NGF exhibits a pro-apoptotic activity for human vascular smooth muscle cells that is inhibited by TGFbeta1

Sortilin expression is essential for pro-nerve growth factor-induced apoptosis of rat vascular smooth muscle cells

Background

Sortilin, a member of the Vps10p-domain receptor family, has been demonstrated a key regulator in mediating cellular response to pro-neurotrophins. In the present study, we investigated the role of sortilin in the apoptotic pathway of vascular smooth muscle cells.

Methods and Principal Findings

Immunohistochemistry revealed that sortilin was barely detectable in human and rat normal young vessels, while its expression was increased in human fibroatheromatous plaques. Sortilin immunodetection was also marked in the neointima of the rat aorta fifteen days after ballooning.In vitro, rat aortic intimal cells expressed higher sortilin levels than normal media SMCs; sortilin was distributed in the cytoplasm and in correspondence of the cell membrane. After 48 h, pro-nerve growth factor (proNGF) induced the strong dose-dependent increase of intimal cell apoptosis and the accumulation of sortilin protein. ProNGF was a more potent apoptotic inducer than equimolar or even higher concentration of NGF, whereas brain derived neutrotrophic factor was ineffective. Targeted interfering RNA-mediated sortilin reduction counteracted proNGF-induced apoptosis without affecting p75^NTR expression. ProNGF-induced apoptosis was associated to NF-κB down-regulation and bax increase. Inhibition of NF-κB activity increased intimal cell apoptosis that did not further increase with the addition of proNGF.

Conclusions

Our results indicate that sortilin expression characterizes human atheromatous lesions and rat aortic post-injury neointima, and suggest that sortilin represents an important regulator of proNGF-induced SMC apoptosis and arterial remodeling.

Proprotein convertase furin enhances survival and migration of vascular smooth muscle cells via processing of pro-nerve growth factor

Maturation of nerve growth factor (NGF) in neuronal cells requires endoproteolytic processing of the precursor protein proNGF to β-NGF by the proprotein convertase furin. Pro- and β-NGF elicit opposite biological functions by differential neurotrophin-receptor binding, leading to apoptosis via sortilin or survival via neurotrophic tyrosine kinase receptor type-1 (TrkA), respectively. The present study was done to investigate the impact of furin-dependent proNGF processing on vascular smooth muscle cell (VSMC) function. We found that β-NGF mRNA and protein expression was upregulated in platelet-derived growth factor-BB/transforming growth factor-β1-stimulated, proliferating rat aortic VSMCs. Although β-NGF itself did not affect VSMC proliferation, it promoted VSMC motility in an autocrine fashion via TrkA/Akt-dependent integrin inside-out signalling. The β-NGF-induced migration of VSMCs required proNGF processing by furin, which was co-regulated with NGF. Furin-inhibition increased proNGF and reduced β-NGF secretion, leading to apoptosis rather than migration. In line with our in vitro demonstration, we found co- and upregulation of NGF, its convertase furin and its high-affinity receptor TrkA in the neointima of balloon-injured rodent arteries. These results indicate that furin determines the balance between proNGF and β-NGF in proliferating VSMCs, thus impacting on VSMC survival and migration and is also important in neointima formation.

Cardiovascular actions of neurotrophins

Neurotrophins were christened in consideration of their actions on the nervous system and, for a long time, they were the exclusive interest of neuroscientists. However, more recently, this family of proteins has been shown to possess essential cardiovascular functions. During cardiovascular development, neurotrophins and their receptors are essential factors in the formation of the heart and critical regulator of vascular development. Postnatally, neurotrophins control the survival of endothelial cells, vascular smooth muscle cells, and cardiomyocytes and regulate angiogenesis and vasculogenesis, by autocrine and paracrine mechanisms. Recent studies suggest the capacity of neurotrophins, via their tropomyosin-kinase receptors, to promote therapeutic neovascularization in animal models of hindlimb ischemia. Conversely, the neurotrophin low-affinity p75(NTR) receptor induces apoptosis of endothelial cells and vascular smooth muscle cells and impairs angiogenesis. Finally, nerve growth factor looks particularly promising in treating microvascular complications of diabetes or reducing cardiomyocyte apoptosis in the infarcted heart. These seminal discoveries have fuelled basic and translational research and thus opened a new field of investigation in cardiovascular medicine and therapeutics. Here, we review recent progress on the molecular signaling and roles played by neurotrophins in cardiovascular development, function, and pathology, and we discuss therapeutic potential of strategies based on neurotrophin manipulation.

