Pleiotropic regulation of vascular smooth muscle tone by cyclic GMP-dependent protein kinase.

Cyclic GMP (cGMP) mediates vascular smooth muscle relaxation in response to nitric oxide and atrial natriuretic peptides. One mechanism by which cGMP decreases vascular tone is by lowering cytosolic Ca2+ levels in smooth muscle cells. Although mechanisms by which cGMP regulates cytosolic Ca2+ are unclear, an important role for the cGMP-dependent dependent protein kinase in regulating Ca2+ has been proposed. Cyclic GMP-dependent protein kinase has been shown to regulate several pathways that control cytosolic Ca2+ levels: inositol 1,4,5-trisphosphate production and action, Ca(2+)-ATPase ATPase activation, and activation of Ca(2+)-activated K+ channels. The pleiotropic action of cGMP-dependent protein kinase is proposed to occur through the phosphorylation of important proteins that control several signaling pathways in smooth muscle cells. One potential target for cGMP-dependent protein kinase is the class of okadaic acid-sensitive protein phosphatases that appears to regulate K+ channels among other potentially important events to reduce cytosolic Ca2+ and tone. In addition, cytoskeletal proteins are targets for cGMP-dependent protein phosphorylation, and it is now appreciated that the cytoskeleton may play a key role in signal transduction.

Evidence that additional mechanisms to cyclic GMP mediate the decrease in intracellular calcium and relaxation of rabbit aortic smooth muscle to nitric oxide.

1. The role of cyclic GMP in the ability of nitric oxide (NO) to decrease intracellular free calcium concentration [Ca2+]i and divalent cation influx was studied in rabbit aortic smooth muscle cells in primary culture. In cells stimulated with angiotensin II (AII, 10(-1) M), NO (10(-10) - 10(-6) M) increased cyclic GMP levels measured by radioimmunoassay and decreased [Ca2+]i and cation influx as indicated by fura-2 fluorimetry. 2. Zaprinast (10(-4) M), increased NO-stimulated levels of cyclic GMP by 3-20 fold. Although the phosphodiesterase inhibitor lowered the level of [Ca2+]i reached after administration of NO, the initial decreases in [Ca2+]i initiated by NO were not significantly different in magnitude or duration from those that occurred in the absence of zaprinast. 3. The guanylyl cyclase inhibitor, H-(1,2,4) oxadiazolo(4,3-a) quinoxallin-1-one (ODQ, 10(-5) M), blocked cyclic GMP accumulation and activation of protein kinase G, as measured by back phosphorylation of the inositol trisphosphate receptor. ODQ and Rp-8-Br-cyclic GMPS, a protein kinase G inhibitor, decreased the effects of NO, 10(-10) - 10(-8) M, but the decrease in [Ca2+]i or cation influx caused by higher concentrations of NO (10(-7) - 10(-6) M) were unaffected. Relaxation of intact rabbit aorta rings to NO (10(-7) - 10(-5) M) also persisted in the presence of ODQ without a significant increase in cyclic GMP. Rp-8-Br-cyclic GMPS blocked the decreases in cation influx caused by a cell permeable cyclic GMP analog, but ODQ and/or the protein kinase G inhibitor had no significant effect on the decrease caused by NO. 4. Although inhibitors of cyclic GMP, protein kinase G and phosphodiesterase can be shown to affect the decrease in [Ca2+]i and cation influx via protein kinase G, these studies indicate that when these mechanisms are blocked, cyclic GMP-independent mechanisms also contribute significantly to the decrease in [Ca2+]i and smooth muscle relaxation to NO.

PI3-kinase/Akt modulates vascular smooth muscle tone via cAMP signaling pathways.

Phosphatidylinositol 3-kinase (PI3-kinase) activates protein kinase B (also known as Akt), which phosphorylates and activates a cyclic nucleotide phosphodiesterase 3B. Increases in cyclic nucleotide concentrations inhibit agonist-induced contraction of vascular smooth muscle. Thus we hypothesized that the PI3-kinase/Akt pathway may regulate vascular smooth muscle tone. In unstimulated, intact bovine carotid artery smooth muscle, the basal phosphorylation of Akt was higher than that in cultured smooth muscle cells. The phosphorylation of Akt decreases in a time-dependent manner when incubated with the PI3-kinase inhibitor, LY-294002. Agonist (serotonin)-, phorbol ester (phorbol 12,13-dibutyrate; PDBu)-, and depolarization (KCl)-induced contractions of vascular smooth muscles were all inhibited in a dose-dependent fashion by LY-294002. However, LY-294002 did not inhibit serotonin- or PDBu-induced increases in myosin light chain phosphorylation or total O(2) consumption, suggesting that inhibition of contraction was not mediated by reversal or inhibition of the pathways that lead to smooth muscle activation and contraction. Treatment of vascular smooth muscle with LY-294002 increased the activity of cAMP-dependent protein kinase and increased the phosphorylation of the cAMP-dependent protein kinase substrate heat shock protein 20 (HSP20). These data suggest that activation of the PI3-kinase/Akt pathway in unstimulated smooth muscle may modulate vascular smooth muscle tone (allow agonist-induced contraction) through inhibition of the cyclic nucleotide/HSP20 pathway and suggest that cyclic nucleotide-dependent inhibition of contraction is dissociated from the myosin light chain contractile regulatory pathways.

