VEGF-integrin interplay controls tumor growth and vascularization

Arginine regulation by myeloid derived suppressor cells and tolerance in cancer: mechanisms and therapeutic perspectives

Patients with cancer have an impaired T-cell response that can decrease the potential therapeutic benefit of cancer vaccines and other forms of immunotherapy. L-arginine (L-Arg) is a conditionally essential amino acid that is fundamental for the function of T lymphocytes. Recent findings in tumor-bearing mice and cancer patients indicate that increased metabolism of L-Arg by myeloid derived suppressor cells (MDSCs) producing arginase I inhibits T-lymphocyte responses. Here we discuss some of the most recent concepts how MDSC expressing arginase I may regulate T-cell function in cancer and other chronic inflammatory diseases and suggest possible therapeutic interventions to overcome this inhibitory effect.

Cancer cell angiogenic capability is regulated by 3D culture and integrin engagement

Three-dimensional culture alters cancer cell signaling; however, the underlying mechanisms and importance of these changes on tumor vascularization remain unclear. A hydrogel system was used to examine the role of the transition from 2D to 3D culture, with and without integrin engagement, on cancer cell angiogenic capability. Three-dimensional culture recreated tumor microenvironmental cues and led to enhanced interleukin 8 (IL-8) secretion that depended on integrin engagement with adhesion peptides coupled to the polymer. In contrast, vascular endothelial growth factor (VEGF) secretion was unaffected by 3D culture with or without substrate adhesion. IL-8 diffused greater distances and was present in higher concentrations in the systemic circulation, relative to VEGF. Implantation of a polymeric IL-8 delivery system into GFP bone marrow-transplanted mice revealed that localized IL-8 up-regulation was critical to both the local and systemic control of tumor vascularization in vivo. In summary, 3D integrin engagement within tumor microenvironments regulates cancer cell angiogenic signaling, and controlled local and systemic blockade of both IL-8 and VEGF signaling may improve antiangiogenic therapies.

Spatial signaling networks converge at the adaptor protein Shc

Endothelial cells, which are located at the interface between the blood and the vessel wall, respond dynamically to a variety of stimuli initiating signaling cascades that regulate cardiovascular development, physiology and pathology. These inputs include soluble factors that bind to their receptors, integrin-matrix interactions, cell-cell contacts and mechanical forces due to the flowing blood. While these stimuli can mediate unique downstream signals, it is well-accepted that signaling pathways are highly interwoven into complex signaling networks with several levels of cross-talk, integration and coordination. Recent studies suggest that several signaling networks coalesce at the adaptor protein Shc.

Integrin and growth factor receptor alliance in angiogenesis

A sequence of events in vascular and stromal cells maintained in a highly coordinated manner regulates angiogenesis and tissue remodeling. These processes are mediated by the ability of cells to respond to environmental cues and activate surface integrins. Physiological and pathological processes in vascular biology are dependent on the specificity of important signaling mechanisms that are activated through the association between growth factors, their receptors, integrins, and their specific extracellular matrix ligands. A large body of evidence from in vitro and in vivo models demonstrates the importance of coordination of signals from the extracellular environment that activates specific tyrosine kinase receptors and integrins in order to regulate angiogenic processes in vivo. In addition to complex formation between growth factor receptors and integrins, growth factors and cytokines also directly interact with integrins, depending upon their concentration levels in the environment, and differentially regulate integrin-related processes. Recent studies from a number of laboratories including ours have provided important novel insights into the involvement of many signaling events that improve our existing knowledge on the cross-talk between growth factor receptors and integrins in the regulation of angiogenesis. In this review, our focus will be on updating the recent developments in the field of integrin-growth factor receptor associations and their implications in the vascular processes.

Multimodality molecular imaging of tumor angiogenesis

Molecular imaging is a key component of 21st-century cancer management. The vascular endothelial growth factor (VEGF)/VEGF receptor signaling pathway and integrin alpha v beta 3, a cell adhesion molecule, play pivotal roles in regulating tumor angiogenesis, the growth of new blood vessels. This review summarizes the current status of tumor angiogenesis imaging with SPECT, PET, molecular MRI, targeted ultrasound, and optical techniques. For integrin alpha v beta 3 imaging, only nanoparticle-based probes, which truly target the tumor vasculature rather than tumor cells because of poor extravasation, are discussed. Once improvements in the in vivo stability, tumor-targeting efficacy, and pharmacokinetics of tumor angiogenesis imaging probes are made, translation to clinical applications will be critical for the maximum benefit of these novel agents. The future of tumor angiogenesis imaging lies in multimodality and nanoparticle-based approaches, imaging of protein-protein interactions, and quantitative molecular imaging. Combinations of multiple modalities can yield complementary information and offer synergistic advantages over any modality alone. Nanoparticles, possessing multifunctionality and enormous flexibility, can allow for the integration of therapeutic components, targeting ligands, and multimodality imaging labels into one entity, termed "nanomedicine," for which the ideal target is tumor neovasculature. Quantitative imaging of tumor angiogenesis and protein-protein interactions that modulate angiogenesis will lead to more robust and effective monitoring of personalized molecular cancer therapy. Multidisciplinary approaches and cooperative efforts from many individuals, institutions, industries, and organizations are needed to quickly translate multimodality tumor angiogenesis imaging into multiple facets of cancer management. Not limited to cancer, these novel agents can also have broad applications for many other angiogenesis-related diseases.

alphavbeta3 integrin and angiogenesis: a moody integrin in a changing environment

In the adult, angiogenesis, the formation of new blood vessels from pre-existing vasculature contributes to the pathogenesis of many disorders including cancer. The role of adhesion molecules, especially integrins, in pathological angiogenesis has long been the subject of investigation, mostly because of their potential as anti-angiogenic targets. Recent studies have highlighted the complexities connected with understanding the roles of one particular integrin, alphavbeta3, in neovascularization. This integrin is notoriously promiscuous and its precise functions in angiogenesis are unclear. Here, I have firstly summarized some of the salient features of the roles played by alphavbeta3 during angiogenesis; secondly attempted to address the apparently conflicting issues surrounding this topic; and finally raised some questions that appear to be unanswered.

