Stress, apoptosis, and mitosis induce phosphorylation of human keratin 8 at Ser-73 in tissues and cultured cells

Desmoplakin Maintains Transcellular Keratin Scaffolding and Protects From Intestinal Injury

BACKGROUND & AIMS

Desmosomes are intercellular junctions connecting keratin intermediate filaments of neighboring cells. The cadherins desmoglein 2 (Dsg2) and desmocollin 2 mediate cell-cell adhesion, whereas desmoplakin (Dsp) provides the attachment of desmosomes to keratins. Although the importance of the desmosome-keratin network is well established in mechanically challenged tissues, we aimed to assess the currently understudied function of desmosomal proteins in intestinal epithelia.

METHODS

We analyzed the intestine-specific villin-Cre DSP (DSP^ΔIEC) and the combined intestine-specific DSG2/DSP^ΔIEC (ΔDsg2/Dsp) knockout mice. Cross-breeding with keratin 8-yellow fluorescent protein knock-in mice and generation of organoids was performed to visualize the keratin network. A Dsp-deficient colorectal carcinoma HT29-derived cell line was generated and the role of Dsp in adhesion and mechanical stress was studied in dispase assays, after exposure to uniaxial cell stretching and during scratch assay.

RESULTS

The intestine of DSP^ΔIEC mice was histopathologically inconspicuous. Intestinal epithelial cells, however, showed an accelerated migration along the crypt and an enhanced shedding into the lumen. Increased intestinal permeability and altered levels of desmosomal proteins were detected. An inconspicuous phenotype also was seen in ΔDsg2/Dsp mice. After dextran sodium sulfate treatment, DSP^ΔIEC mice developed more pronounced colitis. A retracted keratin network was seen in the intestinal epithelium of DSP^ΔIEC/keratin 8-yellow fluorescent protein mice and organoids derived from these mice presented a collapsed keratin network. The level, phosphorylation status, and solubility of keratins were not affected. Dsp-deficient HT29 cells had an impaired cell adhesion and suffered from increased cellular damage after stretch.

CONCLUSIONS

Our results show that Dsp is required for proper keratin network architecture in intestinal epithelia, mechanical resilience, and adhesion, thereby protecting from injury.

Keratin 8 mutations in transgenic mice predispose to lung injury

Keratin 8 (K8) is the cytoskeletal intermediate filament protein of simple-type epithelia. Mutations in K8 predispose the affected individual and transgenic mouse to liver disease. However, the role of K8 in the lung has not been reported in mutant transgenic mouse models. Here, we investigated the susceptibility of two different transgenic mice expressing K8 Gly62-Cys (Gly62 replaced with Cys) or Ser74-Ala (Ser74 replaced with Ala) to lung injury. The mutant transgenic mice were highly susceptible to two independent acute and chronic lung injuries compared with control mice. Both K8 Gly62-Cys mice and K8 Ser74-Ala mice showed markedly increased mouse lethality (∼74% mutant mice versus ∼34% control mice) and more severe lung damage, with increased inflammation and apoptosis, under L-arginine-mediated acute lung injury. Moreover, the K8 Ser74-Ala mice had more severe lung damage, with extensive hemorrhage and prominent fibrosis, under bleomycin-induced chronic lung injury. Our study provides the first direct evidence that K8 mutations predispose to lung injury in transgenic mice.

Alterations of endometrial epithelial-mesenchymal transition and MAPK signalling components in women with PCOS are partially modulated by metformin in vitro

Growing evidence suggests that epithelial-mesenchymal transition (EMT) and its regulator mitogen-activated protein kinase (MAPK) contribute to endometria-related reproductive disorders. However, the regulation of EMT and MAPK signalling components in the endometrium from polycystic ovary syndrome (PCOS) patients has not been systematically investigated and remains elusive. In humans, how metformin induces molecular alterations in the endometrial tissues under PCOS conditions is not completely clear. Here, we recruited 7 non-PCOS patients during the proliferative phase (nPCOS), 7 non-PCOS patients with endometrial hyperplasia (nPCOSEH), 14 PCOS patients during the proliferative phase (PCOS) and 3 PCOS patients with endometrial hyperplasia (PCOSEH). Our studies demonstrated that compared with nPCOS, PCOS patients showed decreased Claudin 1 and increased Vimentin and Slug proteins. Similar to increased Slug protein, nPCOSEH and PCOSEH patients showed increased N-cadherin protein. Western blot and immunostaining revealed increased epithelial phosphorylated Cytokeratin 8 (p-CK 8) expression and an increased p-CK 8:CK 8 ratio in PCOS, nPCOSEH and PCOSEH patients compared to nPCOS patients. Although nPCOSEH and PCOSEH patients showed increased p-ERK1/2 and/or p38 protein levels, the significant increase in p-ERK1/2 expression and p-ERK1/2:ERK1/2 ratio was only found in PCOS patients compared to nPCOS patients. A significant induction of the membrane ERβ immunostaining was observed in the epithelial cells of PCOS and PCOSEH patients compared to nPCOS and nPCOSEH patients. While in vitro treatment with metformin alone increased Snail and decreased Claudin 1, N-cadherin and α-SMA proteins, concomitant treatment with metformin and E2 increased the expression of CK 8 and Snail proteins and decreased the expression of Claudin 1, ZO-1, Slug and α-SMA proteins. Our findings suggest that the EMT contributes to the switch from a healthy state to a PCOS state in the endometrium, which might subsequently drive endometrial injury and dysfunction. We also provide evidence that metformin differentially modulates EMT protein expression in PCOS patients depending on oestrogenic stimulation.

Generation of a tissue-specific transgenic model for K8 phosphomutants: A tool to investigate the role of K8 phosphorylation during skin carcinogenesis in vivo

Keratin 8/18, the predominant keratin pair of simple epithelia, is known to be aberrantly expressed in several squamous cell carcinomas (SCCs), where its expression is often correlated with increased invasion, neoplastic progression, and poor prognosis. The majority of keratin 8/18 structural and regulatory functions are governed by posttranslational modifications, particularly phosphorylation. Apart from filament reorganization, cellular processes including cell cycle, cell growth, cellular stress, and apoptosis are known to be orchestrated by K8 phosphorylation at specific residues in the head and tail domains. Even though deregulation of K8 phosphorylation at two significant sites (Serine⁷³ /Serine⁴³¹ ) has been implicated in neoplastic progression of SCCs by various in vitro studies, including ours, it is reported to be highly context-dependent. Therefore, to delineate the precise role of Kereatin 8 phosphorylation in cancer initiation and progression, we have developed the tissue-specific transgenic mouse model expressing Keratin 8 wild type and phosphodead mutants under Keratin 14 promoter. Subjecting these mice to 7,12-dimethylbenz(a)anthracene/12-O-tetradecanoylphorbol-13-acetate-mediated skin carcinogenesis revealed that Keratin 8 phosphorylation may lead to an early onset of tumors compared to Keratin 8 wild-type expressing mice. Conclusively, the transgenic mouse model developed in the present study ascertained a positive impact of Keratin 8 phosphorylation on the neoplastic transformation of skin-squamous cells.

