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

Transmissible gastroenteritis virus infection decreases arginine uptake by downregulating CAT-1 expression

Transmissible gastroenteritis virus (TGEV) is a coronavirus that causes severe diarrhea in suckling piglets. TGEV primarily targets and infects porcine intestinal epithelial cells, which play an important role in nutrient absorption. However, the effects of TGEV infection on nutrient absorption in swine have not yet been investigated. In this study, we evaluated the impact of TGEV infection on arginine uptake using the porcine small intestinal epithelial cell line IPEC-J2 as a model system. High performance liquid chromatography (HPLC) analyses showed that TGEV infection leads to reduced arginine uptake at 48 hours post-infection (hpi). Expression of cationic amino acid transporter 1 (CAT-1) was attenuated as well. TGEV infection induced activation of phospho-protein kinase C α (p-PKC α), phospho-epidermal growth factor receptor (p-EGFR), and enhanced the expression of caveolin-1, all of which appear to be involved in down-regulating arginine uptake and CAT-1 expression. These results illuminate the relationship between TGEV infection and nutrient absorption, and further our understanding of the mechanisms of TGEV infection.

Molecular and pathophysiological aspects of metal ion uptake by the zinc transporter ZIP8 (SLC39A8)

Zinc ion (Zn2+) is essential for life; its deficiency in the human body could cause stunted growth, anemia and susceptibility to infection. The Zn transporter ZIP8 (also known as SLC39A8) is an important Zn2+ importer; aberrant Zn2+ influx mediated by ZIP8 can lead to the pathogenesis of osteoarthritis and inflammatory diseases. ZIP8 also mediates the cellular uptake of divalent metal ions including iron, manganese, and the toxic heavy metal cadmium. Individuals with SLC39A8 mutations and transgenic mouse models are starting to reveal the critical role that this gene plays in embryonic development and the metabolism of essential metal ions. Here we summarize our current understanding of ZIP8's function and regulation, at both the molecular and biological levels. We also review the association of ZIP8 with various diseases and its linkage with complex disorders like obesity, hypertension, and schizophrenia as revealed by several large genome-wide association studies.

Overactivation of corticotropin-releasing factor receptor type 1 and aquaporin-4 by hypoxia induces cerebral edema

Cerebral edema is a potentially life-threatening illness, but knowledge of its underlying mechanisms is limited. Here we report that hypobaric hypoxia induces rat cerebral edema and neuronal apoptosis and increases the expression of corticotrophin releasing factor (CRF), CRF receptor type 1 (CRFR1), aquaporin-4 (AQP4), and endothelin-1 (ET-1) in the cortex. These effects, except for the increased expression of CRF itself, could all be blocked by pretreatment with an antagonist of the CRF receptor CRFR1. We also show that, in cultured primary astrocytes: (i) both CRFR1 and AQP4 are expressed; (ii) exogenous CRF, acting through CRFR1, triggers signaling of cAMP/PKA, intracellular Ca(2+), and PKCε; and (iii) the up-regulated cAMP/PKA signaling contributes to the phosphorylation and expression of AQP4 to enhance water influx into astrocytes and produces an up-regulation of ET-1 expression. Finally, using CHO cells transfected with CRFR1(+) and AQP4(+), we show that transfected CRFR1(+) contributes to edema via transfected AQP4(+). In conclusion, hypoxia triggers cortical release of CRF, which acts on CRFR1 to trigger signaling of cAMP/PKA in cortical astrocytes, leading to activation of AQP4 and cerebral edema.

Abnormal mitochondrial L-arginine transport contributes to the pathogenesis of heart failure and rexoygenation injury

Background

Impaired mitochondrial function is fundamental feature of heart failure (HF) and myocardial ischemia. In addition to the effects of heightened oxidative stress, altered nitric oxide (NO) metabolism, generated by a mitochondrial NO synthase, has also been proposed to impact upon mitochondrial function. However, the mechanism responsible for arginine transport into mitochondria and the effect of HF on such a process is unknown. We therefore aimed to characterize mitochondrial L-arginine transport and to investigate the hypothesis that impaired mitochondrial L-arginine transport plays a key role in the pathogenesis of heart failure and myocardial injury.

Methods and Results

In mitochondria isolated from failing hearts (sheep rapid pacing model and mouse Mst1 transgenic model) we demonstrated a marked reduction in L-arginine uptake (p<0.05 and p<0.01 respectively) and expression of the principal L-arginine transporter, CAT-1 (p<0.001, p<0.01) compared to controls. This was accompanied by significantly lower NO production and higher 3-nitrotyrosine levels (both p<0.05). The role of mitochondrial L-arginine transport in modulating cardiac stress responses was examined in cardiomyocytes with mitochondrial specific overexpression of CAT-1 (mtCAT1) exposed to hypoxia-reoxygenation stress. mtCAT1 cardiomyocytes had significantly improved mitochondrial membrane potential, respiration and ATP turnover together with significantly decreased reactive oxygen species production and cell death following mitochondrial stress.

Conclusion

These data provide new insights into the role of L-arginine transport in mitochondrial biology and cardiovascular disease. Augmentation of mitochondrial L-arginine availability may be a novel therapeutic strategy for myocardial disorders involving mitochondrial stress such as heart failure and reperfusion injury.

