Mitogen-activated protein kinase phosphatase-1 in rat arterial smooth muscle cell proliferation

Role of p44/p42 MAP kinase in the age-dependent increase in vascular smooth muscle cell proliferation and neointimal formation

OBJECTIVE

Age-dependent increase in vascular smooth muscle cell (VSMC) proliferation is thought to contribute to the pathology of atherosclerotic diseases. In this study, we investigated the role of mitogen-activated protein kinases (MAPKs) on VSMC proliferation and neointimal formation in the context of aging.

METHODS AND RESULTS

VSMCs were isolated from the aorta of young and old rabbits. The proliferative index after serum stimulation was significantly increased in old versus young VSMCs. This was associated with a significant and specific age-dependent increase in p44/p42 MAPK activation. Treatment with MEK inhibitor PD98059 successfully inhibited p44/p42 MAPK activities and VSMC proliferation. These results were confirmed in vivo using a model of balloon injury in rabbit iliac arteries. p44/p42 MAPK activities were rapidly induced by angioplasty in young and old animals. However, the levels of p44/p42 MAPK activities achieved in arteries of old rabbits were significantly higher than those of young rabbits. This was associated with a higher cellular proliferative index and a significant increase in neointimal formation in old animals. Local delivery of PD98059 in old rabbits successfully inhibited p44/p42 MAPK activities after angioplasty, which led to a significant reduction in cellular proliferation and neointimal formation in treated animals.

CONCLUSIONS

Our study suggests for the first time that increased p44/p42 MAPK activation contributes to augmented VSMC proliferation and neointimal formation with aging. p44/p42 MAPK inhibition could represent a novel therapeutic avenue against atherosclerotic diseases.

Down-regulation of the ERK1 and ERK2 mitogen-activated protein kinases using antisense oligonucleotides inhibits intimal hyperplasia in a porcine model of coronary balloon angioplasty

OBJECTIVE

Neointimal hyperplasia is a central feature in the pathogenesis of a variety of vascular pathologies. Mitogen-activated protein kinases (MAPK) are involved in the downstream transduction of signals from receptors for many of the molecules known to be instrumental in this process and thus represent a potential target for the modification of the proliferative response. We examined the hypothesis that down-regulation of MAPK would inhibit neointima formation in a porcine coronary injury model.

METHODS

Balloon angioplasty was performed on 38 coronary arteries from 23 large white pigs. Antisense oligonucleotides to the p42 and p44 MAPK were locally delivered to the site of injury immediately after balloon injury. At 7 or 21 days, arteries were harvested for morphometry, determination of cell proliferation and assessment of MAPK protein levels.

RESULTS

At 7 days, neointima formation was significantly reduced compared to controls (corrected intima/media ratio (IMR) 1.01+/-0.13 vs. 1.61+/-0.07, P<0.01) and this was associated with a 58% and 23% down-regulation of p42 and p44 protein levels, respectively. Intimal and medial proliferation rates were also reduced by 32% and 26%, respectively. At 21 days however, the effect of the treatment on MAPK protein levels was no longer significant and this correlated with a loss of the effects on IMR and cell proliferation.

CONCLUSIONS

Down-regulation of MAPK inhibits early smooth muscle cell (SMC) proliferation and neointimal thickening in response to arterial injury, implying that it plays an important role in determining the early vascular response to injury. Inhibitory effects were less apparent at 21 days after a single delivery of oligonucleotide, implying that more sustained local delivery may be required to achieve longer term therapeutic benefit.

Magnolol suppresses proliferation of cultured human colon and liver cancer cells by inhibiting DNA synthesis and activating apoptosis

Magnolol, a hydroxylated biphenyl compound isolated from the Chinese herb Hou p'u of Magnolia officinalis, has been reported to have anti-cancer activity. In the present study, magnolol at very low concentrations of 3-10 microM inhibited DNA synthesis and decreased cell number in cultured human cancer cells (COLO-205 and Hep-G2) in a dose-dependent manner, but not in human untransformed cells such as keratinocytes, fibroblasts, and human umbilical vein endothelial cells (HUVEC). Magnolol was not cytotoxic at these concentrations and this indicates that it may have an inhibitory effect on cell proliferation in the subcultured cancer cell lines. [(3)H] thymidine incorporation and flow cytometry analyses revealed that magnolol treatment decreased DNA synthesis and arrested the cells at the G0/G1 phase of the cell cycle. Moreover, the magnolol-induced cell cycle arrest occurred when the cyclin-CDK system was inhibited, just as p21 protein expression was augmented. When magnolol concentration was increased to 100 microM, apoptosis was observed in COLO-205 and Hep-G2 cells, but not in cultured human fibroblasts and HUVEC. COLO-205 cells implanted subcutaneously in nude mice formed solid tumors; subsequent daily i.p.-injections of magnolol led to profound regression of these tumors of up to 85%. In these tumors, an increase in the expression of p21 protein level and the occurrence of apoptosis were observed. These findings demonstrate for the first time that magnolol can inhibit the proliferation of tumor cells in vitro and in vivo.

Role of reactive oxygen species in IL-1 beta-stimulated sustained ERK activation and MMP-9 induction

We have recently demonstrated that interleukin-1 beta (IL-1 beta) stimulates matrix metalloproteinase-9 (MMP-9) induction. In this study we have investigated the roles of superoxide and extracellular signal-regulated kinase (ERK) activation in MMP-9 induction following exposure to IL-1 beta. IL-1 beta stimulated biphasic ERK activation in vascular smooth muscle (VSM) cells, a transient activation that reached a maximum at 15 min and declined to baseline levels within 1 h, and a second phase of sustained ERK activation lasting up to 8 h. To determine the role of ERK in IL-1 beta-stimulated MMP-9 induction, we treated cells with the specific ERK pathway inhibitor PD-98059 at different time intervals after IL-1 beta stimulation. Addition of PD-98059 up to 4 h after IL-1 beta stimulation significantly inhibited MMP-9 induction, suggesting a role for sustained ERK activation in MMP-9 induction. IL-1 beta treatment stimulated superoxide production in VSM cells that was inhibited by pretreatment of cells with the superoxide scavenger N-acetyl-L-cysteine (NAC) and also by overexpression of the human manganese superoxide dismutase (MnSOD) gene. Treatment of VSM cells with NAC selectively inhibited the sustained phase of ERK activation without influencing the transient phase, suggesting a role for reactive oxygen species in sustained ERK activation. In addition, both NAC treatment and MnSOD overexpression significantly inhibited IL-1 beta-stimulated MMP-9 induction (P < 0.05). The results demonstrate that IL-1 beta-dependent MMP-9 induction is mediated by superoxide-stimulated ERK activation.