Vitamin D Promotes Skeletal Muscle Regeneration and Mitochondrial Health

Vitamin D is an essential nutrient for the maintenance of skeletal muscle and bone health. The vitamin D receptor (VDR) is present in muscle, as is CYP27B1, the enzyme that hydroxylates 25(OH)D to its active form, 1,25(OH)D. Furthermore, mounting evidence suggests that vitamin D may play an important role during muscle damage and regeneration. Muscle damage is characterized by compromised muscle fiber architecture, disruption of contractile protein integrity, and mitochondrial dysfunction. Muscle regeneration is a complex process that involves restoration of mitochondrial function and activation of satellite cells (SC), the resident skeletal muscle stem cells. VDR expression is strongly upregulated following injury, particularly in central nuclei and SCs in animal models of muscle injury. Mechanistic studies provide some insight into the possible role of vitamin D activity in injured muscle. In vitro and in vivo rodent studies show that vitamin D mitigates reactive oxygen species (ROS) production, augments antioxidant capacity, and prevents oxidative stress, a common antagonist in muscle damage. Additionally, VDR knockdown results in decreased mitochondrial oxidative capacity and ATP production, suggesting that vitamin D is crucial for mitochondrial oxidative phosphorylation capacity; an important driver of muscle regeneration. Vitamin D regulation of mitochondrial health may also have implications for SC activity and self-renewal capacity, which could further affect muscle regeneration. However, the optimal timing, form and dose of vitamin D, as well as the mechanism by which vitamin D contributes to maintenance and restoration of muscle strength following injury, have not been determined. More research is needed to determine mechanistic action of 1,25(OH)D on mitochondria and SCs, as well as how this action manifests following muscle injury in vivo. Moreover, standardization in vitamin D sufficiency cut-points, time-course study of the efficacy of vitamin D administration, and comparison of multiple analogs of vitamin D are necessary to elucidate the potential of vitamin D as a significant contributor to muscle regeneration following injury. Here we will review the contribution of vitamin D to skeletal muscle regeneration following injury.

Acute effects of cell stretch on keratin filaments in A549 lung cells

Keratin filaments (KFs) comprise the intermediate filaments of epithelial cells and are well known for their cytoprotective properties and their mechanical resilience. Although, several studies have demonstrated KFs' remarkable tensile properties relatively little is known about acute implications of mechanical stretch on KFs in living cells. This includes structural effects on the KFs and their higher level assembly structures as well as posttranslational response mechanisms to possibly modify KF's properties. We subjected simple epithelial A549 lung cells to 30% unidirectional stretch and already after 10 seconds we observed morphological changes of the KF-network as well as structural effects on their desmosomal anchor sites-both apparently caused by the tensile strain. Interestingly, the effect on the desmosomes was attenuated after 30 seconds of cell stretch with a concomitant increase in phosphorylation of keratin8-S432, keratin18-S53, and keratin18-S34 without an apparent increase in keratin solubility. When mimicking the phosphorylation of keratin18-S34 the stretch-induced effect on the desmosomes could be diminished and probing the cell surface with atomic force microscopy showed a lowered elastic modulus. We conclude that the stretch-induced KF phosphorylation affects KF's tensile properties, probably to lower the mechanical load on strained desmosomal cell-cell contacts, and hence, preserve epithelial integrity.

Knockdown of cytokeratin 8 overcomes chemoresistance of chordoma cells by aggravating endoplasmic reticulum stress through PERK/eIF2α arm of unfolded protein response and blocking autophagy

Chordoma is a malignant primary osseous spinal tumor with pronounced chemoresistance. However, the mechanisms of how chordoma cells develop chemoresistance are still not fully understood. Cytokeratin 8 (KRT8) is a molecular marker of notochordal cells, from which chordoma cells were believed to be originated. In this study, we showed that either doxorubicin or irinotecan promoted KRT8 expression in both CM319 and UCH1 cell lines, accompanied by an increased unfolded protein response and autophagy activity. Then, siRNA-mediated knockdown of KRT8 chemosensitized chordoma cells in vitro. Mechanistic studies showed that knockdown of KRT8 followed by chemotherapy aggravated endoplasmic reticulum stress through PERK/eIF2α arm of unfolded protein response and blocked late-stage autophagy. Moreover, suppression of the PERK/eIF2α arm of unfolded protein response using PERK inhibitor GSK2606414 partially rescued the apoptotic chordoma cells but did not reverse the blockage of the autophagy flux. Finally, tumor xenograft model further confirmed the chemosensitizing effects of siKRT8. This study represents the first systematic investigation into the role of KRT8 in chemoresistance of chordoma and our results highlight a possible strategy of targeting KRT8 to overcome chordoma chemoresistance.

Liver disease-associated keratin 8 and 18 mutations modulate keratin acetylation and methylation

Keratin 8 (K8) and keratin 18 (K18) are the intermediate filament proteins whose phosphorylation/transamidation associate with their aggregation in Mallory-Denk bodies found in patients with various liver diseases. However, the functions of other post-translational modifications in keratins related to liver diseases have not been fully elucidated. Here, using a site-specific mutation assay combined with nano-liquid chromatography-tandem mass spectrometry, we identified K8-Lys108 and K18-Lys187/426 as acetylation sites, and K8-Arg47 and K18-Arg55 as methylation sites. Keratin mutation (Arg-to-Lys/Ala) at the methylation sites, but not the acetylation sites, led to decreased stability of the keratin protein. We compared keratin acetylation/methylation in liver disease-associated keratin variants. The acetylation of K8 variants increased or decreased to various extents, whereas the methylation of K18-del65-72 and K18-I150V variants increased. Notably, the highly acetylated/methylated K18-I150V variant was less soluble and exhibited unusually prolonged protein stability, which suggests that additional acetylation of highly methylated keratins has a synergistic effect on prolonged stability. Therefore, the different levels of acetylation/methylation of the liver disease-associated variants regulate keratin protein stability. These findings extend our understanding of how disease-associated mutations in keratins modulate keratin acetylation and methylation, which may contribute to disease pathogenesis.-Jang, K.-H., Yoon, H.-N., Lee, J., Yi, H., Park, S.-Y., Lee, S.-Y., Lim, Y., Lee, H.-J., Cho, J.-W., Paik, Y.-K., Hancock, W. S., Ku, N.-O. Liver disease-associated keratin 8 and 18 mutations modulate keratin acetylation and methylation.

KRT1 gene silencing ameliorates myocardial ischemia-reperfusion injury via the activation of the Notch signaling pathway in mouse models

Myocardial ischemia and reperfusion injury (MIRI) includes major drawbacks, such as excessive formation of free radicals and also overload of calcium, which lead to cell death, tissue scarring, and remodeling. The current study aims to explore whether KRT1 silencing may ameliorate MIRI via the Notch signaling pathway in mouse models. Myocardial tissues were used for the determination of the positive rate of KRT1 protein expression, apoptosis of myocardial cells, creatine kinase (CK) and lactate dehydrogenase (LDH) expression, expression of related biomarkers as well as myocardial infarction area. The transfected myocardial cells were treated with KRT1-siRNA, Jagged1, and DAPT (inhibitor of Notch-1 signaling pathway). The expression of KRT1, NICD, Hes1, Bcl-2, and Bax protein was detected. The MTT assay was applied for cell proliferation and flow cytometry was used for cell apoptosis. Mice with MIRI had a higher positive rate of KRT1 protein expression, apoptosis of myocardial cells, CK and LDH expression, myocardial infarction area, increased expression of MDA, NO, SDH, IL-1, IL-6, TNF-α, CRP, KRT1, Bax protein, CK, and LDH, and decreased expression of SOD, NICD, Hes1, and Bcl-2. The downregulation of KRT1 led to decreased expression of KRT1 and Bax protein, increased expression of NICD, Hes1, and Bcl-2, decreased cell apoptosis, and improved cell proliferation. The inhibition of the Notch signaling pathway leads to reduced expression of Bax, increased expression of NICD, Hes1, and Bcl 2, and also decreased cell apoptosis and increased cell proliferation. Our data conclude that KRT1 silencing is able to make MIRI better by activating the Notch signaling pathway in mice.

14-3-3 proteins regulate desmosomal adhesion via plakophilins

Desmosomes are essential for strong intercellular adhesion and are abundant in tissues exposed to mechanical strain. At the same time, desmosomes need to be dynamic to allow for remodeling of epithelia during differentiation or wound healing. Phosphorylation of desmosomal plaque proteins appears essential for desmosome dynamics. However, the mechanisms how context-dependent post-translational modifications regulate desmosome formation, dynamics or stability are incompletely understood. Here, we show that growth factor signaling regulates the phosphorylation-dependent association of plakophilins 1 and 3 with 14-3-3 protein isoforms and uncover unique and partially antagonistic functions of members of the 14-3-3 family in the regulation of desmosomes. 14-3-3γ associated primarily with cytoplasmic plakophilin 1 phosphorylated at S155 and destabilized intercellular cohesion of keratinocytes by reducing its incorporation into desmosomes. In contrast, stratifin/14-3-3σ interacted preferentially with S285-phosphorylated plakophilin 3 to promote its accumulation at tricellular contact sites, leading to stable desmosomes. Taken together, our study identifies a new layer of regulation of intercellular adhesion by 14-3-3 proteins.