Voluntary wheel running augments aortic l-arginine transport and endothelial function in rats with chronic kidney disease

Reduced nitric oxide (NO) synthesis contributes to risk for cardiovascular disease in chronic kidney disease (CKD). Vascular uptake of the NO precursor l-arginine (ARG) is attenuated in rodents with CKD, resulting in reduced substrate availability for NO synthesis and impaired vascular function. We tested the effect of 4 wk of voluntary wheel running (RUN) and/or ARG supplementation on endothelium-dependent relaxation (EDR) in rats with CKD. Twelve-week-old male Sprague-Dawley rats underwent ⅚ ablation infarction surgery to induce CKD, or SHAM surgery as a control. Beginning 4 wk following surgery, CKD animals either remained sedentary (SED) or received one of the following interventions: supplemental ARG, RUN, or combined RUN+ARG. Animals were euthanized 8 wk after surgery, and EDR was assessed. EDR was significantly impaired in SED vs. SHAM animals after 8 wk, in response to ACh (10(-9)-10(-5) M) as indicated by a reduced area under the curve (AUC; 44.56 ± 9.01 vs 100 ± 4.58, P < 0.05) and reduced maximal response (Emax; 59.9 ± 9.67 vs. 94.31 ± 1.27%, P < 0.05). AUC was not improved by ARG treatment but was significantly improved above SED animals in both RUN and RUN+ARG-treated animals. Maximal relaxation was elevated above SED in RUN+ARG animals only. l-[(3)H]arginine uptake was impaired in both SED and ARG animals and was improved in RUN and RUN+ARG animals. The results suggest that voluntary wheel running is an effective therapy to improve vascular function in CKD and may be more beneficial when combined with l-arginine.

Melatonin regulates L-arginine transport and NADPH oxidase in young rats with bile duct ligation: role of protein kinase C

BACKGROUND

Bile duct ligation (BDL) is a commonly used cholestatic liver disease (CLD) model. We recently found that L-arginine levels were significantly raised by melatonin in young rats with BDL. We hypothesized that protein kinase C-α (PKC-α) is involved in the increases of L-arginine in melatonin-treated BDL rats. In addition, we tested whether melatonin prevents nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-induced reactive oxygen species (ROS) production, in rats with BDL, through PKC.

METHODS

Four groups of young male rats were studied: shams (n = 6), untreated BDL rats (n = 9), melatonin-treated shams (n = 6, M), and melatonin-treated BDL rats (n = 6, BDL + M). Melatonin-treated rats received daily melatonin 1 mg/kg/d via i.p. injection. All surviving rats were killed 14 d after surgery.

RESULTS

Melatonin prevented BDL-induced mortality and kidney injury. Melatonin additionally increased L-arginine concentrations in BDL liver, which is correlated with decreased PKC-α translocation. Next, melatonin increased L-arginine levels in BDL kidneys, which was correlated with decreased renal levels of arginase II. In the BDL kidney, melatonin decreased PKC-β translocation, reduced p47phox translocation, and diminished NADPH-dependent superoxide production.

CONCLUSION

Melatonin inhibits PKC-α to increase cationic amino acid transporter-1 (CAT-1)-mediated L-arginine uptake in BDL liver, whereas it inhibits PKC-β to reduce NADPH-dependent superoxide production.

Polydatin attenuates hypoxic pulmonary hypertension and reverses remodeling through protein kinase C mechanisms

Hypoxic pulmonary hypertension is a life-threatening emergency if untreated. Consistent pulmonary hypertension also leads to arteries and ventricular remodeling. The clinical therapeutic strategy for pulmonary hypertension and the corresponding remodeling mainly interacts with NO, angiotensin II (Ang II) and elevated endothelin (ET) targets. In the present study, we evaluated the effects of polydatin on hypoxia-induced pulmonary hypertension. It was observed that polydatin attenuated hypoxic pulmonary hypertension, reversed remodeling, and regulated NO, Ang II, ET contents in the serum and lung samples. However, forced activation of PKC signaling by its selective activator thymeleatoxin (THX) could abate the effects of polydatain. These results suggest that polydatin might be a promising candidate for hypoxic pulmonary treatment through interaction with PKC mechanisms.

In vitro evidence of involvement of the epithelial y+ transporter in β-defensin production on the ocular surface

To analyse the hypothesis as to whether there is a functional relationship between human cationic amino acid transporters (hCATs, y(+) transporter, the main transporter of L-arginine and L-lysine) and human β-defensin (important components of immune function) production on the ocular surface, arginase and nitrate monoxide synthase (NOS), enzymes that compete for L-arginine, were inhibited by norNOHA (N(omega)-hydroxy-nor-L-arginine) and/or L-NAME (NG-nitro-L-arginine methyl ester) in cultured human corneal epithelial cells. In addition, the transport activity of hCAT proteins was inhibited or activated through α-tocopherol or PMA (phorbol myristate acetate), respectively. Concentrations of the human inducible β-defensins (hBD) 2 and 3 were determined by ELISA experiments. The basic expression of hBD3 in non-stimulated HCE cells significantly exceeded that of hBD2. Both β-defensins also differed as to how readily their excretion could be stimulated. HBD2 excretion rate was 3.5 time more by L-NAME, whereas norNOHA had no effect. In contrast, hBD3 excretion was increased by norNOHA by a factor of 1.5 but L-NAME alone had no effect. The excretion of both β-defensins was increased 3- and 6-fold by combined administration of L-NAME, norNOHA and interleukin (IL)-1β. Administration of α-tocopherol increased hBD2 excretion twofold. No effect was observed for hBD3. With PMA, on the other hand, a reduction in secretion for both β-defensins was observed. These in vitro findings provide evidence of a functional association between CAT proteins and β-defensins 2 and 3 opening up a new field of research with pharmacological perspectives for treatment of inflammatory diseases such as keratitis or dry eye disease.