Nerve growth factor as an angiogenic factor

Nerve growth factor (NGF), a neurotrophin that plays a crucial role in promoting neurotrophic and neurotropic effects in sympathetic neurons, has recently been identified as a novel angiogenic molecule, which exerts a variety of effects in the cardiovascular system and on endothelial cells. In fact, NGF may contribute to maintenance, survival, and function of endothelial cells by autocrine and/or paracrine mechanisms. This review summarizes the involvement of NGF in the regulation of angiogenesis in both normal and pathological conditions.

Transforming growth factor-beta activation of phosphatidylinositol 3-kinase is independent of Smad2 and Smad3 and regulates fibroblast responses via p21-activated kinase-2

Transforming growth factor-beta (TGF-beta) stimulates cellular proliferation and transformation to a myofibroblast phenotype in vivo and in a subset of fibroblast cell lines. As the Smad pathway is activated by TGF-beta in essentially all cell types, it is unlikely to be the sole mediator of cell type-specific outcomes to TGF-beta stimulation. In the current study, we determined that TGF-beta receptor signaling activates phosphatidylinositol 3-kinase (PI3K) in several fibroblast but not epithelial cultures independently of Smad2 and Smad3. PI3K activation occurs in the presence of dominant-negative dynamin and is required for p21-activated kinase-2 kinase activity and the increased proliferation and morphologic change induced by TGF-beta in vitro.

Neurotrophin and neurotrophin receptor protein expression in the human lung

Neurotrophins (NTs) promote survival and differentiation of central and peripheral neurons, and display several activities also in non-neuronal cells. Human lungs synthesize and release NTs, which are probably involved in the pathophysiology of pulmonary disturbances. In this article the expression and anatomic localization of nerve growth factor, brain-derived neurotrophic factor, and NT-3 and of corresponding high-affinity receptors TrkA, TrkB (full-length and truncated [TR-] isoforms), TrkC, and of the low-affinity p75 receptor, were assessed in surgical samples from adult human lung by reverse transcriptase-polymerase chain reaction, Western blot, and immunohistochemistry. NTs and their cognate receptor mRNA and protein transcripts were detected by reverse transcriptase-polymerase chain reaction and immunoblotting, respectively, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) mRNA and corresponding protein transcripts being the most expressed. High levels of TrkB-[TR-] mRNA and of its protein transcript were also demonstrated, whereas a low expression of p75 mRNA and of corresponding protein transcript were found. Microanatomic analysis of immunohistochemical study revealed that bronchial epithelial cells were immunoreactive for different NTs, with a higher intensity of BDNF immune staining compared with other NTs, but did not express NT receptor immunoreactivity. Alveolar cells were immunoreactive for TrkA and TrkC receptor protein, but did not display immunoreactivity for NTs or other receptors investigated. Gland cells expressed NT and high-affinity NT receptor immunoreactivity, but not p75 receptor immunoreactivity. NT and low-affinity receptor immunoreactivity was observed within neurons and satellite cells of parasympathetic ganglia as well as in nerve fiber-like structures supplying the bronchopulmonary tree. An obvious immunoreactivity for NTs and NT receptor protein was also observed in intrapulmonary branches of pulmonary artery. Pulmonary lymphocytes and macrophages express nerve growth factor and high-affinity NT receptor immunoreactivity. The role of NTs in non-neuronal tissue including lung has not been clarified yet. The widespread expression of NTs and their receptors in different components of the lung suggests that these factors may contribute to regulate cell function in human lung.

