Induction of cell proliferation in quiescent NIH 3T3 cells by oncogenic c-Raf-1

The role of CRAF in cancer progression: from molecular mechanisms to precision therapies

The RAF family of kinases includes key activators of the pro-tumourigenic mitogen-activated protein kinase pathway. Hyperactivation of RAF proteins, particularly BRAF and CRAF, drives tumour progression and drug resistance in many types of cancer. Although BRAF is the most studied RAF protein, partially owing to its high mutation incidence in melanoma, the role of CRAF in tumourigenesis and drug resistance is becoming increasingly clinically relevant. Here, we summarize the main known regulatory mechanisms and gene alterations that contribute to CRAF activity, highlighting the different oncogenic roles of CRAF, and categorize RAF1 (CRAF) mutations according to the effect on kinase activity. Additionally, we emphasize the effect that CRAF alterations may have on drug resistance and how precision therapies could effectively target CRAF-dependent tumours. Here, we discuss preclinical and clinical findings that may lead to improved treatments for all types of oncogenic RAF1 alterations in cancer.

RAS specific protease induces irreversible growth arrest via p27 in several KRAS mutant colorectal cancer cell lines

Ras-specific proteases to degrade RAS within cancer cells are under active development as an innovative strategy to treat tumorigenesis. The naturally occurring biological toxin effector called RAS/RAP1-specific endopeptidase (RRSP) is known to cleave all RAS within a cell, including HRAS, KRAS, NRAS and mutant KRAS G13D. Yet, our understanding of the mechanisms by which RRSP drives growth inhibition are unknown. Here, we demonstrate, using isogenic mouse fibroblasts expressing a single isoform of RAS or mutant KRAS, that RRSP equally inactivates all isoforms of RAS as well as the major oncogenic KRAS mutants. To investigate how RAS processing might lead to varying outcomes in cell fate within cancer cells, we tested RRSP against four colorectal cancer cell lines with a range of cell fates. While cell lines highly susceptible to RRSP (HCT116 and SW1463) undergo apoptosis, RRSP treatment of GP5d and SW620 cells induces G1 cell cycle arrest. In some cell lines, growth effects were dictated by rescued expression of the tumor suppressor protein p27 (Kip1). The ability of RRSP to irreversibly inhibit cancer cell growth highlights the antitumor potential of RRSP, and further warrants investigation as a potential anti-tumor therapeutic.

Obesity and the risk of Multiple Sclerosis. The role of Leptin

OBJECTIVE

To investigate the effects of leptin on different T-cell populations, in order to gain more insight into the link between leptin and obesity.

METHODS

Three hundred and nine RRMS patients and 322 controls participated in a cross-sectional survey, to confirm whether excess weight/obesity in adolescence or early adulthood increased the risk of MS. Serum leptin levels were determined by ELISA. MBP_83-102 , and MOG_63-87 peptide-specific T cells lines were expanded from peripheral blood mononuclear cells. Leptin receptor expression was measured by RT-PCR and flow cytometry. Bcl-2, p-STAT3, pERK1/2, and p27^kip1 expression were assayed using ELISA, and apoptosis induction was determined by Annexin V detection. Cytokines were assessed by ELISPOT and ELISA, and regulatory T cells (Tregs) by flow cytometry.

RESULTS

Logistic regression analysis, showed excess weight at age 15, and obesity at 20 years of age increased MS risk (OR = 2.16, P = 0.01 and OR = 3.9, P = 0.01). Leptin levels correlated with BMI in both groups. The addition of Leptin increased autoreactive T-cell proliferation, reduced apoptosis induction, and promoted proinflammatory cytokine secretion. Obese patients produced more proinflammatory cytokines compared to overweight/normal/underweight subjects. Inverse correlation was found between leptin levels and circulating Treg cells (r = -0.97, P < 0.0001). Leptin inhibited Treg proliferation. Effects of leptin on CD4⁺ CD25^- effector T cells were mediated by increased STAT3 and ERK1/2 phosphorylation, and down modulation of the cell cycle inhibitor P27^kip1 . In contrast, leptin effects on Tregs resulted from decreased phosphorylation of ERK1/2 and upregulation of p27^kip1 .

INTERPRETATION

Leptin promotes autoreactive T-cell proliferation and proinflammatory cytokine secretion, but inhibits Treg-cell proliferation.

Raf Kinases Are Essential for Phosphate Induction of ERK1/2 Phosphorylation in Hypertrophic Chondrocytes and Normal Endochondral Bone Development

Hypophosphatemia causes rickets by impairing hypertrophic chondrocyte apoptosis. Phosphate induction of MEK1/2-ERK1/2 phosphorylation in hypertrophic chondrocytes is required for phosphate-mediated apoptosis and growth plate maturation. MEK1/2 can be activated by numerous molecules including Raf isoforms. A- and B-Raf ablation in chondrocytes does not alter skeletal development, whereas ablation of C-Raf decreases hypertrophic chondrocyte apoptosis and impairs vascularization of the growth plate. However, ablation of C-Raf does not impair phosphate-induced ERK1/2 phosphorylation in vitro, but leads to rickets by decreasing VEGF protein stability. To determine whether Raf isoforms are required for phosphate-induced hypertrophic chondrocyte apoptosis, mice lacking all three Raf isoforms in chondrocytes were generated. Raf deletion caused neonatal death and a significant expansion of the hypertrophic chondrocyte layer of the growth plate, accompanied by decreased cleaved caspase-9. This was associated with decreased phospho-ERK1/2 immunoreactivity in the hypertrophic chondrocyte layer and impaired vascular invasion. These data further demonstrated that Raf kinases are required for phosphate-induced ERK1/2 phosphorylation in cultured hypertrophic chondrocytes and perform essential, but partially redundant roles in growth plate maturation.

14-3-3β Depletion Drives a Senescence Program in Glioblastoma Cells Through the ERK/SKP2/p27 Pathway

The induction of senescence in cancer cells has recently been implicated as a mechanism of tumor regression in response to various modes of stress. 14-3-3 proteins are conserved scaffolding molecules that are involved in various cellular functions. Among the seven isoforms, 14-3-3β is specifically expressed in astrocytoma in correlation with the malignancy grade. We investigated the possible role of 14-3-3β in the regulation of senescence induction in A172 glioblastoma cells. The knockdown of 14-3-3β by specific small interfering RNA resulted in a significant change in cellular phenotypes and an increase in cells staining positive for senescence-associated β-galactosidase. Western blotting of the 14-3-3β-depleted A172 cells revealed increased p27 expression and decreased SKP2 expression, while the expression of p53 and p21 was not altered. Subsequently, we demonstrated that ERK is a key modulator of SKP2/p27 axis activity in 14-3-3β-mediated senescence based on the following: (1) 14-3-3β knockdown decreased p-ERK levels; (2) treatment with U0126, an MEK inhibitor, completely reproduced the senescence morphology as well as the expression profiles of p27 and SKP2; and (3) the senescence phenotypes induced by 14-3-3β depletion were considerably recovered by constitutively active ERK expression. Our results indicate that 14-3-3β negatively regulates senescence in glioblastoma cells via the ERK/SKP2/p27 pathway. Furthermore, 14-3-3β depletion also resulted in senescence phenotypes in U87 glioblastoma cells, suggesting that 14-3-3β could be targeted to induce premature senescence as a therapeutic strategy against glioblastoma progression.

Cyclin D1 in the Liver: Role of Noncanonical Signaling in Liver Steatosis and Hormone Regulation

BACKGROUND

Cyclin D1 is an important protein for cell cycle progression; however, functions independent of the cell cycle have been described in the liver. Cyclin D1 is also involved in DNA repair, is overexpressed in many cancers, and functions as a proto-oncogene. The lesser-known roles of Cyclin D1, specifically in hepatocytes, impact liver steatosis and hormone regulation in the liver.

