Caveolin-mediated regulation of signaling along the p42/44 MAP kinase cascade in vivo. A role for the caveolin-scaffolding domain

The p42/44 mitogen-activated protein (MAP)-kinase cascade is a well-established signal transduction pathway that is initiated at the cell surface and terminates within the nucleus. More specifically, receptor tyrosine kinases can indirectly activate Raf, which in turn leads to activation of MEK and ERK and ultimately phosphorylation of Elk, a nuclear transcription factor. Recent reports have suggested that some members of p42/44 MAP kinase cascade can be sequestered within plasmalemmal caveolae in vivo. For example, morphological studies have directly shown that ERK-1/2 is concentrated in plasma membrane caveolae in vivo using immunoelectron microscopy. In addition, constitutive activation of the p42/44 MAP kinase cascade is sufficient to reversibly down-regulate caveolin-1 mRNA and protein expression. However, the functional relationship between the p42/44 MAP kinase cascade and caveolins remains unknown. Here, we examine the in vivo role of caveolins in regulating signaling along the MAP kinase cascade. We find that co-expression with caveolin 1 dramatically inhibits signaling from EGF-R, Raf, MEK-1 and ERK-2 to the nucleus. Using a variety of caveolin-1 deletion mutants, we mapped this in vivo inhibitory activity to caveolin-1 residues 32-95. Peptides derived from this region of caveolin 1 also inhibit the in vitro kinase activity of purified MEK-1 and ERK-2. Thus, we show here that caveolin-1 expression can inhibit signal transduction from the p42/44 MAP kinase cascade both in vitro and in vivo. Taken together with previous data, our results also suggest that a novel form of reciprocal negative regulation exists between p42/44 MAP kinase activation and caveolin-1 protein expression, i.e. up-regulation of caveolin-1 protein expression down-modulates p42/44 MAP kinase activity (this report) and up-regulation of p42/44 MAP kinase activity down-regulates caveolin-1 mRNA and protein expression.

Biological aspects of reactive nitrogen species

Nitric oxide (NO) plays an important role as a cell-signalling molecule, anti-infective agent and, as most recently recognised, an antioxidant. The metabolic fate of NO gives rise to a further series of compounds, collectively known as the reactive nitrogen species (RNS), which possess their own unique characteristics. In this review we discuss this emerging aspect of the NO field in the context of the formation of the RNS and what is known about their effects on biological systems. While much of the insight into the RNS has been gained from the extensive chemical characterisation of these species, to reveal biological consequences this approach must be complemented by direct measures of physiological function. Although we do not know the consequences of many of the dominant chemical reactions of RNS an intriguing aspect is now emerging. This review will illustrate how, when specificity and amplification through cell signalling mechanisms are taken into account, the less significant reactions, in terms of yield or rates, can explain many of the biological responses of exposure of cells or physiological systems to RNS.

Differential effect of shear stress on extracellular signal-regulated kinase and N-terminal Jun kinase in endothelial cells. Gi2- and Gbeta/gamma-dependent signaling pathways

Shear stress differentially regulates production of many vasoactive factors at the level of gene expression in endothelial cells that may be mediated by mitogen-activated protein kinases, including extracellular signal-regulated kinase (ERK) and N-terminal Jun kinase (JNK). Here we show, using bovine aortic endothelial cells (BAEC), that shear stress differentially regulates ERK and JNK by mechanisms involving Gi2 and pertussis toxin (PTx)-insensitive G-protein-dependent pathways, respectively. Shear activated ERK with a rapid, biphasic time course (maximum by 5 min and basal by 30-min shear exposure) and force dependence (minimum and maximum at 1 and 10 dyn/cm2 shear stress, respectively). PTx treatment prevented shear-dependent activation of ERK1/2, consistent with a Gi-dependent mechanism. In contrast, JNK activity was maximally turned on by a threshold level of shear force (0.5 dyn/cm2 or higher) with a much slower and prolonged time course (requiring at least 30 min to 4 h) than that of ERK. Also, PTx had no effect on shear-dependent activation of JNK. To further define the shear-sensitive ERK and JNK pathways, vectors expressing hemagglutinin epitope-tagged ERK (HA-ERK) or HA-JNK were co-transfected with other vectors by using adenovirus-polylysine in BAEC. Expression of the mutant (alpha)i2(G203), antisense G(alpha)i2 and a dominant negative Ras (N17Ras) prevented shear-dependent activation of HA-ERK, while that of (alpha)i2(G204) and antisense (alpha)i3 did not. Expression of a Gbeta/gamma scavenger, the carboxyl terminus of beta-adrenergic receptor kinase (betaARK-ct), and N17Ras inhibited shear-dependent activation of HA-JNK. Treatment of BAEC with genistein prevented shear-dependent activation of ERK and JNK, indicating the essential role of tyrosine kinase(s) in both ERK and JNK pathways. These results provide evidence that 1) Gi2-protein, Ras, and tyrosine kinase(s) are upstream regulators of shear-dependent activation of ERK and 2) that shear-dependent activation of JNK is regulated by mechanisms involving Gbeta/gamma, Ras, and tyrosine kinase(s).

