Oncoprotein signalling and mitosis

Stress activated p38 MAPK regulates cell cycle via AP-1 factors in areca extract exposed human lung epithelial cells

Areca nut chewing habits are associated with several oral manifestations like leukoplakia, submucous fibrosis and oral squamous cell carcinoma. Although numerous evidence on areca toxicity is known but the mechanistic pathway of disease causation is to be studied. Aqueous areca nut extract treated A549 cells showed reduced cell viability by 48 h with IC₅₀ value of 0.50%. The toxic nature of areca nut induced the production of reactive oxygen species with decreased anti-oxidant glutathione S transferase levels lead to altered redox homeostasis. PCR studies showed decreased mRNA levels of Jun and Fos AP-1 subunits on extract treatment by 48 h. The protein levels of PCNA, CDK4, RB, p53, c-Jun and c-Fos were found to be downregulated with upregulated CDK inhibitor p21 on extract treatment as compared to control. Results of FACS analysis further confirm G₁/S phase cell cycle arrest on areca nut extract exposure. The regulation of downstream AP-1 subunits by MAPKs was studied by using specific inhibitors of ERK, JNK and p38 along with areca nut extract. Results showed the redox activation of MAP kinases down regulated the mRNA levels of AP-1 subunits in aqueous areca nut extract treated cells. Hence the present study aids in elucidating the role of MAP kinases in regulating the AP-1 subunits and their implications on target genes that are involved regulation of various cellular processes. Further, it would help in understanding the mechanistic aspects of the diseased state which may facilitate in designing of new therapeutic modalities that could help in cancer management.

The involvement of MCT-1 oncoprotein in inducing mitotic catastrophe and nuclear abnormalities

Centrosome amplification and chromosome abnormality are frequently identified in neoplasia and tumorigenesis. However, the mechanisms underlying these defects remain unclear. We here identify that MCT-1 is a centrosomal oncoprotein involved in mitosis. Knockdown of MCT-1 protein results in intercellular bridging, chromosome mis-congregation, cytokinesis delay, and mitotic death. Introduction of MCT-1 oncogene into the p53 deficient cells (MCT-1-p53), the mitotic checkpoint kinases and proteins are deregulated synergistically. These biochemical alterations are accompanied with increased frequencies of cytokinesis failure, multi-nucleation, and centrosome amplification in subsequent cell cycle. As a result, the incidences of polyploidy and aneuploidy are progressively induced by prolonged cell cultivation or further promoted by sustained spindle damage on MCT-1-p53 background. These data show that the oncoprotein perturbs centrosome structure and mitotic progression, which provide the molecular aspect of chromsomal abnormality in vitro and the information for understanding the stepwise progression of tumors under oncogenic stress.

Antiproliferative and pro-apoptotic effects afforded by novel Src-kinase inhibitors in human neuroblastoma cells

Background

Neuroblastoma (NB) is the second most common solid malignancy of childhood that usually undergoes rapid progression with a poor prognosis upon metastasis. The Src-family tyrosine kinases (SFKs) are a group of proteins involved in cancer development and invasiveness that seem to play an important role in the NB carcinogenesis.

Methods

To determine cell proliferation, the growth rate was evaluated by both MTT test and cells counted. Analysis of DNA content was performed for the evaluation of the cell cycle and apoptosis. To characterize the mechanisms underlying the antiproliferative effects induced by SI 34, a novel pyrazolo-pyrimidine derivative provided with Src inhibitory activity, the involvement of some cellular pathways that are important for cell proliferation and survival was investigated by western blot assays. In particular, the contribution of cyclins, Src and ERK were examined. Finally, experiments of cell adhesion and invasiveness were performed.

Results

Treatment of SH-SY5Y human NB cells and CHP100 human neuroepithelioma (NE) cultures with three novel pyrazolo[3,4-d]pyrimidine derivatives, namely SI 34, SI 35 and SI 83, inhibits the cell proliferation in a time and concentration-dependent manner. The maximal effect was obtained after 72 hours incubation with SI 34 10 μM. Fluorescence microscopy experiments, flow cytometry analysis and determination of caspase-3 activity by fluorimetric assays showed that SI 34 induced SH-SY5Y apoptosis. Moreover, SI 34 determined cell cycle arrest at the G0/G1 phase, paralleled by a decreased expression of cyclin D1. Furthermore, our data indicate that SI 34 reduces the SH-SY5Y cells adhesion and invasiveness. Evidence that SI 34 inhibits the Src and the ERK-phosphorylation, suggests the mechanism through which it exerts its effects in SH-SY5Y cells.

Conclusions

Our study shows the ability of this pyrazolo-pyrimidine Src inhibitor in reducing the growth and the invasiveness of human NB cells, suggesting a promising role as novel drug in the treatment of neuroblastoma.

Toggle switches, pulses and oscillations are intrinsic properties of the Src activation/deactivation cycle

Src-family kinases (SFKs) play a pivotal role in growth factor signaling, mitosis, cell motility and invasiveness. In their basal state, SFKs maintain a closed autoinhibited conformation, where the Src homology 2 domain interacts with an inhibitory phosphotyrosine in the C-terminus. Activation involves dephosphorylation of this inhibitory phosphotyrosine, followed by intermolecular autophosphorylation of a specific tyrosine residue in the activation loop. The spatiotemporal dynamics of SFK activation controls cell behavior, yet these dynamics remain largely uninvestigated. In the present study, we show that the basic properties of the Src activation/deactivation cycle can bring about complex signaling dynamics, including oscillations, toggle switches and excitable behavior. These intricate dynamics do not require imposed external feedback loops and occur at constant activities of Src inhibitors and activators, such as C-terminal Src kinase and receptor-type protein tyrosine phosphatases. We demonstrate that C-terminal Src kinase and receptor-type protein tyrosine phosphatase underexpression or their simultaneous overexpression can transform Src response patterns into oscillatory or bistable responses, respectively. Similarly, Src overexpression leads to dysregulation of Src activity, promoting sustained self-perpetuating oscillations. Distinct types of responses can allow SFKs to trigger different cell-fate decisions, where cellular outcomes are determined by the stimulation threshold and history. Our mathematical model helps to understand the puzzling experimental observations and suggests conditions where these different kinetic behaviors of SFKs can be tested experimentally.

