Apelin prevents cardiac fibroblast activation and collagen production through inhibition of sphingosine kinase 1.

AIMS: Activation of cardiac fibroblasts and their differentiation into myofibroblasts is a key event in the progression of cardiac fibrosis that leads to end-stage heart failure. Apelin, an adipocyte-derived factor, exhibits a number of cardioprotective properties; however, whether apelin is involved in cardiac fibroblast activation and myofibroblast formation remains unknown. The aim of this study was to determine the effects of apelin in activated cardiac fibroblasts, the potential related mechanisms and impact on cardiac fibrotic remodelling process. METHODS AND RESULTS: In vitro experiments were performed in mouse cardiac fibroblasts obtained from normal and pressure-overload hearts. Pretreatment of naive cardiac fibroblasts with apelin (1-100 nM) inhibited Transforming growth factor-ß (TGF-ß)-mediated expression of the myofibroblast marker α-smooth muscle actin (α-SMA) and collagen production. Furthermore, apelin decreased the spontaneous collagen production in cardiac fibroblasts isolated from hearts after aortic banding. Knockdown strategy and pharmacological inhibition revealed that prevention of collagen accumulation by apelin was mediated by a reduction in sphingosine kinase 1 (SphK1) activity. In vivo studies using the aortic banding model indicated that pretreatment with apelin attenuated the development of myocardial fibrotic remodelling and inhibited cardiac SphK1 activity and α-SMA expression. Moreover, administration of apelin 2 weeks after aortic banding prevented cardiac remodelling by inhibiting myocyte hypertrophy, cardiac fibrosis, and ventricular dysfunction. CONCLUSION: Our data provide the first evidence that apelin inhibits TGF-ß-stimulated activation of cardiac fibroblasts through a SphK1-dependent mechanism. We also demonstrated that the administration of apelin during the phase of reactive fibrosis prevents structural remodelling of the myocardium and ventricular dysfunction. These findings may have important implications for designing future therapies for myocardial performance during fibrotic remodelling, affecting the clinical management of patients with progressive heart failure.

Condensin positioning at telomeres by shelterin proteins drives sister-telomere disjunction in anaphase.

The localization of condensin along chromosomes is crucial for their accurate segregation in anaphase. Condensin is enriched at telomeres but how and for what purpose had remained elusive. Here, we show that fission yeast condensin accumulates at telomere repeats through the balancing acts of Taz1, a core component of the shelterin complex that ensures telomeric functions, and Mit1, a nucleosome remodeler associated with shelterin. We further show that condensin takes part in sister-telomere separation in anaphase, and that this event can be uncoupled from the prior separation of chromosome arms, implying a telomere-specific separation mechanism. Consistent with a cis-acting process, increasing or decreasing condensin occupancy specifically at telomeres modifies accordingly the efficiency of their separation in anaphase. Genetic evidence suggests that condensin promotes sister-telomere separation by counteracting cohesin. Thus, our results reveal a shelterin-based mechanism that enriches condensin at telomeres to drive in cis their separation during mitosis.

Morphologic and functional renal impact of acute kidney injury after prolonged hemorrhagic shock in mice.

OBJECTIVE: Sparse data are available on renal consequences of hemorrhagic shock in mice. This study aimed to extend the current knowledge on functional and morphologic renal impact of hemorrhagic shock in mice and to determine its ability to stand as an accurate model of acute kidney injury. DESIGN: In vivo study. SETTING: University research unit. SUBJECTS: C57/Bl6 mice. INTERVENTIONS: A model of controlled hemorrhagic shock was adapted to determine the renal impact of hemorrhagic shock in mice. MEASUREMENTS AND MAIN RESULTS: Renal functions and kidney morphology were followed up from 3 hrs to 21 days after hemorrhagic shock. When prolonged up to 2 hrs, hypotension (35 mm Hg mean arterial blood pressure) induced by temporary blood removal was responsible for an early and lasting increase in hypoxia-inducible factor-1α and kidney-inducible molecule-1 gene expression that paralleled acute tubular necrosis and renal failure. Two-hr hypotension induced an important but reversible decrease in glomerular filtration rate up to 6 days after hemorrhagic shock. Other renal dysfunctions included a renal loss of sodium, assessed by the increase in sodium excretion, and a decrease in urine concentration that persists up to day 21. Tissular damages prevailed in the outer medulla 2 days after hemorrhagic shock, being maximal at day 6. At day 21, renal healing was associated with epithelial recovery and a significant interstitial fibrosis. CONCLUSIONS: Our data indicate that apparent recovery of renal function after acute kidney injury can mask persisting dysfunctions and tissular damages that could predispose to chronic kidney disease. Prolonged hemorrhagic shock in mice closely mimics renal effects induced by similar situation in humans, thus providing a useful tool to investigate pathophysiological mechanisms and protection strategies against acute kidney injury in situations such as hemorrhagic shock.

