MEK inhibition augments Raf activity, but has variable effects on mitogenesis, in vascular smooth muscle cells

Mechanisms shaping the role of ERK1/2 in cellular senescence (Review)

Senescence is a result of cellular stress and is a potential mechanism for regulating cancer. As a member of the mitogen-activated protein kinase family, ERK1/2 (extracellular signal-regulated protein kinase) has an important role in delivering extracellular signals to the nucleus, and these signals regulate the cell cycle, cell proliferation and cell development. Previous studies demonstrated that ERK1/2 is closely associated with cell aging; however other previous studies suggested that ERK1/2 exerts an opposite effect on aging models and target proteins, even within the same cell model. Recent studies demonstrated that the effect of ERK1/2 on aging is likely associated with its target proteins and regulators, negative feedback loops, phosphorylated ERK1/2 factors and ERK1/2 translocation from the cytoplasm to the nucleus. The present review aims to examine the mechanism of ERK1/2 and discuss its role in cellular outcomes and novel drug development.

Permissiveness to form pluripotent stem cells may be an evolutionarily derived characteristic in Mus musculus

Mus musculus is the only known species from which embryonic stem cells (ESC) can be isolated under conditions requiring only leukemia inhibitory factor (LIF). Other species are non-permissive in LIF media, and form developmentally primed epiblast stem cells (EpiSC) similar to cells derived from post-implantation, egg cylinders. To evaluate whether non-permissiveness extends to induced pluripotent stem cells (iPSC), we derived iPSC from the eight founder strains of the mouse Collaborative Cross. Two strains, NOD/ShiLtJ and the WSB/EiJ, were non-permissive, consistent with the previous classification of NOD/ShiLtJ as non-permissive to ESC derivation. We determined non-permissiveness is recessive, and that non-permissive genomes do not compliment. We overcame iPSC non-permissiveness by using GSK3B and MEK inhibitors with serum, a technique we termed 2iS reprogramming. Although used for ESC derivation, GSK3B and MEK inhibitors have not been used during iPSC reprogramming because they inhibit survival of progenitor differentiated cells. iPSC derived in 2iS are more transcriptionally similar to ESC than EpiSC, indicating that 2iS reprogramming acts to overcome genetic background constraints. Finally, of species tested for ESC or iPSC derivation, only some M. musculus strains are permissive under LIF culture conditions suggesting that this is an evolutionarily derived characteristic in the M. musculus lineage.

BRAF fusions identified in melanomas have variable treatment responses and phenotypes

Oncogenic BRAF fusions have emerged as an alternate mechanism for BRAF activation in melanomas and other cancers. A number of BRAF fusions with different 5' gene partners and BRAF exon breakpoints have been described, but the effects of different partners and breakpoints on cancer phenotypes and treatment responses has not been well characterized. Targeted RNA sequencing was used to screen 60 melanoma patient-derived xenograft (PDX) models for BRAF fusions. We identified three unique BRAF fusions, including a novel SEPT3-BRAF fusion, occurring in four tumors (4/60, 6.7%), all of which were "pan-negative" (lacking other common mutations) (4/18, 22.2%). The BRAF fusion PDX models showed variable growth rates and responses to MAPK inhibitors in vivo. Overexpression of BRAF fusions identified in our study, as well as other BRAF fusions previously identified in melanomas, resulted in a high degree of variability in 2D proliferation and 3D invasion between the different fusions. While exogenously expressed BRAF fusions all responded to MAPK inhibition in vitro, we observed potential differences in signaling and feedback mechanisms. In summary, BRAF fusions are actionable therapeutic targets, however there are significant differences in phenotypes, treatment responses, and signaling which may be clinically relevant.

