Disruption of the cytoskeleton with cytochalasin D induces c-fos gene expression

Cytochalasin D enhances the accumulation of a protease-resistant form of prion protein in ScN2a cells: involvement of PI3 kinase/Akt signalling pathway

The conversion of a host-encoded PrPsen (protease-sensitive cellular prion protein) into a PrPres (protease-resistant pathogenic form) is a key process in the pathogenesis of prion diseases, but the intracellular mechanisms underlying PrPres amplification in prion-infected cells remain elusive. To assess the role of cytoskeletal proteins in the regulation of PrPres amplification, the effects of cytoskeletal disruptors on PrPres accumulation in ScN2a cells that were persistently infected with the scrapie Chandler strain have been examined. Actin microfilament disruption with cytochalasin D enhanced PrPres accumulation in ScN2a cells. In contrast, the microtubule-disrupting agents, colchicine, nocodazole and paclitaxel, had no effect on PrPres accumulation. In addition, a PI3K (phosphoinositide 3-kinase) inhibitor, wortmannin and an Akt kinase inhibitor prevented the cytochalasin D-induced enhancement of PrPres accumulation. Cytochalasin D-induced extension of neurite-like processes might correlate with enhanced accumulation of PrPres. The results suggest that the actin cytoskeleton and PI3K/Akt pathway are involved in the regulation of PrPres accumulation in prion-infected cells.

Effects of copper on proliferation and autocrine secretion of insulin-like growth factor-1 (IGF-1) and IGF-binding protein-3 (IGFBP-3) in chondrocytes from newborn pigs in vitro

Chondrocytes from the lateral trochlear ridge of the distal femur taken from 1-day-old piglets were cultured in medium supplemented with 0, 7.8, 15.6, 31.2, and 62.5 μmol/L copper. Insulin-like growth factor-1 (IGF-1) and IGF-binding protein 3 (IGFBP-3) levels in culture medium were determined by radioimmunoassay. DNA synthesis in chondrocytes was measured by tritiated thymidine ((3)H-TdR) incorporation. Proliferation-promoting activity and incorporation of (3)H-TdR in chondrocytes were increased in all culture media supplemented with copper and 15% fetal calf serum (FCS). The contents of IGF-1 and IGFBP-3 were also enhanced significantly in culture media containing 15% FCS and supplemented with copper at 15.6, 31.2, and 62.5 μmol/L. The optimal copper concentration for promoting chondrocyte proliferation and autocrine secretion of IGF-1 and IGFBP-3 was 31.2 μmol/L.

Effect of cytoskeleton inhibitors on deadhesion kinetics of HepG2 cells on biomimetic surface

Cytochalasin-D (Cyto-D) and latrunculin-A (Lat-A) are known inhibitors of actin microfilaments and adversely affect the physiological functions of anchorage-dependent cells. Alternatively, doxorubicin (Dox), a chemotherapeutic drug is known to induce apoptosis and cell detachment of tumor cells. However, the intricate interplay between drug administration, cytoskeletal rearrangement and biophysical responses of live cells on immobilized layer of extracellular matrix (ECM) protein remains unknown. In this study, the deadhesion kinetics and actin remodeling of live HepG2 cells following the addition of the three drugs are probed with confocal reflectance interference contrast microscopy (C-RICM) and fluorescence confocal microscopy. First, it is shown that the reduction in two-dimensional spread area of HepG2 cells is 10.5%, 15.4% and 21.9% under the influence of 5 microM of Lat-A, Cyto-D and Dox, respectively. Secondly, C-RICM demonstrates the recession of strong adhesion contact against time of cell seeding upon the addition of the three drugs. Thirdly, the initial cell detachment rate and extent of reduction in the degree of cell deformation (a/R) are dependent on both the drug types and concentration. Lastly, oscillation-like responses of a/R and adhesion energy are uniquely found in Lat-A induced cell detachment. Overall, our biophysical approaches have been proven as a highly quantitative platform for elucidating the interfacial properties of adherent cells on biomimetic surfaces under cytoskeleton disruption.

Non-redundant Role of Shc in Erk Activation by Cytoskeletal Reorganization

We have shown previously that cytoskeletal reorganization (CSR) induced by pharmacological reagents such as colchicine or cytochalasins can up-regulate the urokinase-type plasminogen activator (uPA) gene via the Ras/Erk signaling pathway. In this present study using the small interfering RNA technique, we have found that ShcA adapter proteins play a rather active role in CSR-induced Erk activation, contrary to their mostly redundant role in other signaling pathways, e.g. growth factor-induced Erk activation, where Grb2 can bind directly to the receptor tyrosine kinase and activate Erk in the absence of ShcA. ShcA knockdown abolished CSR-induced activation of both Erk and the uPA promoter. Expression of small interfering RNA-escaping silent mutants of p52 or p46 but not p66 ShcA isoform efficiently rescued CSR-induced Erk activation. Moreover, we have shown that phosphorylation of either Tyr-239/Tyr-240 or Tyr-313 in p52(ShcA) can mediate CSR-induced Erk activation equally well. In a quest for molecules upstream of ShcA in this signaling, we found that CSR-induced ShcA tyrosine phosphorylation, its association with Grb2, Erk activation, and uPA gene expression were all dependent on Rho kinase, p38 mitogen-activated protein kinase, and Src. In summary, we have found a novel, non-redundant role for ShcA in contrast to its redundant role in many other signaling pathways.

Rho protein-mediated changes in the structure of the actin cytoskeleton regulate human inducible NO synthase gene expression

Rho proteins (Rho, Rac, Cdc 42) are known to control the organization of the actin cytoskeleton as well as gene expression. Inhibition of Rho proteins by Clostridium difficile toxin B disrupted the F-actin cytoskeleton and enhanced cytokine-induced inducible nitric oxide synthase (iNOS) expression in human epithelial cells. Also specific inhibition by Y-27632 of p160ROCK, which mediates Rho effects on actin fibers, caused a disruption of the actin cytoskeleton and a superinduction of cytokine-induced iNOS expression. Accordingly, direct disruption of the actin cytoskeleton by cytochalasin D, latrunculin B, or jasplakinolide enhanced cytokine-induced iNOS expression. The transcription factor serum response factor (SRF) has been described as mediating actin cytoskeleton-dependent regulation of gene expression. Direct targets of SRF are activating protein 1 (AP1)-dependent genes. All compounds used inhibited SRF- and AP1-dependent reporter gene expression in DLD-1 cells. However, the enhancing effect of the actin cytoskeleton-disrupting compounds on human iNOS promoter activity was much less pronounced than the effect on iNOS mRNA expression. Therefore, besides transcriptional mechanisms, posttranscriptional effects seem to be involved in the regulation of iNOS expression by the above compounds. In conclusion, our data suggest that Rho protein-mediated changes of the actin cytoskeleton negatively modulate the expression of human iNOS.