Threonine 150 Phosphorylation of Keratin 5 Is Linked to Epidermolysis Bullosa Simplex and Regulates Filament Assembly and Cell Viability

A characteristic feature of the skin blistering disease epidermolysis bullosa simplex is keratin filament (KF) network collapse caused by aggregation of the basal epidermal keratin type II (KtyII) K5 and its type I partner keratin 14 (K14). Here, we examine the role of keratin phosphorylation in KF network rearrangement and cellular functions. We detect phosphorylation of the K5 head domain residue T150 in cytoplasmic epidermolysis bullosa simplex granules containing R125C K14 mutants. Expression of phosphomimetic T150D K5 mutants results in impaired KF formation in keratinocytes. The phenotype is enhanced upon combination with other phosphomimetic K5 head domain mutations. Remarkably, introduction of T150D K5 mutants into KtyII-lacking (KtyII^-/-) keratinocytes prevents keratin network formation altogether. In contrast, phosphorylation-deficient T150A K5 leads to KFs with reduced branching and turnover. Assembly of T150D K5 is arrested at the heterotetramer stage coinciding with increased heat shock protein association. Finally, reduced cell viability and elevated response to stressors is noted in T150 mutant cells. Taken together, our findings identify T150 K5 phosphorylation as an important determinant of KF network formation and function with a possible role in epidermolysis bullosa simplex pathogenesis.

Three-dimensional Organization of Layered Apical Cytoskeletal Networks Associated with Mouse Airway Tissue Development

The cytoskeleton is an essential cellular component that enables various sophisticated functions of epithelial cells by forming specialized subcellular compartments. However, the functional and structural roles of cytoskeletons in subcellular compartmentalization are still not fully understood. Here we identified a novel network structure consisting of actin filaments, intermediate filaments, and microtubules directly beneath the apical membrane in mouse airway multiciliated cells and in cultured epithelial cells. Three-dimensional imaging by ultra-high voltage electron microscopy and immunofluorescence revealed that the morphological features of each network depended on the cell type and were spatiotemporally integrated in association with tissue development. Detailed analyses using Odf2 mutant mice, which lack ciliary basal feet and apical microtubules, suggested a novel contribution of the intermediate filaments to coordinated ciliary beating. These findings provide a new perspective for viewing epithelial cell differentiation and tissue morphogenesis through the structure and function of apical cytoskeletal networks.

Autophagy and KRT8/keratin 8 protect degeneration of retinal pigment epithelium under oxidative stress

Contribution of autophagy and regulation of related proteins to the degeneration of retinal pigment epithelium (RPE) in age-related macular degeneration (AMD) remain unknown. We report that upregulation of KRT8 (keratin 8) as well as its phosphorylation are accompanied with autophagy and attenuated with the inhibition of autophagy in RPE cells under oxidative stress. KRT8 appears to have a dual role in RPE pathophysiology. While increased expression of KRT8 following autophagy provides a cytoprotective role in RPE, phosphorylation of KRT8 induces pathologic epithelial-mesenchymal transition (EMT) of RPE cells under oxidative stress, which is mediated by MAPK1/ERK2 (mitogen-activated protein kinase 1) and MAPK3/ERK1. Inhibition of autophagy further promotes EMT, which can be reversed by inhibition of MAPK. Thus, regulated enhancement of autophagy with concurrent increased expression of KRT8 and the inhibition of KRT8 phosphorylation serve to inhibit oxidative stress-induced EMT of RPE cells as well as to prevent cell death, suggesting that pharmacological manipulation of KRT8 upregulation through autophagy with combined inhibition of the MAPK1/3 pathway may be attractive therapeutic strategies for the treatment of AMD.

Comparative proteomic analysis of paclitaxel resistance-related proteins in human breast cancer cell lines

Paclitaxel is widely used to treat various cancers; however, resistance to this drug is a major obstacle to breast cancer chemotherapy. To identify the proteins involved in paclitaxel resistance, the present study compared the proteomes of MCF-7 human breast cancer cells and its paclitaxel-resistant subclone MCF-7/PTX. Using two-dimensional gel electrophoresis and matrix-assisted laser desorption/ionization time of flight mass spectrometry, 11 upregulated and 12 downregulated proteins were identified in MCF-7/PTX cells compared with the parental cell line. These 23 proteins were functionally classified as stress-induced chaperones, metabolic enzymes and cytoskeletal proteins. The anti-apoptotic proteins, stress-70 protein, 78-kD glucose-regulated protein, peptidyl-prolyl cis-trans isomerase A (PPIA) and heterogeneous nuclear ribonucleoprotein H3, were also upregulated in MCF-7/PTX cells. Notably, knockdown of the stress-response chaperone PPIA using small interfering RNA in MCF-7/PTX cells restored their sensitivity to paclitaxel. These findings indicated that PPIA may have an important role in paclitaxel resistance in MCF-7/PTX cells.

Assays to Study Consequences of Cytoplasmic Intermediate Filament Mutations: The Case of Epidermal Keratins

The discovery of the causative link between keratin mutations and a growing number of human diseases opened the way for a better understanding of the function of the whole intermediate filament families of cytoskeleton proteins. This chapter describes analytical approaches to identification and interpretation of the consequences of keratin mutations, from the clinical and diagnostic level to cells in tissue culture. Intermediate filament pathologies can be accurately diagnosed from skin biopsies and DNA samples. The Human Intermediate Filament Database collates reported mutations in intermediate filament genes and their diseases, and can help clinicians to establish accurate diagnoses, leading to disease stratification for genetic counseling, optimal care delivery, and future mutation-aligned new therapies. Looking at the best-studied keratinopathy, epidermolysis bullosa simplex, the generation of cell lines mimicking keratinopathies is described, in which tagged mutant keratins facilitate live-cell imaging to make use of today's powerful enhanced light microscopy modalities. Cell stress assays such as cell spreading and cell migration in scratch wound assays can interrogate the consequences of the compromised cytoskeletal network. Application of extrinsic stresses, such as heat, osmotic, or mechanical stress, can enhance the differentiation of mutant keratin cells from wild-type cells. To bring the experiments to the next level, 3D organotypic human cultures can be generated, and even grafted onto the backs of immunodeficient mice for greater in vivo relevance. While development of these assays has focused on mutant K5/K14 cells, the approaches are often applicable to mutations in other intermediate filaments, reinforcing fundamental commonalities in spite of diverse clinical pathologies.

Inflammation decreases keratin level in ulcerative colitis; inadequate restoration associates with increased risk of colitis-associated cancer

Background

Keratins are intermediate filament (IF) proteins, which form part of the epithelial cytoskeleton and which have been implicated pathology of inflammatory bowel diseases (IBD).

Methods

In this study biopsies were obtained from IBD patients grouped by disease duration and subtype into eight categories based on cancer risk and inflammatory status: quiescent recent onset (<5 years) UC (ROUC); UC with primary sclerosing cholangitis; quiescent long-standing pancolitis (20–40 years) (LSPC); active colitis and non-inflamed proximal colonic mucosa; pancolitis with dysplasia-both dysplastic lesions (DT) and distal rectal mucosa (DR); control group without pathology. Alterations in IF protein composition across the groups were determined by quantitative proteomics. Key protein changes were validated by western immunoblotting and immunohistochemical analysis.

Result

Acute inflammation resulted in reduced K8, K18, K19 and VIM (all p<0.05) compared to controls and non inflamed mucosa; reduced levels of if– associated proteins were also seen in DT and DR. Increased levels of keratins in LSPC was noted relative to controls or ROUC (K8, K18, K19 and VIM, p<0.05). Multiple K8 forms were noted on immunoblotting, with K8 phosphorylation reduced in progressive disease along with an increase in VIM:K8 ratio. K8 levels and phosphorylation are reduced in acute inflammation but appear restored or elevated in subjects with clinical and endoscopic remission (LSPC) but not apparent in subjects with elevated risk of cancer.

