Platelet bioenergetic screen in sickle cell patients reveals mitochondrial complex V inhibition, which contributes to platelet activation.

Bioenergetic dysfunction, although central to the pathogenesis of numerous diseases, remains uncharacterized in many patient populations because of the invasiveness of obtaining tissue for mitochondrial studies. Although platelets are an accessible source of mitochondria, the role of bioenergetics in regulating platelet function remains unclear. Herein, we validate extracellular flux analysis in human platelets and use this technique to screen for mitochondrial dysfunction in sickle cell disease (SCD) patients, a population with aberrant platelet activation of an unknown mechanism and in which mitochondrial function has never been assessed. We identify a bioenergetic alteration in SCD patients characterized by deficient complex V activity, leading to decreased mitochondrial respiration, membrane hyperpolarization, and augmented oxidant production compared with healthy subjects. This dysfunction correlates with platelet activation and hemolysis in vivo and can be recapitulated in vitro by exposing healthy platelets to hemoglobin or a complex V inhibitor. Further, reproduction of this dysfunction in vitro activates healthy platelets, an effect prevented by attenuation of mitochondrial hyperpolarization or by scavenging mitochondrial oxidants. These data identify bioenergetic dysfunction in SCD patients for the first time and establish mitochondrial hyperpolarization and oxidant generation as potential pathogenic mechanism in SCD as well as a modulator of healthy platelet function.

Nitrite augments glucose uptake in adipocytes through the protein kinase A-dependent stimulation of mitochondrial fusion.

Though it is well accepted that adipose tissue is central in the regulation of glycemic homeostasis, the molecular mechanisms governing adipocyte glucose uptake remain unclear. Recent studies demonstrate that mitochondrial dynamics (fission and fusion) regulate lipid accumulation and differentiation in adipocytes. However, the role of mitochondrial dynamics in glucose homeostasis has not been explored. The nitric oxide oxidation products nitrite and nitrate are endogenous signaling molecules and dietary constituents that have recently been shown to modulate glucose metabolism, prevent weight gain, and reverse the development of metabolic syndrome in mice. Although the mechanism of this protection is unclear, the mitochondrion is a known subcellular target for nitrite signaling. Thus, we hypothesize that nitrite modulates mitochondrial dynamics and function to regulate glucose uptake in adipocytes. Herein, we demonstrate that nitrite significantly increases glucose uptake in differentiated murine adipocytes through a mechanism dependent on mitochondrial fusion. Specifically, nitrite promotes mitochondrial fusion by increasing the profusion protein mitofusin 1 while concomitantly activating protein kinase A (PKA), which phosphorylates and inhibits the profission protein dynamin-related protein 1 (Drp1). Functionally, this signaling augments cellular respiration, fatty acid oxidation, mitochondrial oxidant production, and glucose uptake. Importantly, inhibition of PKA or Drp1 significantly attenuates nitrite-induced mitochondrial respiration and glucose uptake. These findings demonstrate that mitochondria play an essential metabolic role in adipocytes, show a novel role for both nitrite and mitochondrial fusion in regulating adipocyte glucose homeostasis, and have implications for the potential therapeutic use of nitrite and mitochondrial modulators in glycemic regulation.

Nitrite activates protein kinase A in normoxia to mediate mitochondrial fusion and tolerance to ischaemia/reperfusion.

AIMS: Nitrite (NO2(-)), a dietary constituent and nitric oxide (NO) oxidation product, mediates cardioprotection after ischaemia/reperfusion (I/R) in a number of animal models when administered during ischaemia or as a pre-conditioning agent hours to days prior to the ischaemic episode. When present during ischaemia, the reduction of nitrite to bioactive NO by deoxygenated haem proteins accounts for its protective effects. However, the mechanism of nitrite-induced pre-conditioning, a normoxic response which does not appear to require reduction of nitrite to NO, remains unexplored. METHODS AND RESULTS: Using a model of hypoxia/reoxygenation (H/R) in cultured rat H9c2 cardiomyocytes, we demonstrate that a transient (30 min) normoxic nitrite treatment significantly attenuates cell death after a hypoxic episode initiated 1 h later. Mechanistically, this protection depends on the activation of protein kinase A, which phosphorylates and inhibits dynamin-related protein 1, the predominant regulator of mitochondrial fission. This results morphologically, in the promotion of mitochondrial fusion and functionally in the augmentation of mitochondrial membrane potential and superoxide production. We identify AMP kinase (AMPK) as a downstream target of the mitochondrial reactive oxygen species (ROS) generated and show that its oxidation and subsequent phosphorylation are essential for cytoprotection, as scavenging of ROS prevents AMPK activation and inhibits nitrite-mediated protection after H/R. The protein kinase A-dependent protection mediated by nitrite is reproduced in an intact isolated rat heart model of I/R. CONCLUSIONS: These data are the first to demonstrate nitrite-dependent normoxic modulation of both mitochondrial morphology and function and reveal a novel signalling pathway responsible for nitrite-mediated cardioprotection.