Role of G proteins in shear stress-mediated nitric oxide production by endothelial cells

Exposure of cultured endothelial cells to shear stress resulting from well-defined fluid flow stimulates the production of nitric oxide (NO). We have established that an initial burst in production is followed by sustained steady-state NO production. The signal transduction events leading to this stimulation are not well understood. In the present study, we examined the role of regulatory guanine nucleotide binding proteins (G proteins) in shear stress-mediated NO production. In endothelial cells not exposed to shear stress, AIF4-, a general activator of G proteins, markedly elevated the production of guanosine 3',5'-cyclic monophosphate (cGMP). Pretreatment with NO synthase inhibitor N omega-nitro-L-arginine completely blocked this stimulation. Incubation with guanosine 5'-O-(2-thiodiphosphate) (GDP beta S), a general G protein inhibitor, blocked the flow-mediated burst in cGMP production in a dose-dependent manner. Likewise, GDP beta S inhibited NOx (NO2 + NO3) production for the 1st h. However, inhibition was not detectable between 1 and 3 h. Pertussis toxin (PTx) had no effect on the shear response at any time point. The burst in NO production caused by a change in shear stress appears to be dependent on a PTx-refractory G protein. Sustained shear-mediated production is independent of G protein activation.

Cell signaling by reactive nitrogen and oxygen species in atherosclerosis

The production of reactive oxygen and nitrogen species has been implicated in atherosclerosis principally as means of damaging low-density lipoprotein that in turn initiates the accumulation of cholesterol in macrophages. The diversity of novel oxidative modifications to lipids and proteins recently identified in atherosclerotic lesions has revealed surprising complexity in the mechanisms of oxidative damage and their potential role in atherosclerosis. Oxidative or nitrosative stress does not completely consume intracellular antioxidants leading to cell death as previously thought. Rather, oxidative and nitrosative stress have a more subtle impact on the atherogenic process by modulating intracellular signaling pathways in vascular tissues to affect inflammatory cell adhesion, migration, proliferation, and differentiation. Furthermore, cellular responses can affect the production of nitric oxide, which in turn can strongly influence the nature of oxidative modifications occurring in atherosclerosis. The dynamic interactions between endogenous low concentrations of oxidants or reactive nitrogen species with intracellular signaling pathways may have a general role in processes affecting wound healing to apoptosis, which can provide novel insights into the pathogenesis of atherosclerosis.

Plasma membrane cholesterol is a key molecule in shear stress-dependent activation of extracellular signal-regulated kinase

Shear stress, the dragging force generated by fluid flow, differentially activates extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal kinase (JNK) in bovine aortic endothelial cells (BAEC) (Jo, H., Sipos, K., Go, Y. M., Law, R., Rong, J., and McDonald, J. M. (1997) J. Biol. Chem. 272, 1395-1401). Here, we examine whether cholesterol-enriched compartments in the plasma membrane are responsible for such differential regulation. Pretreatment of BAEC with a cholesterol-binding antibiotic, filipin, did not inhibit shear-dependent activation of JNK. In contrast, filipin and other membrane-permeable cholesterol-binding agents (digitonin and nystatin), but not the lipid-binding agent xylazine, inhibited shear-dependent activation of ERK. The effect of cholesterol-binding drugs did not appear to be due to membrane permeabilization, since treatment of BAEC with a detergent, Triton X-100 which also permeabilizes membranes, did not inhibit shear-dependent activation of ERK. Furthermore, shear-dependent activation of ERK, but not JNK, was inhibited by cyclodextrin, a membrane-impermeable cholesterol-binding agent, which removes cell-surface cholesterol. Moreover, the effects of cyclodextrin were prevented by adding cholesterol during the incubation. These results indicate that cholesterol or cholesterol-sensitive compartments in the plasma membrane play a selective and essential role in activation of ERK, but not JNK, by shear stress. Although exposure to shear stress (1 h) increased the number of caveolae by 3-fold, treatment with filipin had no effect in either control or shear-exposed cells suggesting that caveolae density per se is not a crucial determinant in shear-dependent ERK activation. In summary, the current study suggests that cholesterol-sensitive microdomains in the plasma membrane, such as caveolae-like domains, play a critical role in differential activation of ERK and JNK by shear stress.

