Raf-1 regulates Rho signaling and cell migration

Raf kinases in cancer-roles and therapeutic opportunities

Raf are conserved, ubiquitous serine/protein kinases discovered as the cellular elements hijacked by transforming retroviruses. The three mammalian RAF proteins (A, B and CRAF) can be activated by the human oncogene RAS, downstream from which they exert both kinase-dependent and kinase-independent, tumor-promoting functions. The kinase-dependent functions are mediated chiefly by the MEK/ERK pathway, whose activation is associated with proliferation in a broad range of human tumors. Almost 10 years ago, activating BRAF mutations were discovered in a subset of human tumors, and in the past year treatment with small-molecule RAF inhibitors has yielded unprecedented response rates in melanoma patients. Thus, Raf qualifies as an excellent molecular target for anticancer therapy. This review focuses on the role of BRAF and CRAF in different aspects of carcinogenesis, on the success of molecular therapies targeting Raf and the challenges they present.

Spatial phosphoprotein profiling reveals a compartmentalized extracellular signal-regulated kinase switch governing neurite growth and retraction

Brain development and spinal cord regeneration require neurite sprouting and growth cone navigation in response to extension and collapsing factors present in the extracellular environment. These external guidance cues control neurite growth cone extension and retraction processes through intracellular protein phosphorylation of numerous cytoskeletal, adhesion, and polarity complex signaling proteins. However, the complex kinase/substrate signaling networks that mediate neuritogenesis have not been investigated. Here, we compare the neurite phosphoproteome under growth and retraction conditions using neurite purification methodology combined with mass spectrometry. More than 4000 non-redundant phosphorylation sites from 1883 proteins have been annotated and mapped to signaling pathways that control kinase/phosphatase networks, cytoskeleton remodeling, and axon/dendrite specification. Comprehensive informatics and functional studies revealed a compartmentalized ERK activation/deactivation cytoskeletal switch that governs neurite growth and retraction, respectively. Our findings provide the first system-wide analysis of the phosphoprotein signaling networks that enable neurite growth and retraction and reveal an important molecular switch that governs neuritogenesis.

Raf family kinases: old dogs have learned new tricks

First identified in the early 1980s as retroviral oncogenes, the Raf proteins have been the objects of intense research. The discoveries 10 years later that the Raf family members (Raf-1, B-Raf, and A-Raf) are bona fide Ras effectors and upstream activators of the ubiquitous ERK pathway increased the interest in these proteins primarily because of the central role that this cascade plays in cancer development. The important role of Raf in cancer was corroborated in 2002 with the discovery of B-Raf genetic mutations in a large number of tumors. This led to intensified drug development efforts to target Raf signaling in cancer. This work yielded not only recent clinical successes but also surprising insights into the regulation of Raf proteins by homodimerization and heterodimerization. Surprising insights also came from the hunt for new Raf targets. Although MEK remains the only widely accepted Raf substrate, new kinase-independent roles for Raf proteins have emerged. These include the regulation of apoptosis by suppressing the activity of the proapoptotic kinases, ASK1 and MST2, and the regulation of cell motility and differentiation by controlling the activity of Rok-α. In this review, we discuss the regulation of Raf proteins and their role in cancer, with special focus on the interacting proteins that modulate Raf signaling. We also describe the new pathways controlled by Raf proteins and summarize the successes and failures in the development of efficient anticancer therapies targeting Raf. Finally, we also argue for the necessity of more systemic approaches to obtain a better understanding of how the Ras-Raf signaling network generates biological specificity.

Mechanistic principles of RAF kinase signaling

The RAF family of kinases are key components acting downstream of receptor tyrosine kinases and cells employ several distinct mechanisms to strictly control their activity. RAF transitions from an inactive state, where the N-terminal regulatory region binds intramolecularly to the C-terminal kinase domain, to an open state capable of executing the phosphoryl transfer reaction. This transition involves changes both within and between the protein domains in RAF. Many different proteins regulate the transition between inactive and active states of RAF, including RAS and KSR, which are arguably the two most prominent regulators of RAF function. Recent developments have added several new twists to our understanding of RAF regulation. Among others, dimerization of the RAF kinase domain is emerging as a crucial step in the RAF activation process. The multitude of regulatory protein-protein interactions involving RAF remains a largely untapped area for therapeutic applications.

RAF kinase activity regulates neuroepithelial cell proliferation and neuronal progenitor cell differentiation during early inner ear development

Background

Early inner ear development requires the strict regulation of cell proliferation, survival, migration and differentiation, coordinated by the concerted action of extrinsic and intrinsic factors. Deregulation of these processes is associated with embryonic malformations and deafness. We have shown that insulin-like growth factor I (IGF-I) plays a key role in embryonic and postnatal otic development by triggering the activation of intracellular lipid and protein kinases. RAF kinases are serine/threonine kinases that regulate the highly conserved RAS-RAF-MEK-ERK signaling cascade involved in transducing the signals from extracellular growth factors to the nucleus. However, the regulation of RAF kinase activity by growth factors during development is complex and still not fully understood.

Methodology/Principal Findings

By using a combination of qRT-PCR, Western blotting, immunohistochemistry and in situ hybridization, we show that C-RAF and B-RAF are expressed during the early development of the chicken inner ear in specific spatiotemporal patterns. Moreover, later in development B-RAF expression is associated to hair cells in the sensory patches. Experiments in ex vivo cultures of otic vesicle explants demonstrate that the influence of IGF-I on proliferation but not survival depends on RAF kinase activating the MEK-ERK phosphorylation cascade. With the specific RAF inhibitor Sorafenib, we show that blocking RAF activity in organotypic cultures increases apoptosis and diminishes the rate of cell proliferation in the otic epithelia, as well as severely impairing neurogenesis of the acoustic-vestibular ganglion (AVG) and neuron maturation.

Conclusions/Significance

We conclude that RAF kinase activity is essential to establish the balance between cell proliferation and death in neuroepithelial otic precursors, and for otic neuron differentiation and axonal growth at the AVG.

Partner exchange: protein-protein interactions in the Raf pathway

The three-tiered Raf-MEK-ERK kinase module is activated downstream of Ras and has been traditionally linked to cellular proliferation. Mammals have three Raf, two Mek and two Erk genes. Recently, the analysis of protein-protein interactions in the pathway has begun to provide a rationale for the redundancy within each tier. New results show that the MEK-ERK-activating unit consists of Raf hetero- and homodimers; downstream of Raf, MEK1-MEK2 heterodimers and ERK dimers are required for temporal and spatial pathway regulation. Finally, C-Raf mediates pathway crosstalk downstream of Ras by directly binding to and inhibiting kinases engaged in other signaling cascades. Given the roles of these interactions in tumorigenesis, their study will provide new opportunities for molecule-based therapies that target the pathway.