Areca nut-induced micronuclei and cytokinesis failure in Chinese hamster ovary cells is related to reactive oxygen species production and actin filament deregulation

Epidemiological studies have shown a strong association between environmental exposure to betel quid (BQ) and oral cancer. Areca nut (AN), an ingredient of BQ, contains genotoxic and mutagenic compounds. In this study, we found that AN extract (ANE) inhibited the growth of Chinese hamster ovary cells (CHO-K1) in a dose- and time-dependent manner. Intracellular reactive oxygen species (ROS) levels and micronuclei (MN) frequency were significantly increased following ANE treatment in CHO-K1 cells. Addition of catalase markedly inhibited ANE-induced MN formation, indicating that ANE-induced genotoxicity was correlated with intracellular H(2)O(2). Incubation of CHO-K1 cells with ANE (400-800 microg/ml) for 24 hr caused G2/M arrest, and prolonged exposure to ANE (800 microg/ml) significantly induced cell death. Surprisingly, ANE itself caused cytokinesis failure and subsequent increase in binucleated cell formation. Coexposure to catalase (2,000 U/ml) and ANE (800 microg/ml) reduced the generation of binucleated cells, indicating that ANE-induced cytokinesis failure was associated with oxidative stress. Following prolonged exposure to ANE, an accumulation of hyperploid/aneuploid cells concomitant with bi-, micro- or multinucleated cells was found. In summary, our results demonstrate that ANE exposure to CHO-K1 cells caused increased MN frequency, G2/M arrest, cytokinesis failure, and an accumulation of hyperploid/aneuploid cells. These events are associated with an increase in intracellular H(2)O(2) level and actin filament disorganization.

The protein tyrosine phosphatase SHP-2 is required for EGFRvIII oncogenic transformation in human glioblastoma cells

Oncogenic EGFRvIII is a naturally occurring oncoprotein and is expressed in about 40-50% of human glioblastomas, particularly those that arise de novo. To understand the molecular mechanisms by which this oncoprotein alters transforming phenotypes, and since our previous work indicated that SHP-2 protein tyrosine phosphatase activity modulated EGFRvIII activation and downstream signaling, we examined whether SHP-2 plays a role in EGFRvIII-induced oncogenesis by using both PTEN-deficient U87MG.EGFRvIII and PTEN-intact LN229.EGFRvIII cells. Inhibition of SHP-2 expression by Shp-2 siRNA inhibited cell growth, transformation and altered morphology of these EGFRvIII transformed GBM cells. Ectopic expression of a PTPase-inactive form of SHP-2, SHP-2 C459S, but not its wild-type SHP-2 or either of two SH2 domain mutants, abrogated transformation of EGFRvIII-expressing glioblastomas in soft agar and in nude mice. SHP-2 C459S cells grew slower and exhibited a more flattened morphology with more organized actin stress fibers under both full growth and low serum conditions. Furthermore, shp-2+/- and -/- mouse embryonic fibroblasts (MEFs) could not be transformed by EGFRvIII while shp-2+/+ MEFs displayed a fully transformed phenotype upon introduction of EGFRvIII, again indicating a requirement for functional SHP-2 in EGFRvIII transformation. Moreover, the SHP-2 PTPase activity inhibitor NSC-87877 inhibited endogenous SHP-2 activity, Erk phosphorylation and transformation in both GBM cell lines. EGFRvIII expression recruited SHP-2 to the receptor complex to transduce signals and also increased SHP-2 phosphorylation at Tyr542. Inhibition of EGFRvIII-induced cell growth and transformation by SHP-2 C459S or shp-2 siRNA was mediated by its ability to block cell cycle progression at different phases in these GBM cells. These data indicate that differential activation of SHP-2 phosphorylation at Tyr542 in these two GBM cell lines likely results in increased different PTPase activity and distinct mechanisms of cell cycle progression and SHP-2, in particular its PTPase activity, plays a critical role in EGFRvIII-mediated transformation.

Expression of stathmin/op18 as a significant prognostic factor for cervical carcinoma patients

PURPOSE

Stathmin (Oncoprotein18), a ubiquitous and highly conserved 19-kDa cytosolic phosphoprotein, has been reported to play a critical role in mitosis and possibly other cellular processes, which is associated with tumor carcinogenesis and development. The purpose of this study was to examine the involvement of stathmin in human cervical carcinogenesis and to evaluate its prognostic significance in human cervical carcinoma.

METHODS

Using semiquantitative RT-PCR and Western blotting, we detected the expression of stathmin in human normal cervical epithelial cell line, immortalized cervical epithelial cell lines, and cervical carcinoma cell lines. Additionally, we also detected the expression of stathmin protein in 15 cases of cervical carcinoma tissues and adjacent non-carcinomous margin tissues. Furthermore, specimens from 148 patients with different grade and stage cervical carcinoma were investigated by immunohistochemistry for stathmin expression. Correlations between the expression of stathmin and various clinicopathological factors were studied, while statistical analyses were performed to evaluate prognostic and diagnostic associations.

