Keratins as susceptibility genes for end-stage liver disease

Perturbation effect of single polar group substitution on the Self-Association of amphiphilic peptide helices

As an important attempt towards creating hierarchical structures more like nature, the peptide is employed as a building block to build supramolecular architectures. An emerging question is whether the molecular mechanism of self-assembly obtained from the small molecule system, e.g., the driving forces of assembly are conventionally regarded as pairwise-additive, can be manifested in the self-association of biologically relevant amphiphilic peptides. A peptide, KRT-R, was derived from the 120-144 segment of keratin 14. The single cation-to-cation substitution with KRT-R at the site of 125 from arginine (R) to either lysine (K) or histidine (H) results in the peptide helices, KRT-K and KRT-H, sharing 96% sequence identity. These KRT-derived peptides possess similarities in the folding structures but exhibit divergent self-assembled structures. KRT-R and KRT-K self-assemble into sheets and fibrils, respectively. Whereas KRT-H associates into heterogeneous structures, including sheets, particles, and branched networks. The intrinsic tyrosine fluorescence spectroscopy measurements with the KRT-derived peptides within a temperature range of 25 °C to 95 °C reveal that the heating-triggered structural transitions of KRT-derived peptides are divergent. The alternation of single cationic residue changes the thermodynamic signature of peptide assemblies upon heating. A chemical denaturation experiment with KRT-derived peptides indicates that the intermolecular interactions that govern the supramolecular architectures formed by peptides are distinct. Overall, our work demonstrates the contribution of the interplay among various noncovalent interactions to supramolecular assembly.

Conservation and Identity Selection of Cationic Residues Flanking the Hydrophobic Regions in Intermediate Filament Superfamily

The interplay between the hydrophobic interactions generated by the nonpolar region and the proximal functional groups within nanometers of the nonpolar region offers a promising strategy to manipulate the intermolecular hydrophobic attractions in an artificial molecule system, but the outcomes of such modulations in the building of a native protein architecture remain unclear. Here we focus on the intermediate filament (IF) coiled-coil superfamily to assess the conservation of positively charged residue identity via a biostatistical approach. By screening the disease-correlated mutations throughout the IF superfamily, 10 distinct hotspots where a cation-to-cation substitution is associated with a pathogenic syndrome have been identified. The analysis of the local chemical context surrounding the hotspots revealed that the cationic diversity depends on their separation distance to the hydrophobic domain. The nearby cationic residues flanking the hydrophobic domain of a helix (separation <1 nm) are relatively conserved in evolution. In contrast, the cationic residues that are not adjacent to the hydrophobic domain (separation >1 nm) tolerate higher levels of variation and replaceability. We attribute this bias in the conservation degree of the cationic residue identity to reflect the interplay between the proximal cations and the hydrophobic interactions.

Keratin 8/18 Regulate the Akt Signaling Pathway

Keratin 8 and keratin 18 (K8/K18) are intermediate filament proteins that form the obligate heteropolymers in hepatocytes and protect the liver against toxins. The mechanisms of protection include the regulation of signaling pathway associated with cell survival. Previous studies show K8/K18 binding with Akt, which is a well-known protein kinase involved in the cell survival signaling pathway. However, the role of K8/K18 in the Akt signaling pathway is unclear. In this study, we found that K8/K18-Akt binding is downregulated by K8/K18 phosphorylation, specifically phosphorylation of K18 ser7/34/53 residues, whereas the binding is upregulated by K8 gly-62-cys mutation. K8/K18 expression in cultured cell system tends to enhance the stability of the Akt protein. A comparison of the Akt signaling pathway in a mouse system with liver damage shows that the pathway is downregulated in K18-null mice compared with nontransgenic mice. K18-null mice with Fas-induced liver damage show enhanced apoptosis combined with the downregulation of the Akt signaling pathway, i.e., lower phosphorylation levels of GSK3β and NFκB, which are the downstream signaling factors in the Akt signaling pathway, in K18-null mice compared with the control mice. Our study indicates that K8/K18 expression protects mice from liver damage by participating in enhancing the Akt signaling pathway.

Revealing the Roles of Keratin 8/18-Associated Signaling Proteins Involved in the Development of Hepatocellular Carcinoma

Although hepatocellular carcinoma (HCC) is developed with various etiologies, protection of hepatocytes seems basically essential to prevent the incidence of HCC. Keratin 8 and keratin 18 (K8/K18) are cytoskeletal intermediate filament proteins that are expressed in hepatocytes. They maintain the cell shape and protect cells under stress conditions. Their protective roles in liver damage have been described in studies of mouse models, and K8/K18 mutation frequency in liver patients. Interestingly, K8/K18 bind to signaling proteins such as transcription factors and protein kinases involved in HCC development. Since K8/K18 are abundant cytoskeletal proteins, K8/K18 binding with the signaling factors can alter the availability of the factors. Herein, we discuss the potential roles of K8/K18 in HCC development.

From structural resilience to cell specification - Intermediate filaments as regulators of cell fate

During the last decades intermediate filaments (IFs) have emerged as important regulators of cellular signaling events, ascribing IFs with functions beyond the structural support they provide. The organ and developmental stage‐specific expression of IFs regulate cell differentiation within developing or remodeling tissues. Lack of IFs causes perturbed stem cell differentiation in vasculature, intestine, nervous system, and mammary gland, in transgenic mouse models. The aberrant cell fate decisions are caused by deregulation of different stem cell signaling pathways, such as Notch, Wnt, YAP/TAZ, and TGFβ. Mutations in genes coding for IFs cause an array of different diseases, many related to stem cell dysfunction, but the molecular mechanisms remain unresolved. Here, we provide a comprehensive overview of how IFs interact with and regulate the activity, localization and function of different signaling proteins in stem cells, and how the assembly state and PTM profile of IFs may affect these processes. Identifying when, where and how IFs and cell signaling congregate, will expand our understanding of IF‐linked stem cell dysfunction during development and disease.

Keratin 8 Mutations Were Associated With Susceptibility to Chronic Hepatitis B and Related Progression

BACKGROUND

Keratin 8 and 18 (K8/K18) are the exclusively expressed keratins intermediate filaments pair in hepatocytes that protect against liver injuries and viral infection. We aimed to explore the genetic link between keratin variants and chronic hepatitis B virus (CHB) infection in a large cohort from a high-epidemic area.

METHODS

Genomic deoxyribonucleic acid was isolated from patients, and Sanger sequencing was applied to analyze variations in exon regions of K8/18. Biochemical and functional analysis of novel mutations was also performed.

RESULTS

The 713 participants comprised 173 healthy controls and 540 patients, which covered chronic hepatitis (n = 174), decompensated cirrhosis (n = 192), and primary liver carcinoma (n = 174). The frequency of mutations in K8/18 was significantly higher among patients than among controls (8.15% vs 0.58%, P < .001). Significant differences were found between the chronic hepatitis subgroup and controls in multiple comparisons (6.32% vs 0.58%, P = .006). All 21 missense mutations (3.89%) were detected in the keratin 8 (K8), including 4 novel conserved missense variants (R469C, R469H, A447V, and K483T). Multivariate logistic regression analysis demonstrated a higher risk of acute-on-chronic liver failure (ACLF) and missense variants (odds ratio = 4.38, P = .035). Transfection of these variants caused keratin network disruption in vivo.

CONCLUSIONS

Novel K8 cytoskeleton-disrupting variants predispose toward ACLF in CHB.