Platelet mitochondrial function: from regulation of thrombosis to biomarker of disease.

Circulating blood platelets contain small numbers of fully functional mitochondria. Accumulating evidence demonstrates that these mitochondria regulate the pro-thrombotic function of platelets through not only energy generation, but also redox signalling and the initiation of apoptosis. Beyond its regulation of haemostasis, platelet mitochondrial function has also traditionally been used to identify and study mitochondrial dysfunction in human disease, owing to the easy accessibility of platelets compared with other metabolically active tissues. In the present article, we provide a brief overview of what is currently known about the function of mitochondria in platelets and review how platelet mitochondria have been used to study mitochondrial function in human disease.

Novel peptide for attenuation of hypoxia-induced pulmonary hypertension via modulation of nitric oxide release and phosphodiesterase -5 activity.

Pulmonary vascular endothelial nitric oxide (NO) synthase (eNOS)-derived NO is the major stimulant of cyclic guanosine 5'-monophosphate (cGMP) production and NO/cGMP-dependent vasorelaxation in the pulmonary circulation. We recently synthesized multiple peptides and reported that an eleven amino acid (SSWRRKRKESS) peptide (P1) but not scrambled P1 stimulated the catalytic activity but not expression of eNOS and causes NO/cGMP-dependent sustained vasorelaxation in isolated pulmonary artery (PA) segments and in lung perfusion models. Since cGMP levels can also be elevated by inhibition of phosphodiesterase type 5 (PDE-5), this study was designed to test the hypothesis that P1-mediated vesorelaxation is due to its unique dual action as NO-releasing PDE-5 inhibitor in the pulmonary circulation. Treatment of porcine PA endothelial cells (PAEC) with P1 caused time-dependent increase in intracellular NO release and inhibition of the catalytic activity of cGMP-specific PDE-5 but not PDE-5 protein expression leading to increased levels of cGMP. Acute hypoxia-induced PA vasoconstriction ex vivo and continuous telemetry monitoring of hypoxia (10% oxygen)-induced elevated PA pressure in freely moving rats were significantly restored by administration of P1. Chronic hypoxia (10% oxygen for 4 weeks)-induced alterations in PA perfusion pressure, right ventricular hypertrophy, and vascular remodeling were attenuated by P1 treatment. These results demonstrate the potential therapeutic effects of P1 to prevent and/or arrest the progression of hypoxia-induced PAH via NO/cGMP-dependent modulation of hemodynamic and vascular remodeling in the pulmonary circulation.

Arginine and asymmetric dimethylarginine in puromycin aminonucleoside-induced chronic kidney disease in the rat.

