Keratin binding to 14-3-3 proteins modulates keratin filaments and hepatocyte mitotic progression

Keratin 8 and 18 loss in epithelial cancer cells increases collective cell migration and cisplatin sensitivity through claudin1 up-regulation

Keratins 8 and 18 (K8/18) are simple epithelial cell-specific intermediate filament proteins. Keratins are essential for tissue integrity and are involved in intracellular signaling pathways that regulate cell response to injuries, cell growth, and death. K8/18 expression is maintained during tumorigenesis; hence, they are used as a diagnostic marker in tumor pathology. In recent years, studies have provided evidence that keratins should be considered not only as markers but also as regulators of cancer cell signaling. The loss of K8/18 expression during epithelial-mesenchymal transition (EMT) is associated with metastasis and chemoresistance. In the present study, we investigated whether K8/18 expression plays an active role in EMT. We show that K8/18 stable knockdown using shRNA increased collective migration and invasiveness of epithelial cancer cells without modulating EMT markers. K8/18-depleted cells showed PI3K/Akt/NF-κB hyperactivation and increased MMP2 and MMP9 expression. K8/18 deletion also increased cisplatin-induced apoptosis. Increased Fas receptor membrane targeting suggests that apoptosis is enhanced via the extrinsic pathway. Interestingly, we identified the tight junction protein claudin1 as a regulator of these processes. This is the first indication that modulation of K8/18 expression can influence the phenotype of epithelial cancer cells at a transcriptional level and supports the hypothesis that keratins play an active role in cancer progression.

Depolymerization of cytokeratin intermediate filaments facilitates intracellular infection of HeLa cells by Bartonella henselae

Bartonella henselae is capable of invading epithelial and endothelial cells by modulating the function of actin-dependent cytoskeleton proteins. Although understanding of the pathogenesis has been increased by the development of an in vitro infection model involving endothelial cells, little is known about the mechanism of interaction between B. henselae and epithelial cells. This study aims to identify the binding candidates of B. henselae in epithelial cells and explores their effect on B. henselae infection. Pull-down assays and mass spectrometry analysis confirmed that some of the binding proteins (keratin 14, keratin 6, and F-actin) are cytoskeleton associated. B. henselae infection significantly induces the expression of the cytokeratin genes. Chemical disruption of the keratin network by using ethylene glycol tetraacetic acid promotes the intracellular persistence of B. henselae in HeLa cells. However, cytochalasin B and phalloidin treatment inhibits B. henselae invasion. Immunofluorescent staining demonstrates that B. henselae infection induces an F-actin-dependent rearrangement of the cytoskeleton. However, we demonstrated via immunofluorescent staining and whole-mount cell electron microscopy that keratin intermediate filaments are depolymerized by B. henselae. The results indicate that B. henselae achieves an intracellular persistence in epithelial cells through the depolymerization of cytokeratin intermediate filaments that are protective against B. henselae invasion.

14-3-3 proteins interact with neurofilament protein-L and regulate dynamic assembly of neurofilaments

Neurofilament protein-L (NF-L) is the core component of neurofilaments. Recent studies indicate that the NF-L mutations reported in human Charcot-Marie-Tooth (CMT) disease lead to the formation of NF-L aggregates and result in axon degeneration of motor and sensory neurons, which are thought to be the cause of CMT disease type 2E. In the present study, we investigated the dynamic regulation of NF-L assembly and the mechanism of aggregate formation of CMT NF-L mutants. We report that 14-3-3 proteins interact with NF-L in a phosphorylation-dependent manner. Investigation of mutations of phospho-serine sites at the head domain of NF-L revealed that several phosphorylation sites, particularly Ser43 and Ser55, were important for 14-3-3 binding. 14-3-3 overexpression resulted in a significant increase in the dynamic exchange rate of NF-L subunits and induced striking disassembly of neurofilaments. CMT NF-L mutants, particularly those with mutations in the Pro8 and Pro22 sites of the NF-L head domain, led to substantially diminished interaction between 14-3-3 and NF-L, which resulted in the formation of NF-L aggregates and the disruption of the neurofilament co-assembly of NF-L and NF-M. However, aggregate formation in CMT NF-L mutants was downregulated by 14-3-3 overexpression. Taken together, these results suggest the important role of 14-3-3 in the dynamic regulation of NF-L assembly, and in the capacity to prevent the formation of NF-L aggregates. Thus, the 14-3-3 proteins are a possible molecular target for CMT disease therapy.

An integrated workflow for extraction and solubilization of intermediate filaments from colorectal biopsies for proteomic analysis

We report a technique for isolation and solubilization of intermediate filament (IF) proteins from colonic biopsies compatible with both gel electrophoresis and liquid chromatography "shotgun" proteomics using mass spectrometry (MS). This is important because changes in the IF proteome, particularly in keratin expression and modification, are noted in colonic mucosa of patients with colorectal cancer. Though keratins have traditionally been dissolved in high concentration of urea, the latter solvent precludes efficient proteolytic digestion by trypsin prior to gel-free LC-MS/MS approaches. The extraction of cytoskeletal proteins was initially evaluated using MCF-7 cancer cell lines using a published, differential detergent solubilization protocol. IF proteins were extracted from colonic biopsies using a combination of homogenization and sonication. Since comparable efficiency of solubilization was noted on the extracted IF from cell lines between urea and guanidine hydrochloride (GuHCl) in triethylammonium bicarbonate buffer, isolated proteins from endoscopic biopsies were solubilized in GuHCl. Using immunoblotting techniques, we successfully demonstrated isolation of keratins and preservation of posttranslational modifications (phosphorylation, acetylation). Dissolved proteins were tryptically digested and peptides analyzed by MS, showing the functionality of the workflow in shotgun proteomic applications, specifically compatibility of the workflow for isobaric tagging relative and absolute quantification based quantitation approaches.

