The semaphorins: versatile regulators of tumour progression and tumour angiogenesis

The semaphorins and their receptors, the neuropilins and the plexins, were originally characterized as constituents of the complex regulatory system responsible for the guidance of axons during the development of the central nervous system. However, a growing body of evidence indicates that various semaphorins can either promote or inhibit tumour progression through the promotion or inhibition of processes such as tumour angiogenesis, tumour metastasis and tumour cell survival. This Review focuses on the emerging role of the semaphorins in cancer.

The neuropilins: multifunctional semaphorin and VEGF receptors that modulate axon guidance and angiogenesis

The neuropilin-1 (np1) and neuropilin-2 (np2) receptors function as receptors for the axon guidance factors belonging to the class-3 semaphorin subfamily. In addition, both neuropilins are able to bind to certain heparin-binding splice forms of vascular endothelial growth factor (VEGF), indicating that both neuropilins have roles in the cardiovascular system as well. Gene targeting experiments indicate that np1 does indeed function as an important modulator of VEGF function during vasculogenesis and angiogenesis, but the role of np2 in the cardiovascular system has not been studied in detail as yet. This review focuses on the neuropilins, their interactions, and their biological roles in the nervous and cardiovascular systems.

Semaphorin-3F Is an Inhibitor of Tumor Angiogenesis

The neuropilin-1 (np1) and neuropilin-2 (np2) receptors form complexes with type-A plexins. These complexes serve as signaling receptors for specific class-3 semaphorins. Np1 and np2 function in addition as receptors for heparin-binding forms of vascular endothelial growth factor (VEGF), such as VEGF(165). Human umbilical vein endothelial cells (HUVEC) express tyrosine-kinase receptors for VEGF and basic fibroblast growth factor (bFGF), as well as np1, np2, and several type-A plexins. We have found that semaphorin-3F (s3f), a semaphorin which signals through the np2 receptor, was able to inhibit VEGF(165), as well as bFGF-induced proliferation of HUVECs. Furthermore, s3f inhibited VEGF as well as bFGF-induced phosphorylation of extracellular signal-regulated kinase-1/2. Our experiments indicate that bFGF does not bind to neuropilins, nor does s3f inhibit the binding of bFGF to FGF receptors. It is therefore possible that s3f inhibits the activity of bFGF by a mechanism that requires active s3f signal transduction rather than by inhibition of bFGF binding to FGF receptors. s3f also inhibited VEGF(165), as well as bFGF-induced in vivo angiogenesis as determined by the alginate micro-encapsulation and Matrigel plug assays. Overexpression of s3f in tumorigenic human HEK293 cells inhibited their tumor-forming ability but not their proliferation in cell culture. The tumors that did develop from s3f-expressing HEK293 cells developed at a much slower rate and had a significantly lower concentration of tumor-associated blood vessels, indicating that s3f is an inhibitor of tumor angiogenesis.

Semaphorin-3A and semaphorin-3F work together to repel endothelial cells and to inhibit their survival by induction of apoptosis

Semaphorin-3A (sema3A) is a neuropilin-1 (np1) agonist. It inhibits the binding of the 165-amino acid form of VEGF (VEGF(165)) to np1 and was reported to inhibit angiogenesis as a result. However, we find that sema3A concentrations that inhibit the mitogenic effects of VEGF(165) do not inhibit VEGF(165)-induced phosphorylation of VEGF receptor-2 (VEGFR-2). Furthermore, sema3A inhibits the biological effects of VEGF(121), a VEGF form that does not bind to neuropilins and basic fibroblast growth factor, a growth factor whose activity, unlike that of VEGF, is not inhibited by small interfering RNA directed against np1. Therefore, the mechanism by which sema3A inhibits VEGF(165) activity does not depend on competition with VEGF(165) for binding to np1. Sema3A induced rapid disappearance of focal contacts followed by collapse of the actin cytoskeleton in human umbilical vein-derived endothelial cells. HEK293 cells expressing sema3A repel human endothelial cells and at high concentrations induce their death by apoptosis. Furthermore, sema3A inhibited the formation of tubes from endothelial cells in an in vitro angiogenesis assay. Similar effects are induced by the neuropilin-2 (np2) agonist sema3F. These inhibitory effects are abrogated by small interfering RNAs directed against np1 or np2, respectively. The anti-proliferative effects of sema3A and sema3F are additive when the semaphorins are added as pure proteins. However, when sema3A and sema3F were co-expressed in HEK293 cells their pro-apoptotic and cell repellant activities appeared to be synergistic. These observations suggest that combinations of sema3A and sema3F may be able to inhibit tumor angiogenesis more effectively than single semaphorins.

