Angiotensin II stimulates p90rsk in vascular smooth muscle cells. A potential Na(+)-H+ exchanger kinase

Altered glycolysis results in drug-resistant in clinical tumor therapy

Cancer cells undergo metabolic reprogramming, including increased glucose metabolism, fatty acid synthesis and glutamine metabolic rates. These enhancements to three major metabolic pathways are closely associated with glycolysis, which is considered the central component of cancer cell metabolism. Increasing evidence suggests that dysfunctional glycolysis is commonly associated with drug resistance in cancer treatment, and aberrant glycolysis plays a significant role in drug-resistant cancer cells. Studies on the development of drugs targeting these abnormalities have led to improvements in the efficacy of tumor treatment. The present review discusses the changes in glycolysis targets that cause drug resistance in cancer cells, including hexokinase, pyruvate kinase, pyruvate dehydrogenase complex, glucose transporters, and lactate, as well the underlying molecular mechanisms and corresponding novel therapeutic strategies. In addition, the association between increased oxidative phosphorylation and drug resistance is introduced, which is caused by metabolic plasticity. Given that aberrant glycolysis has been identified as a common metabolic feature of drug-resistant tumor cells, targeting glycolysis may be a novel strategy to develop new drugs to benefit patients with drug-resistance.

Structural and Functional Changes in the Na+/H+ Exchanger Isoform 1, Induced by Erk1/2 Phosphorylation

The human Na⁺/H⁺ exchanger isoform 1 (NHE1) is a plasma membrane transport protein that plays an important role in pH regulation in mammalian cells. Because of the generation of protons by intermediary metabolism as well as the negative membrane potential, protons accumulate within the cytosol. Extracellular signal-regulated kinase (ERK)-mediated regulation of NHE1 is important in several human pathologies including in the myocardium in heart disease, as well as in breast cancer as a trigger for growth and metastasis. NHE1 has a N-terminal, a 500 amino acid membrane domain, and a C-terminal 315 amino acid cytosolic domain. The C-terminal domain regulates the membrane domain and its effects on transport are modified by protein binding and phosphorylation. Here, we discuss the physiological regulation of NHE1 by ERK, with an emphasis on the critical effects on structure and function. ERK binds directly to the cytosolic domain at specific binding domains. ERK also phosphorylates NHE1 directly at multiple sites, which enhance NHE1 activity with subsequent downstream physiological effects. The NHE1 cytosolic regulatory tail possesses both ordered and disordered regions, and the disordered regions are stabilized by ERK-mediated phosphorylation at a phosphorylation motif. Overall, ERK pathway mediated phosphorylation modulates the NHE1 tail, and affects the activity, structure, and function of this membrane protein.

RSK2 contributes to myogenic vasoconstriction of resistance arteries by activating smooth muscle myosin and the Na+/H+ exchanger

Smooth muscle contraction is triggered when Ca²⁺/calmodulin-dependent myosin light chain kinase (MLCK) phosphorylates the regulatory light chain of myosin (RLC₂₀). However, blood vessels from Mlck-deficient mouse embryos retain the ability to contract, suggesting the existence of additional regulatory mechanisms. We showed that the p90 ribosomal S6 kinase 2 (RSK2) also phosphorylated RLC₂₀ to promote smooth muscle contractility. Active, phosphorylated RSK2 was present in mouse resistance arteries under normal basal tone, and phosphorylation of RSK2 increased with myogenic vasoconstriction or agonist stimulation. Resistance arteries from Rsk2-deficient mice were dilated and showed reduced myogenic tone and RLC₂₀ phosphorylation. RSK2 phosphorylated Ser¹⁹ in RLC in vitro. In addition, RSK2 phosphorylated an activating site in the Na⁺/H⁺ exchanger (NHE-1), resulting in cytosolic alkalinization and an increase in intracellular Ca²⁺ that promotes vasoconstriction. NHE-1 activity increased upon myogenic constriction, and the increase in intracellular pH was suppressed in Rsk2-deficient mice. In pressured arteries, RSK2-dependent activation of NHE-1 was associated with increased intracellular Ca²⁺ transients, which would be expected to increase MLCK activity, thereby contributing to basal tone and myogenic responses. Accordingly, Rsk2-deficient mice had lower blood pressure than normal littermates. Thus, RSK2 mediates a procontractile signaling pathway that contributes to the regulation of basal vascular tone, myogenic vasoconstriction, and blood pressure and may be a potential therapeutic target in smooth muscle contractility disorders.

Na+/H+ exchanger isoform 1-induced osteopontin expression facilitates cardiac hypertrophy through p90 ribosomal S6 kinase

Cardiovascular diseases are the leading cause of death worldwide. One in three cases of heart failure is due to dilated cardiomyopathy. The Na⁺/H⁺ exchanger isoform 1 (NHE1), a multifunctional protein and the key pH regulator in the heart, has been demonstrated to be increased in this condition. We have previously demonstrated that elevated NHE1 activity induced cardiac hypertrophy in vivo. Furthermore, the overexpression of active NHE1 elicited modulation of gene expression in cardiomyocytes including an upregulation of myocardial osteopontin (OPN) expression. To determine the role of OPN in inducing NHE1-mediated cardiomyocyte hypertrophy, double transgenic mice expressing active NHE1 and OPN knockout were generated and assessed by echocardiography and the cardiac phenotype. Our studies showed that hearts expressing active NHE1 exhibited cardiac remodeling indicated by increased systolic and diastolic left ventricular internal diameter and increased ventricular volume. Moreover, these hearts demonstrated impaired function with decreased fractional shortening and ejection fraction. Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) mRNA was upregulated, and there was an increase in heart cell cross-sectional area confirming the cardiac hypertrophic effect. Moreover, NHE1 transgenic mice also showed increased collagen deposition, upregulation of CD44 and phosphorylation of p90 ribosomal s6 kinase (RSK), effects that were regressed in OPN knockout mice. In conclusion, we developed an interesting comparative model of active NHE1 transgenic mouse lines which express a dilated hypertrophic phenotype expressing CD44 and phosphorylated RSK, effects which were regressed in absence of OPN.

Bacterial Expression, Purification and In Vitro Phosphorylation of Full-Length Ribosomal S6 Kinase 2 (RSK2)

Ribosomal S6 kinases (RSK) play important roles in cell signaling through the mitogen-activated protein kinase (MAPK) pathway. Each of the four RSK isoforms (RSK1-4) is a single polypeptide chain containing two kinase domains connected by a linker sequence with regulatory phosphorylation sites. Here, we demonstrate that full-length RSK2-which is implicated in several types of cancer, and which is linked to the genetic Coffin-Lowry syndrome-can be overexpressed with high yields in Escherichia coli as a fusion with maltose binding protein (MBP), and can be purified to homogeneity after proteolytic removal of MBP by affinity and size-exclusion chromatography. The purified protein can be fully activated in vitro by phosphorylation with protein kinases ERK2 and PDK1. Compared to full-length RSK2 purified from insect host cells, the bacterially expressed and phosphorylated murine RSK2 shows the same levels of catalytic activity after phosphorylation, and sensitivity to inhibition by RSK-specific inhibitor SL0101. Interestingly, we detect low levels of phosphorylation in the nascent RSK2 on Ser386, owing to autocatalysis by the C-terminal domain, independent of ERK. This observation has implications for in vivo signaling, as it suggests that full activation of RSK2 by PDK1 alone is possible, circumventing at least in some cases the requirement for ERK.

Ribosomal S6 kinase (RSK) modulators: a patent review

INTRODUCTION

The p90 ribosomal S6 kinases (RSK) are a family of Ser/Thr protein kinases that are downstream effectors of MEK1/2-ERK1/2. Increased RSK activation is implicated in the etiology of multiple pathologies, including numerous types of cancers, cardiovascular disease, liver and lung fibrosis, and infections.

