Domain mapping studies reveal that the M domain of hsp90 serves as a molecular scaffold to regulate Akt-dependent phosphorylation of endothelial nitric oxide synthase and NO release

Heat-induced increases in endothelial NO synthase expression and activity and endothelial NO release

Endothelial nitric oxide (NO) synthase (eNOS) is regulated by heat shock protein 90 (HSP90), a heat-inducible protein; however, the effect of heat shock on eNOS expression and eNO release is unknown. Bovine aortic endothelial cells were incubated for 1 h at 37 degrees C, 42 degrees C, or 45 degrees C and cell lysates were evaluated with the use of Western blotting. We observed a 2.1 +/- 0.1-fold increase in eNOS protein content, but no change in HSP90 content, HSP70 content, or HSP90/eNOS association, 24 h after heat shock at 42 degrees C. We also observed a 7.7 +/- 1.5-fold increase in HSP70 protein content, but did not observe a change in eNOS or HSP90 24 h after heat shock at 45 degrees C. eNOS activity and maximal bradykinin-stimulated NO release was significantly increased 24 h after heat shock at 42 degrees C. Heat shock in rats (core temperature: 42 degrees C, 15 min) resulted in a significant increase in aortic eNOS, HSP90, and HSP70 protein content. The aorta from heat-shocked rats exhibited a decreased maximal contractile response to phenylephrine, which was abolished by preincubation with NG-nitro-l-arginine. We conclude that prior heat shock is a physical stimulus of increased eNOS expression and is associated with an increase in eNOS activity, agonist-stimulated NO release, and a decreased vasoconstrictor response.

Decreased association of HSP90 impairs endothelial nitric oxide synthase in fetal lambs with persistent pulmonary hypertension

Persistent pulmonary hypertension of newborn (PPHN) is associated with decreased nitric oxide (NO) release and impaired pulmonary vasodilation. We investigated the hypothesis that decreased association of heat shock protein 90 (HSP90) with endothelial NO synthase (eNOS) impairs NO release and vasodilation in PPHN. The responses to the NOS agonist ATP were investigated in fetal lambs with PPHN induced by prenatal ligation of ductus arteriosus, and in sham ligation controls. ATP caused dose-dependent vasodilation in control pulmonary resistance arteries, and this response was attenuated in PPHN vessels. The response of control pulmonary arteries to ATP was attenuated by NG-nitro-l-arginine methyl ester (l-NAME), a NOS antagonist, and geldanamycin, an inhibitor of HSP90-eNOS interaction. The attenuated response to ATP observed in PPHN was improved by pretreatment of vessels with l-NAME or 4,5-dihydroxy-1,3-benzene-disulfonate, a superoxide scavenger. Pulmonary arteries from PPHN lambs had decreased basal levels of HSP90 in association with eNOS. Association of HSP90 with eNOS and NO release increased in response to ATP in control pulmonary artery endothelial cells, but not in cells from PPHN lambs. Decreased HSP90-eNOS interactions may contribute to the impaired NO release and vasodilation observed in the ductal ligation model of PPHN.

Vascular endothelial growth factor-dependent down-regulation of Flk-1/KDR involves Cbl-mediated ubiquitination. Consequences on nitric oxide production from endothelial cells

Ligand-stimulated degradation of receptor tyrosine kinase (RTK) is an important regulatory step of signal transduction. The vascular endothelial growth factor (VEGF) receptor Flk-1/KDR is responsible for the VEGF-stimulated nitric oxide (NO) production from endothelial cells. Cellular mechanisms mediating the negative regulation of Flk-1 signaling in endothelial cells have not been investigated. Here we show that Flk-1 is rapidly down-regulated following VEGF stimulation of bovine aortic endothelial cells (BAECs). Consequently, VEGF pretreatment of endothelial cells prevents any further stimulation of Flk-1, resulting in decreased NO production from subsequent VEGF challenges. Ubiquitination of RTKs targets them for degradation; we demonstrate that activation of Flk-1 by VEGF leads to its polyubiquitination in BAECs. Furthermore, VEGF stimulation of BAECs or COS-7 cells transiently transfected with Flk-1 results in the phosphorylation of the ubiquitin ligase Cbl, the enhanced association of Cbl with Flk-1, and the relocalization of Cbl to vesicular structures in BAECs. Overexpression of Cbl in COS-7 cells enhances VEGF-induced ubiquitination of Flk-1, whereas a Cbl mutant lacking the ubiquitin ligase RING finger domain, 70Z/3-Cbl, does not. Moreover, expression of Cbl in contrast to 70Z/3-Cbl inhibits the Flk-1-dependent activation of eNOS and, thus, NO release. In BAEC overexpressing Cbl, the degradation of Flk-1 upon VEGF stimulation is accelerated compared with cells transfected with a control vector (green fluorescent protein). Our findings demonstrate that Flk-1 is rapidly down-regulated following sustained VEGF stimulation and identify Cbl as a negative regulator of Flk-1 signaling to eNOS. Cbl thus plays a role in the regulation of VEGF signaling by mediating the stimulated ubiquitination and, consequently, degradation of Flk-1 in endothelial cells.

