Disruption of integrin-dependent adhesion and survival of endothelial cells by recombinant adenovirus expressing isolated beta integrin cytoplasmic domains

Are Integrins Still Practicable Targets for Anti-Cancer Therapy?

Correlative clinical evidence and experimental observations indicate that integrin adhesion receptors, in particular those of the αV family, are relevant to cancer cell features, including proliferation, survival, migration, invasion, and metastasis. In addition, integrins promote events in the tumor microenvironment that are critical for tumor progression and metastasis, including tumor angiogenesis, matrix remodeling, and the recruitment of immune and inflammatory cells. In spite of compelling preclinical results demonstrating that the inhibition of integrin αVβ3/αVβ5 and α5β1 has therapeutic potential, clinical trials with integrin inhibitors targeting those integrins have repeatedly failed to demonstrate therapeutic benefits in cancer patients. Here, we review emerging integrin functions and their proposed contribution to tumor progression, discuss preclinical evidence of therapeutic significance, revisit clinical trial results, and consider alternative approaches for their therapeutic targeting in oncology, including targeting integrins in the other cells of the tumor microenvironment, e.g., cancer-associated fibroblasts and immune/inflammatory cells. We conclude that integrins remain a valid target for cancer therapy; however, agents with better pharmacological properties, alternative models for their preclinical evaluation, and innovative combination strategies for clinical testing (e.g., together with immuno-oncology agents) are needed.

Polysaccharide hydrogels with tunable stiffness and provasculogenic properties via α-helix to β-sheet switch in secondary structure

Mechanical aspects of the cellular environment can influence cell function, and in this context hydrogels can serve as an instructive matrix. Here we report that physicochemical properties of hydrogels derived from polysaccharides (agarose, κ-carrageenan) having an α-helical backbone can be tailored by inducing a switch in the secondary structure from α-helix to β-sheet through carboxylation. This enables the gel modulus to be tuned over four orders of magnitude (G' 6 Pa-3.6 × 10(4) Pa) independently of polymer concentration and molecular weight. Using carboxylated agarose gels as a screening platform, we demonstrate that soft-carboxylated agarose provides a unique environment for the polarization of endothelial cells in the presence of soluble and bound signals, which notably does not occur in fibrin and collagen gels. Furthermore, endothelial cells organize into freestanding lumens over 100 μm in length. The finding that a biomaterial can modulate soluble and bound signals provides impetus for exploring mechanobiology paradigms in regenerative therapies.

Ischemia-induced apoptosis of intestinal epithelial cells correlates with altered integrin distribution and disassembly of F-actin triggered by calcium overload

The present study examined intestinal epithelial cell (IEC) integrin distribution and disassembly of actin cytoskeleton in response to ischemia-anoxia. Protective effects of calcium channel blocker(CCB) were further examined to explore underlying mechanisms of cellular injury. Materials and Methods. Primary cultures of rat IECs and an in vitro model of ischemia/anoxia were established. IECs were exposed to ischemia/anoxia in the presence and absence of verapamil. The extent of exfoliation was determined using light microscopy while apoptosis rate was measured using flow cytometry. Changes in intracellular calcium, the distribution of integrins and the morphology of F-actin were assessed by confocal microscopy. Results. Detachment and apoptosis of IECs increased following ischemia/anoxia-induced injury. Treatment with verapamil inhibited the detachment and apoptosis. Under control conditions, the strongest fluorescent staining for integrins appeared on the basal surface of IECs while this re-distributed to the apical membrane in response to ischemic injury. Depolymerization of F-actin was also observed in the injured cells. Verapamil attenuated both changes of integrins and F-actin. Conclusions. Redistribution of integrins and disruption of F-actin under ischemia/anoxia injury is associated with IEC detachment and increased apoptosis. These events appeared to be triggered by an increase in Ca²⁺_i suggesting a potential use for CCB in prevention and treatment of intestinal injury.