Proteomic analyses of lung lysates from short-term exposure of Fischer 344 rats to cigarette smoke

A short-term 5 day mainstream cigarette smoke exposure study was conducted in Fischer 344 rats to identify changes in lung proteins. Groups of 10 male and female rats at 5 weeks of age were assigned to one of four exposure groups. Animals received either nose-only filtered air (Air Control) or 75, 200, or 400 mg total particulate matter (TPM)/m(3) of diluted cigarette smoke. Exposures were conducted for 3 h per day, for 5 consecutive days. One lung per animal was frozen in liquid nitrogen and processed for proteomic analyses. Lung lysates from control verses treated animals were screened with 650 antibodies for changes in signaling protein levels and phosphorylation using antibody microarray technology, and then over 100 of the top protein hits were assessed by immunoblotting. The top smoke-altered proteins were further evaluated using reverse lysate microarrays. Major protein changes showed medium to strong bands on Western blots, depended on dose and gender, and included protein-serine kinases (Cot/Tpl2, ERK1/2, GSK3α/β, MEK6, PKCα/γ, RSK1), protein phosphatases (PP4/A'2, PP1Cβ), and other proteins (caspase 5, CRMP2, Hsc70, Hsp60, Rac1 and STAT2). The most pronounced changes occurred with 75 mg TPM/m(3) exposed females and 200 mg TPM/m(3) exposed males. Smoke-altered proteins regulate apoptosis, stress response, cell structure, and inflammation. Changes in identified proteins may serve as early indicators of lung damage.

Arginine transport in human monocytic leukemia THP-1 cells during macrophage differentiation

L-arginine metabolism in myeloid cells plays a central role in the processes of macrophage activation and in the regulation of immune responses. In this study, we investigated arginine transport activity and the expression of the related transporter genes during the differentiation of monocytes to macrophages. We show here that the induction of THP-1 monocyte differentiation by PMA markedly increases the expression of SLC7A7 mRNA and of y(+)LAT1 protein and consequently, the activity of system y(+)L-mediated arginine transport. Conversely, the activity of system y(+) decreases during macrophage differentiation as a result of a reduction in CAT1 protein expression. The PMA-induced, macrophage-differentiated phenotype and the increased activity of system y(+)L through the induction of SLC7A7 gene are mediated by the specific activation of PKCβ. SLC7A7 gene silencing causes a significant reduction of system y(+)L activity and a subsequent, marked increase of arginine and lysine cell content, thus suggesting that in macrophagic cells, system y(+)L activity is mainly directed outwardly. Differentiating agents other than PMA, i.e., VD3 and ATRA, are equally effective in the stimulation of system y(+)L transport activity through the increased expression of SLC7A7 mRNA and y(+)LAT1 protein. Moreover, we found that also during differentiation of human monocytes from peripheral blood SLC7A7 mRNA and system y(+)L activity are increased. These findings point to SLC7A7 gene as a marker of macrophage differentiation.

Protein kinase C-dependent ubiquitination and clathrin-mediated endocytosis of the cationic amino acid transporter CAT-1

Cationic amino acid transporter 1 (CAT-1) is responsible for the bulk of the uptake of cationic amino acids in most mammalian cells. Activation of protein kinase C (PKC) leads to down-regulation of the cell surface CAT-1. To examine the mechanisms of PKC-induced down-regulation of CAT-1, a functional mutant of CAT-1 (CAT-1-HA-GFP) was generated in which a hemagglutinin antigen (HA) epitope tag was introduced into the second extracellular loop and GFP was attached to the carboxyl terminus. CAT-1-HA-GFP was stably expressed in porcine aorthic endothelial and human epithelial kidney (HEK) 293 cells. Using the HA antibody internalization assay we have demonstrated that PKC-dependent endocytosis was strongly inhibited by siRNA depletion of clathrin heavy chain, indicating that CAT-1-HA-GFP internalization requires clathrin-coated pits. Internalized CAT-1-HA-GFP was accumulated in early, recycling, and late endosomes. PKC activation also resulted in ubiquitination of CAT-1. CAT-1 ubiquitination and endocytosis in phorbol ester-stimulated porcine aorthic endothelial and HEK293 cells were inhibited by siRNA knockdown of NEDD4-2 and NEDD4-1 E3 ubiquitin ligases, respectively. In contrast, ubiquitination and endocytosis of the dopamine transporter was dependent on NEDD4-2 in all cell types tested. Altogether, our data suggest that ubiquitination mediated by NEDD4-2 or NEDD4-1 leading to clathrin-mediated endocytosis is the common mode of regulation of various transporter proteins by PKC.

Regulation of arginine transport and metabolism by protein kinase Calpha in endothelial cells: stimulation of CAT2 transporters and arginase activity

Endothelial metabolism of arginine plays a key role in vascular homeostasis. While it is documented that the availability of extracellular arginine is critical for nitric oxide synthesis by eNOS, little is known about the relationships existing between arginine transport and the activity of arginase, the enzyme responsible for the production of ornithine and urea. The present study aims to characterize the role of PKC in the regulation of arginine transport and metabolism by human umbilical vein (HUVEC) and aortic (HAEC) endothelial cells. The results obtained demonstrate that the activation of PKCalpha by phorbol esters or thymeleatoxin causes a transient increase of arginine transport through system y(+), referable to the induction of SLC7A2 mRNAs and to the increased expression of CAT2 transporters. PKCalpha-dependent stimulation of arginine transport requires the activation of MEK/ERK1/2 cascade, which leads to the stimulation of AP-1 and to the consequent induction of CAT2 expression. In parallel, PKCalpha activation also increases arginase expression and activity and promotes eNOS phosphorylation, resulting in decreased NO production. It is concluded that the activation of PKCalpha stimulates arginine entry in human endothelial cells and shifts the metabolism of the cationic amino acid from NO synthesis to arginase-dependent production of ornithine and urea. This metabolic deviation may contribute to the endothelial dysfunction associated with conditions of PKC overactivity.