AREAS COVERED

The review summarizes the patent and scientific literature on small molecule modulators of RSK and their potential use as therapeutics. The patents were identified using World Intellectual Property Organization and United States Patent and Trademark Office databases. The compounds described are predominantly RSK inhibitors, but a RSK activator is also described. The majority of the inhibitors are not RSK-specific.

EXPERT OPINION

Based on the overwhelming evidence that RSK is involved in a number of diseases that have high mortalities it seems surprising that there are no RSK modulators that have pharmacokinetic properties suitable for in vivo use. MEK1/2 inhibitors are in the clinic, but the efficacy of these compounds appears to be limited by their side effects. We hypothesize that targeting the downstream effectors of MEK1/2, like RSK, are an untapped source of drug targets and that they will generate less side effects than MEK1/2 inhibitors because they regulate fewer effectors.

The RSK Inhibitor BIX02565 Limits Cardiac Ischemia/Reperfusion Injury

AIMS

During ischemia/reperfusion (I/R), ribosomal S6 kinase (RSK) activates Na(+)/H(+) exchanger 1 (NHE1) by phosphorylating NHE1 at serine 703 (pS703-NHE1), which promotes cardiomyocyte death and injury. Pharmacologic inhibition of NHE1 effectively protects animal hearts from I/R. However, clinical trials using NHE1 inhibitors failed to show benefit in patients with acute myocardial infarction (MI). One possible explanation is those inhibitors block both agonist-stimulated activity (increasing I/R injury) and basal NHE1 activity (necessary for cell survival). We previously showed that dominant-negative RSK (DN-RSK) selectively blocked agonist-stimulated NHE1 activity. Therefore, we hypothesized that a novel RSK inhibitor (BIX02565) would blunt agonist-stimulated NHE1 and protect hearts from I/R.

METHODS AND RESULTS

Serum/angiotensin II-stimulated pS703-NHE1 was significantly decreased by BIX02565 in cultured cells. Intracellular pH recovery assay showed that BIX02565 selectively inhibited serum-stimulated NHE1 activity. Ischemia/reperfusion decreased left ventricular-developed pressure (LVDP; inhibited) to 8.7% of the basal level in non-transgenic littermate control (NLC) mouse hearts, which was significantly improved (44.6%) by BIX02565. Similar protection was observed in vehicle-treated, cardiac-specific DN-RSK-Tg mice (43%). No additional protective effect was seen in BIX02565-treated DN-RSK-Tg hearts. BIX02565 also improved LVDP in cardiac-specific wild-type (WT)-RSK-Tg mouse hearts (7.4%-40.9%, P < .01). Finally, Western Blotting results confirmed DN-RSK and BIX02565 significantly decreased I/R-induced pS703-NHE1.

CONCLUSION

The RSK plays a crucial role in I/R-induced activation of NHE1 and cardiac injury. The RSK inhibition may provide an alternative target for patients with MI.

Na+/H+ exchanger isoform 1 induced cardiomyocyte hypertrophy involves activation of p90 ribosomal s6 kinase

Studies using pharmacological and genetic approaches have shown that increased activity/expression of the Na⁺/H⁺ exchanger isoform 1 (NHE1) play a critical role in the pathogenesis of cardiac hypertrophy. Despite the importance of NHE1 in cardiac hypertrophy, severe cerebrovascular side effects were associated with the use of NHE1 inhibitors when administered to patients with myocardial infarctions. p90 ribosomal S6 Kinase (RSK), a downstream regulator of the mitogen-activated protein kinase pathway, has also been implicated in cardiac hypertrophy. We hypothesized that RSK plays a role in the NHE1 induced cardiomyocyte hypertrophic response. Infection of H9c2 cardiomyoblasts with the active form of the NHE1 adenovirus induced hypertrophy and was associated with an increase in the phosphorylation of RSK (P<0.05). Parameters of hypertrophy such as cell area, protein content and atrial natriuretic mRNA expression were significantly reduced in H9c2 cardiomyoblasts infected with active NHE1 in the presence of dominant negative RSK (DN-RSK) (P<0.05). These results confirm that NHE1 lies upstream of RSK. Increased phosphorylation and activation of GATA4 at Ser²⁶¹ was correlated with increased RSK phosphorylation. This increase was reversed upon inhibition of RSK or NHE1. These findings demonstrate for the first time that the NHE1 mediated hypertrophy is accounted for by increased activation and phosphorylation of RSK, which subsequently increased the phosphorylation of GATA4; eventually activating fetal gene transcriptional machinery.

Antibodies in the pathogenesis of hypertension

It has long been known that circulating levels of IgG and IgM antibodies are elevated in patients with essential and pregnancy-related hypertension. Recent studies indicate these antibodies target, and in many cases activate, G-protein coupled receptors and ion channels. Prominent among these protein targets are AT₁ receptors, α₁-adrenoceptors, β₁-adrenoceptors, and L-type voltage operated Ca²⁺ channels, all of which are known to play key roles in the regulation of blood pressure through modulation of vascular tone, cardiac output, and/or Na⁺/water reabsorption in the kidneys. This suggests that elevated antibody production may be a causal mechanism in at least some cases of hypertension. In this brief review, we will further describe the protein targets of the antibodies that are elevated in individuals with essential and pregnancy-related hypertension and the likely pathophysiological consequences of antibody binding to these targets. We will speculate on the potential mechanisms that underlie elevated antibody levels in hypertensive individuals and, finally, we will outline the therapeutic opportunities that could arise with a better understanding of how and why antibodies are produced in hypertension.

The p90 ribosomal S6 kinase (RSK) is a mediator of smooth muscle contractility

In the canonical model of smooth muscle (SM) contraction, the contractile force is generated by phosphorylation of the myosin regulatory light chain (RLC₂₀) by the myosin light chain kinase (MLCK). Moreover, phosphorylation of the myosin targeting subunit (MYPT1) of the RLC₂₀ phosphatase (MLCP) by the RhoA-dependent ROCK kinase, inhibits the phosphatase activity and consequently inhibits dephosphorylation of RLC₂₀ with concomitant increase in contractile force, at constant intracellular [Ca²⁺]. This pathway is referred to as Ca²⁺-sensitization. There is, however, emerging evidence suggesting that additional Ser/Thr kinases may contribute to the regulatory pathways in SM. Here, we report data implicating the p90 ribosomal S6 kinase (RSK) in SM contractility. During both Ca²⁺- and agonist (U46619) induced SM contraction, RSK inhibition by the highly selective compound BI-D1870 (which has no effect on MLCK or ROCK) resulted in significant suppression of contractile force. Furthermore, phosphorylation levels of RLC₂₀ and MYPT1 were both significantly decreased. Experiments involving the irreversible MLCP inhibitor microcystin-LR, in the absence of Ca²⁺, revealed that the decrease in phosphorylation levels of RLC₂₀ upon RSK inhibition are not due solely to the increase in the phosphatase activity, but reflect direct or indirect phosphorylation of RLC₂₀ by RSK. Finally, we show that agonist (U46619) stimulation of SM leads to activation of extracellular signal-regulated kinases ERK1/2 and PDK1, consistent with a canonical activation cascade for RSK. Thus, we demonstrate a novel and important physiological function of the p90 ribosomal S6 kinase, which to date has been typically associated with the regulation of gene expression.

p90RSK targets the ERK5-CHIP ubiquitin E3 ligase activity in diabetic hearts and promotes cardiac apoptosis and dysfunction

RATIONALE

Cardiomyocyte apoptosis is one of the key events in the development and progression of heart failure, and a crucial role for ICER (inducible cAMP early repressor) in this process has been previously reported. ERK5 is known to inhibit cardiac apoptosis after myocardial infarction (MI), especially in hyperglycemic states, via association with CHIP ubiquitin (Ub) ligase and subsequent upregulation of CHIP ligase activity, which induces ICER ubiquitination and subsequent protein degradation. The regulatory mechanism governing ERK5/CHIP interaction is unknown.