METHODS

A comprehensive search of PubMed was conducted using the keywords Cyclin D1, steatosis, lipogenesis, and liver transplantation. In this article, we review the results from this literature search, with a focus on the role of Cyclin D1 in hepatic lipogenesis and gluconeogenesis, as well as the impact and function of this protein in hepatic steatosis.

RESULTS

Cyclin D1 represses carbohydrate response element binding protein (ChREBP) and results in a decrease in transcription of fatty acid synthase (FAS) and acetyl-coenzyme A carboxylase (ACC). Cyclin D1 also inhibits peroxisome proliferator-activated receptor gamma (PPARγ) which is involved in hepatic lipogenesis. Cyclin D1 inhibits both hepatocyte nuclear factor 4 alpha (HNF4α) and peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC1α) and represses transcription of lipogenic genes FAS and liver-type pyruvate kinase (Pklr), along with the gluconeogenic genes phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase).

CONCLUSION

Cyclin D1 represses multiple proteins involved in both lipogenesis and gluconeogenesis in the liver. Targeting Cyclin D1 to decrease hepatic steatosis in patients with nonalcoholic fatty liver disease or alcoholic fatty liver disease may help improve patient health and the quality of the donor liver pool.

Cyclin D1, cancer progression, and opportunities in cancer treatment

Mammalian cells encode three D cyclins (D1, D2, and D3) that coordinately function as allosteric regulators of cyclin-dependent kinase 4 (CDK4) and CDK6 to regulate cell cycle transition from G1 to S phase. Cyclin expression, accumulation, and degradation, as well as assembly and activation of CDK4/CDK6 are governed by growth factor stimulation. Cyclin D1 is more frequently dysregulated than cyclin D2 or D3 in human cancers, and as such, it has been more extensively characterized. Overexpression of cyclin D1 results in dysregulated CDK activity, rapid cell growth under conditions of restricted mitogenic signaling, bypass of key cellular checkpoints, and ultimately, neoplastic growth. This review discusses cyclin D1 transcriptional, translational, and post-translational regulations and its biological function with a particular focus on the mechanisms that result in its dysregulation in human cancers.

Targeting RAF kinases for cancer therapy: BRAF-mutated melanoma and beyond

The identification of mutationally activated BRAF in many cancers altered our conception of the part played by the RAF family of protein kinases in oncogenesis. In this Review, we describe the development of BRAF inhibitors and the results that have emerged from their analysis in both the laboratory and the clinic. We discuss the spectrum of RAF mutations in human cancer and the complex interplay between the tissue of origin and the response to RAF inhibition. Finally, we enumerate mechanisms of resistance to BRAF inhibition that have been characterized and postulate how strategies of RAF pathway inhibition may be extended in scope to benefit not only the thousands of patients who are diagnosed annually with BRAF-mutated metastatic melanoma but also the larger patient population with malignancies harbouring mutationally activated RAF genes that are ineffectively treated with the current generation of BRAF kinase inhibitors.

Modelling mammalian cellular quiescence

Cellular quiescence is a reversible non-proliferating state. The reactivation of 'sleep-like' quiescent cells (e.g. fibroblasts, lymphocytes and stem cells) into proliferation is crucial for tissue repair and regeneration and a key to the growth, development and health of higher multicellular organisms, such as mammals. Quiescence has been a primarily phenotypic description (i.e. non-permanent cell cycle arrest) and poorly studied. However, contrary to the earlier thinking that quiescence is simply a passive and dormant state lacking proliferating activities, recent studies have revealed that cellular quiescence is actively maintained in the cell and that it corresponds to a collection of heterogeneous states. Recent modelling and experimental work have suggested that an Rb-E2F bistable switch plays a pivotal role in controlling the quiescence-proliferation balance and the heterogeneous quiescent states. Other quiescence regulatory activities may crosstalk with and impinge upon the Rb-E2F bistable switch, forming a gene network that controls the cells' quiescent states and their dynamic transitions to proliferation in response to noisy environmental signals. Elucidating the dynamic control mechanisms underlying quiescence may lead to novel therapeutic strategies that re-establish normal quiescent states, in a variety of hyper- and hypo-proliferative diseases, including cancer and ageing.

Heparin-binding epidermal growth factor-like growth factor/diphtheria toxin receptor in normal and neoplastic hematopoiesis

Heparin-binding EGF-like growth factor (HB-EGF) belongs to the EGF family of growth factors. It is biologically active either as a molecule anchored to the membrane or as a soluble form released by proteolytic cleavage of the extracellular domain. HB-EGF is involved in relevant physiological and pathological processes spanning from proliferation and apoptosis to morphogenesis. We outline here the main activities of HB-EGF in connection with normal or neoplastic differentiative or proliferative events taking place primitively in the hematopoietic microenvironment.

ERK1/2 MAP kinases promote cell cycle entry by rapid, kinase-independent disruption of retinoblastoma-lamin A complexes

When in the nucleus, ERK1/2 dislodges the retinoblastoma protein from lamin A, facilitating its rapid phosphorylation.

As orchestrators of essential cellular processes like proliferation, ERK1/2 mitogen-activated protein kinase signals impact on cell cycle regulation. A-type lamins are major constituents of the nuclear matrix that also control the cell cycle machinery by largely unknown mechanisms. In this paper, we disclose a functional liaison between ERK1/2 and lamin A whereby cell cycle progression is regulated. We demonstrate that lamin A serves as a mutually exclusive dock for ERK1/2 and the retinoblastoma (Rb) protein. Our results reveal that, immediately after their postactivation entrance in the nucleus, ERK1/2 dislodge Rb from its interaction with lamin A, thereby facilitating its rapid phosphorylation and consequently promoting E2F activation and cell cycle entry. Interestingly, these effects are independent of ERK1/2 kinase activity. We also show that cellular transformation and tumor cell proliferation are dependent on the balance between lamin A and nuclear ERK1/2 levels, which determines Rb accessibility for phosphorylation/inactivation.

Targets of Raf in tumorigenesis

Some 25 years ago, Raf was discovered as the transforming principle shared by a murine sarcoma and an avian carcinoma virus. Thus, Raf and tumorigenesis have been connected from the very beginning. Ten years later, the work of many groups instated Raf as the link between Ras, the oncogene most frequently mutated in human cancers, and the mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK/ERK) module, which with its manifold substrates can contribute to different aspects of carcinogenesis. Finally, the discovery of activating B-Raf mutations in a subset of human cancers, notably melanomas, conclusively established Raf as a major player in tumor development. Recent studies in animal models now show that endogenous C-Raf is essential for the development and maintenance of Ras-induced epidermal tumors. Surprisingly, the role of C-Raf in this case is not that of an mitogen-activated protein kinase activator, but rather that of an endogenous inhibitor of Rho signaling, expanding the range of tumor-related Raf targets. This review focuses on old and new targets of Raf in tumorigenesis.

Regulation of cellular proliferation, differentiation and cell death by activated Raf

The protein kinases Raf-1, A-Raf and B-Raf connect receptor stimulation with intracellular signaling pathways and function as a central intermediate in many signaling pathways. Gain-of-function experiments shed light on the pleiotropic biological activities of these enzymes. Expression experiments involving constitutively active Raf revealed the essential functions of Raf in controlling proliferation, differentiation and cell death in a cell-type specific manner.