Probing the activation-promoted structural rearrangements in preassembled receptor-G protein complexes.

Activation of heterotrimeric G proteins by their cognate seven transmembrane domain receptors is believed to involve conformational changes propagated from the receptor to the G proteins. However, the nature of these changes remains unknown. We monitored the conformational rearrangements at the interfaces between receptors and G proteins and between G protein subunits by measuring bioluminescence resonance energy transfer between probes inserted at multiple sites in receptor-G protein complexes. Using the data obtained for the alpha(2A)AR-G alpha(i1) beta1gamma2 complex and the available crystal structures of G alpha(i1) beta1gamma2, we propose a model wherein agonist binding induces conformational reorganization of a preexisting receptor-G protein complex, leading the G alpha-G betagamma interface to open but not dissociate. This conformational change may represent the movement required to allow nucleotide exit from the G alpha subunit, thus reflecting the initial activation event.

Transcriptional down-regulation of human alpha(2A)-adrenoceptors by IFNgamma and TNFalpha in intestinal cells.

alpha(2A)-adrenoceptors are expressed on intestinal cells and they participate in the control of epithelial functions such as solute and water transport or cell proliferation. In pathological conditions, pro-inflammatory cytokines secreted by lymphocytes are responsible for modification of intestinal cell characteristics including phenotype switch and changes in the expression of pumps and ion channels. Using the HT29 cell line as a model, the present work examined the effect of two inflammatory cytokines, interferon-gamma (IFNgamma) and tumor necrosis factor-alpha (TNFalpha), on the expression of the human alpha(2A)-adrenoceptor. Exposure of cells to either IFNgamma or TNFalpha resulted in a concentration- and time-dependent diminution of [(3)H]RX821002 binding sites, which is preceded by a large decrease in the amount of alpha(2A)-adrenoceptor mRNA. The cytokines did not affect the receptor mRNA half-life, but inhibited the activity of a luciferase construct containing the promoter region of alpha(2A)-adrenoceptor gene, indicating that a decrease in the transcription rate is primarily responsible for the diminution of receptor expression. Exposure of cells to either IFNgamma or TNFalpha caused increased production of reactive oxygen species and transient phosphorylation of extracellular signal-regulated kinase (Erk1/2). The effect of cytokines was mimicked by H(2)O(2) but was unaffected by the addition of anti-oxidants. The blockade of Erk1/2 activation by PD98059 blunted the effect of TNFalpha but not of IFNgamma. In conclusion, the present findings demonstrate that IFNgamma and TNFalpha diminish the alpha(2A)-adrenoceptor expression in HT29 cells by decreasing the transcription rate without modifying the stability of mRNA. The transcription inhibition is however triggered via different signalling pathways. The results suggest that cytokine-mediated down-regulation of alpha(2A)-adrenoceptor could contribute to the pathogenesis of inflammatory bowel disease.

Body fat reduction without cardiovascular changes in mice after oral treatment with the MAO inhibitor phenelzine.

