Increase of cytokeratin D during liver regeneration: association with the nuclear matrix

Nuclear roles for non-lamin intermediate filament proteins

The nuclear-localized lamins have long been thought to be the only intermediate filaments (IFs) with an impact on the architecture, properties, and functions of the nucleus. Recent studies, however, uncovered significant roles for IFs other than lamins (here referred to as "non-lamin IFs") in regulating key properties of the nucleus in various cell types and biological settings. In the cytoplasm, IFs often occur in the perinuclear space where they contribute to local stiffness and impact the shape and/or the integrity of the nucleus, particularly in cells under stress. In addition, selective non-lamin IF proteins can occur inside the nucleus where they partake in fundamental processes including nuclear architecture and chromatin organization, regulation of gene expression, cell cycle progression, and the repair of DNA damage. This text reviews the evidence supporting a role for non-lamin IF proteins in regulating various properties of the nucleus and highlights opportunities for further study.

Keratins Are Going Nuclear

Previously thought to reside exclusively in the cytoplasm, the cytoskeletal protein keratin 17 (K17) has been recently identified inside the nucleus of tumor epithelial cells with a direct impact on cell proliferation and gene expression. We comment on fundamental questions raised by this new finding and the associated significance.

Identification of cytokeratin 18 as a biomarker of mouse and human hepatosplenic schistosomiasis

Previously, we demonstrated unique protein expression patterns in 20-week-Schistosoma mansoni-infected CBA/J mice with moderate splenomegaly syndrome (MSS) or hypersplemomegaly syndrome (HSS). To better understand the development of severe pathology, we compared the two-dimensional differential in-gel electrophoresis (2D-DIGE) proteomic signatures of livers from uninfected mice and mice infected for 6, 8, 12, or 20 weeks and found significant changes in collagen isoforms, interleukin-2 (IL-2), cytokeratin 18, hydroxyproline, S. mansoni phosphoenolpyruvate carboxykinase, major urinary protein isoforms, and peroxiredoxin 6. Cytokeratin 18, hydroxyproline, and connective tissue growth factor (CTGF) were chosen for analysis in mouse and human sera using targeted biochemical assays. Consistent with the liver analysis, cytokeratin 18, CTGF, and hydroxyproline were significantly elevated in sera from mice with HSS compared to those from uninfected mice or mice with MSS. Moreover, cytokeratin 18 and CTGF were found to be markers for subjects with hepatosplenic and intestinal schistosomiasis, respectively, while serum hydroxyproline was a strong indicator of fibrosis for severe HS. These findings indicate that schistosome-associated changes to the liver can be detected in the serum and reveal the potential for cytokeratin 18 to be used as a diagnostic marker for early detection of hepatosplenic schistosomiasis.

Interaction in vitro of type III intermediate filament proteins with Z-DNA and B-Z-DNA junctions

