Regulation by hypoxia of endothelin-1-stimulated phospholipase D activity in sheep pulmonary artery cultured smooth muscle cells

Novel role for proteinase-activated receptor 2 (PAR2) in membrane trafficking of proteinase-activated receptor 4 (PAR4)

Proteinase-activated receptors 4 (PAR₄) is a class A G protein-coupled receptor (GPCR) recognized through the ability of serine proteases such as thrombin and trypsin to mediate receptor activation. Due to the irreversible nature of activation, a fresh supply of receptor is required to be mobilized to the cell surface for responsiveness to agonist to be sustained. Unlike other PAR subtypes, the mechanisms regulating receptor trafficking of PAR₄ remain unknown. Here, we report novel features of the intracellular trafficking of PAR₄ to the plasma membrane. PAR₄ was poorly expressed at the plasma membrane and largely retained in the endoplasmic reticulum (ER) in a complex with the COPI protein subunit β-COP1. Analysis of the PAR₄ protein sequence identified an arginine-based (RXR) ER retention sequence located within intracellular loop-2 (R¹⁸³AR → A¹⁸³AA), mutation of which allowed efficient membrane delivery of PAR₄. Interestingly, co-expression with PAR₂ facilitated plasma membrane delivery of PAR₄, an effect produced through disruption of β-COP1 binding and facilitation of interaction with the chaperone protein 14-3-3ζ. Intermolecular FRET studies confirmed heterodimerization between PAR₂ and PAR₄. PAR₂ also enhanced glycosylation of PAR₄ and activation of PAR₄ signaling. Our results identify a novel regulatory role for PAR₂ in the anterograde traffic of PAR₄. PAR₂ was shown to both facilitate and abrogate protein interactions with PAR₄, impacting upon receptor localization and cell signal transduction. This work is likely to impact markedly upon the understanding of the receptor pharmacology of PAR₄ in normal physiology and disease.

Novel antagonists for proteinase-activated receptor 2: inhibition of cellular and vascular responses in vitro and in vivo

BACKGROUND AND PURPOSE

Proteinase-activated receptor 2 (PAR(2)) is a G-protein coupled receptor associated with many pathophysiological functions. To date, the development of PAR(2) antagonists has been limited. Here, we identify a number of novel peptide-mimetic PAR(2) antagonists and demonstrate inhibitory effects on PAR(2)-mediated intracellular signalling pathways and vascular responses.

EXPERIMENTAL APPROACH

The peptide-mimetic compound library based on the structures of PAR(2) agonist peptides were screened for inhibition of PAR(2)-induced calcium mobilisation in human keratinocytes. Representative compounds were further evaluated by radioligand binding and inhibition of NFkappaB transcriptional activity and IL-8 production. The vascular effects of the antagonists were assessed using in vitro and in vivo models.

KEY RESULTS

Two compounds, K-12940 and K-14585, significantly reduced SLIGKV-induced Ca(2+) mobilisation in primary human keratinocytes. Both K-12940 and K-14585 exhibited competitive inhibition for the binding of a high-affinity radiolabelled PAR(2)-ligand, [(3)H]-2-furoyl-LIGRL-NH(2), to human PAR(2) with K(i) values of 1.94 and 0.627 microM respectively. NFkappaB reporter activity and IL-8 production were also significantly reduced. Furthermore, relaxation of rat-isolated aorta induced by SLIGRL-NH(2) was inhibited competitively by K-14585. K-14585 also significantly lowered plasma extravasation in the dorsal skin of guinea pigs and reduced salivation in mice.

CONCLUSIONS AND IMPLICATIONS

K-12940 and K-14585 antagonized PAR(2) competitively, resulting in inhibition of PAR(2)-mediated signalling and physiological responses both in vitro and in vivo. These peptide-mimetic PAR(2) antagonists could be useful in evaluating PAR(2)-mediated biological events and might lead to a new generation of therapeutically useful antagonists.

Dual effect of the novel peptide antagonist K-14585 on proteinase-activated receptor-2-mediated signalling

BACKGROUND AND PURPOSE

Here we have examined the effects of the novel peptide antagonist N-[1-(2,6-dichlorophenyl)methyl]-3-(1-pyrrolidinylmethyl)-1H-indol-5-yl]aminocarbonyl]-glycinyl-L-lysinyl-L-phenylalanyl-N-benzhydrylamide (K-14585) on proteinase-activated receptor (PAR)(2)-mediated intracellular signalling events.