Molecular Modeling of Pathogenic Mutations in the Keratin 1B Domain

Keratin intermediate filaments constitute the primary cytoskeletal component of epithelial cells. Numerous human disease phenotypes related to keratin mutation remain mechanistically elusive. Our recent crystal structures of the helix 1B heterotetramer from keratin 1/10 enabled further investigation of the effect of pathologic 1B domain mutations on keratin structure. We used our highest resolution keratin 1B structure as a template for homology-modeling the 1B heterotetramers of keratin 5/14 (associated with blistering skin disorders), keratin 8/18 (associated with liver disease), and keratin 74/28 (associated with hair disorder). Each structure was examined for the molecular alterations caused by incorporating pathogenic 1B keratin mutations. Structural modeling indicated keratin 1B mutations can harm the heterodimer interface (R265P^K5, L311R^K5, R211P^K14, I150V^K18), the tetramer interface (F231L^K1, F274S^K74), or higher-order interactions needed for mature filament formation (S233L^K1, L311R^K5, Q169E^K8, H128L^K18). The biochemical changes included altered hydrophobic and electrostatic interactions, and altered surface charge, hydrophobicity or contour. Together, these findings advance the genotype-structurotype-phenotype correlation for keratin-based human diseases.

Keratins regulate Hsp70-mediated nuclear localization of p38 mitogen-activated protein kinase

Intermediate filament protein keratin 8 (K8) binds to heat shock protein 70 (Hsp70) and p38 MAPK, and is phosphorylated at Ser74 by p38α (MAPK14, hereafter p38). However, a p38 binding site on K8 and the molecular mechanism of K8-p38 interaction related to Hsp70 are unknown. Here, we identify a p38 docking site on K8 (Arg148/149 and Leu159/161) that is highly conserved in other intermediate filaments. A docking-deficient K8 mutation caused increased p38-Hsp70 interaction and enhanced p38 nuclear localization, indicating that the p38 dissociated from mutant K8 makes a complex with Hsp70, which is known as a potential chaperone for p38 nuclear translocation. Comparison of p38 MAPK binding with keratin variants associated with liver disease showed that the K18 I150V variant dramatically reduced binding with p38, which is similar to the effect of the p38 docking-deficient mutation on K8. Because the p38 docking site on K8 (Arg148/149 and Leu159/161) and the K18 Ile150 residue are closely localized in the parallel K8/K18 heterodimer, the K18 I150V mutation might interfere with K8-p38 interaction. These findings show that keratins, functioning as cytoplasmic anchors for p38, modulate p38 nuclear localization and thereby might affect a number of p38-mediated signal transduction pathways.

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

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

High Keratin 8/18 Ratio Predicts Aggressive Hepatocellular Cancer Phenotype

BACKGROUND & AIMS: Steatohepatitis (SH) and SH-associated hepatocellular carcinoma (HCC) are of considerable clinical significance. SH is morphologically characterized by steatosis, liver cell ballooning, cytoplasmic aggregates termed Mallory-Denk bodies (MDBs), inflammation, and fibrosis at late stage. Disturbance of the keratin cytoskeleton and aggregation of keratins (KRTs) are essential for MDB formation. METHODS: We analyzed livers of aged Krt18^−/− mice that spontaneously developed in the majority of cases SH-associated HCC independent of sex. Interestingly, the hepatic lipid profile in Krt18^−/− mice, which accumulate KRT8, closely resembles human SH lipid profiles and shows that the excess of KRT8 over KRT18 determines the likelihood to develop SH-associated HCC linked with enhanced lipogenesis. RESULTS: Our analysis of the genetic profile of Krt18^−/− mice with 26 human hepatoma cell lines and with data sets of >300 patients with HCC, where Krt18^−/− gene signatures matched human HCC. Interestingly, a high KRT8/18 ratio is associated with an aggressive HCC phenotype. CONCLUSIONS: We can prove that intermediate filaments and their binding partners are tightly linked to hepatic lipid metabolism and to hepatocarcinogenesis. We suggest KRT8/18 ratio as a novel HCC biomarker for HCC.

Association of keratin 8/18 variants with non-alcoholic fatty liver disease and insulin resistance in Chinese patients: A case-control study

AIM

To test the hypothesis that K8/K18 variants predispose humans to non-alcoholic fatty liver disease (NAFLD) progression and its metabolic phenotypes.

METHODS

We selected a total of 373 unrelated adult subjects from our Physical Examination Department, including 200 unrelated NAFLD patients and 173 controls of both genders and different ages. Diagnoses of NAFLD were established according to ultrasonic signs of fatty liver. All subjects were tested for population characteristics, lipid profile, liver tests, as well as glucose tests. Genomic DNA was obtained from peripheral blood with a DNeasy Tissue Kit. K8/K18 coding regions were analyzed, including 15 exons and exon-intron boundaries.

RESULTS

Among 200 NAFLD patients, 10 (5%) heterozygous carriers of keratin variants were identified. There were 5 amino-acid-altering heterozygous variants and 6 non-coding heterozygous variants. One novel amino-acid-altering heterozygous variant (K18 N193S) and three novel non-coding variants were observed (K8 IVS5-9A→G, K8 IVS6+19G→A, K18 T195T). A total of 9 patients had a single variant and 1 patient had compound variants (K18 N193S+K8 IVS3-15C→G). Only one R341H variant was found in the control group (1 of 173, 0.58%). The frequency of keratin variants in NAFLD patients was significantly higher than that in the control group (5% vs 0.58%, P = 0.015). Notably, the keratin variants were significantly associated with insulin resistance (IR) in NAFLD patients (8.86% in NAFLD patients with IR vs 2.5% in NAFLD patients without IR, P = 0.043).

CONCLUSION

K8/K18 variants are overrepresented in Chinese NAFLD patients and might accelerate liver fat storage through IR.

Keratin 18-deficiency results in steatohepatitis and liver tumors in old mice: A model of steatohepatitis-associated liver carcinogenesis

Backround: Steatohepatitis (SH)-associated liver carcinogenesis is an increasingly important issue in clinical medicine. SH is morphologically characterized by steatosis, hepatocyte injury, ballooning, hepatocytic cytoplasmic inclusions termed Mallory-Denk bodies (MDBs), inflammation and fibrosis.

RESULTS

17-20-months-old Krt18-/- and Krt18+/- mice in contrast to wt mice spontaneously developed liver lesions closely resembling the morphological spectrum of human SH as well as liver tumors. The pathologic alterations were more pronounced in Krt18-/- than in Krt18+/- mice. The frequency of liver tumors with male predominance was significantly higher in Krt18-/- compared to age-matched Krt18+/- and wt mice. Krt18-deficient tumors in contrast to wt animals displayed SH features and often pleomorphic morphology. aCGH analysis of tumors revealed chromosomal aberrations in Krt18-/- liver tumors, affecting loci of oncogenes and tumor suppressor genes.

MATERIALS AND METHODS

Livers of 3-, 6-, 12- and 17-20-months-old aged wild type (wt), Krt18+/- and Krt18-/- (129P2/OlaHsd background) mice were analyzed by light and immunofluorescence microscopy as well as immunohistochemistry. Liver tumors arising in aged mice were analyzed by array comparative genomic hybridization (aCGH).

CONCLUSIONS

Our findings show that K18 deficiency of hepatocytes leads to steatosis, increasing with age, and finally to SH. K18 deficiency and age promote liver tumor development in mice, frequently on the basis of chromosomal instability, resembling human HCC with stemness features.