Abnormal deposition of collagen around hepatocytes in Wilson's disease is associated with hepatocyte specific expression of lysyl oxidase and lysyl oxidase like protein-2

BACKGROUND/AIMS

Lysyl-oxidases catalyze the oxidation of lysine residues in collagen and elastin thereby promoting their polymerization. We have studied here the expression of four lysyl-oxidases in normal and diseased human liver.

METHODS

The expression of the different lysyl-oxidases in paraffin embedded liver sections was studied using in-situ hybridization and immunohistochemistry. The enzymatic activity of lysyl-oxidase like protein-2 (Loxl2 or LOR-1) using a previously described lysyl-oxidase assay.

RESULTS

We have found that the four lysyl-oxidases which we examined are not significantly expressed in the normal liver. By contrast, Wilson's disease and primary biliary cirrhosis (PBC) patients express lysyl-oxidase (Lox) and lysyl-oxidase like protein-2 (Loxl2 or LOR-1) in hepatocytes, and the expression is accompanied by collagen deposition around the hepatocytes. Lysyl-oxidases are also expressed in additional fibrotic liver diseases such as hepatitis B and C but in these diseases the expression is confined to the fibrotic lesions and collagen does not accumulate around hepatocytes. We have found that Loxl2 is able to oxidize lysine residues of collagen, and behaves in that respect similarly to Lox. The copper chelator D-penicillamine inhibits Loxl2 induced oxidation of collagen but the Lox inhibitor beta-aminopropionitrile did not inhibit the oxidation using a BAPN concentration at which Lox activity was completely inhibited. Loxl2 also catalyzed the oxidation of cell surface proteins on HepG2 hepatoblastoma cells and inhibited their proliferation.

CONCLUSIONS

Upregulation of Lox and Loxl2 in hepatocytes of Wilson's disease and PBC patients may contribute to liver damage by various mechanisms. The upregulation of Lox and Loxl2 in Wilson's disease could perhaps be utilized for diagnostic purposes since their expression is up-regulated in hepatocytes even before the onset of fibrosis.

The interaction of Neuropilin-1 and Neuropilin-2 with tyrosine-kinase receptors for VEGF

The Neuropilin-1 (NRP1) and Neuropilin-2 (NRP2) receptors were initially described as receptors for axon guidance factors belonging to the class-3 Semaphorin sub-family. Subsequently, it was found the Neuropilins also function as receptors for some forms of vascular endothelial growth factor (VEGF). VEGF165 binds to both NRP1 and to NRP2 but VEGF121, does not bind to either of these receptors. VEGF145 on the other hand, binds to NRP2 but not to NRP1. Additional VEGF family members such as the heparin binding form of placenta growth factor (PlGF-2) and VEGF-B bind to NRP1, and it was also shown that both PlGF-2 and VEGF-C bind to NRP2. The intracellular domains of the Neuropilins are short, and do not suffice for independent transduction of biological signals subsequent to Semaphorin or VEGF binding. It was shown that both Neuropilins can form complexes with receptors belonging to the Plexin family, and that such Plexin/Neuropilin complexes are able to transduce signals following the binding of class-3 Semaphorins to Neuropilins. The VEGF165 induced proliferation and migration of cells that express the VEGF tyrosine-kinase receptor VEGFR2 is enhanced in the presence of NRP1, suggesting that Neuropilins may also form complexes with VEGF tyrosine-kinase receptors such as VEGFR2. However, it is not yet clear whether VEGFR2 and NRPI form complexes and contrasting results have been reported with regard to this issue. In contrast, it was recently reported by two laboratories that Neuropilins can form complexes with the second tyrosine-kinase receptor of VEGF, VEGFR1. However, the biological function of these complexes is still unclear.

The neuropilins and their role in tumorigenesis and tumor progression

The neuropilins were originally described as receptors for the six axon guidance factors belonging to the class-3 semaphorins. They were subsequently found to function in addition as receptors for specific splice forms of angiogenic factors belonging to the VEGF family. The neuropilins are expressed in many types of cancer cells, in endothelial cells and in additional many types of normal diploid cell types. Recent findings indicate that the neuropilins and their associated plexin and tyrosine-kinase VEGF receptors play a regulatory role in developmental angiogenesis as well as in tumor angiogenesis. The neuropilin ligands belonging to the semaphorin family as well as the various VEGF's function as modulators of angiogenesis and tumor angiogenesis. Furthermore, since many types of cancer cells express neuropilins and neuropilin associated receptors, it is not surprising that various neuropilin ligands can modulate the behavior of cancer cells directly leading to the potentiation or inhibition of tumor progression.