Identification of rCop-1, a new member of the CCN protein family, as a negative regulator for cell transformation

By using a model system for cell transformation mediated by the cooperation of the activated H-ras oncogene and the inactivated p53 tumor suppressor gene, rCop-1 was identified by mRNA differential display as a gene whose expression became lost after cell transformation. Homology analysis indicates that rCop-1 belongs to an emerging cysteine-rich growth regulator family called CCN, which includes connective-tissue growth factor, CYR61, CEF10 (v-src inducible), and the product of the nov proto-oncogene. Unlike the other members of the CCN gene family, rCop-1 is not an immediate-early gene, it lacks the conserved C-terminal domain which was shown to confer both growth-stimulating and heparin-binding activities, and its expression is lost in cells transformed by a variety of mechanisms. Ectopic expression of rCop-1 by retroviral gene transfers led to cell death in a transformation-specific manner. These results suggest that rCop-1 represents a new class of CCN family proteins that have functions opposing those of the previously identified members.

Activation of mitogen-activated protein kinase pathways by cyclic GMP and cyclic GMP-dependent protein kinase in contractile vascular smooth muscle cells

Vascular smooth muscle cells (VSMC) exist in either a contractile or a synthetic phenotype in vitro and in vivo. The molecular mechanisms regulating phenotypic modulation are unknown. Previous studies have suggested that the serine/threonine protein kinase mediator of nitric oxide (NO) and cyclic GMP (cGMP) signaling, the cGMP-dependent protein kinase (PKG) promotes modulation to the contractile phenotype in cultured rat aortic smooth muscle cells (RASMC). Because of the potential importance of the mitogen-activated protein kinase (MAP kinase) pathways in VSMC proliferation and phenotypic modulation, the effects of PKG expression in PKG-deficient and PKG-expressing adult RASMC on MAP kinases were examined. In PKG-expressing adult RASMC, 8-para-chlorophenylthio-cGMP activated extracellular signal- regulated kinases (ERK1/2) and c-Jun N-terminal kinase (JNK). The major effect of PKG activation was increased activation by MAP kinase kinase (MEK). The cAMP analog, 8-Br-cAMP inhibited ERK1/2 activation in PKG-deficient and PKG-expressing RASMC but had no effect on JNK activity. The effects of PKG on ERK and JNK activity were additive with those of platelet-derived growth factor (PDGF), suggesting that PKG activates MEK through a pathway not used by PDGF. The stimulatory effects of cGMP on ERK and JNK activation were also observed in low-passaged, contractile RASMC still expressing endogenous PKG, suggesting that the effects of PKG expression were not artifacts of cell transfections. These results suggest that in contractile adult RASMC, NO-cGMP signaling increases MAP kinase activity. Increased activation of these MAP kinase pathways may be one mechanism by which cGMP and PKG activation mediate c-fos induction and increased proliferation of contractile adult RASMC.

Caveolin-1 regulates shear stress-dependent activation of extracellular signal-regulated kinase

