Keratins turn over by ubiquitination in a phosphorylation-modulated fashion

Ubiquitination is involved in PKC-mediated degradation of cell surface Kv1.5 channels

The voltage-gated Kv1.5 potassium channel, conducting the ultra-rapid delayed rectifier K+ current (IKur) in human cells, plays important roles in the repolarization of atrial action potentials and regulation of the vascular tone. We previously reported that activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) induces endocytic degradation of cell-surface Kv1.5 channels, and a point mutation removing the phosphorylation site, T15A, in the N terminus of Kv1.5 abolished the PMA-effect. In the present study, using mutagenesis, patch clamp recording, Western blot analysis, and immunocytochemical staining, we demonstrate that ubiquitination is involved in the PMA-mediated degradation of mature Kv1.5 channels. Since the expression of the Kv1.4 channel is unaffected by PMA treatment, we swapped the N- and/or C-termini between Kv1.5 and Kv1.4. We found that the N-terminus alone did not but both N- and C-termini of Kv1.5 did confer PMA sensitivity to mature Kv1.4 channels, suggesting the involvement of Kv1.5 C-terminus in the channel ubiquitination. Removal of each of the potential ubiquitination residue Lysine at position 536, 565, and 591 by Arginine substitution (K536R, K565R, and K591R) had little effect, but removal of all three Lysine residues with Arginine substitution (3K-R) partially reduced PMA-mediated Kv1.5 degradation. Furthermore, removing the cysteine residue at position 604 by Serine substitution (C604S) drastically reduced PMA-induced channel degradation. Removal of the three Lysines and Cys604 with a quadruple mutation (3K-R/C604S) or a truncation mutation (Δ536) completely abolished the PKC activation-mediated degradation of Kv1.5 channels. These results provide mechanistic insight into PKC activation-mediated Kv1.5 degradation.

Human APOE4 Protects High-Fat and High-Sucrose Diet Fed Targeted Replacement Mice against Fatty Liver Disease Compared to APOE3

Recent genome- and exome-wide association studies suggest that the human APOE ε4 allele protects against non-alcoholic fatty liver disease (NAFLD), while ε3 promotes hepatic steatosis and steatohepatitis. The present study aimed at examining the APOE genotype-dependent development of fatty liver disease and its underlying mechanisms in a targeted replacement mouse model. Male mice expressing the human APOE3 or APOE4 protein isoforms on a C57BL/6J background and unmodified C57BL/6J mice were chronically fed a high-fat and high-sucrose diet to induce obesity. After 7 months, body weight gain was more pronounced in human APOE than endogenous APOE expressing mice with elevated plasma biomarkers suggesting aggravated metabolic dysfunction. APOE3 mice exhibited the highest liver weights and, compared to APOE4, massive hepatic steatosis. An untargeted quantitative proteome analysis of the liver identified a high number of proteins differentially abundant in APOE3 versus APOE4 mice. The majority of the higher abundant proteins in APOE3 mice could be grouped to inflammation and damage-associated response, and lipid storage, amongst others. Results of the targeted qRT-PCR and Western blot analyses contribute to the overall finding that APOE3 as opposed to APOE4 promotes hepatic steatosis, inflammatory- and damage-associated response signaling and fibrosis in the liver of obese mice. Our experimental data substantiate the observation of an increased NAFLD-risk associated with the human APOEε3 allele, while APOEε4 appears protective. The underlying mechanisms of the protection possibly involve a higher capacity of nonectopic lipid deposition in subcutaneous adipose tissue and lower hepatic pathogen recognition in the APOE4 mice.

Intermediate, but not average: The unusual lives of the nuclear lamin proteins

The nuclear lamins are polymeric intermediate filament proteins that scaffold the nucleus and organize the genome in nearly all eukaryotic cells. This review focuses on the dynamic regulation of lamin filaments through their biogenesis, assembly, disassembly, and degradation. The lamins are unusually long-lived proteins under homeostatic conditions, but their turnover can be induced in select contexts that are highlighted in this review. Finally, we discuss recent investigations into the influence of laminopathy-linked mutations on the assembly, folding, and stability of the nuclear lamins.

Deacetylation via SIRT2 prevents keratin-mutation-associated injury and keratin aggregation

Keratin (K) and other intermediate filament (IF) protein mutations at conserved arginines disrupt keratin filaments into aggregates and cause human epidermolysis bullosa simplex (EBS; K14-R125C) or predispose to mouse liver injury (K18-R90C). The challenge for more than 70 IF-associated diseases is the lack of clinically utilized IF-targeted therapies. We used high-throughput drug screening to identify compounds that normalized mutation-triggered keratin filament disruption. Parthenolide, a plant sesquiterpene lactone, dramatically reversed keratin filament disruption and protected cells and mice expressing K18-R90C from apoptosis. K18-R90C became hyperacetylated compared with K18-WT and treatment with parthenolide normalized K18 acetylation. Parthenolide upregulated the NAD-dependent SIRT2, and increased SIRT2-keratin association. SIRT2 knockdown or pharmacologic inhibition blocked the parthenolide effect, while site-specific Lys-to-Arg mutation of keratin acetylation sites normalized K18-R90C filaments. Treatment of K18-R90C-expressing cells and mice with nicotinamide mononucleotide had a parthenolide-like protective effect. In 2 human K18 variants that associate with human fatal drug-induced liver injury, parthenolide protected K18-D89H- but not K8-K393R-induced filament disruption and cell death. Importantly, parthenolide normalized K14-R125C-mediated filament disruption in keratinocytes and inhibited dispase-triggered keratinocyte sheet fragmentation and Fas-mediated apoptosis. Therefore, keratin acetylation may provide a novel therapeutic target for some keratin-associated diseases.