The CXC-chemokine CXCL4 interacts with integrins implicated in angiogenesis

The human CXC-chemokine CXCL4 is a potent inhibitor of tumor-induced angiogenesis. Considering that CXCL4 is sequestered in platelet alpha-granules and released following platelet activation in the vicinity of vessel wall injury, we tested the hypothesis that CXCL4 might function as a ligand for integrins. Integrins are a family of adhesion receptors that play a crucial role in angiogenesis by regulating early angiogenic processes, such as endothelial cell adhesion and migration. Here, we show that CXCL4 interacts with alphavbeta3 on the surface of alphavbeta3-CHO. More importantly, human umbilical vein endothelial cells adhere to immobilized CXCL4 through alphavbeta3 integrin, and also through other integrins, such as alphavbeta5 and alpha5beta1. We further demonstrate that CXCL4-integrin interaction is of functional significance in vitro, since immobilized CXCL4 supported endothelial cell spreading and migration in an integrin-dependent manner. Soluble CXCL4, in turn, inhibits integrin-dependent endothelial cell adhesion and migration. As a whole, our study identifies integrins as novel receptors for CXCL4 that may contribute to its antiangiogenic effect.

Therapeutic potential of novel modulators of neovascularization

Neovacularization is an important biological process whereby new blood vessels develop in both health and disease. During development, blood vessels are formed from mesodermal cells in a process called vasculogenesis. The vascular network then expands by the sprouting of new vessel networks from pre-established vessels in a process known as angiogenesis. However, in adult life, undesirable neovascularization is associated with tumor development and a growing list of 'angiogenesis-dependent' diseases, including cardiovascular complications. Furthermore, diseases characterized by ischemia-induced tissue damage cause a neovascularization response to facilitate tissue repair. Recent research has identified novel molecular and cellular mediators of neovascularization that, in adult life, recapitulate angiogenic processes observed during embryonic development. The discovery of vascular progenitor cells and new molecules that display selective functions in modulating endothelial cell fate, migration and patterning, vessel morphogenesis and the amplification of angiogenic signaling by regulating the master signal VEGF, opens the door to new clinical strategies that target angiogenesis-dependent diseases or that can promote therapeutic neovascularization.

Synthesis, characterization, and biological evaluation of integrin alphavbeta3-targeted PAMAM dendrimers

Ligand size and valency strongly influence the receptor uptake and clearance of tumor angiogenesis imaging agents. The structures of successful imaging agents exhibit a high degree of variability, encompassing small monovalent arginine-glycine-aspartic acid (RGD)-containing peptides, multivalent RGD-oligomers, and a monoclonal antibody against integrin alpha-v-beta-3 (alpha-v-beta-3). We have pursued a nanoscale approach to imaging of angiogenesis using rationally designed polyamidoamine (PAMAM) dendrimers covalently adorned with RGD-cyclopeptides. An orthogonal oxime-ligation strategy was applied to chemoselectively effect conjugation of the PAMAM dendrimers with RGD-cyclopeptides for targeting alpha vbeta 3. Fluorescent dyes for optical imaging and chelates for gadolinium-based magnetic resonance (MR) imaging were subsequently appended to create robust multimodal macromolecular imaging agents. Fluorescence microscopy revealed selective binding of the resulting RGD peptide-bearing dendrimer with empty chelates to alpha-v-beta-3-expressing cells, but somewhat reduced selectivity was observed following Gd(III) complexation. The expected incomplete saturation of chelates with Gd(III) ions permitted radiometal complexation, and an in vivo tissue distribution of the resulting agent in M21 melanoma tumor-bearing mice showed mostly renal and reticuloendothelial accumulation, with the tumor:blood ratio peaking (3.30+/-0.03) at 2 h postinjection.

Chapter 7. Molecular imaging of tumor vasculature

Cancer, with more than 10 million new cases a year worldwide, is the third leading cause of death in developed countries. One critical requirement during cancer progression is angiogenesis, the formation of new blood vessels. Structural and functional imaging of tumor vasculature has been studied using various imaging modalities such as magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound. Molecular imaging, a key component of the 21st-century cancer-patient management strategy, takes advantage of these traditional imaging techniques and introduces molecular probes to determine the expression of indicative molecular markers at different stages of cancer development. In this chapter, we will focus on two tumor vasculature-related targets: integrin alpha(v)beta(3) and vascular endothelial growth factor receptor (VEGFR). For imaging of integrin alpha(v)beta(3) on the tumor vasculature, only nanoparticle-based probes will be discussed. VEGFR imaging will be discussed in depth, and we will give a detailed example of positron emission tomography (PET) imaging of VEGFR expression using radio-labeled VEGF(121) protein. Future clinical translation will be critical for maximum patient benefit from these agents. To achieve this goal, multidisciplinary approaches and cooperative efforts from many individuals, institutions, industries, and organizations are needed to quickly translate multimodality tumor vasculature imaging into multiple facets of cancer patient management.

