Serotonin release and cell proliferation are under the control of alpha-bungarotoxin-sensitive nicotinic receptors in small-cell lung carcinoma cell lines

Alpha7 nicotinic acetylcholine receptors in lung inflammation and carcinogenesis: Friends or foes?

The lung tissue expresses the cholinergic system including nicotinic acetylcholine receptors (nAChRs) which included in many physiologic and pathologic processes. Mounting evidence revealed that these receptors have important roles in lung carcinogenesis via modulating either stimulatory or inhibitory signaling pathways. Among different members of nicotinic receptors family, alpha7-subtype of nAChR (α7nAChR) is a critical mediator involved in both inflammatory responses and cancers. Several studies have shown that this receptor is the most powerful regulator of responses that stimulate lung cancer processes such as proliferation, angiogenesis, metastasis, and inhibition of apoptosis. Moreover, aside from its roles in the regulation of cancer pathways, there is growing evidence indicating that α7nAChR has profound impacts on lung inflammation through the cholinergic anti-inflammatory pathway. Regarding such diverse effects as well as the critical roles of nicotine as an activator of α7nAChR on lung cancer pathogenesis, its modulation has emerged as a promising target for drug developments. In this review, we aim to highlight the detrimental as well as the possible beneficial influences of α7nAChR downstream signaling cascades in the control of lung inflammation and cancer-associated properties. Consequently, by considering the significant global burden of lung cancer, delineating the complex influences of α7 receptors would be of great interest in designing novel anticancer and anti-inflammatory strategies for the patients suffering from lung cancer.

Consider the lung as a sensory organ: A tip from pulmonary neuroendocrine cells

While the lung is commonly known for its gas exchange function, it is exposed to signals in the inhaled air and responds to them by collaborating with other systems including immune cells and the neural circuit. This important aspect of lung physiology led us to consider the lung as a sensory organ. Among different cell types within the lung that mediate this role, several recent studies have renewed attention on pulmonary neuroendocrine cells (PNECs). PNECs are a rare, innervated airway epithelial cell type that accounts for <1% of the lung epithelium population. They are enriched at airway branch points. Classical in vitro studies have shown that PNECs can respond to an array of aerosol stimuli such as hypoxia, hypercapnia and nicotine. Recent in vivo evidence suggests an essential role of PNECs at neuroimmunomodulatory sites of action, releasing neuropeptides, neurotransmitters and facilitating asthmatic responses to allergen. In addition, evidence supports that PNECs can function both as progenitor cells and progenitor niches following airway epithelial injury. Increases in PNECs have been documented in a large array of chronic lung diseases. They are also the cells-of-origin for small cell lung cancer. A better understanding of the specificity of their responses to distinct insults, their impact on normal lung function and their roles in the pathogenesis of pulmonary ailments will be the next challenge toward designing therapeutics targeting the neuroendocrine system in lung.

The Role of Nitrosamine (NNK) in Breast Cancer Carcinogenesis

Smoking cigarettes is one of the most concerning issues that leads to tobacco-related cancers and can even result in death. Therefore, these issues should be addressed with a great sense of urgency with low-cost and simple approaches. Over the past several years, the scientific community has attempted to find solutions to overcome this issue. Thus, a large number of excellent studies have been reported in this field, and summarizing these results and providing important roadmaps for future studies is currently of great importance. Finding an outstanding solution to address aforementioned issue would be of great value to the community and to the social. Tobacco contains thousands of chemicals, and sixty-nine compounds have been established as human carcinogens; specifically, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is the strongest carcinogen among the tobacco-specific nitrosamines. Tobacco carcinogens are also linked to mammary gland pathogenesis and increased risk of developing many cancers, including breast cancer, the most common cancer in women worldwide. This mini-review summarizes the role of NNK and the mechanisms of its receptor, nicotine acetylcholine receptor (nAChR), signaling in breast cancer based on publications identified using the keywords "secondhand smoke (SHS)", "Nitrosamines" and "breast cancer". Furthermore, this review considers the risk of NNK to the public in an effort to reduce exposure to SHS in women and their chances of developing breast cancer.

Acetylcholine suppresses shoot formation and callusing in leaf explants of in vitro raised seedlings of tomato, Lycopersicon esculentum Miller var. Pusa Ruby

We present experimental evidence to show that acetylcholine (ACh) causes decrease in shoot formation in leaf explants of tomato (Lycopersicon esculentum Miller var Pusa Ruby) when cultured on shoot regeneration medium. The optimum response was obtained at 10(-4) M ACh-enriched medium. ACh also causes decrease in percentage of cultures forming callus and reduces the callus mass. Inhibitors of enzymatic hydrolysis of ACh, neostigmine and physostigmine, also suppresses callogenesis and caulogenesis. On the other hand, the breakdown products of Ach, choline and acetate, do not alter the morphogenic response induced on the shoot regeneration medium. Neostigmine showed optimal reduction in shoot formation at 10(-5) M. The explants cultured on neostigmine augmented medium showed decline in the activity of ACh hydrolyzing enzyme acetylcholinesterase. ACh and neostigmine added together showed marked reduction in callus mass. These results strongly support the role of ACh as a natural regulator of morphogenesis in tomato plants.

The Oncogenic Functions of Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors (nAChRs) are ion channels that are expressed in the cell membrane of all mammalian cells, including cancer cells. Recent findings suggest that nAChRs not only mediate nicotine addiction in the brain but also contribute to the development and progression of cancers directly induced by nicotine and its derived carcinogenic nitrosamines whereas deregulation of the nAChRs is observed in many cancers, and genome-wide association studies (GWAS) indicate that SNPs nAChRs associate with risks of lung cancers and nicotine addiction. Emerging evidences suggest nAChRs are posited at the central regulatory loops of numerous cell growth and prosurvival signal pathways and also mediate the synthesis and release of stimulatory and inhibitory neurotransmitters induced by their agonists. Thus nAChRs mediated cell signaling plays an important role in stimulating the growth and angiogenic and neurogenic factors and mediating oncogenic signal transduction during cancer development in a cell type specific manner. In this review, we provide an integrated view of nAChRs signaling in cancer, heightening on the oncogenic properties of nAChRs that may be targeted for cancer treatment.

Dysregulated cholinergic network as a novel biomarker of poor prognostic in patients with head and neck squamous cell carcinoma

BACKGROUND

In airways, a proliferative effect is played directly by cholinergic agonists through nicotinic and muscarinic receptors activation. How tumors respond to aberrantly activated cholinergic signalling is a key question in smoking-related cancer. This research was addressed to explore a possible link of cholinergic signalling changes with cancer biology.

METHODS

Fifty-seven paired pieces of head and neck squamous cell carcinoma (HNSCC) and adjacent non-cancerous tissue (ANCT) were compared for their mRNA levels for ACh-related proteins and ACh-hydrolyzing activity.

RESULTS

The measurement in ANCT of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) activities (5.416 ± 0.501 mU/mg protein and 6.350 ± 0.599 mU/mg protein, respectively) demonstrated that upper respiratory tract is capable of controlling the availability of ACh. In HNSCC, AChE and BChE activities dropped to 3.584 ± 0.599 mU/mg protein (p = 0.002) and 3.965 ± 0.423 mU/mg protein (p < 0.001). Moreover, tumours with low AChE activity and high BChE activity were associated with shorter patient overall survival. ANCT and HNSCC differed in mRNA levels for AChE-T, α3, α5, α9 and β2 for nAChR subunits. Tobacco exposure had a great impact on the expression of both AChE-H and AChE-T mRNAs. Unaffected and cancerous pieces contained principal AChE dimers and BChE tetramers. The lack of nerve-born PRiMA-linked AChE agreed with pathological findings on nerve terminal remodelling and loss in HNSCC.

