Heat stress or rotavirus infection of human epithelial cells generates a distinct hyperphosphorylated form of keratin 8

Host-cell dependent role of phosphorylated keratin 8 during influenza A/NWS/33 virus (H1N1) infection in mammalian cells

In this study, we investigated the involvement of keratin 8 during human influenza A/NWS/33 virus (H1N1) infection in semi-permissive rhesus monkey-kidney (LLC-MK2) and permissive human type II alveolar epithelial (A549) cells. In A549 cells, keratin 8 showed major expression and phosphorylation levels. Influenza A/NWS/33 virus was able to subvert keratin 8 structural organization at late stages of infection in both cell models, promoting keratin 8 phosphorylation in A549 cells at early phases of infection. Accordingly, partial colocalizations of the viral nucleoprotein with keratin 8 and its phosphorylated form were assessed by confocal microscopy at early stages of infection in A549 cells. The employment of chemical activators of phosphorylation resulted in structural changes as well as increased phosphorylation of keratin 8 in both cell models, favoring the influenza A/NWS/33 virus's replicative efficiency in A549 but not in LLC-MK2 cells. In A549 and human larynx epidermoid carcinoma (HEp-2) cells inoculated with respiratory secretions from pediatric patients positive for, respectively, influenza A virus or respiratory syncytial virus, the keratin 8 phosphorylation level had increased only in the case of influenza A virus infection. The results obtained suggest that in A549 cells the influenza virus is able to induce keratin 8 phosphorylation thereby enhancing its replicative efficiency.

Keratin 6a reorganization for ubiquitin-proteasomal processing is a direct antimicrobial response

Chan et al. show that epithelial cells respond to bacterial components in the environment by releasing subunits of the keratin 6a (K6a) filament network to the cytosol for degradation by the ubiquitin–proteasome system. This generates antimicrobial peptides from K6a that are important for innate defense of the mucosal surface.

Skin and mucosal epithelia deploy antimicrobial peptides (AMPs) to eliminate harmful microbes. We reported that the intermediate filament keratin 6a (K6a) is constitutively processed into antimicrobial fragments in corneal epithelial cells. In this study, we show that K6a network remodeling is a host defense response that directly up-regulates production of keratin-derived AMPs (KAMPs) by the ubiquitin–proteasome system (UPS). Bacterial ligands trigger K6a phosphorylation at S19, S22, S37, and S60, leading to network disassembly. Mutagenic analysis of K6a confirmed that the site-specific phosphorylation augmented its solubility. K6a in the cytosol is ubiquitinated by cullin-RING E3 ligases for subsequent proteasomal processing. Without an appreciable increase in K6a gene expression and proteasome activity, a higher level of cytosolic K6a results in enhanced KAMP production. Although proteasome-mediated proteolysis is known to produce antigenic peptides in adaptive immunity, our findings demonstrate its new role in producing AMPs for innate immune defense. Manipulating K6a phosphorylation or UPS activity may provide opportunities to harness the innate immunity of epithelia against infection.

Effects of cerulein on keratin 8 phosphorylation and perinuclear reorganization in pancreatic cancer cells: Involvement of downregulation of protein phosphatase 2A and alpha4

Toxicants can perturb cellular homeostasis by modifying phosphorylation-based signaling. In the present study, we examined the effects of cerulein, an inducer of acute pancreatitis, on keratin 8 (K8) phosphorylation. We found that cerulein dose-dependently induced K8 phosphorylation and perinuclear reorganization in PANC-1 cells, thus leading to migration and invasion. The extracellular signal-regulated kinases (ERK) inhibitor U0126 suppressed cerulein-induced phosphorylation of serine 431 and reorganization of K8. Cerulein reduced the expressions of protein phosphatase 2A (PP2A) via ubiqutination and alpha4. PP2A's involvement in K8 phosphorylation of PANC-1 cells was also confirmed by the observation that PP2A gene silencing resulted in K8 phosphorylation and migration of PANC-1 cells. Overall, these results suggest that cerulein induced phosphorylation and reorganization through ERK activation by downregulating PP2A and alpha4, leading to increased migration and invasion of PANC-1 cells. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 2090-2098, 2016.

Epithelial Intermediate Filaments: Guardians against Microbial Infection?