Fluid shear stress activates a member of the mitogen-activated protein (MAP) kinase family, extracellular signal-regulated kinase (ERK), by mechanisms dependent on cholesterol in the plasma membrane in bovine aortic endothelial cells (BAEC). Caveolae are microdomains of the plasma membrane that are enriched with cholesterol, caveolin, and signaling molecules. We hypothesized that caveolin-1 regulates shear activation of ERK. Because caveolin-1 is not exposed to the outside, cells were minimally permeabilized by Triton X-100 (0.01%) to deliver a neutralizing, polyclonal caveolin-1 antibody (pCav-1) inside the cells. pCav-1 then bound to caveolin-1 and inhibited shear activation of ERK but not c-Jun NH(2)-terminal kinase. Epitope mapping studies showed that pCav-1 binds to caveolin-1 at two regions (residues 1-21 and 61-101). When the recombinant proteins containing the epitopes fused to glutathione-S-transferase (GST-Cav(1-21) or GST-Cav(61-101)) were preincubated with pCav-1, only GST-Cav(61-101) reversed the inhibitory effect of the antibody on shear activation of ERK. Other antibodies, including m2234, which binds to caveolin-1 residues 1-21, had no effect on shear activation of ERK. Caveolin-1 residues 61-101 contain the scaffolding and oligomerization domains, suggesting that binding of pCav-1 to these regions likely disrupts the clustering of caveolin-1 or its interaction with signaling molecules involved in the shear-sensitive ERK pathway. We suggest that caveolae-like domains play a critical role in the mechanosensing and/or mechanosignal transduction of the ERK pathway.

The induction of GSH synthesis by nanomolar concentrations of NO in endothelial cells: a role for gamma-glutamylcysteine synthetase and gamma-glutamyl transpeptidase

Nitric oxide protects cells from oxidative stress through a number of direct scavenging reactions with free radicals but the effects of nitric oxide on the regulation of antioxidant enzymes are only now emerging. Using bovine aortic endothelial cells as a model, we show that nitric oxide, at physiological rates of production (1-3 nM/s), is capable of inducing the synthesis of glutathione through a mechanism involving gamma-glutamylcysteine synthetase and gamma-glutamyl transpeptidase. This novel nitric oxide signalling pathway is cGMP-independent and we hypothesize that it makes an important contribution to the anti-atherosclerotic and antioxidant properties of nitric oxide.

Mechanisms of cell signaling by nitric oxide and peroxynitrite: from mitochondria to MAP kinases

Many of the biological and pathological effects of nitric oxide (NO) are mediated through cell signaling pathways that are initiated by NO reacting with metalloproteins. More recently, it has been recognized that the reaction of NO with free radicals such as superoxide and the lipid peroxyl radical also has the potential to modulate redox signaling. Although it is clear that NO can exert both cytotoxic and cytoprotective actions, the focus of this overview are those reactions that could lead to protection of the cell against oxidative stress in the vasculature. This will include the induction of antioxidant defenses such as glutathione, activation of mitogen-activated protein kinases in response to blood flow, and modulation of mitochondrial function and its impact on apoptosis. Models are presented that show the increased synthesis of glutathione in response to shear stress and inhibition of cytochrome c release from mitochondria. It appears that in the vasculature NO-dependent signaling pathways are of three types: (i) those involving NO itself, leading to modulation of mitochondrial respiration and soluble guanylate cyclase; (ii) those that involve S-nitrosation, including inhibition of caspases; and (iii) autocrine signaling that involves the intracellular formation of peroxynitrite and the activation of the mitogen-activated protein kinases. Taken together, NO plays a major role in the modulation of redox cell signaling through a number of distinct pathways in a cellular setting.

Nitric oxide-dependent induction of glutathione synthesis through increased expression of gamma-glutamylcysteine synthetase

The nitric oxide (NO) donors S-nitrosopenicillamine or DetaNONOate, which release NO at a rate of 0-15 nM sec-1, were exposed to rat aortic vascular smooth muscle cells for a period of 0-24 h. This treatment resulted in an increase in total glutathione levels of two- to threefold under conditions where no cytotoxicity was detected. The signaling pathways do not involve activation of protein kinase G Ialpha nor are they cGMP dependent. Oxidation of reduced glutathione (GSH) was found after exposure to NO for 3-4 h at rates of formation at or above 8 nM sec-1. Increased intracellular GSH was due to enhanced expression of the rate-limiting enzyme for GSH synthesis, gamma-glutamylcysteine synthetase. Since NO has been shown previously to protect cells against oxidative stress, we propose that the increase in GSH by NO is a potential mechanism for enhancing the antioxidant defenses of the cell. This result also has important implications for identifying redox-sensitive cell signaling pathways that can be activated by NO.

