Selective requirement for Src kinases during VEGF-induced angiogenesis and vascular permeability

Parathyroid hormone-related peptide is a naturally occurring, protein kinase A-dependent angiogenesis inhibitor

Angiogenesis is a highly regulated process that results from the sequential actions of naturally occurring stimulators and inhibitors. Here, we show that parathyroid hormone-related peptide, a peptide hormone derived from normal and tumor cells that regulates bone metabolism and vascular tone, is a naturally occurring angiogenesis inhibitor. Parathyroid hormone-related peptide or a ten-amino-acid peptide from its N terminus inhibits endothelial cell migration in vitro and angiogenesis in vivo by activating endothelial cell protein kinase A. Activation of protein kinase A inhibits cell migration and angiogenesis by inhibiting the small GTPase Rac. In contrast, inhibition of protein kinase A reverses the anti-migratory and anti-angiogenic properties of parathyroid hormone-related peptide. These studies show that parathyroid hormone-related peptide is a naturally occurring angiogenesis inhibitor that functions by activation of protein kinase A.

Inhibition of Src family kinases blocks epidermal growth factor (EGF)-induced activation of Akt, phosphorylation of c-Cbl, and ubiquitination of the EGF receptor

Stimulation of T47D cells with epidermal growth factor (EGF) results in the activation of the intrinsic tyrosine kinases of the receptor and the phosphorylation of multiple cellular proteins including the receptor, scaffold molecules such as c-Cbl, adapter molecules such as Shc, and the serine/threonine protein kinase Akt. We demonstrate that EGF stimulation of T47D cells results in the activation of the Src protein-tyrosine kinase and that the Src kinase inhibitor PP1 blocks the EGF-induced phosphorylation of c-Cbl but not the activation/phosphorylation of the EGF receptor itself. PP1 also blocks EGF-induced ubiquitination of the EGF receptor, which is presumably mediated by phosphorylated c-Cbl. Src is associated with c-Cbl, and we have previously demonstrated that the Src-like kinase Fyn can phosphorylate c-Cbl at a preferred binding site for the p85 subunit of phosphatidylinositol 3'-kinase. PP1 treatment blocks EGF-induced activation of the anti-apoptotic protein kinase Akt suggesting that Src may regulate activation of Akt, perhaps by a Src --> c-Cbl --> phosphatidylinositol 3'-kinase --> Akt pathway.

Loss of placental growth factor protects mice against vascular permeability in pathological conditions

Vascular leakage contributes to numerous disorders but only a limited number of molecules have been demonstrated to modulate permeability of the vessel wall. The vascular endothelial growth factor (VEGF) is a potent inducer of vascular leakage. Previous studies demonstrated that exogenous administration of placental growth factor (PlGF), a homologue of VEGF, stimulates vascular permeability but the role of endogenous PlGF in plasma extravasation during pathological conditions remains unknown. We recently generated PlGF deficient (PlGF(-/-)) mice and demonstrated that loss of PlGF impaired pathological angiogenesis by attenuating the response to VEGF. Here, we demonstrate that absence of PlGF reduces vascular leakage induced by skin wounding, allergens, and neurogenic inflammation. These findings suggest that inhibition of PlGF might be an attractive tool to reduce vascular leakage in various diseases.

Tiny dancers: the integrin-growth factor nexus in angiogenic signaling

A vital step in growth factor-driven angiogenesis is the coordinated engagement of endothelial integrins with the extracellular matrix. The molecular mechanisms that partner growth factors and integrins are being elucidated, revealing an intricate interaction of surface receptors and their signaling pathways.

Src in cancer: deregulation and consequences for cell behaviour

Considerable evidence now implicates elevated expression and/or activity of Src in cancer development. In cells, endogenous Src is switched from an inactive to an active state by a variety of mechanisms that simultaneously relieve constraints on the kinase and protein-interacting Src homology (SH) domains. As a result, Src is translocated to the cell periphery, often to sites of cell adhesion, where myristylation mediates attachment to the inner surface of the plasma membrane. From these peripheral sites, Src's catalytic activity initiates intracellular signal transduction pathways that influence cell growth and adhesion strength, the latter contributing to control of cell migration. De-regulation in cancer cells may therefore enhance tumour growth and/or stimulate migratory or invasive potential in cells that would normally be relatively non-motile. Evidence now exists to suggest that Src may also influence the life or death decisions that cells make during many biological processes. Thus, Src modulation in cancer cells can alter cell responses that are often perturbed in cancer. Consequently, there is optimism that drugs which inhibit Src's kinase activity, or the activity of its downstream effectors, might have profound effects on cancer cell behaviour and be useful therapeutic agents.

Induction of angiogenesis by a fragment of human tyrosyl-tRNA synthetase

The first step of protein synthesis is catalyzed by aminoacyl-tRNA synthetases. In addition, certain mammalian tRNA synthetases link protein synthesis to cytokine signaling pathways. In particular, human tyrosyl-tRNA synthetase (TyrRS) can be split by proteolysis into two fragments having distinct cytokine activities. One of the TyrRS fragments (mini TyrRS) contains features identical to those in CXC chemokines (like interleukin-8) that also act as angiogenic factors. Here mini TyrRS (but not full-length TyrRS) is shown to stimulate chemotaxis of endothelial cells in vitro and stimulate angiogenesis in each of two in vivo animal models. The angiogenic activity of mini TyrRS can be opposed by anti-angiogenic chemokines like IP-10. Thus, a biological fragment of human tyrosyl-tRNA synthetase links protein synthesis to regulation of angiogenesis.

The role of vascular endothelial growth factor in angiogenesis

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen and an angiogenic inducer as well as a mediator of vascular permeability. The biological effects of VEGF are mediated by two tyrosine kinase receptors, Flt-1 (VEGFr-1) and KDR (VEGFR-2). VEGF is essential for developmental angiogenesis and is also required for female reproductive functions and endochondral bone formation. Substantial evidence also implicates VEGF in tumors and intraocular neovascular syndromes. Currently, several clinical trials are ongoing to test the hypothesis that inhibition of VEGF activity may be beneficial for these conditions.