Ras and Raf pathways in epidermis development and carcinogenesis

The epidermis is the outermost layer of the body and protects it from environmental insults. This crucial function is sustained by a continuous process of self-renewal involving the carefully balanced proliferation and differentiation of progenitor cells constantly replacing the mature cells at the surface of the epidermis. Genetic changes in the signalling pathways controlling keratinocyte proliferation and differentiation disrupt this balance and lead to pathological changes including carcinogenesis. This review discusses the role of Ras, an oncogene critically involved in the development of skin neoplasia, and its downstream effector Raf in epidermal homeostasis and tumourigenesis. In particular, we will focus on the recently established role of Raf-1 as the decisive element that, by restraining keratinocyte differentiation, allows the development and maintenance of Ras-driven tumours.

BRAF inactivation drives aneuploidy by deregulating CRAF

Aspartate-594 is the third most common BRAF residue mutated in human cancer. Mutants of this residue are kinase inactive, and the mechanism(s) by which they contribute to cancer has remained perplexing. Using a conditional knock-in mouse model, we show that the (D594A)Braf mutant does not drive tumor development per se but is able to induce aneuploidy in murine splenocytes and mouse embryonic fibroblasts and contributes to immortalization through the propagation of aneuploid cells. (D594A)Braf lacks kinase activity but induces the related gene product Craf as well as the mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase (MEK)/ERK pathway. Here, we show that the aneuploid phenotype is dependent on Craf. Treatment with the MEK inhibitor U0126 did not attenuate the emergence of aneuploidy but prevented the growth of aneuploid cells. These results provide a previously unidentified link between Craf and chromosomal stability, with important implications for our understanding of the development of cancers with driver mutations that hyperactivate Craf.

Keratinocyte-specific stat3 heterozygosity impairs development of skin tumors in human papillomavirus 8 transgenic mice

Human papillomaviruses (HPV) of the genus β are thought to play a role in human skin cancers, but this has been difficult to establish using epidemiologic approaches. To gain insight into the transforming activities of β-HPV, transgenic mouse models have been generated that develop skin tumors. Recent evidence suggests a central role of signal transducer and activator of transcription 3 (Stat3) as a transcriptional node for cancer cell-autonomous initiation of a tumor-promoting gene signature associated with cell proliferation, cell survival, and angiogenesis. Moreover, high levels of phospho-Stat3 have been detected in tumors arising in HPV8-CER transgenic mice. In this study, we investigate the in vivo role of Stat3 in HPV8-induced skin carcinogenesis by combining our established experimental model of HPV8-induced skin cancer with epidermis-restricted Stat3 ablation. Stat3 heterozygous epidermis was less prone to tumorigenesis than wild-type epidermis. Three of the 23 (13%) Stat3(+/-):HPV8 animals developed tumors within 12 weeks of life, whereas 54.3% of Stat3(+/+):HPV8 mice already exhibited tumors in the same observation period (median age for tumor appearance, 10 weeks). The few tumors that arose in the Stat3(+/-):HPV8 mice were benign and never progressed to a more malignant phenotype. Collectively, these results offer direct evidence of a critical role for Stat3 in HPV8-driven epithelial carcinogenesis. Our findings imply that targeting Stat3 activity in keratinocytes may be a viable strategy to prevent and treat HPV-induced skin cancer.

BAG-1 enhances cell-cell adhesion, reduces proliferation and induces chaperone-independent suppression of hepatocyte growth factor-induced epidermal keratinocyte migration

Cell motility is important in maintaining tissue homeostasis, facilitating epithelial wound repair and in tumour formation and progression. The aim of this study was to determine whether BAG-1 isoforms regulate epidermal cell migration in in vitro models of wound healing. In the human epidermal cell line HaCaT, endogenous BAG-1 is primarily nuclear and increases with confluence. Both transient and stable p36-Bag-1 overexpression resulted in increased cellular cohesion. Stable transfection of either of the three human BAG-1 isoforms p36-Bag-1 (BAG-1S), p46-Bag-1 (BAG-1M) and p50-Bag-1 (BAG-1L) inhibited growth and wound closure in serum-containing medium. However, in response to hepatocyte growth factor (HGF) in serum-free medium, BAG-1S/M reduced communal motility and colony scattering, but BAG-1L did not. In the presence of HGF, p36-Bag-1 transfectants retained proliferative response to HGF with no change in ERK1/2 activation. However, the cells retained E-cadherin localisation at cell-cell junctions and exhibited pronounced cortical actin. Point mutations in the BAG domain showed that BAG-1 inhibition of motility is independent of its function as a chaperone regulator. These findings are the first to suggest that BAG-1 plays a role in regulating cell-cell adhesion and suggest an important function in epidermal cohesion.

From autoinhibition to inhibition in trans: the Raf-1 regulatory domain inhibits Rok-alpha kinase activity

The mechanism by which Raf-1 antagonizes Rok-α to promote migration and tumorigenesis is revealed.

The activity of Raf-1 and Rok-α kinases is regulated by intramolecular binding of the regulatory region to the kinase domain. Autoinhibition is relieved upon binding to the small guanosine triphosphatases Ras and Rho. Downstream of Ras, Raf-1 promotes migration and tumorigenesis by antagonizing Rok-α, but the underlying mechanism is unknown. In this study, we show that Rok-α inhibition by Raf-1 relies on an intermolecular interaction between the Rok-α kinase domain and the cysteine-rich Raf-1 regulatory domain (Raf-1reg), which is similar to Rok-α's own autoinhibitory region. Thus, Raf-1 mediates Rok-α inhibition in trans, which is a new concept in kinase regulation. This mechanism is physiologically relevant because Raf-1reg is sufficient to rescue all Rok-α–dependent defects of Raf-1–deficient cells. Downstream of Ras and Rho, the Raf-1–Rok-α interaction represents a novel paradigm of pathway cross talk that contributes to tumorigenesis and cell motility.

Senescent vs. non-senescent cells in the human annulus in vivo: cell harvest with laser capture microdissection and gene expression studies with microarray analysis

BACKGROUND

Senescent cells are well-recognized in the aging/degenerating human disc. Senescent cells are viable, cannot divide, remain metabolically active and accumulate within the disc over time. Molecular analysis of senescent cells in tissue offers a special challenge since there are no cell surface markers for senescence which would let one use fluorescence-activated cell sorting as a method for separating out senescent cells.

METHODS

We employed a novel laser capture microdissection (LCM) design to selectively harvest senescent and non-senescent annulus cells in paraffin-embedded tissue, and compared their gene expression with microarray analysis. LCM was used to separately harvest senescent and non-senescent cells from 11 human annulus specimens.

RESULTS

Microarray analysis revealed significant differences in expression levels in senescent cells vs non-senescent cells: 292 genes were upregulated, and 321 downregulated. Genes with established relationships to senescence were found to be significantly upregulated in senescent cells vs. non-senescent cells: p38 (MPAK14), RB-Associated KRAB zinc finger, Discoidin, CUB and LCCL domain, growth arrest and DNA-damage inducible beta, p28ING5, sphingosine-1-phosphate receptor 2 and somatostatin receptor 3; cyclin-dependent kinase 8 showed significant downregulation in senescent cells. Nitric oxidase synthase 1, and heat shock 70 kDa protein 6, both of which were significantly down-regulated in senescent cells, also showed significant changes. Additional genes related to cytokines, cell proliferation, and other processes were also identified.