Neuropilin-1-VEGFR-2 complexing requires the PDZ-binding domain of neuropilin-1

Vascular endothelial growth factor (VEGF) acts as a hierarchically high switch of the angiogenic cascade by interacting with its high affinity VEGF receptors and with neuropilin co-receptors. VEGF(165) binds to both Neuropilin-1 (NP-1) and VEGFR-2, and it is believed that ligand binding forms an extracellular bridge between both molecules. This leads to complex formation, thereby enhancing VEGFR-2 phosphorylation and subsequent signaling. We found that inhibition of VEGF receptor (VEGFR) phosphorylation reduced complex formation between NP-1 and VEGFR-2, suggesting a functional role of the cytoplasmic domain of VEGFR-2 for complex formation. Correspondingly, deleting the PDZ-binding domain of NP-1 decreased complex formation, indicating that extracellular VEGF(165) binding is not sufficient for VEGFR-2-NP-1 interaction. Synectin is an NP-1 PDZ-binding domain-interacting molecule. Experiments in Synectin-deficient endothelial cells revealed reduced VEGFR-2-NP-1 complex formation, suggesting a role for Synectin in VEGFR-2-NP-1 signaling. Taken together, the experiments have identified a novel mechanism of NP-1 interaction with VEGFR-2, which involves the cytoplasmic domain of NP-1.

Successful inhibition of tumor development by specific class-3 semaphorins is associated with expression of appropriate semaphorin receptors by tumor cells

The class-3 semaphorins (sema3s) include seven family members. Six of them bind to neuropilin-1 (np1) or neuropilin-2 (np2) receptors or to both, while the seventh, sema3E, binds to the plexin-D1 receptor. Sema3B and sema3F were previously characterized as tumor suppressors and as inhibitors of tumor angiogenesis. To determine if additional class-3 semaphorins such as sema3A, sema3D, sema3E and sema3G possess anti-angiogenic and anti-tumorigenic properties, we expressed the recombinant full length semaphorins in four different tumorigenic cell lines expressing different combinations of class-3 semaphorin receptors. We show for the first time that sema3A, sema3D, sema3E and sema3G can function as potent anti-tumorigenic agents. All the semaphorins we examined were also able to reduce the concentration of tumor associated blood vessels although the potencies of the anti-angiogenic effects varied depending on the tumor cell type. Surprisingly, there was little correlation between the ability to inhibit tumor angiogenesis and their anti-tumorigenic activity. None of the semaphorins inhibited the adhesion of the tumor cells to plastic or fibronectin nor did they modulate the proliferation of tumor cells cultured in cell culture dishes. However, various semaphorins were able to inhibit the formation of soft agar colonies from tumor cells expressing appropriate semaphorin receptors, although in this case too the inhibitory effect was not always correlated with the anti-tumorigenic effect. In contrast, the anti-tumorigenic effect of each of the semaphorins correlated very well with tumor cell expression of specific signal transducing receptors for particular semaphorins. This correlation was not broken even in cases in which the tumor cells expressed significant concentrations of endogenous semaphorins. Our results suggest that combinations of different class-3 semaphorins may be more effective than single semaphorins in cases in which tumor cells express more than one type of semaphorin receptors.

Semaphorin-3B is an angiogenesis inhibitor that is inactivated by furin-like pro-protein convertases

Semaphorin-3B (sema3B) and semaphorin-3F (sema3F) are secreted tumor suppressors of lung cancer. Sema3F functions as an antiangiogenic factor that repels endothelial cells and compromises their proliferation/survival. However, tumor cells expressing either endogenous or recombinant sema3B fail to repel endothelial cells efficiently. Sema3B found in the conditioned medium of such cells is almost completely cleaved by furin-like pro-protein convertases, generating inactive 61- and 22-kDa fragments. We have generated a sema3B variant that was point mutated at the cleavage site (sema3B-m), thereby conferring partial resistance to cleavage. Conditioned medium from HEK293 cells expressing sema3b-m and conditioned medium of HEK293 cells expressing sema3B contained similar concentrations of semaphorin but sema3B-m was cleaved much less than sema3B. In contrast to HEK293 cells expressing native sema3B, cells expressing sema3b-m strongly repel endothelial cells. Conditioned medium from sema3B-m-expressing cells rapidly caused disassembly of focal adhesions and a collapse of the actin cytoskeleton of endothelial cells, inhibited vascular endothelial growth factor-induced phosphorylation of extracellular signal-regulated kinase 1/2, induced apoptosis of endothelial cells, and inhibited the formation of tubes from endothelial cells in an in vitro angiogenesis assay more potently than conditioned medium from cells expressing sema3B. Furthermore, HEK293 cells expressing sema3B-m inhibited basic fibroblast growth factor-induced angiogenesis in vivo much more potently than cells expressing sema3B. Repulsion of human umbilical vascular endothelial cells by sema3B-m was mediated primarily by the neuropilin-1 (np1) receptor but sema3B-m was also able to transduce signals via neuropilin-2 (np2). These results suggest that up-regulation of furin-like pro-protein convertases in malignant cells may enable tumors to evade the antiangiogenic effects of sema3B.

