CXCL8/IL8 stimulates vascular endothelial growth factor (VEGF) expression and the autocrine activation of VEGFR2 in endothelial cells by activating NFkappaB through the CBM (Carma3/Bcl10/Malt1) complex

Vascular endothelial growth factor (VEGF) is a potent mitogen and permeability factor for endothelial cells that plays a central role in angiogenesis, vascular maintenance, inflammation, and cancer. VEGF also mediates the homeostatic adaptation to hypoxic conditions by promoting an increase in vascular density to compensate for decreased oxygenation. This process is triggered by an oxygen-sensitive transcription factor, hypoxia-inducible factor-1 (HIF1alpha), which becomes active in hypoxic tissues, leading to the synthesis and secretion of VEGF. The role of HIF1alpha in other processes that involve angiogenesis such as in inflammation is less clear. Of interest, endothelial cells not only respond to but also store and secrete VEGF, which is required for the maintenance of the integrity of the vascular system. How this intracellular pool of VEGF is regulated is still not understood. Here, we found that CXCL8/IL8, a potent proangiogenic and inflammatory chemokine, up-regulates VEGF mRNA and protein levels in endothelial cells by acting on its cognate receptor, CXCR2, and that this results in the autocrine activation of VEGFR2. Surprisingly, this process does not involve HIF1alpha but instead requires the activation of the transcription factor NFkappaB. Furthermore, we identified the components of the CBM complex, Carma3, Bcl10, and Malt1, as key mediators of the CXCL8/IL8-induced NFkappaB activation and VEGF up-regulation. Together, these findings support the existence of an NFkappaB-mediated pathway by which the proinflammatory chemokine CXCL8/IL8 controls the expression of VEGF in endothelial cells, thereby promoting the activation of VEGF receptors in an autocrine fashion.

Angiopoietin-1 induces Kruppel-like factor 2 expression through a phosphoinositide 3-kinase/AKT-dependent activation of myocyte enhancer factor 2

Angiopoietin-1 (Ang1) regulates both vascular quiescence and angiogenesis through the receptor tyrosine kinase Tie2. We and another group have recently shown that Ang1 and Tie2 form distinct signaling complexes at cell-cell and cell-matrix contacts and further demonstrated that the former selectively induces expression of Krüppel-like factor 2 (KLF2), a transcription factor involved in vascular quiescence. Here, we investigated the mechanism of how Ang1/Tie2 signal induces KLF2 expression to clarify the role of KLF2 in Ang1/Tie2 signal-mediated vascular quiescence. Ang1 stimulated KLF2 promoter-driven reporter gene expression in endothelial cells, whereas it failed when a myocyte enhancer factor 2 (MEF2)-binding site of KLF2 promoter was mutated. Depletion of MEF2 by siRNAs abolished Ang1-induced KLF2 expression, indicating the requirement of MEF2 in KLF2 induction by Ang1. Constitutive active phosphoinositide 3-kinase (PI3K) and AKT increased the MEF2-dependent reporter gene expression by enhancing its transcriptional activity and stimulated the KLF2 promoter activity cooperatively with MEF2. Consistently, inhibition of either PI3K or AKT and depletion of AKT abrogated Ang1-induced KLF2 expression. In addition, we confirmed the dispensability of extracellular signal-regulated kinase 5 (ERK5) for Ang1-induced KLF2 expression. Furthermore, depletion of KLF2 resulted in the loss of the inhibitory effect of Ang1 on vascular endothelial growth factor (VEGF)-mediated expression of vascular cell adhesion molecule-1 in endothelial cells and VEGF-mediated monocyte adhesion to endothelial cells. Collectively, these findings indicate that Ang1/Tie2 signal stimulates transcriptional activity of MEF2 through a PI3K/AKT pathway to induce KLF2 expression, which may counteract VEGF-mediated inflammatory responses.

Inhibition of Mammalian target of rapamycin by rapamycin causes the regression of carcinogen-induced skin tumor lesions

PURPOSE

The activation of Akt/mammalian target of rapamycin (mTOR) pathway represents a frequent event in squamous cell carcinoma (SCC) progression, thus raising the possibility of using specific mTOR inhibitors for the treatment of SCC patients. In this regard, blockade of mTOR with rapamycin prevents the growth of human head and neck SCC cells when xenotransplanted into immunodeficient mice. However, therapeutic responses in xenograft tumors are not always predictive of clinical anticancer activity.

