Staurosporine inhibits inositol phosphate formation in bovine adrenal medullary cells

Protein kinase C delta negatively regulates tyrosine hydroxylase activity and dopamine synthesis by enhancing protein phosphatase-2A activity in dopaminergic neurons

Tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine synthesis, can be regulated by phosphorylation at multiple serine residues, including serine-40. In the present study, we report a novel interaction between a key member of the novel PKC family, protein kinase Cdelta (PKCdelta), and TH, in which the kinase modulates dopamine synthesis by negatively regulating TH activity via protein phosphatase 2A (PP2A). We observed that PKCdelta is highly expressed in nigral dopaminergic neurons and colocalizes with TH. Interestingly, suppression of PKCdelta activity with the kinase inhibitor rottlerin, PKCdelta-small interfering RNA, or with PKCdelta dominant-negative mutant effectively increased a number of key biochemical events in the dopamine pathway, including TH-ser40 phosphorylation, TH enzymatic activity, and dopamine synthesis in neuronal cell culture models. Additionally, we found that PKCdelta not only physically associates with the PP2A catalytic subunit (PP2Ac) but also phosphorylates the phosphatase to increase its activity. Notably, inhibition of PKCdelta reduced the dephosphorylation activity of PP2A and thereby increased TH-ser40 phosphorylation, TH activity, and dopamine synthesis. To further validate our findings, we used the PKCdelta knock-out (PKCdelta-/-) mouse model. Consistent with other results, we found greater TH-ser40 phosphorylation and reduced PP2A activity in the substantia nigra of PKCdelta-/- mice than in wild-type mice. Importantly, this was accompanied by an increased dopamine level in the striatum of PKCdelta-/- mice. Collectively, these results suggest that PKCdelta phosphorylates PP2Ac to enhance its activity and thereby reduces TH-ser40 phosphorylation and TH activity and ultimately dopamine synthesis.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

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.

Transfected adenosine A1 receptor-mediated modulation of thrombin-stimulated phospholipase C and phospholipase A2 activity in CHO cells

Thrombin receptor activation in Chinese hamster ovary (CHO) cells stimulates the hydrolysis of inositol phospholipids and the release of arachidonic acid. Our previous studies have shown that activation of the human transfected adenosine A1 receptor in CHO cells (CHO-A1) potentiates the accumulation of inositol phosphates elicited by endogenous P2U purinoceptors and CCKA receptors. In this study we have investigated whether adenosine A1 receptor activation can modulate thrombin-stimulated arachidonic acid release and/or inositol phospholipid hydrolysis in CHO-A1 cells. Thrombin stimulated [3H]arachidonic acid release and total [3H]inositol phosphate accumulation in CHO-A1 cells. Both these responses to thrombin were were insensitive to pertussis toxin. The protein kinase C activator, phorbol 12-myristate 13-acetate (PMA), potentiated thrombin-stimulated [3H]arachidonic acid. In marked contrast, PMA inhibited thrombin-stimulated [3H]inositol phosphate accumulation. The selective protein kinase C inhibitor Ro 31-8220 (3-¿1-[3-(2-isothioureido)propyl] indol-3-yl¿-4-(1-methylindol-3-yl)-3-pyrrolin-2,5-dione) had no effect on thrombin-stimulated [3H]arachidonic acid release but reversed the potentiation of thrombin-stimulated [3H]arachidonic acid release elicited by PMA. The selective adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) augmented the release of [3H]arachidonic acid produced by thrombin. Co-activation of the adenosine A1 receptor also potentiated thrombin-stimulated [3H]inositol phosphate accumulation. The synergistic interactions between the adenosine A1 receptor and thrombin were abolished in pertussis-toxin-treated cells. The potentiation of [3H]arachidonic acid release by CPA was blocked by the protein kinase C inhibitors Ro 31-8220 and GF 109203X (3-[1-[3-(dimethylamino)propyl]-1 H-indol-3-yl]-4-(1 H-indol-3-yl)- 1H-pyrrole-2,5-dione). In conclusion, thrombin receptor activation in CHO-A1 cells stimulates the accumulation of [3H]inositol phosphates and the release of [3H]arachidonic acid through pertussis-toxin-insensitive G-proteins. Experiments using PMA suggest that protein kinase C differentially regulates thrombin receptor activation of phospholipase C and phospholipase A2. Co-activation of the transfected human adenosine A1 receptor augments thrombin-stimulated phospholipase C and phospholipase A2 activity. Finally, the augmentation of phospholipase A2 activity by the adenosine A1 receptor is inhibited by selective protein kinase C inhibitors, suggesting the involvement of protein kinase C.