狼疮肾炎患者尿蛋白通过p38丝裂素活化蛋白激酶信号途径刺激近端肾小管上皮细胞上调表达骨调素

目的探讨p38丝裂素活化蛋白激酶(MAPK)信号通路在狼疮肾炎(LN)尿蛋白介导近端肾小管上皮细胞(HK-2)表达骨调素(DPN)中的作用。方法收集狼疮肾炎患者24h的尿液提纯总蛋白，体外刺激HK-2细胞，分别用逆转录-聚合酶链式反应(RT-PCR)和Western印迹法观察应用p38MAPK特异性阻断剂SB203580对。HK-2细胞表达OPN mRNA及蛋白的影响：免疫荧光法检测OPN及其受体CD44在细胞中的表达。结果狼疮肾炎患者尿蛋白可刺激HK-2细胞OPN mRNA及蛋白上调表达，并呈时间和浓度依赖性，而SB203580可明显抑制这一作用。结论狼疮肾炎尿蛋白可通过p38MAPK途径刺激HK-2细胞OPN mRNA和蛋白上调表达。     

转化生长因子β1对肾小管上皮细胞中结缔组织生长因子基因启动子活性的影响

目的探讨转化生长因子[β1（TCF-β1）对人近端肾小管上皮细胞系HK-2中结缔组织生长因子（CTGF）基因启动子活性的调控作用，以及丝裂原激活蛋白激酶（MAPK）途径对该生长因子作用的影响。方法构建含有人类CTGF基因启动子的报告基因pCTGF-luc，将其瞬时转染HK-2细胞。通过检测荧光素酶的活性观察TGF-β1和MAPK途径抑制剂对CTGF基因启动子活性的影响。结果TGF-β1以剂量和时间依赖方式上调HK-2中CTGF基因启动子的活性。最佳刺激浓度是5ng／ml，最佳刺激时间为12h，荧光素酶相对活性分别为对照组的1．82倍和2．10倍（P〈0．05）。应用PD98059、SB203580和SP600125分别特异性抑制MAPK途径的胞外信号调节蛋白激酶（ERK）、蛋白激酶p38（p38MAPK）和c-Jun-氨基末端激酶（JNK）通路，对TGF-β1上调CTGF启动子活性的作用有不同影响。PD98059显著增加HK-2中pCTGF-luc的基础活性．并在一定浓度范围内（0．5～10μmol／L）促进TGF-β1的上调作用。SB203580对pCTGF-luc基础活性无影响，但以剂量依赖方式显著抑制TGF-β1的激活效应。而SP600125对基础状态和TGF-β1刺激下CTGF基因启动子活性无影响。结论TGF-β1以剂量和时间依赖方式上调HK-2中CTGF基因启动子活性，在转录水平调节CTGF表达。MAPK途径的ERK和p38MAPK通路可影响TGF-β1的这一调控作用。     

斯伐他汀对糖尿病肾病大鼠肾小球系膜细胞p38信号通路的影响

目的研究糖尿病肾病大鼠肾小球系膜细胞p38丝裂原活化蛋白激酶（MAPK）的表达及斯伐他汀对其的影响。方法分别以高糖、糖基化终产物（AGE）及过氧化氢孵育糖尿病大鼠肾小球系膜细胞（RMC），Western印迹法检测RMC的p38MAPK和TGF—β蛋白表达，p38MAPK特异性抑制剂SB203580及斯伐他汀预处理对其影响。结果高糖、AGE及过氧化氢均可单独激活p38MAPK，增加RMC的磷酸化（P）p38MAPK和TGF—β的蛋白表达；SB203580显著抑制TGF—β的蛋白表达（P〈0．05）；斯伐他汀抑制p38MAPK的活化并减少TGF—β的蛋白表达（P〈0．05）。结论p38MAPK可能是糖尿病肾病发生的始动信号之一。斯伐他汀可能通过抑制p38MAPK磷酸化而减少TGF—β的蛋白表达。     

