Protein Phosphatase 2A Activation Via ApoER2 in Trophoblasts Drives Preeclampsia in a Mouse Model of the Antiphospholipid Syndrome

[Figure: see text].

Supplementation With the Sialic Acid Precursor N-Acetyl-D-Mannosamine Breaks the Link Between Obesity and Hypertension

BACKGROUND

Obesity-related hypertension is a common disorder, and attempts to combat the underlying obesity are often unsuccessful. We previously revealed that mice globally deficient in the inhibitory immunoglobulin G (IgG) receptor FcγRIIB are protected from obesity-induced hypertension. However, how FcγRIIB participates is unknown. Studies were designed to determine if alterations in IgG contribute to the pathogenesis of obesity-induced hypertension.

METHODS

Involvement of IgG was studied using IgG μ heavy chain-null mice deficient in mature B cells and by IgG transfer. Participation of FcγRIIB was interrogated in mice with global or endothelial cell-specific deletion of the receptor. Obesity was induced by high-fat diet (HFD), and blood pressure (BP) was measured by radiotelemetry or tail cuff. The relative sialylation of the Fc glycan on mouse IgG, which influences IgG activation of Fc receptors, was evaluated by Sambucus nigra lectin blotting. Effects of IgG on endothelial NO synthase were assessed in human aortic endothelial cells. IgG Fc glycan sialylation was interrogated in 3442 human participants by mass spectrometry, and the relationship between sialylation and BP was evaluated. Effects of normalizing IgG sialylation were determined in HFD-fed mice administered the sialic acid precursor N-acetyl-D-mannosamine (ManNAc).

RESULTS

Mice deficient in B cells were protected from obesity-induced hypertension. Compared with IgG from control chow-fed mice, IgG from HFD-fed mice was hyposialylated, and it raised BP when transferred to recipients lacking IgG; the hypertensive response was absent if recipients were FcγRIIB-deficient. Neuraminidase-treated IgG lacking the Fc glycan terminal sialic acid also raised BP. In cultured endothelial cells, via FcγRIIB, IgG from HFD-fed mice and neuraminidase-treated IgG inhibited vascular endothelial growth factor activation of endothelial NO synthase by altering endothelial NO synthase phosphorylation. In humans, obesity was associated with lower IgG sialylation, and systolic BP was inversely related to IgG sialylation. Mice deficient in FcγRIIB in endothelium were protected from obesity-induced hypertension. Furthermore, in HFD-fed mice, ManNAc normalized IgG sialylation and prevented obesity-induced hypertension.

CONCLUSIONS

Hyposialylated IgG and FcγRIIB in endothelium are critically involved in obesity-induced hypertension in mice, and supportive evidence was obtained in humans. Interventions targeting these mechanisms, such as ManNAc supplementation, may provide novel means to break the link between obesity and hypertension.

SR-B1 drives endothelial cell LDL transcytosis via DOCK4 to promote atherosclerosis

Atherosclerosis, which underlies life-threatening cardiovascular disorders including myocardial infarction and stroke¹, is initiated by low density lipoprotein cholesterol (LDL) passage into the artery wall and engulfment by macrophages, leading to foam cell formation and lesion development^{2, 2, 3, 3}. How circulating LDL enters the artery wall to instigate atherosclerosis is unknown. Here we show in mice that scavenger receptor, class B type 1 (SR-B1) in endothelial cells mediates LDL delivery into arteries and its accumulation by artery wall macrophages, thereby promoting atherosclerosis. LDL particles are colocalized with SR-B1 in endothelial cell intracellular vesicles in vivo, and LDL transcytosis across endothelial monolayers requires its direct binding to SR-B1 and an 8 amino acid cytoplasmic domain of the receptor that recruits the guanine nucleotide exchange factor dedicator of cytokinesis 4 (DOCK4)⁴. DOCK4 promotes SR-B1 internalization and LDL transport by coupling LDL binding to SR-B1 with Rac1 activation. SR-B1 and DOCK4 expression are increased in atherosclerosis-prone regions of the mouse aorta prior to lesion formation, and in human atherosclerotic versus normal arteries. These findings challenge the long-held concept that atherogenesis involves passive LDL movement across a compromised endothelial barrier. Interventions inhibiting endothelial delivery of LDL into the artery wall may represent a new therapeutic category in the battle against cardiovascular disease.

Selective Nonnuclear Estrogen Receptor Activation Decreases Stroke Severity and Promotes Functional Recovery in Female Mice

Estrogens provide neuroprotection in animal models of stroke, but uterotrophic effects and cancer risk limit translation. Classic estrogen receptors (ERs) serve as transcription factors, whereas nonnuclear ERs govern numerous cell processes and exert beneficial cardiometabolic effects without uterine or breast cancer growth in mice. Here, we determined how nonnuclear ER stimulation with pathway-preferential estrogen (PaPE)-1 affects stroke outcome in mice. Ovariectomized female mice received vehicle, estradiol (E2), or PaPE-1 before and after transient middle cerebral artery occlusion (tMCAo). Lesion severity was assessed with MRI, and poststroke motor function was evaluated through 2 weeks after tMCAo. Circulating, spleen, and brain leukocyte subpopulations were quantified 3 days after tMCAo by flow cytometry, and neurogenesis and angiogenesis were evaluated histologically 2 weeks after tMCAo. Compared with vehicle, E2 and PaPE-1 reduced infarct volumes at 3 days after tMCAo, though only PaPE-1 reduced leukocyte infiltration into the ischemic brain. Unlike E2, PaPE-1 had no uterotrophic effect. Both interventions had negligible effect on long-term poststroke neuronal or vascular plasticity. All mice displayed a decline in motor performance at 2 days after tMCAo, and vehicle-treated mice did not improve thereafter. In contrast, E2 and PaPE-1 treatment afforded functional recovery at 6 days after tMCAo and beyond. Thus, the selective activation of nonnuclear ER by PaPE-1 decreased stroke severity and improved functional recovery in mice without undesirable uterotrophic effects. The beneficial effects of PaPE-1 are also associated with attenuated neuroinflammation in the brain. PaPE-1 and similar molecules may warrant consideration as efficacious ER modulators providing neuroprotection without detrimental effects on the uterus or cancer risk.

Hyposialylated IgG activates endothelial IgG receptor FcγRIIB to promote obesity-induced insulin resistance

Type 2 diabetes mellitus (T2DM) is a common complication of obesity. Here, we have shown that activation of the IgG receptor FcγRIIB in endothelium by hyposialylated IgG plays an important role in obesity-induced insulin resistance. Despite becoming obese on a high-fat diet (HFD), mice lacking FcγRIIB globally or selectively in endothelium were protected from insulin resistance as a result of the preservation of insulin delivery to skeletal muscle and resulting maintenance of muscle glucose disposal. IgG transfer in IgG-deficient mice implicated IgG as the pathogenetic ligand for endothelial FcγRIIB in obesity-induced insulin resistance. Moreover, IgG transferred from patients with T2DM but not from metabolically healthy subjects caused insulin resistance in IgG-deficient mice via FcγRIIB, indicating that similar processes may be operative in T2DM in humans. Mechanistically, the activation of FcγRIIB by IgG from obese mice impaired endothelial cell insulin transcytosis in culture and in vivo. These effects were attributed to hyposialylation of the Fc glycan, and IgG from T2DM patients was also hyposialylated. In HFD-fed mice, supplementation with the sialic acid precursor N-acetyl-D-mannosamine restored IgG sialylation and preserved insulin sensitivity without affecting weight gain. Thus, IgG sialylation and endothelial FcγRIIB may represent promising therapeutic targets to sever the link between obesity and T2DM.