OBJECTIVE

We previously demonstrated increased p90RSK activation in the diabetic heart. As a logical extension of this work, we now investigate whether p90RSK activation inhibits ERK5-mediated CHIP activation, and subsequently increases ICER levels and apoptosis.

METHODS AND RESULTS

p90RSK activation inhibits ERK5/CHIP association and CHIP Ub ligase activity. p90RSK and CHIP share a common binding site in the ERK5 C-terminal domain (aa571-807). Overexpression of either p90RSK or an ERK5 fragment (aa571-807) inhibits ERK5/CHIP association, suggesting that p90RSK and CHIP competes for ERK5 binding and that p90RSK activation is critical for inhibiting ERK5/CHIP interaction. We also identified ERK5-S496 as being directly phosphorylated by p90RSK and demonstrated that an ERK5-S496A mutant significantly impairs Angiotensin II-mediated inhibition of CHIP activity and subsequent increase in ICER levels. In vivo, either cardiac-specific depletion of ERK5 or overexpression of p90RSK inhibits CHIP activity and accelerates cardiac apoptosis after MI-a phenomenon fully reversible by activating ERK5.

CONCLUSIONS

These data suggest a role for p90RSK in inhibiting CHIP activity and promoting cardiac apoptosis through binding to and phosphorylation of ERK5-S496.

cMyBP-C as a promiscuous substrate: phosphorylation by non-PKA kinases and its potential significance

It is now generally accepted that phosphorylation of cMyBP-C is critically important in maintaining normal cardiac function. Although much of the work to date on phospho-regulation of cMyBP-C has focused on the role of protein kinase A (PKA, also known as cAMP-dependent protein kinase), recent evidence suggests that a number of non-PKA serine/threonine kinases, such as Ca(2+)/calmodulin-dependent protein kinase II, protein kinase C, protein kinase D and the 90-kDa ribosomal S6 kinase are also capable of targeting this key regulatory sarcomeric protein. This article reviews such evidence and proposes a hypothetical role for some of the pertinent signalling pathways in phospho-regulation of cMyBP-C in the setting of heart failure.

Activation and function of the MAPKs and their substrates, the MAPK-activated protein kinases

The mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs by relaying extracellular signals to intracellular responses. In mammals, there are more than a dozen MAPK enzymes that coordinately regulate cell proliferation, differentiation, motility, and survival. The best known are the conventional MAPKs, which include the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinases 1 to 3 (JNK1 to -3), p38 (α, β, γ, and δ), and ERK5 families. There are additional, atypical MAPK enzymes, including ERK3/4, ERK7/8, and Nemo-like kinase (NLK), which have distinct regulation and functions. Together, the MAPKs regulate a large number of substrates, including members of a family of protein Ser/Thr kinases termed MAPK-activated protein kinases (MAPKAPKs). The MAPKAPKs are related enzymes that respond to extracellular stimulation through direct MAPK-dependent activation loop phosphorylation and kinase activation. There are five MAPKAPK subfamilies: the p90 ribosomal S6 kinase (RSK), the mitogen- and stress-activated kinase (MSK), the MAPK-interacting kinase (MNK), the MAPK-activated protein kinase 2/3 (MK2/3), and MK5 (also known as p38-regulated/activated protein kinase [PRAK]). These enzymes have diverse biological functions, including regulation of nucleosome and gene expression, mRNA stability and translation, and cell proliferation and survival. Here we review the mechanisms of MAPKAPK activation by the different MAPKs and discuss their physiological roles based on established substrates and recent discoveries.

Activation and Function of the MAPKs and Their Substrates, the MAPK-Activated Protein Kinases

The mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs by relaying extracellular signals to intracellular responses. In mammals, there are more than a dozen MAPK enzymes that coordinately regulate cell proliferation, differentiation, motility, and survival. The best known are the conventional MAPKs, which include the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinases 1 to 3 (JNK1 to -3), p38 (α, β, γ, and δ), and ERK5 families. There are additional, atypical MAPK enzymes, including ERK3/4, ERK7/8, and Nemo-like kinase (NLK), which have distinct regulation and functions. Together, the MAPKs regulate a large number of substrates, including members of a family of protein Ser/Thr kinases termed MAPK-activated protein kinases (MAPKAPKs). The MAPKAPKs are related enzymes that respond to extracellular stimulation through direct MAPK-dependent activation loop phosphorylation and kinase activation. There are five MAPKAPK subfamilies: the p90 ribosomal S6 kinase (RSK), the mitogen- and stress-activated kinase (MSK), the MAPK-interacting kinase (MNK), the MAPK-activated protein kinase 2/3 (MK2/3), and MK5 (also known as p38-regulated/activated protein kinase [PRAK]). These enzymes have diverse biological functions, including regulation of nucleosome and gene expression, mRNA stability and translation, and cell proliferation and survival. Here we review the mechanisms of MAPKAPK activation by the different MAPKs and discuss their physiological roles based on established substrates and recent discoveries.

Novel role for p90 ribosomal S6 kinase in the regulation of cardiac myofilament phosphorylation

In myocardium, the 90-kDa ribosomal S6 kinase (RSK) is activated by diverse stimuli and regulates the sarcolemmal Na(+)/H(+) exchanger through direct phosphorylation. Only limited information is available on other cardiac RSK substrates and functions. We evaluated cardiac myosin-binding protein C (cMyBP-C), a sarcomeric regulatory phosphoprotein, as a potential RSK substrate. In rat ventricular myocytes, RSK activation by endothelin 1 (ET1) increased cMyBP-C phosphorylation at Ser(282), which was inhibited by the selective RSK inhibitor D1870. Neither ET1 nor D1870 affected the phosphorylation status of Ser(273) or Ser(302), cMyBP-C residues additionally targeted by cAMP-dependent protein kinase (PKA). Complementary genetic gain- and loss-of-function experiments, through the adenoviral expression of wild-type or kinase-inactive RSK isoforms, confirmed RSK-mediated phosphorylation of cMyBP-C at Ser(282). Kinase assays utilizing as substrate wild-type or mutated (S273A, S282A, S302A) recombinant cMyBP-C fragments revealed direct and selective Ser(282) phosphorylation by RSK. Immunolabeling with a Ser(P)(282) antibody and confocal fluorescence microscopy showed RSK-mediated phosphorylation of cMyBP-C across the C-zones of sarcomeric A-bands. In chemically permeabilized mouse ventricular muscles, active RSK again induced selective Ser(282) phosphorylation in cMyBP-C, accompanied by significant reduction in Ca(2+) sensitivity of force development and significant acceleration of cross-bridge cycle kinetics, independently of troponin I phosphorylation at Ser(22)/Ser(23). The magnitudes of these RSK-induced changes were comparable with those induced by PKA, which phosphorylated cMyBP-C additionally at Ser(273) and Ser(302). We conclude that Ser(282) in cMyBP-C is a novel cardiac RSK substrate and its selective phosphorylation appears to regulate cardiac myofilament function.

Structural and functional analysis of the Na+/H+ exchanger

The mammalian NHE (Na+/H+ exchanger) is a ubiquitously expressed integral membrane protein that regulates intracellular pH by removing a proton in exchange for an extracellular sodium ion. Of the nine known isoforms of the mammalian NHEs, the first isoform discovered (NHE1) is the most thoroughly characterized. NHE1 is involved in numerous physiological processes in mammals, including regulation of intracellular pH, cell-volume control, cytoskeletal organization, heart disease and cancer. NHE comprises two domains: an N-terminal membrane domain that functions to transport ions, and a C-terminal cytoplasmic regulatory domain that regulates the activity and mediates cytoskeletal interactions. Although the exact mechanism of transport by NHE1 remains elusive, recent studies have identified amino acid residues that are important for NHE function. In addition, progress has been made regarding the elucidation of the structure of NHEs. Specifically, the structure of a single TM (transmembrane) segment from NHE1 has been solved, and the high-resolution structure of the bacterial Na+/H+ antiporter NhaA has recently been elucidated. In this review we discuss what is known about both functional and structural aspects of NHE1. We relate the known structural data for NHE1 to the NhaA structure, where TM IV of NHE1 shows surprising structural similarity with TM IV of NhaA, despite little primary sequence similarity. Further experiments that will be required to fully understand the mechanism of transport and regulation of the NHE1 protein are discussed.