Lactotransferrin acts as a tumor suppressor in nasopharyngeal carcinoma by repressing AKT through multiple mechanisms

LTF (lactotransferrin, also known as lactoferrin) is a key component of innate immune defense. It has recently been found to have anti-tumor and anti-metastatic activity in different cancers. We previously reported LTF to be the most significantly downregulated gene in nasopharyngeal carcinoma (NPC) specimens relative to normal nasopharyngeal epithelial tissues, and it was also negatively associated with the progression and metastasis of NPC. However, the mechanism underlying this remains unclear. In the current study, we revealed that LTF can suppress 3-phosphoinositide-dependent protein kinase 1 expression via the mitogen-activated protein kinase/c-Jun pathway and thus repress AKT signaling. We also showed that LTF interacts with keratin 18 (K18) and so blocks the formation of the K18-14-3-3 complex, leading to downregulation of K18-mediated AKT activation. Thus, LTF suppresses AKT signaling by two separate mechanisms, leading to inhibition of NPC tumorigenesis. This is the first report on the tumor suppressive effects of LTF through repression of AKT signaling in NPC. It suggests that both LTF and AKT signaling merit further study in the field of NPC research.

Contribution of proteomics to the study of the role of cytokeratins in disease and physiopathology

Cytokeratins (CKs), the most abundant group of cytoskeletal intermediate filaments, and proteomics are strongly connected. On the one hand, proteomics has been extremely useful to uncover new features and functions of CKs, on the other, the highly abundant CKs serve as an exceptional tool to test new technological developments in proteomics. As a result, proteomics has contributed to finding valuable associations of CKs with diseases as diverse as cancer, cystic fibrosis, steatohepatitis, viral and bacterial infection, keratoconus, vitreoretinopathy, preeclampsia or the chronic fatigue syndrome, as well as to characterizing their participation in a number of physiopathological processes, including drug resistance, response to toxicants, inflammation, stem cell differentiation, embryo development, and tissue repair. In some cases, like in cystic fibrosis, CKs have been described as potential therapeutic targets. The development of a specific field of proteomics where CKs become the main subject of research aims and hypotheses is suggested.

Synemin promotes AKT-dependent glioblastoma cell proliferation by antagonizing PP2A

Synemin is an intermediate filament protein present in glioblastomas (GBMs) but not in normal brain. In GBM cells synemin interacts with and antagonizes PP2A, which is the phosphatase dephosphorylating Akt. This maintains the phosphorylation status of Akt sites that are substrates for PDPK1 and mTORc2, thereby fostering proliferation.

The intermediate filament protein synemin is present in astrocyte progenitors and glioblastoma cells but not in mature astrocytes. Here we demonstrate a role for synemin in enhancing glioblastoma cell proliferation and clonogenic survival, as synemin RNA interference decreased both behaviors by inducing G1 arrest along with Rb hypophosphorylation and increased protein levels of the G1/S inhibitors p21^Cip1 and p27^Kip1. Akt involvement was demonstrated by decreased phosphorylation of its substrate, p21^Cip1, and reduced Akt catalytic activity and phosphorylation at essential activation sites. Synemin silencing, however, did not affect the activities of PDPK1 and mTOR complex 2, which directly phosphorylate Akt activation sites, but instead enhanced the activity of the major regulator of Akt dephosphorylation, protein phosphatase type 2A (PP2A). This was accompanied by changes in PP2A subcellular distribution resulting in increased physical interactions between PP2A and Akt, as shown by proximity ligation assays (PLAs). PLAs and immunoprecipitation experiments further revealed that synemin and PP2A form a protein complex. In addition, treatment of synemin-silenced cells with the PP2A inhibitor cantharidic acid resulted in proliferation and pAkt and pRb levels similar to those of controls. Collectively these results indicate that synemin positively regulates glioblastoma cell proliferation by helping sequester PP2A away from Akt, thereby favoring Akt activation.

14-3-3 proteins and regulation of cytoskeleton

The proteins of the 14-3-3 family are universal adapters participating in multiple processes running in the cell. We describe the structure, isoform composition, and distribution of 14-3-3 proteins in different tissues. Different elements of 14-3-3 structure important for dimer formation and recognition of protein targets are analyzed in detail. Special attention is paid to analysis of posttranslational modifications playing important roles in regulation of 14-3-3 function. The data of the literature concerning participation of 14-3-3 in regulation of intercellular contacts and different elements of cytoskeleton formed by microfilaments are analyzed. We also describe participation of 14-3-3 in regulation of small G-proteins and protein kinases important for proper functioning of cytoskeleton. The data on the interaction of 14-3-3 with different components of microtubules are presented, and the probable role of 14-3-3 in developing of certain neurodegenerative diseases is discussed. The data of the literature concerning the role of 14-3-3 in formation and normal functioning of intermediate filaments are also reviewed. It is concluded that due to its adapter properties 14-3-3 plays an important role in cytoskeleton regulation. The cytoskeletal proteins that are abundant in the cell might compete with the other protein targets of 14-3-3 and therefore can indirectly regulate many intracellular processes that are dependent on 14-3-3.

14-3-3 proteins as signaling integration points for cell cycle control and apoptosis

14-3-3 proteins play critical roles in the regulation of cell fate through phospho-dependent binding to a large number of intracellular proteins that are targeted by various classes of protein kinases. 14-3-3 proteins play particularly important roles in coordinating progression of cells through the cell cycle, regulating their response to DNA damage, and influencing life-death decisions following internal injury or external cytokine-mediated cues. This review focuses on 14-3-3-dependent pathways that control cell cycle arrest and recovery, and the influence of 14-3-3 on the apoptotic machinery at multiple levels of regulation. Recognition of 14-3-3 proteins as signaling integrators that connect protein kinase signaling pathways to resulting cellular phenotypes, and their exquisite control through feedforward and feedback loops, identifies new drug targets for human disease, and highlights the emerging importance of using systems-based approaches to understand signal transduction events at the network biology level.

Cytokeratins in hepatitis

Experimental and clinical evidence suggests that cytokeratins (CK), among other physiological functions, are expressed in hepatocytes and can be released in the bloodstream after acute or chronic inflammatory liver injury. Interest in CK in viral and nonviral hepatitis has been rapidly increasing during the last years, especially as they have been proposed as circulating biomarkers of hepatocyte necrosis and apoptosis. In the present review, we sought to summarize and discuss the alterations in circulating CK levels in different form viral and nonviral hepatitis, as well as their potential relation with liver histology. Understanding the mechanisms of hepatitis impact on CK and vice versa is a promising area of research that will positively enhance our understanding of the complexity of acute and chronic inflammatory liver injury.

