Transcriptional regulation of neuronal nicotinic acetylcholine receptor genes. Functional interactions between Sp1 and the rat beta4 subunit gene promoter

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.

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.

ASCL1 regulates the expression of the CHRNA5/A3/B4 lung cancer susceptibility locus

Tobacco contains a variety of carcinogens as well as the addictive compound nicotine. Nicotine addiction begins with the binding of nicotine to its cognate receptor, the nicotinic acetylcholine receptor (nAChR). Genome-wide association studies have implicated the nAChR gene cluster, CHRNA5/A3/B4, in nicotine addiction and lung cancer susceptibility. To further delineate the role of this gene cluster in lung cancer, we examined the expression levels of these three genes as well as other members of the nAChR gene family in lung cancer cell lines and patient samples using quantitative reverse transcription-PCR. Overexpression of the clustered nAChR genes was observed in small-cell lung carcinoma (SCLC), an aggressive form of lung cancer highly associated with cigarette smoking. The overexpression of the genomically clustered genes in SCLC suggests their coordinate regulation. In silico analysis of the promoter regions of these genes revealed putative binding sites in all three promoters for achaete-scute complex homolog 1 (ASCL1), a transcription factor implicated in the pathogenesis of SCLC, raising the possibility that this factor may regulate the expression of the clustered nAChR genes. Consistent with this idea, knockdown of ASCL1 in SCLC, but not in non-SCLC, led to a significant decrease in expression of the alpha 3 and beta 4 genes without having an effect on any other highly expressed nAChR gene. Our data indicate a specific role for ASCL1 in regulating the expression of the CHRNA5/A3/B4 lung cancer susceptibility locus. This regulation may contribute to the predicted role that ASCL1 plays in SCLC tumorigenesis.

Temporally- and spatially-regulated transcriptional activity of the nicotinic acetylcholine receptor beta4 subunit gene promoter

Signaling through nicotinic acetylcholine (nACh) receptors underlies a diverse array of behaviors. In order for appropriate signaling to occur via nACh receptors, it is necessary for the genes encoding the receptor subunits to be expressed in a highly regulated temporal and spatial manner. Here we report a transgenic mouse approach to characterize the transcriptional regulation of the gene encoding the nACh receptor beta4 subunit. nACh receptors containing this subunit play critical roles in both the central and peripheral nervous systems. We demonstrate that a 2.3-kilobase pair fragment of the beta4 5'-flanking region is capable of directing reporter gene expression in transgenic animals. Importantly, the transcriptional activity of the promoter region is cell-type-specific and developmentally regulated and overlaps to a great extent with endogenous beta4 mRNA expression. These data indicate that the 2.3-kilobase pair fragment contains transcriptional regulatory elements critical for appropriate beta4 subunit gene expression.

The armadillo repeat-containing protein, ARMCX3, physically and functionally interacts with the developmental regulatory factor Sox10

Sox10 is a member of the group E Sox transcription factor family and plays key roles in neural crest development and subsequent cellular differentiation. Sox10 binds to regulatory sequences in target genes via its conserved high mobility group domain. In most cases, Sox10 exerts its transcriptional effects in concert with other DNA-binding factors, adaptor proteins, and nuclear import proteins. These interactions can lead to synergistic gene activation and can be cell type-specific. In earlier work, we demonstrated that Sox10 transactivates the nicotinic acetylcholine receptor alpha3 and beta4 subunit genes and does so only in neuronal-like cell lines, raising the possibility that Sox10 mediates its effects via interactions with co-regulatory factors. Here we describe the identification of the armadillo repeat-containing protein, ARMCX3, as a Sox10-interacting protein. Biochemical analyses indicate that ARMCX3 is an integral membrane protein of the mitochondrial outer membrane. Others have shown that Sox10 is a nucleocytoplasmic shuttling protein. We extend this observation and demonstrate that, in the cytoplasm, Sox10 is peripherally associated with the mitochondrial outer membrane. Both Sox10 and ARMCX3 are expressed in mouse brain and spinal cord as well as several cell lines. Overexpression of ARMCX3 increased the amount of mitochondrially associated Sox10. In addition, although ARMCX3 does not possess intrinsic transcriptional activity, it does enhance transactivation of the nicotinic acetylcholine receptor alpha3 and beta4 subunit gene promoters by Sox10. These results suggest that Sox10 is a membrane-associated factor whose transcriptional function is increased by direct interactions with ARMCX3 and raise the possibility of a signal transduction cascade between the nucleus and mitochondria through Sox10/ARMCX3 interactions.

