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

Alpha-1 Antitrypsin Inclusions Sequester GRP78 in a Bile Acid-Inducible Manner

BACKGROUND AND AIMS

The homozygous PiZ mutation (PIZZ genotype) constitutes the predominant cause of severe alpha-1 antitrypsin (AAT) deficiency and leads to liver disease via hepatocellular AAT aggregation. We systematically analysed the composition of AAT aggregates and studied the impact of bile acids.

METHODS

AAT inclusions were isolated from livers of PiZ overexpressing mice and PIZZ humans via fluorescence-activated and immunomagnetic sorting (FACS/MACS), while insoluble proteins were obtained via Triton-X extraction. Inclusion composition was evaluated through mass-spectrometry (MS), immunoblotting and immunostaining. Hepatocytes with versus without AAT aggregates were obtained via microdissection. Serum bile acids were assessed in 57 PIZZ subjects and 19 controls. Mice were administered 2% cholic acid (CA)-supplemented chow for 7 days.

RESULTS

MS identified the key endoplasmic reticulum chaperone 78 kDa glucose-regulated protein (GRP78) in FACS/MACS pulldowns. GRP78 was also enriched in insoluble fractions from PiZ mice versus wild types and detected in insoluble fractions/MACS isolates from PIZZ liver explants. In cultured cells/primary hepatocytes, PiZ overexpression was associated with increased GRP78 mRNA/protein levels. In human livers, hepatocytes with AAT aggregates had higher GRP78 levels than hepatocytes without. PIZZ subjects displayed higher serum bile acid levels than controls and the highest levels were seen in individuals with liver injury/fibrosis. In PiZ mice, CA-mediated bile acid challenge resulted in increased liver injury and translocation of GRP78 into the aggregates.

CONCLUSIONS

Our results demonstrate that GRP78 is sequestered within AAT inclusions. Bile acid accumulation, as seen in PIZZ subjects with liver disease, may promote GRP78 segregation and thereby augment liver damage.

TRIAL REGISTRATION

NCT02929940.

Keratin8 Deficiency Aggravates Retinal Ganglion Cell Damage Under Acute Ocular Hypertension

Purpose

Keratin 8/18 (KRT8/18), paired members of the intermediate filament family, have shown vital functions in regulating physiological activities more than supporting the mechanic strength for cells and organelles. However, the KRT8/18 presence in retinal ganglion cells (RGCs) and functions on neuroprotection in a mouse model of acute ocular hypertension (AOH) are unknown and worthy of exploration.

Methods

We identified the existence of KRT8/18 in normal human and mouse retinas and primary RGCs. KRT8/18 levels were detected after AOH modeling. The adeno-associated virus (AAV) system was intravitreally used for selective KRT8 knockdown in RGCs. The histological changes, the loss and dysfunction of RGCs, and the gliosis in retinas were detected. The markers of cell apoptosis and MAPK pathways were investigated.

Results

KRT8/18 existed in all retinal layers and was highly expressed in RGCs, and they increased after AOH induction. The KRT8 knockdown in RGCs caused no histopathological changes and RGC loss in retinas without AOH modeling. However, after the KRT8 deficiency, AOH significantly promoted the loss of whole retina and inner retina thickness, the reduction, apoptosis, and dysfunction of RGCs, and the glial activation. Besides, downregulated Bcl-2 and upregulated cleaved-Caspase 3 were found in the AOH retinas with KRT8 knockdown, which may be caused by the increased phosphorylation level of MAPK pathways (JNK, p38, and ERK).

Conclusions

The KRT8 deficiency promoted RGC apoptosis and neurodegeneration by abnormal activation of MAPK pathways in AOH retinas. Targeting KRT8 may serve as a novel treatment for saving RCGs from glaucomatous injuries.