Novel function of keratins 5 and 14 in proliferation and differentiation of stratified epithelial cells

Keratin expression in stratified epithelia is tightly regulated during squamous cell differentiation. Keratins 5 and 14 are expressed in mitotically active basal layer cells, but their function is not well defined. Reported here is the possible role of K14 in regulation of cell proliferation/differentiation in stratified epithelial cells.

Keratins are cytoplasmic intermediate filament proteins preferentially expressed by epithelial tissues in a site-specific and differentiation-dependent manner. The complex network of keratin filaments in stratified epithelia is tightly regulated during squamous cell differentiation. Keratin 14 (K14) is expressed in mitotically active basal layer cells, along with its partner keratin 5 (K5), and their expression is down-regulated as cells differentiate. Apart from the cytoprotective functions of K14, very little is known about K14 regulatory functions, since the K14 knockout mice show postnatal lethality. In this study, K14 expression was inhibited using RNA interference in cell lines derived from stratified epithelia to study the K14 functions in epithelial homeostasis. The K14 knockdown clones demonstrated substantial decreases in the levels of the K14 partner K5. These cells showed reduction in cell proliferation and delay in cell cycle progression, along with decreased phosphorylated Akt levels. K14 knockdown cells also exhibited enhanced levels of activated Notch1, involucrin, and K1. In addition, K14 knockdown AW13516 cells showed significant reduction in tumorigenicity. Our results suggest that K5 and K14 may have a role in maintenance of cell proliferation potential in the basal layer of stratified epithelia, modulating phosphatidylinositol 3-kinase/Akt–mediated cell proliferation and/or Notch1-dependent cell differentiation.

FoxO transcription factors; Regulation by AKT and 14-3-3 proteins

The forkhead box O (FoxO) transcription factor family is a key player in an evolutionary conserved pathway downstream of insulin and insulin-like growth factor receptors. The mammalian FoxO family consists of FoxO1, 3, 4 and 6, which share high similarity in their structure, function and regulation. FoxO proteins are involved in diverse cellular and physiological processes including cell proliferation, apoptosis, reactive oxygen species (ROS) response, longevity, cancer and regulation of cell cycle and metabolism. The regulation of FoxO protein function involves an intricate network of posttranslational modifications and protein-protein interactions that provide integrated cellular response to changing physiological conditions and cues. AKT was identified in early genetic and biochemical studies as a main regulator of FoxO function in diverse organisms. Though other FoxO regulatory pathways and mechanisms have been delineated since, AKT remains a key regulator of the pathway. The present review summarizes the current knowledge of FoxO regulation by AKT and 14-3-3 proteins, focusing on its mechanistic and structural aspects and discusses its crosstalk with the other FoxO regulatory mechanisms. This article is part of a Special Issue entitled: PI3K-AKT-FoxO axis in cancer and aging.

Nestin negatively regulates postsynaptic differentiation of the neuromuscular synapse

Positive and negative regulation of neurotransmitter receptor aggregation on the postsynaptic membrane is a critical event during synapse formation. Acetylcholine (ACh) and agrin are two opposing signals that regulate ACh receptor (AChR) clustering during neuromuscular junction (NMJ) development. ACh induces dispersion of AChR clusters that are not stabilized by agrin via a cyclin-dependent kinase 5 (Cdk5)-mediated mechanism, but regulation of Cdk5 activation is poorly understood. Here we show that the intermediate filament protein nestin physically interacts with Cdk5 and is required for ACh-induced association of p35, the co-activator of Cdk5, with the muscle membrane. Blockade of nestin-dependent signaling inhibits ACh-induced Cdk5 activation and the dispersion of AChR clusters in cultured myotubes. Similar to the effects of Cdk5 gene inactivation, knockdown of nestin in agrin-deficient embryos significantly restores AChR clusters. These results suggest that nestin is required for ACh-induced, Cdk5-dependent dispersion of AChR clusters during NMJ development.

Keratin 23, a novel DPC4/Smad4 target gene which binds 14-3-3ε

Background

Inactivating mutations of SMAD4 are frequent in metastatic colorectal carcinomas. In previous analyses, we were able to show that restoration of Smad4 expression in Smad4-deficient SW480 human colon carcinoma cells was adequate to suppress tumorigenicity and invasive potential, whereas in vitro cell growth was not affected. Using this cellular model system, we searched for new Smad4 targets comparing nuclear subproteomes derived from Smad4 re-expressing and Smad4 negative SW480 cells.

Methods

High resolution two-dimensional (2D) gel electrophoresis was applied to identify novel Smad4 targets in the nuclear subproteome of Smad4 re-expressing SW480 cells. The identified candidate protein Keratin 23 was further characterized by tandem affinity purification. Immunoprecipitation, subfractionation and immunolocalization studies in combination with RNAi were used to validate the Keratin 23-14-3-3ε interaction.

Results

We identified keratins 8 and 18, heat shock proteins 60 and 70, plectin 1, as well as 14-3-3ε and γ as novel proteins present in the KRT23-interacting complex. Co-immunoprecipitation and subfractionation analyses as well as immunolocalization studies in our Smad4-SW480 model cells provided further evidence that KRT23 associates with 14-3-3ε and that Smad4 dependent KRT23 up-regulation induces a shift of the 14-3-3ε protein from a nuclear to a cytoplasmic localization.

Conclusion

Based on our findings we propose a new regulatory circuitry involving Smad4 dependent up-regulation of KRT23 (directly or indirectly) which in turn modulates the interaction between KRT23 and 14-3-3ε leading to a cytoplasmic sequestration of 14-3-3ε. This cytoplasmic KRT23-14-3-3 interaction may alter the functional status of the well described 14-3-3 scaffold protein, known to regulate key cellular processes, such as signal transduction, cell cycle control, and apoptosis and may thus be a previously unappreciated facet of the Smad4 tumor suppressive circuitry.