Normalization of the ovarian cancer microenvironment by SPARC

Malignant ascites is a major source of morbidity and mortality in ovarian cancer patients. It functions as a permissive reactive tumor-host microenvironment and provides sustenance for the floating tumor cells through a plethora of survival/metastasis-associated molecules. Using a syngeneic, immunocompetent model of peritoneal ovarian carcinomatosis in SP(-/-) mice, we investigated the molecular mechanisms implicated in the interplay between host secreted protein acidic and rich in cysteine (SPARC) and ascitic fluid prosurvival/prometastasis factors that result in the significantly augmented levels of vascular endothelial growth factor (VEGF) and matrix metalloproteinases (MMP). Ascitic fluid-enhanced ID8 invasiveness was mediated through VEGF via a positive feedback loop with MMP-2 and MMP-9 and through activation of alpha(v) and beta(1) integrins. Host SPARC down-regulated the VEGF-MMP axis at the transcriptional and posttranscriptional levels. In vitro, SPARC attenuated the basal as well as VEGF-induced integrin activation in tumor cells. SPARC inhibited the VEGF- and integrin-mediated ID8 proliferation in vitro and significantly suppressed their tumorigenicity in vivo. Relative to SP(+/+), SP(-/-) ascitic fluid contained significantly higher levels of bioactive lipids and exerted stronger chemotactic, proinvasive, and mitogenic effects on ID8 cells in vitro. SP(-/-) ascites also contained high levels of interleukin-6, macrophage chemoattractant protein-1, and 8-isoprostane (prostaglandin F(2)alpha) that were positively correlated with extensive infiltration of SP(-/-) ovarian tumors and ascites with macrophages. In summary, our findings strongly suggest that host SPARC normalizes the microenvironment of ovarian cancer malignant ascites through down-regulation of the VEGF-integrin-MMP axis, decreases the levels and activity of bioactive lipids, and ameliorates downstream inflammation.

Overexpression of vascular endothelial growth factor 189 in breast cancer cells leads to delayed tumor uptake with dilated intratumoral vessels

Vascular endothelial growth factor (VEGF) is essential for breast cancer progression and is a relevant target in anti-angiogenesis. Although VEGF121 and VEGF165, the fully or partially secreted isoforms, respectively, have been the focus of intense studies, the role of the cell-associated VEGF189 isoform is not understood. To clarify the contribution of VEGF189 to human mammary carcinogenesis, we established several clones of MDA-MB-231 cells stably overexpressing VEGF189 (V189) and VEGF165 (V165). V189 and V165 clones increased tumor growth and angiogenesis in vivo. Remarkably, V165 induced the most rapid tumor uptake, whereas V189 increased vasodilation. In vitro overexpression of VEGF165 and VEGF189 increases the proliferation and chemokinesis of these cancer cells. Interestingly, overexpression of VEGF189 increased cell adhesion on fibronectin (1.9-fold) and vitronectin (1.6-fold), as compared to VEGF165, through alpha5beta1 and alphavbeta5 integrins. Using the BIACore system we demonstrated for the first time that VEGF189 binds directly to neuropilin-1, which is strongly expressed in MDA-MB-231 cells. In contrast, VEGF-R2 was not significantly expressed and VEGF-R1 was expressed at low level. Our in vitro results suggest an autocrine effect of VEGF189 on breast cancer cells, probably through neuropilin-1. In conclusion, our data indicate that VEGF189 participates in mammary tumor growth through both angiogenesis and nonangiogenic functions. Whether VEGF189 overexpression is correlated to prognosis in human breast tumors remains to be established.

Synergistic control of cell adhesion by integrins and syndecans

The ability of cells to adhere to each other and to their surrounding extracellular matrices is essential for a multicellular existence. Adhesion provides physical support for cells, regulates cell positioning and enables microenvironmental sensing. The integrins and the syndecans are two adhesion receptor families that mediate adhesion, but their relative and functional contributions to cell-extracellular matrix interactions remain obscure. Recent advances have highlighted connections between the signalling networks that are controlled by these families of receptors. Here we survey the evidence that synergistic signalling is involved in controlling adhesive function and the regulation of cell behaviour in response to the external environment.

MRI of tumor angiogenesis

Angiogenesis has long been established as a key element in the pathophysiology of tumor growth and metastasis. Increasingly, new molecularly targeted antiangiogenic drugs are being developed in the fight against cancer. These drugs bring with them a need for an accurate means of diagnosing tumor angiogenesis and monitoring response to treatment. Imaging techniques can offer this in a noninvasive way, while also providing functional information about the tumor. Among the many clinical imaging techniques available, MRI alone can provide relatively good spatial resolution and specificity, without ionizing radiation and with limited side effects. Arterial spin labeling (ASL) and blood oxygenation level-dependent (BOLD) imaging techniques can be employed to confer indirect measures of angiogenesis, such as blood flow and blood volume, without the need for external contrast agents. Dynamic contrast-enhanced (DCE)-MRI is rapidly emerging as a standard method for directly measuring angiogenesis during angiogenesis-inhibitor drug trials. As macromolecular MR contrast agents become available, they will inevitably be utilized in the assessment of tumor perfusion and vessel permeability. Meanwhile, technological advances have made imaging at a molecular level a possibility. They have brought the potential to directly target MR contrast agents to markers of angiogenesis, such as the alpha(v)beta(3) integrin. Before this is used clinically, however, substantial gains in sensitivity brought about by improved coils, pulse sequences, and contrast agents will be needed. Herein we discuss the techniques currently available for MRI of angiogenesis, along with their respective advantages and disadvantages, and what the future holds for this evolving field of imaging.

Alpha-v integrins as therapeutic targets in oncology

Integrins are heterodimeric cell adhesion receptors that mediate intercellular communication through cell-extracellular matrix interactions and cell-cell interactions. Integrins have been demonstrated to play a direct role in cancer progression, specifically in tumor cell survival, tumor angiogenesis, and metastasis. Therefore, agents targeted against integrin function have potential as effective anticancer therapies. Numerous anti-integrin agents, including monoclonal antibodies and small-molecule inhibitors, are in clinical development for the treatment of solid and hematologic tumors. This review focuses on the role of alpha(v) integrins in cancer progression, the current status of integrin-targeted agents in development, and strategies for the clinical development of anti-integrin therapies.

