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

Raf kinase inhibitor protein combined with phosphorylated extracellular signal-regulated kinase offers valuable prognosis in gastrointestinal stromal tumor

BACKGROUND

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract. Tyrosine kinase inhibitors, such as imatinib, have been used as first-line therapy for the treatment of GISTs. Although these drugs have achieved considerable efficacy in some patients, reports of resistance and recurrence have emerged. Extracellular signal-regulated kinase 1/2 (ERK1/2) protein, as a member of the mitogen-activated protein kinase (MAPK) family, is a core molecule of this signaling pathway. Nowadays, research reports on the important clinical and prognostic value of phosphorylated-ERK (P-ERK) and phosphorylated-MAPK/ERK kinase (P-MEK) proteins closely related to raf kinase inhibitor protein (RKIP) have gradually emerged in digestive tract tumors such as gastric cancer, colon cancer, and pancreatic cancer. However, literature on the expression of these downstream proteins combined with RKIP in GIST is scarce. This study will focus on this aspect and search for answers to the problem.

AIM

To detect the expression of RKIP, P-ERK, and P-MEK protein in GIST and to analyze their relationship with clinicopathological characteristics and prognosis of this disease. Try to establish a new prognosis evaluation model using RKIP and P-ERK in combination with analysis and its prognosis evaluation efficacy.

METHODS

The research object of our experiment was 66 pathologically diagnosed GIST patients with complete clinical and follow-up information. These patients received surgical treatment at China Medical University Affiliated Hospital from January 2015 to January 2020. Immunohistochemical method was used to detect the expression of RKIP, P-ERK, and P-MEK proteins in GIST tissue samples from these patients. Kaplan-Meier method was used to calculate the survival rate of 63 patients with complete follow-up data. A Nomogram was used to represent the new prognostic evaluation model. The Cox multivariate regression analysis was conducted separately for each set of risk evaluation factors, based on two risk classification systems [the new risk grade model vs the modified National Institutes of Health (NIH) 2008 risk classification system]. Receiver operating characteristic (ROC) curves were used for evaluating the accuracy and efficiency of the two prognostic evaluation systems.

RESULTS

In GIST tissues, RKIP protein showed positive expression in the cytoplasm and cell membrane, appearing as brownish-yellow or brown granules. The expression of RKIP was related to GIST tumor size, NIH grade, and mucosal invasion. P-ERK protein exhibited heterogeneous distribution in GIST cells, mainly in the cytoplasm, with occasional presence in the nucleus, and appeared as brownish-yellow granules, and the expression of P-ERK protein was associated with GIST tumor size, mitotic count, mucosal invasion, and NIH grade. Meanwhile, RKIP protein expression was negatively correlated with P-ERK expression. The results in COX multivariate regression analysis showed that RKIP protein expression was not an independent risk factor for tumor prognosis. However, RKIP combined with P-ERK protein expression were identified as independent risk factors for prognosis with statistical significance. Furthermore, we establish a new prognosis evaluation model using RKIP and P-ERK in combination and obtained the nomogram of the new prognosis evaluation model. ROC curve analysis also showed that the new evaluation model had better prognostic performance than the modified NIH 2008 risk classification system.

CONCLUSION

Our experimental results showed that the expression of RKIP and P-ERK proteins in GIST was associated with tumor size, NIH 2008 staging, and tumor invasion, and P-ERK expression was also related to mitotic count. The expression of the two proteins had a certain negative correlation. The combined expression of RKIP and P-ERK proteins can serve as an independent risk factor for predicting the prognosis of GIST patients. The new risk assessment model incorporating RKIP and P-ERK has superior evaluation efficacy and is worth further practical application to validate.

Transcriptional regulation of PEBP1 expression by androgen receptor in mouse testes

Androgen and AR are essential for maintaining spermatogenesis and male fertility. Previous studies have shown that the phosphatidyl ethanolamine binding protein 1 (Pebp1) gene is down-regulated in the selective ablation of the AR in the Sertoli cells of mouse testes compared with wild-type mice, indicating that Pebp1 is a candidate target of AR. The ChIP-PCR data and ChIP-sequencing results of this study verified that Pebp1 is a target gene regulated by AR. Real-time PCR, Western blot analysis, and immunofluorescence data showed that Pebp1 is expressed at all stages of testicular development, with an increasing trend from 1 to 8 weeks of postnatal development. PEBP1 was principally located in the cytoplasm, and high-intensity fluorescence revealed PEBP in the lumen of the testicular tubules. Bioinformatics analysis indicated effective androgen-responsive elements (AREs) located in the promotor of Pepb1 gene. Dual fluorescence assay data showed that androgens and AR could bind to the AREs of Pebp1 and induce an increase of gene expression. These data suggest that Pepb1 is a newfound target gene regulated by androgens and AR in mouse Sertoli cells. However, the detailed molecular mechanism of their role in spermatogenesis still needs to be further studied.Abbreviations: AR: androgen receptor; Pebp1: phosphatidyl ethanolamine binding protein 1; ARKO: androgen receptor knockout; WT: wild type; SCARKO: Sertoli cell-selective androgen receptor knockout; ChIP: chromatin immunoprecipitation; RKIP: Raf kinase inhibitory protein; MAPK: mitogen-activated protein kinase; NF-κB: nuclear factor kappa-light-chain-enhancer of activated B cells; GSK-3: glycogen synthase kinase-3; RT-PCR: reverse transcriptase polymerase chain reaction; SEM: standard error of the mean.

Clinical and prognostic significance of Raf kinase inhibitory protein expression in gastrointestinal stromal tumors

AIM

To detect the expression of Raf kinase inhibitory protein (RKIP) in gastrointestinal stromal tumors (GISTs) and to analyze its relationship with clinicopatholgical characteristics and prognosis of this disease.

METHODS

Sixty-three patients with pathologically diagnosed GISTs who underwent surgical resection at the Shengjing Hospital of China Medical University from January 2011 to January 2015 and had complete clinical, pathological, and follow-up data were included. Immunohistochemical method was used to detect the expression of RKIP in GIST tissue samples from these patients. Kaplan-Meier method was used to calculate the survival rate of 60 patients with complete follow-up data, and Cox regression analysis was performed to identify factors affecting the prognosis of patients GISTs to evaluate further the diagnostic and prognostic value of RKIP in GISTs.

