Cytokeratin expression, fibrillar organization, and subtle function in liver cells

Exploring the Potential of Metal-Based Candidate Drugs as Modulators of the Cytoskeleton

During recent years, accumulating evidence suggested that metal-based candidate drugs are promising modulators of cytoskeletal and cytoskeleton-associated proteins. This was substantiated by the identification and validation of actin, vimentin and plectin as targets of distinct ruthenium(II)- and platinum(II)-based modulators. Despite this, structural information about molecular interaction is scarcely available. Here, we compile the scattered reports about metal-based candidate molecules that influence the cytoskeleton, its associated proteins and explore their potential to interfere in cancer-related processes, including proliferation, invasion and the epithelial-to-mesenchymal transition. Advances in this field depend crucially on determining binding sites and on gaining comprehensive insight into molecular drug-target interactions. These are key steps towards establishing yet elusive structure-activity relationships.

Carbendazim induces death in alveolar epithelial cells: A comparison between submerged and at the air-liquid interface cell culture

The fungicide Carbendazim is widely used in agriculture and preservation of films and fibers. In mammals, it can promote germ cell mutagenicity, carcinogenicity, and reproductive toxicity. However, few data about the effects of this toxicant upon the respiratory system are available. In this work, we evaluated Carbendazim toxicity upon A549 alveolar cells both in monolayer and upon air-liquid interface cell system. Monolayer cell exposed to non-cytotoxic concentrations of this fungicide showed cell arrest at G2/M phase, and did not show additional alterations. On the other hand, alveolar 3D reconstructed epithelial model (air-liquid interface cell system) was characterized and exposed to IC₂₅ of Carbendazim using the Vitrocell® Cloud 12 chamber. Expression of Active Caspase-3, α-tubulin and ROS was significantly increased after such exposure. Mitochondrial activity was also reduced after exposed to Carbendazim. The obtained results indicate that besides the environmental and reproductive toxicity concerns regarding Carbendazim exposure, pulmonary toxicity must be considered for this fungicide. In addition, we observed that the way of exposure impacts considerably on the cell response for in vitro assessment of chemicals inhalation toxicity profile.

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.

Isolation of cytoplasmatic proteins from cultured cells for two-dimensional gel electrophoresis

Cytoplasma is the cell interior place between the cellular membrane and the nucleus, where various intracellular activities take place, including energy production, reactive oxygen species (ROS) detoxification, heme synthesis, nitrogen and lipid metabolism, phosphorylation in signal transduction, and cytoskeletal meshwork construction. The rich cytoplasmatic proteins carrying out these intracellular functions are interesting targets for biochemical and molecular biological studies. The relatively recent discipline of proteomics offers a chance to globally analyze the changes in cytoplasmic proteins corresponding to drug treatments or disease conditions, and thus provide target candidates for further biological validation in drug development and biomarker discovery. Isolation of cytoplasmic proteins from cells is a necessary step for high resolution protein separation by two-dimensional gel electrophoresis (2DE) and specific proteomic analysis.

Dual roles of intermediate filaments in apoptosis

New roles have emerged recently for intermediate filaments (IFs), namely in modulating cell adhesion and growth, and providing resistance to various forms of stress and to apoptosis. In this context, we first summarize findings on the IF association with the cell response to mechanical stress and growth stimulation, in light of growth-related signaling events that are relevant to death-receptor engagement. We then address the molecular mechanisms by which IFs can provide cell resistance to apoptosis initiated by death-receptor stimulation and to necrosis triggered by excessive oxidative stress. In the same way, we examine IF involvement, along with cytolinker participation, in sequential caspase-mediated protein cleavages that are part of the overall cell death execution, particularly those that generate new functional IF protein fragments and uncover neoantigen markers. Finally, we report on the usefulness of these markers as diagnostic tools for disease-related aspects of apoptosis in humans. Clearly, the data accumulated in recent years provide new and significant insights into the multiple functions of IFs, particularly their dual roles in cell response to apoptotic insults.

