Electric Field–directed Cell Motility Involves Up-regulated Expression and Asymmetric Redistribution of the Epidermal Growth Factor Receptors and Is Enhanced by Fibronectin and Laminin

Wounding corneal epithelium establishes a laterally oriented, DC electric field (EF). Corneal epithelial cells (CECs) cultured in similar physiological EFs migrate cathodally, but this requires serum growth factors. Migration depends also on the substrate. On fibronectin (FN) or laminin (LAM) substrates in EF, cells migrated faster and more directly cathodally. This also was serum dependent. Epidermal growth factor (EGF) restored cathodal-directed migration in serum-free medium. Therefore, the hypothesis that EGF is a serum constituent underlying both field-directed migration and enhanced migration on ECM molecules was tested. We used immunofluorescence, flow cytometry, and confocal microscopy and report that 1) EF exposure up-regulated the EGF receptor (EGFR); so also did growing cells on substrates of FN or LAM; and 2) EGFRs and actin accumulated in the cathodal-directed half of CECs, within 10 min in EF. The cathodal asymmetry of EGFR and actin staining was correlated, being most marked at the cell–substrate interface and showing similar patterns of asymmetry at various levels through a cell. At the cell–substrate interface, EGFRs and actin frequently colocalized as interdigitated, punctate spots resembling tank tracks. Cathodal accumulation of EGFR and actin did not occur in the absence of serum but were restored by adding ligand to serum-free medium. Inhibition of MAPK, one second messenger engaged by EGF, significantly reduced EF-directed cell migration. Transforming growth factor β and fibroblast growth factor also restored cathodal-directed cell migration in serum-free medium. However, longer EF exposure was needed to show clear asymmetric distribution of the receptors for transforming growth factor β and fibroblast growth factor. We propose that up-regulated expression and redistribution of EGFRs underlie cathodal-directed migration of CECs and directed migration induced by EF on FN and LAM.

Radiation-induced Release of Transforming Growth Factor α Activates the Epidermal Growth Factor Receptor and Mitogen-activated Protein Kinase Pathway in Carcinoma Cells, Leading to Increased Proliferation and Protection from Radiation-induced Cell Death

Exposure of A431 squamous and MDA-MB-231 mammary carcinoma cells to ionizing radiation has been associated with short transient increases in epidermal growth factor receptor (EGFR) tyrosine phosphorylation and activation of the mitogen-activated protein kinase (MAPK) and c-Jun NH2-terminal kinase (JNK) pathways. Irradiation (2 Gy) of A431 and MDA-MB-231 cells caused immediate primary activations (0–10 min) of the EGFR and the MAPK and JNK pathways, which were surprisingly followed by later prolonged secondary activations (90–240 min). Primary and secondary activation of the EGFR was abolished by molecular inhibition of EGFR function. The primary and secondary activation of the MAPK pathway was abolished by molecular inhibition of either EGFR or Ras function. In contrast, molecular inhibition of EGFR function abolished the secondary but not the primary activation of the JNK pathway. Inhibition of tumor necrosis factor α receptor function by use of neutralizing monoclonal antibodies blunted primary activation of the JNK pathway. Addition of a neutralizing monoclonal antibody versus transforming growth factor α (TGFα) had no effect on the primary activation of either the EGFR or the MAPK and JNK pathways after irradiation but abolished the secondary activation of EGFR, MAPK, and JNK. Irradiation of cells increased pro-TGFα cleavage 120–180 min after exposure. In agreement with radiation-induced release of a soluble factor, activation of the EGFR and the MAPK and JNK pathways could be induced in nonirradiated cells by the transfer of media from irradiated cells 120 min after irradiation. The ability of the transferred media to cause MAPK and JNK activation was blocked when media were incubated with a neutralizing antibody to TGFα. Thus radiation causes primary and secondary activation of the EGFR and the MAPK and JNK pathways in autocrine-regulated carcinoma cells. Secondary activation of the EGFR and the MAPK and JNK pathways is dependent on radiation-induced cleavage and autocrine action of TGFα. Neutralization of TGFα function by an anti-TGFα antibody or inhibition of MAPK function by MEK1/2 inhibitors (PD98059 and U0126) radiosensitized A431 and MDA-MB-231 cells after irradiation in apoptosis, 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT), and clonogenic assays. These data demonstrate that disruption of the TGFα–EGFR–MAPK signaling module represents a strategy to decrease carcinoma cell growth and survival after irradiation.