Conditional switching of VEGF provides new insights into adult neovascularization and pro-angiogenic therapy

To gain insight into neovascularization of adult organs and to uncover inherent obstacles in vascular endothelial growth factor (VEGF)-based therapeutic angiogenesis, a transgenic system for conditional switching of VEGF expression was devised. The system allows for a reversible induction of VEGF specifically in the heart muscle or liver at any selected schedule, thereby circumventing embryonic lethality due to developmental misexpression of VEGF. Using this system, we demonstrate a progressive, unlimited ramification of the existing vasculature. In the absence of spatial cues, however, abnormal vascular trees were produced, a consequence of chaotic connections with the existing network and formation of irregularly shaped sac-like vessels. VEGF also caused a massive and highly disruptive edema. Importantly, premature cessation of the VEGF stimulus led to regression of most acquired vessels, thus challenging the utility of therapeutic approaches relying on short stimulus duration. A critical transition point was defined beyond which remodeled new vessels persisted for months after withdrawing VEGF, conferring a long-term improvement in organ perfusion. This novel genetic system thus highlights remaining problems in the implementation of pro-angiogenic therapy.

EphrinB phosphorylation and reverse signaling: regulation by Src kinases and PTP-BL phosphatase

Ephrins are cell surface-associated ligands for Eph receptors and are important regulators of morphogenic processes such as axon guidance and angiogenesis. Transmembrane ephrinB ligands act as "receptor-like" signaling molecules, in part mediated by tyrosine phosphorylation and by engagement with PDZ domain proteins. However, the underlying cell biology and signaling mechanisms are poorly understood. Here we show that Src family kinases (SFKs) are positive regulators of ephrinB phosphorylation and phosphotyrosine-mediated reverse signaling. EphB receptor engagement of ephrinB causes rapid recruitment of SFKs to ephrinB expression domains and transient SFK activation. With delayed kinetics, ephrinB ligands recruit the cytoplasmic PDZ domain containing protein tyrosine phosphatase PTP-BL and are dephosphorylated. Our data suggest the presence of a switch mechanism that allows a shift from phosphotyrosine/SFK-dependent signaling to PDZ-dependent signaling.

Src-mediated coupling of focal adhesion kinase to integrin alpha(v)beta5 in vascular endothelial growth factor signaling

Vascular endothelial growth factor (VEGF) promotes vascular permeability (VP) and neovascularization, and is required for development. We find that VEGF-stimulated Src activity in chick embryo blood vessels induces the coupling of focal adhesion kinase (FAK) to integrin alpha(v)beta5, a critical event in VEGF-mediated signaling and biological responsiveness. In contrast, FAK is constitutively associated with beta1 and beta3 integrins in the presence or absence of growth factors. In cultured endothelial cells, VEGF, but not basic fibroblast growth factor, promotes the Src-mediated phosphorylation of FAK on tyrosine 861, which contributes to the formation of a FAK/alpha(v)beta5 signaling complex. Moreover, formation of this FAK/alpha(v)beta5 complex is significantly reduced in pp60c-src-deficient mice. Supporting these results, mice deficient in either pp60c-src or integrin beta5, but not integrin beta3, have a reduced VP response to VEGF. This FAK/alpha(v)beta5 complex was also detected in epidermal growth factor-stimulated epithelial cells, suggesting a function for this complex outside the endothelium. Our findings indicate that Src can coordinate specific growth factor and extracellular matrix inputs by recruiting integrin alpha(v)beta5 into a FAK-containing signaling complex during growth factor-mediated biological responses.

Altered vascular permeability and early onset of experimental autoimmune encephalomyelitis in PECAM-1-deficient mice

Platelet/endothelial cell adhesion molecule-1 (PECAM-1, CD31), a 130-kDa glycoprotein member of the Ig superfamily of transmembrane proteins, is expressed on endothelial cells, platelets, and subsets of leukocytes. It functions as a cell adhesion molecule as well as a scaffolding molecule capable of modulating cellular signaling pathways. In this study, using PECAM-1-deficient (KO) mice, as well as cells derived from these mice, we demonstrate that the absence of PECAM-1 expression is associated with an early onset of clinical symptoms during experimental autoimmune encephalomyelitis (EAE), a mouse model for the human autoimmune disease multiple sclerosis. During EAE, mononuclear cell extravasation and infiltration of the CNS occur at earlier time points in PECAM-KO mice than in wild-type mice. In vitro, T lymphocyte transendothelial migration across PECAM-KO endothelial cells is enhanced, regardless of expression of PECAM-1 on transmigrating T cells. Additionally, cultured PECAM-KO endothelial cells exhibit prolonged permeability changes in response to histamine treatment compared with PECAM-1-reconstituted endothelial cells. Lastly, we demonstrate an exaggerated and prolonged CNS vascular permeability during the development of EAE and a delay in restoration of dermal vascular integrity following histamine challenge in PECAM-KO mice.

'Srcasm: a novel Src activating and signaling molecule

The Src family tyrosine kinase, Fyn, can facilitate regulation of cell proliferation and differentiation. Mice with mutations in the fyn gene have defects in the brain, immune system, and epidermal differentiation. To identify molecules that may interact with Fyn in the epidermis, we performed a yeast two-hybrid interaction screen of a murine keratinocyte library. A novel adaptor-like molecule was isolated and termed Srcasm for Src activating and signaling molecule. Murine Srcasm is a 52.7-kDa protein that contains a VHS membrane association domain and a number of tyrosine motifs suggesting that it may be a substrate for Src family kinases and serve as an adaptor protein. Northern blot analysis of murine tissues demonstrates that Srcasm expression is highest in brain and kidney. In situ hybridization analysis reveals that srcasm mRNA is expressed in regions of the epidermis and hair follicle where keratinocyte differentiation occurs. In the brain, srcasm mRNA distribution correlates with that of fyn, with both being highly expressed in the hippocampal and cerebellar Purkinje neurons. Fyn can phosphorylate Srcasm, and association of these molecules relies on cooperative binding between the SH2 and SH3 domains of Fyn and corresponding canonical binding sites in Srcasm. Srcasm is capable of interacting with Grb2 and the regulatory subunit of phosphoinositide 3-kinase, p85, in a phosphorylation-dependent manner. The evidence suggests that Srcasm may help promote Src family kinase signaling in cells.

Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer

The phosphoinositide 3-kinase (PI3K) family of enzymes is recruited upon growth factor receptor activation and produces 3' phosphoinositide lipids. The lipid products of PI3K act as second messengers by binding to and activating diverse cellular target proteins. These events constitute the start of a complex signaling cascade, which ultimately results in the mediation of cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. Therefore, PI3Ks play a central role in many cellular functions. The factors that determine which cellular function is mediated are complex and may be partly attributed to the diversity that exists at each level of the PI3K signaling cascade, such as the type of stimulus, the isoform of PI3K, or the nature of the second messenger lipids. Numerous studies have helped to elucidate some of the key factors that determine cell fate in the context of PI3K signaling. For example, the past two years has seen the publication of many transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated. These models have helped to build a picture of the role of PI3K in physiology and indeed there have been a number of surprises. This review uses such models as a framework to build a profile of PI3K function within both the cell and the organism and focuses, in particular, on the role of PI3K in cell regulation, immunity, and development. The evidence for the role of deregulated PI3K signaling in diseases such as cancer and diabetes is reviewed.

A human aminoacyl-tRNA synthetase as a regulator of angiogenesis

Aminoacyl-tRNA synthetases catalyze the first step of protein synthesis. It was shown recently that human tyrosyl-tRNA synthetase (TyrRS) can be split into two fragments having distinct cytokine activities, thereby linking protein synthesis to cytokine signaling pathways. Tryptophanyl-tRNA synthetase (TrpRS) is a close homologue of TyrRS. A natural fragment, herein designated as mini TrpRS, was shown by others to be produced by alternative splicing. Production of this fragment is reported to be stimulated by IFN-gamma, a cytokine that also stimulates production of angiostatic factors. Mini TrpRS is shown here to be angiostatic in a mammalian cell culture system, the chicken embryo, and two independent angiogenesis assays in the mouse. The full-length enzyme is inactive in the same assays. Thus, protein synthesis may be linked to the regulation of angiogenesis by a natural fragment of TrpRS.

A fragment of human TrpRS as a potent antagonist of ocular angiogenesis

Pathological angiogenesis contributes directly to profound loss of vision associated with many diseases of the eye. Recent work suggests that human tyrosyl- and tryptophanyl-tRNA synthetases (TrpRS) link protein synthesis to signal transduction pathways including angiogenesis. In this study, we show that a recombinant form of a COOH-terminal fragment of TrpRS is a potent antagonist of vascular endothelial growth factor-induced angiogenesis in a mouse model and of naturally occurring retinal angiogenesis in the neonatal mouse. The angiostatic activity is dose-dependent in both systems. The recombinant fragment is similar in size to one generated naturally by alternative splicing and can be produced by proteolysis of the full-length protein. In contrast, the full-length protein is inactive as an antagonist of angiogenesis. These results suggest that fragments of TrpRS, as naturally occurring and potentially nonimmunogenic anti-angiogenics, can be used for the treatment of neovascular eye diseases.

The molecular control of angiogenesis

Angiogenesis is a key event in a broad range of pathological conditions including both diseases with an enhanced and insufficient angiogenesis. Angiogenesis is often initiated with vasodilation accompanied by an increase in vascular permeability. After destabilization of the vessel wall and degradation of the surrounding extracellular matrix, extravasation of plasma proteins provides a provisional scaffold for the migration of endothelial cells. Endothelial cell proliferation and migration themselves are under tight control by a balance of angiogenesis inducers and inhibitors. A large number of angiogenic factors work together in a highly coordinated manner to induce endothelial cell outgrowth and the formation of functional vessels. On the other hand, angiostatic factors may play a critical role in the pathogenesis of ischemic diseases and contribute to the termination of physiological angiogenesis. Angiogenesis ends with the recruitment of pericytes and smooth muscle cells, which stabilize the newly formed vessel. The rapid increase in the knowledge about the molecular mechanisms of angiogenesis has led to first treatment trials in diseases with both enhanced and reduced angiogenesis. Although initial results are promising, much more work has to be done to consider anti-angiogenic or pro-angiogenic approaches as reliable therapeutic tools.

Src mediates stimulation by vascular endothelial growth factor of the phosphorylation of focal adhesion kinase at tyrosine 861, and migration and anti-apoptosis in endothelial cells

Vascular endothelial growth factor (VEGF) stimulates the tyrosine phosphorylation of focal adhesion kinase (FAK), increases focal adhesion formation and is chemotactic for human umbilical-vein endothelial cells (HUVECs). In the present study we identified the major sites of VEGF-induced FAK tyrosine phosphorylation and investigated the mechanism mediating this pathway in the action of VEGF. VEGF increased the focal adhesion localization of FAK phosphorylated at Tyr-397 (Y397) and Y861 but stimulated a marked increase in phosphorylation at Y861 without significantly affecting the total level of phospho-Y397 FAK. Inhibition of Src with the specific inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) completely blocked VEGF-induced Y861 phosphorylation without decreasing the level of phospho-Y397 FAK. We also examined the role of Src in mediating endothelial functions of VEGF in which FAK has been implicated as having a role. PP2 markedly inhibited VEGF-induced chemotaxis and wound-healing cell migration. The Src inhibitor also decreased the anti-apoptotic effect of VEGF determined by surface staining of annexin V but did not increase FAK proteolysis or prevent the VEGF-dependent inhibition of FAK proteolysis. In contrast, the specific PtdIns 3-kinase inhibitor LY294002 induced apoptosis and markedly decreased p125(FAK) expression and increased FAK proteolysis but had little effect on Y861 phosphorylation. These findings identify Src-dependent FAK phosphorylation at Y861 as a novel VEGF-induced signalling pathway in endothelial cells and suggest that this pathway might be involved in the mechanisms mediating VEGF-induced endothelial cell migration and anti-apoptosis.