EXPERIMENTAL APPROACH

Using NCTC2544 cells expressing PAR(2), we assessed the effects of K-14585 on PAR(2)-mediated [(3)H] inositol phosphate accumulation, MAP kinase activation, p65 NFkappaB phosphorylation and DNA binding and IL-8 production.

KEY RESULTS

Pretreatment with K-14585 (5 microM) inhibited [(3)H] inositol phosphate levels stimulated by PAR(2)-activating peptide Ser-Leu-Ile-Gly-Lys-Val (SLIGKV-OH) in PAR(2)-expressing NCTC2544 cells. K-14585 pretreatment did not influence PAR(2)-mediated extracellular regulated kinase activation but inhibited p38 MAP kinase phosphorylation. At a higher concentration (30 microM), K-14585 alone stimulated p38 MAP kinase activation. These effects were replicated in EAhy926 cells, endogenously expressing PAR(2), but not in parental or PAR(4)-expressing NCTC2544 cells, suggesting these effects were PAR(2)-dependent. SLIGKV-mediated stimulation of p38 MAP kinase phosphorylation was substantially reduced by the G(q/11) inhibitor YM-254890, without affecting K-14585-mediated phosphorylation. Pretreatment with K-14585 inhibited PAR(2)-mediated p65 NFkappaB phosphorylation and NFkappaB-DNA binding. K-14585 (30 microM) alone stimulated comparable NFkappaB reporter activity to SLIGKV-OH. K-14585 inhibited SLIGKV-stimulated IL-8 production, but given alone increased IL-8. While SLIGKV-induced IL-8 formation was reduced by both SB203580 and YM-254890, the response to K-14585 was sensitive to SB203580 but not YM-254890.

CONCLUSIONS AND IMPLICATIONS

These data reveal that K-14585 has a duality of action functioning both as an antagonist and agonist due to either partial agonist actions or possible agonist-directed signalling. The data also suggest two modes of p38 MAP kinase activation emanating from PAR(2), one G(q/11)-dependent and the other G(q/11)-independent.

Phospholipase D signaling in serotonin-induced mitogenesis of pulmonary artery smooth muscle cells

We have previously reported the participation of mitogen-activated protein, Rho, and phosphoinositide-3 (PI3) kinases in separate pathways in serotonin (5-HT)-induced proliferation of pulmonary artery smooth muscle cells (SMCs). In this study, we investigated the possible participation of phospholipase D (PLD) and phosphatidic acid (PA) in this growth process. 5-HT stimulated a time-dependent increase in [(3)H]phosphatidylbutanol and PA generation. Exposure of SMCs to 1-butanol or overexpression of an inactive mutant of human PLD1R898R blocked 5-HT-induced proliferation. Furthermore, 1-butanol inhibited 5-HT activation of S6K1 and S6 protein, downstream effectors of mammalian target of rapamycin (mTOR), by 80 and 72%, respectively, and partially blocked activation of extracellular signal-regulated kinase (ERK) by 30% but had no effect on other associated signaling pathways. Exogenous PA caused cellular proliferation and revitalized cyclin D1 expression by 5-HT of the 1-butanol-treated cells. PA also reproduced activations by 5-HT of mTOR, S6K1, and ERK. Transfection with inactive human PLD1 reduced 5-HT-induced activation of S6K1 by approximately 50%. Inhibition of 5-HT receptor 2A (R 2A) with ketaserin blocked PLD activation by 5-HT. Inhibition with PI3-kinase inhibitor failed to block either activation of PLD by 5-HT or PA-dependent S6K1 phosphorylation. Taken together, these results indicate that ligation of the 5-HTR 2A by 5-HT initiates PLD activation in SMCs, and that its product, PA, is an early signaling molecule in 5-HT-induced pulmonary artery SMC proliferation. Signaling by PA produces its downstream effects primarily through the mTOR/S6K1 pathway and to a lesser extent through the ERK pathway. Hydrolysis of cell membrane lipid may be important in vascular effects of 5-HT.

Cytokine upregulation of proteinase-activated-receptors 2 and 4 expression mediated by p38 MAP kinase and inhibitory kappa B kinase beta in human endothelial cells

BACKGROUND AND PURPOSE

Up-regulation of proteinase-activated receptor-2 (PAR2) is a factor in a number of disease states and we have therefore examined the signalling pathways involved in the expression of the receptor.