Revisiting the significance of keratin expression in complex epithelia

A large group of keratin genes (n=54 in the human genome) code for intermediate filament (IF)-forming proteins and show differential regulation in epithelial cells and tissues. Keratin expression can be highly informative about the type of epithelial tissue, differentiation status of constituent cells and biological context (e.g. normal versus diseased settings). The foundational principles underlying the use of keratin expression to gain insight about epithelial cells and tissues primarily originated in pioneering studies conducted in the 1980s. The recent emergence of single cell transcriptomics provides an opportunity to revisit these principles and gain new insight into epithelial biology. Re-analysis of single-cell RNAseq data collected from human and mouse skin has confirmed long-held views regarding the quantitative importance and pairwise regulation of specific keratin genes in keratinocytes of surface epithelia. Furthermore, such analyses confirm and extend the notion that changes in keratin gene expression occur gradually as progenitor keratinocytes commit to and undergo differentiation, and challenge the prevailing assumption that specific keratin combinations reflect a mitotic versus a post-mitotic differentiating state. Our findings provide a blueprint for similar analyses in other tissues, and warrant a more nuanced approach in the use of keratin genes as biomarkers in epithelia.

Extracellular and Intracellular Skeletons: How Do They Involve in Apoptosis

Apoptosis plays a key role in removing abnormal or senescent cells, maintaining the overall health of the tissue, and coordinating individual development. Recently, it has been discovered that the intracellular cytoskeleton plays a role in the apoptotic process. In addition, the regulatory role of extracellular matrix (ECM) fibrous proteins, which can be considered as the extracellular skeleton, in the process of apoptosis is rarely summarized. In this review, we collect the latest knowledge about how fibrous proteins inside and outside cells regulate apoptosis. We describe how ECM fibrous proteins participate in the regulation of death receptor and mitochondrial pathways through various signaling cascades mediated by integrins. We then explore the molecular mechanisms by which intracellular intermediate filaments regulate cell apoptosis by regulating death receptors on the cell membrane surface. Similarly, we report on novel supporting functions of microtubules in the execution phase of apoptosis and discuss their formation mechanisms. Finally, we discuss that the polypeptide fragments formed by caspase degradation of ECM fibrous proteins and intracellular intermediate filament act as local regulatory signals to play different regulatory roles in apoptosis.

Keratins as an Inflammation Trigger Point in Epidermolysis Bullosa Simplex

Epidermolysis bullosa simplex (EBS) is a group of inherited keratinopathies that, in most cases, arise due to mutations in keratins and lead to intraepidermal ruptures. The cellular pathology of most EBS subtypes is associated with the fragility of the intermediate filament network, cytolysis of the basal layer of the epidermis, or attenuation of hemidesmosomal/desmosomal components. Mutations in keratins 5/14 or in other genes that encode associated proteins induce structural disarrangements of different strengths depending on their locations in the genes. Keratin aggregates display impaired dynamics of assembly and diminished solubility and appear to be the trigger for endoplasmic reticulum (ER) stress upon being phosphorylated by MAPKs. Global changes in cellular signaling mainly occur in cases of severe dominant EBS mutations. The spectrum of changes initiated by phosphorylation includes the inhibition of proteasome degradation, TNF-α signaling activation, deregulated proliferation, abnormal cell migration, and impaired adherence of keratinocytes. ER stress also leads to the release of proinflammatory danger-associated molecular pattern (DAMP) molecules, which enhance avalanche-like inflammation. Many instances of positive feedback in the course of cellular stress and the development of sterile inflammation led to systemic chronic inflammation in EBS. This highlights the role of keratin in the maintenance of epidermal and immune homeostasis.

Keratin 7 Is a Constituent of the Keratin Network in Mouse Pancreatic Islets and Is Upregulated in Experimental Diabetes

Keratin (K) 7 is an intermediate filament protein expressed in ducts and glands of simple epithelial organs and in urothelial tissues. In the pancreas, K7 is expressed in exocrine ducts, and apico-laterally in acinar cells. Here, we report K7 expression with K8 and K18 in the endocrine islets of Langerhans in mice. K7 filament formation in islet and MIN6 β-cells is dependent on the presence and levels of K18. K18-knockout (K18^‒/‒) mice have undetectable islet K7 and K8 proteins, while K7 and K18 are downregulated in K8^‒/‒ islets. K7, akin to F-actin, is concentrated at the apical vertex of β-cells in wild-type mice and along the lateral membrane, in addition to forming a fine cytoplasmic network. In K8^‒/‒ β-cells, apical K7 remains, but lateral keratin bundles are displaced and cytoplasmic filaments are scarce. Islet K7, rather than K8, is increased in K18 over-expressing mice and the K18-R90C mutation disrupts K7 filaments in mouse β-cells and in MIN6 cells. Notably, islet K7 filament networks significantly increase and expand in the perinuclear regions when examined in the streptozotocin diabetes model. Hence, K7 represents a significant component of the murine islet keratin network and becomes markedly upregulated during experimental diabetes.