Semaphorins in angiogenesis and tumor progression

The semaphorins were initially described as axon guidance factors, but have recently been implicated in a variety of physiological and developmental functions, including regulation of immune response, angiogenesis, and migration of neural crest cells. The semaphorin family contains more than 30 genes divided into seven subfamilies, all of which are characterized by the presence of a sema domain. The semaphorins transduce their signals by binding to one of the nine receptors belonging to the plexin family, or, in the case of the class 3 semaphorins, by binding to one of the two neuropilin receptors. Additional receptors, which form complexes with these primary semaphorin receptors, are also frequently involved in semaphorin signaling. Recent evidence suggests that some semaphorins can act as antiangiogenic and/or antitumorigenic agents whereas other semaphorins promote tumor progression and/or angiogenesis. Furthermore, loss of endogenous inhibitory semaphorin expression or function on one hand, and overexpression of protumorigenic semaphorins on the other hand, is associated with the progression of some tumor types.

Semaphorins in cancer

The semaphorins are the products of a large family of genes currently containing more than 30 members. These genes are divided into eight classes of which classes 1, 2 and 8 contain invertebrate and viral semaphorins, while classes 3-7 contain the vertebrate semaphorins. The semaphorins have been implicated in diverse developmental processes such as axon guidance during nervous system development and regulation of cell migration. Plexin receptors function as binding and signal transducing receptors for all semaphorins except for the class-3 semaphorins which bind to neuropilins which subsequently activate signaling through associated plexins. The class-3 semaphorins semaphorin-3B (s3b) and semaphorin-3F (s3f) function additionally as potent inhibitors of tumor development in small cell lung carcinoma. Recent evidence indicates that these semaphorins modulate the adhesive and migratory properties of responsive malignant cells. S3f as well as semaphorin-3A (s3a) were also found to function as inhibitors of angiogenesis, and it was shown that the anti-angiogenic properties of s3f contribute significantly to its anti-tumorigenic properties. In contrast with these inhibitory semaphorins, there is some evidence indicating that semaphorins such as semaphorin-3C (s3c), semaphorin-3E (s3e), semaphorin-4D (s4d), semaphorin-5C (s5c) semaphorin-6A (s6a) and semaphorin-6b (s6b) may contribute to tumorigenesis or to tumor progression. In this review we discuss the semaphorins, their receptors and their signal transduction mechanisms, and evidence linking semaphorins to the control of tumorigenesis and tumor progression.

The role of the semaphorins in cancer

The semaphorins were initially characterized as axon guidance factors, but have subsequently been implicated also in the regulation of immune responses, angiogenesis, organ formation, and a variety of additional physiological and developmental functions. The semaphorin family contains more then 20 genes divided into 7 subfamilies, all of which contain the signature sema domain. The semaphorins transduce signals by binding to receptors belonging to the neuropilin or plexin families. Additional receptors which form complexes with these primary semaphorin receptors are also frequently involved in semaphorin signaling. Recent evidence suggests that semaphorins also fulfill important roles in the etiology of multiple forms of cancer. Some semaphorins have been found to function as bona-fide tumor suppressors and to inhibit tumor progression by various mechanisms while other semaphorins function as inducers and promoters of tumor progression.

Pro-angiogenic cytokines and their role in tumor angiogenesis

The development of solid tumors depends upon an adequate supply of blood. This can be achieved by way of co-option of preexisting blood vessels and by the induction of angiogenesis. During the past 30 years, tumor angiogenesis had been found to play a crucial role in the progression of solid tumors. Tumor angiogenesis was found to be induced by a variety of pro-angiogenic cytokines of which the best characterized is vascular endothelial growth factor (VEGF). Indeed, the first FDA approved anti-angiogenic drug for the treatment of cancer is Avastin, a neutralizing antibody directed against VEGF. This review focuses on cytokines which have been reported to induce tumor angiogenesis.

Ambulatory blood pressure monitoring in children with a solitary kidney - a comparison between unilateral renal agenesis and uninephrectomy

BACKGROUND

In experimental animals, a reduction in renal mass leads to glomerulosclerosis of the residual parenchyma and eventual renal failure. In humans, however, data on the influence of nephrectomy or agenesis on future functional parameters and hypertension are as yet controversial.

OBJECTIVE

To evaluate blood-pressure patterns in children with a solitary kidney as a result of either unilateral renal agenesis (URA) or uninephrectomy (UNX) and correlate them to the increase in size of the remaining kidney.