MEK/ERK pathway mediates cell-shape-dependent plasminogen activator inhibitor type 1 gene expression upon drug-induced disruption of the microfilament and microtubule networks

Changes in cellular morphology induced as a consequence of direct perturbation of cytoskeletal structure with network-specific targeting agents(i.e. microfilament- or microtubule-disrupting drugs) results in the stimulated expression of a specific subset of genes. Transcription of c-fos, collagenase, transforming growth factor-β, actin,urokinase plasminogen activator and its type-1 inhibitor (PAI-1) appears to be particularly responsive to shape-activated signaling pathways. Cytochalasin D(CD) or colchicine treatment of contact-inhibited and serum-deprived vascular smooth muscle (R22) cells was used, therefore, as a model system to evaluate morphology-associated controls on PAI-1 gene regulation in the absence of added growth factors. PAI-1 transcript levels in quiescent R22 cells increased rapidly and in a CD-concentration-dependent fashion, with kinetics of expression paralleling the morphological changes. Colchicine concentrations that effectively disrupted microtubule structure and reduced the cellular`footprint' area (to approximately that of CD treatment) also stimulated PAI-1 synthesis. Shape-related increases in PAI-1 mRNA synthesis were ablated by prior exposure to actinomycin D. Unlike the mechanism of induction in growth-factor-stimulated cells, CD- and colchicine-induced PAI-1 expression required on-going protein synthesis (i.e. it was a secondary response). Although PAI-1 is a TGF-β-regulated gene and TGF-β expression is also shape dependent, an autocrine TGF-β loop was not a factor in CD-initiated PAI-1 transcription. Since CD exposure resulted in actin microfilament disruption and subsequent morphological changes, with uncertain effects on interactions between signaling intermediates or `scaffold'structures, a pharmacological approach was selected to probe the pathways involved. Signaling events leading to PAI-1 induction were compared with colchicine-treated cells. CD- as well as colchicine-stimulated PAI-1 expression was effectively and dose dependently attenuated by the MEK inhibitor PD98059 (in the 10 to 25 μM concentration range), consistent with the known MAP kinase dependency of PAI-1 synthesis in growth-factor-stimulated cells. Reduced PAI-1 mRNA levels upon exposure to genistein prior to CD addition correlated with inhibition of ERK1/2 activity, implicating a tyrosine kinase in shape-dependent MEK activation. Src-family kinases,moreover, appeared to be specific upstream elements in the CD- and colchicine-dependent pathways of PAI-1 transcription since both agents effectively activated pp60c-src kinase activity in quiescent R22 cells. The restrictive (src-family) kinase inhibitor PP1 completely inhibited induced, as well as basal, ERK activity in a coupled immunoprecipitation myelin-basic-protein-phosphorylation assay and ablated shape-initiated PAI-1 mRNA expression. These data suggest that PP1-sensitive tyrosine kinases are upstream intermediates in cell-shape-associated signaling pathways resulting in ERK1/2 activation and subsequent PAI-1 transcription. In contrast to the rapid and transient kinetics of ERK activity typical of serum-stimulated cells, the ERK1/2 response to CD and colchicine is both delayed and relatively sustained. Collectively, these data support a model in which MEK is a focal point for the convergence of shape-initiated signaling events leading to induced PAI-1 transcription.

Distinct pathways in the over-expression of matrix metalloproteinases in human fibroblasts by relaxation of mechanical tension

The aim of the work was to analyze, on a comparative basis, the signaling pathways operating in the regulation of a panel of matrix metalloproteinases (MMP) expressed by human dermal fibroblasts submitted to mechanical stress relaxation by cytochalasin D (CD) and in a retracting collagen gel (RCG). The mRNA steady-state level of MMPs was measured by a quantitative RT-PCR procedure using a synthetic RNA as internal standard. In monolayer, most MMPs were barely detected, except MMP-2. Disruption of the actin stress fibers by CD induced a moderate increase of MMP-2 mRNA and a much larger stimulation of MMP-3, -9, -13 and -14 mRNAs. In RCG, a significant up-regulation of these MMPs was also observed although to a lower extent than in CD-treated monolayers. Among the investigated MMPs, the MMP-8 and -11 were not reproducibly detected. MMP-2 was processed to its active form both by CD and in RCG. The CD-induced up-regulation of gene expression was largely repressed by blocking protein synthesis by cycloheximide for all the MMPs, by inhibiting the tyrosine-kinases of the src family by herbimycin A for all MMPs, except MMP-2, and by inhibiting the TPA-inducible PKC isoforms by bisindoyl maleimide for all MMPs, except MMP-14. The up-regulation induced by stress relaxation in RCG was protein synthesis-dependent for MMP-2 and MMP-13, tyrosine kinases-dependent for MMP-3 and MMP-13, as previously described for MMP-1. Inhibiting TPA-inducible PKC did not affect any MMP in RCG except MMP-13, which was strongly induced. The processing of MMP-2 was tyrosine kinases-dependent but PKC-independent. Inhibitors of the ERK1,2 and p38 MAP kinases pathways diversely affected the MMPs expression. Inhibiting the Rho-kinase activity by Y-27632 was inactive. These results point to the potent regulation operated by the status of the cytoskeleton on the cell phenotype, and to distinct regulatory pathways involved in the control of different MMPs expression.

Cartilage damage by matrix degradation products: fibronectin fragments

Catabolic cytokines play a major role in cartilage degradation not only in rheumatoid arthritis but also in osteoarthritis. Although the major source in rheumatoid arthritis may be mononuclear cells and synovial tissue and the cause of release may be multifactorial, the source of cytokines in osteoarthritis would be mostly from chondrocytes. However, there are few explanations of how upregulation of the cytokines might occur in osteoarthritis. One possibility is that degradation products of the extracellular matrix arising from elevated protease levels, substrate, or both, might regulate cytokine activities. Fragments of the extracellular matrix protein, fibronectin, upregulate cytokine expression and induce the events of suppressed matrix synthesis and upregulation of matrix metalloproteinases, characteristic of osteoarthritis. The catabolic aspects of this system are short term, subsequently serve to enhance anabolic processes above untreated levels, and condition the tissue against additional insult. It will be necessary to determine whether in vivo these degradation products precede cytokine expression and act early and are targets for intervention or instead are a consequence of cytokine damage. Whether they regulate anabolism and catabolism, blocking of their activities may not be ideal.

Perturbation of the actin cytoskeleton induces PAI-1 gene expression in cultured epithelial cells independent of substrate anchorage

Perturbation of cellular architecture with agents that alter cytoskeletal organization provides a means to assess the relationship between cell shape and gene expression. Induced transcription of the plasminogen activator inhibitor type-1 (PAI-1) gene in serum-free cultures of normal rat kidney (NRK-52E) cells following disruption of actin microfilament structures with cytochalasin D (CD) provides a simple model to probe mechanisms underlying shape-related expression control. Transition from the typical flat epithelial cell shape to an "arborized" phenotype was a concomitant of the PAI-1 inductive response. Stimulated expression occurred rapidly (i.e., within 2 h of CD addition), involved increases in both PAI-1 mRNA abundance and de novo protein synthesis, and was dependent upon the concentration of CD used. A series of culture conditions were designed (e.g., use of bacteriological surfaces, poly-HEMA coated surfaces, maintenance in suspension on agarose) to discriminate cell shape from adhesive influences on CD-stimulated PAI-1 expression. Cytoskeletal disruption, and not simply changes in cell shape, was a critical aspect of CD-mediated PAI-1 expression in NRK cells cultured under serum-free conditions; induced expression was independent of substrate anchorage. Low concentrations of CD (1-2 microM) failed to cause cell arborization or increase either relative PAI-1 mRNA/protein abundance levels suggesting, however, that cell rounding may be a necessary but not sufficient aspect in CD-mediated PAI-1 induction. Transfection of PAI-1 promoter-CAT reporter constructs into NRK cells followed by stimulation with CD or serum additionally indicated that CD-induced PAI-1 expression did not utilize the same functional complement of serum-responsive promoter sequences, thus, further defining differences in the growth factor- and cytoskeletal-mediated pathways of PAI-1 gene regulation.

