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

Deletions in the KAP6-1 gene are associated with fiber traits in cashmere-producing goats

Keratin-associated proteins (KAPs) are important structural components of fibers that predominantly present in the ortho-cortex. These proteins form a cross-linked network with keratin intermediate filaments (KIFs), thus producing a strong hair shaft. The keratin-associated protein 6-1 gene (KAP6-1) is a member of the KAPs family that has a potential correlation with fiber traits. In this study, we investigated the influence of KAP6-1 sequence polymorphisms on the fiber characteristics of a Chinese cashmere-producing goat breed (n = 844). Two main variants were found, including a three base pair (bp) deletion (namely B) and a 36-bp deletion (namely C), while the reference genotype of KAP6-1 was named A. Among them, the B variant was first reported on cashmere goats. This study then correlated these genotypes with the collected fiber data to investigate the potential association of these variants. The results showed that variant A is associated with decreased fiber diameter (p < 0.01), while variant C is associated with deceased fiber length (p < 0.01). These two related variants of the KAP6-1 gene have potential applications as gene-makers to improve the fiber diameter and length in cashmere-producing goats.

Phosphorylation of keratin 18 serine 52 regulates mother-daughter centriole engagement and microtubule nucleation by cell cycle-dependent accumulation at the centriole

Serine-52 (Ser52) is the major physiologic site of keratin 18 (K18) phosphorylation. Here, we report that serine-52 phosphorylated K18 (phospho-Ser52 K18) accumulated on centrosomes in a cell cycle-dependent manner. Moreover, we found that phospho-Ser52 K18 was located at the proximal end of the mother centriole. Transfection with the K18 Ser52 → Ala (K18 S52A) mutant prevented centriole localization of phospho-Ser52 K18 and resulted in separation of the mother-daughter centrioles. Inhibition of microtubule polymerization led to the disappearance of aggregated phospho-Ser52 K18 on the centrosome; removal of inhibitors resulted in reaccumulation of phospho-Ser52 K18 in microtubule-organizing centers. Transfection with a K18 S52A mutant inhibited microtubule nucleation. These results reveal a cell cycle-dependent change in centrosome localization of phospho-Ser52 k18 and strongly suggest that the phosphorylation status of Ser52 K18 of mother centrioles plays a critical role in maintaining a tight engagement between mother and daughter centrioles and also contributes to microtubule nucleation.

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.

Variation in the Ovine KAP6-3 Gene (KRTAP6-3) Is Associated with Variation in Mean Fibre Diameter-Associated Wool Traits

Polymerase chain reaction-single stranded conformational polymorphism (PCR-SSCP) analysis was used to investigate variation in the ovine KAP6-3 gene (KRTAP6-3) in 383 Merino × Southdown-cross lambs from four sire-lines, and to determine whether this variation affects wool traits. Five PCR-SSCP banding patterns, representing five different nucleotide sequences, were detected, including four previously identified (named A, B, C, and F) variants and one newly identified (named G) variant. A new non-synonymous single nucleotide polymorphism (SNP) and a 45-bp deletion were detected in variant G. Of the three common genotypes (AA, AB, and AG) identified in these sheep, wool from sheep that were AG, on average, had a lower mean fibre diameter (MFD), fibre diameter standard deviation (FDSD), and prickle factor (PF) than wool from AA sheep, whereas wool from AB sheep, on average, had a higher MFD, FDSD, and PF than wool from AA sheep. This suggests that variation in ovine KRTAP6-3 affect MFD, FDSD, and PF, and that this gene may have potential for use as a gene-maker for improving fibre diameter-associated wool traits.

In vitro phosphorylation as tool for modification of silk and keratin fibrous materials

An overview is given of the recent work on in vitro enzymatic phosphorylation of silk fibroin and human hair keratin. Opposing to many chemical "conventional" approaches, enzymatic phosphorylation is in fact a mild reaction and the treatment falls within "green chemistry" approach. Silk and keratin are not phosphorylated in vivo, but in vitro. This enzyme-driven modification is a major technological breakthrough. Harsh chemical chemicals are avoided, and mild conditions make enzymatic phosphorylation a real "green chemistry" approach. The current communication presents a novel approach stating that enzyme phosphorylation may be used as a tool to modify the surface charge of biocompatible materials such as keratin and silk.

Functional Implications of O-GlcNAcylation-dependent Phosphorylation at a Proximal Site on Keratin 18

Keratins 8/18 (K8/18) are phosphoglycoproteins and form the major intermediate filament network of simple epithelia. The three O-GlcNAcylation (Ser(29), Ser(30), and Ser(48)) and two phosphorylation (Ser(33) and Ser(52)) serine sites on K18 are well characterized. Both of these modifications have been reported to increase K18 solubility and regulate its filament organization. In this report, we investigated the site-specific interplay between these two modifications in regulating the functional properties of K18, like solubility, stability, and filament organization. An immortalized hepatocyte cell line (HHL-17) stably expressing site-specific single, double, and triple O-GlcNAc and phosphomutants of K18 were used to identify the site(s) critical for regulating these functions. Keratin 18 mutants where O-GlcNAcylation at Ser(30) was abolished (K18-S30A) exhibited reduced phosphorylation induced solubility, increased stability, defective filament architecture, and slower migration. Interestingly, K18-S30A mutants also showed loss of phosphorylation at Ser(33), a modification known to regulate the solubility of K18. Further to this, the K18 phosphomutant (K18-S33A) mimicked K18-S30A in its stability, filament organization, and cell migration. These results indicate that O-GlcNAcylation at Ser(30) promotes phosphorylation at Ser(33) to regulate the functional properties of K18 and also impact cellular processes like migration. O-GlcNAcylation and phosphorylation on the same or adjacent sites on most proteins antagonize each other in regulating protein functions. Here we report a novel, positive interplay between O-GlcNAcylation and phosphorylation at adjacent sites on K18 to regulate its fundamental properties.

Identification of nuclear phosphoproteins as novel tobacco markers in mouse lung tissue following short-term exposure to tobacco smoke

•

We analyzed nuclear phosphoprotein expression activated by tobacco smoke exposure.

•

253 phosphoproteins were identified in 1-day and 7-day exposure groups.

•

Of these, 33 were significantly differentially expressed in control and exposed groups.

•

Identified proteins were related to inflammation, response to stress and nicotine.

•

OSF3 and spectrin β chain were identified as candidate tobacco smoke markers.

