Phosphorylation of Tyrosine Hydroxylase by cGMP-Dependent Protein Kinase in Intact Bovine Chromaffin Cells

Abstract

The phosphorylation of the enzyme tyrosine hydroxylase by the cGMP pathway was investigated in chromaffin cells from the bovine adrenal medulla. The nitric oxide donor, sodium nitroprusside, and the natriuretic peptide, C-type natriuretic peptide, which are able to increase cGMP levels and cGMP-dependent protein kinase activity, produced significant increases in the phosphorylation level of tyrosine hydroxylase in a time- and concentration-dependent manner. The pretreatment of the cells with the soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one blocked the effect of sodium nitroprusside. This result indicates that cGMP production by this enzyme mediated this effect. Experiments performed with a cGMP-dependent protein kinase inhibitor, the Rp-isomer of 8-(4-chlorophenylthio)-cyclic guanosine monophosphorothioate, which blocked the effects of both sodium nitroprusside and C-type natriuretic peptide, demonstrated that the phosphorylation increases evoked by both compounds were mediated by the activation of cGMP-dependent protein kinase. In cells incubated with the adenylyl cyclase activator, forskolin, an increase in the phosphorylation level of the tyrosine hydroxylase was also found. When cells were treated simultaneously with forskolin and sodium nitroprusside or C-type natriuretic peptide, an additive effect on tyrosine hydroxylase phosphorylation was not observed. This suggests that cAMP- and cGMP-dependent protein kinases may phosphorylate the same amino acid residues in the enzyme. Western blot analysis of soluble extracts from chromaffin cells detected specific immunoreactivity for two different commercial antibodies raised against cGMP-dependent protein kinase (both Iα and Iβ isoforms). Electrophoretic mobility correlates with that of purified PKG Iα. Because the phosphorylation of the tyrosine hydroxylase correlates with increases in its enzymatic activity and thus with augmentation in the cell capacity to synthesize catecholamines, our results indicate that a cGMP-based second messenger pathway participates in catecholamine biosynthesis regulation in chromaffin cells, a mechanism which may be widespread in other catecholamine-synthesizing cells.