CONCLUSIONS

Our LCM-microarray analyses identified a set of genes associated with senescence which were significantly upregulated in senescent vs non-senescent cells in the human annulus. These genes include p38 MAP kinase, discoidin, inhibitor of growth family member 5, and growth arrest and DNA-damage-inducible beta. Other genes, including genes associated with cell proliferation, extracellular matrix formation, cell signaling and other cell functions also showed significant modulation in senescent vs non-senescent cells. The aging/degenerating disc undergoes a well-recognized loss of cells; understanding senescent cells is important since their presence further reduces the disc's ability to generate new cells to replace those lost to necrosis or apoptosis.

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.

Deciphering signaling pathways in vivo: the Ras/Raf/MEK/ERK cascade

The Ras/Raf/MEK/ERK cascade is a highly conserved signal transduction module, whose activation results in a number of different physiological outcomes. Depending on the cell type or the stimulus used, the pathway has been implicated in proliferation, differentiation, apoptosis, and migration. Because of this wide range of activities, these kinases are considered attractive (anticancer) therapeutic targets. However, their essential functions in the context of the whole organism are still incompletely known. Here, we describe immunohistochemical and immunofluorescence methods that can be used to define the essential function(s) and the relevant downstream targets of Raf-1, B-Raf, and Mek-1 in in vivo models of organ development, remodeling, and neoplasia.

Raf-1 and squamous cell carcinoma: Rok-ing the boat

Squamous cell carcinoma (SCC) is the second most common form of nonmelanoma skin cancer. In this issue of Cancer Cell, Ehrenreiter et al. unveil a critical role for the Raf-1/Rok-alpha interaction in the pathogenesis of SCCs, thus paving the way for the development of therapeutic modalities to treat this malignancy.

Raf-1 addiction in Ras-induced skin carcinogenesis

Ras activation is common to many human cancers and promotes cell proliferation and survival by initiating multiple signaling cascades. Accordingly, Ras-transformed cells are generally considered too resourceful to become addicted to a single effector. In contrast to this tenet, we now demonstrate an absolute, cell autonomous requirement for Raf-1 in the development and maintenance of Ras-induced skin epidermis tumors. Mechanistically, Raf-1 functions as an endogenous inhibitor dimming the activity of the Rho-dependent kinase Rok-alpha in the context of a Ras-induced Raf-1:Rok-alpha complex. Raf-1-induced Rok-alpha inhibition allows the phosphorylation of STAT3 and Myc expression and promotes dedifferentiation in Ras-induced tumors. These data link the Raf-1:Rok-alpha complex to STAT3/Myc activation and delineate a pathway crucial for cell fate decision in Ras-induced tumorigenesis.

The fibrotic phenotype induced by IGFBP-5 is regulated by MAPK activation and egr-1-dependent and -independent mechanisms

We have previously shown that insulin-like growth factor (IGF) binding protein- 5 (IGFBP-5) is overexpressed in lung fibrosis and induces the production of extracellular matrix components, such as collagen and fibronectin, both in vitro and in vivo. The exact mechanism by which IGFBP-5 exerts these novel fibrotic effects is unknown. We thus examined the signaling cascades that mediate IGFBP-5-induced fibrosis. We demonstrate for the first time that IGFBP-5 induction of extracellular matrix occurs independently of IGF-I, and results from IGFBP-5 activation of MAPK signaling, which facilitates the translocation of IGFBP-5 to the nucleus. We examined the effects of IGFBP-5 on early growth response (Egr)-1, a transcription factor that is central to growth factor-mediated fibrosis. Egr-1 was up-regulated by IGFBP-5 in a MAPK-dependent manner and bound to nuclear IGFBP-5. In fibroblasts from Egr-1 knockout mice, induction of fibronectin by IGFBP-5 was abolished. Expression of Egr-1 in these cells rescued the extracellular matrix-promoting effects of IGFBP-5. Moreover, IGFBP-5 induced cell migration in an Egr-1-dependent manner. Notably, Egr-1 levels, similar to IGFBP-5, were increased in vivo in lung tissues and in vitro in primary fibroblasts of patients with pulmonary idiopathic fibrosis. Taken together, our findings suggest that IGFBP-5 induces a fibrotic phenotype via the activation of MAPK signaling and the induction of nuclear Egr-1 that interacts with IGFBP-5 and promotes fibrotic gene transcription.

Introducing intermediate filaments: from discovery to disease

It took more than 100 years before it was established that the proteins that form intermediate filaments (IFs) comprise a unified protein family, the members of which are ubiquitous in virtually all differentiated cells and present both in the cytoplasm and in the nucleus. However, during the past 2 decades, knowledge regarding the functions of these structures has been expanding rapidly. Many disease-related roles of IFs have been revealed. In some cases, the molecular mechanisms underlying these diseases reflect disturbances in the functions traditionally assigned to IFs, i.e., maintenance of structural and mechanical integrity of cells and tissues. However, many disease conditions seem to link to the nonmechanical functions of IFs, many of which have been defined only in the past few years.

Increased cell senescence is associated with decreased cell proliferation in vivo in the degenerating human annulus

BACKGROUND CONTEXT

During disc degeneration, there is a well-recognized loss of cells. This puts the remaining cell population at high risk for any further decrease in cell function or cell numbers. Cell senescence has recently been shown to be present in the aging/degenerating human disc. Senescent cell are viable, metabolically active, persist, and accumulate over time, but cannot divide. Little is known about the relationship between renewal of the disc cell population via cell proliferation and disc cell senescence.

PURPOSE

To determine the percentage of senescent cells and proliferating cells in the human annulus in vivo.

STUDY DESIGN/SETTING

Human annulus specimens were obtained from surgical subjects and control donors in a study approved by the authors' Human Subjects Institutional Review Board.

PATIENT SAMPLE

One Thompson Grade I disc, 4 Grade II discs, 9 Grade III discs, and 12 Grade IV discs were studied.

OUTCOME MEASURES

The percentages of senescent cells and the percentage of proliferating cells.

METHODS

Immunohistochemistry was used to detect senescent cells using an antisenescence-associated beta-galactosidase antibody, and an antiproliferation antibody (Ki67). An average of 410 cells/specimens was counted to determine the percent senescence, and an average of 229 cells was counted to determine the percent proliferation.

RESULTS

Cell proliferation was low in both surgical and control normal donor annulus tissue (4.09%+1.77 (26), mean+SD (n)). There was no significant difference in the percentage of proliferating cells for more degenerate discs versus healthier discs (4.7%+1.6 (21) for Grades III and IV vs. 5.3%+1.9 (5) for Grades I and II). More degenerated Grades III and IV discs contained significantly greater percentages of senescent annulus cells than did the healthier Grades I and II discs (44.4%+20.0 (21) vs. 18.8%+11.0 (5), respectively; p=.011). A significant negative correlation was present between the percentage of senescent cells versus the percentage of proliferating cells, r=-0.013, p=.013. No correlation was present between age and the percentage of senescent cells or age and the percentage of proliferating cells.