RESULTS

In GIST tissues, RKIP positive signals, manifesting as brownish yellow or brown granules, were located in the cytoplasm or on the membrane. Of 63 tissue samples included in this study, 34 (54%) were positive and 29 (46%) were negative for RKIP expression. Statistical analysis showed that RKIP expression in GISTs was significantly associated with tumor size, National Institutes of Health (NIH) risk grade, and mucosal invasion, but had no significant association with age, gender, tumor location, or the number of mitotic figures. Univariate Kaplan-Meier analysis revealed that the 1-, 3-, and 5-year survival rates were 94.4%, 89.2%, and 80.5% for patients with positive RKIP expression, and 88.6%, 68.2%, and 48.2% for patients with negative RKIP expression, suggesting that patients with high RKIP expression had significantly higher survival rates than those with low expression (Log-rank test, P = 0.0015). Cox regression analysis demonstrated that NIH risk grade was significantly associated with the prognosis of GISTs (P = 0.037), suggesting that NIH risk grade is a significant predictor of the prognosis of GISTs. RKIP expression had a tendency to predict the survival of GISTs (P = 0.122), suggesting that RKIP expression may have appreciated value to predict the prognosis of GISTs.

CONCLUSION

This study demonstrated that: (1) RKIP expression in GISTs is associated with tumor size, NIH risk grade, and mucosal invasion, and low or no expression of RKIP predicts a high malignancy potential; (2) high RKIP correlates positively with the survival of patients with GISTs; and (3) RKIP expression has appreciated value for predicting the survival of patients with GISTs, although it is not an independent prognostic factor in GISTs.

Activation of RAF1 (c-RAF) by the Marine Alkaloid Lasonolide A Induces Rapid Premature Chromosome Condensation

Lasonolide A (LSA), a potent antitumor polyketide from the marine sponge, Forcepia sp., induces rapid and reversible protein hyperphosphorylation and premature chromosome condensation (PCC) at nanomolar concentrations independent of cyclin-dependent kinases. To identify cellular targets of LSA, we screened 2951 shRNAs targeting a pool of human kinases and phosphatases (1140 RefSeqs) to identify genes that modulate PCC in response to LSA. This led to the identification of RAF1 (C-RAF) as a mediator of LSA-induced PCC, as shRNAs against RAF1 conferred resistance to LSA. We found that LSA induced RAF1 phosphorylation on Serine 338 within minutes in human colorectal carcinoma HCT-116, ovarian carcinoma OVCAR-8, and Burkitt’s lymphoma CA46 cell lines. RAF1 depletion by siRNAs attenuated LSA-induced PCC in HCT-116 and OVCAR-8 cells. Furthermore, mouse embryonic fibroblasts (MEF) with homozygous deletion in Raf1, but not deletion in the related kinase Braf, were resistant to LSA-induced PCC. Complementation of Raf1^−/− MEFs with wild-type human RAF1, but not with kinase-dead RAF1 mutant, restored LSA-induced PCC. Finally, the Raf inhibitor sorafenib, but not the MEK inhibitor AZD6244, effectively suppressed LSA-induced PCC. Our findings implicate a previously unknown, MAPK-independent role of RAF1 in chromatin condensation and potent activation of this pathway by LSA.

Investigation of MEK activity in COS7 cells entering mitosis

Although the mitogen-activated protein kinase (MAPK) pathway has been extensively investigated, numerous events remain unclear. In the present study, we examined mitogen-activated protein kinase kinase (MEK) expression from interphase to mitosis. Following nocodazole treatment, COS7 cells gradually became round as early as 4 h after treatment. Cyclin B1 expression gradually increased from 4 to 24 h in the presence of nocodazole. When cells were treated with nocodazole for 4 h, the level of epidermal growth factor (EGF)-mediated MEK phosphorylation did not significantly change between nocodazole-untreated and -treated (4 h) cells (P>0.05). However, EGF-mediated MEK phosphorylation was significantly inhibited upon treatment with nocodazole for 8 and 24 h compared to nocodazole-untreated cells (P<0.05). MEK phosphorylation levels were comparable between 1, 5, 10 and 50 ng/ml EGF treatments. Phorbol 12-myristic 13-acetate (PMA) did not activate MEK in mitotic cells. Following treatment of COS7 cells at the interphase with AG1478 or U0126, MEK phosphorylation was blocked. In addition, the investigation of the expression of proteins downstream of MEK demonstrated that EGF does not significantly affect the phosphorylation level of extracellular-signal-regulated kinase (ERK), ribosomal protein S6 kinase (RSK) and Elk in mitotic cells (P>0.05). The results showed that MEK expression is gradually inhibited from cell interphase to mitosis, and that MEK downstream signaling is affected by this inhibition, which probably reflects the requirements of cell physiology during mitosis.

PKD controls mitotic Golgi complex fragmentation through a Raf-MEK1 pathway

Before entering mitosis, the stacks of the Golgi cisternae are separated from each other, and inhibiting this process delays entry of mammalian cells into mitosis. Protein kinase D (PKD) is known to be involved in Golgi-to-cell surface transport by controlling the biogenesis of specific transport carriers. Here we show that depletion of PKD1 and PKD2 proteins from HeLa cells by small interfering RNA leads to the accumulation of cells in the G2 phase of the cell cycle and prevents cells from entering mitosis. We further provide evidence that inhibition of PKD blocks mitotic Raf-1 and mitogen-activated protein kinase kinase (MEK) activation, and, as a consequence, mitotic Golgi fragmentation, which could be rescued by expression of active MEK1. Finally, Golgi fluorescence recovery after photobleaching analyses demonstrate that PKD is crucial for the cleavage of the noncompact zones of Golgi membranes in G2 phase. Our findings suggest that PKD controls interstack Golgi connections in a Raf-1/MEK1-dependent manner, a process required for entry of the cells into mitosis.

