Nicotinic and muscarinic acetylcholine receptors are essential for the long-term response of tyrosine hydroxylase gene expression to chronic nicotine treatment in rat adrenal medulla

Novel Pathways and Mechanism of Nicotine-Induced Oral Carcinogenesis

BACKGROUND

Smokeless Tobacco (SLT) contains 9 times more nicotine than Smoked Tobacco (SMT). The carcinogenic effect of nicotine is intensified by converting nicotine-to-nicotine-derived Nitrosamines (NDNs).

METHODS

A review of the literature was conducted with a tailored search strategy to unravel the novel pathways and mechanisms of nicotine-induced oral carcinogenesis.

RESULTS

Nicotine and NDNs act on nicotinic Acetylcholine Receptors (nAChRs) as agonists. Nicotine facilitates cravings through α4β2nAChR and α7nAChR, via enhanced brain dopamine release. Nicotine binding to nAChR promotes proliferation, migration, invasion, chemoresistance, radioresistance, and metastasis of oral cancer cells. Nicotine binding to α7nAChR on keratinocytes triggers Ras/Raf-1/MEK1/ERK cascade promoting anti-apoptosis and pro-proliferative effects. Furthermore, the nicotine-enhanced metastasis is subdued on nAChR blockade through reduced nuclear localization of p-EGFR.

CONCLUSION

Protracted exposure to nicotine/NDN augments cancer-stimulatory α7nAChR and desensitizes cancer inhibitory α4β2nAChR. Since nAChRs dictate both addictive and carcinogenic effects of nicotine, it seems counterintuitive to designate nicotine just as an addictive agent devoid of any carcinogenicity.

Cholinergic Regulation of Morphine Release from Human White Blood Cells: Evidence for a Novel Nicotinic Receptor via Pharmacological and Micro Array Analysis

Recent work from our laboratory has demonstrated that human white blood cells make morphine and that substances of abuse, i.e. nicotine, alcohol and cocaine have the ability to release this endogenous substance, suggesting a common mechanism of action. We now demonstrate that the nicotinic process is more complex than formerly envisioned. The incorporation rate of 125I-labeled morphine into PMN and MN are 7.85±0.36%, 1.42±0.19%, respectfully, suggesting in MN this process is of low activity. Separate incubations of PMN with varying concentrations of nicotine or the nicotine agonist epibatidine resulted in a statistically significant enhancement of 125I-trace labeled morphine released into the extracellular medium. In order to ascertain the specificity of the nicotine stimulated morphine release the following experiments were performed. Co-incubation of hexamethonium dichloride (5 μg/ml and at 10 μg/ml), which preferentially blocks nicotinic receptors at autonomic ganglia, with nicotine, exerted a very weak inhibitory effect. Co-incubation of α-BuTx or atropine or chlorisondamine diiodide or dihydro-β-erythroidine hydrobromide, an α4β2 receptor antagonist, did not block nicotine induced morphine release alone or in combination, suggesting either the response was not specific or it was mediated by a novel nicotinic receptor. Human leukocyte total RNA isolated from whole blood were analyzed, using the Human Genome Survey microarray (Applied Biosystems), for cholinergic receptor expression. PMN nicotinic receptor gene expression was present and contained numerous variants (eight). The number of variants suggests that indeed a novel nicotinic receptor may be mediating this effect, while simultaneously demonstrating the significance of the cholinergic receptor expression in these immune cells.

cAMP-mediated stimulation of tyrosine hydroxylase mRNA translation is mediated by polypyrimidine-rich sequences within its 3'-untranslated region and poly(C)-binding protein 2