Smoking is a risk factor for lung diseases, including chronic obstructive pulmonary disease and lung cancer. However, the molecular mechanisms mediating the progression of these diseases remain unclear. Therefore, we sought to identify signaling pathways activated by tobacco-smoke exposure, by analyzing nuclear phosphoprotein expression using phosphoproteomic analysis of lung tissue from mice exposed to tobacco smoke. Sixteen mice were exposed to tobacco smoke for 1 or 7 days, and the expression of phosphorylated peptides was analyzed by mass spectrometry. A total of 253 phosphoproteins were identified, including FACT complex subunit SPT16 in the 1-day exposure group, keratin type 1 cytoskeletal 18 (K18), and adipocyte fatty acid-binding protein, in the 7-day exposure group, and peroxiredoxin-1 (OSF3) and spectrin β chain brain 1 (SPTBN1), in both groups. Semi-quantitative analysis of the identified phosphoproteins revealed that 33 proteins were significantly differentially expressed between the control and exposed groups. The identified phosphoproteins were classified according to their biological functions. We found that the identified proteins were related to inflammation, regeneration, repair, proliferation, differentiation, morphogenesis, and response to stress and nicotine. In conclusion, we identified proteins, including OSF3 and SPTBN1, as candidate tobacco smoke-exposure markers; our results provide insights into the mechanisms of tobacco smoke-induced diseases.

Epidermal growth factor-induced modulation of cytokeratin expression levels influences the morphological phenotype of head and neck squamous cell carcinoma cells

The migratory ability of tumor cells requires cytoskeletal rearrangement processes. Epidermal growth factor receptor (EGFR)-signaling tightly correlates with tumor progression in head and neck squamous cell carcinomas (HNSCCs), and has previously been implicated in the regulation of cytokeratin (CK) expression. In this study, HNSCC cell lines were treated with EGF, and CK expression levels were monitored by Western blot analysis. Changes in cellular morphology were documented by fluorescence- and atomic force microscopy. Some of the cell lines demonstrated an EGF-dependent modulation of CK expression levels. Interestingly, regression of some CK subtypes or initial up-regulation followed by downregulation at higher EGF-levels could also be observed in the tested cell lines. Overall, the influence of EGF on CK expression levels appeared variable and cell-type-dependent. Real-time cellular analysis of EGF-treated and -untreated HNSCC cell lines demonstrated a rise over time in cellular impedance. In three of the EGF-treated HNSCC cell lines, this rise was markedly higher than in untreated controls, whereas in one of the cell lines the gain of cellular impedance was paradoxically reduced after EGF treatment, which was found to correlate with changes in cellular morphology rather than with relevant changes in cellular viability or proliferation. After treating HNSCC cells with EGF, CK filaments frequently appeared diffusely distributed throughout the cytoplasm, and in some cases were found in a perinuclear localization, the latter being reminiscent to observations by other groups. In summary, the data points to a possible role of EGFR in modulating HNSCC cell morphology.

Characterization of O-GlcNAc cycling and proteomic identification of differentially O-GlcNAcylated proteins during G1/S transition

BACKGROUND

DNA replication represents a critical step of the cell cycle which requires highly controlled and ordered regulatory mechanisms to ensure the integrity of genome duplication. Among a plethora of elements, post-translational modifications (PTMs) ensure the spatiotemporal regulation of pivotal proteins orchestrating cell division. Despite increasing evidences showing that O-GlcNAcylation regulates mitotic events, the impact of this PTM in the early steps of the cell cycle remains poorly understood.

METHODS AND RESULTS

Quiescent MCF7 cells were stimulated by serum mitogens and cell cycle progression was determined by flow cytometry. The levels of O-GlcNAc modified proteins, O-GlcNAc Transferase (OGT) and O-GlcNAcase (OGA) were examined by Western blotting and OGA activity was measured during the progression of cells towards S phase. A global decrease in O-GlcNAcylation was observed at S phase entry, concomitantly to an increase in the activity of OGA. A combination of two-dimensional electrophoresis, Western blotting and mass spectrometry was then used to detect and identify cell cycle-dependent putative O-GlcNAcylated proteins. 58 cytoplasmic and nuclear proteins differentially O-GlcNAcylated through G1/S transition were identified and the O-GlcNAc variations of Cytokeratin 8, hnRNP K, Caprin-1, Minichromosome Maintenance proteins MCM3, MCM6 and MCM7 were validated by immunoprecipitation.

CONCLUSIONS

The dynamics of O-GlcNAc is regulated during G1/S transition and observed on key proteins involved in the cytoskeleton networks, mRNA processing, translation, protein folding and DNA replication.

GENERAL SIGNIFICANCE

Our results led us to propose that O-GlcNAcylation joins the PTMs that take part in the regulation of DNA replication initiation.

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.

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.

Redox regulation of cytokeratin 18 protein by NADPH oxidase 1 in preneoplastic human epithelial cells

INTRODUCTION

A catalytic subunit of NADPH oxidase 1 (Nox1) is implicated to be involved in neoplastic progression in human epithelial cancers. We had previously demonstrated that Nox1 overexpression of immortalized epithelial cells was able to induce the generation of progenitor cells that expressed fetal-type cytokeratins 8 and 18.

PURPOSE

We aimed to investigate the direct effects and underlying mechanisms of Nox1 on expression of cytokeratin 18 (CK18).

METHODS

Immortalized human epithelial GM16 cells with low CK18 were used in Nox1 overexpression experiments. NuB2 cells with high CK18 were used in Nox1 knockdown experiments. Protein expression of CK18, phosphorylated and ubiquitinated CK18 were analyzed by Western blot.

RESULTS

With no effects on the mRNA levels, CK18 protein was increased upon Nox1 overexpression and decreased upon Nox1 knockdown. Treatment with proteasome inhibitor MG132 prevented CK18 degradation and increased CK18 protein indicating translational regulation of CK18. Treatment for NuB2 cells with N-acetyl-L: -cysteine, diphenyleneiodonium, or apocynin decreased CK18 protein levels indicating its regulation involving reactive oxygen species and flavoprotein Nox. It has been known that phosphorylation of CK18 regulates CK18 turnover by ubiquination. Consistently, Nox1 modulated CK18 phosphorylation at ser52. Nox1 knockdown and treatment with diphenyleneiodonium accumulated the levels of ubiquinated CK18 enhancing degradation causing decreased CK18 protein.

CONCLUSION

We demonstrated that Nox1 was able to induce CK18 stabilization by inhibiting CK18 protein degradation in a phosphorylation-dependent manner. CK18 accumulation induced by Nox1 is consistent with the persistence of fetal-type CK18 protein in many epithelial carcinomas.

O-GlcNAcylation determines the solubility, filament organization, and stability of keratins 8 and 18

Keratins 8 and 18 (K8/18) are intermediate filament proteins expressed specifically in simple epithelial tissues. Dynamic equilibrium of these phosphoglycoproteins in the soluble and filament pool is an important determinant of their cellular functions, and it is known to be regulated by site-specific phosphorylation. However, little is known about the role of dynamic O-GlcNAcylation on this keratin pair. Here, by comparing immortalized (Chang) and transformed hepatocyte (HepG2) cell lines, we have demonstrated that O-GlcNAcylation of K8/18 exhibits a positive correlation with their solubility (Nonidet P-40 extractability). Heat stress, which increases K8/18 solubility, resulted in a simultaneous increase in O-GlcNAc on these proteins. Conversely, increasing O-GlcNAc levels were associated with a concurrent increase in their solubility. This was also associated with a notable decrease in total cellular levels of K8/18. Unaltered levels of transcripts and the reduced half-life of K8 and K18 indicated their decreased stability on increasing O-GlcNAcylation. On the contrary, the K18 glycosylation mutant (K18 S29A/S30A/S48A) was notably more stable than the wild type K18 in Chang cells. The K18-O-GlcNAc mutant accumulated as aggregates upon stable expression, which possibly altered endogenous filament architecture. These results strongly indicate the involvement of O-GlcNAc on K8/18 in regulating their solubility and stability, which may have a bearing on the functions of these keratins.