Angiotensin II acutely decreases myocardial stiffness: a novel AT1, PKC and Na+/H+ exchanger-mediated effect

Acute effects of angiotensin II (AngII) on diastolic properties of the myocardium were investigated. Increasing concentrations of AngII (10(-9) to 10(-5) M) were added to rabbit papillary muscles in the absence (n=11) or presence of: (i) AT1 receptor antagonists, losartan (10(-6) M; n=7) or ZD-7155 (10(-7) M; n=8); (ii) ZD-7155 (10(-7) M) plus AT2 receptor antagonist PD-123,319 (2 x 10(-6) M; n=6); (iii) PKC inhibitor, chelerythrine (10(-5) M; n=8); or (iv) Na(+)/H(+) exchanger (NHE) inhibitor, 5-(N-methyl-N-isobutyl)-amiloride (10(-6) M; n=10). Passive length-tension relations were constructed before and after a single concentration of AngII (10(-5) M, n=6). Effects of AngII infusion (10 microg kg(-1) min(-1)) were evaluated in in situ rabbit hearts. AngII concentration dependently increased inotropy and resting muscle length (RL). At 10(-5) M, active tension increased 43.3+/-6.25% and RL 1.96+/-0.4%. Correcting RL to its initial value resulted in a 46+/-4% decrease of resting tension, indicating decreased muscle stiffness, as confirmed by the right and downward shift of the passive length-tension relation promoted by AngII. In the intact heart, at matched systolic pressures of 112 mmHg, AngII decreased end-diastolic pressures from 10.3+/-0.3 to 5.9+/-0.5 mmHg, and minimal diastolic pressures from 8.4+/-0.5 to 4.6+/-0.6 mmHg. AT1 blockade inhibited AngII effects on myocardial inotropy and stiffness, while PKC or NHE inhibition only significantly attenuated its effects on resting length and tension. In conclusion, AngII decreases myocardial stiffness, an effect that requires AT1 receptor activation and is mediated by PKC and NHE. This represents a novel mechanism of acute neurohumoral modulation of diastolic function, suggesting that AngII is a powerful regulator of cardiac filling.

Effects of bisindolylmaleimide PKC inhibitors on p90RSK activity in vitro and in adult ventricular myocytes

1 Bisindolylmaleimide inhibitors of protein kinase C (PKC), such as GF109203X and Ro31-8220, have been used to investigate the roles of PKC isoforms in many cellular processes in cardiac myocytes, but these agents may also inhibit p90RSK activity. 2 In in vitro kinase assays utilising 50 microM [ATP], GF109203X and Ro31-8220 inhibited p90RSK isoforms (IC50 values for inhibition of RSK1, RSK2 and RSK3, respectively, were 610, 310 and 120 nM for GF109203X, and 200, 36 and 5 nM for Ro31-8220) as well as classical and novel PKC isoforms (IC50 values for inhibition of PKCalpha and PKCepsilon, respectively, were 8 and 12 nM for GF109203X, and 4 and 8 nM for Ro31-8220). 3 At physiological [ATP] (5 mM), both GF109203X and Ro31-8220 exhibited reduced potency as inhibitors of RSK2, PKCalpha and PKCepsilon (IC50 values of 7400, 310 and 170 nM, respectively, for GF109203X, and 930, 150 and 140 nM, respectively, for Ro31-8220), with the latter agent retaining its relatively greater potency. 4 To determine the effects of GF109203X and Ro31-8220 on p90RSK activity in cultured adult rat ventricular myocytes (ARVM), phosphorylation of the eukaryotic elongation factor 2 kinase (eEF2K) at Ser366, a known p90RSK target, was used as the index of such activity. Adenoviral expression of a constitutively active form of mitogen-activated protein kinase (MAPK) or extracellular signal-regulated kinase (ERK) kinase 1 (MEK1) was used to induce PKC-independent p90RSK activation and downstream phosphorylation of eEF2K. 5 eEF2K phosphorylation was abolished by U0126 (1 microM), a selective inhibitor of MEK1, and was significantly reduced by GF109203X at > or =3 microM and by Ro31-8220 at > or =1 microM. At 1 microM, both agents inhibited PMA-induced PKC activity in ARVM. 6 These data show that GF109203X and Ro31-8220 inhibit various isoforms of PKC and p90RSK in vitro and in intact ARVM, with the former agent exhibiting relatively greater selectivity for PKC.

Two G protein-coupled receptors activate Na+/H+ exchanger isoform 1 in Chinese hamster lung fibroblasts through an ERK-dependent pathway

The sodium hydrogen exchanger isoform 1 (NHE1) is present in nearly all cells. Regulation of proton flux via the exchanger is a permissive step in cell growth and tumorgenesis and is vital in control of cell volume. The regulation of NHE1 by growth factors involves the Ras-extracellular signal regulated kinase (ERK) pathway, however, the mechanism for G protein-coupled receptor (GPCR) activation of NHE1 is not well established. In this report, the relationship between GPCRs, ERK, and NHE1 in CCL39 cells is investigated. We give evidence that two agonists, the specific alpha(1)-adrenergic agonist, phenylephrine and the water-soluble lipid mitogen, lysophosphatidic acid (LPA) activate NHE1 in CCL39 cells. Activation of ERK by phenylephrine and LPA occurs in a dose- and time-dependent manner. Optimal ERK activation was observed at 10 min and displayed a maximum stimulation at 100 microM phenylephrine and 10 microM LPA. alpha(1)-Adrenergic stimulation also led to a rise in steady-state pH(i) of 0.16+/-0.02 pH units, and incubation with LPA induced a 0.43+/-0.06 pH unit increase in pH(i). Phenylephrine-induced activation of NHE1 transport and ERK activity was inhibited by pretreating the cells with the MEK inhibitor PD98059. While only half of the LPA activatable exchange activity was abolished by PD98059 and U0126. To further demonstrate the specificity of the phenylephrine and LPA regulation of NHE1 and ERK, CCL39 cells were transfected with a kinase inactive MEK. The data indicate that ERK activation is essential for phenylephrine stimulation of NHE1, and that ERK and RhoA are involved in LPA stimulation of NHE1 by more than one mechanism. In addition, evidence of the convergence of these two pathways is shown by the loss of NHE1 activity when both pathways are inhibited and by the partial additivity of the two agonists on ERK and NHE1 activity. These studies indicate a direct involvement of ERK in the alpha(1)-adrenergic activation of NHE1 and a significant role for both ERK and RhoA in LPA stimulation of NHE1 in CCL39 fibroblasts.