Non-nuclear estrogen receptor alpha activation in endothelium reduces cardiac ischemia-reperfusion injury in mice

Steroid hormone receptors including estrogen receptors (ER) classically function as ligand-regulated transcription factors. However, estrogens also elicit cellular effects through binding to extra-nuclear ER (ERα, ERβ, and G protein-coupled ER or GPER) that are coupled to kinases. How extra-nuclear ER actions impact cardiac ischemia-reperfusion (I/R) injury is unknown. We treated ovariectomized wild-type female mice with estradiol or an estrogen-dendrimer conjugate (EDC), which selectively activates extra-nuclear ER, or vehicle interventions for two weeks. I/R injury was then evaluated in isolated Langendorff perfused hearts. Two weeks of treatment with estradiol significantly decreased infarct size and improved post-ischemic contractile function. Similarly, EDC treatment significantly decreased infarct size and increased post-ischemic functional recovery compared to vehicle-treated hearts. EDC also caused an increase in myocardial protein S-nitrosylation, consistent with previous studies showing a role for this post-translational modification in cardioprotection. In further support of a role for S-nitrosylation, inhibition of nitric oxide synthase, but not soluble guanylyl cyclase blocked the EDC mediated protection. The administration of ICI182,780, which is an agonist of G-protein coupled estrogen receptor (GPER) and an antagonist of ERα and ERβ, did not result in protection; however, ICI182,780 significantly blocked EDC-mediated cardioprotection, indicating participation of ERα and/or ERβ. In studies determining the specific ER subtype and cellular target involved, EDC decreased infarct size and improved functional recovery in mice lacking ERα in cardiomyocytes. In contrast, protection was lost in mice deficient in endothelial cell ERα. Thus, extra-nuclear ERα activation in endothelium reduces cardiac I/R injury in mice, and this likely entails increased protein S-nitrosylation. Since EDC does not stimulate uterine growth, in the clinical setting EDC-like compounds may provide myocardial protection without undesired uterotrophic and cancer-promoting effects.

Nonnuclear Estrogen Receptor Activation Improves Hepatic Steatosis in Female Mice

Estrogens have the potential to afford atheroprotection, to prevent excess adiposity and its metabolic complications including insulin resistance, and to lessen hepatic steatosis. Cellular responses to estrogens occur through gene regulation by nuclear estrogen receptors (ERs), and through signal initiation by plasma membrane-associated ER. Leveraging the potentially favorable cardiometabolic actions of estrogens has been challenging, because their reproductive tract and cancer-promoting effects adversely impact the risk to benefit ratio of the therapy. In previous works, we discovered that an estrogen dendrimer conjugate (EDC) comprised of ethinyl-estradiol (E₂) molecules linked to a poly(amido)amine dendrimer selectively activates nonnuclear ER, and in mice, EDC does not invoke a uterotrophic response or support ER-positive breast cancer growth. In the present investigation, we employed EDC to determine how selective nonnuclear ER activation impacts atherosclerosis, adiposity, glucose homeostasis, and hepatic steatosis in female mice. In contrast to E₂, EDC did not blunt atherosclerosis in hypercholesterolemic apoE^-/- mice. Also in contrast to E₂, EDC did not prevent the increase in adiposity caused by Western diet feeding in wild-type mice, and it did not affect Western diet-induced glucose intolerance. However, E₂ and EDC had comparable favorable effect on diet-induced hepatic steatosis, and this was related to down-regulation of fatty acid and triglyceride synthesis genes in the liver. Predictably, only E₂ caused a uterotrophic response. Thus, although nonnuclear ER activation does not prevent atherosclerosis or diet-induced obesity or glucose intolerance, it may provide a potential new strategy to combat hepatic steatosis without impacting the female reproductive tract or increasing cancer risk.

Endothelial Fcγ Receptor IIB Activation Blunts Insulin Delivery to Skeletal Muscle to Cause Insulin Resistance in Mice

Modest elevations in C-reactive protein (CRP) are associated with type 2 diabetes. We previously revealed in mice that increased CRP causes insulin resistance and mice globally deficient in the CRP receptor Fcγ receptor IIB (FcγRIIB) were protected from the disorder. FcγRIIB is expressed in numerous cell types including endothelium and B lymphocytes. Here we investigated how endothelial FcγRIIB influences glucose homeostasis, using mice with elevated CRP expressing or lacking endothelial FcγRIIB. Whereas increased CRP caused insulin resistance in mice expressing endothelial FcγRIIB, mice deficient in the endothelial receptor were protected. The insulin resistance with endothelial FcγRIIB activation was due to impaired skeletal muscle glucose uptake caused by attenuated insulin delivery, and it was associated with blunted endothelial nitric oxide synthase (eNOS) activation in skeletal muscle. In culture, CRP suppressed endothelial cell insulin transcytosis via FcγRIIB activation and eNOS antagonism. Furthermore, in knock-in mice harboring constitutively active eNOS, elevated CRP did not invoke insulin resistance. Collectively these findings reveal that by inhibiting eNOS, endothelial FcγRIIB activation by CRP blunts insulin delivery to skeletal muscle to cause insulin resistance. Thus, a series of mechanisms in endothelium that impairs insulin movement has been identified that may contribute to type 2 diabetes pathogenesis.

Design of pathway preferential estrogens that provide beneficial metabolic and vascular effects without stimulating reproductive tissues

There is great medical need for estrogens with favorable pharmacological profiles that support desirable activities for menopausal women, such as metabolic and vascular protection, but that lack stimulatory activities on the breast and uterus. We report the development of structurally novel estrogens that preferentially activate a subset of estrogen receptor (ER) signaling pathways and result in favorable target tissue-selective activity. Through a process of structural alteration of estrogenic ligands that was designed to preserve their essential chemical and physical features but greatly reduced their binding affinity for ERs, we obtained "pathway preferential estrogens" (PaPEs), which interacted with ERs to activate the extranuclear-initiated signaling pathway preferentially over the nuclear-initiated pathway. PaPEs elicited a pattern of gene regulation and cellular and biological processes that did not stimulate reproductive and mammary tissues or breast cancer cells. However, in ovariectomized mice, PaPEs triggered beneficial responses both in metabolic tissues (adipose tissue and liver) that reduced body weight gain and fat accumulation and in the vasculature that accelerated repair of endothelial damage. This process of designed ligand structure alteration represents a novel approach to develop ligands that shift the balance in ER-mediated extranuclear and nuclear pathways to obtain tissue-selective, non-nuclear PaPEs, which may be beneficial for postmenopausal hormone replacement. The approach may also have broad applicability for other members of the nuclear hormone receptor superfamily.

IgG receptor FcγRIIB plays a key role in obesity-induced hypertension

There is a well-recognized association between obesity, inflammation, and hypertension. Why obesity causes hypertension is poorly understood. We previously demonstrated using a C-reactive protein (CRP) transgenic mouse that CRP induces hypertension that is related to NO deficiency. Our prior work in cultured endothelial cells identified the Fcγ receptor IIB (FcγRIIB) as the receptor for CRP whereby it antagonizes endothelial NO synthase. Recognizing known associations between CRP and obesity and hypertension in humans, in the present study we tested the hypothesis that FcγRIIB plays a role in obesity-induced hypertension in mice. Using radiotelemetry, we first demonstrated that the hypertension observed in transgenic mouse-CRP is mediated by the receptor, indicating that FcγRIIB is capable of modifying blood pressure. We then discovered in a model of diet-induced obesity yielding equal adiposity in all study groups that whereas FcγRIIB(+/+) mice developed obesity-induced hypertension, FcγRIIB(-/-) mice were fully protected. Levels of CRP, the related pentraxin serum amyloid P component which is the CRP-equivalent in mice, and total IgG were unaltered by diet-induced obesity; FcγRIIB expression in endothelium was also unchanged. However, whereas IgG isolated from chow-fed mice had no effect, IgG from high-fat diet-fed mice inhibited endothelial NO synthase in cultured endothelial cells, and this was an FcγRIIB-dependent process. Thus, we have identified a novel role for FcγRIIB in the pathogenesis of obesity-induced hypertension, independent of processes regulating adiposity, and it may entail an IgG-induced attenuation of endothelial NO synthase function. Approaches targeting FcγRIIB may potentially offer new means to treat hypertension in obese individuals.