Keratin impact on PKCδ- and ASMase-mediated regulation of hepatocyte lipid raft size - implication for FasR-associated apoptosis

Keratins are epithelial cell intermediate filament (IF) proteins that are expressed as pairs in a cell-differentiation-regulated manner. Hepatocytes express the keratin 8 and 18 pair (denoted K8/K18) of IFs, and a loss of K8 or K18, as in K8-null mice, leads to degradation of the keratin partner. We have previously reported that a K8/K18 loss in hepatocytes leads to altered cell surface lipid raft distribution and more efficient Fas receptor (FasR, also known as TNFRSF6)-mediated apoptosis. We demonstrate here that the absence of K8 or transgenic expression of the K8 G62C mutant in mouse hepatocytes reduces lipid raft size. Mechanistically, we find that the lipid raft size is dependent on acid sphingomyelinase (ASMase, also known as SMPD1) enzyme activity, which is reduced in absence of K8/K18. Notably, the reduction of ASMase activity appears to be caused by a less efficient redistribution of surface membrane PKCδ toward lysosomes. Moreover, we delineate the lipid raft volume range that is required for an optimal FasR-mediated apoptosis. Hence, K8/K18-dependent PKCδ- and ASMase-mediated modulation of lipid raft size can explain the more prominent FasR-mediated signaling resulting from K8/K18 loss. The fine-tuning of ASMase-mediated regulation of lipid rafts might provide a therapeutic target for death-receptor-related liver diseases.

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.

Human keratin 8 variants promote mouse acetaminophen hepatotoxicity coupled with c-jun amino-terminal kinase activation and protein adduct formation

Keratins 8 and 18 (K8/K18) are the intermediate filaments proteins of simple-type digestive epithelia and provide important cytoprotective function. K8/K18 variants predispose humans to chronic liver disease progression and poor outcomes in acute acetaminophen (APAP)-related liver failure. Given that K8 G62C and R341H/R341C are common K8 variants in European and North American populations, we studied their biological significance using transgenic mice. Mice that overexpress the human K8 variants, R341H or R341C, were generated and used together with previously described mice that overexpress wild-type K8 or K8 G62C. Mice were injected with 600 mg/kg of APAP or underwent bile duct ligation (BDL). Livers were evaluated by microarray analysis, quantitative real-time polymerase chain reaction, immunoblotting, histological and immunological staining, and biochemical assays. Under basal conditions, the K8 G62C/R341H/R341C variant-expressing mice did not show an obvious liver phenotype or altered keratin filament distribution, whereas K8 G62C/R341C animals had aberrant disulphide cross-linked keratins. Animals carrying the K8 variants displayed limited gene expression changes, but had lower nicotinamide N-methyl transferase (NNMT) levels and were predisposed to APAP-induced hepatotoxicity. NNMT represents a novel K8/K18-associated protein that becomes up-regulated after K8/K18 transfection. The more pronounced liver damage was accompanied by increased and prolonged JNK activation; elevated APAP protein adducts; K8 hyperphosphorylation at S74/S432 with enhanced keratin solubility; and prominent pericentral keratin network disruption. No differences in APAP serum levels, glutathione, or adenosine triphosphate levels were noted. BDL resulted in similar liver injury and biliary fibrosis in all mouse genotypes.

CONCLUSION

Expression of human K8 variants G62C, R341H, or R341C in mice predisposes to acute APAP hepatotoxicity, thereby providing direct evidence for the importance of these variants in human acute liver failure.

Prevalence of genetic variants of keratins 8 and 18 in patients with drug-induced liver injury

BACKGROUND

Keratin 8 and 18 (K8/K18) cytoskeletal proteins protect hepatocytes from undergoing apoptosis and their mutations predispose to adverse outcomes in acute liver failure (ALF). All known K8/K18 variants occur at relatively non-conserved residues and do not cause keratin cytoskeleton reorganization, whereas epidermal keratin-conserved residue mutations disrupt the keratin cytoskeleton and cause severe skin disease. The aim of our study was to identify keratin variants in idiosyncratic drug-induced liver injury (DILI).

METHODS

Genomic DNA was isolated from 800 patients enrolled in an ongoing US multicenter study, with DILI attributed to a wide range of drugs. Specific K8/K18 exonic regions were PCR-amplified and screened by denaturing HPLC followed by DNA sequencing. The functional impact of keratin variants was assessed using cell transfection and immune staining.

RESULTS

Heterozygous and compound amino acid-altering K8/K18 variants were identified in 86 DILI patients and non-coding variants in 15 subjects. Five novel amino acid-altering (K8 Lys393Arg, K8 Ala351Val, K8 Ala358Val, K8 Ile346Val, K18 Asp89His) and two non-coding variants were observed. Several variants segregated with specific ethnic backgrounds but were found at similar frequencies in DILI subjects and ethnically matched population controls. Notably, variants in highly conserved residues of K8 Lys393Arg (ezetimibe/simvastatin-related) and K18 Asp89His (isoniazid-related) were found in patients with fatal DILI. These novel variants also led to keratin network disruption in transfected cells.

CONCLUSIONS

Novel K8/K18 cytoskeleton-disrupting variants were identified in two patients and segregated with fatal DILI. Other non-cytoskeleton-disrupting keratin variants did not preferentially associate with DILI.

Emerging roles of sumoylation in the regulation of actin, microtubules, intermediate filaments, and septins

Sumoylation is a powerful regulatory system that controls many of the critical processes in the cell, including DNA repair, transcriptional regulation, nuclear transport, and DNA replication. Recently, new functions for SUMO have begun to emerge. SUMO is covalently attached to components of each of the four major cytoskeletal networks, including microtubule-associated proteins, septins, and intermediate filaments, in addition to nuclear actin and actin-regulatory proteins. However, knowledge of the mechanisms by which this signal transduction system controls the cytoskeleton is still in its infancy. One story that is beginning to unfold is that SUMO may regulate the microtubule motor protein dynein by modification of its adaptor Lis1. In other instances, cytoskeletal elements can both bind to SUMO non-covalently and also be conjugated by it. The molecular mechanisms for many of these new functions are not yet clear, but are under active investigation. One emerging model links the function of MAP sumoylation to protein degradation through SUMO-targeted ubiquitin ligases, also known as STUbL enzymes. Other possible functions for cytoskeletal sumoylation are also discussed.

A role for disulfide bonding in keratin intermediate filament organization and dynamics in skin keratinocytes

Disulfide bonds involving cysteine 367 in K14 play a crucial role in the assembly, dynamics, and organization of K14-containing filaments in epidermal keratinocytes.

We recently reported that a trans-dimer, homotypic disulfide bond involving Cys367 in keratin 14 (K14) occurs in an atomic-resolution structure of the interacting K5/K14 2B domains and in keratinocyte cell lines. Here we show that a sizable fraction of the K14 and K5 protein pools participates in interkeratin disulfide bonding in primary cultures of mouse skin keratinocytes. By comparing the properties of wild-type K14 with a completely cysteine-free variant thereof, we found that K14-dependent disulfide bonding limited filament elongation during polymerization in vitro but was necessary for the genesis of a perinuclear-concentrated network of keratin filaments, normal keratin cycling, and the sessile behavior of the nucleus and whole cell in keratinocytes studied by live imaging. Many of these phenotypes were rescued when analyzing a K14 variant harboring a single Cys residue at position 367. These findings establish disulfide bonding as a novel and important mechanism regulating the assembly, intracellular organization, and dynamics of K14-containing intermediate filaments in skin keratinocytes.