Conclusions

These data suggest that keratin regulation in remission may influence subsequent cancer risk.

Comparative proteomics analysis of differential proteins in respond to doxorubicin resistance in myelogenous leukemia cell lines

Background

Chemoresistance remains a significant challenge in chronic myelogenous leukemia (CML) management, which is one of the most critical prognostic factors. Elucidation the molecular mechanisms underlying the resistance to chemoresistance may lead to better clinical outcomes.

Results

In order to identify potential protein targets involved in the drug-resistant phenotype of leukemia, especially the chronic myelogenous leukemia (CML), we used a high-resolution “ultra-zoom” 2DE-based proteomics approach to characterize global protein expression patterns in doxorubicin-resistant myelogenous leukemia cells compared with parental control cells. Ultra-high resolution of 2DE was achieved by using a series of slightly overlapping narrow-range IPG strips during isoelectric focusing (IEF) separation. A total number of 44 proteins with altered abundances were detected and identified by MALDI-TOF or LC-MS/MS. Among these proteins, enolase, aldolase, HSP70 and sorcin were up-regulated in doxorubicin-resistant myelogenous leukemia cell line, whereas HSP27 was down-regulated. Some of the results have been validated by Western blotting. Both enolase and aldolase were first reported to be involved in chemoresistance, suggesting that process of glycolysis in doxorubicin-resistant myelogenous leukemia cells was accelerated to some extent to provide more energy to survive chemical stress. Possible roles of most of the identified proteins in development of chemoresistance in myelogenous leukemia cells were fully discussed. The results presented here could provide clues to further study for elucidating the mechanisms underlying drug resistance in leukemia.

Conclusions

As a whole, under the chemical stress, the doxorubicin-resistant myelogenous leukemia cells may employ various protective strategies to survive. These include: (i) pumping the cytotoxic drug out of the cells by P-glycoprotein, (ii) increased storage of fermentable fuel, (iii) sophisticated cellular protection by molecular chaperones, (iv) improved handling of intracellular calcium, (v) increased glucose utilization via increased rates of glycolysis. In the present study, proteomic analysis of leukemia cells and their drug resistant variants revealed multiple alterations in protein expression. Our results indicate that the development of drug resistance in doxorubicin-resistant myelogenous leukemia cells is a complex phenomenon undergoing several mechanisms.

Electronic supplementary material

The online version of this article (doi:10.1186/s12953-014-0057-y) contains supplementary material, which is available to authorized users.

Directed expression of a chimeric type II keratin partially rescues keratin 5-null mice

The crucial role of structural support fulfilled by keratin intermediate filaments (IFs) in surface epithelia likely requires that they be organized into cross-linked networks. For IFs comprised of keratins 5 and 14 (K5 and K14), which occur in basal keratinocytes of the epidermis, formation of cross-linked bundles is, in part, self-driven through cis-acting determinants. Here, we targeted the expression of a bundling-competent KRT5/KRT8 chimeric cDNA (KRT8bc) or bundling-deficient wild type KRT8 as a control to the epidermal basal layer of Krt5-null mice to assess the functional importance of keratin IF self-organization in vivo. Such targeted expression of K8bc rescued Krt5-null mice with a 47% frequency, whereas K8 completely failed to do so. This outcome correlated with lower than expected levels of K8bc and especially K8 mRNA and protein in the epidermis of E18.5 replacement embryos. Ex vivo culture of embryonic skin keratinocytes confirmed the ability of K8bc to form IFs in the absence of K5. Additionally, electron microscopy analysis of E18.5 embryonic skin revealed that the striking defects observed in keratin IF bundling, cytoarchitecture, and mitochondria are partially restored by K8bc expression. As young adults, viable KRT8bc replacement mice develop alopecia and chronic skin lesions, indicating that the skin epithelia are not completely normal. These findings are consistent with a contribution of self-mediated organization of keratin IFs to structural support and cytoarchitecture in basal layer keratinocytes of the epidermis and underscore the importance of context-dependent regulation for keratin genes and proteins in vivo.

Post-translational modifications of intermediate filament proteins: mechanisms and functions

Intermediate filaments (IFs) are cytoskeletal and nucleoskeletal structures that provide mechanical and stress-coping resilience to cells, contribute to subcellular and tissue-specific biological functions, and facilitate intracellular communication. IFs, including nuclear lamins and those in the cytoplasm (keratins, vimentin, desmin, neurofilaments and glial fibrillary acidic protein, among others), are functionally regulated by post-translational modifications (PTMs). Proteomic advances highlight the enormous complexity and regulatory potential of IF protein PTMs, which include phosphorylation, glycosylation, sumoylation, acetylation and prenylation, with novel modifications becoming increasingly appreciated. Future studies will need to characterize their on-off mechanisms, crosstalk and utility as biomarkers and targets for diseases involving the IF cytoskeleton.

Changes in protein composition of epidermal mucus in turbot Scophthalmus maximus (L.) under high water temperature

To explore the skin mucous of juvenile turbot (Scophthalmus maximus) protein under thermo-treatment in different temperatures (20, 23, 25, and 27 °C), the corresponding proteome maps were constructed by two-dimensional gel electrophoresis (2-DE), from which the peptide mass map with MALDI-TOF-TOF was obtained, and a novel protein of polypeptide was identified by database retrieval. Results show that the proteome maps varied remarkably with temperature, indicating the increase or decrease in protein spot. Totally, 209 protein spots were matched in five maps in temperature using ImageMaster 2D Platinum 6.0. In addition, six unique protein spots were selected and identified with MALDI-TOF-TOF. By searching database for protein identification and function prediction, five proteins were confirmed, of which lectin and cytokeratin are highly potential as protein marker for further research. The information should be useful for better understanding the role of mucus as a component of innate immune system and for identifying genotypes that suit best to the aquiculture environment. These proteins could be used as potential biomarkers to environmental stressors in mucus for providing early warning when fish suffers in a dangerous situation.

A conserved rod domain phosphotyrosine that is targeted by the phosphatase PTP1B promotes keratin 8 protein insolubility and filament organization

Post-translational modifications are important functional determinants for intermediate filament (IF) proteins. Phosphorylation of IF proteins regulates filament organization, solubility, and cell-protective functions. Most known IF protein phosphorylation sites are serines localized in the variable "head" and "tail" domain regions. By contrast, little is known about site-specific tyrosine phosphorylation or its implications on IF protein function. We used available proteomic data from large scale studies to narrow down potential phospho-tyrosine sites on the simple epithelial IF protein keratin 8 (K8). Validation of the predicted sites using a pan-phosphotyrosine and a site-specific antibody, which we generated, revealed that the highly conserved Tyr-267 in the K8 "rod" domain was basally phosphorylated. The charge at this site was critically important, as demonstrated by altered filament organization of site-directed mutants, Y267F and Y267D, the latter exhibiting significantly diminished solubility. Pharmacological inhibition of the protein-tyrosine phosphatase PTP1B increased K8 Tyr-267 phosphorylation, decreased solubility, and increased K8 filament bundling, whereas PTP1B overexpression had the opposite effects. Furthermore, there was significant co-localization between K8 and a "substrate-trapping" mutant of PTP1B (D181A). Because K8 Tyr-267 is conserved in many IFs (QYE motif), we tested the effect of the paralogous Tyr in glial fibrillary acidic protein (GFAP), which is mutated in Alexander disease (Y242D). Similar to K8, Y242D GFAP exhibited highly irregular filament organization and diminished solubility. Our results implicate the rod domain QYE motif tyrosine as an important determinant of IF assembly and solubility properties that can be dynamically modulated by phosphorylation.

Glucose and SIRT2 reciprocally mediate the regulation of keratin 8 by lysine acetylation

Keratin 8 lysine acetylation, which is enhanced by hyperglycemia and reduced by SIRT2, alters filament organization and reduces solubility.

