Coordinate and differential regulation of proenkephalin A and PNMT mRNA expression in cultured bovine adrenal chromaffin cells: responses to secretory stimuli

Dense-core secretory granule biogenesis

The dense-core secretory granule is a key organelle for secretion of hormones and neuropeptides in endocrine cells and neurons, in response to stimulation. Cholesterol and granins are critical for the assembly of these organelles at the trans-Golgi network, and their biogenesis is regulated quantitatively by posttranscriptional and posttranslational mechanisms.

Protease nexin-1 promotes secretory granule biogenesis by preventing granule protein degradation

Dense-core secretory granule (DCG) biogenesis is a prerequisite step for the sorting, processing, and secretion of neuropeptides and hormones in (neuro)endocrine cells. Previously, chromogranin A (CgA) has been shown to play a key role in the regulation of DCG biogenesis in vitro and in vivo. However, the underlying mechanism of CgA-mediated DCG biogenesis has not been explored. In this study, we have uncovered a novel mechanism for the regulation of CgA-mediated DCG biogenesis. Transfection of CgA into endocrine 6T3 cells lacking CgA and DCGs not only recovered DCG formation and regulated secretion but also prevented granule protein degradation. Genetic profiling of CgA-expressing 6T3 versus CgA- and DCG-deficient 6T3 cells, followed by real-time reverse transcription-polymerase chain reaction and Western blotting analyses, revealed that a serine protease inhibitor, protease nexin-1 (PN-1), was significantly up-regulated in CgA-expressing 6T3 cells. Overexpression of PN-1 in CgA-deficient 6T3 cells prevented degradation of DCG proteins at the Golgi apparatus, enhanced DCG biogenesis, and recovered regulated secretion. Moreover, depletion of PN-1 by antisense RNAs in CgA-expressing 6T3 cells resulted in the specific degradation of DCG proteins. We conclude that CgA increases DCG biogenesis in endocrine cells by up-regulating PN-1 expression to stabilize granule proteins against degradation.

Protein kinase A and protein kinase C signaling pathway interaction in phenylethanolamine N-methyltransferase gene regulation

The protein kinase A (PKA) and protein kinase C (PKC) signaling pathways appear to interact in regulating phenylethanolamine N-methyltransferase (PNMT) promoter-driven gene transcription in PC12 cells. Forskolin treatment of cells transfected with the rat PNMT promoter-luciferase reporter gene construct pGL3RP893 increased promoter activity approximately two-fold whereas phorbol-12-myristate-13 acetate (PMA) treatment had no effect. However, simultaneous forskolin and PMA treatment synergistically activated the PNMT promoter approximately four-fold, suggesting that PKC stimulation requires prior induction of the PKA pathway. Consistent with this possibility the adenylate cyclase inhibitor MDL12,330A, and the PKA inhibitor H-89 prevented PNMT promoter stimulation by the combination of forskolin and PMA. PKA and PKC regulation seems to be mediated in part by Egr-1 and Sp1 through their consensus elements in the PNMT promoter. Forskolin and PMA treatment of PC12 cells increased Egr-1 protein and phosphorylated Egr-1/DNA-binding complex formation to the same extent but only increased phosphorylated Sp1/DNA binding complex formation without altering Sp1 protein levels. Mutation of the - 165 bp Egr-1 and - 48 bp Sp1 sites, respectively, attenuated and abolished combined forskolin and PMA-mediated promoter activation. PNMT promoter analysis further showed that synergistic stimulation by PKA and PKC involves DNA sequences between - 442 and - 392 bp, and potentially a GCM binding element lying within this region.

