Mitogenesis in Glioblastoma Multiforme Cell Lines: A Role for NGF and its TrkA Receptors

Neurotrophins have definitive roles in the growth/maintenance of neuronal populations, but their function in malignant gliomas is unknown. The ability for nerve growth factor (NGF) to serve as a mitogenic agent was investigated in several human glioblastoma multiforme (GBM) cell lines, including U251, U87, and U373. In a serum-free medium, the addition of NGF (200ng/ml) to these cell lines increased cell counts over controls, after 3 days in culture by 9%, 16%, and 33%, respectively. Dose-dependent increases in cell counts and [³H]thymidine uptake were found in the more rigorously investigated U373 cell line. Proteins for both the high affinity NGF-specific tyrosine kinase binding site (p140^TrkA; TrkA) and the low affinity neurotrophin (p75^NTR) receptor were present in all three GBM cell lines. TrkA mRNA was identified in U373 (only cell line studied). NGF-stimulated proliferation was inhibited in a dose-dependent fashion by K252a, a blocker of Trk-induced receptor kinases. NGF, measured by ELISA, was detectable in all GBM cell lines even after 7 days of growth in serum-free medium. These data suggest that GBM cell growth can be enhanced by NGF acting via Trk receptor phosphorylation. Future studies of antiproliferative therapies should consider agents directed against intracellular Trk signaling cascades.     

Therapeutic targets for neuroblastomas

Introduction: Neuroblastoma (NB) is the most common and deadly solid tumor in children. Despite recent improvements, the long-term outlook for high-risk NB is still < 50%. Further, there is considerable short- and long-term toxicity. More effective, less toxic therapy is needed, and the development of targeted therapies offers great promise.

Areas covered: Relevant literature was reviewed to identify current and future therapeutic targets that are critical to malignant transformation and progression of NB. The potential or actual NB therapeutic targets are classified into four categories: i) genes activated by amplification, mutation, translocation or autocrine overexpression; ii) genes inactivated by deletion, mutation or epigenetic silencing; iii) membrane-associated genes expressed on most NBs but few other tissues; or iv) common target genes relevant to NB as well as other tumors.

Expert opinion: Therapeutic approaches have been developed to some of these targets, but many remain untargeted at the present time. It is unlikely that single targeted agents will be sufficient for long-term cure, at least for high-risk NBs. The challenge will be how to integrate targeted agents with each other and with conventional therapy to enhance their efficacy, while simultaneously reducing systemic toxicity.     

Dynamic expression of nerve growth factor and its receptor TrkA after subarachnoid hemorrhage in rat brain

Delayed ischemic neurologic deifcit after subarachnoid hemorrhage results from loss of neural cells. Nerve growth factor and its receptor TrkA may promote regeneration of neural cells, but their expression after subarachnoid hemorrhage remains unclear. In the present study, a rat model of subarachnoid hemorrhage was established using two injections of autologous blood into the cistern magna. Immunohisto-chemical staining suggested that the expression of nerve growth factor and TrkA in the cerebral cortex and brainstem increased at 6 hours, peaked at 12 hours and decreased 1 day after induction of subarachnoid hemorrhage, whereas the expression in the hippocampus increased at 6 hours, peaked on day 1, and decreased 3 days later. Compared with those for the rats in the sham and saline groups, neurobehavioral scores decreased signiifcantly 12 hours and 3 days after subarachnoid hemorrhage (P<0.05). These results suggest that the expression of nerve growth factor and its receptor TrkA is dynamically changed in the rat brain and may thus participate in neuronal survival and nerve regeneration after subarachnoid hemorrhage.     

二苯乙烯苷对脑缺血再灌注大鼠TrkA及Bcl-2表达的影响

目的探讨二苯乙烯苷（TSG）对脑缺血再灌注大鼠的神经保护作用及机制。方法将250～350g健康雄性sD大鼠共4组：对照组、模型组、小剂量TSG组和大剂量TSG组,每组24只。Longa线栓法制备大脑中动脉栓塞模型（MCAO）,按Longa的5级标准评分法评价神经功能缺损。再灌注后6h、24h、48h和7d共4个时间点处死大鼠。采用原位末端脱氧核苷酸转移酶标记法（TUNEL）检测神经细胞凋亡;采用原位杂交方法、免疫组化法检测TrkA、Bcl-2基因／蛋白表达变化。结果神经功能缺损评分显示造模各组各时间点均有明显的神经功能缺损症状,除6h时间点外,两个剂量TSG治疗组其余各时间点神经功能缺损评分明显低于模型组,差异有统计学意义（P〈0．05）;与模型组比较,两个剂量TSG组各时间点凋亡细胞减少,TrkA、Bcl-2基因／蛋白的表达明显上调,差异都有统计学意义（P〈0．05）。结论TSG可能通过增强脑缺血再灌注损伤后TrkA／Bcl-2通路的活性,起到神经保护作用。     