RIP2 mediates LPS-induced p38 and IkappaBalpha signaling including IL-12 p40 expression in human monocyte-derived dendritic cells

IL-12, the critical factor for the generation of the Th1 type immune response, is produced by dendritic cells (DC) upon stimulation with LPS. Different signal pathways mediate LPS-induced expression of IL-12 and involve PI3K, MAPK and the transcription factor NF-kappaB. Here, we show that the kinase Raf is involved in the expression of IL-12 in human DC stimulated by LPS. We demonstrate that Raf regulates the expression of the IL-12 subunit p40 not via the kinase MEK, the major effector of Raf in growth factor-dependent signaling, but via the receptor-interacting protein 2 (RIP2) using specific inhibitors for MAPK pathways. RIP2 is a kinase participating in LPS/Toll-like receptor 4 signaling. Knockdown of RIP2 by siRNA inhibited LPS-dependent expression of IL-12 p40. In addition, knockdown of RIP2 reduced phosphorylation of p38 MAPK, ERK and IkappaBalpha, which are known upstream regulators of IL-12 production. Thus, in human DC LPS stimulates a signal cascade that involves the Raf-dependent activation of RIP2 leading to expression of IL-12 p40.

Raf kinase inhibitory protein regulates aurora B kinase and the spindle checkpoint

Raf kinase inhibitory protein (RKIP or PEBP) is an inhibitor of the Raf/MEK/MAP kinase signaling cascade and a suppressor of cancer metastasis. We now show that RKIP associates with centrosomes and kinetochores and regulates the spindle checkpoint in mammalian cells. RKIP depletion causes decreases in the mitotic index, the number of metaphase cells, and traversal times from nuclear envelope breakdown to anaphase, and an override of mitotic checkpoints induced by spindle poisons. Raf-1 depletion or MEK inhibition reverses the reduction in the mitotic index, whereas hyperactivation of Raf mimics the RKIP-depletion phenotype. Finally, RKIP depletion or Raf hyperactivation reduces kinetochore localization and kinase activity of Aurora B, a regulator of the spindle checkpoint. These results indicate that RKIP regulates Aurora B kinase and the spindle checkpoint via the Raf-1/MEK/ERK cascade and demonstrate that small changes in the MAP kinase (MAPK) pathway can profoundly impact the fidelity of the cell cycle.

Activation of extracellular signal-regulated kinase (ERK) in G2 phase delays mitotic entry through p21CIP1

Extracellular signal-regulated kinase activity is essential for mediating cell cycle progression from G(1) phase to S phase (DNA synthesis). In contrast, the role of extracellular signal-regulated kinase during G(2) phase and mitosis (M phase) is largely undefined. Previous studies have suggested that inhibition of basal extracellular signal-regulated kinase activity delays G(2)- and M-phase progression. In the current investigation, we have examined the consequence of activating the extracellular signal-regulated kinase pathway during G(2) phase on subsequent progression through mitosis. Using synchronized HeLa cells, we show that activation of the extracellular signal-regulated kinase pathway with phorbol 12-myristate 13-acetate or epidermal growth factor during G(2) phase causes a rapid cell cycle arrest in G(2) as measured by flow cytometry, mitotic indices and cyclin B1 expression. This G(2)-phase arrest was reversed by pre-treatment with bisindolylmaleimide or U0126, which are selective inhibitors of protein kinase C proteins or the extracellular signal-regulated kinase activators, MEK1/2, respectively. The extracellular signal-regulated kinase-mediated delay in M-phase entry appeared to involve de novo synthesis of the cyclin-dependent kinase inhibitor, p21(CIP1), during G(2) through a p53-independent mechanism. To establish a function for the increased expression of p21(CIP1) and delayed cell cycle progression, we show that extracellular signal-regulated kinase activation in G(2)-phase cells results in an increased number of cells containing chromosome aberrations characteristic of genomic instability. The presence of chromosome aberrations following extracellular signal-regulated kinase activation during G(2)-phase was further augmented in cells lacking p21(CIP1). These findings suggest that p21(CIP1) mediated inhibition of cell cycle progression during G(2)/M phase protects against inappropriate activation of signalling pathways, which may cause excessive chromosome damage and be detrimental to cell survival.

B-Raf Is Critical For MAPK Activation during Mitosis and Is Regulated in an M Phase-dependent Manner in Xenopus Egg Extracts

Activation of the MAPK cascade during mitosis is critical for spindle assembly and normal mitotic progression. The underlying regulatory mechanisms that control activation of the MEK/MAPK cascade during mitosis are poorly understood. Here we purified and characterized the MEK kinase activity present in Xenopus M phase-arrested egg extracts. Our results show that B-Raf was the critical MEK kinase required for M phase activation of the MAPK pathway. Consistent with this, B-Raf was activated and underwent hyperphosphorylation in an M phase-dependent manner. Interestingly B-Raf hyperphosphorylation at mitosis occurred, at least in part, as a consequence of a feedback loop involving MAPK-mediated phosphorylation within a conserved C-terminal SPKTP motif. The kinase activity of a B-Raf mutant defective at both phosphorylation sites was substantially greater than its wild type counterpart when incubated in Xenopus M phase egg extracts. Furthermore suppression of MAPK feedback at mitosis enhanced B-Raf activity, whereas constitutive activation of MAPK at mitosis strongly suppressed B-Raf activity. These results suggest that feedback phosphorylation by MAPK negatively regulates B-Raf activity at mitosis. Collectively our data demonstrate for the first time a role for B-Raf at mitosis and provide new insight into understanding the regulation and function of B-Raf during cell proliferation.