RESULTS

The levels of stathmin mRNA and protein expression were significantly higher in cervical carcinoma cells and immortalized cervical epithelial cells than in normal cervical epithelial cells (P < 0.05). Moreover, Western blotting revealed high stathmin protein expression in 73.3% (11/15) cervical carcinoma tissues, while stathmin were overexpressed in tumor tissues as compared with adjacent non-carcinomous margin samples (P = 0.017). In addition, immunohistochemical staining revealed stathmin immunoreactivity in 81.1% (120/148) of cervical carcinoma tissues and high stathmin expression was significantly correlated with clinical stage (P = 0.006), T classification (P = 0.012), regional lymph node metastasis (P = 0.005) and hematogenous metastasis (P = 0.021). Kaplan-Meier analysis showed that high stathmin positivity was significantly associated with a shorter survival time (P < 0.001). Clinical stage (P = 0.0022), T classification (P = 0.0035), regional lymph node (P = 0.0008) or hematogenous metastasis (P = 0.0015) were also associated with survival time. Furthermore, by Cox multivariate analysis, only lymph node (P = 0.0052) or hematogenous metastasis (P = 0.0046) maintained their significance as independent prognostic factors, although stathmin was not an independent prognostic factor (risk ratio: 1.45; P = 0.0624).

CONCLUSIONS

Stathmin expression correlates with cervical carcinogenesis and tumor progression. This molecule is a valuable prognostic marker in patients with cervical carcinoma.

Measurements of stathmin-tubulin interaction in solution

Stathmin is an important phosphorylation-controlled regulator of microtubule dynamics and plays a crucial role in cell division and cell proliferation. In its non-phosphorylated form, stathmin is the protein that interacts the most tightly with tubulin, in a 2:1 tubulin-stathmin (T2S) complex that does not participate in microtubule assembly. The importance of stathmin at different levels of phosphorylation in different steps of mitosis This article is a short overview of the different methods that have been or could be used to monitor the kinetic and thermodynamic parameters of tubulin-stathmin interaction and to evaluate the effects of phosphorylation. The author has tried to emphasize how hydrodynamic and spectroscopic methods measuring direct binding of stathmin to tubulin can be complemented by methods that make use of linked functions, measuring how the change in a functional property of tubulin upon binding stathmin provides information on binding parameters.

Inhibiting proliferation and enhancing chemosensitivity to taxanes in osteosarcoma cells by RNA interference-mediated downregulation of stathmin expression

Stathmin (Oncoprotein18), a signal transduction regulatory factor, plays an important role in cell division and malignant tumor development. Stathmin is a ubiquitous intracellular phosphoprotein that is overexpressed in a variety of human malignancies, including osteosarcoma. To investigate the potential use of stathmin as a therapeutic target for human osteosarcomas, we employed RNA interference [small interfering RNA (siRNA)] to reduce stathmin expression in human osteosarcoma cell lines and analyzed their phenotypic changes. Results showed that the downregulation of stathmin expression in human osteosarcoma cells significantly inhibited cell proliferation in vitro and tumorigenicity in vivo. The specific downregulation induced cell arrest in the G(2)/M phase of cell cycle and eventually apoptotic cell death. Taxanes are a group of effective chemotherapeutic agents whose activity is mediated through stabilization of the microtubules of the mitotic spindle. In the present study, we also observed a synergistic enhancement of the cytotoxicity effect by combination use of taxanes and RNA interference-mediated stathmin downregulation. All these experimental data indicate that stathmin downregulation can lead to potent antitumor activity and chemosensitizing activity to taxanes in human osteosarcomas.

Overexpression of stathmin in oral squamous-cell carcinoma: correlation with tumour progression and poor prognosis

Stathmin is an intracellular phosphoprotein that is overexpressed in a number of human malignancies. Our previous study using proteomic profiling showed that significant upregulation of stathmin occurs in oral squamous-cell carcinoma (OSCC)-derived cell lines. In the current study, to determine the potential involvement of stathmin in OSCC, we evaluated the state of stathmin protein and mRNA expression in OSCC-derived cell lines and human primary OSCCs. A significant increase in stathmin expression was observed in all OSCC-derived cell lines examined compared to human normal oral keratinocytes. In immunohistochemistry, 65% of the OSCCs were positive for stathmin, and no immunoreaction was observed in corresponding normal tissues. Real-time quantitative reverse transcriptase-polymerase chain reaction data were consistent with the protein expression status. Moreover, stathmin expression status was correlated with the TNM stage grading. Furthermore, we found a statistical correlation between the protein expression status and disease-free survival (P=0.029). These results suggest that expression of stathmin could contribute to cancer progression/prognosis, and that stathmin may have potential as a biomarker and a therapeutic target for OSCC.

Dopamine regulation of amiloride-sensitive sodium channels in lung cells

Dopamine increases lung fluid clearance. This is partly due to activation of basolateral Na-K-ATPase. However, activation of Na-K-ATPase by itself is unlikely to produce large changes in transepithelial transport. Therefore, we examined apical and basolateral dopamine's effect on apical, highly selective sodium channels [epithelial sodium channels (ENaC)] in monolayers of an alveolar type 2 cell line (L2). Dopamine increased channel open probability (P(o)) without changing the unitary current. The D(1) receptor blocker SCH-23390 blocked the dopamine effect, but the D(2) receptor blocker sulpiride did not. The dopamine-mediated increase in ENaC activity was not a secondary effect of dopamine stimulation of Na-K-ATPase, since ouabain applied to the basolateral surface to block the activity of Na-K-ATPase did not alter dopamine-mediated ENaC activity. Protein kinase A (PKA) was not responsible for dopamine's effect since a PKA inhibitor, H89, did not reduce dopamine's effect. However, cpt-2-O-Me-cAMP, which selectively binds and activates EPAC (exchange protein activated by cAMP) but not PKA, increased ENaC P(o). An Src inhibitor, PP2, and the phosphatidylinositol-3-kinase inhibitor, LY-294002, blocked dopamine's effect on ENaC. In addition, an MEK blocker, U0126, an inhibitor of phospholipase A(2), and a protein phosphatase inhibitor also blocked the effect of dopamine on ENaC P(o). Finally, since the cAMP-EPAC-Rap1 pathway also activates DARPP32 (32-kDa dopamine response protein phosphatase), we confirmed that dopamine phosphorylates DARPP32, and okadaic acid, which blocks phosphatases (DARPP32), also blocks dopamine's effect. In summary, dopamine increases ENaC activity by a cAMP-mediated alternative signaling pathway involving EPAC and Rap1, signaling molecules usually associated with growth-factor-activated receptors.