Negative feedback regulation of the ERK1/2 MAPK pathway

The extracellular signal-regulated kinase 1/2 (ERK1/2) mitogen-activated protein kinase (MAPK) signalling pathway regulates many cellular functions, including proliferation, differentiation, and transformation. To reliably convert external stimuli into specific cellular responses and to adapt to environmental circumstances, the pathway must be integrated into the overall signalling activity of the cell. Multiple mechanisms have evolved to perform this role. In this review, we will focus on negative feedback mechanisms and examine how they shape ERK1/2 MAPK signalling. We will first discuss the extensive number of negative feedback loops targeting the different components of the ERK1/2 MAPK cascade, specifically the direct posttranslational modification of pathway components by downstream protein kinases and the induction of de novo gene synthesis of specific pathway inhibitors. We will then evaluate how negative feedback modulates the spatiotemporal signalling dynamics of the ERK1/2 pathway regarding signalling amplitude and duration as well as subcellular localisation. Aberrant ERK1/2 activation results in deregulated proliferation and malignant transformation in model systems and is commonly observed in human tumours. Inhibition of the ERK1/2 pathway thus represents an attractive target for the treatment of malignant tumours with increased ERK1/2 activity. We will, therefore, discuss the effect of ERK1/2 MAPK feedback regulation on cancer treatment and how it contributes to reduced clinical efficacy of therapeutic agents and the development of drug resistance.

Pathway analysis of kidney cancer using proteomics and metabolic profiling

Background

Renal cell carcinoma (RCC) is the sixth leading cause of cancer death and is responsible for 11,000 deaths per year in the US. Approximately one-third of patients present with disease which is already metastatic and for which there is currently no adequate treatment, and no biofluid screening tests exist for RCC. In this study, we have undertaken a comprehensive proteomic analysis and subsequently a pathway and network approach to identify biological processes involved in clear cell RCC (ccRCC). We have used these data to investigate urinary markers of RCC which could be applied to high-risk patients, or to those being followed for recurrence, for early diagnosis and treatment, thereby substantially reducing mortality of this disease.

Results

Using 2-dimensional electrophoresis and mass spectrometric analysis, we identified 31 proteins which were differentially expressed with a high degree of significance in ccRCC as compared to adjacent non-malignant tissue, and we confirmed some of these by immunoblotting, immunohistochemistry, and comparison to published transcriptomic data. When evaluated by several pathway and biological process analysis programs, these proteins are demonstrated to be involved with a high degree of confidence (p values < 2.0 E-05) in glycolysis, propanoate metabolism, pyruvate metabolism, urea cycle and arginine/proline metabolism, as well as in the non-metabolic p53 and FAS pathways. In a pilot study using random urine samples from both ccRCC and control patients, we performed metabolic profiling and found that only sorbitol, a component of an alternative glycolysis pathway, is significantly elevated at 5.4-fold in RCC patients as compared to controls.

Conclusion

Extensive pathway and network analysis allowed for the discovery of highly significant pathways from a set of clear cell RCC samples. Knowledge of activation of these processes will lead to novel assays identifying their proteomic and/or metabolomic signatures in biofluids of patient at high risk for this disease; we provide pilot data for such a urinary bioassay. Furthermore, we demonstrate how the knowledge of networks, processes, and pathways altered in kidney cancer may be used to influence the choice of optimal therapy.

The Rho GTPase effector ROCK regulates cyclin A, cyclin D1, and p27Kip1 levels by distinct mechanisms

The members of the Rho GTPase family are well known for their regulation of actin cytoskeletal structures. In addition, they influence progression through the cell cycle. The RhoA and RhoC proteins regulate numerous effector proteins, with a central and vital signaling role mediated by the ROCK I and ROCK II serine/threonine kinases. The requirement for ROCK function in the proliferation of numerous cell types has been revealed by studies utilizing ROCK-selective inhibitors such as Y-27632. However, the mechanisms by which ROCK signaling promotes cell cycle progression have not been thoroughly characterized. Using a conditionally activated ROCK-estrogen receptor fusion protein, we found that ROCK activation is sufficient to stimulate G1/S cell cycle progression in NIH 3T3 mouse fibroblasts. Further analysis revealed that ROCK acts via independent pathways to alter the levels of cell cycle regulatory proteins: cyclin D1 and p21(Cip1) elevation via Ras and the mitogen-activated protein kinase pathway, increased cyclin A via LIM kinase 2, and reduction of p27(Kip1) protein levels. Therefore, the influence of ROCK on cell cycle regulatory proteins occurs by multiple independent mechanisms.