Intermediate filaments are abundant cytoskeletal components of epithelial tissues. They have been implicated in overall stress protection. A hitherto poorly investigated area of research is the function of intermediate filaments as a barrier to microbial infection. This review summarizes the accumulating knowledge about this interaction. It first emphasizes the unique spatial organization of the keratin intermediate filament cytoskeleton in different epithelial tissues to protect the organism against microbial insults. We then present examples of direct interaction between viral, bacterial, and parasitic proteins and the intermediate filament system and describe how this affects the microbe-host interaction by modulating the epithelial cytoskeleton, the progression of infection, and host response. These observations not only provide novel insights into the dynamics and function of intermediate filaments but also indicate future avenues to combat microbial infection.

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.

Phosphorylation and Reorganization of Keratin Networks: Implications for Carcinogenesis and Epithelial Mesenchymal Transition

Metastasis is one of hallmarks of cancer and a major cause of cancer death. Combatting metastasis is highly challenging. To overcome these difficulties, researchers have focused on physical properties of metastatic cancer cells. Metastatic cancer cells from patients are softer than benign cancer or normal cells. Changes of viscoelasticity of cancer cells are related to the keratin network. Unexpectedly, keratin network is dynamic and regulation of keratin network is important to the metastasis of cancer. Keratin is composed of heteropolymer of type I and II. Keratin connects from the plasma membrane to nucleus. Several proteins including kinases, and protein phosphatases bind to keratin intermediate filaments. Several endogenous compounds or toxic compounds induce phosphorylation and reorganization of keratin network in cancer cells, leading to increased migration. Continuous phosphorylation of keratin results in loss of keratin, which is one of the features of epithelial mesenchymal transition (EMT). Therefore, several proteins involved in phosphorylation and reorganization of keratin also have a role in EMT. It is likely that compounds controlling phosphorylation and reorganization of keratin are potential candidates for combating EMT and metastasis.

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

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We analyzed nuclear phosphoprotein expression activated by tobacco smoke exposure.

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

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

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

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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.

High-fat diet triggers Mallory-Denk body formation through misfolding and crosslinking of excess keratin 8

Mallory-Denk bodies (MDBs) are protein aggregates consisting of ubiquitinated keratins 8/18 (K8/K18). MDBs are characteristic of alcoholic and nonalcoholic steatohepatitis (NASH) and discriminate between the relatively benign simple steatosis and the more aggressive NASH. Given the emerging evidence for a genetic predisposition to MDB formation and NASH development in general, we studied whether high-fat (HF) diet triggers MDB formation and liver injury in susceptible animals. Mice were fed a high-fat (HF) or low-fat (LF) diet plus a cofactor for MDB development, 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC). Additionally, we fed nontransgenic and K8 overexpressing mice (K8tg) with the HF diet. The presence of MDB and extent of liver injury was evaluated using biochemical markers, histological staining, and immunofluorescence microscopy. In DDC-fed animals, an HF diet resulted in greater liver injury and up-regulation of inflammation-related genes. As a potential mechanism, K8/K18 accumulation and increased ecto-5'-nucleotidase (CD73) levels were noted. In the genetically susceptible K8tg mice, HF diet triggered hepatocellular injury, ballooning, apoptosis, inflammation, and MDB development by way of 1) decreased expression of the major stress-inducible chaperone Hsp72 with appearance of misfolded keratins; 2) elevated levels of the transglutaminase 2 (TG2); 3) increased K8 phosphorylation at S74 with subsequent TG2-mediated crosslinking of phosphorylated K8; and 4) higher production of the MDB-modifier gene CD73.

CONCLUSION

Our data demonstrate that HF diet triggers aggregate formation and development of liver injury in susceptible individuals through misfolding and crosslinking of excess K8.

Increased replication of respiratory syncytial virus in the presence of cytokeratin 8 and 18

Previously, it was reported that productive viral infection, viral protein synthesis, and viral RNA replication of respiratory syncytial virus (RSV) operated efficiently in two human epithelial cell lines (HEp‐2 and A549), but not in a human mast‐cell line, HMC‐1. Based on these observations, it was hypothesized that HMC‐1 cells lack the machinery required for RSV replication. To identify the host factors required for RSV replication, cDNA subtraction using A549, HEp‐2, and HMC‐1 cells was performed, and cytokeratin 18 (C18) was identified as a candidate host factor. Because C18 is generally expressed in simple epithelia with cytokeratin 8 (C8), HMC‐1 cells that constitutively express C18 and C8 (HMC‐1‐C8/18) were established to evaluate the role of C8/18 in RSV replication. In HMC‐1‐C8/18 cells, RSV RNA replication was increased, and the amount of infective virus produced was also increased in the cellular fraction after RSV spinoculation, whereas RSV production was decreased in A549 cells in which C18 expression was knocked down. These data suggest that the replication of RSV increases in the presence of C8/18. J. Med. Virol. 84:365–370, 2012. © 2011 Wiley Periodicals, Inc.