BACKGROUND/AIMS: Reduced renal L-arginine (L-Arg) synthesis/transport, induction of arginases and increased endogenous NOS inhibitor, asymmetric dimethylarginine (ADMA) will inhibit NO production. This study investigated pathways of L-Arg synthesis/uptake/utilization, ADMA degradation and oxidant/antioxidants in puromycin aminonucleoside (PAN) chronic kidney disease (CKD). METHODS: Rats were given low- (LD) or high-dose (HD) PAN and followed for 11 weeks for proteinuria. BP was measured and blood and tissues were harvested and analyzed for abundance of argininosuccinate synthase (ASS) and lyase (ASL), arginase, cationic amino acid transporter (CAT1) and dimethylargininedimethylaminohydrolase (DDAH) in kidney, cortex, aorta and liver. Arginase and DDAH activity, plasma L-Arg and ADMA, renal pathology and creatinine clearances were also measured. RESULTS: PAN caused dose-dependent kidney damage and hypertension and creatinine clearance fell in HD-PAN. Renal ASS fell in HD-PAN, renal cortex and aortic ASL and membrane CAT1 fell in both PAN groups. There was no activation of renal arginase, but aortic arginase increased in LD-PAN. Renal DDAH activity fell moderately in LD-PAN and markedly in HD-PAN where hepatic DDAH activity also fell. Plasma L-Arg was unchanged while ADMA rose moderately and dose-dependently with PAN. There were several indices of oxidative stress which was most prominent in HD-PAN. CONCLUSION: Reduction in renal ASS/ASL and loss of renal cortex CAT1 compromises renal L-Arg synthesis and release. Loss of aortic CAT1 impairs L-Arg uptake. Increased plasma ADMA was associated with progressive loss of renal DDAH activity. However, loss of renal clearance and falls in hepatic DDAH activity in HD-PAN did not have additive effects on plasma ADMA.

Hypoxic upregulation of arginase II in human lung endothelial cells.

Activated arginase has been implicated in many diseases including cancer, immune cell dysfunction, infections, and vascular disease. Enhanced arginase activity has been reported in lungs of patients with pulmonary artery hypertension. We used hypoxia as a model for pulmonary hypertension and studied the effect of exposure to hypoxia on arginase activity in human lung microvascular endothelial cells (HMVEC). Hypoxia induces upregulation of arginase activity as well as mRNA and protein levels of arginase II (Arg II), the only arginase isoform we were able to identify in HMVEC. In endothelial cells, arginase shares and competes for the substrate l-arginine with nitric oxide (NO) synthase (NOS). Through regulation of substrate availability for NOS, arginase is able to modulate NO production. To evaluate the role of Arg II in regulation of NO production under hypoxia, we compared NO output (RFL-6 reporter assay) in cells with normal and silenced Arg II. Exposure to hypoxia led to an increase in NO levels produced by HMVEC. Inhibition of Arg II by specific small interfering RNA or by the pharmacological inhibitor BEC additionally enhanced the levels of NO. Another possible role for activated arginase is involvement in regulation of cell proliferation. However, we showed that hypoxia decreased cell proliferation and upregulated Arg II did not have an effect on cell proliferation. Since hypoxia-inducible factors (HIF) are a family of transcriptional factors activated by hypoxia, we tested the possibility of involvement of HIF-1 and HIF-2 in regulation of Arg II under hypoxia. The silencing of HIF-2 but not HIF-1 prevented the activation of Arg II by hypoxia.

Endothelial arginase II responds to pharmacological inhibition by elevation in protein level.

Arginase is an enzyme which converts arginine to ornithine and urea. Recently, arginase has been implicated in many physiological and pathological processes including vascular diseases. Inhibition of arginase activity by pharmacological inhibitors is a useful tool to study the biology of arginases and their possible role in therapy. There are several arginase-specific inhibitors commercially available. Herein, we show that some of these inhibitors lead to an increase in arginase II protein level and activity. These effects should be anticipated when these inhibitors are in use or during the testing of new arginase inhibitors.

Effects of angiotensin type 1 receptor blockade on arginine and ADMA synthesis and metabolic pathways in fawn-hooded hypertensive rats.