Lysine acetylation is an important posttranslational modification that regulates microtubules and microfilaments, but its effects on intermediate filament proteins (IFs) are unknown. We investigated the regulation of keratin 8 (K8), a type II simple epithelial IF, by lysine acetylation. K8 was basally acetylated and the highly conserved Lys-207 was a major acetylation site. K8 acetylation regulated filament organization and decreased keratin solubility. Acetylation of K8 was rapidly responsive to changes in glucose levels and was up-regulated in response to nicotinamide adenine dinucleotide (NAD) depletion and in diabetic mouse and human livers. The NAD-dependent deacetylase sirtuin 2 (SIRT2) associated with and deacetylated K8. Pharmacologic or genetic inhibition of SIRT2 decreased K8 solubility and affected filament organization. Inhibition of K8 Lys-207 acetylation resulted in site-specific phosphorylation changes of K8. Therefore, K8 acetylation at Lys-207, a highly conserved residue among type II keratins and other IFs, is up-regulated upon hyperglycemia and down-regulated by SIRT2. Keratin acetylation provides a new mechanism to regulate keratin filaments, possibly via modulating keratin phosphorylation.

Keratin 8 phosphorylation regulates keratin reorganization and migration of epithelial tumor cells

Cell migration and invasion are largely dependent on the complex organization of the various cytoskeletal components. Whereas the role of actin filaments and microtubules in cell motility is well established, the role of intermediate filaments in this process is incompletely understood. Organization and structure of the keratin cytoskeleton, which consists of heteropolymers of at least one type 1 and one type 2 intermediate filament, are in part regulated by post-translational modifications. In particular, phosphorylation events influence the properties of the keratin network. Sphingosylphosphorylcholine (SPC) is a bioactive lipid with the exceptional ability to change the organization of the keratin cytoskeleton, leading to reorganization of keratin filaments, increased elasticity, and subsequently increased migration of epithelial tumor cells. Here we investigate the signaling pathways that mediate SPC-induced keratin reorganization and the role of keratin phosphorylation in this process. We establish that the MEK-ERK signaling cascade regulates both SPC-induced keratin phosphorylation and reorganization in human pancreatic and gastric cancer cells and identify Ser431 in keratin 8 as the crucial residue whose phosphorylation is required and sufficient to induce keratin reorganization and consequently enhanced migration of human epithelial tumor cells.

Increased Phospho-Keratin 8 Isoforms in Colorectal Tumors Associated with EGFR Pathway Activation and Reduced Apoptosis

Hyperphosphorylated keratin (K) 8 acts as a phosphate “sponge” for stress-activated protein kinases thereby inhibiting pro-apoptotic molecules and thus apoptosis. MAP kinase/ERK1 has increased activity in colorectal cancer (CRC) and is known to phosphorylate K8. The aims were to identify the K8 isoforms abundantly present in colon tumors, using 2D difference gel electrophoresis (DIGE), to identify the modifications using mass spectrometry, and to validate the differential abundance of these isoforms in tumors relative to matched normal mucosae. 2D DIGE showed 3 isoforms of K8 significantly increased in tumor ≥2-fold in 6/8 pairs. Metal oxide affinity chromatography mass spectrometry and bioinformatics were used to identify phosphorylated serine residues. Levels of PS24, PS432, and PS74 by western blotting were found to be significantly increased in tumor versus matched normal. Blocking of EGFR signaling in Caco2 cells showed a significant decrease (P < 0.0001) in K8 PS74 and PS432 levels by 59% and 66%, respectively, resulting in increased apoptosis.

Loss of keratin 8 phosphorylation leads to increased tumor progression and correlates with clinico-pathological parameters of OSCC patients

BACKGROUND

Keratins are cytoplasmic intermediate filament proteins expressed in tissue specific and differentiation dependent manner. Keratins 8 and 18 (K8 and K18) are predominantly expressed in simple epithelial tissues and perform both mechanical and regulatory functions. Aberrant expression of K8 and K18 is associated with neoplastic progression, invasion and poor prognosis in human oral squamous cell carcinomas (OSCCs). K8 and K18 undergo several post-translational modifications including phosphorylation, which are known to regulate their functions in various cellular processes. Although, K8 and K18 phosphorylation is known to regulate cell cycle, cell growth and apoptosis, its significance in cell migration and/or neoplastic progression is largely unknown. In the present study we have investigated the role of K8 phosphorylation in cell migration and/or neoplastic progression in OSCC.

METHODOLOGY AND PRINCIPAL FINDINGS

To understand the role of K8 phosphorylation in neoplastic progression of OSCC, shRNA-resistant K8 phospho-mutants of Ser73 and Ser431 were overexpressed in K8-knockdown human AW13516 cells (derived from SCC of tongue; generated previously). Wound healing assays and tumor growth in NOD-SCID mice were performed to analyze the cell motility and tumorigenicity respectively in overexpressed clones. The overexpressed K8 phospho-mutants clones showed significant increase in cell migration and tumorigenicity as compared with K8 wild type clones. Furthermore, loss of K8 Ser73 and Ser431 phosphorylation was also observed in human OSCC tissues analyzed by immunohistochemistry, where their dephosphorylation significantly correlated with size, lymph node metastasis and stage of the tumor.

CONCLUSION AND SIGNIFICANCE

Our results provide first evidence of a potential role of K8 phosphorylation in cell migration and/or tumorigenicity in OSCC. Moreover, correlation studies of K8 dephosphorylation with clinico-pathological parameters of OSCC patients also suggest its possible use in prognostication of human OSCC.

Proteomic identification of Hsp70 as a new Plk1 substrate in arsenic trioxide-induced mitotically arrested cells

We previously demonstrated that when arsenic trioxide (ATO)-induced mitotically arrested HeLa S3 cells (AIMACs) were treated with staurosporine (SSP) the cells rapidly exited mitosis. To better define the cellular targets and the underlying mechanisms of AIMACs, we applied 2-D DIGE followed by LC-MS/MS analysis and showed that SSP induced a significant change in the phosphoproteome of AIMACs. Among the proteins whose phosphorylation was modulated by SSP, we identified Hsp70, Rad 23B, and eukaryotic translation initiation factor 4B as potentially new substrates of polo-like kinase 1 (Plk1), an essential serine/threonine kinase with versatile mitotic functions. Since Hsp70 is a stress protein responsible for ATO treatment, we further identified Thr(13) , Ser(362) , Ser(631) , and Ser(633) on Hsp70 intracellularly phosphorylated in AIMACs by combining TiO(2) phospho-peptides enrichment and MS/MS analysis. Using antibody specifically against phosph-Ser(631) Hsp70 and further aided by expression of kinase-dead Plk1 and pharmacological inhibition of Plk1, we concluded that Ser(631) on Hsp70 is phosphorylated by Plk1 in AIMACs. By immnuofluorescent staining, we found the colocalization of Hsp70 and Plk1 in AIMACs but not in interphase cells. In addition, Plk1-mediated phosphorylation of Hsp70 prevented AIMACs from mitotic death. Our results reveal that Hsp70 is a novel substrate of Plk1 and that its phosphorylation contributes to attenuation of ATO-induced mitotic abnormalities.

Type I keratin 17 protein is phosphorylated on serine 44 by p90 ribosomal protein S6 kinase 1 (RSK1) in a growth- and stress-dependent fashion

Keratin 17 (K17) is a type I intermediate filament protein that is constitutively expressed in ectoderm-derived epithelial appendages and robustly induced in epidermis following injury, during inflammation, and in chronic diseases such as psoriasis and cancer. Mutations within K17 are responsible for two rare diseases related to ectodermal dysplasias. Studies in K17-null mice uncovered several roles for K17, including structural support, resistance to TNFα-induced apoptosis, regulation of protein synthesis, and modulation of cytokine expression. Yet, little is known about the regulation of K17 protein via post-translational modification. Here, we report that serine 44 in the N-terminal head domain of K17 (K17-Ser(44)) is phosphorylated in response to extracellular stimuli (serum, EGF, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate) that alter skin keratinocyte growth, and to cellular stresses (sorbitol-induced hyperosmotic shock, UV irradiation, and hydrogen peroxide-induced oxidative stress). It also occurs in basaloid skin tumors in situ. Upon its stimulation in skin keratinocytes, K17-Ser(44) phosphorylation is induced rapidly but stays on transiently. The majority of the phosphorylated K17-Ser(44) pool is polymer-bound and is not obviously related to a change in filament organization. The amino acid sequence surrounding K17-Ser(44) matches the consensus for the AGC family of basophilic kinases. We show that p90 RSK1, an AGC kinase involved in the regulation of cell survival and proliferation, phosphorylates K17-Ser(44) in skin keratinocytes. These findings confirm and expand the tight link that has emerged between K17 up-regulation and growth and stress responses in the skin epithelium.