Tyrosine hydroxylase (TH) plays a critical role in maintaining the appropriate concentrations of catecholamine neurotransmitters in brain and periphery, particularly during long-term stress, long-term drug treatment, or neurodegenerative diseases. Its expression is controlled by both transcriptional and post-transcriptional mechanisms. In a previous report, we showed that treatment of rat midbrain slice explant cultures or mouse MN9D cells with cAMP analog or forskolin leads to induction of TH protein without concomitant induction of TH mRNA. We further showed that cAMP activates mechanisms that regulate TH mRNA translation via cis-acting sequences within its 3'-untranslated region (UTR). In the present report, we extend these studies to show that MN9D cytoplasmic proteins bind to the same TH mRNA 3'-UTR domain that is required for the cAMP response. RNase T1 mapping demonstrates binding of proteins to a 27-nucleotide polypyrimidine-rich sequence within this domain. A specific mutation within the polypyrimidine-rich sequence inhibits protein binding and cAMP-mediated translational activation. UV-cross-linking studies identify a approximately 44-kDa protein as a major TH mRNA 3'-UTR binding factor, and cAMP induces the 40- to 42-kDa poly(C)-binding protein-2 (PCBP2) in MN9D cells. We show that PCBP2 binds to the TH mRNA 3'-UTR domain that participates in the cAMP response. Overexpression of PCBP2 induces TH protein without concomitant induction of TH mRNA. These results support a model in which cAMP induces PCBP2, leading to increased interaction with its cognate polypyrimidine binding site in the TH mRNA 3'-UTR. This increased interaction presumably plays a role in the activation of TH mRNA translation by cAMP in dopaminergic neurons.

Post-transcriptional regulation of tyrosine hydroxylase expression in adrenal medulla and brain

It is well established that long-term stress leads to induction of tyrosine hydroxylase (TH) mRNA and TH protein in adrenal medulla and brain. This induction is usually associated with stimulation of the TH gene transcription rate. However, a number of studies have reported major discrepancies between the stress-induced changes in TH gene transcription, TH mRNA, and TH protein. These discrepancies suggest that post-transcriptional mechanisms also play an important role in regulating TH expression in response to stress and other stimuli. In this report, we summarize some of our findings and literature reports that demonstrate these discrepancies in adrenal medulla, locus ceruleus, and midbrain dopamine neurons. We then describe our recent work investigating the molecular mechanisms that mediate this post-transcriptional regulation in adrenal medulla and midbrain. Our results suggest that trans-acting factors binding to the polypyrimidine-rich region of the 3' untranslated region of TH mRNA play a role in these post-transcriptional mechanisms. A hypothetical cellular model describing this post-transcriptional regulation is proposed.

Glial cell line-derived neurotrophic factor reverses ethanol-mediated increases in tyrosine hydroxylase immunoreactivity via altering the activity of heat shock protein 90

We previously found that glial cell line-derived neurotrophic factor (GDNF) in the midbrain ventral tegmental area (VTA) negatively regulates alcohol drinking (He, D. Y., McGough, N. N., Ravindranathan, A., Jeanblanc, J., Logrip, M. L., Phamluong, K., Janak, P. H., and Ron, D. (2005) J. Neurosci. 25, 619-628). Several studies suggest a role for GDNF in the regulation of tyrosine hydroxylase (TH) levels in the midbrain (Georgievska, B., Kirik, D., and Bjorklund, A. (2004) J. Neurosci. 24, 6437-6445). Up-regulation of TH levels has been reported as a hallmark of biochemical adaptations to in vivo chronic exposure to drugs of abuse, including ethanol (Ortiz, J., Fitzgerald, L. W., Charlton, M., Lane, S., Trevisan, L., Guitart, X., Shoemaker, W., Duman, R. S., and Nestler, E. J. (1995) Synapse 21, 289-298). We hypothesized that GDNF plays an important role in regulating prolonged ethanol-mediated increases in TH protein levels. Using the SH-SY5Y dopaminergic-like cell line, we found that the increase in TH levels in the presence of ethanol required the activation of the cAMP/PKA pathway and was reversed by GDNF. Ethanol treatment did not alter the mRNA level or protein translation of TH, but enhanced the stability of the protein that was decreased by GDNF. Interestingly, we observed that ethanol treatment resulted in an increase in TH association with the chaperone heat shock protein (HSP90) that was mediated by the cAMP/PKA pathway and inhibited by GDNF. Taken together, these data suggest that prolonged ethanol exposure leads to increased association of TH and HSP90 via the cAMP/PKA pathway, resulting in the stabilization and subsequent accumulation of TH. GDNF reverses this ethanol-mediated adaptation by inhibiting the interaction of TH with HSP90.