On the origin of germ cell neoplasia in situ: Dedifferentiation of human adult Sertoli cells in cross talk with seminoma cells in vitro

Germ cell neoplasia in situ (GCNIS) is the noninvasive precursor of testicular germ cell tumors type II, the most common cancer in young men, which originates from embryonic germ cells blocked in their maturation. GCNIS is associated with impaired Sertoli cells (SCs) that express fetal keratin 18 (KRT18) and the pluripotency factor SRY-Box transcription factor 2 (SOX2). According to the current theory concerning the origin of GCNIS, these SCs are prepubertal cells arrested in their maturation due to (epi)genetic anomalies and/or environmental antiandrogens. Thus, they are unable to support the development of germ cells, which leads to their maturational block and further progresses into GCNIS. Alternatively, these SCs are hypothesized to be adult cells dedifferentiating secondarily under the influence of GCNIS. To examine whether tumor cells can dedifferentiate SCs, we established a coculture model of adult human SCs (FS1) and a seminoma cell line similar to GCNIS (TCam-2). After 2 wk of coculture, FS1 cells showed progressive expression of KRT18 and SOX2, mimicking the in vivo changes. TCam-2 cells showed SOX2 expression and upregulation of further pluripotency- and reprogramming-associated genes, suggesting a seminoma to embryonal carcinoma transition. Thus, our FS1/TCam-2 coculture model is a valuable tool for investigating interactions between SCs and seminoma cells. Our immunohistochemical and ultrastructural studies of human testicular biopsies with varying degrees of GCNIS compared to biopsies from fetuses, patients with androgen insensitivity syndrome, and patients showing normal spermatogenesis further suggest that GCNIS-associated SCs represent adult cells undergoing progressive dedifferentiation.

Differential proteome profile of gill and spleen in three pathogen-infected Paralichthys olivaceus

BACKGROUND

Olive flounder (Paralichthys olivaceus) is one of the major cultured fish species in Asia including Korea. However, the mass mortality of olive flounder caused by various pathogens leads to huge economic loss. The pathogens that lead to fish mortality include parasites, bacteria, and viruses that can cause various kinds of diseases.

OBJECTIVE

The purpose of this study was to investigate the protein expression patterns in the gills and spleens of olive flounder after artificial infection. We hypothesized that proteomics levels in gills and spleen may be differentially expressed depending on infectious agents.

METHODS

To investigate the expression pattern of proteins in gills and spleens, olive flounders were experimentally infected with VHSV (virus), S. parauberis (bacteria), or M. avidus (pathogenic ciliate). Proteins were extracted from the gills and spleens of infected olive flounder. We used 2-DE analysis with LC-MS/MS to investigate proteome changes in infected olive flounders.

RESULTS

The results of the LC-MS/MS analyses showed different protein expression profiles depending on pathogenic sources and target organs. Proteins related to cytoskeletal structure like keratin, calmodulin and actin were mostly expressed in the infected gills. Proteins involved in the metabolism pathway like glycolysis were expressed mainly in the spleens. The protein profiles of S. parauberis and VHSV infection groups had many similarities, but the profile of the M. avidus infection group was greatly different in the gill and spleen.

CONCLUSION

Our results indicate that measures according to the characteristics of each pathogen are necessary for disease prevention and treatment of farmed fish.

Keratin intermediate filaments in the colon: guardians of epithelial homeostasis

Keratin intermediate filament proteins are major cytoskeletal components of the mammalian simple layered columnar epithelium in the gastrointestinal tract. Human colon crypt epithelial cells express keratins 18, 19 and 20 as the major type I keratins, and keratin 8 as the type II keratin. Keratin expression patterns vary between species, and mouse colonocytes express keratin 7 as a second type II keratin. Colonic keratin patterns change during cell differentiation, such that K20 increases in the more differentiated crypt cells closer to the central lumen. Keratins provide a structural and mechanical scaffold to support cellular stability, integrity and stress protection in this rapidly regenerating tissue. They participate in central colonocyte processes including barrier function, ion transport, differentiation, proliferation and inflammatory signaling. The cell-specific keratin compositions in different epithelial tissues has allowed for the utilization of keratin-based diagnostic methods. Since the keratin expression pattern in tumors often resembles that in the primary tissue, it can be used to recognize metastases of colonic origin. This review focuses on recent findings on the biological functions of mammalian colon epithelial keratins obtained from pivotal in vivo models. We also discuss the diagnostic value of keratins in chronic colonic disease and known keratin alterations in colon pathologies. This review describes the biochemical properties of keratins and their molecular actions in colonic epithelial cells and highlights diagnostic data in colorectal cancer and inflammatory bowel disease patients, which may facilitate the recognition of disease subtypes and the establishment of personal therapies in the future.