Transcriptional Regulation of Fibroblast Growth Factor-2 Expression in Human Astrocytes: Implications for Cell Plasticity

Induction of the fibroblast growth factor-2 (FGF-2) gene and the consequent accumulation of FGF-2 in the nucleus are operative events in mitotic activation and hypertrophy of human astrocytes. In the brain, these events are associated with cellular degeneration and may reflect release of the FGF-2 gene from cell contact inhibition. We used cultures of human astrocytes to examine whether expression of FGF-2 is also controlled by soluble growth factors. Treatment of subconfluent astrocytes with interleukin-1β, epidermal or platelet-derived growth factors, 18-kDa FGF-2, or serum or direct stimulation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate or adenylate cyclase with forskolin increased the levels of 18-, 22-, and 24-kDa FGF-2 isoforms and FGF-2 mRNA. Transfection of FGF-2 promoter–luciferase constructs identified a unique −555/−513 bp growth factor-responsive element (GFRE) that confers high basal promoter activity and activation by growth factors to a downstream promoter region. It also identified a separate region (−624/−556 bp) essential for PKC and cAMP stimulation. DNA–protein binding assays indicated that novel cis-acting elements and trans-acting factors mediate activation of the FGF-2 gene. Southwestern analysis identified 40-, 50-, 60-, and 100-kDa GFRE-binding proteins and 165-, 112-, and 90-kDa proteins that interacted with the PKC/cAMP-responsive region. The GFRE and the element essential for PKC and cAMP stimulation overlap with the region that mediates cell contact inhibition of the FGF-2 promoter. The results show a two-stage regulation of the FGF-2 gene: 1) an initial induction by reduced cell contact, and 2) further activation by growth factors or the PKC-signaling pathway. The hierarchic regulation of the FGF-2 gene promoter by cell density and growth factors or PKC reflects a two-stage activation of protein binding to the GFRE and to the PKC/cAMP-responsive region, respectively.

Synthesis and Turnover of Embryonic Sea Urchin Ciliary Proteins during Selective Inhibition of Tubulin Synthesis and Assembly

When ciliogenesis first occurs in sea urchin embryos, the major building block proteins, tubulin and dynein, exist in substantial pools, but most 9+2 architectural proteins must be synthesized de novo. Pulse-chase labeling with [3H]leucine demonstrates that these proteins are coordinately up-regulated in response to deciliation so that regeneration ensues and the tubulin and dynein pools are replenished. Protein labeling and incorporation into already-assembled cilia is high, indicating constitutive ciliary gene expression and steady-state turnover. To determine whether either the synthesis of tubulin or the size of its available pool is coupled to the synthesis or turnover of the other 9+2 proteins in some feedback manner, fully-ciliated mid- or late-gastrula stage Strongylocentrotus droebachiensis embryos were pulse labeled in the presence of colchicine or taxol at concentrations that block ciliary growth. As a consequence of tubulin autoregulation mediated by increased free tubulin, no labeling of ciliary tubulin occurred in colchicine-treated embryos. However, most other proteins were labeled and incorporated into steady-state cilia at near-control levels in the presence of colchicine or taxol. With taxol, tubulin was labeled as well. An axoneme-associated 78 kDa cognate of the molecular chaperone HSP70 correlated with length during regeneration; neither colchicine nor taxol influenced the association of this protein in steady-state cilia. These data indicate that 1) ciliary protein synthesis and turnover is independent of tubulin synthesis or tubulin pool size; 2) steady-state incorporation of labeled proteins cannot be due to formation or elongation of cilia; 3) substantial tubulin exchange takes place in fully-motile cilia; and 4) chaperone presence and association in steady-state cilia is independent of background ciliogenesis, tubulin synthesis, and tubulin assembly state.