Transcription factor assembly on the nicotinic receptor beta4 subunit gene promoter

Nicotinic acetylcholine receptors are involved in a plethora of fundamental biological processes ranging from muscle contraction to formation of memories. The receptors are pentameric proteins whose subunits are encoded by distinct genes. Subunit composition of a mature nicotinic receptor is governed in part by the transcriptional regulation of each subunit gene. Here, using chromatin immunoprecipitation assays, we report the interaction of the transcription factors Sp1, Sp3, c-Jun and Sox10 with the beta4 subunit gene promoter in neuronal-like cell lines and rodent brain tissue. Our results corroborate previous in-vitro data demonstrating that these transcription factors interact with the beta4 promoter. Taken together, these data suggest that Sp1, Sp3, c-Jun and Sox10 regulate expression of the beta4 subunit gene in the mammalian brain.

Transcriptional Repression by a Conserved Intronic Sequence in the Nicotinic Receptor α3 Subunit Gene

The genes encoding the nicotinic acetylcholine receptor alpha3, alpha5, and beta4 subunits are genomically clustered. These genes are co-expressed in a variety of cells in the peripheral and central nervous systems. Their gene products assemble in a number of stoichiometries to generate several nicotinic receptor subtypes that have distinct pharmacological and physiological properties. Signaling through these receptors is critical for a variety of fundamental biological processes. Despite their importance, the transcriptional mechanisms underlying their coordinated expression remain to be completely elucidated. By using a bioinformatics approach, we identified a highly conserved intronic sequence within the fifth intron of the alpha3 subunit gene. Reporter gene analysis demonstrated that this sequence, termed "alpha3 intron 5," inhibits the transcriptional activities of the alpha3 and beta4 subunit gene promoters. This repressive activity is position- and orientation-independent. Importantly, repression occurs in a cell type-specific manner, being present in cells that do not express the receptor genes or expresses them at very low levels. Electrophoretic mobility shift assays demonstrate that nuclear proteins specifically interact with alpha3 intron 5 at two distinct sites. We propose that this intronic repressor element is important for the restricted expression patterns of the nicotinic receptor alpha3 and beta4 subunit genes.

Shared long-range regulatory elements coordinate expression of a gene cluster encoding nicotinic receptor heteromeric subtypes

The nicotinic acetylcholine receptor (nAChR) beta4/alpha3/alpha5 gene cluster encodes several heteromeric transmitter receptor subtypes that are essential for cholinergic synaptic transmission in adrenal gland, autonomic ganglia, pineal gland, and several nuclei in the central nervous system. However, the transcriptional mechanisms coordinating expression of these subunit genes in different cell populations are unknown. Here, we used transgenic methods to investigate long-range transcriptional control of the cluster. A 132-kb P1-derived artificial chromosome (PAC) encoding the rat cluster recapitulated the neurally- and endocrine-restricted expression patterns of the endogenous beta4/alpha3/alpha5 genes. Mutation of ETS factor binding sites in an enhancer, beta43', embedded in the beta4 3'-untranslated exon resulted in greatly diminished beta4, alpha3, and alpha5 expression in adrenal gland and to a lesser extent in the superior cervical ganglion (SCG) but not in other tissues. Phylogenetic sequence analyses revealed several conserved noncoding regions (CNRs) upstream of beta4 and alpha5. Deletion of one of them (CNR4) located 20 kb upstream of beta4 resulted in a dramatic decrease in beta4 and alpha3 expression in the pineal gland and SCG. CNR4 was sufficient to direct LacZ transgene expression to SCG neurons, which express the endogenous beta4alpha3alpha5 subunits, and pineal cells, which express the endogenous beta4alpha3 combination. Finally, CNR4 was able to direct transgene expression to major sites of expression of the endogenous cluster in the brain. Together, our findings support a model in which cell type-specific shared long-range regulatory elements are required for coordinate expression of clustered nAChR genes.

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.

Regulation of the estrogen receptor alpha minimal promoter by Sp1, USF-1 and ERalpha

The exact molecular mechanisms regulating estrogen receptor alpha (ERalpha) expression in breast tumors are unclear, but studies suggest that they are partly at the level of transcription. We have focused on the transcription factors that regulate the ERalpha minimal promoter, which we have previously shown to reside within the first 245 bp of the 5'-flanking region of the gene. Within this region are several elements essential for full ERalpha promoter transcriptional activity, including a GC box and an imperfect E box. In earlier studies we demonstrated an essential function for the Sp1 family of transcription factors in the regulation of ERalpha expression. We have now identified both USF-1 and ERalpha itself as components of a multi-protein complex of transcription factors that interacts at the ERalpha minimal promoter and is essential for its full transcriptional activity. Electrophoretic mobility shift assays demonstrated that Sp1 and USF-1, but not ERalpha, bind directly to the ERalpha minimal promoter. We showed by GST pull-down assays that ERalpha is able to interact in vitro with USF-1, suggesting, in addition to a possible interaction between ERalpha and Sp1, a mechanism whereby ERalpha is able to interact with the protein complex. Combined exogenous expression of the components of the complex in MCF-7 breast cancer cells resulted in a synergistic effect on transactivation of the ERalpha minimal promoter, suggesting that the importance of the protein complex is in the interactions among the components. Based upon these findings, we propose a possible model for transcription from the ERalpha minimal promoter.