Gammaherpesvirus 68 infection of endothelial cells requires both host autophagy genes and viral oncogenes for optimal survival and persistence

Gammaherpesvirus-associated neoplasms include tumors of lymphocytes, epithelial cells, and endothelial cells (ECs). We previously showed that, unlike most cell types, ECs survive productive gammaherpesvirus 68 (γHV68) infection and achieve anchorage-independent growth, providing a cellular reservoir for viral persistence. Here, we demonstrated autophagy in infected ECs by analysis of LC3 localization and protein modification and that infected ECs progress through the autophagosome pathway by LC3 dual fluorescence and p62 analysis. We demonstrate that pharmacologic autophagy induction results in increased survival of infected ECs and, conversely, that autophagy inhibition results in death of infected EC survivors. Furthermore, we identified two viral oncogenes, v-cyclin and v-Bcl2, that are critical to EC survival and that modify EC proliferation and survival during infection-induced autophagy. We found that these viral oncogenes can also facilitate survival of substrate detachment in the absence of viral infection. Autophagy affords cells the opportunity to recover from stressful conditions, and consistent with this, the altered phenotype of surviving infected ECs was reversible. Finally, we demonstrated that knockdown of critical autophagy genes completely abrogated EC survival. This study reveals a viral mechanism which usurps the autophagic machinery to promote viral persistence within nonadherent ECs, with the potential for recovery of infected ECs at a distant site upon disruption of virus replication.

Vascular integrins: therapeutic and imaging targets of tumor angiogenesis

Cells, including endothelial cells, continuously sense their surrounding environment and rapidly adapt to changes in order to assure tissues and organs homeostasis. The extracellular matrix (ECM) provides a physical scaffold for cell positioning and represents an instructive interface allowing cells to communicate over short distances. Cell surface receptors of the integrin family emerged through evolution as essential mediators and integrators of ECM-dependent communication. In preclinical studies, pharmacological inhibition of vascular integrins suppressed angiogenesis and inhibited tumor progression. alpha(V)beta(3) and alpha(V)beta(5) were the first integrins targeted to suppress tumor angiogenesis. Subsequently, additional integrins, in particular alpha(1)beta(1), alpha(2)beta(1), alpha(5)beta(1), and alpha(6)beta(4), emerged as potential therapeutic targets. Integrin inhibitors are currently tested in clinical trials for their safety and antiangiogenic/antitumor activity. In this chapter, we review the role of integrins in angiogenesis and present recent advances in the use of integrin antagonists as potential therapeutics in cancer and discuss future perspectives.

Vascular Integrins in Tumor Angiogenesis: Mediators and Therapeutic Targets

The notion that tumor angiogenesis may have therapeutic implications in the control of tumor growth was introduced by Dr. Judah Folkman in 1971. The approval of Avastin in 2004 as the first antiangiogenic systemic drug to treat cancer patients came as a validation of this visionary concept and opened new perspectives to the treatment of cancer. In addition, this success boosted the field to the quest for new therapeutic targets and antiangiogenic drugs. Preclinical and clinical evidence indicate that vascular integrins may be valid therapeutic targets. In preclinical studies, pharmacological inhibition of integrin function efficiently suppressed angiogenesis and inhibited tumor progression. alphaVbeta3 and alphaVbeta5 were the first vascular integrins targeted to suppress tumor angiogenesis. Subsequent experiments revealed that at least four additional integrins (i.e., alpha1beta1, alpha2beta1, alpha5beta1, and alpha6beta4) might be potential therapeutic targets. In clinical studies low-molecular-weight integrin inhibitors and anti-integrin function-blocking antibodies demonstrated low toxicity and good tolerability and are now being tested in combination with radiotherapy and chemotherapy for anticancer activity in patients. In this article the authors review the role of integrins in angiogenesis, present recent development in the use of alphaVbeta3 and alpha5beta1 integrin antagonists as potential therapeutics in cancer, and discuss future perspectives.

Identification of integrin alpha3 as a new substrate of the adenovirus E4orf6/E1B 55-kilodalton E3 ubiquitin ligase complex

