic process

Aurora-B is an evolutionally conserved protein kinase that regulates several mitotic events including cytokinesis. We previously demonstrated the possible existence of a protein kinase that phosphorylates at least Ser-72 on vimentin, the most widely expressed intermediate filament protein, in the cleavage furrow-specific manner. Here we showed that vimentin-Ser-72 phosphorylation occurred specifically at the border of the Aurora-B-localized area from anaphase to telophase. Expression of a dominant-negative mutant of Aurora-B led to a reduction of this vimentin-Ser-72 phosphorylation. In vitro analyses revealed that Aurora-B phosphorylates vimentin at approximately 2 mol phosphate/mol of substrate for 30 min and that this phosphorylation dramatically inhibits vimentin filament formation. We further identified eight Aurora-B phosphorylation sites, including Ser-72 on vimentin, and then constructed the mutant vimentin in which these identified sites are changed into Ala. Cells expressing this mutant formed an unusually long bridge-like intermediate filament structure between unseparated daughter cells. We then identified important phosphorylation sites for the bridge phenotype. Our findings indicate that Aurora-B regulates the cleavage furrow-specific vimentin phosphorylation and controls vimentin filament segregation in cytokinetic process.

Induction of rapid and reversible cytokeratin filament network remodeling by inhibition of tyrosine phosphatases

The cytokeratin filament network is intrinsically dynamic, continuously exchanging subunits over its entire surface, while conferring structural stability on epithelial cells. However, it is not known how cytokeratin filaments are remodeled in situations where the network is temporarily and spatially restricted. Using the tyrosine phosphatase inhibitor orthovanadate we observed rapid and reversible restructuring in living cells, which may provide the basis for such dynamics. By examining cells stably expressing fluorescent cytokeratin chimeras, we found that cytokeratin filaments were broken down and then formed into granular aggregates within a few minutes of orthovanadate addition. After drug removal, gradual reincorporation of granules into the filament network was observed for aggregates that were either part of residual filaments or stayed in close apposition to remaining filaments. Even when cytokeratin filaments were no longer detectable, granules with low mobility were still able to reestablish a cytokeratin filament network. This process took less than 30 minutes and occurred at multiple foci throughout the cytoplasm without apparent correlation to alterations in the actin- and tubulin-based systems. Interestingly, the short-lived and rather small orthovanadate-induced cytokeratin granules contained the cytoskeletal crosslinker plectin but lacked the cytokeratin-solubilising 14-3-3 proteins. By contrast, the long-lived and larger cytokeratin aggregates generated after treatment with the serine/threonine phosphatase inhibitor okadaic acid were negative for plectin but positive for 14-3-3 proteins. Taken together, our observations in living orthovanadate-treated interphase cells revealed modes of cytokeratin remodeling that qualify as basic mechanisms capable of rapidly adapting the cytokeratin filament cytoskeleton to specific requirements.

Protein kinases required for segregation of vimentin filaments in mitotic process

Vimentin, one of type III intermediate filament (IF) proteins, is expressed not only in mesenchymal cells but also in most types of tumor cells. In the present study, we introduced several types of vimentin mutated at putative phosphorylation sites in its amino-terminal head domain into type III IF-negative T24 cells. Site-specific mutation induced the formation of an unusually long bridge-like IF structure between the unseparated daughter cells, although these mutants formed the filament network similar to wild type in interphase cells. Together with sites phosphorylated by Rho-kinase and protein kinase C (PKC), vimentin-Ser72, which can not be phosphorylated by any known vimentin kinase, was one of the mutation sites essential for this phenotype. We further demonstrated that vimentin-Ser72 was phosphorylated specifically at the cleavage furrow during cytokinesis. These observations suggest the existence of a novel protein kinase responsible for vimentin filament separation through the cleavage furrow-specific vimentin phosphorylation. We propose that Rho-kinase, PKC, and an unidentified vimentin-Ser72 kinase may play important roles in vimentin filament separation during cytokinesis.

Regulation of intermediate filament organization during cytokinesis: possible roles of Rho-associated kinase

Intermediate filaments (IFs), which form the structural framework of cytoskeleton, have been found to be dramatically reorganized during mitosis. Some protein kinases activated in mitosis are thought to control spatial and temporal IF reorganization through phosphorylation of IF proteins. Rho-associated kinase (Rho-kinase), one of the putative targets of the small GTPase Rho, does phosphorylate IF proteins, specifically at the cleavage furrow during cytokinesis. This cleavage furrow-specific phosphorylation plays an important role in the local IF breakdown and efficient separation of IF networks. Recent studies on Rho signaling pathways have introduced new models about the molecular mechanism of rearrangements of cytoskeletons including IFs during cytokinesis.

Hepatocyte cytokeratins are hyperphosphorylated at multiple sites in human alcoholic hepatitis and in a mallory body mouse model

Alcoholic hepatitis (AH) is associated with cytokeratin 8 and 18 (CK8/18) accumulation as cytoplasmic inclusion bodies, termed Mallory bodies (MBs). Studies with MB mouse models and cultured hepatocytes suggested that CK8/18 hyperphosphorylation might be involved in MB formation. However, no data exist on phosphorylation of CK8/18 in human AH. In this study, antibodies that selectively recognize phosphorylated epitopes of CK8 or CK18 were used to analyze CK8/18 phosphorylation states in normal human and murine livers, human AH biopsies, and livers of 3,5-diethoxycarbonyl-1, 4-dihydrocollidine (DDC)-intoxicated mice, the last serving as model for MB induction. Hepatocyte cytokeratins become hyperphosphorylated at multiple sites in AH and in DDC-intoxicated mice. Hyperphosphorylation of CK8/18 occurred rapidly, after 1 day of DDC intoxication and preceded architectural changes of the cytoskeleton. In long-term DDC-intoxicated mice as well as in human AH, MBs preferentially contain hyperphosphorylated CK8/18 as compared with the cytoplasmic cytokeratin intermediate filament network suggesting that CK8/18 hyperphosphorylation may play a contributing role in MB pathogenesis. Furthermore, the site-specific phosphorylation of cytokeratin in different stages of MB induction provides indirect evidence for the involvement of a variety of protein kinases known to be activated in stress responses, mitosis, and apoptosis.