Sp1 is essential for estrogen receptor alpha gene transcription

The exact molecular mechanisms regulating estrogen receptor (ER)alpha expression in breast tumors are unclear, but studies suggest that the regulation is at least partly transcriptional. We therefore undertook a detailed analysis of ERalpha promoter activity in a number of breast cancer cell lines. We find that the majority of ERalpha promoter activity lies within the first 245bp of the 5'-flanking region of the gene. Three elements essential for full ERalpha promoter transcriptional activity were identified within the -245 to -192bp region in transient transactivation assays using linker-scanner mutation analysis. These three elements include two binding sites for the Sp1 family of transcription factors as well as a non-consensus E box. We show that both Sp1 and Sp3 bind to this region using electrophoretic mobility shift assays. Exogenous expression of Sp1 or Sp3 in Sp1/3-negative Drosophila Schneider SL2 cells results in transactivation of the -245 to +212bp fragment of the ERalpha promoter. These data demonstrate that transcription of ERalpha is dependent upon the expression of members of the Sp1 family.

Transcription factors NF-Y and Sp1 are important determinants of the promoter activity of the bovine and human neuronal nicotinic receptor beta 4 subunit genes

The beta4 subunit is a component of the neuronal nicotinic acetylcholine receptors which control catecholamine secretion in bovine adrenomedullary chromaffin cells. The promoter of the gene coding for this subunit was characterized. A proximal region (from minus sign99 to minus sign64) was responsible for the transcriptional activity observed in chromaffin, C2C12, and COS cells. Within this region two cis-acting elements that bind transcription factors Sp1 and NF-Y were identified. Mutagenesis of the two elements indicated that they cooperate for the basal transcription activity of the promoter. The human beta4 promoter, that was also characterized, shared structural and functional homologies with the bovine promoter. Thus, two adjacent binding elements for Sp1 and NF-Y were detected. Whereas the Sp1 site was an important determinant of the promoter activity, the NF-Y site may have cell-specific effects. Given that these promoters showed no structural or functional homology with the previously characterized rat beta4 subunit promoter (Bigger, C. B., Casanova, E. A., and Gardner, P. D. (1996) J. Biol. Chem. 271, 32842--32848) except for the involvement of an Sp1 binding element, we propose that constitutive expression of the beta4 subunit gene in these three close species may be controlled by the general transcription factor Sp1. Nevertheless, other components could determine species-specific beta4 subunit expression.

Retinal neuron activity of ETS domain-binding sites in a nicotinic acetylcholine receptor gene cluster enhancer

Nicotinic acetylcholine receptors (nAchRs) mediate amacrine to ganglion cell synaptic transmission in the developing mammalian retina. The clustered neuronal nAchRs subunit genes, alpha 3 and beta 4, are expressed in amacrine and ganglion cells where they are used to assemble functional receptor subtypes. The transcriptional mechanisms underlying expression of these subunits in retina are not yet known but may involve enhancers that are selectively active in retinal neurons. We previously identified a neuron-selective enhancer, beta 43', whose activity in neural cell lines is dependent on ETS domain-binding sites. To determine whether beta 43' is active in retinal neurons that express the alpha 3 and beta 4 genes, we investigated beta 43' activity in primary dissociated rat retinal cultures. We found that beta 43' is selectively active in retinal neurons compared with retinal non-neuronal cells. This activity was derived primarily from amacrine and ganglion neurons, which are the retinal neuron cell types that express the clustered genes. Moreover, beta 43' was selectively active in retinal neurons compared with cerebral cortical neurons suggesting that it is not a pan-neuronal enhancer. ETS factor-binding sites in the enhancer are required for its retinal neuron activity. These findings suggest that ETS factor interactions with beta 43' control retinal neuron expression of certain nAchR subtypes.