Cornification of nail keratinocytes requires autophagy for bulk degradation of intracellular proteins while sparing components of the cytoskeleton

Epidermal keratinocytes undergo cornification to form the cellular building blocks of hard skin appendages such as nails and the protective layer on the surface of the skin. Cornification requires the cross-linking of structural proteins and the removal of other cellular components to form mechanically rigid and inert corneocytes. Autophagy has been proposed to contribute to this intracellular remodelling process, but its molecular targets in keratinocytes, if any, have remained elusive. Here, we deleted the essential autophagy factor Atg7 in K14-positive epithelia of mice and determined by proteomics the impact of this deletion on the abundance of individual proteins in cornified nails. The genetic suppression of autophagy in keratinocytes resulted in a significant increase in the number of proteins that survived cornification and in alterations of their abundance in the nail proteome. A broad range of enzymes and other non-structural proteins were elevated whereas the amounts of cytoskeletal proteins of the keratin and keratin-associated protein families, cytolinker proteins and desmosomal proteins were either unaltered or decreased in nails of mice lacking epithelial autophagy. Among the various types of non-cytoskeletal proteins, the subunits of the proteasome and of the TRiC/CCT chaperonin were most strongly elevated in mutant nails, indicating a particularly important role of autophagy in removing these large protein complexes during normal cornification. Taken together, the results of this study suggest that autophagy is active during nail keratinocyte cornification and its substrate specificity depends on the accessibility of proteins outside of the cytoskeleton and their presence in large complexes.

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.

Corneal keratin aggresome (CKAGG) formation and clearance by proteasome activation

Purpose

To understand the mechanism of corneal keratin expression and clearance in corneal epithelium with Limbal Stem Cell Deficiency (LSCD). The hypothesis is that LSCD-induced proteasome dysfunction is a contributing factor to keratin aggregation, causing corneal keratin aggresome (CKAGG) formation.

Method

LSCD was surgically induced in rabbit corneas. LSCD corneal epithelial cells (D-CEC) were collected to investigate keratin K4 and K13 expression and CKAGG formation. Oral mucosal epithelial cells (OMECS) were isolated and cultured to study K4 and K13 expression. Cultured cells were treated with proteasome inhibitor to induce CKAGG formation.

Results

K4 and K13 were strongly expressed in D-CEC, with additional higher molecular weight bands of K4 and K13, suggesting CKAGG formation. Double staining of K4/K13 and ubiquitin showed co-localization of these keratins with ubiquitin in D-CEC. Proteasome inhibition also showed K4/K13 modification and accumulation in cultured OMECS, similar to D-CEC. Proteasome activation was then performed in cultured OMEC. There was no accumulation of keratins, and levels of unmodified keratins were found significantly reduced.

Conclusion

Results showed an abnormal expression of K4 and K13 after LSCD-induced proteasome dysfunction, which coalesce to form CKAGG in Corneal Epithelial Cells (CEC). We propose that CKAGG formation may be one of the causative factors of morphological alterations in the injured corneal epithelium, and that CKAGG could potentially be cleared by enhancing proteasome activity.

Degradation for better survival? Role of ubiquitination in epithelial morphogenesis

As a prevalent post-translational modification, ubiquitination is essential for many developmental processes. Once covalently attached to the small and conserved polypeptide ubiquitin (Ub), a substrate protein can be directed to perform specific biological functions via its Ub-modified form. Three sequential catalytic reactions contribute to this process, among which E3 ligases serve to identify target substrates and promote the activated Ub to conjugate to substrate proteins. Ubiquitination has great plasticity, with diverse numbers, topologies and modifications of Ub chains conjugated at different substrate residues adding a layer of complexity that facilitates a huge range of cellular functions. Herein, we highlight key advances in the understanding of ubiquitination in epithelial morphogenesis, with an emphasis on the latest insights into its roles in cellular events involved in polarized epithelial tissue, including cell adhesion, asymmetric localization of polarity determinants and cytoskeletal organization. In addition, the physiological roles of ubiquitination are discussed for typical examples of epithelial morphogenesis, such as lung branching, vascular development and synaptic formation and plasticity. Our increased understanding of ubiquitination in epithelial morphogenesis may provide novel insights into the molecular mechanisms underlying epithelial regeneration and maintenance.

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

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

The proteasome helps epithelial cells set up KAMPs

Study describes how epithelial cells process keratin 6a to generate antimicrobial peptides that prevent bacteria from colonizing the cornea.