EXPERIMENTAL DESIGN

As genetically defined and chemically induced animal cancer models often reflect better the complexity of the clinical setting, we used here a two-step chemical carcinogenesis model to explore the effectiveness of rapamycin for the treatment of skin SCC.

RESULTS

Rapamycin exerted a remarkable anticancer activity in this chemically induced cancer model, decreasing the tumor burden of mice harboring early and advanced tumor lesions, and even recurrent skin SCCs. Immunohistochemical studies on tumor biopsies and clustering analysis revealed that rapamycin causes the rapid decrease in the phosphorylation status of mTOR targets followed by the apoptotic death of cancer cells and the reduction in the growth and metabolic activity of the surviving ones, concomitant with a decrease in the population of cancer cells expressing mutant p53. This approach enabled investigating the relationship among molecular changes caused by mTOR inhibition, thus helping identify relevant biomarkers for monitoring the effectiveness of mTOR inhibition in the clinical setting.

CONCLUSIONS

Together, these findings provide a strong rationale for the early evaluation of mTOR inhibitors as a molecular targeted approach to treat SCC.

Role for EPS8 in squamous carcinogenesis

We have investigated the role of the signaling intermediate, EPS8, in tumor progression using a model system and in vivo. HN4 primary tumor cells express low levels of EPS8, similar to normal keratinocytes, and show minimal invasion in vitro in response to epidermal growth factor, whereas HN12 cells express high levels of EPS8 and are highly motile in vitro and tumorigenic in vivo. Additional independent tumor cell lines also showed elevated EPS8 expression compared with normal keratinocytes. Using retroviral transduction, we generated HN4 cell lines expressing EPS8 (HN4/EPS8) at levels equivalent to those present in HN12 cells. HN4/EPS8 cells showed increased proliferation and migration compared with controls, together with elevated expression and activity of matrix metalloprotease (MMP)-9, which was dependent on protein kinase B (AKT) activity. Introduction of plasmids that direct synthesis of EPS8 short hairpin RNA (shRNA) into HN12 cells resulted in decreased EPS8 expression in these cells, which correlated with a decrease in their capacity to migrate and invade in vitro. In addition, shRNA-mediated knockdown of EPS8 reduced expression and activity of MMP-9 produced by these cells and reduced MMP-9 promoter activity. EPS8 knockdown cells showed decreased tumorigenicity in vivo compared with controls and lower MMP-9 expression. Conversely, overexpression of EPS8 in HN4 cells was sufficient to induce growth of these non-tumorigenic cells in orthotopic transplantation assays. Furthermore, EPS8 expression in clinical samples of squamous cell carcinoma showed variable expression levels and broadly paralleled expression of MMP-9. The data support a role for EPS8 in squamous carcinogenesis.

Keratin down-regulation in vimentin-positive cancer cells is reversible by vimentin RNA interference, which inhibits growth and motility

At later stages of tumor progression, epithelial carcinogenesis is associated with transition to a mesenchymal phenotype, which may contribute to the more aggressive properties of cancer cells and may be stimulated by growth factors such as epidermal growth factor and transforming growth factor-beta. Previously, we found that cells derived from a nodal metastatic squamous cell carcinoma are highly proliferative and motile in vitro and tumorigenic in vivo. In the current study, we have investigated the role of vimentin in proliferation and motility. Cells derived from nodal metastasis express high levels of vimentin, which is undetectable in tumor cells derived from a synchronous primary lesion of tongue. Vimentin expression was enhanced by epidermal growth factor and transforming growth factor-beta both independently and in combination. Use of RNA interference resulted in the generation of stable cell lines that express constitutively low levels of vimentin. RNA interference-mediated vimentin knockdown reduced cellular proliferation, migration, and invasion through a basement membrane substitute by 3-fold compared with nontargeting controls. In addition, cells with reduced vimentin reexpressed differentiation-specific keratins K13, K14, and K15 as a result of increased gene transcription as judged by quantitative PCR and promoter-reporter assays. Furthermore, cells in which vimentin expression was reduced showed a greatly decreased tumorigenic potential, as tumors developing from these cells were 70% smaller than those from control cells. The data suggest that reversal of the mesenchymal phenotype by inhibiting vimentin expression results in reexpression of epithelial characteristics and reduced tumor aggressiveness.