VEGF receptor signal transduction

The family of vascular endothelial growth factors (VEGFs) currently includes VEGF-A, -B, -C, -D, -E, and placenta growth factor (PlGF). Several of these factors, notably VEGF-A, exist as different isoforms, which appear to have unique biological functions. The VEGF family proteins bind in a distinct pattern to three structurally related receptor tyrosine kinases, denoted VEGF receptor-1, -2, and -3. Neuropilins, heparan-sulfated proteoglycans, cadherins, and integrin alphavbeta3 serve as coreceptors for certain but not all VEGF proteins. Moreover, the angiogenic response to VEGF varies between different organs and is dependent on the genetic background of the animal. Inactivation of the genes for VEGF-A and VEGF receptor-2 leads to embryonal death due to the lack of endothelial cells. Inactivation of the gene encoding VEGF receptor-1 leads to an increased number of endothelial cells, which obstruct the vessel lumen. Inactivation of VEGF receptor-3 leads to abnormally organized vessels and cardiac failure. Although VEGF receptor-3 normally is expressed only on lymphatic endothelial cells, it is up-regulated on vascular as well as nonvascular tumors and appears to be involved in the regulation of angiogenesis. A large body of data, such as those on gene inactivation, indicate that VEGF receptor-1 exerts a negative regulatory effect on VEGF receptor-2, at least during embryogenesis. Recent data imply a positive regulatory role for VEGF receptor-1 in pathological angiogenesis. The VEGF proteins are in general poor mitogens, but binding of VEGF-A to VEGF receptor-2 leads to survival, migration, and differentiation of endothelial cells and mediation of vascular permeability. This review outlines the current knowledge about the signal transduction properties of VEGF receptors, with focus on VEGF receptor-2.

Integrin and growth factor receptor crosstalk

Crosstalk between integrins and growth factor receptors are an important signaling mechanism to provide specificity during normal development and pathological processes in vascular biology. Evidence from several model systems demonstrates the physiological importance of the coordination of signals from growth factors and the extracellular matrix to support cell proliferation, migration, and invasion in vivo. Several examples of crosstalk between these two important classes of receptors indicate that integrin ligation is required for growth factor-induced biological processes. Furthermore, integrins can directly associate with growth factor receptors, thereby regulating the capacity of integrin/growth factor receptor complexes to propagate downstream signaling. Recent data suggest that antagonists of alpha(v) integrins can provide a therapeutic benefit in human cancer patients, whereas knockout mice lacking specific integrins can provide an interesting insight into the role of integrins during development. This review will focus on the biological importance of integrin and growth factor receptor crosstalk that occurs during cell growth, migration, and invasion as well as in endothelial cells during angiogenesis.

Src mediates stimulation by vascular endothelial growth factor of the phosphorylation of focal adhesion kinase at tyrosine 861, and migration and anti-apoptosis in endothelial cells

Vascular endothelial growth factor (VEGF) stimulates the tyrosine phosphorylation of focal adhesion kinase (FAK), increases focal adhesion formation and is chemotactic for human umbilical-vein endothelial cells (HUVECs). In the present study we identified the major sites of VEGF-induced FAK tyrosine phosphorylation and investigated the mechanism mediating this pathway in the action of VEGF. VEGF increased the focal adhesion localization of FAK phosphorylated at Tyr-397 (Y397) and Y861 but stimulated a marked increase in phosphorylation at Y861 without significantly affecting the total level of phospho-Y397 FAK. Inhibition of Src with the specific inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) completely blocked VEGF-induced Y861 phosphorylation without decreasing the level of phospho-Y397 FAK. We also examined the role of Src in mediating endothelial functions of VEGF in which FAK has been implicated as having a role. PP2 markedly inhibited VEGF-induced chemotaxis and wound-healing cell migration. The Src inhibitor also decreased the anti-apoptotic effect of VEGF determined by surface staining of annexin V but did not increase FAK proteolysis or prevent the VEGF-dependent inhibition of FAK proteolysis. In contrast, the specific PtdIns 3-kinase inhibitor LY294002 induced apoptosis and markedly decreased p125(FAK) expression and increased FAK proteolysis but had little effect on Y861 phosphorylation. These findings identify Src-dependent FAK phosphorylation at Y861 as a novel VEGF-induced signalling pathway in endothelial cells and suggest that this pathway might be involved in the mechanisms mediating VEGF-induced endothelial cell migration and anti-apoptosis.

Optimization of 4-phenylamino-3-quinolinecarbonitriles as potent inhibitors of Src kinase activity

Subsequent to the discovery of 4-[(2,4-dichlorophenyl)amino]-6,7-dimethoxy-3-quinolinecarbonitrile (1a) as an inhibitor of Src kinase activity (IC(50) = 30 nM), several additional analogues were prepared. Optimization of the C-4 anilino group of 1a led to 1c, which contains a 2,4-dichloro-5-methoxy-substituted aniline. Replacement of the methoxy group at C-7 of 1c with a 3-(morpholin-4-yl)propoxy group provided 2c, resulting in increased inhibition of both Src kinase activity and Src-mediated cell proliferation. Analogues of 2c with other trisubstituted anilines at C-4 were also potent Src inhibitors, and the propoxy group of 2c was preferred over ethoxy, butoxy, or pentoxy. Replacement of the morpholine group of 2c with a 4-methylpiperazine group provided 31a, which had an IC(50) of 1.2 nM in the Src enzymatic assay, an IC(50) of 100 nM for the inhibition of Src-dependent cell proliferation and was selective for Src over non-Src family kinases. Compound 31a, which had higher 1 and 4 h plasma levels than 2c, effectively inhibited tumor growth in xenograft models.

Adult tissue angiogenesis: evidence for negative regulation by estrogen in the uterus

Increased uterine vascular permeability and angiogenesis are two major events of embryo implantation and placentation during pregnancy. These latter processes require coordinated, uterine-specific interactions between progesterone (P4) and estrogen (E) signaling. Although roles of these steroids have long been suspected, definitive functions of E and/or P4 in uterine angiogenesis still remain elusive. We have therefore exploited the availability of reporter and mutant mice to explore the regulation of angiogenesis in response to steroid hormonal changes in vivo. We present here molecular, genetic, physiological, and pharmacological evidence that E and P4 have different effects in vivo: E promotes uterine vascular permeability but profoundly inhibits angiogenesis, whereas P4 stimulates angiogenesis with little effect on vascular permeability. These effects of E and P4 are mediated by differential spatiotemporal expression of proangiogenic factors in the uterus.

Adhesion events in angiogenesis

Recent work from several laboratories indicates that the coordination of endothelial cell adhesion events with growth factor receptor inputs regulates endothelial cell responses during angiogenesis. Analyses of the signaling pathways downstream of integrins, cadherins and growth-factor receptors are providing an insight into the molecular basis of known anti-angiogenic strategies, as well as into the design of novel therapies.