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.

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.

Keratins in health and cancer: more than mere epithelial cell markers

Keratins are the intermediate filament (IF)-forming proteins of epithelial cells. Since their initial characterization almost 30 years ago, the total number of mammalian keratins has increased to 54, including 28 type I and 26 type II keratins. Keratins are obligate heteropolymers and, similarly to other IFs, they contain a dimeric central α-helical rod domain that is flanked by non-helical head and tail domains. The 10-nm keratin filaments participate in the formation of a proteinaceous structural framework within the cellular cytoplasm and, as such, serve an important role in epithelial cell protection from mechanical and non-mechanical stressors, a property extensively substantiated by the discovery of human keratin mutations predisposing to tissue-specific injury and by studies in keratin knockout and transgenic mice. More recently, keratins have also been recognized as regulators of other cellular properties and functions, including apico-basal polarization, motility, cell size, protein synthesis and membrane traffic and signaling. In cancer, keratins are extensively used as diagnostic tumor markers, as epithelial malignancies largely maintain the specific keratin patterns associated with their respective cells of origin, and, in many occasions, full-length or cleaved keratin expression (or lack there of) in tumors and/or peripheral blood carries prognostic significance for cancer patients. Quite intriguingly, several studies have provided evidence for active keratin involvement in cancer cell invasion and metastasis, as well as in treatment responsiveness, and have set the foundation for further exploration of the role of keratins as multifunctional regulators of epithelial tumorigenesis.

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

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

E1--E4-mediated keratin phosphorylation and ubiquitylation: a mechanism for keratin depletion in HPV16-infected epithelium

The keratin IF network of epidermal keratinocytes provides a protective barrier against mechanical insult, it is also a major player in absorbing stress in these cells. The human papilloma virus (HPV) type 16 E1--E4 protein accumulates in the upper layers of HPV16-infected epithelium and is known to associate with and reorganise the keratin IF network in cells in culture. Here, we show that this function is conserved amongst a number of HPV alpha-group E1--E4 proteins and that the differentiation-dependent keratins are also targeted. Using time-lapse microscopy, HPV16 E1--E4 was found to effect a dramatic cessation of keratin IF network dynamics by associating with both soluble and insoluble keratin. Network disruption was accompanied by keratin hyperphosphorylation at several sites, including K8 S73, which is typically phosphorylated in response to stress stimuli. Keratin immunoprecipitated from E1--E4-expressing cells was also found to be ubiquitylated, indicating that it is targeted for proteasomal degradation. Interestingly, the accumulation of hyperphosphorylated, ubiquitylated E1--E4-keratin structures was found to result in an impairment of proteasomal function. These observations shed new light on the mechanism of keratin IF network reorganisation mediated by HPV16 E1--E4 and provide an insight into the depletion of keratin co-incident with E1--E4 accumulation observed in HPV-infected epithelium.

Novel insights into changes in biochemical properties of keratins 8 and 18 in griseofulvin-induced toxic liver injury

Keratins 8 and 18 (K8/18) intermediate filament proteins are believed to play an essential role in the protection of hepatocytes against mechanical and toxic stress. This assertion is mainly based on increased hepatocyte fragility observed in transgenic mice deficient in K8/18, or carrying mutations on K8/18. The molecular mechanism by which keratins accomplish their protective functions has not been totally elucidated. Liver diseases such as alcoholic hepatitis and copper metabolism diseases are associated with modifications, in hepatocytes, of intermediate filament organisation and the formation of K8/18 containing aggregates named Mallory-Denk bodies. Treatment of mice with a diet containing griseofulvin induces the formation of Mallory-Denk bodies in hepatocytes. This provides a reliable animal model for assessing the molecular mechanism by which keratins accomplish their protective role in the response of hepatocytes to chemical injuries. In this study, we found that griseofulvin intoxication induced changes in keratin solubility and that there was a 5% to 25% increase in the relative amounts of soluble keratin. Keratin phosphorylation on specific sites (K8 pS79, K8 pS436 and K18 pS33) was increased and prominent in the insoluble protein fractions. Since at least six K8 phosphoepitopes were detected after GF treatment, phosphorylation sites other than the ones studied need to be accounted for. Immunofluorescence staining showed that K8 pS79 epitope was present in clusters of hepatocytes that surrounded apoptotic cells. Activated p38 MAPK was associated with, but not present in K8 pS79-positive cells. These results indicate that griseofulvin intoxication mediates changes in the physicochemical properties of keratin, which result in the remodelling of keratin intermediate filaments which in turn could modulate the signalling pathways in which they are involved by modifying their binding to signalling proteins.

Accelerated liver regeneration and hepatocarcinogenesis in mice overexpressing serine-45 mutant beta-catenin

The Wnt/beta-catenin pathway is implicated in the pathogenesis of hepatocellular cancer (HCC). We developed a transgenic mouse (TG) in the FVB strain that overexpresses Ser45-mutated-beta-catenin in hepatocytes to study the effects on liver regeneration and cancer. In the two independent TG lines adult mice show elevated beta-catenin at hepatocyte membrane with no increase in the Wnt pathway targets cyclin-D1 or glutamine synthetase. However, TG hepatocytes upon culture exhibit a 2-fold increase in thymidine incorporation at day 5 (D5) when compared to hepatocytes from wildtype FVB mice (WT). When subjected to partial hepatectomy (PH), dramatic increases in the number of hepatocytes in S-phase are evident in TG at 40 and WT at 72 hours. Coincident with the earlier onset of proliferation, we observed nuclear translocation of beta-catenin along with an increase in total and nuclear cyclin-D1 protein at 40 hours in TG livers. To test if stimulation of beta-catenin induces regeneration, we used hydrodynamic delivery of Wnt-1 naked DNA to control mice, which prompted an increase in Wnt-1, beta-catenin, and known targets, glutamine synthetase (GS) and cyclin-D1, along with a concomitant increase in cell proliferation. beta-Catenin-overexpressing TG mice, when followed up to 12 months, showed no signs of spontaneous tumorigenesis. However, intraperitoneal delivery of diethylnitrosamine (DEN), a known carcinogen, induced HCC at 6 months in TG mice only. Tumors in TG livers showed up-regulation of beta-catenin, cyclin-D1, and unique genetic aberrations, whereas other canonical targets were unremarkable.