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.

Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia

Historically, the term 'keratin' stood for all of the proteins extracted from skin modifications, such as horns, claws and hooves. Subsequently, it was realized that this keratin is actually a mixture of keratins, keratin filament-associated proteins and other proteins, such as enzymes. Keratins were then defined as certain filament-forming proteins with specific physicochemical properties and extracted from the cornified layer of the epidermis, whereas those filament-forming proteins that were extracted from the living layers of the epidermis were grouped as 'prekeratins' or 'cytokeratins'. Currently, the term 'keratin' covers all intermediate filament-forming proteins with specific physicochemical properties and produced in any vertebrate epithelia. Similarly, the nomenclature of epithelia as cornified, keratinized or non-keratinized is based historically on the notion that only the epidermis of skin modifications such as horns, claws and hooves is cornified, that the non-modified epidermis is a keratinized stratified epithelium, and that all other stratified and non-stratified epithelia are non-keratinized epithelia. At this point in time, the concepts of keratins and of keratinized or cornified epithelia need clarification and revision concerning the structure and function of keratin and keratin filaments in various epithelia of different species, as well as of keratin genes and their modifications, in view of recent research, such as the sequencing of keratin proteins and their genes, cell culture, transfection of epithelial cells, immunohistochemistry and immunoblotting. Recently, new functions of keratins and keratin filaments in cell signaling and intracellular vesicle transport have been discovered. It is currently understood that all stratified epithelia are keratinized and that some of these keratinized stratified epithelia cornify by forming a Stratum corneum. The processes of keratinization and cornification in skin modifications are different especially with respect to the keratins that are produced. Future research in keratins will provide a better understanding of the processes of keratinization and cornification of stratified epithelia, including those of skin modifications, of the adaptability of epithelia in general, of skin diseases, and of the changes in structure and function of epithelia in the course of evolution. This review focuses on keratins and keratin filaments in mammalian tissue but keratins in the tissues of some other vertebrates are also considered.

Keratin overexpression levels correlate with the extent of spontaneous pancreatic injury

Mutation of the adult hepatocyte keratins K8 and K18 predisposes to liver disease. In contrast, exocrine pancreas K8 and K18 are dispensable and are co-expressed with limited levels of membrane-proximal K19 and K20. Overexpression of mutant K18 or genetic ablation of K8 in mouse pancreas is well tolerated whereas overexpression of K8 causes spontaneous chronic pancreatitis. To better understand the effect of exocrine pancreatic keratin overexpression, we compared transgenic mice that overexpress K18, K8, or K8/K18, associated with minimal, modest, or large increases in keratin expression, respectively, with nontransgenic wild-type (WT) mice. Overexpression of the type-II keratin K8 up-regulated type-I keratins K18, K19, and K20 and generated K19/K20-containing neocytoplasmic typical or short filaments; however, overexpression of K18 had no effect on K8 levels. K8- and K18-overexpressing pancreata were histologically similar to WT, whereas K8/K18 pancreata displayed age-enhanced vacuolization and atrophy of the exocrine pancreas and exhibited keratin hyperphosphorylation. Zymogen granules in K8/K18 pancreata were 50% smaller and more dispersed than their normal apical concentration but were twice as numerous as in WT controls. Therefore, modest keratin overexpression has minor effects on the exocrine pancreas whereas significant keratin overexpression alters zymogen granule organization and causes aging-associated exocrine atrophy. Keratin absence or mutation is well tolerated after pancreatic but not liver injury, whereas excessive overexpression is toxic to the pancreas but not the liver when induced under basal conditions.

Reg-II is an exocrine pancreas injury-response product that is up-regulated by keratin absence or mutation

The major keratins in the pancreas and liver are keratins 8 and 18 (K8/K18), but their function seemingly differs in that liver K8/K18 are essential cytoprotective proteins, whereas pancreatic K8/K18 are dispensable. This functional dichotomy raises the hypothesis that K8-null pancreata may undergo compensatory cytoprotective gene expression. We tested this hypothesis by comparing the gene expression profile in pancreata of wild-type and K8-null mice. Most prominent among the up-regulated genes in K8-null pancreas was mRNA for regenerating islet-derived (Reg)-II, which was confirmed by quantitative reverse transcription-polymerase chain reaction and by an anti-Reg-II peptide antibody we generated. Both K8-null and wild-type mice express Reg-II predominantly in acinar cells as determined by in situ hybridization and immunostaining. Analysis of Reg-II expression in various keratin-related transgenic mouse models showed that its induction also occurs in response to keratin cytoplasmic filament collapse, absence, or ablation of K18 Ser52 but not Ser33 phosphorylation via Ser-to-Ala mutation, which represent situations associated with predisposition to liver but not pancreatic injury. In wild-type mice, Reg-II is markedly up-regulated in two established pancreatitis models in response to injury and during the recovery phase. Thus, Reg-II is a likely mouse exocrine pancreas cytoprotective candidate protein whose expression is regulated by keratin filament organization and phosphorylation.

Arsenite induced oxidative damage in mouse liver is associated with increased cytokeratin 18 expression

Cytokeratins (CK) constitute a family of cytoskeletal intermediate filament proteins that are typically expressed in epithelial cells. An abnormal structure and function are effects that are clearly related to liver diseases as non-alcoholic steatohepatitis, cirrhosis and hepatocellular carcinoma. We have previously observed that sodium arsenite (SA) induced the synthesis of CK18 protein and promotes a dose-related disruption of cytoplasmic CK18 filaments in a human hepatic cell line. Both abnormal gene expression and disturbance of structural organization are toxic effects that are likely to cause liver disease by interfering with normal hepatocyte function. To investigate if a disruption in the CK18 expression pattern is associated with arsenite liver damage, we investigated CK18 mRNA and protein levels in liver slices treated with low levels of SA. Organotypic cultures were incubated with 0.01, 1 and 10 microM of SA in the absence and presence of N-acetyl cysteine (NAC). Cell viability and inorganic arsenic metabolism were determined. Increased expression of CK18 was observed after exposure to SA. The addition of NAC impeded the oxidative effects of SA exposure, decreasing the production of thiobarbituric acid-reactive substances and significantly diminishing the up regulation of CK18 mRNA and protein. Liver arsenic levels correlated with increased levels of mRNA. Mice treated with intragastric single doses of 2.5 and 5 mg/kg of SA showed an increased expression of CK18. Results suggest that CK18 expression may be a sensible early biomarker of oxidative stress and damage induced by arsenite in vitro and in vivo. Then, during SA exposure, altered CK expression may compromise liver function.