NSAIDs inhibit alpha V beta 3 integrin-mediated and Cdc42/Rac-dependent endothelial-cell spreading, migration and angiogenesis

Cyclooxygenase-2 (COX-2), a key enzyme in arachidonic acid metabolism, is overexpressed in many cancers. Inhibition of COX-2 by nonsteroidal anti-inflammatory drugs (NSAIDs) reduces the risk of cancer development in humans and suppresses tumor growth in animal models. The anti-cancer effect of NSAIDs seems to involve suppression of tumor angiogenesis, but the underlying mechanism is not completely understood. Integrin alpha V beta 3 is an adhesion receptor critically involved in mediating tumor angiogenesis. Here we show that inhibition of endothelial-cell COX-2 by NSAIDs suppresses alpha V beta 3-dependent activation of the small GTPases Cdc42 and Rac, resulting in inhibition of endothelial-cell spreading and migration in vitro and suppression of fibroblast growth factor-2-induced angiogenesis in vivo. These results establish a novel functional link between COX-2, integrin alpha V beta 3 and Cdc42-/Rac-dependent endothelial-cell migration. Moreover, they provide a rationale to the understanding of the anti-angiogenic activity of NSAIDs.

Soluble Jagged 1 represses the function of its transmembrane form to induce the formation of the Src-dependent chord-like phenotype

We have previously demonstrated that the expression of the soluble extracellular domain of the transmembrane ligand for Notch receptors, Jagged 1 (sJ1), in NIH 3T3 cells results in the formation of a matrix-dependent chord-like phenotype, the loss of contact inhibition of growth, and an inhibition of pro-alpha 1(I) collagen expression. In an effort to define the mechanism by which sJ1 induces this phenotype, we report that sJ1 transfectants display biochemical and cytoskeletal alterations consistent with the activation of Src. Indeed, cotransfection of sJ1 transfectants with a dominant-negative mutant of Src resulted in the loss of matrix-dependent chord formation and correlated with the restoration of type I collagen expression and contact inhibition of growth. We also report that the sJ1-mediated induction of Src activity and related phenotypes, including chord formation, may result from the inhibition of endogenous Jagged 1-mediated Notch signaling since it was not possible to detect an sJ1-dependent induction of CSL-dependent transcription in these cells. Interestingly, NIH 3T3 cells transfected with dominant-negative (but not constitutively active) mutants of either Notch 1 or Notch 2 displayed a similar Src-related phenotype as the sJ1 transfectants. These data suggest that the ability of sJ1 to mediate chord formation is Src-dependent and requires the repression of endogenous Jagged 1-mediated Notch signaling, which is tolerant to the destabilization of the actin cytoskeleton, a mediator of cell migration.

Inhibition of FGF-2-mediated chemotaxis of murine brain capillary endothelial cells by cyclic RGDfV peptide through blocking the redistribution of c-Src into focal adhesions

The alpha(v)beta(3) integrin is essential for fibroblast growth factor (FGF)-induced angiogenesis in vivo. However, the role of this integrin in FGF-2-mediated cellular responses by cultured endothelial cells is largely unknown. Cyclic RGDfV (cRGDfV) peptide is widely used to inhibit the binding of alpha(v)beta(3) integrin to vitronectin. To investigate the role of this integrin in FGF-2-mediated cellular responses, we used immortalized murine brain capillary endothelial cells, denoted IBE cells. Because IBE cells proliferate and migrate in response to FGF-2-treatment, when cultured on fibronectin-coated surface, we first examined the inhibitory activity of this peptide on the binding of alpha(v)beta(3) integrin to fibronectin as well as vitronectin. Solid phase binding assay revealed that cRGDfV peptide strongly inhibited the binding of purified alpha(v)beta(3) integrin to vitonectin- and fibronectin-coated plastic surfaces at a concentration of 50 microM. cRGDfV peptide at 50 microM inhibited spreading as well as adhesion of IBE cells on vitronectin-coated plastic surface but not on fibronectin. On fibronectin-coated substrata, cRGDfV at 50 microM attenuated FGF-2-mediated chemotaxis, but not FGF-2-induced proliferation, of IBE cells. We have previously demonstrated that mitogen-activated protein kinase (MAPK) activation within focal adhesions through c-Src activity was involved in FGF-2-induced chemotaxis of IBE cells. Treatment of cells with cRGDfV peptide was associated with reduced c-Src activity without tyrosine dephosphorylation. Immunofluorescent staining showed that cRGDfV inhibited redistribution of c-Src into focal adhesions. MAPK activation by FGF-2 within focal adhesions was also attenuated in the presence of cRGDfV peptide. Our results indicated that cRGDfV peptide inhibited redistribution of c-Src into focal adhesions, leading to impaired MAPK activation within focal adhesions and motility in FGF-2-treated endothelial cells.

The adapter protein, Grb10, is a positive regulator of vascular endothelial growth factor signaling

Vascular endothelial growth factor (VEGF) is an important regulator of vasculogenesis and angiogenesis. Activation of VEGF receptors leads to the recruitment of SH2 containing proteins which link the receptors to the activation of signaling pathways. Here we report that Grb10, an adapter protein of which the biological role remains unknown, is tyrosine phosphorylated in response to VEGF in endothelial cells (HUVEC) and in 293 cells expressing the VEGF receptor KDR. An intact SH2 domain is required for Grb10 tyrosine phosphorylation in response to VEGF, and this phosphorylation is mediated in part through the activation of Src. In HUVEC, VEGF increases Grb10 mRNA level. Expression of Grb10 in HUVEC or in KDR expressing 293 cells results in an increase in the amount and in the tyrosine phosphorylation of KDR. In 293 cells, this is correlated with the activation of signaling molecules, such as MAP kinase. By expressing mutants of Grb10, we found that the positive action of Grb10 is independent of its SH2 domain. Moreover, these Grb10 effects on KDR seem to be specific since Grb10 has no effect on the insulin receptor, and Grb2, another adapter protein, does not mimic the effect of Grb10 on KDR. In conclusion, we propose that VEGF up-regulates Grb10 level, which in turn increases KDR molecules, suggesting that Grb10 could be involved in a positive feedback loop in VEGF signaling.