BACKGROUND AND PURPOSE: Phenelzine is an antidepressant drug known to increase the risk of hypertensive crisis when dietary tyramine is not restricted. However, this MAO inhibitor inhibits other enzymes not limited to the nervous system. Here we investigated if its antiadipogenic and antilipogenic effects in cultured adipocytes could contribute to decreased body fat in vivo, without unwanted hypertensive or cardiovascular effects. EXPERIMENTAL APPROACH: Mice were fed a standard chow and given 0.028% phenelzine in drinking water for 12 weeks. Body composition was determined by NMR. Cardiovascular dysfunction was assessed by heart rate variability analyses and by evaluation of cardiac oxidative stress markers. MAO activity, hydrogen peroxide release and triacylglycerol turnover were assayed in white adipose tissue (WAT), alongside determination of glucose and lipid circulating levels. KEY RESULTS: Phenelzine-treated mice exhibited lower body fat content, subcutaneous WAT mass and lipid content in skeletal muscles than control, without decreased body weight gain or food consumption. A modest alteration of cardiac sympathovagal balance occurred without depressed aconitase activity. In WAT, phenelzine impaired the lipogenic but not the antilipolytic actions of insulin, MAO activity and hydrogen peroxide release. Phenelzine treatment lowered non-fasting blood glucose and phosphoenolpyruvate carboxykinase expression. In vitro, high doses of phenelzine decreased both lipolytic and lipogenic responses in mouse adipocytes. CONCLUSION AND IMPLICATIONS: As phenelzine reduced body fat content without affecting cardiovascular function in mice, it may be of benefit in the treatment of obesity-associated complications, with the precautions of use recommended for antidepressant therapy.

Alpha2-adrenoreceptors profile modulation. 3.1 (R)-(+)-m-nitrobiphenyline, a new efficient and alpha2C-subtype selective agonist.

To assess the stereochemical requirements for efficient alpha2C-adrenoreceptor activation, the enantiomeric forms of m-nitrobiphenyline [(+/-)-5] were prepared and tested on cells expressing the human alpha2-adrenoreceptor subtypes. The importance of chirality was confirmed, since the enantiomer (R)-(+)-5 was much more efficient than (S)-(-)-5 in producing alpha2C-activation. Surprising reversal of enantioselectivity was observed with respect to structurally similar biphenyline [(+/-)-1] whose (S)-(-)-form proved the preferred alpha2C-configuration.

EGF receptor transactivation and PI3-kinase mediate stimulation of ERK by alpha(2A)-adrenoreceptor in intestinal epithelial cells: a role in wound healing.

Intestinal cells express alpha(2A)-adrenoreceptors that stimulate sodium and peptide absorption and promote cell proliferation. Involved mechanisms are poorly understood and are not fully related to inhibition of cAMP production. Previous study using a clone of CaCo2 cells expressing the human alpha(2A)-adrenoreceptor (CaCo2-3B) showed that alpha(2)-adrenoreceptor agonists cause extracellular signal-regulated kinase (ERK) phosphorylation. Present work examines the signaling pathway triggering ERK activation and investigates the consequence of alpha(2A)-adrenoreceptor stimulation on cell migration. Treatment of CaCo2-3B with the alpha(2)-adrenoreceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino) quinoxaline (UK14304) induces not only ERK, but also Akt phosphorylation. Both effects are strongly attenuated by inhibition or desensitization of epidermal growth factor (EGF) receptor, matrix metalloproteinase (MMP) blockade, heparin-binding-EGF neutralization or phosphatidylinositol 3-kinase (PI3-kinase) inhibitors. Conditioned medium from UK14304-treated CaCo2-3B stimulates ERK in parental CaCo2 by a mechanism sensitive to EGF receptor and PI3-kinase inhibitors. Exposure of CaCo2-3B to UK14304 accelerates the wound healing. This effect is abolished by heparin-binding-EGF neutralization but not by mitomycin C, indicating that it results probably from increased cell spreading and/or migration. In conclusion, alpha(2A)-adrenoreceptor activates ERK and Akt in intestinal cells by a common pathway which depends on PI3-kinase activation and results from EGF receptor transactivation, via an autocrine/paracrine pathway implying MMP activation and heparin-binding-EGF shedding. Therefore, alpha(2A)-adrenoreceptor could have a positive role in intestinal regeneration in vivo.

