Nuclear accumulation of G-actin in isolated rat hepatocytes by adenine nucleotides

Restraint of Human Skin Fibroblast Motility, Migration, and Cell Surface Actin Dynamics, by Pannexin 1 and P2X7 Receptor Signaling

Wound healing is a dynamic process required to maintain skin integrity and which relies on the precise migration of different cell types. A key molecule that regulates this process is ATP. However, the mechanisms involved in extracellular ATP management are poorly understood, particularly in the human dermis. Here, we explore the role, in human fibroblast migration during wound healing, of Pannexin 1 channels and their relationship with purinergic signals and in vivo cell surface filamentous actin dynamics. Using siRNA against Panx isoforms and different Panx1 channel inhibitors, we demonstrate in cultured human dermal fibroblasts that the absence or inhibition of Panx1 channels accelerates cell migration, increases single-cell motility, and promotes actin redistribution. These changes occur through a mechanism that involves the release of ATP to the extracellular space through a Panx1-dependent mechanism and the activation of the purinergic receptor P2X7. Together, these findings point to a pivotal role of Panx1 channels in skin fibroblast migration and suggest that these channels could be a useful pharmacological target to promote damaged skin healing.

Low-dose etoposide-treatment induces endoreplication and cell death accompanied by cytoskeletal alterations in A549 cells: Does the response involve senescence? The possible role of vimentin

Background

Senescence in the population of cells is often described as a program of restricted proliferative capacity, which is manifested by broad morphological and biochemical changes including a metabolic shift towards an autophagic-like response and a genotoxic-stress related induction of polyploidy. Concomitantly, the cell cycle progression of a senescent cell is believed to be irreversibly arrested. Recent reports suggest that this phenomenon may have an influence on the therapeutic outcome of anticancer treatment. The aim of this study was to verify the possible involvement of this program in the response to the treatment of the A549 cell population with low doses of etoposide, as well as to describe accompanying cytoskeletal alterations.

Methods

After treatment with etoposide, selected biochemical and morphological parameters were examined, including: the activity of senescence-associated ß-galactosidase, SAHF formation, cell cycle progression, the induction of p21^{Cip1/Waf1/Sdi1} and cyclin D1, DNA strand breaks, the disruption of cell membrane asymmetry/integrity and ultrastructural alterations. Vimentin and G-actin cytoskeleton was evaluated both cytometrically and microscopically.

Results and conclusions

Etoposide induced a senescence-like phenotype in the population of A549 cells. Morphological alterations were nevertheless not directly coupled with other senescence markers including a stable cell cycle arrest, SAHF formation or p21^{Cip1/Waf1/Sdi1} induction. Instead, a polyploid, TUNEL-positive fraction of cells visibly grew in number. Also upregulation of cyclin D1 was observed. Here we present preliminary evidence, based on microscopic analyses, that suggest a possible role of vimentin in nuclear alterations accompanying polyploidization-depolyploidization events following genotoxic insults.

Ostreocin-D impact on globular actin of intact cells

Ostreocin-D, discovered in the past decade, is a marine toxin produced by dinoflagellates. It shares structure with palytoxin, a toxic compound responsible for the seafood intoxication named clupeotoxism. At the cellular level, the action sites and pharmacological effects for ostreocin-D are still almost unknown. Previously, we demonstrated that these toxins change the filamentous actin cytoskeleton, which is essential for multiple cellular functions. However, nothing has yet been reported about what happens with the unpolymerized actin pool. Here (i) the effects induced by ostreocin-D on unpolymerized actin, (ii) the Ca2+ role in such a process, and (iii) the cytotoxic activity of ostreocin-D on the human neuroblastoma BE(2)-M17 cell line are shown for the first time. Fluorescently labeled DNase I was used for staining of monomeric actin prior to detection with both laser-scanning cytometry and confocal microscopy techniques. Cellular viability was tested through a microplate metabolic activity assay. Ostreocin-D elicited a rearrangement of monomeric actin toward the nuclear region. This event was not accompanied by changes in its content. In addition, the presence or absence of external Ca2+ did not change these results. This toxin was also found to cause a decrease in the viability of neuroblastoma cells, which was inhibited by the specific blocker of Na+/K+-ATPase, ouabain. All these responses were comparable to those obtained with palytoxin under identical conditions. The data suggest that ostreocin-D modulates the unassembled actin pool, activating signal transduction pathways not related to Ca2+ influx in the same way as palytoxin.