Identification of phosphorylated serine-15 and -82 residues of HSPB1 in 5-fluorouracil-resistant colorectal cancer cells by proteomics

To identify the proteins involved in 5-fluorouracil (5-FU) resistance, a comparison of the total and phosphorylated proteins between the human colorectal cancer (CRC) cell line DLD-1 and its 5-FU-resistant subclone DLD-1/5-FU was performed. Using 2-DE and MALDI-TOF/TOF-based proteomics, 17 up-regulated and 19 down-regulated protein spots were identified in the 5-FU-resistant DLD-1/5-FU cells compared with the parent cell lines. In DLD-1/5-FU cells, 7 anti-apoptotic proteins (HSPB1, proteasome subunit α-5, transitional endoplasmic reticulum ATPase, 14-3-3 β, 14-3-3 γ, 14-3-3 σ, and phosphoglycerate kinase 1) were up-regulated and 4 proapoptotic proteins (cofilin-1, pyruvate kinase M2, glyceraldehyde-3-phosphate dehydrogenase, and nucleophosmin) were down-regulated. The results show that the acquired drug resistance of DLD-1/5-FU cells is caused by the prevention of drug-induced apoptosis, in particular through the enhanced constitutive expression of HSPB1 and its phosphorylated form. Short interfering RNA knockdown of endogenous HSPB1 in DLD-1/5-FU cells restored the sensitivity to 5-FU. Furthermore, MALDI-TOF/TOF and 2-DE Western blot analysis identified the phosphorylated residues of HSPB1 as Ser-15 and Ser-82 in the main (diphosphorylated) form and Ser-15, Ser-78, and Ser-82 in the minor (triphosphorylated) form. The current findings indicate that phosphorylated HSPB1 may play an important role in 5-FU resistance.

Keratins in health and cancer: more than mere epithelial cell markers

Keratins are the intermediate filament (IF)-forming proteins of epithelial cells. Since their initial characterization almost 30 years ago, the total number of mammalian keratins has increased to 54, including 28 type I and 26 type II keratins. Keratins are obligate heteropolymers and, similarly to other IFs, they contain a dimeric central α-helical rod domain that is flanked by non-helical head and tail domains. The 10-nm keratin filaments participate in the formation of a proteinaceous structural framework within the cellular cytoplasm and, as such, serve an important role in epithelial cell protection from mechanical and non-mechanical stressors, a property extensively substantiated by the discovery of human keratin mutations predisposing to tissue-specific injury and by studies in keratin knockout and transgenic mice. More recently, keratins have also been recognized as regulators of other cellular properties and functions, including apico-basal polarization, motility, cell size, protein synthesis and membrane traffic and signaling. In cancer, keratins are extensively used as diagnostic tumor markers, as epithelial malignancies largely maintain the specific keratin patterns associated with their respective cells of origin, and, in many occasions, full-length or cleaved keratin expression (or lack there of) in tumors and/or peripheral blood carries prognostic significance for cancer patients. Quite intriguingly, several studies have provided evidence for active keratin involvement in cancer cell invasion and metastasis, as well as in treatment responsiveness, and have set the foundation for further exploration of the role of keratins as multifunctional regulators of epithelial tumorigenesis.

p38 MAP kinase and MAPKAP kinases MK2/3 cooperatively phosphorylate epithelial keratins

The MAPK-activated protein kinases (MAPKAP kinases) MK2 and MK3 are directly activated via p38 MAPK phosphorylation, stabilize p38 by complex formation, and contribute to the stress response. The list of substrates of MK2/3 is increasing steadily. We applied a phosphoproteomics approach to compare protein phosphorylation in MK2/3-deficient cells rescued or not by ectopic expression of MK2. In addition to differences in phosphorylation of the known substrates of MK2, HSPB1 and Bag-2, we identified strong differences in phosphorylation of keratin 8 (K8). The phosphorylation of K8-Ser(73) is catalyzed directly by p38, which in turn shows MK2-dependent expression. Notably, analysis of small molecule p38 inhibitors on K8-Ser(73) phosphorylation also demonstrated reduced phosphorylations of keratins K18-Ser(52) and K20-Ser(13) but not of K8-Ser(431) or K18-Ser(33). Interestingly, K18-Ser(52) and K20-Ser(13) are not directly phosphorylated by p38 in vitro, but by MK2. Furthermore, anisomycin-stimulated phosphorylations of K20-Ser(13) and K18-Ser(52) are inhibited by small molecule inhibitors of both p38 and MK2. MK2 knockdown in HT29 cells leads to reduced K20-Ser(13) phosphorylation, which further supports the notion that MK2 is responsible for K20 phosphorylation in vivo. Physiologic relevance of these findings was confirmed by differences of K20-Ser(13) phosphorylation between the ileum of wild-type and MK2/3-deficient mice and by demonstrating p38- and MK2-dependent mucin secretion of HT29 cells. Therefore, MK2 and p38 MAPK function in concert to phosphorylate K8, K18, and K20 in intestinal epithelia.

E1--E4-mediated keratin phosphorylation and ubiquitylation: a mechanism for keratin depletion in HPV16-infected epithelium

The keratin IF network of epidermal keratinocytes provides a protective barrier against mechanical insult, it is also a major player in absorbing stress in these cells. The human papilloma virus (HPV) type 16 E1--E4 protein accumulates in the upper layers of HPV16-infected epithelium and is known to associate with and reorganise the keratin IF network in cells in culture. Here, we show that this function is conserved amongst a number of HPV alpha-group E1--E4 proteins and that the differentiation-dependent keratins are also targeted. Using time-lapse microscopy, HPV16 E1--E4 was found to effect a dramatic cessation of keratin IF network dynamics by associating with both soluble and insoluble keratin. Network disruption was accompanied by keratin hyperphosphorylation at several sites, including K8 S73, which is typically phosphorylated in response to stress stimuli. Keratin immunoprecipitated from E1--E4-expressing cells was also found to be ubiquitylated, indicating that it is targeted for proteasomal degradation. Interestingly, the accumulation of hyperphosphorylated, ubiquitylated E1--E4-keratin structures was found to result in an impairment of proteasomal function. These observations shed new light on the mechanism of keratin IF network reorganisation mediated by HPV16 E1--E4 and provide an insight into the depletion of keratin co-incident with E1--E4 accumulation observed in HPV-infected epithelium.

Novel insights into changes in biochemical properties of keratins 8 and 18 in griseofulvin-induced toxic liver injury

Keratins 8 and 18 (K8/18) intermediate filament proteins are believed to play an essential role in the protection of hepatocytes against mechanical and toxic stress. This assertion is mainly based on increased hepatocyte fragility observed in transgenic mice deficient in K8/18, or carrying mutations on K8/18. The molecular mechanism by which keratins accomplish their protective functions has not been totally elucidated. Liver diseases such as alcoholic hepatitis and copper metabolism diseases are associated with modifications, in hepatocytes, of intermediate filament organisation and the formation of K8/18 containing aggregates named Mallory-Denk bodies. Treatment of mice with a diet containing griseofulvin induces the formation of Mallory-Denk bodies in hepatocytes. This provides a reliable animal model for assessing the molecular mechanism by which keratins accomplish their protective role in the response of hepatocytes to chemical injuries. In this study, we found that griseofulvin intoxication induced changes in keratin solubility and that there was a 5% to 25% increase in the relative amounts of soluble keratin. Keratin phosphorylation on specific sites (K8 pS79, K8 pS436 and K18 pS33) was increased and prominent in the insoluble protein fractions. Since at least six K8 phosphoepitopes were detected after GF treatment, phosphorylation sites other than the ones studied need to be accounted for. Immunofluorescence staining showed that K8 pS79 epitope was present in clusters of hepatocytes that surrounded apoptotic cells. Activated p38 MAPK was associated with, but not present in K8 pS79-positive cells. These results indicate that griseofulvin intoxication mediates changes in the physicochemical properties of keratin, which result in the remodelling of keratin intermediate filaments which in turn could modulate the signalling pathways in which they are involved by modifying their binding to signalling proteins.