Ca2+ influx and protein scaffolding via TRPC3 sustain PKCbeta and ERK activation in B cells

Ca(2+) signaling mediated by phospholipase C that produces inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] and diacylglycerol (DAG) controls lymphocyte activation. In contrast to store-operated Ca(2+) entry activated by Ins(1,4,5)P(3)-induced Ca(2+) release from endoplasmic reticulum, the importance of DAG-activated Ca(2+) entry remains elusive. Here, we describe the physiological role of DAG-activated Ca(2+) entry channels in B-cell receptor (BCR) signaling. In avian DT40 B cells, deficiency of transient receptor potential TRPC3 at the plasma membrane (PM) impaired DAG-activated cation currents and, upon BCR stimulation, the sustained translocation to the PM of protein kinase Cbeta (PKCbeta) that activated extracellular signal-regulated kinase (ERK). Notably, TRPC3 showed direct association with PKCbeta that maintained localization of PKCbeta at the PM. Thus, TRPC3 functions as both a Ca(2+)-permeable channel and a protein scaffold at the PM for downstream PKCbeta activation in B cells.

Dietary protein intake and stage of lactation differentially modulate amino acid transporter mRNA abundance in porcine mammary tissue

To test the hypothesis that under restricted and surfeit protein intake the mammary gland undergoes adaptive regulation, changes in mammary tissue mRNA abundance of cationic amino acid (AA) transporter (CAT)-1, CAT-2B, alanine/serine/cysteine/threonine transporter 1 (ASCT1), and broad specificity transporter for neutral and cationic AA (ATB(0,+)), and CAT-1 protein abundance were investigated at 2 stages of lactation. Eighteen sows were allocated to a 2 x 3 randomized incomplete block design with 2 stages of lactation (early and peak) and 3 protein levels: deficient (D), adequate (A), or in excess (E) of lactation requirement. In early lactation, compared with A, sows fed E had lower (P = 0.05) piglet growth rate and sows fed D or E had lower (P < or = 0.05) casein yield. In early lactation, piglet growth rate and milk protein and casein yield increased from D to A and decreased from A to E (quadratic, P = 0.095, P < 0.05, and P < 0.01, respectively). Protein intake did not affect CAT-1, ASCT1, ATB(0,+) mRNA abundance, or CAT-1 protein level. Overall, CAT-2B mRNA abundance decreased linearly with increasing protein intake (P < 0.05). Compared with A, E decreased CAT-2B mRNA abundance (P < 0.05). Compared with early lactation, peak lactation did not increase CAT-1 mRNA abundance or relative CAT-1 protein content, but increased abundance of ASCT1 and ATB(0,+) mRNA (P < 0.01). Mammary CAT-2B appears to be adaptively regulated in vivo at the transcription level, whereas ASCT1 and ATB(0,+) mRNA abundances are associated only with stage of lactation. Neither protein intake nor stage of lactation affects porcine mammary CAT-1 gene expression in vivo.

Effect of peroxynitrite on endothelial L-arginine transport and metabolism

Under conditions of oxidative stress it is well known that the bioavailability of nitric oxide (NO) is known to be significantly reduced. This process is in part due to the combination of NO with superoxide radicals to form peroxynitrite (ONOO(-)). While this process inactivates NO per se, it is not certain to which extent this process may also further impair ongoing NO production. Given the pivotal role of arginine availability for NO synthesis we determined the impact of ONOO(-) on endothelial arginine transport and intracellular arginine metabolism. Peroxynitrite reduced endothelial [(3)H]-L-arginine transport and increased the rate of arginine efflux in a concentration-dependent manner (both p<0.05). In conjunction, exposure to ONOO(-) significantly reduced the intracellular concentration of L-arginine, N(G)-hydroxy-L-arginine (an intermediate of NO biosynthesis) and citrulline by 46%, 45% and 60% respectively (all p<0.05), while asymmetric dimethyl arginine (ADMA) levels rose by 180% (p<0.05). ONOO(-) exposure did not alter the cellular distribution of the principal L-arginine transporter, CAT1, rather the effect on CAT1 activity appeared to be mediated by protein nitrosation. Conclusion Peroxynitrite negatively influences NO production by combined effects on arginine uptake and efflux, most likely due to a nitrosative action of ONOO(-) on CAT-1.

