Effects of cyclic nucleotides and phorbol myristate acetate on proliferation of pig aortic endothelial cells

Natriuretic peptides and their receptors in the central nervous system

Natriuretic peptides (NPs), including atrial, brain and C-type NPs, are a family of structurally related but genetically distinct peptides. These peptides, along with their receptors (NPRs), are long known to be involved in the regulation of various physiological functions, such as diuresis, natriuresis, and blood flow. Recently, abundant evidence shows that NPs and NPRs are widely distributed in the central nervous system (CNS), suggesting possible roles of NPs in modulating physiological functions of the CNS. This review starts with a brief summary of relevant background information, such as molecular structures of NPs and NPRs and general intracellular mechanisms after activation of NPRs. We then provide a detailed description of the expression profiles of NPs and NPRs in the CNS and an in-depth discussion of how NPs are involved in neural development, neurotransmitter release, synaptic transmission and neuroprotection through activation of NPRs.

The cyclin-dependent kinase inhibitor p21cip1 mediates the growth inhibitory effect of phorbol esters in human venous endothelial cells

Long-term application of the phorbol ester phorbol 12,13-dibutyrate (PDBu) inhibits the proliferation of human venous endothelial cells. The cyclin-dependent kinase inhibitor p21cip1 is a potential candidate mediating the PDBu-induced delayed entry of the cells into S-phase (by approximately 10 h when compared with cells stimulated with basic fibroblast growth factor (bFGF)). Levels of p21cip1 (protein and mRNA) rapidly rise (within approximately 2 h) in endothelial cells treated with the active isomer beta-PDBu, but not with alpha-PDBu; this effect is blocked by the mitogen-activated protein kinase kinase-1 (Mek1) inhibitor PD098059 and by the protein kinase C (PKC) antagonists GF109203X and rottlerin (selective for PKC-delta), but not Gö 6976 (selective for Ca2+-dependent PKC isoforms). Rapamycin blocks the PDBu-induced accumulation of p21cip1 (but not of the cognate mRNA), indicating an action of PKC on p21(cip1) mRNA translation. If endothelial cells are recruited into the cell cycle by bFGF, p21cip1 mRNA and protein levels rise initially (within 2 h) and decline subsequently such that p21cip1 drops to a minimum prior to the initiation of DNA synthesis (i.e. after approximately 12 h). In bFGF-stimulated cells, changes in p21cip1 mRNA and protein are strictly linked. In contrast, the levels of p21cip1 mRNA decline substantially (>10 h) before the protein decreases in PDBu-stimulated cells. Thus, PKC (presumably PKC-delta) regulates the amounts of p21cip1 in endothelial cells at the level of mRNA accumulation and translation, leading to a rapid and robust induction; following persistent PKC activation, p21(cip1) remains elevated despite reduced mRNA levels, indicating an enhanced stability of the protein. The bFGF-mediated increase in p21cip1 is blocked by the Mek1 inhibitor, but not by GF109203X; hence, in endothelial cells, induction of p21cip1 by PKC- and growth factor-dependent signaling is achieved by distinct pathways that converge and require activation of the mitogen-activated protein kinase cascade. The beta-PDBu-induced delayed S-phase entry and drop in p21cip1 are reversed if GF109203X is added 4 h after beta-PDBu to prevent persistent PKC activation. These observations indicate a cause and effect relation between sustained p21cip1 elevations and the delay in S-phase entry induced by beta-PDBu.

Exogenous, basal, and flow-induced nitric oxide production and endothelial cell proliferation