EXPERIMENTAL APPROACH

We investigated the effects of tumour necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), trypsin and the PAR2 activating peptide, 2-furoyl(2f)-LIGKV-OH on both mRNA and functional expression of PAR2 in human umbilical vein endothelial cells (HUVECs). The effect of specific chemical inhibitors and dominant negative adenovirus constructs of the mitogen-activated protein kinase (MAPK) cascade and the nuclear factor kappa B (NF-kappaB) signalling pathway was assessed. Methods included semi-quantitative and quantitative RT-PCR, [(3)H]inositol phosphate (IP) accumulation and Ca(2+)-dependent fluorescence.

KEY RESULTS

The above agonists induced both mRNA and functional expression of PAR2; PAR4 mRNA, but not that for PAR1 or PAR-3, also increased following TNFalpha treatment. Inhibition of p38 MAP kinase reduced PAR2 and PAR4 expression, whilst inhibition of MEK1/ERK/JNK was without effect. A similar dependency upon p38 MAP kinase was observed for the expression of PAR4. TNFalpha -induced enhancement of PAR2 stimulated [(3)H]-inositol phosphate accumulation (IP) and Ca(2+) signalling was abolished following SB203580 pre-treatment. Infection with adenovirus encoding dominant-negative IKKbeta (Ad.IKKbeta(+/-)) and to a lesser extent dominant-negative IKKalpha (Ad.IKKalpha(+/-)), substantially reduced both control and IL-1beta- induced expression of both PAR2 and PAR4 mRNA and enhancement of PAR2-stimulated IP accumulation and Ca(2+) mobilisation.

CONCLUSIONS AND IMPLICATIONS

These data reveal for the first time the signalling events involved in the upregulation of both PAR2 and PAR4 during pro-inflammatory challenge.

Changes in pulmonary arteriole protein kinase cαexpression associated with supplementalL-arginine in broilers during cool temperature exposure

The present study was conducted to examine the effect of supplemental L-arginine on pulmonary arteriole protein kinase Calpha (PKCalpha) expression in broilers exposed to cool temperature, to investigate further the molecular mechanisms of supplemental L-arginine on modulating pulmonary vascular functions in hypertensive broilers. Broilers were subjected to sub-thermoneutral (cool) temperature to induce pulmonary hypertension syndrome (PHS), and an additional 10 g/kg L-arginine was added to the basal diet to evaluate the effects of supplemental L-arginine on PHS mortality, plasma nitric oxide (NO) production and pulmonary arterioles PKCalpha expression. Supplemental L-arginine reduced PHS mortality but did not affect right/total ventricle (RV/TV) ratios in clinically healthy birds. Birds fed additional L-arginine had increased plasma NO and decreased PKCalpha protein expression in pulmonary arterioles; NO production was negatively correlated with PKCalpha expression. These results demonstrated that supplemental L-arginine diminished PKCalpha expression in birds exposed to cool temperature. It is suggested that NO-induced loss of PKCalpha expression might be partially responsible for its effects on dilating pulmonary vasculature and inhibiting pulmonary vascular remodelling in vivo.

Protein kinases in vascular smooth muscle tone--role in the pulmonary vasculature and hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is an adaptive mechanism that in the normal animal diverts blood away from poorly ventilated areas of the lung, thereby maintaining optimal ventilation-perfusion matching. In global hypoxia however, such as in respiratory disease or at altitude, it causes detrimental increases in pulmonary vascular resistance and pulmonary artery (PA) pressure. The precise intracellular pathways and mechanisms underlying HPV remain unclear, although it is now recognised that both an elevation in smooth muscle intracellular [Ca2+] and a concomitant increase in Ca2+ sensitivity are involved. Several key intracellular protein kinases have been proposed as components of the signal transduction pathways leading to development of HPV, specifically Rho kinase, non-receptor tyrosine kinases (NRTK), p38 mitogen activated protein (MAP) kinase, and protein kinase C (PKC). All of these have been implicated to a greater or lesser extent in pathways leading to Ca2+ sensitisation, and in some cases regulation of intracellular [Ca2+] as well. In this article, we review the role of these key protein kinases in the regulation of vascular smooth muscle (VSM) constriction, applying what is known in the systemic circulation to the pulmonary circulation and HPV. We conclude that the strongest evidence for direct involvement of protein kinases in the mechanisms of HPV concerns a central role for Rho kinase in Ca2+ sensitisation, and a potential role for Src-family kinases in both modulation of Ca2+ entry via capacitative Ca2+ entry (CCE) and activation of Rho kinase, though others are likely to have indirect or modulatory influences. In addition, we speculate that Src family kinases may provide a central interface between the proposed hypoxia-induced generation of reactive oxygen species by mitochondria and both the elevation in intracellular [Ca2+] and Rho kinase mediated Ca2+ sensitisation.