Growth inhibitory and anti-tumour activities of OSU-03012, a novel PDK-1 inhibitor, on vestibular schwannoma and malignant schwannoma cells

BACKGROUND

Vestibular schwannomas (VS) frequently express high levels of activated AKT. Small-molecule inhibitors of AKT signalling may have therapeutic potential in suppressing the growth of benign VS and malignant schwannomas.

METHOD

Primary VS and Schwann cells, human malignant schwannoma HMS-97 cells and mouse Nf2(-/-) Schwann cells and schwannoma cells were prepared to investigate the growth inhibitory and anti-tumour activities of OSU-03012, a celecoxib-derived small-molecule inhibitor of phosphoinositide-dependent kinase-1. Cell proliferation assays, apoptosis, Western blot, in vivo xenograft analysis using SCID mice and immunohistochemistry were performed.

RESULTS

OSU-03012 inhibited cell proliferation more effectively in both VS and HMS-97 cells than in normal human Schwann cells. The IC5) of OSU-03012 at 48h was approximately 3.1 microM for VS cells and 2.6 microM for HMS-97 cells, compared with the IC(50) of greater than 12 microM for human Schwann cells. Similarly, mouse Nf2(-/-) schwannoma and Nf2(-/-) Schwann cells were more sensitive to growth inhibition by OSU-03012 than wild-type mouse Schwann cells and mouse schwannoma cells established from transgenic mice carrying the NF2 promoter-driven SV40 T-antigen gene. Like VS cells, malignant schwannoma HMS-97 cells expressed high levels of activated AKT. OSU-03012 induced apoptosis in both VS and HMS-97 cells and caused a marked reduction of AKT phosphorylation at both the Ser-308 and Thr-473 sites in a dose-dependent manner. In vivo xenograft analysis showed that OSU-03012 was well tolerated and inhibited the growth of HMS-97 schwannoma xenografts by 55% after 9 weeks of oral treatment. The anti-tumour activity correlated with reduced AKT phosphorylation.

CONCLUSION

OSU-03012 is a potential chemotherapeutic agent for VS and malignant schwannomas.

Enhanced clonogenic survival induced by protein tyrosine phosphatase (PTP) inhibition after Cr(VI) exposure is mediated by c-Raf and Ras activity

Our recent studies showed that maintenance of protein tyrosine phosphorylation by PTP inhibition enhanced cell growth, clonogenic survival, and mutagenesis after a single low-level Cr(VI) exposure, thereby suggesting that tyrosine phosphorylation-dependent signaling may govern inappropriate survival in human lung fibroblasts (HLFs). Our goal is to identify specific phospho-tyrosine regulator(s)/ downstream effectors involved in enhanced survival after Cr(VI) exposure and PTP inhibition. Phosphotyrosine profiling array showed that PTP inhibition following Cr(VI) exposure increased tyrosine phosphorylation of specific proteins, such as FGR and ABL, which are upstream regulators of both Erk and Akt pathways. To explore the roles of these pathways in the PTP-induced increase in clonogenic survival after Cr(VI) exposure, we examined the effect of combined Akt1 and Erk1/2 knockdown via siRNA technology. Akt1 and/or Erk1/2 silencing had no effect on the PTP inhibitor-induced increase in survival following Cr(VI) exposure, suggesting the presence of non-Akt/non-Erk-mediated survival signaling. Interestingly, geldanamycin, an HSP90 inhibitor and non-specific Raf inhibitor, abrogated the PTP inhibitor-mediated increase in survival following Cr(VI) exposure and abolished the expression/activity of c-Raf and activity of Mek. These findings prompted us to explore upstream regulators of Erk, i.e., Ras, c-Raf and Mek for their potential roles in clonogenic survival. GW5074, a specific c-Raf kinase inhibitor did not alter the effect of the PTP inhibitor but decreased Cr(VI)-mediated clonogenic lethality, potentially though Mek hyperactivation. A genetic approach with a c/a Mek1 mutant also showed that Mek activity was not directly associated with the PTP inhibitor effect. Finally, a genetic approach with d/n or c/a Ras and c-Raf mutants, showed that Ras and c-Raf activities play a substantive role in enhancing clonogenic survival by PTP inhibition following Cr(VI) insult. In conclusion, these studies highlight a novel pro-survival mechanism for clonogenic survival in the face of genotoxic stress in the presence of PTP inhibition via an Erk/Mek-independent and Ras/c-Raf-dependent regulation in normal human lung fibroblasts.

Activating PTPN11 mutants promote hematopoietic progenitor cell-cycle progression and survival

OBJECTIVE

Mutations in PTPN11, which encodes the protein tyrosine phosphatase Shp2, are commonly found in juvenile myelomonocytic leukemia (JMML). We hypothesized that PTPN11 mutations promote cell-cycle progression and confer enhanced survival to hematopoietic progenitors.

MATERIALS AND METHODS

Murine bone marrow low-density mononuclear cells were transduced with pMIEG3, pMIEG3-WT Shp2, pMIEG3-Shp2D61Y, or pMIEG3-Shp2E76K followed by cell-cycle and survival functional analysis as well as biochemical analysis for key cell-cycle and programmed cell-death regulatory proteins.

RESULTS

A higher proportion of hematopoietic progenitors bearing the gain-of-function Shp2 mutants were residing in the S or G2 phase of the cell cycle in response to low doses of granulocyte-macrophage colony-stimulating factor compared to cells transduced with empty vector (MIEG3) or with WT Shp2. Likewise, Shp2D61Y- or Shp2E76K-expressing hematopoietic cells demonstrated reduced apoptosis based on Annexin-V staining and produced increased progenitor colonies after 48 hours in minimal media compared to cells transduced with empty vector or WT Shp2. To differentiate enhanced survival vs hyperproliferation, cells were stained with PKH26 to distinguish undivided cells from divided progeny. Shp2D61Y- or Shp2E76K-expressing PKH26+ cells similarly demonstrated reduced apoptosis. Upon biochemical analysis, expression of Akt- and Erk-responsive cell-cycle and programmed cell-death regulatory proteins were altered, including increased levels of cyclin D1, Bcl2, and BclXL and reduced levels of p27, p21, and Bim.

CONCLUSION

Collectively, these data demonstrate that gain-of-function Shp2 mutants promote hematopoietic progenitor cell-cycle progression and survival and imply that agents targeting the cell cycle or promoting apoptosis may have therapeutic potential in JMML.

C-RAF activation promotes BAD poly-ubiquitylation and turn-over by the proteasome

BAD, a member of the BCL2 family, exhibits an original mode of regulation by phosphorylation. In the present report, we examine the role of the kinase C-RAF in this process. We show that the inducible activation of C-RAF promotes the rapid phosphorylation of BAD on Serine-112 (Ser-75 in the human protein), through a cascade involving the kinases MEK and RSK. Our findings reveal a new aspect of the regulation of BAD protein and its control by the RAF pathway: we find that C-RAF activation promotes BAD poly-ubiquitylation in a phosphorylation-dependent fashion, and increases the turn-over of this protein through proteasomal degradation.

Isoform-specific interaction of C-RAF with mitochondria

The proteins of the RAF family (A-RAF, B-RAF, and C-RAF) are serine/threonine kinases that play important roles in development, mature cell regulation, and cancer. Although it is widely held that their localization on membranes is an important aspect of their function, there are few data that address this aspect of their mode of action. Here, we report that each member of the RAF family exhibits a specific distribution at the level of cellular membranes and that C-RAF is the only isoform that directly targets mitochondria. We found that the RAF kinases exhibit intrinsic differences in terms of mitochondrial affinity and that C-RAF is the only isoform that binds this organelle efficiently. This affinity is conferred by the C-RAF amino-terminal domain and does not depend on the presence of RAS GTPases on the surface of mitochondria. Finally, we analyzed the consequences of C-RAF activation on mitochondria and observed that this event dramatically changes their morphology and their subcellular distribution. Our observations indicate that: (i) RAF kinases exhibit different localizations at the level of cellular membranes; (ii) C-RAF is the only isoform that directly binds mitochondria; and (iii) through its functional coupling with MEK, C-RAF regulates the shape and the cellular distribution of mitochondria.