Intermediate Filaments at the Junction of Mechanotransduction, Migration, and Development

Mechanically induced signal transduction has an essential role in development. Cells actively transduce and respond to mechanical signals and their internal architecture must manage the associated forces while also being dynamically responsive. With unique assembly-disassembly dynamics and physical properties, cytoplasmic intermediate filaments play an important role in regulating cell shape and mechanical integrity. While this function has been recognized and appreciated for more than 30 years, continually emerging data also demonstrate important roles of intermediate filaments in cell signal transduction. In this review, with a particular focus on keratins and vimentin, the relationship between the physical state of intermediate filaments and their role in mechanotransduction signaling is illustrated through a survey of current literature. Association with adhesion receptors such as cadherins and integrins provides a critical interface through which intermediate filaments are exposed to forces from a cell's environment. As a consequence, these cytoskeletal networks are posttranslationally modified, remodeled and reorganized with direct impacts on local signal transduction events and cell migratory behaviors important to development. We propose that intermediate filaments provide an opportune platform for cells to both cope with mechanical forces and modulate signal transduction.

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

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

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.

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

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

Assays for Posttranslational Modifications of Intermediate Filament Proteins

Intermediate filament (IF) proteins are known to be regulated by a number of posttranslational modifications (PTMs). Phosphorylation is the best-studied IF PTM, whereas ubiquitination, sumoylation, acetylation, glycosylation, ADP-ribosylation, farnesylation, and transamidation are less understood in functional terms but are known to regulate specific IFs under various contexts. The number and diversity of IF PTMs is certain to grow along with rapid advances in proteomic technologies. Therefore, the need for a greater understanding of the implications of PTMs to the structure, organization, and function of the IF cytoskeleton has become more apparent with the increased availability of data from global profiling studies of normal and diseased specimens. This chapter will provide information on established methods for the isolation and monitoring of IF PTMs along with the key reagents that are necessary to carry out these experiments.

Absence of keratin 8 or 18 promotes antimitochondrial autoantibody formation in aging male mice

Human mutations in keratin 8 (K8) and keratin 18 (K18), the intermediate filament proteins of hepatocytes, predispose to several liver diseases. K8-null mice develop chronic liver injury and fragile hepatocytes, dysfunctional mitochondria, and Th2-type colitis. We tested the hypothesis that autoantibody formation accompanies the liver damage that associates with K8/K18 absence. Sera from wild-type control, K8-null, and K18-null mice were analyzed by immunoblotting and immunofluorescence staining of cell and mouse tissue homogenates. Autoantibodies to several antigens were identified in 81% of K8-null male mice 8 mo or older. Similar autoantibodies were detected in aging K18-null male mice that had a related liver phenotype but normal colon compared with K8-null mice, suggesting that the autoantibodies are linked to liver rather than colonic disease. However, these autoantibodies were not observed in nontransgenic mice subjected to 4 chronic injury models. The autoantigens are ubiquitous and partition with mitochondria. Mass spectrometry and purified protein analysis identified, mitochondrial HMG-CoA synthase, aldehyde dehydrogenase, and catalase as the primary autoantigens, and glutamate dehydrogenase and epoxide hydrolase-2 as additional autoantigens. Therefore, absence of the hepatocyte keratins results in production of anti-mitochondrial autoantibodies (AMA) that recognize proteins involved in energy metabolism and oxidative stress, raising the possibility that AMA may be found in patients with keratin mutations that associate with liver and other diseases.

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.

"Panta rhei": Perpetual cycling of the keratin cytoskeleton

The filamentous cytoskeletal systems fulfil seemingly incompatible functions by maintaining a stable scaffolding to ensure tissue integrity and simultaneously facilitating rapid adaptation to intracellular processes and environmental stimuli. This paradox is particularly obvious for the abundant keratin intermediate filaments in epithelial tissues. The epidermal keratin cytoskeleton, for example, supports the protective and selective barrier function of the skin while enabling rapid growth and remodelling in response to physical, chemical and microbial challenges. We propose that these dynamic properties are linked to the perpetual re-cycling of keratin intermediate filaments that we observe in cultured cells. This cycle of assembly and disassembly is independent of protein biosynthesis and consists of distinct, temporally and spatially defined steps. In this way, the keratin cytoskeleton remains in constant motion but stays intact and is also able to restructure rapidly in response to specific regulatory cues as is needed, e.g., during division, differentiation and wound healing.

Piwil2 inhibits keratin 8 degradation through promoting p38-induced phosphorylation to resist Fas-mediated apoptosis

The piwi-like 2 (piwil2) gene is widely expressed in tumors and protects cells from apoptosis induced by a variety of stress stimuli. However, the role of Piwil2 in Fas-mediated apoptosis remains unknown. Here, we present evidence that Piwil2 inhibits Fas-mediated apoptosis. By a bacterial two-hybrid screening, we identify a new Piwil2-interacting partner, keratin 8 (K8), a major intermediate filament protein protecting the cell from Fas-mediated apoptosis. Our results show that Piwil2 binds to K8 and p38 through its PIWI domain and forms a Piwil2/K8/P38 triple protein-protein complex. Thus, Piwil2 increases the phosphorylation level of K8 Ser-73 and then inhibits ubiquitin-mediated degradation of K8. As a result, the knockdown of Piwil2 increases the Fas protein level at the membrane. In addition to our previous finding that Piwil2 inhibits the expression of p53 through the Src/STAT3 pathway, here we demonstrate that Piwil2 represses p53 phosphorylation through p38. Our present study indicates that Piwil2 plays a role in Fas-mediated apoptosis for the first time and also can affect p53 phosphorylation in tumor cells, revealing a novel mechanism of Piwil2 in apoptosis, and supports that Piwil2 plays an active role in tumorigenesis.