Molecular mechanisms of apoptosis induced by magnolol in colon and liver cancer cells

Magnolol has been reported to have anticancer activity. In this study we found that treatment with 100 microm magnolol induced apoptosis in cultured human hepatoma (Hep G2) and colon cancer (COLO 205) cell lines but not in human untransformed gingival fibroblasts and human umbilical vein endothelial cells. Our investigation of apoptosis in Hep G2 cells showed a sequence of associated intracellular events that included (a) increased cytosolic free Ca(2+); (b) increased translocation of cytochrome c (Cyto c) from mitochondria to cytosol; (c) activation of caspase 3, caspase 8, and caspase 9; and (d) downregulation of bcl-2 protein. Pretreatment of the cells with the phospholipase C inhibitor 1-[6-[[(17 beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1 H-pyrrole-2,5-dione (U73122) or the intracellular chelator of Ca(2+) 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA/AM) inhibited the subsequent magnolol augmentation of [Ca(2+)](i) and also the activation of caspase-8 and caspase-9, so that the occurrence of apoptosis in those cells was greatly reduced. Pretreatment of the cells with ZB4 (which disrupts the Fas response mechanism) also decreased the subsequent magnolol-induced caspase-8 activation and reduced the occurrence of apoptosis. We interpreted these findings to indicate that the above-listed sequence of intracellular events led to the apoptosis seen in Hep G2 cells and that [Ca(2+)](i), Cyto c, and Fas function as intracellular signals to coordinate those events.

Decreased mitogen activated protein kinase activities in congenital diaphragmatic hernia-associated pulmonary hypoplasia

BACKGROUND/PURPOSE

The mechanisms that cause pulmonary hypoplasia associated with congenital diaphragmatic hernia (CDH) currently are unknown. The authors proposed that the reduced size and immaturity of these lungs may be associated with differences in the levels of mitogen activated protein (MAP) kinase phosphorylation (extracellular signal regulated protein kinases, ERK-1 and -2).

METHODS

ERK-1 activities were measured using immune-complex kinase assays on fetal whole-lung lysates obtained from both nitrofen and olive oil-treated (control) pregnant rats. In addition, ERK-1 and ERK-2 functional activities were estimated by semiquantitative Western blot analysis, using an antibody specific for the diphosphorylated (dp-ERK, activated) forms of the enzymes.

RESULTS

ERK-1 activities, measured using immune-complex kinase assays, were reduced in CDH lungs compared with olive oil-treated controls (P <.02). In addition, dp-ERK-1 and dp-ERK-2 levels were found to be reduced in CDH lungs compared with controls (dp-ERK-1, P =.003; dp-ERK-2, P =.04), whereas ERK-1 and ERK-2 protein levels were unchanged.

CONCLUSIONS

The lower values of ERK-1 activity and reduced amounts of dp-ERK-1 and dp-ERK-2 in lung tissue from CDH animals, suggests that ERK-1 and ERK-2 activities are reduced in pulmonary hypoplasia associated with CDH. The observed reduction in ERK-1 and ERK-2 activities implicates attenuated cell signaling upstream of the ERK-1 and -2 enzymes.

Activation of MAPK by modified low-density lipoproteins in vascular smooth muscle cells

A high concentration of circulating low-density lipoproteins (LDL) is a major risk factor for atherosclerosis. Native LDL and LDL modified by glycation and/or oxidation are increased in diabetic individuals. LDL directly stimulate vascular smooth muscle cell (VSMC) proliferation; however, the mechanisms remain undefined. The extracellular signal-regulated kinase (ERK) pathway mediates changes in cell function and growth. Therefore, we examined the cellular effects of native and modified LDL on ERK phosphorylation in VSMC. Addition of native, mildly modified (oxidized, glycated, glycoxidized) and highly modified (highly oxidized, highly glycoxidized) LDL at 25 microg/ml to rat VSMC for 5 min induced a fivefold increase in ERK phosphorylation. To elucidate the signal transduction pathway by which LDL phosphorylate ERK, we examined the roles of the Ca(2+)/calmodulin pathway, protein kinase C (PKC), src kinase, and mitogen-activated protein kinase kinase (MEK). Treatment of VSMC with the intracellular Ca(2+) chelator EGTA-AM (50 micromol/l) significantly increased ERK phosphorylation induced by native and mildly modified LDL, whereas chelation of extracellular Ca(2+) by EGTA (3 mmol/l) significantly reduced LDL-induced ERK phosphorylation. The calmodulin inhibitor N-(6-aminohexyl)-1-naphthalenesulfonamide (40 micromol/l) significantly decreased ERK phosphorylation induced by all types of LDL. Downregulation of PKC with phorbol myristate acetate (5 micromol/l) markedly reduced LDL-induced ERK phosphorylation. Pretreatment of VSMC with a cell-permeable MEK inhibitor (PD-98059, 40 micromol/l) significantly decreased ERK phosphorylation in response to native and modified LDL. These findings indicate that native and mildly and highly modified LDL utilize similar signaling pathways to phosphorylate ERK and implicate a role for Ca(2+)/calmodulin, PKC, and MEK. These results suggest a potential link between modified LDL, vascular function, and the development of atherosclerosis in diabetes.

Phenotypic modulation of arterial smooth muscle cells is associated with prolonged activation of ERK1/2

Arterial smooth muscle cells grown in primary culture on a substrate of fibronectin in serum-free medium are converted from a contractile to a synthetic phenotype. This process is dependent on integrin signaling and includes a major structural reorganization with loss of myofilaments and formation of a large secretory apparatus. Functionally, the cells lose their contractility and become competent to migrate, secrete extracellular matrix components, and proliferate in response to growth factor stimulation. Here, it is demonstrated that the mitogen-activated protein kinases ERK1/2 play a vital role in the fibronectin-mediated modification of rat aortic smooth muscle cells. Immunoblotting showed that phosphorylated ERK1/2 (p44/p42) were expressed throughout the period when the change in phenotypic properties of the cells took place. Moreover, phosphorylated ERK1/2 accumulated in the nucleus as revealed by immunocytochemical staining. Additional support for an active role of ERK1/2 in the shift in smooth muscle phenotype was obtained by the finding that PD98059, an inhibitor of the upstream kinase MEK1, potently suppressed both the expression of phosphorylated ERK1/2 and the fine structural rebuilding of the cells. In conclusion, the observations point to an important and multifaceted role of ERK1/2 in the regulation of differentiated properties and growth of vascular smooth muscle cells.

Mechanisms by which bradykinin promotes fibrosis in vascular smooth muscle cells: role of TGF-beta and MAPK

Accumulation of extracellular matrix (ECM) is a hallmark feature of vascular disease. We have previously shown that hyperglycemia induces the expression of B(2)-kinin receptors in vascular smooth muscle cells (VSMC) and that bradykinin (BK) and hyperglycemia synergize to stimulate ECM production. The present study examined the cellular mechanisms through which BK contributes to VSMC fibrosis. VSMC treated with BK (10(-8) M) for 24 h significantly increased alpha(2)(I) collagen mRNA levels. In addition, BK produced a two- to threefold increase in alpha(2)(I) collagen promoter activity in VSMC transfected with a plasmid containing the alpha(2)(I) collagen promoter. Furthermore, treatment of VSMC with BK for 24 h produced a two- to threefold increase in the secretion rate of tissue inhibitor of metalloproteinase 1 (TIMP-1). The increase in alpha(2)(I) collagen mRNA levels and alpha(2)(I) collagen promoter activity, as well as TIMP-1 secretion, in response to BK were blocked by anti-transforming growth factor-beta (anti-TGF-beta) neutralizing antibodies. BK (10(-8) M) increased the endogenous production of TGF-beta1 mRNA and protein levels. Inhibition of the mitogen-activated protein kinase (MAPK) pathway by PD-98059 inhibited the increase of alpha(2)(I) collagen promoter activity, TIMP-1 production, and TGF-beta1 protein levels observed in response to BK. These findings provide the first evidence that BK induces collagen type I and TIMP-1 production via autocrine activation of TGF-beta1 and implicate MAPK pathway as a key player in VSMC fibrosis in response of BK.