Reciprocal keratin 18 Ser48 O-GlcNAcylation and Ser52 phosphorylation using peptide analysis

Phosphorylation and O-GlcNAcylation of keratin 18 (K18) are highly dynamic and involve primarily independent K18 populations. We used in vitro phosphorylation and O-GlcNAcylation of wild-type, phospho-Ser52, glyco-Ser48, and Ser-to-Ala mutant 17mer peptides (K18 amino acids 40-56), which include the major K18 glycosylation (Ser48) and phosphorylation (Ser52) sites, to address whether each modification blocks the other. The glyco-K18 peptide blocks Ser52 phosphorylation by protein kinase C, an in vivo K18 kinase, while the phospho-K18 peptide blocks its O-GlcNAcylation. Our findings support the reciprocity of these two post-translational modifications. Therefore, regulation of protein Ser/Thr phosphorylation and glycosylation at proximal sites can be interdependent and provides a potential mechanism of counter regulation.

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.

Accumulation of ubiquitin-conjugated cytokeratin fragments in tumor cells

Ubiquitin regulates cell functions by modifying various proteins, and cytokeratin (CK) is one of the targets of ubiquitilation. Accumulation of modified CK in various cancers has been demonstrated, and the modified CK increases the aggressiveness of the cancer by disrupting the cytoplasmic CK network and allows them to move freely. The phenotype of the cancer cells may be altered in such a way as to facilitate invasion and metastasis. Modified CK also deregulates mechanisms of mitosis and apoptosis, and leads to immortalization. Therapeutic targeting of ubiquitin or ubiquitilated proteins may reduce the malignant potential of cancer cells.

Keratin 8 and 18 hyperphosphorylation is a marker of progression of human liver disease

Keratin 8 and 18 (K8/18) phosphorylation plays a significant and site-specific role in regulating keratin filament organization, association with binding proteins, and modulation of cell cycle progression. Keratin hyperphosphorylation correlates with exposure to a variety of stresses in cultured cells and in mouse models of liver, pancreatic, and gallbladder injury, and it is found in association with mouse and human Mallory bodies. We asked whether K8/18 phosphorylation correlates with human liver disease progression by analyzing liver explants and biopsies of patients with chronic noncirrhotic hepatitis C virus (HCV) or cirrhosis. We also examined the effect of HCV therapy with interleukin-10 on keratin phosphorylation. Using site-specific antiphosphokeratin antibodies we found keratin hyperphosphorylation on most K8/18 sites in all cirrhotic liver explants tested and in most liver biopsies from patients with chronic HCV infection. Immunofluorescence staining of precirrhotic HCV livers showed focal keratin hyperphosphorylation and limited reorganization of keratin filament networks. In cirrhotic livers, keratin hyperphosphorylation occurred preferentially in hepatic nodule cells adjacent to bridging fibrosis and associated with increased stress kinase activation and apoptosis. Histological and serological improvement after interleukin-10 therapy was accompanied by normalization of keratin hyperphosphorylation on some sites in 7 of 10 patients. In conclusion, site-specific keratin phosphorylation in liver disease is a progression marker when increased and a likely regression marker when decreased.

Cell cycle-dependent protein fingerprint from a single cancer cell: image cytometry coupled with single-cell capillary sieving electrophoresis

Study of cell cycle-dependent protein expression is important in oncology, stem cell research, and developmental biology. In this paper, we report the first protein fingerprint from a single cell with known phase in the cell cycle. To determine that phase, we treated HT-29 colon cancer cells with Hoescht 33342, a vital nuclear stain. A microscope was used to measure the fluorescence intensity from one treated cell; in this form of image cytometry, the fluorescence intensity is proportional to the cell's DNA content, which varies in a predictable fashion during the cell cycle. To generate the protein fingerprint, the cell was aspirated into the separation capillary and lysed. Proteins were fluorescently labeled with 3-(2-furoylquinoline-2-carboxaldehyde, separated by capillary sieving electrophoresis, and detected by laser-induced fluorescence. This form of electrophoresis is the capillary version of SDS-PAGE. The single-cell electropherogram partially resolved approximately 25 components in a 30-min separation, and the dynamic range of the detector exceeded 5000. There was a large cell-to-cell variation in protein expression, averaging 40% relative standard deviation across the electropherogram. The dominant source of variation was the phase of the cell in the cell cycle; on average, approximately 60% of the cell-to-cell variance in protein expression was associated with the cell cycle. Cells in the G1 and G2/M phases of the cell cycle had 27 and 21% relative standard deviations in protein expression, respectively. Cells in the G2/M phase generated signals that were twice the amplitude of the signals generated by G1 phase cells, as expected for cells that are soon to divide into two daughter cells. When electropherograms were normalized to total protein content, the expression of only one component was dependent on cell cycle at the 99% confidence limit. That protein is tentatively identified as cytokeratin 18 in a companion paper.

Identification of Proteins in Single-Cell Capillary Electrophoresis Fingerprints Based on Comigration with Standard Proteins

In the previous paper in this Journal, we reported the use of capillary sieving electrophoresis to characterize proteins expressed by single cancer cells at specific phases in the cell cycle. Analysis of the data revealed one component with cell cycle-dependent changes in expression at the 99% confidence limit. However, the amount of protein present in a single cell is far too small to allow its direct identification by mass spectrometry. In this paper, we report a method by which such proteins can be tentatively identified. We perform standard SDS-PAGE electrophoresis of the proteins contained within a homogenate prepared from an HT29 cell culture. Proteins extracted from bands in the gel are identified by mass spectrometry. The proteins also provide a set of standards that can be used to spike the sample before capillary sieving electrophoresis (CSE) separation; comigration is taken as evidence for the identity of the target protein. In a proof-of-principle experiment, a single band migrating at approximately 47 kDa was isolated from the SDS-PAGE gel generated from the HT29 cell line. Proteins extracted from this band were used to spike a CSE separation of the same extract. This band comigrated with a cell cycle-dependent component identified from single-cell analysis. In-gel digestion and LC/MS/MS were used to identify five proteins, including cytokeratin 18, which is the product of the most highly expressed gene in this cell line.