METHODS

Twenty-eight children with a solitary kidney were divided into two groups: the URA group, with 18 subjects (13 male and five female, with a mean age of 9.6 +/- 3.9 years), and the UNX group, which contained 10 subjects (four male and six female, with a mean age of 14.0 +/- 2.7 years). The mean time since nephrectomy was 8.7 years. Each study group was assigned an age-, weight- and height-matched control group. In all subjects, ambulatory blood pressure monitoring was performed using the SpaceLabs model no. 90207. Daytime and night-time were defined as 0800-2200 h and 2200-0800 h, respectively. Urinalysis, serum creatinine and urea results were recorded for all the study group participants. The size of the remaining kidney was determined by ultrasound examination. The percentage increase in kidney size, as calculated from standard kidney-length-against-age nomograms was correlated to the percentage deviation from normal blood pressure values.

RESULTS

The mean 24 h, daytime and night-time systolic blood pressure (SBP) readings were significantly higher in children in the URA group than in those in the corresponding control group (111.4 +/- 7.5 mmHg versus 106.9 +/- 6.7 mmHg, P < 0.004; 115.2 +/- 3.1 mmHg versus 110.7 +/- 3.4 mmHg, P < 0.004; and 105.2 +/- 2.6 mmHg versus 101.2 +/- 1.8 mmHg, P < 0.002, respectively). Daytime heart rate was found to be significantly reduced (84.4 +/- 5.2 versus 87.8 +/- 8.2 beats/min, P < 0.04). In the UNX group, 24 h mean, systolic and diastolic blood pressure (DBP), as well as heart rate, did not differ from control values. Daytime systolic and diastolic blood pressures were higher than those of the controls (119.8 +/- 4.0 mmHg versus 115.5 +/- 3.8 mmHg, P < 0.02; and 72.7 +/- 4.0 mmHg versus 70.0 +/- 11.0 mmHg, P < 0.02, respectively). Nocturnal dipping was present in all groups and was of equal magnitude in the corresponding control groups. The mean 24 h diastolic blood pressure load was significantly greater in URA patients than in controls (19.6 versus 10.8%, respectively, P < 0.01). In the UNX group, the blood pressure loads were similar to those of controls. The percentage increase in length of the remaining kidney was found to correlate positively with the percentage delta increase in both systolic and diastolic blood pressure.

CONCLUSIONS

Compared with the age-, height- and weight-matched controls, the children with a solitary kidney caused by URA had an elevated mean 24 h SBP. In contrast, those with UNX had mean 24 h blood pressure values similar to those of their controls. A rise in SBP was, however, seen in both groups during the daytime hours. Thus, the presence of a solitary kidney, for whatever reason, may be pathogenetically linked to a raised blood pressure, and this linkage may be more pronounced in URA. The delta increase in size of the remaining kidney may serve as a prognostic indicator of blood pressure elevation.

Tumour growth inhibition and anti-metastatic activity of a mutated furin-resistant Semaphorin 3E isoform

Secreted Semaphorin 3E (Sema3E) promotes cancer cell invasiveness and metastatic spreading. The pro-metastatic activity of Sema3E is due to its proteolytic fragment p61, capable of transactivating the oncogenic tyrosine kinase ErbB2 that associates with the Sema3E receptor PlexinD1 in cancer cells. Here, we show that a mutated, uncleavable variant of Sema3E (Uncl-Sema3E) binds to PlexinD1 like p61-Sema3E, but does not promote the association of PlexinD1 with ErbB2 nor activates the ensuing signalling cascade leading to metastatic spreading. Furthermore, Uncl-Sema3E competes with endogenous p61-Sema3E produced by tumour cells, thereby hampering their metastatic ability. Uncl-Sema3E also acts independently as a potent anti-angiogenic factor. It activates a PlexinD1-mediated signalling cascade in endothelial cells that leads to the inhibition of adhesion to extracellular matrix, directional migration and cell survival. The putative therapeutic potential of Uncl-Sema3E was validated in multiple orthotopic or spontaneous tumour models in vivo, where either local or systemic delivery of Uncl-Sema3E-reduced angiogenesis, growth and metastasis, even in the case of tumours refractory to treatment with a soluble vascular endothelial growth factor trap. In summary, we conclude that Uncl-Sema3E is a novel inhibitor of tumour angiogenesis and growth that concomitantly hampers metastatic spreading.