Identification of four human cDNAs that are differentially expressed by early hematopoietic progenitors

The molecular processes that maintain the stem cell pool are largely unknown. Using polymerase chain reaction-driven subtraction, we examined genes that are differentially expressed by early hematopoietic progenitors. We expected that identifying genes that are uniquely expressed by the earliest precursors would provide insight into the mechanism(s) through which stem cell number is maintained and differentiation is regulated. Using CD34(+)CD38(-) cells as starting material, we identified four mRNAs, expressed by these cells, that are either absent or present in reduced amounts in more mature CD34(+)CD38(+) cells. One of these cDNAs (C40) encodes a known member of the subfamily of protein phosphatases (CL100) that exhibits dual substrate specificity for phosphotyrosine- and phosphoserine/threonine-containing substrates and specifically inactivates MAP kinases. This phosphatase has been shown to play a role in regulating the differentiation of several cell types. The second cDNA (C23) is identical to LR11 (gp250), a member of the low-density lipoprotein receptor family. LR11 is unusual in that, in addition to 11 ligand-binding repeats, it contains a series of fibronectin type III repeats near its carboxyl terminal end that are similar to those found in cytokine receptors. It is highly expressed in developing brain, but hematopoietic expression has not been reported. The 178-bp fragment that we originally cloned is part of a 4,145-bp 3' untranslated region (UTR) that had not been previously sequenced and is among the largest human 3' UTRs ever reported. The other isolates (C21 and C12) do not correspond to known protein sequences. They are homologous to EST sequences from a fetal brain library. C21 encodes a previously unknown gene that is a member of the WD-40 family. An open reading frame encoding a 515 amino acid protein has been identified. Four mRNAs, differentially expressed by CD34(+)CD38(-) human bone marrow cells, have been identified. Although this population is highly enriched for early hematopoietic progenitors, none of these genes encodes a message whose expression is limited to the hematopoietic system. They all are expressed in a variety of tissues, suggesting that they are involved in processes that are fundamental to the development of many cell types. All of these cDNAs possess atypically long 3' UTRs, and one of them is among the longest ever described. Their differential expression by immature hematopoietic cells, in contrast to more mature cells, suggest that long 3' UTRs may be characteristic of genes that play a regulatory role during development.

Fibrinogen interactions with ICAM-1 (CD54) regulate endothelial cell survival

We tested hypothesis that the interaction of fibrinogen (Fg) with intercellular adhesion molecule 1 (ICAM-1) mediates cellular adhesion and cell proliferation. Our results demonstrate that Fg : ICAM-1 ligation mediates endothelial cell survival and has an anti-apoptotic effect via activation of the MAP kinase pathway. Fg : ICAM-1 ligation in endothelial cells treated with tumor necrosis factor (TNF)alpha resulted in the hyperphosphorylation of extracellular signal-regulated kinase (ERK)-1/2 (eightfold to 10-fold) at 5-30 min. The specificity of ERK-1/2 phosphorylation was verified using the recognition peptides Fg-gamma-(117-133) and ICAM-1(8-22). ERK-1/2 hyperphosphorylation was dependent on intact cytoskeleton, as treatment with cytochalasin B and nocodazole blocked this activity. The attachment of TNFalpha-treated endothelial cells to fibrinogen or Fg-gamma-(117-133) resulted in cell survival, as assessed by an annexin V binding assay. ICAM-1(8-22) blocked the survival process. The MEK-1 inhibitor PD 98059 blocked ERK-1/2 phosphorylation, and treatment of endothelial cells with PD 98059 resulted in apoptosis even upon Fg : ICAM-1 ligation. Cells transfected with dominant-negative ERK-1/2 underwent apoptosis upon Fg : ICAM-1 ligation. Cell survival factor A1 was specifically upregulated upon adhesion of TNFalpha-stimulated endothelial cells to Fg. A1 expression was blocked by ICAM-1(8-22) and PD 98059. The Fg : ICAM-1 endothelial cell survival pathway appears to be mediated via the activation and upregulation of ERK-1/2 and A1.

Mechanical stress-initiated signal transductions in vascular smooth muscle cells

Mechanical force is an important modulator of cellular morphology and function in a variety of tissues, and is particularly important in cardiovascular systems. Vascular smooth muscle cell (VSMC) hypertrophy and proliferation contribute to the development of atherosclerosis, hypertension, and restenosis, where mechanical forces are largely disturbed. How VSMCs sense and transduce the extracellular mechanical signals into the cell nucleus resulting in quantitative and qualitative changes in gene expression is an interesting and important research field. Recently, it has been demonstrated that mechanical stress rapidly induced phosphorylation of platelet-derived growth factor (PDGF) receptor, activation of integrin receptor, stretch-activated cation channels, and G proteins, which might serve as mechanosensors. Once mechanical force is sensed, protein kinase C and mitogen-activated protein kinases (MAPKs) were activated, leading to increased c-fos and c-jun gene expression and enhanced transcription factor AP-1 DNA-binding activity. Interestingly, physical forces also rapidly resulted in expression of MAPK phosphatase-1 (MKP-1), which inactivates MAPKs. Thus, mechanical stresses can directly stretch the cell membrane and alter receptor or G protein conformation, thereby initiating signalling pathways, usually used by growth factors. These findings have significantly enhanced our knowledge of the pathogenesis of arteriosclerosis and provided promising information for therapeutic interventions for vascular diseases.

Cycloheximide-induced cPLA(2) activation is via the MKP-1 down-regulation and ERK activation

Extracellular signal-regulated kinase (ERK)-dependent phosphorylation is an important regulator for cytosolic phospholipase A(2) (cPLA(2)). In this study, we found that the protein synthesis inhibitor cycloheximide can potentiate thapsigargin-induced arachidonic acid (AA) release concomitant with ERK phosphorylation from murine RAW 264.7 macrophages. The cycloheximide effect is not due to the activation of p38 mitogen-activated protein kinase (MAPK) nor c-Jun NH(2)-terminal kinase (JNK), because the activator of both MAPKs anisomycin does not elicit AA release. Cycloheximide effect is additive to the tyrosine phosphatase inhibitor orthovanadate since these two stimuli induced sustained ERK activation respectively through inhibition of the translation and activity of MAPK phosphatase-1 (MKP-1).

The mechanisms of coronary restenosis: insights from experimental models

Since its introduction into clinical practice, more than 20 years ago, percutaneous transluminal coronary angioplasty (PTCA) has proven to be an effective, minimally invasive alternative to coronary artery bypass grafting (CABG). During this time there have been great improvements in the design of balloon catheters, operative procedures and adjuvant drug therapy, and this has resulted in low rates of primary failure and short-term complications. However, the potential benefits of angioplasty are diminished by the high rate of recurrent disease. Up to 40% of patients undergoing angioplasty develop clinically significant restenosis within a year of the procedure. Although the deployment of endovascular stents at the time of angioplasty improves the short-term outcome, 'in-stent' stenosis remains an enduring problem. In order to gain an insight into the mechanisms of restenosis, several experimental models of angioplasty have been developed. These have been used together with the tools provided by recent advances in molecular biology and catheter design to investigate restenosis in detail. It is now possible to deliver highly specific molecular antagonists, such as antisense gene sequences, to the site of injury. The knowledge provided by these studies may ultimately lead to novel forms of intervention. The present review is a synopsis of our current understanding of the pathological mechanisms of restenosis.