RCAS/SCL-TVA animal model allows targeted delivery of polyoma middle T oncogene to vascular endothelial progenitors in vivo and results in hemangioma development

PURPOSE

To recapitulate the generation of cancer stem cells in the context of an intact animal using a retroviral vector capable of in vivo delivery of oncogenes to primitive endothelial and hematopoietic stem cells.

EXPERIMENTAL DESIGN

Targeting of these progenitors was achieved using transgenic mice in which the avian TVA retroviral receptor was placed under the control of the stem cell leukemia (scl/tal-1) gene promoter and SCL +19 enhancer.

RESULTS

Injection of an avian retrovirus encoding polyoma middle T (PyMT), an oncogene that transforms endothelial cells, caused rapid lethality in all SCL-TVA mice but not in control TVA(-) littermates. The infected animals exhibited hemorrhagic foci in several organs. Histopathologic analysis confirmed the presence of hemangiomas and the endothelial origin of the PyMT-transformed cells. Surprisingly, the transformed endothelial cells contained readily detectable numbers of TVA(+) cells. By contrast, normal blood vessels had very few of these cells. The presence of TVA(+) cells in the lesions suggests that the cells originally infected by PyMT retained stem cell characteristics. Further analysis showed that the tumor cells exhibited activation of the phosphatidylinositol 3-kinase/Akt and S6/mammalian target of rapamycin pathways, suggesting a mechanism used by PyMT to transform endothelial progenitors in vivo.

CONCLUSIONS

We conclude that this experimental system can specifically deliver oncogenes to vascular endothelial progenitors in vivo and cause a fatal neoplastic disease. This animal model should allow the generation of endothelial cancer stem cells in the natural environment of an immunocompetent animal, thereby enabling the recapitulation of genetic alterations that are responsible for the initiation and progression of human malignancies of endothelial origin.

Protein kinase C-related kinase and ROCK are required for thrombin-induced endothelial cell permeability downstream from Galpha12/13 and Galpha11/q

Increase in vascular permeability occurs under many physiological conditions such as wound repair, inflammation, and thrombotic reactions and is central in diverse human pathologies, including tumor-induced angiogenesis, ocular diseases, and septic shock. Thrombin is a pro-coagulant serine protease, which causes the local loss of endothelial barrier integrity thereby enabling the rapid extravasation of plasma proteins and the local formation of fibrin-containing clots. Available information suggests that thrombin induces endothelial permeability by promoting actomyosin contractility through the Rho/ROCK signaling pathway. Here we took advantage of pharmacological inhibitors, knockdown approaches, and the emerging knowledge on how permeability factors affect endothelial junctions to investigate in detail the mechanism underlying thrombin-induced endothelial permeability. We show that thrombin signals through PAR-1 and its coupled G proteins Galpha(12/13) and Galpha(11/q) to induce RhoA activation and intracellular calcium elevation, and that these events are interrelated. In turn, this leads to the stimulation of ROCK, which causes actin stress-fiber formation. However, this alone is not sufficient to account for thrombin-induced permeability. Instead, we found that protein kinase C-related kinase, a Rho-dependent serine/threonine kinase, is activated in endothelial cells upon thrombin stimulation and that its expression is required for endothelial permeability and the remodeling of cell-extracellular matrix and cell-cell adhesions. Our results demonstrate that the signal initiated by thrombin bifurcates at the level of RhoA to promote changes in the cytoskeletal architecture through ROCK, and the remodeling of focal adhesion components through protein kinase C-related kinase. Ultimately, both pathways converge to cause cell-cell junction disruption and provoke vascular leakage.