Aha1 binds to the middle domain of Hsp90, contributes to client protein activation, and stimulates the ATPase activity of the molecular chaperone

The ATP-dependent molecular chaperone Hsp90 is an essential and abundant stress protein in the eukaryotic cytosol that cooperates with a cohort of cofactors/cochaperones to fulfill its cellular tasks. We have identified Aha1 (activator of Hsp90 ATPase) and its relative Hch1 (high copy Hsp90 suppressor) as binding partners of Hsp90 in Saccharomyces cerevisiae. By using genetic and biochemical approaches, the middle domain of Hsp90 (amino acids 272-617) was found to mediate the interaction with Aha1 and Hch1. Data base searches revealed that homologues of Aha1 are conserved from yeast to man, whereas Hch1 was found to be restricted to lower eukaryotes like S. cerevisiae and Candida albicans. In experiments with purified proteins, Aha1 but not Hch1 stimulated the intrinsic ATPase activity of Hsp90 5-fold. To establish their cellular role further, we deleted the genes encoding Aha1 and Hch1 in S. cerevisiae. In vivo experiments demonstrated that Aha1 and Hch1 contributed to efficient activation of the heterologous Hsp90 client protein v-Src. Moreover, Aha1 and Hch1 became crucial for cell viability under non-optimal growth conditions when Hsp90 levels are limiting. Thus, our results identify a novel type of cofactor involved in the regulation of the molecular chaperone Hsp90.

Compensatory phosphorylation and protein-protein interactions revealed by loss of function and gain of function mutants of multiple serine phosphorylation sites in endothelial nitric-oxide synthase

We examined the influence of individual serine phosphorylation sites in endothelial nitric-oxide synthase (eNOS) on basal and stimulated NO release, cooperative phosphorylation, and co-association with hsp90 and Akt. Mutation of the serine phosphorylation sites 116, 617, and 1179 to alanines affected the phospho-state of at least one other site, demonstrating cooperation between multiple phosphorylation events, whereas mutation of serine 635 to alanine did not cause compensation. Mutation of serines 116 and 617 to alanine promoted a greater protein-protein interaction with hsp90 and Akt and greater phosphorylation on serine 1179, the major site for Akt phosphorylation. More importantly, using alanine substitutions, Ser-116 is important for agonist, but not basal NO release, Ser-635 is important for basal, but not stimulated, Ser-617 negatively regulates basal and stimulated NO release, and Ser-1179 phosphorylation is stimulatory for both basal and agonist-mediated NO release. Using putative "gain of function" mutants (serine to aspartate) serines 635 and 1179 are important positive regulators of basal and stimulated NO release. S635D eNOS is the most efficacious, yielding 5-fold increases in basal and 2-fold increases in stimulated NO release from cells. However, S617A and S617D eNOS both increased NO release with opposite actions in NOS activity assays. Thus, multiple serine phosphorylation events regulate basal and stimulate NO release with Ser-635 and Ser-1179 being important positive regulatory sites and Ser-116 as a negative regulatory. Ser-617 may not be important for directly regulating NO release but is important as a modulator of phosphorylation at other sites and protein-protein interactions.

