Regulation of C-myc and C-Ha-ras oncogene expression by cell shape

Cell Architecture-Dependent Constraints: Critical Safeguards to Carcinogenesis

Animal cells display great diversity in their shape. These morphological characteristics result from crosstalk between the plasma membrane and the force-generating capacities of the cytoskeleton macromolecules. Changes in cell shape are not merely byproducts of cell fate determinants, they also actively drive cell fate decisions, including proliferation and differentiation. Global and local changes in cell shape alter the transcriptional program by a multitude of mechanisms, including the regulation of physical links between the plasma membrane and the nuclear envelope and the mechanical modulation of cation channels and signalling molecules. It is therefore not surprising that anomalies in cell shape contribute to several diseases, including cancer. In this review, we discuss the possibility that the constraints imposed by cell shape determine the behaviour of normal and pro-tumour cells by organizing the whole interconnected regulatory network. In turn, cell behaviour might stabilize cells into discrete shapes. However, to progress towards a fully transformed phenotype and to acquire plasticity properties, pro-tumour cells might need to escape these cell shape restrictions. Thus, robust controls of the cell shape machinery may represent a critical safeguard against carcinogenesis.

Preparation and Reinforcement of Dual-Porous Biocompatible Cellulose Scaffolds for Tissue Engineering

1Biocompatible cellulose-based aerogels composed of nanoporous struts, which embed interconnected voids of controlled micron-size, have been prepared employing temporary templates of fused porogens, reinforcement by interpenetrating PMMA networks and supercritical carbon dioxide drying. Different combinations of cellulose solvent (Ca(SCN)₂/H₂O/LiCl or [EMIm][OAc]/DMSO) and anti-solvent (EtOH), porogen type (paraffin wax or PMMA spheres) and porogen size (various fractions in the range of 100-500 μm) as well as intensity of PMMA reinforcement have been investigated to tailor the materials for cell scaffolding applications. All aerogels exhibited an open and dual porosity (micronporosity >100 μm and nanoporosity extending to the low micrometer range). Mechanical properties of the dual-porous aerogels under compressive stress were considerably improved by introduction of interpenetrating PMMA networks. The effect of the reinforcing polymer on attachment, spreading, and proliferation of NIH 3T3 fibroblast cells, cultivated on selected dual-porous aerogels to pre-evaluate their biocompatibility was similarly positive.

Influence of the cellular redox state on NF-kappaB-regulated gene expression

The redox state has been shown to regulate a variety of biochemical functions including cellular proliferation. Previous studies from our laboratory and others have shown that the binding of many transcription factors to their cognate DNA sequences is sensitive to the redox environment. Therefore, it is likely that redox status serves as an additional regulatory control for the activity of transcription factors and that this may mediate the redox regulation of proliferation. To assess this possibility, the influence of altering the redox state on NF-kappaB-regulated gene expression was studied. A more-reducing environment favored higher levels of expression of gro, an endogenous gene associated with proliferation, when the redox levels were changed either naturally by altering culture density or chemically by treatment with modulators of glutathione synthesis. Furthermore, nuclear runoff studies showed that a more-reducing redox increased transcription of gro. In order to ascertain the singular effect of the redox state on the activity of NF-kappaB, expression of a secreted alkaline phosphatase (SEAP) reporter gene solely under the control of an NF-kappaB response element was measured under varying redox conditions. Changes in the redox state modulated the expression of this reporter system. Taken together, these results suggest the involvement of a redox mechanism regulating signaling events operating through the control of gene expression by transcription factors.

Induction of tyrosine aminotransferase of primary cultured rat hepatocytes depends on the organization of microtubules

We investigated the relationship between the expression of tyrosine aminotransferase (TAT) and cytoskeletal systems of cultured rat hepatocytes by using serum-free culture conditions and changing three factors: 1) the concentration of calcium, 2) the dish-coating material, and 3) the cell-plating density. In hepatocytes in low-calcium medium, induction of TAT by dexamethasone and glucagon was maintained, although cell-cell adhesion was lost. Hepatocytes on Matrigel formed a nonspreading, spherical shape that provided them with the full extent of TAT activity without cell-cell adhesion. Hepatocytes plated on collagen at low cell density spread and changed shape, and the induction of TAT activity was markedly reduced. By using confocal laser-scanning microscopy, we analyzed the three-dimensional organization of cytoplasmic microtubules of hepatocytes maintaining the ability of TAT induction. Hepatocytes plated on collagen at low cell density possessed the radial filamentous structure of cytoplasmic microtubules. When the spherical shape of hepatocytes was maintained by cultivating cells on Matrigel, a ring-like structure of cytoplasmic microtubules beneath the plasma membrane was dominant. Moreover, the induction of TAT activity of hepatocytes in a standard culture system was strongly inhibited by the addition of 1 microM colchicine. These studies suggest that the organization of cytoplasmic microtubules may participate in the shape-related regulation of cell function.