CONCLUSIONS

Our results suggest that the low AChE activity in HNSCC can be used to predict survival in patients with head and neck cancer. So, the ChE activity level can be used as a reliable prognostic marker.

Nicotinic acetylcholine receptors mediate lung cancer growth

Ion channels modulate ion flux across cell membranes, activate signal transduction pathways, and influence cellular transport—vital biological functions that are inexorably linked to cellular processes that go awry during carcinogenesis. Indeed, deregulation of ion channel function has been implicated in cancer-related phenomena such as unrestrained cell proliferation and apoptotic evasion. As the prototype for ligand-gated ion channels, nicotinic acetylcholine receptors (nAChRs) have been extensively studied in the context of neuronal cells but accumulating evidence also indicate a role for nAChRs in carcinogenesis. Recently, variants in the nAChR genes CHRNA3, CHRNA5, and CHRNB4 have been implicated in nicotine dependence and lung cancer susceptibility. Here, we silenced the expression of these three genes to investigate their function in lung cancer. We show that these genes are necessary for the viability of small cell lung carcinomas (SCLC), the most aggressive type of lung cancer. Furthermore, we show that nicotine promotes SCLC cell viability whereas an α3β4-selective antagonist, α-conotoxin AuIB, inhibits it. Our findings posit a mechanism whereby signaling via α3/α5/β4-containing nAChRs promotes lung carcinogenesis.

Nicotine and lung cancer

Tobacco use in cancer patients is associated with increased cancer treatment failure and decreased survival. Nicotine is one of over 7,000 compounds in tobacco smoke and nicotine is the principal chemical associated with addiction. The purpose of this article is to review the tumor promoting activities of nicotine. Nicotine and its metabolites can promote tumor growth through increased proliferation, angiogenesis, migration, invasion, epithelial to mesenchymal transition, and stimulation of autocrine loops associated with tumor growth. Furthermore, nicotine can decrease the biologic effectiveness of conventional cancer treatments such as chemotherapy and radiotherapy. Common mechanisms appear to involve activation of nicotinic acetylcholine receptors and beta-adrenergic receptors leading to downstream activation of parallel signal transduction pathways that facilitate tumor progression and resistance to treatment. Data suggest that nicotine may be an important mechanism by which tobacco promotes tumor development, progression, and resistance to cancer treatment.

Lung carcinoma signaling pathways activated by smoking

Lung cancer is the leading cause of cancer death in men and women worldwide, with over a million deaths annually. Tobacco smoke is the major etiologic risk factor for lung cancer in current or previous smokers and has been strongly related to certain types of lung cancer, such as small cell lung carcinoma and squamous cell lung carcinoma. In recent years, there has been an increased incidence of lung adenocarcinoma. This change is strongly associated with changes in smoking behavior and cigarette design. Carcinogens present in tobacco products and their intermediate metabolites can activate multiple signaling pathways that contribute to lung cancer carcinogenesis. In this review, we summarize the smoking-activated signaling pathways involved in lung cancer.

Role of nicotine in gallbladder carcinoma: a preliminary report

OBJECTIVE

To assess the role of nicotine in gallbladder carcinoma and its association with the stage and degree of cancer differentiation.

METHODS

Tissue samples from gallbladder were obtained from 20 patients with gallbladder cancer and 20 age- and gender-matched patients with cholelithiasis who served as the control group. Gallbladder tissue (2 g) was extracted and analyzed for nicotine content using capillary gas chromatography. Nitrogen was used as the carrier gas. Standard curves of nicotine in methanol were made by injecting the internal standards.

RESULTS

A significantly higher tissue nicotine concentration was observed in the gallbladder carcinoma group than that in the control group (179.63 ng/mg vs 6.00 ng/mg, P < 0.001). The stage and degree of cancer differentiation did not seem to affect the nicotine levels. Gallbladder tissue contained a significantly higher nicotine concentration in smokers with cancer compared with those in the control group (1570.00 ng/mg vs 232.25 ng/mg, P < 0.001). Interestingly, non-smokers in cancer group also had a higher nicotine concentration than the control group (161.50 ng/mg vs 4.00 ng/mg, P = 0.002).

CONCLUSION

Nicotine is selectively concentrated in malignant gallbladder tissue irrespective of smoking status, showing its strong association with gallbladder cancer.

Nicotinic acetylcholine receptor signaling in tumor growth and metastasis

Cigarette smoking is highly correlated with the onset of a variety of human cancers, and continued smoking is known to abrogate the beneficial effects of cancer therapy. While tobacco smoke contains hundreds of molecules that are known carcinogens, nicotine, the main addictive component of tobacco smoke, is not carcinogenic. At the same time, nicotine has been shown to promote cell proliferation, angiogenesis, and epithelial-mesenchymal transition, leading to enhanced tumor growth and metastasis. These effects of nicotine are mediated through the nicotinic acetylcholine receptors that are expressed on a variety of neuronal and nonneuronal cells. Specific signal transduction cascades that emanate from different nAChR subunits or subunit combinations facilitate the proliferative and prosurvival functions of nicotine. Nicotinic acetylcholine receptors appear to stimulate many downstream signaling cascades induced by growth factors and mitogens. It has been suggested that antagonists of nAChR signaling might have antitumor effects and might open new avenues for combating tobacco-related cancer. This paper examines the historical data connecting nicotine tumor progression and the recent efforts to target the nicotinic acetylcholine receptors to combat cancer.

Inhibition of non-neuronal alpha7-nicotinic receptor reduces tumorigenicity in A549 NSCLC xenografts

Nicotinic acetylcholine receptors (nAChR) are expressed on bronchial epithelial and non-small cell lung cancer cells and are involved in cell growth regulation. Nicotine (classical nAChR agonist) induced cell proliferation, whereas nAChR antagonists, d- tubocurarine or alpha-cobratoxin (alpha-CbT), induced cell death. In the current study, we further explored the antitumor potential mechanisms and activities of alpha-CbT. NOD/SCID mice were grafted intraperitoneally or orthotopically and treated with alpha-CbT. alpha-CbT treatment [0.04 ng/kg or 0.12 ng/kg] induced a strong reduction in tumor size ( approximately 90%) in comparison with mice treated with the vehicle alone. Tumor inhibition was related to severe induction of apoptosis. Moreover, neoangiogenesis was strongly inhibited (reduction of cells positive to vascular endothelial growth factor and CD31). Biochemical analyses of the cells, isolated by the primary lung tumor in alpha-CbT-treated mice, showed apoptosis features characterized by: (i) inhibition of BAD phosphorylation at Ser(112) and Ser(136); (ii) BAD dissociation from 14-3-3; (iii) BAD association with BCL-XL; and (iv) cleavage of caspase-9. Moreover, these cells were unable to grow in soft agar and develop tumor, when reinjected into mice. The small interfering RNA-mediated silencing of the alpha7-nAChR gene confirmed that alpha-CbT specifically inhibited the alpha7-nAChR-mediated survival pathway in A549 cells. Furthermore, the specificity of alpha-CbT is reinforced by the lack of effect of short chain toxin (Erabutoxin-a). Once more, the no effect of the low-affinity R33E-modified alpha-CbT strengthened the specificity of this inhibition. Although alpha7-nAChR antagonists, such as alpha-CbT, are unlikely to be a primary therapy, it may provide lead compounds for the design of clinically useful drugs.