High intake of dietary tyramine does not deteriorate glucose handling and does not cause adverse cardiovascular effects in mice.

Tyramine is naturally occurring in food and induces pressor responses. Low-tyramine diets are recommended for patients treated with MAO inhibitors to avoid the fatal hypertensive crisis sadly known as "cheese effect". Hence, tyramine intake is suspected to have toxicological consequences in humans, while its administration to type 1 diabetic rodents has been reported to improve glucose tolerance. We investigated in mice whether prolonged tyramine ingestion could alter glucose homeostasis, insulin sensitivity, adipose tissue physiology or cardiovascular functions. Tyramine was added at 0.04 or 0.14 % in the drinking water since this was estimated to increase by 10- to 40-fold the spontaneous tyramine intake of control mice fed a standard diet. Ten to 12 weeks of such tyramine supplementation did not influence body weight gain, adiposity or food consumption. Both doses (reaching approx. 300 and 1100 µmol tyramine/kg bw/day) decreased nonfasting blood glucose but did not modify glucose tolerance or fasting levels of glucose, insulin or circulating lipids. Blood pressure was not increased in tyramine-drinking mice, while only the higher tested dose moderately increased heart rate without change in its variability. Markers of cardiac tissue injury or oxidative stress remained unaltered, except an increased hydrogen peroxide production in heart preparations. In isolated adipocytes, tyramine inhibited lipolysis similarly in treated and control groups, as did insulin. The lack of serious adverse cardiovascular effects of prolonged tyramine supplementation in normoglycemic mice together with the somewhat insulin-like effects found on adipose cells should lead to reconsider favourably the risk/benefit ratio of the intake of this dietary amine.

Cloning and functional characterization of the rat alpha2B-adrenergic receptor gene promoter region: Evidence for binding sites for erythropoiesis-related transcription factors GATA1 and NF-E2.

In the rat, the alpha2B-adrenergic receptor (alpha2B-AR) is encoded by the rat non-glycosylated (RNG) gene and is primarily expressed in the kidney, brain and liver of adult animals. High levels of alpha2B-AR are also found during fetal life in the placenta, liver and blood, where it is borne by cells of the erythropoietic lineage. As a first step to define the mechanisms responsible for the spatio-temporal pattern of alpha2B-AR expression, a genomic fragment containing 2.8 kb of the 5'-flanking region, the ORF and approximately 20 kb of the 3'-flanking region of the RNG gene was isolated. RNase protection assays performed on RNA from placenta or kidney using a series of riboprobes permitted to locate the transcription start site 372 bases upstream from the start codon. Transient transfection of various cells, including rat proximal tubule in primary culture, with constructs containing luciferase as a reporter gene demonstrated that: (i) the 5'-flanking region exhibited a strong and sense-dependent transcriptional activity and (ii) the 332 bp fragment (-732/-401 relative to the start codon), which lacks a TATA box but contains Sp1 sites, is sufficient to drive expression. Analysis of chromatin susceptibility to DNaseI digestion identified two hypersensitive sites (HS1 and HS2) located 1.7 and 1.0 kb, respectively, upstream from ATG and containing recognition sequences for erythroid transcription factors. EMSA showed specific binding of GATA1 and NF-E2 to these elements. Taken together, the results suggest that the chromatin environment in the vicinity of these boxes plays a critical role for alpha2B-AR expression during fetal life.

Cloning, characterisation and identification of several polymorphisms in the promoter region of the human alpha2B-adrenergic receptor gene.

