The role of protein kinase C in nicotinic responses of bovine chromaffin cells

Nicotinic stimulation of catecholamine synthesis and tyrosine hydroxylase phosphorylation in cervine adrenal medullary chromaffin cells

The synthesis and secretion of catecholamines by the adrenal medulla is of major importance in the stress response. Tyrosine hydroxylase, the rate-limiting enzyme for catecholamine biosynthesis, has been extensively studied in adrenal medullary chromaffin cells from a number of species. Cervine chromaffin cells are of interest because the deer is known to be a relatively stress-prone reactive species. We report the first characterisation of tyrosine hydroxylase regulation in cervine chromaffin cells. Nicotinic receptor activation resulted in a time- and concentration-dependent increase in catecholamine synthesis, which was significantly reduced by the extracellular signal-regulated kinase (ERK)1/2 signalling pathway inhibitor PD98059 and the calcium/calmodulin protein kinase II inhibitor KN-93, but not by H89 or bisindolylmaleimide I, inhibitors of protein kinase A and C, respectively. Nicotinic stimulation also increased the phosphorylation of ERK1/2 and tyrosine hydroxylase. This latter response occurred on serine residues 19, 31 and 40 of the enzyme. The nicotinic-induced phosphorylation of ERK1/2 and serine 31 of tyrosine hydroxylase was suppressed by PD98059 but not bisindolylmaleimide I. These data indicate that nicotinic stimulation of tyrosine hydroxylase involves the phosphorylation of serine 31 via an ERK1/2-dependent, protein kinase C-independent pathway. Protein kinase C activation by phorbol 12-myristate 13-acetate also caused an ERK1/2-dependent increase in the serine 31 phosphorylation of tyrosine hydroxylase but, in contrast to the nicotinic response, was not accompanied by an increase in enzyme activity. Thus, ERK1/2-mediated serine 31 phosphorylation of tyrosine hydroxylase appears necessary but not sufficient for nicotinic activation of catecholamine synthesis in cervine chromaffin cells. These data present potentially important similarities and differences between the regulation of catecholamine synthesis in cervine and the more widely studied bovine adrenal medulla.

Nicotine tolerance to PC12 cell line: acute and chronic exposures modulate dopamine D2 receptor and tyrosine hydroxylase expression

PC12 is a clonal cell line from chromaffin tumor of rat adrenal pheochromocytoma that releases catecholamine including dopamine, which via interaction with its receptor (D(1) and D(2) receptor), is known to be involved in reward and reinforcement properties of many addictive drugs like nicotine. Nicotine tolerance is the key aspect of nicotine addiction. However, nicotine tolerance on dopamine receptors in PC12 cell line is poorly understood. In this paper, we have demonstrated the tolerance to acute and chronic nicotine administrations on PC12 cell line on the basis of the expressions of dopamine receptors and tyrosine hydroxylase, the rate-limiting enzyme of dopamine biosynthesis, by Western blot, immunohistochemistry and in situ hybridization. In vitro treatment of nicotine resulted in similar expressional changes of dopamine D(2) receptor and tyrosine hydroxylase at protein and mRNA levels in dose- and time-dependent manner, whereas dopamine D(1) receptor did not reveal any positive output. Moreover, moderate to strong signals were obtained from 0.1 to 10 microM of nicotine concentrations and the signals were gradually decreased at 100 and 1000 microM nicotine concentrations relative to the untreated control cell line. Therefore, this study implied a new approach towards nicotine tolerance which is likely to be related to the modulation of dopamine D(2) receptor and tyrosine hydroxylase expressions by chronic and acute nicotine exposures in PC12 cell line.

Phospholipase C-mediated signalling is not required for histamine-induced catecholamine secretion from bovine chromaffin cells

A possible role for signalling through phospholipase C in histamine-induced catecholamine secretion from bovine adrenal chromaffin cells has been investigated. Secretion evoked by histamine over 10 min was not prevented by inhibiting inositol-1,4,5-trisphosphate receptors with 2-APB, by blocking ryanodine receptors with a combination of ryanodine and caffeine, or by depleting intracellular Ca(2+) stores by pretreatment with thapsigargin. Inhibition of protein kinase C with Ro31-8220 also failed to reduce secretion. Inhibition of phospholipase C with ET-18-OCH(3) reduced both histamine- and K(+) -induced inositol phosphate responses by 70-80% without reducing their secretory responses. Stimulating phospholipase C with Pasteurella multocida toxin did not evoke secretion or enhance the secretory response to histamine. The secretory response to histamine was little affected by tetrodotoxin or by substituting extracellular Na(+) with N -methyl-d-glucamine(+) or choline(+), or by substituting external Cl(-) with nitrate(-). Blocking various K(+) channels with apamin, charybdotoxin, Ba(2+), tetraethylammonium, 4-aminopyridine, tertiapin or glibenclamide failed to reduce the ability of histamine to evoke secretion. These results indicate that histamine evokes secretion by a mechanism that does not require inositol-1,4,5-trisphosphate-mediated mobilization of stored Ca(2+), diacylglycerol-mediated activation of protein kinase C, or activation of phospholipase C. The results are consistent with histamine acting by depolarizing chromaffin cells through a phospholipase C-independent mechanism.