The role of nitric oxide (NO) from endogenous and exogenous sources in regulating large vessel and microvascular endothelial cell proliferation was investigated. Exogenous NO liberated from five different chemical donors inhibited bovine aortic, bovine retinal microvascular, and human umbilical vein endothelial cell proliferation in a dose-dependent manner as determined by 3H-thymidine incorporation. The potency of the donors varied as a function of the donors' half-lives. Donors with half-lives greater than 30 min were more effective than donors with significantly shorter half-lives. Coincubation of endothelial cells with 0.4 mM deoxyadenosine and 0.4 mM deoxyguanosine reduced the percentage of inhibition due to an NO donor. These data are consistent with a ribonucleotide reductase-dependent mechanism of inhibition. Inhibition of basal NO production with four different inhibitors of nitric oxide synthase (NOS) did not modify proliferation. Laminar flow with a wall shear stress of 22 dyn/cm2 inhibited the proliferation of subconfluent bovine aortic endothelial cells. The addition of a NOS inhibitor did not abrogate the flow-induced inhibition of proliferation, suggesting that flow-stimulated release of NO from endothelial cells did not account for flow-induced inhibition of proliferation. Taken together, these data suggest that relatively large concentrations of exogenous NO inhibit endothelial cell proliferation, while endogenous levels of NO are inadequate to inhibit proliferation.

Stimulation of the mitogen-activated protein kinase via the A2A-adenosine receptor in primary human endothelial cells

Adenosine exerts a mitogenic effect on human endothelial cells via stimulation of the A2A-adenosine receptor. This effect can also be elicited by the beta2-adrenergic receptor but is not mimicked by elevation of intracellular cAMP levels. In the present work, we report that stimulation of the A2A-adenosine receptor and of the beta2-adrenergic receptor activates mitogen-activated protein kinase (MAP kinase) in human endothelial cells based on the following criteria: adenosine analogues and beta-adrenergic agonists cause an (i) increase in tyrosine phosphorylation of the p42 isoform and to a lesser extent of the p44 isoform of MAP kinase and (ii) stimulate the phosphorylation of myelin basic protein by MAP kinase; (iii) this is accompanied by a redistribution of the enzyme to the perinuclear region. Pretreatment of the cells with cholera toxin (to down-regulate Gsalpha) abolishes activation of MAP kinase by isoproterenol but not that induced by adenosine analogues. In addition, MAP kinase stimulation via the A2A-adenosine receptor is neither impaired following pretreatment of the cells with pertussis toxin (to block Gi-dependent pathways) nor affected by GF109203X (1 microM; to inhibit typical protein kinase C isoforms) nor by a monoclonal antibody, which blocks epidermal growth factor-dependent signaling. In contrast, MAP kinase activation is blocked by PD 098059, an inhibitor of MAP kinase kinase 1 (MEK1) activation, which also blunts the A2A-adenosine receptor-mediated increase in [3H]thymidine incorporation. Activation of the A2A-adenosine receptor is associated with increased levels of GTP-bound p21(ras). Thus, our experiments define stimulation of MAP kinase as the candidate cellular target mediating the mitogenic action of the A2A-adenosine receptor on primary human endothelial cells; the signaling pathway operates via p21(ras) and MEK1 but is independent of Gi, Gs, and the typical protein kinase C isoforms. This implies an additional G protein which links this prototypical Gs-coupled receptor to the MAP kinase cascade.

Human endothelial cells bioactivate organic nitrates to nitric oxide: implications for the reinforcement of endothelial defence mechanisms