The selection of DNA fragments containing simple d(GT)(n) and composite d(GT)(m). d(GA)(n) microsatellites during affinity binding of mouse genomic DNA to type III cytoplasmic intermediate filaments (cIFs) in vitro, and the detection of such repeats, often as parts of nuclear matrix attachment region (MAR)-like DNA, in SDS-stable DNA-vimentin crosslinkage products isolated from intact fibroblasts, prompted a detailed study of the interaction of type III cIF proteins with left-handed Z-DNA formed from d(GT)(17) and d(CG)(17) repeats under the topological tension of negatively supercoiled plasmids. Although d(GT)(n) tracts possess a distinctly lower Z-DNA-forming potential than d(CG)(n) tracts, the filament proteins produced a stronger electrophoretic mobility shift with a plasmid carrying a d(GT)(17) insert than with plasmids containing different d(CG)(n) inserts, consistent with the facts that the B-Z transition of d(GT)(n) repeats requires a higher negative superhelical density than that of d(CG)(n) repeats and the affinity of cIF proteins for plasmid DNA increases with its superhelical tension. That both types of dinucleotide repeat had indeed undergone B-Z transition was confirmed by S1 nuclease and chemical footprinting analysis of the plasmids, which also demonstrated efficient protection by cIF proteins from nucleolytic and chemical attack of the Z-DNA helices as such, as well as of the flanking B-Z junctions. The analysis also revealed sensibilization of nucleotides in the center of one of the two strands of a perfect d(CG)(17) insert toward S1 nuclease, indicating cIF protein-induced bending of the repeat. In all these assays, vimentin and glial fibrillary acidic protein (GFAP) showed comparable activities, versus desmin, which was almost inactive. In addition, vimentin and GFAP exhibited much higher affinities for the Z-DNA conformation of brominated, linear d(CG)(25) repeats than for the B-DNA configuration of the unmodified oligonucleotides. While double-stranded DNA was incapable of chasing the Z-DNA from its protein complexes, and Holliday junction and single-stranded (ss)DNA were distinguished by reasonable competitiveness, phosphatidylinositol (PI) and, particularly, phosphatidylinositol 4,5-diphosphate (PIP(2)) turned out to be extremely potent competitors. Because PIP(2) is an important member of the nuclear PI signal transduction cascade, it might exert a regulatory influence on the binding of cIF proteins to Z- and other DNA conformations. From this interaction of cIF proteins with Z- and bent DNA and their previously detected affinities for MAR-like, ss, triple helical, and four-way junction DNA, it may be concluded that the filament proteins play a general role in such nuclear matrix-associated processes as DNA replication, recombination, repair, and transcription.

Cytoplasmic intermediate filaments are stably associated with nuclear matrices and potentially modulate their DNA-binding function

The tight association of cytoplasmic intermediate filaments (cIFs) with the nucleus and the isolation of crosslinkage products of vimentin with genomic DNA fragments, including nuclear matrix attachment regions (MARs) from proliferating fibroblasts, point to a participation of cIFs in nuclear activities. To test the possibility that cIFs are complementary nuclear matrix elements, the nuclei of a series of cultured cells were subjected to the Li-diiodosalicylate (LIS) extraction protocol developed for the preparation of nuclear matrices and analyzed by immunofluorescence microscopy and immunoblotting with antibodies directed against lamin B and cIF proteins. When nuclei released from hypotonically swollen L929 suspension cells in the presence of digitonin or Triton X-100 were exposed to such strong shearing forces that a considerable number were totally disrupted, a thin, discontinuous layer of vimentin IFs remained tenaciously adhering to still intact nuclei, in apparent coalignment with the nuclear lamina. Even in broken nuclei, the distribution of vimentin followed that of lamin B in areas where the lamina still appeared intact. The same retention of vimentin together with desmin and glial IFs was observed on the nuclei isolated from differentiating C2C12 myoblast and U333 glioma cells, respectively. Nuclei from epithelial cells shed their residual perinuclear IF layers as coherent cytoskeletal ghosts, except for small fractions of vimentin and cytokeratin IFs, which remained in a dot-to cap-like arrangement on the nuclear surface, in apparent codistribution with lamin B. LIS extraction did not bring about a reduction in the cIF protein contents of such nuclei upon their transformation into nuclear matrices. Moreover, in whole mount preparations of mouse embryo fibroblasts, DNA/chromatin emerging from nuclei during LIS extraction mechanically and chemically cleaned the nuclear surface and perinuclear area from loosely anchored cytoplasmic material with the production of broad, IF-free annular spaces, but left substantial fractions of the vimentin IFs in tight association with the nuclear surface. Accordingly, double-immunogold electron microscopy of fixed and permeabilized fibroblasts disclosed a close neighborhood of vimentin IFs and lamin B, with a minimal distance between the nanogold particles of ca. 30 nm. These data indicate an extremely solid interconnection of cIFs with structural elements of the nuclear matrix, and make them, together with their susceptibility to crosslinkage to MARs and other genomic DNA sequences under native conditions, complementary or even integral constituents of the karyoskeleton.

Fibronectin regulates morphology, cell organization and gene expression of rat fetal hepatocytes in primary culture

BACKGROUND/AIMS

The extracellular matrix regulates hepatic development and regeneration, modulating the maintenance of liver architecture in the differentiated state. The aim of this work was to analyze how different extracellular matrix molecules modulate fetal hepatocyte morphology, growth and differentiation.