Genetic correlates of brain aging on MRI and cognitive test measures: a genome-wide association and linkage analysis in the Framingham Study

BACKGROUND

Brain magnetic resonance imaging (MRI) and cognitive tests can identify heritable endophenotypes associated with an increased risk of developing stroke, dementia and Alzheimer's disease (AD). We conducted a genome-wide association (GWA) and linkage analysis exploring the genetic basis of these endophenotypes in a community-based sample.

METHODS

A total of 705 stroke- and dementia-free Framingham participants (age 62 +9 yrs, 50% male) who underwent volumetric brain MRI and cognitive testing (1999-2002) were genotyped. We used linear models adjusting for first degree relationships via generalized estimating equations (GEE) and family based association tests (FBAT) in additive models to relate qualifying single nucleotide polymorphisms (SNPs, 70,987 autosomal on Affymetrix 100K Human Gene Chip with minor allele frequency > or = 0.10, genotypic call rate > or = 0.80, and Hardy-Weinberg equilibrium p-value > or = 0.001) to multivariable-adjusted residuals of 9 MRI measures including total cerebral brain (TCBV), lobar, ventricular and white matter hyperintensity (WMH) volumes, and 6 cognitive factors/tests assessing verbal and visuospatial memory, visual scanning and motor speed, reading, abstract reasoning and naming. We determined multipoint identity-by-descent utilizing 10,592 informative SNPs and 613 short tandem repeats and used variance component analyses to compute LOD scores.

RESULTS

The strongest gene-phenotype association in FBAT analyses was between SORL1 (rs1131497; p = 3.2 x 10(-6)) and abstract reasoning, and in GEE analyses between CDH4 (rs1970546; p = 3.7 x 10(-8)) and TCBV. SORL1 plays a role in amyloid precursor protein processing and has been associated with the risk of AD. Among the 50 strongest associations (25 each by GEE and FBAT) were other biologically interesting genes. Polymorphisms within 28 of 163 candidate genes for stroke, AD and memory impairment were associated with the endophenotypes studied at p < 0.001. We confirmed our previously reported linkage of WMH on chromosome 4 and describe linkage of reading performance to a marker on chromosome 18 (GATA11A06), previously linked to dyslexia (LOD scores = 2.2 and 5.1).

CONCLUSION

Our results suggest that genes associated with clinical neurological disease also have detectable effects on subclinical phenotypes. These hypothesis generating data illustrate the use of an unbiased approach to discover novel pathways that may be involved in brain aging, and could be used to replicate observations made in other studies.

Cell-matrix adhesion in vascular development

Vascular development requires correct interactions among endothelial cells, pericytes and surrounding cells. These interactions involve many cell adhesion interactions, including cell-matrix interactions both with basement membranes and with surrounding extracellular matrices. Investigations of the contributions of these various interactions in vascular development and angiogenesis have been rather uneven and incomplete over the past 10-15 years. There has been considerable concentration on a few receptors, matrix proteins and proteolytic fragments with the goal of finding means to control angiogenesis. Many other potential contributors have received much less attention. Even for those molecules that have been subject to intensive investigation, our knowledge is incomplete. This review will survey the spectrum of extracellular matrix (ECM) proteins and cell-matrix adhesion receptors (particularly integrins) that are likely to contribute to angiogenesis and discuss what is known and not known about the roles of each of them.

Increased expression of VEGF121/VEGF165-189 ratio results in a significant enhancement of human prostate tumor angiogenesis

Vascular endothelial growth factor (VEGF) is a proangiogenic factor upregulated in many tumors. The alternative splicing of VEGF mRNA renders 3 major isoforms of 121, 165 and 189 amino-acids in humans (1 less amino-acid for each mouse VEGF isoform). We have designed isoform specific real time QRT-PCR assays to quantitate VEGF transcripts in mouse and human normal and malignant prostates. In the human normal prostate, VEGF(165) was the predominant isoform (62.8% +/- 5.2%), followed by VEGF(121) (22.5% +/- 6.3%) and VEGF(189) (p < 0.001) (14.6% +/- 2.1%). Prostate tumors showed a significant increase in the percentage of VEGF(121) and decreases in VEGF(165) (p < 0.01) and VEGF(189) (p < 0.05). However, the amount of total VEGF mRNA was similar between normal and malignant prostates. VEGF(164) was the transcript with the highest expression in the mouse normal prostate. Unlike human prostate cancer, tumors from TRAMP mice demonstrated a significant increase in total VEGF mRNA levels and in each of the VEGF isoforms, without changes in the relative isoform ratios. Morpholino phosphorodiamide antisense oligonucleotide technology was used to increase the relative amount of VEGF(121) while proportionally decreasing VEGF(165) and VEGF(189) levels in human prostate cell lines, through the modification of alternative splicing, without changing transcription levels and total amount of VEGF. The increase in the VEGF(121)/VEGF(165-189) ratio in PC3 cells resulted in a dramatic increase in prostate tumor angiogenesis in vivo. Our results underscore the importance of VEGF(121) in human prostate carcinoma and demonstrate that the relative expression of the different VEGF isoforms has an impact on prostate carcinogenesis.