Progressive loss of Syk and abnormal proliferation in breast cancer cells

The tumor suppressor gene Syk tyrosine kinase is absent or reduced in invasive breast cancer tissues and cell lines; its loss in breast tissues is linked to poor prognosis and metastasis. Also, evidence shows that in vitro Syk is involved in regulating proliferation. Here, we show by in situ hybridization on breast tissue sections that the loss of Syk expression is progressive during tumor development. Strikingly, Syk is already partially lost in normal epithelial tissue adjacent to the cancer lesion. In vivo, cell proliferation (as measured by the proliferative index Ki67) increased from normal to ductal carcinoma in situ to invasive, whereas Syk in situ staining in the same tissues decreased. In vitro, the presence of Syk was associated with reduced cell proliferation in an epidermal growth factor receptor-overexpressing breast cancer cell line, BT549, whereas changes in apoptosis were undetected. Concomitantly, the kinase activity of the proto-oncogene Src was reduced by approximately 30%. A 5-fold increase in abnormal mitoses was observed in the Syk-transfected cells compared with vector control. We propose that Syk is involved in the regulation of cell proliferation, possibly by controlling mechanisms of mitosis and cytokinesis via Src signal transduction pathway(s). Because of its progressive and early loss during tumor onset and development, monitoring of Syk loss in breast epithelial cells by noninvasive techniques such as ductal lavage may be a powerful tool for screening purposes.

MEK inhibition and phosphorylation of serine 4 on B23 are two coincident events in mitosis

Previous studies have shown that activation of the Raf/MEK/ERK pathway is necessary for G2/M transition. However, as for the activation state of MEK in mitosis the conclusion is not consistent. Here we show that MEK is inhibited in mitosis. In addition, we identify a multifunctional protein named B23 that strongly cross-reacts with a phospho-MEK antibody in mitotic cells. Sequence homology between the N-terminus surrounding Ser 4 of B23 and the Raf phosphorylation site on MEK suggests a mechanism for cross-reaction of the antibody. Thus, mutation of Ser 4 to alanine abolishes cross-reactivity between B23 and the phospho-MEK antibody. Our findings may explain the discrepancy of results obtained with the use of phospho-MEK antibody regarding the activation state of MEK in mitosis.

Growth and EGFR Regulation in Breast Cancer Cells by Vitamin D and Retinoid Compounds

The effect of 1,25-dihydroxyvitamin D(3), analog C (1,25-(OH)(2)-16-en-23-yn-26,27-F(6)-vitamin D(3)), 9-cis retinoic acid, and all-trans retinoic acid on the growth and expression of EGFR in MCF7, T47D, BT474, and BT549 breast cancer cells was examined. Significant growth inhibition was noted in MCF7, T47D, and BT474 cells by 8 days of treatment, while BT549 cells showed none. MCF7, T47D, and BT549 cells treated with 1,25-dihydroxyvitamin D(3) demonstrated a 50% decrease in EGFR mRNA within 2 h which was sustained to 72 h, while BT474 cells demonstrated a 200-500% increase. EGFR protein levels correlated with these mRNA changes in BT474 and BT549 cells. Measurement of mRNA stability in vitamin D treated BT474 cells indicated that there was no change in EGFR mRNA half-life. Transfection of an EGFR promoter containing reporter plasmid demonstrated vitamin D induced changes in reporter gene activity that paralleled the changes observed in EGFR mRNA and protein. Electrophoretic mobility shift assays using a putative vitamin D response element within this region of the EGFR promoter demonstrated specific VDR binding. These results indicate that the vitamin D effect on EGFR expression in breast cancer cells has a transcriptional component likely mediated through a vitamin D responsive promoter sequence. They also suggest that growth inhibition and EGFR down-regulation by vitamin D and retinoids may be related events in some breast cancer cells, but not in all.

Carbazolequinone induction of caspase-dependent cell death in Src-overexpressing cells

We previously reported that RSV-transformed quail neuroretina cells (QNR-ts68) were highly resistant to apoptosis provoked by serum withdrawal, and that this property was due to v-Src kinase activity. The present study investigates the cytotoxic effect and the functional mechanism of carbazolequinone-mediated cell death in this system. QNR-ts68 cells were subjected to carbazolequinone treatment and both growth inhibition and cell death induction were examined using formazan assays. Cell death mechanism (both apoptosis and necrosis) was confirmed through phosphatidyl serine exposure and propidium iodide incorporation. Furthermore, the effect of active carbazolequinone was inhibited by a pan caspase inhibitor. Cytofluorimetric and immunofluorescence data demonstrated the activation of caspase-3 and the involvement of mitochondria. Therefore, this study clearly indicates that carbazolequinones could induce cell death in transformed cells displaying high levels of antiapoptotic tyrosine kinase activity. Further investigations would be necessary to elucidate the mechanisms by which these carbazolequinones act as antitumor agents.

Mechanisms of action of the novel sulfonamide anticancer agent E7070 on cell cycle progression in human non-small cell lung cancer cells

E7070 is a novel sulfonamide antitumor agent that exhibits potent antitumor activity in vitro and in vivo. This compound affects cell cycle progression in human tumor cells. To elucidate the mechanisms by which E7070 inhibits tumor cell growth, we established and characterized an E7070-resistant subline, A549/ER, from a human non-small cell lung cancer cell line A549. Flow cytometric analyses demonstrated an increase in G0/G1 and a decrease in S phase populations in cells treated with E7070 at 20 or 100 microg/ml for 24 h. Longer exposure to E7070, i.e. 48 and 72 h, increased the G2/M phase fraction in A549 cells. These inhibitory actions of E7070 on cell cycle progression were not observed in A549/ER cells. E7070 inhibited the phosphorylation of pRb, decreased expressions of cyclin A, B1, CDK2, and CDC2 proteins, and suppressed CDK2 catalytic activity with the induction of p53 and p21 proteins in A549 cells but not in A549/ER cells. Taken together, these results suggest that E7070 exerts its antitumor effects by disturbing the cell cycle at multiple points, including both the G1/S and the G2/M transition, in human lung cancer cells.