Detection of cytokeratin dynamics by time-lapse fluorescence microscopy in living cells

To monitor the desmosome-anchored cytokeratin network in living cells fusion protein HK13-EGFP consisting of human cytokeratin 13 and the enhanced green fluorescent protein was stably expressed in vulvar carcinoma-derived A-431 cells. It is shown for A-431 subclone AK13-1 that HK13-EGFP emits strong fluorescence in fixed and living cells, being part of an extended cytoplasmic intermediate filament network that is indistinguishable from that of parent A-431 cells. Biochemical, immunological and ultrastructural analyses demonstrate that HK13-EGFP behaves identically to the endogenous cytokeratin 13 and is therefore a reliable in vivo tag for this polypeptide and the structures formed by it. Time-lapse fluorescence microscopy reveals that the cytokeratin 13-containing network is in constant motion, resulting in continuous restructuring occurring in single and migratory cells, as well as in desmosome-anchored cells. Two major types of movement are distinguished: (i) oscillations of mostly long filaments, and (ii) an inward-directed flow of fluorescence originating as diffuse material at the cell periphery and moving in the form of dots and thin filaments toward the deeper cytoplasm where it coalesces with other filaments and filament bundles. Both movements are energy dependent and can be inhibited by nocodazole, but not by cytochalasin D. Finally, disassembly and reformation of cytokeratin filament networks are documented in dividing cells revealing distinct and rapidly occurring stages of cytokeratin organisation and distribution.

The calcium-modulated proteins, S100A1 and S100B, as potential regulators of the dynamics of type III intermediate filaments

The Ca2+-modulated, dimeric proteins of the EF-hand (helix-loop-helix) type, S100A1 and S100B, that have been shown to inhibit microtubule (MT) protein assembly and to promote MT disassembly, interact with the type III intermediate filament (IF) subunits, desmin and glial fibrillary acidic protein (GFAP), with a stoichiometry of 2 mol of IF subunit/mol of S100A1 or S100B dimer and an affinity of 0.5-1.0 microM in the presence of a few micromolar concentrations of Ca2+. Binding of S100A1 and S100B results in inhibition of desmin and GFAP assemblies into IFs and stimulation of the disassembly of preformed desmin and GFAP IFs. S100A1 and S100B interact with a stretch of residues in the N-terminal (head) domain of desmin and GFAP, thereby blocking the head-to-tail process of IF elongation. The C-terminal extension of S100A1 (and, likely, S100B) represents a critical part of the site that recognizes desmin and GFAP. S100B is localized to IFs within cells, suggesting that it might have a role in remodeling IFs upon elevation of cytosolic Ca2+ concentration by avoiding excess IF assembly and/or promoting IF disassembly in vivo. S100A1, that is not localized to IFs, might also play a role in the regulation of IF dynamics by binding to and sequestering unassembled IF subunits. Together, these observations suggest that S100A1 and S100B may be regarded as Ca2+-dependent regulators of the state of assembly of two important elements of the cytoskeleton, IFs and MTs, and, potentially, of MT- and IF-based activities.

Mechanisms of mallory body formation induced by okadaic acid in drug-primed mice

Drug-primed mice form Mallory bodies in their liver after various types of liver injury such as heat shock, drug refeeding, or ethanol ingestion. However, the mechanisms involved that lead to Mallory body formation after these different treatments are unknown. There may be a common pathway of Mallory body formation that is initiated by these different types of injuries. Recently it was shown that the phosphatase 1/2A inhibitor okadaic acid induced Mallory body formation, suggesting that the mechanism of formation involves hyperphosphorylation or oxidative stress-induced NFkappaB activation. To test this hypothesis we exposed drug-primed mice to okadaic acid and measured phosphorylation of Mallory body proteins immunohistochemically and by immunoblot chemiluminescence using an antibody specific for phosphothreonine. NFkappaB activation was measured by a gel shift retardation assay of nuclear lysates. Beginning 15 min after okadaic acid injection, complex changes were progressively seen in the liver cells focally including aggregation of cytokeratins 8 and 18 in hepatocytes which otherwise failed to stain normally with cytokeratin antibody. The aggregates stained positive with ubiquitin and phosphothreonine antibodies. Immunoblots showed a progressive increase in positive staining of the Mallory body band with the antibody to phosphothreonine. NFkappaB activation was progressive up to 2 h after okadaic acid treatment but was downregulated 7 days later. In summary we show for the first time the effect of okadaic acid on the liver cytokeratins in vivo. We conclude that hyperphosphorylation and NFkappaB activation may play a role in the early phases of Mallory body formation.

Phosphorylation of human keratin 18 serine 33 regulates binding to 14-3-3 proteins

Members of the 14-3-3 protein family bind the human intermediate filament protein keratin 18 (K18) in vivo, in a cell-cycle- and phosphorylation-dependent manner. We identified K18 Ser33 as an interphase phosphorylation site, which increases its phosphorylation during mitosis in cultured cells and regenerating liver, and as an in vitro cdc2 kinase phosphorylation site. Comparison of wild-type versus K18 Ser33-->Ala/Asp transfected cells showed that K18 Ser33 phosphorylation is essential for the association of K18 with 14-3-3 proteins, and plays a role in keratin organization and distribution. Mutation of another K18 major phosphorylation site (Ser52) or K18 glycosylation sites had no effect on the binding of K18 to 14-3-3 proteins. The K18 phospho-Ser33 motif is different from several 14-3-3-binding phosphomotifs already described. Antibodies that are specific to K18 phospho-Ser33 or phospho-Ser52 show that although Ser52 and Ser33 phosphorylated K18 molecules manifest partial colocalization, these phosphorylation events reside predominantly on distinct K18 molecules. Our results demonstrate a unique K18 phosphorylation site that is necessary but not sufficient for K18 binding to 14-3-3 proteins. This binding is likely to involve one or more mitotic events coupled to K18 Ser33 phosphorylation, and plays a role in keratin subcellular distribution. Physiological Ser52 or Ser33 phosphorylation on distinct K18 molecules suggests functional compartmentalization of these modifications.