Analysis of endogenous peptides bound by soluble MHC class I molecules: a novel approach for identifying tumor-specific antigens

The Human MHC Project aims at comprehensive cataloging of peptides presented within the context of different human leukocyte antigens (HLA) expressed by cells of various tissue origins, both in health and in disease. Of major interest are peptides presented on cancer cells, which include peptides derived from tumor antigens that are of interest for immunotherapy. Here, HLA-restricted tumor-specific antigens were identified by transfecting human breast, ovarian and prostate tumor cell lines with truncated genes of HLA-A2 and HLA-B7. Soluble HLA secreted by these cell lines were purified by affinity chromatography and analyzed by nano-capillary electrospray ionization-tandem mass spectrometry. Typically, a large peptide pool was recovered and sequenced including peptides derived from MAGE-B2 and mucin and other new tumor-derived antigens that may serve as potential candidates for immunotherapy.

Full-Length Semaphorin-3C Is an Inhibitor of Tumor Lymphangiogenesis and Metastasis

Semaphorins play important regulatory roles in diverse processes such as axon guidance, angiogenesis, and immune responses. We find that semaphorin-3C (sema3C) induces the collapse of the cytoskeleton of lymphatic endothelial cells (LEC) in a neuropilin-2-, plexin-D1-, and plexin-A1-dependent manner, while most other semaphorins, including antiangiogenic semaphorins such as sema3A do not. Sema3C is cleaved, like other class-3 semaphorins, by furin-like pro-protein convertases (FPPC). Cleaved sema3C (p65-Sema3C) was unable to induce the collapse of the cytoskeleton of LEC. FPPC are strongly upregulated in tumor cells. In order to examine the effects of full-length sema3C on tumor progression, we therefore generated an active point mutated furin cleavage-resistant sema3C (FR-sema3C). FR-sema3C inhibited potently proliferation of LEC and to a lesser extent proliferation of human umbilical vein-derived endothelial cells. FR-sema3C also inhibited VEGF-C-induced phosphorylation of VEGFR-3, ERK1/2, and AKT. Expression of recombinant FR-sema3C in metastatic, triple-negative LM2-4 breast cancer cells did not affect their migration or proliferation in vitro. However, tumors derived from FR-sema3C-expressing LM2-4 cells implanted in mammary fat pads developed at a slower rate, contained a lower concentration of blood vessels and lymph vessels, and metastasized much less effectively to lymph nodes. Interestingly, p65-Sema3C, but not FR-sema3C, rendered A549 lung cancer cells resistant to serum deprivation, suggesting that previously reported protumorigenic activities of sema3C may be due to p65-Sema3C produced by tumor cells. Our observations suggest that FR-sema3C may be further developed into a novel antitumorigenic drug.

Neuropilin‐1 and neuropilin‐2 enhance VEGF121stimulated signal transduction by the VEGFR‐2 receptor

The neuropilin-1 (np1) receptor binds the 165 amino-acid form of vascular endothelial growth factor165 (VEGF165) and functions as an enhancer that potentiates VEGF165 signaling via the VEGFR-2 tyrosine-kinase receptor. To study the mechanism by which neuropilins potentiate VEGF activity we produced a VEGF165 mutant (VEGF165KF) that binds to neuropilins but displays a much lower affinity toward VEGFR-1 and VEGFR-2. VEGF165KF failed to induce VEGFR-2 phosphorylation in cells lacking neuropilins. However, in the presence of np1, VEGF165KF bound weakly to VEGFR-2, induced VEGFR-2 phosphorylation, and activated ERK1/2. Interestingly, VEGF165KF did not promote formation of VEGFR-2/np1 complexes nor did high concentrations of VEGF165KF inhibit VEGF165 induced formation of such complexes, suggesting that VEGF165 does not stabilize VEGFR-2/np1 complexes by forming bridges spanning VEGFR-2 and np1. VEGF121 is a VEGF form that does not bind to neuropilins. Surprisingly, both np1 and neuropilin-2 (np2) enhanced VEGF121-induced phosphorylation of VEGFR-2 and VEGF121-induced proliferation of endothelial cells. The enhancement of VEGF121 activity by np1 was accompanied by a 10-fold increase in binding affinity to VEGFR-2 and was not associated with the formation of new VEGFR-2/np1 complexes. These observations suggest that neuropilins enhance the activity of VEGF forms that do not bind to neuropilins, indicate that np2 is a functional VEGF receptor, and imply that spontaneously formed VEGFR-2/np1 complexes suffice for efficient neuropilin mediated enhancement of VEGF activity.