Role of reactive oxygen species in bradykinin-induced mitogen-activated protein kinase and c-fos induction in vascular cells

Bradykinin stimulates proliferation of aortic vascular smooth muscle cells (VSMCs). We investigated the action of bradykinin on the phosphorylation state of the mitogen-activated protein kinases p42(mapk) and p44(mapk) in VSMCs and tested the hypothesis that reactive oxygen species (ROS) might be involved in the signal transduction pathway linking bradykinin activation of nuclear transcription factors to the phosphorylation of p42(mapk) and p44(mapk). Bradykinin (10(-8) mol/L) rapidly increased (4- to 5-fold) the phosphorylation of p42(mapk) and p44(mapk) in VSMCs. Preincubation of VSMCs with either N-acetyl-L-cysteine and/or alpha-lipoic acid significantly decreased bradykinin-induced cytosolic and nuclear phosphorylation of p42(mapk) and p44(mapk). In addition, the induction c-fos mRNA levels by bradykinin was completely abolished by N-acetyl-L-cysteine and alpha-lipoic acid. Using the cell-permeable fluorescent dye dichlorofluorescein diacetate, we determined that bradykinin (10(-8) mol/L) rapidly increased the generation of ROS in VSMCs. The NADPH oxidase inhibitor diphenylene iodonium (DPI) blocked bradykinin-induced c-fos mRNA expression and p42(mapk) and p44(mapk) activation, implicating NADPH oxidase as the source for the generation of ROS. These findings demonstrate that the phosphorylation of cytosolic and nuclear p42(mapk) and p44(mapk) and the expression of c-fos mRNA in VSMCs in response to bradykinin are mediated via the generation of ROS and implicate ROS as important mediators in the signal transduction pathway through which bradykinin promotes VSMC proliferation in states of vascular injury.

Hyperexpression and activation of extracellular signal-regulated kinases (ERK1/2) in atherosclerotic lesions of cholesterol-fed rabbits

A hallmark of hyperlipidemia-induced atherosclerosis is altered gene expression that initiates cell proliferation and (de)differentiation in the intima of the arterial wall. The molecular signaling that mediates this process in vivo has yet to be identified. Extracellular signal-regulated kinases (ERKs) are thought to play a pivotal role in transmitting transmembrane signals required for cell proliferation in vitro. The present studies were designed to investigate the activity, abundance, and localization of ERK1/2 in atherosclerotic lesions of cholesterol-fed rabbits. Immunofluorescence analysis revealed abundant and heterogeneous distribution of ERK1/2, mainly localized in the cap and basal regions of atheromas. A population of ERK-enriched cells was identified as alpha-actin-positive smooth muscle cells (SMCs). ERK1 and 2 were heavily phosphorylated on tyrosyl residues and coexpressed with proliferating cell nuclear antigen in atherosclerotic lesions. ERK1/2 protein levels in protein extracts from atherosclerotic lesions were 2- to 3-fold higher than the vessels of chow-fed rabbits, and their activities were elevated 3- to 5-fold over those of the normal vessel. SMCs derived from atherosclerotic lesions had increased migratory/proliferative ability and higher ERK activity in response to LDL stimulation compared with cells from the normal vessel. Inhibition of ERK activation by PD98059, a specific inhibitor of mitogen-activated protein kinase kinases (MEK1/2), abrogated LDL-induced SMC proliferation in vitro. Taken together, our findings support the proposition that persistent activation and hyperexpression of ERK1/2 may be a critical element to initiate and perpetuate cell proliferation during the development of atherosclerosis.

Balloon catheter vascular injury of the alloxan-induced diabetic rabbit: the role of insulin-like growth factor-1

Neointimal thickening following catheter injury is characterized, in part, by growth factor-induced vascular smooth muscle cell (VSMC) proliferation. It was hypothesized that a reduction in serum insulin-like growth factor-1 (IGF-1), characteristic of chemically-induced diabetes, would result in decreased VSMC proliferation and attenuate neointimal thickening. It was found that alloxan-treated New Zealand White rabbits exhibit varying degrees of glycemia. Rabbits classified as diabetic (glucose > or = 400 mg/dL) had significantly decreased serum concentration of IGF-1 (87.4+/-14 nmol/L vs. 170+/-14 nmol/L) and significantly decreased intimal/medial (I/M) ratios 2, 4, and 8 weeks after aortic injury compared to euglycemic rabbits (13.7+/-2, 21.1+/-2, 32.4+/-3 in euglycemics and 6.6+/-1, 14+/-2, 19+/-5 in diabetics, respectively). The I/M for high hyperglycemic animals (glucose 286-399 mg/dL) was comparable to diabetic animals yet their serum IGF-1 levels were normal rather than depressed. Vascular IGF-1 content similarly increased upon injury in both diabetic and euglycemic animals. In diabetic animals, proliferating cell nuclear antigen (PCNA) immunostaining was present by day 1 peaked by day 5 and returned to control by day 14. In euglycemic animals, staining by day 1 continued to increase through day 14. A similar increase in mitogen-activated protein kinase (MAPK) activity occurred from day 1 through day 5 in both diabetic and euglycemic animals. This is the first demonstration of an association between MAPK activity and VSMC proliferation following vascular injury in diabetic animals as previously reported in euglycemic animals. In conclusion, this study provides evidence against a direct effect of IGF-1 in the reduction in neointimal thickening, VSMC proliferation, and MAPK activity upon catheter injury in chemically-induced diabetic rabbits.

Cyclic strain stress-induced mitogen-activated protein kinase (MAPK) phosphatase 1 expression in vascular smooth muscle cells is regulated by Ras/Rac-MAPK pathways

Recently, we demonstrated that mechanical stress results in rapid phosphorylation or activation of platelet-derived growth factor receptors in vascular smooth muscle cells (VSMCs) followed by activation of mitogen-activated protein kinases (MAPKs) and AP-1 transcription factors (Hu, Y., Bock, G., Wick, G., and Xu, Q. (1998) FASEB J. 12, 1135-1142). Herein, we provide evidence that VSMC responses to mechanical stress also include induction of MAPK phosphatase-1 (MKP-1), which may serve as a negative regulator of MAPK signaling pathways. When rat VSMCs cultivated on a flexible membrane were subjected to cyclic strain stress (60 cycles/min, 5-30% elongation), induction of MKP-1 proteins and mRNA was observed in time- and strength-dependent manners. Concomitantly, mechanical forces evoked rapid and transient activation of all three members of MAPKs, i.e. extracellular signal-regulated kinases (ERKs), c-Jun NH(2)-terminal protein kinases (JNKs), or stress-activated protein kinases (SAPKs), and p38 MAPKs. Suramin, a growth factor receptor antagonist, completely abolished ERK activation, significantly blocked MKP-1 expression, but not JNK/SAPK and p38 MAPK activation, in response to mechanical stress. Interestingly, VSMC lines stably expressing dominant negative Ras (Ras N17) or Rac (Rac N17) exhibited a marked decrease in MKP-1 expression; the inhibition of ERK kinases (MEK1/2) by PD 98059 or of p38 MAPKs by SB 202190 resulted in a down-regulation of MKP-1 induction. Furthermore, overexpressing MKP-1 in VSMCs led to the dephosphorylation and inactivation of ERKs, JNKs/SAPKs, and p38 MAPKs and inhibition of DNA synthesis. Taken together, our findings demonstrate that mechanical stress induces MKP-1 expression regulated by two signal pathways, including growth factor receptor-Ras-ERK and Rac-JNK/SAPK or p38 MAPK, and that MKP-1 inhibits VSMC proliferation via MAPK inactivation. These results suggest that MKP-1 plays a crucial role in mechanical stress-stimulated signaling leading to VSMC growth and differentiation.