Inhibitory effects of cigarette smoke extract on neural crest migration occur through suppression of R-spondin1 expression via aryl hydrocarbon receptor

Although it is known that smoking during pregnancy induces fetal malformations, few basic studies at the molecular level are currently available. Since it is known that neural crest cells (NCC) play an important role in tissue development and differentiation, we investigated the influence of cigarette smoke extract (CSE) on NCC migration. CSE treatment reduced the migration index of NCC in dose- and tar-content-dependent manners without induction of apoptosis or decrease in proliferation of NCC. alpha-Naphthoflavone, an antagonist of aryl hydrocarbon receptor (AhR), prevented the reduction in NCC migration that was otherwise induced by CSE treatment. Overexpression of AhR caused a significant decrease in NCC migration index, implying that CSE can attenuate NCC migration through AhR signaling. Transcriptome analysis revealed that overexpression of AhR led to decreased expression of R-spondin1 in NCC. Furthermore, overexpression of R-spondin1 prevented the inhibitory effect of CSE on NCC. These results suggest that CSE causes suppressed expression of R-spondin1 by activating signals via the AhR, which leads to impaired neural crest cell migration.

Insights into the mechanical properties of epithelial cells: the effects of shear stress on the assembly and remodeling of keratin intermediate filaments

The effects of shear stress on the keratin intermediate filament (KIF) cytoskeleton of cultured human alveolar epithelial (A549) cells have been investigated. Under normal culture conditions, immunofluorescence revealed a delicate network of fine tonofibrils containing KIFs, together with many nonfilamentous, keratin-containing "particles," mostly containing either keratin 8 (K8) or 18 (K18), but not both. Triton X-100 extracted approximately 10% of the cellular keratin, and this was accompanied by a loss of the particles but not the KIFs. Shear stress dramatically reduced the soluble keratin component and transformed the fine bundles of KIFs into thicker, "wavy" tonofibrils. Both effects were accompanied by the disappearance of most keratin particles and by increased phosphorylation of K8 and K18 on serine residues 73 and 33, respectively. The particles that remained after shearing were phosphorylated and were closely associated with KIFs. We suggest that keratin particles constitute a reservoir of protein that can be recruited into KIFs under flow, creating a more robust cytoskeleton able to withstand shear forces more effectively.

Novel Pathways Associated with Quinone-Induced Stress in Breast Cancer Cells

Hormone-dependent breast cancers that overexpress the ligand-binding nuclear transcription factor, estrogen receptor (ER), represent the most common form of breast epithelial malignancy. Exposure of breast epithelial cells to a redox-cycling and arylating quinone induces mitogen-activated protein kinase phosphorylation of the cytoskeletal filament protein, cytokeratin-8, along with thiol arylation of H3 nuclear histones. Exogenous or endogenous quinones can also induce ligand-independent nuclear translocation and phosphorylation of ER; with excess exposure, these quinones can arylate ER zinc fingers, impairing ER DNA-binding and altering ER-inducible gene expression. Immunoaffinity enrichment for low abundance proteins such as ER, coupled with modern mass spectrometry techniques, promises to improve understanding of the protein-modifications produced by endogenous and exogenous quinone exposure and their role in the development or progression of epithelial malignancies such as breast cancer.

Stress-induced recruitment of epiplakin to keratin networks increases their resistance to hyperphosphorylation-induced disruption

Epiplakin is a large (>725 kDa) cytoskeletal protein exclusively expressed in epithelial tissues. It has a unique structure, consisting entirely of plakin repeat domains (PRDs), one of the hallmarks of spectraplakin protein family members. Previous studies, including the phenotypic analyses of knockout mice, failed to reveal the biological function of epiplakin. Using in vitro binding assays, we show here that all but one of the 16 PRDs of mouse epiplakin bind to keratins of basal keratinocytes. Nevertheless, in primary keratinocyte cell cultures, epiplakin only partially colocalized with keratin intermediate filament networks. However, upon application of cellular stress in the form of keratin hyperphosphorylation, osmotic shock or UV irradiation, the entire cytoplasmic epiplakin pool became associated with keratin. In response to such types of stress, epiplakin initially translocated to the still-intact keratin filament network and remained associated with keratin after its disruption and transformation into granular aggregates. Time-course experiments revealed that serine/threonine (okadaic acid) and tyrosine (orthovanadate) phosphatase inhibitor-induced filament disruption in differentiated keratinocytes proceeded faster in epiplakin-deficient cells compared with wild-type cells. Our data suggest that epiplakin plays a role in keratin filament reorganization in response to stress, probably by protecting keratin filaments against disruption in a chaperone-like fashion.

Protein kinase C ε phosphorylates keratin 8 at Ser8 and Ser23 in GH4C1 cells stimulated by thyrotropin-releasing hormone

Protein kinase C epsilon (PKCepsilon) is activated by thyrotropin-releasing hormone (TRH), a regulator of pituitary function in rat pituitary GH(4)C(1) cells. We analyzed the downstream mechanism after PKCepsilon activation. Exposure of GH(4)C(1) cells to TRH or a phorbol ester increased the phosphorylation of three p52 proteins (p52a, p52b and p52c) and decreased the phosphorylation of destrin and cofilin. GF109203X, an inhibitor of protein kinases including PKC, inhibited phosphorylation of the p52 proteins by TRH stimulation. Peptide mapping, amino-acid sequencing, and immunochemical studies indicated that p52a, p52b, and p52c are the differentially phosphorylated isoforms of keratin 8 (K8), an intermediate filament protein. The unphosphorylated K8 (p52n) localized exclusively to the cytoskeleton, whereas the phosphorylated forms (especially p52c), which are increased in TRH-stimulated cells, localized mainly to the cytosol. K8 phosphorylation was enhanced in PKCepsilon-overexpressing clones, and purified recombinant PKCepsilon directly phosphorylated K8 with a profile similar to that observed in TRH-stimulated cells. PKCepsilon and K8 colocalized near the nucleus under basal conditions and were concentrated in the cell periphery and cell-cell contact area after TRH stimulation. MS analyses of phospho-K8 and K8-synthesized peptide (amino acids 1-53) showed that PKCepsilon phosphorylates Ser8 and Ser23 of K8. Phosphorylation of these sites is enhanced in TRH-stimulated cells and PKCepsilon-overexpressing cells, as assessed by immunoblotting using antibodies to phospho-K8. These results suggest that K8 is a physiological substrate for PKCepsilon, and the phosphorylation at Ser8 and Ser23 transduces, at least in part, TRH-PKCepsilon signaling in pituitary cells.

p38 MAPK-dependent shaping of the keratin cytoskeleton in cultured cells

Plasticity of the resilient keratin intermediate filament cytoskeleton is an important prerequisite for epithelial tissue homeostasis. Here, the contribution of stress-activated p38 MAPK to keratin network organization was examined in cultured cells. It was observed that phosphorylated p38 colocalized with keratin granules that were rapidly formed in response to orthovanadate. The same p38^p recruitment was noted during mitosis, in various stress situations and in cells producing mutant keratins. In all these situations keratin 8 became phosphorylated on S73, a well-known p38 target site. To demonstrate that p38-dependent keratin phosphorylation determines keratin organization, p38 activity was pharmacologically and genetically modulated: up-regulation induced keratin granule formation, whereas down-regulation prevented keratin filament network disassembly. Furthermore, transient p38 inhibition also inhibited keratin filament precursor formation and mutant keratin granule dissolution. Collectively, the rapid and reversible effects of p38 activity on keratin phosphorylation and organization in diverse physiological, stress, and pathological situations identify p38-dependent signalling as a major intermediate filament–regulating pathway.