Attenuation of Vascular Smooth Muscle Cell Proliferation by 1-Cyclohexyl-3-dodecyl Urea Is Independent of Soluble Epoxide Hydrolase Inhibition

Epoxyeicosatrienoic acid(s) (EET) have variable hemodynamic, anti-inflammatory, and growth regulatory effects, and inhibitors of their regulatory enzyme, soluble epoxide hydrolase (sEH), can mimic these effects. For this reason, sEH inhibitors are being studied as potential pharmaceuticals for the treatment of hypertension, atherosclerosis, and inflammatory diseases. We now show that a highly selective urea-based sEH inhibitor 1-cyclohexyl-3-dodecyl urea (CDU) attenuates human aortic vascular smooth muscle (HVSM) cell proliferation independently of any effect on sEH. CDU also inhibits endothelial cells when stimulated with basic fibroblast growth factor or serum. In addition, we demonstrate that EET, as well as several newer generation sEH inhibitors and a urea-based weak sEH inhibitor, do not affect proliferation in HVSM cells. Structure-activity relationships demonstrate that the addition of an acid group to the dodecyl carbon chain, changing the cyclohexyl group to an adamantyl group, and shortening the carbon chain to two carbons all abolish the antiproliferative effect. Our finding that a highly selective urea-based inhibitor of sEH can alter biology independently of its putative target enzyme suggests that there may be other useful properties of this class of compounds unrelated to their influence on epoxyeicosanoids. In addition, our results show that caution should be used when attempting to infer conclusions of EET biology based solely on the effects these inhibitors in tissue culture models, especially when used at micromolar concentrations.

Selective activation of the MEK-ERK pathway is regulated by mechanical stimuli in forming joints and promotes pericellular matrix formation

It is well established that local modification of extracellular matrix (ECM) hyaluronan composition is vital in the regulation of cell behavior. Indeed, the formation of articulating chick joint cavities, which requires mechanical stimuli derived from skeletal movement, is dependent upon the accumulation of an ECM rich in hyaluronan (HA). However, the mechanisms responsible for such precise mechano-dependent regulation of cell behavior and the formation of a HA-rich ECM remain undefined. Here we show that extracellular-regulated kinase 1/2 (ERK1/2) is selectively activated in cells at sites of cavity formation and activity diminished by in ovo immobilization that induces cartilaginous fusion across presumptive joint interzones. In vitro analyses offer mechanistic support for the role of mechanical stimuli in promoting a MEK-dependent activation of ERK1/2. In addition, our direct regulation of ERK1/2 phosphorylation status via modulation of its up-stream "classical cascade" activator either pharmacologically or by transfection with dominant negative or constitutively active Mek confirms the essential role for ERK1/2 activation in the elaboration of HA-rich pericellular matrices. Together, our findings demonstrate that the MEK-ERK pathway, regulated by mechanical stimuli, controls HA-rich matrix assembly. The precision of ERK1/2 activation selectively distinguishing cells at the joint line suggests that it directly contributes to the loss of tissue cohesion essential for generating HA-rich cavities between joint elements during their development.

Cytosolic p21Waf1/Cip1 increases cell cycle transit in vascular smooth muscle cells

The intracellular localization of signaling proteins is critical in directing their interactions with both upstream and downstream signaling cascade components. While initially described as a cyclin kinase inhibitor, p21Waf1/Cip1 has since been shown to have bimodal effects on cell cycle progression and cell proliferation, and evidence is emerging that intracellular localization of this protein plays a role in directing its signaling properties by dictating its interactions with downstream molecules. Since we have previously demonstrated a pro-apoptotic and cell cycle inhibitory effect of p21 attenuation after transfection of antisense p21 oligodeoxynucleotides (ODN) in several cell lines, we asked whether cytosolic p21 mediates a positive effect on vascular smooth muscle (VSM) cell cycle transit. We now show that transfection of a nuclear-localization signal deficient (DeltaNLS) p21 construct into VSM cells results in increased cytosolic levels of p21 and causes increased cell cycle transit as measured by [3H]thymidine incorporation. Thus, at least in VSM cells, cytosolic localization of p21 is a means by which this signaling protein transmits pro-mitogenic signals to the proteins responsible for G1/S transition. Furthermore, compartmentalization of p21 may help explain the biphasic nature of p21 in a variety of cell types and may lead to therapeutic advances directed at modulating pathologic cell growth in vascular diseases and cancer.