Snail up-regulates proinflammatory mediators and inhibits differentiation in oral keratinocytes

The transcriptional repressor Snail2 is overexpressed in head and neck squamous cell carcinomas (HNSCC) relative to nonmalignant head and neck mucosal epithelium, and in locally recurrent relative to nonrecurrent HNSCCs. We investigated the mechanisms by which Snails might contribute to the pathogenesis of HNSCCs using cell biological and molecular analyses. Oral keratinocytes that expressed Snails acquired an enhanced ability to attract monocytes and to invade a dense interstitial collagen matrix. They were also found to up-regulate production of proinflammatory cytokines and cyclooxygenase-2 (COX2), which have previously been shown to correlate with malignancy. Induction of nuclear factor-kappaB transcriptional activity by Snails was weak and not sufficient to account for the elevated levels of COX2, interleukin (IL)-6, IL8, or CXCL1. In addition, expression of Snails in oral keratinocytes impaired desquamation in vitro and strongly repressed expression of both ELF3 and matriptase-1, which play important roles in the terminal differentiation of keratinocytes. Reexpression of matriptase-1 in Snail-expressing cells partially rescued desquamation. This implicates Snails as contributing to malignancy both at the early stages, by impeding terminal differentiation, and at later stages, when invasion and inflammation are important.

Proteomic analysis of laser-captured paraffin-embedded tissues: a molecular portrait of head and neck cancer progression

PURPOSE

Squamous cell carcinoma of the head and neck (HNSCC), the sixth most prevalent cancer among men worldwide, is associated with poor prognosis, which has improved only marginally over the past three decades. A proteomic analysis of HNSCC lesions may help identify novel molecular targets for the early detection, prevention, and treatment of HNSCC.

EXPERIMENTAL DESIGN

Laser capture microdissection was combined with recently developed techniques for protein extraction from formalin-fixed paraffin-embedded (FFPE) tissues and a novel proteomics platform. Approximately 20,000 cells procured from FFPE tissue sections of normal oral epithelium and well, moderately, and poorly differentiated HNSCC were processed for mass spectrometry and bioinformatic analysis.

RESULTS

A large number of proteins expressed in normal oral epithelium and HNSCC, including cytokeratins, intermediate filaments, differentiation markers, and proteins involved in stem cell maintenance, signal transduction, migration, cell cycle regulation, growth and angiogenesis, matrix degradation, and proteins with tumor suppressive and oncogenic potential, were readily detected. Of interest, the relative expression of many of these molecules followed a distinct pattern in normal squamous epithelia and well, moderately, and poorly differentiated HNSCC tumor tissues. Representative proteins were further validated using immunohistochemical studies in HNSCC tissue sections and tissue microarrays.

CONCLUSIONS

The ability to combine laser capture microdissection and in-depth proteomic analysis of FFPE tissues provided a wealth of information regarding the nature of the proteins expressed in normal squamous epithelium and during HNSCC progression, which may allow the development of novel biomarkers of diagnostic and prognostic value and the identification of novel targets for therapeutic intervention in HNSCC.

Binding-induced folding of a natively unstructured transcription factor

Transcription factors are central components of the intracellular regulatory networks that control gene expression. An increasingly recognized phenomenon among human transcription factors is the formation of structure upon target binding. Here, we study the folding and binding of the pKID domain of CREB to the KIX domain of the co-activator CBP. Our simulations of a topology-based Gō-type model predict a coupled folding and binding mechanism, and the existence of partially bound intermediates. From transition-path and Phi-value analyses, we find that the binding transition state resembles the unstructured state in solution, implying that CREB becomes structured only after committing to binding. A change of structure following binding is reminiscent of an induced-fit mechanism and contrasts with models in which binding occurs to pre-structured conformations that exist in the unbound state at equilibrium. Interestingly, increasing the amount of structure in the unbound pKID reduces the rate of binding, suggesting a "fly-casting"-like process. We find that the inclusion of attractive non-native interactions results in the formation of non-specific encounter complexes that enhance the on-rate of binding, but do not significantly change the binding mechanism. Our study helps explain how being unstructured can confer an advantage in protein target recognition. The simulations are in general agreement with the results of a recently reported nuclear magnetic resonance study, and aid in the interpretation of the experimental binding kinetics.

A retroinhibition approach reveals a tumor cell-autonomous response to rapamycin in head and neck cancer