CONCLUSION

beta-Catenin overexpression offers growth advantage during liver regeneration. Also, whereas no spontaneous HCC is evident, beta-catenin overexpression makes TG mice susceptible to DEN-induced HCC.

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

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

Keratin 18 phosphorylation as a progression marker of chronic hepatitis B

Background

The intermediate filament proteins keratins 18 (K18) and 8 (K8) polymerize to form the cytoskeletal network in the mature hepatocytes. It has been shown that the phosphorylation of K18 at two serine residues, 33 and 52, correlates with the progression of hepatitis C, but little is known of chronic hepatitis B (CHB). In this study, we examined K18 phosphorylation in relation to CHB.

Results

Site-specific phosphorylation of K18 was determined in livers of twelve healthy donors, and non-cirrhosis (n = 40) and cirrhosis (n = 21) patients. On average, progressively higher level of Ser52 phosphorylation was observed in non-cirrhotic and cirrhotic livers, while elevated Ser33 phosphorylation was detected in both livers but no significant difference. Progressive increase of Ser33 and Ser52 phosphorylation correlated with the elevation of both histological lesions and enzymatic activities of alanine aminotransferase in non-cirrhotic livers. In the hepatocytes of an inactive HBV carrier, strong signals of Ser33 phosphorylation were co-localized with viral infection, while only basal level of Ser52 phosphorylation was detected in infected cells.

Conclusion

Assuming all obtained data, our data suggest that K18 phosphorylation is a progression marker for CHB.

Akt isoforms regulate intermediate filament protein levels in epithelial carcinoma cells

Keratin 8 and 18 are simple epithelial intermediate filament (IF) proteins, whose expression is differentiation- and tissue-specific, and is maintained during tumorigenesis. Vimentin IF is often co-expressed with keratins in cancer cells. Recently, IF have been proposed to be involved in signaling pathways regulating cell growth, death and motility. The PI3K/Akt pathway plays a pivotal role in these processes. Thus, we investigated the role of Akt (1 and 2) in regulating IF expression in different epithelial cancer cell lines. Over-expression of Akt1 increases K8/18 proteins. Akt2 up-regulates K18 and vimentin expression by an increased mRNA stability. To our knowledge, these results represent the first indication that Akt isoforms regulate IF expression and support the hypothesis that IFs are involved in PI3K/Akt pathway.

Proteomic analysis of regenerating mouse liver following 50% partial hepatectomy

Background

Although 70% (or 2/3) partial hepatectomy (PH) is the most studied model for liver regeneration, the hepatic protein expression profile associated with lower volume liver resection (such as 50% PH) has not yet been reported. Therefore, the aim of this study was to determine the global protein expression profile of the regenerating mouse liver following 50% PH by differential proteomics, and thereby gaining some insights into the hepatic regeneration mechanism(s) under this milder but clinically more relevant condition.

Results

Proteins from sham-operated mouse livers and livers regenerating for 24 h after 50% PH were separated by SDS-PAGE and analyzed by nanoUPLC-Q-Tof mass spectrometry. Compared to sham-operated group, there were totally 87 differentially expressed proteins (with 50 up-regulated and 37 down-regulated ones) identified in the regenerating mouse livers, most of which have not been previously related to liver regeneration. Remarkably, over 25 differentially expressed proteins were located at mitochondria. Several of the mitochondria-resident proteins which play important roles in citric acid cycle, oxidative phosphorylation and ATP production were found to be down-regulated, consistent with the recently-proposed model in which the reduction of ATP content in the remnant liver gives rise to early stress signals that contribute to the onset of liver regeneration. Pathway analysis revealed a central role of c-Myc in the regulation of liver regeneration.

Conclusions

Our study provides novel evidence for mitochondria as a pivotal organelle that is connected to liver regeneration, and lays the foundation for further studies on key factors and pathways involved in liver regeneration following 50% PH, a condition frequently used for partial liver transplantation and conservative liver resection.

Proteomic profiling of cancer of the gingivo-buccal complex: Identification of new differentially expressed markers

Tobacco-related oral cancer is the most common cancer among Indian males, gingivo-buccal complex (GBC) being the most affected subsite due to the habit of chewing tobacco. Proteins from the lysates of microdissected normal and transformed epithelium from clinically well-characterized tissue samples of the GBC were separated by two-dimensional gel electrophoresis to identify differentially expressed proteins. Eleven protein spots showed differential expression, which could withstand the stringency of statistical evaluation. The observations were confirmed with additional tissues. Nine of these differentiators were identified by MS as lactate dehydrogenase B, α-enolase, prohibitin, cathepsin D, apolipoprotein A-I, tumor protein translationally controlled-1, an SFN family protein, 14-3-3σ and tropomyosin. Cluster analysis indicated that these proteins, as a coexpressed set, could distinguish normal and transformed epithelium. Functionally, these differentiator molecules are relevant to the pathways and processes that have been previously implicated in oral carcinogenesis and could therefore be investigated further as a panel of markers for management of cancer of the GBC.

Circulating monocytes have the capacity to be transdifferentiated into keratinocyte-like cells

Transdifferentiation is a process in which the original commitment of a cell is changed to give rise to unexpected peripheral mature cells. Our previous report showed that circulating stem cells can generate keratinocyte-like cells (KLCs). However, it remains to be determined whether or not other peripheral blood mononuclear cells (PBMC) subsets have the potential to follow the same cell fate. In this study, the cell transdifferentiation of circulating CD14(+) monocytes into KLCs and their regulatory effect on matrix metalloproteinase-1 (MMP-1) expression in dermal fibroblasts were evaluated. The results showed that monocytes isolated from peripheral blood mononuclear cells have the capacity to generate KLCs. These transdifferentiated cells exhibited, along with a keratinocyte-like morphology, a characteristic profile consisting in stratifin(+), cytokeratins(+) (types I and II), CD14(low), and involucrin(+) on day 21 in culture. Similar to keratinocyte-conditioned media, KLC-derived conditioned media were able to induce an increase in the MMP-1 expression in dermal fibroblasts. This effect was significantly reduced by using 14-3-3 protein-depleted KLC-conditioned media. Our findings show the potential transdifferentiation of circulating CD14(+) monocytes into KLCs and their regulatory effect on MMP-1 expression in dermal fibroblasts.