Absence of keratins 8 and 18 in rodent epithelial cell lines associates with keratin gene mutation and DNA methylation: Cell line selective effects on cell invasion

Epithelial-mesenchymal transition (EMT) in carcinoma is associated with dramatic up-regulation of vimentin and down-regulation of the simple-type keratins 8 and 18 (K8/K18), but the mechanisms of these changes are poorly understood. We demonstrate that two commonly-studied murine (CT26) and rat (IEC-6) intestinal cell lines have negligible K8/K18 but high vimentin protein expression. Proteasome inhibition led to a limited increase in K18 but not K8 stabilization, thereby indicating that K8/K18 absence is not due, in large part, to increased protein turnover. CT26 and IEC-6 cells had <10% of normal K8/K18 mRNA and exhibited decreased mRNA stability, with K8 mRNA levels being higher in IEC-6 versus CT26 and K18 being higher in CT26 versus IEC-6 cells. Keratin gene sequencing showed that KRT8 in CT26 cells had a 21-nucleotide deletion while K18 in IEC-6 cells had a 9-amino acid in-frame insertion. Furthermore, the KRT8 promoter in CT26 and the KRT18 promoter in IEC-6 are hypermethylated. Inhibition of DNA methylation using 5-azacytidine increased K8 or K18 in some but all the tested rodent epithelial cell lines. Restoring K8 and K18 by lentiviral transduction reduced CT26 but not IEC-6 cell matrigel invasion. K8/K18 re-introduction also decreased E-cadherin expression in IEC-6 but not CT26 cells, suggesting that the effect of keratin expression on epithelial to mesenchymal transition is cell-line dependent. Therefore, some commonly utilized rodent epithelial cell lines, unexpectedly, manifest barely detectable keratin expression but have high levels of vimentin. In the CT26 and IEC-6 intestinal cell lines, keratin expression correlates with keratin gene insertion or deletion and with promoter methylation, which likely suppress keratin transcription and mRNA or protein stability.

Keratin 8 is involved in hepatitis B virus replication

Hepatitis B virus (HBV) infection can result in fatal liver diseases, including cirrhosis or liver failure, and its replication and pathogenesis depend on the critical interplay between viral and host factors. This study investigated HBV replication-related host proteins and the effect of candidate proteins on HBV replication. Isobaric tags for relative and absolute quantitation (iTRAQ) were used to measure HBV replication-related proteins in HepG2 cells and HepG2.2.15 cells. KRT8 was up-regulated in HepG2.2.15 cells but not in HepG2 cells, and KRT8 was overexpressed in an HBV-infected patient's liver tissue. This result suggested that KRT8 is involved in HBV replication. To further clarify the relationship between KRT8 and HBV replication, KRT8 gene expression was inhibited by siRNA. The silencing of KRT8 mildly suppressed HBV replication. Moreover, overexpressed KRT8 significantly increased HBV replication, and the inhibition of HBV DNA did not suppress KRT8 expression. Thus, the host protein KRT8 is involved in the replication of HBV DNA, and it dramatically enhances HBV replication.

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

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

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

Keratin 8 variants are infrequent in patients with alcohol-related liver cirrhosis and do not associate with development of hepatocellular carcinoma

Background

Keratins 8/18 (K8/K18) are established hepatoprotective proteins and K8/K18 variants predispose to development and adverse outcome of multiple liver disorders. The importance of K8/K18 in alcoholic liver disease as well as in established cirrhosis remains unknown.

Methods

We analyzed the K8 mutational hot-spots in 261 prospectively followed-up patients with alcoholic cirrhosis (mean follow-up 65 months). PCR-amplified samples were pre-screened by denaturing high-performance liquid chromatography and conspicuous samples were sequenced.

Results

67 patients developed hepatocellular carcinoma (HCC) and 133 died. Fourteen patients harbored amino-acid-altering K8 variants (5xG62C, 8xR341H). The presence of K8 variants did not associate with development of HCC (log-rank=0.5) or death (log-rank=0.7) and no significant associations were obtained for the single K8 variants after a correction for multiple testing was performed.

Conclusions

Keratin variants are expressed in a low percentage of patients with alcoholic cirrhosis and do not influence HCC development. Further studies conducted in larger prospective cohorts are needed to find out whether presence of K8 R341H variant predispose to non-HCC-related liver mortality.

Genetic background effects of keratin 8 and 18 in a DDC-induced hepatotoxicity and Mallory-Denk body formation mouse model

Keratin 8 (K8) and keratin 18 (K18) form the major hepatocyte cytoskeleton. We investigated the impact of genetic loss of either K8 or K18 on liver homeostasis under toxic stress with the hypothesis that K8 and K18 exert different functions. krt8⁻/⁻ and krt18⁻/⁻ mice crossed into the same 129-ola genetic background were treated by acute and chronic administration of 3,5-diethoxy-carbonyl-1,4-dihydrocollidine (DDC). In acutely DDC-intoxicated mice, macrovesicular steatosis was more pronounced in krt8⁻/⁻ and krt18⁻/⁻ compared with wild-type (wt) animals. Mallory-Denk bodies (MDBs) appeared in krt18⁻/⁻ mice already at an early stage of intoxication in contrast to krt8⁻/⁻ mice that did not display MDB formation when fed with DDC. Keratin-deficient mice displayed significantly lower numbers of apoptotic hepatocytes than wt animals. krt8⁻/⁻, krt18⁻/⁻ and control mice displayed comparable cell proliferation rates. Chronically DDC-intoxicated krt18⁻/⁻ and wt mice showed a similarly increased degree of steatohepatitis with hepatocyte ballooning and MDB formation. In krt8⁻/⁻ mice, steatosis was less, ballooning, and MDBs were absent. krt18⁻/⁻ mice developed MDBs whereas krt8⁻/⁻ mice on the same genetic background did not, highlighting the significance of different structural properties of keratins. They are independent of the genetic background as an intrinsic factor. By contrast, toxicity effects may depend on the genetic background. krt8⁻/⁻ and krt18⁻/⁻ mice on the same genetic background show similar sensitivity to DDC intoxication and almost resemble wt animals regarding survival, degree of porphyria, liver-to-body weight ratio, serum bilirubin and liver enzyme levels. This stands in contrast to previous work where krt8⁻/⁻ and krt18⁻/⁻ mice on different genetic backgrounds were investigated.

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.

Energy determinants GAPDH and NDPK act as genetic modifiers for hepatocyte inclusion formation

Differential expression and activity of the cellular energy regulators GAPDH and NDPK underlie reactive oxygen species–induced damage in the mouse liver and may contribute to human liver disease progression.

Genetic factors impact liver injury susceptibility and disease progression. Prominent histological features of some chronic human liver diseases are hepatocyte ballooning and Mallory-Denk bodies. In mice, these features are induced by 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) in a strain-dependent manner, with the C57BL and C3H strains showing high and low susceptibility, respectively. To identify modifiers of DDC-induced liver injury, we compared C57BL and C3H mice using proteomic, biochemical, and cell biological tools. DDC elevated reactive oxygen species (ROS) and oxidative stress enzymes preferentially in C57BL livers and isolated hepatocytes. C57BL livers and hepatocytes also manifested significant down-regulation, aggregation, and nuclear translocation of glyceraldehyde 3-phosphate dehydrogenase (GAPDH). GAPDH knockdown depleted bioenergetic and antioxidant enzymes and elevated hepatocyte ROS, whereas GAPDH overexpression decreased hepatocyte ROS. On the other hand, C3H livers had higher expression and activity of the energy-generating nucleoside-diphosphate kinase (NDPK), and knockdown of hepatocyte NDPK augmented DDC-induced ROS formation. Consistent with these findings, cirrhotic, but not normal, human livers contained GAPDH aggregates and NDPK complexes. We propose that GAPDH and NDPK are genetic modifiers of murine DDC-induced liver injury and potentially human liver disease.