Activation of tyrosine hydroxylase mRNA translation by cAMP in midbrain dopaminergic neurons

During prolonged stress or chronic treatment with neurotoxins, robust compensatory mechanisms occur that maintain sufficient levels of catecholamine neurotransmitters in terminal regions. One of these mechanisms is the up-regulation of tyrosine hydroxylase (TH), the enzyme that controls catecholamine biosynthesis. In neurons of the periphery and locus coeruleus, this up-regulation is associated with an initial induction of TH mRNA. In contrast, this induction either does not occur or it is nominal in mesencephalic dopamine neurons. The reasons for this lack of compensatory TH mRNA induction remain obscure, because so little is known about the regulation of TH expression in these neurons. In this study, we test whether activation of the cAMP signaling pathway regulates TH gene expression in two rodent models of midbrain dopamine neurons, ventral midbrain organotypic slice cultures and MN9D cells. Our results demonstrate that elevation of cAMP leads to induction of TH protein and TH activity in both model systems; however, TH mRNA levels are not up-regulated by cAMP. The induction of TH protein is the result of a novel post-transcriptional mechanism that activates TH mRNA translation. This translational activation is mediated by sequences within the 3' untranslated region (UTR) of TH mRNA. Our results support a model in which cAMP induces or activates trans-factors that interact with the TH mRNA 3'UTR to increase TH protein synthesis. An understanding of this novel regulatory mechanism may help to explain the control of TH gene expression and consequently dopamine biosynthesis in midbrain neurons under different physiological and pathological conditions.

Adrenal adrenoceptors in heart failure: fine-tuning cardiac stimulation

Chronic heart failure (HF) is characterized by sympathetic hyperactivity reflected by increased circulating catecholamines (CAs), which contributes significantly to its morbidity and mortality. Therefore, sympatholytic treatments, that is, treatments that reduce sympathetic hyperactivity, are being pursued currently for the treatment of HF. Secretion of CAs from the adrenal gland, which is a major source of CAs, is regulated by alpha(2)-adrenoceptors (alpha(2)ARs), which inhibit, and by beta-adrenoceptors (betaARs), which enhance CA secretion. All ARs are G-protein-coupled receptors (GPCRs), whose signaling and function are regulated tightly by the family of GPCR kinases (GRKs). Despite the enormous potential of adrenal ARs for the regulation of sympathetic outflow, elucidation of their properties has only begun recently. Here, recent advances regarding the roles of adrenal ARs in the regulation of sympathetic outflow in HF and the regulatory properties of ARs are discussed, along with the potential benefits and challenges of harnessing their function for HF therapy.

Stress-induced catecholaminergic function: transcriptional and post-transcriptional control

This review summarizes knowledge on the effects of stress on two catecholamine biosynthetic enzymes, tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT). Information is presented on differential responses of the enzymes to a variety of stressors as well as differential responses of the enzymes localized to the central nervous system vs. peripheral nervous system and tissues. Changes in mRNA and protein or activity are described, including species- and stressor-specific effects. While temporal changes in these parameters may differ for the particular stressor or enzyme, in general, maximal changes in mRNA and protein content occur at 6-8 and 24 h after stressor exposure, respectively. Elevation of TH and PNMT transcriptional activators prior to mRNA induction and nuclear run-on assays show that stress activates the genes encoding these enzymes. Yet, extents of induction of mRNA, protein and enzyme activity are often discordant depending on the stress, its duration and repetition of exposure. The extremes are concordant changes in mRNA and protein/activity vs. highly elevated mRNA with no change in protein/activity. Post-transcriptional and/or post-translational regulatory influences that may contribute to the complex effects of stress on TH, PNMT and the stress hormone epinephrine are explored.

Evidence for regulation of tyrosine hydroxylase mRNA translation by stress in rat adrenal medulla

Long-term stress leads to the induction of tyrosine hydroxylase (TH) protein and enzymatic activity in the adrenal medulla. This adaptive response is necessary to maintain the catecholamine biosynthetic capacity of adrenal chromaffin cells during periods of sustained catecholamine secretion. In this report we demonstrate that when rats are subjected to short-term stress, TH mRNA is induced for at least 24 h, but TH protein and TH activity (assayed under Vmax conditions) are not increased. In contrast, adrenal TH mRNA, TH protein and TH activity are induced in rats subjected to long-term stress. Using sucrose gradient fractionation, we show that the lack of induction of TH protein after one type of short-term stressor, a single 2-h immobilization stress is associated with a decrease in the percentage of TH mRNA molecules associated with polysomes. In contrast, after repeated immobilizations the polysome profile of TH mRNA is identical to that observed in control animals, even though TH mRNA is induced 2- to 3-fold. These results are consistent with the hypothesis that even though TH mRNA is induced by short-term stressors, mechanisms that control TH mRNA translation must also be appropriately regulated for TH protein to be induced.