BACKGROUND: The fawn-hooded hypertensive (FHH) rat develops spontaneous glomerulosclerosis that is ameliorated by inhibition of the angiotensin II type 1 receptor (AT-1). Since kidney damage is associated with nitric oxide (NO) deficiency, we investigated how AT-1 antagonism influenced nitric oxide synthase (NOS), as well as NOS substrate [L-arginine (L-Arg)] and inhibitor [asymmetric dimethylarginine (ADMA)]. L-Arg is synthesized by renal argininosuccinate synthase/argininosuccinate lyase (ASS/ASL) and then either consumed within the kidney by arginase II or NOS or released into the circulation. L-Arg is then taken up from plasma into cells where it can be utilized by NOS and other pathways. The competitive inhibitor of NOS, ADMA, is degraded by dimethylarginine dimethylaminohydrolase (DDAH). METHODS AND RESULTS: Male FHH rats were put on a 40% casein diet for 13 weeks, and some received AT-1 antagonist which reduced blood pressure and kidney weight and prevented glomerulosclerosis and hyperfiltration. The AT-1 antagonist reduced the expression of DDAH2, increased DDAH1 and increased total DDAH activity in the kidney cortex, although there was no change in plasma or kidney cortex ADMA levels. The AT-1 antagonist caused no change in the expression of renal ASS/ASL, but reduced renal and aortic arginase expression and renal arginase activity, which could explain the increased plasma L-Arg. In separate studies, 1 week of AT-1 blockade in young FHH rats had no effect on any of these parameters. CONCLUSION: Thus, the net result of AT-1 antagonist was an improved L-Arg to ADMA ratio due to the prevention of renal and vascular arginase activation which favours increased NO production. Since 1 week of AT-1 blockade in the absence of kidney damage was without effect on arginases, this suggests that the reduction in arginase activity is secondary to the prevention of structural damage rather than a direct immediate effect of AT-1 antagonism.

Could uric acid be a modifiable risk factor in subjects with pulmonary hypertension?

A high serum uric acid is common in subjects with pulmonary hypertension. The increase in serum uric acid may be a consequence of the local tissue ischemia and/or hypoxia, and it may also result from other factors independent of ischemia or hypoxia that occur in various forms of pulmonary hypertension. While classically viewed as a secondary phenomenon, recent studies suggest that hyperuricemia may also have a role in mediating the local vasoconstriction and vascular remodeling in the pulmonary vasculature. If uric acid does have a contributory role in pulmonary hypertension, we may see an increasing prevalence of pulmonary hypertension as hyperuricemia is common in subjects with obesity and metabolic syndrome. We propose studies to investigate the role of uric acid in pulmonary hypertension and to determine if lowering serum uric acid may have clinical benefit in this condition.

Induction of nitric oxide by erythropoietin is mediated by the {beta} common receptor and requires interaction with VEGF receptor 2.

Vascular endothelial growth factor (VEGF) and erythropoietin (EPO) have profound effects on the endothelium and endothelial progenitor cells (EPCs), which originate from the bone marrow and differentiate into endothelial cells. Both EPO and VEGF have demonstrated an ability to increase the number and performance properties of EPCs. EPC behavior is highly dependent on nitric oxide (NO), and both VEGF and EPO can stimulate intracellular NO. EPO can bind to the homodimeric EPO receptor (EPO-R) and the heterodimeric receptor, EPO-R and the common beta receptor (betaC-R). Although VEGF has several receptors, VEGF-R2 appears most critical to EPC function. We demonstrate that EPO induction of NO is dependent on the betaC-R and VEGF-R2, that VEGF induction of NO is dependent on the expression of the betaC-R, and that the betaC-R and VEGF-R2 interact. This is the first definitive functional and structural evidence of an interaction between the 2 receptors and has implications for the side effects of EPO.

Uric acid decreases NO production and increases arginase activity in cultured pulmonary artery endothelial cells.

Elevated levels of serum uric acid (UA) are commonly associated with primary pulmonary hypertension but have generally not been thought to have any causal role. Recent experimental studies, however, have suggested that UA may affect various vasoactive mediators. We therefore tested the hypothesis that UA might alter nitric oxide (NO) levels in pulmonary arterial endothelial cells (PAEC). In isolated porcine pulmonary artery segments (PAS), UA (7.5 mg/dl) inhibits acetylcholine-induced vasodilation. The incubation of PAEC with UA caused a dose-dependent decrease in NO and cGMP production stimulated by bradykinin or Ca(2+)-ionophore A23187. We explored cellular mechanisms by which UA might cause reduced NO production focusing on the effects of UA on the l-arginine-endothelial NO synthase (eNOS) and l-arginine-arginase pathways. Incubation of PAEC with different concentrations of UA (2.5-15 mg/dl) for 24 h did not affect l-[(3)H]arginine uptake or activity/expression of eNOS. However, PAEC incubated with UA (7.5 mg/dl; 24 h) released more urea in culture media than control PAEC, suggesting that arginase activation might be involved in the UA effect. Kinetic analysis of arginase activity in PAEC lysates and rat liver and kidney homogenates demonstrated that UA activated arginase by increasing its affinity for l-arginine. An inhibitor of arginase (S)-(2-boronoethyl)-l-cysteine prevented UA-induced reduction of A23187-stimulated cGMP production by PAEC and abolished UA-induced inhibition of acetylcholine-stimulated vasodilation in PAS. We conclude that UA-induced arginase activation is a potential mechanism for reduction of NO production in PAEC.