Bacterial lipopolysaccharide induces cytoskeletal rearrangement in small intestinal lamina propria fibroblasts: actin assembly is essential for lipopolysaccharide signaling

Cytoskeletal proteins are major components of the cell backbone and regulate cell shape and function. The purpose of this study was to investigate the effect of lipopolysaccharide (LPS) on the dynamics and organization of the cytoskeletal proteins, actin, vimentin, tubulin and vinculin in human small intestinal lamina propria fibroblasts (HSILPF). A noticeable change in the actin architecture was observed after 30 min incubation with LPS with the formation of orthogonal fibers and further accumulation of actin filament at the cell periphery by 2 h. Reorganization of the vimentin network into vimentin bundling was conspicuous at 2 h. With further increase in the time period of LPS exposure, diffused staining of vimentin along with vimentin bundling was observed. Vinculin plaques distributed in the cell body and cell periphery in the control cells rearrange to cell periphery in LPS-treated cells by 30 min of LPS exposure. However, there was no change in the tubulin architecture in HSILPF in response to LPS. LPS increased the F-actin pool in HSILPF in a concentration-dependent manner with no difference in the level of G-actin. A time-dependent study depicted an increase in the G-actin pool at 10 and 20 min of LPS exposure followed by a decrease at further time intervals. The F-actin pool in LPS-treated cells was lower than the control levels at 10 and 20 min of LPS exposure followed by a sharp increase until 120 min and finally returning to the basal level at 140 and 160 min. Further (35)S-methionine incorporation studies suggested a new pool of actin synthesis, whereas the synthesis of other cytoskeletal filaments was not altered. Cytochalasin B, an actin-disrupting agent, severely affected the LPS induced increased percentage of 'S' phase cells and IL-6 synthesis in HSILPF. We conclude that dynamic and orchestrated organization of the cytoskeletal filaments and actin assembly in response to LPS may be a prime requirement for the LPS induced increase in percentage of 'S' phase cells and IL-6 synthesis

Growth factor-induced signal transduction in adherent mammalian cells is sensitive to gravity

Epidermal growth factor (EGF) activates a well-characterized signal transduction cascade in a wide variety of cells. This activation leads to increased cell proliferation in most cell types. Among the early effects evoked by EGF are receptor clustering, cell rounding, and early gene expression. The influence of gravity on EGF-induced EGF receptor clustering and gene expression as well as on actin polymerization and cell rounding have been investigated in adherent A431 epithelial cells with the use of sounding rockets to create microgravity conditions. EGF-induced c-fos and c-jun expression decreased in microgravity. This was caused by alteration of the EGF receptor and protein kinase C-mediated signal transduction pathways. In contrast, neither the binding of EGF to the receptor nor the receptor clustering were changed under microgravity conditions. Because cell morphology was also modulated under microgravity conditions, and the growth factor-induced signal transduction cascades have been demonstrated to be linked to the actin microfilament system, it is tempting to suggest that the actin microfilament system constitutes the gravity-sensitive cell component.

Actin polymerization is required for negative feedback regulation of epidermal growth factor-induced signal transduction

Epidermal growth factor (EGF) induces rapid actin filament assembly in the membrane skeleton of a variety of cells. To investigate the significance of this process for signal transduction, actin polymerization is inhibited by dihydrocytochalasin B (CB). CB almost completely abolishes EGF-induced actin polymerization, as assessed by quantitative confocal laser scanning microscopy. Under these conditions, EGF induces enhanced EGF receptor (EGFR) tyrosine kinase activity, as well as superinduction of the c-fos proto-oncogene. These data suggest that EGF-induced actin polymerization may be important for negative feedback regulation of signal transduction by the EGFR. The phosphorylation of Thr654 by protein kinase C (PKC) is a well-characterized negative feedback control mechanism for signal transduction by the EGFR tyrosine kinase. A synthetic peptide, corresponding to the regions flanking Thr654 of the EGFR, is used to analyze EGF stimulated PKC activity by incorporation of 32P into the peptide. Cotreatment of cells with CB and EGF results in a complete loss of EGF-induced phosphorylation of the peptide. These data suggest that actin polymerization is obligatory for negative feedback regulation of the EGFR tyrosine kinase through the C-kinase pathway.

Regulation of Wnt5a expression in human mammary cells by protein kinase C activity and the cytoskeleton

The Wnts can be classified into two classes based on their ability to transform cells. The Wnt5a class can antagonize the effects of transforming Wnts partly through effects on cell migration. To understand the mechanisms of regulation of Wnt5a, we investigated its expression in human normal and breast cancer cell lines. Elevation of Wnt5a in HB2, a normal breast epithelial cell line, was linearly correlated with cell density, but this did not occur in cancer cell lines. We examined intracellular events responsible for the regulation of Wnt5a by cell to cell contacts, using various metabolic agents known to affect signal transduction pathways. Agents that selectively blocked protein kinase C (calphostin C) or protein tyrosine kinases (genistein) reduced the level of Wnt5a expression markedly. Protein kinase C activation by phorbol 12-myristate 13-acetate up-regulated Wnt5a partly through prolongation of Wnt5a mRNA half-life. Cytoskeleton reorganization following cytochalasin D treatment caused an induction of Wnt5a, which was associated with changes in cell morphology. Calphostin C did not block these effects, showing that protein kinase C is acting upstream of cytoskeletal modulation. However, the cancer cell lines treated with cytochalasin D showed no changes in cell morphology or Wnt5a induction, suggesting disruption of this regulatory pathway in cancer.

Peripheral nuclear matrix actin forms perinuclear shells

Perinuclear actin shells have been reported in a variety of organisms. The shells have been identified by staining perinuclear material with fluorescently-labelled phalloidin, but have not been localized to a specific subcellular compartment at the ultrastructural level. We show here that the shells of 3T3 cells lie in the peripheral nuclear matrix. Nuclear shells and matrix actin in other parts of the nucleus are not usually detected by immunohistochemical staining because they are inaccessible to antibodies or to phalloidin. Immunohistochemical detection of nuclear actin is only possible during its deposition at the end of mitosis, or in interphase nuclei that have been extracted with detergent, digested with nucleases and washed with high salt buffers.

Cell shape-dependent pathway of plasminogen activator inhibitor type-1 gene expression requires cytoskeletal reorganization

Synthesis of plasminogen activator inhibitor type-1 (PAI-1), a major physiological modulator of plasmin generation, is regulated by growth factors and changes in cell shape. To evaluate the specific relationship between PAI-1 gene expression and cytoarchitecture, serum-free cultures of quiescent rat kidney (NRK) cells were exposed to cytochalasin D (CD) at concentrations that disrupt microfilament structure. Treatment with 1-10 microM CD resulted in an increased 1) incidence of rounded cells, 2) relative PAI-1 mRNA content, and 3) fraction of PAI-1 protein-expressing cells. Abrupt increases in each response were evident at a final concentration of 5 microM CD. Maximal levels of induced PAI-1 transcripts (18-fold that of control) occurred 4 hours post-CD addition and declined thereafter but remained elevated (by at least tenfold) for 24 hours. Assessment of the metabolic requirements for CD-induced PAI-1 expression by using the protein synthesis inhibitors puromycin and cycloheximide indicated that PAI-1 transcripts were regulated in a complex manner in response to CD. The predominant mode of induction reflected secondary (protein synthesis-dependent) metabolic processes, although a minor, albeit significant, primary (protein synthesis-independent) pathway was also evident. PAI-1 mRNA levels in NRK cells maintained in serum- and CD-free agarose suspension culture were low or undetectable. Relative abundance of PAI-1 transcripts in suspended cells cultured in the presence of CD, however, closely approximated that of plastic-adherent, CD-treated cells (13-fold over control). NRK cells in suspension culture with or without CD were morphologically identical, remaining spherical and unattached. It appears, therefore, that cell rounding alone is not a sufficient stimulus to induce PAI-1 expression in quiescent NRK cells and that perturbation of the actin skeleton as a consequence of CD treatment is a critical event in the inductive response. A protein tyrosine kinase is likely involved in the CD-mediated signal-transduction cascade, since induced PAI-1 expression can be down-regulated by genistein and herbimycin A but not by calphostin C or tyrphostin B46.