Cdc2-mediated Inhibition of Epidermal Growth Factor Activation of the Extracellular Signal-regulated Kinase Pathway during Mitosis

Inhibition of general transcription and translation occurs during mitosis to preserve the high energy requirements needed for the dynamic structural changes that are occurring at this time of the cell cycle. Although the mitotic kinase Cdc2 appears to directly phosphorylate and inhibit key proteins directly involved in transcription and translation, the role of Cdc2 in regulating up-stream growth factor receptor-mediated signal transduction pathways is limited. In the present study, we examined mechanisms involved in uncoupling receptor-mediated activation of the extracellular signal-regulated (ERK) signaling pathway in mitotic cells. Treatment with epidermal growth factor (EGF) failed to activate the ERK pathway in mitotic cells, although partial activation of ERK could be achieved in mitotic cells treated with phorbol 12-myristate 13-acetate (PMA). The discrepancy between EGF and PMA-mediated ERK activation suggested that multiple events in the ERK pathway were regulated during mitosis. We show that Cdc2 inhibits EGF-mediated ERK activation through direct interaction and phosphorylation of several ERK pathway proteins, including the guanine nucleotide exchange factor, Sos-1, and Raf-1 kinase. Inhibition of Cdc2 activity with roscovitine in mitotic cells restored ERK activation by EGF and PMA. Similarly, mitotic inhibition of ERK activity in cells expressing active mutants of H-Ras and Raf-1 kinase could also be reversed following Cdc2 inhibition. In contrast, ERK activation in cells expressing active MEK1 was not inhibited during mitosis or affected by roscovitine. These data suggest that Cdc2 inhibits growth factor receptor-mediated ERK activation during mitosis by primarily targeting signaling proteins that are upstream of MEK1.

CNK1 is a scaffold protein that regulates Src-mediated Raf-1 activation

Raf-1 is a regulator of cellular proliferation, differentiation, and apoptosis. Activation of the Raf-1 kinase activity is tightly regulated and involves targeting to the membrane by Ras and phosphorylation by various kinases, including the tyrosine kinase Src. Here we demonstrate that the connector enhancer of Ksr1, CNK1, mediates Src-dependent tyrosine phosphorylation and activation of Raf-1. CNK1 binds preactivated Raf-1 and activated Src and forms a trimeric complex. CNK1 regulates the activation of Raf-1 by Src in a concentration-dependent manner typical for a scaffold protein. Down-regulation of endogenously expressed CNK1 by small inhibitory RNA interferes with Src-dependent activation of ERK. Thus, CNK1 allows cross-talk between Src and Raf-1 and is essential for the full activation of Raf-1.

Antitumor mechanism of evodiamine, a constituent from Chinese herb Evodiae fructus, in human multiple-drug resistant breast cancer NCI/ADR-RES cells in vitro and in vivo

Drug resistance is one of the main obstacles to the successful treatment of cancer. The availability of agents that are highly effective against drug-resistant cancer cells is therefore essential. The present study was performed to examine the anticancer effects of evodiamine, a major constituent of the Chinese herb Evodiae fructus, in adriamycin-resistant human breast cancer NCI/ADR-RES cells. Evodiamine inhibited the proliferation of NCI/ADR-RES cells in a concentration-dependent manner with a GI50 of 0.59 +/- 0.11 microM. This agent also caused a substantial apoptosis at 1 microM. FACScan flow cytometric analysis of cell cycle progression revealed that a G2/M arrest was initiated after a 12-h exposure to the drug. Evodiamine increased tubulin polymerization as determined by the immunocytochemical and in vivo tubulin polymerization analyses. In a time- and concentration-dependent manner, evodiamine also promoted the phosphorylations of Raf-1 kinase and Bcl-2. The phosphorylation site of Raf-1 kinase was identified to be serine338. The in vivo anticancer effects of evodiamine were evaluated in Balb-c/nude mice following a tumor xenograft implantation of NCI/ADR-RES cells. The antitumor activity of evodiamine against the human multiple-drug resistant tumor xenograft was found to be superior to that of paclitaxel. Evodiamine therefore represents a highly promising chemotherapeutic agent in the treatment of human multiple-drug resistant cancer cells.

Drosophila PI3 kinase and Akt involved in insulin-stimulated proliferation and ERK pathway activation in Schneider cells

We have characterized the role of Drosophila PI3K and AKT in ERK pathway activation involving insulin-induced proliferation using Drosophila Schneider cells. After insulin treatment, dPI3K and dAKT activities were both increased along with activation of the dERK pathway components dMEK and dERK. The insulin-induced activations of dERK and dAKT were blocked by LY294002, dPTEN, and by an AKT inhibitor, indicating involvement of dPI3K and dAKT in the insulin-induced dERK and dAKT activations. Proliferation and the G1 to S phase cell cycle progression due to insulin were also blocked by PI3K and AKT inhibitors, indicating that the Drosophila PI3K-AKT pathway involves insulin-mediated cell proliferation. The insulin-stimulated size increase was blocked by both LY294002 and AKT inhibitor, not by U0126, indicating that insulin-mediated size control by dPI3K and dAKT occurs independently of the ERK pathway. This study indicates that dPI3K and dAKT are involved in insulin-induced ERK pathway activation leading to proliferation in Drosophila Schneider cells.

Src Tyrosine Kinase Inhibitor PP2 Markedly Enhances Ras-independent Activation of Raf-1 Protein Kinase by Phorbol Myristate Acetate and H2O2

Recently we reported that simultaneous treatment of NIH 3T3 cells with the combination of phorbol myristate acetate (PMA) and hydrogen peroxide (H2O2) resulted in synergistic activation of Raf-1 kinase (Lee, M., Petrovics, G., and Anderson, W. B. (2003) Biochem. Biophys. Res. Commun. 311, 1026-1033). In this study we have demonstrated that PP2 (4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine), a potent and selective inhibitor of the Src-family tyrosine kinase, greatly potentiated the ability of PMA and/or H2O2 to activate Raf-1 kinase, whereas it blocked the tyrosine phosphorylation of Raf-1. Unlike PMA/H2O2 treatment, which showed transient activation, PP2-mediated Raf-1 activation was sustained and continued to increase through 4 h of treatment. Transient transfection studies with a dominant-negative mutant of Ras (N19Ras) indicated that this PP2-induced activation of Raf-1 was Ras-independent. Moreover, PP2 showed no effect on platelet-derived growth factor-induced Raf-1 activation. Interestingly, mutation of the reported Raf-1 Src family tyrosine kinase phosphorylation site by conversion of tyrosines 340 and 341 to phenylalanine (YY340/341FF Raf) had limited effect on the ability of PP2 to induce significant stimulation of Raf-1 kinase activity. Taken together, our results suggest that a tyrosine phosphorylation event is involved in the negative feedback regulation of Raf-1. Inhibition of a Src family tyrosine kinase by PP2 appears to alleviate this tyrosine kinase-mediated inhibition of Raf-1 and allow activating modification(s) of Raf-1 to proceed. This PP2 effect resulted in significant and sustained Ras-independent activation of Raf-1 by PMA and H2O2.