Cell-type-specific activation of PAK2 by transforming growth factor beta independent of Smad2 and Smad3

Transforming growth factor beta (TGF-beta) causes growth arrest in epithelial cells and proliferation and morphological transformation in fibroblasts. Despite the ability of TGF-beta to induce various cellular phenotypes, few discernible differences in TGF-beta signaling between cell types have been reported, with the only well-characterized pathway (the Smad cascade) seemingly under identical control. We determined that TGF-beta receptor signaling activates the STE20 homolog PAK2 in mammalian cells. PAK2 activation occurs in fibroblast but not epithelial cell cultures and is independent of Smad2 and/or Smad3. Furthermore, we show that TGF-beta-stimulated PAK2 activity is regulated by Rac1 and Cdc42 and dominant negative PAK2 or morpholino antisense oligonucleotides to PAK2 prevent the morphological alteration observed following TGF-beta addition. Thus, PAK2 represents a novel Smad-independent pathway that differentiates TGF-beta signaling in fibroblast (growth-stimulated) and epithelial cell (growth-inhibited) cultures.

BDNF heightens the sensitivity of motor neurons to excitotoxic insults through activation of TrkB

The survival promoting and neuroprotective actions of brain-derived neurotrophic factor (BDNF) are well known but under certain circumstances this growth factor can also exacerbate excitotoxic insults to neurons. Prior exploration of the receptor through which BDNF exerts this action on motor neurons deflects attention away from p75. Here we investigated the possibility that BDNF acts through the receptor tyrosine kinase, TrkB, to confer on motor neurons sensitivity to excitotoxic challenge. We blocked BDNF activation of TrkB using a dominant negative TrkB mutant or a TrkB function blocking antibody, and found that this protected motor neurons against excitotoxic insult in cultures of mixed spinal cord neurons. Addition of a function blocking antibody to BDNF to mixed spinal cord neuron cultures is also neuroprotective indicating that endogenously produced BDNF participates in vulnerability to excitotoxicity. We next examined the intracellular signaling cascades that are engaged upon TrkB activation. Previously we found that inhibition of the phosphatidylinositide-3'-kinase (PI3'K) pathway blocks BDNF-induced excitotoxic sensitivity. Here we show that expression of a constitutively active catalytic subunit of PI3'K, p110, confers excitotoxic sensitivity (ES) upon motor neurons not incubated with BDNF. Parallel studies with purified motor neurons confirm that these events are likely to be occuring specifically within motor neurons. The abrogation of BDNF's capacity to accentuate excitotoxic insults may make it a more attractive neuroprotective agent.

Nerve growth factor promotes angiogenesis and arteriogenesis in ischemic hindlimbs

BACKGROUND

The neurotrophin nerve growth factor (NGF) regulates neuron survival and differentiation. Implication in neovascularization is supported by statement of NGF and its high-affinity receptor at vascular level and by NGF property of stimulating vascular endothelial cell proliferation. The present study investigated the involvement of endogenous NGF in spontaneous reparative response to ischemia. Mechanisms and therapeutic potential of NGF-induced neovascularization were examined.

METHODS AND RESULTS

Unilateral limb ischemia was produced in CD1 mice by femoral artery resection. By ELISA and immunohistochemistry, we documented that statement of NGF and its high-affinity receptor is upregulated in ischemic muscles. The functional relevance of this phenomenon was assessed by means of NGF-neutralizing antibody. Chronic NGF blockade abrogated the spontaneous capillarization response to ischemia and augmented myocyte apoptosis. Then we tested whether NGF administration may exert curative effects. Repeated NGF injection into ischemic adductors increased capillary and arteriole density, reduced endothelial cell and myofiber apoptosis, and accelerated perfusion recovery, without altering systemic hemodynamics. In normoperfused muscles, NFG-induced capillarization was blocked by vascular endothelial growth factor-neutralizing antibodies, dominant-negative Akt, or NO synthase inhibition.