Type II keratins are phosphorylated on a unique motif during stress and mitosis in tissues and cultured cells

Epithelial cell keratins make up the type I (K9-K20) and type II (K1-K8) intermediate filament proteins. In glandular epithelia, K8 becomes phosphorylated on S73 ((71)LLpSPL) in human cultured cells and tissues during stress, apoptosis, and mitosis. Of all known proteins, the context of the K8 S73 motif (LLS/TPL) is unique to type II keratins and is conserved in epidermal K5/K6, esophageal K4, and type II hair keratins, except that serine is replaced by threonine. Because knowledge regarding epidermal and esophageal keratin regulation is limited, we tested whether K4-K6 are phosphorylated on the LLTPL motif. K5 and K6 become phosphorylated in vitro on threonine by the stress-activated kinase p38. Site-specific anti-phosphokeratin antibodies to LLpTPL were generated, which demonstrated negligible basal K4-K6 phosphorylation. In contrast, treatment of primary keratinocytes and other cultured cells, and ex vivo skin and esophagus cultures, with serine/threonine phosphatase inhibitors causes a dramatic increase in K4-K6 LLpTPL phosphorylation. This phosphorylation is accompanied by keratin solubilization, filament reorganization, and collapse. K5/K6 LLTPL phosphorylation occurs in vivo during mitosis and apoptosis induced by UV light or anisomycin, and in human psoriatic skin and squamous cell carcinoma. In conclusion, type II keratins of proliferating epithelia undergo phosphorylation at a unique and conserved motif as part of physiological mitotic and stress-related signals.

Keratin-mediated resistance to stress and apoptosis in simple epithelial cells in relation to health and disease

Epithelial cells such as hepatocytes exhibit highly polarized properties as a result of the asymmetric distribution of subsets of receptors at unique portions of the surface membrane. While the proper targeting of these surface receptors and maintenance of the resulting polarity depend on microtubules (MTs), the Golgi sorting compartment, and different actin-filament networks, the contribution of keratin intermediate filaments (IFs) has been unclear. Recent data show that the latter cytoskeletal network plays a predominant role in providing resistance to various forms of stress and to apoptosis targeted to the surface membrane. In this context, we first summarize our knowledge of the domain- or assembly-related features of IF proteins and the dynamic properties of IF networks that may explain how the same keratin pair K8/K18 can exert multiple resistance-related functions in simple epithelial cells. We then examine the contribution of linker protein(s) that integrate interactions of keratin IFs with MTs and the actin-cytoskeleton network, polarity-dependent surface receptors and cytoplasmic organelles. We next address likely molecular mechanisms by which K8/K18 can selectively provide resistance to a mechanical or toxic stress, or to Fas-mediated apoptosis. Finally, these issues on keratin structure-function are examined within a context of pathological anomalies emerging in tissue architecture as a result of natural or targeted mutations, or posttranslational modifications at specific amino acid residues. Clearly. the data accumulated in recent years provide new and significant insights on the role of K8/K18, particularly under conditions where polarized cells resist to stressful or apoptotic insults.

Keratin 8 phosphorylation by p38 kinase regulates cellular keratin filament reorganization: modulation by a keratin 1-like disease causing mutation

Keratin 8 (K8) serine 73 occurs within a relatively conserved type II keratin motif ((68)NQSLLSPL) and becomes phosphorylated in cultured cells and organs during mitosis, cell stress, and apoptosis. Here we show that Ser-73 is exclusively phosphorylated in vitro by p38 mitogen-activated protein kinase. In cells, Ser-73 phosphorylation occurs in association with p38 kinase activation and is inhibited by SB203580 but not by PD98059. Transfection of K8 Ser-73 --> Ala or K8 Ser-73 --> Asp with K18 generates normal-appearing filaments. In contrast, exposure to okadaic acid results in keratin filament destabilization in cells expressing wild-type or Ser-73 --> Asp K8, whereas Ser-73 --> Ala K8-expressing cells maintain relatively stable filaments. p38 kinase associates with K8/18 immunoprecipitates and binds selectively with K8 using an in vitro overlay assay. Given that K1 Leu-160 --> Pro ((157)NQSLLQPL --> (157)NQSPLQPL) leads to epidermolytic hyperkeratosis, we tested and showed that the analogous K8 Leu-71 --> Pro leads to K8 hyperphosphorylation by p38 kinase in vitro and in transfected cells, likely due to Ser-70 neo-phosphorylation, in association with significant keratin filament collapse upon cell exposure to okadaic acid. Hence, K8 Ser-73 is a physiologic phosphorylation site for p38 kinase, and its phosphorylation plays an important role in keratin filament reorganization. The Ser-73 --> Ala-associated filament reorganization defect is rescued by a Ser-73 --> Asp mutation. Also, disease-causing keratin mutations can modulate keratin phosphorylation and organization, which may affect disease pathogenesis.

Cytokeratin expression patterns in jaw cyst linings with metaplastic epithelium

BACKGROUND

Cytokeratin (CK) expression patterns have been studied in numerous intact and diseased oral tissues. However, CK expression in metaplastic squamous cells has not been explored in depth and the origin of metaplastic epithelial linings of the jaw cysts has not been sufficiently investigated.

METHODS

We examined CK expression in 46 postoperative maxillary cysts (POMCs) which were lined with pseudostratified columnar cells only, columnar and squamous cells, and squamous cells only, in 13, 30 and 3 cases, respectively.

RESULTS

The expression of CK8, CK13 and CK18 were observed in 39, 9 and all 43 of the columnar epithelial linings, respectively. Metaplastic squamous epithelia expressed more CK13, and less CK18 and CK8. Of the 33 metaplastic linings, 24 expressed CK8, 23 CK13 and 26 linings expressed CK18. The patterns of expression of CK13 and CK18 observed were CK18(+)-CK13(-) in 10 metaplastic linings, CK18(+)-CK13(+) in 16, and CK18(-)-CK13(+) in 7. The expression of CK13- and CK18-mRNA was generally correlated with level of protein expressed. CK18-mRNA expression was observed by in situ hybridization, not only in the 26 metaplastic linings which were positive for CK18 protein, but also in five of the seven metaplastic linings which did not express CK18 protein. In addition, RT-PCR revealed an expression of CK18-mRNA in all metaplastic squamous linings, although the expression level was weaker than that in the columnar epithelial linings. The CK13-mRNA was expressed inversely to the CK18-mRNA.

CONCLUSIONS

These results indicate that CK18-mRNA is preserved through metaplasia, although the protein expression decreased. Metaplastic squamous cells differentiate with a decrease of CK18 and an increase of CK13 expression.

The interaction of Src and RACK1 is enhanced by activation of protein kinase C and tyrosine phosphorylation of RACK1

RACK1 is an intracellular receptor for the serine/ threonine protein kinase C. Previously, we demonstrated that RACK1 also interacts with the Src protein-tyrosine kinase. RACK1, via its association with these protein kinases, may play a key role in signal transduction. To further characterize the Src-RACK1 interaction and to analyze mechanisms by which cross-talk occurs between the two RACK1-linked signaling kinases, we identified sites on Src and RACK1 that mediate their binding, and factors that regulate their interaction. We found that the interaction of Src and RACK1 is mediated, in part, by the SH2 domain of Src and by phosphotyrosines in the sixth WD repeat of RACK1, and is enhanced by serum or platelet-derived growth factor stimulation, protein kinase C activation, and tyrosine phosphorylation of RACK1. To the best of our knowledge, this is the first report of tyrosine phosphorylation of a member of the WD repeat family of proteins. We think that tyrosine phosphorylation of these proteins is an important mechanism of signal transduction in cells.