Interleukin-6- and cyclic AMP-mediated signaling potentiates neuroendocrine differentiation of LNCaP prostate tumor cells

Neuroendocrine (NE) differentiation in prostatic adenocarcinomas has been reported to be an early marker for development of androgen independence. Secretion of mitogenic peptides from nondividing NE cells is thought to contribute to a more aggressive disease by promoting the proliferation of surrounding tumor cells. We undertook studies to determine whether the prostate cancer cell line LNCaP could be induced to acquire NE characteristics by treatment with agents that are found in the complex environment in which progression of prostate cancer towards androgen independence occurs. We found that cotreatment of LNCaP cells with agents that signal through cyclic AMP-dependent protein kinase (PKA), such as epinephrine and forskolin, and with the cytokine interleukin-6 (IL-6) promoted the acquisition of an NE morphological phenotype above that seen with single agents. Convergent IL-6 and PKA signaling also resulted in potentiated mitogen-activated protein kinase (MAPK) activation without affecting the level of signal transducer and activator of transcription or PKA activation observed with these agents alone. Cotreatment with epinephrine and IL-6 synergistically increased c-fos transcription as well as transcription from the beta4 nicotinic acetylcholine receptor subunit promoter. Potentiated transcription from these elements was shown to be dependent on the MAPK pathway. Most importantly, cotreatment with PKA activators and IL-6 resulted in increased secretion of mitogenic neuropeptides. These results indicate that PKA and IL-6 signaling participates in gene transcriptional changes that reflect acquisition of an NE phenotype by LNCaP cells and suggest that similar signaling mechanisms, particularly at sites of metastasis, may be responsible for the increased NE content of many advanced prostate carcinomas.

The signal transduction pathway underlying ion channel gene regulation by SP1-C-Jun interactions

During neuronal differentiation, an exquisitely controlled program of signal transduction events takes place, leading to the temporally and spatially regulated expression of genes associated with the differentiated phenotype. A critical class of genes involved in this phenomenon is that made up of genes encoding neurotransmitter-gated ion channels that play a central role in signal generation and propagation within the nervous system. We used the well established PC12 cell line to investigate the molecular details underlying the expression of the neuronal nicotinic acetylcholine receptor class of ion channels. Neuronal differentiation of PC12 cells can be induced by nerve growth factor, leading to an increase in neuronal nicotinic acetylcholine receptor gene expression. Nerve growth factor initiates several signal transduction cascades. Here, we show that the Ras-dependent mitogen-activated protein kinase and phosphoinositide 3-kinase pathways are critical for the nerve growth factor-mediated increase in the transcriptional activity of a neuronal nicotinic acetylcholine receptor gene promoter. In addition, we show that a component of the Ras-dependent mitogen-activated protein kinase pathway, nerve growth factor-inducible c-Jun, exerts its effects on receptor gene promoter activity most likely through protein-protein interactions with Sp1. Finally, we demonstrate that the target for nerve growth factor signaling is an Sp1-binding site within the neuronal nicotinic acetylcholine receptor gene promoter.

Characterization of a rat neuronal nicotinic acetylcholine receptor alpha7 promoter

Neuronal nicotinic acetylcholine receptors (nAChRs) containing the alpha7 subunit are expressed in the central nervous system, autonomic nervous system, retina, adrenal medulla, and PC12 cells. alpha7 nAChRs have been implicated in several important biological activities apart from synaptic transmission such as mediating neurite growth and presynaptic control of neurotransmitter release. A 178-base pair promoter was sufficient to drive high level expression of the alpha7 gene in PC12 cells. The alpha7 promoter was also cell-specific, expressing in PC12 cells but not in L6 rat muscle cells. Within our minimal rat alpha7 nAChR promoter we identified two sequences important for basal level expression. Mutation of a GC-rich sequence at -172 relative to the translational start site led to an increase in activity of the promoter, indicating the presence of a negative regulatory element. Upstream stimulatory factor-1 acted to regulate alpha7 expression positively by binding to an E-box at -116. A site directly adjacent to the upstream stimulatory factor-1 binding site was shown to bind Egr-1. Sp1 and Sp3 binding also occurred downstream from or overlapping the Egr-1 binding site in the rat alpha7 promoter. Several transcription factors interact in close proximity to control expression of the rat alpha7 nicotinic receptor gene.

Sp1 and Sp3 transactivate the RET proto-oncogene promoter

The RET proto-oncogene plays an important role in the initiation and progression of tumors derived from the neural crest. The cis-regulatory elements responsible for RET basal promoter activity have not been identified. To characterize these elements, a RET promoter DNA fragment (-453 to +227bp) was fused to a luciferase reporter and introduced into TT, a neural crest-derived cell line. Sequential 5' deletions of the promoter revealed that optimal expression of the RET promoter in TT cells required only 70bp of sequence upstream of the transcription start site, and contains two Sp1 binding sites. DNase I footprinting, electrophoretic mobility shift analysis (EMSA), and supershift assays revealed that this region binds both Sp1 and its related protein, Sp3. Additionally, RET basal promoter activity was abrogated by removal of these Sp1/Sp3 binding sites. The proximal two GC boxes were sufficient to allow transactivation of the RET promoter in Drosophila SL2 cells. Sp3 expression in these cells caused an additional activation of the promoter. These results demonstrate that the transactivation of the RET promoter within a neural crest-derived cell line is dependent on Sp1 and Sp3.