Src inhibitors: genomics to therapeutics

Following the milestone discoveries that identified Src as the first known protein tyrosine kinase and as a prototype oncogene, as well as Src transgenic studies to validate it as a promising therapeutic target for osteoporosis, intense efforts are being made to create Src inhibitor drugs. Drug discovery strategies focused on both the non-catalytic and catalytic domains of Src have successfully resulted in promising Src inhibitor lead compounds with potential therapeutic applications for osteoporosis, cancer, and other diseases. Some noteworthy examples of Src inhibitors are described, and their chemical diversity, structure-based design, and biological activities in vitro and in vivo are illustrated. The potency, selectivity, and in vivo efficacy of key Src inhibitors are being investigated in molecular, cellular and animal models. Consequently, Src inhibitor drug development is imminent, and current studies are well-poised to achieve the ultimate milestone of a Src inhibitor therapeutic.

c-abl is required for the development of hyperoxia-induced retinopathy

The requirement for the nonreceptor tyrosine kinase c-abl in the pathogenesis of retinopathy of prematurity (ROP) was examined using the mouse model for ROP and c-abl-deficient mice. Hyperoxia-induced retinal neovascularization was observed in wild-type and heterozygous mice but animals that were homozygous null for c-abl did not develop a vasoproliferative retinopathy in response to hyperoxia. Two gene products, endothelin-1 (ET-1) and vascular endothelial growth factor (VEGF), have been implicated in the pathogenesis of ROP. The mRNA expression of ET-1 and VEGF was assessed in mice maintained in normoxia and in hyperoxia-exposed mice. ET-1 mRNA levels were unchanged in wild-type mice throughout the hyperoxia treatment, suggesting that ET-1 mRNA expression is not regulated by the increase in inspired oxygen. In wild-type mice maintained in room air, VEGF mRNA levels rose threefold from postnatal day 6 (P6) to P17. When wild-type mice were treated with the hyperoxia regimen, a fivefold decrease in VEGF mRNA expression was observed from P7 to P16. However, retinal VEGF expression in hyperoxia-treated homozygous null mice did not decrease and remained at control levels. These data suggest that c-abl is required for the hyperoxia-induced retinal neovascularization and hyperoxia-induced decrease in VEGF mRNA levels.

pp60(c-src) modulates microvascular endothelial phenotype and in vitro angiogenesis

The tyrosine kinase c-src associates with the platelet-derived growth factor (PDGF) receptor. Overexpression of wild-type c-src, a kinase-negative c-src mutant, and v-src in microvascular endothelial cells modulated the mitogenic effect of PDGF, suggesting that c-src kinase activity inhibits PDGF signals. Analyses of cell morphology in two-dimensional culture revealed changes in cell shape and size induced by the overexpression of c-src proteins. Investigations in three-dimensional culture unveiled a modulatory role of c-src during in vitro angiogenesis. Overexpression of c-src resulted in an increased diameter of tube-like structures, and the number of branching segments was decreased. Expression of the kinase-negative c-src mutant resulted in abortive tube formation consisting of disconnected multicellular fragments. These results indicate that the c-src tyrosine kinase exerts regulatory effects on endothelial proliferation, size, and cytoskeletal organization in two-dimensional culture and on the formation of a differentiated multicellular network in three-dimensional culture.

Role of vascular endothelial growth factor in regulation of physiological angiogenesis

Evidence accumulating over the last decade has established the fundamental role of vascular endothelial growth factor (VEGF) as a key regulator of normal and abnormal angiogenesis. The biological effects of VEGF are mediated by two tyrosine kinase receptors, Flt-1 (VEGFR-1) and KDR (VEGFR-2). The signaling and biological properties of these two receptors are strikingly different. VEGF is essential for early development of the vasculature to the extent that inactivation of even a single allele of the VEGF gene results in embryonic lethality. VEGF is also required for female reproductive functions and endochondral bone formation. Substantial evidence also implicates VEGF as an angiogenic mediator in tumors and intraocular neovascular syndromes, and numerous clinical trials are presently testing the hypothesis that inhibition of VEGF may have therapeutic value.

Regulation of c-myc expression by PDGF through Rho GTPases

Src family protein-tyrosine kinases have a central role in several biological functions, including cell adhesion and spreading, chemotaxis, cell cycle progression, differentiation and apoptosis. Surprisingly, these kinases also participate in mitogenic signalling by receptors that themselves exhibit an intrinsic protein-tyrosine kinase activity, including those for platelet-derived growth factor (PDGF), epidermal growth factor and colony-stimulating factor-1. Indeed, Src kinases are strictly required for the nuclear expression of the c-myc proto-oncogene and thus for DNA synthesis in response to PDGF. However, the nature of the signalling pathways by which Src kinases participate in the induction of c-myc expression by tyrosine kinase receptors is still unknown. Here we show that PDGF enhances c-myc expression and stimulates the c-myc promoter in a Src-dependent manner, and that neither Ras nor the mitogen-activated protein kinase pathway mediate these effects. In contrast, we present evidence that PDGF stimulates Vav2 through Src, thereby initiating the activation of a Rac-dependent pathway that controls the expression of the c-myc proto-oncogene.

Synergism between vascular endothelial growth factor and placental growth factor contributes to angiogenesis and plasma extravasation in pathological conditions

Vascular endothelial growth factor (VEGF) stimulates angiogenesis by activating VEGF receptor-2 (VEGFR-2). The role of its homolog, placental growth factor (PlGF), remains unknown. Both VEGF and PlGF bind to VEGF receptor-1 (VEGFR-1), but it is unknown whether VEGFR-1, which exists as a soluble or a membrane-bound type, is an inert decoy or a signaling receptor for PlGF during angiogenesis. Here, we report that embryonic angiogenesis in mice was not affected by deficiency of PlGF (Pgf-/-). VEGF-B, another ligand of VEGFR-1, did not rescue development in Pgf-/- mice. However, loss of PlGF impaired angiogenesis, plasma extravasation and collateral growth during ischemia, inflammation, wound healing and cancer. Transplantation of wild-type bone marrow rescued the impaired angiogenesis and collateral growth in Pgf-/- mice, indicating that PlGF might have contributed to vessel growth in the adult by mobilizing bone-marrow-derived cells. The synergism between PlGF and VEGF was specific, as PlGF deficiency impaired the response to VEGF, but not to bFGF or histamine. VEGFR-1 was activated by PlGF, given that anti-VEGFR-1 antibodies and a Src-kinase inhibitor blocked the endothelial response to PlGF or VEGF/PlGF. By upregulating PlGF and the signaling subtype of VEGFR-1, endothelial cells amplify their responsiveness to VEGF during the 'angiogenic switch' in many pathological disorders.