Mitotic reorganization of the intermediate filament protein nestin involves phosphorylation by cdc2 kinase

The intermediate filament protein nestin is expressed during early stages of development in the central nervous system and in muscle tissues. Nestin expression is associated with morphologically dynamic cells, such as dividing and migrating cells. However, little is known about regulation of nestin during these cellular processes. We have characterized the phosphorylation-based regulation of nestin during different stages of the cell cycle in a neuronal progenitor cell line, ST15A. Confocal microscopy of nestin organization and (32)P in vivo labeling studies show that the mitotic reorganization of nestin is accompanied by elevated phosphorylation of nestin. The phosphorylation-induced alterations in nestin organization during mitosis in ST15A cells are associated with partial disassembly of nestin filaments. Comparative in vitro and in vivo phosphorylation studies identified cdc2 as the primary mitotic kinase and Thr(316) as a cdc2-specific phosphorylation site on nestin. We generated a phosphospecific nestin antibody recognizing the phosphorylated form of this site. By using this antibody we observed that nestin shows constitutive phosphorylation at Thr(316), which is increased during mitosis. This study shows that nestin is reorganized during mitosis and that cdc2-mediated phosphorylation is an important regulator of nestin organization and dynamics during mitosis.

Simple epithelial keratins are dispensable for cytoprotection in two pancreatitis models

Pancreatic acinar cells express keratins 8 and 18 (K8/18), which form cytoplasmic filament (CF) and apicolateral filament (ALF) pools. Hepatocyte K8/18 CF provide important protection from environmental stresses, but disruption of acinar cell CF has no significant impact. We asked whether acinar cell ALF are important in providing cytoprotective roles by studying keratin filaments in pancreata of K8- and K18-null mice. K8-null pancreas lacks both keratin pools, but K18-null pancreas lacks only CF. Mouse but not human acinar cells also express apicolateral keratin 19 (K19), which explains the presence of apicolateral keratins in K18-null pancreas. K8- and K18-null pancreata are histologically normal, and their acini respond similarly to stimulated secretion, although K8-null acini viability is reduced. Absence of total filaments (K8-null) or CF (K18-null) does not increase susceptibility to pancreatitis induced by caerulein or a choline-deficient diet. In normal and K18-null acini, K19 is upregulated after caerulein injury and, unexpectedly, forms CF. As in hepatocytes, acinar injury is also associated with keratin hyperphosphorylation. Hence, K19 forms ALF in mouse acinar cells and helps define two distinct ALF and CF pools. On injury, K19 forms CF that revert to ALF after healing. Acinar keratins appear to be dispensable for cytoprotection, in contrast to hepatocyte keratins, despite similar hyperphosphorylation patterns after injury.

Site-specific phosphorylation of neurofilament-L is mediated by calcium/calmodulin-dependent protein kinase II in the apical dendrites during long-term potentiation

Neurofilament-L (NF-L), one subunit of the neuronal intermediate filaments, is a major element of neuronal cytoskeletons. The dynamics of NF-L are regulated by phosphorylation of its head domain. The phosphorylation sites of the NF-L head domain by protein kinase A, protein kinase C, and Rho-associated kinase have been previously identified, and those by calcium/calmodulin-dependent protein kinase II (CaMKII) were identified in this study. A series of site- and phosphorylation state-specific antibodies against NF-L was prepared to investigate NF-L phosphorylation in neuronal systems. Long-term potentiation (LTP) is a cellular model of neuronal plasticity that is thought to involve the phosphorylation of various proteins. NF-L is considered a possible substrate for phosphorylation. During LTP stimulation of mouse hippocampal slices, the series of antibodies demonstrated the increase in the phosphorylation level of Ser(57) in NF-L and the visualization of the localized distribution of Ser(57) phosphorylation in a subpopulation of apical dendrites of the pyramidal neurons. Furthermore, Ser(57) phosphorylation during LTP is suggested to be mediated by CaMKII. Here we show that NF-L is phosphorylated by CaMKII in a subpopulation of apical dendrites during LTP, indicating that Ser(57) is a novel phosphorylation site of NF-L in vivo related to the neuronal signal transduction.

Keratin filament deployment and cytoskeletal networking in a sensory epithelium that vibrates during hearing

The intricate and spatially precise ways in which keratin intermediate filaments are deployed in certain cochlear epithelial cells, called supporting cells, suggests that these filaments make a micromechanically important contribution to the functional design of the guinea pig organ of Corti. Filament arrays that include keratins 8, 18, and 19 are confined mainly to regions close to the ends of large transcellular microtubule bundles in supporting cells. These cells and their microtubule bundles link sensory hair cells to a specialized basement membrane that vibrates during hearing. The keratin filament arrays apparently help anchor the ends of the microtubule bundles to cell surfaces. Filaments are concentrated at the apices and bases of most cells that contact hair cells. Substantial arrays of adherens junctions link the apices of these cells. Hence, keratin filaments may contribute to a cytoskeletal network that distributes mechanical forces from cell to cell and that coordinates the displacement of neighboring hair cells. However, high concentrations of keratin filaments have not been detected at the apices of one of the supporting cell types, which apparently has a mechanical role that is different from that of the others. Transmission electron microscopy has revealed previously undescribed filament networks at all the locations where the binding of antibodies to keratins is most marked. There is evidence that intercellular linkage of the keratin networks via their association with actin-containing meshworks and adherens junctions is more extensive than linkage provided by desmosomes.

Keratins turn over by ubiquitination in a phosphorylation-modulated fashion

Keratin polypeptides 8 and 18 (K8/18) are intermediate filament (IF) proteins that are expressed in glandular epithelia. Although the mechanism of keratin turnover is poorly understood, caspase-mediated degradation of type I keratins occurs during apoptosis and the proteasome pathway has been indirectly implicated in keratin turnover based on colocalization of keratin-ubiquitin antibody staining. Here we show that K8 and K18 are ubiquitinated based on cotransfection of His-tagged ubiquitin and human K8 and/or K18 cDNAs, followed by purification of ubiquitinated proteins and immunoblotting with keratin antibodies. Transfection of K8 or K18 alone yields higher levels of keratin ubiquitination as compared with cotransfection of K8/18, likely due to stabilization of the keratin heteropolymer. Most of the ubiquitinated species partition with the noncytosolic keratin fraction. Proteasome inhibition stabilizes K8 and K18 turnover, and is associated with accumulation of phosphorylated keratins, which indicates that although keratins are stable they still turnover. Analysis of K8 and K18 ubiquitination and degradation showed that K8 phosphorylation contributes to its stabilization. Our results provide direct evidence for K8 and K18 ubiquitination, in a phosphorylation modulated fashion, as a mechanism for regulating their turnover and suggest that other IF proteins could undergo similar regulation. These and other data offer a model that links keratin ubiquitination and hyperphosphorylation that, in turn, are associated with Mallory body deposits in a variety of liver diseases.