Caffeine stimulates Ca(2+) entry through store-operated channels to activate tyrosine hydroxylase in bovine chromaffin cells

The ability of caffeine-induced store Ca(2+) mobilization to activate tyrosine hydroxylase was studied in bovine adrenal chromaffin cells. Caffeine increased tyrosine hydroxylase activity over 10 min with an EC(50) of 3 mm and maximum effect at 20 mm. The maximum response to caffeine was substantial, being almost one third that of the strongest agonists acetylcholine and PACAP-27, about half that for K(+) and similar to that for histamine. In contrast, catecholamine secretion evoked by caffeine was small, being less than 10% of the response to strong agonists. Caffeine-induced tyrosine hydroxylase activation was not mimicked or prevented by phosphodiesterase inhibition with isobutylmethylxanthine, nor was it mimicked by an equimolar concentration of sucrose. However, the effect of caffeine was prevented by depleting intracellular Ca(2+) stores by thapsigargin pretreatment, and reduced substantially by removing extracellular Ca(2+), by blocking Ca(2+) channels with Co(2+) or Ni(2+), or by inhibiting store-operated channels with 2-aminoethyl diphenylborate. It was not affected by inhibiting Ca(2+) entry through voltage-operated Ca(2+)-channels or by tetrodotoxin. The effect of caffeine was mimicked by acute thapsigargin treatment or by depleting intracellular Ca(2+) stores in Ca(2+)-free buffer and then reintroducing extracellular Ca(2+). The results indicate that mobilizing store Ca(2+) with caffeine is a very effective mechanism for activating tyrosine hydroxylase and that the majority of this response depends on extracellular Ca(2+) entry through store-operated channels. They also suggest that extracellular Ca(2+) entry through such channels regulates cellular responses differently to Ca(2+) entry through voltage-operated Ca(2+) channels.

Ca2+ -dependent activation of tyrosine hydroxylase involves MEK1

Tyrosine hydroxylase (TOH) activity is regulated acutely by phosphorylation of serines 8, 19, 31 and 40. The only kinases known to phosphorylate Ser31 are the mitogen-activated protein kinases MAPK-1 and 2. The involvement of these kinases in TOH activation in situ was therefore investigated using intact bovine chromaffin cells. Nicotine, K+ and A23187 increased TOH activity over 10 min in a Ca2+-dependent manner. The response to all three was reduced by PD098059, a selective inhibitor of the upstream activator of MAPK, MEK1. In contrast, TOH activation by forskolin and phorbol dibutyrate were unaffected by PD098059. The results support a key role for MEK1/MAPK in the acute activation of TOH by nicotinic receptors and by other agonists that increase cytosolic Ca2+.

Protein kinase C effectors bind to multidrug ABC transporters and inhibit their activity

P-Glycoprotein and homologous multidrug transporters contain a phosphorylatable linker sequence that was proposed to control drug efflux on the basis that it was indeed phosphorylated in vitro and in vivo, and that inhibitors of protein kinase C (PKC) inhibited both P-glycoprotein phosphorylation and activity. However, site-directed mutagenesis of all phosphorylatable residues did not alter the drug resistance. The present work shows that PKC effectors are able to bind directly to multidrug transporters, from either cancer cells (mouse P-glycoprotein), yeast (Saccharomyces cerevisiae Pdr5p), or protozoan parasite (Leishmania tropica ltmdr1), and to inhibit their energy-dependent drug-efflux activity. The binding of staurosporine and derivatives such as CGP 41251 is prevented by preincubation with ATP, suggesting at least partial interaction at the ATP-binding site. In contrast, more hydrophobic compounds such as calphostin C and CGP 42700 bind outside the ATP-binding site and strongly interfere with drug interaction. A direct correlation is obtained between the efficiencies of PKC effectors to inhibit energy-dependent interaction of rhodamine 6G with yeast Pdr5p, to promote intracellular drug accumulation in various multidrug resistant cells, and to chemosensitize growth of resistant cells. The noncompetitive inhibition by PKC effectors of rhodamine 6G interaction with Pdr5p suggests that the binding might interfere with signal transduction between nucleotide hydrolysis and drug interaction. The overall results indicate that the multidrug transporters from different species display common features for interaction with PKC inhibitors. The hydrophobic derivative of staurosporine, CGP 42700, constitutes a potentially powerful modulator of P-glycoprotein-mediated multidrug resistance.