The human adenovirus E4orf6 and E1B55K proteins promote viral replication by targeting several cellular proteins for degradation. The E4orf6 product has been shown by our group and others to form an E3 ubiquitin ligase complex that contains elongins B and C and cullin family member Cul5. E1B55K associates with this complex, where it is believed to function primarily to introduce bound substrates for degradation via proteasomes. In addition to p53, its first known substrate, the E4orf6/E1B 55-kDa complex (E4orf6/E1B55K) was shown to promote the degradation of Mre11 and DNA ligase IV; however, additional substrates are believed to exist. This notion is strengthened by the fact that none of these substrates seems likely to be associated with additional functions shown to be mediated by the E4orf6-associated E3 ubiquitin ligase complex, including export of late viral mRNAs and blockage of export of the bulk cellular mRNAs from the nucleus. In an attempt to identify new E4orf6/E1B55K substrates, we undertook a proteomic screen using human p53-null, non-small-cell lung carcinoma H1299 cells expressing either E4orf6 protein alone or in combination with E1B55K through infection by appropriate adenovirus vectors. One cellular protein that appeared to be degraded by E1B55K in combination with the E4orf6 protein was a species of molecular mass approximately 130 kDa that was identified as the integrin alpha3 subunit (i.e., very late activation antigen 3 alpha subunit). Preliminary analyses suggested that degradation of alpha3 may play a role in promoting release and spread of progeny virions.

Circulating endothelial cells: a novel marker of endothelial damage

Circulating endothelial cells (CECs) were first described over 30 years ago in smears of peripheral blood. Since then, more sophisticated techniques for CEC isolation have become available. In particular, immunomagnetic isolation and fluorescence-activated cell sorting (FACS) have been employed with success. We provide a short historical perspective and a comprehensive review on the subject. We review isolation and enumeration of CECs with an emphasis on CD146-driven immunomagnetic isolation and FACS. We describe, in great detail, advantages and pitfalls of both approaches and compare their specificity. Moreover, we provide a comprehensive list of clinical studies in this field and describe the possible clinical use of CECs. We also describe the phenotype of these cells and list typical surface markers. In addition, we review the phenotype of CECs and discuss mechanisms of detachment. We speculate about potential interactions between CECs and other cell subsets. We also describe other serum markers of endothelial damage and compare CECs with these markers. Finally, we highlight differences between circulating endothelial cells and endothelial progenitor cells. In summary, CECs must now be regarded as a sensitive and specific marker of endothelial damage. We emphasize that use of CECs in a clinical setting is on the horizon and pathogenetic clues may also be obtained.

beta1-integrin-mediated signaling essentially contributes to cell survival after radiation-induced genotoxic injury

Integrin-mediated adhesion to extracellular matrix proteins confers resistance to radiation- or drug-induced genotoxic injury. To analyse the underlying mechanisms specific for beta1-integrins, wild-type beta1A-integrin-expressing GD25beta1A cells were compared to GD25beta1B cells, which express signaling-incompetent beta1B variants. Cells grown on fibronectin, collagen-III, beta1-integrin-IgG or poly-l-lysine were exposed to 0-6 Gy X-rays in presence or depletion of growth factors and phosphatidylinositol-3 kinase (PI3K) inhibitors (LY294002, wortmannin). In order to test the relevance of these findings in tumor cells, human A-172 glioma cells were examined under the same conditions after siRNA-mediated silencing of beta1-integrins. We found that beta1A-integrin-mediated adhesion to fibronectin, collagen-III or beta1-IgG was essential for cell survival after radiation-induced genotoxic injury. Mediated by PI3K, pro-survival beta1A-integrin/Akt signaling was critically involved in this process. Additionally, the beta1-integrin downstream targets p130Cas and paxillin-impaired survival-regulating PI3K-dependent JNK. In A-172 glioma cells, beta1-integrin knockdown and PI3K inhibition confirmed the central role of beta1-integrins in Akt- and p130Cas/paxillin-mediated prosurvival signaling. These findings suggest beta1-integrins as critical regulators of cell survival after radiation-induced genotoxic injury. Elucidation of the molecular circuitry of prosurvival beta1-integrin-mediated signaling in tumor cells may promote the development of innovative molecular-targeted therapeutic antitumor strategies.