Phosphorylation of the inositol 1,4,5-trisphosphate receptor by cyclic GMP-dependent protein kinase.

Cyclic GMP (cGMP) inhibits intracellular calcium ([Ca2+]i) mobilization in vascular smooth muscle cells by a mechanism that is not well understood. Because several studies suggest that cGMP inhibits inositol 1,4,5-trisphosphate (IP3) action, we examined the effects of cGMP-dependent protein kinase on IP3 receptor phosphorylation. The purified IP3 receptor was phosphorylated using either the cGMP- or cAMP-dependent protein kinase in vitro. Phosphorylation was time-dependent and stoichiometric using both kinases. Two-dimensional phosphopeptide mapping, high performance liquid chromatography analysis, and amino acid analysis showed that identical sites were phosphorylated using either kinase, and identified serine 1755 as the site of phosphorylation. The synthetic peptide corresponding to serine 1755 (GRRESLTSFG) was phosphorylated with aKm in the range of 30-40 microM by both kinases. The kinetic analysis revealed that this peptide substrate is the best substrate described for cGMP kinase to date. Vascular smooth muscle cells prelabeled with [32P]orthophosphate and treated with atrial natriuretic peptide or sodium nitroprusside to elevate cGMP also resulted in increased labeling of the IP3 receptor. Phosphorylation of IP3 receptor by cGMP kinase may regulate the function of IP3 receptor in vascular smooth muscle cells and contribute to the effect of cGMP to regulate intracellular calcium levels.

Phosphorylation of the inositol 1,4,5-trisphosphate receptor. Cyclic GMP-dependent protein kinase mediates cAMP and cGMP dependent phosphorylation in the intact rat aorta.

The effects of cyclic GMP (cGMP) and activation of cGMP-dependent protein kinase (PKG) on the phosphorylation of the inositol 1,4, 5-trisphosphate (IP3) receptor were examined in intact rat aorta using the technique of back phosphorylation. Aorta treated with the nitric oxide donors, S-nitroso-N-acetylpenicillamine and sodium nitroprusside, or the selective PKG activator, 8-(4-para-chlorophenylthio)-cGMP (8-CPT-cGMP), demonstrated increased IP3 receptor phosphorylation in situ, which was both time- and concentration-dependent with a stoichiometry of 0.5 mol of phosphate/mol of receptor above control. Treatment of aorta with the adenyl cyclase activator, forskolin, also demonstrated increased phosphorylation of the IP3 receptor on the PKG site, although the selective cAMP-dependent protein kinase activator, 8-(4-para-chlorophenylthio)-cAMP (8-CPT-cAMP), did not increase the phosphorylation of the IP3 receptor. Moreover, the PKG selective inhibitor, KT 5823, inhibited both sodium nitroprusside and forskolin-induced IP3 receptor phosphorylation more potently than the selective cAMP-dependent protein kinase inhibitor, KT 5720, suggesting that PKG mediates the increase in IP3 receptor phosphorylation by both cyclic nucleotides in intact aorta. These results provide further support for the notion that PKG is activated by both cAMP and cGMP in intact vascular smooth muscle and that PKG performs a critical role in cyclic nucleotide-dependent relaxation of blood vessels.

Arabinogalactan-proteins from the suspension culture medium and plasma membrane of rose cells.

Arabinogalactan-proteins (AGPs) that bind to beta-glycosyl Yariv antigens have been purified from the culture medium and plasma membrane of "Paul's Scarlet" rose cells. Starting from culture medium or from plasma membrane vesicles prepared by aqueous two-phase partitioning, the purification procedure involved Yariv antigen-induced precipitation and subsequent chromatographic steps. Two fractions, AGP-(a) and AGP-(b), were obtained from the culture medium, and one AGP fraction was obtained from the plasma membrane. The glycosyl compositions of all three fractions were dominated by arabinosyl and galactosyl residues and included glucuronosyl and other minor residues. Methylation analysis showed that AGP-(a) and AGP-(b) were both highly branched 3,6-galactans with terminal arabinofuranosyl substituents. The amino acid compositions of all three AGPs were high in alanine, hydroxyproline, and serine and/or threonine. The amino-terminal sequence of AGP-(b) contained an alanine-hydroxyproline repeat. While sharing general structural similarity, the AGPs from the plasma membrane and the culture medium were distinguishable by composition and by size and charge, with the plasma membrane AGPs being larger and more negatively charged than the culture medium AGPs.

Analysis of expression of cGMP-dependent protein kinase in rabbit heart cells.

