Regulation of proenkephalin gene expression by angiotensin in bovine adrenal medullary cells: Molecular mechanisms and nature of the second messenger systems

Regulation of tyrosine hydroxylase gene expression in depolarized non-transformed bovine adrenal medullary cells: second messenger systems and promoter mechanisms

Activation of the tyrosine hydroxylase (TH) gene in the adrenal medulla during stress is mediated by trans-synaptic mechanisms and may involve cholinergic receptors. Stimulation of nicotinic receptors in adrenal medullary cells induces cell depolarization, influx of Ca2+ ions and increases levels of cAMP. We have shown that both cAMP and membrane depolarization produce an increase in the expression of the TH gene in cultured bovine adrenal medullary cells (BAMC). Others have proposed that transcriptional activation of the TH gene by cAMP is mediated through the sequence homologous to a cAMP responsive element (CRE) located in the proximal region of the TH gene promoter. In the present study we have examined the mechanisms by which membrane depolarization increases the TH gene activity. Treatment of serum-free BAMC cultures with the depolarizing agent, veratridine, increased the extracellular concentration of catecholamines, Met5-enkephalin, and the relative abundance of TH mRNA. Veratridine treatment also increased the levels of mRNAs for the catecholamine biosynthetic enzyme phenylethanolamine N-methyltransferase (PNMT), and proenkephalin A (PEK). Treatment for longer than 3 h was required to increase TH mRNA levels. By contrast, our previous studies indicated that cAMP stimulation for 2 h produces a maximal increase in TH mRNA levels in BAMC. The effects of veratridine and forskolin on TH mRNA levels were additive, further indicating that depolarization and cAMP activate TH gene expression via different pathways. Calmidazolium, an antagonist of calmodulin, had no effect on the veratridine-induced increase in TH mRNA levels. Similarly sphingosine treatment or preincubation with PMA, which reduce protein kinase C (PKC) activity and attenuate the induction of TH mRNA by PMA or the hormone, angiotensin II, did not affect the induction by veratridine. To identify promoter mechanisms of TH gene activation in depolarized cells we transfected BAMC with a plasmid pTHgoodLuc and treated with veratridine for 24 h. pTHgoodLUC contains a luciferase reporter gene linked to a -428/+21 bp fragment of the bovine TH gene promoter (relative to the transcription start site). Veratridine increased the expression of luciferase from the TH promoter 2.5-fold. Deletion of the -194/-54 bp promoter region containing SP-1 and POU/Oct sites reduced veratridine stimulation by 40%. Additional deletion of the -269 to -190 bp promoter segment, including an AP-1 element, further reduced veratridine stimulation to a statistically non-significant level. In conclusion, activation of TH gene expression upon depolarization is not mediated by calmodulin and PKC. Promoter sequences involved in this activation are located upstream from the CRE. Depolarization may activate TH gene transcription by acting on more than one regulatory region.

Basic fibroblast growth factor (bFGF) regulates tyrosine hydroxylase and proenkephalin mRNA levels in adrenal chromaffin cells

bFGF is a neurotrophic protein expressed in various regions of the adult peripheral and central nervous system. The present study was undertaken to examine the role of bFGF in multihormonal, catecholaminergic and enkephalinergic cells of the adrenal medulla (AM). Western blot analysis revealed the presence of at least three bFGF isoforms (18, 22/23, and 24 kDa) in cultured bovine AM cells. Incubation of AM cells with the exogenous 18 kDa bFGF produced time-dependent increases in tyrosine hydroxylase (TH) and proenkephalin (PEK) mRNA, with maximal changes occurring at 12 h (TH) or 24 h (PEK) of bFGF exposure. Effects of bFGF on TH and PEK mRNA were non-additive with increases induced by exposure of AM cells to nicotine, the depolarizing agent veratridine, or the adenylate cyclase activator forskolin. These data indicate that bFGF effects may occur through intracellular pathways accessed during transsynaptic induction of TH and PEK genes. The increases in PEK mRNA induced by nicotine or bFGF were inhibited by the calcium antagonist TMB-8. TMB-8 also inhibited bFGF-induced increases in TH mRNA as well. However, treatment with TMB-8 increased basal levels of TH mRNA. The addition of bFGF increased endogenous levels of c-fos mRNA, c-Fos and c-Fos-related proteins, suggesting that bFGF may activate TH and PEK gene expression through a calcium-AP1 transcriptional regulatory pathway. Immunohistochemical analysis revealed the presence of bFGF-immunoreactivity (bFGF-IR) in the cytoplasm and in the nucleus of AM cells. Incubation of cells with exogenous bFGF produced time-dependent increases of nuclear bFGF-IR.(ABSTRACT TRUNCATED AT 250 WORDS)

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.