Adverse effects of the classic antioxidant uric acid in adipocytes: NADPH oxidase-mediated oxidative/nitrosative stress.

Uric acid is considered a major antioxidant in human blood that may protect against aging and oxidative stress. Despite its proposed protective properties, elevated levels of uric acid are commonly associated with increased risk for cardiovascular disease and mortality. Furthermore, recent experimental studies suggest that uric acid may have a causal role in hypertension and metabolic syndrome. All these conditions are thought to be mediated by oxidative stress. In this study we demonstrate that differentiation of cultured mouse adipocytes is associated with increased production of reactive oxygen species (ROS) and uptake of uric acid. Soluble uric acid stimulated an increase in NADPH oxidase activity and ROS production in mature adipocytes but not in preadipocytes. The stimulation of NADPH oxidase-dependent ROS by uric acid resulted in activation of MAP kinases p38 and ERK1/2, a decrease in nitric oxide bioavailability, and an increase in protein nitrosylation and lipid oxidation. Collectively, our results suggest that hyperuricemia induces redox-dependent signaling and oxidative stress in adipocytes. Since oxidative stress in the adipose tissue has recently been recognized as a major cause of insulin resistance and cardiovascular disease, hyperuricemia-induced alterations in oxidative homeostasis in the adipose tissue might play an important role in these derangements.

Peptides modified by myristoylation activate eNOS in endothelial cells through Akt phosphorylation.

1. Myristoylated pseudosubstrate of PKCzeta (mPS) - a synthetic myristoylated peptide with a sequence (13 amino acids) mimicking the endogenous PKCzeta pseudosubstrate region -- is considered a selective cell-permeable inhibitor of PKCzeta. We present strong evidence that in endothelial cells the action of mPS is not limited to inhibition of PKC activity and that myristoylation of certain peptides can activate eNOS (endothelial nitric oxide synthase) through Akt phosphorylation. 2. mPS at micromolar concentrations (1-10 microM) induced profound phosphorylation of eNOS, Akt, ERK 1/2, and p38 MAPK in cultured pulmonary artery endothelial cells (PAEC). The same changes were observed after treatment of PAEC with a myristoylated scrambled version of mPS (mScr), whereas a cell-permeable version of PKCzeta pseudosubstrate fused to the HIV-TAT membrane-translocating peptide did not induce analogous changes, suggesting that myristoylation confers new properties on the peptides consisting of activation of different signaling pathways in endothelial cells. 3. In addition to mPS and mScr, a number of other myristoylated peptides induced phosphorylation of eNOS suggesting that myristoylation of peptides can activate eNOS by mechanisms unrelated to inhibition of PKC. All active myristoylated peptides contained basic amino acids motif and were longer than six amino acids. 4. Activation of eNOS by myristoylated peptides was dependent on the PI3K/Akt pathway and the rise of intracellular calcium and was associated with an elevation of cGMP levels in PAEC and with relaxation of precontracted isolated pulmonary artery segments. 5. Myristoylated peptides can be considered a new class of activators of NO production in endothelial cells and that using mPS as a specific inhibitor of PKC should be done with caution, especially in endothelial cells.

A causal role for uric acid in fructose-induced metabolic syndrome.