Protein kinase B/Akt activates c-Jun NH(2)-terminal kinase by increasing NO production in response to shear stress

Laminar shear stress activates c-Jun NH(2)-terminal kinase (JNK) by the mechanisms involving both nitric oxide (NO) and phosphatidylinositide 3-kinase (PI3K). Because protein kinase B (Akt), a downstream effector of PI3K, has been shown to phosphorylate and activate endothelial NO synthase, we hypothesized that Akt regulates shear-dependent activation of JNK by stimulating NO production. Here, we examined the role of Akt in shear-dependent NO production and JNK activation by expressing a dominant negative Akt mutant (Akt(AA)) and a constitutively active mutant (Akt(Myr)) in bovine aortic endothelial cells (BAEC). As expected, pretreatment of BAEC with the PI3K inhibitor (wortmannin) prevented shear-dependent stimulation of Akt and NO production. Transient expression of Akt(AA) in BAEC by using a recombinant adenoviral construct inhibited the shear-dependent stimulation of NO production and JNK activation. However, transient expression of Akt(Myr) by using a recombinant adenoviral construct did not induce JNK activation. This is consistent with our previous finding that NO is required, but not sufficient on its own, to activate JNK in response to shear stress. These results and our previous findings strongly suggest that shear stress triggers activation of PI3K, Akt, and endothelial NO synthase, leading to production of NO, which (along with O(2-), which is also produced by shear) activates Ras-JNK pathway. The regulation of Akt, NO, and JNK by shear stress is likely to play a critical role in its antiatherogenic effects.

The effect of varying albumin concentration and hydrostatic pressure on hydraulic conductivity and albumin permeability of cultured endothelial monolayers

An in vitro model of the endothelial transport barrier was developed using bovine aortic endothelial cell monolayers cultured on a porous polycarbonate substrate. Hydraulic conductivity (Lp) was measured by a bubble tracking technique at varying pressure differentials and albumin concentrations. The effective albumin permeability (Pc) was determined by measuring the flux of fluorescent-labeled albumin across monolayers at varying hydrostatic pressures. Lp determined at pressure differentials between 5.0 and 10 cm H2O demonstrated a strong dependence on albumin concentration, decreasing approximately 10-fold from 21.3 x 10(-7) +/- 3.18 x 10(-7) cm/sec/cm H2O (mean +/- SEM) at 0.0 g/dl to 2.35 x 10(-7) +/- 0.20 x 10(-7) cm/sec/cm H2O at 1.0 g/dl albumin. Increasing the albumin concentration from 1.0 to 4.0 g/dl reduced Lp by an additional 16% to 1.97 x 10(-7) +/- 0.17 x 10(-7) cm/sec/cm H2O. Furthermore, Lp was moderately dependent on the pressure differential, increasing by about a factor of two with a doubling of the pressure differential. The effective permeability (Pc) was also dependent on the pressure differential. At an albumin concentration of 4.0 g/dl, Pc increased from 1.37 x 10(-6) +/- 0.26 x 10(-6) cm/sec at 0.0 cm H2O to 5.06 x 10(-6) +/- 1.92 x 10(-6) cm/sec at 10 cm H2O. Analysis of Pc and Jv data, however, demonstrates that water and albumin do not share a common pathway in crossing the endothelial monolayer. These data suggest the existence of a large pore pathway for albumin. Thus, the in vitro system has many of the transport characteristics of intact vessels in vivo and should be useful for physiological studies of the endothelial transport barrier.

NF-kappa B is required for H-ras oncogene induced abnormal cell proliferation and tumorigenesis

Oncogenic mutations in ras lead to constitutive activation of downstream signaling pathways that modulate the activities of transcription factors. In turn, these factors control the expression of a subset of genes responsible for neoplastic cell transformation. Recent studies suggest that transcription factor NF-kappa B contributes to cell transformation by inhibiting the cell death signal activated by oncogenic Ras. In this study, inhibition of NF-kappa B activity by forced expression of a super-repressor form of I kappa B alpha, the major inhibitor of NF-kappa B, markedly decreased the growth rate, saturation density and tumorigenicity of oncogenic H-Ras transformed rat embryo fibroblasts. Such clonally isolated cells overexpressing I kappa B alpha super-repressor not only were viable but also exhibited no sign of spontaneous apoptosis. Inhibition of NF-kappa B in these cells was functionally demonstrated by both the loss of cytokine induced DNA binding activity and a profoundly increased sensitivity to cell death in response to TNF-alpha treatment. In contrast, inhibition of NF-kappa B activity in non-transformed fibroblasts had minimal effect on growth, but rendered the cells resistant to a subsequent transformation by H-ras oncogene. Similar results were also obtained with rat intestinal epithelial cells harboring an inducible ras oncogene. Taken together, these findings suggest that NF-kappa B activity is essential for abnormal cell proliferation and tumorigenicity activated by the ras oncogene and highlight an alternative functional role for NF-kappa B in oncogenic Ras-mediated cell transformation that is distinct from its anti-apoptotic activity. Oncogene (2000) 19, 841 - 849.