p38有丝分裂素激活蛋白激酶在白蛋白刺激肾小管上皮细胞表达调节激活正常T细胞表达和分泌细胞因子中的作用

目的探讨p38有丝分裂素激活蛋白激酶（p38MAPK）对白蛋白诱导的大鼠肾小管上皮细胞表达调节激活正常T细胞表达和分泌细胞因子（RANTES）及核因子-κB（NF-κB）活性的影响。方法培养大鼠肾小管上皮细胞（NRK-52E），分别加入不同浓度白蛋白（5、15、30g／L）或／和SB203580（p38MAPK抑制剂），应用逆转录．聚合酶链反应（RT-PCR）和Western印迹法分别检测RANTESmRNA及蛋白表达水平；应用凝胶迁移率变动分析检测NF-κB的活性变化；p38MAPK的磷酸化水平分析采用Western印迹法。结果白蛋白呈浓度和时间依赖性上调RANTESmRNA及蛋白表达水平，呈时间依赖性激活NF-κB活性，白蛋白明显刺激了p38MAPK磷酸化水平增高，SB203580可抑制白蛋白刺激的RANTES表达与NF-κB活性。结论白蛋白通过p38MAPK信号通路刺激RANTES的表达和NF-κB的活性。     

p38MAPK与 ERK1/2的协同效应及对成骨细胞分化的调控

 目的探讨骨髓间质干细胞(bone marrow mesenchymal stem cells,BMSCs)向成骨细胞分化过程中 p38MAPK与 ERK1/2的协同效应及其机制。方法以成骨细胞分化添加剂诱导小鼠 BMSCs向成骨细胞分化,测定碱性磷酸酶活性和钙沉积量。检测磷酸化 p38MAPK和磷酸化 ERK1/2(p-ERK1/2)的表达水平评估通路的激活状况。以 SB203580或 PD98059阻断 p38MAPK或 ERK1/2通路,观察对成骨细胞分化的影响。以 SB203580或亚砷酸钠阻断或激活 p38MAPK通路,观察 p-ERK1/2的变化。以冈田酸抑制蛋白磷酸酯酶 2A(protein phosphatases type 2A,PP2A)活性,观察 p-ERK1/2的变化及对成骨细胞分化的影响。通过免疫共沉淀实验观察 PP2A和 ERK1/2间的结合及 SB203580对结合的影响。结果成骨细胞分化添加剂诱导 BMSCs向成骨细胞分化的过程伴有 ERK1/2和 p38MAPK通路的激活, SB203580剂量±赖性抑制成骨细胞分化,PD98059剂量±赖性增强成骨细胞分化。 SB203580使 p-ERK1/2表达增加,亚砷酸钠减弱其表达。冈田酸使 p-ERK1/2表达增加,并使成骨细胞分化受到抑制。 PP2A可直接与 ERK1/2结合,SB203580使 PP2A与 ERK1/2的结合减弱。结论 p38MAPK可通过 PP2A与 ERK1/2产生协同效应,并调节 BMSCs向成骨细胞分化。     