Although in therapeutic use for more than a century, the mode of cellular action of organic nitrates remains incompletely understood. Despite ample experimental evidence from animal studies to show that nitrates are metabolized to NO in the vascular smooth muscle, direct demonstration of such an activity in human vascular cells is still lacking. Moreover, the role of the endothelium in modulating the pharmacodynamic action of nitrates is far from clear. We therefore aimed to investigate whether or not human endothelial cells are capable of bioactivating these drugs to NO and whether the amounts generated are sufficient to elicit any biological effects. Using cultured human umbilical vein endothelial cells (HUVECs) as an established model system a combination of three different methods was used to address this issue: (1) quantification of NO formation upon endothelial nitrate metabolism using the oxyhaemoglobin technique; (2) evaluation of the second messenger response using radioimmunoassay for cGMP; and (3) assessment of mechanism and extent of potentiation of the anti-aggregatory effect of nitrates in the presence of endothelial cells as a relevant bioassay. We now show that superfusion of cultured human endothelial cells on microcarrier beads with either glyceryl trinitrate (GTN) or isosorbide dinitrate (ISDN; both at 0.1-100 mumol L-1) results in a concentration-dependent formation of NO. NO generation from isosorbide 5-mononitrate (IS-5-N) was below the detection limit. The amounts of NO produced (maximally 2.97 +/- 0.98 pmoles NO min-1 x mg protein with 100 mumol L-1 GTN; n = 8) were similar to those elicited upon challenge of the cells with 100 nM bradykinin. NO formation from either organic nitrate was accompanied, in a concentration-dependent and methylene blue-inhibitable manner, by stimulation of endothelial soluble guanylyl cyclase with consequent increases in the intracellular level of cGMP (maximally 32-fold over basal levels with ISDN), a significant portion of which was released into the extracellular space. Upon continuous 30 min superfusion or repeated application of high concentrations of GTN (100 mumol L-1) nitrate bioactivation to NO was subject to partial tachyphylaxis. Co-incubation of washed human platelets with HUVECs potentiated the anti-aggregatory action of nitrates in a cell number dependent and oxyhaemoglobin-sensitive manner and this effect, too, was accompanied by increases in intraplatelet cGMP levels. The potentiating effect was largely inhibited after blockade of sulfhydryl groups by pre-incubation of HUVECs with N-ethylmaleimide and completely abrogated after pretreatment of cells with the tissue fixative glutaraldehyde. These results demonstrate that human endothelial cells are capable of bioactivating organic nitrates to NO by an enzymatic, apparently thiol-sensitive pathway, in quantities sufficient to influence endothelial and platelet function. Besides the well known vasorelaxant action of organic nitrates, which is mainly due to their metabolism in the smooth muscle compartment, these drugs may therefore be endowed with a hitherto underestimated potential to directly influence endothelial functions via the NO/cGMP pathway. Through specific bioactivation in the endothelium itself organic nitrates can thus mimic and reinforce protective functions normally served by a functional endothelium such as the modulation of blood cell/vessel wall interactions and inhibition of cell proliferation.

Stimulation of human umbilical vein endothelial cell proliferation by A2-adenosine and beta 2-adrenoceptors

1. Adenosine is known to stimulate capillary outgrowth and endothelial cell proliferation, but the underlying mechanism has not been identified. In order to identify the receptor subtype involved, the effects of adenosine receptor agonists and antagonists on human umbilical vein endothelial cell (HUVEC) proliferation were investigated. 2. Raising intracellular adenosine levels by use of the adenosine transport inhibitor, 4-nitrobenzylthioinosine (NBMPR) did not affect cell growth. This observation suggests that stimulation of an extracellular adenosine receptor generates the mitogenic signal. 3. In the presence of adenosine deaminase (ADA), which was used to remove adenosine present in the culture medium, the adenosine receptor agonists N-ethylcarboxamidoadenosine (NECA, non-selective) and CGS21680 (A2A-receptor-selective) stimulated [3H]-thymidine incorporation with a half-maximum effect at about 10 nM, while N6-cyclopentyladenosine (CPA, A1-selective) was about 100 fold less potent. The adenosine receptor antagonist, xanthine amine congener (XAC) produced a concentration-dependent decrease in endothelial cell proliferation with a half-maximum effect at about 10 nM. Hence, stimulation of an endothelial A2A-adenosine receptor seems responsible for the mitogenic signal. 4. In the presence of ADA, isoprenaline is also able to stimulate [3H]-thymidine incorporation with a half maximal effect of about 3 nM, an effect, which is reversed by the highly beta 2-selective antagonist, ICI 118,551. In the absence of ADA, isoprenaline exerts only a minor stimulatory effect. Combination of A2A adenosine and beta 2-adrenoceptor agonists did not further enhance [3H]-thymidine incorporation when compared to the sole addition of each agonist. We therefore conclude that both receptors stimulate endothelial cell proliferation via a common signal transduction pathway. 5. Both receptors are coupled to stimulation of adenylyl cyclase via the stimulatory G protein G8.However, direct activation of downstream effectors in the cyclic AMP-signalling cascade (G8 with cholera toxin, adenylyl cyclase with forskolin, protein kinase A with 8Br-cyclic AMP) not only failed to mimic the action of receptor-activation, but even reduced cell proliferation.6. Similarly, pertussis toxin-treatment which inactivated the Gi 2 protein present in HUVEC and thus inhibited cell proliferation per se, did not impair the ability of A2A-receptor agonists to stimulate cell proliferation. This suggests that the A2A-adenosine and beta2-adrenoceptor-mediated stimulation of endothelial cell proliferation occurs via a mechanism that is independent of G8 and Gi.