Endothelial albumin permeability is shear dependent, time dependent, and reversible

Altered permeability of vascular endothelium to macromolecules may play a role in vascular disease as well as vascular homeostasis. Because the shear stress of flowing blood on the vascular wall is known to influence many endothelial cell properties, an in vitro system to measure transendothelial permeability (Pe) to fluorescein isothiocyanate conjugated bovine serum albumin under defined physiological levels of steady laminar shear stress was developed. Bovine aortic endothelial cells grown on polycarbonate filters pretreated with gelatin and fibronectin constituted the model system. Onset of 1 dyn/cm2 shear stress resulted in a Pe rise from 5.1 +/- 1.3 x 10(-6) cm/s to 21.9 +/- 4.6 X 10(-6) cm/s at 60 min (n = 6); while 10 dyn/cm2 shear stress increased Pe from 4.8 +/- 1.5 X 10(-6) cm/s to 50.2 +/- 6.8 X 10(-6) cm/s at 30 min and 49.6 +/- 8.9 X 10(-6) cm/s at 60 (n = 9). Pe returned to preshear values within 120 and 60 min after removal of 1 and 10 dyn/cm2 shear stress, respectively. The data show that endothelial cell Pe in vitro is acutely sensitive to shear stress.

Effects of pyrrolidine dithiocarbamate on endothelial cells: protection against oxidative stress

The dithiocarbamates are well known for their antioxidant properties and effects on cellular transcriptional events. For example, pyrrolidine dithiocarbamate (PDTC) is widely used as an inhibitor of nuclear factor kappa B (NFkappaB) and this, or related compounds may have therapeutic potential in inhibiting atherosclerosis. However, the precise molecular mechanisms through which PDTC could elicit antioxidant or cell signaling effects in a cellular setting remain unclear. Furthermore, the mechanisms for the effects of PDTC on NFkappaB are likely to involve inhibition of binding of the transcription factor to DNA rather than an effect on the activation process as first proposed. In relation to pharmacological applications of such compounds, little is known of their interaction with endothelial cells, the anticipated site of action for inhibition of vascular related diseases. Until recently, PDTC was generally classified as an antioxidant but evidence for pro-oxidant effects have been reported. In this study, we have addressed this issue in bovine aortic endothelial cells and identified two mechanisms through which PDTC can exert antioxidant effects. At low concentrations (0-25 microM), PDTC induces a concentration dependent increase in cellular GSH levels through the increased activity of gamma-glutamylcysteine synthetase. At higher concentrations, GSH oxidation and apoptotic cell death occur. Using 2,3 dimethoxy-1,4-napthoquinone (DMNQ) as an intracellular generator of superoxide radicals, we find PDTC (10 microM) protects against the cytotoxicity of this agent through a GSH-independent mechanism.

Evidence for peroxynitrite as a signaling molecule in flow-dependent activation of c-Jun NH(2)-terminal kinase

The c-Jun NH(2)-terminal kinase (JNK), also known as stress-activated protein kinase, is a mitogen-activated protein kinase that determines cell survival in response to environmental stress. Activation of JNK involves redox-sensitive mechanisms and physiological stimuli such as shear stress, the dragging force generated by blood flow over the endothelium. Laminar shear stress has antiatherogenic properties and controls structure and function of endothelial cells by mechanisms including production of nitric oxide (NO) and superoxide (O(-)(2)). Here we show that both NO and O(-)(2) are required for activation of JNK by shear stress in endothelial cells. The present study also demonstrates that exposure of endothelial cells to shear stress increases tyrosine nitration, a marker of reactive nitrogen species formation. Furthermore, inhibitors or scavengers of NO, O(-)(2), or reactive nitrogen species prevented shear-dependent increase in tyrosine nitration and activation of JNK. Peroxynitrite alone, added to cells as a bolus or generated over 60 min by 3-morpholinosydnonimine, also activates JNK. These results suggest that reactive nitrogen species, in this case most likely peroxynitrite, act as signaling molecules in the mechanoactivation of JNK.