Priming, Triggering, Adaptation and Senescence (PTAS): A Hypothesis for a Common Damage Mechanism of Steatohepatitis

Understanding the pathomechanism of steatohepatitis (SH) is hampered by the difficulty of distinguishing between causes and consequences, by the broad spectrum of aetiologies that can produce the phenotype, and by the long time-span during which SH develops, often without clinical symptoms. We propose that SH develops in four phases with transitions: (i) priming lowers stress defence; (ii) triggering leads to acute damage; (iii) adaptation, possibly associated with cellular senescence, mitigates tissue damage, leads to the phenotype, and preserves liver function at a lower level; (iv) finally, senescence prevents neoplastic transformation but favours fibrosis (cirrhosis) and inflammation and further reduction in liver function. Escape from senescence eventually leads to hepatocellular carcinoma. This hypothesis for a pathomechanism of SH is supported by clinical and experimental observations. It allows organizing the various findings to uncover remaining gaps in our knowledge and, finally, to provide possible diagnostic and intervention strategies for each stage of SH development.

Cytoskeleton and Associated Proteins: Pleiotropic JNK Substrates and Regulators

This review extensively reports data from the literature concerning the complex relationships between the stress-induced c-Jun N-terminal kinases (JNKs) and the four main cytoskeleton elements, which are actin filaments, microtubules, intermediate filaments, and septins. To a lesser extent, we also focused on the two membrane-associated cytoskeletons spectrin and ESCRT-III. We gather the mechanisms controlling cytoskeleton-associated JNK activation and the known cytoskeleton-related substrates directly phosphorylated by JNK. We also point out specific locations of the JNK upstream regulators at cytoskeletal components. We finally compile available techniques and tools that could allow a better characterization of the interplay between the different types of cytoskeleton filaments upon JNK-mediated stress and during development. This overview may bring new important information for applied medical research.

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.

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.

Leukamenin E Induces K8/18 Phosphorylation and Blocks the Assembly of Keratin Filament Networks Through ERK Activation

Leukamenin E is a natural ent-kaurane diterpenoid isolated from Isodon racemosa (Hemsl) Hara that has been found to be a novel and potential keratin filament inhibitor, but its underlying mechanisms remain largely unknown. Here, we show that leukamenin E induces keratin filaments (KFs) depolymerization, largely independently of microfilament (MFs) and microtubules (MTs) in well-spread cells and inhibition of KFs assembly in spreading cells. These effects are accompanied by keratin phosphorylation at K8-Ser73/Ser431 and K18-Ser52 via the by extracellular signal-regulated kinases (ERK) pathway in primary liver carcinoma cells (PLC) and human umbilical vein endothelial cells (HUVECs). Moreover, leukamenin E increases soluble pK8-Ser73/Ser431, pK18-Ser52, and pan-keratin in the cytoplasmic supernatant by immunofluorescence imaging and Western blotting assay. Accordingly, leukamenin E inhibits the spreading and migration of cells. We propose that leukamenin E-induced keratin phosphorylation may interfere with the initiation of KFs assembly and block the formation of a new KFs network, leading to the inhibition of cell spreading. Leukamenin E is a potential target drug for inhibition of KFs assembly.

The Metastasis Potential Promoting Capacity of Cancer-Associated Fibroblasts Was Attenuated by Cisplatin via Modulating KRT8

Background

Cancer-associated fibroblasts (CAFs) are an essential component of tumor microenvironment. They are attracting increasing attentions due to their crucial role in tumor growth, drug-resistance and metastasis. Cisplatin is a first-line chemotherapy drug applying in various types of cancer. There are intensive studies on cisplatin's effect on tumor cells, however, its effect on CAFs remains poorly understood. In the present study, we investigated the effect of cisplatin on CAFs.

Methods

Cell migration was detected by wound healing assay. Cell invasion was performed by the transwell assay. mRNA expression was detected by quantitative PCR, and protein expression was detected by Western blotting. Tumor growth was measured using BALB/c nude mice tumor models.

Results

Cisplatin attenuated the promoting capacity of CAFs on lung cancer cell migration and invasion, via suppressing CAFs' effect on metastasis-related genes including Twist1, vascular endothelial growth factor receptor (VEGFR), MMP2, and AKT signaling pathway. Keratin 8 (KRT8) was identified as a target of cisplatin. KRT8 upregulation in CAFs is responsible for the inhibitory effect of cisplatin on lung cancer cells metastasis potential through AKT pathway suppression. The stimulation of AKT by AKT activator SC79 reversed KRT8's effect on cell migration. Importantly, in vivo study also showed that CAFs enhanced tumor growth significantly, and cisplatin effectively abrogated the promoting effect of CAFs on tumor growth.