Increased c-fos mRNA expression by human fibroblasts contracting stressed collagen matrices

We studied early changes in gene expression during fibroblast contraction of stressed collagen matrices. The level of c-fos mRNA increased dramatically and peaked 50 to 60 min after matrix contraction was initiated. This response did not require serum and could not be accounted for simply by disruption of the actin cytoskeleton. Increased c-fos mRNA levels required Ca2+ influx but not the cyclic AMP or extracellular signal-regulated kinase (ERK 1/2) signaling pathways, both of which are activated when fibroblasts contract stressed collagen matrices. The levels of two other immediate-early genes, fosb and c-jun, also increased transiently after fibroblast contraction, whereas the levels of fra-1, fra-2, c-myc, and the transcription factor NF-kappaB remained the same, indicating that fibroblast contraction caused changes in a selective group of genes. The increase in c-fos mRNA during contraction of stressed collagen matrices may reflect a unique role for c-fos in mechanoregulated events at the end of wound repair.

Differential nuclear matrix-intermediate filament expression in human prostate cancer in respect to benign prostatic hyperplasia

We have characterized the changes in composition of the nuclear matrix-intermediate filament complex (NM-IF) isolated from prostate cancer (PCa), compared with benign prostatic hyperplasia (BPH). We prepared the NM-IF from ten patients undergoing radical retropubic prostectomy; the benign hyperplastic tissue was obtained from the prostate lobe contralateral to the cancer zone. Several quantitative and qualitative changes have been identified. Three new proteins of molecular weight 48, 47 and 29 kDa and isoelectric point 6.0, 4.9 and 6.4, respectively, were detected in PCa, referred to here as P8, P5 and NM-1, P8 was present in all ten of the tumors examined, P5 was expressed in 9/10 PCa; conversely, they were present in only one and two BPH, respectively; NM-1 was found in eight tumors out of nine and never in BPH. These proteins are expressed in moderately differentiated malignant cells, suggesting that the proteins of the NM-IF complex can be interesting biomarkers for prostate cancer. Immunoblot analysis shows that P8 and P5 proteins belong to the IF superfamily. This observation, taken together with previous data obtained by our and other groups, suggests that the characterization of NM-IF protein changes could also shed light on mechanistic aspects of cancer progression.

Estrogen regulation of nuclear matrix-intermediate filament proteins in human breast cancer cells

The tissue matrix consists of linkages and interactions of the nuclear matrix, cytoskeleton, and extracellular matrix. This system is a dynamic structural component of the cell that organizes and processes structural and functional information to maintain and coordinate cell function and gene expression. We have studied estrogen regulation of nuclear matrix associated proteins, including the intimately connected cytoskeletal intermediate filaments, in T-47D5 human breast cancer cells. Three proteins (identified as cytokeratins 8, 18, and 19) present in the nuclear matrix-intermediate filament fraction (NM-IF) of cells grown in estrogen-replete conditions were dramatically reduced when the cells were grown in acute (1 week) estrogen-depleted conditions. Replacing estrogen in the medium of acute estrogen-depleted cells restored expression of these proteins. T-47D5 cells that are chronically depleted of estrogen (T5-PRF) are estrogen-nonresponsive in culture. These cells overexpressed these three proteins, compared to parent cells grown in the presence of estrogen. Treatment of the T5-PRF cells with estrogen did not lead to further up-regulation of these proteins. Treating T-47D5 cells in estrogen-replete conditions with the antiestrogens 4-hydroxytamoxifen and ICI 164 384 (100 nM, 3 days) resulted in a significant reduction in these proteins, while no effect was seen in long-term chronic estrogen-depleted T-47D5 cells. In conclusion, we have identified NM-IF proteins (cytokeratins 8, 18, and 19) in human breast cancer cells that are estrogen regulated and may play a role in estrogen action in human breast cancer cells.

The pruned gene encodes the Drosophila serum response factor and regulates cytoplasmic outgrowth during terminal branching of the tracheal system

We identified a Drosophila gene, pruned, that regulates formation of the terminal branches of the tracheal (respiratory) system. These branches arise by extension of long cytoplasmic processes from terminal tracheal cells towards oxygen-starved tissues, followed by formation of a lumen within the processes. The pruned gene is expressed in terminal cells throughout the period of terminal branching. pruned encodes the Drosophila homologue of serum response factor (SRF), which functions with an ETS domain ternary complex factor as a growth-factor-activated transcription complex in mammalian cells. In pruned loss of function mutants, terminal cells fail to extend cytoplasmic projections. A constitutively activated SRF drives formation of extra projections that grow out in an unregulated fashion. An activated ternary complex factor has a similar effect. We propose that the Drosophila SRF functions like mammalian SRF in an inducible transcription complex, and that activation of this complex by signals from target tissues induces expression of genes involved in cytoplasmic outgrowth.

F-actin forms transient perinuclear shells at the mitosis-interphase transition

Intermediate filaments and microtubules are known to be involved in establishing and maintaining nuclear shape. F-actin may also be involved in determining nuclear shape, since we have found it associated with reforming nuclei very briefly after cell division. We stained cells from vertebrate tissue cultures (3T3 and NRK-49F) and epidermal cells from an insect with rhodamine-phalloidin and Hoechst #33342 to localize F-actin in relation to the nucleus. We found that F-actin forms shells only around nuclei during reorganization in late mitosis and early interphase. We suggest that perinuclear F-actin shells may be generally present in eukaryotes, but that they are easily missed because of their delicacy and transience.

The Rho family GTPases RhoA, Rac1, and CDC42Hs regulate transcriptional activation by SRF

The c-fos serum response element (SRE) forms a ternary complex with the transcription factors SRF (serum response factor) and TCF (ternary complex factor). By itself, SRF can mediate transcriptional activation induced by serum, lysophosphatidic acid, or intracellular activation of heterotrimeric G proteins. Activated forms of the Rho family GTPases RhoA, Rac1, and CDC42Hs also activate transcription via SRF and act synergistically at the SRE with signals that activate TCF. Functional Rho is required for signaling to SRF by several stimuli, but not by activated CDC42Hs or Rac1. Activation of the SRF-linked signaling pathway does not correlate with activation of the MAP kinases ERK, SAPK/JNK, or MPK2/p38. Functional Rho is required for regulated activity of the c-fos promoter. These results establish SRF as a nuclear target of a novel Rho-mediated signaling pathway.

Components of the nuclear signaling cascade that regulate collagenase gene expression in response to integrin-derived signals

We have shown previously that the expression of collagenase is upregulated in rabbit synovial fibroblasts cultured on a substrate of antibody to the alpha 5 chain of the alpha 5 beta 1 integrin fibronectin receptor or on the 120-kD cell-binding chymotryptic fragment of plasma fibronectin, but remains at basal levels in cells plated on intact plasma fibronectin. We now have identified some of the components of a signaling pathway that couples the fibronectin receptor to the induction of collagenase transcription. We studied the control of collagenase gene expression in cells adhering to the 120-kD fragment of fibronectin, to antifibronectin receptor antibody, or to plasma fibronectin by transiently introducing promoter-reporter constructs into rabbit synovial fibroblasts before plating cells on these matrices. The constructs contained segments of the human collagenase promoter regulating transcription of chloramphenicol acyl transferase. Expression of constructs containing the -1200/-42-bp segment or the -139/-42-bp segment of the collagenase promoter inserted upstream from the reporter gene was induced to similar extents in cells plated on the 120-kD fragment of fibronectin or on anti-fibronectin receptor antibody, relative to that in fibroblasts plated on fibronectin. The expression of the construct containing the -66/-42-bp segment of the promoter was not regulated and was similar to that of the parent pBLCAT2 plasmid, suggesting that the -139/-67 region of the collagenase promoter, which contains PEA3- and AP1-binding sites, regulates the transcription of collagenase caused by integrin-derived signals. Expression of a reporter construct containing only the PEA3 and AP1 sites in the collagenase promoter (-90/-67) also increased in cells plated on the 120-kD fragment of fibronectin or on anti-fibronectin receptor antibody, relative to that in cells plated on fibronectin. Mutations in either the AP1 or PEA3 site of this minimal promoter abrogated its activity in cells plated on these inductive ligands. Expression of c-fos mRNA increased within 1 h of plating cells on the 120-kD fibronectin fragment or on anti-fibronectin receptor antibody, relative to that in cells plated on fibronectin. c-Fos protein accumulated in the nuclei of fibroblasts within 10 min of plating on the 120-kD fibronectin fragment. The increase in c-Fos was required for the increase in collagenase in cells plated on the 120-kD fibronectin fragment: incubation of cells with antisense, but not sense, c-fos oligonucleotides diminished both basal and induced expression of the -139/-42 collagenase promoter-reporter construct and decreased expression of the endogenous collagenase gene.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell-shape-dependent modulation of p52(PAI-1) gene expression involves a secondary response pathway