Suppression of breast cancer growth and angiogenesis by an antisense oligodeoxynucleotide to p21(Waf1/Cip1)

Under some conditions, p21(Waf1/Cip1) plays an assembly factor role for the cyclins and cyclin-dependent kinases, and recent reports demonstrate that p21 can act as an anti-apoptotic protein. Thus, it is logical to exploit this function of p21 as an anti-cancer target. We have performed a pilot study showing that daily subcutaneous injection of a phosphorothioate antisense p21 oligodeoxynucleotide, which we have previously shown to attenuate p21 levels in vitro, into nude mice who have been implanted with highly metastatic breast cancer cells results in inhibition of tumor growth and angiogenesis. Inhibition of in vitro endothelial capillary formation confirms that these oligodeoxynucleotides have a direct effect upon tumor angiogenesis. The attractiveness of our novel approach to breast cancer therapy, which capitalizes on the anti-apoptotic function of p21, derives from the ease of transfection of antisense oligodeoxynucleotides as well as the observations that p21(-/-) mice do not develop spontaneous tumors, making techniques exploiting the assembly factor and anti-apoptotic role of p21 worthy of further study against breast cancer.

Overexpression of p73 causes apoptosis in vascular smooth muscle cells

Abnormal vascular smooth muscle (VSM) cell proliferation contributes to the development of atherosclerosis and its associated disorders, including angioplasty restenosis. The tumor-suppressor protein p53 has been linked to the development of atherosclerotic lesions, and its homolog, p73, is proving to have contrasting functions in a variety of tissues. As an outgrowth of our previous finding that p73 is increased in serum-stimulated VSM cells and human atherosclerotic tissue, we examined p73 overexpression in VSM cells to elucidate causality of p73 expression with growth response. Overexpression of p73 results in decreased cell cycle transit and is accompanied by apoptosis. The apoptotic changes in p73 overexpressing VSM cells are independent of p53 and are associated with a decrease in levels of p21(waf1/cip1). In conjunction with our previous data finding that p73 is increased in serum-stimulated VSM cells, this work suggests a role for p73 in vascular proliferative diseases.

Attenuation of matrix protein secretion by antisense oligodeoxynucleotides to the cyclin kinase inhibitor p21(Waf1/Cip1)

Progressive fibrosis in major organs, including the heart, the kidney and the vascular tree, plays an important role in mediating chronic disease and atherosclerosis. Production of extracellular matrix proteins, in many cases regulated by the growth factor TGF-beta is an essential component of this process. In a parallel manner to TGF-beta, the cyclin kinase inhibitors (CKIs; which are induced by TGF-beta) regulate transit through the cell cycle, and their effect on growth has been shown to be bimodal in the case of vascular smooth muscle (VSM) cells. Using an antisense oligodeoxynucleotide to the CKI p21(Waf1/Cip1), developed in our laboratory and shown to specifically inhibit p21(Waf1/Cip1) protein levels, we asked whether attenuation of the CKI p21(Waf1/Cip1) by transfection of this oligodeoxynucleotide results in the abolition of TGF-beta-mediated growth inhibition and/or diminished matrix protein production and secretion in the presence or absence of TGF-beta. Specific inhibition of p21(Waf1/Cip1) protein with the antisense oligodeoxynucleotide markedly reduces the production and secretion of the matrix proteins fibronectin and laminin, both in the presence and absence of TGF-beta stimulation, in VSM cells as observed by Western blotting of cell lysate and conditioned medium. In addition, TGF-beta-mediated cell growth inhibition, though attenuated by this oligo, is preserved. Due to the relative ease and safety of transfecting antisense oligodeoxynucleotides into VSM, we believe that this work unmasks a potentially powerful technique for inhibition of matrix protein synthesis in VSM and related cell lines, and may lead to new treatment strategies for atherosclerotic as well as other systemic diseases characterized by aberrant matrix protein secretion.

Inhibitors of soluble epoxide hydrolase attenuate vascular smooth muscle cell proliferation