Phosphorylation of human keratin 8 in vivo at conserved head domain serine 23 and at epidermal growth factor-stimulated tail domain serine 431

Dynamic phosphorylation is one mechanism that regulates the more than 20 keratin type I and II intermediate filament proteins in epithelial cells. The major type II keratin in "simple type" glandular epithelia is keratin 8 (K8). We used biochemical and mutational approaches to localize two major in vivo phosphorylation sites of human K8 to the head (Ser-23) and tail (Ser-431) domains. Since Ser-23 of K8 is highly conserved among all type II keratins, we also examined if the corresponding Ser-59 in stratified epithelial keratin 6e is phosphorylated. Mutation of K6e Ser-59 abolished its phosphorylation in 32PO4-labeled baby hamster kidney cell transfectants. With regard to K8 phosphorylation at Ser-431, it increases dramatically upon stimulation of cells with epidermal growth factor (EGF) or after mitotic arrest and is the major K8 phosphorylated residue after incubating K8 immunoprecipitates with mitogen-activated protein or cdc2 kinases. A monoclonal antibody that specifically recognizes phosphoserine 431-K8 manifests increased reactivity with K8 and recognizes reorganized K8/18 filaments after EGF stimulation. Our results suggest that in vivo serine phosphorylation of K8 and K6e within the conserved head domain motif is likely to reflect a conserved phosphorylation site of most if not all type II keratins. Furthermore, K8 Ser-431 phosphorylation occurs after EGF stimulation and during mitotic arrest and is likely to be mediated by mitogen-activated protein and cdc2 kinases, respectively.

Implications of intermediate filament protein phosphorylation

Intermediate filament (IF) proteins, a large family of tissue specific proteins, undergo several posttranslational modifications, with phosphorylation being the most studied modification. IF protein phosphorylation is highly dynamic and involves the head and/or tail domains of these proteins, which are the domains that impart most of the structural heterogeneity and hence presumed tissue specific functions. Although the function of IF proteins remains poorly understood, several regulatory roles for IF protein phosphorylation have been identified or are emerging. Those roles include filament disassembly and reorganization, solubility, localization within specific cellular domains, association with other cytoplasmic or membrane associated proteins, protection against physiologic stress and mediation of tissue-specific functions. Understanding the mechanistic and functional aspects of IF protein phosphorylation is providing insights not only regarding the function of this modification, but also regarding the function of IF proteins.

Characterization of type III intermediate filament regulatory protein target epitopes: S-100 (beta and/or alpha) binds the N-terminal head domain; annexin II2-p11(2) binds the rod domain

We have investigated the interaction of S-100 proteins (beta and/or alpha) and annexin II2-p11(2) with glial fibrillary acidic protein (GFAP) and desmin to have further information on the mechanisms whereby S-100 proteins and annexin II2-p11(2) affect assembly/disassembly of GFAP and desmin intermediate filaments (IFs). Analyses were conducted on either native IF subunits, GFAP or desmin rod domain, or headless GFAP or desmin. Our data indicate that: (i) S-100 proteins bind to GFAP and desmin N-terminal head domain; (ii) annexin II2-p11(2) binds to GFAP rod domain; (iii) annexin II2-p11(2) does not interact with desmin nor affects desmin assembly. The present data suggest that the ability of S-100 proteins to inhibit GFAP and desmin assemblies and to promote the disassembly of preformed GFAP and desmin IFs depends on occupation of a site on the N-terminal head domain of these IF subunit. It is known that the N-terminal head domain is critical for the progression from the stage of GFAP and desmin dimers/tetramers to that of large oligomers. On the other hand, the ability of annexin II2-p11(2) to stimulate GFAP assembly under conditions where this latter is normally hampered (e.g., at alkaline pH values) might depend on annexin II2-p11(2)-induced changes in the structure of GFAP rod domain, possibly as a consequence of charge modifications. By contrast, the inability of annexin II2-p11(2) to bind to desmin would depend on desmin resistance to charge modifications.

Role of vimentin in early stages of neuritogenesis in cultured hippocampal neurons

Vimentin is expressed initially by nearly all neuronal precursors in vivo, and is replaced by neurofilaments shortly after the immature neurons become post-mitotic. Moreover, both vimentin and neurofilaments can be detected transiently within the same neurite, leaving open the possibility that vimentin may play a role in the early stages of neuritogenesis. In the present study, cultured hippocampal neurons, which transiently express vimentin in culture, were treated with sense- and antisense-oriented deoxyoligonucleotides encoding regions of the vimentin sequence that overlap the translation initiation codon. Antisense oligonucleotide treatment reduced vimentin-immunoreactivity to background levels. Moreover, while 90-100% of cultured hippocampal neurons elaborated neurites within the first 24 hr following plating, only 24-30% did so in the presence of vimentin antisense oligonucleotides. Inhibition of neurite outgrowth was reversible following removal of antisense oligonucleotide. These findings substantiate earlier studies in neuroblastoma cells, indicating a possible role for vimentin in the initiation of neurite outgrowth.