CONCLUSIONS

Because senescent cells cannot divide, senescence may reduce the disc's ability to generate new cells to replace cells lost to necrosis or apoptosis. Senescent cells also accumulate in the disc over time, such that their metabolic patterns may contribute to the pathologic changes seen in degenerating discs. Novel data presented here show a significant negative correlation between the percentage of senescent cells and the percentage of proliferating cells during disc degeneration. Molecular work is underway in our lab to help us determine whether senescent cells in the disc secrete factors that can result in decreased proliferation in neighboring cells.

BLNK binds active H-Ras to promote B cell receptor-mediated capping and ERK activation

Cross-linked B cell receptor (BCR) aggregates on the cell surface, then assembles into the "cap" where Ras is co-localized, and transduces various intracellular signals including Ras-ERK activation. BCR signals induce proliferation, differentiation, or apoptosis of B cells depending on their maturational stage. The adaptor protein BLNK binds various signaling proteins and Igalpha, a signaling subunit of the BCR complex, and plays an important role in the BCR signal transduction. BLNK was shown to be required for activation of ERK, but not of Ras, after BCR cross-linking, raising a question how BLNK facilitates ERK activation. Here we demonstrate that BLNK binds the active form of H-Ras, and their binding is facilitated by BCR cross-linking. We have identified a 10-amino acid Ras-binding domain within BLNK that is necessary for restoration of BCR-mediated ERK activation in BLNK-deficient B cells and for anti-apoptotic signaling. The Ras-binding domain fused with a CD8alpha-Igalpha chimeric receptor could induce prolonged ERK phosphorylation, transcriptional activation of Elk1, as well as the capping of the receptor in BLNK-deficient B cells. These results indicate that BLNK recruits active H-Ras to the BCR complex, which is essential for sustained surface expression of BCR in the form of the cap and for the signal leading to functional ERK activation.

An essential role for p120-catenin in Src- and Rac1-mediated anchorage-independent cell growth

p120-catenin regulates epithelial cadherin stability and has been suggested to function as a tumor suppressor. In this study, we used anchorage-independent growth (AIG), a classical in vitro tumorigenicity assay, to examine the role of p120 in a different context, namely oncogene-mediated tumorigenesis. Surprisingly, p120 ablation by short hairpin RNA completely blocked AIG induced by both Rac1 and Src. This role for p120 was traced to its activity in suppression of the RhoA-ROCK pathway, which appears to be essential for AIG. Remarkably, the AIG block associated with p120 ablation was completely reversed by inhibition of the downstream RhoA effector ROCK. Harvey-Ras (H-Ras)-induced AIG was also dependent on suppression of the ROCK cascade but was p120 independent because its action on the pathway occurred downstream of p120. The data suggest that p120 modulates oncogenic signaling pathways important for AIG. Although H-Ras bypasses p120, a unifying theme for all three oncogenes is the requirement to suppress ROCK, which may act as a gatekeeper for the transition to anchorage independence.

The pro-fibrotic factor IGFBP-5 induces lung fibroblast and mononuclear cell migration

We have previously shown that insulin-like growth factor-binding protein-5 (IGFBP-5) is overexpressed in fibrotic lung tissues and that it induces production of extracellular matrix components such as collagen and fibronectin both in vitro and in vivo. We recently observed mononuclear cell infiltration in lung tissues of mice expressing IGFBP-5. We therefore examined the role of IGFBP-5 on the migration of immune cells. Migration assays demonstrated that IGFBP-5 induced migration of peripheral blood mononuclear cells (PBMCs) in a dose-dependent manner. Preferential migration of monocytes/macrophages, natural killer cells, and T cells was observed. Moreover, the CD4/CD8 ratio of migrating cells was significantly higher in vitro and in vivo in response to IGFBP-5. IGFBP-5 resulted in preferential migration of activated CD4(+) T cells and monocytes. Interestingly, IGFBP-5 also induced migration of primary human lung fibroblasts. Exogenous administration of IGFBP-5 induced activation of mitogen-activated protein kinase (MAPK) signaling cascade but not PI3K in PBMCs. IGFBP-5-induced migration was blocked by the MEK1/2 inhibitor U0126, suggesting that IGFBP-5-induced migration occurs via MAPK activation. Furthermore, monocytes treated with recombinant IGFBP-5 expressed the mesenchymal markers alpha-smooth muscle actin and fibronectin in vitro and in vivo, suggesting that IGFBP-5 can induce the transformation of monocytes into mesenchymal cells. Collectively, our results suggest that IGFBP-5 induces cell migration via MAPK-dependent and IGF-I-independent mechanisms.

Cell and molecular biology of the novel protein tyrosine-phosphatase-interacting protein 51

This chapter examines the current state of knowledge about the expression profile, as well as biochemical properties and biological functions of the evolutionarily conserved protein PTPIP51. PTPIP51 is apparently expressed in splice variants and shows a particularly high expression in epithelia, skeletal muscle, placenta, and germ cells, as well as during mammalian development and in cancer. PTPIP51 is an in vitro substrate of Src- and protein kinase A, the PTP1B/TCPTP protein tyrosine phosphatases and interacts with 14-3-3 proteins, the Nuf2 kinetochore protein, the ninein-interacting CGI-99 protein, diacylglycerol kinase alpha, and also with itself forming dimers and trimers. Although the precise cellular function remains to be elucidated, the current data implicate PTPIP51 in signaling cascades mediating proliferation, differentiation, apoptosis, and motility.

Long-term ethanol exposure impairs neuronal differentiation of human neuroblastoma cells involving neurotrophin-mediated intracellular signaling and in particular protein kinase C

BACKGROUND

Revealing the molecular changes in chronic ethanol-impaired neuronal differentiation may be of great importance for understanding ethanol-related pathology in embryonic development but also in the adult brain. In this study, both acute and long-term effects of ethanol on neuronal differentiation of human neuroblastoma cells were investigated. We focused on several aspects of brain-derived neurotrophic factor (BDNF) signaling because BDNF activates the extracellular signal-regulated kinase (ERK) cascade, promoting neuronal differentiation including neurite outgrowth.

METHODS

The effects of ethanol exposure on morphological differentiation, cellular density, neuronal marker proteins, basal ERK activity, and ERK responsiveness to BDNF were measured over 2 to 4 weeks. qRT-PCR and Western blotting were performed to investigate the expression of neurotrophin receptor tyrosin kinase B (TrkB), members of the ERK-cascade, protein kinase C (PKC) isoforms and Raf-Kinase-Inhibitor-Protein (RKIP).