Keratin 8 modulation of desmoplakin deposition at desmosomes in hepatocytes

Keratins, the intermediate filament proteins of epithelial cells, connect to desmosomes, the cell-cell adhesion structures at the surface membrane. The building elements of desmosomes include desmoglein and desmocollin, which provide the actual cell adhesive properties, and desmoplakins, which anchor the keratin intermediate filaments to desmosomes. In the work reported here, we address the role of keratin 8 in modulating desmoplakin deposition at surface membrane in mouse hepatocytes. The experimental approach is based on the use of keratin 8- and keratin 18-null mouse hepatocytes as cell models. In wild-type mouse hepatocytes, desmoplakin is aligned with desmoglein and keratin 8 at the surface membrane. In keratin 8-null hepatocytes, the intermediate filament loss leads to alterations in desmoplakin distribution at the surface membrane, but not of desmoglein. Intriguingly, a significant proportion of keratin 18-null hepatocytes express keratin 8 at the surface membrane, associated with a proper desmoplakin alignment with desmoglein at desmosomes. A Triton treatment of the monolayer reveals that most of the desmoplakin present in either wild-type, keratin 8- or keratin 18-null hepatocytes is insoluble. Deletion analysis of keratin 8 further suggests that the recovery of desmoplakin alignment requires the keratin 8 rod domain. In addition, similarly to other works revealing a key role of desmoplakin phosphorylation on its interaction with intermediate filaments, we find that the phosphorylation status of the keratin 8 head domain affects desmoplakin distribution at desmosomes. Together, the data indicate that a proper alignment/deposition of desmoplakin with keratins and desmoglein in hepatocytes requires keratin 8, through a reciprocal phosphoserine-dependent process.

Keratins modulate hepatic cell adhesion, size and G1/S transition

Keratins (Ks) are the intermediate filament (IF) proteins of epithelial cells. Hepatocyte IFs are made solely of keratins 8 and 18 (K8/K18), the hallmark of all simple epithelia. While K8/K18 are essential for maintaining structural integrity, there is accumulating evidence indicating that they also exert non-mechanical functions. We have reported recently that K8/K18-free hepatocytes from K8-null mice are more sensitive to Fas-mediated apoptosis, in line with an increased Fas density at the cell surface and an altered c-Flip regulation of the anti-apoptotic ERK1/2 signaling pathway. In the present study, we show that K8-null hepatocytes attach more rapidly but spread more slowly on a fibronectin substratum and undergo a more efficient G1/S transition than wild-type hepatocytes. Moreover, plectin, an IF associated protein, receptor for activated C kinase 1 (RACK1), a plectin partner, and vinculin, a key component of focal adhesions, distribute differently in spreading K8-null hepatocytes. Cell seeding leads to no differential activation of ERK1/2 in WT versus K8-null hepatocytes, whereas a stronger Akt activation is detected in K8-null hepatocytes. Insulin stimulation also leads to a differential Akt activation, implying altered Akt signaling capacity as a result of the K8/K18 loss. In addition, a delayed autophosphorylation of FAK, a target for integrin beta1 signaling, was obtained in seeding K8-null hepatocytes. These alterations in cell cycle-related events in hepatocytes in primary culture are also found in a K8-knockdown H4-II-E-C3 rat hepatoma cell line. Besides, K8/K18-free cells are smaller and exhibit a reduced rate of protein synthesis. In addition, a distinctive cyclin interplay is observed in these K8/K18-free hepatic cells, namely a more efficient cyclin A-dependent G1/S phase transition. Furthermore, K8 re-expression in these cells, following transfer of a human K8 cDNA, restores proper cell size, spreading and growth. Together, these results suggest new interrelated signaling roles of K8/18 with plectin/RACK1 in the modulation of cell attachment/spreading, size/protein synthesis and G1/S transition.

Keratin-mediated resistance to stress and apoptosis in simple epithelial cells in relation to health and disease

Epithelial cells such as hepatocytes exhibit highly polarized properties as a result of the asymmetric distribution of subsets of receptors at unique portions of the surface membrane. While the proper targeting of these surface receptors and maintenance of the resulting polarity depend on microtubules (MTs), the Golgi sorting compartment, and different actin-filament networks, the contribution of keratin intermediate filaments (IFs) has been unclear. Recent data show that the latter cytoskeletal network plays a predominant role in providing resistance to various forms of stress and to apoptosis targeted to the surface membrane. In this context, we first summarize our knowledge of the domain- or assembly-related features of IF proteins and the dynamic properties of IF networks that may explain how the same keratin pair K8/K18 can exert multiple resistance-related functions in simple epithelial cells. We then examine the contribution of linker protein(s) that integrate interactions of keratin IFs with MTs and the actin-cytoskeleton network, polarity-dependent surface receptors and cytoplasmic organelles. We next address likely molecular mechanisms by which K8/K18 can selectively provide resistance to a mechanical or toxic stress, or to Fas-mediated apoptosis. Finally, these issues on keratin structure-function are examined within a context of pathological anomalies emerging in tissue architecture as a result of natural or targeted mutations, or posttranslational modifications at specific amino acid residues. Clearly. the data accumulated in recent years provide new and significant insights on the role of K8/K18, particularly under conditions where polarized cells resist to stressful or apoptotic insults.