Cytokeratin dynamics during oocyte maturation in the hamster requires reaching of metaphase I

Cytoskeletal components like microfilaments and microtubules are known to play important roles during the processes of oocyte maturation, fertilization and early embryonic development in mammals. However, the roles of other components such as cytoplasmic intermediate filaments, during these critical events remain largely unknown. Oocyte maturation is the final step of oogenesis, immediately before ovulation. Several cytological changes involving the cytoskeleton take place during the maturation process, including meiotic spindle formation, redistribution of cell organelles, membrane polarization and first polar body emission. In this study we determined the organization and rearrangements of cytokeratins during hamster oocyte maturation. Fully grown oocytes were cultured and then visualised using microscopic immunolabelling techniques to monitor the cytokeratin dynamics at specific meiotic stages of the maturation process. In prophase-I-arrested fully grown hamster oocytes, cytokeratins are confined to 4-10 large cortical aggregates, corresponding to extensive meshworks of intermediate filaments. These large aggregates disperse into multiple small spots starting at metaphase I until the end of the maturation period at metaphase II, where cytokeratin exhibits a homogeneously distributed spotted pattern. However, meiotic progression to metaphase II is not necessary for cytokeratin redistribution to occur, since precociously arrested metaphase I oocytes also exhibit dispersed cytoplasmic foci at the end of the culture period. The redistribution of cytokeratins is insensitive to nocodazole and cytochalasin D suggesting it occurs independent of microtubules and microfilaments. In contrast, both cumulus cells and protein synthesis are required for cytokeratin modifications to take place during oocyte maturation. These results show that cytokeratin intermediate filaments are present in the fully grown hamster oocyte, and that a striking reorganization of cytokeratins, triggered by attainment of the metaphase I stage, occurs during maturation.

14-3-3 proteins associate with phosphorylated simple epithelial keratins during cell cycle progression and act as a solubility cofactor

14-3-3 is a ubiquitous protein family that interacts with several signal transduction kinases. We show that 14-3-3 proteins associate with keratin intermediate filament polypeptides 8 and 18 (K8/18) that are expressed in simple-type epithelia. The association is stoichiometrically significant (> or = one 14-3-3 molecule/keratin tetramer), occurs preferentially with K18, and is phosphorylation- and cell cycle-dependent in that it occurs during S/G2/M phases of the cell cycle when keratins become hyperphosphorylated. Binding of phospho-K8/18 to 14-3-3 can be reconstituted in vitro using recombinant 14-3-3 or using total cellular cytosol. Phosphatase treatment results in dissociation of 14-3-3, and dephosphorylation of phospho-K8/18 prevents reconstitution of the binding. Three cellular keratin subpopulations were analyzed that showed parallel gradients of keratin phosphorylation and 14-3-3 binding. Incubation of 14-3-3 with keratins during or after in vitro filament assembly results in sequestering of additional soluble keratin, only in cases when the keratins were hyperphosphorylated. Our results demonstrate a stoichiometrically significant cell cycle- and phosphorylation-regulated binding of 14-3-3 proteins to K18 and in vitro evidence of a simple epithelial keratin sequestering role for 14-3-3 proteins.

Dynamics of human keratin 18 phosphorylation: polarized distribution of phosphorylated keratins in simple epithelial tissues

Phosphorylation of keratin polypeptides 8 and 18 (K8/18) and other intermediate filament proteins results in their reorganization in vitro and in vivo. In order to study functional aspects of human K18 phosphorylation, we generated and purified a polyclonal antibody (termed 3055) that specifically recognizes a major phosphorylation site (ser52) of human K18 but not dephosphorylated K18 or a ser52-->ala K18 mutant. Pulse-chase experiments followed by immunoprecipitation and peptide mapping of in vivo 32PO4-labeled K8/18 indicated that the overall phosphorylation turnover rate is faster for K18 versus K8, and that ser52 of K18 is a highly dynamic phosphorylation site. Isoelectric focusing of 32PO4 labeled K18 followed by immunoblotting with 3055 showed that the major phosphorylated K18 species contain ser52 phosphorylation but that some K18 molecules exist that are preferentially phosphorylated on K18 sites other than ser52. Immunoblotting of total cell lysates obtained from cells at different stages of the cell cycle showed that ser52 phosphorylation increases three to fourfold during the S and G2/M phases of the cell cycle. Immunofluorescence staining of cells at different stages of mitosis, using 3055 or other antibodies that recognize the total keratin pool, resulted in preferential binding of the 3055 antibody to the reorganized keratin fraction. Staining of human tissues or tissues from transgenic mice that express human K18 showed that the phospho-ser52 K18 species are located preferentially in the basolateral and apical domains in the liver and pancreas, respectively, but no preferential localization was noted in other simple epithelial organs examined. Our results support a model whereby phosphorylated intermediate filaments are localized in specific cellular domains depending on the tissue type and site(s) of phosphorylation. In addition, ser52 of human K18 is a highly dynamic phosphorylation site that undergoes modulation during the S and G2/M phases of the cell cycle in association with filament reorganization.

Chronic hepatitis, hepatocyte fragility, and increased soluble phosphoglycokeratins in transgenic mice expressing a keratin 18 conserved arginine mutant

The two major intermediate filament proteins in glandular epithelia are keratin polypeptides 8 and 18 (K8/18). To evaluate the function and potential disease association of K18, we examined the effects of mutating a highly conserved arginine (arg89) of K18. Expression of K18 arg89-->his/cys and its normal K8 partner in cultured cells resulted in punctate staining as compared with the typical filaments obtained after expression of wild-type K8/18. Generation of transgenic mice expressing human K18 arg89-->cys resulted in marked disruption of liver and pancreas keratin filament networks. The most prominent histologic abnormalities were liver inflammation and necrosis that appeared at a young age in association with hepatocyte fragility and serum transaminase elevation. These effects were caused by the mutation since transgenic mice expressing wild-type human K18 showed a normal phenotype. A relative increase in the phosphorylation and glycosylation of detergent solubilized K8/18 was also noted in vitro and in transgenic animals that express mutant K18. Our results indicate that the highly conserved arg plays an important role in glandular keratin organization and tissue fragility as already described for epidermal keratins. Phosphorylation and glycosylation alterations in the arg mutant keratins may account for some of the potential changes in the cellular function of these proteins. Mice expressing mutant K18 provide a novel animal model for human chronic hepatitis, and for studying the tissue specific function(s) of K8/18.