Philinopside E, a New Sulfated Saponin from Sea Cucumber, Blocks the Interaction between Kinase Insert Domain-Containing Receptor (KDR) and αvβ3 Integrin via Binding to the Extracellular Domain of KDR

Vascular endothelial growth factor (VEGF) signaling pathway is essential for tumor angiogenesis and has long been recognized as a promising target for cancer therapy. Current view holds that physical interaction between alpha(v)beta(3) integrin and kinase insert domain-containing receptor (KDR) is important in regulating angiogenesis and tumor development. We have reported previously that a new marine-derived compound, philinopside E (PE), exhibited the antiangiogenic activity via inhibition on KDR phosphorylation and downstream signaling. Herein, we have further demonstrated that PE specifically interacts with KDR extracellular domain, which is distinct from conventional small-molecule inhibitors targeting cytoplasmic kinase domain, to block its interaction with VEGF and the downstream signaling. We also noted that PE markedly suppresses alpha(v)beta(3) integrin-driven downstream signaling as a result of disturbance of the physical interaction between KDR and alpha(v)beta(3) integrin in HMECs, followed by disruption of the actin cytoskeleton organization and decreased cell adhesion to vitronectin. All of these findings substantiate PE to be an unrecognized therapeutic class in tumor angiogenesis and, more importantly, help appeal the interest of the therapeutic potential in angiogenesis and cancer development via targeting integrin-KDR interaction in the future.

Antisense integrin alphaV and beta3 gene therapy suppresses subcutaneously implanted hepatocellular carcinomas

BACKGROUND

Integrin alphaVbeta3 plays a critical role in tumour angiogenesis and metastasis formation, and is recognized as a key therapeutic target in the treatment of cancer.

AIM

To investigate whether antisense alphaV and beta3 gene therapy has utility in the treatment of hepatocellular carcinomas.

METHODS

Antisense expression plasmids targeting integrin alphaV or beta3 were constructed, and examined by immunohistochemistry and Western blot analyses for their ability to inhibit alphaV and beta3 expression. The antisense alphaV and beta3 expression vectors, either alone or in combination, were injected into HepG2 hepatomas established subcutaneously in nude mice and tumour growth, angiogenesis and apoptosis were monitored.

RESULTS

Antisense alphaV and beta3 downregulated the alphaV and beta3 subunits expressed by human umbilical vein endothelial cells, and the alphaV subunit expressed by HepG2 cells. Gene transfer of antisense alphaV and beta3 expression vectors downregulated alphaV and beta3 in HepG2 tumours established in nude mice, inhibited tumour vascularization and growth, and enhanced tumour cell apoptosis. Antisense alphaV suppressed tumour growth more strongly than antisense beta3; however antisense therapy that simultaneously targeted both integrin subunits was more effective than the respective monotherapies. Antisense alphaV and beta3 inhibited tumour angiogenesis to similar extents, by a process that is independent of vascular endothelial growth factor.

CONCLUSIONS

Antisense gene therapy targeting alphaV integrins warrants consideration as an approach to treat hepatocellular carcinomas.

Secreted protein acidic and rich in cysteine (SPARC) inhibits integrin-mediated adhesion and growth factor-dependent survival signaling in ovarian cancer

The matricellular glycoprotein SPARC (secreted protein acidic and rich in cysteine) has been accorded major roles in regulation of cell adhesion and proliferation, as well as tumorigenesis and metastasis. We have recently reported that in addition to its potent antiproliferative and proapoptotic functions, SPARC also abrogates ovarian carcinoma cell adhesion, a key step in peritoneal implantation. However, the underlying molecular mechanism through which SPARC ameliorates peritoneal ovarian carcinomatosis seems to be multifaceted and has yet to be delineated. Herein, we show that SPARC significantly inhibited integrin-mediated ovarian cancer cell adhesion to extracellular matrix proteins, as well as to peritoneal mesothelial cells. This counteradhesive effect of SPARC was shown to be mediated in part through significant attenuation of cell surface expression and clustering of alpha(v)-integrin subunit, alpha(v)beta(3)- and alpha(v)beta(5)-heterodimers, and beta(1)-subunit, albeit to a lesser extent, in ovarian cancer cells. Moreover, SPARC significantly suppressed both anchorage-dependent and -independent activation of AKT and mitogen-acti-vated protein kinase survival signaling pathways in ovarian cancer cells in response to serum and epidermal growth factor stimulation. In summary, we have identified a novel role of SPARC as a negative regulator of both integrin-mediated adhesion and growth factor-stimulated survival signaling pathways in ovarian cancer.

Prostate cancer specific integrin alphavbeta3 modulates bone metastatic growth and tissue remodeling

The management of pain and morbidity due to the spreading and growth of cancer within bone remains to be a paramount problem in clinical care. Cancer cells actively transform bone, however, the molecular requirements and mechanisms of this process remain unclear. This study shows that functional modulation of the alphavbeta3 integrin receptor in prostate cancer cells is required for progression within bone and determines tumor-induced bone tissue transformation. Using histology and quantitative microCT analysis, we show that alphavbeta3 integrin is required not only for tumor growth within the bone but for tumor-induced bone gain, a response resembling bone lesions in prostate cancer patients. Expression of normal, fully functional alphavbeta3 enabled tumor growth in bone (incidence: 4/4), whereas alphavbeta3 (-), inactive or constitutively active mutants of alphavbeta3 did not (incidence: 0/4, 0/6 and 1/7, respectively) within a 35-day-period. This response appeared to be bone-specific in comparison to the subcutis where tumor incidence was greater than 60% for all groups. Interestingly, bone residing prostate cancer cells expressing normal or dis-regulated alphavbeta3 (either inactive of constitutively active), but not those lacking beta3 promoted bone gain or afforded protection from bone loss in the presence or absence of histologically detectable tumor 35 days following implantation. As bone is replete with ligands for beta3 integrin, we next demonstrated that alphavbeta3 integrin activation on tumor cells is essential for the recognition of key bone-specific matrix proteins. As a result, prostate cancer cells expressing fully functional but not dis-regulated alphavbeta3 integrin are able to control their own adherence and migration to bone matrix, functions that facilitate tumor growth and control bone lesion development.