Keratins in oral cancer: necessity of mass spectrometry for validation of antibody based identifications

Keratins are intermediate filament family proteins which are predominantly expressed in the epithelial cells. Most of the studies which evaluate the status of keratins in clinical samples of the oral cavity are based on the identification of their presence and localization by immunohistochemistry using monoclonal antibodies. It is very well known that many monoclonal/polyclonal antibodies show cross-reactivity with the other closely related or non-related proteins. This cross-reactivity might be the result of epitope similarity, but it is not always necessary. Therefore studies done with only antibody based techniques can mislead interpretation unless they are validated with additional techniques like mass-spectrometry. In this investigation we have evaluated the status of keratin 18 in cancer of buccal mucosa using 1DE, 2DE and western blotting with monoclonal antibody to keratin 18. The patterns emerging showed aberrant as well as differential expression of K18 in adjacent normal versus tumor tissue samples of buccal mucosa. Mass spectrometry analysis of the immunodetected spots however revealed that it is keratin 13. Thus this study emphasizes the necessity of validation of antibody based findings when dealing with proteins of a large family having similarity/homology in amino acid sequence.

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.

The roles of O-linked β-N-acetylglucosamine in cardiovascular physiology and disease

More than 1,000 proteins of the nucleus, cytoplasm, and mitochondria are dynamically modified by O-linked β-N-acetylglucosamine (O-GlcNAc), an essential post-translational modification of metazoans. O-GlcNAc, which modifies Ser/Thr residues, is thought to regulate protein function in a manner analogous to protein phosphorylation and, on a subset of proteins, appears to have a reciprocal relationship with phosphorylation. Like phosphorylation, O-GlcNAc levels change dynamically in response to numerous signals including hyperglycemia and cellular injury. Recent data suggests that O-GlcNAc appears to be a key regulator of the cellular stress response, the augmentation of which is protective in models of acute vascular injury, trauma hemorrhage, and ischemia-reperfusion injury. In contrast to these studies, O-GlcNAc has also been implicated in the development of hypertension and type II diabetes, leading to vascular and cardiac dysfunction. Here we summarize the current understanding of the roles of O-GlcNAc in the heart and vasculature.

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.

Intermediate filaments take the heat as stress proteins

Intermediate filament (IF) proteins and heat shock proteins (HSPs) are large multimember families that share several features, including protein abundance, significant upregulation in response to a variety of stresses, cytoprotective functions, and the phenocopying of several human diseases after IF protein or HSP mutation. We are now coming to understand that these common elements point to IFs as important cellular stress proteins with some roles akin to those already well-characterized for HSPs. Unique functional roles for IFs include protection from mechanical stress, whereas HSPs are characteristically involved in protein folding and as chaperones. Shared IF and HSP cytoprotective roles include inhibition of apoptosis, organelle homeostasis, and scaffolding. In this report, we review data that corroborate the view that IFs function as highly specialized cytoskeletal stress proteins that promote cellular organization and homeostasis.

Mechanical strain of alveolar type II cells in culture: changes in the transcellular cytokeratin network and adaptations

Mechanical forces exert multiple effects in cells, ranging from altered protein expression patterns to cell damage and death. Despite undisputable biological importance, little is known about structural changes in cells subjected to strain ex vivo. Here, we undertake the first transmission electron microscopy investigation combined with fluorescence imaging on pulmonary alveolar type II cells that are subjected to equibiaxial strain. When cells are investigated immediately after stretch, we demonstrate that curved cytokeratin (CK) fibers are straightened out at 10% increase in cell surface area (CSA) and that this is accompanied by a widened extracellular gap of desmosomes-the insertion points of CK fibers. Surprisingly, a CSA increase by 20% led to higher fiber curvatures of CK fibers and a concurrent return of the desmosomal gap to normal values. Since 20% CSA increase also induced a significant phosphorylation of CK8-ser431, we suggest CK phosphorylation might lower the tensile force of the transcellular CK network, which could explain the morphological observations. Stretch durations of 5 min caused membrane injury in up to 24% of the cells stretched by 30%, but the CK network remained surprisingly intact even in dead cells. We conclude that CK and desmosomes constitute a strong transcellular scaffold that survives cell death and hypothesize that phosphorylation of CK fibers is a mechano-induced adaptive mechanism to maintain epithelial overall integrity.