Survival signaling by C-RAF: mitochondrial reactive oxygen species and Ca2+ are critical targets

Survival signaling by RAF occurs through largely unknown mechanisms. Here we provide evidence for the first time that RAF controls cell survival by maintaining permissive levels of mitochondrial reactive oxygen species (ROS) and Ca(2+). Interleukin-3 (IL-3) withdrawal from 32D cells resulted in ROS production, which was suppressed by activated C-RAF. Oncogenic C-RAF decreased the percentage of apoptotic cells following treatment with staurosporine or the oxidative stress-inducing agent tert-butyl hydroperoxide. However, it was also the case that in parental 32D cells growing in the presence of IL-3, inhibition of RAF signaling resulted in elevated mitochondrial ROS and Ca(2+) levels. Cell death is preceded by a ROS-dependent increase in mitochondrial Ca(2+), which was absent from cells expressing transforming C-RAF. Prevention of mitochondrial Ca(2+) overload after IL-3 deprivation increased cell viability. MEK was essential for the mitochondrial effects of RAF. In summary, our data show that survival control by C-RAF involves controlling ROS production, which otherwise perturbs mitochondrial Ca(2+) homeostasis.

In melanoma, RAS mutations are accompanied by switching signaling from BRAF to CRAF and disrupted cyclic AMP signaling

Melanocytes require the RAS/RAF/MEK/ERK and the cyclic AMP (cAMP) signaling pathways to maintain the fine balance between proliferation and differentiation. We have investigated how cross-talk between these pathways affects melanoma progression. We show that cAMP suppresses CRAF activity in melanocytes and that this is essential to suppress the oncogenic potential of CRAF in these cells. As a consequence, BRAF alone is responsible for signaling to MEK. However, when RAS is mutated in melanoma, the cells switch their signaling from BRAF to CRAF. This switch is accompanied by dysregulated cAMP signaling, a step that is necessary to allow CRAF to signal to MEK. Thus, a fundamental switch in RAF isoform usage occurs when RAS is mutated in melanoma, and this occurs in the context of disrupted cAMP signaling. These data have important implications for the development of therapeutic strategies to treat this life-threatening disease.

What's taking so long? S-phase entry from quiescence versus proliferation

There is a short window in the mammalian cell cycle during which cells can respond to extracellular cues by withdrawing temporarily from the cell cycle. When these cells re-enter the cell cycle, they require several extra hours in the G1 phase before they replicate their DNA compared with their cycling counterparts. More than 20 years after this initial observation, we still do not understand what is taking so long.

Changes in the antioxidant defense and hepatic drug metabolizing enzyme and isoenzyme levels, 8-hydroxydeoxyguanosine formation and expressions of c-raf.1 and insulin-like growth factor II genes during the stages of development of hepatocellular carcinoma in rats

This is an extensive study in a defined initiation-promotion hepatocellular carcinoma model of hepatocarcinogenesis (in rats) in which many important marker enzymes and isoenzymes and 8-hydroxydeoxyguanosine formation have been studied together with two very important cellular proliferating genes, insulin-like growth factor II and c-raf.1, known for their role in hepatocellular cancer development. Experiments were carried out on hepatic tissues of male Sprague-Dawley rats. Variations in different enzyme/isoenzyme activities/contents/expression pattern and 8-hydroxydeoxyguanosine-positive cells were studied. Insulin-like growth factor II and c-raf.1 gene expressions were monitored. A direct shift with increase in size and numbers of lesions was found to occur in different experimental groups. In this study, glutathione peroxidase (1.14 and 1.46-fold) and reduced triphosphopyridine nucleotide (TPNH)-cytochrome-c-reductase (1.94 and 2.94-fold) activities, cytochrome b5 (1.57 and 3.28-fold) and P-450 contents (1.45 and 1.22-fold), glutathione content (1.27 and 1.45-fold) and superoxide dismutase and catalase (1.16 and 1.39-fold) activities in group A animals were found to be lower than those in initiation and promotion studies, respectively. 8-Hydroxydeoxyguanosine-positive nuclei count showed that oxidative damage of nuclear DNA enhanced with the progress of the disease. The insulin-like growth factor II expression was found to be predominant in hepatocellular carcinoma and in early preneoplastic lesions. Unlike insulin-like growth factor II, c-raf.1 expression was located in the late basophilic lesions associated with hepatocellular carcinoma. During the various stages of the development of hepatocellular carcinoma, the enzymes played a significant role in metabolizing carcinogens and thereby scavenging various toxic metabolites or free radicals produced. A sequence of cellular changes starting from the appearance of glycogen storage foci to basophilic foci leading to hepatocellular carcinoma via mixed cell foci varied the activity/content or expression pattern of the enzymes and isoenzymes and in 8-hydroxydeoxyguanosine formation. It has been established that c-raf.1-induced signaling pathways activated by insulin-like growth factor II is implicated in the late stage of development of cancer.

The ERK1/2 mitogen-activated protein kinase pathway as a master regulator of the G1- to S-phase transition

The Ras-dependent extracellular signal-regulated kinase (ERK)1/2 mitogen-activated protein (MAP) kinase pathway plays a central role in cell proliferation control. In normal cells, sustained activation of ERK1/ERK2 is necessary for G1- to S-phase progression and is associated with induction of positive regulators of the cell cycle and inactivation of antiproliferative genes. In cells expressing activated Ras or Raf mutants, hyperactivation of the ERK1/2 pathway elicits cell cycle arrest by inducing the accumulation of cyclin-dependent kinase inhibitors. In this review, we discuss the mechanisms by which activated ERK1/ERK2 regulate growth and cell cycle progression of mammalian somatic cells. We also highlight the findings obtained from gene disruption studies.

Activation of the MAP kinase pathway induces apoptosis in the Merkel cell carcinoma cell line UISO

Merkel cell carcinoma (MCC) is a rare but highly aggressive tumor of the skin. Recently, we have shown that MCC cells in situ are characterized by a complete absence of mitogen-activated protein kinase (MAPK) pathway signaling, which is preserved in the MCC cell line UISO. Here we present data suggesting that silencing of the MAPK pathway is essential for the survival of MCC cells. Activation of the MAPK pathway could be achieved by inducing a regulatable form of the c-Raf-1 kinase domain in UISO cells. Consequently, MAPK signaling led to morphological changes, loss of actin stress fibers, and induction of apoptosis, which could be prevented by the MAP kinase kinase-specific inhibitor U0126. Hence, despite the fact that activation of the MAPK pathway contributes to oncogenesis in many cancers, it seems to be a negative selection factor for MCC cells. Since ERK phosphorylation was also inducible by the Raf-activating pharmacological agent ZM336372, these results provide new perspectives for potential therapeutics for this highly aggressive tumor.