Unique amino acid signatures that are evolutionarily conserved distinguish simple-type, epidermal and hair keratins

Keratins (Ks) consist of central α-helical rod domains that are flanked by non-α-helical head and tail domains. The cellular abundance of keratins, coupled with their selective cell expression patterns, suggests that they diversified to fulfill tissue-specific functions although the primary structure differences between them have not been comprehensively compared. We analyzed keratin sequences from many species: K1, K2, K5, K9, K10, K14 were studied as representatives of epidermal keratins, and compared with K7, K8, K18, K19, K20 and K31, K35, K81, K85, K86, which represent simple-type (single-layered or glandular) epithelial and hair keratins, respectively. We show that keratin domains have striking differences in their amino acids. There are many cysteines in hair keratins but only a small number in epidermal keratins and rare or none in simple-type keratins. The heads and/or tails of epidermal keratins are glycine and phenylalanine rich but alanine poor, whereas parallel domains of hair keratins are abundant in prolines, and those of simple-type epithelial keratins are enriched in acidic and/or basic residues. The observed differences between simple-type, epidermal and hair keratins are highly conserved throughout evolution. Cysteines and histidines, which are infrequent keratin amino acids, are involved in de novo mutations that are markedly overrepresented in keratins. Hence, keratins have evolutionarily conserved and domain-selectively enriched amino acids including glycine and phenylalanine (epidermal), cysteine and proline (hair), and basic and acidic (simple-type epithelial), which reflect unique functions related to structural flexibility, rigidity and solubility, respectively. Our findings also support the importance of human keratin 'mutation hotspot' residues and their wild-type counterparts.

Keratin 18, Apoptosis, and Liver Disease in Children

Keratins, a major component of epithelial cell intermediate filaments, provide structural support to the cell and are important for the maintenance of structural integrity. Beyond its role of structural integrity in hepatocytes, keratin 18 (K18) is a known marker of apoptosis and has been proposed as an indicator of progression in chronic liver diseases such as nonalcoholic fatty liver disease (NAFLD). NAFLD is the most common cause of chronic liver disease in children and adolescents in the United States and throughout the world and comprises a wide spectrum of disease ranging from simple steatosis (fatty liver) to nonalcoholic steatohepatitis (NASH) and cirrhosis. While simple steatosis is typically benign in nature, NASH is a more serious condition that may progress to end-stage liver disease and liver failure. Currently, liver biopsy is considered the most reliable method of assessing the histological severity of disease and differentiating between simple steatosis and NASH. Because biopsy is invasive in nature, expensive, and subject to sampling error and/or variability in interpretation, it is not suitable as a screening test. Therefore, it is necessary to examine known mechanisms associated with the progression of liver disease, such as hepatocellular apoptosis, and identify potential biomarkers that could be used as a diagnostic tool in NASH. This review will focus on the role of apoptosis in pediatric liver disease and how K18, an early marker of apoptosis, can be utilized as a noninvasive biomarker to diagnose NASH.

Mallory-Denk bodies are associated with outcomes and histologic features in patients with chronic hepatitis C

BACKGROUND & AIMS

Mallory-Denk bodies (MDBs) are inclusions found in hepatocytes of patients with chronic liver diseases. Their clinical significance and prognostic value are not understood.

METHODS

We performed cross-sectional and longitudinal analyses of patients with chronic hepatitis C (CHC) enrolled in the Hepatitis C Antiviral Long-Term Treatment against Cirrhosis (HALT-C) trial to identify clinical features associated with MDBs and changes in MDBs over time. Biopsy specimens were obtained at baseline and 1.5 and 3.5 years after patients were assigned to groups for the HALT-C trial; and patients were followed up to assess clinical and histologic outcomes.

RESULTS

Of biopsy samples collected from 1050 patients, MDBs were present in 15%. They were associated with insulin resistance and laboratory and histologic markers of advanced liver disease (higher levels of periportal fibrosis, pericellular fibrosis, steatosis, and inflammation). After adjusting for disease severity (the ratio of aspartate aminotransferase to alanine aminotransferase, albumin, platelets, fibrosis, steatosis), the presence of MDBs was associated with histologic progression (odds ratio, 1.97; P = .04). Of the 844 patients from whom serial biopsy samples were collected, 61 (7.2%) developed MDBs (MDB gain) and 101 (12.0%) lost MDBs (MDB loss). The presence or absence of diabetes mellitus was associated with MDB gain (P = .006) or loss (P = .024), respectively. Development of MDBs was associated with decompensation (adjusted hazard ratio, 2.81; P < .001) and histologic signs of progression (adjusted odds ratio, 4.02; P = .004).

CONCLUSIONS

The presence of MDBs in liver biopsy samples from patients with CHC is associated independently with fibrosis progression. Gain of MDBs over time is associated with decompensation and progression to cirrhosis; and occurs most frequently among diabetic patients. MDBs might be used as prognostic factors for patients with CHC.

Constitutional telomerase mutations are genetic risk factors for cirrhosis

Some patients with liver disease progress to cirrhosis, but the risk factors for cirrhosis development are unknown. Dyskeratosis congenita, an inherited bone marrow failure syndrome associated with mucocutaneous anomalies, pulmonary fibrosis, and cirrhosis, is caused by germline mutations of genes in the telomerase complex. We examined whether telomerase mutations also occurred in sporadic cirrhosis. In all, 134 patients with cirrhosis of common etiologies treated at the Liver Research Institute, University of Arizona, between May 2008 and July 2009, and 528 healthy subjects were screened for variation in the TERT and TERC genes by direct sequencing; an additional 1,472 controls were examined for the most common genetic variation observed in patients. Telomere length of leukocytes was measured by quantitative polymerase chain reaction. Functional effects of genetic changes were assessed by transfection of mutation-containing vectors into telomerase-deficient cell lines, and telomerase activity was measured in cell lysates. Nine of the 134 patients with cirrhosis (7%) carried a missense variant in TERT, resulting in a cumulative carrier frequency significantly higher than in controls (P = 0.0009). One patient was homozygous and eight were heterozygous. The allele frequency for the most common missense TERT variant was significantly higher in patients with cirrhosis (2.6%) than in 2,000 controls (0.7%; P = 0.0011). One additional patient carried a TERC mutation. The mean telomere length of leukocytes in patients with cirrhosis, including six mutant cases, was shorter than in age-matched controls (P = 0.0004).

CONCLUSION

Most TERT gene variants reduced telomerase enzymatic activity in vitro. Loss-of-function telomerase gene variants associated with short telomeres are risk factors for sporadic cirrhosis.