RESULTS

Chronic ethanol interfered with the development of a neuronal network consisting of cell clusters and neuritic bundles. Furthermore, neuronal and synaptic markers were reduced, indicating impaired neuronal differentiation. BDNF-mediated activation of the ERK cascade was found to be continuously impaired by ethanol. This could not be explained by expressional changes monitored for TrkB, Raf-1, MEK, and ERK. However, BDNF also activates PKC signaling which involves RKIP, which finally leads to ERK activation as well. Therefore, we hypothesized that ethanol impairs this branch of BDNF signaling. Indeed, both PKC and RKIP were significantly down-regulated.

CONCLUSIONS

Chronic ethanol exposure impaired neuronal differentiation of neuroblastoma cells and BDNF signaling, particularly the PKC-dependent branch. RKIP, acting as a signaling switch at the merge of the PKC cascade and the Raf/MEK/ERK cascade, was associated with neuronal differentiation and significantly reduced in ethanol treatment. Moreover, PKC expression itself was even more strongly reduced. In contrast, members of the Raf-1/MEK/ERK cascade were less affected and the observed changes were not associated with impaired differentiation. Thus, reduced RKIP and PKC levels and subsequently reduced positive feedback on ERK activation provide an explanation for the striking effects of long-term ethanol exposure on BDNF signal transduction and neuronal differentiation, respectively.

Cell migration by a FRS2-adaptor dependent membrane relocation of ret receptors

During development neural progenitor cells migrate with extraordinary precision to inhabit tissues and organs far from their initial position. Little is known about the cellular basis for directional guidance by tyrosine kinase receptors (RTKs). RET is a RTK with important functions in guiding the migration of neuronal cells, and RET dysregulation leads to clinical disease such as agangliosis of the colon. We show here that RET migration in neuroepitheliomal and non-neuronal cells is elicited by the activation of specific signaling pathways initiated by the competitive recruitment of the FRS2 adaptor molecule to tyrosine 1062 (Y1062) in RET. FRS2 selectively recruited RET to focal complexes and led to activation of SRC family kinases and focal adhesion kinase (FAK). Activation of SRC depended on its direct interaction with RET at a different intracellular tyrosine (Y981) and activation of molecular signaling from these two separate sites in concert regulated migration. Our data suggest that an important function for FRS2 is to concentrate RET in membrane foci, leading to an engagement of specific signaling complexes localized in these membrane domains.

BAG1 promotes axonal outgrowth and regeneration in vivo via Raf-1 and reduction of ROCK activity

Improved survival of injured neurons and the inhibition of repulsive environmental signalling are prerequisites for functional regeneration. BAG1 (Bcl-2-associated athanogene-1) is an Hsp70/Hsc70-binding protein, which has been shown to suppress apoptosis and enhance neuronal differentiation. We investigated BAG1 as a therapeutic molecule in the lesioned visual system in vivo. Using an adeno-associated viral vector, BAG1 (AAV.BAG1) was expressed in retinal ganglion cells (RGC) and then tested in models of optic nerve axotomy and optic nerve crush. BAG1 significantly increased RGC survival as compared to adeno-associated viral vector enhanced green fluorescent protein (AAV.EGFP) treated controls and this was independently confirmed in transgenic mice over-expressing BAG1 in neurons. The numbers and lengths of regenerating axons after optic nerve crush were also significantly increased in the AAV.BAG1 group. In pRGC cultures, BAG1-over-expression resulted in a approximately 3-fold increase in neurite length and growth cone surface. Interestingly, BAG1 induced an intracellular translocation of Raf-1 and ROCK2 and ROCK activity was decreased in a Raf-1-dependent manner by BAG1-over-expression. In summary, we show that BAG1 acts in a dual role by inhibition of lesion-induced apoptosis and interaction with the inhibitory ROCK signalling cascade. BAG1 is therefore a promising molecule to be further examined as a putative therapeutic tool in neurorestorative strategies.

Elevated CRAF as a potential mechanism of acquired resistance to BRAF inhibition in melanoma

Activating BRAF kinase mutations arise in approximately 7% of all human tumors, and preclinical studies have validated the RAF-mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase-ERK signaling cascade as a potentially important therapeutic target in this setting. Selective RAF kinase inhibitors are currently undergoing clinical development, and based on the experience with other kinase-targeted therapeutics, it is expected that clinical responses to these agents, if observed, will lead to the eventual emergence of drug resistance in most cases. Thus, it is important to establish molecular mechanisms underlying such resistance to develop effective therapeutic strategies to overcome or prevent drug resistance. To anticipate potential mechanisms of acquired resistance to RAF inhibitors during the course of treatment, we established drug-resistant clones from a human melanoma-derived cell line harboring the recurrent V600E activating BRAF mutation, which exhibits exquisite sensitivity to AZ628, a selective RAF kinase inhibitor. We determined that elevated CRAF protein levels account for the acquisition of resistance to AZ628 in these cells, associated with a switch from BRAF to CRAF dependency in tumor cells. We also found that elevated CRAF protein levels may similarly contribute to primary insensitivity to RAF inhibition in a subset of BRAF mutant tumor cells. Interestingly, AZ628-resistant cells demonstrating either primary drug insensitivity or acquired drug resistance exhibit exquisite sensitivity to the HSP90 inhibitor geldanamycin. Geldanamycin effectively promotes the degradation of CRAF, thereby revealing a potential therapeutic strategy to overcome resistance to RAF inhibition in a subset of BRAF mutant tumors.

IGF-1 rescues human intervertebral annulus cells from in vitro stress-induced premature senescence

The aging human intervertebral disc contains a sizeable population of senescent cells. Since senescent cells cannot divide, senescence reduces the disc's ability to generate new cells to replace existing ones lost to necrosis or apoptosis. The objectives of the present work were: (1) to develop a reliable in vitro model for stress-induced premature senescence in human annulus cells, and (2) to investigate the potential for insulin-like growth factor-1 (IGF-1) to prevent or ameliorate senescence in vitro. The developed experimental model employs a 2 h exposure to 50 microM hydrogen peroxide; immunocytochemical localization of senescence associated-beta-galactosidase at pH 6.0 was used as the marker for senescent cells, and the percentage of senescent cells quantified after 3 days of culture. Nine sets of annulus cells were obtained from eight human surgical disc specimens; cells were tested with 0, 50, 100 or 500 ng/ml IGF-1. Although 50 or 100 ng/ml IGF-1 did not significantly alter the percentage of senescent cells, a significant reduction was present following exposure to 500 ng/ml IGF-1 (control, 56.3% +/- 8.5 (9); mean +/- SEM, (n) vs. treated, 39.6% +/- 6.6 (9), p = 0.0009). These novel findings point to the value of continued research towards development of future biologic therapies designed to reduce cell senescence in degenerating human discs.