Critical requirement for cell cycle inhibitors in sustaining nonproliferative states

In adult vertebrates, most cells are not in the cell cycle at any one time. Physiological nonproliferation states encompass reversible quiescence and permanent postmitotic conditions such as terminal differentiation and replicative senescence. Although these states appear to be attained and maintained quite differently, they might share a core proliferation-restricting mechanism. Unexpectedly, we found that all sorts of nonproliferating cells can be mitotically reactivated by the sole suppression of histotype-specific cyclin-dependent kinase (cdk) inhibitors (CKIs) in the absence of exogenous mitogens. RNA interference–mediated suppression of appropriate CKIs efficiently triggered DNA synthesis and mitosis in established and primary terminally differentiated skeletal muscle cells (myotubes), quiescent human fibroblasts, and senescent human embryo kidney cells. In serum-starved fibroblasts and myotubes alike, cell cycle reactivation was critically mediated by the derepression of cyclin D–cdk4/6 complexes. Thus, both temporary and permanent growth arrest must be actively maintained by the constant expression of CKIs, whereas the cell cycle–driving cyclins are always present or can be readily elicited. In principle, our findings could find wide application in biotechnology and tissue repair whenever cell proliferation is limiting.

ERK activity and G1 phase progression: identifying dispensable versus essential activities and primary versus secondary targets

The ERK subfamily of MAP kinases is a critical regulator of S phase entry. ERK activity regulates the induction of cyclin D1, and a sustained ERK signal is thought to be required for this effect, at least in fibroblasts. We now show that early G1 phase ERK activity is dispensable for the induction of cyclin D1 and that the critical ERK signaling period is restricted to 3-6 h after mitogenic stimulation of quiescent fibroblasts. Similarly, early G1 phase ERK activity is dispensable for entry into S phase. Moreover, if cyclin D1 is expressed ectopically, ERK activity becomes dispensable throughout the G1 phase. In addition to its effect on cyclin D1, ERK activity is thought to contribute to the down-regulation of p27kip1. We found that this effect is restricted to late G1/S phase. Mechanistic analysis showed that the ERK effect on p27kip1 is mediated by Skp2 and is secondary to its effect on cyclin D1. Our results emphasize the importance of mid-G1 phase ERK activity and resolve primary versus secondary ERK targets within the G1 phase cyclin-dependent kinases.

Macrophage-colony-stimulating factor-induced proliferation and lipopolysaccharide-dependent activation of macrophages requires Raf-1 phosphorylation to induce mitogen kinase phosphatase-1 expression

Macrophages are key regulators of immune responses. In the absence of an activating signal, murine bone marrow-derived macrophages undergo proliferation in response to their specific growth factor, namely M-CSF. The addition of bacterial LPS results in macrophage growth arrest and their engagement in a proinflammatory response. Although participation of ERKs is required for both macrophage proliferation and activation, ERK phosphorylation follows a more delayed pattern in response to activating agents. In primary macrophages, mitogen kinase phosphatase-1 (MKP-1) is a key regulator of the time course of MAPK activity. Here we showed that MKP-1 expression is dependent on Raf-1 activation. The time course of Raf-1 activation correlated with that of ERK-1/2. However, whereas ERK phosphorylation in response to M-CSF is Raf-1 dependent, in response to LPS, an alternative pathway directs the activation of these kinases. Inhibition of Raf-1 activity increased the expression of cyclin-dependent kinase inhibitors and growth arrest. In contrast, no effect was observed in the expression of proinflammatory cytokines and inducible NO synthase following LPS stimulation. The data reported here reveal new insights into how signaling determines opposing macrophage functions.

Identification of mutations that disrupt phosphorylation-dependent nuclear export of cyclin D1

Although cyclin D1 is overexpressed in a significant number of human cancers, overexpression alone is insufficient to promote tumorigenesis. In vitro studies have revealed that inhibition of cyclin D1 nuclear export unmasks its neoplastic potential. Cyclin D1 nuclear export depends upon phosphorylation of a C-terminal residue, threonine 286, (Thr-286) which in turn promotes association with the nuclear exportin, CRM1. Mutation of Thr-286 to a non-phosphorylatable residue results in a constitutively nuclear cyclin D1 protein with significantly increased oncogenic potential. To determine whether cyclin D1 is subject to mutations that inhibit its nuclear export in human cancer, we have sequenced exon 5 of cyclin D1 in primary esophageal carcinoma samples and in cell lines derived from esophageal cancer. Our work reveals that cyclin D1 is subject to mutations in primary human cancer. The mutations identified specifically disrupt phosphorylation of cyclin D1 at Thr-286, thereby enforcing nuclear accumulation of cyclin D1. Through characterization of these mutants, we also define an acidic residue within the C-terminus of cyclin D1 that is necessary for recognition and phosphorylation of cyclin D1 by glycogen synthase kinase-3 beta. Finally, through construction of compound mutants, we demonstrate that cell transformation by the cancer-derived cyclin D1 alleles correlates with their ability to associate with and activate CDK4. Our data reveal that cyclin D1 is subject to mutations in primary human cancer that specifically disrupt phosphorylation-dependent nuclear export of cyclin D1 and suggest that such mutations contribute to the genesis and progression of neoplastic growth.

BRAF mutation predicts sensitivity to MEK inhibition

The kinase pathway comprising RAS, RAF, mitogen-activated protein kinase kinase (MEK) and extracellular signal regulated kinase (ERK) is activated in most human tumours, often through gain-of-function mutations of RAS and RAF family members. Using small-molecule inhibitors of MEK and an integrated genetic and pharmacologic analysis, we find that mutation of BRAF is associated with enhanced and selective sensitivity to MEK inhibition when compared to either 'wild-type' cells or cells harbouring a RAS mutation. This MEK dependency was observed in BRAF mutant cells regardless of tissue lineage, and correlated with both downregulation of cyclin D1 protein expression and the induction of G1 arrest. Pharmacological MEK inhibition completely abrogated tumour growth in BRAF mutant xenografts, whereas RAS mutant tumours were only partially inhibited. These data suggest an exquisite dependency on MEK activity in BRAF mutant tumours, and offer a rational therapeutic strategy for this genetically defined tumour subtype.

Molecular determinants of kinase pathway activation by Apo2 ligand/tumor necrosis factor-related apoptosis-inducing ligand

Apo2 ligand/tumor necrosis factor (TNF)-related apoptosis-inducing ligand (Apo2L/TRAIL) mainly activates programmed cell death through caspases. By contrast, TNF primarily induces gene transcription through the inhibitor of kappaB kinase (IKK), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase pathways. Apo2L/TRAIL also can stimulate these kinases, albeit less strongly; however, the underlying mechanisms of this stimulation and its relation to apoptosis are not well understood. Here we show that Apo2L/TRAIL activates kinase pathways by promoting the association of a secondary signaling complex, subsequent to assembly of a primary, death-inducing signaling complex (DISC). The secondary complex retained the DISC components FADD and caspase-8, but recruited several factors involved in kinase activation by TNF, namely, RIP1, TRAF2, and NEMO/IKKgamma. Secondary complex formation required Fas-associated death domain (FADD), as well as caspase-8 activity. Apo2L/TRAIL stimulation of JNK and p38 further depended on RIP1 and TRAF2, whereas IKK activation required NEMO. Apo2L/TRAIL induced secretion of interleukin-8 and monocyte chemoattractant protein-1, augmenting macrophage migration. Thus, Apo2L/TRAIL and TNF organize common molecular determinants in distinct signaling complexes to stimulate similar kinase pathways. One function of kinase stimulation by Apo2L/TRAIL may be to promote phagocytic engulfment of apoptotic cells.