Stimulus coupling to transcription versus secretion in pheochromocytoma cells. Convergent and divergent signal transduction pathways and the crucial roles for route of cytosolic calcium entry and protein kinase C

How do chromaffin cell secretory stimuli program resynthesis of secreted peptides and amines? We previously showed that the physiologic nicotinic cholinergic signal for secretion also activates the biosynthesis of chromogranin A, the major protein released with catecholamines. Here, we examine signal transduction pathways whereby secretory stimuli influence exocytotic secretion versus chromogranin A transcription. Both secretion and transcription depended on initial nicotinic-triggered sodium entry into the cytosol, followed by calcium entry through -type voltage-gated channels. When calcium entered through -type channels, activation of secretion paralleled activation of transcription (r = 0.897, P = 0.002). Calcium entry from intracellular stores or through calcium ionophore channels activated secretion, though not transcription. Nicotinic-stimulated transcription depended upon protein kinase C activation; nicotine caused translocation of protein kinase C to the cell membrane fraction, and inhibition of protein kinase C blocked activation of transcription, while activation of protein kinase C mimicked nicotine effects. Transcriptional responses to both nicotine and protein kinase C mapped principally onto the chromogranin A promoter's cAMP response element (TGACGTAA; CRE box). KCREB, a dominant negative mutant of the CRE-binding protein CREB, blunted activation of chromogranin A transcription by nicotine, phorbol ester, or membrane depolarization. We conclude that activation of chromogranin A transcription by secretory stimulation in chromaffin cells is highly dependent upon precise route of calcium entry into the cytosol; transcription occurred after entry of calcium through -type channels on the cell surface, and was mediated by protein kinase C activation. The trans-acting factor CREB ultimately relays the secretory signal to the chromogranin A promoter's CRE box in cis.

Expression of catecholamine-synthesizing enzymes, peptidylglycine alpha-amidating monooxygenase, and neuropeptide Y mRNA in the rat adrenal medulla after acute systemic nicotine

The expression of catecholamine-synthesizing enzymes in the adrenal medulla is upregulated in parallel by stress and pharmacological treatments. In this study we examined whether a neuropeptide and its processing enzyme are regulated in parallel with catecholamine enzyme genes after drug treatment. Because the main effect of stress on the adrenal medulla is via splanchnic nerve stimulation of nicotinic receptors, we used nicotine to stimulate the medulla and visualized expression of catecholamine enzyme genes, the medullary peptide neuropeptide Y (NPY), and the neuropeptide-processing enzyme peptidylglycine alpha-amidating monooxygenase (PAM) by in situ hybridization quantified by image analysis of autoradiographic images. Rats received a single injection of nicotine (0, 1, or 5 mg/kg sc). Six hours later, rats were transcardially perfused. Free-floating adrenal gland sections were hybridized with 35S-labeled cDNA probes for tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), phenylethanolamine N-methyltransferase (PNMT), PAM, and NPY. Nicotine treatment upregulated the expression of TH, PNMT, and NPY genes in a dose-dependent fashion. Small but nonsignificant increases were observed in DBH and PAM mRNA levels. These results suggest that common transcriptional activation mechanisms may upregulate both catecholamine and neuropeptide synthesis in the adrenal medulla after nicotinic stimulation.

Stimulus-transcription coupling in pheochromocytoma cells. Promoter region-specific activation of chromogranin a biosynthesis

To explore stimulus-transcription coupling in pheochromocytoma cells, we studied the biosynthetic response of chromogranin A, the major soluble protein co-stored and co-released with catecholamines, to chromaffin cells' physiologic nicotinic cholinergic secretory stimulation. Chromogranin A mRNA showed a time-dependent 3.87-fold response to nicotinic stimulation, and a nuclear run-off experiment indicated that the response occurred at a transcriptional level. Transfected chromogranin A promoter/luciferase reporter constructs were activated by nicotinic stimulation, in time- and dose-dependent fashions, in both rat PC12 pheochromocytoma cells and bovine chromaffin cells. Cholinergic subtype agents indicated that nicotinic stimulation was required. Promoter deletions established both positive and negative nicotinic response domains. Transfer of candidate promoter domains to a heterologous (thymidine kinase) promoter conferred region-specific nicotinic responses onto that promoter. A proximal promoter domain (from -93 to -62 base pairs) was activated in copy number- and distance-dependent fashion, and thus displayed features of a promoter element. Its activation was sufficient to account for the overall positive response to nicotine. Within this proximal region, a cAMP response element (CRE) was implicated as a major nicotinic response element, since a CRE point-gap mutation decreased nicotinic induction, transfer of CRE to a thymidine kinase promoter augmented the promoter's response to nicotine, and nicotine activated the CRE-binding protein CREB through phosphorylation at serine 133. We conclude that secretory stimulation of pheochromocytoma cells also activates the biosynthesis of the major secreted protein (chromogranin A), that the activation is transcriptional, and that a small proximal domain, including the CRE box, is, at least in part, both necessary and sufficient to account for the positive response to nicotine.