Antiangiogenic peptides and proteins: from experimental tools to clinical drugs

The formation of a 'tumor-associated vasculature', a process referred to as tumor angiogenesis, is a stromal reaction essential for tumor progression. Inhibition of tumor angiogenesis suppresses tumor growth in many experimental models, thereby indicating that tumor-associated vasculature may be a relevant target to inhibit tumor progression. Among the antiangiogenic molecules reported to date many are peptides and proteins. They include cytokines, chemokines, antibodies to vascular growth factors and growth factor receptors, soluble receptors, fragments derived from extracellular matrix proteins and small synthetic peptides. The polypeptide tumor necrosis factor (TNF, Beromun) was the first drug registered for the regional treatment of human cancer, whose mechanisms of action involved selective disruption of the tumor vasculature. More recently, bevacizumab (Avastin), an antibody against vascular endothelial growth factor (VEGF)-A, was approved as the first systemic antiangiogenic drug that had a significant impact on the survival of patients with advanced colorectal cancer, in combination with chemotherapy. Several additional peptides and antibodies with antiangiogenic activity are currently tested in clinical trials for their therapeutic efficacy. Thus, peptides, polypeptides and antibodies are emerging as leading molecules among the plethora of compounds with antiangiogenic activity. In this article, we will review some of these molecules and discuss their mechanism of action and their potential therapeutic use as anticancer agents in humans.

Gingipains from Porphyromonas gingivalis W83 induce cell adhesion molecule cleavage and apoptosis in endothelial cells

The presence of Porphyromonas gingivalis in the periodontal pocket and the high levels of gingipain activity detected in gingival crevicular fluid could implicate a role for gingipains in the destruction of the highly vascular periodontal tissue. To explore the effects of these proteases on endothelial cells, we exposed bovine coronary artery endothelial cells and human microvascular endothelial cells to gingipain-active extracellular protein preparations and/or purified gingipains from P. gingivalis. Treated cells exhibited a rapid loss of cell adhesion properties that was followed by apoptotic cell death. Cleavage of N- and VE-cadherin and integrin beta1 was observed in immunoblots of cell lysates. There was a direct correlation between the kinetics of cleavage of N- and VE-cadherin and loss of cell adhesion properties. Loss of cell adhesion, as well as N- and VE-cadherin and integrin beta1 cleavage, could be inhibited or significantly delayed by preincubation of P. gingivalis W83 gingipain-active extracellular extracts with the cysteine protease inhibitor Nalpha-p-tosyl-l-lysine chloromethylketone. Furthermore, purified gingipains also induced endothelial cell detachment and apoptosis. Apoptosis-associated events, including annexin V positivity, caspase-3 activation, and cleavage of the caspase substrates poly(ADP-ribose) polymerase and topoisomerase I (Topo I), were observed in endothelial cells after detachment. All of the effects observed were correlated with the different levels of cysteine-dependent proteolytic activity of the extracts tested. Taken together, these results indicate that gingipains from P. gingivalis can alter cell adhesion molecules and induce endothelial cell death, which could have implications for the pathogenicity of this organism.

Integrin α4β1 function is required for cell survival in developing retina

In the retina, integrins in the beta1 family have been shown to be important in many phases of neuronal development, particularly neuroblast migration and axon outgrowth. However, the functions of specific integrin heterodimers are not well defined. In this study, we investigated the functions of beta1 integrins in developing chicken retina by expression of a dominant-negative beta1A construct using a replication-competent retrovirus. Inhibition of integrins using this approach resulted in alteration of cell morphology and increased apoptosis, but did not preclude migration and axon elongation. In an attempt to identify which specific beta1 heterodimer was important, expression and function of the alpha4beta1 heterodimer were also investigated. At early developmental stages, alpha4 protein and mRNA were detected in undifferentiated neuroblasts throughout the retina. At later stages, expression was confined to retinal ganglion cells (RGCs) and amacrine cells. A small molecule antagonist of alpha4 integrins was shown to inhibit neurite outgrowth on recombinant soluble vascular cell adhesion molecule-1 (VCAM-1), a known ligand of alpha4beta1. Introduction of alpha4 antagonist in vivo gave rise to increased apoptosis and led to a thinning of the retina and reduced numbers of retinal ganglion cells (RGCs). We conclude that the integrin alpha4beta1 is important for survival of developing retinal neurons, including RGCs.