Active erk regulates microtubule stability in H-ras-transformed cells

Increasing evidence suggests that activated erk regulates cell functions, at least in part, by mechanisms that do not require gene transcription. Here we show that the map kinase, erk, decorates microtubules (MTs) and mitotic spindles in both parental and mutant active ras-transfected 10T1/2 fibroblasts and MCF10A breast epithelial cells. Approximately 20% of total cellular erk decorated MTs in both cell lines. A greater proportion of activated erk was associated with MTs in the presence of mutant active H-ras than in parental cells. Activation of erk by the ras pathway coincided with a decrease in the stability of MT, as detected by a stability marker. The MKK1 inhibitor, PD98059 and transfection of a dominant negative MKK1 blocked ras-induced instability of MTs but did not modify the association of erk with MTs or affect MT stability of the parental cells. These results indicate that the subset of active erk kinase that associates with MTs contributes to their instability in the presence of a mutant active ras. The MT-associated subset of active erk likely contributes to the enhanced invasive and proliferative abilities of cells containing mutant active H-ras.

Perforant path lesion induces up-regulation of stathmin messenger RNA, but not SCG10 messenger RNA, in the adult rat hippocampus

In this study, we performed in situ hybridization analysis of the expression pattern of two growth-associated proteins, stathmin and SCG10, in the hippocampus after unilateral lesion of the perforant pathway, the main excitatory input from the entorhinal cortex to the hippocampus. Stathmin is one of the major neural-enriched cytosolic phosphoproteins and a potential target of cyclic-AMP-dependent kinases [Jin L. W. et al. (1996) Neurobiol. Aging 17, 331-341; Leighton I. A. et al. (1993) Molec. Cell Biochem. 127/128, 151-156]. Three days after the lesion, stathmin messenger RNA was up-regulated ipsilaterally in the hilus, in the granule cell layer of the dentate gyrus and in the pyramidal cell layer of the CA1 region. Simultaneously, the hilar region of the contralateral dentate gyrus showed an increased stathmin messenger RNA expression. This altered expression pattern was observed until 15 days after lesion. Stathmin messenger RNA expression returned to a normal level until 21 days after lesion in all regions analysed. SCG10, a membrane-bound neuronal growth-associated protein belonging to the SCG10/stathmin gene family, did not show any alteration of messenger RNA expression after perforant path lesion. The temporal changes of stathmin messenger RNA expression in the ipsilateral hippocampus correspond well to the process of reactive synaptogenesis. The enhanced messenger RNA expression in the hilar region of the contralateral dentate gyrus might suggest a role in neurite elongation, since this region is the origin of commissural fibres involved in the sprouting response in the deafferented hippocampus. The present study provides evidence that the induction of specific growth-associated proteins is differentially regulated in the hippocampus.

Oncoprotein 18 overexpression increases the sensitivity to vindesine in the human lung carcinoma cells

BACKGROUND

Oncoprotein 18 (op18) was first isolated as a molecule overexpressed in several malignant cells, suggesting a function of op18 in malignant processes, such as differentiation in hematologic malignancies, op18 also was found to enhance microtubule deassembly in the cells. Antimitotic agents that bind to tubulin have been used for chemotherapy to treat solid tumors, such as lung carcinoma. Vinca alkaloids, such as vindesine and vincristine, have commonly been used for chemotherapy of nonsmall cell lung carcinoma. The authors examined the role of op18 in the sensitivity of human lung carcinoma cells to antimitotic agents.

METHODS

Expression of op18 mRNA was detected in all 17 lung carcinoma cell lines tested by Northern blotting. Oncoprotein 18 cDNA was transfected to SBC-3 human lung carcinoma cells, and the stable transfectants, SBC-3/op1-3, were isolated. The sensitivity of these transfectants against antimitotic agents were examined by the MTT assay in vitro. Cell cycle distribution of the transfectants on DNA histogram was analyzed by flow cytometry.

RESULTS

Oncoprotein 18-transfected cells showed higher sensitivity to vindesine and vincristine, but not to taxanes. Vindesine-exposure increased the G2/M population of the cell cycle in the Mock transfectants, but not in SBC-3/op1, suggesting that the cell cycle dynamics were altered by op18 expression in SBC-3/op1.

CONCLUSION

Oncoprotein 18 expression is associated with lung carcinoma cell sensitivity to vindesine and may be able to serve as a surrogate marker for the chemosensitivity to Vinca alkaloids in human lung carcinomas.

Neoplastic transformation and tumorigenesis associated with sam68 protein deficiency in cultured murine fibroblasts

Sam68 is a multimeric 68-kDa RNA-binding nuclear protein of unknown function that interacts with, and is tyrosine-phosphorylated by, the oncogenic protein Src during mitosis. Random homozygous knock-out (RHKO) is a retroviral-based antisense RNA strategy that can identify chromosomal genes whose functional disablement leads to reversible tumorigenic capabilities. Here we report that RHKO-induced Sam68 deficiency results in neoplastic transformation of murine NIH3T3 fibroblasts. Whereas simple haploinsufficiency of Sam68 produced by insertion mutagenesis in a single chromosomal allele did not detectably affect cell growth, reduction of Sam68 protein to <25% of the wild type level was associated with anchorage-independent growth, defective contact inhibition, and the ability to form metastatic tumors in nude mice. These properties were reversed by cessation of RHKO inactivation. Our findings, which indicate that the Sam68 protein level can prominently affect cell proliferation, implicate Sam68 function in tumorigenesis. Consistent with these results is evidence that cells undergoing mitosis show a dramatic reduction in the level of Sam68 protein.