Altered NGF response but not release in the aged septo-hippocampal cholinergic system

An important aspect of aging and Alzheimer's disease (AD) pathology includes the degeneration of basal forebrain cholinergic neurons (BFCNs), possibly due to disrupted nerve growth factor (NGF) signaling. Previous studies on disrupted NGF signaling have focused on changes in retrograde transport. This study focuses on two other possible mechanisms for loss of trophic support: diminished release of NGF from hippocampal neurons or diminished TrkA receptor response of BFCNs to NGF. We measured NGF levels in the effluent of hippocampal slices from young and aged rats in response to potassium chloride and glutamate. We found that release of NGF was not altered in aged hippocampal slices compared to slices from young controls. To measure the in situ response of the BFCNs to NGF, we injected NGF intraparenchymally into the right hippocampus of young and aged rats. Injections of cytochrome C served as controls. Fifteen minutes post-administration, a dramatic increase in TrkA immunoreactivity was found in the cell bodies of medial septal neurons. We found that this rapid response was blunted in aged rats compared to young adult controls. To determine whether retrograde transport was necessary for this rapid response, we injected colchicine prior to NGF injection. The NGF-induced upregulation was not blocked by colchicine, suggesting that this acute response was not dependent on classical retrograde transport. Since cholinergic degeneration coupled with altered levels of NGF and TrkA receptors are also seen in human aging and AD, the loss of acute responsivity to NGF in the BFCNs may also play a role in these processes.     

Expression of NGF in hepatocellular carcinoma cells with its receptors in non-tumor cell components

Nerve growth factor (NGF) is suggested to have a role in tumor progression in addition to its role in differentiation and survival of neuronal cells. We investigated expression of NGF and its receptors, TrkA and p75NTR, in hepatocellular carcinomas (HCCs). Although hepatocytes and hepatic stellate cells (HSCs) showed respectively weak and intense NGF immunostaining in the background livers of patients suffering from liver cirrhosis (LC) or chronic hepatitis (CH), intense staining was demonstrated in HCC cells of 33 of 54 (61.1%) tumors. RT-PCR detected NGF mRNA in 7 freshly-isolated HCC samples, and in 2 of 4 cases, in which both background livers and tumors could be analyzed, NGF mRNA was more abundant in the tumors than the background livers. TrkA was detected in the smooth muscle cells of hepatic arteries, but it was negative in tumor cells as well as non-neoplastic hepatocytes. p75NTR and alpha-smooth muscle actin (alphaSMA) was expressed in HSCs in the background liver and fibroblast-like cells in stromal septa, whereas HSCs within the HCC tissues were mostly negative for p75NTR but positive for alphaSMA. This suggests that HSCs in HCC have a different property from those in background livers. Furthermore, the stromal septa contained abundant nerve fibers, which may be related to the increased NGF expression in HCC cells. NGF and its receptors are then thought to have a role in cellular interactions involving HCC cells, HSCs, arterial cells and nerve cells in HCC tissues.     

NGF-induced reduction of an outward-rectifying TRPM7-like current in rat CA1 hippocampal neurons

Transient receptor potential melastatin 7 (TRPM7) channels are widely expressed in the nervous system; however, their function and regulation are largely unknown. This study aimed to explore whether the current mediated by TRPM7-like channels in rat CA1 hippocampal neurons could be modulated by nerve growth factor (NGF). Using whole-cell patch clamp techniques, we identified an outward-rectifying TRPM7-like current in hippocampal neurons freshly isolated from postnatal (10-day-old) rats. The outward component of this current was reversibly reduced by NGF in dose- and time-dependent manners, and this effect was substantially blocked by K252a, an inhibitor of TrkA. In addition, NGF-induced reduction of the TRPM7-like current was abolished by U73122, a phopholipase C inhibitor. In light of the abundance of NGF in hippocampus that express both TrkA and TRPM7, these results suggest that the function of TRPM7-like channels in hippocampal neurons may be regulated by NGF.     