Identification and mutational analysis of the glycosylation sites of human keratin 18

Keratin polypeptides 8 and 18 (K8/18) are intermediate filament phosphoglycoproteins that are expressed preferentially in glandular epithelia. We previously showed that K8/18 phosphorylation occurs on serine residues and that K8/18 glycosylation consists of O-linked single N-acetylglucosamines (O-GlcNAc) that are linked to Ser/Thr. Since the function of these modifications is unknown, we sought as a first step to identify the precise modification sites and asked if they play a role in keratin filament assembly. For this, we generated a panel of K18 Ser and Thr-->Ala mutants at potential glycosylation sites followed by expression in a baculovirus-insect cell system. We identified the major glycosylation sites of K18 by comparing the tryptic 3H-glycopeptide pattern of the panel of mutant and wild type K18 expressed in the insect cells with the glycopeptides of K18 in human colonic cells. The identified sites occur on three serines in the head domain of K18. The precise modified residues in human cells were verified using Edman degradation and confirmed further by the lack of glycosylation of a K18 construct that was mutated at the molecularly identified sites then transfected into NIH-3T3 cells. Partial or total K18 glycosylation mutants transfected into mammalian cells manifested nondistinguishable filament assembly to cells transfected with wild type K8/18. Our results show that K18 glycosylation sites share some features with other already identified O-GlcNAc sites and may together help predict glycosylation sites of other intermediate filament proteins.

Identification of the major physiologic phosphorylation site of human keratin 18: potential kinases and a role in filament reorganization

There is ample in vitro evidence that phosphorylation of intermediate filaments, including keratins, plays an important role in filament reorganization. In order to gain a better understanding of the function of intermediate filament phosphorylation, we sought to identify the major phosphorylation site of human keratin polypeptide 18 (K18) and study its role in filament assembly or reorganization. We generated a series of K18 ser-->ala mutations at potential phosphorylation sites, followed by expression in insect cells and comparison of the tryptic 32PO4-labeled patterns of the generated constructs. Using this approach, coupled with Edman degradation of the 32PO4-labeled tryptic peptides, and comparison with tryptic peptides analyzed after labeling normal human colonic tissues, we identified ser-52 as the major K18 physiologic phosphorylation site. Ser-52 in K18 is not glycosylated and matches consensus sequences for phosphorylation by CAM kinase, S6 kinase and protein kinase C, and all these kinases can phosphorylate K18 in vitro predominantly at that site. Expression of K18 ser-52-->ala mutant in mammalian cells showed minimal phosphorylation but no distinguishable difference in filament assembly when compared with wild-type K18. In contrast, the ser-52 mutation played a clear but nonexclusive role in filament reorganization, based on analysis of filament alterations in cells treated with okadaic acid or arrested at the G2/M stage of the cell cycle. Our results show that ser-52 is the major physiologic phosphorylation site of human K18 in interphase cells, and that its phosphorylation may play an in vivo role in filament reorganization.

Mechanism of S100 protein-dependent inhibition of glial fibrillary acidic protein (GFAP) polymerization

S100 protein, a subfamily of Ca(2+)-binding proteins of the EF-hand type, was recently shown to bind to and to inhibit the polymerization of the glial fibrillary acidic protein (GFAP), the intermediate filament component of astroglial cells, in the presence of micromolar levels of Ca2+ (J. Biol. Chem. 268, 12669-12674). By a sedimentation assay and viscometry we show here that S100 protein interferes with the very early steps of GFAP polymerization (nucleation) and with the GFAP polymer growth, thereby retarding the onset of GFAP assembly, reducing the rate and the extent of GFAP assembly, and increasing the critical concentration of GFAP assembly. Moreover, S100 protein disassembles preformed glial filaments. All the above effects can be explained by sequestration of soluble GFAP by S100 protein, as also indicated by the stoichiometry of S100 protein binding to GFAP and of S100 protein effects on GFAP assembly. Our data suggest that S100 protein might serve the function of avoiding excess GFAP polymerization and might participate in remodeling of glial filaments following elevation of the intracellular free Ca2+ concentration. Also, our data lend support to the notion that intermediate filaments are dynamic cytoskeleton structures that assemble and disassemble, and to the existence of cytoplasmic factors implicated in the regulation of the state of assembly of intermediate filaments.

Glial fibrillary acidic protein: dynamic property and regulation by phosphorylation

Glial fibrillary acidic protein (GFAP) is an intermediate filament (IF) protein of astroglia, and belongs to the type III subclass of IF proteins. IF proteins are composed of an amino-terminal HEAD domain, a central ROD domain and a carboxyterminal TAIL domain. GFAP, with a molecular mass of approximately 50 KDa, has the smallest HEAD domain among type III IF proteins. Despite its insolubility, GFAP is in dynamic equilibrium between assembled filaments and unassembled subunits, as demonstrated using fluorescently labeled GFAP molecules. Like other IF proteins, assembly of GFAP is regulated by phosphorylation-dephosphorylation of the HEAD domain by altering its charge. This regulation of GFAP assembly contributes to extensive remodeling of glial frameworks in mitosis. Another type III IF protein, vimentin, colocalizes with GFAP in immature, reactive or radial glia, thereby indicating that vimentin has an important role in the build up of the glial architecture.