Synergistic transcriptional activation by Sox10 and Sp1 family members

Neuronal nicotinic acetylcholine receptors (nAChR) are expressed at specific times during development and in discrete neuronal populations. Transcriptional regulation of the receptor genes clearly plays a key role in the molecular pathway underlying the expression of these critical synaptic components. In an effort to understand this regulation, we focus upon the genes encoding three receptor subunits: alpha3, alpha5 and beta4. These subunits are genomically clustered and constitute the predominant nAChR subtype expressed in the peripheral nervous system. We and others demonstrated that the general transcription factors, Sp1 and Sp3, can transactivate the promoter of each subunit gene. Further, we showed that the regulatory factor Sox10 transactivates the alpha3 and beta4 promoters and does so in a cell-type-specific manner. Interestingly, the Sp- and Sox10-binding sites on the beta4 promoter are located immediately adjacent to each other, raising the possibility that the two sets of factors functionally interact to regulate receptor gene expression. Consistent with this hypothesis, we demonstrated that the proteins can directly interact. Here, we extend these observations and show that Sox10 and the Sp factors functionally interact, leading to synergistic transcriptional activation in a cholinergic cell line. Finally, evidence for the existence of cell-type-specific co-regulators for Sp1 and Sox10 is presented.

Puralpha: a multifunctional single-stranded DNA- and RNA-binding protein

Puralpha is a ubiquitous, sequence-specific DNA- and RNA-binding protein which is highly conserved in eukaryotic cells. Puralpha has been implicated in diverse cellular functions, including transcriptional activation and repression, translation and cell growth. Moreover, this protein has been shown to be involved in regulating several human viruses which replicate in the central nervous system (CNS), including human immunodeficiency virus type I (HIV-1) and JC virus (JCV). Puralpha exerts part of its activity by interacting with cellular proteins, including pRb, E2F, cyclin A, Sp1 and members of the Y-box family of proteins, including YB-1 and MSY1, as well as viral proteins such as polyomavirus large T-antigen and HIV-1 Tat. The ability of Puralpha to interact with its target DNA sequence and to associate with several viral and cellular proteins is modulated by RNA. Puralpha has also been shown to be involved in cell growth and proliferation. Its association with pRb, E2F and cyclin A coupled with its fluctuating levels throughout the cell cycle, position Puralpha as a crucial factor in the cell cycle. Moreover, microinjection studies demonstrate that Puralpha causes either a G(1) or G(2) arrest depending on the cell cycle time of injection. The gene encoding Puralpha has been localized to a human locus which is frequently deleted in myelogenous leukemias and other cancers and Puralpha gene deletions have been detected in many cases of lymphoid cancers. The following review details the structural characteristics of Puralpha, its family members and the involvement of this protein in regulating various cellular and viral genes, viral replication and cell growth.

Regulation of alpha3 nicotinic acetylcholine receptor subunit mRNA levels by nerve growth factor and cyclic AMP in PC12 cells

To investigate the effects of nerve growth factor (NGF) and cyclic AMP (cAMP) on the level of the nicotinic acetylcholine receptor subunit alpha3 mRNA, we used PC12h cells, PC12 cells expressing dominant-negative Ras protein, and the parental PC12 cells. PC12h cells have NGF-responsive tyrosine hydroxylase activity. Expression of dominant-negative Ras protein prevents the signaling through the Ras-mitogen-activated protein kinase cascade. The morphological changes of the parental PC12 cells in response to NGF and 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPTcAMP), a cell-penetrating cAMP analogue, were similar to those of PC12h cells. NGF up-regulated the alpha3 mRNA level in PC12h cells and down-regulated the alpha3 mRNA level in the parental PC12 cells. Expression of dominant-negative Ras protein and an inhibitor of mitogen-activated protein kinase kinase inhibited the effects of NGF on alpha3 mRNA level. CPTcAMP down-regulated the alpha3 mRNA level in all three PC12 cell lines. An inhibitor of protein kinase A inhibited the CPTcAMP-induced down-regulation of alpha3 mRNA. The alpha3 mRNA down-regulation required prolonged treatment with CPTcAMP even after cAMP response element binding protein phosphorylation was decreased. Membrane depolarization with high K+ had no effect on the alpha3 mRNA level in PC12h cells. Based on these results, we propose that at least two unknown effectors regulate alpha3 mRNA levels in PC12 cells.