Keratin hypersumoylation alters filament dynamics and is a marker for human liver disease and keratin mutation

Keratin polypeptide 8 (K8) associates noncovalently with its partners K18 and/or K19 to form the intermediate filament cytoskeleton of hepatocytes and other simple-type epithelial cells. Human K8, K18, and K19 variants predispose to liver disease, whereas site-specific keratin phosphorylation confers hepatoprotection. Because stress-induced protein phosphorylation regulates sumoylation, we hypothesized that keratins are sumoylated in an injury-dependent manner and that keratin sumoylation is an important regulatory modification. We demonstrate that K8/K18/K19, epidermal keratins, and vimentin are sumoylated in vitro. Upon transfection, K8, K18, and K19 are modified by poly-SUMO-2/3 chains on Lys-285/Lys-364 (K8), Lys-207/Lys-372 (K18), and Lys-208 (K19). Sumoylation affects filament organization and stimulus-induced keratin solubility and is partially inhibited upon mutation of one of three known K8 phosphorylation sites. Extensive sumoylation occurs in cells transfected with individual K8, K18, or K19 but is limited upon heterodimerization (K8/K18 or K8/K19) in the absence of stress. In contrast, keratin sumoylation is significantly augmented in cells and tissues during apoptosis, oxidative stress, and phosphatase inhibition. Poly-SUMO-2/3 conjugates are present in chronically injured but not normal, human, and mouse livers along with polyubiquitinated and large insoluble keratin-containing complexes. Notably, common human K8 liver disease-associated variants trigger keratin hypersumoylation with consequent diminished solubility. In contrast, modest sumoylation of wild type K8 promotes solubility. Hence, conformational changes induced by keratin natural mutations and extensive tissue injury result in K8/K18/K19 hypersumoylation, which retains keratins in an insoluble compartment, thereby limiting their cytoprotective function.

Keratin variants predispose to acute liver failure and adverse outcome: race and ethnic associations

BACKGROUND & AIMS

Keratins 8 and 18 (K8/K18) provide anti-apoptotic functions upon liver injury. The cytoprotective function of keratins explains the overrepresentation of K8/K18 variants in patients with cirrhosis. However, K8/K18 variant-associated susceptibility to acute liver injury, which is well-described in animal models, has not been studied in humans.

METHODS

We analyzed the entire coding regions of KRT8 and KRT18 genes (15 total exons and their exon-intron boundaries) to determine the frequency of K8/K18 variants in 344 acute liver failure (ALF) patients (49% acetaminophen-related) and 2 control groups (African-American [n = 245] and previously analyzed white [n = 727] subjects).

RESULTS

Forty-five ALF patients had significant amino-acid-altering K8/K18 variants, including 23 with K8 R341H and 11 with K8 G434S. K8 variants were significantly more common (total of 42 patients) than K18 variants (3 patients) (P < .001). We found increased frequency of variants in white ALF patients (9.1%) versus controls (3.7%) (P = .01). K8 R341H was more common in white (P = .01) and G434S was more common in African-American (P = .02) ALF patients versus controls. White patients with K8/K18 variants were less likely to survive ALF without transplantation (P = .02). K8 A333A and G434S variants associated exclusively with African Americans (23% combined frequency in African American but none in white controls; P < .0001), while overall, K18 variants were more common in non-white liver-disease subjects compared to whites (2.8% vs 0.6%, respectively; P = .008).

CONCLUSIONS

KRT8 and KRT18 are important susceptibility genes for ALF development. Presence of K8/K18 variants predisposes to adverse ALF outcome, and some variants segregate with unique ethnic and race backgrounds.

Cytoskeletal keratin glycosylation protects epithelial tissue from injury

Keratins 8 and 18 (K8 and K18) are heteropolymeric intermediate filament phosphoglycoproteins of simple-type epithelia. Mutations in K8 and K18 predispose the affected individual to liver disease as they protect hepatocytes from apoptosis. K18 undergoes dynamic O-linked N-acetylglucosamine glycosylation at Ser 30, 31 and 49. We investigated the function of K18 glycosylation by generating mice that overexpress human K18 S30/31/49A substitution mutants that cannot be glycosylated (K18-Gly(-)), and compared the susceptibility of these mice to injury with wild-type and other keratin-mutant mice. K18-Gly(-) mice are more susceptible to liver and pancreatic injury and apoptosis induced by streptozotocin or to liver injury by combined N-acetyl-D-glucosaminidase inhibition and Fas administration. The enhanced apoptosis in the livers of mice that express K18-Gly(-) involves the inactivation of Akt1 and protein kinase Ctheta as a result of their site-specific hypophosphorylation. Akt1 binds to K8, which probably contributes to the reciprocal hyperglycosylation and hypophosphorylation of Akt1 that occurs on K18 hypoglycosylation, and leads to decreased Akt1 kinase activity. Therefore, K18 glycosylation provides a unique protective role in epithelial injury by promoting the phosphorylation and activation of cell-survival kinases.

Quantitation of tissue polypeptide antigen (TPA) in hepatic and systemic circulation in patients with chronic liver diseases

BACKGROUND AND AIM

Abnormal serum tissue polypeptide antigen (TPA) values are commonly found in patients with chronic liver damage and liver cirrhosis even in the absence of malignancies. The aim of this study was to compare serum TPA levels in patients with cirrhosis, to examine correlations between TPA levels and the degree of portal hypertension, and to evaluate TPA concentrations in paired hepatic and peripheral blood samples.

METHODS

A total of 128 patients with chronic liver disease of various severity were studied prospectively. TPA concentrations in hepatic vein and peripheral blood were determined, and Hepatic Vein Pressure Gradient (HVPG) was measured.

RESULTS

TPA levels were significantly higher in patients with cirrhosis than in those with chronic hepatitis, and in systemic circulation than in hepatic vein blood. Peripheral but not hepatic TPA levels did correlate with the HVPG. Subjects with clinically significant portal hypertension (HVPG > 10 mmHg) showed significantly higher peripheral TPA levels than those with HVPG < 10 mmHg.

CONCLUSIONS

Our data suggest that the increased TPA levels observed in cirrhotic patients and the high systemic-to-hepatic blood TPA gradient are probably due to the presence of portal-systemic shunts rather than to hepatic necro-inflammatory activity. In clinical practice, TPA determination could help us to identify and to follow up cirrhotic patients with more severe portal hypertension.

The role of the hepatocyte cytokeratin network in bile formation and resistance to bile acid challenge and cholestasis in mice

The intermediate filament cytoskeleton of hepatocytes is composed of keratin (K) 8 and K18 and has important mechanical and nonmechanical functions. However, the potential role of the K8/K18 network for proper membrane targeting of hepatocellular adenosine triphosphate-binding cassette transporters and bile formation is unknown. We therefore designed a comparative study in K8 and K18 knockout mice and respective wild-type controls to test the hypothesis that intermediate filaments of hepatocytes play a role in normal bile formation. In addition, we challenged mice either with a 1% cholic acid-supplemented diet or a diet containing the porphyrinogenic xenobiotic 3,5-diethoxycarbonyl-1,4-dihydrocollidine to determine the effect of K8/K18 loss on bile flow/composition and liver injury under different physiological and toxic stress stimuli. Protein expression levels and membrane localization of various transporters and anion exchangers were compared using western blotting and immunofluorescence microscopy, respectively, and bile flow and composition were determined under various experimental conditions. Our results demonstrate that loss of the intermediate filament network had no significant effect on bile formation and composition, as well as expression levels and membrane targeting of key hepatobiliary transporters under baseline and stress conditions. However, loss of K8 significantly increased liver injury in response to toxic stress.

CONCLUSION

The intermediate filament network of hepatocytes is not specifically required for proper bile formation in mice.