Phenotype dictates the growth response of vascular smooth muscle cells to pulse pressure in vitro

The objective of this study was to determine the effect of phenotype on pulse pressure-induced signaling and growth of vascular smooth muscle cells in vitro. Using a perfused transcapillary culture system, cells were exposed to increases in pulsatile flow and hence pulse pressure and maintained for 72 h before cells were harvested. Cell proliferation was determined by cell number, DNA synthesis, and proliferating cell nuclear antigen expression. Mitogen-activated protein kinase (MAPK) levels were determined by immunoblot and kinase activity by phosphorylation of myelin basic protein. Cell phenotype was determined by immunoblot and immunocytofluorescence using antisera specific for the differentiation markers alpha-actin, myosin, calponin, osteopontin, and phospholamban. In cells that highly expressed these differentiation markers, there was a significant increase in cell growth in response to chronic increases in pulse pressure without a significant change in MAPK activity in these cells. In contrast, in cells that weakly expressed SMC differentiation markers, there was a significant decrease in cell growth concomitant with a significant decrease in MAPK signaling in these cells. We conclude that SMC phenotype dictates the growth response of SMC to mechanical force in vitro.

Effect of glucose on stress-activated protein kinase activity in mesangial cells and diabetic glomeruli

BACKGROUND

We have reported that hyperglycemia increases c-jun mRNA levels in isolated glomeruli of diabetic rats. The transcriptional activity of c-jun can be modified by phosphorylation of serine residues in the regulatory domain of the protein by stress-activated protein kinases (SAPKs), but the effect of high glucose concentrations on SAPK expression and activity is unknown. Accordingly, we studied p42/44 MAPK, p38 MAPK, and SAPK expression and activity in primary mesangial cells exposed to high glucose concentrations, as well as SAPK expression and activity in glomeruli of normal and streptozotocin-induced diabetic rats.

METHODS

Mesangial cells were incubated in 40 mM glucose for 30 and 60 minutes and 6, 12, 24, and 48 hours, whereas glomeruli of streptozotocin-induced diabetic rats were isolated one day and one and two weeks after the onset of hyperglycemia (blood glucose levels more than 15 mmol/liter), and were compared with age-matched normal rats. Cell lysates were subjected to Western blot analysis of SAPK and phosphorylated SAPK and an in vitro SAPK assay using recombinant c-jun.

RESULTS

Western blot analysis revealed that SAPK was expressed, but unphosphorylated, in unstimulated mesangial cells and whole glomerular lysates from normal rats. In accord with these observations, no SAPK activity was detected in lysates from mesangial cells or whole glomeruli from normal rats, although mesangial cell SAPK activity was readily induced in vitro by sorbitol. High glucose concentrations did not increase SAPK activity or lead to detectable phosphorylated SAPK either in vitro or in vivo. In contrast, short-term exposure to 40 mM of glucose activated both p42/44 MAPK and p38 MAPK.

CONCLUSIONS

We conclude that high glucose concentrations do not activate SAPK in primary cultured mesangial cells or in diabetic glomeruli during the early phase of diabetic renal hypertrophy.

Sequences within fibrinogen and intercellular adhesion molecule-1 (ICAM-1) modulate signals required for mitogenesis

The interaction of fibrinogen (Fg) with intercellular adhesion molecule-1 (ICAM) on B-lymphoid Raji cells results in mitogenesis (Gardiner, E. E., and D'Souza, S. E. (1997) J. Biol. Chem. 272, 15474-15480). Incubation of Raji with Fg resulted in the increased tyrosine phosphorylation of the receptor-associated tyrosine kinase, pp60(Src) and extracellular signal-regulated kinase-1 (ERK). The increase in ERK-1 phosphorylation was blocked by a peptide with sequence matching ICAM-1-(8-22) and corresponded to a decrease in ERK-1 enzymatic activity. 100 microM amounts of Fg peptide gamma-(117-133) caused an increase in tyrosine phosphorylation of ERK-1. These results are consistent with our previous report wherein ICAM-1-(8-22) blocked Fg-induced mitogenesis and Fg-gamma-(117-133) induced proliferation in Raji. The specific inhibitor of MEK, PD98059 (25 microM), abrogated the increased phosphorylation of ERK-1 and blocked Raji mitogenesis by >50%. Inhibitors of pp60(Src), geldanamycin (62 nM), and herbimycin A (2.5 microM) blocked >50% of Raji proliferation. These results indicate that the proliferation induced by Fg interactions with ICAM-1 is mediated in part by receptor-associated tyrosine kinases and ERK-1, and that the recognition sequences within Fg and ICAM-1 participate in the signaling process.

Polyamine depletion arrests cell cycle and induces inhibitors p21(Waf1/Cip1), p27(Kip1), and p53 in IEC-6 cells

The polyamines spermidine and spermine and their precursor putrescine are intimately involved in and are required for cell growth and proliferation. This study examines the mechanism by which polyamines modulate cell growth, cell cycle progression, and signal transduction cascades. IEC-6 cells were grown in the presence or absence of DL-alpha-difluoromethylornithine (DFMO), a specific inhibitor of ornithine decarboxylase, which is the first rate-limiting enzyme for polyamine synthesis. Depletion of polyamines inhibited growth and arrested cells in the G1 phase of the cell cycle. Cell cycle arrest was accompanied by an increase in the level of p53 protein and other cell cycle inhibitors, including p21(Waf1/Cip1) and p27(Kip1). Induction of cell cycle inhibitors and p53 did not induce apoptosis in IEC-6 cells, unlike many other cell lines. Although polyamine depletion decreased the expression of extracellular signal-regulated kinase (ERK)-2 protein, a sustained increase in ERK-2 isoform activity was observed. The ERK-1 protein level did not change, but ERK-1 activity was increased in polyamine-depleted cells. In addition, polyamine depletion induced the stress-activated protein kinase/c-Jun NH2-terminal kinase (JNK) type of mitogen-activated protein kinase (MAPK). Activation of JNK-1 was the earliest event; within 5 h after DFMO treatment, JNK activity was increased by 150%. The above results indicate that polyamine depletion causes cell cycle arrest and upregulates cell cycle inhibitors and suggest that MAPK and JNK may be involved in the regulation of the activity of these molecules.