Emerging evidence supporting the activation of the Akt-mammalian target of rapamycin (mTOR) signaling network in head and neck squamous cell carcinoma (HNSCC) progression has provided the rationale for exploring the therapeutic potential of inhibiting this pathway for HNSCC treatment. Indeed, rapamycin, a clinically relevant mTOR inhibitor, promotes the rapid regression of HNSCC-tumor xenografts in mice. However, rapamycin does not affect the growth of HNSCC cells in vitro, thus raising the possibility that, as for other cancer types, rapamycin may not target cancer cells directly but may instead act on a component of the tumor microenvironment, such as tumor-associated vasculature. Here, we used a retroinhibition approach to assess the contribution of cancer cell-autonomous actions of rapamycin to its antitumor activity in HNSCC. A rapamycin-resistant form of mTOR (mTOR-RR) was expressed in HNSCC cells while retaining the wild-type (rapamycin-sensitive) mTOR (mTOR-WT) alleles in host-derived endothelial and stromal cells. Expression of mTOR-RR prevented the decrease in phospho-S6 levels caused by rapamycin through mTOR in HNSCC cells but not in stromal cells, and rendered HNSCC xenografts completely resistant to the antitumoral activity of rapamycin. This reverse pharmacology strategy also enabled monitoring the direct consequences of inhibiting mTOR in cancer cells within the complex tumor microenvironment, which revealed that mTOR controls the accumulation of hypoxia-inducible factor-1 alpha (HIF-1 alpha) and the consequent expression of vascular endothelial growth factor and a glucose transporter, Glut-1, in HNSCC cells. These findings indicate that HNSCC cells are the primary target of rapamycin in vivo, and provide evidence that its antiangiogenic effects may represent a downstream consequence of mTOR inhibition in HNSCC cells.

An essential role for Rac1 in endothelial cell function and vascular development

Numerous cell surface receptors, including tyrosine kinase and G protein-coupled receptors, play critical roles in endothelial cell function and blood vessel development. These receptors share the ability of stimulating an intricate network of intracellular signaling pathways, including the activation of members of the Ras and Rho family of small GTPases. However, the contribution of these signaling molecules to the numerous biological activities performed by endothelial cells is still not fully understood. Here, we have used a conditional Cre/Flox approach, enabling the deletion of the Rac1 gene in endothelial cells, to examine the role of the Rho-related GTPase Rac1 in endothelial cell function and vascular development. Rac1 excision in primary endothelial cells in vitro revealed that Rac1 plays a central role in endothelial cell migration, tubulogenesis, adhesion, and permeability in response to vascular endothelial growth factor (VEGF) and sphingosine-1-phosphate (S1P), which is likely due to the inability of Rac1-deficient endothelial cells to form lamellipodial structures and focal adhesions, and to remodel their cell-cell contacts. Importantly, endothelial-specific excision of Rac1 results in embryonic lethality in midgestation (around E9.5), and defective development of major vessels and complete lack of small branched vessels was readily observed in these endothelial Rac1-deficient embryos and their yolk sacs. These findings provide direct evidence that the activity of Rac1 in endothelial cells is essential for vascular development and suggest that Rac1 and its downstream targets may represent promising therapeutic targets for the treatment of numerous human diseases that involve aberrant neovascularization.

c-Abl activates p38 MAPK independently of its tyrosine kinase activity: Implications in cisplatin-based therapy

Activation of p38 MAPK is a critical requisite for the therapeutics activity of the antitumor agent cisplatin. In this sense, a growing body of evidences supports the role of c-Abl as a major determinant of p38 MAPK activation, especially in response to genotoxic stress when triggered by cisplatin. Here, we demonstrate that p38 MAPK activation in response to cisplatin does not require the tyrosine kinase activity of c-Abl. Indeed, c-Abl can activate the p38 MAPK signaling pathway by a mechanism that is independent of its tyrosine kinase activity, but that instead involves the ability of c-Abl to increase the stability of MKK6. Similar results were obtained in chronic myeloid leukemia-derived cell lines, in which a chimeric Bcr/Abl protein mimics the effects of c-Abl overexpression on p38 MAPK activation. These findings may explain why a clinically used c-Abl inhibitor, imatinib mesylate, fails to inhibit the p38 MAPK pathway alone or in combination with cisplatin, and provide evidence of a novel signaling mechanism in which these antitumor agents act.

Multiple PPPS/TP motifs act in a combinatorial fashion to transduce Wnt signaling through LRP6

Binding of Wnt to Frizzled, and either of two members of the low-density-lipoprotein receptor-related protein family, LRP5/6, leads to beta-catenin activation by a poorly understood mechanism. LRP5/6 exhibit five highly conserved PPPS/TP motifs in their intracellular region, among which the first PPPS/TP site is rapidly phosphorylated upon Wnt stimulation. By the use of full-length LRP6 mutants harboring multiple mutations involving the five PPPS/TP motifs, we found that this first PPPS/TP phosphoacceptor site is alone not sufficient or strictly necessary for beta-catenin activation. Instead, we show that each LRP6 PPPS/TP motif contributes in a combinatorial fashion to activate the canonical Wnt-beta-catenin pathway.