Stimulation of perivascular nitric oxide synthesis by oxygen

We hypothesized that elevated partial pressures of O(2) would increase perivascular nitric oxide (*NO) synthesis. Rodents with O(2)- and.NO-specific microelectrodes implanted adjacent to the abdominal aorta were exposed to O(2) at partial pressures from 0.2 to 2.8 atmospheres absolute (ATA). Exposures to 2.0 and 2.8 ATA O(2) stimulated neuronal (type I) NO synthase (nNOS) and significantly increased steady-state.NO concentration, but the mechanism for enzyme activation differed at each partial pressure. At both pressures, elevations in.NO concentration were inhibited by the nNOS inhibitor 7-nitroindazole and the calcium channel blocker nimodipine. Enzyme activation at 2.0 ATA O(2) appeared to be due to an altered cellular redox state. Exposure to 2.8 ATA O(2), but not 2.0 ATA O(2), increased nNOS activity by enhancing nNOS association with calmodulin, and an inhibitory effect of geldanamycin indicated that the association was facilitated by heat shock protein 90. Infusion of superoxide dismutase inhibited.NO elevation at 2.8 but not 2.0 ATA O(2). Hyperoxia increased the concentration of.NO associated with hemoglobin. These findings highlight the complexity of oxidative stress responses and may help explain some of the dose responses associated with therapeutic applications of hyperbaric oxygen.

L-4F, an apolipoprotein A-1 mimetic, restores nitric oxide and superoxide anion balance in low-density lipoprotein-treated endothelial cells

BACKGROUND

Low-density lipoprotein (LDL) impairs endothelial cell function by uncoupling endothelial nitric oxide synthase (eNOS) activity, which allows superoxide anion (O2*-)) to be generated rather than nitric oxide (*NO). Recent reports indicate that apolipoprotein (apo) A-1 mimetics inhibit the development of atherosclerotic lesions in LDL receptor-null mice. Here we hypothesize that L-4F, an apoA-1 mimetic that inhibits atherosclerosis induced by hypercholesterolemia, protects endothelial cell function by preventing LDL from uncoupling eNOS activity.

METHODS AND RESULTS

Bovine aortic endothelial cells were incubated with LDL+/-L-4F, and changes in A23187-stimulated.NO and O2*- generation were determined by ozone chemiluminescence and superoxide dismutase-inhibitable ferricytochrome c reduction, respectively. Western analysis of eNOS immunoprecipitates was used to determine effects of LDL and L-4F on heat shock protein 90 (hsp90) interactions with eNOS. LDL decreased.NO production and increased eNOS-dependent O2*- generation. Pretreatment of LDL with L-4F increased.NO and decreased O2*- generation. By itself, L-4F had no effect on O2*- but did increase *NO generation. Stimulation of endothelial cells incubated with LDL decreased the association of hsp90 with eNOS. Pretreatment of LDL with L-4F prevented a decrease in hsp90 association with eNOS and often enhanced association on stimulation.

CONCLUSIONS

These data demonstrate that L-4F protects endothelial cell function by preventing LDL from uncoupling eNOS activity. L-4F allows endothelial cell to maintain coupled eNOS activity to generate.NO even in the face of atherogenic concentrations of LDL.

Calmodulin-dependent and -independent activation of endothelial nitric-oxide synthase by heat shock protein 90

Endothelial nitric oxide synthase (eNOS), which generates the endogenous vasodilator, nitric oxide (NO), is highly regulated by post-translational modifications and protein interactions. Heat shock protein 90 (HSP90) binds directly to eNOS, augmenting NO production. We have used purified proteins to characterize further the mechanism by which HSP90 increases eNOS activity at low (100 nm) and high (10 microm) Ca(2+) levels. In the presence of calmodulin (CaM), HSP90 increased eNOS activity dose dependently at both low and high Ca(2+) concentrations. This effect was abolished by the specific HSP90 inhibitor geldanamycin (GA) at both calcium concentrations. The EC(50) values of eNOS for both Ca(2+) and CaM were decreased in the presence of HSP90. HSP90 also significantly increased the rate of NADPH-dependent cytochrome c reduction by eNOS at both low and high Ca(2+) concentrations. HSP90 bound to eNOS in a dose-dependent manner, and the amount of bound HSP90 also increased with increasing Ca(2+)/CaM. At 100 nm Ca(2+), HSP90 promoted dose-dependent CaM binding to eNOS that was fully inhibitable by GA. At high calcium, HSP90 did not affect CaM binding to eNOS, but GA inhibited HSP90 binding to eNOS. At high Ca(2+), HSP90 caused the V(max) of eNOS for l-arginine to increase by 2-fold, but the K(m) of eNOS was unchanged. HSP90 bound preferentially to CaM-prebound eNOS and significantly increased both its NO synthesis and reductase activities. These data support that HSP90 promotes eNOS activity by two mechanisms: (i) a CaM-dependent mechanism operative at low Ca(2+) concentrations, characterized by an increase in the affinity of eNOS for CaM and (ii) a CaM-independent mechanism apparent at high Ca(2+) concentrations, characterized by stimulation of eNOS reductase activity without further change in CaM binding. These studies contribute to our understanding of eNOS activation by HSP90 and provide a basis for in vitro studies of other eNOS-interacting proteins.