Atherosclerosis, in its myriad incarnations the foremost killer disease in the industrialized world, is characterized by aberrant proliferation of vascular smooth muscle (VSM) cells in part as a result of the recruitment of inflammatory cells to the blood vessel wall. The epoxyeicosatrienoic acids are synthesized from arachidonic acid in a reaction catalyzed by the cytochrome P450 system and are vasoactive substances. Metabolism of these compounds by epoxide hydrolases results in the formation of compounds that affect the vasculature in a pleiotropic manner. As an outgrowth of our observations that urea inhibitors of the soluble epoxide hydrolase (sEH) reduce blood pressure in spontaneously hypertensive rats as well as the findings of other investigators that these compounds possess antiinflammatory actions, we have examined the effect of sEH inhibitors on VSM cell proliferation. We now show that the sEH inhibitor 1-cyclohexyl-3-dodecyl urea (CDU) inhibits human VSM cell proliferation in a dose-dependent manner and is associated with a decrease in the level of cyclin D1. In addition, cis-epoxyeicosatrienoic acid mimics the growth-suppressive activity of CDU; there is no evidence of cellular toxicity or apoptosis in CDU-treated cells when incubated with 20 microM CDU for up to 48 h. These results, in light of the antiinflammatory and antihypertensive properties of these compounds that have been demonstrated already, suggest that the urea class of sEH inhibitors may be useful for therapy for diseases such as hypertension and atherosclerosis characterized by exuberant VSM cell proliferation and vascular inflammation.

Regulation of Raf-1 activation and signalling by dephosphorylation

The Raf-1 kinase is regulated by phosphorylation, and Ser259 has been identified as an inhibitory phosphorylation site. Here we show that the dephosphorylation of Ser259 is an essential part of the Raf-1 activation process, and further reveal the molecular role of Ser259. The fraction of Raf-1 that is phosphorylated on Ser259 is refractory to mitogenic stimulation. Mutating Ser259 elevates kinase activity because of enhanced binding to Ras and constitutive membrane recruitment. This facilitates the phosphorylation of an activating site, Ser338. The mutation of Ser259 also increases the functional coupling to MEK, augmenting the efficiency of MEK activation. Our results suggest that Ser259 regulates the coupling of Raf-1 to upstream activators as well as to its downstream substrate MEK, thus determining the pool of Raf-1 that is competent for signalling. They also suggest a new model for Raf-1 activation where the release of repression through Ser259 dephosphorylation is the pivotal step.

p73 is a growth-regulated protein in vascular smooth muscle cells and is present at high levels in human atherosclerotic plaque

p73 is a newly described homologue of the tumour suppressor p53 that was cloned serendipitously and subsequently shown to possess considerable homology in the most evolutionarily conserved p53 domains. Yet despite the fact that p53 and p73 have extensive structural similarities, their functions are proving to be quite different. We now show that p73 is a growth-regulated protein in the vasculature, being markedly increased in cultured vascular smooth muscle (VSM) cells stimulated with 10% serum, with no significant change in p73 mRNA levels. Stability of p73 is increased after serum stimulation and, probably contributing to this increase in p73 stability, the c-Abl oncogene protein displays a higher molecular weight species and is probably phosphorylated and activated in serum-stimulated VSM cells. The serum-mediated induction of p73 is not altered when the cells are incubated with inhibitors of the MAP/ERK pathway or tyrosine kinases, and is not stimulated by PDGF-BB, demonstrating that the mechanism of the increase in p73 does not involve this classical receptor tyrosine kinase growth factor signalling cascade. p73 is markedly increased in plaque tissue taken from atherosclerotic human carotid arteries, but not in comparable intimal scrapings from normal human arteries. Our data indicate that the tumour suppressor homologue p73 probably plays a role in VSM cell cycle progression, being mediated by a specific, as yet unidentified, serum component, and identifies a new function for this protein as being important in the pathogenesis of human atherosclerosis as well as other vascular diseases.

Molecular mechanisms of ATP and insulin synergistic stimulation of coronary artery smooth muscle growth

Coronary artery disease (CAD) is the major cause of death in diabetics. Abnormal proliferation of coronary artery smooth muscle cells (CASMC) leads to intimal thickening in CAD. We examined signaling mechanisms involved in the mitogenic effect of ATP and insulin on CASMC. ATP and insulin individually stimulated DNA synthesis by 4- and 2-fold, respectively; however, they acted synergistically to stimulate an increase of 17-fold over basal. A similar synergistic stimulation of extracellular signal-regulated kinase (ERK) and mitogen-activated protein or ERK kinase activities was observed (ATP, 7-fold; insulin, 2-fold; and ATP + insulin, 16-fold over basal). However, the combination of ATP and insulin stimulated only an additive activation of Raf (ATP, 5-fold; insulin, <2-fold; and ATP + insulin, 8-fold over basal) and Ras (ATP, 5-fold; insulin, 2-fold; and ATP + insulin, 8-fold over basal). Thus convergence of ATP and insulin signals appears to be at the level of Ras and Raf. In addition, insulin stimulated activation of Akt (also known as protein kinase B) (10-fold over basal), whereas ATP had little effect. However, when ATP and insulin were added in combination, ATP dramatically reduced the insulin-stimulated Akt activation (2-fold above basal). Thus these results are consistent with ATP relieving an insulin-induced Akt-dependent inhibitory effect on the ERK signaling pathway, leading to synergistic stimulation of CASMC proliferation.