The worldwide epidemic of metabolic syndrome correlates with an elevation in serum uric acid as well as a marked increase in total fructose intake (in the form of table sugar and high-fructose corn syrup). Fructose raises uric acid, and the latter inhibits nitric oxide bioavailability. Because insulin requires nitric oxide to stimulate glucose uptake, we hypothesized that fructose-induced hyperuricemia may have a pathogenic role in metabolic syndrome. Four sets of experiments were performed. First, pair-feeding studies showed that fructose, and not dextrose, induced features (hyperinsulinemia, hypertriglyceridemia, and hyperuricemia) of metabolic syndrome. Second, in rats receiving a high-fructose diet, the lowering of uric acid with either allopurinol (a xanthine oxidase inhibitor) or benzbromarone (a uricosuric agent) was able to prevent or reverse features of metabolic syndrome. In particular, the administration of allopurinol prophylactically prevented fructose-induced hyperinsulinemia (272.3 vs.160.8 pmol/l, P < 0.05), systolic hypertension (142 vs. 133 mmHg, P < 0.05), hypertriglyceridemia (233.7 vs. 65.4 mg/dl, P < 0.01), and weight gain (455 vs. 425 g, P < 0.05) at 8 wk. Neither allopurinol nor benzbromarone affected dietary intake of control diet in rats. Finally, uric acid dose dependently inhibited endothelial function as manifested by a reduced vasodilatory response of aortic artery rings to acetylcholine. These data provide the first evidence that uric acid may be a cause of metabolic syndrome, possibly due to its ability to inhibit endothelial function. Fructose may have a major role in the epidemic of metabolic syndrome and obesity due to its ability to raise uric acid.

Hyperuricemia induces endothelial dysfunction.

BACKGROUND: Hyperuricemia has been linked to cardiovascular and renal diseases, possibly through the generation of reactive oxygen species (ROS) and subsequent endothelial dysfunction. The enzymatic effect of xanthine oxidase is the production of ROS and uric acid. Studies have shown that inhibiting xanthine oxidase with allopurinol can reverse endothelial dysfunction. Furthermore, rat studies have shown that hyperuricemia-induced hypertension and vascular disease is at least partially reversed by the supplementation of the nitric oxide synthase (NOS) substrate, L-arginine. Therefore, we hypothesized that uric acid induces endothelial dysfunction by inhibiting nitric oxide production. METHODS: Hyperuricemia was induced in male Sprague-Dawley rats with an uricase inhibitor, oxonic acid, by gavage; control rats received vehicle. Allopurinol was placed in drinking water to block hyperuricemia. Rats were randomly divided into four groups: (1) control, (2) allopurinol only, (3) oxonic acid only, and (4) oxonic acid + allopurinol. Rats were sacrificed at 1 and 7 days, and their serum analyzed for serum uric acid and nitrites/nitrates concentrations. The effect of uric acid on nitric oxide production was also determined in bovine aortic endothelial cells. RESULTS: Oxonic acid induced mild hyperuricemia at both 1 and 7 days (P < 0.05). Allopurinol reversed the hyperuricemia at 7 days (P < .001). Serum nitrites and nitrates (NO(X)) were reduced in hyperuricemic rats at both 1 and 7 days (P < .001). Allopurinol slightly reversed the decrease in NO(X) at 1 day and completely at 7 days (P < .001). There was a direct linear correlation between serum uric acid and NO(X) (R(2)= 0.56) and a trend toward higher systolic blood pressure in hyperuricemic rats (P= NS). Uric acid was also found to inhibit both basal and vascular endothelial growth factor (VEGF)-induced nitric oxide production in bovine aortic endothelial cells. CONCLUSION: Hyperuricemic rats have a decrease in serum nitric oxide which is reversed by lowering uric acid levels. Soluble uric acid also impairs nitric oxide generation in cultured endothelial cells. Thus, hyperuricemia induces endothelial dysfunction; this may provide insight into a pathogenic mechanism by which uric acid may induce hypertension and vascular disease.

Molecular cloning, characterization, and chromosomal assignment of porcine cationic amino acid transporter-1.

We have cloned and characterized the gene encoding the porcine cationic amino acid transporter, member 1 (CAT-1) (HGMW-approved gene symbol SLC7A1) from porcine pulmonary artery endothelial cells. The porcine SLC7A1 encodes 629 deduced amino acid residues showing a higher degree of sequence similarity with the human counterpart (91.1%) than with the rat (87.3%) and mouse (87.6%) counterparts. Confocal microscopic examination of porcine CAT-1-GFP-expressing HEK293 cells revealed that porcine CAT-1 localizes on the plasma membrane. Amino acid uptake studies in Xenopus oocytes injected with cRNA encoding this protein demonstrated transport properties consistent with system y(+). Radiation hybrid mapping data indicate that the porcine SLC7A1 maps to the distal end of the short arm of pig chromosome 11 (SSC11). This map location is consistent with the known conservation of genome organization between human and pig and provides further confirmation that we have characterized the porcine orthologue of the human SLC7A1.