Dominant negative inhibition by fragments of a monomeric enzyme

Dominant negative inhibition is most commonly seen when a mutant subunit of a multisubunit protein is coexpressed with the wild-type protein so that assembly of a functional oligomer is impaired. By analogy, it should be possible to interfere with the functional assembly of a monomeric enzyme by interfering with the folding pathway. Experiments in vitro by others suggested that fragments of a monomeric enzyme might be exploited for this purpose. We report here dominant negative inhibition of bacterial cell growth by expression of fragments of a tRNA synthetase. Inhibition is fragment-specific, as not all fragments cause inhibition. An inhibitory fragment characterized in more detail forms a specific complex with the intact enzyme in vivo, leading to enzyme inactivation. This fragment also associated stoichiometrically with the full-length enzyme in vitro after denaturation and refolding, and the resulting complex was catalytically inactive. Inhibition therefore appears to arise from an interruption in the folding pathway of the wild-type enzyme, thus suggesting a new strategy to design dominant negative inhibitors of monomeric enzymes.

Identification of a novel p53 functional domain that is necessary for efficient growth suppression

Activation of the p53 tumor suppressor protein has been demonstrated to block cell growth by inducing either a transient cell cycle arrest or programmed cell death (apoptosis). Although evidence exists linking p53’s function as an activator of transcription to its ability to effect cell cycle arrest, the role of this activity in the induction of apoptosis remains unclear. To gain insight into the molecular mechanisms underlying p53-mediated antiproliferative pathways, a study was initiated to explore the functions of a putative p53 signaling domain. This region of the human p53 protein is localized between amino acids 61 and 94 (out of 393) and is noteworthy in that it contains five repeats of the sequence PXXP (where P represents proline and X any amino acid). This motif has been shown to play a role in signal transduction via its SH3 domain binding activity. A p53 cDNA deletion mutant (ΔproAE), which lacks this entire proline-rich domain (deleted for amino acids 62–91), was created and characterized for a variety of p53 functions. The entire domain has been shown to be completely dispensable for transcriptional activation. On the other hand, this deletion of the p53 proline-rich domain impairs p53’s ability to suppress tumor cell growth in culture. Amino acid substitution mutations at residues 22 and 23 of p53 (eliminates transcriptional activity) also impair p53-mediated inhibition of cell growth in culture. Unlike wild-type p53, the ΔproAE mutant cDNA can be stably expressed in tumor derived cell lines with few immediate detrimental effects. These cells express physiologic levels of p53 protein that are induced normally in response to DNA damage, indicating that removal of the proline-rich domain does not disrupt p53’s upstream regulation by DNA damage. These data indicate that, in addition to the transcriptional activation domain, the p53 proline-rich domain plays a critical role in the transmission of antiproliferative signals downstream of the p53 protein and may link p53 to a direct signal transduction pathway.

Involvement of protein kinase A in fibroblast growth factor-2-activated transcription

Polypeptide growth factors activate common signal transduction pathways, yet they can induce transcription of different target genes. The mechanisms that control this specificity are not completely understood. Recently, we have described a fibroblast growth factor (FGF)-inducible response element, FiRE, on the syndecan-1 gene. In NIH 3T3 cells, the FiRE is activated by FGF-2 but not by several other growth factors, such as platelet-derived growth factor or epidermal growth factor, suggesting that FGF-2 activates signaling pathways that diverge from pathways activated by other growth factors. In this paper, we report that the activation of FiRE by FGF-2 requires protein kinase A (PKA) in NIH 3T3 cells. The PKA-specific inhibitor H-89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide) blocked the FGF-2-induced activation of FiRE, the transcription of the syndecan-1 gene, and cell proliferation. Also, expression of a dominant-negative form of PKA inhibited the FGF-2-induced FiRE activation and the transcription of the syndecan-1 gene. The binding of activator protein-1 transcription-factor complexes, required for the activation of FiRE, was blocked by inhibition of PKA activity before FGF-2 treatment. In accordance with the growth factor specificity of FiRE, the activity of PKA was stimulated by FGF-2 but not by platelet-derived growth factor or epidermal growth factor. Furthermore, a portion of the PKA catalytic subunit pool was translocated to the nucleus by FGF-2. Noticeably, the total cellular cAMP concentration was not affected by FGF-2 stimulus. We propose that the FGF-2-selective transcriptional activation through FiRE is caused by the ability of FGF-2 to control PKA activity.