Differential changes in phospholipase D and phosphatidate phosphohydrolase activities in ischemia–reperfusion of rat heart

Phospholipase D (PLD2) produces phosphatidic acid (PA), which is converted to 1,2 diacylglycerol (DAG) by phosphatidate phosphohydrolase (PAP2). Since PA and DAG regulate Ca(2+) movements, we examined PLD2 and PAP2 in the sarcolemma (SL) and sarcoplasmic reticular (SR) membranes from hearts subjected to ischemia and reperfusion (I-R). Although SL and SR PLD2 activities were unaltered after 30 min ischemia, 5 min reperfusion resulted in a 36% increase in SL PLD2 activity, whereas 30 min reperfusion resulted in a 30% decrease in SL PLD2 activity, as compared to the control value. SR PLD2 activity was decreased (39%) after 5 min reperfusion, but returned to control levels after 30 min reperfusion. Ischemia for 60 min resulted in depressed SL and SR PLD2 activities, characterized with reduced V(max) and increased K(m) values, which were not reversed during reperfusion. Although the SL PAP2 activity was decreased (31%) during ischemia and at 30 min reperfusion (28%), the SR PAP2 activity was unchanged after 30 min ischemia, but was decreased after 5 min reperfusion (25%) and almost completely recovered after 30 min reperfusion. A 60 min period of ischemia followed by reperfusion caused an irreversible depression of SL and SR PAP2 activities. Our results indicate that I-R induced cardiac dysfunction is associated with subcellular changes in PLD2 and PAP2 activities.

Activation of PKCalpha and pulmonary vascular remodelling in broilers

OBJECTIVE

The present study was conducted to examine the presence of protein kinase Calpha (PKCalpha) in the pulmonary arterioles of broilers during the development of pulmonary hypertension and pulmonary vascular remodelling.

METHOD

One hundred and sixty day-old Avian-2000 broilers were divided equally into a control group and a cold temperature group. All the birds were reared in normal temperatures up to day 14, with the lighting schedule at 24 h per day. Thereafter, birds in the cold temperature group were subjected to low temperature by lowering 1-2 degrees C per day to 12-14 degrees C, and then kept constant until day 49, while birds in the control group were still brooded at normal temperatures. All the birds were fed a diet of pellets throughout the study. Samples of blood were taken from the wing vein, and of heart and lung collected after the birds were killed with an overdose of sodium pentobarbitial, at days 24, 32, 39 and 45 of age, respectively. Right ventricle to total ventricle ratio (RV/TV) and packed cell volume (PCV) were measured. Vessel wall area to vessel total area ratio (WA/TA) and mean media thickness in pulmonary arterioles (mMTPA) was examined using computer-image analytic software. Expression of PKC in pulmonary muscular arterioles was assessed by immunohistochemistry and quantified by measuring optical density (OD) using computer-image analytic software.

RESULTS

The incidence of pulmonary hypertension syndrome (PHS) was 12.5% in birds exposed to cold, and 3.75% in the control group (P<0.05). PCV in the cold temperature group was elevated after day 32 (P<0.05), and RV/TV ratio increased on day 45 (P<0.05). Both the WA/TA and mMTPA of birds subjected to cold were significantly elevated (P<0.05). The OD values were not significantly increased before day 32 (P>0.05), however, one week later (at day 39 of age), the difference between the two groups was significant (P<0.05). The increased PKCalpha expression was positively correlated with the values of mMTPA and WA/TA.

CONCLUSION

PKCalpha expression was up-regulated during the development of pulmonary hypertension. The activation of PKCalpha might be involved in the development of pulmonary vascular remodelling.