Interactions between regulatory proteins that bind to the nicotinic receptor beta4 subunit gene promoter

The genes encoding the alpha3, alpha5 and beta4 subunits of nicotinic acetylcholine receptors are tightly clustered within the genome. As these three subunits constitute the predominant acetylcholine receptor subtype expressed in the peripheral nervous system, their genomic proximity suggests a regulatory mechanism ensuring their coordinate expression. We previously identified two transcriptional regulatory elements within the beta4 promoter. One of these elements, a CT box, interacts with the regulatory factors heterogeneous nuclear ribonucleoprotein K and Puralpha. Another element, a CA box, interacts with Sp1 and Sp3. The binding site for a fifth factor, Sox10, overlaps the CT and CA boxes. As the CT and CA boxes are adjacent, we postulated that the proteins that bind to the elements interact. Here we report that the CT box-binding factors interact with each other as do the CA box-binding factors. However, there are no direct associations between the two pairs of proteins. Interestingly though, Sox10 directly interacts with all four proteins, suggesting a central role in beta4 gene expression for this member of the Sox family of regulatory factors.

Cell type-specific activation of neuronal nicotinic acetylcholine receptor subunit genes by Sox10

The regulatory factor Sox10 is expressed in neural crest derivatives during development as well as in the adult CNS and peripheral nervous system. Mutations of the human Sox10 gene have been identified in patients with Waardenburg-Hirschsprung syndrome that is characterized by defects in neural crest development. Previous studies suggested that Sox10 might function as an important transcriptional regulator of neural crest development. No natural target genes of Sox10 have yet been identified. Although human Sox10 activates a synthetic promoter consisting of a TATA box and multiple Sox consensus sequences, no transcriptional activity of the rat Sox10 homolog has been detected. Here we report that the neuronal nicotinic acetylcholine receptor beta4 and alpha3 subunit gene promoters are transactivated by rat Sox10 in a cell type-specific manner. The alpha3 and beta4 subunits, in combination with the alpha5 subunit, make up the predominant nicotinic receptor subtype expressed in the peripheral nervous system. Transfections using Sox10 mutants indicate that the C-terminal region is dispensable for its ability to activate the beta4 and alpha3 promoters. Rat Sox10 was originally identified as an accessory protein of the POU domain protein Tst-1/Oct6/SCIP in glial cells. Tst-1/Oct6/SCIP was shown previously to activate the alpha3 promoter. We now demonstrate that it can transactivate the beta4 promoter as well. However, we were unable to detect any synergistic effects of Sox10 and Tst-1/Oct6/SCIP on beta4 or alpha3 promoter activity. Finally, we present data suggesting that recombinant Sox10 protein can directly interact with a previously characterized regulatory region of the beta4 gene.

Transcriptional activation of the murine CTP:phosphocholine cytidylyltransferase gene (Ctpct): combined action of upstream stimulatory and inhibitory cis-acting elements

CTP:phosphocholine cytidylyltransferase plays a key role in regulating the rate of phosphatidylcholine biosynthesis. However, the proximal regulatory elements for the gene (Ctpct) that encode this enzyme and the cognate transcription factors involved have not been characterized. Ctpct promoter activities were deduced from promoter deletion constructs linked to a luciferase reporter and transiently transfected into C3H10T1/2 and McArdle RH7777 cells. Positive regulatory elements were located between -130 and -52 bp from the transcription start site. Basal expression resided downstream between -52 and +38 bp. DNase I protection and electromobility-shift assays indicated that Sp1-related nuclear factors bind to a stimulatory, a possible inhibitory and minimal promoter element. Gel-shift assays confirmed that all three regulatory regions bound Sp1. Sp1 was further implicated when Sp1-deficient Drosophila cells were co-transfected with promoter-reporter constructs and an Sp1 construct. DNase I assays also indicated that the Ap1 binding elements could be occupied in the proximal activator and minimal promoter regions. Gel-shift assays demonstrated that the distal activator region could bind Ap1 and an unknown transcription factor. We conclude that Sp1, Ap1 and an unknown transcription factor have important roles in regulating expression of the Ctpct gene.