Calcium-activated RAF/MEK/ERK signaling pathway mediates p53-dependent apoptosis and is abrogated by alpha B-crystallin through inhibition of RAS activation

The ocular lens is the only organ that does not develop spontaneous tumor. The molecular mechanism for this phenomenon remains unknown. Through examination of the signaling pathways mediating stress-induced apoptosis, here we presented evidence to show that different from most other tissues in which the extracellular signal-regulated kinases (ERKs) pathway is generally implicated in mediation of survival signals activated by different factors, the RAF/MEK/ERK signaling pathway alone plays a key role in stress-activated apoptosis of lens epithelial cells. Treatment of N/N1003A cells with calcimycin, a calcium mobilizer, activates the RAF/MEK/ERK pathway through RAS, which is indispensable for the induced apoptosis because inhibition of this pathway by either pharmacological drug or dominant negative mutants greatly attenuates the induced apoptosis. Calcimycin also activates p38 kinase and JNK2, which are not involved in calcium-induced apoptosis. Downstream of ERK activation, p53 is essential. Activation of RAF/MEK/ERK pathway by calcimycin leads to distinct up-regulation of p53. Moreover, overexpression of p53 enhances calcimycin-induced apoptosis, whereas inhibition of p53 expression attenuates calcimycin-induced apoptosis. Up-regulation of p53 directly promotes Bax expression, which changes the integrity of mitochondria, leading to release of cytochrome c, activation of caspase-3 and eventually execution of apoptosis. Overexpression of alphaB-crystallin, a member of the small heat-shock protein family, blocks activation of RAS to inhibit ERK1/2 activation, and greatly attenuates calcimycin-induced apoptosis. Together, our results provide 1) a partial explanation for the lack of spontaneous tumor in the lens, 2) a novel signaling pathway for calcium-induced apoptosis, and 3) a novel antiapoptotic mechanism for alphaB-crystallin.

The priming/completion paradigm to explain growth factor-dependent cell cycle progression

Approximately 50 years ago, researchers established conditions to maintain cells in tissue culture: Likely et al. (1952), Scherer et al. (1953), Eagle (1955). This simple model system set the stage for discovery of growth factors and the signaling systems that they engage to mediate cellular responses such as proliferation. The purpose of this review is to present the original view of how growth factors regulate cell cycle progression and an updated (priming/completion) version of how growth factors advance resting cells through the cell cycle.

Apoptosis induced by the kinase inhibitor BAY 43-9006 in human leukemia cells involves down-regulation of Mcl-1 through inhibition of translation

BAY 43-9006 is a kinase inhibitor that induces apoptosis in a variety of tumor cells. Here we report that treatment with BAY 43-9006 results in marked cytochrome c and AIF release into the cytosol, caspase-9, -8, -7, and -3 activation, and apoptosis in human leukemia cells (U937, Jurkat, and K562). Pronounced apoptosis was also observed in blasts from patients with acute myeloid leukemia. These events were accompanied by ERK1/2 inactivation and caspase-independent down-regulation of Mcl-1. Inducible expression of a constitutively active MEK1 construct did not prevent Mcl-1 down-regulation, suggesting that this event is not related to MEK/ERK pathway inactivation. Furthermore, BAY 43-9006 did not induce major changes in Mcl-1 mRNA levels monitored by real-time PCR or Mcl-1 promoter activity demonstrated by luciferase reporter assays, but it did enhance Mcl-1 down-regulation in actinomycin D-treated cells. Inhibition of protein synthesis by cycloheximide or proteasome function with MG132 and pulse-chase studies with [35S]methionine demonstrated that BAY 43-9006 did not diminish Mcl-1 protein stability, nor did it enhance Mcl-1 ubiquitination, but instead markedly attenuated Mcl-1 translation in association with the rapid and potent dephosphorylation of the eIF4E translation initiation factor. Finally, ectopic expression of Mcl-1 in leukemic cells markedly inhibited BAY 43-9006-mediated cytochrome c cytosolic release, caspase-9, -7, and -3 activation, as well as cell death, indicating that Mcl-1 operates upstream of cytochrome c release and caspase activation. Together, these findings demonstrate that BAY 43-9006 mediates cell death in human leukemia cells, at least in part, through down-regulation of Mcl-1 via inhibition of translation.

A-Raf and Raf-1 work together to influence transient ERK phosphorylation and Gl/S cell cycle progression

The Raf/MEK/ERK (extracellular regulated kinase) signal transduction pathway controls the ability of cells to respond to proliferative, apoptotic, migratory and differentiation signals. We have investigated the combined contribution of A-Raf and Raf-1 isotypes to signalling through this pathway by generating mice with knockout mutations of both A-raf and raf-1 genes. Double knockout (DKO) mice have a more severe phenotype than single null mutations of either gene, dying in embryogenesis at E10.5. The DKO embryos show no changes in apoptosis, but staining for Ki67 indicates a generalized reduction in proliferation. DKO mouse embryonic fibroblasts (MEFs) exhibit a delayed ability to enter S phase of the cell cycle. This is associated with a reduction in levels of transiently induced MEK and ERK phosphorylation and reduced expression of c-Fos and cyclin Dl. Levels of sustained ERK phosphorylation are not significantly altered. Thus, Raf-1 and A-Raf have a combined role in controlling physiological transient ERK activation and in maintenance of cell cycle progression at its usual rate.

Very-KIND is a novel nervous system specific guanine nucleotide exchange factor for Ras GTPases

The kinase non-catalytic c-lobe domain (KIND) evolved from the catalytic protein kinase fold into a potential protein interaction module for signalling proteins. Spir family actin organizers and the non-receptor phosphatase type 13 (PTP type 13) encode a KIND domain in the very N-terminal parts of the proteins. Here we report the characterization and cloning of a third member of the KIND protein family, which we have named very-KIND (VKIND) because of its two KIND domains. Like the other members of the protein family, VKIND has a KIND domain at the N-terminus. A second KIND domain is located in the central part of the protein. The C-terminal half encodes a guanine nucleotide exchange factor motif for Ras-like GTPases (RasGEF) and a RasGEF N-terminal module (RasGEFN). There is only one VKIND gene in the mammalian genomes and up to now we have found the gene only in vertebrates. During mouse embryogenesis the VKIND gene was specifically expressed in the developing nervous system. In adult mice Northern hybridizations revealed high expression only in brain. Low expression could be detected in ovary. In situ hybridizations showed a specific expression of VKIND in neuronal cells of the granular and Purkinje cell layers of the cerebellum.

Phase I safety and pharmacokinetics of BAY 43-9006 administered for 21 days on/7 days off in patients with advanced, refractory solid tumours

BAY 43-9006 is a novel dual-action Raf kinase and vascular endothelial growth factor receptor (VEGFR) inhibitor that targets tumour cell proliferation and tumour angiogenesis. This Phase I study was undertaken to determine the safety profile, maximum tolerated dose (MTD), dose-limiting toxicities (DLTs), pharmacokinetics, and tumour response profile of oral BAY 43-9006 in patients with advanced, refractory solid tumours. BAY 43-9006 was administered daily for repeated cycles of 21 days on/7 days off. A total of 44 patients were enrolled at doses from 50 to 800 mg b.i.d. Pharmacokinetic profiles of BAY 43-9006 in plasma were determined during the first treatment cycle. The most frequently reported adverse events over multiple cycles were gastrointestinal (75%), dermatologic (71%), constitutional (68%), pain (64%), or hepatic (61%) related. A MTD of 400 mg b.i.d. BAY 43-9006 was defined. BAY 43-9006 was absorbed rapidly; steady-state conditions were reached within 7 days. BAY 43-9006 exposure increased nonproportionally with increasing dose. In all, 32 patients were evaluated for tumour response: 15 patients showed tumour progression, 16 patients experienced stable disease (>6 months in eight patients), and one patient with renal cell carcinoma achieved a partial response. BAY 43-9006 given for 21 days with 7 days off treatment was safe, well tolerated, and showed antitumour activity.