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.

The Bcr kinase downregulates Ras signaling by phosphorylating AF-6 and binding to its PDZ domain

The protein kinase Bcr is a negative regulator of cell proliferation and oncogenic transformation. We identified Bcr as a ligand for the PDZ domain of the cell junction and Ras-interacting protein AF-6. The Bcr kinase phosphorylates AF-6, which subsequently allows efficient binding of Bcr to AF-6, showing that the Bcr kinase is a regulator of the PDZ domain-ligand interaction. Bcr and AF-6 colocalize in epithelial cells at the plasma membrane. In addition, Bcr, AF-6, and Ras form a trimeric complex. Bcr increases the affinity of AF-6 to Ras, and a mutant of AF-6 that lacks a specific phosphorylation site for Bcr shows a reduced binding to Ras. Wild-type Bcr, but not Bcr mutants defective in binding to AF-6, interferes with the Ras-dependent stimulation of the Raf/MEK/ERK pathway. Since AF-6 binds to Bcr via its PDZ domain and to Ras via its Ras-binding domain, we propose that AF-6 functions as a scaffold-like protein that links Bcr and Ras to cellular junctions. We suggest that this trimeric complex is involved in downregulation of Ras-mediated signaling at sites of cell-cell contact to maintain cells in a nonproliferating state.

Serine 338 phosphorylation is dispensable for activation of c-Raf1

Numerous extracellular agonists induce consecutive stimulation of Ras guanine nucleotide exchange factors, Ras and c-Raf1, as the starting point of the intracellular mitogen-activated protein kinase cascade. Recent data point to a more complex reaction pattern of this simple sequence. This study was aimed at elucidating the activation process of endogenous c-Raf1 in U937 cells. Treatment of permeabilized U937 cells with the nonhydrolyzable nucleotide guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) induced prolonged stimulation of Ras and c-Raf1 activity. Intriguingly, both signaling proteins expressed differential responses toward specific inhibitors of phosphoinositide 3-kinases and tyrosine kinases, which indicates diverse signaling reactions feeding into Ras and cRaf-1. Phosphorylation of c-Raf1 serine 338 by p21-activated kinase has been recently reported to contribute to phosphoinositide 3-kinase-dependent activation of c-Raf1. However, in U937 cells stimulation of c-Raf1 activity by GTPgammaS did not correlate with p21-activated kinase activity and Ser-338 phosphorylation. Thus Ser-338 phosphorylation appears dispensable for c-Raf1 activation under the conditions used. Together these data deny an essential role for serine 338 phosphorylation in c-Raf1 activation and disclose divergent signaling connections of Ras and c-Raf1 in U937 cells.

RAF1-activated MEK1 is found on the Golgi apparatus in late prophase and is required for Golgi complex fragmentation in mitosis

Amitotically activated mitogen-activated protein kinase 1 (MEK1) fragments the pericentriolar Golgi stacks in mammalian cells. We show that activated MEK1 is found on the Golgi apparatus in late prophase. The fragmented and dispersed Golgi membranes in prometaphase and later stages of mitosis do not contain activated MEK1. MEK1-dependent Golgi complex fragmentation is through activation by RAF1 and not MEK1 kinase 1. We propose that a RAF1-dependent activation of MEK1 and its presence on the Golgi apparatus in late prophase is required for Golgi complex fragmentation.

Ras-MAP kinase signaling pathways and control of cell proliferation: relevance to cancer therapy

The mitogen-activated protein (MAP) kinase pathways represent several families of signal transduction cascades that mediate information provided by extracellular stimuli. MAP kinase pathways regulate a wide range of physiological responses, including cell proliferation, apoptosis, cell differentiation, and tissue development. Constitutive activation of MAP kinase proteins in experimental models has been shown to cause cell transformation and is implicated in tumorigenesis. Of clinical importance, MAP kinase pathways are regulated by Ras G-proteins, which are found to be mutated and constitutively active in approximately 30% of all human cancers. Thus, a major goal in the treatment of cancer is the development of specific compounds that target Ras and critical downstream signaling proteins responsible for uncontrolled cell growth. A variety of biochemical, molecular, and structural approaches have been used to develop drug compounds that target signaling proteins important for MAP kinase pathway activation. These compounds have been useful tools for identifying the mechanisms of MAP kinase pathway signaling and hold promise for clinical use. This review will present an overview of the major proteins involved in Ras and MAP kinase signaling pathways and their function in regulating cell cycle events and proliferation. In addition, some of the relevant compounds that have been developed to inhibit the activities of these proteins and MAP kinase signaling are discussed.

CCAAT/enhancer binding protein alpha uses distinct domains to prolong pituitary cells in the growth 1 and DNA synthesis phases of the cell cycle

BACKGROUND

A number of transcription factors coordinate differentiation by simultaneously regulating gene expression and cell proliferation. CCAAT/enhancer binding protein alpha (C/EBPalpha) is a basic/leucine zipper transcription factor that integrates transcription with proliferation to regulate the differentiation of tissues involved in energy balance. In the pituitary, C/EBPalpha regulates the transcription of a key metabolic regulator, growth hormone.

RESULTS

We examined the consequences of C/EBPalpha expression on proliferation of the transformed, mouse GHFT1-5 pituitary progenitor cell line. In contrast to mature pituitary cells, GHFT1-5 cells do not contain C/EBPalpha. Ectopic expression of C/EBPalpha in the progenitor cells resulted in prolongation of both growth 1 (G1) and the DNA synthesis (S) phases of the cell cycle. Transcription activation domain 1 and 2 of C/EBPalpha were required for prolongation of G1, but not of S. Some transcriptionally inactive derivatives of C/EBPalpha remained competent for G1 and S phase prolongation. C/EBPalpha deleted of its leucine zipper dimerization functions was as effective as full-length C/EBPalpha in prolonging G1 and S.