Is cancer triggered by altered signalling of nicotinic acetylcholine receptors?

Nicotinic acetylcholine receptors (nAChRs) are the central regulators of stimulatory and inhibitory neurotransmitters that control the synthesis and release of growth, angiogenic and neurotrophic factors in cancer cells, the cancer microenvironment and distant organs. Data discussed in this Review suggests that smoking and possibly other environmental and lifestyle factors increase the function of nAChRs that stimulate cancer cells and reduce the function of nAChRs that inhibit cancer cells. This novel paradigm necessitates the development of marker-guided cancer intervention strategies that aim to restore the balance between nAChR-mediated stimulatory and inhibitory neurotransmitters and their downstream effectors.

Serotonin metabolism is dysregulated in cholangiocarcinoma, which has implications for tumor growth

Cholangiocarcinoma is a devastating cancer of biliary origin with limited treatment options. Symptoms are usually evident after blockage of the bile duct by the tumor, and at this late stage, they are relatively resistant to chemotherapy and radiation therapy. Therefore, it is imperative that alternative treatment options are explored. We present novel data indicating that the metabolism of serotonin is dysregulated in cholangiocarcinoma cell lines, compared with normal cholangiocytes, and tissue and bile from cholangiocarcinoma patients. Specifically, there was an increased expression of tryptophan hydroxylase 1 and a suppression of monoamine oxidase A expression (enzymes responsible for the synthesis and degradation of serotonin, respectively) in cholangiocarcinoma. This resulted in an increased secretion of serotonin from cholangiocarcinoma and increased serotonin in the bile from cholangiocarcinoma patients. Increased local serotonin release may have implications on cholangiocarcinoma cell growth. Serotonin administration increased cholangiocarcinoma cell growth in vitro, whereas inhibition of serotonin synthesis decreases tumor cell growth both in vitro and in vivo. The data presented here represent the first evidence that serotonin metabolism is dysregulated in cholangiocarcinoma and that modulation of serotonin synthesis may represent an alternative target for the development of therapeutic strategies.

Characterization of a multiple ligand-gated ion channel cellular membrane affinity chromatography column and identification of endogenously expressed receptors in astrocytoma cell lines

Cellular membranes obtained from the 1321N1 and A172 astrocytoma cell lines were immobilized on a chromatographic phase to create cellular membrane affinity chromatography (CMAC) columns, CMAC(1321N1) and CMAC(A172). The columns were characterized using frontal affinity chromatography with [(3)H]-epibatidine as the marker ligand and epibatidine, nicotine, and methyllycaconitine as the displacers. The results indicated that the columns contained homomeric alpha7 nicotinic acetylcholine receptors (alpha7 nAChR) and heteromeric nicotinic acetylcholine receptors (alpha(x)beta(y) nAChRs), which was confirmed by the addition of subtype-specific inhibitors, alpha-bungarotoxin (alpha7 nAChR) and kappa-bungarotoxin (alpha(x)beta(y) nAChR) to the mobile phase. The presence of two additional ligand-gated ion channels (LGICs), gamma-aminobutyric acid (GABA(A)) and N-methyl-D-aspartic acid (NMDA), was established using frontal affinity chromatography with flunitrazepam and diazepam (GABA(A) receptor) and MK-801 and NMDA (NMDA receptor). The presence of the four LGICs was confirmed using confocal microscopy and flow cytometry. The results indicate that the CMAC(1321N1) and CMAC(A172) columns contain four independently functioning LGICs, that the columns can be used to characterize binding affinities of small molecules to each of the receptors, and that the CMAC approach can be used to probe the expression of endogenous membrane receptors.

Receptor-mediated tobacco toxicity: alterations of the NF-kappaB expression and activity downstream of alpha7 nicotinic receptor in oral keratinocytes

To gain a mechanistic insight into nicotinic receptor-dependent morbidity of tobacco products in the oral cavity, we studied effects of exposures of normal human oral keratinocytes (KCs) for 24 h to environmental tobacco smoke (ETS) vs. equivalent concentration of pure nicotine. The exposed KCs showed a multifold increase of nuclear factor-kappaB (NF-kappaB) at the mRNA and protein levels, which could be significantly (p<0.05) diminished by alpha-bungarotoxin or transfection with anti-alpha7 small interfering RNA. An increased protein-binding activity of NF-kappaB also could be prevented by blocking alpha7 signaling. The use of pathway inhibitors demonstrated that the Ras/Raf-1/MEK1/ERK steps mediated alpha7-dependent upregulation of NF-kappaB. Thus, exposure of KCs to tobacco may lead to the pathobiologic effects via an intracellular signaling pathway downstream of alpha7 that proceeds through the Ras/Raf-1/MEK1/ERK steps leading to upregulated expression and transactivation of NF-kappaB.

Cyclic adenosine monophosphate-dependent cell type-specific modulation of mitogenic signaling by retinoids in normal and neoplastic lung cells

BACKGROUND

Lung cancer is the leading cause of cancer death worldwide. A diet rich in fruit and vegetables has been shown to reduce the lung cancer risk. However, clinical trials with beta-carotene and retinoids have disappointed, resulted in increased mortality from lung cancer and cardiovascular disease.

METHODS

We have investigated the effects of the two major retinol metabolites, 9-cis-retinoic acid (9-Cis-RA), and 13-cis-retinoic acid (13-Cis-RA), on cell proliferation (MTT assays), intracellular cAMP (cAMP immunoassays), PKA activation (non-radioactive PKA activation assays), and ERK1/2 phosphorylation (Western blots) in immortalized human small airway epithelial cells, HPL1D, a human lung adenocarcinoma cell line, NCI-H322, immortalized human bronchial epithelial cells, BEAS-2B, and in the human small cell lung carcinoma cell line, NCI-H69.

RESULTS

Both retinoids increased intracellular cAMP and PKA activation in all cell lines. In BEAS-2B and NCI-H69 cells, the stimulation of cAMP/PKA reduced the phosphorylation of ERK1/2 and inhibited cell proliferation whereas phosphorylation of ERK1/2 and cell proliferation were increased in HPL1D and NCI-H322 cells.

CONCLUSIONS

Our data have identified a novel mechanism of action of 9-Cis-RA and 13-Cis-RA: activation of PKA in response to increased cAMP. The observed stimulation of cAMP/PKA may inhibit the development of small cell lung carcinoma and other tumors derived from large airway epithelia whereas it may selectively promote the development of lung tumors derived from small airway epithelial cells, such as adenocarcinoma.