Active caspases and cleaved cytokeratins are sequestered into cytoplasmic inclusions in TRAIL-induced apoptosis

Tumor necrosis factor-related apoptosis- inducing ligand (TRAIL) -induced apoptosis, in transformed human breast epithelial MCF-7 cells, resulted in a time-dependent activation of the initiator caspases-8 and -9 and the effector caspase-7. Cleavage of caspase-8 and its preferred substrate, Bid, preceded processing of caspases-7 and -9, indicating that caspase-8 is the apical initiator caspase in TRAIL-induced apoptosis. Using transient transfection of COOH-terminal-tagged green fluorescent protein fusion constructs, caspases-3, -7, and -8 were localized throughout the cytoplasm of MCF-7 cells. TRAIL-induced apoptosis resulted in activation of caspases-3 and -7, and the redistribution of most of their detectable catalytically active small subunits into large spheroidal cytoplasmic inclusions, which lacked a limiting membrane. These inclusions, which were also induced in untransfected cells, contained cytokeratins 8, 18, and 19, together with both a phosphorylated form and a caspase-cleavage fragment of cytokeratin 18. Similarly, in untransfected breast HBL100 and lung A549 epithelial cells, TRAIL induced the formation of cytoplasmic inclusions that contained cleaved cytokeratin 18 and colocalized with active endogenous caspase-3. We propose that effector caspase-mediated cleavage of cytokeratins, resulting in disassembly of the cytoskeleton and formation of cytoplasmic inclusions, may be a characteristic feature of epithelial cell apoptosis.

Effects of keratin filament disruption on exocrine pancreas-stimulated secretion and susceptibility to injury

Disruption or absence of hepatocyte keratins 8 and 18 is associated with chronic hepatitis, marked hepatocyte fragility, and a significant predisposition to stress-induced liver injury. In contrast, pancreatic keratin disruption in transgenic mice that express keratin 18 Arg89 --> Cys (K18C) is not associated with an obvious pancreatic pathology. We compared the effects of keratin filament disruption on pancreatic acini or acinar cell viability, and on cholecystokinin (CCK)-stimulated secretion, in transgenic mice that overexpress wild-type keratin 18 and harbor normal extended keratin filaments (TG2) and K18C mice. We also compared the response of these mice to pancreatitis induced by a choline-deficient ethionine-supplemented diet or by caerulein. Despite extensive cytoplasmic keratin filament disruption, the apicolateral keratin filament bundles appear intact in the acinar pancreas of K18C mice, as determined ultrastructurally and by light microscopy. No significant pancreatitis-associated histologic, serologic, or F-actin/keratin apicolateral redistribution differences were noted between TG2 and K18C mice. Acinar cell viability and yield after collagenase digestion were lower in K18C than in TG2 mice, but the yields of intact acini and their (125)I-CCK uptake and responses to CCK-stimulated secretion were similar. Our results indicate that keratin filament reorganization is a normal physiologic response to pancreatic cell injury, but an intact keratin cytoplasmic filament network is not as essential in protection from cell injury as in the liver. These findings raise the possibility that the abundant apicolateral acinar keratin filaments, which are not as evident in hepatocytes, may play the cytoprotective role that is seen in liver and other tissues. Alternatively, identical keratins may function differently in different tissues.

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.

alphaB-crystallin interacts with cytoplasmic intermediate filament bundles during mitosis

The small heat-shock protein alphaB-crystallin interacts with intermediate filament proteins. Using cosedimentation assay, we showed previously that in vitro binding of alphaB-crystallin to peripherin and vimentin was temperature-dependent. Furthermore, when NIH 3T3 cells were submitted to different stress conditions a dynamic reorganization of the intermediate filament network was observed concomitantly with the recruitment of alphaB-crystallins on the intermediate filament proteins. Thus, the intracellular state of alphaB-crystallin correlated directly with the remodeling of the intermediate filament network in response to stress. Here, we show data suggesting that alphaB-crystallin is implicated in remodeling of intermediate filaments during cell division. We investigated the intracellular distribution of alphaB-crystallin in naturally occurring mitotic NIH 3T3 cells and in neuroblastoma N2a and N1E115 cells. In NIH 3T3 cells, alphaB-crystallin remained diffused throughout the cell cycle. Subcellular fractionation of alphaB-crystallin showed that alphaB-crystallin remained in the cytosolic compartment during mitosis. Furthermore, alphaB-crystallin accumulated in mitotically arrested NIH 3T3 cells. This increased level of alphaB-crystallin protein was due to an increased level of alphaB-crystallin mRNA in mitotic NIH 3T3 cells. In the neuroblastoma cells, the intermediate filaments were rearranged into thick cable-like structures and alphaB-crystallin was recruited onto them. In neuroblastoma N2a cells the level of expression did not change during the cell cycle. However, a small fraction of alphaB-crystallin switched onto the insoluble fraction in mitotically arrested N2a cells. Our results suggested that depending on the state of rearrangement of the intermediate filament network during mitosis alphaB-crystallin was either recruited onto the intermediate filaments or upregulated in the cytosolic compartment.

Pervanadate-mediated tyrosine phosphorylation of keratins 8 and 19 via a p38 mitogen-activated protein kinase-dependent pathway

Glandular epithelia express the keratin intermediate filament (IF) polypeptides 8, 18 and 19 (K8/18/19). These proteins undergo significant serine phosphorylation upon stimulation with growth factors and during mitosis, with subsequent modulation of their organization and interaction with associated proteins. Here we demonstrate reversible and dynamic tyrosine phosphorylation of K8 and K19, but not K18, upon exposure of intact mouse colon or cultured human cells to pervanadate. K8/19 tyrosine phosphorylation was confirmed by metabolic 32PO4-labeling followed by phosphoamino acid analysis, and by immunoblotting with anti-phosphotyrosine antibodies. Pervanadate treatment increases keratin solubility and also indirectly increases K8/18 serine phosphorylation at several known sites, some of which were previously shown to be associated with EGF stimulation, extracellular signal-regulated kinase (ERK), or p38 kinase activation. However, K8/19 tyrosine phosphorylation is independent of EGF signaling or ERK activation while inhibition of p38 kinase activity blocks pervanadate-induced K8/19 tyrosine phosphorylation. Our results demonstrate tyrosine phosphatase inhibitor-mediated in vivo tyrosine phosphorylation of K8/19, but not K18, and suggest that tyrosine phosphorylation may be a general modification of other IF proteins. K8/19 tyrosine phosphorylation involves a pathway that utilizes the p38 mitogen-activated protein kinase, but appears independent of EGF signaling or ERK kinase activation.