Reduced l-arginine transport and nitric oxide synthesis in human umbilical vein endothelial cells from intrauterine growth restriction pregnancies is not further altered by hypoxia

Intrauterine growth restriction (IUGR) is associated with chronic fetal hypoxia, altered placental vasodilatation and reduced endothelial nitric oxide synthase (eNOS) activity. In human umbilical vein endothelial cells (HUVEC) from pregnancies complicated with IUGR (IUGR cells) and in HUVEC from normal pregnancies (normal cells) cultured under hypoxia l-arginine transport is reduced; however, the mechanisms leading to this dysfunction are unknown. We studied hypoxia effect on l-arginine transport and human cationic amino acid transporters 1 (hCAT-1) expression, and the potential NO and protein kinase C alpha (PKCalpha) involvement. Normal or IUGR HUVEC monolayers were exposed (0-24h) to 5% O(2) (normoxia), and 1 or 2% O(2) (hypoxia). l-Arginine transport and hCAT-1 expression, phosphorylated and total PKCalpha or eNOS protein and mRNA expression were quantified. eNOS involvement was tested using a siRNA against eNOS (eNOS-siRNA) adenovirus. IUGR cells in normoxia or hypoxia, and normal cells in hypoxia exhibited reduced l-arginine transport, hCAT-1 expression, NO synthesis and eNOS phosphorylation at Serine(1177), effects reversed by calphostin C (PKC inhibitor) and S-nitroso-N-acetyl-l,d-penicillamine (SNAP, NO donor). However, N(G)-nitro-l-arginine methyl ester (l-NAME, NOS inhibitor) reduced hCAT-1 expression only in normal cells in normoxia. Increased Thr(638)-phosphorylated PKCalpha was exhibited by IUGR cells in normoxia or hypoxia and normal cells in hypoxia. The effects of hypoxia in normal cells were mimicked in eNOS-siRNA transduced cells; however, IUGR phenotype was unaltered by eNOS knockdown. Thus, IUGR- and hypoxia-reduced l-arginine transport could result from increased PKCalpha, but reduced eNOS activity leading to a lower hCAT-1 expression in HUVEC. In addition, IUGR endothelial cells are either not responsive or maximally affected by hypoxia. These mechanisms could be responsible for placental dysfunction in diseases where fetal endothelium is chronically exposed to hypoxia, such as IUGR.

Protein kinase C mediated inhibition of endothelial L-arginine transport is mediated by MARCKS protein

The endothelium plays a vital role in the maintenance of vascular tone and structural vascular integrity, principally mediated via the actions of nitric oxide (NO). L-arginine is the immediate substrate for NO synthesis, and the availability of extracellular L-arginine is critical for the production of NO. Activation of protein kinase C (PKC) dependent signalling pathways are a feature of a number of cardiovascular disease states, and in this study we aimed to systematically evaluate the mechanism by which PKC regulates L-arginine transport in endothelial cells. In response to PKC activation (PMA 100 nM, 30 min), [(3)H]L-arginine uptake by bovine aortic endothelial cells (BAEC) was reduced to 45+4% of control (p<0.05). This resulted from a 53% reduction in the Vmax (p<0.05), with no change in the K(m) for L-arginine. Western blot analysis and confocal microscopy revealed no change in the expression or membrane distribution of CAT-1, the principal BAEC L-arginine transporter. Moreover in (32)P-labeling studies, PMA exposure did not result in CAT-1 phosphorylation. We therefore explored the possibility that PKC altered and interaction with MARCKS protein, a candidate membrane associated protein. By co-immunoprecipitation we show that CAT-1 interacts with, a membrane associated protein, that was significantly inhibited by PKC activation (p<0.05). Moreover antisense inhibition of MARCKS abolished the PMA effect on L-arginine transport. PKC dependent mechanisms regulate the transport of L-arginine, mediated via process involving MARCKS.

l-Arginine transporters in cardiovascular disease: A novel therapeutic target

The amino acid l-arginine participates in a variety of key biochemical and physiological activities, including its well-recognized role as the key substrate for nitric oxide (NO) biosynthesis. The current review describes the cellular influences on arginine metabolism with particular focus on the transport of l-arginine in the endothelium. It details the processes by which intracellular and extracellular levels of l-arginine may influence nitric oxide production and further documents the imbalance that is evident in various cardiovascular disease states. In man, impairment of l-arginine transport has been observed in hypertension, heart failure, and renal disease, and it may thus be a relevant therapeutic target for rectification of nitric oxide pathogenesis in these conditions.

Chronic nitroglycerine administration reduces endothelial nitric oxide production in rabbit mesenteric resistance artery

We investigated whether 10 days' in vivo treatment with nitroglycerine (NTG) would inhibit nitric oxide production by the endothelial cells of resistance arteries ex vivo and, if so, what the underlying mechanism might be. ACh increased the intracellular nitric oxide concentration ([NO]i; estimated using the nitric oxide-sensitive fluorescent dye diaminofluorescein-2) within the endothelial cells of rabbit mesenteric resistance arteries. This effect was significantly smaller in arteries isolated from NTG-treated rabbits than in those from control rabbits. The reduction in endothelial [NO]i in NTG-treated rabbits was prevented when olmesartan (blocker of type 1 angiotensin II receptors (AT1Rs)) was coadministered in vivo with NTG and also when the superoxide scavenger manganese (III) tetrakis-(4-benzoic acid) porphyrin (Mn-TBAP), the protein kinase C (PKC) inhibitor GF109203X or L-arginine (with or without the active form of folate (5-methyltetrahydrofolate)) was incubated with the arteries in vitro. Endothelial cell superoxide production (estimated by ethidium fluorescence) was greatly increased in arteries from NTG-treated rabbits. This was normalized by in vivo coadministration of olmesartan with NTG and also by in vitro application of Mn-TBAP or GF109203X (but not of 5-methyltetrahydrofolate+L-arginine). ACh increased the intracellular Ca2+ concentration (estimated using the Ca2+-sensitive dye Fura 2) within endothelial cells, the increase being not significantly different between NTG-treated rabbits and control rabbits. We conclude that in NTG-treated rabbits, endothelial nitric oxide production in mesenteric resistance arteries is reduced, possibly through a reduction in the bioavailability of L-arginine via an action mediated by superoxide. Activation of the AT1R-PKC pathway may be involved in increasing superoxide production.

