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

Nicotine-induced expression of low-density lipoprotein receptor in oral epithelial cells

Background

Nicotine use is one of the most important risk factors for the development of cardiovascular and periodontal diseases. Numerous reports have suggested the possible contribution of disturbed lipid metabolism for the development of both disease groups. Despite these observations, little is known about the relationship between tobacco smoking and the development of these diseases. Our previous microarray data revealed that nicotine induced low-density lipoprotein receptor (LDLR) expression in oral epithelial cells (OECs). The aim of the present study was to confirm nicotine-mediated LDLR induction and to elucidate the signaling mechanisms leading to the augmented expression of LDLR in OECs.

Methods and Results

LDLR and nicotinic acetylcholine receptor (nAChR) subunit expression was detected by real-time PCR. The production of LDLR was demonstrated by immunofluorescence staining. nAChR-mediated LDLR induction was examined by pre-incubation of the cells with its specific inhibitor, α-bungarotoxin (α-BTX). The functional importance of transcription factor specific protein 1 (Sp1) was examined by luciferase assay, mithramycin pre-incubation or by small interfering RNA (siRNA) transfection. The specific binding of Sp1 to R3 region of LDLR 5’-untranslated region was demonstrated with electrophoretic mobility shift assay (EMSA) and streptavidin-agarose precipitation assay followed by western blotting. The results confirmed that nicotine induced LDLR expression at the transcriptional level. Nicotine was sensed by nAChR and the signal was transduced by Sp1 which bound to the R3 region of LDLR gene. Augmented production of LDLR in the gingival epithelial cells was further demonstrated by immunofluorescence staining using the gingival tissues obtained from the smoking patients.

Conclusions

Taken together, the results suggested that nicotine might contribute to the development of both cardiovascular and periodontal diseases by inducing the LDLR in OECs thereby disturbing lipid metabolism.

Neuregulin upregulates microglial α7 nicotinic acetylcholine receptor expression in immortalized cell lines: implications for regulating neuroinflammation

Neuregulin, previously known as ARIA, is a signaling protein involved in cell survival, synaptic plasticity, cell communication and differentiation. Neuregulin has also been described as a potent inducer of acetylcholine receptor transcription in muscle and although both neuregulin and acetylcholine have been individually described to have neuroprotective roles, their relationship in the cholinergic anti-inflammatory pathway of the brain has not been examined. Using three cell lines, BV-2, EOC-20 and RAW 264.7, we investigated the role that neuregulin signaling through the Erb family of tyrosine kinases may play in the anti-inflammatory process mediated by the α7 nicotinic acetylcholine receptors. Here we show that ErbB4 is expressed in all of our cell lines and is phosphorylated upon treatment with neuregulin. Neuregulin treatment further increases the expression of α7 nicotinic acetylcholine receptors in the microglial lines tested. Given the central role of α7 nicotinic acetylcholine receptors in regulating system inflammation we analyzed the expression of several pro-inflammatory cytokines in our system. Using ELISAs for TNF-α and IL-6 we show that treatment with NRG can produce a nearly a 33% decrease in the levels of tumor necrosis factor-α secreted by activated microglia and a nearly 88% decrease in IL-6. Given these results we propose a neuroprotective role for neuregulin wherein it modulates the expression of TNF-α and thus inflammation in the CNS via the upregulation of α7 nicotinic acetylcholine receptor expression in microglia in vitro. We suggest that the disregulation of neuregulin expression may be pivotal in neurological disorders characterized by inflammation.

Promoter methylation and tissue-specific transcription of the α7 nicotinic receptor gene, CHRNA7

The α7 nicotinic acetylcholine receptor is known to regulate a wide variety of developmental and secretory functions in neural and non-neural tissues. The mechanisms that regulate its transcription in these varied tissues are not well understood. Epigenetic processes may play a role in the tissue-specific regulation of mRNA expression from the α7 nicotinic receptor subunit gene, CHRNA7. Promoter methylation was correlated with CHRNA7 mRNA expression in various tissue types and the role of DNA methylation in regulating transcription from the gene was tested by using DNA methyltransferase (DNMT1) inhibitors and methyl donors. CHRNA7 mRNA expression was silenced in SH-EP1 cells and bisulfite sequencing PCR revealed the CHRNA7 proximal promoter was hypermethylated. The proximal promoter was hypomethylated in the cell lines HeLa, SH-SY5Y, and SK-N-BE which express varying levels of CHRNA7 mRNA. Expression of CHRNA7 mRNA was present in SH-EP1 cells after treatment with the methylation inhibitor, 5-aza-2-deoxycytidine (5-Aza-CdR), and increased in SH-EP1 and HeLa cells using another methylation inhibitor, zebularine (ZEB). Transcription from the CHRNA7 promoter in HeLa cells was increased when the methyl donor methionine (MET) was absent from the media. Using methylation-sensitive restriction enzyme analysis (MSRE), there was a strong inverse correlation between CHRNA7 mRNA levels and promoter DNA methylation across several human tissue types. The results support a role for DNA methylation of the proximal promoter in regulation of CHRNA7 transcription.