Bazedoxifene and conjugated estrogen prevent diet-induced obesity, hepatic steatosis, and type 2 diabetes in mice without impacting the reproductive tract

Despite the capacity of estrogens to favorably regulate body composition and glucose homeostasis, their use to combat obesity and type 2 diabetes is not feasible, because they promote sex steroid-responsive cancers. The novel selective estrogen receptor modulator (SERM) bazedoxifene acetate (BZA) uniquely antagonizes both breast cancer development and estrogen-related changes in the female reproductive tract. How BZA administered with conjugated estrogen (CE) or alone impacts metabolism is unknown. The effects of BZA or CE + BZA on body composition and glucose homeostasis were determined in ovariectomized female mice fed a Western diet for 10-12 wk. In contrast to vehicle, estradiol (E₂), CE, BZA, and CE + BZA equally prevented body weight gain by 50%. In parallel, all treatments caused equal attenuation of the increase in body fat mass invoked by the diet as well as the increases in subcutaneous and visceral white adipose tissue. Diet-induced hepatic steatosis was attenuated by E₂ or CE, and BZA alone or with CE provided even greater steatosis prevention; all interventions improved pyruvate tolerance tests. Glucose tolerance tests and HOMA-IR were improved by E₂, CE, and CE + BZA. Whereas E₂ or CE alone invoked a uterotrophic response, BZA alone or CE + BZA had negligible impact on the uterus. Thus, CE + BZA affords protection from diet-induced adiposity, hepatic steatosis, and insulin resistance with minimal impact on the female reproductive tract in mice. These combined agents may provide a valuable new means to favorably regulate body composition and glucose homeostasis and combat fatty liver.

Recent insights into non-nuclear actions of estrogen receptor alpha

Estrogen receptors (ER) classically function as transcription factors regulating gene expression. More recently, evidence has continued to accumulate that ER additionally serve numerous important functions remote from the nucleus in a variety of cell types, particularly in non-reproductive tissues. The identification of post-translational modifications of ERα and protein-protein interactions with the receptor that are critical to its non-nuclear functions has afforded opportunities to gain greater insights into these novel non-genomic roles of the receptor. The development of a stable ligand that selectively activates non-nuclear ER has also been invaluable. In this review focused on ERα, recent new understanding of the processes underlying non-nuclear ER action and their in vivo consequences will be discussed. Further research into the non-nuclear capacities by which ER modulate cellular behavior is essential to ultimately harnessing these processes for therapeutic gain in numerous disease contexts.

Non-nuclear-initiated actions of the estrogen receptor protect cortical bone mass

Extensive evidence has suggested that at least some of the effects of estrogens on bone are mediated via extranuclear estrogen receptor α signaling. However, definitive proof for this contention and the extent to which such effects may contribute to the overall protective effects of estrogens on bone maintenance have remained elusive. Here, we investigated the ability of a 17β-estradiol (E2) dendrimer conjugate (EDC), incapable of stimulating nuclear-initiated actions of estrogen receptor α, to prevent the effects of ovariectomy (OVX) on the murine skeleton. We report that EDC was as potent as an equimolar dose of E2 in preventing bone loss in the cortical compartment that represents 80% of the entire skeleton, but was ineffective on cancellous bone. In contrast, E2 was effective in both compartments. Consistent with its effect on cortical bone mass, EDC partially prevented the loss of both vertebral and femoral strength. In addition, EDC, as did E2, prevented the OVX-induced increase in osteoclastogenesis, osteoblastogenesis, and oxidative stress. Nonetheless, the OVX-induced decrease in uterine weight was unaltered by EDC but was restored by E2. These results demonstrate that the protection of cortical bone mass by estrogens is mediated, at least in part, via a mechanism that is distinct from the classic mechanism of estrogen action on reproductive organs.

Coupling of Fcγ receptor I to Fcγ receptor IIb by SRC kinase mediates C-reactive protein impairment of endothelial function

RATIONALE

Elevations in C-reactive protein (CRP) are associated with increased cardiovascular disease risk and endothelial dysfunction. CRP antagonizes endothelial nitric oxide synthase (eNOS) through processes mediated by the IgG receptor Fcγ receptor IIB (FcγRIIB), its immunoreceptor tyrosine-based inhibitory motif, and SH2 domain-containing inositol 5'-phosphatase 1. In mice, CRP actions on eNOS blunt carotid artery re-endothelialization.

OBJECTIVE

How CRP activates FcγRIIB in endothelium is not known. We determined the role of Fcγ receptor I (FcγRI) and the basis for coupling of FcγRI to FcγRIIB in endothelium.

METHODS AND RESULTS

In cultured endothelial cells, FcγRI-blocking antibodies prevented CRP antagonism of eNOS, and CRP activated Src via FcγRI. CRP-induced increases in FcγRIIB immunoreceptor tyrosine-based inhibitory motif phosphorylation and SH2 domain-containing inositol 5'-phosphatase 1 activation were Src-dependent, and Src inhibition prevented eNOS antagonism by CRP. Similar processes mediated eNOS antagonism by aggregated IgG used to mimic immune complex. Carotid artery re-endothelialization was evaluated in offspring from crosses of CRP transgenic mice (TG-CRP) with either mice lacking the γ subunit of FcγRI (FcRγ(-/-)) or FcγRIIB(-/-) mice. Whereas re-endothelialization was impaired in TG-CRP vs wild-type, it was normal in both FcRγ(-/-); TG-CRP and FcγRIIB(-/-); TG-CRP mice.

CONCLUSIONS

CRP antagonism of eNOS is mediated by the coupling of FcγRI to FcγRIIB by Src kinase and resulting activation of SH2 domain-containing inositol 5'-phosphatase 1, and consistent with this mechanism, both FcγRI and FcγRIIB are required for CRP to blunt endothelial repair in vivo. Similar mechanisms underlie eNOS antagonism by immune complex. FcγRI and FcγRIIB may be novel therapeutic targets for preventing endothelial dysfunction in inflammatory or immune complex-mediated conditions.

Non-nuclear estrogen receptor alpha signaling promotes cardiovascular protection but not uterine or breast cancer growth in mice

Steroid hormone receptors function classically in the nucleus as transcription factors. However, recent data indicate that there are also non-nuclear subpopulations of steroid hormone receptors, including estrogen receptors (ERs), that mediate membrane-initiated signaling of unclear basis and significance. Here we have shown that an estrogen-dendrimer conjugate (EDC) that is excluded from the nucleus stimulates endothelial cell proliferation and migration via ERalpha, direct ERalpha-Galphai interaction, and endothelial NOS (eNOS) activation. Analysis of mice carrying an estrogen response element luciferase reporter, ER-regulated genes in the mouse uterus, and eNOS enzyme activation further indicated that EDC specifically targets non-nuclear processes in vivo. In mice, estradiol and EDC equally stimulated carotid artery reendothelialization in an ERalpha- and G protein-dependent manner, and both agents attenuated the development of neointimal hyperplasia following endothelial injury. In contrast, endometrial carcinoma cell growth in vitro and uterine enlargement and MCF-7 cell breast cancer xenograft growth in vivo were stimulated by estradiol but not EDC. Thus, EDC is a non-nuclear selective ER modulator (SERM) in vivo, and in mice, non-nuclear ER signaling promotes cardiovascular protection. These processes potentially could be harnessed to provide vascular benefit without increasing the risk of uterine or breast cancer.