Structural and functional analysis of the middle segment of hsp90: implications for ATP hydrolysis and client protein and cochaperone interactions

Activation of client proteins by the Hsp90 molecular chaperone is dependent on binding and hydrolysis of ATP, which drives a molecular clamp via transient dimerization of the N-terminal domains. The crystal structure of the middle segment of yeast Hsp90 reveals considerable evolutionary divergence from the equivalent regions of other GHKL protein family members such as MutL and GyrB, including an additional domain of new fold. Using the known structure of the N-terminal nucleotide binding domain, a model for the Hsp90 dimer has been constructed. From this structure, residues implicated in the ATPase-coupled conformational cycle and in interactions with client proteins and the activating cochaperone Aha1 have been identified, and their roles functionally characterized in vitro and in vivo.

Endothelial nitric oxide synthase: the Cinderella of inflammation?

A hallmark of inflammation is increased vascular permeability. Increases in vascular permeability and the migration of inflammatory cells are linked to complex interactions of inflammatory mediators with the vascular endothelium. Normally, endothelial nitric oxide synthase (eNOS) produces a tonic amount of nitric oxide (NO), which is responsible for the homeostasis between the endothelium and surrounding tissues. However, most agonists that act on endothelial cells cause a series of post-translational modifications that influence eNOS activity. Furthermore, stimulation by shear stress, autacoids or growth factors either induces eNOS or shifts it to a more active state, which produces a burst of NO. Here, we highlight recent findings about eNOS and propose how new pharmacological tools can be used to dissect the involvement and contribution of eNOS to inflammatory responses.

Endothelium-dependent and -independent relaxation and VASP serines 157/239 phosphorylation by cyclic nucleotide-elevating vasodilators in rat aorta

Endothelium-dependent vasodilation is thought to be mediated primarily by the NO/cGMP signaling pathway whereas cAMP-elevating vasodilators are considered to act independent of the endothelial cell layer. However, recent functional data suggest that cAMP-elevating vasodilators such as beta-receptor agonists, adenosine or forskolin may also be endothelium-dependent. Here we used functional and biochemical assays to analyze endothelium-dependent, cGMP- and cAMP-mediated signaling in rat aorta. Acetylcholine and sodium nitroprusside (SNP) induced a concentration-dependent relaxation of phenylephrine-precontracted aorta. This response was reflected by the phosphorylation of the vasodilator-stimulated phosphoprotein (VASP), a validated substrate of cGMP- and cAMP-dependent protein kinases (cGK, cAK), on Ser(157) and Ser(239). As expected, the effects of acetylcholine were endothelium-dependent. However, relaxation induced by the beta-receptor agonist isoproterenol was also almost completely impaired after endothelial denudation. At the biochemical level, acetylcholine- and isoproterenol-evoked cGK and cAK activation, respectively, as measured by VASP Ser(239) and Ser(157) phosphorylation, was strongly diminished. Furthermore, the effects of isoproterenol were repressed by eNOS inhibition when endothelium was present. We also observed that the relaxing and biochemical effects of forskolin were at least partially endothelium-dependent. We conclude that cAMP-elevating vasodilators, i.e. isoproterenol and to a lesser extent also forskolin, induce vasodilation and concomitant cyclic nucleotide protein kinase activation in the vessel wall in an endothelium-dependent way.