FGF and VEGF function in angiogenesis: signalling pathways, biological responses and therapeutic inhibition

Angiogenic growth factors such as fibroblast growth factors (FGFs) and vascular endothelial growth factors (VEGFs) are currently targets of intense efforts to inhibit deregulated blood vessel formation in diseases such as cancer. FGFs and VEGFs exert their effects via specific binding to cell surface-expressed receptors equipped with tyrosine kinase activity. Activation of the receptor kinase activity allows coupling to downstream signal transduction pathways that regulate proliferation, migration and differentiation of endothelial cells. Inhibitors of FGF and VEGF signalling are currently in clinical trials. In this article, the current knowledge of FGF- and VEGF-induced signal transduction that leads to specific biological responses will be summarized. Furthermore, the manner in which this knowledge is being exploited to regulate angiogenesis will be discussed.

alpha 2 integrin subunit cytoplasmic domain-dependent cellular migration requires p38 MAPK

The alpha(2) integrin subunit cytoplasmic domain uniquely supported epidermal growth factor (EGF)-stimulated migration on type I collagen. p38 MAP kinase- and phosphatidylinositol 3-kinase-specific inhibitors, but not a MEK-specific inhibitor, eliminated EGF-stimulated and unstimulated alpha(2)-cytoplasmic domain-dependent migration. Following adhesion to collagenous matrices, cells expressing the full-length alpha(2) integrin subunit, but not cells expressing a chimeric alpha(2) integrin subunit in which the alpha(2)-cytoplasmic domain was replaced by the cytoplasmic domain of the alpha(1)-subunit, exhibited sustained and robust phosphorylation of p38 MAP kinase. Expression of dominant negative p38 MAP kinase inhibited alpha(2)-cytoplasmic domain-dependent, EGF-stimulated migration as well as unstimulated migration on collagen. Expression of constitutively active Rac1(Val-12) augmented p38 MAP kinase activation and alpha(2)-cytoplasmic domain-dependent migration. It also rescued the ability of cells expressing the alpha(1)-cytoplasmic domain to activate p38 MAPK and to migrate. These results suggest that the alpha(2) integrin cytoplasmic domain uniquely stimulates the p38 MAP kinase pathway that is required for unstimulated and EGF-stimulated migration on type I collagen.

Potential therapeutic application of antisense oligonucleotides in the treatment of ocular diseases

Antisense oligonucleotides are a class of compounds being developed as therapeutic agents for many types of diseases. Although still relatively early in the clinical characterisation, the power of this technology lies in the ability to utilise genetic information and the known molecular mechanisms of disease to foster efficient and rational drug design. Consideration of novel approaches to treating ocular diseases is of interest because there are many ocular diseases with no satisfactory treatments. The recent availability of animal models of many ocular diseases provides the opportunity to use antisense oligonucleotides to understand the mechanisms of disease pathology and to potentially intervene therapeutically in ocular disease. There are already a number of examples where antisense oligonucleotides have been applied to the study of ocular physiology and disease and there is an antisense oligonucleotide approved for the treatment of cytomegalovirus (CMV) retinitis. We summarise current research in this area and highlight the properties of these compounds that are favourable for use as ocular therapeutics.

Src deficiency or blockade of Src activity in mice provides cerebral protection following stroke

Vascular endothelial growth factor (VEGF), an angiogenic factor produced in response to ischemic injury, promotes vascular permeability (VP). Evidence is provided that Src kinase regulates VEGF-mediated VP in the brain following stroke and that suppression of Src activity decreases VP thereby minimizing brain injury. Mice lacking pp60c-src are resistant to VEGF-induced VP and show decreased infarct volumes after stroke whereas mice deficient in pp59c-fyn, another Src family member, have normal VEGF-mediated VP and infarct size. Systemic application of a Src-inhibitor given up to six hours following stroke suppressed VP protecting wild-type mice from ischemia-induced brain damage without influencing VEGF expression. This was associated with reduced edema, improved cerebral perfusion and decreased infarct volume 24 hours after injury as measured by magnetic resonance imaging and histological analysis. Thus, Src represents a key intermediate and novel therapeutic target in the pathophysiology of cerebral ischemia where it appears to regulate neuronal damage by influencing VEGF-mediated VP.

Simvastatin promotes osteoblast differentiation and mineralization in MC3T3-E1 cells

The cholesterol-lowering drug, simvastatin, is a pro-drug of a potent 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor and inhibits cholesterol synthesis in humans and animals. In addition, the bone effects of statins including simvastatin are being studied. We assessed the effects of simvastatin on osteoblastic differentiation in nontransformed osteoblastic cells (MC3T3-E1) and rat bone marrow cells. Simvastatin enhanced alkaline phosphatase (ALP) activity and mineralization in a dose- and time-dependent fashion. This stimulatory effect of the statin was observed at relatively low doses (significant at 10(-8) M and maximal at 10(-7) M). Northern blot analysis showed that the statin (10(-7) M) increased in bone morphogenetic protein-2 as well as ALP mRNA concentrations in MC3T3-E1 cells. Simvastatin (10(-7) M) slightly increased in type I collagen mRNA abundance throughout the culture period, whereas it markedly inhibited the gene expression of collagenase-1 between days 14 and 22 of culture. These results indicate that simvastatin has anabolic effects on bone through the promotion of osteoblastic differentiation, suggesting that it could be used for the treatment of common metabolic bone diseases such as osteoporosis.

Genetic evidence for a role for Src family kinases in TNF family receptor signaling and cell survival

Mutant src(-/-) mice have osteopetrosis resulting from defective osteoclasts, the cells that resorb bone. However, signaling pathways involving Src family members in osteoclasts remain unclear. We demonstrate that expression of a truncated Src molecule, Src251, lacking the kinase domain, induces osteopetrosis in wild-type and src(+/-) mice and worsens osteopetrosis in src(-/-) mice by a novel mechanism, increased osteoclast apoptosis. Induction of apoptosis by Src251 requires a functional SH2, but not an SH3, domain and is associated with reduced AKT kinase activity. Expression of Src251 dramatically reduces osteoclast survival in response to RANKL/TRANCE/OPGL, providing evidence that Src family kinases are required in vivo for survival signaling pathways downstream from TNF family receptors.