Multiple functional Sp1 domains in the minimal promoter region of the neuronal nicotinic receptor alpha5 subunit gene

The alpha5 subunit is a component of the neuronal nicotinic acetylcholine receptors, which are probably involved in the activation step of the catecholamine secretion process in bovine adrenomedullary chromaffin cells. The promoter of the gene coding for this subunit was isolated, and its proximal region was characterized, revealing several GC boxes located close to the site of transcription initiation (from -111 to -40). Deletion analysis and transient transfections showed that a 266-base pair region (-111 to +155) gave rise to approximately 77 and 100% of the maximal transcriptional activity observed in chromaffin and SHSY-5Y neuroblastoma cells, respectively. Site-directed mutagenesis of five different GC motifs indicated that all of them contribute to the activity of the alpha5 gene, but in a different way, depending on the type of transfected cell. Thus, in SHSY-5Y cells, alteration of the most promoter-proximal of the GC boxes decreased alpha5 promoter activity by approximately 50%, whereas single mutations of the other GC boxes had no effect. In chromaffin cells, by contrast, modification of any of the GC boxes produced a similar decrease in promoter activity (50-69%). In both cell types, however, activity was almost abolished when four GC boxes were suppressed simultaneously. Electrophoretic mobility shift assays using nuclear extracts from either chromaffin or SHSY-5Y cells showed the specific binding of Sp1 protein to fragment -111 to -27. Binding of Sp1 to the GC boxes was also demonstrated by DNase I footprint analysis. This study suggests that the general transcription factor Sp1 plays a dominant role in alpha5 subunit expression, as has also been demonstrated previously for alpha3 and beta4 subunits. Since these three subunits have their genes tightly clustered and are expressed in chromaffin cells, probably as components of the same receptor subtype, we propose that Sp1 constitutes the key factor of a regulatory mechanism common to the three subunits.

Regulation of ion channel expression in neural cells by hormones and growth factors

Voltage-and ligand-gated ion channels are key players in synaptic transmission and neuron-glia communication in the nervous system. Expression of these proteins can be regulated at several levels (transcriptional, translational, or posttranslational) and by multiple extracellular factors in the developing and mature nervous system. A wide variety of hormones and growth factors have been identified as important in neural cell differentiation, which is a complex process involving the acquisition of cell-type-specific ion channel phenotypes. Much literature has already accumulated describing the structural and functional characteristics of ion channels, but relatively little is known about the factors that influence their synthesis and cell surface expression, although this area has generated considerable interest in the context of neural cell development. This article reviews several examples of regulated expression of these channels by cellular factors, namely peptide growth factors and steroid hormones, and discusses, where applicable, current understanding of molecular mechanisms underlying such regulation of voltage-and neurotransmitter-gated ion channels.

GC- and E-box motifs as regulatory elements in the proximal promoter region of the neuronal nicotinic receptor alpha7 subunit gene

The alpha7 subunit is a component of alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors expressed in bovine adrenomedullary chromaffin cells. The proximal promoter of the gene coding for this subunit contains several GC-boxes and one E-box. Deletion analysis and transient transfections showed that a 120-base pair region (-77 to +43) including all of these elements gave rise to approximately 70 and 95% of the maximal transcriptional activity observed in chromaffin and SHSY-5Y neuroblastoma cells, respectively. Site-directed mutagenesis of the different elements indicated that both GC and E motifs contribute to the activity of the alpha7 gene in a very prominent way. Using electrophoretic mobility shift assays, the upstream stimulatory factor (USF) was shown to be a component of the complexes that interacted with the E-box when nuclear extracts from chromaffin and SHSY-5Y cells were used. Binding of the early growth response gene transcription factor (Egr-1) to three different GC-boxes was also demonstrated by shift assays and DNase I footprint analysis. Likewise, alpha7 promoter activity increased by up to 5-fold when alpha7 constructs and an Egr-1 expression vector were cotransfected into chromaffin cell cultures. Mutagenesis of individual GC-boxes had little effect on Egr-1 activation. By contrast, pairwise suppression of GC-boxes abolished activation, especially when the most promoter-proximal of the Egr-1 sites was removed. Taken together, these studies indicate that the alpha7 gene is likely to be a target for multiple signaling pathways, in which various regulatory elements are involved.

Differential effects of heterogeneous nuclear ribonucleoprotein K on Sp1- and Sp3-mediated transcriptional activation of a neuronal nicotinic acetylcholine receptor promoter

The neuronal nicotinic acetylcholine receptor gene family consists of 11 members, alpha2-alpha9 and beta2-beta4. Three of the genes, those encoding the alpha3, alpha5, and beta4 subunits, are clustered tightly within the genome. These three subunits constitute the predominant acetylcholine receptor subtype expressed in the peripheral nervous system. The genomic proximity of the three genes suggests a regulatory mechanism ensuring their coordinate expression. However, it is likely that gene-specific regulatory mechanisms are also functioning because the expression patterns of the three genes, although similar, are not identical. Previously we identified regulatory elements within the beta4 promoter region and demonstrated that these elements interact specifically with nuclear proteins. One of these elements, E1, interacts with the regulatory factor Puralpha as well as three other unidentified DNA-binding proteins with molecular masses of 31, 65, and 114 kDa. Another element, E2, interacts with Sp1 and Sp3. Because E1 and E2 are immediately adjacent to one another, we postulated that the proteins that bind to the elements interact to regulate beta4 gene expression. Here we report the identification of the 65-kDa E1-binding protein as heterogeneous nuclear ribonucleoprotein K and demonstrate that it affects the transactivation of beta4 promoter activity by Sp1 and Sp3 differentially.