Structural changes in intermediate filament networks alter the activity of insulin-degrading enzyme

The intermediate filament (IF) protein nestin coassembles with vimentin and promotes the disassembly of these copolymers when vimentin is hyperphosphorylated during mitosis. The aim of this study is to determine the function of these nonfilamentous particles by identifying their interacting partners. In this study, we report that these disassembled vimentin/nestin complexes interact with insulin degrading enzyme (IDE). Both vimentin and nestin interact with IDE in vitro, but vimentin binds IDE with a higher affinity than nestin. Although the interaction between vimentin and IDE is enhanced by vimentin phosphorylation at Ser-55, the interaction between nestin and IDE is phosphorylation independent. Further analyses show that phosphorylated vimentin plays the dominant role in targeting IDE to the vimentin/nestin particles in vivo, while the requirement for nestin is related to its ability to promote vimentin IF disassembly. The binding of IDE to either nestin or phosphorylated vimentin regulates IDE activity differently, depending on the substrate. The insulin degradation activity of IDE is suppressed approximately 50% by either nestin or phosphorylated vimentin, while the cleavage of bradykinin-mimetic peptide by IDE is increased 2- to 3-fold. Taken together, our data demonstrate that the nestin-mediated disassembly of vimentin IFs generates a structure capable of sequestering and modulating the activity of IDE.

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.

Shear stress induced reorganization of the keratin intermediate filament network requires phosphorylation by protein kinase C zeta

Keratin intermediate filaments (KIFs) form a fibrous polymer network that helps epithelial cells withstand external mechanical forces. Recently, we established a correlation between the structure of the KIF network and its local mechanical properties in alveolar epithelial cells. Shear stress applied across the cell surface resulted in the structural remodeling of KIF and a substantial increase in the elastic modulus of the network. This study examines the mechanosignaling that regulates the structural remodeling of the KIF network. We report that the shear stress-mediated remodeling of the KIF network is facilitated by a twofold increase in the dynamic exchange rate of KIF subunits, which is regulated in a PKC zeta and 14-3-3-dependent manner. PKC zeta phosphorylates K18pSer33, and this is required for the structural reorganization because the KIF network in A549 cells transfected with a dominant negative PKC zeta, or expressing the K18Ser33Ala mutation, is unchanged. Blocking the shear stress-mediated reorganization results in reduced cellular viability and increased apoptotic levels. These data suggest that shear stress mediates the phosphorylation of K18pSer33, which is required for the reorganization of the KIF network, resulting in changes in mechanical properties of the cell that help maintain the integrity of alveolar epithelial cells.

Contrasting expression of keratins in mouse and human embryonic stem cells

RNA expression data reveals that human embryonic stem (hES) cells differ from mouse ES (mES) cells in the expression of RNAs for keratin intermediate filament proteins. These differences were confirmed at the cellular and protein level and may reflect a fundamental difference in the epithelial nature of embryonic stem cells derived from mouse and human blastocysts. Mouse ES cells express very low levels of the simple epithelial keratins K8, K18 and K19. By contrast hES cells express moderate levels of the RNAs for these intermediate filament proteins as do mouse stem cells derived from the mouse epiblast. Expression of K8 and K18 RNAs are correlated with increased c-Jun RNA expression in both mouse and human ES cell cultures. However, decreasing K8 and K18 expression associated with differentiation to neuronal progenitor cells is correlated with increasing expression of the Snai2 (Slug) transcriptional repression and not decreased Jun expression. Increasing K7 expression is correlated with increased CDX2 and decreased Oct4 RNA expression associated with the formation of trophoblast derivatives by hES cells. Our study supports the view that hES cells are more similar to mouse epiblast cells than mouse ES cells and is consistent with the epithelial nature of hES cells. Keratin intermediate filament expression in hES cells may modulate sensitivity to death receptor mediated apoptosis and stress.

Maintenance of the Salmonella-containing vacuole in the juxtanuclear area: a role for intermediate filaments

Until recently, intermediate filaments (IF) were thought to be only involved in resistance to physical stress and mechanical integrity of cells and tissues. Recent data indicate that IF play a much more important role in cellular physiology including organelle structure and positioning within the cell. Here, we show that Salmonella enterica serovar Typhimurium (S. typhimurium) induces in epithelial cells and macrophages the formation of an aggresome-like structure with a dramatic remodelling of cytoplasmic IF (vimentin and cytokeratin) networks and the adaptor proteins 14-3-3 which are recruited around intracellular S. typhimurium microcolonies. These rearrangements are not necessary for bacterial replication. Depletion of vimentin and cytokeratin by siRNA indicates that IF remodelling is required to maintain Salmonella microcolonies in the juxtanuclear area.

The human keratins: biology and pathology

The keratins are the typical intermediate filament proteins of epithelia, showing an outstanding degree of molecular diversity. Heteropolymeric filaments are formed by pairing of type I and type II molecules. In humans 54 functional keratin genes exist. They are expressed in highly specific patterns related to the epithelial type and stage of cellular differentiation. About half of all keratins—including numerous keratins characterized only recently—are restricted to the various compartments of hair follicles. As part of the epithelial cytoskeleton, keratins are important for the mechanical stability and integrity of epithelial cells and tissues. Moreover, some keratins also have regulatory functions and are involved in intracellular signaling pathways, e.g. protection from stress, wound healing, and apoptosis. Applying the new consensus nomenclature, this article summarizes, for all human keratins, their cell type and tissue distribution and their functional significance in relation to transgenic mouse models and human hereditary keratin diseases. Furthermore, since keratins also exhibit characteristic expression patterns in human tumors, several of them (notably K5, K7, K8/K18, K19, and K20) have great importance in immunohistochemical tumor diagnosis of carcinomas, in particular of unclear metastases and in precise classification and subtyping. Future research might open further fields of clinical application for this remarkable protein family.