Keratin variants are overrepresented in primary biliary cirrhosis and associate with disease severity

Keratins (K) 8 and 18 variants predispose carriers to the development of end-stage liver disease and patients with chronic hepatitis C to disease progression. Hepatocytes express K8/K18, whereas biliary epithelia express K8/K18/K19. K8-null mice, which are predisposed to liver injury, spontaneously develop anti-mitochondrial antibodies (AMA) and have altered hepatocyte mitochondrial size and function. There is no known association of K19 with human disease and no known association of K8/K18/K19 with human autoimmune liver disease. We tested the hypothesis that K8/K18/K19 variants associate with primary biliary cirrhosis (PBC), an autoimmune cholestatic liver disease characterized by the presence of serum AMA. In doing so, we analyzed the entire exonic regions of K8/K18/K19 in 201 Italian patients and 200 control blood bank donors. Five disease-associated keratin heterozygous variants were identified in patients versus controls (K8 G62C/R341H/V380I, K18 R411H, and K19 G17S). Four variants were novel and included K19 G17S/V229M/N184N and K18 R411H. Overall, heterozygous disease-associated keratin variants were found in 17 of 201 (8.5%) PBC patients and 4 of 200 (2%) blood bank donors (P < 0.004, odds ratio = 4.53, 95% confidence interval = 1.5-13.7). Of the K19 variants, K19 G17S was found in three patients but not in controls and all K8 R341H (eight patients and three controls) associated with concurrent presence of the previously described intronic K8 IVS7+10delC deletion. Notably, keratin variants associated with disease severity (12.4% variants in Ludwig stage III/IV versus 4.2% in stages I/II; P < 0.04, odds ratio = 3.25, 95% confidence interval = 1.02-10.40), but not with the presence of AMA.

CONCLUSION

K8/K18/K19 variants are overrepresented in Italian PBC patients and associate with liver disease progression. Therefore, we hypothesize that K8/K18/K19 variants may serve as genetic modifiers in PBC.

Toward unraveling the complexity of simple epithelial keratins in human disease

Simple epithelial keratins (SEKs) are found primarily in single-layered simple epithelia and include keratin 7 (K7), K8, K18-K20, and K23. Genetically engineered mice that lack SEKs or overexpress mutant SEKs have helped illuminate several keratin functions and served as important disease models. Insight into the contribution of SEKs to human disease has indicated that K8 and K18 are the major constituents of Mallory-Denk bodies, hepatic inclusions associated with several liver diseases, and are essential for inclusion formation. Furthermore, mutations in the genes encoding K8, K18, and K19 predispose individuals to a variety of liver diseases. Hence, as we discuss here, the SEK cytoskeleton is involved in the orchestration of several important cellular functions and contributes to the pathogenesis of human liver disease.

Keratin 18 provides resistance to Fas-mediated liver failure in mice

BACKGROUND

Keratins are intermediate filament proteins of epithelial cells with pivotal functions for cell integrity. They comprise keratins 18 [K18] and 8 [K8] in hepatocytes. Keratins are of major importance for an intact cellular microarchitecture and have protective functions in human liver diseases. In mice, K8 has been demonstrated to protect against Fas-antibody-induced liver failure by direct interaction with apoptotic regulators, while the role of K18 remains unresolved.

MATERIALS AND METHODS

We analysed effects of K18 deficiency on Fas-induced liver failure in mice. We determined survival and analysed induction of apoptosis after injection of the agonistic Fas antibody Jo2 into K18(-/-) and wild-type control mice by TUNEL assay and fluorometrically analysed caspase-3, -8 and -9 activities 1, 2 and 3 h after Jo2 injection.

RESULTS

In K18(-/-) mice, survival of Fas-antibody treated mice was significantly shorter than that of wild-type controls (P = 0.02). However, shortened survival of K18(-/-) mice was caused by increased hepatic damage but was not correlated to enhanced induction of apoptotic pathways, as neither numbers of TUNEL positive apoptotic cells nor activities of caspases-3, -8 and -9 differed between K18(-/-) and K18(+/+) mice at any point of time.

CONCLUSION

K18(-/-) mice are significantly more susceptible to Fas-antibody-induced liver failure. The cytoprotective effect of K18 is not explained by a differential activation of caspases-3, -8 and -9, suggesting that K18 does not directly interfere with apoptotic regulators. Importantly, however, K18 exerts significant protective functions by other mechanisms.

Intermediate filament cytoskeleton of the liver in health and disease

Intermediate filaments (IFs) represent the largest cytoskeletal gene family comprising approximately 70 genes expressed in tissue specific manner. In addition to scaffolding function, they form complex signaling platforms and interact with various kinases, adaptor, and apoptotic proteins. IFs are established cytoprotectants and IF variants are associated with >30 human diseases. Furthermore, IF-containing inclusion bodies are characteristic features of several neurodegenerative, muscular, and other disorders. Acidic (type I) and basic keratins (type II) build obligatory type I and type II heteropolymers and are expressed in epithelial cells. Adult hepatocytes contain K8 and K18 as their only cytoplasmic IF pair, whereas cholangiocytes express K7 and K19 in addition. K8/K18-deficient animals exhibit a marked susceptibility to various toxic agents and Fas-induced apoptosis. In humans, K8/K18 variants predispose to development of end-stage liver disease and acute liver failure (ALF). K8/K18 variants also associate with development of liver fibrosis in patients with chronic hepatitis C. Mallory-Denk bodies (MDBs) are protein aggregates consisting of ubiquitinated K8/K18, chaperones and sequestosome1/p62 (p62) as their major constituents. MDBs are found in various liver diseases including alcoholic and non-alcoholic steatohepatitis and can be formed in mice by feeding hepatotoxic substances griseofulvin and 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC). MDBs also arise in cell culture after transfection with K8/K18, ubiquitin, and p62. Major factors that determine MDB formation in vivo are the type of stress (with oxidative stress as a major player), the extent of stress-induced protein misfolding and resulting chaperone, proteasome and autophagy overload, keratin 8 excess, transglutaminase activation with transamidation of keratin 8 and p62 upregulation.

Keratin mutation predisposes to mouse liver fibrosis and unmasks differential effects of the carbon tetrachloride and thioacetamide models

BACKGROUND & AIMS

Keratins 8 and 18 (K8/K18) are important hepatoprotective proteins. Animals expressing K8/K18 mutants show a marked susceptibility to acute/subacute liver injury. K8/K18 variants predispose to human end-stage liver disease and associate with fibrosis progression during chronic hepatitis C infection. We sought direct evidence for a keratin mutation-related predisposition to liver fibrosis using transgenic mouse models because the relationship between keratin mutations and cirrhosis is based primarily on human association studies.

METHODS

Mouse hepatofibrosis was induced by carbon tetrachloride (CCl(4)) or thioacetamide. Nontransgenic mice, or mice that over express either human Arg89-to-Cys (R89C mice) or wild-type K18 (WT mice) were used. The extent of fibrosis was evaluated by quantitative real-time reverse-transcription polymerase chain reaction of fibrosis-related genes, liver hydroxyproline measurement, and Picro-Sirius red staining and collagen immunofluorescence staining.

RESULTS

Compared with control animals, CCl(4) led to similar liver fibrosis but increased injury in K18 R89C mice. In contrast, thioacetamide caused more severe liver injury and fibrosis in K18 R89C as compared with WT and nontransgenic mice and resulted in increased messenger RNA levels of collagen, tissue inhibitor of metalloproteinase 1, matrix metalloproteinase 2, and matrix metalloproteinase 13. Analysis in nontransgenic mice showed that thioacetamide and CCl(4) have dramatically different molecular expression responses involving cytoskeletal and chaperone proteins.