Extracellular receptor kinase and cAMP response element binding protein activation in the neonatal rat heart after perinatal cocaine exposure

Prenatal exposure to cocaine has been shown to induce an increase in the myocardial expression and activation of the cAMP response binding protein (CREB), a transcriptional factor that has been shown to regulate gene expression. Several different kinases, including protein kinase A, calcium calmodulin kinase II, and mitogen-activated protein kinase can induce phosphorylation of CREB at serine 133, a necessary step for CREB activation. We examined whether the mitogen-activated protein kinase-extracellular receptor kinase (ERK) pathway may be involved in mediating the serine 133 CREB phosphorylation in cardiac nuclei after perinatal cocaine exposure. Pregnant rats were treated daily with saline or cocaine at 60 mg/kg (C60) by intragastric administration during the entire gestational period, and treatment was continued in the nursing dams after delivery until the time of the study. Nuclear extracts were isolated from hearts of 1-d- and 7-d-old neonatal rats. We performed immunoblotting experiments using an antibody that recognized CREB with phosphorylation specifically at the serine 133 site and an antibody that recognized both the phosphorylated and the unphosphorylated forms of CREB, as well as antibodies for total ERK, phospho-ERK, total ribosomal S6 kinase 1 (RSK1), RSK2, and phospho-RSK. We assessed the interaction of RSK with CREB or CREB-binding protein by performing co-immunoprecipitation experiments. We found that perinatal cocaine exposure increased both phospho-ERK and phospho-RSK expression, indicative of an increased activity of these two enzymes. Furthermore, we demonstrated that phospho-RSK was immunoprecipitated with CREB in all neonatal cardiac nuclei and that the greatest interaction was found in day 7 hearts after perinatal cocaine exposure. Our results thus illustrate that the ERK-RSK pathway was active in the postnatal rat heart at 1 and 7 d of age and that this pathway may mediate the increase in myocardial CREB activation after perinatal cocaine exposure in the day 7 hearts.

Jak2 and Ca2+/calmodulin are key intermediates for bradykinin B2 receptor-mediated activation of Na+/H+ exchange in KNRK and CHO cells

Na(+)/H(+) exchangers are ubiquitous in mammalian cells, carrying out key functions, such as cell volume defense, acid-base homeostasis, and regulation of the cytoskeleton. We used two screening technologies (FLIPR and microphysiometry) to characterize the signal transduction pathway used by the bradykinin B(2) receptor to activate Na(+)/H(+) exchange in two cell lines, KNRK and CHO. In both cell types, B(2) receptor activation resulted in rapid increases in the rate of proton extrusion that were sodium-dependent and could be blocked by the Na(+)/H(+) exchange inhibitors EIPA and MIA or by replacing extracellular sodium with TMA. Activation of Na(+)/H(+) exchange by bradykinin was concentration-dependent and could be blocked by the selective B(2) receptor antagonist HOE140, but not by the B(1) receptor antagonist des-Arg10-HOE140. Inhibitors of Jak2 tyrosine kinase (genistein and AG490) and of CAM (W-7 and calmidazolium) attenuated bradykinin-induced activation of Na(+)/H(+) exchange. Bradykinin induced formation of a complex between CAM and Jak2, supporting a regulatory role for Jak2 and CAM in the activation of Na(+)/H(+) exchange in KNRK and CHO cells. We propose that this pathway (B(2) receptor --> Jak2 --> CAM --> Na(+)/H(+) exchanger) is a fundamental regulator of Na(+)/H(+) exchange activity.

ERK and p38 MAPK-activated protein kinases: a family of protein kinases with diverse biological functions

Conserved signaling pathways that activate the mitogen-activated protein kinases (MAPKs) are involved in relaying extracellular stimulations to intracellular responses. The MAPKs coordinately regulate cell proliferation, differentiation, motility, and survival, which are functions also known to be mediated by members of a growing family of MAPK-activated protein kinases (MKs; formerly known as MAPKAP kinases). The MKs are related serine/threonine kinases that respond to mitogenic and stress stimuli through proline-directed phosphorylation and activation of the kinase domain by extracellular signal-regulated kinases 1 and 2 and p38 MAPKs. There are currently 11 vertebrate MKs in five subfamilies based on primary sequence homology: the ribosomal S6 kinases, the mitogen- and stress-activated kinases, the MAPK-interacting kinases, MAPK-activated protein kinases 2 and 3, and MK5. In the last 5 years, several MK substrates have been identified, which has helped tremendously to identify the biological role of the members of this family. Together with data from the study of MK-knockout mice, the identities of the MK substrates indicate that they play important roles in diverse biological processes, including mRNA translation, cell proliferation and survival, and the nuclear genomic response to mitogens and cellular stresses. In this article, we review the existing data on the MKs and discuss their physiological functions based on recent discoveries.

ERK Is Regulated by Sodium-Proton Exchanger in Rat Aortic Vascular Smooth Muscle Cells

The purposes of this study were to test 1) the relationship between two widely studied mitogenic effector pathways, and 2) the hypothesis that sodium-proton exchanger type 1 (NHE-1) is a regulator of extracellular signal-regulated protein kinase (ERK) activation in rat aortic smooth muscle (RASM) cells. Angiotensin II (Ang II) and 5-hydroxytryptamine (5-HT) stimulated both ERK and NHE-1 activities, with activation of NHE-1 preceding that of ERK. The concentration-response curves for 5-HT and Ang II were superimposable for both processes. Inhibition of NHE-1 with pharmacological agents or by isotonic replacement of sodium in the perfusate with choline or tetramethylammonium greatly attenuated ERK activation by 5-HT or Ang II. Similar maneuvers significantly attenuated 5-HT- or Ang II-mediated activation of MEK and Ras but not transphosphorylation of the epidermal growth factor (EGF) receptor. EGF receptor blockade attenuated ERK activation, but not NHE-1 activation by 5-HT and Ang II, suggesting that the EGF receptor and NHE-1 work in parallel to stimulate ERK activity in RASM cells, converging distal to the EGF receptor but at or above the level of Ras in the Ras-MEK-ERK pathway. Receptor-independent activation of NHE-1 by acute acid loading of RASM cells resulted in the rapid phosphorylation of ERK, which could be blocked by pharmacological inhibitors of NHE-1 or by isotonic replacement of sodium, closely linking the proton transport function of NHE-1 to ERK activation. These studies identify NHE as a new regulator of ERK activity in RASM cells.

Pyk2- and Src-dependent tyrosine phosphorylation of PDK1 regulates focal adhesions

3-Phosphoinositide-dependent protein kinase 1 (PDK1) is a signal integrator that activates the AGC superfamily of serine/threonine kinases. PDK1 is phosphorylated on tyrosine by oxidants, although its regulation by agonists that stimulate G-protein-coupled receptor signaling pathways and the physiological consequences of tyrosine phosphorylation in this setting have not been fully identified. We found that angiotensin II stimulates the tyrosine phosphorylation of PDK1 in vascular smooth muscle in a calcium- and c-Src-dependent manner. The calcium-activated tyrosine kinase Pyk2 acts as a scaffold for Src-dependent phosphorylation of PDK1 on Tyr9, which permits phosphorylation of Tyr373 and -376 by Src. This critical function of Pyk2 is further supported by the observation that Pyk2 and tyrosine-phosphorylated PDK1 colocalize in focal adhesions after angiotensin II stimulation. Importantly, infection of smooth muscle cells with a Tyr9 mutant of PDK1 inhibits angiotensin II-induced tyrosine phosphorylation of paxillin and focal adhesion formation. These observations identify a novel interaction between PDK1 and Pyk2 that regulates the integrity of focal adhesions, which are major compartments for integrating signals for cell growth, apoptosis, and migration.

Nuclear translocation of p90Rsk and phosphorylation of CREB is induced by ionomycin in a Ras-independent manner in PC12 cells

In the present study we examined the possible role of p90Rsk in pathways leading to neuronal differentiation of PC12 cells induced by nerve growth factor (NGF) and the calcium ionophore ionomycin. PC12-M17 cells, expressing a dominant inhibitory Ras protein, do not undergo neuronal differentiation in response to NGF like wild-type PC12 cells, but exhibit neurite outgrowth when treated with NGF in combination with ionomycin. However, the blockade of Ras in these cells results in failure of activation of mitogen-activated protein kinase (MAPK)/extracellular signal regulation kinase (ERK) (MEK) and ERK activation as well, therefore kinases other than those of the ERK pathway might play a role in the induction of neuronal differentiation in this case. Here we show that p90Rsk translocates to the nucleus in response to ionomycin in both wild-type PC12 and PC12-M17 cells, and this spatial distribution is followed by increased phosphorylation of the cAMP response element binding protein (CREB). Since CREB is believed to be the transcription factor that can integrate Ca2+, growth factor and cAMP-induced signals, we suggest that p90Rsk may be one of the kinases which is able to replace ERKs under certain circumstances, thereby participating in Ras-independent neuronal differentiation induced by NGF plus ionomycin.