Expression of the rat p52(PAI-1) gene is positively regulated by agents that influence cellular microfilament organization and/or cell-to-substrate adhesion [e.g. cytochalasin D (CD) and sodium n-butyrate (NaB)] [Higgins, Chaudhari and Ryan (1991) Biochem. J. 273, 651-658; Higgins, Ryan and Providence (1994) J. Cell. Physiol. 159, 187-195]. As shape-responsive genes may be subject to inducer-specific controls, the biochemical mechanisms underlying the shape-dependent pathway of p52(PAI-1) gene regulation were examined in v-ras-transformed rat kidney (KNRK) cells. NaB and/or CD effectively stimulated p52(PAI-1) run-off transcription and augmented de novo p52(PAI-1) mRNA and protein synthesis in KNRK cells; induction at both the mRNA and protein levels was inhibited by actinomycin D. Pretreatment with cycloheximide (CX) markedly attenuated NaB- and/or CD-stimulated p52(PAI-1) expression. CX alone, however, induced low levels of p52(PAI-1) mRNA; increased p52(PAI-1) protein synthesis was evident after release of KNRK cells from CX blockade. Such CX-mediated induction was also sensitive to actinomycin D. Full stimulation of p52(PAI-1) expression in KNRK cells in response to the shape modulators NaB and/or CD involves transcriptional activation of the p52(PAI-1) gene, requires de novo RNA synthesis and occurs through a secondary-response (i.e. protein-synthesis-dependent) pathway.

Integrity of intermediate filaments is associated with the development of acquired thermotolerance in 9L rat brain tumor cells

Withangulatin A (WA), a newly discovered withanolide isolated from an antitumor Chinese herb, has been shown to be a vimentin intermediate filament-targeting drug by using immunofluorescence microscopy. Together with cytochalasin D and colchicine, these drugs were employed to investigate the importance of vimentin intermediate filaments, actin filaments, and microtubules in the development of acquired thermotolerance in 9L rat brain tumor cells treated at 45 degrees C for 15 min (priming heat-shock). Acquired thermotolerance was abrogated in cells incubated with WA before the priming heat-shock but it could be detected in cells treated with WA after the priming heat-shock. In contrast, cytochalasin D and colchicine do not interfere with the development of thermotolerance at all. The intracellular localizations of vimentin and the constitutive heat-shock protein70 (HSC70) in treated cells were examined by using immunofluorescence microscopy and detergent-extractability studies. In cells treated with WA before the priming heat-shock, vimentin IFs were tightly aggregated around the nucleus and unable to return to their normal organization after a recovery under normal growing conditions. In contrast, the IF network in cells treated with WA after the priming heat-shock was able to reorganize into filamentous form after a recovery period, a behavior similar to that of the cells treated with heat-shock only. HSC70 was found to be co-localized with vimentin during these changes. It is suggested that the integrity of intermediate filaments is important for the development of thermotolerance and that HSC70 may be involved in this process by stabilizing the intermediate filaments through direct or indirect binding.

Alterations in cellular gene expression without changes in nuclear matrix protein content

Cell metabolism and function are modulated in part by cell and nuclear shape. Nuclear shape is controlled by the nuclear matrix, the RNA-protein skeleton of the nucleus, and its interactions with cytoskeletal systems such as intermediate filaments and actin microfilaments. The nuclear matrix plays an important role in cell function and gene expression because active genes are bound to the nuclear matrix whereas inactive genes are not. It is unknown, however, how genes move on and off the matrix, and whether these events require compositional protein changes, i.e., alterations in protein content of the nuclear matrix, or other, more subtle alterations and/or modifications. The purpose of this investigation was to begin to determine how nuclear matrix protein composition is related to gene expression. We demonstrate that gene expression can change without apparent changes in the protein composition of the nuclear matrix in MCF10A breast epithelial cells.

Signalling the molecular stress response to nephrotoxic and mutagenic cysteine conjugates: differential roles for protein synthesis and calcium in the induction of c-fos and c-myc mRNA in LLC-PK1 cells

Nephrotoxic and mutagenic cysteine conjugates (NCC) are activated by the enzyme cysteine conjugate, beta-lyase, to reactive acylating species which bind covalently to cellular macromolecules. We now show that an early event after treatment of LLC-PK1 cells with NCC is the induction of mRNA for both c-fos and c-myc. Treatment with S-(1,2-dichlorovinyl)-L-cysteine (DCVC) induced c-fos (53-fold) and c-myc mRNA (20-fold) and increased transcription about 3-fold for both genes. Covalent binding was required for induction of both mRNAs. Dithiothreitol partially prevented induction of both c-fos and c-myc RNA. Buffering the DCVC-induced increase in cytosolic free calcium had no effect on c-fos mRNA, but partially blocked c-myc mRNA induction. Cycloheximide blocked the induction of c-myc mRNA in the absence of an effect on c-fos induction. The data suggest that the increase in c-fos mRNA is a primary response to DCVC toxicity and occurs without a requirement for protein synthesis or an increase in intracellular free calcium. In contrast, c-myc induction requires protein synthesis, suggesting that the presence of another primary response factor may regulate induction either transcriptionally or posttranscriptionally. The data suggest that different signalling pathways regulate induction of c-fos and c-myc mRNA in response to stress caused by reactive acylating species.

Cytochalisin D exerts stimulatory and inhibitory effects on insulin-induced glucokinase mRNA expression in hepatocytes

The microfilament cytoskeleton is postulated to have a role in the localization, transport and anchorage of certain specific mRNAs. We investigated the effects of cytochalasin D, a fungal metabolite that binds to actin and disrupts the microfilament structure, on insulin-induced expression of glucokinase mRNA in rat hepatocyte cultures. Cytochalasin-D significantly potentiates insulin-induced glucokinase mRNA expression at 100 nM concentration but counteracts glucokinase expression at 2-20 microM. The latter effect is at least in part due to an increase in glucokinase mRNA degradation. This effect of cytochalasin D cannot be accounted for by an increase in cAMP and is also not due to a non-specific effect on mRNA degradation since albumin mRNA levels were not affected by cytochalasin-D and actin mRNA and tubulin mRNA levels were increased. Measurement of glucokinase mRNA release from digitonin-permeabilized hepatocytes suggests that cytochalasin D does not cause acute dissociation of glucokinase mRNA from its binding site. The increased degradation of glucokinase mRNA suggests involvement of the cytoskeleton in glucokinase mRNA stability. However, an additional effect of cytochalasin D on the insulin signalling mechanism cannot be excluded.