Recent progress in angiotensin II type 2 receptor research in the cardiovascular system

Angiotensin II (Ang II) plays an important role in regulating cardiovascular hemodynamics and structure. Multiple lines of evidence have suggested the existence of Ang II receptor subtypes, and at least 2 distinct receptor subtypes have been defined on the basis of their differential pharmacological and biochemical properties and designated as type 1 (AT1) and type 2 (AT2) receptors. To date, most of the known effects of Ang II in adult tissues are attributable to the AT1 receptor. Recent cloning of the AT2 receptor contributes to reveal its physiological functions, but many functions of the AT2 receptor are still an enigma. AT1 and AT2 receptors belong to the 7-transmembrane, G protein-coupled receptor family. However, accumulating evidence demonstrates that the function and signaling mechanisms of these receptor subtypes are quite different, and these receptors may exert opposite effects in terms of cell growth and blood pressure regulation. We will review the role of the AT2 receptor in the cardiovascular system and the molecular and cellular mechanisms of AT2 receptor action.

Mitogen-activated protein kinase phosphatase: a negative regulator of the mitogen-activated protein kinase cascade

Mitogen-activated protein kinases (MAPKs) are activated by various stimuli, such as growth factors, cytokines, or stress, and are considered to be important mediators in intracellular signal transduction networks. The dual-specificity kinases, MAPK kinases (MKKs), which phosphorylate the TXY motif in the catalytic domain of MAPKs, can cause the activation of MAPKs. Recently, a family of dual-specificity phosphatases has been identified, members of which are able to dephosphorylate and inactivate MAPKs. The studies cited in this review have revealed that these MAPK phosphatases might play an important role in various cellular functions by downregulating the MAPK cascade.

Expression of the AT2 receptor developmentally programs extracellular signal-regulated kinase activity and influences fetal vascular growth

Angiotensin II type 2 (AT2) receptor is abundantly expressed in vascular smooth muscle cells (VSMC) of the fetal vasculature during late gestation (embryonic day 15-20), during which the blood vessels undergo remodeling. To examine directly the influence of AT2 receptor expression in the developmental biology of VSMC, we studied cultures of VSMC from fetal and postnatal wild-type (Agtr2(+)) and AT2 receptor null (Agtr2(-)) mice. Consistent with in vivo data, AT2 receptor binding in cultured Agtr2(+) VSMC increased by age, peaking at embryonic day 20, and decreased dramatically after birth. Angiotensin II-induced growth in Agtr2(+) VSMC (embryonic day 20) was increased by the AT2 receptor blocker PD123319, indicating that the AT2 receptors are functional and exert an antigrowth effect in Agtr2(+) VSMC. Growth of VSMC in response to serum decreased age dependently and was higher in Agtr2(-) than in Agtr2(+), inversely correlating with AT2 receptor expression. However, serum-induced growth in Agtr2(+) and Agtr2(-) VSMC and the exaggerated Agtr2(-) VSMC growth was maintained even in the presence of PD123319 or losartan, an AT1 receptor blocker. Moreover, Agtr2(-) VSMC showed greater growth responses to platelet-derived growth factor and basic fibroblast growth factor, indicating that Agtr2(-) cells exhibit a generalized exaggerated growth phenotype. We studied the mechanism responsible for this phenotype and observed that extracellular signal-regulated kinase (ERK) activity was higher in Agtr2(-) VSMC at baseline and also in response to serum. ERK kinase inhibitor PD98059 inhibited both growth and ERK phosphorylation dose-dependently, while the regression lines between growth and ERK phosphorylation were identical in Agtr2(+) and Agtr2(-) VSMC, suggesting that increased ERK activity in Agtr2(-) VSMC is pivotal in the growth enhancement. Furthermore, the difference in ERK phosphorylation between Agtr2(+) and Agtr2(-) was abolished by vanadate but not by okadaic acid, implicating tyrosine phosphatase in the difference in ERK activity. These results suggest that the AT2 receptor expression during the fetal vasculogenesis influences the growth phenotype of VSMC via the modulation of ERK cascade.

Role of tyrosine phosphorylation in excitation-contraction coupling in vascular smooth muscle

Increasingly it is recognized that tyrosine phosphorylation plays an important part in the regulation of function in differentiated contractile vascular smooth muscle. Tyrosine kinases and phosphatases are present in large amounts in vascular smooth muscle and have been reported to influence a number of processes crucial to contraction, including ion channel gating, calcium homeostasis and sensitization of the contractile process to [Ca2+]i. This review summarizes current understanding regarding the role of tyrosine phosphorylation in excitation-contraction coupling in blood vessels.

MAP kinases and vascular smooth muscle function

The mitogen-activated protein (MAP) kinase family members are ubiquitously expressed protein kinases activated in response to a variety of extracellular stimuli and shown to be involved in cell growth, transformation, differentiation and apoptosis. MAP kinases have been implicated in both growth and apoptosis of vascular smooth muscle cells (VSMC) which suggests that they play important roles in cardiovascular diseases such as essential hypertension, atherosclerosis, and restenosis followed angioplasty. The MAP kinases are themselves components of specific kinase cascades characterized by activation by specific stimuli, families of related serine and threonine kinases and downstream substrates that include other kinases, transcription factors, membrane receptors and other cell mediators. Cross-talk among the different MAP kinases results in direct modulation of signal transduction. In addition, increased expression and activation of MAP kinase phosphatases plays an important role in MAP kinase inactivation. Our laboratory has used angiotensin II (AngII), a potent activator of all MAP kinases in VSMC, to study mechanisms by which MAP kinases are regulated by vasoactive peptides. In this review, we describe the mechanisms by which AngII activates MAP kinases, and potential roles for MAP kinases in AngII-dependent effects on VSMC function.

Regulation of mitogen-activated protein kinase phosphatase-1 induction by insulin in vascular smooth muscle cells. Evaluation of the role of the nitric oxide signaling pathway and potential defects in hypertension

In this study, we examined the regulation of mitogen-activated protein kinase phosphatase (MKP-1) expression by insulin in primary vascular smooth muscle cell cultures. Insulin caused a rapid time- and dose-dependent induction of MKP-1 mRNA and protein expression. Blockade of nitric-oxide synthase (NOS) with NG-monomethyl-L-arginine acetate, and cGMP with RpcGMP, completely inhibited MKP-1 expression. Insulin-mediated MKP-1 expression was preceded by inducible NOS (iNOS) induction and cGMP production. Blockade of phosphatidylinositol 3-kinase (PI3-kinase) signaling with wortmannin inhibited insulin-mediated iNOS protein induction, cGMP production, and MKP-1 expression. To evaluate potential interactions between NOS and the mitogen-activated protein kinase (MAPK) signaling pathways, we employed PD98059 and SB203580, two specific inhibitors of ERKs and p38 MAPK. These inhibitors abolished the effect of insulin on MKP-1 expression. Only PD98059 inhibited insulin-mediated iNOS protein induction. Vascular smooth muscle cells from spontaneous hypertensive rats exhibited a marked decrease in MKP-1 induction due to defects in insulin-induced iNOS expression because of reductions in PI3-kinase activity. Treatment with sodium nitroprusside and 8-bromo-cGMP restored MKP-1 mRNA expression to levels comparable with controls. We conclude that insulin-induced MKP-1 expression is mediated by PI3-kinase-initiated signals, leading to the induction of iNOS and elevated cGMP levels that stimulates MKP-1 expression.