Clinical relevance of the small intestine as an organ of drug elimination: drug-fruit juice interactions

Most drugs are taken orally. For those intended to act systemically, a significant fraction of the dose can be eliminated during its first passage through a sequence of organs before entry into the general circulation. For some drugs, the degree of first-pass elimination can be large enough such that oral bioavailability is significantly reduced, with the consequent potential for a reduced clinical response. Of these first-pass eliminating organs, the small intestine and liver are the most commonly implicated, in part because they express the highest levels of drug-metabolizing enzymes. For several drugs whose major route of elimination occurs via CYP3A-mediated metabolism, the extent of first-pass metabolism in the small intestine can rival that in the liver. As such, alterations in enteric CYP3A activity alone can significantly influence oral bioavailability. The most extensively studied xenobiotic shown to inhibit only enteric CYP3A is grapefruit juice, the consequences of which can be clinically significant. Although much information has been gained regarding the grapefruit juice effect, progress in the relatively understudied area of drug-diet interactions continues to be sluggish and reactive. In stark contrast, the potential for drug-drug interactions involving any new therapeutic agent must be evaluated, prospectively, before market introduction. To prospectively elucidate mechanisms underlying drug-diet interactions, a multidisciplinary, translational research approach is required, which capitalizes on the collective expertise of drug metabolism scientists and natural products chemists. Such an approach would allow proper between-study comparisons, and ultimately provide conclusive information as to whether specific dietary substances can be taken safely with certain medications.

Thyroid hormone is a MAPK-dependent growth factor for thyroid cancer cells and is anti-apoptotic

Thyroid hormone (l-thyroxine, T(4), or 3,5,3'-triiodo-l-thyronine, T(3)) treatment of human papillary and follicular thyroid cancer cell lines resulted in enhanced cell proliferation, measured by proliferating cell nuclear antigen (PCNA). Thyroid hormone also induced activation of the Ras/MAPK (ERK1/2) signal transduction pathway. ERK1/2 activation and cell proliferation caused by thyroid hormone were blocked by an iodothyronine analogue, tetraiodothyroacetic acid (tetrac), that inhibits binding of iodothyronines to the cell surface receptor for thyroid hormone on integrin alphaVbeta3. A MAPK cascade inhibitor at MEK, PD 98059, also blocked hormone-induced cell proliferation. We then assessed the possibility that thyroid hormone is anti-apoptotic. We first established that resveratrol (10 microM), a pro-apoptotic agent in other cancer cells, induced p53-dependent apoptosis and c-fos, c-jun and p21 gene expression in both papillary and follicular thyroid cancer cells. Induction of apoptosis by the stilbene required Ser-15 phosphorylation of p53. Resveratrol-induced gene expression and apoptosis were inhibited more than 50% by physiological concentrations of T(4). T(4) activated MAPK in the absence of resveratrol, caused minimal Ser-15 phosphorylation of p53 and did not affect c-fos, c-jun and p21 mRNA abundance. Thus, plasma membrane-initiated activation of the MAPK cascade by thyroid hormone promotes papillary and follicular thyroid cancer cell proliferation in vitro.

Evaluating integrin function in models of angiogenesis and vascular permeability

All vascular biological processes are influenced to some degree by integrins expressed on endothelial cells, vascular smooth muscle cells, fibroblasts, platelets, or other circulating cells. In particular, angiogenesis requires cells to process signals from their microenvironment and respond by altering their cell-cell and cell-matrix adhesion, events which allow migration and vascular remodeling over the period of days to weeks. On the other hand, endothelial cells can respond to a permeability stimulus and alter their junctional adhesion molecules or vesicular transport machinery within seconds or minutes. This chapter will discuss the current understanding of how integrins participate in these processes, and explore the in vitro and in vivo models available to study the role of integrin function during angiogenesis and vascular leak.

Molecular imaging of angiogenesis

Angiogenesis (the growth of new blood vessels) is a complex multistep process that involves multiple cell types, numerous growth factors, and complex regulatory checks and balances. Tight control of vascular remodeling evolved to ensure stability of the vasculature while maintaining the body's ability to rapidly mount an angiogenic response requiring a high degree of plasticity. Angiogenesis is critical not only for physiological development, but also for the progression of pathologies, and is thus a target for therapeutic intervention. The importance of the process coupled with the ease of access for delivery of contrast agents makes the vasculature at large, and angiogenesis in particular, a favorable target of functional and molecular imaging. Recent developments in molecular imaging tools have expanded our views to encompass many components of the process. Functional imaging of blood volume, vessel permeability, and vasoreactivity is complemented by novel contrast agents that reveal specific targets on endothelial cells. Methods have been developed to label vascular cells so as to track their recruitment to sites of angiogenesis, and new "smart" contrast agents have been designed to reveal the activity of enzymatic reactions in altering the extracellular matrix (ECM) during angiogenesis.

How molecular imaging is speeding up antiangiogenic drug development

Drug development is a long process that generally spans about 10 to 15 years. The shift in recent drug discovery to novel agents against specific molecular targets highlights the need for more robust molecular imaging platforms. Using molecular probes, molecular imaging can aid in many steps of the drug development process, such as providing whole body readout in an intact system, decreasing the workload and speeding up drug development/validation, and facilitating individualized anticancer treatment monitoring and dose optimization. The main focus of this review is the recent advances in tumor angiogenesis imaging, and the targets include vascular endothelial growth factor and vascular endothelial growth factor receptor, integrin alpha(v)beta(3), matrix metalloproteinase, endoglin (CD105), and E-selectin. Through tumor angiogenesis imaging, it is expected that a robust platform for understanding the mechanisms of tumor angiogenesis and evaluating the efficacy of novel antiangiogenic therapies will be developed, which can help antiangiogenic drug development in both the preclinical stage and the clinical settings. Molecular imaging has enormous potential in improving the efficiency of the drug development process, including the specific area of antiangiogenic drugs.