The Possible Role of Cytokeratin 8 in Cadmium-Induced Adaptation and Carcinogenesis

Chronic exposures to cadmium compounds are carcinogenic. It was hypothesized that the development of resistance to cadmium may drive carcinogenesis. This is achieved by selection of resistant cells in which the apoptotic response is significantly attenuated. The induction of cadmium resistance in rat lung epithelial cells (LEC) was used to explore the mechanisms of cadmium-induced adaptation and carcinogenesis. Our previous results showed that LECs developed resistance to apoptosis during cadmium adaptation possibly due to perturbation of the c-Jun NH(2)-terminal kinase pathway. Here, we further study these cells by comparative proteomics. Interestingly, we showed that two intermediate filament proteins, cytokeratin 8 (CK8) and cytokeratin 14 (CK14), were increased significantly and stably maintained only in the adapted cells but not in cadmium-treated parental cells. It has been documented that CK8/cytokeratin 18 provided resistance to tumor necrosis factor (TNF)-induced apoptosis and CK14 may function as an inhibitor of TNF-TNF receptor 1 (TNFR1) signaling through an association with TNFR1-associated death domain protein, suggesting that up-regulation of CK8 and CK14 may be responsible for apoptotic resistance. Finally, we showed that small interfering RNA-specific knockdown of CK8 in cadmium-adapted cells attenuated the cadmium resistance, indicating the potential role of CK8 in cadmium resistance. This acquired self-resistance to apoptosis could account for cadmium-induced carcinogenesis, as this promotes neoplastic cell survival as well as subsequent clonal expansion and then progression of tumor development. Thus, increased expression of these cytokeratins represents an adaptive survival mechanism that resists cadmium-induced apoptosis and it is unprecedented that cells respond to long-term cadmium exposure by modulating keratin dynamics.

Periplakin-dependent re-organisation of keratin cytoskeleton and loss of collective migration in keratin-8-downregulated epithelial sheets

Collective migration of epithelial sheets requires maintenance of cell-cell junctions and co-ordination of the movement of the migrating front. We have investigated the role of keratin intermediate filaments and periplakin, a cytoskeletal linker protein, in the migration of simple epithelial cells. Scratch wounding induces bundling of keratins into a cable of tightly packed filaments adjacent to the free wound edge. Keratin re-organisation is preceded by a re-distribution of periplakin away from the free wound edge. Periplakin participates with dynamic changes in the keratin cytoskeleton via its C-terminal linker domain that co-localises with okadaic-acid-treated keratin granules. Stable expression of the periplakin C-terminal domain increases keratin bundling and Ser431 keratin phosphorylation at wound edge resulting in a delay in wound closure. Ablation of periplakin by siRNA inhibits keratin cable formation and impairs wound closure. Knockdown of keratin 8 with siRNA results in (1) a loss of desmoplakin localisation at cell borders, (2) a failure of MCF-7 epithelial sheets to migrate as a collective unit and (3) accelerated wound closure in vimentin-positive HeLa and Panc-1 cell lines. Thus, keratin 8 is required for the maintenance of epithelial integrity during migration and periplakin participates in the re-organisation of keratins in migrating cells.

Keratin 8 modulation of desmoplakin deposition at desmosomes in hepatocytes

Keratins, the intermediate filament proteins of epithelial cells, connect to desmosomes, the cell-cell adhesion structures at the surface membrane. The building elements of desmosomes include desmoglein and desmocollin, which provide the actual cell adhesive properties, and desmoplakins, which anchor the keratin intermediate filaments to desmosomes. In the work reported here, we address the role of keratin 8 in modulating desmoplakin deposition at surface membrane in mouse hepatocytes. The experimental approach is based on the use of keratin 8- and keratin 18-null mouse hepatocytes as cell models. In wild-type mouse hepatocytes, desmoplakin is aligned with desmoglein and keratin 8 at the surface membrane. In keratin 8-null hepatocytes, the intermediate filament loss leads to alterations in desmoplakin distribution at the surface membrane, but not of desmoglein. Intriguingly, a significant proportion of keratin 18-null hepatocytes express keratin 8 at the surface membrane, associated with a proper desmoplakin alignment with desmoglein at desmosomes. A Triton treatment of the monolayer reveals that most of the desmoplakin present in either wild-type, keratin 8- or keratin 18-null hepatocytes is insoluble. Deletion analysis of keratin 8 further suggests that the recovery of desmoplakin alignment requires the keratin 8 rod domain. In addition, similarly to other works revealing a key role of desmoplakin phosphorylation on its interaction with intermediate filaments, we find that the phosphorylation status of the keratin 8 head domain affects desmoplakin distribution at desmosomes. Together, the data indicate that a proper alignment/deposition of desmoplakin with keratins and desmoglein in hepatocytes requires keratin 8, through a reciprocal phosphoserine-dependent process.

A disease- and phosphorylation-related nonmechanical function for keratin 8

Keratin 8 (K8) variants predispose to human liver injury via poorly understood mechanisms. We generated transgenic mice that overexpress the human disease-associated K8 Gly61-to-Cys (G61C) variant and showed that G61C predisposes to liver injury and apoptosis and dramatically inhibits K8 phosphorylation at serine 73 (S73) via stress-activated kinases. This led us to generate mice that overexpress K8 S73-to-Ala (S73A), which mimicked the susceptibility of K8 G61C mice to injury, thereby providing a molecular link between K8 phosphorylation and disease-associated mutation. Upon apoptotic stimulation, G61C and S73A hepatocytes have persistent and increased nonkeratin proapoptotic substrate phosphorylation by stress-activated kinases, compared with wild-type hepatocytes, in association with an inability to phosphorylate K8 S73. Our findings provide the first direct link between patient-related human keratin variants and liver disease predisposition. The highly abundant cytoskeletal protein K8, and possibly other keratins with the conserved S73-containing phosphoepitope, can protect tissue from injury by serving as a phosphate “sponge” for stress-activated kinases and thereby provide a novel nonmechanical function for intermediate filament proteins.

Protein phosphatase-2A associates with and dephosphorylates keratin 8 after hyposmotic stress in a site- and cell-specific manner

Keratins 8 and 18 (K8 and K18) are regulated by site-specific phosphorylation in response to multiple stresses. We examined the effect and regulation of hyposmotic stress on keratin phosphorylation. K8 phospho-Ser431 (Ser431-P) becomes dephosphorylated in HT29 cells, but hyperphosphorylated on other K8 but not K18 sites in HRT18 and Caco2 cells and in normal human colonic ex vivo cultures. Hyposmosis-induced dephosphorylation involves K8 but not K18, K19 or K20, occurs preferentially in mitotically active cells, and peaks by 6-8 hours then returns to baseline by 12-16 hours. By contrast, hyperosmosis causes K8 Ser431 hyperphosphorylation in all tested cell lines. Hyposmosis-induced dephosphorylation of K8 Ser431-P is inhibited by okadaic acid but not by tautomycin or cyclosporine. The PP2A catalytic subunit co-immunoprecipitated with K8 and K18 after hyposmotic stress in HT29 cells, but not in HRT18 or Caco2 cells where K8 Ser431 becomes hyperphosphorylated. K8 Ser431-P dephosphorylation after hyposmosis was independent of PP2A levels but correlated with increased PP2A activity towards K8 Ser431-P. Therefore, hyposmotic stress alters K8 phosphorylation in a cell-dependent manner, and renders K8 Ser431-P a physiologic substrate for PP2A in HT29 cells as a result of PP2A activation and the physical association with K8 and K18. The divergent hyposmosis versus hyperosmosis K8 Ser431 phosphorylation changes in HT29 cells suggest that there are unique signaling responses to osmotic stress.