Vascular smooth muscle cell growth and insulin regulation of mitogen-activated protein kinase in hypertension

Hyperinsulinemia (HI) and insulin resistance (IR) are frequently associated with hypertension and atherosclerosis. However, the exact roles of HI and IR in the development of hypertension are unclear. Mitogen-activated protein kinases (MAPK) are well-characterized intracellular mediators of cell proliferation. In this study, we examined the contribution of MAPK pathway in insulin-stimulated mitogenesis using primary vascular smooth muscle cells (VSMCs) isolated from aortas of normotensive Wistar-Kyoto rats (WKY) and spontaneous hypertensive rats (SHR). VSMCs were grown to confluence in culture, serum starved, and examined for DNA synthesis (using [3H]thymidine (TDR), immunoprecipitated MAPK activity, and MAPK phosphatase (MKP-1) induction). Basal rate of TDR incorporation into DNA was twofold higher in SHR compared with WKY (P < 0.005). Insulin caused a dose-dependent increase in TDR incorporation (150% over basal levels with 100 nM in 12 h). Stimulation was sustained for 24 h with a decline toward basal in 36 h. Pretreatment with insulin-like growth factor I (IGF-I) receptor antibody did not abolish mitogenesis mediated by 10-100 nM insulin, suggesting that insulin effect is mediated via its own receptors. Insulin had a small mitogenic effect in WKY (33% over basal). Insulin-stimulated mitogenesis was accompanied by a dose-dependent increase in MAPK activity in SHR, with a peak activation (>2-fold over basal) between 5 and 10 min with 100 nM insulin. Insulin had very small effects on MAPK activity in WKY. In contrast, serum-stimulated MAPK activation was comparable in WKY and SHR. Pretreatment with MEK inhibitor, PD-98059, completely blocked insulin's effect on MAPK activation and mitogenesis. Inhibition of phosphatidylinositol 3-kinase with wortmannin also prevented insulin's effects on MAPK activation and mitogenesis. In WKY, insulin and IGF-I treatment resulted in a rapid induction of MKP-1, the dual-specificity MAPK phosphatase. In contrast, VSMCs from SHR were resistant to insulin with respect to MPK-1 expression. We conclude that insulin is mitogenic in SHR, and the effect appears to be mediated by sustained MAPK activation due to impaired insulin-mediated MKP-1 mRNA expression, which may act as an inhibitory feedback loop in attenuating MAPK signaling.

Cell replication in the arterial wall: activation of signaling pathway following in vivo injury

This study examined intracellular signal events of arterial cells following balloon catheter injury to rat carotid artery. Within 30 minutes, a marked increase in extracellular signal-regulated kinase-1/2 (ERK1/2) activity was observed. This activity remained elevated for 12 hours but had decreased to control levels by day 1. No increase in ERK1/2 was detected at any later times. Injection of anti-fibroblast growth factor 2 antibody (60 mg i.v.) significantly inhibited the activation of ERK1/2 at 30 minutes after the injury. PD98059 (80 micromol/L), a selective inhibitor of mitogen-activated protein kinase/ERK kinase-1 (MEK1), decreased ERK1/2 activity in injured arteries and also reduced the medial cell replication. In contrast, PD98059 did not block the intimal cell replication at day 8. Mitogen-activated protein kinase phosphatase-1 (MKP-1) was expressed within hours after injury but only weakly at later times; MKP-1 was again expressed after 7 and 14 days. The expression of MKP-1 was associated with an activation of c-Jun amino-terminal kinase. Injury to the arterial wall also stimulated the activity of p70 S6 kinase from 30 minutes to 12 hours, suggesting an alternative pathway in mitogenic signaling of early cell replication. These findings demonstrate that fibroblast growth factor 2-induced ERK1/2 activation promotes medial cell replication after balloon injury; however, signaling of intimal cell replication may not be linked to the MEK1-dependent ERK pathway.

Activation of MAP kinase in vivo follows balloon overstretch injury of porcine coronary and carotid arteries

Vascular restenosis involves contraction, proliferation, and remodeling of the arterial wall in response to overstretch injury. Mitogen-activated protein kinases (MAPKs) are implicated in both contraction and proliferation of vascular smooth muscle (VSM), and studies of porcine carotid arterial muscle strips have shown that mechanical stretch leads to the activation of the extracellular signal-regulated kinase (ERK) family of MAPKs in vivo. We, therefore, analyzed the acute effect of mechanical overstretch injury on ERK-MAPK (herein referred to simply as MAPK) activity in porcine coronary and carotid arteries in vivo. Balloon angioplasty catheters were inflated to 6 atm three times over 5 minutes at a balloon-artery ratio of 1.2:1 in either porcine coronary or carotid arteries. The arteries were snap-frozen after angioplasty, and MAPK activity was measured. Angioplasty of the left anterior descending (LAD, n = 5), left circumflex (LCx, n = 5), and carotid (n = 5) arteries effected an increase in MAPK activity compared with the activity in uninstrumented right coronary arteries (RCAs) or carotid arteries from the same animals used for controls. Balloon angioplasty of carotid arteries led to an increase in MAPK activity that was 7.7-fold over the activity in control arteries and comparable to the activity in stretched carotid arterial muscle strips in vivo. The increase in coronary artery kinase activity on angioplasty was variable from animal to animal. The increase in MAPK activity over that in control arteries ranged from 4.5- to 31.7-fold (mean +/- SEM, 10.7 +/- 5.3) in the LAD and 1.8- to 31.3-fold (mean +/- SEM, 9.7 +/- 5.7) in the LCx. There were no apparent inherent differences in the levels of MAPK activity in the three different types of coronary arteries (RCA, LAD, and LCx) without instrumentation. MAPK activation occurs rapidly during angioplasty, suggesting that this kinase may play an early role in initiating the injury response in both porcine coronary and carotid arteries. MAPKs may be key enzymes targeted to treat or prevent restenosis.

Activation of mitogen-activated protein kinases (ERK/JNK) and AP-1 transcription factor in rat carotid arteries after balloon injury

Smooth muscle cell proliferation is a key event in neointimal formation after balloon angioplasty. The molecular signals that mediate this process have yet to be identified. Mitogen-activated protein (MAP) kinases are thought to play a pivotal role in transmitting transmembrane signals required for cell proliferation in vitro. The present studies were designed to investigate whether the signal transduction pathways of MAP kinases were involved in the development of restenosis in the injured arteries. Rat carotid arteries were isolated at various time points after balloon injury, and activities of MAP kinases, including extracellular signal-regulated kinases (ERK), and stress activated protein kinases (SAPK)/c-Jun N-terminal protein kinases (JNK), were determined in protein extracts of the vasculature using protein kinase assay and Western blot analysis. After balloon angioplasty, ERK2 and JNK1 activities in the vessel wall increased rapidly, reached a high level in 5 minutes and maintained for 1 hour. A sustained increase in ERK2 kinase activity was observed over the next 7 days in the arterial wall and 14 days in neointima after injury. In contrast, opposite and uninjured arteries did not show significant changes in these kinase activities. Concomitantly, Western blot analysis confirmed that the ERK2 kinase in the injured vessels was indeed activated or phosphorylated, showing a slowly migrating species of a 42-kDa protein containing phosphorylated tyrosine. Kinase activation is followed by an increase in c-fos and c-jun gene expression and enhanced activator protein 1 (AP-1) DNA-binding activity. Thus, balloon injury rapidly activates the MAP kinases in rat carotid arteries. These kinase activations may be crucial in mediating smooth muscle cell proliferation in response to vascular angioplasty.