CONCLUSION

We found that C/EBPalpha utilizes mechanistically distinct activities to prolong the cell cycle in G1 and S in pituitary progenitor cells. G1 and S phase prolongation did not require that C/EBPalpha remained transcriptionally active or retained the ability to dimerize via the leucine zipper. G1, but not S, arrest required a domain overlapping with C/EBPalpha transcription activation functions 1 and 2. Separation of mechanisms governing proliferation and transcription permits C/EBPalpha to regulate gene expression independently of its effects on proliferation.

p60(v-src) and serum control cell shape and apoptosis via distinct pathways in quail neuroretina cells

We made use of QNR cells transformed by a thermosensitive (tsNY68) strain of the Rous sarcoma virus (RSV) to compare the effect of p60(v-src) and serum in cultured nerve cells. In this system, both p60(v-src) heat inactivation and serum removal resulted in growth arrest in G1. In both cases, growth arrest was reversible since cell proliferation was rapidly re-induced following respectively p60v-src renaturation or serum re-addition. However, cells did not fully recover their ability to grow in soft agar, suggesting that, in contrast to the cell cycle machinery, the transforming capacities of these cells have been irreversibly altered. We found that p60(v-src) kinase activity prevented detachment from the substratum and cell death following serum removal. Thermal inactivation of p60(v-src) at restrictive temperature (41.5 degrees C), but not serum removal, resulted in dramatic morphological changes, which occurred 4 h after temperature shift up to 41.5 degrees C. Later on, typical features of apoptotic cells could be observed. Cell death was greatly reduced by the caspase-3 inhibitor ZVAD.FMK, but not by the caspase-1 inhibitor Ac-YVAD.CHO. Together, these results suggested that p60(v-src) and serum factors act on distinct pathways, at least in part. In an attempt to identify the signalling pathways involved in the cell response to p60(v-src) down regulation, we found that Erk and Rac were rapidly inactivated following temperature shift up to 41.5 degrees C. Thus, the combined effects of p60(v-src) and serum factors on the cytoskeleton dynamics and the apoptosis machinery are essential for full neoplastic transformation of neuroretina cells.

Cell cycle checkpoint efficiency and cellular response to paclitaxel in prostate cancer cells

BACKGROUND

Defects in the cell cycle machinery of prostate cancer cells might impair the efficiency of cell cycle checkpoints and affect the cell response to chemotherapeutic drugs. We examined the relationship between the status of microtubule damage-activated checkpoints and the response of hormone-refractory prostate cancer cells to paclitaxel.

METHODS

The two cell lines DU145 and PC3 harboring defects at proteins involved in the regulation of checkpoints activated by microtubule damage were examined for cell sensitivity, apoptotic response, and efficiency of checkpoints in response to paclitaxel.

RESULTS

In spite of a comparable sensitivity to the antiproliferative effects of paclitaxel, DU145 and PC3 cells exhibited different cell cycle control at checkpoints activated by microtubule damage. A transient mitotic arrest was induced by the taxane in both cell lines. However, PC3 cells underwent a rapid mitotic slippage and displayed a defective postmitotic checkpoint as evidenced by the appearance of polyploid cells. In this cell line, paclitaxel-induced cell death was a slow and delayed event, occurring also after S-phase re-entry. The mitotic checkpoint appeared to be more stringent in DU145 cells compared to PC3 cells. Moreover, despite the expression of mutated proteins involved in the prevention of DNA endoreduplication (p53, pRb, and p16(INK4A)), these cells did not progress into the cell cycle but efficiently underwent apoptosis by 24 hr. Such a response of DU145 cells was associated with phosphorylation of the p21(WAF1) protein.

CONCLUSIONS

These observations evidence that activation of checkpoints following microtubule damage in prostate cancer may be regulated through complex mechanisms possibly involving p21(WAF1). Our findings support that the status of cell cycle checkpoints might affect the modality of cell death. However, the relevance of the mode of cell death for the sensitivity to taxanes remains to be determined.

MEK, ERK, and p90RSK are present on mitotic tubulin in Swiss 3T3 cells: a role for the MAP kinase pathway in regulating mitotic exit

The mitogen-activated protein (MAP) kinase pathway has been implicated in cell cycle control for some time. Several reports have suggested a role for this pathway in growth factor stimulation of DNA synthesis, while other reports have proposed a role in the transition of cells through mitosis. Here, we have examined the potential involvement of the extracellular signal-related kinase (ERK)1/2 MAP kinases, their upstream regulators, and downstream effectors in the regulation of mitosis. Inhibition of MAP kinase/ERK kinase (MEK) activity reduced the serum-stimulated DNA synthesis and proliferation of Swiss 3T3 cells. To study the potential mechanisms of this effect, we examined the subcellular localization of members of the MAP kinase pathway including regulators (MEK1/2), substrates (90-kDa ribosomal S6 kinases (RSKs): RSK1, RSK2 and RSK3), and ERK itself. We show that there is enrichment of ERK, MEK, and the RSK enzymes on both the spindle and midbody tubulin of dividing cells. Inhibition of MEK1/2 activity in cells released from mitotic arrest results in an inability of cells to complete mitosis. This failure to exit mitosis correlated with altered cyclin-dependent kinase (cdk) activities. Thus, the MAP kinase pathway may act to coordinate passage through mitosis in Swiss 3T3 fibroblasts by regulation of cdk activity.