The in vitro effects of dehydroepiandrosterone on human osteoarthritic chondrocytes

OBJECTIVE

To investigate the in vitro effects of dehydroepiandrosterone (DHEA) on human osteoarthritic chondrocytes.

DESIGN

Chondrocytes isolated from human osteoarthritic knee cartilage were three-dimensionally cultured in alginate beads, except for cell proliferation experiment. Cells were treated with DHEA in the presence or absence of IL-1beta. The effects on chondrocytes were analyzed using a 3-(4,5-dimethylthiazol-2yl)-5-(3-carboxymethoxy-phenyl)-2-(4-sulfophenyl)-2H-tetrazolium inner salt (MTS) assay (for chondrocyte proliferation), a dimethylmethylene blue (DMB) assay (for glycosaminoglycan (GAG) synthesis), and an indole assay (for DNA amount). Gene expressions of type I and II collagen, metalloproteinase-1 and -3 (MMP-1 and -3), and tissue inhibitor of metalloproteinase-1 (TIMP-1) as well as the IL-1beta-induced gene expressions of MMP-1 and -3 were analyzed by reverse transcription-polymerase chain reaction (RT-PCR). The protein synthesis of MMP-1 and -3 and TIMP-1 was determined by Western blotting.

RESULTS

The treatment of chondrocytes with DHEA did not affect chondrocyte proliferation or GAG synthesis up to 100 micro M of concentration. The gene expression of type II collagen increased in a dose-dependent manner, while that of type I decreased. DHEA suppressed the expression of MMP-1 significantly at concentrations exceeding 50 micro M. The gene expression of MMP-3 was also suppressed, but this was without statistical significance. The expression of TIMP-1 was significantly increased by DHEA at concentrations exceeding 10 micro M. The effects of DHEA on the gene expressions of MMP-1 and -3 were more prominent in the presence of IL-1beta, in which DHEA suppressed not only MMP-1, but also MMP-3 at the lower concentrations, 10 and 50 micro M, respectively. Western blotting results were in agreement with RT-PCR, which indicates that DHEA acts at the gene transcription level.

CONCLUSIONS

Our study demonstrates that DHEA has no toxic effect on chondrocytes up to 100 micro M of concentration and has an ability to modulate the imbalance between MMPs and TIMP-1 during OA at the transcription level, which suggest that it has a protective role against articular cartilage loss.

Phosphatidylinositol 3-kinase gamma mediates shear stress-dependent activation of JNK in endothelial cells

Shear stress differentially activates extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal kinase (JNK) by mechanisms involving Galphai2 and Gbeta/gamma proteins, respectively, in bovine aortic endothelial cells (BAEC). The early events in this signaling mechanism by which G proteins regulate ERK and JNK in response to shear stress have not been defined. Here we show that BAEC endogenously express a G protein-dependent form of phosphatidylinositol 3-kinase, PI3Kgamma, and its activity is stimulated by shear stress. PI3Kgamma activity was measured in vitro using BAEC that were transiently transfected with an epitope-tagged PI3Kgamma (vsv-PI3Kgamma). Exposure of BAEC to shear stress rapidly and transiently stimulated the activity of vsv-PI3Kgamma (maximum by 15 s, with a return to basal after 1-min exposure to 5 dyn/cm2 shear stress). Activity of vsv-PI3Kgamma was stimulated by shear stress intensities as low as 0.5 dyn/cm2. Treatment of BAEC with an inhibitor of PI3K, wortmannin, inhibited shear-dependent activation of JNK but had no effect on that of ERK. Furthermore, expression of a kinase-inactive mutant (PI3KgammaK799R) in BAEC inhibited the shear-dependent activation of JNK but not ERK. Taken together, these results suggest that PI3Kgamma selectively regulates the shear-sensitive JNK pathway. This differential and novel signaling pathway may be responsible for coordinating various mechanosensitive events in endothelial cells.