A small molecule disruptor of Rb/Raf-1 interaction inhibits cell proliferation, angiogenesis, and growth of human tumor xenografts in nude mice

Although it is well established that cyclin-dependent kinases phosphorylate and inactivate Rb, the Raf-1 kinase physically interacts with Rb and initiates the phosphorylation cascade early in the cell cycle. We have identified an orally active small molecule, Rb/Raf-1 disruptor 251 (RRD-251), that potently and selectively disrupts the Rb/Raf-1 but not Rb/E2F, Rb/prohibitin, Rb/cyclin E, and Rb/HDAC binding. The selective inhibition of Rb/Raf-1 binding suppressed the ability of Rb to recruit Raf-1 to proliferative promoters and inhibited E2F1-dependent transcriptional activity. RRD-251 inhibited anchorage-dependent and anchorage-independent growth of human cancer cells and knockdown of Rb with short hairpin RNA or forced expression of E2F1 rescued cells from RRD-251-mediated growth arrest. P.o. treatment of mice resulted in significant tumor growth suppression only in tumors with functional Rb, and this was accompanied by inhibition of angiogenesis, inhibition of proliferation, decreased phosphorylated Rb levels, and inhibition of Rb/Raf-1 but not Rb/E2F1 binding in vivo. Thus, selective targeting of Rb/Raf-1 interaction seems to be a promising approach for developing novel chemotherapeutic agents.

Fibronectin-tissue transglutaminase matrix rescues RGD-impaired cell adhesion through syndecan-4 and beta1 integrin co-signaling

Heterotropic association of tissue transglutaminase (TG2) with extracellular matrix-associated fibronectin (FN) can restore the adhesion of fibroblasts when the integrin-mediated direct binding to FN is impaired using RGD-containing peptide. We demonstrate that the compensatory effect of the TG-FN complex in the presence of RGD-containing peptides is mediated by TG2 binding to the heparan sulfate chains of the syndecan-4 cell surface receptor. This binding mediates activation of protein kinase Calpha (PKCalpha) and its subsequent interaction with beta(1) integrin since disruption of PKCalpha binding to beta(1) integrins with a cell-permeant competitive peptide inhibits cell adhesion and the associated actin stress fiber formation. Cell signaling by this process leads to the activation of focal adhesion kinase and ERK1/2 mitogen-activated protein kinases. Fibroblasts deficient in Raf-1 do not respond fully to the TG-FN complex unless either the full-length kinase competent Raf-1 or the kinase-inactive domain of Raf-1 is reintroduced, indicating the involvement of the Raf-1 protein in the signaling mechanism. We propose a model for a novel RGD-independent cell adhesion process that could be important during tissue injury and/or remodeling whereby TG-FN binding to syndecan-4 activates PKCalpha leading to its association with beta(1) integrin, reinforcement of actin-stress fiber organization, and MAPK pathway activation.

Real-time imaging reveals that noninvasive mammary epithelial acini can contain motile cells

To determine how extracellular signal–regulated kinases (ERK) 1/2 promote mammary tumorigenesis, we examined the real-time behavior of cells in an organotypic culture of the mammary glandular epithelium. Inducible activation of ERK1/2 in mature acini elicits cell motility and disrupts epithelial architecture in a manner that is reminiscent of ductal carcinoma in situ; however, motile cells do not invade through the basement membrane and branching morphogenesis does not take place. ERK1/2-induced motility causes cells to move both within the cell monolayer that contacts the basement membrane surrounding the acinus and through the luminal space of the acinus. E-cadherin expression is reduced after ERK1/2 activation, but motility does not involve an epithelial–mesenchymal transition. Cell motility and the disruption of epithelial architecture require a Rho kinase– and myosin light chain kinase–dependent increase in the phosphorylation of myosin light chain 2. Our results identify a new mechanism for the disruption of architecture in epithelial acini and suggest that ERK1/2 can promote noninvasive motility in preinvasive mammary tumors.

Selective Raf inhibition in cancer therapy

Over the past 5 years, the Raf kinase family has emerged as a promising target for protein-directed cancer therapy development. The goal of this review is to first provide a concise summary of the data validating Raf proteins as high-interest therapeutic targets. The authors then outline the mode of action of Raf kinases, emphasizing how Raf activities and protein interactions suggest specific approaches to inhibiting Raf. The authors then summarize the set of drugs, antisense reagents and antibodies available or in development for therapeutically targeting Raf or Raf-related proteins, as well as existing strategies combining these and other therapeutic agents. Finally, the authors discuss recent results from systems biology analyses that have the potential to increasingly guide the intelligent selection of combination therapies involving Raf-targeting agents and other therapeutics.

SteC is a Salmonella kinase required for SPI-2-dependent F-actin remodelling

Salmonella enterica serovar Typhimurium (S. Typhimurium) replicates inside mammalian cells within membrane-bound compartments called Salmonella-containing vacuoles. Intracellular replication is dependent on the activities of several effector proteins translocated across the vacuolar membrane by the Salmonella pathogenicity island 2 (SPI-2)-type III secretion system (T3SS). This is accompanied by the formation in the vicinity of bacterial vacuoles of an F-actin meshwork, thought to be involved in maintaining the integrity of vacuolar membranes. In this study, we investigated the function of the SPI-2 T3SS effector SteC. An steC mutant strain was not defective for intracellular replication or attenuated for virulence in mice. However, the steC mutant was defective for SPI-2-dependent F-actin meshwork formation in host cells, although the vacuolar membranes surrounding mutant bacteria appeared to be normal. Expression of SteC in fibroblast cells following transfection caused extensive rearrangements of the F-actin cytoskeleton. Sequence analysis identified amino acid similarity between SteC and the human kinase Raf-1. A His-tagged SteC fusion protein had kinase activity in vitro and a point mutant lacking kinase activity was unable to induce F-actin rearrangements in vivo. We conclude that SPI-2-dependent F-actin meshwork formation depends on the kinase activity of SteC, which resembles more closely eukaryotic than prokaryotic kinases.

MAP kinase signalling pathways in cancer

Cancer can be perceived as a disease of communication between and within cells. The aberrations are pleiotropic, but mitogen-activated protein kinase (MAPK) pathways feature prominently. Here, we discuss recent findings and hypotheses on the role of MAPK pathways in cancer. Cancerous mutations in MAPK pathways are frequently mostly affecting Ras and B-Raf in the extracellular signal-regulated kinase pathway. Stress-activated pathways, such as Jun N-terminal kinase and p38, largely seem to counteract malignant transformation. The balance and integration between these signals may widely vary in different tumours, but are important for the outcome and the sensitivity to drug therapy.

Raf kinases: function, regulation and role in human cancer

The Ras-Raf-MAPK pathway regulates diverse physiological processes by transmitting signals from membrane based receptors to various nuclear, cytoplasmic and membrane-bound targets, coordinating a large variety of cellular responses. Function of Raf family kinases has been shown to play a role during organism development, cell cycle regulation, cell proliferation and differentiation, cell survival and apoptosis and many other cellular and physiological processes. Aberrations along the Ras-Raf-MAPK pathway play an integral role in various biological processes concerning human health and disease. Overexpression or activation of the pathway components is a common indicator in proliferative diseases such as cancer and contributes to tumor initiation, progression and metastasis. In this review, we focus on the physiological roles of Raf kinases in normal and disease conditions, specifically cancer, and the current thoughts on Raf regulation.