Activation of classical protein kinase C reduces the expression of human cationic amino acid transporter 3 (hCAT-3) in the plasma membrane

We have previously shown that activation of PKC (protein kinase C) results in internalization of hCAT-1 [human CAT-1 (cationic amino acid transporter 1)] and a decrease in arginine transport [Rotmann, Strand, Martiné and Closs (2004) J. Biol. Chem. 279, 54185-54192]. However, others found increased transport rates for arginine in response to PKC activation, suggesting a differential effect of PKC on different CAT isoforms. Therefore we investigated the effect of PKC on hCAT-3, an isoform expressed in thymus, brain, ovary, uterus and mammary gland. In Xenopus laevis oocytes and human U373MG glioblastoma cells, hCAT-3-mediated L-arginine transport was significantly reduced upon treatment with compounds that activate classical PKC. In contrast, inactive phorbol esters and an activator of novel PKC isoforms had no effect. PKC inhibitors (including the PKCalpha-preferring Ro 31-8280) reduced the inhibitory effect of the PKC-activating compounds. Microscopic analyses revealed a PMA-induced reduction in the cell-surface expression of fusion proteins between hCAT-3 and enhanced green fluorescent protein expressed in X. laevis oocytes and glioblastoma cells. Western-blot analysis of biotinylated surface proteins demonstrated a PMA-induced decrease in hCAT-3 in the plasma membrane, but not in total protein lysates. Pretreatment with a PKC inhibitor also reduced this PMA effect. It is concluded that similar to hCAT-1, hCAT-3 activity is decreased by PKC via reduction of transporter molecules in the plasma membrane. Classical PKC isoforms seem to be responsible for this effect.

Expression and regulation of SLC39A family zinc transporters in the developing mouse intestine

Several ZIP genes (SLC39A family of metal transporters) play roles in zinc homeostasis. Herein, the temporal and spatial patterns of expression of the mouse ZIP1, 3, 4, and 5 genes in the developing intestine and the effects of maternal dietary zinc deficiency on these patterns of expression were examined. ZIP1 and ZIP3 genes, conserved members of the ZIP subfamily II, were found to be coexpressed during development. Expression of these genes was detected on day 14 of gestation in smooth muscle and the pseudostratified endoderm. By 5 days post-partum, prominent expression became restricted to muscle and connective stroma. In contrast, expression of ZIP4 and ZIP5 genes, members of the ZIP subfamily called LIV-1, coincided with epithelial morphogenesis. ZIP5 expression was detected on d16 of gestation and localized to the basolateral membranes of the single-layered epithelium. ZIP4 expression was detected on d18 of gestation and localized to the apical membrane of villus epithelial cells. When dams were fed a zinc-deficient diet beginning at parturition, ZIP4 expression in the nursing neonate was greatly induced. In contrast, neonatal ZIP5 expression remained unchanged, but this protein was removed from the basolateral membrane of the enterocyte. These responses to dietary zinc deficiency mimic those found in the adult intestine. These studies reveal cell-type-specific expression of ZIP genes during development of the intestine, and suggest that the mouse intestine can elicit an adaptive response to dietary zinc availability at birth.

MouseZIP1 andZIP3 genes together are essential for adaptation to dietary zinc deficiency during pregnancy

Subfamily II of the solute-linked carrier 39A superfamily contains three well-conserved zinc transporters (ZIPs1, 2, 3) whose physiological functions are unknown. We generated mice homozygous for knockout alleles of ZIP1 and both ZIP1 and ZIP 3 (double-knockout). These mice were apparently normal when dietary zinc was replete, but when dietary zinc was limited during pregnancy embryos from ZIP1 or ZIP3 knockout mice were two to three times more likely to develop abnormally than those in wildtype mice, and 91% (71/78) of embryos developed abnormally in ZIP1, ZIP3 double-knockout mice. Analysis of the patterns of expression of these genes in mice revealed predominate expression in intestinal stromal cells, nephric-tubular epithelial cells, pancreatic ductal epithelial cells, and hepatocytes surrounding the central vein. This suggests that these zinc transporters function, at least in part, in the redistribution and/or retention of zinc rather than its acquisition from the diet. In conclusion, mutations in the ZIP1 and ZIP3 zinc transporter genes are silent when dietary intake of zinc is normal, but can dramatically compromise the success of pregnancy when dietary intake of zinc is limiting.

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.

Mu and kappa opioid receptors activate ERK/MAPK via different protein kinase C isoforms and secondary messengers in astrocytes