Heparin-II domain of fibronectin is a vascular endothelial growth factor-binding domain: enhancement of VEGF biological activity by a singular growth factor/matrix protein synergism

We describe extracellular interactions between fibronectin (Fn) and vascular endothelial growth factor (VEGF) that influence integrin-growth factor receptor crosstalk and cellular responses. In previous work, we found that VEGF bound specifically to fibronectin (Fn) but not vitronectin or collagens. Herein we report that VEGF binds to the heparin-II domain of Fn and that the cell-binding and VEGF-binding domains of Fn, when physically linked, are necessary and sufficient to promote VEGF-induced endothelial cell proliferation, migration, and Erk activation. Using recombinant Fn domains, the C-terminal heparin-II domain of Fn (type III repeats 13 to 14) was identified as a key VEGF-binding site. Mutation of the heparin-binding residues on FnIII(13-14) abolished VEGF binding, and peptides corresponding to the heparin-binding sequences in FnIII(13-14) inhibited VEGF binding to Fn. Fn fragments containing both the alpha5beta1 integrin-binding domain (III 9 to 10) and the VEGF-binding domain (III 13 to 14) significantly enhanced VEGF-induced EC migration and proliferation and induced strong phosphorylation of the VEGF receptor and Erk. Neither the cell-binding or VEGF-binding fragment of Fn alone had comparable VEGF-promoting effects. These results suggest that the mechanism of VEGF/Fn synergism is mediated extracellularly by the formation of a novel VEGF/Fn complex requiring both the cell-binding and VEGF-binding domains linked in a single molecular unit. These data also highlight a new function for the Fn C-terminal heparin-binding domain that may have important implications for angiogenesis and tumor growth.

Semaphorin 3C regulates endothelial cell function by increasing integrin activity

Class 3 semaphorins (sema 3) are secreted guidance proteins. Sema 3A expressed by endothelial cells controls vascular morphogenesis through integrin inhibition. Sema 3C is required for normal cardiovascular patterning. Here we examined the potential role of sema 3C as regulator of endothelial cell function in vitro using mouse glomerular endothelial cells (MGEC). We determined that MGEC express sema 3C mRNA and protein and its receptors mRNA. Recombinant sema 3C induced MGEC proliferation 18 +/- 2% above control, as assessed by bromodeoxyuridine (BrdU) incorporation, and reduced starvation-induced apoptosis by 46 +/- 3%, as indicated by an in situ marker of activated caspase 3. Sema 3C increased MGEC adhesion to fibronectin 79 +/- 13% and to collagen 55 +/- 12% as compared with control. Sema 3C-induced MGEC adhesion was prevented by integrin blocking antibodies and involved beta1 integrin serine phosphorylation. Sema 3C-induced MGEC adhesion and proliferation were similar to those induced by vascular endothelial growth factor (VEGF)-A. Sema 3C induced a 44 +/- 11% increase in MGEC directional migration and stimulated MGEC capillary-like network formation on collagen I gels. Collectively, our data indicate that sema 3C promotes glomerular endothelial cell proliferation, adhesion, directional migration, and tube formation in vitro by stimulating integrin phosphorylation and VEGF120 secretion, functions that are similar to VEGF-A and opposite to sema 3A.

The role of Src kinase in insulin-like growth factor-dependent mitogenic signaling in vascular smooth muscle cells

Activation of the MAPK pathway mediates insulin-like growth factor-I (IGF-I)-dependent proliferation in vascular smooth muscle cells (SMC). Our previous studies have shown that IGF-I-induced Shc phosphorylation is necessary for sustained activation of MAPK and increased cell proliferation of SMCs, and both Shc and the tyrosine phosphatase SHP-2 must be recruited to the membrane protein SHPS-1 in order for Shc to be phosphorylated. These studies were undertaken to determine whether Src kinase activity is required to phosphorylate Shc in response to IGF-I in SMC and because SHP-2 binds to Src whether their interaction was also required for IGF-I-stimulated mitogenesis. Our results show that IGF-I induces activation of Src kinase and that is required for Shc phosphorylation and for optimal MAPK activation. We tested whether Shc is a substrate of c-Src in SMC by disrupting Src/Shc association using a peptide containing a YXXL (Tyr328) motif sequence derived from Src. The peptide blocked the binding of Src and Shc in vitro and in vivo. Cells expressing a mutant Src (Src-FF) that had Tyr328/Tyr358 substituted with phenylalanines (Src-FF) showed defective Src/Shc binding, impaired IGF-I-dependent Shc phorylation, and impaired mitogenesis. This supports the conclusion that Src phosphorylates Shc. IGF-I induced both Src/SHP-2 and Src/SHPS-1 association. SMCs expressing an SHP-2 mutant that had the polyproline-rich region of SH2 deleted (SHP-2Delta10) had disrupted SHP-2/Src association, impaired IGF-I-dependent Shc phosphorylation, and an attenuated mitogenic response. IGF-I-induced association of Src and SHPS-1 was also impaired in SHP-2Delata10-expressing cells, although SHP-2/SHPS-1 association was unaffected. Upon IGF-I stimulation, a complex assembles on SHPS-1 that contains SHP-2, c-Src, and Shc wherein Src phosphorylates Shc, a signaling step that is necessary for an optimal mitogenic response.

Intrinsic FAK activity and Y925 phosphorylation facilitate an angiogenic switch in tumors

Elevated focal adhesion kinase (FAK) expression occurs in advanced cancers, yet a signaling role for FAK in tumor progression remains undefined. Here, we suppressed FAK activity in 4T1 breast carcinoma cells resulting in reduced FAK Y925 phosphorylation, Grb2 adaptor protein binding to FAK, and signaling to mitogen-activated protein (MAP) kinase (MAPK). Loss of a FAK-Grb2-MAPK linkage did not affect 4T1 cell proliferation or survival in culture, yet FAK inhibition reduced vascular endothelial growth factor (VEGF) expression and resulted in small avascular tumors in mice. This FAK-Grb2-MAPK linkage was essential in promoting angiogenesis as reconstitution experiments using Src-transformed FAK-null fibroblasts revealed that point mutations affecting FAK catalytic activity (R454) or Y925 phosphorylation (F925) disrupted the ability of FAK to promote MAPK- and VEGF-associated tumor growth. Notably, in both FAK-inhibited 4T1 and Src-transformed FAK-null cells, constitutively activated (CA) mitogen-activated protein kinase kinase 1 (MEK1) restored VEGF production and CA-MEK1 or added VEGF rescued tumor growth and angiogenesis. These studies provide the first biological support for Y925 FAK phosphorylation and define a novel role for FAK activity in promoting a MAPK-associated angiogenic switch during tumor progression.