丝裂素激活蛋白激酶活化与肾小管上皮细胞生物学行为的关系

目的探讨丝裂素激活蛋白激酶(MAPK)信号通路对血小板源生长因子(PDGF)诱导肾小管上皮细胞表型转化的作用及其对细胞移行能力的影响.方法以PDGF(20 ng/ml)刺激培养的人近端肾小管上皮HK-2细胞,分别观察细胞形态、增殖和移行能力的变化.采用蛋白免疫印记法检测MAPK各亚类活性及α-平滑肌肌动蛋白(SMA)的表达,同时观察分别采用细胞外信号调节激酶(ERK)、c-Jun氨基末端激酶(JNK)和p38信号通路特异阻断剂PD98059、SP600125和SB203580后上述指标的变化.结果 PDGF刺激6～12 h使α-SMA表达上调近2倍,但刺激24～48 h反而使α-SMA表达低于基础水平.PDGF刺激24 h后HK-2细胞从立方形转变为梭形,细胞虽尚无增殖反应但移行能力增强至3倍(P＜0.01).PDGF诱导的ERK、JNK和p38活性均在2 min时迅速增高,分别在2～5 min达高峰,分别为基础水平的5.7倍、2.6倍和1.6倍(P＜0.05),ERK和JNK活性增高可持续至120 min,而p38活性则于60 min以后恢复至基础水平.在PDGF刺激24 h的情况下,PD98059和SP600125不影响基础状态及PDGF对α-SMA表达的作用,但SP600125可抑制PDGF上调的细胞移行能力(P＜0.01);而SB203580既可抑制α-SMA的基础表达(P＜0.01)及PDGF下调的α-SMA表达(P＜0.001),同时还可显著抑制PDGF诱导的细胞移行能力(P＜0.05).结论 PDGF可刺激MAPK的不同亚类信号通路磷酸化,并可诱导肾小管上皮细胞形态改变及其移行能力增强.在PDGF诱导的表型转化和移行功能改变中,ERK信号通路无介导作用,JNK通路主要与细胞移行能力变化相关,而p38可能是调控α-SMA表达和细胞移行改变的主要信号通路.     

丝裂原活化蛋白激酶类和蛋白激酶C在转化生长因子β1诱导肾小管细胞结缔组织生长因子表达中的作用

目的了解转化生长因子β1（TGF-β1）诱导肾小管细胞结缔组织生长因子（CTGF）表达的机制，特别是蛋白激酶C（PKC）和丝裂原活化蛋白激酶（MAPK）在CTGF基因表达中的作用及其对Smad磷酸化的影响。方法分别应用PKC抑制剂G06850以及MAPK的3个组成成分ERK、JNK和p38MAPK的抑制剂PD98059、U0126、SP600125和SB203580阻断相应通路，观察其对TGF．131诱导的CTGF表达以及Smad2／Smad3磷酸化的影响。结果TGF-β1（5μg／L）以时间依赖方式诱导HK-2细胞中Smad2／Smad3的磷酸化，从基础值0．87±0．09上升至2h时高峰2．350±0.11。PKC抑制剂G06850（5μmol／L）和ERK抑制剂PD98059（10μmol／L）、U0126（10μmol／L）可部分抑制TGF-β1诱导的CTGF表达，而p38MAPK抑制剂SB203580（20μmol／L）和JNK抑制剂SP600125（10μmol／L）对TGF-β1诱导的CTGF的表达无影响。PKC抑制剂G06850（5μmol／L）可减少TGF-β1诱导的Smad2／Smad3磷酸化，而ERK抑制剂PD98059（10μmol／L）和U0126（10μmol／L）对Smad2／Smad3的磷酸化没有影响。结论在肾小管上皮细胞中，TGF-β1诱导CTGF的表达需要PKC和Ras／MEK／ERK的参与。PKC以Smad依赖的方式参与肾小管上皮细胞中TGF-β1诱导的CTGF的表达，而Ras／MEK／ERK对CTGF表达的调节不依赖于Smads。     