Cleavage of the TrkA neurotrophin receptor by multiple metalloproteases generates signalling-competent truncated forms

The ectodomain of the neurotrophin receptor TrkA has been recovered as a soluble fragment from the culture media of cells by a process that involves endoproteolytic cleavage. This cleavage may be upregulated by several treatments, including NGF treatment or protein kinase C activation. In this report we have investigated the cellular site and proteolytic activities involved in TrkA cleavage, and the effects of ectodomain truncation on signalling. Cleavage occurs when the receptor is at, or near, the cell surface, and it can be prevented by agents that affect protein sorting. Cleavage generates several cell-bound fragments, and their generation can be differentially blocked by inhibitors, documenting the involvement of multiple plasma membrane metalloendoproteases. The major cell-bound receptor fragment (i) is tyrosine-phosphorylated in vivo; (ii) does autophosphorylate in vitro; and (iii) is able to associate with intracellular signalling substrates. Artificial deletion of the TrkA ectodomain results in an active receptor that induced neurite outgrowth in pheochromocytoma cells. Cleavage by this natural cellular mechanism appears thus to serve not only as an outlet of receptor binding fragments, but also to generate signalling-competent cell-bound receptor fragments. In the nervous system this ligand-independent receptor activation could play important roles in the development and survival of neurons.     

盆痛灵方对子宫内膜异位症大鼠神经生长因子及其受体表达的影响

目的:探讨盆痛灵方对子宫内膜异位症(EMs)大鼠异位内膜神经生长因子(NGF)及其受体酪氨酸激酶受体A(TrkA)和神经营养因子受体p75(p75NTR)表达的影响。方法:采用自体内膜移植法建立大鼠EMs模型。取造模成功的大鼠随机分为模型组、消炎痛组和盆痛灵低、中、高剂量组,每组10只;另设假手术组10只(仅切除单侧结扎段子宫)。消炎痛组大鼠给予3 mg·kg~(-1)·d~(-1)消炎痛混悬液,盆痛灵低、中、高剂量组大鼠分别给予10.2、20.4、40.8 g·kg~(-1)·d~(-1)盆痛灵方水煎液,各组均采用直肠给药方式,每日1次,连续4周;模型组和假手术组给予等体积0.9%NaCl溶液。末次给药后,采集各组大鼠的子宫内膜组织。PCR检测各组大鼠异位内膜NGF、p75NTR、TrkA的mRNA表达,Western blot和免疫组化法检测异位内膜NGF、p75NTR、TrkA蛋白表达。结果:①PCR检测显示,模型组大鼠内膜组织NGF、TrkA、p75NTR的mRNA表达较假手术组显著升高(P0.01);与模型组相比,消炎痛组和盆痛灵中、高剂量组NGF和TrkA的mRNA表达明显降低(P0.05,P0.01),消炎痛组和盆痛灵高剂量组p75NTR的mRNA表达亦明显降低(P0.01);与盆痛灵低剂量组相比,高剂量组大鼠的内膜组织NGF、TrkA、p75NTR的mRNA表达均显著降低(P0.01),中剂量组TrkA mRNA表达亦明显降低(P0.05);与盆痛灵中剂量组相比,高剂量组p75NTR mRNA表达明显降低(P0.01)。②Western blot检测显示,模型组大鼠内膜组织NGF、TrkA、p75NTR蛋白表达水平较假手术组显著升高(P0.01);与模型组相比,消炎痛组及盆痛灵高、中、低剂量组NGF、TrkA、p75NTR蛋白表达水平均明显降低(P0.01);与盆痛灵低剂量组相比,中、高剂量组大鼠子宫内膜NGF、TrkA、p75NTR蛋白表达水平均显著降低(P0.01);与盆痛灵中剂量组相比,高剂量组NGF、TrkA、p75NTR蛋白表达水平亦显著降低(P0.01)。③免疫组化检测显示,与假手术组相比,模型组NGF及其受体p75NTR、TrkA在异位内膜的腺上皮细胞与间质细胞胞浆呈高表达,仅少量表达于胞核;与模型组相比,消炎痛组及盆痛灵中、高剂量组NGF、p75NTR、TrkA的阳性表达明显减弱;与盆痛灵低剂量组相比,盆痛灵中、高剂量组NGF、p75NTR、TrkA的阳性表达明显减弱。结论:盆痛灵方可能通过降低异位内膜NGF及其受体p75NTR、TrkA的表达,对子宫内膜异位症起治疗作用,且其作用具有一定的剂量依赖性。