Lysyl oxidase-like-2 promotes tumour angiogenesis and is a potential therapeutic target in angiogenic tumours

Lysyl oxidase-like 2 (LOXL2), a secreted enzyme that catalyzes the cross-linking of collagen, plays an essential role in developmental angiogenesis. We found that administration of the LOXL2-neutralizing antibody AB0023 inhibited bFGF-induced angiogenesis in Matrigel plug assays and suppressed recruitment of angiogenesis promoting bone marrow cells. Small hairpin RNA-mediated inhibition of LOXL2 expression or inhibition of LOXL2 using AB0023 reduced the migration and network-forming ability of endothelial cells, suggesting that the inhibition of angiogenesis results from a direct effect on endothelial cells. To examine the effects of AB0023 on tumour angiogenesis, AB0023 was administered to mice bearing tumours derived from SKOV-3 ovarian carcinoma or Lewis lung carcinoma (LLC) cells. AB0023 treatment significantly reduced the microvascular density in these tumours but did not inhibit tumour growth. However, treatment of mice bearing SKOV-3-derived tumours with AB0023 also promoted increased coverage of tumour vessels with pericytes and reduced tumour hypoxia, providing evidence that anti-LOXL2 therapy results in the normalization of tumour blood vessels. In agreement with these data, treatment of mice bearing LLC-derived tumours with AB0023 improved the perfusion of the tumour-associated vessels as determined by ultrasonography. Improved perfusion and normalization of tumour vessels after treatment with anti-angiogenic agents were previously found to improve the delivery of chemotherapeutic agents into tumours and to result in an enhancement of chemotherapeutic efficiency. Indeed, treatment with AB0023 significantly enhanced the anti-tumourigenic effects of taxol. Our results suggest that inhibition of LOXL2 may prove beneficial for the treatment of angiogenic tumours.

Effects of nonsense mutations on nuclear and cytoplasmic adenine phosphoribosyltransferase RNA

We have analyzed Chinese hamster ovary (CHO) cell mutants bearing nonsense codons in four of the five exons of the adenine phosphoribosyltransferase (aprt) gene and have found a pattern of mRNA reduction similar to that seen in systems studied previously: a decrease in steady-state mRNA levels of 5- to 10-fold for mutations in exons 1, 2, and 4 but little effect for mutations in the 3'-most exon (exon 5). Nuclear aprt mRNA levels showed a similar decrease. Nonsense-containing aprt mRNA decayed at the same rate as wild-type mRNA in these cell lines after inhibition of transcription with actinomycin D. Nonsense-containing aprt mRNA is associated with polysomes, ruling out a model in which stable residual mRNA escapes degradation by avoiding translation initiation. A tetracycline-responsive form of the aprt gene was used to compare the stability of nonsense-containing and wild-type aprt mRNAs without globally inhibiting transcription. In contrast to measurements made in the presence of actinomycin D, after inhibition of aprt transcription with tetracycline, a nonsense-mediated destabilization of aprt mRNA was indeed demonstrable. The increased rate of decay of cytoplasmic aprt mRNA seen here could account for the nonsense-mediated reduction in steady-state levels of aprt mRNA. However, the low levels of nonsense-bearing aprt mRNA in the nucleus suggest a sensibility of mRNA to translation or translatability before it exits that compartment. Quantitation of the steady-state levels of transcripts containing introns revealed no accumulation of partially spliced aprt RNA and hence no indication of nonsense-mediated aberrancies in splicing. Our results are consistent with a model in which translation facilitates the export of mRNA through a nuclear pore. However, the mechanism of this intriguing nucleocytoplasmic communication remains to be determined.

Semaphorin 3A is a marker for disease activity and a potential immunoregulator in systemic lupus erythematosus

Introduction

Semaphorin 3A (sema3A) and neuropilin-1 (NP-1) play a regulatory role in immune responses and have a demonstrated effect on the course of collagen induced arthritis. This study was undertaken to evaluate the role of sema3A and NP-1 in the pathogenesis of systemic lupus erythematosus (SLE) and the specific effect of sema3A on the auto-reactive properties of B cells in SLE patients.

Methods

Thirty two SLE and 24 rheumatoid arthritis (RA) patients were assessed and compared with 40 normal individuals. Sema3A serum levels were measured and correlated with SLE disease activity. The in vitro effect of sema3A in reducing Toll-like receptor 9 (TLR-9) expression in B cells of SLE patients was evaluated.

Results

Sema3A serum levels in SLE patients were found to be significantly lower than in RA patients (55.04 ± 16.30 ng/ml versus 65.54 ± 14.82 ng/ml, P = 0.018) and lower yet than in normal individuals (55.04 ± 16.30 ng/ml versus 74.41 ± 17.60 ng/ml, P < 0.0001). Altered serum sema3A levels were found to be in inverse correlation with SLE disease activity, mainly with renal damage. The expression of both sema3A and NP-1 on B cells from SLE patients was significantly different in comparison with normal healthy individuals. Finally, when sema3A was co-cultured with cytosine-phosphodiester-guanine oligodeoxynucleotides (CpG-ODN)-stimulated B cells of SLE patients, their TLR-9 expression was significantly reduced, by almost 50% (P = 0.001).