IL-1 beta-stimulated leucocyte-endothelial adhesion is regulated, in part, by the cyclic-GMP-dependent signal transduction pathway

It is well known that the exposure of endothelial cells to IL-1 beta induces an increase in endothelial cell adhesiveness for leucocytes. Using rat heart endothelial cells we found that exposure of endothelial cells to IL-1 beta (100 U/ml) induces a 133-fold increase in the intracellular concentration of cyclic-GMP; from 11.5 +/- 0.2 fM to 1530 +/- 117.8 fM (per 10(6) cells). Therefore, we examined whether cyclic-GMP is involved in the regulation of endothelial adhesiveness for leucocytes. Cyclic-GMP analogue, dibutyryl cyclic-GMP Methylene blue, an inhibitor of guanylate cyclaese, and KT5823, a specific inhibitor of cyclic-GMP-dependent protein kinase, inhibited both basal as well as IL-1 beta-induced endothelial cell adhesiveness for leucocytes, and KT5823 abolished the dibutyryl-cyclic-GMP-induced increase in endothelial adhesiveness. The effect of cyclic-GMP, induced by IL-1 beta treatment, on the endothelial adhesiveness may be either direct or indirect because of the time-gap between the rise in cyclic-GMP level and the increase of endothelial adhesiveness. IL-1 beta (100 U/ml) and dibutyryl-cyclic-GMP (0.01 mM) both induced an increase in the expression of intercellular adhesion molecule-1 by endothelial cells. However, the fact that KT5823 failed to prevent this increase, suggests that, although the IL-1 beta-induced increase in adhesiveness is caused by the increase in intracellular levels of cyclic-GMP, it may not be mediated through intercellular adhesion molecule-1. In conclusion, the results obtained indicate that endothelial cell adhesiveness for leucocytes is, in part, regulated by the cyclic-GMP-dependent signal transduction pathway.

Effects of adenine nucleotides on the proliferation of aortic endothelial cells

The effects of adenine nucleotides and adenosine on DNA synthesis and cell growth have been studied in bovine aortic endothelial cells (BAECs). ATP produced a small but significant (+44%) increase of the fraction of BAECs whose nuclei are labeled by [3H]thymidine. This mitogenic effect was mimicked by ADP, the phosphorothioate analogues ATP gamma S and ADP beta S, and the nonhydrolyzable analogue adenosine 5'-(beta, gamma-imido)triphosphate (APPNP), whereas adenosine 5'-(alpha, beta-methylene)triphosphate (APCPP), a selective agonist of P2x-purinoceptors, had no effect at 10 microM and a small one at 100 microM; this profile is consistent with the involvement of P2y-receptors. Adenosine induced a mitogenic response of a magnitude similar to that of ATP. This effect was not reproduced by R-phenylisopropyl adenosine, by 5'-N-ethylcarboxamide adenosine, or by 2',5'-dideoxyadenosine, selective ligands of the A1- and A2-receptors and the P site, respectively, nor was it inhibited by 8-phenyltheophylline, an antagonist of both A1- and A2-receptors. The mechanism of this adenosine action thus remains unclear. ATP and ATP gamma S did not enhance the proliferation of BAECs cultured in the presence of fetal calf serum concentrations ranging from 0.5% to 10%. They inhibited the growth-promoting effect of basic fibroblast growth factor; among the various nucleotides tested, APCPP was the least effective to reproduce the action of ATP, suggesting the possible involvement of P2y-receptors.(ABSTRACT TRUNCATED AT 250 WORDS)