The differential effects of poly(2-hydroxyethyl methacrylate) and poly(2-hydroxyethyl methacrylate)/poly(caprolactone) polymers on cell proliferation and collagen synthesis by human lung fibroblasts

Because of its chemical versatility and demonstrated biocompatibility, poly(2-hydroxyethyl methacrylate) (pHEMA) has been widely used as a polymer for biomedical applications. Since this hydrophilic material shows a poor interface with cells, blendings with other polymers were done to improve cytocompatibility. In our polymer, the presence of hydrophobic dominions on the material surface, due to the interpenetrating polymerization of pHEMA with poly(caprolactone) (PCL), seems to ameliorate the cytocompatibility in terms of cell adhesion and metabolism. For our experiments, we used IMR-90 human fibroblasts, as these cells strongly regulate DNA, RNA, and protein synthesis as anchorage-dependent variables. Cell attachment on a pHEMA/PCL interpenetrating polymer network was optimal, suggesting a strong adhesion between the cells and the polymer surface. Cell adhesion was weaker on pHEMA, as a significant fraction of the fibroblasts revealed a lack of spreading, with most cells remaining spherical. Moreover, only fibroblasts seeded on pHEMA significantly decreased mRNA synthesis; collagen production and cell shapes ranged from fully flat and proliferating, to minimally spread and nonproliferating. Finally, DNA synthesis, as a measure of cell proliferation, was markedly inhibited in cells cultured on pHEMA but not on pHEMA/PCL. In conclusion, our results suggest that control of cell growth and metabolism by biomedical polymers is based on physicochemical mechanism(s) in which the hydrophilicity/hydrophobicity ratio of the material surfaces may play an important role.

Cell spreading and the regulation of ornithine decarboxylase

The aim of this study was to investigate the effect of cell spreading on the induction of ornithine decarboxylase and the rate of putrescine uptake in anchorage-dependent and anchorage-independent cells. Plating non-transformed IEC-6 epithelial cells at high versus low cell density restricted cell spreading from 900 microns 2 to approximately 140 microns 2, blunted the transient induction of ornithine decarboxylase activity from 202 to 32 pmol 14CO2/mg protein per hour and reduced the rate of [14C] putrescine uptake from 46 to 23 pmol/10(5) cells per hour. The mean spreading area of the cell population was controlled by coating tissue culture dishes with the nonadhesive polymer, polyHEMA. Ornithine decarboxylase activity and putrescine uptake correlated with cell spreading with minimal spreading (263 microns 2) corresponding to an 83% decrease in ornithine decarboxylase activity and 51% decrease in the rate of putrescine uptake. Adding the RGD peptide, Gly-Arg-Gly-Glu-Ser-Pro to the medium of sparsely plated cells resulted in rapid reductions in cell spreading concomitant with dose-dependent decreases in ornithine decarboxylase activity and putrescine uptake. Finally, minimizing cell spreading by depriving cells of substratum contact completely abolished serum-induced increases in ornithine decarboxylase and reduced the rate of putrescine uptake by 47%. In contrast to IEC-6 cells, ornithine decarboxylase of neoplastic HTC-116 cells was constitutively expressed with basal and stimulated activity (193 and 982 pmol 14CO2/mg protein per hour, respectively) completely independent of cell adhesion. Putrescine uptake, however, was abolished in the absence of cell adhesion. These data suggest that the induction of ornithine decarboxylase activity and the rate of putrescine uptake correlate with spreading of anchorage-dependent IEC-6 cells and that ornithine decarboxylase activity but not putrescine uptake, appears to be independent of spreading of neoplastic HTC-116 cells.

Integrin binding and cell spreading on extracellular matrix act at different points in the cell cycle to promote hepatocyte growth

This study was undertaken to determine the importance of integrin binding and cell shape changes in the control of cell-cycle progression by extracellular matrix (ECM). Primary rat hepatocytes were cultured on ECM-coated dishes in serum-free medium with saturating amounts of growth factors (epidermal growth factor and insulin). Integrin binding and cell spreading were promoted in parallel by plating cells on dishes coated with fibronectin (FN). Integrin binding was separated from cell shape changes by culturing cells on dishes coated with a synthetic arg-gly-asp (RGD)-peptide that acts as an integrin ligand but does not support hepatocyte extension. Expression of early (junB) and late (ras) growth response genes and DNA synthesis were measured to determine whether these substrata induce G0-synchronized hepatocytes to reenter the growth cycle. Cells plated on FN exhibited transient increases in junB and ras gene expression (within 2 and 8 h after plating, respectively) and synchronous entry into S phase. Induction of junB and ras was observed over a similar time course in cells on RGD-coated dishes, however, these round cells did not enter S phase. The possibility that round cells on RGD were blocked in mid to late G1 was confirmed by the finding that when trypsinized and replated onto FN-coated dishes after 30 h of culture, they required a similar time (12-15 h) to reenter S phase as cells that had been spread and allowed to progress through G1 on FN. We have previously shown that hepatocytes remain viable and maintain high levels of liver-specific functions when cultured on these RGD-coated dishes. Thus, these results suggest that ECM acts at two different points in the cell cycle to regulate hepatocyte growth: first, by activating the G0/G1 transition via integrin binding and second, by promoting the G1/S phase transition and switching off the default differentiation program through mechanisms related to cell spreading.

Changes in oncogene expression in ascite tumour cells during ageing

The expression of two oncogenes, c-myc and c-fos, was studied in an ascitic tumour (ATPC+) at different times after implantation. The specific mRNA synthesis was analysed by Northern blot analysis. The presence of the oncogene proteins was shown by immunofluorescence using flow cytometry and referred to the distribution of the cells in the different cell phases. The results show that both oncogenes are expressed by ATPC+ tumour cells. c-myc is expressed 5, 8 and 12 days after implantation, although with a different intensity, and the protein is mainly present in S or S+G2 phase cells. The c-fos oncogene is expressed only 12 days after tumour implantation and the cells labelled with the specific antibody are mainly in G1 phase. We conclude that c-myc is principally correlated with proliferative activity, whereas c-fos is expressed by non-cycling cells.