Stimulation of catecholamine synthesis in cultured bovine adrenal medullary cells by leptin

Recently, we characterized leptin receptors in bovine adrenal medullary cells (Yanagihara et al. 2000). Here we report the stimulatory effect of leptin on catecholamine synthesis in the cells. Incubating cells with leptin (10 nM) for 20 min increased the synthesis of 14C-catecholamines from [14C]tyrosine, but not from L-3,4-dihydroxyphenyl [3-14C]alanine. The stimulation of catecholamine synthesis in the cells by leptin was associated with the phosphorylation and activation of tyrosine hydroxylase, the rate-limiting enzyme of catecholamine biosynthesis. The incubation of cells with leptin resulted in a rapid activation of the mitogen-activated protein kinases (MAPKs). An inhibitor of MAPK kinase, U0126, nullified the stimulatory effect of leptin on the synthesis of 14C-catecholamines. Leptin potentiated the stimulatory effect of acetylcholine on 14C-catecholamine synthesis, whereas leptin failed to enhance the phosphorylation and activation of tyrosine hydroxylase induced by acetylcholine. These findings suggest that leptin stimulates catecholamine synthesis via the activation of tyrosine hydroxylase by two different mechanisms, i.e., one is dependent on tyrosine hydroxylase phosphorylation mediated through the MAPK pathway and the second is independent of enzyme phosphorylation.

Activation of tyrosine hydroxylase by histamine in bovine chromaffin cells

Acute activation of tyrosine hydroxylase by histamine has been studied in cultured bovine chromaffin cells. Tyrosine hydroxylase activity was determined in situ by measuring 14CO2 release following the hydroxylation and rapid decarboxylation of 14C-tyrosine offered to the cells. Histamine increased tyrosine hydroxylase activity 2-fold over 10 min with an EC50 of 0.3 microM and maximal response at 10 microM. Tyrosine hydroxylase activation was detectable within 1-2 min and maintained for at least 10 min. The effect of histamine was fully blocked by the H1 antagonist mepyramine, but unaffected by H2 (cimetidine) and H3 (thioperamide) antagonists. It was mimicked by Nalpha-methylhistamine and the H1 agonist 2-thiazolylethylamine, but not by H2 (dimaprit) or H3 (R)alpha-methylhistamine) agonists. The response to histamine was reduced by 70% by removing extracellular Ca2+ and abolished by removing extracellular Ca2+ and chelating intracellular Ca2+ with BAPTA. Tyrosine hydroxylase activation by histamine was unaffected by the protein kinase C inhibitor Ro 31-8220 but was completely blocked by the protein kinase A inhibitor H89. The results indicate that histamine activates tyrosine hydroxylase and that this effect is mediated through H1 receptors by a mechanism that depends on both extracellular and intracellular Ca2+ and that requires protein kinase A.

Imaging of intracellular calcium during desensitization of nicotinic acetylcholine receptors of rat chromaffin cells

1. The possible role of intracellular Ca2+ levels ([Ca2+]i) in desensitization of nicotinic acetylcholine receptors (AChRs) was investigated in rat cultured chromaffin cells by use of combined whole-cell patch clamping and confocal laser scanning microscopy with the fluorescent dye fluo-3. 2. On cells held at -70 mV, pressure-application of nicotine elicited inward currents with associated [Ca2+]i rises mainly due to influx through nicotinic AChRs. These responses were blocked by (+)-tubocurarine (10 microM) but were insensitive to alpha-bungarotoxin (1 microM) or Cd2+ (0.1 mM). 3. Pressure applications of 1 mM nicotine for 2 s (conditioning pulse) evoked inward currents which faded biexponentially to a steady state level due to receptor desensitization and were accompanied by a sustained increase in [Ca2+]i. Inward currents evoked by subsequent application of brief test pulses of nicotine were depressed but recovered with a time course reciprocal to the decay of the [Ca2+]i transient induced by the conditioning pulse. 4. Omission of intracellular Ca2+ chelators or use of high extracellular Ca2+ solution (10 mM) lengthened recovery of nicotinic AChRs from desensitization while adding BAPTA or EGTA intracellularly had the opposite effect. When the patch pipette contained fluo-3 or no chelators, after establishing whole cell conditions the rate of recovery became progressively longer presumably due to dialysis of endogenous Ca2+ buffers. None of these manipulations of external or internal Ca2+ had any effect on onset or steady state level of desensitization. 5. High spatial resolution imaging of [Ca2+]i in intact cells (in the presence of 0.1 mM Cd2+) showed that its level in the immediate submembrane area decayed at the same rate as in the rest of the cell, indicating that Ca2+ was in a strategic location to modulate (directly or indirectly) AChR desensitization. 6. The present data suggest that desensitized nicotinic AChRs are stabilized in their conformation by raised [Ca2+]i and that this phenomenon retards their recovery to full activity.