Schlafen-1 causes a cell cycle arrest by inhibiting induction of cyclin D1

Schlafen-1 (Slfn-1), the prototypic member of the Schlafen family of proteins, was described as an inducer of growth arrest in T-lymphocytes and causes a cell cycle arrest in NIH3T3 fibroblasts prior to the G1/S transition. How Slfn-1 exerts its effects on the cell cycle is not currently known. We report that synchronized murine fibroblasts expressing Slfn-1 do not exit G1 when stimulated with fetal calf serum, platelet-derived growth factor BB (PDGF-BB) or epidermal growth factor (EGF). The induction of cyclin D1 by these stimuli was blocked in the presence of Slfn-1 as were all downstream cell cycle processes. Overexpression of cyclin D1 in growth-arrested, Slfn-1-expressing cells induced an increase in cell growth consistent with this protein being the biological target of Slfn-1. Activation of the mitogen-activated protein kinase pathway by EGF or phorbol 12-myristate 13-acetate was unaffected by Slfn-1 expression. PDGF signaling was, however, almost completely blocked. This was due to a lack of PDGF receptor expression in Slfn-1-expressing cells consistent with Slfn-1 blocking the cell cycle in G1 where PDGF receptor expression is normally down-regulated. Finally, overexpression of Slfn-1 inhibited the activation of the cyclin D1 promoter. Slfn-1 therefore causes a cell cycle arrest during G1 by inhibiting induction of cyclin D1 by mitogens.

The MAPK signalling pathways and colorectal cancer

There are three major subfamilies of mitogen-activated protein kinases (MAPK): the extracellular-signal-regulated kinases (ERK MAPK); the c-jun N-terminal kinase or stress-activated protein kinases (JNK or SAPK); and MAPK14. The ERK MAPK pathway is one of the most important for cell proliferation. The MAPK pathways are located downstream of many growth-factor receptors, including that for epidermal growth factor. Overexpression and activation of this receptor are commonly detected in colorectal cancer, and several lines of evidence indicate that overexpression and activation of ERK MAPK play an important part in progression of this cancer. ERK MAPK could be a molecular target for treatment of the disorder. This review focuses on the ERK MAPK signal-transduction pathway, the consequences of its dysregulation in colorectal cancer, and its potential as an approach to cancer treatment. Future challenges for the assessment of these targeted agents in the clinic are also presented.

B-Raf and C-Raf signaling investigated in a simplified model of the mitogenic kinase cascade

Signaling pathways based on the reversible phosphorylation of proteins control most aspects of cellular life in higher organisms. Extracellular stimuli can induce growth, differentiation, survival and the stress response through a number of highly conserved signaling pathways. We discuss how the intensity and duration of signals may have dramatic consequences on the way cells respond to stimuli. Picking the central Ras-Raf-MEK-ERK signal cascade, we developed a mathematical model of how stimuli induce different signal patterns and thereby different cellular responses, depending on cell type and the ratio between B-Raf and C-Raf. Based on biochemical data for activation and dephosphorylation, as well as the differential equations of our model, we suggest a different signaling pattern and response result for B-Raf (strong activation, sustained signal) and C-Raf (steep activation, transient signal). We further support the significance of such differential modulatory signaling by showing different Raf isoform expression in various cell lines and experimental testing of the predicted kinase activities in B-Raf, C-Raf and mutated versions.

Phase I clinical and pharmacokinetic study of the Novel Raf kinase and vascular endothelial growth factor receptor inhibitor BAY 43-9006 in patients with advanced refractory solid tumors

PURPOSE

BAY 43-9006 is a novel dual-action Raf kinase and vascular endothelial growth factor receptor inhibitor that inhibits tumor cell proliferation and angiogenesis. This study established the safety and pharmacokinetics of BAY 43-9006 in 69 patients with advanced refractory solid tumors.

PATIENTS AND METHODS

BAY 43-9006 (50 to 800 mg) was administered once or twice daily on a varying weekly schedule. Pharmacokinetic sampling was performed in all patients; preliminary tumor response was also assessed. The effect of BAY 43-9006 on phorbol myristate acetate-stimulated ERK phosphorylation in peripheral blood lymphocytes was studied using flow cytometry.

RESULTS

Mild to moderate diarrhea was the most common (55%) treatment-related adverse event. The maximum-tolerated dose was 400 mg bid continuous. Dose-limiting toxicities were grade 3 diarrhea and fatigue at 800 mg bid, and grade 3 skin toxicity at 600 mg bid. BAY 43-9006 pharmacokinetics were highly variable for single and multiple dosing, and toxicity did not appear to be dose dependent. Significant decreases of phorbol myristate acetate-stimulated ERK phosphorylation (P < .01) were identified at doses >/= 200 mg bid continuous. Forty-five patients were assessable for efficacy; one patient had a partial response (hepatocellular carcinoma at 400 mg bid continuous), 25 patients had stable disease, with eight lasting > 6 months and five for >12 months. Eighteen patients had progressive disease, and tumor response could not be evaluated in one patient.

CONCLUSION

Oral BAY 43-9006 was well tolerated and appeared to provide some clinical benefits. Based on the results of this study, BAY 43-9006 at 400 mg bid continuous is recommended for ongoing and future studies.

Overlapping expression pattern of the actin organizers Spir-1 and formin-2 in the developing mouse nervous system and the adult brain

The Wiskott-Aldrich homology domain 2 (WH2) family protein Spir and the formin Cappuccino belong to two distinct classes of actin organizers. Despite their functional classification as actin organizers, a major defect of Drosophila spire and cappuccino mutant oocytes is a failure in the orientation of microtubule plus ends towards the posterior pole. Mammalian homologues of spire are the spir-1 and spir-2 genes. The mouse and human formin-1 and formin-2 genes have high similarity to the cappuccino gene. The mouse formin-2 gene has been found to be expressed in the developing nervous system and in neuronal cells of the adult brain. By analyzing the expression of the spir-1 gene we show that spir-1 and formin-2 have a nearly identical expression pattern during mouse embryogenesis and in the adult brain. In mouse embryos both genes are expressed in the developing nervous system. In the adult brain high expression of the genes was found in the Purkinje cells of the cerebellum and in neuronal cells of the hippocampus and dentate gyrus.

Cooperation of Ras- and c-Myc-dependent pathways in regulating the growth and invasiveness of synovial fibroblasts in rheumatoid arthritis

OBJECTIVE

To study the specific contribution of MAP kinase activator c-Raf-1 and one of its downstream transcription factors, c-Myc, to the growth and invasive behavior of rheumatoid arthritis synovial fibroblasts (RASFs).

METHODS

RASFs were transduced with retroviral constructs expressing dominant-negative mutants of c-Raf-1 or c-Myc (DN c-Raf-1 or DN c-Myc, respectively) or with the mock vector. The expression of wild-type and mutant proteins was confirmed by Western blotting. Growth curves of RASFs were recorded, and apoptosis was measured by flow cytometry. Invasiveness of RASFs was assessed in the SCID mouse model of RA. Immunohistochemistry was used to study the effects of DN c-Raf-1 on phosphorylated c-Jun and matrix metalloproteinase 1 (MMP-1) in RASFs implanted into SCID mice. The phosphorylation of ERK and JNK in DN c-Raf-1- and mock-transduced RASFs was determined in vitro by Western blotting. The levels of MMPs in these cells were measured by quantitative polymerase chain reaction (PCR).

RESULTS

Neither DN c-Raf-1 alone nor DN c-Myc alone significantly altered proliferation or apoptosis of RASFs, but both mutants together rapidly induced apoptosis. Inhibition of c-Raf-1 or c-Myc significantly reduced the invasiveness of RASFs in the SCID mouse model. DN c-Raf-1 decreased the phosphorylation of ERK and JNK in vitro and reduced the in vivo expression of phosphorylated c-Jun as well as the expression of disease-relevant MMPs. As determined by quantitative PCR, the inhibition was most pronounced for MMP-1 and MMP-3.