Coupling of cell structure to cell metabolism and function

The fact that cells make directed decisions regarding how to use energy, i.e., where to direct intracellular particles or where to move, suggests that energy can be, and is, harnessed in specific ways. It is now well established that the chemical reactions of the cell do not occur in nonorganized soup, but rather in the context of ordered structure. The physical components that make up this ordered structure of the cell are part of the tissue matrix, which consists of the dynamic linkages between the skeletal networks of the nucleus (the nuclear matrix), the cytoplasm (the cytoskeleton), and the extracellular environment (the extracellular matrix). To understand gene function and how the energy of the cell is directed towards accomplishing the tasks directed by DNA (gene expression), a further understanding of how cell structure is tied to cellular energy and function is required. We propose that the structural components of the cell harness cellular energy to direct cell functions by providing a dynamic bridge between thermodynamics and gene expression.

Induced expression of p52(PAI-1) in normal rat kidney cells by the microfilament-disrupting agent cytochalasin D

In established normal rat kidney (NRK) cells, synthesis of the 52 kDa type-1 inhibitor of plasminogen activator [p52(PAI-1)] is stimulated by the cell shape-modulating fungal metabolite cytochalasin D (CD). Induction paralleled the time course of morphologic change and reflected relatively specific increases in saponin-resistant p52(PAI-1) protein accumulation (approximating ten- to thirty-fold over control) and mRNA abundance (seven- to nine-fold). Augmented p52 (PAI-1) mRNA levels closely correlated with increases in 43 kDa p52(PAI-1) core protein biosynthesis. Sensitivity to tunicamycin indicated that N-linked post-translational modifications to this 43 kDa core species generated the full complement of 50 kDa (intermediate) and 52 kDa (mature) p52(PAI-1) glycosylated isoforms. CD-induced p52(PAI-1) expression occurred efficiently in quiescent NRK cells maintained under serum-free conditions as well as in fully serum-supplemented actively growing cultures. While 8-bromo-cAMP reduced both constitutive and transforming growth factor-beta-induced p52(PAI-1) synthesis by > 50%, no such inhibition was evident in short-term (4 h) CD-stimulated cultures. Long-term (24 h) exposure of NRK/CD cells to 8-bromo-cAMP did result in an approximately 34% reduction in stimulated p52(PAI-1) expression, however, levels expressed by NRK/CD+cAMP populations remained markedly elevated relative to control values. These data suggest the existence of a cell shape-dependent aspect of p52(PAI-1) expression control distinct from both the constitutive and growth factor-mediated pathways of gene regulation.

Effects of gravity on the cellular response to epidermal growth factor

EGF and related polypeptides are involved in the regulation of cell growth and differentiation of continuously regenerating tissues, in tissue repair processes and in placental and fetal development. Their initial mode of action generally constitutes binding to specific plasma membrane localized receptors, transduction of the signal across the plasma membrane, subsequent activation of signalling pathways in the cell, and the induction of early nuclear gene expression. EGF-induced signal transmission from the plasma membrane to the nucleus has been studied in microgravity in order to gain insight in the molecular mechanisms that constitute the effects of gravity on cell growth. Exposure of human A431 cells to microgravity strongly suppresses EGF- and PMA-induced c-fos and c-jun expression. In contrast, forskolin- and A23187-induced c-fos expression and constitutive beta-2 microglobulin expression remain unaffected. This suggests that microgravity differentially modulates EGF-induced signal transduction pathways. Since both EGF and PMA are known to be activators of PKC, which is not the case for forskolin and A23187, PKC-mediated signal transduction may be a cellular target for microgravity. Inhibition of EGF-induced c-fos expression by microgravity occurs downstream of the initiation of EGF-induced signal transduction, i.e., EGF binding and EGFR redistribution. In addition to PKC signaling, actin microfilament organization appears to be sensitive to microgravity. Therefore, the inhibition of signal transduction by microgravity may be related to alterations in actin microfilament organization. The fact that early gene expression is affected by agents that alter the organization of the actin microfilament system supports this hypothesis. The decrease in c-fos and c-jun expression in microgravity may result in the decreased formation of the FOS and JUN proteins. Consequently, a short-term reduction in gene expression in microgravity may have a more dramatic effect over the long term, since both the JUN and FOS protein families are required for normal cell cycle progression. However, since more than 20 years of manned spaceflight have shown that humans can survive in microgravity for prolonged periods, it appears that cells in the human body can partly or completely overcome gravitational stress. Although some insight in the molecular basis on human cells has been obtained, future studies will be needed for a better understanding of the grounds for alterations in the cellular biochemistry due to altered gravity conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Coordinated expression of five tropomyosin isoforms and beta-actin in astrocytes treated with dibutyryl cAMP and cytochalasin D

Cytochalasin D and dBcAMP cause cultured astrocytes to change from flat cells to retracted process-bearing cells. F-actin was present throughout cells stimulated with dBcAMP for 16 h, whereas cytochalasin D caused F-actin to form massive aggregates at the tips of the cell processes. The two drugs differently regulated the expression of both beta-actin and tropomyosin genes in astrocytes cultured in the presence or absence of serum: dBcAMP caused down-regulation and cytochalasin D caused up-regulation. Northern blot analyses indicated that: (1) serum deprivation halved the concentration of all tropomyosin transcripts (TM-1, TM-2, TM-4, TMBr-1, TMBr-2). Serum induced TM-4 via transcriptional activation, independent of protein synthesis, (2) dBcAMP induced down-regulation of beta-actin (-50%) and tropomyosin transcripts (-35 to 52%) even in the presence of serum. The concentration of profilin mRNA decreased in dBcAMP-reactive astrocytes (-46%). The decrease in beta-actin mRNA concentration was not blocked by cycloheximide, whereas down-regulation of tropomyosin transcripts was completely reversed when protein synthesis was inhibited, and (3) cytochalasin D induced an increase in the concentration of tropomyosin transcripts (+69 to 185%) which was cumulative with serum stimulation. Cytochalasin D induction of both beta-actin and TM-4 operated through transcriptional activation, independent of protein synthesis. The production of all tropomyosin transcripts examined here were strictly coordinated with beta-actin expression in serum-, dBcAMP- and cytochalasin D-treated astrocytes. This indicates that the differential expression of tropomyosin isoforms occurring during astrocyte maturation is due to more complex regulation than that involved in serum- or cAMP-stimulated astrocytes.

Differential expression of P-glycoprotein genes in primary rat hepatocyte culture

The multidrug resistance (MDR)-associated protein, P-glycoprotein (Pgp), is expressed on the bile canalicular surface of hepatocytes, where it is thought to function in the detoxification of xenobiotics or in the transport of specific metabolites. Several studies have shown that Pgp expression in rat liver can be perturbed in vivo and in vitro; however, it is not known which of the 3 Pgp genes (class I, II, or III) are involved. In rodents, the class I and II Pgp genes have been shown to mediate MDR while the class III gene apparently does not. In this report, we have used gene-specific probes generated from the 3'-untranslated regions of the three rat Pgp genes (Deuchars et al.: Biochim. Biophys. Acta, 1130:157-165, 1992) to investigate Pgp gene expression in primary rat hepatocytes. We observed that the class II Pgp mRNA, the least abundant in the intact liver, is dramatically increased in culture over a 48 h period, while the class I Pgp showed only a modest increase in mRNA level. In contrast, the class III Pgp mRNA, which is the most abundant in the intact liver, exhibited a gradual decline. In rat liver hepatocytes, different culture conditions, as well as drugs such as cytochalasin D and colchicine, appear to affect the level of the class II Pgp gene expression. Moreover, under all these conditions, there is a strong correlation between the level of the class II Pgp and cytoskeletal (actin and tubulin) mRNAs. Thus, there may be a common mechanism regulating the expression of cytoskeletal protein genes and the class II Pgp gene. These findings have implications for our understanding of the regulation of Pgp gene expression in normal and malignant tissues.