Conclusions

This is the first study in which a reduced serum level of sema3A was found in association with SLE disease activity. It also raises the possibility that sema3A may have a regulatory function in SLE.

Order of intron removal during splicing of endogenous adenine phosphoribosyltransferase and dihydrofolate reductase pre-mRNA

Using a strategy based on reverse transcription and the polymerase chain reaction, we have determined the order of splicing of the four introns of the endogenous adenine phosphoribosyltransferase (aprt) gene in Chinese hamster ovary cells. The method involves a pairwise comparison of molecules that retain one intron and have either retained or spliced another intron(s). A highly preferred order of removal was found: intron 3 > 2 > 4 = 1. This order did not represent a linear progression from one end of the transcript to the other, nor did it correlate with the conformity of the splice site sequences to the consensus sequences or to the calculated energy of duplex formation with U1 small nuclear RNA. By using actinomycin D to inhibit RNA synthesis, the in vivo rate of the first step in splicing was estimated for all four introns; a half-life of 6 min was found for introns 2, 3, and 4. Intron 1 was spliced more slowly, with a 12-min half-life. A substantial amount of RNA that retained intron 1 as the sole intron was exported to the cytoplasm. In the course of these experiments, we also determined that intron 3, but not intron 4, is spliced before 3'-end formation is complete, probably on nascent transcripts. This result is consistent with the idea that polyadenylation is required for splicing of the 3'-most intron. We applied a similar strategy to determine the last intron to be spliced in a very large transcript, that of the endogenous dihydrofolate reductase (dhfr) gene in Chinese hamster ovary cells (25 kb). Here again, intron 1 was the last intron to be spliced.

The enzymatic activity of lysyl oxidas-like-2 (LOXL2) is not required for LOXL2-induced inhibition of keratinocyte differentiation

Lysyl oxidase-like-2 (LOXL2) induces tumor progression and fibrosis. It also inhibits the differentiation of keratinocytes promoting development of squamous cell carcinomas. Stimulation of HaCaT skin keratinocytes with exogenous LOXL2 or overexpression of LOXL2 in these cells inhibits their differentiation as manifested by inhibition of calcium or vitamin D-induced involucrin expression. The inhibition was abrogated by the LOXL2 function-blocking monoclonal antibody AB0023 as well as by an anti-LOXL2 polyclonal antibody. Surprisingly, a point-mutated form of LOXL2 (LOXL2(Y689F)) lacking enzymatic activity, as well as a LOXL2 deletion mutant lacking the entire catalytic domain, also inhibited calcium or vitamin D-induced up-regulation of involucrin expression, suggesting that the enzymatic activity of LOXL2 is not required for this activity. This conclusion was supported by experiments that showed that β-aminoproprionitrile, an irreversible competitive inhibitor of the enzymatic activity of all lysyl oxidases, is unable to abolish the LOXL2-induced inhibition of HaCaT cell differentiation. The activity of LOXL2(Y689F) required the presence of the fourth scavenger receptor-cysteine-rich (SRCR) domain of LOXL2, which is also the binding target of AB0023. Epitope-tagged LOXL2(Y689F) was internalized at 37 °C by HaCaT cells. The internalization was inhibited by AB0023 and by competition with unlabeled LOXL2, suggesting that these cells may express a LOXL2 receptor. Our results suggest that agents that inhibit the enzymatic activity of LOXL2 may not suffice to inhibit completely the effects of LOXL2 on complex processes that involve altered states of cellular differentiation.

Localized LoxL3-Dependent Fibronectin Oxidation Regulates Myofiber Stretch and Integrin-Mediated Adhesion

For muscles to function, myofibers have to stretch and anchor at the myotendinous junction (MTJ), a region rich in extracellular matrix (ECM). Integrin signaling is required for MTJ formation, and mutations affecting the cascade lead to muscular dystrophies in mice and humans. Underlying mechanisms for integrin activation at the MTJ and ECM modifications regulating its signaling are unclear. We show that lysyl oxidase-like 3 (LoxL3) is a key regulator of integrin signaling that ensures localized control of the cascade. In LoxL3 mutants, myofibers anchor prematurely or overshoot to adjacent somites, and are loose and lack tension. We find that LoxL3 complexes with and directly oxidizes Fibronectin (FN), an ECM scaffold protein and integrin ligand enriched at the MTJ. We identify a mechanism whereby localized LoxL3 secretion from myofiber termini oxidizes FN, enabling enhanced integrin activation at the tips of myofibers and ensuring correct positioning and anchoring of myofibers along the MTJ.