Expression of phospholipase D isozymes in scar and viable tissue in congestive heart failure due to myocardial infarction

The phospholipase D (PLD) associated with the cardiac sarcolemmal (SL) membrane hydrolyses phosphatidylcholine to produce phosphatidic acid, an important phospholipid signaling molecule known to influence cardiac function. The present study was undertaken to examine PLD isozyme mRNA expression, protein contents and activities in congestive heart failure (CHF) subsequent to myocardial infarction (MI). MI was induced in rats by occlusion of the left anterior descending coronary artery. At 8 weeks after the surgical procedure, hemodynamic assessment revealed that these experimental rats were at a moderate stage of CHF. Semi-quantitative reverse transcriptase-polymerase chain reaction revealed that PLD1 and PLD2 mRNA amounts were unchanged in viable left ventricular (LV) tissue of the failing heart. Furthermore, this technique demonstrated the presence of PLD1 and PLD2 mRNA in the scar tissue. While SL PLD1 and PLD2 protein contents were elevated in the viable LV tissue of the failing heart, SL PLD1 activity was significantly decreased, whereas SL PLD2 activity was significantly increased. On the other hand, although PLD1 protein was undetectable, PLD2 protein and activity were detected in the scar tissue. Our findings suggest that differential changes in PLD isozymes may contribute to the pathophysiology of CHF and may also be involved in the processes of scar remodeling.

Oxidant-induced pHi/Ca2+ changes in rat aortic smooth muscle cells. The role of atrial natriuretic peptide

The aim of this study was to investigate the effects of oxidative stress on PLD activity, [Ca2+]i and pHi levels and the possible relationship among them. Moreover, since atrial natriuretic peptide (ANP) protects against oxidant-induced injury, we investigated the potential protective role of the hormone in rat aortic smooth muscle (RASM) cells exposed to oxidative stress. Water-soluble 2,2'-Azobis (2-amidinopropane) dihydrochloride (AAPH) was used as free radical generating system, since it generates peroxyl radicals with defined reaction and the half time of peroxyl radicals is longer than other ROS. A significant increase of PLD activity was related to a significant decrease in pHi, while [Ca2+]i levels showed an increase followed by a decrease after cell exposure to AAPH. [Ca2+]i changes and pHi fall induced by AAPH were prevented by cadmium which inhibits a plasma membrane Ca2+ ATPase coupled to Ca2+/H+ exchanger, that operates the efflux of Ca2+ coupled to H+ influx. The involvement of PLD in pHi and [Ca2+]i changes was confirmed by calphostin-c treatment, a potent inhibitor of PLD, which abolished all AAPH-induced effects. Pretreatment of RASM cells with pharmacological concentrations of ANP attenuated the AAPH effects on PLD activity as well as [Ca2+]i and pHi changes, while no effects were observed with physiological ANP concentrations, suggesting a possible role of the hormone as defensive effector against early events of the oxidative stress.

The role of the C-terminal tail in protease-activated receptor-2-mediated Ca2+ signalling, proline-rich tyrosine kinase-2 activation, and mitogen-activated protein kinase activity

C-terminal truncation mutants were made to investigate the role of the C-terminus in coupling proteinase-activated receptor-2 (PAR-2) to various signalling pathways. Membrane expression of the delta15, delta34, delta43, and delta34-43 mutants was similar; however, expression of deltatail was lost, as was agonist-mediated internalisation of deltatail, delta43, and delta34-43. Additionally, trypsin and SLIGKV-stimulated [3H]IP accumulation was abrogated in cells transiently expressing delta43 or delta34-43 truncations, but remained unaffected in cells expressing delta34 or delta15. PAR-2 agonist-stimulated intracellular Ca(2+) mobilisation and PYK-2 activity were also abolished by deltatail, delta43, and delta34-43 mutants. However, trypsin-stimulated stress-activated protein kinases (SAPKs) or extracellular signal-regulated kinase (ERK) activities were unaffected by the delta34-43 mutation, although activity was abrogated following delta43 or deltatail truncations, suggesting that Ca(2+) mobilisation, PYK-2, or receptor internalisation are not requied for activation of SAPKs or ERK. These studies identify a novel sequence within the PAR-2 C-terminus essential for InsP(3) generation and PYK-2 activity but not mitogen-activated protein kinase (MAPK) activation.