Receptor-mediated tobacco toxicity: cooperation of the Ras/Raf-1/MEK1/ERK and JAK-2/STAT-3 pathways downstream of alpha7 nicotinic receptor in oral keratinocytes

The use of tobacco products is associated with an increased incidence of periodontal disease, poor response to periodontal therapy, and a high risk for developing head and neck cancer. Nicotine and tobacco-derived nitrosamines have been shown to exhibit their pathobiologic effects due in part to activation of the nicotinic acetylcholine (ACh) receptors (nAChRs), mainly alpha7 nAChR, expressed by oral keratinocytes (KCs). This study was designed to gain mechanistic insight into alpha7-mediated morbidity of tobacco products in the oral cavity. We investigated the signaling pathways downstream of alpha7 nAChR in monolayers of oral KCs exposed for 24 h to aged and diluted sidestream cigarette smoke (ADSS) or an equivalent concentration of pure nicotine. By both real-time polymerase chain reaction (PCR) and In-cell Western, the KCs stimulated with ADSS or nicotine showed multifold increases of STAT-3. These effects could be completely blocked or significantly (P<0.05) diminished if the cells were pretreated with the alpha7 antagonist alpha-bungarotoxin (alphaBTX) or transfected with anti-alpha7 small interfering RNA (siRNA-alpha7). The use of pathway inhibitors revealed that signaling through the Ras/Raf-1/MEK1/ERK steps mediated alpha7-dependent up-regulation of STAT-3. Targeted mutation of the alpha7 gene prevented ERK1/2 activation by nicotine. Using the gel mobility shift assay, we demonstrated that an increased protein binding activity of STAT-3 caused by ADSS or pure nicotine was mediated by janus-activated kinase (JAK)-2. Activation of JAK-2/STAT-3 pathway could be prevented by alphaBTX or siRNA-alpha7. Thus, nuclear transactivation of STAT-3 in KCs exposed to tobacco products is mediated via intracellular signaling downstream from alpha7, which proceeds via two complementary pathways. The Ras/Raf-1/MEK1/ERK cascade culminates in up-regulated expression of the gene encoding STAT-3, whereas recruitment and activation of tyrosine kinase JAK-2 phosphorylates it. Elucidation of this novel mechanism of nicotine-dependent nuclear transactivation of STAT-3 identifies oral alpha7 nAChR as a promising molecular target to prevent, reverse, or retard tobacco-related periodontal disease and progression of head and neck cancer by receptor inhibitors.

Control by cholinergic mechanisms

In the respiratory tract acetylcholine is neurotransmitter in ganglia and postganglionic parasympathetic nerves, but in addition is paracrine mediator released from various non-neuronal cells. Almost every cell type present in the respiratory tract expresses nicotinic and muscarinic receptors and therefore appears to be a target for acetylcholine. The present review describes the mechanisms of synthesis and release of acetylcholine from neuronal and non-neuronal cells and the differential control mechanisms. The different cholinoceptors, multiple nicotinic and muscarinic receptors and their signalling are outlined and their involvement in the modulation of the function of various target cells, smooth muscles, nerves, surface epithelial, secretory cells, fibroblasts and inflammatory cells is discussed in detail.

The role of nicotinic acetylcholine receptors in lymphocyte development

The sizes of lymphocyte populations in lymphoid organs of nicotinic acetylcholine receptor knockout and chimera (knockout/wild-type) mice were studied by flow cytometry. The absence of beta2 subunit decreased, while nicotine treatment increased B lymphocyte numbers in the bone marrow. In chimera mice, either beta2 or alpha7 subunits influenced lymphocyte populations in primary lymphoid organs, while in the spleen, only alpha7 receptors were critical. More annexin V-positive B cells were found in the bone marrow of knockout than wild-type animals. We conclude that nicotinic receptors are involved in regulating lymphocyte development and control the B lymphocyte survival.

Temporal-spatial expression and transcriptional regulation of alpha7 nicotinic acetylcholine receptor by thyroid transcription factor-1 and early growth response factor-1 during murine lung development

Nicotinic acetylcholine receptors are ligand-gated ion channels formed by five homologous subunits that are involved in processes including signal transduction, proliferation, and apoptosis. The developmental role of these receptors, however, is unclear. In the present investigation, alpha(7) nicotinic acetylcholine receptor expression was assessed by immunohistochemistry in mouse lungs from embryonic day (E)13.5 to postnatal day (PN)20. Transcriptional mechanisms that regulate alpha(7) were assessed by the transfection of murine bronchiolar cells with a reporter containing 1.1 kb of the mouse alpha(7) promoter, TTF-1, and Egr-1. alpha(7) was initially detected at E13.5 in pulmonary mesenchymal cells and in the epithelium of the primitive tubules at E15.5. From E18.5 to PN1, alpha(7) was expressed in conducting airway and saccule epithelial cells. By PN10, expression was observed in the peripheral epithelium and on luminal membranes of bronchiolar epithelial cells in the proximal lung, a pattern that continued through PN20. From E15.5 to PN20, type II alveolar cells expressed both prosurfactant protein C and alpha(7). From E18.5 to PN20, Clara cells in the bronchiolar epithelium co-expressed Clara cell secretory protein and alpha(7). TTF-1 dose-dependently activated alpha(7) transcription in vitro by binding specific TTF-1 regulatory elements in the mouse alpha(7) promoter. Furthermore, alpha(7) was not detected in TTF-1-null mice and markedly increased in TTF-1-overexpressing mice. Conversely, Egr-1 inhibited alpha(7) expression. Temporal-spatial alpha(7) expression supports the concept that these receptors function during normal pulmonary morphogenesis. A model is also supported whereby alpha(7) is induced by the essential pulmonary transcription factor TTF-1 and suppressed by Egr-1 during pulmonary development.

Neuronal nicotinic receptors: from structure to pathology

Neuronal nicotinic receptors (NAChRs) form a heterogeneous family of ion channels that are differently expressed in many regions of the central nervous system (CNS) and peripheral nervous system. These different receptor subtypes, which have characteristic pharmacological and biophysical properties, have a pentameric structure consisting of the homomeric or heteromeric combination of 12 different subunits (alpha2-alpha10, beta2-beta4). By responding to the endogenous neurotransmitter acetylcholine, NAChRs contribute to a wide range of brain activities and influence a number of physiological functions. Furthermore, it is becoming evident that the perturbation of cholinergic nicotinic neurotransmission can lead to various diseases involving nAChR dysfunction during development, adulthood and ageing. In recent years, it has been discovered that NAChRs are present in a number of non-neuronal cells where they play a significant functional role and are the pathogenetic targets in several diseases. NAChRs are also the target of natural ligands and toxins including nicotine (Nic), the most widespread drug of abuse. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining their regional and cellular localisation and the molecular basis of their functional diversity mainly in pharmacological and biochemical terms. The recent availability of mice with the genetic ablation of single or double nicotinic subunits or point mutations have shed light on the role of nAChRs in major physiological functions, and we will here discuss recent data relating to their behavioural phenotypes. Finally, the role of NAChRs in disease will be considered in some details.

Expression of the alpha7 nicotinic acetylcholine receptor in human lung cells

Background

We and others have shown that one of the mechanisms of growth regulation of small cell lung cancer cell lines and cultured pulmonary neuroendocrine cells is by the binding of agonists to the α7 neuronal nicotinic acetylcholine receptor. In addition, we have shown that the nicotine-derived carcinogenic nitrosamine, 4(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), is a high affinity agonist for the α7 nicotinic acetylcholine receptor. In the present study, our goal was to determine the extent of α7 mRNA and protein expression in the human lung.

Methods

Experiments were done using reverse transcription polymerase chain reaction (RT-PCR), a nuclease protection assay and western blotting using membrane proteins.

Results

We detected mRNA for the neuronal nicotinic acetylcholine receptor α7 receptor in seven small cell lung cancer (SCLC) cell lines, in two pulmonary adenocarcinoma cell lines, in cultured normal human small airway epithelial cells (SAEC), one carcinoid cell line, three squamous cell lines and tissue samples from nine patients with various types of lung cancer. A nuclease protection assay showed prominent levels of α7 in the NCI-H82 SCLC cell line while α7 was not detected in SAEC, suggesting that α7 mRNA levels may be higher in SCLC compared to normal cells. Using a specific antibody to the α7 nicotinic receptor, protein expression of α7 was determined. All SCLC cell lines except NCI-H187 expressed protein for the α7 receptor. In the non-SCLC cells and normal cells that express the α7 nAChR mRNA, only in SAEC, A549 and NCI-H226 was expression of the α7 nicotinic receptor protein shown. When NCI-H69 SCLC cell line was exposed to 100 pm NNK, protein expression of the α7 receptor was increased at 60 and 150 min.