Acute mu and kappa opioids activate the ERK/MAPK phosphorylation cascade that represents an integral part of the signaling pathway of growth factors in astrocytes. By this cross-talk, opioids may impact neural development and plasticity among other basic neurobiological processes in vivo. The mu agonist, [D-ala2,mephe4,glyol5]enkephalin (DAMGO), induces a transient stimulation of ERK phosphorylation, whereas kappa agonist, U69,593, engenders sustained ERK activation. Here we demonstrate that acute U69,593 and DAMGO stimulate ERK phosphorylation by utilization of different secondary messengers and protein kinase C (PKC) isoforms upstream of the growth factor pathway. Immortalized astrocytes transfected with either antisense calmodulin (CaM), a mutant mu opioid receptor that binds CaM poorly or a dominant negative mutant of PKCepsilon were used as a model system to study mu signaling. Evidence was gained to implicate CaM and PKCepsilon in DAMGO stimulation of ERK. DAMGO activation of PKCepsilon and/or ERK was insensitive to selective inhibitors of Ca2+ mobilization, but it was blocked upon phospholipase C inhibition. These results suggest a novel mechanism wherein, upon DAMGO binding, CaM is released from the mu receptor and activates phospholipase C. Subsequently, phospholipase C generates diacylglycerides that activate PKCepsilon. In contrast, U69,593 appears to act via phosphoinositide 3-kinase, PKCzeta, and Ca2+ mobilization. These signaling components were implicated based on studies with specific inhibitors and a dominant negative mutant of PKCzeta. Collectively, our findings on acute opioid effects suggest that differences in their mechanism of signaling may contribute to the distinct outcomes on ERK modulation induced by chronic mu and kappa opioids.

Role of mitogen activated protein kinases and protein kinase C in cadmium-induced apoptosis of primary epithelial lung cells

Cadmium acetate (CdAc) induced apoptosis in primary alveolar type 2 cells and Clara cells from rat lung. Phosphorylation of the MAPKs ERK1/2, p38 and JNK was markedly increased in both cell types 15 min to 2 h after start of exposure to 10 microM CdAc. The phosphorylation of all the MAPKs remained elevated or was progressively increased up to 12 h. The p38 inhibitor SB202190 reduced the Cd-induced apoptosis, whereas the ERK and JNK inhibitors, PD98059 and JNKI1, respectively, did not have any significant effect. The activity of total PKC and the isoforms PKC(alpha) and PKC(delta) seemed initially to be high in type 2 cells and Clara cells. Exposure to 10 microM CdAc did not further increase the total PKC activity or phosphorylation levels of the specific isoforms. However, the PKC inhibitors, GF109203X and rottlerin partially reduced the Cd-induced apoptosis. Furthermore, exposure to GF109203X reduced the phosphorylation of p38 in Clara cells. In conclusion, the MAPK p38 seemed to be involved in the Cd-induced apoptosis in Clara cells and type 2 cells. The activity of PKC isoforms is suggested to have a permissive role in the apoptotic process, located upstream of p38 phosphorylation.

Regulation of cationic amino acid transport: the story of the CAT-1 transporter

The discovery of the function of the receptor for the ecotropic retrovirus as a membrane transporter for the essential amino acids lysine and arginine was a landmark finding in the field of molecular nutrition. This finding indicated that cationic amino acid transporters (CATs) act pathologically as viral receptors. The importance of this transporter was further supported by knockout mice that were not viable after birth. CAT-1 was the first amino acid transporter to be cloned; several other CATs were later characterized biochemically and molecularly. These transporters mediate the bidirectional transport of cationic amino acids, thus supporting important metabolic functions, such as synthesis of proteins, nitric oxide (NO) synthesis, polyamine biosynthesis, and interorgan amino acid flow. This review briefly describes the advances in the regulation of cationic amino acid transport, focusing on the molecular mechanisms that regulate the CAT-1 transporter. Of particular interest to this review is the regulation of CAT-1 by nutritional stresses, such as amino acid availability. The studies that are reviewed conclude that the CAT-1 gene is essential for cell survival during stress because it allows cells to resume growth as soon as amino acids become available.

Plasma membrane transporters for arginine

The supply of arginine may become rate limiting for enzymatic reactions that use this semiessential amino acid as a substrate (e.g., nitric oxide, agmatine, creatine, and urea synthesis), particularly under conditions of high demand such as growth, sepsis, or wound healing. In addition, arginine acts as a signaling molecule that regulates essential cellular functions such as protein synthesis, apoptosis, and growth. In the past decade, a number of carrier proteins for amino acids have been identified on the molecular level. They belong to different gene families, exhibit overlapping but distinctive substrate specificities, and can further be distinguished by their requirement for the cotransport or countertransport of inorganic ions. A number of these transporters function as exchangers rather than uniporters. Uptake of amino acids by these transporters therefore depends largely on the intracellular substrate composition. Hence, there is a complex crosstalk between transporters for cationic and neutral amino acids as well as for peptides. This article briefly reviews current knowledge regarding mammalian plasma membrane transporters that accept arginine as a substrate.

The adaptive response to dietary zinc in mice involves the differential cellular localization and zinc regulation of the zinc transporters ZIP4 and ZIP5

The ZIP5 gene encodes a protein closely related to ZIP4, a zinc transporter mutated in the human genetic disorder acrodermatitis enteropathica. Herein, we demonstrate that mouse ZIP5 and ZIP4 genes are co-expressed in several tissues involved in zinc homeostasis (intestine, pancreas, embryonic yolk sac). However, unlike expression of the ZIP4 gene, which is induced during periods of zinc deficiency, ZIP5 gene expression is unaltered by dietary zinc. Immunohistochemistry localizes ZIP5 to the basolateral surfaces of enterocytes, acinar cells, and visceral endoderm cells in mice fed a zinc-adequate diet. However, this protein is removed from these cell surfaces and internalized during dietary zinc deficiency. In contrast, ZIP4 is induced and recruited to the apical surface of enterocytes and endoderm cells during zinc deficiency. In the pancreas, ZIP4 is expressed in beta-cells, whereas ZIP5 is expressed in acinar cells. These results suggest that the function of ZIP5 is antagonistic to that of ZIP4 in the control of zinc homeostasis; rather than functioning in the acquisition of dietary zinc, as does ZIP4, ZIP5 may function in the removal of zinc from the body. Thus, during periods when dietary zinc is replete, ZIP5 may function to remove zinc from the blood via the pancreas and intestine, the major sites of zinc excretion in mammals, whereas the acquisition of dietary zinc by intestinal ZIP4 would be minimal. In contrast, during periods of dietary zinc deficiency when secretion of zinc by the pancreas and intestine is minimized, ZIP5 is removed from the cell surface, and the intestinal uptake of zinc is augmented by induction of ZIP4.