Genomic DNA sequence, promoter expression, and chromosomal mapping of rat muscle carnitine palmitoyltransferase I

Carnitine palmitoyltransferase I (CPT-I) is a key enzyme involved in the regulation of fatty acid oxidation. CPT-IA and CPT-IB are isoforms of carnitine palmitoyltransferase I, of which CPT-IA is expressed in liver, kidney, fibroblasts, and heart and CPT-IB is expressed in skeletal muscle, heart, brown and white adipocytes, and testes. Although the genomic DNA sequence of human CPT-IB is available, the transcription start site and upstream regulatory sequences are not known. For rat CPT-IB, only the cDNA sequence has been published. We have cloned the entire rat CPT-IB gene from a Lambda fix II rat kidney genomic library. The genomic structure contains 19 exons, with the transcription start site for CPT-IB located in a short first exon, which is a 13-bp extension to the previously published cDNA 5' sequence. The coding sequence is identical with the rat muscle cDNA. The rat CPT-IB gene contains 18 introns and 19 exons, the latter 18 exons showing 85% homology to the human CPT-IB cDNA. CPT-IB maps to rat chromosome 7 at band q34. A putative promoter region was identified to within 391 bp of the transcription start site. The muscle specificity of the 5' flanking region was verified by comparison of luciferase expression to that of beta-galactosidase in cardiac myocytes and in HepG2 cells.

Sp1 and Sp3 regulate expression of the neuronal nicotinic acetylcholine receptor beta4 subunit gene

Neuronal nicotinic acetylcholine receptors play important roles in signal transduction within the nervous system. The receptors exist in a variety of functionally distinct subtypes that are determined by their subunit structures. The subunits are encoded by 11 genes, alpha2-alpha9 and beta2-beta4. Three of the genes, alpha3, alpha5, and beta4, are tightly clustered, and their encoded proteins make up the predominant receptor subtype in the peripheral nervous system. The tight linkage of the genes suggests there may be a common regulatory mechanism underlying their expression. However, although their expression patterns significantly overlap, they are not identical, indicating that independent regulatory mechanisms must also exist. Our studies have focused upon the gene encoding the beta4 subunit for which we have identified several transcriptional regulatory elements. One of these elements, E2, specifically interacts with the general transcription factor Sp1. Here we show that another member of the Sp family of factors, Sp3, can specifically interact with E2 whereas two other members, Sp2 and Sp4, cannot. Co-transfection experiments indicate that Sp3 can transactivate a beta4 promoter/reporter gene construct and, furthermore, that Sp1 and Sp3 can transactivate the beta4 reporter construct synergistically. The transactivation is dependent upon an intact E2 and may involve direct interactions between Sp1 and Sp3.

Transcriptional regulation of neuronal nicotinic acetylcholine receptor genes. A possible role for the DNA-binding protein Puralpha

Nicotinic acetylcholine receptors constitute a multigene family (alpha2-alpha9, beta2-beta4) expressed in discrete temporal and spatial patterns within the nervous system. The receptors are critical for proper signal transmission between neurons and their targets. The molecular mechanisms underlying receptor gene expression have not been completely elucidated but clearly involve regulation at the level of transcription. We previously identified a novel 19-base pair (bp) transcriptional regulatory element in the promoter region of the rat beta4 subunit gene. This 19-bp element interacts specifically with DNA-binding proteins enriched in nuclear extracts prepared from adult rat brain. Using a combination of cellulose-phosphate, DNA-cellulose, and DNA sequence-specific affinity chromatographies, we purified the 19-bp element binding activity approximately 19,000-fold. Analysis by denaturing gel electrophoresis revealed the presence of four polypeptides in the most purified fraction, ranging in molecular masses between 31 and 114 kDa. Peptide sequence analysis revealed that one of the polypeptides is the bovine homologue of the transcriptional regulatory factor, Puralpha. Electrophoretic mobility shift assays indicated that Puralpha interacts directly and specifically with the 19-bp element. In addition, mobility shift assays using an anti-Puralpha monoclonal antibody revealed the presence of Puralpha, or an immunologically related protein, in nuclear extracts prepared from brain tissue. We hypothesize that the interaction between Puralpha and the 19-bp element is critical for proper expression of the beta4 subunit gene.