Involvement of p44/42 mitogen-activated protein kinases in regulating angiotensin II- and endothelin-1-induced contraction of rat thoracic aorta

In order to elucidate the signal transduction pathway of vascular smooth muscle contraction induced by the activation of receptors for angiotensin II and endothelin-1, we examined whether tyrosine kinases and mitogen-activated protein (MAP) kinases are involved in the development of force of contraction in the rat aorta. Isolated aortic smooth muscles without endothelium were incubated in a modified Krebs-Henseleit solution and stimulated with angiotensin II (100 nM) or endothelin-1 (10 nM). A tyrosine kinase inhibitor genistein (10 microM) reduced the angiotensin II- and endothelin-1-induced aortic contraction, while 10 microM of daidzein (an inactive analogue of genistein) did not. The K(+) depolarization-induced contraction was not attenuated by 10 microM of genistein. Selective inhibitors of MAP kinase/extracellular signal-regulated kinase (Erk) kinase (MEK) such as PD98059 [2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one] and U0126 [1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene] inhibited the angiotensin II- and endothelin-1-induced vasocontraction. The p44/42 MAP kinases were phosphorylated in cultured aortic smooth muscle cells and in physiologically contracted aortic vessels stimulated with angiotensin II and endothelin-1 for 5 min. The angiotensin II- and endothelin-1-induced phosphorylations of p44/42 MAP kinases were inhibited by PD98059 as well as U0126 in the intact aorta. These results suggest that the activation of genistein-sensitive tyrosine kinases and p44/42 MAP kinases is involved in the angiotensin II- and endothelin-1-induced rat aortic contraction.

Critical role of cAMP-response element-binding protein for angiotensin II-induced hypertrophy of vascular smooth muscle cells

We reported previously an important role of cyclic AMP-response element (CRE) for the induction of interleukin-6 gene expression by angiotensin II (AngII). We examined signaling pathways that are responsible for AngII-induced phosphorylation of CRE-binding protein (CREB) at serine 133 that is a critical marker for the activation in rat vascular smooth muscle cells (VSMC). AngII time dependently induced phosphorylation of CREB with a peak at 5 min. The AngII-induced phosphorylation of CREB was blocked by CV11974, an AngII type I receptor antagonist, suggesting that AngII type I receptor may mediate the phosphorylation of CREB. Inhibition of extracellular signal-regulated protein kinase (ERK) by PD98059 or inhibition of p38 mitogen-activated protein kinase (MAPK) by SB203580 partially inhibited AngII-induced CREB phosphorylation. A protein kinase A inhibitor, H89, also partially suppressed AngII-induced CREB phosphorylation. Inhibition of epidermal growth factor-receptor by AG1478 suppressed the AngII-induced CREB phosphorylation as well as activation of ERK and p38MAPK. Overexpression of the dominant negative form of CREB by an adenovirus vector suppressed AngII-induced c-fos expression and incorporation of [(3)H]leucine to VSMC. These findings suggest that AngII may activate multiple signaling pathways involving two MAPK pathways and protein kinase A, all of which contribute to the activation of CREB. Transactivation of epidermal growth factor-receptor is also critical for AngII-induced CREB phosphorylation. Activation of CREB may be important for the regulation of gene expression and hypertrophy of VSMC induced by AngII.

Src family kinase and adenosine differentially regulate multiple MAP kinases in ischemic myocardium: modulation of MAP kinases activation by ischemic preconditioning

Recent studies suggest that ischemia activates Src and members of the mitogen-activated protein (MAP) kinase superfamily and their downstream effectors, including big MAP kinase 1 (BMK1) and p90 ribosomal S6 kinase (p90RSK). It has also been reported that adenosine is released during ischemia and involved in triggering the protective mechanism of ischemic preconditioning. To assess the roles of Src and adenosine in ischemia-induced MAP kinases activation, we utilized the Src inhibitor PP2 (4-Amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine) and the adenosine receptor antagonist 8-(p-sulfophenyl) theophylline (SPT) in perfused guinea pig hearts. PP2 (1 microm) inhibited ischemia-induced Src, BMK1 and JNK activation but not JAK2 and p38 activation. SPT inhibited ischemia-mediated p38 and JNK activation. These results demonstrate that Src family kinase and adenosine regulate MAP kinases by parallel pathways. Preconditioning significantly improved both recovery of developed pressure and dp/dt in isolated guinea pig hearts. Since the protective effect of preconditioning was blocked by PP2 (1 microm) and SPT (50 microm), we next investigated the regulation of Src, MAP kinases and p90RSK during preconditioning. The activity and time course of ERK1/2 was not changed, but p90RSK activation by reperfusion was completely inhibited by preconditioning. In contrast, the activation by ischemia of Src, BMK1, p38 and JNK was significantly faster in preconditioned hearts. Maximal BMK1 activation by ischemia was also significantly enhanced by preconditioning. These data suggest important roles for Src family kinases and adenosine in mediating preconditioning, and suggest specific roles for individual MAP kinases in preconditioning.

Src and multiple MAP kinase activation in cardiac hypertrophy and congestive heart failure under chronic pressure-overload: comparison with acute mechanical stretch

Activation of members of the mitogen-activated protein (MAP) kinase family and their downstream effectors has been proposed to play a key role in the pathogenesis of cell survival, ischaemic preconditioning, cardiac hypertrophy and heart failure. This study investigated the responses of Src kinase and multiple MAP kinases during the transition from compensated pressure-overload hypertrophy to decompensated congestive heart failure. Extracellular signal-regulated protein kinase (ERK) 1/2, p38, and Src were activated by chronic pressure-overload and their activity was sustained for 8 weeks after aortic banding. In contrast, while p90 ribosomal S6 kinase (90RSK) and big MAP kinase 1 (BMK1) were activated in compensated hypertrophy, their activities were significantly decreased in hearts with heart failure. No changes were found in C-Jun NH2 terminal kinase (JNK) activity after aortic banding. These data suggest that differential activation of MAP kinase family members may contribute to the transition from compensated to decompensated hypertrophy. We also examined acute effects of mechanical stretch on the activation of these kinases in normal and hypertrophied hearts. In the isolated coronary-perfused heart, a balloon in the left ventricle was inflated to achieve minimum end-diastolic pressure of 25 mmHg for 10-20 min. In normal guinea pig hearts, stretch activated ERK1/2, p90RSK, p38, Src, and BMK1 but not JNK. However in hypertrophied hearts, further activation of these kinases was not observed by acute mechanical stretch. Mechanical stretch-induced activation of ERK1/2 and p38 kinase in normal hearts was attenuated significantly by a protein kinase C inhibitor, chelerythrine. We demonstrate that ERK1/2, p90RSK, p38, Src, and BMK1 are activated by chronic pressure-overload and by acute mechanical stretch. These data suggest that Src, BMK1 and p90RSK play a role as novel signal transduction pathways leading to cardiac hypertrophy. In addition, the differential inhibition of p90RSK and BMK1 in hearts with congestive heart failure suggests the specific role of these two kinases to maintain cardiac function under chronic pressure-overload.