Microtubule integrity regulates Pak leading to Ras-independent activation of Raf-1. insights into mechanisms of Raf-1 activation

Growth factors activate Raf-1 by engaging a complex program, which requires Ras binding, membrane recruitment, and phosphorylation of Raf-1. The present study employs the microtubule-depolymerizing drug nocodazole as an alternative approach to explore the mechanisms of Raf activation. Incubation of cells with nocodazole leads to activation of Pak1/2, kinases downstream of small GTPases Rac/Cdc42, which have been previously indicated to phosphorylate Raf-1 Ser(338). Nocodazole-induced stimulation of Raf-1 is augmented by co-expression of small GTPases Rac/Cdc42 and Pak1/2. Dominant negative mutants of these proteins block activation of Raf-1 by nocodazole, but not by epidermal growth factor (EGF). Thus, our studies define Rac/Cdc42/Pak as a module upstream of Raf-1 during its activation by microtubule disruption. Although it is Ras-independent, nocodazole-induced activation of Raf-1 appears to involve the amino-terminal regulatory region in which the integrity of the Ras binding domain is required. Surprisingly, the Raf zinc finger mutation (C165S/C168S) causes a robust activation of Raf-1 by nocodazole, whereas it diminishes Ras-dependent activation of Raf-1. We also show that mutation of residues Ser(338) to Ala or Tyr(340)-Tyr(341) to Phe-Phe immediately amino-terminal to the catalytic domain abrogates activation of both the wild type and zinc finger mutant Raf by both EGF/4beta-12-O-tetradecanoylphorbol-13-acetate and nocodazole. Finally, an in vitro kinase assay demonstrates that the zinc finger mutant serves as a better substrate of Pak1 than the wild type Raf-1. Collectively, our results indicate that 1) the zinc finger exerts an inhibitory effect on Raf-1 activation, probably by preventing phosphorylation of (338)SSYY(341); 2) such inhibition is first overcome by an unknown factor binding in place of Ras-GTP to the amino-terminal regulatory region in response to nocodazole; and 3) EGF and nocodazole utilize different kinases to phosphorylate Ser(338), an event crucial for Raf activation.

Unwinding the loop of Bcl-2 phosphorylation

Recent evidence indicates that anti-apoptotic functions of BcI-2 can be regulated by its phosphorylation. According to the 'mitotic arrest-induced' model, multi-site phosphorylation of the BcI-2 loop domain is followed by cell death. In contrast, in cytokine-dependent cell lines, cytokines mediate phosphorylation of BcI-2 on S70, preventing apoptosis. As discussed in this review, these models are not mutually exclusive but reflect different cellular contexts. During mitotic arrest, signal transduction is unique and is fundamentally different from classical mitogenic signaling, since the nucleus membrane is dissolved, gene expression is reduced, and numerous kinases and regulatory proteins are hyperphosphorylated. Hyperphosphorylation of BcI-2 mediated by paclitaxel and other microtubule-active drugs is strictly dependent on targeting microtubules that in turn cause mitotic arrest. In addition to serine-70 (S70), microtubule-active agents promote phosphorylation of S87 and threonine-69 (T69), inactivating BcI-2. A major obstacle for identification of the mitotic BcI-2 kinase(s) is that inhibition of putative kinase(s) by any means (dominant-negative mutants, antisense oligonucleotides, pharmacological agents) may arrest cycle, preventing mitosis and BcI-2 phosphorylation. The role of BcI-2 phosphorylation in cell death is discussed.

Effects of stimulation of AMP-activated protein kinase on insulin-like growth factor 1- and epidermal growth factor-dependent extracellular signal-regulated kinase pathway

AMP-activated protein kinase (AMPK) is tightly regulated by the cellular AMP:ATP ratio and plays a central role in the regulation of energy homeostasis. Previously, AMPK was reported to phosphorylate serine 621 of Raf-1 in vitro. In the present study, we investigated a possible role of AMPK in extracellular signal-regulated kinase (Erk) cascades, using 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR), a cell-permeable activator of AMPK and antisense RNA experiments. Activation of AMPK by AICAR in NIH-3T3 cells resulted in drastic inhibitions of Ras, Raf-1, and Erk activation induced by insulin-like growth factor 1 (IGF-1). Expression of an antisense RNA for the AMPK catalytic subunit decreased the AMPK activity and significantly diminished the AICAR effect on IGF-1-induced Ras activation and the subsequent Erk activation, indicating that its effect is indeed mediated by AMPK. Phosphorylation of Raf-1 serine 621, however, was not involved in AMPK-mediated inhibition of Erk cascades. In contrast to IGF-1, AICAR did not block epidermal growth factor (EGF)-dependent Raf-1 and Erk activation, but our results demonstrated that multiple Raf-1 upstream pathways induced by EGF were differentially affected by AICAR: inhibition of Ras activation and simultaneous induction of Ras-independent Raf activation. The activities of IGF-1 and EGF receptor were not affected by AICAR. Taken together, our results suggest that AMPK differentially regulate Erk cascades by inhibiting Ras activation or stimulating the Ras-independent pathway in response to the varying energy status of the cell.

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.

A different function for a critical tryptophan in c-Raf and Hck

The similarity of the catalytic domains of Raf and Src family members suggests that functions of homologous residues may be similar in both kinase families. A tryptophan residue, W260, in the WEI region of the Src family kinase Hck has an important role in regulating ATP binding. We tested the hypothesis that the tryptophan, W342, in the WEI region of c-Raf may have a similar role to the W260 of Hck. Mutation of W260 to A in Hck activates kinase activity, but we found that mutation of W342 to A in c-Raf inactivates the kinase activity. Mutating W342 to aspartate (D), lysine (K) or histidine (H) also inactivated c-Raf whether assayed as a purified immunoprecipitate or when recruited to the plasma membrane. A constitutively active c-Raf can be generated by mutating two regulatory tyrosines to aspartate. When placed into this active c-Raf mutant, mutation of W342 to D, K or H enabled phosphorylation and activation of the c-Raf substrate MEK at the plasma membrane but not in an immunoprecipitation assay. We conclude that (1) Tryptophan has a different role in the WEI regions of c-Raf and Hck, (2) W342 is not directly involved in MEK binding as both positive and negative residues at 342 are permissive for MEK activation at the membrane in a constitutively active c-Raf mutant, (3) Factors at the membrane are capable of potentiating activation of c-Raf containing mutated W342 in a hyperactivated c-Raf, but not in a wild type c-Raf and (4) There is a stringent structural requirement for W at residue 342 in c-Raf.