Characterization of the major physiologic phosphorylation site of human keratin 19 and its role in filament organization

Keratin polypeptide 19 (K19) is a type I intermediate filament protein that is expressed in stratified and simple-type epithelia. Little is known regarding K19 regulation or function, and the only other type I keratin that has been studied in terms of regulation is keratin 18 (K18). We characterized K19 phosphorylation as a handle to study its function. In vivo, serine is the major phosphorylated residue, and phosphopeptide mapping of 32PO4-labeled K19 generates one major phosphopeptide. Edman degradation suggested that the radiolabeled phosphopeptide represents K19 Ser-10 and/or Ser-35 phosphorylation. Mutation of Ser-10 or Ser-35 followed by transfection confirmed that Ser-35 is the major K19 phosphorylation site. Transfection of Ser-35 --> Ala K19 showed a filament assembly defect as compared with normal or with Ser-10 --> Ala K19. Comparison of K18 and K19 phosphorylation features in interphase cells showed that both are phosphorylated primarily at a single site, preferentially in the soluble versus the insoluble keratin fractions. K19 has higher basal phosphorylation, whereas K18 phosphorylation is far more sensitive to phosphatase type I and IIA inhibition. Our results demonstrate that Ser-35 is the major K19 interphase phosphorylation site and that it plays a role in keratin filament assembly. K19 and K18 phosphorylations share some features but also have distinct properties that suggest different regulation of type I keratins within the same cells.

Mutation of a major keratin phosphorylation site predisposes to hepatotoxic injury in transgenic mice

Simple epithelia express keratins 8 (K8) and 18 (K18) as their major intermediate filament (IF) proteins. One important physiologic function of K8/18 is to protect hepatocytes from drug-induced liver injury. Although the mechanism of this protection is unknown, marked K8/18 hyperphosphorylation occurs in association with a variety of cell stresses and during mitosis. This increase in keratin phosphorylation involves multiple sites including human K18 serine-(ser)52, which is a major K18 phosphorylation site. We studied the significance of keratin hyperphosphorylation and focused on K18 ser52 by generating transgenic mice that overexpress a human genomic K18 ser52--> ala mutant (S52A) and compared them with mice that overexpress, at similar levels, wild-type (WT) human K18. Abrogation of K18 ser52 phosphorylation did not affect filament organization after partial hepatectomy nor the ability of mouse livers to regenerate. However, exposure of S52A-expressing mice to the hepatotoxins, griseofulvin or microcystin, which are associated with K18 ser52 and other keratin phosphorylation changes, resulted in more dramatic hepatotoxicity as compared with WT K18-expressing mice. Our results demonstrate that K18 ser52 phosphorylation plays a physiologic role in protecting hepatocytes from stress-induced liver injury. Since hepatotoxins are associated with increased keratin phosphorylation at multiple sites, it is likely that unique sites aside from K18 ser52, and phosphorylation sites on other IF proteins, also participate in protection from cell stress.

Functional differences between keratins of stratified and simple epithelia

Dividing populations of stratified and simple epithelial tissues express keratins 5 and 14, and keratins 8 and 18, respectively. It has been suggested that these keratins form a mechanical framework important to cellular integrity, since their absence gives rise to a blistering skin disorder in neonatal epidermis, and hemorrhaging within the embryonic liver. An unresolved fundamental issue is whether different keratins perform unique functions in epithelia. We now address this question using transgenic technology to express a K16-14 hybrid epidermal keratin transgene and a K18 simple epithelial keratin transgene in the epidermis of mice null for K14. Under conditions where the hybrid epidermal keratin restored a wild-type phenotype to newborn epidermis, K18 partially but not fully rescued. The explanation does not appear to reside in an inability of K18 to form 10-nm filaments with K5, which it does in vitro and in vivo. Rather, it appears that the keratin network formed between K5 and K18 is deficient in withstanding mechanical stress, leading to perturbations in the keratin network in regions of the skin that are subjected either to natural or to mechanically induced trauma. Taken together, these findings suggest that the loss of a type I epidermal keratin cannot be fully compensated by its counterpart of simple epithelial cells, and that in vivo, all keratins are not equivalent.

Adducin is an in vivo substrate for protein kinase C: phosphorylation in the MARCKS-related domain inhibits activity in promoting spectrin-actin complexes and occurs in many cells, including dendritic spines of neurons

Adducin is a heteromeric protein with subunits containing a COOH-terminal myristoylated alanine-rich C kinase substrate (MARCKS)-related domain that caps and preferentially recruits spectrin to the fast-growing ends of actin filaments. The basic MARCKS-related domain, present in alpha, beta, and gamma adducin subunits, binds calmodulin and contains the major phosphorylation site for protein kinase C (PKC). This report presents the first evidence that phosphorylation of the MARCKS-related domain modifies in vitro and in vivo activities of adducin involving actin and spectrin, and we demonstrate that adducin is a prominent in vivo substrate for PKC or other phorbol 12-myristate 13-acetate (PMA)-activated kinases in multiple cell types, including neurons. PKC phosphorylation of native and recombinant adducin inhibited actin capping measured using pyrene-actin polymerization and abolished activity of adducin in recruiting spectrin to ends and sides of actin filaments. A polyclonal antibody specific to the phosphorylated state of the RTPS-serine, which is the major PKC phosphorylation site in the MARCKS-related domain, was used to evaluate phosphorylation of adducin in cells. Reactivity with phosphoadducin antibody in immunoblots increased twofold in rat hippocampal slices, eight- to ninefold in human embryonal kidney (HEK 293) cells, threefold in MDCK cells, and greater than 10-fold in human erythrocytes after treatments with PMA, but not with forskolin. Thus, the RTPS-serine of adducin is an in vivo phosphorylation site for PKC or other PMA-activated kinases but not for cAMP-dependent protein kinase in a variety of cell types. Physiological consequences of the two PKC phosphorylation sites in the MARCKS-related domain were investigated by stably transfecting MDCK cells with either wild-type or PKC-unphosphorylatable S716A/S726A mutant alpha adducin. The mutant alpha adducin was no longer concentrated at the cell membrane at sites of cell-cell contact, and instead it was distributed as a cytoplasmic punctate pattern. Moreover, the cells expressing the mutant alpha adducin exhibited increased levels of cytoplasmic spectrin, which was colocalized with the mutant alpha adducin in a punctate pattern. Immunofluorescence with the phosphoadducin-specific antibody revealed the RTPS-serine phosphorylation of adducin in postsynaptic areas in the developing rat hippocampus. High levels of the phosphoadducin were detected in the dendritic spines of cultured hippocampal neurons. Spectrin also was a component of dendritic spines, although at distinct sites from the ones containing phosphoadducin. These data demonstrate that adducin is a significant in vivo substrate for PKC or other PMA-activated kinases in a variety of cells, and that phosphorylation of adducin occurs in dendritic spines that are believed to respond to external signals by changes in morphology and reorganization of cytoskeletal structures.