Functional module search in protein networks based on semantic similarity improves the analysis of proteomics data

The continuously evolving field of proteomics produces increasing amounts of data while improving the quality of protein identifications. Albeit quantitative measurements are becoming more popular, many proteomic studies are still based on non-quantitative methods for protein identification. These studies result in potentially large sets of identified proteins, where the biological interpretation of proteins can be challenging. Systems biology develops innovative network-based methods, which allow an integrated analysis of these data. Here we present a novel approach, which combines prior knowledge of protein-protein interactions (PPI) with proteomics data using functional similarity measurements of interacting proteins. This integrated network analysis exactly identifies network modules with a maximal consistent functional similarity reflecting biological processes of the investigated cells. We validated our approach on small (H9N2 virus-infected gastric cells) and large (blood constituents) proteomic data sets. Using this novel algorithm, we identified characteristic functional modules in virus-infected cells, comprising key signaling proteins (e.g. the stress-related kinase RAF1) and demonstrate that this method allows a module-based functional characterization of cell types. Analysis of a large proteome data set of blood constituents resulted in clear separation of blood cells according to their developmental origin. A detailed investigation of the T-cell proteome further illustrates how the algorithm partitions large networks into functional subnetworks each representing specific cellular functions. These results demonstrate that the integrated network approach not only allows a detailed analysis of proteome networks but also yields a functional decomposition of complex proteomic data sets and thereby provides deeper insights into the underlying cellular processes of the investigated system.

Group B streptococcal infection of the choriodecidua induces dysfunction of the cytokeratin network in amniotic epithelium: a pathway to membrane weakening

Early events leading to intrauterine infection remain poorly defined, but may hold the key to preventing preterm delivery. To determine molecular pathways within fetal membranes (chorioamnion) associated with early choriodecidual infection that may progress to preterm premature rupture of membranes (PPROM), we examined the effects of a Group B Streptococcus (GBS) choriodecidual infection on chorioamnion in a nonhuman primate model. Ten chronically catheterized pregnant monkeys (Macaca nemestrina) at 118–125 days gestation (term = 172 days) received choriodecidual inoculation of either GBS (n = 5) or saline (n = 5). Cesarean section was performed in the first week after GBS or saline inoculation. RNA extracted from chorioamnion (inoculation site) was profiled by microarray. Single gene, Gene Set, and Ingenuity Pathway Analysis results were validated using qRT-PCR (chorioamnion), Luminex (amniotic fluid, AF), immunohistochemistry, and transmission electron microscopy (TEM). Despite uterine quiescence in most cases, significant elevations of AF cytokines (TNF-α, IL-8, IL-1β, IL-6) were detected in GBS versus controls (p<0.05). Choriodecidual infection resolved by the time of cesarean section in 3 of 5 cases and GBS was undetectable by culture and PCR in the AF. A total of 331 genes were differentially expressed (>2-fold change, p<0.05). Remarkably, GBS exposure was associated with significantly downregulated expression of multiple cytokeratin (CK) and other cytoskeletal genes critical for maintenance of tissue tensile strength. Immunofluorescence revealed highly significant changes in the CK network within amniocytes with dense CK aggregates and retraction from the cell periphery (all p = 0.006). In human pregnancies affected by PPROM, there was further evidence of CK network retraction with significantly shorter amniocyte foot processes (p = 0.002). These results suggest early choriodecidual infection results in decreased cellular membrane integrity and tensile strength via dysfunction of CK networks. Downregulation of CK expression and perturbations in the amniotic epithelial cell intermediate filament network occur after GBS choriodecidual infection, which may contribute to PPROM.

Group B Streptococcus (GBS) is one cause of preterm birth, stillbirth, and fetal brain injury. GBS is present in the vagina and is thought to ascend into the uterus of some women where it can cause placental inflammation and preterm birth. Understanding the earliest events in the placenta that lead to preterm birth is elusive in humans, because the placenta cannot be studied until after birth. Here, we use a nonhuman primate model to show that an early GBS infection can damage the structural support of the fetal membranes, specifically the cytokeratin network in the epithelium of the amnion (one part of the membranes). Next, we obtained human placentas to show that this cytokeratin network was also damaged in human patients that had preterm premature rupture of the membranes, a major cause of preterm birth. Our work is important in understanding why fetal membranes may rupture prematurely, which may lead to early interventions to prevent membrane damage after placental infection and preterm birth.

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

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

Krtap11-1, a hair keratin-associated protein, as a possible crucial element for the physical properties of hair shafts

BACKGROUND

The physical properties of the hair are predominantly determined by the assembly of keratin bundles. The keratin-associated proteins (Krtaps) are thought to be involved in keratin bundle assembly, however, the functional role of the individual member still remains largely unknown.