Essential role of IL-6 signaling pathway in keloid pathogenesis

Cytokines are pleiotropic substances that are known to participate in inflammatory and immune responses as well as cell differentiation and proliferation. Interleukin-6 (IL-6) is a key cytokine with pro-inflammatory function. Wound healing is a complex cascade of physiologic events comprising inflammation, proliferation and remodeling, and proceeds with the integrated actions of different cells, cytokines, and the extracellular matrix. Aberrant wound healing results in keloid formation which causes disfigured appearance, discomfort, psychological stress, and patient frustration. In this review, the role of IL-6 signaling pathway in the pathogenesis of keloid is assessed and its potential as a therapeutic target is addressed. The existing data suggest that IL-6 mediated inflammation is a key player and may be considered as a common causative factor for development of keloid. Furthermore, in a recent comprehensive study, we confirmed the functional role of IL-6 signaling in keloid pathogenesis. Accordingly, inhibitory strategies of IL-6 signaling pathway by targeting the IL-6 receptors, its downstream effecters, or other molecules influencing this pathway appear to have considerable potential as new therapeutic or preventive challenges for keloid. Hopefully, several IL-6 blocking agents including a humanized antibody to IL-6 receptor have been developed and successfully used in clinical trials of inflammatory diseases. It is likely that these agents may prove worthy in the treatment or prevention of keloid as well. Future in-depth exploration of such challenges will shed light on their efficacy and safety for clinical application in keloid.

New insight into BRAF mutations in cancer

There has been much recent progress in our understanding of the role played by the RAS-RAF-MEK-ERK cascade in human cancer. RAS is an oncogene and this pathway is known to promote proliferation and malignant transformation. More recently, however, RAF has become the focus of attention, particularly in melanoma, where approximately 70% of cases carry mutations in the BRAF gene. The majority of the mutations in BRAF in cancer are activating, but rare mutants that cannot activate MEK have provided new insight into RAF signalling networks that exist in cancer and normal cells. Surprisingly, germline mutations in BRAF that occur in rare genetic syndromes have also recently been described. The induction of BRAF mutations in melanoma depends on the type of UV exposure that the skin receives, and some studies have suggested the existence of susceptibility loci that make it more likely that some individuals will acquire these mutations. Importantly, genetic profiling and microarray studies have provided insight into the spectrum of melanomas in which BRAF plays a role and also revealed intriguing new data that could be important for the diagnosis and treatment of human cancers.

Raf-1 and B-Raf promote protein kinase C θ interaction with BAD

PKCtheta regulates the proliferation, survival and differentiation of T-cells. Here we show that PKCtheta interacts with Raf-1 and B-Raf kinases. Raf-1 enhanced the kinase activity of associated PKCtheta, while PKCtheta reduced the catalytic activity of associated Raf-1. In contrast, B-Raf binding did not affect PKCtheta kinase activity, and PKCtheta did not change B-Raf activity. Coexpression of mutationally activated Raf-1 in cells enhanced the phosphorylation of T538 in the PKCtheta activation loop. PKCtheta and Raf cooperated in terms of binding to BAD, a pro-apoptotic Bcl-2 family protein that is inactivated by phosphorylation. While neither Raf-1 nor B-Raf could phosphorylate BAD, they enhanced the ability of PKCtheta to interact with BAD and to phosphorylate BAD in vitro and in vivo, suggesting a new role for Raf proteins in T-cells by targeting PKCtheta to interact with and phosphorylate BAD.

Senescence in cells of the aging and degenerating intervertebral disc: immunolocalization of senescence-associated beta-galactosidase in human and sand rat discs

STUDY DESIGN

Human intervertebral disc anulus tissue was obtained in a prospective study of cell senescence. Localization of the senescence biomarker beta-galactosidase (senescence associated beta-galactosidase, SA-beta-gal) was used for quantitative determination of the % senescent cells. Discs were obtained from surgical specimens or control donors. Discs were also studied from the lumbar spine of the sand rat. Experimental studies were approved by the authors' Human Subjects Institutional Review Board and animal use committee.

OBJECTIVES

To determine the incidence of cell senescence in human discs with Thompson Grades I through V using immunocytochemistry to quantify the percentage of cells positive for the senescence biomarker SA-beta-gal.

SUMMARY OF BACKGROUND DATA

Cell senescence has been recognized as a potential factor playing a role age-related disc degeneration. Senescent cells are viable but have lost the ability to divide. Senescence cells, however, are metabolically active.

METHODS

Fifty-seven discs specimens from 54 subjects were examined with immunocytochemistry for anti-SA-beta-gal immunocytochemical localization to identify senescent cells. The fraction of positive cells was determined with quantitative histomorphometry.

RESULTS

Quantitative histomorphometry of human discs show an overall incidence of SA-beta-gal-positive cells of 29.9% (+/-24.8, SD), with a range from 0 to 92.01%. Analysis by ANOVA of the % senescent cells grouped by Thompson grade showed significant increases in senescence with increasing disc degeneration (P < 0.0001). Further analysis with Tukey's test showed significant differences between the % senescent cells in Grades I/II versus IV, and versus V. SA-beta-gal-positive cells were also present in discs of the aging sand rat spine.

CONCLUSIONS

Quantitative analysis of immunohistochemical localization of SA-beta-gal identified a sizeable population of senescent cells in the aging/degenerating disc. It is important to discover more about the senescent disc cell population because these cells persist and accumulate over time within the disc. Since senescent cells cannot divide, senescence may reduce the disc's ability to generate new cells to replace existing ones lost to necrosis or apoptosis.

In vivo functions of mitogen-activated protein kinases: conclusions from knock-in and knock-out mice

Multicellular organisms achieve intercellular communication by means of signalling molecules whose effect on the target cell is mediated by signal transduction pathways. Such pathways relay, amplify and integrate signals to elicit appropriate biological responses. Protein kinases form crucial intermediate components of numerous signalling pathways. One group of protein kinases, the mitogen-activated protein kinases (MAP kinases) are kinases involved in signalling pathways that respond primarily to mitogens and stress stimuli. In vitro studies revealed that the MAP kinases are implicated in several cellular processes, including cell division, differentiation, cell survival/apoptosis, gene expression, motility and metabolism. As such, dysfunction of specific MAP kinases is associated with diseases such as cancer and immunological disorders. However, the genuine in vivo functions of many MAP kinases remain elusive. Genetically modified mouse models deficient in a specific MAP kinase or expressing a constitutive active or a dominant negative variant of a particular MAP kinase offer valuable tools for elucidating the biological role of these protein kinases. In this review, we focus on the current status of MAP kinase knock-in and knock-out mouse models and their phenotypes. Moreover, examples of the application of MAP kinase transgenic mice for validating therapeutic properties of specific MAP kinase inhibitors, and for investigating the role of MAP kinase in pathogen-host interactions will be discussed.