Essential role for proteinase-activated receptor-2 in arthritis

Using physiological, pharmacological, and gene disruption approaches, we demonstrate that proteinase-activated receptor-2 (PAR-2) plays a pivotal role in mediating chronic inflammation. Using an adjuvant monoarthritis model of chronic inflammation, joint swelling was substantially inhibited in PAR-2-deficient mice, being reduced by more than fourfold compared with wild-type mice, with virtually no histological evidence of joint damage. Mice heterozygous for PAR-2 gene disruption showed an intermediate phenotype. PAR-2 expression, normally limited to endothelial cells in small arterioles, was substantially upregulated 2 weeks after induction of inflammation, both in synovium and in other periarticular tissues. PAR-2 agonists showed potent proinflammatory effects as intra-articular injection of ASKH95, a novel synthetic PAR-2 agonist, induced prolonged joint swelling and synovial hyperemia. Given the absence of the chronic inflammatory response in the PAR-2-deficient mice, our findings demonstrate a key role for PAR-2 in mediating chronic inflammation, thereby identifying a novel and important therapeutic target for the management of chronic inflammatory diseases such as rheumatoid arthritis.

Alterations of sarcolemmal phospholipase D and phosphatidate phosphohydrolase in congestive heart failure

Phospholipase D 2 (PLD2) is the major PLD isozyme associated with the cardiac sarcolemmal (SL) membrane. Hydrolysis of SL phosphatidylcholine (PC) by PLD2 produces phosphatidic acid (PA), which is then converted to 1,2 diacylglycerol (DAG) by the action of phosphatidate phosphohydrolase type 2 (PAP2). In view of the role of both PA and DAG in the regulation of Ca(2+) movements and the association of abnormal Ca(2+) homeostasis with congestive heart failure (CHF), we examined the status of both PLD2 and PAP2 in SL membranes in the infarcted heart upon occluding the left coronary artery in rats for 1, 2, 4, 8 and 16 weeks. A time-dependent increase in both SL PLD2 and PAP2 activities was observed in the non-infarcted left ventricular tissue following myocardial infarction (MI); however, the increase in PAP2 activity was greater than that in PLD2 activity. Furthermore, the contents of both PA and PC were reduced, whereas that of DAG was increased in the failing heart SL membrane. Treatment of the CHF animals with imidapril, an angiotensin-converting enzyme (ACE) inhibitor, attenuated the observed changes in heart function, SL PLD2 and PAP2 activities, as well as SL PA, PC and DAG contents. The results suggest that heart failure is associated with increased activities of both PLD2 and PAP2 in the SL membrane and the beneficial effect of imidapril on heart function may be due to its ability to prevent these changes in the phospholipid signaling molecules in the cardiac SL membrane.

Proteinase-activated receptor-2-mediated activation of stress-activated protein kinases and inhibitory kappa B kinases in NCTC 2544 keratinocytes

In this study we examined the regulation of the stress-activated protein (SAP) kinases and inhibitory kappa B kinases (IKKs) through stimulation of the novel G-protein-coupled receptor proteinase-activated receptor-2 in the human keratinocyte cell line NCTC2544. Trypsin and the peptide SLIGKV stimulated a time-dependent increase in both c-Jun N-terminal kinase and p38 mitogen-activated protein kinase activity. Trypsin also stimulated NF kappa B-DNA binding and the activation of the upstream kinases IKK alpha and -beta. Phorbol 12-myristate 13-acetate also strongly activated both SAP kinases and IKK isoforms, suggesting the potential for a protein kinase C-mediated regulatory mechanism underlying the effects of trypsin. Pre-incubation with selective protein kinase C (PKC) inhibitors GF109203X and Gö6983, or transfection of dominant negative (DN)-PKC alpha, abolished phorbol 12-myristate 13-acetate-mediated c-Jun N-terminal kinase activity, although it only partially inhibited the response to trypsin. In contrast, Gö6983 reduced trypsin-stimulated p38 mitogen-activated protein kinase activity to a greater extent than GF109203X, although DN-PKC alpha or PKC zeta had no substantial effect. Additionally, inhibitors of PKC partially reduced trypsin-stimulated IKK alpha activity but abolished that of IKK beta, whereas DN-PKC alpha but not DN-PKC zeta substantially reduced trypsin-stimulated Flag-IKK beta activity. This study shows for the first time proteinase-activated receptor-2-mediated stimulation of both SAP kinase and IKK signaling and differing roles for PKC isoforms in the regulation of each pathway.