Effects of keratin filament disruption on exocrine pancreas-stimulated secretion and susceptibility to injury

Disruption or absence of hepatocyte keratins 8 and 18 is associated with chronic hepatitis, marked hepatocyte fragility, and a significant predisposition to stress-induced liver injury. In contrast, pancreatic keratin disruption in transgenic mice that express keratin 18 Arg89 --> Cys (K18C) is not associated with an obvious pancreatic pathology. We compared the effects of keratin filament disruption on pancreatic acini or acinar cell viability, and on cholecystokinin (CCK)-stimulated secretion, in transgenic mice that overexpress wild-type keratin 18 and harbor normal extended keratin filaments (TG2) and K18C mice. We also compared the response of these mice to pancreatitis induced by a choline-deficient ethionine-supplemented diet or by caerulein. Despite extensive cytoplasmic keratin filament disruption, the apicolateral keratin filament bundles appear intact in the acinar pancreas of K18C mice, as determined ultrastructurally and by light microscopy. No significant pancreatitis-associated histologic, serologic, or F-actin/keratin apicolateral redistribution differences were noted between TG2 and K18C mice. Acinar cell viability and yield after collagenase digestion were lower in K18C than in TG2 mice, but the yields of intact acini and their (125)I-CCK uptake and responses to CCK-stimulated secretion were similar. Our results indicate that keratin filament reorganization is a normal physiologic response to pancreatic cell injury, but an intact keratin cytoplasmic filament network is not as essential in protection from cell injury as in the liver. These findings raise the possibility that the abundant apicolateral acinar keratin filaments, which are not as evident in hepatocytes, may play the cytoprotective role that is seen in liver and other tissues. Alternatively, identical keratins may function differently in different tissues.

C-CAM1 expression: differential effects on morphology, differentiation state and suppression of human PC-3 prostate carcinoma cells

Studies in rat prostate and liver have suggested that C-CAM1 is involved in the formation and maintenance of histotypic associations in tissues and possibly tumors. Most recently, C-CAM1 has been shown to suppress tumorigenicity of prostate and colon carcinoma cells. However, the mechanisms whereby C-CAM1 suppresses growth and the relationship of this activity to its proposed role in histotypic interactions remain largely unknown. In the present study, we have analysed the growth, phenotypic, morphological and ultrastructural characteristics of four human PC-3 prostate carcinoma cell lines transduced with C-CAM1 retrovirus. We report that three of four lines regained their tumorigenic phenotype in vivo while maintaining high levels of C-CAM1 expression and a growth retarded phenotype in vitro. These findings suggested that high levels of C-CAM1 expression were negatively influencing recovery during reconstitution after freezing or during the latency period after subcutaneous injection and that loss of suppression resulted from changes in expression of other molecules required for full disclosure of C-CAM1 mediated growth inhibition. Results from Northern blot and immunofluorescence analyses of tumor nodules demonstrated that C-CAM1 decreased rather than enhanced phenotypic differentiation and induced ultrastructural and morphological changes that occurred independently of tumor suppression.

Protein phosphatase inhibition in normal and keratin 8/18 assembly-incompetent mouse strains supports a functional role of keratin intermediate filaments in preserving hepatocyte integrity

The function and regulation of keratin 8 (K8) and 18 (K18), intermediate filament (IF) proteins of the liver, are not fully understood. We employed the liver damage induced by microcystin-LR (MC-LR), a liver-specific inhibitor of type-1 and type-2A protein phosphatases, in normal and in keratin assembly-incompetent mouse strains as a model to elucidate the roles of IF phosphorylation in situ. The mouse strains used were wild-type (wt) mice and mice with abnormal filament assembly, caused by a targeted null mutation of the K8 gene or caused by expression of a point-mutated dominant negative human K18. In vivo 32P-labeled wt mice, subsequently injected with a lethal dose of MC-LR, showed hyperphosphorylation, disassembly, and reorganization of K8/K18, in particular K18, indicating high phosphate turnover on liver keratins in situ. At lethal doses, the keratin assembly-incompetent mice displayed liver lesions faster than wt mice, as indicated histopathologically and by liver-specific plasma enzyme elevations. The histological changes included centrilobular hemorrhage in all mouse strains. The assembly-incompetent mice showed a marked vacuolization of periportal hepatocytes. Indistinguishable MC-LR-induced reorganization of microfilaments was observed in all mice, indicating that this effect on microfilaments is not dependent on the presence of functional K8/K18 networks. At sublethal doses of MC-LR, all animals had the same potential to recover from the liver damage. Our study shows that K8/K18 filament assembly is regulated in vivo by serine phosphorylation. The absence or occurrence of defective K8/K18 filaments render animals more prone to liver damage, which supports the previously suggested roles of keratin IFs in maintenance of structural integrity.