Effects of Raf-1 siRNA on human cerebral microvascular endothelial cells: A potential therapeutic strategy for inhibition of tumor angiogenesis

The serine/threonine kinase Raf-1 is involved in the regulation of tumor cell survival, proliferation and metastasis formation, and has therefore emerged as a promising target for cancer therapy. In addition, Raf-1 activity mediates proliferation of endothelial cells thereby promoting angiogenesis and invasive growth of various tumors, including highly vascularized malignant glioblastoma. The aim of this study was to evaluate the effects of small inhibitory RNA (siRNA) directed against Raf-1 on viability, proliferation and motility in glioma cells and cerebral endothelial cells. Half-quantitative RT-PCR and Western blotting revealed efficient siRNA-mediated Raf-1 down regulation in glioma cells (U373, U251) and in human cerebral microvascular endothelial cells (HCMEC). Surprisingly, Raf-1 gene silencing failed to affect cell survival, proliferation or migration activity in the glioblastoma cell lines. In HCMEC, however, pronounced decrease of cell survival and significant inhibition of tube formation was achieved by Raf-1 siRNA compared to non-functional siRNA or vehicle controls. In conclusion, Raf-1 silencing appears as a potential therapeutic strategy to inhibit brain tumor angiogenesis and thereby outgrowth of highly vascularized glioblastoma multiforme, whereas direct cytotoxic effects of Raf-1 knockdown in tumor cells may vary.

Functional implications of the IL-6 signaling pathway in keloid pathogenesis

The molecular mechanism(s) behind keloid pathogenesis remains unclear. Previously by global gene expression analysis of keloid fibroblasts (KFs), we implicated the IL-6 signaling pathway in keloid pathogenesis. Here, we determine a functional role of IL-6 signaling in keloid scars. Primary cultures of KFs and surrounding nonlesional fibroblasts (NFs) were subjected to induction or inhibition of IL-6 or its specific receptor IL-6 receptor alpha (IL-6R alpha) and detection of their effects on extracellular matrix gene expression. The levels of gp130 and several downstream targets in IL-6 signaling were also examined. IL-6 secretion was significantly higher in KFs than NFs. Addition of IL-6 peptide to NFs culture or inhibition of IL-6 or its receptor IL-6R alpha by their corresponding antibodies in KFs culture revealed a dose-dependent increase or decrease in collagen type I alpha 2 and fibronectin 1 mRNAs, respectively. Induction of IL-6 by IL-1beta peptide and stimulation by IL-6 peptide in NFs, or inhibition of IL-6 or IL-6R alpha in KFs cultures demonstrated a dose-dependent increase or decrease in procollagen I synthesis, respectively. The mRNA and protein expressions of gp130 and several downstream targets in IL-6 signaling (JAK1, STAT3, RAF1, and ELK1) were upregulated in KFs versus NFs. Our results indicate that IL-6 signaling may play an integral role in keloid pathogenesis and provide clues for development of IL-6 receptor blocking strategies for therapy or prophylaxis of keloid scars.

Regulation of epidermal homeostasis and repair by phosphoinositide 3-kinase

The epidermis undergoes continuous self-renewal to maintain its protective function. Whereas growth factors are known to modulate overall skin homeostasis, the intracellular signaling pathways, which control the delicate balance between proliferation and differentiation in keratinocytes, are largely unknown. Here we show transient upregulation of the phosphoinositide 3-kinase (PI3K) catalytic subunits p110α and p110β in differentiating keratinocytes in vitro, expression of these subunits in the epidermis of normal and wounded skin, and enhanced Akt phosphorylation in the hyperproliferative wound epidermis. Stimulation of PI3K activity in cultured keratinocytes by stable expression of an inducible, constitutively active PI3K mutant promoted cell proliferation and inhibited terminal differentiation in keratinocyte monocultures and induced the formation of a hyperplastic, disorganized and poorly differentiated epithelium in organotypic skin cultures. Activation of PI3K signaling also caused reorganization of the actin cytoskeleton and induced keratinocyte migration in vitro and in skin organ cultures. The identification of 122 genes, which are differentially expressed after induction of PI3K signaling provides insight into the molecular mechanisms underlying the observed effects of active PI3K on keratinocytes and indicates that hyperproliferation may be achieved at the expense of genome integrity. These results identify PI3K as an important intracellular regulator of epidermal homeostasis and repair.

Disassembly of the vimentin cytoskeleton disrupts articular cartilage chondrocyte homeostasis

Articular cartilage functions in dissipating forces applied across joints. It comprises an extracellular matrix containing primarily collagens, proteoglycans and water to maintain its functional properties, and is interspersed with chondrocytes. The chondrocyte cytoskeleton comprises actin microfilaments, tubulin microtubules and vimentin intermediate filaments. Previous studies have determined the contribution of actin and tubulin in regulating the synthesis of the extracellular matrix components aggrecan and type II collagen. The contribution of vimentin to extracellular matrix biosynthesis in any cell type has not previously been addressed. Therefore the aim of this study was to assess the role of vimentin in cartilage chondrocyte metabolism. Vimentin intermediate filaments were disrupted in high-density monolayer articular chondrocyte cultures using acrylamide for 7 days. De novo protein and collagen synthesis were measured by adding [3H]-proline, and sulphated glycosaminoglycan (sGAG) synthesis measured by adding [35S]-sulphate to cultures. Vimentin disruption resulted in decreased collagen synthesis, whilst sGAG synthesis was unaffected. In addition, there was a significant reduction in type II collagen and aggrecan gene transcription suggesting that the effects observed occur at both the transcriptional and translational levels. A 3-day cold chase demonstrated a significant inhibition of collagen and sGAG degradation; the reduction in collagen degradation was corroborated by the observed reduction in both pro-MMP 2 expression and activation. We have demonstrated that an intact vimentin intermediate filament network contributes to the maintenance of the chondrocyte phenotype and thus an imbalance favouring filament disassembly can disturb the integrity of the articular cartilage, and may ultimately lead to the development of pathologies such as osteoarthritis.

Migration of F9 parietal endoderm cells is regulated by the ERK pathway

Cell migration is regulated by the action of many signaling pathways that are activated in specific regions of migrating cells. Extracellular regulated kinase 1/2 (ERK) signaling can modulate the migration of cells by controlling the turnover of focal adhesions and the dynamics of actin polymerization. Focal adhesion turnover is necessary for cell migration, and the formation of strong actin stress fibers and mature focal adhesions puts the brakes on cell migration. We used F9 wild-type and vinculin null (vin-/-) parietal endoderm (PE) outgrowth to study the role of the ERK signaling pathway in cell migration. Upon plating of F9 embryoid bodies (EBs) onto laminin-coated dishes, PE cells migrate away from the EBs, providing an in vitro model for studying directed migration of this embryonic cell type. Our results suggest that the ERK pathway regulates PE cell migration by affecting the formation of focal adhesions and lamellipodia through the action of myosin light chain kinase (MLCK).