Selective up-regulation of alpha 1-chimaerin mRNA in SK-N-SH neuroblastoma cells by K+/-induced depolarisation

The expression of alpha 1-chimaerin, which encodes a neuron-specific GTPase-activating protein for p21rac, is spatially and temporally regulated in vivo. In vitro, expression of the mRNA of both alpha 1-chimaerin and its alternative spliced form, alpha 2-chimaerin, was up-regulated when human neuroblastoma SK-N-SH cells underwent neuronal-type differentiation in a serum-free medium. KCl-induced membrane depolarisation also specifically up-regulated alpha 1-chimaerin mRNA expression in SK-N-SH cells at the transcriptional level. The up-regulation of alpha 1-chimaerin expression by membrane depolarisation is not an immediate early event, and occurs 3 h after KCl treatment. It does not require de novo protein synthesis. The increase in calcium influx via the L-type voltage-sensitive calcium channel as the result of depolarisation is a key event leading to the up-regulation of alpha 1-chimaerin mRNA. alpha 1-Chimaerin expression was also found to respond positively to the hypertonic osmolarity changes. These results suggest that in vivo expression of alpha 1-chimaerin, a potential signal transduction molecule, may be regulated by neuronal/synaptic activity.

Role of transcription factor Egr-1 in phorbol ester-induced phenylethanolamine N-methyltransferase gene expression

Transfection of PC12-variant RS1 cells with an Egr-1 expression construct has previously been shown to stimulate phenylethanolamine N-methyltransferase (PNMT) promoter activity, thus suggesting a putative role of Egr-1 as a factor regulating PNMT gene expression. To elucidate the physiological implication of this finding, the effects of phorbol 12-myristate 13-acetate (PMA) on PNMT promoter activity and Egr-1 expression were examined. PMA stimulated luciferase expression in RS1 cells transfected with a rat PNMT promoter-luciferase reporter gene construct, and also elevated both Egr-1 mRNA and Egr-1 protein levels in the untransfected cells. Further study on the concentration dependence of PMA action showed that the stimulation of luciferase expression correlated with the elevation of Egr-1 mRNA level. Finally, the stimulatory action of PMA on luciferase expression was dramatically diminished in the cells transfected with a mutant construct in which the Egr-1 binding site in PNMT promoter was mutated. These findings suggest that PMA-stimulated PNMT gene expression requires the enhancement of Egr-1 expression, thus providing further evidence for the physiological role of Egr-1 in the regulation of PNMT gene expression in the adrenergic cell.

Brief cortisol exposure elevates adrenal phenylethanolamine N-methyltransferase after a necessary lag period

The present study, using bovine adrenal medullary cells, characterized in detail the time course of regulation of phenylethanolamine N-methyltransferase activity following brief glucocorticoid exposure. Cortisol pulses (10(-4) and 10(-5) M), as short as 15 min, increased phenylethanolamine N-methyltransferase activity measured 2 days following cortisol exposure, with a required lag period of 18 h or more. Phenylethanolamine N-methyltransferase activity was increased 2 days following brief (2 h) exposure to cortisol in concentrations that reach the medulla in vivo (10(-6) to 10(-4) M). Phenylethanolamine N-methyltransferase activity following both continuous and 2 h pulses of 10(-5) M cortisol were reduced by the glucocorticoid receptor antagonist, RU 38486. A 2 h pulse of nicotine (10(-5) M) increased phenylethanolamine N-methyltransferase activity with a lag period of at least 18 h, while combination treatment of nicotine and cortisol (10(-4) M) produced significantly higher increases in phenylethanolamine N-methyltransferase compared to either treatment alone. Therefore, this study provides novel in vitro evidence for the regulation of adrenomedullary phenylethanolamine N-methyltransferase activity, following a necessary lag period, by acute changes in both cortisol and nicotine.