Conclusion

Expression of mRNA for the neuronal nicotinic acetylcholine receptor α7 seems to be ubiquitously expressed in all human lung cancer cell lines tested (except for NCI-H441) as well as normal lung cells. The α7 nicotinic receptor protein is expressed in fewer cell lines, and the tobacco carcinogen NNK increases α7 nicotinic receptor protein levels.

The airway cholinergic system: physiology and pharmacology

The present review summarizes the current knowledge of the cholinergic systems in the airways with special emphasis on the role of acetylcholine both as neurotransmitter in ganglia and postganglionic parasympathetic nerves and as non-neuronal paracrine mediator. The different cholinoceptors, various nicotinic and muscarinic receptors, as well as their signalling mechanisms are presented. The complex ganglionic and prejunctional mechanisms controlling the release of acetylcholine are explained, and it is discussed whether changes in transmitter release could be involved in airway dysfunctions. The effects of acetylcholine on different target cells, smooth muscles, nerves, surface epithelial and secretory cells as well as mast cells are described in detail, including the receptor subtypes involved in signal transmission.

Nicotine promoted colon cancer growth via epidermal growth factor receptor, c-Src, and 5-lipoxygenase-mediated signal pathway

Nicotine [3-(1-methyl-2-pyrrolidinyl)-pyridine], a major alkaloid in tobacco, has been implicated as playing a role in carcinogenesis. Our previous study showed that passive cigarette smoking promoted inflammation-associated colonic adenoma formation in mice, and 5-lipoxygenase (5-LOX) plays an important role in this process. In the present study, we aimed to investigate whether nicotine could stimulate colon cancer cell proliferation and tumor growth in nude mice xenograft model and the possible mechanisms involved. Results showed that nicotine stimulated SW1116 colon cancer cell proliferation in a dose-dependent manner. Epidermal growth factor receptor (EGFR) and c-Src phosphorylation levels together with protein expression of 5-LOX were also significantly enhanced in this proliferation process. Inhibitors of EGFR and c-Src alleviated the actions of nicotine on cell proliferation and 5-LOX protein expression. Combination of both agents produced additive effect. In contrast, 5-LOX inhibitor had no direct effect on the phosphorylation levels of EGFR and c-Src and yet inhibited cell proliferation. In the colon cancer xenograft model, nicotine also significantly enhanced tumor growth. This acceleration of tumor growth corresponded well with increased vascularization and its proangiogenic factors. Inhibitors of EGFR, c-Src, and 5-LOX all significantly impeded the tumor growth induced by nicotine. Together, nicotine can promote colonic tumorigenesis both in vitro and in vivo. Activation of the phosphorylated form of EGFR and c-Src followed by an increased 5-LOX expression are the prime pathogenic mechanisms in the tumorigenic process in the colon.

Functional Nicotinic Acetylcholine Receptors Are Expressed in B Lymphocyte-Derived Cell Lines

Nicotine has been shown to affect B lymphocyte immune response. In this study, we have explored the presence of nicotinic receptors in B lymphocyte-derived cell lines, myeloma X63-Ag8 and hybridoma 1D6. We found that myeloma expressed on average 10,170 +/- 1,100 [3H]epibatidine and 6,730 +/- 370 125I-alpha-bungarotoxin binding sites per cell, thus reflecting the presence of both homomeric and heteromeric nicotinic receptors. More specifically, the presence of alpha4- and alpha7-containing nicotinic receptor subunits was demonstrated in both myeloma and hybridoma cells with subunit-specific antibodies. It was significantly higher in dividing than in resting cells. Long-term exposure to nicotine, at physiological concentration found in smokers, resulted in up-regulation of both alpha4 and alpha7 subunits in hybridoma cells. Additionally, nicotine stimulated hybridoma cell proliferation, whereas it decreased antibody production. In contrast, alpha7-specific snake toxins inhibited cell proliferation but increased antibody production. It is concluded that myeloma and hybridoma cells express alpha4- and alpha7-containing nicotinic receptors, which participate in regulating cell proliferation and function.

Comparison of alpha7 nicotinic acetylcholine receptor development in the hippocampal formation of C3H and DBA/2 mice

A strain-specific restriction fragment length polymorphism in the alpha7 receptor gene locus has been reported to significantly affect the expression of the alpha7 subtype of nicotinic receptor in adult mouse hippocampus. The goal of the present study was to characterize the development of the alpha7 receptor in hippocampus from two mouse strains (C3H and DBA/2) with different alleles of the alpha7 receptor gene locus by using alpha-bungarotoxin (alpha-BTX) autoradiography. Binding of alpha-BTX was initially detected in fetal C3H mice on embryonic day 13 (E13) in the dorsal portion of the hippocampal anlage. In contrast, alpha-BTX binding was initially detected primarily in hippocampal area CA3 in the DBA/2 strain on E16. Binding of alpha-BTX was absent from the neuroepithelium in both strains. A marked increase in alpha-BTX binding was observed in hippocampal area CA1 and to a lesser extent in area CA3 between E18 and postnatal day 5 (P5) in neonatal C3H mice, an increase that was not observed in the DBA/2 mice. By the end of the first postnatal month, hippocampal alpha-BTX binding appeared adult-like in each strain. These data suggest that variations in the alpha7 receptor gene locus differentially influence the developmental expression of the alpha7 receptor in murine hippocampus. Therefore, the potential influence of the alpha7 receptor on developmental processes such as cell migration, dendritic elaboration and/or axonal connectivity may exhibit strain-selective differences because of the dissimilar time courses of alpha7 receptor expression in C3H and DBA/2 mice.

Immunization with neuronal nicotinic acetylcholine receptor induces neurological autoimmune disease

Neuronal nicotinic AChRs (nAChRs) are implicated in the pathogenesis of diverse neurological disorders and in the regulation of small-cell lung carcinoma growth. Twelve subunits have been identified in vertebrates, and mutations of one are recognized in a rare form of human epilepsy. Mice with genetically manipulated neuronal nAChR subunits exhibit behavioral or autonomic phenotypes. Here, we report the first model of an acquired neuronal nAChR disorder and evidence for its pertinence to paraneoplastic neurological autoimmunity. Rabbits immunized once with recombinant alpha3 subunit (residues 1-205) develop profound gastrointestinal hypomotility, dilated pupils with impaired light response, and grossly distended bladders. As in patients with idiopathic and paraneoplastic autoimmune autonomic neuropathy, the severity parallels serum levels of ganglionic nAChR autoantibody. Failure of neurotransmission through abdominal sympathetic ganglia, with retention of neuronal viability, confirms that the disorder is a postsynaptic channelopathy. In addition, we found ganglionic nAChR protein in small-cell carcinoma lines, identifying this cancer as a potential initiator of ganglionic nAChR autoimmunity. The data support our hypothesis that immune responses driven by distinct neuronal nAChR subtypes expressed in small-cell carcinomas account for several lung cancer-related paraneoplastic disorders affecting cholinergic systems, including autoimmune autonomic neuropathy, seizures, dementia, and movement disorders.