Inhibition of heat shock protein 90 (hsp90) in proliferating endothelial cells uncouples endothelial nitric oxide synthase activity

Dual increases in nitric oxide ((*)NO) and superoxide anion (O(2)(*-)) production are one of the hallmarks of endothelial cell proliferation. Increased expression of endothelial nitric oxide synthase (eNOS) has been shown to play an important role in maintaining high levels of (*)NO generation to offset the increase in O(2)(*-) that occurs during proliferation. Although recent reports indicate that heat shock protein 90 (hsp90) associates with eNOS to increase (*)NO generation, the role of hsp90 association with eNOS during endothelial cell proliferation remains unknown. In this report, we examine the effects of endothelial cell proliferation on eNOS expression, hsp90 association with eNOS, and the mechanisms governing eNOS generation of (*)NO and O(2)(*-). Western analysis revealed that endothelial cells not only increased eNOS expression during proliferation but also hsp90 interactions with the enzyme. Pretreatment of cultures with radicicol (RAD, 20 microM), a specific inhibitor that does not redox cycle, decreased A23187-stimulated (*)NO production and increased L(omega)-nitroargininemethylester (L-NAME)-inhibitable O(2)(*-) generation. In contrast, A23187 stimulation of controls in the presence of L-NAME increased O(2)(*-) generation, confirming that during proliferation eNOS generates (*)NO. Our findings demonstrate that hsp90 plays an important role in maintaining (*)NO generation during proliferation. Inhibition of hsp90 in vascular endothelium provides a convenient mechanism for uncoupling eNOS activity to inhibit (*)NO production. This study provides new understanding of the mechanisms by which ansamycin antibiotics inhibit endothelial cell proliferation. Such information may be useful in the development and design of new antineoplastic agents in the future.

Molecular targeting therapy of cancer: drug resistance, apoptosis and survival signal

Recent progress in the development of molecular cancer therapeutics has revealed new types of antitumor drugs, such as Herceptin, Gleevec, and Iressa, as potent therapeutics for specific tumors. Our work has focused on molecular cancer therapeutics, mainly in the areas of drug resistance, apoptosis and apoptosis resistance, and survival-signaling, which is related to drug resistance. In this review, we describe our research on molecular cancer therapeutics, including molecular mechanisms and therapeutic approaches. Resistance to chemotherapeutic drugs is a principal problem in the treatment of cancer. P-Glycoprotein (P-gp), encoded by the MDR1 gene, is a multidrug transporter and has a major role in multidrug resistance (MDR). Targeting of P-gp by small-molecular compounds and/or antibodies is an effective strategy to overcome MDR in cancer, especially hematologic malignancies. Several P-gp inhibitors have been developed and are currently under clinical phased studies. In addition to the multidrug transporter proteins, cancer cells have several drug resistance mechanisms. Solid tumors are often placed under stress conditions, such as glucose starvation and hypoxia. These conditions result in topo II poison resistance that is due to proteasome-mediated degradation of DNA topoisomerases. Proteasome inhibitors effectively prevent this stress-induced drug resistance. Glyoxalase I, which is often elevated in drug- and apoptosis-resistant cancers, offers another possibility for overcoming drug resistance. It plays a role in detoxification of methylglioxal, a side product of glycolysis, which is highly reactive with DNA and proteins. Inhibitors of glyoxalase I selectively kill drug-resistant tumors that express glyoxalase I. Finally, the susceptibility of tumor cells to apoptosis induced by antitumor drugs appears to depend on the balance between pro-apoptotic and survival (anti-apoptotic) signals. PI3K-Akt is an important survival signal pathway, that has been shown to be the target of various antitumor drugs, including UCN-01 and geldanamycin, new anticancer drugs under clinical evaluation. Our present studies provide novel targets for future effective molecular cancer therapeutics.

Molecular mechanisms involved in the regulation of the endothelial nitric oxide synthase

The endothelial nitric oxide synthase (eNOS), the expression of which is regulated by a range of transcriptional and posttranscriptional mechanisms, generates nitric oxide (NO) in response to a number of stimuli. The physiologically most important determinants for the continuous generation of NO and thus the regulation of local blood flow are fluid shear stress and pulsatile stretch. Although eNOS activity is coupled to changes in endothelial cell Ca(2+) levels, an increase in Ca(2+) alone is not sufficient to affect enzyme activity because the binding of calmodulin (CaM) and the flow of electrons from the reductase to the oxygenase domain of the enzyme is dependent on protein phosphorylation and dephosphorylation. Two amino acids seem to be particularly important in regulating eNOS activity and these are a serine residue in the reductase domain (Ser(1177)) and a threonine residue (Thr(495)) located within the CaM-binding domain. Simultaneous alterations in the phosphorylation of Ser(1177) and Thr(495) in response to a variety of stimuli are regulated by a number of kinases and phosphatases that continuously associate with and dissociate from the eNOS signaling complex. eNOS associated proteins, such as caveolin, heat shock protein 90, eNOS interacting protein, and possibly also motor proteins provide the scaffold for the formation of the protein complex as well as its intracellular localization.