Proteome and cytoskeleton responses in osteosarcoma cells with reduced OXPHOS activity

We have recently shown disorganization of the vimentin network in cultured cells deficient in oxidative phosphorylation (OXPHOS). We describe here the cellular responses to OXPHOS deficiency in osteosarcoma cells upon complex I (CI) and complex IV (CIV) inhibition, and upon the lack of mitochondrial DNA (rho0 cells). We examined the cytoskeletal organization and the distribution of mitochondria and analysed total proteome by 2-DE and vimentin expression by ELISA. Upon CIV inhibition and in rho0 cells, the vimentin network had collapsed around the nucleus and formed thick bundles. The mitochondria formed a perinuclear crescent upon CIV inhibition, whereas they accumulated around the nucleus in the rho0 cells, where the amount of vimentin was increased. Analysis of the total proteome revealed that a lack of mitochondrial DNA or inhibition of CI or CIV led to changes in the expression of cytoskeletal and cytoskeleton-associated proteins and proteins involved in apoptosis, OXPHOS, glycolysis, the tricarboxylic acid cycle, and oxidative stress responses. Our findings suggest that a deficiency in the energy converting system and oxidative stress can lead to cytoskeletal changes.

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.

Keratin 8 phosphorylation by protein kinase C delta regulates shear stress-mediated disassembly of keratin intermediate filaments in alveolar epithelial cells

Phosphorylation of keratin intermediate filaments (IF) is known to affect their assembly state and organization; however, little is known about the mechanisms regulating keratin phosphorylation. In this study, we demonstrate that shear stress, but not stretch, causes disassembly of keratin IF in lung alveolar epithelial cells (AEC) and that this disassembly is regulated by protein kinase C delta-mediated phosphorylation of keratin 8 (K8) Ser-73. Specifically, in AEC subjected to shear stress, keratin IF are disassembled, as reflected by their increased solubility. In contrast, AEC subjected to stretch showed no changes in the state of assembly of IF. Pretreatment with the protein kinase C (PKC) inhibitor, bisindolymaleimide, prevents the increase in solubility of either K8 or its assembly partner K18 in shear-stressed AEC. Phosphoserine-specific antibodies demonstrate that K8 Ser-73 is phosphorylated in a time-dependent manner in shear-stressed AEC. Furthermore, we showed that shear stress activates PKC delta and that the PKC delta peptide antagonist, delta V1-1, significantly attenuates the shear stress-induced increase in keratin phosphorylation and solubility. These data suggested that shear stress mediates the phosphorylation of serine residues in K8, leading to the disassembly of IF in alveolar epithelial cells. Importantly, these data provided clues regarding a molecular link between mechanically induced signal transduction and alterations in cytoskeletal IF.

Requirement for ERK activation in acetone extract identified from Bupleurum scorzonerifolium induced A549 tumor cell apoptosis and keratin 8 phosphorylation

We previously demonstrated that the crude acetone extract of Bupleurum scorzonerifolium (AE-BS) 60 microg/ml has anti-proliferation activity and apoptosis effects to A549 human lung cancer cells. They can also cause tumor cell arrest in G2/M phase. To better understand its target protein in A549 cell, two-dimensional electrophoresis and liquid chromatography-tandem mass spectrometry were applied. The modification of keratin 8 was identified. By immunoblot, the expression of phosphorylated keratin 8 at Ser-73 was increased from 2.0 to 3.0-fold after AE-BS treatment 24 to 48 hr respectively as compared with untreated A549 control cells. Furthermore, the A549 cells were pretreated with 50 microM PD98059, a specific inhibitor of the upstream regulator of ERK1/2, or with the p38 kinase inhibitor 20 microM SB203580 or JNK inhibitor 20 microM SP600125 for 30 min, followed by 24 h of incubation with AE-BS, PD98059 can inhibit K8-Ser-73 hyperphosphorylation and prevented cell apoptosis which was induced by AE-BS significantly. By immunoblot, AE-BS also can induce ERK 1/2 phosphorylation. In conclusion, our data indicate that the AE-BS induced tumor apoptosis in A549 cells was related to ERK 1/2 activation. The molecular mechanism of hyperphosphorylation of K8 on Ser-73 was associated with ERK 1/2 activation rather than JNK and p38 kinase. The apoptosis induced by AE-BS may be related to K8 phosphorylation.