Disruption of Raf-1/heat shock protein 90 complex and Raf signaling by dexamethasone in mast cells

Antigen stimulation of mast cells via the IgE receptor, FcepsilonRI, results in the recruitment of the cytosolic tyrosine kinase, Syk, and the activation of various signaling cascades. One of these, the extracellular signal-regulated kinase (ERK2) cascade, is inhibited by low concentrations of the immunosuppressant drug, dexamethasone, probably at a step prior to the activation of Raf-1 (Rider, L. G., Hirasawa, N., Santini, F., and Beaven, M. A. (1996) J. Immunol. 157, 2374-2380). We now show that treatment of cultured RBL-2H3 mast cells with nanomolar concentrations of dexamethasone causes dissociation of the Raf-1.heat shock protein 90 (Hsp90) complex. Raf-1 bereft of this protein fails to associate with the membrane or Ras in antigen-stimulated cells. Upstream events such as the Syk-dependent phosphorylation of Shc, the engagement of Shc with the adapter protein, Grb2, and the activation of Ras itself are unaffected. Interestingly, the counterpart of Raf-1 in the c-Jun N-terminal kinase (JNK) cascade, MEKK-1 (mitogen-activated protein kinase/ERK kinase), is similarly associated with Hsp90, and this association as well as the activation of MEKK-1 are disrupted by dexamethasone treatment. Disruption of the ERK and JNK cascades at the level of Raf-1 and MEKK-1 could account for the inhibitory action of dexamethasone on the generation of inflammatory mediators in stimulated mast cells.

Regulation of the protein kinase Raf-1 by oncogenic Ras through phosphatidylinositol 3-kinase, Cdc42/Rac and Pak

Activation of the protein kinase Raf-1 is a complex process involving association with the GTP-bound form of Ras (Ras-GTP), membrane translocation and both serine/threonine and tyrosine phosphorylation (reviewed in [1]). We have reported previously that p21-activated kinase 3 (Pak3) upregulates Raf-1 through direct phosphorylation on Ser338 [2]. Here, we investigated the origin of the signal for Pak-mediated Raf-1 activation by examining the role of the small GTPase Cdc42, Rac and Ras, and of phosphatidylinositol (PI) 3-kinase. Pak3 acted synergistically with either Cdc42V12 or Rac1V12 to stimulate the activities of Raf-1, Raf-CX, a membrane-localized Raf-1 mutant, and Raf-1 mutants defective in Ras binding. Raf-1 mutants defective in Ras binding were also readily activated by RasV12. This indirect activation of Raf-1 by Ras was blocked by a dominant-negative mutant of Pak, implicating an alternative Ras effector pathway in Pak-mediated Raf-1 activation. Subsequently, we show that Pak-mediated Raf-1 activation is upregulated by both RasV12C40, a selective activator of PI 3-kinase, and p110-CX, a constitutively active PI 3-kinase. In addition, p85Delta, a mutant of the PI 3-kinase regulatory subunit, inhibited the stimulated activity of Raf-1. Pharmacological inhibitors of PI 3-kinase also blocked both activation and Ser338 phosphorylation of Raf-1 induced by epidermal growth factor (EGF). Thus, Raf-1 activation by Ras is achieved through a combination of both physical interaction and indirect mechanisms involving the activation of a second Ras effector, PI 3-kinase, which directs Pak-mediated regulatory phosphorylation of Raf-1.

Localization of endogenous Grb10 to the mitochondria and its interaction with the mitochondrial-associated Raf-1 pool

Grb10 belongs to a small family of adapter proteins that are known to interact with a number of receptor tyrosine kinases and signaling molecules. We have recently demonstrated that the Grb10 SH2 domain interacts with both the Raf-1 and MEK1 kinases. Overexpression of Grb10 genes with mutations in their SH2 domains promotes apoptosis in cultured cells, a phenotype that is reversed by concomitant overexpression of the wild type gene. Using immunofluorescence microscopy and subcellular fractionation we now show that most of the Grb10 molecules are peripherally associated with mitochondria. Following insulin-like growth factor I or serum treatment, small pools of Grb10 can also be found at the plasma membrane and in actin-rich membrane ruffles, whereas overexpression of Grb10 leads to its mislocalization to the cytosol. Two-hybrid analysis shows that the Grb10-binding site on Raf-1 co-localizes with its Ras-binding domain. Finally, we show that the endogenous Grb10 and Raf-1 proteins can be co-immunoprecipitated from a partially purified mitochondrial extract, an interaction that is enhanced following the activation of Raf-1 by ultraviolet radiation. Thus, we infer that Grb10 may regulate signaling between plasma membrane receptors and the apoptosis-inducing machinery on the mitochondrial outer membrane by modulating the anti-apoptotic activity of mitochondrial Raf-1.

Molecular effects of paclitaxel: myths and reality (a critical review)

Recent studies on paclitaxel (Taxol), a microtubule-stabilizing agent and effective anti-cancer drug, have identified numerous cellular and molecular effects, such as induction of cytokines and tumor-suppressor genes, indirect cytotoxicity due to secretion of tumor necrosis factor, vast activation of signal-transduction pathways and selective activity against cells lacking functional p53. Some of these results, including the immediate activation of signaling pathways and gene expression, have been observed only with paclitaxel concentrations 1,000-fold higher than those required for mitotic arrest and apoptosis. The effects of loss of p53 on paclitaxel cytotoxicity depend on cell type (normal murine fibroblasts vs. human cancer cells) and duration of exposure to paclitaxel; p53 status marginally affects paclitaxel sensitivity in human cancer. Although the biochemistry of mitosis and meiosis has been studied independently of research on the mechanism of action of anti-cancer drugs, it eventually provided insight into the effects of paclitaxel. For example, serine protein phosphorylation, which occurs during mitotic arrest or meiosis, explains paclitaxel-induced hyperphosphorylation of Bcl-2 and Bcl-xL. Although some observations are disputed, such mitotic arrest correlates with paclitaxel cytotoxicity, while there is currently no evidence that any paclitaxel effect at clinically relevant concentrations is independent of its tubulin-binding properties. Thus, paclitaxel exerts two types of effect: mitotic arrest with coincidental serine protein phosphorylation and cytotoxicity at clinically relevant concentrations as well as immediate activation of tyrosine kinase pathways and activation of gene expression at much higher concentrations.

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.