From smoking to lung cancer: the CHRNA5/A3/B4 connection

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that modulate key physiological processes ranging from neurotransmission to cancer signaling. These receptors are activated by the neurotransmitter, acetylcholine, and the tobacco alkaloid, nicotine. Recently, the gene cluster encoding the alpha3, alpha5 and beta4 nAChR subunits received heightened interest after a succession of linkage analyses and association studies identified multiple single-nucleotide polymorphisms in these genes that are associated with an increased risk for nicotine dependence and lung cancer. It is not clear whether the risk for lung cancer is direct or an effect of nicotine dependence, as evidence for both scenarios exist. In this study, we summarize the body of work implicating nAChRs in the pathogenesis of lung cancer, with special focus on the clustered nAChR subunits and their emerging role in this disease state.

A transcriptional regulatory element critical for CHRNB4 promoter activity in vivo

Genome-wide association studies have underscored the importance of the clustered neuronal nicotinic acetylcholine receptor (nAChR) subunit genes with respect to nicotine dependence as well as lung cancer susceptibility. CHRNB4, which encodes the nAChR β4 subunit, plays a major role in the molecular mechanisms that govern nicotine withdrawal. Thus, elucidating how expression of the β4 gene is regulated is critical for understanding the pathophysiology of nicotine addiction. We previously identified a CA box regulatory element, (5'-CCACCCCT-3') critical for β4 promoter activity in vitro. We further demonstrated that a 2.3-kb fragment of the β4 promoter region containing the 5'-CCACCCCT-3' regulatory element in the β4 gene promoter (CA box) is capable of directing cell-type specific expression of a reporter gene to a myriad of brain regions that endogenously express the β4 gene. To test the hypothesis that the CA box is critical for β4 promoter activity in vivo, transgenic animals expressing a mutant form of the β4 promoter were generated. Reporter gene expression was not detected in any tissue or cell type at embryonic day 18.5 (ED 18.5). Similarly, we observed drastically reduced reporter gene expression at postnatal day 30 (PD30) when compared to wild type (WT) transgenic animals. Finally, we demonstrated that CA box mutation results in decreased interaction of the transcription factor Sp1 with the mutant β4 promoter. Taken together these results demonstrate that the CA box is critical for β4 promoter activity in vivo.

The nicotinic acetylcholine receptor CHRNA5/A3/B4 gene cluster: dual role in nicotine addiction and lung cancer

More than 1 billion people around the world smoke, with 10 million cigarettes sold every minute. Cigarettes contain thousands of harmful chemicals including the psychoactive compound, nicotine. Nicotine addiction is initiated by the binding of nicotine to nicotinic acetylcholine receptors, ligand-gated cation channels activated by the endogenous neurotransmitter, acetylcholine. These receptors serve as prototypes for all ligand-gated ion channels and have been extensively studied in an attempt to elucidate their role in nicotine addiction. Many of these studies have focused on heteromeric nicotinic acetylcholine receptors containing α4 and β2 subunits and homomeric nicotinic acetylcholine receptors containing the α7 subunit, two of the most abundant subtypes expressed in the brain. Recently however, a series of linkage analyses, candidate-gene analyses and genome-wide association studies have brought attention to three other members of the nicotinic acetylcholine receptor family: the α5, α3 and β4 subunits. The genes encoding these subunits lie in a genomic cluster that contains variants associated with increased risk for several diseases including nicotine dependence and lung cancer. The underlying mechanisms for these associations have not yet been elucidated but decades of research on the nicotinic receptor gene family as well as emerging data provide insight on how these receptors may function in pathological states. Here, we review this body of work, focusing on the clustered nicotinic acetylcholine receptor genes and evaluating their role in nicotine addiction and lung cancer.

Receptor-mediated tobacco toxicity: acceleration of sequential expression of alpha5 and alpha7 nicotinic receptor subunits in oral keratinocytes exposed to cigarette smoke