Nonnuclear actions of estrogen

Estrogen has long been observed to endow cardiovascular protective effects, as evidenced by sex-specific differences in the incidence of hypertensive and coronary artery disease, the development of atherosclerosis, and myocardial remodeling after infarction. To exert its tissue-specific effects, the classic estrogen receptor (ER) functions as a ligand-dependent transcription factor. However, there is growing evidence that in response to 17beta-estradiol and heterologous signals, the ER can also mediate signaling cascades at the membrane and in the cytoplasm via various second messengers, such as receptor-mediated protein kinases. This review summarizes the current understanding of nonnuclear ER signaling and discusses the relevance to eliciting the beneficial cardiovascular effects of estrogen. These include vasodilation, inhibition of response to vessel injury, limiting myocardial injury after infarction, and attenuating cardiac hypertrophy. Defining the full repertoire of ER function promises to expose novel, highly specific targets for pharmacological interventions and may ultimately lead to the primary and secondary prevention of cardiovascular diseases.

Post-translational mechanisms of endothelial nitric oxide synthase regulation by bradykinin

The endothelial nitric oxide synthase (eNOS) plays a key role in blood pressure regulation and vascular homeostasis. Among the more potent inducers of eNOS activity in vascular endothelial cells is bradykinin (BK). This brief review summarizes the current state of knowledge with regard to regulation of eNOS through several distinct molecular mechanisms, each of which acts in concert with Ca2+-calmodulin (CaM) signaling in post-translational activation of eNOS. These mechanisms include alterations in protein-protein interactions with caveolin-1, the BK B2 receptor, and heat shock protein 90 (Hsp90). In addition, BK stimulates an increase in eNOS activity through phosphorylation of the enzyme at three specific amino acid residues as well as through dephosphorylation at a fourth residue.

Endothelial function. From vascular biology to clinical applications

The endothelium, by releasing nitric oxide (NO), promotes vasodilation and inhibits inflammation, thrombosis, and vascular smooth muscle cell proliferation. These biological actions of NO make it an important component in the endogenous defense against atherosclerosis and its overt clinical complications. Loss of the functional integrity of the endothelium, as seen commonly in the milieu of cardiovascular risk factors, plays an integral role in all stages of atherosclerosis from lesion initiation to plaque rupture. A number of established techniques can assess endothelial function in human vascular beds. The outcome of endothelial testing has profound prognostic implications and is an independent predictor of atherosclerosis disease progression and cardiovascular event rates. The large clinical benefit of statins and angiotensin-converting enzyme inhibitors in patients with atherosclerosis involves favorable effects of endothelial function. Studies of endothelial function represent a prime example of a successful application of insights derived from vascular biology at the bedside.

PKB binding proteins. Getting in on the Akt

Protein kinase B (PKB) has emerged as the focal point for many signal transduction pathways, regulating multiple cellular processes such as glucose metabolism, transcription, apoptosis, cell proliferation, angiogenesis, and cell motility. In addition to acting as a kinase toward many substrates involved in these processes, PKB forms complexes with other proteins that are not substrates, but rather act as modulators of PKB activity and function. In this review, we discuss the implications of these data in understanding the multitude of functions predicted for PKB in cells.

Akt forms an intracellular complex with heat shock protein 90 (Hsp90) and Cdc37 and is destabilized by inhibitors of Hsp90 function

Hsp90 is a chaperone required for the conformational maturation of certain signaling proteins including Raf, cdk4, and steroid receptors. Natural products and synthetic small molecules that bind to the ATP-binding pocket in the amino-terminal domain of Hsp90 inhibit its function and cause the degradation of these client proteins. Inhibition of Hsp90 function in cells causes down-regulation of an Akt kinase-dependent pathway required for D-cyclin expression and retinoblastoma protein-dependent G(1) arrest. Intracellular Akt is associated with Hsp90 and Cdc37 in a complex in which Akt kinase is active and regulated by phosphatidylinositol 3-kinase. Functional Hsp90 is required for the stability of Akt in the complex. Occupancy of the ATP-binding pocket by inhibitors is associated with the ubiquitination of Akt and its targeting to the proteasome, where it is degraded. This results in a shortening of the half-life of Akt from 36 to 12 h and an 80% reduction in its expression. Akt and its activating kinase, PDK1, are the only members of the protein kinase A/protein kinase B/protein kinase C-like kinase family that are affected by Hsp90 inhibitors. Thus, transduction of growth factor signaling via the Akt and Raf pathways requires functional Hsp90 and can be coordinately blocked by its inhibition.