低渗非离子造影剂对肾小管上皮细胞凋亡的作用及其机制

目的 探讨低渗非离子型造影剂碘必乐诱导体外培养的人肾小管上皮细胞（HK-2细胞）凋亡的作用及机制。 方法 体外培养的HK-2细胞,分为阴性对照组、不同剂量（1.16、4.63、18.5、74、296 gI/L）的碘必乐作用组（作用时间为6 h）。通过流式细胞仪、Hoechst 33258染色观察细胞凋亡的比例和形态学变化;Western印迹方法检测凋亡蛋白天冬氨酸半胱氨酸蛋白酶3（caspase-3）的表达水平,并选取作用最强的剂量（296 gI/L）刺激2、4、6、12 h,观察caspase-3表达变化。选取不同剂量碘必乐作用1 h,与阴性对照组比较,观察细胞外信号调节激酶（ERK）和p38丝裂原活化蛋白激酶（MAPK）信号通路磷酸化水平的变化,并观察不同剂量p38MAPK抑制剂SB203580对碘必乐诱导的caspase-3和Bcl-2表达的影响。 结果 流式细胞仪检测结果示74、296 gI/L碘必乐作用下细胞凋亡率较阴性对照显著升高（均P < 0.05）。Hoechst 33258染色结果示296 gI/L碘必乐可致明显的细胞凋亡形态学改变,凋亡细胞核呈致密浓染或核碎裂。Western印迹检测方法表明,碘必乐以剂量和时间依赖方式诱导细胞内caspase-3表达,以296 gI/L作用12 h表达最强;各剂量碘必乐作用下细胞内ERK1/2磷酸化水平与阴性对照组的差异均无统计学意义（均P > 0.05）,而一定剂量的碘必乐处理组细胞内p38MAPK磷酸化水平较阴性对照组显著升高（均P < 0.05）。应用p38MAPK特异性抑制剂（30 μmol/L）预处理2 h可以阻断细胞内p38MAPK信号通路的磷酸化,抑制碘必乐诱导的细胞内caspase-3表达并部分上调Bcl-2表达,与碘必乐阳性对照组的差异均有统计学意义（P < 0.05）。 结论 低渗非离子型造影剂碘必乐以剂量和时间依赖方式诱导体外培养的HK-2细胞凋亡,其机制可能与上调caspase-3和下调抗凋亡蛋白Bcl-2有关,而p38MAPK信号通路的激活可能参与了该过程的调控。     

匹格列酮对高糖时肾小球系膜细胞p38信号通路和TGF-β的影响

目的：研究匹格列酮对高糖培养下人肾小球系膜细胞P38促分裂原活化蛋白激酶（p38MAPK）信号通路及转化生长因子（TGF-β）的影响,进一步探讨匹格列酮抗糖尿病肾病的作用机制。方法：实验分组：正常对照组、甘露醇组、高糖组、高糖＋SB203580组、高糖＋匹格列酮组,观察匹格列酮对高糖培养下的人肾小球系膜细胞p38MAPK信号通路和TGF-β蛋白表达的影响。结果：与高糖组相比,匹格列酮降低系膜细胞p38MAPK信号通路和TGF-β蛋白表达水平。结论：匹格列酮抗糖尿病肾病的作用可能与其抑制p38MAPK信号通路激活和减少TGF-β的表达密切相关。     

白细胞介素1β通过p38信号通路上调大鼠肾小球系膜细胞骨调素mRNA的表达

目的 探讨p38信号通路(1938MAPK)在白细胞介素1(IL-1)β介导的大鼠肾小球系膜细胞表达骨调素(OPN)中的作用。方法 应用Western印迹检测p38MAPK在IL-1β诱导的肾小球系膜细胞炎症反应中的活化程度。应用逆转录-聚合酶链式反应(RT-PCR)法观察p38MAPK特异性阻断剂SB203580对IL-1β诱导的系膜细胞促炎症介质OPNmRNA的影响。结果 IL-1β以时间和剂量依赖方式刺激系膜细胞引起p38MAPK的活化，并明显上调系膜细胞OPNmRNA的表达。p38MAPK特异性抑制剂SB203580以剂量依赖性方式显著抑制IL-1β诱导的OPNmRNA的表达。结论 p38MAPK在IL-16介导的肾小球系膜细胞上调表达黏附分子OPN中起重要作用。     

Urinary proteins from patients with nephrotic syndrome alters the signalling proteins regulating epithelial–mesenchymal transition