Tobacco products and nicotine alter the cell cycle and lead to squamatization of oral keratinocytes (KCs) and squamous cell carcinoma. Activation of nicotinic acetylcholine receptors (nAChRs) elicits Ca(2+) influx that varies in magnitude between different nAChR subtypes. Normal differentiation of KCs is associated with sequential expression of the nAChR subtypes with increasing Ca(2+) permeability, such as alpha5-containing alpha3 nAChR and alpha7 nAChR. Exposure to environmental tobacco smoke (ETS) or an equivalent concentration of nicotine accelerated by severalfold the alpha5 and alpha7 expression in KCs, which could be abolished by mecamylamine and alpha-bungarotoxin with different efficacies, suggesting the following sequence of autoregulation of the expression of nAChR subtypes: alpha3(beta2/beta4) > alpha3(beta2/beta4)alpha5 > alpha7 > alpha7. This conjecture was corroborated by results of quantitative assays of subunit mRNA and protein levels, using nAChR-specific pharmacologic antagonists and small interfering RNAs. The genomic effects of ETS and nicotine involved the transcription factor GATA-2 that showed a multifold increase in quantity and activity in exposed KCs. Using protein kinase inhibitors and dominant negative and constitutively active constructs, we characterized the principal signaling cascades mediating a switch in the nAChR subtype. Cumulative results indicated that the alpha3(beta2/beta4) to alpha3(beta2/beta4)alpha5 nAChR transition predominantly involved protein kinase C, alpha3(beta2/beta4)alpha5 to alpha7 nAChR transition-Ca(2+)/calmodulin-dependent protein kinase II and p38 MAPK, and alpha7 self-up-regulation-the p38 MAPK/Akt pathway, and JAK-2. These results provide a mechanistic insight into the genomic effects of ETS and nicotine on KCs and characterize signaling pathways mediating autoregulation of stepwise overexpression of nAChR subtypes with increasing Ca(2+) permeability in exposed cells. These observations have salient clinical implications, because a switch in the nAChR subunit composition can bring about a corresponding switch in receptor function, leading to profound pathobiologic effects observed in KCs exposed to tobacco products.

alpha7 nicotinic receptor gene promoter polymorphisms in inbred mice affect expression in a cell type-specific fashion

Inbred mouse strains display significant differences in their levels of brain alpha7 nicotinic acetylcholine receptor (alpha7 nAChR) expression, as measured by binding of the alpha7-selective antagonist alpha-bungarotoxin. Variations in alpha-bungarotoxin binding have been shown to correlate with an animal's sensitivity to nicotine-induced seizures and sensory gating. In two inbred mouse strains, C3H/2Ibg (C3H) and DBA/2Ibg (DBA/2), the inter-strain binding differences are linked to a restriction length polymorphism in the alpha7 nAChR gene, Chrna7. Despite this finding, the molecular mechanism(s) through which genetic variability in Chrna7 may contribute to alpha7 nAChR expression differences remains unknown. However, studies of the human alpha7 nAChR gene (CHRNA7) previously have demonstrated that CHRNA7 promoter polymorphisms are associated with differences in promoter activity as well as differences in sensory processing. In the present study, a 947-base pair region of the Chrna7 promoter was cloned from both the C3H and DBA/2 inbred mouse strains in an attempt to identify polymorphisms that may underlie alpha7 nAChR differential expression. Sequence analysis of these fragments identified 14 single nucleotide polymorphisms (SNPs). A combination of two of these SNPs affects promoter activity in an in vitro luciferase reporter assay. These results suggest a mechanism through which the Chrna7 promoter genotype may influence interstrain variations in alpha7 nAChR expression.

Cell specificity of a rat neuronal nicotinic acetylcholine receptor α7 subunit gene promoter

Neuronal nAChRs are pentameric transmembrane proteins which function as ligand-gated ion channels and are composed of multiple alpha and beta subunits. Nine neuronal nAChR alpha subunit genes (alpha2-alpha10) and three nAChR beta subunit genes (beta2-beta4) have been identified. nAChR subtypes are heteromers, composed of various combinations of nAChR subunits or homomers composed of alpha7, alpha8, or alpha9 subunits. nAChR subtypes are widely expressed in the nervous system, yet each subunit has a distinct and unique pattern of expression. This report focuses on the expression of the nAChR alpha7 gene since homomeric nAChRs can be formed from this one subunit, simplifying a study of the expression of a specific nAChR subtype. Alpha7 nAChRs are involved in several important biological activities in addition to synaptic transmission including mediating neurite outgrowth, neuronal development and cell death, and in presynaptic control of neurotransmitter release. Transcriptional regulation of alpha7 gene expression may be important to control the location and timing of these events. We previously isolated a rat alpha7 nAChR promoter and studied expression in PC12 cells. In this study we examined the expression of the alpha7 promoter in PC12, HEK293, L6, SN17 and Neuro-2A cells in order to define elements necessary for cell-specific expression. Elements promoting expression of alpha7 in muscle and fibroblasts were identified. We also demonstrated that several other nAChR genes are also expressed in SN 17 and Neuro-2A cells, supporting use of these cell lines as models to study transcriptional control of nAChR genes.