A peptide derived from the non-receptor-binding region of urokinase plasminogen activator inhibits glioblastoma growth and angiogenesis in vivo in combination with cisplatin

The urokinase plasminogen activator system is involved in angiogenesis and tumor growth of malignant gliomas, which are highly neovascularized and so may be amenable to antiangiogenic therapy. In this paper, we describe the activity of Å6, an octamer capped peptide derived from the non-receptor-binding region of urokinase plasminogen activator. Å6 inhibited human microvascular endothelial cell migration but had no effect on the proliferation of human microvascular endothelial cells or U87MG glioma cells in vitro. In contrast, Å6 or cisplatin (CDDP) alone suppressed subcutaneous tumor growth in vivo by 48% and 53%, respectively, and, more strikingly, the combination of Å6 plus CDDP inhibited tumor growth by 92%. Such combination treatment also greatly reduced the volume of intracranial tumor xenografts and increased survival of tumor-bearing animals when compared with CDDP or Å6 alone. Tumors from the combination treatment group had significantly reduced neovascularization, suggesting a mechanism involving Å6-mediated inhibition of endothelial cell motility, thereby eliciting vascular sensitivity to CDDP-mediated toxicity. These data suggest that the combination of an angiogenesis inhibitor that targets endothelial cells with a cytotoxic agent may be a useful therapeutic approach.

Fibroblast growth factors: An epigenetic mechanism of broad spectrum resistance to anticancer drugs

Based on the observation that removal of tumors from metastatic organs reversed their chemoresistance, we hypothesized that chemoresistance is induced by extracellular factors in tumor-bearing organs. By comparing chemosensitivity and proteins in different tumors (primary vs. metastases) and different culture systems (tumor fragment histocultures vs. monolayer cultures derived from the same tumor), we found elevated levels of acidic (aFGF) and basic (bFGF) fibroblast growth factors in the conditioned medium (CM) of solid and metastatic tumors. These CM induced broad spectrum resistance to drugs with diverse structures and action mechanisms (paclitaxel, doxorubicin, 5-fluorouracil). Inhibition of bFGF by mAb and its removal by immunoprecipitation resulted in complete reversal of the CM-induced chemoresistance, whereas inhibition/removal of aFGF resulted in partial reversal. Using CM that had been depleted of aFGF and/or bFGF and subsequently reconstituted with respective human recombinant proteins, we found that bFGF but not aFGF induced chemoresistance whereas aFGF amplified the bFGF effect. aFGF and bFGF fully accounted for the CM effect, indicating these proteins as the underlying mechanism of the chemoresistance. The FGF-induced resistance was not due to reduced intracellular drug accumulation or altered cell proliferation. We further showed that an inhibitor of aFGF/bFGF (suramin) enhanced the in vitro and in vivo activity of chemotherapy, resulting in shrinkage and eradication of well established human lung metastases in mice without enhancing toxicity. These results indicate elevated levels of extracellular aFGF/bFGF as an epigenetic mechanism by which cancer cells elude cytotoxic insult by chemotherapy, and provide a basis for designing new treatment strategies.