Mitotic arrest with anti-microtubule agents or okadaic acid is associated with increased glycoprotein terminal GlcNAc's

The two major intermediate filament glycoproteins in human simple epithelia are keratins 8 and 18 (K8/18). A dramatic increase in terminal N-acetylglucosamine (GlcNAc) residues in K8/18 was previously noted after arresting cells in G2/M using anti-microtubule agents. Here we use in vitro galactosylation to show that increased terminal GlcNAc's is a general phenomenon that occurs in glycoproteins isolated from nuclear and plasma membrane fractions after cells are arrested in mitosis using colcemid, nocodazole, or okadaic acid. All three agents also resulted in a hyperphosphorylated form of K8 as determined by phosphatase treatment and tryptic phosphopeptide mapping. The altered glycosylation was found to be independent of microtubule disassembly, and was not directly related to the G2/M phase of the cell cycle after aphidicolin synchronization. Staurosporine (1 microM) inhibited K8/18 phosphorylation in okadaic acid- or nocodazole-treated cells, and inhibited the increase in K8/18 glycosylation without inhibiting the increase in terminal GlcNAc's of membrane-associated glycoproteins. In contrast, brefeldin A resulted in a dramatic increase in terminal GlcNAc's of membrane-associated but not intermediate filament proteins. Golgi complex-related staining using anti-beta-COP antibody showed significant fragmentation under conditions associated with altered membrane protein glycosylation. Our results suggest that Golgi disruption may be involved in the observed increase in terminal GlcNAc's of membrane but not intermediate filament glycoproteins. The mechanism of increased glycoprotein terminal GlcNAc's in association with mitotic arrest appears to be distinct for intermediate filaments and membrane-associated proteins, and in the case of intermediate filament proteins, phosphorylation may play an important role. Some of the effects of agents that induce mitotic arrest may be mediated by glycosylation changes.

Transient increase in vimentin phosphorylation and vimentin-HSC70 association in 9L rat brain tumor cells experiencing heat-shock

Characteristic changes in vimentin were studied in 9L rat brain tumor cells treated at 45 degrees C. During heat-shock treatment, vimentin molecules were rapidly phosphorylated and reorganized from a filamentous form into a perinuclear higher-order structure that was less extractable by nonionic detergent. These effects were found to be highly transient, peaked at 30 min after the onset of heat-shock treatment, and subsided thereafter. Simultaneously, the solubility of the constitutively expressed heat-shock protein 70 (HSC70) was also temporarily decreased and the kinetics was identical to that of vimentin. The results indicated that HSC70 and vimentin were co-insolubilized during the heat-shock treatment. We propose that the reorganization of the intermediate filaments resulted from enhanced phosphorylation of vimentin leads to the concurrent association of HSC70 to the intermediate filaments. This process may play an essential role in regulating heat-shock genes.

Characterization of keratin densities in mitotic WISH cells

Three dimensional (3-D) reconstruction of four mitotic WISH cells from ultrathin sections gave an informative representation of the spatial distribution of keratin densities in these cells. The correspondence between the densities as studied by transmission electron microscopy (TEM) and the keratin bodies initially revealed by immunoflourescent colabeling of cultures, was confirmed by immunoelectronmicroscopy. The smaller, and sometimes more elongated densities, were relatively abundant just beneath the subplasmalemmal microfilament band; and at certain levels of the mitotic cell they were observed to be connected to neighboring densities by intact intermediate filaments (IFs). The larger and more spherical densities appeared to be somewhat more discrete and randomly distributed. Other observed associations of the keratin densities included the telophase contractile ring of microfilaments, chromosomes, the reformed telophase nucleus, and desmosomal junctions with neighboring interphase cells. Cytochalasin D (CD) treatment of cells displaced the peripheral keratin densities toward the cell membrane. The density volume constituted 0.52% to 1.57% of the total cell volume, and the proportional density size was decreased in the cells that had progressed into anaphase and telophase. The observed formation and subsequent dissolution of keratin densities during mitosis may represent a dynamic mechanism of restructuring the keratin cytoskeleton in an unpolymerized form in order to allow for rapid reformation of interphase cell junctions. The physical associations observed between intact IFs and the keratin densities may provide support at certain depths of the mitotic cell, and the juxtaposition of densities with nuclear components suggests a possible source of and role for keratin IFs during nuclear events.

Confocal and conventional immunofluorescence and ultrastructural localisation of intracellular strength-giving components of human amniochorion

Key cytoskeletal polypeptides of human fetal membranes have been localised at subcellular level using confocal and conventional indirect immunofluorescence microscopy. Correlation with electron microscope data has allowed us to examine how cellular compartments of this multilaminar tissue maintain their mechanical integrity until the time of membrane rupture at parturition. Evidence is presented for myofibroblastic characteristics of cells in both the fibroblast and reticular layers which may therefore have tension-generating, position-adjustment and wound-healing roles in the amniochorion. Desmin and vimentin are coexpressed in these cells, but a small localised population of cells in the fibroblast layer contains vimentin alone. An interaction of cytokeratin filaments with nuclei and desmosomes of amniotic epithelium in vivo is demonstrated, indicating that nuclei of cells of ectodermal origin are integrated into a mechanical structure extending throughout the tissue as a whole. Cells of the basal 1 or 2 layers of trophoblast have been shown to have a more extensive and better integrated cytoskeletal organisation than those overlying and forming the boundary with decidua. Structures within the trophoblast, identified previously as degenerate villi, contain cells with intermediate filaments with similar immunofluorescence properties to those of the neighbouring reticular layer and thus may represent papillae that prevent shearing at this interface.