C-reactive protein inhibits insulin activation of endothelial nitric oxide synthase via the immunoreceptor tyrosine-based inhibition motif of FcgammaRIIB and SHIP-1

Insulin promotes the cardiovascular protective functions of the endothelium including NO production by endothelial NO synthase (eNOS), which it stimulates via Akt kinase which phosphorylates eNOS Ser1179. C-reactive protein (CRP) is an acute-phase reactant that is positively correlated with cardiovascular disease risk in patients with type 2 diabetes. We previously showed that CRP inhibits eNOS activation by insulin by blunting Ser1179 phosphorylation. We now elucidate the underlying molecular mechanisms. We first show in mice that CRP inhibits insulin-induced eNOS phosphorylation, indicating that these processes are operative in vivo. In endothelial cells we find that CRP attenuates insulin-induced Akt phosphorylation, and CRP antagonism of eNOS is negated by expression of constitutively active Akt; the inhibitory effect of CRP on Akt is also observed in vivo. A requirement for the IgG receptor FcgammaRIIB was demonstrated in vitro using blocking antibody, and reconstitution experiments with wild-type and mutant FcgammaRIIB in NIH3T3IR cells revealed that these processes require the ITIM (immunoreceptor tyrosine-based inhibition motif) of the receptor. Furthermore, we find that endothelium express SHIP-1 (Src homology 2 domain-containing inositol 5'-phosphatase 1), that CRP induces SHIP-1 stimulatory phosphorylation in endothelium in culture and in vivo, and that SHIP-1 knockdown by small interfering RNA prevents CRP antagonism of insulin-induced eNOS activation. Thus, CRP inhibits eNOS stimulation by insulin via FcgammaRIIB and its ITIM, SHIP-1 activation, and resulting blunted activation of Akt. These findings provide mechanistic linkage among CRP, impaired insulin signaling in endothelium, and greater cardiovascular disease risk in type 2 diabetes.

The scavenger receptor class B type I adaptor protein PDZK1 maintains endothelial monolayer integrity

Circulating levels of high-density lipoprotein (HDL) cholesterol are inversely related to the risk of cardiovascular disease, and HDL and the HDL receptor scavenger receptor class B type I (SR-BI) initiate signaling in endothelium through src that promotes endothelial NO synthase activity and cell migration. Such signaling requires the C-terminal PDZ-interacting domain of SR-BI. Here we show that the PDZ domain-containing protein PDZK1 is expressed in endothelium and required for HDL activation of endothelial NO synthase and cell migration; in contrast, endothelial cell responses to other stimuli, including vascular endothelial growth factor, are PDZK1-independent. Coimmunoprecipitation experiments reveal that Src interacts with SR-BI, and this process is PDZK1-independent. PDZK1 also does not regulate SR-BI abundance or plasma membrane localization in endothelium or HDL binding or cholesterol efflux. Alternatively, PDZK1 is required for HDL/SR-BI to induce Src phosphorylation. Paralleling the in vitro findings, carotid artery reendothelialization following perivascular electric injury is absent in PDZK1-/- mice, and this phenotype persists in PDZK1-/- mice with genetic reconstitution of PDZK1 expression in liver, where PDZK1 modifies SR-BI abundance. Thus, PDZK1 is uniquely required for HDL/SR-BI signaling in endothelium, and through these mechanisms, it is critically involved in the maintenance of endothelial monolayer integrity.

Direct Interactions with Gαi and Gβγ Mediate Nongenomic Signaling by Estrogen Receptor α

Estrogen induces G protein-dependent nongenomic signaling in a variety of cell types via the activation of a plasma membrane-associated subpopulation of estrogen receptor α (ERα). Using pull-down experiments with purified recombinant proteins, we now demonstrate that ERα binds directly to Gαi and Gβγ. Mutagenesis and the addition of blocking peptide reveals that this occurs via amino acids 251–260 and 271–595 of ERα, respectively. Studies of ERα complexed with heterotrimeric G proteins further show that estradiol causes the release of both Gαi and Gβγ without stimulating GTP binding to Gαi. Moreover, in COS-7 cells, the disruption of ERα-Gαi interaction by deletion mutagenesis of ERα or expression of blocking peptide, as well as Gβγ sequestration with β-adrenergic receptor kinase C terminus, prevents nongenomic responses to estradiol including src and erk activation. In endothelial cells, the disruption of ERα-Gαi interaction prevents estradiol-induced nitric oxide synthase activation and the resulting attenuation of monocyte adhesion that contributes to estrogen-related cardiovascular protection. Thus, through direct interactions, ERα mediates a novel mechanism of G protein activation that provides greater diversity of function of both the steroid hormone receptor and G proteins.

High-density lipoprotein promotes endothelial cell migration and reendothelialization via scavenger receptor-B type I

Vascular disease risk is inversely related to circulating levels of high-density lipoprotein (HDL) cholesterol. However, the mechanisms by which HDL provides vascular protection are unclear. The disruption of endothelial monolayer integrity is an important contributing factor in multiple vascular disorders, and vascular lesion severity is tempered by enhanced endothelial repair. Here, we show that HDL stimulates endothelial cell migration in vitro in a nitric oxide-independent manner via scavenger receptor B type I (SR-BI)-mediated activation of Rac GTPase. This process does not require HDL cargo molecules, and it is dependent on the activation of Src kinases, phosphatidylinositol 3-kinase, and p44/42 mitogen-activated protein kinases. Rapid initial stimulation of lamellipodia formation by HDL via SR-BI, Src kinases, and Rac is also demonstrable. Paralleling the in vitro findings, carotid artery reendothelialization after perivascular electric injury is blunted in apolipoprotein A-I(-/-) mice, and reconstitution of apolipoprotein A-I expression rescues normal reendothelialization. Furthermore, reendothelialization is impaired in SR-BI(-/-) mice. Thus, HDL stimulates endothelial cell migration via SR-BI-initiated signaling, and these mechanisms promote endothelial monolayer integrity in vivo.

Molecular basis of estrogen-induced cyclooxygenase type 1 upregulation in endothelial cells

Estrogen upregulates cyclooxygenase-1 (COX-1) expression in endothelial cells. To determine the basis of this process, studies were performed in ovine endothelial cells transfected with the human COX-1 promoter fused to luciferase. Estradiol (E2) caused activation of the COX-1 promoter with maximal stimulation at 10(-8) mol/L E2, and the response was mediated by either ERalpha or ERbeta. Mutagenesis revealed a primary role for a putative Sp1 binding motif at -89 (relative to the ATG codon) and lesser involvement of a consensus Sp1 site at -111. Electrophoretic mobility shift assays yielded a single complex with the site at -89, and supershift analyses implicated AP-2alpha and ERalpha, and not Sp1, in protein-DNA complex formation. In endothelial cells with minimal endogenous ER, the transfection of ERalpha mutants lacking the DNA binding domain or primary nuclear localization signals caused 4-fold greater stimulation of promoter activity with E2 than wild-type ERalpha. In contrast, mutant ERalpha lacking the A-B domains was inactive. Thus, estrogen-mediated upregulation of COX-1 in endothelium is uniquely independent of direct ERalpha-DNA binding and instead entails protein-DNA interaction involving AP-2alpha and ERalpha at a proximal regulatory element. In addition, the process may be initiated by cytoplasmic ERalpha, and critical receptor elements reside within the amino terminus.