Aim:   Proteinuria plays an important role in the progression of tubulointerstitial fibrosis, but the mechanism for the differential renal damage induced by proteinuria is unknown. This study examined the effects of urinary proteins from patients with idiopathic minimal change disease (MCD) and focal segmental glomerulosclerosis (FSGS) on several epithelial–mesenchymal transition (EMT)-related marker proteins in cultured proximal tubular HK-2 cells.
Methods:   Urinary proteins from MCD and FSGS patients were extracted by ultrafiltration and incubated with HK-2 cells; the expression of the cytokeratin-18, α-smooth muscle actin (α-SMA) and vimentin were assessed. p38 and extracellular regulated kinase (ERK) activation were measured by western blotting, and SB203580 (a p38 inhibitor) and PD98059 (an ERK1/2 inhibitor) were used to inhibit their activation.
Results:   It was observed that urinary proteins from FSGS patients more significantly induced the expression of α-SMA and vimentin and reduced cytokeratin-18 expression than those from MCD patients in HK-2 cells. Both ERK1/2 and p38 were activated by urinary proteins from MCD or FSGS patients. Pretreatment of the cells with SB203580 or PD98059 abolished the effect of urinary proteins from FSGS patients on the expression of α-SMA, vimentin and cytokeratin-18, while only SB203580 elicited this effect when cells were treated with urinary proteins from MCD patients.
Conclusion:   The urinary proteins from MCD and FSGS patients induced significant changes of EMT-related proteins through activation of distinct mitogen-activated protein kinase-related signalling pathways. Quality of proteinuria may play an important role in determining the severity and progression of tubular injury associated with different kidney diseases.     

ERK and p38 MAP kinase are required for rat renal development

BACKGROUND: We previously demonstrated that extracellular signal-regulated protein kinase (ERK) and p38 mitogen-activated protein (MAP) kinase (p38) are strongly expressed in the embryonic kidney. In the present study, we investigated the role of ERK and p38 during kidney development. METHODS: Rat metanephroi were cultured from 15-day-old embryos, and exposed to inhibitors of MEK, an activator of ERK, PD98059 (300 micromol/L), U0126 (10 micromol/L), or a p38 inhibitor SB203580 (30 micromol/L) 24 to 120 hours after the start of culture. Growth of metanephroi was measured by surface area and thymidine incorporation. Ureteric buds and glomeruli were identified by labeling with Dolichos biflorus lectin and peanut agglutinin, respectively. PCNA staining and TUNEL assay were performed on kidney sections. The level of apoptosis was evaluated by examining DNA ladder formation. RESULTS: Growth of metanephroi was significantly inhibited by SB203580 but not by PD98059 or U0126. Ureteric bud branching was not affected by SB203580 or MEK inhibitors. Glomerular number was markedly reduced by SB203580 and to a lesser extent by U0126 (14 +/- 2 and 48 +/- 10% of controls, respectively). On histological examination, the number of tubuloglomerular structures was reduced in MEK inhibitor-treated metanephroi compared to controls. Very few mesenchymal condensates were observed in kidneys incubated with SB203580. PCNA-positive cells were reduced in SB203580-treated metanephroi compared to control and PD98059-treated kidneys. Apoptosis was increased in SB203580-treated kidneys and to a lesser extent in PD98059-treated cultures. CONCLUSIONS: Both ERK and p38 are required for renal development. ERK appears to play a role in nephrogenesis and p38 for kidney growth and nephrogenesis.     

Role of MAP kinase pathways in mediating IL-6 production in human primary mesangial and proximal tubular cells.