Heat shock protein 70 expression, keratin phosphorylation and Mallory body formation in hepatocytes from griseofulvin-intoxicated mice

Background

Keratins are members of the intermediate filaments (IFs) proteins, which constitute one of the three major cytoskeletal protein families. In hepatocytes, keratin 8 and 18 (K8/18) are believed to play a protective role against mechanical and toxic stress. Post-translational modifications such as phosphorylation and glycosylation are thought to modulate K8/18 functions. Treatment of mouse with a diet containing griseofulvin (GF) induces, in hepatocytes, modifications in organization, expression and phosphorylation of K8/18 IFs and leads, on the long term, to the formation of K8/18 containing aggregates morphologically and biochemically identical to Mallory bodies present in a number of human liver diseases. The aim of the present study was to investigate the relationship between the level and localization of the stress inducible heat shock protein 70 kDa (HSP70i) and the level and localization of K8/18 phosphorylation in the liver of GF-intoxicated mice. The role of these processes in Mallory body formation was studied, too. The experiment was carried out parallely on two different mouse strains, C3H and FVB/n.

Results

GF-treatment induced an increase in HSP70i expression and K8 phosphorylation on serines 79 (K8 S79), 436 (K8 S436), and K18 phosphorylation on serine 33 (K18 S33) as determined by Western blotting. Using immunofluorescence staining, we showed that after treatment, HSP70i was present in all hepatocytes. However, phosphorylated K8 S79 (K8 pS79) and K8 S436 (K8 pS436) were observed only in groups of hepatocytes or in isolated hepatocytes. K18 pS33 was increased in all hepatocytes. HSP70i colocalized with MBs containing phosphorylated K8/18. Phophorylation of K8 S79 was observed in C3H mice MBs but was not present in FVB/n MBs.

Conclusions

Our results indicate that GF intoxication represents a stress condition affecting all hepatocytes, whereas induction of K8/18 phosphorylation is not occurring in every hepatocyte. We conclude that, in vivo, there is no direct relationship between GF-induced stress and K8/18 phosphorylation on the studied sites. The K8/18 phosphorylation pattern indicates that different cell signaling pathways are activated in subpopulations of hepatocytes. Moreover, our results demonstrate that, in distinct genetic backgrounds, the induction of K8/18 phosphorylation can be different.

A cyclic AMP protein kinase A-dependent mechanism by which rotavirus impairs the expression and enzyme activity of brush border-associated sucrase-isomaltase in differentiated intestinal Caco-2 cells

We undertook a study of the mechanism by which rhesus monkey rotavirus (RRV) impairs the expression and enzyme activity of brush border-associated sucrase isomaltase (SI) in cultured, human, fully differentiated, intestinal Caco-2 cells. We provide evidence that the RRV-induced defects in the expression and enzyme activity of SI are not related to the previously observed, RRV-induced, Ca2+ -dependent, disassembly of the F-actin cytoskeleton. This conclusion is based on the facts that: (i) the intracellular Ca2+ blocker, BAPTA/AM, which antagonizes the RRV-induced increase in [Ca2+](i), fails to inhibit the RRV-induced decrease in SI expression and enzyme activity; and (ii) Jasplakinolide (JAS) treatment, known to stabilize actin filaments, had no effect on the RRV-induced decrease in SI expression. Results reported here demonstrate that the RRV-induced impairment in the expression and enzyme activity of brush border-associated SI results from a hitherto unknown mechanism involving PKA signalling. This conclusion is based on the observations that (i) intracellular cAMP was increased in RRV-infected cells and (ii) treatment of RRV-infected cells with PKA blockers resulted in the reappearance of apical SI expression, accompanied by the restoration of the enzyme activity at the brush border. In addition, in RRV-infected cells a twofold increase of phosphorylated form of cytokeratin 18 was observed after immunopurification and Western Blot analysis, which was antagonized by exposing the RRV-infected cells to the PKA blockers.