G12-G13-LARG-mediated signaling in vascular smooth muscle is required for salt-induced hypertension

The tone of vascular smooth muscle cells is a primary determinant of the total peripheral vascular resistance and hence the arterial blood pressure. Most forms of hypertension ultimately result from an increased vascular tone that leads to an elevated total peripheral resistance. Regulation of vascular resistance under normotensive and hypertensive conditions involves multiple mediators, many of which act through G protein-coupled receptors on vascular smooth muscle cells. Receptors that mediate vasoconstriction couple with the G-proteins G(q)-G11 and G12-G13 to stimulate phosphorylation of myosin light chain (MLC) via the Ca2+/MLC kinase- and Rho/Rho kinase-mediated signaling pathways, respectively. Using genetically altered mouse models that allow for the acute abrogation of both signaling pathways by inducible Cre/loxP-mediated mutagenesis in smooth muscle cells, we show that G(q)-G11-mediated signaling in smooth muscle cells is required for maintenance of basal blood pressure and for the development of salt-induced hypertension. In contrast, lack of G12-G13, as well as of their major effector, the leukemia-associated Rho guanine nucleotide exchange factor (LARG), did not alter normal blood pressure regulation but did block the development of salt-induced hypertension. This identifies the G12-G13-LARG-mediated signaling pathway as a new target for antihypertensive therapies that would be expected to leave normal blood pressure regulation unaffected.

Matrix metalloproteinase-activated anthrax lethal toxin demonstrates high potency in targeting tumor vasculature

Anthrax lethal toxin (LT), a virulence factor secreted by Bacillus anthracis, is selectively toxic to human melanomas with the BRAF V600E activating mutation because of its proteolytic activities toward the mitogen-activated protein kinase kinases (MEKs). To develop LT variants with lower in vivo toxicity and high tumor specificity, and therefore greater potential for clinical use, we generated a mutated LT that requires activation by matrix metalloproteinases (MMPs). This engineered toxin was less toxic than wild-type LT to mice because of the limited expression of MMPs by normal cells. Moreover, the systemically administered toxin produced greater anti-tumor effects than wild-type LT toward human xenografted tumors. This was shown to result from its greater bioavailability, a consequence of the limited uptake and clearance of the modified toxin by normal cells. Furthermore, the MMP-activated LT had very potent anti-tumor activity not only to human melanomas containing the BRAF mutation but also to other tumor types, including lung and colon carcinomas regardless of their BRAF status. Tumor histology and in vivo angiogenesis assays showed that this anti-tumor activity is due largely to the indirect targeting of tumor vasculature and angiogenic processes. Thus, even tumors genetically deficient in anthrax toxin receptors were still susceptible to the toxin therapy in vivo. Moreover, the modified toxin also displayed lower immunogenicity compared with the wild-type toxin. All these properties suggest that this MMP-activated anti-tumor toxin has potential for use in cancer therapy.

Plexin-B1 Utilizes RhoA and Rho Kinase to Promote the Integrin-dependent Activation of Akt and ERK and Endothelial Cell Motility

The semaphorins are a family of proteins originally identified as axon-guiding molecules in the developing nervous system that have been recently shown to regulate many cellular functions, including motility, in a variety of cell types. We have previously shown that in endothelial cells Semaphorin 4D acts through its receptor, Plexin-B1, to elicit a pro-angiogenic phenotype that involves the activation of the phosphatidylinositol 3-kinase (PI3K)-Akt signaling pathway. Here we show through the use of a receptor chimeric approach, Plexin-B1 mutants, and dominant negative and pharmacological inhibitors that this response is dependent upon the activation of RhoA and its downstream target, Rho kinase (ROK). Indeed, we demonstrate that in endothelial cells, Semaphorin 4D promotes the formation of focal adhesion complexes, stress fibers, and the phosphorylation of myosin light chain, a response that was abolished by the use of ROK inhibitors and absent from cells expressing Plexin-B1 mutant constructs incapable of signaling to RhoA. Stress fiber polymerization and contraction are in turn necessary for RhoA-dependent pro-angiogenic signaling through Plexin-B1. Furthermore, we observed that in endothelial cells Plexin-B1 promotes the integrin-mediated activation of Pyk2, resulting in the stimulation of PI3K, Akt, and ERK. These findings provide evidence that Plexin-B1 promotes endothelial cell motility through RhoA and ROK by regulating the integrin-dependent signaling networks that result in the activation of PI3K and Akt.