Dissecting the basis of nongenomic activation of endothelial nitric oxide synthase by estradiol: role of ERalpha domains with known nuclear functions

Estradiol stimulates endothelial nitric oxide synthase (eNOS) via the activation of plasma membrane (PM)-associated estrogen receptor (ER) alpha. The process requires Src and erk signaling and eNOS phosphorylation by phosphoinositide 3-kinase (PI3 kinase)-Akt kinase, with Src and PI3 kinase associating with ERalpha upon ligand activation. To delineate the basis of nongenomic eNOS stimulation, the potential roles of ERalpha domains necessary for classical nuclear function were investigated in COS-7 cells. In cross-linking studies, estradiol-17beta (E2) caused PM-associated ERalpha to form dimers. However, eNOS activation by E2 was unaltered for a dimerization-deficient mutant ERalpha (ERalphaL511R). In contrast, ERalpha mutants lacking the nuclear localization signals (NLS), NLS2,3 (ERalphaDelta250-274) or the DNA binding domain (ERalphaDelta185-251), which targeted normally to PM and caveolae/rafts, were incapable of activating eNOS. The loss of NLS2/NLS3 prevented Src and erk activation, and it altered ligand-induced PI3 kinase-ERalpha interaction and prevented eNOS phosphorylation. Loss of the DNA binding domain did not change E2 activation of Src or erk, but ligand-induced PI3 kinase-ERalpha binding and eNOS phosphorylation did not occur. Thus, dimerization is not required for ERalpha coupling to eNOS; however, NLS2/NLS3 plays a role in Src activation, and the DNA binding region is involved in the dynamic interaction between ERalpha and PI3 kinase.

Estrogen causes dynamic alterations in endothelial estrogen receptor expression

Estrogen receptor (ER)alpha mediates many of the effects of estrogen on the vascular endothelium. The purpose of the present study was to determine whether estrogen modifies endothelial ERalpha expression. In experiments in cultured ovine endothelial cells, physiological concentrations of 17beta-estradiol (E2, 10(-10) to 10(-8) mol/L) caused an increase in ERalpha protein abundance that was evident after 6 hours of hormone exposure. Shorter (2-hour) E2 treatment caused ERalpha downregulation. In contrast to the upregulation in ERalpha after long-term E2, the expression of the other ER isoform, ERbeta, was downregulated. Both nonselective ER antagonism with ICI 182,780 and the inhibition of gene transcription with actinomycin D blocked the increase in ERalpha with E2. In studies using the human ERalpha gene promoter P-1 coupled to luciferase, an increase in ERalpha gene transcription was evident in endothelial cells within 4 hours of E2 exposure. The transcriptional activation was fully blocked by ICI 182,780, whereas the specific ERbeta antagonist RR-tetrahydrochrysene yielded partial blockade. Overexpression of ERalpha or ERbeta caused comparable 10- and 8-fold increases, respectively, in ERalpha promoter activation by E2. Thus, long-term exposure to E2 upregulates ERalpha expression in endothelial cells through the actions of either ERalpha or ERbeta on ERalpha gene transcription; in contrast, E2 causes ERbeta downregulation in the endothelium. We postulate that E2-induced changes in ERalpha and ERbeta expression modify the effects of the hormone on vascular endothelium.

Estrogen modulation of endothelial nitric oxide synthase

Over the past decade, clinical and basic research has demonstrated that estrogen has a dramatic impact on the response to vascular injury and the development of atherosclerosis. Further work has indicated that this is at least partially mediated by an enhancement in nitric oxide (NO) production by the endothelial isoform of NO synthase (eNOS) due to increases in both eNOS expression and level of activation. The effects on eNOS abundance are primarily mediated at the level of gene transcription, and they are dependent on estrogen receptors (ERs), which classically serve as transcription factors, but they are independent of estrogen response element action. Estrogen also has potent nongenomic effects on eNOS activity mediated by a subpopulation of ERalpha localized to caveolae in endothelial cells, where they are coupled to eNOS in a functional signaling module. These observations, which emphasize dependence on cell surface-associated receptors, provide evidence for the existence of a steroid receptor fast-action complex, or SRFC, in caveolae. Estrogen binding to ERalpha on the SRFC in caveolae leads to G(alphai) activation, which mediates downstream events. The downstream signaling includes activation of tyrosine kinase-MAPK and Akt/protein kinase B signaling, stimulation of heat shock protein 90 binding to eNOS, and perturbation of the local calcium environment, leading to eNOS phosphorylation and calmodulin-mediated eNOS stimulation. These unique genomic and nongenomic processes are critical to the vasoprotective and atheroprotective characteristics of estrogen. In addition, they serve as excellent paradigms for further elucidation of novel mechanisms of steroid hormone action.

ERbeta has nongenomic action in caveolae

ERalpha and ERbeta serve classically as transcription factors, and ERalpha also mediates nongenomic responses to E2 such as the activation of endothelial nitric oxide synthase (eNOS). In contrast, the nongenomic capacities of endogenous ERbeta are poorly understood. We evaluated eNOS activation by E2 in cultured endothelial cells that express endogenous ERbeta to determine whether the ERbeta isoform has nongenomic action and to reveal the subcellular locale of that function. A subpopulation of ERbeta was localized to the endothelial cell plasma membrane, overexpression of ERbeta enhanced rapid eNOS stimulation by E2, and the response to endogenous ER activation was inhibited by the ERbeta-selective antagonist RR-tetrahydrochrysene (THC). eNOS activation through ERbeta was reconstituted and shown to occur independent of ERalpha in COS-7 cells, and ERbeta protein in COS-7 was directed to the plasma membrane. THC also blunted E2 activation of eNOS in isolated endothelial cell plasma membranes. Furthermore, ERbeta protein was detected and THC attenuated E2 stimulation of eNOS in isolated endothelial cell caveolae, and functional ERbeta-eNOS coupling was recapitulated in caveolae from transfected COS-7 cells. These findings in the ER-eNOS signaling paradigm indicate that endogenous ERbeta has nongenomic action in caveolae.

Rapid activation of endothelial NO synthase by estrogen: evidence for a steroid receptor fast-action complex (SRFC) in caveolae

Estrogen has important atheroprotective and vasoactive properties related to its capacity to stimulate nitric oxide (NO) production by endothelial NO synthase. Previous work has shown that these effects are mediated by estrogen receptor (ER) alpha functioning in a nongenomic manner via calcium-dependent, MAP kinase-dependent mechanisms. Recent studies have demonstrated that estradiol (E(2)) activates eNOS in isolated endothelial plasma membranes in the absence of added calcium, calmodulin or eNOS cofactors. Studies of blockade by ICI 182,780 and by ER alpha antibody, and also immunoidentification experiments indicate that the process is mediated by a subpopulation of plasma membrane-associated ER alpha. Fractionation of endothelial cell plasma membranes has further revealed that ER alpha protein is localized to caveolae, and that E(2) causes stimulation of eNOS in isolated caveolae which is ER-dependent and calcium-dependent, whereas noncaveolae membranes are insensitive. Furthermore, in intact endothelial cells the activation of eNOS by E(2) is prevented by pertussis toxin, and exogenous GDP beta S inhibits the response in isolated plasma membranes. Coimmunoprecipitation studies have shown that E(2) exposure causes interaction between ER alpha and G(alpha i) on the plasma membrane, and eNOS activation by E(2) is enhanced by overexpression of G(alpha i) and attenuated by expression of a protein regulator of G protein signaling (RGS), RGS4. Thus, a subpopulation of ER alpha is localized to caveolae in endothelial cells, where they are coupled via G(alpha i) to eNOS in a functional signaling module. Emphasizing the dependence on cell surface-associated receptors, these observations provide evidence for the existence of a steroid receptor fast-action complex, or SRFC, in caveolae.