The Role of Neurotransmitters in Neurite Outgrowth and Synapse Formation

Besides a well-established role in neuronal communication in the adult central nervous system, neurotransmitters have diverse tasks in the embryonic brain, ranging from early developmental functions in morphogenesis /13/, to later functions in target selection and synapse formation /87/. For example, growth cones of developing neurons are known to release transmitters /26,36,88,110,115/ and respond to transmitters released from other neurons /35,44,59, 61,70/. Moreover, depletion of transmitters during embryonic development results in developmental deficits of the brain /21,48,84,109/, suggesting that transmitters have crucial roles as morphogens and/or neurotrophic factors. Although recently the idea of neurotransmitters being important for neural development has been challenged /99/, there is a vast amount of literature that seems to support the hypothesis that neurotransmitter release in the developing central nervous system is crucial for proper brain development. In this review we focus on the roles that neurotransmitters play in neurite outgrowth, target selection and synapse formation, with particular emphasis on the effects of the transmitters serotonin and dopamine.

Receptor-mediated effects of nicotine and its nitrosated derivative NNK on pulmonary neuroendocrine cells

Pulmonary neuroendocrine cells (PNECs) have been implicated in the development of small cell lung carcinoma (SCLC) and pediatric asthma, and smoking is a risk factor for both diseases. We as well as others have shown that the alpha(7) nicotinic acetylcholine receptor (alpha(7) nAChR) regulates the release of 5-hydroxytryptamine (5-HT, serotonin) in PNECs and SCLC. Serotonin is an autocrine growth factor for PNECs and SCLC and acts as broncho-constrictor. We found that nicotine and its nitrosated carcinogenic derivative 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) bind to the alpha(7) nAChR in SCLC and PNECs, resulting in the influx of Ca(2+), release of 5-HT, and activation of a mitogenic pathway mediated by protein kinase C (PKC), Raf-1, mitogen activated protein kinase (MAPK) and c-myc. Exposure to 10% CO(2) acted synergistically. Unstimulated SCLC cells from smokers demonstrated high base levels of 5-HT release and of individual downstream signaling components in comparison to PNECs. Subchronic exposure of PNECs to NNK up-regulated the alpha(7) nAChR and its associated serotonergic mitogenic pathway in PNECs, an effect that may contribute to the development of SCLC in smokers and pediatric asthma in children of mothers who smoke.

Development of the alpha7 nicotinic cholinergic receptor in rat hippocampal formation

The alpha7 nicotinic receptor has been implicated in the regulation of a variety of developmental processes. The goal of the present study was to assess whether the alpha7 receptor might participate in the regulation of hippocampal ontogeny by describing the spatiotemporal development of alpha7 mRNA and alpha-bungarotoxin binding in rat hippocampal formation. Message for the alpha7 receptor was initially observed in the hippocampal neuroepithelium at embryonic day 13 and in the anlage of the hippocampal formation on embryonic day 14. Binding of alpha-bungarotoxin was initially seen on embryonic day 15 in the dorsal portion of the anlage of stratum oriens and stratum radiatum-lacunosum moleculare, but was never observed in the neuroepithelium. Dramatic elevations in both alpha7 mRNA and alpha-bungarotoxin binding were observed in most regions of the hippocampal formation neonatally. The levels of both alpha7 message and protein gradually decreased during the first three postnatal weeks to adult levels in most regions. The lack of alpha-bungarotoxin binding in the neuroepithelium suggests that the alpha7 receptor does not influence neurogenesis. The early appearance and complex, prolonged pattern of development of the alpha7 receptor suggest that it may influence processes as diverse as cell migration, dendritic elaboration and apoptosis during hippocampal maturation.

Autoimmune diseases involving nicotinic receptors

The antibody-mediated autoimmune response to alpha1 muscle nicotinic acetylcholine receptors that causes myasthenia gravis is one of the best characterized autoimmune diseases. Antibody-mediated autoimmune responses to neuronal nicotinic receptors are just beginning to be discovered and characterized. One of these causes dysautonomia through antibodies to alpha 3 nicotinic receptors of autonomic ganglia. Another causes pemphigus through antibodies to alpha 9 nicotinic receptors in skin. Other autoimmune responses to nicotinic receptors may be discovered as the many functional roles of nicotinic receptors are revealed.

Nicotine modulates the effects of retinoids on growth inhibition and RAR beta expression in lung cancer cells

Epidemiological and animal studies have demonstrated that vitamin A and its natural and synthetic derivatives, retinoids, are effective agents in preventing the development of tobacco-associated cancers. Unfortunately, clinical trials of retinoids on cigarette smokers have shown lack of efficacy in preventing lung cancer. In our study, we investigated the effect of nicotine on the anti-cancer activity of all trans-retinoic acid (trans-RA) in human lung cancer cells. Our results demonstrated that nicotine could abrogate the growth inhibitory effect of trans-RA by suppressing its ability to induce the expression of RA receptor beta (RAR beta), a tumor suppressor. The inhibitory effect of nicotine was accompanied with induction of orphan receptor TR3. Inhibition of TR3 expression by overexpression of TR3 anti-sense RNA in H460 lung cancer cells strongly prevented the suppressive effect of nicotine on trans-RA activity. Treatment with nicotine or the cotransfection of TR3 expression vector inhibited the induction of RAR beta promoter activity by trans-RA in transient transfection assays. The inhibition of RAR beta promoter activity was due to the interaction of TR3 with orphan receptor COUP-TF, resulting in inhibition of COUP-TF DNA binding and transactivation on the RAR beta promoter. Furthermore, we found that nicotine failed to suppress the effect of a retinoid X receptor (RXR)-selective retinoid SR11237 on inducing both growth inhibition and RAR beta promoter activity, due to the ability of SR11237 to activate the RAR beta promoter through the RXR/TR3 heterodimer. Together, our results demonstrate that nicotine suppresses the growth inhibitory effects of trans-RA by inhibiting RAR beta expression through its induction of TR3 expression and suggest that RXR-selective retinoids may be more effective than classical retinoids for preventing and treating tobacco-associated cancers.

Functional alterations in CNS catecholamine systems in adolescence and adulthood after neonatal chlorpyrifos exposure

Chlorpyrifos (CPF), one of the most widely used pesticides, is a neurobehavioral teratogen in animals. We administered CPF to neonatal rats on postnatal days (PN) 1-4 (1 mg/kg) or PN11-14 (5 mg/kg), regimens devoid of overt systemic toxicity. We then examined the impact on catecholaminergic systems in adolescence (PN30) and adulthood (PN60), assessing basal neurotransmitter content and transmitter utilization rates (turnover) in brain regions comprising the major noradrenergic and dopaminergic projections. Although CPF had only sporadic effects on basal norepinephrine and dopamine content, it profoundly suppressed norepinephrine turnover across multiple regions, indicative of net reductions in presynaptic activity. Dopamine turnover showed less consistent effects, with subnormal turnover in some regions and activation in others. We also evaluated whether CPF exposure altered the ability of catecholamine systems to respond to acute cholinergic stimulation, elicited by administration of a single challenge dose of nicotine. In the normal brain, nicotine increases the utilization of norepinephrine and dopamine. With only a few exceptions, animals receiving neonatal CPF exposure showed lasting desensitization of the nicotine response; not only was the activation by nicotine blunted in the CPF group, but in some regions the nicotine response was reversed, eliciting a reduction in transmitter turnover. These results indicate that neonatal CPF exposure produces widespread deficiencies in catecholaminergic synaptic function that persist into adulthood, and that are best revealed by dynamic measures of synaptic activity and responsiveness, as opposed to static markers like basal transmitter levels. The effects seen here are likely to contribute to alterations in behavioral performance that persist or emerge long after the termination of CPF exposure.