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

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

The intermediate filament protein keratin 8 is a novel cytoplasmic substrate for c-Jun N-terminal kinase

Keratins 8 (K8) and 18 are the primary intermediate filaments of simple epithelia. Phosphorylation of keratins at specific sites affects their organization, assembly dynamics, and their interaction with signaling molecules. A number of keratin in vitro and in vivo phosphorylation sites have been identified. One example is K8 Ser-73, which has been implicated as an important phosphorylation site during mitosis, cell stress, and apoptosis. We show that K8 is strongly phosphorylated on Ser-73 upon stimulation of the pro-apoptotic cytokine receptor Fas/CD95/Apo-1 in HT-29 cells. Kinase assays showed that c-Jun N-terminal kinase (JNK) was also activated with activation kinetics corresponding to that of K8 phosphorylation. Furthermore, K8 was also phosphorylated on Ser-73 by JNK in vitro, yielding similar phosphopeptide maps as the in vivo phosphorylated material. In addition, co-immunoprecipitation studies revealed that part of JNK is associated with K8 in vivo, correlating with decreased ability of JNK to phosphorylate the endogenous c-Jun. Taken together, K8 is a new cytoplasmic target for JNK in Fas receptor-mediated signaling. The functional significance of this phosphorylation could relate to regulation of JNK signaling and/or regulation of keratin dynamics.

Keratin 8 mutations in patients with cryptogenic liver disease

BACKGROUND

About 10 percent of patients who undergo liver transplantation have cryptogenic liver disease. In animal models, the absence of heteropolymeric keratins 8 and 18 or the presence of mutant keratins in hepatocytes causes or promotes liver disease. We have previously described a mutation in the keratin 18 gene in a patient with cryptogenic cirrhosis, but the importance of mutations in the keratin 8 and keratin 18 genes in such patients is unclear.

METHODS

We tested for mutations in the keratin 8 and keratin 18 genes in purified genomic DNA isolated from 150 explanted livers and 89 peripheral-blood specimens from three groups of patients: 55 patients with cryptogenic liver disease; 98 patients with noncryptogenic liver disease, with causes that included alcohol use, autoimmunity, drug use, and viral infections; and 86 randomly selected inpatients and outpatients who provided blood to the hematology laboratory.

RESULTS

Of the 55 patients with cryptogenic liver disease, 3 had glycine-to-cysteine mutations at position 61 (a highly conserved glycine) of keratin 8, and 2 had tyrosine-to-histidine mutations at position 53 of keratin 8. These mutations were not detected in the patients with other liver diseases or in the randomly selected patients. We verified the presence of the mutations in specimens of explanted livers by protein analysis and by the detection of unique restriction-enzyme cleavage sites. In transfected cells, the glycine-to-cysteine mutation limited keratin-filament reorganization when the cells were exposed to oxidative stress. In contrast, the tyrosine-to-histidine mutation destabilized keratin filaments when transfected cells were exposed to heat or okadaic acid stress.

CONCLUSIONS

Mutations in the keratin 8 gene may predispose people to liver disease and may account for cryptogenic liver disease in some patients.

Rotavirus infection induces cytoskeleton disorganization in human intestinal epithelial cells: implication of an increase in intracellular calcium concentration

Rotavirus infection is the most common cause of severe infantile gastroenteritis worldwide. In vivo, rotavirus exhibits a marked tropism for the differentiated enterocytes of the intestinal epithelium. In vitro, differentiated and undifferentiated intestinal cells can be infected. We observed that rotavirus infection of the human intestinal epithelial Caco-2 cells induces cytoskeleton alterations as a function of cell differentiation. The vimentin network disorganization detected in undifferentiated Caco-2 cells was not found in fully differentiated cells. In contrast, differentiated Caco-2 cells presented Ca(2+)-dependent microtubule disassembly and Ca(2+)-independent cytokeratin 18 rearrangement, which both require viral replication. We propose that these structural alterations could represent the first manifestations of rotavirus-infected enterocyte injury leading to functional perturbations and then to diarrhea.

Cytokeratin 8 protects from hepatotoxicity, and its ratio to cytokeratin 18 determines the ability of hepatocytes to form Mallory bodies

In alcoholic hepatitis, a severe form of alcohol-induced toxic liver injury, as well as in experimental intoxication of mice with the porphyrinogenic drugs griseofulvin and 3,5-diethoxycarbonyl-1, 4-dihydrocollidine, hepatocytes form cytoplasmic protein aggregates (Mallory bodies; MBs) containing cytokeratins (CKs) and non-CK components. Here we report that mice lacking the CK8 gene and hence CK intermediate filaments in hepatocytes, but still expressing the type I partner, ie, the CK18 gene, do not form MBs but suffer from extensive porphyria and progressive toxic liver damage, leading to the death of a considerable number of animals (7 of 12 during 12 weeks of intoxication). Our observations show that 1) in the absence of CK8 as well as in the situation of a relative excess of CK18 over CK8 no MBs are formed; 2) the loss of CK8 is not compensated by other type II CKs; and 3) porphyria and toxic liver damage are drastically enhanced in the absence of CK8. Our results point to a protective role of CKs in certain types of toxic liver injury and suggest that MBs by themselves are not harmful to hepatocytes but may be considered as a product of a novel defense mechanism in hepatocytes.