BACKGROUND: Both interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) are pleiotropic cytokines that have been implicated in the development of glomerular and tubular injury in various forms of immune-mediated renal disease, including glomerulonephritis. Although TNF-alpha has been shown to stimulate IL-6 production in renal cells in culture, the signaling mechanisms that regulate IL-6 production are not fully understood. The aim of this study was to examine the role of the p38 and extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) pathways in regulating TNF-alpha-mediated IL-6 production from both primary human mesangial cells (HMCs) and human proximal tubular (HPT) cells. METHODS: Primary mesangial and proximal tubular cells were prepared from nephrectomized human kidney tissue. Cells were treated for 24 hours with TNF-alpha in the presence and absence of the specific p38 and ERK1,2 MAPK inhibitors SB203580 and PD98059, respectively, either alone or in combination. IL-6 levels in the cell culture media were measured by enzyme-linked immunosorbent assay. MAPK activation was demonstrated by immunoblot for the active kinase (tyrosine/threonine phosphorylated) in whole cell extracts using phospho-specific antibodies. p38 MAPK activity in HPT cells was measured using an in vitro immunokinase assay using ATF2 as the substrate. RESULTS: TNF-alpha (0.1 to 100 ng/ml) stimulated a dose-dependent increase in IL-6 production in both renal cell types. The activation of the p38 and the ERK1,2 MAPKs occurred following TNF-alpha stimulation. The role of these activations in IL-6 production was confirmed by the ability of both inhibitors SB203580 (1 to 30 microM) and PD98059 (0.01 to 10 microM) to inhibit basal and TNF-alpha-stimulated IL-6 production in both cell types. The addition of both inhibitors in combination caused greater decreases in IL-6 production compared with either inhibitor alone. Pretreatment with SB203580 (10 microM) had no effect on basal or TNF-alpha-stimulated phosphorylation of p38 MAPK but completely abolished TNF-alpha-stimulated p38 MAPK activity. PD98059 decreased both basal and TNF-alpha-stimulated phosphorylation of ERK1,2. CONCLUSIONS: This study provides evidence that both the p38 and ERK MAPK pathways are important for the regulation of the production of IL-6 from the proximal tubular and glomerular mesangial regions of the nephron. In response to TNF-alpha, the activation of both pathways leads to IL-6 production. These findings could aid in an understanding of the cellular mechanisms that regulate IL-6 production and could provide insights into possible pharmacological strategies in inflammatory renal disease.     

Modulation of renal epithelial barrier function by mitogen-activated protein kinases (MAPKs): mechanism of cyclosporine A-induced increase in transepithelial resistance

BACKGROUND: Cyclosporine A (CsA) has been shown to increase transepithelial resistance in Madin-Darby canine kidney (MDCK) cells, and the mechanism may involve altered phosphorylation of junctional proteins. In this study, we examine the effect of the extracellular signal-regulated protein kinase (ERK) 1/2 and p38 mitogen-activated protein kinase (MAPK) pathways on the basal transepithelial resistance (TER) and on the CsA-induced increase in TER across MDCK monolayers. Here we present evidence that CsA may be mediating some of its effects through activation of the ERK 1/2 MAPK pathway. METHODS: MDCK cells were treated with CsA (4.2 micromol/L) and paracellular permeability was assessed by measuring TER. The role of the ERK 1/2 and the p38 MAPK pathways in modulating TER was investigated using the inhibitors PD98059 and U0126 for ERK 1/2 and SB203580 for p38. ERK 1/2 and p38 phosphorylation/activation was also examined by Western blot analysis. RESULTS: CsA (4.2 micromol/L) increased the TER of MDCK monolayers. The ERK 1/2 inhibitor PD98059 decreased basal TER and also ameliorated the CsA-induced increase in TER. Similar results were found with the U0126 inhibitor of ERK 1/2. The p38 inhibitor SB203580 had no effect on the basal TER of the monolayers, however, SB203580 significantly augmented the CsA-induced increase in TER. CsA was shown to significantly activate ERK 1/2 and this activation by CsA was prevented by PD98059. Inhibition of the p38 pathway by SB203580 also resulted in activation of ERK 1/2 and this activation of ERK 1/2 was further enhanced by CsA. No effect of CsA or the inhibitors PD98059 or SB203580 on p38 phosphorylation was detected. CONCLUSION: The results presented here suggest that activation of the ERK 1/2 MAPK cascade is important in the regulation of the paracellular permeability in MDCK cells. Activation of this pathway appears to be pivotal to the CsA-induced increase in TER.     