Screening of a foetal brain genomic DNA library allowed to isolate a 10-kb fragment of the gene encoding the human alpha2B-adrenergic receptor, that contained 5.5 kb of the 5'-flanking region, the open reading frame and 2.9 kb of the 3'-flanking region. The 1-kb fragment upstream from the start codon was rich in GC, lacked consensus TATA or CAAT box, but contained several Sp1-binding sites. Other potential cis-regulatory elements found in the 5'-flanking region included AP2, USF, Stat-6, NFkappaB and Olf-1. A single canonical polyadenylation signal (AATAAA) was found at position +3252/+3257 and the polyadenylation site was 3274 nucleotides downstream from ATG. Transfection experiments with chimeric luciferase constructs containing various truncated fragments of the 5'-region showed that the fragment -3160/+3 exhibited promoter activity in all tested cell lines and permitted the definition of a minimal 200-bp promoter (-603/-411) containing three putative Sp1-binding sites and two initiator elements. Transcriptional activity of this region was inhibited by the addition of mithramycin, a specific inhibitor of Sp1 binding to GC-rich sequences. The search for sequence variants within a fragment covering 1.7 kb of 5'-flanking region and the coding region allowed us to identify five novel single nucleotide polymorphisms. Interestingly, the G/C substitution at position -98 relative to the start codon was common and in complete linkage with a previously identified insertion/deletion polymorphism in the coding region which was showed to affect alpha2B-adrenergic receptor function. Based on transfection data and computer-assisted sequence analysis, the -98 G/C single nucleotide polymorphism was located within a portion of the 5'-UTR (-127/+3) affecting luciferase activity and it created additional putative binding site for Sp1. However, G/C substitution had no significant incidence on promoter activity in BHK-21 or HeLa cells.

Type 2 diabetes mellitus in mice aggravates the renal impact of hemorrhagic shock.

The objectives of this study were to determine whether type 2 diabetic mice would exhibit a more severe renal impact of hemorrhagic shock (HS) based on a recently described model of acute kidney injury and to determine the impact of HS on renal responses to hypoxia. We induced HS or sham procedure in type 2 diabetic and obese db/db mice. Creatininemia, glomerular filtration rate, urine output, histologic injury score, and kidney inductible molecule 1 mRNA were used to investigate the renal impact of HS. Tissular hypoxia and its impact were quantified using pimonidazole immunostaining and mRNA of hypoxic inducible factor, vascular endothelial growth factor receptors 1 and 2, Tie-2, endothelial nitric oxide synthase, and inducible nitric oxide synthase. Diabetic mice exhibiting mild diabetic nephropathy express hypoxic signals at baseline. The renal impact of HS was more severe in diabetic mice, with a worsening of tissular hypoxia and an altered response to hypoxia. Furthermore, endothelial nitric oxide synthase was highly overexpressed in diabetic shocked mice when compared with nondiabetic shocked mice. Renal impact of HS in type 2 diabetic mice is more intense than in nondiabetic ones. Preexisting hypoxia during diabetes could result in a renal preconditioning that modifies endothelial and tissular responses to acute kidney injury.

Identification of a novel 12-nucleotide insertion polymorphism in the promoter region of ADRA2B: full linkage with the 9-nucleotide deletion in the coding region and influence on transcriptional activity.

The alpha(2B)-adrenoceptor (alpha(2B)-AR) mediates vasoconstriction and a common polymorphism (+901 Ins/Del), located in the coding region of the human alpha(2B)-AR gene (ADRA2B), has been demonstrated to affect receptor function in vitro. In this study, we have identified a novel polymorphism corresponding to the insertion of 12-nucleotides (GGGACGGCCCTG) at position -4825 relative to the start codon (-4825 del/ins) in the far upstream region of the ADRA2B promoter. The genotyping of 71 unrelated Finnish individuals showed that the -4825 ins polymorphism is common and in complete linkage with the Del polymorphism at position +901 and a G/C substitution at position -98. Transfection of various cell lines with luciferase constructs containing a 5.5 kb fragment of the ADRA2B promoter region indicated that the 12-nucleotide insertion at -4825 resulted in a large reduction of transcriptional activity. Electrophoretic mobility shift assays with oligonucleotide probes corresponding to the two ADRA2B alleles demonstrated that the region where the -4825 del/ins variation occurs binds nuclear proteins and that the 12-nucleotide insertion affects the pattern of bound transcription factors. Altogether, the present findings show that the previously identified +901 Del polymorphism is linked with a variation in the ADRA2B promoter that affects transcriptional activity in vitro. The molecular mechanisms underlying this effect are still unclear but a possible impact of the -4825 ins polymorphism on alpha(2B)-AR expression would merit to be examined in vivo as a diminution of promoter activity may limit the functional consequences of the +901 Del polymorphism.