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.

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.

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

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

Association of glucose-regulated protein (grp78) with human keratin 8

Keratin polypeptides 8 and 18 (K8/18) are intermediate filament proteins that are expressed in 'simple-type' epithelial cells. They associate with several proteins including the 70 kDa cytoplasmic heat shock proteins (hsp70). We identified the human 78 kDa glucose-regulated protein (grp78) as a keratin-associated protein. Keratin-grp78 association was noted after co-immunoprecipitation of K8/18 from HT29 detergent solubilized cell lysates, and appears to involve non-posttranslationally modified grp78. The grp78-K8/18 association is induced by culturing cells in the presence of tunicamycin or after glucose starvation. K8/18-bound grp78 can be dissociated by Mg-ATP and the association can be reconstituted in vitro using purified grp78, then redissociated again by Mg-ATP. Binding of grp78 occurs preferentially with K8, and when reconstituted does not depend on the posttranslational modification state of K8/18. Co-incubation of K8/18 with hsp70 and grp78 shows preferential association with hsp70. Our results demonstrate a direct association of grp78 with K8 under conditions that induce grp78 expression.

Caspase cleavage of keratin 18 and reorganization of intermediate filaments during epithelial cell apoptosis

Keratins 8 (K8) and 18 (K18) are major components of intermediate filaments (IFs) of simple epithelial cells and tumors derived from such cells. Structural cell changes during apoptosis are mediated by proteases of the caspase family. During apoptosis, K18 IFs reorganize into granular structures enriched for K18 phosphorylated on serine 53. K18, but not K8, generates a proteolytic fragment during drug- and UV light-induced apoptosis; this fragment comigrates with K18 cleaved in vitro by caspase-6, -3, and -7. K18 is cleaved by caspase-6 into NH2-terminal, 26-kD and COOH-terminal, 22-kD fragments; caspase-3 and -7 additionally cleave the 22-kD fragment into a 19-kD fragment. The cleavage site common for the three caspases was the sequence VEVD/A, located in the conserved L1-2 linker region of K18. The additional site for caspases-3 and -7 that is not cleaved efficiently by caspase-6 is located in the COOH-terminal tail domain of K18. Expression of K18 with alanine instead of serine at position 53 demonstrated that cleavage during apoptosis does not require phosphorylation of serine 53. However, K18 with a glutamate instead of aspartate at position 238 was resistant to proteolysis during apoptosis. Furthermore, this cleavage site mutant appears to cause keratin filament reorganization in stably transfected clones. The identification of the L1-2 caspase cleavage site, and the conservation of the same or very similar sites in multiple other intermediate filament proteins, suggests that the processing of IFs during apoptosis may be initiated by a similar caspase cleavage.

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

Simple epithelia express keratins 8 (K8) and 18 (K18) as their major intermediate filament proteins. We previously showed that several types of cell stress such as heat and virus infection result in a distinct hyperphosphorylated form of K8 (termed HK8). To better characterize K8/18 phosphorylation, we generated monoclonal antibodies by immunizing mice with hyperphosphorylated keratins that were purified from colonic cultured human HT29 cells pretreated with okadaic acid. One antibody specifically recognized HK8, and the epitope was identified as 71LLpSPL which corresponds to K8 phosphorylation at Ser-73. Generation of HK8 occurs in mitotic HT29 cells, basal crypt mitotic cells in normal mouse intestine, and in regenerating mouse hepatocytes after partial hepatectomy. Prominent levels of HK8 were also generated in HT29 cells that were induced to undergo apoptosis using anisomycin or etoposide. In addition, mouse hepatotoxicity that is induced by chronic feeding with griseofulvin resulted in HK8 formation in the liver. Our results demonstrate that a "reverse immunological" approach, coupled with enhancing in vivo phosphorylation using phosphatase inhibitors, can result in the identification of physiologic phosphorylation states. As such, K8 Ser-73 phosphorylation generates a distinct HK8 species under a variety of in vivo conditions including mitosis, apoptosis, and cell stress. The low steady state levels of HK8 during mitosis, in contrast to stress and apoptosis, suggest that accumulation of HK8 may represent a physiologic stress marker for simple epithelia.