CONCLUSION

The data demonstrate that Ras- and c-Myc-dependent signaling events cooperate to regulate the growth and invasiveness of RASFs. Targeting of both c-Raf-1 and c-Myc may constitute an interesting therapeutic approach in RA.

The p75NTR-interacting protein SC1 inhibits cell cycle progression by transcriptional repression of cyclin E

Schwann cell factor 1 (SC1), a p75 neurotrophin receptor–interacting protein, is a member of the positive regulatory/suppressor of variegation, enhancer of zeste, trithorax (PR/SET) domain-containing zinc finger protein family, and it has been shown to be regulated by serum and neurotrophins. SC1 shows a differential cytoplasmic and nuclear distribution, and its presence in the nucleus correlates strongly with the absence of bromodeoxyuridine (BrdU) in these nuclei. Here, we investigated potential transcriptional activities of SC1 and analyzed the function of its various domains. We show that SC1 acts as a transcriptional repressor when it is tethered to Gal4 DNA-binding domain. The repressive activity requires a trichostatin A–sensitive histone deacetylase (HDAC) activity, and SC1 is found in a complex with HDACs 1, 2, and 3. Transcriptional repression exerted by SC1 requires the presence of its zinc finger domains and the PR domain. Additionally, these two domains are involved in the efficient block of BrdU incorporation by SC1. The zinc finger domains are also necessary to direct SC1's nuclear localization. Lastly, SC1 represses the promoter of a promitotic gene, cyclin E, suggesting a mechanism for how growth arrest is regulated by SC1.

Different effects of point mutations within the B-Raf glycine-rich loop in colorectal tumors on mitogen-activated protein/extracellular signal-regulated kinase kinase/extracellular signal-regulated kinase and nuclear factor kappaB pathway and cellular transformation

Recently, mutations in the B-Raf gene have been identified in a variety of human cancers, such as melanoma and colorectal carcinoma, and more than 80% of the B-Raf mutations have been V599E. Although other mutations have been reported, their functional consequences are poorly understood. In our earlier study, we demonstrated that colon tumor-associated B-Raf mutations within the kinase activation segment are not necessarily associated with an increase in mitogen-activated protein/extracellular signal-regulated kinase kinase/extracellular signal-regulated kinase (MEK/Erk) or nuclear factor kappaB (NFkappaB) signaling activity or in NIH3T3-transforming ability. In this study, we examined the effect of colon tumor-associated mutations within the B-Raf glycine-rich loop (G loop) on MEK/Erk and NFkappaB signaling and on the transformation of NIH3T3 fibroblasts or IEC-6 intestinal epithelial cells. Of the six G loop mutations examined, only the B-Raf G468A significantly increased MEK/Erk and NFkappaB signaling and NIH3T3 transformation. Only this mutation induced transformed phenotypes of IEC-6 cells. In contrast, the B-Raf G468E mutation significantly decreased MEK/Erk signaling and NIH3T3 transformation and had no effect on NFkappaB signaling. The B-Raf F467C mutation moderately elevated MEK/Erk signaling and NIH3T3 transformation. The other three B-Raf mutations, R461I, I462S, and G463E, did not increase MEK/Erk or NFkappaB signaling or NIH3T3 transformation. Except for F467C, none of the tumors with B-Raf mutations examined in this study had K-Ras mutations. These results suggest that some of the B-Raf G loop mutations reported in colorectal tumors do not increase kinase or transforming activities but might contribute to carcinogenesis via other mechanisms or be irrelevant to carcinogenesis.

Successful growth and characterization of mouse pancreatic ductal cells: functional properties of the Ki-RAS(G12V) oncogene

BACKGROUND & AIMS

The Ki-RAS oncogene is altered in pancreatic ductal neoplasms. Pancreatic ductal cells (PDCs) were purified from cytokeratin 19 (K19)-Ki-RAS(G12V) transgenic mice and control littermates to identify properties of Ki-Ras activation in a cell-type-specific context. Because Ki-RAS mutation has prognostic significance in patients treated with radiation, we studied the influence of Ki-RAS status on radiation survival.

METHODS

Pancreatic ductal fragments from mice with Ki-RAS(G12V) mutation or wild-type (WT)-Ki-RAS were cultured. Growth curves, electron microscopy, flow cytometry, and analysis of signaling and cell-cycle proteins were established. Farnesyltransferase inhibitor (FTI) treatment with R115777 before and after irradiation was used to determine the effect of Ki-Ras farnesylation on cell survival.

RESULTS

PDCs from WT and K19-Ki-RAS(G12V) mice had features of ductal cells with formation of 3-dimensional structures on collagen without differences in morphology, growth, and cell-cycle distribution. This may result from up-regulation of p16INK4 and p27(Kip1) and lack of hyperstimulation of the mitogen-activated protein kinase pathway in Ki-RAS(G12V) PDCs. No differences in radiation survival between Ki-RAS(G12V) PDCs and WT PDCs were observed. However, Ki-RAS(G12V) PDCs expressing mutant p53(V143A) had enhanced survival compared with WT PDCs transduced with p53(V143A). R115777 treatment sensitized Ki-RAS(G12V) PDCs and Ki-RAS(G12V)/p53(V143A) PDCs, but not WT PDCs.

CONCLUSIONS

Novel characterization of murine WT PDCs and Ki-RAS(G12V) PDCs is described. Induction of cell-cycle regulators and lack of mitogen-activated protein kinase hyperstimulation likely are responsible for constraining activated Ki-RAS(G12V)-mediated proliferation. Because its activation was required for sensitization by an FTI, R115777 may be useful against pancreatic tumors expressing oncogenic Ki-Ras.

Murine coronavirus replication induces cell cycle arrest in G0/G1 phase

Mouse hepatitis virus (MHV) replication in actively growing DBT and 17Cl-1 cells resulted in the inhibition of host cellular DNA synthesis and the accumulation of infected cells in the G(0)/G(1) phase of the cell cycle. UV-irradiated MHV failed to inhibit host cellular DNA synthesis. MHV infection in quiescent 17Cl-1 cells that had been synchronized in the G(0) phase by serum deprivation prevented infected cells from entering the S phase after serum stimulation. MHV replication inhibited hyperphosphorylation of the retinoblastoma protein (pRb), the event that is necessary for cell cycle progression through late G(1) and into the S phase. While the amounts of the cellular cyclin-dependent kinase (Cdk) inhibitors p21(Cip1), p27(Kip1), and p16(INK4a) did not change in infected cells, MHV infection in asynchronous cultures induced a clear reduction in the amounts of Cdk4 and G(1) cyclins (cyclins D1, D2, D3, and E) in both DBT and 17Cl-1 cells and a reduction in Cdk6 levels in 17Cl-1 cells. Infection also resulted in a decrease in Cdk2 activity in both cell lines. MHV infection in quiescent 17Cl-1 cells prevented normal increases in Cdk4, Cdk6, cyclin D1, and cyclin D3 levels after serum stimulation. The amounts of cyclin D2 and cyclin E were not increased significantly after serum stimulation in mock-infected cells, whereas they were decreased in MHV-infected cells, suggesting the possibility that MHV infection may induce cyclin D2 and cyclin E degradation. Our data suggested that a reduction in the amounts of G(1) cyclin-Cdk complexes in MHV-infected cells led to a reduction in Cdk activities and insufficient hyperphosphorylation of pRb, resulting in inhibition of the cell cycle in the G(0)/G(1) phase.