Inhibition of growth and p21ras methylation in vascular endothelial cells by homocysteine but not cysteine

Although hyperhomocysteinemia has been recognized recently as a prevalent risk factor for myocardial infarction and stroke, the mechanisms by which it accelerates arteriosclerosis have not been elucidated, mostly because the biological effects of homocysteine can only be demonstrated at very high concentrations and can be mimicked by cysteine, which indicates a lack of specificity. We found that 10-50 microM of homocysteine (a range that overlaps levels observed clinically) but not cysteine inhibited DNA synthesis in vascular endothelial cells (VEC) and arrested their growth at the G1 phase of the cell cycle. Homocysteine in this same range had no effect on the growth of vascular smooth muscle cells (VSMC) or fibroblasts. Homocysteine decreased carboxyl methylation of p21(ras) (a G1 regulator whose activity is regulated by prenylation and methylation in addition to GTP-GDP exchange) by 50% in VEC but not VSMC, a difference that may be explained by the ability of homocysteine to dramatically increase levels of S-adenosylhomocysteine, a potent inhibitor of methyltransferase, in VEC but not VSMC. Moreover, homocysteine-induced hypomethylation in VEC was associated with a 66% reduction in membrane-associated p21(ras) and a 67% reduction in extracellular signal-regulated kinase 1/2, which is a member of the mitogen-activated protein (MAP) kinase family. Because the MAP kinases have been implicated in cell growth, the p21(ras)-MAP kinase pathway may represent one of the mechanisms that mediates homocysteine's effect on VEC growth. VEC damage is a hallmark of arteriosclerosis. Homocysteine-induced inhibition of VEC growth may play an important role in this disease process.

The mitogen-activated protein kinase pathway can mediate growth inhibition and proliferation in smooth muscle cells. Dependence on the availability of downstream targets

Activation of the classical mitogen-activated protein kinase (MAPK) pathway leads to proliferation of many cell types. Accordingly, an inhibitor of MAPK kinase, PD 098059, inhibits PDGF-induced proliferation of human arterial smooth muscle cells (SMCs) that do not secrete growth-inhibitory PGs such as PGE2. In striking contrast, in SMCs that express the inducible form of cyclooxygenase (COX-2), activation of MAPK serves as a negative regulator of proliferation. In these cells, PDGF-induced MAPK activation leads to cytosolic phospholipase A2 activation, PGE2 release, and subsequent activation of the cAMP-dependent protein kinase (PKA), which acts as a strong inhibitor of SMC proliferation. Inhibition of either MAPK kinase signaling or of COX-2 in these cells releases them from the influence of the growth-inhibitory PGs and results in the subsequent cell cycle traverse and proliferation. Thus, the MAPK pathway mediates either proliferation or growth inhibition in human arterial SMCs depending on the availability of specific downstream enzyme targets.

Increased MAPK expression and activity in primary human hepatocellular carcinoma

We investigated the expression and activity of mitogen-activated protein kinase (MAPK) in human hepatocellular carcinoma (HCC). MAPK expression was determined in five human tumors and five normal tissues (adjacent non-neoplastic liver) by Western blotting using specific antisera raised against four MAPK pathway intermediates: Erk-1, Erk-2 (extracellular-signal regulated kinases), Mek-1 and Mek-2 (mitogen activated protein kinase kinases). There was a significant increase in Erk-1, Erk-2, Mek-1 and Mek-2 expression in particulate and cytosolic fractions prepared from tumor specimens as compared with the adjacent normal control tissues. The functional activity of both membrane and cytosolic Erk-2, determined by phosphorylation of myelin basic protein (MBP), was significantly increased in tumor specimens as compared to normal (membrane: 321%+/-50%, p<0.05; and cytosol: 597%+/-233%, p<0.05 percent of normal tissue). These data demonstrate for the first time a significant increase in MAPK expression and functional activity in human HCC. Because of the important role that the MAPK pathway plays in cellular growth and differentiation, overexpression of MAPK may be of critical importance to the formation and maintenance of human hepatocellular carcinoma.

Induction of mitogen-activated protein kinase phosphatase-1 during acute hypertension

Recently, we demonstrated that elevated blood pressure activates mitogen-activated protein (MAP) kinases in rat aorta. Here we provide evidence that the vascular response to acute hypertension also includes induction of MAP kinase phosphatase-1 (MKP-1), which has been shown to function in the dephosphorylation and inactivation of MAP kinases. Restraint or immobilization stress, which leads to a rapid rise in blood pressure, resulted in a rapid and transient induction of MKP-1 mRNA followed by elevated MKP-1 protein expression in rat aorta. That the induction of MKP-1 by restraint was due to the rise in blood pressure was supported by the finding that several different hypertensive agents (phenylephrine, vasopressin, and angiotensin II) were likewise capable of eliciting the response, and sodium nitroprusside, a nonspecific vasodilator agent that prevented the acute rise in blood pressure in response to the hypertensive agents, abrogated MKP-1 mRNA induction. The in vivo effects could not be mimicked by treatment of cultured aortic smooth muscle cells with similar doses of the hypertensive agents. These findings support a role for MKP-1 in the in vivo regulation of MAP kinase activity during hemodynamic stress.

The regulation of p42/p44 mitogen-activated protein kinases in the injured rat carotid artery

Arterial smooth muscle cell (SMC) proliferation is an important factor in the development of atherosclerotic plaques and restenotic lesions following arterial reconstruction. Basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF), and thrombin are known to induce SMC proliferation and migration in vitro and in vivo. In cultured cells the proliferative responses to these mitogens depend on the activation of the p42/p44 mitogen-activated protein kinases (MAPKs), whereas the role of these kinases in vivo has yet to be established. We tested whether MAPK activity is induced following vessel injury and whether activity is dependent on the release of bFGF, PDGF, and thrombin. Following balloon injury of the left carotid of male Sprague-Dawley rats, arteries were removed and analyzed with respect to MAPK activity, BrdU-labeled nuclei, and/or luminal, medial, and intimal areas. MAPK activity is induced in the rat carotid artery following balloon-catheter injury with a maximum activation at 30 min with a return to just above baseline at 11 hr after injury. Intravenous administration of heparin or neutralizing antibodies to bFGF or PDGF prior to injury reduced SMC proliferation and neointimal lesional formation but did not affect the early induction of MAPK activity. Administration of a tissue factor inhibitor or thrombin inhibitor also did not affect MAPK activity, although it impaired the initiation of the coagulation cascade.

IN CONCLUSION

(1) MAPK is activated in a time-dependent manner in response to injury; (2) the antiproliferative effect of heparin in vivo is not mediated through the inhibition of MAPK activity induced 30 min after injury; (3) the activation of MAPK after 30 min is not dependent on PDGF, bFGF, or thrombin following vessel injury in the rat.