Evidence for the involvement of diacylglycerol kinase in the activation of hypoxia-inducible transcription factor 1 by low oxygen tension

Hypoxia-inducible factor 1 (HIF-1) induces a gene expression program essential for the cellular adaptation to lowered oxygen environments. The intracellular mechanisms by which hypoxia induces HIF-1 remain poorly understood. Here we show that exposure of various cell types to hypoxia raises the intracellular level of phosphatidic acid primarily through the action of diacylglycerol kinase (DGK). Pharmacological inhibition of DGK activity through use of the specific DGK inhibitors and abrogated specifically HIF-1-dependent transcription analyzed with a HIF-1-responsive reporter plasmid. A more detailed analysis revealed that pharmacological inhibition of DGK activity prevented the hypoxia-dependent accumulation of the HIF-1alpha subunit and the subsequent HIF-1-DNA complex formation as well as hypoxia-induced activity of the HIF-1 transactivation domains localized to amino acids 530-582 and 775-826 of the HIF-1alpha subunit. Our results demonstrate for the first time that accumulation of phosphatidic acid through DGK underlines oxygen sensing and provide evidence for the involvement of this lipid kinase in the intracellular signaling that leads to HIF-1 activation.

Chronic hypoxia alters effects of endothelin and angiotensin on K+ currents in pulmonary arterial myocytes

We tested the hypothesis that chronic hypoxia alters the regulation of K+ channels in intrapulmonary arterial smooth muscle cells (PASMCs). Charybdotoxin-insensitive, 4-aminopyridine-sensitive voltage-gated K+ (K(V,CI)) and Ca2+-activated K+ (KCa) currents were measured in freshly isolated PASMCs from rats exposed to 21 or 10% O2 for 17-21 days. In chronically hypoxic PASMCs, K(V, CI) current was reduced and KCa current was enhanced. 4-Aminopyridine (10 mM) depolarized both normoxic and chronically hypoxic PASMCs, whereas charybdotoxin (100 nM) had no effect in either group. The inhibitory effect of endothelin (ET)-1 (10(-7) M) on K(V,CI) current was significantly reduced in PASMCs from chronically hypoxic rats, whereas inhibition by angiotensin (ANG) II (10(-7) M) was enhanced. Neither ET-1 nor ANG II altered K(Ca) current in normoxic PASMCs; however, both stimulated K(Ca) current at positive potentials in chronically hypoxic PASMCs. These results suggest that although modulation of K(V,CI) and KCa channels by ET-1 and ANG II is altered by chronic hypoxia, the role of these channels in the regulation of resting membrane potential was not changed.

Mechanisms whereby nerve growth factor increases diacylglycerol levels in differentiating PC12 cells

We previously showed indirectly that the increase in diacylglycerol (DAG) levels caused by exposing differentiating PC12 cells to nerve growth factor (NGF) must derive mainly from de novo synthesis and, to a lesser and transient extent, from the hydrolysis of [3H]phosphatidylinositol (PI). To explore further the biochemical mechanisms of this increase, we measured, in PC12 cells, DAG synthesis from glycerol or various fatty acids; its liberation from phosphatidylcholine (PC); and the activities of various enzymes involved in DAG production and metabolism. Among cells exposed to NGF (0-116 h), the labeling of DAG from [3H]glycerol peaked earlier than that of [3H]PC, and the specific radioactivity of [3H]glycerol-labeled DAG was much higher than those of the [3H]phospholipids, indicating that [3H]DAG synthesis precedes [3H]phospholipid synthesis. NGF treatment also increased (by 50-330%) the incorporation of monounsaturated ([3H]oleic acid) and polyunsaturated ([14C]linoleic acid or [3H]arachidonic acid) fatty acids into DAG, and, by 15-70%, into PC. NGF treatment increased the activities of long chain acyl-CoA synthetases (LCASs), including oleoyl-CoA synthetase and arachidonoyl-CoA synthetase, by 150-580% over control, but cholinephosphotransferase activity rose by only 60%, suggesting that the synthesis of DAG in the cells was increased to a greater extent than its utilization. NGF did not promote the breakdown of newly formed [3H]PC to [3H]DAG, nor did it consistently affect the activities of phospholipase C or D. NGF did increase phospholipase A2 activity, however the hydrolysis catalyzed by this enzyme does not liberate DAG. Hence the major source of the increased DAG levels in PC12 cells exposed to NGF appears to be enhanced de novo DAG synthesis, probably initiated by the activation of LCASs, rather than the breakdown of PC or PI.