Inhibitory effect of pentalenolactone on vascular smooth muscle cell proliferation

The effect of pentalenolactone, an inhibitor of glyceraldehyde-3-phosphate dehydrogenase, on rat vascular smooth muscle cell proliferation was studied. Addition of pentalenolactone together with serum to quiescent cells dose-dependently inhibited cell proliferation and DNA synthesis. This inhibition was not associated with cell death. When quiescent cells were stimulated with serum and then treated with pentalenolactone, the inhibitory effect on the DNA synthesis declined gradually. A similar result was obtained when PD 98059 (2'-amino-3'-methoxyflavone), an inhibitor of extracellular signal-regulated kinase1/2 (ERK1/2) kinase (MEK1/2), was added to the cells after serum stimulation. Pentalenolactone inhibited serum or protein kinase C activator (phorbol 12,13-dibutyrate)-induced phosphorylation of ERK1/2 and MEK1/2. In contrast, pentalenolactone had little effect on platelet-derived growth factor receptor autophosphorylation. Taken together, these results indicate that pentalenolactone inhibits vascular smooth muscle cell proliferation, and that this inhibition appears to be mediated by inhibition of the ERK1/2 cascade.

The permissive effect of p21(Waf1/Cip1) on DNA synthesis is dependent on cell type: effect is absent in p53-inactive cells

The cyclin-dependent kinase inhibitors (CKI) interact with cyclin-cdk complexes to arrest mitogen-stimulated transit through the cell cycle, but we and others have recently shown that these molecules can exert permissive effects on cell cycle transit as well. The p53 protein induces transcription of the p21(Waf1/Cip1) gene, but whether p53 has any effect on the stimulatory versus inhibitory state of p21(Waf1/Cip1) toward cell growth is not known. The focus of the current study was to examine the effect of p21(Waf1/Cip1) inhibition on growth in cells which possess an inactive p53 protein. We found that there was significant and specific inhibition of p21(Waf1/Cip1) protein transcription in human squamous carcinoma A431 cells after transfection of an antisense p21(Waf1/Cip1) oligodeoxynucleotide, yet there was no significant growth inhibition in these cells after stimulation with 10% serum or with PDGF-BB, in contrast to what was observed in vascular smooth muscle (VSM) cells. Furthermore, there was no attenuation of either cyclinD/cdk4 association or of Rb hyperphosphorylation after antisense p21(Waf1/Cip1) oligodeoxynucleotide transfection, suggesting that an alternate pathway exists to allow association and phosphorylation of these cell cycle components in the absence (or with lower levels) of p21(Waf1/Cip1). Thus, the permissive effect of p21(Waf1/Cip1) toward growth is dependent on cell type, and active p53 is likely required for this effect.

p21(Waf1/Cip1) is an assembly factor required for platelet-derived growth factor-induced vascular smooth muscle cell proliferation

The cyclin-dependent kinase inhibitors interact with cyclin-cdk complexes to arrest mitogen-stimulated transit through the cell cycle, but these proteins have recently been shown to have positive regulatory effects on cyclin-cdk complex activity as well. Most of the previous work in this area has focussed on the finding that overexpressed p21(Waf1/Cip1) causes growth arrest. However, mice lacking p21(Waf1/Cip1) showed normal development with no aberrancy in their cell cycles, and antisense p21(Waf1/Cip1) has only been shown to prevent cell cycle arrest, leading to the conclusion that the cyclin kinase inhibitors may not be required for cell cycle progression. We found that transfection of several lines of vascular smooth muscle cells with antisense oligodeoxynucleotide specific to p21(Waf1/Cip1) correlates with decreased cyclin D1/cdk 4, but not cyclin E/cdk 2, association, yet, unexpectedly, results in dose-dependent inhibition of platelet-derived growth factor-BB-stimulated DNA synthesis and cell proliferation. Our finding that p21(Waf1/Cip1) exhibits permissive effects on growth factor-induced vascular smooth muscle cell cycle progression, such that its presence is required for growth factor-induced proliferation, is the first such report and opens up a fertile area of research relevant to diseases involving vascular cell proliferation.