Nuclear targeting determinants of the far upstream element binding protein, a c-myc transcription factor

FUSE binding protein (FBP) binds in vivo and in vitro with the single-stranded far upstream element (FUSE) upstream of the c-myc gene. In addition to its transcriptional role, FBP and its closely related siblings FBP2 (KSRP) and FBP3 have been reported to bind RNA and participate in various steps of RNA processing, transport or catabolism. To perform these diverse functions, FBP must traffic to different nuclear sites. To identify determinants of nuclear localization, full-length FBP or fragments thereof were fused to green fluorescent protein. Fluorescent-FBP localized in the nucleus in three patterns, diffuse, dots and spots. Each pattern was conferred by a distinct nuclear localization signal (NLS): a classical bipartite NLS in the N-terminal and two non-canonical signals, an alpha-helix in the third KH-motif of the nucleic acid binding domain and a tyrosine-rich motif in the C-terminal transcription activation domain. Upon treatment with the transcription inhibitor actinomycin D, FBP completely re-localized into dots, but did not exit from the nucleus. This is in contrast to many general RNA-binding proteins, which shuttle from the nucleus upon treatment with actinomycin D. Furthermore, FBP co-localized with transcription sites and with the general transcription factor TFIIH, but not with the splicing factor SC-35. Taken together, these data reveal complex intranuclear trafficking of FBP and support a transcriptional role for this protein.

Raf-1/MEK/MAPK pathway is necessary for the G2/M transition induced by nocodazole

The dynamic balance between polymerization and depolymerization of microtubules is critical for cells to enter and exit mitosis, and drugs that disrupt this balance, such as taxol, colchicine, and nocodazole, arrest the cell cycle in mitosis. Although the Raf/MEK/MAPK pathway can be activated by these drugs, its role in mitosis has not been addressed. Here, we characterize activation of Raf/MEK/MAPK by nocodazole when mitosis is induced. We find that at early time points (up to 3 h) in nocodazole induction, Raf/MEK/MAPK is activated, and inhibition of MAPK activation by a MEK inhibitor, PD98059 or U0126, reduces the number of cells entering mitosis by creating a block at G(2). At later time points and in mitosis, activation of MEK/MAPK is severely inhibited, even though Raf-1 activity remains high and can be further increased by growth factor. This inhibition is reversed when cells are released from metaphase and enter G(0)/G(1) phase. In addition, we find that binding of Raf-1 to 14-3-3 is progressively induced by nocodazole, reaching a maximum in mitosis, and that this binding is necessary to maintain mitotic Raf-1 activity. Our present study indicates that activation of the Raf/MEK/MAPK pathway is necessary for the G(2)/M progression.

Lack of effect of TP53 status on fluorodeoxyuridine-mediated radiosensitization

Substantial evidence suggests that TP53 (also known as p53) status can influence the response of cells to chemotherapy and radiation. We wished to determine if TP53 function affected the response of cells to fluoropyrimidines and radiation, a combination used for tens of thousands of patients each year. To assess the role of TP53 in fluoropyrimidine-mediated radiosensitization, we carried out experiments using RKO parental cells (wild-type TP53) and RKO cells overexpressing mutant TP53 (which blocks TP53 function) or expressing E6 (which degrades TP53). We found that TP53 function had no effect on the ability of fluorodeoxyuridine to increase radiation sensitivity. These findings are consistent with the hypothesis that the late G(1)-phase checkpoint, which is mediated by TP53, is not crucial to radiosensitization. Rather, the ability of cells to progress in to S phase in the presence of the drug, which is independent of TP53, is more closely associated with increased radiation sensitivity.

Growth factor-dependent activation of the Ras-Raf-MEK-MAPK pathway in the human pancreatic carcinoma cell line PANC-1 carrying activated K-ras: implications for cell proliferation and cell migration

Human ductal adenocarcinoma of the pancreas frequently carry activating point mutations in the K-ras protooncogene. We have analysed the activity of the Ras-Raf-MEK-MAPK cascade in the human pancreatic carcinoma cell line PANC-1 carrying an activating K-ras mutation. Serum-starved cells and cells grown in medium with serum did not show constitutively activated c-Raf, MEK-1, or p42 MAPK. Stimulation of cells with epidermal growth factor (EGF) or fetal calf serum (FCS) resulted in activation of N-Ras, but not K-Ras, as well as activation of c-Raf, MEK-1, and p42 MAPK. Preincubation of serum-starved cells with MEK-1 inhibitor PD98059 abolished EGF- and FCS-induced MAPK activation, identifying MEK as the upstream activator of MAPK. PANC-1 cells exhibited marked serum-dependence of anchorage-dependent and -independent cell growth as well as cell migration. EGF, alone or in combination with insulin and transferrin, did not induce cell proliferation of serum-starved PANC-1 cells, indicating that activation of MAPK alone was not sufficient to induce cell proliferation. FCS-induced DNA synthesis was inhibited by 40% by the MEK-1 inhibitor. On the other hand, treatment with either FCS or EGF alone resulted in marked, MEK-dependent increase of directed cell migration. Collectively, our results show that the activating K-ras mutation in PANC-1 cells does not result in constitutively increased Raf-MEK-MAPK signaling. Signal transduction via the Ras-Raf-MEK-MAPK cascade is maintained in these cells and is required for growth factor-induced cell proliferation and directed cell migration. Oncogene (2000).