CONCLUSIONS

These results indicate that NGF plays a functional role in reparative neovascularization. Furthermore, supplementation of the growth factor promotes angiogenesis through a vascular endothelial growth factor-Akt-NO-mediated mechanism. In the setting of ischemia, potentiation of NGF pathway stimulates angiogenesis and arteriogenesis, thereby accelerating hemodynamic recovery. NGF might be envisaged as a utilitarian target for the treatment of ischemic vascular disease.

Expression and function of Xenopus laevis p75(NTR) suggest evolution of developmental regulatory mechanisms

Neurotrophins signal through two different classes of receptors, members of the trk family of receptor tyrosine kinases, and p75 neurotrophin receptor (p75(NTR)), a member of the tumor necrosis factor receptor family. While neurotrophin binding to trks results in, among other things, increased cell survival, p75(NTR) has enigmatically been implicated in promoting both survival and cell death. Which of these two signals p75(NTR) imparts depends on the specific cellular context. Xenopus laevis is an excellent system in which to study p75(NTR) function in vivo because of its amenability to experimental manipulation. We therefore cloned partial cDNAs of two p75(NTR) genes from Xenopus, which we have termed p75(NTR)a and p75(NTR)b. We then cloned two different cDNAs, both of which encompass the full coding region of p75(NTR)a. Early in development both p75(NTR)a and p75(NTR)b are expressed in developing cranial ganglia and presumptive spinal sensory neurons, similar to what is observed in other species. Later, p75(NTR)a expression largely continues to parallel p75(NTR) expression in other species. However, Xenopus p75(NTR)a is additionally expressed in the neuroepithelium of the anterior telencephalon, all layers of the retina including the photoreceptor layer, and functioning axial skeletal muscle. Finally, misexpression of full length p75(NTR) and each of two truncated mutants in developing retina reveal that p75(NTR) probably signals for cell survival in this system. This result contrasts with the reported role of p75(NTR) in developing retinae of other species, and the possible implications of this difference are discussed.

Nerve growth factor levels and mast cell distribution in human coronary atherosclerosis

Nerve growth factor (NGF), in addition to its neurotrophic function, acts on a variety of non-neuronal cells including immune cells and vascular smooth muscle cells. The aim of the present study was to determine the NGF levels and the distribution of NGF and low-affinity NGF receptor (p75NGFR) and mast cells (MC) in human atherosclerotic coronary arteries. Specimens of human coronary arteries obtained from autopsy cases (n=12, subjects with atherosclerotic lesions; n=9, subjects without atherosclerotic lesions/controls) were used. The present study showed that in the atherosclerosis-lesioned arteries, the amount of NGF decreased, whereas the expression of p75NGFR immunoreactivity and the number, both of MC and vasa vasorum, particularly in the adventitia, significantly increased, compared with the control arteries. Cumulatively, our findings help to set the neurotrophic theory and its currently extended neuroimmune framework into the context of pathobiology of atherosclerosis, suggesting that altered presence of NGF, p75NGFR, and MC may play a role in neuroimmune mechanisms of human coronary atherosclerosis.

Role of apoptosis and transforming growth factor beta1 in fibroblast selection and activation in systemic sclerosis

OBJECTIVE

We hypothesized that pathophysiologic events during the development of systemic sclerosis (SSc) may lead to selection and propagation of certain apoptosis-resistant fibroblast subpopulations. The aim of this study was to examine a possible role for apoptosis in fibroblast selection in SSc and the role of transforming growth factor beta1 (TGFbeta1).

METHODS

We compared SSc and normal fibroblasts for their susceptibility to anti-Fas-induced apoptosis and analyzed 2 models that might lead to fibroblast resistance to apoptosis in this process: long-term exposure to either anti-Fas or TGFbeta1.