Short-term pravastatin mediates growth inhibition and apoptosis, independently of Ras, via the signaling proteins p27Kip1 and P13 kinase

Growth factor-stimulated DNA synthesis in a variety of cell lines has been shown to be decreased after overnight (or longer) treatment with the 3-hydroxy-3-methylglutaryl CoA reductase inhibitors, the statins. Although this anti-mitogenic effect had been presumed to be the result of the impairment of Ras lipidation, a stable modification (T1/2 approximately 20 h), this study provides new data demonstrating that brief (approximately 1 h) pretreatment of rat vascular smooth muscle cells with 100 microM pravastatin before platelet-derived growth factor-BB (PDGF-BB) stimulation results in attenuation of DNA synthesis through a Ras-independent mechanism. PDGF-BB-stimulated PDGF-beta receptor tyrosine phosphorylation, Ras activity, and mitogen-activated protein/extracellular signal-regulated kinase activity are unaffected by from 10 min to 1 h of pravastatin incubation, while Raf activity is markedly increased after 1 h of pravastatin. Phosphatidylinositol-3 kinase activity and phosphorylation of its downstream effector Akt are decreased after 1 h pravastatin incubation. Rho is stabilized by pravastatin, and ADP-ribosylation of Rho by C3 exoenzyme decreases PDGF-stimulated phosphatidylinositol-3 kinase activity, mimicking the effect of pravastatin on this signaling protein. Levels of the cyclin-dependent kinase inhibitor p27Kip1 are increased when cells were preincubated with pravastatin for 1 h and then exposed to PDGF, and apoptosis is induced by pravastatin incubation times as short as 1 to 4 h. Thus, short-term, high-dose pravastatin inhibits vascular smooth muscle cell growth and induces apoptosis independently of Ras, likely by means of the drug's effect on p27Kip1, mediated by Rho and/or phosphatidylinositol-3 kinase. This work demonstrates for the first time that the statins may be therapeutically useful when applied for short periods of time such that potential toxicity of long-term statin use (such as chronic Ras inhibition) may be avoided, suggesting future therapeutic directions for statin research.

Protein kinase Cdelta mediates neurogenic but not mitogenic activation of mitogen-activated protein kinase in neuronal cells

In several neuronal cell systems, fibroblast-derived growth factor (FGF) and nerve growth factor (NGF) act as neurogenic agents, whereas epidermal growth factor (EGF) acts as a mitogen. The mechanisms responsible for these different cellular fates are unclear. We report here that although FGF, NGF, and EGF all activate mitogen-activated protein (MAP) kinase (extracellular signal-related kinase [ERK]) in rat hippocampal (H19-7) and pheochromocytoma (PC12) cells, the activation of ERK by the neurogenic agents FGF and NGF is dependent upon protein kinase Cdelta (PKCdelta), whereas ERK activation in response to the mitogenic EGF is independent of PKCdelta. Antisense PKCdelta oligonucleotides or the PKCdelta-specific inhibitor rottlerin inhibited FGF- and NGF-induced, but not EGF-induced, ERK activation. In contrast, EGF-induced ERK activation was inhibited by the phosphatidylinositol-3-kinase inhibitor wortmannin, which had no effect upon FGF-induced ERK activation. Rottlerin also inhibited the activation of MAP kinase kinase (MEK) in response to activated Raf, but had no effect upon c-Raf activity or ERK activation by activated MEK. These results indicate that PKCdelta functions either downstream from or in parallel with c-Raf, but upstream of MEK. Inhibition of PKCdelta also blocked neurite outgrowth induced by FGF and NGF in PC12 cells and by activated Raf in H19-7 cells, indicating a role for PKCdelta in the neurogenic effects of FGF, NGF, and Raf. Interestingly, the PKCdelta requirement is apparently cell type specific, since FGF-induced ERK activation was independent of PKCdelta in NIH 3T3 murine fibroblasts, in which FGF is a mitogen. These data demonstrate that PKCdelta contributes to growth factor specificity and response in neuronal cells and may also promote cell-type-specific differences in growth factor signaling.