ATM-dependent expression of the insulin-like growth factor-I receptor in a pathway regulating radiation response

The ATM gene is mutated in the syndrome of ataxia telangiectasia (AT), associated with neurologic dysfunction, growth abnormalities, and extreme radiosensitivity. Insulin-like growth factor-I receptor (IGF-IR) is a cell surface receptor with tyrosine kinase activity that can mediate mitogenesis, cell transformation, and inhibition of apoptosis. We report here that AT cells express low levels of IGF-IR and show decreased IGF-IR promoter activity compared with wild-type cells. Complementation of AT cells with the ATM cDNA results in increased IGF-IR promoter activity and elevated IGF-IR levels, whereas expression in wild-type cells of a dominant negative fragment of ATM specifically reduces IGF-IR expression, results consistent with a role for ATM in regulating IGF-IR expression at the level of transcription. When expression of IGF-IR cDNA is forced in AT cells via a heterologous viral promoter, near normal radioresistance is conferred on the cells. Conversely, in ATM cells complemented with the ATM cDNA, specific inhibition of the IGF-IR pathway prevents correction of the radiosensitivity. Taken together, these results establish a fundamental link between ATM function and IGF-IR expression and suggest that reduced expression of IGF-IR contributes to the radiosensitivity of AT cells. In addition, because IGF-I plays a major role in human growth and metabolism and serves as a survival and differentiation factor for developing neuronal tissue, these results may provide a basis for understanding other aspects of the AT syndrome, including the growth abnormalities, insulin resistance, and neurodegeneration.

Anti-PSCA mAbs inhibit tumor growth and metastasis formation and prolong the survival of mice bearing human prostate cancer xenografts

Prostate stem-cell antigen (PSCA) is a cell-surface antigen expressed in normal prostate and overexpressed in prostate cancer tissues. PSCA expression is detected in over 80% of patients with local disease, and elevated levels of PSCA are correlated with increased tumor stage, grade, and androgen independence, including high expression in bone metastases. We evaluated the therapeutic efficacy of anti-PSCA mAbs in human prostate cancer xenograft mouse models by using the androgen-dependent LAPC-9 xenograft and the androgen-independent recombinant cell line PC3-PSCA. Two different anti-PSCA mAbs, 1G8 (IgG1κ) and 3C5 (IgG2aκ), inhibited formation of s.c. and orthotopic xenograft tumors in a dose-dependent manner. Furthermore, administration of anti-PSCA mAbs led to retardation of established orthotopic tumor growth and inhibition of metastasis to distant sites, resulting in a significant prolongation in the survival of tumor-bearing mice. These studies suggest PSCA as an attractive target for immunotherapy and demonstrate the therapeutic potential of anti-PSCA mAbs for the treatment of local and metastatic prostate cancer.

Inhibition of poly(ADP-ribose) polymerase activity is insufficient to induce tetraploidy

Poly(ADP-ribose) polymerase (PARP) knockout mice are resistant to murine models of human diseases such as cerebral and myocardial ischemia, traumatic brain injury, diabetes, Parkinsonism, endotoxic shock and arthritis, implicating PARP in the pathogenesis of these diseases. Potent selective PARP inhibitors are therefore being evaluated as novel therapeutic agents in the treatment of these diseases. Inhibition or depletion of PARP, however, increases genomic instability in cells exposed to genotoxic agents. We recently demonstrated the presence of a genomically unstable tetraploid population in PARP–/– fibroblasts and its loss after stable transfection with PARP cDNA. To elucidate whether the genomic instability is attributable to PARP deficiency or lack of PARP activity, we investigated the effects of PARP inhibition on development of tetraploidy. Immortalized wild-type and PARP–/– fibroblasts were exposed for 3 weeks to 20 µM GPI 6150 (1,11b-dihydro-[2H]benzopyrano[4,3,2-de]isoquinolin-3-one), a novel small molecule specific competitive inhibitor of PARP (Ki = 60 nM) and one of the most potent PARP inhibitors to date (IC50 = 0.15 µM). Although GPI 6150 initially decreased cell growth in wild-type cells, there was no effect on cell growth or viability after 24 h. GPI 6150 inhibited endogenous PARP activity in wild-type cells by ∼91%, to about the residual levels in PARP–/– cells. Flow cytometric analysis of unsynchronized wild-type cells exposed for 3 weeks to GPI 6150 did not induce the development of tetraploidy, suggesting that, aside from its catalytic function, PARP may play other essential roles in the maintenance of genomic stability.