Differential regulation of actin polymerization following activation of resting T lymphocytes from young and aged mice

Actin polymerization accompanies receptor-mediated responses and is correlated with motility-related events. In T lymphocytes, there is a lateral redistribution of surface receptors into caps and aggregation of actin-myosin in cytoplasmic subcaps, and these are impaired in T cells from aged individuals. This study documents marked changes in age-related cytoskeletal actin filament function which may account for the reduced motility. Basal levels of filamentous actin (F-actin) are significantly higher in purified G(o) T cells from aged C57BL/6 mice, due to a preferential increase in the CD8+ subpopulation. Following activation of the resting T cells with Concanavalin A (Con A), F-actin depolymerized in cells from young mice for 2 min, followed by rapid polymerization, reaching a plateau 200% above resting levels. In cells from 15-17-month-old mice, an attenuated depolymerization phase was seen for 45 sec, followed by little polymerization. No depolymerization or polymerization phases occurred in cells from aged mice. Phorbol 12 myristate 13-acetate (PMA), which activates protein kinase C (PKC), bypassing receptor mediated signals, induced actin polymerization to 57% of the levels of that after Con A stimulation in cells from both young and old animals and partially eliminated the differences in actin filament assembly due to age. Perturbation of the cytoskeleton with cytochalasin E (CE) potentiated proliferation of Con A-stimulated T cells from aged mice but did not completely restore the deficit attributed to immunosenescence. The results show an age-related impairment of cytoskeletal functions and suggest that differences in early signal transduction events contribute to the decrements in surface receptor motility and subsequent proliferation of T lymphocytes from older individuals.

Cytoskeletal reorganization and TPA differently modify AP-1 to induce the urokinase-type plasminogen activator gene in LLC-PK1 cells

Urokinase-type plasminogen activator (uPA) is an extracellular protease and expressed in various cells that exhibit dynamic changes in cell morphology, suggesting a link between cytoskeletal reorganization (CSR) and uPA expression. CSR can be induced by pharmacological agents, such as by colchicine for microtubule cytoskeleton and by cytochalasin for microfilament cytoskeleton. Using these agents, we previously showed that CSR induced the uPA gene in LLC-PK1 cells independently of the protein kinase C and cAMP-dependent protein kinase. Here we show that the induction of the uPA gene by CSR is mediated by the activation of c-Jun which interacts with an AP-1-like site located 2 kb upstream of the uPA gene. 12-O-tetradecanoylphorbol 13-acetate (TPA) induces the uPA gene through the same elements, but additionally utilizes an adjacent PEA3 element and induces c-fos. Furthermore, CSR induces a greater accumulation and a more pronounced phosphorylation of c-Jun than TPA induction. AP-1 is a positive regulator of growth and oncogenesis, and CSR is an integral part of these processes. Our results provide a view how CSR and AP-1 could be coupled in these processes. We also show that TPA and CSR act synergistically, suggesting a model where an initial activation signal could be amplified by CSR.

Induction of vimentin modification and vimentin-HSP72 association by withangulatin A in 9L rat brain tumor cells

Withangulatin A induced cell rounding up and the morphological alteration resulted from the reorganization of all of the major cytoskeletal components, i.e., vimentin, tubulin, and actin, as revealed by immunofluorescence techniques. When the withangulatin A-treated cells changed to a round-up morphology, vimentin intermediate filaments were found to be collapsed and clustered around the nucleus. The alteration was accompanied by characteristic changes of vimentin molecules, including augmentation of phosphorylation, retardation of electrophoretic mobility, and decrease in detergent extractability. The levels of vimentin phosphorylation were augmented by 2.5- and 1.8-fold in cells incubated with 50 microM withangulatin A for 1 and 3 h, respectively. The electrophoretic mobility of vimentin was partially retarded in cells treated with withangulatin A for 1 h at 10 microM and a completely upshift mobility was observed after 5 h treatment at 50 microM. In addition, vimentin molecules became less extractable by nonident P-40 after the cells were treated with withangulatin A and this effect was dose dependent. The decrease in solubility of vimentin was accompanied by the redistribution of HSP72 into the detergent nonextractable fraction and these two events were well correlated. Our results suggest that withangulatin A induced the modification of vimentin, which resulted in the alteration of cell morphology and redistribution of intracellular HSP72, an event that may play an important role in the induction of heat-shock response.

Nonsense codons can reduce the abundance of nuclear mRNA without affecting the abundance of pre-mRNA or the half-life of cytoplasmic mRNA

The abundance of the mRNA for human triosephosphate isomerase (TPI) is decreased to approximately 20% of normal by frameshift and nonsense mutations that cause translation to terminate at a nonsense codon within the first three-fourths of the reading frame. Results of previous studies inhibiting RNA synthesis with actinomycin D suggested that the decrease is not attributable to an increased rate of cytoplasmic mRNA decay. However, the step in TPI RNA metabolism that is altered was not defined, and the use of actinomycin D, in affecting all polymerase II-transcribed genes, could result in artifactual conclusions. In data presented here, the nonsense codon-mediated reduction in the level of TPI mRNA is shown to be characteristic of both nuclear and cytoplasmic fractions of the cell, indicating that the altered metabolic step is nucleus associated. Neither aberrancies in gene transcription nor aberrancies in RNA splicing appear to contribute to the reduction since there were no accompanying changes in the amount of nuclear run-on transcription, the level of any of the six introns in TPI pre-mRNA, or the size of processed mRNA in the nucleus. Deletion of all splice sites that reside downstream of a nonsense codon does not abrogate the reduction, indicating that the reduction takes place independently of the splicing of a downstream intron. Experiments that placed TPI gene expression under the control of the human c-fos promoter, which can be transiently activated by the addition of serum to serum-deprived cells, verified that there is no detectable effect of a nonsense codon on the turnover of cytoplasmic mRNA.

Nonsense codons can reduce the abundance of nuclear mRNA without affecting the abundance of pre-mRNA or the half-life of cytoplasmic mRNA

The abundance of the mRNA for human triosephosphate isomerase (TPI) is decreased to approximately 20% of normal by frameshift and nonsense mutations that cause translation to terminate at a nonsense codon within the first three-fourths of the reading frame. Results of previous studies inhibiting RNA synthesis with actinomycin D suggested that the decrease is not attributable to an increased rate of cytoplasmic mRNA decay. However, the step in TPI RNA metabolism that is altered was not defined, and the use of actinomycin D, in affecting all polymerase II-transcribed genes, could result in artifactual conclusions. In data presented here, the nonsense codon-mediated reduction in the level of TPI mRNA is shown to be characteristic of both nuclear and cytoplasmic fractions of the cell, indicating that the altered metabolic step is nucleus associated. Neither aberrancies in gene transcription nor aberrancies in RNA splicing appear to contribute to the reduction since there were no accompanying changes in the amount of nuclear run-on transcription, the level of any of the six introns in TPI pre-mRNA, or the size of processed mRNA in the nucleus. Deletion of all splice sites that reside downstream of a nonsense codon does not abrogate the reduction, indicating that the reduction takes place independently of the splicing of a downstream intron. Experiments that placed TPI gene expression under the control of the human c-fos promoter, which can be transiently activated by the addition of serum to serum-deprived cells, verified that there is no detectable effect of a nonsense codon on the turnover of cytoplasmic mRNA.