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.

Stress-induced recruitment of epiplakin to keratin networks increases their resistance to hyperphosphorylation-induced disruption

Epiplakin is a large (>725 kDa) cytoskeletal protein exclusively expressed in epithelial tissues. It has a unique structure, consisting entirely of plakin repeat domains (PRDs), one of the hallmarks of spectraplakin protein family members. Previous studies, including the phenotypic analyses of knockout mice, failed to reveal the biological function of epiplakin. Using in vitro binding assays, we show here that all but one of the 16 PRDs of mouse epiplakin bind to keratins of basal keratinocytes. Nevertheless, in primary keratinocyte cell cultures, epiplakin only partially colocalized with keratin intermediate filament networks. However, upon application of cellular stress in the form of keratin hyperphosphorylation, osmotic shock or UV irradiation, the entire cytoplasmic epiplakin pool became associated with keratin. In response to such types of stress, epiplakin initially translocated to the still-intact keratin filament network and remained associated with keratin after its disruption and transformation into granular aggregates. Time-course experiments revealed that serine/threonine (okadaic acid) and tyrosine (orthovanadate) phosphatase inhibitor-induced filament disruption in differentiated keratinocytes proceeded faster in epiplakin-deficient cells compared with wild-type cells. Our data suggest that epiplakin plays a role in keratin filament reorganization in response to stress, probably by protecting keratin filaments against disruption in a chaperone-like fashion.

Intermediate filaments in smooth muscle

The intermediate filament (IF) network is one of the three cytoskeletal systems in smooth muscle. The type III IF proteins vimentin and desmin are major constituents of the network in smooth muscle cells and tissues. Lack of vimentin or desmin impairs contractile ability of various smooth muscle preparations, implying their important role for smooth muscle force development. The IF framework has long been viewed as a fixed cytostructure that solely provides mechanical integrity for the cell. However, recent studies suggest that the IF cytoskeleton is dynamic in mammalian cells in response to various external stimulation. In this review, the structure and biological properties of IF proteins in smooth muscle are summarized. The role of IF proteins in the modulation of smooth muscle force development and redistribution/translocation of signaling partners (such as p130 Crk-associated substrate, CAS) is depicted. This review also summarizes our latest understanding on how the IF network may be regulated in smooth muscle.

Keratin inclusions alter cytosolic protein localization in hepatocytes

AIM

Mallory bodies have been observed in various liver diseases, however, the precise mechanism and significance of these structures have yet to be determined.

METHODS

Previously we reported on the redistribution of cytosolic proteins to keratin inclusions in mutant keratin 18-transfected cells. In this study, we treated green fluorescent protein-tagged wild-type keratin 18-transfected cells with several proteasome inhibitors and performed immunofluorescent analyses.

RESULTS

Proteasome inhibitors induced intracellular keratin inclusions, and desmoplakin, zonula occludens-1 and beta-catenin were relocated to keratin inclusions, while theintegral membrane proteins were intact. The cytosolic proteins, 14-3-3 zeta protein and glucose-6-phosphate dehydrogenase were also relocated to inclusions. Moreover, E-cadherin, a basolateral membrane protein, was present on both the apical and basolateral domains in inclusion-containing cells.

CONCLUSION

These data are identical to those in the mutant keratin 18 transfection study and suggest that keratin inclusions induced by different treatments affect localization of various cytosolic components, which may influence cellular functions performed by these proteins.

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.

Transdifferentiation of peripheral blood mononuclear cells into epithelial-like cells

Bone marrow-derived stem cells have the potential to transdifferentiate into unexpected peripheral cells. We hypothesize that circulating bone marrow-derived stem cells might have the capacity to transdifferentiate into epithelial-like cells and release matrix metalloproteinase-1-modulating factors such as 14-3-3varsigma for dermal fibroblasts. We have characterized a subset of peripheral blood mononuclear cells (PBMCs) that develops an epithelial-like profile. Our findings show that these cells develop epithelial-like morphology and express 14-3-3varsigma and keratin-5, -8 as early as day 7 and day 21, respectively. When compared with control, conditioned media collected from PBMCs in advanced epithelial-like differentiation (cultures on days 28, 35, and 42) increased the matrix metalloproteinase-1 expression in dermal fibroblasts (P </= 0.01). The depletion of 14-3-3varsigma from these conditioned media by immunoprecipitation reduced the effect by 39.5% (P value, 0.05). Therefore, the releasable 14-3-3varsigma from PBMC-derived epithelial-like cells is involved in this process. Our findings provide new insights into the PBMC transdifferentiation to generate epithelial-like cells and subsequently release of 14-3-3varsigma that will disclose new therapeutic alternatives for different dermal clinical settings.

Intermediate filament scaffolds fulfill mechanical, organizational, and signaling functions in the cytoplasm

Intermediate filaments (IFs) are cytoskeletal polymers whose protein constituents are encoded by a large family of differentially expressed genes. Owing in part to their properties and intracellular organization, IFs provide crucial structural support in the cytoplasm and nucleus, the perturbation of which causes cell and tissue fragility and accounts for a large number of genetic diseases in humans. A number of additional roles, nonmechanical in nature, have been recently uncovered for IF proteins. These include the regulation of key signaling pathways that control cell survival, cell growth, and vectorial processes including protein targeting in polarized cellular settings. As this discovery process continues to unfold, a rationale for the large size of this family and the context-dependent regulation of its members is finally emerging.