14-3-3 Binding to Na+/H+ exchanger isoform-1 is associated with serum-dependent activation of Na+/H+ exchange

Na(+)/H(+) exchanger isoform-1 (NHE1), the ubiquitous form of the Na(+)/H(+) exchanger, has increased activity in hypertensive patients and in animal models of hypertension. Furthermore, NHE1 is activated in cells stimulated with growth factors. We showed previously that activation of the exchanger is dependent on phosphorylation of serine 703 (Ser(P)(703)) by p90 ribosomal S6 kinase (RSK). Because the NHE1 sequence at Ser(P)(703) (RIGSDP) is similar to a consensus sequence (RSXSXP) specific for 14-3-3 ligands, we evaluated whether serum stimulated 14-3-3 binding to NHE1. Five different GST-NHE1 fusion proteins spanning amino acids 515-815 were phosphorylated by RSK and used as ligands in a far Western analysis; only those containing Ser(P)(703) exhibited high affinity 14-3-3 binding. In PS127A cells (NHE1-overexpressing Chinese hamster fibroblasts) stimulated with 20% serum, NHE1 co-precipitation with GST-14-3-3 fusion protein increased at 5 min (5.2 +/- 0.4-fold versus control; p < 0.01) and persisted at 40 min (3.9 +/- 0.3-fold; p < 0.01). We confirmed that binding occurs at the RIGSDP motif using PS120 (NHE1 null) cells transfected with S703A-NHE1 or P705A-NHE1 (based on data indicating that 14-3-3 binding requires phosphoserine and +2 proline). Serum failed to stimulate association of 14-3-3 with these mutants. A GST-NHE1 fusion protein was phosphorylated by RSK and used as a ligand to assess the effect of 14-3-3 on protein phosphatase 1-mediated dephosphorylation of Ser(P)(703). GST-14-3-3 limited dephosphorylation (66% of initial state at 60 min) compared with GST alone (27% of initial state; p < 0.01). The protective effect of GST-14-3-3 was lost in the GST-NHE1 P705A mutant. Finally, the base-line rate of pH recovery in acid-loaded cells was equal in unstimulated cells expressing wild-type or P705A-NHE1. However, activation of NHE1 by serum was dramatically inhibited in cells expressing P705A-NHE1 compared with wild-type (0.13 +/- 0.02 versus 0.48 +/- 0.06 mmol of H(+)/min/liter, p < 0.01). These data suggest that 14-3-3 binding to NHE1 participates in serum-stimulated exchanger activation, a new function for 14-3-3.

The molecular basis of hypertension

More than 50 million Americans display blood pressures outside the safe physiological range. Unfortunately for most individuals, the molecular basis of hypertension is unknown, in part because pathological elevations of blood pressure are the result of abnormal expression of multiple genes. This review identifies a number of important blood pressure regulatory genes including their loci in the human, mouse, and rat genome. Phenotypes of gene deletions and overexpression in mice are summarized. More detailed discussion of selected gene products follows, beginning with proteins involved in ion transport, specifically the epithelial sodium channel and sodium proton exchangers. Next, proteins involved in vasodilation/natriuresis are discussed with emphasis on natriuretic peptides, guanylin/uroguanylin, and nitric oxide. The renin angiotensin aldosterone system has an important role antagonizing the vasodilatory cyclic GMP system.

Reactive oxygen species activate p90 ribosomal S6 kinase via Fyn and Ras

Reactive oxygen species and growth factors stimulate similar intracellular signal transduction events including activation of Src kinase family members and extracellular signal-regulated kinases (ERK1/2). A potentially important downstream effector of Src and ERK1/2 is p90 ribosomal S6 kinase (p90RSK), which plays an important role in cell growth by activating several transcription factors as well as the Na(+)/H(+) exchanger. In the present study, we determined whether H(2)O(2) activates p90RSK to gain insight into signal transduction mechanisms activated by reactive oxygen species. H(2)O(2) (200 microM) stimulated ERK1/2 and p90RSK activity in lymphocytes, endothelial cells, and fibroblasts. The MEK-1 inhibitor, PD98059 (30 microM), inhibited H(2)O(2)-mediated activation of ERK1/2 but not of p90RSK. An essential role for Fyn and Ras in p90RSK activation was suggested by five findings. 1) The tyrosine kinase inhibitor, herbimycin A, and the specific Src kinase family inhibitor, PP1, blocked p90RSK activation by H(2)O(2) in a concentration-dependent manner. 2) p90RSK activation by H(2)O(2) was significantly reduced in fibroblasts derived from transgenic mice deficient in Fyn, but not c-Src. 3) H(2)O(2) rapidly activated Ras (peak at 2-5 min), which preceded p90RSK activation (peak at 20 min). 4) Dominant negative Ras completely blocked H(2)O(2)-induced activation of p90RSK. 5) In Fyn-/- fibroblasts, activation of Ras by H(2)O(2) was significantly attenuated. These results show essential roles for Fyn and Ras in H(2)O(2)-mediated activation of p90RSK and establish redox-sensitive regulation of Ras and p90RSK as a new function for Fyn.

NRG-1-induced cardiomyocyte hypertrophy. Role of PI-3-kinase, p70(S6K), and MEK-MAPK-RSK

Neuregulins are a family of growth-promoting peptides known to be important in neural and mesenchymal tissue development. Targeted disruption of neuregulin (NRG)-1 or one of two of its cognate receptors, ErbB2 or ErbB4, results in embryonic lethality because of failure of the heart to develop. Although expression of NRGs and their receptors declines after midembryogenesis, both ErbB2 and ErbB4 are present in cardiac myocytes, and NRG-1 expression remains inducible in primary cultures of coronary microvascular endothelial cells from adult rat ventricular muscle. In neonatal rat ventricular myocytes, a soluble NRG-1, recombinant human glial growth factor-2, increased [(3)H]phenylalanine uptake and induced expression of atrial natriuretic factor (ANF) and sarcomeric F-actin polymerization. The effect of NRG-1 on [(3)H]phenylalanine uptake and sarcomeric F-actin polymerization was maximal at 20 ng/ml but declined at higher concentrations. NRG-1 activated p42/p44 mitogen-activated protein kinase (MAPK) [extracellular signal-regulated kinase (ERK)-2/ERK1] and ribosomal S6 kinase (RSK)-2 (90-kDa ribosomal S6 kinase), both of which could be inhibited by the MAPK/ERK kinase-1 antagonist PD-098059. NRG-1 also activated 70-kDa ribosomal S6 kinase, which was inhibited by either rapamycin or wortmannin. Activation of these pathways exhibited the same "biphasic" response to increasing NRG-1 concentrations. Wortmannin and LY-294002 blocked sarcomeric F-actin polymerization but not [(3)H]phenylalanine uptake or ANF expression, whereas PD-098059 consistently blocked both [(3)H]phenylalanine uptake and ANF expression but not actin polymerization. In contrast, rapamycin inhibited [(3)H]phenylalanine uptake and F-actin polymerization but not ANF expression. Thus NRG-ErbB signaling triggers multiple nonredundant pathways in postnatal ventricular myocytes.

Angiotensin II type 2 receptor overexpression activates the vascular kinin system and causes vasodilation

Angiotensin II (Ang II) is a potent vasopressor peptide that interacts with 2 major receptor isoforms - AT1 and AT2. Although blood pressure is increased in AT2 knockout mice, the underlying mechanisms remain undefined because of the low levels of expression of AT2 in the vasculature. Here we overexpressed AT2 in vascular smooth muscle (VSM) cells in transgenic (TG) mice. Aortic AT1 was not affected by overexpression of AT2. Chronic infusion of Ang II into AT2-TG mice completely abolished the AT1-mediated pressor effect, which was blocked by inhibitors of bradykinin type 2 receptor (icatibant) and nitric oxide (NO) synthase (L-NAME). Aortic explants from TG mice showed greatly increased cGMP production and diminished Ang II-induced vascular constriction. Removal of endothelium or treatment with icatibant and L-NAME abolished these AT2-mediated effects. AT2 blocked the amiloride-sensitive Na(+)/H(+) exchanger, promoting intracellular acidosis in VSM cells and activating kininogenases. The resulting enhancement of aortic kinin formation in TG mice was not affected by removal of endothelium. Our results suggest that AT2 in aortic VSM cells stimulates the production of bradykinin, which stimulates the NO/cGMP system in a paracrine manner to promote vasodilation. Selective stimulation of AT2 in the presence of AT1 antagonists is predicted to have a beneficial clinical effect in controlling blood pressure.