Nerve growth factor stimulates diacylglycerol de novo synthesis and phosphatidylinositol hydrolysis in pheochromocytoma cells

Induction of neurite outgrowth by treating pheochromocytoma cells (PC12 cells) with nerve growth factor (NGF) is associated with major increases in cellular levels of diacylglycerol (DAG), an essential and probably limiting precursor in phosphatidylcholine (PC) and phosphatidylethanolamine (PE) syntheses. To identify the sources of this DAG we examined the effects of NGF treatment on the conversion of [3H]oleic acid (OA) or [3H]glycerol to [3H]glycerolipids, and the turnover of these products in PC12 cells. In kinetic studies on [3H]OA incorporation, most of the radioactivity in the cells initially was free [3H]OA; then it appeared predominantly as [3H]DAG and, eventually, as large amounts of [3H]phospholipids (PLs). In NGF pre-treated cells, the increases in the levels of [3H]DAG (which were most prominent) and PLs were similar to those in unlabeled DAG and PLs. These effects of NGF could be partially blocked by an inhibitor (triacsin C) of long chain acyl-CoA synthetase. NGF pre-treatment also significantly enhanced the incorporation of [3H]glycerol into lipids, a pathway for de novo synthesis of glycerolipids. In studies on the degradation of [3H]OA-labeled lipids, the disappearance of [3H]OA-labeled neutral lipids exhibited an initial rapid phase and a subsequent stable phase. NGF treatment transiently promoted the hydrolysis of [3H]PI to [3H]DAG. These data suggest that the increases in DAG levels observed in PC12 cells exposed to NGF derive mainly from de novo synthesis and, to a lesser and transient extent, from the hydrolysis of [3H]PI.

Muscarinic regulation of phospholipase D and its role in arachidonic acid release in rat submandibular acinar cells

The characteristics of muscarinic cholinergic-induced phospholipase D (PLD) activation, and the involvement of the enzyme in the release of arachidonic acid were examined in rat submandibular acinar cells. Carbachol produced a dose-related activation of PLD to around fivefold control values at 100 microM agonist concentration. This was associated with the appearance of free choline, phosphatidic acid and arachidonic acid, indicating that the PLD substrate was phosphatidylcholine. The response to carbachol was inhibited by 60% by U73122, a blocker of a phospholipase C (PLC) specific to phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], suggesting that the cleavage of phosphatidylcholine by PLD was, at least in part, secondary to agonist-coupled hydrolysis of PtdIns(4,5)P2 by PLC. Consistent with this, PLD was also activated to levels comparable to those induced by carbachol, by the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), and the Ca2+ mobilizer, thapsigargin, two agents that respectively mimic the activation of protein kinase C (PKC) by diacylglycerol and the elevation of cytosolic Ca2+ by inositol 1,4,5-triphosphate [Ins(1,4,5)P3] in the phosphoinositide effect. The cell-permeant Ca2+ chelator 1,2-bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester (BAPTA/AM) abolished the thapsigargin-induced activation of PLD and inhibited the responses of PLD to carbachol and TPA by 60%. The PKC inhibitor, Ro-31-8220, also inhibited the activation of PLD by carbacol and TPA to a level of approximately double control values, but had no effect on the thapsigargin-induced elevation of PLD. A role for both the PKC-associated and Ca(2+)-mobilizing arms of the PtdIns(4,5)P2-PLC pathway in PLD regulation is thus suggested. Pretreatment of cells with the phosphatidate phosphohydrolase blocker, propranolol, significantly enhanced the carbachol-induced elevation of phosphatidic acid, but decreased agonist-stimulated production of diacylglycerol and arachidonic acid, indicating that phosphatidlycholine was the likely source of arachidonic acid. We therefore propose that, in submandibular mucous acinar cells, muscarinic activation of the PtdIns(4,5)P2-PLC pathway regulates phosphatidylcholine-specific PLD through both the PKC- and Ca(2+)-mobilizing arms of the phosphoinositide response, and that diacylglycerol, derived from phosphatidylcholine via phosphatidic acid, is a source of free arachidonic acid.