CONCLUSIONS

Over expression of K18 R89C predisposes transgenic mice to thioacetamide- but not CCl(4)-induced liver fibrosis. Differences in the keratin mutation-associated fibrosis response among the 2 models raise the hypothesis that keratin variants may preferentially predispose to fibrosis in unique human liver diseases. Findings herein highlight distinct differences in the 2 widely used fibrosis models.

Keratins let liver live: Mutations predispose to liver disease and crosslinking generates Mallory-Denk bodies

Keratin polypeptides 8 and 18 (K8/K18) are the cytoskeletal intermediate filament proteins of hepatocytes while K8/K18/K19 are the keratins of hepatobiliary ductal cells. Hepatocyte K8/K18 are highly abundant and behave as stress proteins with injury-inducible expression. Human association studies show that K8/K18 germline heterozygous mutations predispose to end-stage liver disease of multiple etiologies ( approximately 3 fold increased risk), and to liver disease progression in patients with chronic hepatitis C infection. These findings are supported by extensive transgenic mouse and ex vivo primary hepatocyte culture studies showing that K8 or K18 mutations predispose the liver to acute or subacute injury and promote apoptosis and fibrosis. Mutation-associated predisposition to liver injury is likely related to mechanical and nonmechanical keratin functions including maintenance of cell integrity, protection from apoptosis and oxidative injury, serving as a phosphate sponge, regulation of mitochondrial organization/function and protein targeting. These functions are altered by mutation-induced changes in keratin phosphorylation, solubility and filament organization/reorganization. Keratins are also the major constituents of Mallory-Denk bodies (MDBs). A toxin-induced K8>K18 ratio, and keratin crosslinking by transglutaminase-2 play essential roles in MDB formation. Furthermore, intracellular or cell-released K18 fragments, generated by caspase-mediated proteolysis during apoptosis serve as markers of liver injury. Therefore, K8 and K18 are cytoprotective stress proteins that play a central role in guarding hepatocytes from apoptosis. Keratin involvement in liver disease is multi-faceted and includes modulating disease progression upon mutation, formation of MDBs in response to unique forms of injury, and serving as markers of epithelial cell death.

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.

Analysis of keratin polypeptides 8 and 19 variants in inflammatory bowel disease

BACKGROUND/AIMS

Keratin-8 (KRT8)-null mice develop spontaneous colitis and predisposition to liver injury. Human studies show that some KRT8 variants predispose to end-stage liver disease and progression and suggest that such variants might associate with UC or CD. We asked whether mutations in KRT8 or KRT19, the major intestinal keratins, are associated with UC/CD.

METHODS

Exonic regions of the KRT8/KRT19 genes were polymerase chain reaction-amplified using genomic DNA from 2 independent groups. Group I included 91 unrelated patients with CD, 93 unrelated patients with UC, and 70 unrelated/unaffected volunteers. KRT8 variants were also tested with pyrosequencing in Group II that included 682 independent nuclear families with both parents and at least 1 CD/UC-affected offspring and 273 unaffected controls. Both cohorts were enriched for familial IBD.

RESULTS

In Group I, KRT19 variants were identified in CD/UC patients within the promoter and exons 1+2, with similar mutation frequencies in the control/CD/UC groups. In contrast, 16 of 184 CD+UC patients harbored KRT8 heterozygous variants involving Gly62-to-Cys and Arg341-to-His and a novel Arg341-to-Cys, which were noted in 4 volunteers (Arg341-to-His) and correlated with extensive UC (P = .005). One family with unaffected parents had 3 pediatric-affected siblings with severe disease, 2 of whom are compound heterozygous (Gly62-to-Cys/Arg341-to-His). However, there was no significant departure from random transmission of the 3 alleles in Group II IBD families.

CONCLUSIONS

KRT8 and KRT19 variants are not overtransmitted or associated with familial IBD, although a potential role in sporadic IBD cannot be excluded. A novel but rare keratin-8 Arg341-to-Cys is identified in IBD patients.

Genetics in liver diseases

In recent years, few fields in medicine have witnessed discoveries as momentous as those pertaining to the liver. Dramatic advances have been made, particularly in the areas of molecular biology and genetics. A joint EASL/AASLD Monothematic Conference was held on June 23rd-24th, 2006, in Modena, Italy, to bring the latest breakthroughs in different fields of genetics to hepatologists. This article reports the highlights of the conference and summarizes the main conclusions and implications for clinical and experimental hepatology.

Keratin 18 overexpression but not phosphorylation or filament organization blocks mouse Mallory body formation

Several human liver diseases are associated with formation of Mallory body (MB) inclusions. These hepatocyte cytoplasmic deposits are composed primarily of hyperphosphorylated keratins 8 and 18 (K8/K18). Feeding a 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-containing diet is a well-established mouse model of MBs. K8 overexpression, and K8-null or K18-null mouse models, indicate that a K8-greater-than-K18 expression ratio is critical for MB formation. We used established transgenic mouse models to study the effect of K18 overexpression and phosphorylation, or keratin filament disorganization, on MB formation. Five mouse lines were used: nontransgenic, those that overexpress wild-type K18 or the K18 phosphorylation mutants Ser33-to-Ala (S33A) or Ser52-to-Ala (S52A), and mice that overexpress K18 Arg89-to-Cys, which causes collapse of the keratin filament network into dots. DDC feeding induced MBs in nontransgenic livers, but MBs were rarely seen in any of the K18 transgenic mice. Wild-type K18 overexpression protected mice from DDC-induced liver injury.

CONCLUSION

K18 overexpression protects mice from MB formation and from DDC-induced liver injury, which supports the importance of the K8-to-K18 ratio in MB formation. The effect of K18 on MB formation is independent of hepatocyte keratin filament organization or K18 Ser33/Ser52 phosphorylation. Keratin filament collapse, which is a major risk for acute liver injury, is well tolerated in the context of chronic DDC-mediated liver injury.

Serum levels of cytokeratin-18 (tissue polypeptide-specific antigen) in liver diseases

OBJECTIVE

The tissue polypeptide-specific antigen (TPS, cytokeratin-18, a normal constituent of the hepatocyte cytoskeleton) is a standard tumour marker. This study aimed to evaluate serum TPS levels in patients with liver disease.

METHODS

Serum TPS was measured with a commercial immunoassay in 884 individuals (753 outpatients from a liver disease clinic, 131 patients admitted to the hospital with acute liver disease).

RESULTS

Abnormally high (> 80 U/l) TPS levels were found in 57.7% (95% CI 54.0-61.3%) of outpatients with liver disease. Elevated TPS levels were observed for all liver diseases, including fatty liver, alcoholic disease, chronic viral hepatitis, autoimmune hepatitis, cholestasis, transplantation, and hepatocarcinoma. TPS levels correlated with liver markers, particularly serum AST. In addition, TPS levels correlated with Knodell's score in patients with chronic hepatitis. TPS was increased in one-third of patients with normal liver enzyme values. Serum TPS levels decreased after specific therapy in patients with hepatitis C and autoimmune hepatitis. Abnormally high TPS levels were found in the vast majority of patients admitted to the hospital, with markedly high (> 800 U/l) values being observed in 47.5% (95% CI 36.1-55.7%) of patients with alcoholic liver disease and in 80.8% (95% CI 60.0-92.7%) of patients with acute hepatitis.

CONCLUSIONS

Serum TPS (cytokeratin-18) is elevated in patients with non-malignant liver diseases, particularly in those with prominent cytolysis. Further studies are needed to evaluate the use of TPS as a marker of liver disease.

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

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