Protein kinase-mediated regulation of the Na(+)/H(+) exchanger in the rat myocardium by mitogen-activated protein kinase-dependent pathways

We examined regulation of the Na(+)/H(+) exchanger isoform 1 by phosphorylation in the rat myocardium. We utilized cell extracts from adult rat hearts, adult rat extracts fractionated by fast performance liquid chromatography, and extracts from cultured neonatal cardiac myocytes. The carboxyl-terminal 178 amino acids of the Na(+)/H(+) exchanger were expressed in Escherichia coli fused with glutathione S-transferase. The purified protein was used as a substrate for in vitro phosphorylation and in-gel kinase assays. Unfractionated extracts from neonatal myocytes or adult hearts phosphorylated the COOH-terminal domain of the antiporter. Western blot analysis revealed that mitogen-activated protein (MAP) kinase (44 and 42 kDa) and p90(rsk) (90 kDa) were present in specific fractions of cardiac extracts that phosphorylated the COOH-terminal protein. In-gel kinase assays confirmed that protein kinases of approximately 44 and 90 kDa could phosphorylate this domain. MAP kinase and p90(rsk)-dependent phosphorylation of the antiporter could be demonstrated by immunoprecipitation of these kinases from extracts of neonatal cardiac myocytes. PD98059, a mitogen-activated protein kinase kinase inhibitor, decreased MAP kinase and p90(rsk) phosphorylation of the antiporter and abolished serum and endothelin 1-stimulated increases in steady-state pH(i). These results confirm the presence of MAP kinase-dependent phosphorylation in the regulation of the Na(+)/H(+) exchanger in the rat myocardium and suggest an important role for p90(rsk) phosphorylation in regulation of the protein by endothelin-mediated stimulation of the antiporter.

p90(RSK) is a serum-stimulated Na+/H+ exchanger isoform-1 kinase. Regulatory phosphorylation of serine 703 of Na+/H+ exchanger isoform-1

The Na+/H+ exchanger isoform-1 (NHE-1) is the key member of a family of exchangers that regulates intracellular pH and cell volume. Activation of NHE-1 by growth factors is rapid, correlates with increased NHE-1 phosphorylation and cell alkalinization, and plays a role in cell cycle progression. By two-dimensional tryptic peptide mapping of immunoprecipitated NHE-1, we identify serine 703 as the major serum-stimulated amino acid. Mutation of serine 703 to alanine had no effect on acid-stimulated Na+/H+ exchange but completely prevented the growth factor-mediated increase in NHE-1 affinity for H+. In addition, we show that p90 ribosomal S6 kinase (p90(RSK)) is a key NHE-1 kinase since p90(RSK) phosphorylates NHE-1 serine 703 stoichiometrically in vitro, and transfection with kinase-inactive p90(RSK) inhibits serum-induced phosphorylation of NHE-1 serine 703 in transfected 293 cells. These findings establish p90(RSK) as a serum-stimulated NHE-1 kinase and a mediator of increased Na+/H+ exchange in vivo.

Functional and cellular regulation of the myocardial Na+/H+ exchanger

The Na(+)/H(+) exchanger is a pH-regulatory protein present in the plasma membrane of cardiomyocytes and other cell types. In response to intracellular acidosis, the protein removes one intracellular proton in exchange for an extracellular sodium. The protein consists of a membrane transporting domain and a regulatory cytosolic domain. The regulatory cytosolic domain mediates the stimulation of the membrane domain. Hormonal stimulation of myocardial cells results in activation of the antiporter, possibly through protein kinases and other regulatory proteins. Several hormones and growth factors have been shown to stimulate the antiporter in the myocardium, including endothelin, thrombin, angiotensin II, and alpha(1)-adrenergic stimulation. The exact mechanisms involved in this stimulation are as yet unclear, and may be important in regulation of the Na(+)/H(+) exchanger during ischemia and reperfusion.

Sodium-hydrogen exchange and platelet function

On stimulation of platelets with agonists, for example, thrombin, a rapid rise in intracellular pH is observed. This alkalinization is mediated by an increase in transport activity of the Na(+)/H(+) exchanger isoform NHE1. In addition to this Na(+)/H(+) exchange mechanism, platelets express bicarbonate/chloride exchangers, which also contribute to pH(i) homeostasis. The main functions of NHE1 in platelets include pH(i) control, volume regulation, and participation in cell signaling. The isoform NHE1 is highly sensitive toward inhibition by EIPA, Hoe694, and Hoe642. The regulation of NHE1 activity is complex and is not completely understood. It includes the MAP kinase cascade, the Ca/calmodulin system, several heterotrimeric G proteins (Galpha12, Galpha13, Galphaq, and Galphai), small G proteins (ras, cdc42, rhoA), and downstream kinases (e.g., p160ROCK). Volume challenges stimulate tyrosine phosphorylation of cytoplasmic proteins, which ultimately activate NHE1. Thrombin, thromboxane, platelet-activating factor, angiotensin II, endothelin, phorbol ester, and Ca(2+) ionophors stimulate NHE1 activity in platelets. Blockade of platelet NHE1 can inhibit platelet activation. With the development of highly specific NHE1 inhibitors, detailed investigation of the relationships between NHE1 activity and platelet activation now becomes feasible.

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.

Mechanisms of pH preservation during global ischemia in preconditioned rat heart: roles for PKC and NHE

Ischemic preconditioning (PC) attenuates cardiac acidosis during global ischemia. This adaptation to ischemia is detectable before other better known indexes of PC are manifested. Clarification of the endogenous mechanisms may provide insights into how protein kinase C (PKC) signaling might be linked to altered intracellular biochemistry. 31P NMR studies of isolated, buffer-perfused rat heart were performed to determine whether functionally cardioprotective PC by cyclic ischemia (CI) and alpha1-adrenergic stimuli [phenylephrine (PE)] attenuated acidosis during ischemia and, if so, whether this 1) involves a PKC-dependent pathway and is due to 2) decreased glycolytic proton production, 3) an increase in proton buffering, or 4) proton extrusion. At the end of 20 min of global ischemia, both CI-PC (pH = 6.86 +/- 0.14) and PE-PC (pH = 6.90 +/- 0.13) attenuated end-ischemic acidosis (control pH = 6.54 +/- 0.1). PKC blockade with chelerythrine (Chel) prevented the attenuation of ischemic acidosis by PC stimuli (end-ischemic pH: CI + Chel, 6.43 +/- 0.06; PE + Chel, 6.17 +/- 0.17). End-ischemic lactate accumulation was decreased in CI-PC hearts (7.54 +/- 0.5 vs. control, 14.61 +/- 2.1 micromol/g wet wt) but not in those preconditioned through the alpha1-adrenergic receptor (12.25 +/- 0.9 micromol/g wet wt). Physiologically relevant buffers were not increased in the preconditioned groups. Blockade of the Na+/H+ exchanger [NHE; with 5-(N-ethyl-N-isopropyl) amiloride (EIPA) or HOE-694] eliminated the attenuation of ischemic acidosis seen with PC stimuli (pH: CI + EIPA, 6.5 +/- 0.1; PE + EIPA, 6.46 +/- 0.2; PE + HOE-694, 6.26 +/- 0.15; not significantly different from control). We conclude that CI and alpha1-adrenergic PC stimuli attenuate ischemic acidosis, and this may involve the cardiac amiloride-sensitive NHE. The signaling pathways of both these two stimuli appear to involve PKC.