Mechanism in the Sequential Control of Cell Morphology and S Phase Entry by Epidermal Growth Factor Involves Distinct MEK/ERK Activations

Cell shape plays a role in cell growth, differentiation, and death. Herein, we used the hepatocyte, a normal, highly differentiated cell characterized by a long G1 phase, to understand the mechanisms that link cell shape to growth. First, evidence was provided that the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) cascade is a key transduction pathway controlling the hepatocyte morphology. MEK2/ERK2 activation in early G1 phase did not lead to cell proliferation but induced cell shape spreading and demonstration was provided that this MAPK-dependent spreading was required for reaching G1/S transition and DNA replication. Moreover, epidermal growth factor (EGF) was found to control this morphogenic signal in addition to its mitogenic effect. Thus, blockade of cell spreading by cytochalasin D or PD98059 treatment resulted in inhibition of EGF-dependent DNA replication. Our data led us to assess the first third of G1, is exclusively devoted to the growth factor-dependent morphogenic events, whereas the mitogenic signal occured at only approximately mid-G1 phase. Moreover, these two growth factor-related sequential signaling events involved successively activation of MEK2-ERK2 and then MEK1/2-ERK1/2 isoforms. In addition, we demonstrated that inhibition of extracellular matrix receptor, such as integrin β1 subunit, leads to cell arrest in G1, whereas EGF was found to up-regulated integrin β1 and fibronectin in a MEK-ERK–dependent manner. This process in relation to cytoskeletal reorganization could induce hepatocyte spreading, making them permissive for DNA replication. Our results provide new insight into the mechanisms by which a growth factor can temporally control dual morphogenic and mitogenic signals during the G1 phase.

Change in Apoplastic Aluminum during the Initial Growth Response to Aluminum by Roots of a Tolerant Maize Variety1

Root elongation, hematoxylin staining, and changes in the ultrastructure of root-tip cells of an Al-tolerant maize variety (Zea mays L. C 525 M) exposed to nutrient solutions with 20 μm Al (2.1 μm Al3+ activity) for 0, 4, and 24 h were investigated in relation to the subcellular distribution of Al using scanning transmission electron microscopy and energy-dispersive x-ray microanalysis on samples fixed by different methods. Inhibition of root-elongation rates, hematoxylin staining, cell wall thickening, and disturbance of the distribution of pyroantimoniate-stainable cations, mainly Ca, was observed only after 4 and not after 24 h of exposure to Al. The occurrence of these transient, toxic Al effects on root elongation and in cell walls was accompanied by the presence of solid Al-P deposits in the walls. Whereas no Al was detectable in cell walls after 24 h, an increase of vacuolar Al was observed after 4 h of exposure. After 24 h, a higher amount of electron-dense deposits containing Al and P or Si was observed in the vacuoles. These results indicate that in this tropical maize variety, tolerance mechanisms that cause a change in apoplastic Al must be active. Our data support the hypothesis that in Al-tolerant plants, Al can rapidly cross the plasma membrane; these data clearly contradict the former conclusions that Al mainly accumulates in the apoplast and enters the symplast only after severe cell damage has occurred.

Arachidonic Acid Alters Tomato HMG Expression and Fruit Growth and Induces 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase-Independent Lycopene Accumulation1

Regulation of isoprenoid end-product synthesis required for normal growth and development in plants is not well understood. To investigate the extent to which specific genes for the enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) are involved in end-product regulation, we manipulated expression of the HMG1 and HMG2 genes in tomato (Lycopersicon esculentum) fruit using arachidonic acid (AA). In developing young fruit AA blocked fruit growth, inhibited HMG1, and activated HMG2 expression. These results are consistent with other reports indicating that HMG1 expression is closely correlated with growth processes requiring phytosterol production. In mature-green fruit AA strongly induced the expression of HMG2, PSY1 (the gene for phytoene synthase), and lycopene accumulation before the normal onset of carotenoid synthesis and ripening. The induction of lycopene synthesis was not blocked by inhibition of HMGR activity using mevinolin, suggesting that cytoplasmic HMGR is not required for carotenoid synthesis. Our results are consistent with the function of an alternative plastid isoprenoid pathway (the Rohmer pathway) that appears to direct the production of carotenoids during tomato fruit ripening.