Lovastatin decreases mortality and improves liver functions in fulminant hepatic failure from 90% partial hepatectomy in rats

BACKGROUND/AIMS

Liver insufficiency occurs when the liver cannot perform critical functions such as ammonia metabolism, gluconeogenesis, or production of coagulation factors The hypothesis of this study was that decreased function of existing hepatocytes may contribute to hepatic failure, and that the function of these cells might be increased pharmacologically. Lovastatin is a 3-hydroxy-3-methylglutaryl CoA reductase inhibitor that inhibits cholesterol biosynthesis and affects the activity of some signal transduction pathways and liver transcription factors. Changes in hepatic transcription factors during liver regeneration might result in decreased liver functions, and lovastatin might prevent these changes

METHODS

Rats received 90% partial hepatectomy (90% PH), and either lovastatin or vehicle alone daily. Survival and liver functions were assessed.

RESULTS

Lovastatin increased survival to 58% (vs. 6% in controls that received 90% PH without drug), decreased the peak ammonia level to 427 microM (vs. 846 microM in controls), increased the nadir of glucose to 88 mg/dl (vs. 57 mg/dl in controls), decreased the peak prothrombin time to 23 s (vs 29 s in controls), and decreased the peak activated partial thromboplastin time to 29 s (vs. 39 s in controls). The full survival and metabolic benefits were observed when lovastatin was started at 30 min after 90% PH, but lovastatin was less efficacious when started at later times.

CONCLUSIONS

Lovastatin increases the function of existing hepatocytes and might be used to improve liver function after extensive hepatic resection.

Roles for basal and stimulated p21(Cip-1/WAF1/MDA6) expression and mitogen-activated protein kinase signaling in radiation-induced cell cycle checkpoint control in carcinoma cells

We investigated the role of the cdk inhibitor protein p21(Cip-1/WAF1/MDA6) (p21) in the ability of MAPK pathway inhibition to enhance radiation-induced apoptosis in A431 squamous carcinoma cells. In carcinoma cells, ionizing radiation (2 Gy) caused both primary (0-10 min) and secondary (90-240 min) activations of the MAPK pathway. Radiation induced p21 protein expression in A431 cells within 6 h via secondary activation of the MAPK pathway. Within 6 h, radiation weakly enhanced the proportion of cells in G(1) that were p21 and MAPK dependent, whereas the elevation of cells present in G(2)/M at this time was independent of either p21 expression or MAPK inhibition. Inhibition of the MAPK pathway increased the proportion of irradiated cells in G(2)/M phase 24-48 h after irradiation and enhanced radiation-induced apoptosis. This correlated with elevated Cdc2 tyrosine 15 phosphorylation, decreased Cdc2 activity, and decreased Cdc25C protein levels. Caffeine treatment or removal of MEK1/2 inhibitors from cells 6 h after irradiation reduced the proportion of cells present in G(2)/M phase at 24 h and abolished the ability of MAPK inhibition to potentiate radiation-induced apoptosis. These data argue that MAPK signaling plays an important role in the progression/release of cells through G(2)/M phase after radiation exposure and that an impairment of this progression/release enhances radiation-induced apoptosis. Surprisingly, the ability of irradiation/MAPK inhibition to increase the proportion of cells in G(2)/M at 24 h was found to be dependent on basal p21 expression. Transient inhibition of basal p21 expression increased the control level of apoptosis as well as the abilities of both radiation and MEK1/2 inhibitors to cause apoptosis. In addition, loss of basal p21 expression significantly reduced the capacity of MAPK inhibition to potentiate radiation-induced apoptosis. Collectively, our data argue that MAPK signaling and p21 can regulate cell cycle checkpoint control in carcinoma cells at the G(1)/S transition shortly after exposure to radiation. In contrast, inhibition of MAPK increases the proportion of irradiated cells in G(2)/M, and basal expression of p21 is required to maintain this effect. Our data suggest that basal and radiation-stimulated p21 may play different roles in regulating cell cycle progression that affect cell survival after radiation exposure.

Mitogen-activated protein kinase kinase activity is required for the G(2)/M transition of the cell cycle in mammalian fibroblasts

The mitogen-activated protein kinase (MAPK) cascade is required for mitogenesis in somatic mammalian cells and is activated by a wide variety of oncogenic stimuli. Specific roles for this signaling module in growth were dissected by inhibiting MAPK kinase 1 (MAPKK1) activity in highly synchronized NIH 3T3 cells. In addition to the known role of this kinase in cell-cycle entry from G(0), the level of MAPKK activity was observed to affect the kinetics of progression through both the G(1) and G(2) phases of the cell cycle in NIH 3T3 cells. Ectopic expression of dominant-negative forms of MAPKK1, which was previously shown to inhibit G(0)/G(1) progression, was found to also delay progression of cells through G(2). In addition, treatment of cells with the specific MAPKK inhibitor PD 98059 during a synchronous S phase arrested the cells in the following G(2) phase. These data demonstrate a novel role for the MAPK cascade in progression from G(2) into mitosis in NIH 3T3 cells.

Fyn Membrane Localization Is Necessary to Induce the Constitutive Tyrosine Phosphorylation of Sam68 in the Nucleus of T Lymphocytes

A close relationship between Sam68, a tyrosine and proline-rich RNA-binding protein, and Src protein tyrosine kinases (PTK) has already been established, also in T lymphocytes. A constitutive phosphorylation of the molecule has also been documented in various transformed T cells, which probably reflects an increased expression of PTK of the Src family. Using the hybridoma T cell line, T8.1, or Jurkat T cells, we investigated the respective contribution of the two Src kinases Fyn and Lck, expressed in T cells, in this phenomenon. By overexpressing the two proteins, we show that the constitutive phosphorylation of Sam68 in vivo directly correlates with cellular Fyn levels, but not with Lck expression, despite the capacity of the PTK to strongly phosphorylate the molecule in vitro. Overexpressed Fyn is mainly localized at the cell membrane. We find that Sam68 phosphorylation, including in the nuclear fraction in which the molecule is predominantly expressed, is lost with a delocalized Fyn mutant deleted of its N-terminal membrane-anchoring domain. Finally, we demonstrate, using a construct encoding a Sam68 molecule without its nuclear localization signal, that nuclear expression of Sam68 is not required for phosphorylation. We conclude that the constitutive phosphorylation of Sam68 in T cells is a Fyn-dependent process occurring in a cell-membrane compartment from which phospho-Sam68 molecules can thereafter accumulate into the nucleus.