RESULTS

SSc-derived fibroblasts were resistant to anti-Fas-induced apoptosis, showing 5.5 +/- 17.2% (mean +/- SD) apoptosis, compared with 32.1 +/- 14.0% among normal fibroblasts (P < 0.05). Anti-Fas-selected normal fibroblasts showed 9.0 +/- 3.7% apoptosis, compared with 21.6 +/- 5.9% for sham-treated cells, which is consistent with the elimination of apoptosis-susceptible subpopulations. Normal fibroblasts subjected to 6 weeks of TGFbeta1 treatment showed not only resistance to apoptosis, but also proliferation (118.5 +/- 35.4%), after anti-Fas treatment, compared with sham-treated cells (35.1 +/- 11.1% apoptotic cell death). TGFbeta1 treatment also increased the proportion of myofibroblasts (47% versus 28% in controls). Cultured SSc fibroblasts had a greater proportion of myofibroblasts (32-83%) than did normal fibroblasts (4-25%). We also examined the relationship between collagen gene expression and the myofibroblast phenotype in normal and SSc skin sections. Only 2 of 7 normal sections had alpha-smooth muscle actin (a-SMA)-positive cells (mean +/- SD score 0.29 +/- 0.49 on a scale of 0-3), but all SSc sections were positive for alpha-SMA, with a mean score of 1.90 +/- 0.88 for lesional and 1.50 +/- 0.71 for nonlesional sections. Scores for alpha1(I) procollagen messenger RNA (mRNA) in lesional skin (mean +/- SD 3.30 +/- 0.82 on a scale of 1-4) were significantly higher than in normal (1.43 +/- 0.79) or nonlesional (1.40 +/- 0.52) skin, but scores varied, and there was no correlation between collagen mRNA and alpha-SMA levels.

CONCLUSION

Our results show that resistance to apoptosis is an important part of the SSc phenotype. TGFbeta1 may play a role by inducing apoptosis-resistant fibroblast populations, and also by inducing myofibroblasts and by enhancing extracellular matrix synthesis.

p75(NTR) mediates neurotrophin-induced apoptosis of vascular smooth muscle cells

The development of atherosclerotic lesions results from aberrant cell migration, proliferation, and extracellular matrix production. In advanced lesions, however, cellular apoptosis, leading to lesion remodeling, predominates. During lesion formation, the neurotrophins and the neurotrophin receptor tyrosine kinases, trks B and C, are induced and mediate smooth muscle cell migration. Here we demonstrate that a second neurotrophin receptor, p75(NTR), is expressed by established human atherosclerotic lesions and late lesions that develop after balloon injury of the rat thoracic aorta. The p75(NTR), a member of the tumor necrosis factor/FAS receptor family, can modulate trk receptor function as well as initiate cell death when expressed in cells of the nervous system that lack kinase-active trk receptors. p75(NTR) expression colocalizes to neointimal cells, which express smooth muscle cell alpha-actin and are expressed by cultured human endarterectomy-derived cells (HEDC). Areas of the plaque expressing p75(NTR) demonstrate increased TUNEL positivity, and HEDC undergo apoptosis in response to the neurotrophins. Finally, neurotrophins also induced apoptosis of a smooth muscle cell line genetically manipulated to express p75(NTR), but lacking trk receptor expression. These studies identify the regulated expression of neurotrophins and p75(NTR) as an inducer of smooth muscle cell apoptosis in atherosclerotic lesions.

The expression of transforming growth factor-beta1 (TGF-beta1) in hippocampal neurons: a temporary upregulated protein level after transient forebrain ischemia in the rat