缺氧时丝裂原激活蛋白激酶类对人肾小管上皮细胞富含半胱氨酸蛋白61基因转录活性的调控

目的 探讨丝裂原激活蛋白激酶类（MAPKs）对缺氧条件下人近端肾小管上皮细胞（HKC）中富含半胱氨酸蛋白61（Cyr61）基因转录活性的调控机制。方法 缺氧培养HKC，Northern印迹检测Cyr61mRNA表达；Western印迹检测Cyr61、p38、细胞外信号调节激酶（ERK1／2）、c—Jun—N末端蛋白激酶（JNK）以及缺氧诱导因子1c（HIF-1α）的表达。构建含有人Cyr61基因启动子的报告基因Cyr61-luc质粒，将其单独或者分别与表达活性MAPKs的质粒Ca—MEK1和Ca—MKK6共同瞬时转染HKC。通过荧光素酶活性检测观察缺氧、MAPKs抑制剂和MAPKs活性酶对Cyr61基因转录活性的调控。结果 缺氧时HKC表达cyr61、HIF-1α增高，ERK1／2、JNK、p38总量不变，而其各自的磷酸化形式均明显增加。HKC转染Cyr—luc后，p38通路抑制剂SB203580和ERK通路抑制剂PD98059显著抑制缺氧时Cyr61的转录活性，两者协同作用时抑制作用显著增强。Ca—MEK1与Cyr—luc共转染HKC后，Cyr61转录活性无改变；而Ca—MKK6与Cyr—luc共转染后，Cyr61转录活性显著增高。对缺氧培养的HKC，PD98059处理使HIF-1α和Cyr61蛋白表达显著降低；SB203580处理可显著降低Cyr61蛋白表达，但对HIF-1α无影响。结论 在HKC中，缺氧可通过p38通路直接上调Cyr61基因启动子活性，也可通过ERK1／2途径促进HIF-1α表达，间接调节Cyr61基因启动子活性。     

Sustained orbital shear stress stimulates smooth muscle cell proliferation via the extracellular signal-regulated protein kinase 1/2 pathway

OBJECTIVE: Nonlaminar shear stress stimulates smooth muscle cell (SMC) proliferation and migration in vivo, especially after an endothelial-denuding injury. To determine whether sustained shear stress directly stimulates SMC proliferation in vitro, the effect of orbital shear stress on SMC proliferation, phenotype, and extracellular signal-regulated protein kinase 1/2 (ERK1/2) phosphorylation was examined. METHODS: Bovine SMCs were exposed to orbital shear stress (210 rpm) for up to 10 days, with and without the ERK1/2 upstream pathway inhibitor PD98059 (10 microM) or the p38 pathway inhibitor SB203580 (10 microM). Proliferation was directly counted and assessed with proliferation cell nuclear antigen. Western blotting was used to assess activation of SMC ERK1/2 and SMC phenotype markers. RESULTS: SMCs exposed to sustained orbital shear stress (10 days) had 75% increased proliferation after 10 days compared with static conditions. Expression of markers of the contractile phenotype (alpha-actin, calponin) was decreased, and markers of the synthetic phenotype (vimentin, beta-actin) were increased. ERK1/2 was phosphorylated in the presence of orbital shear stress, and orbital shear-stress-stimulated SMC proliferation was inhibited in the presence of PD98059 but sustained in the presence of SB203580. Orbital shear-stress-induced changes in SMC phenotype were also inhibited in the presence of PD98059. CONCLUSION: Orbital shear stress directly stimulates SMC proliferation in long-term culture in vitro and is mediated, at least partially, by the ERK1/2 pathway. The ERK1/2 pathway may also mediate the orbital shear-stress-stimulated switch from SMC contractile to synthetic phenotype. These results suggest that shear-stress-stimulated SMC proliferation after vascular injury is mediated by a pathway amenable to pharmacologic manipulation.