Hsp90 interactions and acylation target the G protein Galpha 12 but not Galpha 13 to lipid rafts

The heterotrimeric G proteins, G(12) and G(13), are closely related in their sequences, signaling partners, and cellular effects such as oncogenic transformation and cytoskeletal reorganization. Yet G(12) and G(13) can act through different pathways, bind different proteins, and show opposing actions on some effectors. We investigated the compartmentalization of G(12) and G(13) at the membrane because other G proteins reside in lipid rafts, membrane microdomains enriched in cholesterol and sphingolipids. Lipid rafts were isolated after cold, nonionic detergent extraction of cells and gradient centrifugation. Galpha(12) was in the lipid raft fractions, whereas Galpha(13) was not associated with lipid rafts. Mutation of Cys-11 on Galpha(12), which prevents its palmitoylation, partially shifted Galpha(12) from the lipid rafts. Geldanamycin treatment, which specifically inhibits Hsp90, caused a partial loss of wild-type Galpha(12) and a complete loss of the Cys-11 mutant from the lipid rafts and the appearance of a higher molecular weight form of Galpha(12) in the soluble fractions. These results indicate that acylation and Hsp90 interactions localized Galpha(12) to lipid rafts. Hsp90 may act as both a scaffold and chaperone to maintain a functional Galpha(12) only in discrete membrane domains and thereby explain some of the nonoverlapping functions of G(12) and G(13) and control of these potent cell regulators.

Chronic hypoxia increases endothelial nitric oxide synthase generation of nitric oxide by increasing heat shock protein 90 association and serine phosphorylation

Chronic hypoxia increases endothelial nitric oxide synthase (eNOS) production of nitric oxide (*NO) and cardioprotection in neonatal rabbit hearts. However, the mechanism by which this occurs remains unclear. Recent studies suggest that heat shock protein 90 (hsp90) alters eNOS function. In the present study, we examined the role of hsp90 in eNOS-dependent cardioprotection in neonatal rabbit hearts. Chronic hypoxia increased recovery of postischemic left ventricular developed pressure (LVDP). Geldanamycin (GA), which inhibits hsp90 and increases oxidative stress, decreased functional recovery in normoxic and hypoxic hearts. To determine if a loss in *NO, afforded by GA, decreased recovery, GA-treated hearts were perfused with S-nitrosoglutathione (GSNO) as a source of *NO. GSNO increased recovery of postischemic LVDP in GA-treated normoxic and hypoxic hearts to baseline levels. Although chronic hypoxia decreased phosphorylated eNOS (S1177) levels by approximately 4- to 5-fold and total Akt and phosphorylated Akt by 4- and 5-fold, it also increased hsp90 association with eNOS by more than 3-fold. Using hydroethidine (HEt), a fluorescent probe for superoxide, we found that hypoxic hearts contained less ethidine (Et) staining than normoxic hearts. Normoxic hearts generated 3 times more superoxide by an N(omega)-nitro-L-arginine methyl ester (L-NAME)-inhibitable mechanism than hypoxic hearts. Taken together, these data indicate that the association of hsp90 with eNOS is important for increasing *NO production and limiting eNOS-dependent superoxide anion generation. Such changes in eNOS function appear to play a critical role in protecting the myocardium against ischemic injury.

Role of Akt signaling in vascular homeostasis and angiogenesis

Akt is a serine/threonine protein kinase that is activated by a number of growth factors and cytokines in a phosphatidylinositol-3 kinase-dependent manner. Although antiapoptotic activity of Akt is well known, it also regulates other aspects of cellular functions, including migration, glucose metabolism, and protein synthesis. In this review, Akt signaling in endothelial cells and its critical roles in the regulation of vascular homeostasis and angiogenesis will be discussed.