Effects of [Sar1]angiotensin II on proenkephalin gene expression and secretion of [Met5]enkephalin in bovine adrenal medullary chromaffin cells

We have studied the effect of [Sar1]angiotensin II [S1-AII; a degradation-resistant analogue of angiotensin II (AII) on the release of [Met5]enkephalin (ME) and proenkephalin A (proENK) gene expression. Short-term (15-min to 1-h) stimulation of bovine adrenal medullary chromaffin (BAMC) cells with S1-AII at concentrations from 0.1 to 100 nM had no significant effect on secretion of ME, whereas high concentrations of S1-AII (3 to 100 microM) produced a concentration-dependent increase in the concentration of ME in the incubation media. In contrast, long-term (3- to 24-h) stimulation with low concentrations (0.1 nM-1 microM) of S1-AII increased the secretion of ME in a concentration-dependent manner (EC50 = 1 nM). The intracellular level of ME was not changed by long-term treatment with S1-AII (100 nM). In addition to increased ME secretion, long-term (24-h) stimulation with S1-AII increased the expression of proENK mRNA in a concentration-dependent manner (EC50 = 4 nM). Losartan (2-n-butyl-4 chloro-5-hydroxymethyl-1-[(2'-(1 H-tetrazol-5-yl)biphenyl-4-yl)- methyl]imidazole potassium salt, a type 1 AII receptor antagonist) inhibited these effects, whereas PD123319 (50 microM, a type 2 AII receptor antagonist) was inactive. Our results suggest that AII in BAMC cells exerts a major effect on the long-term regulation of expression of proENK mRNA and secretion of ME. These effects appear to be mediated by type 1-like AII receptors.

Long-term activation of protein kinase C by nicotine in bovine adrenal chromaffin cells

Previous results from our laboratory suggest that long-term treatment of primary cultured bovine adrenal medullary (BAM) chromaffin cells with nicotine or phorbol 12-myristate 13-acetate, either of which directly activates protein kinase C (PKC), increases the mRNA levels encoding catecholamine-synthesizing enzymes and proenkephalin. In the present study, we have examined the effects of nicotine on BAM cell PKC activity with special emphasis on long-term effects. Nicotine increased particulate PKC activity in a concentration-dependent manner when measured using in vitro enzyme assay with histone as the substrate. This effect is mediated through nicotinic cholinergic receptors, because 1,1-dimethylphenylpiperazinium, a nicotinic agonist, had a similar effect. In addition, chlorisondamine, a specific nicotine-receptor blocking drug, antagonized the effect of nicotine. Nicotine also increased specific [3H]phorbol 12,13-dibutyrate ([3H]PdBu) binding within 1 min, the effect of which was maximal between 3 and 12 min. This effect was reversed by chlorisondamine similarly after 12 min and after 18 h of nicotine treatment, indicating that continual nicotinic-receptor occupancy is required for persistent PKC activation. Compared to PKC activation, the onset of nicotine-stimulated diacylglycerol production was slow, and it was observed after 12 min of incubation with nicotine. The diacylglycerol levels, specific [3H]PdBu binding, and PKC activity remained significantly elevated for at least 18 h with continuous nicotine incubation. Furthermore, nicotine increased the PKC immunoreactivity of a particulate protein with a molecular mass of 82 kDa in the western blot. These results suggest that nicotinic-receptor activation increases PKC activity and immunoreactivity in BAM cells.(ABSTRACT TRUNCATED AT 250 WORDS)