Pertussis toxin activates L-arginine uptake in pulmonary endothelial cells through downregulation of PKC-alpha activity.

Pertussis toxin (PTX) induces activation of l-arginine transport in pulmonary artery endothelial cells (PAEC). The effects of PTX on l-arginine transport appeared after 6 h of treatment and reached maximal values after treatment for 12 h. PTX-induced changes in l-arginine transport were not accompanied by changes in expression of cationic amino acid transporter (CAT)-1 protein, the main l-arginine transporter in PAEC. Unlike holotoxin, the beta-oligomer-binding subunit of PTX did not affect l-arginine transport in PAEC, suggesting that Galpha(i) ribosylation is an important step in the activation of l-arginine transport by PTX. An activator of adenylate cyclase, forskolin, and an activator of protein kinase A (PKA), Sp-cAMPS, did not affect l-arginine transport in PAEC. In addition, inhibitors of PKA or adenylate cyclase did not change the activating effect of PTX on l-arginine uptake. Long-term treatment with PTX (18 h) induced a 40% decrease in protein kinase C (PKC)-alpha but did not affect the activities of PKC-epsilon and PKC-zeta in PAEC. An activator of PKC-alpha, phorbol 12-myristate 13-acetate, abrogated the activation of l-arginine transport in PAEC treated with PTX. Incubation of PTX-treated PAEC with phorbol 12-myristate 13-acetate in combination with an inhibitor of PKC-alpha (Go 6976) restored the activating effects of PTX on l-arginine uptake, suggesting PTX-induced activation of l-arginine transport is mediated through downregulation of PKC-alpha. Measurements of nitric oxide (NO) production by PAEC revealed that long-term treatment with PTX induced twofold increases in the amount of NO in PAEC. PTX also increased l-[(3)H]citrulline production from extracellular l-[(3)H]arginine without affecting endothelial NO synthase activity. These results demonstrate that PTX increased NO production through activation of l-arginine transport in PAEC.

Classical isoforms of PKC as regulators of CAT-1 transporter activity in pulmonary artery endothelial cells.

We examined which isoforms of protein kinase C (PKC) may be involved in the regulation of cationic amino acid transporter-1 (CAT-1) transport activity in cultured pulmonary artery endothelial cells (PAEC). An activator of classical and novel isoforms of PKC, phorbol 12-myristate-13-acetate (PMA; 100 nM), inhibited CAT-1-mediated l-arginine transport in PAEC after a 1-h treatment and activated l-arginine uptake after an 18-h treatment of cells. These changes in l-arginine transport were not related to the changes in the expression of the CAT-1 transporter. The inhibitory effect of PMA on l-arginine transport was accompanied by a translocation of PKCalpha (a classical PKC isoform) from the cytosol to the membrane fraction, whereas the activating effect of PMA on l-arginine transport was accompanied by full depletion of the expression of PKCalpha in PAEC. A selective activator of Ca(2+)-dependent classical isoforms of PKC, thymeleatoxin (Thy; 100 nM; 1-h and 18-h treatments), induced the same changes in l-arginine uptake and PKCalpha translocation and depletion as PMA. The effects of PMA and Thy on l-arginine transport in PAEC were attenuated by a selective inhibitor of classical PKC isoforms Go 6976 (1 micro M). Phosphatidylinositol-3,4,5-triphosphate-dipalmitoyl (PIP; 5 micro M), which activates novel PKC isoforms, did not affect l-arginine transport in PAEC after 1-h and 18-h treatment of cells. PIP (5 micro M; 1 h) induced the translocation of PKCepsilon (a novel PKC isoform) from the cytosolic to the particulate fraction and did not affect the translocation of PKCalpha. These results demonstrate that classical isoforms of PKC are involved in the regulation of CAT-1 transport activity in PAEC. We suggest that translocation of PKCalpha to the plasma membrane induces phosphorylation of the CAT-1 transporter, which leads to inhibition of its transport activity in PAEC. In contrast, depletion of PKCalpha after long-term treatment with PMA or Thy promotes dephosphorylation of the CAT-1 transporter and activation of its activity.