The role of stratifin in fibroblast-keratinocyte interaction

Stratifin is a member of 14-3-3 protein family, a highly conserved group of proteins constituted by seven isoforms. They are involved in numerous crucial intracellular functions such as cell cycle and apoptosis, regulation of signal transduction pathways, cellular trafficking, cell proliferation and differentiation, cell survival, and protein folding and processing, among others. At epidermal level, stratifin (also called 14-3-3 sigma) has been described as molecule with relevant functions. For instance, this isoform is a marker associated with keratinocyte differentiation. In this maturation process, the presence of dominant negative molecules of p53 induces a "stemness condition" of keratinocyte precursor cells and suppression of stratifin expression. In addition, the recently described keratinocyte-releasable form of stratifin is involved in dermal fibroblast MMP-1 over-expression through c-Fos and c-Jun activity. This effect is mediated, at least in part, by p38 mitogen-activated protein kinase (MAPK). Other MMP family members such as stromelysin-1 (MMP-3), stromelysin-2 (MMP-10), neutrophil collagenase (MMP-8), and membrane-type MMP-24 (MT5-MMP) are also up-regulated by stratifin. Within fibroproliferative disorder of skin, hypertrophic scar and keloids exhibit a high content of collagen, proteoglycans, and fibronectin. Thus, the MMP profile induced by stratifin is an interesting starting point to establish new therapeutic tools to control the process of wound healing. In this review, we will focus on site of synthesis and mode of action of stratifin in skin and wound healing.

Role of Scarf and Its Binding Target Proteins in Epidermal Calcium Homeostasis

The novel Ca2+-binding protein, Scarf (skin calmodulin-related factor) belongs to the calmodulin-like protein family and is expressed in the differentiated layers of the epidermis. To determine the roles of Scarf during stratification, we set out to identify the binding target proteins by affinity chromatography and subsequent analysis by mass spectrometry. Several binding factors, including 14-3-3s, annexins, calreticulin, ERp72 (endoplasmic reticulum protein 72), and nucleolin, were identified, and their interactions with Scarf were corroborated by co-immunoprecipitation and co-localization analyses. To further understand the functions of Scarf in epidermis in vivo, we altered the epidermal Ca2+ gradient by acute barrier disruption. The change in the expression levels of Scarf and its binding target proteins were determined by immunohistochemistry and Western blot analysis. The expression of Scarf, annexins, calreticulin, and ERp72 were up-regulated by Ca2+ gradient disruption, whereas the expression of 14-3-3s and nucleolin was reduced. Because annexins, calreticulin, and ERp72 have been implicated in Ca2+-induced cellular trafficking, including the secretion of lamellar bodies and Ca2+ homeostasis, we propose that the interaction of Scarf with these proteins might be crucial in the process of barrier restoration. On the other hand, down-regulation of 14-3-3s and nucleolin is potentially involved in the process of keratinocyte differentiation and growth inhibition. The calcium-dependent localization and up-regulation of Scarf and its binding target proteins were studied in mouse keratinocytes treated with ionomycin and during the wound-healing process. We found increased expression and nuclear presence of Scarf in the epidermis of the wound edge 4 and 7 days post-wounding, entailing the role of Scarf in barrier restoration. Our results suggest that Scarf plays a critical role as a Ca2+ sensor, potentially regulating the function of its binding target proteins in a Ca2+-dependent manner in the process of restoration of epidermal Ca2+ gradient as well as during epidermal barrier formation.

The motility and dynamic properties of intermediate filaments and their constituent proteins

Intermediate filament (IF) proteins exist in multiple structural forms within cells including mature IF, short filaments or 'squiggles', and non-filamentous precursors called particles. These forms are interconvertible and their relative abundance is IF type, cell type- and cell cycle stage-dependent. These structures are often associated with molecular motors, such as kinesin and dynein, and are therefore capable of translocating through the cytoplasm along microtubules. The assembly of mature IF from their precursor particles is also coupled to translation. These dynamic properties of IF provide mechanisms for regulating their reorganization and assembly in response to the functional requirements of cells. The recent findings that IF and their precursors are frequently associated with signaling molecules have revealed new functions for IF beyond their more traditional roles as mechanical integrators of cells and tissues.

Overexpression of Angiopoietin-Like Protein 4 Alters Mitochondria Activities and Modulates Methionine Metabolic Cycle in the Liver Tissues of db/db Diabetic Mice

Angiopoietin-like protein 4 (ANGPTL4) is a circulating protein predominantly produced from fat tissue and liver. Recent data from others and our laboratory have demonstrated this protein to be an important player in energy metabolism and insulin sensitivity. However, the molecular mechanisms underlying its metabolic actions remain elusive. In this study, we have employed a two-dimensional fluorescence difference gel electrophoresis technique to study the protein profiles in the livers of db/db mice treated with or without ANGPTL4. When compared with those of lean mice, 118 proteins were found to be up- or down-regulated in db/db mice. Adenovirus-mediated overexpression of ANGPTL4 could reverse a large portion of the up- or down-regulated proteins to control levels. Especially, a number of mitochondria proteins were down-regulated by ANGPTL4 to a great extent. Chronic treatment with ANGPTL4 resulted in an elevated activity of mitochondria respiratory chain complexes II-III and IV in db/db mice. Additionally, several key enzymes in the methionine/homocysteine metabolic cycle were found to be increased in db/db diabetic mice but decreased by ANGPTL4 treatment. HPLC analysis consistently revealed that ANGPTL4 could significantly restore the augmented S-adenosylmethionine levels and S-adenosylmethionine/S-adenosylhomocysteine ratios in livers of db/db mice. In summary, our results suggest that ANGPTL4 might elicit its metabolic effects through modulating the mitochondria functions and methionine metabolic cycles in the liver tissue.

Structural and regulatory functions of keratins

The diversity of epithelial functions is reflected by the expression of distinct keratin pairs that are responsible to protect epithelial cells against mechanical stress and to act as signaling platforms. The keratin cytoskeleton integrates these functions by forming a supracellular scaffold that connects at desmosomal cell-cell adhesions. Multiple human diseases and murine knockouts in which the integrity of this system is destroyed testify to its importance as a mechanical stabilizer in certain epithelia. Yet, surprisingly little is known about the precise mechanisms responsible for assembly and disease pathology. In addition to these structural aspects of keratin function, experimental evidence accumulating in recent years has led to a much more complex view of the keratin cytoskeleton. Distinct keratins emerge as highly dynamic scaffolds in different settings and contribute to cell size determination, translation control, proliferation, cell type-specific organelle transport, malignant transformation and various stress responses. All of these properties are controlled by highly complex patterns of phosphorylation and molecular associations.