Activity of the nicotinic acetylcholine receptor alpha5 and alpha7 subunit promoters in muscle cells

The acetylcholine receptor alpha5 and alpha7 subunits are components of different nicotinic receptor subtypes expressed in the nervous system. However, they are also present in non-neuronal tissues. We have detected alpha5 and alpha7 transcripts in mouse C2C12 muscle cells. Moreover, on differentiation of myoblasts into myotubes, the amount of alpha7 transcripts increased significantly, whereas alpha5 remained unchanged. In order to explore how the expression of these neuronal genes is regulated in muscle, we have characterized their promoter activities. Deletion and mutagenesis analysis with transfected reporter genes showed that transcriptional activity was controlled by regulatory elements also operative in neuronal-like cells. Thus, the activity of the alpha5 subunit core promoter decreased to approximately 50% on alteration of one, two, or three of the five Sp1 binding sites present in this region and was almost abolished when four or five sites were mutated simultaneously. In the case of the alpha7 subunit promoter, the upstream stimulatory factor and the early growth response gene transcription factor were involved in regulating its transcriptional activity. In addition, the alpha7 promoter was activated during the differentiation process, in a mechanism partially dependent on the mentioned factors.

Fetal and adolescent nicotine administration: effects on CNS serotonergic systems

Nicotine is a neuroteratogen that targets synaptic function during critical developmental stages and recent studies indicate that CNS vulnerability extends into adolescence, the time that smoking typically commences. We administered nicotine to pregnant or adolescent rats via continuous minipump infusions, using dose rates that replicate the plasma nicotine levels found in smokers. Fetal nicotine exposure (gestational days 4-21) decreased the cerebrocortical binding of paroxetine (PXT), a marker for the serotonin (5HT) transporter, likely indicative of a decrease in nerve terminals in that region; the effect lasted into adulthood. There was a corresponding increase in PXT binding in the midbrain/brainstem, the region containing the 5HT cell bodies that project to the cerebral cortex, a pattern typical of reactive sprouting in response to nerve terminal damage. After adolescent nicotine treatment (postnatal days 30-47), PXT binding was reduced in the hippocampus and striatum instead of the cerebral cortex, again accompanied by increased binding in the midbrain and brainstem; the patterns of effects within each region were gender-selective, although both males and females displayed abnormalities. Superimposed on this overall effect, there were transient increases in PXT binding, likely due to acute stimulant effects of nicotine. We also assessed 5HT presynaptic activity (5HIAA/5HT ratio). Withdrawal from adolescent nicotine treatment led to suppression of activity in the cerebral cortex and activation in the midbrain. These results indicate that both fetal and adolescent nicotine exposure elicit apparent damage to 5HT projections with reactive increases in regions containing 5HT cell bodies. Long-term changes in 5HT innervation and/or synaptic activity may play a role in the subsequent development of depression in the offspring of women who smoke during pregnancy or in adolescent smokers.

Alterations in serotonin transporter expression in brain regions of rats exposed neonatally to chlorpyrifos

Chlorpyrifos (CPF), one of the most widely-used organophosphate pesticides, is a suspected neuroteratogen. We administered CPF to neonatal rats on postnatal days (PN) 1-4 (1 mg/kg) or PN11-14 (5 mg/kg), treatments devoid of overt toxicity. At the end of the treatment period (PN5 and 15, respectively) and 5-7 days later, we then examined the effects on paroxetine (PXT) binding to the presynaptic 5HT high-affinity transporter, a marker for serotonin (5HT) projections. In males, we found a persistent decrease in PXT binding across the two different treatment regimens, with deficits apparent in a brain region containing 5HT terminal fields (forebrain) as well as in a region containing 5HT cell bodies (brainstem). In contrast, females given the early treatment regimen (PN1-4) showed deficits in the brainstem but transient elevations in the forebrain; the later treatment regimen (PN11-14) had no significant effect on PXT binding in females. These data are consistent with earlier work showing brainstem cell injury resulting from neonatal CPF exposure, and indicate specific damage to 5HT neurons, with a consequent loss of transporter expression in both terminal fields and perikarya. In females, the damage may be temporarily offset by initial trophic effects in the terminal region, consequent to the cholinergic stimulation evoked by cholinesterase inhibition via the active metabolite, CPF oxon. The gender-selective effects on 5HT systems are likely to contribute to similar gender dimorphism in behavioral performance. Because the CPF effects involve 5HT, a neurotransmitter intimately involved in the control of mood, we suggest the need to evaluate behaviors that typify animal models of depression.

Thymic epithelial cell line expresses transcripts encoding alpha-3, alpha-5 and beta-4 subunits of acetylcholine receptors, responds to cholinergic agents and expresses choline acetyl transferase. An in vitro system to investigate thymic cholinergic mechanisms

Transcriptional and immunocytological characterization of thymic epithelial (TE) cell line TE750 shows that these cells, like primary TE cell cultures, transcribe alpha-3, alpha-5 and beta-4 acetylcholine receptor (AcChR) subunit genes while expressing cortical, medullary and epithelial differentiation thymic markers. Incubation of TE750 cells with nicotine decreases cell adherence and growth as measured through direct cytological observation and nucleic acid quantification, respectively. Physostigmine, a traditional cholinesterase inhibitor that also activates nicotinic AcChRs, reproduces the effects of nicotine. Strengthening the hypothesis that cholinergic receptors mediate the effects of physostigmine, acetylcholinesterase (AcChase) activity is not detected in TE750 cells. Also, like thymocytes, TE750 cells express choline acetyltransferase (ChAT), indicating that the natural transmitter AcCh can be produced locally within the thymic parenchyma. Taken together these findings indicate that TE750 cells in culture represent a suitable in vitro system for the analysis of cholinergic mechanisms operational in the thymic epithelium.

Nicotine effects on proliferation and the bombesin-like peptide autocrine system in human small cell lung carcinoma SHP77 cells in culture

OBJECTIVES

To determine whether nicotine affects the proliferation and expression of the bombesin-like peptide autocrine system in human small cell lung carcinoma (SCLC) SHP77 cells compared with nonmalignant human bronchial epithelial BEAS 2B cells as non-neuroendocrine controls.

METHODS

Human lung cells were cultured in defined serum-free medium with various concentrations of nicotine added for various times. Proliferation was measured by cell counts and colorimetric assay, bombesin-like peptide receptor expression was assayed by specific binding assays and quantitative competitive PCR, and bombesin-like peptides determined by ELISA.

RESULTS

Nicotine significantly stimulated the growth of human SCLC SHP77 and NCI-H865 cells, but not BEAS 2B cells. Bombesin-like peptide receptor specific binding and mRNA expression were not affected by nicotine exposure in SHP77 cells or BEAS 2B cells. An increase in SHP77 cellular bombesin-like peptide content was observed.

CONCLUSIONS

Human SCLC SHP77 cells express the components of the bombesin-like peptide autocrine system. Increased proliferation in the presence of nicotine may be due in part to increased levels of bombesin-like peptides in SHP77 cultured in nicotine. Nicotine effects on nonmalignant pulmonary neuroendocrine cells may provide additional insight into how nicotine itself may promote lung carcinogenesis.