ERbeta has nongenomic action in caveolae

ERalpha and ERbeta serve classically as transcription factors, and ERalpha also mediates nongenomic responses to E2 such as the activation of endothelial nitric oxide synthase (eNOS). In contrast, the nongenomic capacities of endogenous ERbeta are poorly understood. We evaluated eNOS activation by E2 in cultured endothelial cells that express endogenous ERbeta to determine whether the ERbeta isoform has nongenomic action and to reveal the subcellular locale of that function. A subpopulation of ERbeta was localized to the endothelial cell plasma membrane, overexpression of ERbeta enhanced rapid eNOS stimulation by E2, and the response to endogenous ER activation was inhibited by the ERbeta-selective antagonist RR-tetrahydrochrysene (THC). eNOS activation through ERbeta was reconstituted and shown to occur independent of ERalpha in COS-7 cells, and ERbeta protein in COS-7 was directed to the plasma membrane. THC also blunted E2 activation of eNOS in isolated endothelial cell plasma membranes. Furthermore, ERbeta protein was detected and THC attenuated E2 stimulation of eNOS in isolated endothelial cell caveolae, and functional ERbeta-eNOS coupling was recapitulated in caveolae from transfected COS-7 cells. These findings in the ER-eNOS signaling paradigm indicate that endogenous ERbeta has nongenomic action in caveolae.

Estrogen acutely activates prostacyclin synthesis in ovine fetal pulmonary artery endothelium

Prostacyclin (PGI(2)) is a key mediator of pulmonary vasodilation during perinatal cardiopulmonary transition, at a time when fetal plasma estrogen levels are rising. We have previously shown that estradiol-17beta (E(2)) rapidly stimulates nitric oxide production by ovine fetal pulmonary artery endothelial cells (PAEC), and that this occurs through nongenomic mechanisms which are calcium- and tyrosine kinase-mitogen-activated protein (MAP) kinase-dependent. In the present study, we determined if E(2) acutely activates PGI(2) production in PAEC. E(2) (10(-8) M for 15 min) caused a 52% increase in PGI(2), the threshold concentration was 10(-10) M E(2), the effect occurred within 5 min, and it was not related to changes in cyclooxygenase type 1 (COX-1) or COX-2 abundance. Estrogen receptor (ER) alpha and ER beta proteins and mRNAs were found to be constitutively expressed in PAEC, and PGI(2) stimulation with E(2) was fully blocked by both ER antagonism with ICI 182,780, which is not selective for either ER isoform, and the ER beta-specific antagonist RR-tetrahydrochrysene. The rapid response to E(2) was also inhibited by calcium chelation, whereas genistein- or PD98059-induced inhibition of tyrosine kinase and MAP kinase kinase, respectively, had no effect. Thus, E(2) causes rapid stimulation of PGI(2) synthesis in fetal PAEC, this process is mediated by ER beta, and it is calcium-dependent and tyrosine kinase-MAP kinase-independent. These mechanisms may play a role in pulmonary vasodilation in the perinatal period.

Plasma Membrane Estrogen Receptors Are Coupled to Endothelial Nitric-oxide Synthase through Gαi

Estrogen causes rapid endothelial nitric oxide (NO) production because of the activation of plasma membrane-associated estrogen receptors (ER) coupled to endothelial NO synthase (eNOS). In the present study, we determined the role of G proteins in eNOS activation by estrogen. Estradiol-17beta (E(2), 10(-8) m) and acetylcholine (10(-5) m) caused comparable increases in NOS activity (15 min) in intact endothelial cells that were fully blocked by pertussis toxin (Ptox). In addition, exogenous guanosine 5'-O-(2- thiodiphosphate) inhibited E(2)-mediated eNOS stimulation in isolated endothelial plasma membranes, and Ptox prevented enzyme activation by E(2) in COS-7 cells expressing ERalpha and eNOS. Coimmunoprecipitation studies of plasma membranes from COS-7 cells transfected with ERalpha and specific Galpha proteins demonstrated E(2)-stimulated interaction between ERalpha and Galpha(i) but not between ERalpha and either Galpha(q) or Galpha(s); the observed ERalpha-Galpha(i) interaction was blocked by the ER antagonist ICI 182,780 and by Ptox. E(2)-stimulated ERalpha-Galpha(i) interaction was also demonstrable in endothelial cell plasma membranes. Cotransfection of Galpha(i) into COS-7 cells expressing ERalpha and eNOS yielded a 3-fold increase in E(2)-mediated eNOS stimulation, whereas cotransfection with a protein regulator of G protein signaling, RGS4, inhibited the E(2) response. These findings indicate that eNOS stimulation by E(2) requires plasma membrane ERalpha coupling to Galpha(i) and that activated Galpha(i) mediates the requisite downstream signaling events. Thus, novel G protein coupling enables a subpopulation of ERalpha to initiate signal transduction at the cell surface. Similar mechanisms may underly the nongenomic actions of other steroid hormones.

Estrogen receptor alpha and endothelial nitric oxide synthase are organized into a functional signaling module in caveolae

Estrogen causes nitric oxide (NO)-dependent vasodilation due to estrogen receptor (ER) alpha-mediated, nongenomic activation of endothelial NO synthase (eNOS). The subcellular site of interaction between ERalpha and eNOS was determined in studies of isolated endothelial cell plasma membranes. Estradiol (E(2), 10(-8) mol/L) caused an increase in eNOS activity in plasma membranes in the absence of added calcium, calmodulin, or eNOS cofactors, which was blocked by ICI 182,780 and ERalpha antibody. Immunoidentification studies detected the same 67-kDa protein in endothelial cell nucleus, cytosol, and plasma membrane. Plasma membranes from COS-7 cells expressing eNOS and ERalpha displayed ER-mediated eNOS stimulation, whereas membranes from cells expressing eNOS alone or ERalpha plus a myristoylation-deficient mutant eNOS were insensitive. Fractionation of endothelial cell plasma membranes revealed ERalpha protein in caveolae, and E(2) caused stimulation of eNOS in isolated caveolae that was ER-dependent; noncaveolae membranes were insensitive. Acetylcholine and bradykinin also activated eNOS in isolated caveolae. Furthermore, the effect of E(2) on eNOS in caveolae was prevented by calcium chelation. Thus, a subpopulation of ERalpha is localized to endothelial cell caveolae where they are coupled to eNOS in a functional signaling module that may regulate the local calcium environment. The full text of this article is available at http://www.circresaha.org.

Molecular characterization of methylmalonate semialdehyde dehydrogenase deficiency

Three patients have been reported with (putative) methylmalonic semialdehyde dehydrogenase (MMSDH) deficiency. The urine metabolic pattern was strikingly different in all, including beta-alanine, 3-hydroxypropionic acid, both isomers of 3-amino- and 3-hydroxyisobutyric acids in one and 3-hydroxyisobutyric and lactic acids in a second, and mild methylmalonic aciduria in a third patient. In an effort to clarify these disparate metabolite patterns, we completed the cDNA structure, and characterized the genomic structure of human MMSDH gene in order to undertake molecular analysis. Only the first patient had alterations in the MMSDH coding region, revealing homozygosity for a 1336G > A transversion, which leads to substitution of arginine for highly conserved glycine at amino acid 446. No abnormalities of the MMSDH cDNA were detected in the other patients. These data provide the first molecular characterization of an inborn error of metabolism specific to the L-valine catabolic pathway.