Dissecting the Akt/mammalian target of rapamycin signaling network: emerging results from the head and neck cancer tissue array initiative

PURPOSE

As an approach to evaluate the expression pattern and status of activation of signaling pathways in clinical specimens from head and neck squamous cell carcinoma (HNSCC) patients, we established the Head and Neck Cancer Tissue Array Initiative, an international consortium aimed at developing a high-density HNSCC tissue microarray, with a high representation of oral squamous cell carcinoma.

EXPERIMENTAL DESIGN

These tissue arrays were constructed by acquiring cylindrical biopsies from multiple individual tumor tissues and transferring them into tissue microarray blocks. From a total of 1,300 cases, 547 cores, including controls, were selected and used to build the array.

RESULTS

Emerging information by the use of phosphospecific antibodies detecting the activated state of signaling molecules indicates that the Akt-mammalian target of rapamycin (mTOR) pathway is frequently activated in HNSCC, but independently from the activation of epidermal growth factor receptor or the detection of mutant p53. Indeed, we identified a large group of tissue samples displaying active Akt and mTOR in the absence of epidermal growth factor receptor activation. Furthermore, we have also identified a small subgroup of patients in which the mTOR pathway is activated but not Akt, suggesting the existence of an Akt-independent signaling route stimulating mTOR.

CONCLUSIONS

These findings provide important information about the nature of the dysregulated signaling networks in HNSCC and may also provide the rationale for the future development of novel mechanism-based therapies for HNSCC patients.

The Galpha12/13 family of heterotrimeric G proteins and the small GTPase RhoA link the Kaposi sarcoma-associated herpes virus G protein-coupled receptor to heme oxygenase-1 expression and tumorigenesis

Heme oxygenase-1 (HO-1), an inducible enzyme that metabolizes the heme group, is highly expressed in human Kaposi sarcoma lesions. Its expression is up-regulated by the G protein-coupled receptor from the Kaposi sarcoma-associated herpes virus (vGPCR). Although recent evidence shows that HO-1 contributes to vGPCR-induced tumorigenesis and vascular endothelial growth factor (VEGF) expression, the molecular steps that link vGPCR to HO-1 remain unknown. Here we show that vGPCR induces HO-1 expression and transformation through the Galpha(12/13) family of heterotrimeric G proteins and the small GTPase RhoA. Targeted small hairpin RNA knockdown expression of Galpha(12), Galpha(13), or RhoA and inhibition of RhoA activity impair vGPCR-induced transformation and ho-1 promoter activity. Knockdown expression of RhoA also reduces vGPCR-induced VEFG-A secretion and blocks tumor growth in a murine allograft tumor model. NIH-3T3 cells expressing constitutively activated Galpha(13) or RhoA implanted in nude mice develop tumors displaying spindle-shaped cells that express HO-1 and VEGF-A, similarly to vGPCR-derived tumors. RhoAQL-induced tumor growth is reduced 80% by small hairpin RNA-mediated knockdown expression of HO-1 in the implanted cells. Likewise, inhibition of HO-1 activity by chronic administration of the HO-1 inhibitor tin protoporphyrin IX to mice reduces RhoAQL-induced tumor growth by 70%. Our study shows that vGPCR induces HO-1 expression through the Galpha(12/13)/RhoA axes and shows for the first time a potential role for HO-1 as a therapeutic target in tumors where RhoA has oncogenic activity.