OBJECTIVE

The aim of this study is to clarify the role of a unique class of Krtaps, Krtap11-1, in the development and physical properties of the hair.

METHODS

The expression regulation of Krtap11-1 was analyzed and its binding partners in the hair cortex were determined. Also, the effects of the forcible expression of this protein on the hair follicle development were analyzed in culture.

RESULTS

The expression pattern of Krtap11-1 was concentrically asymmetric in the faulty hair that develops in Foxn1nu mice. In cultured keratinocytes, the expression of Krtap11-1 transgene product was strictly regulated by the keratinization process and proteasome-dependent protein elimination. While the association with keratin as well as the cohesive self-assembly of Krtap11-1 appeared to be stabilized by disulfide cross-links, the biotinylated Krtap11-1 probe enabled the adherence to certain type I keratins in the hair cortex, including K31, 33 and 34, in the absence of disulfide formation. When embryonic upper lip rudiments were forcibly introduced with Krtap11-1, the hair follicles formed irregularly arranged globular hair keratin-clumps surrounded by multilayered epithelial cells in culture.

CONCLUSION

Krtap11-1 may play an important role on keratin-bundle assembly in the hair cortex and this study provides insight into the physical properties of the hair shaft.

Antibody-based analysis reveals "filamentous vs. non-filamentous" and "cytoplasmic vs. nuclear" crosstalk of cytoskeletal proteins

To uncover the molecular composition and dynamics of the functional scaffold for the nucleus, three fractions of biochemically-stable nuclear protein complexes were extracted and used as immunogens to produce a variety of monoclonal antibodies. Many helix-based cytoskeletal proteins were identified as antigens, suggesting their dynamic contribution to nuclear architecture and function. Interestingly, sets of antibodies distinguished distinct subcellular localization of a single isoform of certain cytoskeletal proteins; distinct molecular forms of keratin and actinin were found in the nucleus. Their nuclear shuttling properties were verified by the apparent nuclear accumulations under inhibition of CRM1-dependent nuclear export. Nuclear keratins do not take an obvious filamentous structure, as was revealed by non-filamentous cytoplasmic keratin-specific monoclonal antibody. These results suggest the distinct roles of the helix-based cytoskeletal proteins in the nucleus.

Measuring the regulation of keratin filament network dynamics

The organization of the keratin intermediate filament cytoskeleton is closely linked to epithelial function. To study keratin network plasticity and its regulation at different levels, tools are needed to localize and measure local network dynamics. In this paper, we present image analysis methods designed to determine the speed and direction of keratin filament motion and to identify locations of keratin filament polymerization and depolymerization at subcellular resolution. Using these methods, we have analyzed time-lapse fluorescence recordings of fluorescent keratin 13 in human vulva carcinoma-derived A431 cells. The fluorescent keratins integrated into the endogenous keratin cytoskeleton, and thereby served as reliable markers of keratin dynamics. We found that increased times after seeding correlated with down-regulation of inward-directed keratin filament movement. Bulk flow analyses further revealed that keratin filament polymerization in the cell periphery and keratin depolymerization in the more central cytoplasm were both reduced. Treating these cells and other human keratinocyte-derived cells with EGF reversed all these processes within a few minutes, coinciding with increased keratin phosphorylation. These results highlight the value of the newly developed tools for identifying modulators of keratin filament network dynamics and characterizing their mode of action, which, in turn, contributes to understanding the close link between keratin filament network plasticity and epithelial physiology.

Plakophilin3 loss leads to an increase in PRL3 levels promoting K8 dephosphorylation, which is required for transformation and metastasis

The desmosome anchors keratin filaments in epithelial cells leading to the formation of a tissue wide IF network. Loss of the desmosomal plaque protein plakophilin3 (PKP3) in HCT116 cells, leads to an increase in neoplastic progression and metastasis, which was accompanied by an increase in K8 levels. The increase in levels was due to an increase in the protein levels of the Phosphatase of Regenerating Liver 3 (PRL3), which results in a decrease in phosphorylation on K8. The increase in PRL3 and K8 protein levels could be reversed by introduction of an shRNA resistant PKP3 cDNA. Inhibition of K8 expression in the PKP3 knockdown clone S10, led to a decrease in cell migration and lamellipodia formation. Further, the K8 PKP3 double knockdown clones showed a decrease in colony formation in soft agar and decreased tumorigenesis and metastasis in nude mice. These results suggest that a stabilisation of K8 filaments leading to an increase in migration and transformation may be one mechanism by which PKP3 loss leads to tumor progression and metastasis.

Neurofilament Phosphorylation during Development and Disease: Which Came First, the Phosphorylation or the Accumulation?