Inhibition of the epidermal growth factor receptor family of tyrosine kinases as an approach to cancer chemotherapy: progression from reversible to irreversible inhibitors

The rationale to inhibit the epidermal growth factor receptor (EGFR) tyrosine kinase family as an approach to cancer chemotherapy has continued to grow stronger over the last 10 years. Both preclinical and clinical data strongly support the involvement of these receptors in the formation and progression of human cancers, as well as establish a high correlation in cancer patients between receptor/ ligand expression and poor prognosis. During the past 4 years, significant progress has been made in the area of EGFR tyrosine kinase inhibitors, and new structural classes have emerged that exhibit enormous improvements with regard to potency, specificity, and in vitro and in vivo activity. Very recently, further advancements in this field have been made whereby very specific, irreversible inhibitors of the EGFR family have been synthesized that provide unique pharmacological properties and exceptional efficacy. The in vivo performance of these modern kinase inhibitors has improved to the point where several compounds are either in clinical trials or very near to that point in their development. This review will briefly address the justification for targeting the EGFR family for cancer therapeutics, and then will highlight some of the more promising kinase inhibitors that are in development.

Characterization of Raf-1 activation in mitosis

We have used site-directed mutagenesis to explore the mechanisms underlying Raf-1 activation in mitosis, and we have excluded most previously characterized activating interactions. Our results indicate that the primary locus of activation lies in the carboxyl-half of the molecule, although the extent of activation can be influenced by the amino-proximal region, particularly by the Raf-1 zinc finger. We also found that Raf-1 is hyperphosphorylated in mitosis at multiple sites within residues 283-302 and that these hyperphosphorylations are not required for activation. In addition, neither Mek1 nor Mek2 are stably activated in coordination with Raf-1 in nocodazole-arrested cells. Overall, the data suggest that the mechanism(s) responsible for activating Raf-1 during mitosis, and the subsequent downstream effects, are distinct from those involved in growth factor stimulation.

Active MAP kinase in mitosis: localization at kinetochores and association with the motor protein CENP-E

To investigate possible involvement of the mitogen-activated protein (MAP) kinases ERK1 and ERK2 (extracellular signal-regulated kinases) in somatic cell mitosis, we have used indirect immunofluorescence with a highly specific phospho-MAP kinase antibody and found that a portion of the active MAP kinase is localized at kinetochores, asters, and the midbody during mitosis. Although the aster labeling was constant from the time of nuclear envelope breakdown, the kinetochore labeling first appeared at early prometaphase, started to fade during chromosome congression, and then disappeared at midanaphase. At telophase, active MAP kinase localized at the midbody. Based on colocalization and the presence of a MAP kinase consensus phosphorylation site, we identified the kinetochore motor protein CENP-E as a candidate mitotic substrate for MAP kinase. CENP-E was phosphorylated in vitro by MAP kinase on sites that are known to regulate its interactions with microtubules and was found to associate in vivo preferentially with the active MAP kinase during mitosis. Therefore, the presence of active MAP kinase at specific mitotic structures and its interaction with CENP-E suggest that MAP kinase could play a role in mitosis at least in part by altering the ability of CENP-E to mediate interactions between chromosomes and microtubules.

Mitotic Raf-1 is stimulated independently of Ras and is active in the cytoplasm

Raf-1 is a Ser/Thr protein kinase that is involved in regulation of proliferation, differentiation, and apoptosis. Recently, we and others showed that Raf-1 is not only activated in mitogenic pathways leading to cell cycle entry but also during mitosis. Transient expression studies in COS cells now demonstrate that, in contrast to growth factor-dependent activation of Raf-1, mitotic activation of Raf-1 is Ras-independent. Dominant negative RasS17N does not interfere with mitotic activation of Raf-1, whereas epidermal growth factor-dependent stimulation of Raf-1 is inhibited. In addition, the Raf-1 mutant RafR89L, which cannot bind to activated Ras, is still stimulated in mitotic cells. Mitotic activation of Raf-1 seems to be partially dependent on tyrosine phosphorylation since the kinase activity of the Raf mutant RafYY340/341FF, which can no longer be activated by Src, is reduced in mitotic cells. Surprisingly, cell fractionation experiments showed that mitotic-activated Raf-1 is predominantly located in the cytoplasm in contrast to the mitogen-activated Raf-1 that is bound to the plasma membrane. In addition, mitotic activation of Raf-1 does not lead to stimulation of the mitogen-activated protein kinase kinase (MAPKK or MEK) and the extracellular signal-regulated protein kinase (ERK). These data demonstrate that in mitotic cells a Ras-independent mechanism results in a cytoplasmic active Raf-1 kinase which does not signal via the MEK/ERK pathway. These data demonstrate that in mitotic cells a Ras-independent mechanism results in a cytoplasmic active Raf-1 kinase which does not signal via the MEK/ERK pathway.

Human T cell leukemia virus type 1 oncoprotein Tax targets the human mitotic checkpoint protein MAD1

In searching for cellular targets of the HTLV-I oncoprotein Tax, we identified TXBP181, which we characterized as the human homolog of yeast mitotic checkpoint MAD1 protein. Evidence supporting TXBP181 as HsMAD1 includes sequence conservation with yeast MAD1, hyperphosphorylation during S/G2/M phases and upon treatment of cells with nocodazole, and binding to HsMAD2. HsMAD1 functions as a homodimer. It localizes to the centrosome during metaphase and to the spindle midzone and the midbody during anaphase and telophase. Expression of either Tax or a transdominant-negative TXBP181 results in multinucleated cells, a phenotype consistent with a loss of HsMAD1 function. We propose a model of viral transformation in which Tax targets TXBP181, thereby abrogating a mitotic checkpoint.