Cell-shape-associated transcriptional activation of the p52(PAI-1) gene in rat kidney cells

The microfilament-disrupting agent cytochalasin D (CD) increased (by 10-22-fold) the synthesis de novo and extracellular matrix deposition of plasminogen-activator inhibitor type-1 [p52(PAI-1)] in normal rat kidney (NRK) cells. Transition from a flat to a round phenotype occurred concomitantly with, and may actually precede, p52(PAI-1) induction; both the morphological and p52(PAI-1) responses were dose-dependent. Augmented synthesis became evident between 4 and 5 h of treatment of NRK cells with 100 microM-CD, correlating with a transition from 25 to more than 60% rounded cells. CD-associated increases in p52(PAI-1) mRNA abundance and protein biosynthesis were maximal between 6 and 8 h of continuous CD exposure, declined by 50% thereafter, but remained elevated (by at least 6-21-fold respectively over control values) for 24 h. Changes in p52(PAI-1) mRNA abundance at this 24 h point reflected an approx. 5-fold increase in p52(PAI-1)-gene transcription. These data confirm previous suggestions, based on actinomycin D-sensitivity of the inductive response [Higgins & Ryan (1992) Biochem. J. 284, 433-439], that CD-mediated increases in p52(PAI-1) expression are at least partly due to transcription-level events. Since CD also augments specific cellular responses to growth factors or cytokines, the potential effectiveness of this inducer was evaluated both in the presence and absence of serum growth factors using quiescent NRK cells [a growth state in which p52(PAI-1) is not expressed] as a model system. Induction of p52(PAI-1) synthesis and matrix deposition in CD-stimulated quiescent NRK cells was as efficient under growth-factor-deficient conditions as when CD was added simultaneously with serum. CD alone is thus a complete inducer of p52(PAI-1) expression in NRK cells, an observation that supports the contention that cell shape is an important regulatory element in p52(PAI-1)-gene control.

Growth-regulated expression of vimentin in hamster fibroblasts is a result of increased transcription

We have previously shown that vimentin is a growth-regulated gene whose mRNA levels increase after serum stimulation of quiescent hamster fibroblasts. In this study, the control of the growth-regulated expression of vimentin was determined in ts13 cells induced to proliferate by serum. Both transcriptional and post-transcriptional mechanisms of regulation were examined by determining transcriptional rates, cytoplasmic transcript abundance, transcript stability, and protein abundance. We observed a fourfold increase in vimentin transcripts in the cytoplasm of serum-stimulated ts13 cells. Since transcripts are stable in both quiescent and stimulated cells, this induction of vimentin expression is a result of a fivefold increase in vimentin-specific transcriptional activity. As a result of this increased transcript availability, the abundance of polymerized vimentin protein increased following serum stimulation of quiescent fibroblasts. Overall, the induction of vimentin expression in fibroblasts by serum is a consequence of increased vimentin-specific transcriptional activity. The significance of this with regard to cytoskeletal organization and cell division is discussed.

Nuclear-cytoskeletal interactions: evidence for physical connections between the nucleus and cell periphery and their alteration by transformation

The overall coordination of cell structure and function that results in gene expression requires a spatial and temporal precision that would be unobtainable in the absence of structural order within the cell. Cells contain extensive and elaborate three-dimensional skeletal networks that form integral structural components of the plasma membrane, cytoplasm, and nucleus. These skeletal networks form a dynamic tissue matrix are composed of the nuclear matrix, cytoskeleton, and extracellular matrix. The tissue matrix is an interactive network which undergoes dynamic changes as cells move and change shape. Pathologists have long recognized cancer in pathologic specimens based on the altered morphology of tumor cells compared to their normal counterparts. The structural order of cells appears to be altered in transformed cells. This structural order is reflected in the altered morphology and motility observed in transformed cells compared to their normal counterparts, however, it is unclear whether the structural changes observed in cancer cells have any functional significance. We report here on the nature of the physical connections between the nucleus and cell periphery in nontransformed cells and demonstrate that the nucleus is dynamically coupled to the cell periphery via actin microfilaments. We also demonstrate that the dynamic coupling of the nucleus to the cell periphery via actin microfilaments is altered in Kirsten-ras transformed rat kidney epithelial cells. This loss of structure-function relationship may be an important factor in the process of cell transformation.

Phorbol esters induce transient changes in the accessibility of the carboxy-terminal domain of nuclear lamin A

Treatment of human epithelial cells in culture with phorbol esters (TPA) gives rise to a transient and reversible loss of accessibility to antibodies of the nonhelical carboxy-terminal domain of nuclear lamin A that distinguishes it from lamin C. No change in the accessibility of epitopes present in the common domain of lamins A and C was observed. Loss of accessibility of lamin A was not due to proteolytic degradation nor to modification of the isoelectric point of lamin A and did not depend upon protein kinase C activation nor protein synthesis. Perturbation of desmosome organization by growth in low calcium blocked the effect of TPA on lamin A. Prolonged exposure to nocodazole, one of the effects of which is a perinuclear collapse of intermediate filaments, also blocked the effect of TPA on lamin A. These results suggest that the initial target of TPA may be at the level of cell-cell contacts and that the perturbation induced by TPA may be propagated via the structural link formed by intermediate filaments between the cell surface and the nucleus, giving rise to a change in conformation of the carboxy-terminal domain of lamin A or to an interaction of this domain with another nuclear component. These results form the basis for the hypothesis that the interphase nuclear lamina may play an active role in the process of mechanochemical signal transduction.

Flagellar microtubule dynamics in Chlamydomonas: cytochalasin D induces periods of microtubule shortening and elongation; and colchicine induces disassembly of the distal, but not proximal, half of the flagellum

To study the mechanisms responsible for the regulation of flagellar length, we examined the effects of colchicine and Cytochalasin D (CD) on the growth and maintenance of Chlamydomonas flagella on motile wild type cells as well as on pf 18 cells, whose flagella lack the central microtubules and are immobile. CD had no effect on the regeneration of flagella after deflagellation but it induced fully assembled flagella to shorten at an average rate of 0.03 microns-min. Cells remained fully motile in CD and even stubby flagella continued to move, indicating that flagellar shortening did not selectively disrupt machinery necessary for motility. To observe the effects of the drug on individual cells, pf 18 cells were treated with CD and flagella on cells were monitored by direct observation over a 5-hour period. Flagella on control pf 18 cells maintained their initial lengths throughout the experiment but flagella on CD-treated cells exhibited periods of elongation, shortening, and regrowth suggestive of the dynamic behavior of cytoplasmic microtubules observed in vitro and in vitro. Cells behaved individually, with no two cells exhibiting the same flagellar behavior at any given time although both flagella on any single cell behaved identically. The rate of drug-induced flagellar shortening and elongation in pf 18 cells varied from 0.08 to 0.17 microns-min-1, with each event occurring over 10-60-min periods. Addition of colchicine to wild type and pf 18 cells induced flagella to shorten at an average rate of 0.06 microns-min-1 until the flagella reached an average of 73% of their initial length, after which they exhibited no further shortening or elongation. Cells treated with colchicine and CD exhibited nearly complete flagellar resorption, with little variation in flagellar length among cells. The effects of these drugs were reversible and flagella grew to normal stable lengths after drug removal. Taken together, these results show that the distal half to one-third of the Chlamydomonas flagellum is relatively unstable in the presence of colchicine but that the proximal half to two-thirds of the flagellum is stable to this drug. In contrast to colchicine, CD can induce nearly complete flagellar microtubule disassembly as well as flagellar assembly. Flagellar microtubules must, therefore, be inherently unstable, and flagellar length is stabilized by factors that are sensitive, either directly or indirectly, to the effects of CD.

Induction of tyrosinase in human melanoma cells by l-tyrosine phosphate and cytochalasin D

Pigmentation of RVH 421 human melanoma cells is induced when cell division is inhibited by cytochalasin D or L-tyrosine phosphate. Increased pigmentation correlates with increased tyrosinase activity when this is monitored over a time-course. Parallel measurements show that the amount of tyrosinase mRNA correlates with enzyme activity in cells growing without these additives. In contrast, in the presence of cytochalasin D or L-tyrosine phosphate, the increase in amount of tyrosinase mRNA is not sufficient to account for the increase in enzyme activity, indicating that these compounds act mainly at a post-transcriptional level.