The interplay of nitric oxide and peroxynitrite with signal transduction pathways: implications for disease

Since the discovery that at least one form of endothelium derived relaxing factor is nitric oxide (NO), numerous studies have uncovered diverse roles for this free radical in a variety of physiological and pathophysiological processes. NO production, a process mediated by a family of enzymes termed NO synthases, has been detected in most cell types. Many of the effects of NO are thought to be mediated through its direct interaction with specific and defined cell signaling pathways. The nature of such interactions are highly dependent on the concentration of NO and cell type. Furthermore, specific NO derived reaction products, such as peroxynitrite, also have the potential to effect cell signal transduction events. As with NO, this can occur through diverse mechanisms and depends on concentration and cell type. It is perhaps not surprising that the reported effects of NO in different disease states are often conflicting. In this brief overview, a framework for placing these apparently disparate properties of NO will be described and will focus on the effects of NO and peroxynitrite on signaling pathways.

Endothelial NOS-dependent activation of c-Jun NH(2)- terminal kinase by oxidized low-density lipoprotein

Oxidized low-density lipoprotein (oxLDL) is known to activate a number of signal transduction pathways in endothelial cells. Among these are the c-Jun NH(2)-terminal kinase (JNK), also known as stress-activated protein kinase, and extracellular signal-regulated kinase (ERK). These mitogen-activated protein kinases (MAP kinase) determine cell survival in response to environmental stress. Interestingly, JNK signaling involves redox-sensitive mechanisms and is activated by reactive oxygen and nitrogen species derived from both NADPH oxidases, nitric oxide synthases (NOS), peroxides, and oxidized low-density lipoprotein (oxLDL). The role of endothelial NOS (eNOS) in the activation of JNK in response to oxLDL has not been examined. Herein, we show that on exposure of endothelial cells to oxLDL, both ERK and JNK are activated through independent signal transduction pathways. A key role of eNOS activation through a phosphatidylinositol-3-kinase-dependent mechanism leading to phosphorylation of eNOS is demonstrated for oxLDL-dependent activation of JNK. Moreover, we show that activation of ERK by oxLDL is critical in protection against the cytotoxicity of oxLDL.

Gene expression of central and peripheral renin-angiotensin system components upon dietary sodium intake in rats

The effects of dietary sodium intake on the gene expression of the renin-angiotensin system (RAS) were investigated in rat central and peripheral tissues in a single set of experiment. Northern and reverse transcriptase-polymerase chain reaction (RT-PCR) techniques were used to detect mRNA expression in rats fed a low- or a high-sodium diet (5 or 500 mmol Na+/kg diet) for 20 days. Plasma and renal renin levels were elevated in rats maintained on the low-sodium diet. Sodium deprivation enhanced the expression of angiotensinogen, renin, AT1A and AT1B receptor subtypes in the hypothalamus, but suppressed them in the brainstem. Kidney and adrenal levels of those mRNAs were also enhanced in the sodium-restricted rats. Both AT1A and AT1B mRNAs changed in a similar magnitude in each tissue examined upon dietary sodium intake. AT1A was the predominant receptor subtype of AT1 in all the tissues examined in the present study except the adrenal gland. The present study demonstrated that dietary sodium modulated the gene expression of the RAS components in the central and peripheral tissues. It also showed that the RAS components in the brainstem and hypothalamus were differentially expressed upon sodium deprivation. This suggests different roles of the RAS in these tissues in maintaining body fluid homeostasis in response to different sodium intakes.

Tyrosine-phosphorylated calmodulin has reduced biological activity

Calmodulin is phosphorylated by the purified insulin receptor on tyrosine residues with a maximum stoichiometry of 1 mol phosphate/mol of calmodulin. Isolated tryptic phosphopeptides were sequenced by manual Edman degradation and demonstrated that calmodulin is equally phosphorylated on tyrosine 99 and tyrosine 138. Phosphorylated calmodulin has a decreased affinity (K0.5 = 4.2 nM) for the 63-kDa isozyme of cyclic nucleotide phosphodiesterase compared to nonphosphorylated calmodulin (K0.5 = 2.1 nM). The K0.5 for Ca2+ is marginally increased from 2.8 to 3.2 microM in the presence of phosphotyrosyl calmodulin. The effect of the calmodulin antagonist, mastoparan, was investigated to determine whether mastoparan would differentially inhibit calmodulin- or phosphocalmodulin-dependent enzyme activity. The IC50 of mastoparan is fourfold lower for phosphotyrosyl calmodulin compared to nonphosphorylated calmodulin. Phosphorylation of calmodulin may provide a mechanism for the differential regulation of calmodulin-dependent enzymes. These observations further support a potentially important regulatory function of calmodulin phosphorylation in signal transduction.