Nicotine induces tyrosine hydroxylase plasticity in the neurodegenerating striatum

It has been shown that nicotine prevents the loss of dopamine (DA) in the corpus striatum (CS) after 6-hydroxydopamine injection in the substantia nigra. To study the role of the enzyme tyrosine hydroxylase (TH; EC 1.14.16.2) in this experimental paradigm, we have examined its activity by assessing the accumulation of l-3,4-dihydroxyphenylalanine after inhibiting the subsequent enzyme in the DA synthetic pathway, aromatic l-amino acid decarboxylase, with 3-hydroxybenzylhydrazine. In addition the amount of TH protein was assessed by western blotting and its distribution in the CS was examined using immunohistochemical methods. 6-hydroxydopamine injection produced a significant decrease in DA levels and l-3,4-dihydroxyphenylalanine accumulation, as well as decreases in TH protein and TH immunoreactive fibres in the CS. After nicotine treatment, the decrease in TH protein in the CS was significantly reduced, with a concomitant preservation of TH activity, but nicotine did not alter the number of TH immunoreactive fibres. The activity and amount of TH did not change in the contralateral (intact) CS. Thus, nicotine induces long lasting TH plasticity in the degenerating CS. A synergistic action of nicotine-activated and lesion-originated signals appears necessary for the expression of this neuronal molecular plasticity.

Chronic nicotine administration. Analysis of the development of hypertension and glutamatergic neurotransmission

Among numerous neurotransmitters involved in central cardiovascular control, glutamate is one of the most studied transmitters that are related to nicotine considering its release and its postsynaptic regulation. However, there are no conclusive studies about nicotine effects on glutamatergic system and its relevance on hypertension development, which can help to understand the role of these two systems in that pathology. In this context, the objective of the present study is to evaluate the effects of systemic chronic nicotine exposure on hypertension development as well as the interaction between nicotine and the glutamatergic system in normotensive and neurogenic hypertensive rats. By means of high performance liquid chromatograph, immunohistochemistry, in situ hybridization and binding techniques, glutamatergic system was evaluated in SHR and Wistar Kyoto (WKY) rats treated with nicotine, delivered subcutaneously through nicotine pellets, for 8 weeks. The most important findings in this study were that (1) moderate doses of nicotine accelerated the onset and increased blood pressure in SHR but not in WKY rats, (2) the nicotine dosage and time of treatment employed did not affect body weight, (3) chronic nicotine treatment differentially affected glutamatergic system in normotensive and hypertensive rats, and (4) spontaneously hypertensive rats seem to be more sensitive to peripherally administered nicotine than Wistar Kyoto rats considering blood pressure and glutamatergic neurotransmission changes. In conclusion, we have demonstrated that a moderate dose of nicotine accelerates the onset and exacerbates hypertension in the SHR and that might be, at least in part, related to the modulation of glutamatergic neurotransmission.

Induction of tyrosine hydroxylase in the locus coeruleus of transgenic mice in response to stress or nicotine treatment: lack of activation of tyrosine hydroxylase promoter activity

Prolonged stress or chronic nicotine administration leads to induction of tyrosine hydroxylase (TH) in adrenal medulla and locus coeruleus (LC) of the rat. In this study we use mice that express a transgene encoding 4.5 kb of TH gene 5'-flanking region fused upstream of the reporter gene, human alkaline phosphatase (hAP) to test whether TH gene promoter activity is stimulated by immobilization stress, cold exposure or nicotine administration in adrenal medulla and LC. TH-hAP transgene expression is increased in response to all three stimuli in the adrenal medulla. In contrast, TH-hAP expression does not increase in response to either immobilization stress or nicotine administration in the LC and only a small induction of LC TH-hAP mRNA is observed in response to cold exposure. TH mRNA is induced 2-3 fold and TH activity is increased significantly by all three stimuli in both the adrenal and LC. These results support the hypothesis that TH expression is induced by stress or nicotine treatment in both the adrenal medulla and LC of the mouse. The induction in the adrenal is dependent primarily on transcriptional mechanisms, whereas that in the LC is apparently dependent primarily on post-transcriptional mechanisms.