Cell and molecular biology of nuclear actin

Actin is a highly conserved protein and one of the major components of the cytoplasm and the nucleus in eukaryotic cells. In the nucleus, actin is involved in a variety of nuclear processes that include transcription and transcription regulation, RNA processing and export, intranuclear movement, and structure maintenance. Recent advances in the field of nuclear actin have established that functions of actin in the nucleus are versatile, complex, and interconnected. It also has become increasingly evident that the cytoplasmic and nuclear pools of actin are functionally linked. However, while the biological significance of nuclear actin has become clear, we are only beginning to understand the mechanisms that lie behind the regulation of nuclear actin. This review provides an overview of our current understanding of the functions of actin in the nucleus.

Changes of G-actin localisation in the mitotic spindle region or nucleus during mitosis and after heat shock: a histochemical study of G-actin in various cell lines with fluorescent labelled vitamin D-binding protein

The presence and localisation of G-actin in various cell lines was studied using the highly G-actin specific, fluorescence-labelled vitamin D-binding protein. In various cell-types, pig kidney-derived cells (LLC-PK1), Chinese hamster ovary (CHO) cells, SV-40 transformed African green monkey kidney (COS) cells and human hepatoma (HepG2) cells, G-actin was only visible in the cytoplasm of interphase cells. However, in mitotic cells, depending on the mitotic phase, intense G-actin staining was found associated with the mitotic spindle (early mitosis) or overlapping the DNA-staining pattern (late mitosis). Also after heat shock (60-180 min at 43 degrees C), an intense nuclear staining of G-actin was observed. In LLC-PK1 cells, the increase of nuclear G-actin staining disappeared again after 24 h at 37 degrees C, but in COS, CHO and HepG2 cells, it was still present in the nucleus after 24 h at 37 degrees C, indicating that the process was not rapidly reversible in these cells; the increased nuclear G-actin was not associated with cell division. Comparison of the amount of G-actin present in the nucleus and in the cytosol before and after heat shock using Western blotting demonstrated that the total amount of G-actin in both nucleus and cytosol was unchanged after heat shock. This indicates that the increased G-actin staining is not a result of import of G-actin into the nucleus. These observations suggest a rearrangement of G-actin in the nucleus during both mitosis and heat shock, which may be due to changes in interaction of G-actin with chromosomes.

Induction of apoptosis and changes in nuclear G-actin are mediated by different pathways: the effect of inhibitors of protein and RNA synthesis in isolated rat hepatocytes

Stressor-induced changes in the cytoskeleton, of which actin is a major component, may lead to apoptosis. The role of drug-induced changes in nuclear G-actin and apoptosis was studied in freshly isolated hepatocytes. Several protein synthesis inhibitors, cycloheximide, puromycin, and emetine, induced 10 to 15% apoptosis in hepatocytes after 4 h, as was determined by changes in nuclear morphology and flow cytometric analysis of Annexin V-positive cells. Apoptosis induced by protein synthesis inhibition could be prevented by the caspase inhibitors Z-Val-Ala-DL-Asp fluormethylketone (zVAD-fmk) and Ac-Asp-Glu-Val-Asp-aldehyde (DEVD-cho). Several (chemical) stressors cause a rapid increase in nuclear G-actin staining in hepatocytes or cell lines (Meijerman et al., Biochem. Biophys. Res. Commun. 240, 697-700, 1997). The protein synthesis inhibitors also increased G-actin staining in nuclei after 2 h; this could not be inhibited by zVAD-fmk or DEVD-cho. Changes in the cytosolic F-actin pattern did not occur until nuclear G-actin staining had already increased. The mRNA synthesis inhibitor actinomycin D, also increased nuclear G-actin staining, but did not induce apoptosis within the studied time frame. The results suggest that the induction of apoptosis and the increased nuclear staining of G-actin by protein synthesis inhibition are differently controlled.

Polyamine depletion alters the relationship of F-actin, G-actin, and thymosin beta4 in migrating IEC-6 cells

The cause of reduced migration ability in polyamine-deficient cells is not known, but their actin cytoskeleton is clearly abnormal. We depleted polyamines with alpha-difluoromethylornithine (DFMO) in migrating cells with or without stimulation by epidermal growth factor (EGF) and investigated filamentous (F-) actin, monomeric (G-) actin, and thymosin beta4 (Tbeta4), using immunofluorescent confocal microscopy, DNase assay, and immunoblot analysis. DFMO reduced F-actin in the cell interior, increased it in the cell cortex, redistributed G-actin, and increased nuclear staining of Tbeta4. However, DFMO did not affect the amount of Tbeta4 mRNA. EGF caused a rapid increase in the staining of F-actin in control cells, but DFMO prevented this response to EGF. Despite the visible changes shown by immunocytochemistry, statistically significant changes in the amount of either actin isoform or of total actin did not occur. We propose that DFMO reduces migration by interfering with the sequestration of G-actin by Tbeta4 and the association of F-actin with activated EGF receptors.