Molecular regulation of lymphangiogenesis and targets for tissue oedema

New insight has recently been obtained into the molecular mechanisms regulating the function of lymphatic endothelial cells. Vascular endothelial growth factors-C and -D have been shown to stimulate lymphangiogenesis, and their receptor VEGFR-3 has been linked to human hereditary lymphoedema, although there is evidence that other genes are also involved. These data suggest that it may become possible to stimulate lymphatic growth and function and to treat tissue oedema involved in many diseases.

VEGF: an update on biological and therapeutic aspects

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen and an angiogenic inducer as well as a mediator of vascular permeability. VEGF is essential for developmental angiogenesis and is also required for female reproductive functions and endochondral bone formation. Substantial evidence also implicates VEGF in tumors and intraocular neovascular syndromes. Currently, several clinical trials are ongoing to test the hypothesis that the inhibition of VEGF activity may be beneficial for these conditions.

Src inhibitors: drugs for the treatment of osteoporosis, cancer or both?

Src was one of the first proto-oncogenes to be identified and is a prototype of non-receptor type tyrosine kinases. The role of Src in bone metabolism first became apparent in Src-deficient mice and has been confirmed using low-molecular-weight Src inhibitors in animal models of osteoporosis. At the cellular level, it is well established that Src plays an important role in proliferation, and adhesion and motility. In addition, recent data indicate an involvement of Src in cell survival and intracellular trafficking in various specialized cell types. These new findings suggest that Src inhibitors might have therapeutic value in the suppression of tumor growth, tumor angiogenesis and bone resorption.

Endothelial cell apoptosis in angiogenesis and vessel regression

The programmed form of cell death (apoptosis) is essential for normal development of multicellular organisms. In the past few years, compelling evidence accumulated that dysregulation of apoptosis can lead to embryonal death and is involved in the pathophysiology of various inflammatory and degenerative diseases. Specifically, the occurrence of endothelial cell apoptosis has deleterious effects on the development of the cardiovascular system leading to embryonal death. Moreover, endothelial cell apoptosis counteracts neovascularization in the adult organism. On the basis of these findings, one may consider the regulation of endothelial cell apoptosis as a potential therapeutic target. The induction of endothelial cell apoptosis may limit unwanted neovascularization of tumors. In contrast, the prevention of endothelial cell apoptosis may improve angiogenesis and vasculogenesis in patients with ischemia. The present work critically reviews the existing data that supports a role of endothelial cell apoptosis for vascular growth and remodeling and provides insights into the mechanisms and the potential therapeutic consequences.

Src-mediated activation of alpha-diacylglycerol kinase is required for hepatocyte growth factor-induced cell motility

Diacylglycerol kinases are involved in cell signaling, either as regulators of diacylglycerol levels or as intracellular signal-generating enzymes. However, neither their role in signal transduction nor their biochemical regulation has been elucidated. Hepatocyte growth factor (HGF), upon binding to its tyrosine kinase receptor, activates multiple signaling pathways stimulating cell motility, scattering, proliferation and branching morphogenesis. Herein we demonstrate that: (i) the enzymatic activity of alpha-diacylglycerol kinase (alphaDgk) is stimulated by HGF in epithelial, endothelial and alphaDgk-transfected COS cells; (ii) cellular expression of an alphaDgk kinase-defective mutant inhibits activation of endogenous alphaDgk acting as dominant negative; (iii) specific inhibition of alphaDgk prevents HGF-induced cell movement of endothelial cells; (iv) HGF induces the association of alphaDgk in a complex with Src, whose tyrosine kinase activity is required for alphaDgk activation by HGF; (v) Src wild type stimulates alphaDgk activity in vitro; and (vi) alphaDgk can be tyrosine phosphorylated in intact cells.

Both src-dependent and -independent mechanisms mediate phosphatidylinositol 3-kinase regulation of colony-stimulating factor 1-activated mitogen-activated protein kinases in myeloid progenitors

Colony-stimulating factor 1 (CSF-1) supports the proliferation, survival, and differentiation of bone marrow-derived cells of the monocytic lineage. In the myeloid progenitor 32D cell line expressing CSF-1 receptor (CSF-1R), CSF-1 activation of the extracellular signal-regulated kinase (ERK) pathway is both Ras and phosphatidylinositol 3-kinase (PI3-kinase) dependent. PI3-kinase inhibition did not influence events leading to Ras activation. Using the activity of the PI3-kinase effector, Akt, as readout, studies with dominant-negative and oncogenic Ras failed to place PI3-kinase downstream of Ras. Thus, PI3-kinase appears to act in parallel to Ras. PI3-kinase inhibitors enhanced CSF-1-stimulated A-Raf and c-Raf-1 activities, and dominant-negative A-Raf but not dominant-negative c-Raf-1 reduced CSF-1-provoked ERK activation, suggesting that A-Raf mediates a part of the stimulatory signal from Ras to MEK/ERK, acting in parallel to PI3-kinase. Unexpectedly, a CSF-1R lacking the PI3-kinase binding site (DeltaKI) remained capable of activating MEK/ERK in a PI3-kinase-dependent manner. To determine if Src family kinases (SFKs) are involved, we demonstrated that CSF-1 activated Fyn and Lyn in cells expressing wild-type (WT) or DeltaKI receptors. Moreover, CSF-1-induced Akt activity in cells expressing DeltaKI is SFK dependent since Akt activation was prevented by pharmacological or genetic inhibition of SFK activity. The docking protein Gab2 may link SFK to PI3-kinase. CSF-1 induced Gab2 tyrosyl phosphorylation and association with PI3-kinase in cells expressing WT or DeltaKI receptors. However, only in DeltaKI cells are these events prevented by PP1. Thus in myeloid progenitors, CSF-1 can activate the PI3-kinase/Akt pathway by at least two mechanisms, one involving direct receptor binding and one involving SFKs.

Mechanisms of angiogenesis and arteriogenesis

Endothelial and smooth muscle cells interact with each other to form new blood vessels. In this review, the cellular and molecular mechanisms underlying the formation of endothelium-lined channels (angiogenesis) and their maturation via recruitment of smooth muscle cells (arteriogenesis) during physiological and pathological conditions are summarized, alongside with possible therapeutic applications.