Genetic variation of human adrenergic receptors: from molecular and functional properties to clinical and pharmacogenetic implications.

Adrenergic receptors (ARs) are directly or indirectly involved in the control of a large panel of physiological functions and are the targets of drugs for the treatment of several common diseases including congestive heart failure, asthma or benign prostatic hyperplasia. The genotyping of human populations with diverse ethnicity has revealed that the genes encoding alpha(1A)-, alpha(1B)-, alpha(2A)-, alpha(2B)-, alpha(2C)-, beta(1)-, beta(2)- and beta(3)-AR are polymorphic in their coding region as well as in their regulatory domains and non-coding regions. The functional consequences of these genetic variations include changes in expression at transcriptional or translational level, modification of coupling to heterotrimeric G-proteins resulting in a gain or a loss in function, and alteration of GRK-mediated receptor phosphorylation/desensitization or of agonist-promoted down-regulation. None of the mutations identified so far is per se a major risk factor for acquired or inherited disease; however, variants of alpha(2C)-AR and beta(1)-AR may act in synergy to determine the progression of heart failure and certain combinations of polymorphisms on beta(2)-AR correlate with asthmatic phenotypes or response to beta(2)-agonist therapy. Herein we summarize the present knowledge on AR gene polymorphisms, and discuss the putative consequences of variations resulting in receptor malfunction on pharmacogenomics and disease predisposition.

High intake of dietary tyramine does not deteriorate glucose handling and does not cause adverse cardiovascular effects in mice

Tyramine is naturally occurring in food and induces pressor responses. Low-tyramine diets are recommended for patients treated with MAO inhibitors to avoid the fatal hypertensive crisis sadly known as "cheese effect". Hence, tyramine intake is suspected to have toxicological consequences in humans, while its administration to type 1 diabetic rodents has been reported to improve glucose tolerance. We investigated in mice whether prolonged tyramine ingestion could alter glucose homeostasis, insulin sensitivity, adipose tissue physiology or cardiovascular functions. Tyramine was added at 0.04 or 0.14 % in the drinking water since this was estimated to increase by 10- to 40-fold the spontaneous tyramine intake of control mice fed a standard diet. Ten to 12 weeks of such tyramine supplementation did not influence body weight gain, adiposity or food consumption. Both doses (reaching approx. 300 and 1100 μmol tyramine/kg bw/day) decreased nonfasting blood glucose but did not modify glucose tolerance or fasting levels of glucose, insulin or circulating lipids. Blood pressure was not increased in tyramine-drinking mice, while only the higher tested dose moderately increased heart rate without change in its variability. Markers of cardiac tissue injury or oxidative stress remained unaltered, except an increased hydrogen peroxide production in heart preparations. In isolated adipocytes, tyramine inhibited lipolysis similarly in treated and control groups, as did insulin. The lack of serious adverse cardiovascular effects of prolonged tyramine supplementation in normoglycemic mice together with the somewhat insulin-like effects found on adipose cells should lead to reconsider favourably the risk/benefit ratio of the intake of this dietary amine (AU)

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Accelerated lipid absorption in mice overexpressing intestinal SR-BI.