Dual Role of the RIC-3 Protein in Trafficking of Serotonin and Nicotinic Acetylcholine Receptors

The ric-3 gene is required for maturation of nicotinic acetylcholine receptors in Caenorhabditis elegans. The human homolog of RIC-3, hRIC-3, enhances expression of alpha7 nicotinic receptors in Xenopus laevis oocytes, whereas it totally abolishes expression of alpha4beta2 nicotinic and 5-HT3 serotonergic receptors. Both the N-terminal region of hRIC-3, which contains two transmembrane segments, and the C-terminal region are needed for these differential effects. hRIC-3 inhibits receptor expression by hindering export of mature receptors to the cell membrane. By using chimeric proteins made of alpha7 and 5-HT3 receptors, we have shown that the presence of an extracellular isoleucine close to the first transmembrane receptor fragment is responsible for the transport arrest induced by hRIC-3. Enhancement of alpha7 receptor expression occurs, at least, at two levels: by increasing the number of mature receptors and facilitating its transport to the membrane. Certain amino acids of a putative amphipathic helix present at the large cytoplasmic region of the alpha7 subunit are required for these actions. Therefore, hRIC-3 can act as a specific regulator of receptor expression at different levels.

Glucocorticoid activation of the neuronal nicotinic acetylcholine receptor alpha7 subunit gene: involvement of transcription factor Egr-1

The alpha7 subunit is a component of nicotinic acetylcholine receptors expressed in bovine chromaffin cells. The peculiar localization of these receptors at adrenomedullary areas adjacent to the adrenal cortex suggests that factors, probably glucocorticoids, arising from the cortex might diffuse and regulate alpha7 receptor expression. In reporter gene transfection experiments, dexamethasone increased alpha7 promoter activity by up to fivefold in a concentration- and time-dependent manner despite the absence of consensus glucocorticoid receptor elements at the alpha7 promoter. Transcriptional activation induced by glucocorticoids was abolished through simultaneous mutation of at least two of the three sites for the immediate early transcription factor Egr-1, present in the proximal promoter region of the alpha7 subunit gene. Therefore, glucocorticoids activate the alpha7 subunit gene through Egr-1 in an indirect way.

Transcriptional regulation by activation and repression elements located at the 5'-noncoding region of the human alpha9 nicotinic receptor subunit gene

The alpha9 subunit is a component of the neuronal nicotinic acetylcholine receptor gene superfamily that is expressed in very restricted locations. The promoter of the human gene has been analyzed in the human neuroblastoma SH-SY5Y, where alpha9 subunit expression was detected, and in C2C12 cells that do not express alpha9. A proximal promoter region (from -322 to +113) showed maximal transcriptional activity in SH-SY5Y cells, whereas its activity in C1C12 cells was much lower. Two elements unusually located at the 5'-noncoding region exhibited opposite roles. A negative element located between +15 and +48 appears to be cell-specific because it was effective in C2C12 but not in SH-SY5Y cells, where it was counterbalanced by the presence of the promoter region 5' to the initiation site. An activating element located between +66 and +79 and formed by two adjacent Sox boxes increased the activity of the alpha9 promoter about 4-fold and was even able to activate other promoters. This element interacts with Sox proteins, probably through a cooperative mechanism in which the two Sox boxes are necessary. We propose that the Sox complex provides an initial scaffold that facilitates the recruiting of the transcriptional machinery responsible for alpha9 subunit expression.

Mouse muscle denervation increases expression of an alpha7 nicotinic receptor with unusual pharmacology

Neuronal nicotinic alpha7 subunits have been found in chick and rat skeletal muscle during development and denervation. In the present study, reverse transcriptase-polymerase chain reaction was used to detect alpha7 subunit mRNA in denervated mouse muscle. To determine whether the alpha7 subunit forms functional nicotinic acetylcholine receptors (nAChRs) in muscle, choline was used to induce a membrane depolarization because choline has been considered a specific agonist of alpha7-containing (alpha7*) nAChRs. We found, however, that choline (3-10 mM) also weakly activates muscle nAChRs. After inhibiting muscle nAChRs with a specific muscle nAChR inhibitor, alpha-conotoxin GI (alphaCTxGI), choline was used to activate the alpha7* nAChRs on muscle selectively. Four weeks after denervation, rapid application of choline (10 mM) elicited a substantial depolarization in the presence of alphaCTxGI (0.1 microM). This component of the depolarization was never present in denervated muscles obtained from mutant mice lacking the alpha7 subunit (i.e. alpha7-null mice). The depolarization component that is resistant to alphaCTxGI was antagonized by pancuronium (3-10 microM) and by a 4-oxystilbene derivative (F3, 0.1-0.5 microM) at concentrations considered highly specific for alpha7* nAChRs. Another selective alpha7 antagonist, methyllycaconitine (0.05-5 microM), did not strongly inhibit this choline-induced depolarization. Furthermore, the choline-sensitive nAChRs showed little desensitization over 10 s of application with choline (10-30 mM). These results indicate that functional alpha7* nAChRs are significantly present on denervated muscle, and that these receptors display unusual functional and pharmacological characteristics.