Peptide-based Molecules in Angiogenesis

Angiogenesis refers to the process of remodeling the vascular tissue characterized by the branching out of a new blood vessel from a pre-existing vessel. Angiogenesis is particularly active during embryogenesis, while during adult life it is quiescent and limited to particular physiologic phenomena. Recently, the study of molecular mechanisms of angiogenesis has stirred renewed interest due to the recognition of the role played by angiogenesis in several pathologies of significant medical impact, such as cancer and cardiovascular disease, and due to the pharmacologic interest rising from the possibility of modulating these phenomena. Antibodies, peptides and small molecules targeting active endothelial cells represent an innovative tool in therapeutic and diagnostic fields. In this study, we reviewed the literature of peptide and peptidomimetics in angiogenesis and their potential applications. Two specific protein systems, namely the vascular endothelial growth factor and its receptor and integrins, will be discussed in detail.

Quantitative PET imaging of tumor integrin alphavbeta3 expression with 18F-FRGD2

The development of noninvasive methods to visualize and quantify integrin alpha(v)beta(3) expression in vivo appears to be crucial for the success of antiangiogenic therapy based on integrin antagonism. Precise documentation of integrin receptor levels will allow appropriate selection of patients who will most likely benefit from an antiintegrin treatment regimen. Imaging can also be used to provide an optimal dosage and time course for treatment based on receptor occupancy studies. In addition, imaging integrin expression will be important to evaluate antiintegrin treatment efficacy and to develop new therapeutic drugs with favorable tumor targeting and in vivo kinetics. We labeled the dimeric RGD peptide E[c(RGDyK)](2) with (18)F and evaluated its tumor-targeting efficacy and pharmacokinetics of (18)F-FB-E[c(RGDyK)](2) ((18)F-FRGD2).

METHODS

E[c(RGDyK)](2) was labeled with (18)F by conjugation coupling with N-succinimidyl-4-(18)F-fluorobenzoate ((18)F-SFB) under a slightly basic condition. The in vivo metabolic stability of (18)F-FRGD2 was determined. The diagnostic value after injection of (18)F-FRGD2 was evaluated in various xenograft models by dynamic microPET followed by ex vivo quantification of tumor integrin level.

RESULTS

Starting with (18)F(-) Kryptofix 2.2.2./K(2)CO(3) solution, the total reaction time for (18)F-FRGD2, including final high-performance liquid chromatography purification, is about 200 +/- 20 min. Typical decay-corrected radiochemical yield is 23% +/- 2% (n = 20). (18)F-FRGD2 is metabolically stable. The binding potential extrapolated from graphical analysis of PET data and Logan plot correlates well with the receptor density measured by sodium dodecyl sulfate polyacrylamide electrophoresis and autoradiography in various xenograft models. The tumor-to-background ratio at 1 h after injection of (18)F-FRGD2 also gives a good linear relationship with the tumor tissue integrin level.

CONCLUSION

The dimeric RGD peptide tracer (18)F-FRGD2, with high integrin specificity and favorable excretion profile, may be translated into the clinic for imaging integrin alpha(v)beta(3) expression. The binding potential calculated from simplified tracer kinetic modeling such as the Logan plot appears to be an excellent indicator of tumor integrin density.

Targeting integrins for the control of tumour angiogenesis

The crucial role of cell extracellular matrix communication in angiogenesis is well established; thus, it is not surprising that integrins have gained considerable attention as targets for the treatment of neovascular disease. Given the diversity of ligands and complexity of integrin signalling, a new appreciation for the divergent roles of integrins in angiogenesis is emerging. It is becoming clear that integrins regulate angiogenesis in both a positive and negative manner. New studies have provided a better understanding of integrin structure as it relates to ligand binding and signalling. This new insight has opened exciting possibilities for the design of novel inhibitors for clinical applications. In this review, studies concerning the cooperative interactions between integrins and regulatory molecules and possible new strategies for controlling angiogenesis will be discussed.

Akt1 regulates pathological angiogenesis, vascular maturation and permeability in vivo

Akt kinases control essential cellular functions, including proliferation, apoptosis, metabolism and transcription, and have been proposed as promising targets for treatment of angiogenesis-dependent pathologies, such as cancer and ischemic injury. But their precise roles in neovascularization remain elusive. Here we show that Akt1 is the predominant isoform in vascular cells and describe the unexpected consequences of Akt1 knockout on vascular integrity and pathological angiogenesis. Angiogenic responses in three distinct in vivo models were enhanced in Akt1(-/-) mice; these enhanced responses were associated with impairment of blood vessel maturation and increased vascular permeability. Although impaired vascular maturation in Akt1(-/-) mice may be attributed to reduced activation of endothelial nitric oxide synthase (eNOS), the major phenotypic changes in vascular permeability and angiogenesis were linked to reduced expression of two endogenous vascular regulators, thrombospondins 1 (TSP-1) and 2 (TSP-2). Re-expression of TSP-1 and TSP-2 in mice transplanted with wild-type bone marrow corrected the angiogenic abnormalities in Akt1(-/-) mice. These findings establish a crucial role of an Akt-thrombospondin axis in angiogenesis.