Dietary cholesterol absorption contributes to a large part of the circulating cholesterol. However, the mechanism of sterol intestinal uptake is not clearly elucidated. Scavenger receptor class B type I (SR-BI), major component in the control of cholesterol homeostasis, is expressed in the intestine, but its role in this organ remains unclear. We have generated transgenic mice overexpressing SR-BI primarily in the intestine by using the mouse SR-BI gene under the control of intestinal specific "apoC-III enhancer coupled with apoA-IV promoter." We found SR-BI overexpression with respect to the natural protein along the intestine and at the top of the villosities. After a meal containing [(14)C]cholesterol and [(3)H]triolein, SR-BI transgenic mice presented a rise in intestinal absorption of both lipids that was not due to a defect in chylomicron clearance nor to a change in the bile flow or the bile acid content. Nevertheless, SR-BI transgenic mice showed a decrease of total cholesterol but an increase of triglyceride content in plasma without any change in the high density lipoprotein apoA-I level. Thus, we described for the first time a functional role in vivo for SR-BI in cholesterol but also in triglyceride intestinal absorption.

alpha 2B-adrenergic receptor activates MAPK via a pathway involving arachidonic acid metabolism, matrix metalloproteinases, and epidermal growth factor receptor transactivation.

We have investigated the mechanisms whereby alpha(2B)-adrenergic receptor (alpha(2B)-AR) promotes MAPK activation in a clone of the renal tubular cell line, LLC-PK1, transfected with the rat nonglycosylated alpha(2)-AR gene. Treatment of LLC-PK1-alpha(2B) with UK14304 or dexmedetomidine caused arachidonic acid (AA) release and ERK2 phosphorylation. AA release was abolished by prior treatment of the cells with pertussis toxin, quinacrine, or methyl arachidonyl fluorophosphonate but not by the addition of the MEK inhibitor U0126. The effects of alpha(2)-agonists on MAPK phosphorylation were mimicked by cell exposure to exogenous AA. On the other hand, quinacrine abolished the effects of UK14304, but not of AA, suggesting that AA released through PLA2 is responsible for MAPK activation by alpha(2B)-AR. The effects of alpha(2)-agonists or AA were PKC-independent and were attenuated by indomethacin and nordihydroguaiaretic acid. Treatment with batimastat, CRM 197, or tyrphostin AG1478 suppressed MAPK phosphorylation promoted by alpha(2)-agonist or AA. Furthermore, conditioned culture medium from UK14304-treated LLC-PK1-alpha(2B) induced MAPK phosphorylation in wild-type LLC-PK1. Based on these data, we propose a model whereby activation of MAPK by alpha(2B)-AR is mediated through stimulation of PLA2, AA release, generation of AA derivatives, activation of matrix metalloproteinases, release of heparin-binding EGF-like growth factor, transactivation of epidermal growth factor receptor, and recruitment of Shc. Whether this pathway is particular to alpha(2B)-AR and LLC-PK1 or whether it can be extended to other cell types and/or other G-protein-coupled receptors remains to be established.

alpha(2B)-Adrenergic receptors activate MAPK and modulate proliferation of primary cultured proximal tubule cells.

In the rat proximal tubule, the alpha(2B)-adrenergic receptor (alpha(2B)-AR) enhances Na(+) reabsorption by increasing the activity of Na(+)/H(+) exchanger isoform NHE3. The mechanisms involved are unclear, and inhibition of cAMP production remains controversial. In this study, we reinvestigated alpha(2B)-AR signaling pathways using rat proximal tubule cells (PTC) in primary culture and LLC-PK(1) cells permanently transfected with the RNG gene (rat nonglycosylated alpha(2)-AR). Binding experiments indicated that PTC express substantial amounts of alpha(2B)-AR (130 fmol/mg protein), and only RNG transcripts were detected. In both cell types, the alpha(2B)-AR is coupled to G protein, and its stimulation by dexmedetomidine, but not by UK-14304, provoked a significant inhibition of the accumulation of cAMP induced by forskolin or parathyroid hormone. Exposure to alpha(2)-agonists increased arachidonic acid release and caused extracellular signal-regulated kinase (ERK)1/2 phosphorylation, which correlated with enhanced mitogen-activated protein kinse (MAPK) activity and nuclear translocation. MAPK phosphorylation was blunted by pertussis toxin but not by protein kinase C desensitization, and it coincided with transient phosphorylation of Shc. Finally, treatment with UK-14304 accelerated cell growth. Further studies will be necessary to clarify the precise mechanism of MAPK activation, but the present data suggest that alpha(2B)-AR may play a positive role during tubular regeneration.