Posttranslational modification of proteins is a ubiquitous cellular mechanism for regulating protein function. Some of the most heavily modified neuronal proteins are cytoskeletal proteins of long myelinated axons referred to as neurofilaments (NFs). NFs are type IV intermediate filaments (IFs) that can be composed of four subunits, neurofilament heavy (NF-H), neurofilament medium (NF-M), neurofilament light (NF-L), and α-internexin. Within wild type axons, NFs are responsible for mediating radial growth, a process that determines axonal diameter. NFs are phosphorylated on highly conserved lysine-serine-proline (KSP) repeats located along the C-termini of both NF-M and NF-H within myelinated axonal regions. Phosphorylation is thought to regulate aspects of NF transport and function. However, a key pathological hallmark of several neurodegenerative diseases is ectopic accumulation and phosphorylation of NFs. The goal of this review is to provide an overview of the posttranslational modifications that occur in both normal and diseased axons. We review evidence that challenges the role of KSP phosphorylation as essential for radial growth and suggests an alternative role for NF phosphorylation in myelinated axons. Furthermore, we demonstrate that regulation of NF phosphorylation dynamics may be essential to avoiding NF accumulations.

Cytoskeleton in motion: the dynamics of keratin intermediate filaments in epithelia

Epithelia are exposed to multiple forms of stress. Keratin intermediate filaments are abundant in epithelia and form cytoskeletal networks that contribute to cell type–specific functions, such as adhesion, migration, and metabolism. A perpetual keratin filament turnover cycle supports these functions. This multistep process keeps the cytoskeleton in motion, facilitating rapid and protein biosynthesis–independent network remodeling while maintaining an intact network. The current challenge is to unravel the molecular mechanisms underlying the regulation of the keratin cycle in relation to actin and microtubule networks and in the context of epithelial tissue function.

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

INTRODUCTION

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Shotgun proteomics and network analysis of ubiquitin-related proteins from human breast carcinoma epithelial cells

Protein ubiquitination via the covalent attachment of ubiquitin (Ub) plays an important role in the regulation of the stability, function or localization of multiple proteins in eukaryotic cells. Comprehensive investigation of the proteomics related to ubiquitination will gain the insight into the Ub-mediated regulatory mechanism. In the present study, the combination of polyUb affinity purification, SDS-PAGE separation, and liquid chromatography-tandem mass spectrometry analysis (GeLC-MS/MS) was employed to analyze the Ub-related proteins in human MDA-MB-231 breast carcinoma epithelial cells after treatment with the proteasome inhibitor MG132. A total of 260 non-redundant Ub-related proteins were identified from the cells. These proteins were shown to be involved in a host of critical cellular functions and processes, including transcription, translation, Ub-proteasome pathway, cell cycle, heat shock response, transport, etc. The interaction network analysis by STRING indicated that the identified Ub-related proteins formed eleven clusters, the three most highly ranked network clusters were mainly involved in protein translation, RNA transcription and processing, and Ub-proteasome pathway, suggesting that there were obvious ubiquitination-mediated alternations in gene expression of human MDA-MB-231 cells. The proteomic profiling and their interaction network analysis in this study would help to our systematic understanding of the Ub-related cellular protein functions and the related biological processes in human disease tissue cells.

Tip30 deletion in MMTV-Neu mice leads to enhanced EGFR signaling and development of estrogen receptor-positive and progesterone receptor-negative mammary tumors

Estrogen receptor-positive and progesterone receptor-negative (ER+/PR-) breast cancers account for 15% to 25% of all human breast cancers and display more aggressive malignant characteristics than ER+/PR+ cancers. However, the molecular mechanism underlying development of ER+/PR- breast cancers still remains elusive. We show here that Tip30 deletion dramatically accelerated the onset of mammary tumors in the MMTV-Neu mouse model of breast cancer. The mammary tumors arising in Tip30(-/-)/MMTV-Neu mice were exclusively ER+/PR-. The growth of these ER+/PR- tumors depends not only on estrogen but also on progesterone despite the absence of detectable PR. Tip30 is predominantly expressed in ER+ mammary epithelial cells, and its deletion leads to an increase in the number of phospho-ERα-positive cells in mammary glands and accelerated activation of Akt in MMTV-Neu mice. Moreover, we found that Tip30 regulates the EGFR pathway through controlling endocytic downregulation of EGFR protein level and signaling. Together, these findings suggest a novel mechanism in which loss of Tip30 cooperates with Neu activation to enhance the activation of Akt signaling, leading to the development of ER+/PR- mammary tumors.

Comprehensive proteomic and transcriptomic characterization of hepatic expression signatures affected in p14 liver conditional knockout mice

Scaffold proteins regulate intracellular MAP kinase signaling by providing critical spatial and temporal specificities. We have shown previously that the scaffold protein MEK1 partner (MP1) is localized to late endosomes by the adaptor protein p14. Using conditional gene disruption of p14 in livers of mice (p14(Δhep) ) we analyzed protein and transcript signatures in tissue samples. Further biological network analysis predicted that the differentially expressed transcripts and proteins are involved in cell cycle progression and regulation of cellular proliferation. Although some of the here identified signatures were previously linked to phospho-ERK activity, most of them were novel targets of the late endosomal p14/MP1/MEK/ERK signaling module. Finally, the proliferation defect was confirmed in a chemically induced liver regeneration model in p14(Δhep) knockout mice.