Complementary roles of intracellular and pericellular collagen degradation pathways in vivo

Collagen degradation is essential for cell migration, proliferation, and differentiation. Two key turnover pathways have been described for collagen: intracellular cathepsin-mediated degradation and pericellular collagenase-mediated degradation. However, the functional relationship between these two pathways is unclear and even controversial. Here we show that intracellular and pericellular collagen turnover pathways have complementary roles in vivo. Individual deficits in intracellular collagen degradation (urokinase plasminogen activator receptor-associated protein/Endo180 ablation) or pericellular collagen degradation (membrane type 1-matrix metalloproteinase ablation) were compatible with development and survival. Their combined deficits, however, synergized to cause postnatal death by severely impairing bone formation. Interestingly, this was mechanistically linked to the proliferative failure and poor survival of cartilage- and bone-forming cells within their collagen-rich microenvironment. These findings have important implications for the use of pharmacological inhibitors of collagenase activity to prevent connective tissue destruction in a variety of diseases.

Augmented Wnt signaling in a mammalian model of accelerated aging

The contribution of stem and progenitor cell dysfunction and depletion in normal aging remains incompletely understood. We explored this concept in the Klotho mouse model of accelerated aging. Analysis of various tissues and organs from young Klotho mice revealed a decrease in stem cell number and an increase in progenitor cell senescence. Because klotho is a secreted protein, we postulated that klotho might interact with other soluble mediators of stem cells. We found that klotho bound to various Wnt family members. In a cell culture model, the Wnt-klotho interaction resulted in the suppression of Wnt biological activity. Tissues and organs from klotho-deficient animals showed evidence of increased Wnt signaling, and ectopic expression of klotho antagonized the activity of endogenous and exogenous Wnt. Both in vitro and in vivo, continuous Wnt exposure triggered accelerated cellular senescence. Thus, klotho appears to be a secreted Wnt antagonist and Wnt proteins have an unexpected role in mammalian aging.

Inhibition of Pin1 reduces glutamate-induced perikaryal accumulation of phosphorylated neurofilament-H in neurons

Under normal conditions, the proline-directed serine/threonine residues of neurofilament tail-domain repeats are exclusively phosphorylated in axons. In pathological conditions such as amyotrophic lateral sclerosis (ALS), motor neurons contain abnormal perikaryal accumulations of phosphorylated neurofilament proteins. The precise mechanisms for this compartment-specific phosphorylation of neurofilaments are not completely understood. Although localization of kinases and phosphatases is certainly implicated, another possibility involves Pin1 modulation of phosphorylation of the proline-directed serine/threonine residues. Pin1, a prolyl isomerase, selectively binds to phosphorylated proline-directed serine/threonine residues in target proteins and isomerizes cis isomers to more stable trans configurations. In this study we show that Pin1 associates with phosphorylated neurofilament-H (p-NF-H) in neurons and is colocalized in ALS-affected spinal cord neuronal inclusions. To mimic the pathology of neurodegeneration, we studied glutamate-stressed neurons that displayed increased p-NF-H in perikaryal accumulations that colocalized with Pin1 and led to cell death. Both effects were reduced upon inhibition of Pin1 activity by the use of an inhibitor juglone and down-regulating Pin1 levels through the use of Pin1 small interfering RNA. Thus, isomerization of lys-ser-pro repeat residues that are abundant in NF-H tail domains by Pin1 can regulate NF-H phosphorylation, which suggests that Pin1 inhibition may be an attractive therapeutic target to reduce pathological accumulations of p-NF-H.

MAP kinases and the control of nuclear events

The mitogen-activated protein kinases (MAPKs) are a family of serine/threonine kinases that play an essential role in signal transduction by modulating gene transcription in the nucleus in response to changes in the cellular environment. They include the extracellular signal-regulated protein kinases (ERK1 and ERK2); c-Jun N-terminal kinases (JNK1, JNK2, JNK3); p38s (p38alpha, p38beta, p38gamma, p38delta) and ERK5. The molecular events in which MAPKs function can be separated in discrete and yet interrelated steps: activation of the MAPK by their upstream kinases, changes in the subcellular localization of MAPKs, and recognition, binding and phosphorylation of MAPK downstream targets. The resulting pattern of gene expression will ultimately depend on the integration of the combinatorial signals provided by the temporal activation of each group of MAPKs. This review will focus on how the specificity of signal transmission by MAPKs is achieved by scaffolding molecules and by the presence of structural motifs in MAPKs that are dynamically regulated by phosphorylation and protein-protein interactions. We discuss also how MAPKs recognize and phosphorylate their target nuclear proteins, including transcription factors, co-activators and repressors and chromatin-remodeling molecules, thereby affecting an intricate balance of nuclear regulatory molecules that ultimately control gene expression in response to environmental cues.