Molecular basis of cell-specific endothelial nitric-oxide synthase expression in airway epithelium

Nitric oxide (NO) plays an important role in airway function, and endothelial NO synthase (eNOS) is expressed in airway epithelium. To determine the basis of cell-specific eNOS expression in airway epithelium, studies were performed in NCI-H441 human bronchiolar epithelial cells transfected with the human eNOS promoter fused to luciferase. Transfection with 1624 base pairs of sequence 5' to the initiation ATG (position -1624) yielded a 19-fold increase in promoter activity versus vector alone. No activity was found in lung fibroblasts, which do not express eNOS. 5' deletions from -1624 to -279 had modest effects on promoter activity in H441 cells. Further deletion to -248 reduced activity by 65%, and activity was lost with deletion to -79. Point mutations revealed that the GATA binding motif at -254 is mandatory for promoter activity and that the positive regulatory element between -248 and -79 is the Sp1 binding motif at -125. Electrophoretic mobility shift assays yielded two complexes with the GATA site and three with the Sp1 site. Immunodepletion with antiserum to GATA-2 prevented formation of the slowest migrating GATA complex, and antiserum to Sp1 supershifted the slowest migrating Sp1 complex. An electrophoretic mobility shift assay with H441 versus fibroblast nuclei revealed that the slowest migrating GATA complex is unique to airway epithelium. Thus, cell-specific eNOS expression in airway epithelium is dependent on the interaction of GATA-2 with the core eNOS promoter, and the proximal Sp1 binding site is also an important positive regulatory element.

Establishment of an immortalized fetal intrapulmonary artery endothelial cell line

The investigation of fetal pulmonary endothelial cell gene expression and function has been limited by the requirement for primary cells. In an effort to establish an immortalized cell line, ovine fetal pulmonary artery endothelial cells (PAECs; passage 5) were permanently transfected with the E6 and E7 open reading frames of human papillomavirus type 16, and phenotypes related to nitric oxide (NO) production were evaluated up to passage 28. Acetylated low-density lipoprotein uptake, endothelial NO synthase (eNOS) expression, and proliferation rates were unaltered by immortalization. Acetylcholine-stimulated eNOS activity was 218-255% above basal levels in immortalized cells, and this was comparable to the 250% increase seen in primary PAECs (passage 6). eNOS was also acutely activated by estradiol to levels 197-309% above basal, paralleling the stimulation obtained in primary cells. In addition, the expression of estrogen receptor-alpha, which has recently been shown to mediate the acute response in primary PAECs, was conserved. Thus fetal PAECs transfected with E6 and E7 show no signs of senescence with passage, and mechanisms of NO production, including those mediated by estradiol, are conserved. Immortalized PAECs will provide an excellent model for further studies of eNOS gene expression and function in fetal pulmonary endothelium.

Characterization of phosphomevalonate kinase: chromosomal localization, regulation, and subcellular targeting

Phosphomevalonate kinase catalyzes the conversion of mevalonate-5-phosphate to mevalonate-5-diphosphate and was originally believed to be a cytosolic enzyme. In this study we have localized the phosphomevalonate kinase gene to chromosome 1p13-1q22-23 and present a genomic map indicating that the gene spans more than 8.4 kb in the human genome. Furthermore, we show that message levels and enzyme activity of rat liver phosphomevalonate kinase are regulated in response to dietary sterol levels and that this regulation is coordinate with 3-hydroxy-3-methylglutaryl coenzyme A reductase, the rate-limiting enzyme of cholesterol biosynthesis. In addition, we demonstrate that phosphomevalonate kinase is a peroxisomal protein which requires the C-terminal peroxisomal targeting signal, Ser-Arg-Leu, for localization to the organelle.

Two exon-skipping mutations as the molecular basis of succinic semialdehyde dehydrogenase deficiency (4-hydroxybutyric aciduria)

Succinic semialdehyde dehydrogenase (SSADH) deficiency, a rare metabolic disorder of 4-aminobutyric acid degradation, has been identified in approximately 150 patients. Affected individuals accumulate large quantities of 4-hydroxybutyric acid, a compound with a wide range of neuropharmacological activities, in physiological fluids. As a first step in beginning an investigation of the molecular genetics of SSADH deficiency, we have utilized SSADH cDNA and genomic sequences to identify two point mutations in the SSADH genes derived from four patients. These mutations, identified by standard methods of reverse transcription, PCR, dideoxy-chain termination, and cycle sequencing, alter highly conserved sequences at intron/exon boundaries and prevent the RNA-splicing apparatus from properly recognizing the normal splice junction. Each family segregated a mutation in a different splice site, resulting in exon skipping and, in one case, a frameshift and premature termination and, in the other case, an in-frame deletion in the resulting protein. Family members, including parents and siblings of these patients, were shown to be heterozygotes for the splicing abnormality, providing additional evidence for autosomal recessive inheritance. Our results provide the first evidence that 4-hydroxybutyric aciduria, resulting from SSADH deficiency, is the result of genetic defects in the human SSADH gene.

Characterization of the mevalonate kinase 5'-untranslated region provides evidence for coordinate regulation of cholesterol biosynthesis

Using a probe derived from the 5'-untranslated region of the human mevalonate kinase (MK) cDNA, we screened a lambda gt 11 genomic library and obtained a single clone containing the 5' untranslated region of the gene. Nucleotide sequencing identified several putative regulatory elements, including two Sp1 (GC box) elements and a CCAAT box. A canonical TATA box was not detected. Directly adjacent to one Sp1 element was a sterol regulatory element (SRE), 5'-CACCCCAG-3', which was a 7/8 base pair match to the consensus sequences identified in the genes encoding 3-hydroxy-3-methyl-glutaryl-coenzyme A synthase and reductase, and the LDL receptor. There was no Sp1 element upstream of the SRE. Northern blot analysis in human CRL1508T cells revealed that quantities of MK poly A+ RNA increased for cells grown in the presence of lipid-deficient calf serum, and further increased upon addition of 1 microM lovastatin. Primer extension analysis with human poly A+ RNA suggested at least 4 transcription initiation sites downstream from the CCAAT box. To assess sterol responsiveness of transcription initiation, a 1.4 kb genomic fragment upstream of the translational start site was fused to the pSV2cat vector for transient expression in COS-7 cells, with chloramphenicol acetyltransferase (CAT) as the reporter gene. This construct demonstrated modest levels of CAT expression which was induced > 2-fold when cells were grown in lipoprotein-deficient calf serum. Our data provide further evidence for coordinate regulation of cholesterol biosynthesis in response to sterol.

Post-translational regulation of mevalonate kinase by intermediates of the cholesterol and nonsterol isoprene biosynthetic pathways

To assess the potential for feedback inhibition by isoprene intermediates in the cholesterol and nonsterol isoprene biosynthetic pathway, we expressed human cDNAs encoding mevalonate kinase (MKase), phosphomevalonate kinase (PMKase), and mevalonate diphosphate decarboxylase (MDDase) as fusion proteins in Escherichia coli DH5alpha, and purified these proteins by affinity chromatography. Several phosphorylated and non-phosphorylated isoprenes were analyzed as inhibitors of the enzymes using a standard spectrophotometric assay. Of the three proteins, only MKase was inhibited through competitive interaction at the ATP-binding site. The intermediates studied (and their relative inhibitory capacity) were: geranylgeranyl-diphosphate (GGPP, C20) > farnesyl-diphosphate (FPP, C15) > geranyl-diphosphate (GPP, C10) > isopentenyl-diphosphate (IPP, C5) > or = 3,3-dimethylallyl-diphosphate (DMAPP, C5) > farnesol (C15) > dolichol-phosphate (DP, C(80-100)). Mevalonate-diphosphate, geraniol, and dolichol were not inhibitors. Our data further define the spectrum of physiologic inhibitors of MKase, and provide the first evidence for feedback inhibition of MKase by a nonsterol isoprene produced by the branched pathway, dolichol-phosphate. These results provide additional evidence that MKase may occupy a central regulatory role in the control of cholesterol and nonsterol isoprene biosynthesis.