Conclusion

Our results revealed a novel mechanism that cisplatin attenuated the metastasis promoting effect of CAFs via KRT8/AKT signaling pathway. This finding highlights KRT8 in CAFs as a potential therapeutic candidate for metastasis treatment.

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

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

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

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

Thymoquinone Attenuates Acetaminophen Overdose-Induced Acute Liver Injury and Inflammation Via Regulation of JNK and AMPK Signaling Pathway

Thymoquinone (TQ) is a main aromatic component of Nigella sativa L. seeds or Agastache rugosa (Fisch. & C.A.Mey.) Kuntze. The protective mechanism of TQ against acute liver injury induced by acetaminophen (APAP), however, remains unclear. We aimed to investigated the hepato-protective mechanism of TQ on the development of APAP-induced acute liver injury. Male kunming mice were pretreated with TQ or N-acetylcysteine (NAC) before a single APAP injection. Human Chang liver cells were incubated with TQ, SP600125 or AICAR in presence of APAP for 24 h. TQ pretreatment reduced levels of serum aminotransferases and increased hepatic glutathione and glutathione peroxidase activities via inhibiting CYP2E1 expression. TQ inhibited JNK, ERK and P38 phosphorylation induced by APAP. Meanwhile, TQ inhibited PI3K/mTOR signaling activation and activated AMPK phosphorylation. Moreover, TQ prevented APAP-induced hepatocytes apoptosis regulated by Bcl-2 and Bax. Furthermore, TQ inhibited STAT3 phosphorylation on APAP-induced acute liver injury. In addition, TQ significantly inhibited P2X7R protein expression and IL-1 β release. APAP-enhanced JNK phosphorylation and APAP-suppressed AMPK phosphorylation were also observed in Chang liver cells, and these changes were recovered by pretreatment with TQ, SP600125 and AICAR. Our findings suggest that TQ may actively prevent APAP-induced acute liver injury, and the effect may be mediated by JNK and AMPK signaling pathways.

The role of keratins in the digestive system: lessons from transgenic mouse models

Keratins are the largest subfamily of intermediate filament proteins. They are either type I acidic or type II basic keratins. Keratins form obligate heteropolymer in epithelial cells and their expression patterns are tissue-specific. Studies have shown that keratin mutations are the cause of many diseases in humans or predispose humans to acquiring them. Using mouse models to study keratin-associated human diseases is critical, because they allow researchers to get a better understanding of these diseases and their progressions, and so many such studies have been conducted. Acknowledging the importance, researches with genetically modified mice expressing human disease-associated keratin mutants have been widely done. Numerous studies using keratin knockout mice, keratin-overexpressed mice, or transgenic mice expressing keratin mutants have been conducted. This review summarizes the mouse models that have been used to study type I and type II keratin expression in the digestive organs, namely, the liver, pancreas, and colon.

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

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

Intermediate filament proteins of digestive organs: physiology and pathophysiology

Intermediate filament proteins (IFs), such as cytoplasmic keratins in epithelial cells and vimentin in mesenchymal cells and the nuclear lamins, make up one of the three major cytoskeletal protein families. Whether in digestive organs or other tissues, IFs share several unique features including stress-inducible overexpression, abundance, cell-selective and differentiation state expression, and association with >80 human diseases when mutated. Whereas most IF mutations cause disease, mutations in simple epithelial keratins 8, 18, or 19 or in lamin A/C predispose to liver disease with or without other tissue manifestations. Keratins serve major functions including protection from apoptosis, providing cellular and subcellular mechanical integrity, protein targeting to subcellular compartments, and scaffolding and regulation of cell-signaling processes. Keratins are essential for Mallory-Denk body aggregate formation that occurs in association with several liver diseases, whereas an alternate type of keratin and lamin aggregation occurs upon liver involvement in porphyria. IF-associated diseases have no known directed therapy, but high-throughput drug screening to identify potential therapies is an appealing ongoing approach. Despite the extensive current knowledge base, much remains to be discovered regarding IF physiology and pathophysiology in digestive and nondigestive organs.

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.