Vimentin serves as a phosphate sink during the apparent activation of protein kinases by okadaic acid in mammalian cells

The vimentin contents of four mammalian cell lines originating from rat and human tissues were determined by immunoblotting and scanning densitometry. On per cell volume basis, vimentin content in 9L, KD, and HeLa cells was found to be 206.6, 151.6, and 19.1 ng/microliters, respectively. A431 cells were devoid of vimentin. Protein phosphorylation was augmented by treatment of 600 nM okadaic acid for 1 h in these cells. During the apparent activation of protein kinases, vimentin became hyperphosphorylated and the phosphorylation level of other nonvimentin phosphoproteins was relatively little affected in 9L and KD cells. In contrast, cytokeratins and other nonvimentin proteins were heavily phosphorylated in OA-treated HeLa and A431 cells. Regression analysis indicated that the relative increase in phosphorylation level of nonvimentin phosphoproteins was inversely correlated to the contents of vimentin in the four cell lines [r2 = -0.985]. These observations strongly suggest that vimentin acts as a phosphate sink by which the effects of "excess kinase activity" inflicted by phosphatases inhibition was attenuated.

Transient requirement for vimentin in neuritogenesis: intracellular delivery of anti-vimentin antibodies and antisense oligonucleotides inhibit neurite initiation but not elongation of existing neurites in neuroblastoma

Vimentin is initially expressed by nearly all neuronal precursors in vivo, and is gradually replaced by neurofilaments shortly after the immature neurons become postmitotic (Cochard and Paulin, 1984, J Neurosci 4:2080; Tapscott et al., 1981, Dev Biol 86:40). A transient increase in neuritic vimentin filaments occurs within the first day of dbcAMP-mediated neurite induction in NB2a/d1 neuroblastoma, after which vimentin levels rapidly decline and neurofilaments increase (Shea, 1990, Brain Res 521:343). In the present study, we tested the possibility that vimentin filaments may function in neurite elaboration by inducing neuritogenesis under conditions where vimentin expression and assembly was inhibited. Intracellular delivery of anti-vimentin antiserum into transiently permeabilized NB2a/d1 cells prevented the initial elaboration of neurites, but did not retract existing neurites. By contrast, intracellular delivery of antiserum directed against the low molecular weight neurofilament subunit or normal rabbit antiserum did not affect neurite outgrowth. Treatment with vimentin antisense oligonucleotides reversibly depleted vimentin synthesis and steady-state levels, and prevented neurite initiation, but did not induce retraction of existing neurites. These findings point toward an hitherto undetected role for vimentin in the initiation of neurite outgrowth.

Protein phosphatase inhibitor calyculin A induces hyperphosphorylation of cytokeratins and inhibits amylase exocytosis in the rat parotid acini

Calyculin A, a protein phosphatase inhibitor with a chemical structure completely different from that of okadaic acid, reproduced the inhibitory effect of okadaic acid on cyclic AMP-mediated amylase release from rat parotid acinar cells. Calyculin A markedly enhanced phosphorylation of cytokeratins in the cytoskeletal fraction of the cells, whereas cAMP had apparently no effect on the phosphorylation. Microscopic observations showed that parotid acini incubated with 100 nM calyculin A for 15 min had large vacuoles in the cytoplasm and conspicuous blebs on the basal plasma membrane. K252a, a nonselective protein kinase inhibitor, clearly reduced calyclin A-induced phosphorylation of cytokeratins, and it markedly blocked the inhibition of amylase release and morphological changes evoked by calyculin A. These results suggest that hyperphosphorylation of cytokeratins profoundly affects the morphology and secretory activity of parotid acinar cells.

Role of phosphorylation in keratin and vimentin filament integrity in cultured thyroid epithelial cells

Cytokeratin and vimentin intermediate filaments (IFs) possess relatively stable polymeric properties which can be affected by phosphorylation. The present study, using cultures of thyroid epithelial cells, shows by indirect immunofluorescence that these cells contain both keratin tonofilament and vimentin IF complexes. Immunoblots of Triton X-100 insoluble cytoskeletal fractions show vimentin, and approximately 52 kDa type II and 40/38 kDa type I keratins. Under "basal" conditions, following prelabeling of cells with [32PO4], vimentin is not significantly phosphorylated, while both type II and I keratins are phosphorylated. Treatment of cells for 20 min with 1 mM dbcAMP or 0.4 microM 12-O-tetradecanoyl-phorbol-13-acetate (TPA), to stimulate protein kinase A and C, respectively, has no effect on either the phosphorylation state or cytoplasmic filament integrity of vimentin. However, while dbcAMP also does not affect keratin filaments, TPA increases both type II and I phosphorylation approximately 3-fold, and concomitantly disrupts tonofilament complexes associated with the nucleus, cytoplasm, and desmosomes. TPA-treated cells also show dramatic shape changes and protrusive activity. Tryptic peptide mappings show phosphorylations of at least 6 and approximately 2 additional sites for type II and I keratins, respectively, vs. [32P]-peptides from control cells. Treatment of [32PO4]-labeled cells with 0.4 microM calyculin A to inhibit types 1 and 2A phosphatase activity causes hyperphosphorylation of both vimentin and keratin, disruption of IF complexes, and actomyosin/cell contraction within 20 min. Quantitatively, approximately 50% of the type II/I keratin hyperphosphorylations are at some sites apparently also phosphorylated after TPA treatment. Thus, in these cells, IFs are specifically and differentially affected and regulated by the activity of several kinases.