Endothelial cell integrins and COX-2: mediators and therapeutic targets of tumor angiogenesis

Vascular integrins are essential regulators and mediators of physiological and pathological angiogenesis, including tumor angiogenesis. Integrins provide the physical interaction with the extracellular matrix (ECM) necessary for cell adhesion, migration and positioning, and induce signaling events essential for cell survival, proliferation and differentiation. Integrins preferentially expressed on neovascular endothelial cells, such as alphaVbeta3 and alpha5beta1, are considered as relevant targets for anti-angiogenic therapies. Anti-integrin antibodies and small molecular integrin inhibitors suppress angiogenesis and tumor progression in many animal models, and are currently tested in clinical trials as anti-angiogenic agents. Cyclooxygense-2 (COX-2), a key enzyme in the synthesis of prostaglandins and thromboxans, is highly up-regulated in tumor cells, stromal cells and angiogenic endothelial cells during tumor progression. Recent experiments have demonstrated that COX-2 promotes tumor angiogenesis. Chronic intake of nonsteroidal anti-inflammatory drugs and COX-2 inhibitors significantly reduces the risk of cancer development, and this effect may be due, at least in part, to the inhibition of tumor angiogenesis. Endothelial cell COX-2 promotes integrin alphaVbeta3-mediated endothelial cell adhesion, spreading, migration and angiogenesis through the prostaglandin-cAMP-PKA-dependent activation of the small GTPase Rac. In this article, we review the role of integrins and COX-2 in angiogenesis, their cross talk, and discuss implications relevant to their targeting to suppress tumor angiogenesis.

beta1-integrin-ligand disengagement induces in vitro capillary tube disruption mediated by p38 MAPK activity

BACKGROUND

Disrupting cell-matrix interactions may lead to capillary injury as seen in sepsis and transplant rejection. Previously, we demonstrated capillary disruption mediated by beta1-integrin-ligand disengagement. We now determine whether p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK) pathways are involved in this capillary injury.

METHODS

Endothelial capillaries on Matrigel were preincubated with a p38 MAPK inhibitor (SB203580), ERK pathway inhibitor (PD98059), or dimethyl sulfoxide. Subsequently, a beta1-integrin blocking (P5D2) or an irrelevant antibody was added. After 24 hours, capillary integrity was quantified as capillary intersections/well. Antibody-treated cell lysates then were immunoprecipitated with either a phospho-p38 MAPK or phospho-ERK1/2 antibody. Kinase activity was measured with ATF-2 and Elk-1 fusion proteins as substrates for p38 MAPK and ERK, respectively, followed by Western blotting.

RESULTS

P5D2 disrupted capillary tubes. Increased p38 MAPK activity at 8 hours and ERK activity at 2 and 8 hours were seen in P5D2-treated lysates. Preincubation with SB203580, but not with PD98059 or DSMO, significantly reduced capillary tube disruption.

CONCLUSIONS

The beta1-integrin-ligand disengagement resulted in capillary disruption and stimulated p38 MAPK and ERK activity. In spite of activation of both pathways, the p38 MAPK but not the ERK pathway inhibitor prevented beta1-integrin antibody effects. Inhibiting p38 MAPK may mitigate capillary injury associated with sepsis and transplant rejection.

Suppression of tumor angiogenesis through the inhibition of integrin function and signaling in endothelial cells: which side to target?

Tumor angiogenesis is an essential step in tumor progression and metastasis formation. Suppression of tumor angiogenesis results in the inhibition of tumor growth. Recent evidence indicates that vascular integrins, in particular alpha V beta 3, are important regulators of angiogenesis, including tumor angiogenesis. Integrin alpha V beta 3 antagonists, such as blocking antibodies or peptides, suppress tumor angiogenesis and tumor progression in many preclinical tumor models. The potential therapeutic efficacy of extracellular integrin antagonists in human cancer is currently being tested in clinical trials. Selective disruption of the tumor vasculature by high doses of tumor necrosis factor (TNF) and interferon gamma (IFN-gamma), and the antiangiogenic activity of nonsteroidal anti-inflammatory drugs are associated with the suppression of integrin alpha V beta 3 function and signaling in endothelial cells. Furthermore, expression of isolated integrin cytoplasmic domains disrupts integrin-dependent adhesion, resulting in endothelial cell detachment and apoptosis. These results confirm the critical role of vascular integrins in promoting endothelial cell survival and angiogenesis and suggest that intracellular targeting of integrin function and signaling may be an alternative strategy to extracellular integrin antagonists for the therapeutic inhibition of tumor angiogenesis.