Interaction of tobacco-specific toxicants with the neuronal alpha(7) nicotinic acetylcholine receptor and its associated mitogenic signal transduction pathway: potential role in lung carcinogenesis and pediatric lung disorders

Pulmonary neuroendocrine cells function as hypoxia-sensitive chemoreceptors, and they release peptides and biogenic amines that are important mediators of pulmonary neonatal adaptation. Some of these products additionally act as autocrine growth factors. Increased numbers of pulmonary neuroendocrine cells have been observed in several smoking-associated pediatric lung disorders such as bronchopulmonary dysplasia, cystic fibrosis, sudden infant death syndrome, and asthma. Disturbed pulmonary neuroendocrine function has been implicated in the etiology of this disease complex. One of the most common smoking-associated lung cancer types, small cell lung carcinoma, expresses phenotypic and functional features of pulmonary neuroendocrine cells. We, as well as others, have shown that the release of the autocrine growth factors 5-hydroxytryptamine (5-HT, serotonin) and mammalian bombesin/gastrin releasing peptide (MB/GRP) by cell lines derived from human small cell lung carcinoma or fetal hamster pulmonary neuroendocrine cells are regulated by a neuronal nicotinic acetylcholine receptor comprised of alpha(7) subunits. In radio-receptor assays, nicotine and the nicotine-derived carcinogenic nitrosamines NNNN. Binding of nicotine or NNK to the alpha(7) receptor resulted in calcium influx and overexpression and activation of the serine-threonine protein kinase Raf-1. In turn, this event lead to overexpression and activation of the mitogen activated (MAP) kinases extracellular signal regulated kinase 1 (ERK1) and extracellular signal regulated kinase 2 (ERK2) and stimulation of DNA synthesis accompanied by an increase in cell numbers in fetal pulmonary neuroendocrine cells and small cell carcinoma cells. Exposure of fetal pulmonary neuroendocrine cells for 6 days to NNK caused a prominant up-regulation of Raf-1. Our findings suggest that chronic exposure to nicotine and NNK in pregnant women who smoke may up-regulate the alpha(7) nicotinic receptor as well as components of its associated mitogenic signal transduction pathway, thus increasing the susceptibilities of the infants for the development of pediatric lung disorders. Similarly, up-regulation of one or several components of this nicotinic receptor pathway in smokers may be an important factor for the development of small cell lung carcinoma.

Agonist activation and alpha-bungarotoxin inhibition of wild type and mutant alpha7 nicotinic acetylcholine receptors

The properties of wild type and mutant rat nicotinic alpha7 receptors expressed in Xenopus oocytes were investigated using electrophysiology and site-directed mutagenesis. When compared at individual agonist concentrations, neither the normalised nicotinic, nor acetylcholine, responses of the wild type receptors were significantly different from the corresponding responses obtained from a first extracellular domain mutant, phenylalanine(189)tyrosine (P0.05). The dissociation constants (K(D)) of the wild type (4.7 nM) and Phe(189)Tyr mutant (5.2 nM) receptors for alpha-bungarotoxin were estimated by an electrophysiological approach. The similarity of the results suggests that the mutation did not lead to a widespread disruption of structure-function relationships, although a slight change in nicotine sensitivity may have occurred. In contrast, the mutations (Tyr(190)Gln, first extracellular domain), (Glu(261)Ala, M2 region) severely compromised receptor function. An additional mutation was made in a negatively charged motif of the second extracellular domain which is conserved in homomeric nicotinic receptors. This mutation, Asp(268)Ala, also caused a loss of function. Thus the structure-function relationships in nicotinic alpha7 receptors have parallels with heteromeric nicotinic receptors, but there may also be some marked differences.

The alpha7 nicotinic acetylcholine receptor in neuronal plasticity

A growing body of evidence indicates that neuronal nicotinic acetylcholine receptors (nAChRs), in addition to promoting fast cholinergic transmission, may modulate other neuronal activities within the central nervous system (CNS). In particular, the alpha7 nAChR is highly permeable to Ca2+ and may serve a distinct role in regulating neuronal plasticity. By elevating intracellular Ca2+ levels in discrete neuronal locations, these ligand-gated ion channels may influence numerous physiological processes in developing and adult CNS. In this article, we review evidence that both pre- and postsynaptic alpha7 nAChRs modulate transmitter release in the brain and periphery through Ca2+-dependent mechanisms. The possible role of alpha7 nAChRs in regulating neuronal growth and differentiation in developing CNS is also evaluated. We consider an interaction between cholinergic and glutamatergic transmission and propose a hypothesis on the possible coregulation of intracellular Ca2+ by N-methyl-D-aspartate (NMDA) receptors and alpha7 nAChRs. Finally, the clinical significance of alterations in the normal function of alpha7 nAChRs is discussed as it pertains to prenatal nicotine exposure, schizophrenia, and epilepsy.

Nicotinic receptor-induced apoptotic cell death of hippocampal progenitor cells

Nicotine has many effects on CNS functions, presumably through its action on neuronal nicotinic acetylcholine receptors (AChRs). One subclass of AChRs that binds the snake venom toxin alpha-bungarotoxin (alpha-Bgt-AChRs) has been shown to modulate neurotransmission in the brain. We now show that alpha-Bgt-AChR activation by low doses of nicotine results in apoptotic cell death of both primary and immortalized hippocampal progenitor cells. In HC2S2-immortalized hippocampal progenitors, nicotine is cytotoxic to undifferentiated cells, whereas it spares the same cells once differentiation has been induced. The activation of alpha-Bgt-AChRs by nicotine results in the induction of the tumor suppressor protein p53 and the cdk inhibitor p21. The cytotoxic effect of nicotine is dependent on extracellular calcium levels and is probably attributable to the poor ability of undifferentiated progenitors to buffer calcium loads. The major calcium buffer in these cells, calbindin D28K, is present only after differentiation has been induced. Furthermore transfection of undifferentiated cells with calbindin results in dramatic protection against the cytotoxic effects of nicotine. These results show that nicotine abuse could have significant effects on the survival of progenitor populations in the developing and adult brain and also suggest an endogenous role for alpha-Bgt-AChRs in neuronal development and differentiation.

4-Oxystilbene compounds are selective ligands for neuronal nicotinic alphaBungarotoxin receptors

1. Starting from the structure of an old 4-oxystilbene derivate with ganglioplegic activity (MG624), we synthesized two further derivates (F2 and F3) and two stereoisomers of F3 (F3A and F3B), and studied their selective effect on neuronal nicotinic acetylcholine receptor (AChR) subtypes. 2. MG 624, F3, F3A and F3B inhibited of 125I-alphaBungarotoxin (alphaBgtx) binding to neuronal chick optic lobe (COL) membranes, with nM affinity, but inhibited 125I-alphaBgtx binding to TE671 cell-expressed muscle-type AChR only at much higher concentrations. 3. We immobilized the alpha7, beta2 and beta4 containing chick neuronal nicotinic AChR subtypes using anti-subunit specific antibodies. MG 624, F3, F3A and F3B inhibited 125I-alphaBgtx binding to the alpha7-containing receptors with nM affinity, but inhibited 3H-Epi binding to beta2-containing receptors only at very high concentrations (more than 35 microM); their affinity for the beta4-containing receptors was ten times more than for the beta2-containing subtype. 4. Both MG624 and F3 compounds inhibited the ACh evoked currents in homomeric oocyte-expressed chick alpha7 receptors with an IC50 of respectively 94 and 119 nM. 5. High doses of both MG 624 and F3 depressed the contractile response to vagus nerve stimulation in guinea pig nerve-stomach preparations although at different IC50s (49.4 vs 166.2 microM) The effect of MG624 on rat nerve-hemidiaphragm preparations was 33 times less potent than that of F3 (IC50 486 vs 14.5 microM). 6. In conclusion, MG624 and F3 have a high degree of antagonist selectivity for neuronal nicotinic alphaBgtx receptors containing the alpha7 subunit.