METHODS

We cultured fetal hepatocytes either on plastic or on different extracellular matrix proteins, i.e., collagen I, fibronectin or E-C-L (entactin-collagen IV-laminin) and we analyzed cell attachment, morphological organization, proliferative response and gene expression.

RESULTS

Cell attachment was increased by all the extracellular matrix proteins to a similar extent. However, only fibronectin facilitated the formation of elongated cord-like structures, reminiscent of liver plate organization. Immunocytochemical analysis of the cells in these structures revealed high levels of albumin and cytokeratin 18, phenotypical markers of parenchymal hepatocytes. Fibronectin did not block the mitogenic stimuli induced by epidermal growth factor in these cells and the elongated structures appeared either in the absence or in the presence of the mitogen. Cells cultured on fibronectin, regardless of whether epidermal growth factor was present or not, also presented the maximal levels of expression for liver specific genes, such as albumin or alpha-fetoprotein. This expression was coincident with an increased expression of hepatocyte nuclear factor (HNF)-4 and a higher HNF-1alpha/HNF-1beta ratio, when compared with those cells that were cultured on collagen or E-C-L extracellular matrix.

CONCLUSIONS

These results suggest that fibronectin might play a differential role, as compared to other extracellular matrix proteins, in fetal hepatocyte organization and gene expression.

Heterogeneous nuclear ribonucleoprotein A2 interacts with protein kinase CK2

The catalytic subunit of protein kinase CK2 (CK2alpha) was found associated with heterogeneous nuclear ribonucleoprotein particles (hnRNPs) that contain the core proteins A2 and C1-C2. High levels of CK2 activity were also detected in these complexes. Phosphopeptide patterns of hnRNP A2 phosphorylated in vivo and in vitro by protein kinase CK2 were similar, suggesting that this kinase can phosphorylate hnRNPA2 in vivo. Binding experiments using human recombinant hnRNP A2, free human recombinant CK2alpha or CK2beta subunits, reconstituted CK2 holoenzyme and purified native rat liver CK2 indicated that hnRNP A2 associated with both catalytic and regulatory CK2 subunits, and that the interaction was independent of the presence of RNA. However, the capability of hnRNP A2 to bind to CK2 holoenzyme was lower than its binding to the isolated subunits. These data indicate that the association of CK2alpha with CK2beta interferes with the subsequent binding of hnRNP A2. HnRNP A2 inhibited the autophosphorylation of CK2beta. This effect was stronger with reconstituted human recombinant CK2 than with purified native rat liver CK2.

Transforming growth factor-beta (TGF-beta) and EGF promote cord-like structures that indicate terminal differentiation of fetal hepatocytes in primary culture

When fetal hepatocytes were cultured in the presence of transforming growth factor-beta (TGF-beta 1) and epidermal growth factor (EGF), some morphological changes were observed. Under these conditions, cells migrated, from typical clusters that hepatocytes adopt in culture, to form elongated, cord-like structures similar to the hepatic acinus organization. Immunocytochemical analysis of these cells revealed high levels of albumin and cytokeratin 18, phenotypic markers of parenchymal hepatocytes. Although some of the cells in the cord-like structures presented a cortical ring distribution of F-actin filaments, the cord also presented thick peripheral bundles and cells of the tips showed thin stress fibers oriented to the cell edges, typical of a migratory phenotype. In addition to these morphological effects, flow cytometric analysis of the cells revealed a larger size, granularity and intracellular lipid content (as a parameter related to liver metabolic function), in TGF-beta + EGF-treated hepatocytes. Western blot analysis of the albumin levels revealed that both expression and secretion of albumin were increased in EGF + TGF-beta-treated cells. Finally, all these changes were coincident with an enhancement in the DNA-binding activity for hepatocyte nuclear factors (HNF1, HNF3, and HNF4), as revealed in gel-shift experiments with nuclear extracts. We conclude that a cooperative action between TGF-beta and EGF might modulate terminal maturation of fetal hepatocytes.