Identification of a novel keratin epitope: evidence for association between non-helical sub-domains L12 during filament assembly

Keratin filaments in simple epithelial cells are heteropolymers of keratin 8 (K8) and keratin 18 (K18) polypeptides. The assembly of these polypeptides into intermediate filaments is a complex multi-stage phenomenon that involves several levels of associations. These molecular associations are not very well characterized. Monoclonal antibodies (MAbs) with defined specificities can be used to probe these associations and to isolate various intermediates in the assembly pathway. Here we describe the specificity of a MAb LE65 that has been widely used in keratin expression studies. We report that the MAb LE65 does not recognize individual keratin polypeptides but it instead reacts with a complex of K8 with K18. The MAb also did not react with complexes of K8 or K18 with other keratins. By allowing the antibody to react with complexes reconstituted from keratin fragments plus the complementary keratin, we have mapped the MAb LE65 epitope on the L12 sub-domains of K18, residues 214-231, and K8, residues 234-265, which must associate together to achieve antibody binding. These results suggest that the non-helical linkers, L12, of complementary keratins associate directly during filament assembly. This would explain why microinjection of MAb LE65 has been shown to disrupt keratin filaments. Furthermore, it may also help to explain the mechanism of filament disruption in some skin blistering syndromes induced by spontaneous mutations in the L12 region.

Mitotic arrest with nocodazole induces selective changes in the level of O-linked N-acetylglucosamine and accumulation of incompletely processed N-glycans on proteins from HT29 cells

O-Linked N-acetylglucosamine (O-GlcNAc) is a ubiquitous and abundant protein modification found on nuclear and cytoplasmic proteins. Several lines of evidence suggest that it is a highly dynamic modification and that the levels of this sugar on proteins may be regulated. Previous workers (Chou, C. F., and Omary, M. B. (1993) J. Biol. Chem. 268, 4465-4472) have shown that mitotic arrest with microtubule-destabilizing agents such as nocodazole causes an increase in the O-GlcNAc levels on keratins in the human colon cancer cell line HT29. We have sought to determine whether this increase in glycosylation is a general (i.e. occurring on many proteins) or a limited (i.e. occurring only on the keratins) process. A general increase would suggest that the microtubule-destabilizing agents were somehow affecting the enzymes responsible for addition and/or removal of O-GlcNAc. Our results suggest that the changes in O-GlcNAc induced by nocodazole are selective for the keratins. The levels of O-GlcNAc on other proteins, including the nuclear pore protein p62 and the transcription factor Sp1, are not significantly affected by this treatment. In agreement with these findings, nocodazole treatment caused no change in the activity of the enzymes responsible for addition or removal of O-GlcNAc as determined by direct in vitro assay. Interestingly, nocodazole treatment did cause a dramatic increase in modification of N-glycans with terminal GlcNAc residues on numerous proteins. Potential mechanisms for this and the change in glycosylation of the keratins are discussed.

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

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

Implications of intermediate filament protein phosphorylation

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

Susceptibility to hepatotoxicity in transgenic mice that express a dominant-negative human keratin 18 mutant

Keratins 8 and 18 (K8/18) are intermediate filament phosphoglycoproteins that are expressed preferentially in simple-type epithelia. We recently described transgenic mice that express point-mutant human K18 (Ku, N.-O., S. Michie, R.G. Oshima, and M.B. Omary. 1995. J. Cell Biol. 131:1303-1314) and develop chronic hepatitis and hepatocyte fragility in association with hepatocyte keratin filament disruption. Here we show that mutant K18 expressing transgenic mice are highly susceptible to hepatotoxicity after acute administration of acetaminophen (400 mg/Kg) or chronic ingestion of griseofulvin (1.25% wt/wt of diet). The predisposition to hepatotoxicity results directly from the keratin mutation since nontransgenic or transgenic mice that express normal human K18 are more resistant. Hepatotoxicity was manifested by a significant difference in lethality, liver histopathology, and biochemical serum testing. Keratin glycosylation decreased in all griseofulvin-fed mice, whereas keratin phosphorylation increased dramatically preferentially in mice expressing normal K18. The phosphorylation increase in normal K18 after griseofulvin feeding appears to involve sites that are different to those that increase after partial hepatectomy. Our results indicate that hepatocyte intermediate filament disruption renders mice highly susceptible to hepatotoxicity, and raises the possibility that K18 mutations may predispose to drug hepatotoxicity. The dramatic phosphorylation increase in nonmutant keratins could provide survival advantage to hepatocytes.

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

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

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

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

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

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