The stimulation of arginine transport by TNFalpha in human endothelial cells depends on NF-kappaB activation

In human saphenous vein endothelial cells (HSVECs), tumor necrosis factor-alpha (TNFalpha) and bacterial lipopolysaccharide (LPS), but neither interferon gamma (IFNgamma) nor interleukin 1beta (IL-1beta), stimulate arginine transport. The effects of TNFalpha and LPS are due solely to the enhancement of system y+ activity, whereas system y+L is substantially unaffected. TNFalpha causes an increased expression of SLC7A2/CAT-2B gene while SLC7A1/CAT-1 expression is not altered by the cytokine. The suppression of PKC-dependent transduction pathways, obtained with the inhibitor chelerytrhine, the inhibitor peptide of PKCzeta isoform, or chronic exposure to phorbol esters, does not prevent TNFalpha effect on arginine transport. Likewise, ERK, JNK, and p38 MAP kinases are not involved in the cytokine effect, since arginine transport stimulation is unaffected by their specific inhibitors. On the contrary, inhibitors of NF-kappaB pathway hinder the increase in CAT2B mRNA and the stimulation of arginine uptake. These results indicate that in human endothelial cells the activation of NF-kappaB pathway mediates the TNFalpha effects on arginine transport.

Role of adenosine transport in gestational diabetes-induced L-arginine transport and nitric oxide synthesis in human umbilical vein endothelium

Gestational diabetes is associated with increased L-arginine transport and nitric oxide (NO) synthesis, and reduced adenosine transport in human umbilical vein endothelial cells (HUVEC). Adenosine increases endothelial L-arginine/NO pathway via A(2) purinoceptors in HUVEC from normal pregnancies. It is unknown whether the effect of gestational diabetes is associated with activation of these purinoceptors or altered expression of human cationic amino acid transporter 1 (hCAT-1) or human equilibrative nucleoside transporter 1 (hENT1), or endothelial NO synthase (eNOS) in HUVEC. Cells were isolated from normal or gestational diabetic pregnancies and cultured up to passage 2. Gestational diabetes increased hCAT-1 mRNA expression (2.4-fold) and activity, eNOS mRNA (2.3-fold), protein level (2.1-fold), and phosphorylation (3.8-fold), but reduced hENT1 mRNA expression (32%) and activity. Gestational diabetes increased extracellular adenosine (2.7 microM), and intracellular L-arginine (1.9 mM) and L-citrulline (0.7 mM) levels compared with normal cells (0.05 microM, 0.89 mM, 0.35 mM, respectively). Incubation of HUVEC from normal pregnancies with 1 microM nitrobenzylthioinosine (NBMPR) mimicked the effect of gestational diabetes, but NBMPR was ineffective in diabetic cells. Gestational diabetes and NBMPR effects involved eNOS, PKC and p42/44(mapk) activation, and were blocked by the A(2a) purinoceptor antagonist ZM-241385. Thus, gestational diabetes increases the L-arginine/NO pathway involving activation of mitogen-activated protein (MAP) kinases, protein kinase C (PKC) and NO cell signalling cascades following activation of A(2a) purinoceptors by extracellular adenosine. A functional relationship is proposed between adenosine transport and modulation of L-arginine transport and NO synthesis in HUVEC, which could be determinant in regulating vascular reactivity in diabetes mellitus.

Cell signalling-mediating insulin increase of mRNA expression for cationic amino acid transporters-1 and -2 and membrane hyperpolarization in human umbilical vein endothelial cells

Insulin induces vasodilatation in human subjects and increases L-arginine transport and NO synthesis in human umbilical vein endothelial cells (HUVEC). Cell signalling events associated with insulin effects on activity and mRNA expression of the human cationic amino acid transporters 1 (hCAT-1) and 2B (hCAT-2B) are unknown. L-arginine transport and eNOS activity were determined in HUVEC exposed to insulin. mRNA levels for hCAT-1, hCAT-2B and eNOS were quantitated by real time RT-PCR and endothelial NO synthase (eNOS) protein was identified by Western blot analysis. Intracellular Ca2+, L-arginine and L-citrulline levels, L-[3H]citrulline formation from L-[(3)H]arginine, cGMP formation, nitrite level, ATP release and membrane potential were determined. Insulin increased L-arginine transport and the mRNA levels for hCAT-1 and hCAT-2B and eNOS expression and activity. Insulin also induced membrane hyperpolarization and increased intracellular Ca2+, L-[3H]citrulline, cGMP and nitrite formation. Insulin-mediated stimulation of the L-arginine/NO pathway is thus associated with increased hCAT-1 and hCAT-2B mRNA, and eNOS expression, via mechanisms involving membrane hyperpolarization, mitogen-activated protein kinases p42 and p44, phosphatidylinositol 3-kinase, NO and protein kinase C. We have characterized a cell signalling pathway by which hyperinsulinaemia could lead to vasodilatation in human subjects, and which could have implications in patients in whom plasma insulin levels are altered, such as in diabetes mellitus.