Exogenous TGF-beta1 has been shown to protect neurons from damage induced in vitro and in vivo. In this study we attempted to examine the expression of endogenous TGF-beta1 mRNA and protein in the hippocampus of non-ischemic and ischemic rats, and to localize TGF-beta1 protein and DNA fragmentation by double-staining. Transient ischemia was induced for 10 min in Wistar rats by clamping both common carotid arteries and lowering blood pressure to 40 mmHg. Bioactive TGF-beta1 was selectively determined in CA1 pyramidal neurons of non-ischemic rats. It was upregulated after 3 h and 6 h of reperfusion corresponding to the increase in TGF-beta1 mRNA level detected by RT-PCR. Lectin and GFAP staining showed no detectable activated microglial cells and astrocytes in the hippocampus 3 h and 6 h after ischemia. When neuronal damage proceeded through day 2 to day 4 after ischemia as demonstrated by TUNEL-staining, TGF-beta1 immunoreactivity (ir) disappeared in damaged neurons but persisted in viable neurons although TGF-beta1 mRNA levels continuously increased. Double-staining revealed that TUNEL-positive neurons did not express TGF-beta1, while TUNEL-negative neurons in the CA1 subfield exhibited a distinct TGF-beta1 ir. These data indicate that hippocampal CA1 neurons can express TGF-beta1 under physiological conditions and upregulate its expression during the first hours after ischemia, that is independent of the activation of glial cells. The endogenous TGF-beta1 expressed in neurons may play a role in the pathological process of DNA degradation and delayed neuronal death after transient forebrain ischemia.

Damage to the enteric nervous system in experimental colitis

Inflammation of the intestine causes pain and altered motility, at least in part through effects on the enteric nervous system. While these changes may be reversed with healing, permanent damage may contribute to inflammatory bowel disease (IBD) and post-enteritis irritable bowel syndrome. Since little information exists, we induced colitis in male Sprague-Dawley rats with dinitrobenzene sulfonic acid and used immunocytochemistry to examine the number and distribution of enteric neurons at times up to 35 days later. Inflammation caused significant neuronal loss in the inflamed region by 24 hours, with only 49% of neurons remaining by days 4 to 6 and thereafter, when inflammation had subsided. Eosinophils were found within the myenteric plexus at only at the earliest time points, despite a general infiltration of neutrophils into the muscle wall. While the number of myenteric ganglia remained constant, there was significant decrease in the number of ganglia in the submucosal plexus. Despite reduced neuronal number and hyperplasia of smooth muscle, the density of axons among the smooth muscle cells remained unchanged during and after inflammation. Intracolonic application of the topical steroid budesonide caused a dose-dependent prevention of neuronal loss, suggesting that evaluation of anti-inflammatory therapy in inflammatory bowel disease should include quantitative assessment of neural components.

Transforming growth factor-beta inhibits apoptosis induced by beta-amyloid peptide fragment 25-35 in cultured neuronal cells

Previously, we demonstrated that transforming growth factor-beta (TGF-beta) pretreatment protects neuroblastoma cell lines, human hNT neurons, and primary rat embryo hippocampal neurons (REHIPs) from degeneration caused by incubation with beta-amyloid peptide (Abeta). Here we present evidence suggesting that TGF-beta interferes with an apoptotic pathway induced by Abeta. TGF-beta preteatment decreases the amount of DNA laddering seen following Abeta treatment in neuroblastoma cells, while in REHIPs, TGF-beta decreases the number of positive cells detected in situ by Klenow labelling following Abeta treatment. RT-PCR shows that in REHIPs, Abeta decreases mRNA expression of Bcl-2, as well as the ratio of Bcl-xL/Bcl-xS, with little effect on Bax expression. These changes are expected to promote apoptosis. When REHIPs are incubated with TGF-beta before addition of Abeta, the Bcl-xL/Bcl-xS ratio and Bcl-2 levels are increased compared to cells treated with Abeta alone. Again there is little effect on Bax expression. Western blotting and immunohistochemistry experiments also show that TGF-beta maintains increased levels of Bcl-2 and Bcl-xL protein in REHIPs even in the presence of Abeta. This pattern of gene expression should function to decrease apoptosis. Similarly, RT-PCR analysis of mRNA prepared from hNT cells shows that TGF-beta pretreatment before addition of Abeta maintains a higher level of Bcl-2 expression and an increased Bcl-xL/Bcl-xS ratio as compared to cells treated with Abeta alone. In neuronal cell types treated with Abeta, TGF-beta appears to regulate expression of genes in the Bcl-2 family to favor an anti-apoptotic pathway.