Identification of an active site alanine in mevalonate kinase through characterization of a novel mutation in mevalonate kinase deficiency

Sequencing of polymerase chain reaction-amplified cDNAs from cultured cells of three patients with mevalonate kinase deficiency revealed a G --> A transversion at nucleotide 1000 of the coding region, converting alanine to threonine at position 334 (A334T). To characterize this defect, we expressed wild-type and mutant cDNAs in Escherichia coli as the glutathione S-transferase fusion proteins, with purification by affinity chromatography. SDS-polyacrylamide gel electrophoresis analysis for wild-type and mutant fusion proteins indicated an expected molecular mass of 42-43 kDa. Kinetic characterization of the wild-type fusion protein yielded Km values of 150 +/- 23 and 440 +/- 190 microM (mean +/- S.E.) for substrates (RS)-mevalonate and ATP, respectively. Expressed wild-type mevalonate kinase (MKase) had a maximum velocity of 13.6 +/- 1.4 units/mg of protein (n = 22, +/-S.E.), whereas the A334T mutation yielded an enzyme with average Vmax of 0.26 +/- 0.02 unit/mg of protein (n = 6, +/-S.E.), representing a decrease to 1.4% of control Vmax. Restriction digestion with HhaI, in conjunction with direct sequencing of cDNAs, revealed that two patients were homozygous and one heterozygous for the A334T allele, establishing autosomal recessive inheritance within families. Although the A334T enzyme had a normal Km for ATP of 680 +/- 226 microM (n = 3, +/-S.E.), the Michaelis constant for (RS)-mevalonate was increased >30-fold to 4623 +/- 1167 microM (n = 4, +/-S.E.) under standard assay conditions. Comparable kinetic results were obtained using extracts of lymphoblasts, which were homozygous for the A334T allele. Alanine 334 is invariant in MKase from bacteria to man and located in a glycine-rich region postulated to have homology with ATP-binding sequences. Our results indicate that the bacterial expression system for human MKase will provide a useful model system in which to analyze inherited mutations and identify the first active site residue in MKase associated with stabilization of mevalonate binding.

Molecular cloning of human phosphomevalonate kinase and identification of a consensus peroxisomal targeting sequence

Two overlapping cDNAs which encode human liver phosphomevalonate kinase (PMKase) were isolated. The human PMKase cDNAs predict a 191-amino acid protein with a molecular weight of 21,862, consistent with previous reports for mammalian PMKase (Mr = 21,000-22,500). Further verification of the clones was obtained by expression of PMKase activity in bacteria using a composite 1024-base pair cDNA clone. Northern blot analysis of several human tissues revealed a doublet of transcripts at approximately 1 kilobase (kb) in heart, liver, skeletal muscle, kidney, and pancreas and lower but detectable transcript levels in brain, placenta, and lung. Analysis of transcripts from human lymphoblasts subcultured in lipid-depleted sera (LDS) and LDS supplemented with lovastatin indicated that PMKase gene expression is subject to regulation by sterol at the level of transcription. Southern blotting indicated that PMKase is a single copy gene covering less than 15 kb in the human genome. The human PMKase amino acid sequence contains a consensus peroxisomal targeting sequence (PTS-1), Ser-Arg-Leu, at the C terminus of the protein. This is the first report of a cholesterol biosynthetic protein which contains a consensus PTS-1, providing further evidence for the concept that early cholesterol and nonsterol isoprenoid biosynthesis may occur in the peroxisome.

Enzymatic and immunologic identification of succinic semialdehyde dehydrogenase in rat and human neural and nonneural tissues

We have identified succinic semialdehyde dehydrogenase protein in rat and human neural and nonneural tissues. Tissue localization was determined by enzymatic assay and by western immunoblotting using polyclonal antibodies raised in rabbit against the purified rat brain protein. Although brain shows the highest level of succinic semialdehyde dehydrogenase activity, substantial amounts of enzyme activity occur in mammalian liver, pituitary, heart, and ovary. We further demonstrate the absence of succinic semialdehyde dehydrogenase enzyme activity and protein in brain, liver, and kidney tissue samples from an individual affected with succinic semialdehyde dehydrogenase deficiency, thereby verifying the specificity of our antibodies.

Molecular cloning of the mature NAD(+)-dependent succinic semialdehyde dehydrogenase from rat and human. cDNA isolation, evolutionary homology, and tissue expression

Three rat brain cDNA clones approximately 3500, 1465, and 1135 base pairs in length encoding succinic semialdehyde dehydrogenase (SSADH; EC 1.2.1.24) were isolated from two cDNA libraries using a polymerase chain reaction derived probe. Restriction mapping and DNA sequencing revealed that the 3.5-kilobase clone contained an 84-base pair (28 amino acid) insert in the coding region. Composite clones encoding mature SSADH predicted proteins with 488 amino acids (M(r) = 52,188) when including the insert and 460 amino acids (M(r) = 48,854) without the insert. The cDNA clones were confirmed by expression of enzyme activity in bacteria and protein sequence data obtained from sequencing purified rat brain SSADH. Two human liver SSADH cDNA clones of 1091 and 899 base pairs were also isolated. Human and rat SSADH share 83 and 91% identity in nucleotide and protein sequence, respectively. Northern blot analysis revealed two differentially expressed SSADH transcripts of approximately 2.0 and 6.0 kilobases in both rat and human tissues. Human genomic Southern blots indicate that the two SSADH transcripts are encoded by a greater than 20-kilobase single copy gene. Mammalian SSADH contains significant homology to bacterial NADP(+)-succinic semialdehyde dehydrogenase (EC 1.2.1.16) and conserved regions of general aldehyde dehydrogenases (EC 1.2.1.3), suggesting it is a member of the aldehyde dehydrogenase superfamily of proteins.

Succinic semialdehyde dehydrogenase from mammalian brain: subunit analysis using polyclonal antiserum

1. NAD(+)-dependent succinic semialdehyde dehydrogenase was purified to apparent homogeneity from rat brain and highly purified from human brain. 2. Molecular exclusion chromatography of the purified enzymes on Sephadex G-150 and G-200 revealed M(r) values of 203,000 and 191,000 for rat and human, respectively. 3. Electrophoresis on sodium dodecylsulfate polyacrylamide gels revealed a single subunit of M(r) 54,000 for rat and 58,000 for human. Isoelectric focusing of the purified rat enzyme yielded a pI of 6.1. 4. For both proteins, Km values for short-chain aldehydes acetaldehyde and propionaldehyde ranged from 0.33 to 2.5 mM; Km values for succinic semialdehyde were in the 2-4 microM range. 5. The subunit structure of both enzymes was investigated in brain extracts and purified preparations by immunoblotting, using a polyclonal rabbit antiserum against the purified rat brain enzyme. 6. For rat and human extracts, single bands were detected at M(r) 54,000 and 58,000, comparable to findings in the purified preparations. Immunoblotting analyses in other species (guinea pig, hamster, mouse and rabbit) revealed single subunits of M(r) 54,000-56,500.