PKC mediates 12(S)-HETE-induced cytoskeletal rearrangement in B16a melanoma cells

The fatty acid 12(S)-HETE may be a new second messenger capable of activating PKC. In tumor cells 12(S)-HETE stimulates cytoskeleton-dependent cellular responses such as adhesion and spreading. Analysis of 12(S)-HETE effects on B16a melanoma cell cytoskeleton revealed reversible rearrangement of microtubules, microfilaments, the actin-binding proteins, vinculin, myosin heavy (MHC) and light chains (MLC), as well as bundling of vimentin intermediate filaments. The alterations in microfilaments and intermediate filaments occurred very rapidly, i.e., 5 min after exposure of tumor cells to 12(S)-HETE. The 12(S)-HETE-induced cytoskeletal alterations were accompanied by centrifugal organelle-translocation. Interestingly, MLC exhibited clear association with the cytoplasmic organelles. Biochemical analysis of the 12(S)-HETE effect indicated a PKC-mediated reversible hyperphosphorylation of MLC, vimentin, and a 130 kD cytoskeletal-associated protein. Optimal effects were obtained after 5 min treatment with 12(S)-HETE at 0.1 microM concentration. 12(S)-HETE pretreatment induced tumor cell spreading on a fibronectin matrix which required the intactness of all three major cytoskeletal components. The spreading process was dependent upon the activity of PKC. Our data suggest that 12(S)-HETE is a physiological stimulant of PKC. Further, it induces rearrangement of the cytoskeleton of tumor cells in interphase resulting in the stimulation of cytoskeleton-dependent cell activity such as spreading.

Cell cycle-dependent changes in the organization of an intermediate filament-associated protein: correlation with phosphorylation by p34cdc2

During mitosis in BHK-21 baby hamster kidney cells the hyperphosphorylation of the type III intermediate filament (IF) protein vimentin is accompanied by the disruption of the IF network into punctate, protofilamentous structures. In this study, the morphological and biochemical changes of IFAP 300, a 300-kDa IF-crossbridging protein, are examined during mitosis. Double-label immunofluorescence shows that the distribution of IFAP 300 coincides with the typical filamentous pattern displayed by vimentin in interphase cells, whereas in mitotic cells it is reorganized into a punctate, nonfilamentous pattern. Accompanying these latter morphological changes, IFAP 300 is phosphorylated at a unique, mitosis-specific site. Comparison of the sites phosphorylated in cultured cells with those phosphorylated in vitro by various kinases suggests that IFAP 300 is phosphorylated by the same two kinases that phosphorylate vimentin during mitosis. One of these is p34cdc2 protein kinase, which appears to be responsible for the phosphorylation of the mitosis-specific site. The other kinase phosphorylates IFAP 300 in vitro at a site that is also found in the protein immunoprecipitated from either mitotic or interphase cells. In contrast to vimentin, the phosphorylation levels of IFAP 300 are not obviously altered between interphase and mitosis. Our results show that IFAP 300 is a physiological substrate for p34cdc2 and that this kinase may be involved in the mitotic reorganization of IFAP 300 by phosphorylating a mitosis-specific site. Taken together with our previous results, this study suggests that the activation of p34cdc2 coordinates the mitotic reorganization of the vimentin IF network both by severing IF-IF connections mediated by IFAP 300 and by disassembling individual IFs into protofilaments.

Host cell factors controlling vimentin organization in the Xenopus oocyte

To study vimentin filament organization in vivo we injected Xenopus oocytes, which have no significant vimentin system of their own, with in vitro-synthesized RNAs encoding Xenopus vimentins. Exogenous vimentins were localized primarily to the cytoplasmic surface of the nucleus and to the subplasma membrane "cortex." In the cortex of the animal hemisphere, wild-type vimentin forms punctate structures and short filaments. In contrast, long anastomosing vimentin filaments are formed in the vegetal hemisphere cortex. This asymmetry in the organization of exogenous vimentin is similar to that of the endogenous keratin system (Klymkowsky, M. W., L. A. Maynell, and A. G. Polson. 1987. Development (Camb.). 100:543-557), which suggests that the same cellular factors are responsible for both. Before germinal vesicle breakdown, in the initial stage of oocyte maturation, large vimentin and keratin filament bundles appear in the animal hemisphere. As maturation proceeds, keratin filaments fragment into soluble oligomers (Klymkowsky, M. W., L. A. Maynell, and C. Nislow. 1991. J. Cell Biol. 114:787-797), while vimentin filaments remain intact and vimentin is hyperphosphorylated. To examine the role of MPF kinase in the M-phase reorganization of vimentin we deleted the conserved proline of vimentin's single MPF-kinase site; this mutation had no apparent effect on the prophase or M-phase behavior of vimentin. In contrast, deletion of amino acids 19-68 or 18-61 of the NH2-terminal "head" domain produced proteins that formed extended filaments in the animal hemisphere of the prophase oocyte. We suggest that the animal hemisphere cortex of the prophase oocyte contains a factor that actively suppresses the formation of extended vimentin filaments through a direct interaction with vimentin's head domain. During maturation this "suppressor of extended filaments" appears to be inactivated, leading to the formation of an extended vimentin filament system.

Differential phosphorylation of CK8 and CK18 by 12-O-tetradecanoyl-phorbol-13-acetate in primary cultures of mouse hepatocytes

The phosphorylation of cytokeratin was investigated in primary cultures of hepatocytes. The two hepatocyte cytokeratins CK8 and CK18 (55,000 and 49,000 M(r) respectively) were phosphorylated, CK8 being more phosphorylated than CK18. Treatment of the hepatocytes with 150 nM 12-O-tetradecanoyl-phorbol-13-acetate (TPA) an activator of protein kinase C induced a transient increase in the level of phosphorylation of CK8 but not CK18. This effect was maximal after 15 min of TPA treatment and was maintained for up to 3 h. After 22 h of treatment with TPA, which down-regulates protein kinase C, CK8 phosphorylation was returned to the basal level. Further addition of TPA to the 22-h treated cells did not cause an increase in CK8 phosphorylation. Indirect immunofluorescence microscopy with a monoclonal antibody to CK8 indicated that while the addition of TPA induced the formation of granular cytokeratin aggregates in some hepatocytes, in most hepatocytes no major changes in the intermediate filament network were observed. Staining for actin showed that actin microfilaments were rapidly reorganized after the treatment and a loss of stress fibres were observed. We propose that CK8 is an in vivo substrate for protein kinase C and that the specific phosphorylation of CK8 plays a role in protein kinase C signal transduction.