We previously showed that stimulation of cGMP-dependent protein kinase (PKG) stimulates L-type calcium current in newborn but not in adult rabbit ventricular myocytes. We have now isolated rabbit PKG type Ialpha cDNA (+1 to 2095), determined the sequence, and analyzed specific expression of PKG in adult and newborn rabbit heart by Western and Northern analyses to elucidate the developmental decline in the significance of PKG in cardiac function. We obtained full-length cDNA of PKG Ialpha from newborn rabbit heart mRNA with reverse transcription-polymerase chain reaction. The coding region of rabbit PKG Ialpha showed 94% homology to sequences of human and bovine PKG Ialpha. The deduced amino acid sequence of 671 amino acids showed seven substitutions between rabbit and either human or bovine PKG Ialpha. The major substitutions were found in the cGMP-binding domain. The cloned PKG 1alpha cDNA was expressed in COS cells. Expressed PKG showed cGMP stimulated PKG activity and immunoreactivity. Northern blot analysis of cardiac tissue demonstrated PKG Ialpha mRNA of 6.8 kb, with much higher levels in newborn than in adult cells. Western analysis in homogenates from ventricular tissues and isolated ventricular myocytes of rabbit heart showed much higher expression of PKG type I protein in newborn compared with adult cells. These findings suggest that PKG is developmentally regulated in rabbit heart and is expressed at a much higher level in newborn than in adult cells. The greater expression of PKG in newborn cells could be responsible for differences in the significance of cGMP in adult and newborn rabbit cells.

Activation of mitogen-activated protein kinase pathways by cyclic GMP and cyclic GMP-dependent protein kinase in contractile vascular smooth muscle cells.

Vascular smooth muscle cells (VSMC) exist in either a contractile or a synthetic phenotype in vitro and in vivo. The molecular mechanisms regulating phenotypic modulation are unknown. Previous studies have suggested that the serine/threonine protein kinase mediator of nitric oxide (NO) and cyclic GMP (cGMP) signaling, the cGMP-dependent protein kinase (PKG) promotes modulation to the contractile phenotype in cultured rat aortic smooth muscle cells (RASMC). Because of the potential importance of the mitogen-activated protein kinase (MAP kinase) pathways in VSMC proliferation and phenotypic modulation, the effects of PKG expression in PKG-deficient and PKG-expressing adult RASMC on MAP kinases were examined. In PKG-expressing adult RASMC, 8-para-chlorophenylthio-cGMP activated extracellular signal- regulated kinases (ERK1/2) and c-Jun N-terminal kinase (JNK). The major effect of PKG activation was increased activation by MAP kinase kinase (MEK). The cAMP analog, 8-Br-cAMP inhibited ERK1/2 activation in PKG-deficient and PKG-expressing RASMC but had no effect on JNK activity. The effects of PKG on ERK and JNK activity were additive with those of platelet-derived growth factor (PDGF), suggesting that PKG activates MEK through a pathway not used by PDGF. The stimulatory effects of cGMP on ERK and JNK activation were also observed in low-passaged, contractile RASMC still expressing endogenous PKG, suggesting that the effects of PKG expression were not artifacts of cell transfections. These results suggest that in contractile adult RASMC, NO-cGMP signaling increases MAP kinase activity. Increased activation of these MAP kinase pathways may be one mechanism by which cGMP and PKG activation mediate c-fos induction and increased proliferation of contractile adult RASMC.

Protein kinase G is not essential to NO-cGMP modulation of basal tone in rat pulmonary circulation.

Nitric oxide (NO) is important in modulating increased pulmonary vascular tone. Whereas in other systems it is believed that the action of NO is mediated through guanosine 3',5'-cyclic monophosphate (cGMP) and protein kinase G (PKG), the validity of this pathway in the pulmonary circulation has not been established. Using isolated salt-perfused normotensive and hypertensive rat lungs, we studied the effects of the soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), and the PKG inhibitors, KT5823, Rp-8-pCPT-cGMPS, and (N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide) (H-8), on pulmonary vascular resistance. In isolated normotensive lungs, ODQ-mediated inhibition of soluble guanylyl cyclase augmented hypoxic pulmonary vasoconstriction, whereas the PKG inhibitors had no effect. Despite the marked differences in the physiological effect, ODQ and Rp-8-pCPT-cGMPS inhibited PKG activity to a similar degree as determined by a back-phosphorylation assay showing decreased PKG-mediated phosphorylation of serine 1755 on the D-myo-inositol 1,4,5-trisphosphate receptor. In hypertensive lungs, inhibition of soluble guanylyl cyclase by ODQ increased perfusion pressure by 101 +/- 20% (P < 0.05), an increase similar to that seen with inhibition of NO synthase (NOS), confirming an essential role for cGMP. In contrast, KT5823, Rp-8-pCPT-cGMPS, and H-8 (used in doses 5- to 100-fold in excess of their reported inhibitory concentrations for PKG) caused only a small increase in baseline perfusion pressure (14 +/- 2%, P = not significant from vehicle control). Effectiveness of PKG inhibition in the hypertensive lungs was also confirmed with the back-phosphorylation assay. These studies suggest that whereas NO-mediated modulation of vascular tone in the normotensive and hypertensive pulmonary circulation is dependent on cGMP formation, activation of PKG may not be essential.